Impact of nitrogen doping on triazole-based graphitic carbon Nitride-TiO2 (P25) S-scheme heterojunction for improved photocatalytic hydrogen production

Establishing an S-scheme heterojunction is a promising method for increasing the photocatalytic activity of synthetic materials. In this study, nitrogen-doped g-C3N5/TiO2 S-scheme photocatalysts have been synthesized and examined for photocatalytic hydrogen production using thermal decomposition methods. Nitrogen-doped g-C3N5/TiO2 composites performed better than pure nitrogen-doped g-C3N5 and TiO2 alone. Using experiments and density functional theory (DFT) calculations, nitrogen (N) doping was identified as being introduced by replacing the carbon (C) atoms in the matrix of g-C3N5. In addition to its narrow band gap, N-doped g-C3N5 showed efficient carrier separation and charge transfer, resulting in the enhanced absorption of visible light and photocatalytic activity. DFT, XPS, optical property characteristics, and PL spectra confirmed these findings, which were attributed to the successful nitrogen doping, and the composite was proven to be a potential candidate for photocatalytic hydrogen generation under light irradiation. The quantity of H2 produced from the nitrogen-doped g-C3N5/TiO2 composite for 3 hours (3515.1 μmol g-1) was about three times that of N-doped g-C3N5. The H2 production percentage of the nitrogen-doped g-C3N5/TiO2 catalyst with Pt as the cocatalyst was improved by nearly ten times as compared to N-doped g-C3N5/TiO2 without a cocatalyst. Herein, we report the successful preparation of the N-doped g-C3N5/TiO2 S-scheme heterojunction and highlight a simple and efficient catalyst for energy storage requirements and environmental monitoring.

Ni/TiO2 heterostructures derived from phase separation for enhanced electrocatalysis of hydrogen evolution and biomass oxidative upgrading in anion exchange membrane electrolyzers

The construction of heterostructures is an effective strategy to enhance electrocatalysis for hydrogen evolution reactions (HERs) and biomass oxidative upgrading. In this work, a Ni/TiO2 heterostructure prepared by a phase-separation strategy was adopted as a bifunctional electrocatalyst for HERs and biomass oxidation in alkaline media. Due to the optimized hydrogen adsorption energetics as well as the interfacial water structure and hydrogen bond connectivity in the electrical double layer, Ni/TiO2 exhibited high activity for HERs with an overpotential of 28 mV at 10 mA cm-2 and good durability at 1000 mA cm-2 for over 100 h in an anion exchange membrane (AEM) electrolyzer. In addition, Ni/TiO2 showed high catalytic performance for the oxidation of biomass-based platform compound 5-hydroxymethylfurfural (HMF) to high-value added compound 2,5-furandicarboxylic acid (FDCA). Continuous production of FDCA with a yield >95% was achieved in the AEM electrolyzer for over 50 h. The superior HMF oxidation performance on the Ni/TiO2 heterostructure compared to Ni resulted from stronger HMF adsorption, lower Ni3+-O formation potential, longer Ni3+-O bond and smaller Ni crystal size.

Investigation of TiO2 Deposit on SiO2 Films: Synthesis, Characterization, and Efficiency for the Photocatalytic Discoloration of Methylene Blue in Aqueous Solution

TiO₂-SiO₂ thin films were created on Corning glass substrates using a simple method. Nine layers of SiO₂ were deposited; later, several layers of TiO₂ were deposited, and their influence was studied. Raman spectroscopy, high resolution transmission electron spectroscopy (HRTEM), an X-ray diffractometer (XRD), ultraviolet-visible spectroscopy (UV-Vis), a scanning electron microscope (SEM), and atomic force microscopy (AFM) were used to describe the sample's shape, size, composition, and optical characteristics. Photocatalysis was realized through an experiment involving the deterioration of methylene blue (MB) solution exposed to UV-Vis radiation. With the increase of TiO₂ layers, the photocatalytic activity (PA) of the thin films showed an increasing trend, and the maximum degradation efficiency of MB by TiO₂-SiO₂ was 98%, which was significantly higher than that obtained by SiO₂ thin films. It was found that an anatase structure was formed at a calcination temperature of 550 °C; phases of brookite or rutile were not observed. Each nanoparticle's size was 13-18 nm. Due to photo-excitation occurring in both the SiO₂ and the TiO₂, deep UV light (λ = 232 nm) had to be used as a light source to increase photocatalytic activity.

