Sintering of Fe2O3 thin film from natural micaceous hematite powders as photoanodes in a photoelectrochemical water reduction

Fe2O3 (mineral name: hematite) has attracted research interest as a photoanode in photoelectrochemical (PEC) water reduction devices. Its structural, electronic and optical properties allow to promote an oxygen evolution reaction in a water-splitting reaction. Apart from its properties, hematite’s components of Fe and O are also abundant and non-toxic, guaranteeing a sustainable materials supply for envisioned green hydrogen production. Fe2O3 also exists in nature in the form of micaceous hematite. It has a platelet or lamellae-like ore morphology with relatively high Fe and O purities. In this contribution, a natural micaceous hematite Fe2O3 was employed as a feedstock for providing photoactive Fe2O3 thin film as a photoanode in a PEC water reduction device. To prepare hematite thin films, the hematite powders underwent a powder-to-film approach by firstly grinding the ore into sub-micron powder by using industrial jet milling, followed by ink formulation, thin film deposition by using spin coating and finalized by quartz tube sintering in air at 600 – 1000 °C for 30 minutes. The original hematite raw powders demonstrate lamellae morphologies with a nominal particle size as large as 100 µm through electron microscopy analysis. Jet milling for the grinding process was able to grind the powder with D50 of 0.69 µm and cumulative D90 of 1.43 µm. The hematite powder composition taken by EDX shows a main element of Fe and O with traceable impurities of C, Na, Mg, Al, Si, S, K, Ca, Ti, Mn, Co. Following XRD qualitative analysis, the original and as-grinded hematite powders resemble the hematite phase (Fe2O3) as the primary phase, with the possible occurrence of two iron oxide phases; magnetite (Fe3O4) and goethite (FeO2). The Fe2O3 ink formulation as the next step towards thin film preparation was prepared by employing an environmentally friendly binder of Gum Arabicum and de-ionized water as the solvent. The spin coating deposition was performed using a dynamic spinning procedure where the 100 µl hematite ink was dropped on the 25 x 25 mm2 substrates during the substrate spin at 1000 rpm for 10 sec., followed by a gradual spin rotation from 1500 rpm for 30 sec. and 2000 rpm, for another 30 sec. To allow high-temperature sintering of powders to form dense and packed hematite layers, titanium plates were employed as substrates. The as-spin coated hematite films were sintered successfully at temperatures > 800 °C to form compact hematite films with a thickness of 3-5 µm. Films with a large grain of hematite could be prepared at 1000 °C sintering temperature.