Effect of particle size on the photocatalytic activity of modified rutile sand (TiO2) for the discoloration of methylene blue in water

Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid by electrospun TiO2 nanofibres synthesised from two different titanium molecular precursors

The evaluation of the photocatalytic properties of electrospun TiO2 nanofibres (TiO2-NFs) synthesised in the same experimental conditions using two distinct precursors, tetraisopropyl orthotitanate (TTIP) and tetrabutyl orthotitanate (TNBT), with morphology and crystalline structure controlled by annealing at 460 °C for 3 h is presented. The presence of circular-shaped TiO2-NFs was corroborated by scanning electron microscopy (SEM). By using X-ray photoelectron spectroscopy (XPS), the chemical binding energies and their interactions of the TiO2 with the different incorporated impurities were determined; the most intense photoelectronic transitions of Ti 2p3/2 (458.39 eV), O 1 s (529.65 eV) and C 1 s (284.51 eV) were detected for TTIP and slightly blue-shifted for TNBT. By using energy-dispersive X-ray spectroscopy (EDS), the chemical element percentages in TiO2 were determined. Using X-ray diffraction, it was found that the annealed electrospun TiO2-NFs presented the anatase crystalline phase and confirmed by Raman scattering. Bandgap energies were determined by diffuse reflectance spectroscopy at room temperature. The photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide under exposure to ultraviolet light was studied using the TiO2-NFs obtained with the two molecular precursors. The results showed that the catalyst, prepared with the TTIP precursor, turned out to be the one that presented the highest photocatalytic activity with a half-life time (t1/2) of 28 min and a degradation percentage of 93%. The total organic carbon (TOC) in the solutions resulting from the 2,4-D degradation by the TiO2-NFs was measured, which showed a TOC removal of 50.67% for the TTIP sample and 36.14% for the TNBT sample. Finally, by using FTIR spectroscopy, the final chemical compounds of the degradation were identified as H2O and CO2.

Preparation and characterization of sulphur and zinc oxide Co-doped graphitic carbon nitride for photo-assisted removal of Safranin-O dye

Recently, there has been significant interest in photocatalytic reactions involving graphitic carbon nitride (g-C3N4) due to its sp2-hybridized carbon and nitrogen content and it is an ideal candidate for blending with other materials to enhance performance. Here, we have synthesized and analyzed both doped and undoped g-C3N4 nanoparticles. Specifically, we co-doped sulfur (S) into g-C3N4, integrated it with ZnO particles, and investigated the photocatalytic potential of these nanocomposites to remove Safranin-O dye. The initial step involved the preparation of pure g-C3N4 through calcination of urea. Subsequently, S-g-C3N4 was synthesized by calcining a mixture of urea and thiourea with a 3 : 1 ratio. Finally, the ZnO-S-g-C3N4 composite was synthesized using the liquid exfoliation technique, with distilled water serving as the exfoliating solvent. These samples were characterized by advanced techniques, including UV-Vis spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray Diffraction (XRD), energy dispersive X-ray (EDX) and scanning electron microscopy (SEM), to assess their crystallinity, morphology, optical properties, and phase purity. Subsequently, these nanocomposites were employed in catalytic and photocatalytic processes to remove the Safranin-O dye (SO). The results highlighted the formation of Z-scheme junction responsible for ZnO-S-g-C3N4's significant performance improvement. The comparison of results demonstrated that S-g-C3N4 and ZnO-S-g-C3N4 composites revealed an effective removal of Safranin-O dye in the presence of UV-light as compared to pure g-C3N4, as it was attributed to the phenomenon of improved separation of photogenerated charge carriers as a result of heterojunction formation between S-g-C3N4 and ZnO interfaces. In addition to improving photocatalytic performance, this study presents a facile route for producing ZnO-S-g-C3N4 composite with superior adsorption capabilities and selectivity.

