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Ni(OH)2 precursor for efficiently photocatalytic H2 evolution
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 9 8 e1 0 0 4
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Nickels/CdS photocatalyst prepared by flowerlike Ni/Ni(OH)2 precursor for efficiently photocatalytic H2 evolution Xiaoping Chen a, Shu Chen a, Caifang Lin a, Zhi Jiang a,b, Wenfeng Shangguan a,b,* a
Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China b Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China
article info
abstract
Article history:
Nickels/CdS photocatalyst was prepared by a simple photocatalytic reaction of CdS and
Received 29 April 2014
flowerlike Ni/Ni(OH)2 nanocomposite. The photocatalyst shows effectively photocatalytic
Received in revised form
H2 production activity, and reaches to the maximum rate of 373.5 mmol h1 when the mass
6 November 2014
ratio of CdS and flowerlike Ni/Ni(OH)2 is 2, even higher than the one of CdS loaded with 1 wt
Accepted 10 November 2014
% Pt. Characterization results (FESEM, TG, FT-IR, XPS, etc.) indicate that the effective
Available online 4 December 2014
photocatalytic H2-production activity was predominantly attributed to the porous flowerlike superstructures, in which CdS could be well fixed. Thus, CdS could contact fully with
Keywords:
flowerlike Ni/Ni(OH)2. Ni is an excellent cocatalyst, which can transfer electrons efficiently.
Flowerlike Ni/Ni(OH)2
Furthermore, Ni2þ of Ni(OH)2 were effectively reduced to Ni. These nickels could also act as
CdS
cocatalyst to promote the separation and transfer of photogenerated electrons. Therefore,
Photocatalysis
the flowerlike superstructures was gradually broken during the photocatalytic progress
Hydrogen production
because of Ni(OH)2 reduction, while resulted in effective photocatalytic H2 production activities. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasingly serious energy crisis and the environmental problems caused by the burning of fossil fuels have led to an aggressive search for renewable and environmental-friendly energy resources [1]. Hydrogen, a new renewable and nopolluting energy, has been considered as a promising
candidate for the future energy [2]. Since the discovery of photocatalytic hydrogen evolution from TiO2 by Fujishima and Honda in 1972 [3], many efficient photocatalysts have been found for photocatalytic hydrogen production [4,5]. However, most of them only respond to ultraviolet (UV) light which only accounts for about 4% of the solar radiation energy, while the visible light contributes to about 43%.
* Corresponding author. Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China. E-mail address: [email protected] (W. Shangguan). http://dx.doi.org/10.1016/j.ijhydene.2014.11.058 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 9 8 e1 0 0 4
As compared to their wide bandgap counterparts, CdS particles are attractive photocatalytic materials for the conversion of solar energy into chemical energy under visible light irradiation. Specifically, it has a band-gap energy (Eg) of 2.4 eV that suits very well with the solar spectrum and the conduction band edge is much more negative than the Hþ/H2 reduction potential [1,2,6]. However, CdS alone shows low photocatalytic hydrogen production activity because of the rapid recombination of excited electrons and holes. Many approaches have been proposed to enhance the photocatalytic activity of CdS particles, including synthesis of CdS with specific morphology [7e9], deposition of cocatalysts such as Pt, Au, metal sulfides etc. [10e14], incorporation of CdS in the interlayer region of layered compounds [15], and preparation of nanocomposite with other materials such as TiO2, ZnO, graphene etc. [1,16,17]. Ni and its oxides have been proved to be low-cost and efficient cocatalysts for photocatalytic H2 production [6], and Ni(OH)2 has also been found that it can significantly improve photocatalytic H2 production of photocatalysts [18,19]. In this work, high efficiency of the visible-light-driven photocatalytic H2 production was achieved by a simple photocatalytic reaction of CdS and flowerlike Ni/Ni(OH)2, which were synthesized via a simple hydrothermal method. Furthermore, a mechanism for the photocatalytic reaction of the composite is proposed.
Experimental Sample preparation All of the reagents were of analytical grade and used without further purification. Deionized water was used in all experiments. Flowerlike Ni/Ni(OH)2 was prepared by a simple chemical reduction and hydrothermal method [20,21]. 3 mmol of NiCl2$6H2O was dissolved in a 30 ml mixed solution of ethylene glycol and N, N-dimethylformamide with the volume ratio 3:2 to form a homogeneous solution. Then, 1 mmol of NaBH4 was added into the above mixed solution under stirring for 10 min. After that, the mixed system was poured into a
999
Teflon-lined stainless steel autoclave with the capacity of 100 ml and maintained at 170 C for 8 h. Finally, the products were washed with ethanol and distilled water several times, and dried in air at 80 C for 12 h. Then, 0.1 g flowerlike Ni/Ni(OH)2 prepared as above was dispersed in 50 ml deionized water and certain amount of Cd(CH3COO)2$2H2O was added in the mixed system. After stirring for 2 h, the same mole ratio of Na2S (0.14 M) was added in the mixed system. After stirring for another 2 h, the mixed system was poured into a Teflon-lined stainless steel autoclave with the capacity of 100 ml and maintained at 160 C for 12 h. The products were washed with distilled water several times and dried in air 80 C for 12 h. CdS was synthesized as the same process except without flowerlike Ni/Ni(OH)2. NC(R) was referred as CdS and Ni/Ni(OH)2 composite (R was the mass ratio of CdS and Ni/Ni(OH)2).
Characterization The crystal structure of the photocatalytic materials was confirmed by X-ray diffraction (Rigaku D/max-2200/PC Japan) with Cu Ka (40 kV, 20 mA). The UVevis diffuse reflection spectra (DRS) were determined by a UVevis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the KubelkaeMunk method. The surface areas of the samples were determined by BET measurement (Tristar II, USA). The surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis Ultra DLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C1s ¼ 284.6 eV) as a reference. The TG of the samples was obtained on the differential thermal analyzer (NETZSCH, STA 449 F3) with a heating rate of 10 C min1 in flowing Ar. The FTIR spectrums of the products were obtained on Fourier Transform Infrared spe (ThermoFisher, Nicolet 6700). FESEM images and EDS were obtained on Hitachi SP2600 field emission scanning electron microscope, employing an accelerating voltage of 5 kV and 15 kV, respectively.
