ZnO heterostructure for photocatalytic hydrogen generation

Accepted Manuscript Oxygen Vacancies Promoted Interfacial Charge Carrier Transfer of CdS/ZnO Heterostructure for Photocatalytic Hydrogen Generation Ying Peng Xie, Yongqiang Yang, Guosheng Wang, Gang Liu PII: DOI: Reference:

S0021-9797(17)30517-9 http://dx.doi.org/10.1016/j.jcis.2017.05.006 YJCIS 22313

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

7 March 2017 28 April 2017 3 May 2017

Please cite this article as: Y.P. Xie, Y. Yang, G. Wang, G. Liu, Oxygen Vacancies Promoted Interfacial Charge Carrier Transfer of CdS/ZnO Heterostructure for Photocatalytic Hydrogen Generation, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.05.006

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Oxygen Vacancies Promoted Interfacial Charge Carrier Transfer of CdS/ZnO Heterostructure for Photocatalytic Hydrogen Generation Ying Peng Xie,a, Yongqiang Yang,b Guosheng Wang,a Gang Liub,c a College

of Chemical Engineering, The key Laboratory of the Inorganic Molecule-Based

Chemistry of Liaoning Province, Shenyang University of Chemical Technology, No. 11 St. Economical and Technological Development Zone, Shenyang 110142, China E-mail: [email protected] (Y. P. Xie) b Shenyang

National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, 72 Wenhua RD, Shenyang 110016, China c

School of Materials Science and Engineering, University of Science and Technology of

China, 96 Jinzhai Road, Hefei 230026, China E-mail: [email protected] (G. Liu)

ABSTRACT. The solid-state Z-scheme trinary/binary heterostructures show the advantage of utilizing the high-energy photogenerated charge carriers in photocatalysis. However, the key factors controlling such Z-scheme in the binary heterostructures are still unclear. In this paper, we showed that oxygen vacancies could act as an interface electron transfer mediator to promote the direct Z-scheme charge transfer process in binary semiconductor heterostructures of CdS/ZnS. Increasing the concentration of surface oxygen vacancies of ZnO crystal can greatly enhance photocatalytic hydrogen generation of CdS/ZnO heterostructure. This was attributed to the strengthened direct Z-scheme charge transfer process in CdS/ZnO, as evidenced by steady-state/time-resolved photoluminescene spectroscopy and selective photodeposition of metal particles on the heterostructure. Keywords: direct Z-scheme, oxygen vacancies, interface electron transfer, CdS/ZnO, hydrogen 1. Introduction 1

Solar photocatalytic hydrogen evolution from water splitting is considered to be a promising way to obtain clean energy carrier hydrogen. [1-5] Constructing heterostructures [6-10] together with loading cocatalysts [11,12] is widely used to promote the transfer of photogenerated charge carriers for high photocatalytic activity. The type-II heterostructures consisting of two semiconductor components with staggered band alignments widely studied can realize effective charge transfer between two components via their interface (Fig. 1a). Specifically, the photogenerated electrons (holes) are transferred from one semiconductor with higher conduction (lower valence) band to that with lower conduction (higher valence) band. Thus, one intrinsic drawback associated with the type-II heterostructure is that only the photogenerated low-energy charge carriers can be utilized in photocatalysis. In contrast to the type-II heterostructures, all-solid-state Z-scheme heterostructures show the advantage of utilizing the photogenerated high-energy charge carriers in photocatalysis reaction by selectively recombining the electrons from the semiconductor with low conduction band with holes from the semiconductor with high valence band at the metal particle located at the interface of two semiconductors (namely, vectorial Z-scheme charge carrier transfer process), as shown in Fig. 1b. This idea was realized for the first time in trinary CdS/Au/TiO2 [13] where Au located at the interface of CdS and TiO2 plays the role of mediating the selective recombination of the electrons from TiO 2 and holes from CdS. The model was then imitated in other trinary heterostructures using Au, [14,15] Ag, [16] Rh [17,18] or graphene [19-22] as the interface electron transfer mediator. On the other hand, a lot of binary heterostructures consisting of two semiconductors (e.g., CdS/ZnO, [23] WO3/CaFe2O4, [24] CdS/WO3 [25] and BiOI/g-C3N4 [26]) have been reported to have the ability of inducing the occurrence of direct Z-scheme charge transfer process (Fig. 1c) in photocatalysis since the first such example of CdS/ZnO [23,27]. However, the key factor of controlling the occurrence of direct Z-scheme in the binary heterostructures has been not fully revealed. The fact that oxygen vacancies could act as interface electron transfer mediator to 2

promote the direct Z-scheme charge transfer in binary semiconductor heterostructures was revealed in this paper. a

b

c Red.

