Photocatalytic Films Research Articles

Preparation, kinetics, mechanism and properties of semi-transparent photocatalytic stable films active in dye degradation

The photocatalytic activity, mechanism and characterization of polyethylene-TiO2 films (PE-TiO2) during the discoloration/degradation of methylene blue (MB) are reported in this study. PE-TiO2 films were prepared at moderate temperatures (∼90°C). TiO2 did not leach out during discoloration/degradation of MB. This is a significant finding for a low temperature catalyst-polymer preparation leading to a stable repetitive MB degradation kinetics. MB discoloration on PE-TiO2 films proceeded showing a quantum yield of 0.25%. Band-gap irradiation of TiO2 was applied by way of a mercury medium pressure lamp to excite only TiO2 and avoid MB photosensitization. The nature of the oxidative radicals intervening in MB discoloration was investigated by scavenging experiments. The relative importance of the oxidative species during MB discoloration is: TiO2vbh+>HO2/O2−>1O2>OH. Evidence is presented for the superoxide anion-radical O2− at pH 6, as the main radical species leading to MB-discoloration. The MB- discoloration kinetic trend on PE-TiO2 followed the behavior observed for diffusion-controlled reactions. The diffusion distance of the HO2 and OH radicals during the discoloration of a solution MB 1×10−5molL−1 were estimated as 71 and 2.2μm respectively. The OH formation was assessed quantitatively during MB degradation by fluorescence measurements. The PE-TiO2 film at time zero presented a contact angle (CA) of ∼92°, decreasing within the five hour MB discoloration period to <5°. Reversibility from the hydrophilic nature of the film to the initial hydrophobic state proceeded in the dark within few days. The implications of the reversibility time on the MB-discoloration performance are discussed. Evidence for redox catalysis during MB-discoloration was found by XPS measurements. The surface properties of the (PE-TiO2) films were characterized by X-ray fluorescence (XRF), UV–vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass-spectrometry (ICP-MS).

Oxygen-Deficient Titania with Adjustable Band Positions and Defects; Molecular Layer Deposition of Hybrid Organic–Inorganic Thin Films as Precursors for Enhanced Photocatalysis

Molecular layer deposition (MLD) of TiCl4 and ethylene-glycol (Ti-EG) was recently demonstrated as a vapor phase synthetic route for preparation of photocatalytic thin films via hybrid organic–inorganic thin films. The organic moieties of the hybrid material function as sacrificial components that undergo controlled decomposition during thermal annealing. Anneal temperature was shown to be an important factor determining the overall photocatalytic performances of the treated films with 650 °C optimal for photodegradation of dye molecules and anneal at 520 °C showing optimal performance for the direct photocatalytic production of H2O2. Both systems exhibit activities that are not typically attainable by titania, yet a fundamental understanding of the underlying details leading to these improved reactivities for the specific cases is still lacking. Here we demonstrate that thermal anneal of hybrid organic–inorganic thin films prepared by MLD yield oxygen-deficient titania with controllable levels of oxygen vacancies (OVs) and defect states that are adjusted by the temperature of the anneal process, performed under air. The anneal process result in nonstoichiometric oxide films with unique electronic properties including the tuning of band positions, accessibility to significantly deeper valence band position and controlled formation of electronic defect states that assist in charge separation for Au-titania catalyst. We correlate the oxygen deficiency and electronic structure of the annealed film with the photocatalytic activity for shedding light on the details that lead to the improved reactivity. These results extend the scope of using MLD in the context of photocatalysis with new routes for obtaining nonstoichiometric oxides which are key for enhancing and tailoring the reactivity of metal oxide (photo)catalysis.

