Nanostructured TiO2 Photocatalysts Research Articles

Surface Defect Engineering in Colored TiO2 Hollow Spheres Toward Efficient Photocatalysis

AbstractNanostructured TiO2 is one of the best materials for photocatalysis, thanks to its high surface area and surface reactivity, but its large energy bandgap (3.2 eV) hinders the use of the entire solar spectrum. Here, it is proposed that defect‐engineered nanostructured TiO2 photocatalysts are obtained by hydrogenation strategy to extend its light absorption up to the near‐infrared region. It is demonstrated that hydrogenated or colored TiO2 hollow spheres (THS) composed of hierarchically assembled nanoparticles result in much broader exploitation of the solar spectrum (up to 1200 nm) and the engineered surface enhances the photogeneration of charges for photocatalytic processes. In turn, when applied for photodegradation of a targeted drug (Ciprofloxacin) this results in 82% degradation after 6 h under simulated sunlight. Valence band analysis by photoelectron spectroscopy revealed the presence of oxygen vacancies, whose surface density increases with the hydrogenation rate. Thus, a tight correlation between degree of hydrogenation and photocatalytic activity is directly established. Further insight comes from electron paramagnetic resonance, which evidences bulk Ti3+ centers only in hydrogenated THS. The results are anticipated to disclose a new path toward highly efficient photocatalytic titania in a series of applications targeting water remediation and solar fuel production.

Electroless Ni plated nanostructured TiO2 as a photocatalyst for solar hydrogen production.

Herein, we have demonstrated a facile electroless Ni coated nanostructured TiO2 photocatalyst for the first time. More significantly the photocatalytic water splitting shows excellent performance for hydrogen production which is hitherto unattempted. The structural study exhibits majorly the anatase phase along with the minor rutile phase of TiO2. Interestingly, electroless nickel deposited on the TiO2 nanoparticles of size 20 nm shows a cubic structure with nanometer scale Ni coating (1-2 nm). XPS supports the existence of Ni without any oxygen impurity. The FTIR and Raman studies support the formation of TiO2 phases without any other impurities. The optical study shows a red shift in the band gap due to optimum nickel loading. The emission spectra show variation in the intensity of the peaks with Ni concentration. The vacancy defects are pronounced in lower concentrations of Ni loading which shows the formation of a huge number of charge carriers. The electroless Ni loaded TiO2 has been used as a photocatalyst for water splitting under solar light. The primary results manifest that the hydrogen evolution of electroless Ni plated TiO2 is 3.5 times higher (1600 μmol g-1 h-1) than pristine TiO2 (470 μmol g-1 h-1). As shown in the TEM images, nickel is completely electroless plated on the TiO2 surface, which accelerates the fast transport of electrons to the surface. It suppresses the electron-hole recombination drastically which is responsible for higher hydrogen evolution using electroless Ni plated TiO2. The recycling study exhibits a similar amount of hydrogen evolution at similar conditions which shows the stability of the Ni loaded sample. Interestingly, Ni powder loaded TiO2 did not show any hydrogen evolution. Hence, the approach of electroless plating of nickel over the semiconductor surface will have potential as a good photocatalyst for hydrogen evolution.

Advanced titanium dioxide fluidizable nanowire photocatalysts.

In photocatalytic water splitting, fluidization is known to minimize the adverse effects of mass-transfer, poor radiation distribution, parasitic back-reactions and photocatalyst handling difficulties, which limit the scalability of immobilized-film and suspended slurry photocatalysts. Fluidization of one-dimensional TiO2 photocatalyst particles, such as nanorods, -wires and -ribbons, is highly desired as it further enhances the efficiency of photocatalytic reaction, due to their peculiar photo-electrochemical characteristics that result in more effective separation of photo-generated charges and absorption of photons. However, the harsh physical environment of a fluidized bed reactor does not readily allow for nanostructured TiO2 photocatalysts, as the fine features would be quickly removed from the particle surface. Here, we propose a scalable method for fabrication of rutile TiO2 nanorods on porous glass beads as a 3D protective substrate to reduce the attrition rate caused by fluidization. The quality of the synthesized nanorod films was optimized through controlling a growth quality factor, Rq, allowing for good quality films to be grown in different batch amounts and different hydrothermal reactor sizes. The utilization of porous glass beads substrate has reduced the attrition rate, and the protective features of the particles reduced the rate of attrition by an order of magnitude, compared to a particulate photocatalyst, to near negligible levels. Such considerably reduced attrition makes the as-developed porous glass beads supported rutile TiO2 nanorods a viable fluidizable photocatalyst candidate for various applications, including water splitting and degradation of organic compounds.

