Colloidal oxide-based heterostructured nanocrystals

Colloidal oxide-based heterostructured nanocrystals

Colloidal oxide-based heterostructured nanocrystals 13 P. Davide Cozzolia,b,c, Concetta Nobilea a CNR NANOTEC—Institute of Nanotechnology, UOS of Le...

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Colloidal oxide-based heterostructured nanocrystals

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P. Davide Cozzolia,b,c, Concetta Nobilea a CNR NANOTEC—Institute of Nanotechnology, UOS of Lecce, Lecce, Italy, bDepartment of Mathematics and Physics “E. De Giorgi”, University of Salento, Lecce, Italy, cUdR INSTM of Lecce, University of Salento, Lecce, Italy

13.1

Introduction

The discovery of the dimensionality dependence of the chemical-physical behavior of nanoscale solids has stimulated efforts toward the development of approaches for the controllable synthesis of nanostructures with programmable compositional and geometric features, and of advanced investigative tools and techniques for their characterization. At the heart of the current scientific revolution of nanoscience and nanotechnology are colloidal inorganic nanocrystals (NCs), functional crystalline nanoparticles (<100–150 nm) grown in liquid media. NCs distinguish from other classes of nanomaterials because of the high degree of precision and flexibility with which their crystal habit, size, shape, and surface functionalities, hence their properties, can be engineered in the fabrication stage, and of the versatility with which they can be processed and exploited for delivering artificial mesoscopic materials, innovative processes, and devices [1–5]. Valuable technological applications have already been proposed in disparate fields, including optoelectronics, catalysis, energy conversion and production, sensing, environmental remediation, and biomedicine. Commercialization of some of the envisaged applications is likely forthcoming [4–7]. Wet-chemistry routes permit judicious regulation of the thermodynamics and kinetics of NC formation in liquid media upon regulation of temperature and adjustment of the growth environment with the aid of selected solvents, ligands, surfactants, or other additives. They have emerged because of their capability to produce a wide variety of semiconductor, metal, and oxide NCs with precisely engineered structural and quality features across diverse size-morphological regimes [1–5, 8–17]. Recently, in response to the growing request for nanostructured entities capable of exhibiting enhanced or even unconventional physical-chemical properties as well as diversified capabilities for multitask applications, nanochemistry research has propelled significant progress in synthetic methodologies, enabling access to novel breeds of complex multicomponent NCs broadly referred to as heterostructured nanocrystals (HNCs). These are sophisticate freestanding multicomponent nanoparticles characterized by a spatially controlled distribution of their composition and crystal structure, which comprise two or more different material modules permanently interconnected through epitaxial interfaces. Available examples include concentric or eccentric onion-like Colloidal Metal Oxide Nanoparticles. https://doi.org/10.1016/B978-0-12-813357-6.00016-4 © 2020 Elsevier Inc. All rights reserved.

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HNCs with [email protected] and yolk[email protected] topologies as well as hetero-oligomer HNCs in which segregated material domains with defined geometries are arranged controllably in space through small-area heterojunctions [4, 5, 18–29]. On-purpose design and synthetic development of HNCs aim to realize an efficient paradigm in nanochemistry, whereby the elaboration of artificial hybrid nanostructures with an increasingly high level of structural-architectural and compositional complexity is being envisioned as a means of boosting the technological potential of conventional nanoparticles beyond the limitations imposed by their intrinsic compositional and geometric features. The creation of HNCs allows anticipating that enhanced, diversified, or entirely unprecedented properties and functionalities could be achievable through combining control over the structural and geometric features of their constituent modules, on one side, with suitable engineering of their three-dimensional connectivity on the other side. HNCs promise to disclose incredibly broad and exciting horizons in both fundamental science and future technology. Primarily, freestanding, liquid-phase processable HNCs made of fused inorganic sections, each offering distinctive optoelectronic, magnetic, and/or chemical properties, represent appealing multifunctional nanoplatforms upon which new processes and applications can be built. These include, for example, the possibility to assemble “superstructures” made of nanoscale building blocks, to perform cooperative catalytic conversions, to implant an anisotropic surface distribution of functional molecules, and to devise multimodal techniques for biomedical diagnostics and therapeutics [4, 5, 18–24, 26–29]. Additionally, in HNCs, electronic communication is naturally established across interconnected material domains, which may underlie exchange-coupling interactions between nonhomologous properties, leading to modulated, synergistically enhanced, or even entirely unprecedented properties and functionality. This represents an opportunity obviously prohibited to any of the single material components alone, or their physical mixture counterparts. For example, HNCs based on semiconductors and/or noble metals may exhibit anomalous absorption/emission and/or conductivity due to modifications in electronic structure, confinement, recombination, separation, and relocation dynamics of photo-stimulated charge carriers, and/or to plasmon-to-exciton coupling [24, 26–28]. In the case of HNCs incorporating magnetic phases and plasmonic materials, abnormally modified or mutually switchable magnetic and optical responses may reflect the synergistic interplay of magnetic, magneto-optical, and surface plasmon resonance properties through various exchange-coupling mechanisms, some of which have yet to be clarified [5, 18, 20, 21, 30, 31]. Changes in electronic structure, the formation of heterointerfaces, and the possibility to induce a programmable destination of the photoexcited charge carrier across potential barriers of tunable height and widths have proven to have a profound impact on the catalytic and photocatalytic performances of HNCs [28, 32–35]. These findings clearly indicate that the establishment of effective bonding junctions among dissimilar nanoscale materials may be exploited as an effective strategy to engineer the chemical-physical behavior and functional capabilities of modular HNCs constructed from appropriate materials associations and spatial arrangements [18, 20, 21, 24, 25, 28, 30].

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The controllable synthesis of colloidal HNCs with predictable topologies requires a high level of creativity in architectural nanostructure design and exquisite synthetic ingenuity. Indeed, the formation of multimaterial architectures occurs at a critical thermodynamic-kinetic crossover, whereby the delicate dynamics that govern the structural and geometric evolution of individual components inevitably interplay with heteroepitaxial deposition or transformative growth pathways. These are, in turn, heavily affected by facet-dependent reactivity, interfacial strain, solid-state atomic diffusivity, and/or amenability to lattice-ion exchange. Given the current limited availability of general mechanistic knowledge, control over such complex dynamics is particularly difficult to achieve, and has to be optimized for the specific material association concerned within the context of the particular reaction conditions applied. This chapter will provide a comprehensive overview of recent progress (covering approximately the past 12 years) made in the development of new breeds of HNCs constructed of domains of oxide and nonoxide materials permanently fused together into topologically controlled heterostructures. The mechanisms by which magnetic HNCs can be accessed by epitaxial seeded-growth techniques in nonequivalent configurations, spanning from concentric/eccentric [email protected] and yolk-shell geometries to more spatially intricate hetero-oligomer architectures, will be systematically illustrated and discussed for a rich selection of material associations. The distinctive properties and functionalities offered by such generations of complex nanomaterials will also be succinctly emphasized.

13.2

Synthesis of HNCs: Basic concepts and formation mechanisms

13.2.1 Synthesis of single-material nanocrystals Colloidal NCs evolve upon the reaction of molecular precursors in a liquid medium that may be composed of coordinating solvents and some stabilizing agents, such as ligands, polymers, surfactants, or soft self-assembled nanostructured templates (e.g., micelles). The synthesis is initiated at a suitable temperature at which highly reactive species, commonly referred to as the “monomers,” are generated and, above a critical supersaturation threshold, are induced to condense, thus triggering the nucleation of NCs and sustaining their subsequent growth. The organic ligands, surfactants, and/or coordinating solvents in the reaction environment play several key roles along the course of NC formation. They can (i) regulate the solution supersaturation degree upon forming complexes with the monomers, thus tuning their actual chemical potential in the solution; (ii) dynamically adsorb onto/desorb from the surface of the growing clusters, preventing irreversible aggregation; (iii) allow the steady addition of monomers to guarantee continuous growth; and (iv) act as size- and shape-regulating agents [4, 5, 8, 10]. Judicious adjustment of temperature, the type, and the relative concentrations of precursors and organic stabilizers impact on numerous processes and chemicalphysical parameters underlying nanocrystal formation in liquid media, among which

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are solution supersaturation, reactant diffusion, size-dependent relative polymorph stability, and crystallographic-direction-dependent lattice development [4, 5, 8]. Experimental and theoretical mechanistic insights have indicated that, in most cases, the key to producing monodisperse NCs relies on combining a discrete, temporally restricted nucleation event with time-separated diffusion-controlled growth [9, 10, 36]. Systematic size tuning along with size distribution focusing can be achieved by balancing the relative consumption of monomers between the nucleation and the growth stages. Depending on the particular material target, this dynamic may be realized by applying appropriate reactant delivery techniques (e.g., a primary swift “hot injection” combined with secondary slow additions of extra reactants) by manipulating the unique reactivity of the system (e.g., a “delayed” nucleation event followed by fast “autocatalytic” growth), or deliberate promotion of digestive ripening to promote dissolution of the smallest unstable nanoparticles to the benefit of the larger ones, the size variance of which will consistently shrink over time [9, 10, 36, 37]. In addition, it has to be recalled that the dynamic binding of organic stabilizers can significantly modify the relative stability of the surface facets enclosing the growing NCs, thereby determining their evolution into nonspherical shapes (e.g., cubes, polyhedrons, rods, wires, polypods). In particular, anisotropic lattice development and branching, most frequently observed for materials forming in a low-symmetric crystal structured and/or exhibiting polytypism, may be promoted within kinetically driven growth regimes, propelled by high monomer fluxes as well as by other conditions breaking growth symmetry (e.g., formation of soft surfactant or polymer lamellar templates, occurrence of crystal-oriented attachment, presence of foreign particle catalysts, or the application of external electric/magnetic fields) [4, 5, 8, 15–17, 38–42].

13.2.2 Thermodynamics of heterostructure formation From a thermodynamic viewpoint, regardless of the specific mechanisms underlying their evolution (e.g., heterogeneous deposition, thermally driven phase segregation), the construction of HNCs may be formally viewed as resulting from the sequential addition of one or more secondary material domains to a preexisting (hetero)nanocrystal substrate, accompanied by the formation of corresponding bonding heterointerfaces. In general, in addition to being chemically dissimilar, the material modules that are grouped together are likely to be structurally dissimilar (i.e., crystallize in different crystallographic phases and/or with dissimilar lattice parameters). At a first approximation, the energy balance underlying this process can be taken as being analogous to that explaining the epitaxial growth of thin-film heterostructures and strained quantum dots onto large-area oriented substrates performed by vapor-phase techniques (e.g., MBE, CVD, etc.). This is illustrated in Scheme 13.1. The sign of the total Gibbs free surface energy change function, ΔGS, that accounts for the heterogeneous deposition of a secondary material (2) over a preformed substrate of a different composition/structure (1), will essentially dictate the growth mode of the former [21, 43]: ΔGS ¼ γ 1  γ 2 + γ 1,2

(13.1)

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Scheme 13.1 Illustration of the possible growth modes by which a secondary material (referred to as “2”) may be deposited and evolve over a preformed substrate of a different primary seed material (referred to as “1”): (A) Franck-van der Merwe; (B) Volmer-Weber; and (C) StranskiKrastanov regimes. Reproduced from M. Casavola, R. Buonsanti, G. Caputo, P.D. Cozzoli, Colloidal strategies for preparing oxide-based hybrid nanocrystals, Eur. J. Inorg. Chem. (6) (2008) 837–854 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA.

where γ 1 and γ 2 are the solid/solution interfacial energies of the bare surfaces (facets) of the substrate and of the secondary material through which the heterojunction will be attained, and γ 1,2 is the solid/solid interfacial energy associated with the latter heterointerface. Assuming that these parameters are not noticeably affected by the heterodeposition process itself, γ 1and γ 2 terms can be expected to be mainly influenced by the binding or adsorption of solution species (e.g., surfactants, ligands, reactive

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monomers) and, to a lower extent, by the extension and structure of all other facets exposed to the liquid environment, while γ 1,2 will strictly depend on the bonding strength and degree of structural similarity of the concerned lattices at the interfacial region. If the secondary material exposes lower energy surfaces (i.e., γ 2 < γ 1) and/or can crystallographically match with the substrate to a good extent (hence, γ 1,2 is small), then its deposition will likely proceed layer by layer, resulting in a continuous and uniform coverage (ΔGS > 0: Frank-van der Merwe mode in Scheme 13.1A). As opposed, if the secondary material features higher energy surfaces (i.e., γ 2 > γ 1) and/or is significantly lattice-mismatched (hence, γ 1,2 is high), then it will tend to deposit as an array of island-to-droplet-like domains as a means of minimizing the overall interfacial area (hence, the interfacial misfit strain) shared with the seed substrate underneath (ΔGS < 0: Volmer-Weber mode in Scheme 13.1B). The mean interisland distance would approximately scale with the extent of excess strain that has to be accommodated. An evolutionary mixed-deposition regime could also be observed (Stranski-Krastanov mode in Scheme 13.1C). In the early stages, the secondary material grows according to a layer-by-layer mode (ΔGS > 0). Subsequently, as the deposited layer exceeds a threshold thickness and/or reaches a critical composition (in those cases in which it chemically reacts with the substrate underneath, forming an alloy or solid solution), and/or an excess of thermal energy is provided (e.g., reaction temperature is increased), subsequent growth continuation will proceed in the form of segregated domains protruding out of the initially deposited layer (ΔGS < 0) in response to an intensification of interfacial strain fields. An exceedingly high strain field, combined with a strong cohesive energy of the secondary material, may also lead to a complete dewetting of the initially deposited thin layer into discrete domains [44, 45].

13.2.3 Liquid-phase epitaxy via seeded growth The most widely exploited strategy to synthesize HNCs relies on the so-called “seeded growth” approach, which represents the solution-phase analogue of vapor-phase heteroepitaxial deposition. According to this scheme, the liquid growth environment contains preformed size- and shape-controlled NCs of a suitable material, which serve as primary “seeds” for accommodating secondary inorganic domains of different materials upon reaction of the respective molecular precursors. This approach relies on a key principle of the Classical Nucleation Theory (CNT) [2, 5, 8–10, 20, 21], according to which the energy barrier, ΔG∗het, that has to be surpassed for a given secondary material to nucleate heterogeneously onto a preexisting condensed phase (e.g., the seeds) is lower than the activation energy, ΔG∗hom, required to induce corresponding independent homogeneous nucleation of distinct crystal embryos, according to Eq. (13.2): ΔG∗het ¼ f ðθÞΔG∗hom

(13.2)

where the “wetting” function f(θ) f ðθ Þ ¼

ð2 + cos θÞð1  cos θÞ2 4

(13.3)

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(with 0 < f(θ) < 1, where the contact angle θ depends on the surface-tension equilibrium (Eq. 13.4) attained at the three-boundary seed/“droplet”/solution region (which will depend on the nature of the materials, the extension and structure of the exposed surfaces, and the geometry of the substrate seed and the secondary domain deposited thereon) and on (Eq. 13.1) [2, 21, 43, 46]: γ 1 ¼ γ 2 cos θ + γ 1,2

(13.4)

Note that the barrier for the growth of a heterogeneously nucleated domain, ΔG∗growth, is far smaller than both ΔG∗hom and ΔG∗het and corresponds to the limiting case of complete wetting (f(θ) ! 0 for θ ! 0). In an equivalent way, heterogeneous nucleation can be understood as requiring a much lower chemical potential of solution monomers (proportional to their concentration) to be triggered, relative to homogenous nucleation: Δμhet < Δμhom

(13.5)

The deposition regimes predicable on the basis of Eq. (13.1) can be equally identified within the context of a seeded-growth synthesis, whereby the energy change accounting for the preference for a given topological configuration arises from a balancing mechanism by which the surface and interfacial energy terms (Eq. 13.1) conveniently compensate for each other. For instance, on an NC seed with well-defined faceting, a secondary material can either form a continuous shell (thus, leading to an onion-like HNC) or grow as a discrete domain (thus, leading to a heterodimer habit), if the condition for a Frank-van der Merwe regime is either met for all facets exposed or selectively for just one of them, respectively (Scheme 13.1A). On the other hand, under circumstances favoring a Volmer-Weber regime, one or more sufficiently extended facets of the original seeds may accommodate multiple domains of the foreign material (Scheme 13.1B). In the intermediate case of a Stranski-Krastanov or dewetting regime, a transformation from metastable architectures, where the secondary material has initially formed an ubiquitous thin coverage on some or all facets of the seed, to a phase-segregated heterodimer/hetero-oligomer heterostructure could be observed as a convenient pathway toward lowering interfacial strain (Scheme 13.1C). At this point, it is important to remark that the creation of nanoscale heterointerfaces in solution can greatly benefit from the binding of organic stabilizers or other solution species, which can significantly change the surface energy terms (i.e., γ 1 and γ 2 terms), to the point that large γ 1, 2 may be offset to a noticeable extent. This opportunity lays the basis for the superior synthetic flexibility of colloidal epitaxial routes, which can open access to HNCs even made of highly structurally dissimilar materials, which would otherwise be difficult to envisage or predict.

