[en] Highlights: • Precisely-controlled Fe₂TiO₅ layers of IOS is fabricated via a layer-by-layer self-assembly. • Hybrid microwave annealing yields highly transparent and crystalline Fe₂TiO₅ IOS photoelectrode. • IOS provides enhanced light harvesting and higher density of catalytically active crystal planes. • The modified IOS photoanode shows a high PEC water splitting activity. -- Abstract: Iron titanate (Fe₂TiO₅) is a promising photoanode material due to a narrow band gap, appropriate band edges, robustness and abundance. However, its performance is limited because of its low conductivity and short hole diffusion length. Precisely controlled, a few Fe₂TiO₅ layers of inverse opal structure (IOS) is fabricated via a layer-by-layer self-assembly and then treated by hybrid microwave annealing to produce a highly crystalline, yet IOS morphology-preserved Fe₂TiO₅ photoanode film for solar water splitting. The highly transparent Fe₂TiO₅ IOS film shows a greatly enhanced visible light harvesting, higher density of catalytically more active crystal planes, and many single crystalline nanoplates grown on the IOS architecture, relative to a reference planar film prepared under similar conditions. As a result, the optimized ‘exactly’ three Fe₂TiO₅ layers IOS electrode with a sacrificial gallium oxide underlayer and a ternary (Ni₂CoFe)OOH co-catalyst records 2.08 mA cm⁻² at 1.23 V_RHE under 1 sun (100 mW cm⁻²) irradiation, which is the highest photocurrent density produced by Fe₂TiO₅ photoanode up to date.