No abstract

No abstract

Using the electron paramagnetic resonance technique, we have elucidated the multiple roles of water and carbonates in the overall photocatalytic reduction of carbon dioxide to methane over titania nanoparticles. The formation of H atoms (reduction product) and (•)OH radicals (oxidation product) from water, and CO(3)(-) radical anions (oxidation product) from carbonates, was detected in CO(2)-saturated titania aqueous dispersion under UV illumination. Additionally, methoxyl, (•)OCH(3), and methyl, (•)CH(3), radicals were identified as reaction intermediates. The two-electron, one-proton reaction proposed as an initial step in the reduction of CO(2) on the surface of TiO(2) is supported by the results of first-principles calculations.

We report the results of a comprehensive study on the structural, electronic, and optical properties of Ga 2 O 3 in its ambient, monoclinic ͑␤͒ and high-pressure, hexagonal ͑␣͒ phases in the framework of all-electron density functional theory. In both phases, the conduction band minimum is at the zone center while the valance band maximum is rather flat in the k space. The calculated electron effective mass m e * / m 0 comes out to be 0.342 and 0.276 for ␤-Ga 2 O 3 and ␣-Ga 2 O 3 , respectively. The dynamic dielectric function, reflectance, and energy-loss function for both phases are reported for a wide energy range of 0-50 eV. The subtle differences in electronic and optical properties can be attributed to the higher symmetry, coordination number of Ga atoms, and packing density in ␣-Ga 2 O 3 relative to that in ␤-Ga 2 O 3 .

The design of low-cost yet high-efficiency electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) over a wide pH range is highly challenging. We now report a hierarchical co-assembly of interacting MoS 2 and Co 9 S 8 nanosheets attached on Ni 3 S 2 nanorod arrays which are supported on nickel foam (NF). This tiered structure endows high performance toward HER and OER over a very broad pH range. By adjusting the molar ratio of the Co:Mo precursors, we have created CoMoNiS-NF-xy composites (x:y means Co:Mo molar ratios ranging from 5:1 to 1:3) with controllable morphology and composition. The three-dimensional composites have an abundance of active sites capable of universal pH catalytic HER and OER activity. The CoMoNiS-NF-31 demonstrates the best electrocatalytic activity, giving ultralow overpotentials (113, 103, and 117 mV for HER and 166, 228, and 405 mV for OER) to achieve a current density of 10 mA cm −2 in alkaline, acidic, and neutral electrolytes, respectively. It also shows a remarkable balance between electrocatalytic activity and stability. Based on the distinguished catalytic performance of CoMoNiS-NF-31 toward HER and OER, we demonstrate a two-electrode electrolyzer performing water electrolysis over a wide pH range, with low cell voltages of 1.54, 1.45, and 1.80 V at 10 mA cm −2 in alkaline, acidic, and neutral media, respectively. First-principles calculations suggest that the high OER activity arises from electron transfer from Co 9 S 8 to MoS 2 at the interface, which alters the binding energies of adsorbed species and decreases overpotentials. Our results demonstrate that hierarchical metal sulfides can serve as highly efficient all-pH (pH = 0−14) electrocatalysts for overall water splitting.

No abstract

Binding configurations of CO 2 and CO 2 -on perfect and oxygen-deficient anatase (101) surfaces were explored using first-principles calculations on both cluster and periodic models. The solvent effect was taken into account via the polarizable continuum model. Analysis of molecular orbitals, charge, and spin density distributions was used to help identify the radical anion CO 2 -adsorbed on the surface. On defect-free surfaces, it is found to bind as a bridging bidentate configuration with both oxygens coordinating to the 5-fold Ti ions. Analysis of vibrational frequencies provides a specific signature of the CO 2 anion to distinguish it from other species in experiments. The reduction potential of adsorbed CO 2 on a (101) surface is lower by 0.24 V than the reduction potential of a CO 2 molecule, both in aqueous solution, due to the formation of hybridized orbitals, which facilitates charge transfer to CO 2 . The reduced (101) surface of TiO 2 is much more favorable for CO 2 binding with accompanying charge transfer to CO 2 .

Perovskite oxides are an important class of oxygen evolution reaction (OER) catalysts in alkaline media, despite the elusive nature of their active sites. Here, we demonstrate that the origin of the OER activity in a La 1−x Sr x CoO 3 model perovskite arises from a thin surface layer of Co hydr(oxy)oxide (CoO x H y ) that interacts with trace-level Fe species present in the electrolyte, creating dynamically stable active sites. Generation of the hydr(oxy)oxide layer is a consequence of a surface evolution process driven by the A-site dissolution and O-vacancy creation. In turn, this imparts a 10-fold improvement in stability against Co dissolution and a 3-fold increase in the activity−stability factor for CoO x H y / LSCO when compared to nanoscale Co-hydr(oxy)oxides clusters. Our results suggest new design rules for active and stable perovskite oxide-based OER materials.