Heterophase Polymorph of TiO2 (Anatase, Rutile, Brookite, TiO2 (B)) for Efficient Photocatalyst: Fabrication and Activity

TiO2 exists naturally in three crystalline forms: Anatase, rutile, brookite, and TiO2 (B). These polymorphs exhibit different properties and consequently different photocatalytic performances. This paper aims to clarify the differences between titanium dioxide polymorphs, and the differences in homophase, biphase, and triphase properties in various photocatalytic applications. However, homophase TiO2 has various disadvantages such as high recombination rates and low adsorption capacity. Meanwhile, TiO2 heterophase can effectively stimulate electron transfer from one phase to another causing superior photocatalytic performance. Various studies have reported the biphase of polymorph TiO2 such as anatase/rutile, anatase/brookite, rutile/brookite, and anatase/TiO2 (B). In addition, this paper also presents the triphase of the TiO2 polymorph. This review is mainly focused on information regarding the heterophase of the TiO2 polymorph, fabrication of heterophase synthesis, and its application as a photocatalyst.

Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm

Anatase TiO2 is a widely investigated photocatalyst; however, it can only work under ultraviolet (UV) light with wavelengths less than 390 nm (band gap 3.2 eV). Rutile TiO2 can absorb visible light at wavelengths less than 410 nm (band gap 3.0 eV); however, its photocatalytic activity is not high. Herein, we activated rutile TiO2, which was prepared from Evonik TiO2 P 25 through calcination at 800 °C using hydrogen reduction treatment at 700 °C. The photocatalytic activity of the hydrogen-treated TiO2 was as high as P 25 under UV irradiation at 380 nm, which was significantly higher than P 25 under violet light irradiation at 405 nm for the oxidative decomposition of acetic acid in water. Electron spin resonance studies indicate that charge separation is enhanced in reduced TiO2, and their oxygen reduction pathways differ between anatase and rutile. The formation of H2O2 was observed on rutile TiO2; however, it was consumed during photocatalysis to accelerate acetic acid decomposition.

Copper-Modified Titania-Based Photocatalysts for the Efficient Hydrogen Production under UV and Visible Light from Aqueous Solutions of Glycerol

In this study, we have proposed titania-based photocatalysts modified with copper compounds for hydrogen evolution. Thermal pre-treatment of commercial TiO₂ Degussa P25 (DTiO₂) and Hombifine N (HTiO₂) in the range from 600 to 800 °C was carried out followed by the deposition of copper oxides (1-10 wt. % of Cu). The morphology and chemical state of synthesized photocatalysts were studied using X-ray diffraction, UV-Vis diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and XANES/EXAFS X-ray absorption spectroscopy. Photocatalytic activity was tested in the hydrogen evolution from aqueous solutions of glycerol under ultraviolet (λ = 381 nm) and visible (λ = 427 nm) light. The photocatalysts 2% CuO_x/DTiO₂ T750 and 5% CuO_x/DTiO₂ T700 showed the highest activity under UV irradiation (λ = 380 nm), with the rate of H₂ evolution at the level of 2.5 mmol (H₂) g^-1 h^-1. Under the visible light irradiation (λ = 427 nm), the highest activity of 0.6 mmol (H₂) g^-1 h^-1 was achieved with the 5% CuO_x/DTiO₂ T700 photocatalyst. The activity of these photocatalysts is 50% higher than that of the platinized 1% Pt/DTiO₂ sample. Thus, it was shown for the first time that a simple heat treatment of a commercial titanium dioxide in combination with a deposition of non-noble metal particles led to a significant increase in the activity of photocatalysts and made it possible to obtain materials that were active in hydrogen production under visible light irradiation.

Enhanced visible-light photodegradation of fluoroquinolone-based antibiotics and E. coli growth inhibition using Ag-TiO2 nanoparticles

Antibiotics in wastewater represent a growing and worrying menace for environmental and human health fostering the spread of antimicrobial resistance. Titanium dioxide (TiO₂) is a well-studied and well-performing photocatalyst for wastewater treatment. However, it presents drawbacks linked with the high energy needed for its activation and the fast electron–hole pair recombination. In this work, TiO₂ nanoparticles were decorated with Ag nanoparticles by a facile photochemical reduction method to obtain an increased photocatalytic response under visible light. Although similar materials have been reported, we advanced this field by performing a study of the photocatalytic mechanism for Ag–TiO₂ nanoparticles (Ag–TiO₂ NPs) under visible light taking in consideration also the rutile phase of the TiO₂ nanoparticles. Moreover, we examined the Ag–TiO₂ NPs photocatalytic performance against two antibiotics from the same family. The obtained Ag–TiO₂ NPs were fully characterised. The results showed that Ag NPs (average size: 23.9 ± 18.3 nm) were homogeneously dispersed on the TiO₂ surface and the photo-response of the Ag–TiO₂ NPs was greatly enhanced in the visible light region when compared to TiO₂ P25. Hence, the obtained Ag–TiO₂ NPs showed excellent photocatalytic degradation efficiency towards the two fluoroquinolone-based antibiotics ciprofloxacin (92%) and norfloxacin (94%) after 240 min of visible light irradiation, demonstrating a possible application of these particles in wastewater treatment. In addition, it was also proved that, after five Ag–TiO₂ NPs re-utilisations in consecutive ciprofloxacin photodegradation reactions, only a photocatalytic efficiency drop of 8% was observed. Scavengers experiments demonstrated that the photocatalytic mechanism of ciprofloxacin degradation in the presence of Ag–TiO₂ NPs is mainly driven by holes and ˙OH radicals, and that the rutile phase in the system plays a crucial role. Finally, Ag–TiO₂ NPs showed also antibacterial activity towards Escherichia coli (E. coli) opening the avenue for a possible use of this material in hospital wastewater treatment.

