Surface modification of TiO2 photocatalyst for environmental applications

Comparative study of the physicochemical and photocatalytic properties of water-soluble polymer-capped TiO2 nanoparticles

TiO₂ shows great potential as ideal and powerful photocatalyst to degrade and eliminate harmful organic pollutants from waste water. It is desirable to prepare anatase TiO₂ with good aqueous solubility and photocatalytic activity for water decontamination. In this paper, water-soluble TiO₂ nanoparticles were successfully prepared using polytetramethylene ether glycol (PTMG), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) as stabilizers. The anatase phase of TiO₂ was well controlled by introduction of HCl in the reaction system. The results showed that all the polymer-capped TiO₂ were well dissolved in water and their aqueous solutions could maintain stable for more than 2 months. The photocatalytic activities of polymer-capped TiO₂ were evaluated by monitoring the degradation of Rhodamine B (RhB) with Degussa P25 as a control. The results showed that all the polymer-capped TiO₂ presented better photocatalytic activity than that of Degussa P25. PTMG capped TiO₂ exhibited the longest average lifetime of charge carriers, indicating the lowest charge recombination rate. Thus, PTMG capped TiO₂ presented the best photocatalytic activity. In summary, PTMG, PEG, and PPG can be used as stabilizers to prepare water-soluble TiO₂. PTMG is an ideal stabilizer for the synthesis of water-soluble TiO₂ with good photocatalytic activity.

Active {001} Facet Exposed TiO2 Nanotubes Photocatalyst Filter for Volatile Organic Compounds Removal: From Material Development to Commercial Indoor Air Cleaner Application

TiO₂ nanotubes (TNT) have a highly ordered open structure that promotes the diffusion of dioxygen and substrates onto active sites and exhibit high durability against deactivation during the photocatalytic air purification. Herein, we synthesized {001} facet-exposed TiO₂ nanotubes (001-TNT) using a new and simple method that can be easily scaled up, and tested them for the photocatalytic removal of volatile organic compounds (VOCs) in both a laboratory reactor and a commercial air cleaner. While the surface of TNT is mainly composed of {101} facet anatase, 001-TNT's outer surface was preferentially aligned with {001} facet anatase. The photocatalytic degradation activity of toluene on 001-TNT was at least twice as high as that of TNT. While the TNT experienced a gradual deactivation during successive cycles of photocatalytic degradation of toluene, the 001-TNT did not exhibit any sign of catalyst deactivation under the same test conditions. Under visible light irradiation, the 001-TNT showed degradation activity for acetaldehyde and formaldehyde, while the TNT did not exhibit any degradation activity for them. The 001-TNT filter was successfully scaled up and installed on a commercial air cleaner. The air cleaner equipped with the 001-TNT filters achieved an average VOCs removal efficiency of 72% (in 30 min of operation) in a 8-m³ test chamber, which satisfied the air cleaner standards protocol (Korea) to be the first photocatalytic air cleaner that passed this protocol.

Conformal Titanyl Phosphate Surface Passivation for Enhancing Photocatalytic Activity

A conformal titanyl phosphate passivation with the thickness of ca. 5 nm on the surface of TiO2 nanoparticles for enhancing the photocatalytic degradation of organic pollutants and hydrogen production is described. The phosphate anion species bound to the surface of TiO2 promote the favorable kinetics of photocatalytic activity and influence the catalytic reaction pathway. By using a facile surfactant-assisted sol-gel process, the surface defects of TiO2 associated with deep traps was reduced and passivated by the phosphate anion species to form the titanyl phosphate. The strong bonds between the titanyl phosphate shell and TiO2 core provided a long-term photochemical stability in aqueous electrolytes with enhanced photocatalytic activities. The titanyl phosphate contributed to the production and stabilization of hydroxyl radicals on the surface of photocatalyst, which facilitated the efficient photooxidation of the organic pollutants. Further, enhancing the photocatalytic hydrogen production was achieved by the titanyl phosphate modified TiO2 (TP-TiO2). Consequently, the conformal titanyl phosphate passivation enhanced photocatalytic activity of TiO2. Comparing to the bare TiO2 nanoparticles, approximately two-fold higher photocatalytic H2 production rate was achieved by the TP-TiO2.

