Boosting photocatalytic activity of WO3 nanorods with tailored surface oxygen vacancies for selective alcohol oxidations

Accepted Manuscript Full Length Article Boosting photocatalytic activity of WO3 nanorods with tailored surface oxygen vacancies for selective alcohol oxidations Jinguo Wang, Zimei Chen, Guangjun Zhai, Yong Men PII: DOI: Reference:

S0169-4332(18)32329-8 https://doi.org/10.1016/j.apsusc.2018.08.181 APSUSC 40218

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

2 June 2018 29 July 2018 21 August 2018

Please cite this article as: J. Wang, Z. Chen, G. Zhai, Y. Men, Boosting photocatalytic activity of WO3 nanorods with tailored surface oxygen vacancies for selective alcohol oxidations, Applied Surface Science (2018), doi: https:// doi.org/10.1016/j.apsusc.2018.08.181

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Boosting photocatalytic activity of WO3 nanorods with tailored surface oxygen vacancies for selective alcohol oxidations

Jinguo Wang*, Zimei Chen, Guangjun Zhai, Yong Men*

College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, P. R. China

*Author to

whom correspondence should

be addressed.

[email protected] and [email protected]

E-mail address:

Abstract: Developing metal oxides with tailored surface oxygen vacancies is very important for heterogeneous photocatalysis. In this work, uniform WO3 nanorods with tailored surface oxygen vacancies were prepared by a facile hydrothermal route, which exhibited highly enhanced photocatalytic activity for selective oxidation of alcohols to corresponding ketones in water medium. The physicochemical structural properties of uniform WO3 nanorods have been characterized and correlated with their photocatalytic activities. Results indicated that the enhanced photocatalytic activity of uniform WO3 nanorods was ascribed to the synergistic effect of the following three aspects: firstly, the high surface area favored the adsorption and diffusion of alcohol molecules; secondly, the small crystallite size shortened the transfer distance of photoelectrons and thus facilitated the fast transfer of photoelectrons, thereby restrained the recombination rate of photoelectron-hole pairs; finally, the surface oxygen vacancies acted as the photoelectron sinks could capture photoelectrons and promote their separation from holes. Meanwhile, the uniform WO3 nanorods also demonstrated good stability owning to its stable crystal phase and robust morphological structure.

Keywords: Photocatalysis; WO3 nanorods; oxygen vacancy; aromatic alcohols; selective oxidation

1. Introduction Selective oxidation of alcohols to corresponding ketones is a significant functional group transformation reaction in fine chemicals and organic synthesis [1-4], but conventional oxidation processes are generally conducted at high temperature and high pressure in environment-unfriendly organic solvents by using stoichiometric oxidants (e.g. Mn and Cr salts or V2O5), which not only are noxious compounds but also generate lots of harmful wastes [1-6]. Therefore, it is highly urgent to explore an environment-benign route for the high-efficient conversion of alcohols to their corresponding

ketones.

Fortunately,

heterogeneous

photocatalysis

has

been

considered as one of the most environment-benign techniques for organic synthesis, which usually occurs in water medium under mild conditions without employing noxious solvents or oxidants [7-14], but the central task is to develop high-efficient photocatalysts. Recent years, considerable efforts have been conducted to develop high-efficient photocatalysts with visible-light activities such as C-TiO2, N-TiO2, S-TiO2, CdS, WO3, Ag3PO4, Bi2MoO6 and Bi2 WO6 [15-23]. Among them, WO3 with intrinsic narrow band gap at around 2.6 eV has been recognized as one of the most effective candidates because of its highly stable physicochemical characteristics and broad practical applications in electrolysis and heterogeneous photocatalysis, which extends the light-harvesting ability into the visible-light region and thus makes it more suitable for heterogeneous photocatalysis [11,20,24-28]. However, pure WO3 demonstrates relatively poor photocatalytic activity due to its high recombination rate

