The positions of electronic band edges are one important metric for determining a material's capability to function in a solar energy conversion device that produces fuels from sunlight. In particular, the position of the valence band maximum (conduction band minimum) must lie lower (higher) in energy than the oxidation (reduction) reaction free energy in order for these reactions to be thermodynamically favorable. We present first principles quantum mechanics calculations of the band edge positions in five transition metal oxides and discuss the feasibility of using these materials in photoelectrochemical cells that produce fuels, including hydrogen, methane, methanol, and formic acid. The band gap center is determined within the framework of DFT+U theory. The valence band maximum (conduction band minimum) is found by subtracting (adding) half of the quasiparticle gap obtained from a non-self-consistent GW calculation. The calculations are validated against experimental data where possible; results for several materials including manganese(ii) oxide, iron(ii) oxide, iron(iii) oxide, copper(i) oxide and nickel(ii) oxide are presented.

Practical implementation of solar-powered water splitting and CO 2 reduction to fuels requires the discovery of efficient and inexpensive photocatalytic (PC) materials. One possible materials design strategy aims to tune properties of relatively inexpensive transition metal oxide catalysts to increase sunlight absorption while preserving potential redox reactivity.Here we consider MnO for PC use by alloying it with ZnO in solid solutions. A combined density functional theory and GW scheme is used to study the band gap and band edge placements as a function of composition. We predict that alloying MnO with ZnO in varying amounts reduces MnO's band gap for more efficient light absorption while maintaining advantageous band edge placements. The 1:1 alloy of MnO and ZnO is identified as a new (2.6 eV band gap) visible-light-absorbing material with band edges suitably placed with respect to both water-oxidation and CO 2 -reduction reactions, making it a compelling candidate for solar PC chemistry.

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Rationally engineering photocatalytic devices that power water splitting or CO 2 reduction reactions requires identifying economical materials that efficiently absorb sunlight and have suitable band edge placements. Recent theoretical investigations have predicted that a 1 : 1 alloy of MnO and ZnO meets these criteria. However, poor hole conductivity in undoped MnO:ZnO alloys (with up to 10% ZnO) severely limits this material's utility in electronic devices, and its electron conductivity has not yet been characterized. Here we investigate carrier transport in pure and doped MnO and MnO:ZnO with ab initio quantum chemistry calculations. Electrostatically embedded clusters are used to compute and compare relative electron/hole transfer barriers within the small polaron model. We assess the effects of Al, Ga, In, Sc, Y, Ti, Sb, Gd, F (n-type dopants) and Li (a p-type dopant) to determine which may enhance conductivity in MnO:ZnO. Our findings indicate that Ga, Sc, Ti, F, and Sb dopants create deep traps whereas In forms shallower traps that merit further investigation. Y, Al, Gd, and Li dopants should increase the carrier concentration while maintaining favorable electron and hole transport pathways.The latter are recommended for increasing the conductivity of MnO:ZnO and its effectiveness for solar energy conversion.

We have performed a rigorous theoretical study of the quantum translation-rotation (T-R) dynamics of one and two H2 and D2 molecules confined inside the large hexakaidecahedral (5(12)6(4)) cage of the sII clathrate hydrate. For a single encapsulated H2 and D2 molecule, accurate quantum five-dimensional calculations of the T-R energy levels and wave functions are performed that include explicitly, as fully coupled, all three translational and the two rotational degrees of freedom of the hydrogen molecule, while the cage is taken to be rigid. In addition, the ground-state properties, energetics, and spatial distribution of one and two p-H2 and o-D2 molecules in the large cage are calculated rigorously using the diffusion Monte Carlo method. These calculations reveal that the low-energy T-R dynamics of hydrogen molecules in the large cage are qualitatively different from that inside the small cage, studied by us recently. This is caused by the following: (i) The large cage has a cavity whose diameter is about twice that of the small cage for the hydrogen molecule. (ii) In the small cage, the potential energy surface (PES) for H2 is essentially flat in the central region, while in the large cage the PES has a prominent maximum at the cage center, whose height exceeds the T-R zero-point energy of H2/D2. As a result, the guest molecule is excluded from the central part of the large cage, its wave function localized around the off-center global minimum. Peculiar quantum dynamics of the hydrogen molecule squeezed between the central maximum and the cage wall manifests in the excited T-R states whose energies and wave functions differ greatly from those for the small cage. Moreover, they are sensitive to the variations in the hydrogen-bonding topology, which modulate the corrugation of the cage wall.

The demand for renewable hydrogen derived from CO2‐neutral water‐splitting processes spurs efforts to develop new catalysts, including those inspired by nature. A first‐principles quantum mechanics (Kohn–Sham density functional theory + U) approach has been used to model electrocatalytic water oxidation on the visible‐light‐absorbing transition‐metal oxide alloy, MnO:ZnO; a material that can be considered a heterogeneous analogue to the photosystem II photocatalyst. Ab‐initio‐derived U values were used to correct self‐interaction errors in the highly correlated material. It has been confirmed that previously established scaling relationships between the binding energies of reaction intermediates are valid. The predicted electrochemical overpotential for water oxidation under experimentally relevant conditions (0.82 V versus the standard hydrogen electrode) is slightly higher than those values reported for manganese oxides and comparable to those previously calculated values for hematite photoanodes.

MnO:ZnO was recently proposed as a novel, potentially visible-light absorbing catalyst with many advantages for water splitting. However, little is known about its optical absorption properties and whether they are suited for solar applications. We address this issue by examining excitations in MnO:ZnO and pure MnO using electrostatically embedded cluster models treated with complete active space second order perturbation (CASPT2) theory. The lowest-lying excitations in MnO are single Mn d → d ligand field excitations (∼2.6 eV, ∼108 s lifetime), followed by double d → d excitations (∼5.2 eV, ∼106 s lifetime), Mn 3d → 4s excitations (∼6.3 eV, ∼10–3 s lifetime), and higher-lying O 2p → Mn 3d ligand-to-metal charge-transfer (LMCT) excitations (∼10.1 eV, ∼10–4 s lifetime). The longer-lived transitions should exhibit better electron–hole pair separation and enhance photoconductivity depending on ease of carrier transport. Alloying MnO with ZnO notably lowers the LMCT transition to ∼8.3 eV with the metal orbital comprised mainly of Zn 4s states, leaving most other absorption properties relatively unchanged. We find near degeneracies among spin-allowed and spin-forbidden LMCT states that could facilitate intersystem crossing resulting in longer lifetimes. We suggest seeking other materials that exhibit similar LMCT excitations but that are visible-light activated as a design strategy for further enhancing photon conversion efficiencies.

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