Monodisperse spherical sandwiched core-shell structured SiO2AuTa2O5 and SiO2AuTa3N5 composites as visible-light plasmonic photocatalysts

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Monodisperse spherical sandwiched core-shell structured SiO2eAueTa2O5 and SiO2eAueTa3N5 composites as visible-light plasmonic photocatalysts Xiaoming Liu a,b,c, Jingchun Feng a, Binquan Wu a, Yizu Li a, Weijie Xie a, Jun Lin b,*, Xia Zheng a, Xubiao Luo a,**, Abdulaziz A. Al Kheraif d a

School of Environment and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, PR China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China c Jiangxi Province Engineering & Technology Research Center for Paper Chemicals, Nanchang Hangkong University, Nanchang 330063, PR China d Dental Health Department, College of Applied Medical Sciences, King Saud University, Riyadh, Saudi Arabia b

article info

abstract

Article history:

Au nanoparticles and fine Ta2O5 nanocrystallines were deposited on nonaggregated,

Received 19 March 2018

spherical, monodisperse SiO2 micorspheres to form a kind of sandwiched core-shell

Received in revised form

structured SiO2eAueTa2O5 composite plasmonic photocatalyst by an in-situ reduction

6 September 2018

process integrated with a sol-gel process. The prepared SiO2eAueTa2O5 composite

Accepted 10 September 2018

plasmonic photocatalyst was nitrided in ammonia flow at an appropriate temperature to

Available online 6 October 2018

obtain SiO2eAueTa3N5 composite plasmonic photocatalyst. Benefitting from Au nanoparticles' surface plasmon resonance effect, the obtained SiO2eAueTa2O5 composite

Keywords:

plasmonic photocatalyst presents a novel photochemical activity of H2 production from

Plasmonic photocatalyst

H2O with visible light response, and the prepared SiO2eAueTa3N5 composite plasmonic

Hydrogen production

photocatalyst exhibits a higher photochemical activity of H2 production from H2O with

SiO2eAueTa2O5 (Ta3N5) composites

visible light response than that of blank Ta3N5 nanocrystalline. Furthermore, the excellent morphologies of SiO2eAueTa2O5 and SiO2eAueTa3N5 composites, e.g., sandwiched core-shell structure, monodisperse, and spherical, drastically improve their dispersion in solution, light harvesting, charge separation and transportation, which might do some contributions to improve their photocatalytic activities accordingly. This investigation might set a good example for designing highly efficient plasmonic photocatalysts for photochemical solar H2 production with visible light response. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Lin), [email protected] (X. Luo). https://doi.org/10.1016/j.ijhydene.2018.09.052 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Introduction Photocatalytic water-splitting for H2 production by utilizing solar power represents a promising scheme for the increasing demands of sustainable and clean energy [1e11]. One of the key challenges for the application of such technology is the development of high efficient photocatalysts [12e20]. Although several kinds of semiconductor photocatalysts for photochemical water-splitting have been discovered, however, due to the limitation of their wide bandgaps (>3.0 eV), most of them merely response to ultraviolet light (UV) [21e23], which is only around 3e5% of the whole solar spectrum. However, for the visible light, it accounts for around 44e46% of the whole solar spectrum, which is far beyond the abundant of UV. Therefore, it is of great importance and interest to explore new and high efficient photocatalysts with visible light response for practical application in photocatalytic water-splitting for H2 production by utilizing solar power [24e37]. Noble-metal nanoparticles (NPs), e.g., Ag and Au, have attracted tremendous interests since they exhibit a strong and wide absorption band in visible light region on account of their localized surface plasmon resonance (SPR) effect, deriving from the collective oscillation of its surface electrons [31,38e47]. The enhanced near-field amplitude of SPR close to noble-metal NPs can promote the semiconductor's photocatalytic activity to some extent. Therefore, combining noblemetals NPs with semiconductors can get a better catalytic properties of semiconductor photocatalytic materials [31,43,44,48,49]. Liu et al. had reported an efficient plasmonic photocatalyst for H2 evolution from H2O with visible light response by combining TiO2 nanotubes with Au NPs [43]. Tian et al. had proposed an improved photocatalytic activity for the oxidation of methanol and ethanol by decorating a TiO2 films with Au nanoparticles [44]. Awazu et al. had reported a new type of AgeSiO2eTiO2 plasmonic photocatalyst with quite high photocatalytic activity for the decomposition of methylene blue [50]. Up to now, three mechanisms, e.g., photo scattering, near-field electromagnetic, and charge transfer, have been put forwarded to illustrate how the SPR effects improve the photocatalytic activity of the relevant plasmonic photocatalyst [31,45]. Different structured plasmonic photocatalysts may have different mechanisms. Metal-core semiconductor-shell structures can increase the efficiency of charge separation through direct hot electrons transport, resonant energy transmission, and local electromagnetic field improvement [31,38,41,45,51,52]. Therefore, the metalcore semiconductor-shell structured photocatalyst usually possesses enhanced photocatalytic activity. Wu et al. had reported a sandwiched nanostructure AueSiO2eCu2O photocatalyst with excellent photocatalytic activity, e.g., its methyl orange decomposition rate with visible-light response was around five times to that of Cu2O alone [45]. Yin et al. had proposed a AueSiO2 core-shell structure composite with a high photocatalytic activity in catalyzing reduction of 4nitrophenol. Also, Yu et al. had prepared a AueTiO2 coreshell composite with high photochemical activity in photodegrading acetaldehyde [53].

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Tantalum oxide (Ta2O5), as a semiconductor with a bandgap of around 4.0 eV, has been explored as a photocatalyst for photochemical decomposition of H2O into H2 and O2 because of its excellent properties, e.g., chemical stability, suitable bandedge position, light absorption, and reasonable price [54e56]. Although, due to its large bandgap energy (4.0 eV), the solar energy conversion efficiency is much low for practical application. Therefore, it would be very interesting if it could be modified to be a visible-light-active semiconductor photocatalyst with high efficiency [32]. Fortunately, SPR has provided a possibility of exploring a typically UV-active semiconductor photocatalyst to be a visible-light-active semiconductor photocatalyst by utilizing a plasmonic effect [32,43e46]. On the other hand, the shapes, sizes, and morphologies of the photocatalysts are also the key factors that could regulate their photocatalytic activity to some extent. Taking control of the photocatalyst's shape, morphology and size, enables us to regulate its optical properties and electronic, and finally to regulate its photocatalytic activity to some extents [57e61]. In a previous study, we had reported the photocatalytic activities of plasmonic photocatalysts of Nano Au/Ta2O5 and Nano Au/Ta3N5 composites, where Au nanoparticles were dispersed at random in Ta2O5 and Ta3N5 semiconductor host lattices [32]. But a detailed investigated of how the structure, size, and morphology of plasmonic photocatalyst regulates their electronic, optical properties and photocatalytic activities has not been performed. So as a continuation of our previous work, accordingly, in this manuscript we have designed and synthesized a sort of SiO2eAueTa2O5 and SiO2eAueTa3N5 composites with sandwiched core-shell structure, monodisperse, and submicron spherical morphologies. Moreover, we have investigated the structure, morphologies, optical properties, photocatalytic performances of the prepared SiO2eAueTa2O5 and SiO2eAueTa3N5 plasmonic photocatalyst composites with a comparison of the Nano Au/Ta2O5 and Nano Au/Ta3N5 composites in more detail. The monodisperse SiO2 microsphere (around 550 nm in diameter) was firstly decorated with Au NPs on its surface to introduce SPR effect. After decorating SiO2 microsphere with Au NPs on its surface, the SiO2eAu microsphere was coated by a thin layer of Ta2O5 nanocrystalline by a famous Pechini-type sol-gel method, which would produce a sandwiched coreshell structured SiO2eAueTa2O5 plasmonic photocatalyst composite with Au NPs anchoring between the SiO2 microsphere core and Ta2O5 nanocrystallines shell. The SiO2eAue Ta3N5 plasmonic photocatalyst was prepared by nitriding SiO2eAueTa2O5 plasmonic photocatalyst composite with ammonia at around 850
C for 8 h. It is very interesting to find that the obtained SiO2eAueTa2O5 plasmonic photocatalyst composite presents a novel photochemical activity for H2 production from H2O with visible light response, and the prepared SiO2eAueTa3N5 composite plasmonic photocatalyst exhibits a higher photochemical activity for H2 production from H2O with visible light response than that of blank Ta3N5 nanocrystalline and Nano Au/Ta3N5 composites. The current investigation might set a good example for designing highly efficient plasmonic photocatalysts for photochemical solar H2 production with visible light response.

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Experimental The synthesis of SiO2 microspheres SiO2 microspheres (around 550 nm in diameter) were syn€ ber method of base-catalyzed hythesized by a famous Sto drolysis of TEOS (tetraethoxysilane) [61]. Typically, 18 ml deionized water, 9.0 ml TEOS (A.R., Sinopharm), and 98 ml ammonium hydroxide (28 wt%, NH3 in water, A.R., Sinopharm) were added to 78 ml anhydrous ethanol (A.R., Sinopharm) in 500 ml beaker. After stirring for 4 h, a SiO2 microspheres suspension was obtained. The SiO2 microspheres were separated from above suspension by centrifugation. After that, they are washed with deionized water and anhydrous ethanol in turns at least 4 times. Finally, the obtained SiO2 microspheres were dried at 60
C in vacuum for 10 h.

The synthesis of SiO2eAu composites Citric acid monohydrate (A.R.), trisodium citrate dihydrate (A.R.), and chloroauric acid tetrahydrate (A.R.) was obtained from Sinopharm. The Au NPs was deposited on SiO2 microspheres' surface through a citrate-reduction process [62]. Briefly, a suitable amount of SiO2 microspheres, 2 ml H[AuCl4]$ 4H2O aqueous solution (2% w/v), and 100 ml deionized H2O were put into a 150 ml round-bottom flask. After SiO2 microspheres dispersed homogeneously in water by ultrasonication, the mixture solution was heated to boil. After keep boiling for a few minutes, 2.0 ml mixture solution containing citric acid and trisodium citrate was poured into 150 ml roundbottom flask with rapid stirring. After keep boiling for 2 min, the whole reaction system was cooled to room temperature. The SiO2eAu composites were separated from suspension by centrifugation. After washing with deionized water and anhydrous ethanol in turns at least 4 times, the obtained SiO2eAu composites were dried at 60
C in vacuum for 10 h.

