One-step reductive synthesis of Ti3+ self-doped elongated anatase TiO2 nanowires combined with reduced graphene oxide for adsorbing and degrading waste engine oil

Efficient Cr(VI) remediation by electrospun composite porous nanofibers incorporating biomass with metal oxides and metal-organic framework

To develop a highly efficient adsorbent to remediate and remove hexavalent chromium ions (Cr(VI)) from polluted water, cellulose acetate (CA) and chitosan (CS), along with metal oxides (titanium dioxide (TiO2) and ferroferric oxide (Fe3O4)), and a zirconium-based metal-organic framework (UiO-66) were used to fabricate the composite porous nanofiber membranes through electrospinning. The adsorption performance, influencing factors, adsorption kinetics and isotherms of composite nanofiber membranes were comprehensively investigated. The multi-layer membrane with interpenetrating nanofibers and surface functional groups enhanced the natural physical adsorption and provided potential chemical sites. The thermal stability was improved by introducing TiO2 and UiO-66. CA/CS/UiO-66 exhibited the highest adsorption capacity (118.81 mg g-1) and removal rate (60.76%), which were twice higher than those of the control. The correlation coefficients (R2) of all the composite nanofibers regressed by the Langmuir model were significantly higher than those by the Freundlich model. The pseudo-first-order kinetic curve of CA/CS composite nanofibers showed the highest R2 (0.973), demonstrating that the whole adsorption process involved a combination of strong physical adsorption and weak chemical adsorption by the amino groups of CS. However, the R2 values of the pseudo-second-order kinetic model increased after incorporating TiO2, Fe3O4, and UiO-66 into the CA/CS composite nanofiber membranes since an enhanced chemical reaction with Cr (VI) occured during the adsorption.

Fe2O3/TiO2/reduced graphene oxide-driven recycled visible-photocatalytic Fenton reactions to mineralize organic pollutants in a wide pH range

Photocatalytic Fenton reactions combined the advantages from both photocatalysis and Fenton reaction in mineralizing organic pollutants. The key problems are the efficiency and recycling stability. Herein, we reported a novel Fe2O3/TiO2/reduced graphene oxide (FTG) nanocomposite synthesized by a facile solvothermal method. The TiO2 in FTG degraded organic pollutants and mineralized intermediates via photocatalysis under visible light irradiation, which could also promote Fenton reaction by accelerating Fe3+-Fe2+ recycle. Meanwhile, the Fe2O3 rapidly degraded organic pollutants via Fenton reactions, which also promoted photocatalysis by enhancing visible light absorbance and diminishing photoelectron-hole recombination. The high distribution of TiO2 and Fe2O3 on rGO, together with their strong interaction resulted in enhanced synergetic cooperation between photocatalysis and Fenton reactions, leading to the high mineralization efficiency of organic pollutants. More importantly, it could also inhibit the leaching of Fe species, leading to the long lifetime of FTG during photocatalytic Fenton reactions in a wide pH range from 3.4 to 9.2.

Assessment of surface and electrical properties of the TiO2@zeolite hybrid materials

Degradation of pollutants in aqueous medium is of high interest due to the impact on environment and human health, therefore, design and study of the physico-chemical properties of photocatalysts for water remediation are of major significance. Among properties of photocatalyst, those related to the surface and electrical mechanism are crucial to the photocatalyst´s performance. Here we report the chemical and morphological characteristics of TiO₂@zeolite photocatalyst by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) respectively, and a coherent electrical conduction mechanism was proposed based on data obtained from assisted laser impedance spectroscopy (ALIS), in which the zeolite was synthesized from recycled coal fly ash. The results obtained by SEM and XPS verified the presence of spherical particles of TiO₂ anatase with presence of Ti³⁺ state. ALIS results showed that impedance of the entire system increases when the amount of TiO₂ increases and the samples with lower capacitive performance allowed a larger transfer of the charges between the solid-liquid interface. All results showed that higher photocatalytic performance of TiO₂ growth over hydroxysodalite with 8.7 wt% and 25 wt% of TiO₂ can be explained in terms of the morphology of TiO₂ and the interactions between substrate-TiO₂ mainly.

