ZnIn2S4 for promoting photocatalytic selective oxidation of aromatic alcohol

Journal Pre-proofs Full Length Article Dual interfacial synergism in Au-Pd/ZnIn2S4 for promoting photocatalytic selective oxidation of aromatic alcohol Chenjie Feng, Xiaolong Yang, Zhaoli Sun, Jian Xue, Luoran Sun, Jiahui Wang, Zilong He, Jianqiang Yu PII: DOI: Reference:

S0169-4332(19)32834-X https://doi.org/10.1016/j.apsusc.2019.144018 APSUSC 144018

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

14 July 2019 30 August 2019 13 September 2019

Please cite this article as: C. Feng, X. Yang, Z. Sun, J. Xue, L. Sun, J. Wang, Z. He, J. Yu, Dual interfacial synergism in Au-Pd/ZnIn2S4 for promoting photocatalytic selective oxidation of aromatic alcohol, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144018

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Dual interfacial synergism in Au-Pd/ZnIn2S4 for promoting

photocatalytic

selective

oxidation

of

aromatic alcohol Chenjie Fenga, Xiaolong Yanga, b, *, Zhaoli Suna, Jian Xuea, Luoran Suna, Jiahui Wanga, Zilong Hea, Jianqiang Yua, b, * a.

College of Chemistry and Chemical Engineering, Qingdao University, No. 308

Ning-Xia Road, Qingdao 266071, P. R. China b.

Weihai Institute of Innovation, Qingdao University, No. 59 Wenhua Middle Road,

Weihai 264200 *Corresponding author: E-mail address: [email protected] (X. L. Yang) [email protected] (J. Q. Yu)



Abstract Visible-light-responsive semiconductor decorated with bi-metal nanoparticles synergistic photocatalysts are promising in photocatalysis. Herein, bi-metal nanoparticles Au-Pd decorated ZnIn2S4 nanosheets photocatalytic system was constructed and applied in reaction of photocatalytic selective oxidation of of aromatic alcohols. Various parameters including loading ratio of two metal, loading amount, solvents, time, reactant were investigated. The optimal 0.5 wt% Au-Pd/ZnIn2S4 photocatalysts was founded to exhibit the highest photocatalytic activity, which is 1.5, 2.0 and 1.3 times higher than pristine ZnIn2S4, Au/ZnIn2S4 and Pd/ZnIn2S4, respectively. Characterization results confirmed enhanced visible-light harvesting capability as well as superior photoinduced carriers’s separation and transfer behavior, enhanced O2 adsorption and reduction ability of Pd and surface reaction kinetics account for enhanced photocatalytic activity, in addition, which ascribed to dual metal synergistic effect and metal-semiconductor interaction in Au-Pd/ZnIn2S4 system. Finally, the corresponding reactive radical species was confirmed by ESR and other method. Based on the experimental data and analysis, possible reaction mechanism is proposed. The photogenerated h+, •O2- and carbon centered radicals are responsible for the reaction. This systemic work shed light on the bi-metal decorated semiconductor photocatalyst, where metal-metal interaction as well as metal-semiconductor cooperated together to improve the performance of



catalysts in visible-light-driven organic transformations.

Keywords: Metal nanoparticles, Gold, Palladium, Selective alcohol oxidation, ZnIn2S4, Visible light photocatalysis

1. Introduction Organic synthesis technology plays an extremely important role in today’s chemical

engineering

process[1,2].

Various

organic

compounds

such

as

pharmaceuticals, pesticides, additives, perfume, resin and rubber etc are ubiquitous in our lives. In this case, aromatic aldehydes are important organic intermediate, which can be used to synthesize pharmaceuticals, cosmetics, spices and so on[3]. Aromatic aldehydes are often synthesized by catalytic selective oxidation in industry. The strong stoichiometric oxidants like potassium dichromate, potassium permanganate, potassium hypochlorite and the like are inevitably used in the traditional industrial production process[4], however, low selectivity of the oxidation products, strong corrosiveness, high energy-consumption, harsh operation conditions and serious heavy metal pollution for environment make it urgent to develop a “green” synthetic route of aromatic aldehydes[5]. Thus, the development and exploration of a new type of synthesis routes avoiding the shortcoming listed above is highly desirable. Since Fujishima’s pioneering work in 1972, photocatalytic technology has attracted the attention of researchers from all over the world[6]. The photocatalytic technology utilize sunlight to drive chemical reactions, which does not involve environmental



pollution to some extent. It is the most potential technology to achieve “green” production. Up to now, a large amount of photocatalysts as well as photocatalytic process has been employed to solve the problem of energy shortage and environmental remediation[7,8]. For instances, overall water splitting, hydrogen or oxygen evolution, H2O2 generation, CO2 reduction, N2 fixation, organic synthesis, degradation of organic pollutants[9-16], NOx abatement, metal ion redox etc. Among them, semiconductor photocatalytic organic synthesis promote chemical reaction with solar energy, which means greater economical and environmental merits[17]. People pay much attention to photocatalytic selective oxidation since pioneering work of Fujihira et al

[18].

Great effort have been make for utilizing dye-sensitized

TiO2 as effective photocatalyst for selective oxidation of alcohol[19]. Besides wide band-gap type of TiO2-based photocatalysts, a large number of narrow band-gap semiconductors materials were also exploited to satisfy the requirement of selective oxidation reaction, such as ZnIn2S4, BiVO4, Bi2MoO6, g-C3N4, BiOX (X=Cl, Br, I), MOF[20-25], CdS, In2S3, non-stoichiometric W18O49[26]and many others[27]. As a typical visible light responsive photocatalyst, ternary sulfides like ZnIn2S4 (n-type direct-bandgap semiconductor, bandgap of 2.06-2.85 eV) have attracted more and more attention due to its unique two dimension assembling structure, which has been widely used in the filed of hydrogen evolution

