1. INTRODUCTION:
One of the greatest problems that the world is facing today is that of environmental pollution increasing with every passing year and causing grave and irreparable damage to the earth. The industrial and textile waste waters is a considerable source of non-aesthetic pollution in the environment and can create dangerous by-products through oxidation, hydrolysis or other chemical reactions that takes place in the waste water phase[1]. Many industries use dyes extensively in different operation such as textile, paper, plastic, leather, tanning etc. These industries discharge verity of pollutants in different process. Out of which textile industries discharge large quantity of dyes into water bodies, possess serious ecological problems[2]. Therefore removing such contaminants is a major focus in reducing environmental pollution. A facile and cheap method for removing inorganic and organic pollutants from wastewater has much relevance in modern world[3].
A lot of techniques have been employed to accomplish this task. Among AOPs, one of the most efficient and economical method is probably photo catalysis under visible-light irradiation, which does not require additional chemicals and the main component in the solar spectra and indoor illuminations is visible light[4]. Chemical treatment of wastewaters by AOPs can result in the complete mineralization of the pollutants to carbon dioxide, water, inorganic compounds or atleast, in their transformation to harmless end products[5].
Titanium dioxide (TiO2) is one of the most efficient photo catalyst due to its high stability, chemical inertness, non-toxicity and low cost[6-7]. Photo catalytic water treatment using titanium dioxide (NTO) is a well-known advanced oxidation process (AOP) for environmental remediation[8]. The use of TiO2 as a semiconductor photocatalysis is an efficient methods for elimination of environmental pollutants especially for degradation of organic pollutants from water. TiO2 has the band gap energy of 3.2 eV of which the absorption corresponds to 380 nm. The photocatalytic process requires the energy of light equal or greater than the band gap energy of TiO2 for the complete degradation of the dye[9-10]. Doping Titania with transition metal ions has been tested as a promising way of improving the photocatalytic activity of semiconductor oxides. The incorporation of metal ions into Titania crystal lattice can significantly extend the absorption by the photocatalysts into visible region[11-13].
In this work, the metal doped titania based photocatalysts were prepared by liquid impregnation and sol- gel method and characterized by UV-Visible, XRD and SEM analysis. Finally the decolourization of methyl orange (MO) dye in the presence of visible light was investigated for all the synthesized photocatalysts.
2. EXPERIMENTAL:
2.1 Materials:
All the chemicals used in this work were analytical grade reagents and used without purification. The commercial azo anionic dye Methyl orange was purchased from ACROS organics. Titanium dioxide (TiO2) powder (Analytical grade) was purchased from RFCL Limited. AgNO3, Cu(NO3)2.3H2O, Titanium (1V) isopropoxide, Ferric chloride, 90% ethanol, 4M Ammonia solution were purchased from Merck and QReC Companies Limited. Deionized water was used to prepare all solutions.
2.2 Synthesis of photocatalysts:
2.2.1 Liquid impregnation method:
Ag and Cu doped TiO2 photocatalysts was synthesized by Liquid impregnation method. Firstly, 3g of TiO2 was added to 100 ml deionized water. Then the required amount of relevant reagent for doping was added to TiO2 suspension, where the metal concentration was of 3% (mole ratio) versus TiO2. The slurry was stirred well and allowed to rest for 24hr and then dried in an air oven at specific temperature. The dried solids were grounded in an agate mortar and calcined at a particular temperature in a furnace.
Table 1.
|
Photocatalysts |
Reagent |
Calcination Temperature |
|
Ag-TiO2 |
AgNO3 |
4000 C for 6 hr |
|
Cu-TiO2 |
Cu(NO3) 2.3H 2O |
6000 C for 6 hr |
2.2.2 Synthesis of Fe3+- doped TiO2 photocatalyst:
The Fe3+ doped TiO2 powder was prepared by conventional sol-gel method. In this method, 5ml of Titanium(IV) isopropoxide was dissolved in 50ml ethanol, mixed with 5g FeCl3 by stirring for 15 min, at room temperature and followed by adding droplets of 4M NH3 into the solution until pH about 3-4. Finally, distilled water was slowly added to the solution by stirring for 30 min. The solution was dried at 1050C for 24hr and calcined at 4000C for 2hr.
3. CHARACTERIZATION TECHNIQUES:
3.1.1 UV-Visible spectroscopy:
Electronic spectra of compounds were recorded in suitable solvents in the range of 200-700 nm using UV-Visible spectrophotometer.