Carbamazepine degradation using a N-doped TiO2 coated photocatalytic membrane reactor: Influence of physical parameters

Commercial α-Al2O3 photocatalytic membranes with a pore size of 200 and 800-nm were coated with N-doped TiO2 photocatalytic film using a sol-gel technique for concurrent bottom-up filtration and photocatalytic oxidation. X-ray diffraction confirmed that the deposited N-doped TiO2 films are in the form of anatase with 78–84% coverage of the membrane surface. The concentration of N found by X-ray photoelectron spectroscopy was in the range of 0.3–0.9 atomic percentage. Membrane permeability after coating decreased by 50% and 12% for the 200- and 800-nm membrane substrates, respectively. The impact of operational parameters on the photocatalytic activity (PCA) of the N-doped TiO2-coated membranes was examined in a laboratory flow cell based on degradation of the model micropollutant carbamazepine, using a solar simulator as the light source. The significant gap in degradation rate between flow through the membrane and flow on the surface of the membrane was attributed both to the hydraulic effect and in-pore PCA. N-doped TiO2-coated membranes showed enhanced activity for UV wavelengths, in addition to activity under visible light. Experiments of PCA under varying flow rates concluded that the process is in the mass-transfer control regime. Carbamazepine removal rate increased with temperature, despite the decrease in dissolved oxygen concentration.

Metal ions doped and polythiophene coated nanophotocatalysts: Synthesis and spectroscopic characterization for H2 production and dye degradation

Transition metal ions doped cadmium sulphide nanomaterials (Cu/CdS, Co/CdS, Zn/CdS), polythiophene (PTh) nanoparticles (NPs) and CdS/PTh nanocomposites were prepared by aqueous chemical method. The synthesized materials were subjected to structural and optical characterizations to be utilized as photocatalysts. The efficiency of the above photocatalysts was determined by employing it in dye degradation and hydrogen evolution studies. The results show that Zn/CdS nanomaterials have the highest efficiency of degrading methylene blue (MB) upto an extent of 94.14% and the hydrogen production rate of 205.16μmolh−1. The overall photocatalytic efficiency of Zn/CdS nanomaterials was found to be 6.32%. So, this specific nanomaterial was cast into thin films by solution casting method using PVA as binder material. Two different films were designed to test our hypothesis possessing their own unique properties. The hydrogen production rate for the films were observed and found to be decreasing with the addition of the binder material. However, the efficiency of the Zn/CdS film was higher than the other photocatalytic films. In order to extend its application, impedance studies were also made.

Preparation of anatase phase titanium dioxide film by non-aqueous electrodeposition

The electrodeposition–annealing route to fabricating thin film of the promising photocatalyst material anatase-titanium dioxide (anatase-TiO2) has been studied. The sample was deposited with a solution of N,N-dimethylformamide containing titanium compound by controlled-potential technique. SEM image showed the annealed sample at 600°C for 1h under air provided a continuous film with a thickness of ca. 350nm. In this sample, X-ray photoelectron spectrum corresponding to the Ti 2p peak assigned to a chemical bond of TiO2 and X-ray diffraction peaks assigned to the anatase phase were observed, respectively. Electrochemical oxidation in sodium sulfate solution on this annealed film was enhanced in the presence of UV light radiation. These results confirm the successful synthesis of photocatalytic anatase-TiO2 film by the electrodeposition and annealing process.

Photocatalytic TiO2 thin films synthesized by the post-discharge of an RF atmospheric plasma torch

Thin films of titanium oxide (TiO2) are synthesized at room temperature by the post-discharge of an RF atmospheric plasma torch supplied with argon and oxygen. Vapours of titanium tetraisopropoxide (TTIP) precursor are injected in the post-discharge by an argon flow rate bubbling in the liquid precursor. Without any external substrate heating, the coatings are amorphous and characterized by a thin film upon which agglomerates can be observed. The annealing of the coatings at 450 °C is efficient to (partially) crystallize the TiO2 since bands characteristic of the TiO2 anatase structure are observed in Raman spectra. The films are super-hydrophilic and present excellent photocatalytic activity; two properties of particular interest for self-cleaning applications. Nevertheless, annealed coating presents a higher photocatalytic activity.

Deposition of Visible Light Active Photocatalytic Bismuth Molybdate Thin Films by Reactive Magnetron Sputtering.

