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Jul 11, 2017 - Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, School of. Electrical ...
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Highly Efficient Photocatalyst Based on a CdS Quantum Dots/ZnO Nanosheets 0D/2D Heterojunction for Hydrogen Evolution from Water Splitting Dandan Ma, Jian-Wen Shi,* Yajun Zou, Zhaoyang Fan, Xin Ji, and Chunming Niu Center of Nanomaterials for Renewable Energy, State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China

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S Supporting Information *

ABSTRACT: A novel CdS/ZnO heterojunction constructed of zero-dimensional (0D) CdS quantum dots (QDs) and twodimensional (2D) ZnO nanosheets (NSs) was rationally designed for the first time. The 2D ZnO NSs were assembled into ZnO microflowers (MFs) via an ultrasonic-assisted hydrothermal procedure (100 °C, 12 h) in the presence of a NaOH solution (0.06 M), and CdS QDs were deposited on both sides of every ZnO NS in situ by using the successive ionic-layer absorption and reaction method. It was found that the ultrasonic treatment played an important role in the generation of ZnO NSs, while NaOH was responsible to the assembly of a flower-like structure. The obtained CdS/ZnO 0D/2D heterostructures exhibited remarkably enhanced photocatalytic activity for hydrogen evolution from water splitting in comparison with other CdS/ZnO heterostructures with different dimensional combinations such as 2D/2D, 0D/three-dimensional (3D), and 3D/0D. Among them, CdS/ZnO-12 (12 deposition cycles of CdS QDs) exhibited the highest hydrogen evolution rate of 22.12 mmol/g/h, which was 13 and 138 times higher than those of single CdS (1.68 mmol/g/h) and ZnO (0.16 mmol/g/h), respectively. The enhanced photocatalytic activity can be attributed to several positive factors, such as the formation of a Z-scheme photocatalytic system, the tiny size effect of 0D CdS QDs and 2D ZnO NSs, and the intimate contact between CdS QDs and ZnO NSs. The formation of a Z-scheme photocatalytic system remarkably promoted the separation and migration of photogenerated electron−hole pairs. The tiny size effect effectively decreased the recombination probability of electrons and holes. The intimate contact between the two semiconductors efficiently reduced the migration resistance of photogenerated carriers. Furthermore, CdS/ZnO-12 also presented excellent stability for photocatalytic hydrogen evolution without any decay within five cycles in 25 h. KEYWORDS: photocatalysis, H2 evolution, water splitting, heterojunction, quantum dots

1. INTRODUCTION Hydrogen is considered to be a new ecofriendly energy source because of its high combustion heat and clean combustion product. Hydrogen generation from photocatalytic water splitting has been widely explored as a promising technology to solve the global energy crisis and environmental pollution.1−6 Many semiconductor materials have been utilized as photocatalysts to convert solar energy to hydrogen energy by water splitting, such as TiO2,7−9 transition-metal sulfide,10−13 carbon nitride,14−16 ZnO,17,18 and composite materials of these semiconductors. Among them, ZnO with a direct band gap of 3.2 eV is one of the most attractive semiconductor photocatalysts because of its high photosensitivity, nontoxic nature, low cost, and environmental sustainability.19−24 However, the photocatalytic efficiency of ZnO is limited by several factors, such as restrictive light absorption [only ultraviolet (UV) light can be absorbed because of its wide band gap] and fast recombination of charge carriers.20,24 Several strategies have © 2017 American Chemical Society

been explored to improve these shortcomings for enhanced photocatalytic efficiency, such as ion doping,22 noble-metal deposition,25 and the construction of a heterojunction structure.26−28 Among these strategies, coupling ZnO with other visible-light-responsive semiconductors to form a heterojunction structure has been proven to be highly effective. CdS is a visible-light-responsive photocatalyst with a narrow band gap (2.4 eV).29 Many publications have proven that CdS is a promising candidate for coupling ZnO to construct a CdS/ ZnO heterojunction structure with high photocatalytic activity.26,30−32 For example, Zhao et al. synthesized a CdS/ ZnO heterostructure that exhibited about 9.2 and 34.5 times higher hydrogen evolution rate than those of single ZnO and CdS, respectively.21 Liu et al. synthesized a nanotree-like CdS/ Received: June 12, 2017 Accepted: July 11, 2017 Published: July 11, 2017 25377