The synthesis of SiO2eAueTa2O5 and SiO2eAueTa3N5 core-shell composites The sandwiched core-shell structured SiO2eAueTa2O5 composites were synthesized through a famous Pechini-type solgel method [61,63]. The TaCl5 solution was obtained by dissolving a certain amount of anhydrous TaCl5 in anhydrous alcohol. Then, the TaCl5 solution was poured into the mixture solution containing water and ethanol (Vwater:Vethanol ¼ 1:7). Then an appropriate amount of citric acid and polyethylene glycol (Sinopharm, PEG with Mol. wt. ¼ 10000), which were utilized as chelating agent and cross-linking agent, respectively, were added into the above mixture solution. After stirring for another 2 h, a transparent sol was obtained. After that, the prepared SiO2eAu composites were added into the above transparent sol with rapid stirring. After stirring for 4 h, the SiO2eAu composites were separated from sol by a centrifugation process. After drying at 110
C for 1 h, they are annealed at 500
C for 2 h to get SiO2eAueTa2O5 precursor. In order to increase the thickness of Ta2O5 nanocrystalline shell, the above synthesis process need to be repeated for desired

times. Usually, one time coating will produce a single layer of Ta2O5 nanocrystalline on SiO2eAu composites' surface. After a few times of coating, finally, the obtained SiO2eAueTa2O5 composite precursors were annealed at 750
C for 3 h to get SiO2eAueTa2O5 composites. The SiO2eAueTa3N5 composite was prepared by a nitridation process. The details were shown as follows. The SiO2eAueTa2O5 composite was put into a tube furnace. After that it was nitrided in ammonia flow at 850
C for around 8 h until the final SiO2eAueTa3N5 composite was obtained [32,61]. The formation process for SiO2eAueTa2O5 and SiO2eAueTa3N5 composites is shown in Fig. 1. For comparisons, the Nano Au/Ta2O5 and Nano Au/Ta3N5 composites with Au nanoparticles homogeneously embedded in Ta2O5 and Ta3N5 host lattices were also synthesized by same process, e.g., a famous Pechini-type sol-gel method [32,63]. During the sol-gel process, the Au NPs stock solution was injected into the sol precursor to get homogenous mixture sol with deep red color. Then the deep red sol was heated in a water bath at around 85
C to obtain deep red gel. After it was dried at 120
C in an oven for 10 h, the deep red gel was presintered in air at 450
C to get precursor. Finally, the precursor was fully ground into powder, then it was calcinated at 750
C for 3 h to get Nano Au/Ta2O5 composite. The Nano Au/Ta3N5 composite was obtained by nitriding the prepared Nano Au/Ta2O5 composite in ammonia flow at 850
C for 8 h [32].

Photocatalytic activity measurements Photocatalytic activity measurements were carried out in a photochemical reaction system [32]. The whole reactor was purged and evacuated several times with argon gas to remove air completely before starting a photochemical reaction. A 300 W Xe-lamp assembled with a UV long pass filter (L420) was used as a visible-light source. For H2 evolution, 50 mg photocatalyst power was put into reaction vessel containing 100 ml methyl alcohol aqueous solution (10 vol%). A certain amount of Pt NPs cocatalyst was deposited onto photocatalysts power's surface through an impregnation process, employing an aqueous solution of H2PtCl6 as precursor. After impregnating with H2PtCl6, the photocatalysts was then reduced in an H2 flow at 300
C for 2 h to get final Pt NPs loaded photocatalysts [64]. For O2 production, a certain amount of AgNO3 aqueous solution was utilized as a cathode sacrificial agent. In order to stabilize the reaction solution's pH value, around 50 mg La2O3 (Sinopharm, 99.99%) was added into the reaction solution. Beyond that, the other experimental conditions and process were basically the same as those for H2 production. The produced H2 (or O2) was analyzed by a gas chromatography (Agilent 7890b GC System) with high purity argon as carrier gas.

Characterizations The crystalline structure of the prepared samples were investigated by a powder X-ray diffractometer (XRD) using CuKa (l ¼ 0.15406 nm) radiation. The prepared samples' morphologies were characterized by a field-emission scanning electronic microscope (JSF-6700, FE-SEM), and sphericalaberration-corrected field-emission scanning transmission

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Fig. 1 e Formation process of SiO2eAueTa2O5 and SiO2eAueTa3N5 composites plasmonic photocatalyst with sandwiched core-shell structure.

electron microscopy (FE-STEM, FEI Titan ST electron microscope) in combination with energy-dispersive X-ray spectroscopy (EDS). The X-ray photoelectron spectroscopy (XPS) of the prepared samples was obtained from a PHI 5000C ESCA system. The diffuse reflectance spectra (DRS) of the prepared samples were acquired through a JASCO spectrometer (V-560). All measurements were performed at room temperature. Photoelectrochemical properties of the prepared samples were characterized by a CHI660E electrochemical workstation equipped with a standard 3-electrode system. The Pt wire and Hg/Hg2Cl2 were utilized as a counter and reference electrodes, respectively. A 300 W Xe-lamp assembled with a UV long pass filter (L420) was used as a visible-light source. A certain amount of Na2SO4 aqueous solution was served as the electrolyte. For the preparation of working electrode, 5 mg photocatalyst powder was put into the mixture solution containing 20 ml Nafion solution (0.25%) and 2 ml ethanol. Then, the above mixture was changed into a homogeneous electrolyte by ultrasonic dispersion. After that, 500 ml homogeneous electrolyte was cast onto a 2  6 cm2 FTO glass substrate to prepare electrode. After drying out in air, the prepared electrodes were baked in a N2 flow at 150
C for 1 h. Sample's electrochemical impedance spectroscopy was measured by applying a 5 mV amplitude AC voltage in the frequency range of from 0.01 Hz to 106 Hz in Na2SO4 solution.

nanocrystalline, besides the XRD patterns for SiO2eAu composite (shown in Fig. 2b), all characteristic diffraction patterns assigning to Ta2O5 were presented in Fig. 2c, which agree well with pure Ta2O5 phase's XRD patterns (Fig. 2e), suggesting that the crystallized Ta2O5 nanocrystalline was formed on SiO2e Au composite. No other impurity phase can be observed in Fig. 2c. It is obvious that there is no any chemical reaction happened between the SiO2eAu composite core and the Ta2O5 nanocrystalline shell in the course of the coating and heating treatment processes. However, for Nano Au/Ta2O5 composite (Fig. 2d), all diffraction patterns can be basically indexed to Ta2O5 phase. No diffraction peaks of Au nanoparticles were detected. This probably is ascribed to the low content of embedded Au nanoparticles [32]. Similarly, Au nanoparticles' diffraction patterns is not detected in Nano Au/Ta3N5 composite (shown in Fig. 2g). After nitriding the SiO2eAueTa2O5 composite in ammonia flow at 850
C for 8 h, besides the XRD patterns for amorphous SiO2 and Au nanoparticles, XRD patterns assigning to Ta3N5 crystal structure (shown in Fig. 2h) can be found in Fig. 2f, indicating the Ta3N5 is well-crystallized on SiO2eAu composite's surface. For Nano Au/Ta3N5

Results and discussion Crystalline structure of samples Fig. 2 shows the typically X-ray diffractometer patterns of bare SiO2 microspheres, SiO2eAu, SiO2eAueTa2O5 and SiO2eAue Ta3N5 composites. For the purpose of comparisons, the representative XRD patterns of Ta2O5, Ta3N5, Nano Au/Ta2O5 and Nano Au/Ta3N5 are also presented in Fig. 2. As shown in Fig. 2, there is only one broadband diffraction peak peaking at 2q ¼ 22.0
was acquired for bare SiO2 microspheres, assigning to the amorphous silica's characteristic peak [61]. However, for SiO2eAu composite (Fig. 2b), besides the broadband diffraction peak at 2q ¼ 22.0
, three metallic gold's diffraction peaks are presented, indicating that the deposited Au nanoparticles on amorphous SiO2 microspheres' surface are wellcrystallized. After coating SiO2eAu composite with Ta2O5

Fig. 2 e XRD patterns for as formed SiO2 microspheres (a), SiO2eAu (b), SiO2eAueTa2O5 (c), Nano Au/Ta2O5 (d), Ta2O5 (e), SiO2eAueTa3N5 (f), Nano Au/Ta3N5 (g), and Ta3N5 (h).

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composite (Fig. 2g), its diffraction patterns are basically in line with those of SiO2eAueTa3N5 composite and blank Ta3N5 nanocrystalline. The XRD patterns for SiO2eAueTa2O5 composites with different coating layers of Ta2O5 nanocrystallines are shown in Supporting Information 1 (SP1). With increasing the coating layers, the XRD patterns signal for Ta2O5 crystalline increases to some extent. Due to Au NPs were covered by layers of Ta2O5 nanocrystalline on SiO2 surface, it is normal that the XRD patterns signal assigning to Au nanoparticles decreases with increasing the coating Ta2O5 nanocrystalline layers (shown in SP1). For XRD patterns of the SiO2eAueTa2O5 (Fig. 2c) and SiO2e AueTa3N5 (Fig. 2f) composites, the broadening of their diffraction patterns can be due to the Ta2O5 and Ta3N5 nanocrystallines' small size. The crystallite sizes of the Ta2O5 and Ta3N5 nanocrystallines could be calculated by the Scherrer equation, that is D ¼ 0.941l/bcosq, here, b and q are the fullwidth at half-maximum and diffraction angle of an observed peak, l is the X-ray wavelength, and D is the average grain size, respectively [65]. The highest peaks of 2q ¼ 24.5
(for SiO2eAueTa3N5) and 2q ¼ 22.9
(for SiO2eAueTa2O5) were utilized to calculate the average crystallize size (D) of Ta3N5 and Ta2O5 nanocrystallines. The average crystallite sizes are estimated to be around 10.6 nm and 10.2 nm for Ta3N5 and Ta2O5 nanocrystallines on SiO2eAu composite's surface, respectively, which basically agree with their TEM measurement results (shown in Fig. 4f and i).

