for Enhanced Photoelectrochemical Water Splitting - American

Mar 21, 2014 - Hefei, Anhui 230026, China. ABSTRACT: Doping with anion and cation impurities is an effective approach to tune the photoelectrochemical...
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Band Structure Tuning of TiO2 for Enhanced Photoelectrochemical Water Splitting Jiajun Wang,† Haifeng Sun,† Jing Huang,†,‡ Qunxiang Li,*,† and Jinlong Yang†,§ †

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China ‡ School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei, Anhui 230022, China § Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ABSTRACT: Doping with anion and cation impurities is an effective approach to tune the photoelectrochemical properties of TiO2. Here, we explore the Rh monodoping and (Rh + F) codoping effect on electronic structures and photocatalytic activities of anatase TiO2 by performing extensive density functional theory calculations. Upon Rh monodoping, the band gap of TiO2 can be effectively reduced. But this cationic dopant creates an unoccupied intermediate localized state within the band gap, which will act as photogenerated carrier recombination center, which reduces the photocatalytic efficiency. Fortunately, we find that the stable charge-compensated donor−acceptor pair (Rh + F) codoping in TiO2 can effectively reduce the band gap by forming a delocalized intermediate band within the band gap. Moreover, the band edge alignment in the (Rh + F) codoped TiO2 is desirable for water splitting. The calculated optical absorption curve of (Rh + F) codoped TiO2 verifies that it has significantly improved visible light absorption. These findings imply that the (Rh + F) codoped TiO2 is a promising visible light photocatalyst for water splitting.

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INTRODUCTION Over the past several decades, oxide semiconductor photocatalysts have attracted much research attention due to their promising applications in the efficient conversion of sunlight to environmentally friendly and renewable energy and the treatment of environmental pollution.1,2 It is well-known that titanium dioxide (TiO2) is a promising photocatalyst for water splitting and hydrogen production because of its high photocatalytic activity, resistance to photocorrosion, low cost, and nontoxicity.3−7 However, the photoreaction efficiency of TiO2 is severely limited by its large intrinsic band gap (e.g., for the anatase phase, 3.20 eV8) capable of absorbing only the ultraviolet portion of the solar spectrum.9,10 To enhance the solar energy conversion efficiency, an effective approach is to reduce the band gap of TiO2 below 2.0 eV; then it can absorb the more abundant visible light.11−13 Since the demonstration of water splitting under visible light by Fujishima and Honda,3 numerous attempts have been made to effectively tune the band gap of TiO2 by different doping schemes.9,11−16 Previous investigations have suggested that doping with elements including nonmetals9,14 or transition metals17,18 would be a promising way to modify the band edges and enhance the photocatalytic activity of TiO2 under visible light. However, the photoelectrochemical efficiency of these nonmetal-doped TiO2 materials is still limited by the relative high recombination rate of the photogenerated electron−hole pairs, accounting for the loss of the electron−hole pairs in experiments.19,20 Moreover, the transition metal doping could © 2014 American Chemical Society

reduce the carrier mobility by the formation of strongly localized d states within the band gap. In addition, the doped ions act as the recombination centers of the photogenerated charge carriers.21 In order to avoid these problems, several recent reports have demonstrated that the use of donor− acceptor codoping can effectively improve the solubility of dopants and semiconducting material quality, separate the photogenerated carriers, and thus enhance the photocatalytic hydrogen production efficiency of TiO2.11−13 Now it is wellknown that the dopants in TiO2 need to satisfy the follow four criteria: (i) the soluble impurities can effectively reduce the energy gap of the host and then enhance optical absorption, (ii) the position of conduction band minimum (CBM) cannot be lowered by the dopants, (iii) the dopants are able to shed the photoexcited electrons transferring to the CBM of the host, and (iv) the impurities can act as active sites for H2 evolution. Note that the photocatalytic activity strongly depends on the cocatalyst (such as Pt, Pd, Rh, Ru, Ir) and the loading method or process. In general, photocatalysts without cocatalyst have poor photocatalytic activity since the cocatalysts can introduce active sites and promote charge separation.22,23 Recently, TiO2 photocatalysts modified with molecular metal oxide species and the noble metals as cocatalyst have been extensively reported. By contrast, theoretical investigations on doping mechanism of Received: January 15, 2014 Revised: March 20, 2014 Published: March 21, 2014 7451