DOI: 10.1021/acsami.7b08407 ACS Appl. Mater. Interfaces 2017, 9, 25377−25386

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QDs on ZnO MFs using the SILAR method.40 The amount of CdS QDs was controlled by the number of deposition cycles. One deposition cycle included the following two steps: (1) ZnO MFs were first immersed in a 0.1 M Cd(CH3COO)2 solution for 5 min and then washed with deionized water to remove the excrescent Cd2+ ions. (2) After drying in air, the sample was immersed in a 0.1 M Na2S solution for another 5 min and then washed with deionized water. A series of CdS/ZnO heterostructures with different amounts of CdS QDs were obtained by adjusting the number of deposition cycles as 4, 8, 12, and 16 cycles, respectively. For convenience, the obtained CdS/ZnO heterostructures were denoted as CdS/ZnO-x, where x was the number of deposition cycles. 2.4. Synthesis of CdS Nanoparticles. For comparison, CdS nanoparticles were synthesized according to a previous report.41 A total of 200 mL of a Na2S (0.14 M) solution was added to 250 mL of a Cd(CH3COO)2 (0.14 M) solution. After stirring for 24 h, the mixture was placed in a quiescent condition for another 24 h and then filtered. The obtained yellow solid was washed with deionized water several times and then transferred to a 100-mL Teflon-lined autoclave. The autoclave was heated at 200 °C for 72 h. After cooling to ambient temperature, the resultant precipitate was washed with deionized water and ethanol several times and then dried at 80 °C, resulting in CdS nanoparticles. 2.5. Photocatalytic Hydrogen Generation. The photocatalytic water splitting reaction was carried out in a top-irradiation quartz cell connected to a closed glass gas circulation system (GEL-SPH2N, Beijing), and a 225 W xenon arc lamp (320−780 nm, 70 mW/cm2) was used as the simulated solar light source. A total of 10 mg of the assynthesized photocatalyst was dispersed in 25 mL of deionized water, and 0.35 M Na2SO4 and 0.25 M Na2S·9H2O were added as sacrificial agents; 0.1 mL of H2PtCl6 (1 mg/mL) was added to the suspension and then was irradiated with the xenon lamp under stirring for 0.5 h to realize deposition of the Pt cocatalyst (1 wt %) on the photocatalyst. The amount of hydrogen evolved under irradiation was monitored via a TCD gas chromatograph (GC-9720). Magnetic stirring was used during the water splitting experiments to ensure homogeneity of the suspension. In the long-time stability test, the reaction system was evacuated with nitrogen after every 5 h of irradiation to remove evolved hydrogen, and then the mixture solution was refreshed with Na2S and Na2SO3. All of the reaction conditions in the long-time stability test were the same as those in the above-mentioned photocatalytic hydrogen evolution. 2.6. Characterization. X-ray diffraction (XRD) patterns were recorded on a Philips X’pert multipurpose XRD system using Cu Kα radiation in step mode between 10° and 60°. UV−vis absorption spectra were recorded with a UV−vis−near-IR spectrometer (JASCO, V-670). The morphology and energy-dispersive spectrometry (EDS) were studied using field-emission scanning electron microscopy (SEM; Quanta 250 FEG). X-ray photoelectron spectroscopy (XPS) analyses were performed on a PHI 5300 Versa Probe analyzer with Al Kα radiation. The specific surface area of the samples was characterized using Micromeritics Tristar 2000 instrument at 77 K and calculated by the Brunauer−Emmett−Teller (BET) method. The photoluminescence (PL) spectra were measured by a fluorescence spectrometer (Edinburgh Instruments FLS 980). Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100F microscope operating at a voltage of 200 kV. Transient photocurrent responses were carried out in a standard three-compartment cell. Ag/AgCl was used as the reference electrode, and Pt was the counter electrode. The working electrode was made by loading samples on the fluorine-doped tin oxide (FTO; 7Ω per square) glasses. A 0.1 mol/L of Na2SO4 solution was used as the electrolyte. Light on and off was controlled by a shutter every 15 s, and the response of the transient photocurrent was recorded on a CHI660E electrochemical workstation (Shanghai, China).36,42

