To improve the photoelectrochemical activity of ${\mathrm{TiO}}_2$ for hydrogen production through water splitting, the band edges of ${\mathrm{TiO}}_2$ should be tailored to match with visible light absorption and the hydrogen or oxygen production levels. By analyzing the band structure of ${\mathrm{TiO}}_2$ and the chemical potentials of the dopants, we propose that the band edges of ${\mathrm{TiO}}_2$ can be modified by passivated codopants such as ($\mathrm{Mo}+\mathrm{C}$) to shift the valence band edge up significantly, while leaving the conduction band edge almost unchanged, thus satisfying the stringent requirements. The design principle for the band-edge modification should be applicable to other wide-band-gap semiconductors.

Using first-principles calculations, we investigate the adsorption of various gas molecules (H₂, O₂, H₂O, NH₃, NO, NO₂, and CO) on monolayer MoS₂. The most stable adsorption configuration, adsorption energy, and charge transfer are obtained. It is shown that all the molecules are weakly adsorbed on the monolayer MoS₂ surface and act as charge acceptors for the monolayer, except NH₃ which is found to be a charge donor. Furthermore, we show that charge transfer between the adsorbed molecule and MoS₂ can be significantly modulated by a perpendicular electric field. Our theoretical results are consistent with the recent experiments and suggest MoS₂ as a potential material for gas sensing application.

The photoelectrical properties of multilayer WS₂ nanoflakes including field-effect, photosensitive and gas sensing are comprehensively and systematically studied. The transistors perform an n-type behavior with electron mobility of 12 cm²/Vs and exhibit high photosensitive characteristics with response time (τ) of <20 ms, photo-responsivity (R_λ) of 5.7 A/W and external quantum efficiency (EQE) of 1118%. In addition, charge transfer can appear between the multilayer WS₂ nanoflakes and the physical-adsorbed gas molecules, greatly influencing the photoelectrical properties of our devices. The ethanol and NH₃ molecules can serve as electron donors to enhance the R_λ and EQE significantly. Under the NH₃ atmosphere, the maximum R_λ and EQE can even reach 884 A/W and 1.7 × 10⁵%, respectively. This work demonstrates that multilayer WS₂ nanoflakes possess important potential for applications in field-effect transistors, highly sensitive photodetectors, and gas sensors, and it will open new way to develop two-dimensional (2D) WS₂-based optoelectronics.

The origin of ferromagnetism in $d^0$ semiconductors is studied using first-principles methods with ZnO as a prototype material. We show that the presence of spontaneous magnetization in nitrides and oxides with sufficient holes is an intrinsic property of these first-row $d^0$ semiconductors and can be attributed to the localized nature of the $2p$ states of O and N. We find that acceptor doping, especially doping at the anion site, can enhance the ferromagnetism with much smaller threshold hole concentrations. The quantum confinement effect also reduces the critical hole concentration to induce ferromagnetism in ZnO nanowires. The characteristic nonmonotonic spin couplings in these systems are explained in terms of the band coupling model.