WO3 2D-heterojunctional

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Synthesis of Si-O bridged g-C3N4/WO3 2D-heterojunctional nanocomposites as efficient photocatalysts for aerobic alcohol oxidation and mechanism insight Liqun Sun, Bin Li, Xiaoyu Chu, Ning Sun, Yang Qu, Xuliang Zhang, Imran Khan, Linlu Bai, and Liqiang Jing ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00711 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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ACS Sustainable Chemistry & Engineering

Synthesis of Si-O bridged g-C3N4/WO3 2D-heterojunctional nanocomposites as efficient photocatalysts for aerobic alcohol oxidation and mechanism insight Liqun Sun,

†§

Bin Li, † Xiaoyu Chu, †‡ Ning Sun,

†

Yang Qu,

†

Xuliang Zhang,

†

Imran Khan,

†

Linlu Bai,*†‡ Liqiang Jing*†

†Key

Laboratory of Functional Inorganic Material Chemistry (Ministry of Education), School of

Chemistry and Materials Science, International Joint Research Center for Catalytic Technology, Heilongjiang University, Harbin 150080, PR China.

‡School

of Chemical and Environmental Engineering, Harbin University of Science and

Technology, Harbin 150080, PR China.

§Heilongjiang

Provincial Key Laboratory of Oilfield Applied Chemistry and Technology, College

of Chemical Engineering, Daqing Normal University, Daqing, 163712, P.R. China.

* To whom correspondence should be addressed. Email address: [email protected] (Linlu Bai); [email protected] (Liqiang Jing).

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ABSTRACT: It’s highly desired to promote the charge transfer and separation of WO3

for highly efficient green process for the photocatalytic aerobic selective alcohol oxidation. In this work, 2D WO3 nanoplates (2D WO) with enlarged surface area and favorable charge separation were successfully fabricated by the phase-separated hydrolysis-solvothermal method. The photoactivities of resulting 2D WO were improved by coupling with g-C3N4 nanosheets (2D CN) to construct the 2D/2D type nanocomposites, and then they were further enhanced by introducing silicate bridges between 2D WO and CN via a facile wet-chemical method. The amount-optimized Si-O bridged 2D/2D g-C3N4/WO3 nanocomposite exhibited excellent photoactivity and high selectivity for the aerobic selective alcohol oxidation by 5-fold enhancement compared to bare 2D WO under full-light irradiation, which is attributed to the significantly improved charge separation by Z-scheme between 2D WO and CN and facilitated charge transfer through the silicate bridges. The Z-scheme charge transfer mechanism was clearly verified by the single-wavelength photocurrent action spectra and single-wavelength fluorescence spectra. Moreover, the subsequent charge carrier-induced photocatalytic reaction mechanism was investigated by the scavenger trapping and 18O2 isotope-labeled experiments. It’s evidenced that the photogenerated holes mainly dominate the selective alcohol oxidation. This present work has shown superior photocatalytic performance among all reported works on the WO3 based photocatalysts for the aerobic selective alcohol oxidation to date. The research basis could be provided by this work for the synthesis of highly active WO3 based photocatalysts for the aerobic selective organic oxidation.


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KEYWORDS: 2D/2D g-C3N4/WO3 nanocomposite, Si-O bridge, Z-scheme mechanism,

Photogenerated charge separation, Aerobic selective alcohol oxidation. INTRODUCTION Selective oxidation of alcohols is a useful and fundamental organic reaction, which could provide the products like carbonyl compounds, aldehydes and ketones as important building blocks for the synthesis of fine chemicals and pharmaceuticals.1 However, traditional synthetic processes still feature severe drawbacks. In industry, harsh reaction conditions like high temperature and pressure might be needed during the oxidation process. Moreover, noble metal catalysts such as Au, Pd and Pt, etc are comprehensively applied leading to high economic cost. Noteworthily, toxic or corrosive agents such as Br2, CrO3, ClO- and KMnO4 are usually involved in the oxidation process as the oxidants, which are harmful to the environment.2-4 Therefore, it’s highly desired to develop an economic catalytic route to selectively oxidize alcohol with oxygen as the green oxidant under mild conditions. Photocatalysis has emerged as a reliable, green and promising technology in the fields of energy and environment, which has been utilized to realize various types of reactions like H2 evolution, CO2 conversion and pollutant degradation, etc.5-8 More recently, to utilize the photocatalytic technology to realize the organic transformation, especially the selective oxidation with O2 as the oxidant, to obtain abundant organic products has become an attractive research direction.9 To realize the photocatalytic aerobic selective oxidation of alcohol, the key is to design and fabricate the targeted



photocatalysts of high efficiency. Hitherto, TiO2 is considered as one of the most applied photocatalysts for selective organic oxidation, including the alcohol oxidation.10 However, TiO2 has limited light adsorption due to its band gap of 3.2 eV. Furthermore, its rather positive valence band (VB) of 2.7 eV would easily cause the over oxidation of alcohol to produce CO2, water and mineral acid, resulting in the low selectivity towards desired products.11 Moreover, additional agents such as organic dyes or hole scavenger might be necessary to enhance the limited photoactivity,12,13 which complicates the photocatalytic reaction system. Besides, supported Au nanoparticles/nanorods capable of absorbing visible light due to the surface plasmon resonance effect, especially Au supported on TiO2, are also reported to catalyze the photocatalytic aerobic alcohol oxidation with favorable catalytic performance and high selectivity.14,15 While noble metals undoubtedly would increase the economic cost. Therefore, a great vision has come towards the development of the low-cost photocatalyst candidates which could utilize full solar light meanwhile possess high photoactivity and selectivity for this target reaction. In recent years, narrow band-gap semiconductors such as metal oxide,16 metal sulfide17 and polymer like g-C3N4,18 etc have been reported effective for the photocatalytic aerobic selective alcohol oxidation. Among these candidates, WO3 has received great interest, mainly due to its narrow band gap (2.4-2.8 eV), low cost, non-toxicity and stable physicochemical properties.19,20 Noteworthily, its relatively less positive VB would avoid the over oxidation so as to enable WO3 to provide high selectivity for the photocatalytic alcohol oxidation. Nevertheless, the morphology


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characteristics including small surface area and poor charge separation of bulk WO3 obviously

limit

its

photoactivity.

