TiO2 Thin Film Photocatalysts Research Articles

Characterization and photocatalytic activity of TiO2 thin films prepared by sol-gel method for NOx degradation

Titanium dioxide (TiO2) thin film photocatalysts for the elimination of nitrogen oxides (NOx) were prepared by the dip-coating process using the sol–gel method. To prepare the sols, titanium (IV) butoxide (Ti(OBu)4), diethanolamine (DEA), butanol, and HCl were chosen. The TiO2 coatings were prepared using sols with varying volume ratios of Ti(OBu)4:DEA:Butanol. Additionally, three withdrawal speeds from the sol were employed. Subsequently, the TiO2 thin films were sintered at 500 and 700 °C. The obtained thin films were characterized using SEM to determine thickness, AFM to determine roughness, and the Step 500 platform for the adhesion test. NOx was found to be efficiently eliminated by TiO2 thin film photocatalyst. It is possible to obtain a TiO2 coating that is capable of degrading 300 ppb of NO in real time under specific conditions: a relative humidity of 30 %, a UV intensity of 20 W/m2, a gas flow rate of 1 dm3/min.

Fabrication of an in situ-grown TiO2 nanowire thin film and its enhanced photocatalytic activity.

TiO2 is a promising photocatalyst used in practical environmental remediation. TiO2 photocatalysts are usually implemented in two forms: suspended powder and immobilized thin films. A simple technique for fabricating TiO2 thin film photocatalyst was developed in this work. The fabricated TiO2 thin film photocatalyst featured a homogeneous nanowire layer grown in situ on the parent Ti plate. The optimized fabrication protocol was to soak the ultrasonically cleaned and acid-washed Ti plate in 30% H2O2 solution containing 3.2mM melamine and 0.29M HNO3 at 80°C for 72h and then anneal at 450°C for 1h. TiO2 nanowires with uniform diameters were homogeneously arrayed on the Ti plate surface. The thickness of the TiO2 nanowire array layer was 1.5μm. The pore properties of the TiO2 thin film were close to those of P25. The band gap of the fabricated photocatalyst was 3.14eV. The photocatalytic activity of the fabricated photocatalyst toward 10mg/L RhB and 1mg/L CBZ demonstrated greater than 60% degradation under 2h UVC irradiation. The RhB and CBZ degradation efficiencies remained at a good level after 5 consecutive cycles. Mechanical wearing, such as 2min sonication, will not lead to significant suppression of the photocatalytic activity. Photocatalytic RhB and CBZ degradation using the fabricated photocatalyst favored an acidic > alkaline > neutral environment. The presence of Cl- slightly suppressed the photocatalytic degradation kinetics. However, RhB and CBZ photocatalytic degradation kinetics were promoted in the copresence of SO42- or NO3-.

Mo-doped, Cr-Doped, and Mo–Cr codoped TiO2 thin-film photocatalysts by comparative sol-gel spin coating and ion implantation

Uniformly codoped anatase TiO2 thin films of varying (equal) Mo and Cr concentrations (≤1.00 mol% for each dopant) were fabricated using sol-gel spin coating and deposited on fused silica substrates. All films were annealed at 450 °C for 2 h to recrystallise anatase. Undoped anatase films have been subjected to dual ion implantation for the first time, using Mo, Cr, and sequential Mo + Cr at 1 × 1014 atoms/cm2. The films were characterised by GAXRD, AFM, SIMS, XPS, and UV–Vis and the performance was assessed by dye degradation. Despite the volumetric doping by sol-gel and the directional doping by ion implantation, neither method resulted in homogeneous dopant distributions. Both methods caused decreasing crystallinities and associated partial amorphisation. The XPS signal of the uniformly codoped films is dominated by undissolved dopant ions, which is not the case for the ion-implanted films. Increasing Ti valences are attributed to the fully oxidised condition of the Ti4+ ions that diffuse to the surface from Ti vacancy formation compared to the Ti valence of the bulk lattice, which contains Ti3+. Increasing O valence is attributed to the electronegativity of O2−, which is higher than that of Ti4+. Detailed structural mechanisms for the solubility and energetics mechanisms involve the initial formation of Mo and Cr interstitials that fill the two voids adjacent to the central Ti ion in the TiO6 octahedron, followed by integrated solid solubility (ISS) and intervalence/multivalence charge transfer (IVCT/MVCT). The sequential order of the last two is reversed for the two different doping methods. These two effects are likely to be the source of synergy, if any, between the two dopant ions. The photocatalytic performances of the uniformly codoped films are relatively poor and correlate well with the band gap (Eg). The performances of the ion-implanted films do not correlate with the Eg, where TiO2–Mo performs poorly but TiO2–Cr and TiO2–Mo–Cr outperform the undoped film. These results are interpreted in terms of the competition between the effects of Mo doping, which causes partial amorphisation and/or blockage of active sites, and Cr doping, which may cause Mo–Cr synergism, Cr-based heterojunction formation, and/or improved charge-carrier separation owing to the surface-deposition nature of ion implantation.

