First-principles study on transition metal-doped anatase TiO2
© Wang et al.; licensee Springer. 2014
Received: 1 November 2013
Accepted: 5 December 2013
Published: 28 January 2014
The electronic structures, formation energies, and band edge positions of anatase TiO2 doped with transition metals have been analyzed by ab initio band calculations based on the density functional theory with the planewave ultrasoft pseudopotential method. The model structures of transition metal-doped TiO2 were constructed by using the 24-atom 2 × 1 × 1 supercell of anatase TiO2 with one Ti atom replaced by a transition metal atom. The results indicate that most transition metal doping can narrow the band gap of TiO2, lead to the improvement in the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, the preparation of Co-, Cr-, and Ni-doped TiO2 becomes relatively easy in the experiment due to their negative impurity formation energies, which suggests that these doping systems are easy to obtain and with good stability. The theoretical calculations could provide meaningful guides to develop more active photocatalysts with visible light response.
The discovery of water photolysis on a TiO2 electrode by Fujishima and Honda in 1972  has been recognized as a landmark event. Since then, TiO2 has attracted extensive attention as an ideal photocatalytic material because of its excellent properties such as high activity, good stability, nontoxicity and low cost. Thus, it has been widely used in the fields of renewable energy and ecological environmental protection [2–4]. However, as a wide band gap oxide semiconductor (Eg = 3.23 eV), anatase TiO2 can only show photocatalytic activity under UV light irradiation (λ < 387.5 nm) that accounts for only a small portion of solar energy (approximately 5%), in contrast to visible light for a major part of solar energy (approximately 45%). Therefore, how to effectively utilize sunlight is the most challenging subject for the extensive application of TiO2 as a photocatalyst. In the past decades, many efforts have been devoted to extending the spectral response of TiO2 to visible light, including energy band modulation by doping with elements [5–11], the construction of heterojunctions by combining TiO2 with metals such as Pt or Pd [12, 13] and other semiconductors (such as MnO2, RuO2, and WO3), and the addition of quantum dots  or dyes  on the surface of TiO2 for better light sensitization.
Because of the unique d electronic configuration and spectral characteristics of transition metals, transition metal doping is one of the most effective approaches to extend the absorption edge of TiO2 to visible light region, which either inserts a new band into the original band gap or modifies the conduction band (CB) or valence band (VB), improving the photocatalytic activity of TiO2 to some degree [19–24]. For example, Umebayashi et al.  showed that the localized energy level due to Co doping was sufficiently low to lie at the top of the valence band, while the dopants such as V, Mn, Fe, Cr, and Ni produced the mid-gap states. Yu et al.  reported that the density functional theory (DFT) calculation further confirmed the red shift of absorption edges and the narrowing of the band gap of Fe-TiO2 nanorods. Hou et al.  showed that new occupied bands were found in the band gap of Ag-doped anatase TiO2. The formation of these new bands results from the hybridization of Ag 4d and Ti 3d states, and they were supposed to contribute to visible light absorption. Guo and Du  showed that Cu could lead to the enhancement of d states near the uppermost part of the valence band of TiO2 and the Ag or Au doping caused some new electronic states in the band gap.
Even though the effects of the transition metal-doped TiO2 have been investigated frequently, it remains difficult to make direct comparisons and draw conclusions due to the various experimental conditions and different methods for sample preparation and photoreactivity testing. At the same time, because of the lack of the detailed information about the effects of metal doping on crystal structures and electronic structures, there is still much dispute about these issues. In comparison with the experimental investigation, the theoretical analysis by computer simulation can be a proper method to clarify the effects of transition metal doping in detail.
