Nanostructured AgBr loaded TiO2: An efficient sunlight active photocatalyst for degradation of Reactive Red 120
© Velmurugan et al 2011
Received: 10 March 2011
Accepted: 30 July 2011
Published: 30 July 2011
The AgBr loaded TiO2 catalyst was prepared by a feasible approach with AgBr and tetraisopropyl orthotitanate and characterized by BET surface area measurement, diffuse reflectance spectra (DRS), scanning electron microscope (SEM), energy dispersive spectra (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM) and atomic force microscope (AFM) analysis. The results of characterization reveal that AgBr loaded TiO2 has a nanostructure. Formation of the nanostructure in AgBr loaded TiO2 results in substantial shifting of the absorption edge of TiO2 to red and enhancement of visible light absorption. Electrochemical impedance spectroscopy measurements reveal that AgBr loaded TiO2 has a higher photoconductivity than prepared TiO2 due to higher separation efficiency of electron-hole pairs. Cyclic voltammetric studies reveal enhanced conductivity in AgBr loaded TiO2, which causes an increase in its photocatalytic activity. AgBr loaded TiO2 exhibited a higher photocatalytic activity than TiO2-P25 and prepared TiO2 in the photodegradation of Reactive Red 120 (RR 120).
However, AgBr could maintain its stability and photocatalytic activity if it is well dispersed on support materials. Al-MCM-41  (Aluminium loaded Mobile Crystalline Material) and titanium dioxide (TiO2)  were used as the support to deposit AgBr and these catalysts were active and stable while applied in gas phase or aqueous solution under UV or visible irradiation for the degradation of organic compounds.
In this study, we report the synthesis of AgBr loaded TiO2 photocatalyst without using any surfactant or solvent at moderate temperature and its characterization. Reactive Red 120 (RR 120) was chosen as the model pollutant to evaluate the photoactivity and stability of the synthesized catalyst under direct sunlight. RR 120 is one of the most widely used synthetic azo dye in textile industries. Direct sunlight was employed to illustrate the possibility of solar energy utilization.
2. Results and discussion
2.1 Characterization of AgBr loaded TiO2 photocatalyst
2.1.1 XRD analysis
where D is the crystal size of the catalyst, K is dimensionless constant (0.9). λ is the wavelength of X-ray, β is the full width at half-maximum (FWHM) of the diffraction peak and θ is the diffraction angle. The average crystalline size of AgBr loaded TiO2 is found to be 31.8 nm which is less than the size of prepared TiO2 (34.7 nm).
2.1.2 BET surface area analysis
Surface properties of AgBr loaded TiO2.
BET surface area
55.4 (m2 g-1)
Total pore volume (single point)
0.146 (cm3 g-1)
BJH desorption average pore diameter (2 V/A)
Median pore width
2.1.3 SEM and EDS analysis
2.1.4 TEM analysis of AgBr loaded TiO2
2.1.5 AFM analysis of AgBr loaded TiO2
2.1.6 DRS analysis
2.1.7 Electrochemical impedance spectroscopy
2.1.8 Cyclic voltammetry analysis
2.2 Photodegradability of RR 120
Degradation efficiency of AgBr-TiO2 was also tested with a colorless toxic chemical 4-nitrophenol. It was found that 83.0% degradation occurred with UV light where as only 66% degradation was observed with solar light in 60 min. Higher efficiency in solar light for dye degradation reveals the presence of dye sensitization mechanism along with Ag-Br-TiO2 sensitization. Since this dye sensitization is common for AgBr-TiO2, TiO2-P25 and prepared TiO2, higher efficiency of AgBr-TiO2 may be due to the plasmonic photocatalytic mechanism reported for AgCl-TiO2 system . Further work in the study of mechanism is in progress.
2.3 Catalyst recyclability
Nanostructured AgBr loaded TiO2 catalyst has been prepared at room temperature by the facile deposition-precipitation method. The UV-Vis spectra indicate that the range of visible-light photoresponse of AgBr loaded TiO2 system is broadened increasing its visible-light-driven photocatalytic activity. The formation of AgBr loaded TiO2 nanocluster has been revealed by the XRD, SEM, TEM and AFM analysis. Electron-hole recombination in TiO2 is reduced by AgBr loading as revealed by Nyquest plots of Eelectrochemical Impedance Spectrascopy. Cyclic voltammetric studies reveal enhanced conductivity in AgBr loaded TiO2. The increase of electro conductivity of AgBr loaded TiO2 enhances its photocatalytic activity. Nano AgBr loaded TiO2 is found to be more efficient than prepared TiO2 and TiO2-P25 for degradation of RR 120. Nano AgBr loaded TiO2 is found to be a stable, recyclable and efficient photocatalyst at pH 5, for the degradation of RR 120 under solar light.