[28],

degradation of organic

pollutants[29], organic synthesis[30]and others[31]. Previous works demonstrated that the valence band of ZnIn2S4 located at 1.48 V (vs. NHE), which not only possess 

sufficient selective oxidizing capability from alcohol to aldehyde, but also inhibit efficiently the overoxidation reaction. Simultaneously, the conduction band position loacated at -0.90 V (vs. NHE) is beneficial for generation of •O2-, which is confirmed as the reactive radical for the selective oxidation reaction[32]. Chen et al firstly synthesized marigold-like ZnIn2S4 photocatalyst by ethanol-thermal technique, which present superior photocatalytic activity for selective oxidation of aromatic alcohol to aldehyde[33]. Chen group further pointed out the improved activity was ascribed to exposed high energy {0001} crystal facets for ZnIn2S4 microsphere prepared by one-pot solvothermal method[34]. However, limited light harvesting ability, serious recombination behavior of photoinduced carriers of ZnIn2S4 significantly imprison the photocatalytic performance of selective oxidation reaction[35,36]. Researchers have developed various modification strategies to enhance the lifetime of photogenerated charges and holes for ZnIn2S4 to improve its photocatalytic activity, such as structure and morphology controlling, element doping, co-catalyst modifying [37] and heterojunction construction etc. Recently, semiconductor interfaced with bimetallic nanoparticles exhibits better photocatalytic selective oxidation performance owing to their synergistic effects on selective oxidation reaction

[38].

Li

group synthesized Au-Pd/BiVO4 ultrathin nanoflakes and found its superior photocatalysis than the pristine counterparts, which can be ascribed to enhancing of light-adsorption capability, charge separation efficiency and surface reaction kinetics assisted by the bimetallic electronic coupling and interfacial synergism effect



[11].

Czelej et al found bimetallic Pd-Au/TiO2 system exhibited highly active and selective activity for transformation from methanol to methyl formate[39]. Xue et al synthesized Au-Pd decorated TiO2 nanofiber and found its excellent activity for H2 production from formic acid[40]. Sun et al prepared Au-Cu@CeO2 bimetallic-core CeO2-shell nanocomposite, which exhibits high activity, high selectivity and stability for the selective oxidation reaction[41]. Furthermore, Parida et al prepared a bimetallic Au-Ag /LDH/RGO nanocomposite, which exhibited excellent catalytic activity for tandem photoredox reactions[38]. Being motivated by the above-mentioned works, bimetallic nanoparticles decorated ZnIn2S4 is a promising choice for photocatalytic selective oxidation because of its multi-component synergistic effect between metals and between metals and semiconductor support. It should be noted that many merits would contributed to the enhanced catalytic activity like Schottky barrier at metal/semiconductor interface, cocatalysis synergistic effect of metal nanoparticles, and improved light harvesting capability, etc. To our knowledge, there is no abundant information regarding such work for bimetallic nanoparticles anchored on ZnIn2S4 system toward the photocatalytic selective oxidation reaction. Hence, herein, Au, Pd, Au-Pd decorated marigold-like ZnIn2S4 microsphere photocatalysts were synthesized and utilized as selective oxidation photocatalysts for aromatic alcohol. The as-prepared pristine ZnIn2S4, Au/ZnIn2S4, Pd/ZnIn2S4 and a series of x wt% Au-Pd (molar ratio=m:n)/ZnIn2S4 were 

studied in reaction under visible light irradiation. It was demonstrated the 0.5 wt% Au-Pd (2:8)/ZnIn2S4 showed the highest photocatalytic activity and selectivity in whole catalysts series. Various physicochemical characterization technique were thoroughly conducted to investigate photocatalysts. Furthermore, various reaction parameters were investigated related with the reactant type, solvents and reaction time to optimize the reaction results. Finally, combined with ESR technique as well as scavengers experiments, a possible mechanism aiming at uncover the effective reactive radicals as well as corresponding selective oxidation mechanism over the surface of bimetallic Au-Pd decorated ZnIn2S4 was proposed.

2. Experimental section 2.1. Materials ZnCl2, InCl3·4H2O, Thiacetamide, (NH4)2PdCl4, HAuCl4·4H2O, NaBH4, benzyl alcohol

(C7H8O),

4-chlorobenzyl

methyl

alcohol

benzyl

(C8H10O),

(C7H7ClO),

methoxy

4-nitrobenzyl

benzyl alcohol

(C8H10O2), (C7H7NO3),

1-phenylethanol (C8H10O), trifluorotoluene (BTF), absolute ethanol were obtained from Shanghai Macklin Biochemical Co., Ltd (P. R. China). CH3CN, 1, 3, 5-trimethylbenzene (C9H12), 1, 4-dioxane (C4H8O2), 2-phenylethanol (C8H10O) were obtain from Sinopharm Chemical Reagent Co., Ltd (P. R. China). All reagents are analytically pure and used as received. Deionized water is obtained from an ultra-pure purification equipment.



2.2. Synthesis of Au/ZnIn2S4, Pd/ZnIn2S4 and Au-Pd/ZnIn2S4 photocatalysts Marigold-like ZnIn2S4 microsphere were synthesized through a facile solvothermal method found anywhere [36]. Certain amount of (NH4)2PdCl4 (54.8 mg) was dissolved in aqueous solution (20 mL) to prepare (NH4)2PdCl4 solution of 1 mg/mL. Similarly, HAuCl4·4H2O (34.5 mg) was dissolved in deionized water (20 mL) to prepare HAuCl4 solution of 1 mg/mL. A series of Au/ZnIn2S4, Pd/ZnIn2S4 as well as Au-Pd/ZnIn2S4 photocatalysts were synthesized by the facile NaBH4 reduction method as previous reports [32]. Briefly, for 0.1 wt% Au-Pd(molar ratio = 1:9)/ZnIn2S4 photocatalyst, 200 mg ZnIn2S4 microsphere, 34 μL HAuCl4 solution of 1 mg/mL and 166 μL (NH4)2PdCl4 solution of 1 mg/mL were added into the 60 mL deionized water under stirring for 12 h, obtained product was re-dispersed in 30 mL deizonized water after washing, a certain amount of NaBH4 (8 mg) was added. The dark-grey precipitate was obtained after 2 h under vigorous stirring, which was collected by centrifugation, wash and drying. Different volume of HAuCl4 solution and (NH4)2PdCl4 solution was used for a series of Au-Pd/ZnIn2S4 photocatalysts. The Au/ZnIn2S4 (Pd/ZnIn2S4) photocatalyst was prepared by the same procedure above and the difference was using HAuCl4 (1 mg/mL) ((NH4)2PdCl4 (1 mg/mL)) solution as metal precursor only. 2.3. Characterization techniques XRD were carried out on DX-2700 advance X-ray diffractometer with Cu Kα