Bismuth molybdate thin films were deposited by reactive magnetron co-sputtering from two metallic targets in an argon/oxygen atmosphere, reportedly for the first time. Energy dispersive X-ray spectroscopy (EDX) analysis showed that the ratio of bismuth to molybdenum in the coatings can be effectively controlled by varying the power applied to each target. Deposited coatings were annealed in air at 673 K for 30 min. The crystalline structure was assessed by means of Raman spectroscopy and X-ray diffraction (XRD). Oxidation state information was obtained by X-ray photoelectron spectroscopy (XPS). Photodegradation of organic dyes methylene blue and rhodamine B was used for evaluation of the photocatalytic properties of the coatings under a visible light source. The photocatalytic properties of the deposited coatings were then compared to a sample of commercial titanium dioxide-based photocatalytic product. The repeatability of the dye degradation reactions and photocatalytic coating reusability are discussed. It was found that coatings with a Bi:Mo ratio of approximately 2:1 exhibited the highest photocatalytic activity of the coatings studied; its efficacy in dye photodegradation significantly outperformed a sample of commercial photocatalytic coating.

Solar-activated tandem thin films based on CuInS2, TiO2 and SnO2 in optimized wastewater treatment processes

Tandem structures involving CuInS2, TiO2 and SnO2 thin films were prepared by Robotic spray pyrolysis and were optimized in terms of composition and deposition temperatures. The compounds have structural compatibility (crystalline structures and crystallites sizes) and good homogeneity. The crystalline structures were investigated by X-ray diffraction and the morphology by atomic force microscopy and scanning electron microscopy. The wettability properties and the surface energy were calculated based on contact angle measurements using two liquids with different polarities (glycerol and ethylene glycol). The components in the tandem systems have suitably positioned the energy of the conduction and valance bands, limiting charge recombination, therefore, the sample containing all three semiconductors has the highest mineralization efficiency (95.3% removal of total organic carbon) under UV+Vis irradiation, mimicking solar radiation. Long term use of the photocatalytic films (up to 36h) proved the need to design the process with interim washing steps for preserving the photocatalytic efficiency by avoiding surface clogging.

Optical, Structural and Morphological Properties of Photocatalytic ZnO Thin Films Deposited by Pray Pyrolysis Technique

Photocatalytic ZnO thin films have been deposited onto glass substrate by spray pyrolysis technique. The sprayed solution consists of 0.1 M of zinc acetate dihydrate dissolved in double distilled water and sprays onto ultrasonically cleaned glass substrates maintained at 350°C, through an air-atomizing nozzle. The X-ray diffraction (XRD), scanning electron microscopy (SEM), EDX and UV-VIS spectrophotometer were applied to describe the structural, morphological, compositional and optical properties of ZnO catalyst. XRD analysis confirms that the films were found to be single phase hexagonal wurtzite structure. The SEM micrograph of the films is shown highly uniform, crack free and found to be fiber like structures. The optical transmittance spectra of the ZnO thin films were found to be transparent to visible light and the average optical transmittance was greater than 85%. The direct optical band gap energy values of the films shift towards the lower energy as a consequence of the thermal annealing. The Urbach energy of the films was found to increase with annealing temperature. The refractive index of the films was calculated and the refractive index dispersion curve of the films obeys the single oscillator model. The values of oscillatory energy Eo, dispersion energy Ed, and static dielectric constant es for the ZnO thin films were determined. The films were evaluated for their ability to degrade methylene blue. The Langmuir-Hinshelwood kinetic model was used to interpret quantitatively the observed kinetic experimental result. The photocatalytic activity of ZnO thin films was enhanced by annealing temperature.

Atomic layer stack deposition-annealing synthesis of CuWO4

Stack deposition-annealing (SDA) synthesis of photocatalytic CuWO4 thin films via sequential ALD of WO3/CuO and post-ALD annealing.

Ultrathin titanium oxide nanosheets film with memory bactericidal activity.

