Modified Solvothermal Strategy for Straightforward Synthesis of Cubic

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Article

Modified-Solvothermal Strategy for Straightforward Synthesis of Cubic NaNbO Nanowires with Enhanced Photocatalytic H Evolution 3

2

Qilin Gu, Kongjun Zhu, Ningsi Zhang, Qiaomei Sun, Pengcheng Liu, Jinsong Liu, Jing Wang, and Zhaosheng Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08018 • Publication Date (Web): 04 Nov 2015 Downloaded from http://pubs.acs.org on November 5, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Modified-Solvothermal Strategy for Straightforward Synthesis of Cubic NaNbO3 Nanowires with Enhanced Photocatalytic H2 Evolution Qilin Gu,ab Kongjun Zhu,a* Ningsi Zhang,c Qiaomei Sun,

ab

Pengcheng Liu,

ab

Jinsong Liu,bd Jing Wanga and Zhaosheng Lic a

State Key Laboratory of Mechanics and Control of Mechanical Structures, College

of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing, 210016, China. b

College of Materials Science and Engineering, Nanjing University of Aeronautics

and Astronautics, Nanjing 210016, China c

National Laboratory of Solid State Microstructures, Collaborative Innovation Center

of Advanced Microstructures, College of Engineering and Applied Science, Ecomaterials and Renewable Energy Research Center, Nanjing University, Nanjing 210093, China d

Department of Materials Science and Engineering, University of California Los

Angeles, California 90095, USA *Corresponding Author, E-mail: [email protected] (Kongjun Zhu), Fax: +86 25 84895759, Tel: +86 25 84895982

ACS Paragon Plus Environment


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Abstract: To further improve the photocatalytic H2 evolution activity, NaNbO3 photocatalyts simultaneously possessing cubic crystal structure and 1D morphology have been successfully synthesized via a modified-solvothermal strategy. During the process of synthesis employing ethylene glycol as solvent, a temperature-fluctuation during the autoclaving period is proposed to regulate the grain growth without any other additives or calcinations. It’s demonstrated that the structure-directing effect of the solvent is enhanced in the condition of the temperature fluctuation, contributing to the formation of 1D nanostructure. Otherwise, the irregular NaNbO3 nanoparticles with severe aggregation are resulted. Photocatalytic H2 evolution activities of samples under ultraviolet light irradiation with 0.5 wt% of Pt co-catalyst indicate that NaNbO3 nanowires expectedly exhibit an enhanced activity of 699 µmol·h-1·g-1, approaching twice that of NaNbO3 nanoparticles. The higher photocatalytic activity of NaNbO3 nanowires is attributed to their large specific surface area, high chemical purity and powerful

reduction

ability,

which

have

been

confirmed

by

the

further

characterizations and analysis based on crystal structure, valence state, elemental composition and energy band structure. The modified-solvothermal strategy provides an alternative pathway to regulate the crystal growth, which can effectively integrate the unique morphology with desired crystalline structure towards increasing photocatalytic activity.


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1. Introduction The efficient conversion of solar energy into chemical energy makes photocatalytic H2 evolution (PHE) among the best potential solutions towards the increasingly global energy and environmental crisis.1 Generally, PHE involves (i) the generation of electrons and holes by photo-excitation, (ii) the charge separation and migration of the photo-generated carriers to the surface, (iii) the recombination of the photo-generated electron–hole pairs during migration, and (iv) the reduction of the adsorbed reactants,2 which enlists both crystal structure and morphology of the photocatalysts among the crucial factors in their PHE efficiency. Unfortunately, for most of the photocatalysts, such as TiO23,4 and CdS, 5 their thermodynamically stable phase structure and intrinsic morphology at room temperature (RT) are not always the best choice for PHE. Therefore, exploring the synthesis strategy for the rational design of photocatalysts with thermodynamically metastable phase structure and extrinsic morphology that simultaneously preferred for PHE becomes an urgent and challenging research hotspot.3,6 Among the promising photocatalysts, NaNbO3 (NN) plays a noticeable role in organic-pollutant degradation, CO2 reduction and PHE.7,8 NN exhibits a sequence of temperature-induced phase transitions and the most stable phase at RT is orthorhombic (Pbcm), while the cubic phase (Pm3m) is stable only at high temperature (> 913 K). Recently, Peng Li et al. reported that metastable cubic phase NN nanoparticles (NPs) could be stabilized through the surface coordination effect and such a high symmetry structure showed superior activities in both PHE and CO2 reduction.9,10 Nevertheless, in their work, expensive metal alkoxide were used as raw



