Ni(OH)2 loaded on TaON for enhancing photocatalytic water splitting activity under visible light irradiation

Applied Surface Science 324 (2015) 432–437

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Ni(OH)2 loaded on TaON for enhancing photocatalytic water splitting activity under visible light irradiation Wei Chen a,b , Mingchao Chu c , Li Gao a , Liqun Mao a , Jian Yuan b , Wenfeng Shangguan b,∗ a b c

College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475001, PR China Research Center for Combustion and Environmental Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Henan Province Institute of Scientific and Technical Information, Zhengzhou 450003, PR China

a r t i c l e

i n f o

Article history: Received 9 September 2014 Received in revised form 16 October 2014 Accepted 18 October 2014 Available online 24 October 2014 Keywords: Hydrogen production Photocatalysis Ni(OH)2 cocatalyst TaON

a b s t r a c t The noble-metal-free Ni(OH)2 as a cocatalyst was loaded on TaON photocatalyst by a precipitation method. The characterization of XRD, SEM, HRTEM and XPS revealed that Ni species was loaded on TaON in the form of ␤-Ni(OH)2 . The composite Ni(OH)2 /TaON showed higher photocatalytic activity for hydrogen evolution (3.15 ␮mol/h) than that of naked TaON and even higher than 0.5 wt% Pt/TaON (1.48 ␮mol/h) under visible light ( > 400 nm) in the presence of methanol as a sacrificial reagent, while the weight ratio of TaON: Ni(OH)2 reached 5. The enhancement of photocatalytic activity of Ni(OH)2 /TaON is attributed to the rapid electrons migration from CB of TaON to Ni(OH)2 , which restrains the recombination of charge carriers generated by TaON and facilitates the hydrogen evolution. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The development of photocatalysts for hydrogen production is an important research area to solve the energy and environmental problems we are facing up today [1,2]. Since the discovery of photocatalytic splitting of water on TiO2 electrodes in 1972 by Honda and Fujishima [3], many promising photocatalysts such as TiO2 [4], NaTaO3 [5], K4 Nb5 O17 [6], GaN:ZnO [7] have been found for hydrogen production. However, due to the too wide band gaps, most of them only can response to the UV light whose energy accounts for only 4% in the full solar spectrum [8]. Therefore, the development of visible-light-driven photocatalytic is extremely urgent. Based on the energy band theory, (oxy)nitrides were considered as very potential visible-light-response candidates because their valence bands (VB) consisting of N 2p are more negative than that of oxides and result in a red-shift of onset to the visible light range [9,10]. Among them, TaON has a band gap of 2.4 eV and its conduction band (CB) edge locates at around −0.3 V (vs. SHE, pH 0), which is more negative than water reduction potential (0 V vs. SHE, pH 0)[11–13]. On account of a narrow band gap and favorable band gap position, TaON has been extensively studied for hydrogen production in the presence of a sacrificial reagent (e.g. methanol) as well as for the overall water splitting reaction under Z scheme under visible light irradiation [14]. However, to achieve high photocatalytic

activity for hydrogen evolution, the surface of TaON must be loaded with noble metal such as Pt or Ru to inhabit the swift recombination of photogenerated carriers [15], which would greatly increase the cost of hydrogen production and go against the practical application in the future. Very recently, Yu et al. [16,17] reported that Ni(OH)2 , Cu(OH)2 (so called noble-metal-free) cocatalysts could significantly improve the hydrogen evolution activity on TiO2 and CdS. We also found that the hydrogen evolution rate of Ni/CdS was 12 times of pure CdS [18], indicating noble-metal-free cocatalysts such as Ni, Ni(OH)2 or Cu(OH)2 could be used as substitutes for noble metal cocatalysts in photocatalytic water splitting for hydrogen production. Considering the advantages of TaON in photocatalytic water splitting such as non-toxic, high stability, visible-light-driven, it is very necessary to investigate the effect of Ni(OH)2 loading on TaON. Herein, we report Ni(OH)2 as a cocatalyst loaded on the surface of TaON for photocatalytic hydrogen production. In this work, TaON was synthesized by Ta2 O5 under NH3 flowing at high temperature and then Ni(OH)2 was deposited on its surface by using Ni(CH3 COO)2 and NaOH. The photocatalytic activity for hydrogen evolution of Ni(OH)2 /TaON with different contents of Ni loading was tested under visible light irradiation. The influence of interfacial charge transfer after loading Ni(OH)2 was also discussed. 2. Experimental 2.1. Preparation of TaON

∗ Corresponding author. E-mail address: [email protected] (W. Shangguan). http://dx.doi.org/10.1016/j.apsusc.2014.10.114 0169-4332/© 2014 Elsevier B.V. All rights reserved.

