Photocatalytic performance and dispersion stability of nanodispersed TiO2 hydrosol in electrolyte solutions with different cations

Delineating the Effects of Molecular and Colloidal Interactions of Dissolved Organic Matter on Titania Photocatalysis

In the face of significant challenges to practical applications of photocatalysis for water treatment, recent reports revealed a potential route to overcome a problem posed by dissolved organic matter (DOM). These studies showed that inhibition of photocatalytic processes by DOM is driven largely by competition for active surface sites on TiO₂ or other catalysts, and controlling the type of DOM present in solution could significantly mitigate DOM fouling. Whether or not control of solution parameters could achieve the same preventative action is not known. Here, a series of DOM isolates, including humic acid (HA) and transphilic (TPI), hydrophobic (HPO), or colloidal fractions of organic matter from a membrane bioreactor mixed liquor supernatant, were tested for inhibitory activity under a range of pH values (3, 5, 7, and 9) and ionic compositions (NaCl, CaCl₂, and Al₂(SO₄)₃ with ionic strengths (IS) ranging from 0 to 3 M). The resulting TiO₂-DOM agglomerates were monitored for size and ζ-potential. Inhibitory profiles were generated using para-chlorobenzoic acid (pCBA) as probe with varying concentrations of inhibitory DOM for each solution condition to discern the extent of surface-phase quenching of radicals. Manipulation of pH clearly impacted inhibition, and the effect varied by DOM type; for example, interference occurred at all pHs for HA, at neutral or basic pHs for TPI, and only at pH 7 for HPO. Particle sizes did not correlate with inhibitory action of DOM. Increases in ionic strength induced growth of TiO₂ and TiO₂-DOM agglomerates, but again, particle sizes did not correlate to inhibition by DOM. The changes to IS, regardless of ion type, were not affected by the presence of TPI or HPO. Since particle stability did not correlate directly with photocatalytic activity, we suggest that surface-based quenching reactions arise from site-specific adsorption rather than generalized particle destabilization and aggregation.

An effective strategy for improving charge separation efficiency and photocatalytic degradation performance using a facilely synthesized oxidative TiO2 catalyst

Titanium dioxide (TiO₂) has attracted enormous interest in abundant photocatalytic reactions, but its photocatalytic efficiency is limited by its wide bandgap and the rapid recombination of electron-hole pairs. To overcome the disadvantages of its rapid electron-hole recombination rate, herein, oxidative TiO₂ was one-step fabricated using potassium permanganate (KMnO₄), exhibiting improved charge separation efficiency and photocatalytic degradation performance towards methyl orange (MO). Remarkably, the first-order photodegradation rate of oxidative TiO₂ is 3.68 times higher than that of pristine TiO₂ under the irradiation of simulated sunlight and 2.15 times higher under ultraviolet light. This exceptional photocatalytic activity is attributed to the additional oxygen doped into the interstices of the TiO₂ lattice, creating impurity states in the bandgap acting as trapping sites, thus facilitating charge separation. This work provides a promising strategy for the insertion of O atoms into the TiO₂ lattice and expands the photocatalytic application of the related materials.

Exploring the photocatalytic inactivation mechanism of Microcystis aeruginosa under visible light using Ag3PO4/g-C3N4

In this work, a series of Ag₃PO₄/g-C₃N₄ (AG) photocatalysts were synthesized. After characterizing the properties, the effects of mass ratio, light intensity, and material dosages on photodegradation were investigated. The material with a 1/2 mass ratio of Ag₃PO₄/g-C₃N₄ showed the highest photocatalytic activity under visible light, and the removal efficiency reached 90.22% for an initial suspended algae concentration of 2.7 × 10⁶ cells/mL, 0.1 g of AG, and 3 h of irradiation. These results showed that the conductivity was increased while the total protein and COD contents of the algae suspension were declined rapidly. In contrast, the variations in the malondialdehyde (MDA) level suggested that the algae cell wall was severely damaged and that selective permeability of the membrane was significantly affected. A possible photocatalytic mechanism was proposed and •O₂^- was shown to be the major reactive oxygen species in the photocatalysis. In summary, during the visible light photocatalytic process, the cell structure was destroyed, which caused the leakage of electrolyte, the inactivation of protein, and the inhibition of photosynthesis; finally, the cells died. This study provides a reference for photodegradation of algae pollution in water bodies.

Recent progress in g-C3N4, TiO2 and ZnO based photocatalysts for dye degradation: Strategies to improve photocatalytic activity

Water contamination by dyes is a matter of concern for human health and the environment. Various methods (membrane separation, coagulation and adsorption) have been explored to remove/degrade dyes. However, now the exploitation of semiconductor assisted materials using renewable solar energy has emerged as a potential candidate to resolve the issue. Although, single component photocatalysts (ZnO, TiO₂, ZrO₂) were experimented, due to their low efficiency and stability due to the high recombination rate electron-hole pair and inefficient visible light absorption, composites of semiconductor materials are being used. Semiconductor heterojunction systems are developed by coupling two or more semiconductor components. The synergistic effect of their properties, such as adsorption and improved charge carrier migration, is observed to increase overall stability. This review covers recent progress in advanced nanocomposite materials based on g-C₃N₄, TiO₂ and ZnO used as photocatalysts with details of enhancing the photocatalytic properties by heterojunctions, crystallinity and doping. The conclusion at the end displays a summary, research gaps and future outlook. A holistic analysis of recent progress to demonstrate the efficient heterojunctions for photodegradation with optimal conditions, this review will be helpful for the development of efficient heterostructured systems for photodegradation. This review covers references from the year 2017-2020.

