Controlling Photoactivity in Ultrathin Hematite Films for Solar Water‐Splitting

Abstract

A promising route to increase the performance of hematite (α‐Fe2O3) photoelectrodes for solar hydrogen production through water‐splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1 V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm−2 at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiOx is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum‐dot and extremely‐thin‐absorber (ETA)‐type solar cells.

Dynamics of photogenerated holes in surface modified α-Fe 2 O 3 photoanodes for solar water splitting

“…A thin layer of amorphous CoO x was electrodeposited on the surface of these photoanodes from a Co(II) solution in aqueous phosphate buffer. For the second type of electrodes, ultrathin hematite films (approximately 20 nm) were prepared by ultrasonic spray pyrolysis (USP) (hereafter designated USP hematite) (35). These electrodes were subsequently modified with a Ga 2 O 3 overlayer by chemical bath deposition (CBD) (19).…”

Activation of Hematite Nanorod Arrays for Photoelectrochemical Water Splitting

“…[2,21] The J OER was measured at the thermodynamic water splitting potential (V = 1.23 v. RHE), [1,2,21] while V on was determined based on the derivative of the J-V curve using the methodology adopted by the Grätzel group. [22] Activated photoanodes displayed onset potentials less than +0.75 V versus RHE. These are among the lowest onset potentials achieved for hematite photoanodes without an OER catalyst, [9,23] about 250 mV lower than many reports.…”

Immobilization of a molecular cobalt electrocatalyst by hydrophobic interaction with a hematite photoanode for highly stable oxygen evolution

“…For the Co-Pi/hematite sample (Figure 3), an anodic current spike is also observed during this charge accumulation phase, which decreases rapidly with time and forward potential sweep because the accumulated holes disorder the charge dissemination in the space charge layer. 23 Ultimately, a steady-state current vs. potential regime is achieved between carrier recombination and OER. 3 Conversely, the anodic current spikes for the CoFPc/hematite system are not significant, and they almost completely disappear at potential >1.1 V vs. RHE (Figure 3c).…”