Photocatalytic degradation of the acetaminophen by nanocrystal-engineered TiO2 thin film in batch and continuous system

Un-biodegradable pharmaceuticals are one of the major growing threats in the wastewaters. In the current study, TiO2 thin film photocatalysts were designed by nanocrystal engineering and fabricated for degradation of the acetaminophen (ACE) in a photocatalytic reaction under UV light irradiation in batch and continuous systems. The photocatalyst was prepared by sputtering and then engineered by thermal treatment (annealing at 300°C (T300) and 650°C (T650)). The annealing effects on the crystallinity and photocatalytic activity of the TiO2 film were completely studied; it was found that annealing at higher temperatures increases the surface roughness and grain size which are favorable for photocatalytic activity due to the reduction in the recombination rate of photo-generated electron-hole pairs. For the continuous system, a flat plate reactor (FPR) was designed and manufactured. The photocatalytic performance was decreased with the increase of flow rate because the higher flow rate caused to form the thicker film of the liquid in the reactor and reduced the UV light received by photocatalyst. The reusability and durability of the samples after 6 h of photocatalytic reaction showed promising performance for the T650 sample (annealed samples in higher temperatures).

Effect of the sonication and coating time on the photocatalytic degradation of TiO2, TiO2-Ag, and TiO2-ZnO thin film photocatalysts

Today, the ultrasound utilizing for material synthesis has been extensively investigated. The unusual acoustic cavitation phenomenon caused by ultrasonic waves has created a new world for the production of high efficiency photocatalysts with new structures. In this study, TiO2, TiO2-Ag, and TiO2-ZnO thin film photocatalysts were prepared using titanium isopropoxide Ti[OCH(CH3)2]4, zinc acetate dehydrates (CH3COO)2Zn·2H2O, and silver nitrate AgNO3 by a sol–gel method under the ultrasonic irradiation. The prepared photocatalysts were characterized by UV–vis diffuse reflectance spectroscopy, X-ray diffraction, scanning electron microscopy (SEM), and energy dispersive spectroscopy. The SEM images showed that the Ag and ZnO particles were evenly dispersed in the photocatalysts due to the ultrasonic irradiation, and Ag particles were approximately 90 nm, which is relatively small compared to the photocatalysts which is not treated with ultrasonic irradiation. The catalytic activity of the photocatalysts was determined using Acid Red 27 dye. The most excellent catalytic degradation was obtained with TiO2-ZnO thin film photocatalyst. In comparison to the conventional photocatalyst, the efficiency of photocatalytic activity of the photocatalyst produced under ultrasonication has been increased due to the reduced size of Ag and ZnO and its uniform dispersion.

Reduction of Peroxide Value and Free Fatty Acid Value of Used Frying Oil Using TiO2 Thin Film Photocatalyst

The quality of used frying oil degraded due to the presence of products degradation, such as; PV and FFA which formed during the frying process. PV and FFA are harmful to human health. The photocatalytic activity of TiO2 thin film has been applied in various fields, especially in the environment. The aim of this study is to evaluate photocatalytic activity of TiO2 thin film for reducing PV and FFA in used frying oil. The TiO2 thin films were deposited on glass substrate by spray coating method at a temperature of 450 oC. The TiO2 precursor solution was prepared by mixing TTiP, AcAc, and ethanol. The thin films were varied into two conditions; as-deposited and annealed at a temperature of 500 oC. The morphology, crystalline structure, and optical properties of the thin films were characterized by scanning electron microscope (SEM), X-ray Diffraction (XRD), and UV-VIS spectrophotometer, respectively. The photocatalytic process was carried out by putting TiO2 thin film in used frying oil and irradiated by sunlight. The result showed that both of TiO2 thin films were still amorphous in nature. However, there was a peak with low intensity for annealed TiO2 thin film which corresponding to the TiO2 anatase crystals plane of (101). Annealing process improved crystallinity and changed the shape morphology of TiO2 thin films. The band gap was found to be 3.59 eV for as-deposited TiO2 thin film and 3.49 eV for the annealed-TiO2 thin film. The photocatalytic process shows that TiO2 thin films reduced FFA and PV of used frying oil up to 67.10% and 79.15%, respectively. Photocatalytic activity of annealed TiO2 thin film was higher than as-deposited TiO2 thin film. The results indicated that TiO2 thin film photocatalyst potential as the new alternative method to purify used frying oil.

