Roles of various Ni species on TiO₂ in enhancing photocatalytic H₂ evolution

Abstract

Low-cost nickels can be used as cocatalyst to improve the performance of photocatalysts, which may be promising materials applied in the field of photocatalytic water splitting. In this study, different nickel species Ni, Ni(OH)₂, NiO, NiO_x, and NiS are used to modified titanium dioxide (P25) to investigate their roles on the photocatalytic hydrogen evolution activities. UV-visible, X-ray diffraction (XRD), Brunner-Emmet-Teller (BET) measurements, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) analysis etc. are employed to characterize the physical and chemical properties of catalysts. The results indicate that all the nickel species can improve the photocatalytic hydrogen production activity of P25. The P25 modified with NiO_x and NiS has more superior photocatalytic hydrogen evolution activities than those modified with other nickel species. The reason for this is that NiO_x and NiS can form p-n junctions with P25 respectively. In addition, NiO_x can be selectively deposited on the active sites of P25 via in situ the photodeposition method and NiS is beneficial for H⁺ reacting with photo-excited electrons.

Keywords

nickel species / TiO₂ / photocatalytic hydrogen evolution

Introduction

Photocatalytic water splitting using solar energy has been regarded as a promising way to solve the serious global energy and environmental problems. Since the evolution of hydrogen and oxygen on TiO₂ and Pt counter electrodes under the irradiation of ultraviolet (UV) light was found in 1972 [¹], many efficient photocatalysts have been found for water splitting as a potentially efficient utilization of solar energy [²].

The main processes of photocatalytic water splitting include the absorption of photons to generate electron-hole pairs, the charge separation and migration, and the surface chemical reactions (active sites) for H₂ and O₂ evolution [³,⁴]. The first step can be improved by developing photocatalysts with broader absorption of the solar energy spectrum [⁵–⁸]. Designing various nanostructured semiconductors [⁹–¹⁴] and the formation of semiconductor heterojunctions [¹⁵,¹⁶] can be applied to improve the charge separation and migration. Usually, the third step is promoted via modification with cocatalysts [¹⁷,¹⁸]. Meanwhile, cocatalysts can also be beneficial for charge separation and transfer.

Many noble metals are used as cocatalysts such as Pt, PdS, Au, Rh, RuO₂, etc. [¹⁹–²¹] to improve the photocatalytic water splitting activities of photocatalysts. However, their practical application is hindered by their high cost. Thus, various low-cost and effective cocatalysts such as low cost metals, carbon dots etc. [²²–²⁵] have been developed to improve photocatalytic hydrogen evolution in recent years.

Nickel species are low-cost cocatalysts compared with other noble metals. Nickel species including metal nickel [²⁶], nickel oxides [²⁷,²⁸], nickel hydroxide [²⁹], and nickel sulfide [³⁰,³¹] have been reported for improving photocatalytic hydrogen production activities of photocatalysts respectively, which will be promising materials applied in the field of photocatalytic water splitting. However, the roles of these nickel species in enhancing the photocatalytic hydrogen production are still not clear. In this work, the influence of different nickel species on commercial titanium dioxide (P25) for photocatalytic hydrogen production has been systematically studied, which may shed some light on effective photocatalytic hydrogen evolution using non-noble metal cocatalysts.

Experimental

Sample preparation

All of the reagents were of analytical grade and used without further purification. Deionized water was used in all experiments. Titanium dioxide (P25) was purchased from Degussa.

Ni, Ni(OH)₂, NiO, and NiS are used to modify P25 to investigate their roles in the photocatalytic hydrogen evolution, which are denoted as P-Ni, P-Ni(OH)₂, P-NiO, and P-NiS respectively. P-in situ, as P25, is modified with nickels via the in situ photodeposition method. The amount of nickel of all samples is 5 mol% (the ratio of Ni/Ti). The synthesis details of samples are as follows:

P-Ni: 0.2 g of P25 were dispersed in 50 mL of glycol with 1.565 mL of 0.08 M Ni(NO₃)₂ added in. The solution was stirred for 1 h. Then, 0.5 g of NaOH was added to the solution and stirred for 0.5 h. The mixed system was poured into a Teflonlined stainless steel autoclave with a capacity of 100 mL and maintained at 200°C for 10 h. Finally, the products were washed with ethanol and distilled water several times, and dried in air at 60°C for 12 h.

P-Ni(OH)₂: 0.2 g of P25 was dispersed in 50 mL of 1M NaOH solution. 1.565 mL of 0.08 M Ni(NO₃)₂ was added in and stirred for 12 h. Then, the samples were filtrated and washed with distilled water till the wash water was neutral, and dried at 60°C under vacuum conditions for 12 h.

P-NiO: 0.2 g of P25 was dispersed in 50 mL of distilled water and 1.565 mL of 0.08 M Ni(NO₃)₂ was added in. Then, it was dried at 60°C via the water bath method. The collected powder was calcined at 350°C for 3h.

