Photocatalytic TiO2 Thin Films Research Articles

Non-blockage of atomic-scale active sites in photocatalytic TiO2 thin films deposited on silica-based substrates

The present work delivers an in-depth analysis on contamination and active site blockage of anatase thin films deposited on three types of SiO2-based glass substrates using high-resolution XPS and ICP-MS. Further, the photocatalytic performance of the thin films before and after purification were analysed by UV–Vis spectrophotometry. The results reveal that ionic contaminants including alkalies (Na), alkaline earths (Mg, Ca), Al, Si, and F (and probably B) diffuse from the substrates into the grain boundaries of the thin films and represent potential sources of active site blockage. The long-term leaching in aqueous-based medium showed the entire removal of the contaminants. It was also revealed that the kinetics of leaching are controlled not by the ionic sizes or valences of the contaminant ions but by their solubilities, which are confirmed by theoretical thermodynamic study and speciation diagrams for soluble species deriving from the three types of substrates.

Photocatalytic TiO2 thin films and coatings prepared by sol–gel processing: a brief review

Photocatalytic TiO2 thin films and coatings prepared by sol–gel processing: a brief review

Unveiling a critical thickness in photocatalytic TiO2 thin films grown by plasma-enhanced chemical vapor deposition using real time in situ spectroscopic ellipsometry

TiO2 thin films of various thicknesses have been deposited on silicon at low-temperature by PECVD operating in continuous mode (T < 130 °C) and pulsed mode (T < 80 °C) using oxygen/titanium isopropoxide low-pressure inductively coupled plasma. The study of the crystallinity and microstructure of the films by atomic force microscopy, scanning electron microscopy and x-ray diffraction allowed showing that the roughness and amount of anatase are closely related to the film thickness. The coalescence of large polycrystalline columns emerging from an assembly of thin columns was found to happen at a critical thickness, at about 150 nm in the continuous mode and 250 nm in the pulsed mode. Real time in situ spectroscopic ellipsometry study allowed to monitor the growth and to determine this critical thickness for both plasma modes. The change of morphology type to large columnar structure is associated with an important increase in the photocatalytic activity. The determination of this coalescence thickness by ellipsometry may provide an interesting method to evaluate the impact of process parameters on TiO2 thin films characteristics.

Anti-Fogging, Photocatalytic and Self-Cleaning Properties of TiO2-Transparent Coating

Transparent, photocatalytic, and self-cleaning TiO2 thin film is developed by TiO2 sol-gel coating on glass and polycarbonate (PC) substrates. Acetyl acetone (AcAc) suppresses the precipitation of TiO2 by forming a yellowish (complex) transparent sol-gel. XPS analysis confirms the presence of Ti2p and O1s in the thin films on glass and PC substrates. The TiO2- sol is prepared by stabilizing titanium (IV) isopropoxide (TTIP) with diethylamine and methyl alcohol. The addition of AcAcsilane coupling solution to the TiO2-sol instantaneously turns to yellowish color owing to the complexing of titanium with AcAc. The AcAc solution substantially improves the photocatalytic property of the TiO2 coating layer in MB solutions. The coated TiO2 film exhibits super hydrophilicity without and with light irradiation. The TiO2 thin film stabilized by adding 8.7 wt% AcAc shows the highest photo-degradation for methylene blue (MB) solution under UV light irradiation. Also, the optimum photocatalytic activity is obtained for the 8.7 wt% AcAc-stabilized TiO2 coating layer calcined at 450 oC. The thin-films on glass exhibit fast self-cleaning from oleic acid contamination within 45 min of UV-light irradiation. The appropriate curing time at 140 oC improves the anti-fogging and thermal stability of the TiO2 film coated on PC substrate. The watermark-free PC substrate is particularly beneficial to combat fogging problems of transparent substrates.

Composition and properties of RF-sputter deposited titanium dioxide thin films.

