Boosting Charge Transfer Efficiency by Simultaneously Tuning Double

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Boosting Charge Transfer Efficiency by Simultaneously Tuning Double Effects of Metal Nanocrystal in Z-Scheme Photocatalytic Redox System Keyi Jiang, Xiaocheng Dai, Yan Yu, Qiaoling Mo, and Fang-Xing Xiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02895 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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The Journal of Physical Chemistry

Boosting Charge Transfer Efficiency by Simultaneously Tuning Double Effects of Metal Nanocrystal in Z-Scheme Photocatalytic Redox System Ke-Yi Jianga,b,# Xiao-Cheng Daia,# Yan Yu*a,b, Qiao-Ling Moa,b, Fang-Xing Xiao*a a

College of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350108, China. b

Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), China. *Corresponding author. Fax: +86 591 22866534 E-mail address: [email protected] [email protected] # These two authors contribute equally to this work.

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Abstract Integrating individual functional materials into elegant nano-architectures holds great promise for creating high-efficiency photosynthesis systems with unique structure-directing merits. Herein, an allsolid-state metal-based Z-scheme photocatalytic system consisting of well-defined one-dimensional (1D) WO3@Au@CdS core–shell heterostructure has been progressively and rationally designed by a green and facile two-step wet-chemistry approach. Significantly, it was uncovered that Au ingredient sandwiched in-between the interfacial domain of WO3 and CdS layer play simultaneous dual roles in boosting the visible-light-driven photoactivities of core-shell ternary heterostructure, that is, interfacial charge transfer mediator to expedite vectorial Z-scheme electron transfer between CdS and WO3 and plasmonic photosensitizer to trigger the generation of plasmon-induced hot electrons, thereby substantially augmenting the photoelectron density in photoredox catalytic system. Such cooperative concurrent dual roles of Au nanocrystal in Z-scheme photocatalytic system results in the versatile and considerably

enhanced

photoredox

performances

of

plasmonic

WO3@Au@CdS

core-shell

heterostructure toward anaeronic reduction of aromatic nitro compounds to corresponding aminos and mineralization of organic pollutants under visible light irradiation at ambient conditions. Moreover, predominant active species during the photoredox catalysis were accurately determined, based on which photocatalytic mechanism was reasonably deduced and clearly elucidated. This work would provide a quintessential paradigm to uncover the essential roles of metal nanocrystals along with their cooperative synergy in Z-scheme photocatalytic system for substantial solar energy conversion.

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1. Introduction Heterogeneous photocatalysis based on semiconductors represents a fascinating technique to achieve highly efficient solar energy conversion to chemical energy. The efficiency of solar energy conversion is heavily dependent upon light absorption, charge separation, migration and recombination in photocatalysts, with an ideal photocatalyst excelling in all these four aspects.1, 2 In recent years, worldwide efforts have been devoted to developing diverse strategies to fabricate efficient photocatalysts with high solar conversion efficiency.3-5 Among which, Z-scheme photocatalytic systems have been gaining particular attention due to their great potential for mimicking natural photosynthesis process.6-10 Up to date, there have been primarily three categories of Z-scheme photosynthetic systems, including direct Z-scheme without an electron mediator,11, 12 Zscheme with redox mediators in solution,13, 14 and three-component (3C) all-solid-state Z-scheme.6, 15 Particularly, 3C all-solid-state Z-scheme photocatalytic system demonstrates unique merits in facilitating the vectorial electron transfer process16 and avoiding backward reactions involving redox mediators17 over its two counterparts, which endows it with more possibilities to alleviate the energy and environmental crises.8 Metal nanocrystals (NCs) have been regarded as a pivotal class of nanomaterials for photocatalysis,18-22 owing to their unique surface plasmon resonance (SPR) effect without the bandgap restriction of traditional semiconductors as well as their essential roles as electron traps for capturing photoelectrons during the photocatalytic reactions.23-28 Thus far, a large variety of metalsemiconductors nanomaterials have been explored for photocatalytic applications such as hydrogen production, mineralization of organic pollutants, CO2 reduction and selective organic 3

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transformation.2, 29-37 Albeit the progress, how to fully harness their intrinsic dual roles of metal NCs as charge transfer mediators and SPR photosensitizers and, simultaneously, maneuvering their cooperativity in a Z-scheme photocatalytic reaction system is still challenging. Tungsten oxide (WO3), as a direct n-type semiconductor with a favorable band gap (2.4-2.8 eV)38, 39

has been extensively investigated for photocatalytic applications due to its intrinsic merits

including nontoxicity,40 easy synthesis,41 stable physicochemical properties,42, 43 resistance to photocorrosion24 and high oxidation power of valence band (VB) holes. In particular, one-dimensional (1D) WO3 nanostructures hold great promise for constructing varied photocatalytic systems in comparison with nanoparticulate and bulk counterparts due to 1D nanostructure-directing advantages, such as large specific surface area,44 efficient light harvesting owing to the strong internal light absorption and scattering properties,37 minority carrier diffusion,45-47 Nevertheless, photocatalytic application of WO3 is hindered by several drawbacks: (i) relatively wide band gap (~2.85 eV) makes it response to narrow light absorption range of solar spectrum; (ii) defective vacancies on the WO3 inevitably lead to high recombination rate of surface charge carrier resulting in decreased photoactivity; (iii) slow charge transfer rate and sluggish kinetics of charge carriers.48-51 These issues remarkably retard the potential applications of WO3 in photocatalysis. Therefore, it is of paramount importance to develop effective strategies to surmount these disadvantages of WO3 especially in an integrated system for boosted photocatalytic performances. To this end, deposition with noble metal, hybridization with second semiconductors, doping with metal element or non-metal elements, and encapsulation with carbon materials have been developed to reinforce the light absorption of WO3.49,

52, 53

As

corroborated by our previous work,29 combination of WO3 nanorods (NRs) with plasmonic metal 4


