Jiajia Cai

4d transition-metal doped hematite for enhancing photoelectrochemical activity: theoretical prediction and experimental confirmation

To explore the photoelectrochemical efficiency of hematite as a photoanode, we comprehensively investigate the electronic structures of hematite doped with 4d transition-metal X (X = Y, Zr, Mo, Tc, Rh, and Ru) based on the density-functional theory (DFT). The results indicate that the bandgap of hematite can be reduced by doping with the transition metal atoms, which leads to the enhanced absorption coefficient of long-wavelength photons in the visible light region. In addition, the carrier concentration can be improved by Zr, Mo, Tc, and Ru dopants. More interesting, the incorporation of Ru can also modify the conduction band edge and hence reduce the effective electron mass, leading to better electron mobility. Subsequent experiments confirm that the photoelectrochemical (PEC) activity of Ru doped hematite film is significantly improved. For example, the highest photocurrent density value of 9 at.% Ru doped hematite is 4.7 times that of the undoped material at E = 1.23 V. Based on both calculations and experiments, the enhanced PEC activities of Ru doped hematite are derived from the improved electrical conductivity and increased visible light absorption coefficient.

Thermal Oxidation Preparation of Doped Hematite Thin Films for Photoelectrochemical Water Splitting

Sn- or Ge-doped hematite thin films were fabricated by annealing alloyed films for the purpose of photoelectrochemical (PEC) water splitting. The alloyed films were deposited on FTO glass by magnetron sputtering and their compositions were controlled by the target. The morphology, crystalline structure, optical properties, and photocatalytic activities have been investigated. The SEM observation showed that uniform, large area arrays of nanoflakes formed after thermal oxidation. The incorporation of doping elements into the hematite structure was confirmed by XRD. The photocurrent density-voltage characterization illustrated that the nanoflake films of Sn-doped hematite exhibited high PEC performance and the Sn concentration was optimized about 5%. The doped ions were proposed to occupy the empty octahedral holes and their effect on PEC performance of hematite is smaller than that of tin ions.

c-In2O3/α-Fe2O3 heterojunction photoanodes for water oxidation

Hematite (α-Fe2O3) is supposed to be one of the most promising photoanode candidates for solar-driven water splitting. However, the photoelectrochemical (PEC) performance of α-Fe2O3 is limited by fast recombination of carriers. In this work, we demonstrate that the recombination of α-Fe2O3 films could be suppressed by forming the heterojunction structure with cubic-In2O3. By utilizing the magnetron sputtering method, the In2O3/α-Fe2O3 films were prepared when the In concentration exceeded its solubility in α-Fe2O3 matrix, which was confirmed by the XRD and TEM analysis. The dependence of charge separation on heterojunction structure was evidenced by Mott–Schottky and EIS analyses. It was found that the enhanced separation of holes and electrons in α-Fe2O3 films contributed to higher PEC performance.

Structural and Morphological Modulation of BiOCl Visible-light Photocatalyst Prepared via an In situ Oxidation Synthesis

The evolution of morphology and heterostructure of BiOCl was investigated during an in situ oxidation reaction. Morphology and structure transformation of regular 2D nanoflake, 0D nanosphere or 3D nanoflower was achieved by adjusting the ratio of reagent concentration or reaction temperature, respectively. The enhanced photocatalytic degradation ability and the photocurrent intensity of BiOCl nanomaterials may be attributed to the improved degree of crystallinity and the formation of Bi/BiOCl heterostructure. The photocurrent density of Schottky battery was increased due to enhancing the optical pathway and assisting during charge separation. Crystallinity also contributed to the improvement of the photoelectric conversion efficiency and reduction of the recombination rate of photogenerated electron-hole pairs.