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The roles of various Ni species over SnO2 in enhancing the photocatalytic properties for hydrogen generation under visible light irradiation
Applied Surface Science 305 (2014) 235–241
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The roles of various Ni species over SnO2 in enhancing the photocatalytic properties for hydrogen generation under visible light irradiation Quanchao Du a,b , Gongxuan Lu a,∗ a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 2 January 2014 Received in revised form 26 February 2014 Accepted 5 March 2014 Available online 15 March 2014 Keywords: Photocatalysis Hydrogen evolution Ni(OH)2 Ni2 O3 PN junction Visible light irradiation
a b s t r a c t The photocatalysts containing various nickel species were successfully fabricated via loading Ni(OH)2 and/or Ni2 O3 species on the surface of SnO2 nanoparticles, and their roles in the photo-absorb ability, photo-induced charge transfer and photocatalytic hydrogen evolution performances were also investigated under visible light irradiation. Pt/Ni2 O3 –SnO2 catalyst was not active for hydrogen evolution reaction, while the photocatalytic hydrogen evolution rates of Pt/Ni(OH)2 –SnO2 and Pt/Ni(OH)2 –Ni2 O3 –SnO2 were 0.9 and 10.8 mol g−1 h−1 , respectively. It indicated that the photocatalytic activity of Pt/Ni(OH)2 –Ni2 O3 –SnO2 catalyst had been improved evidently in the presence of Ni2 O3 . The UV–vis DRS results showed that Ni(OH)2 enhanced light absorption of catalyst significantly in visible light region. The i–t curves confirmed that the co-existence of Ni(OH)2 and Ni2 O3 improved significantly the photocurrent of Pt/Ni(OH)2 –Ni2 O3 –SnO2 photo-electrode. In addition, the long term experiments revealed that Pt/Ni(OH)2 –Ni2 O3 –SnO2 catalyst was quite stable under given conditions. A proposed reaction mechanism for hydrogen evolution was also present. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Kudo et al. [1] found that water could be decomposed into H2 and O2 over NiO/TiO2 powder in 1987. Since then, NiO as hydrogen evolution sites had been paid much attention in photocatalytic reactions under UV irradiation [2–7]. In recent years, the roles of other Ni species in catalytic reactions were studied, such as Ni(OH)2 [8,9], Ni2 O3 [10,11], NiOOH [12,13] and so on. These Ni species had mainly been used in the elimination organic pollutions and water splitting under UV irradiation by loading on the surface of TiO2 in photocatalytic reactions. Yu et al. [14] reported that the photocatalytic hydrogen production activity of TiO2 was significantly enhanced by loading Ni(OH)2 clusters under UV irradiation. Zhang et al. [15] found that the photosensitized electrolytic oxidation of Ni(OH)2 exhibited a much lower rate than the corresponding electrochemical oxidation. The as-prepared 3D nanostructured TiO2 –Ni(OH)2 composite film electrode, coupled with platinum, showed excellent UV-induced oxidative energy storage ability, and the apparent quantum yield was as high as 10.6%. Most of these
∗ Corresponding author. Tel.: +86 931 4968178. E-mail address: [email protected] (G. Lu). http://dx.doi.org/10.1016/j.apsusc.2014.03.043 0169-4332/© 2014 Elsevier B.V. All rights reserved.
reactions were carried out on the surface of wide band gap catalysts after loading trace amounts of Ni species, and the catalysts showed much higher photocatalytic activity under ultraviolet irradiation, which was explained by the fact that Ni species could enhance the photocatalytic activity of catalysts. They focused on the higher photocatalytic activity while ignored the utilization of visible light. As a typical n-type semiconductor material, SnO2 also showed excellent optical, gas-sensing properties and chemical stability, which was widely used in sensors [16], solar cells [17,18] and lithium batteries [19]. However, it was difficult to respond to visible light because of its wide band gap, so the studies on photocatalytic water splitting over SnO2 under visible light irradiation had rarely been reported. Fortunately, the composite of SnO2 and nickel species could absorb visible light. Therefore, a series of nickel catalysts were firstly prepared by loading Ni(OH)2 and/or Ni2 O3 on the surface of SnO2 nanoparticles, then the photocatalytic activities of the catalysts were evaluated by the variation of Ni species content under visible light irradiation in this paper. The roles of various Ni species over SnO2 in enhancing the photocatalytic properties for hydrogen generation under visible light irradiation were discussed in detail. Hydrogen generation activity of the catalyst was highly depended on the variation of Ni species content, and a possible reaction
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mechanism on photocatalytic hydrogen production was also proposed based on experimental results and the characterization results of XRD, TEM and XPS. 2. Experiment section 2.1. Preparation of catalysts containing various Ni species 2.1.1. Preparation of SnO2 SnO2 powder was prepared by hydrolysis method. Proper SnCl4 ·5H2 O used as starting materials was dissolved in 250 mL double-distilled water, and the pH value of solution was adjusted to 9.0 by adding proper amount NaOH aqueous solution under vigorous stirring; the white slurry was filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained powder was fired 4 h at 550 ◦ C in muffle furnace, heating rate 2 ◦ C min−1 . The white SnO2 powder was obtained, which was denoted as S1. 2.1.2. Preparation of Ni(OH)2 –SnO2 1.51 g SnO2 powder was dispersed in 100 mL 0.01 mol L−1 NiSO4 aqueous solution, then added dropwise 0.01 mol L−1 NaOH solution under vigorous stirring, filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained powder was Ni(OH)2 –SnO2 (the molar ratio of Ni(OH)2 /SnO2 was 0.1:1), which was denoted as S2. 2.1.3. Preparation of Ni2 O3 –SnO2 1.51 g SnO2 powder was dispersed in 100 mL 0.01 mol L−1 NiSO4 aqueous solution, then added dropwise 0.01 mol L−1 (NH4 )2 CO3 solution under vigorous stirring, filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained green powder was heated 4 h at 350 ◦ C in muffle furnace, and the Ni2 O3 –SnO2 sample (the molar ratio of Ni2 O3 /SnO2 was 0.05:1) was obtained, which was denoted as S3. 2.1.4. Preparation of Ni(OH)2 –Ni2 O3 –SnO2 1.55 g Ni2 O3 –SnO2 powder was dispersed in 100 mL 0.01 mol L−1 NiSO4 aqueous solution, then added dropwise 0.01 mol L−1 NaOH solution under vigorous stirring, filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained powder was Ni(OH)2 /Ni2 O3 /SnO2 (the molar ratio of Ni(OH)2 /Ni2 O3 /SnO2 was 0.1:0.05:1), which was denoted as S4. Moreover, [Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 catalysts were also prepared by the same method, using S3 as starting material. Pt/Ni(OH)2 , Pt/Ni2 O3 , Pt/S1, Pt/S2, Pt/S3 and Pt/S4 were prepared by the photo-reduction method in ethanol/water (v/v = 2/3) mixed solution, and the Pt loading content was 1%.