of photoelectron-hole pairs. Until today, two methods have been mainly implemented to enhance the photocatalytic activity of WO3: one is the rational design of WO3 doped with metals or nonmetals and the other is to construct WO3-heterojunctions with another semiconductor while they usually exhibit poor photocatalytic activity because of the limited amount and easy leaching of dopants or semiconductors [28-32]. As an alternative strategy, the controllable creation of intrinsic defects (e.g. oxygen vacancies or low valence metal centers) in metal oxides has been considered as a novel strategy to manipulate their optical and electronic properties and thus, regulate their photocatalytic performances [33-35]. For instance, Li et al. reported that the oxygen vacancies can be created by annealing the TiO2 nanowire arrays at evaluated temperatures in H2 atmosphere due to that Ti4+ cations could be reduced to Ti3+ cations in a reducing atmosphere and consequently generated oxygen vacancies. The H2-treated TiO2 nanowire arrays showed highly enhanced photoelectrochemical performances owning to the improvement in the separation efficiency of photoelectron-hole pairs as a result of the created oxygen vacancies [34]. Chen et al. synthesized black TiO2 nanoparticles by annealing TiO2 at about 200oC for 120 h in H2 atmosphere, which exhibited superior photocatalytic performances for pollutant degradation and hydrogen generation due to that the surface disorder and oxygen vacancies of TiO2 narrowed its energy band gap [35]. Later, Wang et al. revealed that H2-treated

WO3

nanoflakes

showed

highly

enhanced

performances

for

photoelectrochemical oxidation of water in neutral medium because of the controllable generation of oxygen vacancies by hydrogen treatment [36]. After that,

the similar strategy for creating oxygen vacancies has been successfully extended into other metal oxides like as Fe2O3, ZnO, In2O3, BiPO4 and BiVO4 [37-40], and all these treated metal oxides demonstrated higher photoelectrochemical performances than their untreated metal oxides. In addition to create oxygen vacancies or low valence metal centers in metal oxides treated by H2 atmosphere, the rapid development of synthetic technologies has offered new opportunities for creating oxygen vacancies or low valence metal centers in metal oxides. Recently, many synthetic technologies have been implemented to create oxygen vacancies or low valence metal centers in metal oxides including H2 treatment, aluminothermic reduction, electrochemical reduction, flame reduction and chemical reduction [33-43]. However, these synthetic technologies not only introduce impurities (e.g. H, Al, Na and B) into metal oxides but also are conducted under harsh conditions especially the H 2 treatment requiring at high temperature and high pressure [33-43]. Therefore, it is highly imperative to develop a facile route for creating oxygen vacancies or low valence metal centers in metal oxides under mind conditions. Herein, uniform WO3 nanorods with tailored surface oxygen vacancies have been synthesized by a facile hydrothermal route under mind conditions, aiming to significantly increase the photocatalytic activity by creating surface oxygen vacancies. As expected, the uniform WO3 nanorods exhibited highly enhanced photocatalytic activity for selective oxidation of alcohols to corresponding ketones in water medium, which can be ascribed to the synergistic effect of the high surface area, small crystallite size and moderate amount of surface oxygen vacancies. Meanwhile, the

uniform WO3 nanorods also demonstrated good stability due to its stable crystal phase and robust morphological structure.

2. Experimental 2.1 Catalyst preparation The uniform WO3 nanorods with tailored surface oxygen vacancies were prepared by a facile hydrothermal route. Typically, 2.0 g of (NH4)10(H2W12O42)xH2O and desired amount of citric acid were dissolved in 80 mL distilled H2O under stirring at room temperature, then transferred into a 100 mL Teflon-lined autoclave and kept at 160oC for 24 h. After naturally cooling to room temperature, the products were filtered and washed completely with distilled H2O, and then dried at 60oC for 10 h. The obtained sample was denoted as WNRX, where X referred to 1, 2, 3, 4 and 5 corresponding to the amount of citric acid from 0.50, 0.70, 0.80, 0.90 to 1.0 g, respectively. Commercial WO3 denoted as WCM was supplied by Aladdin Industrial Corporation and used as the reference catalyst. 2.2 Catalyst characterization Powder X-ray diffraction (XRD) was conducted on a diffractometer (BRUKER D2 PHASER) with Cu Kα radiation. Raman spectra were conducted using Dilor Super LabRam Π at room temperature. The morphological structure was observed by Hitachi S4800 field emission scanning electron microscopy (FESEM) operating at the accelerating voltage of 5.0 kV and JEM-2100 transmission electron microscopy (TEM: JEOL, Japan) operating at the accelerating voltage of 300 kV. N2 sorption isotherms were measured with an accelerated surface area and porosimetry

