Article pubs.acs.org/JPCC
Artificial Photosynthetic Z‑scheme Photocatalyst for Hydrogen Evolution with High Quantum Efficiency Hong-Li Guo, Hong Du, Yi-Fan Jiang, Nan Jiang, Cong-Cong Shen, Xiao Zhou, Ya-Nan Liu, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China S Supporting Information *
ABSTRACT: As global energy consumption continues to rise, it is imperative to develop clean and renewable sources of energy as alternatives to the fossil fuel energy sources. Photocatalytic hydrogen generation or photochemical water splitting is a promising carbon-free technology producing H2 and O2 from water. Here, we report an exciting advance in the photocatalytic hydrogen evolution from water under visible light, using newly developed direct Z-scheme heterostructures constructed by oxygen deficient ZnO1−x nanorods and Zn0.2Cd0.8S nanoparticles by calcination. The highest rate for hydrogen production reaches 2518 μmol h−1 over optimal ZnO1−x (10 wt %)/Zn0.2Cd0.8S Z-scheme photocatalyst with a high apparent quantum efficiency (AQY) of 49.5% at 420 nm, which is 20 times higher than bare Zn0.2Cd0.8S and 25 times higher than ZnO1−x sample. From the results of photocurrent response, electrochemical impedance spectroscopy and time-resolved PL spectra, we demonstrate that the high increase in the photocatalytic hydrogen generation arises from the formation of artificial photosynthetic Z-scheme system and oxygen vacancy abundant ZnO1−x in the heterojunction. Direct Z-scheme ZnO1−x/ Zn0.2Cd0.8S nanoheterostructures result in an efficient charge carrier separation and strong reduction ability for enhanced H2 production; additionally, the presence of oxygen vacancies in the sample significantly enhances visible light absorption, these synergistic effects lead to highly efficient photocatalytic hydrogen production with an exceptional high quantum efficiency under visible light irradiation. Our findings provide possibilities for creating other high-efficiency photocatalysts mimicking natural photosynthetic Z-scheme system.
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INTRODUCTION Due to the increasing worldwide energy crisis and environmental pollution, hydrogen, an attractive sustainable clean energy source with zero emission, has become an alternative solution.1 An attractive environmentally friendly method, which captures available solar energy and converts it into valuable hydrogen, is the photocatalytic generation of hydrogen. Since the pioneering work of Fujishima and Honda2 in 1972, a large number of photocatalytic materials such as oxides, nitrides, and sulfides have been synthesized and explored for producing hydrogen from water.3 However, most of the developed photocatalysts capable of photocatalytic hydrogen generation from water can only utilize ultraviolet irradiation constituting only 4% of the incoming solar energy and thus rendering the overall process impractical. In this case, the development of the © XXXX American Chemical Society
photocatalysts with visible light response is of great importance to H2 production because the visible light accounts for about 43% of the solar light.4 Many researchers have taken up the research on semiconductor-type heterogeneous photocatalysts5 with cocatalysts loaded on the surface of semiconductor,6 promoting photocatalytic activities. Noble metals such as Ag,7 Pt,8 Ru,9 and Rh10 and metal oxides such as WO3,11 NiO,12 RuO213 and MoO214 were reported to be effective cocatalysts for photocatalytic hydrogen evolution. Polymeric carbon nitride (CN) semiconductor g-C3N4 has attracted significant attention because this catalyst exhibits stable performance in the Received: October 4, 2016 Revised: December 15, 2016
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Figure 1. TEM and HRTEM images of ZnO1−x (10 wt %)/Zn0.2Cd0.8S Z-scheme nanoheterostructure.