—

Ox.

—

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Red.

—

— —

Ox.

Red. +

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+

Ox.

+

Ox.

+

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Red.

Fig. 1. Schematic of the (a) type-II, (b) vectorial Z-scheme and (c) direct Z-scheme charge transfer process in heterostructures.

Oxygen vacancy as intrinsic defect has a great influence on electronic structures and related functionalities of metal oxides. The substantial role of oxygen vacancies in affecting metal oxide photocatalysts has been investigated theoretically and experimentally in single component. [28-32] However, their potential role in controlling the properties of the heterostructured photocatalysts is much less concerned. Here, the influence of oxygen vacancy on the charge transfer properties in typical binary CdS/ZnO heterostructure was investigated carefully through steady-state and time-resolved photoluminescene (PL) spectroscopy. Photodeposition (PD) of Au and MnOx particles was used to distinguish the reducing and oxidizing active sites in CdS/ZnO. The coexistence of type-II and direct Zscheme charge transfer processes in CdS/ZnO was evidenced by the PL and PD results. With the increase of oxygen vacancy concentration of ZnO, the direct Z-scheme charge transfer process between ZnO and CdS can be strengthened to improve the photocatalytic hydrogen generation of CdS/ZnO. 2. Experimental details 2.1. Materials and reagents

3

All reagents used in this study were of analytical reagent quality, purchased form Sinopharm Chemical Reagent Co., Ltd, China. The purity of reagents is as follows: zinc acetate (Zn(CH3COO)2·2H2O, purity > 99%), sodium hydroxide (NaOH, purity > 96%), sodium borohydride (NaBH4, purity > 98%), cadmium acetate (Cd(CH3COO)2·2H2O, purity > 99%), sodium sulfide (Na2S, purity > 98%), chloroauric acid (HAuCl4, purity > 99%), manganese sulfate (MnSO4, purity >99%). Deionized water was produced by an ultrapure water producing system (TAOSHI water equipment Engineering Co., Ltd, China). 2.2. Sample preparation procedure Preparation of ZnO disks by a hydrothermal process. 4.39 g of Zn(CH3COO)2·2H2O with a concentration of 0.5 mol·L-1, 0.4 g of NaOH with a concentration of 0.25 mol·L -1 were added in 40 mL deionized water under stirring to form an emulsion. The emulsion was then transferred to a Teflon-lined autoclave with a volume of 80 mL, and treated at 120 oC for 12 h. After the reaction, a white ZnO product was collected by centrifugation and washed with deionized water several times to remove dissolvable ionic impurities. The sample was then dried at 80 oC in air. Increasing the surface oxygen vacancy concentration of ZnO. The surface oxygen vacancy concentration of ZnO was increased by a chemical reduction route with NaBH4 as reducing agent. 300 mg of ZnO disks was firstly dispersed in 40 mL deionized water under ultrasonication for 30 min. 150 mg of NaBH4 was then added in the ZnO suspensions under strong stirring for 15 min. A light grey product after reduction was collected by centrifugation and washed with deionized water several times to remove dissolvable ionic impurities. The final product collected was dried at 60 oC in a vacuum oven, and the product obtained was denoted as OV-ZnO. Loading CdS on ZnO or OV-ZnO. CdS loading was conducted by chemical deposition as follows. 200 mg of the ZnO or OV-ZnO powder was ultrasonically dispersed in 20 mL of an aqueous solution containing 18.5 mg of Cd(CH3 COO)2·2H2O (5 wt% CdS vs. ZnO or OV4

