Accepted Manuscript Scale synthesized cubic NaNbO3 nanoparticles with recoverable adsorption and photodegradation for prompt removal of methylene blue Lin Wang, Haoshuang Gu, Jian He, Tingting Zhao, Xuewen Zhang, Chuan Xiao, Hui Liu, Xianghui Zhang, Yuebin Li PII:
To appear in:
Journal of Alloys and Compounds
Received Date: 7 June 2016 Revised Date:
23 October 2016
Accepted Date: 26 October 2016
Please cite this article as: L. Wang, H. Gu, J. He, T. Zhao, X. Zhang, C. Xiao, H. Liu, X. Zhang, Y. Li, Scale synthesized cubic NaNbO3 nanoparticles with recoverable adsorption and photodegradation for prompt removal of methylene blue, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.10.327. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
M AN U
Scale synthesized cubic NaNbO3 nanoparticles with recoverable adsorption and photodegradation for prompt removal of methylene blue
Lin Wang 1, Haoshuang Gu 1, Jian He 1, Tingting Zhao 1, Xuewen Zhang 1, Chuan Xiao 2, Hui Liu 1, Xianghui Zhang 1, and Yuebin Li 1,* 1
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials ― Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices, Faculty of Physics & Electronic Sciences, Hubei University, Wuhan 430062, China 2 The Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0356, USA
* Corresponding author. E-mail addresses: [email protected]
M AN U
The adsorption and photodegradation performance are of paramount importance to photocatalysts in environmental hazards, especially in serious organic pollutants leaking emergency in the absence of natural light illumination. Here we demonstrated a surface ligand assisted localized crystallization ( SLALC) method to scale synthesis of cubic phase NaNbO3 (c-NaNbO3) photocatalyst at a low temperature as 350
. The as synthesized c-NaNbO3 nanoparticles could
adsorb 95% of methylene blue (MB) in 3 minutes and photodegrade 99.3% of MB in 180 minutes. Moreover, the as prepared c-NaNbO3 nanoparticles exhibit both outstanding adsorption and photodegradation recoverability, which shows
great potential in actual environmental conservation.
Adsorption, Photodegradation, Recoverable, c-NaNbO3 nanoparticles, Prompt removal, Methylene blue
1. Introduction During the past decade, the burgeoning semiconductor nanomaterials with varied composition, morphology, size and structure have been developed to work as light-driven photocatalysts for the degradation of organic pollutants due to their synthetic feasibility and charming optical performance [1-3]. Among the explored semiconductor photocatalysts, the newly reported NaNbO3 perovskite nanostructures have attracted noticeable attention currently in alleviating organic pollutants due to its rich useful properties including good physicochemical stability, high crystallinity, low cost, abundance, and low environmental impact [4-7]. NaNbO3 normally belongs to the orthorhombic system at room temperature and exhibits unique complex sequence of temperature-, pressure-, and particle-size-
driven phase transitions from cubic, tetragonal to orthorhombic [8,9]. As a result of the rich variation of crystal phase, NaNbO3 exhibits additional modulation strategy of light response other than element and size. Theoretically, the cubic phase (Pm3m) NaNbO3 (c-NaNbO3) possesses unique light response due to the high crystallographic symmetry induced electronic structures [9,10]. Nevertheless, most of the reported photocatalytic investigations were carried out over the common orthorhombic (Pbcm) NaNbO3 which are the most stable phase at room temperature, and the c-NaNbO3 is stable only at high temperature (>913 K) [8-11]. Jinhua Ye et al. reported that c-NaNbO3 nanoparticles could be stabilized through the surface coordination effect, and such a high-symmetry structure showed superior photocatalytic activities [12,13].
ACCEPTED MANUSCRIPT 2.3. Adsorption ability evaluation of the c-NaNbO3 nanoparticles.