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NCs (e.g., Au) is conducive to expedite the photoelectron transfer in the CB of WO3 and meanwhile producing highly active hot electrons54, which circumvents the sluggish kinetic of photoelectrons in the CB of WO3 and increases the electron density in reaction system, thus concurrently contributing to the significantly enhanced photoredox catalytic performances of metal/WO3 heterostructure in comparison with pristine 1D WO3 under simulated solar and visible light irradiation. Nonetheless, visible-light-driven photocatalytic performances of plasmonic metal/WO3 heterostructure are still far from satisfactory. Inspired by Tada’s work 6 which proposed the concept of all-solid-state Z-scheme photocatalysis by creatively designing a sandwich-structured CdS/Au/TiO2 photocatalyst, in which Au ingredient was deposited in the interfacial region of CdS and TiO2 and acted as electron transfer mediator. In this Z-scheme configuration, electrons in conduction band (CB) of TiO2 could transfer through the Au and recombine with the holes left in the valance band (VB) of CdS. As a result, electrons and holes in the CB of CdS and VB of TiO2 are involved in the reduction and oxidation reactions, respectively. Compared with conventional p-n heterojunction photocatalysts, Z-scheme photocatalytic system remains the strong reduction and oxidation capability of the both semiconductors.55-58 There are general three points that should be considered to rationally design the Z-scheme photocatalytic system, (i) two semiconductors and metals NCs used in Z-scheme system should possess visible light response ability which enables them to capture more photons; (ii) the semiconductors in this system should have high redox stability in photocatalytic reaction; (iii) metal component anchored in-between the interfacial region of two semiconductors should be intimately integrated with both semiconductors. Considering the intrinsic merits of 1D WO3 and plasmonic metal NCs, it is highly desirable to construct well-defined metal/1D WO3 based Z-scheme 5



photocatalytic system for multifarious photocatalytic applications. Herein, we have reported the fabrication of plasmonic metal-based Z-scheme photocatalytic system composed of core-shell WO3 NRs@Au@CdS ternary heterostructure which was progressively designed via a facile and green two-step wet chemistry approach. In this ternary hybrids, WO3 NRs were used as the fundamental scaffold and Au NPs were first deposited on the WO3 NRs substrate via an electrostatic self-assembly strategy and, subsequently, CdS layer was insitu grown on the WO3 NRs@Au framework via an efficient photo-deposition approach forming 1D core–shell ternary heterostructure. It was unveiled that WO3 NRs@Au@CdS heterostructure was unraveled to exhibit significantly enhanced and versatile photoredox catalytic performances toward selective reduction of aromatic nitro compounds and mineralization of organic pollutants under visible light irradiation as compared with single and binary counterparts. Synergistic effect dual roles of Au NPs as “electron traps” and SPR photosensitizers facilitate the vectorial Z-scheme electron transfer in the interfacial region and, simultaneously, production of energetic hot electrons in such ternary heterostructured system. It is anticipated that our work could offer a conceptual avenue to rationally construct highly efficient plasmonic metal-based Z-scheme photocatalytic systems for solar energy conversion.

2. Experimental section 2.1 Synthesis of Au@4-Dimethylaminopyridine (DMAP) nanoparticles (NPs) Synthesis of Au@DMAP was referred to a previously published method.58 Prior to experiment, all glassware were cleaned thoroughly with aqua (3:1 in volume for HCl and HNO3) for 12 h and thoroughly washed with Deionized (DI) H2O. A 30 mM aqueous chloroauric acid solution (HAuCl4, 6


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30 mL) was added to a 25 mM tetraoctylammonium bromide (TOAB) solution in toluene (80 mL). The transfer of the gold salt to the toluene phase occurs within a few seconds. A freshly prepared 0.4 M NaBH4 (25 mL) aqueous solution was added to the above mixture under vigorous stirring, which caused an immediate color change from light yellow to dark red. After 30 min the two phases were separated and the toluene phase was subsequently washed with 0.1 M H2SO4, 0.1 M NaOH, and DI H2O for three times, and then dried with anhydrous Na2SO4. Afterwards, an aqueous 0.1 M DMAP solution (80 mL) was added to aliquots (80 mL) of the as-prepared nanoparticle mixtures. Direct phase transfer across the organic/aqueous interface was completed within 2 h with no stirring or agitation required. Finally, Au-dissolved water phase was separated by a separatory funnel and Au@DMAP aqueous solution (2.22 mgꞏmL-1) was thus obtained. 2.2 Preparation of WO3 nanorods (NRs) WO3 NRs were synthesized by a one-step hydrothermal treatment. Typically, 1.056 g of Na2WO4ꞏ2H2O and 0.935 g of NaCl were dissolved in 30 mL of DI H2O and kept stirring for 20 min. The pH value of the mixed solution was adjusted to be ca. 2 using HCl (3.0 M) aqueous solution and then the solution was vigorously stirred for 2 h. After that, the solution was transferred to a Teflonlined autoclave (100 mL) and maintained at 180 °C for 24 h. Afterwards, the products were cooled to room temperature, separated by centrifugation, and washed with DI H2O. Eventually, the sediment was dried at 60 °C. 2.3 Self-assembly of binary WO3 NRs@Au heterostructure Au@DMAP-WO3 NRs nanocomposites were fabricated by an electrostatic self-assembly method. Specifically, the as-prepared Au@DMAP aqueous solution (2.22 mgꞏmL-1) was diluted to 0.05 7