Defective Anatase TiO2-x Mesocrystal Growth In Situ on g-C3 N4 Nanosheets: Construction of 3D/2D Z-Scheme Heterostructures for Highly Efficient Visible-Light Photocatalysis

Environmental remediation by employing visible-light-active semiconductor heterostructures provides effective solutions for handling emerging contaminants by a much greener and lower cost approach compared with other methods. This report demonstrates that the in situ growth of nanosized single-crystal-like defective anatase TiO_2-x mesocrystals (DTMCs) on g-C₃ N₄ nanosheets (NSs) can produce a 3D/2D DTMC/g-C₃ N₄ NS heterostructure with the two components held together by chemical bonds to form tight interfaces. This nanostructured heterostructure displayed remarkably improved photocatalytic activity toward the removal of the model pollutants Methyl Orange (MO) and Cr^VI under visible-light irradiation in comparison with the pristine DTMC and g-C₃ N₄ NS components, which suggests that both the oxidation and reduction abilities of the DTMC/g-C₃ N₄ NSs were simultaneously enhanced after fabrication. On the basis of the results of a systematic characterization, a reasonable mechanism for the photocatalytic activity based on a direct Z-scheme heterojunction is proposed and further verified by the measurement of ^. OH. This novel Z-scheme heterojunction endows the heterostructure with improved photogenerated electron/hole pair separation and a strong redox ability for the efficient degradation of wastewater pollutants. This work will be useful for the design and fabrication of direct Z-scheme heterostructured photocatalysts with novel architectures for applications in energy conversion and environmental remediation.

Band Gap Implications on Nano-TiO₂ Surface Modification with Ascorbic Acid for Visible Light-Active Polypropylene Coated Photocatalyst

The effect of surface modification using ascorbic acid as a surface modifier of nano-TiO₂ heterogeneous photocatalyst was studied. The preparation of supported photocatalyst was made by a specific paste containing ascorbic acid modified TiO₂ nanoparticles used to cover Polypropylene as a support material. The obtained heterogeneous photocatalyst was thoroughly characterized (scanning electron microscope (SEM), RAMAN, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Diffuse Reflectance Spectra (DRS) and successfully applied in the visible light photodegradation of Alizarin Red S in water solutions. In particular, this new supported TiO₂ photocatalyst showed a change in the adsorption mechanism of dye with respect to that of only TiO₂ due to the surface properties. In addition, an improvement of photocatalytic performances in the visible light photodegration was obtained, showing a strict correlation between efficiency and energy band gap values, evidencing the favorable surface modification of TiO₂ nanoparticles.

UV-visible photocurrent enhancement using metal-semiconductor-metal with symmetric and asymmetric double Schottky barriers

We experimentally report improved photocurrent performance under the illumination of both UV and visible light in the metal-semiconductor-metal nanostructures with double Schottky barriers, consisting of a monolayer of Au nanoparticles and a planar Au film separated by a thin WO3 layer. In addition to the near-unit broadband optical absorption causing the increased population of hot electrons, we demonstrate that the occurrence of the electron trapping effect at the bottom Schottky barrier is also responsible for the photocurrent enhancement under visible light illumination. Under the direct UV bandgap excitation of WO3, the electron trapping effect at the bottom barrier plays a crucial role in the anodic photocurrent enhancement. Furthermore, we demonstrate that by replacing the Au film with the metal Pt of higher work function to increase the barrier height of the bottom Schottky barrier, and hence strengthening its ability for electron trapping, the photocurrent is found to gain a further significant enhancement under the illumination of both visible and UV light.

The impact of TiO2 modifications on the effectiveness of photocatalytic processes [review]

This paper outlines the recent studies on the application of photocatalysis using semiconductors, with modified titanium dioxide (TiO2) in the process of reducing chemical contamination of surface and ground waters. During the last forty years, an increasing interest in catalysts of this type is noticeable. Hence, a wide range of methods of TiO2 modifications have been proposed so far by using its various polymorphs, composites with metals and non-metals and polymer-coatings or impregnating it with dyes that effectively absorb sunlight.