Photocatalytic hydrogen production The photocatalytic reactions were carried out in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. 0.1 g catalyst was suspended in 100 ml of aqueous solution containing 30 ml methanol. For comparison, 0.1 g CdS was suspended in 100 ml of aqueous solution containing 30 ml methanol and a certain amount of H2PtCl6 solution that the mass ratio of Pt is 1 wt%. The suspension was then thoroughly degassed and irradiated by a Xe lamp (300 W), equipped with an optical cutoff filter (l > 400 nm, containing 1 M NaNO2). The activity of H2 evolution was analyzed using an online gas chromatography.
Results and discussion Phase structures and morphology Fig. 1 e XRD patterns of samples CdS, flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
The phase of the products obtained under the present system was characterized by XRD. As shown in Fig. 1, three diffraction
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Fig. 2 e Representative FESEM (a) flowerlike Ni/Ni(OH)2, (b) NC(1/2), (c) NC(1/2)-5, (d) EDS analysis of NC(1/2).
peaks centered at 44.5, 51.8 and 76.3 can be indexed as the (111), (200) and (220) planes of the fcc Ni form respectively. The other two weak diffraction peaks centered at 33.2, 59.3 correspond to the (101) and (110) planes of the rhombohedra Ni(OH)2$0.75H2O form [21]. Namely, the product is a composite consisting of Ni and Ni(OH)2. However, the diffraction peaks of Ni and Ni(OH)2 disappeared after incorporating with CdS as shown in Fig. 1, and the peaks of Ni and Ni(OH)2 did not appear even after 5 h photocatalytic reaction. This was probably due to the shielding effect of CdS. No peaks of NiS was observed for NC(1/2), indicating that NiS did not form during preparation process of CdS by hydrothermal method. There was little Ni2þ in the solution because the solubility product (Ksp) of Ni(OH)2 was 2.0 1015. On the other hand, from the solubility products (Ksp) of CdS (8.0 1027) and NiS (1.1 1021), it also could be inferred that NiS could not form during the hydrothermal process. The morphology of samples were analyzed by FESEM to directly observe the hybrid nanostructures of CdS and flowerlike Ni/Ni(OH)2. As shown in Fig. 2a, a great deal of porous superstructures with different size can be seen clearly, which are consisted of abundant thin nano-plates. After hybrid with CdS, some CdS nanoparticles are inserted into the superstructures, while other CdS particles agglomerate on the surface of flowerlike Ni/Ni(OH)2 as shown in Fig. 2b, which also can be observed from TEM images. TEM images of NC(1/2) was shown in Fig. 3, plate-like nanostructure could be easily found (Fig. 3A), and the lattice spacing (0.332 nm) was observed in Fig. 3B indicated that the existence of CdS. The component of the hybrid nanostructures was investigated by EDS. As shown
in Fig. 2d, strong O, Ni, Cd and S peaks can be clearly found. Hydrogen peaks cannot be detected owing to the limitation of the instrument. However, the superstructures of flowerlike Ni/Ni(OH)2 disappeared after 5 h photocatalytic reaction as shown in Fig. 2c. The reasons will be explained in following sections.
TG, FT-IR and XPS analyses To investigate the changes of the hybrid nanostructures during the photocatalytic process, TG spectra were collected from samples flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5 (Fig. 4). The loss weight below 300 C of flowerlike Ni/(OH)2 is about 17.0%, which is attributed to desorption of adsorbed or/and crystal water of the composite. A weight loss of 12.2% located from 300 to 400 C is ascribed to the decomposition of Ni(OH)2 [21,22]. Thus, the mass ratio of Ni(OH)2 and Ni in the flowerlike superstructure composite can be calculated to be about 1.8. In the same way, the contents of Ni(OH)2 in NC(1/2) and NC(1/2)-5 are calculated to be 22.1 wt% and 7.2 wt% respectively. From the results above, it can be inferred easily that the Ni(OH)2 content in NC(1/2) decreased apparently after photocatalytic reaction for 5 h. The content of Ni(OH)2 reduced during photocatalytic process, which could also be identified by FT-IR. Fig. 5 shows the FT-IR spectrum of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/ 2)-5 in the range of 400e4000 cm1. The broad and strong band centered at 3200e3650 cm1 is assigned to the OeH stretching vibration of the interlayer water molecules and the H-bound OH group. Another peak observed at 1628 cm1 is also
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Fig. 5 e FTIR spectrum of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
assigned to the bending vibration of water molecules [21,23]. The peaks observed at 1398 cm1 and 1268 cm1 are assigned to the various vibrational modes of the carbonate groups originating from the adsorption of atmospheric CO2 [24]. Additionally, the peak at 660 cm1 corresponds to d (NieOeH)
[25]. It can be seen clearly that the peak at 660 cm1 disappeared as shown in spectra for NC(1/2)-5 (Fig. 5), which indicated that Ni(OH)2 were consumed during photocatalytic reaction. Fig. 6 shows the high-resolution XPS spectra of the Ni2p peaks for samples flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)5. For the as-prepared flowerlike Ni/Ni(OH)2, the measured binding energies of Ni 2p3/2 and Ni 2p1/2 are equal to 855.2 and 873.0 eV respectively. The Ni 2p3/2 peak at 855.2 eV suggests that Ni(OH)2 existed in flowerlike compound [18]. A further analysis of the spectrum for flowerlike Ni/Ni(OH)2, a weak peak appears at 852.0 eV, indicating the presence of metallic Ni [19]. This result provides an additional confirmation that Ni exist in the flowerlike composite. After incorporating with CdS nanoparticles, the peak at 852.0 eV of flowerlike Ni/Ni(OH)2 disappears as shown in Fig. 6, which is probably because of a shielding effect from CdS. There is little changes about binding energies of Ni 2p3/2 and Ni 2p1/2 after incorporating with CdS nanoparticles and photocatalytic reaction for 5 h, which indicates that Ni(OH)2 still exists in the composite. However, the peak at 852.0 eV appears again after photocatalytic
Fig. 4 e The TG analysis of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
Fig. 6 e High-resolution XPS spectra of Ni 2p of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
Fig. 3 e (A) TEM image of NC(1/2), (B) Magnification of marked area of (A).