ZnO nanocomposite that exhibited excellent photocatalytic properties because of the driving force of the Z-scheme chargetransfer mechanism between ZnO and CdS.30 However, up to now, the hydrogen evolution rate over a CdS/ZnO heterostructure is still far below the level of practical application, mainly because of the rapid recombination of photogenerated electrons and holes in the inner parts of bulk ZnO and bulk CdS.33 Compared with bulk materials, zero-dimensional (0D) nanomaterials, such as quantum dots (QDs), are very useful for the rapid migration of photogenerated electrons and holes from the inner part to the surface because of the tiny particle size,34 and two-dimensional (2D) nanosheets (NSs), such as graphene,35,36 g-C3N4,37 generally possess many extraordinary properties, such as high specific surface area and outstanding electronic conductivity.38,39 Therefore, it can be certainly predicted that the photocatalytic activity of a CdS/ZnO heterostructure can be significantly enhanced if one semiconductor is prepared as QDs and the other semiconductor is controlled in NSs; meanwhile, intimate contact between the two semiconductors can be successfully realized. In this study, a novel CdS/ZnO heterojunction constructed of 0D CdS QDs and 2D ZnO NSs was rationally designed for the first time. The 2D ZnO NSs were assembled into ZnO microflowers (MFs) via an ultrasonic-assisted hydrothermal procedure, and CdS QDs were deposited on both sides of every ZnO NS in situ by using the successive ionic-layer absorption and reaction (SILAR) method. Furthermore, intimate contact between CdS QDs and ZnO NSs was well realized by means of this in situ deposition method. As expected, the obtained CdS/ ZnO heterostructure exhibited highly enhanced photocatalytic activity for hydrogen evolution in comparison with single ZnO and CdS.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Hexamethylenetetramine (HMTA), sodium sulfite anhydrous (Na2SO3), sodium sulfate (Na2SO4), and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Co., Ltd. Zinc nitrate [Zn(NO3)2·6H2O] was obtained from Chengdu Kelon Chemical Reagent Factory, China. Sodium sulfide (Na2S·9H2O) and cadmium acetate dihydrate [Cd(CH3COO)2·2H2O] were obtained from Tianjin Tianli Chemical Reagent Co., Ltd., China, and Tianjin Guangfu Fine Chemical Research Institute, China, respectively. A chloroplatinic acid solution (H2PtCl6, 8 wt % in H2O) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd., China. All reagents were of analytical grade and were used as received without further purification. Deionized water with a resistivity of 18.2 MΩ·cm was used throughout the experiments. 2.2. Synthesis of ZnO MFs. ZnO MFs were synthesized with an ultrasonic-assisted hydrothermal method. Typically, 1.5 mmol of Zn(NO3)2·6H2O, 1.5 mmol of HMTA, and 50 mL of deionized water were added to a beaker. After dissolution, 25 mL of a NaOH solution (0.06 M) was added, and the resulting solution was stirred for 5 min (300 rpm/min) to form a white turbid suspension. Subsequently, the suspension was treated ultrasonically for 60 min, then transferred to a 100 mL Teflon-lined autoclave, and treated at 100 °C for 12 h. After cooling to room temperature naturally, the obtained white solid was washed with deionized water several times. Finally, the products were dried at 60 °C, resulting in ZnO MFs (the yield was about 82%). In order to explore the roles of NaOH and ultrasonic treatment on the formation of ZnO MFs, we prepared two counterparts by the same procedure except without NaOH addition or without ultrasonic treatment (labeled as ZnO-without-NaOH and ZnO-without-ultrasonic, respectively). 2.3. Synthesis of CdS/ZnO Heterostructures. CdS/ZnO heterostructures were synthesized by the in situ deposition of CdS 25378

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Figure 1. SEM (a and b) and TEM (c) images and the XRD pattern (d) of the as-synthesized ZnO MFs (the inset in part c is the HRTEM image of one ZnO NS).

Figure 2. SEM images of ZnO (a), CdS/ZnO-4 (b), CdS/ZnO-8 (c), CdS/ZnO-12 (d), and CdS/ZnO-16 (e) and the EDS spectrum of CdS/ZnO12 (f).

