ABSTRACT
The space charge layer in hematite photoelectrodes has been analyzed by means of Poisson-Boltzmann equations, the Stern model, and density functional theory, in view of its application for photoelectrochemical water oxidation. The width of the space charge layer can be smaller than â¼10 Å under experimental conditions. In this regime, a substantial part of the potential drop takes place in the Helmholtz layer, leading to important corrections to the Mott-Schottky behavior of the space charge layer capacitance. These results shed light on an unexpected regime of high photoelectrocatalytic efficiency, different from the classical picture of the electrochemical interface of a semiconducting photocatalyst, which is also amenable to direct study by quantum-mechanical atomistic simulations. Density functional theory has been used to calculate the band bending (BB) in the space charge layer in atomistic models of pristine stoichiometric and hydroxylated surfaces. These surface terminations display BBs of 0.14 and 0.49 eV, respectively, with an increasing width of the space charge layer, however still in the sub-nanometer regime. This work shows that, at high doping, the width of the space charge layer of a hematite photoelectrode can become comparable with interatomic distances.
ABSTRACT
A study on the influence of oxygen vacancies on the anatase (001) surface on the CO2 adsorption process is presented. For its realization, density functional theory (DFT) was used under the Perdew-Burke-Ernzerhof (PBE) generalized gradient and the spin-polarized approximations. Hubbard-U corrections and van der Waals interactions were also included. Three different types of oxygen vacancies were investigated at different sites on the anatase (001) surface; the formation energies in each case were 67.05, 113.84, and 93.16 kcal/mol, respectively. We identified a type of oxygen vacancy that could favor both the CO2 adsorption and dissociation. The differences on CO2 adsorption properties are due to electronic and structural causes, such as midgap states (Ti3+ polarons species) and changes in the structural properties on the TiO2 surface, generated upon the introduction of an oxygen vacancy. It is concluded that oxygen vacancies can play an important role in both CO2 adsorption and dissociation.
ABSTRACT
Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. Here, we describe a self-assembled two-dimensional metal-organic crystal, composed of graphene-supported macrocycles, each hosting a single FeN4 center, where a single carbon monoxide molecule can adsorb. In this heme-like biomimetic model system, excitons are generated by visible laser light upon a spin transition associated with the layer 2D crystallinity, and are simultaneously detected via the carbon monoxide ligand stretching mode at room temperature and near-ambient pressure. The proposed mechanism is supported by the results of infrared and time-resolved pump-probe spectroscopies, and by ab initio theoretical methods, opening a path towards the handling of exciton dynamics on 2D biomimetic crystals.
