Photocatalytic Reaction Rate Constant Research Articles

Preparation and characterization of ZrO2/TiO2 composite photocatalytic film by micro-arc oxidation

ZrO2/TiO2 composite photocatalytic film was produced on the pure titanium substrate using in-situ Zr(OH)4 colloidal particle by the micro-arc oxidation technique and characterized by scanning electron microscope (SEM), energy dispersive X-ray (EDX), X-ray diffraction (XRD) and ultraviolet-visible (UV-Vis) spectrophotometer. The composite film shows a lamellar and porous structure which consists of anatase, rutile and ZrO2 phases. The optical absorption edge of film is shifted to longer wavelength when ZrO2 is introduced to TiO2. Furthermore, the photocatalytic reaction rate constants of degradation of rhodamine B solution with ZrO2/TiO2 composite film and pure TiO2 film under ultraviolet irradiation are measured as 0.0442 and 0.0186 h−1, respectively.

Enhanced photocatalytic efficiency in zirconia buffered n-NiO/p-NiO single crystalline heterostructures by nanosecond laser treatment

We report the formation of NiO based single crystalline p-n junctions with enhanced photocatalytic activity induced by pulsed laser irradiation. The NiO epilayers were grown on Si(001) substrates buffered with cubic yttria-stabilized zirconia (c-YSZ) by using pulsed laser deposition. The NiO/c-YSZ/Si heterostructures were subsequently laser treated by 5 pulses of KrF excimer laser (pulse duration = 25 × 10−9 s) at lower energies. Microstructural studies, conducted by X-ray diffraction (θ-2θ and φ techniques) and high resolution transmission electron microscope, showed a cube-on-cube epitaxial relationship at the c-YSZ/Si interface; the epitaxial relationship across the NiO/c-YSZ interface was established as NiO⟨111 ⟩||c-YSZ⟨001⟩ and in-plane NiO⟨110⟩||c-YSZ⟨100⟩. Electron microscopy studies showed that the interface between the laser annealed and the pristine region as well as the NiO/c-YSZ interface was atomically sharp and crystallographically continuous. The formation of point defects, namely oxygen vacancies and NiO, due to the coupling of the laser photons with the NiO epilayers was confirmed by XPS. The p-type electrical characteristics of the pristine NiO epilayers turned to an n-type behavior and the electrical conductivity was increased by one order of magnitude after laser treatment. Photocatalytic activity of the pristine (p-NiO/c-YSZ/Si) and the laser-annealed (n-NiO/p-NiO/c-YSZ/Si) heterostructures were assessed by measuring the decomposition rate of 4-chlorophenol under UV light. The photocatalytic reaction rate constants were determined to be 0.0059 and 0.0092 min−1 for the as-deposited and the laser-treated samples, respectively. The enhanced photocatalytic efficiency was attributed to the suppressed charge carrier recombination in the NiO based p-n junctions and higher electrical conductivity. Besides, the oxygen vacancies ease the adsorption of 4-chlorophenol, hydroxyl, and water molecules to the surface. Thus, n-NiO/p-NiO single crystalline catalysts can be introduced as a potent candidate to remediate the environmental pollution.

Analysis of photocatalytic performance of nanostructured pyrogenic titanium dioxide powders in view of their polydispersity and phase transition: Critical anatase particle size as a factor for suppression of charge recombination

Series of TiO2 nanopowders from flame aerosol synthesis with average particle size from 7.1 to 21nm with increased rutile content were studied for particle size distribution (PSD) and anatase-to-rutile transition to be correlated with their photocatalytic performance. The certain particles’ growth regime favours the formation of defect-rich anatase nanoparticles which were found to condition the overall reactivity of polydispersed nanostructured powders. Two crucial roles of oxygen vacancies as key characteristics of nanoparticles are discussed: (a) they are responsible for anatase-to-rutile transition and (b) are vital for the adsorption of oxygen as electron acceptor, i.e. for the photoelectrochemical efficiency. The linear dependence of the photocatalytic oxidation reaction rate constant on the content of the specific anatase particles fraction amenable to phase transition was revealed by the regression analysis.

