The electronic structure and phase stability of MgO, ZnO, CdO, and related alloys in the rocksalt ͑B1͒, zincblende ͑B3͒, and wurtzite ͑B4͒ crystal structures were examined within first-principles band structure theory; the thermodynamically stable phases are reproduced for each material. The band alignment and bandgap deformation potentials were analyzed, showing an increase in the valence band maximum from Mg to Zn to Cd. Ternary alloy formation was explored through application of the special quasirandom structure method. The B1 structure is stable over all ͑Mg,Cd͒O compositions, as expected from the preferences of the binary oxides. The ͑Mg,Zn͒O alloy undergoes a tetrahedral to octahedral transition above 34% Mg content, in agreement with experiment. For ͑Zn,Cd͒O, a transition is predicted above 62% Cd content. These results imply that band-gap manipulation of ZnO from alloying with Mg ͑Cd͒ will be limited to 4.0 eV ͑1.6 eV͒, while preserving the tetrahedral coordination of the host.

Solar-driven water evaporation and valuable fuel generation is an environmentally friendly and sustainable way for clean water and energy production. However, a few bottlenecks for practical applications are high-cost, low productivity, and severe sunlight angle dependence. Herein, solar evaporation with enhanced photocatalytic capacity that is light direction insensitive and of efficiency breakthrough by virtue of a three-dimensional (3D) photothermal catalytic spherical isotopic evaporator is demonstrated. A homogeneous layer of microfluidic blow spun polyamide nanofibers loaded with efficient light absorber of polypyrrole nanoparticles conformally wraps onto a lightweight, thermal insulating plastic sphere, featuring favorable interfacial solar heating and efficient water transportation. The 3D spherical geometry not only guarantees the omnidirectional solar absorbance by the light-facing hemisphere, but also keeps the other hemisphere under shadow to harvest energy from the warmer environment. As a result, the light-to-vapor efficiency exceeds the theoretical limit, reaching 217% and 156% under 1 and 2 sun, respectively. Simultaneously, CO 2 photoreduction with generated steam reveals a favorable clean fuels production rate using photocatalytic spherical evaporator by secondary growth of Cu 2 O nanoparticles. Finally, an outdoor demonstration manifests a high evaporation rate and easy-to-perform construction on-site, providing a promising opportunity for efficient and decentralized water and clean fuel production.

The adsorption structures and mechanisms of water adsorption on ZnO(0001) and GaN(0001) surface are investigated by using the first-principles methods. It is found that the stable adsorption structure at full monolayer (ML) coverage is (2 × 1) reconstructed. A (2 × 1) molecular adsorption is definite for ZnO, and a (2 × 1) dissociative adsorption is also possible for GaN. For these structures the hydrogen bonds between adsorbates are significant besides the covalent interaction with substrate. For the coverage below 0.5 ML for GaN and 0.25 ML for ZnO, the individually adsorbed H2O can easily decompose to OH and H. Both covalent and electrostatic attractions contribute to the stability of dissociative adsorption. For the coverage between the above two cases, molecular adsorption is found to be stable in theory, but the real structure may be greatly dependent on the chemical condition. These results give a detailed description of the interaction between the first water adlayer and ZnO(GaN)(0001) surface.

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First-principles calculations are performed to study the adsorption of oxygen at wurtzite AlN ͑0001͒ and ͑0001͒ surfaces as a function of oxygen coverage. We find that the adsorption of oxygen at the AlN ͑0001͒ surface has a larger binding energy than at the AlN ͑0001͒ surface. The hollow site ͑H3͒ is preferred for the ͑0001͒ surface, whereas the site directly above the Al sublayer and the H3 site are almost degenerate in energy for the ͑0001͒ surface. The trend of the adsorption energy as a function of the oxygen coverage for the AlN ͑0001͒ surface is similar to that of GaN, whereas for the AlN ͑0001͒ surface it is different from GaN. The asymmetry of the oxygen adsorption at the two surfaces is explained using the electron counting rule and the resulting surface electronic states.Surface study is an indispensable subdiscipline of materials research. 1 A microscopic determination of semiconductor surface structures is crucial in understanding its material properties and can provide important guidance to tune material properties during crystal growth. AlN is one of the IIInitride semiconductors, which has great potential for highpower, high-temperature UV photoelectronic devices. 2 Some work has been done in the past to study AlN surface properties, such as surface reconstruction, 3-7 surface band structure, 8 and surface phonons. 9,10 Oxygen is one of the most abundant unintended impurities incorporated in crystal lattice or adsorbed at surface in III-nitrides. Unlike in GaN where O behaves as a shallow donor, it acts as a deep center in AlN due to the wide band gap of AlN. 11 Experimental studies have also shown that O absorption at the AlN surface has effects on the surface roughness, 12 surface chemical stability, 13 surface contact with metal, 14 surface acoustic properties, 15,16 and thermal conductivity. 17 However, there is a lack of theoretical study of oxygen adsorption at the AlN surface. It is not clear where the stable oxygen absorption site is and what is the dependence of the stability and surface electronic structures on the surface polarity and coverage.In this work, we performed first-principles calculation to investigate the adsorption of oxygen at wurtzite AlN ͑0001͒ and ͑0001͒ surfaces, which are the two most common surfaces of all III-nitrides. We calculated the adsorption energies of various oxygen coverages at three possible adsorption sites. We show that the adsorption of oxygen at the Aladlayer-terminated ͑0001͒ surface is more exothermic than at the AlN ͑0001͒ surface. The hollow site ͑H3͒ is preferred for the ͑0001͒ surface, whereas the site directly above the sublayer ͑T4͒ and the H3 site are almost degenerate in energy for the ͑0001͒ surface. Electronic structure calculations show that the calculated results can be understood using simple electron counting rules applied to these semiconductor surfaces.The calculations are performed employing the general gradient approximation 18 within the first-principles densityfunctional theory. We used the plane wave basis and the Vanderbilt ultr...