Michael E. A. Warwick

Advances in thermochromic vanadium dioxide films

Vanadium dioxide is a thermochromic material that undergoes a semiconductor to metal transitions at a critical temperature of 68 °C. This phase change from a low temperature monoclinic structure to a higher temperature rutile structure is accompanied by a marked change in infrared reflectivity and change in resistivity. This review presents the fundamental chemical principles that describe the electronic structure and properties of solids, and the chronological developments in the theory behind the thermochromic transitions such as, the effects of electron–electron interactions and structural phase changes due to lattice distortions. An extensive discussion and observations on the current understanding of the nature of the semiconductor-to-metal transition exhibited by vanadium dioxide is detailed. The possibility of manipulating the transition temperature by introducing various dopants, additional layers or by size effects into the vanadium dioxide lattice are examined. Thermochromic vanadium dioxide materials may be exploited in areas such as microelectronics, data storage, or intelligent architectural glazing, thus are required to be synthesised as thin films for use in such applications. The numerous synthetic techniques (physical vapour deposition, sol–gel method, pulsed laser deposition, chemical vapour deposition), for making metal oxide thermochromic thin films are described in reference to the production of vanadium dioxide with a particular focus on recent results.

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.

The Application of Electric Fields to Aerosol Assisted Chemical Vapor Deposition Reactions

Chemical vapor deposition methodologies are widely employed in a variety of fields such as microelectronics and glazing. Control of film growth and microstructure, and hence film properties, may be limited by precursor properties such as volatility or decomposition chemistry. In this paper we report how the incorporation of an applied electric field to aerosol assisted chemical vapor deposition reactions of vanadyl acetylacetonate in alcohols can influence the microstructure and growth of thin films of vanadium dioxide in unusual and sometimes unexpected ways. The films were characterized using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy.

Fe2O3–TiO2 nanosystems by a hybrid PE-CVD/ALD approach: controllable synthesis, growth mechanism, and photocatalytic properties

Supported Fe2O3–TiO2 nanocomposites are fabricated by an original vapor phase synthetic strategy, consisting of the initial growth of Fe2O3 nanosystems on fluorine-doped tin oxide substrates by plasma enhanced-chemical vapor deposition, followed by atomic layer deposition of TiO2 overlayers with variable thickness, and final thermal treatment in air. A thorough characterization of the target systems is carried out by X-ray diffraction, atomic force microscopy, field emission-scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. High purity nanomaterials characterized by the co-presence of Fe2O3 (hematite) and TiO2 (anatase), with an intimate Fe2O3–TiO2 contact, are successfully obtained. In addition, photocatalytic tests demonstrate that, whereas both single-phase oxides do not show appreciable activity, the composite systems are able to degrade methyl orange aqueous solutions under simulated solar light, and even visible light, with an efficiency directly dependent on TiO2 overlayer thickness. This finding opens attractive perspectives for eventual applications in wastewater treatment.

Electric field assisted chemical vapour deposition – a new method for the preparation of highly porous supercapacitor electrodes

Nanostructured thin films of vanadium oxides were deposited using electric field assisted chemical vapour deposition. The films were characterised using scanning electron microscopy, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy. It was found that the films had open and porous morphologies with extremely small (5 nm) surface features. The films were made into supercapacitor cells and tested using cyclic voltammetry. It was found that stable asymptotic values specific capacitance values as high as 3700 μF cm−2 could be obtained with good cycling behaviour. Electrodes synthesized in this way show promise for applications in fields such as supercapacitors.

