Robert G. Palgrave

Band alignment of rutile and anatase TiO2

The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO₂. The discovery of the photolysis of water on the surface of TiO₂ in 1972 launched four decades of intensive research into the underlying chemical and physical processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO₂ (ref. ). We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments, that a type-II, staggered, band alignment of ~ 0.4 eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust separation of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.

Self-cleaning coatings

This review summarises the key topics in the field of self-cleaning coatings, concentrating on the materials that have been used in commercial applications and recent research that aims to improve these materials. Hydrophobic and hydrophilic coatings are discussed, and the various mechanisms of self-cleaning are described and related to the material properties of the coatings. Although several multinational companies have released products incorporating self-cleaning coatings, there remains much potential in this field.

Titania and silver–titania composite films on glass—potent antimicrobial coatings

Titania (anatase) and Ag-doped titania (anatase) coatings were prepared on glass microscope slides by a sol - gel dip-coating method. The resultant coatings were characterised by X-ray diffraction, X-ray absorption near edge structure (XANES), Raman, scanning electron microscopy (SEM), wavelength dispersive X-ray (WDX) analysis, X-ray photoelectron spectroscopy (XPS) and UV-vis techniques and shown to consist of anatase with ca. 0.2 - 1 atom% Ag2O. Photocatalytic activity of the coatings was determined by photomineralisation of stearic acid, monitored by FT-IR spectroscopy. Photocatalytically-active coatings were screened for their antibacterial efficacy against Staphylococcus aureus (NCTC 6571), Escherichia coli ( NCTC 10418) and Bacillus cereus (CH70-2). Ag-doped titania coatings were found to be significantly more photocatalytically and antimicrobially active than a titania coating. No antimicrobial activity was observed in the dark - indicating that silver ion diffusion was not the mechanism for antimicrobial action. The mode of action was explained in terms of a charge separation model. The coatings also demonstrated significantly higher activity against the Gram-positive organisms than against the Gram-negative. The Ag2O - TiO2 coating is a potentially useful coating for hard surfaces in a hospital environment due to its robustness, stability to cleaning and reuse, and its excellent antimicrobial response.

Triazine‐Based Graphitic Carbon Nitride: a Two‐Dimensional Semiconductor

Graphitic carbon nitride has been predicted to be structurally analogous to carbon-only graphite, yet with an inherent bandgap. We have grown, for the first time, macroscopically large crystalline thin films of triazine-based, graphitic carbon nitride (TGCN) using an ionothermal, interfacial reaction starting with the abundant monomer dicyandiamide. The films consist of stacked, two-dimensional (2D) crystals between a few and several hundreds of atomic layers in thickness. Scanning force and transmission electron microscopy show long-range, in-plane order, while optical spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations corroborate a direct bandgap between 1.6 and 2.0 eV. Thus TGCN is of interest for electronic devices, such as field-effect transistors and light-emitting diodes.

Visible Light Photo-oxidation of Model Pollutants Using CaCu3Ti4O12: An Experimental and Theoretical Study of Optical Properties, Electronic Structure, and Selectivity

Charge transfer between metal ions occupying distinct crystallographic sublattices in an ordered material is a strategy to confer visible light absorption on complex oxides to generate potentially catalytically active electron and hole charge carriers. CaCu3Ti4O12 has distinct octahedral Ti4+ and square planar Cu2+ sites and is thus a candidate material for this approach. The sol−gel synthesis of high surface area CaCu3Ti4O12 and investigation of its optical absorption and photocatalytic reactivity with model pollutants are reported. Two gaps of 2.21 and 1.39 eV are observed in the visible region. These absorptions are explained by LSDA+U electronic structure calculations, including electron correlation on the Cu sites, as arising from transitions from a Cu-hybridized O 2p-derived valence band to localized empty states on Cu (attributed to the isolation of CuO4 units within the structure of CaCu3Ti4O12) and to a Ti-based conduction band. The resulting charge carriers produce selective visible light photodegradation of 4-chlorophenol (monitored by mass spectrometry) by Pt-loaded CaCu3Ti4O12 which is attributed to the chemical nature of the photogenerated charge carriers and has a quantum yield comparable with commercial visible light photocatalysts.