Nanopillars TiO2 thin film photocatalyst application in the remediation of aquatic environment

We assessed the photocatalytic behavior of Nanopillars-TiO2 thin films obtained onto a borosilicate glass in the degradation of 17α-ethynylestradiol (EE2) from aqueous solution under batch reactor operations. The thin films were characterized by the XRD, SEM, AFM analytical methods. BET specific surface area and pore sizes were also obtained. The photocatalytic degradation of EE2 using the UV light was studied at wide range of physico-chemical parametric studies to determine the mechanism of degradation as well the practical implacability of the technique. The batch reactor operations were conducted at varied pH (pH 4.0 to 10.0), EE2 initial concentration (1.0 to 5.0mg/L) and presence of several interfering ions, i.e., cadmium nitrate, copper sulfate, zinc chloride, sodium chloride, sodium nitrate, sodium nitrite, glycine, oxalic acid and EDTA in the photocatalytic degradation of EE2. The time dependence photocatalytic degradation of EE2 was demonstrated with the pseudo-first-order rate kinetics. The mineralization of EE2 was assessed using the total organic carbon analysis conducted at varied initial pH and EE2 concentrations. Further, the repeated use of the photocatalyst enhanced the applicability of thin films in the successive photocatalytic operations.

Photocatalytic decolourisation of dyes using TiO2 thin film photocatalysts

This research investigates the photocatalytic decolourisation of two selected dyes using multiple thin film coatings (three, four and five layers) of a TiO2 photocatalyst under a low intensity ultraviolet A (UVA) light source. The TiO2 photocatalyst was prepared using a simplified sol–gel process, and the decolourisation efficiencies of the two experimental dyes, indigo carmine and reactive black 5, were studied. The kinetics of the photocatalytic decolourisation reactions under low intensity UVA were described by a pseudofirst order kinetic model, and the photocatalytic kinetic rate constants were reported. An increase in the number of coating layers contributed to the TiO2 photocatalyst's smaller grain size, higher surface area and narrower optical band gap. The findings also revealed a positive relationship between the number of TiO2 coating layers and the efficiencies and kinetics of dye decolourisation in the experimental photocatalytic process. In addition, the TiO2 thin film photocatalyst induced high photocatalytic activity, even under low intensity UVA light.

Production of fine chemicals by integrated photocatalytical degradation of alkali lignin solution in corrugated plate reactor and cyclic extraction technology

This paper presented a procedure which includes the photocatalytical degradation of alkali lignin solution in corrugated plate reactor and the recovery of degraded products in organic solvent with cyclic extraction. It was prepared a Fe–TiO2 thin film photocatalyst by dip-coating method successfully immobilized on the surface of copper corrugated plate. Lignin was dissolved in sodium hydroxide (0.5mol/L) solution and the concentration of lignin solution in range of 10–40g/L. Lignin was degraded on the surface of catalyst and over irradiation by ultraviolet (UV) lamp (power-21W with wavelength of 254nm). By GC–MS analysis, it confirmed that there were more than 20 kinds of compounds in degradation products of alkali lignin. In addition to this, some commercial value chemicals including 2,6-bis(1,1-dimethylethyl)-4-methylphenol (BHT), dibutyl phthalate (DBP) and diisobutyl phthalate (DIBP) were also observed.