13.3

Heterostructures with core/shell geometries

The configuration in which HNCs have most frequently been engineered is the so-called [email protected] topology. In such systems, an inner NC “core” is evenly enwrapped within a “shell” made of one or more layers of other materials, which

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ultimately governs or mediates all interactions with the external environment. Semiconductors, metals, and oxides arranged in centrosymmetric onion-like or eccentric [email protected] configurations share large connecting heterointerfaces, across which direct electronic communication and hybridization may lead not only to chemicalphysical properties distinct from those inherent to the individual components, but also to exchange coupling of nonhomologous properties, depending on the specific material combination. HNCs with coherently grown epitaxial interfaces may be attained when the core and shell materials are characterized by a similar crystal structure and closely matching lattice parameters, which permits misfit strain to be kept acceptably low and prevents the generation of dislocations or other defects as long as the coating thickness is sufficiently small. However, various circumstances may occur under which the requirements of lattice compatibility can be fairly less prohibitive [5, 21, 27]. For example, pathways enabling plastic strain relaxation may be available when the deposition proceeds nonepitaxially, for example, when the shell grows polycrystalline or even partially amorphous. Misfit strain constraints could easily be circumvented under kinetically overdriven conditions, albeit at the cost of incorporation of a large density of crystal defects at the interfacial regions and/or within the shell. In such cases, the core/shell interface may ultimately entail a number of small-area coherent heterojunctions at which nonequivalent crystallographic relationships locally hold between the relevant lattices. Alternatively, the ligand environment may allow the interfacial energy to be efficiently counterbalanced by a proportional decrease in surface energy associated with the outermost exposed shell surfaces. Obviously, the quality features of the shell as well as the structure of the connecting interfaces may strongly impact the ultimate chemical-physical properties of the heterostructures. Over the past decade, magnetic [email protected] HNCs accessible by colloidal seededgrowth routes have reached an unprecedented level of synthetic diversity and structural perfection. A convenient classification of preparation schemes may be made according to the relevant mechanisms leading to shell formation, as sketched in Scheme 13.2: (A and B) direct heterogeneous deposition on geometry- and crystalphase-controlled seeds; (C) silica shell growth upon priming of the seed surface and subsequent polymerization; (D–E) cation-exchange reaction or sacrificial redox replacement involving the outer seed surface layers, eventually followed by hollowing; (F) self-regulated nucleation-growth dynamics; and (G) solid-state diffusion and thermally driven crystal-phase segregation.

13.3.1 Direct heterogeneous deposition Building upon the vast synthetic knowledge that has been acquired on the growth of NCs of optoelectronically active semiconductor materials over the past decade [5, 21, 24, 27], a broad selection of magnetic-based [email protected] HNCs based on disparate combinations of metals, semiconductors, and oxides has been delivered by exploitation and manipulation of direct-deposition pathways. These routes generally involve inducing and governing sequential heterogeneous nucleation and growth of one or more secondary material layers onto preformed NC seeds that serve as freestanding

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Scheme 13.2 Mechanisms that may lead to the formation of [email protected] HNCs: (A and B) Direct heterogeneous nucleation and growth of the shell material onto preformed nanocrystal seeds with controlled shape and crystal structure; (C) silica shell growth by priming of the seed surface and subsequent polymerization; (D) direct red-ox replacement or cation-exchange reaction involving the outer seed layer; (E) surface-confined red-ox reaction followed by hollowing via the Kirkendall effect; (F) self-controlled sequential nucleation growth; (G) solidstate diffusion and hermetically driven crystal-phase segregation. Reproduced from M. Casavola, R. Buonsanti, G. Caputo, P.D. Cozzoli, Colloidal strategies for preparing oxide-based hybrid nanocrystals, Eur. J. Inorg. Chem. (6) (2008) 837–854 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA.

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substrate “cores” in the liquid phase (Scheme 13.2A and B). Simultaneously, independent parasitic homogeneous nucleation of nanocrystals of the shell material has to be inhibited. [email protected] heterostructured systems, in which large heterointerfaces have to be created and stabilized, are found to be accessible when the concerned material components feature close structural similarity (identical crystal phase and/or similar lattice parameters). This prerequirement can naturally promote epitaxial deposition of the outer protecting shell and consequent efficient healing of defects and/or dangling bonds on the surface of the starting core seeds. However, within the context of colloidal epitaxy routes, there may be found numerous case exceptions demonstrating that particular growth kinetics regimes and/or the unpredicted involvement of nanoscale strain-relief mechanisms may allow alleviating or conveniently circumventing prohibitive growth limitations posed by the emergence of misfit strain at the intervening core/shell interface [5, 21, 24]. Synthetic strategies to [email protected] systems aim not only at controlling the composition, structure, and geometry of the starting seeds, but also at precisely regulating the thickness and shape of the shell section. Practical techniques to realize these objectives heavily rely on the selection of suitable shell molecular precursors and organic stabilizers, on one side, and their programmed addition to the seed-containing medium at a judiciously slow rate. Experimental synthesis parameters have to be empirically identified on the basis of the inherent chemical reactivity of the seeds within a coordinating liquid environment appropriate for the growth of the shell material. Regulation of the reaction temperature will ultimately make a decisive impact on the temporal evolution of the solution supersaturation degree, in turn dictating whether a shell will be formed under thermodynamically or kinetically controlled growth regimes. In the following, we will briefly review the most relevant synthetic achievements in the field, highlighting various aspects concerned with the colloidal preparation of [email protected] heterostructured systems based on several functional material associations. A broad library of HNCs based on core-conformal [email protected] architectures, in which the shell habit follows the symmetry as well as the shape of the starting core, has been constructed by direct seeded growth across single or multiple deposition steps (Scheme 13.2A and B). Yet, within this family, HNCs incorporating transition-metal oxide components are still underrepresented when compared to HNCs based on nonoxide materials, due to the inherent challenges posed by their controllable synthesis [21, 24, 47]. A selection of low-magnification transmission electron microscopy (TEM) and phase-contrast high-resolution TEM (HRTEM) images, collected in Figs. 13.1–13.3, illustrates the degree of progress achieved so far in the field. [email protected] HNCs made of an Au or Ag core covered with either a spherical or flower-like shell of TiO2, ZrO2, or SnO2 were obtained by manipulating the hydrolysis-condensation reactions of corresponding metal alkoxides in the presence of surfactant-capped Au or Ag NC seeds in mixed organic/aqueous media or in microemulsions [20, 48–50] (Fig. 13.1A and B). These routes typically yielded amorphous

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Fig. 13.1 Examples of [email protected] HNCs based on nonmagnetic oxide materials, synthesized by direct single- or multiple-step heterogeneous nucleation and growth of the shell material onto preformed nanocrystal seed cores (cf. Scheme 13.2A and B). The panels report representative low-magnification TEM overviews and HRTEM images of: (A) [email protected] nanoreactors with inner void space. (B) [email protected] HNCs; (C–E) [email protected] HNCs with variable core and shell shapes (the smaller bottom panels report cross-sectional views of the relevant interfacial regions). (A) Reproduced from J. Li, H.C. Zeng, Size tuning, functionalization, and reactivation of Au in TiO2 nanoreactors, Angew. Chem. Int. Ed. 44 (28) (2005) 4342–4345 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA. (B) Reproduced from H. Sakai, T. Kanda, H. Shibata, T. Ohkubo, M. Abe, Preparation of highly dispersed core/shell-type titania nanocapsules containing a single Ag nanoparticle, J. Am. Chem. Soc. 128 (15) (2006) 4944–4945 with permission, copyright American Chemical Society. (C–E) Reproduced from C.-H. Kuo, T.-E. Hua, M. H. Huang, Au nanocrystal-directed growth of Au-Cu2O Core-shell heterostructures with precise morphological control, J. Am. Chem. Soc. 131 (49) (2009) 17871–17878 with permission, copyright American Chemical Society.

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and/or highly porous oxide coatings that affected the plasmonic properties of the noble-metal core while guaranteeing chemical accessibility to the latter. Owing to these favorable features, these HNCs have been explored as catalytically active nanoreactors, capacitors, and sacrificial templates for the preparation of hollow oxide capsules by selective metal core etching techniques [20]. More recently, allcrystalline [email protected] HNCs with different shapes have been synthesized by a room-temperature aqueous approach that relies on alkaline CuCl2 reduction with NH2OH in a sodium dodecyl sulfate surfactant environment loaded with shapecontrolled Au NC seeds [51]. Irrespective of the appreciable differences in lattice parameters between the two materials, [email protected] heterostructures with epitaxial interfaces were formed in which the shell could be either grown morphologically conformal to the Au core underneath or be almost independently shaped over a broad size interval (Fig. 13.1C–E). Several prototypes of [email protected] [email protected] HNCs were afforded by performing chemical [20, 52–55] or photocatalyzed reduction [20, 56–59] of metal ion precursors on weakly organic-passivated oxide NCs in alcohol media, which resulted in either a discontinuous or uninterrupted metal (Au, Ag, Pt, or Cu) layer onto ZnO or TiO2 cores. These architectures have been widely acknowledged as platforms for solar energy conversion processes, where the metal to semiconductor band alignment promotes separation of electron-hole pairs under band-gap photoexcitation of the oxide section. Also, excitation of the surface plasmon oscillation of the

Fig. 13.2 Examples of [email protected] HNCs based on magnetic oxide materials, synthesized by direct heterogeneous nucleation and growth of the shell material (cf. Scheme 13.2A). The panels report representative low-magnification TEM overviews and HRTEM images of: (A and B) Spherical and flower-like [email protected] HNCs; (C) spherical [email protected] HNCs; (D) nearly spherical [email protected] HNCs; (E) cube-shaped [email protected]; (F, G and H) cubeshaped Fe1 [email protected] HNCs observed at different resolution. (A and B) Reproduced from W. Shi, H. Zeng, Y. Sahoo, T.Y. Ohulchanskyy, Y. Ding, Z.L. Wang, M. Swihart, P.N. Prasad, A general approach to binary and ternary hybrid nanocrystals, Nano Lett. 6 (4) (2006) 875–881 and F. Pineider, C. de Julia´n Ferna´ndez, V. Videtta, E. Carlino, A. al Hourani, F. Wilhelm, A. Rogalev, P.D. Cozzoli, P. Ghigna, C. Sangregorio, Spinpolarization transfer in colloidal magnetic-plasmonic Au/iron oxide hetero-nanocrystals, ACS Nano 7 (1) (2013) 857–866 with permission, copyright American Chemical Society. (C) Reproduced from X. Teng, H. Yang, Synthesis of magnetic nanocomposites and alloys from platinum-iron oxide core-shell nanoparticles, Nanotechnology 16 (7) (2005) S554–S561 with permission, copyright IOP Publishing. (D) Reproduced from T. Zhou, M. Lu, Z. Zhang, H. Gong, W.S. Chin, B. Liu, Synthesis and characterization of multifunctional FePt/ZnO core/shell nanoparticles, Adv. Mater. 22 (3) (2010) 403–406 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA. (E) Reproduced from S.-h. Noh, W. Na, J.-t. Jang, J.-H. Lee, E.J. Lee, S.H. Moon, Y. Lim, J.-S. Shin, J. Cheon, Nanoscale magnetism control via surface and exchange anisotropy for optimized ferrimagnetic hysteresis, Nano Lett. 12 (7) (2012) 3716–3721 with permission, copyright American Chemical Society. (F, G and H) Reproduced from B.P. Pichon, O. Gerber, C. Lefevre, I. Florea, S. Fleutot, W. Baaziz, M. Pauly, M. Ohlmann, C. Ulhaq, O. Ersen, V. r. Pierron-Bohnes, P. Panissod, M. Drillon, S. Begin-Colin, Microstructural and magnetic investigations of w€ustite-spinel core-shell cubic-shaped nanoparticles, Chem. Mater. 23 (11) (2011) 2886–2900 with permission, copyright American Chemical Society.