Aerosol assisted chemical vapour deposition of photochromic tungsten oxide and doped tungsten oxide thin films

Thin films of tungsten oxide, titanium doped tungsten oxide and overlayers of titanium oxide/tungsten oxide have been produced by aerosol assisted CVD (AACVD) at 400 °C. The films were crystalline monoclinic WO3 by XRD, showing strong preferred orientation in the (0 2 0) plane. Films were blue on deposition (WO3−x; x ≈ 0.1). Fully oxidised yellow WO3 films were produced by annealing the blue films in air at 500 °C. The annealed films showed a photochromic transition to a coloured blue state when irradiated with 254 nm or 365 nm UV light. The colour change was reversible; the films returned to their original yellow colour when left in the dark, after which the blue colour could be regained by further UV irradiation. The WO3 and layered WO3/TiO2 films showed significant photochromism, while comparable thickness titanium doped WO3 films showed reduced photochromism, proportional to the level of Ti doping.

Aerosol assisted chemical vapour deposition of WO3 thin films from tungsten hexacarbonyl and their gas sensing properties

Aerosol assisted chemical vapour deposition (AACVD) reactions of tungsten hexacarbonyl, [W(CO)6], in acetone, methanol, acetonitrile and a 50 : 50 mixture of acetone and toluene resulted in the deposition of blue partially reduced WO3−x films which showed preferred orientation along the (0 1 0) direction. Films deposited solely from toluene, however, were composed of a mixture of tungsten metal and W3O. All films could be annealed to yellow randomly orientated crystalline monoclinic WO3. The films deposited from methanol had a morphology comprising of a network of randomly orientated needles, which is strikingly different to the agglomeration of spherical particles observed with the other solvents. The WO3 films functioned as gas sensitive resistors for the detection of NO2. Responses were recorded at minimum concentrations of 1.03 ppm of NO2, significantly exceeding those of commercial screen printed sensors.

Scalable route to CH3NH3PbI3 perovskite thin films by aerosol assisted chemical vapour deposition

Methyl-ammonium lead iodide is the archetypal perovskite solar cell material. Phase pure, compositionally uniform methyl-ammonium lead iodide thin films on large glass substrates were deposited using ambient pressure aerosol assisted chemical vapour deposition. This opens up a route to efficient scale up of hybrid perovskite film growth towards industrial deployment.

A Polar Corundum Oxide Displaying Weak Ferromagnetism at Room Temperature

Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO3 (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO3 has a weak ferromagnetic ground state below 356 K—this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO3.

Aerosol-assisted chemical vapour deposition of WO3 thin films using polyoxometallate precursors and their gas sensing properties

Aerosol-assisted chemical vapour deposition (AACVD) of polytungstates in acetonitrile or water yielded thin films of tungsten oxide on glass. AACVD reactions of [NH4]6[W12O39] and [NH4]10H2[W2O7]6 and of [nBu4N]4[W10O32] and [nBu4N]2[W6O19] at substrate temperatures exceeding 500 °C resulted in the formation of yellow films comprised of randomly-orientated crystalline monoclinic WO3. In contrast, the films deposited from [nBu4N]3[WO4] were blue and showed preferred orientation along the direction, with the direction of crystallites becoming increasingly randomised with increasing deposition temperature. Annealing these films in air for 30 minutes at 550 °C yielded yellow films in which the crystallites were randomly orientated. The WO3 films functioned as gas sensors showing a linear change in electrical resistance upon exposure to trace amounts of ethanol and nitrogen dioxide vapour in air, with responses comparable to that of screen-printed sensors and a faster speed of response. Furthermore, the CVD sensors gave a maximum response to nitrogen dioxide at a significantly lower temperature (250 °C) than the screen-printed sensor.