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noble metals are limited so far. Recently, several experimental attempts have been made to dope SrTiO3 and TiO2 with Rh atoms to enhance the photocatalytic activity.24,25 The doped Rh atoms are used to tune the concentration of electrons and/or to improve the visible light response. Okamoto et al. found that the Rh-doped nanosheets exhibit high photocatalytic activity and the RhO6 units in the nanosheets probably act as reaction sites for H2 evolution.26 Motivated by these experiments, here we present extensive density functional theory (DFT) calculations to explore the Rh-monodoping and (Rh + F)codoping effect on the electronic structure and photocatalytic activities of anatase TiO2. Our results clearly reveal that (Rh + F) codoping not only can effectively narrow the band gap by introducing an intermediate delocalized band but also can create the ideal band edge alignment for photoelectrochemical water splitting. These findings indicate that (Rh + F) codoped anatase TiO2 is a promising visible-light photocatalyst.

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COMPUTATIONAL METHODS AND MODEL Our extensive first-principles calculations are conducting by using the Vienna ab initio simulation package (VASP).27,28 The interaction between the core and valence electrons is described using the frozen-core projector augmented wave (PAW) approach.29 The PAW potentials with the 4s and 3d valence states for Ti atoms, 2s and 2p for O and F atoms, and 4d and 5s for Rh atoms are employed. All geometric optimizations are carried out with the Perdew−Burke−Ernzerhof (PBE) functional,30 and all calculations of the presented electronic structures are performed with the Heyd−Scuseria−Ernzerhof (HSE06)31,32 hybrid functional. The Monkhorst−Pack mesh33 of 2 × 2 × 2 and 3 × 3 × 3 k-points are used to sample the Brillouin zone for geometry optimizations and electronic structure calculations, respectively. In our calculations, a plane-wave cutoff energy of 520 eV is used, all atomic positions are relaxed at the PBE level until the atomic forces are less than 0.01 eV/Å, and the tolerance for energy convergence was set to 10−5 eV. To explore the optical properties, the optical absorption spectra are simulated by converting the complex dielectric function to the absorption coefficient αabs according to the relation34 αabs =

Figure 1. (a) Computational model (a 3 × 3 × 1 supercell) for the doped anatase TiO2. The gray and red spheres stand for Ti and O atoms, respectively. The color labeled on Ti and O atoms are used to illustrate the doped sites. (b) Calculated total and partial DOS of pure anatase TiO2 at the HSE06 level. Here, the vertical dashed line stands for the Fermi level.

small. The neighboring Rh−O and Ti−F distances are 1.97 and 2.02 Å, respectively, while Rh−F distances is 2.06 Å.

2 ω( ε12(ω) + ε2 2(ω) − ε1(ω))1/2

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RESULTS AND DISCUSSION We start with pristine anatase TiO2. The HSE06 calculated total and partial density of states (DOS) of pure anatase TiO2 are plotted in Figure 1b. Clearly, similar to pervious results,11,12 we find that the valence band (VB) edge is mainly contributed by the O 2p states while the conduction band (CB) edge originates from Ti 3d states. The VB edge is deep enough (by 1.67 eV) relative to the water oxidation potential. Here, the calculated band gap is about 3.37 eV, which slightly overestimates experiment by 0.17 eV.8 Previous investigations9,11−16 have suggested that anions can tune the VB edge due to their different atomic p orbital energies compare with the O p orbitals, while cations can modify the CB edge due to their different atomic d orbital energies comparing with Ti d states. Moreover, the compensated and noncompensated n−p codopings have been adopted to enhance the visible-light photoactivity of anatase TiO2 by reducing its intrinsic band gap.11,12 In what follows, we try to explore the cation and anion monodoping and cation−anion codoping effect on the