P-NiS: 0.2 g P25 was dispersed in 50 mL of distilled water. 1.565 mL of 0.08 M Ni(NO₃)₂ was added in and stirred for 1 h. Then, 2.0 g of Na₂S·9H₂O was added in and stirred for another 1 h. The mixed system was poured into a Teflon lined stainless steel autoclave with a capacity of 100 mL and maintained at 160°C for 10 h. Finally, the products were washed with ethanol and distilled water several times, and dried in air at 60°C for 12 h.

P-in situ: 0.2 g of P25 was dispersed in 100 mL of CH₃OH solution (30 vol%) containing 1.565 mL of 0.08 M Ni(NO₃)₂. It was under irradiation of a Xe lamp (300 W) for 2 h. The sample was filtrated and washed with distilled water, and dried in air at 60°C for 12 h.

Ni, Ni(OH)₂, NiO, and NiS were also synthesized via the above method without adding P25 respectively.

Characterization

The crystal structure of the photocatalytic materials was confirmed by X-ray diffraction (Rigaku D/max-2200/PC Japan) with Cu Ka (40 kV, 20 mA). The UV-visible diffuse reflection spectra (DRS) were determined by using a UV-visible spectrophotometer UV-2450 (Shimadzu, Japan) and were converted to absorbance by using the Kubelka-Munk method. The surface areas of the samples were determined by BET measurement (TristarII, USA). The surface electronic state was analyzed by using X-ray photoelectron spectroscopy (XPS, Shimadzu-Kratos, Axis Ultra DLD, Japan). All the binding energy (BE) values were calibrated by using the standard BE value of contaminant carbon (C1s= 284.6 eV) as a reference. The transmission electron microscopy (TEM) measurements were conducted using a JEM-2100F (Japan).

Photocatalytic hydrogen production

The photocatalytic reactions were carried out in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. 0.1 g of catalyst was suspended in 100 mL of aqueous solution containing 30 mL methanol. The suspension was then thoroughly degassed and irradiated by a Xe lamp (300 W). The activity of H₂ evolution was analyzed using an online gas chromatography.

Results and discussion

XRD analyses

The phase of the samples obtained under the present system was characterized by using XRD. As shown in Fig. 1, three diffraction peaks centered at 44.5°, 51.8°, and 76.3° can be indexed as the (111), (200), and (220) planes of the fcc Ni form respectively. The diffraction peaks centered at 33.1°, 38.5°, 59.0° and 62.6° correspond to the (100), (101), (110), and (003) planes of b-Ni(OH)₂ [³²]. The XRD pattern at the peaks of 2q = 37°, 43.4°, 62.8°, and 75.5° correspond to the (111), (200), (220), and (311) planes of the NiO [³³] and the peaks of 2q = 30.2°, 34.7°, 46.2°, and 53.6° correspond to the (100), (101), (102), and (110) planes of the NiS [³⁴]. This information indicates that Ni, NiO, Ni(OH)₂, and NiS have been synthesized successfully by using the methods in the experimental section. The fact that diffraction peaks center at 25.2°, 27.0°, 37.9°, 47.8°, 53.8°, 55.0°, and 62.1° confirm that P25 contains anatase and rutile phases [³⁵]. However, no detection of nickels was found by XRD when they modified titanium dioxide (P25) as shown in Fig. 1, which was mainly due to the low content and high dispersion on the surface of P25.

BET, ICP, and UV-visible analyses

BET surface area and ICP results of P25 modified with different nickel species are listed in Table 1. The value of the BET surface area of P25 modified with different nickel species reduced a little compared with that of P25, due to the effect of nickel species deposited on P25. The actual nickel content of P25 modified with Ni, NiO, and Ni(OH)₂ were close to the calculative value (5 mol% of Ni/Ti). However, the loaded NiS content measured by ICP was relatively lower than the theoretical value. It may attribute to the loss during the NiS loading process and this phenomenon has also been found in Ref. [³¹]. The nickel content loaded on P25 via in situ photodeposition was also lower than the theoretical value. The reason for this is that nickel could notbe deposited on P25 totally under light irradiation just for 2 h.

The UV-visible spectra of P25 modified with different nickel species are shown in Fig. 2. It can be seen clearly that P25 is only responsive to ultraviolet light. After modification with different nickels, the absorptions in ultraviolet light region increased, which could be beneficial for improving its photocatalytic activity. The absorption in the region above 400 nm also increased, which was ascribed to the color changes of the samples after modification with different nickels. The absorption in the region above 400 nm of P-Ni and P-in situ were higher than those of other samples and their colors were correspondingly dark and gray. It should be noted that the absorption variation in the region of ultraviolet light and visible light was not the same. This was not the deciding factor although it might have an effect on the hydrogen evolution activities of catalysts in this experiment, which could be concluded from the results of their photocatalytic performance in Subsection 3.4.

