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NiS photocatalysts for high H2 evolution from water under visible light
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One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light Xiaozhou Zhou, He Sun, Huaizhang Zhang, Weixia Tu* State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
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abstract
Article history:
The development of efficient and stable noble-metal-free photocatalysts is crucial for
Received 19 December 2016
hydrogen evolution from water splitting as clean energy. This study reports uniform CdS/NiS
Received in revised form
spherical nanoparticles through a simple one-pot hydrothermal method with the aid of
24 March 2017
KOH. The prepared CdS/NiS composites show superior photocatalytic activities toward the
Accepted 25 March 2017
water splitting under visible light. A suitable amount of KOH in the synthesis benefits to form
Available online xxx
CdS/NiS photocatalysts with the improved activity. The CdS/NiS composite including 10 mol % metal percentage of Ni exhibits the highest photocatalytic activity. The high hydrogen
Keywords:
evolution rate of 24.37 mmol h1 g1 is achieved over the CdS/NiS composite photocatalyst.
Hydrogen evolution
The CdS/NiS photocatalyst has good photocatalytic stability in the recycling uses. The pre-
Photocatalyst
sent CdS/NiS as a noble-metal-free photocatalyst provides the superior visible-light driven
Non-noble metal
catalytic activity for hydrogen evolution.
Metal sulfide
© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Visible light
Introduction Hydrogen (H2) from water splitting over photocatalysts using solar energy has received much attention due to its sustainability and environmental friendliness [1e3]. In order to improve the utilization of solar energy, high visible-lightresponsive photocatalysts should be developed. Up to now, kinds of visible-light-driven photocatalysts have been obtained, such as CdS [4], In2S3 [5], Ta3N5 [6] and g-C3N4 [7]. Thereinto, CdS has elicited more and more attention because of its appropriate band gap (2.4 eV). However, pure CdS is usually not active in hydrogen production because of the rapid recombination of photo-generated charge carriers [8e10].
Although this problem can be solved by doping some noble metal co-catalysts [11e13], the high cost limits their further application in photocatalyst. Therefore, it is eager to develop highly efficient and inexpensive cocatalysts for the replacement of noble metals. Earth abundant element-based compounds as co-catalysts have been attempted to improve the photocatalytic activity of CdS. Zhou et al. synthesized Co(OH)2/CdS nanowires through precipitation method and the H2 evolution rate is 14.43 mmol h1 g1 [14]. Ran et al. modified CdS nanorods with Ni(OH)2 resulting in a 5.08 mmol h1 g1 of H2-production rate [15]. Recently, extensive studies have indicated that transition metal sulfides can be ideal choices of the alternative to noble
* Corresponding author. P. O. Box 100, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail address: [email protected] (W. Tu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.179 0360-3199/© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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metals, such as MoS2 [16,17], NiS [18], CuS [19,20], WS2 [21]. Among them, nickel sulfide (NiS) has received increasing attention owing to its low-cost, good electrical properties, and other unique properties. Zhang et al. prepared NiS modified TiO2 using solvothermal method for the photocatalytic H2 production and the H2 evolution rate is 0.7 mmol h1 g1 [22]. Chen et al. synthesized g-C3N4/NiS hybrid photocatalysts through in situ template-free ion-exchange process and the H2 production rate is 0.45 mmol h1 g1 [23]. Furthermore, Xu et al. reported that NiS can be used as cocatalyst of CdS for photocatalytic water splitting via hydrothermal loading method, and the H2 evolution rate can reach 7.3 mmol h1 g1 [24]. Yu et al. also developed NiS modified CdS nanorod photocatalysts with a two-step hydrothermal method, exhibiting an H2 evolution rate of 1.13 mmol h1 g1 [25]. Guo et al. reported an efficient Cd0.5Zn0.5S photocatalyst with unanchored NiSx co-catalyst for photogeneration of hydrogen [26]. Meng et al. used PdS to modify the CdS/NiS composite as a photocatalyst for H2 evolution under visible light [27]. For CdS/NiS photocatalyst, different synthesis methods are still necessary to be explored for improving its catalytic activity. In this study, we present a simple one-pot hydrothermal method to synthesize CdS/NiS composites. The uniform CdS/ NiS nanoparticles are obtained with the aid of KOH and structure promoter (CTAB). Their photocatalytic performances are evaluated for H2 evolution from water splitting under visible light irradiation. The synthesis conditions for CdS/NiS nanocomposites are optimized for the superior photocatalytic H2-production activity. A possible mechanism for the enhanced photocatalytic activity is also discussed.
Experimental Synthesis of CdS/NiS composites CdS/NiS samples were prepared by a one-pot hydrothermal method. Typically, X mmol nickel acetate tetrahydrate (Ni(Ac)2$4H2O), Y mmol cadmium acetate (Cd(Ac)2$2H2O) together with 2 mmol thiourea (NH2CSNH2) were dissolved into 80 mL aqueous solution of cetyltrimethyl ammonium bromide (CTAB) under the strong magnetic stirring (X þ Y ¼ 1, 0 < X < 1). Ten minutes later, 10 mmol potassium hydroxide (KOH) was added, and kept stirring for 24 h. Then, the mixtures were transferred into a 100 mL Teflon autoclave and heated at 200 C for 24 h in a vacuum oven. The resulting mixtures were filtered and washed three times with absolute ethanol. Finally, the precipitate was dried overnight at 60 C. The obtained powder was the CdS/NiS composite. The nominal molar percentages of Ni/(Cd þ Ni), designated as R, were 5, 7.5, 10, 15, 20, 50, and 80 mol%, and the resulting samples were named as CdS/NiS-5, CdS/NiS-7.5, CdS/NiS-10, CdS/NiS-15, CdS/NiS-20, CdS/NiS-50, CdS/NiS-80, respectively. Pure CdS and NiS samples were also prepared using the same method for comparison.