where ε1(ω) and ε2(ω) are the real and imaginary parts of frequency-dependent complex dielectric function ε(ω), respectively. Taking into account the tensor nature of the dielectric function, ε1(ω) and ε2(ω) are averaged over three polarization vectors (along x, y, and z directions). To examine these doped anatase TiO2 systems, a 3 × 3 × 1 supercell with 108 atoms (36 Ti and 72 O atoms) is adopted, as shown in Figure 1a. Here, in our calculations, the doped Rh and F atoms substitute Ti and O sites in TiO2 labeled with blue and green colors, respectively. The PBE optimized lattice parameters of pure TiO2 are a = 3.82 Å and c = 9.69 Å, and two kinds of Ti−O separation are about 1.96 and 2.0 Å, which are close to the previous experimental and theoretical results.35−37 As for the Rh or F atom monodoping cases, the neighboring O or Ti atoms slightly distort around the doping positions. Compared to the Ti−O distance in pure TiO2, the averaged Rh−O bond length (1.99 Å) just changes slightly, while the averaged Ti−F bond is 2.06 Å (longer about 5.0%). As for the codoping case, the lattice constants changes rather 7452

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state of Rh dopant can be formally described as Rh3+;25 namely, TiO2 shows the net p-type character upon Rh monodoping. Upon Rh monodoping, unoccupied localized states are created in the band gap of the Rh-doped TiO2 system. These unoccupied localized states reduce the carrier mobility and may also act as recombination centers for photogenerated electron− hole pairs and thus reduce the photoelectrochemical efficiency.21 In this sense, Rh-doped TiO2 should not have high photocatalytic activity. Fortunately, the anion−cation codoping is an effective method to tune the band-edge of wideband-gap semiconductors including TiO2.11,12 Now we turn to examine codoped TiO2. Following the previous investigation,11 we substitute the O atom (labeled with green color in Figure 1a) connecting to Rh atom with one F atom to introduce charge compensated donor−acceptor pair (Rh + F) in TiO2, in which the donor electrons exactly passivate the acceptor holes, and so the semiconductor character can kept in the compensated codoped systems. After (Rh + F) codoping in TiO2, the corresponding ground state is spin-restricted and the observed magnetism in Rh-doped TiO2 is quenched. The HSE06 calculated total DOS (Rh + F) codoped TiO2 and partial DOS of O, Ti, F, and Rh atoms are shown in parts a, b, c, d, and e of Figure 3, respectively. It is clear that the (Rh + F) codoping leads to a significantly narrowed band gap of 2.31 eV, which is a desirable band gap for efficient visible light

electronic structures and photocatalytic activities of anatase TiO2. First Rh-doped TiO2 is considered. One of the Ti atoms (labeled with blue color) in a 3 × 3 × 1 supercell of TiO2, as shown in Figure 1a, is replaced by one Rh atom, which corresponds to a doping concentration of 2.80%. Through the total energies calculations, we find that the ground state of Rhdoped TiO2 is spin-polarized, and the magnetic moment of Rhdoped TiO2 is 1.0 μB per supercell. The spin-resolved total DOS of Rh-doped TiO2 and partial DOS of O, Ti, and Rh atoms are plotted in parts a, b, c, and d of Figure 2, respectively.

Figure 2. HSE06 calculated total DOS and partial DOS of Rh-doped TiO2: (a) total DOS; (b, c, d) partial DOS of O, Ti, and Rh atoms, respectively. Here, the red vertical dashed line represents the Fermi level, and the black line stands for the total DOS of pure anatase TiO2 for clarity.