Photocatalytic dye degradation and antibacterial activities of CeO2/g-C3N4 nanomaterials for environmental applications

The uncontrolled dumping of synthetic dyes into water sources has posed severe hazards to the ecosystem. For decades, several materials with low cost and high efficiency have been investigated for dye degradation. Photocatalytic degradation is regarded as a successful strategy since it utilizes sunlight to transform harmful pollutants into nontoxic compounds without using oxidative agents. The photocatalytic potentials of CeO2/g-C3N4 (CG) were investigated in this work using a simplistic ultrasonication process. Here, the amount of CeO2 was fixed, and g-C3N4 was varied in the ratio (1:x, where x = 1, 2, and 3) and abbreviated as CG1, CG2, and CG3. Characterization techniques such as Fourier transforms-infrared spectroscopy, thermal gravimetric analysis (TGA), powdered X-ray diffraction, ultraviolet-visible spectroscopy, etc. were used to characterize structural analysis, optical properties, particle size, and chemical bonds of the prepared nanocomposites. The photocatalytic results showed that CG2 effectively degraded rose bengal (RB) and crystal violet (CV) dyes when exposed to visible light irradiation as compared to pure GCN and CeO2. The antibacterial activity analysis further supported the potential application of prepared photocatalyst as a disinfectant agent against both gram-positive (Staphylococcus aureus and Bacillus cereus) and gram-negative (Salmonella abony and Escherichia coli) pathogenic strains of bacteria.

Visible light enhanced photocatalytic degradation of organic pollutants with SiO2/g-C3N4 nanocomposite for environmental applications

The development of eco-friendly photocatalysts is gaining attention as an effective approach for degrading organic pollutants. In the present study, the composite materials are composed of various components with varying structures that combine to enhance their characteristics and widen their applications. This work uses the hydrothermal method for the fabrication of a novel and steady SiO2/g-C3N4 photocatalyst. The amount of SiO2 was fixed, and graphitic carbon nitride (g-C3N4) was varied in the ratio (1:x, where x = 1, 2, 3) and abbreviated as SCN1, SCN2, and SCN3. The optical properties, surface morphology, and structural analysis of the prepared nanocomposites were studied using various techniques such as FTIR, TGA, X-ray diffraction, and ultraviolet-visible spectroscopy. The results show that SCN2 nanocomposites significantly improved the photocatalytic activity, with a degradation efficiency of 70% for auramine O and 84.6% for xylenol orange dye under visible light irradiation, which is a result of their large surface area and efficient electron-hole separation rate.

Hydrogen Bond-Induced Activation of Photocatalytic and Piezophotocatalytic Properties in Calcium Nitrate Doped Electrospun PVDF Fibers

In this study, polyvinylidene fluoride (PVDF) fibers doped with hydrated calcium nitrate were prepared using electrospinning. The samples were analyzed using scanning electron microscopy (SEM), X-ray diffraction (XRD), optical spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Raman, and photoluminescence (PL) spectroscopy. The results are complementary and confirm the presence of chemical hydrogen bonding between the polymer and the dopant. Additionally, there was a significant increase in the proportion of the electroactive polar beta phase from 72 to 86%. It was shown that hydrogen bonds acted as a transport pathway for electron capture by the conjugated salt, leading to more than a three-fold quenching of photoluminescence. Furthermore, the optical bandgap of the composite material narrowed to the range of visible light energies. For the first time, it the addition of the salt reduced the energy of the PVDF exciton by a factor of 17.3, initiating photocatalytic activity. The calcium nitrate-doped PVDF exhibited high photocatalytic activity in the degradation of methylene blue (MB) under both UV and visible light (89 and 44%, respectively). The reaction rate increased by a factor of 2.4 under UV and 3.3 under visible light during piezophotocatalysis. The catalysis experiments proved the efficiency of the membrane design and mechanisms of catalysis are suggested. This study offers insight into the nature of chemical bonds in piezopolymer composites and potential opportunities for their use.

Current perspectives of metal-based nanomaterials as photocatalytic antimicrobial agents and their therapeutic modes of action: A review

Efforts to restrict the emergence and progression of multidrug-resistant bacterial strains should heavily involve the scientific community, including government bodies, researchers, and industries, in developing new and effective photocatalytic antimicrobial agents. Such changes warrant the modernization and upscaling of materials synthesis laboratories to support and expedite the mass production of materials at the industrial scale for the benefit of humankind and the environment. Despite the massive volume of publications reporting the potential usage of different types of metal-based nanomaterials as antimicrobial agents, reviews uncovering the similarities and differences among the various products remain lacking. This review details the basic and unique properties of metal-based nanoparticles, their use as photocatalytic antimicrobial agents, and their therapeutic modes of action. It shall be noted that compared to traditional antibiotics, the mode of action of photocatalytic metal-based nanomaterials for killing microorganisms are completely different, despite displaying promising performance against antibiotic-resistant bacteria. Besides, this review uncovers the differences in the mode of actions of metal oxide nanoparticles against different types of bacteria, as well as towards viruses. Last but not least, this review comprehensively describes previous published clinical trials and medical usages involving contemporary photocatalytic antimicrobial agents.

