Facile Fabrication and Mechanism of Single-Crystal Sodium Niobate

Oct 31, 2017 - Finally, the total transformation of wires to high purity cubes occurred at 180 min (Figure 1F), and its XRD patterns suggest that ther...
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Article Cite This: J. Phys. Chem. C 2017, 121, 25898-25907

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Facile Fabrication and Mechanism of Single-Crystal Sodium Niobate Photocatalyst: Insight into the Structure Features Influence on Photocatalytic Performance for H2 Evolution Qianqian Liu,† Lu Zhang,† Yuanyuan Chai,† and Wei-Lin Dai*,† †

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Single-crystal sodium niobate with wire- and cube-like structures has been created by a facile and eco-friendly route including a hydrothermal as well as in-site self-assembled process via optimizing thermal treatment temperature. Comparative studies were carried out to evaluate the structure and phase transformation during the fabrication course. A possible growth mechanism of NaNbO3 with controllable morphologies was elucidated based on the time- and temperature-dependent evolution experiments. In addition, special attention was paid to figure out the mutual relationship between the unique structure features and photocatalytic performance toward photocatalytic H2 evolution. The results illustrated that the NaNbO3 nanowires formed at 500 °C calcinations exhibited the highest photocatalytic activity compared with other materials and presented super stability as well. The superior photocatalytic performance could be interpreted in terms of the better crystalline, fewer defects, and perfect 1D nanowire morphology as demonstrated by TEM and SEM images, FT-IR, and Raman spectroscopy analysis. Additionally, a deep insight into the underlying of the photocatalytic reaction mechanism was proposed. These findings shed light on an efficient and facile pathway for the creation and formation mechanisms of photocatalytic materials, which provided new opportunities for solar-energy conversion.

1. INTRODUCTION With the aggravation of global energy and environmental crisis, photocatalytic splitting of water, one of the most effective strategies to achieve the conversion of solar energy into chemical energy, has attracted extensive attention in recent years.1 Many kinds of photocatalysts with special electronic structure, such as oxides,2−5 nitrides,6−8 and sulfides,9,10 have been reported to achieve efficient solar water splitting, whereas seeking a photocatalyst with exceptional performance is still urgent to satisfy the requirement of potential application. Perovskite niobates, especially NaNbO3, as one of the most promising candidates, play a conspicuous role owing to their outstanding properties, such as ionic conductive, ferroelectric, piezoelectric, and photocatalytic properties.11−16 Some of the niobates, including NaNbO3,17 SrNb2O6,18 and Ba5Nb4O15,19 have been demonstrated as efficient photocatalytic materials for water splitting and presented excellent performance as well. It is well known that the performance of a photocatalyst greatly depends on its morphology and microstructure.20 For example, Park et al. reported that the ultrathin Ba5Nb4O15 2D nanosheets showed improved photocatalytic activity compared to micrometer-sized particles for the evolution of H2 from water splitting.19 The morphologies dramatically affected their optical capture ability, active site number, and the accessibility to the active sites as well,21 among which one-dimensional (1D) materials have attracted great interest because of their © 2017 American Chemical Society

diverse advantages such as quantum refinement effects, much more active sites, and higher aspect ratio which are beneficial to photocatalytic reactions.22,23 For NaNbO3, to date, the 1D nanostructure has been gained through ion exchange based on molten-salt reaction.24 However, the method requires ultrahigh calcination temperature and complicated preparation procedures, greatly limiting its large-scale applications. However, it was recently found in our group that the NaNbO3 nanowires were easily obtained through thermal treatment at lower temperature using a Na2Nb2O6·nH2O nanowire precursor which was synthesized first under a hydrothermal process, following NaNbO 3 nanocubes formed by extending the hydrothermal time.25 Meanwhile, Liu and co-workers also confirmed the NaNbO3 nanowires could be successfully prepared via the thermal dehydration and solid-phase transformation of Na2Nb2O6· nH2O nanowires.26 Therefore, NaNbO3 with controllable morphology (especially NaNbO3 nanowires) can be prepared simply by changing the hydrothermal time or calcination temperature. However, to date, comparative studies on the morphology evolution of NaNbO3 are still lacking, and the comprehensive understanding of the growth mechanism of Received: September 5, 2017 Revised: October 30, 2017 Published: October 31, 2017 25898