mgꞏmL-1, after which given volume of positively charged Au@DMAP aqueous suspension (0.05 mgꞏmL−1, pH=10) was added dropwise to the negatively charged WO3 NRs aqueous dispersion (0.1 mgꞏmL−1, pH=10). The mixture was stirred for 2 h and the precipitation was centrifuged, washed with DI H2O, and dried in an oven at 60 °C. Finally, the samples were calcinated at 300 °C in air for 1 h. 2.4 Synthesis of ternary WO3 NRs@Au@CdS core-shell heterostructure CdS shell layer was coated on the binary WO3 NRs@Au by a photodeposition approach. In a typical procedure, 40 mg of CdCl2ꞏ2.5H2O and 10 mg of S8 were dispersed in 100 mL of ethanol to form a stable suspension and bubbled with N2 for 30 min in the dark. The Au/WO3 NRs nanocomposite was immersed in the above suspension and irradiated for different time under simulated solar light (300 W Xe arc lamp). The samples were rinsed with ethanol and DI H2O for three times and dried in an vacuum oven at 60 oC. 2.5 Characterization Zeta potentials (ξ) were probed by dynamic light scattering analysis (ZetasizerNano ZS-90). Crystal structure was studied by X-ray diffraction (XRD, X'Pert Pro MPD, Philips, Holland) using Cu Kα as radiation source in the 2θ range from 10 to 80o at a scan rate of 2o s-1 under 40 kV and 40 mA. Transmission electron microscopy (TEM) and high-resolution (HR) TEM images and energy dispersive X-ray spectrum (EDX) were collected on a JEOL-2010 transmission electron microscope with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a photoelectron spectrometer (Escalab 250, Thermo Scientific, America), binding energy (BE) of the elements was calibrated based on the BE of carbon (284.60 eV). UV-vis diffuse 8


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reflectance spectra (DRS) (Varian Cary 500 UV-vis spectrophotometer, Varian, America) were obtained using BaSO4 as the reflectance background ranging from 300 to 600 nm. Morphologies were probed by a field-emission scanning electron microscopy (FESEM, Philips XL-30, Philips, Holland). Electron paramagnetic resonance spectra (EPR) were recorded on a Bruker A-300-EPR Xband spectrometer. Brunauer-Emmett-Teller (BET) specific surface areas were determined by a quantachrome autosorb-1-C-TCD automated gas sorption analyzer. Photoluminescence (PL) spectra were collected on a Varian Cary Eclipse spectrometer. The generation of hydroxyl radicals in aqueous solution was detected by a photoluminescence (PL) technique with terephthalic acid (TA) as a probe molecule. Hydrogen peroxide (H2O2) was determined by a photometric method in which N, N-diethyl-p-phenylenediamine (DPD) is oxidized by a peroxidase (POD)-catalyzed reaction. 2.6 Photoelectrochemical (PEC) measurements PEC measurements were performed on an electrochemical workstation (CHI660E, IM6, Zahner Germany). The electrochemical setup is composed of conventional three electrodes, a quartz cell containing 40 mL Na2SO4 (0.2 M) aqueous solution and a potentiostat. A platinum wire electrode was used as counter electrode and Ag/AgCl as reference electrode. The sample films (5 mm × 5 mm) were vertically dipped into the electrolyte and irradiated with visible light (λ≥420 nm). 2.7 Photoredox catalytic performances (a) Photocatalytic reduction performances Photocatalytic selective reduction of aromatic nitro compounds to the corresponding amines was performed in a quartz vial at ambient conditions. 20 mg of the catalyst was uniformly dispersed in 40 mL of aromatic nitro compounds (10 ppm) aqueous solution with adding 40 mg of ammonium 9



oxalate as hole scavenger. Before light irradiation, the suspension was stirred in the dark for 0.5 h to establish the adsorption–desorption equilibrium between reactant and the sample. Then, the above mixture was irradiated with a 300W Xe arc lamp with a UV-CUT filter to cut-off the light with wavelength less than 420 nm. During the reaction, 3 mL of the sample solution was taken from the reaction system at every 3 min interval and centrifuged at 11 000 rpm for 5 min to remove the catalyst. The remaining supernatant was analyzed on a Varian UV-vis spectrophotometer (UV-3600, Shimadzu). The entire experimental process was carried out under N2 bubbling at a flow rate of 60 mL min-1 under ambient conditions. Action spectra were obtained by probing the photoreduction performances of samples under different monochromatic light (365, 380, 400, 450, 500, 550 and 600 nm) with light density of ca. 150 mW/cm2. (b) Photocatalytic oxidation performances Photocatalytic oxidation performances were evaluated by degradation of Rhodamine B (RhB) which was frequently used as a model organic dye pollutant at ambition conditions. Specifically, 20 mg of catalyst was dispersed in 40 mL of RhB aqueous solution (5 ppm). Prior to irradiation, the reaction system was stirred in dark for 0.5 h to achieve the adsorption-desorption equilibrium between the catalyst and the reactant. Then, the suspension was irradiated with simulated solar (300 W Xe lamp with wavelength ranging from 200 to 800 nm) or visible light (λ ≥ 420 nm). During the photocatalytic reaction, 5 mL of the sample solution was withdrawn at every 30 min interval and centrifuged to remove the catalyst. The residual RhB solution was analyzed by monitoring the variation of its characteristic light absorption at 554 nm using a UV-vis/NIR spectrophotometer (UV3600, Shimadzu). Conversion of the reaction was determined according to the formula below: 10


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Where C0 was the original concentration of the reactant and C is the concentration of the reactant at every 30 min interval during the reaction. 2.8 Determination of •OH radicals and H2O2. (a) Determination of •OH radicals by a photoluminescence (PL) technique The experimental procedure was similar to the photo activity test for dye degradation and the only difference is the dye solution was replaced by 5 × 10-3 M terephthalic acid aqueous solution containing 0.01 M NaOH. After reaction for every 30 min, the sample solution was centrifuged and analysis by PL spectrum. (b) Determination of H2O2 by a photometric method 20 mg catalyst was added into 40 mL of DI H2O and stirred in dark for 30 min to achieve adsorptiondesorption equilibrium. Afterward, the reaction was irradiated under visible light (λ > 420 nm) for 150 min. The sample was collected and centrifuged (1000 rpm) for every 30 min to obtain the supernant solution, in which 50 µL of DPD was added and shaked manually followed by adding 50 µL of POD. After 2 min, the final solution was analyzed by UV-absorption spectra to determine whether two peaks at 510 and 551 nm appear in the UV-vis region ranging from 400 to 600 nm.

