Improved Interfacial Charge Transfer Dynamics and Onset Shift in Nanostructured Hematite Photoanodes via Efficient Ti⁴⁺/Sn⁴⁺ Heterogeneous Self-Doping Through Controlled TiO₂ Underlayers

Abstract

We introduce a simple strategy to unintentional heterogeneous Ti4+/Sn4+ doping and surface passivation of hematite via TiO2 underlayers at high temperature quenching. The effects of the controlled TiO2 underlayer thickness and high temperature quenching process on the interfacial diffusion of Ti4+/Sn4+ and TiO2 passivation of hematite nanorod arrays have been carefully studied. The improved photoelectrochemical water oxidation performance of the TiO2 underlayered hematite nanorod photoanodes after high-temperature quenching (800 °C for 10 min) suggests enhanced interfacial Ti4+ diffusion, blocking of electron back transfer, and reduced interfacial charge recombination. The TiO2 underlayers led to more inclined growth of hematite (α-Fe2O3) nanorods on the fluorine-doped tin oxide (FTO) substrates. Ti4+ and Sn4+ diffusion and formation of the TiO2 passivation layer on the α-Fe2O3 surface are confirmed by HRTEM and X-ray photoelectron spectroscopy (XPS) analyses. As a result, the TU2 photoanode displayed higher donor density and enhanced photocurrent density of (1.45 mA·cm–2) than the pristine hematite photoelectrode (1.0 mA·cm–2). The improved photoelectrochemical performance of TU2 is attributed to the high separation efficiency of photoinduced carriers via TiO2 underlayer, the Ti4+/Sn4+ diffusion, and surface passivation of hematite at high-temperature annealing. The thickness of the TiO2 underlayer has great influence on the surface passivation as well as resistance on FTO/hematite interfaces than the diffused Sn.

Engineering surface states of hematite based photoanodes for boosting photoelectrochemical water splitting

“…[106][107][108][109][110][111][112][113][114][115][116] Meanwhile, other researchers spared no effort in correlating PEC performance improvement with the surface states evolution upon doping or coating of a secondary semiconductor onto hematite photoanodes. [117][118][119][120][121][122][123][124][125][126][127][128] For instance, D. Monllor-Satoca et al prepared mesoporous hematite-titania composite films by mixing the respective preformed nanoparticles obtained by a non-aqueous solgel route in a wide range of loading levels (0-20 mol %). Voltammetric and EIS were performed observing an optimum 10% doping, with a 15-fold photocurrent increase (up to 1.3 mA cm -2 at 1.23 V vs RHE) and a 100-fold decrease of the charge transfer resistance.…”

Unraveling the role of Ti3C2 MXene underlayer for enhanced photoelectrochemical water oxidation of hematite photoanodes

“…Therein, the decreased photocurrent density is probably because that the thicker Ti 3 C 2 MXene underlayer will hinder the diffusion of Sn 4 + from FTO substrate and electron forward transfer [24 , 39] . As reported in previous study, Park et al found that the thick TiO 2 underlayer could greatly hinder the diffusion of Sn 4 + , consequently acting as a dominant factor in reducing the photocurrent density of hematite [39] . While, for the anodic shift in onset potential, it is possibly due to the surface effects of Ti species via the diffusion through bulk hematite to surface, which will increase the surface trapping states of hematite photoanode [39 , 40] .…”

“…While, for the anodic shift in onset potential, it is possibly due to the surface effects of Ti species via the diffusion through bulk hematite to surface, which will increase the surface trapping states of hematite photoanode [39 , 40] . This conclusion strongly indicates that the optimum thickness of Ti 3 C 2 MXene underlayer should balance the suppression of interfacial charge recombination and electron transfer from hematite to FTO substrate [39] . Meanwhile, the thickness of hematite nanostructure may affect the PEC performance of Ti-Fe 2 O 3 photoanode, and consequently which has also been optimized by controlling the concentration of FeCl 3 •6H 2 O in the precursor solution for the hydrothermal growth of FeOOH film.…”

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

“…In this vein, the tin diffusion length limit has been studied by different research groups 35,62,69–71 . For instance, ionic exchanges can be observed in Figure 4A, where the behavior of Sn diffusion in the semiconductor matrix was detailed by the line profile obtained from cross‐section TEM‐EDS analysis (Figure 4B) of hematite nanoceramics.…”