In

recent

years,

two-dimensional

(2D)

nanostructured materials have attracted much attention due to their relatively large surface area and shorten charge transfer distance.21,22 Thus, 2D nano-structured WO3 is preferred for efficient photocatalysis. While even with 2D morphology, WO3 would still suffer its instinct weakness. On one aspect the charge separation of pristine WO3 needs to be further improved. Moreover, the less negative CB position of WO3 would make the photogenerated electrons lack sufficient thermodynamic energy, which could not induce the reduction reaction. Therefore, it’s obliged to further improve the charge separation of WO3 meanwhile to elevate the energy level of the photogenerated electrons. One of the most feasible strategies to improve the charge separation is to construct effective heterojunction. Moreover, considering to effectively utilize the oxidation ability of the holes of WO3 and elevate the energy level of electrons, it’s rational to construct the WO3 based Z-scheme heterojunction by simulating the natural photosynthesis to couple another narrow band-gap semiconductor.23 To successfully construct a Z-scheme photocatalyst, the CB and VB bottom levels of the coupled semiconductor should be emphatically considered referring to the energy band structure of WO3. Carbon nitride (g-C3N4, CN) is a vital type of semiconductors attracting comprehensive interest due to its advantageous properties such as visible light adsorption, robust texture and non toxicity, etc.24,25 Its narrow optical band structure (its band gap is 2.70 eV with theoretical calculated CB and VB edge potentials are -1.13 and 1.57 eV, respectively) and tunable electronic



structure make it suitable semiconductor to form an effective Z-scheme photocatalytic system with WO3.26 On the basis of the planning to fabricate the 2D WO, the dimension matching between different components for constructing an effective heterojunction should be considered, however, which is always neglected. To achieve more efficient charge transfer through the dimension matching, g-C3N4 should also be fabricated to the 2D morphology then coupled with 2D WO. Noteworthily, it’s the first time that the Z-scheme g-C3N4/WO3 photocatalysts are applied in the aerobic selective alcohol oxidation. In other reported works on the Z-scheme 2D/2D g-C3N4/WO3 photocatalytic systems the dimension matching between g-C3N4 and WO3 is rarely discussed. Moreover, the charge transfer mechanism for the Z-scheme system is still ambiguous, which needs to be further clarified. Notably, although the Z-scheme system has shown great advantages and the dimension matching could be achieved by constructing the 2D/2D heterojunction to some extent, its charge separation is still limited by the lattice mismatch between the two semiconductors. Therefore, it’s necessary to build the electron shuttling bridges between the two components in the Z-scheme system so as to enhance the charge separation. Metals like Pd, Au and Ag as well as graphene are applied as the effective electron shuttles, while they are still not ideal owing to the relatively high cost.27 Therefore, it’s desired to develop the low-cost and effective electron shuttle bridge alternatives. Based on our previous works, inorganic groups like -P-OH and -Si-OH could function as effective electron bridges for heterojunctional nanocomposites to promote the charge transfer.28,29 Thus, it is extremely promising to further improve the


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charge transfer between 2D WO and 2D CN by introducing the inorganic bridges. Besides, the electron transfer mechanism for the Z-scheme photocatalyst with polyhydroxy group bridges has seldom been investigated, which therefore is urged to be revealed. In addition, the reaction path of the photocatalytic aerobic oxidation of alcohol depends on the specific instinct nature of the photocatalyst materials, which is kind of controversial in reported relevant works.30-32 Generally, the photo-induced holes, hydroxyl radicals and superoxide radicals are taken for the major active species generated during the photocatalysis.33 While for g-C3N4/WO3 photocatalysts, the reaction path is far less investigated, which needs to be further clarified. Based on above designing strategy, the Si-O bridged 2D/2D g-C3N4/WO3 nanocomposites were successfully fabricated as the efficient photocatalysts for the aerobic selective benzyl alcohol oxidation as the typical reaction. It is clearly demonstrated that the charge transfer and separation of 2D/2D g-C3N4/WO3 nanocomposites were efficiently promoted due to the introduction of Si-O bridges as electron transfer paths, which is evidenced by atmosphere-controlled transient-state surface photovoltage technique and measurement of produced ·OH amounts. The Z-scheme mechanism is proved by the single-wavelength photocurrent action spectra and single-wavelength ·OH-mediated fluorescence spectra. Synchronously, with the assistance of the scavenger experiments and isotopic experiments with 18O2, this direct hole-induced selective oxidation pathway has been proposed. To the best of our knowledge, this novel Si-O bridged Z-scheme 2D/2D g-C3N4/WO3 photocatalyst



addressing the dimension matching has never been reported for the selective aerobic alcohol oxidation. Moreover, it exhibited advantageous photoactivity as well as high selectivity among reported WO3 based photocatalysts for the same reaction under the full light irradiation. The work provides a stepping stone towards the design and practical application of Z-scheme WO3 based 2D/2D photocatalysts for the aerobic selective organic transformation. EXPERIMETNAL SECTION Materials and methods All the reagents were of analytical grade and used as-received without further purification. Deionized water was used throughout the reaction. Synthesis of WO3 nanoplates: WO3 nanoplates were prepared by a modified phase-separated hydrolysis-solvothermal method. The key point to the synthetic reaction is to select to n-butanol with a little higher boiling point than water as organic phase to dissolve WCl6 and also as hydrothermal solvent, and introduce the water in system to modulate the hydrolysis of W ions in the organic phase. In a typical procedure, 10 mL of water and 8 mL of n-butanol, which contains 0.04 g WCl6, was placed in the weighing bottle separately. Then, the sealed device was kept at 120 °C for 15 h, followed by naturally cooling to room temperature. The resulting yellow precipitate was collected by separating from n-butanol and rinsed with deionized water and absolute ethanol several times, and then dried in air at 80 °C for 12 h. Synthesis of g-C3N4 nanosheets: g-C3N4 was synthesized by directly heating urea in the crucible with a cover at 550 °C for 4 h with a heating rate of 0.5°C/min in a


