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Photocatalytic Activity of - Composites
- photocatalysts were prepared in a vacuum evaporator by impregnation of with dissolved in an solution (30%) and followed by calcination at 400 and . XRD analyses showed that at monoclinic phase of was dominated whereas at both monoclinic and regular phases of were present. Modification of by caused increasing in the absorption of light to the visible range. and photocatalysts modified with low amount of (1–5 wt.%) showed high adsorption of Acid Red (AR) on their surface and enhanced photocatalytic activity under UV irradiation. Under visible light irradiation, - photocatalysts prepared at were more active for AR decomposition than those prepared at suggesting that monoclinic phase of is more active under visible light than regular . Although - photocatalysts appeared to be active under visible light for decomposition of AR, the UV irradiation was more efficient.
The photocatalytic process using photocatalyst is very promising for application in the water purification, because many organic compounds can be decomposed and mineralized by the proceeding oxidation and reduction processes on surface. The most commonly tested compounds for decomposition through the photocatalysis are phenols, chlorophenols, pesticides, herbicides, benzenes, alcohols, dyes, pharmaceutics, humic acids, organic acids, and others [1–3].
is the most commonly used photocatalyst, because it is nontoxic, chemically stable, cheap, and very efficient. However it has some disadvantages, from which one of the most important is a relatively high value of the band gap, around 3.2 eV, which limits its using to the UV light. Therefore nowadays a lot of research work is focused on the preparation of visible light active photocatalysts in order to utilize the solar light more efficiently and stop using UV lamps as a driving force of photocatalytic process, because it consumes a lot of energy.
A lot of works have been done in preparation of anion-doped photocatalysts, such as nitrogen, carbon, and sulphur-doped , which showed the photocatalytic activity under visible light [4–6]. The other solution is coupling of with semiconductor, which has sensitivity to the visible light. Such semiconductor can be , which has a band gap of 2.8 eV and can absorb the light in the visible range. However it also shows high speed of recombination process between generated free carriers. It was reported that by coupling of these two semiconductors, and , an efficient charge separation can be achieved which results in enhanced photocatalytic activity of photocatalyst [7–9].
In presented results the preparation of - photocatalysts was performed by dissolving of in hydrogen peroxide, wet impregantaion on the anatase particles, and calcination at 400 and 600. The impregnation method was selected for preparation to get well dispersion of tangsten oxide particles on the used material, which as original exhibits high BET surface area, around 300 /g. Addition of tangsten oxide before calcination can also retard the phase transformation of anatase to less active rutile. The influence of doping to on its photocatalytic activity under UV and visible light irradiations was tested.
2. Preparation of - Photocatalysts
As a source of a raw material of white was used, which was supported by the Police S.A. Company in Poland. This has BET surface area of 300 /g and contains poorly crystalized anatase phase and some nuclei of rutile. - composites were prepared by mixing of with dissolved in an solution (30%) in a vacuum evaporator at 70 for 1 hour. Then the solution was heated up to 100 to evaporate the water. The obtained powder was dried in a dryer overnight and then was subjected to calcination at 400 and 600. The amount of in the prepared - samples ranged from 1 to 90 wt.%.
3. Analytical Methods
The phase composition of and - composites was measured by XRD in X’Pert PRO diffractometer of Philips Company, with CuKα lamp (35 kW, 30 mA). The obtained XRD patterns were compared with (JCPDSs Joint Committee on Powder Diffraction Standards) cards. The morphology of the photocatalysts surface and content of in - composites were evaluated by SEM measurements with EDS analysis. The particle size was measured in Zetasizer Nano ZS of Malvern Company by (DLS Dynamic Light Scattering) method. For measurements photocatalyst samples were suspended in ultra pure water solution with dispersant and were treated with ultrasonic vibrations.
UV-Vis spectra of and - powders were taken in UV-Vis spectrometer Jasco V-540. These spectra were transformed to Kubelka-Munk equation for indirect semiconductor, and the band gap was calculated.