TaON was prepared by a nitrogenization method at high temperature according to the previous report [11]. Ta2 O5 (99.99%) was

W. Chen et al. / Applied Surface Science 324 (2015) 432–437

Ni(OH)2 PDF # 14-0117 Ni(OH)2 TaON: Ni(OH)2=2:1

Intensity/a.u.

nitrogenized in a quartz tube under NH3 (20–25 ml/min) flowing at 1123 K for 10 h. To avoid being oxidized again, the NH3 keeps flowing prior to the cooling process. Ni(OH)2 /TaON was synthesized by a precipitation method. 0.3 g TaON was immersed in a glass beaker with 50 ml deionized water and then a certain volume of 0.01 mol/l Ni(CH3 COO)2 aqueous solution was added. After stirring for 1 h, 0.5 mol/l NaOH was added dropwise under stirring until the pH value reach 10. The mixture solutions were stirred continuously for 2 h and then the precipitates were washed with deionized water and dried at 70 ◦ C for 10 h. In this paper, the ratios of TaON: Ni(OH)2 (denoted as T:N) referred to proportion of weight. Pt as a referenced cocatalyst was prepared by an impregnation method [19]. 0.4 g TaON powder was immersed in 5 ml 0.0021 mol/l H2 PtCl6 aqueous solution in a water bath. After the solution was dried, the resultant powder was collected and reduced with 10% H2 gas at 473 K for 1 h to obtain 0.5 wt% Pt/TaON.

433

TaON: Ni(OH)2=4:1 TaON: Ni(OH)2=20:1

TaON TaON PDF # 20-1235

10

15

20

25

30

35

2θ/ο

40

45

50

55

60

2.2. Characterizations The crystal structure of the photocatalytic materials was confirmed by X-ray diffraction (Rigaku D/max-2200/PC Japan) with Cu K␣ (40 kV, 20 mA). The UV–vis diffuse reflection spectra (DRS) were determined by a UV-vis spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by the Kulbelkae–Munk method. The morphology of the samples was studied by scanning electron microscopy (FEI SIRION 200, USA). The high resolution transmission electron microscopy (HR-TEM) measurements were conducted using a JEM-2100F (Japan). The specific areas of TaON, Pt/TaON and Ni(OH)2 /TaON were determined by a BET method from N2 absorption isotherms at 77 K (Micromeritics TriStarII3020 USA). The Time-Resolved Photoluminescence (TRPL) was measured by Steady-State&Time Resolved Spectrofluorometer (Photo Technology international USA). The surface electronic state was analyzed by X-ray photoelectron spectroscopy (XPS, ShimadzuKratos, Axis UltraDLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C1s = 284.6 eV) as a reference. 2.3. Photocatalytic reaction Photocatalytic reactions for water splitting were carried out in a 350 ml top irradiation reaction quartz cell at room temperature. The catalyst powder (0.1 g) was suspended in 60 ml methanol solution (10 vol%) under magnetic stirring. The reaction cell was connected to a vacuum system, and a 300 W Xe lamp was used as a light source. The UV light ( < 400 nm) was removed by a NaNO2 aqueous solution (1 mol/l). The gases evolved were analyzed by GC with a TCD detector (Huaai, GC9160, China, MS-5A, argon as carrier gas).

Fig. 1. XRD patterns of TaON, Ni(OH)2 and Ni(OH)2 /TaON composite materials with different weight ratios of Ni(OH)2 .

