Ionic liquids on oxide surfaces

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, SnO₂and 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.

Water-induced reordering in ultrathin ionic liquid films

Water-induced reordering in ultrathin ionic liquid films has been observed using in situ x-ray photoelectron spectroscopy. An ultrathin layer of 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄C₁Im][BF₄]) was deposited on a rutile TiO₂ (1 1 0) single crystal and exposed to water vapour at a relative humidity of ~70% in an in situ cell. Water was found to adsorb onto the ionic liquid surface, causing a reordering of the ions at the interface. Water initially remained trapped on the ionic liquid surface as the in situ cell was evacuated. This could have negative implications for supported ionic liquid phase catalysis, where reactants and products move in and out of an ionic liquid containing the catalyst. This insight into the behaviour at the water/ionic liquid interface provides a basis for understanding interfacial behaviour in more complex gas/ionic liquid systems.

Interactions and stabilisation of acetone, sulfur dioxide and water with 1-octyl-3-methylimidazolium tetrafluoroborate [OMIM][BF4] at low temperatures

The interactions between three small molecules, water (H₂O), sulfur dioxide (SO₂) and acetone ((CH₃)₂CO), with the ionic liquid (IL) 1-octyl-3-methylimidazolium tetrafluoroborate, [OMIM][BF₄], have been determined using line of sight temperature programmed desorption (LOSTPD) from a gold surface. Multilayers of the IL were deposited by physical vapour deposition with multilayers of the small molecular species (adsorbed from the gas phase) at 90 K. LOSTPD was then carried out with the small molecular species desorbing first from the mixed multilayer, followed at higher temperatures by desorption of the IL from the gold surface. The IL had a high activation energy for desorption of 126(6) kJ mol^-1. Pure acetone showed a desorption activation energy of 38(2) kJ mol^-1, which increased to 45-61 kJ mol^-1 when it was pre-adsorbed below an overlying porous layer of the ionic liquid at 90 K. The stabilised acetone is thought to be associated with pores containing ionic moieties. Destabilised acetone was also observed and thought to originate from pores containing octyl chains. The quantity of stabilised acetone scaled with the amount of IL, being ≈1.1 molecules per IL ion pair. SO₂ and H₂O were co-adsorbed with the IL at 90 K leading to an intimate mixture of the two. For pure SO₂ the desorption energy was 32(2) kJ mol^-1, which increased to 40-52 kJ mol^-1 for relative concentrations up to 6 SO₂ molecules per IL ion pair. For pure water the activation energies were 49(5) kJ mol^-1 and 43(1) kJ mol^-1 for amorphous and crystalline ice respectively. When co-adsorbed with the IL the stabilisation energies were 42-49 kJ mol^-1, but up to 505 water molecules per IL ion pair could be stabilised to some degree. The desorption mechanisms and the reasons for these interactions are discussed.

Ionic Liquid Ordering at an Oxide Surface

The interaction of the ionic liquid [C₄ C₁ Im][BF₄ ] with anatase TiO₂ , a model photoanode material, has been studied using a combination of synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The system is of interest as a model for fundamental electrolyte-electrode and dye-sensitized solar cells. The initial interaction involves degradation of the [BF₄ ]^- anion, resulting in incorporation of F into O vacancies in the anatase surface. At low coverages, [C₄ C₁ Im][BF₄ ] is found to order at the anatase(101) surface via electrostatic attraction, with the imidazolium ring oriented 32±4° from the anatase TiO₂ surface. As the coverage of ionic liquid increases, the influence of the oxide surface on the topmost layers is reduced and the ordering is lost.

