Probing Long‐Lived Plasmonic‐Generated Charges in TiO₂/Au by High‐Resolution X‐ray Absorption Spectroscopy

Abstract

Exploiting plasmonic Au nanoparticles to sensitize TiO 2 to visible light is aw idely employed route to produce efficient photocatalysts.H owever,adescription of the atomic and electronic structure of the semiconductor sites in which charges are injected is still not available.S uch ad escription is of great importance in understanding the underlying physical mechanisms and to improve the design of catalysts with enhanced photoactivity.W ei nvestigated changes in the local electronic structure of Ti in pure and N-doped nanostructured TiO 2 loaded with Au nanoparticles during continuous selective excitation of the Au localized surface plasmon resonance with X-ray absorption spectroscopy( XAS) and resonant inelastic X-ray scattering (RIXS). Spectral variations strongly support the presence of long-lived charges localized on Ti states at the semiconductor surface,g iving rise to new laser-induced lowcoordinated Ti sites. Thesearchforinnovativeandefficientschemesfortheuseof solar energy is motivated by the increasing demand for clean energy.M aterials used in artificial photosynthesis,f or example,inwater splitting/CO 2 reduction, [1,2] also find application in important chemical processes,s uch as wastewater treatment, pollutant removal, and production of fine chemicals. [3, 4] Wide band gap semiconductors,t hat is,T iO 2 ,h ave al ow conversion efficiency owing to their poor absorption of solar light. Among the new concepts introduced to increase light harvesting,t he use of plasmonics is particularly promising. [3] Plasmonic nanoparticles (NPs) have extremely high absorption cross-sections as ar esult of localized surface plasmon resonance (LSPR), which is easily tailored across the solar spectrum by the NPs shape and size.U pon illumination, plasmonic NPs can sensitize semiconductors to below bandgap light and create charge-separated states with prolonged lifetime.[4] Them ain sensitization mechanism is the generation of hot electrons (e À )w hich have sufficient energy to overcome the Schottky barrier at the metal/TiO 2 interface and be injected into the TiO 2 conduction band (CB). In parallel, the unique ability of plasmonic NPs to concentrate electromagnetic fields in nanoscale volumes can induce as econdary process where plasmon oscillation resonates with the semiconductor band gap,that is,plasmonic resonantenergy transfer (PRET).[4] Recently,p lasmonic driven processes (whether charge or energy transfer) have been the focus of intense research. [3, 4] Fore xample,M ubeen et al. reported aw ater-splitting device based on Au/TiO 2 in which all the charge carriers involved in the reaction were Au hot e À .[2] Tailored composite materials and bimetallic plasmonic NPs have also shown high efficiencyi nd riving extensive number of selective chemical reactions. [3, 4] In metal oxides that are used to convert solar light into chemical energy,t he catalytic process is driven by transient changes in the metal oxide'sp roperties,s uch as the metal oxidation state and/or the local reconstruction of cata...

Solar-Powered Plasmon-Enhanced Heterogeneous Catalysis

“…With synchrotronbased x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering spectroscopy (RIXS), some of us recently provided atomistic insights into the electronic and structural localization of plasmon-generated charges in TiO 2 /Au ( Figure 2B). Our investigation indicated that a part of the injected hot electrons remained trapped for a long time (ms to sec) near the composite interface in Ti dp states and generated low-coordinated sites that proved active in plasmonic photocatalysis [58].…”

“…Hot electron transfer occurred preferentially when Au was deposited on stoichiometric TiO 2 ( Figure 5A). By contrast, Au/TiO 2 with intragap defect states was shown to drive the formic acid photodegradation via both hot electron transfer and PIRET [37,50,58,103]. From finite element method simulations, Au NPs coupling was shown to be progressively more efficient as the distance between plasmonic carriers was reduced.…”

“…However, plasmon-induced heating can be neglected under low intensity illumination. Experimental and theoretical analysis have shown that a 2-fold increment in the reaction rate due to plasmonic heating can be achieved only under 10 6 mW cm −2 (corresponding to 10 4 times the intensity of solar irradiation) [39,58,115]. On the other hand, the use of hot carriers can activate unconventional reaction pathways.…”

“…Despite the few theoretical models and experimental observations that have recently appeared, a systematic study of the fundamental physics of LSPR and of the interaction between LSPR and a nearby semiconductor or molecule has yet to be provided. In this scenario, we are convinced that time-resolved x-ray spectroscopies, both with synchrotron and free-electron laser source as well as on table top stations, will play a fundamental role in the advancement of the knowledge on energetic carrier distribution and lifetime, and on their interaction with the electronic levels of molecules and semiconductors [58,82,142,143]. Indeed, through this basic information, it will be possible to op- timize the efficiency and selectivity of plasmon-assisted chemical transformations.…”

Time resolved X-ray absorption spectroscopy in condensed matter: A road map to the future

“…In this regard, the slicing scheme has convincingly demonstrated to be an ideal tool to investigate the nature of transient electronic and structural transient phenomena in a variety of systems where the overall dynamics extend in the picosecond time regime. Indeed, a good amount of recent papers have been published reporting the effectiveness of time resolved XAS applied tostudy, e.g., photoinduced plasmon dynamics in functionalized gold nanoparticles[28,67,68], photoinduced hot electron transfer in Au:TiO 2 nanopowders[69][70][71][72], the electronic rearrangements occurring during the photoinduced insulator-metal phase transition in VO 2[73] and ultrafast demagnetization dynamics in ferromagnets[47,[74][75][76][77].…”