Enhanced Photoinduced-Stability and Photocatalytic Activity of CdS by

Feb 5, 2016 - Wei YuanZhen ZhangXiaoling CuiHuarong LiuChen TaiYuanrui Song. ACS Sustainable Chemistry & Engineering 2018 Article ASAP...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Enhanced Photoinduced-Stability and Photocatalytic Activity of CdS by Dual Amorphous Cocatalysts: Synergistic Effect of Ti(IV)-Hole Cocatalyst and Ni(II)-Electron Cocatalyst Huogen Yu,*,†,‡ Xiao Huang,† Ping Wang,† and Jiaguo Yu§ †

School of Chemistry, Chemical Engineering and Life Sciences, ‡State Key Laboratory of Silicate Materials for Architectures, and State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China §

S Supporting Information *

ABSTRACT: CdS is one of the most well-known and important visible-light photocatalytic materials for water splitting to produce hydrogen energy. Owing to its serious photocorrosion property (poor photoinduced stability), however, CdS photocatalyst can unavoidably be oxidized to form S0 by its photogenerated holes, causing an obviously decreased photocatalytic performance. In this study, to improve the photoinduced stability of CdS photocatalyst, amorphous TiO2 (referred to as Ti(IV)) as a hole cocatalyst was successfully loaded on the CdS surface to prepare Ti(IV)/CdS photocatalysts. It was found that the resultant Ti(IV)/CdS photocatalyst exhibited an obviously enhanced photocatalytic stability, namely, its deactivation rate clearly decreased from 37.9% to 13.5% after five cycles of photocatalytic reactions. However, its corresponding photocatalytic activity only showed a very limited increase (ca. 37.4%) compared with the naked CdS. To further improve its photocatalytic performance, the amorphous Ni(II) as an electron cocatalyst was subsequently modified on the Ti(IV)/CdS surface to prepare the dual amorphous-cocatalyst modified Ti(IV)−Ni(II)/CdS photocatalyst. In this case, the resultant Ti(IV)−Ni(II)/CdS photocatalyst not only exhibited a significantly improved photocatalytic activity and stability, but also could maintain the excellent photoinduced stability of CdS surface structure. Based on the experimental results, a synergistic effect of dual amorphous Ti(IV)−Ni(II) cocatalysts is proposed, namely, the amorphous Ti(IV) works as a hole-cocatalyst to rapidly capture the photogenerated holes from CdS surface, causing the less oxidation of surface lattice S2− ions in CdS, while the amorphous Ni(II) functions as an electron-cocatalyst to rapidly transfer the photogenerated electrons and then promote their following interfacial H2-evolution reaction. Compared with the traditional noble metal cocatalysts (such as Pt and RuO2), the present amorphous Ti(IV) and Ni(II) cocatalysts are apparently low-cost, nontoxic, and earth-abundant, which can widely be applied in the design and development of highly efficient photocatalytic materials.

1. INTRODUCTION Hydrogen, a new renewable and environmentally friendly energy, has been considered one of the most promising candidates to resolve the serious energy crisis.1,2 Since the discovery of photocatalytic hydrogen evolution from TiO2 by Fujishima and Honda in 1972,3 TiO2 has been extensively investigated as a photocatalyst for hydrogen production. Although TiO2 possesses excellent photocatalytic activity and stability,4 it requires near-ultraviolet (UV) irradiation (about 4% of the solar spectrum) for effective photocatalytic reactions, which severely limits its practical applications.5,6 Therefore, it is © XXXX American Chemical Society

highly desirable to develop visible-light-driven photocatalysts for hydrogen production under sunlight irradiation.7 Recently, various visible-light driven photocatalysts, including multicomponent oxides,8 sulfides,9 oxynitrides,10 and polymers,11 have widely been applied for hydrogen generation. Among them, CdS, one of the most important sulfides, has been extensively demonstrated to be a fascinating photocatalyst Received: January 6, 2016 Revised: January 24, 2016

A

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Graphical illustration for the synthesis of various samples: (a) CdS, (b) Ti(IV)/CdS, (c) Ni(II)/CdS, and (d) Ti(IV)−Ni(II)/CdS.

performance of TiO2 can be significantly improved by loading amorphous TiO2 as a hole-trapping center on the rutile TiO2 surface.29 Further investigations suggested that the amorphous TiO2 film could work as an excellent cocatalyst to promote the rapid transfer for photogenerated holes on BiVO4 and GaAs semiconductor photoelectrodes.30,31 Considering its low-cost, stable, and highly efficient properties, it is highly desirable to investigate whether the amorphous TiO2 can act as a holecocatalyst to improve the photoinduced stability of CdS photocatalyst, with the aim of improving its photocatalytic H2-evolution activity. In this study, to improve the photoinduced stability of the CdS photocatalyst, the amorphous Ti(IV) nanoparticles were successfully loaded on the CdS surface to form Ti(IV)/CdS photocatalyst by a simple impregnation method, and their photocatalytic activity and stability were evaluated by the photocatalytic hydrogen production under visible-light irradiation. It was found that amorphous Ti(IV) can be used as a hole-cocatalyst to greatly improve the photoinduced stability of CdS photocatalyst. However, owing to the lack of hydrogenevolution active sites, the resultant Ti(IV)/CdS showed a limited enhancement of photocatalytic activity. To improve its photocatalytic performance, the amorphous Ni(II) electroncocatalyst was further modified on the Ti(IV)/CdS surface by an impregnation method. As a consequence, the photoinduced stability and photocatalytic activity of CdS can be greatly improved by simultaneous modification of amorphous Ti(IV) and Ni(II), which is ascribed to the synergistic effect of Ti(IV) as a hole cocatalyst and Ni(II) as an electron cocatalyst. To the best of our knowledge, there is no report about the simultaneous modification of amorphous Ti(IV) and Ni(II) cocatalysts on the CdS surface to enhance its photoinduced stability and photocatalytic activity. This work may provide new insight for the development of highly stable and efficient photocatalytic materials for hydrogen production.