The photocatalytic properties of titania (TiO2) have prompted research utilising its useful ability to convert solar energy into electron–hole pairs to drive novel chemistry. The aim of the present work is to examine the properties required for a synthetic method capable of producing thin TiO2 films, with well defined, easily modifiable characteristics. Presented here is a method of synthesis of TiO2 nanoparticulate thin films generated using RF plasma capable of homogenous depositions with known elemental composition and modifiable properties at a far lower cost than single-crystal TiO2. Multiple depositions regimes were examined for their effect on overall chemical composition and to minimise the unwanted contaminant, carbon, from the final film. The resulting TiO2 films can be easily modified through heating to further induce defects and change the electronic structure, crystallinity, surface morphology and roughness of the deposited thin film.

Anatase TiO2 deposited at low temperature by pulsing an electron cyclotron wave resonance plasma source

Photocatalytic surfaces have the potentiality to respond to many of nowadays societal concerns such as clean H2 generation, CO2 conversion, organic pollutant removal or virus inactivation. Despite its numerous superior properties, the wide development of TiO2 photocatalytic surfaces suffers from important drawbacks. Hence, the high temperature usually required (> 450 °C) for the synthesis of anatase TiO2 is still a challenge to outreach. In this article, we report the development and optimisation of an ECWR-PECVD process enabling the deposition of anatase TiO2 thin films at low substrate temperature. Scanning of experimental parameters such as RF power and deposition time was achieved in order to maximise photocatalytic activity. The careful selection of the deposition parameters (RF power, deposition time and plasma gas composition) enabled the synthesis of coatings exhibiting photocatalytic activity comparable to industrial references such as P25 Degussa and Pilkington Activ at a substrate temperature below 200 °C. In addition, to further decrease the substrate temperature, the interest of pulsing the plasma RF source was investigated. Using a duty cycle of 50%, it is thus possible to synthesise photocatalytic anatase TiO2 thin films at a substrate temperature below 115 °C with a deposition rate around 10 nm/min.

Plasmonic gold-embedded TiO2 thin films as photocatalytic self-cleaning coatings

Transparent photocatalytic TiO2 thin films hold great potential in the development of self-cleaning glass surfaces, but suffer from a poor visible light response that hinders the application under actual sunlight. To alleviate this problem, the photocatalytic film can be modified with plasmonic nanoparticles that interact very effectively with visible light. Since the plasmonic effect is strongly concentrated in the near surroundings of the nanoparticle surface, an approach is presented to embed the plasmonic nanostructures in the TiO2 matrix itself, rather than deposit them loosely on the surface. This way the interaction interface is maximised and the plasmonic effect can be fully exploited. In this study, pre-fabricated gold nanoparticles are made compatible with the organic medium of a TiO2 sol-gel coating suspension, resulting in a one-pot coating suspension. After spin coating, homogeneous, smooth, highly transparent and photoactive gold-embedded anatase thin films are obtained.

Pathways to Tailor Photocatalytic Performance of TiO2 Thin Films Deposited by Reactive Magnetron Sputtering.

TiO2 thin films are used extensively for a broad range of applications including environmental remediation, self-cleaning technologies (windows, building exteriors, and textiles), water splitting, antibacterial, and biomedical surfaces. While a broad range of methods such as wet-chemical synthesis techniques, chemical vapor deposition (CVD), and physical vapor deposition (PVD) have been developed for preparation of TiO2 thin films, PVD techniques allow a good control of the homogeneity and thickness as well as provide a good film adhesion. On the other hand, the choice of the PVD technique enormously influences the photocatalytic performance of the TiO2 layer to be deposited. Three important parameters play an important role on the photocatalytic performance of TiO2 thin films: first, the different pathways in crystallization (nucleation and growth); second, anatase/rutile formation; and third, surface area at the interface to the reactants. This study aims to provide a review regarding some strategies developed by our research group in recent years to improve the photocatalytic performance of TiO2 thin films. An innovative approach, which uses thermally induced nanocrack networks as an effective tool to enhance the photocatalytic performance of sputter deposited TiO2 thin films, is presented. Plasmonic and non-plasmonic enhancement of photocatalytic performance by decorating TiO2 thin films with metallic nanostructures are also briefly discussed by case studies. In addition to remediation applications, a new approach, which utilizes highly active photocatalytic TiO2 thin film for micro- and nanostructuring, is also presented.

