Environmentally friendly synthesis of supportless Pt based nanoreactors in aqueous solution

Environmentally friendly synthesis of supportless Pt based nanoreactors in aqueous solution

Chemical Physics Letters 612 (2014) 309–312 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/lo...

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Chemical Physics Letters 612 (2014) 309–312

Contents lists available at ScienceDirect

Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Environmentally friendly synthesis of supportless Pt based nanoreactors in aqueous solution Michael N. Groves a,1 , Cecile Malardier-Jugroot a,∗ , Manish Jugroot b a b

Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, ON, Canada K7K7B4 Department of Mechanical and Aerospace Engineering, Royal Military College of Canada, Kingston, ON, Canada K7K7B4

a r t i c l e

i n f o

Article history: Received 30 May 2014 In final form 7 August 2014 Available online 14 August 2014

a b s t r a c t Using the amphiphilic alternating copolymer poly(styrene-alt-maleic anhydride), the hydrophobic salt PtCl2 , is reduced into platinum nanoparticles that are less than 3 nm in an aqueous biocompatible environment. The nanoparticles are characterized by transmission electron microscopy and X-ray diffraction. The formation of unsupported nanoparticles embedded in the polymer nanotemplate take advantage of the confinement effect in reactions due to the dynamic formation of the poly(styrene-alt-maleic anhydride) nanotube. This enhanced catalytic effect is demonstrated with the polymerization of pyrrole. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

Nanocatalysts are high surface-to-volume particles useful for many reactions including reactions in polymer electrolyte fuel cells [1], methanol and ethanol electrooxidation [2,3], and in a number of single step hydrogenation reactions [4]. They show a higher catalytic activity than bulk structures made of the same metal and can come in a variety of shapes [5,6] which can have an effect on their relative activities [7]. The high surface to volume ratio ensures an optimal use of the catalyst. The most common method to synthesize nanocatalysts is by chemical reduction methods including alcohol reduction [8], hydrogen reduction [9] and sodium borohydride reduction [10]. Other reduction techniques include electrochemical [11], photochemical [12,13] and sonochemical [14] methods. In general, the use of these reduction techniques are not environmentally friendly. Creating and employing catalysts in environmentally friendly conditions has many advantages. For instance, employing water instead of organic solvents such as benzene, toluene and acetone reduces toxicity and volatility concerns [15]. Ideally, the movement toward greener synthesis and usage of nanoparticle catalysts should also work toward maximizing their activity. To this end it is desirable to form higher order crystallographic facets which are more active [16] given that they contain more edge and corner sites [17,18]. In a work by Chen et al. [19] involving the polyol synthesis of platinum nanoparticles, the speed

∗ Corresponding author. E-mail address: [email protected] (C. Malardier-Jugroot). 1 Present address: Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark. http://dx.doi.org/10.1016/j.cplett.2014.08.017 0009-2614/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

of the reduction of the platinum precursor was controlled by including molecular oxygen and an iron species (FeII or FeIII ) to the reaction. By slowing the reaction rate they observed an increase in the growth of the (1 1 1) crystal face which lead to the formation of highly anisotropic structures. This indicates that the speed of the reduction plays some role in the morphology of the nanoparticle. Enhancements to a reaction rate can arise from macromolecular crowding, or spatial confinement. They have both been shown to enhance reactions depending on the relative sizes and shapes of the concentrated crowding species and on the diluted reactants and products [20]. In general the confinement effect is expected to increase reaction rates that are slow, transition statelimited association reactions and decrease the reaction rate of fast, diffusion-limited association reactions [21]. For example, spatial confinement physically restricts the available conformations that a protein can form which can make the active folded state more favorable. Macromolecular crowding can also be employed to achieve the same outcome but since the boundary is not rigid and interstitial voids are present, there is a probability that the reactants and intermediates will escape [22]. Poly(styrene-alt-maleic anhydride) (SMA) is an amphiphilic alternating copolymer that forms nanotubes with a 2.8 nm interior diameter in water at pH 7 [23]. This biocompatible copolymer can be used as a template for the synthesis of metal nanoparticles with a diameter less than 3 nm. Indeed, hydrophobic metal salts such as PtCl2 can be solubilized within the nanotube hydrophobic cavity. The hydrophobic property of this salt makes it favorable for it to reside in the interior of the SMA nanotubes where the reduction occurs. The Pt0 nanoparticles will be characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and

