The effects of firing conditions on the properties of electrophoretically deposited titanium dioxide films on graphite substrates
Introduction
The development of novel approaches to water purification is of increasing importance as population growth and climate change place a growing strain on water resources.1, 2 Photocatalysis is an attractive approach to water treatment as this technique does not involve the consumption of chemical reagents, enables the removal of a variety of pollutants, is effective across a wide range of pollutant concentration levels and can be achieved using solar irradiation as the sole energy input.3, 4, 5
Owing to the distinct levels of its valence and conduction bands, TiO2 has emerged as the leading material in photocatalytic applications.6, 7 TiO2 photocatalysis takes place through the photo-generation of electron-hole pairs (excitons) by irradiation exceeding the band gap of the material. This leads to the generation of surface adsorbed radicals and subsequent oxidation of organic pollutants on TiO2 surfaces.8 The two phases of titanium dioxide most commonly used in photocatalysis are anatase and rutile. Despite the slightly larger band gap of anatase (∼3.2 eV vs. ∼3.0 eV), this phase is widely considered to exhibit superior photocatalytic activity as a result of greater levels of surface adsorbed radicals.9, 10, 11, 12 It has been frequently reported that mixed-phase TiO2 exhibiting low levels of rutile alongside anatase exhibits enhanced performance through reduced electron-hole recombination.13, 14, 15, 16, 17
As photocatalyzed destruction of pollutants takes place at close proximity to TiO2 surfaces, a high surface area is advantageous for effective rates of pollutant removal.18 For this reason studies of water purification by TiO2 photocatalysts are often carried out using aqueous suspensions of powder.19, 20 The disadvantage of using TiO2 in the form of a powder suspended in the treated water is the required catalyst recovery processes, for this reason the immobilisation of TiO2 is frequently carried out.21, 22, 23, 24
Electrophoretic deposition (EPD) is a practical method for immobilising TiO2 photocatalysts as it enables rapid sample fabrication from suspensions of low solids loading.25, 26, 27 The current work examines the effects of firing conditions on the microstructure and performance of thick films prepared by anodic electrophoretic deposition of anatase TiO2 onto graphite substrates. As carbon has been reported to enhance the anatase to rutile phase transformation and lower the band gap in TiO2,28, 29, 30 this method of fabrication may improve photocatalytic performance by yielding bi-phasic TiO2 at lower temperatures and through carbon doping of the photocatalyst layer.
Section snippets
EPD
Thick TiO2 films were prepared through anodic EPD from acidic aqueous suspensions adjusted to pH ∼ 3 using oxalic acid. As reported elsewhere, the use of oxalic acid imparts a negative zeta potential to TiO2 particles in suspension and thus facilitates anodic EPD from acidic aqueous suspensions with lower levels of water electrolysis.31 Using a solids loading of 1%, a deposition voltage of 10 V and a deposition time of 10 min, TiO2 anatase powder (>99%, Merck Chemicals) with a BET evaluated surface
Microstructure
The prefiring microstructure of all thick films prepared in this work exhibited gas-bubble damage resulting from the parasitic process of water electrolysis as shown in Fig. 2. Holes resulting from bubble damaged ranged from ∼5 to ∼50 μm in size. Samples fired in air at 700 °C exhibited complete oxidation of the graphite substrate, leaving behind a fragile unsupported TiO2 film. Samples fired in air at 500 and 600 °C exhibited substrate deterioration through partial oxidation, resulting in
Effect of firing conditions on microstructure
Graphite is generally reported to exhibit rapid oxidation in air around 700 °C36, 37 and thus the oxidation of the substrates fired at 700 °C in air in this work was anticipated. As would be expected, the oxidation of the graphite substrate for films fired in air has a detrimental effect on the structure and adhesion of the deposited thick film. This is evident from increased spalling and cracking, shown in Fig. 3, and the low resilience of the air-fired films to abrasion, suggesting firing in
Conclusions
Porous thick films of TiO2 can be fabricated on graphite substrates by using a method of anodic aqueous EPD. When such fabrication methods are combined with firing in a nitrogen atmosphere a well adhered film exhibiting enhanced photocatalytic activity can be obtained.
The anatase to rutile transformation is enhanced in thick films on graphite substrates as a result of increased levels of oxygen vacancies created by the diffusion of carbon atoms into the TiO2 lattice. The diffusion of carbon and
Acknowledgements
The authors acknowledge access to the UNSW node of the Australian Microscopy and Microanalysis Research Facility (AMMRF), support from the Australian Institute of Nuclear Science and Engineering (AINSE) and the assistance of Anne Rich of the spectroscopy lab at the Mark Wainwright analytical centre at UNSW.
References (54)
Climate change and global water resources: SRES emissions and socio-economic scenarios
Global Environmental Change
(2004)- et al.
Photocatalytic oxidation of organic pollutants in water
Catalysis Today
(1998) - et al.
Supported titanium oxide as photocatalyst in water decontamination: state of the art
Catalysis Today
(1997) - et al.
Immobilisation of TiO2 powder for the treatment of polluted water
Applied Catalysis B: Environmental
(1998) - et al.
On the structural properties and optical transmittance of TiO2 rf sputtered thin films
Applied Surface Science
(2000) - et al.
Effect of rutile phase on the photocatalytic properties of nano-crystalline titania
Applied Catalysis B
(1998) - et al.
Synergism between rutile and anatase TiO2 particles in photocatalytic oxidation of naphtalene
Applied Catalysis A
(2003) - et al.
Photocatalytic labyrinth flow reactor with immobilized P25 TiO2 bed for removal of phenol from water
Applied Catalysis B: Environmental
(2003) - et al.
Linear correlation between inactivation of E. coli and OH radical concentration in TiO2 photocatalytic disinfection
Water Research
(2004) - et al.
Evaluating the activities of immobilized TiO2 powder films for the photocatalytic degradation of organic contaminants in water
Applied Catalysis B: Environmental
(2004)
Enhanced photo-efficiency of immobilized TiO2 catalyst via intense anodic bias
Electrochemistry Communications
Morphology and photocatalytic activity of highly oriented mixed phase titanium dioxide thin films
Surface and Coatings Technology
A review on fundamentals and applications of electrophoretic deposition (EPD)
Progress in Materials Science
EPD kinetics: a review
Journal of the European Ceramic Society
Synthesis of visible-light-active TiO2-based photocatalysts by carbon and nitrogen doping
Journal of Catalysis
Anodic aqueous electrophoretic deposition of titanium dioxide using carboxylic acids as dispersing agents
Journal of the European Ceramic Society
Photoluminescence in TiO2 anatase single crystals
Solid State Communications
Optical properties of anatase (TiO2)
Solid State Communications
Kinetics and mechanisms of non-isothermal oxidation of graphite in air
Corrosion Science
Low-firing of CuO-doped anatase
Materials Research Bulletin
High visible-light photocatalytic activity of nitrogen-doped titania prepared from layered titania/isostearate nanocomposite
Catalysis Today
UV solar irradiance from broadband radiation and other meteorological data
Atmospheric Research
Applications of photoluminescence techniques to the characterization of solid surfaces in relation to adsorption, catalysis, and photocatalysis
Advances in Catalysis
The application of TiO2 photocatalysis for disinfection of water contaminated with pathogenic micro-organisms: a review
Research on Chemical Intermediates
Water purification by semiconductor photocatalysis
Chemical Society Reviews
Electronic band structure of titanium dioxide
Physical Review B
Heterogeneous catalysis
Chemical Reviews
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