Photocatalytic Kinetics Research Articles

In situ formation of Ag/ZnO heterostructure arrays during synergistic photocatalytic process for SERS and photocatalysis

Abstract ZnO nanorod arrays were used to oxidize rhodamine B (RhB) and reduce Ag+ ions simultaneously under UV or visible light. The photocatalytic kinetics of RhB degradation followed a pseudo-first order with two steps, and during the synergistic photocatalysis process, Ag/ZnO heterostructural nanocomposites were generated in situ based on ZnO nanorod arrays. The Ag/ZnO heterostructure arrays were composed of Ag nanosheets with (111) plane on the basis of ZnO nanorods. The growth process of Ag nanosheets experienced a similar two-stage manner as the degradation kinetics of RhB, indicating that the change in apparent rate constants for RhB degradation was ascribed to the variation of photoinduced electrons and holes. The microstructure, composition, chemical states, and UV–visible light adsorption of reclaimed Ag/ZnO heterostructure arrays were investigated; their multifunctional applications on surface enhanced Raman scattering and photocatalytic activity under UV or simulated solar light were also discussed.

Preparation of nano-TiO2/diatomite-based porous ceramics and their photocatalytic kinetics for formaldehyde degradation

Diatomite-based porous ceramics were adopted as carriers to immobilize nano-TiO2 via a hydrolysis-deposition technique. The thermal degradation of as-prepared composites was investigated using thermogravimetric–differential thermal analysis, and the phase and microstructure were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and transmission electron microscopy. The results indicated that the carriers were encapsulated by nano-TiO2 with a thickness of 300–450 nm. The main crystalline phase of TiO2 calcined at 650°C was anatase, and the average grain size was 8.3 nm. The FT-IR absorption bands at 955.38 cm−1 suggested that new chemical bonds among Ti, O, and Si had formed in the composites. The photocatalytic (PC) activity of the composites was investigated under UV irradiation. Furthermore, the photodegradation kinetics of formaldehyde was investigated using the composites as the cores of an air cleaner. A kinetics study showed that the reaction rate constants of the gas-phase PC reaction of formaldehyde were κ = 0.576 mg·m–3·min–1 and K = 0.048 m3/mg.

Facile synthesis of BiFeO3 nanosheets with enhanced visible-light photocatalytic activity

Single-crystalline BiFeO3 nanosheets with rectangular shape and exposed {101} facets were successfully synthesized via a facile hydrothermal method with low reaction temperature and short time (130 °C for 14 h). The process has advantages of energy saving, template and surfactant free, and no additional equipment in required. The average side lengths of BiFeO3 nanosheets are around 140 and 230 nm, and thickness is about 30 nm. As a result, the BiFeO3 nanosheets photocatalyst reaches as high as 89% of photodegradation efficiency of rhodamine B under 180 min visible light irradiation, which is about 4.68 and 2.41 times that of BiFeO3 powders prepared by solid-state reaction and sol–gel process respectively. The BiFeO3 nanosheets photocatalyst also exhibits a high reusability and storage stability for the photodegradation reaction. The internal electric fields produced due to the ferroelectric nature are perpendicular to the surfaces of BiFeO3 nanosheets, which can promote the separation efficiency of photoinduced charges along [101] direction. While the nanoscale thickness structure can shorten the separation distance of photoinduced charges along [101] direction. These two factors all greatly suppress the recombination rate of e−/h+ pairs, then leading to the improved photocatalytic kinetics of BiFeO3 nanosheets.

Connecting contact angle evolution to photocatalytic kinetics of self cleaning surfaces

We demonstrate that the Cassie-Baxter model of contact angle for a composite surface consisting of “bare” and “covered” components (typically adventitious or purposeful carbonaceous deposits) can be used for quantitative determination of the kinetics of “self-cleaning” photocatalyst surfaces. In particular, literature data is analyzed to demonstrate examples of zero and first order reaction, as well as quantitative determination of rate constant values for two key processes: the initial activation of new photocatalysts, and the removal of purposeful fatty acid deposits such as that required in the ISO standard 27448: 2009 for activity determination of photocatalytic self-cleaning surfaces.