In order to systematically investigate the influence of transition metal doping into anatase TiO2, we adopted the planewave ultrasoft pseudopotential method within the framework of density functional theory (DFT) to calculate the electronic structures, formation energies, and band edge positions of supercells, in which a Ti atom was substituted by a transition metal atom. Considering the accessibility of the doping metals, the 3d transition metal atoms (M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and the 4d transition metal atoms (M = Y, Zr, Nb, Mo, and Ag) were studied in the present work. Moreover, the present calculation results were compared with the experimental results reported in the literatures. The conclusions are important to understand the reactive mechanism and optimize the performance of TiO2 photocatalysts that are active under visible light irradiation.
DFT calculations  were carried out using Cambridge Sequential Total Energy Package (CASTEP, Accelrys Company, San Diego, CA, USA) [26, 27], with the planewave ultrasoft pseudopotential approach. Our geometry optimizations employed a local density approximation (LDA) exchange-correlation functional, while the Perdew-Burke-Ernzerh (PBE) of the generalized gradient approximation (GGA) was chosen to perform calculations to obtain the electronic structures and accurate formation energies. In these calculations, the cutoff energy of the planewave basis set was 380 eV. The Monkhorst-Pack scheme k-point grid sampling was set as 5 × 5 × 2 for the irreducible Brillouin zone. The Pulay density mixing method was used in the computations of self-consistent field, and the self-consistent accuracy was set to the degree that every atomic energy converges to 2.0 × 10-6 eV. The force on every atom was smaller than 0.05 eV/nm. We calculated the total energy and electronic structures in the supercell under these conditions.
Results and discussion
Optimized structural parameters for anatase TiO 2 compared with experimental and previous theoretical results
For TiO2 doped with V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, and Ag, considering the underestimation of the calculations, the band gaps of the transition metal-doped anatase TiO2 are corrected by scissors operator. Scissors operator is used for a purpose as correction to the band gap, which has a clear separation between the CB and VB. For these calculations, the scissors operator is set at 1.02 eV, accounting for the difference between the experimental band gap (3.23 eV) and the calculated band gap (2.21 eV) for pure anatase TiO2. Then, the band gaps of TiO2 doped with V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, and Ag, are determined as 2.84, 3.26, 3.35, 2.86, 2.80, 3.25, 3.20, 2.69, 3.15, 3.25, 3.33, 2.96, and 3.20 eV, respectively. It should be noted that the band gap of transition metal-doped TiO2 is not related to the band gap between the Ti t2g (d xy , d xz , d yz ) and eg (, ) bands, but to the energy separation between the O 2p and the Ti t2g bands of TiO2 that is modified by doping atoms.
In comparison with pure TiO2, the calculation results of the electronic structures of Ti7MO16 can be classified into six groups according to the position of the IELs in Figures 3 and 4: (1) Ti7VO16 and Ti7MoO16; (2) Ti7CrO16; (3) Ti7MnO16, Ti7FeO16, Ti7CoO16, Ti7NiO16, and Ti7AgO16; (4) Ti7CuO16; (5) Ti7ZnO16 and Ti7YO16; and (6) Ti7ZrO16 and Ti7NbO16.
Ti7VO16 and Ti7MoO16. The IELs are located at the bottom of the CB and mixed with the Ti 3d states to form a new CBM, which leads to an obvious band gap narrowing. The position of the IELs might result in a red shift, which gives an explanation of the experimental optical absorption spectra of V-doped TiO2. The positions of the IELs in the Mo-doped system in Figure 4 are similar to those in V-doped TiO2, which may also result in red shift of absorption spectra in experiments.
Ti7CrO16. The IELs are located below the CBM with a small distance. For Cr-doped TiO2, the IELs act as a shallow donor, and their occurrence is mainly due to the Cr 3d states that lie at the bottom of CB as shown in Figure 3. As the EF crosses it, it is partially filled with electrons at the ground state. In this case, the optical transitions are expected to be two transitions. One is the acceptor transition from the VBM to the IELs. The other is a donor transition from the IELs into the CBM. Meanwhile, VB holes and CB electrons appear. The former contributes to the anodic photocurrent, and the latter contributes to the cathodic photocurrent under visible light. Then, the Cr-doped system can serve as a remarkably better photocatalyst.