ZnO). 3 mL of 0.05 mol·L-1 Na2S aqueous solution was added to the suspension under stirring for 15 min. The product after chemical deposition was collected by centrifugation and washed with deionized water several times to remove dissolvable ionic impurities. The products obtained by drying the samples at 60 oC in a vacuum oven were denoted as CdS/ZnO and CdS/OV-ZnO, respectively. Photodeposition Au and MnOx particles on CdS/ZnO or CdS/OV-ZnO. 50 mg CdS/ZnO or CdS/OV-ZnO powder, 2.5 mL of HAuCl4 solution with a concentration of 1 mg·mL -1 referring to Au, and 2.5 mL of MnSO4 solution with a concentration of 1 mg·mL -1 referring to MnO2 were mixed in 100 mL deionized water. The suspension was then irradiated by a 300 W Xe lamp under continuous stirring for 5 h. The suspension after the irradiation was filtered, washed with deionized water several times, and finally dried at 60 oC in a vacuum oven. 2.3. Characterization X-ray diffraction patterns of the samples were recorded on a Rigaku diffractometer using Cu irradiation (λ = 1.54056 Å). The sample morphology was determined by scanning and transmission electron microscopy performed on Nova NanoSEM 430 and Tecnai F30 electron microscopes, respectively. The Nova NanoSEM 430, equipped with energy dispersive X-ray spectroscopy systems, was also used for composition mapping analysis. The electron spin resonance spectra were recorded on a Bruker ESP300E spectrometer in atmospheric condition at the temperature 77 K. The optical absorption spectra of the samples were recorded in a UVvisible spectrophotometer (JASCO-550). Chemical composition and chemical state of the samples were analyzed by X-ray photoelectron spectroscopy (Thermo Escalab 250, a monochromatic Al Kα X-ray source, 1486.6 eV). All binding energies were referenced to the C 1s peak (284.6 eV) arising from adventitious carbon. Photoluminescence emission and photoluminescence decay spectra were recorded at room temperature with a fluorescence spectrophotometer (Edinburgh Instruments, FLSP-920). 2.4. Hydrogen evolution measurements 5

Photocatalytic water splitting reactions were carried out in a top-irradiation vessel connected to a glass enclosed gas circulation system. 100 mg of the photocatalyst powder was dispersed in 270 mL aqueous solution of 0.1 mol·L -1 Na2SO3 and 0.1 mol·L-1 Na2S as the sacrificial reagents. The light source used for irradiation was a 300 W Xe lamp (Beijing Trusttech Co. Ltd, PLS-SXE-300UV). The reaction temperature was maintained around 20 oC. The amount of H2 evolved was determined using a Shimadzu gas chromatography system (GC-2014). The apparent quantum yield (AQY) for H2 evolution was measured by using band-pass filter (380, 435, 450, 500, 550 and 600 nm) equipped with Xe lamp, and the AQY was estimated as: AQY (%) = (number of reacted electrons)/(number of incident photons) × 100 = (number of evolved H2 molecules × 2)/(number of incident photons) × 100 Where the irradiation intensity of band-pass light was measured with a spectroradiometer. 3. Results and discussion 3.1. Crystal structure and morphology CdS/ZnO heterostructure consists of micron-sized ZnO crystals and CdS nanoparticles. ZnO disks with a particle size of around 5 μm were synthesized by a hydrothermal route using zinc acetate and sodium hydroxide as the precursors. Scanning electron microscopy (SEM) images in Fig. 2a and Fig. S1a show that ZnO crystals obtained have the morphology of hexagonal disks. X-ray diffraction (XRD) patterns confirm that these crystals are pure ZnO phase with a wurtzite structure (Fig. 3). To increase surface oxygen vacancy concentration of ZnO crystals, the crystals were treated with sodium borohydride (NaBH 4). The crystal structure (Fig. 3) and morphology (Fig. 2d and Fig. S1d) of these treated ZnO crystals with a high concentration of oxygen vacancies (denoted as OV-ZnO) is well retained. 5 wt% CdS nanoparticles with the particle size ranging from tens to hundreds of nanometers were deposited on ZnO and OV-ZnO crystals by the precipitation reaction of cadmium acetate and sodium sulfide. Fig. 2b, e and Fig. S1b, e show SEM images of CdS/ZnO and CdS/OV-ZnO 6

heterostructures. Transmission electron microscopy (TEM) images in Fig. 2c, f and Fig. S1c, f also show the formation of CdS/ZnO and CdS/OV-ZnO heterostructures. The presence of CdS on ZnO disks can be further confirmed by the strong XPS of Cd and S species (Fig. S2).

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Fig. 2. (a), (b) SEM images of ZnO and CdS/ZnO samples; (c) TEM image of CdS/ZnO sample; (d), (e) SEM images of OV-ZnO and CdS/OV-ZnO samples; (f) TEM image of CdS/OV-ZnO sample.

Intensity / a.u.

