α-Fe2O3/TiO2 stratified photoanodes

Atomic layer deposited films of Al2O3 on fluorine-doped tin oxide electrodes: stability and barrier properties

Al₂O₃ layers were deposited onto electrodes by atomic layer deposition. Solubility and electron-transport blocking were tested. Films deposited onto fluorine-doped tin oxide (FTO, F:SnO₂/glass) substrates blocked electron transfer to redox couples (ferricyanide/ferrocyanide) in aqueous media. However, these films were rapidly dissolved in 1 M NaOH (≈100 nm/h). The dissolution was slower in 1 M H₂SO₄ (1 nm/h) but after 24 h the blocking behaviour was entirely lost. The optimal stability was reached at pH 7.2 where no changes were found up to 24 h and even after 168 h of exposure the changes in the blocking behaviour were still minimal. This behaviour was also observed for protection against direct reduction of FTO.

Enhanced visible light photocatalytic activity of Fe2O3 modified TiO2 prepared by atomic layer deposition

In this work, commercial anatase TiO2 powders were modified using ultrathin Fe2O3 layer by atomic layer deposition (ALD). The ultrathin Fe2O3 coating having small bandgap of 2.20 eV can increase the visible light absorption of TiO2 supports, at the meantime, Fe2O3/TiO2 heterojunction can effectively improve the lifetime of photogenerated electron-hole pairs. Results of ALD Fe2O3 modified TiO2 catalyst, therefore, showed great visible light driven catalytic degradation of methyl orange compared to pristine TiO2. A 400 cycles of ALD Fe2O3 (~ 2.6 nm) coated TiO2 powders exhibit the highest degradation efficiency of 97.4% in 90 min, much higher than pristine TiO2 powders of only 12.5%. Moreover, an ultrathin ALD Al2O3 (~ 2 nm) was able to improve the stability of Fe2O3-TiO2 catalyst. These results demonstrate that ALD surface modification with ultrathin coating is an extremely powerful route for the applications in constructing efficient and stable photocatalysts.

Semiconducting p-Type Copper Iron Oxide Thin Films Deposited by Hybrid Reactive-HiPIMS + ECWR and Reactive-HiPIMS Magnetron Plasma System

A reactive high-power impulse magnetron sputtering (r-HiPIMS) and a reactive high-power impulse magnetron sputtering combined with electron cyclotron wave resonance plasma source (r-HiPIMS + ECWR) were used for the deposition of p-type CuFexOy thin films on glass with SnO2F conductive layer (FTO). The aim of this work was to deposit CuFexOy films with different atomic ratio of Cu and Fe atoms contained in the films by these two reactive sputtering methods and find deposition conditions that lead to growth of films with maximum amount of delafossite phase CuFeO2. Deposited copper iron oxide films were subjected to photoelectrochemical measurement in cathodic region in order to test the possibility of application of these films as photocathodes in solar hydrogen production. The time stability of the deposited films during photoelectrochemical measurement was evaluated. In the system r-HiPIMS + ECWR, an additional plasma source based on special modification of inductively coupled plasma, which works with an electron cyclotron wave resonance ECWR, was used for further enhancement of plasma density ne and electron temperature Te at the substrate during the reactive sputtering deposition process. A radio frequency (RF) planar probe was used for the determination of time evolution of ion flux density iionflux at the position of the substrate during the discharge pulses. Special modification of this probe to fast sweep the probe system made it possible to determine the time evolution of the tail electron temperature Te at energies around floating potential Vfl and the time evolution of ion concentration ni. This plasma diagnostics was done at particular deposition conditions in pure r-HiPIMS plasma and in r-HiPIMS with additional ECWR plasma. Generally, it was found that the obtained ion flux density iionflux and the tail electron temperature Te were systematically higher in case of r-HiPIMS + ECWR plasma than in pure r-HiPIMS during the active part of discharge pulses. Furthermore, in case of hybrid discharge plasma excitation, r-HiPIMS + ECWR plasma has also constant plasma density all the time between active discharge pulses ni ≈ 7 × 1016 m−3 and electron temperature Te ≈ 4 eV, on the contrary in pure r-HiPIMS ni and Te were negligible during the “OFF” time between active discharge pulses. CuFexOy thin films with different atomic ration of Cu/Fe were deposited at different conditions and various crystal structures were achieved after annealing in air, in argon and in vacuum. Photocurrents in cathodic region for different achieved crystal structures were observed by chopped light linear voltammetry and material stability by chronoamperometry under simulated solar light and X-ray diffraction (XRD). Optimization of depositions conditions results in the desired Cu/Fe ratio in deposited films. Optimized r-HiPIMS and r-HiPIMS + ECWR plasma deposition at 500 °C together with post deposition heat treatment at 650 °C in vacuum is essential for the formation of stable and photoactive CuFeO2 phase.

TiO2/Fe2O3 heterostructures with enhanced photocatalytic reduction of Cr(vi) under visible light irradiation

We report a study on the synthesis of TiO₂/Fe₂O₃ (TF) nanocomposites and their photocatalytic performance under visible-light irradiation. The characterization of structure and morphology shows that hematite Fe₂O₃ was deposited on anatase TiO₂ nanoparticles with particle sizes in the range of 20–100 nm. In contrast to pure TiO₂ and pure Fe₂O₃, the nanocomposites exhibited remarkable photocatalytic activity. For example, the photoreduction efficiency of TF0.5 reaches 100% for a 100 ppm Cr(vi) solution within 160 minutes. The photochemical properties were studied by various methods. Finally, we conclude that the excellent performance of the photocatalysts is mainly attributed to two aspects: the enhanced absorption of visible light and the synergistic effect of an internal electric field at the heterojunction and citric acid for promoting the separation of electron–hole pairs.

A TiO₂/Fe₂O₃ heterojunction with an internal electric field was constructed for enhancing photocatalytic reduction efficiency of Cr(vi).