Intermittent photocatalytic activity of single CdS nanoparticles

Semiconductor photocatalysis holds promising keys to address various energy and environmental challenges. Most studies to date are based on ensemble analysis, which may mask critical photocatalytic kinetics in single nanocatalysts. Here we report a study of imaging photocatalytic hydrogen production of single CdS nanoparticles with a plasmonic microscopy in an in operando manner. Surprisingly, we find that the photocatalytic reaction switches on and off stochastically despite the fact that the illumination is kept constant. The on and off states follow truncated and full-scale power-law distributions in broad time scales spanning 3-4 orders of magnitude, respectively, which can be described with a statistical model involving stochastic reactions rates at multiple active sites. This phenomenon is analogous to fluorescence photoblinking, but the underlying mechanism is different. As individual nanocatalyst represents the elementary photocatalytic platform, the discovery of the intermittent nature of the photocatalysis provides insights into the fundamental photochemistry and photophysics of semiconductor nanomaterials, which is anticipated to substantially benefit broad application fields such as clean energy, pollution treatment, and chemical synthesis.

Photocatalytic performance of Pt-TiO2, Pt-N-TiO2 and Pt-N/F-TiO2 towards simultaneous Cr(VI) reduction/benzoic acid oxidation: Insights into photogenerated charge carrier dynamics and catalyst properties

Abstract Pt-TiO2, Pt-N-TiO2 and Pt-N/F-TiO2 photocatalysts were prepared with different Pt contents (0.5%, 1.0% and 1.5% wt.) via a sol-gel impregnation method. In all cases, XRD analysis showed mainly (>90%) formation of anatase phase with ca. 10 nm particle diameter and about 10% brookite phase fraction with 6–8 nm particle diameter. UV–vis Diffuse Reflectance Spectroscopy (DRS) showed that visible light photoactivity is influenced by the type of doping atoms. N/F co-doped TiO2 presented higher visible light absorption than N-doped-TiO2. XPS revealed that platinum existed mainly in Pt2+ and Pt2+, Pt4+ for Pt-TiO2 and Pt-N(N/F)-TiO2 catalysts, respectively. Electron Paramagnetic Resonance (EPR) spectroscopy was used to investigate the photodynamics of hole-electron (h+-e−) pairs. In all cases, the amount of Pt-loading affected decisively the photoinduced e− (both surface and lattice electrons) and simultaneously enhanced the life-time of holes (h+). The optimal h+/e− balance ratio was detected for 0.5% wt. Pt-loading in all TiO2, N-TiO2 and N/F-TiO2 materials. The EPR data on the e−/h+ ratio was consistent with the observed photocatalytic kinetics towards the simultaneous Cr(VI) reduction and benzoic acid oxidation. Pt showed to improve the visible-light absorption efficiency as well as the e− transfer to Cr(VI). Accordingly, this mechanism explains the non-obvious observed trend for Cr(VI) reduction, i.e. Pt-N-TiO2 > Pt-N/F-TiO2 > Pt-TiO2.

In-situ photocalorimetry-fluorescence spectroscopy studies of RhB photocatalysis over Z-scheme g-C3N4@Ag@Ag3PO4 nanocomposites: A pseudo-zero-order rather than a first-order process

