Computational design of graphitic carbon nitride photocatalysts for water splitting

A series of structures based on graphitic carbon nitride (g-C3N4), a layered material composed of linked carbon-nitrogen heterocycles arranged in a plane, were investigated by density functional theory calculations. g-C3N4 is a semiconductor that absorbs UV light and visible light at the blue end of the visible spectrum, and is widely studied as a photocatalyst for water splitting; however, its photocatalytic efficiency is limited by its poor light-harvesting ability and low charge mobilities. Modifications to the g-C3N4 structure could greatly improve its optical and electronic properties and its photocatalytic efficiency. In this work, the g-C3N4 structure was modified by replacing the nitrogen linker with heteroatoms (phosphorus, boron) or aromatic groups (benzene, s-triazine and substituted benzenes). Two-dimensional (2D) sheets and three-dimensional (3D) multilayer structures with different stacking types were modelled. Several new structures were predicted to have electronic properties superior to g-C3N4 for use as water splitting photocatalysts. In particular, introduction of benzene and s-triazine groups led to band gaps smaller than in the standard g-C3N4 (down to 2.4 eV, corresponding to green light). Doping with boron in the linker positions dramatically reduced the band gap (to 1.7 eV, corresponding to red light); the doped material had the valence band position suitable for water oxidation. Our computational study showed that chemical modification of g-C3N4 is a powerful method to tune this material’s electronic properties and improve its photocatalytic activity

π-Conjugated Indole Dyads with Strong Blue Emission Made Possible by Stille Cross-Coupling and Double Fischer Indole Cyclisation

Small fluorescent π-conjugated indolyl-based molecules 4–6, 23 are prepared through direct Fischer synthesis or/and Stille cross-coupling method in appreciable yields. Our synthetic results have shown the benefits using Stille approach when Fischer double cyclisation method to access the bisindole dyads 4–6, 23 is not efficient. The synthetic routes to these materials have been designed to investigate the substrate requirements for the respective cyclisation and CC-coupling reactions and to evaluate their wider synthetic applicability. Comparative analysis of the different substituents and the different π-bridging units e. g. pyridine, thiophene and thiazole on the electronic and photophysical properties of the final compounds 4–6, 23 has been carried out. The structure−property relationship of the final bisindole dyads has been investigated via photophysical characterisation, and computational modelling. The obtained compounds absorb near-UV and visible (blue) light, with the spectral range dependent on the nature of the π-bridging units, and are capable of bright blue emission

Temperature control in molecular dynamic simulations of non-equilibrium processes

Thermostats are often used in various condensed matter problems, e.g. when a biological molecule undergoes a transformation in a solution, a crystal surface is irradiated with energetic particles, a crack propagates in a solid upon applied stress, two surfaces slide with respect to each other, an excited local phonon dissipates its energy into a crystal bulk, and so on. In all of these problems, as well as in many others, there is an energy transfer between different local parts of the entire system kept at a constant temperature. Very often, when modelling such processes using molecular dynamics simulations, thermostatting is done using strictly equilibrium approaches serving to describe the NV T ensemble. In this paper we critically discuss the applicability of such approaches to non-equilibrium problems, including those mentioned above, and stress that the correct temperature control can only be achieved if the method is based on the generalized Langevin equation (GLE). Specifically, we emphasize that a meaningful compromise between computational efficiency and a physically appropriate implementation of the NV T thermostat can be achieved, at least for solid state and surface problems, if the so-called stochastic boundary conditions (SBC), recently derived from the GLE (Kantorovich and Rompotis 2008 Phys. Rev. B 78 094305), are used. For SBC, the Langevin thermostat is only applied to the outer part of the simulated fragment of the entire system which borders the surrounding environment (not considered explicitly) serving as a heat bath. This point is illustrated by comparing the performance of the SBC and some of the equilibrium thermostats in two problems: (i) irradiation of the Si(001) surface with an energetic CaF2 molecule using an ab initio density functional theory based method, and (ii) the tribology of two amorphous SiO2 surfaces coated with self-assembled monolayers of methyl-terminated hydrocarbon alkoxylsilane molecules using a classical atomistic force field. We discuss the differences in behaviour of these systems due to applied thermostatting, and show that in some cases a qualitatively different physical behaviour of the simulated system can be obtained if an equilibrium thermostat is used

