Theoretical treatment of semiconductor heterojunctions for photocatalysis: the WO3/BiVO4 interface

Band Edges Engineering of 2D/2D Heterostructures: The C3 N4 /Phosphorene Interface

We investigate the interface between carbon nitride (C₃ N₄ ) and phosphorene nanosheets (P-ene) by means of Density Functional Theory (DFT) calculations. C₃ N₄ /P-ene composites have been recently obtained experimentally showing excellent photoactivity. Our results indicate that the formation of the interface is a favorable process driven by Van der Waals forces. The thickness of P-ene nanosheets determines the band edges offsets and the charge carriers' separation. The system is predicted to pass from a nearly type-II to a type-I junction when the thickness of P-ene increases, and the conduction band offset is particularly sensitive. Last, we apply the Transfer Matrix Method to estimate the efficiency for charge carriers' migration as a function of the P-ene thickness.

Rational Design of Semiconductor Heterojunctions for Photocatalysis

Electronic structure calculations provide a useful complement to experimental characterization tools in the atomic-scale design of semiconductor heterojunctions for photocatalysis. The band alignment of the heterojunction is of fundamental importance to achieve an efficient charge carrier separation, so as to reduce electron/hole recombination and improve photoactivity. The accurate prediction of the offsets of valence and conduction bands in the constituent units is thus of key importance but poses several methodological and practical problems. In this Minireview we address some of these problems by considering selected examples of binary and ternary semiconductor heterojunctions and how these are determined at the level of density functional theory (DFT). The atomically precise description of the interface, the consequent charge polarization, the role of quantum confinement, the possibility to use facet engineering to determine a specific band alignment, are among the effects discussed, with particular attention to pros and cons of each one of these aspects. This analysis shows the increasingly important role of accurate electronic structure calculations to drive the design and the preparation of new interfaces with desired properties.

WO3/BiVO4 Photoanodes: Facets Matching at the Heterojunction and BiVO4 Layer Thickness Effects

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO₃/BiVO₄ heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO₃/BiVO₄ interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO₄ layer over a WO₃ scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO₄ growth over the WO₃ layer along the BiVO₄ {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO₃/BiVO₄ photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO₄ layer, a 75 nm thick BiVO₄ layer on WO₃ being best performing.

Role of surface termination in forming type-II photocatalyst heterojunctions: the case of TiO2/BiVO4

In this work we investigate TiO₂ and BiVO₄ nanostructures by means of density functional theory (DFT) calculations, to provide an estimate of the band alignment in TiO₂/BiVO₄ interfaces, highly active in photo-electrochemistry and photocatalytic water splitting. Calculations were carried out with both DFT range separated and self-consistent dielectric dependent hybrid functionals (HSE06 and PBE0_DD). The impact of systems' size has been investigated. The converged electronic levels of TiO₂ and BiVO₄ surfaces have been used to predict the band alignment in TiO₂/BiVO₄ heterostructures. Results indicated that when TiO₂ (101) surface is matched with BiVO₄ (110), a type-II alignment is obtained, where the band edges of BiVO₄ are higher in energy that those of TiO₂. This picture is favorable for charge-carriers separation upon photoexcitation, where electrons move toward TiO₂ and holes toward BiVO₄. On the contrary, if TiO₂ (101) is interfaced to BiVO₄ (010) the offset between the band edges is negligible, thus reducing the driving force toward separation of charge carriers. These results rationalize the dependence on the facet's exposure of the observed photocatalytic performances of TiO₂/BiVO₄ composites, where the TiO₂ (101)/BiVO₄ (110) interface outperforms the TiO₂ (101)/BiVO₄ (010) one.

Band Gap in Magnetic Insulators from a Charge Transition Level Approach

The theoretical description of the electronic structure of magnetic insulators and, in particular, of transition-metal oxides (TMOs), MnO, FeO, CoO, NiO, and CuO, poses several problems due to their highly correlated nature. Particularly challenging is the determination of the band gap. The most widely used approach is based on density functional theory (DFT) Kohn-Sham energy levels using self-interaction-corrected functionals (such as hybrid functionals). Here, we present a different approach based on the assumption that the band gap in some TMOs can have a partial Mott-Hubbard character and can be defined as the energy associated with the process Mm+(3dn) + Mm+(3dn) → M(m+1)+(3dn-1) + M(m-1)+(3dn+1). The band gap is thus associated with the removal (ionization potential, I) and addition (electron affinity, A) of one electron to an ion of the lattice. In fact, due to the hybridization of metal with ligand orbitals, these energy contributions are not purely atomic in nature. I and A can be computed accurately using the charge transition level (CTL) scheme. This procedure is based on the calculation of energy levels of charged states and goes beyond the approximations inherent to the Kohn-Sham (KS) approach. The novel and relevant aspect of this work is the extension of CTLs from the domain of point defects to a bulk property such as the band gap. The results show that the calculation based on CTLs provides band gaps in better agreement with experiments than the KS approach, with direct insight into the nature of the gap in these complex systems.