photoelectron spectrometer with an MgK␣ X-ray resource. All the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. We used the Gauss fitted data for multi-peaks in calculating the various Ni species content and relative-content. The Shirley-type background was subtracted from the recorded spectra and the curve fitting and deconvolution of overlapped peaks were done by non-linear least-square fitting with a Gauss–Lorentz (ratio 60:40) curve. 2.3. Preparation of working electrodes and electrochemical experiments For the preparation of working electrodes for electrochemical measurements, a homogenous catalyst ink was firstly prepared by dispersing 4 mg of catalyst material and 80 l of 5 wt% Nafion solution in 2 mL of H2 O by ultrasonication. Then 400 l of catalyst ink dispersions were drop-coated directly onto the precleaned indium tin oxide (ITO) glass surface (ca. 2 cm2 ) by microsyringe and dried under an infrared heat lamp. Photocurrent responses of photocatalyst samples were measured using an electrochemical analyzer (CHI660A) in a homemade standard three-compartment cell. Platinum foil was used as counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The geometrical surface areas of working electrode exposed to the electrolyte was a circular film of 1.6 cm2 . 0.1 M K2 SO4 solution (20 vol.% EtOH) was used as supporting electrolyte. A 300 W Xenon lamp with optical cut-off filter ( ≥ 420 nm) was used for excitation light. 2.4. Photocatalytic activity test The photocatalytic reactions were carried out in a 148 mL Pyrex reactor with a side flat window, sealed by silicone rubber on bottle mouth. 200 mg photocatalyst powder was dispersed in 100 mL ethanol–deionized water (v:v = 20:80) mixed solution. Suspension system formed by ultrasonication was pumped into Ar gas 40 min to remove the air before irradiation. The light source was a 300 W tungsten halogen lamp. The intensity of incident light was 1.16 × 1020 photons s−1 m−2 using a cut off filter (420 nm) to remove the UV light. The photocatalytic activities of Pt/Ni(OH)2 –Ni2 O3 –SnO2 samples were determined by measuring amount of generated hydrogen. The gas evolved was analyzed by an Agilent 6820 gas chromatograph (TCD, Ar carrier gas) per 30 min. 3. Results and discussion 3.1. Characterizations of XRD, TEM and UV–vis DRS
2.2. Structure and composition characterizations Transmission electron microscopy (TEM) and high-resolution TEM images were taken with a Tecnai-G2-F30 field emission transmission electron microscope operated at accelerating voltage of 300 kV. The samples were dispersed in anhydrous ethanol by ultrasonic treatment for 5 min, and the obtained suspension was dropped on the surface of the micro mesh copper grid for TEM characterization. To determine the phase structures of the samples, X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (type Rigaku D/max-RB) using Cu K␣ irradi˚ and the accelerating voltage, applied current, ation ( = 1.5406 A), scan rate, step pace, and scan range were 40 kV, 30 mA, 0.02◦ s−1 , 0.0167◦ , and 15∼90◦ , respectively. UV–vis diffused reflectance spectra (UV–vis DRS) were obtained using a UV–vis spectrophotometer (UV-2550, Shimadzu), and the BaSO4 was used as a reflectance standard. X-ray photoelectron spectroscopy (XPS) analysis was performed using a VG Scientific ESCALAB250-XPS
The XRD patterns of SnO2 (S1), Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4) have been investigated, as shown in Fig. 1. The three main peaks of 26.6◦ , 33.9◦ and 51.8◦ for SnO2 are assigned to the diffraction planes of (1 1 0), (1 0 1) and (2 1 1), and several other widely and slightly weak intensity peaks are corresponding to (2 2 0), (3 1 0), (3 0 1), (2 0 2), (3 2 1) and (2 2 2) planes of SnO2 . Obviously, no other diffraction peak is detected in the XRD pattern of S1, which suggests S1 is composed of pure SnO2 . The peaks at 19.2◦ and 38.4◦ are assigned to the diffraction planes (0 0 1) and (0 1 1) of Ni(OH)2 , while the 56.9◦ peak is corresponded to Ni2 O3 (2 0 2) plane. Therefore, S2 is composed of Ni(OH)2 and SnO2 ; S3 is composed of Ni2 O3 and SnO2 ; while S4 is composed of Ni(OH)2 , Ni2 O3 and SnO2 . Moreover, compared with the JCPDS standard card (88-0287), no shift in the peak positions is evident in each of the collected samples, which indicates that the Ni species such as Ni(OH)2 and Ni2 O3 are not incorporated into the lattice of SnO2 , and are probably adsorbed on the surface of SnO2 particles.
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Fig. 2 shows a typical TEM image of the sample Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4). Many Ni-containing matter clusters can be clearly observed in Fig. 2a, and these Ni-containing matter clusters are uniformly dispersed on the surface of SnO2 nanoparticles. The HRTEM image (Fig. 2b) shows that the distances of the lattice fringes over a large area are 0.333 and 0.263 nm, which are in agreement with the lattice spacing of SnO2 (1 1 0) and (1 0 1) plane, respectively; the lattice fringes of 0.233 and 0.271 nm are corresponding to Ni(OH)2 (0 1 1) and (1 0 0) planes respectively; and the lattice fringes of 0.323 nm is corresponding to Ni2 O3 (1 0 1) planes. It indicates that Ni species on the surface of SnO2 nanoparticles are not a single compound, but composed of Ni(OH)2 and Ni2 O3 . The energy dispersive X-ray (EDX) spectrum further confirms the presence of Ni and Pt in the sample Pt/S4. The typical ultraviolet–visible diffuse reflectance spectra of the four catalysts are shown in Fig. 3. As can be seen, naked SnO2 has a good light absorption only in the ultraviolet region; the catalyst S3 has a slight absorption at 420 nm; and the photocatalysts S2 and S4 show better absorption performance in the range of 400–600 nm. It indicates that Ni(OH)2 enhances significantly visible light absorption of SnO2 catalyst. Fig. 1. XRD patterns of SnO2 (S1), Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4).
Fig. 2. Low- (a), High- (b) magnification TEM image and energy dispersive X-ray (c) spectrum of Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4). A, B and C represent SnO2 , Ni(OH)2 and Ni2 O3 respectively in Fig. 2b.
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Fig. 3. UV–vis DRS of SnO2 (S1), Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4).