Morphologies of samples Fig. 3a-d shows the FESEM micrographs of bare SiO2 microspheres, SiO2eAu, SiO2eAueTa2O5, and SiO2eAueTa3N5 composites, respectively. For comparison, the representative FESEM micrographs of Nano Au/Ta2O5 and Nano Au/Ta3N5 composites are also presented in Fig. 3e,f. It can be seen clearly from Fig. 3a that the bare SiO2 microspheres are in average size of around 550 nm in diameter. These microspheres are monodisperse and nonaggregated, with a smooth surface and a narrow size distribution. After decorating SiO2 microspheres with some Au NPs on the surface, as shown in Fig. 3b, the SiO2eAu composites still keep the original morphologies of SiO2 microspheres, e.g., spherical, nonaggregated, and monodisperse. Similarly, as shown in Fig. 3c, after Ta2O5 nanocrystalline was coated on SiO2eAu composites' surface, the obtained core-shell structured SiO2eAue Ta2O5 composites always keep the original morphologies of SiO2eAu composites, i.e., spherical, nonaggregated, and monodisperse, but a little bit larger in size than that of SiO2e Au composites. The SiO2eAueTa2O5 composites' surface is rough in texture comparing with that of SiO2eAu composites. After nitridation of SiO2eAueTa2O5 composites, the prepared SiO2eAueTa3N5 composites have similar morphologies to these of SiO2eAueTa2O5 composites, i.e. spherical, nonaggregated, monodisperse, uneven surface, a litter bit larger in size than that of SiO2eAu composites. All of these evidences suggest that the Ta2O5 and Ta3N5 nanocrystallines have been successfully coated on SiO2eAu composites' surface, respectively. From Fig. 3e, it also can be observed that the Nano Au/ Ta2O5 composites consisted of small particles in different sizes with aggregation. After nitridation, the obtained Nano

Au/Ta3N5 composites are serious aggregated with irregular shapes (shown in Fig. 3f). Fig. 4 shows the HAAD-STEM micrographs of SiO2eAu (Fig. 4a), SiO2eAueTa2O5 (Fig. 4d and e), and SiO2eAueTa3N5 (Fig. 4g and h) composites, and the HRTEM micrographs of Au NPs (Inset in Fig. 4a), SiO2eAueTa2O5 (Fig. 4f) and SiO2eAue Ta3N5 composites (Fig. 4i). For comparison, the Nano Au/ Ta2O5, Nano Au/Ta3N5 composites are also shown in Fig. 4b and c, respectively. The bare SiO2 micorspheres are monodisperse, nonaggregated, spherical, with smooth surface in the size of around 550 nm. The deposited Au NPs (10e20 nm in diameter) were detected as bright spots in SiO2eAu composite, indicating Au NPs are homogeneously anchored on SiO2 microspheres' surface. The Au NPs' HRTEM image (the inset in Fig. 4a) shows clearly diffraction fringes, which indicate its high crystallinity. Similarly, the Au nanoparticles can also be detected as bright spots in Nano Au/Ta2O5 and Nano Au/Ta3N5 composites (shown in Fig. 4b and c), which indicate that Au NPs are distributed in Ta2O5 and Ta3N5 host lattices at random. The SiO2eAueTa2O5 composite's average size was estimated to be around 650 nm in diameter, which is larger than that of bare SiO2 microspheres (550 nm in diameter). From the magnified HAADF-STEM micrograph of an individual SiO2eAueTa2O5 particle (shown in Fig. 4e), it is seen clearly that the SiO2 microsphere's surface was coated by uneven, rough Ta2O5 nanocrystallines. A supplementary HRTEM examination at the interface of the SiO2eAu and Ta2O5 nanocrystalline is presented in Fig. 4f. From Fig. 4f, it is seen clearly that the Ta2O5 nanocrystallines shell consisted of aggregated fine nanoparticles with high crystallinity in the thickness of around 10 nm, which can be proved by well-defined lattice fringes in Fig. 4f. The Ta2O5 nanocrystalline shell's average thickness is determined to be around 50 nm for four times coating cycles (shown in Fig. 4a). Therefore, one time coating will produce a layer of Ta2O5 nanocrystalline shell with the thickness around 10 nm on SiO2eAu composite's surface. As discussed before, after nitridation, the prepared SiO2eAueTa3N5 composites keep the original morphologies of SiO2eAue Ta2O5 composites, i.e. spherical, nonaggregated, monodisperse, uneven surface. A representative enlarged HAADF-STEM image for a single SiO2eAueTa3N5 particle (shown in Fig. 4h) confirmed the above characterizations. The HRTEM micrograph obtained from the interface of Ta3N5 nanocrystalline and SiO2eAu composite are shown in Fig. 4i. From Fig. 4i, it can be concluded that the Ta3N5 nanocrystalline consists of aggregated small nanoparticles with high crystallinity in the average size of around 10 nm. Similarly to SiO2eAueTa2O5 composite, the whole thickness of Ta3N5 nanocrystalline shell is determined to be around 50 nm for four times coating. Herein, the EDS element mappings of STEM were performed to make sure the existence of Au, Ta, N component in SiO2eAueTa2O5 and SiO2eAueTa3N5 composites. The results are shown in SP2. The Au, Ta, and N component comes from the Au nanoparticles, Ta2O5 and Ta3N5 nanocrystallines, respectively. The EDS elemental mappings from SP2a and 2f for SiO2eAueTa2O5 and SiO2eAueTa3N5 composites clearly show that the Au, Ta, and N element is evenly distributed in

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Fig. 3 e FE-SEM micrographs of SiO2 microspheres (a), SiO2eAu (b), SiO2eAueTa2O5 (c), SiO2eAueTa3N5 (d), Nano Au/Ta2O5 (e), and Nano Au/Ta3N5 (f).

the composites (SP 2b-e, SP2g-j). Since N component only comes from Ta3N5, therefore, it can be deduced that the Ta2O5 nanocrystalline has been successfully nitrided to be Ta3N5 nanocrystalline on the surface of spherical SiO2@Au composites. The XPS is utilized to make sure of the prepared SiO2eAue Ta2O5 and SiO2eAueTa3N5 composites' chemical states and surface composition. The corresponding measurement results are shown in Fig. 5. The Si, O, Au, Ta and N elements are clearly shown in Fig. 5a, which agree well with the constituents of the SiO2eAueTa2O5 and SiO2eAueTa3N5 composites. The high resolution XPS spectrum of Au 4f, O 1s, Si 2p, N 1s

and Ta 4f are presented in Fig. 5bef. The signals of Ta and Au in the XPS spectrum of SiO2eAueTa2O5 composites indicates that the Au nanoparticles and Ta2O5 nanocrystalline have been successfully anchored on the surface of SiO2 microsphere. Besides Au and Ta, the presence of N in the XPS spectrum of SiO2eAueTa3N5 composites further shows that the SiO2eAueTa2O5 composites have been successful nitrided to be SiO2eAueTa3N5 composites. The XPS spectra of SiO2e AueTa2O5 and SiO2eAueTa3N5 composites are in line with the results of XRD, TEM and EDX, which further confirm the formation of sandwiched core-shell structured SiO2eAueTa2O5 and SiO2eAueTa3N5 composites.

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Fig. 4 e HADF-STEM images of SiO2eAu (a), Nano Au/Ta2O5 (b), Nano Au/Ta3N5 (c), and SiO2eAueTa2O5 (d), the single magnified image of SiO2eAueTa2O5 (e), HRTEM image of SiO2eAueTa2O5 (f), HADF-field STEM images of SiO2eAueTa3N5 (g), the single magnified image of SiO2eAueTa3N5 (h), HRTEM image of SiO2eAueTa3N5 (i). The HRTEM image of Au NPs in SiO2eAu composite is shown in inset in Fig. 4(a). The density of states (DOS) for SiO2eAueTa2O5 and SiO2e AueTa3N5 composites' valence band can also be estimated by valence band XPS. The results are shown in SP4. From SP4, it is clearly seen that the SiO2eAueTa2O5 composite shows a typical valence band DOS characteristic of Ta2O5. The maximum energy value at the edge of the valence band (EVB) is estimated to be around 3.76 eV. The band gap (Eg) of SiO2eAue Ta2O5 (Ta2O5) is determined to be around 3.87 eV estimated by UVevisible diffuse reflectance spectra (shown in Fig. 6). Therefore, its minimum value at the edge of conduction band (ECB) is determined to be around 0.11 eV. For SiO2eAueTa3N5 composite, similarly, it shows typical valence band DOS characteristic of Ta3N5. Its maximum energy (EVB) value at the edge of the valence band is estimated to be around 2.02 eV. The band gap (Eg) of SiO2eAueTa3N5 (Ta3N5) estimated from UVevisible absorption spectra (shown in Fig. 6) is determined to be around 2.04 eV. Thus for the minimum value at the edge of conduction band (ECB), it is determined to be around 0.13 eV. The EVB and ECB of SiO2eAueTa2O5 and SiO2eAue Ta3N5 composites, estimated from valence band XPS spectra and UVevisible diffuse reflectance spectra, basically agree with the theoretical values of Ta2O5 and Ta3N5, e.g., 3.83 eV for

EVB of Ta2O5, 1.91 eV for EVB of Ta3N5, and 0.17 eV for the ECB of both Ta2O5 and Ta3N5 [32,54e56].