owing to its suitable bandgap (2.4 eV) for visible-light absorption and strong reduction potential (−0.51 V, vs SHE) for hydrogen evolution.12 However, the bare CdS usually shows a very low hydrogen-evolution activity because of the rapid recombination of photogenerated electrons and holes.13,14 To overcome this shortcoming, numerous efforts have been developed, including nanostructure modulation (e.g., nanoparticles,15 mesoporous structures,16 hollow structures,17 and one-dimensional structures18), coupling with other semiconductors (e.g., SnO2/CdS19 and TiO2/CdS20) and deposition of noble metals (e.g., Pt/CdS21 and Au/CdS22). In fact, the surface modification by loading cocatalyst has been regarded as one of the most important strategies for the improved photocatalytic performance of CdS owing to its facial synthetic method and the requirement of a very small amount of cocatalyst. Considering the expensive cost of the above noblemetal cocatalysts, it is highly required to develop a new, lowcost and efficient cocatalyst to further improve the H2-evolution performance of CdS photocatalyst. As an excellent photocatalytic material, possessing a stable structure during photocatalytic reactions is highly required in addition to its high-efficiency photocatalytic activity. However, compared with the well-known TiO2 photocatalyst, CdS material usually shows a serious photocorrosion during photocatalytic reactions under visible-light irradiation,23 which can cause an obviously decreased photocatalytic performance. The main reason can be ascribed to the fact that excess photogenerated holes are accumulated on the valence band (VB) of the CdS surface and subsequently induce the rapid oxidation of surface lattice S2− ions to form S0 phase. Therefore, enhancing its photoinduced stability (or preventing its photocorrosion) is highly required in the CdS system for practical applications. In this case, one of the effective strategies is to develop the CdS-coupled composite photocatalysts such as CdS−Cu2O,24 CdS−Fe3O4,25 and CdS−C3N4,26 and the principal mechanism for improved photoinduced stability of CdS can be attributed to the rapid transfer of photogenerated holes from CdS VB to the coupled semiconductors. On the other hand, the surface modification by loading hole-cocatalyst is also an efficient and important strategy to prevent the photocorrosion of CdS photocatalyst via the rapid transfer and separation of photogenerated holes. For example, Yan et al. reported that the photoinduced stability and photocatalytic performance of CdS photocatalyst could be greatly improved by loading PdS as a hole-cocatalyst and Pt as an electron cocatalyst.27 Zhang et al. also found that the antiphotocorrosion performance of CdS photocatalyst can be obviously enhanced by PdS-cocatalyst modification.28 However, the above-reported PdS hole-cocatalyst is very expensive and scarce, thus, largely limiting its practical applications. Hence, it is very necessary to develop new, highly efficient, and economical hole-cocatalyst materials. Recently, Liu et al. reported that the photocatalytic

2. EXPERIMENTAL SECTION 2.1. Preparation of CdS Sample. CdS sample was synthesized by a typical precipitation method. Briefly, 250 mL of 0.1 mol·L−1 Na2S solution was added dropwise into 250 mL of 0.1 mol·L−1 Cd(NO3)2 solution, and the resulting suspension was vigorously stirred for 2 h at room temperature. After stirring, the obtained yellow precipitate was filtrated, rinsed with deionized water several times, and dried at 60 °C for 12 h. Subsequently, the products were collected and ground into powder by an agate mortar. Finally, the resulting product was calcined at 550 °C for 4 h in a N2 atmosphere to obtain the CdS sample (Figure 1a). 2.2. Preparation of Amorphous Ti(IV)-Modified CdS Photocatalyst. The amorphous Ti(IV)-modified CdS (Ti(IV)/CdS) photocatalyst was prepared by an impregnation method via the hydrolysis of TiCl4. In a typical preparation, 0.2 B

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

cutoff filter (λ≥ 420 nm), which was positioned 20 cm away from the reactor, served as the irradiation light source to trigger the photocatalytic reaction. The focused intensity on the flask was about 180 mW cm−2. In a typical photocatalytic experiment, 50 mg of catalyst was dispersed in 80 mL of a mixed aqueous solution containing 0.35 mol·L−1 Na2S and 0.25 mol·L−1 Na2SO3, and the system was then bubbled with nitrogen for 30 min to remove the dissolved oxygen. In the process of irradiation, continuous stirring was applied to keep the photocatalyst particles in suspension state. Gas (0.4 mL) was intermittently sampled through the septum, and hydrogen was analyzed by a gas chromatograph (Shimadzu GC-1240, Japan, with nitrogen as a carrier gas) equipped with a 5 Å molecular sieve column and a thermal conductivity detector.

g of CdS was uniformly dispersed into 10 mL of distilled water under constant magnetic stirring. Subsequently, a known amount of TiCl4 solution with a pH of 2 (adjusted by 1 mol· L−1 HCl solution) was injected into the above suspension quickly. After stirring for 5 min, the suspension solution was maintained at 80 °C for 2 h. Finally, the resulting sample was filtrated, rinsed with deionized water, and dried at 60 °C to obtain the Ti(IV)/CdS photocatalysts (Figure 1b). To investigate the effect of Ti(IV) content on the photocatalytic performance of the CdS photocatalyst, the amount of Ti(IV) (the weight ratio of Ti to CdS) was controlled to be 0, 0.1, 0.5, 1, and 5 wt %, and the resultant Ti(IV)/CdS samples were referred to as CdS, Ti(IV)/CdS(0.1 wt %), Ti(IV)/CdS(0.5 wt %), Ti(IV)/CdS(1 wt %), and Ti(IV)/CdS(5 wt %), respectively. According to our present results, it was found that the Ti(IV)/CdS(1 wt %) showed the highest photocatalytic activity. Therefore, to further simply the sample same, the Ti(IV)/CdS(1 wt %) photocatalyst was referred to as Ti(IV)/CdS in this study. 2.3. Preparation of Amorphous Ni(II)-Modified CdS Photocatalyst. The amorphous Ni(II)-modified CdS (Ni(II)/ CdS) photocatalyst was also prepared by an impregnation method via the hydrolysis of Ni(NO3)2, similar to our previous routes about Fe(III)32,33 and Cu(II)34 cocatalysts. Briefly, 0.2 g of CdS was dispersed into 10 mL of Ni(NO3)2 solution to form a suspension solution. After stirring at 75 °C for 2 h, the resulting sample was filtrated, rinsed with deionized water, and finally dried at 60 °C to obtain the Ni(II)/CdS photocatalyst (Figure 1c). According to the previously reported results,35,36 when the amount of Ni (the weight ratio of Ni to CdS) was about 0.5 wt %, the resultant samples usually showed the highest photocatalytic activity. Therefore, in this study, the amount of Ni(II) to CdS was controlled to be 0.5 wt %. 2.4. Preparation of Ti(IV)−Ni(II)/CdS Photocatalyst. The dual amorphous Ti(IV)−Ni(II)-comodified CdS (Ti(IV)− Ni(II)/CdS) photocatalyst was prepared by using the methods described in sections 2.2 and 2.3. First, the Ti(IV)/CdS sample was prepared according to section 2.2, and then Ni(II) cocatalyst was further loaded onto the Ti(IV)/CdS(1 wt %) surface to form the Ti(IV)−Ni(II)/CdS photocatalyst (Figure 1d) according to section 2.3. 2.5. Characterization. Morphological observations were conducted on a JSM-7500 field emission scanning electron microscope (FESEM, JEOL, Japan) equipped with an X-Max 50 energy-dispersive X-ray spectrometer (EDS, Oxford Instruments, Britain) and a JEM-2100F transmission electron microscope (TEM, JEOL, Japan). X-ray diffraction (XRD) patterns were obtained on a Rigaku Ultima III X-ray Diffractometer (Japan) using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements were done on a KRATOA XSAM800 XPS system with a Mg Kα source. All the binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon. Raman spectra were collected using an INVIA spectrophotometer (Renishaw, U.K.). UV−vis absorption spectra were obtained using a UV−visible spectrophotometer (UV-2450, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV−vis diffuse reflectance experiment. 2.6. Photocatalytic H2-Production Activity. The photocatalytic hydrogen-production experiments were conducted in a 100 mL three-necked Pyrex flask at ambient temperature and atmospheric pressure, and the outlets of the flask were sealed with a silicone rubber septum. A 350 W xenon lamp with a UV

