Geoffrey Hyett

Doped and un-doped vanadium dioxide thin films prepared by atmospheric pressure chemical vapour deposition from vanadyl acetylacetonate and tungsten hexachloride: the effects of thickness and crystallographic orientation on thermochromic properties

The atmospheric pressure chemical vapour deposition reaction of vanadyl acetylacetonate and tungsten hexachloride with oxygen led to the production of thin films of tungsten doped monoclinic vanadium dioxide on glass substrates. Scanning electron microscopy and X-ray diffraction indicated that the films had different morphologies and crystallinities depending on the deposition conditions used. Transmission and reflectance measurements showed a significant change in properties in the near infra-red either side of the metal to semiconductor transition. Variable temperature transmission studies show that the metal to semiconductor transition was lowered by tungsten doping. The effect of film thickness was studied with un-doped and doped films. It was found that film thickness limited the intensity of light passing through the film and the extent of the thermochromic transition but was found not to influence the hysteresis width or temperature of transition. Different film growth conditions led to a range of film morphologies which profoundly affected the resulting optical properties of the films. It was found that film morphology and preferred crystallographic orientation had a marked influence on the width and switching temperature of the thermochromic transition.

A critical analysis of calcium carbonate mesocrystals.

The term mesocrystal has been widely used to describe crystals that form by oriented assembly, and that exhibit nanoparticle substructures. Using calcite crystals co-precipitated with polymers as a suitable test case, this article looks critically at the concept of mesocrystals. Here we demonstrate that the data commonly used to assign mesocrystal structure may be frequently misinterpreted, and that these calcite/polymer crystals do not have nanoparticle substructures. Although morphologies suggest the presence of nanoparticles, these are only present on the crystal surface. High surface areas are only recorded for crystals freshly removed from solution and are again attributed to a thin shell of nanoparticles on a solid calcite core. Line broadening in powder X-ray diffraction spectra is due to lattice strain only, precluding the existence of a nanoparticle sub-structure. Finally, study of the formation mechanism provides no evidence for crystalline precursor particles. A re-evaluation of existing literature on some mesocrystals may therefore be required.

The interaction between gold nanoparticles and cationic and anionic dyes: enhanced UV-visible absorption.

The interactions of charge stabilised gold nanoparticles with cationic and anionic dyes are reported. The nanoparticles were synthesised by the Turkevich citrate reduction method. It was found that when a solution of thiazine dye is titrated against gold citrate hydrosol, at a critical concentration of dye there is an enhanced maximum absorption in the dye. The extinction coefficient is increased up to ten-fold. This enhancement was observed for a number of cationic thiazine dyes, of which methylene blue and toluidine blue are established light-activated antimicrobial agents. The same enhancement was not observed for anionic, acidic dyes such as rose bengal which showed no communication with the gold nanoparticles and showed UV-visible titration experiments with well formed isosbestic points. By studying the interaction of the dye and gold nanoparticles under conditions of different ionic strength and by using a zetasizer and TEM to measure the gold nanoparticle size it was demonstrated that the cause of enhancement was not due to nanoparticle aggregation. It is proposed that thiazine cationic dyes coordinate around a gold nanoparticle and give significantly enhanced UV-visible absorptions.

High-throughput continuous hydrothermal synthesis of an entire nanoceramic phase diagram.

A novel High-Throughput Continuous Hydrothermal (HiTCH) flow synthesis reactor was used to make directly and rapidly a 66-sample nanoparticle library (entire phase diagram) of nanocrystalline Ce(x)Zr(y)Y(z)O(2-delta) in less than 12 h. High resolution PXRD data were obtained for the entire heat-treated library (at 1000 degrees C/1 h) in less than a day using the new robotic beamline I11, located at Diamond Light Source (DLS). This allowed Rietveld-quality powder X-ray diffraction (PXRD) data collection of the entire 66-sample library in <1 day. Consequently, the authors rapidly mapped out phase behavior and sintering behaviors for the entire library. Out of the entire 66-sample heat-treated library, the PXRD data suggests that 43 possess the fluorite structure, of which 30 (out of 36) are ternary compositions. The speed, quantity and quality of data obtained by our new approach, offers an exciting new development which will allow structure-property relationships to be accessed for nanoceramics in much shorter time periods.

