ARTICLE pubs.acs.org/JPCC
Effects of H-, N-, and (H, N)-Doping on the Photocatalytic Activity of TiO2 Hui Pan,*,† Yong-Wei Zhang,† Vivek B. Shenoy,‡ and Huajian Gao‡ † ‡
Institute of High Performance Computing, 1 Fusionopolis Way, Singapore 138632 Division of Engineering, Brown University, 610 barus & Holley, 182 Hope Street, Providence, Rhode Island 02912, United States ABSTRACT: We present a first-principles study on the phase-dependent doping effects on the photocatalytic activity of TiO2. Three TiO2 polymorphs—anatase, rutile, and brookite—doped with H, N, or both of them were systematically investigated to understand the mechanism for the improvement of photocatalytic performance induced by the doping. Our calculations revealed that the doping effects on the electronic properties of TiO2 are phase-dependent. The n-type TiO2 was formed by the introduction of hydrogen interstitial or substitution, although interstitial state is stable for hydrogen in all of the TiO2 structures, revealing that hydrogen is one of important sources for the n-type character of TiO2. All of the bandgaps of TiO2 polymorphs were slightly narrowed by interstitial hydrogen, while only the bandgaps of brookite and rutile TiO2 were narrowed by substitutional hydrogen. Nitrogen also prefers the interstitial state in all of the TiO2 polymorphs. However, the effects of interstitial nitrogen on the electronic properties of TiO2 are phase-dependent: anatase and brookite TiO2 with interstitial nitrogen keep intrinsic and their bandgaps are unchanged, while rutile TiO2 with interstitial nitrogen shows p-type conductivity and bandgap narrowing. Our calculation further predicted that nitrogen substitution is enhanced in the presence of hydrogen interstitial bonded to nitrogen due to the reduced formation energy. The narrowing of the bandgaps, the improved mobility, intrinsic semiconducting characters, and reduced trapping centers in anatase and brookite TiO2 by the (H, N)-codoping result in the improvement of their photocatalytic performance because of the enhancement of absorption in visible light and reduction of recombination centers, while the performance of rutile TiO2 has not been improved because its bandgap is almost unaffected by the codoping.
1. INTRODUCTION Solar energy is considered to be one of the most important candidates for sustainable alternative energy sources because it is abundant, clean, and renewable. A variety of technologies have been developed to take advantage of solar energy, such as photovoltaic cell (PV) and photoelectrochemical cell (PEC). The PV can directly convert the solar energy to electrical energy. The PEC splits water into hydrogen and oxygen, so as to convert the solar energy into chemical energy. Because of the versatile applications of hydrogen and oxygen gases, the PEC has attracted substantial attention. The development of efficient photocatalyst for water splitting should satisfy (1) an optimal band structure for maximal utilization of solar energy,13 (2) efficient photoinduced electronhole separation and high carrier mobility,3 (3) high surface activity and large contacting surface area with the electrolyte,4 and (4) high stability in extreme environments.4 Titanium dioxide (TiO2) has been widely used in pigment, photocatalyst, photovoltaic materials, gas sensor, electrical circuit varistor, biocompatible material for bone implants, and spacer material for magnetic spin valve systems.13,5,6 TiO2, as a photocatalyst, has attracted extensive interest since the discovery of water splitting property on the TiO2 electrodes under ultraviolet (UV) illumination because of its corrosion resistance and high surface activity.6 However, its efficiency of water splitting is limited by its wide bandgap (∼3.0 eV) because the UV irradiation only counts for 5% of the solar spectrum. To reduce the bandgap and improve the efficiency, considerable effort has been r 2011 American Chemical Society
carried out, such as doping714 and defect engineering.1521 Compared to the metal doping, nonmetal doping, especially nitrogen doping, has been extensively investigated experimentally.2,8,2226 Theoretically, first-principles calculations based on density functional theory have been used to reveal the origin of the visible-light absorption of N-doped TiO2.8,22,2529 The mechanism have been attributed to the N-substitution, substitutional and interstitial N-doping configurations, the localized N 2p levels above the valence band maximum, or the 3d states of Ti3þ below the conduction band.8,22,2530 On the one hand, although N-doped TiO2 has been widely studied experimentally and theoretically, almost all of the studies were focused on the anatase TiO2, and the effects of TiO2 phases on the doping and the corresponding photocatalytic activity have been rarely touched. However, recently, brookite has been shown to be more photochemically active than anatase and rutile phases.15,16,3135 On the other hand, although the conversion efficiency has been improved by the various methods, the effect of hydrogen, which is always present in TiO2 introduced in the synthesis process or by immersing in water, on the photocatalytic activity has seldom been studied.3639 However, as a very active atom, hydrogen exists in all organic and many inorganic materials4042 and exhibits qualitatively different behavior depending on the host.43 Especially Received: March 14, 2011 Revised: May 11, 2011 Published: May 13, 2011 12224