Much effort has been expended recently to apply photocatalysis to address the environmental crises and energy shortage crises. However, it is controversial to demonstrate that photocatalytic kinetics follows a pseudo-first-order process by common spectroscopy, because it only reflects the variation of chromophoric groups vs. time in the initiating photocatalysis, ignoring the subsequent intermediates oxidation. In this study, we used a photocalorimetry-fluorescence spectrum, coupling of a photocalorimeter and a laser-induced fluorescence spectroscopy, to demonstrate it is a pseudo-zero-order rather than a first-order process. It shows the feasibility to investigate not only photocatalytic kinetics but also reaction pathway and rate-determining step. During RhB photocatalysis over g-C3N4@Ag@Ag3PO4, three main heat changes occurred: (ab) the photoresponse of Rhodamine B molecules and the photocatalyst, consequently generating both hydroxyl and superoxide radicals, (bc) the balance between endothermic photoresponse and exothermic RhB photodegradation by O2−/OH and finally (cd) a stable exothermic period of RhB photodegradation. Specifically, the photocatalysis underwent an initial endothermic reaction and a subsequent exothermic stage, and eventually maintained a stable exothermic rate of −(0.2345±0.0971)mJs−1 with a pseudo-zero-order process. With Ag NPs working as a bridge, the g-C3N4@Ag@Ag3PO4 system followed a Z-scheme mechanism, simultaneously generating both hydroxyl and superoxide radicals. Therefore, N-demethylation and chromophore cleavage rapidly occurred within 10min. Then the ring cleavage and the oxidation of intermediates which were the rate-determining step eventually dominated, corresponding to the cd stage with a pseudo-zero-order process, and resulting in a pseudo-zero-order rather than a first-order process in RhB photocatalysis over g-C3N4@Ag@Ag3PO4.

The Photocatalytic Kinetics of the Methyl Orange Degradation in the Aqueous Suspension of Irradiated TiO2

The Photocatalytic Kinetics of the Methyl Orange Degradation in the Aqueous Suspension of Irradiated TiO2

Enhanced removal of azo dye using modified PAN nanofibrous membrane Fe complexes with adsorption/visible-driven photocatalysis bifunctional roles

A series of polyacrylonitrile (PAN) nanofibrous membrane Fe complexes as the Fenton heterogeneous catalysts were fabricated through surface modification with different ratio of hydrazine hydrate (HH) and hydroxylamine (HA) and subsequent coordination with Fe3+ ions for the synergistic removal of a typical azo dye, Reactive Red 195 (RR 195) via adsorption and visible-driven photocatalytic oxidation. Effect of molar ratio of HH and HA on surface structure characteristics of the resulting complexes were examined. Their adsorptive or photocatalytic activity was also compared by changing molar ratio of HH and HA. The results indicated that three PAN nanofibrous membrane Fe complexes prepared with simultaneous modification of HA and HH exhibited much higher adsorption and visible photocatalytic activities than the complex modified solely with HA or HH due to their distinctive surface structures containing more active sites. Their adsorption and visible photocatalytic kinetics of RR 195 followed pseudo-second-order model equation. Their high photocatalytic rate constant and large amount of dye adsorption were regarded as the main reasons for better dye removal efficiency and durability in cyclic reuse by means of the synergistic adsorption-photocatalysis process.

Boron doped titaniumdioxide nanotube arrays: production, characterization and photocatalytic properties

In numerous studies, regularly oriented anatase titanium dioxide nanotube arrays (TNTAs), obtained through electrochemical anodization, have been employed as catalyst surfaces for photocatalytical applications. However, in many practical applications the phocatalytical activities are restricted due to their wide band gaps. This work shows that photocatalytical activity of TNTAs can be improved by a novel approach which provides boron incorporation into TiO2 structure during anodization process without any further treatment. Anodization was performed in an aqueous solution containing hydrofluoric acid, oxalic acid and sodium fluoroborate (NaBF4) at room temperature on titanium (Gr2) substrates. The anodized samples were annealed at 480 °C for 2 h in air in order to obtain anatase transformation and intended crystalline structure. As-prepared B-doped TNT structure was characterized by field emission scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction, atomic force microscopy and ultraviolet–visible light diffuse absorbance spectroscopy. The photocatalytic kinetics of B-doped TNTAs for the degradation of methylene blue (MB) under visible light irradiation were evaluated in details.