3. Results and discussion

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Scheme 1 Schematic illustration of the fabrication of WO3 NRs@Au@CdS core-shell ternary heterostructures. WO3 NRs@Au@CdS ternary core-shell heterostructures were constructed by a facile two-step wet-chemistry approach including electrostatic self-assembly of Au NPs on the WO3 NRs framework followed by in-situ photo-deposition of CdS layer on the WO3 NRs@Au surface, as illustrated in Scheme 1. As shown in Fig. S1(a & b), Au@DMAP prepared by phase transformation method exhibited a mean diameter of ca. 6.0 nm. Specifically, WO3 NRs and Au NPs were selected as the building blocks in the first step in terms of their characteristic intrinsic oppositely charged surfaces. Notably, WO3 NRs and Au@DMAP colloidal suspensions used in our work are substantially negatively and positively charged (Fig. S2), respectively. In particular, DMAP ligands capped on the Au NPs surface can form a labile donor-acceptor complex with surface Au atoms via endocyclic nitrogen atoms and surface charge arising from partial protonation of the exocyclic nitrogen atom,59 which induces intrinsic positively charged surface of Au NPs. In this regard, attracted by the substantial electrostatic interaction, Au@DMAP can be spontaneously and uniformly self-assembled on the WO3 NRs with deposition percentage of Au NPs finely tuned by addition amount. Subsequently, Au@DMAP-WO3 NRs nanocomposite was calcined to reinforce the interfacial interaction, thus resulting in the WO3 NRs@Au binary heterostructures. Finally, uniform growth of 12


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CdS layer on the WO3 NRs@Au scaffold was achieved by a facile photo-deposition method under simulated sunlight irradiation, which gives rise to well-defined WO3 NRs@Au@CdS core-shell ternary heterostructures. 3.1 Characterizations of core-shell nanocomposites

Fig. 1 XRD patterns of (a) pristine WO3 NRs, (b) WO3 NRs@Au, (c) WO3 NRs@CdS binary and (d) WO3 NRs@Au@CdS ternary heterostructures. (W: WO3, A: Au, C: CdS) Crystalline structures were probed by X-ray diffraction (XRD). As shown in Fig. 1a, the peaks at 2θ values of 14.0°, 22.7°, 24.3°, 26.8°, 28.2°, 36.6°, 50.0° and 55.3° correspond to the (100), (001), (110), (101), (200), (201), (220), and (202) crystal planes of hexagonal-phase WO3 (a=b=7.298 Å and c=3.899 Å, JCPDS card no. 75–2187.), respectively. The strong and sharp diffraction peaks suggested high crystallinity of WO3 NRs. Note that diffraction peaks of WO3 can be seen in the XRD patterns of all samples and this indicates WO3 NRs structure was retained after Au NPs and CdS deposition. Besides, the peaks at 38.2° and 44.4° corresponding to the (111) and (200) crystallographic planes of cubic Au (JCPDS Card No. 04-0784) in the XRD patterns of WO3 13



NRs@Au (Fig. 1b) and WO3 NRs@Au@CdS (Fig. 1d) were clearly observed, confirming Au NPs have been self-assembled on the WO3 NRs. Notably, the peaks at 43.9o in the XRD patterns of WO3 NRs@CdS (Fig. 1c) and WO3 NRs@Au@CdS (Fig. 1d) can be accurately indexed to the (220) crystallographic planes of wurtzite CdS (JCPDS card No. 65-2887), corroborating the successful encapsulation of CdS layer on the surfaces of WO3 NRs and WO3 NRs@Au.

Fig. 2 FESEM images of (a) WO3 NRs@Au binary and (d) WO3 NRs@Au@CdS ternary heterostructures; TEM and HRTEM images of (b & c) WO3 NRs@Au binary and (e & f) WO3 NRs@Au@CdS ternary heterostructures; (g & h) STEM image of WO3 NRs@Au@CdS ternary heterostructure with corresponding elemental mapping results. Microscopic structures and morphologies were probed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Typical FESEM (Fig. S4a) and low-magnified TEM images (Fig. S3b-c) show that pristine WO3 NRs possess mean diameter and length of ca. 100 nm and 5 µm, respectively. As displayed in Fig. 2(a & b), Au NPs with diameter of 14


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ca. 6 nm were uniformly distributed on the WO3 NRs surface and this indicates electrostatic selfassembly strategy developed in our work is efficacious to combineWO3 NRs with Au.29 HRTEM image in Fig. 2c reveals the intimate interfacial contact between Au NPs and WO3 NRs and the lattice fringes of 0.235 and 0.384 nm correspond to the (111) and (001) crystallographic planes of Au and WO3, respectively. FESEM and TEM images of WO3 NRs@Au@CdS in Fig. 2(d & e) clearly demonstrate the encapsulation of a thin and uniform CdS layer (ca. 15 nm) on the WO3 NRs@Au surface forming a core-shell heterostructure. As displayed in Fig. 2(f & g), HRTEM image of WO3 NRs@Au@CdS demonstrates well-defined core-shell structure with CdS layer uniformly enwrapped on the outer surface and Au NPs integrated in-between the interfacial layer of CdS and WO3. The lattice fringes of ca. 0.235, 0.245 and 0.384 nm are accurately attributed to the (111), (220) and (001) crystal planes of cubic Au, wurtzite CdS and hexagonal-phase WO3, respectively. Elemental mapping results (Fig. 2h) corroborate the co-existence of W, O, Au, Cd and S elements in ternary heterostructure and moreover, distribution patterns of these elements additionally verify the coreshell architecture of WO3 NRs@Au@CdS ternary heterostructure.