photocatalytic water-splitting reaction under visible light.15−21 Although an increasing number of photocatalysts have been developed, suitable materials with sufficiently small band gap, a suitable band gap position for photocatalytic hydrogen generation from water and adequate stability are still desired. Water splitting is an uphill reaction with a large positive change in Gibbs free energy (ΔGo = 238 kJ/mol), additionally, it is necessary to avoid the back reaction between hydrogen and oxygen.22,23 Therefore, the development of new materials exhibiting characteristic nanostructures could be useful for the design of the spatially separated active sites to achieve highly active photocatalysts for hydrogen evolution, which represents a central challenge of photocatalytic research. CdS absorbs visible light because of its narrow band gap of 2.4 eV, it alone shows activity in visible light photocatalytic H2 evolution, but the rate of H2 evolution is very low (ca. 15 μmol h−1).24 CdS loaded with cocatalyst or coupled with other semiconductor exhibits good performance in photocatalytic hydrogen production.24,25 However, it still suffers from disadvantages of a low efficiency of photogenerated electron− hole pair separation and photocorrosion in aqueous media when exposed to visible light.26 Recently, much efforts have been devoted to the incorporation of Zn2+ ions into CdS to form Zn1−xCdxS solid solution nanocrystals aiming to boost photocatalytic hydrogen production.27 The band gap of ZnxCd1−xS possesses a continuous modulation from 2.40 to 3.60 eV (x = 0−1), which influences the hydrogen production.28,29 Development of new catalysts containing ZnxCd1−xS with improved photocatalytic hydrogen production activity and stability is thus highly valued. ZnO is one of the most important wide band gap materials (3.20 eV) with versatile properties; coupling ZnO with a narrow band gap semiconductor is excepted to be an effective method to accelerate the electron−hole pair separation and, finally, enhances photoelectric conversion.30 Chen et al. reported the photoanodes consisting of CdS quantum dots sensitized ZnO− ZnS applied for solar cells.31 Rakshit et al. fabricated a hybrid solar cell using CdS decorated ZnO nanorods.32 The fabrication of a heterojunction in the right structure has been widely employed in photocatalysis, as it is an effective way to enhance the efficiency of photoinduced charge carrier separation, thus leading to better photocatalytic performance. Z-scheme photocatalytic system is one of the photocatalytic hydrogen evolution systems under visible light irradiation by introducing heterogeneous semiconductors, which applies a two-step excitation mechanism using two different photocatalysts. This system is inspired by natural photosynthesis in green plants, two different photocatalysts are combined using an appropriate shuttle redox mediator.33 Since Bard et al.
introduced the concept of Z-scheme hydrogen evolution in 1979,34 successful hydrogen evolution via two-step excitation using various combinations of photocatalysts has been reported.35−38 However, the photocatalytic efficiency of previously reported Z-scheme hydrogen evolution systems is still low. Our aim is to develop direct Z-scheme hydrogen evolution systems to boost photocatalytic performance. In this study, we present an advance in the photocatalytic hydrogen generation from water by using solar energy based on a new developed catalyst that is a unique Z-scheme nanoheterostructure comprising oxygen deficient ZnO1−x nanorods and Zn0.2Cd0.8S solid solution nanocrystals. The photocatalytic hydrogen production activity of the as developed ZnO1−x/Zn0.2Cd0.8S Z-scheme heterojunction significantly increases. The H2 evolution rate obtained using the optimized photocatalyst is as high as 2518 μmol h−1 (λ > 420 nm) and the AQY is 49.5% at 420 nm from aqueous solution containing Na2S and Na2SO3 as sacrificial reagents. To our knowledge, this is the highest AQY among Z-scheme visible light driven hydrogen evolution photocatalyst reported to date. Additionally, when using a very small portion of Pd nanoparticles (0.1 wt %) as a cocatalyst, the photocatalytic H2 evolution rate further increases up to 3779 μmol h−1 (λ > 420 nm) and the AQY reaches 62.1% at 420 nm.