The adsorption ability of the c-NaNbO3 nanoparticles was evaluated using MB as the organic pollutant model in the dark and at room temperature. Firstly, 60 mL methylene blue solution with a concentration of 15 mg/mL was mixed into 30 mL aqueous solutions containing 9-160 mg c-NaNbO3 nanoparticles. The mixture solution was then kept in the dark and sampled every 3 min until the adsorption process kept saturated and equilibrated. The adsorption efficiency values of as synthesized c-NaNbO3 nanoparticles were derived from the absorption spectra measured by a UV-vis absorption spectrometer. Photodegradation ability c-NaNbO3 nanoparticles
The photodegradation activity of the c-NaNbO3 nanoparticles was also evaluated using MB as the pollutant target at room temperature. Firstly, methylene blue solution with a concentration of 10 mg/mL were added into a Pyrex glass vessel containing 9-160 g c-NaNbO3 nanoparticles, and the mixture solution was stirred for 30 min to reach the equilibrium between adsorption and desorption in the dark. Then, the solution was sampled every 30 min under irradiation of a 100 W Hg arc lamp with a wavelength ranging from 365 to 600 nm (corresponding spectrogram profiled in Fig. S-1). The photocatalytic efficiency values of as synthesized c-NaNbO3 nanoparticles were derived from the absorption spectra measured by an UV-vis absorption spectrometer. After the photocatalysis reaction persisted for 3 h, the resulting c-NaNbO3 powders were collected by centrifugation and washed with deionized water for several times, and then dried at 60-80‐oC for recycle using in the degradation of MB.
M AN U
Whereas, expensive unstable metal alkoxides were used as precursor and a 500−900 °C calcination treatment was also required for about 10 h to perform the crystallization. Hence the facile fabrication of c-NaNbO3 nanoparticles is essential to the fundamental properties investigation and applications. Herein, c-NaNbO3 were synthesized via a designed surface ligand assisted localized crystallization (SLALC) method where stable and economical niobium oxalate and NaCl worked as the precursor or Nb5+ and Na+. Beside feasibility of synthesis, the excess Na+ also endows the as prepared c-NaNbO3 nanoparticles with abundant surface hydroxyl groups to improve the adsorptive capability. The as prepared c-NaNbO3 nanophotocatalyst exhibits both advanced adsorption and photodegradation performances, which was explored as adsorptive photocatalysts for the rapid removal of methylene blue (MB) dye from the aqueous solution. 2. Experimental 2.1. Chemical reagents
Ethanol (C2H6OH) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium chloride (NaCl, 99.5%) was purchased from Aladdin Chemistry Co. Ltd. Methylene blue (C16H18ClN3S· 3H2O) was purchased from Tianjin Hongyan reagent factory. All of these reagents were used without further purification. And solutions were prepared using water from a Millipore Waters MilliQ purification system.
2.2. Synthesis of c-NaNbO3 nanoparticles
The NaNbO3 nanocrystals with pure cubic phase were synthesized in gram scale via a low temperature localized crystallization method. In a typical synthesis process, 1.0-5.0 g of niobium oxalate and 0.1-5.0 g of sodium chloride firstly dissolved homogeneously in ethanol/DI water solution to form a white slurry. Subsequently, the slurry was ground for 30-50 min and dried in a vacuum oven. The resulted white powder was then moved into a crucible and heated up to 400-500 °C, and maintain 1-4 h to produce a white powder. NaNbO3 nanoparticles were eventually obtained by rinsing the white powder with ethanol/DI water solution for three times and then dried in a vacuum oven.
2.5. Apparatus and measurements The morphology of the as prepared c-NaNbO3 nanoparticles was characterized using a Field Emission Transmission Electron Microscope (Tecnai G20, FEI Co., USA) operated at 200 kV. The crystal phase was identified by an X-ray Diffractometer (D8A25, Bruker Co., Germany) equipped with a Cu Kα radiation (λ=0.154178 nm). The UV-Vis absorption spectra of c-NaNbO3 and concentration of MB was measured by a UV-Vis spectrometer (UV-3600, Shimadzu Co., Japan).
3. Results and discussion
M AN U
3.1. Synthesis and Characterization of the c-NaNbO3 nanoparticles
and sodium. The oxalate groups between the Nb5+ and Na+ ions were then cleaved by the further calcination, which resulted in the uniformly nucleation and growth of c-NaNbO3 nanocrystals due to the coordination effects of the oxalate groups and steric hindrance of the excessive surrounding NaCl with a melting point higher than 800 o C. The TEM image and HRTEM image (inset) shown in Fig. 1b reveals that the as prepared c-NaNbO3 white powder are composed of rice-like nanoparticles with an average size of 27 nm. As compared with traditional solid-state reaction route, the proposed SLALC method is more attractive to produce uniform small particles due to the synergetic effects of ligand coordination and spatial hindrance of excessive accumulated NaCl surrounding the c-NaNbO3 nuclei. Even though the reaction temperature of SLALC is low than that of typical solid-state reaction, the as prepared c-NaNbO3 nanoparticles are well crystallized, and the clear marked lattice-fringe with the spacing of 0.373 nm and 0.282 nm are corresponding to (100) and (110) planes of c-NaNbO3, respectively [15,16]. The XRD diffraction pattern (Fig. 1c) demonstrates that the as prepared c-NaNbO3 nanoparticles are highly crystallized and the obviously broadened diffraction pattern agreed well with that of the cubic NaNbO3 according to JCPDS card no.75-2102. Through the richly detailed analysis, one should find that the orthorhombic phase NaNbO3 with similar diffraction pattern usually exhibit apparent peaks splitting at 32.5o and 46.5o in main diffraction pattern . Nevertheless, The top right magnified (110) and (200) diffraction peaks (insets in Fig. 1c) present smooth unimodal shape, which is the typical characteristic of cubic structured NaNbO3
The FTIR spectra of the sample were measured by a Fourier Transform Infrared Spectrophotometer (SPECTRUM, USA). The XPS spectra of the sample were further measured by an X-ray Photoelectron Spectrometer (ESCALAB 250Xi, Thermo Scientific, UK) using Kα monochromatic X-ray radiation. The N2 adsorption-desorption curves were measured on an automated surface areas & pore size analyzer (QDS-MP-30, Quantachrome Instrument, USA). The zeta potentials were measured with Malvern Zetasizer Nano HPPS5001. The pH values of the samples for zeta potential measurement were adjusted with HNO3 or NaOH solutions.