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reaction for 5 h as shown in Fig. 6. This probably because the Ni2þ in Ni(OH)2 are reduced to Ni, which will be further explained in tentative mechanism section.
BET analyses and UVevis diffuse reflection spectra Generally, the BET surface area of catalyst plays an important role in catalytic performance [26,27]. As shown in Table 1, the BET surface area of flowerlike Ni/Ni(OH)2 is highly to 265.8 m2/ g. However, the BET surface area of NC(1/2) is 86.8 m2/g which is due to the low BET surface area of CdS (52.9 m2/g). Further, the pore volume of NC(1/2) is lower than flowerlike Ni/Ni(OH)2 as shown in Table 1, indicating that CdS nanoparticles are inserted into the superstructure of flowerlike Ni/Ni(OH)2, which has been confirmed by FESEM (Fig. 2b). After 5 h photocatalytic reaction, both of the BET surface area and pore volume of NC(1/2)-5 reduced compared with the ones of NC(1/2). This is probably because that the porous superstructure of flowerlike Ni/Ni(OH)2 was broken during photocatalytic process. A comparison of the UVevis diffuse reflectance spectra of samples CdS, flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5 is displayed in Fig. 7. As can be seen in this figure, the absorption intensity of CdS starts to increase rapidly at 520 nm, indicating a band gap of 2.4 eV, which coincides with the reported value [6]. In comparison to pure CdS, the absorption edge of the asprepared NC(1/2) has no obvious change. However, there is an enhanced absorbance in the visible-light region (>530 nm). This is also observed as a color change of the samples from yellow to pale green. After photocatalytic reaction for 5 h, the absorbance in the visible-light region (>530 nm) increased apparently as shown in Fig. 7 and the color of the catalyst changed from pale green to dark green, which was ascribed to metallic Ni formed during photocatalytic H2 production.
Photocatalytic activity and tentative mechanism of photocatalytic reaction The hybrid nanostructures of CdS and flowerlike Ni/Ni(OH)2 were prepared by a simple hydrothermal method which has been shown in sample preparation section. The nominal mass ratios of Ni/Ni(OH)2 to CdS were 0.25, 0.5, 1 and 2. Fig. 8 displays a comparison of the photocatalytic H2 production activities of CdS and NC(R) samples. As can be seen from this figure, R has a great influence on the photocatalytic H2 production activity. The pure CdS shows a low photocatalytic activity (about 2.8 mmol h1), mainly due to the rapid recombination of photogenerated charge carriers. However, the H2production rate noticeably increases after hybrid with flowerlike Ni/Ni(OH)2 and the sample NC(1/2) shows the highest
Fig. 7 e UVevis diffuse reflection, spectra of CdS, flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5. average activity about 373.5 mmol h1,which is 1.7 times of CdS loaded with noble metal 1 wt% Pt via in situ photodeposition method. The quantum efficiency at 400 nm is 2.2% when methanol was as sacrificial agent. The photocatalytic H2 production activity of NC(1/2) is stable even after 10 h irradiation. From the TG results, the hydrogen content of Ni(OH)2 in NC(1/ 2) is calculated to be 238.4 mmol, which is only 0.0638 times of the photocatalytic H2 content for 10 h. Thus, the high photocatalytic H2 production activity is not due to the decomposition of Ni(OH)2. This is attributed to the superstructure of flowerlike Ni/Ni(OH)2, which CdS nanoparticles can be well inserted in. Therefore, they can contact fully with each other. Ni is an effective cocatalyst for photocatalytic H2 production, which can transfer electrons efficiently [2,6]. Furthermore, the potential (0.23 V vs. SHE, pH ¼ 0) of Ni2þ/Ni is less negative than the conduct band level of CdS (about 0.7 V), the photoinduced electrons in the conduct band can transfer to Ni(OH)2 and then effectively reduce some Ni2þ to Ni. These nickels can also act as cocatalysts to promote the separation and transfer of photogenerated electrons from CdS conduct band to Ni/ Ni(OH)2, where Hþ is reduced to H2 [19]. Thus, the
Table 1 e BET surface areas and pore volume analysis of CdS, flowerlike Ni/ni(OH)2, NC(1/2) and NC(1/2)-5. Sample Ni/Ni(OH)2 NC(1/2) NC(1/2)-5 CdS
BET (m2/g)
Pore volume (cm3/g)
265.8 86.8 43.6 52.9
0.72 0.38 0.29 0.20
Fig. 8 e Comparison of the photocatalytic H2-production activity of samples CdS, NC (R, R ¼ 2/1, 1/1, 1/2 and 1/4) under visible light irradiation.
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the National Key Basic Research and Development Program (2009CN220000) and the International Cooperation Project of Shanghai Municipal Science and Technology Commission (12160705700).
references
Fig. 9 e Volume of hydrogen evolved in three cycles every 5 h.
superstructure of flowerlike Ni/Ni(OH)2 are broken during the photocatalytic process because Ni(OH)2 are reduced to Ni. For the samples with R > 0.5, the photocatalytic H2 production activity decreases. This is probably because of the shielding effect from flowerlike Ni/Ni(OH)2. As a consequence, a suitable ratio of CdS and flowerlike Ni/Ni(OH)2 is crucial for optimizing the photocatalytic activity of the hybrid nanostructures. In comparison, no hydrogen was detected when flowerlike Ni/ Ni(OH)2 was used as the photocatalyst, suggesting that the bare flowerlike Ni/Ni(OH)2 without CdS was likely not active for photocatalytic H2 production under the experimental conditions studied. Fig. 9 showed that the hydrogen production activity of NC(1/2) could be keep stable in three recycled runs. This indicates that photocatalytic hydrogen evolution in aqueous methanol solution on NC(1/2) can be recycled and reused without apparent deactivation.
