Tubular morphology preservation and doping engineering of Sn/P-codoped hematite for photoelectrochemical water oxidation

Tubular hematite with high-concentration, uniform doping is regarded as a promising material for photoelectrochemical water oxidation. However, the high-temperature annealing commonly used for activating doped hematite inevitably causes deformation of the tubular structure and an increase in the trap states. In the present work, Sn-doped tubular hematite on fluorine-doped tin oxide (FTO) is successfully obtained at 750 °C from a Sn-coated FeOOH tube precursor. Sn/P codoping, which is rarely considered for hematite, is also achieved via a gas phase reaction in phosphide atmosphere. The tubular morphology allows the dopant to diffuse from both the inner and outer surfaces, thus decreasing the doping profile in the radial direction. The even distribution of Sn and P synergetically increases the carrier density of hematite by one order of magnitude, which shortens the width of the depletion layer to ca. 2.3 nm (compared with 19.3 nm for the pristine sample) and leads to prolonged carrier lifetime and efficient charge separation. In addition, this codoping protocol does not introduce additional surface trap states, as evidenced by the increased charge injection efficiency and surface kinetic analysis using intensity modulated photocurrent spectroscopy (IMPS). As a result, the morphology- and doping-engineered hematite exhibits photocurrents of 0.9 mA cm-2 at 1.23 V and 3.8 mA cm-2 at 2.0 V vs. RHE under AM 1.5 G illumination (100 mW cm-2) in 1.0 M NaOH, representing 4.5-fold and 4.8-fold enhancements, respectively, compared with the photocurrents of undoped hematite. The present method is shown to be effective for preparing multi-element-doped hematite nanotubes and may find broad application in the development of other nanotubular photoelectrodes with or without doping for efficient and robust water oxidation.