Chong Wanga,b, Qianqian Hua,b, Jiquan Huanga, Lan Wuc, Zhonghua Denga, Zhuguang Liua, Yang Liua,b, Yongge Caoa,c,∗

Abstract

N-doped TiO2 film was deposited by RF reactive magnetron sputtering in a mixture gas of N2, O2 and Ar. The experimental results show that the crystal structure is anatase phase, and the concentration of substitutional nitrogen is 4.91 at.% which leads to a narrow optical band gap of 2.65 eV. The H2 production rate of the N-doped TiO2 film is about 601 μmol g−1 h−1, far higher than that of the undoped TiO2 film and even about 50 times higher than that of dispersive P25 powder.

Experimental

N-doped TiO2 films were deposited on quartz glass substrates (2 × 4 cm) by RF reactive magnetron sputtering. These substrates were beforehand cleaned, sequentially with acetone, alcohol and de-ionized water, respectively for 20 min in the ultrasonic bath. The working gas was a mixture of N2 (99.999% pure), O2 (99.99% pure) and Ar (99.999% pure). The target was a 2-inch-diameter Ti metal plate (99.999% pure). The target-substrate distance was fixed at 70 mm and the base vacuum was 5.8 × 10−3 Pa. Before deposition, the substrate temperature was maintained at 400 ◦C for 2 h to degas and the target was pre-sputtered for 30 min by argon plasma. Afterwards, working gas was introduced into the chamber through three mass flow controllers and flow rate fixed at 5.0sccm (standard cubic centimeter per minute), 3.9sccm, 40sccm for N2, O2 and Ar respectively. The total working pressure was set at 0.3 Pa and the RF power was fixed at 130W. The whole deposition process was conducted at 450 ◦C for 4 h, during which the substrate rotated around its axis at 8 rotations per minute. Pure TiO2 film was also deposited under the same condition without N2 as reference. The crystalline structure of the as-deposited N-doped TiO2 films was identified by X-ray diffraction (XRD, Model D/Max 2550V, Rigaku, Japan). The thickness and morphology were determined by field emission scanning electron microscopy (FE-SEM, JSM6700F, JEOL, Japan). The surface roughness was estimated by atomic force microscopy (AFM, Nanoscope, NS3A-02, Veeco, USA). The transmittance spectra of the films were measured by a UV–vis spectrophotometer (Lambda 900, PerkinElmer, USA). The chemical compositions and valence-band spectra were determinated by X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific, USA). The hydrogen production was examined by the photocatalytic water splitting testing system (CEL-SPH2N, AULTT, China), irradiated by a 300W Xe lamp (CEL-HXF300, AULTT, China). The total output of the lamp was 50W and only 2.6W of incoming irradiance was obtained below 390 nm. The photocatalytic reaction was carried out in a quartz cell containing 100 mL aqueous methanol solution (CH3OH: H2O = 1:10 v/v). Before reaction,the as-deposited film was immersed in the aqueous solution and then oxygen was removed by a mechanical pump. The film-lamp distance was fixed at 12 cm. The amount of H2 produced was analyzed by a gas chromatograph (SP7800, AULTT, China) using N2 as carrier gas.

Results and discussion

XRD patterns of pure TiO2 and N-doped TiO2 films prepared by RF reactive magnetron sputtering.

Hydrogen evolution profiles of N-doped TiO2 film, P25 powder, TiO2 film and P25 film.

Conclusions

The N-doped TiO2 film was successfully synthesized by RF reactive magnetron sputtering, with anatase phase and a large percentage of exposed (0 0 4), (1 1 2), (2 0 0), and especially (2 1 1) facets. The film, about 900 nm in thickness, is composed of columnar structures. The optical band gap decreases from 3.28 eV (for undoped anatase TiO2 sample) to 2.65 eV when 4.91 at.% nitrogen is incorporated into the anatase structure. H2 is photocatalytically produced and the amount of H2 increases linearly with the exposure time when immersing this sample into 10% aqueous methanol solution and irradiating under the Xe lamp. The H2 production rate of the N-doped TiO2 film is about 601 mol g−1 h−1, far higher than that of the undoped TiO2 film (H2 evolution was hardly detected) and even dispersive P25 powder (about 12 mol H2 g−1 h−1), which is attributed to the red shift of the absorbance edge and the large percentage of exposed high surface energy facets. Acknowledgment This work was supported by the National Natural Science Foundation of China (20901079）.

Efficient hydrogen production by photocatalytic water splitting usingN-doped TiO2film.pdf