SiO2-TiO2-B2O3-BaO glass-ceramic system with Fe2O3 or CuO additives as photocatalyst
Introduction
The most popular and promising photocatalyst is crystalline TiO2 because of its high efficiency, low cost and wide gap which can be activated by UV light from solar or artificial radiation sources [1], [2]. Two main criteria were investigated for improving the efficiency of TiO2: minimizing photogenerated electron–hole recombination rates and expanding the range of operation into visible light wavelengths [3]. Both processes can be achieved by coupling TiO2 with other materials such as semiconductor oxides that show a clear energy difference between their valence bands as well as between their conduction bands (the two differences-shifts being to the same direction) [2], [3].
The strong absorptivity of Fe2O3 in the visible range, along with its abundance and low cost, narrow band gap (Eg ≈ 2.2 eV [4]) has stimulated considerable interest in its use as a photocatalyst and a photoelectrode [5]. But, the position of the valence band of Fe2O3 does not allow the generation of OH radical from water making it an ineffective photocatalyst for organics oxidation, when used by itself [6]. Therefore, its combination with other semiconductors (e.g. TiO2) is obligatory [7]. TiO2–Fe2O3 binary mixed oxides were reported to be a good catalyst with improved photocatalytic properties and enhanced visible light response. Studies with TiO2–Fe2O3 mixed oxide catalyst have shown an increased photocatalytic activity for dichloroacetic acid destruction at 450 nm [8]. Recently, Wodka et al. [9] synthesized Fe2O3/P25 composite containing 1 wt.% of the iron(III) oxide nanoparticles on the Evonic-Degussa P25 titania surface using precipitation method. According to their findings, doping with Fe(III) enhanced the activity of TiO2 toward the removal of oxalic acid (OxA) and formic acid (FA) from water under UV or artificial sunlight (halogen lamp). Also, Liu et al. [10] prepared Fe2O3–TiO2 composite photocatalyst using hydrothermal method, which showed excellent photocatalytic activity for the degradation of auramine under visible and solar light irradiation.
Cu2O (and CuO) is also an abundant p-type semiconductor with band-gap energy of 1.8–2.5 eV (and 1.21–2.00 eV, respectively) [11] that absorb visible light. CuO/TiO2 nanorods has been prepared via electrospinning process and showed excellent antibacterial abilities under visible light [12]. In a similar study, TiO2/CuO electrospun nanofibers was successfully fabricated and displayed efficient concurrent photocatalytic organic degradation and clean energy production from dye wastewater [13].
Because photocatalysis usually occurs on the surface of photocatalyst, the higher the surface area is, the better is the efficiency of the photocatalyst due to presence of more active sites for the adsorption of water and contaminants as well as for the formation of hydroxyl radicals [14]. One method for increasing the surface area is to diminish the particle size as possible using surfactants and polymers as structure-directing agents [14]. However, this small particle size leads to difficulties in separation of the photocatalyst from the treated solution. Precipitation of photoactive crystals in a glassy matrix in the form of glass-ceramic ensures the confinement of crystal growth and hence small crystals could be obtained. In mean time, the glass-ceramic composite as a whole possesses large particle size which allows its separation by simple methods [15].
In this study, we propose a glass-ceramic composition in which TiO2, as photocatalyst, was coupled with another low band gap semiconductor (Fe2O3 or CuO) to enhance TiO2 activity. The glass composition is SiO2, TiO2, B2O3, Na2O, K2O, P2O5, Li2O and BaO with either Fe2O3 or CuO. The prepared photocatalytic glass-ceramic materials were investigated for the degradation of Humic acid (HA) which is the major constituent of natural organic matter found in all surface waters [16], [17] and represent the major precursor of the carcinogenic halogenated disinfection by-products (DBPs) [18].
Section snippets
Preparation of glass samples
The glass samples were prepared using reagent-grade chemicals (Loba Chemie, India). All chemicals were used as received without further purifications. The composition (24.69 SiO2, 24.29 TiO2, 12.95 B2O3, 7.69 Na2O, 4K2O, 1.2 P2O5, 0.8 Li2O, 24.28 BaO in wt.%) was selected because of its good transparency and photocatalytic activity according to our previous study [15]. Fe2O3 or CuO was added to the mentioned composition with 0.25, 0.5 and 1% over the batch to study their effects on the final
Characterization of the prepared materials
Differential scanning calorimetry (DSC) is a rapid tool for studying the crystallization nature through the determination of temperature range of crystallization and the suitable heat-treatment schedule. This is achieved by finding out the onset of glass transition temperature (Tg), onset of first crystallization peak (Tp) and endotherm endpoint liquidus temperature (Tliq) [19]).
Fig. 2 shows the DSC curves of TF glass samples containing various amounts of Fe2O3. It is obvious that adding Fe2O3
Conclusion
In this study, photocatalytic transparent nanocrystalline materials based on SiO2-TiO2-B2O3-Na2O-K2O-P2O5-Li2O-BaO glass ceramic system with the addition of Fe2O3 or CuO were prepared successfully. XRD analysis of the materials before and after heat-treatment showed no peaks which can be credited to the small crystallite sizes and dispersion of the formed crystals observed in SEM and TEM images. All materials showed strong absorption in UV and reasonable absorption in the visible light region.
Acknowledgements
The authors would like to extend a special acknowledgment to Science and Technology Development Fund (STDF) for supporting this research through the project “Glass-ceramics as innovative photocatalyst materials for water/wastewater purification ID:4230”.
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