TEM analysis of photocatalytic TiO2 thin films deposited on polymer substrates by low-temperature ICP-PECVD

Abstract TiO2 thin films were deposited on polymer substrates (polyethylene terephthalate and polystyrene) at low substrate temperature (

Role of UV Plasmonics in the Photocatalytic Performance of TiO2 Decorated with Aluminum Nanoparticles

We present a facile method, combining sputtering and gas aggregation techniques, to prepare a photocatalytic TiO2 thin film decorated with stable aluminum plasmonic nanoparticles (Al NPs) to reveal the localized surface plasmon resonance (LSPR) effect on TiO2 photocatalysis under UV irradiation. We demonstrate for the first time the negative and positive influences of LSPR on UV photocatalysis by irradiating Al NPs/TiO2 hybrid structures at two different UV wavelengths: both at and above the plasmonic absorption of Al NPs. These findings open the door to design low-cost Al/TiO2 photocatalytic hybrid surfaces that function in a broad spectral range from deep-ultraviolet to visible wavelengths.

A comparative study of photocatalysis on highly active columnar TiO2 nanostructures in-air and in-solution

While there is continuous progress in development of new photocatalytic thin films and coatings, the lack of a reliable and standard procedure for measuring the photocatalytic performance of such active surfaces makes it difficult to compare results between research groups and different measurement setups. Here, a comparative study was carried out to demonstrate the high photocatalytic activity of sputter-deposited TiO2 film with self-organized nanocrack networks by using two different analytical approaches: (i) bleaching of a thin Methylene Blue (MB) solid layer on photocatalytic TiO2 thin film (in-air) and (ii) the decolorization of a MB aqueous solution in the presence of TiO2 thin film (in-solution). While the decolorization of aqueous MB solution provides an indirect observation of the photocatalytic effect imposed by the TiO2 film, the use of a solid MB layer as an indicator allows monitoring of photocatalytic reactions at the solid-air interface directly. We showed the applicability of this approach as a complementary and a fast analysis method to reveal the photocatalytic efficiency of thin films by comparing it with the state of the art inks (based on MB and other similar organic dyes) used as photocatalysis indicator.

XANES study of vanadium and nitrogen dopants in photocatalytic TiO2 thin films.

We report an X-ray absorption near edge structure (XANES) study of vanadium (V) and nitrogen (N) dopants in anatase TiO2 thin films deposited by radio-frequency magnetron sputtering. Measurements at the Ti K and V K edges were combined with soft X-ray experiments at the Ti L2,3, O K and N K edges. Full potential ab initio spectral simulations of the V, O and N K-edges were carried out for different possible configurations of substitutional and interstitial dopant-related point defects in the anatase structure. The comparison between experiments and simulations demonstrates that V occupies substitutional cationic sites (replacing Ti) irrespective of the film structure and dopant concentration (up to 4.5 at%). On the other hand, N is found both in substitutional anionic sites (replacing O) and as N2 dimers within TiO2 interstices. The dopants' local structures are discussed with reference to the enhanced optical absorption and photocatalytic activity achieved by (co)doping.

Self-Cleaning and Photocatalytic Performance of TiO2 Coating Films Prepared by Peroxo Titanic Acid

Self-cleaning and photocatalytic TiO2 thin films were prepared by a facile sol-gel method followed by spin coating using peroxo titanic acid as a precursor. The as-prepared thin films were heated at low temperature(110 °C) and high temperature (400 °C). Thin films were characterized by X-ray diffraction(XRD), Field-emission scanning electron microscopy(FESEM), UVVisible spectroscopy and water contact angle measurement. XRD analysis confirms the low crystallinity of thin films prepared at low temperature, while crystalline anatase phase was found the for high temperature thin film. The photocatalytic activity of thin films was studied by the photocatalytic degradation of methylene blue dye solution. Self-cleaning and photocatalytic performance of both low and high temperature thin films were compared.

Photocatalytic Anatase TiO2 Thin Films on Polymer Optical Fiber Using Atmospheric-Pressure Plasma.