[20,21]. For any of Ni(OH)2 /TaON composite samples, no characteristic diffraction peaks of Ni(OH)2 were observed, regardless of Ni(OH)2 /TaON weight ratios, which should be ascribed to weak crystallization and/or small size of Ni(OH)2 particles. In addition, there is no shift of peak positions in each of the composite materials, suggesting Ni(OH)2 only deposited on the surface of TaON instead of incorporating into its lattice (Table 1). 3.2. Electron microscopic and BET SEM images of the TaON and Ni(OH)2 /TaON samples are shown in Fig. 2. From Fig. 2a and b comparing to the smooth surface of TaON, many particles with the diameter of about 100 nm were observed on the surface of the composite material. TEM image (Fig. 2c) clearly shows that these particles are about 10 nm in diameter. The selected area electron diffraction (SAED) pattern of the particles was shown in Fig. 2c. The diffraction rings correspond to the diffractions of (1 1 0), (1 0 0), and (1 0 1) and (1 0 3) planes of ␤-Ni(OH)2 , which proves the multicrystalline nature of the Ni(OH)2 [22]. HRTEM image in Fig. 2d exhibits the lattice space of 0.237 nm, which corresponds to the (1 0 1) plane of hexagonal phase Ni(OH)2 based on the data of JCPD 14-0117, further confirming that nanoparticles on the surface of TaON are

b

Absorbance/a.u.

Fig. 1 shows XRD patterns of Ni(OH)2 /TaON composite materials with various Ni(OH)2 /TaON weight ratios. The data of TaON and Ni(OH)2 are shown for comparison. The XRD pattern of TaON exhibits a monoclinic structure and all the diffraction peaks correspond to that of ␤-TaON [11]. No peaks attributed to Ta2 O5 or Ta3 N5 were observed, implying that the pure phase of TaON was obtained by using the nitrogenization method and the disturbance of Ta3 N5 was also excluded in photocatalytic reaction for hydrogen production. The XRD pattern of pure Ni(OH)2 can be indexed as (0 0 1), (1 0 0), (1 0 1) and (1 1 0) planes of hexagonal phase with weak crystallization according to the main diffraction peaks locating at 18.9◦ , 33.0◦ , 38.3◦ and 59.1◦ , indicating the Ni(OH)2 is ␤-Ni(OH)2

a) TaON b) TaON: Ni(OH)2=20:1 c) TaON: Ni(OH)2=5:1

3. Results and discussion 3.1. X-ray diffraction

c

d) TaON: Ni(OH)2=2:1

a

e) Pt/TaON f ) Ni(OH)2

c

a b 600

625

650

675

700

f e

300

350

400

450

500

550

600

650

d

700

750

800

Wavelength/ nm Fig. 2. UV–vis diffuse reflectance spectra (DRS) of the photocatalytic samples in this study.

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W. Chen et al. / Applied Surface Science 324 (2015) 432–437

Fig. 3. SEM (a and b) and (HR)TEM (c and d) images of TaON (a) and Ni(OH)2 /TaON (b–d).

␤-Ni(OH)2 . The specific surface areas of Ni(OH)2 /TaON, Pt/TaON and pure TaON were shown in Fig. 5. The pure TaON reveals a small specific surface area, only about 2.5 (m2 /g), which is due to the treatment at high temperature (850 ◦ C) during the nitrogenization process. It can bee seen that, after laoding of Pt or Ni(OH)2 , the specific surface areas of Pt/TaON and Ni(OH)2 /TaON are larger than that of pure TaON. This should be attributed to the contributions of Pt or Ni(OH)2 nanoparticles on the surface of TaON. 3.3. DRS and photoluminescence spectroscopy The UV–visible diffuse reflectance spectra (DRS) for TaON, Ni(OH)2 /TaON, Pt/TaON and Ni(OH)2 are shown in Fig. 3. The absorption edge of TaON is near at 520 nm. This is due to electron transition from the valence band (hybridization orbits of N 2p and O 2p) to the conduction band (Ta 5d orbits), which is consistent with the previous report by Domen et al. [23]. Compared to the pure TaON, the absorption band edges of composite materials and Pt/TaON have no obvious change because Pt or Ni loading on the surface of samples does not participate in the formation of energy band of TaON. However, with the increase of Ni(OH)2 loading