Light-induced degradation of rhodamine B by tellurium quantum dots

Tellurium quantum dots (Te QDs) were prepared using bulk tellurium as the precursor. Te QDs can be a highly active photocatalyst for boosting the photocatalytic degradation of rhodamine B (RhB) under visible light irradiation. The morphology and composition of Te QDs were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that in the presence of H₂O₂, the photocatalytic efficiency of Te QDs on RhB could achieve a good degradation effect within a very short time (30 min). The effects of initial dye concentration, pH value, light intensity, catalyst dosage and H₂O₂ concentration on dye degradation were successively studied. The effects of inorganic ions (NO₃^-, Cl^-, SO₄^2-, Ca²⁺, Mg²⁺ and Fe³⁺) on photocatalytic degradation were also discussed. Experimental results of free radical capture showed that OH^• and O₂^•- played important roles in photocatalytic degradation. More importantly, Te QDs efficiency still remained above 85% after four cycles of use, indicating good stability, recyclability and utility. This work may inspire further design of other semiconductor QDs for highly efficient dye degradation.

Highly-efficient green photocatalytic cementitious materials with robust weathering resistance: From laboratory to application

The combined use of nano-TiO2 with cementitious materials offers an environmentally-friendly way to combat the air pollution problem. However, a trade-off between a high efficiency and a robust weathering resistance has often to be made for most of the attempted nano-TiO2 incorporation methods. This paper developed a simple and effective "spraying" method to coat nano-TiO2 particles on the surface of concrete surface layers (CSL). The results showed that the NOx removal rate of the samples increased with an increase in both the concentrations of nano-TiO2 solutions and the number of times of the spraying action. And the conditions for preparation of the Spray AB (the CSL were first sprayed with the 30 g L-1 TiO2-solution 20 times, followed by mechanical compaction, and for another 20 times after the compaction) were found to be optimal in terms of NOx removal performance and weathering resistance. The Spray AB was superior to the 5% TiO2-intermixed samples with respect to photocatalytic NOx removal ability. Compared with TiO2-dip-coated samples, the Spray AB samples had better and robust weathering resistance. A case study on the factory-fabricated green Eco-blocks (produced by the laboratory-developed spray method and the conventional intermix method) was performed. Examination and comparison on their respective photocatalytic NOx removal further verified the advantages of the spray method over the intermix method.

Biomorphic Fibrous TiO2 Photocatalyst Obtained by Hydrothermal Impregnation of Short Flax Fibers with Titanium Polyhydroxocomplexes

A biomimetic solution technology for producing a photocatalytic material in the form of biomorphic titanium oxide fibers with a hierarchical structure using short flax fiber as a biotemplate is proposed. The impregnation of flax fibers intensified under hydrothermal conditions with a precursor was performed in an autoclave to activate the nucleation of the photoactive TiO2 phases. The interaction between precursor and flax fibers was studied by using infrared spectroscopy (IR) and differential scanning calorimetry/thermogravimetry analysis (DSC/TG). The morphology, structure, and textural properties of the TiO2 fibers obtained at annealing temperatures of 500–700 °C were determined by X-ray diffraction analysis, scanning electron microscopy, and nitrogen adsorption/desorption. It is shown that the annealing temperature of the impregnated biotemplates significantly affects the phase composition, crystallite size, and porous structure of TiO2 fiber samples. The photocatalytic activity of the obtained fibrous TiO2 materials was evaluated by using the decomposition of the cationic dye Rhodamine B in an aqueous solution (concentration 12 mg/L) under the influence of ultraviolet radiation (UV). The maximum photodegradation efficiency of the Rhodamine B was observed for TiO2 fibers annealed at 600 °C and containing 40% anatase and 60% rutile. This sample ensured 100% degradation of the dye in 20 min, and this amount significantly exceeds the photocatalytic activity of the commercial Degussa P25 photocatalyst and TiO2 samples obtained previously under hydrothermal conditions by the sol-gel method.

Enhanced Activation of Persulfate by Co-Doped Bismuth Ferrite Nanocomposites for Degradation of Levofloxacin Under Visible Light Irradiation

In this study, magnetic visible light driven photocatalysts (bismuth ferrite, Bi₂Fe₄O₉, BFO and Co-doped bismuth ferrite, Co-BFO) were successfully prepared by the facile hydrothermal method. The catalyst was used in the application of heterogeneous persulfate (PS) system under visible LED light irradiation for the degradation of levofloxacin (LFX), proving to be an excellent photocatalyst when evaluated by various characterization methods. The effect of Co-doping in the BFO structure was investigated that the decrease of band gap width and the generated photoelectrons and holes would effectively reduce the recombination of photogenerated electron-hole pairs, leading to the enhancement photocatalytic activity. The results demonstrated that Co-BFO catalyst had a high photodegradation efficiency over a wide pH range of 3.0-9.0 and the Co-BFO-2 composite displayed the optimal catalytic performance. It was found that the degradation rate of LFX by Co-BFO-2 catalyst was 3.52 times higher than that of pure BFO catalyst under visible light condition. The free radical trapping experiments and EPR tests demonstrated that superoxide, photogenerated holes and sulfate radicals were the main active species in the photocatalytic degradation of LFX. And a possible photocatalytic degradation mechanism of LFX was proposed in the Vis/Co-BFO/PS process. These findings provided new insight of the mechanism of heterogeneous activation of persulfate by Co-BFO under visible light irradiation.