radiation λ = 0.154050 nm with scan speed of 3°·min-1. ICP-MS was utilized to determine the metal weight percentage of photocatalysts (Agilent 7700ce, USA). SEM images were obtained by JSM-6700F field emission scanning electron microscope. TEM, HRTEM images, and elemental mapping results were taken on an JEM, 2200FS electron microscope operating at 300 kV. UV-vis-DRS was conducted on Hitachi, UH4150 spectrophotometer by using BaSO4 as the reflection standard. The

photoluminescence

were

taken

on

PerkinElmer

FLs980

fluorescence

spectrometer with excitation wavelength of 432 nm at room temperature. XPS was conducted on ESCALAB 250Xi system with a monochromatic X-ray Mg Ka (hα = 1486.6 eV) source. ESR spectra were recorded with Bruker model JEOL JES FA200 spectrometer. Photoelectrochemical properties were tested on electrochemical workstation (B22118, Ivium) using standard three-electrode system. 2.4. Photocatalytic selective oxidation reaction Photocatalytic selective oxidation reaction for aromatic alcohol was operated in side-irradiation-type

three-neck

flask

under

illumination

of

visible

light

(PLS-SXE300) at room temperature. Balloon was used to provide 1 bar O2 reaction atmosphere. 50 mg photocatalyst, 10 mL reaction solvent and 0.25 mmol aromaic alcohol reactant were used unless otherwise specified. Adsorption-desorption equilibrium was built by O2 saturation 30 min and stirring in dark for another 30 min. After 3 h of reaction, the products were analyzed by gas chromatograph (Shimaduzi, GC-14C) equipped with DA-CARBONWAX column.



The conversion of alcohols, selectivity for aldehydes product and yield of aldehydes were calculated as follows: Conversion (%) = [(C0 − Calcohol)/C0] × 100

(1)

Selectivity (%) = [Caldehyde/(C0 − Calcohol)] × 100

(2)

Yield (%) = Conversion (%) × Selectivity (%)

(3)

C0 is initial concentration of aromatic alcohols, Calcohol as well as Caldehyde is concentration of aromatic alcohols and aromatic aldehyde, respectively.

3. Results and discussion 3.1. Characterization of photocatalysts

3.1.1. XRD analysis The XRD patterns of pure ZnIn2S4, Au/ZnIn2S4, Pd/ZnIn2S4 and a series of Au-Pd/ZnIn2S4 photocatalysts are shown in Fig. 1. For pristine ZnIn2S4, characteristic diffraction peaks appeared at about 7.2 °, 21.5 °, 27.7 °, 30.5°, 47.8 ° and 52.2 ° can be attributed to the (002), (006), (102), (104), (112) and (1012) lattice planes of hexagonal

ZnIn2S4

(PDF#72-0773),

respectively[42].

As

compared,

single

metal/ZnIn2S4 as well as binary metal/ZnIn2S4 composites have the similar XRD patterns with the pristine ZnIn2S4, and no diffraction peaks ascribed to Au or Pd phase could be observed. Notably, even the loading amount of metal increased to 3.0 wt%, no corresponding signals could be detected. This is mainly attributed to the ultra-low metal content lower than the detection limits of XRD and the small size of metal



nanoparticles[21]. Furthermore, the crystallographic structure of ZnIn2S4 in composites is not changed by the corporation of single or binary metal as show in Fig. 1. Furthermore, the composition of 0.5 wt% Au-Pd (2:8)/ZnIn2S4 photocatalyst were analyzed with ICP-MS. As a result, actual mass percentage of metallic Au as well as Pd is measured to be approximately 0.13 wt% and 0.30 wt% in 0.5 wt% Au-Pd (2:8)/ZnIn2S4 photocatalyst, which is in accordance with the initial theoretical calculation.

3.1.2. SEM and TEM analysis

The microstructure and morphology of series of photocatalysts are examined by SEM and TEM as shown in Fig. 2 and Fig. 3. The pristine ZnIn2S4 exhibited the morphology of marigold-like microsphere with diameter ranging from 6 to 9 μm, which are assembled by the interlaced two dimensional nanosheets as shown in Fig. 2a-2c, Fig. 3a[35,

36].

The introduction of Au, Pd or different weight ratio Au-Pd

nanoparticles together have no obvious influence on the corresponding composites as shown in Fig. 2d-2f. Notably, Fig. 2f shows the micro-morphology of 3.0 wt% Au-Pd (2:8)/ZnIn2S4 sample remain the same as that of the pure ZnIn2S4 even the loading amount of Au-Pd reached to 3.0 wt%. The corresponding Au-Pd nanoparticles size distribution was counted by measuring more than 100 particles all over the screen from TEM images (Fig. 3b). As a result, the TEM images of 0.5 wt% Au-Pd



(2:8)/ZnIn2S4 revealed that metal nanoparticles with diameter of 2~7 nm are dispersed uniformly on the nanosheets of hexagonal ZnIn2S4 as shown in Fig. 3b, 3c, respectively. Average diameter of metal nanoparticles centered at 3.9 nm (Inset in Fig. 3b). The HRTEM was further conducted to observe the lattice fringe of Au-Pd metal as well as ZnIn2S4 support. As shown in Fig. 3d, the interplanar spacing is found to be 0.235 nm for Au nanoparticle, which is consistence with (111) lattice plane of of Au. And the corresponding value is 0.225 nm, corresponding to (111) plane of Pd. The distance of 0.328 nm was ascribed to (102) plane of hexagonal ZnIn2S4, which is consistence with the XRD results (Fig. 3d).The SAED analysis also showed that 0.5 wt% Au-Pd (2:8)/ZnIn2S4 sample were highly-crystallized polycrystalline, with continuous and clear diffraction ring at surface such as (102) and (112), which is in accordance with XRD analysis (Fig. 1). TEM elements mapping exhibited that Au, Pd, Zn, In and S elements distributed uniformly in the 0.5 wt% Au-Pd(2:8)/ZnIn2S4 composite (Fig. 3e). 3.1.3. UV-Vis-DRS analysis Light adsorption capability of the ZnIn2S4, Au/ZnIn2S4, Pd/ZnIn2S4 and Au-Pd/ZnIn2S4 photocatalysts were investigated by the UV-Vis-DRS technique. The optical adsorption edge of pure ZnIn2S4 is at ~496 nm, corresponding to optical bandgap of ~2.5 eV, which is consistent with previous results (Fig. 4a)

[36].

