Gas-phase photocatalytic activity of sputter-deposited anatase TiO2 films: Effect of 〈0 0 1〉 preferential orientation, surface temperature and humidity

Photocatalytic Degradation of Methyl Orange and Methylene Blue Dyes by Engineering the Surface Nano-Textures of TiO2 Thin Films Deposited at Different Temperatures via MOCVD

TiO₂ thin films were deposited on quartz substrates by metal-organic chemical vapor deposition (MOCVD) at temperatures of 250, 350, and 450 °C. X-ray diffraction (XRD) data revealed the production of a pure anatase phase, a decrease in crystallite size, and a textural change as deposition temperature increased. Atomic force microscopy (AFM) was used to study the morphological properties and confirm XRD results. UV-Vis.-NIR spectroscopy was used to investigate the optical properties of the samples. The effect of deposition temperature on wettability was investigated using contact angle measurements. Sunlight photocatalytic properties increased with the increase in deposition temperature for methyl orange and methylene blue. Films were post-annealed at 500 °C for 2 h. The effect of annealing on all the above-mentioned properties was explored. The kinetic analysis demonstrated superb agreement with the kinetic pseudo-first-order model. The rate of photocatalytic degradation of MB was ~8, 13, and 12 times that of MO using 250, 350, and 450 °C deposited films, respectively. Photodegradation was found to depend on the specific surface area, type of pollutant, and annealing temperature.

Calcination-Induced Oxygen Vacancies Enhancing the Photocatalytic Performance of a Recycled Bi2O3/BiOCl Heterojunction Nanosheet

With the rapid development of industry, bismuth-based semiconductors have been widely used for the photocatalytic degradation of organic contaminants discharged into wastewater. Herein, a Bi₂O₃/BiOCl (BBOC) heterojunction was constructed with high photocatalytic activity toward Rhodamine B (RhB) in the first cycle of the photocatalysis test, while the photocatalytic performance was drastically reduced after repeated testing. The adsorbed RhB molecules occupying the facial active sites of BBOC contributed to the decline of photocatalytic activity. The spent BBOC can be reactivated by the decomposition of the adsorbed RhB and the introduction of oxygen vacancies during calcination under an air atmosphere. The BBOC thus recovered exhibited a superior apparent rate constant of 0.08087 min^-1 compared with 0.05228 min^-1 of pristine BBOC. This study provided an effective strategy to investigate the deactivation/activation mechanism of bismuth-based heterojunction photocatalysts.

Synthesis Control of Charge Separation at Anatase TiO2 Thin Films Studied by Transient Surface Photovoltage Spectroscopy

For the efficient photocatalytic oxidation of organic pollutants at surfaces of semiconductors, photogenerated holes shall be separated toward the surface and transferred to reactive surface sites, whereas the transfer of photogenerated electrons toward the surface shall be minimized. In this Research Article, the identification of suitable synthesis control of charge separation combined with an in-depth understanding of charge kinetics and trapping passivation mechanisms at the related surfaces can provide tremendous opportunities for boosting the photocatalytic performance. In this work, a comprehensive transient surface photovoltage spectroscopy study of charge separation at anatase TiO₂ thin films, synthesized by ultrasonic spray pyrolysis from titanium(IV) isopropoxide (TTIP)-acetylacetone (AcacH) based precursor is reported. By varying the amount of AcacH in the precursor solution, an experimental approach of synthesis control of the charge transfer toward TiO₂ surface is provided for the first time. An increased amount of AcacH in the precursor promotes transition from preferential fast electron to preferential fast hole transfer toward anatase surface, correlating with a strong increase of the photocatalytic decomposition rate of organic pollutants. Suitable mechanisms of AcacH-induced passivation of electron traps at TiO₂ surfaces are analyzed, providing a new degree of freedom for tailoring the properties of photocatalytic systems.

Recent advances on inactivation of waterborne pathogenic microorganisms by (photo) electrochemical oxidation processes: Design and application strategies

Compared with other conventional water disinfection processes, (photo) electrochemical oxidation (P/ECO) processes have the characteristics of environmental friendliness, convenient installation and operation, easy control and high efficiency of inactivating waterborne pathogenic microorganisms (PMs), so that more and more research work has been focused on this topic, but there is still a huge gap between the research and practical application. Here, the research network of inactivating PMs by P/ECO processes has been comprehensively summarized, and the electrode/reactor/process design strategies based on strengthening direct and indirect oxidation, enhancing mass transfer efficiency and electron transfer efficiency, and improving the effective dose of electrogenerated oxidants are discussed. Furthermore, the factors affecting the inactivation of PMs and the issues regarding to stability and lifetime of the electrode are discussed respectively. Finally, the important research priorities and possible research challenges of P/ECO processes are put forward to make significant progress of this technology.