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an enhanced catalytic activity will also be demonstrated by using UV–vis spectroscopy to monitor the polymerization of pyrrole. 1 wt% SMA solutions were prepared by mixing Poly(styrenealt-maleic anhydride), partial methyl ester with an average Mw = 350 000 (Sigma–Aldrich), with deionized water. An aqueous solution of NaOH was used to raise the pH to 7 and the mixture was sonicated until the polymer had completely dissolved. 0.1 g of platinum (II) chloride, 98% (Sigma–Aldrich), was weighed and put into 3.0 g of the 1 wt% SMA solution. These samples were sonicated for 90 min at room temperature to break up the aggregated PtCl2 which would form when mixed into the aqueous solution. They were left to sit for a week for the color of the solution to start changing from a pale green to black with a precipitate on the bottom of the vial. A second sample was synthesized using 1 mL of a 0.5 M NaBH4 solution to reduce the PtII salt in an identical PtCl2 /SMA solution. In this second sample the color change was identical when the reducing agent was added. XRD was performed at beamline 08B1-1 at the Canadian Macromolecular Crystallography Facility at the Canadian Light Source. A rotary evaporator was used to dry five samples: PtCl2 in deionized (DI) water, PtCl2 in DI water reduced with NaBH4 , PtCl2 in 1 wt% SMA reduced with NaBH4 , PtCl2 in 1 wt% SMA, and the precipitate formed from the previously mentioned sample. The resulting ˚ powders were examined using an X-ray wavelength of 0.6888 A. Two-dimensional scattering data was collected and the intensities were integrated as a function of 2. Transmission electron microscopy (TEM) was performed on two samples at the Canadian Center for Electron Microscopy at McMaster University. Aqueous solutions of the PtCl2 reduced in 1 wt% SMA and in 1 wt% SMA plus NaBH4 were deposited onto copper TEM grids and left to dry in air at room temperature. Images were then recorded using a FEI Titan 80-300 LB TEM. To demonstrate how the Pt nanoparticles embedded in the SMA nanotemplate can further catalyze a reaction under confinement, the polymerization of pyrrole monomers was observed using UV–vis spectroscopy. Using a UV-240 spectrometer, the growth of the ␲ to ␲* excitation among the polypyrrole oglimer chain which occurs at 480 nm [24,25] was measured. Polypyrrole has already been demonstrated to spontaneously polymerize in SMA at pH 7 due to the confinement effect [26]. Thus, the synthesis of polypyrrole in 1 wt% SMA in water will be compared to 1 wt% SMA with embedded Pt nanoparticles. Two samples were prepared consisting of 10 ␮L of 1 wt% SMA and 10 ␮L of a 1 wt% SMA/Pt nanoparticle solutions each mixed with 2 mL of 18 M water and 10 ␮L of pyrrole monomer and left at room temperature in quartz UV–vis cuvettes in ambient light. The progress of the polymerization reaction was then recorded every eight days. The results from the XRD and TEM analyses confirm that the SMA reduced the PtII to Pt0 nanoparticles. It has been shown previously that the thermal decomposition of hexachloroplatinic acid yields PtCl2 with good stability between the temperatures of 350 ◦ C and 410 ◦ C. Above this temperature it decomposes into Pt(s) and Cl2(g) [27]. It is this decomposition that is proposed here as a potential mechanism, which now occurs at room temperature due to the confinement of PtCl2 in the hydrophobic interior of the SMA polymer. The radially integrated XRD results are presented in Figure 1. The PtCl2 sample that was mixed into DI water only and then dried matches the crystallography profile for PtCl2 (curve B in Figure 1a). The second PtCl2 sample mixed into DI water only but was then reduced using NaBH4 (curve A in Figure 1b) shows two large peaks which correspond to the Pt (1 1 1) crystal face at 17.49◦ and the (2 0 0) crystal face at 20.22◦ (JCPDS 04-0802). These two peaks were also observed in both samples with 1 wt% SMA and PtCl2 regardless of if NaBH4 was used (curves B and C in Figure 1b). This remarkable result, that Pt nanoparticles are present throughout the sample

Figure 1. XRD results from the five PtCl2 samples using an X-ray wavelength of ˚ Curves have been offset for clarity. 0.6888 A.

when no reducing agent is used, can only be due to the presence of SMA nanotubes. Finally, the precipitate from the SMA/PtCl2 solution where no reducing agent was added (curve A in Figure 1a) matched the profile from the original PtCl2 sample. This shows that the SMA is only able to solubilize a certain amount of the PtCl2 while the remaining precipitate can be reclaimed and reduced with a fresh batch of SMA. The remaining phases in the XRD data will be fully characterized in a future publication along with ab initio descriptions of these phases. The TEM images presented in Figure 2 also shows that platinum nanoparticles have been formed even when the NaBH4 reducing agent was not used. Regardless of if NaBH4 is employed the nanoparticles are found in polymer matrices. The average particle size was determined by averaging the width, in number of pixels, of the dark circles seen in the TEM images when compared to the 2 nm scale bar at the bottom left of both images. When NaBH4 is used the average size is 1.94 ± 0.09 nm while in the case where no NaBH4 is used the average size is 1.89 ± 0.09 nm. Given that there is no statistical difference in average nanoparticle size between the two implies that the upper bound on the rate of PtCl2 uptake into the SMA is on the order of a couple of minutes. This is based on the fact that the NaBH4 solution was applied within minutes of the addition of PtCl2 to the SMA solution. This is not surprising as PtCl2

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Figure 3. UV–vis absorbance data taken every eight days of the pyrrole polymerization reaction.