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metal may be used to boost the photocatalytic activity of the oxide and/or control of the electron storage capabilities across the heterostructure [59]. A variety of [email protected] HNCs incorporating at least one magnetic transition-metal oxide component have been accessed by direct heterogeneous nucleation routes (Scheme 13.2A). Examples are reported in Fig. 13.2. In organic-phase synthetic protocols, surfactant-capped Fe, Fe3O4, or FePt NCs were used as templates for the reduction of Au(III)- or Ag(I)-salts in the presence of oleic acid (OLAC) and/or oleyl amine (OLAM) at 170–180°C [60–64]. Such a high temperature was needed to weaken surfactant binding to the surface of the seeds so as to render the latter accessible to the reactive metal species. Other authors demonstrated that OLAC/OLAM-capped Fe3O4 and unstable thiol-capped Au NCs in toluene in the presence of an ammonium bromide salt at 150°C could be thermally driven to coalesce to [email protected] HNCs, thereby decreasing the overall surface energy of the system [65]. There have also been reports on aqueous synthesis under mild conditions, which, however, enables modest control over the size distribution and dispersibility of the heterostructures that, in fact, tend to aggregate. Exceptions are represented by [email protected] HNCs with thick Au or Ag shells grown upon reducing Au(III) salts with Fig. 13.3 Examples of [email protected] HNCs synthesized by preactivating the surface of the seed cores for silica shell growth (cf. Scheme 13.2C). The panels report representative low-magnification TEM overviews of: (A and B) [email protected] HNCs with mesoporous SiO2 shell; (C) [email protected] HNCs; (D) [email protected] HNCs; (E) ternary [email protected]@SiO2 HNCs; (F) ternary [email protected] HNCs; (G and H) ternary [email protected] HNCs with asymmetric SiO2 shell HNCs. (A and B) Reproduced from T. Kim, E. Momin, J. Choi, K. Yuan, H. Zaidi, J. Kim, M. Park, N. Lee, M.T. McMahon, A. Quinones-Hinojosa, J.W.M. Bulte, T. Hyeon, A.A. Gilad, Mesoporous silica-coated hollow manganese oxide nanoparticles as positive T1 contrast agents for labeling and MRI tracking of adipose-derived mesenchymal stem cells, J. Am. Chem. Soc. 133 (9) (2011) 2955–2961 with permission, copyright American Chemical Society (https://pubs.acs. org/doi/abs/10.1021/ja1084095; further permissions related to the material excerpted should be directed to the American Chemical Society). (C) Reproduced from D.C. Lee, F.V. Mikulec, J.M. Pelaez, B. Koo, B.A. Korgel, Synthesis and magnetic properties of silica-coated FePt nanocrystals, J. Phys. Chem. B 110 (23) (2006) 11160–11166 with permission, copyright American Chemical Society. (D) Reproduced from C. Cannas, A. Musinu, A. Ardu, F. Orru`, D. Peddis, M. Casu, R. Sanna, F. Angius, G. Diaz, G. Piccaluga, CoFe2O4 and CoFe2O4/SiO2 core/ shell nanoparticles: magnetic and spectroscopic study, Chem. Mater. 22 (11) (2010) 3353–3361, with permission, copyright American Chemical Society. (E) Reproduced from Y.J. Kim, J.K. Choi, D.-G. Lee, K. Baek, S.H. Oh, I.S. Lee, Solid-state conversion chemistry of multicomponent nanocrystals cast in a hollow silica nanosphere: morphology-controlled syntheses of hybrid nanocrystals, ACS Nano 9 (11) (2015) 10719–10728 with permission, copyright American Chemical Society. (F) Reproduced from I. Schick, S. Lorenz, D. Gehrig, A.-M. Schilmann, H. Bauer, M. Panth€ofer, K. Fischer, D. Strand, F. Laquai, W. Tremel, Multifunctional two-photon active silica-coated [email protected] janus particles for selective dual functionalization and imaging, J. Am. Chem. Soc. 136 (6) (2014) 2473–2483 with permission, copyright American Chemical Society. (G and H) Reproduced from B. Wu, S. Tang, M. Chen, N. Zheng, Amphiphilic modification and asymmetric silica encapsulation of hydrophobic Au-Fe3O4 dumbbell nanoparticles, Chem. Commun. 50 (2) (2014) 174–176 with permission, copyright Royal Chemical Society.

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hydroxylamine [66] or AgNO3 with borohydride [65], respectively, on weakly organic-stabilized FexOy seeds in suspension or a water-in-oil microemulsion. [email protected] HNCs with a star-like Au shell have been documented [67]. More recently, highly monodisperse spherical [email protected] HNCs with tunable sizes were successfully obtained utilizing triblock copolymer nanotemplates, inside which the core and shell sections were sequentially generated [68]. Based on the characterization data supplied, all aforementioned [email protected] [email protected] HNCs exhibited shell thickness-dependent surface plasmon absorption and altered magnetic properties (e.g., lower blocking temperature, higher coercivity) relative to those of their bare magnetic components, which was explained by invoking the attainment of different interparticle coupling degrees as a consequence of the metal shell screening [18, 60–64]. By proper structural control and surface functionalization, these nanoheterostructures, in which an optically active and biocompatible noble-metal shell is combined with a ferro- or ferri-magnetic material core, have been proposed as efficient nanoscale platforms for catalysis, bioassay, and magnetically assisted bioseparation purposes; as bactericidal killers; and as magnetic resonance imaging (MRI) and photothermal agents for biomedical diagnostics and therapeutic actions [18, 60, 61, 63, 65, 67, 69, 70]. HNCs based on a reverse [email protected] configuration have been accessed by utilizing metal seeds within the context of a solvothermal precipitation reaction [71], or, more frequently, of nonhydrolytic sol-gel routes to transition-metal oxides, which are reaction pathways inherently characterized by a high activation barrier for homogeneous nucleation [10, 20, 72–78]. For example, monodisperse [email protected] HNCs with spherical, flower-like, or cubic-shaped Fe3O4 shells epitaxially connected with the Au core (Fig. 13.2A and B) were synthesized by decomposing Fe(CO)5 at 200–300°C in the presence of small Au seeds in a noncoordinating octadecene (ODE) or dioctyl ether (DOE) loaded with OLAC and OLAM surfactants [79, 80]. [email protected] (Me ¼ Fe, Ni, Co) and [email protected] Fe3O4 HNCs were synthesized by analogous strategies [81, 82]. Uniform onion-like [email protected] HNCs were obtained in suitable carboxylic acid/ alkyl amine mixtures in a one-pot approach, whereby Pt or FePd NCs were first nucleated in situ by alkyldiol reduction of platinum acetylacetonate and then combined with the iron precursor for shell deposition [76, 83] (Fig. 13.2C). Exotic urchin-like [email protected] nanocomposites, entailing spherical clusters of FePd nanoparticles with spikes of Fe3O4 nanorods, were derived by manipulating the thermal decomposition of Fe(CO)5 and the reduction of palladium acetylacetonate [84]. These nanostructures could be converted to strongly exchange-coupled L10-FePd-Fe nanocomposite magnets upon solid-state reductive annealing. Very recently, within the framework of a two-step seeded growth route, the reaction of Pt seeds with iron palmitate or myristate precursors at high temperature has been exploited to generate epitaxial [email protected] HNCs in which the Pt cores are embedded in a controlled triangular nanoprism-shaped shell [85]. Exotic heterostructures composed of Au nanorods asymmetrically coated with hematite α-Fe2O3 have also been documented [86]. A few magnetic/semiconductor-oxide associations have been successfully tackled. [email protected] HNCs with a large shell-to-core volume ratio were synthesized by

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OLAM-driven aminolysis of zinc acetate in FePt seed/DOE mixtures at 260°C [75]. These heterostructures, which exhibited both superparamagnetic behavior and the typical fluorescence of ZnO, a valuable piezoelectric material, have been envisioned to provide a basis for realizing future magnetically controlled electromechanical devices (Fig. 13.2D). A case of phase-dispersible magneto-opto-fluorescent [email protected] HNCs has recently been documented [87]. The attainment of fluorescent [email protected] HNCs has also been claimed [88]. For the latter heterostructures, significant abatement of the photoluminescence quantum yield was verified. The temperature dependence of the magnetization showed the characteristic features of two coexisting and interacting magnetic (Fe3O4) and nonmagnetic (ZnSe) phases. Compared to a reference bare Fe3O4 sample, the room-temperature Nee`l relaxation time in the [email protected] HNCs was found to be much longer [88]. Finally, the recent availability of unique rattle-type [email protected] HNCs is worthy of being highlighted [89]. These heteronanostructures, which entailed covellite CuS, a self-doped metallic-like semiconductor with plasmonic properties [90], were exploited as magnetically guided photo-induced hyperthermia agents within the first and second near-infrared biological windows, and as MRI contrast agents to identify tumor cells [89]. A broad library of all-oxide [email protected] HNCs has been developed. Among these, several Fe3O4-based HNCs incorporating nonmagnetic oxides have been reported. For example, spherical [email protected] HNCs were obtained by pyrolysis of zinc acetate in the presence of Fe3O4 seeds that had previously been decapped from their original oleate ligands by a pyridine treatment [91]. These HNCs were magnetically responsive, although no fluorescence from the ZnO shell was detectable. Other protocols were proposed by which magnetically recordable, aggregated [email protected] heterostructures showing photocatalytic activity were obtained [92]. More interesting properties were found to be associated with porous [email protected] and [email protected] nanorods, which were fabricated upon reductive annealing of the cores of their α[email protected] and α[email protected] parent heterostructures with shells grown by hydrothermal treatment of potassium stannate and zinc acetate at 120–170°C, respectively [93–95]. These HNCs exhibited multinonlinear dielectric loss behavior and excellent electromagnetic wave absorption capability, which were ascertained to result from favorably complementary dielectric and magnetic losses associated with the respective materials [93–95]. Other valuable examples of a multifunctional nanoheterostructure are represented by spherical to spindle-like α[email protected] and α[email protected] HNCs with a core-shape conformal porous shell [96, 97]. Investigations on all-oxide bimagnetic heterostructures have nicely demonstrated that rational design of the core and shell composition, their structure, and their volume ratio can allow for independent control of their magnetic properties, including blocking temperature, coercivity, surface/lattice anisotropy, and magnetic exchange coupling through the relevant heterointerfaces. Thermal decomposition of metal acetates in DOE (or ODE) diluted tri-noctylamine/OLAC mixtures at 320°C was used to deposit a shell of antiferromagnetic (AFM) CoO, MnO, or NiO onto monodisperse FiM cores of spinel-phase MeFe2O4 (Me ¼ Zn, Co, Fe) [98–100]. The resulting bimagnetic [email protected] HNCs exhibited the signature of magnetic exchange coupling between interfacial spins of

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the FM (or FiM) and AFM sections, which produced enhanced unidirectional anisotropy with consequent distinctive alterations in the hysteresis cycles (e.g., horizontal loop shift by an “exchange bias” field) and the extent of coercivity of the FM (or FiM) phase [18, 99, 101–103]. Other examples of exchange-biased bimagnetic heterostructures obtained by slightly modified chemical protocols include hard-soft shape-controlled [email protected] HNCs of [email protected] [104, 105], Zn0.4Fe2.6 [email protected] (Fig. 13.2E) [106], [email protected] [107], [email protected] [108], and [email protected]γ[email protected] [109] and [email protected] nanocomposites obtained by thermal annealing of colloidal [email protected] HNCs [110]. These nanoheterostructures, for which either conventional or anomalous exchange-bias effects can underlie enhancement of the thermal stability of the magnetization, are appealing candidates for highdensity magnetic recording media [30, 103], as hyperthermia agents for biomedical purposes [105, 106], and as magnetically recyclable catalysts [100]. Several prototypes of inverted AFM/ferrimagnetic(FiM) have been developed. [email protected] HNCs were derived from particle surface-confined oxidation of MnO NCs prepared by surfactant-assisted pyrolysis of manganese acetylacetonate in the presence of OLAM and/or 1,2-hexadecandiol [111] or by pyrolysis of manganese oleate [112]. Similarly, upon exploiting known nonhydrolytic reactions of metal carboxylate precursors in OLAM/OLAC/alkyldiol mixtures to phase-controlled iron and manganese oxides, binary spherical and cubic-shaped Fe1 [email protected] [113–115] [email protected] HNCs (Fig. 13.2F–H) as well as ternary Fe1[email protected]@MnO and quaternary multishell Fe1[email protected]@[email protected] HNCs [113] were controllably accessed. All these types of HNCs promise to be technologically appealing as they indeed feature the signature of FM (or FiM)-AFM exchange bias [30]. The latter could be modulated by proper topological design and size tailoring of the core and shell domains to meet requirements for device implementation [30]. In another account, CoFe2O4 NCs were used to seed Fe3O4 overgrowth, unexpectedly leading to [email protected] HNCs with different degrees of interfacial coherence [73]. Bimagnetic [email protected] HNCs (where Me ¼ Fe, Co) with a coherent core/shell interface were derived directly by a surfactant-assisted two-step solution synthesis [73, 77, 78]. In the reported procedure, FexPt1x seeds, obtained by simultaneous platinum acetylacetonate reduction and iron carbonyl decomposition, were covered with an MFe2O4 upon codecomposition of cobalt and iron acetylacetonate in a suitable proportion in the presence of OLAC and OLAM stabilizers. In most of the aforementioned cases, the geometric parameters of the HNCs were easily engineered by adjusting the reactant concentration ratio, with the seeds governing the relative balance of shell precursor consumption in the heterogeneous nucleation and growth stages. The relevant magnetic analyses indicated that effective exchange interactions were established between the relevant magnetic phases, the relative volume proportions of which dictated the overall coercivity value. By using a slight modification of the above-reported procedures to shell growth, other material combinations were successfully combined. For example, biocompatible [email protected] HNCs (Me ¼ Co, SmCo5.2) were surface functionalized and thereby applied in the selective separation histidine-tagged proteins [72, 74]. A technologically appealing approach to hard nanomagnets was devised, which

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combined solution-phase processing with a solid-state reaction. The proposed methodology involved thermal reductive annealing of preformed colloidal [email protected] HNCs, which converted to hexagonal close-packed nanocrystalline SmCo5 with large magnetic coercivity [74].

13.3.2 Silica coating Procedures to enwrap NCs of a variety of materials within a biocompatible and hydrophilic SiO2 shell are now remarkably advanced. Such techniques generally involve a “priming” step, which consists of creating a primary polymerization layer of alkoxide groups on the surface of the NC seeds via attaching bifunctional coupling agents, such as silicon organometallic molecules, polymers, or gelatine [116–129]. Controlled hydrolysis and condensation reactions in organic media by judicious supply of a silicon alkoxide and water results in the built-up of an SiO2 network covering the seeds [116–119, 129–136] (Scheme 13.2C). The outer shell surface can further serve as platform for implanting other chemical functionalities that would be otherwise difficult to anchor on the original NCs. These types of metal oxide shells can be grown to large thicknesses because they are usually characterized by an amorphous and porous structure that may easily accommodate possible interfacial strain. Examples of [email protected] HNCs prepared by means of this aforementioned technique are reported in Fig. 13.3. A variety of materials have been enwrapped within a SiO2 shell. For example, SiO2 coating of organic-coated (initially hydrophobic) semiconductor magnetic NCs, such as of MnO, FexOy, Co, FePt, and FeAuPd (Fig. 13.3A–D) was used to transfer such nanomaterials to biological environments for specific cell targeting/sorting, labeling, and/or MRI imaging purposes while minimizing the release of toxic heavy metals into aqueous media [116, 118, 122, 123, 134–145]. Exploitation of the synergistically combined dielectric and magnetic losses associated with the SiO2 shell and magnetic components, respectively, was envisaged as a means of generating excellent electromagnetic wave absorption properties [93]. Among the most recent achievements, hollow [email protected] HNCs with a mesoporous SiO2 shell were synthesized, starting from oleic-acid-stabilized MnO nanoparticles. These were coated with a mesoporous SiO2 upon a sol-gel reaction of tetraethyl orthosilicate in aqueous solution containing cetyltrimethylammonium bromide (CTAB) under basic conditions [142]. The CTAB molecules acted not only as the stabilizing agent to transfer hydrophobic nanoparticles to the aqueous phase, but also as an organic structure-directing template to create mesopores on the silica shell. The formation of a hollow interior in the MnO cores and simultaneous template removal was achieved by acid etching and refluxing in ethanol solutions (Fig. 13.3A) More recently, the silica-coating approach has been successfully extended to the enwrapping of multimaterial heterostructures. For example, preformed [email protected] or Au-MnO heterodimer HNCs were used as the starting cores on which a chemically accessible concentric or asymmetric SiO2 shell was precisely grown [145, 146] (Fig. 13.3E–H). As a means toward increased functionality, other composite silica-embedded HNC systems, for example based on magneticluminescent materials (e.g., FePt/Fe3O4 and CdSe), have been tackled [147].

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Interestingly, it has been demonstrated that protecting SiO2 shells can serve as confining walls within which compositional and structural transformations (e.g., reductive annealing, phase conversion, solid-state diffusion) of the cores trapped in can be carried out, leading to nanostructures with otherwise hardly accessible crystalphase composition and/or topology [143, 144, 148–150]. In the hollow interior of [email protected] [email protected] nanoreactors or inside the continuous channels of the mesoporous SiO2 shell of [email protected] [email protected] HNCs, thermally activated catalytic conversions can efficiently take place at the metal core surface, with the core remaining preserved against irreversible coalescence [100, 124, 126].