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Figure 3. TEM (a) and HRTEM (b) images of CdS/ZnO-12 and XRD patterns (c), N2 adsorption−desorption isotherms (d), and the SBET data (inset in part d) of the samples.

3. RESULTS AND DISCUSSION The morphology and crystal phase of the as-synthesized ZnO MFs were investigated with SEM, TEM, and XRD. It can be observed from the SEM images (Figure 1a,b) that the morphology exhibited a flower-like structure, which was selfassembled by many uniform 2D ZnO NSs, and the diameter of the ZnO MFs was about 2.5 μm. By contrast, the ZnO NSs were not assembled into a flower-like structure in the absence of NaOH (Figure S1a,b), suggesting that the addition of NaOH is important to the formation of ZnO MFs. Furthermore, when the ultrasonic treatment was omitted, the obtained ZnO presented large-sized bulk without NS generation (Figure S1c,d). These results demonstrate that both the NaOH addition and the ultrasonic treatment are indispensable for the formation of ZnO MFs. Figure 1c displays the lowmagnification TEM image of the as-synthesized ZnO MFs. It can be observed that the thickness of every ZnO NS was about 20 nm. The fringe spacing of 0.26 nm displayed in the inset in Figure 1c was consistent with the (002) plane of hexagonal ZnO. The result of XRD analysis (Figure 1d) also confirmed that the crystal phase of the ZnO MFs was hexagonal ZnO (JCPDS 36-1451),43 and the sharp diffraction peaks demonstrated their good crystallinity. Figure 2a further displays the structure of ZnO MFs by SEM images with low and high magnification. It can be seen from the inset in Figure 2a that both sides of the ZnO NSs are very smooth. After the deposition procedure was carried out for four cycles, it can be observed from Figure 2b that a large number of CdS QDs with tiny size appeared on the surface of every ZnO NS, which makes the surfaces of the ZnO NSs to look very rough. The density of CdS QDs increased with an increase in the number of deposition cycles from 4 to 8 to 12 to 16 (Figure 2b−e). When the number of deposition cycles reached to 16, it can be seen that there were too many CdS QDs deposited on every ZnO NS, which partly destroyed the flower structure, indicating excessive loading of CdS QDs (Figure 2e). Figure 2f

shows the EDS spectrum of CdS/ZnO-12. It can be observed that the peaks assigned to Zn, O, Cd, and S appeared, further confirming the successful deposition of CdS QDs on the surface of ZnO NSs. Compositional data of CdS/ZnO-x measured by EDS are shown in Table S1, suggesting that the atom percentages of Cd and S increased with an increase in the number of deposition cycles from 4 to 8 to 12 to 16. These results demonstrate that the CdS QDs have been successfully deposited on the surfaces of ZnO NSs. As for the assynthesized CdS nanoparticles, they exhibited uniform nanospheres with a small size of about 30 nm (SEM and TEM images are presented in Figure S2). As a typical sample, CdS/ZnO-12 was further characterized by TEM and high-resolution TEM (HRTEM) to ascertain the loading of CdS QDs on the surface of ZnO NSs. As shown in Figure 3a, many nanoparticle-like CdS QDs were highly dispersed on the surface of every ZnO NS. Two different lattice fringes with interplanar spacings of 0.26 and 0.34 nm could be observed from the HRTEM image of CdS/ZnO-12 (Figure 3b), which are consistent with the (002) plane of hexagonal ZnO and the (002) lattice plane of hexagonal CdS, respectively. The recognized size of CdS QDs was about 5 nm, and the intimate interface between CdS and ZnO could be clearly observed. XRD patterns of ZnO and CdS/ZnO-x are given in Figure 3c. Besides the characteristic diffraction peaks of hexagonal ZnO (JCPDS 36-1451), a weak wide diffraction peak in the range of 24−30° appeared in the XRD patterns of CdS/ ZnO-x samples, and the position of the peak is matched with the (100), (002), and (101) faces of hexagonal CdS (JCPDS 41-1049). In combination with other characterization results, such as HRTEM, where the (002) lattice plane of hexagonal CdS was detected, and EDS and XPS, where S and Cd were detected, we think that the peak can be assigned to CdS QDs,26 further demonstrating the successful deposition of CdS QDs on ZnO NSs. It is worth mentioning that the hexagonal structure of both CdS and ZnO results in the heterojunction with less 25380