3. RESULTS AND DISCUSSION 3.1. Morphology and Microstructures of Ti(IV)−Ni(II)/ CdS. The preparation of various photocatalysts such as CdS, Ti(IV)/CdS, Ni(II)/CdS and Ni(II)−Ti(IV)/CdS can be easily controlled by a facile solution route, as shown in Figure 1. The Ti(IV)/CdS (Figure 1b) and Ni(II)/CdS (Figure 1c) samples can be easily prepared by an impregnation method, similar to our reported Fe(III) or Cu(II)-modified silver-based photocatalysts,37−39 while the preparation of Ni(II)−Ti(IV)/ CdS sample (Figure 1d) is the simple combination of the above methods. Since all the cocatalyst-modified samples are prepared from a low-temperature modification process (75−80 °C), it is quite believed that these Ti(IV) and Ni(II) cocatalysts are in amorphous structure and can only be loaded on the CdS surface, but not doped into its lattices. The controlled preparation of various cocatalyst-modified CdS photocatalysts can first be demonstrated by FESEM, TEM, and XRD results. Figure 2 shows the FESEM images of the CdS, Ti(IV)/CdS, Ni(II)/CdS, and Ti(IV)−Ni(II)/CdS. It can be seen that the CdS sample is composed of irregular particles with a size range of 50−200 nm due to a simple precipitation method, and the particle surface is smooth (Figure 2A). After their surface is modified with Ti(IV) cocatalyst, the resulting Ti(IV)/CdS (Figure 2B) shows a similar morphology as the CdS sample due to a very low amount of Ti(IV) cocatalyst (Ti/ CdS = 1 wt %). Further observation indicates that many nanoparticles with a size of 5−10 nm can clearly be observed on its surface. To investigate the components of those nanoparticles, the EDS analysis was performed and the corresponding results were shown in the inset in Figure 2B. It is clear that, in addition to the main Cd and S signals from the CdS, the Ti and O signals are clearly observed, implying that the Ti(IV) cocatalyst nanoparticles have been successfully loaded on the CdS surface. As for the Ni(II)/CdS (Figure 2C) and Ti(IV)−Ni(II)/CdS (Figure 2D) samples, they also show very similar particle morphologies with the pure CdS. In addition, it was found that the Ni(II) and Ti(IV)−Ni(II) cocatalyst were homogeneously loaded on the CdS particle surface, which can be well confirmed by their corresponding EDS results (Figure 2C,D). The amounts of Ti(IV) and Ni(II) can be calculated to be about 1.84 and 0.48 at% in the Ti(IV)− Ni(II)/CdS sample, respectively. Therefore, the above results strongly demonstrate the successful preparation of Ti(IV)− Ni(II)/CdS photocatalyst. The TEM results of Ti(IV)−Ni(II)/ CdS are shown in Figure 2E,F. It is clear that the Ti(IV) and Ni(II) cocatalysts with a very small size (ca. 5−10 nm) are homogeneously dispersed on the surface of the CdS sample (showed by the red arrows in Figure 2E), suggesting that the C

