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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.

Dehalogenation of aromatic halides is one important reaction for detoxification and organic synthesis. Photocatalytic dehalogenation with alcohol, a safe hydrogen source, is one promising method; however, systems reported earlier need UV irradiation. We found that Pt–Pd bimetallic alloy nanoparticles (ca. 4 nm) supported on Ta2O5 (PtPd/Ta2O5), on absorption of visible light (λ > 450 nm), efficiently promote dehalogenation with 2-PrOH as a hydrogen source. Catalytic dehydrogenation of 2-PrOH on the alloy in the dark produces hydrogen atoms (H) on the particles. Photoexcitation of d electrons on the alloy particles by absorbing visible light produces hot electrons (ehot–). They efficiently reduce the adsorbed H atoms and produce hydride species (H–) active for dehalogenation. The catalytic activity depends on the Pt/Pd mole ratio; alloy particles consisting of 70 mol % of Pt and 30 mol % of Pd exhibit the highest activity for dehalogenation.

Generating hydrogen peroxide (H2O2) from water and dioxygen (O2) by photocatalysis is one ideal artificial photosynthesis for solar fuel production. Several early reported powdered photocatalysts, however, produce small amounts of H2O2 ( 420 nm) in pure water with O2, successfully produces millimolar levels of H2O2 via water oxidation by valence band holes and selective two-electron reduction of O2 by conduction band electrons. The incorporation of triply branched MTI units creates a condensed melem layer. This facilitates efficient intra- and interlayer transfer of photogenerated charge carriers and shows high electrical conductivity. The solar-to-chemical conversion efficiency for H2O2 production on the catalyst is 0.18%, which is higher than that of natural photosynthesis (∼0.1%) ...

Au nanoparticles loaded on semiconductor TiO

Colloidal suspensions of gold nanoparticles (AuNPs) in water can be prepared by heating water containing HAuCl4 with citric acid as reductant and surface stabilizing reagent. This thermal reduction method, however, promotes aggregation of the formed AuNPs, resulting in bimodal distribution of AuNPs. Here we report that UV (254 nm) irradiation of water containing HAuCl4 with citric acid at room temperature successfully reduces HAuCl4 and produces monodispersed AuNPs, while suppressing aggregation of the formed AuNPs. Changing the intensity of UV light or the amount of citric acid added successfully produces AuNPs with tunable sizes and narrow size distributions.

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.

The design of a safe and sustainable process for the synthesis of hydrogen peroxide (H2O2) is a very important subject from the viewpoint of green chemistry. Photocatalytic H2O2 production with earth-abundant water and molecular oxygen (O2) as resources is an ideal process. A successful system based on an organic semiconductor has been proposed; however, it suffers from poor photostability. Here we report an inorganic photocatalyst for H2O2 synthesis. Visible light irradiation (λ >420 nm) of the semiconductor BiVO4 loaded with Au nanoparticles (Au/BiVO4) in pure water with O2 successfully produces H2O2. The bottom of the BiVO4 conduction band (0.02 V vs NHE, pH 0) is more positive than the one-electron reduction potential of O2 (−0.13 V) while more negative than the two-electron reduction potential of O2 (0.68 V). This thus suppresses one-electron reduction of O2 and selectively promotes two-electron reduction of O2, resulting in efficient H2O2 formation.

Photocatalytic production of hydrogen peroxide (H2O2) from ethanol (EtOH) and molecular oxygen (O2) was carried out by visible light irradiation (λ > 420 nm) of mesoporous graphitic carbon nitride (GCN) catalysts with different surface areas prepared by silica-templated thermal polymerization of cyanamide. On these catalysts, the photoformed positive hole oxidize EtOH and the conduction band electrons localized at the 1,4-positions of the melem unit promote two-electron reduction of O2 (H2O2 formation). The GCN catalysts with 56 and 160 m2 g–1 surface areas exhibit higher activity for H2O2 production than the catalyst prepared without silica template (surface area: 10 m2 g–1), but a further increase in the surface area (228 m2 g–1) decreases the activity. In addition, the selectivity for H2O2 formation significantly decreases with an increase in the surface area. The mesoporous GCN with larger surface areas inherently contain a larger number of primary amine moieties at the surface of mesopores. These def...