DOI: 10.1021/acs.jpcc.7b08819 J. Phys. Chem. C 2017, 121, 25898−25907

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Figure 1. SEM images and XRD patterns of the morphology and phase evolution of niobate prepared at different reaction times: the starting Nb2O5 powder (A, a), 120 min (B, b), 135 min (C, c), 150 min (D, d), 165 min (E, e) and 180 min (F, f).

2. EXPERIMENTAL SECTION

NaNbO3 remains unexplored. More importantly, few studies have focused on their inter-relationship between structure features and photocatalytic performance, as it is acknowledged that the structural features directly influence the catalytic performance of a photocatalyst.27−30 In this work, the NaNbO3 nanowires and nanocubes were synthesized by a facile hydrothermal or direct annealing approach. Material characterizations were performed to figure out the growth mechanism of NaNbO3 with different morphologies. The photocatalytic water splitting for H2 evolution was chosen as the probe reaction to reveal the correlation between the unique structural features and photocatalytic performances, and the stability of NaNbO3 nanowires during the photocatalytic reaction was also investigated.

Materials. Nb2O5 (99.99%, AR) and NaOH (99.99%, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Methanol (99.0%, AR) was purchased from Aladdin Industrial Inc. All of the reagents used were of analytical grade and were used without further purification. All aqueous solutions were prepared with deionized water. Catalyst Preparation. The NaNbO 3 samples were prepared by hydrothermal treatment of mixtures containing Nb2O5, NaOH, and water. Typically, 3.8 mmol of Nb2O5 and 10 M sodium hydroxide were dissolved into 60 mL of deionized water. After stirring for 1 h, the mixtures were transferred into 100 mL Teflon-lined stainless-steel autoclaves and placed in the oven at the temperature of 180 °C for a period between 2 and 3 h to yield white niobate solids. Soon afterward, the white precipitates were collected by centrifugation, then washed with deionized water and absolute ethanol 25899

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Figure 2. FT-IR spectrum (a) and Raman spectra (b) of the phase evolution of niobate prepared at different reaction times.

several times, and finally dried at 100 °C overnight. Additionally, the products obtained at 2 h were calcined at 300−600 °C for 12 h to investigate the influence of postheating temperature. Characterization. The morphologies were investigated by scanning electron micrographs (SEMs) and transmission electron microscopy (TEM) images, which used a PHILIPS XL 30 microscope and JEOL JEM 2010 transmission electron microscope, respectively. X-ray diffraction (XRD) patterns were performed on a Bruker D8 advance spectrometer equipped with Cu Kα radiation (λ = 0.154 nm). The dehydration and phase transition were determined by thermogravimetry (TG) and differential scanning calorimetry (DSC), respectively, and the temperature was slowly increased by 10 °C min−1. The FTIR and Laser Raman spectra were performed on a Nicolet Avatar-360 FT-IR spectrometer and Horiba Jobin Yvon XploRA Raman spectrometer with a holographic notch filter and CCD detector, respectively. Ultraviolet visible (UV−vis) diffuse reflectance spectra (DRS) spectra were performed on a SHIMADZU UV-2450 instrument using BaSO4 as the reference, and the collection speed was 40 nm min−1. X-ray photoelectron spectroscopy (XPS) was collected using a RBD 147 upgraded PerkinElmer PHI 5000C ESCA system with a dual X-ray source. The Mg Kα (1253.6 eV) anode and a hemispherical energy analyzer were used. All binding energies were calibrated by contaminant carbon (C 1s = 284.6 eV). Photoluminescence (PL) spectra were investigated on a JASCO FP-6500 type fluorescence spectrophotometer. The lowtemperature EPR spectra were measured on a JES-FA200 ESR spectrometer at 103 K. Photocatalytic Activity Test. The water-splitting reaction was evaluated in a glass-closed gas circulation system using a 300 W Xe arc lamp (CeauLight, CEL-HXF300) at room temperature. The photocatalyst (100 mg) was dispersed in 80 mL of distilled water and 20 mL of CH3OH under magnetic stirring. The deposition of 0.5% Pt cocatalyst was performed via dissolving H2PtCl6 in the above solution which was degassed several times to remove air and then irradiated. The amount of H2 was analyzed using a gas chromatograph with a thermal conductivity detector (TCD) and 5 Å molecular sieve column.

