Plasmonic enhanced photocatalytic water splitting under UV and visible light

Abstract

Solar-driven water splitting is a promising way to produce hydrogen (H2), which can be used in fuel cells to generate clean electricity. Au/TiO2 composite photocatalysts are widely studied for photoelectrochemical and photocatalytic water splitting because TiO2 is a good semiconductor photocatalyst under UV light and Au nanostructures can extend titania’s photocatalysis into the visible region through localized surface plasmon resonances (LSPRs). In this thesis, the photocatalytic activity of Au/TiO2 systems for solar-driven water splitting were systematically investigated across the UV-visible region as a function of Au nanoparticles (AuNPs) loading. A series of 0-4 wt.% AuNP/TiO2 powders were prepared by the depositon-precipitation method from P25 TiO2, and then formulated into pastes and applied as thin films on fluorine tin oxide (FTO) coated glass slides (DP AuNP/TiO2 photoelectrodes). Two other sets of samples were fabricated using the spin coating method based on a commercially available TiO2 support with 0-4 cycles of Au deposition (SC-CM and SC AuNP/TiO2 photoelectrodes). These different fabrication methods allowed us to control the Au loading and the TiO2 film thickness in each sample, whilst keeping AuNP size similar within each sample series (7-8 nm for the DP AuNP/TiO2 photoelectrodes and 12-13 nm for the SC-CM and SC AuNP/TiO2 photoelectrodes). Under visible-light irradiation, all photoelectrodes functionalized with AuNPs exhibited superior photoactivity compared to bare TiO2 photoelectrodes. The highest photocurrent density was about 9.76 μA/cm2 achieved after 3 cycles of Au deposition and 10.41 μA/cm2 achieved at 3 wt.% of nominal Au loading for the SC-CM and DP AuNP/TiO2 photoelectrodes, respectively. The maximum photocurrent density (7.18, 8.67, or 11.58 μA/cm2) for the SC AuNP/TiO2 was achieved after 2-3 cycles of Au deposition depending on the TiO2 film thickness. Monochromatic incident photon-to-current efficiency (IPCE) measurements from 450 nm to 700 nm indicated that localized surface plasmon resonances (LSPRs) of the AuNPs accounted for the enhanced photoactivity of AuNP/TiO2 photoelectrodes under visible light. Whilst hot electron injection was considered to be the dominant mechanism for the photocurrent enhancement, finite-integration time domain simulation results suggested that such enhancement could also be attributed to improved charge separation by amplified electric fields at the interface between AuNPs and TiO2. Under UV irradiation, electron-hole recombination was observed in all photoelectrodes with bare TiO2 electrodes having the greatest recombination rate, which adversely impacted their photocatalytic performance. Chronoamperometric studies revealed that AuNPs effectively suppressed elelctron-hole recombination in TiO2 by acting as electron traps, yet caused the overall photocurrent to drop. This phenomenon was amplified with increasing Au loading regardless of the TiO2 film thickness. H2 production tests under UV and visible illumination were conducted for the 0-4 wt.% AuNP/TiO2 powders in methanol-water mixtures. Using 50 vol.% methanol, the highest H2 production rate achieved for Au-loaded samples (41.56 mmol g-1 h-1 for 3 wt.% Au) was ~17 times higher than the bare TiO2 sample under UV excitation. H2 production rates were slightly lower when 10 vol.% methanol-water mixtures were used, with the optimal Au loading still being 3 wt.% (27.62 mmol g-1 h-1). Under visible light illumination, the highest H2 production rate was achieved with the 3 wt.% Au sample (0.50 and 0.48 mmol g-1 h-1 for 50 vol.% and 10 vol.% methanol, respectively). These rates were about twice those of bare TiO2 samples, whose visible photoactivity was attributed to the existence of intrinsic defect states. The small enhancement in H2 production rates resulted from the addition of AuNPs, with the same rates achieved with both 10 and 50 vol.% methanol-water mixtures. Results indicate a weak effect of the AuNP LSPRs on improving the photoactivity of TiO2 for H2 evolution under visible light. In conclusion, this study demonstrates highly controllable and reproducible strategies for fabricating effective AuNP/TiO2 photocatalysts for solar-driven water splitting, whilst contributing to a better mechanistic understanding of the roles of plasmonic AuNPs in promoting the photoactivity of TiO2 across the UV-visible spectrum.