Synergetic effect of Sn addition and oxygen-deficient atmosphere to fabricate active hematite photoelectrodes for light-induced water splitting

Abstract

This work describes a microwave-assisted hydrothermal conditions method to design pure and Sn-hematite photoelectrodes at different synthesis time with additional thermal treatment under air and N2 atmosphere. The hematite photoelectrode designed under N2 atmosphere and Sn deposited on its surface, which is represented by material synthesized at 4 hours exhibit the highest performance. Hence, the Sn-addition followed by high annealing temperature conducted at oxygen-deficient atmosphere seems to create of oxygen vacancies and have prevented the dopant segregation to form SnO2 phase at the hematite crystal reducing its energy and suppressing the grain growth. The increased number of donor density provided by the oxygen vacancies (confirmed by X-ray photoelectron data) and a possible reduction in the grain boundary energy or hematite crystal interface might favor the charge separation and increase the electron transfer through the hematite into the back contact (FTO substrate). As consequence, the light-induced water oxidation reaction efficiency of Sn-hematite photoelectrode was significantly increased in comparison with pure ones, even though the vertical rod morphology was not preserved. This finding provides a novel insight on the intentional Sn-addition revealing that the dopant segregation at the hematite crystal surface (or at the grain boundaries) could be the more relevant factor for developing active hematite photoelectrodes by increasing the electron mobility than the columnar morphology control.

Revealing the synergy of Sn insertion in hematite for next‐generation solar water splitting nanoceramics

“…Uncomplicated strategies for Sn modification of hydrothermally designed photoanodes are often reported, among which the dropping/immersion of akaganeite in a Sn‐based solution followed by high‐temperature annealing process is one of the simplest. Different reports in the literature presented similar results using the iron oxyhydroxide immersion in Sn 4+ precursor solution, either in aqueous or ethanol‐based solution 24,27,29 . From SEM images, in most cases, the annealing step necessary to phase conversion reveals considerable changes showing a less well‐defined morphology, with grain coalescence, defects and the thick layer of SnO 2 on the nanorods’ (NRs) surface 30,32,72,84 …”

“…Moreover, new trends showed that Sn incorporation on hematite can lead to its segregation on grain boundaries; besides that only some doping of superficial hematite sites may also occur. Thus, the final effect is no‐dominant change on the density or conductivity; that is, the final contribution is a weak dopant behavior 27,38,94 …”

“…The Sn‐hematite curve illustrated a common behavior for the photoanodes intentionally modified with tin. For example, more positive onset potentials (at least 100 mV) were obtained for Sn‐hematite prepared by hydrothermal treatment with subsequent dip coating in water‐based SnCl 4 solution, 27 dropping 35 and dipping 104 in ethanol‐based SnCl 4 solution. Furthermore, from the correlation between the capacitance of surface states ( C ss ), charge transfer resistance ( R ct ), and the photocurrent onset displayed in Figure 9B, it is revealed that turn‐on voltage is mediated by intermediate states on the surface, in other words, associated with their charging process 100 …”

“…Furthermore, after years of research aimed at understanding the effects of intentional Sn addition on hematite, it can be seen that, in many cases, Sn addition creates surface states on solid/interface that positively shift the E onset , hindering the reaction kinetics because it demands greater overpotential 27,35 . Despite being indicative that substitutional doping did not occur, this disadvantage resulting from the Sn incorporation is controllable by using co‐doping elements with distinct roles or cocatalysts that negatively shift the E onset , making water oxidation more spontaneous and optimizing the photoelectrochemical process.…”

“…The presence of natural mineral reserves and mining activity represents an advantage if hematite is used in PEC photoanodes, since the demand could be supplied by the countries that explore iron sources. To overcome well‐known intrinsic deficiencies of hematite associated with their electronic limitation that make the real response fall short of the predicted theoretical efficiency into solar to hydrogen conversion, different elements, such as: Ti, 18 V, 19 Sb, 20,21 P, 22 Cr, 23 B, 24,25 Ge, 26 and Sn, 27–32 and combinations like Nb‐Sn, 33 Ti‐Mg, 34 and Co‐Sn, 35 have been successfully employed. Sn has been highlighted as a potential modifier for this semiconductor polycrystalline ceramic, since it has been reported as able to improve the separation of photogenerated charges, achieving the highest photocurrent values among hematite photoanodes with different morphologies 36–38 .…”