Integrated Adsorption-Photodegradation of Organic Pollutants by Carbon Xerogel/Titania Composites

Recent studies on the removal of pollutants via adsorption include the use of carbon-based adsorbents, due to their high porosity and large surface area; however, such materials lack photoactive properties. This study evaluates the synergistic effect of integrated mesoporous carbon xerogel (derived from resorcinol formaldehyde) and titanium dioxide (TiO₂) for combined adsorption and photodegradation application. The complex formed between carbon xerogel and TiO₂ phase was investigated through FTIR, proving the presence of a Ti-O-C chemical linkage. The physicochemical properties of the synthesised adsorbent-photocatalyst were probed using FESEM, BET analysis and UV-Vis analysis. The kinetics, equilibrium adsorption, effect of pH, and effect of adsorbent dosage were investigated. The expansion of the absorbance range to the visible range was verified, and the corresponding band gap evaluated. These properties enabled a visible light response when the system was exposed to visible light post adsorption. Hence, an assistive adsorption-photodegradation phenomenon was successfully executed. The adsorption performance exhibited 85% dye degradation which improved to 99% following photodegradation. Further experiments showed the reduction of microorganisms under visible light, where no microbial colonies were observed after treatment, indicating the potential application of these composite materials.

Photocatalytic oxidation of Mn(II) on the surface of Bi2.15WO6via the ligand-to-metal charge transfer (LMCT) pathway

Biotic and abiotic oxidation of Mn(II) in aqueous environments is an important process for the cycling of many elements. However, the mechanism involved in photocatalytic oxidation of Mn(II) has not been clearly elucidated yet. In this study, the photocatalytic oxidation of Mn(II) on the surface of self-doped Bi_2+xWO₆ (Bi_2.15WO₆) under visible light was conducted. Kinetics results show that visible light apparently accelerates the oxidation of Mn(II) to Mn(III, IV) oxides on Bi_2.15WO₆. The average oxidation states (AOS) of manganese reach 2.18 after 80 min of reaction under visible light at pH 8.50. Characterizations indicate the formation of Bi(III)-O-Mn(II) surface complexes between Mn(II) and surface Bi(III) on Bi_2.15WO₆, which then decreases the bandgap of [Bi_2.15WO₆ + Mn(II)]_light (2.53 eV) compared with those of [Bi_2.15WO₆ + Mn(II)]_dark (2.72 eV) and pure Bi_2.15WO₆ (2.86 eV), suggesting the contribution of the ligand-to-metal charge transfer (LMCT) pathway to the photocatalytic oxidation of Mn(II). Moreover, the addition of inorganic oxidants with strong oxidizing capacities (such as Cr₂O₇^2-, NO₃^- or NO₂^-) significantly increases the oxidation rate of Mn(II), further verifying the contribution of the LMCT pathway to Mn(II) oxidation. We therefore suggest that the LMCT pathway is one of the important oxidation routes for Mn(II) oxidation on Bi_2.15WO₆ under visible light.

Linkage Dependence of Interfacial Charge-Transfer Transitions in ZnO: Carboxylate versus Sulfur Linker

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted much attention due to the unique features of a wide range of visible light absorption with colorless organic molecules and direct interfacial charge separation for their potential applications in photoenergy conversions and chemical sensing. As the research on ICTT has almost been limited to titanium oxide semiconductors such as TiO₂, the exploration of ICTT in other inorganic semiconductors is a high-priority issue. Recently, we demonstrated that ICTT is strongly induced by chemisorption of aromatic thiols on ZnO nanoparticles via the sulfur atom. Here, we report on ICTT in ZnO nanoparticles adsorbed with benzoic acid derivatives and the linkage dependence of ICTT in ZnO. We observed ICTT bands in the visible region upon adsorption of 4-(dimethylamino)benzoic acid (4-DMABA) and 3,4-dimethoxybenzoic acid (3,4-DMOBA) on ZnO nanoparticles via the carboxylate group. Notably, the ICTT absorption intensities are about 1 order of magnitude lower than those in the ZnO surface complexes with aromatic thiol compounds. Time-dependence density functional theory (TD-DFT) calculations well reproduce the linkage dependence of ICTT. This characteristic linkage dependence of ICTT in ZnO is attributed to the difference in the valence orbital of bridging atoms. The sulfur bridging atom with the larger 3p valence orbitals gives rise to strong electronic couplings between ZnO and adsorbates for ICTT, in contrast to very weak electronic couplings via the smaller 2p valence orbitals of the oxygen bridging atoms in the carboxylate linkage. Our research reveals the important linkage dependence of ICTT in ZnO and elucidates the mechanism.

