Visible light with 515 nm there was negligible signal for conversion of terephthalic acid, indicating the plasmon response for hole-mediated reactions is not more significant compared to just using TiO2 under UV light. In contrast, as observed in Figure and Figure , electron-mediated pathways under solar light irradiation are much more pronounced for Au@CloseLarge compared to TiO2, Au@CloseSmall and Au@DistantSmall, as concluded from the XTT assay. When the light source is changed to just UV light, pristine TiO2 again dominates the XTT conversion. When using only visible light of 532 nm, there is negligible conversion of XTT. The higher contribution of electron and superoxide generation that was probed for Au@CloseLarge under solar light corresponds well with the observed trend in photocatalytic hydrogen peroxide generation. This leads us to an understanding that the H2O2 generation on such SPs is occurring mainly through electron-mediated (i.e. 2-or 4-electron) O2 reduction pathways. This is well in line with former studies that have indicated H2O2 generation using Au-TiO2 composites occurs mainly through 2-electron reduction of O2. , , Combining all previous information, we hypothesize that the generation of hot electrons upon plasmonic excitation of the SP is a crucial factor for explaining the superior performance of these materials. Hot electrons are generated at the Au metal sites and are subsequently transferred to the TiO2 conduction band, leading to an increase in the hydrogen-peroxide generation rate through electron-mediated ORR. For Au@CloseLarge SPs, the generation and transfer of hot electrons from Au to TiO2 are enhanced because of 1) the close packing of Au nanoparticles, which increases the near-electric field through plasmonic coupling (as shown in the SERS experiments), intensifying hot electron generation, and 2) the packing of Au nanoparticles into a single assembly, which promotes charge separation and charge transfer into the TiO2 conduction band due to the overflow of electrons from multiple Au nanoparticles. For Au@CloseLarge SPs, the photocatalytic hydrogen peroxide from electron-based reactions could be enhanced because: 1) the close packing of Au nanoparticles can lead to collective lattice resonances which increases the near-electric field through plasmonic coupling, correlating it to a more efficient hot electron generation, , 2) the packing of Au nanoparticles into a single assembly, has shown to promote plasmon enhanced resonant energy transfer (PIRET), albeit in Au-CdSe self-assembly of 100 nm in size and micrometer sized Au-CuS nanochains, which can corelate similarly to promote charge separation and charge transfer into the TiO2 conduction band, , and 3) the packing of Au nanoparticles into a single assembly of TiO2 in multiple layers can promote the dark plasmon modes and therefore hot electron injection through Landau damping. We further argue that the generation of hot electrons is the pivotal plasmonic mechanism for the present system, as other plasmonic effects are less likely to occur under the present set of experimental conditions. Indeed, the size of Au nanoparticles used in this work (15 nm) implies that scattering of light is not a significant factor, as it only becomes important for nanoparticles largely exceeding 50 nm. Additionally, plasmonic local thermal heating within the Au composites is expected to be minimal, given the light intensity used is only (at most) 100 mW cm -2 , while it has been shown that for photothermal effects to occur, much higher irradiance levels are required. Furthermore, the optical absorption band of TiO2 and the LSPR band of Au do not align, hence the direct effect of increased charge carrier generation at the semiconductor boosted by the enhanced near-field is hampered. It should be noted that these hypotheses remain theoretical and we would recommend this to be the subject of follow-up experimental studies using time-resolved transient absorption spectroscopy to understand electron transfer dynamics, as well as theoretical investigations using density functional theory to validate the electron transfer dynamics and detect the source of enhanced electron generation.