system (ASAP) 2460 at the temperature of -196oC, and the specific surface area (SBET), pore volume (VP) and pore diameter (DP) were calculated by using Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halender (BJH) models from the desorption branches. X-ray photoelectron spectra (XPS) was recorded on PerkinElmer PHI-5000C XPS system and all binding energies were charge-corrected with C1s = 284.6 eV. The fourier transform infrared (FTIR) spectra was conducted on NEXUS-470 and the ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) was carried out on MC-2530. Photoluminescence spectroscopy (PLS) was taken on Varian Cary-Eclipse 500. Photoelectrochemical tests were performed in a single-compartment quartz cell on an electrochemical station CHI660E coupled with a 300 W Xe arc lamp irradiated by full wavelengths and a conventional three-electrode with the applied bias of -0.5 V including the work electrode, the Pt plate as the counter electrode and the Ag/AgCl as the reference electrode. The work electrode was prepared by depositing 10 mg WO3 catalyst onto the fluoride-tin oxide (FTO) conductor glass and 0.5 mol/L Na2SO4 aqueous solution was used as the electrolyte. The linear sweeps voltammogram measurements of all the catalysts were performed in the above standard three-electrode system with the applied bias of -0.4 V and 0.5 mol/L CH3HSO3 aqueous solution was used as the electrolyte. The photocatalytic products of selective alcohol oxidations were analyzed by using Gas Chromatography-Mass Spectrometer (Agilent 6890N-5973I). 2.3 Photocatalytic activity test Selective oxidation of alcohols to corresponding ketones by photocatalysis was

conducted as follows: typically, 0.10 g catalysts, 10 mL distilled H2O and 0.10 mmol 1-phenylethanol or its derivatives were added into a 100 mL quartz reactor (see Fig. S1) and stirred for 1.0 h to make the catalysts blend evenly in the solution at 25oC, then the suspensions were irradiated by a 300 W Xe arc lamp with full wavelengths for 7.0 h. After that, the products were extracted with desired amount of diethyl ether and analyzed by using Shimadzu GC-2014C coupled with a flame ionization detector (FID) and a TB-1 column. The comparative experiments indicated that no measurable alcohol oxidation happened without either light irradiation or WO3 catalyst. The active species by capturing experiments were carried out by adding 40 mmol different captures.

3. Results and discussion 3.1 Physicochemical characteristics The XRD patterns in Fig. 1 showed that all the WNRX catalysts were existed in hexagonal WO3 phase with the characteristic diffraction peaks at 2θ of 14.0 o, 23.2o, 28.1o and 36.8o corresponding to the (100), (002), (200) and (202) facets (JCPDS No.85-2459), respectively. No other diffraction peaks assigned to impurities can be observed, demonstrating that all the WNRX catalysts were present in pure hexagonal WO3 phase. However, WCM was existed in monoclinic WO3 phase coupled with the diffraction peaks at 2θ of 23.1o, 23.6o, 24.4o and 33.3o indicative of the (002), (020), (200) and (022) facets (JCPDS No.43-1035), respectively. Based on the principal (002) peaks of all the WO3 catalysts, the crystallite size was calculated according to Scherrer’s equation. From Table 1, it should be noted that the crystallite size of all the

WNRX catalysts decreased gradually with the increasing amount of citric acid during the preparation process, suggesting that the citric acid inhibited the crystal growth along the [001] direction of hexagonal WO3 phase. Meanwhile, WCM displayed much larger crystallite size than those of WNRX catalysts, which could be attributed to their different crystal phases and preparation methods. The FTIR spectra in Fig. S2 demonstrated that all the WO3 catalysts exhibited three distinct peaks at the wavelengths of 1622 cm-1, 1405 cm-1 and 804 cm-1, which corresponded to the W-OH bending vibration mode as a result of the adsorbed water molecules, the W-O stretching vibration and the O-W-O stretching mode [11,24], respectively. No extra peaks from other impurities were detected, which further verified that all the WO3 catalysts were existed in pure WO3 phase. The Raman spectra in Fig. S3 exhibited that all the catalysts showed four major peaks in the range from 200 to 900 cm-1. For WCM, two peaks at 273 cm-1 and 327 cm-1 corresponded to the O-W-O bending vibration modes of the bridging oxygen and other two peaks at 715 cm-1 and 807 cm-1 represented the symmetric and asymmetric O-W-O stretching vibration modes, respectively, suggesting that WCM was present in monoclinic WO3 phase [27]. For all the WNRX catalysts, two peaks at 293 cm-1 and 341 cm-1 were assigned to the O-W-O bending vibration modes of the bridging oxygen and other two peaks at 692 cm-1 and 778 cm-1 corresponded to the symmetric and asymmetric O-W-O stretching vibration modes, respectively, implying that all the WNRX catalysts were existed in hexagonal WO3 phase [44,45]. Interestingly, it can be noted that the full width at half-maximum values of all the WNRX catalysts decreased