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

card No. 77−2306). In addition, all the cocatalyst-modified samples show a comparable diffraction peak intensity and full width at half-maximum compared with the pure CdS sample, suggesting that the crystallization and crystallite size of CdS are not affected by the different modification processes owing to their mild conditions, in good agreement with the results observed in FESEM images. However, it should be noted that no diffraction peaks of Ti(IV) and Ni(II) nanoparticles can be observed in the XRD pattern, owing to its very limited amount, amorphous phases, and well dispersion. XPS, an analytic technique about surface properties, is employed to further demonstrate the successful loading of Ti(IV) and Ni(II) cocatalyst on the surface of CdS samples. Figure 4A shows the XPS survey spectra for various samples. It can be concluded that Cd and S elements can be mainly ascribed to the CdS phase, while the O element is most likely from adsorbed H2O or amorphous Ti(IV) and Ni(II) compounds. Compared with pure CdS (Figure 4A-a), new XPS peaks of Ti and Ni elements are found in the Ti(IV)/CdS (Figure 4A-b) and Ni(II)/CdS (Figure 4A-c), respectively. To further reveal the existence of Ti and Ni elements and their chemical state, the high-resolution XPS spectra of the Ti(IV)/ CdS and Ni(II)/CdS samples are investigated. From Figure 4B,C, it is clear that the Ti(IV)/CdS sample shows obvious Ti 2p peaks at about 459.0 and 464.4 eV, and the Ni(II)/CdS sample shows obvious Ni 2p peaks at about 856.2 and 873.8 eV, which suggests that the Ti and Ni elements are in the +4 and +2 states,2,40 respectively. Considering a low-temperature hydrolysis process in this study, it is suggested that the Ti(IV) and Ni(II) cocatalysts are in the amorphous TiO(OH)2 and NiO(OH)-like structures on the CdS surface, respectively, similar to our previous studies.37,38 In addition, according to the element component analysis based on the XPS results (Table 1), it is clear that the amount of Ti(IV) in the Ti(IV)/CdS and Ni(II) in the Ni(II)/CdS are about 6.66 at% and 5.96 at%, while that of Ti(IV) and Ni(II) in the Ti(IV)−Ni(II)/CdS are 6.80 at% and 4.49 at%, respectively. Figure S1 shows the Cd 3d and S 2p XPS results. It is found that all samples show the Cd 3d peaks at 405.2 and 411.9 eV and the S 2p peaks at 161.5 and 162.7 eV, implying that Cd and S elements exist mainly in the form of Cd2+ and S2− in the CdS,26 respectively. Moreover, the peak intensities and positions of Cd and S show no shift, suggesting that the Ti(IV) and Ni(II) cocatalyst modification cannot obviously affect the surface microstructures of the CdS phase, but only loading on its surface. Figure 5A shows the UV−vis spectra of CdS, Ti(IV)/CdS, Ni(II)/CdS, and Ti(IV)−Ni(II)/CdS samples. Pure CdS nanoparticles exhibit a strong and broad absorption in the visible light region of 400−524 nm.41 After its surface modification with Ti(IV) and Ni(II) cocatalyst, the resultant Ti(IV)/CdS, Ni(II)/CdS, and Ti(IV)−Ni(II)/CdS photocatalysts show a similar absorption curve as the pure CdS, owing to the very limited amount of Ti(IV) and Ni(II) cocatalysts. Hence, it is clear that the modification of Ti(IV) and Ni(II) cocatalysts does not significantly affect the optical absorption feature of CdS nanoparticles.31 Figure 5B presents a comparison of the Raman patterns of CdS, Ti(IV)/CdS, Ni(II)/CdS, and Ti(IV)−Ni(II)/CdS samples. It is found that all samples show the same Raman shift as the pure CdS, which suggests that the microstructures of CdS can be well maintained after coupling with Ti(IV) and Ni(II) cocatalysts. Further observation indicates that, after modification with Ti(IV) and Ni(II) cocatalysts, the resultant Ti(IV)/CdS,

Figure 2. FESEM images of various samples: (A) CdS, (B) Ti(IV)/ CdS, (C) Ni(II)/CdS, (D) Ni(II)−Ti(IV)/CdS; TEM (E) and HRTEM (F) images of Ti(IV)−Ni(II)/CdS.

present impregnation method is an excellent strategy for the loading of Ti(IV) and Ni(II) cocatalysts. In addition, it is found that Ti(IV) and Ni(II) nanoparticles are in an amorphous phase on the CdS surface (shown by the red arrows in Figure 2F) owing to a low temperature modification progress in this study. Moreover, it also shows that there is an intimate contact between the amorphous cocatalysts and the CdS particles. In fact, the well interfacial-coupling interaction is beneficial to the effective transfer of photogenerated charges between the cocatalysts and CdS photocatalyst, which is significant for the enhancement of photocatalytic performance and photoinduced stability. The crystal structures of different samples are revealed by XRD patterns (Figure 3). It is clear that all the diffraction peaks of the CdS, Ti(IV)/CdS, Ni(II)/CdS, and Ti(IV)−Ni(II)/CdS samples can be indexed to the hexagonal CdS phase (JCPDS

Figure 3. XRD patterns of various samples: (a) CdS, (b) Ti(IV)/CdS, (c) Ni(II)/CdS, and (d) Ti(IV)−Ni(II)/CdS. D

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 4. (A) XPS survey spectra and (B, C) the high-resolution XPS spectra of (B) Ti 2p and (C) Ni 2p for various samples: (a) CdS, (b) Ti(IV)/ CdS, (c) Ni(II)/CdS, and (d) Ti(IV)−Ni(II)/CdS.

Table 1. Element Components of Various Samples Based on the XPS Results samples

Cd

S

O

Ni

Ti

CdS CdS(after) Ti(IV)/CdS Ti(IV)/CdS(after) Ni(II)/CdS Ni(II)/CdS(after) Ti(IV)−Ni(II)/CdS Ti(IV)−Ni(II)/CdS(after)

41.75 40.25 22.12 24.36 32.32 35.65 28.28 29.81

31.47 32.33 23.56 23.90 25.44 31.76 23.96 25.51

26.78 27.42 47.66 43.89 36.28 30.41 36.46 35.30

0 0 0 0 5.96 2.18 4.49 1.67

0 0 6.66 7.85 0 0 6.81 7.71

Raa (%) 15.9 2.3 28.2 3.1

a a

R represents the atom ratio of S0 to total S (S0 + S2−).

catalytic reactions. Especially, in the fifth cycle, the deactivation rate of the CdS sample reaches to 37.9%. However, after modification with Ti(IV) cocatalyst, the deactivation rate (13.5%) of Ti(IV)/CdS can clearly be decreased compared with the pure CdS sample, suggesting that the Ti(IV) cocatalyst not only can improve the photocatalytic activity of CdS photocatalyst, but also contribute to a more stable photocatalytic performance. It is very interesting to investigate the potential mechanism about the stable photocatalytic performance and improved photocatalytic activity of CdS by loading amorphous Ti(IV) cocatalyst. In fact, amorphous Ti(IV) has been demonstrated to act as a hole-cocatalyst to improve the photocatalytic performance of TiO2 via effectively capturing photogenerated holes.29 In the present work, it is believed that the amorphous Ti(IV) can also work as a hole-cocatalyst to modify the photocatalytic activity and stability of CdS. Figure 7a and b show the possible photocatalytic mechanism of CdS and Ti(IV)/CdS photocatalysts, respectively. When the CdS photocatalyst is illuminated by visible light, photogenerated