Effects of Anodic Aluminum Oxide Substrate Pore Geometry on the Gas-Phase Photocatalytic Activity of ZnO/Al2O3 Composites Prepared by Atomic Layer Deposition

We report on the photocatalytic activity of ZnO layers deposited by atomic layer deposition on a porous anodic aluminum oxide substrate with hexagonal pore symmetry and varied pore dimensions. ZnO/Al2O3 composites were prepared with pore diameters in the range 93–134 nm and interpore distance in the range 185–286 nm, and their photocatalytic activity was measured for gas-phase photocatalytic oxidation of acetaldehyde at varying UV illumination intensities (0.08–3.94 mW cm−2). The results show that substrates with narrower pore diameters (<115 nm, in the case of this study) have a detrimental effect on the photocatalyst performance, despite their higher effective surface. The results are explained on the basis of limited mass transfer inside the porous structure and can be used as a guideline in the purposeful design of photocatalysts with a nanoporous or nanotubular structure.

Ammonia and methane oxidation on TiO2 supported on glass fiber mesh under artificial solar irradiation

In this work, we present the application of solar photocatalysis for air purification including toxic substances such as ammonia and methane normally related to emissions from agriculture (e.g., poultry and cattle farms), landfills, etc. The study was done in three different laboratory and semi-pilot scale reactors: annular reactor (AR), mini-photocatalytic wind tunnel (MPWT), and photocatalytic wind tunnel (PWT). Reactors present a physical model for estimation of air-borne pollutant degradation over TiO₂-based photocatalytic layer in respect to optimal operating conditions (relative humidity, air/gas flow, and feed concentration). All studies were performed under artificial solar irradiation with different portions of UVB and UVA light. The application of solar photocatalysis for air purification was evaluated based on thorough monitoring of pollutants in inlet and outlet streams. The kinetic study resulted with intrinsic reaction rate constants: k_p,int,NH3 = (3.05 ± 0.04) × 10^-3 cm^4.5 mW^-0.5 g^-1 min^-1 and k_p,int,CH4 = (1.81 ± 0.02) × 10^-2 cm^4.5 mW^-0.5 g^-1 min^-1, calculated using axial dispersion model including mass transfer considerations and first-order reaction rate kinetics with photon absorption effects. The results of photocatalytic oxidation of NH₃ and CH₄ confirmed continuous reduction of pollutant content in the air stream due to the oxidation of NH₃ to N₂ and CH₄ to CO and CO₂, respectively. The application of solar photocatalysis in outdoor air protection is still a pioneering work in the field, and the results obtained in this work represent a good basis for sizing large-scale devices and applying them to prevent further environmental pollution. In the current study, a TiO₂ P25 supported on a glass fiber mesh was prepared from commercially available materials. The system designed in this way is easy to perform, operate, and relatively inexpensive.

Photocatalytic Decomposition of Acetaldehyde on Different TiO2-Based Materials: A Review

Purification of air from the organic contaminants by the photocatalytic process has been confirmed to be very perspective. Although many various photocatalysts have been prepared and studied so far, TiO2 is still the most commonly used, because of its advantageous properties such as non-toxicity, relatively low cost and high stability. Surface modifications of TiO2 were extensively proceeded in order to increase photocatalytic activity of the photocatalyst under both UV and visible light activations. The intention of this review paper was to summarize the scientific achievements devoted to developing of TiO2-based materials considered as photocatalysts for the photocatalytic degradation of acetaldehyde in air. Influence of the preparation and modification methods on the parameters of the resultant photocatalyst is reviewed and discussed in this work. Affinity of the photocatalyst surfaces towards adsorption of acetaldehyde will be described by taking into account its physicochemical parameters. Impact of the contact time of a pollutant with the photocatalyst surface is analyzed and discussed with respect to both the degradation rate and mineralization degree of the contaminant. Influence of the photocatalyst properties on the mechanism and yield of the photocatalytic reactions is discussed. New data related to the acetaldehyde decomposition on commercial TiO2 were added, which indicated the different mechanisms occurring on the anatase and rutile structures. Finally, possible applications of the materials revealing photocatalytic activity are presented with a special attention paid to the photocatalytic purification of air from Volatile Organic Compounds (VOCs).

Enhanced Visible and Ultraviolet Light-Induced Gas-Phase Photocatalytic Activity of TiO2 Thin Films Modified by Increased Amount of Acetylacetone in Precursor Solution for Spray Pyrolysis

TiO2 thin films, modified by acetylacetone (AcacH) in solution, were deposited on glass substrate by ultrasonic spray pyrolysis and tested for photocatalytic activity in a multi-section continuous flow reactor by degradation of acetone and acetaldehyde under ultraviolet and visible light. The increase in molar ratio of AcacH in respect of titanium (IV) isopropoxide (TTIP) from 1:5 to 1:8 modified the electronic structure of the films, favoring enhanced photocatalytic activity. The photocatalytic activity was enhanced approximately twofold on the film with molar ratio 1:8 under both irradiations; the film completely oxidized 10 ppm of acetone and acetaldehyde. The photocatalytic efficacy of TiO2 films in oxidation of air pollutants was three times higher compared to the industrial glass Pilkington ActivTM. Moreover, all the synthesized films indicate antibacterial efficiency against E. coli of over 99% under ultraviolet. TiO2 film, with TTIP:AcacH molar ratio 1:8 having great possibility for its commercial use as a material for indoor air purification.