Characterization X-ray diffraction (XRD) patterns were obtained through the measurement on a Bruker D8 Advance diffractometer
(Germany Bruker AXS Ltd.) using Cu-Ka radiation at a scan rate of 0.02 /s. S-4700F electron microscope (SEM, Japan JEOL Ltd.) was used to know about the morphology of the samples. The HRTEM analysis was taken on a J-3010 electron microscope (Japan JEOL Ltd.). The UVeVis diffuse-reflectance spectra were recorded by a UVeVis spectrophotometer (Tu-1901, Beijing Persee General Instrument Co. Ltd.), using BaSO4 as a reflectance standard. The photoluminescence (PL) spectra were conducted using FL spectrophotometer (Hitachi F-7000). The XPS measurements were taken on a VG ESCALAB 250 electron spectrometer using a multichannel detector. ICP-AES results were determined on an inductively coupled plasma spectrometry-atomic emission spectrometer (ULTIMA, JY Inc.). The BrunauereEmmetteTeller (BET) surface areas of the powders were measured using a Micrometrics ASAP 2020HD88 nitrogen adsorption apparatus.
Photocatalytic H2-production activity A 500 mL Reactor equipped with a 500 W Xe lamp was used to conduct the photocatalytic reactions. 60 mg of CdS/NiS photocatalyst was dispersed in a 400 mL mixed solution of lactic acid (40 mL) and water (360 mL) by ultrasonic treatment for 10 min. Then, the solution was decanted into a 500 mL Pyrex reactor under a constant stirring. The reaction system was maintained at room temperature and atmospheric pressure during the reaction under Xe lamp irradiation. 1 mL of gas was intermittently sampled every 1 h, and was analyzed by gas chromatography (TCD with TDX-01 molecular sieve column, N2 carrier). The quantum efficiency (QE) of photocatalyst was measured through the photocatalytic reaction in a 65 mL Pyrex flask. The opening of the flask was sealed with a silicone rubber septum. 3 mg of the catalyst was sonically dispersed in an aqueous solution (20 mL) containing 10 vol% lactic acid. A 300 W Xe lamp coupled with a mono-tone filter (l ¼ 420 nm) served as the light source, and was placed at 15 cm away from the reactor. Before irradiation, the system was bubbled with N2 for 20 min to ensure anaerobic conditions. The focused intensity was determined to be 28.92 mW cm2 and the irradiation area was 12.5 cm2. The QE is equal to the ratio of the reacted electrons number to the incident photons number, while the reacted electrons number is twice of the evolved H2 molecules number.
Results and discussion Photocatalytic H2-evolution activity of the CdS/NiS composites Through adjusting the ratio of metal precursors (R), a series of the CdS/NiS composites were synthesized by the one-pot hydrothermal method detailed above. Photocatalytic reactions over the CdS/NiS composites were carried out under a 500 W Xe lamp irradiation with lactic acid as sacrificial agent. Fig. 1 shows the H2 evolution rate in 4-h water splitting reaction over the photocatalysts with different R. As can be seen, the pure NiS (R ¼ 100) sample shows no appreciable photocatalytic activity for H2 production. The pure CdS (R ¼ 0) has photocatalytic activity with the H2 evolution rate of 2.30 mmol h1 g1. Pure CdS
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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Fig. 1 e Photocatalytic activity of the CdS/NiS composites synthesized with the different molar percentages of Ni/(Cd þ Ni).
photocatalysts prepared by different methods show different photocatalytic H2-production activities. CdS nanoparticles synthesized by a thermolysis method exhibited a H2 evolution rate of 0.31 mmol h1 g1 [28]. CdS nanoparticles obtained by calcining the commercial CdS in Ar atmosphere showed a H2 production activity of 0.88 mmol h1 g1 [2]. CdS nanowires prepared by a solvothermal method displayed a H2 evolution of 0.07 mmol h1 g1 [14]. It is evident that the CdS obtained by our present method has superior performance in H2 production. Moreover, the CdS/NiS composites photocatalysts exhibit much higher H2 production activity than the pure CdS and NiS alone as shown in Fig. 1. The photocatalytic activity of CdS/NiS composites changes with the molar percentage of Ni. The CdS/ NiS-5 shows a H2 production activity of 12.73 mmol h1 g1. With the increase of the Ni amount in CdS/NiS composites (R ¼ 5e10), the H2 evolution rates increase obviously. The highest hydrogen production rate of 24.37 mmol h1 g1 is achieved over CdS/NiS-10, which is about 11 times higher than that over pure CdS. A further increase of R (15e80) causes a decrease of the H2 evolution rate that is still higher than that of pure CdS. Therefore, NiS plays an important role in the H2 production promotion of CdS and shows good synergistic effect with CdS. The present CdS/NiS-10 demonstrates an impressive H2 evolution rate among the non-noble metal photocatalysts under visible light irradiation. In order to investigate the effect of the hydrothermal temperature on photocatalytic performance of the CdS/NiS-10 composites for H2 production, a series of samples were prepared under different hydrothermal temperature changing from 140 to 200 C. Fig. 2 displays the H2 production rate in water splitting over these CdS/NiS-10 samples as the catalysts. It is clear that the H2 evolution rate goes up with the increasing hydrothermal temperature. The high H2 evolution rate is observed for the CdS/NiS-10 catalyst obtained by the hydrothermal treatment with high temperature 200 C. It is reasonable that the decomposition rate of thiourea can be accelerated with the temperature increasing. Thus, the high hydrothermal temperature makes for the rapid and complete
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Fig. 2 e Photocatalytic activity of CdS/NiS-10 composites synthesized under different hydrothermal temperature.