Optical properties Fig. 6 presents the corresponding UVevisible absorption spectra of bare SiO2 microspheres, SiO2eAu, Ta2O5, SiO2eAue Ta2O5, Ta3N5 and SiO2eAueTa3N5 samples. For comparison, Au NPs stock solution's absorption spectrum is also presented in Fig. 6. For bare SiO2 microspheres, it only shows a weak absorption from 250 to 350 nm peaking at 280 nm (black line in Fig. 6), which can be seen clearly when enlarge 50 times (olive dot line in Fig. 6). On account of strong SPR effects, Au NPs stock solution shows a strong absorption band ranging from 200 to 900 nm with a maximum at 518 nm shouldering at around 360 nm (Fig. 6, wine dot line). With Au NPs anchoring on the SiO2 microspheres, the SiO2eAu composites show the characteristic absorptions of Au NPs in range of from 200 nm (UV) to 900 nm (visible), i.g., a wide band absorption from 400 to 900 nm in visible region peaking at 534 nm and 423 nm with shoulders from 200 to 400 nm peaking at 246, 305, 350 nm in UV region (Fig. 6, blue line). As it is reported that the SPR's

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Fig. 5 e XPS survey spectra of the SiO2eAueTa2O5 and SiO2eAueTa3N5 composites (a), high resolution XPS Si 2p (b), O1s (c), Au 4f (d), Ta 4f (e), and N 1s (f) spectra of SiO2eAueTa2O5 and SiO2eAueTa3N5 composites.

frequency can be regulated by tuning the shape, size, composition of the noble metal nanoparticles together with the surrounding medium's dielectric properties [43,47]. Comparing with Au NPs stock solution, the SPR absorption band of SiO2e Au composites show a red-shift from 518 nm (Au NPs stock solution) to 534 nm (SiO2eAu composites), which might be ascribed to the variation of the dielectric environment of Au NPs, e.g. change from water to SiO2 and air environments. The UVevisible absorption spectrum of Ta2O5 nanocrystalline (Fig. 6, red line) shows a strong wide band absorption in UV light range peaking at 256 nm. The band gap of Ta2O5 is determined to be 4.0 eV, therefore, there is no any absorption in visible range for Ta2O5. After Ta2O5 nanocrystalline was coated on SiO2eAu composite, the obtained SiO2eAueTa2O5 composite presents the characteristic absorption of Au NPs and Ta2O5 nanocrystalline (pink line in

Fig. 6). Comparing with SiO2eAu composites, the Au NPs' SPR absorption band in SiO2eAueTa2O5 composite further show a red-shift, e.g. shift from 534 nm in SiO2eAu composites to 580 nm in SiO2eAueTa2O5 composite. From Fig. 6, it can be seen clearly that the UV absorptions band from around 305 to 350 nm (shown in SiO2eAu composites, blue line in Fig. 6) disappeared in SiO2eAueTa2O5 composite. Obviously, the further red-shift of SPR absorption and the disappearance of UV absorptions in SiO2eAueTa2O5 composites are ascribed to the variation of the dielectric environments of Au NPs, e.g., for SiO2eAu composites, the Au NPs are surrounded by SiO2 and air, while for SiO2eAueTa2O5 composites, the Au NPs are surrounded only by SiO2 and Ta2O5. The different dielectric constant of air (1.0), SiO2 (3.9) and Ta2O5 (26.0) lead to the further red-shift of SPR absorption and the disappearance of UV absorptions in SiO2eAueTa2O5 composites.

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situation hold for SiO2eAueTa2O5 composite. As shown in SP3b, the absorption intensities of Au NPs' SPR in SiO2eAue Ta2O5 composite increases with increasing Au NP's content until it up to the maximum at 0.788 wt % of SiO2. For SiO2eAue Ta3N5 composites, the absorbance in range from 200 to 518 nm and new absorption bands beyond 598 nm in the UVevisible absorption (shown in SP 3c) increases with increasing Au NPs' contents to some extents. The variation of SiO2eAue Ta3N5 composite's absorbance basically agrees with that of SiO2eAu and SiO2eAueTa2O5 composites. The highest absorbance of SiO2eAueTa3N5 composite is the sample with Au NP' contents around 0.788 wt % of SiO2.

Photocatalytic properties of SiO2eAueTa2O5 and SiO2eAue Ta3N5 composites Fig. 6 e UVevis diffuse reflectance spectra of SiO2, Ta2O5, SiO2eAu, SiO2eAueTa2O5, Ta3N5, and SiO2eAueTa3N5. The Au NPs stock solution's absorption spectrum is also shown here for comparison.

For Ta3N5 nanocrystalline, its absorption spectrum (Fig. 6, green line) shows a strong absorption from 200 to 650 nm. On account of its high crystallinity, the background absorption is very weak beyond 650 nm. The strong absorption in visible range from 400 to 650 nm is ascribed to its narrow energy band gap (around 2.06 eV). After Ta3N5 nanocrystalline was coated on SiO2eAu composite, similarly to SiO2eAueTa2O5 composite, the obtained SiO2eAueTa3N5 composite presents the characteristic absorption of Au NPs and Ta3N5 nanocrystalline (deep blue line in Fig. 6). When Au NPs are covered by Ta3N5 nanocrystalline on the surface of SiO2 particle, the strong coupling effect of Au NPs, SiO2 particle, and Ta3N5 nanocrystalline as well as the high dielectric constant of the Ta3N5 (110.0 for Ta3N5, 30.0 for Ta2O5, 3.9 for SiO2 and 1.0 for H2O) has leaded to a big red-shift of Au NPs' SPR and the increase absorbance of Ta3N5 in the UVevisible range [43,47]. For example, in UVevisible region, the SiO2eAueTa3N5 composite's absorption intensity is much stronger than Ta3N5 nanocrystalline alone, and there is an obviously increase of absorption beyond 598 nm comparing with absorption of Ta3N5 nanocrystalline. The SPR absorption peaks of Au NPs on SiO2 surface (SiO2eAu composite) at 305, 350, and 423 nm have been shifted to 345, 413, and 520 nm, respectively. Obviously, the anchored Au NPs between the SiO2 particles core and Ta3N5 nanocrystallines shell are responsible for the increased absorption range from 200 to 518 nm and new absorption bands beyond 598 nm in the UVevisible absorption of SiO2eAueTa3N5 composites, which will contribute to its higher photocatalytic activities accordingly. The absorption intensities of SiO2eAu, SiO2eAueTa2O5 and SiO2eAueTa3N5 composites as a function of the Au NPs' contents are shown in SP3. From SP3a, it is seen clearly that in visible range the SiO2eAu composite's absorption intensity increases with increasing Au NP's anchoring contents firstly, after reaching the maximum when Au NP's content up to 0.788 wt% of SiO2, it starts to decrease with increasing Au NP's anchoring contents, so the optimum Au NP's anchoring content is determined to be around 0.788 wt% of SiO2. The same

After SiO2eAueTa2O5 composite was loaded with Pt NPs as cocatalyst, its photocatalytic activity with response to visible light had been examined. Fig. 7 shows the H2 production as a function of time course for a series of SiO2eAueTa2O5 composites anchored with different contents of Au NPs (Nano Au 0e0.985 wt% of SiO2) calcined at 750
C. For comparison, the photocatalytic activities for blank Ta2O5 nanocrystalline, SiO2eAu and SiO2eTa2O5 composites have also been measured in similar experimental conditions as that of SiO2e AueTa2O5 composites. As shown in Fig. 7, no H2 can be evolved from blank Ta2O5, SiO2eAu and SiO2eTa2O5 composites with response to visible light irradiation. However, for the SiO2eAueTa2O5 composite, it presents a novel photocatalytic activity for H2 production with response to visible light irradiation. Obviously, the Au NPs, sandwiched between the SiO2 particle core and Ta2O5 nanocrystallines shell, are the key elements that determine SiO2eAueTa2O5 composite's novel photocatalytic activity. Control experiments show that neither Au NPs nor Ta2O5 nanocrystallines alone show any photocatalytic activity for H2 production with visible light

Fig. 7 e The time course of H2 evolution for Ta2O5, SiO2eAu, SiO2eTa2O5 and SiO2eAueTa2O5 (Au nanoparticle 0e0.985 wt% of SiO2). The vertical lines are errors bars. The inset shows the photocatalytic activity of SiO2eAueTa2O5 composite as a function of anchoring concentration of Au nanoparticles.

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response. Therefore, the novel photocatalytic activity of SiO2e AueTa2O5 composite for H2 production with visible light response is the results of the synergistic effect of Au NPs and Ta2O5 nanocrystallines. The photocatalytic activities of SiO2e AueTa2O5 composites for H2 evolution from water anchored with different contents of Au nanoparticles are also shown in the inset in Fig. 7. As shown in inset, the SiO2eAueTa2O5 composites' photocatalytic efficiency of increases with increasing the anchored Au NPs' content until it reaches to a maximum of 78.12 mmol g1 h1. So the optimized Au NPs' content is determined to be 0.788 wt% of SiO2 [2,13]. Overpassing that critical point, higher Au NP's content will lead to the decrease of photocatalytic activity. This might be ascribed to the light scattering effect caused by high content of Au NPs in SiO2eAueTa2O5 composites. As a chain reaction, the light scatting effect will lessen the light absorption and lower photocatalytic activity accordingly [32]. The changing trend of photocatalytic activity of SiO2eAueTa2O5 composite (shown in Fig. 7) basically agrees with that of the absorbance of SiO2e Au composite (shown in SP3). The highest absorption of SiO2e Au composite leads to the highest photocatalytic activity of SiO2eAueTa2O5 composite. Therefore, the highest photocatalytic activity of SiO2eAueTa2O5 composite is the one with anchoring Au NPs contents of around 0.788 wt% of SiO2. Usually, Pt NPs were often utilized as cocatalyst to improve catalytic activity in photocatalytic water-splitting system [1,2,13]. Therefore, it is needed to optimize the amount of Pt in Pt-deposited SiO2eAueTa2O5 composite. Here, how the Pt NPs' content influence the SiO2eAueTa2O5 composites' photocatalytic activities was investigated to optimize the loading amount of Pt NPs. SP5 shows the photocatalytic activity of SiO2eAueTa2O5 composites (Nano Au 0.788 wt% of SiO2, 10 layers) as a function of Pt NPs' content on Pt/SiO2eAueTa2O5 composites. The SiO2eAueTa2O5 composite's photocatalytic activity increases drastically with increasing Pt NPs' content until Pt NPs' content reaches to 0.5 wt% of SiO2eAueTa2O5 composites, then it starts to decrease with increasing Pt NPs' content. Therefore, the optimized Pt NPs' content is determined to be around 0.5 wt% of the SiO2eAueTa2O5 plasmonic photocatalyst composites. The number of coating layers (times, one time one layer) is also another elements that will influence the SiO2eAueTa2O5 composite's photocatalytic activity. SP6 shows the SiO2eAue Ta2O5 composite's photocatalytic activity as a function of the numbers of coating layers. Basically, the SiO2eAueTa2O5 composite's photocatalytic activity increases with increasing the coating layers of Ta2O5 nanocrystalline. In current case, the thickness of one layer of Ta2O5 nanocrystalline shell is determined to be around 10 nm (As discussed in front part). At low number of coating layers, the Ta2O5 nanocrystallines are distributed on SiO2eAu particles in discontinuous island morphology with low thickness, some of Au nanoparticles on SiO2 are not covered or half covered by Ta2O5 nanocrystalline shell, so some of Au nanoparticles can not have a close contact with Ta2O5 nanocrystallines. Therefore, part of excited SPR electrons can not be transferred from Au nanoparticles to conduction band of Ta2O5 nanocrystallines shell smoothly, which will lead to low photocatalytic activity accordingly. When increasing thickness of Ta2O5 nanocrystallines shell at appropriate level, the Au NPs will be fully covered and will

