Solar active nano-TiO₂ for mineralization of Reactive Red 120 and Trypan Blue

2.1 Materials

The commercial azo dyes, Trypan Blue (Fig. 1a), from SD Fine and Reactive Red 120 (Fig. 1b), from Balaji Colour Company, Dyes and Auxiliaries (Chennai) were used as such. A gift sample of TiO₂–P25 was received from Degussa (Germany). It is a mixture of 80% anatase and 20% rutile with the particle size of 30 nm and BET specific area 50 m² g⁻¹. AnalaR grade tetraisopropyl orthotitanate (Himedia, 98.0%), isopropanol (99.5%, Spectrochem) were used as received. The double distilled water was used to prepare experimental solutions.

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Figure 1

The chemical structures and absorption maxima of (a) TB and (b) RR 120.

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2.2 Preparation of nano-TiO₂

Nano-TiO₂ was prepared by sol–gel method. 12.5 mL of tetraisopropyl orthotitanate was dissolved in 100 mL of isopropanol (Spectrochem ⩾99.5%) and to this solution 3 mL of water was added drop wise under vigorous stirring. The resulting colloidal suspension was stirred for 4 h. The gel obtained was filtered, washed and dried in an air oven at 100 °C for 5 h. The sample was calcinated at 400 °C in a muffle furnace for 12 h.

2.3 Irradiation procedure

Solar photocatalytic degradation experiments were carried out under similar conditions on sunny days between 11 am to 2 pm in the month of May, 2010. An open borosilicate glass tube of 50 mL capacity, 40 cm height and 12.6 mm diameter was used as the reaction vessel. Fifty milliliters of RR 120 (2 × 10⁻⁴ M) or TB (1 × 10⁻⁴ M) with the appropriate amount of catalyst was stirred for 30 min in the dark prior to illumination in order to achieve maximum adsorption of dye onto the semiconductor surface. Irradiation was carried out in the open air with continuous aeration by a pump to provide oxygen and for the complete mixing of reaction solution. During the illumination time no volatility of the solvent was observed. In all cases, 50 mL of reaction mixture was irradiated. At specific time intervals, 2–3 mL of the sample was withdrawn and centrifuged to separate the catalyst. One milliliter of the sample was suitably diluted and dye concentration was determined from the absorbance at the analytical wavelength (RR 120–285 nm, TB −313 nm).

2.4 Solar light intensity measurements

Solar light intensity was measured for every 30 min and the average light intensity over the duration of each experiment was calculated. The sensor was always set in the position of maximum intensity. The intensity of solar light was measured using LT Lutron LX-10/A Digital Lux meter and the intensity was 1250 × 100 lux. The intensity was nearly constant during the experiments.

2.5 Chemical oxygen demand (COD) measurements

COD was determined using the following procedure. Sample was refluxed with HgSO₄, known volume of standard K₂Cr₂O₇, AgSO₄ and H₂SO₄ for two hours and titrated with standard Ferrous Ammonium Sulfate (FAS) using ferroin indicator. A blank titration was carried out with distilled water instead of dye sample. COD was determined using the following equation

2.6 Analytical methods

The specific surface area of the catalyst was determined through nitrogen adsorption at 77 K on the basis of BET equation using a micrometrics ASAP 2020 V3.00 H. Scanning electron microscope (SEM) images were taken on gold coated samples using a JEOL-JSM 5610 LV, equipped with OXFORD energy dispersive spectroscopy (EDS). The grids were dried under natural conditions and examined using a TEM Hitachi H-7500. Powder X-ray diffraction patterns of TiO₂ and TiO₂–P25 catalyst were obtained using X’Per PRO diffractometer equipped with a Cu Kα radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identity the crystalline phase. Avatar-330 FT-IR spectrophotometer was used for recording IR spectra. Diffuse reflectance spectra were recorded using Shimadzu UV-2450. UV spectral measurements were done using Hitachi-U-2001 spectrometer. The pH of the solution was measured by ELICO (LI-10T model) digital pH meter.

3.1 Characterization of catalyst

Prepared TiO₂ was characterized by XRD, SEM, TEM, BET surface area and DRS analysis. XRD pattern of TiO₂–P25 and prepared TiO₂ are given in Fig. 2a and b, respectively. All the marked diffraction peaks of prepared TiO₂ in Fig. 2b can coincidently be indexed by the known anatase phase TiO₂. Characteristic peaks of TiO₂ at 25.3, 37.8, 48.0, 53.9, 54.9, 62.7, 68.7, 69.7 and 75.1 corresponds to (1 0 1), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0) and (2 1 5) diffraction peaks of anatase phase TiO₂. Fig. 2a displays the XRD pattern of Degussa TiO₂–P25 since it is a mixture of 80% anatase and 20% rutile XRD pattern shows both anatase and rutile lines. The crystalline size of prepared TiO₂ was determined using the following Debye–Scherrer equation:

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Figure 2

XRD patterns of (a) TiO₂–P25 and (b) prepared TiO₂.

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The structure and morphology of the catalyst are very important parameters as they influence the photocatalytic activity. The SEM images of TiO₂–P25 and prepared TiO₂ are shown in Fig. 3a and b. Fig. 3a and b depict that the crystallinity is found to be more in prepared TiO₂. Fig. 4 shows the typical bright-field TEM image of the prepared TiO₂. TEM image reveals that the size of the TiO₂ nanoparticles is in the range of 10–50 nm, which is consistent with the average crystallite size (15.2 nm) estimated from XRD analysis.

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Figure 3

SEM images of (a) TiO₂–P25 and (b) prepared TiO₂.