release of sulfur from thiourea, which is beneficial for the ionexchange between S2 and OH to form CdS/NiS composites [17]. The sample with higher temperature is not prepared because of the use limit of Teflon vessel. As for the present one-pot hydrothermal method, the introducing of KOH into the reactant solution is a crucial procedure for obtaining the CdS/NiS composites with high photocatalytic activity. When there is no KOH added in the reactant solution, the photocatalytic activity of the obtained CdS/NiS for H2 production is almost keep the same as the pure CdS. A small quantity of KOH added can effectively enhance the photocatalytic efficiency as seen in Fig. 3. With the optimum amount of KOH (10 mmol) added, the synthesized CdS/ NiS-10 demonstrates superior photocatalytic activity with a high H2 evolution rate of 24.37 mmol h1 g1. A more addition of KOH is unfavorable to the system and leads to a decrease of the photocatalytic activity. Two possible effects could be considered on the synthesis system after the addition of KOH. One is that the hydroxides intermediate Cd(OH)2 is formed
Fig. 3 e Photocatalytic activity of CdS/NiS-10 composites synthesized with different amount of KOH.
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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immediately as soon as the KOH is added and its nucleation rate can be controlled by changing the amount of KOH. The other is that thiourea is not stable, and is easy to decompose into sulfur ions in the presence of OH, the generated S2 can further react with the hydroxides to form metal sulfides with the assistance of a hydro-thermal treatment.
The photocatalytic stability of CdS/NiS composites It is known that the stability of catalyst is a vital requirement and the stability of pure CdS is not enough because of its photocorrosion. To study the H2 evolution stability of CdS/NiS composites, the photocatalytic experiment was conducted in the presence of 0.06 g CdS/NiS-10 in a 400 mL aqueous solution containing 10 vol% of lactic acid, the produced H2 was evacuated every 4 h. As shown in Fig. 4, the H2 evolution rate of each cycle has no significant change, and the total amounts of H2 production are almost equal in every 4 h. It is proven that the CdS/NiS composites exhibit good photocatalytic stability. The addition of NiS leads to effective transfer of the photoexcited electrons from CdS to NiS and the fast reduction of Hþ to H2. Meanwhile, the holes oxidize the sacrificial agents, thus prevent the reaction between the holes and CdS, which benefits to the stability of CdS. Therefore, there is no distinct photocorrosion during the photocatalytic reaction of this CdS/ NiS material. Furthermore, the apparent quantum efficiency (AQE) of CdS/NiS-10 can reach 12.78% at 420 nm.
Morphologies and crystalline properties of the CdS and CdS/ NiS composites As known from above results, the presence of Ni can significant improves the photocatalytic activity and stability of CdS. Morphologies and crystalline properties were characterized to further understand the CdS and CdS/NiS composites. Fig. 5a shows the XRD patterns of CdS, CdS/NiS-10, CdS/NiS-20, and NiS. The diffraction peaks of the pure NiS can be indexed to the mixture of rhombohedral [JCPDS No. 02-0693] and hexagonal [JCPDS No. 89-1956] phases, and the pure CdS was indexed to the hexagonal wurtzite structure [JCPDS No. 65-
Fig. 4 e Cycling runs of the CdS/NiS composite for photocatalytic hydrogen evolution.
3414]. The samples of CdS/NiS-10 and CdS/NiS-20 show the diffraction signals of both CdS and NiS (2q ¼ 30.7 , 37.8 and 54.9 ). Compared to those diffraction signals of CdS as shown in the inset of Fig. 5a, the (100), (002), and (101) diffraction peaks of CdS/NiS-10 and CdS/NiS-20 have slight shifts toward high 2q values. The shifts may be attributed to the Ni2þ ions that are doped into the CdS lattice and occupy the substitutional cationic sites because the effective ionic radius of Cd2þ (6 coordinated radius: 0.95 A) is larger than that of Ni2þ (6 coordinated radius: 0.69 A) [29]. The morphologies of CdS and CdS/NiS-10 were observed by SEM. The image in Fig. 5b demonstrates that the CdS involves uniform spherical particles with an average diameter of 30 nm. Fig. 5c displays CdS/NiS-10 also keeps the morphology of uniform spherical particle but its average diameter is a little bit larger than that of CdS. HRTEM was used to further investigate the structure of CdS/NiS-10. The HRTEM image given in Fig. 5d exhibits lattice spacing of approximately 3.37 A and 5.42 A, which correspond to the (002) plane of hexagonal wurtzite-structured CdS and (111) plane of rhombohedral NiS, respectively. The HRTEM observation indicates that NiS intimately contacts with the surface of CdS nanoparticles. The intimate contacting is believed to be beneficial to the separation of the photogenerated electronseholes, thus leads to the enhancement of photocatalytic efficiency [15]. Moreover, the BET surface areas of pure CdS and CdS/NiS-10 composite are measured to be 11.08 m2 g1 and 56.21 m2 g1, respectively. The specific surface areas of CdS/NiS are four times larger than that of CdS, which means more activity sites and more capture of light. Both of them can result in the enhancement of the photocatalytic reactivity.