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Figure 4. XPS survey spectra of CdS/ZnO-12 (a) and high-resolution XPS spectra of Zn 2p for ZnO and CdS/ZnO-12 (b).

interface defects due to integration of the heterocomponents at the atomic level, which is very helpful to the transfer of photogenerated carriers. Figure 3d shows the N2 adsorption−desorption isotherms of ZnO MFs with different deposition cycles of CdS QDs. Both ZnO and CdS/ZnO-12 exhibited a type IV isotherm (IUPAC classification) with a H3 hysteresis loop, indicating the presence of slitlike pores resulting from the assembly of ZnO NSs,44 which is greatly coincident with the results of SEM and TEM. The specific surface area (SBET) of ZnO MFs was 16.65 m2/g, which gradually increased with an increase in the number of deposition cycles of CdS QDs and then reached to the maximum value (33.83 m2/g) when the number of deposition cycles was 12, which was twice as much as that of ZnO MFs. When the number of deposition cycles further increased to 16, SBET decreased to 24.58 m2/g. The reason can be attributed to the blocking effect caused by redundant CdS QDs, as confirmed by SEM (Figure 2e). Surface compositions and chemical states of the assynthesized composites were measured by XPS (Figure 4). Four peaks belonging to the Cd, S, Zn, and O elements, respectively, can be distinguished from the full XPS spectrum (Figure 4a). The high-resolution XPS spectra of Zn 2p for ZnO and CdS/ZnO-12 are shown in Figure 4b. For pure ZnO, two peaks at binding energies of 1021.9 and 1044.9 eV were observed. After CdS QDs were deposited, the two peaks shifted to lower binding energies of 1021.4 and 1044.5 eV, respectively, indicating that strong interaction and possible chemical bonding between ZnO and CdS QDs have formed,45−47 which is meaningful to the transfer of photogenerated charge carriers and to improvement of the photocatalytic activity.45 The surface concentrations of S, Cd, O, and Zn, calculated from XPS data, are listed in Table S2. It can be clearly observed that the percentages of Cd and S gradually enhanced with an increase in the deposition cycles from 4 to 8 to 12 to 16, which coincides with the results measured by EDS shown in Table S1. The optical properties of semiconductors are closely related to their structure and inner electron behavior and thus are recognized as key factors in the assessment of their photocatalytic activities.45 The Tauc plot for optical-band-gap determination is illustrated in Figure 5. The band gap (Eg) can be calculated from the equation (Rhν)2 = A(hν − Eg), where R is the absorption coefficient, hν is the photon energy, A is a constant for the material.48−50 The estimated values of direct Eg for ZnO, CdS/ZnO-4, CdS/ZnO-8, CdS/ZnO-12, CdS/ZnO-16, and CdS are 3.19, 3.12, 3.02, 2.62, 2.88, and 2.25 eV, respectively, suggesting that the Eg of ZnO can be reduced

Figure 5. Tauc plot for optical-band-gap determination and UV−vis absorption spectra (the inset) of all samples.

by the deposition CdS QDs, which can also be proven by the UV−vis absorption spectra of these samples (the inset in Figure 5). ZnO MFs exhibited strong absorption in the UV region (200−400 nm) because of its broad band gap (3.19 eV), while CdS presented strong absorption from the UV to visible light region (200−600 nm) because of its narrow band gap (2.25 eV). After CdS QDs were deposited, the absorption edge of CdS/ZnO overally shifted toward longer wavelength in comparison with ZnO MFs, suggesting that intimate contact between CdS QDs and ZnO NSs was successfully realized by this in situ deposition procedure.17,51 For comparison, a sample labeled as ZnO + CdS was also prepared by the simple mechanical mixing of ZnO MFs and CdS nanoparticles, which only presented a shoulder-like tail without changing the intrinsic optical absorption edge, indicating that mechanical mixing cannot realize intimate contact between the two semiconductors. The photocatalytic hydrogen evolution rates over these samples were measured under simulated solar light irradiation by using Na2S and Na2SO3 as sacrificial agents. As shown in Figure 6, ZnO MFs presented a relatively low photocatalytic activity with a hydrogen evolution rate of 0.16 mmol/g/h, which can be attributed to its restricted absorption to simulated sunlight. Compared with ZnO MFs, CdS nanoparticles exhibited a slightly higher hydrogen evolution rate (1.69 μmol/g/h) because of its narrow band gap. ZnO + CdS, the sample prepared by mechanical mixing, also presented a low hydrogen evolution rate (0.76 μmol/g/h), which can be ascribed to worse contact between CdS and ZnO. By contrast, 25381