Theoretically, c-NaNbO3 exhibits better light response activity and photochemical performance owing to their higher crystal symmetry, while the past reported NaNbO3 photocatalysts were mainly orthorhombic phase because that bulk c-NaNbO3 is only stable at the high temperature above 913 K due to the particular Brillouin-zone boundary phonon vibration [8,9]. As illustrated in Formula (1)-(2) and Fig. 1a, a facile surface ligand assisted localized crystallization ( SLALC) was proposed to synthesize c-NaNbO3 nanophotocatalyst with remarkable adsorptive properties . Initially, Na+ ions were coordinated to the carboxylic groups of niobium oxalate via an ion-exchange reaction, which resulted in a niobium sodium oxalate compound denoted as C10H5-xNaxNbO20. It worth pointing out that the as prepared C10H5-xNaxNbO20 compound could function as the single-source molecular precursor for both niobium
H2 O C10 H 5 NbO 20 + xNaCl → C10 H 5-x Na x NbO 20 + xHCl (1)
2C10 H5-x Na x NbO20 +5O2 → 2NaNbO3 + (x -1)Na 2CO3 + (21- x)CO2 + (5- x)H2O (2) Calcination
M AN U
Fig. 1 Growth mechanism, morphology, and crystal structure of the as prepared c-NaNbO3 nanoparticles. (a) Schematic illustration of
the LTLC method for the synthesis of c-NaNbO3 nanoparticles. (b) The TEM image and HRTEM image (the top right inset) of the as prepared c-NaNbO3 nanoparticles. (c) XRD pattern of as the prepared c-NaNbO3 nanopowder. (d) EDS spectrum and corresponding elements contents of the as prepared c-NaNbO3 nanopowder. (e) FTIR spectra of the as prepared c-NaNbO3 nanoparticles.
crystals . Fig. 1d presents the EDS spectrum of the white nanopowder, and the signal of Cu and C are originated from the TEM copper grid and carbon support film, respectively. Therefore, it can be concluded that the as prepared c-NaNbO3 nanoparticles are composed of Na, Nb and O elements with a molar ratio of 1.04:1:4.5, and the evident excess Na and relative higher oxygen contents were attributed to the surface oxygen atoms bonded Na+ ions and abundant surface hydroxyl groups of the c-NaNbO3 nanoparticles, respectively. Fig. 1e shows the FTIR spectrum of the as prepared c-NaNbO3 nanoparticles recorded in the region 450-4000 cm-1, and a strong peak located at 654 cm-1 in fingerprint region could be ascribed to the stretching vibrations of Nb-O bonds of the NbO6 octahedra in perovskite NaNbO3 structure. The weak peaks in Fig. 1e at 2344 cm-1, 2185 cm-1 and 1115 cm-1 in fundamental frequency region
could be attributed to the stretching vibrations of O=C=O, C=O and C-O of the surface adsorbed CO2 . The prominent peaks at 3430 cm-1, 1631 cm-1, and 1384 cm-1 are assigned to the asymmetric stretching vibrations and the bending vibrations of hydroxyl groups, which indicating there are abundant hydroxyl groups on the surface of the c-NaNbO3 nanoparticles . As shown in Fig. S-2, all peaks in the Raman spectrum are associated to the internal vibrational modes of NbO6 octahedrons. The weak peak at about 330 cm-1 can be assigned to Nb-O modes (A1g) and the distinct peak at 480 cm-1 can be assigned to Nb-O-Nb stretching modes (Eg) . However, the peaks related to the short Nb=O vibrational modes (580-700 cm-1) are not so prominent as those peaks reported about other phases of NaNbO3 [21-23], which is mainly due to the small nanoparticle size of the as prepared c-NaNbO3 (about 27 nm). The smaller
ACCEPTED MANUSCRIPT plot , which is smaller than that of orthorhombic NaNbO3 nanoparticles with a typical band gap of 3.45 eV . Moreover, the as prepared c-NaNbO3 nanoparticles exhibit an obvious absorption tail extended to 500 nm, which present high light responsive capability than common semiconductor photocatalysts. This intense and broad light absorption ability are mainly originated from the enhanced transfer capability of photogenerated carriers due to the high crystal symmetry of c-NaNbO3.
