Synthesis and peferentially loading of nickel nanoparticle on CdS surface and its photocatalytic performance for hydrogen evolution under visible light

Accepted Manuscript Title: Synthesis and peferentially loading of nickel nanoparticle on CdS surface and its photocatalytic performance for hydrogen evolution under visible light Author: Xiying Li Hui Wang Tingting Chu Danzhen Li Liqun Mao PII: DOI: Reference:

S0025-5408(14)00280-3 http://dx.doi.org/doi:10.1016/j.materresbull.2014.05.016 MRB 7460

To appear in:

MRB

Received date: Revised date: Accepted date:

29-12-2013 4-5-2014 5-5-2014

Please cite this article as: Xiying Li, Hui Wang, Tingting Chu, Danzhen Li, Liqun Mao, Synthesis and peferentially loading of nickel nanoparticle on CdS surface and its photocatalytic performance for hydrogen evolution under visible light, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2014.05.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and Preferentially Loading of Nickel Nanoparticle on CdS Surface and Its Photocatalytic Performance for Hydrogen Evolution under Visible

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Light Xiying Lia, Hui Wanga, Tingting Chua, Danzhen Lib, Liqun Maoa*

Laboratory of Fine Chemistry and Industry, Henan University, Kaifeng 475004, P. R.

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a

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China; bNational Engineering Center of Environmental Photocatalysis, Fuzhou

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University, Fuzhou 350002, P. R. China

* Corresponding author: e-mail address: [email protected], Tel:+86 13513781969;

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Fax:+86 371 2388 1589

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Graphical abstract Ni nanoparticles were prepared via chemical reduction of aqueous NiCl2 by borohydride reducing agent in the presence of polyvinlylpyrolidone as a modifier to prevent fast growth of Ni crystals and their aggregation, and then preferentially deposited on (100), (002), and (101) crystal planes of CdS by photo-induced electrons while water splitting reaction occurred simultaneously. Resultant nickel nanoparticles have a size of about 3 nm, and contributes to decreasing the photoluminescence peak intensity of CdS, which means that nickel functions as the trapper of photo-generated electrons thereby quenching the photoluminescence of CdS. Therefore, nano-Ni/CdS photocatalyst with a Ni loading of 2.5% possesses the best visible-light catalytic activity for water splitting-hydrogen evolution and provides a hydrogen production rate of up to 9050 μmol·h-1·g-1, while it exhibits stabilized activity towards H2 evolution as well. Highlights

Ni nanoparticles are prepared by chemical reduction and then loaded on CdS surface by photo-reduction. ► Non-noble metal Ni nanoparticles (size: about 3 nm) act as co-catalyst for photocatalytic H2 evolution. ► Nano-Ni/CdS exhibits high activity (9050 μmol·h-1·g-1) and perfect stability.

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Abstract : Ni nanoparticles were prepared via chemical reduction of NiCl2 by NaBH4 in the presence of polyvinlylpyrolidone (PVP), and loaded on the surface of CdS by

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photo-induced electrons while water splitting reaction occurred simultaneously. Resultant Ni/CdS was characterized by high-resolution transmission electron

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microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet–visible

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light diffuse reflectance spectrometry, and photoluminescence spectrometry. It was

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found that as-prepared Ni nanoparticles are about 3 nm, and preferentially deposited on (100), (002), and (101) crystal planes of CdS. Meanwhile, loading nickel decreases

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the photoluminescence intensity of CdS, which means nickel functions as the trapper

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of photo-generated electrons. Therefore, nano-Ni/CdS photocatalyst with a Ni loading

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of 2.5% possesses the best visible-light catalytic activity for water splitting-hydrogen evolution and provides a hydrogen production rate of up to 9050 μmol·h-1·g-1, while it exhibits stabilized activity towards H2 evolution as well.

Key words: A. nanostructures, A. semiconductors, B. chemical synthesis, B. epitaxial growth, D. catalytic properties, D. crystal structure

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1. Introduction The researchers at the Florida Solar Energy Center, by making use of a new class of metal sulfate ammonia (MSO4-NH3) based hybrids, recently developed water splitting

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cycles for hydrogen production [1-4]. These cycles employ the photonic component of

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sunlight for hydrogen production and its thermal component (i.e., infrared) for oxygen generation; and the total energy efficiency of these cycles exceeds 60%. However, the

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photocatalysts used for these cycles are of very high cost, which is thought to be a

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serious impediment for the practical application in water splitting.