Clearly, compared with the pure anatase TiO2, the Rh dopant almost does not affect the CBM, while the substitution of one Rh for Ti atom introduces some impurity states. These impurity states at the valence band maximum (VBM) primarily consist of Rh 4d and O 2p orbitals. A narrow and small DOS peak at approximately 2.0 eV appears in the band gap; this empty intermediate state is mainly contributed by Rh 4d localized orbitals. This peak results in a significantly narrowed band gap of Rh-doped TiO2 (about 2.05 eV). This feature is similar to the previous noncompensated n−p codoped TiO2.12 As seen in Figure 2b−d, the filling differences between two spin channels of the partial DOS for Ti and O atoms are very small, while the spin-down states of Rh atom are filled more than the spin-up states, which gives the main contribution to the predicted magnetic moment. Actually, the chemical valence

Figure 3. (a) Calculated total DOS of (Rh + F) codoped TiO2. (b, c, d, e) Partial DOS of O, Ti, F, and Rh atoms in the codoped system, respectively. Here, the red vertical dashed line represents the Fermi level, and the black line stands for the total DOS of pure anatase TiO2 for clarity. (f) Charge density distribution of the VB, IB, and CB states of (Rh + F) codoped TiO2. 7453

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in TiO2 with F doping concentration of 1.4%, which is not suitable for H2 production. Note that the F-doped TiO2 has strong oxidation ability due to the almost unchanged VBM. (c) For Rh-doped TiO2, the VBM shifts upward by 0.63 eV compared with pure TiO2, but its water oxidation ability is still good. The CBM slightly shifts (about 0.02 eV) with respect to that of pure TiO2. The main feature is that a localized IB appears within the band gap. The IB localized nature leads to low photoelectrochemical efficiency. (d) For the (Rh + F) codoping case, there is an intermediate band in the band gap. We find that TiO2 with (Rh + F) codoping has the highest merit for the photoelectrochemical water splitting because it not only is the band gap narrowed (reducing 1.06 eV from 3.37 eV for pure TiO2), which is ideal for absorbing visible light, but also it just slightly perturbs the position of both VBM and CBM. Note that the (Rh + F) codoped TiO2 under visible light is thermodynamically feasible for the water reduction and oxidation. As we know that there are three steps in the water-splitting reaction on a semiconductor photocatalyst surface: (1) When the absorption photon energy is greater than the band gap of the photocatalyst, the electron−hole pairs in the bulk are photogenerated. (2) These photoexcited electron−hole pairs separate and migrate to the surface without recombination. (3) Then the photogenerated electrons and holes reduce and oxidize the adsorbed H2O molecules into H2 and O2, respectively.43 Since the electrons jumping from VB to CB via the intermediate states depend on its localized or delocalized nature, we plot the electron density distributions of the VB, IB, and CB states of (Rh + F) codoped TiO2 in Figure 3f. Clearly, the VB and CB states averagely disperse in regions of nonbonding Ti 3d and O 2p orbitals, respectively, and both VB and CB near the band edges are highly delocalized. The observation also verifies that (Rh + F) codoping does not obviously impact on the VB and CB of pure TiO2, which is consistent with the calculated DOS result, as shown in Figure 3a. This implies that the carrier mobility in the VBs and CBs bands is almost unperturbed. For the intermediate bands, as shown in Figure 3f, is mainly contributed by Rh 4d and O 2p orbitals. The hybridization of O 2p and Ti 3d with Rh 4d orbitals delocalizes the intermediate states within the band gap. So, as stated by Zhang et al.,12,44 the delocalized nature indicates that these intermediate states play an important role as stepping stones to bridge the VB top and the CB bottom. Electrons excited by the visible region light can quickly jump from the VB to the CB via these intermediate states. Figure 5 demonstrates the HSE06 calculated optical spectra of pure and (Rh + F) codoped TiO2. Clearly, a steep absorption edge around 360 nm in the UV region appears in pure TiO2 system. This finding agrees well with previous reports.45−47 In comparison, a red-shifted absorption onset in optical spectra extending into the visible region is observed in (Rh + F) codoped TiO2. That is to say, the band position alignment in (Rh + F) codoped TiO2 is highly required for the visible light photocatalysis, as shown in Figure 4. Clearly, the (Rh + F) codoped system should be a promising visible light photocatalysis, since it can effectively harvest the visible light spectrum as compared to the pristine TiO2. In search for the optimal growth conditions in experiments, we calculate the formation energies (Eform) of Rh and F monodoping and (Rh + F) codoping, which provides useful information to evaluate the relative difficulty for the