Figure 2. TEM images showing the platinum nanoparticles. The scale bar in both cases is 2 nm.

is hydrophobic and would tend to associate within the hydrophobic cavity of the SMA nanotube. With the PtCl2 in the SMA nanotube in both cases, the reduction occurs gradually on its own or quickly using the NaBH4 . In both cases no platinum was observed outside the polymer matrix. The addition of NaBH4 to the PtCl2 /SMA mixture also seems to modify the nanoparticle size limits according to the TEM images. In both cases the nanoparticle sizes are restricted to be below 3 nm which is consistent with the previously reported 2.8 nm interior diameter of the SMA nanotube. In the case where no reducing agent is used, however, a greater frequency of larger nanoparticles is observed. This can be attributed to the slower reduction rate. When the reducing agent is employed, the reduction rate is increased which tends to limit nanoparticle growth. This is consistent with the results presented in Chen et al. [19].

The lower limit on the nanoparticle sizes also seem to have an abrupt floor when reduced with NaBH4 . This might also have to do with the speed at which the reduction occurs. When the reduction is allowed to proceed using only the SMA the smaller nanoparticles measured might still be growing. At this point, PtCl2 available to be reduced will be scarce as the excess will have precipitated out of solution as shown in the XRD data. The NaBH4 used to reduce the other sample would be able to utilize this excess PtCl2 , as it is still in solution, to build a minimum size nanoparticle. Finally, from the TEM images, the dominant crystal face can be determined. In the case where the platinum salt was reduced with NaBH4 the average d-spacing was 0.198 ± 0.007 nm. In the case where no NaBH4 is used the d-spacing is 0.197 ± 0.007 nm. Given that there is no significant difference between the two they will be considered identical and corresponds to the (2 0 0) miller index [28]. The slower reduction rate was previously found to form highly anisotropic structures. This is not the case here. Instead, circular nanoparticles were formed. While synthesizing CdSe nanoparticles, Peng [29] showed that the amount of precursor in solution also determines the morphology of the nanoparticle. The amount of monomer in solution is depleted by the nucleation and growth of the particle. The concentration of precursor needs to remain high to form elongated and thermodynamically metastable nanocrystals. If the monomer concentration drops then dot-shaped nanoparticles

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form with lower order crystal faces [29]. Due to the hydrophobic nature of PtCl2 the concentration of reactant to be reduced and ultimately form the nanoparticles will be limited to what has migrated inside the SMA nanotube. This means that in this case, the precursor concentration determines the shape and crystal structure of the nanoparticles instead of the reaction rate. This raises the potential for the formation of nanorods/wires and other structures if the monomer concentration is increased. The synthesis of high surface to volume Pt nanoparticles in the SMA polymer template can also be shown to help catalyze reactions that already can benefit from the enhancement of the confinement effect. SMA has previously been reported to trigger the polymerization of pyrrole in about a month [26]. The rate of the reaction was increased by the presence of Pt nanoparticles. Indeed, it only takes a third of the time to start to show a measurable change when the Pt nanoparticles are embedded according to UV–vis spectroscopy as shown in Figure 3. According to the data, the start of the characteristic polypyrrole peak occurs after only eight days with the presence of the Pt nanoparticles in 1 wt% SMA. This peak does not appear for the 1 wt% SMA sample until 24 days after the start of the reaction. This result clearly demonstrates the additional catalytic activity enjoyed by the SMA nanoreactor with the inclusion of platinum. In addition, as the position of the peak does not change during the polymerization, the clearer peak observed in the presence of a catalyst could mean that the chain length of the polymer inside the confined environment is well-defined. Given the environmentally friendly synthesis and biocompatibility of this system, applications seem to only be limited by having hydrophobic reactants. This work demonstrates that the metal salt, PtCl2 can be reduced to platinum nanoparticles with a size of less than 3 nm inside a SMA nanotube in aqueous environment at pH 7. While this process is only expected to work with hydrophobic metal precursors, it creates a supportless catalyst which can benefit from the confinement effect in reactions that take place within the controlled environment of the polymeric template. This was demonstrated by observing the higher polymerization rate of pyrrole upon inclusion of the Pt nanoparticles with a 1 wt% SMA nanotemplate. Furthermore, the Pt nanoparticle synthesis occurs in a biocompatible environment. This environmentally friendly synthesis allows the fabrication of monodisperse nanostructures in a controlled environment with potential applications ranging from biological systems to nanoelectronics. Author contributions All authors contributed equally to the presented work.