Optical properties of the CdS and CdS/NiS composites To identify the key factors responsible for the high H2 evolution rate, the light-absorption properties were measured and the results are shown in Fig. 6. Fig. 6a shows the UVeVis diffuse reflectance spectra of the CdS, CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 samples. It is seen that CdS nanoparticles can absorb visible light with the absorption edge at 560 nm, which corresponds to the band gap 2.22 eV. Compared to the spectra of CdS, the spectra of CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 demonstrate an enhanced absorption in the 550e800 nm regions. The enhancement can be assigned to the synergistic effect between the CdS and NiS phases. Furthermore, the absorption edges of CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 have an obvious red-shift, indicating that there is a decrease in energy band gap (Eg) of these samples. The absorption edges of CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 are at 565 nm, 620 nm, and 585 nm, respectively. The shift of the absorption edges is attributed to the Ni2þ incorporating into the lattice of CdS, which coincides with the XRD analysis. Among the samples, Eg of CdS/NiS-10 is the narrowest as 2.00 eV. The absorption intensity of CdS/NiS-10 is also stronger than those of CdS/NiS-20 and CdS/NiS-5 above 550 nm, which indicates the addition of NiS with an appropriate amount is necessary. Fig. 6b demonstrates the photoluminescence (PL) spectra of CdS, CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20. The PL spectra are excitated at the wavelength of 364 nm, and the distinct emission band at 417 nm can be observed. The PL intensity of
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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Fig. 5 e X-ray diffraction patterns of CdS, CdS/NiS-10, CdS/NiS-20 and NiS (a); SEM images of CdS (b) and CdS/NiS-10 (c); HRTEM image of CdS/NiS-10 (d).
CdS/NiS-5, CdS/NiS-10, or CdS/NiS-20 is weaker than that of CdS. Results above are attributed to the fast transfer of electrons from CdS to NiS. Thus, the electronehole recombination is suppressed and the photocatalytic activity can be enhanced. That the CdS/NiS-10 has the lowest PL intensity suggests its best photocatalytic activity, which is consistent with its H2 evolution rate.
XPS and ICP analysis of the CdS and CdS/NiS composites To characterize the chemical states of CdS and CdS/NiS-10, the samples were investigated by X-ray photoelectron spectroscopy (XPS). Fig. 7 provides the XPS spectra of CdS and CdS/NiS10 with the binding energy (BE) of C 1s (285.0 eV) referenced.
Fig. 7a shows typical high resolution XPS spectra of Cd 3d. Two peaks at 404.8 and 411.5 eV are assigned to Cd 3d5/2 and 3d3/2, respectively, which is a characteristic of Cd2þ in CdS. For CdS/ NiS-10, the BE values of Cd 3d are 0.4 eV higher than those of CdS. For the CdS/NiS-10, the BEs of S 2p3/2 and S 2p1/2 are equal to 161.1 and 162.3 eV, respectively, and the S 2p1/2 peak becomes more distinct than that of CdS as can be seen in Fig. 7b. The BE shift of Cd 3d and the altered S peaks may be attributed to the presence of Ni in CdS/NiS-10. The BE peaks of Ni 2p are observed at 856.5 and 874.2 eV as shown in Fig. 7c. The XPS results show the existence of Cd, Ni and S elements, and further confirm the formation of CdS and CdS/NiS. Moreover, the presence of Ni has great influence on the chemical states of Cd and S in the CdS/NiS atoms. The actual contents of Ni in the
Fig. 6 e UVeVis diffuse-reflectance spectra (a) and photoluminescence spectra (b) of CdS, CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20. Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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Fig. 7 e XPS spectra of CdS and CdS/NiS-10 with Cd 3d (a), S 2p (b) and Ni 2p (c) binding energies.
sample of CdS/NiS-10 is 10.73 mol% metal percentage of Ni/(Cd þ Ni) (measured by ICP-AES), which well corresponds to the content of Ni added in the starting reactants.
Mechanism for the photocatalytic hydrogen evolution of the CdS/NiS composites From the above results of XRD and UVevis spectra, it can be found that there are some Ni2þ ions doped into the lattice of CdS. The substitution of Ni2þ at CdS lattice sites creates an electron donor level in bandgap, thus narrows the band gap. Indeed, the samples of CdS/NiS-10 and CdS/NiS-20 have energy gaps of 2.00 eV and 2.12 eV, respectively, both are lower than that of pure CdS (2.22 eV). The decreased band gap is more favorable to the harvest of visible light and the generation of electrons and holes pairs [29]. What is more, there is
Scheme 1 e Mechanism for the photocatalytic H2-production of CdS/NiS systems under visible-light irradiation.
intimately contact between NiS and CdS as seen from HRTEM. The existence of the heterojunction can make the photogenerated electrons transfer to the surface of NiS easily, which suppresses the recombination of electronehole pairs. On the basis of the above results, the proposed photocatalytic mechanism in the CdS/NiS systems is illustrated in Scheme 1. After trapping electrons, NiS can accelerate the electrochemical adsorption and desorption kinetics for H2 evolution [23,24]. The source of Hþ can be gained from both water and lactic acid. Therefore, it is reasonable that high photocatalytic H2 production activity is achieved.
Conclusion Nanosized CdS/NiS spherical nanoparticles are synthesized via a one-pot hydrothermal method. The CdS/NiS composites as visible-light-driven photocatalysts exhibit high photocatalytic activity for H2 evolution from water, which show the superior synergistic photocatalysis effect differing from pure CdS and NiS. A suitable amount of KOH presented in the hydrothermal synthesis system is essential for achieving the highly active CdS/NiS photocatalyst. A highest H2 evolution rate of 24.37 mmol h1 g1 is obtained from water over the CdS/NiS composite including 10 mol% of Ni. Several kinds of characterizations confirm the existence of the CdS/NiS composites and the interaction between CdS and NiS. The introducing of NiS in the CdS/NiS composites not only narrows energy gap of the CdS/NiS photocatalyst but also accelerates
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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the isolation of electronehole pairs. The CdS/NiS photocatalyst has excellent photochemical stability accompanying with the superior H2 evolution rate, which exhibits the prominent activity among non-noble metal photocatalyst. The present one-pot synthesized CdS/NiS composites provide a kind of the promising and efficient photocatalysts for the practical conversion of solar energy to hydrogen.
