TiO2 photocatalysis: Design and applications

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.

Light induced DNA-functionalized TiO2 nanocrystalline interface: Theoretical and experimental insights towards DNA damage detection

Owing to the emerging applications of DNA-functionalized TiO₂ nanocrystals towards DNA damage detection, it is inevitable to understand the better chemistry as well as in-depth molecular interaction phenomena. Fundamentally, energy difference underlies the layer-by-layer construction, resulted in the increase of the interaction energy and thus, altering the electrochemical behavior. Herein, Density functional theory (DFT) calculations were performed using DMol3 and DFTB+ codes successfully to elucidate the structural, electronics, and vibrational properties of the layer-by-layer components composing ss-DNA/dopamine/TiO₂/FTO. The obtained results are in good agreement with the experimental findings. The band gaps of FTO and TiO₂ were computationally obtained at 3.335 and 3.136 eV which are comparable with the experimental data (3.500 eV; FTO and 3.200 eV; TiO₂). Frontier orbital analysis is also considered to elucidate their electron transfer phenomena. Further, a 100 ns MD simulations are carried out using canonical ensemble embedded with COMPASS-Universal Forcefields generating useful thermodynamics parameters. Binding energies indicate increasing interaction energies for the layer-by-layer nanosystem, in agreement with the increasing diameter of electrochemical impedance spectroscopy (EIS) semicircle. Our results reveal the fundamental understanding of the DNA-functionalized TiO₂ nanocrystals down to molecular and electronic level and further, paving a way of its application towards nanoelectrochemical DNA biosensors.

Silver-modified octahedral anatase particles as plasmonic photocatalyst

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Octahedral anatase particles (OAPs) modified with silver NPs by photodeposition.

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Ag/OAPs with enhanced photocatalytic activity under both UV and vis irradiation.

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Electron traps as nucleation sites for silver NPs.

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Polydispersity of silver NPs results in broad LSPR and thus enhanced vis activity.

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TRMC data correlate with photocatalytic activity.

Octahedral anatase particles (OAPs) were modified with silver nanoparticles (NPs) by photodeposition method. The properties of OAPs influenced the properties of silver deposits, and thus the photocatalytic activity of the obtained silver-modified OAPs. Photocatalytic activities were tested under UV and vis irradiation for oxidative decomposition of acetic acid and oxidation of 2-propanol, respectively. The properties of silver-modified OAPs were investigated by XRD, STEM, DRS, XPS and time-resolved microwave conductivity (TRMC) method. It was found that electron traps (ETs) worked as nucleation sites for silver, resulting in formation of smaller silver NPs on smaller OAPs with larger content of ETs. The modification with silver resulted in enhanced photocatalytic activity under both UV and vis irradiation. It was found that larger crystallite size of silver NPs, and thus larger polydispersity of silver deposits resulted in broad and intense plasmon resonance peak causing enhanced visible activity. The correlation between photocatalytic activity and TRMC data, e.g., slower decay of TRMC signal for more active samples, allowed discussion on property-governed photocatalytic activities of silver-modified titania.

Characterization and Applications of Nanoparticles Modified in-Flight with Silica or Silica-Organic Coatings

Nanoparticles are coated in-flight with a plasma-enhanced chemical vapor deposition (PECVD) process at ambient or elevated temperatures (up to 300 °C). Two silicon precursors, tetraethyl orthosilicate (TEOS) and hexamethyldisiloxane (HMDSO), are used to produce inorganic silica or silica-organic shells on Pt, Au and TiO₂ particles. The morphology of the coated particles is examined with transmission electron microscopy (TEM) and the chemical composition is studied with Fourier-transform infrared spectroscopy (FTIR), Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). It is found that both the precursor and certain core materials have an influence on the coating composition, while other parameters, such as the precursor concentration, aerosol residence time and temperature, influence the morphology, but hardly the chemical composition. The coated particles are used to demonstrate simple applications, such as the modification of the surface wettability of powders and the improvement or hampering of the photocatalytic activity of titania particles.

