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The product selectivity of the CO2 reduction reaction occurring on transition metal-doped ZnS monolayers was theoretically studied.

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.

In this work, pure and Fe-doped ZnO were investigated in both experimental and theoretical aspects. The Zn1-x Fe x O (x=0.000, 0.0625, and 0.125) nanoparticles were prepared by a combustion method. The crystal structures were characterized by the X-ray diffraction (XRD) and selected area electron diffraction (SAED) analysis, morphology by the scanning electron microscope (SEM) and transmission electron microscopy (TEM) techniques, elemental analysis or chemical characterization by energy-dispersive X-ray spectroscopy (EDS or EDX), magnetic behavior by vibrating sample magnetometer (VSM), and optical band gap by ultraviolet-visible (UV-Vis) spectroscopy. In the first principle calculation, the structural properties, density of states (DOS), electronic band structure, and magnetic property of pure ZnO and Zn1-x Fe x O have been investigated by means of density functional theory with local density approximation (LDA), general gradient approximation (GGA), as well as LDA and GGA with Hubbard model scheme (LDA + U and GGA + U), packaged in the Vienna Ab initio Simulation Package (VASP). The calculation was performed using self-consistent projected augmented plane wave (PAW). The zinc oxide was modeled using 2×2×2 super-cell in ideal hexagonal wurtzite structure. The prepared samples of pure ZnO and Zn1-x Fe x O with iron concentration of 6.25 and 12.5 % by mole have a phase of the hexagonal wurtzite structure with particle size in nanometer scale. The calculation results indicate that the pure ZnO has direct energy band gap of 2.24 eV for GGA + U calculation in the scheme of Perdew–Burke–Ernzerh of PBE, which are underestimated when compared to the results from the experiment part, E g =.17 eV. The calculated magnetic dipole moments of the Zn1-x Fe x O when the iron contents (x) are 0.000, 0.0625, and 0.125 equal to 0.00, 3.91, and 7.83 μ b respectively. The density of states of dopant systems shows an intermediate band from d orbital of iron atoms located near the valence band. This indicates that small amount of doped iron engineers the band structure. These results show that the doped iron atoms seem to play an important role for the appearance of intermediate band and magnetism.

The terahertz (THz) frequency range is very important in various practical applications, such as terahertz imaging, chemical sensing, biological sensing, high-speed telecommunications, security, and medical applications. Based on the density functional theory (DFT), this work presents electronic and optical properties of N-doped ZnO/ZnO/N-doped ZnO quantum well and quantum wire nanostructures. The density of states (DOS), the band structures, effective masses, and the band offsets of ZnO and N-doped ZnO were calculated as the input parameters for the subsequent modeling of the ZnO/N-doped ZnO heterojunctions. The results show that the energy gaps of the component materials are different, and the conduction and valence band offsets at the ZnO/N-doped ZnO heterojunction give type-II alignment. Furthermore, the optical characteristics of N-doped ZnO/ZnO/N-doped ZnO quantum well were studied by calculating the absorption coefficient from transitions between the confined states in the conduction band under the applied electric field (Stark effect). The results indicate that N-doped ZnO/ZnO/N-doped ZnO quantum wells, quantum wires, and quantum cascade structures could offer the absorption spectrum tunable in the THz range by varying the electric field and the quantum system size. Therefore, our work indicates the possibility of using ZnO as a promising candidate for infrared and terahertz applications.