Christopher S. Blackman

APCVD of thermochromic vanadium dioxide thin films—solid solutions V2−xMxO2 (M = Mo, Nb) or composites VO2 : SnO2

Atmospheric pressure chemical vapour deposition of V2−xMxO2 (M = Mo, Nb; X = 0.01–0.003) thin films was achieved on glass substrates from the reaction of VOCl3, H2O and MCl5. Comparable reactions with SnCl4 formed SnO2 : VO2 composites. The ease with which solid solutions or composite films formed was related to the relative reaction rates. The films were characterised by X-ray diffraction, Raman, X-ray photoelectron spectroscopy and scanning electron microscopy. Doping of the VO2 phase was shown to affect both the growth morphology and the thermochromic properties of the films. The Mo-doped VO2 films showed a thermochromic switch of 47 °C with a narrow hysteresis (4–6 °C).

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.

MOCVD of crystalline Bi2O3 thin films using a single-source bismuth alkoxide precursor and their use in photodegradation of water

Bismuth(III) tert-butoxide [Bi(OtBu)3] was utilised as a single-source precursor to controllably deposit thin films of different phases of bismuth oxide (Bi2O3) on glass substrates via low-pressure chemical vapour deposition (LPCVD). Band gaps for the different phases have been measured (Eg = 2.3–3.0 eV) and the films displayed excellent photodegradation of water under near-UV irradiation.

Thermochromic Coatings for Intelligent Architectural Glazing

Thermochromic glazing is a type of intelligent glazing; one where the properties of the glazing change according to some external stimulus. More particularly a thermochromic window is a device that changes its transmission and reflectance properties at a critical temperature (Tc). Atthis specific temperature the material undergoes a semi-conductor to metal transition. At temperatures lower than Tc the window lets all of the solar energy that hits it through. At emperatures above Tc the window reflects the infra-red portion of solar energy. In such a way thermochromic windows may help reduce air conditioning and heating costs leading to more energy efficient buildings. This review details the nature of the semi-conductor to metal transition and indicates how substitutional doping within a crystal lattice can be used to manipulate and fine tune the critical temperature. Also detailed is the underlying science and methodologies so far employed in the production of thermochromic thin films.

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.

A simple, low-cost CVD route to thin films of BiFeO3 for efficient water photo-oxidation

A novel method for preparation of BiFeO3 films via a simple solution-based CVD method is reported using for the first time a single-source heterobimetallic precursor [CpFe(CO)2BiCl2]. BiFeO3 films display ferroelectric and ferromagnetic ordering at room temperature and possess direct band-gaps between 2.0 and 2.2 eV. Photocatalytic testing for water oxidation revealed high activities under UVA (365 nm) and simulated solar irradiation, superior to that exhibited by a commercial standard (Pilkington Activ® TiO2 film) resulting in an apparent quantum yield of ∼24%.

Nanostructured tungsten oxide gas sensors prepared by electric field assisted aerosol assisted chemical vapour deposition

Nanostructured thin films of tungsten trioxide were deposited on to gas sensor substrates at 600 °C from the aerosol assisted chemical vapour deposition reaction of tungsten hexaphenoxide solutions in toluene under the influence of electric fields. The electric fields were generated by applying a potential difference between the inter-digitated electrodes of the gas sensor substrates during the deposition. The deposited films were characterised using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The application of an electric field, encouraged formation of enhanced nanostructured morphologies, with an increase in needle length and reduction in needle diameter being observed. The film gas sensor properties were also examined; it was found that the highest response of 110 to 800 ppb NO2 was given by a sensor grown under the influence of a 1.8 × 104 V m−1 electric field and operated at 250 °C, a 2.5 times enhancement compared to a sensor grown in the absence of an electric field under its optimal operating conditions.

Aerosol assisted chemical vapour deposition of MoO3 and MoO2 thin films on glass from molybdenum polyoxometallate precursors; thermophoresis and gas phase nanoparticle formation

Aerosol assisted chemical vapour deposition (AACVD) of molybdenum polyoxometallates dissolved in acetonitrile or water yielded adhesive thin films of molybdenum oxides on glass. At substrate temperatures of 300–350 °C single phase MoO3 was obtained, from 350–500 °C mixed phases of MoO3–MoO2 were formed and at 500–550 °C single phase MoO2 was observed. The morphology of the as-deposited molybdenum oxide films was found to be dependent upon a number of factors including the nature of the precursor used, the deposition temperature and position of the film within the reactor. Needles, spheres, agglomerates and platelets formed depending on the conditions. The films with a needle-like microstructure displayed enhanced hydrophobicity to water droplets (125° contact angle). X-Ray diffraction showed that the MoO3 films had typical cell constants of a = 3.96, b = 13.85, c = 3.69 A and the MoO2 films had typical cell constants of a = 5.62, b = 4.84, c = 5.56 A, β = 119.32°. The MoO2 films were readily converted to MoO3 by annealing in air for 30 minutes at 600 °C. The MoO3 films functioned as gas sensors showing a linear change in electrical resistance upon exposure to trace amounts of ethanol vapour in air.

Aerosol-Assisted CVD-Grown PdO Nanoparticle-Decorated Tungsten Oxide Nanoneedles Extremely Sensitive and Selective to Hydrogen

We report for the first time the successful synthesis of palladium (Pd) nanoparticle (NP)-decorated tungsten trioxide (WO3) nanoneedles (NNs) via a two-step aerosol-assisted chemical vapor deposition approach. Morphological, structural, and elemental composition analysis revealed that a Pd(acac)2 precursor was very suitable to decorate WO3 NNs with uniform and well-dispersed PdO NPs. Gas-sensing results revealed that decoration with PdO NPs led to an ultrasensitive and selective hydrogen (H2) gas sensor (sensor response peaks at 1670 at 500 ppm of H2) with low operating temperature (150 °C). The response of decorated NNs is 755 times higher than that of bare WO3 NNs. Additionally, at a temperature near that of the ambient temperature (50 °C), the response of this sensor toward the same concentration of H2 was 23, which is higher than that of some promising sensors reported in the literature. Finally, humidity measurements showed that PdO/WO3 sensors displayed low-cross-sensitivity toward water vapor, compared to bare WO3 sensors. The addition of PdO NPs helps to minimize the effect of ambient humidity on the sensor response.

The gas-sensing properties of WO3−x thin films deposited via the atmospheric pressure chemical vapour deposition (APCVD) of WCl6 with ethanol

The use of the APCVD reaction of WCl6 with ethanol to produce tungsten oxide films (3600?6700 nm thick) for use as gas sensors is presented. The response of these films to varying concentrations of ethanol and NO2 at varying temperatures has been examined. A comparison of the CVD sensors to a thick film (~60 ?m) screen-printed sensor has shown that the CVD sensors have a faster rate of response, a stable sensing response and faster return to baseline with no drift at the conditions examined.