Ni(II)/CdS, and Ti(IV)−Ni(II)/CdS samples show a weaker peak intensity compared to the pure CdS, owing to the good surrounding of CdS particles by the Ti(IV) and Ni(II) cocatalysts, which can further demonstrate that the Ti(IV) and Ni(II) cocatalysts are only loaded on the CdS surface. 3.2. Enhanced Photocatalytic Activity of Ti(IV)/CdS. The photocatalytic hydrogen-production performances of Ti(IV)/CdS photocatalysts are evaluated under visible light (λ ≥ 420 nm) irradiation in a Na2SO3−Na2S mixed solution. Figure S2 shows the photocatalytic H2-production rate of various Ti(IV)/CdS samples. It can be seen that all the Ti(IV)/ CdS samples exhibit higher photocatalytic activities than the pure CdS, and the Ti(IV)/CdS(1 wt %) photocatalyst shows the highest photocatalytic performance with a H2-production rate of 2270.41 μmol h−1 g−1. To investigate the effect of Ti(IV) cocatalyst on the performance stability of the CdS photocatalyst, five cycles of the photocatalytic tests are performed, and their corresponding results are displayed in Figure 6. It is found that the pure CdS sample shows a gradually decreased performance during the repeating photoE

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 7. Photocatalytic mechanism of various samples: (a) CdS, (b) Ti(IV)/CdS, (c) Ni(II)/CdS, and (d) Ti(IV)−Ni(II)/CdS.

then are rapidly consumed by the Na2SO3−Na2S scavenger, causing the less oxidation of surface S2− ions (Figure 7b). On the other hand, owing to the rapid capture of photogenerated holes by the Ti(IV) cocatalyst, more photogenerated electrons are generated on the CB of CdS, which can lead to a higher photocatalytic H2-evolution performance. 3.3. Further Improved Photocatalytic Activity of Ti(IV)/CdS by Loading Amorphous Ni(II) Cocatalyst. It is found that, although the performance stability of CdS sample can be greatly enhanced by loading amorphous Ti(IV), its improved photocatalytic activity with a value of 37.4% was still very limited. To further enhance the photocatalytic performance of Ti(IV)/CdS, the Ni(II) cocatalyst was loaded on its surface to prepare Ti(IV)−Ni(II)/CdS by a facile impregnation method (Figure 1d). For comparison, we also prepared the Ni(II)-modified CdS photocatalyst (Ni(II)/CdS) under an identical condition (Figure 1c). According to the photocatalytic results (Figure 6c), the resultant Ni(II)/CdS clearly shows an improved photocatalytic activity with a rate constant of 3161.18 μmol h−1 g−1 compared with the CdS and Ti(IV)/CdS photocatalysts. After the Ni(II) cocatalyst is further loaded on the Ti(IV)/CdS surface, the resulting Ti(IV)−Ni(II)/CdS photocatalyst shows the highest photocatalytic activity (3437.95 μmol h−1g−1), which is obviously higher than that of the pure CdS by a factor of 2.1. Moreover, even for the fifth cycle, the deactivation rate (ca. 10.1%) of Ti(IV)−Ni(II)/CdS photocatalyst is clearly lower than that (37.9%) of the CdS sample. Therefore, the Ti(IV)−Ni(II)/CdS photocatalyst not only shows a high photocatalytic activity, but also can maintain a stable photocatalytic performance, which is highly required and necessary for practical applications. In view of the above interesting results, it is very meaningful to investigate the possible photocatalytic mechanism of Ni(II)modified photocatalysts. For the Ni(II)/CdS (Figure 7c), its photocatalytic mechanism is completely different from the Ti(IV)/CdS photocatalyst (Figure 7b). It is well-known that the Ni(OH)242,43 and NiO44−46 have widely been demonstrated to be an efficient electron-cocatalyst for the improved photocatalytic performance via quickly capturing photogenerated electrons and promoting the interfacial H2-evolution reaction. In this study, when the CdS surface is modified with Ni(II) cocatalyst, the photogenerated electrons of CdS can rapidly transfer to the Ni(II) cocatalyst owing to the more positive potential (−0.23 V vs SHE, pH = 0) of Ni2+/Ni0 than

Figure 5. (A) UV−vis diffuse reflectance spectra and (B) Raman spectra for various samples: (a) CdS, (b) Ti(IV)/CdS, (c) Ni(II)/ CdS, and (d) Ti(IV)−Ni(II)/CdS.

Figure 6. Cycling runs of the photocatalytic activity for various samples: (a) CdS, (b) Ti(IV)/CdS, (c) Ni(II)/CdS, and (d) Ti(IV)− Ni(II)/CdS.

electrons are produced on the CB of CdS particles, while photogenerated holes remain on its VB. Subsequently, the photogenerated electrons transfer to the CdS surface to produce hydrogen, and the photogenerated holes are accumulated on the VB of CdS and then can be consumed by the Na2SO3−Na2S scavenger gradually. Considering a comparable hole-capture ability of the lattice S2− on the CdS surface and the S2− ions in the Na2SO3−Na2S solution, the photogenerated holes tend to accumulate on the CdS surface, causing the rapid oxidation of surface lattice S2− ions to form the S0 phase (the right side in Figure 7a). As a result, CdS photocatalyst shows a gradually decreased photocatalytic performance during repeating tests (Figure 6a). To suppress the deactivation rate of CdS, it is quite necessary to promote the rapid transfer and separation of photogenerated holes from the CdS surface. In this case, when the Ti(IV) cocatalyst is loaded on the CdS surface, the photogenerated holes on the VB of CdS would first transfer to the Ti(IV) hole-cocatalyst and F