Ag nanoparticles decorated-TiO₂ P25 are a viable alternative for the degradation, through a rutile-mediated mechanism, of fluoroquinolone-based antibiotics under visible light irradiation and, at the same time, for bacteria inactivation in water.

Long-term and stable antimicrobial properties of immobilized Ni/TiO2 nanocomposites against Escherichia coli, Legionella thermalis, and MS2 bacteriophage

Nickel has been extensively used as a high work function metal because of its abundance, low cost, relatively non-toxic nature, and environmentally benign characteristics. However, it has rarely been extended in a form of immobilized composite, which is a practical strategy applicable for photocatalytic antimicrobial activities. In this study, a composite of nickel and TiO₂ (Ni/TiO₂) was prepared using a photodeposition method, and its antibacterial properties were investigated using Escherichia coli (E. coli). To optimize Ni/TiO₂ synthesis, the effect of various photodeposition conditions on antibacterial performance were investigated, such as the light irradiation time, metal content, TiO₂ crystalline structure, and presence or absence of electron donors (i.e., methanol). The optimized 2 wt% Ni/TiO₂ exhibited an antibacterial efficiency of 3.74 log within 7 min, which is more than 10-fold higher than that of pristine TiO₂ (2.54 log). Based on this optimized weight ratio, Ni/TiO₂ was immobilized on a steel mesh using an electrospray/thermal compression method, and its antibacterial performance was further assessed against E. coli, MS2 bacteriophage virus (MS2 phage), and a common pulmonary pathogen (Legionella thermalis, L. thermalis). Within 70 min, all target microorganisms achieved an inactivation that exceeded 4 log. Furthermore, the long-term stability and sustainable usability of the Ni/TiO₂ mesh were confirmed by performing more than 50 antibacterial evaluation cycles using E. coli. The results of this study facilitate the successful utilization of immobilized Ni/TiO₂ mesh in water disinfection applications.

Promoting hydrogen evolution of a g-C3N4-based photocatalyst by indium and phosphorus co-doping

An indium and phosphorus co-doped g-C₃N₄ photocatalyst (In,P-g-C₃N₄) was prepared by K₂HPO₄ post-treatment of an indium-doped g-C₃N₄ photocatalyst (In-g-C₃N₄) derived from in situ copolymerization of dicyandiamide and indium chloride.

Effect of diatomite addition on crystalline phase formation of TiO2 and photocatalytic degradation of MDMA

Different composites TiO₂@SiO₂ were obtained by in situ synthesis of TiO₂ on Algerian diatomite. Our results show that there is an optimum amount of diatomite which leads to mixed TiO₂ phase with enhanced photocatalytic activity.

Synergistic effect of Ni-Ag-rutile TiO2 ternary nanocomposite for efficient visible-light-driven photocatalytic activity

P25 comprising of mixed anatase and rutile phases is known to be highly photocatalytically active compared to the individual phases. Using a facile wet chemical method, we demonstrate a ternary nanocomposite consisting of Ni and Ag nanoparticles, decorated on the surface of XTiO2 (X: P25, rutile (R)) as an efficient visible-light-driven photocatalyst. Contrary to the current perspective, RTiO2-based Ni-Ag-RTiO2 shows the highest activity with the H2 evolution rate of ∼86 μmol g-1 W-1 h-1@535 nm. Together with quantitative assessment of active Ni, Ag and XTiO2 in these ternary systems using high energy synchrotron X-ray diffraction, transmission electron microscopy coupled energy dispersive spectroscopy mapping evidences the metal to semiconductor contact via Ag. The robust photocatalytic activity is attributed to the improved visible light absorption, as noted by the observed band edge of ∼2.67 eV corroborating well with the occurrence of Ti3+ in Ti 2p XPS. The effective charge separation due to intimate contact between Ni and RTiO2 via Ag is further evidenced by the plasmon loss peak in Ag 3d XPS. Moreover, density functional theory calculations revealed enhanced adsorption of H2 on Ti8O16 clusters when both Ag and Ni are simultaneously present, owing to the hybridization of the metal atoms with d orbitals of Ti and p orbitals of O leading to enhanced bonding characteristics, as substantiated by the density of states. Additionally, the variation in the electronegativity in Bader charge analysis indicates the possibility of hydrogen evolution at the Ni sites, in agreement with the experimental observations.