contents, the new absorption of composite materials at 600–700 nm can be observed, which should be assigned to the Ni d–d transition [17]. The result also indicates the Ni(OH)2 are loaded on the photocatalysts successfully. In order to check the contribution of Ni(OH)2 to the separation of photogenerated charge carriers in Ni(OH)2 /TaON, PL lifetime of fluorescence was examined by time-resolved photoluminescence (TRPL), for the PL lifetime can reflect the carrier transfer rate [24]. The curves of fluorescence attenuation of TaON with time delay are shown in Fig. 4. The PL lifetimes of fluorescence based on TRPL curves of TaON for TaON, Pt/TaON and Ni(OH)2 /TaON are retrieved to be 2.45 ns, 2.28 ns and 1.90 ns, respectively. Among these three samples, Ni(OH)2 /TaON has the shortest TaON PL lifetime, suggesting the injection rate of electrons from TaON to Ni(OH)2 is the fastest [25–28]. Therefore, this suggests that Ni(OH)2 /TaON possesses the most efficient charge separation. 3.4. Photocatalytic water splitting activities The rate of hydrogen evolution over all the prepared catalysts is shown in Fig. 5. The reaction is performed in the methanol solution containing 0.1 g catalyst under visible light ( > 400 nm)

W. Chen et al. / Applied Surface Science 324 (2015) 432–437

435

50

700

40

TaON

500

H 2 evolution/μmol

Luminescence Intensity/a.u.

600

400 Pt/TaON

300 200

30

20

100 10

Ni(OH)2/TaON

0 144

146

148

150

152

154

156

158

160

162

Time/ ns

0

0

4

8

12

16

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24

28

32

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44

48

Reaction time/ h

Fig. 4. TRPL curves of TaON from TaON, Pt/TaON and Ni(OH)2 /TaON. The excitation wavelength is 400 nm, and the PL wavelength is 530 nm.

Fig. 6. Time courses of photocatalytic hydrogen evolution on Ni(OH)2 /TaON (TaON:Ni(OH)2 = 5) under visible light ( > 400 nm). Reaction conditions: catalyst, 0.1 g; 60 ml 10 vol% methanol; light source, 300 W Xe lamp.

Table 1 PL lifetimes of the samples based on the data from Fig. 4. Samples

Excitation wavelength (nm)

Detection wavelength (nm)

Lifetimes (ns)

Chi2

TaON Pt/TaON Ni(OH)2 /TaON

400 400 400

530 530 530

2.45 2.28 1.90

0.9767 1.268 1.161

H2 evolution(μmol/h)

2.5 2.0 1.5

4 3

1.0

2

160 140 120 100 80 60 40 20

3.0

BET (m /g)

irradiation. TaON shows almost no photocatalytic activity even though its band structure meets the requirement of water splitting, which is because of the strong recombination of electrons and holes on the surface of TaON. After 0.5 wt% Pt is loaded, the hydrogen evolution rate of Pt/TaON becomes 1.48 ␮mol/h because Pt on the surface of TaON can promote the transfer of electrons from TaON to Pt and result in the separation between electrons and holes, indicating that modification is crucial to the preparation of photocatalytic materials with high photocatalytic. It is noteworthy that, although Ni(OH)2 exhibits no photocatalytic activity as well, the Ni(OH)2 /TaON shows high photocatalytic activity for hydrogen production. Besides the photocatalytic activity is even higher than that of Pt/TaON, suggesting Ni(OH)2 is beneficial to the separation of charge carriers. The highest hydrogen evolution rate was obtained when the weight ratio of TaON: Ni(OH)2 is 5, reaching 3.15 ␮mol/h, even 2 times of Pt/TaON. But further increase of Ni(OH)2 content would lead to a decrease in photocatalytic hydrogen evolution activity, which is because the excessive Ni(OH)2 loading on the

2 0.5 0.0

Trace

0

TaON Pt/TaONT:N=20 T:N=7 T:N=5 T:N=3 T:N=2 Ni(OH)2

1 0

Fig. 5. BET and rates of hydrogen evolution of photocatalysts with experiment errors of 3–5% from methanol solution under visible light ( > 400 nm). Reaction conditions: catalyst, 0.1 g; 60 ml 10 vol% methanol, light source, 300 W Xe lamp.