13.3.3 Shell growth by red-ox replacement or conversion reactions Another commonly pursued strategy toward [email protected] HNCs relies on the sacrificial conversion of the outermost exposed layers of a starting NC core into a different material by a galvanic replacement reaction [151–160] (Scheme 13.2D). In this respect, many transition-metal NCs are potentially useful substrates because they can undergo transmetalation reactions in organic media [153–155] or be easily oxidized when exposed to air, solvated oxygen species, or other oxidizing reagents. Red-ox conversions enable the creation of oxide [email protected] HNCs where the mean oxidation state of metal atoms located in the core is different from that in the shell section. Examples of as-synthesized HNCs are assembled in Fig. 13.4. [email protected] systems achieved by partial oxidation of their respective metal cores have been extensively addressed. For example, monodisperse [email protected] HNCs [156] and [email protected] nanoparticles with amorphous or crystalline cores [157] were generated upon room-temperature air exposure of surfactant-capped Ni seeds and Fe seeds, respectively, and used for biomolecule tagging, magnetic separation, and/ or MRI imaging purposes [156, 161–164]. Temperature and reactant manipulations enabled tuning the crystal-phase compositions of the cores, for example, allowing switching from [email protected] to [email protected] [email protected]γ-Fe2O3 [email protected] HNCs [115, 165, 166]. Control of oxidation conditions through the calibrated O2 supply was explored as a means to synthesize ferromagnetic (FM)/antiferromagnetic (AFM) [email protected] HNCs with a CoO shell of adjustable thickness at nearly constant particle volume, up to the limiting case of all-oxidized CoxOy NCs [152, 158]. Alternative oxidizing agents other than O2 could be successfully used to design these types of [email protected] HNCs [167]. Unprecedented progress toward simultaneous morphological and compositional control has been made with the synthesis of complex [email protected] HNCs via a mechanism known in metallurgy as the “Kirkendall effect,” an atomic diffusion process that takes place through vacancy exchange rather than by direct interchange of atoms [11–13, 22, 23]. It is indeed established that, in a nanoscale object, where the inner core region hosts fast-diffusing species and the outer region acts as a reservoir of slower diffusing species (e.g., metal cations and oxygen anions in oxide-passivated metallic nanoparticles, respectively), a net transport of matter from the core outward

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Fig. 13.4 Examples of magnetic [email protected] and [email protected] HNCs synthesized by performing red-ox replacement reactions on metal seed cores (cf. Scheme 13.2D and E). The panels report low-magnification TEM galleries and selected HRTEM images of: (A and B) [email protected] [email protected] HNCs and corresponding [email protected] [email protected] HNCs thereof, respectively; (C) [email protected] [email protected] HNCs; (D and E) [email protected] and [email protected] [email protected] HNCs. (A and B) Reproduced from A. Cabot, V.F. Puntes, E. Shevchenko, Y. Yin, L. Balcells, M.A. Marcus, S.M. Hughes, A.P. Alivisatos, Vacancy coalescence during oxidation of iron nanoparticles, J. Am. Chem. Soc. 129 (34) (2007) 10358–10360 with permission, copyright American Chemical Society. (C) Reproduced from E.V. Shevchenko, M.I. Bodnarchuk, M.V. Kovalenko, D.V. Talapin, R.K. Smith, S. Aloni, W. Heiss, A.P. Alivisatos, Gold/iron oxide core/ hollow-shell nanoparticles, Adv. Mater. 20 (22) (2008) 4323–4329 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA. (D and E) Reproduced from J. Gao, G. Liang, J.S. Cheung, Y. Pan, Y. Kuang, F. Zhao, B. Zhang, X. Zhang, E.X. Wu, B. Xu, Multifunctional yolkshell nanoparticles: a potential MRI contrast and anticancer agent, J. Am. Chem. Soc. 130 (35) (2008) 11828–11833 with permission, copyright American Chemical Society.

can take place along with coalescence of the generated vacancies into a single large void [11, 12] (Scheme 13.2E). For example, during the temperature-controlled oxidation of amorphous Fe nanoparticles with either O2/Ar flow or trimethylamine N-oxide in octadecene/OLAM solution, the entire evolution from amorphous [email protected]

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[email protected] [81, 157, 168] to [email protected] [email protected] nanostructures (Fig. 13.4A and B), and finally to hollow Fe3O4 NCs was tracked [159, 160]. These findings suggested further smart routes to achieve increasingly sophisticated heterostructure design. Actually, selective application of the Kirkendall mechanism to the shell section of preformed [email protected] HNCs has opened access to more topologically complex [email protected] architectures, in which a void space intervenes between the core and shell portions of different chemical composition. Relevant demonstrations include [email protected], [email protected], and [email protected] [email protected] HNCs, obtained upon reacting [email protected], [email protected], and [email protected] [email protected] HNCs with HAuCl4, O2, or S, respectively, in organic media at moderate temperatures [169–174] (Fig. 13.4C–E). The internal void structure of the [email protected] heterostructures has been assessed by dedicated probe experiments. For instance, [email protected] [email protected] HNCs were found to be catalytically active in ethylene hydrogenation, thus confirming that small molecular reactants and products could permeate through grain boundaries of the CoO shell and reach the surface of the inner Pt core [171, 175]. Following prototypical biological tests, in which [email protected] and [email protected] [email protected] HNCs were demonstrated to act as powerful killing agents for HeLa cells [172] or as carriers for gene delivery [169], respectively, [email protected] [email protected] HNCs were studied as both MRI contrast agents and anticancer drugs [170]. These achievements promise to inspire future development of nanomedicine approaches for cancer diagnosis and therapy. All-oxide made [email protected] heterostructures have also been developed with a high degree of precision. α[email protected] [176] and [email protected] [177] [email protected] HNCs were obtained of corresponding α[email protected] and [email protected] [email protected] particles initially formed upon induced internal ripening under solvothermal conditions at 200°C or upon air calcination at 300°C, respectively. [email protected](OH)x nanorattles with various shapes and dimensions were obtained by template-engaged red-ox etching of shape-controlled Cu2O crystals, followed by thermal annealing to generate nanorattles of different compositions, such as of [email protected] and [email protected] [178]. In another case, selective etching of the inner SiO2 shell in [email protected]@TiO2 architectures yielded [email protected] [email protected] HNCs with a highly porous urchin-like TiO2 shell [179]. Along a similar strategy, a three-step approach was introduced to synthesize [email protected] [email protected] HNCs upon selective removal of SiO2 from ternary [email protected]@ZrO2 double-shell particle precursors [180]. Depending on the specific combination, these [email protected] HNCs have been proven to hold potential for application in lithium-ion storage, electrocatalysis, photocatalysis, and microwave absorption. Very recently, galvanic replacement reactions have been proved to take place on transition-metal oxide nanocrystals [181]. When Mn3O4 or Co3O4 nanocrystals were reacted with Fe(ClO4)2 or SnCl2 precursors, hollow [email protected]γ-Fe2O3, [email protected], or Co3O4/SnO2 [email protected] nanoboxes were produced, respectively, which ultimately transformed into hollow γ-Fe2O3 or SnO2 nanocages, respectively. Because of their nonequilibrium compositions and hollow interior, the accessed nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries [181].

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13.3.4 Shell transformation via cation-exchange reactions A flexible route to manipulate both the composition and crystal-phase distribution of inorganic nanostructures, which would otherwise be hard to achieve by other pathways, is represented by reversible transformation mediated by solid-state cation exchange [21, 182] (Scheme 13.2D). Partial cation exchange reactions can allow preformed NCs of a given composition to be either transformed to alloy NCs or to various types of nanoheterostructures possessing core/shell, segmented, or striped architectures. Several metal-chalcogenide NCs have proven to be interconvertible by such a mechanism (CdX ! MexEy, where E ¼ S, Se, Te; Me ¼ Pd, Pt, Cu, Pb), the driving force of which depends on the solvation energies of the entering and leaving cations as well as on the presence of suitable metal-coordinating species [21, 182]. In contrast to the galvanic replacement pathways addressed earlier, where substantial morphology changes are frequently observed, the shape of the starting nanocrystal template can largely be preserved owing to conservation of the anionic framework sublattice, which might result from the larger ionic size, hence a slower diffusion rate, of the anions in the crystal (as compared to that of the cations). In particular, core/shell HNCs represent a particularly interesting set of templates over which spatially selective atomic exchange reactions can be exploited to induce partial compositional-structural changes selectively, either in the core or the shell section. However, the reported cases of cation-exchange treatment applied to magnetic nanostructures are still quite sparse, as illustrated in Fig. 13.5. Reacting preformed OLAC-stabilized Fe3O4 nanocrystals, magneto-plasmonic [email protected], and bimagnetic Fe1[email protected] [email protected] HNCs, respectively, with a CoCl2-OLAM complex in the presence of tri-n-octyl phosphine (TOP) serving as a weak Fe2+ sequestering agent at 200–220°C, resulted in homogeneously alloyed ferrite CoxFe2xO4 nanocrystals, [email protected]xO4, and Fe1[email protected] core @shell HNCs, respectively [183]. Structural-compositional analyses and modifications in the magnetic properties of the nanostructures strongly corroborated the occurrence of Co2+ ion diffusion and incorporation (heavy doping) into the lattice of the metal ferrite (Fe3O4 or CoFe2O4) components of the original seeds (Fig. 13.5A–G).

13.3.5 Self-controlled nucleation growth, thermally induced solid-state atomic diffusion, and phase segregation There have been successful efforts to identify one-pot single-step approaches to [email protected] HNCs, in which all necessary material precursors can simultaneously be present in the same growth solution, and the nucleation growth of the core and the shell-coating stage may be self-regulated due to the operation of less conventional mechanistic pathways. HNC systems accessed by such routes are here highlighted on the basis of the representative examples reported in Fig. 13.6. (i) Self-Controlled Nucleation Growth: Under appropriate conditions, the inherent reactivities of the molecular precursors introduced for building the core and shell section, or, equally,

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Fig. 13.5 Examples of magnetic [email protected] HNCs derived by partial cation-exchange reactions (cf. Scheme 13.2D) on preformed [email protected] heterostructure seeds: (A–C) Lowresolution TEM images of the starting seeds homogeneous spherical Fe3O4 nanocrystals, [email protected] [email protected] HNCs, and Fe1 [email protected] [email protected] HNCs, respectively; (D) Scheme of the cation exchange mechanism operating on the three different nanocrystal seeds shown in panels A–C (blue balls represent oxygen atoms; red balls, Fe3+; orange, Fe2+; green, Co2+; gold, Au atoms). (E–G) High-angle annular dark-field STEM images of the nanocrystals and HNCs in panels A–C, respectively, after Fe2+ to Co2+ cation exchange. The ion concentration distributions for Fe (red) and Co (green), and Au (golden) across the nanostructures, shown by the EDX line scans (obtained for transitions from the K or M shell respectively), evidence the Co doping after the cation-exchange treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this chapter.) Reproduced from M. Sytnyk, R. Kirchschlager, M.I. Bodnarchuk, D. Primetzhofer, D. Kriegner, H. Enser, J. Stangl, P. Bauer, M. Voith, A.W. Hassel, F. Krumeich, F. Ludwig, A. Meingast, G. Kothleitner, M.V. Kovalenko, W. Heiss, Tuning the magnetic properties of metal oxide nanocrystal heterostructures by cation exchange, Nano Lett. 13 (2) (2013) 586–593 with permission, American Chemical Society (https://pubs.acs.org/doi/10.1021/nl304115r; further permissions related to the material excerpted should be directed to the American Chemical Society).

the energy activation barriers for the homogeneous nucleation of NCs of the individual material components may diverge to such an extent that: (i) the two different materials nucleate or grow appreciably at distinct times and/or different temperatures; and (ii) the shell material develops as a result of catalyzed heterogeneous deposition on preexisting seeds that have formed in situ in a previous stage (Scheme 13.2F). Such dynamics allow avoiding the use of the classical “hot-injection” technique of precursor delivery, which is frequently exploited to temporally separate the nucleation and growth processes [9, 10, 37].

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Fig. 13.6 Examples of magnetic [email protected] HNCs achieved by less conventional mechanisms (cf. Scheme 13.2F and G). The panels show: (A–C) TEM and (D) HRTEM images of variablesized cube-shaped [email protected] [email protected]; (E and F) TEM images of [email protected] HNCs with rhombohedral-shaped shell. (G–I) TEM, high-angle annular dark-field STEM and HRTEM images of [email protected]@ZnO HNCs made of flower-like multipetal ZnO shell covering asymmetric [email protected] [email protected] cores. (A–C) Reproduced from H.T. Hai, H.T. Yang, H. Kura, D. Hasegawa, Y. Ogata, M. Takahashi, T. Ogawa, Size control and characterization of wustite (core)/spinel (shell) nanocubes obtained by decomposition of iron oleate complex, J. Colloid Interface Sci. 346 (1) (2010) 37–42 with permission, copyright Elsevier. (E and F) Reproduced from B. Nakhjavan, M.N. Tahir, M. Panthofer, H. Gao, T. Gasi, V. Ksenofontov, R. Branscheid, S. Weber, U. Kolb, L.M. Schreiber, W. Tremel, Controlling phase formation in solids: rational synthesis of phase separated [email protected] heteroparticles and CoFe2O4 nanoparticles, Chem. Commun. 47 (31) (2011) 8898–8900 with permission, copyright Royal Society of Chemistry. (G–I) Reproduced from Y. Chen, D. Zeng, M.B. Cortie, A. Dowd, H. Guo, J. Wang, D.-L. Peng, Seed-induced growth of flower-like Au–Ni–ZnO metal–semiconductor hybrid nanocrystals for photocatalytic applications, Small 11 (12) (2015) 1460–1469 with permission, copyright WileyVCH Verlag GmbH & Co. KGaA.

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As an additional practical advantage, they do not require performing repeated slow precursor injections at later synthesis stages to facilitate the focusing of the size distribution [9, 10, 37]. Examples that illustrate the concept of a self-controlled nucleation/growth mechanism are provided by the unique circumstances under which magnetic [email protected]γ-Fe2O3 [184], [email protected] [185], and [email protected] [186, 187] HNCs (Fig. 13.6A–D) were generated. For example, biphasic [email protected] [186, 187] and [email protected] [185] HNCs were synthesized by thermolysis of metal-oleate complexes: primary homogeneous nucleation and early growth of the FexO phase took place until a critical size was reached, at which deposition of a Fe3O4 or CoFe2O4 shell, respectively, became favored over continued volume expansion of the core. Under solvothermal conditions (160–200°C) the one-pot preparation of [email protected] HNCs with a thick Fe3O4 shell involved AgNO3 reduction (with ethylene glycol or sodium citrate) at 120–160°C, followed by reduction and precipitation of Fe(NO3)3 to mixed-valence Fe3O4 on the Ag seeds generated in the initial heating stages [188, 189]. Analogously, differences in the rate of decomposition of the corresponding precursors were exploited to achieve the formation of [email protected]γ-Fe2O3 and [email protected]γ-Fe2O3 [190] (Fig. 13.6E and F), [email protected] [191] [email protected] HNCs, and hollow [email protected]α-Fe2O3 ring-like nanoprisms [192].