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have a close contact with Ta2O5 nanocrystalline shell, which will offer facilities for SPR electrons transferring from Au NPs to conduction band of Ta2O5 nanocrystalline shell and lead to the higher photocatalytic efficiency. From SP6, it can be seen clearly that the SiO2eAueTa2O5 composite's photocatalytic activity increase with increasing the coating layers up to getting to 10 layers (ten times of coating processes). However, when the coating layer is higher than 10, the photocatalytic activity decreases slowly with increasing the coating layers. This might be ascribed to after ten times of coating process, the SiO2eAueTa2O5 composites start to aggregate, and the aggregation become more serious when further increasing the coating layers (>10 layers). Therefore, it is difficult to get a monodisperse, spherical morphology, which might influence its photocatalytic activity accordingly. Therefore, the optimized coating layers of Ta2O5 nanocrystalline shell on SiO2e Au composite is determined to be 10 with around 100 nm in thickness. As discussed before, three mechanisms, e.g., photo scattering, near-field electromagnetic, and charge transfer, have been put forwarded to illustrate how the SPR effects improve the photocatalytic activity of the relevant plasmonic photocatalyst [31,45]. However, photon scattering and near-field electromagnetic effects can only be observed in plasmon photocatalysts when pure photocatalyst's absorption spectrum has an overlap with the SPR spectrum of noble metal NPs [45,66]. From Fig. 6 it can be seen clearly that there is no any overlap for pure Ta2O5 photocatalyst's absorption spectrum and Au NPs' SPR spectrum in UVevis region, which eliminates the resonant photon scattering and near-field electromagnetic effects. Therefore, the novel photocatalytic activity of SiO2eAueTa2O5 plasmonic photocatalyst composite might be due to the charge transfer mechanism, such as, energetic electrons are directly transferred from Au NPs to Ta2O5 nanocrystalline for photocatalytic reaction [45,67,68]. In SiO2eAueTa2O5 plasmonic photocatalyst composite, SiO2 only acts as a morphology modulation support material for the monodisperse, submicron spherical and special coreshell structured morphology of SiO2eAueTa2O5 composite. It is believed that Au NP serves as a photosensitizer in SiO2eAue Ta2O5 plasmonic photocatalyst composite, e.g. Au NP absorbs resonant photons, generates excited electrons upon SPR excitation, then these excited electrons with high plasmon resonance energy are transferred to adjacent Ta2O5 nanocrystalline's conduction band for photocatalytic reactions. With an efficient electrons injection from Au NPs upon SPR excitation, the Ta2O5 nanocrystalline in SiO2eAueTa2O5 composite presents a novel photoactivity with visible light response for H2 production. Usually, the component, morphology, structure of the plasmon photocatalyst composites will influence their photocatalytic activities to some extents [57e60]. To illustrate this issue, we have measured the absorbance and photocatalytic activity of SiO2eAueTa2O5 composite with a comparison with those of Nano Au/Ta2O5 composite, in which a certain amount of Au NPs are homogenously dispersed in Ta2O5 host lattice [32]. As we known, except for the component, morphology, structure, there are a lot of variates between the SiO2eAue Ta2O5 and Nano Au/Ta2O5 plasmonic photocatalyst composites, e.g., the amount of Au NPs, Ta2O5, the relative ratio of Au

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NPs to Ta2O5, and the absorbance in visible range, etc. To compare the photocatalytic activity of SiO2eAueTa2O5 and Nano Au/Ta2O5 plasmonic photocatalyst composites qualitatively or semi-quantitatively, we have selected the representative samples of SiO2eAueTa2O5 and Nano Au/Ta2O5 plasmonic photocatalyst composites with similar absorbance in visible light range. The corresponding UVevis absorption spectra for SiO2e AueTa2O5 composite, blank Ta2O5 nanocrystallines and Nano Au/Ta2O5 composite are presented in Fig. 8a. For comparison, the Au NPs stock solution's absorption spectrum is presented in Fig. 8a also., It is seen clearly from Fig. 8a that the SiO2eAue Ta2O5 composite's UVevis absorption spectrum presents similar intensity to Nano Au/Ta2O5 composite, but it shows a smaller red-shift of absorption band (peaking at 580 nm) than that of SiO2eAueTa2O5 composite (peaking at 600 nm), which might be ascribed to the different surrounding environment for Au nanoparticles in SiO2eAueTa2O5 and Nano Au/Ta2O5 composites. For Nano SiO2eAueTa2O5 composite, the Au nanoparticles are surrounded by SiO2 and Ta2O5, while for Nano Au/Ta2O5 composite, the Au nanoparticles are surrounded only by Ta2O5. The different dielectric constant of SiO2 (3.9) and Ta2O5 (26.0) lead to different red-shift of absorption band. The UVevis absorption spectra of the SiO2e

Fig. 8 e The UVevis diffuse reflectance spectra for Ta2O5 nanocrystalline, SiO2eAueTa2O5, Nano Au/Ta2O5 composites (a). The time course for H2 production of SiO2e AueTa2O5 and Nano Au/Ta2O5 composites (b). The vertical lines are error bars.

AueTa2O5 and Nano Au/Ta2O5 composites in Fig. 8a indicate that the selected SiO2eAueTa2O5 and Nano Au/Ta2O5 composites have similar absorbance in visible range. Fig. 8b presents the time course for H2 production from H2O of SiO2eAue Ta2O5 and Nano Au/Ta2O5 composites. Usually, the hydrogen evolution rate of photocatalyst can be calculated by straight slope. The hydrogen evolution rate of SiO2eAueTa2O5 and Nano Au/Ta2O5 composites are determined to be around 79.10 and 65,66 mmol g1 h1, respectively. It is found from Fig. 8b that even the SiO2eAueTa2O5 and Nano Au/Ta2O5 composites show similar absorbance in visible range, but the SiO2eAue Ta2O5 composite's photocatalytic activity is higher than the Nano Au/Ta2O5 composite with visible light response. All these results indicate that the component, structure, and morphologies of plasmon photocatalyst composites are vital elements to their photocatalytic activities. The possible reasons are discussed as follows. Firstly, the monodisperse, spherical and core-shell structured SiO2eAueTa2O5 composites show highly dispersancy in solution ascribe to their properties of monodisperse, narrowsized distribution and submicron size. Accordingly, these properties will contribute to build a high efficient photocatalytic reactor, such as improve the absorption of visible light, and enlarge the unit surface of photocatalyst [1]. In order to evaluate the prepared composites' dispersancy in reaction solution, Nano Au/Ta2O5, SiO2eAueTa2O5, Nano Au/Ta3N5 and SiO2eAueTa3N5 composites in equal amount were dispersed in water after ultrasonication for 5 min, and then, they were placed in a quiet place for a long time to examine their dispersancy. Photographs for Nano Au/Ta2O5, SiO2eAue Ta2O5, Nano Au/Ta3N5 and SiO2eAueTa3N5 composites dispersed in water with different standing time are shown in SP7. For Nano/Ta2O5 (SP7a) and Nano Au/Ta3N5 (SP7d) composites, due to its larger particle size and wide size distribution, the suspended particles are easy to sedimentates in the solution, and settled down completely after 2 h' standing. However, for the suspensions of the SiO2eAueTa2O5 (SP7b) and SiO2eAueTa3N5 (SP7c) composites, they almost do not show any change after 2 h' standing. The control experiment indicates that the SiO2eAueTa2O5 and SiO2eAueTa3N5 composites' suspensions show higher dispersancy in water than Nano Au/Ta2O5 and Nano Au/Ta3N5 composites. Secondly, the sandwiched core-shell structure of SiO2e AueTa2O5 composites might be another element of benefitting to their photocatalytic activity. The Au NPs are anchored in the inner side of Ta2O5 (Ta3N5) nanocrystalline shell, while the Pt NPs, as co-catalyst, are anchored on the outside of nanocrystalline shell. Therefore, once excitation, the energetic electrons in Au NPs will migrate directionally from inner side Au NPs, across the Ta2O5 (Ta3N5) nanocrystalline shell, until up to the outside Pt NPs for H2 evolution. The directional migration of energetic electrons will certainly increase the efficiency of charge separation and migration processes. Furthermore, SPR effect in Au NPs will produce a new local electric field in SiO2eAueTa2O5 composites, which will promote the charge migration and separation, and restrain charge recombination to some extent [45,66]. Finally, the thin layer of Ta2O5 nanocrystalline shell allows a faster transport of photogenerated charges from inner side to outsite, which will lower charge recombination and

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increase photocatalytic activities accordingly [1,2]. However, for Nano Au/Ta2O5 composites, the photogenerated charges are easy to get quenched by defects or nearby Au NPs (act as defects, sometimes) in Ta2O5 host lattice because they have to travel a long distance before reaching Ta2O5 nanocrystalline's surface for photochemical reaction [32]. Furthermore, due to Au NPs are homogeneously dispersed in Ta2O5 host lattice, when Au NPs' content exceed the optimized datum, the Au NPs in outside layer Ta2O5 will hinder the visible light absorption of Nano Au/Ta2O5 composites to some extents. Therefore, it is reasonable that the SiO2eAueTa2O5 composite's photocatalytic activity is higher than Nano Au/Ta2O5 composite under the similar experimental conditions. As discussed before, the prepared SiO2eAueTa3N5 composite has inherited the excellent morphologies of the SiO2e AueTa2O5 composite, e.g., special sandwiched core-shell structure, spherical, monodisperse, and nonaggregation, etc. Accordingly, its excellent morphologies and strong SPR of Au NPs has leaded to its high photocatalytic activity comparing with blank Ta3N5. Fig. 9 shows the H2 production of a series of SiO2eAueTa3N5 composites (Au nanoparticles content is 0e0.985 wt% of SiO2) as a function of time course. For comparison, the photocatalytic activities of blank Ta3N5, SiO2eAu and SiO2eTa3N5 composites are also shown in Fig. 9. For blank SiO2eAu, there is no H2 evolution, indicating that Au NPs do not show any photocatalytic activity. And for SiO2eTa3N5 composite, it shows comparable photocatalytic activity to the blank Ta3N5 nanocrystallines with visible light response. However, for SiO2eAueTa3N5 composites, all of them show higher photocatalytic activity than Ta3N5 nanocrystalline and SiO2eTa3N5 composite. The inset in Fig. 9 also shows visually that with the increase of Au NPs' content, the SiO2eAueTa3N5 composite's H2 evolution rates increase firstly, after reaching a maximum, it start to decrease when further increasing Au NPs' content. The optimized Au NPs' content in SiO2eAue

Fig. 9 e The time course of H2 evolution for Ta3N5, SiO2eAu, SiO2eTa3N5 and SiO2eAueTa3N5 (Au nanoparticle 0e0.985 wt% of SiO2). The vertical lines are errors bars. The inset shows the photocatalytic activity for SiO2eAueTa3N5 composite as a function of Au NPs' content.

