Synthesis and characterization of nanostructured hematite for photoelectrochemical water splitting

Abstract

This study aims to synthesize nanostructured hematite films using spray pyrolysis at different deposition temperatures. L-arginine was used to transform the irregular shaped nanoparticles to uniform nanospheres by chemical bath deposition at 90°C for 48 h. We also investigated the variation of L-arginine: iron precursor concentrations from 1:1 to 3:1, respectively. Likewise, hematite films doped with zinc (Zn), silver (Ag), and Zn/Ag were synthesized using spray pyrolysis. All the films were annealed at temperatures ranging from 450 to 500°C for complete hematite phase transformation. The films were used as photoanodes in photoelectrochemical (PEC) water splitting experiments. X-ray diffraction confirmed the formation of the corundum hexagonal structure of hematite with space group. Raman spectroscopy further confirmed the polycrystalline hematite symmetry with two Eg and five A1g vibrational phonon modes. UV-Vis absorption showed a variation of absorbance with bandgaps that ranged from 2.10 to 1.90 eV. Scanning electron microscopy reported the shape transformation of nanoparticles to nanospheres that ranged in size from 6 to 100 nm. The study showed that the nanostructured films synthesized at temperatures of 430 and 400°C have the highest photocurrent densities of 6 and 1.52 µAcm-2, respectively. There was an improvement of the photocurrent density from 6.4 to 10 µAcm-2 after the transformation of pristine irregularly shaped hematite nanoparticles to spherical hematite. However, on the variation of L-arginine: iron precursor concentrations, a photocurrent of 9.8 µAcm-2 was obtained for 3:1 sample. Also, an improvement of photocurrent from 17 to 89 µAcm-2 was observed for films prepared at 30 and 50 mM iron precursor concentration, respectively. In addition, there was a significant increase in the photocurrent density from 40 to 813 µAcm-2 for pristine and Zn/Ag hematite films, respectively. Ultrafast transient absorption spectroscopy was used to study the electron-hole recombination rates and lifetimes. The results indicated four lifetimes obtained from global analysis with a reduction in the electron-hole recombination rate in the femtosecond and nanosecond range, both for L-arginine/hematite and doped samples. From this study, we were able to prove that the nanostructured and doped hematite films had a longer charge carrier lifetime compared to bulk hematite.