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C the CB level of CdS (ca. −0.7 V),47 as shown in Figure 7c. When both of the Ni(II) and Ti(IV) cocatalysts are simultaneously modified on the CdS surface (Figure 7d), the Ni(II) cocatalyst can first serve as an effective capture site to accept the photogenerated electrons and then act as the reduction active site to promote the interfacial H2-evolution reactions, while the Ti(IV) cocatalyst can work as the hole cocatalyst to rapidly transfer the photogenerated holes and boot their oxidation reaction. Therefore, the Ti(IV)−Ni(II)/CdS photocatalyst exhibits an obviously enhanced photocatalytic activity and stability owing to the synergistic action of Ti(IV)hole cocatalyst and Ni(II)-electron cocatalyst. 3.4. Dependence of Microstructures on the Photocatalytic Performance of Cocatalyst-Modified CdS. It is well-known that the photocatalytic performance of a semiconductor material strongly depends on its surface microstructures since the photocatalytic reactions belongs to heterogeneous catalysis that can only be conducted on its interface. To further investigate the effect of surface microstructures on the photocatalytic performance (including their activity and stability) of CdS photocatalysts, the various samples such as CdS(after), Ti(IV)/CdS(after), Ni(II)/CdS(after), and Ti(IV)−Ni(II)/CdS(after) were obtained after their repeated photocatalytic reactions, and their morphologies and microstructures were characterized by FESEM, XRD, UV− vis, and XPS technologies. From the FESEM (Figure S3), XRD (Figure S4), and XPS (Figure S5) results, it is clear that the main phase structure and surface microstructures of various CdS samples show no obvious change before and after repeated photocatalytic reactions, suggesting a stable structure of CdSbased photocatalysts. However, from the UV−vis spectra (Figure 8), it is clear that the various CdS samples (after

Figure 9. High-resolution XPS spectra of S 2p for the various samples after photocatalytic reactions: (a) CdS(after), (b) Ti(IV)/CdS(after), (c) Ni(II)/CdS(after), and (d) Ti(IV)−Ni(II)/CdS(after).

ascribed to its photoinduced instability (Figure 7a), namely, the destruction of surface structure and the formation of S0 (S0/ S(total) = 15.9 at%). After surface modification by Ti(IV), the resultant Ti(IV)/CdS(after) exhibits a similar UV−vis spectra and color as the as-prepared CdS sample, suggesting the negligible formation of S0 during light irradiation (Figure 8b). In fact, the above results can further be demonstrated by its XPS results (Figure 9b), where the less amount of S0 is produced during repeating photocatalytic tests. In this case, the photoinduced stability of CdS photocatalyst can be greatly improved by loading Ti(IV) hole cocatalyst (Figure 7b). For the Ni(II)-modified CdS, owing to the rapid transfer of photogenerated electrons, more photogenerated holes can be accumulated on the VB of CdS, causing the formation of more S0 (S0/S(total) = 28.2%, Figure 9c) and a darker color (Figure 8d). Therefore, the as-prepared Ni(II)/CdS photocatalyst is instable during photocatalytic reaction although it shows a high photocatalytic activity (Figure 7c). When both of the amorphous Ti(IV) and Ni(II) are simultaneously loaded on the CdS surface, the Ti(IV)−Ni(II)/CdS photocatalyst shows a highly efficient and stable photocatalytic performance due to its formation of stable microstructure (Figure 7d), which can be well explained by the similar UV−vis spectrum (Figure 8e) and XPS result (Figure 9d) as the Ti(IV)/CdS sample (Figure 8c and Figure 9b).

4. CONCLUSION In summary, amorphous Ti(IV) as an effective hole cocatalyst was successfully loaded on the CdS surface by a facile impregnation method and the resultant Ti(IV)/CdS photocatalysts exhibited an obviously enhanced photocatalytic stability (a decreased deactivation rate from 37.9% to 13.5%) in addition to its limited improvement (37.4%) of photocatalytic activity. To further improve its photocatalytic performance, the amorphous Ni(II) as an electron cocatalyst was modified on the surface of Ti(IV)/CdS to prepare the dual amorphous-cocatalyst modified Ti(IV)−Ni(II)/CdS photocatalyst. As a consequence, the photocatalytic activity and stability of CdS can be greatly improved by simultaneous modification of amorphous Ti(IV) and Ni(II) due to their synergistic effect, namely, the Ti(IV) hole-cocatalyst can effectively reduce the photocorrosion effect of CdS by rapidly transferring the interfacial photogenerated holes, while the Ni(II) electron-cocatalyst first can rapidly capture the photogenerated electrons and then function as the reduction active

Figure 8. UV−vis diffuse reflectance spectra of (a) as-prepared CdS and (b−e) the various samples after photocatalytic reactions: (b) CdS(after), (c) Ti(IV)/CdS(after), (d) Ni(II)/CdS(after), and (e) Ti(IV)−Ni(II)/CdS(after). Inset showing their corresponding photographs.

photocatalytic reactions) show different light-absorption ability owing to their different surface structures. Compared with the as-prepared CdS (Figure 8a), the CdS(after) (Figure 8b) shows an obviously higher absorption in the range of 520−700 nm and a darker color (the inset in Figure 8), which can be attributed to the formation of S0 via the oxidation of surface lattice S2− by the photogenerated holes (Figure 7a). The production of S0 can be well confirmed by its corresponding XPS results (Figure 9a) where, in addition to the main S2− in CdS, a small amount of S0 is clearly observed.48 As a consequence, it is suggested that the deactivation (37.9%) of CdS sample during repeating photocatalytic tests can be G