[13]
[14]
[15]
Acknowledgment [16]
Financial support from the National Natural Science Foundation of China (21106006) is acknowledged. [17]
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Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light Xiaozhou Zhou, He Sun, Huaizhang Zhang, Weixia Tu* State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China
article info
abstract
Article history:
The development of efficient and stable noble-metal-free photocatalysts is crucial for
Received 19 December 2016
hydrogen evolution from water splitting as clean energy. This study reports uniform CdS/NiS
Received in revised form
spherical nanoparticles through a simple one-pot hydrothermal method with the aid of
24 March 2017
KOH. The prepared CdS/NiS composites show superior photocatalytic activities toward the
Accepted 25 March 2017
water splitting under visible light. A suitable amount of KOH in the synthesis benefits to form
Available online xxx
CdS/NiS photocatalysts with the improved activity. The CdS/NiS composite including 10 mol % metal percentage of Ni exhibits the highest photocatalytic activity. The high hydrogen
Keywords:
evolution rate of 24.37 mmol h1 g1 is achieved over the CdS/NiS composite photocatalyst.
Hydrogen evolution
The CdS/NiS photocatalyst has good photocatalytic stability in the recycling uses. The pre-
Photocatalyst
sent CdS/NiS as a noble-metal-free photocatalyst provides the superior visible-light driven
Non-noble metal
catalytic activity for hydrogen evolution.
Metal sulfide
© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
Visible light
Introduction Hydrogen (H2) from water splitting over photocatalysts using solar energy has received much attention due to its sustainability and environmental friendliness [1e3]. In order to improve the utilization of solar energy, high visible-lightresponsive photocatalysts should be developed. Up to now, kinds of visible-light-driven photocatalysts have been obtained, such as CdS [4], In2S3 [5], Ta3N5 [6] and g-C3N4 [7]. Thereinto, CdS has elicited more and more attention because of its appropriate band gap (2.4 eV). However, pure CdS is usually not active in hydrogen production because of the rapid recombination of photo-generated charge carriers [8e10].
Although this problem can be solved by doping some noble metal co-catalysts [11e13], the high cost limits their further application in photocatalyst. Therefore, it is eager to develop highly efficient and inexpensive cocatalysts for the replacement of noble metals. Earth abundant element-based compounds as co-catalysts have been attempted to improve the photocatalytic activity of CdS. Zhou et al. synthesized Co(OH)2/CdS nanowires through precipitation method and the H2 evolution rate is 14.43 mmol h1 g1 [14]. Ran et al. modified CdS nanorods with Ni(OH)2 resulting in a 5.08 mmol h1 g1 of H2-production rate [15]. Recently, extensive studies have indicated that transition metal sulfides can be ideal choices of the alternative to noble
* Corresponding author. P. O. Box 100, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail address: [email protected] (W. Tu). http://dx.doi.org/10.1016/j.ijhydene.2017.03.179 0360-3199/© 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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metals, such as MoS2 [16,17], NiS [18], CuS [19,20], WS2 [21]. Among them, nickel sulfide (NiS) has received increasing attention owing to its low-cost, good electrical properties, and other unique properties. Zhang et al. prepared NiS modified TiO2 using solvothermal method for the photocatalytic H2 production and the H2 evolution rate is 0.7 mmol h1 g1 [22]. Chen et al. synthesized g-C3N4/NiS hybrid photocatalysts through in situ template-free ion-exchange process and the H2 production rate is 0.45 mmol h1 g1 [23]. Furthermore, Xu et al. reported that NiS can be used as cocatalyst of CdS for photocatalytic water splitting via hydrothermal loading method, and the H2 evolution rate can reach 7.3 mmol h1 g1 [24]. Yu et al. also developed NiS modified CdS nanorod photocatalysts with a two-step hydrothermal method, exhibiting an H2 evolution rate of 1.13 mmol h1 g1 [25]. Guo et al. reported an efficient Cd0.5Zn0.5S photocatalyst with unanchored NiSx co-catalyst for photogeneration of hydrogen [26]. Meng et al. used PdS to modify the CdS/NiS composite as a photocatalyst for H2 evolution under visible light [27]. For CdS/NiS photocatalyst, different synthesis methods are still necessary to be explored for improving its catalytic activity. In this study, we present a simple one-pot hydrothermal method to synthesize CdS/NiS composites. The uniform CdS/ NiS nanoparticles are obtained with the aid of KOH and structure promoter (CTAB). Their photocatalytic performances are evaluated for H2 evolution from water splitting under visible light irradiation. The synthesis conditions for CdS/NiS nanocomposites are optimized for the superior photocatalytic H2-production activity. A possible mechanism for the enhanced photocatalytic activity is also discussed.
Experimental Synthesis of CdS/NiS composites CdS/NiS samples were prepared by a one-pot hydrothermal method. Typically, X mmol nickel acetate tetrahydrate (Ni(Ac)2$4H2O), Y mmol cadmium acetate (Cd(Ac)2$2H2O) together with 2 mmol thiourea (NH2CSNH2) were dissolved into 80 mL aqueous solution of cetyltrimethyl ammonium bromide (CTAB) under the strong magnetic stirring (X þ Y ¼ 1, 0 < X < 1). Ten minutes later, 10 mmol potassium hydroxide (KOH) was added, and kept stirring for 24 h. Then, the mixtures were transferred into a 100 mL Teflon autoclave and heated at 200 C for 24 h in a vacuum oven. The resulting mixtures were filtered and washed three times with absolute ethanol. Finally, the precipitate was dried overnight at 60 C. The obtained powder was the CdS/NiS composite. The nominal molar percentages of Ni/(Cd þ Ni), designated as R, were 5, 7.5, 10, 15, 20, 50, and 80 mol%, and the resulting samples were named as CdS/NiS-5, CdS/NiS-7.5, CdS/NiS-10, CdS/NiS-15, CdS/NiS-20, CdS/NiS-50, CdS/NiS-80, respectively. Pure CdS and NiS samples were also prepared using the same method for comparison.