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muffle furnace. After that, the furnace was cooled to the room temperature naturally. The obtained light-yellow product was grinded into fine powders for further experimental work. Synthesis of g-C3N4/WO3 nanocomposites: g-C3N4/WO3 nanocomposites were fabricated by a simple wet-chemical process. WO3 of 0.5 g was added into a mixed solution consisting of 50 mL of 50% ethanol solution, then treated by an ultrasonic process for 10 min and stirring for 10 min. Similarly, certain amounts of g-C3N4 was dispersed respectively in 50 mL of 50% ethanol solution. Subsequently, the g-C3N4-containing solution was added into WO3 solution gradually under stirring. Finally, the mixed solution was stirred for 1 h and dried at 80°C, followed by calcination at 400 °C for 1h. The as-prepared samples were represented by XCN-WO, in which X means the different mass percent (5%, 10%, and 15%) of g-C3N4 to WO3, CN stands for g-C3N4, and WO means WO3 nanoplates. Synthesis of silicate-bridged g-C3N4/WO3 nanocomposites: The silicate-bridged g-C3N4/WO3 nanocomposites were prepared by a two-step wet chemical method with TEOS (tetraethoxysilane) solution as the Si-O source. Firstly, 0.5 g resulting WO3 nanosheets was put into a certain content of TEOS solution (50 mL) under vigorously stirring for 1 h. Subsequently, the mixture was kept on the hot plate and heated at 80 °C under continuous stirring until it is dried. The silicate-modified WO3 obtained is denoted as YSi-WO, in which Y is the mole ratio percentage of Si to WO. And where after, the resulting silicate-modified 2D WO was added into a mixed solution containing anhydrous ethanol (25 mL), water (25 mL) and g-C3N4 (0.05 g) with



continuous stirring for 1h. The mixture was dried at 80 °C in air. Finally, the silicate-bridged g-C3N4/WO3 nanocomposites were obtained by calcining the dried mixture at 400 °C for 1 h. The obtained nanocomposites are represented by 10CN-YSi-WO, in which 10 represents 10% mass ratios of used g-C3N4 to WO3. Characterization Crystallographic phases of the samples were investigated by X-ray diffraction (XRD) (Bruker D8, Japan). UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded on a Model Shimadzu UV2550 spectrophotometer using BaSO4 as a reference standard. Chemical compositions and states were analyzed through X-ray photoelectron spectroscopy (XPS) using a Kratos-Axis Ultra DLD apparatus with an Al (mono) X-ray source, and the binding energies were calibrated with respect to the signal for adventitious carbon. Transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM) images were obtained on an electron microscope (Hitachi H7650, Japan) with a high acceleration voltage of 200 kV. The atmosphere-controlled SS-SPS measurements were carried out with a home-built apparatus equipped with a lock-in amplifier (SR830) and synchronized with a light chopper (SR540). The powder sample was sandwiched between two indium-tin oxide (ITO) glass electrodes, and the electrodes were kept in an atmosphere-controlled sealed container. Radiations from a 500 W xenon lamp (CHF XQ 500 W, Global Xe lamp power) were passed through a double-prism monochromator (SBP300) to get the monochromatic light. The transient-state surface photovoltage (TS-SPV) characterization is carried out


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on a home-made instrument. The sample is excited with a laser radiation pulse (wavelength of 355 nm and pulse width of 10 ns) from a second-harmonic Nd: YAG laser (Lab-130-10H, Newport, Co.). Intensity of the pulse was measured by a high-energy pyroelectric sensor (PE50BF-DIF-C, Ophir Photonics Group). The signals were amplified with a preamplier and then registered by a 1 GHz digital phosphor oscilloscope (DPO 4104B, Tektronix). The TS-TPV measurements were performed in air atmosphere and at room temperature. Photoelectrochemical (PEC) measurements: PEC measurements were performed on a CHI660D electrochemical workstation (Chenhua Instrument, Shang-hai, China) in a conventional three-electrode configuration. The working electrode prepared with the sample has an active area of ca. 1 cm2. A Pt foil and a Ag/AgCl electrode were used as the counter and reference electrode, respectively. Aqueous Na2SO4 solution with the concentration of 0.5 M was used as the electrolyte. PEC I-V curves were measured at set potential of -0.2 V to 1.8 V. The photocurrent densities at different excitation wavelengths were measured with the applied potential of 0.4 V, in which monochromatic light was obtained by passing light from a 500 W Xenon lamp through a monochromator (CM110, Spectral Products). Electrochemical impedance spectra (EIS) were performed using a three-electrode configuration with the Princeton Applied Research Versa STAT 3 and carried out over the frequency range from 102 to 105 Hz with an amplitude of 10 mV (Root Mean Square) at the applied bias of 0.4 V in a 0.5 M Na2SO4 solution, using a 300 W Xenon light as the illumination source.



Evaluation of produced ·OH amount: for ·OH measurement, 0.02 g of each sample was dispersed in 20 mL of 0.001 M coumarin solution. Prior to irradiation, the mixture was magnetically stirred for 30 min in dark to ensure the establishment of an adsorption/desorption equilibrium. After irradiation for 1 h, a certain amount of the solution was transferred into a Pyrex glass cell for the fluorescence measurement of 7-hydroxycoumarin at around 456 nm with 332 nm excitation through a spectrofluorometer (Perkin-Elmer LS55). Photocatalytic oxidation Standard photocatalytic reaction: the photocatalyst (0.02 g) and the reactant benzyl alcohol (BA, 0.2 mmol) was added to acetonitrile (10 mL) within a three-neck flask (25 mL), and the flask was sealed with rubber septum caps. The photocatalyst was dispersed well by ultrasonication for 5 min, and O2 was bubbled through the solution for 20 min. The flask with the reflux of cooling water was immersed in a temperature-controlled water bath (30 2 oC) and photoirradiated using a 300 W Xe lamp with magnetic stirring. After reaction, the same moles of 1, 2-diclorobezene with BA was added in the mixture as the external standard. The mixture was then diluted with ethyl acetate to 25 mL. The photocatalyst powder was separated from the liquid by filtration. The liquid mixture was analyzed using an Agilent gas chromatograph 6890 equipped with a HP-5 capillary column (30 m long and 0.32 mm in diameter, packed with silica-based supel cosil) and flame ionization detector (FID). The injector temperature was 250 oC and the spilt is 0.1 μL. The column head


