Vapor Phase Processing of α-Fe₂O₃Photoelectrodes for Water Splitting: An Insight into the Structure/Property Interplay

Abstract

Harvesting radiant energy to trigger water photoelectrolysis and produce clean hydrogen is receiving increasing attention in the search of alternative energy resources. In this regard, hematite (α-Fe2O3) nanostructures with controlled nano-organization have been fabricated and investigated for use as anodes in photoelectrochemical (PEC) cells. The target systems have been grown on conductive substrates by plasma enhanced-chemical vapor deposition (PE-CVD) and subjected to eventual ex situ annealing in air to further tailor their structure and properties. A detailed multitechnique approach has enabled to elucidate the interrelations between system characteristics and the generated photocurrent. The present α-Fe2O3 systems are characterized by a high purity and hierarchical morphologies consisting of nanopyramids/organized dendrites, offering a high contact area with the electrolyte. PEC data reveal a dramatic response enhancement upon thermal treatment, related to a more efficient electron transfer. The reasons underlying such a phenomenon are elucidated and discussed by transient absorption spectroscopy (TAS) studies of photogenerated charge carrier kinetics, investigated on different time scales for the first time on PE-CVD Fe2O3 nanostructures.

Key Strategies to Advance the Photoelectrochemical Water Splitting Performance of α‐Fe2O3 Photoanode

“…The F‐ and Rh treatment resulted in 230 mV cathodic shift in comparison to Fe 2 TiO 5 incorporated α‐Fe 2 O 3 photoanode. A detailed interpretation of the dependence of PEC performance on the system conditions was reported by Tendeloo and co‐workers . The role of the passivation layer in enhancing the PEC water oxidation performance has also been reviewed by Yang and co‐workers …”

Interplay of thickness and photoelectrochemical properties in nanostructured α-Fe2 O3 thin films

“…Two further auxiliary gas‐lines were used to introduce Ar (flow rate = 15 sccm) and electronic grade O 2 (flow rate = 20 sccm) directly into the reactor. After a preliminary optimization , growth experiments were performed at 300 °C, using an RF‐power = 10 W and a total pressure = 1.0 mbar, on FTO‐coated glass slides (Aldrich ® , ≈7 Ω square; FTO thickness ≈600 nm), suitably cleaned prior to each deposition . Three different deposition times (10, 30, or 60 min) were used in order to tailor the thickness and features of the resulting nanodeposits, that were subjected to ex situ thermal treatment in air for 1 h at 650 °C using a Carbolite HST 12/200 tubular oven (heating rate = 20 °C min −1 ).…”

Nickel Oxide Nanosheets for Enhanced Photoelectrochemical Water Splitting by Hematite (α‐Fe2O3) Nanowire Arrays

“…With the continuous demand for clean and sustainable energy sources, electrochemical and photoelectrochemical (PEC) cells have been intensively investigated as unique and effective approaches towards water‐splitting . Among various photoactive semiconductor materials in the field of PEC cells, hematite (α‐Fe 2 O 3 ) has attracted much attention owing to its stability, nontoxicity, low costs, and suitable UV‐Vis absorption (band gap: 2.0–2.2 eV) . However, because of the position of its conduction band energy level (more positive vs. the normal hydrogen electrode E NHE ) with respect to hydrogen evolution, an external bias is needed for water splitting .…”

“…The nanostructured system (NiO@α‐Fe 2 O 3 ) shows an enhanced photocurrent compared to pristine hematite. Although we are aware that higher performances have been reported for photoanodes based on hematite, these were achieved by doping of tin into hematite by annealing at high temperature (800 °C) . In our experiments, we focus only on the effect of NiO on the performance of hematite photoanodes.…”