Due to the undeniable industrial advantages of low-temperature atmospheric-pressure plasma processes, such as low cost, low temperature, easy implementation, and in-line process capabilities, they have become the most promising next-generation candidate system for replacing thermal chemical vapor deposition or wet chemical processes for the deposition of functional coatings. In the work detailed in this article, photocatalytic anatase TiO2 thin films were deposited at a low temperature on polymer optical fibers using an atmospheric-pressure plasma process. This method overcomes the challenge of forming crystalline transition metal oxide coatings on polymer substrates by using a dry and up-scalable method. The careful selection of the plasma source and the titanium precursor, i.e., titanium ethoxide with a short alkoxy group, allowed the deposition of well-adherent, dense, and crystalline TiO2 coatings at low substrate temperature. Raman and XRD investigations showed that the addition of oxygen to the precursor's carrier gas resulted in a further increase of the film's crystallinity. Furthermore, the films deposited in the presence of oxygen exhibited a better photocatalytic activity toward methylene blue degradation assumedly due to their higher amount of photoactive {101} facets.

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.

Incorporation of nitrogen into TiO2 thin films during PVD processes

In this paper we investigate the possibility of incorporating nitrogen into amorphous, photocatalytic TiO2 thin films, prepared at room temperature, during the growth process. The aim is to reduce the bandgap of the UV active thin films. Physical vapor deposition experiments employing a titanium vacuum arc with gas backfill ranging from pure oxygen to pure nitrogen, are carried out. The resulting films are characterized for chemical composition, phase composition, optical properties and hydrophilicity in order to determine a correlation between gas composition and thin film properties. The experimental results point that a visible change in the band structure of the deposited layers is achieved.

Aging of the photocatalytic TiO2 thin films modified by Ag and Pt

This work focuses on the investigation of the photocatalytic activity of the TiO2 thin films (either modified or unmodified by noble metal) and on its stability during 16 months after deposition. RF discharge was used to deposit the TiO2 thin films by PECVD from the mixture of titanium isopropoxide (TTIP) vapors and oxygen. The surface of TiO2 films was subsequently modified by Ag or Pt by means of magnetron sputtering. Several analytical methods were used to characterize the thin films properties. XPS was used to investigate the chemical composition of the surface, SEM to study the surface morphology. The photocatalytic activity was expressed in terms of photocatalytic efficiency and it was determined by decomposition of the organic dye Acid orange 7 (AO7). The results show that the enhancement of the photocatalytic efficiency achieved by the modification by Ag or Pt decreases with time, but it is still several times higher after 16 months aging than in case of unmodified TiO2 layers.

Structural control and photocatalytic properties of photocatalytic TiO2 thin films prepared using two-step deposition including radical-assisted sputtering

TiO2 films were deposited using a two-step growth process, in which an ultra-thin nucleation layer was first produced by reactive-mode sputtering, followed by a thicker growth layer by radical-assisted sputtering. The effect of the structure of the nucleation layer on the density, crystallinity and photocatalytic properties of the final films was investigated. The kinetic energy of sputtered particles reaching the substrate during the first growth step was also considered. This was found to affect the amount of three-dimensional island growth that occurred, and thus the number of seed particles and gaps between them. The optimum structure is found to be one in which few seed particles have undergone island growth, so that there are a large number of gaps. This not only produces a dense final structure, but the crystallinity is improved due to chemical annealing by radicals during the second growth step, leading to a film with excellent photocatalytic properties. This indicates the importance of an initial structure that facilitates absorption and diffusion of radicals.

Effect of Nucleation Layer of Photocatalytic TiO2 Thin Film Prepared by Two-Step Deposition Using Radical-Assisted Sputtering

A nucleation layer forms in the nucleation step (the first step) of a two-step deposition method that we have developed for forming TiO2 thin films. We found that the nucleation layer affects the crystallinity of TiO2 thin films during the growth step (the second step). The crystallinity of the growth layer can be controlled by varying the kinetic energy of the sputter particles when they arrive at the substrate in the nucleation step. Since the nucleation density varies with the kinetic energy, the crystallinity is thought to improve, owing to the improvement in the nucleation density.

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.