Abstract
Abstract Hematite-based photoanodes have been intensively studied for photoelectrochemical water oxidation. The n-type dopant Sn has been shown to benefit the activity of hematite anodes. We demonstrate in this study that Sn-doped hematite thin films grown by atomic layer deposition can achieve uniform doping across the film thickness up to at least 32 mol%, far exceeding the equilibrium solubility limit of less than 1 mol%. On the other hand, with the introduction of Sn doping, the hematite crystallite size decreases and many twin boundaries form in the film, which may contribute to the low photocurrent observed in these films. Density functional theory calculations with a Hubbard U term show that Sn doping has multiple effects on the hematite properties. With increasing Sn4+ content, the Fe2+ concentration increases, leading to a reduction of the band gap and finally to a metallic state. This goes hand in hand with an increase of the lattice constant.
Highlights
Hematite (α-Fe2O3) has long been studied for photoelectrochemical (PEC) water oxidation [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]
We present a study on the effect of Sn concentration on the microstructure and PEC water oxidation activity of hematite photoanodes
It is demonstrated that by atomic layer deposition (ALD), up to 32 mol% Sn can be homogeneously incorporated into the hematite phase, which leads to reduction of a fraction of Fe3+ to Fe2+
Summary
Hematite (α-Fe2O3) has long been studied for photoelectrochemical (PEC) water oxidation [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. The fundamental limit of hematite, like many other transition metal oxides, is its poor conductivity Both types of charge carriers, electrons and holes, are highly localized in space, and transport occurs by a phonon-assisted polaron mechanism. The conductivity may be further compromised by the surface and by crystallographic defects in the bulk [6], and quantum confinement may alter the band structure and alignment [7] Another possibility is to improve the transport properties of hematite by substituting other elements [8], such as Al, Si, Ti, Cr, Ni, Cu, Zn, Zr, Nb, Mo, Sn, Pt, as has been summarized in a previous report [9]. The photocurrents derived from hematite photoanodes peak at low Sn concentration, and vanish as the Sn concentration exceeds 32 mol%
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