Conflict of interest The authors declare no competing financial interests. Acknowledgments The authors would like to thank the National Science and Engineering Research Council of Canada (NSERC) (NSERC Discovery Grant, 355595), the Canadian Center for Electron Microscopy (McMaster University) and the Canadian Macromolecular Crystallography Facility at the Canadian Light Source for their generous support. References [1] J.W. Guo, T.S. Zhao, J. Prabhuram, C.W. Wong, Electrochim. Acta 50 (2005) 1973. [2] S.B. Han, Y.J. Song, J.M. Lee, J.Y. Kim, K.W. Park, Electrochem. Commun. 10 (2008) 1044. [3] P. Maity, C.S. Gopinath, S. Bhaduri, G.K. Lahiri, Green Chem. 11 (2009) 554. [4] J.M. Thomas, B.F.G. Johnson, R. Raja, G. Sankar, P.A. Midgley, Acc. Chem. Res. 36 (2003) 20. [5] T.S. Ahmadi, Z.L. Wang, T.C. Green, A. Henglein, M.A. El-Sayed, Science 272 (1996) 1924. [6] M.A. Mahmoud, C.E. Tabor, M.A. El-Sayed, Y. Ding, Z.L. Wang, J. Am. Chem. Soc. 130 (2008) 4590. [7] R. Narayanan, M.A. El-Sayed, Nano Lett. 4 (2004) 1343. [8] T. Teranishi, M. Miyake, Chem. Mater. 10 (1998) 594. [9] C.H. Yen, X. Cui, H.-B. Pan, S. Wang, Y. Lin, C.M. Wai, J. Nanosci. Nanotechnol. 5 (2005) 1852. [10] R.M. Modibedi, T. Masombuka, M.K. Mathe, Int. J. Hydrogen Energy 36 (2011) 4664. [11] M.T. Reetz, W. Helbig, J. Am. Chem. Soc. 116 (1994) 7401. [12] N. Toshima, T. Takahashi, H. Hirai, Chem. Lett. 14 (1985) 1245. [13] K. Kurihara, J. Kizling, P. Stenius, J.H. Fendler, J. Am. Chem. Soc. 105 (1983) 2574. [14] T. Fujimoto, S.-Y. Terauchi, H. Umehara, I. Kojima, W. Henderson, Chem. Mater. 13 (2001) 1057. [15] G. Zhan, Y. Hong, V.T. Mbah, J. Huang, A.-R. Ibrahim, M. Du, Q. Li, Appl. Catal. A 439–440 (2012) 179. [16] Y. Xiong, B.J. Wiley, Y. Xia, Angew. Chem. Int. Ed. 46 (2007) 7157. [17] G. Somorjai, D. Blakely, Nature 258 (1975) 580. [18] R. Narayanan, M. El-Sayed, J. Phys. Chem. B 109 (2005) 12663. [19] J. Chen, T. Herricks, Y. Xia, Angew. Chem. Int. Ed. 44 (2005) 2589. [20] M.A. Mahmoud, F. Saira, M.A. El-Sayed, Nano Lett. 10 (2010) 3764. [21] H.-X. Zhou, G. Rivas, A.P. Minton, Annu. Rev. Biophys. 37 (2008) 375. [22] H.-X. Zhou, Acc. Chem. Res. 37 (2004) 123. [23] C. Malardier-Jugroot, T.G.M. van de Ven, T. Cosgrove, R.M. Richardson, M.A. Whitehead, Langmuir 21 (2005) 10179. [24] G. Appel, D. Schmei er, J. Bauer, M. Bauer, H. Egelhaaf, D. Oelkrug, Synth. Met. 99 (1999) 69. [25] M.C. Henry, C.-C. Hsueh, B.P. Timko, M.S. Freund, J. Electrochem. Soc. 148 (2001) D155. [26] X. Li, C. Malardier-Jugroot, Macromolecules 46 (2013) 2258. [27] A.E. Schweizer, G.T. Kerr, Inorg. Chem. 17 (1978) 2326. [28] L. Zhang, M. Ichiki, R. Maeda, J. Eur. Ceram. Soc. 24 (2004) 1673. [29] X. Peng, Adv. Mater. 15 (2003) 459.