Photoirradiation (λ > 300 nm) of TiO2 loaded with Pd nanoparticles (ca. 2 wt%, 5 nm diameter) in water containing alcohols and azobenzene derivatives at room temperature successfully produces the corresponding secondary amines with high yields. This is facilitated via consecutive photocatalytic and catalytic reactions: (i) photocatalytic oxidation of alcohols (aldehyde formation) and reduction of azobenzenes (aniline formation); (ii) catalytic condensation of the formed aldehydes with anilines on the TiO2 surface (imine formation); and (iii) photocatalytic hydrogenation of the formed imines (secondary amine formation). This catalytic system successfully produces several kinds of secondary amines with >80% yields.

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

An amino-substituted spirothiopyran, when added to an aqueous solution containing gold nanoparticles (AuNPs), promotes aggregation of AuNPs at room temperature in the dark. The sulfur atom of the spirothiopyran has high nucleophilicity due to the substitution of electron-donating amino groups and is bound to the AuNP surface via covalent Au–S binding. This promotes spontaneous spirocycle opening of the spirothiopyran and produces a positively charged indolinium moiety on the AuNP surface. This reduces electrostatic repulsion between the AuNPs and promotes their aggregation. Changing the amount of the spirothiopyran added successfully produces AuNP aggregates with tunable sizes (40–1300 nm) and narrow size distributions with standard deviation being less than 30%. This system therefore has potential to be a new simple method for self-assembly of AuNPs.

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.

Visible light absorption of plasmonic Au nanoparticles supported on semiconductor TiO

Platinum (Pt) nanoparticles with4 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 (λ420 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.

Visible light irradiation (λ > 450 nm) of Pt nanoparticles supported on Degussa P25 TiO2 (Pt/P25 catalyst) promotes efficient and selective aerobic oxidation of aniline to nitrosobenzene. This is facilicated via the interband excitation of Pt atoms by visible light followed by the transfer of activated electrons to the TiO2 conduction band. The positive charges formed on the Pt surface oxidize substrates, and the conduction band electrons reduce O2, promoting photocatalytic cycles. The high activity of Pt/P25 is due to the high electron density of Pt particles. They behave as Lewis base sites for reductive deprotonation of aniline, thus promoting efficient aniline oxidation. The Pt/P25 catalyst, when photoirradiated at a low temperature (∼283 K), suppresses subsequent condensation of aniline and the formed nitrosobenzene, successfully producing nitrosobenzene with a very high selectivity (90%).

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

Visible light irradiation (λ450 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.

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 (λ420 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.

An easily recoverable and recyclable heterogeneous catalyst renders possible the synthesis of secondary amines from azobenzenes (I) and alcohols (II) or (IV) under mild reaction conditions.

Photoirradiation (λ > 300 nm) of TiO2 loaded with Pd nanoparticles (2 wt%, ca. 5 nm diameter) in water containing benzyl alcohol and nitrobenzene at room temperature successfully produces the corresponding secondary amine (N-benzylaniline) with 96% yield. This is achieved via three consecutive catalytic reactions: (i) photocatalytic oxidation of alcohol (aldehyde formation) and reduction of nitrobenzene (aniline formation); (ii) catalytic condensation of the formed aldehyde with aniline on the TiO2 surface (imine formation); and, (iii) photocatalytic hydrogenation of the formed imine (secondary amine formation). This catalytic system successfully produces several kinds of secondary amines, even those containing reducible substituents such as –CN, –COOH, or –CHO with >76% yields.

Aerobic oxidation on a heterogeneous catalyst driven by visible light (λ400 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.

Pyromellitic diimide is incorporated into graphitic carbon nitride, g-C3N4 by calcination of a mixture of melem and pyromellitic dianhydride at 598 K for 4 h.

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 (

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 (λ400 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.