Ternary flower-like [email protected]@ZnO HNCs were synthesized by a facile one-pot twostep strategy [193]. First, HAuCl4(CO)8 and Ni(acac)2 were coreduced by OLAM in benzyl ether (BE) at 200°C, resulting in asymmetric [email protected] [email protected] nanocrystals. Then, the medium containing the bimetallic seeds was combined with a zinc stearate/ OLAM/BE mixture and annealed to 290°C, at which a multispectral ZnO shell was selectively overgrown on the seeds, leading to flower-like [email protected]@ZnO architectures exploitable as magnetically recoverable photocatalytically active platforms (Fig. 13.6F–I). More recently, an analogous strategy was implemented to generate [email protected] HNCs [194]. Finally, freestanding Na2TiO3O7 nanosheets epitaxially decorated with Fe3O4 crystallites were accessed by a modified route to alkali-metal titanate, involving the solvothermal treatment of TiO2 powders, NaOH, and Fe nanoparticles [195]. Notably, the resulting [email protected] nanosheets were distinguished by improved Na+ ion exchange performances and superior stability against spontaneous scrolling into tubular structures more than their bare Na2TiO3O7 counterparts. An unconventional example of chain-like magnetic Ni/Ni3C [email protected] heterostructures was delivered upon NiCl2 reduction and Ni-core-catalyzed organics decomposition to carbonaceous species in boiling ethylene glycol [196]. (ii) Thermally Induced Solid-State Diffusion and Phase Segregation: It has been reported that the reaction of substoichiometric amounts of white phosphorus (P4) with monodisperse Ni nanoparticles in solution at T < 220°C can lead to the crystallization of an Ni2P core surrounded by a Ni shell rather than the single-phase Ni3P that would be favored at the bulk scale [197]. The formation of such [email protected] [email protected] HNCs (Fig. 13.6F) was considered to involve solid-state P diffusion into the Ni lattice, followed by coalescence and segregation of distinct Ni2P and Ni core and shell domains as a consequence of their lower energy of formation of nanoscale Ni2P and Ni, compared to their Ni3P phase counterpart (Scheme 13.2G). The observed formation of a core/shell structure rather than a heterodimer configuration is also likely to be permitted by the tolerable incommensurability of the crystal structures of the Ni and Ni2P phases.

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Analogous reaction dynamics were deliberately exploited to create electrocatalytically active [email protected] HNCs that could not otherwise be obtained by direct heterogeneous deposition of an MnO shell on Au seeds [198]. In this approach, crystalline AuxMny nanoparticles were first synthesized by a KBEt3H-driven reduction of manganese acetylacetonate in ODE/OLAM media loaded with preformed Au nanoparticle seeds at 240°C, during which fast Mn diffusion into the Au structure took place, leading to the AuxMny alloy nanoparticles. The latter seeds could then be induced to extrude Mn upon air annealing at 170°C in a distinct step, which resulted in Mn segregating toward the surface and converting to MnO upon reaction with atmospheric O2. Ultimately, either [email protected] [email protected] HNCs or multidomain Au-MnO composite HNCs were formed, depending on the Au:Mn composition of the starting alloy seeds [198]. In other circumstances, simple treatment of bimetallic Au-Ni heterodimer HNCs with tri-n-phenylphosphine serving as both a capping agent (for Au) and a phosphorous source, was shown to lead to corresponding [email protected] [email protected] HNCs at 230–270°C across a sequence including Ni phosphorylation, crystallization, and reshaping of the resulting Ni12P phase into a regular shell around the Au domain [199]. In this environment, the tri-n-phenylphosphine ligand was understood as being essential to guaranteeing stabilization of the Au domains against diffusion and alloying with Ni.

13.4

Hetero-oligomer architectures

A broad library of distinguished HNCs comprises nanoarchitectures featuring a spatially asymmetric distribution of their composition and crystal structure. These are heterodimer and hetero-oligomer HNCs that incorporate distinct size- and shapecontrolled domains of dissimilar materials interconnected through one or multiple heterointerfaces of limited extension. Distinct from their [email protected] counterparts, dimeric/oligomeric nanoheterostructures can be regarded as the inorganic analogues of complex organic molecules equipped with a number of functional moieties [25]. While grouping the properties of the component materials and allowing their electronic interactions, these HNCs offer a diversified set of surface platforms onto which a topologically controlled distribution of functional moieties may eventually be anchored [5, 18, 20, 21, 24, 25]. In seeded-growth synthesis, various circumstances can make HNC growth switch from a [email protected] to a phase-segregated development regime. Under thermodynamically controlled conditions, the topology will be dictated by the ultimate surface energy balance accompanying the deposition event or heterostructuring process (Eq. 13.1). For example, materials that do not form alloys and/or are strongly lattice-uncorrelated can evolve into oligomer-type heterostructures as a pathway toward minimizing the overall interfacial strain at a proportionally smaller cost of increased surface energy (associated with the multiple material surfaces exposed). Equally, small bonding junctions may be induced to form among preexisting NCs as a means of alleviating the high surface energy that would otherwise characterize their physical mixtures, for example, in the case of ineffective stabilization by the

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surface ligands. Other growth circumstances favoring the formation of [email protected] HNCs may concern the introduction of seeds characterized by site-preferential accessibility or chemical reactivity that could arise from varying degrees of lattice matching at the exposed facets, or from kinetically overdriven deposition conditions [5, 18, 20, 21, 24, 25].

13.4.1 Heterostructures based on nearly isotropic-shaped material domains Heterodimers and hetero-oligomers grouping two or more isotropically shaped modules have been readily accessed by seeded-growth based techniques. The developed preparation schemes may be classified according to the relevant mechanism underlying the heterostructure formation, as sketched in Scheme 13.3: (A) direct

Scheme 13.3 Mechanisms that may underlie formation of oligomer-type HNCs: (A) Direct heterogeneous nucleation; (B and C) nonepitaxial deposition followed by thermally driven coalescence/crystallization and/or solid-state atomic diffusion; (D) reactions at liquid/liquid interfaces; and (E) induced attachment of preformed heterodimers. Reproduced from M. Casavola, R. Buonsanti, G. Caputo, P.D. Cozzoli, Colloidal strategies for preparing oxide-based hybrid nanocrystals, Eur. J. Inorg. Chem. (6) (2008) 837–854 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA.

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heterogeneous nucleation; (B) nonepitaxial deposition followed by thermally driven coalescence-crystallization and/or solid-state atomic diffusion; (C) reactions at liquid/ liquid interfaces; (D) self-regulated homogeneous-heterogeneous nucleation; and (E) induced attachment. Representative TEM images that illustrate the level of synthetic sophistication achievable by such mechanistic pathways can be found in Figs. 13.7–13.10.

13.4.1.1 Direct heterogeneous nucleation Direct heterogeneous nucleation (Scheme 13.3A) is among the most frequently exploited mechanisms through which HNCs can be tailored as binary and ternary oligomers composed of diverse associations of magnetic, metal, and materials. Such HNCs have been envisioned as key elements on which new technological solutions, especially in areas where multifunctionality and multitasking are sought (e.g., sensing, imaging, and therapy in biomedicine), may be founded. For example, it has been proposed that distinct material modules of an HNC can be used as platforms for the site-specific anchoring of biomolecules (e.g., DNA strands, proteins, peptides) by exploiting the affinity of specific chemical moieties toward inorganic surfaces [18, 200–202]. In addition, while a metal or semiconductor module can enable optical detection, a magnetic domain can be utilized for complementary purposes, such as for MRI/optical imaging and magnetic separation [18, 200–207]. Synergistic interactions coming into play through the heterointerfaces upon assembly of dissimilar materials into HNCs have clearly been recognized to affect their magnetic [30, 80, 207–212], optical [213–215], transport [208], magneto-optical [31, 216], and (electro)catalytic [217–225] properties as well as their energy-storing capabilities [226]. Several protocols based on the thermal decomposition of metallorganic precursors in the presence of preformed Me1, Me2, Me3, Fe3O4, or FePt seeds in ODE or phenyl ether solvents containing OLAC, OLAM, and/or TOP surfactants at 200–300°C at atmospheric pressure have enabled access to heterodimer HNCs with epitaxially joint nearly spherical and/or cubic-shaped domains, such as of Me1-Fe3O4 (Me1 ¼ Au, AuAg, PtPd, AuPd, AuPt, AuCu, Pt, Ni, Cu), Me2-CoO (Me2 ¼ Au, AuAg, Pt), Me3-MnO (Me3 ¼ Au, Ag), Fe3O4-PbS, Fe3O4-CdSe, FePt-YZ (YZ ¼ CoFe2O4, PbSe, CdSe, In2O3, Fe3O4, MgO), Me4-In-doped CdO (Me4 ¼ Au, FePt, Pt, Pd), and Fe3O4-MnO, respectively [79, 110, 205, 206, 208, 210, 211, 219, 220, 223–225, 227–239]. Synthesis under solvothermal conditions in polar media has also been explored [233]. Depending on the geometric features of the two component domains, these heterostructures showed a peanut-, dumbbell-, brick- or flower-like morphological profile (Fig. 13.7A–D). In most cases, the appreciable difference in lattice parameters of the concerned materials was assumed to be the main driving force that promoted segregation into discrete domains oriented relative to one another so as to guarantee coherent interfaces with minimal strain. The formation mechanism was investigated in detail for the Au-Fe3O4 association, for which a fundamental role of the solvent in regulating the Fe3O4 nucleation sites on the Au seeds was identified [228, 229]. The dumbbell-like configuration yielded by reactions performed in nonpolar media was explained by invoking the induction of charge polarization at those

Fig. 13.7 See figure legend on next page.

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regions of the Au seeds where Fe3O4 had initially been deposited, which depleted the electron density elsewhere and thus inhibited further nucleation events. In contrast, in syntheses carried out in a more polar electron-donor solvent, any electron deficiency generated over the Au surface could be smeared out by the medium, which made the seed a more suitable ground either for the accommodation of multiple Fe3O4 “petals” or even for the attainment of uniform oxide coverage [220, 228, 229]. Recently, it has been discovered that the formation of AuPt-Fe3O4 heterodimers grown from AuPt seeds involved the initial evolution of [email protected] [email protected] HNCs (at a relatively Fig. 13.7 Examples of magnetic binary and ternary hetero-oligomer HNCs synthesized by direct epitaxial heterogeneous nucleation onto preformed seeds (cf. Scheme 13.3A). The panel shows low-resolution TEM images galleries of: (A) Peanut-shaped Au-MnO HNCs; (B) dumbbell-like Au-Fe3O4 HNCs; (C) peanut-shaped FePt-In2O3 heterodimer HNCs with cubic-shaped FePt domains; (D) peanut-shaped Pt-Fe3O4 HNCs; (E and F) asymmetric Fe3O4-Ag heterodimer HNCs; (G–I) FePt-Au trimer and high-order oligomer HNCs grown starting from cubic FePt seeds; (J–L) evolution from [email protected] hetero-oligomer HNCs to lower-order [email protected] hetero-oligomers to hollow-FexOy-Ag heterodimers via ripening; (M) Au-Fe3O4-PbS heterotrimer HNCs obtained by nucleating a rod-shaped PbS section on the Au domain of Au-Fe3O4 heterodimer seeds. (N and O) Ag-Pt-Fe3O4 and Cu9S5-Pt-Fe3O4 hetero-trimer HNCs obtained by chemoselective nucleation of Au or Cu9S5 on the Pt domain of Pt-Fe3O4 heterodimer seeds. (A) Reproduced from S.-H. Choi, H.B. Na, Y.I. Park, K. An, S.G. Kwon, Y. Jang, M.-h. Park, J. Moon, J.S. Son, I.C. Song, W.K. Moon, T. Hyeon, Simple and generalized synthesis of oxidemetal heterostructured nanoparticles and their applications in multimodal biomedical probes, J. Am. Chem. Soc. 130 (46) (2008) 15573–15580 with permission, copyright American Chemical Society. (B) Reproduced from H. Yu, M. Chen, P.M. Rice, S.X. Wang, R.L. White, S. Sun, Dumbbell-like bifunctional Au-Fe3O4 nanoparticles, Nano Lett. 5 (2) (2005) 379–382 with permission, copyright American Chemical Society. (C) Reproduced from H.M. Wu, O. Chen, J.Q. Zhuang, J. Lynch, D. LaMontagne, Y. Nagaoka, Y.C. Cao, Formation of heterodimer nanocrystals: UO2/In2O3 and FePt/In2O3, J. Am. Chem. Soc. 133 (36) (2011) 14327–14337 with permission, copyright American Chemical Society. (D) Reproduced from C. Wang, H. Yin, S. Dai, S. Sun, A general approach to noble metal-metal oxide dumbbell nanoparticles and their catalytic application for CO oxidation, Chem. Mater. 22 (10) (2010) 3277–3282 with permission, copyright American Chemical Society. (E and F) Reproduced from J. Jiang, H. Gu, H. Shao, E. Devlin, G.C. Papaefthymiou, J.Y. Ying, Bifunctional Fe3O4–Ag heterodimer nanoparticles for two-photon fluorescence imaging and magnetic manipulation, Adv. Mater. 20 (23) (2008) 4403–4407 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA. (G–I) Reproduced with permission from J. Zhu, J. Wu, F. Liu, R. Xing, C. Zhang, C. Yang, H. Yin, Y. Hou, Controlled synthesis of FePt-Au hybrid nanoparticles triggered by reaction atmosphere and FePt seeds, Nanoscale 5 (19) (2013) 9141–9149, copyright Royal Society of Chemistry. (J–L) Reproduced from S. Peng, C. Lei, Y. Ren, R.E. Cook, Y. Sun, Plasmonic/magnetic bifunctional nanoparticles, Angew. Chem. Int. Ed. 50 (14) (2011) 3158–3163 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA. (M) Reproduced from W. Shi, H. Zeng, Y. Sahoo, T.Y. Ohulchanskyy, Y. Ding, Z.L. Wang, M. Swihart, P.N. Prasad, A general approach to binary and ternary hybrid nanocrystals, Nano Lett. 6 (4) (2006) 875–881 with permission, copyright American Chemical Society. (N and O) Reproduced from M.R. Buck, J.F. Bondi, R.E. Schaak, A total-synthesis framework for the construction of high-order colloidal hybrid nanoparticles, Nat. Chem. 4 (1) (2012) 37–44 with permission, copyright Nature Publishing Group.