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results indicate that, compared with tiny-sized materials, charge carriers produced in bulk materials are easy to encounter and recombine during their long migrations from the inner part to the surface, and this demonstrates that the separation and transfer of photogenerated carriers in the 2D/0D heterostructure are more efficient than those in bulk materials. Taking the actual application into consideration, the longtime stability is very important for photocatalysts. To investigate the photocatalytic stability of the as-prepared CdS/ZnO heterostructures, the recycling photocatalytic performance of CdS/ZnO-12 was measured. As shown in Figure 7a, no obvious decrease in the hydrogen evolution rate was detected within five cycles in 25 h, indicating that the asprepared CdS/ZnO-12 possesses excellent stability for photocatalytic hydrogen evolution. The XRD patterns of CdS/ZnO12 before and after 25 h irradiation were also measured to investigate the structure stability. No distinguishable change can be observed from the XRD patterns (Figure 7b), indicating that the CdS/ZnO-12 catalyst has good photostability. The separation and transfer efficiency of photogenerated charge carriers are crucial to the photocatalytic hydrogen evolution over photocatalysts. To investigate the separation and transfer efficiency of the photogenerated electrons and holes over the CdS/ZnO heterostructures, the photoelectrochemical properties of the as-synthesized samples were tested. The separation and transfer behavior of photoinduced electrons and holes can be reflected by the PL spectrum and photocurrent response.24 The PL spectra of ZnO and CdS/ZnO-x heterostructures at 325 nm excitation are shown in Figure 8a. ZnO showed the strongest emission at the wavelength of 590 nm because of the quick recombination of photogenerated electrons and holes. After CdS QDs were loaded, CdS/ZnO-x heterostructures exhibited quenched PL intensity, indicating the significant inhibition effect on the recombination of photogenerated electrons and holes due to the formation of a CdS/ZnO heterojunction.53−57 Figure 8b shows the photocurrent response of the samples under irradiation of simulated sunlight. Both CdS and ZnO showed low photocurrent response because of the quick recombination of photogenerated charge carriers. CdS/ZnO-x heterostructures showed an enhanced photocurrent response to the on/off of the light source, indicating more efficient separation and transfer of photogenerated carriers due to the heterojunction structure, which is crucial in improvement of the photocatalytic activity. In order to explore the possible photocatalytic mechanism, active species, hydroxyl radicals (•OH), over CdS/ZnO-12 was

Figure 6. Comparison of the photocatalytic hydrogen evolution rate over different samples during 5 h [25 mL of an aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3, 10 mg of catalysts, simulated-light irradiation (xenon lamp, 225 W)].

CdS/ZnO-x heterostructures showed remarkably enhanced hydrogen evolution rates because of the improved light absorption efficiency and quick separation and migration of photogenerated carriers resulting from the 0D/2D heterojunction structure. The hydrogen evolution rate gradually increased with an increase in the number of deposition cycles and then reached the highest value (22.12 mmol/g/h) when the number of deposition cycles was 12, which is 13 and 138 times higher than those of CdS (1.69 mmol/g/h) and ZnO (0.16 mmol/g/h), respectively. However, the hydrogen evolution rate over CdS/ZnO-16 sharply decreased to 9.74 mmol/g/h, which can be attributed to the destruction of the flower-like structure resulting from the overloading of CdS QDs.26,52 In order to highlight the superiority of the 0D/2D heterojunction structure, we further synthesized ZnO MFs/ CdS NSs (2D/2D), ZnO nanoparticles/CdS bulk (0D/3D), and ZnO bulk/CdS QDs (3D/0D) (the synthesized procedures are presented in the Supporting Information) and compared their photocatalytic performances with that of ZnO MFs/CdS QDs (2D/0D). As shown in Figure S3a, the hydrogen evolution rates over ZnO MFs/CdS NSs, ZnO nanoparticles/CdS bulk, and ZnO bulk/CdS QDs were 9.74, 3.19, and 1.32 mmol/g/h, respectively, which are far lower than that of ZnO MFs/CdS QDs (2D/0D; 22.12 mmol/g/h). From the results of the transient photocurrent responses (Figure S3b), it can be seen that ZnO MFs/CdS QDs (2D/0D) presented the highest response to light irradiation. These

