7.2.1 Production of the functionalised nanofibres
Two methods of functionalisation are being used in this chapter. The TiO2
nanoparticles will occur throughout the matrix of the nanofibre material when the TiO2 is added to the spinning solution. Functionalisation after the spinning process,
such as dip-coating, will result in the presence of TiO2 on the surface of the
nanofibres. Functionalisation with nTiO2 was carried out using two different types of TiO2: a colloidal suspension that was used as received by the University of Belgrade,
and commercially available Degussa P25.
The colloid solution supplied by the University of Belgrade consisting of TiO2
nanoparticles was prepared by acidic hydrolysis of TiCl4 in a manner analogous to the
one proposed by Rajh at al. (1996). The solution of TiCl4 cooled down to -20 °C was
added drop-wise to cooled water (at 4 °C) under vigorous stirring and then kept at this temperature for 30 min. The pH of the solution was between 0 and 1, depending on TiCl4 concentration. Slow growth of the particles was achieved by dialysis against
water at 4°C until the pH of the solution reached 3.5. The concentration of colloid was determined from the concentration of the peroxide complex obtained after dissolving the particles in concentrated sulphuric acid (Thompson 1984). In order to enhance the crystallinity and overall efficiency of generated TiO2 nanoparticles the
colloid was thermally treated in reflux at 60 °C for 16 h. The synthesized colloid comprises of faceted, single crystalline, anatase TiO2 nanoparticles with an average
size of 6 nm (Mihailović et al. 2010) and are further denoted as “colloidal TiO2
nanoparticles”.
Degussa P25 (Aldrich Co., USA), a standard material in the field of photocatalytic reactions, is a TiO2 nanopowder containing anatase and rutile phases in a ratio of
about 3:1 (Ohno et al. 2001) with a molecular weight of 79.87 g/mol, specific surface area 35-65 m2/g and average particle size of 21 nm. Degussa P25 was used as a commercial available reference system (Markowska-Szczupak et al. 2011). Moreover its application allowed preparation of suspensions with higher concentration of TiO2
nanoparticles then possible with our 0.12 M colloidal TiO2 nanoparticles. In this study
Degussa P25 TiO2 nanoparticles were used in an aqueous suspension of 0.12 M and
0.50 M. These nanoparticles are further denoted as “commercial TiO2 nanoparticles”.
Inline functionalised nanofibres were obtained by adding a colloid/suspension of TiO2 nanoparticles to the PA-6 solution prior to the electrospinning process. Five
different colloid/suspension volumes were added in the electrospun solution as to find the optimal dose for the two types of TiO2 nanoparticles (6 nm colloidal and 21
nm commercial TiO2). For the commercial TiO2 nanoparticles it was possible to use
both 0.12 and 0.50 M stock-solution, the colloidal TiO2 nanoparticles allowed only for
a 0.12 M colloid stock-solution since higher concentrations during synthesis are not yet possible. The colloids/suspensions were gently stirred with a magnetic stirrer for 10 minutes at room temperature after which the electrospinning process was immediately started.
Post-functionalisation was accomplished by dipping the electrospun nanofibre membranes in the 0.12 M colloidal TiO2 nanoparticles suspension or the 0.50 M
commercial TiO2 nanoparticles suspension (ratio of membrane mass (mg) to liquor
volume (ml) was 1:400) for 5 min. Subsequently the membranes were dried at room temperature (24h). After 30 min of curing at 100°C, the samples were rinsed twice (5 min) with deionized water and again dried at room temperature (24h).
7.2.2 Experiments on membrane properties
First membranes produced via single nozzle electrospinning were used for screening to determine the optimum concentration of TiO2 nanoparticles. After this screening,
the tests were repeated on the multi-nozzle set-up with the selected concentrations on membranes with a density of 10 g/m².
Prior to electrospinning the viscosity and conductivity of the electrospinning solutions were examined. The morphology of the electrospun nanofibres was examined using a scanning electron microscope.
The CWP was measured on the functionalized membranes and compared to the non- functionalized membrane. Previous work was done on non-functionalised membranes. This CWP test was performed to have an idea on a possible change in CWP due to a potential modification in morphology or pore structure as a result of the functionalisation techniques. The CWP was thus measured on the functionalised membranes and compared to the non-functionalised membrane. The membranes used in this chapter, were further developed since previous chapters allowing for production of thinner membranes, which resulted in higher CWP values than earlier described.
7.2.3 Photodegradation experiments
Photocatalytic activity of the functionalised nanofibres was assessed by photodegradation of methylene blue under UV irradiation by measuring the absorbance at 663 nm wavelength on a UV–vis spectrophotometer (UV-Vis spectrophotometer Shimadzu UV-1601). The experiment is described more in detail in chapter 3. Identical colloidal nTiO2 particles were previously tested by Mihailovic
et al. (2010) on functionalised PES fibres. To obtain a correct comparison between the functionalised PES fibres (diameter 30 µm) and nanofibres (diameter 160 nm) used in this study, exactly the same method for evaluation of the photocatalytic activity of the TiO2 functionalised fibres, was used.
Stability of the fibres was evaluated by immerging the non-functionalised and TiO2
nanoparticles functionalised multi-nozzle membranes in distilled water during 20 days. After this period, photocatalytic activity was tested on these membranes and the results on removal of methylene blue were compared to the results obtained with new membranes to see whether leaching of TiO2 nanoparticles had occurred