Photonic Band Gap and Bactericide Performance of Amorphous Sol-Gel Titania: An Alternative to Crystalline TiO₂

In addition to its traditional application in white pigments, nanocrystalline titania (TiO₂) has optoelectronic and photocatalytic properties (strongly dependent on crystallinity, particle size, and surface structure) that grant this naturally occurring oxide new technological applications. Sol-gel is one of the most widely used methods to synthesize TiO₂ films and NPs, but the products obtained (mostly oxy-hydrated amorphous phases) require severe heat-treatments to promote crystallization, in which control over size and shape is difficult to achieve. In this work, we obtained new photocatalytic materials based on amorphous titania and measured their electronic band gap. Two case studies are reported that show the enormous potential of amorphous titania as bactericide or photocatalyst. In the first, amorphous sol-gel TiO₂ thin films doped with N (TiO2−xNx, x = 0.75) were designed to exhibit a photonic band gap in the visible region. The identification of Ti-O-N and N-Ti-O bindings was achieved by XPS. The photonic band gaps were found to be 3.18 eV for a-TiO₂ and 2.99 eV for N-doped a-TiO₂. In the second study, amorphous titania and amine-functionalized amorphous titania nanoparticles were synthetized using a novel base-catalysed sol-gel methodology. All the synthesized amorphous TiO₂ nanoparticles exhibit bactericide performance (E. coli, ASTME 2149-13).

Mitigation of harmful indoor organic vapors using plug-flow unit coated with 2D g-C3N4 and metallic Cu dual-incorporated 1D titania heterostructure

Herein, a plug-flow reactor coated with one-dimensional (1D) TiO₂ nanotube (TNT) heterostructures incorporated with g-C₃N₄ (CN) and metallic Cu (CN/Cu/TNT) nanocomposite and irradiated by a daylight lamp was newly applied for the mitigation of harmful indoor organic vapors. The CN/Cu/TNT catalyst showed high mitigation efficiency for all target pollutants, followed by Cu-incorporated TNT (Cu/TNT), CN-incorporated TNT (CN/TNT), TNT, and TiO₂, in that order. The order of their photocatalytic activities agrees with that of the electron‒hole separation rates determined from their photoluminescence emission spectra. The mitigation efficiency of the CN/Cu/TNT catalyst increased as the CN-to-Cu/TNT percentage was increased from 1% to 10%, but subsequently decreased as the CN-to-Cu/TNT percentage increased to 20%. The mitigation efficiencies of the CN/Cu/TNT catalyst decreased with increasing relative humidity, feed pollutant concentrations, and airstream flow rates. However, in most cases, the reaction rates of the target compounds increased when the feed concentration was increased from 1 to 5 ppm. The mineralization rates of all target pollutants were lower than the corresponding photocatalytic mitigation rates, which could be ascribed to the production of CO and organic intermediates observed during the photocatalysis of the target pollutants. Nevertheless, the intermediates formed during the photocatalytic mitigation process would not cause significant adverse health effects to building occupants, because their concentrations were far below their exposure or threshold limit values. A probable mechanism for the photocatalytic mitigation of the organic vapors by the CN/Cu/TNT catalyst under daylight illumination was also proposed.

Evaluation of SnO2 for sunlight photocatalytic decontamination of water

The broad bandgap tin (IV) oxide (SnO₂) is the least investigated semiconductor material for photocatalytic water decontamination in sunlight exposure. A detailed study covering the synthesis, characterization and the evaluation of photocatalytic activity of SnO₂, in the natural sunlight exposure, is presented. The structural characterization by XRD revealed the formation of phase pure tetragonal SnO₂ with the average crystallite size of ∼41.5 nm whereas minor Sn²⁺ states in the material were identified by XPS analysis. As explored by diffuse reflectance (DR) and photoluminescence (PL) spectroscopy, the material exhibited a distinct absorption edge at ∼3.4 eV. The morphological and microstructure analysis of the synthesized SnO₂ was carried out by FESEM and HRTEM. The electrochemical impedance spectroscopy (EIS) and chronopotentiometry (CP) predicted the better charge transport and retention ability of the material under illumination whereas the Mott-Schottky extrapolation prophesied the n-type behavior with the flat-band potential of -0.60 V. The photocatalytic activity of SnO₂ was assessed in the exposure of complete spectrum natural sunlight for the removal of 2,4,6-trichlorophenol. The HPLC and TOC analysis monitored the progress of degradation and mineralization whereas the released chloride ions were evaluated by ion chromatography. The effect of the transition metal ions (Fe³⁺, Cu²⁺, Ni^2+, and Zn²⁺) as electron capture agents and H₂O₂ as ROS generator was explored during the degradation process. The utility of the material for the simultaneous removal of chlorophenols in the mixture was also investigated. The SnO₂ exhibited sustained activity in the repeated use. Based on experimental evidence congregated, the mechanism of the removal process and the efficacy of SnO₂ for sunlight photocatalytic decontamination of water was established.