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Fig. 3. XRD patterns of the ZnO, OV-ZnO, CdS/ZnO and CdS/OV-ZnO samples. 3.2. Oxygen Vacancies and Optical Absorption Spectra The existence of oxygen vacancies in ZnO and OV-ZnO crystals was confirmed by electron spin resonance (ESR) spectroscopy (Fig. 4). The peak with a g-factor of 1.957 can be assigned to surface oxygen vacancies in ZnO. [33,34] The intensity of the ESR signal for OV7

ZnO is higher than that of ZnO, suggesting that the oxygen vacancy concentration in OV-ZnO was increased after NaBH4 reduction.

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1.92

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Fig. 4. ESR spectra of ZnO and OV-ZnO measured at 77 K.

X-ray photoelectron spectroscopy (XPS) was used to study the surface properties of ZnO and OV-ZnO crystals. In ZnO, the core lines of Zn 2p3/2 and Zn 2p1/2 are located at 1022.2 eV and 1045.3 eV, respectively. The binding energy of Zn 2p3/2 (1022.7 eV) and Zn 2p1/2 (1045.8 eV) is shifted towards high energy in OV-ZnO (Fig. 5a). This peak shift could be ascribed to the reduction of the surface band bending. The chemical state of Zn species was detected by the Auger spectra because the difference of the Zn L3M45M45 peak between Zn2+ and Zn0 is much larger than that of the Zn 2p peak between Zn2+ and Zn0. The Zn LMM peaks in ZnO and OV-ZnO were all assigned to the Zn2+ form (kinetic energy, 989.0 eV) according to the literature (Fig. 5b). [35] The O 1s spectra of ZnO and OV-ZnO can be fitted with three components (Fig. 5c). The O 1s binding energy peaks at 530.0 eV, 531.5 eV and 532.5 eV are originated from the core electrons of lattice oxygen, O2- ions around the oxygen-deficient regions and surface hydroxyl groups or water, respectively. [36] Based on the surface areas of these O 1s peaks, the concentration ratio of oxygen vacancy to all oxygen species can be determined to increase from 0.32 in ZnO to 0.37 in OV-ZnO (Table S1). XPS valence band spectra were also recorded in Fig. 5d. ZnO and OV-ZnO have the same valence band 8

maximum. UV-visible absorption spectra in Fig. 6 show that the absorption beyond 400 nm for OV-ZnO increases as a result of the formation of oxygen vacancy related shallow donor levels in the band gap of ZnO. [37]

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Fig. 5. (a) Zn 2p XPS spectra, (b) Zn LMM Auger spectra, (c) O 1s XPS spectra and (d) XPS valence band spectra of ZnO and OV-ZnO.

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Fig. 6. UV-visible absorption spectra of ZnO and OV-ZnO.

3.3. Photophysical properties Photophysical properties of ZnO, OV-ZnO, CdS/ZnO and CdS/OV-ZnO were investigated by steady-state and time-resolved PL spectroscopy. As shown in the steady-state PL spectra (Fig. 7a), pure ZnO sample exhibits a broad green emission with its peak at 520 nm, which is originated from defect emission of ZnO crystals. [37-42] In contrast, the PL emission intensity of OV-ZnO is much weakened due to surface oxygen vacancy induced charge trapping in ZnO. The accelerated charge transfer effect can be achieved by loading CdS on ZnO, despite the band center of PL emission is shifted to yellow region at 580 nm. Based on the band structure analysis of CdS/ZnO (Fig. S3), the possible emission features are proposed in Fig. S4: (1) the 580 nm emission might be attributed to the charge transfer from the conduction band of ZnO to the valence band of CdS rather than the defect emission of CdS with a very low percentage in the heterostructure; (2) the 520 nm emission probably originated from the type-II charge transfer between CdS and ZnO still exists. These features indicate that both direct Z-scheme and type-II charge transfer processes in CdS/ZnO proceed under light irradiation. CdS/OV-ZnO with a higher concentration of oxygen vacancies shows

10

a weaker PL emission than CdS/ZnO, suggesting the accelerated charge transfer in CdS/OVZnO.

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Fig. 7. (a) Steady-state PL spectra of ZnO, OV-ZnO, CdS/ZnO and CdS/OV-ZnO samples; (b) PL decay curves of ZnO, OV-ZnO, CdS/ZnO and CdS/OV-ZnO particles monitored at their strongest steady PL peak (ZnO and OV-ZnO, 520 nm; CdS/ZnO and CdS/OV-ZnO, 580 nm). A picosecond pulsed light emitting diode laser (334.6 nm) with pulse width of 799.6 ps was used as the excitation.