particle size is responsible for the decrease of the Raman scattering intensity and the shift of the Raman peaks [21,22]. Besides, in the ideal c-NaNbO3 structure, there is no NbO6 octahedrons lattice distortion to provide the high enough polarizability in the Raman scattering processes for folded phonons [18, 23-26]. X-ray photoelectron spectroscopy (XPS) was employed to further characterize the elemental composition and electronic states of the c-NaNbO3
3.2. Adsorption properties
M AN U
The adsorption equilibrium is a prerequisite step for the degradation of organic compounds by photocatalysis process. To evaluate the photocatalysis performance accurately, the adsorption capability of as prepared c-NaNbO3 nanoparticles was firstly evaluated through the adsorption equilibrium of MB (10 mg/L) at room temperature in the dark. As shown in Fig. 3a, after the addition of as prepared c-NaNbO3 nanoparticles, the bluish color of MB solution faded quickly within 3 min, which means the concentration of MB molecules are substantially reduced and it could be demonstrated by the UV-vis absorption evolution of the residual MB solutions). And this resulted from the adsorption effects of the c-NaNbO3 nanoparticles, rather than the reduction reactions induced colorless leucomethylene blue (LMB) (Fig. S-3a).To investigate the concentration effects of c-NaNbO3 nanoparticles on the adsorption behavior, the varied mass of c-NaNbO3 nanoparticles were added into MB solution, and the corresponded concentration dependent adsorption efficiency were profiled in Fig. 3b, where η = (C0 − Ct ) / C0 ∗100% , in which η is the adsorption efficiency, C0 and Ct are the initial concentration and concentration of the MB at time t (mg/L). The adsorption quantity of c-NaNbO3 nanoparticles (0.1-2.2 g/L) could reach a maximum value within about 3 min, and the adsorption reaction turns into the equilibrium state. The adsorption efficiency of MB was increased with the increase of the c-NaNbO3 nanoparticles, and the maximum adsorption efficiency value could be up to 95% at the concentration of 1.8 g/L. However, the efficiency reduced gradually with the further increase of the c-NaNbO3 to 81% (2.0 g/L) and 72%
Fig. 2 The electronic and optical properties of the as prepared
c-NaNbO3 nanoparticles. (a) Survey XPS spectrum of the as
prepared c-NaNbO3 nanoparticles. (b) High-resolution XPS spectra of the Nb 3d and Na 1s. (c) High-resolution XPS
spectrum of the O 1s. (d) UV-vis absorption spectrum and the c-NaNbO3 nanoparticles.
corresponding (αhν)1/2-hν curve (top right inset) of the
nanoparticles. Fig. 2a-c presents the XPS spectra of the c-NaNbO3 nanoparticles calibrated by C 1s peak at 284.8 eV. The binding energies of Nb 3d5/2 (206.9 eV) and Nb 3d3/2 eV (209.6 eV) present obvious red shift to those of orthorhombic NaNbO3, which could be attributed to the high symmetry, surface charging effects and the bonding effect between surface hydroxy groups (–OH) and lattice Nb atoms [7,27,28]. As is shown in Fig. 2c, the O 1s presents two peaks at 530.0 eV and 531.7 eV, which are ascribed to lattice oxygen (O2-) and surface hydroxyl (-OH) groups, respectively. As is revealed in Fig. 2d, the as prepared c-NaNbO3 nanoparticles present a strong light absorption band from 200 nm to 400 nm with an indirect band gap of 3.42 eV derived from the (αhν)1/2-hν
ACCEPTED MANUSCRIPT quantity of MB molecules by unit mass of the as prepared c-NaNbO3 nanoparticles (qads, mg/g) was employed to estimate the adsorption capacity and it is described by qads = (C0 - Ct )V m , in which C0 and C
M AN U
(2.2 g/L), respectively. This diminish was ascribed to the excess of the reaction sites versus the certain amount of MB molecules, which was also reflected by the adsorption capacity of each unit mass of the nanoparticles. As is shown in Fig. 3c, the adsorption
Fig. 3 The adsorption performances of as prepared c-NaNbO3 nanoparticles. (a) UV-vis absorption spectra of residual MB solutions
adsorbed by 1.8 g/L c-NaNbO3 nanoparticles and sampled every 3 min in 15 min. (right inset: the corresponding photographs of residual MB solutions). (b) The MB adsorption efficiency curves with different dosages of c-NaNbO3 in 15 min. (c) The MB adsorption efficiency and the adsorption capacity of c-NaNbO3 nanoparticles with varied concentration. (d) The adsorption stability of as prepared c-NaNbO3 nanoparticles in five cycles.