The

loading process of metal significantly changes the optical properties of ZnIn2S4 support compared with pure ZnIn2S4. Specifically, loading of Au nanoparticles lead to the 

decrease of optical absorption capacity in UV region, which may be ascribed to occlusion of Au nanoparticles over boundary of the ZnIn2S4. The increased optical adsorption phenomenon is observed for loading of Pd nanoparticles. Also, the 0.5 wt% Au-Pd(2:8)/ZnIn2S4 possess superior light harvesting ability ranging from 460 to 800 nm than the pure support, which may account for enhanced catalytic performance. In addition, Kubelka-Munk transformation was used to calculate band gap energy (Eg) of samples. For ZnIn2S4 sample, it is an indirect semiconductor and Eg=2.50 eV, which is in accordance with the previous literature[36]. For the metal loading system, the Eg of Au/ZnIn2S4, Pd/ZnIn2S4 as well as Au-Pd/ZnIn2S4 is estimated to be 2.25, 2.30 and 2.19 eV, respectively. 3.1.4. XPS analysis XPS measurements was conducted to study element compositions as well as valence states of the loading metal as shown in Fig. 5. Obviously pure ZnIn2S4 exhibits the elemental signals of Zn, In and S without other impurities (Fig. 5a). Additionally, C 1s peak centered at 284.8 eV may be ascribed to adventitious carbon of environment, O 1s with BE of 532.0 eV is ascribed to adsorbed water molecular over the surface of samples [30] . After the decoration process, the signals of Au and Pd are prominent although the intensity is very weak due to its low loading content. High resolution XPS spectra are further presented to investigate the electronic states of the corresponding elements. As shown in Fig. 5b, Zn 2p3/2 and Zn 2p1/2 located at 1022.4 eV and 1045.5 eV, which are consistent with Zn2+. The In 3d3/2 and In 3d5/2 centered 

at 452.6 as well as 445.0 eV, respectively, corresponding to In3+. The peak of S 2p at the binding energy of 161.9 (2p3/2) and 163.2 (2p1/2) eV are observed, which belongs to S2-. Their binding energy peaks are all well assigned to ZnIn2S4 sample, which is consistent with previous report [43]. Au in 0.5 wt% Au-Pd(2:8)/ZnIn2S4 sample present two doublet peaks: Au 4f7/2 84.5 eV and Au 4f5/2 88.9 eV; Au 4f7/2 86.1 eV and Au 4f5/2 91.9 eV, the former is attributed to Au0 while the latter are ascribed to Au3+ [20,44]. The presence of two different valence state maybe attributed to the incomplete reduction of AuCl4-[32]. Furthermore, it should be noted that the Au species was mainly existed as Au0 due to its larger peak intensity than Au3+, which indicated that Au nanoparticles has been decorated successfully onto the surface of ZnIn2S4 with NaBH4 reduction method. Pd 3d species divided into two peaks at 336.9 eV for 3d5/2 and 342.1 eV for 3d3/2 for 0.5 wt% Au-Pd(2:8)/ZnIn2S4, the BE value of Pd2+ decrease compared with the 0.5 wt% Pd/ZnIn2S4 sample, which indicates that the electronic density of Pd nanoparticles increase after the decoration of Au component, it is consistent with the positive shift for 0.5 wt% Au-Pd(2:8)/ZnIn2S4 compared with 0.5 wt% Au/ZnIn2S4 [20]. It is understandable charge would inject into Pd from Au metal because of the difference of work function values when two metal nanoparticles was close enough [32]. Such charge migrating phenomenon possibly make Pd nanoparticles negatively charged, which is desirable for the activation of nucleophilic reactant molecules, such as oxygen molecules. Therefore, it is expected that bimetallic photocatalyst possess higher photocatalytic activity for selective oxidation reaction.



3.1.5. Photoelectrochemical analysis The separation as well as recombination efficiency of photo-generated carriers for

different

characterization

photocatalysts

was

investigated

by

photoelectrochemical

as shown in Fig. 6. The 0.5 wt% Au-Pd (2:8)/ZnIn2S4 exhibited the

largest photocurrent intensity, which is two times larger than pure ZnIn2S4 and the order is as follows: 0.5 wt% Au-Pd (2:8)/ZnIn2S4>0.5 wt% Pd (2:8)/ZnIn2S4>0.5 wt% Au/ZnIn2S4 ≈ pure ZnIn2S4. It indicates that the decoration of bimetal nanoparticles effectively inhibited recombination behavior of the photoexcited carriers

[45].

The

photocurrent intensity of Au/ZnIn2S4 is possibly attributed to covering effect of Au nanoparticles over ZnIn2S4 or incomplete reduction of Au3+. The corresponding conclusion was also supplemented by the EIS Nyqusit results. The semicircular diameter of EIS corresponds to the electrical resistance[46]. Always smaller semi-circle arc radius means more effective interface charge transfer and effective separation of photo-generated carriers [47]. As shown in Fig 6b, it is obviously 0.5 wt% Au-Pd/ZnIn2S4 have the smallest arc radius of Nyquist circle, indicating electronic resistance is weakest, which boost the speed of separation as well as transfer of photoinduced carriers[48]. While the inferior photocurrent and EIS results may be ascribed to the incomplete reduction of Au precursor. In addition, flat-band potentials of related photocatalysts are investigated by Mott-Schottky method as shown in Fig. 6c. All of the samples exhibited the characteristics of n-type semiconductors due to its positive value of tangent slope. Further, flat-band potential values can be estimated by intercept across the X-axis of fitting line in Mott-Schottky plots. Flat-band potentials of ZnIn2S4, Au/ZnIn2S4, Pd/ZnIn2S4 as well as Au-Pd/ZnIn2S4 are -0.52, -0.58, -0.33, -0.48 V vs. Ag/AgCl,