Adsorption, Desorption, and Reaction of 1-Octyl-3-methylimidazolium Tetrafluoroborate, [C₈C₁Im][BF₄], Ionic Liquid Multilayers on Cu(111)

Multilayers of 1-octyl-3-methylimidazolium tetrafluoroborate [C8C1Im][BF4] have been deposited on a Cu(111) surface by evaporation in UHV. XPS shows that [C8C1Im][BF4] adsorbs without decomposition for substrate temperatures < 300 K. XPS and UPS data indicate that ionic liquid (IL) deposition onto a 120 K Cu(111) surface results in the IL forming multilayers by a simultaneous-multilayer growth process. IL deposition onto a room temperature Cu(111) surface results in a different arrangement where at a coverage of one monolayer the IL forms droplets of about 100 Å height covering only about 1/10th of the surface. Multilayers deposited at 120 K convert to the room temperature arrangement upon heating. Further heating above room temperature causes the IL multilayer droplets to desorb leaving an IL monolayer of ≈6 Å thickness at ≈430 K. At higher temperatures, this monolayer reacts with the surface and BF3 is emitted, leaving products containing C, N, and some F on the surface. We propose a surface reaction where [BF4](-) ions react to form chemisorbed fluorine (Cu-F) and gaseous BF3, with the remaining [C8C1Im](+) decomposing on the Cu(111) in an unidentified manner.

Surface properties of nanocrystalline PbS films deposited at the water-oil interface: a study of atmospheric aging

Nanocrystalline thin films of PbS are obtained in a straightforward reaction by precipitation at the interface between toluene (containing a Pb precursor) and water (containing Na2S). Lead thiobiuret [Pb(SON(CN(i)Pr2)2)2] and lead diethyldithiocarbamate [Pb(S2CNEt2)2] precursors are used. The films are characterized by X-ray diffraction and electron microscopy, revealing typical particle sizes of 10-40 nm and preferred (200) orientation. Synchrotron-excited depth-profiling X-ray photoelectron spectroscopy (XPS) is used to determine the depth-dependent chemical composition as a function of surface aging in air for periods of up to 9 months. The as-synthesized films show a 1:1 Pb/S composition. Initial degradation occurs to form lead hydroxide and small quantities of surface-adsorbed -SH species. A lead-deficient Pb1-xS phase is produced as the aging proceeds. Oxidation of the sulfur occurs later to form sulfite and sulfate products that are highly localized at the surface layers of the nanocrystals. These species show logarithmic growth kinetics, demonstrating that the sulfite/sulfate layer acts to passivate the nanocrystals. Our results demonstrate that the initial reaction of the PbS nanocrystals (forming lead hydroxide) is incongruent. The results are discussed in the context of the use of PbS nanocrystals as light-harvesting elements in next-generation solar technology.

Growth of nanocrystalline thin films of metal sulfides [CdS, ZnS, CuS and PbS] at the water–oil interface

Films of Nanocrystalline CuS, PbS, CdS and ZnS at water toluene interface.

Adsorption of dopamine on rutile TiO2 (110): a photoemission and near-edge X-ray absorption fine structure study

Synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the adsorption of dopamine on a rutile TiO2 (110) single crystal. Photoemission results suggest that dopamine bonds through the oxygen molecules in a bidentate fashion. From the data, it is ambiguous whether the oxygens bond to the same 5-fold coordinated surface titanium atom or bridges across two, although based on the bonding of pyrocatechol on rutile TiO2 (110), it is likely that the dopamine bridges two titanium atoms. Using the searchlight effect, the carbon K-edge near-edge X-ray absorption fine structure NEXAFS spectra recorded for dopamine on rutile TiO2 (110) show the phenyl ring to be oriented at 78° ± 5° from the surface and twisted 11 ± 10° relative to the (001) direction.

Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101̄0)

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101̄0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS–ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.

Adsorption of organic molecules on rutile TiO2 and anatase TiO2 single crystal surfaces

The interaction of organic molecules with titanium dioxide surfaces has been the subject of many studies over the last few decades. Numerous surface science techniques have been utilised to understand the often complex nature of these systems. The reasons for studying these systems are hugely diverse given that titanium dioxide has many technological and medical applications. Although surface science experiments investigating the adsorption of organic molecules on titanium dioxide surfaces is not a new area of research, the field continues to change and evolve as new potential applications are discovered and new techniques to study the systems are developed. This tutorial review aims to update previous reviews on the subject. It describes experimental and theoretical work on the adsorption of carboxylic acids, dye molecules, amino acids, alcohols, catechols and nitrogen containing compounds on single crystal TiO(2) surfaces.