A one step electrospinning process for the preparation of polyaniline modified TiO2/polyacrylonitile nanocomposite with enhanced photocatalytic activity

A one step electrospinning approach was utilized for the preparation of polyaniline (PANI) modified titanium dioxide/polyacrylonitrile (PAN) composite nanofiber using a mixture of dimethylformamde (DMF) and N-methyl-2-pyrrolidone (NMP) as solvents. Several techniques were carried out to characterize the resulting electrospun fibers including SEM, FTIR, XRD, TGA and DRUV-Vis absorption spectroscopy. The later method showed an optical band gap shifting for polyaniline modified titanium dioxide/polyacrylonitrile composites in comparison to unmodified one. Due to these optical absorptions, the fibers can be considered promising candidates as visible light photocatalysts to generate more electron-hole pairs. Photodegradation was accomplished using methyl orange (MO) as a model organic pollutant. The degradation of MO, extremely increased using photocatalysts which contained PANI and decolorization in the presence of visible light achieved up to 90% in less than 60 min in comparison with the neat nanofibers (about 10%). This advantage promises a bright future for these fibers as useful photocatalysts.The effect of reaction parameters such as photocatalyst dose, concentration of the starting substrate solution and pH were studied and photocatalytic kinetics was investigated. The stability of photocatalyst was also investigated. The photocatalyst retained 87% of its initial activity after 10 replicate uses.

Preparation and enhanced photocatalytic activity of S-doped TiO2/ palygorskite composites

Using tetrabutyl titanate and thiourea as titanium and sulphur source, S-doped TiO2/palygorskite (S–TiO2–Pal) composites were prepared by a solgel method. The XRD patterns of S–TiO2–Pal photocatalysts exhibited the characteristic reflection at 2θ values of 8.3° of Pal gradually disappear, while those of the quartz become more intense with the increase in calcination temperature (400–600 °C). The photocatalytic activity of composites was evaluated by photocatalytic degradation of Reactive Black KN-B (KN-B), which is exposed under UV light at the wavelength of 254 nm and visible light of 11 W. The results showed that when the mass ratio of S to TiO2 was 1:3, calcination temperature 500 °C and pH value of dye solution 2.00, the photocatalytic degradation rate of S–TiO2–Pal catalyst under UV and visible lights increased significantly relative to that of TiO2, respectively. Furthermore, the photocatalytic kinetics could be well fitted with the classical first-order Langmuir–Hinshelwood model.

Photodegradation of aqueous bisphenol A using boric acid-doped titanium dioxide

A high-photoactivity boric acid-doped titanium dioxide (B-doped TiO2) photocatalyst of the degradation of aqueous bisphenol A (BPA) under sunlight was synthesized and characterized by scanning electron microscope, X-ray photoelectron spectroscopy and UV-vis diffuse reflectance. The band gap energies of TiO2 and B-doped TiO2 (B/Ti mole ratio = 0.2%) were 3.01 and 2.98, showing that B-doped TiO2 could narrow the band gap of pure TiO2. In the photocatalytic kinetics of the photodegradation, the BPA photodegradation rate constants were 1.67 and 1.08 h−1, respectively. The BPA removal rate satisfies pseudo-first-order kinetics. Results showed that photocatalysts doped with boron displayed greater photodegradation (88% BPA removal) than pure TiO2 (65% BPA removal). Experimental results indicated that B-doped TiO2 not only was an effective photocatalyst, but also had considerable mineralization effects. The recycling test revealed that the photocatalyst remained effective after 10 uses, revealing the stability and reusability.