Oxygen vacancies promoted direct Z-scheme charge transfer process is further supported by time-resolved PL spectra analysis (Fig. 7b). The lifetimes of defective emission in ZnO change from several hundreds of picoseconds to microseconds or even to milliseconds, which depends strongly on the properties of ZnO obtained by different methods. [39-42] The relationship between PL lifetime and charge transfer process in semiconductors has two 11

features: (1) if the effective charge transfer is unidirectional, for example, from photocatalyst to cocatalyst, the PL lifetime would be shortened; [43-45] (2) if the effective charge transfer is bidirectional, e.g., in type-II heterostructure, the PL lifetime would be prolonged. [23,46,47] These features can also be clearly observed in our case (Table S2): (1) the average PL lifetime in OV-ZnO is much shorter than that in ZnO (611 ns vs. 725 ns), indicating that the surface oxygen vacancy in ZnO does play the role in capturing the photogenerated electrons; (2) compared to ZnO, the average PL lifetime in CdS/ZnO is prolonged to 893 ns due to the existence of type-II charge transfer process; (3) the average PL lifetime in CdS/OV-ZnO is thus shortened to 766 ns, which could be attributed to the direct Z-scheme charge transfer induced by oxygen vacancy. Monitoring the deposition of metals and metal oxides from the reduction and oxidation reactions induced by the photogenerated electrons and holes provides a straightforward method for observing the distribution of reducing and oxidizing active sites on semiconductor photocatalysts. [48,49] Here, Au and MnOx particles were deposited simultaneously on CdS/ZnO or CdS/OV-ZnO using HAuCl4 and MnSO4 as precursors (Au3+ + e− → Au; Mn2+ + h+ + OH− → MnOx). SEM images, energy dispersive X-ray spectroscopy (EDX) images and corresponding elemental maps in Fig. S5 show that Au and MnOx particles were mainly deposited on CdS and ZnO, respectively. This indicates the selective distribution of photogenerated electrons and holes in CdS and ZnO parts of CdS/ZnO, respectively, as a result of the direct Z-scheme charge transfer process. Similar distribution of Au or MnO x depositions was also observed in CdS/OV-ZnO (Fig. S6). 3.4. Photocatalytic hydrogen evolution activity Photocatalytic hydrogen evolution of photocatalysts were measured in an aqueous solution containing Na2S/Na2SO3 sacrificial agents under simulated solar light irradiation. As shown in Fig. 8a, ZnO exhibits a low activity in photocatalytic hydrogen evolution (2.3 µmol·h-1). 12

Photocatalytic activity can be greatly enhanced by loading CdS on ZnO (58.4 µmol·h-1 for CdS/ZnO). It is worth noting that the increased oxygen vacancy concentration in OV-ZnO shows a negligible effect in improving the activity of ZnO. However, oxygen vacancy can substantially improve photocatalytic activity of CdS/OV-ZnO (132.9 µmol·h-1) as a consequence of promoted interfacial charge transfer process. After the photocatalytic reaction, the crystal structure of CdS/ZnO and CdS/OV-ZnO is well retained without the formation of new phase (Fig. S7). The dependence of apparent quantum yield (AQY) in photocatalytic H 2 evolution on the wavelength of light was investigated, as shown in Fig. 8b and Table S3. The results show that the AQY for CdS/ZnO and CdS/OV-ZnO in ultraviolet region (380 nm) is much higher than that in visible light region (435, 450, 500, 550 and 600 nm). The AQY for CdS/ZnO and CdS/OV-ZnO beyond 500 nm is negligible. This indicates that photocatalytic activity of CdS/ZnO and CdS/OV-ZnO is dominantly contributed by the direct Z-scheme charge transfer process requiring the co-excitation of ZnO and CdS by ultraviolet, instead of the sensitization effect of CdS on ZnO under visible light (namely, the type-II charge transfer process). Moreover, the AQY for CdS/OV-ZnO (19.01%) is higher than that for CdS/ZnO (12.94%) at 380 nm, whereas the AQY difference between two samples becomes negligible with the increase of the light wavelength in visible light region. The enhanced AQY for CdS/OV-ZnO at 380 nm could be attributed to the accelerated direct Z-scheme charge transfer by the increased oxygen vacancy concentration.