Hydroxyl radicals were detected by using fluorescence techinque. Coumarine can easily react with hydroxyl radicals to form highly fluorescence compound, 7-hydroxycoumarine, which is determined in the Fluorescence Spectrometer Hitachi F-2500. For these measurements the photocatalyst samples were irradiated under UV in the coumarine solution ( M), and then the solution after separation from a photocatalyst was taken to analysis. The fluorescence measurements were performed at the excitation wavelength of 332 nm and the emission of 335–600 nm with maximum peak at 460 nm. The detailed procedure has been described elsewhere .
For photocatalytic test, azo dye, Acid Red (AR) was decomposed, 30 mg/L AR in 500 mL solution and catalyst loading 0.2 g/L under UV irradiation with UV intensity 154 W/ and Vis 100 W/. Experiments of AR decomposition were also performed under fluorescence light irradiation with intensity of W/, for that photocatalytic test lower concentration of AR solution was used, 10 mg/L. Fading of AR solution was monitored by UV-Vis spectroscopy.
4. Photocatalytic Activity Test
The photoactivities of prepared samples were tested for decomposition of azo dye Acid Red under irradiation of two different sources: UV and fluorescence lamps. UV was emited from UV six lamps of Philips Company with power of 20 W each. These lamps emit the radiation at UV range of 154 W/ and at the visible region of about 100 W/ in the range of 312–553 nm with a maximum at around 350 nm. The fluorescence lamps used as a source of visible light ( W) emit light in the visible region with intensity of 715 W/ and insignificant amount of UV with intensity of 0.22 W/m2.
Each time, for the photocatalytic test, the beaker with 500 mL of a dye solution of concentrations around 0.03 g/L under UV, and 0.01 g/L under visible light and 0.1 g of photocatalyst was used. The solutions were first magnetically stirred in a dark for 30 minutes in order to estimate the adsorption of dye on the photocatalyst surface and then were irradiated under UV or visible lights from the top of the beaker. The concentration of a dye solution was analyzed in UV-Vis spectrophotometer,
5. Results and Discussion
In general modification of by caused increasing the absorption of light to the visible range; however heat treatment caused almost complete reduction of absorption in the range of 400–700 nm, and only a few percentage of light absorption in the range of 500–700 nm could be noticed for modified samples; the exception is - with doped amount of 5 and 90 wt.%, which exhibited higher absorption of visible light even after heat treatment.
XRD measurements of and - photocatalysts were performed. Phase was difficult to observe in the prepared samples with doping less than 50%. In Figure 2 XRD patterns of and with different amounts of doped photocatalysts as received and calcined at different temperatures are presented.
Original consists of poorly crystalized antase phase with insignificant amount of rutile. Heating of anatase- caused narrowing of the diffraction peaks of anatase phase due to the growing of its crystals. The additional reflexes from anatase phase such as 103, 112, 116, 220, 215, and 301 were clearly observed for-well crystalized antase. In case of --90% photocatalyst monoclinic phase of appeared at 400 whereas at 600 additionally regular phase was present.
Doping to caused inhibition of growing anatase crystals during heat treatment, and narrowing of the anatase reflex (101) was insignificant, mostly for the samples with doped amount up to 3%. Above 3% of doped the anatase reflexes 103 and 112 were not observed, and some reflexes as 105 and 211 were not distinguished due to the presence of broad peaks, even after heated at 600. The exception is with doped amount of 10%, in which narrowing of anatase 101 reflex was significant, and reflexes 105 and 211 were clearly identified. In this sample probably distribution of particles on was not homogoneous.
Photoactivity of and prepared photocatalysts in direction of OH radicals formation was tested by the fluorescence technique. In Figure 3 there are presented results from OH radicals measurements on and - photocatalysts during UV irradiation.
The linear correlation of OH radicals formation from the irradiation time can be noticed. Doping WO3 to and higher calcination temperature caused increasing in the amount of OH radicals formation. This tendency was observed for modified samples with doping up to 3%; for higher amount of doped the photocatalysts heat treated at 400 showed higher amount of OH radicals formation than those as received and calcined at 600. The highest photoactivity toward OH radicals formation was noted for --3% heat treated at 600. In Figure 4 OH radicals formation on the photocatalysts under visible light are presented.