Nanostructured TiO2 photocatalyst modified with Cu for improved imidacloprid degradation

We investigated the role of Cu(II) ions, their local atomic structure and chemical state in the Cu-modified immobilized nanostructured TiO2 photocatalysts used for imidacloprid degradation under UV/VIS-radiation. Catalyst samples were prepared via electrochemical anodization and modified with different Cu concentrations (0.2–1 M). The catalysts’ structure and morphology were characterized by X-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopy, UV–VIS-NIR spectroscopy, Photoluminescence spectroscopy and X-ray absorption spectroscopy: extended X-ray absorption fine structure and X-ray absorption near edge structure. The EXAFS results indicate formation of Ti–O–Cu connections, suggesting that the Cu(II) cations are attached to TiO2 surface which contributes to the photocatalytic activity improvement, i.e. Cu(II) ions are free electron traps that reduce high recombination rate of photo generated charge carriers. The results here demonstrate the role of Cu(II) cations on photocatalytic activity of the Cu@TiO2 photocatalyst.

A Comparative Study of Microcystin-LR Degradation by UV-A, Solar and Visible Light Irradiation Using Bare and C/N/S-Modified Titania

In an endeavor to tackle environmental problems, the photodegradation of microcystin-LR (MC-LR), one of the most common and toxic cyanotoxins, produced by the cyanobacteria blooms, was examined using nanostructured TiO2 photocatalysts (anatase, brookite, anatase–brookite, and C/N/S co-modified anatase–brookite) under UV-A, solar and visible light irradiation. The tailoring of TiO2 properties to hinder the electron–hole recombination and improve MC-LR adsorption on TiO2 surface was achieved by altering the preparation pH value. The highest photocatalytic efficiency was 97% and 99% with degradation rate of 0.002 mmol L−1 min−1 and 0.0007 mmol L−1 min−1 under UV and solar irradiation, respectively, using a bare TiO2 photocatalyst prepared at pH 10 with anatase to brookite ratio of ca. 1:2.5. However, the bare TiO2 samples were hardly active under visible light irradiation (25%) due to a large band gap. Upon UV, solar and vis irradiation, the complete MC-LR degradation (100%) was obtained in the presence of C/N/S co-modified TiO2 with a degradation rate constant of 0.26 min−1, 0.11 min−1 and 0.04 min−1, respectively. It was proposed that the remarkable activity of co-modified TiO2 might originate from its mixed-phase composition, mesoporous structure, and non-metal co-modification.

Fabrication of a nanostructured TiO2 photocatalyst using He plasma-irradiated tungsten and ethylene gas decomposition

Decomposition of ethylene gas by a photocatalyst is an environmentally friendly and sustainable process. We fabricated titanium dioxide thin films deposited on nanostructured tungsten substrates formed by exposure to helium plasma and evaluated their photocatalytic performance. The nanostructures were maintained even after sputtering and annealing, and the proportion of tungsten inside the thin film increased with increasing oxidation temperature. The photocatalytic activity was improved due to formation of the nanostructure.

Formation of Micro- and Nanostructured TiO2 Films by Anodic Oxidation for Enhanced Photocatalytic Activities

This paper presents growth of micro- and nanostructured TiO2 layers on Ti6Al4 V by thermal and anodic spark oxidation for photocatalysis. Effects of anodization parameters on surface morphology and photocatalytic behavior were investigated. The photocatalysts divided into two groups consisting of four samples were prepared for growth of micro- and nanostructured TiO2 layer on Ti6Al4 V. The microstructured surfaces were obtained in 1 M H2SO4 electrolyte in 5 min at the anodization voltage in the range of 100–400 V, whilst the nanostructured TiO2 layers were prepared in 1% HF solution in the range of 10–40 V in equal time. Surface morphology, phase structure, band gap and surface roughness values of the micro- and nanostructured TiO2 photocatalysts were analyzed by scanning electron microscope, X-ray diffraction, UV–Vis diffuse reflectance spectroscopy and surface profilometer, respectively. Photocatalytic performances of TiO2 photocatalysts obtained with different anodization parameters were investigated via methylene blue aqueous solution. It was observed that the anodization voltage has a great effect on photocatalytic performances due to the formation of different crystallinity and morphology. Among the samples, the layer prepared at 100 V exhibited the best photocatalytic activity with 91.15% degradation efficiency and 95.5 × 10−4 min−1 kinetic rate constant. Moreover, the further investigation determining active species were carried out using ammonium oxalate, benzoquinone, and isopropyl alcohol. In this process, the photocatalytic mechanism revealed that the superoxide radicals $$(^{ \cdot } {\text{O}}_{ 2}^{ - } )$$ play a very important role during the photodegradation process. The TiO2 layer on Ti6Al4 V anodized at 100 V also showed high activity and stability, suggesting its potential for wastewater treatment.