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pressure of the carrier gas (helium) during the analysis was maintained at 22.57 psi. Temperature program: 50 oC to 180 oC; 20 oC · min-1; hold for 4 min. The by-products were identified by a gas chromatography-mass spectrometry (GC-MS, Agilent, GC 6890N, MS 5973 inert). The conversion of BA and selectivity of BA/BAD were defined as: conversion = [(C0 – CBA)/C0] × 100% and selectivity = [CBAD/(C0 – CBA)] × 100%, where C0 is the initial concentration of BA, CBA and CBAD are the concentrations of the detected BA and BAD, respectively. Recycle test: photocatalyst was recovered by centrifugation and washed with 50 mL acetone and dried at 60 oC overnight. Upon drying, the same amount of the recovered photocatalyst was reused in a standard photocatalytic reaction as described above. Photocatalytic aerobic oxidation of BA with scavengers: tiny amounts of AgNO3 ， triethanolamine (TEA), 1,4-benzoquinone (BQ), isopropanol (IPA) and butylated hydroxytoluene (BHT) were added in the photocatalytic oxidation system (the concentration of each scavenger is 4 mM), respectively, to find out the active species in the conversion process of BA.33,34 Photocatalytic oxidation of BA in the 18O

2-labeling

18O

2

isotope-labeled oxygen atmosphere:

experiments were conducted under similar conditions to the typical

photocatalytic oxidation of BA except that the normal O2 was replaced by

18O

2

atmosphere. The identification of the oxidation products was conducted on the GC-MS (Agilent Technologies, GC6890N, MS5973) that equipped with a HP-5MS capillary column (30 m × 0.25 mm × 0.50 m). The temperature of sample injector was



maintained at 260 °C. The column temperature began at 50 °C for 3 min, then increased to 250 °C with the heating rate of 10 °C /min. According to the NIST mass spectral database, the products were deduced from MS/MS mode and analyzed by scan mode. RESULTS AND DISCUSSION Structural Characterization 2D WO were successfully fabricated by the phase-separated hydrolysis-solvothermal method. The X-ray diffraction (XRD) pattern (Fig. S1a) verified its crystalline structure and relevant diffraction peaks are indexed on the basis of a WO3 monoclinic phase (JCPDS 43-1035).35 For the CN/WO samples, the peaks at approximately 27.6° are attributed to the (002) planes of graphitic materials, reflecting the interlayer stacking of a conjugated aromatic system (JCPDS 87-1526).36 For the silicate-bridged 2D/2D g-C3N4/WO3, both the crystalline structure of 2D WO and CN were well preserved (Fig. S1b). While no diffraction peaks related to silicate appeared, which might be due to the limited introduced amounts. The UV-vis diffuse reflectance spectra (DRS) of the fabricated samples were measured and shown in Fig. S2. It is observed that 2D WO displays a reflection band edge at around 470 nm corresponding to the intrinsic band gap energy of approximately 2.7 eV.19,37 The optical absorbance of 2D WO remains unchanged after coupling with different amounts of CN then introducing silicate groups. The TEM images clearly confirm the 2D morphology of WO3 with plate-like structure (Fig. 1a) and g-C3N4 nanosheets (Fig. 1b) with wrinkled surface,


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respectively. The TEM (Fig. 1c) and HRTEM image (Fig. 1d) of an individual 2D WO on the CN surface shows the crystal lattices with the interplanar distance of ≈ 0.368 nm, matching well with the lattice spacing of the (200) facet of pristine WO3.38 Moreover, the TEM image along with the corresponding elemental mapping (Fig. S3) indicates the WO3 nanoplates are uniformly decorated on the g-C3N4 nanosheets surface. Based above, the 2D/2D heterojunction between the WO3 nanoplate and the g-C3N4 nanosheet has been successfully constructed, which is inferred to benefit the charge transfer. Further introduced silicate groups could not be directly observed in TEM image due to its high dispersion. Notably the Si-O groups didn’t change the morphology of the 2D/2D g-C3N4/WO3. While the corresponding EDX elemental mapping of Si element clearly verified the uniform distribution of as-introduced silicate groups. Figure 1 To further illustrate the chemical states of each element, the XPS spectra were obtained as shown in Fig. 2 and Fig. S4. The W 4f spectrum of pristine 2D WO in Fig. S4a demonstrates two major peaks at 35.2 (W 4f

7/2)

and 37.3 eV (W 4f

5/2),

which

can be assigned to the W6+ ions of WO6 octahedral in monoclinic WO3.39 For 10CN-WO and 10CN-0.5Si-WO, as the introduction of CN the peaks of W 4f consistently shift towards lower binding energies, indicating a decrease in the W oxidation state. Figure 2 Moreover, for 0.5Si-WO the major peaks of W 4f showed no obvious shift



compared to 2D WO, which further proves that the oxidation state change of W originates from the interaction between 2D WO and CN. As shown in Fig. 4Sb, for 10CN-WO and 10CN-0.5Si-WO the major O 1s peaks attributed to the surficial lattice O atoms of WO slightly move towards higher binding energies compared to that of pristine 2D WO. Both the binding energy changes of W 4f and O 1s peaks originate from the interaction between 2D WO and CN nanosheets, indicating the formation of the 2D/2D heterojunction. Considering the synthetic conditions, it’s inferred that 2D WO interact with CN nanosheets through the dehydration of surfacial hydroxyl groups. Moreover, compared to pristine CN nanosheets, the C 1s peaks for 10CN-WO and 10CN-0.5Si-WO at ca. 284.5 eV are more obvious, which are attributed to adventitious carbon (Fig. 4Sc). Except for the peak intensity, the C 1s and N 1s spectra of 10CN-WO and 10CN-0.5Si-WO show negligible differences from those of pristine CN nanosheets (Fig. 4Sc, d). As shown in Fig. 2, for the samples with silicate group introduced, the Si 2p peaks at the 102.7 eV could be observed, which further verified the successful introduction of silicate groups.40 Evaluation of Photocatalytic Activities The photocatalytic activities of the series of samples were examined with the aerobic selective oxidation of benzyl alcohol (BA) as the typical reaction at 25 °C under the irradiation of full light with the 300 W Xe lamp. As shown in Fig. S5a, 2D WO could convert BA to certain extent with the BA conversion of 12.2%. While the the XCN-WO nanocomposites with the 2D/2D heterojunction exhibited enhanced photoactivities (Fig. 3a and Fig. S5a), among which 10CN-WO with the optimum CN