Conclusions A high efficiency of the photocatalytic H2 production has been achieved over the nickels/CdS prepared by a simple photocatalytic reaction of CdS and flowerlike Ni/Ni(OH)2. The optimal mass ratio of flowerlike Ni/Ni(OH)2 and CdS was found to be 0.5, which resulted in a high visible light photocatalytic H2 production rate of 373.5 mmol h1. It was even higher than the one of CdS loaded with noble metal 1 wt% Pt. CdS can be well inserted into the superstructure of flowerlike Ni/Ni(OH)2 and contact fully with each other. Through photocatalytic reaction, the Ni2þ in Ni(OH)2 can be reduced to metal nickel and form effective photocatalyst nickels/CdS. This work provides a new method to prepare composite photocatalyst with effective photocatalytic properties, especially for these materials with unique features.
Acknowledgments This work was supported by the National High Technology Research and Development Program of China (2012AA051501),
[1] Li Q, Guo BD, Yu JG, Ran J, Zhang BH, Yan HJ, et al. Highly efficient visible-light driven photocatalytic hydrogen production of CdS-cluster-decorated grapheme nanosheets. J Am Chem Soc 2011;133:10878e84. [2] Chen XP, Shangguan WF. Hydrogen production from water splitting on CdS-based photocatalysts using solar light. Front Energy 2013;7:111e8. [3] Honda K, Fujishima A. Photolysis-decomposition of water at the surface of an irradiated semiconductor. Nature 1972;238:37e8. [4] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. [5] Maeda K, Domen KJ. Photocatalytic water splitting: recent progress and future challenges. J Phys Chem Lett 2010;1:2655e61. [6] Chen XP, Chen W, Lin PB, Yang Y, Gao HY, Yuan J, et al. In situ photodeposition of nickel oxides on CdS for highly efficient hydrogen production via visible-light-driven photocatalysis. Catal Commun 2013;36:104e8. [7] Li CL, Yuan J, Han BY, Shangguan WF. Synthesis and photochemical performance of morphology-controlled CdS photocatalysts for hydrogen evolution under visible light. Int J Hydrogen Energy 2011;36:4271e9. [8] Bao NZ, Shen LM, Takata T, Domen K. Self-templated synthesis of nanoporous CdS nanostructures for highly efficient photocatalytic hydrogen production under visible light. Chem Mater 2008;20:110e7. [9] Liu JK, Luo CX, Yang XH, Zhang XY. Ultrasonic-template method synthesis of CdS hollow nanoparticle chains. Mater Lett 2009;63:124e6. [10] Yang JH, Yan HJ, Wang XL, Wen FY, Wang ZJ, Fan DY, et al. Roles of cocatalysts in PtePdS/CdS with exceptionally high quantum efficiency for photocatalytic hydrogen production. J Catal 2012;290:151e7. [11] Yang TT, Chen WT, Hsu YJ, Wei KH, Lin TY, Lin TW. Interfacial charge carrier dynamics in coreshell AueCdS nanocrystals. J Phys Chem C 2010;114:11414e20. [12] Zong X, Han JF, Ma GJ, Yan HJ, Wu GP, Li C. Photocatalytic H2 evolution on CdS loaded with WS2 as cocatalyst under visible light irradiation. J Phys Chem C 2011;115:12202e8. [13] Zong X, Wu GP, Yan HJ, Ma GJ, Shi JY, Wen F, et al. Photocatalytic H2 evolution on MoS2/CdS catalysts under visible light irradiation. J Phys Chem C 2010;114:1963e8. [14] Zhang J, Qiao SZ, Qi LF, Yu JG. Fabrication of NiS modified CdS nanorod pen junction photocatalysts with enhanced visible-light photocatalytic H2-production activity. Phys Chem Chem Phys 2013;15:12088e94. [15] Shangguan WF. Hydrogen evolution from water splitting on nanocomposite photocatalysts. Sci Adv Mater 2007;8:76e81. [16] Li CL, Yuan J, Han BY, Jiang L, Shangguan WF. TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation. Int J Hydrogen Energy 2010;35:7073e9. [17] Barpuzary D, Khan Z, Vinothkumar N, De M, Qureshi M. Hierarchically grown urchinlike CdS@ZnO and CdS@Al2O3 heteroarrays for efficient visible-light-driven photocatalytic hydrogen generation. J Phys Chem C 2012;116:150e6.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 9 8 e1 0 0 4
[18] Yu JG, Hai Y, Cheng B. Enhanced photocatalytic H2production activity of TiO2 by Ni(OH)2 cluster modification. J Phys Chem C 2011;115:4953e8. [19] Ran J, Yu JG, Jaroniec M. Ni(OH)2 modified CdS nanorods for highly efficient visible-light-driven photocatalytic H2 generation. Green Chem 2011;13:2708e13. [20] Wang M, Ni YH, Cao L, Zhao D, Ma X. Porous Ni/b-Ni(OH)2 superstructures: rapid solvothermal synthesis, characterization, and electrochemical property. J Colloid Interface Sci 2013;401:8e13. [21] Cao L, Ni YH, Wanga M, Mab X. Magnetic Ni/a-Ni(OH)2 porous superstructures: synthesis, influencing factors and applications in the removal of heavy metals. RSC Adv 2013;3:3585e91. [22] Wang Y, Zhu QS, Zhang HG. Fabrication of b-Ni(OH)2 and NiO hollow spheres by a facile template-free process. Chem Commun 2005:5231e3.