out and analyzed by SEM images and XRD patterns. As exhibited in Figure 1, the reactant Nb2O5 was composed of large-scale nanoparticles with a diameter of about 800 nm (Figure 1A). The corresponding XRD patterns match well with the standard PDF card of the orthorhombic phase (JCPDS 271003) (Figure 1a). When the reaction time was 120 min, huge changes of the morphologies were detected. As shown in Figure 1B, high purity nanowires with a diameter of 80−120 nm and a length of several tens of micrometers were observed at 120 min. The XRD patterns revealed the nanowires had a pure phase of sandia octahedral molecular sieve (SOMS) structure, with a formula of Na2Nb2O6·nH2O, whose unit cell parameters were a ∼ 17.05 Å, b ∼ 5.03 Å, and c ∼ 16.49 Å (Figure 1b). However, with the reaction extended to 135 min, a small number of cubes emerged on the surface of wires. Meanwhile, the crack and tangle of wires occurred by contrast with 120 min, indicating the cubes grew at the expense of wires (Figure 1C). The corresponding XRD turned out the same as the one at 120 min (Figure 1c). Further enhancing the hydrothermal time to 150 min, the number of cubes increased gradually (Figure 1D). Besides, new peaks could be observed from the XRD of 150 min, which were originated from a coexistent phase of Na2Nb2O6·nH2O and NaNbO3 (Figure 1d). Upon further extending the time to 165 min, the wires dramatically disappeared and were replaced by a large number of cubes (Figure 1E). Meanwhile, the corresponding XRD patterns belonging to Na2Nb2O6·1.3H2O almost vanished and changed into the phase of NaNbO3 except for the peaks at 11−14° (Figure 1e). Finally, the total transformation of wires to high purity cubes occurred at 180 min (Figure 1F), and its XRD patterns suggest that there was only one crystal phase of NaNbO3 in this sample, which matched well with the standard card of the orthorhombic phase (JCPDS 33-1270) (Figure 1f). To further investigate the structure evolution of niobate, we carried out the FT-IR investigation of samples at different hydrothermal treatment times (Figure 2a). As the reaction proceeded, noticeable changes of the spectrum occurred with respect to the reactant Nb2O5. In the FT-IR spectrum of 120 min, the bands between 3500 and 3000 cm−1 were assigned to the O−H stretching of the hydroxyl group and water molecules. The adsorption band around 1650 cm−1 was the H−O−H bending of the water molecules bonded to Na sites, and the adsorption peaks positioned at 870, 760, 645, 520, and 445 cm−1 were attributed to the vibrations of the Na−niobate framework including Nb−O bands, Nb−O−Nb bending, and lattice vibrations.31 With the reaction time increasing to 150

3. RESULTS AND DISCUSSION Effect of the Hydrothermal Treatment Time on Phase Evolution. In order to reveal the formation process of NaNbO3 in detail, time-dependent experiments were carried 25900

DOI: 10.1021/acs.jpcc.7b08819 J. Phys. Chem. C 2017, 121, 25898−25907

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Figure 3. (a) XRD patterns of the products acquired at different postheat temperature (RT referred to Na2Nb2O6·nH2O) and (b) thermogravimetry (TG) and differential scanning calorimetry (DSC) results of Na2Nb2O6·nH2O with the increase of temperature.