OH radicals formation under visible light was much lower than under UV, when we compare Figures 3 and 4, but again doped with showed higher amount of OH radicals formation than . From Figure 4 it can be seen that --10% heat treated at 400 was much more photoactive than heat treated at 600 and no calncinated one. The same tendency was kept for the other photocatalysts with doping from 5%–90%. The highest OH radicals formation under visible light was obtained on the - photocatalyst with doping amount of 10% heat treated at 400. However this sample did not show significant absorption of light in the visible range; coupling of and could occur at small amount of visible light absorption by and absorption of UV light by , even although the energy of UV light was insignificant. with 90% of doped showed much higher absorption of visible light than the other samples with lower content of but had low activity under visible light. It is concluded that activity is much more powerful than in generation of OH radicals, and can serve as a support in OH radicals formation by transfer electrons to the conductive band of under visible light or can retard the recombination reaction occurring in .
The influence of doping to on the particles size of photocatalysts was measured. The results from the measurements of particles size of and - samples are listed in Table 1.
In general particles size of doped photocatalysts were lower than . Calcination caused growing of crystals, and so some heat-treated samples exhibited higher size of particles than those as received ones.
The structure of photocatalysts and particles size were observed on SEM photos. For comparison SEM of not modified and --3% calcinated at 600 are presented in Figure 5.
Some agglomerates of primary particles of can be seen with size over 1 m in Figure 5(a) whereas doped with comprises of much smaller particles.
Both measurements, DLS and SEM, showed that doping to cause, reduction of its particles size, mostly because of reducing tendency of particles to form agglomerates. Smaller particle size of - composites in comparison to prepared by the sol-gel method was also reported by Li et al. .
From EDS analysis the measured amount of Ti was 93 wt.%, W – 6 wt.%, and S – 1 wt.%. Sulphur came from the production process of .
Photocatalytic activity of prepared samples was tested for Acid Red decomposition under UV and visible light irradiations. Preliminary adsorption of this dye on the photocatalysts surface was performed. The results from the adsorption measurements are presented in Table 2. The initial concentrations of AR used in case of UV and Vis radiations were 30 and 10 mg/L, respectively.
Noncalcined samples of and doped with low amount of up to 5% showed quantitatively adsorption of AR on their surface, which generaly was decreasing with heat treatment temperature; only - photocatalyst with doping amount of 5% showed opposite tendency, that is increased adsorption of AR after heat treatment.
Photocatalysts which exhibited high adsorption of AR on their surface showed no linear correlation of ln (/C) from time of irradiation during AR decomposition. The high acceleration of AR decomposition with time of irradiation on these photocatalysts could be caused by occurring sensitized photocatalysis. Therefore doping to , which caused their increased absorption of light to the visible range and high adsorption of AR, appeared to be beneficial for decomposition of AR, as it can be seen especially in case of noncalcined samples used under UV irradiation and --5% heat treated at 600 used under visible light. Although both and - photocatalysts were active under visible light irradiation, UV light was more powerful in AR degradation.
Under visible light irradiation - photocatalysts prepared at 400 were more active for AR decomposition than those prepared at 600 and noncalcined one.
Doping to caused increasing its absorption of light to the visible range; however it was observed mostly for noncalcined samples. Although OH radicals formation on prepared - photocatalysts was higher than on it was not a key factor affecting the rate of AR decomposition. Both high adsorption of AR on the photocatalyst surface and their ability to absorption of visible light were responsible for the photocatalytic properties of photocatalysts, and therefore the - photocatalysts with low amount of (1–5 wt.%) were more active than the others. Doping to caused also reduction of its particles size, which could improve ability of for dispersion in water and increase the accessible surface for adsorbates. Under visible light irradiation - photocatalysts prepared at 400 were more active for AR decomposition than those prepared at 600 suggesting that monoclinic phase of is more active under visible light than regular . Although the photocatalysts were active under both UV and visible light irradiations, UV light was more powerful for decomposition of AR than visible light, but the latter had important meaning during occurring sensitized photocatalysis.
This work was supported by the research project from the Ministry of Science and Higher Education no.COST/299/2006 for 2007-2010.
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