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Fig. 3 Survey spectrum and high-resolution XPS spectra of (b) W 4f, (c) O 1s, (d) Cd 3d, (e) S 2p, and (f) Au 4f for WO3 NRs@Au@CdS ternary heterostructure. Surface composition and elemental chemical states were explored by X-ray photoelectron spectroscopy (XPS). Survey spectrum of WO3 NRs@Au@CdS ternary heterostructure (Fig. 3a) reveals the substantial W, O, Cd, S and Au signals, which suggests the co-deposition of Au NPs and CdS on the WO3 substrate and this is in line with elemental mapping results aforementioned. As 16


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shown in Fig. 3b, high-resolution W 4f spectrum of WO3 NRs@Au@CdS heterostructure showed two characteristic peaks at 35.36 and 37.5 eV for W 4f7/2 and W 4f5/2 and this indicated W element is in the +6 oxidation state.53 High-resolution O 1s spectrum (Fig. 3c) shows that the peak at 530.09 eV is associated with lattice oxygen (W-O) and the other at 531.58 eV is attributable to hydroxyl groups arising from H2O adsorption.60 The analogous results were observed in the high-resolution O 1s (Fig. S4b) and W 4f (Fig. S4c) spectra of WO3, WO3@Au and WO3@CdS nanostructures, implying structure and elemental chemical states of WO3 scaffold were retained during the self-assembly and photo-deposition process. As shown in Fig. 3d, high-resolution Cd 3d spectrum of WO3 NRs@Au@CdS heterostructure displayed doublet peaks at 404.76 and 411.50 eV with a spin-orbit separation of 6.74 eV, which corresponds well to the Cd2+. The peaks at 161.55 and 162.70 eV (Fig. 3e) in the high-resolution S 2p spectrum of WO3 NRs@Au@CdS heterostructure is attributed to the S2−. Both the high-resolution spectra of Cd 3d and S 2p signify in-situ growth of CdS on the WO3 NRs@Au via a photo-deposition approach. Fig. 3f shows the high-resolution Au 4f spectrum of WO3 NRs@Au@CdS heterostructure in which two pronounced peaks at 83.9 and 87.6 eV corresponding to Au 4f7/2 and Au 4f5/2 can be accurately ascribed to metallic Au0 resulting from Au NPs attachment via electrostatic interaction.18, 61, 62 Obviously, elemental chemical state of Au ingredient was not altered after CdS coating (Fig. S4d).

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Fig. 4 UV-vis diffuse reflectance spectra (DRS) of pristine WO3 NRs, WO3 NRs@5% Au, WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructures. Fig. 4 shows the UV-vis diffuse reflectance spectra (DRS) of different samples. It is apparent that pristine WO3 NRs demonstrate a pronounced peak in the region ranging from 200 to 420 nm which is ascribed to its band-gap-photoexcitation. After deposition with Au NPs, light absorption range of WO3@Au is remarkably extended as compared with pristine WO3 NRs, accompanied with the appearance of a conspicuous peak at ca.530 nm which is attributable to the SPR peak of Au NPs, As for the DRS spectra of WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructures, apart from the typical peaks of WO3 and Au, an additional absorption band in the range of 400-500 nm was clearly observed, which is assigned to the characteristic light absorption of CdS. It is noteworthy that SPR peak of Au NPs in WO3 NRs@5% Au binary heterostructure shifted from 530 nm to 550 nm in WO3 NRs@5% Au@30 CdS ternary heterostructure, which might be due to the strong electromagnetic coupling between Au NPs and CdS.63-68 Apparently, WO3 NRs@Au@CdS ternary heterostructure exhibited the most extended light absorption range in visible 18


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region as compared with binary and pristine counterparts. Consistently, light absorption features of these different samples can also be reflected by their different color (Fig. S5). The remarkably broadened light response region of WO3 NRs@Au@CdS ternary heterostructure is beneficial for producing more charge carriers for enhanced visible-light-driven photoactivities. 3.2 Photoreduction performances

Fig. 5 (a) Photocatalytic performances of WO3 NRs@9% Au@x CdS (x=15, 30, 45, 60, 90 min) ternary heterostructure with different deposition time (x) for CdS encapsulation toward reduction of 4-nitroaniline (4-NA) with the addition of ammonium formate as hole scavenger and N2 purge at ambient conditions under visible light (λ≥420 nm) irradiation; (b) photocatalytic performances of WO3 NRs@y Au@30 CdS (y=1, 3, 5, 7, 9 %) ternary heterostructure with different addition percentage (y) of Au NPs under the same conditions; (c) comparison on the apparent kinetic constants of different samples and (d) cyclic reaction of WO3 NRs@5% Au@30 CdS ternary heterostructure under the same conditions. Photocatalytic performances were evaluated by anaerobic selective reduction of aromatic nitro compounds to corresponding amino compounds, an important class of industrial intermediates for the synthesis of various industrial chemicals,69, 70 According to our previous work, optimal weight 19



addition percentage of Au NPs to WO3 NRs in WO3 NRs@Au nanocomposite is 9 %.29 Therefore, WO3@9% Au is chosen as the substrate for fabricating WO3 NRs@9% Au@x CdS (x=15, 30, 45, 60, 90 min) ternary heterostructures with different loading amount of CdS by controlling the deposition time. As shown in Fig. 5a, pristine WO3 NRs and WO3 NRs@Au binary heterostructure exhibited negligible photoactivity toward 4-NA reduction, which is ascribed to the low CB potential of WO3 and thus the low reduction capability of photoelectrons to reduce the 4-NA. Note that photocatalytic performance of WO3 NRs@Au binary heterostructure was markedly enhanced by incorporation of an appropriate amount of CdS via photodeposition and the result highlights the crucial role of CdS in boosting the photocatalytic performances of WO3 NRs@Au@CdS ternary nanocomposites. This is understandable in terms of the high reduction potential of CdS in comparison with WO3. Noteworthily, photocatalytic performances of WO3 NRs@Au@CdS ternary heterostructures are heavily dependent on the photodepositon time by which loading amount of CdS layer is finely tuned. More specifically, photocatalytic performances of WO3 NRs@Au@CdS ternary heterostructure increase with prolonging the photo-deposition time from 15 to 30 min and subsequently deteriorate with further prolonging the irradiation time to 90 min. Among which, WO3 NRs@Au@30 CdS ternary heterostructure with photo-deposition time of 30 min exhibited the optimal visible-lightdriven photoactivity toward 4-NA reduction in comparison with pristine, binary and ternary counterparts under the same conditions. Note that WO3 NRs@9% Au@30 CdS demonstrates similar photoactivity to WO3 NRs@9% Au@45 CdS with similar decay profile and this can be ascribed to different reasons. First of all, it should be emphasized that deposition amount of CdS on the Au/WO3 NRs can be well-controlled by deposition time in our current reaction system. Thus, apparently, 20


