Water reduction under two different visible-light ranges (λ > 400 nm and λ > 435 nm) was investigated in gold-loaded titanium dioxide (Au-TiO2) heterostructures with different sizes of Au nanoparticles (NPs). Our study clearly demonstrates the essential role played by Au NP size in plasmon-driven H2O reduction and reveals two distinct mechanisms to clarify visible-light photocatalytic activity under different excitation conditions. The size of the Au NP governs the efficiency of plasmon-mediated electron transfer and plays a critical role in determining the reduction potentials of the electrons transferred to the TiO2 conduction band. Our discovery provides a facile method of manipulating photocatalytic activity simply by varying the Au NP size and is expected to greatly facilitate the design of suitable plasmonic photocatalysts for solar-to-fuel energy conversion.

Ideal solar-to-fuel photocatalysts must effectively harvest sunlight to generate significant quantities of long-lived charge carriers necessary for chemical reactions. Here we demonstrate the merits of augmenting traditional photoelectrochemical cells with plasmonic nanoparticles to satisfy these daunting photocatalytic requirements. Electrochemical techniques were employed to elucidate the mechanics of plasmon-mediated electron transfer within Au/TiO2 heterostructures under visible-light (λ>515 nm) irradiation in solution. Significantly, we discovered that these transferred electrons displayed excited-state lifetimes two orders of magnitude longer than those of electrons photogenerated directly within TiO2 via UV excitation. These long-lived electrons further enable visible-light-driven H2 evolution from water, heralding a new photocatalytic paradigm for solar energy conversion.

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The reaction between Ti and methanol (CH OH) is a model system for competition between activation of C-O, C-H, and O-H bonds and of the role of excited electronic pathways in catalytic processes. Herein, we use experimental kinetics, quantum chemical calculations, and statistical modeling to identify the critical features of the reaction's potential energy surface. Experimental kinetics data between 300 and 600 K shows the reaction largely proceeds through C-O bond activation, yielding TiOH and TiO . Products of the O-H activation pathway, TiOCH and TiOCH are minor, whereas C-H bond activation is not observed at thermal energies. Statistical modeling well-reproduces the experimental results and offers insight into the reaction mechanism. Notably, efficient spin-crossing along the C-O activation pathway is required to produce the observed product distribution, in contrast to a previous estimate of inefficient crossing based on calculation of a small spin-orbit coupling constant. This discrepancy highlights a potential limitation of simple models within the Landau-Zener framework, which are commonly used to calculate surface-crossing probabilities in reactive systems.

Ideal solar-to-fuel photocatalysts must effectively harvest sunlight to generate significant quantities of long-lived charge carriers necessary for chemical reactions. Here we demonstrate the merits of augmenting traditional photoelectrochemical cells with plasmonic nanoparticles to satisfy these daunting photocatalytic requirements. Electrochemical techniques were employed to elucidate the mechanics of plasmonmediated electron transfer within Au/TiO 2 heterostructures under visible-light (l > 515 nm) irradiation in solution. Significantly, we discovered that these transferred electrons displayed excited-state lifetimes two orders of magnitude longer than those of electrons photogenerated directly within TiO 2 via UV excitation. These long-lived electrons further enable visible-light-driven H 2 evolution from water, heralding a new photocatalytic paradigm for solar energy conversion.Solar photocatalysis for chemical fuel production represents a promising approach to sustainably satisfy escalating global energy demands. [1] Unfortunately, current photocatalysts exhibit limited solar-to-fuel efficiencies because the lifetimes of photogenerated electron-hole pairs (ps-ms) are often incommensurate with the prolonged timescales required to facilitate photocatalytic reactions (ms-s) at the semiconductor surface. [1, 2a] This kinetic discrepancy hinders the realization of efficient photosynthetic devices for solar energy conversion. [1] Recently, the integration of Au or Ag nanoparticles (NPs) into traditional photoelectrochemical (PEC) cells has been shown to endow the plasmonic composite with enhanced photocatalytic performance by utilizing the surface plasmon resonance (SPR) of plasmonic-metal NPs. [2][3][4] These SPs predominantly relax non-radiatively via Landau damping, yielding highly energetic "hot" electrons that transiently populate electronic states above the metal Fermi level. [5] When plasmonic-metal NPs are proximal to a semiconductor with a dense manifold of conduction band (CB) states, the hot electrons can be directly injected into the energetically accessible CB levels of the semiconductor. [2,3, 5, 6] This plasmon-mediated electron transfer (PMET) process constitutes a unique photosensitization strategy enabling these plasmonic NPs to serve as light-harvesting assemblies when anchored to a semiconductor scaffold. [2,3, 5] Although PMET occurs on an ultrafast timescale (t < 240 fs), [6a] the temporal evolution of these transferred electrons within the semiconductor CB after PMET deserves further study. The average lifetime of hot electrons within the semiconductor CB represents an important catalytic parameter, as these photoexcited electrons must persist long enough to facilitate surface reactions. [7] Previous dynamics studies were conducted in air using femtosecond pulsed-laser excitation, [6a,b] and thus did not adequately capture the excited-state dynamics relevant to PEC systems operating in solution. [8] Knowledge of these hot electron dynamics within a plasmonic device under simulated s...