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amount offered the largest BA conversion of 40.5%. Noteworthily, the possibility that the enhanced photoactivity originates from the contribution of added CN could be excluded, since under the atmospheric pressure, CN shows negligible photoactivity for the aerobic selective alcohol oxidation. Subsequently, additional introduction of silicate to the sample 10CN-WO further enhanced the photoactivity (Fig. S5b). The photocatalytic activity of 10CN-YSi-WO has been greatly improved compared to that of the non-silicate-bridged one (10CN-WO). The amount-optimized 10CN-0.5Si-WO exhibited the largest BA conversion of 85.6%, which is approximately 5 times higher than that of bare 2D WO in identical conditions. With the same modification amount of silicate with 10CN-0.5Si-WO, the photoactivities of 0.5Si-CN and 0.5Si-WO showed no obvious enhancement compared with pristine CN and WO, respectively. This indicates that as-introduced silicate groups could not directly lead to any activity enhancement. Noteworthily, for all the 2D WO based samples, the selectivity towards benzylaldehyde (BAD) all surpass 99.0%, indicating the superiority of the WO3 based nanomaterials as highly selective photocatalysts for the aerobic alcohol oxidation. Figure 3 To further study the excellent photocatalytic effeciency of the silicate-bridged 2D/2D g-C3N4/WO3 nanocomposties, the values of quantum efficiency (QE) of WO, 10CN-WO and 10CN-0.5Si-WO were calculated for the BA conversion (Fig. 3b).41 Noteworthily, it’s the first time that the QE is studied for the photocatalytic alcohol oxidation for the WO3 based photocatalysts. The specific values of QE are 0.11, 0.36 and 0.68% for WO, 10CN-WO and 10CN-0.5Si-WO, respectively, which accounts



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for the superiority of the photoactivity of silicate-bridged 2D/2D g-C3N4/WO3 for the aerobic selective alcohol oxidation. The photoactivity and selectivity of the best sample 10CN-0.5Si-WO was compared with the TiO2 and WO3 based photocatalysts as reported for the selective oxidation of BA with oxygen as the oxidant at the atmospheric pressure (Table 1). Although for the same reaction distinct reaction conditions including the solvent, light source and duration, etc, were applied in these works, here we use the index that the produced moles of BAD per hour per gram of the photocatalyst to evaluate the photocatalytic performances of different photocatalysts. As shown in Table 1, obviously as this defined index as the evaluation parameter, 10CN-0.5Si-WO shows the best performance among the representative photocatalysts for comparison. Noteworthily, the photoactivity of 10CN-0.5Si-WO surpasses all the peer WO3 based photocatalysts (entry 3, 4 and 5) by almost one or two orders of magnitude. Especially even the Pd oxide or Pt modified WO3 could not rival 10CN-0.5Si-WO in the aspect of photoactivity. Besides, among all the peer photocatalysts, 10CN-0.5Si-WO shows the

largest

selectivity

towards

BAD.

Therefore,

it’s

clearly

found

that

10CN-0.5Si-WO has demonstrated significant superioty among the low-cost metal oxide based photocatalysts for the aerobic selective alcohol oxidation. Table 1 The stability is another vital index to evaluate the effective photocatalyst. Under identifical reaction conditions, 10CN-0.5Si-WO could maintain the photoactivity for 5 runs without obvious loss of photoactivity (Fig. S6). Based above, the excellent


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photoactivity and stability of the silicate-bridged 2D/2D g-C3N4/WO3 nanocomposites make them promising photocatalysts for the aerobic selective organic oxidation. Photogenerated charge separation Normally the photoactivity is tightly associated with the charge separation situation of the photocatalysts, the well understanding of which might uncover the reason of the enhanced

photocatalytic

activity.46

The

steady-state

surface

photovoltage

spectroscopy (SS-SPS), transient-state surface photovoltage (TS-SPV), coumarin fluorescent method, PEC I-V curves and electrochemical impedance spectroscopy (EIS) techniques were integrated to be employed for carefully investigating the photogenerated charge separation and transition properties of the WO3 based photocatalysts. Figure 4 Generally, the SS-SPS response is in direct proportion to the charge separation.47 Fig. 4a and Fig. S7 showed the SS-SPS of as-fabricated samples. It is clearly seen, pure 2D WO and 2D CN have scarcely SS-SPS responses. As the amount of coupled g-C3N4 increases, the intensity of SS-SPS response becomes gradually stronger, especially for the 10CN-WO. Further introducing g-C3N4 leads to the decrease of the intensity of SS-SPS signal. Apparently, the improved intensity of SS-SPS response of 10CN-WO compared with that of WO is attributed to charge transfer between 2D WO and coupled 2D CN, leading to the prolonged charge lifetime. Noteworthily, the intensity of SS-SPS signal further increases significantly by constructing Si-O bridges between 2D WO and 2D CN. The highest response was observed for 10CN-0.5Si-WO



nanocomposite with the optimum amount of silicate groups, corresponding to the most favorable charge separation. Moreover, as shown in Fig. S7c and d single modification of silicate groups leads to no improved charge separation for 0.5Si-CN and 0.5Si-WO compared with pristine WO and CN. Therefore, it is suggested that as introduced Si-O bridges be favorable to transport charge so as to promote the electron immigration in the fabricated nanocomposite. To further explore the properties of photogenerated charges, TS-SPV was used to investigate the dynamic processes of the photogenerated charge carriers, which signal is also in direct proportion to the charge separation.48 According to the TS-SPV responses shown in Fig. 4b, the TS-SPV responses are positive for 2D WO, 10CN-WO and 10CN-0.5Si-WO nanocomposites under laser pulse irradiation with 350 nm light. The intensities of the TS-SPV responses that the charge separation situation is consistent with the SS-SPS results. The 10CN-WO nanocomposite with the optimum CN amount exhibits rather longer carrier lifetime and higher charge separation than 2D WO. For the sample 10CN-0.5Si-WO, it shows the strongest TS-SPV intensity. From the TS-SPV spectra, we can also estimate the time of the whole separation and recombination process of photogenerated electron-hole pairs in the 10CN-0.5Si-WO. As can be seen, there is a remarkable feature that TS-SPV response moves towards the longer timescale for 10CN-0.5Si-WO, approximately by millisecond, compared with those for 10CN-WO and WO. This indicates that Si-O bridge could effectively inhibit the recombination and prolong the lifetime of the photogenerated charge. Based on the SS-SPS and TS-SPV results, it is confirmed that