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RESULTS AND DISCUSSION Oxygen vacancy-rich ZnO1−x nanorods (NRs) were prepared by hydrothermal treatment, Zn0.2Cd0.8S nanoparticles (NPs) were precipitated in aqueous solution at room temperature, and ZnO1−x/Zn0.2Cd0.8S Z-scheme nanoheterostructures were fabricated by annealing a mixture of ZnO1−x and Zn0.2Cd0.8S at 400 °C in N2 (see Supporting Information). X-ray diffraction (XRD) patterns of obtained samples are displayed in Figure S1, all diffraction peaks of ZnO1−x and ZnO1−x/Zn0.2Cd0.8S are well indexed either as the ternary compound Zn0.2Cd0.8S (JCPDS: 49−1302) or as hexagonal wurtzite structure of zinc oxide crystals (JCPDS: 36−1451), indicating the formation of ZnO1−x nanorods and ZnO1−x/ Zn0.2Cd0.8S nanojunctions. The morphology and structure of ZnO1−x nanorods, Zn0.2Cd0.8S nanoparticles, and ZnO1−x (10 wt %)/Zn0.2Cd0.8S photocatalyst were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure S2a shows typical SEM images of the as-grown ZnO1−x and Zn0.2Cd0.8S, the observed ZnO1−x nanorods are grown in a high density with the mean diameters of 40 nm. Figure S2b consists of irregular Zn0.2Cd0.8S nanoparticles with an average size of 15 nm. As shown in Figure 1a, Zn0.2Cd0.8S NPs grow on ZnO1−x NRs by calcination at 400 °C in N2, the intimate B
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Figure 2. XPS spectra of Zn 2p (a), Cd 3d (b), S 2p (c), and O 1s (d) for ZnO1−x (10 wt %)/Zn0.2Cd0.8S Z-scheme photocatalyst.
from the XPS data, is less than the Zn content (59%) in ZnO1−x (x = 0.31) sample, which demonstrates the existence of abundant oxygen vacancies in ZnO1−x NRs. Electron paramagnetic resonance (EPR) spectra of c-ZnO (c-ZnO is commercial zinc oxide), obtained a mechanical mixture of ZnO1−x and Zn0.2Cd0.8S, ZnO 1−x, ZnO1−x / Zn0.2Cd0.8S, and ZnO1−x (10 wt %)/Zn0.2Cd0.8S samples are displayed in Figure S3 and Figure 3, these sample exhibit a
interfacial contact between ZnO1−x and Zn0.2Cd0.8S is observed in the form of defined grain boundaries. The lattice fringes can be clearly observed in high-resolution TEM (HRTEM) image (Figure 1b), and the lattice spacing of ca. 0.52 nm can be attributed to ZnO wurtzite crystal, suggesting that each ZnO1−x nanorod grows along the (0001) direction.39 The lattice spacing of ca. 0.33 nm can be indexed as the (002) plane of the wurtzite CdS.40 These results clearly demonstrate that ZnO1−x/ Zn0.2Cd0.8S Z-scheme nanoheterostructures with strong interface coupling were successfully fabricated at 400 °C, which is favorable for improving the charge carrier separation between the photocatalysts and thus the photocatalytic activity. X-ray photoelectron spectroscopy (XPS) analysis was carried out to investigate the chemical states and compositions of the ZnO1−x/Zn0.2Cd0.8S Z-scheme nanoheterostructure. XPS data (Figure 2) reveals the presence of Zn, Cd, S, and O in the asprepared sample, and further confirms strong phase interaction by the binding energy shift of Zn 2p3/2, Cd 3d5/2, S 2p and O 1s in ZnO1−x/Zn0.2Cd0.8S heterostructure as compared to bare ZnO1−x and Zn0.2Cd0.8S.30,41 As shown in Figure 2a, the Zn 2p peaks located at 1023.1 eV for 2p3/2, and 1045.9 eV for 2p1/2. The Cd 3d spectrum (Figure 2b) shows two sharp peaks located at 411.8 and 405.1 eV, exhibiting a peak separation of 6.7 eV, which corresponds to the spin−orbit split components. The S 2p spectrum shown in Figure 2c exhibits a peak at 161.3 eV, corresponding to S2−.42 The typical O 1s peak can be consistently fitted by two Gaussian curves centered at 532.6 and 531.4 eV, respectively (Figure 2d). The low binding energy component accounting for about 38% of the O 1s spectrum is ascribed to the O2− ions in the wurtzite structure with hexagonal Zn2+ ions arranged periodically. The component at the high binding energy region, accounting for the remaining 62%, is related to the O 1s in the zinc hydroxide species and adsorbed or chemisorbed oxygen species due to surface hydroxyl groups.31 The atomic O content (41%) calculated
Figure 3. X-band EPR spectrum of ZnO1−x (10 wt %)/Zn0.2Cd0.8S sample measured at 140 K.