Removal of 2-phenylbenzimidazole-5-sulfonic acid using heterogeneous photocatalysis

UV filters are classified as environmental pollutants (emerging pollutants). One of the most frequently detected UV filters in real samples is 2-phenylbenzimidazole-5-sulfonic acid (PBSA). It has been shown that conventional technologies applied in sewage treatment plants are not adapted for complete removal of sunscreen agents. Therefore, there is a trend to undertake activities leading to improvement of water quality by enhancing treatment methods. This is important due to the fact that in an aqueous environment, in the presence of UV radiation or sunlight irradation, PBSA generates reactive oxygen species that can damage the DNA of living organisms. The aim of study was to investigate an effect of pH and TiO2 on PBSA stability in the presence of UV radiation. It was found that the rate of PBSA degradation depends on the catalyst dose and pH of solution. The photocatalysis reaction was carried out in a Heraeus laboratory exposure set equipped with a 150 W mediumpressure mercury lamp. The course of PBSA degradation process as a function of time was monitored using UV/VIS spectrophotometer and liquid chromatograph equipped with UV-Vis detector.

Recent progress of metal-graphene nanostructures in photocatalysis

Metal-graphene nanostructures (NSs) as photocatalysts, prepared using simple and scalable synthesis methods, are gaining heightened attention as novel materials for water treatment and environmental remediation applications. Graphene, the unique few layers sheet-like arrangement of sp2 hybridized carbon atoms, has an inimitable two-dimensional (2D) structure. The material is highly conductive, has high electron mobility and an extremely high surface area, and can be produced on a large scale at low cost. Accordingly, it has been considered as an essential base component for producing various metal-based NSs. In particular, metal-graphene NSs as photocatalysts have attracted considerable attention because of their special surface plasmon resonance (SPR) effect that can improve their performance for the removal of toxic dyes and other pollutants. This review summarizes the recent and advanced progress for the easy fabrication and design of graphene-based NSs as photocatalysts, as a novel tool, using a range of approaches, including green and biogenic approaches.

Controlled Synthesis of CuS/TiO2 Heterostructured Nanocomposites for Enhanced Photocatalytic Hydrogen Generation through Water Splitting

Photocatalytic hydrogen (H₂) generation through water splitting has attracted substantial attention as a clean and renewable energy generation process that has enormous potential in converting solar-to-chemical energy using suitable photocatalysts. The major bottleneck in the development of semiconductor-based photocatalysts lies in poor light absorption and fast recombination of photogenerated electron-hole pairs. Herein we report the synthesis of CuS/TiO₂ heterostructured nanocomposites with varied TiO₂ contents via simple hydrothermal and solution-based process. The morphology, crystal structure, composition, and optical properties of the as-synthesized CuS/TiO₂ hybrids are evaluated in detail. Controlling the CuS/TiO₂ ratio to an optimum value leads to the highest photocatalytic H₂ production rate of 1262 μmol h^-1 g^-1, which is 9.7 and 9.3 times higher than that of pristine TiO₂ nanospindles and CuS nanoflakes under irradiation, respectively. The enhancement in the H₂ evolution rate is attributed to increased light absorption and efficient charge separation with an optimum CuS coverage on TiO₂. The photoluminescence and photoelectrochemical measurements further confirm the efficient separation of charge carriers in the CuS/TiO₂ hybrid. The mechanism and synergistic role of CuS and TiO₂ semiconductors for enhanced photoactivity is further delineated.

Porous Electrospun Fibers Embedding TiO2 for Adsorption and Photocatalytic Degradation of Water Pollutants

