Effects of Surface Defects on Photocatalytic H₂O₂ Production by Mesoporous Graphitic Carbon Nitride under Visible Light Irradiation

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 defects behave as the active sites for four-electron reduction of O₂, thus decreasing the H₂O₂ selectivity. Furthermore, these defects also behave as the active sites for photocatalytic decomposition of the formed H₂O₂. Consequently, the GCN catalysts with relatively large surface area but with a small number of surface defects promote relatively efficient H₂O₂ formation

Graphitic Carbon Nitride Doped with Biphenyl Diimide: Efficient Photocatalyst for Hydrogen Peroxide Production from Water and Molecular Oxygen by Sunlight

Photocatalytic hydrogen peroxide (H₂O₂) production from water and molecular oxygen (O₂) by sunlight is a promising strategy for green, safe, and sustainable H₂O₂ synthesis. We prepared graphitic carbon nitride (g-C₃N₄) doped with electron-deficient biphenyl diimide (BDI) units by a simple calcination procedure. The g-C₃N₄/BDI catalyst, when photoirradiated by visible light (λ >420 nm) in pure water with O₂, successfully promotes water oxidation by the photogenerated valence band holes and selective two-electron reduction of O₂ by the conduction band electrons, resulting in successful production of millimolar levels of H₂O₂. Electrochemical analysis, Raman spectroscopy, and ab initio calculation results revealed that, upon photoexcitation of the catalyst, the photogenerated positive holes are localized on the BDI unit while the conduction band electrons are localized on the melem unit. This spatial charge separation suppresses rapid recombination of the hole–electron pairs and facilitates efficient H₂O₂ production. The solar-to-chemical energy conversion efficiency for H₂O₂ production is 0.13%, which is comparable to that for photosynthetic plants. This metal-free photocatalysis therefore shows potential as an artificial photosynthesis for clean solar fuel production

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

Mellitic Triimide-Doped Carbon Nitride as Sunlight-Driven Photocatalysts for Hydrogen Peroxide Production

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 (<0.1 mM). We prepared graphitic carbon nitride (g-C₃N₄) doped with mellitic triimide (MTI) units by thermal condensation of melem and mellitic acid anhydride. The g-C₃N₄/MTI photocatalyst, when irradiated by visible light (λ > 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%) and similar to the highest values obtained by semiconductor water-splitting catalysts

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

Nitrogen Fixation with Water on Carbon-Nitride-Based Metal-Free Photocatalysts with 0.1% Solar-to-Ammonia Energy Conversion Efficiency

Ammonia (NH₃), which is an indispensable chemical, is produced by the Haber–Bosch process using H₂ and N₂ under severe reaction conditions. Although photocatalytic N₂ fixation with water under ambient conditions is ideal, all previously reported catalysts show low efficiency. Here, we report that a metal-free organic semiconductor could provide a new basis for photocatalytic N₂ fixation. We show that phosphorus-doped carbon nitride containing surface nitrogen vacancies (PCN-V), prepared by simple thermal condensation of the precursors under H₂, produces NH₃ from N₂ with water under visible light irradiation. The doped P atoms promote water oxidation by the photoformed valence-band holes, and the N vacancies promote N₂ reduction by the conduction-band electrons. These phenomena facilitate efficient N₂ fixation with a solar-to-chemical conversion (SCC) efficiency of 0.1%, which is comparable to the average solar-to-biomass conversion efficiency of natural photosynthesis by typical plants. Thus, this metal-free catalyst shows considerable potential as a new method of artificial photosynthesis