are the initial concentration and concentration of the MB at time t, m and V are the mass of adsorbents and the volume of MB solution. The adsorption capacity of as prepared c-NaNbO3 nanoparticle could be up to 17.5 mg/g at the concentration of 0.1 g/L, which are more than that of previously reported chitin nanoparticles and silica nanosheets [30,31]. Besides, the adsorption recover stability is another critical factor in practical applications. As is shown in Fig. 3d, the adsorption capacity of as prepared c-NaNbO3 nanoparticles kept prominent stability and the adsorption efficiency keep a steady value above 86% after recycling 4 times. There is a weak decrease in the 5th recycle was due to the weight loss accumulation of the c-NaNbO3 nanoparticles in centrifugation during the desorption processes,
which could be avoided by further structure designing, for example, combined with magnetic nanoparticle for magnetic enrichment and recovery. The adsorption capability and efficiency are usually influenced by the specific surface area, porosity and the surface charge distribution of materials [30-32]. As is illustrated in Fig. 4a. the N2 adsorption-desorption isotherm exhibits a typical type II adsorption isotherm with H3 hysteresis loop, which indicates the as prepared c-NaNbO3 nanoparticles are not well-defined mesoporous structures but regular nanoparticles [29,33]. The specific surface area value is 14.49 m2/g, which is much larger than that of conventional solid-state reaction prepared NaNbO3 (1.13 m2/g) due to the smaller particle size benefited from this LTLC
absorption decreased along the photocatalysis process. As shown in Fig. S-3b, the residual reaction solution was maintained colorless, even if it was exposed to pure O2, which indicates the color fade was due to the decomposition rather than the reduction. As is depicted in Fig. 5c, the photocatalysis efficiency increase from 18% to 99.3% with the increase of c-NaNbO3 nanoparticles from 0.1 to 1.8 g/L, which is comparable to that of commercial P25-TiO2 (Fig. S-6b) and higher than that of current reported BiPO4/BiVO4 photocatalysts . The photocatalytic activity of c-NaNbO3 nanoparticles was also assessed by the photodegradation rate constant and photodegradation efficiency (ηcat), as is shown in Fig. 5d. The photodegradation rate constant (κcat) was computed using the pseudo-first-order approximation . Both the κcat and ηcat increase first, and then decrease with the increasing of c-NaNbO3 nanoparticles. The photodegradation rate constant and efficiency evolution behavior suggests that more nanoparticles provide more active sites to degrade the organic compounds, while excess nanoparticles would intrigue a light screening effect to UV light impeding the generation of charge carriers (e-–h+), leading to the decrease of the photocatalytic efficiency.