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deposition amount of CdS on WO3 NRs@9% Au@45 CdS is much larger than that on WO3 NRs@9% Au@15 CdS. In terms of WO3 NRs@9% Au@45 CdS, although deposition amount of CdS on the surface is larger than WO3 NRs@9% Au@30 CdS, we believe over-deposition of CdS remarkably shields the light absorption of WO3 and Au NPs, which retards the photo-excitation of WO3 and plasmonic excitation of Au NPs, thus leading to reduced photoactivity in comparison with the optimal WO3 NRs@9% Au@30 CdS. Furthermore, over-deposited CdS may also act as recombination center. With regard to WO3 NRs@9% Au@15 CdS, although photoexcitation of WO3 and plasmonic excitation of Au NPs were not influenced, we believe low loading amount of CdS is detrimental to the light absorption of ternary system, thus also resulting in decreased photocatalytic performances as compared with the optimal WO3 NRs@9% Au@30 CdS. It is mainly these two effects caused by the different deposition amount of CdS that makes the WO3 NRs@9% Au@x CdS (x=15, 45) show the similar decay profile. Fig. 5b shows the photocatalytic performances of WO3 NRs@y Au@30 CdS (y=1, 3, 5, 7, 9%) heterostructures with different loading percentage of Au NPs under visible light irradiation toward 4NA reduction. The results suggested that photocatalytic performances of WO3 NRs@Au@CdS ternary heterostructures increase with Au NPs loading percentage increasing from 1 to 5 % and then decreases gradually upon further boosting the percentage, thereby unveiling the close correlation of photocatalytic performances of ternary heterostructures with loading percentage of metal. Among which, WO3 NRs@5% Au@30 CdS ternary heterostructure demonstrated the optimal photoactivity compared with other counterparts. Based on which, optimal photo-deposition time of CdS and loading percentage of Au NPs are rationally determined for designing the most efficient WO3 21



NRs@Au@CdS ternary heterostructure. It has been well-established that photo-reduction of 4-NA agrees with the first-order reaction kinetic,71 based on which kinetic rate constants of different samples were calculated. As reflected in Fig. 5c, photocatalytic efficiency (0.3800 min-1) of WO3 NRs@5% Au@30 CdS ternary heterostructure is ca. 3954 and 1610 times higher than pristine WO3 (0.0000961 min-1) and WO3@9% Au (0.000236 min-1) binary heterostructure, and even 1.54 times higher than WO3@30 CdS (0.2467 min-1) binary heterostructure. Fig. 5d shows that WO3 NRs@5% Au@30 CdS ternary heterostructure demonstrated good photostability with negligible decay of photoactivity even for successive five cyclic reactions under visible light irradiation, implying the favorable photostability of ternary heterostructure. This can also be confirmed by the elemental mapping results of ternary heterostructure after cyclic reactions (Fig. S6). Consistently, XRD (Fig. S7) and XPS results (Fig. S8) of WO3 NRs@5% Au@30 CdS ternary heterostructure before and after cyclic photocatalytic reactions manifested that crystal structure and elemental chemical states of the samples were the same as those of freshly prepared samples, thereby additionally substantiating the WO3 NRs@5% Au@30 CdS ternary heterostructure is stable and can be reusable, which is of paramount importance for future practical applications.

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Fig. 6. Photoreduction of other substituted aromatic nitro compounds including (a) 2-NA, (b) 3-NA, (c) 4-nitrotoluene, (d) nitrobenzene (NB), (e) 2-nitrophenol (2-NP), and (f) 3-nitrophenol (3-NP) over pristine WO3 NRs, WO3 NRs@5% Au, WO3 NRs@30 CdS and WO3 NRs@5% Au@30 CdS ternary heterostructure under visible light irradiation (λ≥420 nm) with addition of ammonium formate as hole scavenger and N2 purge at ambient conditions. Apart from 4-NA, as mirrored by Fig. 6, similar photoactivity enhancement of WO3 NRs@5% Au@30 CdS ternary heterostructure as compared with pristine WO3 NRs, WO3 NRs@5% Au and WO3 NRs@30 CdS counterparts under the same conditions has also been observed toward photoreduction of other aromatic nitro compounds under the same conditions. Consistently, photocatalytic performances of these samples follow the same order of WO3 NRs@5% Au@30 CdS>WO3 NRs@30 CdS>WO3 NR@5% Au>WO3 NRs.