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Fig. 8. (a) Hydrogen evolution as a function of time from an aqueous solution containing Na2S/Na2SO3 agents by ZnO (black line), OV-ZnO (red line), CdS/ZnO (blue line) and CdS/OV-ZnO (green line); (b) The dependence of apparent quantum yield in photocatalytic H2 evolution on the wavelength of light for CdS/ZnO (black sphere) and CdS/OV-ZnO (red sphere). 4. Conclusions The photocatalytic hydrogen generation of ZnO/CdS heterostructure was greatly enhanced by increasing surface oxygen vacancies of ZnO. The oxygen vacancies at the heterostructure interface were considered to act as an effective interfacial mediator to promote the direct Zscheme charge transfer process in CdS/ZnO. The results obtained may shed some light on understanding the essence of the direct Z-scheme in type-II semiconductor heterostructures.

Acknowledgements 14

We are grateful to acknowledge financial support from National Natural Science Foundation of China (No. 51402199, 51422210), and Key Laboratory Project of Education Department of Liaoning Province (No. LZ2015060). Appendix A. Supporting material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2017.00.000. References [1] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253−278. [2] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503−6570. [3] K. Zhang, L. Guo, Metal sulphide semiconductors for photocatalytic hydrogen production, Catal. Sci. Technol. 3 (2013) 1672−1690. [4] X. Chen, C. Li, M. Grätzel, R. Kostecki, S.S. Mao, Nanomaterials for renewable energy production and storage, Chem. Soc. Rev. 41 (2012) 7909−7937. [5] T. Hisatomi, J. Kubota, K. Domen, Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014) 7520−7535. [6] M.R. Gholipour, C.T. Dinh, F. Béland, T.O. Do, Nanocomposite heterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting, Nanoscale 7 (2015) 8187−8208. [7] Y.P. Yuan, L.W. Ruan, J. Barber, S.C.J. Loo, C. Xue, Hetero-nanostructured suspended photocatalysts for solar-to-fuel conversion, Energy Environ. Sci. 7 (2014) 3934−3951. [8] J. Tian, Z. Zhao, A. Kumar, R.I. Boughton, H. Liu, Recent progress in design, synthesis, and applications of one-dimensional TiO2 nanostructured surface heterostructures: a review, Chem. Soc. Rev. 43 (2014) 6920−6937. [9] Y. Xu, Y. Huang, B. Zhang, Rational design of semiconductor-based photocatalysts for advanced photocatalytic hydrogen production: the case of cadmium chalcogenides, Inorg. Chem. Front. 3 (2016) 591−615. [10] H. Wang, L. Zhang, Z. Chen, J. Hu, S. Li, Z. Wang, J. Liu, X. Wang, Semiconductor heterojunction photocatalysts: design, construction, and photocatalytic performances, Chem. Soc. Rev. 43 (2014) 5234−5244. 15

[11] J. Ran, J. Zhang, J. Yu, M. Jaroniecc, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chem. Soc. Rev. 43 (2014) 7787−7812. [12] J. Yang, D. Wang, H.X. Han, C. Li, Roles of cocatalysts in photocatalysis and photoelectrocatalysis, Acc. Chem. Res. 46 (2013) 1900−1909. [13] H. Tada, T. Mitsui, T. Kiyonaga, T. Akita, K. Tanaka, All-solid-state Z-scheme in CdS– Au–TiO2 three-component nanojunction system, Nature Mater. 5 (2006) 782−786. [14] Z.B. Yu, Y.P. Xie, G. Liu, G.Q. Lu, X.L. Ma, H.M. Cheng, Self-assembled CdS/Au/ZnO heterostructure induced by surface polar charges for efficient photocatalytic hydrogen evolution, J. Mater. Chem. A 1 (2013) 2773−2776. [15] H.J. Yun, H. Lee, N.D. Kim, D.M. Lee, S. Yu, J. Yi, A combination of two visible-light responsive photocatalysts for achieving the Z-scheme in the solid state, ACS Nano 5 (2011) 4084−4090. [16] R. Kobayashi, S. Tanigawa, T. Takashima, B. Ohtani, H. Irie, Silver-inserted heterojunction photocatalysts for Z-scheme overall pure-water splitting under visiblelight irradiation, J. Phys. Chem. C 118 (2014) 22450−22456. [17] Y. Sasaki, H. Nemoto, K. Saito, A. Kudo, Solar water splitting using powdered photocatalysts driven by Z-schematic interparticle electron transfer without an electron mediator, J. Phys. Chem. C 113 (2009) 17536−17542. [18] Q. Wang, T. Hisatomi, S.S.K. Ma, Y. Li, K. Domen, Core/shell structured La- and Rhcodoped SrTiO3 as a hydrogen evolution photocatalyst in Z-scheme overall water splitting under visible light irradiation, Chem. Mater. 26 (2014) 4144−4150. [19] A. Iwase, Y.H. Ng, Y. Ishiguro, A. Kudo, R. Amal, Reduced graphene oxide as a solidstate electron mediator in Z-scheme photocatalytic water splitting under visible light, J. Am. Chem. Soc. 133 (2011) 11054−11057. [20] W.K. Jo, T.S. Natarajan, Facile synthesis of novel redox-mediator-free direct Z-scheme CaIn2S4 marigold-flower-like/TiO2 photocatalysts with superior photocatalytic efficiency, ACS Appl. Mater. Interfaces 7 (2015) 17138−17154. [21] X. Wang, L. Yin, G. Liu, Light irradiation-assisted synthesis of ZnO–CdS/reduced graphene oxide heterostructured sheets for efficient photocatalytic H 2 evolution, Chem. Commun. 50 (2014) 3460−3463. [22] J. Xian, D. Li, J. Chen, X. Li, M. He, Y. Shao, L. Yu, J. Fang, TiO2 nanotube arraygraphene-CdS quantum dots composite film in Z-scheme with enhanced photoactivity and photostability, ACS Appl. Mater. Interfaces 6 (2014) 13157−13166. 16