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Ta3N5 composite is determined to be around 0.788 wt % of SiO2. Obviously, the Au NPs' SPR effect in SiO2eAueTa3N5 composite has led to the improvement of Ta3N5 nanocrystalline's photocatalytic activity with visible light response. Similar to SiO2eAueTa2O5 composite, three mechanisms, namely, photo scattering, near-field electromagnetic, and charge transfer, have been put forwarded to explain the improved photocatalytic activities of SiO2eAueTa3N5 plasmonic photocatalyst composite [31,45,66]. As mentioned above, photon scattering and near-field electromagnetic effects can only be observed when the SPR spectrum of noble metal nanoparticles overlaps with the absorption spectrum of semiconductor photocatalyst. It is obvious in Fig. 6 that Au NPs' SPR spectrum (SiO2@Au) has a big overlap with the absorption spectrum of blank Ta3N5 nanocrystalline in visible light range. Generally speaking, photon scattering effect can only be observed when plasmonic metal NPs' size is larger than 50 nm in diameter [45,67,68]. The Au NPs' size in SiO2e AueTa3N5 composite is estimated to be around 15e20 nm in diameter, which eliminates the contribution of the photon scattering effect. Therefore, for SiO2eAueTa3N5 composite, its improved photocatalytic activity might be the results of nearfield electromagnetic effect caused by Au NPs' SPR effect. For near-field electromagnetic effect mechanism, the oscillations of energetic electrons in Au NPs couples with the electromagnetic field (incident light), and just this coupling has lead to the improvement of the local electromagnetic fields in SiO2eAueTa3N5 composite [43,45,47]. The improved local electromagnetic field is much stronger than the applied electromagnetic field [69]. Therefore, once the incident light's frequency matches Au NPs' SPR, the resonance electrons will be produced, as if forming an extra electromagnetic field, which will enhance light absorption, facilitate the production of electron-hole pairs in SiO2eAueTa3N5 composite. Also, the improved local electronmagnetic field will promote the migration and separation of charge carriers (i.g., holes and electrons) in SiO2eAueTa3N5 composite, which will finally result in the improvement of SiO2eAueTa3N5 composite's photocatalytic activity accordingly [41,45e47]. Due to the conduction bands of Ta3N5 and Ta2O5 semiconductors nanocrystallines are mainly consisted of Ta 5d orbitals, therefore, the potentials at the bottom of conduction bands of Ta3N5 and Ta2O5 nanocrystallines should be same [70]. So, the energetic (or excited) electrons in Au NPs could be transmitted directly from Au NPs to Ta3N5 semiconductor for hydrogen production in SiO2eAueTa3N5 composite, as happened in SiO2eAueTa2O5 composite. Therefore, the plasmon-assisted direct energy transfer effect might be another mechanism in SiO2eAueTa3N5 composite. Similar to SiO2eAueTa2O5 plasmonic photocatalyst composites, we have also measured and compared the absorbance and photocatalytic activity for hydrogen evolution of SiO2e AueTa3N5 plasmonic photocatalyst composites with those of Nano Au/Ta3N5 plasmonic photocatalyst composites to investigate how the component, morphology, structure of the SiO2eAueTa3N5 plasmon photocatalyst composite influences its photocatalytic activity. The corresponding UVevis absorption spectra of blank Ta3N5 nanocrystalline, Nano Au/ Ta3N5 composite and SiO2eAueTa3N5 composite are shown in Fig. 10a. For comparison, the Au NPs stock solution's

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Fig. 10 e The UVevis diffuse reflectance spectra of Ta3N5 nanocrystalline, SiO2eAueTa3N5 and Nano Au/Ta3N5 composites (a). The time course of H2 production for Ta3N5 nanocrystalline, SiO2eAueTa3N5 and Nano Au/Ta3N5 composites.

absorption spectrum also is presented in Fig. 10a. From Fig. 10a, it can be seen clearly that the SiO2eAueTa3N5 composite's UVevis absorption spectrum showed higher intensity than that of the Nano Au/Ta3N5 composite and blank Ta3N5 from 200 to 600 nm. But beyond 600 nm, the Nano Au/Ta3N5 composite showed higher absorption intensity than that of SiO2eAueTa3N5 composite, and the SiO2eAueTa3N5 composite showed higher absorption intensity than that of Nano Au/Ta3N5. Therefore, the SiO2eAueTa3N5 and Nano Au/Ta3N5 composites have similar absorption intensity in whole visible light region. Fig. 10b shows the H2 production of the SiO2eAue Ta3N5 and Nano Au/Ta3N5 composites as a function of time course. For the purpose of comparison, the similar experimental results for blank Ta3N5 nanocrystalline is also shown in Fig. 10b. The corresponding hydrogen production rate of SiO2eAueTa3N5 and AueTa3N5 composites are determined to be around 136.32 and 116.88 mmol g1 h1, respectively. From Fig. 10b, it is found that the SiO2eAueTa3N5 composite's photocatalytic activity is higher than that of the Nano Au/ Ta3N5 composite in same experiment condition, although SiO2eAueTa3N5 and Nano Au/Ta3N5 composites show similar absorption intensity in visible range as a whole. Promoted by Au NPs' SPR effect, the SiO2eAueTa3N5 and Nano Au/Ta3N5

composites' photocatalytic activities are higher than blank Ta3N5 nanocrystallines. It is obvious that the obtained SiO2eAueTa3N5 composite has inherited the excellent morphologies from SiO2eAue Ta2O5 composite, e.g., special sandwiched core-shell structure, sub-micrometer size, spherical, monodisperse, narrow size distribution, as well as nonaggregated etc. Similar to SiO2eAueTa2O5 composite, all these excellent morphologies of SiO2eAueTa3N5 composite will do some contributions to efficient light absorption, electron injection, charge separation and charge transformation, and it will promote its photocatalytic activity accordingly. Therefore, the improved photocatalytic activity of SiO2eAueTa3N5 composite (comparing to Nano Au/Ta3N5 and blank Ta3N5) might be due to the synergetic effect of charge transfer, near-field electromagnetic effect, as well as its excellent morphologies, e.g., special sandwiched core-shell structure, sub-micrometer size, spherical, monodisperse, narrow size distribution, as well as nonaggregation etc. [32,45,47]. The SiO2eAueTa2O5 and SiO2eAueTa3N5 composites' photocatalytic activities for O2 production with visible light response have also be examinated. Unfortunately, similar to Nano Au/Ta2O5 and Nano Au/Ta3N5 composites, SiO2eAue Ta2O5 and SiO2eAueTa3N5 composites do not show any photocatalytic activity for O2 production with UVevis light response [32]. Obviously, its photocatalytic activity for O2 production has been quenched by Au NPs. The Au NPs in SiO2eTa2O5 and SiO2eTa3N5 composites has brought in some new energy levels [32]. However, the Ta2O5 and Ta3N5 nanocrystallines' valance band do not match well with these new energy levels well, therefore, as has happened in Nano Au/ Ta2O5 and Nano Au/Ta3N5 composites, the SiO2eAueTa2O5 and SiO2eAueTa3N5 composites' photocatalytic activities for O2 production with visible light response has been quenched [32]. The transient photocurrent-time (I-t) curves of the SiO2e AueTa2O5 and SiO2eAueTa3N5 composite electrodes were measured with visible light response for several on-off cycles. The results are shown in Fig. 11. For both SiO2eAueTa2O5 (Fig. 11a) and SiO2eAueTa3N5 (Fig. 11b) composite electrodes, when turn on visible light source, its photocurrent response increases quickly, and then it decays to a constant value. The photocurrent's decay shows that there is a recombination of holes and electrons. For example, the holes accumulated on SiO2eAueTa2O5 and SiO2eAueTa3N5 composite electrodes' surface would like to recombine competitively with electrons from conduction band, rather than to be captured or trapped by reduction species in electrolyte, and it will lead to the decay of photocurrent accordingly [71]. The photocurrent decay quickly to zero once the light source is turned off. For both SiO2eAueTa2O5 and SiO2eAueTa3N5 composite electrodes, the photocurrent density increases with increasing the content of Au nanoparticles until it reach to the maximum at 0.788 wt % of SiO2. Usually, the higher photocurrent density, the longer lifetime of living photogenerated electron and holes. Accordingly, it will lead to high efficient separation of electron-holes, and high photocatalytic activity [72]. Here, for SiO2eAueTa2O5 composite, due to Ta2O5 is inert to visible light irradiation, so the Au nanoparticle with a strong SPR effect has played a decisive role. With the

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response in 5 h cycled for five times (Fig. 12). As shown in Fig. 12, the photocatalytic activity only shows a slightly decrease, e.g., 0.9 and 7.6 mmol g1 h1 for SiO2eAueTa2O5 and SiO2eAueTa3N5 composite, respectively. The experiment results indicate that the SiO2eAueTa2O5 and SiO2eAueTa3N5 composites has high stability during photocatalytic process. To better understand the mechanism of the novel photocatalytic activity of sandwiched core-shell structured of SiO2e AueTa2O5 composites and a significantly improved photocatalytic activity of SiO2eAueTa3N5 composites, a schematic illustrating the direct energetic electron transfer effect and near-field electromagnetic effect are shown in Fig. 13. In SiO2e AueTa2O5 (SiO2eAueTa3N5) composites, with the excitation

Fig. 12 e Cycle runs of SiO2eAueTa2O5 and SiO2eAue Ta3N5 composites for H2 evolution. The reaction system was purged with Ar for 20 min to remove H2 every 5 h. Fig. 11 e The transient photocurrent e time (I-t) curves of SiO2eAueTa2O5 and SiO2eAueTa3N5 composites electrodes under visible light irradiation.

irradiation of visible light, abundant energetic electrons contained in Au NPs are generated. To keep the electric neutrality, a lot of quasi holes (or energetic positive charges) are formed for photoelectrochemical reaction on SiO2eAueTa2O5 composite's surface [32]. The higher contents of Au NPs, the more energetic electrons and quasi holes will be generated, and it will lead to stronger photocurrent density accordingly. However, the overloading Au NPs will act as quenching centers of holes and electrons. Therefore, the optimized Au NPs' content is determined to be around 0.788 wt % of SiO2 with the maximum photocurrent density of 0.268 mm cm2. For SiO2e AueTa3N5 composite, the increase in photon-to-current could be ascribed to the increased light absorption in visible light range. Furthermore, as discussed before, the near-field electromagnetic effect might do some positive contributions to high efficient charges separation. In order to evaluate the stability of the SiO2eAueTa2O5 and SiO2eAueTa3N5 composites, a series of experiments were executed for H2 production from H2O with visible light

Fig. 13 e Proposed mechanism for hydrogen evolution of Ta2O5, Ta3N5, SiO2eAueTa3N5 and SiO2eAueTa2O5 composites.