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3.3 Photooxidation performances

Fig. 7. (a) Photocatalytic performances of pristine WO3 NRs, WO3 NRs@5% Au, WO3 NRs@30 CdS binary, and WO3 NRs@5% Au@30 CdS ternary heterostructures toward mineralization of RhBunder visible light irradiation (λ≥420 nm) and (b) cyclic photocatalytic reactions of WO3 NRs@5% Au@30 CdS ternary heterostructure under the same conditions. Besides the photoreduction reactions, photo-oxidation reaction toward mineralization of organic pollutant (RhB) over different samples were also performed. As displayed in Fig. 7a, photooxidation performance of pristine WO3 NRs was considerably enhanced by depositing an appropriate amount of Au NPs and this highlights the crucial role of Au NPs in boosting the photocatalytic performances of WO3 NRs@Au binary nanocomposite under visible light irradiation. Moreover, photocatalytic performance of WO3 NRs@30 CdS binary heterostructure is higher than pristine WO3 NRs under the same conditions, implying the contributing role of CdS for photoactivity enhancement. Note that WO3 NRs@5% Au@30 CdS core-shell ternary heterostructure exhibited the substantially improved photo-oxidation performances relative to WO3 NRs@30 CdS counterpart toward RhB degradation under visible light irradiation. The improved photoactivity of WO3 NRs@5% Au@30 CdS ternary heterostructure arises from the pivotal role of Au NPs as efficient interfacial electron transfer mediator and surface plasmon resonance (SPR) photosensitizer, which will be discussed in latter part. As displayed in Fig. 7b, WO3 NRs@5% Au@30 CdS ternary heterostructure demonstrated good photostability with negligible decay of photoactivity even for successive five 24


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cyclic reactions, suggesting favorable photostability of tenary heterostructure. To probe the underlying reasons accounting for the significantly enhanced photocatalytic performances of WO3 NRs@5% Au@30 CdS ternary heterostructure, Brunauer-Emmett-Teller (BET) measurements for probing the specific surface area and porosity of the samples were first explored. As revealed in Fig. S9, pristine WO3 NRs, WO3 NRs@5 % Au, WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructures showed a type IV adsorption-desorption isotherm with a typical H3 hysteresis loop according to IUPAC classification.70 Specific surface area and pore volume of WO3 NRs@5% Au@30 CdS ternary heterostructure were determined to be 21.86 m2/g and 0.090 cm3•g−1, which are larger than WO3 NRs@5% Au (i.e., 14.54 m2/g and 0.059 cm3•g−1), which indicates CdS encapsulation is beneficial for increasing the specific surface area of ternary nanocomposite. The larger specific surface area and pore volume of WO3 NRs@5% Au@30 CdS ternary heterostructure in comparison with WO3 NRs@5% Au binary counterpart could afford more adsorption capacity toward reactant molecules, which contributed partially to improved photoactivity. However, it should be stressed that although specific surface area of WO3 NRs@5% Au@30 CdS is large than WO3 NRs@5% Au the enhancement is rather limit (ΔS=7.32 m2•g−1). In this regard, we believe such a substantial photoactivity enhancement of WO3 NRs@5% Au@30 CdS compared with WO3 NRs@5% Au is not caused by their different specific surface area but rather improved separation and prolonged lifetime of photogenerated electron-hole charge carriers, which will be corroborated by the following PEC and PL results. For comparison, BET results of different samples were listed in Table S1.

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Fig. 8 (a) Control experiments by adding K2S2O8 as electron scavenger for photoreduction of 4-NA over WO3 NRs@5% Au@30 CdS ternary heterostructure under visible light irradiation (λ≥420 nm), (b) Control experiments by adding different radical scavengers for capturing various active species during the photodegradation of RhB over WO3 NRs@5% Au@30 CdS ternary heterostructure under the same conditions, ESR spectra of (c) ꞏOH radicals and (d) ꞏO2- radicals trapped by DMPO (DMPO-ꞏOH, DMPO-ꞏO2-) for WO3 NRs@5% Au@30 CdS ternary heterostructure aqueous suspension (I) under visible light irradiation (λ≥420 nm) and (II) in the dark, (e) PL spectra of WO3 NRs@5% Au@30 CdS ternary heterostructure with an excitation wavelength of 350 nm under visible light irradiation (λ≥420 nm) using TA as a probe molecule, and (f) detection of H2O2 in WO3 NRs@5% Au@30 CdS ternary heterostructure aqueous dispersion under visible light irradiation (λ≥420 nm) using a DPD-POD method with corresponding plots depicting peak intensity vs. irradiation time displayed in the inset. With a view to highlighting the crucial role of photoelectrons in the photoreduction reaction, 26


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control experiments using K2S2O8 as an electron scavenger were carried out. As shown in Fig. 8a and Fig. S10, photoreduction performances of WO3 NRs@5% Au@30 CdS ternary and WO3 NRs@30 CdS binary heterostructures were completely refrained when K2S2O8 was added into the reaction system, strongly confirming photoreduction reaction is driven by photoelectrons.73 In a similar way, tert-butyl alcohol (TBA), ammonium oxalate (AO), K2S2O8 and benzoquinone (BQ) acting as scavengers for quenching •OH radicals, holes (h+), electrons (e-) and ꞏO2- radicals were added in the reaction system for determining the predominant active species during the photooxidation reaction. As shown in Fig. 8b, photocatalytic performances of WO3 NRs@5% Au@30 CdS ternary heterostructure substantially decreased when all these scavengers were added in the reaction system, suggesting •OH radicals, holes (h+), electrons (e-) and ꞏO2- radicals all contribute to the significantly enhanced photo-oxidation performance of WO3 NRs@5% Au@30 CdS ternary heterostructure and the relative contributing role of them follows ꞏO2->•OH>h+>e-, among which ꞏO2radicals played the most pronounced influence. It should be emphasized that holes generally play dual roles in photo-oxidation reaction including direct mineralization of organic dye pollutant and oxidation of water/or –OH groups to •OH radicals, both of which contribute to the promising photoactivity. Formation of •OH and ꞏO2- radicals in reaction system can be further evidenced by electron spin resonance (ESR) results. As displayed in Fig. 8(c & d), obvious •OH and ꞏO2- radicals were clearly observed, confirming in-situ generation of these two active species in reaction system. Generation of •OH radicals were also corroborated by photoluminescence (PL) technique using terephthalic acid (TA) as a probe molecule. The principle of this technique is based on the fact that TA molecule is able to react with •OH radicals which rapidly transform to highly fluorescent 227



