Doping of Nb5+ Species at the Au-TiO2 Interface for Plasmonic Photocatalysis Enhancement.

Au nanoparticles loaded on semiconductor TiO2 absorb visible light due to their surface plasmon resonance (SPR) and inject the photogenerated hot electrons (ehot-) into the conduction band of TiO2. The separated charges promote oxidation and reduction reactions. The step that determines the rate of the plasmonic photocatalysis on the Au/TiO2 system is the ehot- injection through the Schottky barrier created at the Au-TiO2 interface. In the present work, niobium (Nb5+) oxide species were doped at the Au-TiO2 interface by loading Nb5+ onto the TiO2 surface followed by deposition of Au particles (2 wt % of TiO2). Visible light irradiation of the Au/Nb5+/TiO2 catalysts promotes aerobic oxidation of alcohols with much higher efficiency than that of undoped Au/TiO2. Lewis acidity of the Nb5+ species located at the interface cancels the negative charges of Au and creates a barrier with a narrower depletion layer, promoting tunneling ehot- injection. Efficiency of the ehot- injection depends on the amount of Nb5+ doped. Loading small amounts of Nb5+ (∼0.1 wt % of TiO2) creates mononuclear NbO4 species and shows large activity enhancement. In contrast, loading larger amounts of Nb5+ creates aggregated polynuclear Nb2O5 species. They decrease the electron density of Au particles and weaken their SPR absorption. This suppresses the ehot- generation on the Au particles and decreases the activity of plasmonic photocatalysis.

Synthesis of Au Nanoparticles with Benzoic Acid as Reductant and Surface Stabilizer Promoted Solely by UV Light.

Photoreductive synthesis of colloidal gold nanoparticles (AuNPs) from Au3+ is one important process for nanoprocessing. Several methods have been proposed; however, there is no report of a method capable of producing AuNPs with inexpensive reagents acting as both reductant and surface stabilizer, promoted solely under photoirradiation. We found that UV irradiation of water with Au3+ and benzoic acid successfully produces monodispersed AuNPs, where thermal reduction does not occur in the dark condition even at elevated temperatures. Photoexcitation of a benzoate-Au3+ complex reduces Au3+ while oxidizing benzoic acid. The benzoic acid molecules are adsorbed on the AuNPs and act as surface stabilizers. Change in light intensity and benzoic acid amount successfully creates AuNPs with controllable sizes. The obtained AuNPs can easily be redispersed in an organic solvent or loaded onto a solid support by simple treatments.

Carbon Nitride-Aromatic Diimide-Graphene Nanohybrids: Metal-Free Photocatalysts for Solar-to-Hydrogen Peroxide Energy Conversion with 0.2% Efficiency.

Solar-to-chemical energy conversion is a challenging subject for renewable energy storage. In the past 40 years, overall water splitting into H2 and O2 by semiconductor photocatalysis has been studied extensively; however, they need noble metals and extreme care to avoid explosion of the mixed gases. Here we report that generating hydrogen peroxide (H2O2) from water and O2 by organic semiconductor photocatalysts could provide a new basis for clean energy storage without metal and explosion risk. We found that carbon nitride-aromatic diimide-graphene nanohybrids prepared by simple hydrothermal-calcination procedure produce H2O2 from pure water and O2 under visible light (λ > 420 nm). Photoexcitation of the semiconducting carbon nitride-aromatic diimide moiety transfers their conduction band electrons to graphene and enhances charge separation. The valence band holes on the semiconducting moiety oxidize water, while the electrons on the graphene moiety promote selective two-electron reduction of O2. This metal-free system produces H2O2 with solar-to-chemical energy conversion efficiency 0.20%, comparable to the highest levels achieved by powdered water-splitting photocatalysts.

Hot-Electron-Induced Highly Efficient O2 Activation by Pt Nanoparticles Supported on Ta2O5 Driven by Visible Light.

Aerobic oxidation on a heterogeneous catalyst driven by visible light (λ >400 nm) at ambient temperature is a very important reaction for green organic synthesis. A metal particles/semiconductor system, driven by charge separation via an injection of "hot electrons (e(hot)(-))" from photoactivated metal particles to semiconductor, is one of the promising systems. These systems, however, suffer from low quantum yields for the reaction (<5% at 550 nm) because the Schottky barrier created at the metal/semiconductor interface suppresses the e(hot)(-) injection. Some metal particle systems promote aerobic oxidation via a non-e(hot)(-)-injection mechanism, but require high reaction temperatures (>373 K). Here we report that Pt nanoparticles (∼5 nm diameter), when supported on semiconductor Ta2O5, promote the reaction without e(hot)(-) injection at room temperature with significantly high quantum yields (∼25%). Strong Pt-Ta2O5 interaction increases the electron density of the Pt particles and enhances interband transition of Pt electrons by absorbing visible light. A large number of photogenerated e(hot)(-) directly activate O2 on the Pt surface and produce active oxygen species, thus promoting highly efficient aerobic oxidation at room temperature.

Sunlight-driven hydrogen peroxide production from water and molecular oxygen by metal-free photocatalysts.

Design of green, safe, and sustainable process for the synthesis of hydrogen peroxide (H2 O2 ) is a very important subject. Early reported processes, however, require hydrogen (H2 ) and palladium-based catalysts. Herein we propose a photocatalytic process for H2 O2 synthesis driven by metal-free catalysts with earth-abundant water and molecular oxygen (O2 ) as resources under sunlight irradiation (λ>400 nm). We use graphitic carbon nitride (g-C3 N4 ) containing electron-deficient aromatic diimide units as catalysts. Incorporating the diimide units positively shifts the valence-band potential of the catalysts, while maintaining sufficient conduction-band potential for O2 reduction. Visible light irradiation of the catalysts in pure water with O2 successfully produces H2 O2 by oxidation of water by the photoformed valence-band holes and selective two-electron reduction of O2 by the conduction band electrons.