Characterization X-ray diffraction (XRD) patterns were obtained through the measurement on a Bruker D8 Advance diffractometer
(Germany Bruker AXS Ltd.) using Cu-Ka radiation at a scan rate of 0.02 /s. S-4700F electron microscope (SEM, Japan JEOL Ltd.) was used to know about the morphology of the samples. The HRTEM analysis was taken on a J-3010 electron microscope (Japan JEOL Ltd.). The UVeVis diffuse-reflectance spectra were recorded by a UVeVis spectrophotometer (Tu-1901, Beijing Persee General Instrument Co. Ltd.), using BaSO4 as a reflectance standard. The photoluminescence (PL) spectra were conducted using FL spectrophotometer (Hitachi F-7000). The XPS measurements were taken on a VG ESCALAB 250 electron spectrometer using a multichannel detector. ICP-AES results were determined on an inductively coupled plasma spectrometry-atomic emission spectrometer (ULTIMA, JY Inc.). The BrunauereEmmetteTeller (BET) surface areas of the powders were measured using a Micrometrics ASAP 2020HD88 nitrogen adsorption apparatus.
Photocatalytic H2-production activity A 500 mL Reactor equipped with a 500 W Xe lamp was used to conduct the photocatalytic reactions. 60 mg of CdS/NiS photocatalyst was dispersed in a 400 mL mixed solution of lactic acid (40 mL) and water (360 mL) by ultrasonic treatment for 10 min. Then, the solution was decanted into a 500 mL Pyrex reactor under a constant stirring. The reaction system was maintained at room temperature and atmospheric pressure during the reaction under Xe lamp irradiation. 1 mL of gas was intermittently sampled every 1 h, and was analyzed by gas chromatography (TCD with TDX-01 molecular sieve column, N2 carrier). The quantum efficiency (QE) of photocatalyst was measured through the photocatalytic reaction in a 65 mL Pyrex flask. The opening of the flask was sealed with a silicone rubber septum. 3 mg of the catalyst was sonically dispersed in an aqueous solution (20 mL) containing 10 vol% lactic acid. A 300 W Xe lamp coupled with a mono-tone filter (l ¼ 420 nm) served as the light source, and was placed at 15 cm away from the reactor. Before irradiation, the system was bubbled with N2 for 20 min to ensure anaerobic conditions. The focused intensity was determined to be 28.92 mW cm2 and the irradiation area was 12.5 cm2. The QE is equal to the ratio of the reacted electrons number to the incident photons number, while the reacted electrons number is twice of the evolved H2 molecules number.
Results and discussion Photocatalytic H2-evolution activity of the CdS/NiS composites Through adjusting the ratio of metal precursors (R), a series of the CdS/NiS composites were synthesized by the one-pot hydrothermal method detailed above. Photocatalytic reactions over the CdS/NiS composites were carried out under a 500 W Xe lamp irradiation with lactic acid as sacrificial agent. Fig. 1 shows the H2 evolution rate in 4-h water splitting reaction over the photocatalysts with different R. As can be seen, the pure NiS (R ¼ 100) sample shows no appreciable photocatalytic activity for H2 production. The pure CdS (R ¼ 0) has photocatalytic activity with the H2 evolution rate of 2.30 mmol h1 g1. Pure CdS
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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Fig. 1 e Photocatalytic activity of the CdS/NiS composites synthesized with the different molar percentages of Ni/(Cd þ Ni).
photocatalysts prepared by different methods show different photocatalytic H2-production activities. CdS nanoparticles synthesized by a thermolysis method exhibited a H2 evolution rate of 0.31 mmol h1 g1 [28]. CdS nanoparticles obtained by calcining the commercial CdS in Ar atmosphere showed a H2 production activity of 0.88 mmol h1 g1 [2]. CdS nanowires prepared by a solvothermal method displayed a H2 evolution of 0.07 mmol h1 g1 [14]. It is evident that the CdS obtained by our present method has superior performance in H2 production. Moreover, the CdS/NiS composites photocatalysts exhibit much higher H2 production activity than the pure CdS and NiS alone as shown in Fig. 1. The photocatalytic activity of CdS/NiS composites changes with the molar percentage of Ni. The CdS/ NiS-5 shows a H2 production activity of 12.73 mmol h1 g1. With the increase of the Ni amount in CdS/NiS composites (R ¼ 5e10), the H2 evolution rates increase obviously. The highest hydrogen production rate of 24.37 mmol h1 g1 is achieved over CdS/NiS-10, which is about 11 times higher than that over pure CdS. A further increase of R (15e80) causes a decrease of the H2 evolution rate that is still higher than that of pure CdS. Therefore, NiS plays an important role in the H2 production promotion of CdS and shows good synergistic effect with CdS. The present CdS/NiS-10 demonstrates an impressive H2 evolution rate among the non-noble metal photocatalysts under visible light irradiation. In order to investigate the effect of the hydrothermal temperature on photocatalytic performance of the CdS/NiS-10 composites for H2 production, a series of samples were prepared under different hydrothermal temperature changing from 140 to 200 C. Fig. 2 displays the H2 production rate in water splitting over these CdS/NiS-10 samples as the catalysts. It is clear that the H2 evolution rate goes up with the increasing hydrothermal temperature. The high H2 evolution rate is observed for the CdS/NiS-10 catalyst obtained by the hydrothermal treatment with high temperature 200 C. It is reasonable that the decomposition rate of thiourea can be accelerated with the temperature increasing. Thus, the high hydrothermal temperature makes for the rapid and complete
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Fig. 2 e Photocatalytic activity of CdS/NiS-10 composites synthesized under different hydrothermal temperature.