As a candidate catalyst for photochemical water splitting, CdS is relatively cheap, but it

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exhibits low activity, and poor stability. Therefore, it is usually needed to combine CdS

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with noble metal co-catalysts such as platinum (Pt), palladium (Pd), and rhodium (Ru)

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so as to increase the photocatalytic activity [5-7]. This is feasible, since the work function of noble metal co-catalyst is lower than the energy level of conduction band (ECB) of CdS, and noble metal co-catalyst can act as the trapping center of excited electrons to provide redox active sites for water splitting. Therefore, from the viewpoint of practical application, it is imperative to develop cheap and efficient co-catalysts which are noble metal free.

Early in 1990’s, Rufus and his coworkers developed Ni/CdS photocatalyst for photoelectrochemical cell via photodeposition method; and they found that Ni/CdS photocatalyst possess better photocatalytic activity and aerial oxidation resistance than CdS [8]. Recently, several nickel compounds such as NiO [9-11], NiS [12-14], 3

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Ni(OH)2 [15, 16], and Ni@C/CdS [17] were reported to function as co-catalysts for water splitting affording hydrogen and oxygen or for organic pollutant degrading in water and air (Table 1). These co-catalysts could increase the photocatalytic activity

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and avoid aerial oxidation of CdS, because they are able to enhance the visible light absorbance of CdS and promote the separation of light-induced carriers, as evidenced

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by ultraviolet-visible light diffuse reflection spectrometry (denoted as UV-Vis DRS),

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surface photovoltage spectroscopy and fluorescence spectrometry. However, the

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possible photocatalytic mechanism of these nickel compounds remains unknown. With NiS as a typical example, its conduct band (CB) is about 0.93 eV, lower than the

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reduction potential of H+/H2. Then a question arises how NiS induces the reduction of

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NiS.

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H+ after electrons are transferred from the CB of semiconductor CdS and accepted by

Herein, we report the preparation of nickel nanoparticle-loaded cadmium sulfide (denoted as nano-Ni/CdS) and its catalytic performance for water splitting-hydrogen evolution under visible light irradiation. As listed in Table 1, as-prepared nano-Ni/CdS photocatalyst exhibits a high hydrogen evolution rate of 9050 μmol·h-1·g-1. 2. Experimental sections

2.1 Preparation of nano-Ni/CdS photocatalyst Nickel chloride was used as the precursor for preparing Ni nanoparticles. Briefly, aqueous solution of NiCl2 with a pre-set concentration was prepared while 6 drops of 1% (mass fraction; the same hereafter) polyvinlylpyrolidone (denoted as PVP, K 30)

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were added to modify the reduction of Ni2+. Into resultant mixed solution was added a proper amount of sodium borohydride (NaBH4) to reduce Ni2+ generating colloidal-metal particles. As-formed Ni nanoparticles were finally mixed with 0.1 g of

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commercial CdS (Alfa Company, purity99.999%, particle size ca. 20 nm, hexagonal) under stirring to afford nano-Ni/CdS photocatalyst for water splitting-hydrogen

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generation reaction. During the reaction process, NaBH4 releases H atoms in aqueous

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solution, and H atoms react with Ni2+ to form colloidal-metal particles. In the

2.2 Characterization

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nickel crystals and their aggregation as well.

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meantime, PVP added in the solution functions as a modifier to prevent fast growth of

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The crystalline structure and size of CdS were determined by powder X-ray

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diffraction (XRD; X’Pert Pro, Philips Company; Cu K radiation) at an applied current of 40 mA and a voltage of 40 kV. The XRD data were collected in a 2 range of 10~90° at a scan rate of 0.05o per second. The chemical state of Ni nanoparticles deposited on CdS surface was analyzed by X-ray photoelectron spectroscopy (XPS; Kratos， AXISULTRA spectrometer, UK), and Al K (hv = 1486.69 eV) radiation from a Mg/Al dual anode X-ray source (15 kV, 10 mA) was used. During XPS measurements, the base pressure in the analysis chamber was kept well below 110-9 torr. The grain size and microstructure of nano-Ni/CdS after photocatalytic reaction were analyzed with a transmission electron microscope (TEM; JEM-2010, Jeol Company, Japan). The absorption characteristics of CdS were 5

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measured with an ultraviolet-visible light spectrophotometer (UV–vis; U4100, Hitachi Company Limited, Japan) and an X-ray fluorescence spectrometer (X-FS; F7000, Hitachi Company Limited, Japan) under 380 nm excitation.