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materials, and a crystallization treatment at 500 ~ 900 °C for 10 h was also required. Given that NN is a phase transition compound,11 the metastable cubic NN would easily change into stable orthorhombic structure during the calcination process.12 Thus, a direct synthesis process without any thermal treatment is desired. However, till now, the direct synthesis of cubic NN NPs has never been reported. At the same time, 1D nanostructures has gained intensive attentions and interests, due to their high aspect ratio, large surface area and quantum refinement effects.13-16 These characters make them remarkable not only in photocatalysis17,18 but also in piezoelectricity19,20 and energy storage.21 For a long time, NN with 1D nanostructure has been synthesized through the combination of hydrothermal treatment and post-calcination,22-24

and

their

outstanding

photocatalytic

activities

are

well-recognized compared with granulated ones.25-27 Recently, some novel approaches such as electrospinning technology28,29 and molten-salt reaction30 were also adopted to prepare 1D NN nanofibers. It’s noteworthy that all the previously reported 1D NN nanostructures were orthorhombic rather than cubic structure. Admittedly, complex template synthesis30-33 and cost-efficiency wet-chemical methods were also extensively modified to yield the 1D nanostructure. The asymmetric growth was facilitated by the passivation effect of capping agents34,35 and surfactants.36 However, additional treatments (e.g. calcination) should be carried out to remove the templates and organic additives, which could in turn complicate the process, increase the grain size and deteriorate their properties. Hence, the straightforward synthesis of 1D nanostructure without any additives is highly desirable, and in this way, it’s possible to prepare NaNbO3 simultaneously possessing the cubic phase and 1D nanostructure


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towards better PHE activity. Distinguishing from the previous crystal growth regulation (Table S1), in the present work, we developed a modified-solvothermal strategy involving no additives or post-treatments. In this way, the direct synthesis of cubic NN nanowires (NWs) was addressed for the first time. Through simultaneously integrating the 1D morphology with cubic crystal structures, the PHE activity of NaNbO3 was improved to a new stage. Also, detailed discussions about the formation process of NN NWs and the underlying mechanism for the improved PHE efficiency were addressed.

2. Experimental Methods 2.1. Chemicals. Sodium hydroxide (NaOH, 96% min) and niobium oxide (Nb2O5, 99.5%min) were purchased from Sinopharm Chemical Reagent Co., Ltd., and Ethylene Glycol (C2H6O2, 99%) from Guangzhou Jinhuada Chemical Reagent Co., Ltd. All chemicals were of analytical grade and used as received without further purification. 2.2. Materials Synthesis. In a typical solvothermal procedure for synthesizing NN NWs, 1.67g NaOH was added into a breaker containing 40ml EG, and stirred vigorously for 30min to dissolve NaOH as far as possible. 0.5g Nb2O5 was then slowly added into the breaker, followed by continuous stirring for another 30min to form a milky mixture. The prepared EG/NaOH/Nb2O5 mixture was ready for the following solvothermal treatment. For the solvothermal synthesis of NN NWs, the EG/NaOH/Nb2O5 mixture was transferred into Teflon-lined autoclave and kept at 200°C for 4h. To obtain NN NWs, a “disturbance” was conducted during the autoclaving process; that is, as the