Ti7MnO16, Ti7FeO16, Ti7CoO16, Ti7NiO16, and Ti7AgO16. The IELs occur in the middle of the band gap, namely the intermediate level. They may reduce the energy required for electron transition, lower the threshold of photoexcitation, and thus expand the optical absorption spectrum without reducing the energy of electrons or holes. The electrons in the VB can be excited to the IELs and then subsequently excited to the CB by the visible light irradiation. So, IELs are beneficial for extending the sensitive light wavelength. The result gives a good explanation of the red shift [31–34]. However, for these kinds of IELs, high impurity doping concentration might form a recombination center for photoexcited electron–hole pairs and results in a decrease in the quantum yield for the photocatalytic reactions . Therefore, we must control the doping concentration to avoid them to act as the recombination center of photo-generated electrons and holes.
Ti7CuO16. The IELs are located above the VB and partially overlap with the VBM. These kinds of IELs could act as trap centers for photoexcited holes, which can also reduce the recombination rate of charge carriers . The holes generated in the VB produce an anodic photocurrent. Because the Cu t2g level is close to the VB, the holes easily overlap in highly impure media .
Ti7ZnO16 and Ti7YO16. The IELs are located at the top of the VB and completely mixed with the O 2p states to form a new VBM (seen in Figures 3, 4, and 5). The band gaps of Zn- and Y-doped anatase TiO2 are narrowed to 2.69 and 3.15 eV, respectively, and smaller than that of pure TiO2, which is consistent with the experimental data on the red shift of the absorption edge [35, 36].
Ti7ZrO16, Ti7NbO16. The IELs are not situated at band gap. The electronic structure of Zr-doped TiO2 exhibits similar to that of pure TiO2. Therefore, we can infer that the t2g level due to Zr does not contribute to the photo-response. Similarly, the band gap of Nb-doped anatase TiO2 is larger than that of undoped TiO2 by 0.09 eV, which may result in a blue shift of the absorption edge.
where is the energy of the most stable oxide for doping atoms at room temperature.
Impurity formation energies of 3 d and 4 d transition metal-doped TiO 2 supercells under O-rich condition
Metal doping system
Band edge position
Transition metal-doped TiO2 has been studied using first-principles density functional theory. The calculated results show that owing to the formation of the impurity energy levels, which is mainly hybridized by 3d or 4d states of impurities with O 2p states or Ti 3d states, the response region in spectra could be extended to the visible light region. The position of the impurity energy levels in the band gap determines the effects of metal doping on the photocatalytic performance of TiO2. Most transition metal doping could narrow the band gap of TiO2, lead to the improvement of the photoreactivity of TiO2, and simultaneously maintain strong redox potential. Under O-rich growth condition, formation energies of anatase TiO2 doped with various metals are different. Particularly, the formation energies of TiO2 doped with Cr, Co, and Ni are found to be negative, showing that it is energetically more favorable to substitute Co, Ni, or Cr to a Ti site than other metals. These doping systems can be easily obtained and with good stability.
Theoretical research on transition metal-doped TiO2 is of great importance to develop the photocatalytic applications. First-principles calculation of doped TiO2 is still an ongoing subject, and a few challenging problems require further investigation in an urgent demand. One is the influence of the transition metal doping on the phase transition of TiO2 from anatase to rutile. A theoretical understanding on its mechanism will be useful to optimize the performance of TiO2 in photocatalytic and other applications. Another one is the question about using the virtual crystal approximation method to calculate the doping system for very low concentration, which can cut down the calculation time. With the solution of these problems, one could provide more accurate theoretical models to simulate the practical doping approaches which could lead to important implications in the optimization of the performance of transition metal-doped TiO2 photocatalysts.
This work was supported by the National Nature Science Foundation of China (51162007 and 51202050), Hainan Natural Science Foundation (511110), and Tsinghua University Initiative Scientific Research Program.
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