3.2. Activity tests for hydrogen production To investigate the roles of Ni species in water splitting reaction, hydrogen production performances over Pt/SnO2 , Pt/Ni(OH)2 , Pt/Ni2 O3 , Pt/Ni(OH)2 –SnO2 , Pt/Ni2 O3 –SnO2 and Pt/Ni(OH)2 –Ni2 O3 –SnO2 catalysts are evaluated under visible light irradiation, as shown in Fig. 4a and b. No hydrogen is detected over Pt/SnO2 , Pt/Ni(OH)2 and Pt/Ni2 O3 catalysts in 3 h irradiation, which indicates that the catalysts have no hydrogen generation activity in the absence of SnO2 or Ni species. In addition, Fig. 4a also displays that no hydrogen is detected over Pt/Ni2 O3 –SnO2 catalyst, while the photocatalytic hydrogen evolution rates of Pt/Ni(OH)2 –SnO2 and Pt/Ni(OH)2 –Ni2 O3 –SnO2 are 0.9 and 10.8 mol g−1 h−1 under visible light irradiation, respectively. Obviously, the photocatalyst containing nickel species is not active in the presence of single Ni2 O3 , while a small amount of hydrogen is detected in the presence of single Ni(OH)2 ; and Ni2 O3 can improve significantly photocatalytic activity of SnO2 in the presence of Ni(OH)2 . It indicates that Ni2 O3 is quite necessary for the catalyst to maintain higher photocatalytic activity. To give further evidence to support the above statements, the transient photocurrent responses of Ni(OH)2 –SnO2 , Ni2 O3 –SnO2 and Ni(OH)2 –Ni2 O3 –SnO2 are also investigated for several on–off cycles of intermittent irradiation (30 s) in mixed solution of 20% EtOH and 0.1 M K2 SO4 . Fig. 5a displays i–t curves for the aforementioned three samples. As can be seen, Ni2 O3 –SnO2 shows extremely weak photocurrent response under visible light irradiation; while the transient photocurrent of Ni(OH)2 –SnO2 and Ni(OH)2 –Ni2 O3 –SnO2 are 0.04 and 0.1 A, respectively. It also confirms that the photocurrent of Pt/Ni(OH)2 –Ni2 O3 –SnO2 photoelectrode is improved evidently when both Ni(OH)2 and Ni2 O3 are coexistent. In addition, the effect of Ni(OH)2 content on photocurrent has also been studied under the same conditions, as shown in Fig. 5b. It can be seen clearly that the photocurrent increases gradually with the increasing of Ni(OH)2 content. Based on above experimental results, the Ni species can improve evidently the photocatalytic activity of catalyst, which may be caused by two reasons. One is that Ni(OH)2 enhances the photoabsorption performance of catalysts in visible light region; on the other hand, Ni2 O3 acts as electron traps, which facilitates the excited electron transfer and achieves efficient separation of photo-generated electrons and holes.
Fig. 4. (a) and (b) Comparison of the photocatalytic hydrogen generation activities of Pt/SnO2 (Pt/S1), Pt/Ni(OH)2 , Pt/Ni2 O3 , Pt/Ni(OH)2 –SnO2 (Pt/S2), Pt/Ni2 O3 –SnO2 (Pt/S3) and Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) (Pt loading 1%). For S2, Ni(OH)2 /SnO2 = 0.1:1; For S3, Ni2 O3 /SnO2 = 0.05:1; For S4, Ni(OH)2 /Ni2 O3 /SnO2 = 0.1:0.05:1.
The effect of Ni(OH)2 and Ni2 O3 content on photocatalytic activity has been investigated in this paper. Ni2 O3 and Ni(OH)2 are loaded successively on the surface of SnO2 with different molar ratio, and hydrogen production activities of 1% Pt/[Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 catalysts are also evaluated respectively in ethanol–water solution, as shown in Fig. 6. As can be seen, photocatalytic hydrogen production rate of 1% Pt/[Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 increases stably with the increasing of Ni(OH)2 component at 0 < x < 0.02, and increases sharply at x > 0.02. It may be related to site occupancy of Ni(OH)2 in the deposition process. Ni(OH)2 particles mainly deposit on the surface of bare SnO2 at 0 < x < 0.02, and photo-induced electron transfer directly to the surface of SnO2 under visible light irradiation; while partial Ni(OH)2 particle may deposit on the surface of Ni2 O3 at x > 0.02, and Ni2 O3 as electron traps can facilitate photo-generated electron transfer to achieve efficient separation of photo-generated electrons and holes. Therefore the photocatalytic activity is significantly improved in the presence of Ni2 O3 and Ni(OH)2 . The stability of Pt/Ni(OH)2 –Ni2 O3 –SnO2 is evaluated by performing the recycle experiments with a period of 6 h under similar conditions (seen in Fig. 7). An interesting phenomenon that the average hydrogen production rate increases gradually in former 13 cycles and almost reaches a stable value is easily found. It indicates
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Fig. 7. The effect of irradiation time on the average H2 generation rate over Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) (Pt loading 1%, Ni(OH)2 /Ni2 O3 /SnO2 = 0.1:0.05:1).
Table 1 XPS data of 1% Pt/Ni(OH)2 –Ni2 O3 –SnO2 before and after reaction (102 h of catalysis experiment).
Fig. 5. (a) Transient photocurrent–time profiles of Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4) respectively in mixed solution of 20% EtOH and 0.1 M K2 SO4 ; (b) Transient photocurrent–time profiles of [Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 in mixed solution of 20% EtOH and 0.1 M K2 SO4 .
that this material has good stability in a long time (at least 100 h). In order to investigate the reasons for this phenomenon, XPS spectra and XRD patterns of Pt/Ni(OH)2 –Ni2 O3 –SnO2 before and after the reaction (102 h of catalysis experiment) are analyzed in detail.
Name
at.% before reaction
at.% after reaction
Ni2p O1s Pt4f Sn3d
2.77 61.92 0.46 34.85
3.39 62.61 0.43 33.81
The surface concentrations of four elements before and after reaction are shown in Table 1. The surface atom concentration of Ni element increases by 22.4% after the reaction, and the surface atom concentrations of other elements are almost no change. It indicates that hydrogen generation rate increases with the increasing of Ni species surface atom concentrations. Fig. 8a and b shows high-resolution XPS spectra of Ni 2p before and after reaction, respectively. The main electronic peaks of Ni 2p3/2 are resolved and the detailed results obtained are listed in Table 2. The peaks of 855.2 and 855.9 eV are attributed to Ni(OH)2 and Ni2 O3 , respectively; while the peaks of 857.2 is attributed to NiOOH [20,21]. Obviously, Ni species are composed of Ni(OH)2 and Ni2 O3 before the reaction, but Ni species are composed of Ni(OH)2 , Ni2 O3 and NiOOH after the reaction. Moreover, the surface atom concentrations of various Ni species have also been changed after the reaction. The surface atom concentrations of Ni(OH)2 decrease by 7.8%, but that of Ni2 O3 increases 7.0%; while NiOOH is newly generated Ni species after the reaction. It may be caused that the reconstruction of Ni species occurs on the catalyst surface in the reaction process [20,22,23]. Moreover, XRD patterns of Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) are also investigated before and after the reaction, shown in Fig. 9. As can be seen easily, the crystalline phase after reaction is unchanged as compared to the starting Pt/S4. No diffraction peaks of NiOOH are observed in XRD pattern of Pt/S4 after the reaction, which may be due to trace amount of NiOOH generated in the photoreaction.