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.

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.

Combinatorial atmospheric pressure chemical vapour deposition (cAPCVD) of a mixed vanadium oxide and vanadium oxynitride thin film

A novel combinatorial atmospheric pressure chemical vapour deposition (cAPCVD) technique was used to synthesise numerous vanadium oxide and vanadium oxynitride phases on a single film. This is the first example of cAPCVD having been used to synthesise a gradating mixed anion system. The film was characterised by X-ray diffraction (XRD) mapping, Raman, wavelength dispersive X-ray analysis (WDX) and X-ray photoelectron spectroscopy (XPS) analysis of positions along the films front edge allowed the chemical composition to be determined and correlated with XRD data. Film thicknesses were determined using side-on scanning electron microscopy (SEM). Functional property mapping of the optical transmittance/reflectance and electrical resistance allowed systematic investigation on the effects of oxygen content within a vanadium oxynitride film. cAPCVD used in conjunction with mapping analysis tools is a shortcut for identifying numerous, phases, compositions and properties and their relationships on a single film, and offers a rapid method for analysis of phase-space.

An Investigation into the Optimum Thickness of Titanium Dioxide Thin Films Synthesized by Using Atmospheric Pressure Chemical Vapour Deposition for Use in Photocatalytic Water Oxidation

Twenty eight films of titanium dioxide of varying thickness were synthesised by using atmospheric pressure chemical vapour deposition (CVD) of titanium(IV) chloride and ethyl acetate onto glass and titanium substrates. Fixed reaction conditions at a substrate temperature of 660 °C were used for all depositions, with varying deposition times of 5-60 seconds used to control the thickness of the samples. A sacrificial electron acceptor system composed of alkaline sodium persulfate was used to determine the rate at which these films could photo-oxidise water in the presence of 365 nm light. The results of this work showed that the optimum thickness for CVD films on titanium substrates for the purposes of water oxidation was ≈200 nm, and that a platinum coating on the reverse of such samples leads to a five-fold increase in the observed rate of water oxidation.

Synthesis and characterization of mixed phase anatase TiO2 and sodium-doped TiO2(B) thin films by low pressure chemical vapour deposition (LPCVD)

TiO2 thin films were synthesized using Low Pressure Chemical Vapour Deposition (LPCVD) onto glass substrates. Titanium isopropoxide (TTIP) and N2 gas were used as the precursor and carrier gas respectively. The effects of reaction temperature, carrier gas flow rate and deposited area were studied. SEM, TEM, powder XRD and UV-Vis and Raman spectroscopy were employed to characterize the phase and morphology of the synthesized materials. The results show that a dual phase (sodium-doped TiO2(B) and anatase) nanocrystalline thin film was successfully prepared by LPCVD with needle- and polygonal plate-shape crystallites respectively. At the interface with the substrate, the thin film deposit exhibited a preferred orientation of TiO2(B) needles in the [001] direction with an average crystallite size of 50–80 nm in length and 5–10 nm in width, whilst the crystallite size of anatase polygonal-plates was around 200 nm. The optimal LPCVD condition for preparing this mixed phase of TiO2 was 550 °C (actual temperature) with a 1 mL s−1 N2 flow rate. A possible mechanism for the mixed-phase formation by LPCVD on the glass substrates is described as well as the implications for the production of self-cleaning structures.

Photomagnetic studies on spin-crossover solid solutions containing two different metal complexes, [Fe(1-bpp)2]x[M(terpy)2]1−x[BF4]2 (M = Ru or Co)

The photomagnetic properties of two series of spin-crossover solid solutions, [Fe(1-bpp)(2)](x)[Ru(terpy)(2)](1-x)(BF(4))(2) and [Fe(1-bpp)(2)](x)[Co(terpy)(2)](1-x)(BF(4))(2) (1-bpp = 2,6-bis[pyrazol-1-yl]pyridine), have been investigated. For all the materials, the evolution of the T(LIESST) value, the high-spin → low-spin relaxation parameters and the LITH loops were thoroughly studied. Interestingly in the Fe:Co series, along the photo-excitation, cobalt ions are concomitantly converted from low-spin to high-spin states with the iron centres, and also fully relax after light excitation.