sharp EPR signal at g = 2.001, suggesting the presence of strong singly ionized oxygen vacancies.43,44 As compared to graycolored ZnO1−x sample alone (Figure S3), the EPR signal of ZnO1−x/Zn0.2Cd0.8S sample is significantly intensified, suggesting more oxygen vacancies at the interface are generated during calcination at 400 °C in N2. Figure 4 shows UV−visible diffuse reflectance spectra of ZnO1−x/Zn0.2Cd0.8S, Zn0.2Cd0.8S and ZnO1−x samples. It can be clearly seen that the oxygen-deficient ZnO1−x sample displays a deep gray color and a wide intense absorption in the visible C
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shows a good visible light photocatalytic activity with a H2 production rate of 98 μmol h−1, in contrast to commercial oxygen-vacancy-free c-ZnO with a negligible photocatalytic H2 evolution, implying the presence of oxygen vacancies in ZnO1−x leads to good visible photocatalytic activity. ZnO1−x (10 wt %)/Zn0.2Cd0.8S Z-scheme heterojunction exhibits a high H2 generation rate of 2518 μmol h−1 with an apparent quantum efficiency of 49.5% at 420 nm, which is 25 times higher than bare ZnO1−x sample and 20 times higher than Zn0.2Cd0.8S alone. Our ZnO1−x/Zn0.2Cd0.8S Z-scheme nanoheterostructures exhibit the highest H2 evolution rate among Z-scheme hydrogen generation systems without noble metal cocatalyst.45 Experiments prove that the ingenious design of nanoheterojunction offers suitable band gaps to substantially boost the photocatalytic performance. It is noted that a mechanical mixture of ZnO1−x (10 wt %) and Zn0.2Cd0.8S only produced a H2 rate of 120 μmol h−1, indicating the strong interaction at the interface of ZnO1−x/Zn0.2Cd0.8S induced by annealing leads to extremely higher photocatalytic activity than simply mixed ZnO1−x and Zn0.2Cd0.8S sample. While commercial c-ZnO coupled with Zn0.2Cd0.8S as catalyst under the same conditions, the hydrogen generation rate of c-ZnO/Zn0.2Cd0.8S is 186 μmol h−1, indicating abundant oxygen vacancies in the sample play a key role in high H2 production rate. The wavelength dependence of the quantum efficiency (QE) of H2 production over ZnO1−x (10 wt %)/Zn0.2Cd0.8S photocatalyst was measured with band-pass filters of different wavelengths, as shown in Figure S4. The QE of H2 production decreases with increasing incident wavelength. A maximum QE value of 49.5% was recorded at 420 nm, corresponding to the higher light absorption of the sample in the visible light range. ZnO1−x/ Zn0.2Cd0.8S heterostructure is active for H2 evolution even at 620 nm with a moderate QE of 7.1%. The hydrogen evolution rates of ZnO1−x/Zn0.2Cd0.8S photocatalysts with different amounts of ZnO1−x are presented in Figure S5, the results show that the content of ZnO1−x nanorods has an important influence on the photocatalytic activity. In the presence of a small amount of ZnO1−x NRs such as 5 and 7.5 wt %, the H2 production evolution rates are much higher than Zn0.2Cd0.8S alone. When increased the content of ZnO1−x up to 10 wt %, the catalyst achieved a maximum of H2 production rate. while the photocatalytic activity decreases when the content of ZnO1−x nanorods further increases (Figure S5). This is due to a fact that Zn0.2Cd0.8S is a major component of Z-scheme system contributing to H2 production. These results demonstrate that a suitable content of ZnO1−x nanorods is crucial for optimizing the photocatalytic activity of the ZnO1−x/Zn0.2Cd0.8S catalyst. Metal sulfides usually exhibit a decline in activity during the prolonged photocatalytic process due to photocorrosion.46 In order to investigate the stability of the as-prepared photocatalyst, the time courses of photocatalytic H2 evolution on ZnO1−x/Zn0.2Cd0.8S Z-scheme sample were carried out. As shown in Figure 6, no obvious decrease in photocatalytic activity is observed for five recycles, suggesting an excellent stability of our developed Z-scheme photocatalyst. During the photocatalytic reactions, photoinduced holes from the valence band of Zn0.2Cd0.8S and photogenerated electrons from the unoccupied states induced by oxygen vacancies of ZnO1−x can combine at the recombination center formed at the heterointerface,30,43,44 thus suppressing the oxidation of Zn0.2Cd0.8S. Transient photocurrent response curves of Ti foils coated with ZnO1−x/Zn0.2Cd0.8S, ZnO1−x, and Zn0.2Cd0.8S samples
Figure 4. UV−visible diffuse reflectance spectra of ZnO1−x/ Zn0.2Cd0.8S, Zn0.2Cd0.8S, and ZnO1−x sample. The inset is the photographs of ZnO 1−x (a), Zn 0.2 Cd 0.8 S (b), and ZnO 1−x / Zn0.2Cd0.8S (c).