that the postheat treatment temperature played a key role in the phase transformation from Na2Nb2O6·nH2O to NaNbO3. To deeply understand the influence of postheat treatment temperature as well as to make a contrast with the effect of hydrothermal time, we compared the XRD patterns (Figure 3a) of the samples at various synthesis temperatures. Similarly, the phase transformation was also observed with the increase of calcination temperature. However, unlike the effect of hydrothermal time, the intensity of the characteristic diffraction peaks of Na2Nb2O6·nH2O decreased drastically and even almost disappeared except for the peaks at 11−14° at 300 °C, suggesting the phase transformation from SOMS to the amorphous structure occurred. Interestingly, upon further increasing the calcination temperature to 400 °C, recrystallization emerged again, and the representative peaks for NaNbO3 formed. It eventually transformed into the perovskite NaNbO3 phase at 500 °C. Further enhancing the calcination temperature to 600 °C did not result in noticeable changes in the patterns but the increased intensity of the peaks. Meanwhile, the corresponding SEM images exhibited pronounced difference with the effect of hydrothermal times. As shown in Figure 1b and Figure S1, there were no significant changes in the morphology with enhancing postheat treatment temperatures. Namely, all samples were composed of wire-like structures, and no cubes could be observed. However, the twist and even breakage along the length direction of the nanowires occurred by increasing the calcination temperature, which may be attributed to the internal stress aroused by the recrystallization process in the inner structures of nanowires at high temperatures.36 Additionally, to further figure out the phase transformation during the heating process, the thermal analysis was carried out by using TG-DSC analysis. According to the results from Figure 3b, the endothermal peak appeared in the temperature at 292 °C, and the weight of the sample decreased rapidly, about 6.5%, indicating the loss of H2O molecules of Na2Nb2O6· nH2O at this temperature. Meanwhile, from this weight loss, the number of water molecules in the possible formula was about 1.3 per unit. Therefore, the formula for Na2Nb2O6·nH2O was confirmed as Na2Nb2O6·1.3H2O. In addition, the strongest exothermic peak was observed at 474 °C, without significant change of mass at the same time, which corresponded well to the structural transformation from Na2Nb2O6 to NaNbO3. Therefore, based on the results of XRD and TG-DSC, we could infer that the dehydration of H2O from Na2Nb2O6·1.3H2O to

min, the intensity of the peaks concerning O−H bends sharply reduced and then totally vanished at 180 min. This is probably due to the loss of water molecules of Na2Nb2O6·nH2O to turn into NaNbO3. Additionally, the adsorption peaks below 1000 cm−1 were gradually indiscernible and replaced by a broad absorption band over time, indicating the cleavage of Nb−O bonds in the Na-niobate framework during the hydrothermal course. Moreover, Raman spectra were explored to make more accurate structural information on the samples (Figure 2b). By contrast with the spectrum of 120 min, the major band of Nb2O5 at 680 cm−1 belonging to the characteristic band of NbO6 octahedra-sharing corners32 disappeared, indicating the reaction with concentrated NaOH solution ruptured the corner-sharing Nb−O polyhedral in crystalline Nb2O5.33 By further prolonging the reaction time (from 120 to 180 min), the region below 300 cm−1 gradually split and gave rise to a more complicated line shape. Taking the spectra of 180 min, for example, the region from 170 to 300 cm−1 was attributed to the triply degenerate v6 (F2u) and v5 (F2g) modes, and the bands below 180 cm−1 were assigned to Na+ translational modes against the NbO6 octahedrons and the librational modes of the same one.34,35 Combined with the results of XRD and the FT-IR spectrum, the Na2Nb2O6·nH2O gradually lost water during the reaction process, and the vaporized free H2O molecules left the space for the oriented rearrangement of NaNbO3, which possibly induced the octahedral tilting and the displacements of Na+ ions. Therefore, the more complicated Raman scattering profile was possibly due to the distortion related to the rotation of octahedra on different crystallographic layers. Additionally, compared with the time in early reaction stages, the band at 600 cm−1 of 180 min corresponding to the corner-sharing NbO6 octahedra became much broader and stronger, suggesting the reforming of NbO6 octahedra during the reaction process. Therefore, based on the results discussed above, the hydrothermal time has significant consequences on the morphology and microstructure evolution of NaNbO3. Namely, Nb2O5 reacted with concentrated NaOH solution to form Na2Nb2O6·nH2O nanowires at the first stage. Then the structure of Na2Nb2O6·nH2O nanowires could be disturbed and changed into the NaNbO3 nanocubes with prolonged time. The corresponding growth mechanism of NaNbO3 nanocubes is discussed in the mechanism section. Effect of the Post-Heat Treatment Temperature on Phase Evolution. On the other hand, experiments showed 25901