Photocatalytic Degradation of Different VOCs in the Gas-Phase over TiO2 Thin Films Prepared by Ultrasonic Spray Pyrolysis

In this study, we deposited TiO2 thin films onto borosilicate glass by ultrasonic spray pyrolysis at 350 and 450 °C. The aim of study is to determine the effect of deposition temperature on photocatalytic activity of TiO2 thin films and to investigate the performance of TiO2 thin films on photocatalytic degradation of methyl tert-butyl ether (MTBE), acetone, acetaldehyde, and heptane as functions of different operating parameters. TiO2 thin films deposited at 350 and 450 °C have a thickness value of 190 and 330 nm, respectively. All as-prepared TiO2 films possess an anatase crystalline structure. According to the X-ray photon spectroscopy (XPS) study, the TiO2 thin film deposited at 350 °C showed a higher amount of oxygen vacancies and hydroxyl groups on the film surface after UV treatment. The aged-TiO2 thin film deposited at 350 °C showed a water contact angle (WCA) value of 0° after 10 min UV irradiation, showing superhydrophilic surface behavior. The TiO2 film deposited at 350 °C exhibited the highest amount of conversion of MTBE (100%). The results also showed that TiO2 films are capable of photocatalytic degradation of MTBE (100%) and acetaldehyde (approx. 80%) in humid air conditions and high airflow rate. The visible-light-activity of TiO2 thin films was tested with 5 ppm MTBE and acetone. TiO2 thin films deposited at 350 °C with a surface area of 600 cm2 showed 60% of MTBE and 33% of acetone degradation under VIS light.

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.

TiO2 thin films by ultrasonic spray pyrolysis as photocatalytic material for air purification

In this study, we showed that the TiO2 thin films deposited onto window glass are practicable for air purification and self-cleaning applications. TiO2 films were deposited onto window glass by ultrasonic spray pyrolysis method. Different deposition temperatures were used in the range of 250-450°C. The structural, morphological, optical properties and surface chemical composition were investigated to understand probable factors affecting photocatalytic performance and wettability of the TiO2 thin films. The TiO2 thin films were smooth, compacted and adhered adequately on the substrate with a thickness in the range of 100-240 nm. X-ray diffraction patterns revealed that all the TiO2 thin films consisted of anatase phase structure with the mean crystallite size in the range of 13-35 nm. The optical measurements showed that the deposited films were highly transparent (approx. 85%). The wettability test results showed that the TiO2 thin films sprayed at 350°C and 450°C and annealed at 500°C for 1 h were superhydrophilic. The photocatalytic activity of the films was tested for the degradation of methyl tert-butyl ether (MTBE) in multi-section plug-flow reactor. The TiO2 film deposited at 350°C exhibited the highest amount of conversion of MTBE, approximately 80%.

Fabrication of MoS2/TiO2 heterostructures with enhanced photocatalytic activity

Two-dimensional nanophotocatalysts MoS₂/TiO₂ with a tunable decoration amount of MoS₂ nanosheets were fabricated through a hydrothermal route.

Flower-shaped TiO2clusters for highly efficient photocatalysis

TiO₂clusters can be synthesized by a simple two-step process combining laser interference ablation with a hydrothermal reaction.

Enhanced simultaneous PEC eradication of bacteria and antibiotics by facilely fabricated high-activity {001} facets TiO2 mounted onto TiO2 nanotubular photoanode

Biohazards and coexisted antibiotics are two groups of emerging contaminants presented in various aquatic environments. They can pose serious threat to the ecosystem and human health. As a result, inactivation of biohazards, degradation of antibiotics, and simultaneous removal of them are highly desired. In this work, a novel photoanode with a hierarchical structured {001} facets exposed nano-size single crystals (NSC) TiO2 top layer and a perpendicularly aligned TiO2 nanotube array (NTA) bottom layer (NSC/NTA) was successfully fabricated. The morphology and facets of anatase TiO2 nanoparticles covered on the top of NTA layer could be controlled by adjusting precalcination temperature and heating rate as the pure NTA was clamped with glasses. Appropriate recalcination can timely remove surface F from {001} facets, and the photocatalytic activity of the resultant photoanode was subsequently activated. NSC/NTA photoanode fabricated under 500 °C precalcination with 20 °C min(-1) followed by 550 °C recalcination possessed highest photoelectrocatalytic efficiency to simultaneously remove bacteria and antibiotics. Results suggest that two-step calcination is necessary for fabrication of high photocatalytic activity NSC/NTA photoanode. The capability of simultaneous eradication of bacteria and antibiotics shows great potential for development of a versatile approach to effectively purify various wastewaters contaminated with complex pollutants.