[23] Yang ST, Chen GR, Yin YH, Chen HJ. Preparation,Structure and performances of submicronic spherical a-Ni(OH)2. Battery Bimon 2001;31:107e9. [24] Mustafa A, Ahmad NG, Mehdi G. Synthesis, characterization, and electrochemical properties of ultrafine b-Ni(OH)2 nanoparticles. Int J Hydrogen Energy 2011;36:8674e9. [25] Zhao YL, Wang JM, Chen H, Pan T, Zhang JQ, Cao CN. Alsubstituted a-nickel hydroxide prepared by homogeneous precipitation method with urea. Int J Hydrogen Energy 2004;29:889e96. [26] Yu JG, Wang GH, Cheng B, Zhou MH. Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders. Appl Catal B 2007;69:171e80. [27] Yu JG, Zhang LJ, Cheng B, Su YR. Hydrothermal preparation and photocatalytic activity of hierarchically sponge-like macro-/mesoporous titania. J Phys Chem C 2007;111:10582e9.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Nickels/CdS photocatalyst prepared by flowerlike Ni/Ni(OH)2 precursor for efficiently photocatalytic H2 evolution Xiaoping Chen a, Shu Chen a, Caifang Lin a, Zhi Jiang a,b, Wenfeng Shangguan a,b,* a
Research Center for Combustion and Environment Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China b Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China
article info
abstract
Article history:
Nickels/CdS photocatalyst was prepared by a simple photocatalytic reaction of CdS and
Received 29 April 2014
flowerlike Ni/Ni(OH)2 nanocomposite. The photocatalyst shows effectively photocatalytic
Received in revised form
H2 production activity, and reaches to the maximum rate of 373.5 mmol h1 when the mass
6 November 2014
ratio of CdS and flowerlike Ni/Ni(OH)2 is 2, even higher than the one of CdS loaded with 1 wt
Accepted 10 November 2014
% Pt. Characterization results (FESEM, TG, FT-IR, XPS, etc.) indicate that the effective
Available online 4 December 2014
photocatalytic H2-production activity was predominantly attributed to the porous flowerlike superstructures, in which CdS could be well fixed. Thus, CdS could contact fully with
Keywords:
flowerlike Ni/Ni(OH)2. Ni is an excellent cocatalyst, which can transfer electrons efficiently.
Flowerlike Ni/Ni(OH)2
Furthermore, Ni2þ of Ni(OH)2 were effectively reduced to Ni. These nickels could also act as
CdS
cocatalyst to promote the separation and transfer of photogenerated electrons. Therefore,
Photocatalysis
the flowerlike superstructures was gradually broken during the photocatalytic progress
Hydrogen production
because of Ni(OH)2 reduction, while resulted in effective photocatalytic H2 production activities. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasingly serious energy crisis and the environmental problems caused by the burning of fossil fuels have led to an aggressive search for renewable and environmental-friendly energy resources [1]. Hydrogen, a new renewable and nopolluting energy, has been considered as a promising
candidate for the future energy [2]. Since the discovery of photocatalytic hydrogen evolution from TiO2 by Fujishima and Honda in 1972 [3], many efficient photocatalysts have been found for photocatalytic hydrogen production [4,5]. However, most of them only respond to ultraviolet (UV) light which only accounts for about 4% of the solar radiation energy, while the visible light contributes to about 43%.
* Corresponding author. Key Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, PR China. E-mail address: [email protected] (W. Shangguan). http://dx.doi.org/10.1016/j.ijhydene.2014.11.058 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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As compared to their wide bandgap counterparts, CdS particles are attractive photocatalytic materials for the conversion of solar energy into chemical energy under visible light irradiation. Specifically, it has a band-gap energy (Eg) of 2.4 eV that suits very well with the solar spectrum and the conduction band edge is much more negative than the Hþ/H2 reduction potential [1,2,6]. However, CdS alone shows low photocatalytic hydrogen production activity because of the rapid recombination of excited electrons and holes. Many approaches have been proposed to enhance the photocatalytic activity of CdS particles, including synthesis of CdS with specific morphology [7e9], deposition of cocatalysts such as Pt, Au, metal sulfides etc. [10e14], incorporation of CdS in the interlayer region of layered compounds [15], and preparation of nanocomposite with other materials such as TiO2, ZnO, graphene etc. [1,16,17]. Ni and its oxides have been proved to be low-cost and efficient cocatalysts for photocatalytic H2 production [6], and Ni(OH)2 has also been found that it can significantly improve photocatalytic H2 production of photocatalysts [18,19]. In this work, high efficiency of the visible-light-driven photocatalytic H2 production was achieved by a simple photocatalytic reaction of CdS and flowerlike Ni/Ni(OH)2, which were synthesized via a simple hydrothermal method. Furthermore, a mechanism for the photocatalytic reaction of the composite is proposed.
Experimental Sample preparation All of the reagents were of analytical grade and used without further purification. Deionized water was used in all experiments. Flowerlike Ni/Ni(OH)2 was prepared by a simple chemical reduction and hydrothermal method [20,21]. 3 mmol of NiCl2$6H2O was dissolved in a 30 ml mixed solution of ethylene glycol and N, N-dimethylformamide with the volume ratio 3:2 to form a homogeneous solution. Then, 1 mmol of NaBH4 was added into the above mixed solution under stirring for 10 min. After that, the mixed system was poured into a
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Teflon-lined stainless steel autoclave with the capacity of 100 ml and maintained at 170 C for 8 h. Finally, the products were washed with ethanol and distilled water several times, and dried in air at 80 C for 12 h. Then, 0.1 g flowerlike Ni/Ni(OH)2 prepared as above was dispersed in 50 ml deionized water and certain amount of Cd(CH3COO)2$2H2O was added in the mixed system. After stirring for 2 h, the same mole ratio of Na2S (0.14 M) was added in the mixed system. After stirring for another 2 h, the mixed system was poured into a Teflon-lined stainless steel autoclave with the capacity of 100 ml and maintained at 160 C for 12 h. The products were washed with distilled water several times and dried in air 80 C for 12 h. CdS was synthesized as the same process except without flowerlike Ni/Ni(OH)2. NC(R) was referred as CdS and Ni/Ni(OH)2 composite (R was the mass ratio of CdS and Ni/Ni(OH)2).
Characterization The crystal structure of the photocatalytic materials was confirmed by X-ray diffraction (Rigaku D/max-2200/PC Japan) with Cu Ka (40 kV, 20 mA). The UVevis diffuse reflection spectra (DRS) were determined by a UVevis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the KubelkaeMunk method. The surface areas of the samples were determined by BET measurement (Tristar II, USA). The surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis Ultra DLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C1s ¼ 284.6 eV) as a reference. The TG of the samples was obtained on the differential thermal analyzer (NETZSCH, STA 449 F3) with a heating rate of 10 C min1 in flowing Ar. The FTIR spectrums of the products were obtained on Fourier Transform Infrared spe (ThermoFisher, Nicolet 6700). FESEM images and EDS were obtained on Hitachi SP2600 field emission scanning electron microscope, employing an accelerating voltage of 5 kV and 15 kV, respectively.