Adsorption and Photocatalytic Kinetics of Visible-Light Response N-Doped TiO2Nanocatalyst for Indoor Acetaldehyde Removal under Dark and Light Conditions

Understanding the removal nature of the indoor volatile organic compounds under realistic environment conditions would give clear guidance for the development of air purification devices. The study investigated the removal of indoor acetaldehyde using visible-light-responsive N-doped TiO2(N-TiO2) photocatalyst under visible-light irradiation (light) and in the absence of light (dark). The adsorption kinetics of acetaldehyde onto N-TiO2followed a pseudo-second-order model. The magnitude of acetaldehyde adsorption is proportional to temperature, and the results were fitted to the Langmuir isotherm model. Moreover, the effect of initial acetaldehyde concentration and visible-light intensity on the photooxidation of acetaldehyde was well described by the Langmuir-Hinshelwood model. Results show that the mesoporous N-TiO2catalyst had a high ability to absorb acetaldehyde in the dark condition, and then acetaldehyde was subsequently photooxidized under visible-light irradiation. The adsorption capacity was found to increase with decreasing temperature. The negative value ofΔG° and the positive value ofΔS° indicate that the adsorption of acetaldehyde onto N-TiO2was a spontaneous process. Finally, a reaction scheme for removal process of indoor acetaldehyde by N-TiO2was proposed.

Kinetics Study on Photocatalytic Degradation of Methyl Orange Catalyzed by Sea Urchin-Like Cu2O

Sea urchin-like cuprous oxide with hollow glass microsphere as core was prepared using sodium sulfite as the reducing agent and sodium acetate-acetic acid as buffer solution in copper sulfate solution. Methyl orange was selected as degradation target for photocatalytic experiments. The photocatalytic activities were investigated by visible spectro- photometer. Photocatalytic kinetics parameters were studied by the Langmuir-Hinshelwood model and Arrhenius formula. It was observed that the sea urchin-like morphology dramatically improved the photocatalytic activity of cuprous oxide. The photo-degradation belongs to the first-order reaction and the maximum degradation rate could reach 94.37%. The activation energy and pre-exponential factor are 41.18 KJ·mol-1 and 1.07 × 106, respectively. After seven times recycling, the sample still showed high photo-catalytic efficiency and stability.

Monte-Carlo modelling of nano-material photocatalysis: bridging photocatalytic activity and microscopic charge kinetics.

In photocatalysis, it is known that light intensity, organic concentration, and temperature affect the photocatalytic activity by changing the microscopic kinetics of holes and electrons. However, how the microscopic kinetics of holes and electrons relates to the photocatalytic activity was not well known. In the present research, we developed a Monte-Carlo random walking model that involved all of the charge kinetics, including the photo-generation, the recombination, the transport, and the interfacial transfer of holes and electrons, to simulate the overall photocatalytic reaction, which we called a "computer experiment" of photocatalysis. By using this model, we simulated the effect of light intensity, temperature, and organic surface coverage on the photocatalytic activity and the density of the free electrons that accumulate in the simulated system. It was seen that the increase of light intensity increases the electron density and its mobility, which increases the probability for a hole/electron to find an electron/hole for recombination, and consequently led to an apparent kinetics that the quantum yield (QY) decreases with the increase of light intensity. It was also seen that the increase of organic surface coverage could increase the rate of hole interfacial transfer and result in the decrease of the probability for an electron to recombine with a hole. Moreover, the increase of organic coverage on the nano-material surface can also increase the accumulation of electrons, which enhances the mobility for electrons to undergo interfacial transfer, and finally leads to the increase of photocatalytic activity. The simulation showed that the temperature had a more complicated effect, as it can simultaneously change the activation of electrons, the interfacial transfer of holes, and the interfacial transfer of electrons. It was shown that the interfacial transfer of holes might play a main role at low temperature, with the temperature-dependence of QY conforming to the Arrhenius model. The activation of electrons from the traps to the conduction band might become important at high temperature, which accelerates the electron movement for recombination and leads to a temperature dependence of QY that deviates from the Arrhenius model.