surface of TaON would block the absorption of light photon and become the recombination centers of carriers [29]. If a photocatalytic reaction occurred, the photocatalytic materials need to meet the requirement of band gaps in addition to excellent separation efficiency of electrons and holes. Although the CB potential of TaON (−0.3 V vs NHE, pH 0) is more negative than the potential of H+ reduction, TaON does not exhibit photocatalytic activity because of the rapid recombination of charge carriers on its surface before used for the reduction of H+ . After loading Ni(OH)2 , the electrons in CB of TaON inspired by visible light could migrate to Ni(OH)2 rapidly, while the holes remain on the surface of TaON, where the assembling of electrons and holes on different material prevent the combination and improve photocatalytic activity for hydrogen evolution [30]. Moreover, the photocatalytic activity is related to the rate of migration of electrons. The faster the transfer rate of electron from TaON to Ni(OH)2 , the higher the photocatalytic activity of photcatalyst. The electrons in Ni(OH)2 /TaON possess faster transfer rate than that in TaON and Pt/TaON according to the results of TRPL, thus Ni(OH)2 /TaON has the highest photocatalytic activity. In order to check the stability of the composition material in photocatalytic reaction processes, a time course of hydrogen evolution on Ni(OH)2 /TaON(T/N = 5) was shown in Fig. 6. It is found that, with the reaction proceeding, no obvious decrease of hydrogen evolution was observed during the experiment time (50 h), which indicates the excellent stability of Ni(OH)2 /TaON for photocatalytic hydrogen production. Correspondingly, to further investigate the chemical states of elements, especially Ni, in the reaction process, the binding energies of Ta 4f, N 1s and Ni 2p of Ni(OH)2 /TaON before and after reaction with different reaction time were measured by XPS (Fig. 7). As shown in Fig. 7, the bingding energy peaks of Ta 4f and N 1s have not change, indicating the photocatalyst TaON keep good stability during the reaction. Additionally, the binding energy peak of Ni 2p of pure Ni(OH)2 locates at ca. 855.4 eV, demonstrating the Ni compound prepared is Ni(OH)2 [22]. The binding energy peaks of Ni 2p of Ni(OH)2 /TaON (Fig. 7b) have the same position as the pure Ni(OH)2 , indicating the Ni species on the surface of TaON is also ␤-Ni(OH)2 , which is in good agreement with the results of XRD and TEM. In addition, there is little change about Ni chemical state after 4 h reaction. However, after 30 h, the binding energy peak value of Ni 2p has shifted to ca. 854.4 eV, which should be ascribed to that of NiO [31]. The phenomena that hydroxides cocatalysts turn into oxides or metal after photocatalytic reactions were also reported by previous studies [32,33]. This might be because the potential of Ni2+ /Ni is about −0.23 V (vs NHE, pH 0), which

W. Chen et al. / Applied Surface Science 324 (2015) 432–437

N 1s

Ni(OH)2

c b

c b

c b a

a

Ni(OH)2

a 36

34

32

30

28

26

24

B.E./ eV

22

20

18

Ni 2p NiO

Intensity/a.u.

Intensity/a.u.

Ta 4f

Intensity/a.u.

436

16

412 410 408 406 404 402 400 398 396 394 392

B.E./ eV

890 885 880 875 870 865 860 855 850 845 840

B.E./ eV

Fig. 7. XPS spectra for Ta 4f, N 1s and Ni 2p of Ni(OH)2 /TaON: (a) Ni(OH)2 /TaON before reaction, (b) Ni(OH)2 /TaON after reaction 4 h, (c) Ni(OH)2 /TaON after reaction 30 h.