gradually from WNR1 to WNR5 with the increasing amount of citric acid during hydrothermal process, showing an inverse trend to the variation in the crystallization degree as revealed by XRD analysis. This fact can be attributed to that the Raman characterization is surface sensitive while the XRD characterization is a representation of bulk material. Furthermore, the decrease in full width at half-maximum values from WNR1 to WNR5 also indicated that the amount of surface oxygen vacancies decreased gradually from WNR1 to WNR5 [48]. It has been known that the surface oxygen vacancies favored the adsorption and activation of oxygen species to participate in oxidation reactions [12,48,51]. The FESEM images in Fig. 2 displayed that the morphological structures of all the WNRX catalysts were uniform nanorods. The average length of uniform WO3 nanorods decreased gradually from 2.0 μm, 1.2 μm, 1.0 μm, 0.6 μm to 0.2 μm while the average width of uniform WO3 nanorods increased gradually from 15 nm, 21 nm, 27 nm, 32 nm to 40 nm with the increasing amount of citric acid during the preparation process for WNR1, WNR2, WNR3, WNR4 and WNR5, respectively. This result manifested that the aspect ratio of uniform WO 3 nanorods could be easily controlled in the range of 133-5.0. However, WCM was present in irregular nanoparticles with an average size at about 200 nm. The TEM images in Fig. 3 confirmed the consistent morphology and corresponding sizes of nanorods and nanoparticles for all the WO3 catalysts as disclosed by FESEM analysis. Meanwhile, the clear lattice fingerprints in HRTEM images suggested that all the WO3 catalysts possessed the high crystallization degree. It can be also noted that all the crystalline

grains arranged in the same crystallographic direction, indicating that all the WO3 catalysts were present in the single-crystalline structures. The attached SAED images further validated that all the WO3 catalysts owned a single-crystalline structure as revealed by the separated bright dots. The HRTEM images also showed that the reflections with d-spacing values of 0.32 nm and 0.38 nm were assigned to the (200) and (002) lattice planes of hexagonal WO3 phase, respectively, suggesting that all the WNRX catalysts mainly exposed the (100) and (001) facets. The reflections with d-spacing values of 0.38 nm and 0.36 nm corresponded to the (020) and (200) lattice planes of monoclinic WO3 phase, respectively, implying that the WCM mainly exposed the (001) facet. The N2 sorption isotherms in Fig. 4 showed that all the WO3 catalysts exhibited the type-IV sorption isotherms coupled with a typical H3 hysteresis loop in the P/P0 range of 0.85-1.0 according to the IUPAC classifications [11,46,47], implying the existence of slit-shape pores which were generally related to rod- or sheet-like particles. This result was in good compliance with the interconnected nanorods as confirmed by FESEM and TEM analysis. It could be seen that no saturated adsorption at high P/P0 range of all the WO3 catalysts was achieved until P/P0 = 1.0, suggesting the presence of large mesopores and macropores [11,46,47]. These facts could be further verified by their wide pore size distribution curves in which a pore size distribution from mesoporous to macroporous region can be observed, which was mainly originated from the self-assembly of the nanorod or nanoparticle units in good agreement with their non-porous structures as revealed by HRTEM analysis. Based on

N2 sorption isotherms, the SBET, VP and DP of all the WO3 catalysts have been calculated by using BET and BJH models, respectively. As shown in Table 1, it was obvious that the SBET of all the WNRX catalysts increased slightly from WNR1, WNR2, WNR3, WNR4 to WNR5, which could be attributed to the decrease in the crystallite size of uniform WO3 nanorods. It should be noted that WCM exhibited the lowest SBET among all the catalysts, which could be mainly ascribed to its largest particle size. The surface compositions and chemical states of all the WO3 catalysts were investigated by XPS technique as shown in Fig. 5. The XPS spectra of W4f for all the catalysts in Fig. 5a demonstrated two strong peaks corresponding to the W4f7/2 at low binding energy and the W4f5/2 at high binding energy, respectively, confirming that tungsten species were present in the form of WO3 [31,32]. The W5+ and W6+ signals can be observed after the deconvolution of W4f spectra by using Gaussian fitting methods. The W4f peaks at about 35.4 eV and 37.7 eV were assigned to the low oxidation state of characteristic W5+ cations while other major peaks located at 36.1 eV and 38.1 eV represented the high oxidation state of typical W6+ cations [31,32]. It is generally accepted that the presence of W5+ cations implied the formation of oxygen vacancies, which could improve the oxygen mobility on the catalyst’s surface and thus promote the oxidation reactions [47]. Meanwhile, the O1s spectra of all the catalysts in Fig. 5b can be decomposed into three major peaks at around 530.7 eV, 531.0 eV and 532.4 eV corresponding to the lattice oxygen (OL) species, surface hydroxyl species (OOH) and surface oxygen vacancies (OV) [34,49], respectively.