Using a bipolymer system consisting of polyvinylpyrrolidone (PVP) and poly(vinylidene fluoride) (PVDF), P25-TiO₂ was immobilized into thin film mats of porous electrospun fibers. Pores were introduced by dissolving sacrificial PVP to increase surface area and enhance access to TiO₂. The highest photocatalytic activity was achieved using a PVDF:PVP weight ratio of 2:1. Methylene blue (MB) was used to visualize contaminant removal, assess the sorption capacity (5.93 ± 0.23 mg/g) and demonstrate stable removal kinetics ( k_MB > 0.045 min^-1) under UVA irradiation (3.64 × 10^-9 einstein/cm²/s) over 10 cycles. Treatment was also accomplished via sequential MB sorption in the dark and subsequent photocatalytic degradation under UVA irradiation, to illustrate that these processes could be uncoupled to overcome limited light penetration. The photocatalytic mat degraded bisphenol A and 17α-ethynylestradiol in secondary wastewater effluent (17 mg TOC/L), and (relative to TiO₂ slurry) immobilization of TiO₂ in the mat mitigated performance inhibition by co-occurring organics that scavenge oxidation capacity. This significantly lowered the electrical energy-per-order of reaction (EEO) needed to remove such endocrine disruptors in the presence of oxidant scavenging/inhibitory organics. Thus, effective TiO₂ immobilization into polymers with affinity toward specific priority pollutants could both increase the efficiency and reduce energy requirements of photocatalytic water treatment.

Mechanism of UV-driven Photoelectrocatalytic Degradation of Berberine Chloride Form Using the ESR Spin-trapping Method

Photoelectrocatalytic (PEC) system greatly improves the migration of photoexcited charges, retards the fast recombination of electron-hole and increases the lifetime of photogenerated holes. In this article, we constructed a novel PEC system to degrade berberine chloride form (BCF). XRD patterns indicated aging time was an important condition for the crystal type of TiO_2, and the best proportion of anatase/rutile was 80/20. The band gap energy of the TiO₂ was 3.01 eV. We demonstrated that a synergistic effect existed between photocatalysis (PC) and electrocatalysis (EC). Besides, we discussed the influence of pH, current density, the amount of catalyst and initial BCF concentration on the degradation of BCF. Electron spin resonance (ESR) through spin trap 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) and the scavenging experiments suggested that the reactive oxygen species (ROS) were superoxide radicals (O2·-), hydroxyl radicals (·OH) and sulfate radical (SO4·-) in PEC system. Furthermore, we compared the two pathway of formation DMPO-SO₄ adducts and found that SO4·- was the most major oxidized species in degrading BCF. We proposed a plausible reaction mechanism. Intriguingly, even under lower current and weaker light conditions, the PEC process maintained its effectiveness, demonstrating the feasibility of the PEC approach.

Synthesis, Structural Property, Photophysical Property, Photocatalytic Property of Novel ZnBiErO₄ under Visible Light Irradiation

A novel photocatalyst ZnBiErO₄ was firstly synthesized by solid-state reaction method and its structural and photocatalytic properties were analyzed by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and UV-Vis diffuse reflectance. The results demonstrated that ZnBiErO₄ crystallized with tetragonal crystal structure with space group I41/A. The lattice parameters for ZnBiErO₄ were proved to be a = b = 10.255738 Å and c = 9.938888 Å. The band gap of ZnBiErO₄ was estimated to be about 1.69 eV. Compared with nitrogen doped TiO₂, ZnBiErO₄ showed excellent photocatalytic activities for degrading methyl blue during visible light irradiation. The photocatalytic degradation of methyl blue with ZnBiErO₄ or N-doped TiO₂ as catalyst followed the first-order reaction kinetics. Moreover, the apparent first-order rate constant of ZnBiErO₄ or N-doped TiO₂ was 0.01607 min⁻¹ or 0.00435 min⁻¹. The reduction of total organic carbon, formation of inorganic products, such as SO₄²⁻ and NO₃⁻ and the evolution of CO₂ revealed the continuous mineralization of methyl blue during the photocatalytic process. ZnBiErO₄ photocatalyst had great potential to purify textile industry wastewater.

Improving the Optoelectronic Properties of Mesoporous TiO2 by Cobalt Doping for High-Performance Hysteresis-free Perovskite Solar Cells

We for the first time report the incorporation of cobalt into a mesoporous TiO₂ electrode for application in perovskite solar cells (PSCs). The Co-doped PSC exhibits excellent optoelectronic properties; we explain the improvements by passivation of electronic trap or sub-band-gap states arising due to the oxygen vacancies in pristine TiO₂, enabling faster electron transport and collection. A simple postannealing treatment is used to prepare the cobalt-doped mesoporous electrode; UV-visible spectroscopy, X-ray photoemission spectroscopy, space charge-limited current, photoluminescence, and electrochemical impedance measurements confirm the incorporation of cobalt, enhanced conductivity, and the passivation effect induced in the TiO₂. An optimized doping concentration of 0.3 mol % results in the maximum power conversion efficiency of 18.16%, 21.7% higher than that of a similar cell with an undoped TiO₂ electrode. Also, the device shows negligible hysteresis and higher stability, retaining 80.54% of the initial efficiency after 200 h.