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Fig. 13.8 Examples of hetero-oligomer HNCs synthesized by postdeposition coalescencecrystallization and/or solid-state atomic diffusion (cf. Scheme 13.3B). The panels show lowmagnification TEM galleries and selected HRTEM images of: (A and B) Binary and ternary Fe3O4-ZnS oligomers; (C and D) Fe3O4-CdS hetero-oligomer HNCs with one or multiple shaped CdS sections departing from a single Fe3O4 seed. (A and B) Reproduced from K.W. Kwon, M. Shim, γ-Fe2O3/II-VI sulfide nanocrystal heterojunctions, J. Am. Chem. Soc. 127 (29) (2005) 10269–10275 with permission, copyright American Chemical Society. (C and D) Reproduced from H. McDaniel, M. Shim, Size and growth rate dependent structural diversification of Fe3O4/CdS anisotropic nanocrystal heterostructures, ACS Nano 3 (2) (2009) 434–440 with permission, copyright American Chemical Society.

low temperature, 190°C), followed by partial dewetting of the Fe3O4 shell and its reshaping into a discrete spherical Fe3O4 domain attached aside (at much higher temperatures, 280–320°C) [224]. In this process, the AuPt domain ultimately retained a thin discontinuous Fe3O4 shell on the hemisphere region that was diametrically opposite to

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Fig. 13.9 Examples of heterodimers synthesized at liquid/liquid interfaces (cf. Scheme 13.3C). The panels report variable-magnification TEM images of: (A) Starting Fe3O4 seeds and (B–D) Fe3O4-Ag heterodimers thereof with different Ag domain size; (E and F) Fe3O4-Ag heterodimers with hollow Fe3O4 domain. (A–D) Reproduced from H.W. Gu, Z.M. Yang, J.H. Gao, C.K. Chang, B. Xu, Heterodimers of nanoparticles: formation at a liquid-liquid interface and particle-specific surface modification by functional molecules, J. Am. Chem. Soc. 127 (1) (2005) 34–35 with permission, copyright American Chemical Society. (E and F) Reproduced from Y. Pan, J.H. Gao, B. Zhang, X.X. Zhang, B. Xu, Colloidosome-based synthesis of a multifunctional nanostructure of silver and hollow iron oxide nanoparticles, Langmuir 26 (6) (2010) 4184–4187 with permission, copyright American Chemical Society.

the location of the major Fe3O4 domain. A similar mechanism relying on the dewetting of an unstable Fe3O4 shell formed on the metal seeds at intermediate stages under relatively mild conditions may be expected to be involved in the formation of other coinage-metal-Fe3O4 heterodimers, as demonstrated by the outcome of interesting “tug-of-war” etching-destabilization experiments aimed at probing the chemical reactivity of these heterostructures [221, 224, 240]. Actually, Au could be selectively leached out from Me-Fe3O4 (Me ¼ Au, AuPt) heterodimers with profiles as diverse as peanut- to dumbbell-shaped, upon oxidation with I2 at near room temperature. As a result, either nearly spherical Fe3O4 NCs, each bearing a concavity (which corresponded to the volume occupied by the pristine Au domain) or peanut-/dumbbell-like solid-hollow heterodimer Fe3O4 NCs were obtained. These NCs featured a nanocontainer section (i.e., where the Au or AuPt domain was originally accommodated) that encased a residual Pt domain [221, 224, 237, 240]. The production of such

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Fig. 13.10 Examples of [email protected] HNCs synthesized by self-controlled nucleationgrowth mechanism (cf. Scheme 13.3D). (A–C) Low-magnification TEM and HRTEM images of FePt-Fe3O4 heterodimer HNCs. Examples of hetero-oligomer HNCs synthesized by induced fusion of preformed heterodimer seeds (cf. Scheme 13.3E). (D and E) TEM and HRTEM images of ternary Fe3O4-Au-Fe3O4 HNCs with a Au section bridging two Fe3O4 domains. (A–C) Reproduced from A. Figuerola, A. Fiore, R. Di Corato, A. Falqui, C. Giannini, E. Micotti, A. Lascialfari, M. Corti, R. Cingolani, T. Pellegrino, P.D. Cozzoli, L. Manna, One-pot synthesis and characterization of size-controlled bimagnetic FePt-iron oxide heterodimer nanocrystals, J. Am. Chem. Soc. 130 (4) (2008) 1477–1487 with permission, copyright American Chemical Society. (D and E) Reproduced from W. Shi, H. Zeng, Y. Sahoo, T.Y. Ohulchanskyy, Y. Ding, Z.L. Wang, M. Swihart, P.N. Prasad, A general approach to binary and ternary hybrid nanocrystals, Nano Lett. 6 (4) (2006) 875–881 with permission, copyright American Chemical Society.

exotic nanostructures with concave surfaces or cavities indirectly revealed that the metal (hemi)domain of these heterodimers, which appeared to protrude and be exposed to the external environment, should indeed be either naked or covered with a thin, porous, or discontinuous (hence, chemically accessible) shell of Fe3O4. These findings corroborate the dewetting-participated mechanistic picture highlighted earlier. Other exotic prototypes of solid-hollow heterodimers have been obtained by exploiting sulfidization pathways to chemically and structurally modify the oxide domains of metal/metal-oxide heterodimers [239]. In additional mechanistic investigations on other metal/metal oxide and metal/ semiconductor systems, the transition from dumbbell- to flower-like geometry was achieved by increasing the temperature and/or relative precursor to seed proportions

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[79, 204–206, 227, 229, 233]. In some cases, the nature of ligands bound to the surface of the starting seeds or added during the seeding stage was found to be critical to driving the preference for a heterodimer topology over a [email protected] one, which pointed to the influence of kinetic processes on topology selection [204–206, 208, 227–229, 233, 235]. In an attempt to rationalize the formation of peanut-shaped FePt-In2O3 heterodimer HNCs [234], the crystallographic relationships holding between two materials were investigated on the basis of the Coincidence Site Lattice Theory (CSLT) [241–244]. Upon studying the degree of matching between points of the FePt and In2O3 lattices, and the frequency at which this correspondence occurred along the relevant heterojunctions, several couples of facets of the two materials were identified to be satisfactorily coincident at the interfaces, in addition to those few assessed experimentally. An empirical law (bonding energy criterion) for explaining the preference for the relative FePt to In2O3 lattice orientations observed experimentally in the heterodimers was proposed: instead of assuming that In2O3 overgrowth took place on the facets of the FePt seeds, where lattice mismatch could be minimized, the epitaxial deposition of In2O3 was considered to most favorably take place on the FePt crystal facets at which the first atomic monolayer of the deposited In2O3 had the strongest chemical affinity for the seed [234]. By a reverse reaction scheme, successfully applied to the synthesis of all-metal heterodimers [79, 118, 203, 227, 245–252], Fe3O4-Ag was prepared via reduction of Ag(I)-ligand complexes onto preformed Fe3O4 seeds by mild reducing agents (alkyl amines, alkyl diols, Ar/H2 atmosphere) at modest temperatures (<120°C) [204, 253–257] (Fig. 13.7E and F). With the introduction of an extra reaction step, γ-Fe2O3-Cu2O heterodimer HNCs were derived upon postsynthesis air oxidation of the Cu domains in parent γ-Fe2O3-Cu heterodimers [258]. The properties of these metal/semiconductor HNCs clearly diverge from those of the individual components alone. For example, the semiconductor luminescence was abated due to the metal contact favoring electron migration while the plasmon resonance was largely shifted due to the effect of the proximal dielectric environment [79, 118, 227, 237, 247–250, 252, 253, 259–261]. Also, the relevant magnetic parameters were observed to deviate from those of the isolated magnetic components [203, 232, 237, 245, 246, 255, 262], indirectly denoting that interfacial electron communication established in HNCs across inorganic heterojunctions of variable structurecomposition and extension may strongly impact their electronic structure and, in turn, their ultimate chemical-physical behavior (e.g., in catalysis) [79, 204, 237, 238, 251–254, 259]. The formation of heterodimer HNCs with independently adjustable domain sizes has been rationalized mechanistically. For many heterodimer systems, the seeds utilized in the respective cases may be regarded as acting as red-ox active heterogeneous catalysts with a degree of reactivity (e.g., toward reduction of the relevant metalcomplex precursor) that depends on their size and shape (faceting). The seed to precursor concentration ratio and temperature regulate the size to which the secondary domains could ultimately be grown by governing the rate and extent of secondary metal-ion addition at their surfaces [204, 246, 251, 255–257]. When interfacial strain

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is not prohibitive, smaller-sized seeds may accommodate a continuous thick shell of the foreign materials as a means of reducing their high surface energy [100]. On the other hand, when the seeds expose crystallographically and/or chemically inequivalent facets and/or the emergent interfacial strain, being highly dependent on surface curvature, may not be alleviated, heterodimer-type topologies are preferred over [email protected] geometries [204, 246, 252]. When relatively large faceted seeds enclosed by stable facets with comparable surface structure and/or reactivity are utilized, growth of multiple domains of the secondary components may occur at multiple equivalent facets or in proximity of selected sites (defects, edge grooves), where nucleation may be kinetically favored and/or interfacial strain may be minimized. These circumstances may lead to heterooligomer HNCs with binary composition, namely heterostructures comprising flower-like assemblies of multiple domains of the secondary materials arranged around a single central core that essentially retains the geometry and structure of the original seeds [100, 233, 242, 243, 256, 257, 263, 264]. The selection of appropriate surface-binding surfactants [257] or calibrated reducing conditions (e.g., under Ar/H2 atmosphere [246]) to either accentuate or level off the reactivity of the different seed facets exposed has been identified as a valuable strategy to control the frequency of heterogeneous nucleation events (Fig. 13.7G–I). In an interesting case study, during heterogeneous Ag deposition on spherical [email protected] seeds in organic media, an evolution from high-order multiple Ag-decorated [email protected] hetero-oligomer HNCs to lower-order [email protected] hetero-oligomers and, finally, to hollow-FexOy-Ag heterodimers was observed [256] (Fig. 13.7J– L). Across this sequence, the average Ag domain and FexOy shell size increased while the inner Fe cores of the seeds shrunk until their complete disappearance. The time evolution of the density and size of the Ag domains was found to be consistent with the classical LaMer nucleation and Ostwald ripening mechanisms, according to which the growth of nanoparticles is initiated from a temporally short nucleation burst-like event followed by subsequent growth of the larger, more stable particles at the expense of the consumption of the smaller ones by diffusion of released monomer species. Parallel to this process, the [email protected] substrate seeds underwent progressive oxidation and Kirkendall diffusion (cf. Scheme 13.2E) (most likely driven by trace O2, Ag+, and/or NO 3 ions), which eventually generated hollow FexOy domains with thick walls [256]. Conceptually mimicking the total-synthesis approach used to construct complex organic molecules bearing multiple functional moieties, elaborate multicomponent hetero-oligomer HNCs with progressively higher nuclearity orders have been created by performing consecutive chemoselective seeding steps on heterostructured seeds (Scheme 13.3A). To this purpose, well-known chemical pathways for growing the target materials were rationally combined and applied under appropriate conditions. For example, Fe3O4-Au-PbSe hetero-trimer HNCs (Fig. 13.7M) were synthesized by directing the heterogeneous nucleation of PbSe onto binary Fe3O4-Au dumbbellshaped seeds upon reaction of Pb/Se-surfactant complexes [79, 265]. Interestingly, PbSe nanostructures could develop anisotropically out of the Au domains (Fig. 13.7C) via a solution-liquid-solid (SLS) growth mechanism [5] and eventually

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detach from the heterodimer seeds and transfer into the solution [265]. Similarly, linear Fe3O4-Pt-Me (Me ¼ Au, Ag, Ni, Pd) and Fe3O4-Pt-MexSy (Me ¼ Pb, Cu) heterotrimer HNCs (Fig. 13.7N and O) were constructed by exclusive deposition of the desired tertiary metal or metal sulfide component on the Pt domain of Fe3O4-Pt heterodimer seeds, a process that was not accompanied by the formation of other isomer side products [266]. These reaction dynamics are intriguing in that they can be regarded as a nanocrystal-related analogue of regiospecificity in molecular systems, in which out of several products that may be formed with different spatial arrangements of their functional moieties, only one is observed. On the basis of competently designed control experiments, the observed chemoselectivity was preliminarily understood as correlating with an electron enrichment of Pt in the Fe3O4-Pt seeds due to charge transfer from the bound Fe3O4 domain (a process not achievable in isolated Pt nanoparticles). Further microscopic investigation of chemoselectivity in the formation of Ag-Pt-Fe3O4 heterotrimer HNCs [267] revealed an initial indiscriminate Ag nucleation onto both the Pt and Fe3O4 surfaces of Fe3O4-Pt seeds, followed by surface diffusion and coalescence of Ag onto the Pt surface to yield the final Fe3O4-Pt-Ag heterotrimer product. The size of the Ag domain of Fe3O4-Pt-Ag correlated with the overall surface area of the Fe3O4-Pt seeds, which was consistent with a mechanism of Ag coalescence through a surface-mediated process. Additionally, small iron oxide islands on the Pt surface of the Fe3O4-Pt seeds (deposited during their synthesis) were identified as defining the morphology of the Ag domain [267]. To obtain other ternary HNC isomers, the concept of a solid-state protecting group was introduced [268]. A thin amorphous iron oxide shell was installed onto the Pt domain of preformed Fe3O4-Pt heterodimers to serve as a solid-state protecting group that isolates the Pt moiety, thus redirecting the nucleation of a third domain of Ag or Au to an otherwise disfavored site, namely the Fe3O4 domain. This strategy thus allowed producing the distinct and otherwise inaccessible Ag-Fe3O4-Pt and Au-Fe3O4-Pt heterotrimer isomer HNCs, respectively. Occasionally, heterogeneous nucleation processes have been identified to interplay and/or compete with red-ox replacement, cation exchange, and the Kirkendall reaction pathways, leading to heterostructures that, however, may embody an irregular distribution of their chemical composition or contain voids in one of the component domains [174, 250, 259, 269]. In the realm of magnetic-based HNCs, an exception is provided by the outcome of the reaction of presynthesized metallic α-Fe NCs with nickel acetylacetonate [270]. Mechanistic investigations indicated the occurrence of a galvanic replacement of Fe in the α-Fe NCs for Ni, followed by rapid oxidation of both Ni and Fe to yield multidomain alloyed-FexNiy-Me3O4 (Me ¼ Fe, Ni) heterostructures.

13.4.1.2 Postdeposition coalescence-crystallization and solid-state diffusion A systematic investigation of how the minimization of interfacial strain energy governs the topological evolution of HNCs was conducted for heterostructures based on either FePt or γ-Fe2O3 and metal chalcogenides, MeX (Me ¼ Cd, Zn, Hg; X ¼ S, Se)

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[118, 227, 241–243, 271, 272]. Examples of HNCs synthesized by this mechanism are reported in Fig. 13.8. The synthesis of these HNCs was seeded with either γ-Fe2O3 or FePt seeds, onto which a highly defective and amorphous MeX layer was initially deposited upon sequential addition of suitable organometallic precursors at low temperature. Upon prolonged heating at 280°C, the amorphous MeX shell gradually crystallized, causing an induction of strain across the lattice-mismatched seed/MeX heterojunctions. Over time, the shell coalesced and segregated into a discrete MeX grain aside the γ-Fe2O3 seed [118, 227, 241, 242, 271, 272] (Scheme 13.3B). Such evolution was explained by considering that the large junction tension in the annealed [email protected] nanostructure could be greatly relieved upon reduction of the interfacial area between the two materials. This energy gain could therefore be large enough to compensate for the proportionally smaller increase in the overall surface energy that eventually accompanies heterodimer formation. The topological evolution of the γ-Fe2O3-MeX system was rationalized on the basis of the CSLT theory [241–244]. Indeed, the mean number of MeX domains that could be accommodated on each γ-Fe2O3 seed strictly correlated with the degree of lattice match achievable at the γ-Fe2O3/MeX heterointerfaces and with the seed size (Fig. 13.8A and B) [241–243]. Adjustment of the ligand environment and of the growth kinetics regime could facilitate implantation of either one or multiple CdS sections that developed anisotropically out of γ-Fe2O3 seeds [243] (Fig. 13.8C and D). These magnetic/semiconductor HNCs could still exhibit appreciable fluorescent emission from the MeX domains while retaining the typical superparamagnetic behavior of magnetic modules [118, 227, 241, 242, 271, 272], which suggested their utilization as bifunctional probes for dual-mode bioimaging [118, 200].