Quantum tunneling injection of hot electrons in Au/TiO2 plasmonic photocatalysts.

Visible light absorption of plasmonic Au nanoparticles supported on semiconductor TiO2 leads to injection of their photoactivated "hot electrons (ehot-)" into the TiO2 conduction band. This charge separation facilitates several oxidation and reduction reactions. These plasmonic systems, however, suffer from low quantum yields because the Schottky barrier created at the Au-TiO2 interface suppresses ehot- injection. Here we report that Au nanoparticles supported on the anatase particles isolated from Degussa (Evonik) P25 TiO2 promote ehot- injection with much higher efficiency than those supported on other commercially-available TiO2 and catalyze aerobic oxidation with very high quantum yield (7.7% at 550 nm). Photoelectrochemical and spectroscopic analysis revealed that the number of Ti4+ atoms located at the Au-TiO2 interface is the crucial factor. These Ti4+ atoms neutralize the negative charge of the Au particles and create a Schottky barrier with narrower depletion layer. This facilitates efficient ehot- injection by "quantum tunneling" through the Schottky barrier without overbarrier energy. The ehot- injection depends on several factors, and loading of 2 wt% Au particles with 3.5-4 nm diameters at around room temperature exhibits the highest activity of plasmonic photocatalysis.

Photocatalytic hydrogenolysis of epoxides using alcohols as reducing agents on TiO2 loaded with Pt nanoparticles.

Photoexcitation (λ > 300 nm) of TiO2 loaded with Pt particles promotes selective hydrogenolysis of epoxides using alcohols as reducing agents.

One-pot synthesis of imines from nitroaromatics and alcohols by tandem photocatalytic and catalytic reactions on Degussa (Evonik) P25 titanium dioxide.

Photoirradiation (λ > 300 nm) of Degussa (Evonik) P25 TiO2, a mixture of anatase and rutile particles, in alcohols containing nitroaromatics at room temperature produces the corresponding imines with very high yields (80-96%). Other commercially available anatase or rutile TiO2 particles, however, exhibit very low yields (<30%). The imine formation involves two step reactions on the TiO2 surface: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of nitrobenzene (aniline formation) and (ii) condensation of the formed aldehyde and aniline on the Lewis acid sites (imine formation). The respective anatase and rutile particles were isolated from P25 TiO2 by the H2O2/NH3 and HF treatments to clarify the activity of these two step reactions. Photocatalysis experiments revealed that the active sites for photocatalytic reactions on P25 TiO2 are the rutile particles, promoting efficient reduction of nitrobenzene on the surface defects. In contrast, catalysis experiments showed that the anatase particles isolated from P25 TiO2 exhibit very high activity for condensation of aldehyde and aniline, because the number of Lewis acid sites on the particles (73 µmol g(-1)) is much higher than that of other commercially available anatase or rutile particles (<15 µmol g(-1)). The P25 TiO2 particles therefore successfully promote tandem photocatalytic and catalytic reactions on the respective rutile and anatase particles, thus producing imines with very high yields.

Platinum nanoparticles strongly associated with graphitic carbon nitride as efficient co-catalysts for photocatalytic hydrogen evolution under visible light.

Platinum (Pt) nanoparticles with <4 nm diameter loaded on graphitic carbon nitride (g-C3N4) by reduction at 673 K behave as efficient co-catalysts for photocatalytic hydrogen evolution under visible light (λ >420 nm). This is achieved by strong Pt-support interaction due to the high temperature treatment, which facilitates efficient transfer of photoformed conduction band electrons on g-C3N4 to Pt particles.

Spiropyran-modified gold nanoparticles: reversible size control of aggregates by UV and visible light irradiations.

UV or visible light irradiation of gold nanoparticles (AuNPs) modified with a thiol-terminated spiropyran dye promotes reversible aggregation or dispersion of AuNPs. This is facilitated by the electrostatic repulsion/attraction between the AuNPs controlled by the ring-opening/closing photoisomerization of the surface dyes. This photochemical method successfully produces aggregates of AuNPs with tunable sizes (20-340 nm) and narrow size distributions (standard deviation <34%) in a reversible manner. In addition, the formed aggregates, even when left in the dark condition, scarcely change their sizes because the stacking interaction between the ring-opened forms of surface dyes suppresses thermal reverse isomerization and maintains the attractive force between the AuNPs.

Pt-Cu bimetallic alloy nanoparticles supported on anatase TiO2: highly active catalysts for aerobic oxidation driven by visible light.

Visible light irradiation (λ > 450 nm) of Pt-Cu bimetallic alloy nanoparticles (~3-5 nm) supported on anatase TiO2 efficiently promotes aerobic oxidation. This is facilicated via the interband excitation of Pt atoms by visible light followed by the transfer of activated electrons to the anatase conduction band. The positive charges formed on the nanoparticles oxidize substrates, and the conduction band electrons reduce molecular oxygen, promoting photocatalytic cycles. The apparent quantum yield for the reaction on the Pt-Cu alloy catalyst is ~17% under irradiation of 550 nm monochromatic light, which is much higher than that obtained on the monometallic Pt catalyst (~7%). Cu alloying with Pt decreases the work function of nanoparticles and decreases the height of the Schottky barrier created at the nanoparticle/anatase heterojunction. This promotes efficient electron transfer from the photoactivated nanoparticles to anatase, resulting in enhanced photocatalytic activity. The Pt-Cu alloy catalyst is successfully activated by sunlight and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.