light region. The obtained Zn0.2Cd0.8S NPs can absorb visible light with a wavelength of 492 nm, corresponding to a band gap of 2.52 eV. While ZnO1−x/Zn0.2Cd0.8S exhibits a wide and strong visible light absorption, revealing that the incorporation of ZnO1−x into Zn0.2Cd0.8S increases the UV−visible−NIR absorption, which is beneficial for the photocatalytic performance of the composites. Our previous studies demonstrate that this red shift can be attributed to abundant oxygen vacancies of gray color ZnO1−x in the hybrid.30 The color of samples changing from orange to deep orange (see inset photographs in Figure 4). As compared to bare ZnO1−x, ZnO1−x in ZnO1−x/ Zn0.2Cd0.8S sample exhibits a blue shift in absorption edge, this most likely arises from S doping in ZnO1−x at the interface layer of ZnO1−x/Zn0.2Cd0.8S during calcination at 400 °C in N2. The ZnO1−x/Zn0.2Cd0.8S sample has a larger red shift than bare ZnO1−x, owing to additional oxygen vacancies, which is in agreement with their EPR data. Figure 5 compares the visible photocatalytic H2 evolution performance of the ZnO1−x/Zn0.2Cd0.8S, c-ZnO/Zn0.2Cd0.8S, a mechanical mixture of ZnO1−x and Zn0.2Cd0.8S, Zn0.2Cd0.8S, and ZnO1−x samples in the presence of Na2S and Na2SO3 as sacrificial reagents. Zn0.2Cd0.8S alone exhibits a moderate photocatalytic activity with H2 evolution rate of 125 μmol h−1, while the obtained oxygen vacancy ZnO1−x nanorods
Figure 5. Photocatalytic hydrogen evolution rate of different samples under visible−light irradiation (λ > 420 nm). Reaction conditions: 0.1 g of catalyst in 100 mL aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3. D
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ductivity between ZnO 1−x and Zn 0.2 Cd 0.8 S boosts H 2 production rates, proving a key role of rapid electron transfer at the interface of the Z-scheme system.49 This result is consistent with the transient photocurrent response and demonstrates that the high photocatalytic performance of ZnO1−x/Zn0.2Cd0.8S Z-scheme nanoheterostructure originates from an efficient separation and nobility of photoinduced charge carriers.50 Steady-state photoluminescence (PL) spectra of Zn0.2Cd0.8S and ZnO1−x/Zn0.2Cd0.8S sample were measured and presented in Figure S6. As compared to Zn0.2Cd0.8S, the intensity of PL emission of ZnO1−x/Zn0.2Cd0.8S declines, suggesting the recombination of photogenerated charge carriers is reduced.51 Moreover, time-resolved photoluminescence (TRPL) decay spectra were measured to reveal the fast charge carrier transfer process of ZnO1−x/Zn0.2Cd0.8S sample. Figure 8 presents TRPL
Figure 6. Time-cycle photocatalytic hydrogen production over ZnO1−x/Zn0.2Cd0.8S with visible-light illumination (λ ≥ 420 nm).