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solvothermal treatment proceeded for 80min, abrupt decreasing of autoclaving temperature to 150 ~ 170 °C were conducted by open the door of autoclave for 5 min. This disturbance were repeated every 20 ~ 30 min. For comparison, a control experiment was conducted without the temperature change during the solvothermal process. After autoclaving for 4 h, the resultant was naturally cooled to room temperature. The product was rinsed with de-ionized water and then anhydrous alcohol, and precipitated with centrifugation for 10 min at 3000 rpm to yield a white powder. Rinsing was repeated thrice to remove excess ions from the final product, and the precursor was dried at 80 °C overnight. 2.3. Materials Characterization. The crystal phases of the as-prepared samples were characterized by a Bruker X-ray diffractometer (XRD, Bruker D8 Advance, Germany) with a Cu Kα radiation source (40 kV, 40 mA, λ = 0.154178 nm) at a scanning rate of 10 o/min in the 2θ range of 5 ~ 60 °. The morphology and microstructure of the as-synthesized samples were observed using a Hitachi S-4800 field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan) with energy-dispersive X-ray (EDX) analysis of the composition. The microstructures of the resultant products were analyzed with high-resolution transmission electron microscopy (HR-TEM; JEOL JEM-2100, Japan). Fourier transform infrared (FT-IR) spectra from 400 to 4000 cm-1 were acquired using a FT-IR spectrophotometer (NEXUS670, Nicollet, USA) to analyze the surface condition of the as-obtained samples. Thermogravimetric analysis−mass spectrometry (TGA–MS) measurements were carried out using a 409PC thermal analyzer (Netzsch, Germany) coupled with a QMS403C instrument (Netzsch, Germany). About 10 mg of each sample was heated from 40 to 700 °C at a


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heating rate of 10 °C/min under Ar atmosphere. Mass scanning was carried out over the range m/z = 2 ~ 200. Using a 514.5 nm laser, Raman spectra were measured by a confocal laser micro-Raman spectroscopy system (Raman, LABRAM HR800) within the range of 100 ~ 1000 cm-1.The diffuse reflection spectra were measured with an integrating sphere equipped UV-visible recording spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) using BaSO4 as a reference, and the optical absorptions were converted from the reflection spectra according to the Kubelka-Munk equation. Photoelectron spectroscopy were obtained with an PHI 5000 VersaProbe II scanning XPS Microprobe equipped with an Al Kα (1486.6 eV) monochromatic source at base pressures less than 10-8 Torr with a perpendicular take-off angle. Nitrogen isothermal adsorption–desorption measurements are performed to determine the Brunauer– Emmett–Teller (BET) surface areas using an automatic surface area and pore analyzer (SSA-4300, Beijing Builder Corp., China) after treating the samples at 150 °C for 10 h. Photoluminescence spectra were measured with a Varian Cary Eclipse instrument. 2.4. PHE Measurements. The H2 evolution experiments were carried out in a gas-closed circulation system. The NN powder (0.1 g) was dispersed using a magnetic stirrer in CH3OH aqueous solution (220 mL distilled water + 50 mL CH3OH) in a Pyrex cell with a side window. Considering its frequent usage in the previous work, co-catalyst Pt (0.5 wt%) was photodeposited on the NN catalyst by adding H2PtCl6 solution to the reaction solution. The light source was a 300 W Xe arc lamp without filter. The H2 evolution was measured by an on-line gas chromatograph (GC-8A, Shimadzu Co., Japan) with a thermal conductivity detector (TCD), according to the standard curve.



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3. Results and Discussion 3.1. Crystalline Structure and Morphology

Fig. 1 (a) FE-SEM image, (b) XRD patterns, (c) TEM image, (d) SAED pattern, (e) HRTEM image, and (f) FFT pattern of NN NWs synthesized at 200 °C for 4 h with temperature fluctuation. The inset red lines in (b) are standard pattern of cubic NaNbO3 (JCPDS No. 75-2102). As shown in Fig. 1a, typical cotton-like shapes, assembled from myriad of NWs are observed. The diameter of NWs is about 10 nm, while the length cannot be distinguished clearly. The cotton-like architecture is quite loosened, remaining the


