Recent theory has found that native defects such as the O vacancy $V_{\mathrm{O}}$ and Zn interstitial ${\mathrm{Zn}}_I$ have high formation energies in $n$-type ZnO and, thus, are not important donors, especially in comparison to impurities such as H. In contrast, we use both theory and experiment to show that, under N ambient, the complex ${\mathrm{Zn}}_I\mathrm{\text{−}}{\mathrm{N}}_{\mathrm{O}}$ is a stronger candidate than H or any other known impurity for a 30 meV donor commonly found in bulk ZnO grown from the vapor phase. Since the Zn vacancy is also the dominant acceptor in such material, we must conclude that native defects are important donors and acceptors in ZnO.

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 SrTiO_3−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 SrTiO_3−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 SrTiO_3−δ leads to the absence of oxygen evolution which therefore results in the oxidation of SrTiO_3−δ (Ti³⁺ → Ti⁴⁺).

First-principles density-functional theory is employed to investigate the role of hydrogen impurities in ${\text{In}}_2{\text{O}}_3$. We find that both interstitial hydrogen $\left({\text{H}}_i\right)$ and substitutional hydrogen $\left({\text{H}}_{\text{O}}\right)$ act as shallow donors. Our results support recent experiments by Koida et al. [Jpn. J. Appl. Phys. 46, L685 (2007)], which found hydrogen-doped ${\text{In}}_2{\text{O}}_3$ to be a good candidate for transparent conducting films.

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.