A Review on the Electrochemically Self-organized Titania Nanotube Arrays: Synthesis, Modifications, and Biomedical Applications

Titania nanotubes grown by anodic oxidation have intrigued the material science community by its many unique and potential properties, and the synthesis of technology is merging to its mature stage. The present review will focus on TiO2 nanotubes grown by self-organized electrochemical anodization from Ti metal substrate, which critically highlights the synthesis of this type of self-organized titania nanotube layers and the means to influence the size, shape, the degree of order, and crystallized phases via adjusting the anodization parameters and the subsequent thermal annealing. The relationship between dimensions and properties of the anodic TiO2 nanotube arrays will be presented. The latest progress and significance of the research on formation mechanism of anodic TiO2 nanotubes are briefly discussed. Besides, we will show the most promising applications reported recently in biomedical directions and modifications carried out by doping, surface modification, and thermal annealing toward improving the properties of anodically formed TiO2 nanotubes. At last, some unsolved issues and possible future directions of this field are indicated.

Self-Cleaning Ceramic Tiles Produced via Stable Coating of TiO₂ Nanoparticles

The high photocatalytic power of TiO₂ nanoparticles has drawn great attention in environmental and medical applications. Coating surfaces with these particles enables us to benefit from self-cleaning properties and decomposition of pollutants. In this paper, two strategies have been introduced to coat ceramic tiles with TiO₂ nanoparticles, and the self-cleaning effect of the surfaces on degradation of an organic dye under ultraviolent (UV) exposure is investigated. In the first approach, a simple one-step heat treatment method is introduced for coating, and different parameters of the heat treatment process are examined. In the second method, TiO₂ nanoparticles are first aminosilanized using (3-Aminopropyl)triethoxysilane (APTES) treatment followed by their covalently attachment onto CO₂ plasma treated ceramic tiles via N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-Hydroxysuccinimide (NHS) chemistry. We monitor TiO₂ nanoparticle sizes throughout the coating process using dynamic light scattering (DLS) and characterize developed surfaces using X-ray photoelectron spectroscopy (XPS). Moreover, hydrophilicity of the coated surfaces is quantified using a contact angle measurement. It is shown that applying a one-step heat treatment process with the optimum temperature of 200 °C for 5 h results in successful coating of nanoparticles and rapid degradation of dye in a short time. In the second strategy, the APTES treatment creates a stable covalent coating, while the photocatalytic capability of the particles is preserved. The results show that coated ceramic tiles are capable of fully degrading the added dyes under UV exposure in less than 24 h.

Three-Dimensional CdS/Au Butterfly Wing Scales with Hierarchical Rib Structures for Plasmon-Enhanced Photocatalytic Hydrogen Production

Localized surface plasmon resonance (LSPR) of plasmonic metals (e.g., Au) can help semiconductors improve their photocatalytic hydrogen (H₂) production performance. However, an artificial synthesis of hierarchical plasmonic structures down to nanoscales is usually difficult. Here, we adopt the butterfly wing scales from Morpho didius to fabricate three-dimensional (3D) CdS/Au butterfly wing scales for plasmonic photocatalysis. The as-prepared materials well-inherit the pristine hierarchical biostructures. The 3D CdS/Au butterfly wing scales exhibit a high H₂ production rate (221.8 μmol·h^-1 within 420-780 nm), showing a 241-fold increase over the CdS butterfly wing scales. This is attributed to the effective potentiation effect of LSPR introduced by multilayer metallic rib structures and a good interface bonding state between Au and CdS nanoparticles. Thus, our study provides a relatively simple method to learn from nature and inspiration for preparing highly efficient plasmonic photocatalysts.

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.