Kinetics of photocatalytic degradation of organic compounds: a mini-review and new approach

Organic compounds are widespread pollutants in wastewater, causing significant risks for living organisms. In terms of advanced oxidation processes, photocatalysis is known as an effective technology for the oxidation and mineralization of numerous non-biodegradable organic contaminants. The underlying mechanisms of photocatalytic degradation can be explored through kinetic studies. In previous works, Langmuir-Hinshelwood and pseudo-first-order models were commonly applied to fit batch-mode experimental data, revealing critical kinetic parameters. However, the application or combination conditions of these models were inconsistent or ignored. This paper briefly reviews kinetic models and various factors influencing the kinetics of photocatalytic degradation. In this review, kinetic models are also systemized by a new approach to establish a general concept of a kinetic model for the photocatalytic degradation of organic compounds in an aqueous solution.

Ti4+-dopamine/sodium alginate multicomponent complex derived N-doped TiO2@carbon nanocomposites for efficient removal of methylene blue

A one-pot route for the preparation of TiO₂@carbon nanocomposite from Ti⁴⁺/polysaccharide coordination complex has been developed and shown advantages in operation, cost, environment, etc. However, the photodegradation rate of methylene blue needs to be improved. N-doping has been proven as an efficient means to enhance photodegradation performance. Thus, the present study upgraded the TiO₂@carbon nanocomposite to N-doped TiO₂@carbon nanocomposite (N-TiO₂@C) from Ti⁴⁺-dopamine/sodium alginate multicomponent complex. The composites were characterized by FT-IR, XRD, XPS, UV-vis DRS, TG-DTA, and SEM-EDS. TiO₂ was a typical rutile phase, and the carboxyl groups existed on N-TiO₂@C. The photocatalyst consequently showed high removal efficiency of methylene blue (MB). The cycling experiment additionally indicated the high stability of N-TiO₂@C. The present work provided a novel route for preparing N-TiO₂@C. Moreover, it can be extended to prepare N-doped polyvalent metal oxides@carbon composites from all water-soluble polysaccharides such as cellulose derivatives, pectin, starch, and guar gum.

Selective Photooxidation of Valencene and Thymol with Nano-TiO2 and O2 as Oxidant

The selective photocatalytic oxidation with O₂ as oxidant of valencene and thymol was evaluated using nanostructured TiO₂ under UV-Vis radiation at atmospheric conditions. The effect of the morphology and optical properties of TiO₂ nanotubes and aminate nanoparticles was studied. Different scavengers were used to detect the presence of positive holes (h⁺), electrons (e^-), hydroxyl radicals (•OH), and the superoxide radical anion (O₂^-) during the photooxidation reaction. Superoxide anion radical is the main oxidizing specie formed, which is responsible for the selective formation of nootkatone and thymoquinone using aminated TiO₂ nanoparticles under 400 nm radiation.

Recent advances in TiO2/ZnS-based binary and ternary photocatalysts for the degradation of organic pollutants

Semiconductor-mediated photocatalysis plays a pivotal role in the elimination of organic pollutants from water systems. Titanium dioxide (TiO₂) and zinc sulphide (ZnS) semiconductors are commonly utilized as photocatalysts in water purification due to their physical and chemical stability and also large band gap. The drawbacks of both semiconductors, nevertheless, prevent them from being used in real and large-scale treatments. Therefore, binary and ternary-based TiO₂/ZnS nanostructured materials may be a promising solution to improve the quantum efficiency, structural, and electrical features of pure TiO₂ and ZnS semiconductors for improved photoefficiency. This review aims to unravel the development of binary TiO₂/ZnS and the modification of ternary photocatalysts (TiO₂/ZnS-X, X = metal, non-metal, and dye sensitization) by various approaches. The engineered TiO₂/ZnS-based ternary nanostructured materials have exhibited exceptional performance to accelerate the degradation of organic pollutants in wastewater. These materials were fabricated by modifying TiO₂/ZnS binary composite and embedding co-catalysts like carbonaceous material, polymeric material, transition metal, metal oxide, and metal. The relationship between the properties of the resulting nanomaterials and their photocatalytic performances has been examined. This review has also placed a special focus on the synthetic routes applied to derive the binary and ternary TiO₂/ZnS composites. Another aim of this review is to scrutinize the factors that influence the performance of binary and ternary-based TiO₂/ZnS composites on the degradation of organic pollutants. Opportunities for further investigation have been also outlined, along with limitations and impediments based on the current findings.