An ultrathin photocatalytic film of titanium oxide was fabricated using two dimensional (2D) titanium oxide nanosheets (TONs) as building blocks. The as-prepared film was found be able to store photoelectrons upon UV irradiation due to the reduction/oxidation of T4+/Ti3+ on the 2D TONs. Post-illumination discharging of the stored electrons produced antibacterial radicals, and as a result, the as-prepared film showed memory bactericidal activity toward E. faecalis and E. coli in the dark.

Fabrication of a flexible graphene–TiO2/PDMS photocatalytic film by combining air atmospheric pressure glow discharge treatment

This work reported a universal and effective route to fabricate a kind of flexible photocatalyst film to broaden the scope of application (wearable photocatalyst devices for example). This technology roadmap is premised on the air atmospheric pressure glow discharge (AAPGD) pretreatment. Through treating poly (dimethyl siloxane) (PDMS) with AAPGD technique, the PDMS surface is functionalized by hydroxyl groups and the contact angle gains a minimum value of about 26.0, which make it easy to be decorated for the desired efficiency. Then the PDMS surface can be endowed with photocatalytic activity through wet-chemical method without the bondage of inherent hydrophobic-nature. Owing to the hydrophilic surface, the graphene can be inserted to improve the photocatalytic performance and the flexibility of graphene was kept. Compared with the photocatalysts films without AAPGD pretreatment, the photocatalytic performance of films suffering from AAPGD pretreatment is much higher. Therefore this flexible film strategy, based on AAPGD pretreatment, is of great significance to design flexible photocatalysts films, pushing forward the development of wearable photocatalyst devices.

Continuous-reactor, pH-controlled synthesis of multifunctional mesoporous nanocrystalline anatase aggregates

Anatase is an important functional nanomaterial in a wide variety of applications. Despite a breadth of research on novel synthesis methods, there is a lack of studies geared towards developing scalable processes for nanoscale anatase crystalline materials. In this study, anatase secondary nanoparticles are produced in a continuous stirred-tank reactor (CSTR) through forced hydrolysis of pre-diluted aqueous TiCl4 solution by controlling pH. The steady-state supersaturation regime of the CSTR process is shown to be inducive to the formation of phase-pure nanocrystalline anatase aggregates in the order of ∼500–1000nm possessing a very high surface area ∼200–250m2/g. The aggregates made of ∼6nm individual primary nanocrystallites are mesoporous – a highly desirable property in building photocatalytic and thin film electrode (Li-ion and solar) devices with enhanced interfacial functionalities. The new process was tested at a range of conditions, 70–90°C, 0.1–0.5M TiCl4, pH 2–5, and 30–60min. pH control was vital in stabilizing the metastable anatase nanocrystallites and promoting their aggregation that facilitates their handling and application. The process achieves >98% steady-state conversion of TiCl4(aq) to nanoanatase without the use of toxic organic chemicals constituting a cost-effective and green alternative to commonly used sol–gel or hydrothermal technologies.

Polyvinylidene fluoride photocatalytic films embedded by porous Zr x Si 1− x O 2 shell/void/TiO 2 core particles

Abstract In order to recycle TiO 2 photocatalysts and prevent TiO 2 from damaging polyvinylidene fluoride (PVDF) films, porous Zr x Si 1− x O 2 shell/void/TiO 2 core particles (Zr-SVTs) were firstly synthesized by the synergistic effect between nonionic surfactant P123 ((EO) 20 (PO) 70 (EO) 20 ) and oleic acid (CH 3 (CH 2 ) 7 CH = CH(CH 2 ) 7 COOH) and cohydrolysis between TEOS and ZrOCl 2 ·8H 2 O, and then embedded them into PVDF to prepare PVDF photocatalytic films (Zr-SVTP films) by phase inversion process. Zr-SVTs were characterized by SEM, TEM and EDX while Zr-SVTP films were characterized by SEM. The preparation condition and recycle of Zr-SVTP films were investigated and determined. The photocatalytic activities of Zr-SVTP films were investigated by cleaning oily wastewater. The results show that Zr-SVTs hold porous characteristic, and Zr-SVTP films perform the better photocatalytic activities compared with PVDF films (SVTP) embed by SiO 2 shell/void/TiO 2 core without doping Zr in SiO 2 shell, this is because the mass transfer inside channels of Zr-SVTs is promoted by Zr x Si 1− x O 2 shell, ensuring the better photocatalytic activities of Zr-SVTP films. Therefore, Zr-SVTP films structure characteristic will have a promising application in recycle of TiO 2 photocatalysts and cleaning oily wastewater.