Solar-light-driven ternary MgO/TiO2/g-C3N4 heterojunction photocatalyst with surface defects for dinitrobenzene pollutant reduction

Defect engineering and heterojunction are promising strategies to improve the photocatalytic performance of particular catalyst through effective charge carrier separation and transport. Herein, we developed Z-scheme MgO/TiO₂/g-C₃N₄ ternary heterojunction photocatalyst with surface defects and effective charge separation for reduction of recalcitrant dinitrobenzene isomers under simulated solar light irradiation. Mott-Schottky (MS) plot analysis and electron spin resonance (ESR) radical trapping experiment suggested the formation of Z-scheme heterojunction at the interface of TiO₂/g-C₃N₄, which played a crucial role in the electron-hole separation. Incorporating MgO into the structure further enhances charge separation via Ti³⁺ and oxygen vacancy (OV) defects formation at the TiO₂/MgO interface as confirmed by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) analyses. Besides, the surface basicity of MgO enhanced conversion of dinitrobenzene (DNB) isomers through formation of nitrophenylhydroxylamine intermediate which can easily be reduced to phenylenediamines (PDAs). As confirmed by high performance liquid chromatography (HPLC) analysis, excellent selectivity for PDAs (95-98%) was achieved in 90 min with ternary MgO/TiO₂/g-C₃N₄ composite compared to the binary MgO/TiO₂ and TiO₂/g-C₃N₄. A possible reaction pathway and photocatalytic reduction mechanism were proposed and elucidated. This work demonstrated an effective strategy to reduce recalcitrant dinitrobenzene isomers using efficient, low-cost, and environmental benign photocatalyst with a facile identification of reaction intermediates.

GO/TiO2-Related Nanocomposites as Photocatalysts for Pollutant Removal in Wastewater Treatment

Water pollution has been a prevalent issue globally for some time. Some pollutants are released into the water system without treatment, making the water not suitable for consumption. This problem may lead to more grave problems in the future including the destruction of the ecosystem along with the organisms inhabiting it, and illness and diseases endangering human health. Conventional methods have been implemented to remove hazardous pollutants such as dyes, heavy metals, and oil but are incapable of doing so due to economic restraints and the inability to degrade the pollutants, leading to secondary pollution. Photocatalysis is a more recently applied concept and is proven to be able to completely remove and degrade pollutants into simpler organic compounds. Titanium dioxide (TiO₂) is a fine example of a photocatalyst owing to its cost-effectiveness and superb efficiency. However, issues such as the high recombination rate of photogenerated electrons along with positive holes while being only limited to UV irradiation need to be addressed. Carbonaceous materials such as graphene oxide (GO) can overcome such issues by reducing the recombination rate and providing a platform for adsorption accompanied by photocatalytic degradation of TiO₂. The history and development of the synthesis of GO will be discussed, followed by the methods used for GO/TiO₂ synthesis. The hybrid of GO/TiO₂ as a photocatalyst has received some attention in the application of wastewater treatment due to its efficiency and it being environmentally benign. This review paper thereby aims to identify the origins of different pollutants followed by the sickness they may potentially inflict. Recent findings, including that GO/TiO₂-related nanocomposites can remove pollutants from the water system, and on the photodegradation mechanism for pollutants including aromatic dyes, heavy metal and crude oil, will be briefly discussed in this review. Moreover, several crucial factors that affect the performance of photocatalysis in pollutant removal will be discussed as well. Therefore, this paper presents a critical review of recent achievements in the use of GO/TiO₂-related nanocomposites and photocatalysis for removing various pollutants in wastewater treatment.

Photocatalytic Activity of TiO2/g-C3N4 Nanocomposites for Removal of Monochlorophenols from Water

This research employed g-C₃N₄ nanosheets in the hydrothermal synthesis of TiO₂/g-C₃N₄ hybrid photocatalysts. The TiO₂/g-C₃N₄ heterojunctions, well-dispersed TiO₂ nanoparticles on the g-C₃N₄ nanosheets, are effective photocatalysts for the degradation of monochlorophenols (MCPs: 2-CP, 3-CP, and 4-CP) which are prominent water contaminants. The removal efficiency of 2-CP and 4-CP reached 87% and 64%, respectively, after treatment of 25 ppm CP solutions with the photocatalyst (40TiO₂/g-C₃N₄, 1 g/L) and irradiation with UV-Vis light. Treatment of CP solutions with g-C₃N₄ nanosheets or TiO₂ alone in conjunction with irradiation gave removal efficiencies lower than 50%, which suggests the two act synergically to enhance the photocatalytic activity of the 40TiO₂/g-C₃N₄ nanocomposite. Superoxide and hydroxyl radicals are key active species produced during CP photodegradation. In addition, the observed nitrogen and Ti³⁺ defects and oxygen vacancies in the TiO₂/g-C₃N₄ nanocomposites may improve the light-harvesting ability of the composite and assist preventing rapid electron-hole recombination on the surface, enhancing the photocatalytic performance. In addition, interfacial interactions between the MCPs (low polarity) and thermally exfoliated carbon nitride in the TiO₂/g-C₃N₄ nanocomposites may also enhance MCP degradation.