M AN U
strategy. As is revealed in Fig. 4b that the NaNbO3 nanoparticles exhibit pH-dependent ζ-potential behavior and the nanoparticles surface were negatively charged over a wide range of pH values. It's worth noting that the ζ-potential value of c-NaNbO3 nanoparticles could be up to -37.6 mV with a pH of 7, a very common situation in a natural environment, which endows the as prepared c-NaNbO3 nanoparticles with robust water soluble stability. Furthermore, the aforementioned results of EDS, FTIR, and XPS analysis point out that there are abundant hydroxyl groups on the surface of as prepared c-NaNbO3 nanoparticles, which was also reflected in the TG analysis (Fig. S-4). The total weight loss observed in TG curve is about 16%. The first loss of 7% occurs from room temperature to 540 o C, which was due to the evaporation of adsorbed water and the loss of the abundant hydroxyl groups on the surface of the c-NaNbO3 nanoparticles , and the second loss of 9% occurs from 540 oC to 700 o C, which can be attributed to the decomposition of NaNbO3 to Nb2O5 under high temperature + ( 2 NaNbO3 → 2 Na + 2 Nb2 O5 ). Besides, a schematic negative surface charges mechanism were proposed in Fig. S-5. The abundant hydroxyl groups are assisted by the SLALC synthesis strategy, during which the excess electrophilic Na+ ions prefer to bond to the surface oxygen atoms in a form of Nb-O-Na. When the precursors reacting with water molecules, the Nb-O-Na tend to form Nb-OH, which are inclined to loss the hydrogen atoms and show strong electronegativity. The intense surface negative charges of as prepared c-NaNbO3 can not only benefit the water solubility but also enhance the adsorption capability of positively charged MB dye molecules . Hence, the rapid and effective adsorption performance mainly resulted from the small size induced large specific area and the negatively charged surface induced electrostatic interactions. 3.3 Photodegradation of methylene blue
The photocatalytic properties of as prepared c-NaNbO3 nanoparticles were evaluated through the photodegradation of the MB molecules. As is shown in Fig. 5a-b the bluish color of residual MB solution faded gradually and the corresponded characteristic UV-vis
3.4 Recoverable adsorption degradation of methylene blue
Recoverability is another crucial factor for actually sustainable applications of photocatalyst and adsorbent in naturally polluted environment. A serious leakage of organic dyes was simulated to assess the recoverability of the as prepared c-NaNbO3 nanoparticles during the adsorption and photocatalysis process. As shown in Fig. 6c, the adsorption reactions were firstly proceeded by adding a high concentration MB solution and when the adsorption was completed, the photocatalytisis reactions were then conducted under the Hg arc lamp (100 W, λ=365-600 nm, 60 mW/cm2). During the adsorption reactions, the white c-NaNbO3 nanoparticles powder turned into dark purple with the surface adsorption of MB (Fig. 6a: 0 h, 3.2 h, and 6.4 h), and then the dark purple color fades gradually to white (Fig. 6a: 3 h, 6.2h, and 9.4 h) with the MB decomposition induced by a sequence of reducing reactions and oxidization reactions (e– + O2 →·O2–,
ACCEPTED MANUSCRIPT c-NaNbO3 means that more surrounding free MB molecules were allowed to adsorb onto the surface for another photodegradation cycle. The
M AN U
h+ + OH– → ·OH, ·OH + MB → products, ·O2– + MB → products, ·O2– + H+ + e– → H2O2, ·O2 – + H+ + e– → H2O2) in the photocatalytic processes (Fig. 6c), the products components contained mainly the small molecules such CO2, H2O, NO3- and SO42- and so on . In addition, the white color of the
Fig. 4 Surface properties of the as prepared c-NaNbO3 nanoparticles. (a) N2 adsorption-desorption isotherm of the as prepared c-NaNbO3 nanoparticles (Top left Inset: the pore size distribution of c-NaNbO3 nanoparticles.). (b) pH-dependent ζ-potential of the
as prepared c-NaNbO3 nanoparticles.
Fig. 5 The photodegradation capability of the as prepared c-NaNbO3 nanoparticles on MB. (a) Photo images MB solution after adsorption equilibrium with light-driven photodegradation and sampled every 30 min. (b) The corresponding UV-vis absorption spectra of MB solutions during the photodegradation process with 0.36 g/L c-NaNbO3 nanoparticles. (c) The Photocatalytic efficiency curves of c-NaNbO3 with different concentration. (d) Photodegradation rate constant and efficiency of c-NaNbO3 with different concentration.
M AN U
Fig. 6 Recoverable adsorption and photocatalytic degradation performances of the as prepared c-NaNbO3 nanoparticles. (a) Adsorption recyclability of c-NaNbO3 nanoparticles within three cycles. (b) Photocatalysis recyclability of c-NaNbO3 nanoparticles within five cycles. (c) Schematic mechanism of adsorption and photocatalysis recycling. heavy adsorbed c-NaNbO3 nanoparticles slurry could be recycled by the irradiation of 3 h. As shown in Fig.