Photocatalytic hydrogen production The photocatalytic reactions were carried out in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. 0.1 g catalyst was suspended in 100 ml of aqueous solution containing 30 ml methanol. For comparison, 0.1 g CdS was suspended in 100 ml of aqueous solution containing 30 ml methanol and a certain amount of H2PtCl6 solution that the mass ratio of Pt is 1 wt%. The suspension was then thoroughly degassed and irradiated by a Xe lamp (300 W), equipped with an optical cutoff filter (l > 400 nm, containing 1 M NaNO2). The activity of H2 evolution was analyzed using an online gas chromatography.
Results and discussion Phase structures and morphology Fig. 1 e XRD patterns of samples CdS, flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
The phase of the products obtained under the present system was characterized by XRD. As shown in Fig. 1, three diffraction
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Fig. 2 e Representative FESEM (a) flowerlike Ni/Ni(OH)2, (b) NC(1/2), (c) NC(1/2)-5, (d) EDS analysis of NC(1/2).
peaks centered at 44.5, 51.8 and 76.3 can be indexed as the (111), (200) and (220) planes of the fcc Ni form respectively. The other two weak diffraction peaks centered at 33.2, 59.3 correspond to the (101) and (110) planes of the rhombohedra Ni(OH)2$0.75H2O form [21]. Namely, the product is a composite consisting of Ni and Ni(OH)2. However, the diffraction peaks of Ni and Ni(OH)2 disappeared after incorporating with CdS as shown in Fig. 1, and the peaks of Ni and Ni(OH)2 did not appear even after 5 h photocatalytic reaction. This was probably due to the shielding effect of CdS. No peaks of NiS was observed for NC(1/2), indicating that NiS did not form during preparation process of CdS by hydrothermal method. There was little Ni2þ in the solution because the solubility product (Ksp) of Ni(OH)2 was 2.0 1015. On the other hand, from the solubility products (Ksp) of CdS (8.0 1027) and NiS (1.1 1021), it also could be inferred that NiS could not form during the hydrothermal process. The morphology of samples were analyzed by FESEM to directly observe the hybrid nanostructures of CdS and flowerlike Ni/Ni(OH)2. As shown in Fig. 2a, a great deal of porous superstructures with different size can be seen clearly, which are consisted of abundant thin nano-plates. After hybrid with CdS, some CdS nanoparticles are inserted into the superstructures, while other CdS particles agglomerate on the surface of flowerlike Ni/Ni(OH)2 as shown in Fig. 2b, which also can be observed from TEM images. TEM images of NC(1/2) was shown in Fig. 3, plate-like nanostructure could be easily found (Fig. 3A), and the lattice spacing (0.332 nm) was observed in Fig. 3B indicated that the existence of CdS. The component of the hybrid nanostructures was investigated by EDS. As shown
in Fig. 2d, strong O, Ni, Cd and S peaks can be clearly found. Hydrogen peaks cannot be detected owing to the limitation of the instrument. However, the superstructures of flowerlike Ni/Ni(OH)2 disappeared after 5 h photocatalytic reaction as shown in Fig. 2c. The reasons will be explained in following sections.
TG, FT-IR and XPS analyses To investigate the changes of the hybrid nanostructures during the photocatalytic process, TG spectra were collected from samples flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5 (Fig. 4). The loss weight below 300 C of flowerlike Ni/(OH)2 is about 17.0%, which is attributed to desorption of adsorbed or/and crystal water of the composite. A weight loss of 12.2% located from 300 to 400 C is ascribed to the decomposition of Ni(OH)2 [21,22]. Thus, the mass ratio of Ni(OH)2 and Ni in the flowerlike superstructure composite can be calculated to be about 1.8. In the same way, the contents of Ni(OH)2 in NC(1/2) and NC(1/2)-5 are calculated to be 22.1 wt% and 7.2 wt% respectively. From the results above, it can be inferred easily that the Ni(OH)2 content in NC(1/2) decreased apparently after photocatalytic reaction for 5 h. The content of Ni(OH)2 reduced during photocatalytic process, which could also be identified by FT-IR. Fig. 5 shows the FT-IR spectrum of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/ 2)-5 in the range of 400e4000 cm1. The broad and strong band centered at 3200e3650 cm1 is assigned to the OeH stretching vibration of the interlayer water molecules and the H-bound OH group. Another peak observed at 1628 cm1 is also
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Fig. 5 e FTIR spectrum of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
assigned to the bending vibration of water molecules [21,23]. The peaks observed at 1398 cm1 and 1268 cm1 are assigned to the various vibrational modes of the carbonate groups originating from the adsorption of atmospheric CO2 [24]. Additionally, the peak at 660 cm1 corresponds to d (NieOeH)
[25]. It can be seen clearly that the peak at 660 cm1 disappeared as shown in spectra for NC(1/2)-5 (Fig. 5), which indicated that Ni(OH)2 were consumed during photocatalytic reaction. Fig. 6 shows the high-resolution XPS spectra of the Ni2p peaks for samples flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)5. For the as-prepared flowerlike Ni/Ni(OH)2, the measured binding energies of Ni 2p3/2 and Ni 2p1/2 are equal to 855.2 and 873.0 eV respectively. The Ni 2p3/2 peak at 855.2 eV suggests that Ni(OH)2 existed in flowerlike compound [18]. A further analysis of the spectrum for flowerlike Ni/Ni(OH)2, a weak peak appears at 852.0 eV, indicating the presence of metallic Ni [19]. This result provides an additional confirmation that Ni exist in the flowerlike composite. After incorporating with CdS nanoparticles, the peak at 852.0 eV of flowerlike Ni/Ni(OH)2 disappears as shown in Fig. 6, which is probably because of a shielding effect from CdS. There is little changes about binding energies of Ni 2p3/2 and Ni 2p1/2 after incorporating with CdS nanoparticles and photocatalytic reaction for 5 h, which indicates that Ni(OH)2 still exists in the composite. However, the peak at 852.0 eV appears again after photocatalytic
Fig. 4 e The TG analysis of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
Fig. 6 e High-resolution XPS spectra of Ni 2p of flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5.