release of sulfur from thiourea, which is beneficial for the ionexchange between S2 and OH to form CdS/NiS composites [17]. The sample with higher temperature is not prepared because of the use limit of Teflon vessel. As for the present one-pot hydrothermal method, the introducing of KOH into the reactant solution is a crucial procedure for obtaining the CdS/NiS composites with high photocatalytic activity. When there is no KOH added in the reactant solution, the photocatalytic activity of the obtained CdS/NiS for H2 production is almost keep the same as the pure CdS. A small quantity of KOH added can effectively enhance the photocatalytic efficiency as seen in Fig. 3. With the optimum amount of KOH (10 mmol) added, the synthesized CdS/ NiS-10 demonstrates superior photocatalytic activity with a high H2 evolution rate of 24.37 mmol h1 g1. A more addition of KOH is unfavorable to the system and leads to a decrease of the photocatalytic activity. Two possible effects could be considered on the synthesis system after the addition of KOH. One is that the hydroxides intermediate Cd(OH)2 is formed
Fig. 3 e Photocatalytic activity of CdS/NiS-10 composites synthesized with different amount of KOH.
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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immediately as soon as the KOH is added and its nucleation rate can be controlled by changing the amount of KOH. The other is that thiourea is not stable, and is easy to decompose into sulfur ions in the presence of OH, the generated S2 can further react with the hydroxides to form metal sulfides with the assistance of a hydro-thermal treatment.
The photocatalytic stability of CdS/NiS composites It is known that the stability of catalyst is a vital requirement and the stability of pure CdS is not enough because of its photocorrosion. To study the H2 evolution stability of CdS/NiS composites, the photocatalytic experiment was conducted in the presence of 0.06 g CdS/NiS-10 in a 400 mL aqueous solution containing 10 vol% of lactic acid, the produced H2 was evacuated every 4 h. As shown in Fig. 4, the H2 evolution rate of each cycle has no significant change, and the total amounts of H2 production are almost equal in every 4 h. It is proven that the CdS/NiS composites exhibit good photocatalytic stability. The addition of NiS leads to effective transfer of the photoexcited electrons from CdS to NiS and the fast reduction of Hþ to H2. Meanwhile, the holes oxidize the sacrificial agents, thus prevent the reaction between the holes and CdS, which benefits to the stability of CdS. Therefore, there is no distinct photocorrosion during the photocatalytic reaction of this CdS/ NiS material. Furthermore, the apparent quantum efficiency (AQE) of CdS/NiS-10 can reach 12.78% at 420 nm.
Morphologies and crystalline properties of the CdS and CdS/ NiS composites As known from above results, the presence of Ni can significant improves the photocatalytic activity and stability of CdS. Morphologies and crystalline properties were characterized to further understand the CdS and CdS/NiS composites. Fig. 5a shows the XRD patterns of CdS, CdS/NiS-10, CdS/NiS-20, and NiS. The diffraction peaks of the pure NiS can be indexed to the mixture of rhombohedral [JCPDS No. 02-0693] and hexagonal [JCPDS No. 89-1956] phases, and the pure CdS was indexed to the hexagonal wurtzite structure [JCPDS No. 65-
Fig. 4 e Cycling runs of the CdS/NiS composite for photocatalytic hydrogen evolution.
3414]. The samples of CdS/NiS-10 and CdS/NiS-20 show the diffraction signals of both CdS and NiS (2q ¼ 30.7 , 37.8 and 54.9 ). Compared to those diffraction signals of CdS as shown in the inset of Fig. 5a, the (100), (002), and (101) diffraction peaks of CdS/NiS-10 and CdS/NiS-20 have slight shifts toward high 2q values. The shifts may be attributed to the Ni2þ ions that are doped into the CdS lattice and occupy the substitutional cationic sites because the effective ionic radius of Cd2þ (6 coordinated radius: 0.95 A) is larger than that of Ni2þ (6 coordinated radius: 0.69 A) [29]. The morphologies of CdS and CdS/NiS-10 were observed by SEM. The image in Fig. 5b demonstrates that the CdS involves uniform spherical particles with an average diameter of 30 nm. Fig. 5c displays CdS/NiS-10 also keeps the morphology of uniform spherical particle but its average diameter is a little bit larger than that of CdS. HRTEM was used to further investigate the structure of CdS/NiS-10. The HRTEM image given in Fig. 5d exhibits lattice spacing of approximately 3.37 A and 5.42 A, which correspond to the (002) plane of hexagonal wurtzite-structured CdS and (111) plane of rhombohedral NiS, respectively. The HRTEM observation indicates that NiS intimately contacts with the surface of CdS nanoparticles. The intimate contacting is believed to be beneficial to the separation of the photogenerated electronseholes, thus leads to the enhancement of photocatalytic efficiency [15]. Moreover, the BET surface areas of pure CdS and CdS/NiS-10 composite are measured to be 11.08 m2 g1 and 56.21 m2 g1, respectively. The specific surface areas of CdS/NiS are four times larger than that of CdS, which means more activity sites and more capture of light. Both of them can result in the enhancement of the photocatalytic reactivity.
Optical properties of the CdS and CdS/NiS composites To identify the key factors responsible for the high H2 evolution rate, the light-absorption properties were measured and the results are shown in Fig. 6. Fig. 6a shows the UVeVis diffuse reflectance spectra of the CdS, CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 samples. It is seen that CdS nanoparticles can absorb visible light with the absorption edge at 560 nm, which corresponds to the band gap 2.22 eV. Compared to the spectra of CdS, the spectra of CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 demonstrate an enhanced absorption in the 550e800 nm regions. The enhancement can be assigned to the synergistic effect between the CdS and NiS phases. Furthermore, the absorption edges of CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 have an obvious red-shift, indicating that there is a decrease in energy band gap (Eg) of these samples. The absorption edges of CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20 are at 565 nm, 620 nm, and 585 nm, respectively. The shift of the absorption edges is attributed to the Ni2þ incorporating into the lattice of CdS, which coincides with the XRD analysis. Among the samples, Eg of CdS/NiS-10 is the narrowest as 2.00 eV. The absorption intensity of CdS/NiS-10 is also stronger than those of CdS/NiS-20 and CdS/NiS-5 above 550 nm, which indicates the addition of NiS with an appropriate amount is necessary. Fig. 6b demonstrates the photoluminescence (PL) spectra of CdS, CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20. The PL spectra are excitated at the wavelength of 364 nm, and the distinct emission band at 417 nm can be observed. The PL intensity of
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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Fig. 5 e X-ray diffraction patterns of CdS, CdS/NiS-10, CdS/NiS-20 and NiS (a); SEM images of CdS (b) and CdS/NiS-10 (c); HRTEM image of CdS/NiS-10 (d).