6b, the photocatalysis efficiency of the five recycle are 99%, 97%, 92%, 90% and 88%, respectively. And
there was a little decrease in the efficiency due to the
weight loss accumulation of c-NaNbO3 nanoparticles
in centrifugal separation between adjacent cycles which could be improved by composited with magnetic
separation. It is worth noted that the crystal structure of the as prepared c-NaNbO3 nanoparticles keeps stable after five cycles (Fig. S-7) . 4. Conclusions
In summary, a facile surface ligand assisted localized crystallization (SLALC) strategy was proposed to scale synthesize high crystalized c-NaNbO3 nanophotocatalyst with a small average size of 27 nm. The as synthesized c-NaNbO3 nanoparticles exhibit both remarkable adsorption and photocatalysis performance due to the abundant surface hydroxyl groups. The adsorption efficiency
of the c-NaNbO3 nanoparticles could be achieved up to about 95% and the photocatalysis efficiency of the c-NaNbO3 nanoparticles could reach up to 99.3% which is superior to the adsorption efficiency of 2% for P25-TiO2. Moreover, these c-NaNbO3 nanoparticles show stable recoverable adsorption and photodegradation performance, which is especially pronounced for some actual applications, such as toxic pollutants at low concentration and serious environmental emergency in the area that lacking of light irradiation. Besides, the c-NaNbO3 nanoparticles could also hybrid with some other metal nanoparticles to enhance the visible light response and photocatalysis . Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No.51173038, 11274127, 51303046), Natural Science Foundation of Hubei Province (Grant No. 2013CFB014) and Ph.D. Programs Foundation of Ministry of Education of China (No.20134208120001). Appendix A. Supplementary data
Supplementary data related to this article can be found
ACCEPTED MANUSCRIPT at http://dx.doi.org/10.1016/j.jallcom.2016.0x.xxx.
enhancement of CO2 photoreduction, J. Mater. Chem. A 2 (2014) 5606-5609.
 Li, Xiukai, Qing Li, and Liya Wang.
The effects of
 M.M. Khin, A.S. Nair, V.J. Babu, R. Murugan, S.
NaNbO3 particle size on the photocatalytic activity for
Ramakrishna, A review on nanomaterials for environmental
2-propanol photodegradation, Phys. Chem. Chem. Phys. 15
remediation, Energy Environ. Sci. 5 (2012) 8075-8109.
Challenges, Adv. Mater. 24 (2012) 229-251.
 Xu X, Hu L, Gao N, et al. Controlled growth from ZnS nanoparticles to ZnS–CdS nanoparticle hybrids with enhanced photoactivity. Adv. Funct. Mater. 25 (2015)
 X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891-2959.
 H. Pan, G.S. Zhu, X.L. Chao, L.L. Wei, Z. Yang, Properties of NaNbO3 powders and ceramics prepared by
 Baeissa E S. Photocatalytic degradation of malachite green dye using Au/NaNbO3 nanoparticles, J. Alloys Compd. 672 (2016) 564-570.
 H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J.
hydrothermal reaction, Mater. Chem. Phys. 126 (2011) 183-187.
 H. Shi, X. Li, H. Iwai, Z. Zou, J. Ye, 2-Propanol photodegradation over nitrogen-doped NaNbO3 powders
of triangular pyramid NaNbO3, Appl. Catal. B-Environ. 199
under visible-light irradiation, J. Phys. Chem. Solids 70
M AN U
 Yu, Qiaonan, et al. Preparation and photocatalytic activity
 Zlotnik S, Tobaldi D M, Seabra P, et al. Alkali Niobate and Tantalate
ChemPhysChem 17 (2016) 1-7.
growth of Na2Nb2O6·H2O nanowires and fabrication of NaNbO3 nanostructures with controlled morphologies, Appl.
 Shi H, Li X, Wang D, et al. NaNbO3 nanostructures: facile
synthesis, characterization, and their photocatalytic properties, Chem. Eng. J. 132(2009) 205-212.
 A. Yu, J. Qian, L. Liu, H. Pan, X. Zhou, Surface sprouting
 P. Li, S. Ouyang, G. Xi, T. Kako, J. Ye, The Effects of
Surf. Sci. 258 (2012) 3490-3496.
 Li X, Zhuang Z, Li W, et al. Hard Template Synthesis of Nanocrystalline NaNbO3 with Enhanced Photocatalytic Performance. Catal. Lett. 142(2012) 901-906.
 Boukriba M, Sediri F, Gharbi N. Hydrothermal synthesis
H2 Evolution and CO2 Reduction over Two Phases of
and electrical properties of NaNbO3. Mater. Res. Bull.
Perovskite-Structured NaNbO3, J. Phys. Chem. C 116 (2012)
Crystal Structure and Electronic Structure on Photocatalytic
 Cheng, Zhenxiang, et al. Low Temperature Synthesis of
 S.K. Mishra, R. Mittal, V.Y. Pomjakushin, S.L. Chaplot,
Phase stability and structural temperature dependence in sodium niobate: A high-resolution powder neutron diffraction study, Phys. Rev. B: Condens. Matter 83 (2011) 134105.