DOI: 10.1021/acs.jpcc.7b08819 J. Phys. Chem. C 2017, 121, 25898−25907

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Figure 4. TEM image (a), HRTEM image of Na2Nb2O6·1.3H2O nanowires and inset of a SAED pattern (b), HRTEM image at 300 °C (c), 400 °C (d), 500 °C and inset of a SAED pattern (e), and 600 °C and inset of a SAED pattern (f).

Figure 5. FT-IR spectra (a) and Raman spectra (b) of the products acquired at different postheat treatment temperature.

Na2Nb2O6 started at 292 °C and was totally converted into NaNbO3 nanowires at 474 °C. Moreover, TEM and high-resolution TEM (HRTEM) were used to determine the more detailed structure of niobate nanowires during the postheat treatment process. As shown in Figure 4, the TEM image of Na2Nb2O6·1.3H2O further illustrated its wire-like structure (Figure 4a), consistent with the results of the SEM image (Figure 1b), and the typical diameter was ∼150 nm (inset of Figure 4a). The HRTEM image (Figure 4b), taken from the region marked in Figure 4a, demonstrated the Na2Nb2O6·1.3H2O nanowires were well crystallized, as evidenced by the clear and orderly lattice fringes. In addition, a selected area electron diffraction (SAED) pattern of Na2Nb2O6·1.3H2O revealed a diffraction pattern corresponding to a single crystal (inset of Figure 4b). However, the clear lattice fringes became blurred and gradually converted into messily amorphous nanostructure when it was heat-treated at 300 °C (Figure 4c), suggesting the Na2Nb2O6·1.3H2O nanowires partly dissolved and that the crystallinity reduced due to the vaporized H2O molecule with increasing treatment temperature. This finding was confirmed with the results of

XRD (Figure 3a). However, the lattice fringes appeared again and became more clear and orderly with the temperature elevated from 300 to 500 °C (Figure 4d,e), indicating the recrystallization occurred again and the amorphous nanostructure began to nucleate and turn into well-crystalline NaNbO3. The preferable crystallinity of NaNbO3 was still maintained when the annealing temperature was increased from 500 to 600 °C (Figure 4f), evidenced by the SAED patterns as the inset of Figure 4e and 4f. Likewise, FT-IR and Raman spectra (Figure 5) were also employed to analyze the influence of subsequent calcination temperature on the crystal structural evolution of niobate. With the temperature prolonged, the peaks at 1650 cm−1 and 3000− 3500 cm−1 petered out due to the vaporized water molecules of Na2Nb2O6·1.3H2O (Figure 5a), and the peaks of Raman spectra became complicated caused by the tilting of NbO6 octahedra (Figure 5b). It is noted that the results were very similar to that of hydrothermal time (Figure 2), indicating both the hydrothermal time and calcination temperature could change the radical structure of Na2Nb2O6·1.3H2O and finally turn into NaNbO3. However, different NaNbO3 morphologies 25902

DOI: 10.1021/acs.jpcc.7b08819 J. Phys. Chem. C 2017, 121, 25898−25907

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The Journal of Physical Chemistry C Scheme 1. Schematic Diagram of the Growth Process of NaNbO3 Nanowires and NaNbO3 Nanocubes