absorption, and an intermediate band (IB) appears in the intrinsic band gap. According to the calculated partial DOS, we find that (Rh + F) codoping just slightly perturbs the position of both VBM and CBM. The VBM is mainly composed of admixture of the O 2p and Ti 3d states, while the CBM is dominated by Ti 3d orbitals. The IB locating just below the Fermi level is mainly composed of O 2p orbital, Rh 4d orbitals, and somewhat F 2p and Ti 3d orbitals. Because of this strong hybridization, the IB is substantially broadened, and its width is about 0.40 eV. This result is different from the Rh monodoped system, which has a localized impurity state within the band gap. In general, the band energy positions of a semiconductor with respect to the redox potential of the adsorbed species on its surface dominate the photoinduced electron transfer between the semiconductor and the adorbates.38,39 Thermodynamically, to donate an electron to the vacant hole, the valence band potential of the semiconductor needs to be below the potential level of the donor dopants, while the conduction band potential of the semiconductor should be above the relevant potential level of the acceptor species.40,41 To evaluate the doping effect on the photocatalytic activity of TiO2, we illustrate the alignment of the redox potential (including the reduction and oxidation potentials) of water with respect to the band edges of Rh/F monodoped and (Rh + F) codoped TiO2 systems in Figure 4. Here, the values of the VB edge and CB

Figure 4. Band edge alignment of the pure and doped TiO2 systems with respect to the water reduction and oxidation potentials. Here, the gray and light blue regions stand for the CBs and VBs, while the localized and delocalized intermediate bands are labeled with the narrow and broad red lines, respectively.

edge positions of the pure TiO2 with respect to the normal hydrogen electrode (NHE) potential are taken from the experimental values.42 For the doped TiO2 systems, the CB edge positions are obtained from the band plots according to the relative positions as compared with that of pure anatase TiO2, and the VB edge positions are deduced from the absolute band gap values. We observe the following main features: (a) TiO2 has a strong reducing ability ascribed to its relative higher CBM (about 0.3 eV lower than the H+/H2 potential) as compared to the NHE potential, while its VBM is more positive (about 1.8 eV) than the O2/H2O potential. (b) Compared with the pure TiO2, the CB edge moves downward more than 0.3 eV 7454

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of noble metal substitutional impurities can be easily realized under the O-rich condition. As for the cation−anion codoped system, the formation energies are positive under the O-poor condition, meaning that (Rh + F) codoping are endothermic reactions, and thus they are relative difficult to achieve in experiments under the O-poor condition. However, the formation energy of (Rh + F) codoped TiO2 system under the O-rich condition is predicted to be about −2.60 eV. This value is obviously less than that of doping with cation or anion alone. This (Rh + F) codoping reduced formation energy suggests that this kind of cation−anion codoping could be easily realized due to the electrostatic attraction of two doped impurities with opposite charge states. To check this cation−anion pair is stable or not in TiO2, we calculate the defect pair binding energy (Eb), which is defined as Eb = EXTi + EYO − EXTi+YO − Epure, where E is the calculated total energy of the indicated systems using the same supercell. The positive Eb implies that the defect pair prefers to bind to each other when both cation and anion impurities are codoped in sample. The binding energy (Eb) for the (Rh + F) pair is predicted to be 0.87 eV, as list in Table 1. This relative large defect pair binding energy originates from the charge transfer from donor dopant to acceptor dopant, which is associated with the strong Coulomb interaction between positively charged donor and negatively charged acceptor. Anyway, this observation indicates that the (Rh + F) defect pair is more stable than the doped isolated Rh and F impurities in TiO2.