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of visible light, the energetic electrons in Au NPs will be transferred to the conduction band of Ta2O5 (Ta3N5) nanocrystalline (as discussed before, the potentials at the bottom of conduction bands of the Ta3N5 and Ta2O5 nanocrystallines are in same level). After traveling across the thin shell of Ta2O5 (Ta3N5) nanocrystalline, these energetic electrons reach to the surface active center, e.g., Pt NPs, for photocatalytic H2 generation. Due to the loss of energetic electrons, the Au NPs are positively charged. In order to keep the electric neutrality, the Au NPs with positive charges will receive electrons from donors, e.g., methanol, ethanol, aldehyde, etc. [32]. The special monodisperse, spherical, sub-micrometer size of SiO2eAueTa2O5 (SiO2eAueTa3N5) composites help themselves with good suspension in solution and efficient light absorption. Also, the sandwiched core-shell structure, thin layer of high crystallinity Ta2O5 (Ta3N5) shell will facilitate the electron injection, charge separation and transformation. For SiO2eAueTa3N5 composites, as shown in Fig. 13, the near-field electromagnetic effect will do extra contribution to its photocatalytic activity. Finally, these synergistic effects have contributed to the high efficient photocatalytic activity of SiO2eAueTa2O5 and SiO2eAueTa3N5 composites with visible light response.

Conclusions Au nanoparticles and a thin layer of Ta2O5 (Ta3N5) nanocrystallines were deposited on monodisperse spherical SiO2 microspheres by an in-situ reduction combined with a sol-gel method. The obtained SiO2eAueTa2O5 and SiO2eAueTa3N5 plasmonic photocatalyst composites maintain a spherical morphology with sandwiched core-shell structure, a submicrometer size in diameter, monodisperse, and narrow size distribution. Promoted by strong SPR, the prepared SiO2eAue Ta2O5 plasmonic photocatalyst composite demonstrates a novel photochemical photocatalytic activity of H2 production from H2O with visible light response, and the obtained SiO2e AueTa3N5 plasmonic photocatalyst composite presents a significantly improved photochemical activity of H2 production from H2O with visible light response. Furthermore, the excellent morphologies of SiO2eAueTa2O5 and SiO2eAue Ta3N5 composites, e.g., monodisperse, submicron spherical and core-shell structure, etc., drastically improve their dispersion in solution, light harvesting, charge transportation and separation, in return, it will improve their photocatalytic activities accordingly. The current methodology and approach might set a good example for designing highly efficient plasmonic photocatalysts for photochemical solar H2 production with visible light response.

Acknowledgements This project is financially supported by the National Natural Science Foundation of China (NSFC 51762035, 21161015, 21366024, 51468043), the Natural Science Foundation of the Jiangxi Province of China (20152ACB20011), Jiangxi Province Special funds of Condition Platform of Science and

Technology (No. 20164BCD40098), the Natural Science Foundation of the Jiangxi Higher Education Institutions of China (GJJ09180, GJJ14513), and the Distinguished Scientist Fellowship Program of King Saud University as well as the Deanship of Scientific Research at King Saud University for funding this work through research group No (RG- 1939-038).

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.09.052.

references

[1] Grewe T, Meggouh M, Tu¨ysu¨z H. Nanocatalysts for solar water splitting and a perspective on hydrogen economy. Chem Asian J 2016;11:22e4. [2] Chen X, Shen S, Guo L, Mao S. Semiconductor-based photocatalytic hydrogen generation. Chem Rev 2010;110:6503e70. [3] Wang Z, Jin Z, Wang G, Ma B. Efficient hydrogen production over (ZIF-67) and g-C3N4 boosted with MoS2 nanoparticles. Int J Hydrogen Energy 2018;43:13039e50. [4] Ding Z, Hu H, Xu J, Lin P, Cui C, Qian D, et al. Hierarchical spheres assembled from large ultrathin anatase TiO2 nanosheets for photocatalytic dydrogen evolution from water splitting. Int J Hydrogen Energy 2018;43:13190e9. [5] Wang P, Chen P, Kostka A, Marschall R, Wark M. Control of phase coexistence in calcium tantalate composite photocatalysts for highly efficient hydrogen production. Chem Mater 2013;25:4739e45. [6] Saranya D, Selvaraj V. Double metal oxide based nickel hybrid nanocatalyst for electrooxidation and alkaline fuel cell device fabrication. Int J Hydrogen Energy 2018;43:13450e61. [7] Xia C, Qiao Z, Shen L, Liu X, Cai Y, Xu Y, et al. Semiconductor electrolyte for low-operating-temperature solid oxide fuel cell: Li-diped ZnO. Int J Hydrogen Energy 2018;43:12825e34. [8] Yuan Y, Wang F, Hu B, Lu H, Yu Z, Zou Z. Significant enhancement in photocatalytic hydrogen evolution from water using a MoS2 nanosheet-coat ZnO heterostructure photocatalyst. Dalton Trans 2015;44:10997e1003. [9] Cheng Y, Zerhouni N, Lu C. A hybrid remaining useful life prognostic method for proton exchange membrane fuel cell. Int J Hydrogen Energy 2018;43:12314e27. [10] Zhou C, Shi R, Shang L, Zhao Y, Waterhouse GIN, Wu Li-Z, et al. A sustainable strategy for the synthesis of pyrochlore H4Nb2O7 hellow microspheres as photocatalysts for overall water splitting. ChemPlusChem 2017;82:181e5. [11] Harrabi N, Souissi M, Aitouche A, Chaabane M. Modeling and control of photovoltaic and fuel cell based alternative power systems. Int J Hydrogen Energy 2018;43:11442e51. [12] Yousefi R, Beheshtian J, Seyed-Talebi SM, Azimi HR, JamaliSheini F. Experimental and theoretical study of enhanced photocatalytic activity of Mg-doped ZnO NPs and ZnO/rGO nanocomposites. Chem Asian J 2018;13:194e203. [13] Manchala Nagappagari L, Venkatakrishnan S, Shanker V. Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructure for highly efficient photocatalytic hydrogen evolution under solar light irradiation. Int J Hydrogen Energy 2018;43:13145e57. [14] Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M, Ye J. Nanophotocatalytic materials: possiblities and challenges. Adv Mater 2012;24:229e51.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 0 5 4 6 e2 0 5 6 2

[15] Wu Y, Dong B, Zhang Jing, Song H, Yan C. The synthesis of ZnO/SrTiO3 composite for high-efficient photocatalytic dydrogen and electricity conversion. Int J Hydrogen Energy 2018;43:12627e36. [16] Zhao W, Xie L, Zhang M, Ai Z, Xi H, Li Y, et al. Enhanced photocatalytic activity of all-solid-state g-C3N4/Au/P25 Zscheme system for visible-light-driven H2 evolution. Int J Hydrogen Energy 2016;41:6277e87. [17] Chehade G, Demir ME, Dincer I, Yuzer B, Selcuk H. Experimental Investigation and analysis of a new photoelectrochemical reactor for hydrogen production. Int J Hydrogen Energy 2018;43:12049e58. [18] Zhang G, Zhang M, Ye X, Qiu X, Lin S, Wang X. Iodine modified carbon nitride semiconductors as visible light photocatalysts for hydrogen evolution. Adv Mater 2014;26:805e9. [19] Wang X, Yang X, Miao L, Gao J, Wu L, Wang N, et al. Decoration of Bi2Se3 nanosheets with a thin Bi2SeO2 layer for visible-light-driven overall water splitting. Int J Hydrogen Energy 2018;43:10950e8. [20] Weller T, Sann J, Marschall R. Pore structure controlling the activity of mesoporous crystalline CsTaWO6 for photocatalytic hydrogen generation. Adv Energy Mater 2016;6:1600208. [21] Hou Y, Laursen AB, Zhang J, Zhang G, Zhu Y, Wang X, et al. Layered nanojunctions for hydrogen-evolution catalysis. Angew Chem Int Ed 2013;52:3621e5. [22] Shi H, Zhang S, Zhu X, Liu Y, Wang T, Jiang T, et al. Uniform gold-nanoparticle-decorated {001}-faceted anatase TiO2 nanosheets for enhanced solar-light photocatalytic reactions. ACS Appl Mater Interface 2017;9:36907e16. [23] Zhao M, Xu H, Ouyang S, Tong H, Chen H, Li Y, et al. Fabricating a Au@TiO2 plasmonic system to elucidate Alkaliinduced enhanced of photocatalytic H2 evolution: surface Potential shift or Methanol oxidation acceleration? ACS Catal 2018;8:4266e77. [24] Cai X, Zhu M, Elbanna OA, Fujitsuka M, Kim S, Mao L, et al. Au nanorod photosensitized La2Ti2O7 nanosteps: successive surface heterojunctions boosting visible to near-infrared photocatalytic H2 evolution. ACS Catal 2018;8:122e31. [25] Hisatomi T, Kubota J, Domen K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem Soc Rev 2014;43:7520e35. [26] Shang L, Bian T, Yu H, Waterhouse GIN, Zhou C, Zhao Y, et al. CdS nanoparticle-decorated Cd nanosheets for efficient visible light-driven photocatalytic hydrogen evolution. Adv Energy Mater 2016;6:201501241. [27] Li Z, Luo W, Zhang M, Feng J, Zou Z. Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook. Energy Environ Sci 2013;6:347e70. [28] Yousefi R, Azimi HR, Mahmoudian MR, Cheraghizade M. Highly enhanced photocatalytic performance of Zn(1x)MgxO/rGO nanostars under sunlight irradiation synthesized by one-pot refluxing method. Adv Powder Technol 2018;29:78e85. [29] Zhou C, Zhao Y, Shang L, Shi R, Wu LZ, Tung CH, et al. Facile synthesis of ultrathin SnNb2O6 nanosheets towards improved visible-light photocatalytic H2-production activity. Chem Commun 2016;52:8239e42. [30] Sivula K, Le-Formal F, Grau¨tzel M. Solar water splitting: progress using hematite a-Fe2O3 photoelectrodes. ChemSusChem 2011;4:432e49. [31] Shiravizadeh AG, Yousefi R, Elahi SM, Sebt SA. Effects of annealing atmosphere and rGO concentration on the optical properties and enhanced photocatalytic performance of