Based on the XPS analysis, the quantified surface compositions and chemical states of all the WO3 catalysts have been calculated and listed in Table 1. It was obvious that the molar ratio of W5+/W6+ for all the WNRX catalysts decreased gradually from WNR1 to WNR5. The molar ratio of OV/OL also decreased gradually from WNR1 to WNR5, which can be attributed to that the existence of surface oxygen vacancies was mainly contributed by characteristic W5+ cations, agreeing well with the Raman analysis. Moreover, the molar ratio of OOH/OL exhibited the same trend to the variation in the molar ratios of OV/OL and W5+/W6+, which can be associated with that the surface oxygen vacancies favored the adsorption of hydroxyl species. The WCM demonstrated the lowest molar ratios of OV/OL and OOH/OL due to its lowest molar ratio of W5+/W6+. To further confirm the presence of surface oxygen vacancies in all the WO3 catalysts, the UV-vis DRS spectra in Fig. 6 provided solid evidences. It could be clearly seen that the considerably large absorption tail in the visible range of 460-800 nm was present in all the WNRX catalysts, which was originated from the formation of surface oxygen vacancies as revealed by previous studies [29,33,50]. No appreciable absorption tail was observed over WCM due to its low amount of surface oxygen vacancies. Meanwhile, the relative intensity of the absorption tail for all the WNRX catalysts decreased gradually in the order of WNR1 > WNR2 > WNR3 > WNR4 > WNR5 corresponding to the decrease in the amount of surface oxygen vacancies, which agreed well with the XPS analysis. Furthermore, the formation mechanism of surface oxygen vacancies can be attributed to the different coordination

numbers of W in the (001) and (100) facets as depicted in Fig. 7. It can be seen that each W atom is coordinated with five oxygen atoms in the (001) facet, while each W coordinated is with four oxygen atoms in the (100) facet. Thus, the (100) facet displayed an increased number of surface oxygen vacancies in comparison with the (001) facet due to the lower coordination number of W with oxygen atoms [51]. Moreover, the XRD analysis in Fig. 1 has revealed that the citric acid inhibited the crystal growth along the [001] direction of hexagonal WO 3 phase due to the preferable adsorption of citric acid onto the (001) facet [20], indicating that the percentage of exposed (001) facet increased gradually with the increase in the amount of citric acid from WNR1 to WNR5 during hydrothermal process while the percentage of exposed (100) facet decreased [51]. Therefore, the amount of surface oxygen vacancies decreased gradually with the increasing amount of citric acid from WNR1 to WNR5 during hydrothermal process. The photocurrent response of all the WO3 catalysts was investigated under light irradiation with full wavelengths as shown in Fig. 8, which reflected the separation ability of photoelectron-hole pairs. It could be seen that the photocurrent response of all the WO3 catalysts generated immediately when light irradiation was switched on, and then decreased quickly to zero when light irradiation was switched off. Notably, the photocurrent intensity firstly increased from WNR1 to WNR3 and then decreased gradually from WNR3 to WNR5. The photocurrent intensity of WCM was slightly higher than that of WNR1. Meanwhile, the linear sweeps voltammogram measurements for all the catalysts have been also conducted according to previous

research [27]. As shown in Fig. S4, it can be seen that the photocurrent density of all the catalysts demonstrated the same variation order to the photocurrent response. The photocurrent density increased gradually from WNR1 to WNR3, and then decreased greatly from WNR3 to WNR5. The WCM only showed higher photocurrent density than that of WNR1. Therefore, the WNR3 was identified as the catalyst with the highest photocurrent intensity under present conditions, implying that it owned the highest separation efficiency of photoelectron-hole pairs. It should be pointed out that all the catalysts were pure WO3 with similar intrinsic band gap of about 2.6 eV, thus the photocurrent intensity was mainly depended on the light-harvesting capability and the separation efficiency of photoelectron-hole pairs as follows. First of all, the UV-vis DRS spectra in Fig. 6 demonstrated that all the WNRX catalysts exhibited almost the same light absorption curve along with a sharp absorption edge at around 470 nm, which represented the characteristic WO3 phase with intrinsic band gap of 2.6 eV. Meanwhile, it can be noted that all the catalysts except WCM exhibited similar light-harvesting ability in the range from 200 to 460 nm, suggesting that the light-harvesting ability was not the critical factor in determining the photocurrent intensity. Secondly, the PLS spectra in Fig. 9 showed that the relative intensity of PL emission peak at about 740 nm initially decreased from WNR1 to WNR3 corresponding to the decreasing recombination rate of photoelectron-hole pairs, and then increased gradually from WNR3 to WNR5 indicative of the increasing recombination rate of photoelectron-hole pairs, which agreed well with the variation in photocurrent intensity. Thus, the WNR3 was recognized as the catalyst with the

