Title : Plasma modification of alpha Fe2O3 supported nanomaterials for photocatalytic and photoelectrochemical applications
Abstract:
Hematite (α-Fe2O3), the most stable phase of iron(III) oxide, is an appealing active material for various photocatalytic and photoelectrochemical applications concerning sustainable energy generation and environmental remediation. In fact, beyond its favorable catalytic and electronic properties, as well as its band gap of 2.1 eV which allows absorption of Vis photons, this semiconductor exhibits an appreciable chemical stability and is cheap, abundant and nontoxic. Nevertheless, various hematite drawbacks, such as the relatively low absorption coefficient, short lifetime of photogenerated charge carriers (e-/h+), small hole diffusion length and slow reaction kinetics, have prevented up to now the achievement of satisfactory efficiencies.
An amenable strategy to circumvent these drawbacks and obtain improved performances, involves functionalization of α-Fe2O3 by doping or surface modification with suitable nanoparticles or (ultra)thin layers. In this way, additive or synergistic effects arising from the combination of two or more material components come into play. These effects might include, among others, an increased light harvesting, a higher surface reactivity, a reduced recombination of photogenerated e-/h+pairs and their more effective exploitation in the target chemical processes.
In order to fully exploit the advantages originating from α-Fe2O3 modification, a proper design of the final material is imperative to control its morphology, surface area, defect content and interface quality, all these characteristics being directly interrelated with the ultimate functional properties.
In the field of inorganic nanosystems, plasma-assisted routes such as sputtering and plasma enhanced-chemical vapor deposition (PE-CVD) offer a high control over topological, structural and compositional material features, impacting, in turn, a broad variety of technological applications.
In this contribution, attention will be focused on selected case studies demonstrating the high potential of plasma processing in the tailored fabrication of Fe2O3-based functional nanostructures. Representative examples will include:
i) the synthesis of Pt/α-Fe2O3 nanocomposites by a hybrid synthetic route, consisting in the PE-CVD of iron(III) oxide followed by platinum radiofrequency (RF-) sputtering and eventual annealing in air. Material characteristics such as Pt oxidation state and hematite nano-organization could be finely tuned as a function of the adopted processing conditions and strongly affected the system performances in sunlight-assisted photoelectrochemical water splitting;
ii) α-Fe2O3-TiO2-Au composites, fabricated by a three-step plasma-assisted strategy, and tested in the solar-driven H2 generation via photoreforming of ethanol aqueous solutions. Compared to bare hematite, Fe2O3-TiO2-Au photocatalysts displayed an improved functional behavior, rationalized in terms of an enhanced interfacial charge carrier separation and an improved light harvesting;
iii) supported Fe2O3/TiO2 nanocomposites, prepared by a sequential PE-CVD/RF-sputtering approach and successfully tested in gas-phase photocatalytic abatement of NOx (NO + NO2) driven by solar illumination. In this case, a good interfacial quality and an intimate Fe2O3/TiO2 contact was achieved, of key importance to exploit the chemical and electronic coupling between the two oxides. As a consequence, the obtained composites featured a remarkable activity in NOx removal, candidating them as valuable photocatalysts for the abatement of harmful atmospheric pollutants.