Preparation and Application of Fe(III) Doped Titania Photocatalyst with Hollow Structure

The Fe3+ doped titania (TiO2) photocatalyst with hollow structure was synthesized by sol–gel template method followed by calcation at certain temperature for template removal. The as-prepared Fex-TiO2 photocatalysts were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD) and UV–visible diffuse reflectance spectroscopy (DRS). The photocatalytic performances and kinetics of the Fex-TiO2 photocatalysts with hollow structure were evaluated by photocatalytic degradation of rhodamine B (RhB) under UV light irradiation. The results showed that the hollow tube structures TiO2 catalysts, which had an average external diameter of 1.5 μm, were composed of numerous TiO2 nanoparticles. The photocatalytic degradation experiments indicated that the sample of 0.03% Fe/Ti molar ratio shows the best photocatalytic ability. The effect of initial dye concentration and catalyst dosages on the photocatalytic activity were studied respectively and the obtained results were fitted with Langmuir–Hinshelwood model.The kinetic parameters were discussed and syudied according to the experiment results in detail.

Fabrication of thermo responsive fibrous ZnO/PNIPAM nanocomposites with switchable photocatalytic activity

Abstract In this paper, thermo responsive ZnO/PNIPAM nanocomposites were fabricated via electrospinning with the dispersing of ZnO nanoparticles into PNIPAM nanofibers. The effect of the electrospinning parameters on morphology of the nanofibers and the temperatures on the morphology of the nanocomposites was both investigated by SEM–EDS. Furthermore, the photocatalytic kinetics of the nanocomposites was studied by the evaluation of the photocatalytic activities of the nanocomposites above and/or below the lower critical solution temperature (LCST) of PNIPAM. The nanofibers remained the original sizes and shapes above the LCST values, while dissolved obviously and released the embedded ZnO nanoparticles below the LCST values of PNIPAM. The existence of PNIPAM contributed not only the thermo responsive abilities to the nanocomposites, but also the enhanced photocatalytic activities to ZnO nanoparticles, by the detected results of UV–vis spectra of the degradation rates of Rhodamine B and by the calculation results of EPR measurements for the photo induced reactive oxygen species (ROS) of the nanocomposites. Thus, fibrous ZnO/PNIPAM nanocomposites could exhibit switchable and controlled photocatalytic activities through the changing of the temperatures.

Non-ultraviolet photocatalytic kinetics of NaYF4:Yb,Tm@TiO2/Ag core@comby shell nanostructures

Enhanced photocatalysis of NaYF4:Yb,Tm@TiO2/Ag under non-ultraviolet irradiation was demonstrated by monitoring the photocatalytic kinetics with the SERS technique.

Synthesis and Characterization of Urchin-like Mischcrystal TiO2 and Its Photocatalysis

The urchin-like mischcrystal TiO2 using acid attapulgite as an introducer was synthesized after a subsequent low-temperature hydrolyzation and crystallization followed by removal of acid attapulgite. The samples were characterized by transmission electron microscope, X-ray diffraction, Fourier transform infrared spectra and X-ray photoelectron spectroscopy. Acid attapulgite plays a critical role in the morphology and crystal structure of TiO2. The results suggest that the perfect urchin-like mischcrystal TiO2 is fabricated when the mass ratio of TiO2 and acid attapulgite is 0.7:1. The single urchin-like TiO2 is comprised of a nanosphere and plentiful nanoneedles. The nanoneedles grow radially on the surface of the nanosphere. The urchin-like TiO2 is around 100nm, and the nanoneedles have a diameter ranging from 2 to 5nm. It has been confirmed that the chemical groups of acid attapulgite have a significant influence on the growth of TiO2. In addition, the urchin-like mischcrystal TiO2 exhibits excellent activity to assist photodegradation of Rhodamine B aqueous solution under ultraviolet light, and the degradation rate is about 94.15% for 80min. The photocatalytic kinetics can be well described by the pseudo-first rate equation.