were measured on a photoelectrochemical test device under visible light irradiation (λ ≥ 420 nm). As clearly shown in Figure 7a, when the visible light irradiation turns on, the photocurrent rapidly increases to a constant value for these samples. Notably, ZnO1−x/Zn0.2Cd0.8S Z-scheme sample exhibits a much higher photocurrent than bare Zn0.2Cd0.8S sample and ZnO1−x sample, indicating that Zn0.2Cd0.8S nanoparticles coupled with ZnO1−x nanorods can facilitate the transport of photoinduced charge carriers under visible light irradiation. Additionally, the on−off light cycles of the photocurrent response curves show good stability and reproducibility from the current vs time curve. ZnO1−x/ Zn0.2Cd0.8S Z-scheme photocatalyst with higher photocurrent implies a lower recombination rate of electron−hole pairs and more efficient photoelectron emigration, resulting in higher photocatalytic performance.14,47 In addition, electrochemical impedance spectroscopy (EIS) was employed to further investigate the interface charge separation efficiency. EIS curves of Ti foils coated with ZnO1−x/Zn0.2Cd0.8S, a mechanical mixture of ZnO1−x and Zn0.2Cd0.8S, ZnO1−x and Zn0.2Cd0.8S samples were measured on a photoelectrochemical test device. The EIS Nyquist plots of as-prepared ZnO1−x/Zn0.2Cd0.8S, simply mixed ZnO1−x and Zn0.2Cd0.8S, ZnO1−x and Zn0.2Cd0.8S samples are displayed in Figure 7b. The semicircle at the high frequency characterizes the process of charge transfer, and the smaller arc radius represents an efficient separation of photogenerated electron−hole pairs and a fast interface charge transfer process.48 ZnO1−x/Zn0.2Cd0.8S shows one arc/semicircle, implying that only the surface charge transfer step is involved in photocatalytic reactions. The arc for ZnO1−x/ Zn0.2Cd0.8S sample is smaller than the other samples, suggesting that the annealing-induced increase in the electrical con-
Figure 8. Fluorescence decay traces measured at room temperature for ZnO1−x/Zn0.2Cd0.8S, Zn0.2Cd0.8S, and ZnO1−x samples recorded at an excitation wavelength of 320 nm and probe at 500 nm.
decay spectra of ZnO1−x (black line), Zn0.2Cd0.8S (blue line), and ZnO1−x/Zn0.2Cd0.8S sample (red line), the results display that the lifetime of the excited states in ZnO1−x/Zn0.2Cd0.8S sample is much longer than that of other two samples, implying that the rapid transfer of photoexcited electrons and holes at the interface of ZnO1−x and Zn0.2Cd0.8S occurs, thus retarding the electron−hole pair recombination.45,52 The TRPL signal of ZnO1−x/Zn0.2Cd0.8S sample is manifested as positively-valued photoinduced absorption and can be described by two time constants: τ1 = 1.89 ns and τ2 = 18.17 ns. The observed two components in the nanosecond domain comes from quasi-free electron−hole pair recombination (fast component) and
Figure 7. (a) Photocurrent response vs time for ZnO1−x/Zn0.2Cd0.8S, ZnO1−x, and Zn0.2Cd0.8S samples (λ ≥ 420 nm). (b) EIS curves of as-prepared ZnO1−x/Zn0.2Cd0.8S, simply mixed ZnO1−x and Zn0.2Cd0.8S, and ZnO1−x and Zn0.2Cd0.8S samples. E
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electric resistance. These characters prompt the interface to be apt to serve as a recombination center of electron−hole pairs.45 Therefore, the photogenerated electrons in the CB of ZnO1−x rapidly transfer to the VB of Zn0.2Cd0.8S through the heterointerface and then recombine with the local holes, thus accelerating the separation of photogenerated electrons from the CB of Zn0.2Cd0.8S. Consequently, the electrons on the CB of Zn0.2Cd0.8S participate in the efficient reduction of H+ to H2. Meanwhile, the photogenerated holes on the surface of ZnO1−x could quickly move and are consumed by the sacrificial reagents, which would facilitate the process of charge separation and finally boost the photocatalytic activity with an exceptional high quantum efficiency (Scheme 1).
