Research results of the Centre NANOPIN
The complex study of methods of preparation of various forms of nanocrystalline photoactive materials (mostly based on titanium oxide), their physical and chemical characterisation and their functional characterisation in practical photocatalytic processes is carried out in the Research Centre for Nanosurface Engineering "NANOPIN" in accordance with the Research Program of the Centre.
1. Synthesis of photocatalytic nanoparticles
For synthesis of highly active and resistant photocatalysts a number of methods of preparation based on metal oxides (mostly titanium dioxide) is designed. Preparation of colloidal solutions of quantum-sized particles either pure Q-TiO2 or doped by various amounts of three valence iron Q-TiO2 (Fe3+) are under development at the J.Heyrovsky Institute of Physical Chemistry. This process employs the controlled hydrolysis of TiCl4 in aqueous solutions containing corresponding concentrations of FeCl3 [1]. Transparent particles (aqueous colloidal solutions of Q -TiO2 particles) prepared by hydrolysis of TiCl3 solution in the presence polyethyleneglycol [2] and preparation of TiO2 by homogeneous precipitation of the aqueous solution of titanyl sulfate with urea [3] are studied at the Institute of Inorganic Chemistry. Prepared raw powder (Tit30) consists of pure anatase nanoparticles (3-4 nm in diameter) forming spherical agglomerates (1-2 µm) with specific surface area of 300 m2/g (Fig. 1) with the photocatalytic activity about half of TiO2 P25 (Degussa). This parameter has been markedly improved performing additional physical treatment [3]. The resulted most active photocatalyst consists of both anatase and rutile phases.
Fig. 1. Comparison of P25 powder (left) and synthesised Tit30 powder dried at 80°C (right).
Another approach that is under development comprises preparation of sodium titanate nanowires and nanorods (diameter around 50 nm, lengths up to 2-3 µm) using a precursor synthesized by the reaction of sodium titanate with ethylene glycol [4]. During subsequent heating (900°C) of this precursor the glycolate complex of sodium titanate is decomposed and a crystalline product of chemical composition Na2Ti6O13 with morphology of nanorods is formed (Fig. 2). Prepared nanorods showed a good photocatalytic activity for the decomposition of 4-chlorophenol in aqueous slurry under UV radiation. Advantage of this method is that as a raw material is utilized a by-product from the production of white titania pigment by sulfate technology (Precheza Přerov) and, therefore, this raw material is significantly cheaper and more advantageous for technological use in comparison with titanium alkoxide-based materials.
Fig. 2.TEM micrographs of sodium titanate glycolate complex nanoparticles: (a) initial precursor sample; (b) sample heated at 550°C/2h; (c) sample heated at 900°C/2h.
2. Preparation of photoactive thin layers
Preparation of particulate titanium oxide layers of various thickness is under development at the Institute of Inorganic Chemistry and the Institute of Chemical Technology, Prague. Layers on glass and conductive glass were prepared by sedimentation from aqueous suspension of TiO2 particles P25 (Degussa) and particles Tit30 synthesized by homogenous precipitation of TiOSO4 aqueous solutions with urea [5]. Final fixation of the layer associated with the improvement of the layers` mechanical properties was achieved by controlled heating. Morphology of immobilized layers of P25 and Tit30 is shown in Fig. 3.
Fig. 3.Comparison of P25 layer (left) and Tit30 (right) layer on glass plate after sedimentation and thermal treatment at 300°C.
Alternative procedures of preparation of TiO2 layers are also under development. Among particulate, sol-gel and thermally produced layers [6], the plasma deposition of nanocrystalline titanium dioxide on various substrates by using PECVD and/or PVD methods is investigated at Technical University of Liberec. The films properties are optimised by systematic variation of the deposition conditions (chemical composition of the precursor, pressure and composition of the working gas, substrate temperature, electric discharge parameters, geometrical configuration and the type of the discharge, deposition time etc). Plasma diagnostics are used to ensure the reproducible deposition conditions and to clarify the dependence of the film properties on the microscopic discharge conditions. The principal advantages of the plasma based processes, mainly the low substrate temperature, deposition of compact films and good film adhesion (even to hydrophobic substrates) including plastics are the main motivations for these experiments.
3. Design of standard photoactivity testing methods
For assessment of activity of different photocatalysts standard testing methods are necessary. Therefore, standard procedures are proposed by Nanopin centre for estimation of photocatalytic activity of colloidal solutions of quantum-sized particles, aqueous suspensions of powder materials and nanoparticulate thin layers deposited on various supports. The proposed photoactivity tests are based on measuring of the initial transformation rates of appropriate model reactants. Powder materials are tested in a water-jacketed tube photoreactor. As a model compound, 4-chlorophenol is degraded in aerated aqueous suspension, magnetically stirred and irradiated by monochromatic light of wavelength 365 nm. Evaluation of photocatalytic activity is based on the measurements of change of pH caused by produced hydrochloric acid and concentration of 4-chlorophenol by HPLC/TOC techniques. Another method employs model organic dye Orange II. Change of absorbance at 483 nm caused by photocatalytic decomposition of dye is measured. Orange II, sodium salt of sulphonated azo dye, has several advantages. First, as an anion, it is not adsorbed on the negatively charged surface of titanium dioxide in neutral aqueous solutions. Second, because Orange II has an absorption band with maximum at 483 nm and an absorption minimum at 350 nm, Orange II absorbs only small part of the emitted radiation while the majority is absorbed by the photocatalyst. Examples of apparatuses used for testing of photocatalytic activity of powders and layers is shown in Fig. 4.
Fig. 4.(a) Flow-through reactor for estimation of photoactivity of large photocatalytic layers; (b) simple photoreactor for estimation of photoactivity of photocatalytic powders or layers.
REFERENCES
[1] K. Macounová, H. Krýsová, J. Ludvík and J. Jirkovský, Journal of Photochemistry and Photobiology A: Chemistry 156 (2003) 273-282
[2] V. Štengl, S. Bakardjeva, N. Murafa, J. Šubrt, H. Měšťánková and J. Jirkovský, Mat. Chem. Phys. submitted for publication
[3] J. Krýsa, M. Keppert, J. Jirkovský, V. Štengl and J. Šubrt, Mater. Chem. Phys. 86 (2004) 333-339
[4] V. Štengl, S. Bakardjeva, J. Šubrt, E. Večerníková, L. Szatmary, M. Klementová and V. Balek, Applied Catalysis B-Enviromental 63 (2005) 20-30
[5] J. Krýsa, M. Keppert, G. Waldner and J. Jirkovský, Electrochem. Acta Vol. 50 (2005) 5255-5260
[6] J. Krýsa and G. Waldner, Chem. Listy 99 (2005) s512-s513