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Polymer Heterojunction Photocatalyst: The Significance of Sacrificial Reagent. ACS Sustainable Chem. Eng. 2015, 3, 1501−1509. (12) Zhao, Z. G.; Liu, Z. F.; Miyauchi, M. Tailored Remote Photochromic Coloration of in situ Synthesized CdS Quantum Dot Loaded WO3 Films. Adv. Funct. Mater. 2010, 20, 4162−4167. (13) Shi, Y.; Li, H.; Wang, L.; Shen, W.; Chen, H. Novel α-Fe2O3/ CdS cornlike nanorods with enhanced photocatalytic performance. ACS Appl. Mater. Interfaces 2012, 4, 4800−4806. (14) Nascimento, C. C.; Andrade, G. R.; Neves, E. C.; Barbosa, C. D. A. E. S.; Costa, L. P.; Barreto, L. S.; Gimenez, I. F. Nanocomposites of CdS nanocrystals with montmorillonite functionalized with thiourea derivatives and their use in photocatalysis. J. Phys. Chem. C 2012, 116, 21992−22000. (15) Zhang, W.; Wang, Y.; Wang, Z.; Zhong, Z.; Xu, R. Highly efficient and noble metal-free NiS/CdS photocatalysts for H2 evolution from lactic acid sacrificial solution under visible light. Chem. Commun. 2010, 46, 7631−7633. (16) Li, Y.; Zhu, Y.; Yang, X.; Li, C. Mesoporous silica spheres as microreactors for performing CdS nanocrystal synthesis. Cryst. Growth Des. 2008, 8, 4494−4498. (17) Luo, M.; Liu, Y.; Hu, J.; Liu, H.; Li, J. One-pot synthesis of CdS and Ni-doped CdS hollow spheres with enhanced photocatalytic activity and durability. ACS Appl. Mater. Interfaces 2012, 4, 1813− 1821. (18) Wang, L.; Wei, H.; Fan, Y.; Gu, X.; Zhan, J. One-dimensional CdS/α-Fe2O3 and CdS/Fe3O4 heterostructures: epitaxial and nonepitaxial growth and photocatalytic activity. J. Phys. Chem. C 2009, 113, 14119−14125. (19) Liu, Y.; Zhang, P.; Tian, B.; Zhang, J. Core−Shell Structural CdS@SnO2 Nanorods with Excellent Visible-Light Photocatalytic Activity for the Selective Oxidation of Benzyl Alcohol to Benzaldehyde. ACS Appl. Mater. Interfaces 2015, 7, 13849−13858. (20) Liu, S.; Zhang, N.; Tang, Z.-R.; Xu, Y.-J. Synthesis of onedimensional CdS@TiO2 core-shell nanocomposites photocatalyst for selective redox: the dual role of TiO2 shell. ACS Appl. Mater. Interfaces 2012, 4, 6378−6385. (21) Wang, Y.; Wang, Y.; Xu, R. Photochemical deposition of Pt on CdS for H2 evolution from water: markedly enhanced activity by controlling Pt reduction environment. J. Phys. Chem. C 2013, 117, 783−790. (22) Demortière, A.; Schaller, R. D.; Li, T.; Chattopadhyay, S.; Krylova, G.; Shibata, T.; dos Santos Claro, P. C.; Rowland, C. E.; Miller, J. T.; Cook, R. In Situ Optical and Structural Studies on Photoluminesence Quenching in CdSe/CdS/Au Heterostructures. J. Am. Chem. Soc. 2014, 136, 2342−2350. (23) Yang, M.-Q.; Han, C.; Xu, Y.-J. Insight into the Effect of Highly Dispersed MoS2 versus Layer-Structured MoS2 on the Photocorrosion and Photoactivity of CdS in Graphene-CdS-MoS2 Composites. J. Phys. Chem. C 2015, 119, 27234−27246. (24) Cheng, W.-Y.; Yu, T.-H.; Chao, K.-J.; Lu, S.-Y. Cu2O-decorated CdS nanostructures for high efficiency visible light driven hydrogen production. Int. J. Hydrogen Energy 2013, 38, 9665−9672. (25) Liu, X.; Fang, Z.; Zhang, X.; Zhang, W.; Wei, X.; Geng, B. Preparation and characterization of Fe3O4/CdS nanocomposites and their use as recyclable photocatalysts. Cryst. Growth Des. 2009, 9, 197− 202. (26) Yu, H.; Chen, F.; Chen, F.; Wang, X. In situ self-transformation synthesis of g-C3N4-modified CdS heterostructure with enhanced photocatalytic activity. Appl. Surf. Sci. 2015, 358, 385−392. (27) Yan, H.; Yang, J.; Ma, G.; Wu, G.; Zong, X.; Lei, Z.; Shi, J.; Li, C. Visible-light-driven hydrogen production with extremely high quantum efficiency on Pt-PdS/CdS photocatalyst. J. Catal. 2009, 266, 165−168. (28) Zhang, S.; Chen, Q.; Jing, D.; Wang, Y.; Guo, L. Visible photoactivity and antiphotocorrosion performance of PdS-CdS photocatalysts modified by polyaniline. Int. J. Hydrogen Energy 2012, 37, 791−796. (29) Liu, M.; Inde, R.; Nishikawa, M.; Qiu, X.; Atarashi, D.; Sakai, E.; Nosaka, Y.; Hashimoto, K.; Miyauchi, M. Enhanced photoactivity with

site to promote the interfacial H2-evolution reaction. Compared with the traditional noble metal cocatalyst (such as Pt and RuO2), the present amorphous Ti(IV) and Ni(II) cocatalysts with low-cost, nontoxic, and earth-abundant properties can offer various applications for the high-efficiency photocatalytic materials.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00126. Additional supporting figures (PDF).

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21277107, 51472192, and 21477094) and 973 Program (2013CB632402). This work was also financially supported by a program for new century excellent talents in university (NCET-13-0944), and the Fundamental Research Funds for the Central Universities (WUT 2015IB002).

■

REFERENCES

(1) Song, J.; Huang, Z.-F.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Oxygen-Deficient Tungsten Oxide as Versatile and Efficient Hydrogenation Catalyst. ACS Catal. 2015, 5, 6594−6599. (2) Wang, Z.; Liu, G.; Ding, C.; Chen, Z.; Zhang, F.; Shi, J.; Li, C. Synergetic Effect of Conjugated Ni(OH)2/IrO2 Cocatalyst on Titanium-Doped Hematite Photoanode for Solar Water Splitting. J. Phys. Chem. C 2015, 119, 19607−19612. (3) Fujishima, A.; Honda, K. TiO2 photoelectrochemistry and photocatalysis. Nature 1972, 238, 37−38. (4) Liu, R.; Wang, P.; Wang, X. F.; Yu, H. G.; Yu, J. G. UV-and Visible-Light Photocatalytic Activity of Simultaneously Deposited and Doped Ag/Ag(I)-TiO2 Photocatalyst. J. Phys. Chem. C 2012, 116, 17721−17728. (5) Jiang, J. J.; Long, M. C.; Wu, D. Y.; Cai, W. M. Preparation of FModified Nanosized TiO2 and Its Methyl Orange Photodegradation Mechanism. Acta Phys-Chim. Sin. 2011, 27, 1149−1156. (6) Liu, Z. F.; Li, Y. J.; Zhao, Z. G.; Cui, Y.; Hara, K.; Miyauchi, M. Block copolymer templated nanoporous TiO2 for quantum-dotsensitized solar cells. J. Mater. Chem. 2010, 20, 492−497. (7) Zuo, F.; Wang, L.; Wu, T.; Zhang, Z.; Borchardt, D.; Feng, P. Self-doped Ti3+ enhanced photocatalyst for hydrogen production under visible light. J. Am. Chem. Soc. 2010, 132, 11856−11857. (8) Kim, H. G.; Hwang, D. W.; Lee, J. S. An undoped, single-phase oxide photocatalyst working under visible light. J. Am. Chem. Soc. 2004, 126, 8912−8913. (9) Wang, W.; Ng, T. W.; Ho, W. K.; Huang, J.; Liang, S.; An, T.; Li, G.; Yu, J. C.; Wong, P. K. CdIn2S4 microsphere as an efficient visiblelight-driven photocatalyst for bacterial inactivation: Synthesis, characterizations and photocatalytic inactivation mechanisms. Appl. Catal., B 2013, 129, 482−490. (10) Brancho, J. J.; Bartlett, B. M. Challenges in Co-Alloyed Titanium Oxynitrides, a Promising Class of Photochemically Active Materials. Chem. Mater. 2015, 27, 7207−7217. (11) Zhang, X.; Peng, B.; Zhang, S.; Peng, T. Robust Wide VisibleLight-Responsive Photoactivity for H2 Production over a Polymer/ H