CdS/NiS-5, CdS/NiS-10, or CdS/NiS-20 is weaker than that of CdS. Results above are attributed to the fast transfer of electrons from CdS to NiS. Thus, the electronehole recombination is suppressed and the photocatalytic activity can be enhanced. That the CdS/NiS-10 has the lowest PL intensity suggests its best photocatalytic activity, which is consistent with its H2 evolution rate.
XPS and ICP analysis of the CdS and CdS/NiS composites To characterize the chemical states of CdS and CdS/NiS-10, the samples were investigated by X-ray photoelectron spectroscopy (XPS). Fig. 7 provides the XPS spectra of CdS and CdS/NiS10 with the binding energy (BE) of C 1s (285.0 eV) referenced.
Fig. 7a shows typical high resolution XPS spectra of Cd 3d. Two peaks at 404.8 and 411.5 eV are assigned to Cd 3d5/2 and 3d3/2, respectively, which is a characteristic of Cd2þ in CdS. For CdS/ NiS-10, the BE values of Cd 3d are 0.4 eV higher than those of CdS. For the CdS/NiS-10, the BEs of S 2p3/2 and S 2p1/2 are equal to 161.1 and 162.3 eV, respectively, and the S 2p1/2 peak becomes more distinct than that of CdS as can be seen in Fig. 7b. The BE shift of Cd 3d and the altered S peaks may be attributed to the presence of Ni in CdS/NiS-10. The BE peaks of Ni 2p are observed at 856.5 and 874.2 eV as shown in Fig. 7c. The XPS results show the existence of Cd, Ni and S elements, and further confirm the formation of CdS and CdS/NiS. Moreover, the presence of Ni has great influence on the chemical states of Cd and S in the CdS/NiS atoms. The actual contents of Ni in the
Fig. 6 e UVeVis diffuse-reflectance spectra (a) and photoluminescence spectra (b) of CdS, CdS/NiS-5, CdS/NiS-10, and CdS/NiS-20. Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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Fig. 7 e XPS spectra of CdS and CdS/NiS-10 with Cd 3d (a), S 2p (b) and Ni 2p (c) binding energies.
sample of CdS/NiS-10 is 10.73 mol% metal percentage of Ni/(Cd þ Ni) (measured by ICP-AES), which well corresponds to the content of Ni added in the starting reactants.
Mechanism for the photocatalytic hydrogen evolution of the CdS/NiS composites From the above results of XRD and UVevis spectra, it can be found that there are some Ni2þ ions doped into the lattice of CdS. The substitution of Ni2þ at CdS lattice sites creates an electron donor level in bandgap, thus narrows the band gap. Indeed, the samples of CdS/NiS-10 and CdS/NiS-20 have energy gaps of 2.00 eV and 2.12 eV, respectively, both are lower than that of pure CdS (2.22 eV). The decreased band gap is more favorable to the harvest of visible light and the generation of electrons and holes pairs [29]. What is more, there is
Scheme 1 e Mechanism for the photocatalytic H2-production of CdS/NiS systems under visible-light irradiation.
intimately contact between NiS and CdS as seen from HRTEM. The existence of the heterojunction can make the photogenerated electrons transfer to the surface of NiS easily, which suppresses the recombination of electronehole pairs. On the basis of the above results, the proposed photocatalytic mechanism in the CdS/NiS systems is illustrated in Scheme 1. After trapping electrons, NiS can accelerate the electrochemical adsorption and desorption kinetics for H2 evolution [23,24]. The source of Hþ can be gained from both water and lactic acid. Therefore, it is reasonable that high photocatalytic H2 production activity is achieved.
Conclusion Nanosized CdS/NiS spherical nanoparticles are synthesized via a one-pot hydrothermal method. The CdS/NiS composites as visible-light-driven photocatalysts exhibit high photocatalytic activity for H2 evolution from water, which show the superior synergistic photocatalysis effect differing from pure CdS and NiS. A suitable amount of KOH presented in the hydrothermal synthesis system is essential for achieving the highly active CdS/NiS photocatalyst. A highest H2 evolution rate of 24.37 mmol h1 g1 is obtained from water over the CdS/NiS composite including 10 mol% of Ni. Several kinds of characterizations confirm the existence of the CdS/NiS composites and the interaction between CdS and NiS. The introducing of NiS in the CdS/NiS composites not only narrows energy gap of the CdS/NiS photocatalyst but also accelerates
Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179
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the isolation of electronehole pairs. The CdS/NiS photocatalyst has excellent photochemical stability accompanying with the superior H2 evolution rate, which exhibits the prominent activity among non-noble metal photocatalyst. The present one-pot synthesized CdS/NiS composites provide a kind of the promising and efficient photocatalysts for the practical conversion of solar energy to hydrogen.
[13]
[14]
[15]
Acknowledgment [16]
Financial support from the National Natural Science Foundation of China (21106006) is acknowledged. [17]
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Please cite this article in press as: Zhou X, et al., One-pot hydrothermal synthesis of CdS/NiS photocatalysts for high H2 evolution from water under visible light, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.179