Aluminium doped ZnO nanostructures for efficient photodegradation of indigo carmine and azo carmine G in solar irradiation

Aluminium doped zinc oxide (AZO) nanomaterials (AlxZn1-xO) with x fraction varying as 0.02 and 0.04 were synthesized using the auto-combustion method using glycine as a fuel. The synthesized catalysts were characterized with X-ray diffraction (XRD), UV–Visible Spectroscopy (UV–Vis), Raman spectroscopy, Photoluminescence (PL) spectroscopy, and High Resolution Transmission Electron Microscopy (HR-TEM). XRD results showed that synthesized materials possessed good crystallinity, while UV–VIS was employed to find the band gaps of synthesized materials. Raman was used to determine the vibrational modes in the synthesized nanoparticles, while TEM analysis was performed to study the morphology of the samples. Industrial effluents such as indigo carmine and azo carmine G were used to test the photodegradation ability of synthesised catalysts. Parameters such as the effect of catalyst loading, dye concentration and pH were studied. The reduction in crystallite size, band gap and increased lattice strain for the 4% AZO was the primary reason for the degradation in visible irradiation, degrading 97 and 99% equimolar concentrations of indigo carmine and azo carmine G in 140 min. The Al doped ZnO was found to be effective in faster degradation of dyes as compared to pure ZnO in presence of natural sunlight.

Recent advances in structural engineering of photocatalysts for environmental remediation

Photocatalysis appears to be an appealing approach for environmental remediation including pollutants degradation in water, air, and/or soil, due to the utilization of renewable and sustainable source of energy, i.e., solar energy. However, their broad applications remain lagging due to the challenges in pollutant degradation efficiency, large-scale catalyst production, and stability. In recent decades, massive efforts have been devoted to advance the photocatalysis technology for improved environmental remediation. In this review, the latest progress in this aspect is overviewed, particularly, the strategies for improved light sensitivity, charge separation, and hybrid approaches. We also emphasize the low efficiency and poor stability issues with the current photocatalytic systems. Finally, we provide future suggestions to further enhance the photocatalyst performance and lower its large-scale production cost. This review aims to provide valuable insights into the fundamental science and technical engineering of photocatalysis in environmental remediation.

A TiO2@MWCNTs nanocomposite photoanode for solar-driven water splitting

A TiO₂@MWCNTs (multi-wall carbon nanotubes) nanocomposite photoanode is prepared for photoelectrochemical water splitting in this study. The physical and photoelectrochemical properties of the photoanode are characterized using field emission-scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and linear sweep voltammetry. The results show that the TiO₂@MWCNTs nanocomposite has an optical bandgap of 2.5 eV, which is a significant improvement in visible-light absorption capability compared to TiO₂ (3.14 eV). The cyclic voltammograms show that incorporating TiO₂ with the MWCNTs leads to a decrease in the electrical double layer, thereby facilitating the electron transfer rate in the TiO₂@MWCNTs electrode. Moreover, the current density of the photoelectrochemical electrode formed by TiO₂@MWCNTs under solar irradiation is significantly higher than that prepared by TiO₂ (vs Ag/AgCl). The low charge capacity of the TiO₂@MWCNTs electrode-electrolyte interface hinders the recombination of the photogenerated electrons and holes, which contributes to the enhancement of the solar-to-hydrogen (STH) conversion efficiency. The average STH conversion efficiency of the TiO₂@MWCNTs electrode under solar exposure from 6 AM to 5 PM is 11.1%, 8.88 times higher than that of a TiO₂ electrode. The findings suggested TiO₂@MWCNTs is a feasible nanomaterial to fabricate the photoanode using photoelectrochemical water splitting under solar irradiation.