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the separation of photogenerated charge carriers of 2D WO could be effectively improved by coupling with a proper amount of 2D CN then introducing Si-O bridges. It is generally accepted that the amount of produced hydroxyl radicals (·OH) can reflect the separation of photogenerated charges in the photocatalytic process.49 Thus, it is meaningful to test the ·OH amount to further verify the charge separation in a photochemical sense. Since the coumarin easily reacts with ·OH to produce luminescent 7-hydroxy-coumarin, the coumarin fluorescent method is often utilized to measure the ·OH amount.25 The fluorescence intensity related to the amounts of produced ·OH for different WO3-based samples are shown in Fig. 4c and Fig. S8. Generally, the fluorescent signal is proportional to the amount of ·OH. It is clear that the amount of ·OH produced by WO is considerably increased after coupling 2D CN. The sample 10CN-0.5Si-WO produces the largest amount of ·OH, indicating that the Si-O bridges were beneficial for the charge transfer, which is in good agreement with the SS-SPS and TS-SPV measurements. Moreover, the charge separation situation of as fabricated samples is further confirmed by the PEC I-V curves.50 As shown in Fig.4d (inset), it is well demonstrated that the photocurrent responses of resulting 2D WO is gradually enhanced after coupling CN and then introducing the silicate bridges. Noteworthily the sample 10CN-0.5Si-WO possesses the highest photocurrent intensity among the fabricated nanocomposite films, at the bias voltage of 0.4 V (vs Ag/AgCl electrode). Both the low value of photocurrent and the quick decay character in 2D WO are assigned to its fast charge recombination kinetics.51 The greatly enhanced



photocurrent intensity of 10CN-0.5Si-WO suggests that the electron injection from WO to CN through Si-O bridges effectively slow down the recombination of charge carriers. Besides, the increased charge separation is further supported by EIS under the full light irradiation (Fig. 4d) and in dark (Fig. S9). Apparently, the introduction of Si-O bridges can effectively facilitate the interfacial charge transfer from 2D WO to CN, as reflected by the decreased semicycle arc at high frequencies in the EIS Nyquist plots. This implies that 10CN-0.5Si-WO nanocomposite possesses smaller electronic resistance than those of 2D WO and 10CN-WO, due to more effective charge immigration.52 Combining the SS-SPS, TS-SPV, coumarin fluorescent, PEC and EIS results, it could be deduced that the fabricated 10CN-0.5Si-WO nanocomposite exhibits the highest photogenerated charge separation, accounting for its superior photoactivity. Notably, as constructed 2D heterojunction between the 2D WO and CN nanosheets could effectively improve the charge separation. Especially, additional introduced silicate groups with appropriate amount could function as bridges for the charge immigration between the 2D WO and CN nanosheets, which further enhance the charge separation. DISCUSSION Verification of Z-scheme mechanism of charge separation The measurements and analysis on the charge separation of 2D WO based samples have clarified the 2D/2D heterojunction along with the silicate bridges effectively


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improve the charge transfer between WO and CN as the two main components of the hybrid nanocomposites. For more specific charge transfer mode, it’s always suggested to be the Z scheme for the WO3/g-C3N4 nanocomposites as reported. To clearly verify the Z-scheme mechanism of the Si-O bridged 2D/2D g-C3N4/WO3, the single-wavelength fluorescent spectra (FS) related to the ·OH amounts of WO, 10CN-WO, and 10CN-0.5SiO-WO were collected under the wavelength of 520 nm, 450 nm and 405 nm, respectively (Fig. 5a). The optical absorption thresholds of WO3 and g-C3N4 are both ca. 460 nm as reported.37 Under the illumination at the wavelength of 520 nm the three samples all showed rather limited fluorescent responses, which is reasonable since neither 2D WO nor CN could be excited in this situation. While the FS intensities of the three samples significantly increase when decreasing the excitation wavelength from 520 nm to 450 nm and 405 nm. Furthermore, at the wavelength of 450 nm and 405 nm, the FS intensities of 2D WO obviously increase after coupling CN and further increase after introducing the silicate groups. The results are consistent with the Z-scheme mechanism that it’s the facilitated electron transfer from the CB of WO to the VB of CN that results in the greatly increased FS intensities. Apparently, the Si-O bridges further contribute to the charge transfer at interface of 2D WO and CN. Figure 5 To further confirm this Z-scheme mechanism, the single-wavelength photocurrent action spectra of WO, 10CN-WO and 10CN-0.5Si-WO were recorded at 0.4 V bias vs Ag/AgCl in 0.5 M Na2SO4 electrolyte in Fig. 5b. Noticeably, the photocurrent



densities for all the three photocatalysts are rather weak above 450 nm, since neither 2D WO nor CN could be excited due to their energy bandgaps. Under the illumination at the wavelength of 450 nm, both 2D WO and CN would be excited simultaneously. While the photocurrent density of 10CN-WO is much higher than that of WO, suggesting the injection of photogenerated electrons from WO to CN effectively inhibit the charge recombination. Therefore, when WO and CN are excited at the same time, it can be concluded naturally that the sharp increase of the photocurrent density is attributed to the Z-scheme mechanism. The photocurrent density will still increase when the excitation wavelength is further decreased. Interestingly, further introducing the silicate groups to obtain 10CN-0.5Si-WO leads to much increased photocurrent density compared to that of 10CN-WO for all the excitation wavelengths below 450 nm. This indicates that the Si-O bridges existing between the 2D-2D interfaces could effectively facilitate the photogenerated electron immigration from the CB of WO to the VB of CN. Photocatalytic selective alcohol oxidation mechanism The photoactivity is decided not only by the favorable charge separation but also the subsequent catalytic efficiency induced by the charge carriers. To investigate the photocatalytic reaction mechanism of 10CN-0.5Si-WO nanocomposite for the aerobic selective alcohol oxidation, the scavenger experiments were performed to find out the dominating active species during the reaction. In specific, for the typical photocatalytic reaction with 10CN-0.5Si-WO as the photocatalyst, additional scavengers aiming at distinct active species, which might be involved in the reaction,


