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.