Construction of Spindle-Shaped Ti3+ Self-Doped TiO2 Photocatalysts Using Triethanolamine-Aqueous as the Medium and Its Photoelectrochemical Properties

To enhance the utilization efficiency of visible light and reduce the recombination of photogenerated electrons and holes, spindle-shaped TiO2 photocatalysts with different Ti3+ concentrations were fabricated by a simple solvothermal strategy using low-cost, environmentally friendly TiH2 and H2O2 as raw materials and triethanolamine-aqueous as the medium. The photocatalytic activities of the obtained photocatalysts were investigated in the presence of visible light. X-ray diffraction (XRD), Raman spectra, transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectra were applied to characterize the structure, morphologies, and chemical compositions of as-fabricated Ti3+ self-doped TiO2. The concentration of triethanolamine in the mixed solvent plays a significant role on the crystallinity, morphologies, and photocatalytic activities. The electron–hole separation efficiency was found to increase with the increase in the aspect ratio of as-fabricated Ti3+ self-doped TiO2, which was proved by transient photocurrent response and electrochemical impedance spectroscopy.

Enhanced Photoelectrocatalytic Activity of TiO2 Nanowire Arrays via Copolymerized G-C3N4 Hybridization

Photoelectrocatalytic (PEC) oxidation is an advanced technology that combines photocatalytic oxidation (PC) and electrolytic oxidation (EC). PEC activity can be greatly enhanced by the PC and EC synergy effect. In this work, novel copolymerized g-C3N4 (denoted as CNx)/TiO2 core-shell nanowire arrays were prepared by chemical vapor deposition. CNx were deposited on the surface of TiO2 nanowire arrays using organic monomer 4,5-dicyanidazole and dicyandiamide as copolymerization precursor. TiO2 nanowire arrays provide a direct and fast electron transfer path, while CNx is a visible light responsive material. After CNx deposition, the light response range of TiO2 is broadened to 600 nm. The deposition of CNx shell effectively improves the PC efficiency and PEC efficiency of TiO2. Under visible light irradiation and 1 V bias potential, the rate constant k of PEC degradation of CNx/TiO2 core-shell nanowire arrays is 0.0069 min−1, which is 72% higher than that of pure TiO2 nanowires. The built-in electric field formed in the interface between TiO2 core and CNx shell would effectively promote photogenerated charge separation and PEC activity.

Multifunctional nanofibrous membranes with sunlight-driven self-cleaning performance for complex oily wastewater remediation

Developing multifunctional, efficient and durable membrane for long-term usage for treating complex oily wastewater is highly desirable but still a challenge due to the severe membrane fouling. Herein, a hierarchical structured superhydrophilic/underwater superoleophobic nanofibrous with antifouling and visible-light-induced self-cleaning performance was manufactured by a facile combination of electrospun silver/β-cyclodextrin/polyacrylonitrile (Ag/β-CD/PAN) nanofibers and then the in-situ growth of a zinc oxide (ZnO) layer. The formed micro/nano sized hierarchical structure greatly increased the roughness and improved the underwater superoleophobic ability of the membrane. Therefore, the resultant ZnO/Ag/β-CD/PAN membrane displays splendid separation performance for oil/dye/water complex emulsions and high flux recovery (>90%). Meanwhile, the permeation flux of a variety of oil/water emulsions was higher than 619 L m^-2h^-1 with a separation efficiency above 99.7% under the action of gravity. Furthermore, the as-fabricated membrane exhibits excellent stability towards different harsh conditions (e. g. corrosive solution, high temperature, UV irradiation and ultrasound washing). The robust mechanical and chemical stability, outstanding separation capabilities as well as excellent flux recovery capabilities makes the self-cleaning membrane a good candidate for the remediation of complex oily wastewater.