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2.3 Evaluation of photocatalytic activity

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A water-jacketed stainless vessel (diameter 12.5 cm, height: 14.0 cm) fitted with a

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quartz window (diameter: 6.4 cm, exposed area: 32 cm2) was used as the photoreactor. The photolyte temperature was maintained at 298  0.1 K by circulating isothermal

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water in the photoreactor water jacket. A 300 W Xe lamp (Beijing Perfectlight

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Company; Beijing, China) equipped with a cut-off filter ( 420 nm) was used as the light source. Before photoreaction was commenced, 0.1 g of nano-Ni/CdS

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photocatalyst was added to 50 mL of 1 M (NH4)2SO3 solution and transferred into the

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photo-reactor which was purged with high purity N2 flow for 50 min. 3. Results and discussion

3.1 Photo-catalytic properties of nano-Ni/CdS photocatalyst Fig. 1 shows the H2 evolution rates of nano-Ni/CdS photocatalysts with different loadings of nickel nanoparticles. When hydrogen evolution reaction is performed with CdS catalyst alone at room pressure, almost no H2 is detected. Loading of a small amount of nickel leads to a significant increase of activity for H2 evolution. With increasing Ni loading, the rate of H2 evolution on nano-Ni/CdS rises quickly, and the maximum rate of H2 evolution (up to 9050 μmol·h-1·g-1, QE=9.4% at =420 nm ) is achieved at a Ni loading of about 2.5%. This implies that when Ni loading is less than 6

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2.5%, the amount of H2 evolution sites increases with elevating Ni loading thereby increasing H2 evolution rate. When Ni loading is more than 2.5%, however, the incident light is partly screened by Ni particles loaded on CdS surface thereby decreasing H2

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evolution rate. Moreover, as shown in Fig. 2, nano-Ni/CdS photocatalyst does not show obvious loss

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of activity after 16.5 h of photocatalytic reaction (the first circle costs 5 h, and the other

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three circles cost 3.8~4.0 h), and the total H2 evolution rate for each circle is 3683

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μmol·h-1·g-1. Furthermore, at the beginning of photocatalytic reaction, H2 evolution rate increases with elevating loading of photo-induced Ni nanoparticles on CdS

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surface, and it tends to stabilize around 8292 μmol·h-1·g-1 after 2 h of photocatalytic

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reaction, while the H2 evolution rate in follow-up three circles remains around 9590,

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9660 and 8897 μmol·h-1·g-1, respectively.

It should be pointed out that photocatalyst CdS is very sensitive to oxygen in the reaction system. In the present research, when light is turned off, the temperature inside the reactor decreases obviously thereby facilitating vacuum environment inside the reactor. Besides, the switch of nitrogen flow increases the risk of introducing oxygen to the reaction system. As a result, once CdS applied in the reaction is partly oxidized, the H2 evolution rate will seriously decline, as shown in the final circle. 3.2 Characterization of nano-Ni/CdS photocatalyst Figs. 3a and 3b display the low magnification TEM images of nano-Ni/CdS after photocatalytic hydrogen generation reaction. It can be seen that CdS with a particle 7

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size of about 20 nm are of chain-like shape, and evenly decorated with Ni nanoparticles (the black dots in Fig. 3a). Fig. 3c is the high magnification TEM (HRTEM) image of nano-Ni/CdS. The lattices with a distance of 0.176 nm on the

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surface of the CdS particle could be assigned to the (200) plane of Ni (JCPDS card No. 04-0850), and the lattices with a distance of 0.280 nm correspond to the (002) plane

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of Ni2O3 (JCPDS card No. 14-0481). Moreover, a large amount of Ni2O3 and a small

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amount metallic Ni nanoparticles are detected in nano-Ni/CdS photocatalyst, which is

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further confirmed by XPS (Fig. 4).

The XPS survey spectra of Ni/CdS sample indicate the existence of Cd, O, Ni, S, and

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C elements. Fig. 4a depicts the XPS spectra of Ni/CdS sample showing two peaks with

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binding energy of Ni2p3/2 of 852.58 eV and 856.07 eV. The small peak at 852.58 eV is

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assigned to Ni, and the large one at 856.07 eV is for Ni2O3. Meanwhile, the binding energy of Ni2O3 is seen at 531.8 eV in Fig. 4b. These results are consistent with the HRTEM data described above. Moreover, no obvious peak attributed to NiS is found in Fig. 4a and Fig. 4d.

