The photoactivity of both un-doped and carbon-doped titanium dioxide (TiO 2 ) coatings has been widely reported. In this paper, the use of a microwave plasma as a novel oxidation treatment for the fabrication of these coatings is evaluated. The photoactivity performance of the microwave plasma-formed coatings is benchmarked against those fabricated through air furnace oxidation as well as those deposited using reactive magnetron sputtering. The un-doped and carbon-doped TiO 2 coatings were prepared respectively by microwave plasma-oxidizing titanium metal sheets and sputter deposited titanium carbide thin films. The resulting oxides were characterized using XPS, XRD, FEG-SEM, and optical profilometry. The oxide layer thicknesses achieved over the 15 to 45 minute oxidation times were in the range of 0.15 to 3.44 µm. These coatings were considerably thicker than those obtained by air furnace oxidation. The microwave plasma-formed oxides also exhibited significantly higher surface roughness values compared with the magnetron-sputtered coatings. The photoactivity performance of both un-doped and carbon-doped coatings was assessed using photocurrent density measurements. Comparing the un-doped TiO 2 coatings, it was observed that those obtained using the microwave plasma oxidation route yielded photocurrent density measurements that were 4.3 times higher than the TiO 2 coatings of the same thickness that were deposited by sputtering. The microwave plasmaoxidized titanium carbide coatings did not perform as well as the un-doped TiO 2 probably due to the presence of un-oxidized carbide in the coatings, which reduced their photoactivity.
Crystalline titanium dioxide (TiO 2 ) coatings have been widely used in photoelectrochemical solar cell applications. In this study, TiO 2 and carbon-doped TiO 2 coatings were deposited onto unheated titanium and silicon wafer substrates using a DC closed-field magnetron 2 sputtering system. The resultant coatings had an amorphous structure and a post-deposition heat treatment is required to convert this amorphous structure into the photoactive crystalline phase(s) of TiO 2 . This study investigates the use of a microwave plasma heat treatment as a means of achieving this crystalline conversion. The treatment involved placing the sputtered coatings into a 2.45 GHz microwave-induced nitrogen plasma where they were heated to approximately 550°C.It was observed that for treatment times as short as 1 minute, the 0.25-µm thick coatings were converted into the anatase crystalline phase of TiO 2 . The coatings were further transformed into the rutile crystalline phase after treatments at higher temperatures. The doping of TiO 2 with carbon was found to result in a reduction in this phase transformation temperature, with higher level of doping (up to 5.8% in this study) leading to lower anatase-to-rutile transition temperature. The photoactivity performance of both doped and un-doped coatings heat-treated using both furnace and microwave plasma was compared. The carbon-doped TiO 2 exhibited a 29% increase in photocurrent density compared to that observed for the un-doped coating.Comparing carbon-doped coatings heat-treated using the furnace and microwave plasma, it was observed that the latter yielded a 19% increase in photocurrent density. This enhanced performance may be correlated to the differences in the coatings' surface morphology and band gap energy, both of which influence the coatings' photoabsorption efficiency.
The photoactivity of carbon-incorporated titanium dioxide (TiO 2 ) has been widely reported. This study involves a novel approach to the incorporation of carbon into TiO 2 through the use of microwave plasma processing. The process involved thermally treating printed TiO 2 nanoparticle coatings in a microwave-induced argon-oxygen plasma containing low concentrations of methane. The resulting deposited carbon layer was characterized using XRD, XPS, Raman, UV-VIS, ellipsometry, and optical profilometry. It was found that methane gas had been dissociated in the microwave plasma into carbon species, which were then deposited as a nm-thick layer onto the TiO 2 coatings, most likely in the form of graphite. The photovoltaic performances of both the TiO 2 and the carbon-incorporated TiO 2 were assessed through J-V and IPCE measurements of the N719-sensitized solar cells using the titania as their photoanodes. Up to a 72% improvement in the maximum power density (P d-max ) was observed for the carbonincorporated TiO 2 samples as compared to the TiO 2, onto which no carbon was added. This improvement was found to be mainly associated with an increase in the short-circuit current density (J sc ), but independent from the open-circuit voltage (V oc ), the filter factor (FF), and the level of dye adsorption. Possible contributory factors to the improved performance of the carbonincorporated TiO 2 were the enhanced electron conductivity and electron lifetime, both of which were elucidated through electrochemical impedance spectroscopy (EIS). When the surface layer was examined using XPS, the optimal carbon content on the TiO 2 coating surface was found to be 8.4%, beyond which there was a reduction in the DSSC efficiency.
The photocatalytic splitting of water into hydrogen and oxygen using a photoelectrochemical (PEC) cell containing titanium dioxide (TiO2) photoanode is a potentially renewable source of chemical fuels. However, the size of the band gap (-3.2 eV) of the TiO2 photocatalyst leads to its relatively low photoactivity toward visible light in a PEC cell. The development of materials with smaller band gaps of approximately 2.4 eV is therefore necessary to operate PEC cells efficiently. This study investigates the effect of dopant (C or N) and co-dopant (C+N) on the physical, structural and photoactivity of TiO2 nano thick coating. TiO2 nano-thick coatings were deposited using a closed field DC reactive magnetron sputtering technique, from titanium target in argon plasma with trace addition of oxygen. In order to study the influence of doping such as C, N and C+N inclusions in the TiO2 coatings, trace levels of CO2 or N2 or CO2+N2 gas were introduced into the deposition chamber respectively. The properties of the deposited nano-coatings were determined using Spectroscopic Ellipsometry, SEM, AFM, Optical profilometry, XPS, Raman, X-ray diffraction UV-Vis spectroscopy and tri-electrode potentiostat measurements. Coating growth rate, structure, surface morphology and roughness were found to be significantly influenced by the types and amount of doping. Substitutional type of doping in all doped sample were confirmed by XPS. UV-vis measurement confirmed that doping (especially for C doped sample) facilitate photoactivity of sputtered deposited titania coating toward visible light by reducing bandgap. The photocurrent density (indirect indication of water splitting performance) of the C-doped photoanode was approximately 26% higher in comparison with un-doped photoanode. However, coating doped with nitrogen (N or N+C) does not exhibit good performance in the photoelectrochemical cell due to their higher charge recombination properties.
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