[23] X. Wang, G. Liu, Z.G. Chen, F. Li, L. Wang, G.Q. Lu, H.M. Cheng, Enhanced photocatalytic hydrogen evolution by prolonging the lifetime of carriers in ZnO/CdS heterostructures, Chem. Commun. (2009) 3452−3454. [24] M. Miyauchi, Y. Nukui, D. Atarashi, E. Sakai, Selective growth of n-type nanoparticles on p-type semiconductors for Z-scheme photocatalysis, ACS Appl. Mater. Interfaces 5 (2013) 9770−9776. [25] L.J. Zhang, S. Li, B.K. Liu, D.J. Wang, T.F. Xie, Highly efficient CdS/WO3 photocatalysts: Z-scheme photocatalytic mechanism for their enhanced photocatalytic H2 evolution under visible light, ACS Catal. 4 (2014) 3724−3729. [26] J.C. Wang, H.C. Yao, Z.Y. Fan, L. Zhang, J.S. Wang, S.Q. Zang, Z.J. Li, Indirect Zscheme BiOI/g-C3N4 photocatalysts with enhanced photoreduction CO2 activity under visible light irradiation, ACS Appl. Mater. Interfaces 8 (2016) 3765−3775. [27] G. Liu, L.Z. Wang, H.G. Yang, H.M. Cheng, G.Q. (Max) Lu, Titania-based photocatalysts-crystal growth, doping and heterostructuring, J. Mater. Chem. 20 (2010) 831−843. [28] X. Pan, M.Q. Yang, X. Fu, N. Zhang, Y.J. Xu, Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale 5 (2013) 3601−3614. [29] G. Wang, Y. Ling, Y. Li, Oxygen-deficient metal oxide nanostructures for photoelectrochemical water oxidation and other applications, Nanoscale 4 (2012) 6682−6691. [30] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals, Science 331 (2011) 746−750. [31] A.M. Deml, V. Stevanovic, C.L. Muhich, C.B. Musgrave, R. O'Hayre, Oxide enthalpy of formation and band gap energy as accurate descriptors of oxygen vacancy formation energetics, Energy Environ. Sci. 7 (2014) 1996−2004. [32] R.A. Eichel, Structural and dynamic properties of oxygen vacancies in perovskite oxides—analysis of defect chemistry by modern multi-frequency and pulsed EPR techniques, Phys. Chem. Chem. Phys. 13 (2011) 368−384. [33] T. Xia, P. Wallenmeyer, A. Anderson, J. Murowchick, L. Liu, X. Chen, Hydrogenated black ZnO nanoparticles with enhanced photocatalytic performance, RSC Adv. 4 (2014) 41654−41658. [34] X. Xue, T. Wang, X. Jiang, J. Jiang, C. Pan, Y. Wu, Interaction of hydrogen with defects in ZnO nanoparticles–studied by positron annihilation, raman and photoluminescence spectroscopy, CrystEngComm 16 (2014) 1207−1216. 17