respectively. Thus, calculated CB values of corresponding photocatalysts are -0.32, -0.38, -0.13 and -0.28 V vs. NHE, respectively. As shown in Fig. 4b, Eg of the as-synthesized photocatalystt are 2.50, 2.25, 2.30 and 2.19 eV, respectively. Consequently, VB values of could be obtained to be 2.18, 1.87, 2.17 and 1.91 V vs. NHE as shown in Table. 1. The oxidation potential for benzyl alcohol to benzaldehyde is ca. +0.68 V (vs. NHE) according to previous reports[49]. Thus, selective oxidation of BA can be thermodynamically feasible in both monometallic and bimetallic nanoparticles supported ZnIn2S4 systems. Photoluminescence (PL) technique is a powerful tool to study separation and transfer efficiency of photoexcited electron-hole pairs. The higher the PL intensity, the stronger the recombination degree

[50].

The room temperature of PL spectra were

conducted with excitation wavelength of 432 nm. As demonstrated in Fig. 6d, Bare ZnIn2S4 shows many peaks due to its direct recommbination of excitons[11]. The intensity of Au/ZnIn2S4 remain almost the same as that of pristine ZnIn2S4 sample, which is consistent with the photoelectronchemical results and may be attributed to the incomplete reduction of Au3+. While the intensity of Pd/ZnIn2S4 as well as Au-Pd/ZnIn2S4 are much weaker than the pure ZnIn2S4, which indicated the introduction

of

bimetallic

nanoparticles

components

greatly

decrease

the

recombination probability of photoexcited carriers on ZnIn2S4. The formation of heterostructure between Au and Pd, and between Au-Pd nanoparticles and ZnIn2S4 support can effectively boost separation and transfer of the photoinduced carriers and thus inhibited recombination behavior, which can enhance the utilization of



photoinduced carriers for photocatalytic selective oxidation. [34]

3.2. Evaluation of Photocatalytic Activity and Stability

Bi-metal decorated ZnIn2S4 systems were used as photocatalysts for selective oxidation of benzyl alcohol under visible light irradiation. The conversion, selectivity and yield for oxidation reaction after 3 h visible light illumination over different photocatalysts with various metal molar ratio are summarized and presented in Table 2 and it should be noted that all decorated ZnIn2S4 photocatalysts show high selectivity (>99%) (Table 2). No overoxidation product like benzoic acid was generated. Both Au-Pd/ZnIn2S4 and Pd/ZnIn2S4 exhibited enhanced photocatalytic activity than that of pure ZnIn2S4. In addition, bimetallic photocatalysts Au-Pd/ZnIn2S4 showed the rather higher activity than single metal supported catalyst and the order is as follows: Au-Pd/ZnIn2S4  Pd/ZnIn2S4  ZnIn2S4  Au/ZnIn2S4 (Table 2). The restrained photocatalytic activity for 0.5 wt% Au/ZnIn2S4 can be ascribed to the incomplete reduction of Au3+, which resulted in the serious recombination of photoinduced carriers as proved by photoelectronchemical experimental results as well as PL results (Fig. 6a, 6b, 6d). Among bimetallic photocatalysts, 0.5 wt% Au-Pd(2:8)/ZnIn2S4 sample exhibited the highest conversion of 55.4%, which was 17.8%, 28.3% and 12.3% higher than pure ZnIn2S4, Au/ZnIn2S4 and Pd/ZnIn2S4, respectively. Furthermore, the addition amounts of Au-Pd nanoparticles have influence on the photocatalytic activities as illustrated in Table 3.



Among them, 0.5 wt% Au-Pd/ZnIn2S4 exhibits the highest conversion. In addition, conversion increase with increase of loading amount of Au-Pd when the total loading amount is below 0.5 wt%. However, photocatalytic activity decrease sharply with increase of Au-Pd amount.This results indicated that the excessive Au-Pd loading lead to the formation of recombination center of photoinduced carriers [49]. Furthermore, reaction solvents plays an important role for organic synthesis because of its polarity, dielectric constant, steric-hindrance, adsorption-desorption behavior of reactant and product as well as acid-base property. Various solvents with different

polarity

such

as

benzotrifluoride,

acetonitrile,

1,4-dioxane

and

sym-trimethylbenzene were investigated and the results were presented in Table 4 detailly. Of the four solvents tested, the highest conversion and selectivity (61.4%, 99.0%) can be reached using benzotrifluoride (PhCF3) as solvent (Table 4, entries 1-5), which could be attributed to its less polarity as well as better ability to dissolve oxygen molecule[51]. With increase of reaction time, the conversion increases gradually (Table 4, entries 2-5), the highest conversion of 90.6% can be achieved after 10 h (Table 4, entries 5). In contrast, employing polar solvents like acetonitrile lead to a drop in conversion, which may be ascribed to its competition effect with reactant for active site of photocatalysts

[52]

(Table 4, entries 6-10). Specifically,

conversion of 42.6% was achieved using acetonitrile as solvent for 3 h, final conversion of 85.6% was reached after reaction time of 10 h (Table 4, entries 6, 10). Moderate conversions of benzyl alcohol for benzyl aldehyde were obtained when