localized exciton recombination caused by detrapping of carriers (slow component).53 In general, prolonged carrier lifetime of ZnO1−x/Zn0.2Cd0.8S sample represents the efficient charge transfer between ZnO1−x NRs and Zn0.2Cd0.8S NPs in Zscheme heterojunction, which can retard the recombination of electron−hole pairs, in line with the results of photocatalytic activity. Noble metal nanoparticles are generally loaded on the photocatalyst as cocatalyst to further enhance the photocatalytic activity. Here we chose Pd as a very efficient cocatalyst. With an ultralow loading of Pd nanoparticles (0.1 wt %) as cocatalyst; the obtained Pd−ZnO1−x/Zn0.2Cd0.8S Z-scheme photocatalyst exhibits the extremely high activity in H2 evolution as high as 3779 μmol h−1. The maximum H2 production rate results in an apparent quantum efficiency of 62.1% at 420 nm. A Schottky junction is formed between metal Pd and ZnO1−x/Zn0.2Cd0.8S semiconductor: the transfer of photogenerated electrons to Pd cocatalyst further suppresses recombination of electron−hole pairs and significantly enhances kinetics of photocatalytic hydrogen generation.14 Intrinsic ZnO with a band gap of 3.20 eV can only absorb ultraviolet light and is insensitive to the visible part of the spectrum. Thanks to oxygen-vacancy-rich ZnO1−x that can be excited by visible light, we have successfully constructed a direct Z-scheme ZnO1−x/Zn0.2Cd0.8S photocatalyst with an unexpected high quantum efficiency in hydrogen production. Based on the above results, a direct Z-scheme mechanism of the ZnO1−x/Zn0.2Cd0.8S heterostructured photocatalyst is proposed. Scheme 1 displays the schematic diagram of photocatalytic hydrogen evolution over ZnO1−x/Zn0.2Cd0.8S photocatalyst under visible light irradiation.
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CONCLUSIONS In summary, we have developed unique direct Z-scheme nanoheterostructures made of oxygen deficient ZnO1−x nanorods and Zn0.2Cd0.8S nanoparticles by calcination. As compared to pristine ZnO1−x and Zn0.2Cd0.8S, the obtained Z-scheme sample shows extremely high activity and good stability in the photocatalytic hydrogen evolution from water under visible light irradiation. This artificial photosynthetic Z-scheme heterojunction exhibits an exceptionally high H2 generation rate of 2518 μmol h−1 with an apparent quantum efficiency of 49.5% (420 nm) at an optimal (10 wt %) ZnO1−x. Increased visible light absorption and efficient interfacial charge separation of ZnO1−x/Zn0.2Cd0.8S Z-scheme heterostructures are attributed to high efficiency photocatalytic hydrogen production under visible light. A direct Z-scheme mechanism is proposed and further confirmed by PL spectra and photoelectrochemical measurements. Overall, the H2 production rate of our ZnO1−x/Zn0.2Cd0.8S is exceptionally high without any noble metal cocatalyst, which endows our material more attractive for practical applications. We anticipate that this work will open up a prospect of utilizing available oxygenvacancy-rich metal oxide semiconductors for the construction of artificial photosynthetic Z-scheme heterostructures at the nanoscale and developing highly efficient innovative photocatalysts that mimic photosynthesis system.
Scheme 1. Schematic Illustrating Charge Separation and Photocatalytic Hydrogen Generation Process over Direct Zscheme ZnO1−x/Zn0.2Cd0.8S Heterojunction
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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/acs.jpcc.6b10013. Additional figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
Our previous study and theoretical calculations found that the oxygen vacancy (Vo••) defect energy level is below the conduction band (CB) edge and has no obvious effect on the band gap of ZnO1−x when the oxygen vacancy concentration is low.54,55 In the present case, the concentration of oxygen vacancies in ZnO1−x is high (x = 0.31); therefore, the oxygen vacancy level becomes more delocalized and overlaps with the CB edge (Scheme 1, left). Under visible-light illumination, both ZnO1−x and Zn0.2Cd0.8S are excited, leading to the excitation of electrons from their respective valence band (VB) to CB, leaving holes in their VB. It is noted that a large amounts of defects could be readily generated at the solid−solid heterointerface, which exhibits some similar properties to those of a conductor such as analogous energy levels and low
ORCID
An-Wu Xu: 0000-0002-4950-0490 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The special funding support from the National Natural Science Foundation of China (51572253, 21271165), Scientific Research Grant of Hefei Science Center of CAS (2015SRGHSC048), and cooperation between NSFC and Netherlands Organization for Scientific Research (51561135011) is acknowledged. F
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DOI: 10.1021/acs.jpcc.6b10013 J. Phys. Chem. C XXXX, XXX, XXX−XXX