highest separation efficiency of photoelectron-hole pairs, which can be attributed to the following two aspects: one is that the small crystallite size shortened the transfer distance of photoelectrons and facilitated the rapid transfer of photoelectrons, thereby restraining their recombination with holes [9-12,51-53]; and the other is that the moderate amount of surface oxygen vacancies acted as the photoelectron sinks to capture photoelectrons and thus promote their separation from holes. The WNR1 and WNR2 exhibited lower separation efficiency of photoelectron-hole pairs than that of WNR3 although they contained more amounts of surface oxygen vacancies, mainly due to their larger crystallite sizes inhibited the rapid transfer of photoelectrons and thus decreased the separation efficiency of photoelectron-hole pairs [10,33,51]. The WNR4 and WNR5 also showed lower separation efficiency of photoelectron-hole pairs than that of WNR3 in spite of their smaller crystallite sizes, which could be explained by that their lower amounts of surface oxygen vacancies couldn’t have enough ability to separate the photoelectrons from holes. Furthermore, WCM exhibited the lowest separation efficiency of photoelectron-hole pairs owning to its biggest crystallite size and lowest amount of surface oxygen vacancies. However, WCM displayed higher photocurrent intensity than that of WNR1, which could be originated from its strong light-harvesting ability and distinguishing nature of crystal phase [54-56]. 3.2 Catalytic performances The photocatalytic selective oxidation of 1-phenylethanol and its derivatives to their corresponding ketones is selected as a model reaction to evaluate the

photocatalytic activity of all the WO3 catalysts. The underlying photocatalytic mechanism of selective oxidation of 1-phenylethanol was initially explored over WNR3 by using different scavengers to capture active species. Meanwhile, the comparative experiments revealed that no detectable oxidation products of 1-phenylethanol generated without either light irradiation or WNR3 photocatalyst. From Fig. 10, it could be clearly seen that no substantial decline in photocatalytic activity was observed when terephthalic acid was added to capture •OH radicals, suggesting the •OH radicals were not the key active species in present reaction. Interestingly, the photocatalytic activity decreased greatly by using either methanol to capture holes (h+), or diphenylamine to capture photoelectrons (e-), or benzoquinone to capture the •O2- radicals, or bubbling N2 to remove the dissolved O2 in the reaction solution [10-12]. The above results offered direct evidences that the photoelectrons (e-), holes (h+) and •O2- radicals served as the key active species in present reaction except •OH radicals. Taking the above results into account, a plausible photocatalytic mechanism for selective oxidation of 1-phenylethanol to corresponding ketone was proposed and briefly depicted in Fig. 11. Initially, the WO3 catalyst produced holes (h+) and photoelectrons (e-) under light irradiation. Then, the holes (h+) reacted with the 1-phenylethanol adsorption on the surface of WO3 nanorods to form 1-phenylethanol cation radicals, and the photoelectrons (e-) captured by surface oxygen vacancies activated the dissolved O2 in the solution to produce the •O2radicals. Lastly, the 1-phenylethanol cation radicals reacted with the •O2 - radicals to produce the corresponding ketone.

