Effect of Doping (C or N) and Co-Doping (C+N) on the Photoactive Properties of Magnetron Sputtered Titania Coatings for the Application of Solar Water-Splitting

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.