Table 2 XPS data of Ni species before and after reaction (102 h of catalysis experiment).
Fig. 6. The effect of Ni(OH)2 and Ni2 O3 content on photocatalytic activity (Ni(OH)2 /Ni2 O3 /SnO2 = x:0.05:1).
Ni species
Peak area before reaction
Peak area after reaction
Change rate %
Ni(OH)2 Ni2 O3 NiOOH
1313.86 2657.97 –
1212.00 2845.75 404.85
−7.8 7.0 –
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Fig. 10. A possible mechanism of photocatalytic hydrogen production. (a) no Ni2 O3 ; (b) in the presence of Ni2 O3 .
Fig. 8. (a) and (b) are high-resolution XPS spectra of Ni 2p before and after reaction (102 h of catalysis experiment), respectively (Pt loading 1%).
A possible mechanism on photocatalytic hydrogen production is also proposed, as shown in Fig. 10. First, PN junctions may be formed at the interface of SnO2 –Ni(OH)2 [24–27] in the presence of single Ni(OH)2 , and an inner electric field is produced at the interface under visible light irradiation. At the equilibrium, the p-type semiconductor and the n-type one have negative and positive charges, respectively. Electrons in the Ni(OH)2 valence band are excited to the conduction band under visible light irradiation. Thus holes are generated in the valence band. Because of the existence of the inner electric field, the photo-generated electrons and holes flow into SnO2 and Ni(OH)2 regions, respectively. The reduction of H+ to H2 occurs at Pt nanoparticles loaded on the surface of SnO2 , and electron transfer is conducted by route I in the absence of Ni2 O3 ; while the electron donor is oxidized on the surface of Ni(OH)2 by holes and hydroxyl radicals (Fig. 10a). However, transition mechanism of photo-generated electrons has changed in the presence of Ni2 O3 (Fig. 10b). Ni2 O3 species connecting to Ni(OH)2 and SnO2 can act as electron traps and facilitate the excited electron transfer, thus suppress efficiently the recombination of photo-generated electrons and holes. Therefore, electron transfer is mainly conducted by route II in the presence of Ni2 O3 species, which improves greatly photocatalytic hydrogen generation activity of the catalyst [10,28,29]. At the same time, the oxidation of Ni(OH)2 to Ni2 O3 or NiOOH also occurs at the interface or Ni(OH)2 phase. On the one hand, Ni(OH)2 deposited on the surface of SnO2 or Ni2 O3 nanoparticles is oxidized to Ni2 O3 through Eq. (1). Ni(OH)2 + h = 1/2Ni2 O3 + 1/2H2 O + H+
Fig. 9. XRD patterns of Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) before and after the reaction (102 h of catalysis experiment).
(1)
The newly generated Ni2 O3 at the interface of Ni(OH)2 and SnO2 can act as electron traps and achieve efficient separation of photo-induced electrons and holes. Thus, the activity for hydrogen production can be improved greatly with the extension of irradiation time. On the other hand, partial Ni(OH)2 may be also oxidized to NiOOH by enriched photo-generated holes under visible light irradiation through Eq. (2). It leads to the formation of NiOOH after the reaction. Ni(OH)2 + OH− + h+ = NiOOH + H2 O
(2)
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4. Conclusions Photocatalytic water splitting over the catalysts containing various nickel species has been systematically studied in this paper. The results show that the photocatalyst Pt/Ni(OH)2 –Ni2 O3 –SnO2 has higher photocatalytic activity in the presence of Ni(OH)2 and Ni2 O3 , which may be related with the roles of various Ni species in photocatalytic reaction. Ni(OH)2 enhances evidently light absorption of SnO2 in visible light region; while Ni2 O3 acts as electron traps and facilitates the excited electron transfer, and suppresses efficiently the recombination of photo-generated electrons and holes. In addition, a small amount of Ni(OH)2 deposited on the surface of SnO2 may be oxidized to Ni2 O3 , and the newly generated Ni2 O3 at the interface of Ni(OH)2 and SnO2 also enhances the photocatalytic hydrogen evolution activity evidently. Acknowledgements This work is supported by the 973 Program and 863 Program of Department of Sciences and Technology of China (Grant Nos.2013CB632404, 2009CB220003, and 2012AA051501) and National Science Foundation of China (21173242). References [1] A. Kudo, K. Domen, K. Maruya, Photocatalytic activities of TiO2 loaded with NiO, Chem. Phys. Lett. 133 (6) (1987) 517–519. [2] A. Kudo, A. Tanaka, K. Domen, Photocatalytic decomposition of water over NiO/K4 Nb6 O17 catalyst, J. Catal. 111 (1988) 67–76. [3] S.Z. Kang, L.L. Chen, X.Q. Li, J. Mu, Composite photocatalyst containing Eosin Y and multiwalled carbon nanotubes loaded with CuO/NiO: mixed metal oxide as an active center of H2 evolution from water, Appl. Surf. Sci. 258 (2012) 6029–6033. [4] B. Cheng, Y. Le, W.Q. Cai, J.G. Yu, Synthesis of hierarchical Ni(OH)2 and NiO nanosheets and their adsorption kinetics and isotherms to Congo red in water, J. Hazard. Mater. 185 (2011) 889–897. [5] T. Sreethawong, Y. Suzuki, S. Yoshikawa, Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template, Int. J. Hydrogen Energy 30 (2005) 1053–1062. [6] F.S. Tian, Y.L. Liu, Synthesis of p-type NiO/n-type ZnO heterostructure and its enhanced photocatalytic activity, Scripta Mater. 69 (2013) 417–419. [7] J.J. Zou, C.J. Liu, Y.P. Zhang, Control of the metal-support interface of NiO-loaded photocatalysts via cold plasma treatment, Langmuir 22 (2006) 2335–2339. [8] F. Zhang, Y.J. Liu, Y. Cai, H. Li, X.Y. Cai, I. Djerdj, Y.D. Wang, A facial method to synthesize Ni(OH)2 nanosheets for improving the adsorption properties of Congo red in aqueous solution, Powder Technol. 235 (2013) 121–125. [9] Z.H. Xu, J.G. Yu, G. Liu, B. Cheng, P. Zhou, X.Y. Li, Microemulsion-assisted synthesis of hierarchical porous Ni(OH)2 /SiO2 composites toward efficient removal of formaldehyde in air, Dalton Trans. 42 (2013) 10190–10197. [10] W. Zhao, W.H. Ma, C.C. Chen, J.C. Zhao, Z.G. Shuai, Efficient degradation of toxic organic pollutants with Ni2 O3 /TiO2−x Bx under visible irradiation, J. Am. Chem. Soc. 126 (2004) 4782–4783.