Optimized Nanostructured TiO2 Photocatalysts

Titania is the most widely studied photocatalyst. In it’s mixed-phase configuration (anatase-rutile form) -as manifested in the commercially available P25 Degussa material- titania was previously found to exhibit the best photocatalytic properties reported for the pure system. A great deal of published research by various workers in the field have not fully explained the underlying mechanism for the observed behavior of mixed-phase titania photocatalysts. One of the prevalent hypothesis in the literature that is tested in this work involves the presence of small, active clusters of interwoven anatase and rutile crystallites or “catalytic “hot-spots””. Therefore, non-woven nanofibrous mats of titania were produced and upon calcination the mats consisted of nanostructured fibers with different anatase-rutile ratios. By assessing the photocatalytic and photoelectrochemical properties of these samples the optimized photocatalyst was determined. This consisted of TiO2 nanostructures annealed at 500˚C with an anatase /rutile content of 90/10. Since the performance of this material exceeded that of P25 complete structural characterization was employed to understand the catalytic mechanism involved. It was determined that the dominant factors controlling the photocatalytic behavior of the titania system are the relative particle size of the different phases of titania and the growth of rutile laths on anatase grains which allow for rapid electron transfer between the two phases. This explains how to optimize the response of the pure system.

Synthesis of Nanostructured TiO<sub>2</sub> Photocatalyst with Ultrasonication at Low Temperature

A thin film TiO2 in hierarchical nano-structure with high photocatalytic activities was synthesized in simple steps with ultrasonication. The crystal structure and morphology of the photocatalyst were investigated by X-ray diffraction (XRD) and high-resolution field emission scanning electron microscope (FE-SEM). In the present work, nanostructured TiO2 was directly formed onto a Ti substrate via a solution approach. This nanostructured TiO2 photocatalyst can be reused and will not generate secondary contamination to treated water. The photocatalytic activity of the synthesized TiO2 photocatalyst was evaluated by the degradation of phenol under UVC irradiation in water and was compared with the general sol-gel derived TiO2 films as well as a commercial DP-25 TiO2 coating. It was found that the synthesized nanostructured TiO2has significantly high and stable photocatalytic activity.

Hydrogenation of Nano-Structured TiO2 Photocatalyst Through an Electrochemical Method.

Nano-structured photocatalyst P25 was electrochemically hydrogenated through being immersed in 0.01 M NaOH solution to act as cathode to electrolyze water. With its color changed from white to bluish, its absorption in visible light range was obviously enhanced after the treatment, and its solar-driven photocatalytic activity was significantly improved for photo-degradation of methylene blue and rhodamine B. XPS spectra analyses were conducted and the photocurrent was measured, which indicate the formation of Ti3+ and oxygen vacancies, and increased separation efficiency of photoactivated electrons and holes in electrochemically hydrogenated P25. These changes are proposed responsible for the improved photocatalytic activity of P25. Electrochemical hydrogenation has been proven a facile and effective method for improving the photocatalytic activity of nano-structured TiO2 photocatalyst.

Novel urchin-like Fe2O3@SiO2@TiO2 microparticles with magnetically separable and photocatalytic properties

A novel 3D spiny nanostructured TiO2 photocatalyst with a γ-Fe2O3 core possesses good activity under UVC light, reusability and magnetically separable properties.