Synthesis of Nanostructured TiO2 Coatings by Sol-Gel Method: Structural and Morphological Studies

Immobilized TiO2 photocatalytic films were produced by the sol-gel method on stainless steel substrates from four different sols. Starting TiO2 solutions were prepared with and without polyethylene glycol (PEG) and p-25 nano powder. PEG and p-25 were added to sols to improve the photocatalytic properties and homogeneity of the titanium oxide films. The surface morphology of the films was studied by scanning electron microscopy. Decomposition of the organic compounds from the gels was studied using thermal gravimetry and differential thermal analysis. Structural properties of the films prepared at different calcination temperatures were studied by X-ray diffraction.

Visible-light active thin-film WO3 photocatalyst with controlled high-rate deposition by low-damage reactive-gas-flow sputtering

A process based on reactive gas flow sputtering (GFS) for depositing visible-light active photocatalytic WO3 films at high deposition rates and with high film quality was successfully demonstrated. The deposition rate for this process was over 10 times higher than that achieved by the conventional sputtering process and the process was highly stable. Furthermore, Pt nanoparticle-loaded WO3 films deposited by the GFS process exhibited much higher photocatalytic activity than those deposited by conventional sputtering, where the photocatalytic activity was evaluated by the extent of decomposition of CH3CHO under visible light irradiation. The decomposition time for 60 ppm of CH3CHO was 7.5 times more rapid on the films deposited by the GFS process than on the films deposited by the conventional process. During GFS deposition, there are no high-energy particles bombarding the growing film surface, whereas the bombardment of the surface with high-energy particles is a key feature of conventional sputtering. Hence, the WO3 films deposited by GFS should be of higher quality, with fewer structural defects, which would lead to a decrease in the number of centers for electron-hole recombination and to the efficient use of photogenerated holes for the decomposition of CH3CHO.

Development of visible light activated TiO2 thin films on stainless steel via sol spraying with emphasis on microstructural evolution and photocatalytic activity

Visible light activated nitrogen doped TiO2 thin films were developed on 304 stainless steel by sol spraying method using a common painting airbrush. Thin films with different thickness were prepared and calcined at various temperatures from 400 to 600°C. The samples were studied using ellipsometry, XRD, GIXRD, XPS, DRS, SEM and FESEM. Photocatalytic activities of the films were investigated by measuring their ability to degrade methylene blue solution under visible light irradiation. Results revealed that uniform nanostructured films with a thickness range of 29–150nm were successfully prepared on stainless steel by sol spraying. Doping nitrogen into TiO2 structure restricted the crystallite growth of anatase phase and reduced the band gap energy to 2.85eV and therefore, activated TiO2 in visible light region. Increasing calcination temperature not only promoted crack formation in thin films, but also encouraged Fe diffusion from substrate into thin films structure. However, the N doped TiO2 film calcined at 500°C with a thickness of 150nm indicated a significant photocatalytic activity in visible light with 22% higher efficiency in comparison with undoped TiO2 film. Development of TiO2 based photocatalytic thin films by a simple method of airbrushing, builds up the hope for industrial scale applications in future.