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The roles of various Ni species over SnO2 in enhancing the photocatalytic properties for hydrogen generation under visible light irradiation Quanchao Du a,b , Gongxuan Lu a,∗ a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 2 January 2014 Received in revised form 26 February 2014 Accepted 5 March 2014 Available online 15 March 2014 Keywords: Photocatalysis Hydrogen evolution Ni(OH)2 Ni2 O3 PN junction Visible light irradiation
a b s t r a c t The photocatalysts containing various nickel species were successfully fabricated via loading Ni(OH)2 and/or Ni2 O3 species on the surface of SnO2 nanoparticles, and their roles in the photo-absorb ability, photo-induced charge transfer and photocatalytic hydrogen evolution performances were also investigated under visible light irradiation. Pt/Ni2 O3 –SnO2 catalyst was not active for hydrogen evolution reaction, while the photocatalytic hydrogen evolution rates of Pt/Ni(OH)2 –SnO2 and Pt/Ni(OH)2 –Ni2 O3 –SnO2 were 0.9 and 10.8 mol g−1 h−1 , respectively. It indicated that the photocatalytic activity of Pt/Ni(OH)2 –Ni2 O3 –SnO2 catalyst had been improved evidently in the presence of Ni2 O3 . The UV–vis DRS results showed that Ni(OH)2 enhanced light absorption of catalyst significantly in visible light region. The i–t curves confirmed that the co-existence of Ni(OH)2 and Ni2 O3 improved significantly the photocurrent of Pt/Ni(OH)2 –Ni2 O3 –SnO2 photo-electrode. In addition, the long term experiments revealed that Pt/Ni(OH)2 –Ni2 O3 –SnO2 catalyst was quite stable under given conditions. A proposed reaction mechanism for hydrogen evolution was also present. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Kudo et al. [1] found that water could be decomposed into H2 and O2 over NiO/TiO2 powder in 1987. Since then, NiO as hydrogen evolution sites had been paid much attention in photocatalytic reactions under UV irradiation [2–7]. In recent years, the roles of other Ni species in catalytic reactions were studied, such as Ni(OH)2 [8,9], Ni2 O3 [10,11], NiOOH [12,13] and so on. These Ni species had mainly been used in the elimination organic pollutions and water splitting under UV irradiation by loading on the surface of TiO2 in photocatalytic reactions. Yu et al. [14] reported that the photocatalytic hydrogen production activity of TiO2 was significantly enhanced by loading Ni(OH)2 clusters under UV irradiation. Zhang et al. [15] found that the photosensitized electrolytic oxidation of Ni(OH)2 exhibited a much lower rate than the corresponding electrochemical oxidation. The as-prepared 3D nanostructured TiO2 –Ni(OH)2 composite film electrode, coupled with platinum, showed excellent UV-induced oxidative energy storage ability, and the apparent quantum yield was as high as 10.6%. Most of these
∗ Corresponding author. Tel.: +86 931 4968178. E-mail address: [email protected] (G. Lu). http://dx.doi.org/10.1016/j.apsusc.2014.03.043 0169-4332/© 2014 Elsevier B.V. All rights reserved.
reactions were carried out on the surface of wide band gap catalysts after loading trace amounts of Ni species, and the catalysts showed much higher photocatalytic activity under ultraviolet irradiation, which was explained by the fact that Ni species could enhance the photocatalytic activity of catalysts. They focused on the higher photocatalytic activity while ignored the utilization of visible light. As a typical n-type semiconductor material, SnO2 also showed excellent optical, gas-sensing properties and chemical stability, which was widely used in sensors [16], solar cells [17,18] and lithium batteries [19]. However, it was difficult to respond to visible light because of its wide band gap, so the studies on photocatalytic water splitting over SnO2 under visible light irradiation had rarely been reported. Fortunately, the composite of SnO2 and nickel species could absorb visible light. Therefore, a series of nickel catalysts were firstly prepared by loading Ni(OH)2 and/or Ni2 O3 on the surface of SnO2 nanoparticles, then the photocatalytic activities of the catalysts were evaluated by the variation of Ni species content under visible light irradiation in this paper. The roles of various Ni species over SnO2 in enhancing the photocatalytic properties for hydrogen generation under visible light irradiation were discussed in detail. Hydrogen generation activity of the catalyst was highly depended on the variation of Ni species content, and a possible reaction
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mechanism on photocatalytic hydrogen production was also proposed based on experimental results and the characterization results of XRD, TEM and XPS. 2. Experiment section 2.1. Preparation of catalysts containing various Ni species 2.1.1. Preparation of SnO2 SnO2 powder was prepared by hydrolysis method. Proper SnCl4 ·5H2 O used as starting materials was dissolved in 250 mL double-distilled water, and the pH value of solution was adjusted to 9.0 by adding proper amount NaOH aqueous solution under vigorous stirring; the white slurry was filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained powder was fired 4 h at 550 ◦ C in muffle furnace, heating rate 2 ◦ C min−1 . The white SnO2 powder was obtained, which was denoted as S1. 2.1.2. Preparation of Ni(OH)2 –SnO2 1.51 g SnO2 powder was dispersed in 100 mL 0.01 mol L−1 NiSO4 aqueous solution, then added dropwise 0.01 mol L−1 NaOH solution under vigorous stirring, filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained powder was Ni(OH)2 –SnO2 (the molar ratio of Ni(OH)2 /SnO2 was 0.1:1), which was denoted as S2. 2.1.3. Preparation of Ni2 O3 –SnO2 1.51 g SnO2 powder was dispersed in 100 mL 0.01 mol L−1 NiSO4 aqueous solution, then added dropwise 0.01 mol L−1 (NH4 )2 CO3 solution under vigorous stirring, filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained green powder was heated 4 h at 350 ◦ C in muffle furnace, and the Ni2 O3 –SnO2 sample (the molar ratio of Ni2 O3 /SnO2 was 0.05:1) was obtained, which was denoted as S3. 2.1.4. Preparation of Ni(OH)2 –Ni2 O3 –SnO2 1.55 g Ni2 O3 –SnO2 powder was dispersed in 100 mL 0.01 mol L−1 NiSO4 aqueous solution, then added dropwise 0.01 mol L−1 NaOH solution under vigorous stirring, filtered, washed with distilled water adequately, and then dried at 80 ◦ C. The obtained powder was Ni(OH)2 /Ni2 O3 /SnO2 (the molar ratio of Ni(OH)2 /Ni2 O3 /SnO2 was 0.1:0.05:1), which was denoted as S4. Moreover, [Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 catalysts were also prepared by the same method, using S3 as starting material. Pt/Ni(OH)2 , Pt/Ni2 O3 , Pt/S1, Pt/S2, Pt/S3 and Pt/S4 were prepared by the photo-reduction method in ethanol/water (v/v = 2/3) mixed solution, and the Pt loading content was 1%.