Figure 7. Simulated-sunlight-driven photocatalytic hydrogen evolution over CdS/ZnO-12 during 25 h with evacuation every 5 h (a) and XRD spectra of the CdS/ZnO-12 composite before and after 25 h irradiation (b). 25382

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Figure 8. PL spectra of these samples under 325 nm excitation (a) and transient photocurrent responses of these samples at 0.5 V with on/off cycles under simulated sunlight (b) (a 0.4 M Na2SO4 solution was used as the electrolyte).

Figure 9. (a) Fluorescence spectra of CdS/ZnO-12 in a TA solution irradiated by simulated sunlight at different irradiation times (excitation at 315 nm). A total of 0.2 mL of solution was taken out after every 10 min irradiation for fluorescence spectral measurements. (b) Schematic diagram of the conventional heterojunction type and direct Z-scheme mechanisms over the CdS/ZnO heterostructures.

Figure 10. Proposed mechanism for the photocatalytic hydrogen evolution over the CdS/ZnO heterostructures.

negative than E(•OH/OH−) (+2.38 V vs NHE) and E(•OH/ H2O) (+2.27 V vs NHE),26 the holes in the VB of CdS cannot react with adsorbed water molecules (or surface hydroxyls) to form •OH radicals.26,60 In contrast, the holes in the VB of ZnO can react with these groups to produce •OH radicals because the VB position of ZnO (+2.89 eV) is more positive than E(•OH/OH−) and E(•OH/H2O). The generation of a large number of •OH radicals, confirmed by the fluorescence experiment, suggests that the holes must come from the VB of ZnO. Therefore, it can certainly be concluded that the photogenerated holes tend to remain in the VB of ZnO, while the photogenerated electrons transfer from the conduction band (CB) of ZnO to the VB of CdS, constructing a direct Z-

detected by the PL method.58 Terephthalic acid (TA) is generally utilized as a probe molecule because it can react with the formed •OH to generate the fluorescent agent 2hydroxyterephthalic acid (TAOH).52,59 The fluorescence intensity of TAOH is proportional to the generated amount of •OH. The detected fluorescence spectra of TAOH at a wavelength of 425 nm are shown in Figure 9a. No fluorescence was detected before irradiation, which indicates that there was no •OH in the original solution. The fluorescence intensity increased gradually with prolonged irradiation time, indicating that more and more •OH radicals were generated under irradiation of simulated sunlight. Because of the fact that the valence-band (VB) position of CdS (+1.88 eV) is more 25383

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CdS/ZnO-12 exhibited the highest hydrogen generation rate of 22.12 mmol/g/h, which was 13 and 138 times higher than those of CdS (1.68 mmol/g/h) and ZnO (0.16 mmol/g/h), respectively. The enhanced photocatalytic activity can be attributed to several positive factors, such as the formation of a Z-scheme photocatalytic system, the synergistic effect between the Z-scheme mechanism and the effective capture and transfer of electrons via the Pt cocatalyst, the tiny size effect of 2D ZnO NSs and 0D CdS QDs, and intimate contact between CdS QDs and ZnO NSs. Furthermore, CdS/ZnO-12 also presented excellent stability for photocatalytic hydrogen evolution without any decay within five cycles in 25 h. This work may provide a promising strategy to construct intimate contact heterostructure photocatalysts with the Z-scheme mechanism for highly efficient solar-to-fuel conversion.