3.1.2 X-ray diffraction analysis (XRD):
The crystalline phase and particle size of the samples were analyzed by Panalytical X’pert powder X’celerator Diffractometer using Cu Ka radiation in the 2q range of 0-900 diffraction angle.
3.1.3 Scanning electron microscopy (SEM):
The surface morphology and elemental composition of the samples were studied using JOEL model JSM-6390LV for SEM. It provides exact knowledge regarding the particle size and characterization of the synthesized samples.
3.2 Procedure for photocatalytic degradation (Measurement of photocatalytic activity):
The photocatalytic activity of the as-prepared photocatalysts in visible light was investigated by monitoring the decomposition rate of methyl orange in an aqueous solution. Photocatalytic degradation of methyl orange dye under different concentrations was used to find the effect of initial dye concentration on colour removal. Analogous control experiments were performed without the photocatalysts (blank). The photocatalytic experiments were conducted using various concentrations of catalysts suspended in 250ml of methyl orange solution.
During sunlight irradiation, agitation was maintained by a magnetic stirrer to attain the suspension homogeneous equilibrium. The degradation of methyl orange was monitored by taking 2ml of the suspension at the irradiation time intervals (30, 60,90 min etc). Each time the suspension was centrifuged to separate the photocatalyst from the MO solution. The change in intensity of the dye solution was measured by photoelectric colourimeter. Absorption peak corresponding to methyl orange appeared at 460nm. The degree of photo-decolourization (Degradation efficiency, %) of MO as a function of time was calculated by,
%
of Degradation = 
x 100
Where ‘C0’ is the initial concentration of dye, ‘C’ is the concentration of dye at time‘t’.
4. RESULTS AND DISCUSSIONS:
4.1 Characterization of TiO2 and metal doped TiO2 nanoparticles:
4.1.1 UV-Visible spectral analysis:
UV-Vis spectra of synthesized photocatalysts were shown in fig:1. The absorption of pure TiO2 is around 390 nm. Presence of dopants has increased TiO2’s photocatalytic activity due to the shift in optical absorption towards visible region (400-600 nm). This increase in concentration of dopant not only shifts the absorption edge towards the visible region but also increases the absorption of TiO2 in the whole visible range causing the shift.
Fig.1 UV-Visible spectrum of TiO2
Fig.2 UV-Visible spectrum of Ag doped TiO2
Fig.3 UV-Visible spectrum of Cu doped TiO2
Fig.4 UV-Visible spectrum of Fe doped TiO2
4.1.2 XRD spectral analysis:
The crystalline size of the compounds could be estimated from XRD patterns using the scherrer’s formula,
Where, is the wavelength of X-rays, is the diffraction angle and is the full width at half maximum of the peak. When the size of the individual crystal is less than 100nm, the term nanocrystal is used.
The XRD pattern of TiO2 (fig:2), shows five primary peaks at 25.20, 380, 48.20, 550 and 62.50 which can be attributed to different diffraction planes of anatase TiO2. Four different peaks at 27.50, 360, 540 and 690 can be attributed to different diffraction planes of rutile TiO2. These results show that the pure TiO2 is almost 80% anatase and 20% rutile. The XRD patterns of silver doped TiO2, almost coincide with that of pure TiO2 showing no extra diffraction peaks due to silver doping thus suggesting that silver dopants are merely placed on the surface of the TiO2. The XRD result shows that the Ag doped TiO2 photocatalyst has the average crystalline size of 43.28 nm, Cu doped TiO2 photocatalyst has the average crystalline size of 52.8nm, confirming the nanocrystalline nature of the catalyst.
Fig.5 XRD spectrum of pure TiO2 photocatalyst
Fig.6 XRD spectrum of pure Ag-doped TiO2
Fig.7 XRD spectrum of pure Cu-doped TiO2
4.1.3 Morphological study using SEM
SEM images of pure TiO2 and metal doped TiO2 shows irregular shaped particles which are the aggregation of tiny crystals on the TiO2 lattice. Also SEM images confirms the nanocrystalline nature of the photocatalysts.