Scheme 2. Schematic Illustration of the Synthetic Procedure and Growth Mechanism of NaNbO3 Nanowires and NaNbO3 Nanocubesa

a

A refers to the influence of hydrothermal time, and B refers to the influence of post-heat temperature.

also lose a H2O molecule with increasing postheat temperatures, resulting in the formation of amorphous nanostructure and free water molecule (evidenced by TG-DSC results in Figure 3b and HRTEM images in Figure 4). With the elevation of the temperature, the free water molecule gradually evaporated to leave space for the formation of the NaNbO3 nuclei. Then the crystallinity of NaNbO3 gradually increased due to the rapid growth of nuclei with increased temperature. Interestingly, with great difference with the hydrothermal condition, the growth of the NaNbO3 nuclei did not affect its 1D wire-like structure. It was possibly caused by the controlled short-range diffusion of ions in a restricted space under calcination conditions, which meant the nuclei did not grow in aqueous solution (free environment) to form the nanocrystals with their natural behavior, such as cubes. Therefore, the Na2Nb2O6·1.3H2O nanowires could be served as self-sacrificing templates for in-site self-assembled NaNbO3 while successfully maintaining their one-dimensional wire-like structures (shown in Figure S1). Photocatalytic Activity Tests. To investigate the interrelationship between the structure features and photocatalytic performance, we chose the photocatalytic water splitting of H2 as a probe reaction and compared the performance of NaNbO3 nanowires ranging from 400 to 600 °C and NaNbO 3 nanocubes acquired at 180 min (namely, wire-400, wire-500, wire-600, and cube-180, respectively). As shown in Figure 6a and 6b, the highest H2 evolution rate of 330.3 μmol g−1 h−1 was achieved on wire-500, appropriately 3.4 times higher than that

formed through these two processes. The detailed growth mechanism is discussed in the mechanism section. Growth Mechanism of NaNbO3 Nanocubes and NaNbO3 Nanowires. Based on the experiments discussed above, a possible growth mechanism of NaNbO3 nanocubes and NaNbO3 nanowires was elucidated in Schemes 1 and 2. At the first stage, the Nb2O5 nanoparticles could be dissolved and reacted with concentrated NaOH solution to form Na2Nb2O6· 1.3H2O nanowires under the hydrothermal conditions. The framework of Na2Nb2O6·1.3H2O consisted of NbO6 and NaO6 octahedra with the remaining Na ions occupying sites in channels.37 As confirmed by the results of TG-DSC (Figure 3b), the Na2Nb2O6·1.3H2O is a metastable phase. When the hydrothermal time was enhanced, the Na2Nb2O6·1.3H2O nanowires gradually lost water molecules, giving rise to the cleavage of Nb−O bonds in the Na-niobate framework and the displacement of Na ions (confirmed by the results of FT-IR and Raman spectra in Figure 2). Subsequently, the crystal nuclei of NaNbO3 nanocubes began to appear on the surface of wires due to the assembly of NbO6 or NaO6 units (identified by the SEM image in Figure 1C), as the NaNbO3 nanocubes were thermodynamically more stable than the crystal of the wires.38 The majority of cubes kept crystallizing and rapidly grew at the expense of wires due to the Ostwald ripening process. Finally, the well-crystal NaNbO3 nanocubes formed after 180 min of the hydrothermal process. Meanwhile, for the growth of NaNbO3 nanowires, it is noted that the well-crystalline Na2Nb2O6·1.3H2O nanowires could 25903

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Figure 6. Photocatalytic H2 generation under sunlight irradiation (a) and the H2 generation rates of different products (b); reusability of wire-500 for the photocatalytic H2 production under sunlight irradiation (c); and EPR spectrum of wire-500 and wire-600 at 103 K (d).

