Ionic liquids (ILs) supported on oxide surfaces are being investigated for numerous applications including catalysis, batteries, capacitors, transistors, lubricants, solar cells, corrosion inhibitors, nanoparticle synthesis and biomedical applications. The study of ILs with oxide surfaces presents challenges both experimentally and computationally. The interaction between ILs and oxide surfaces can be rather complex, with defects in the oxide surface playing a key role in the adsorption behaviour and resulting electronic properties. The choice of the cation/anion pair is also important and can influence molecular ordering and electronic properties at the interface. These controllable interfacial behaviours make ionic liquid/oxide systems desirable for a number of different technological applications as well as being utilised for nanoparticle synthesis. This topical review aims to bring together recent experimental and theoretical work on the interaction of ILs with oxide surfaces, including TiO₂, ZnO, Al₂O₃, SnO₂ and transition metal oxides. It focusses on the behaviour of ILs at model single crystal surfaces, the interaction between ILs and nanoparticulate oxides, and their performance in prototype devices.

Superbasic ionic liquids (SBILs) are being investigated as potential carbon dioxide (CO₂) gas capture agents, however, the presence of H₂O in the flue stream can inhibit the uptake of CO₂. In this study a thin film of the SBIL trihexyltetradecylphosphonium 1,2,4-triazolide ([P₆₆₆₁₄][124Triz]) was deposited onto rutile TiO₂ (110) using in situ electrospray deposition and studied upon exposure to CO₂ and H₂O using in situ near-ambient pressure x-ray photoelectron spectroscopy (NAP-XPS). The molar uptake ratio of gas in the electrosprayed SBIL (n _gas :n _IL) was calculated to be 0.3:1 for CO₂, 0.7:1 for H₂O, and 0.9:1 for a CO₂/H₂O mixture. NAP-XPS taken at two different depths reveals that the competitive absorption of CO₂ and H₂O in [P₆₆₆₁₄][124Triz] varies with sampling depth. A greater concentration of CO₂ absorbs in the bulk layers, while more H₂O adsorbs/absorbs at the surface. The presence of H₂O in the gas mixture does not inhibit the absorption of CO₂. Measurements taken during exposure and after the removal of gas indicate that CO₂ absorbed in the bulk does so reversibly, whilst CO₂ adsorbed/absorbed at the surface does so irreversibly. This is contrary to the fully reversible CO₂ reaction shown for bulk ionic liquids (ILs) in literature and suggests that irreversible absorption of CO₂ in our highly-structured thin films is largely attributed to reactions at the surface. This has potential implications on IL gas capture and thin film IL catalysis applications.

The adsorption of 4-mercaptobenzoic acid (4-MBA) on anatase (101) and rutile (110) TiO₂ surfaces has been studied using synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy techniques. Photoelectron spectroscopy results suggest that the 4-MBA molecule bonds to both TiO₂ surfaces through the carboxyl group, following deprotonation in a bidentate geometry. Carbon K-edge NEXAFS spectra show that the phenyl ring of the 4-MBA molecule is oriented at 70° ± 5° from the surface on both the rutile (110) and anatase (101) surfaces, although there are subtle differences in the electronic structure of the molecule following adsorption between the two surfaces.

The thermal and moisture stability of halide perovskites is significantly improved by combining imidazolium ionic liquid additives (BMIMBF ₄ and BMIMCl) with a SnO ₂ electron transport layer.

Ionic liquids (ILs) have been explored as a way of improving the performance of ZnO-based optoelectronic devices; however, there are few fundamental studies of the IL/ZnO interface. Here, the adsorption of the IL 1-octyl-3-methylimidazolium tetrafluoroborate [C₈C₁Im][BF₄] on ZnO (0001) and ZnO ($10\overline{1}0$) has been studied using synchrotron-based soft X-ray photoelectron spectroscopy. The results indicate that [C₈C₁Im][BF₄] is deposited intact on the ZnO (0001) surface; however, there is some dissociation of [BF₄]⁻ anions, resulting in boron atoms attaching to the oxygen atoms in the ZnO surface and forming B₂O₃. In contrast, the deposition of [C₈C₁Im][BF₄] on the ZnO ($10\overline{1}0$) surface at −150 °C results in the appearance of more chemical environments in the spectra. We propose that the high temperature of the IL evaporator causes some conversion of [C₈C₁Im][BF₄] to a carbene–borane adduct, resulting in the deposition of both the IL and adduct onto the ZnO surface. The adsorption and desorption of the analogous IL 1-butyl-3-methylimidazolium tetrafluoroborate [C₄C₁Im][BF₄] was investigated on ZnO (0001) using synchrotron-based soft X-ray photoelectron spectroscopy. The results indicate that [C₄C₁Im][BF₄] is deposited largely intact at −150 °C and forms islands when heated to room temperature. When heated to over 80 °C, it begins to react with the ZnO surface and decomposes. This is a much lower temperature than the long-term thermal stability of the pure IL, quoted in the literature as ~400 °C, and of IL on powdered ZnO, quoted in the literature as ~300 °C. This indicates that the ZnO surface may catalyse the thermal decomposition of [C₄C₁Im][BF₄] at lower temperatures. This is likely to have a negative impact on the potential use of ILs in ZnO-based photovoltaic applications, where operating temperatures can routinely reach 80 °C.