Synthesis of high surface area TiO2 nanoparticles by mild acid treatment with HCl or HI for photocatalytic propene oxidation

Nanostructured TiO2 photocatalysts with small crystalline sizes have been synthesized by sol–gel using the amphiphilic triblock copolymer Pluronic P123 as template. A new synthesis route, based on the treatment of TiO2 xerogels with acid–ethanol mixtures in two different steps, synthesis and extraction–crystallization, has been investigated, analyzing two acids, hydrochloric and hydriodic acid. As reference, samples have also been prepared by extraction–crystallization in ethanol, being these TiO2 materials amorphous and presenting higher porosities. The prepared materials present different degrees of crystallinity depending on the experimental conditions used. In general, these materials exhibit high surface areas, with an important contribution of microporosity and mesoporosity, and with very small size anatase crystals, ranging from 5 to 7nm.The activity of the obtained photocatalysts has been assessed in the oxidation of propene in gas phase at low concentration (100ppmv) using a UVA lamp with a wavelength of 365nm. Under the conditions studied, these photocatalysts show different activities in the oxidation of propene which do not depend on their surface areas, but on their crystallinity and band gap energies. Sample prepared with HCl both during synthesis and in extraction–crystallization steps is the most active one, with superior performance than Evonik P25.

Solvothermal synthesis of nanostructured TiO2 photocatalyst in supercritical CO2 fluids

Nano-sized titanium dioxide TiO2 photocatalysts were successfully prepared by solvothermal method under various supercritical conditions. A solvothermal process with the starting precursor of tetrabutyl orthotitanate (TBOT) and ethanol mixture in supercritical CO2 (SC–CO2) environment under different reaction temperature and pressure has been developed for the fabrication of nanostructured TiO2. Nano-sized TiO2 samples obtained exhibited various morphologies and different photocatalytic ability. The influence of reaction temperature and pressure on morphologies and photocatalytic activity of TiO2 products was discussed and analyzed according to the Ostwald Ripening Rules. Characterization of the TiO2 products consisted of XRD, SEM and TEM methods and photocatalytic activity was evaluated by the degradation of methyl orange (MO).

Metal Doped TiO<sub>2</sub> Photocatalysts for CO<sub>2</sub> Photoreduction

Greenhouse gases such as CO2, CH4 and CFCs are the primary causes of global warming. Worldwide, people are exploring techniques to reduce, capture, store CO2 gas and even convert this gas in to some useful chemicals. CO2 can be transformed into hydrocarbons in a photocatalytic reaction. The advantage of photo reduction of CO2 is to use inexhaustible solar energy. Knowledge of elementary steps in photocatalytic CO2 reduction under UV irradiation is required in order to improve the photo efficiency of the photocatalyst. A semiconductor photocatalyst mediating CO2 reduction and water oxidation needs to absorb light energy, generate electron hole pairs, spatially separate them, transfer them to redox active species across the interface and minimize electron hole recombination. This requires the semiconductor to have its conduction band electrons at higher energy compared to the CO2 reduction potential while the holes in the valence band need to be able to oxidize water to O2. A single semiconductor does not usually satisfy these requirements. Some recent developments in this field have been moves towards rational photocatalyst design, the use of highly active isolated Ti-species in mesoporous and microporous materials, metal-doping of TiO2, development of catalysts active at longer wavelengths than can be achieved with commercially available titania etc. The use of transition-metal loaded titanium dioxide (TiO2) has been extensively studied as a photocatalyst in photoreactions. Unlike traditional catalysts drive chemical reactions by thermal energy, semiconducting photocatalysts can induce chemical reactions by inexhaustible sunlight and convert CO2 in to the useful hydrocarbons. In this review article we will cover different aspects of metal doped nano structured TiO2 photocatalysts, used to convert/reduce CO2 in to useful hydrocarbons.

TiO2 modifications by hydrothermal treatment and doping to improve its photocatalytic behaviour under visible light

Heterogeneous photocatalysis based on TiO2 materials is an interesting method for the treatment of polluted water because it allows degradation of a wide variety of organic contaminant compounds. The performance of TiO2 based photocatalysts was studied by aqueous solutions of methylene blue (MB) as a model contaminant compound by using 50mgL−1 as initial water contaminant concentration in order to compare the photocatalytic behaviour of TiO2 Degussa P25 and some synthesized photocatalysts by hydrothermal treatment (nanostructured TiO2 and metal-doped nanostructured TiO2 photocatalysts: nanostructured Co–TiO2, nanostructured Fe–TiO2 and nanostructured Mn–TiO2) under UV and visible light irradiation. Photocatalytic materials characterization was carried out by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption–desorption, mercury porosimetry, XPS and diffuse reflectance UV–vis spectra. An improvement of the photocatalytic activity has been observed when nanostructured and doped photocatalysts were used under visible light irradiation, concluding that the hydrothermal treatment produces a nanostructuration of commercial titania by the formation of nanoparticles, which results in a large decrease of average crystallite size of undoped and doped titania, and in an important increase of BET specific area and total pore area.