[35] H. Ma, L. Yue, C. Yu, X. Dong, X. Zhang, M. Xue, X. Zhang, Y. Fu, Synthesis, characterization and photocatalytic activity of Cu-doped Zn/ZnO photocatalyst with carbon modification, J. Mater. Chem. 22 (2012) 23780−23788. [36] W. Liu, X. Tang, Z. Tang, Effect of oxygen defects on ferromagnetism of Mn doped ZnO, J. Appl. Phys. 114 (2013) 123911. [37] B. ambandam, R.J.V. Michael, P.T. Manoharan, Oxygen vacancies and intense luminescence in manganese loaded ZnO microflowers for visible light water splitting, Nanoscale 7 (2015) 13935−13942. [38] S. Kundu, S. Sain, B. Satpati, S.R. Bhattacharyya, S.K. Pradhan, Structural interpretation, growth mechanism and optical properties of ZnO nanorods synthesized by a simple wet chemical route, RSC Adv. 5 (2015) 23101−23113. [39] M. Li, G. Xing, L.F.N.A. Qune, G. Xing, T. Wu, C.H.A. Huan, X. Zhang, T.C. Sum, Tailoring the charge carrier dynamics in ZnO nanowires: the role of surface hole/electron traps, Phys. Chem. Chem. Phys. 14 (2012) 3075−3082. [40] H. He, Z. Ye, S. Lin, B. Zhao, J. Huang, H. Tang, Negative thermal quenching behavior and long luminescence lifetime of surface-state related green emission in ZnO nanorods, J. Phys. Chem. C 112 (2008) 14262−14265. [41] Y. Zhong, A.B. Djurišić, Y.F. Hsu, K.S. Wong, G. Brauer, C.C. Ling, W.K. Chan, Exceptionally long exciton photoluminescence lifetime in ZnO tetrapods, J. Phys. Chem. C 112 (2008) 16286−16295. [42] K. Kodama, T. Uchino, Variations in decay rate of green photoluminescence in ZnO under above- and below-band-gap excitation, J. Phys. Chem. C 118 (2014) 23977−23985. [43] N.T. Khoa, S.W. Kim, D.V. Thuan, H.N. Tien, S. Hyun Hur, E.J. Kim, S.H. Hahn, Fast and effective electron transport in a Au–graphene–ZnO hybrid for enhanced photocurrent and photocatalysis, RSC Adv. 5 (2015) 63964−63969. [44] S. Sarkar, A. Makhal, T. Bora, S. Baruah, J. Dutta, S.K. Pal, Photoselective excited state dynamics in ZnO–Au nanocomposites and their implications in photocatalysis and dyesensitized solar cells, Phys. Chem. Chem. Phys. 13 (2011) 12488−12496. [45] N.T. Khoa, S.W. Kim, D.H. Yoo, S. Cho, E.J. Kim, S.H. Hahn, Fabrication of Au/graphene-wrapped ZnO-nanoparticle-assembled hollow spheres with effective photoinduced charge transfer for photocatalysis, ACS Appl. Mater. Interfaces 7 (2015) 3524−3531.

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[46] F. Xu, V. Volkov, Y. Zhu, H. Bai, A. Rea, N.V. Valappil, W. Su, X. Gao, I.L. Kuskovsky, H. Matsui, Long electron−hole separation of ZnO-CdS core−shell quantum dots, J. Phys. Chem. C 113 (2009) 19419−19423. [47] S. Khanchandani, S. Kundu, A. Patra, A.K. Ganguli, Shell thickness dependent photocatalytic properties of ZnO/CdS core–shell nanorods, J. Phys. Chem. C 116 (2012) 23653−23662. [48] R. Li, F. Zhang, D. Wang, J. Yang, M. Li, J. Zhu, X. Zhou, H. Han, C. Li, Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4, Nat. Commun. 4 (2013) 1432. [49] Y. Yang, G. Liu, J.T.S. Irvine, H.M. Cheng, Enhanced photocatalytic H2 production in core–shell engineered rutile TiO2, Adv. Mater. 28 (2016) 5850−5856.

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Graphical Abstract Oxygen vacancy could act as an interface electron transfer mediator (route 3) to promote the vectorial Z-scheme charge transfer process (route 2) in the competition with type-II charge transfer process (route 1). This greatly improves the photocatalytic hydrogen evolution of CdS/ZnO heterostructure.

1 e-

CB

3 VO

e- C B 2

3 h+ V B

VB ZnO

h+

1 CdS

20