Alizarin red S–TiO2-catalyzed cascade C(sp3)–H to C(sp2)–H bond formation/cyclization reactions toward tetrahydroquinoline derivatives under visible light irradiation

A very low amount of organic dye (Alizarin red S) sensitized TiO₂ and it was successfully used to catalyze cascade C(sp³)–H to C(sp²)–H bond formation/cyclization reactions under visible light irradiation.

Enhanced Photodecomposition of Methylene Blue in Water with Sr1−xKxTiO3−δ@PC-polyHIPEs under UV and Visible Light

Photocatalytic method was investigated to remove water pollutant methylene blue (MB) produced in textile, plastic, and dye industries. PC-polyHIPEs were prepared by light-induced polymerization of dopamine in transparent polyHIPEs which were synthesized by polymerization within high internal phase emulsions. Sr1-xKxTiO3-δ (x = 0–0.5) nanoparticles were incorporated and adhered to PC-polyHIPEs to form Sr1-xKxTiO3-δ@PC-polyHIPEs for the first time. The catalysts were characterized by XRD, FTIR, TGA, UV-Vis DRS, and SEM and their photocatalytic properties for MB decomposition were measured over UV-Vis spectrometer. The PC-polyHIPEs were of interconnected porous structure with around 100 μm pores and 30 μm windows. Sr1−xKxTiO3−δ@PC-polyHIPEs showed excellent MB decomposition activity under either UV or visible light although Sr1−xKxTiO3−δ alone worked only under UV light. When x = 0.3, Sr1−xKxTiO3−δ@PC-polyHIPEs showed the highest photocatalytic performance due to the existence of more oxygen vacancies. When the water solution with 50 mg L−1 MB and 1.6 gcat. L−1 Sr0.7K0.3TiO3−δ@PC-polyHIPEs was exposed to visible light for 160 min at room temperature, 88.3% of MB was decomposed. After being used for eight cycles, 87.6% activity of fresh Sr0.7K0.3TiO3−δ@PC-polyHIPEs still remained. The influences of salinity, temperature, and catalyst concentration on the catalytic activity were studied. For MB decomposition under visible light, the activation energy of Sr0.7K0.3TiO3−δ@PC-polyHIPEs was calculated to be 12.3 kJ mol−1 and the kinetics analysis revealed that the photocatalysis followed the second-order reaction. These findings demonstrated that Sr1−xKxTiO3−δ@PC-polyHIPEs were an effective candidate for real application in decomposition of MB in water.

Sequential ionic layer adsorption and reaction (SILAR) deposition of Bi4Ti3O12 on TiO2: an enhanced and stable photocatalytic system for water purification

A new method to produce bismuth titanate – titanium dioxide composites by modification of a TiO₂ film deposited on a variety of different glass substrates is reported.

Carboxymethyl-β-cyclodextrin functionalization of TiO2 doped with lanthanum: characterization and enhancement of photocatalytic activity

This paper proposes a lanthanum-doping and carboxymethyl-β-cyclodextrin functionalization mechanism for TiO₂ nanoparticles to increase the photocatalytic activity.

Sunlight-charged heterojunction TiO2and WO3particle-embedded inorganic membranes for night-time environmental applications

A metal oxide-heterojunction photocatalyst is developed to harvest sunlight, store the energy in electrons, and apply the stored energy in water treatment.