TiO2-Fe2O3 and Co3O4-Fe2O3 nanocomposites analyzed by X-ray Photoelectron Spectroscopy

The present work is focused on the characterization of TiO2-Fe2O3 and Co3O4-Fe2O3 nanocomposites, of potential interest as photoanodes for photoelectrochemical (PEC) water splitting triggered by solar light. In particular, Fe2O3 nanostructures were deposited onto fluorine-doped tin oxide (FTO)-coated glass substrates by plasma enhanced-chemical vapor deposition (PE-CVD), and functionalized by either TiO2 or Co3O4, obtained via atomic layer deposition (ALD) or radio frequency (RF)-sputtering, respectively. The resulting systems were investigated by complementary techniques in order to obtain detailed information on their structure and morphological organization. In particular, their chemical composition was analyzed through the use of X-ray Photoelectron and X-ray Excited-Auger Electron Spectroscopies (XPS and XE-AES). To this regard, detailed spectra for C 1s, O 1s, Fe 2p and Ti 2p (or Co 2p and Co LMM) regions are reported and discussed.

A study of Pt/α-Fe2O3 Nanocomposites by XPS

α-Fe2O3 matrices were deposited on Fluorine-doped Tin Oxide (FTO) substrates by Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) from Fe(hfa)2TMEDA (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine). The obtained nanosystems were subsequently functionalized by platinum nanoparticles (NPs) via Radio Frequency (RF)-sputtering, exposing samples either to a pre- or post-sputtering thermal treatment at 650 °C for one hour in air. Interestingly, Pt oxidation state in the final composite systems strongly depended on the adopted processing conditions. In this work, a detailed X-ray Photoelectron Spectroscopy (XPS) analysis was carried out in order to investigate the material chemical composition, with particular regard to the relative Pt(0)/Pt(II)/Pt(IV) content. The obtained results evidenced that, when annealing is performed prior to sputtering, only PtO and PtO2 are revealed in the final Pt/α-Fe2O3 nanocomposite. In a different way, annealing after sputtering resu...

XPS analysis of Fe2O3-TiO2-Au nanocomposites prepared by a plasma-assisted route

Fe2O3 nanodeposits have been grown on fluorine-doped tin oxide (FTO) substrates by plasma enhanced-chemical vapor deposition (PE-CVD). Subsequently, the obtained systems have been functionalized through the sequential introduction of TiO2 and Au nanoparticles (NPs) by means of radio frequency (RF)-sputtering. The target nanocomposites have been specifically optimized in view of their ultimate functional application in solar-driven H2 generation. In the present study, our attention is focused on a detailed X-ray photoelectron spectroscopy (XPS) characterization of the surface composition for a representative Fe2O3-TiO2-Au specimen. In particular, this report provides a detailed discussion of the analyzed C 1s, O 1s, Fe 2p, Ti 2p, and Au 4f regions. The obtained results point to the formation of pure Fe2O3-TiO2-Au composites, with gold present only in its metallic state and each of the constituents maintaining its chemical identity.

Electric Field–Assisted Chemical Vapor Deposition for Nanostructured Thin Films

Nanostructured thin films of tungsten, vanadium, and titanium oxides were deposited on gas sensor substrates from the electric field–assisted chemical vapor deposition reaction of tungsten hexaphenoxide, vanadyl acetylacetonate, and titanium tetraisopropoxide, respectively. The electric fields were generated by applying a potential difference between the interdigitated electrodes of the gas sensor substrates during the deposition. The deposited films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The application of an electric field encouraged the formation of interesting and unusual nanostructured morphologies, with a change in scale length and island packing. It was also noted that crystallographic orientation of the films could be controlled as a function of electric field type and strength. The gas sensor properties of the films were also examined; it was found that a two- to threefold enhancement in the gas response could be observed from sensors with enhanced morphologies compared to control sensors grown without application of an electric field.

Electric Fields and Chemical Vapor Deposition

Chemical vapor deposition methodologies are widely employed in a variety of fields such as microelectronics and glazing. Control of film growth and microstructure, and hence film properties, may be limited by precursor properties such as volatility or decomposition chemistry. In this paper we report how the incorporation of an applied electric field to AACVD reactions of vanadyl acetylacetonate in alcohols can influence the microstructure and growth of thin films of vanadium dioxide in unusual and sometimes unexpected ways. The films were characterized using scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman Spectroscopy.