Quantum dot-decorated semiconductor micro- and nanoparticles: A review of their synthesis, characterization and application in photocatalysis

Quantum dot (QD)-decorated semiconductor micro- and nanoparticles are a new class of functional nanomaterials that have attracted considerable interest for their unique structural, optical and electronic properties that result from the large surface-to-volume ratio and the quantum confinement effect. In addition, because of QDs' excellent light-harvesting capacity, unique photoinduced electron transfer, and up-conversion behaviour, semiconductor nanoparticles decorated with quantum dots have been used widely in photocatalytic applications for the degradation of organic pollutants in both the gas and aqueous phases. This review is a comprehensive overview of the recent progress in synthesis methods for quantum dots and quantum dot-decorated semiconductor composites with an emphasis on their composition, morphology and optical behaviour. Furthermore, various approaches used for the preparation of QD-based composites are discussed in detail with respect to visible and UV light-induced photoactivity. Finally, an outlook on future development is proposed with the goal of overcoming challenges and stimulating further research into this promising field.

Chemical Cross-Linking of Anatase Nanoparticle Thin Films for Enhanced Mechanical Properties

Titania nanoparticle-based thin films are highly attractive for a vast range of commercial applications. Although their application on polymer-based substrates is particularly appealing, the requirement of low process temperatures results in low mechanical stability. Highly crystalline anatase nanoparticles were used as the building blocks for coatings through a two-stage process. The main benefits of this method, over the more common sol-gel ones, are the relatively low temperature required for the production of metal oxide coatings, allowing the use of polymer-based substrates, and the defined crystallinity of the resulting thin films. Although in several cases moderate temperatures can be utilized for drying the films, the mechanical stability of the respective coatings remains a critical issue. In this contribution, we present a strategy to achieve network formation between TiO₂ nanoparticles in a preformed thin film on the basis of the cross-linking of the functionalized nanoparticles. In the first stage, the nanoparticles were functionalized by dicarboxylic acids, concurrently leading to a stable colloidal dispersion that could be utilized for dip-coating to obtain TiO₂ thin films with high homogeneity and optical transparence. During the second stage, the films were immersed in a solution of a diamine as the linker molecule, to achieve cross-linking between the nanoparticles within the film. It is demonstrated that indeed covalent bonding was realized and functional coatings with significantly enhanced mechanical properties were obtained by our strategy.

Novel synthesis of WSe2-Graphene-TiO2 ternary nanocomposite via ultrasonic technics for high photocatalytic reduction of CO2 into CH3OH

In the present work, we report the development of a WSe₂-Graphene-TiO₂ ternary nanocomposite via ultrasonic techniques and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The band-gap of the WSe₂-Graphene-TiO₂ was estimated to be about 1.62 eV which is suitable for photodegradation process under under ultraviolet UV/Visible light irradiation. The photocatalytic potential of nanocomposites is investigated for CO₂ reduction to CH₃OH. WSe₂-G-TiO₂ with an optimum loading of graphene of 8 wt% showed the high photoactivity, gaining a total CH₃OH yield of 6.3262 µmol g^-1 h^-1 after 48 h. This excellent photoreduction activity is owing to the progressive synergistic relation between WSe₂/TiO₂ and graphene components in our heterogeneous system.

Enhanced Photocatalytic Hydrogen Evolution from Transition-Metal Surface-Modified TiO2