dx.doi.org/10.1021/jp202385q | J. Phys. Chem. C 2011, 115, 12224–12231
The Journal of Physical Chemistry C for metal oxide, hydrogen can bonds equally to all of the surrounding metal atoms by substituting oxygen, which are remarkably strong and lead to n-type conductivity.42,43 Recently, Spahr, et al. reported that the enhanced hydrogen transport in TiO2 played an important role in the applications in hydrogen energy research.44 Most recently, Chen et al. reported that the photocatalytic activity of TiO2 can be greatly enhanced by hydrogenation.45 Therefore, for understanding the photocatalytic activity of doped TiO2 and designing the new photocatalyst, it is essential to investigate the effects of hydrogen and phase-dependent doping on the electronic property of TiO2. In this work, we systematically studied the effects of the doping, including hydrogen, nitrogen, and both of them, in three TiO2 polymorphs on the electronic properties of TiO2 by first-principles calculations. Our calculations predict that hydrogen leads to n-type conductivity in TiO2, regardless of in substitutional or interstitial state. We show that the doping in TiO2 is phase-dependent. We further show that the substitutional energy of nitrogen in TiO2 is reduced with the presence of hydrogen, indicating that the concentration of substitutional N is enhanced by the codoping and the (H, N) pair may coexist in the N-doped TiO2 that is synthesized by wet chemistry methods. The narrowing of the bandgap and reduction of recombination centers result in the improvement of photocatalytic activity of (H, N)-codoped TiO2.
2. CALCULATION METHODS The first-principles calculation based on the density functional theory (DFT)46 and the PerdewBurkeEznerhof generalized gradient approximation (PBE-GGA)47 was carried out to investigate the phase-dependent doping effects on the photocatalytic ability of TiO2. The projector augmented wave (PAW) scheme48,49 as incorporated in the Vienna ab initio simulation package (VASP),50 was used in the study. The Monkhorst and Pack scheme of k-point sampling was used for integration over the first Brillouin zone.51 A 3 3 3 grid for k-point sampling for geometry optimization and an energy cutoff of 450 eV were consistently used in our calculations. The density of states (DOS) have been obtained with a 7 7 7 mesh. Good convergence was obtained with these parameters, and the total energy was converged to 2.0 105 eV/atom. The bulk anatase, brookite, and rutile TiO2 structures (a-TiO2, b-TiO2, and r-TiO2) are modeled with a 3 3 1, 1 2 2, and 2 2 3 supercell containing 36 Ti atoms and 72 O atoms, 32 Ti atoms and 64 O atoms, and 24 Ti atoms and 48 O atoms, respectively. The substitution is modeled by replacing oxygen with hydrogen or nitrogen in the supercell. The interstitial hydrogen or nitrogen is realized by additionally adding them into the TiO2 supercell. The (H, N)-codoping is realized by substituting oxygen with nitrogen, where the interstitial hydrogen is bonded to the nitrogen. The GGAþU method was used to treat 3d electrons of Ti with the Hubbard on-site Coulomb interaction parameter (U-J) to calculate the electronic structures of TiO2 with and without doping. A value of 7 eV for the (U-J) is consistently used in our calculations because the calculated bandgap of anatase TiO2 is very close to the experimental data by using this value. 3. RESULTS AND DISCUSSION 3.1. H-Doped TiO2. There are two possible nonequivalent sites for an interstitial hydrogen atom bonded to oxygen (Figure 1). Interestingly, the hydrogen atom is stable to be perpendicular to
ARTICLE
Figure 1. Orientation of the H atom with respect to a TiOTi plane: (a) perpendicular and (b) parallel.
Figure 2. Local structures around hydrogen interstitials in three TiO2 polymorphs.
the TiOTi plane in all of the three TiO2 polymorphs (Figure 1a). The energies for the perpendicular bonding states (Figure 1a) are lower than those for the parallel bonding states (Figure 1a) by 0.48 and 1.01 eV for a-TiO2 and r-TiO2, respectively. The interstitial hydrogen prefers to bond with the oxygen with a bond-length of 0. 99 Å in all of the TiO2 structures (Table 1). The case on anatase TiO2 agrees very well with refs 36 and 37. However, the difference of the total energies between the two bonding configurations in b-TiO2 is very small or even negligible (