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were also introduced for comparison. As shown in Fig. S10 and Fig. 6a, the scavenger experiments were performed on WO, 10CN-WO and 10CN-0.5Si-WO, respectively. For all three samples, when TEA as the photogenerated holes capturer added into the photocatalytic reaction system, the BA conversion sharply decreased, indicating it’s the holes of WO that dominate the photocatalytic selective oxidation of BA to produce BAD. While when AgNO3 as the electron scavenger was added, for WO and 10CN-WO, the BA conversion was significantly increased. This is mainly because the trapping of photogenerated electrons could prolong the lifetime of the photogenerated holes so as to facilitate the oxidation of BA. However, for 10CN-0.5Si-WO, after AgNO3 added only slight increase of BA conversion could be observed. This is because the charge separation has been fully improved by introducing the silicate groups, or rather which is not the main limitation for the photocatalytic activity. Moreover, for all three samples the introduction of BQ as the scavenger of ·O2- all lead to the increase of BA conversion to different extents. This indicates the ·O2might be the predominant reduction product of O2 by the photogenerated electrons. In addition, as-introduced PA and BHT have neglected effects on the BA conversion, indicating that neither carbon-centered radicals nor ·OH plays vital roles during the whole photocatalytic reaction process. Based on the results of the scavenger experiments, it’s firmly proved that the photogenerated holes of WO3 oxidized BA to produce BAD, while the photogenerated electrons were consumed by O2. Therefore, as constructed 2D/2D heterojunction by introducing CN nanosheets and silicate groups greatly enhanced the photoactivity by prolonging the lifetime of holes through



Page 26 of 50

improving the charge separation basing on the Z-scheme. Moreover, this results also account for the high selectivity for all WO based photocatalysts, since compared with the semiconductors like TiO2, etc, the suitable VB position of WO endowed it mild oxidation ability which offered high selectivity towards BAD by avoiding the over oxidation.43 Figure 6 To further verify the reaction mechanism dominated by the photogenerated holes as inferred from the results of the scavenger experiments, the isotopic experiments were also performed with 10CN-0.5Si-WO as the photocatalyst. In specific,

18O

2

were

utilized as the oxidant to replace the normal O2 for the typical photocatalytic oxidation of BA for comparison. If BA were directly oxidized by oxygen or derivative oxygen species, then produced BAD must contain

18O

which could be detected by

GC-MS. It could be seen that after the typical reaction for certain period, similar as the situation using normal O2 (Fig. 6b), 18O-labeled BAD in the final reaction solution could be hardly detected in Fig. 6c. This result is in agreement with the inference of scavenger experiments, which verified the oxidation of BA be conducted by the photogenerated holes of WO while ruled out the direct oxidation of BA by the molecular oxygen and derivative oxygen species. Based on the charge separation and oxidation mechanism, the schematic of selective BA oxidation with O2 as the oxidant over 10CN-0.5Si-WO nanocomposite is depicted in Scheme 1. Under the irradiation with the wavelength below 460 nm, 2D WO and g-C3N4 nanosheet could be excited synchronously, obeying the Z-scheme


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that the excited electrons in the CB of WO would combine with the photo-induced holes in the VB of g-C3N4. The silicate groups between WO and CN function as bridges to further facilitate the electron transfer. For the subsequent catalysis process, the photogenerated electrons on the CB of g-C3N4 would be trapped by electrophilic O2, leaving photogenerated holes on the VB of WO with the mild oxidation ability to oxidize alcohols to produce corresponding aldehydes. Scheme 1 Except for the selected benchmarked reactant BA, the applicability of 10CN-0.5Si-WO has been examined for other types of alcohols as listed in Table 2. All alcohols including the aromatic ones and aliphatic ones, could be effectively converted to corresponding aldehyde with rather high selectivity. Notably, 10CN-0.5Si-WO is more active for the substrates with the electron-donating

group

like

methyl.

Compared

with

BA,

when

an

electron-donating group substitutes the p-position (entry 1), the electron density would be enriched on α-C atom, resulting in the accelerated electron transfer to the photogenerated holes of WO. On the contrast, the electron density of α-C atom is decreased by the electron-withdrawing group like Cl and NO2 groups (entry 2&3), so that the photoactivity of corresponding alcohol is relatively low. As entry 4-6, it could be seen that for the aliphatic alcohols, which are difficult to be oxidized compared with aromatic alcohols, have also been selectively oxidized to some extent with 10CN-0.5Si-WO as the photocatalyst. Table 2



The photocatalytic results of various alcohols indicate that silicate bridged g-C3N4/WO3 2D-heterojunctional nanocomposites are applicable for efficient aerobic selective alcohol oxidation with high selectivity and high conversion rate. CONCLUSION In summary, by constructing the Z-scheme 2D/2D g-C3N4/WO3 nanocomposites and subsequent introduction of Si-O groups as electron bridges, the photoactivity of bare 2D WO has been enhanced to ca. 3-fold and 5-fold for the aerobic selective oxidation of BA under the full light irradiation, respectively. The great enhancement of photoactivity was attributed to the improved charge transfer by the as constructed Z-scheme 2D/2D heterojunction as well as the effecient electron bridging effect of Si-O groups. With the assistance of SS-SPS and single-wavelength TS-TPV, it’s clearly proved the charge transfer and separation of the silicate-bridged 2D/2D g-C3N4/WO3 obey the Z-scheme mechanism. Moreover, the scavenger experiments along with the

18O

isotopic experiments verified that it’s the photo-induced holes of

WO with the mild oxidation ability that dominates the oxidation of alcohol to produce aldehyde with high selectivity. This work provides a feasible strategy to fabricate Z-scheme WO3-based photocatalysts for aerobic selective organic transformation. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.


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AUTHOR INFORMATION * To whom correspondence should be addressed. Email address: [email protected] (Linlu Bai); [email protected] (Liqiang Jing).

Author Contributions L. Q. S performed the photocatalyst synthesis, activity testing and material characterizations; B. L, X. Y. C, N. S, X. L. Z and I. K. assisted to characterize the materials; Y. Q performed HRTEM observation and analysis; L. L. B wrote the main text; All authors discussed and reviewed this paper. L. L. B, and L. Q. J planned, supervised and led the project.