Separate-type Pt-free photofuel cell based on a visible light-responsive TiO2 photoanode: Effect of hydrofluoric acid treatment of the photoanode

The present study deals with the effect of a hydrofluoric acid (HF) treatment of visible light-responsive TiO2 (Vis-TiO2) thin film photocatalysts on their photoelectrochemical performances. The Vis-TiO2 thin film photocatalysts have been prepared on Ti metal substrates by a magnetron sputtering deposition method and then treated by an HF solution. The HF treatment of Vis-TiO2 thin film photocatalysts leads to the dramatic enhancement of their photoelectrochemical performances under UV light and visible light irradiation. Moreover, separate-type Pt-free photofuel cells (SPFCs) constructed using the Vis-TiO2 thin films as photoanodes can generate electricity under light irradiation and simultaneously decompose methanol. The SPFC based on the Vis-TiO2 thin film photoanode etched by HF solution shows the enhanced cell performances with two times higher short-circuit current density and higher energy conversion efficiency than that based on the untreated Vis-TiO2 thin film photoanode. In addition to the increase of surface area and surface roughness, the enhanced conductivity by HF treatment, which is confirmed by Mott–Schottky analyses, is found to be responsible for improvement of cell performances of the SPFC.

Ultra-long Pt nanolawns supported on TiO2-coated carbon fibers as 3D hybrid catalyst for methanol oxidation

In this study, TiO2 thin film photocatalyst on carbon fibers was used to synthesize ultra-long single crystalline Pt nanowires via a simple photoreduction route (thermally activated photoreduction). It also acted as a co-catalytic material with Pt. Taking advantage of the high-aspect ratio of the Pt nanostructure as well as the excellent catalytic activity of TiO2, this hybrid structure has the great potential as the active anode in direct methanol fuel cells. The electrochemical results indicate that TiO2 is capable of transforming CO-like poisoning species on the Pt surface during methanol oxidation and contributes to a high CO tolerance of this Pt nanowire/TiO2 hybrid structure.

Ion engineering techniques for the preparation of the highly effective TiO2 photocatalysts operating under visible light irradiation

The successful application of ion engineering techniques for the development of TiO2 photocatalysts operating under visible and/or solar light irradiations has been summarized in this review article. First, we have physically doped various transition metal ions within a TiO2 lattice on an atomic level by using an advanced metal ion implantation method. The metal ion implanted TiO2 could efficiently work as a photocatalyst under visible light irradiation. Some field tests under solar light irradiation clearly revealed that the Cr or V ions implanted TiO2 samples showed 2–3 times higher photocatalytic reactivity than the un-implanted TiO2. Second, we have developed the visible light responsive TiO2 thin film photocatalyst by a single process using an RF-magnetron sputtering (RF-MS) deposition method. The vis-type TiO2 thin films showed high photocatalytic reactivity for various reactions such as reduction of NOx, degradation of organic compounds, and splitting of H2O under visible and/or solar light irradiations.

Enhanced photocatalytic activity of TiO2 thin film coating on microstructured silicon substrate

Photocatalytic TiO2 thin film is prepared by sol-gel technique on microstructured silicon substrate produced by femtosecond laser cumulative irradiation. The photocatalytic activity is evaluated by the degradation of methylene blue (MB) solution under ultraviolet (UV) irradiation. For 6-ml MB solution with initial concentration of 3.0 \times 10^{-5} mol/L, the degradation rate caused by TiO2 thin film of 2-cm2 area is higher than 70% after 10-h UV irradiation. Microstructured silicon substrate is found to enhance photocatalytic activity of the TiO2 thin film remarkably. The femtosecond laser microstructured silicon substrate is suitable to support TiO2 thin film photocatalysts.

Photocatalytic degradation of dyes in water using porous nanocrystalline titanium dioxide

The degradation of the dyes methylene blue (MB), methyl orange (MO), and indigo (Ind) in water was carried out via a photocatalytic reaction. The porous nanocrystalline TiO2 photocatalysts used in the photocatalytic degradation were prepared using polyethylene glycol (PEG) as a structure-directing agent in the sol–gel system. The Taguchi experimental design method (L9(34)) was applied to determine the preparation conditions of the sol–gel process. The TiO2 photocatalyst powders obtained at the optimal preparation conditions through analysis of variance (ANOVA) could degrade methylene blue, methyl orange, and indigo in water by 57.06–84.14%, 35.15–50.09%, and 69.09–99.09%, respectively, after 10h of UV irradiation. TiO2 thin-film photocatalyst was also prepared at the optimal preparation conditions; it exhibited good performance for the photodegradation of methylene blue, methyl orange, and indigo solution under UV light irradiation.

