Ultraviolet and Visible Photochemistry of Methanol at 3D Mesoporous Networks: TiO₂ and Au–TiO₂

Comparison of methanol photochemistry at three-dimensionally (3D) networked aerogels of TiO₂ or Au–TiO₂ reveals that incorporated Au nanoparticles strongly sensitize the oxide nanoarchitecture to visible light. Methanol dissociatively adsorbs at the surfaces of TiO₂ and Au–TiO₂ aerogels under dark, high-vacuum conditions. Upon irradiation of either ultraporous material with broadband UV light under anaerobic conditions, adsorbed methoxy groups act as hole-traps and extend conduction-band and shallow-trapped electron lifetimes. A higher excited-state electron density arises for UV-irradiated TiO₂ aerogel relative to commercial nanoparticulate TiO₂, indicating that 3D networked TiO₂ more efficiently separates electron–hole pairs. Upon excitation with narrow-band visible light centered at 550 nm, long-lived excited-state electrons are evident on CH₃OH-exposed Au–TiO₂ aerogelsbut not on identically dosed TiO₂ aerogelsverifying that incorporated Au nanoparticles sensitize the networked oxide to visible light. Under aerobic conditions (20 Torr O₂) and broadband UV illumination, surface-sited formates accumulate as adsorbed methoxy groups oxidize, at similar rates, on Au–TiO₂ and TiO₂ aerogels. Moving to excitation wavelengths longer than ∼400 nm (i.e., the low-energy range of UV light) dramatically decreases methoxy photoconversion for methanol-saturated TiO₂ aerogel, while Au–TiO₂ aerogel remains highly active for methanol photooxidation. The wavelength dependence of formate production on Au–TiO₂ tracks the absorbance spectrum for this material, which peaks at λ = 550 nm due to resonance with the surface plasmon in the Au particles. The photooxidation rate for Au–TiO₂ aerogel at 550 nm is comparable to that for TiO₂ aerogel under broadband UV illumination, indicating efficient energy transfer from Au to TiO₂ in the 3D mesoporous nanoarchitecture

Plasmonic Aerogels as a Three-Dimensional Nanoscale Platform for Solar Fuel Photocatalysis

We use plasmonic Au–TiO₂ aerogels as a platform in which to marry synthetically thickened particle–particle junctions in TiO₂ aerogel networks to Au∥TiO₂ interfaces and then investigate their cooperative influence on photocatalytic hydrogen (H₂) generation under both broadband (i.e., UV + visible light) and visible-only excitation. In doing so, we elucidate the dual functions that incorporated Au can play as a water reduction cocatalyst and as a plasmonic sensitizer. We also photodeposit non-plasmonic Pt cocatalyst nanoparticles into our composite aerogels in order to leverage the catalytic water-reducing abilities of Pt. This Au–TiO₂/Pt arrangement in three dimensions effectively utilizes conduction−band electrons injected into the TiO₂ aerogel network upon exciting the Au SPR at the Au∥TiO₂ interface. The extensive nanostructured high surface-area oxide network in the aerogel provides a matrix that spatially separates yet electrochemically connects plasmonic nanoparticle sensitizers and metal nanoparticle catalysts, further enhancing solar-fuels photochemistry. We compare the photocatalytic rates of H₂ generation with and without Pt cocatalysts added to Au–TiO₂ aerogels and demonstrate electrochemical linkage of the SPR-generated carriers at the Au∥TiO₂ interfaces to downfield Pt nanoparticle cocatalysts. Finally, we investigate visible light–stimulated generation of conduction band electrons in Au–TiO₂ and TiO₂ aerogels using ultrafast visible pump/IR probe spectroscopy. Substantially more electrons are produced at Au–TiO₂ aerogels due to the incorporated SPR-active Au nanoparticle, whereas the smaller population of electrons generated at Au-free TiO₂ aerogels likely originate at shallow traps in the high surface-area mesoporous aerogel

Synthesis and Characterization of PbS/ZnS Core/Shell Nanocrystals

We demonstrate a synthetic method to add a ZnS shell, with controlled thickness, to PbS nanocrystals using Zn oleate and thioacetamide as Zn and S precursors. The ZnS shell reaction is self-limiting and deposits approximately a monolayer of ZnS per shell reaction without causing the PbS nanocrystals to Ostwald ripen. The reaction is self-limiting because the sulfur precursor, thioacetamide, is less reactive toward the PbS/ZnS core/shell nanocrystal surface as compared to the Zn oleate precursor present in the reaction solution. To increase the ZnS shell thickness beyond a monolayer, subsequent ZnS shell reactions are modified by adding the thioacetamide 10 minutes before the Zn oleate. This gives the thioacetamide time to react at the PbS/ZnS core/shell nanocrystal surface before the Zn oleate is added. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) shows most ZnS shells lack crystalline order. However, select core/shell nanocrystals have epitaxial crystalline (zinc-blende) ZnS shells or crystalline (zinc-blende) shells with no obvious epitaxial relationship to the PbS core. The PbS core 1S_h–1S_e absorbance and photoluminescence peak energies redshift upon shell addition due to relief of a ligand-induced tensile strain and wave function leakage into the shell. The photoluminescence quantum yield decreases after ZnS shell addition likely due to nonradiative defect states at the core/shell interface

Correlating Changes in Electron Lifetime and Mobility on Photocatalytic Activity at Network-Modified TiO₂ Aerogels

We use intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) to characterize carrier dynamics in titania (TiO₂) aerogels under photocatalytic conditions. By systematically increasing the weight fraction of the sol–gel precursor during TiO₂ sol–gel synthesis, we are able to impart drastic changes in carrier transport/trapping and improve the photocatalytic activity of TiO₂ aerogels for two mechanistically divergent photochemical reactions: reductive water splitting (H₂ generation) and oxidative degradation of dichloroacetate (DCA). The lifetimes of photogenerated electrons increase in going from lowest-to-highest precursor concentrations, as measured by IMVS, indicating increasing site density for electron trapsa trend that correlates with an 8× improvement for photocatalytic H₂ generation. Electron mobility in the aerogel films, as measured by IMPS, decreases with increasing trap density, further implicating the trapping sites as reactive sites. In contrast, photocatalytic DCA degradationdriven primarily by direct hole transfer to adsorbed DCAdepends only weakly on the electron dynamics in the film. Transient infrared spectroscopy shows no difference in carrier decay among the aerogel samples on picosecond time scales, indicating that changes in carrier dynamics within these networked nanomaterials are only observable at time scales measured by IMPV and IMPS. Correlating hole-mediated and electron-mediated photocatalytic activity with direct measurement of electron dynamics under photocatalytically relevant conditions and time scales comprises a powerful approach to determine how synthetic modifications to networked nanostructured photocatalysts affect the relevant physicochemical phenomena underlying their photocatalytic performance