Titanate Nanotubes Decorated Graphene Oxide Nanocomposites: Preparation, Flame Retardancy, and Photodegradation

Most polymers exhibit high flammability and poor degradability, which restrict their applications and causes serious environmental problem like "white pollution." Thus, titanate nanotubes (TNTs) were adopted to decorate graphene oxide (GO) by a facile solution method to afford TNTs/GO nanocomposites with potential in improving the flame retardancy and photodegradability of flexible polyvinyl chloride (PVC). Results show that the as-prepared TNTs/GO can effectively improve the thermal stability and flame retardancy than TNTs and GO, especially, the peak heat release rate and total heat release were reduced by 20 and 29% with only 2.5 wt.% loading. And more, the TNTs/GO also improve the photodegradability of PVC compared with the neat PVC. The reasons can be attributed to synergistic flame-retardant and photocatalytic effects between TNTs and GO. The present research could contribute to paving a feasible pathway to constructing polymer-matrix composites with desired flame retardancy and photodegradability, thereby adding to the elimination of white pollution caused by polymers.

Challenges in photocatalytic reduction of nitrate as a water treatment technology

Management of ubiquitous nitrate contamination in drinking water sources is a major engineering challenge due to its negative impacts from eutrophication to immediate risk to human health. Several water treatment technologies exist to manage nitrate pollution in water sources. However, the most widely used technologies are phase separation treatments. In this context, nanoscale photocatalysis emerges as a highly promising transformative technology capable of reducing nitrate to innocuous nitrogen with noticeable selectivity. This critical review describes the photocatalytic reduction mechanisms of nitrate towards undesirable products (nitrite, ammonium) and the more desirable product (dinitrogen). The mechanisms are based on the standard reduction potential of each individual species and highlight the contribution of reducing species (e.g. CO₂^-) radicals formed from different hole scavengers. The strategic use of different pure, doped, and composite nanoscale photocatalysts is discussed on the basis of reduction mechanisms' overall conversion, kinetic rates, and selectivity towards N₂. The choice of light source affects pathways and influences by-product selectivity because direct photolysis of N-intermediates, which has been overlooked in the literature. In addition, the re-oxidation of nitrite and ammonia as drawback process is explained. Finally, an exhaustive analysis presents the photocatalytic reduction applications for treating real water matrices and the competing effect of other species. Overall, this critical review aims to contribute to the understanding of the potential application/constraints of photocatalysis in inorganic nitrogen management, and guide researchers towards future efforts required for widespread implementation.

Nanotechnology in agriculture: Opportunities, toxicological implications, and occupational risks

Nanotechnology has the potential to make a beneficial impact on several agricultural, forestry, and environmental challenges, such as urbanization, energy constraints, and sustainable use of resources. However, new environmental and human health hazards may emerge from nano-enhanced applications. This raises concerns for agricultural workers who may become primarily exposed to such xenobiotics during their job tasks. The aim of this review is to discuss promising solutions that nanotechnology may provide in agricultural activities, with a specific focus on critical aspects, challenging issues, and research needs for occupational risk assessment and management in this emerging field. Eco-toxicological aspects were not the focus of the review. Nano-fertilizers, (nano-sized nutrients, nano-coated fertilizers, or engineered metal-oxide or carbon-based nanomaterials per se), and nano-pesticides, (nano-formulations of traditional active ingredients or inorganic nanomaterials), may provide a targeted/controlled release of agrochemicals, aimed to obtain their fullest biological efficacy without over-dosage. Nano-sensors and nano-remediation methods may detect and remove environmental contaminants. However, limited knowledge concerning nanomaterial biosafety, adverse effects, fate, and acquired biological reactivity once dispersed into the environment, requires further scientific efforts to assess possible nano-agricultural risks. In this perspective, toxicological research should be aimed to define nanomaterial hazards and levels of exposure along the life-cycle of nano-enabled products, and to assess those physico-chemical features affecting nanomaterial toxicity, possible interactions with agro-system co-formulants, and stressors. Overall, this review highlights the importance to define adequate risk management strategies for workers, occupational safety practices and policies, as well as to develop a responsible regulatory consensus on nanotechnology in agriculture.