is lower than the potential of TaON [33]. Therefore, the CB electrons of TaON are able to transfer to Ni(OH)2 and reduce H+ to H2 . On the other hand, owing to the more positive reduction potential of Ni2+ /Ni than CB potential of TaON, partial Ni2+ is reduced to Ni metal(Ni0 ) by electrons from TaON during the hydrogen evolution process. These Ni0 can also act as a cocatalyst to promote the separation of electrons and holes like Ni(OH)2 [18,34]. Moreover, due to very active property, Ni metal is easily oxidized when it is exposed to the air, so the Ni species after reaction was detected in the form of NiO. 4. Conclusions The visible-light-driven composite photocatalysts TaON with noble-metal-free Ni(OH)2 cocatalyst were prepared by a precipitation method. The Ni(OH)2 loaded on TaON enhanced remarkably the photocatalytic activity for hydrogen production, and the hydrogen evolution rate of Ni(OH)2 /TaON is 2 times of 0.5% Pt/TaON, while the weight ratio of TaON: Ni(OH)2 is 5. The rapid electrons transfer from CB of TaON to Ni(OH)2 promotes the photocatalytic activity of the composite materials through preventing the combination of electrons and holes. This result may give an idea to synthesize oxynitrides photocatalysts with high activity for hydrogen production without noble metal loading. Acknowledgements This work was supported by the National High Technology Research and Development Program of China (2012AA051501), the National Natural Science Foundation of China (51072116) and the International Cooperation Project of Shanghai Municipal Science and Technology Commission (12160705700). References [1] P. Zhang, J.J. Zhang, G.J. Gong, Tantalum-based semiconductors for solar water splitting, Chem. Soc. Rev. 43 (2014) 4359–4422. [2] J. Yang, D. Wang, H.X. Han, C. Li, Role of cocatalysts in photocatalysis and photoelectrocatalysis, Acc. Chem. Res. 46 (2013) 1900–1909. [3] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [4] K. Maeda, N. Murakmi, T. Ohno, Dependence of activity of rutile titanium(IV) oxide powder for photocatalytic overall water splitting on structural properties, J. Phys. Chem. C 118 (2014) 9093–9100. [5] C.C. Hu, H. Teng, Structural features of p-type semiconducting NiO as a cocatalyst for photocatalytic water splitting, J. Catal. 272 (2010) 1–8. [6] K. Sayama, K. Yase, H. Arakawa, K. Asakura, A. Tanaka, K. Domen, T. Onishi, Photocatalytic activity and reaction mechanism of Pt-intercalated K4 Nb6 O17 catalyst on the water splitting in carbonate salt aqueous solution, J. Photochem. Photobiol. A: Chem. 114 (1998) 125–135. [7] K. Maeda, A. Xiong, T. Yoshinaga, T. Ikeda, N. Sakamoto, T. Hisatomi, M. Takashima, D. Lu, M. Kanehara, T. Setoyama, T. Teranishi, K. Domen, Photocatalytic overall water splitting promoted by two different cocatalysts for hydrogen and oxygen evolution under visible light, Angew. Chem. Int. Ed. 49 (2010) 4096–4099.