Origin of charge trapping in TiO2/reduced graphene oxide photocatalytic composites : insights from theory

Composites of titanium dioxide (TiO2) and reduced graphene oxide (RGO) have proven to be much more effective photocatalysts than TiO2 alone. However, little attention has been paid so far to the chemical structure of TiO2/RGO interfaces and to the role that the unavoidable residual oxygen functional groups of RGO play in the photocatalytic mechanism. In this work, we develop models of TiO2 rutile (110)/RGO interfaces by including a variety of oxygen functional groups known to be present in RGO. Using hybrid density functional theory calculations, we demonstrate that the presence of oxygen functional groups and the formation of interfacial cross-links (Ti–O–C covalent bonds and strong hydrogen bonds between TiO2 and RGO) have a major effect on the electronic properties of RGO and RGO-based composites. The electronic structure changes from semimetallic to semiconducting with an indirect band gap, with the lowest unoccupied band positioned below the TiO2 conduction band and largely localized on RGO oxygen and carbon orbitals, with some contributions of RGO-bonded Ti atoms. We suggest that this RGO-based lowest unoccupied band acts as a photoelectron trap and the indirect nature of the band gap hinders electron–hole recombination. These results can explain the experimentally observed extended lifetimes of photoexcited charge carriers in TiO2/RGO composites and the enhancement of photocatalytic efficiency of these composites

Electronic Structure and Charge Transfer in the TiO2 Rutile (110)/Graphene Composite Using Hybrid DFT Calculations

Composite systems of TiO2 with nanocarbon materials, such as graphene, graphene oxide and carbon nanotubes, have proven to be efficient photocatalyst materials. However, detailed understanding of their electronic structure and the mechanisms of the charge transfer processes is still lacking. Here, we use hybrid density functional theory calculations to analyse the electronic properties of the ideal rutile (110)-graphene interface, in order to understand experimentally observed trends in photoinduced charge transfer. We show that the potential energy surface of pristine graphene physisorbed above rutile (110) is relatively flat, enabling many possible positions of graphene above the rutile (110) surface. We verify that tensile and compressive strain has negligible effect on the electronic properties of graphene at low levels of strain. By analysing the band structure of this composite material and the composition of the valence and conduction band edges, we show that both the highest occupied states and the lowest unoccupied states of this composite are dominated by graphene, and that there is also a significant contribution of Ti orbitals to the two lowest unoccupied bands. We suggest that a transition from graphene-dominated occupied bands to mixed graphene and TiO2-based unoccupied bands is responsible for the experimentally observed photoinduced charge transfer from graphene to TiO2 under visible light irradiation; however, the most stable state for an excess (e.g. photoexcited) electron is localised on the carbon orbitals, which make up the lowest-energy conduction band. This separation of photogenerated electrons and holes makes TiO2-graphene an efficient photocatalyst material

Modelling the strength of mineral–organic binding: organic molecules on the α-Al2O3(0001) surface

Organic carbon (OC) is an essential component of soil. Sorption of OC to oxide mineral surfaces is a key process in soil preservation due to its ability to protect OC from microbial degradation. To understand the sorption of OC in soils and obtain a quantitative description of the binding of organic molecules to soil minerals, we investigated the binding of water and small organic molecules, typical building blocks of OC, on α-Al2O3, a common soil mineral. α-Al2O3 was modelled using (0001)-oriented periodic slabs, using density functional theory calculations with empirical dispersion correction. For water, dissociative adsorption was energetically preferred to molecular adsorption. Amine, amide and carboxylic acid functional groups were found to bind more strongly to this surface compared to water. Alcohol, ether, thiol and ester functional groups had adsorption energies very similar to that of water, while hydrocarbons were found to bind less strongly. Carboxylic acids were the strongest bound surface adsorbates in this study. Dissociated adsorption configurations (where allowed by the molecules' chemical nature) were usually more favourable than molecular adsorption. Hydrogen bonding was found to be a major contributor to the stability of adsorption configurations. This work shows that a number of organic functional groups, in particular amine, amide and carboxylic acids, bind to the α-Al2O3(0001) surface more strongly than water; thus they are likely to be adsorbed on this mineral surface under ambient conditions and to provide stability of adsorbed OC