Funding Sources We are grateful to financial support from NSFC (21706044, 91622119, U1805255), the Program for Innovative Research Team in Chinese Universities (IRT1237), General Financial Grant from the China Postdoctoral Science Foundation (2017M621316) and the Natural Science Foundation of Heilongjiang Province, China (B2017006, QC2017004). ABBREVIATIONS REFERENCES (1) Miyamura, H.; Matsubara, R.; Miyazaki, Y.; Kobayashi, S. Aerobic oxidation of alcohols at room temperature and atmospheric conditions catalyzed by reusable gold



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For Table of Contents Use Only. Si-O bridged 2D/2D g-C3N4/WO3 nanocomposites for highly efficient and selective photocatalytic aerobic alcohol oxidation



Figure Captions Table 1. Comparative photocatalytic aerobic selective oxidation of BA by the representative heterogeneous photocatalysts. Table 2. Photocatalytic activities of 10CN-0.5Si-WO for selective oxidation of various alcohols.a Figure 1. TEM and HRTEM images of 2D WO (a) and CN (b); TEM image (c) and area-selected HRTEM (d) of 10CN-0.5Si-WO. Figure 2. The XPS spectra of Si 2p for the silicate bridged 2D WO based samples. Figure 3. Conversion of benzyl alcohol under 300W Xe lamp (a) and quantum efficiency vs different excitation wavelengths (b) for WO, 10CN-WO and 10CN-0.5Si-WO, respectively. WO 10CN-WO 10CN-0.5SiO-WO

%

80

A

60

Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 50

40 20 0

/h

S3

time

Samples S2 S1

0.51.01.52.02.53.0 Irradiation

Figure 4. SS-SPS responses in air atmosphere (a), TR-SPV responses in air atmosphere (b), I-V curves (c) and electrochemical impedance spectra (EIS) of WO, 10CN-WO and 10CN-0.5SiO-WO (d). Figure 5. FS intensities related to the ·OH amounts vs different excitation wavelengths (a) and photocurrent action spectra as a function of different excitation wavelengths of WO, 10CN-WO and 10CN-0.5SiO-WO at 0.4 V bias vs Ag/AgCl in 0.5 M Na2SO4 electrolytes (b). Figure 6. Trapping experiments with 10CN-0.5Si-WO as the photocatalyst for the aerobic selective oxidatio of benzyl alcohol with different scavengers (a) ; the gas mass spectra of the benzaldehyde produced in

16O


2

atmosphere (b) and

18O

2

Page 41 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


atmosphere (c) over 10CN-0.5Si-WO. Scheme 1. Mechanism schematic for charge transfer, separation and the induced photochemical

reactions

under

300W

Xe

lamp

2D-heterojunctional g-C3N4/WO3 nanocomposites.


under

Si-O

bridged

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

Page 42 of 50

Table 1

Produced moles of BAD/ Entry

Photocatalyst

Conv.(%)

Sel.(%)

Time

Reaction conditions

(mass of photocatalyst×duration)

Ref.

(mol/g×h ) 1

TiO2

0.2

50

6

solvent-free, 250 W high

3.25×10-6

42

2

WO3 /TiO2

50

56

3

H2O, light >350 nm pressure Hg lamp (315–420

1.87×10-3

43

3

yolk-shell WO3

>65%

>99

5

H O, full light nm, main2 wavelength at 365

1.27×10-4

44

7.70×10-5

45

1.11×10-4

46

2.82×10-3

This work

4

PdOx/WO3

30.6

63.1

3

H2O, light source, 300 W Xe nm) lamp (300 < λ < 500 nm) H2O, light source, 300 W Xe

5

Pt/WO3

55.8

55.7

3 lamp (300 < λ < 500 nm)

6

10CN-0.5Si-WO

85.6

>99

3

acetonitrile, full light


Page 43 of 50 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Entry

Substrate

Product

CH2 OH

CHO

CH3

CH3

Time/h

Conv.(%)

Sel.(%)

3

81.2

99.1

3

76.8

96.2

3

39.1

88.5

3

12.6

86.2

CHO

3

14.2

72.1

CH3

3

11.5

71.5

1

CHO

CH2 OH

2 Cl

Cl

CHO

CH2 OH

3 NO2

NO2

OH

O

4

H3 C

H3 C

5

OH

H3 C

H3 C

6

a Reaction

CH3 HO

O

conditions: alcohol (0.2 mmol), CH3CN (10 mL) as the solvent, 10CN-0.5Si-WO as the

photocatalyst (20 mg) with 0.1 MPa O2 as the oxidant under full light irradiation with a 300 W Xe lamp.



Figure 1


Page 44 of 50

Page 45 of 50

Figure 2 10CN-0.5Si-WO 10Si-CN 0.5Si-WO WO

Si 2p

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


96

98

100

102

104

106

Binding Energy / eV


108


Figure 3

WO 10CN-WO 10CN-0.5Si-WO

60 40 20 0

3. 2.5 0 / h 2.0 e im

t ion t a 0.5 di a r Ir 1.0

1.5

4.0

Quantum efficiency %

(a)

80

n% Conversio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 50

3.5

(b)

3.0 2.5

10CN-0.5Si-WO 10CN-WO WO

2.0 1.5 1.0 0.5 0.0 460 440 420 400 380 360 340 320 300


Wavelength / nm

Page 47 of 50

Figure 4

1.2 (b)

(a)

Photovoltage / mV

Photovoltage / a.u.



1.0 0.8 0.6 0.4 0.2 0.0

350

400

450

500

550

1E-6

600

1E-5

Wavelength / nm

400

1E-3

0.01

Time / s 30.0k


(d) WO 10CN-WO 10CN-0.5Si-WO

-Z'' /Ω

25.0k 20.0k

2.0

10CN-0.5Si-WO 10CN-0.5Si-WO 10CN-WO 10CN-WO WO

2

(c)

1E-4

Current density (mA/cm )

300

Fluorescent Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15.0k

1.6

WO

1.2 0.8 0.4 0.0 0.0

0.4 0.8 1.2 1.6 Applied potential(V) vs Ag/AgCl

10.0k 5.0k 450

500

550

Emission wavelength / nm

600

0.0

0

Figure 5


10k

20k

30k

Z' /Ω

40k

50k


WO 10CN-WO 10CN-0.5SiO-WO

%

80

0.8

A

60

Conversion

40 20

/h

S3

time

Samples S2 0.51.01.52.02.53.0 Irradiation

(a)

(b)

2

S1

Current density ( mA/cm )

0

Fluorescent Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


520nm

450nm

Wavelength / nm

405nm

Page 48 of 50

380nm


0.6

0.4

400nm 420nm

0.2

450nm 600nm 550nm 500nm

0.0

0

50

100

150

200

Time / s

Figure 6


250

300

350

Page 49 of 50

Conversion %

100

(a)

80 60 40 20 0

+

No scavenger Ag

5x105

5x105

(b) 105.1 106.1

IPA

BHT

(c)105.1106.1

4x105

Abundance

3x105 2x105 1x105 102

BQ

TEA

4x105

Abundance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


106 108

m/z

2x105 1x105

107.1 108.1 104

3x105

110 112

102

104

Scheme 1


107.1 108.1 109.1 106 108 110

m/z

112



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WO3 2D-heterojunctional

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