RESUMO
The electronic structures and optical characteristics of yttrium (Y)-doped ZnO monolayers (MLs) with vacancy (zinc vacancy, oxygen vacancy) were investigated by the first-principles density functional theory. Calculations were performed with the GGA+U (generalized gradient approximation plus U) approach, which can accurately estimate the energy of strong correlation semiconductors. The results show that the formation energy values of Y-doped ZnO MLs with zinc or oxygen vacancy (VZn, VO) are positive, implying that the systems are unstable. The bandgap of Y-VZn-ZnO was 3.23 eV, whereas that of Y-VO-ZnO was 2.24 eV, which are smaller than the bandgaps of pure ZnO ML and Y-doped ZnO MLs with or without VO. Impurity levels appeared in the forbidden band of ZnO MLs with Y and vacancy. Furthermore, Y-VZn-ZnO will result in a red-shift of the absorption edge. Compared with the pure ZnO ML, ZnO MLs with one defect (Y, VZn or VO), and Y-VZn-ZnO, the absorption coefficient of Y-VO-ZnO was significantly enhanced in the visible light region. These findings demonstrate that Y-VO-ZnO would have great application potential in photocatalysis.
RESUMO
The structural stability, electronic structure, and optical properties of an iron-adsorbed ZnO (0001) surface with three high-symmetry adsorption sites are investigated with first-principle calculations on the basis of density functional theory and the Hubbard-U method. It is found that the iron adatom in the H3 adsorption site of ZnO (0001) surface has the lowest adsorption energy of -5.665 eV compared with T4 and Top sites. For the Top site, compared with the pristine ZnO (0001) surface, the absorption peak located at 1.17 eV has a red shift, and the elevation of the absorption coefficient is more pronounced in the visible-light region, because the Fe-related levels are introduced in the forbidden band and near the Fermi level. The electrostatic potential computation reveals that the work function of the ZnO (0001) surface is significantly decreased from 2.340 to 1.768 eV when iron is adsorbed on the Top site. Furthermore, the degradation mechanism based on the band structure is analyzed. It can be concluded that the adsorption of iron will promote the separation of photoinduced carriers, thus improving the photocatalytic activity of ZnO (0001) surface. Our study benefits research on the photocatalytic activity of ZnO and the utilization rate of solar energy.
RESUMO
With high percentage of washing water, swine wastewater is characterized by large volume and low concentration of total solids. Thus, in treating swine wastewater, it is relatively difficult to heat digesters, resulting in low methane production at low ambient temperatures (ATs). To increase methane production from swine wastewater, this study proposed a novel "two-step heating (TSH)" mode with the same energy consumption as a one-step process for anaerobic digestion. Compared with the traditionally heated digesters (one-step heating), the digestion temperature in TSH digesters increased by 3.50-10.50 °C under the assumption of no heat dissipation and by 3.30-9.25 °C in the actual experiments. Although methane production of the TSH digesters improved by 15% in our experiments, the improvement was far less than theoretically estimated. This was mainly caused by short hydraulic retention time and sludge washout in the digesters. Moreover, the acetoclastic methanogenesis, accomplished by genus Methanosaeta, was the major methanogenesis pathway at low temperatures in both the TSH and conventional heating modes. However, the relative abundance of syntrophic bacteria and hydrogenotrophic methanogens in TSH mode were both higher than in the digesters operation in conventional heating mode when the atmospheric temperature was below 10 °C.