photoelectron spectrometer with an MgK␣ X-ray resource. All the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. We used the Gauss fitted data for multi-peaks in calculating the various Ni species content and relative-content. The Shirley-type background was subtracted from the recorded spectra and the curve fitting and deconvolution of overlapped peaks were done by non-linear least-square fitting with a Gauss–Lorentz (ratio 60:40) curve. 2.3. Preparation of working electrodes and electrochemical experiments For the preparation of working electrodes for electrochemical measurements, a homogenous catalyst ink was firstly prepared by dispersing 4 mg of catalyst material and 80 l of 5 wt% Nafion solution in 2 mL of H2 O by ultrasonication. Then 400 l of catalyst ink dispersions were drop-coated directly onto the precleaned indium tin oxide (ITO) glass surface (ca. 2 cm2 ) by microsyringe and dried under an infrared heat lamp. Photocurrent responses of photocatalyst samples were measured using an electrochemical analyzer (CHI660A) in a homemade standard three-compartment cell. Platinum foil was used as counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The geometrical surface areas of working electrode exposed to the electrolyte was a circular film of 1.6 cm2 . 0.1 M K2 SO4 solution (20 vol.% EtOH) was used as supporting electrolyte. A 300 W Xenon lamp with optical cut-off filter ( ≥ 420 nm) was used for excitation light. 2.4. Photocatalytic activity test The photocatalytic reactions were carried out in a 148 mL Pyrex reactor with a side flat window, sealed by silicone rubber on bottle mouth. 200 mg photocatalyst powder was dispersed in 100 mL ethanol–deionized water (v:v = 20:80) mixed solution. Suspension system formed by ultrasonication was pumped into Ar gas 40 min to remove the air before irradiation. The light source was a 300 W tungsten halogen lamp. The intensity of incident light was 1.16 × 1020 photons s−1 m−2 using a cut off filter (420 nm) to remove the UV light. The photocatalytic activities of Pt/Ni(OH)2 –Ni2 O3 –SnO2 samples were determined by measuring amount of generated hydrogen. The gas evolved was analyzed by an Agilent 6820 gas chromatograph (TCD, Ar carrier gas) per 30 min. 3. Results and discussion 3.1. Characterizations of XRD, TEM and UV–vis DRS
2.2. Structure and composition characterizations Transmission electron microscopy (TEM) and high-resolution TEM images were taken with a Tecnai-G2-F30 field emission transmission electron microscope operated at accelerating voltage of 300 kV. The samples were dispersed in anhydrous ethanol by ultrasonic treatment for 5 min, and the obtained suspension was dropped on the surface of the micro mesh copper grid for TEM characterization. To determine the phase structures of the samples, X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (type Rigaku D/max-RB) using Cu K␣ irradi˚ and the accelerating voltage, applied current, ation ( = 1.5406 A), scan rate, step pace, and scan range were 40 kV, 30 mA, 0.02◦ s−1 , 0.0167◦ , and 15∼90◦ , respectively. UV–vis diffused reflectance spectra (UV–vis DRS) were obtained using a UV–vis spectrophotometer (UV-2550, Shimadzu), and the BaSO4 was used as a reflectance standard. X-ray photoelectron spectroscopy (XPS) analysis was performed using a VG Scientific ESCALAB250-XPS
The XRD patterns of SnO2 (S1), Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4) have been investigated, as shown in Fig. 1. The three main peaks of 26.6◦ , 33.9◦ and 51.8◦ for SnO2 are assigned to the diffraction planes of (1 1 0), (1 0 1) and (2 1 1), and several other widely and slightly weak intensity peaks are corresponding to (2 2 0), (3 1 0), (3 0 1), (2 0 2), (3 2 1) and (2 2 2) planes of SnO2 . Obviously, no other diffraction peak is detected in the XRD pattern of S1, which suggests S1 is composed of pure SnO2 . The peaks at 19.2◦ and 38.4◦ are assigned to the diffraction planes (0 0 1) and (0 1 1) of Ni(OH)2 , while the 56.9◦ peak is corresponded to Ni2 O3 (2 0 2) plane. Therefore, S2 is composed of Ni(OH)2 and SnO2 ; S3 is composed of Ni2 O3 and SnO2 ; while S4 is composed of Ni(OH)2 , Ni2 O3 and SnO2 . Moreover, compared with the JCPDS standard card (88-0287), no shift in the peak positions is evident in each of the collected samples, which indicates that the Ni species such as Ni(OH)2 and Ni2 O3 are not incorporated into the lattice of SnO2 , and are probably adsorbed on the surface of SnO2 particles.
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Fig. 2 shows a typical TEM image of the sample Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4). Many Ni-containing matter clusters can be clearly observed in Fig. 2a, and these Ni-containing matter clusters are uniformly dispersed on the surface of SnO2 nanoparticles. The HRTEM image (Fig. 2b) shows that the distances of the lattice fringes over a large area are 0.333 and 0.263 nm, which are in agreement with the lattice spacing of SnO2 (1 1 0) and (1 0 1) plane, respectively; the lattice fringes of 0.233 and 0.271 nm are corresponding to Ni(OH)2 (0 1 1) and (1 0 0) planes respectively; and the lattice fringes of 0.323 nm is corresponding to Ni2 O3 (1 0 1) planes. It indicates that Ni species on the surface of SnO2 nanoparticles are not a single compound, but composed of Ni(OH)2 and Ni2 O3 . The energy dispersive X-ray (EDX) spectrum further confirms the presence of Ni and Pt in the sample Pt/S4. The typical ultraviolet–visible diffuse reflectance spectra of the four catalysts are shown in Fig. 3. As can be seen, naked SnO2 has a good light absorption only in the ultraviolet region; the catalyst S3 has a slight absorption at 420 nm; and the photocatalysts S2 and S4 show better absorption performance in the range of 400–600 nm. It indicates that Ni(OH)2 enhances significantly visible light absorption of SnO2 catalyst. Fig. 1. XRD patterns of SnO2 (S1), Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4).
Fig. 2. Low- (a), High- (b) magnification TEM image and energy dispersive X-ray (c) spectrum of Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4). A, B and C represent SnO2 , Ni(OH)2 and Ni2 O3 respectively in Fig. 2b.
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Fig. 3. UV–vis DRS of SnO2 (S1), Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4).
