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From corrosion inhibition to lubrication, a detailed understanding of the interactions between surfactants and iron oxide surfaces is critical for a range of industrial applications. However, there is still limited understanding of this behavior at the atomic-level, which hinders the design of improved surfactant molecules. In this study, the adsorption of three surfactants which are commonly employed as lubricant additives (carboxylic acid, amide, monoglyceride) on a α-Fe2O3(0001) surface is studied with density functional theory. The nature and strength of the adsorption for the different surfactants, as well as their propensity to deprotonate on the surface, is studied at a range of surface coverages. In agreement with the available experiments, strong chemisorption on α-Fe2O3(0001) is observed for all cases considered. Dissociation is energetically favorable for carboxylic acid and glyceride surfactants through the formation of a surface hydroxyl group, whereas this is not the case for amides. Glycerides form the most strongly adsorbed films at both low and high surface coverage due to the presence of multiple functional groups, which can all act as binding sites. However, the large size of the glyceride headgroup also means that adsorption is stronger at low coverage, where the formation of multiple bonds with the surface is possible, than at high coverage. Conversely, carboxylic acid films have similar stability at low and high coverage, where van der Waals forces between proximal tailgroups stabilize the adsorption structures. The results of this study provide atomic-level insights which help to explain friction results from previous macroscopic tribology experiments and classical molecular dynamics simulations. They also facilitate the molecular design of new surfactants to maximize the adsorption energy, surface coverage, and ultimately friction reduction on iron oxide surfaces., The Journal of Physical Chemistry C, 122 (36), ISSN:1932-7455, ISSN:1932-7447

Designing new catalysts that can efficiently utilize multiple energy sources can contribute to solving the current challenges of environmental remediation and increasing energy demands. In this work, we fabricated single-crystalline BiFeO3 (BFO) nanosheets and nanowires that can successfully harness visible light and mechanical vibrations and utilize them for degradation of organic pollutants. Under visible light both BFO nanostructures displayed a relatively slow reaction rate. However, under piezocatalysis both nanosheets and nanowires exhibited higher reaction rates in comparison with photocatalytic degradation. When both solar light and mechanical vibrations were used simultaneously, the reaction rates were elevated even further, with the BFO nanowires degrading 97% of RhB dye within 1 hr (k-value 0.058 min−1). The enhanced degradation under mechanical vibrations can be attributed to the promotion of charge separation caused by the internal piezoelectric field of BFO. BFO nanowires also exhibited good reusability and versatility toward degrading four different organic pollutants. ISSN:2589-0042

A detailed understanding of the behaviour of confined fluids is critical to a range of industrial applications, for example to control friction in engineering components. In this study, a combination of tribological experiments and confined nonequilibrium molecular dynamics simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5–2.0 GPa) and strain rates (104–1010 s−1) typical of the elastohydrodynamic lubrication regime, reveals clear relationships between the friction and flow behaviour. Lubricants, which are flexible, broadly linear molecules, give low friction coefficients that increase with strain rate and pressure in both the experiments and the simulations. Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, give high friction coefficients that only weakly depend on strain rate and pressure. The observed differences in friction behaviour can be rationalised through the stronger shear localisation which is observed for the traction fluids in the simulations. Higher pressures lead to more pronounced shear localisation, whilst increased strain rates lead to a widening of the sheared region. The methods utilised in this study have clarified the physical mechanisms of important confined fluid behaviour and show significant potential in both improving the prediction of elastohydrodynamic friction and developing new molecules to control it., Physical Chemistry Chemical Physics, 19 (27), ISSN:1463-9084, ISSN:1463-9076

Molybdenum(VI) oxide (MoO3) is used in a number of technical processes such as gas filtration and heterogeneous catalysis. In these applications, the adsorption and dissociation of water on the surface can influence the chemistry of MoO3 and thus the course of heterogeneous reactions. We use ambient pressure X-ray photoelectron spectroscopy to study the interaction of water with a stoichiometric MoO3 surface and a MoO3 surface that features oxygen defects and hydroxyl groups. The experimental results are supported by density functional theory calculations. We show that on a stoichiometric MoO3(010) surface, where Mo sites are unavailable, water adsorption is strongly disfavored. However, the introduction of surface species, which can interact with the lone pairs on the water O atom, e.g., Mo5+ atoms or surface OH groups, promotes water adsorption. Dissociation of water is favored at unsaturated Mo sites, i.e., at oxygen vacancies, while water adsorbs molecularly at hydroxyl sites., The Journal of Physical Chemistry C, 123 (27), ISSN:1932-7455, ISSN:1932-7447

We review the concept of surface charge, first, in the context of the polarization in ferroelectric materials and, second, in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability—the so-called “polar catastrophe”—if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO3, which is unusual in that its spontaneous ferroelectric polarization and the polarization arising from its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface termination, its surface charge is self-compensating. We use density functional calculations of BiFeO3 slabs and superlattices, analysis of high-resolution transmission electron micrographs, and examples from the literature to explore the consequences of this peculiarity., The Journal of Chemical Physics, 154 (15), ISSN:0021-9606, ISSN:1089-7690

Physical Review Letters, 123 (14), ISSN:0031-9007, ISSN:1079-7114

We demonstrate the existence of a polarizing field in thin films of insulators with charged ionic layers. The polarizing field derives from the same physics as the well-known depolarizing field that suppresses ferroelectric polarization in thin-film ferroelectrics, but instead drives thin films of materials that are centrosymmetric and paraelectric in their bulk form into a noncentrosymmetric, polar state. We illustrate the behavior using density-functional computations for perovskite-structure potassium tantalate, KTaO3, which is of considerable interest for its high dielectric constant, proximity to a quantum critical point, and superconductivity. We then provide a simple recipe to identify whether a particular material and film orientation will exhibit the effect and develop an electrostatic model to estimate the critical thickness of the induced polarization in terms of basic material parameters. Our results provide practical guidelines for exploiting the electrostatic properties of thin-film ionic insulators to engineer novel functionalities for nanoscale devices., Physical Review Research, 4 (3), ISSN:2643-1564

Designing new catalysts that can efficiently utilize multiple energy sources can contribute to solving the current challenges of environmental remediation and increasing energy demands. In this work, we fabricated single-crystalline BiFeO3 (BFO) nanosheets and nanowires that can successfully harness visible light and mechanical vibrations and utilize them for degradation of organic pollutants. Under visible light both BFO nanostructures displayed a relatively slow reaction rate. However, under piezocatalysis both nanosheets and nanowires exhibited higher reaction rates in comparison with photocatalytic degradation. When both solar light and mechanical vibrations were used simultaneously, the reaction rates were elevated even further, with the BFO nanowires degrading 97% of RhB dye within 1 hr (k-value 0.058 min−1). The enhanced degradation under mechanical vibrations can be attributed to the promotion of charge separation caused by the internal piezoelectric field of BFO. BFO nanowires also exhibited good reusability and versatility toward degrading four different organic pollutants., iScience, 4, ISSN:2589-0042

We review the concept of surface charge, first in the context of the polarization in ferroelectric materials, and second in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community, and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability - the so-called "polar catastrophe" - if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO3, which is unusual in that its spontaneous ferroelectric polarization and its layer charges can be of the same magnitude. As a result, for certain combinations of polarization orientation and surface terminationits surface charge is self-compensating. We use density functional calculations of BiFeO3 slabs and superlattices, analysis of high-resolution transmission electron micrographs as well as examples from the literature to explore the consequences of this peculiarity., Submitted to the Journal of Chemical Physics