Figure 5. Calculated optical absorption spectra for pure and (Rh + F) codoped anatase TiO2 systems.

incorporation of these doped elements into the host lattice in experiments.12 Actually, Eform is also strongly related to the relative stability of doped TiO2 systems. The formation energy of substitutional dopants of Rh and F atoms and (Rh + F) codoping in TiO2 systems is defined as Eform = EXTi − Epure − μTi + μX, Eform = EYO − Epure − μO + μY, and Eform = EXTi+YO − Epure − μTi − μO + μX + μY, respectively. Here, Epure, EXTi, EYO, and EXTi+YO are the total energy of pure, Rh and F monodoped, and (Rh + F) codoped TiO2 systems, while the μTi, μO, μRh, and μF denote the chemical potentials of the Ti, O, Rh, and F atoms, respectively. The formation energies for these doped systems vary as a function of the oxygen chemical potential, which stands for the oxygen environment during experimental synthesis. The low and high values of μO correspond to the Opoor and O-rich growth conditions, respectively. For pure TiO2, the μTi and μO satisfy the following relationships: μTi + 2μO = μTiO2, μO ≤ μO2/2, and μTi ≤ μTiMetal. In our calculations, the μO in the O-rich growth condition is simulated by the binding energy of an O2 molecule, while μTi is determined by μTiO2 − 2μO. In the O-poor growth condition, μTi amounts to the energy of one Ti atom in Ti bulk and the μO is calculated from μO = μTiO2 − 2 μTi. The calculated formation energies for these examined doped TiO2 systems are summarized in Table 1. The formation energies for one F atom substituting O atom

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CONCLUSIONS In conclusion, we examine the Rh monodoping and (Rh + F) codoping effect on the electronic structures and photocatalytic activities of anatase TiO2 at the HSE06 hybrid functional level. For Rh-doped TiO2, the band gap can be effectively reduced, and the Rh cationic dopant creates an unoccupied impurity band, which will act as a recombination center. Fortunately, we find that the stable (Rh + F) codoping can not only effectively reduce the band gap by forming the delocalized intermediate bands within the band gap but also lead to the nice band edge alignment. The corresponding calculated optical absorption curve of (Rh + F) codoped TiO2 verifies that this codoped system has the improved visible-light absorption. These findings suggest that the (Rh + F) codoped TiO2 is a promising visible-light photocatalyst for water splitting.

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Table 1. Formation Energies of the Substitutional Rh and F Dopants in TiO2 in the O-Rich and O-Poor Conditions O‑poor (EO‑rich form and Eform in eV, Respectively); Defect Pair Binding Energies (Eb, in eV) of (Rh + F) Codoped TiO2 System dopants

EO‑rich form (eV)

EO‑poor form (eV)

F Rh (Rh + F)

−0.74 −1.00 −2.60

−5.18 7.88 1.84

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-551-63607125; Fax +86551-63603748 (Q.L.). Notes

The authors declare no competing financial interest.

Eb (eV)

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ACKNOWLEDGMENTS We thank Professor Zhenyu Zhang for a helpful discussion. This work was partially supported by National Key Basic Research Program (Nos. 2011CB921400 and 2014CB921101), by the Strategic Priority Research Program (B) of the CAS (No. XDB01020000), by the National Natural Science Foundation of China (Nos. 21273208, 21121003, 20903003, and 11034006), by the Fundamental Research Funds for the Central Universities, by China Postdoctoral Science Foundation (No. 2012M511409), and by the SCCAS, Shanghai and USTC Supercomputer Centers.

0.87

in anatase TiO2 under the O-poor and O-rich conditions are −5.18 and −0.74 eV, respectively. The negative value of the formation energy indicates that F monodoping in TiO2 is relative easy to achieve under both O-poor or O-rich conditions. In contrast, the calculated formation energies for the substitutions of Ti by Rh atom under the O-rich condition are −1.00 eV, while the corresponding value is 7.88 eV under the O-poor condition. This result indicates that the formation 7455

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