As mentioned in experimental section, NaBH4 was used to reduce Ni2+ generating colloidal-metal particles, and then Ni nanoparticles was transferred into the photo-reactor, immediately mixed with the slurry of CdS photo-catalyst and reductant (NH4)2SO3, finally purged with nitrogen gas with high purity for 50 min. The preparation process of Ni nanoparticles and the use in the following photo-reaction is in the absence of oxygen. Therefore, we think that the photocatalyst applied in 8

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hydrogen evolution reaction is Ni/CdS in all probability, in other words, metallic Ni works as co-catalyst and enhances the hydrogen production rate. We have to emphasize the difficulties in the characterization of Ni/CdS. As we all

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know, Ni is more active, and its activity tends to rise with declining particle size. Once

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nickel nanoparticles are separated from the reaction solution and exposed to

more active to oxygen is a technique problem.

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atmosphere, they are quickly oxidized. The characterization of metal sample which is

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Fig. 5a and 5b show the XRD patterns of CdS and nano-Ni/CdS, respectively. All

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diffraction peaks of CdS can be indexed to hexagonal CdS phase (space group: P63mc; JCPDS card No. 41-1049), and no other characteristic peaks of impurities are detected.

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Moreover, no characteristic peak of nickel nanoparticles is found in Fig. 5b, which

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might be due to the low loading capacity of nickel on CdS surface. Besides, CdS and nano-Ni/CdS show differences in the relative peak intensity of each crystal plane, as listed in Table 2. Namely, the peak intensity of crystal planes (100), (002), and (101) of nano-Ni/CdS declines, while that of crystal planes (110), (103), (200), (112) and (201) increases significantly as compared with those of CdS. This reveals that Ni nanoparticles tend to preferentially deposit on the surface of CdS under visible light irradiation. In other words, Ni nanoparticles tend to load on the crystal planes (100), (002) and (101) of CdS, thereby reducing the opportunity for these crystal planes to be exposed and decreasing the intensity of corresponding XRD peaks. Such an orientated deposition of Ni nanoparticles on CdS might be related to the transfer 9

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mechanism of photo-induced carriers inside the semiconductor, and it awaits further intensive investigations to reveal the relationship between the electron transfer and the structure of semiconductor photocatalyst.

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3.3 Absorption properties UV–Vis diffuse reflectance spectra of CdS and nano-Ni/CdS are shown in Figure 6. It

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is seen that CdS nanoparticles can absorb visible light; and the absorption threshold

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and band gap are about 530 nm and 2.4 eV, respectively. Figure 7a shows the PL

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spectrum of CdS nanocrystals measured at room temperature under an excitation wavelength of 380 nm. The fluorescence spectrum of CdS exhibits three emission

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peaks. One is the narrow intensive emission band in the wavelength range of 480~495

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nm, respectively.

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nm, and the other two are the broad weak band emissions located at 530 nm and 600

The PL behaviors of CdS nano-structured materials have been studied intensively [18-21]. According to previous studies, CdS nanostructures generally exhibit two PL emissions: band-edge emission and surface-defect emission. Due to the quantum confinement effect, the PL peak positions of the band-edge emission of nanostructures are strongly size-dependent; and in particular, the PL peak position of the band-edge emission of CdS nanostructures is usually located in the wavelength range of 420~500 nm [20-23]. Besides, the surface-defect emission is attributed to surface states such as sulfur vacancies or/and sulfur dangling bonds; and specifically, the PL peak positions of the surface-defect emission of CdS nanostructures is usually located in the 10

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wavelength range of 530~680 nm [18, 19, 24]. However, after Ni nanoparticles are deposited on CdS surface (Fig. 7b), the intensity of the three emission peaks of CdS, especially the peak at 480 nm, decreases significantly. This is because Ni

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nanoparticles loaded on CdS surface function as the trapper of electrons, thereby allowing light-induced electrons to be transferred from the conduction band of CdS to

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4. Conclusions

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nickel and leading to fluorescence quenching of CdS.

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A novel method has been established to load Ni nanoparticles on CdS surface affording nano-Ni/CdS photocatalyst for water splitting-hydrogen evolution under

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visible light irradiation. Since the preparation of Ni nanoparticles and their loading on

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CdS surface are separately conducted at the chemical reduction step and light-induced

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reduction step, respectively, it is feasible to well control the morphology and size of as-fabricated Ni nanoparticles. Resultant Ni nanoparticles have a size of about 3 nm and are uniformly loaded on CdS surface. Moreover, Ni nanoparticles under light-induced reduction tend to be preferentially deposited on the (100), (002) and (101) crystal planes of CdS. As-prepared nano-Ni/CdS photocatalyst exhibits good visible light catalytic activity towards water splitting-hydrogen evolution. Particularly, nano-Ni/CdS photocatalyst with a Ni loading of 2.5% possesses the best visible-light catalytic activity for water splitting-hydrogen evolution and provides a hydrogen production rate of up to 9050 μmol·h-1·g-1, while it exhibits stabilized activity towards H2 evolution as well. 11

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Acknowledgments The authors are grateful for the financial support from International Cooperation

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Program (Science & Technology Department of Henan Province, No.130602). This research is also financially supported by Chinese National Engineering Center of

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Environmental Photocatalysis. (K-092001).

