Pakpoom Reunchan

Recent advances in TiO2-based photocatalysis

Semiconductor photocatalysis is a promising approach to combat both environmental pollution and the global energy shortage. Advanced TiO2-based photocatalysts with novel photoelectronic properties are benchmark materials that have been pursued for their high solar-energy conversion efficiency. In general, the photocatalytic efficiency is affected by the degree of light absorption, charge separation, and surface reactivity. Consequently, in this review we first discuss a series of interesting studies that aim to extend the light absorption of TiO2 from UV wavelengths into the visible or even the near-infrared region. We next focus on attempts to overcome the drawback that dopants usually act as charge recombination centres. We discuss the use of either selective local doping or the introduction of disorder together with doping, which aims to facilitate charge separation while preserving the visible-light response. We also show that crystal facet engineering can endow TiO2 with superior physicochemical properties, thus yielding high surface reactivity in photocatalytic reactions. Finally, we examine the recent theoretical advances of TiO2-based photocatalysis.

Self-doped SrTiO3−δ photocatalyst with enhanced activity for artificial photosynthesis under visible light

Self-doped SrTiO3−δ was prepared through a carbon-free one-step combustion method followed by a series of heat treatments in Ar at temperatures ranging from 1200 to 1400 °C. X-Ray Photoelectron Spectroscopy (XPS), Electron Paramagnetic Resonance (EPR) and High-Resolution TEM confirm the presence of Ti3+ in samples with oxygen vacancy accommodated in perovskite by forming Ruddlesden–Popper crystallographic shears. The UV-vis spectra and electronic structure calculations show that the oxygen vacancy and Ti3+ together induce an in-gap band to enhance the visible light absorption. Pulsed Adsorption of CO2 and Temperature Programmed Desorption (TPD) experiments show that the higher oxygen deficiency tends to improve the chemical adsorption of CO2 on the surface as well as in the bulk of SrTiO3−δ, especially the accommodation of CO2 molecule in the oxygen vacancy. It is the synergetic effect of visible light absorption and chemical adsorption of CO2 that improves the artificial photosynthesis to generate hydrocarbon fuels from CO2/H2O under visible light irradiation. We also demonstrated that the incorporation of oxygen from CO2/H2O into the oxygen vacancy of SrTiO3−δ leads to the absence of oxygen evolution which therefore results in the oxidation of SrTiO3−δ (Ti3+ → Ti4+).

Theoretical design of highly active SrTiO3-based photocatalysts by a codoping scheme towards solar energy utilization for hydrogen production

SrTiO3 is a promising photocatalyst for the production of hydrogen from water splitting under solar light. Cr doping is an effective treatment for adjusting its absorption edge to the visible-light range, although the performance of Cr-doped SrTiO3 is strongly affected by the oxidation number of the Cr ions. In this study, we theoretically predict that elevating the Fermi level, i.e., n-type carrier doping in SrTiO3, can stabilize the desirable oxidation number of chromium (Cr3+), contributing to a higher activity for H2 evolution. Our computational results, based on hybrid density-functional calculations, reveal that such an n-type condition is realized by substituting group-V metals (Ta, Sb, and Nb), group-III metals (La and Y), and fluorine atoms for the Ti, Sr, and O sites in SrTiO3, respectively. From our systematic study of the capability of each dopant, we conclude that La is the most effective donor for stabilizing Cr3+. This prediction is successfully evidenced by experiments showing that the La and Cr codoped SrTiO3 dramatically increases the amount of H2 gas evolved from water under visible-light irradiation, which demonstrates that our guiding principle based on Fermi level tuning by the codoping scheme is valid for the design of advanced photocatalysts.

Mechanism of photocatalytic activities in Cr-doped SrTiO3 under visible-light irradiation: an insight from hybrid density-functional calculations

We used hybrid density-functional calculations to clarify the effect of substituting chromium for titanium (Cr(Ti)) on photocatalytic activities of Cr-doped SrTiO(3). A singly negative Cr(Ti)⁻, which is relevant to a lower oxidation state of Cr, is advantageous for the visible light absorption without forming electron trapping centers, while other charge states are inactive for the photocatalytic reaction. Stabilizing the desirable charge state (Cr(Ti)⁻) is feasible by shifting the Fermi level towards the conduction band. Our theory sheds light on the photocatalytic properties of metal-doped semiconductors.

Energetics of native defects in anatase TiO2: a hybrid density functional study

The energetics and electronic structures of native defects in anatase TiO2 are comprehensively studied using hybrid density functional calculations. We demonstrate that oxygen vacancies (VO) and titanium interstitials (Tii) act as shallow donors, and can form at substantial concentrations, giving rise to free electrons with carrier densities from 1011 to 1019 cm-3 under oxygen-rich and oxygen-poor conditions, respectively. The titanium vacancies (VTi), identified as deep acceptors and induced hole carriers, are incapable of fully compensating for the free electrons originating from the donor-type defects at any oxygen chemical potential. Even under extreme oxygen-rich conditions, the Fermi level, which is determined from the charge neutrality condition among charge defects, electron and hole carriers, is located 2.34 eV above the valence band maximum, indicating that p-type conductivity can never be realized under any growth conditions without external doping. This is consistent with common observations of intrinsic n-type conductivity of TiO2. At a typical annealing temperature and under a typical oxygen partial pressure, the carrier concentration is found to be approximately 5 × 1013 cm-3.

Chapter 2:Theoretical Design of PEC Materials

This chapter discusses the computational design of inorganic photoelectrochemical (PEC) materials. The electronic structure of photoanodes and photocathodes significantly affect photoabsorption, carrier transport, and water redox properties. Adjusting the band edge positions with respect to redox potentials is, therefore, an important task for the design of photocatalysts for PEC application. We present our recent attempts on the band edge engineering of semiconductor photocatalysts using density functional theory (DFT) calculations. First, we discuss the effects of doping on visible light absorption as well as introduction of conductive carriers. Second, we demonstrate how the band edge positions are controlled by biaxial strain. Third, we present our studies on an evolutional crystal structure search for predicting novel photofunctional materials. These results demonstrate that modern DFT-based computational materials science is a powerful tool for finding promising PEC materials.