Study on plasmonic enhancement for photocatalytic activity of TiO2-based materials and the underlying mechanism

Abstract

The Surface Plasmon Resonance (SPR) enhanced visible-light driven photocatalytic efficiency of TiO2-based photocatalysts has been extensively investigated in the past. The contribution of SPR to the enhancement of photocatalytic activity as well as the underlying mechanism have not yet been fully understood and are controversially discussed. Two possible reaction mechanisms are considered as operative in SPR photocatalysis with noble metal modified metal oxides, namely, the resonance energy transfer (RET) and the direct electron transfer (DET). This work focuses on the elucidation of the SPR effect on the photocatalytic water splitting utilizing Au and Cu modified TiO2 as photocatalysts. The research was comprised of three major parts. The first part deals with the photocatalytic performance of Au modified anatase TiO2 (UV100). Au-TiO2 photocatalysts were tested for photocatalytic H2 gas formation upon visible light illumination. By means of EPR spectroscopy and Laser Flash Photolysis, it was confirmed that bare anatase TiO2 can be excited by visible light illumination at wavelength > 420 nm (most likely due to pre-existing defects, such as oxygen vacancies) and the excited electrons migrate to the surface-loaded Au nanoparticles. Here it has been shown that Au serves as a catalyst rather than as SPR sensitizer. In the second part, the Au-SPR effect on rutile: anatase TiO2 sample (P25) was studied. Analysis of the data obtained from EPR and laser flash photolysis spectroscopy in combination with DFT calculation evinced that, two pathways cooperatively contribute to the SPR-induced visible-driven photocatalytic H2 production ability of Au–TiO2 photocatalyst upon visible light illumination. Au NPs can inject electrons into the conduction band of rutile: anatase TiO2 and that the electrons can directly be transferred from the valence band of TiO2 to the surface of Au, process called Interfacial Charge Transfer. Moreover, the DFT calculation analysis clearly shows how the d orbitals of Au clusters create impurity energy levels within the band gap of TiO2 and thus contribute to enhancement of photocatalytic efficiency of Au–TiO2. Low cost plasmonic metals, such as copper, should be investigated aiming at cost-effective photocatalysts. Hence, Cu-TiO2 (P25) photocatalysts were prepared and the photocatalytic H2 production rates were evaluated. It was found that the Cu was regenerated during the photocatalytic reaction through the reduction of CuO, thus minimizing the negative effect of CuO formed at ambient conditions. Moreover, the results evidenced that the visible light induced charge carrier formation in the Cu–TiO2 photocatalysts consists of two distinct pathways: the direct excitation of TiO2 and the induced excitation SPR effect of the Cu nanoparticles on the TiO2 surface. Both pathways are present when the full visible range of the spectrum is used (≥420 nm), while for illumination at longer wavelengths (≥500 nm), the photocatalytic activity is solely promoted by the Cu-SPR effect.