Figure 7. UV−vis diffuse-reflectance spectra (a) and photoluminescence emission spectra of different products with the excitation wavelength at 280 nm (b).

of wire-400 (103.9 μmol g−1 h−1), which was probably due to the more ordered lattice fringes and improved crystallinity with increased calcination temperature. However, further increasing the treatment temperature to 600 °C, the H2 evolution rate decreased to 261.1 μmol g−1 h−1, less than 30% compared with that of wire-500. From the SEM images shown in Figure S1, a more obvious bent and tangled wire-like structure appeared when the temperature was raised from 500 to 600 °C, which would be contributed to more defects in wire-600. It is wellknown that the curved nanowires will generate localized defects at the bending regions and surface.39,40 Meanwhile, according to the results of HRTEM images of wire-500 and wire-600 (Figure S2), it was clearly shown that the wire-600 had a significantly higher defect concentration (namely, stepped

surface or the stacking-fault-created step facet defects) than wire-500. Furthermore, the low-temperature EPR was also performed to explore the relative content of the defects in wire500 and wire-600. It is acknowledged that the ESR peak height corresponds well to the concentration of paramagnetic species. Thus, Figure 6d indicates that there were more defects in the wire-600 sample. While a steep decrease in the concentration of defect sites occurs on wire-500, it did not show any ESR signals. Therefore, it could be concluded that prolonging the treatment temperature significantly increased the defect concentration of NaNbO3 nanowires than wire-500, which is consistent with the results of HRTEM images (Figure S2). The defect concentration increased the number of charge recombination centers, which greatly hampered the photoactivity. Thus, fewer defects 25904

DOI: 10.1021/acs.jpcc.7b08819 J. Phys. Chem. C 2017, 121, 25898−25907

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Figure 8. Schematic illustration of band structures of different products (a) and schematic illustration of the charge separation and transformation of wire-500 during photocatalytic H2 production under sunlight irradiation (b).

of wire-500 may account for its better photocatalytic performance than wire-600. Meanwhile, compared to wire-like structure of NaNbO3, the activity of cube-180 decreased drastically, which is only 7.5 μmol g−1 h−1, because the perfect 1D nanowire morphology possessed preferable light-harvesting ability and easier accessibility to the active sites than the cubelike structure. Additionally, as an indispensable factor for practical application, the stability of a photocatalyst should be explored. As illustrated in Figure 6c, the catalytic stability of wire-500 was carried out under a time-circle H2 production experiment. No noticeable decrease of H2 evolution was observed after three successive cycles, suggesting its superior stability and durability against photocorrosion during the photocatalytic H2 production from water. Mechanism of the Enhanced Photocatalytic Activity. It is acknowledged that the optical absorption and efficient charge separation of a photocatalyst significantly determined its photocatalytic activity.41 Thus, the UV−vis diffuse reflectance spectra were used to explore the optical absorption properties of the samples. As shown in Figure 7a, a broad absorption band in the UV range could be observed, corresponding to the charge transfer from O 2p (VB) to Nb 4d orbitals (CB). The positions of the adsorption edge showed a slight difference after changing the hydrothermal time or postheat treatment temperature, among which the wire-500 exhibited the longest wavelengths (red shift) with respect to other samples, indicating its better light adsorption performance. Moreover, based on the Tauc plot of the (KM energy)n (n = 1/2 for the indirect bandgap) vs the photon energy,42 the derived band gaps for the samples from wire-400 to cube-180 were calculated to be 2.98, 2.99, 3.06, and 3.09 eV, respectively (Figure S3). The different crystallinity and morphology of the samples may be responsible for this observation.43−45 Moreover, the valence band (VB) XPS was carried out to determine the electronic structure of the catalysts. As shown in Figure S4, the EVB of the samples from wire-400 to cube-180 were 1.56, 1.85, 1.66, and 1.69 eV, respectively, suggesting the band structure was influenced by the changes of the hydrothermal time and the calcination temperature. It has been reported that the O 2p and Nb 4d orbitals mainly contribute to the formation of the valence and conduction bands of NaNbO3.46 The difference of VB top value was possibly ascribed to the tilting of the NbO6

