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.
Highlights
Improvement of the electrical conductivity of semiconductor metal oxides is one of the most profound challenges in the development of high performance photoanodes for photoelectrochemical (PEC) water splitting[1,2]
Structural information obtained by extended X-ray absorption fine structure (EXAFS) and micro-strain analysis from the synchrotron X-ray diffraction (XRD) studies give a clearer picture of the micro- and macro- changes in the doped photoanodes
Both pristine and doped α –Fe2O3 photoanodes were sintered at 800 °C and show very similar nanorod morphology, with diameters of 30–50 nm and lengths of approximately 400 nm, roughly vertical to FTO substrates[13]
Summary
The Sn4+ and Be2+ ex-situ co-doping method for α –Fe2O3 photoanodes is illustrated in the Fig. 1. Despite of the increased charge carrier density, the newly created impurity levels might still act as recombination centers for electron-hole pairs, removing those localized states is a key step towards a photoanode with improved PEC performance Sn/Be co-doping, resulting in further enhancement of electrical conductivity This is done by improving charge carrier density while mobility remains unchanged, leading to improved bond-ordering, reduced micro-strain and further enhancement in photocurrent (1.7 mA/cm2) with minimal transport resistance for Sn-Be co-doped α -Fe2O3 photoanodes
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