Photocatalytic activity of TiO2 films prepared by surfactant-mediated sol–gel methods over commercial polymer substrates

Commercially available transparent polymers are evaluated as support for photoactive TiO2 and tested for their application in gas phase photocatalytic oxidation processes. The polymers were selected on the basis of their optical properties and the TiO2 film adhesion. Two low-temperature surfactant-mediated sol–gel synthesis methods of TiO2–SiO2 nanocomposites are investigated to provide acid TiO2–basic SiO2 interactions for the formation of photocatalytic films of higher quality. These procedures are expected to improve photocatalytic properties compared to those obtained by direct coating of TiO2 on the polymers. One method consists in modifying the polymer surface with a layer of poly(diallyldimethylammonium) chloride, which provides a positively charged surface for the fixation of the alkaline SiO2 sol. The other method is based on reducing the surface tension of the SiO2 sol using perfluorobutane sulfonates. Both methods are compared to direct deposition in terms of homogeneity of the layers and photocatalytic activity for trichloroethylene oxidation. Among the commercial polymers employed poly(methyl methacrylate) processed by sheet moulding showed the best optical properties, TiO2 adhesion, and photocatalytic activity, which was promoted by the deposition of the silica interlayer by both preparation methods.

Evaluation of a solar/UV annular pilot scale reactor for 24 h continuous photocatalytic oxidation of n-decane

A pilot scale single-pass continuous-flow annular photoreactor (rannulus=21.3mm) was designed and manufactured featuring a compound parabolic collector (CPC) to capture both direct and diffuse solar radiation and, simultaneously, artificial UVA lamps, in order to examine the possibility of working continuously day and night. Cheap, lightweight and easily shaped cellulose acetate materials, which are transparent in the TiO2 activation range, were used as supports, as an alternative to classic porous solid monolithic structures. Such configuration allows that only 1% of the reactor volume is filled with photoactive material.The photocatalytic oxidation (PCO) of n-decane (Cdec, feed=10ppm, Qfeed=2Lmin−1, τ=44s) over cellulose acetate monolithic structures coated with different TiO2-based photocatalytic films (Lcatalytic bed=144cm) was studied under solar and artificial UVA radiation. Gas-phase n-decane conversions up to 100% were attained using P25 or PC500 photocatalytic films under solar irradiances starting from 16WUVm−2 in the morning (sunrise, increasing temperature) and down to 3WUVm−2 in the afternoon (sunset, decreasing temperature). An increase of the UV irradiance at total n-decane conversion promotes the formation of CO2. The excess of photons reaching the photocatalytic bed favours the direct reaction pathway of CO2 generation. The PCO of n-decane enhanced 29% resulting in 100% of conversion using PC500 film instead of P25 film, under artificial UVA radiation (I=29Wm−2). Results indicate that combining both radiation sources, a 24h continuous PCO process towards the removal of n-decane can be accomplished.

Cu-doped TiO2/reduced graphene oxide thin-film photocatalysts: Effect of Cu content upon methylene blue removal in water

Photocatalytic thin films on quartz substrates were simply prepared by spraying a sol of copper metal (Cu)-doped titanium dioxide (TiO2) combined with reduced graphene oxide (RGO). Cu-doped TiO2/RGO powders containing different amounts of Cu were synthesized to investigate the effect of Cu content upon the photodegradation of methylene blue in water, and were used in the preparation of sols of Cu-doped TiO2/RGO. The Cu-doped TiO2/RGO film photocatalysts showed better performance in the photodegradation of methylene blue than an undoped TiO2/RGO film. After 180min of UV irradiation, 7.5-CTG-5 (7.5wt% of Cu and 5wt% of RGO) photocatalyst removed 63% of methylene blue from aqueous solution with an initial concentration of 10ppm, whereas 0-CTG-5 (0wt% of Cu and 5wt% of RGO) decomposed only 28% of the methylene blue, indicating that the presence of Cu enhanced the photocatalytic activity. Doped Cu could narrow the bandgap of TiO2, thereby influencing the reduction of graphene oxide and enhancing the hydrophilicity of the materials, and thus producing the observed increase in photocatalytic activity. The relationships between hydrophilicity and photocatalytic activity are discussed herein.