The strategy for organic dye and antibiotic photocatalytic removal for water remediation in an example of Co-SnO2 nanoparticles

A challenging problem to create an efficient photocatalyst suitable for industrial water remediation, aiming to remove cyclic organic compounds attracts increasing attention. The current study aimed to clarify a few "dark spots" in the field, namely to find out if it is possible to make an efficient photocatalyst activated with visible light by using a simple and cheap strategy and what are the key factor impacting its efficiency. In this work, a new procedure to obtain spherical nanoparticles with the same average size but different amounts of oxygen vacancies and defects and dopant concentrations was developed. The approach based on hydrothermal treatment was suggested to obtain rod-shaped nanoparticles. The systematic study of photocatalytic behavior on the example of oxytetracycline and methylene blue degradation under visible light of widely available LED lamp was performed. Based on chemical and computational experiments the main factor affecting the process efficiency was determined.

Mechanical Stability Is Key for Large-Scale Implementation of Photocatalytic Surface-Attached Film Technologies in Water Treatment

Replacement of classical tertiary water treatment by chemical-free sunlight-driven photocatalytic units has been often proposed. Photocatalysts are required to be cost-effective, inert, chemically stable, reusable, and easy to separate and also that they are mechanically stable. The effect of mechanical stress on a photoactive TiO2 layer, and on its effectivity for degradation of phenol as a model pollutant, has been studied during photocatalytic water treatment using NUV–vis light. Sol–gel (SG) and liquid phase deposition (LPD) methods have been used to coat spherical glass beads with the photocatalyst (TiO2). Physicochemical characterization of coated glass beads has been performed by N2 adsorption–desorption isotherms, SEM, EDXS, and AFM. Phenol photocatalyzed degradation was carried out both in stirred batch and flow reactors irradiated with a medium-pressure Hg-vapor lamp (λ > 350 nm). Phenol concentration was determined by HPLC, and its photoproducts were identified using HPLC/MS. In the stirred batch reactor, all LPD-coated glass beads displayed higher catalytic activity than SG-coated ones, which increased with calcination temperature, 700°C being the most efficient temperature. Preliminary etching of the glass beads surface yielded dissimilar results; whereas, phenol photodegradation with SG-coated etched glass beads is twice faster than with unetched SG ones, the rate reduces to one-third using LPD etched instead of unetched LPD glass beads. Phenol photodegradation using LPD is similar both in stirred batch and flow reactors, despite the latter uses a lower catalyst load. LPD-etched catalyst was recovered and reused in the stirred batch reactor; its activity reduced sharply after the first use, and it also lost activity in successive runs, ca. 10% of activity after each “use and recover” cycle. In the flow reactor, activity loss after the first experiment and recycling (ca. 30%) was much larger than in the following runs, where the activity remained rather constant through several cycles. LPD is more adequate than SG for TiO2 immobilization onto glass beads, and their calcination at 700°C leads to relatively strong and reactive photocatalytic films. Still, TiO2-coated glass beads exhibited very low photoactivity compared to TiO2-P25 nanoparticles, though their separation is much easier and almost costless. The durability of the catalytic layer increases when using a flow reactor, with the pollutant solution flowing in a laminar regime through the photocatalyst bed. In this way, the abrasion of the photocatalytic surface is largely reduced and its photoactivity is better maintained.

Size dependence of CdS nanoparticles on the precursor concentration and visible light driven photocatalytic degradation of methylene blue

The effect of precursor concentration on the size of the nanoparticles and their visible light driven photocatalytic degradation of methylene blue are demonstrated.

Photodegradation of Herbicide Imazapyr and Phenol over Mesoporous Bicrystalline Phases TiO2: A Kinetic Study

Mesoporous TiO2 nanoparticles were synthesized at different temperatures (400–800 °C). The resulting mesoporous anatase–rutile TiO2 mixtures between 27 and 82% were found to have different structural properties (morphology, mesoporosity, crystallite phases, and sizes) affected through the calcination process. They were tested for the photocatalytic degradation of the herbicides imazapyr and phenol, compared with the nonporous TiO2 P-25. The present work is an extension of a previously published study discussing the influence of the rutile content on the photocatalytic performance of the nanocrystals, based on the modified first order kinetic model, where the degradation rate is a function of the specific surface area of the material. The apparent degradation rate using T-800 is 10-fold higher than in the case using TiO2 P-25. The material with the lowest anatase content (T-800) exhibits the highest photocatalytic activity in terms of initial reaction rate per unit surface area. It is considered that mixed-phase photocatalysts with rutile–anatase exhibit enhanced photoactivity with the increase of the rutile proportion.