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[7] E. Borgarello, N. Serpone, E. Pelizzetti, M. Barbeni, J. Photochem. 33 (1986) 35-48. [8] I.B. Rufus, B. Viswanathan, V. Ramakrishnan, J. Kuriacose, J. Photochem.

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[19] G.Q. Xu, B. Liu, S.J. Xu, C.H. Chew, S.J. Chua, L.M. Gana, J Phys Chem Solids. 61 (2000) 829-836. [20] Q. Pan, K. Huang, S. Ni, Q. Wang, F. Yang, D. He, Mater. Lett. 61 (2007)

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Fig. 1 H2 evolution rate on nano-Ni/CdS at a Ni loading of a-1%, b-1.5%, c-2.5%, d-3.0%, and e-5.0%. Reaction conditions: 0.1 g Ni/CdS, 50 mL aqueous solution containing 1 M (NH4)2SO3; 300 W Xe lamp (  420 nm). Fig. 2 Time course of photocatalytic hydrogen production over nano-Ni/CdS photocatalyst (2.5% Ni). In darkness, the reaction system is switched to N2 flow for 30 min to purge H2. Fig. 3 TEM and HRTEM images of nano-Ni/CdS photocatalyst. Fig. 4 Curve-fitted XPS spectra of typical elements of Ni/CdS after hydrogen evolution reaction

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Fig. 5 XRD spectra of (a) CdS and (b) nano-Ni/CdS photocatalyst. Fig. 6 UV-vis DRS of (a) CdS and (b) 2.5% nano-Ni/CdS.

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Fig. 7 PL spectra of (a) CdS and (b) 2.5% nano-Ni/CdS.

Photocatalyst

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Table 1 Nickel compound-based photocatalysts and their performance in H2 evolution. Reaction conditions

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Light: 300 W Hg-lamp Sacrificer: 200 mL 30 vol% CH3OH Photocatalyst: 0.2 g Light: 300 W Xe-lamp (  420 nm) NiS/CdS [12] Aqueous solution: 100 mL 30 vol% lactic acid Photocatalyst: 0.3 g Light: 250 W Xe NiS/CdS [13] Aqueous: 160 mL 0.4 M Na2SO3 & 0.57 M Na2S Photocatalyst: 0.260 g Light: 300 W Xe-lamp (  420 nm) NiS/CdS [14] Sacrificer: 80 mL 0.25 M Na2SO3 & 0.35 M Na2S Photocatalyst: 0.05 g Light: 350 W Xe NiS/TiO2 [16] Aqueous: 100 mL 30 vol% lactic acid Photocatalyst: 0.2 g Light: 300 W Xe-lamp (  420 nm) Ni(OH)2/CdS [15] Aqueous: 0.25 M Na2SO3 & 0.35 M Na2S Light: 300 W Xe-lamp (  420 nm) Ni@C/CdS [17] Aqueous: 0.25 M Na2SO3 & 0.35 M Na2S Photocatalyst: 0.1 g As-prepared Ni/CdS Light: 300 W Xe-lamp (  420 nm) Aqueous: 50 mL 1 M (NH4)2SO3 Photocatalyst: 0.1 g NiO/TiO2 [9]

H2 -1 -1 [μmol·h ·g ] 813

2180

1517

1131

698 5084 3328

9050

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Table 2 XRD analyses of nano-Ni/CdS photocatalyst.

0.76 0.84 0.93 1.10 1.44 1.49 1.41 1.52 1.54

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ICdS/INi/CdS

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CdS Ni/CdS 4134.1 3127.0 2984.7 2501.0 5957.9 5528.8 1599.0 1763.2 2240.9 3237.3 2020.8 3002.0 417.03 586.2 1633.6 2484.1 737.0 1134.9

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25.07 26.63 28.33 36.86 43.95 48.07 50.05 52.05 53.04

I

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Lattice plane (100) (002) (101) (102) (110) (103) (200) (112) (201)

2 /°

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Figure 1

70

c

10 8 6 4 2 0 0

R2 (mmol/h/g)

H2 Yield (mmol/g)

60 50 40 30

b

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Figure 2 off

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0.176 nm Ni (200)

50 nm

200 nm

0.280 nm Ni2O3 (002)

5 nm

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Figure 4

Ni

2000 1000 0

0 890

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1500

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Ni2O3

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Figure 6 100

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