using mesitylene and 1,4-dioxane (Table 4, entries 11-20). The conversion using mesitylene was apparently higher than that of 1,4-dioxane, while its selectivity was lowest among all the four chosen solvents, which were more than 99%. A series of aromatic alcohol with different substitutes were conducted to study applicability of as-synthesized Au-Pd/ZnIn2S4 photocatalyst under visible light irradiation. Binary metallic nanoparticles decorated ZnIn2S4 photocatalyst exhibited excellent catalytic activity for various aromatic alcohols (4-methylbenzyl alcohol, 4-methoxybenzyl alcohol, 4-chlorobenzyl alcohol, p-nitrobenzyl alcohol and alpha-methylbenzyl alcohol) to corresponding aldehydes, suggesting 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalysts is a versatile catalyst for the reaction (Table 5). Conversion rate decreases in the following order : benzyl alcohol > 4-methoxybenzyl alcohol > 4-chlorobenzylalcohol > alpha-methylbenzyl alcohol > 4-methylbenzyl alcohol > 4-nitrobenzyl alcohol > n-butanol. Among them, the hydrogen of secondary alcohols such as alpha-methylbenzyl alcohol are reduced, which inhibited the formation of carbon centered radicals and prevented the process of selective oxidation of alcohol (Table 5, Entry 6)[53] . The conversion of -OCH3 is much larger than that of -CH3 due to its strong electron donating conjugation effect. While the conversion of -Cl is much larger than that of -NO2 due to its strong electron withdrawing inducing effect. However, the electron effect was weakened because of the competition interaction of electron conjugation effect with electron inducing effect each other because of the presence of lone pairs of electrons of oxygen atom [54]. On the other



hand, p-π conjugation was constructed due to the activation of delocalized large π bond on the benzene ring by the lone pair of electrons on the oxygen atom, thus the conversion of phenylethyl alcohol is slightly higher than that of n-butanol, which is hardly oxidized owing to the lack of π bond (Table 5, entry 7-8). The hydrogen of secondary alcohol such as 1-Phenethyl alcohol are reduced, which inhibited the formation of carbon centered radicals and prevented the process of selective oxidation reaction (Table 5, entry 6) [37]. The photocatalysts before and after reaction were conducted by XRD and TEM. As shown in Fig. 7(a), the XRD diffraction peaks of used 0.5 wt% Au-Pd/ZnIn2S4 remain the same as that of fresh photocatalysts. Also, noble metal nanoparticles are uniformly dispersed on the hexagonal ZnIn2S4 nanosheets, no aggregation phenomenon could be observed, which demonstrate that nanoparticles morphology of 0.5 wt% Au-Pd/ZnIn2S4 photocatalysts is well-maintained. In addition, the actual mass ratio of Au and Pd in used 0.5 wt% Au-Pd/ZnIn2S4 photocatalysts were tested as shown in Table 6, no loss of Au and Pd is observed.

3.3. Mechanism of enhanced photocatalytic activity

It is generally believed that direct hole oxidation and indirect hydroxyl radicals and/or superoxide radical oxidation are the main ways of photocatalytic selective oxidation on metal oxide photocatalysts [55]. To this end, scavenger experiments were conducted to reveal possible reaction mechanism for 0.5 wt% Au-Pd(2:8)/ZnIn2S4



photocatalyts. Various radical scavenger were employed including p-benzoquinone (p-BQ), ethylenediaminetetraacetate disodium (EDTA-2Na), isopropanol (IPA), butylated hydroxytoluene (BHT) ,CuCl2 and Catalase to remove the corresponding reactive radicals such as superoxide radicals •O2-, photoexcited holes h+, hydroxyl radicals •OH, carbon-centered radicals electron e- and H2O2, respectively[56]. O2 is an excellent photoelectron acceptor, which combines with electrons to produce superoxide radicals-•O2-. Therefore, it can inhibit the recombination behavior of photogenerated carriers

[57].

As shown in Table 7, the conversion was

tremendously inhabited by the addition of p-BQ, revealing extremely important role of •O2- (Table 7, entry 2). Photogenerated h+ also have important influence on conversion (Table 7, entry 3). The addition of IPA make conversion slightly decreased (Table 7, entry 4), indicating •OH would not play a necessary role in this system. It is believed carbon-centered radicals is necessary intermediate in photocatalytic selective oxidation. It was found the addition of BHT immensely restrained the conversion (55.4 % to 39.5 %, Table 7, entry 5). The addition of CuCl2 result in the slight decrease of conversion rate, indicating photoexcited hole would play alternative role beside photoexcited electron. A slight decrease to 49.9 % in the presence of catalase as H2O2 scavenger, suggesting the formed H2O2 did not involve in the reaction. Moreover, the presence of H2O2 species was confirmed by the spectrophotometric method as shown in Fig. 8, which is consistence with the previous reports



[45].

The above experimental results

suggested selective oxidation on surface of Au-Pd/ZnIn2S4 is governed by cooperative effects of •O2-, h+ and carbon-centered radicals. Furthermore, ESR technique were employed to deeply investigate reaction process

over

0.5

wt%

Au-Pd(2:8)/ZnIn2S4

photocatalyst,

5,

5-dimethyl-1-pyrroline-N-oxide (DMPO) as well as TEMPO were used as spin-trap reagent for ESR signals as shown in Fig. 9. There is no obvious ESR signals in the dark condition for DMPO adducts. As show in Fig. 9a, four-fold peaks of DMPO-•O2over pure ZnIn2S4 and 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalyst appeared under visible light illumination, obviously, intensity of ESR signals for 0.5 wt% Au-Pd(2:8)/ZnIn2S4 is much stronger than pure ZnIn2S4. It is consistent with previous trap experimental results, which further testify the important role of active oxygen radical of •O2- produced via one-electron reduction process from O2 [11]. Fig. 9b shows six-fold peaks with similar intensity of carbon centered radical for 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalyst[20]. Interestingly, no corresponding signals appeared for the pure ZnIn2S4 under same conditions, implying that carbon-centered radicals are necessary intermediate during reaction. Fig. 9c shows characteristic three-fold peak of the TEMPO-h+ adducts for pure ZnIn2S4 and 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalyst. It should be noted TEMPO is a paramagnetic material, which is often used to detect the hole signal in ESR. If a hole appears, TEMPO will be oxidized to diamagnetic material, which has no signal in ESR spectrum. Therefore, if TEMPO signal is weakened in ESR spectra, it indicates



that there are holes. The stronger the weakening intensity is, the higher the concentration of holes is. As shown in Fig. 9c, in the dark state, the signal peak belongs to TEMPO, the intensity of the corresponding adducts decrease after the light on for 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalyst obviously, in contrast, the peak intensity decreased by pure ZnIn2S4 photocatalyst was not obvious. It indicated that more