[8] A. Kudo, Y. Miseki, Heterogeneous photocatalyst materials for water splitting, Chem. Soc. Rev. 38 (2009) 253–278. [9] M. Higashi, R. Abe, T. Takata, K. Domen, Photocatalytic overall water splitting under visible light using ATaO2 N (A = Ca, Sr, Ba) and WO3 in a IO3 − /I− shuttle redox mediated system, Chem. Mater. 21 (2009) 1543–1549. [10] Y. Zhong, Z. Wang, J. Feng, S. Yan, H. Zhang, Z. Li, Z. Zou, Improvement in photocatalytic H2 evolution over g-C3 N4 prepared from protonated melamine, Appl. Surf. Sci. 295 (2014) 253–259. [11] G. Hitoki, T. Takata, J. Kondo, M. Hara, H. Kobayashi, K. Domen, An oxynitride TaON, as an efficient water oxidation photocatalyst under visible light irradiation ( ≤ 500 nm), Chem. Commun. (2002) 1698–1699. [12] M. Tsang, N. Pridmore, L. Gillie, Y. Chou, R. Brydson, R. Douthwaite, Enhanced photocatalytic hydrogen generation using polymorphic macroporous TaON, Adv. Mater. 24 (2012) 3406–3409. [13] W. Chun, A. Ishikawa, H. Fujisawa, T. Takata, J. Kondo, M. Hara, M. Kawai, Y. Matsumoto, K. Domen, J. Phys. Chem. B 107 (2003) 1798–1803. [14] R. Abe, M. Higashi, K. Domen, Overall water splitting under visible light through a two-step photoexcitation between TaON and WO3 in the presence of an lodate-iodide shuttle redox mediator, ChemSusChem 4 (2011) 228–237. [15] M. Hara, J. Nunoshige, T. Takata, J. Kondo, K. Domen, Unusual enhancement of H2 evolution by Ru on TaON photocatalyst under visible light irradiation, Chem. Commun. (2003) 3000–3001. [16] J. Yu, J. Ran, Facile preparation and enhanced photocatalytic H2 -production activity of Cu(OH)2 cluster modified TiO2 , Energy Environ. Sci. 4 (2011) 1364–1371. [17] J. Ran, J. Yu, M. Jaroniec, Ni(OH)2 modified CdS nanorods for highly efficient visible-light-driven photocatalytic H2 generation, Green Chem. 13 (2011) 2708–2713. [18] S. Chen, X. Chen, Q. Jiang, J. Yuan, C. Li, W. Shangguan, Promotion effect of nickel loaded on CdS for photocatalytic H2 production in lactic acid solution, Appl. Surf. Sci. 316 (2014) 590–594. [19] K. Maeda, M. Higashi, D. Lu, R. Abe, K. Domen, Efficient nonsacrificial water splitting through two-step photoexcitation by visible light using a modified oxynitride as a hydrogen evolution photocatalyst, J. Am. Chem. Soc. 132 (2010) 5858–5868. [20] M.U.A. Prathap, B. Satpati, R. Srivastava, Facile preparation of ␤-Ni(OH)2 NiCo2 O4 hybrid nanostructure and its application in the electro-catalytic oxidation of methanol, Electrochim. Acta 130 (2014) 368–380. [21] Q. Du, G. Lu, The roles of various Ni species over SnO2 in enhancing the photocatalytic properties for hydrogen generation under visible light irradiation, Appl. Surf. Sci. 305 (2014) 235–241. [22] G. Zhou, Q. Yao, X. Wang, J. Yu, Preparation and characterization of nanoplatelets of nickel hydroxide and nickel oxide, Mater. Chem. Phys. 98 (2006) 267–272. [23] M. Hara, G. Hitoki, T. Takata, J. Kondo, H. Kobayashi, K. Domen, Catal. Today 78 (2003) 555–560. [24] K. Lin, C. Chuang, Y. Lee, F. Li, Y. Chang, I. Liu, S. Chou, Y.L. Lee, Charge transfer in the heterointerfaces of CdS/CdSe cosensitized TiO2 photoelectrode, J. Phys. Chem. C 116 (2012) 1550–1555. [25] W. Chen, H.Y. Gao, J. Yuan, W.F. Shangguan, J. Su, Y. Sun, Structure characteristics of CdS/H1.9 K0.3 La0.5 Bi0.1 Ta2 O7 and photocatalytic activity for hydrogen evolution under visible light, Int. J. Hydrogen Energy 38 (2013) 10754–10760. [26] I. Robel, M. Kuno, P. Kamat, Size-dependent electron injection form excited CdSe quantum dots into TiO2 nanoparticles, J. Am. Chem. Soc. 129 (2007) 4136–4137. [27] M. Hartel, S. Chen, B. Swerdlow, H. Hsu, J. Mander, K. Schanze, F. So, Defectinduced loss mechanisms in polymer-inorganic planar heterojunction solar cells, ACS Appl. Mater. Interfaces 5 (2013) 7215–7218. [28] P.N. Kumar, R. Narayanan, M. Deepa, A. Srivastava, Au@poly(acrylic acid) plasmons and C60 improve the light larvesting capability of a TiO2 /CdS/CdSeS photoanode, J. Mater. Chem. A 2 (2014) 9771–9783. [29] W. Chen, H. Wang, L. Mao, X. Chen, W. Shangguan, Influence of loading Pt, RhO2 co-catalysts on photocatalytic overall water splitting over H1.9 K0.3 La0.5 Bi0.1 Ta2 O7 , Catal. Commun. 57 (2014) 115–118. [30] R. Liu, P. Hu, S. Chen, Photocatalytic activity of Ag3 PO4 nanoparticle/TiO2 nanobelt heterostructures, Appl. Surf. Sci. 258 (2012) 9805–9809.

W. Chen et al. / Applied Surface Science 324 (2015) 432–437 [31] Y. Noda, B. Lee, K. Domen, J. Kondo, Synthesis of crystallized mesoporous tantalum oxide and its photocatalytic activity for overall water splitting under ultraviolet light irradiation, Chem. Mater. 20 (2008) 5361–5367. [32] J. Yu, Y. Hai, B. Cheng, Enhanced photocatalytic H2 -production activity of TiO2 by Ni(OH)2 cluster modification, J. Phys. Chem. C 115 (2011) 4953–4958.

437

[33] X. Zhou, Z. Luo, P. Tao, B. Jin, Z. Wu, Y. Huang, Facile preparation and enhanced photocatalytic H2 -production activity of Cu(OH)2 nanospheres modified porous g-C3 N4 , Mater. Chem. Phys. 143 (2014) 1462–1468. [34] J. Yu, S. Wang, B. Cheng, Z. Lin, F. Huang, Noble metal-free Ni(OH)2 –g-C3 N4 composite photocatalyst with enhanced visible-light photocatalytic H2 -production activity, Catal. Sci. Technol. 3 (2013) 1782–1789.