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intrinsic advantages of NWs, which are beneficial for the photocatalytic process. All the peaks in the XRD patterns (Fig. 1b) are identified to the standard database (JCPDS No. 075-2102) of cubic NN. The average crystallite size of 8.7 nm is calculated by fitting the diffraction data. The intensity of (100) is higher than that of (110), while in standard card (100) plane is relatively weaker as compared with (110) plane. This result indicates the preferred orientations, which agrees well with the morphology observation. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HR-TEM) were used to clarify the morphology and crystal structure of cubic NN NWs. One-dimensional nanostructure of NN can be identified unambiguously in the TEM image (Fig. 1c), and the lengths of NN NWs approach several hundred nanometers. The statistical distribution of the diameter of NN NWs from TEM images is shown in Fig. S1, and the average diameter size is 7.8 nm, which is consistent with the XRD and SEM results. The SAED patterns (Fig. 1d) show an obvious polycrystalline feature, which are indexed to be (100), (110), (111), (200) and (210), respectively. The HR-TEM image in Fig. 1e indicates that the spacing between adjacent lattice fringes is 4.076 Å, which approaches the lattice spacing of the (100) planes of cubic NN. The slight expansion can be understood as the result of size effect. Fig. 1f presents the Fast Fourier Transform (FFT) of Fig. 1e, suggesting that the individual nanowire is of single crystal. EDS results in Fig. S2a manifest that NN NWs are composed of Na, Nb and O elements, and no other impurity can be detected. The stoichiometric ratio of the sample is quite closed to the standard value of NN (Na/Nb/O = 1/1/3) with homogenous distribution, as shown in Fig. S2b.



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Fig. 2 (a) FE-SEM image, (b) TEM image and (c) XRD patterns of the NN NPs synthesized at 200 °C for 4 h without temperature fluctuation. The inset lines in (c) are standard pattern of cubic NaNbO3 (JCPDS No. 75-2102). 3.2. Formation Process Studies Although NN with orthorhombic,37 rhombohedral38,39 and monoclinic40 structures have been selectively synthesized in hydrothermal condition, the direct synthesis of cubic NN NWs via solvothermal route is unprecedented. Therefore, it is believed that the organic solvent EG may play an important role in forming the cubic NN NWs. As an excellent directing reagent, EG has already been used to prepare 1D metal oxides,41,42 carbonates43 and sulfides.44 The control experiment has also been carried out to demonstrate the crucial role of temperature fluctuation. As the FE-SEM image shown in Fig. 2(a), samples synthesized at 200 °C for 4 h without temperature fluctuation are aggregations composed of numerous irregular nanoparticles, and no


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NWs can be observed. Further, the representative TEM image is provided as shown in Fig. 2(b). It could be seen that the primary particles of the aggregations is 20~30 nm in size. The crystal structure of the samples is indexed to be cubic NaNbO3 (JCPDS No. 75-2102), as shown in Fig. 2(c). And, no obvious preferred orientation is observed. The results indicate that the formation of cubic NN NWs is the co-effects of organic solvent EG and the temperature-fluctuation process.

Fig. 3 Schemes proposed for the formation of NN NPs and NN NWs at different solvothermal processes: (a) raw material Nb2O5 particles; (b) intermediates formed on the surface of reactant Nb2O5; (c) perovskite structure NN crystal nucleus; (d) NN crystals attached with each other induced by the structure-guiding effect of EG molecules under the introduction of temperature fluctuation; (e) NN NWs; (f) the aggregations of NN crystals; (g) NN NPs. It’s telling that the modified-solvothermal process is efficient, time-saving and energy-economic. To explore the formation mechanism of NN NWs, we have examined the effect of autoclaving period at the fixed temperature of 200 °C and the



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evolution of crystal structure and morphology are presented in Fig. S3. Based on the experimental results above, a plausible formation process of NN NWs is proposed, as illustrated in Fig. 3. Due to relative high dielectric strength of EG (~37), Nb2O5 would dissolve in NaOH nonaqueous solution. With the proceeding of solvothermal treatment, Nb2O5 gradually transforms to intermediates (Fig. 3b). The transform process takes place on the surface, owing to the relative high viscosity of EG. When the temperature fluctuation is abruptly conducted, EG molecules tend to absorb on the surface of the fresh formed NN crystal nucleus in Fig. 3c, and the primary chain-like structure is constructed due to the directing effect of EG (Fig. 3d). The chain-like NN nanocrystals disperse in the nonaqueous surroundings due to size effect and flexibility. The NN samples are of cotton-like morphology, assembled from NWs as shown in Fig. 3e. The formation process has been further evidenced by the selected NN NWs with poor crystallized defects, as shown in Fig. S4. Without the thermal fluctuation, as shown in Figs. 3f and 3g, the fresh NN crystal nucleus tends to aggregate irregularly in order to minimize surface energy. Note that the previously reported NWs synthesized in EG were often conducted at a temperature lower than 200 °C.41,42 It’s the relative low driving force associated with a slow crystal growth rate that promote the asymmetric development.45 We can see that a temperature as high as 200 °C is not beneficial for the structure-directing effect of EG. Meanwhile, since the formation of perovskite-type NN is also thermodynamically controlled, only intermediate phase is obtained when the initial temperature is lower than 200 °C (e.g. 180 °C). It’s therefore concluded that the thermal fluctuation plays a critical role in the formation of subsequent NN NWs. In addition, the cubic crystal structure may relate to the C-H