NaNbO3 Nanopowders and their Thin Films from a Novel Carbon
Free Precursor, J. Am. Ceram. Soc. 89(2006)
1188-1192.  Li, Xiukai, Qing Li, and Liya Wang. The effects of
 Gu, Qilin, et al. Modified Solvothermal Strategy for
NaNbO3 particle size on the photocatalytic activity for
Straightforward Synthesis of Cubic NaNbO3 Nanowires with
2-propanol photodegradation, Phys. Chem. Chem. Phys.
Enhanced Photocatalytic H2 Evolution, J. Phys. Chem. C
(2015) 25956-25964.  Shi H, Zou Z. Photophysical and photocatalytic properties of ANbO3 (A= Na, K) photocatalysts, J. Phys. Chem. Solids 73 (2012) 788-792.  Li, Peng, et al. Constructing cubic–orthorhombic
 Hyun Chul, Young Mee Jung, and Seung Bin Kim. Size effects in the Raman spectra of TiO2 nanoparticles. Vib. Spectrosc. 371 (2005): 33-38.  Yuzyuk, Yu I., et al. Modulated phases in NaNbO3: Raman scattering, synchrotron x-ray diffraction, and dielectric
surface-phase junctions of NaNbO3 towards significant
ACCEPTED MANUSCRIPT investigations. J. Phys. Condens. Matter. 17 (2005) 4977-4991.
Methylene Blue, Bromophenol Blue and Coomassie Brilliant
 Zhu, Kongjun, et al. Hydrothermal synthesis of sodium with
CrystEngComm 14(2012) 411-416.
Blue by α-chitin nanoparticles, J. Adv. Res. 7 (2016) 113-124.  Liu H, Gao N, Liao M, et al. Hexagonal-like Nb2O5
 Chang, Yunfei, et al. Phase structure, morphology, and Raman characteristics of NaNbO3 particles synthesized by different methods, Mater. Res. Bull. 44(2009) 538-542.
nanoplates-based photodetectors and photocatalyst with high performances. Sci. Rep. 5 (2015) 7716.
 S. Dhananasekaran, R. Palanivel, S. Pappu, Adsorption of
 Brown M A, Duyckaerts N, Redondo A B, et al. Effect of
 Shanker, Vishnu, et al. Nanocrystalline NaNbO3 and
surface charge density on the affinity of oxide nanoparticles
NaTaO3: rietveld studies, Raman spectroscopy and dielectric
properties, Solid State Sci. 11(2009) 562-569.
 H. Lin, H. Ye, S. Chen, Y. Chen, One-pot hydrothermal
Z. Li, Modified Solvothermal Strategy for Straightforward
synthesis of BiPO4/BiVO4 with enhanced visible-light
Synthesis of Cubic NaNbO3 Nanowires with Enhanced
photocatalytic activities for methylene blue degradation,
Photocatalytic H2 Evolution, J. Phys. Chem. C 119 (2015)
RSC Adv. 4 (2014) 10968-10974.
 Q. Gu, K. Zhu, N. Zhang, Q. Sun, P. Liu, J. Liu, J. Wang,
M AN U
 Han S, Hu L, Gao N, et al. Efficient Self
 Gu Q, Zhu K, Zhang N, et al. Modified Solvothermal
Synthesis of Uniform CdS Spherical Nanoparticles
Strategy for Straightforward Synthesis of Cubic NaNbO3
Nanoparticles Hybrids with Enhanced Photoactivity. Adv.
Nanowires with Enhanced Photocatalytic H2 Evolution. J.
Funct. Mater. 24(2014) 3725-3733.
Phys. Chem. C 119 (2015) 25956-25964.
 Xun S, Zhang Z, Wang T, et al. Synthesis of novel metal
nanoparitcles/SnNb2O6 nanosheets plasmonic nanocomposite
 Baeissa E S. Photocatalytic degradation of malachite green dye using Au/NaNbO3 nanoparticles. J. Alloys Compd. 672
photocatalysts with enhanced visible-light photocatalytic
activity and mechanism insight. J. Alloys Compd. 685(2016) 647-655.
 M. Zhao, Z. Tang, P. Liu, Removal of methylene blue from aqueous solution with silica nano-sheets derived from
vermiculite, J. Hazard. Mater. 158 (2008) 43-51.
Low temperature scale synthesis of c-NaNbO3 nanoparticles by a surface ligand assisted localized crystallization method.
The unique absorption performance relies on the excess electrophilic Na+ ions.
M AN U
c-NaNbO3 nanoparticles exhibit both unique adsorption and photodegradation recyclability.