Prolonged Hot Electron Dynamics in Plasmonic‐Metal/Semiconductor Heterostructures with Implications for Solar Photocatalysis

Abstract

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...

Insight into plasmonic hot-electron transfer and plasmon molecular drive: new dimensions in energy conversion and nanofabrication

“…37 Figures 4a-d shows the structure of the Au/TiO 2 heterojunctions. Au NPs (d = 20 ± 5 nm) were photochemically grafted directly onto the TiO 2 nanowires to construct Au/TiO 2 heterojunctions.…”

Solar-Powered Plasmon-Enhanced Heterogeneous Catalysis

“…The semiconductor/metal interface drives the ultrafast injection (about 240 fs) of hot electrons into the semiconductor CB [53]. In Au/TiO 2 photoelectrodes, these transferred electrons can display excited-state lifetimes two orders of magnitude longer than those of electrons photogenerated directly within TiO 2 via UV excitation [54]. In addition, the instantaneous photogeneration of a charge-separated state (electron in the semiconductor CB and hot holes in the plasmonic nanostructure) on a sub-100 fs timescale has been recently predicted in Au/TiO 2 and experimentally demonstrated for Au/CdSe heterostructures [55,56].…”

Towards full-spectrum photocatalysis: Achieving a Z-scheme between Ag2S and TiO2 by engineering energy band alignment with interfacial Ag

“…4(a)). The OCVD has been previously demonstrated as a tool to resolve charge kinetics [26][27][28]. As revealed by the OCVD characterizations, the lifetimes of the carriers that are photogenerated in the hybrid structures under λ < 400 nm and λ > 400 nm were comparable to those of bare TiO 2 and bare Ag 2 S, respectively ( Fig.…”