photogenerated

holes

was

generated

in

the

system

of

0.5

wt%

Au-Pd(2:8)/ZnIn2S4 photocatalyst compared with pristine ZnIn2S4. As shown in Fig. 9d, characteristic four-fold peaks of the DMPO-•OH adducts for 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalyst appeared clearly, while the peak intensity of pure ZnIn2S4 is almost invisible, which is attributed to the fact VB value of ZnIn2S4 (+2.18 V vs. NHE) (Table 1) is too negative to produce •OH (Eθ (OH-/•OH) (+2.40 V vs. NHE)) [58]. This suggests that •OH was indirectly produced from •O2-[59]. Possible photocatalytic selective oxidation mechanism of aromatic alcohols on Au-Pd/ZnIn2S4 photocatalyst based on results above is proposed as show in the Scheme 1. When the visible light irradiation is imposed to the Au-Pd/ZnIn2S4 composite, ZnIn2S4 could be excited to produce photogenerated electron and holes, electron of ZnIn2S4 would subsequently migrate to Au-Pd nanoparticles across Schottky barrier, which is beneficial for separation of photoexcited carriers and boost the catalytic activity [11]. On the other hand, hot electron of Au because of SPR effect would inject into Pd nanoparticles, which possess superior adsorption and activation ability for molecule oxygen [45, 60, 61] (Scheme 1a). Thus, molecule oxygen is reduced



to •O2- radicals by capture of excess electron. In addition, carbon-center radical are formed by the oxidation and deprotonation reaction of photoexcited holes with alkoxide anions[62] . Simultaneously, •O2- trap a proton to produce HO2• and further trapping of proton to generate H2O2. The H2O2 intermediate is either testified directly by spectrophotometric method[63]. Finally, the carbon radicals is oxidized by photogenerated holes and produce benzaldehyde as sole product (Scheme 1b). Benzaldehyde would desorb from the surface of Au-Pd/ZnIn2S4 photocatalyst and complete the recycling of catalyst.

4. Conclusion A series of Au-Pd/ZnIn2S4 composite photocatalysts with different metal amounts were prepared and applied for photocatalytic selective oxidation of aromatic alcohol. UV-vis-DRS, PL as well as photoelectrochemical tests demonstrated its enhanced visible-light harvesting capability as well as superior photoinduced carriers’s separation and transfer behavior. The 0.5 wt% Au-Pd/ZnIn2S4 photocatalysts exhibit the highest photocatalytic activity, specifically, it exhibited higher benzaldehyde yield of 54.85%, which is 1.5, 2.0 and 1.3 times higher than pristine ZnIn2S4, Au/ZnIn2S4 and Pd/ZnIn2S4 respectively. Various reaction parameters such as metal loading ratio and amount, solvents, time and reactant were also investigated to optimize the results. Deeply, scavengers experiments and ESR technique confirmed the necessary role of h+, •O2- and carbon centered radicals for the photocatalytic selective oxidation over 

Au-Pd/ZnIn2S4. Possible reaction mechanism is proposed based on experimental results. Therefore, superior photocatalytic performance can be attributed to improved visible light adsorption capacity, photoinduced carrier’s separation efficiency, enhanced O2 adsorption and activation ability of Pd and surface reaction kinetics, which is attributed to dual interfacial synergism between Au and Pd, Au-Pd and ZnIn2S4 semiconductor. Consequently, construction of dual interfacial synergism photocatalysts is an effective strategy for developing high-performance photocatalyst, which promote the production, separation and utilization of photoexcited carriers in solar energy driven organic synthesis.

Acknowledgements We are grateful for financial support from Natural Science Foundation of Shandong Province, China (ZR2019BB044); NSFC (21403257, 21503243); Qingdao University Scientific Research Fund for Young Excellent Talents (DC1900003174); Postdoctoral Research Project of Qingdao (861605040004), China.

Conflict of Interest The authors declare no conflict of interest.



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Figure Captions Fig. 1. XRD patterns of pristine ZnIn2S4, 0.5 wt% Au/ZnIn2S4, 0.5 wt% Pd/ZnIn2S4 and a range of different molar ratios of 0.5 wt% Au-Pd/ZnIn2S4 photocatalysts. Fig. 2 SEM images of photocatalysts: (a) pure ZnIn2S4; (b) 0.5 wt% Au/ZnIn2S4; (c) 0.5 wt% Pd/ZnIn2S4; (d) 0.5 wt% Au-Pd (2:8)/ZnIn2S4; (e) 1.0 wt% Au-Pd (2:8)/ZnIn2S4; (f) 3.0 wt% Au-Pd (2:8)/ZnIn2S4. Fig. 3 TEM and HRTEM images of photocatalysts: (a) pure ZnIn2S4; (b), (c) 0.5 wt% Au-Pd (2:8)/ZnIn2S4; (d) HRTEM of 0.5 wt% Au-Pd (2:8)/ZnIn2S4. (e) elemental mapping images of 0.5 wt% Au-Pd (2:8)/ZnIn2S4 sample. Fig. 4. (a) UV-Vis DRS spectra (b) Estimated Eg plots of corresponding photocatalysts. Fig. 5. XPS spectra of ZnIn2S4 and 0.5 wt% Au-Pd(2:8)/ZnIn2S4 samples. (a) Survey spectra; (b) high-resolution XPS spectra of Zn 2p; (c) In 3d; (d) S 2p; (e) Au 4f; (f) Pd 3d. Fig. 6 (a) Photocurrent response; (b) EIS Nyquist plots; (c) Mott-Schottky plots under visible light irradiation (d) PL spectra. Fig. 7 (a) XRD spectra of 0.5 wt% Au-Pd/ZnIn2S4 photocatalyst before and after reaction and (b, c, d) TEM images after reaction. Fig. 8. (a) UV-vis absorption spectra. (b) photograph of reactant solution with adding C8H5KO4 and KI aqueous solution before (1) and after (2) irradiation for 5 min.



Fig 9. in situ ESR of reactive radicals in benzyl alcohol suspensions with pure ZnIn2S4 and 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalysts, respectively. Scheme 1 (a) A schematic diagram of the formation of Au-Pd/ZnIn2S4 composites; (b) Proposed possible photocatalytic selective oxidation reaction pathway for Au-Pd/ZnIn2S4 composites.

