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ZnO is a widely studied material for several applications, such as a photocatalyst, a working electrode for dye-sensitized solar cells, and for thermoelectric devices. This work studies the effects of an increase in the number of carriers by doping ZnO with Al and Ga. The 6.25 mol% Al-doped ZnO, 6.25 mol% Ga-doped ZnO, and 12.5 mol% (Al, Ga)-co-doped ZnO nanoparticles were prepared using the combustion method. The prepared samples were then characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and UV–visible spectroscopy techniques. Moreover, the density functional theory (DFT) was also employed for computational study of Al and Ga doped ZnO. Optimized crystal structures, density of states (DOS) and band structure of these systems were calculated using Vienna Ab initio Simulation Package code. From this study, Al and Ga are found to play an important role in both the morphology and optical properties of the ZnO: Al and Ga doping can change the band gap and the Fermi level position in the ZnO. The prepared samples were characterized for their thermoelectric properties, and these were also modelled, using BolzTraP code, for ZnO, Al-doped ZnO, Ga-doped ZnO and (Al, Ga)-co-doped ZnO. The Seebeck coefficient, electrical conductivity, relaxation time, electronic thermal conductivity and power factor were all analysed. The experimental and computational results all point in the same direction, indicating that the thermoelectric properties of ZnO change because the semiconductor ZnO transforms into metallic ZnO when doped with Al and Ga. This leads to ZnO showing different thermoelectric properties, particularly Ga-doped ZnO and (Al, Ga)-co doped ZnO: they provide a high electrical conductivity and power factor. Therefore, it is expected that these favorable properties might promote the ZnO to be a potential candidate for improved efficiency thermoelectric devices.

This work presents electronic and thermoelectric properties of ZnO and SrTiO3 in the presence of nitrogen impurity have been investigated by means of the theoretical first-principles calculations. It is found that N-doped ZnO has a smaller band gap compared to the pure counterpart. This material also exhibits the enhanced thermoelectric properties of the positive Seebeck coefficient and higher electrical conductivity per relaxation time. Furthermore, SrTiO3 with nitrogen impurity possess the smaller energy gaps. For thermoelectric properties, ZT of undoped SrTiO3 decreases as temperature increases. Nevertheless, ZT of N-doped SrTiO3 increases with temperature. Therefore, the introduction of nitrogen impurity to ZnO and SrTiO3 is an alternative way to improve their thermoelectric efficiencies.

SrTiO3 (STO) is an attractive material that offers a wide range of technological applications, e.g., ferroelectricity, solar cell and photocatalysis. An application that the STO might be utilized is diluted magnetic semiconductors. Here, we would like to improve magnetic property of the STO by Ti site substitution using Co atoms. In this work, we present the structural, optical and magnetic properties of perfect and oxygen defect structures of STO and Co-doped SrTiO3 via experimental and theoretical aspects. In first-principles calculation, the structural properties, electronic band structure and magnetic properties of undoped STO and Co-doped STO supercells have been investigated by density functional theory using GGA with Hubbard model scheme (GGA+U) on Vienna Ab initio Simulation Package (VASP). In calculation detail, pure phase of STO with nanometer scale size of undoped STO and Co-doped STO have been synthesized using hydrothermal technique. The findings obtained from DFT computation reveal that the new states in gap between the valence band and conduction band of the STO were induced after Co atom was doped into the host structure. These impurity states narrow the band gap corresponding to experimental results. In addition, band splitting was observed on O defect and dopant systems, indicating that missing O and doping Co on STO could induce magnetization on none-magnetic material of STO. In case of synthesized powder, ferromagnetic behaviors are determined in the dopant system annealed in Ar. Additionally, another appreciated point of Co doping is that surface area of the STO is improved. Thus, it is expected that the surface activity, such as photocatalytic performance, of the STO will be enhanced. From all referred results, they introduce that the Co-doped STO might be a potential candidate to be a photocatalyst for the high photocatalytic performance under visible light radiation and the diluted magnetic semiconductor in spintronic devices.

CO2 conversion to valuable products on ZnO (0001) monolayer doped by transition metals (TM-ZnO where TM is Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) was investigated by density functional theory calculation. The results show that doping TMs can reduce the overpotential for CO2 reduction reaction (CRR) compared to pristine ZnO. Significantly, the oxidation state of TMs by different d-orbital occupancy results in a change of the electronic properties of the catalysts, leading to a difference in reactivity, reaction pathway, and selectivity of the final products. Early TMs (Sc to Cr) showing oxidation state 3+ prefer CH4 as a product while late TMs (Mn to Cu) showing oxidation state 2+ can make HCOOH. Remarkably, Co-ZnO can produce HCOOH with ultra-low overpotential at 0.02 V and can further produce CH3OH with an overpotential of only 0.45 V. Therefore, Co-ZnO monolayer is suggested as a promising CRR catalyst for experimental research. This work sheds light on the rational design of low-cost metal oxides with high stability, activity, and product selectivity for CRR and other reactions.