Identification of Active Species in Photodegradation of Aqueous Imidacloprid over g-C3N4/TiO2 Nanocomposites

In this work, g-C3N4/TiO2 composites were fabricated through a hydrothermal method for the efficient photocatalytic degradation of imidacloprid (IMI) pesticide. The composites were fabricated at varying loading of sonochemically exfoliated g-C3N4 (denoted as CNS). Complementary characterization results indicate that the heterojunction between the CNS and TiO2 formed. Among the composites, the 0.5CNS/TiO2 material gave the highest photocatalytic activity (93% IMI removal efficiency) under UV-Vis light irradiation, which was 2.2 times over the pristine g-C3N4. The high photocatalytic activity of the g-C3N4/TiO2 composites could be ascribed to the band gap energy reduction and suppression of photo-induced charge carrier recombination on both TiO2 and CNS surfaces. In addition, it was found that the active species involved in the photodegradation process are OH• and holes, and a possible mechanism was proposed. The g-C3N4/TiO2 photocatalysts exhibited stable photocatalytic performance after regeneration, which shows that g-C3N4/TiO2 is a promising material for the photodegradation of imidacloprid pesticide in wastewater.

Antimicrobial TiO2 nanocomposite coatings for surfaces, dental and orthopaedic implants

Engineering of self-disinfecting surfaces to constrain the spread of SARS-CoV-2 is a challenging task for the scientific community because the human coronavirus spreads through respiratory droplets. Titania (TiO₂) nanocomposite antimicrobial coatings is one of the ideal remedies to disinfect pathogens (virus, bacteria, fungi) from common surfaces under light illumination. The photocatalytic disinfection efficiency of recent TiO₂ nanocomposite antimicrobial coatings for surfaces, dental and orthopaedic implants are emphasized in this review. Mostly, inorganic metals (e.g. copper (Cu), silver (Ag), manganese (Mn), etc), non-metals (e.g. fluorine (F), calcium (Ca), phosphorus (P)) and two-dimensional materials (e.g. MXenes, MOF, graphdiyne) were incorporated with TiO₂ to regulate the charge transfer mechanism, surface porosity, crystallinity, and the microbial disinfection efficiency. The antimicrobial activity of TiO₂ coatings was evaluated against the most crucial pathogenic microbes such as Escherichia coli, methicillin-resistant Staphylococcus aureus, Pseudomonas aeruginosa, Bacillus subtilis, Legionella pneumophila, Staphylococcus aureus, Streptococcus mutans, T2 bacteriophage, H1N1, HCoV-NL63, vesicular stomatitis virus, bovine coronavirus. Silane functionalizing agents and polymers were used to coat the titanium (Ti) metal implants to introduce superhydrophobic features to avoid microbial adhesion. TiO₂ nanocomposite coatings in dental and orthopaedic metal implants disclosed exceptional bio-corrosion resistance, durability, biocompatibility, bone-formation capability, and long-term antimicrobial efficiency. Moreover, the commercial trend, techno-economics, challenges, and prospects of antimicrobial nanocomposite coatings are also discussed briefly.

Recent Developments of Advanced Ti3+-Self-Doped TiO2 for Efficient Visible-Light-Driven Photocatalysis

Research into the development of efficient semiconductor photocatalytic materials is a promising approach to solving environmental and energy problems worldwide. Among these materials, TiO2 photocatalysts are one of the most commonly used due to their efficient photoactivity, high stability, low cost and environmental friendliness. However, since the UV content of sunlight is less than 5%, the development of visible light-activated TiO2-based photocatalysts is essential to increase the solar energy efficiency. Here, we review recent works on advanced visible light-activated Ti3+-self-doped TiO2 (Ti3+–TiO2) photocatalysts with improved electronic band structures for efficient charge separation. We analyze the different methods used to produce Ti3+–TiO2 photocatalysts, where Ti3+ with a high oxygen defect density can be used for energy production from visible light. We categorize advanced modifications in electronic states of Ti3+–TiO2 by improving their photocatalytic activity. Ti3+–TiO2 photocatalysts with large charge separation and low recombination of photogenerated electrons and holes can be practically applied for energy conversion and advanced oxidation processes in natural environments and deserve significant attention.