hydroxyterephthalic acid product whose PL peak intensity can be in-situ probed.20 Normally, PL intensity of the product is proportional to the •OH radicals amount produced. As reflected in Fig. 8e, an obvious peak at 435 nm was clearly observed in the PL results of WO3 NRs@5% Au@30 CdS ternary heterostructure under visible light irradiation and its peak intensity is proportional to the irradiation time (inset, Fig. 8e), once again indicative of the generation of •OH radicals in the reaction system. This is consistent with ESR results (Fig. 8c) and quenching experiments (Fig. 8b). Alternatively, H2O2 has also been deemed as an important intermediate which plays a crucial role for degradation of organic dye pollutant.74 Formation of H2O2 can be in-situ detected by a DPD-POD method.75 As shown in Fig. 8f, two absorption maxima at ca. 510 nm and 551 nm were clearly observed in the UV-absorption spectra of WO3 NRs@5% Au@30 CdS ternary heterostructure aqueous dispersion under visible light irradiation and the peak intensity gradually increases with prolonging the irradiation time, verifying the production of H2O2 in reaction system. 3.4 PEC performances

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Fig. 9 (a) On-off transient photocurrent responses, (b) decay of open circuit potential, (c) linearsweep voltammograms (LSV, scan rate: 5 mV/s) and (e) electrochemical impedance spectra (EIS) results of pristine WO3 NRs, WO3 NRs@5% Au, WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructure under visible light irradiation (λ≥420 nm); (d) photostability of WO3 NRs@5% Au@30 CdS ternary heterostructure and (f) Mott-Schottky results of WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructure. PEC measurements were performed to probe the charge separation and transfer in the interfacial region of photocatalysts.76 Fig. 9(a & c) shows the on-off transient photocurrent responses and LSV results of different samples, from which it is clearly seen that WO3 NRs@5% Au@30 CdS ternary heterostructure demonstrated the most enhanced photocurrent dentistry in comparison with other counterparts under visible light irradiation (λ≥420 nm), suggesting the most efficient separation of photogenerated charge carriers over ternary nanocomposite.77 Moreover, as mirrored by Fig. 9d, WO3 NRs@5% Au@30 CdS ternary heterostructure exhibited the favorable photostability and no photocurrent decay was observed under continuous light irradiation. Electron lifetime of different samples was estimated through open circuit voltage decay by turning off light irradiation at a steady state and then monitoring its decay with time.78 As shown in Fig. 9b, WO3 NRs@5%Au@30CdS ternary heterostructure exhibited much more prolonged electron lifetime and larger photovoltage as compared with pristine WO3 NRs, WO3 NRs@5% Au and WO3 NRs@30 CdS counterpart, which once again corroborated the most enhanced separation of photogenerated electron-hole pairs over ternary nanocomposite. EIS measurements were performed to explore the interfacial charge transfer resistance of photoelectrode.79 Fig. 9e shows that Nyquist plot of WO3 NRs@5% Au@30 CdS ternary heterostructure showed the smallest semi-circle arc radius in high frequency region under visible light irradiation, followed by WO3 NRs@30 CdS, WO3 NRs@5% Au and pristine WO3 NRs, strongly implying the interfacial charge transfer resistance of ternary nanocomposite is remarkably 29



lower than other counterparts, thus resulting in the most efficient separation of electron-hole pairs. Mott-Schottky (M-S) plots of WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructures were further probed to determine the charge carrier density. As apparently displayed in Fig. 9f, M-S plot slop of WO3 NRs@5% Au@30 CdS ternary heterostructure is smaller than WO3 NRs@30 CdS binary counterpart, which suggests WO3 NRs@5% Au@30 CdS showed faster charge transfer rate than WO3 NRs@30 CdS. Charge carrier density (ND) can be determined based on the formula below:

where e=1.6 × 10 −19 C, ε0=8.86×10 −12 F•m−1, ε=300 for hexagonal WO352 and C is the capacitance. Based on which, ND values of WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructure was calculated to be 3.04×1019 and 6.23 × 1019 cm−3, respectively. Obviously, ND of WO3 NRs@5% Au@30 CdS is almost 2 times larger than WO3 NRs@30 CdS and this confirms much more efficient charge transfer rate of WO3 NRs@5% Au@30 CdS than WO3 NRs@30 CdS, which agrees with their EIS results in dark (Fig. S11). Consequently, PEC results suggested that incorporation of Au NPs in-between the interfacial domain of WO3 NRs and CdS not only prolong the lifetime of photogenerated charge carriers, but also remarkably promotes the charge transfer efficiency across the interfacial region.

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Fig. 10 Photoluminescence (PL) results of pristine WO3 NRs, WO3 NRs@5% Au, WO3 NRs@30 CdS binary and WO3 NRs@5% Au@30 CdS ternary heterostructure with an excitation wavelength of 380 nm (450 nm for the inset). Photoluminescence (PL) spectra of the samples have also been explored at room temperature with two different excitation wavelengths (380 and 450 nm). As shown in Fig. 10, PL spectrum of pristine WO3 NRs exhibits a broaden emission peak with an excitation wavelength of 380 nm and the emission band is attributed to the direct recombination of excitons through an exciton-exciton collision process in WO3.80 It is clearly seen that PL intensity of this band in pristine WO3 decreases after Au or CdS deposition. Especially, when Au and CdS were simultaneously deposited on the WO3 NRs, PL intensity of WO3 NRs@30 CdS and WO3 NRs@5% Au binary heterostructures further decreases, indicating simultaneous deposition of an appropriate amount of Au and CdS can efficaciously inhibit the recombination of charge carriers photogenerated from WO3 NRs. PL intensity of the different samples follows the order of WO3 NRs@5% Au@30 CdS

Boosting Charge Transfer Efficiency by Simultaneously Tuning Double

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