A facile approach for preparing densely-packed individual p-NiO/n-Fe₂O₃ heterojunction nanowires for photoelectrochemical water splitting

Abstract

Innovative design of electrode materials is crucial for efficient conversion of solar energy into chemical fuel through photoelectrochemical (PEC) water splitting. Herein, we report the development of a p-n heterojunction nanowire (NW) based photoanode made of low cost earth-abundant materials. Densely-packed and freestanding individual p-NiO/n-Fe2O3 heterojunction NWs are fabricated through consecutive electrodeposition of Fe and Ni NWs inside the pores of the anodic alumina template followed by controlled oxidation. Heterojunction formation in individual NWs is confirmed through energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), along with elemental mapping on individual NWs through electron energy loss spectroscopy (EELS). An inverted 'V' shape nature of the Mott-Schottky curve suggests p-n diode like characteristics of the heterojunction NWs. These p-n heterojunction NWs demonstrate a significantly enhanced photocurrent density (∼24 times at a potential of 1.23 V vs. RHE) and a cathodic shift (∼0.4 V) of the photocurrent onset potential compared to the pristine Fe2O3 NW electrode, which can be attributed to the synergistic combination of n-Fe2O3 with the co-catalyst p-NiO facilitating the generation and transfer of photogenerated holes into the electrolyte for water oxidation. This study validates the feasibility of developing Fe2O3 based heterojunction photoelectrodes for efficient PEC water splitting.

Recent Development of Ni/Fe‐Based Micro/Nanostructures toward Photo/Electrochemical Water Oxidation

“…Recently, Singh and Sarkar report the design of p-NiO/n-Fe 2 O 3 materials, which is a p-n heterojunction nanowire based photoanode (Figure 9A). [95] Compared to the original Fe 2 O 3 nanowire electrode, these p-NiO/n-Fe 2 O 3 heterojunction nanowires indicate a greatly enhanced photocurrent density (≈24 times at a potential of 1.23 V vs RHE) and a cathodic shift of the photocurrent onset potential (≈0.4 V). This is due to the synergistic combination of n-Fe 2 O 3 with the cocatalyst p-NiO, which promotes the generation of photogenerated holes and transfer to the electrolyte for water oxidation.…”

“…Reproduced with permission. [95] Copyright 2018, The Royal Society of Chemistry. B) Schematic of the one-step hydrothermal process for depositing Ni:FeOOH OECs on anisotropic nanostructures, such as WO 3 /BiVO 4 core/shell nanowires.…”

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar‐Driven Water Splitting

“…[30] Sarkar and Singh reported that the axial junction of p-NiO x /n-Fe 2 O 3 significantly enhanced the photocurrent density and resulted in cathodic shift of the onset potential of the photocurrent. [31] Due to the formation of a p-n junction and the existence of NiO x as an OER catalyst, the time constant of the transient photocurrent of NiO x /Fe 2 O 3 was five times longer than that of bare Fe 2 O 3 , suggesting the CR was remarkably suppressed at electrode/electrolyte interface by NiO x .…”

“…[23] Copyright 2018, Springer Nature Publishing AG. forming a p-n junction [28][29][30][31] or a metal-insulator-semiconductor (MIS) junction [32] at the semiconductor/electrocatalyst interface.…”

Enhanced Light Absorption and Charge Carrier Management in Core‐Shell Fe2O3@Nickel Nanocone Photoanodes for Photoelectrochemical Water Splitting

“…Since its invention in 1972, a host of metal oxides have been explored as efficient catalysts for PEC water splitting. Among these, hematite (Fe 2 O 3 ) has drawn particular attention as a promising photo‐electrode material because of its suitable band gap for significant light absorption in the visible region of the solar spectrum, high abundance and astounding chemical stability in aqueous electrolytes . However, it's performance is critically limited by a very short hole diffusion length (∼2–4 nm) and poor majority carrier conductivity which lead to recombination losses of photogenerated charge carriers in the bulk .…”