Thin-Film Epitaxy and Enhancement of Photocatalytic Activity of Anatase/Zirconia Heterostructures by Nanosecond Excimer Laser Treatment

We present a novel method to improve the photocatalytic efficiency of epitaxial c-axis anatase TiO2 thin films by a factor of 2 by using nanosecond laser annealing. The anatase films were epitaxially grown by pulsed laser deposition on Si(001) substrates, where a tetragonal yttria-stabilized zirconia (t-YSZ) buffer was used to effectively remove the native SiOx layer from the substrates prior to deposition of anatase. With the information from X-ray and TEM diffraction patterns, the epitaxial relationship across the interfaces was shown to be: (001)[110]anatase||(001)[110]t-YSZ||(001)[001]silicon. Performing high-temperature XRD, we observed that the anatase epilayers were stable up to 1100 °C, far beyond the normal anatase-to-rutile transition temperature (approximately 600–700 °C). The samples were subsequently laser-annealed in air by a single pulse of KrF excimer laser beam at an energy density of ∼0.3 J.cm–2. On the basis of the detailed HRTEM studies, the interface between the laser annealed and the pristine region as well as the anatase/t-YSZ interface were crystallographically continuous. The XPS results revealed the presence of point defects, and the XRD patterns showed some reduction and broadening after laser treatment. Photocatalytic activity of the pristine and the laser-annealed heterostructures was assessed by measuring the decomposition rate of 4-chlorophenol under UV light. The photocatalytic reaction rate constants were determined to be 0.0077 and 0.0138 min–1 for the as-deposited and the laser-treated samples, respectively. Such an appreciable enhancement was attributed to the formation of point defects, in particular, oxygen vacancies, near the surface of the anatase thin films. Oxygen vacancies facilitate adsorption of 4-chlorophenol, hydroxyl, and water molecules to the titania surface. In addition, these defects trap the photogenerated electrons, giving rise to charge separation and, hence, improvement of the photocatalytic efficiency. The nanosecond photochemical enhancement may lead to fabrication of smart photocatalytic materials to address the needs of future photochemical devices.

Growth mechanism and photocatalytic properties of flower-like Bi2WO6 powders synthesized by a microwave hydrothermal method

Pure orthorhombic phase Bi2WO6 powders were synthesized by a microwave hydrothermal method in the absence of surfactants and templates, using Bi(NO3)3·5H2O and Na2WO4·2H2O as raw materials. Photocatalytic properties of the samples prepared at different reaction temperatures were also studied with Rhodamine B (RhB) solution as the target catabolite under visible light. The results indicate that flower-like Bi2WO6 powders can be obtained by controlling the microwave reaction temperatures in the absence of any additives. The growth of flower-like Bi2WO6 powders is a multistage layer assembly process, in which the flower-like Bi2WO6 self-assembling with the uniform size about 2 μm is synthesized at 180 °C. At the same time, the photocatalytic reaction rate constant (k) gets up to 0.04167/min and the degradation rate of RhB solution is more than 96 % after being irradiated under visible light for 70 min.

Defect mediated photocatalytic decomposition of 4-chlorophenol on epitaxial rutile thin films under visible and UV illumination

We show that pure rutile TiO2 can be photo-responsive even under low energy visible light after annealing in vacuum where we envisage that the point defects, i.e. oxygen vacancies and titanium interstitials, serve an important role. In this study, single crystal rutile films were grown by the pulsed laser deposition technique and then vacuum annealed under different oxygen pressures to introduce defects into their lattices. 4-chlorophenol was selected as a model material and decomposed by the annealed TiO2 films where the maximum photocatalytic reaction rate constants were determined as 0.0107 and 0.0072 min−1 under UV and visible illumination. Epitaxial growth along the [200] direction was confirmed by φ-scan and 2θ-scan XRD and the epitaxial relationship between the rutile film and the c-sapphire substrate was explained as . The formation of atomically sharp interfaces and the epitaxial growth were ascertained by annular dark-field STEM imaging. Based on the XPS, UV–vis and PL spectroscopy results, it was found that the defect concentration increased after annealing under lower pressures, e.g. 5 × 10−6 Torr. In contrast, more perfect crystals were obtained when the films were annealed under high oxygen pressures, namely 5 × 101 Torr. The morphology of the films was also investigated by employing an AFM technique. It was observed that increase of the annealing pressure results in the formation of larger grains. It was also found that the electrical resistivity of the rutile films strongly increased by about three orders of magnitude when the annealing pressure increased from 5 × 10−4 to 5 × 101 Torr.

