(Invited) Efficient Charge Ggeneration Dynamics in Hematite Photoanodes Decorated with Gold Nanostructures

Abstract

Photochemical water splitting offers a useful solution for efficient energy conversion into hydrogen gas. Hematite has been focused on this purpose due to the advantage of low-cost, chemical stability, and suitable band gap. This photocatalytic ability, however, is limited by the short-lived carriers, the poor conductivity, and the short hole diffusion length. As one solution, combination with noble metal can enhance the photocatalytic performance. In particular, gold nanoparticles (AuNP) indicate characteristic absorption in the visible region and injection of electrons into the semiconductor. In this study, a hybrid material of hematite photoanodes with gold nanostructures was fabricated and the carrier dynamics was elucidated by femtosecond transient absorption (TA) spectroscopy. The hybrid material was prepared by immersing the photoanode into the AuNP solution for 16 h, followed by heating at 400℃ for 4 h to remove the organic matter. Femtosecond pump-probe TA spectroscopy was carried out with a Ti:sapphire laser (800 nm wavelength, 130 fs fwhm pulse width, 0.7 mJ/pulse intensity, 1 kHz repetition; Spectra Physics, Hurricane) using pump pulses at 400 nm and probe pulses in the Vis/NIR region. FE-SEM images of hematite and hematite decorated with AuNP (Au/Hematite) indicated that 40 nm AuNPs were decorated on the hematite surface. TA signal of hematite exhibited the positive absorption peak at the 585 nm, while the signal of Au/Hematite did bleaching at 580-620 nm overlapping with broad absorption whose apparent peak shifts from 670 nm towards 580 nm with time. The negative signal at the plasmon band indicates heat generation within the time resolution and cooling in the picosecond timescale. The observed strong positive absorption is induced by the increased electron density in the hematite due to electron transfer from AuNP, demonstrating efficient charge carrier generation in hematite by the decorated gold nanostructure.