Interaction of organic pollutants with TiO2: a density functional theory study of carboxylic acids on the anatase (101) surface

Understanding the interaction of the carboxylic group with TiO2 is crucial for photocatalytic degradation of pollutants, because aromatic molecules containing carboxylic acid groups are among the most common micropollutants. This study investigated the interactions of the anatase TiO2 (101) surface with several aromatic carboxylic acids: benzoic, nicotinic, salicylic and anthranilic acid, using dispersion-corrected density functional theory (DFT) calculations. For all the molecules studied, we found higher stabilities of monodentate adsorbed configurations over bidentate. Dispersion was found to have a significant effect on adsorption energies. In particular, dispersion gave rise to a tilted monodentate adsorption configuration, where adsorption through interfacial covalent and hydrogen bonds was additionally stabilised by dispersion interactions of the aromatic ring with the surface. Comparative calculations using DFT with empirical dispersion correction and van der Waals-corrected functionals found the relative stabilities of adsorbed structures to be independent of the method of describing dispersion. Thermodynamic probabilities of different adsorption configurations were evaluated using the Boltzmann distribution, and the tilted dispersion-stabilised structures were predicted to be by far the most abundant. Finally, the optical absorption of TiO2-acid systems was modelled, and TiO2-aromatic acid complexes were found to have their optical absorption extended into the visible range

Atomic-scale modelling of organic matter in soil: adsorption of organic molecules and biopolymers on the hydroxylated α-Al2O3 (0001) surface

Binding of organic molecules on oxide mineral surfaces is a key process which impacts the fertility and stability of soils. Aluminium oxide and hydroxide minerals are known to strongly bind organic matter. To understand the nature and strength of sorption of organic carbon in soil, we investigated the binding of small organic molecules and larger polysaccharide biomolecules on -Al2O3 (corundum). We modelled the hydroxylated -Al2O3 (0001) surface, since these minerals’ surfaces are hydroxylated in the natural soil environment. Adsorption was modelled using density functional theory (DFT) with empirical dispersion correction. Small organic molecules (alcohol, amine, amide, ester and carboxylic acid) were found to adsorb on the hydroxylated surface by forming multiple hydrogen bonds with the surface, with carboxylic acid as the most favourable adsorbate. A possible route from hydrogen-bonded to covalently bonded adsorbates was demonstrated, through co-adsorption of the acid adsorbate and a hydroxyl group to a surface aluminium atom. Then we modelled the adsorption of biopolymers, fragments of polysaccharides which naturally occur in soil: cellulose, chitin, chitosan and pectin. These biopolymers were able to adopt a large variety of hydrogen-bonded adsorption configurations. Cellulose, pectin and chitosan could adsorb particularly strongly, and therefore are likely to be stable in soil

Thermodynamics of 4,4 '-stilbenedicarboxylic acid monolayer self-assembly at the nonanoic acid-graphite interface

A direct calorimetric measurement of the overall enthalpy change associated with self-assembly of organic monolayers at the liquid–solid interface is for most systems of interest practically impossible. In previous work we proposed an adapted Born–Haber cycle for an indirect assessment of the overall enthalpy change by using terephthalic acid monolayers at the nonanoic acid–graphite interface as a model system. To this end, the sublimation enthalpy, dissolution enthalpy, the monolayer binding enthalpy in vacuum, and a dewetting enthalpy are combined to yield the total enthalpy change. In the present study the Born–Haber cycle is applied to 4,40 -stilbenedicarboxylic acid monolayers. A detailed comparison of these two aromatic dicarboxylic acids is used to evaluate and quantify the contribution of the organic backbone for stabilization of the monolayer at the nonanoic acid–graphite interface