Nanostructured TiO2 photocatalysts for the determination of organic pollutants

Owing to the inherent advantages of nanostructured TiO2 photocatalysts, including high photocatalytic activity, strong oxidation power, low cost, environmental benignity and excellent stability, TiO2 photocatalyts have recently attracted extensive attention from scientific researchers, technology developers and investors for use in sensing applications. The TiO2 sensors can be used for lab-based analyses, on-line and on-site determination of organic pollutants in wastewater. This work reviews the application of TiO2 nanomaterials in photocatalytic and photoelectrocatalytic monitoring of aggregative organic parameters such as total organic carbon (TOC) and chemical oxygen demand (COD), as well as individual organic compounds in aqueous solution.

Characterization, activity and mechanisms of a visible light driven photocatalyst: Manganese and iron co-modified TiO2 nanoparticles

An attempt was made to prepare Mn,Fe-codoped nanostructured TiO2 photocatalyst for visible light assisted degradation of an azo dye (methylene blue) in aqueous solutions by a sol-gel process. The asprepared nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL) techniques. The photocatalytic activity of Mn,Fe-codoped TiO2 catalyst was evaluated by measuring degradation rates of methylene blue (MB) under visible light. The results showed that doping with the manganese and iron ions significantly enhanced the photocatalytic activity for MB degradation under visible light irradiation. This was ascribed to the fact that a small amount of manganese and iron dopants simultaneously increased MB adsorption capacity and separation efficiency of electron-hole pairs. The results of DRS showed that Mn,Fe-codoped TiO2 had significant absorption between 400 and 500 nm, which increased with the increase of manganese ion content. It is found that the stronger the PL intensity, the higher the photocatalytic activity. This could be explained by the points that PL spectra mainly resulted from surface oxygen vacancies and defects during the process of PL, while surface oxygen vacancies and defects could be favorable in capturing the photoinduced electrons during the process of photocatalytic reactions, so that the recombination of photoinduced electrons and holes could be effectively inhibited.

Effect of thermal spray processes on anatase–rutile phase transformation in nano-structured TiO2 photo-catalyst coatings

TiO2 photo-catalytic coatings were deposited using three different thermal spray techniques, namely, flame, plasma and high velocity oxy-fuel (HVOF). TiO2 nanopowders were used as feedstock. X-ray diffraction analysis and scanning electron microscopy were used to study the deposited TiO2 coating. It was concluded that a high anatase content of 80 vol.% can be achieved for the coating deposited by HVOF if kerosene is used as fuel. On the contrary, the anatase contents in the coating deposited by the flame and plasma spray processes were reduced to 29 and 5 vol.% respectively due to their structural transformation of anatase to rutile. Microstructure shows that the as sprayed HVOF TiO2 coating had nanostructure characteristics with smaller grain size than feedstock powder. It was found that the structural transformation of anatase to rutile is significantly influenced by the thermal spray processes. Liquid fuelled HVOF technique can prevent this transformation.

TiO2 Nanopowder Catalyzed Microwave-Induced One-Pot Synthesis of Novel Quinoline/Benzo[h]quinoline3-carbonitrile Under Solvent Free Conditions

A rapid and efficient method for the synthesis of various carbonitrile quinoline/benzo[h]quinolines has been developed through the Knoevenagel condensation, Michael addition of aromatic aldehyde/amine with carbonyl compounds in the presence of a nanostructured TiO2 photocatalyst by microwave irradiation under solvent-free conditions.

Nanostructured TiO2 photocatalyst and pump-probe spectroscopic study

Titanium dioxide (TiO2), with its large band gap, has attracted much attention due to its excellent photocatalytic activity. TiO2 ball-shaped nano-particles were deposited on silicon substrates by a thermal oxidation approach. With an increase in the annealing temperature the surface morphology and the structure of TiO2 remained stable, exhibiting good heat stability; meanwhile, the hydrogen production rate also increased. The femtosecond pump-probe spectroscopic study showed that the lifetime of carriers of the samples as- deposited and post-annealed at different temperatures were longer than 20 ps.