Chong Wanga,b, Qianqian Hua,b, Jiquan Huanga,∗, Chen Zhuc, Zhonghua Denga,b, Hongling Shi c, Lan Wuc, Zhuguang Liua, Yongge Caoa,c,∗

Introduction 

Hydrogen has been considered to be an attractive energy carrier because it only generates water when combusted and has high energy contentperunitmass. Since the electrochemical water splitting using TiO2 was first reported by Honda and Fujishima in 1972 [1], electrochemical hydrogen production using TiO2-based catalyst has attracted significant interest and provides a promising method for solar energy conversion and storage. Nonetheless, the recombination of photoexcited electron–hole pairs results in the low photocatalytic activity of pure TiO2 [2]. To overcome its intrinsic drawback, doping TiO2 with transition metals, such as Cu, Cr, Ni, Zn and Co [3–7], is an effective method. Among these transition metals, copper [8–11] has drawn much attention because it can enhance the photocatalytic activity of TiO2 by preventing the photoexcited electron–hole recombination. In addtion, surface energy is another important factor to influence the performance of TiO2-based catalyst [12–15]. It is well known that the (0 0 1) facet of anatase TiO2 is more active because its average surface energy is higher than that of (1 0 1) and (1 0 0) facet [16–20]. Thus, it is believed thatthe combination of Cu-doping and large percentage of exposed (0 0 1) facets can lead to high performance of TiO2 catalyst. To date, many techniques have been adopted to synthesize anatase TiO2 with exposed (0 0 1) facets, while the study on the water splitting by Cu-doped TiO2 film with dominant (0 01) facets deposited by sputtering is lack. In this work, Cu-doped TiO2 film with preferred(0 0 1) orientation wasdepositedbyRFmagnetronsputtering successfully. Hydrogen production was tested by immersing the films into 10%aqueousmethanol solution. The experimental results showed that the photocatalytic activity of Cu-doped TiO2 film with preferred (0 0 1) orientation was far higher than that of undoped TiO2 film and even Degussa P25 powder.

Experimental

Cu-doped TiO2 film was synthesized on quartz glass substrates (2 × 4 cm) by RF reactive magnetron sputtering. The targets were 2- inch-diameter metallic plates of Ti (99.999% pure) and Cu (99.99% pure). The target-substrate distance was fixed at 70 mm and the base vacuum was 5.8 × 10−3 Pa. Before deposition, the targets were pre-sputtered by argon plasma for 30 min. The working gas was a mixture of Ar (99.999% pure) and O2 (99.99% pure) and the total working pressure was set at 0.3 Pa. The flow rate of O2 and Ar were 3.9 sccm (standard cubic centimeter per minute) and 40 sccm,respectively. The RF power was fixed at 130W and 50W for Ti and Cu, respectively. The whole deposition was carried out for 4 h at a fixed temperature of 450 ◦C. Pure TiO2 film was deposited as reference on the same condition without using Cu target. The crystalline structure of the samples was identified by X-ray diffraction (XRD, Model D/Max 2550V, Rigaku, Japan). The morphology and thickness were estimated by field emission scanning electron microscopy (FE-SEM, JSM6700F, JEOL, Japan).The chemical compositions were determined by using X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Scientific, USA). The hydrogen production was tested by the photocatalytic water splitting testing system (CEL-SPH2N, AULTT, China). The irradiation source was a 300-W Xe lamp (CEL-HXF300, AULTT, China)and the total output of the lamp was 50W. The film–lamp distance was fixed at 15 cm. Withoutthe assistant of metal cathode and bias, the photocatalytic reaction was carried out in a quartz cell containing 100 mL aqueous methanol solution (CH3OH:H2O = 1:10 v/v). The amount of H2 was detected by a gas chromatograph (SP7800, AULTT, China).

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

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

Conclusions 

In summary, Cu-doped TiO2 film with preferred (0 0 1) orientation was successfully deposited by RF magnetron sputtering. It is found that the introduction of copper has a great influence on the preferred growth orientation [from (1 0 1) to (0 0 1) facet] of the TiO2 film and leads to the presence of minor rutile phase. XPS results indicate that the chemical state of copper is Cu1+, which can work as electron trap and thus suppress the recombination of electron–hole pairs. Due to the large percentage of exposed high energy (0 0 1) facets, Cu1+ ions and mixed phases (major anatase and minor rutile phase), the hydrogen production rate of Cu-doped TiO2 film is far higher than that of undoped film, and even about 67 times to P25 powder. Therefore, in this study, it has been revealed that the introduction of copper during the sputtering can enhance the photocatalytic activity of TiO2-based catalyst effectively.