Contaminants of emerging concern: a review of new approach in AOP technologies

The presence of contaminants of emerging concern (CECs) such as pharmaceuticals and personal care products (PPCPs), endocrine-disrupting compounds (EDCs), flame retardants (FRs), pesticides, and artificial sweeteners (ASWs) in the aquatic environments remains a major challenge to the environment and human health. In this review, the classification and occurrence of emerging contaminants in aquatic environments were discussed in detail. It is well documented that CECs are susceptible to poor removal during the conventional wastewater treatment plants, which introduce them back to the environment ranging from nanogram per liter (e.g., carbamazepine) up to milligram per liter (e.g., acesulfame) concentration level. Meanwhile, a deep insight into the application of advanced oxidation processes (AOPs) on mitigation of the CECs from aquatic environment was presented. In this regard, the utilization of various treatment technologies based on AOPs including ozonation, Fenton processes, sonochemical, and TiO₂ heterogeneous photocatalysis was reviewed. Additionally, some innovations (e.g., visible light heterogeneous photocatalysis, electro-Fenton) concerning the AOPs and the combined utilization of AOPs (e.g., sono-Fenton) were documented.

Highly-efficient photocatalytic degradation of methylene blue by PoPD-modified TiO 2 nanocomposites due to photosensitization-synergetic effect of TiO2 with PoPD

Poly-o-phenylenediamine modified TiO₂ nanocomposites were successfully synthesized via an ‘in situ’ oxidative polymerization method. The modified nanocomposites were characterized by BET, XRD, TEM, FT-IR, TGA, XPS, EA and UV-Vis DRS. The photocatalytic degradation of methylene blue was chosen as a model reaction to evaluate the photocatalytic activities of TiO₂ and PoPD/TiO₂. The results indicated that PoPD/TiO₂ nanocomposites exhibited good photocatalytic activity and stability. The photocatalytic activity of PoPD/TiO₂ increased as the initial pH increased because of electrostatic adsorption between the photocatalyst and MB as well as the generation of ·OH, whereas it exhibited an earlier increasing and later decreasing trend as the concentration of the photocatalyst increased owing to the absorption of visible light. The photocatalytic stability of the PoPD/TiO₂ nanocomposite was dependent on the stability of its structure. Based on radical trapping experiments and ESR measurements, the origin of oxidizing ability of PoPD/TiO₂ nanocomposites on photocatalytic degradation of MB was proposed, which taking into account of ·OH and ·O₂⁻ were the first and second important ROS, respectively. The possible photocatalytic mechanism and photocatalytic activity enhanced mechanism has been proposed, taking into account the photosensitization effect and synergetic effect of TiO₂ with PoPD.

Photoelectrochemical Degradation of Organic Compounds Coupled with Molecular Hydrogen Generation Using Electrochromic TiO2 Nanotube Arrays

Vertically aligned TiO₂ nanotube arrays (TNTs) were prepared by electrochemical anodization, and then cathodically polarized with dark blue coloration for the dual-functional photoelectrochemical water treatment of organic substrates degradation and accompanying H₂ generation. The resulting Blue-TNTs (inner diameter: ∼40 nm; length: ∼9 μm) showed negligible shift in X-ray diffraction pattern compared with the intact TNTs, but the X-ray photoelectron spectra indicated a partial reduction of Ti⁴⁺ to Ti³⁺ on the surface. The electrochemical analyses of Blue-TNTs revealed a marked enhancement in donor density and electrical conductivity by orders of magnitude. Degradations of test organic substrates on Blue-TNTs were compared with the intact TNTs in electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions (potential bias: 1.64 V_NHE; λ > 320 nm). The degradation of 4-chlorophenol was greatly enhanced on Blue-TNTs particularly in PEC condition, whereas the PC activities of the Blue- and intact TNTs were similar. The potential bias of 1.64 V_NHE did not induce any noticeable activity in EC condition. Similar trends were observed for the degradation of humic acid and fulvic acid, where main working oxidants were found to be the surface hydroxyl radical as confirmed by hydroxyl radical probe and scavenger tests. H₂ generation coupled with the organic degradation was observed only in PEC condition, where the H₂ generation rate with Blue-TNTs was more than doubled from that of intact TNTs. Such superior PEC activity was not observed when a common TiO₂ nanoparticle film was used as a photoanode. The enhanced electric conductivity of Blue-TNTs coupled with a proper band bending in PEC configuration seemed to induce a highly synergic enhancement.