Crack-free hematite inverse opal photo-anodes for enhancing photo-electrochemical water splitting

Abstract

Cracking is a severe problem in photonic crystals and various functional colloidal coatings, greatly plaguing their applications in energy, optics, catalysis, filtration, etc. Although several elaborate self-assembly approaches have demonstrated great success in avoiding crack formation in photonic materials, there is still no efficient method that can heal the cracks after their occurrence. In this work, crack-free hematite inverse opal photo-anodes were fabricated by directly “sewing” the micro-cracks in opal templates via in situ sequential chemical reactions and nanostructure transformations among iron-based species. The in situ growth of β-FeO(OH) nanoneedles from the hydrolysis of the infiltrated ferric ions bridged the cracks and the shrinking force during their change into nanoparticles under calcination contributed to the healing of the cracks. The crack-free feature of the inverse opals was characterized by scanning electron microscopy, optical microscopy, and reflectance spectrometry. Furthermore, through the solid-state reactions between iron oxides/hydroxides, a variety of other catalysts (Fe₃O₄, γ-Fe₂O₃, and FeP) in the form of crack-free inverse opal structures were obtained, demonstrating the robustness of this method and facilitating their catalytic applications. Based on the fine-tuning of the photonic bandgap of the inverse opals across the entire visible spectrum by controlling their pore sizes, the crack-free hematite photo-anodes with an optimized photonic bandgap of ∼428 nm exhibited a photocurrent density of 2.08 mA cm⁻² at 1.23 V versus the reversible hydrogen electrode for photo-electrochemical (PEC) water splitting, over 10 times that of non-porous photo-anodes. The result is so far the highest record for pristine nanostructured hematite anodes without any dopants, co-catalysts or conductive matrices, and is even superior to many doped hematite anodes. The outstanding PEC performance was due to the enhanced light absorption by the pronounced photonic bandgap, the large catalytic area enabled by the interconnected porosity, and low impedance associated with the crack elimination. This work has solved the serious cracking problem that has accompanied the fabrication of inverse opals via the facile template method for decades, and would pave the way for their practical applications in environment-and-energy-related catalysis.