octahedron and changes of defects of NaNbO3 during the treatment processes. Thus, according to the results of the UV− vis diffuse reflectance spectra and VB-XPS, the band structures of the prepared samples were illustrated in Figure 8a. We could observe that the potentials of conduction band edge of all the samples were more negative than the water reduction potential, indicating the thermodynamically permissibility for the transformation of photoinduced electrons into H2O to produce H2. Besides, PL spectroscopy was carried out to explore the electron−hole pair separation efficiency of a photocatalyst, which is a criterion for evaluating the performance of photocatalysis. It is generally acknowledged that the lower intensity of the PL spectrum is consistent with the enhanced efficiency of electron−hole pair separation.47 As illustrated in Figure 7b, wire-500 exhibited the lowest intensity in all samples, implying the relatively lower recombination of electron−hole pairs than other samples. It should be noted that the trend was similar to the catalytic activity of the samples. From the results discussed above, we know that though the XRD of wire-400 was approaching the peaks of NaNbO3 it completely transformed into the well-crystallinity of NaNbO3 at 474 °C (seen in Figure 3b). Therefore, the crystallinity of wire-400 was much inferior by contrast with wire-500 (confirmed by the HRTEM images in Figure 4). However, with prolonging the treatment temperature to 600 °C, the appearing twist and crack significantly increased the defects of NaNbO3 nanowires compared to wire-500, which also generated new sites for electron−hole pair recombination. As a result, the better activity of wire-500 was possibly due to the superior crystallinity and much fewer defects than wire-400 and wire-600. For cube180, the poorest separation efficiency of the photoinduced electron−hole pair was possibly attributed to the inferior accessibility to the active sites by contrast with the wire-like structure, which would in turn hinder the separation and transportation efficiency of electron−hole pairs. Therefore, based on the aforementioned discussions, a possible mechanism for photocatalytic H2 evolution was illustrated in Figure 8b. Under the mimic of sunlight irradiation, the electron−hole pairs were generated. As the existence of Pt cocatalyst, the photoinduced electrons could be captured effectively to reduce protons to hydrogen, prolonging the lifetime of charge carriers. Meanwhile, the holes reacted with methanol (sacrificial reagent) to impede the electron−hole 25905

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The Journal of Physical Chemistry C

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recombination. The improved photocatalytic performance of wire-500 could be ascribed to several factors: (i) the better crystalline and fewer defects of wire-500 significantly facilitated the electron transformation and further increased the separation of electron−hole pairs; (ii) the red shift of wire500 in the UV−vis diffuse reflectance spectra was beneficial for the absorbance of sunlight as well as the improvement of lightutilization efficiency; and (iii) nanowire with 1D structure provided more active sites and improved light-harvesting ability.

4. CONCLUSIONS In summary, the single-crystal NaNbO3 with controlled morphologies have been successfully prepared by a simple hydrothermal as well as postheat calcination method. Comparative studies were explored to figure out the phase evolution and growth mechanism of NaNbO3 during the hydrothermal and calcination process. Experimental results proved that the hydrothermal treatment time or calcination temperature made a great difference in the morphology and phase transformation of the products which significantly determined their photocatalytic activity as well. Moreover, it was found that a remarkably improved photocatalytic activity of wire-500 for hydrogen production was demonstrated, which showed good stability as well. The enhanced activity was mainly attributed to the better crystalline, fewer defects, and perfect 1D nanowire morphology. Hence, this work opens a new avenue for the development of novel photocatalytic materials with potential applications in solar energy conversion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08819. SEM images of niobate nanowires, XPS valence band spectra and the plot of (ahv)1/2 vs photon energy (hv) of the UV−vis diffuse reflectance spectra of the NaNbO3 samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 5566 5572. E-mail: [email protected]. ORCID

Wei-Lin Dai: 0000-0003-4838-5678 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support by NSFC (Project 21373054, 21173052) and the Natural Science Foundation of Shanghai Science and Technology Committee (08DZ2270500).

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DOI: 10.1021/acs.jpcc.7b08819 J. Phys. Chem. C 2017, 121, 25898−25907