A gift sample of TiO2-P25 (80:20 mixture of anatase and rutile) was obtained from Degussa (Germany). It has the particle size of 30 nm and BET specific surface area of 50 m2 g-1. Titaniumisopropoxide (Himedia), sodium bromide (99.0%) and silver nitrate (99.5%) analytical grade from Merck were used as received. The model pollutant, azo dye (Reactive Red 120), obtained from Balaji Colour Chem, Chennai, was used without further purification. Double distilled water was used for all the experiments.
4.2 Irradiation experiments
All photocatalytic experiments were carried out under similar conditions on sunny days of April-May 2010 between 11 am and 2 pm. An open borosilicate glass tube of 50 mL capacity, 40 cm height and 20 mm diameter was used as the reaction vessel. The suspensions were magnetically stirred in the dark for 30 min to attain adsorption-desorption equilibrium between dye and AgBr loaded TiO2. Irradiation was carried out in the open-air condition. Fifty mL of dye solution with AgBr loaded TiO2 was continuously aerated 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.
After dark adsorption the first sample was taken. At specific time intervals 2-3 mL of the sample was withdrawn and centrifuged to separate the catalyst. One mL of the centrifugate was diluted to 10 mL and its absorbance 285 nm was measured. The absorbance at 285 nm represents the aromatic content of RR 120 and its decrease indicates the degradation of dye.
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 ± 100 lux. The intensity was nearly constant during the experiments.
4.3 Preparation of photocatalyst
AgBr loaded TiO2 was prepared by the deposition-precipitation method . NaBr-ethanol solution was obtained by dissolving 0.1028 g of NaBr in 4 mL of ethanol. AgNO3-ethanol solution was obtained by dissolving 0.1690 g of AgNO3 in 15 mL of ethanol by sonication. The NaBr solution was mixed with the AgNO3 solution under magnetic stirring. AgBr formed was added to a mixture of 10 mL of Ti(OBu)4, 80 mL of 2-propanol and 10 mL water under magnetic stirring. Pale yellow precipitate was obtained and it was filtered, washed thoroughly with distilled water and then with acetone, dried in air oven for 5 h at 90°C and then calcined in muffle furnace at 450°C for 2 h. AgBr loaded TiO2 was formed as a greenish yellow powder. This catalyst contained 42.4 wt% of AgBr. Pure titania (TiO2) was prepared by a similar procedure without the addition of AgBr.
4.4 Analytical methods
The specific surface areas of the samples were 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) analysis was performed on gold coated samples using a JEOL-JSM 5610 LV, equipped with OXFORD energy dispersive X-ray microanalysis (EDS). A quantity of AgBr loaded TiO2 suspensions were dropped onto copper grids with holey carbon film. The grids were dried under natural conditions and examined using a TEM Hitachi H-7500. The surface morphology and particle shape were obtained from atomic force microscope (AFM, JSPM-5200TM, JEOL). Powder X-ray diffraction patterns of AgBr loaded TiO2 catalyst was obtained using X'Per PRO diffractometer equipped with a CuK α radiation (wavelength 1.5406 Å) at 2.2 kW Max. Peak positions were compared with the standard files to identify the crystalline phase. Diffuse reflectance spectra were recorded using Shimadzu UV-2450. The solution containing beaker was then kept in sonication bath (33 KHz, 350 W) at room temperature. UV spectral measurements were done using Hitachi-U-2001 spectrometer. The pH of the solution was measured by using ELICO (LI-10T model) digital pH meter.
4.5 Electrochemical impedance spectroscopy analysis
where A is the area of cross section, t is the thickness of the sample, σ is conductivity and Rb is resistance of the catalyst material.
4.6 Preparation of AgBr loaded TiO2 nanocomposite electrode for cyclic voltammety study
A predetermined amount of AgBr loaded TiO2 was dispersed in a 0.1% nafion in ethanol solution for 1 h in an ultrasonic bath to form a stable suspension. In this case, a uniformly dispersible solution containing up to 1.0 mg/1 mL (AgBr loaded TiO2 solution) is stable. AgBr loaded TiO2 nanocomposite was deposited on the glassy carbon electrode by droplet evaporation for 15 min and then drying in nitrogen atmosphere for 20 minutes. Finally the electrodes were washed with water, before use. For comparison experiments, Prepared TiO2/GCE electrode was prepared, using the the same procedure.
Brunauer, Emmett and Teller
Transmission Electron Microscopy
Scanning Electron Microscopy
Energy Dispersive X-ray microanalysis
Electrochemical Impedance Spectroscopy
Atomic Force Microscope
- RR 120:
Reactive Red 120
The authors thank the Ministry of Environment and Forests (MOEF), New Delhi, for the financial support through research grant No. 315-F-36, F. No. 19/9/2007-RE. We thank Catalysis Laboratory, IIT Madras, Chennai for BET and XRD measurements.
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