Fig. 3 e (A) TEM image of NC(1/2), (B) Magnification of marked area of (A).
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reaction for 5 h as shown in Fig. 6. This probably because the Ni2þ in Ni(OH)2 are reduced to Ni, which will be further explained in tentative mechanism section.
BET analyses and UVevis diffuse reflection spectra Generally, the BET surface area of catalyst plays an important role in catalytic performance [26,27]. As shown in Table 1, the BET surface area of flowerlike Ni/Ni(OH)2 is highly to 265.8 m2/ g. However, the BET surface area of NC(1/2) is 86.8 m2/g which is due to the low BET surface area of CdS (52.9 m2/g). Further, the pore volume of NC(1/2) is lower than flowerlike Ni/Ni(OH)2 as shown in Table 1, indicating that CdS nanoparticles are inserted into the superstructure of flowerlike Ni/Ni(OH)2, which has been confirmed by FESEM (Fig. 2b). After 5 h photocatalytic reaction, both of the BET surface area and pore volume of NC(1/2)-5 reduced compared with the ones of NC(1/2). This is probably because that the porous superstructure of flowerlike Ni/Ni(OH)2 was broken during photocatalytic process. A comparison of the UVevis diffuse reflectance spectra of samples CdS, flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5 is displayed in Fig. 7. As can be seen in this figure, the absorption intensity of CdS starts to increase rapidly at 520 nm, indicating a band gap of 2.4 eV, which coincides with the reported value [6]. In comparison to pure CdS, the absorption edge of the asprepared NC(1/2) has no obvious change. However, there is an enhanced absorbance in the visible-light region (>530 nm). This is also observed as a color change of the samples from yellow to pale green. After photocatalytic reaction for 5 h, the absorbance in the visible-light region (>530 nm) increased apparently as shown in Fig. 7 and the color of the catalyst changed from pale green to dark green, which was ascribed to metallic Ni formed during photocatalytic H2 production.
Photocatalytic activity and tentative mechanism of photocatalytic reaction The hybrid nanostructures of CdS and flowerlike Ni/Ni(OH)2 were prepared by a simple hydrothermal method which has been shown in sample preparation section. The nominal mass ratios of Ni/Ni(OH)2 to CdS were 0.25, 0.5, 1 and 2. Fig. 8 displays a comparison of the photocatalytic H2 production activities of CdS and NC(R) samples. As can be seen from this figure, R has a great influence on the photocatalytic H2 production activity. The pure CdS shows a low photocatalytic activity (about 2.8 mmol h1), mainly due to the rapid recombination of photogenerated charge carriers. However, the H2production rate noticeably increases after hybrid with flowerlike Ni/Ni(OH)2 and the sample NC(1/2) shows the highest
Fig. 7 e UVevis diffuse reflection, spectra of CdS, flowerlike Ni/Ni(OH)2, NC(1/2) and NC(1/2)-5. average activity about 373.5 mmol h1,which is 1.7 times of CdS loaded with noble metal 1 wt% Pt via in situ photodeposition method. The quantum efficiency at 400 nm is 2.2% when methanol was as sacrificial agent. The photocatalytic H2 production activity of NC(1/2) is stable even after 10 h irradiation. From the TG results, the hydrogen content of Ni(OH)2 in NC(1/ 2) is calculated to be 238.4 mmol, which is only 0.0638 times of the photocatalytic H2 content for 10 h. Thus, the high photocatalytic H2 production activity is not due to the decomposition of Ni(OH)2. This is attributed to the superstructure of flowerlike Ni/Ni(OH)2, which CdS nanoparticles can be well inserted in. Therefore, they can contact fully with each other. Ni is an effective cocatalyst for photocatalytic H2 production, which can transfer electrons efficiently [2,6]. Furthermore, the potential (0.23 V vs. SHE, pH ¼ 0) of Ni2þ/Ni is less negative than the conduct band level of CdS (about 0.7 V), the photoinduced electrons in the conduct band can transfer to Ni(OH)2 and then effectively reduce some Ni2þ to Ni. These nickels can also act as cocatalysts to promote the separation and transfer of photogenerated electrons from CdS conduct band to Ni/ Ni(OH)2, where Hþ is reduced to H2 [19]. Thus, the
Table 1 e BET surface areas and pore volume analysis of CdS, flowerlike Ni/ni(OH)2, NC(1/2) and NC(1/2)-5. Sample Ni/Ni(OH)2 NC(1/2) NC(1/2)-5 CdS
BET (m2/g)
Pore volume (cm3/g)
265.8 86.8 43.6 52.9
0.72 0.38 0.29 0.20
Fig. 8 e Comparison of the photocatalytic H2-production activity of samples CdS, NC (R, R ¼ 2/1, 1/1, 1/2 and 1/4) under visible light irradiation.
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the National Key Basic Research and Development Program (2009CN220000) and the International Cooperation Project of Shanghai Municipal Science and Technology Commission (12160705700).
references
Fig. 9 e Volume of hydrogen evolved in three cycles every 5 h.
superstructure of flowerlike Ni/Ni(OH)2 are broken during the photocatalytic process because Ni(OH)2 are reduced to Ni. For the samples with R > 0.5, the photocatalytic H2 production activity decreases. This is probably because of the shielding effect from flowerlike Ni/Ni(OH)2. As a consequence, a suitable ratio of CdS and flowerlike Ni/Ni(OH)2 is crucial for optimizing the photocatalytic activity of the hybrid nanostructures. In comparison, no hydrogen was detected when flowerlike Ni/ Ni(OH)2 was used as the photocatalyst, suggesting that the bare flowerlike Ni/Ni(OH)2 without CdS was likely not active for photocatalytic H2 production under the experimental conditions studied. Fig. 9 showed that the hydrogen production activity of NC(1/2) could be keep stable in three recycled runs. This indicates that photocatalytic hydrogen evolution in aqueous methanol solution on NC(1/2) can be recycled and reused without apparent deactivation.