This study describes the UV solution photodeposition of several earth-abundant 3d transition metals (Co, Ni, and Cu) onto the surface of nanoparticulate TiO2. Irradiated methanolic metal dichloride solutions with suspended Degussa P25-TiO2 (1-2 wt % metal to TiO2) yield visibly colored titanias, whereas the bulk TiO2 structure is unchanged; X-ray photoelectron spectroscopy confirms that metals are present on the titania surface in either reduced metal (Cu/Cu+) or metal cation states (Co2+ and Ni2+), and UV-vis diffuse reflectance spectroscopy shows new visible absorbance features. The analyzed bulk metal contents (∼0.04-0.6 at. %, highest for copper) are lower than the nominal metal solution content. Mixed-metal solution photodeposition reactions roughly parallel observations for single metals, with copper deposition being most favored. These 3d metal surface-modified titanias show significant (∼5-15×) improvement in UV photocatalytic H2 evolution versus unmodified TiO2. H2 evolution rates as high as 85 μmol/h (8500 μmol h-1 g-1) were detected for Cu-coated TiO2 using continuous monitoring of reactor headspace gases by portable mass spectrometry. Control experiments verify the necessity of the methanol sacrificial oxidant in both metal deposition and H2 evolution. In situ metal surface deposition is quickly followed by enhanced H2 evolution relative to TiO2, but at lower levels than isolated metal surface-modified titanias. The photodeposited 3d metal species on the TiO2 surface likely act to reduce electron-hole recombination by facilitating the transfer of photoinduced TiO2 conduction band electrons to protons in solution that are reduced to H2. This study demonstrates a facile method to modify photoactive TiO2 nanoparticles with inexpensive 3d transition metals to improve photocatalytic hydrogen evolution, and it shows the utility of quantitative real-time gas evolution monitoring by portable mass spectrometry.

N-rich covalent organic polymer in situ modified TiO2 for highly efficient photocatalytic hydrogen evolution

TiO₂ has been everlastingly employed as popular photocatalyst for water splitting. However, the wide band gap (3.0-3.2 eV) and poor absorption to visible light of TiO₂ result in a low utilization of solar energy and limit its large-scale application. To decrease its band gap and promote the utilization of full solar energy, we here modified TiO₂ by in situ growth of N-rich covalent organic polymer (termed as COP_HM). During the in situ growth of COP_HM on the surface of TiO₂, intimate contacts between TiO₂ and COP_HM were built and core-shell structures were finally formed. The derived TiO₂@COP_HM demonstrated a narrower band gap (2.53 eV) compared to raw TiO₂ (3.13 eV) and improved absorption to visible light. The optimal TiO₂@COP_HM hybrid exhibited excellent hydrogen evolution performance of 162.7 μmol h^-1 under simulated sunlight which was more than 3 times higher than raw TiO₂ (51.3 μmol h^-1). Particularly, visible light hydrogen evolution rate of TiO₂@COP_HM reached 0.65 μmol h^-1 while non-hydrogen generation was observed using raw TiO₂.

Plasmonic photocatalyst-like fluorescent proteins for generating reactive oxygen species

The recent advances in photocatalysis have opened a variety of new possibilities for energy and biomedical applications. In particular, plasmonic photocatalysis using hybridization of semiconductor materials and metal nanoparticles has recently facilitated the rapid progress in enhancing photocatalytic efficiency under visible or solar light. One critical underlying aspect of photocatalysis is that it generates and releases reactive oxygen species (ROS) as intermediate or final products upon light excitation or activation. Although plasmonic photocatalysis overcomes the limitation of UV irradiation, synthesized metal/semiconductor nanomaterial photocatalysts often bring up biohazardous and environmental issues. In this respect, this review article is centered in identifying natural photosensitizing organic materials that can generate similar types of ROS as those of plasmonic photocatalysis. In particular, we propose the idea of plasmonic photocatalyst-like fluorescent proteins for ROS generation under visible light irradiation. We recapitulate fluorescent proteins that have Type I and Type II photosensitization properties in a comparable manner to plasmonic photocatalysis. Plasmonic photocatalysis and protein photosensitization have not yet been compared systemically in terms of ROS photogeneration under visible light, although the phototoxicity and cytotoxicity of some fluorescent proteins are well recognized. A comprehensive understanding of plasmonic photocatalyst-like fluorescent proteins and their potential advantages will lead us to explore new environmental, biomedical, and defense applications.

Preparation of water-compatible molecularly imprinted thiol-functionalized activated titanium dioxide: Selective adsorption and efficient photodegradation of 2, 4-dinitrophenol in aqueous solution