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C nanocluster-grafted titanium dioxide photocatalysts. ACS Nano 2014, 8, 7229−7238. (30) Eisenberg, D.; Ahn, H. S.; Bard, A. J. Enhanced photoelectrochemical water oxidation on bismuth vanadate by electrodeposition of amorphous titanium dioxide. J. Am. Chem. Soc. 2014, 136, 14011−14014. (31) Hu, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science 2014, 344, 1005−1009. (32) Wang, X.; Yu, R.; Wang, P.; Chen, F.; Yu, H. Co-modification of F− and Fe(III) ions as a facile strategy towards effective separation of photogenerated electrons and holes. Appl. Surf. Sci. 2015, 351, 66−73. (33) Wang, X.; Wang, K.; Feng, K.; Chen, F.; Yu, H.; Yu, J. Greatly enhanced photocatalytic activity of TiO2‑xNx by a simple surface modification of Fe(III) cocatalyst. J. Mol. Catal. A: Chem. 2014, 391, 92−98. (34) Yu, H.; Irie, H.; Hashimoto, K. Conduction band energy level control of titanium dioxide: toward an efficient visible-light-sensitive photocatalyst. J. Am. Chem. Soc. 2010, 132, 6898−6899. (35) Liu, L.; Ji, Z.; Zou, W.; Gu, X.; Deng, Y.; Gao, F.; Tang, C.; Dong, L. In situ loading transition metal oxide clusters on TiO2 nanosheets as co-catalysts for exceptional high photoactivity. ACS Catal. 2013, 3, 2052−2061. (36) Yu, J.; Wang, S.; Cheng, B.; Lin, Z.; Huang, F. Noble metal-free Ni(OH)2-gC3N4 composite photocatalyst with enhanced visible-light photocatalytic H2-production activity. Catal. Sci. Technol. 2013, 3, 1782−1789. (37) Yu, H.; Cao, G.; Chen, F.; Wang, X.; Yu, J.; Lei, M. Enhanced photocatalytic performance of Ag3PO4 by simutaneous loading of Ag nanoparticles and Fe(III) cocatalyst. Appl. Catal., B 2014, 160, 658− 665. (38) Wang, P.; Xia, Y.; Wu, P.; Wang, X.; Yu, H.; Yu, J. Cu(II) as a General Cocatalyst for Improved Visible-Light Photocatalytic Performance of Photosensitive Ag-Based Compounds. J. Phys. Chem. C 2014, 118, 8891−8898. (39) Yu, H.; Xu, L.; Wang, P.; Wang, X.; Yu, J. Enhanced photoinduced stability and photocatalytic activity of AgBr photocatalyst by surface modification of Fe(III) cocatalyst. Appl. Catal., B 2014, 144, 75−82. (40) Singh, J.; Gusain, A.; Saxena, V.; Chauhan, A.; Veerender, P.; Koiry, S.; Jha, P.; Jain, A.; Aswal, D.; Gupta, S. XPS, UV−vis, FTIR, and EXAFS studies to investigate the binding mechanism of N719 dye onto oxalic acid treated TiO2 and its implication on photovoltaic properties. J. Phys. Chem. C 2013, 117, 21096−21104. (41) Shen, S.; Guo, L.; Chen, X.; Ren, F.; Mao, S. Effect of Ag2S on solar-driven photocatalytic hydrogen evolution of nanostructured CdS. Int. J. Hydrogen Energy 2010, 35, 7110−7115. (42) Ran, J.; Zhang, J.; Yu, J.; Qiao, S. Z. Enhanced Visible-Light Photocatalytic H2 Production by ZnxCd1−xS Modified with EarthAbundant Nickel-Based Cocatalysts. ChemSusChem 2014, 7, 3426− 3434. (43) Yu, J.; Hai, Y.; Cheng, B. Enhanced Photocatalytic H2Production Activity of TiO2 by Ni(OH)2 Cluster Modification. J. Phys. Chem. C 2011, 115, 4953−4958. (44) Khan, Z.; Khannam, M.; Vinothkumar, N.; De, M.; Qureshi, M. Hierarchical 3D NiO-CdS heteroarchitecture for efficient visible light photocatalytic hydrogen generation. J. Mater. Chem. 2012, 22, 12090− 12095. (45) Xu, Y.; Xu, R. Nickel-based cocatalysts for photocatalytic hydrogen production. Appl. Surf. Sci. 2015, 351, 779−793. (46) Li, L.; Cheng, B.; Wang, Y.; Yu, J. Enhanced photocatalytic H2production activity of bicomponent NiO/TiO2 composite nanofibers. J. Colloid Interface Sci. 2015, 449, 115−121. (47) Ran, J.; Yu, J.; Jaroniec, M. Ni(OH)2 modified CdS nanorods for highly efficient visible-light-driven photocatalytic H2 generation. Green Chem. 2011, 13, 2708−2713.

(48) Li, Y.; Du, J.; Peng, S.; Xie, D.; Lu, G.; Li, S. Enhancement of photocatalytic activity of cadmium sulfide for hydrogen evolution by photoetching. Int. J. Hydrogen Energy 2008, 33, 2007−2013.

I

DOI: 10.1021/acs.jpcc.6b00126 J. Phys. Chem. C XXXX, XXX, XXX−XXX