1. Introduction
TiO2, a promising semiconductor photocatalytic material, has attracted extensive attention due to its advantages, such as the biological and chemical inertness, stability to corrosion, nontoxicity, relatively low cost, and high photoactivity [1–3]. In nature, there exist three phases of TiO2: rutile, anatase, and brookite TiO2. Among them anatase TiO2is of the best photocatalytic activity and is widely applied in solar energy conversion, environmental purification, and particularly the degradation of harmful organic pollutants in water [4–6]. However, the popular application of anatase TiO2 is severely restricted due to the following two reasons [7]: (i) The band gapEg of 3.23 eV is so large that it leads to the low utilization efficiency of visible light; (ii) The quantum yield is very low because of the low electron transfer rate and the high recombination rate of photoexcited electron-hole pairs. Therefore, it is still urgent to improve the photocatalytic activity of TiO2.
In the past decades, much effort has been devoted to extending the optical response range of TiO2 to the visible-light region, by means of impurity doping [8–14], noble metal loading [15], semiconductor compounding [16–18], and organic sensitizing [19,20]. Among all these methods, impurity doping is considered to be one of the most convenient and efficient methods. In particular, transition metal (TM) atoms have been proven to be the ideal dopants to promote the photocatalytic performance of TiO2 catalysts because of their d electronic configuration and unique characteristics. Quantum-sized TiO2 doped with 0.5 at.% Fe3+, Mo5+, Ru3+, Os3+, Re5+, V4+, and Rh3+ are found to induce the red shift of absorption edge and can improve photoactivity for both CHCl3oxidation and CCl4 reduction [21]. Mesoporous TiO2 impregnated by Ag+, Co2+, Cu2+, Fe3+, and Ni2+ can accelerate the degradation of acetophenone and nitrobenzene in aqueous solution [22]. TiO2 powders doped by a small amount of Fe3+ can clearly enhance the intensity of absorption in the UV-visible light region and the photocatalytic oxidation of acetone in air [23]. Moreover, Ni 8 wt%-doped TiO2has a high photocatalytic activity for the decomposition of 4-chlorophenol in aqueous solution in the presence of both UV and visible light [24]. All the above researches have demonstrated that the TM dopants can extend the optical response range of TiO2 into the visible-light region. However, we want to stress that the above dopings are mainly performed in bulk TiO2.
In fact, surface structure will play an important role in the photocatalytic activity because usually photocatalytic reactions mainly occur on the catalyst surface [25]. It is especially necessary to study the effect of the surface manipulation on the catalytic activity. Hebenstreit et al. [26] found that anatase TiO2(101) surface (TiOS) with few point defects is stable and the fourfold coordinated Ti atoms at step edges are the preferred adsorption sites. By investigating some low-index TiO2 surfaces Labat et al. [27] demonstrated that the TiOS is the most stable surface. Also, Ma et al. [28] found that the TiOS structure with its outermost and second layers terminated by twofold coordinated O atoms and fivefold coordinated Ti atoms, respectively, is much more stable. Although many researches on the TiOS have been carried out, a systematic investigation on the photocatalytic activities of TiOS doped by all 4d TM atoms is still absent. A comparative study of all the 4d TM dopings on TiOS is required to clarify the intrinsic relation between the macroscopic photocatalytic activities and the microscopic TiO2 details.
In this work, we will systematically investigate the surface modification of TiOS doped with 4d TM atoms to uncover how the doping will improve the photocatalytic performance of TiOS. We will first study the geometric structure, the doping manners, and the optical properties of the doped TiOS. Then, the relation of the macroscopic catalytic activity with the microscopic structure is discussed according to the density of states, energy bands, and the effective masses of carriers.