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Figure 4

TEM image of prepared TiO₂.

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In general the surface area of the catalysts is the most important factor influencing the catalytic activity. The surface area of prepared TiO₂ was determined using the nitrogen gas adsorption method. The N₂ adsorption–desorption isotherms and BET surface area plot for the synthesized sample are given in Fig. 5a and b, respectively. Fig. 5a shows the hysteresis loops of the sol–gel TiO₂, which is characteristic of the typical macroporous structures in type IV with almost vertical and nearly parallel adsorption and desorption branches. This implies that the synthesized TiO₂ sample consists of agglomerates or compacts of approximately uniform spheres. The BET surface area of prepared TiO₂ is found to be 86.5 m² g⁻¹ which is higher than that of TiO₂–P25 (50 m² g⁻¹). The single point adsorption total pore volume of pores less than 952.3 Å radius at P/P_o is 0.98:0.22 cm³/g.

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Figure 5

BET surface analysis of (a) N₂ adsorption–desorption isotherm and (b) BET surface area plot.

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The diffuse reflectance spectrum of prepared TiO₂ is shown in Fig. 6. The edge absorption is slightly extended towards visible region. The band gap energy of prepared TiO₂ is found to be 3.1 eV which is equivalent to the wavelength of 397 nm.

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Figure 6

DRS of prepared TiO₂.

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3.2 Photodegradability of TB and RR 120

Photocatalytic degradation of Trypan Blue with TiO₂–P25 and prepared TiO₂ using solar light is shown in Fig. 7. When the dye is irradiated without catalyst there is negligible degradation and for the same experiment performed with the catalyst in dark 47% decrease in dye concentration occurred. This is due to adsorption of the dye on the catalyst. Dye undergoes almost complete degradation in presence of prepared TiO₂ and solar light at 90 min irradiation, whereas 81% degradation takes place at 90 min with TiO₂–P25. This shows that prepared TiO₂ process is more efficient in TB degradation than TiO₂–P25 in solar light.

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Figure 7

Photodegradability of TB; dye concentration = 1 × 10⁻⁴ mol L⁻¹, catalyst suspended = 4 g L⁻¹, pH 6.8, airflow rate = 8.1 mL s⁻¹.

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The photocatalytic efficiency of this catalyst in solar light was also tested with RR 120 dye degradation. Photocatalytic degradation of RR 120 with Commercial TiO₂–P25 and prepared TiO₂ is shown in Fig. 8. When the dye is irradiated without catalyst there is negligible degradation and for the same experiment performed with the catalysts in dark, dye concentration decreases by 16.1% due to adsorption. Dye undergoes almost complete degradation with prepared TiO₂ on solar irradiation in 70 min, whereas TiO₂–P25 takes 90 min for complete degradation. This reveals the higher efficiency of prepared TiO₂ in RR 120 degradation than TiO₂–P25.

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Figure 8

Photodegradability of RR 120; dye concentration = 2 × 10⁻⁴ mol L⁻¹, catalyst suspended = 4 g L⁻¹, pH 5.5, airflow rate = 8.1 mL s⁻¹.

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It is seen from Fig. 7 that the initial adsorption of dye molecules in dark by prepared TiO₂ (47.0%) is higher than the adsorption by commercial TiO₂–P25 (5.5%). After complete degradation (90 min of irradiation) dye free water was obtained with prepared TiO₂. The dark adsorption and the degradation efficiency of the used TiO₂ were found to be similar to those of fresh catalyst. These observations reveal that the dye molecules adsorbed in dark underwent complete degradation during irradiation. In order to confirm the degradation of adsorbed dye molecules, FT-IR spectra of the prepared TiO₂ with the adsorbed dye (TB) molecules before and after irradiation were taken and are shown in Fig. 9b and c along with the FT-IR spectra of the fresh catalyst (Fig. 9a). Fig. 9b shows the characteristic peaks of the dye (1552 and 1501 cm⁻¹ due to azo group of the dye) indicating the adsorption of dye on the catalyst. FT-IR spectrum of catalyst after irradiation, i.e., after complete degradation (Fig. 9c) reveals that the characteristic peaks of dye disappeared and spectrum in Fig. 9c is almost similar to the spectrum of fresh catalyst (Fig. 9a). This confirms that the dye molecules adsorbed on the prepared TiO₂ in dark have been completely degraded at the time of 90 min irradiation. Though TiO₂ can be excited only by UV light, dye adsorbed TiO₂ can be activated by solar light. In this case solar light excited dye molecule generates an electron, which goes to conduction band of TiO₂. This electron produces oxide ion radicals for the oxidation of dye molecules. This dye sensitized mechanism has already been reported (Kyung et al., 2005).

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Figure 9

FT-IR spectra of (a) prepared TiO₂, (b) prepared TiO₂ after TB dye adsorption and (c) prepared TiO₂ after complete degradation of TB.

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3.3 COD measurements

The mineralization of TB and RR 120 using prepared TiO₂ was also confirmed by COD measurements. The COD value of 1251.5 ppm for 1 × 10⁻⁴ mol L⁻¹ of TB concentration gradually decreases to 848 ppm (67.8%) and 3.4 ppm (0.1%) after 30 and 90 min of irradiation, respectively. This indicates 99.9% mineralization of TB in 90 min.

For 2 × 10⁻⁴ mol L⁻¹ of RR 120 the COD value of 2064.4 ppm gradually decreases to 1145.8 ppm (55.6%) and 102.3 ppm (4.95%) after 30 and 60 min of irradiation, respectively. This indicates 95% degradation of RR 120 in 60 min, which is almost equal to complete mineralization.