The photocatlytic activity of all the WO3 catalysts for selective oxidation of 1-phenylethanol to its corresponding ketone was shown in Fig. 12. It was obvious that the photocatalytic activity initially increased from WNR1 to WNR3 and then decreased gradually from WNR3 to WNR5. Meanwhile, WCM only displayed slightly higher photocatalytic activity than that of WNR1. The activity data in Table 2 also showed the same trend in selective oxidation of 1-phenylethanols with different p-substituents to their corresponding ketones for all the WO3 catalysts. Therefore, the WNR3 was identified as the optimal photocatalyst in present photocatalytic reactions. The superior photocatalytic activity of WNR3 could be ascribed to the synergistic effect of the following three aspects: initially, the high surface area favored the adsorption and diffusion of alcohol molecules; secondly, the small crystallite size could effectively shorten the transfer distance of photoelectrons and promote the fast transfer of photoelectrons, thereby improving their separation from holes [9-12,51-53]; finally, the moderate amount of surface oxygen vacancies acted as the photoelectron sinks to capture photoelectrons and thus improve the separation efficiency of photoelectron-hole pairs. The WNR1 and WNR2 displayed lower photocatalytic activities than that of WNR3 although they contained more amounts of surface oxygen vacancies. This appearance could be mainly attributed to that their lower surface area and larger crystallite sizes restrained the adsorption and diffusion of alcohol molecules and increased the recombination probability of photoelectron-hole pairs. Meanwhile, the WNR4 and WNR5 possessed smaller crystallite sizes and higher surface area than that of WNR3 while they also showed lower photocatalytic

activities, which could be ascribed to that their lower amounts of surface oxygen vacancies couldn’t have enough ability to separate the photoelectrons from holes and thus decrease the separation efficiency of photoelectron-hole pairs. Furthermore, WCM displayed higher photocatalytic activity than that of WNR1 although it owned the largest crystallite size, the lowest surface area and the lowest amount of surface oxygen vacancies, which was mainly originated from its strong light-harvesting ability and distinguishing nature of crystal phase [54-56]. Moreover, the activity data in Table 2 also demonstrated that the 1-phenylethanols with electron-donating groups such as p-CH3 and p-OCH3 exhibited higher photocatalytic activities than those with electron-withdrawing groups (e.g. p-F, p-Cl and p-Br) in good agreement with previous studies [10-12]. This fact could be ascribed to that the electron-donating groups endowed the -CHOH-CH3 moieties with higher electron cloud density than those with electron-withdrawing groups and thus, the holes (h+) much more easily activated the -CHOH-CH3 to generate the corresponding cation radicals, thereby further validating the rationality about the underlying mechanism for photocatalytic selective oxidation of 1-phenylethanol as proposed in Fig. 11. The stability of photocatalysts is one of the most important parameters for their practical applications. In order to evaluate the stability, the WNR3 was selected to investigate the stability in the recycled photocatalytic selective oxidation of 1-phenylethanol. The WNR3 was separated from the solution by centrifugation after each photocatalytic reaction, followed by washing thoroughly with absolute ethanol and distilled H2O. Each photocatalytic reaction was conducted by recharging with

fresh reactants into the recycled catalyst for ensuring each photocatalytic reaction under identical conditions. From Fig. 13, it could be seen that the WNR3 could be recycled at least six times and no appreciable decrease in photocatalytic activity was observed, indicating its good stability. This good stability of WNR3 could be attributed to its stable crystal phase and robust morphological structure since its crystal phase and morphology almost kept unchanged after the 6th recycle as confirmed by XRD pattern and FESEM image in Fig. S5.

4. Conclusions In summary, this work developed a facile strategy to synthesize uniform WO3 nanorods with tailored surface oxygen vacancies, which exhibited highly enhanced photocatalytic activity for selective oxidation of alcohols to their corresponding ketones in water medium. The highly enhanced photocatalytic activity of uniform WO3 nanorods was originated from the synergistic effect of the high surface area, small crystallite size and moderate amount of surface oxygen vacancies. Meanwhile, the uniform WO3 nanorods also demonstrated good stability because of its stable crystal phase and robust morphological structure. This work might provide deep insights for guiding the rational design of high-efficient photocatalysts applied in organic synthesis.

Acknowledgements This work is supported by National Natural Science Foundation of China (21503133), Shanghai Talent Development Foundation (2017076), Municipal Education of Shanghai (ZZGCD15031), the Education Ministry Key Lab of Resource

Chemistry of Shanghai (2016No.3), Shanghai Automotive Industry Science and Technology Development Foundation (1721), Zhanchi Plan (nhrc-2015-12), Innovation Fund for Graduate Students (17KY0406) and Startup Foundation (2015-20) of Shanghai University of Engineering Science.

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Table Captions Table 1 Physicochemical structural parameters of different catalysts. Table 2 Photocatalytic performances of different catalysts for selective oxidation of 1-phenylethanols with different p-substituents.