A novel water-compatible surface molecularly imprinted thiol-functionalized titanium dioxide (TiO₂) material (CMIP-coated TiO₂) was prepared in water, using 2, 4-dinitrophenol (2, 4-DNP) as template molecule and o-phenylenediamine (OPDA) as both functional monomer and cross-linker. The as-synthesized materials were characterized by FESEM, FTIR, XRD, BET and UV-vis DRS. Moreover, we have investigated the adsorption capacity, adsorption selectivity and photodegradation activity of the CMIP-coated TiO₂ and non-molecular imprinted materials (CNIP-coated TiO₂). Additionally, the effects of pH and concentration of 2, 4-DNP on the degradation rate of 2, 4-DNP were also investigated. Results showed that CMIP-coated TiO₂ exhibited higher adsorption capacity, greater selectivity and faster photodegradation activity for 2, 4-DNP compared with the CNIP-coated TiO₂. Meanwhile, the specific selectivity to 2, 4-DNP over its structural analogue 4-nitrophenol (4-NP) and the enhanced photodegradation capacity were mainly attributed to the imprinted cavities on the surface of CMIP-coated TiO₂. Taking advantage of efficient removal capacity, high reusability and no-additional chemicals in imprinted process, the prepared materials can be potentially applied to "green" removal of 2, 4-DNP in wastewater.

Surface modified TiO2 floating photocatalyst with PDDA for efficient adsorption and photocatalytic inactivation of Microcystis aeruginosa

Microcystis aeruginosa, as the most common cyanobacteria, often grows uncontrollably in eutrophic lakes with the accumulation of microcystin-LR (MC-LR) in water, which heavily pollutes water and hence imposes tremendous threat to aquatic animals and human beings. To remediate the harmful algae polluted water, here we synthesize a series of poly dimethyl diallyl ammonium chloride (PDDA) modified TiO₂ floating photocatalysts, PDDA@NPT-EGC, and apply them as a visible light driven multifunctional material. The fabricated PDDA@NPT-EGC composites have a worm-like structure with PDDA particles distributed on their surfaces, and the concentration of PDDA can affect the agglomerative condition and distribution of PDDA particles and the photoelectric properties of catalysts. Among these catalysts, the PDDA@NPT-EGC with 0.2 wt% PDDA (0.2PDDA@NPT-EGC) shows the highest adsorption and photocatalytic activity. Compared with the NPT-EGC, the dark adsorption efficiency for the 0.2PDDA@NPT-EGC after 3 h increases from 70.4% to 88.9%, and the total removal efficiency after visible light irradiation for 2 h increases from 77.8% to 92.6%. In addition, the 0.2PDDA@NPT-EGC exhibits a removal efficiency of 96.55% for photocatalytic degradation of MC-LR after irradiation for 3 h. The Adda side chain of MC-LR molecule is found to degradate gradually in the photocatalytic degradation process, indicative of the elimination of biotoxicity for MC-LR molecule in the reaction. We demonstrate that the 0.2PDDA@NPT-EGC is remarkably competitive in both algae inactivation and MC-LR removal, which shall hold substantial promise in remediation of algae pollution in eutrophic waters.

Immobilized TiO2 on glass spheres applied to heterogeneous photocatalysis: photoactivity, leaching and regeneration process

Heterogeneous photocatalysis using titanium dioxide as catalyst is an attractive advanced oxidation process due to its high chemical stability, good performance and low cost. When immobilized in a supporting material, additional benefits are achieved in the treatment. The purpose of this study was to develop a simple protocol for impregnation of TiO₂-P25 on borosilicate glass spheres and evaluate its efficiency in the photocatalytic degradation using an oxidizable substrate (methylene blue), in a Compound Parabolic Concentrator (CPC) reactor. The assays were conducted at lab-scale using radiation, which simulated the solar spectrum. TiO₂ leaching from the glass and the catalyst regeneration were both demonstrated. A very low leaching ratio (0.03%) was observed after 24 h of treatment, suggesting that deposition of TiO₂ resulted in good adhesion and stability of the photocatalyst on the surface of borosilicate. This deposition was successfully achieved after calcination of the photocatalyst at 400 °C (TiO₂-400 °C). The TiO₂ film was immobilized on glass spheres and the powder was characterized by scanning electron microscopy (SEM), X-ray diffraction and BET. This characterization suggested that thermal treatment did not introduce substantial changes in the measured microstructural characteristics of the photocatalyst. The immobilized photocatalyst degraded more than 96% of the MB in up to 90 min of reaction. The photocatalytic activity decreased after four photocatalytic cycles, but it was recovered by the removal of contaminants adsorbed on the active sites after washing in water under UV-Vis irradiation. Based on these results, the TiO₂-400 °C coated on glass spheres is potentially a very attractive option for removal of persistent contaminants present in the environment.

