Obtaining high performance of hematite (α-Fe2O3) in a photoelectrochemical (PEC) water splitting cell is a challenging task because of its poor electrical conductivity and extremely short carrier lifetime. Here, we introduce a new hydrothermal method, called gap hydrothermal synthesis (GAP-HS), to obtain textured hematite thin films with an outstanding PEC water oxidation performance. GAP-HS proceeds in a precursor-solution-filled narrow gap to induce an anisotropic ion supply. This gives rise to an interesting phenomenon associated with the growth of nanomaterials that reflect the texture of the used substrates. Also, GAP-HS causes the preferential growth of hematite crystal along the [110] direction, leading to improved electrical conductivity within the (001) basal plane. The hematite thin films obtained via GAP-HS exhibit a very high photocurrent of more than 1.3 mA cm–2 at 1.23 V with respect to the reversible hydrogen electrode with 550 °C annealing only. It is the highest photocurrent, to the best of our knowledge, obtained for the hydrothermally synthesized pristine hematite photoanode. Because the low-temperature annealing allows avoiding of substrate deformation, the hematite thin films obtained via GAP-HS are expected to be advantageous for tandem-cell configuration.

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State-of-the-art microdevice fabrication requires patterned growth of functional nanomaterials on the desired position of the desired substrate. However, it is challenging, particularly in conventional hydrothermal synthesis, due to difficulties generating a local high-temperature field at the desired place. We introduce a laser-induced hydrothermal growth (LIHG) process for the rapid and selective synthesis of iron oxide nanoparticles (NPs). The substrates absorb the laser energy to generate a local high-temperature field necessary for the growth of iron oxide NPs. On various substrates, a dome-like structure comprising many iron oxide NPs is selectively synthesized within a localized temperature field. The LIHG process has several advantages for iron oxide NP growth, including rapidity, seedless growth, substrate compatibility, position selectivity, and patterning availability. Using its advantages, the LIHG process is used to fabricate flexible micro-supercapacitors based on laser-carbonized colorless polyimide films with iron oxide NPs.

The laser-induced photoreduction process is proposed to obtain patterned magnetite (Fe3O4) nanorod (NR) arrays or the oxygen vacancy (OV) engineering of the hematite (α-Fe2O3) NRs. A continuous-wave laser beam, with a wavelength of 532 nm, is focused on the hematite NRs that are in contact with a liquid reducing agent to provide the thermal energy for triggering the reduction reaction. The path of the laser beam can be controlled through computer software, enabling the reduction reaction to occur in the arbitrary desired area. At lower laser powers than that at which the direct transformation of hematite into magnetite occurs, hematite NRs with a high concentration of OV are produced. The high OV concentration contributes to improving the electrical conductivity of the hematite NRs by increasing the donor density. The OV-abundant hematite NR array is applied to a photoanode in a photoelectrochemical (PEC) water-splitting cell. It exhibits an enhanced PEC performance due to its donor density being higher in comparison with the bare hematite NRs.