Sn/Be Sequentially co-doped Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation: Effect of Be2+ as co-dopant

Abstract

For ex-situ co-doping methods, sintering at high temperatures enables rapid diffusion of Sn4+ and Be2+ dopants into hematite (α–Fe2O3) lattices, without altering the nanorod morphology or damaging their crystallinity. Sn/Be co-doping results in a remarkable enhancement in photocurrent (1.7 mA/cm2) compared to pristine α–Fe2O3 (0.7 mA/cm2), and Sn4+ mono-doped α-Fe2O3 photoanodes (1.0 mA/cm2). From first-principles calculations, we found that Sn4+ doping induced a shallow donor level below the conduction band minimum, which does not contribute to increase electrical conductivity and photocurrent because of its localized nature. Additionally, Sn4+-doping induce local micro-strain and a decreased Fe-O bond ordering. When Be2+ was co-doped with Sn4+-doped α–Fe2O3 photoanodes, the conduction band recovered its original state, without localized impurities peaks, also a reduction in micro-strain and increased Fe-O bond ordering is observed. Also the sequence in which the ex-situ co-doping is carried out is very crucial, as Be/Sn co-doping sequence induces many under-coordinated O atoms resulting in a higher micro-strain and lower charge separation efficiency resulting undesired electron recombination. Here, we perform a detailed systematic characterization using XRD, FESEM, XPS and comprehensive electrochemical and photoelectrochemical studies, along with sophisticated synchrotron diffraction studies and extended X-ray absorption fine structure.

The effect of Fe2+ state in electrical property variations of Sn‐doped hematite powders

“…Figure shows the core‐level X‐ray photoelectron spectra of (A) Fe 2 p , (B) Sn 3 d , and (C) O 1 s states of the Sn‐doped Fe 2 O 3 powders. The vertical dotted lines indicate the reference binding energies . In order to obtain quantitative information, the peaks have to be deconvoluted after proper background subtraction .…”

“…(A) Fe2 p , (B) Sn 3 d , and (C) O 1 s core‐level X‐ray photoelectron spectra of Sn‐doped Fe 2 O 3 powders. The vertical dotted lines indicate the reference binding energy (711.0 eV for Fe 2 p 3/2 , 724.4 eV for Fe 2 p 1/2 , 486.3 eV for Sn 3 d 5/2 , 494.7 eV for Sn 3 d 3/2 , 529.6 eV for metal oxide O 1 s , 531.2 eV for oxygen vacancies O 1 s , and 533.0 eV for carbon contamination O 1 s ) [Color figure can be viewed at wileyonlinelibrary.com]…”

“…The operation of a PEC cell is based on clean energy and the hydrogen byproduct from the water splitting process facilitates the development of future energy sources. For better utilization of a PEC cell, the physical properties of the anode materials have to be optimized to ensure efficient absorption in the visible light range, large charge transport, and durability of the materials …”

“…Common metal oxide semiconductors (eg, TiO 2 , CdWO 4 , WO 3 , α‐Fe 2 O 3 ) are used as the photoanode material. Among them, α‐Fe 2 O 3 (hematite) has attracted much attention owing to its chemical stability in an aqueous environment and a high theoretical solar to hydrogen conversion efficiency (14%‐17%) . Furthermore, it has an optical band gap of 2.2 eV with 40% absorption of solar light .…”

“…Furthermore, it has an optical band gap of 2.2 eV with 40% absorption of solar light . However, the relatively low electrical conductivity, short charging lifetime (10‐12 μs), and high rate of carrier recombination prohibits its practical usage as the PEC cell photoanode material . To overcome these difficulties, various substitutes such as Ti, Sn, Si, Pt, Ge, Cr, and Zn were experimented with .…”