Table Captions Table 1. Physical Parameters in photocatalysts. Table 2. Effect of decorated metal ratio on catalytic activitya. Table 3. Effect of decorated metal amount on catalytic activitya. Table 4. Effect of solvent on catalytic activity over 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalystsa. Table 5. Applicability of 0.5 wt% Au-Pd(2:8)/ZnIn2S4 photocatalysts to different substratesa. Table 6. Actual mass ratio of Au and Pd before and after reaction. Table 7. Effect of various radicals quencher on reaction performancea.



Photocatalysts

VB

CB (V

Flat-band

Fermi

Band

(Vvs.

vs.

potential (V

Level

gap (eV)

NHE )

NHE)

vs. Ag/AgCl)

(vs. NHE)

ZnIn2S4

2.18

-0.32

-0.52

-0.12

2.50

0.5 wt% Au/ZnIn2S4

1.87

-0.38

-0.58

-0.18

2.25

0.5 wt% Pd/ZnIn2S4

2.17

-0.13

-0.33

0.07

2.30

0.5 wt%

1.91

-0.28

-0.48

-0.08

2.19

Au-Pd(2:8)/ZnIn2S4

Table 1



Entry

Photocatalyst

Conversion (%)

Selectivity

Yield (%)

(%)

a

1

ZnIn2S4

37.6

99

37.2

2

Pd/ZnIn2S4

43.1

99

42.7

3

Au-Pd(1:9)/ZnIn2S4

40.8

99

40.4

4

Au-Pd(2:8)/ZnIn2S4

55.4

99

54.9

5

Au-Pd(3:7)/ZnIn2S4

49.0

99

48.5

6

Au-Pd(4:6)/ZnIn2S4

49.4

99

48.9

7

Au-Pd(5:5)/ZnIn2S4

45.8

99

45.3

8

Au-Pd(6:4)/ZnIn2S4

42.3

99

41.8

9

Au-Pd(8:2)/ZnIn2S4

37.7

99

37.4

10

Au/ZnIn2S4

27.1

99

26.8

Reaction conditions: 10 mL benzotrifluoride, 50 mg photocatalyst, 0.25 mmol benzyl

alcohol, visible light, 3 h, O2 atmosphere. 

Table 2

Entry

a

Photocatalyst

Conversion

Selectivity

(%)

(%)

Yield (%)

1

0.05 wt% Au-Pd(2:8)/ZnIn2S4

38.1

99

37.7

2

0.10 wt% Au-Pd(2:8)/ZnIn2S4

51.9

99

51.3

3

0.50 wt% Au-Pd(2:8)/ZnIn2S4

55.4

99

54.8

4

1.00 wt% Au-Pd(2:8)/ZnIn2S4

35.1

99

34.8

5

3.00 wt% Au-Pd(2:8)/ZnIn2S4

27.5

99

27.2

Reaction conditions: 10 mL benzotrifluoride, 50 mg photocatalyst, 0.25 mmol benzyl

alcohol, visible light, 3 h, O2 atmosphere. Table 3



Entry

Solvent

Time/h

Conversion (%)

Selectivity (%)

Yield (%)

1

BTF

3

61.4

99

60.7

2

BTF

4

71.6

99

70.9

3

BTF

6

79.1

99

78.3

4

BTF

8

84.3

99

83.5

5

BTF

10

90.6

99

89.7

6

ACN

3

42.6

99.0

42.2

7

ACN

4

53.6

99.0

53.1

8

ACN

6

67.7

99.0

67.0

9

ACN

8

77.8

99.0

77.0



a

10

ACN

10

85.6

99.0

84.7

11

Mesitylene

3

41.6

25.0

10.4

12

Mesitylene

4

46.6

24.6

11.5

13

Mesitylene

6

53.4

23.8

12.7

14

Mesitylene

8

60.2

23.0

13.8

15

Mesitylene

10

64.4

22.0

14.2

16

1,4-dioxane

3

0

0

0

17

1,4-dioxane

4

9.2

99.0

9.1

18

1,4-dioxane

6

17.2

99.0

17.0

19

1,4-dioxane

8

23.0

99.0

22.8

20

1,4-dioxane

10

28.6

99.0

28.3

Reaction conditions: 10mL benzotrifluoride, 80 mg photocatalyst, 0.25 mmol benzyl

alcohol, visible light for 3~10 h, O2 atmosphere. Table 4



Substrate

Production

1 OH

2

H3C OH

H3C

Yield(%)

90.6

100

90.6

57.7

100

57.7

O

82.1

93.6

76.8

69.4

94.5

65.6

O

H3C

O

OH

4

Selectivity(%)

O

H3C

3

Conversion (%)

O

Cl

Cl OH

OH



5

O O

O

N

O

CH3

OH

H 3C

a

OH

59.2

88.1

52.2

4.4

100

4.4

0

0

0

O

O

OH

8

39.5

O

CH3

7

100

N

OH

6

39.5



Reaction conditions: aromatic alcohol (0.25 mmol), 0.5 wt% Au-Pd/ZnIn2S4

photocatalyst (80mg), benzotrifluoride (10 mL), 1 bar O2, visible light, RT, 10 h. Table 5

Entry

Au/wt%

Pd/wt%

Theoretical content

0.15

0.34

ICP-MS (before)

0.13

0.30

ICP-MS (after)

0.12

0.30

Table 6



Entry

Scavenger

Conversion (%)

Selectivity (%)

1

-

55.4

99

2

BQ

2.0

99

3

EDTA-2Na

36.1

99

4

IPA

47.8

99

5

BHT

39.5

99



a

6

CuCl2

50.0

99

8

Catalase

49.9

99

Reaction conditions: 10 mL benzotrifluoride, 50 mg photocatalyst, 0.25 mmol benzyl

alcohol, visible light, 3 h, 1 bar O2. Table 7

Graphical Abstract





Highlights

 Au-Pd/ZnIn2S4 photocatalyst was synthesized by the NaBH4 reduction method.  Photocatalyst showed excellent activity for photocatalytic selective oxidation.  Interfacial synergism effect was found to be beneficial for reaction.  The reactive radicals was confirmed by ESR and trap experiments.  Possible mechanisms for enhanced photocatalytic activity were proposed.