TEM analyses

Commercial titanium dioxide (P25) is the mixture of TiO₂ with rutile and anatase phases, which can also be verified from the TEM results. As shown in Fig. 3(a), the lattice spacing of 0.352 nm is corresponding to the (101) plane of anatase titanium dioxide [¹⁰] and the lattice spacing of 0.206 nm is corresponding to the (110) plane of rutile titanium dioxide [³⁵]. Ni(OH)₂ particles can be observed on the surface of TiO₂ as shown in Fig. 3(b). The lattice spacing of 0.200 nm is corresponding to the (111) of NiS as shown in Fig. 3(c) and it can be seen clearly that NiS are highly dispersed on the surface of TiO₂. The contrast between nickel species and TiO₂ in TEM images is not obvious because the atomic weights of Ti and Ni are close to each other. In addition, P25 have many exposed crystal faces, thus, it is difficult to find Ni and nickel oxides in the samples of P-Ni, P-NiO, and P-in situ via TEM images.

XPS analyses

XPS is used to confirm the nickel state on P25. As shown in Fig. 4, the observed binding energy of P-Ni shows a small absorption at 852.0 eV, which indicates that Ni is deposited on P25. However, there is also an obvious absorption at 855.2 eV. The reason for this is that Ni nanoparticles are easily oxidized to NiO, as is the case in Ref. [²⁶]. The binding energy of Ni 2p_3/2 at 855.2 eV confirms the fact that NiO are formed on the P-NiO sample. The absorption of P-Ni(OH)₂ at 855.4 eV means that nickel species are deposited on P25 in the form of Ni(OH)₂ [³⁵]. After irradiation, a small absorption at 852.0 eV appears as shown in Fig. 5, which is caused by the fact that Ni²⁺ ions in Ni(OH)₂ are reduced to Ni nanoparticles. Reference [²⁹] has also proved that Ni(OH)₂ would be decomposed during the photocatalytic process. Absorptions of P-in situ at both 852.0 eV and 855.2 eV appear. This is probably due to the fact that Ni²⁺ ions are first reduced to Ni nanoparticles by the photo-excited electrons. But these Ni nanoparticles are not stable and can be easily oxidized to NiO [²⁸]. The small absorption of P-NiS at 853.6 eV indicates that NiS is successfully deposited on P25 [³⁶].

Photocatalytic performance of P25 modified with different nickel species

The photocatalytic activities for H₂ evolution over P25 modified with different nickel species are shown in Fig. 6. It can be observed that the samples P-NiS and P-in situ have higher activities than the P25 modified with other nickel species. This is because that p-n junctions can be formed when P25 is modified with NiS and NiO_x respectively. In addition, NiS can absorb H⁺ more easily, which can react with electrons for H₂ evolution as the following steps [³⁶]

\begin{array}{l}{\mathrm{NiS+H}}^{+}{\mathrm{+e}}^{-}{\hspace{0.17em}}^{\underrightarrow{\mathrm{Ni}}}\mathrm{HNiS}\\ {\mathrm{HNiS+H}}^{+}{\mathrm{+e}}^{-}{\hspace{0.17em}}^{\underrightarrow{\mathrm{Ni}}}\hspace{0.17em}{\mathrm{NiS+H}}_2\end{array}

NiO_x can be deposited selectively on the active sites of photocatalysts through in situ photodeposition for an efficient electron transfer [²⁷,²⁸].The photocatalytic hydrogen evolution activity of P25 also improves effectively after modification with NiO, which is due to the p-n junctions formed between P25 and NiO. The photocatalytic activities of P25 modified with Ni or Ni(OH)₂ also improved although they are not as good as those modified with NiS or NiO_x as shown in Fig. 6. Metallic Ni nanoparticles can enhance the activity of TiO₂ due to its unique properties similar to some noble metals such as Pt, Au etc., which can efficiently improve the separation and transfer of charge carriers [²⁶]. The Ni²⁺ of Ni(OH)₂ can be reduced to metallic Ni which can help the charge separation and Ni(OH)₂ itself can also act as cocatalyst for water reduction. Thus, the photocatalytic H₂ evolution activity of P25 has also been improved after modification with Ni(OH)₂ [²⁹,³⁵].

Conclusions

Different nickel species including Ni, Ni(OH)₂, NiO, NiO_x, and NiS were used as cocatalysts to modified P25 to investigate their roles in photocatalytic hydrogen evolution. All the nickel species can effectively improve the photocatalytic hydrogen evolution activity of P25. Nican enhance the photocatalytic activity of P25. The improvement effect of Ni(OH)₂ is caused by the fact that Ni²⁺ in Ni(OH)₂ is reduced to Ni nanoparticles and Ni(OH)₂ itself can also act as cocatalyst to improve charge separation and transfer. NiO, NiO_x, and NiS can form p-n heterojunctions with P25 respectively, which facilitates charge separation and transfer. In addition, NiO_x can be deposited on the active sites of P25 selectively via the in situ deposition method and NiS is beneficial for H⁺ reacting with photo-excited electrons. Thus, P-in situ and P-NiS have higher photocatalytic activities than those modified with other nickel species. This work may shed some light on low-cost nickels used as cocatalysts in the field of photocatalytic water splitting.

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