Interfacial charge-transfer transitions in SnO2 functionalized with benzoic acid derivatives

Interfacial charge-transfer transitions (ICTTs) between organic compounds and inorganic semiconductors have recently attracted increasing attention for their potential applications in solar energy conversions and chemical sensing due to the unique functions of visible-light absorption with colourless organic molecules and direct charge separation. However, inorganic semiconductors available for ICTT are quite limited to a few kinds of metal-oxide semiconductors (TiO₂, ZnO, etc.). Particularly, the exploration of ICTT in inorganic semiconductors with a lower-energy conduction band such as SnO₂ is an important issue for realizing a wide range of visible-light absorption for organic adsorbates with the deep highest occupied molecular orbital (HOMO) such as benzoic acid derivatives. Here, we report the first observation of ICTT in SnO₂. SnO₂ nanoparticles show a broad absorption band in the visible region by chemisorption of 4-dimethylaminobenzoic acid (4-DMABA) and 4-aminobenzoic acid (4-ABA)) via the carboxylate group. The wavelength range of the ICTT band significantly changes depending on the kind of substituent group. The ionization potential measurement and density functional theory (DFT) analysis reveal that the absorption band is attributed to ICTT from the HOMO of the adsorbed benzoic acid derivatives to the conduction band of SnO₂. In addition, we clarify the mechanism of ICTT in SnO₂ computationally. Our research opens up a way to the fundamental research on ICTT in SnO₂ and applications in solar energy conversions and chemical sensing.

First observation of interfacial charge-transfer transitions in SnO₂.

Preparation of p-aminobenzoic acid grafted TiOx semiconductive aerogels for photocatalytic reduction of CO2 to methanol

A PABA grafted Ti_x aerogel has both porous and semiconductive properties. The porosity, band gap, conductivity and even PL intensity of the aerogel are adjustable. The aerogel exhibited efficient photocatalytic performance in reduction of CO₂.

Interfacial charge-transfer transitions in ZnO induced exclusively by adsorption of aromatic thiols

Interfacial charge-transfer transitions useful for applications in optical biosensing and photoenergy conversion were first observed in ZnO by adsorption of aromatic thiols.

Dye-sensitized photocatalyst for effective water splitting catalyst

Renewable hydrogen production is a sustainable method for the development of next-generation energy technologies. Utilising solar energy and photocatalysts to split water is an ideal method to produce hydrogen. In this review, the fundamental principles and recent progress of hydrogen production by artificial photosynthesis are reviewed, focusing on hydrogen production from photocatalytic water splitting using organic-inorganic composite-based photocatalysts.

Catechol–TiO2 hybrids for photocatalytic H2 production and photocathode assembly

Charge-transfer dyes are explored for photocatalytic H₂ evolution (DSP) and dye-sensitised photoelectrochemical applications (DSPEC).

Synthesis of highly visible light active TiO2-2-naphthol surface complex and its application in photocatalytic chromium(vi) reduction

Visible light active 2-naphthol functionalized TiO₂ (TiO₂-2-NAP) photocatalytic system is developed for the 100% reduction of Cr(vi) in aqueous medium.

Modification of TiO2 with sulfate and phosphate for enhanced eosin Y-sensitized hydrogen evolution under visible light illumination

TiO2 was modified with sulfate and phosphate (denoted as S/TiO2 and P/TiO2) through a simple sulfuric or phosphoric acid treatment. A strong coordination bond forms between sulfate or phosphate and Ti(4+) of TiO2. Eosin Y (EY)-sensitized S/TiO2 and P/TiO2 (Pt as a co-catalyst and triethanolamine as a sacrificial electron donor) exhibit enhanced photocatalytic activity for hydrogen evolution under visible light illumination (λ > 420 nm) compared to that of EY-sensitized TiO2. The conduction band (CB) edges of S/TiO2 and P/TiO2 shift toward the negative, and the hydrogen bond interaction between the reduced radical EY˙-H and S/TiO2 or P/TiO2 is enhanced due to the inducing effect of the bound sulfate and phosphate. Thus, the photocatalytic hydrogen evolution is promoted. The effects of the concentration of the sulfuric or phosphoric acid as well as concentration of EY on the sensitization hydrogen evolution were investigated. The possible mechanism was discussed.