Figure Captions Fig. 1. XRD patterns of different catalysts. Fig. 2. FESEM images of different catalysts. Fig. 3. TEM, HRTEM, and SAED (inset) images of different catalysts. Fig. 4. N2 adsorption-desorption isotherms (a) and pore size distribution curves (b) of different catalysts. Fig. 5. XPS spectra of different catalysts: W4f of (a) and O1s of (b). Fig. 6. UV-vis DRS spectra of different catalysts. Fig. 7. Atomic structural model of hexagonal WO3 (001) and (100) surfaces. Fig. 8. Photocurrent response of different catalysts. Fig. 9. PLS spectra of different catalysts. Fig. 10. Photocatalytic performances for selective oxidation of 1-phenylethanol in the presence of different capturers over WNR3. Fig. 11. Photocatalytic mechanism for selective oxidation of 1-phenylethanols with different p-substituents to their corresponding ketones over WO3 catalyst. Fig. 12. Photocatalytic activities for selective oxidation of 1-phenylethanol over different catalysts.

Fig. 13. Stability test of WNR3 for photocatalytic selective oxidation of 1-phenylethanol. Fig. S1. Photograph of the reaction system for photocatalytic selective oxidation of alcohols. Fig. S2. FTIR spectra of different catalysts. Fig. S3. Raman spectra of different catalysts. Fig. S4. The linear sweeps voltammogram measurements of different catalysts. Fig. S5. XRD pattern and FESEM image (inset) of WNR3 after the 6 th recycle.

Fig. 1. XRD patterns of different catalysts.

Fig. 2. FESEM images of different catalysts.

Fig. 3. TEM, HRTEM and SAED (inset) images of different catalysts.

Fig. 4. N2 adsorption-desorption isotherms (a) and pore size distribution curves (b) of different catalysts.

Fig. 5. XPS spectra of different catalysts: W4f of (a) and O1s of (b).

Fig. 6. UV-vis DRS spectra of different catalysts.

Fig. 7. Atomic structural model of hexagonal WO3 (001) and (100) surfaces.

Fig. 8. Photocurrent response of different catalysts.

Fig. 9. PLS spectra of different catalysts.

Fig. 10. Photocatalytic performances for selective oxidation of 1-phenylethanol in the presence of different capturers over WNR3.

Fig. 11. Photocatalytic mechanism for selective oxidation of 1-phenylethanols with

different p-substituents to their corresponding ketones over WO3 catalyst.

Fig. 12. Photocatalytic activities for selective oxidation of 1-phenylethanol over different catalysts.

Fig. 13. Stability test of WNR3 for photocatalytic selective oxidation of 1-phenylethanol.

Table 1 Physicochemical structural parameters of different catalysts. XPS

SBET

Vp

Dp

Crystallite size

(m2·g-1)

(cm3·g-1)

(nm)

(nm)

W5+/W6+

OOH/OL

OV/OL

WCM

5.4

0.01

34

42

0.12

0.14

0.18

WNR1

42

0.12

29

12

0.51

0.34

0.39

WNR2

43

0.12

27

11

0.46

0.31

0.32

WNR3

45

0.12

25

10

0.40

0.22

0.30

WNR4

47

0.12

20

9.5

0.35

0.20

0.29

WNR5

47

0.12

17

9.2

0.26

0.18

0.20

Catalyst

Table 2 Photocatalytic performances of different catalysts for selective oxidation of 1-phenylethanols with different p-substituents. Conversion (%) Reactant

O

WCM

WNR1

WNR2

WNR3

WNR4

WNR5

52

38

68

84

75

61

40

28

58

72

64

54

28

17

36

55

35

30

20

15

28

40

32

25

12

10

15

18

16

15

OH

OH

F OH

Cl OH

Br OH

Highlights: · Uniform WO3 nanorods synthesized by a facile hydrothermal method. · Uniform WO3 nanorods with tailored surface oxygen vacancies. · Surface

oxygen

vacancies

inhibiting

the

recombination

rate

photoelectron-hole. · Photocatalytic selective oxidation of alcohols to ketones in water medium.

of

Boosting photocatalytic activity of WO3 nanorods with tailored surface oxygen vacancies for selective alcohol oxidations

Jinguo Wang*, Zimei Chen, Guangjun Zhai, Yong Men*

Graphical Abstract

Uniform WO3 nanorods with tailored surface oxygen vacancies were synthesized by a facile hydrothermal route, which exhibited highly enhanced photocatalytic activity for selective oxidation of alcohols in water medium owning to the synergistic effect of the

high surface area, small crystallite size and moderate amount of surface oxygen vacancies.