Phase Tuning, Thin Film Epitaxy, Interfacial Modeling, and Properties of YSZ-Buffered TiO2 on Si(001) Substrate

We have studied systematically the influence of pulsed laser deposition variables on microstructure and properties of TiO2 epitaxial thin films where integration with Si(100) substrate was achieved by cubic yttria-stabilized-zirconia (c-YSZ) buffer layer. Details of crystallographic and atomic arrangements across the interfaces are discussed in the light of the domain matching epitaxy paradigm. The single crystalline rutile films were obtained at higher substrate temperatures and lower oxygen pressures; whereas, the growth of epitaxial anatase films was promoted by decreasing the temperature and increasing the oxygen pressure. We showed that crystallographic structure of the TiO2 films is determined mainly by the termination structure of the c-YSZ layer and the bonding characteristics across the TiO2/c-YSZ interface. Using 2θ and φ scans of XRD, the epitaxial relationship between Si(100) substrate and the zirconia buffer layer was shown to be cube-on-cube: (001)[100]YSZ∥(001)[100]Si. Furthermore, the epitaxial relationships at the rutile/zirconia and the anatase/zirconia interfaces were determined as (100)[01̅1]rutile∥(001)[010]YSZ and (001)[110]anatase∥(001)[100]YSZ, respectively. The proposed crystallographic arrangements as well as the epitaxial growth were confirmed by high resolution TEM diffraction and imaging. XPS and XRD studies showed less defect content and favorable crystallinity in the films grown at higher temperatures. AFM results revealed that the finest domain size and the smoothest surface were obtained at the intermediate deposition temperatures. Based on the four-point electrical measurements, the heterostructures deposited at 500 °C were more conductive than those grown at 300 and 700 °C. Photocatalytic activity of the films was studied through decomposition of 4-chlorophenol under UV illumination. The maximum photocatalytic reaction rate constants were determined as 0.0124 and 0.0087 min–1 for the anatase and the rutile films grown at 500 °C.

UV and visible photodecomposition of organic pollutants over micro arc oxidized Ag-activated TiO2 nanocrystalline layers

Although titania is one of the most important metal oxide photocatalysts, its practical applications are to some extent limited by its relatively wide band gap. Doping TiO2 by metallic species has been expressed as a promising solution to decrease its band gap energy and enhance its photocatalytic performance. In this research, we employed micro arc oxidation technique to grow Ag-doped TiO2 porous layers where influence of the electrolyte composition and the applied voltage on the surface morphology, topography, phase structure, chemical composition, and optical properties was investigated for the first time. The photocatalytic efficiency of the layers was studied using different model materials namely methylene blue and 4-chlorophenol solutions as well as gaseous benzene. A porous morphology with a rough surface was revealed by the SEM and AFM techniques. The pore size varied depending on the voltage and the electrolyte concentration. Compositional studies, conducted by XRD and XPS methods, showed that the fabricated layers mainly consisted of anatase phases. A small amount of rutile phase was also detected at high voltages, i.e. 500V. Considering the XRD patterns, the anatase average crystalline size was determined as 67.8 and 21.5nm for the pure TiO2 and silver doped TiO2 layers, respectively. A red-shift was observed in the absorbance edge of the layers when Ag was introduced into the titania lattice giving rise to a visible-light response. Comparing photocatalytic reaction rate constants of pure and Ag-doped titania, the doped layers exhibited more favorable potential to decompose the pollutants in aqueous phase under ultraviolet and visible lights. Moreover, the doped layers were useful to decompose the pollutants in gaseous phase under ultraviolet light.

In situ growth of vanadia–titania nano/micro-porous layers with enhanced photocatalytic performance by micro-arc oxidation

Micro-arc oxidation process was used to synthesize V 2O 5–TiO 2 porous layers for the first time. Surface morphology and topography of the layers were investigated by scanning electron microscope (SEM) and atomic force microscope (AFM). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques were also employed to evaluate phase structure and chemical composition of the layers. It was found that the V 2O 5–TiO 2 layers consisted of anatase, rutile, and vanadium pentoxide phases fraction of which varied with the applied voltage and the electrolyte concentration. It was also revealed that pore size and surface roughness increased with the applied voltage and the electrolyte concentration. Optical properties of the layers were studied by a UV–vis spectrophotometer, and the band gap energies of the MAO-grown pure TiO 2 and V 2O 5–TiO 2 layers were respectively calculated as 3.21 and 2.56 eV. Furthermore, the composite layers exhibited a significantly enhanced photo-activity when compared to pure TiO 2 layers. The photocatalytic reaction rate constants of degradation of methylene blue on the surface of the V 2O 5–TiO 2 layers under ultraviolet and visible irradiations were measured as 0.0228 and 0.0117 min −1, respectively. As a consequence, micro-arc oxidation was deduced to be an appropriate and efficient method for synthesis of V 2O 5–TiO 2 porous layers.