Fig.8 SEM images of pure TiO2
Fig.9 SEM images of Ag doped TiO2
Fig.10 SEM images of Cu doped TiO2
Fig.11 SEM images of Fe doped TiO2
5. DEGRADATION STUDIES:
5.1 Degradation behaviour of methyl orange:
The photocatalytic activity of the synthesized photocatalysts in visible light was investigated by monitoring the decomposition rate of methyl orange in an aqueous solution. Model aqueous solutions with various initial concentrations of MO and different concentration of metal doped catalysts were used for the study of the photocatalytic activity of the prepared photocatalysts. In order to study the photocatalytic activity of MO under visible light irradiation, the photolysis of MO was carried without any catalyst.
Fig.12 Degradation behaviour of Methyl orange dye without photocatalyst
From the figure, it can be clearly noticed that there is no significant changes of the degradation of MO after 6 hr irradiation which indicated that pure MO solution cannot be easily degraded under UV-Visible light.
Fig.13 UV-Visible spectrum of (a)Methyl orange
Fig.14 UV-Visible spectrum of MO +Ag doped TiO2 catalyst under 2hr Sunlight irradiation.
5.2 Effect of change in catalyst concentration:
In this experiment, two different concentrations of catalysts [(0.025g MO + 0.25g catalyst) and (0.025g MO + 0.5g catalyst)] were used for the study. Photocatalytic degradation procedure was conducted. The photodegradation of MO dye mixed with each synthesized catalysts were examined in terms of change in absorbance using photoelectric colorimeter.
Fig 15. Effect of TiO2 based photocatalyst at catalyst Concentration I (0.025gm MO + 0.25g Catalyst)
Fig 16. Effect of TiO2 based photocatalyst at (b) catalyst Concentration II (0.025gm MO+0.5g Catalyst) on Photocatalytic degradation of MO.
5.3 Effect of change in dye concentration:
In this experiment, two different concentrations of MO dye [(0.025g MO + 0.25g catalyst) and (0.05g MO + 0.25g catalyst)] were used for the study.
Fig 17. Effect of TiO2 based photocatalyst at (a) dye Concentration I (0.025gm MO + 0.25g Catalyst)
Fig 18. Effect of TiO2 based photocatalyst at b) dye Concentration II (0.05gm MO+0.25g Catalyst) on Photocatalytic degradation of MO.
5.4 Comparison of different photocatalysts:
Photocatalytic activity of the synthesized samples were evaluated by the photodegradation of methyl orange dye at 300 minutes under visible light irradiation. As a comparison, the photolysis of MO was also carried out at the same condition, but without any catalyst.
Table-2 Efficiency of different photocatalysts
|
Catalysts |
Concentration I 0.025g MO + 0.25g catalyst |
Concentration II 0.025g MO + 0.5g catalyst |
|
TiO2 |
20% |
30% |
|
Ag-TiO2 |
86% |
50% |
|
Cu-TiO2 |
32% |
18% |
|
Fe-TiO2 |
46% |
14% |
Table-3 Efficiency of different photocatalysts at catalyst concentration I and II at dye concentration I and II
|
Catalysts |
Concentration I 0.025g MO + 0.25g catalyst |
Concentration II 0.05g MO + 0.25g catalyst |
|
TiO2 |
20% |
36% |
|
Ag-TiO2 |
86% |
44% |
|
Cu-TiO2 |
32% |
38% |
|
Fe-TiO2 |
46% |
23% |
5.5 Effect of catalyst dosage:
A series of experiments were conducted to study the effect of catalyst dosage on the decolourization of methyl orange under visible light irradiations. All the synthesized photocatalysts namely Ag/TiO2, Cu/TiO2, Fe/TiO2 along with pure TiO2 photocatalyst were subjected to MO photodegradation. The rate of degradation increases with increase of catalyst loading up to 0.25g. Beyond the limit the dye molecules are not sufficient for adsorption by increased number of catalyst molecules. Hence the additional catalyst powder is not effectively involved in the photocatalytic activity rather increase in the turbidity of the solution, which interfere with penetration of light transmission, thereby reducing the number of photogenerated electron-hole pairs, leading to the decreasing of TiO2 photocatalytic activity. The photoactivity of all the supported photocatalyst is higher as compared to pure TiO2 photocatalyst. Ag doped TiO2 shows remarkable increase in the photoactivity as compared to the undoped TiO2.