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bonds and hydroxyl,9 as evidenced by the strong FT-IR absorption peak in the range of 2800~3000 cm-1 and 3200~3500 cm-1, respectively (Fig. S5).

Fig. 4 (a) UV-vis absorption spectra of NN NWs (red) and NN NPs (dark). The inset in figure (a) shows the corresponding (ahv)1/2~hv curves. (b) photocatalytic H2 evolution from aqueous methanol solution over NN NWs (red) and NN NPs (dark) with 0.5 wt% Pt loading. 3.3. PHE Activities and Underlying Mechanism Investigation Fig. 4a shows the UV-visible absorption spectra of NN NWs and NN NPs. The band gaps (Eg) of NN NWs and NN NPs are determined by equation (αhν)n = A(hν - Eg), where α, n, A and Eg are the absorption coefficient, light frequency, proportionality constant and band gap, respectively.46 From the inset of Fig. 4a, the band gaps of NN NWs and NN NPs are determined to be 3.54 and 3.37 eV, respectively. As plotted in Fig. 4b, the cubic NN NWs exhibit a significantly higher activity (70 µmol h-1) than that of NN particles (29 µmol h-1). To make a direct comparison with the previous reports, the amount of H2 generated per gram catalyst was evaluated. The PHE rate over NN NWs (699 µmol h-1g-1) is higher than that of previously reported cubic NN NPs (453 µmol h-1g-1).9,10 The NN NPs exhibit inferior photocatalytic activity (289



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µmol h-1g-1) as compared to the reported ones,9 due to the aggregation and insufficient effective surface areas. The pH values of reaction solution were measured before and after the photocatalytic process. The results reveal that initial reaction solution is neutral and would change to be slightly acidic (pH = 6.86 ± 0.1), because methanol could be partially oxidized to methanoic acid during the reaction.47 In view of practical application, the catalysts after reaction were recycled through centrifugation and drying. The PHE activity of as-recycled catalysts was evaluated under the same conditions. At each interval, the ultraviolet light was turn off and the gas-closed circulation system was evacuated. As plotted in Fig. S6, the normalized H2 evolution efficiency of NN NWs drops to 60% in the 2nd round, and the reduction from 2nd to 3rd round is yet relatively slight (less than 10%). As for NN NPs, a gradual reduction is observed. The factors that weaken the activity of catalysts may be related to the physical separation between Pt coctalyst and NN catalyst as well as the undesired by-product (e.g. methanoic acid).48


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Fig. 5 Rietveld-XRD plots of (a) NN NPs and (b) NN NWs. (c) XPS narrow scan spectrum of the Nb 3d core level of as-prepared NN NWs and NN NPs. (d) TG curves of NN NWs (red) and NN NPs (dark) from room temperature to 700 °C. In this work, systematic investigation on crystal structure, valence state, chemical composition and energy band structure of the samples were further addressed, aiming to clarify the contributors to the enhanced photocatalytic activity. Raman spectra in Fig. S7 indicate that both NN NWs and NN NPs are perovskite structure with evident Nb-O vibration modes. In addition, structural refinement results are presented in Figs. 5a and 5b, where the evaluation index Rp, Rwp, Rexp and Chi2 for NN NW and NN NPs are 10.9, 11.1, 7.84, 2.02 and 9.01, 9.4, 6.93, 1.842, respectively. This suggests that NPs actually exhibit a higher symmetry than NWs. Earlier work has demonstrated that high structure symmetry may benefit for the electron excitation and