13.4.1.3 Reactions at liquid/liquid interfaces A technique devised to synthesize heterodimers made of one magnetic and one noble metal domain relies on performing seeded growth at a liquid/liquid interface under mild conditions [273, 274]. Examples of HNCs synthesized by this biphasic strategy are shown in Fig. 13.9. In the reported procedure (Scheme 13.3C), an aqueous metal salt solution is brought in contact with an immiscible organic solvent (such as dichlorobenzene, dichloromethane, hexane, or DOE) that contains surfactant-capped γ-Fe2O3/Fe3O4 or FePt seeds. Upon ultrasonic irradiation under an inert atmosphere, an emulsion formed that consisted of a continuous aqueous phase containing organic microdroplets stabilized by the hydrophobic seeds that had self-assembled at the organic/water interfaces (“colloidosomes”) [273]. Under these conditions, the seed NCs offered catalytically active sites onto which the Ag+ or AuCl 4 ions were reduced to the respective Ag or Au upon mild sonication. As the seeds were only partially exposed to the aqueous phase, metal deposition was spatially restricted to a small surface region and proceeded self-catalytically, thus resulting in a single metal domain on each seed (Fig. 13.9A–D). The colloidosome-based approach was extended to the synthesis of solid Ag-hollow γ-Fe2O3 heterodimers starting from hollow γ-Fe2O3 seeds prepared by manipulating the Kirkendall mechanism [274] (Fig. 13.9E and F).

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These heterodimers were used to accommodate a site-differential surface distribution of biomolecules to be exploitable for biomedical purposes [200, 274].

13.4.1.4 Self-regulation of homogeneous versus heterogeneous nucleation There have been a few reports demonstrating the preparation of heterodimer HNCs by one-pot methods. In these cases, the reagents required to build up the heterostructures are introduced together into the same surfactant medium (no separate seed preparation step is necessary). Conditions may be serendipitously discovered under which both homogeneous and heterogeneous nucleation processes that underlie the formation of different material modules self-govern and occur consecutively, with negligible temporal overlapping. A few examples of heterodimer HNCs afforded by such a mechanism have been reported, as illustrated in the top part of Fig. 13.10. One interesting case is represented by the evolution of FePt-FexOy heterodimers with tunable geometric features (Fig. 13.10A–C), which were synthesized by reacting platinum acetylacetonate and Fe(CO)5 precursors in an OLAM/OLAC/ODE environment [275] (Scheme 13.3D). The HNC modules formed sequentially across two steps. Initially, homogeneous nucleation and growth of FePt NCs took place at T  200°C; then, a thin polycrystalline FexOy shell was deposited onto the in situ generated FePt seeds at T  295°C, which rapidly dewetted and grew as a separate domain attached aside as a means of alleviating the intervening interfacial strain. Because each reaction stage was selectively activated under distinct temperature conditions, then regulation of both the temperature and the heating time guaranteed that the two material sections of the HNCs formed at distinct stages of the synthesis course. As a consequence of the magnetic exchange coupling holding between the two soft and hard materials, the HNCs exhibited tunable single-phase-like magnetic behavior and superior capabilities as MRI contrast agents, not achievable by their individual components [275].

13.4.1.5 Reactions between preformed HNCs A sophisticated strategy with the potential to access increasingly complex HNCs envisions utilization of preformed HNCs as multifunctional inorganic seed precursors that can be combined with each other to form larger molecules. Examples of as-synthesized HNCs are shown in the bottom part of Fig. 13.10. Ternary Fe3O4Au-Fe3O4 HNCs, in which an Au section bridged two Fe3O4 domains (Fig. 13.10D and E), were obtained by inducing welding of the Au domains that belonged to separate Au-Fe3O4 heterodimer peanuts in the presence of elemental sulfur [79] (Scheme 13.3E). Due to its high affinity to Au surfaces, S was presumed to adsorb on the Au domains of the heterodimers, thereby displacing the capping surfactants thereon and triggering their fusion as a means of lowering their total surface energy. Extension of this strategy has allowed access to higher-order oligomers with linear or variable branched topology, for example, made of Fe3O4-Pt-Au-Au-Pt-Fe3O4 upon S-participated selective coalescence of preformed Au-Pt-Fe3O4 hetero-trimers through their outer Au domains [266].

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13.4.2 Heterostructures based on anisotropic-shaped domains HNCs with a spatially asymmetric arrangement of their component domains have been created by programming heterogeneous growth reactions onto anisotropically shaped seeds, such as nanorods, nanowires, and branched NCs, which commonly crystallize in noncentrosymmetric phases [5, 8]. This ultimately transcribes into a pronounced facet-dependent reactivity. Thus, in addition to offering peculiar shape-dependent properties, anisotropic substrate seeds represent interesting platforms over which pathways to spatially selective distribution of secondary domainscan be studied and rationalized. On a thermodynamic basis, the preference for secondary material deposition to take place at selected sites of asymmetrically shaped seeds should correspond to the topological configuration allowed by the overall surface interfacial energy balance accompanying the formation of all involved heterointerfaces (Eq. 13.1) [5, 8, 24, 28, 276]. For example, selective heterogeneous nucleation may be viewed as being energetically convenient when it leads to the elimination of unstable facets of the seeds (e.g., the apexes of nanorods) at a proportionally smaller interfacial strain energy cost associated with the newly formed heterojunctions. However, when nucleation and growth occur under kinetically controlled regimes and/or other chemical or physical transformative pathways come into play (e.g., intraparticle ripening, atomic diffusion, ion exchange), understanding the mechanisms underlying topology selection may be less straightforward [5, 8, 20, 24, 28, 276–282]. The following paragraphs describe major synthetic strategies that have been developed to construct magnetic-based HNCs incorporating anisotropic (e.g., linear, branched) material sections [47]. The main growth mechanisms that have been exploited, as sketched in Scheme 13.4, include: (i) site-specific heterogeneous nucleation; (ii) surfactant-controlled site-selective deposition; and (iii) strain-driven heteroepitaxial growth. Examples that demonstrate the degree of synthetic control achievable can be found in Figs. 13.11–13.13.

13.4.2.1 Site-specific heterogeneous nucleation The most developed breeds of anisotropically shaped HNCs are represented by nanoheterostructures based on archetypal metal chalcogenides (MeX, Me ¼ Cd, Zn, Mn; X ¼ S, Se) and transition metal oxides (ZnO, TiO2) for which a high level of synthetic expertise has matured over the past two decades [5, 8, 21, 26, 28, 276, 283]. Nanoplatelets, nanowires, nanorods, or polypods of these materials are commonly trapped in a low-symmetric crystal structure (e.g., hexagonal, tetragonal, orthorhombic) that preferentially extends along or is perpendicular to their axis of higher symmetry. Shape anisotropy has important consequences as to the behavior of these NCs when used as starting seeds. The facets at the apex, edges, and longitudinal/basal sidewalls generally exhibit a distinct atomic arrangement, hence a dissimilar chemical reactivity. In nanorods/wires, the lack of a plane of symmetry perpendicular to their major elongation axis implies that the two terminal basal facets at the extremities are crystallographically, hence chemically, nonequivalent. Narrowing at the nanorod termini may further exacerbate such dissimilarities [243, 277, 279–281, 284–289].

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Scheme 13.4 Possible pathways that may lead to nanocrystal heterostructures, starting from anisotropically shaped seeds: Functionalization of a nanorod (A) or of a tetrapod (B) with spherical domains of a different material at selected locations. From R. Buonsanti, M. Casavola, G. Caputo, P.D. Cozzoli, Advances in the chemical fabrication of complex multimaterial nanocrystals, Recent Pat. Nanotechnol. 1 (3) (2007) 224–232 with permission, copyright 2007 Bentham Science Publisher.

In line with the mechanism of their anisotropic growth, the longitudinal sidewalls and the two tips of nanorods can be expected to exhibit significantly different propensities to accommodate secondary material domains in seeded-growth strategies (Scheme 13.4A). Furthermore, depending on the specific case, nanorods for which the lattice

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Fig. 13.11 Gallery of TEM images showing examples of oxide-based anisotropic HNCs synthesized by site-selective heterogeneous deposition on preformed nanorods seeds (cf. Scheme 13.4). (A) Nanodumbbells of [email protected] CdS nanorods obtained upon Co deposition on the tips of preformed Pt-tipped CdS nanorods and (B) nanodumbbells of corresponding [email protected] CdS nanorods with [email protected] [email protected] termini obtained upon Co oxidation of their parent [email protected] CdS nanorods; (C) TEM image, (D–G) elemental EXD maps, (H) HAADF-STEM image and elemental EDX profiles (I and J) of nanodumbbell-like HNCs individually composed of one Au nanorods capped with extended TiO2 domains at the apexes; (K) [email protected] [email protected] tetrapods asymmetrically decorated with a single [email protected]/CoxOy [email protected] tip domain; (L) nanomatchsticks made of

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deviates from the ideal bulk crystallographic structure or that have been heterostructured into [email protected] architectures can hold an intrinsic dipole moment or accommodate defective strained interior surface regions, which may influence their seeding capabilities [28, 276, 284, 286, 287, 290]. Demonstration of these concepts is illustrated by the fabrication of HNCs with matchstick-like or dumbbell-like topologies, in which either one or both apexes or the longitudinal sidewalls of the nanorod seeds are decorated with domains of other materials [26, 28, 243, 248, 276, 277, 279–282, 284–287, 289, 291–301]. Exemplary cases are selected in Fig. 13.11. At the relevant bonding heterointerfaces, the mismatched lattice arrangement can break the crystal periodicity to a varying extent, giving rise to noticeable interfacial strain that may, in turn, affect their electronic-band structure [26, 28, 276, 283] as well as the type and extent of exchange coupling between different magnetic properties or between the optical and magnetic functionalities [278–280, 286, 301, 302]. Misfit strains may be alleviated: (i) through induced interfacial defects, such as dislocations, stacking faults, or interfacial lattice adaptation (coherency strain) [283, 303] that may be easily activated under the typical high-temperature conditions in which colloidal synthesis is carried out; and/or (ii) through gradual change of the chemical composition of the heterojunction regions, across which constituent atoms can interdiffuse [297, 304]. Experimental results [278, 286, 297, 301, 302, 304] and thermodynamic modeling [283, 305] have indicated that the temperature and the particular chemical pathways being operative in the synthesis can be decisive in determining the attainment of either alloyed, graded, or abrupt interfaces.

Fig. 13.11 cont’d sigle-γ-Fe2O3-tipped brookite TiO2 nanorods; (M) hetero-oligomers made of multiply γ-Fe2O3-decorated brookite TiO2 nanorods. (A and B) Reproduced from L.J. Hill, M.M. Bull, Y. Sung, A.G. Simmonds, P.T. Dirlam, N.E. Richey, S.E. DeRosa, I.-B. Shim, D. Guin, P.J. Costanzo, N. Pinna, M.-G. Willinger, W. Vogel, K. Char, J. Pyun, Directing the deposition of ferromagnetic cobalt onto Pt-tipped [email protected] nanorods: synthetic and mechanistic insights, ACS Nano 6 (10) (2012) 8632–8645 with permission, copyright American Chemical Society. (C–J) Reproduced from B.H. Wu, D.Y. Liu, S. Mubeen, T.T. Chuong, M. Moskovits, G.D. Stucky, Anisotropic growth of TiO2 onto gold nanorods for plasmon-enhanced hydrogen production from water reduction, J. Am. Chem. Soc. 138 (4) (2016) 1114–1117 with permission; copyright American Chemical Society. (K) Reproduced from N.G. Pavlopoulos, J.T. Dubose, N. Pinna, M.-G. Willinger, K. Char, J. Pyun, Synthesis and assembly of dipolar heterostructured tetrapods: colloidal polymers with “giant tert-butyl” groups, Angew. Chem. Int. Ed. 55 (5) (2016) 1787–1791 with permission, copyright Wiley-VCH Verlag GmbH & Co. KGaA. (L) Reproduced from R. Buonsanti, V. Grillo, E. Carlino, C. Giannini, F. Gozzo, M. Garcia-Hernandez, M.A. Garcia, R. Cingolani, P.D. Cozzoli, Architectural control of seeded-grown magnetic-semicondutor iron oxide-TiO2 nanorod heterostructures: the role of seeds in topology selection, J. Am. Chem. Soc. 132 (7) (2010) 2437–2464 with permission, copyright American Chemical Society. (M) from R. Buonsanti, E. Snoeck, C. Giannini, F. Gozzo, M. Garcia-Hernandez, M.A. Garcia, R. Cingolani, P.D. Cozzoli, Colloidal semiconductor/magnetic heterostructures based on iron-oxide-functionalized brookite TiO2 nanorods, Phys. Chem. Chem. Phys. 11 (19) (2009) 3680–3691 with permission, copyright Royal Society of Chemistry.

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Fig. 13.12 Examples of anisotropic HNCs synthesized by surfactant-controlled selective heterogeneous nucleation mechanism (cf. Scheme 13.4). The panels show: (A and B) HRTEM image (where “1” indicates the TiO2 section, while “2” indicates the Co termination) and TEM image of HNCs made of Co-tipped anatase TiO2 nanorods; (C–F) variable-magnification TEM images of oligomer-type HNCs made of Co-decorated γ-Fe2O3 tetrapods; (G) a TEM image of binary HNCs made of Ag-decorated TiO2 nanorods. (A and B) Reproduced from M. Casavola, V. Grillo, E. Carlino, C. Giannini, F. Gozzo, E. Fernandez Pinel, M.A. Garcia, L. Manna, R. Cingolani, P.D. Cozzoli, Topologically controlled growth of magnetic-metal-functionalized semiconductor oxide nanorods, Nano Lett. 7 (5) (2007) 1386–1395 with permission, copyright American Chemical Society. (C–F) Reproduced with permission from M. Casavola, A. Falqui, M.A. Garcia, M. Garcia-Hernandez, C. Giannini, R. Cingolani, P.D. Cozzoli, Exchange-coupled bimagnetic cobalt/iron Oxide branched nanocrystal heterostructures, Nano Lett. 9 (1) (2009) 366–376, copyright American Chemical Society. (G) Synthesized according to Q. Lu, Z. Lu, Y. Lu, L. Lv, Y. Ning, H. Yu, Y. Hou, Y. Yin, Photocatalytic synthesis and photovoltaic application of Ag-TiO2 nanorod composites, Nano Lett. 13 (11) (2013) 5698–5702 with permission, copyright American Chemical Society.

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Fig. 13.13 TEM and HRTEM examples of HNCs achieved by strain-driven heteroepitaxial growth (cf. Scheme 13.4). (A and B) Binary heterostructure, each made of one anatase TiO2 nanorod bound to a single γ-Fe2O3 nanocrystal. (C) Binary heterostructures made of asymmetrically [email protected] FexOy nanorods and (D) corresponding all-oxide nanostructures obtained upon Au leaching. (A and B) Reproduced with permission from R. Buonsanti, V. Grillo, E. Carlino, C. Giannini, M.L. Curri, C. Innocenti, C. Sangregorio, K. Achterhold, F.G. Parak, A. Agostiano, P.D. Cozzoli, Seeded growth of asymmetric binary nanocrystals made of a semiconductor TiO2 rodlike section and a magnetic γ-Fe2O3 spherical domain, J. Am. Chem. Soc. 128 (51) (2006) 16953–16970, copyright American Chemical Society. (C and D) Reproduced with permission from C. George, A. Genovese, F. Qiao, K. Korobchevskaya, A. Comin, A. Falqui, S. Marras, A. Roig, Y. Zhang, R. Krahne, L. Manna, Optical and electrical properties of colloidal (spherical Au)-(spinel ferrite nanorod) heterostructures, Nanoscale 3 (11) (2011) 4647–4654, copyright Royal Society of Chemistry.