5.6 Effect of methyl orange dye concentration:
Photocatalytic degradation increases with increase in the concentration of dye up to 0.025g. This may be attributed to the fact that as the concentration of the dye increased, more dye molecules will be available for excitation and energy transfer, which increases the percentage of degradation. But beyond certain limit of the dye concentration, it may adversely affect the percentage of degradation. This is due to the fact that at higher concentration dyes start covering the surface of photocatalyst from light intensity. At 0.25g of catalyst dosage and the 0.025g of the MO initial concentration, the rate of degradation is high when compared to other concentration. The highest photocatalytic performance of the Ag doped TiO2 catalyst may be due to its high surface area. Experimental data show that the Ag doped catalysts display higher catalytic activity than other metal doped TiO2.
Fig.19 Degraded Methyl orange solution under 8hr exposure to sunlight over Ag-doped TiO2 photocatalyst.
5.7 Mechanism of photodegradation of MO using Ag doped TiO2
Based on the experimental results, silver doped TiO2 are more efficient than undoped TiO2 at decolourization of MO dye. TiO2 under irradiation of light with wavelength lower than 390 nm produces e--h+ pairs. Recombination of e--h+ pairs reduces the rate of photocatalytic degradation. The positive effect of silver on the photoactivity of TiO2 at degradation of MO may be explained by its ability to trap electrons. This process reduces the recombination of light generated e--h+ at TiO2 surface. Therefore a more effective electron transfer occurs to the electron acceptors and donors adsorbed on the surface of the particle than in the case of undoped TiO2. Oxygen adsorbed on photocatalyst surface traps the electrons and produces superoxide anion. On the other hand holes at the TiO2 surface can oxidize adsorbed water or hydroxide ions to produces hydroxyl radicals.
The overall photocatalytic reaction is due to the oxidation reaction between these generated reactive oxygen species (O2•-and OH•) and pollutant i.e., synthetic dye. The enhancement of photocatalytic activity based on metal doped TiO2 depend upon the additional rate of O2•-and OH• formations. In addition the rate of electron transfer from VB of TiO2 to deposited Ag should be faster than those of electron-hole pair recombination. It has been concluded that Ag particle deposited on TiO2 surface acting as electron traps to effectively separate the excited electron-hole pairs. The formed hydroxyl radicals and intermediates are degraded to inorganic products, thus eventually resulting in the complete degradation of the dye.
The photocatalytic process of Ag-TiO2 is represented as,
Ag-TiO2 + hv → dye*ads
dyeads + TiO2 → dye•+ads + TiO2 (e′)
e′CB + O2 → O2•–
O2•– + H+ → OOH•
OOH• + O2•– + H+ → O2 + H2O2
H2O2 + O2•–→ OH• + H– + O2
dyeads + Ag-TiO2 + (OH•, O2•–, H2O2, O2)
CO2+ H2O+etc….
Fig.20 Mechanism of photocatalysis in Ag deposited TiO2 under UV light irradiation
6. CONCLUSION:
In this study solar photocatalytic decolourization of methyl orange dye has been investigated using newly synthesized TiO2 photocatalysts. Experimental results revealed that Ag-TiO2 has been found to be most active catalyst, exhibiting high photocatalytic activity than other synthesized photocatalysts under visible light irradiation. The photocatalytic efficiency was increased upto 86% with the use of 0.25g Ag doped TiO2. That is, at concentration I(0.025g MO + 0.25g catalyst), the best photocatalytic degradation of MO was observed. Also complete decolourization of MO dye was observed after 8hr sunlight irradiation. XRD patterns showed the presence of mixed anatase and rutile phases of TiO2 structure. The particle size and morphology investigated by SEM images revealed irregular shaped particles in the nanocrystalline range. The results clearly showed that doping could greatly enhance the photocatalytic activity of TiO2 photocatalyst. An increase of absorption band edge towards visible region and a decrease of TiO2 band gap observed in UV-Visible study could be the possible reasons explaining for an increased photocatalytic efficiency of synthesized Ag-TiO2 photocatalyst.
7. ACKNOWLEDGEMENT:
The authors are thankful to the Department of Chemistry, NMCC, Marthandam for providing the laboratory facilities. The Regional Research Institute (RRI), Trivandrum and SFRC, Sivakasi are gratefully acknowledged for SEM and XRD analysis.
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Received on 14.03.2015 Modified on 19.03.2015
Accepted on 15.04.2015 © AJRC All right reserved
Asian J. Research Chem 8(5): May 2015; Page 327-334
DOI: 10.5958/0974-4150.2015.00054.1