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charge carrier migration, giving rise to the improved photocatalytic activity.9 X-ray photoelectron spectroscopy (XPS) was employed to investigate the elemental composition and chemical states of as-obtained samples. XPS survey spectra in Fig. S8 indicate that both NN NPs and NN NWs are mainly composed of Na, Nb, O and C. The C 1s peak located around 284.4 eV may be assigned to adventitious carbonaceous species, such as physical absorbed carbon oxide and residual organic molecules. Quantitatively, elements Na, Nb, O and C in NN NWs are 18.67, 16.88, 54.17 and 10.28 %, respectively; while in NN NPs are 19.40, 11.83, 51.02 and 17.75 %, respectively. There exist more carbon elements (7.47 %) in the surface of NN NPs. As shown in Fig. 5c, two typical Nb 3d5/2 and Nb 3d3/2 peaks associated with a slight peak shift are observed, which may attribute to the reduction effect of EG solvent.49 From the viewpoint of Zhou et al. that the reduction of Nb5+ can actually harvest the light energy more effectively,50 the NN NPs should have exhibited a higher PHE activity. According to the discussion above, the possibility that crystal structure and chemical state contributed to the improved PHE activity of NWs has been excluded. As the TGA curves shown in Fig. 5d, it’s evident that NN NWs present a smaller weight loss compared with NN NPs ranging from 25 to 700 °C. In low temperature region (< 100 °C), there exist no obvious difference in weight loss rate between NN NPs and NWs, and the weight loss are mainly resulted from physically absorbed solvents. In the high temperature region (400 ~ 700 °C), both NN NPs and NN NWs exhibit a slight weight loss, resulting from the volatilization of alkali elements. Similar results were previously observed in alkali niobates synthesized in solvothermal condition using IPA as the solvent.40 Special attentions should be paid to


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the medium temperature region (100 ~ 400 °C), where a large weigh loss about 8 % is observed for NN NPs. Especially, a distinct drop around 300 °C is noticed. While the weight loss of NN NWs is relative slow and can be understood as a result of the natural transition between low- and high-temperature regions. TGA coupled with mass spectrometry is an effective tool to study nanoparticles prepared in organic solvents.51 In particular, the residual organic molecules can be well-identified according to the fragments. To get detail information about the weight loss, TG-MS was heated from 40 to 700 °C under Ar atmosphere scanning through m/z in the range of 2 ~ 200. Only an intense ion signal at m/z = 44 is detected for NN NPs (Fig. S9). According to the preparation condition, fragment with m/z = 44 is attributed to CO2,52 which is originated from the decomposition of residual EG (OHCH2CH2OH) molecules. The results indicate that there are more organic molecules covered on the surface of NN NPs. This observation is in good accordance with the XPS results, where carbon content in NN NPs is higher than that in NN NWs. The measured specific surface areas of NWs and NPs are 64.258 m2/g and 80.134 m2/g, respectively, which are about three times higher than the reported values.9,10 Even so, the residual organic molecules would not only decrease the active site but also discount the effective mass of photocatalysts, thus deteriorating their activity of NN NPs. Usually, the intriguing 1D nanostructure is considered beneficial for the transportation of photo-generated carriers and can effectively inhibit their recombination, thereby increasing their photacatalytic activity.53,54 In order to determine the transfer and separation efficiency of the photogenerated charge carriers, the samples were characterized by photoluminescence spectroscopy (PL), as shown in Fig. S11. Both



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their shape and position are similar to the previous results.55 However, the difference of PL intensity between NN NWs and NN NPs is unconspicuous. Based on the TG-MS results in Fig. 5(d), we think it’s the existence of minor impurity in NN NPs that probably facilitate the separation of charge carriers, thus rendering their PL intensity comparable to that of NN NWs. Subsequently, by linearly extrapolating the low binding energy edge of the valence band intersecting with the background, the valence band maximum (VBM) of NN NWs and NN NPs is deduced to be 2.23 ± 0.05 and 2.15 ± 0.05 eV, respectively (Fig. S10a). The observed similar potential is accountable, because the valence band of semiconductor oxides including d0 transition metals is usually formed by O 2p orbital.56 Afterwards, the energy band structures of NN NWs and NN NPs are schematically illustrated in Fig. S10b. It’s seen that both samples possess sufficient conduct band (CB) for the H2 evolution, and the energy level of CB bottom in NN NWs is more negative than that in NN NPs. Generally, the more negative CB potential means the higher reduction ability, thus benefiting for the improved PHE activity.57 Therefore, the energy band structure of NN NWs is responsible for their superior PHE activity.