Conclusions A high efficiency of the photocatalytic H2 production has been achieved over the nickels/CdS prepared by a simple photocatalytic reaction of CdS and flowerlike Ni/Ni(OH)2. The optimal mass ratio of flowerlike Ni/Ni(OH)2 and CdS was found to be 0.5, which resulted in a high visible light photocatalytic H2 production rate of 373.5 mmol h1. It was even higher than the one of CdS loaded with noble metal 1 wt% Pt. CdS can be well inserted into the superstructure of flowerlike Ni/Ni(OH)2 and contact fully with each other. Through photocatalytic reaction, the Ni2þ in Ni(OH)2 can be reduced to metal nickel and form effective photocatalyst nickels/CdS. This work provides a new method to prepare composite photocatalyst with effective photocatalytic properties, especially for these materials with unique features.
Acknowledgments This work was supported by the National High Technology Research and Development Program of China (2012AA051501),
[1] Li Q, Guo BD, Yu JG, Ran J, Zhang BH, Yan HJ, et al. Highly efficient visible-light driven photocatalytic hydrogen production of CdS-cluster-decorated grapheme nanosheets. J Am Chem Soc 2011;133:10878e84. [2] Chen XP, Shangguan WF. Hydrogen production from water splitting on CdS-based photocatalysts using solar light. Front Energy 2013;7:111e8. [3] Honda K, Fujishima A. Photolysis-decomposition of water at the surface of an irradiated semiconductor. Nature 1972;238:37e8. [4] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. [5] Maeda K, Domen KJ. Photocatalytic water splitting: recent progress and future challenges. J Phys Chem Lett 2010;1:2655e61. [6] Chen XP, Chen W, Lin PB, Yang Y, Gao HY, Yuan J, et al. In situ photodeposition of nickel oxides on CdS for highly efficient hydrogen production via visible-light-driven photocatalysis. Catal Commun 2013;36:104e8. [7] Li CL, Yuan J, Han BY, Shangguan WF. Synthesis and photochemical performance of morphology-controlled CdS photocatalysts for hydrogen evolution under visible light. Int J Hydrogen Energy 2011;36:4271e9. [8] Bao NZ, Shen LM, Takata T, Domen K. Self-templated synthesis of nanoporous CdS nanostructures for highly efficient photocatalytic hydrogen production under visible light. Chem Mater 2008;20:110e7. [9] Liu JK, Luo CX, Yang XH, Zhang XY. Ultrasonic-template method synthesis of CdS hollow nanoparticle chains. Mater Lett 2009;63:124e6. [10] Yang JH, Yan HJ, Wang XL, Wen FY, Wang ZJ, Fan DY, et al. Roles of cocatalysts in PtePdS/CdS with exceptionally high quantum efficiency for photocatalytic hydrogen production. J Catal 2012;290:151e7. [11] Yang TT, Chen WT, Hsu YJ, Wei KH, Lin TY, Lin TW. Interfacial charge carrier dynamics in coreshell AueCdS nanocrystals. J Phys Chem C 2010;114:11414e20. [12] Zong X, Han JF, Ma GJ, Yan HJ, Wu GP, Li C. Photocatalytic H2 evolution on CdS loaded with WS2 as cocatalyst under visible light irradiation. J Phys Chem C 2011;115:12202e8. [13] Zong X, Wu GP, Yan HJ, Ma GJ, Shi JY, Wen F, et al. Photocatalytic H2 evolution on MoS2/CdS catalysts under visible light irradiation. J Phys Chem C 2010;114:1963e8. [14] Zhang J, Qiao SZ, Qi LF, Yu JG. Fabrication of NiS modified CdS nanorod pen junction photocatalysts with enhanced visible-light photocatalytic H2-production activity. Phys Chem Chem Phys 2013;15:12088e94. [15] Shangguan WF. Hydrogen evolution from water splitting on nanocomposite photocatalysts. Sci Adv Mater 2007;8:76e81. [16] Li CL, Yuan J, Han BY, Jiang L, Shangguan WF. TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation. Int J Hydrogen Energy 2010;35:7073e9. [17] Barpuzary D, Khan Z, Vinothkumar N, De M, Qureshi M. Hierarchically grown urchinlike CdS@ZnO and CdS@Al2O3 heteroarrays for efficient visible-light-driven photocatalytic hydrogen generation. J Phys Chem C 2012;116:150e6.
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[18] Yu JG, Hai Y, Cheng B. Enhanced photocatalytic H2production activity of TiO2 by Ni(OH)2 cluster modification. J Phys Chem C 2011;115:4953e8. [19] Ran J, Yu JG, Jaroniec M. Ni(OH)2 modified CdS nanorods for highly efficient visible-light-driven photocatalytic H2 generation. Green Chem 2011;13:2708e13. [20] Wang M, Ni YH, Cao L, Zhao D, Ma X. Porous Ni/b-Ni(OH)2 superstructures: rapid solvothermal synthesis, characterization, and electrochemical property. J Colloid Interface Sci 2013;401:8e13. [21] Cao L, Ni YH, Wanga M, Mab X. Magnetic Ni/a-Ni(OH)2 porous superstructures: synthesis, influencing factors and applications in the removal of heavy metals. RSC Adv 2013;3:3585e91. [22] Wang Y, Zhu QS, Zhang HG. Fabrication of b-Ni(OH)2 and NiO hollow spheres by a facile template-free process. Chem Commun 2005:5231e3.
[23] Yang ST, Chen GR, Yin YH, Chen HJ. Preparation,Structure and performances of submicronic spherical a-Ni(OH)2. Battery Bimon 2001;31:107e9. [24] Mustafa A, Ahmad NG, Mehdi G. Synthesis, characterization, and electrochemical properties of ultrafine b-Ni(OH)2 nanoparticles. Int J Hydrogen Energy 2011;36:8674e9. [25] Zhao YL, Wang JM, Chen H, Pan T, Zhang JQ, Cao CN. Alsubstituted a-nickel hydroxide prepared by homogeneous precipitation method with urea. Int J Hydrogen Energy 2004;29:889e96. [26] Yu JG, Wang GH, Cheng B, Zhou MH. Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders. Appl Catal B 2007;69:171e80. [27] Yu JG, Zhang LJ, Cheng B, Su YR. Hydrothermal preparation and photocatalytic activity of hierarchically sponge-like macro-/mesoporous titania. J Phys Chem C 2007;111:10582e9.