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

20561

SnSe/rGO nanocomposites. Phys Chem Chem Phys 2017;19:18089e98. Luo Y, Liu X, Tang X, Luo Y, Zeng Q, Deng X, et al. Gold nanoparticles embedded in Ta2O5/Ta3N5 as active visiblelight plasmonic photocatalysts for solar hydrogen evolution. J Mater Chem A 2014;2:14927e39. Yu H, Shi R, Zhao Y, Bian T, Zhao Y, Zhou C, et al. Alkaliassisted synthesis of nitrogen deficient graphitic carbon nitride with tunable band structures for efficient visiblelight-driven hydrogen evolution. Adv Mater 2017;29:1605148. Patel P, Ghadge S, Hanumantha P, Datta M, Gattu B, Shanthi P, et al. Active and robust novel bilayer photoanode architectures for hydrogen generation via direct non-electric bias induced photo-electrochemical water splitting. Int J Hydrogen Energy 2018;43:13158e76. Huang Y, Wang C, Song H, Bao Y, Lei X. Carbon-coated molybdenum carbide nanosheets derived from molybdenum disulfide for hydrogen evolution reaction. Int J Hydrogen Energy 2018;43:12610e7. Chang G, Cai Z, Jia H, Zhang Z, Liu X, Liu Z, et al. High electrocatalytic performance of a graphene-supported PtAu nanoalloy for methanol oxidation. Int J Hydrogen Energy 2018;43:12803e10. Mao B, Wang B, Yu F, Zhang K, Zhang Z, Hao J, et al. Hierarchical MoS2 nanoflowers on carbon cloth as an efficient cathode electrode for hydrogen evolution under all pH values. Int J Hydrogen Energy 2018;43:11038e46. Wu N. Plasmonic metal-semiconductor photocatalysts and photoelectrochemical cells; a review. Nanoscale 2018;10:2679e96. Yu H, Shi R, Zhao Y, Waterhouse GIN, Wu L-Z, Tung C-H, et al. Self-assembled Au/cdse nanocrystal clusters for plasmon-mediated photocatalytic hydrogen evolution. Adv Mater 2017;29:170083. Pu Y, Wang G, Chang K, Ling Y, Lin Y, Fitzmorris B, et al. Au nanostructure-decorated TiO2 nanowire exhibiting photoactivity across entire UV-visible region for photoelectrochemical water splitting. Nano Lett 2013;13:3817e23. Ingram D, Linic S. Water splitting on composite plasmonicmetal/semiconductor photoelectrodes: evidence for selective plasmon-induced formation of charge carriers near the semiconductor surface. J Am Chem Soc 2011;133:5202e5. Brown M, Suteewong T, Kumar RS, D'Innocenzo V, Petrozza A, Lee MM, et al. Plasmonic dye-sensized solar cells using core-shell metal-insulator nanoparticles. Nano Lett 2011;11:438e45. Liu Z, Hou W, Pavaskar P, Aykol M, Cronin SB. Plasmon resonant enhancement of photocatalytic water splitting under visible illumination. Nano Lett 2011;11:1111e6. Tian Y, Tatsuma T. Mechanisms and applications of plasmon-induced charge separation at TiO2 film loaded with gold nanoparticles. J Am Chem Soc 2005;127:7632e7. Cushing S, Li J, Meng F, Senty T, Suri S, Zhi M, et al. Photocatalytic activity enhanced by plasmonic resonant energy transfer from metal to semiconductor. J Am Chem Soc 2012;134:15033e41. Zhang Q, Lima DQ, Lee I, Zaera F, Chi M, Li Y. A highly active titanium dioxe based visible-light photocatalyst with nonmetal doping and plasmonic metal decoration. Angew Chem Int Ed 2011;31:7088e92. Thomann I, Pinaud B, Chen Z, Clemens B, Jaramillo T, Brongersma M. Plasmon enhanced solar-to-fuel energy conversion. Nano Lett 2011;11:3440e6. Wang H, You T, Shi W, Li J, Guo L. Au/TiO2/Au as a plasmonic coupling photocatalyst. J Phys Chem C 2012;116:6490e4.

20562

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 0 5 4 6 e2 0 5 6 2

[49] Chen J, Wu J, Wu P, Tsai D. Improved photocatalytic activity of shell-isolated plasmonic photocatalyst Au@SiO2/TiO2 by promoted LSPR. J Phys Chem C 2012;116:26535e42. [50] Awazu K, Fujimaki M, Rockstuhl C, Tominaga J, Murakami H, Ohki Y, et al. A plasmonic photocatalyst consisting of siliver nanoparticles embedded in titanium dioxide. J Am Chem Soc 2008;130:1676e80. [51] Shiraishi Y, Tsukamoto D, Sugano Y, Shiro A, Ichikawa S, Tanaka S, et al. Platinum nanoparticles supported on anatase titanium dioxide as highly active catalysts for aerobic oxidation under visible light irradiation. ACS Catal 2012;2:1984e92. [52] Ge J, Zhang Q, Zhang T, Yin Y. Core-satellite nanocomposite catalysts protected by a porous silica shell: controllable reactivity, high stability, and magnetic recyclability. Angew Chem Int Ed 2008;47:8924e8. [53] Wu X, Song H, Yoon J, Yu Y, Chen Y. Synthesis of core-shell Au@TiO2 nanoparticles with truncated wedge-shaped morphology and their photocatalytic properties. Langmuir 2009;25:6438e46. [54] Tu H, Xu L, Mou F, Guan J. Highly active Ta2O5 microcubic single crystals: facet energy calculation, facile fabrication and enhanced photocatalytic activity of hydrogen production. J Mater Chem A 2016;4:16562e8. [55] Cherevan AS, Gebhardt P, Shearer CJ, Matsukawa M, Domen K, Eder D. Interface engineering in nanocarbon-Ta2O5 hybrid photocatalysts. Energy Environ Sci 2014;7:791e6. [56] Goncalves RV, Migowski P, Wender H, Eberhardt D, Weibel DE, Sonaglio FC, et al. Ta2O5 nanotube obtained by anodization: effect of thermal treatment on the photocatalytic activity for hydrogen production. J Phys Chem C 2012;116:14022e30. [57] Lin L, Yang Y, Men L, Wang X, He D, Chai Y, et al. A highly efficient TiO2@ZnO n-p-n heterojunction nanorod photocatalyst. Nanoscale 2013;5:588e93. [58] Cui Z, Ge Y, Chu H, Baines R, Dong P, Tang J, et al. Controlled synthesis of Mo-doped Ni3S2 nano-rods: an efficient and stable electro-catalyst for water splitting. J Mater Chem A 2017;5:1595e602. [59] Kargar A, Sun K, Jing Y, Choi C, Jeong H, Jung GY, et al. 3D branched nanowire photoelectrochemical electrodes for efficient solar water splitting. ACS Nano 2013;7:9407e15. [60] Anandhababu G, Huang Y, Babu DD, Wu M, Wang Y. Oriented growth of ZIF-67 to derive 2D porous CoPO

[61]

[62]

[63]

[64]

[65]

[66]

[67]

[68]

[69]

[70]

[71]

[72]

nanosheets for electrochemical-/photovoltage-driven overall water splitting. Adv Funct Mater 2018;28:1706120. Liu X, Zhao L, Domen K, Takanabe K. Photocatalytic hydrogen production using visible-light-responsive Ta3N5 photocatalyst supported on monodisperse spherical SiO2 particulates. Mater Res Bull 2014;49:58e65. Ziegler C, Eychmu¨uller A. Seeded growth synthesis of uniform gold nanoparticles with diameters of 15-300 nm. J Phys Chem C 2011;115:4502e6. Liu X, Lu¨ Y, Chen C, Luo S, Zeng Y, Zhang X, et al. Synthesis and luminescence properties of YNbO4:A (A ¼ Eu3þ and/or Tb3þ) nanocrystalline phosphors via a solegel process. J Phys Chem C 2014;118:27516e24. Ma G, Chen S, Kuang Y, Akiyama S, Hisatomi T, Nakabayashi M, et al. Water splitting using oxysulfide H2 evolution photocatalysts. J Phys Chem Lett 2016;7:3892e6. Liu X, Xie W, Lu¨ Y, Feng J, Tang X, Lin J, et al. Multichannel luminescence properties of mixed-valent Eu2þ/Eu3þ coactivated SrAl3BO7 nanocrystalline phosphors for near-UV LEDs. Inorg Chem 2017;56:13829e41. Panayotov DA, Desario P, Pietron J, Brintlinger T, Szymczak L, Rolison D, et al. Ultraviolet and visible photochemistry of methanol at 3D mesoporous networks: TiO2 and Au-TiO2. J Phys Chem C 2013;117:15035e49. Shi X, Ueno K, Takabayashi N, Misawa H. Plasmon-enhanced photocurrent generation and water oxidation with a gold nanoisland-loaded titanium dioxide photoelectrode. J Phys Chem C 2013;117:2494e9. Christopher P, Ingram DB, Linic S. Enhancing photochemical activity of semiconductor nanoparticles with optically active Ag nanostructures: photochemistry mediated by Ag surface plasmons. J Phys Chem C 2010;114:9173e7. Chen J, Wu J, Wu P, Tsai D. Plasmonic photocatalyst for H2 evolution in photocatalytic water splitting. J Phys Chem C 2011;115:210e6. Maeda K, Domen K. New non-oxide photocatalysts designed for overall water splitting under visible light. J Phys Chem C 2007;111:7851e61. Liu Y, Su F, Yu Y, Zhang W. Nano g-C3N4 modified Ti-Fe2O3 vertivally arrays for efficient photoelectrochemical generation of hydrogen under visible light. Int J Hydrogen Energy 2016;41:7270e9. Zhang J, Huang F. Enhanced visible light photocatalytic H2 production activity of g-C3N4 via carbon fiber. Appl Surf Sci 2015;358:287e95.