3.2. Activity tests for hydrogen production To investigate the roles of Ni species in water splitting reaction, hydrogen production performances over Pt/SnO2 , Pt/Ni(OH)2 , Pt/Ni2 O3 , Pt/Ni(OH)2 –SnO2 , Pt/Ni2 O3 –SnO2 and Pt/Ni(OH)2 –Ni2 O3 –SnO2 catalysts are evaluated under visible light irradiation, as shown in Fig. 4a and b. No hydrogen is detected over Pt/SnO2 , Pt/Ni(OH)2 and Pt/Ni2 O3 catalysts in 3 h irradiation, which indicates that the catalysts have no hydrogen generation activity in the absence of SnO2 or Ni species. In addition, Fig. 4a also displays that no hydrogen is detected over Pt/Ni2 O3 –SnO2 catalyst, while the photocatalytic hydrogen evolution rates of Pt/Ni(OH)2 –SnO2 and Pt/Ni(OH)2 –Ni2 O3 –SnO2 are 0.9 and 10.8 mol g−1 h−1 under visible light irradiation, respectively. Obviously, the photocatalyst containing nickel species is not active in the presence of single Ni2 O3 , while a small amount of hydrogen is detected in the presence of single Ni(OH)2 ; and Ni2 O3 can improve significantly photocatalytic activity of SnO2 in the presence of Ni(OH)2 . It indicates that Ni2 O3 is quite necessary for the catalyst to maintain higher photocatalytic activity. To give further evidence to support the above statements, the transient photocurrent responses of Ni(OH)2 –SnO2 , Ni2 O3 –SnO2 and Ni(OH)2 –Ni2 O3 –SnO2 are also investigated for several on–off cycles of intermittent irradiation (30 s) in mixed solution of 20% EtOH and 0.1 M K2 SO4 . Fig. 5a displays i–t curves for the aforementioned three samples. As can be seen, Ni2 O3 –SnO2 shows extremely weak photocurrent response under visible light irradiation; while the transient photocurrent of Ni(OH)2 –SnO2 and Ni(OH)2 –Ni2 O3 –SnO2 are 0.04 and 0.1 A, respectively. It also confirms that the photocurrent of Pt/Ni(OH)2 –Ni2 O3 –SnO2 photoelectrode is improved evidently when both Ni(OH)2 and Ni2 O3 are coexistent. In addition, the effect of Ni(OH)2 content on photocurrent has also been studied under the same conditions, as shown in Fig. 5b. It can be seen clearly that the photocurrent increases gradually with the increasing of Ni(OH)2 content. Based on above experimental results, the Ni species can improve evidently the photocatalytic activity of catalyst, which may be caused by two reasons. One is that Ni(OH)2 enhances the photoabsorption performance of catalysts in visible light region; on the other hand, Ni2 O3 acts as electron traps, which facilitates the excited electron transfer and achieves efficient separation of photo-generated electrons and holes.
Fig. 4. (a) and (b) Comparison of the photocatalytic hydrogen generation activities of Pt/SnO2 (Pt/S1), Pt/Ni(OH)2 , Pt/Ni2 O3 , Pt/Ni(OH)2 –SnO2 (Pt/S2), Pt/Ni2 O3 –SnO2 (Pt/S3) and Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) (Pt loading 1%). For S2, Ni(OH)2 /SnO2 = 0.1:1; For S3, Ni2 O3 /SnO2 = 0.05:1; For S4, Ni(OH)2 /Ni2 O3 /SnO2 = 0.1:0.05:1.
The effect of Ni(OH)2 and Ni2 O3 content on photocatalytic activity has been investigated in this paper. Ni2 O3 and Ni(OH)2 are loaded successively on the surface of SnO2 with different molar ratio, and hydrogen production activities of 1% Pt/[Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 catalysts are also evaluated respectively in ethanol–water solution, as shown in Fig. 6. As can be seen, photocatalytic hydrogen production rate of 1% Pt/[Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 increases stably with the increasing of Ni(OH)2 component at 0 < x < 0.02, and increases sharply at x > 0.02. It may be related to site occupancy of Ni(OH)2 in the deposition process. Ni(OH)2 particles mainly deposit on the surface of bare SnO2 at 0 < x < 0.02, and photo-induced electron transfer directly to the surface of SnO2 under visible light irradiation; while partial Ni(OH)2 particle may deposit on the surface of Ni2 O3 at x > 0.02, and Ni2 O3 as electron traps can facilitate photo-generated electron transfer to achieve efficient separation of photo-generated electrons and holes. Therefore the photocatalytic activity is significantly improved in the presence of Ni2 O3 and Ni(OH)2 . The stability of Pt/Ni(OH)2 –Ni2 O3 –SnO2 is evaluated by performing the recycle experiments with a period of 6 h under similar conditions (seen in Fig. 7). An interesting phenomenon that the average hydrogen production rate increases gradually in former 13 cycles and almost reaches a stable value is easily found. It indicates
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Fig. 7. The effect of irradiation time on the average H2 generation rate over Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) (Pt loading 1%, Ni(OH)2 /Ni2 O3 /SnO2 = 0.1:0.05:1).
Table 1 XPS data of 1% Pt/Ni(OH)2 –Ni2 O3 –SnO2 before and after reaction (102 h of catalysis experiment).
Fig. 5. (a) Transient photocurrent–time profiles of Ni(OH)2 –SnO2 (S2), Ni2 O3 –SnO2 (S3) and Ni(OH)2 –Ni2 O3 –SnO2 (S4) respectively in mixed solution of 20% EtOH and 0.1 M K2 SO4 ; (b) Transient photocurrent–time profiles of [Ni(OH)2 ]x –[Ni2 O3 ]0.05 –SnO2 in mixed solution of 20% EtOH and 0.1 M K2 SO4 .
that this material has good stability in a long time (at least 100 h). In order to investigate the reasons for this phenomenon, XPS spectra and XRD patterns of Pt/Ni(OH)2 –Ni2 O3 –SnO2 before and after the reaction (102 h of catalysis experiment) are analyzed in detail.
Name
at.% before reaction
at.% after reaction
Ni2p O1s Pt4f Sn3d
2.77 61.92 0.46 34.85
3.39 62.61 0.43 33.81
The surface concentrations of four elements before and after reaction are shown in Table 1. The surface atom concentration of Ni element increases by 22.4% after the reaction, and the surface atom concentrations of other elements are almost no change. It indicates that hydrogen generation rate increases with the increasing of Ni species surface atom concentrations. Fig. 8a and b shows high-resolution XPS spectra of Ni 2p before and after reaction, respectively. The main electronic peaks of Ni 2p3/2 are resolved and the detailed results obtained are listed in Table 2. The peaks of 855.2 and 855.9 eV are attributed to Ni(OH)2 and Ni2 O3 , respectively; while the peaks of 857.2 is attributed to NiOOH [20,21]. Obviously, Ni species are composed of Ni(OH)2 and Ni2 O3 before the reaction, but Ni species are composed of Ni(OH)2 , Ni2 O3 and NiOOH after the reaction. Moreover, the surface atom concentrations of various Ni species have also been changed after the reaction. The surface atom concentrations of Ni(OH)2 decrease by 7.8%, but that of Ni2 O3 increases 7.0%; while NiOOH is newly generated Ni species after the reaction. It may be caused that the reconstruction of Ni species occurs on the catalyst surface in the reaction process [20,22,23]. Moreover, XRD patterns of Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) are also investigated before and after the reaction, shown in Fig. 9. As can be seen easily, the crystalline phase after reaction is unchanged as compared to the starting Pt/S4. No diffraction peaks of NiOOH are observed in XRD pattern of Pt/S4 after the reaction, which may be due to trace amount of NiOOH generated in the photoreaction.