As found for their [email protected] counterparts, in anisotropic HNCs based on associations of dissimilar semiconductors, (magnetic) metals, and metal oxides, the energy level structure across the interface can be purposely engineered upon proper heterostructure design so as to promote confinement or spatial separation of the

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photogenerated charge carriers [24, 26, 28, 276, 283]. The overall magnetic anisotropy (and the magnetic behavior in general) may also be strongly affected, even when magnetic modules are connected with nonmagnetic ones [278–280, 286, 301, 302]. Such hybrid nanoarchitectures hold promise as key elements for the realization of optoelectronic devices, efficient photocatalytic systems [24, 26, 28, 276], spintronic, and magnetic recording devices [278–280, 286, 301, 302]. Interesting breeds of HNCs obtained by the heterogeneous nucleation/growth are represented by semiconductormetal heterostructures, whereby anisotropic semiconductor or metal NCs have been exploited as platforms for the growth of a different material (Scheme 13.4A). Among the most controlled examples of these architectures are the earlier-developed prototypes of CdX-Me and Me-CdX-Me HNCs, based on CdX (X ¼ Se, S) nanorods functionalized with metal (Me ¼ Au, Pt, PtNi, PtCo, Co, Ni) domains [248, 280, 291, 295–298]. For these systems, a high degree of selectivity for secondary nucleation at the seed apexes has been achieved [295, 296, 298, 306]. Leveraging on this prior knowledge, several cases of oxide-based HNCs have been reported, as summarized below. As a proof of concept toward oxide-based HNCs with increased structural and compositional complexity, secondary metal domains on preformed heterostructures have been considered to be potentially exploitable as highly reactive sites where exclusive deposition of a third component may be directed. This route has actually been proven to be feasible within the context of rationally designed multiseeding-step reaction schemes [299, 300]. In one case, Pt-tipped [email protected] [email protected] nanorods were exploited to direct the otherwise unfeasible overgrowth of Co domains onto the Pt apexes, leading to corresponding [email protected]@[email protected] nanodumbbells with [email protected] terminations; the latter could then be converted into [email protected]@[email protected] heterostructures with [email protected] [email protected] termini upon selective oxidation of the Co sections [299] (Fig. 13.11A and B). In another protocol [300], asymmetrically Au-tipped [email protected] tetrapods were first fabricated by accommodating one single Au sphere at one of the four arm termini of preformed [email protected] [email protected] tetrapods through a photodeposition process that proceeded through the initial attainment of multiple Au-decorated tetrapod arms, followed by intraparticle ripening of the nucleated Au patches into a single Au tip (this evolution is analogous to the one underlying the topological transition of double-tipped Au-CdSe-Au nanodumbbells into their single-tipped Au-CdSe nanomatchstick counterparts [248]) (Scheme 13.4B). Subsequently, thermolysis of Co2(CO)8 in the presence of the [email protected] heterostructures and carboxylic-acid-terminated polystyrene ligands in 1,2,4-trichlorobenzene at T ¼ 140°C enabled selective overgrowth of an easy-to-oxidize Co shell around the previously implanted Au domains, leading to [email protected] [email protected] tetrapod heterostructures asymmetrically decorated with a single [email protected]/CoxOy [email protected] domain at one apex (Fig. 13.11C). In another account, the hydrolysis of TiCl3 in aqueous media via adjustment of pH was manipulated to provide CTAB-capped Au nanorods with TiO2 domains at their extremities, which grew extending partly over the rod section [307] (Fig. 13.11D–J). In most circumstances, the proximity between the semiconductor and the metal sections resulted in electronic band coupling and/or charge-density redistribution,

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which transcribed into luminescence quenching or modified plasmonic absorption features, reflecting into increased charge-carrier separating capabilities [26, 28, 276, 307, 308]. Localized defect states at the relevant interfaces were also invoked to account for the dramatically altered optoelectronic properties of such HNCs [26, 28, 276, 295, 308, 309]. Depending on the reaction medium (e.g., the presence of hole/electron scavengers) and the irradiation conditions, the metal section could either act a sink for the electrons photogenerated in the semiconductor or help shuttling them to the solution, which was transcribed, for instance, in efficient charge retention or enhanced photocatalytic reduction yield [26, 28, 276, 298, 307, 308, 310, 311]. The magnetic properties [280, 295] were also found to be modified upon heterojunction attainment. Interestingly, selective functionalization with magnetic metal domains was proven to be a valuable strategy toward enhancing the propensity of heterostructures to self-assemble through tip-to-tip dipolar magnetic interactions [299, 300]. Examples of all-oxide-based HNCs have been documented. A nonaqueous approach to magnetic-semiconductor γ-Fe2O3-TiO2 HNCs with a switchable hetero-oligomer to heterodimer topologies and tunable geometric parameters was developed [289, 292] (Fig. 13.11L and M). The synthesis exploited brookite TiO2 nanorods with either rectangular- or tapered-shaped profiles, the latter terminating into arrow-like apexes with high-index facets [312] as seeds for accommodating γ-Fe2O3 upon decomposition of Fe(CO)5 in OLAM/OLAC/hexadecan-1,2-diol mixtures at 280–300°C. The TiO2 seeds exhibited size- and shape-dependent anisotropic reactivity, which allowed producing HNCs individually made of a single TiO2 section functionalized with either one or multiple spherical γ-Fe2O3 domains at distinct locations in a controlled manner (Scheme 13.4A, path 1, 3). The surface-interface energy balance associated with the formation of the different architectures, examined on the basis of comparative experimental strain mapping of individual HNCs and on CSLTbased analysis of TiO2/γ-Fe2O3 heteroepitaxy relationships [244], did not provide a conclusive explanation regarding the mechanism underlying the topological selectivity observed under specified conditions. Alternatively, the synthesis outcome was rationalized within the framework of a model assuming rapid establishment of diffusion-limited growth conditions for γ-Fe2O3. According to this model, γ-Fe2O3 overgrowth on the TiO2 seeds could switch from a thermodynamically controlled (corresponding to a nonselective γ-Fe2O3 deposition on the nanorod seeds) to a kinetically driven regime (corresponding to selective γ-Fe2O3 nucleation and growth on the seed apex) when highly anisotropically reactive seeds (those with arrow-like terminations) were utilized under conditions (high iron oxide precursor to seed ratios) favoring establishment of steep monomer gradients across the diffusion layer around each seed [289]. The as-synthesized γ-Fe2O3-TiO2 offered a rich scenario of photocatalytic properties or modified magnetic responses, which clearly diverged from those exhibited by their individual components and physical mixture counterparts [289, 292]. Intriguing branched α-Fe2O3-SnO2 HNCs were built from crystallographicoriented epitaxial SnO2 growth on shaped α-Fe2O3 seeds upon hydrothermal dehydration of Sn(OH)2 6 species [313, 314]. For example, when sixfold symmetrical

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spindle-shaped α-Fe2O3 seeds were used, multiple SnO2 nucleated onto each (110) facet, developing into small adjacent rods that progressively merged together laterally to decrease the overall surface energy. On the other hand, when SnO2 overgrowth took place on cubic-shaped seeds, the secondary oxide nucleated on the α-Fe2O3 facets slantwise at a fixed angle of 65 degrees, an arrangement that reduced the interfacial lattice mismatch to 1% and reduced the generation of misfit dislocations. A similar tendency toward branching was observed on employing hexahedron-shaped α-Fe2O3 seeds [313]. Compared to the bare α-Fe2O3 particles, branched α-Fe2O3SnO2 HNCs showed prominent photocatalytic activity toward organic dye degradation under both visible and UV light irradiation [314]. It is also worth mentioning that hyperbranched α-Fe2O3-SnO2 heterostructures with a sixfold symmetry, composed of an SnO2 nanowire stem and α-Fe2O3 nanorod branches, which were generated by hydrothermal overgrowth of iron oxide on preformed SnO2 nanowire substrates prepared by a vapor-transport deposition route [315]. Such architectures demonstrated superior performance as anode materials for lithium-ion batteries as a result of the synergistic effect of the α-Fe2O3 and SnO2 components arranged in branched architectures. Finally, it deserves mentioning that mixed-dimensionality heterostructures, based on a combination of rod sections and other shaped domains, have also been accessed by various empirically set wet-chemical approaches [316–319].

13.4.2.2 Surfactant-controlled selective heterogeneous nucleation For several material associations, careful investigation of the conditions underlying formation of HNCs with nonequivalent topologies has allowed inferring the involvement of the already postulated mechanism of facet-preferential adhesion of surfactants or ligands as the main pathway governing the site-dependent accessibility and chemical reactivity of shaped seeds (Scheme 13.4). Examples of HNCs that were elaborated by this route are reported in Fig. 13.12. A clear influence of the growing environment on the ultimate location of secondary material domains was identified in the formation of magnetic/semiconductor Co-decorated TiO2 nanorods [278], which were synthesized by thermal decomposition of Co2(CO)8 under assistance of octanoic acid (OCAC) and OLAM at 250–280°C (Fig. 13.12A and B). The temporal variation of the OCAC/OLAM concentration along the synthesis course was judiciously programmed to adjust the approachability of the cobalt precursor to the different TiO2 nanorod facets explored, which enabled switching heterogeneous Co nucleation from a tip-preferential to a nonselective decoration regime in which multiple metal domains formed on the longitudinal sidewalls of the seeds (Scheme 13.4A, path 1 versus path 3). According to CSLT-based arguments [244], both types of TiO2/Co HNCs were interpreted as being nearly equivalent in terms of interfacial strain, regardless of the different types of heterojunctions formed. Hence, site-preferential Co overgrowth was rationalized as being a process that compensated for the increase in the overall surface tension caused by progressive depletion of the organic passivation layer on the respective seed facets at controllably low nominal surfactant concentration. As the two materials

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communicated through rather extended interfaces, proximity effects at the interface unusually influenced the magnetic anisotropy of the Co domains [278]. Another case study pertains to HNCs made of a γ-Fe2O3 tetrapod skeleton randomly decorated with multiple Co domains (Fig. 13.12C–F) (Scheme 13.4B, path 3). In these branched heterostructures, FiM-FM exchange coupling was established, leading to a rich variety of unique magnetic properties, including noticeable exchange bias, increase of the saturation magnetization and coercivity, and enhancement of the magnetization thermal stability [302]. In a more recent report, HNCs with analogous topology, namely multiple Co-decorated CoO tetrapods, were generated through consecutive nucleation growth events that took place at different temperatures during the one-pot thermal processing of Co(acac)2 and 1,2-dodecanediol as the reducing agent in ODE at 200°C and 320°C, respectively [320]. To achieve Ag deposition on anatase TiO2 nanorods, the latter were UV irradiated in a mixed toluene/methanol medium in the presence of an AgNO3 precursor dissolved with the aid of OLAM [321]. The seeds became activated toward heterogeneous nucleation due to partial displacement of their original capping molecules and the availability of reducing electrons produced photocatalytically. According to control experiments and careful monitoring of the synthesis products, the initially multiple Ag-decorated TiO2 nanorods evolved into binary HNCs, each made of a single Ag particle located on the longitudinal side of its supporting nanorod, as a consequence of hole-driven dissolution of the smaller, unstable Ag domains in the later stages of the UV irradiation process (Fig. 13.12G). This was consistent with the propensity of the anisotropic TiO2 seeds toward accommodating foreign material domains at locations guaranteeing higher thermodynamic stability (Scheme 13.4A, path 1-2 versus 3).

13.4.2.3 Strain-driven Heterostructure formation Interesting NC heterostructures have been engineered by taking advantage of the generation of strain fields during heteroepitaxial growth. The cases in Fig. 13.13 illustrate how strain may influence heteroepitaxial growth and the topology of the final heterostructures. Asymmetric binary HNCs, each made of one rod-shaped TiO2 section and one sizetunable γ-Fe2O3 spherical domain attached longitudinally, were obtained by decomposing Fe(CO)5 on anatase TiO2 nanorod seeds in a ternary mixture of OLAC, OLAM, and 1,2-hexadecandiol at 240–300°C [322] (Fig. 13.13A and B). The γ-Fe2O3 deposition likely proceeded as a means of selectively eliminating high-energy edges on the longitudinal stepped sidewalls of the TiO2 seeds (Scheme 13.4A, path 2). However, constraints imposed by the huge interfacial strain (8%–11%) limited extensive “wetting” of the (011)/(101) facets underneath the edges, causing the TiO2 section to be deformed and curved toward the γ-Fe2O3 sphere. The TiO2 and γ-Fe2O3 lattices were coherently connected via a rather limited junction area at which the near-surface planes of the respective materials were locally bent. This allowed the strain accumulated at the interface to be relieved to a great extent, paying only a proportionally smaller cost of additional surface energy. These factors jointly accounted for the

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inhibition of reiterated deposition events, on one side, and for the ultimate size (15 nm) to which the γ-Fe2O3 domains could be enlarged. Due to the limited contact area shared with TiO2, the magnetic properties of the HNCs essentially resembled those of isolated γ-Fe2O3 particles [322]. In another report, HNCs with analogous topologies, made of a spinel FexOy nanorod section asymmetrically decorated with a spherical [email protected] [email protected] domain (Fig. 13.13C and D), were synthesized through an opposite reaction sequence [323]. Primary Au seeds were reacted with Fe(CO)5 in ODE-diluted mixtures of OLAM, OLAC, and dodecyldimethylammonium bromide (DDAB) at 300°C. A thin, nearly ubiquitous yet discontinuous shell of spinel FexOy initially formed on the spherical Au seeds. However, upon prolonged annealing at high temperature, the combined effects of the strain increasing at the Au/FexOy interface and of the facet-selective binding of DDAB on FexOy promoted breaking of the FexOy growth symmetry, leading to tangential evolution of an FexOy nanorod out of each [email protected] [email protected] HNC intermediate. This mechanism was corroborated by topological and structural analysis of the all-oxide nanostructures that were derived by performing selective etching of gold (with iodine) on the [email protected] nanorod heterostructures (Fig. 13.13D).

13.5

Conclusions

The construction of modular nanocrystal heterostructures that integrate sections of epitaxially connected oxide and nonoxide materials represents a frontier research area in which nanochemistry approaches have progressed tremendously over the past few years. The wet-chemical synthesis of colloidal HNCs is inherently challenging, as the ability to tailor the target material modules requires being consolidated by the understanding of the thermodynamic parameters and kinetic processes that underpin their sequential assembly in space via heteroepitaxial growth pathways in liquid media. Achievements documented so far suggest that an increasingly higher degree of topological sophistication and regioselectivity in HNC engineering should be achievable by leveraging the knowledge of the solid-state and liquid-phase formation mechanisms that govern HNC evolution. As of today, realization of the technological potential of oxide-based HNCs is made difficult by the level of synthetic precision and property control with which these nanoheterostructures can be deliberately accessed to meet specific purposes. Unfortunately, the search for multifunctionality often conflicts with some inevitable degradation of the pristine properties of the component modules due, for example, to unfavorable changes in electronic structure and/or to attainment of defective interfaces. In many cases, it remains difficult to unambiguously discriminate mere proximity effects from the emergence of truly new or exotic chemical-physical properties as a consequence of the electronic interactions and exchange coupling via the relevant heterojunctions. The support of theoretical investigations, which are still sparse, is highly needed in an effort toward decoupling the genuine role played by heterointerfaces.

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For the future, it can be envisioned that the development of design and synthetic capabilities in the elaboration of HNCs will pave the way to both fundamental understanding and practical exploitation of unconventional properties and functionalities, particularly in the continually expanding fields of optoelectronics, catalysis, and biomedicine.

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