Fabrication and photoactivity of ionic liquid-TiO2 structures for efficient visible-light-induced photocatalytic decomposition of organic pollutants in aqueous phase

To investigate the effect of the ionic liquid (IL) chain length on the surface properties and photoactivity of TiO₂, a series of TiO₂ microspheres have been synthesized via a solvothermal method assisted by 1-methyl-3-octadecylimidazolium chloride ([ODMIM][Cl]) and 1-methyl-3-tetradecylimidazolium chloride ([TDMIM][Cl]). All as-prepared samples were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), scanning transmission microscopy (STEM) and the Brunauer-Emmett-Teller (BET) surface area method, whereas the photocatalytic activity was evaluated by the degradation of phenol in aqueous solution under visible light irradiation (λ > 420 nm). The highest photoefficiency (four times higher than pristine TiO₂) was observed for the TiO₂ sample obtained in the presence of [TDMIM][Cl] for a IL to TiO₂ precursor molar ratio of 1:3. It was revealed that interactions between the ions of the ionic liquid and the surface of the growing titanium dioxide spheres results in a red-shift of absorption edge for the IL-TiO₂ semiconductors. In this regard, the direct increase of the photoactivity of IL-TiO₂ in comparison to pristine TiO₂ was observed. The active species trapping experiments indicated that O₂^•- is the main active species, created at the surface of the IL-TiO₂ material under visible-light illumination, and is responsible for the effective phenol degradation.

Titania nanowires functionalized polyester fabrics with enhanced photocatalytic and antibacterial performances

Flexible organic fabrics coated with titania find wide applications in pollutant degradations and antibiosis. Because of the enhanced charge separations, TiO₂ with one-dimensional nanostructures exhibits photocatalytic activity superior to that of nanoparticulate films; however, only the later has been achieved on organic substrates through commonly sol-gel techniques till now. In this study, radially aligned TiO₂ nanowires were precipitated on polyester fabrics through multi-steps of surface roughening, sol-gel TiO₂ seeding, hydrogen titanate nanobelts precipitation, and finally sulfuric acid treatment. Both mesoporous anatase TiO₂ nanowires and single-crystalline rutile TiO₂ nanorods have been achieved, which, together with some unchanged titanate nanobelts, exhibited an overall narrowed band gap of ca. 2.50eV. The TiO₂ nanowires on flexible PET fabrics showed higher photocatalytic activity towards degradations of not only rhodamine B in water but also toluene gas in air under UV light illumination, when compared with either TiO₂ nanotube array or commercial Degussa P25 nanoparticulate films on metallic Ti substrates. Remarkable sterilization of E. coli and S. epidermidis under visible light irradiation was also achieved. The excellent photocatalytic and antibacterial performances were attributed to the unique mixed 1D nanostructures, phase junctions, abundant surface hydroxyl groups, and the narrowed band gap.

Compact light-emitting diode optical fiber immobilized TiO2 reactor for photocatalytic water treatment

A key barrier to implementing photocatalysis is delivering light to photocatalysts that are in contact with aqueous pollutants. Slurry photocatalyst systems suffer from poor light penetration and require post-treatment to separate the catalyst. The alternative is to deposit photocatalysts on fixed films and deliver light onto the surface or the backside of the attached catalysts. In this study, TiO₂-coated quartz optical fibers were coupled to light emitting diodes (OF/LED) to improve in situ light delivery. Design factors and mechanisms studied for OF/LEDs in a flow-through reactor included: (i) the influence of number of LED sources coupled to fibers and (ii) the use of multiple optical fibers bundled to a single LED. The light delivery mechanism from the optical fibers into the TiO₂ coatings is thoroughly discussed. To demonstrate influence of design variables, experiments were conducted in the reactor using the chlorinated pollutant para-chlorobenzoic acid (pCBA). From the degradation kinetics of pCBA, the quantum efficiencies (Φ) of oxidation and electrical energies per order (E_EO) were determined. The use of TiO₂ coated optical fiber bundles reduced the energy requirements to deliver photons and increased available surface area, which improved Φ and enhanced oxidative pollutant removal performance (E_EO).