Table 2 XPS data of Ni species before and after reaction (102 h of catalysis experiment).
Fig. 6. The effect of Ni(OH)2 and Ni2 O3 content on photocatalytic activity (Ni(OH)2 /Ni2 O3 /SnO2 = x:0.05:1).
Ni species
Peak area before reaction
Peak area after reaction
Change rate %
Ni(OH)2 Ni2 O3 NiOOH
1313.86 2657.97 –
1212.00 2845.75 404.85
−7.8 7.0 –
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Fig. 10. A possible mechanism of photocatalytic hydrogen production. (a) no Ni2 O3 ; (b) in the presence of Ni2 O3 .
Fig. 8. (a) and (b) are high-resolution XPS spectra of Ni 2p before and after reaction (102 h of catalysis experiment), respectively (Pt loading 1%).
A possible mechanism on photocatalytic hydrogen production is also proposed, as shown in Fig. 10. First, PN junctions may be formed at the interface of SnO2 –Ni(OH)2 [24–27] in the presence of single Ni(OH)2 , and an inner electric field is produced at the interface under visible light irradiation. At the equilibrium, the p-type semiconductor and the n-type one have negative and positive charges, respectively. Electrons in the Ni(OH)2 valence band are excited to the conduction band under visible light irradiation. Thus holes are generated in the valence band. Because of the existence of the inner electric field, the photo-generated electrons and holes flow into SnO2 and Ni(OH)2 regions, respectively. The reduction of H+ to H2 occurs at Pt nanoparticles loaded on the surface of SnO2 , and electron transfer is conducted by route I in the absence of Ni2 O3 ; while the electron donor is oxidized on the surface of Ni(OH)2 by holes and hydroxyl radicals (Fig. 10a). However, transition mechanism of photo-generated electrons has changed in the presence of Ni2 O3 (Fig. 10b). Ni2 O3 species connecting to Ni(OH)2 and SnO2 can act as electron traps and facilitate the excited electron transfer, thus suppress efficiently the recombination of photo-generated electrons and holes. Therefore, electron transfer is mainly conducted by route II in the presence of Ni2 O3 species, which improves greatly photocatalytic hydrogen generation activity of the catalyst [10,28,29]. At the same time, the oxidation of Ni(OH)2 to Ni2 O3 or NiOOH also occurs at the interface or Ni(OH)2 phase. On the one hand, Ni(OH)2 deposited on the surface of SnO2 or Ni2 O3 nanoparticles is oxidized to Ni2 O3 through Eq. (1). Ni(OH)2 + h = 1/2Ni2 O3 + 1/2H2 O + H+
Fig. 9. XRD patterns of Pt/Ni(OH)2 –Ni2 O3 –SnO2 (Pt/S4) before and after the reaction (102 h of catalysis experiment).
(1)
The newly generated Ni2 O3 at the interface of Ni(OH)2 and SnO2 can act as electron traps and achieve efficient separation of photo-induced electrons and holes. Thus, the activity for hydrogen production can be improved greatly with the extension of irradiation time. On the other hand, partial Ni(OH)2 may be also oxidized to NiOOH by enriched photo-generated holes under visible light irradiation through Eq. (2). It leads to the formation of NiOOH after the reaction. Ni(OH)2 + OH− + h+ = NiOOH + H2 O
(2)
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4. Conclusions Photocatalytic water splitting over the catalysts containing various nickel species has been systematically studied in this paper. The results show that the photocatalyst Pt/Ni(OH)2 –Ni2 O3 –SnO2 has higher photocatalytic activity in the presence of Ni(OH)2 and Ni2 O3 , which may be related with the roles of various Ni species in photocatalytic reaction. Ni(OH)2 enhances evidently light absorption of SnO2 in visible light region; while Ni2 O3 acts as electron traps and facilitates the excited electron transfer, and suppresses efficiently the recombination of photo-generated electrons and holes. In addition, a small amount of Ni(OH)2 deposited on the surface of SnO2 may be oxidized to Ni2 O3 , and the newly generated Ni2 O3 at the interface of Ni(OH)2 and SnO2 also enhances the photocatalytic hydrogen evolution activity evidently. Acknowledgements This work is supported by the 973 Program and 863 Program of Department of Sciences and Technology of China (Grant Nos.2013CB632404, 2009CB220003, and 2012AA051501) and National Science Foundation of China (21173242). References [1] A. Kudo, K. Domen, K. Maruya, Photocatalytic activities of TiO2 loaded with NiO, Chem. Phys. Lett. 133 (6) (1987) 517–519. [2] A. Kudo, A. Tanaka, K. Domen, Photocatalytic decomposition of water over NiO/K4 Nb6 O17 catalyst, J. Catal. 111 (1988) 67–76. [3] S.Z. Kang, L.L. Chen, X.Q. Li, J. Mu, Composite photocatalyst containing Eosin Y and multiwalled carbon nanotubes loaded with CuO/NiO: mixed metal oxide as an active center of H2 evolution from water, Appl. Surf. Sci. 258 (2012) 6029–6033. [4] B. Cheng, Y. Le, W.Q. Cai, J.G. Yu, Synthesis of hierarchical Ni(OH)2 and NiO nanosheets and their adsorption kinetics and isotherms to Congo red in water, J. Hazard. Mater. 185 (2011) 889–897. [5] T. Sreethawong, Y. Suzuki, S. Yoshikawa, Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template, Int. J. Hydrogen Energy 30 (2005) 1053–1062. [6] F.S. Tian, Y.L. Liu, Synthesis of p-type NiO/n-type ZnO heterostructure and its enhanced photocatalytic activity, Scripta Mater. 69 (2013) 417–419. [7] J.J. Zou, C.J. Liu, Y.P. Zhang, Control of the metal-support interface of NiO-loaded photocatalysts via cold plasma treatment, Langmuir 22 (2006) 2335–2339. [8] F. Zhang, Y.J. Liu, Y. Cai, H. Li, X.Y. Cai, I. Djerdj, Y.D. Wang, A facial method to synthesize Ni(OH)2 nanosheets for improving the adsorption properties of Congo red in aqueous solution, Powder Technol. 235 (2013) 121–125. [9] Z.H. Xu, J.G. Yu, G. Liu, B. Cheng, P. Zhou, X.Y. Li, Microemulsion-assisted synthesis of hierarchical porous Ni(OH)2 /SiO2 composites toward efficient removal of formaldehyde in air, Dalton Trans. 42 (2013) 10190–10197. [10] W. Zhao, W.H. Ma, C.C. Chen, J.C. Zhao, Z.G. Shuai, Efficient degradation of toxic organic pollutants with Ni2 O3 /TiO2−x Bx under visible irradiation, J. Am. Chem. Soc. 126 (2004) 4782–4783.
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