scheme photocatalytic system without an electron mediator (Figure 9b). To specifically demonstrate the function of the Pt cocatalyst and the importance of the formed heterojunction between CdS and ZnO, the hydrogen evolution rates over ZnO + CdS + Pt, ZnO/CdS + Pt, and ZnO/CdS were measured (Figure S4). After ZnO and CdS were mechanically mixed, the resultant ZnO + CdS + Pt catalyst exhibited a hydrogen evolution rate of 0.78 mmol/g/h, which is far lower than that of CdS/ZnO + Pt (22.12 mmol/g/h), demonstrating that the formation of a heterojunction is crucial for the photocatalytic activity. Meanwhile, the CdS/ZnO catalyst exhibited a low catalyst activity (3.88 mmol/g/h) in the absence of Pt cocatalyst. These results suggest that the enhanced photocatalyst activity of ZnO/CdS + Pt can be attributed to the synergistic effect of both the formed heterojunction between ZnO and CdS and the effective capture and transfer of electrons via the Pt cocatalyst. On the basis of the experimental results and discussion above, a reasonable mechanism for the enhanced photocatalytic performance of the CdS QDs/ZnO NSs (0D/2D) heterojunction is proposed. As shown in Figure 10, when the CdS/ ZnO photocatalyst was irradiated by simulated sunlight, CdS QDs and ZnO NSs could absorb visible and UV light, respectively, to produce the photogenerated electrons and holes. The photogenerated electrons in the CB of ZnO could quickly transfer to the VB of CdS according to the direct Zscheme mechanism. As a result, the photogenerated electrons left in the CB of CdS could be captured by the formed Schottky barrier between Pt and CdS, which acts as an electron trap,61 and then be transferred to prevent their recombination, and thus dramatically boost the generation of hydrogen, while the photogenerated holes that remained in the VB of ZnO could be captured by the sacrificial agent. Thus, the Z-scheme photocatalytic system not only promotes the efficient separation of photogenerated electrons and holes but also retains the strong redox capability of the charge carriers, resulting in remarkably enhanced photocatalytic activity. Meanwhile, both 0D CdS QDs and 2D ZnO NSs effectively reduce the migration distance of electrons and holes from the inner part to the surface, which significantly decreases the recombination probability of electrons and holes. Furthermore, CdS QDs are densely distributed on both sides of every ZnO NS, resulting in the rapid migration of photogenerated carriers following the direct Z-scheme mechanism. Moreover, intimate contact between CdS QDs and ZnO NSs effectively reduces the migration resistance of photogenerated carriers. All of these positive factors effectively enhance the photocatalytic activity of the CdS QDs/ZnO NSs heterojunction for hydrogen evolution from photocatalytic water splitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08407. Experimental section, SEM images of ZnO-withoutNaOH and ZnO-without-ultrasonic, SEM and TEM images of CdS nanoparticles, photocatalytic H2 evolution rate and the transient photocurrent responses over ZnO nanoparticles/CdS bulk, ZnO MFs/CdS NS, ZnO MFs/ CdS QDs, and ZnO bulk/CdS QDs, a comparison of the photocatalytic H2 production rate over ZnO/CdS, ZnO/ CdS + Pt, and ZnO + CdS + Pt, and tables of the composition measured by EDS and XPS analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-W.S.). ORCID

Jian-Wen Shi: 0000-0002-2377-7491 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National Natural Science Fund Committee−Baosteel Group Corporation Steel Joint Research Fund, China (Grant U1460105), the National Science Foundation of China (Grant 51521065), and the Fundamental Research Funds for the Central Universities, China (Grant 2015gjhz06). EDS was carried out at the Center for Advancing Materials Performance from the Nanoscale (CAMP NANO). SEM and TEM were carried out at the International Center for Dielectric Research. We thank Chaowei Guo, Yanzhu Dai, Chuansheng Ma, and Lu Lu for their help in using EDS, SEM, and TEM.

4. CONCLUSIONS In summary, a novel CdS/ZnO heterojunction constructed of 0D CdS QDs and 2D ZnO NSs was rationally designed for the first time. The 2D ZnO NSs were assembled in ZnO MFs via an ultrasonic-assisted hydrothermal procedure (100 °C, 12 h) in the presence of a NaOH solution (0.06 M), and CdS QDs were deposited on both sides of every ZnO NS in situ by using the SILAR method. It was found that the ultrasonic treatment played an important role in the generation of ZnO NSs, while NaOH was responsible for the assembly of a flower-like structure. The obtained CdS/ZnO heterostructures presented remarkably enhanced photocatalytic activity for hydrogen evolution. Among these prepared CdS/ZnO heterostructures,

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