A Facile Surface Passivation of Hematite Photoanodes with TiO₂ Overlayers for Efficient Solar Water Splitting

Abstract

The surface modification of semiconductor photoelectrodes with passivation overlayers has recently attracted great attention as an effective strategy to improve the charge-separation and charge-transfer processes across semiconductor-liquid interfaces. It is usually carried out by employing the sophisticated atomic layer deposition technique, which relies on reactive and expensive metalorganic compounds and vacuum processing, both of which are significant obstacles toward large-scale applications. In this paper, a facile water-based solution method has been developed for the modification of nanostructured hematite photoanode with TiO2 overlayers using a water-soluble titanium complex (i.e., titanium bis(ammonium lactate) dihydroxide, TALH). The thus-fabricated nanostructured hematite photoanodes have been characterized by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Photoelectrochemical measurements indicated that a nanostructured hematite photoanodes modified with a TiO2 overlayer exhibited a photocurrent response ca. 4.5 times higher (i.e., 1.2 mA cm(-2) vs RHE) than that obtained on the bare hematite photoanode (i.e., 0.27 mA cm(-2) vs RHE) measured under standard illumination conditions. Moreover, a cathodic shift of ca. 190 mV in the water oxidation onset potential was achieved. These results are discussed and explored on the basis of steady-state polarization, transient photocurrent response, open-circuit potential, intensity-modulated photocurrent spectroscopy, and impedance spectroscopy measurements. It is concluded that the TiO2 overlayer passivates the surface states and suppresses the surface electron-hole recombination, thus increasing the generated photovoltage and the band bending. The present method for the hematite electrode modification with a TiO2 overlayer is effective and simple and might find broad applications in the development of stable and high-performance photoelectrodes.

Hematite Photoanode with Complex Nanoarchitecture Providing Tunable Gradient Doping and Low Onset Potential for Photoelectrochemical Water Splitting

“…[13][14][15] However, PEC performance of a-Fe 2 O 3 has been hindered by intrinsic material limitations resulting in al ate onsetp otential [0.9-1.1 Vv s. the reversible hydrogen electrode (V RHE )], [16] short hole-diffusion length (L h % 2-4 nm), [17] poor OER performance, [18] and high charge-recombination rate. [3,21] For instance,v arious layers containing TiO 2 , [22] Al 2 O 3 , [23] Ti-doped SiO x , [9] Ga 2 O 3 , [24] and amorphous phosphorus [25] enhanced PEC performance by passivating the surface states of a-Fe 2 O 3 . Various strategies, including the use of passivation layers and/or co-catalysts have shown to reduce the onset potential of a-Fe 2 O 3 .…”

Enhancing Solar‐Driven Water Splitting with Surface‐Engineered Nanostructures

“…As a result, transitions to and from the associated electronic states of engineered transition metal oxide‐based hetero‐nanostructures could be potentially utilized for PEC‐based solar fuel generation. Consequently, a variety of α‐Fe 2 O 3 /TiO 2 hetero‐nanostructures have shown considerable improvement in PEC water splitting …”

Water Oxidation Kinetics of Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate Law Analysis

“…Si 64 and Cu2O 11 ) and inhibit surface recombination (e.g. α-Fe2O3 65 and BiVO4 66 ) in a range of photoanode materials. Therefore, increasing our understanding of the catalytic properties of TiO2 remains an important topic.…”