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Fig. 6 Schematic illustration of energy band structures of (a) Pt cocatalyst and (b) NN catalyst; (c) energy band alignment at the interface NN and Pt. Eg, Evac, EV, EC, EF and qφ are energy gap, vacuum energy level, valence band maximum, conduction band minimum, Fermi level and work function, respectively. Also, contribution of loaded Pt cocatalyst is considered. Nobel metal Pt is commonly used as cocatalyst in the PHE, which can effectively hinder the electron-hole recombination58 and promote the H2 generation.9 In our work, control experiments without Pt-loading were also carried out, and H2 evolution under ultraviolet irradiation is quite feeble. According to the secondary electron cutoff recorded with UPS using HeI (Fig. S12), the work function of NN NPs and NN NWs is estimated to be about 4.98 and 4.86 eV, respectively. As a commonly used noble metal, the work function of Pt is known to be about 5.65 eV.59 Without any further consideration of the specific measurement conditions, an energy band diagram of the NN–Pt hetero-junction showing the band bending at the interface between NN and Pt is proposed in Fig. 6. The Schottky barrier formed at the interface could decrease the energy position of NaNbO3 conduction band to its minimum potential level. The



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photogenerated electrons accumulated at CB tend to transfer to Pt, due to their energy level difference. In this way, the electron-hole recombination process can be effectively inhibited. At the same time, Pt cocatalyst can supply the reactive site where H2 could generate easily because of the low H2 overpotential on their surface, which also contributes to the PHE activity. In our present work, the smaller work function of NN NWs would more easily facilitate the photogenerated electron transfer from NN to Pt, giving rise to the higher H2 evolution efficiency compared with NN NPs. On the basis of aforementioned discussions, three factors can be considered to explain the improved PHE activity of NN NWs. Firstly, NN NWs with 1D nanostructure and smaller grain size can provide relatively large specific surface area for PHE process. Secondly, the higher chemical purity in NN NWs not only ensures the effective mass of the photocatalyst, but also enables the exposed surfaces more active. In contrast, some organic molecules remain on the surfaces of NN NPs, and the active sites will be partially covered, which make the measured specific surface area mendacious. Thirdly, the energy level of conduction band in NN NWs is more negative, and the stronger reduction ability would facilitate the H2 evolution. 4. Conclusion In the present work, cubic NN NWs of several nanometers in width and hundred nanometers in length have been directly synthesized for the first time. The temperature fluctuation during the nucleation period is crucial for the formation of such 1D nanostructure. Otherwise, only NN nanoparticles with severe aggregation are obtained. The abrupt and momentary temperature variation induce the construction of


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chain-like structure directed by the organic solvent, prevent the irregular aggregation and

promote

the

formation

of

NN

NWs.

Importantly,

the

present

temperature-fluctuation strategy is expected to provide an alternative pathway to the crystal growth regulation in the solution-based syntheses. With the loading of 0.5 wt% Pt as cocatalyst, the as-synthesized NN NWs exhibit an excellent PHE activity (699 µmol·h-1·g-1). On the basis of systematic characterization results, it’s concluded that the improved activity are mainly attributed to 1D nanostructure with large specific surface area, pure chemical compositions and powerful reduction ability. Acknowledgements The authors are grateful for the financial support from the National Nature Science Foundation of China (NSFC No. 51172108), the Research Fund of State Key Laboratory of Mechanics and Control of Mechanical Structures (Nanjing University of Aeronautics and astronautics) (Grant No. 0514Y01), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors also gratefully acknowledge the financial support from Funding of Jiangsu Innovation Program for Graduate Education (No. KYLX_0260) and the Fundamental Research Funds for the Central Universities. Supporting Information Available Additional information and figures concerning size distribution, elemental composition, XRD, SEM, FT-IR, Raman, (VB)-XPS, MS, PL, UPS spectra and PHE activity of recycled catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.



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TOC graphic


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Modified Solvothermal Strategy for Straightforward Synthesis of Cubic

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