TiO2 as an interfacial-charge-transfer-bridge to construct eosin Y-mediated direct Z-scheme electron transfer over a Co9S8 quantum dot/TiO2 photocatalyst

A novel eosin Y-mediated Z-scheme Co₉S₈ QDs/TiO₂ photocatalytic system was constructed and a high AQE of 37.4% is obtained at 470 nm for 20%Co₉S₈/TiO₂ heterojunction.

Micro-kinetic modelling of photocatalytic CO2 reduction over undoped and N-doped TiO2

The experimental data of photoreduction of CO₂ using undoped and N-doped TiO₂ photocatalysts were used for micro-kinetic modelling.

Ion-Exchange Loading Promoted Stability of Platinum Catalysts Supported on Layered Protonated Titanate-Derived Titania Nanoarrays

Supported metal catalysts are one of the major classes of heterogeneous catalysts, which demand good stability in both the supports and catalysts. Herein, layered protonated titanate-derived TiO₂ (LPT-TiO₂) nanowire arrays were synthesized to support platinum catalysts using different loading processes. The Pt ion-exchange loading on pristine LPTs followed by thermal annealing resulted in superior Pt catalysts supported on the LPT-TiO₂ nanoarrays with excellent hydrothermal stability and catalytic performance toward CO and NO oxidations as compared to the Pt catalysts through wet-impregnation on the anatase TiO₂ (ANT-TiO₂) nanoarrays resulted from thermal annealing of LPT nanoarrays. Both loading processes resulted in highly dispersed Pt nanoparticles (NPs) with average sizes smaller than 1 nm at their pristine states. However, after hydrothermal aging at 800 °C for 50 h, highly dispersed Pt NPs were only retained on the ion-exchanged LPT-TiO₂ nanoarrays with the support structure consisting of a mixture of 74% anatase and 26% rutile TiO₂. For the wet-impregnation loading directly on anatase TiO₂ nanoarrays derived from LPT, the Pt catalysts experienced severe agglomeration after hydrothermal aging, with the nanoarray supports consisting of 86% anatase and 14% rutile TiO₂. Spectroscopy analysis suggested that Pt²⁺ cations intercalated into the interlayers of the titanate frameworks through ion-exchange impregnation procedure, which altered the chemical and electronic structures of the catalysts, resulting in the shifts of the electronic binding energy, Raman bands, and optical energy bandgap. The ion-exchangeable nature of LPT nanoarrays clearly provides a structural modification in Pt-doped LPT that has resulted in a strong interaction between the Pt catalysts and LPT-TiO₂ nanoarray supports, leading to the enhanced hydrothermal stability of the catalysts. Considering the wide applications of the LPT and TiO₂ nanomaterials as supports for catalysts, this finding provides a new pathway to design highly stable supported metal catalysts for different reactions.

Water Splitting on Rutile TiO2 -Based Photocatalysts

Water splitting using a semiconductor photocatalyst with sunlight has long been viewed as a potential means of large-scale H₂ production from renewable resources. Different from anatase TiO₂ , rutile enables preferential water oxidation, which is useful for the construction of a Z-scheme water-splitting system. The combination of rutile TiO₂ with a suitable H₂ -evolution photocatalyst such as a Pt-loaded BaZrO₃ -BaTaO₂ N solid solution enables solar-driven water splitting into H₂ and O₂ . While rutile TiO₂ is a wide-gap semiconductor with a bandgap of 3.0 eV, co-doping of rutile TiO₂ with certain metal ions and/or nitrogen produces visible-light-driven photocatalysts, which are also useful as a component for water oxidation in visible-light-driven Z-scheme water splitting. The key to achieving highly efficient water oxidation is to maintain a charge balance of dopants in the rutile, because single doping typically produces trap states that capture photogenerated electrons and/or holes. Here we provide a concise summary of rutile TiO₂ -based photocatalysts for water-splitting systems.

Structures, electron density and characterization of novel photocatalysts, (BaTaO2N)1−x(SrWO2N)xsolid solutions

W⁵⁺doping in BaTaO₂N enhances (Ta,W)–(O,N) covalent bonding and can improve the photocatalytic activity.