Design of macroporous TiO2 thin film photocatalysts with enhanced photofunctional properties

TiO2 thin films with uniform macropores were successfully prepared on quartz substrates by a dip-coating method using poly(methyl methacrylate) (PMMA) microspheres as a template. SEM observations revealed that the diameter of the macropores showed a good correspondence with the size of the PMMA microspheres. These thin films had anatase crystalline structures and maintained relatively high transparency. The degradation of organic pollutants such as 2-propanol and acetaldehyde under UV light was significantly enhanced by the construction of the macropores. It was also found that TiO2 thin films with macropores exhibited good photoinduced hydrophilicity after a short period of UV light irradiation, and a slow recovery of water contact angles in the dark as compared to those of non-porous TiO2 thin films.

Noble metal-modified TiO2 thin film photocatalyst on porous steel fiber support

Through-porous steel fiber matrix with high specific surface area and self-support strength was examined as the support of TiO2 film photocatalyst. TiO2 was uniformly and conformally deposited onto the surface of the metal skeleton of the porous steel fiber matrix via atomic layer deposition method so that the porosity as well as gas permeability of the matrix is maintained. For further improving the photocatalytic activity, noble metals (Au and Pt/Pd alloy) were sputtered onto the surface of TiO2 photocatalyst. The photodegradation of methyl orange was used for evaluating the photocatalytic properties. The results show that compared with TiO2 films deposited on flat Si wafers, TiO2 films on the porous support display higher photocatalytic activities, owing to their higher specific surface areas. The photocatalytic activity of TiO2 on porous support was not enhanced by Au, while it was remarkably improved by Pt/Pd. The TiO2 photocatalysts were also analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy.

Photocatalytic Decomposition of Water on Double-Layered Visible Light-Responsive TiO2 Thin Films Prepared by a Magnetron Sputtering Deposition Method

A radio-frequency magnetron sputtering (RF-MS) deposition method was applied to prepare a unique double-layered visible light-responsive TiO2 thin film photocatalyst on a Ti foil substrate (DL-TiO2/Ti). The DL-TiO2/Ti consists of a UV light-responsive TiO2 thin film (UV-TiO2) deposited as the inner block layer on a Ti foil substrate and a visible light-responsive TiO2 thin film (Vis-TiO2) deposited as the outer layer. DL-TiO2/Ti exhibited higher photoelectrochemical and photocatalytic performance under both UV and visible light irradiation than a single-layered Vis-TiO2 thin film photocatalyst deposited on a Ti foil substrate. The optimal thickness of the UV-TiO2 film as the inner block layer of DL-TiO2/Ti was found be 100 nm in terms of the photoelectrochemical and photocatalytic performance. Expanding on this work, a novel double-layered TiO2 thin film device, DL-TiO2/Ti/Pt, was also prepared by the RF-MS method where DL-TiO2 was deposited on one side of a Ti foil substrate and Pt was deposited on the other side. The separate evolution of H2 and O2 from H2O was successfully achieved by using an H-type glass cell consisting of two aqueous phases separated by the DL-TiO2/Ti/Pt and a proton-exchange membrane. The separate evolution rate of H2 and O2 was found to be dramatically enhanced on DL-TiO2/Ti/Pt as compared with a single-layered Vis-TiO2 thin film device under white light irradiation from a solar simulator.

Recent Progress on Development of TiO2 Thin Film Photocatalysts for Pollutant Removal

Recent Progress on Development of TiO2 Thin Film Photocatalysts for Pollutant Removal

Photoinduced hydrophilicity of TiO2 film as the effect of H2O2 addition

TiO2 thin film photocatalysts coated onto glass plates were prepared by thermal decomposition of tetraisopropyl orthotitanate with a dip-coating process. In this Letter, the mechanism for photoinduced hydrophilicity of TiO2 film is clarified by adding the extra H2O2 on TiO2 surface. Results show that the photoinduced hydrophilicity of TiO2 film is inhibited with the presence of H2O2. The amount of Ti–OH bonds on the surface oppositely increased by the addition of H2O2. It indicates the photoinduced hydrophilicity of TiO2 results from surface oxygen vacancy, not Ti–OH bonds.

Preparation of TiO2 Thin Film Photocatalyst by High-rate Low-temperature Sputtering Method

After depositing an ultra-thin metallic Ti film by sputtering, we repeated the process of exposing the still active plasma gas for high-rate low-temperature deposition of a metal compound film, thereby successfully fabricating a thin film of TiO2 with photocatalytic properties. The TiO2 thin film produced using the present method exhibited approximately 15 times the deposition rate of DC reactive magnetron sputtering, yielding a crystalline structure using an unheated substrate (approximately 40°C).