Optimization of preparation conditions of ZnO–SiO2 xerogel by sol–gel technique for photodegradation of methylene blue dye
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The ZnO–SiO2 xerogel photocatalyst was prepared via the sol–gel technique and applied for photodegradation of methylene blue (MB) dye. The optimum conditions for preparation of ZnO–SiO2 gel is 30:70 ZnO:SiO2 molar ratio and TEOS:C2H5OH:H2O:HNO3 is 1:16:12:0.04 molar ratios at 30°C for 30 min, at these conditions the photoactivity of ZnO–SiO2 xerogel was 99% at a surface area of 500 m2/g after 60 min. The optimum loading of ZnO–SiO2 photocatalyst was 0.050 wt% that gives 99% MB dye removal efficiency after 40 min. The overall kinetics of photodegradation of MB dye using ZnO–SiO2 photocatalyst was found to be of the first order.
KeywordsSol–gel Nanoparticles ZnO–SiO2 Photocatalysis Methylene blue dye
With increasing demands on the coloured textile industry, a large amount of toxic wastewater is produced and released into the aqueous ecosystem. Moreover, modern dye wastewater consists of high concentrations, stable colour and a complicated composition. Hence, many traditional treatment methods are limited because of low degradation efficiencies, consumption of chemicals and the generation of secondary pollution (Height et al. 2006; Gao et al. 2011). Recently, advanced oxidation processes have been used to degrade natural and synthetic dyes efficiently. In particular, semiconductor-mediated photocatalytic oxidation can be conveniently applied towards the degradation of dye pollutants using only light, catalyst, and air (Minero et al. 2005; Meng and Juan 2008; Qourzal et al. 2009). The photocatalytic process creates an electronic charge carrier in the conduction band (e−) and an electron vacancy in the valance band (h+). Because the valence band edge of ZnO occurs at approximately 3.37 eV, the hole is a very powerful oxidising agent and is capable of oxidising a variety of organic molecules as well as generating hydroxyl radicals in water (Meng and Juan 2008; Qourzal et al. 2009; Anpo et al. 1980). Among the various semiconductors recently studied, zinc oxide (ZnO) stands out for use in decomposition of organic pollutants because of its high photosensitivity, excellent mechanical characteristics, low cost and environmentally safe nature (Singhal et al. 2008; Shen et al. 2008). The use of ZnO for photocatalytic degradation of organic pollutants has been studied extensively. Examples of such studies may include degradation of trichloroethylene (Jung et al. 1997), anthraquinone sulphonic acid (Sivakumar et al. 2000), 2-chlorophenol (Abdel Aal et al. 2008), Rhodamine dyes (Yu et al. 2004), azo-reactive dyes (Fouad et al. 2006), Congo Red (Movahedi et al. 2009), and Methylene Blue (MB) (Chu et al. 2010). However, silicon dioxide (SiO2) has been coupled with semiconductor photocatalysts to enhance the photocatalytic process. SiO2 has high thermal stability, excellent mechanical strength and helps to create new catalytic active sites due to the interaction between semiconductor photocatalysts and SiO2. Additionally, SiO2 helps to obtain a large surface area as well as a suitable porous structure (Abd Aziz and Sopyan 2009; Anderson and Bard 1995; Ruetten and Thomas 2003; Chun et al. 2001). The present study aims to determine the optimal conditions for the preparation of the ZnO–SiO2 xerogel photocatalyst via the sol–gel technique. The photocatalyst was used in the photocatalytic degradation of methylene blue.
Materials and procedure
All chemicals used namely are: zinc nitrate hexahydrated [Zn (NO3)2 6H2O] from BDH Laboratories Supplies and tetraethylorthosilicate [TEOS, Si(OC2H5)4, 98%] from ACROS organics Laboratories Supplies. A total of 20 ml TEOS was mixed with ethyl alcohol (C2H5OH), ultra pure water (H2O) and nitric acid (HNO3) as catalyst under magnetic stirring for 60 min. Then calculated amount of Zn (NO3)2 6H2O was added simultaneously and slowly into the previous mixture with continuous stirring for 30 min. The prepared sol was left to stand the formation of gel. The gel sample was calcined at 550°C for 5 h in air to obtain the ZnO–SiO2 xerogel.
Methylene blue was selected as a model for the photocatalytic degradation experiments because it is a common contaminant in industrial textile wastewater.
All of the experiments were carried out with a horizontal cylinder annular batch reactor. A black light fluorescent bulb (F18 W-BLB) was positioned at the axis of the reactor to supply UV illumination. The wavelength of the used UV lamp was 365 nm. The experiments were performed by suspending 0.1 g of ZnO–SiO2 xerogel into the reactor with 300 ml aqueous MB solution (50 ppm). The reaction was carried out at room temperature and pH 7, and a sample of the reaction mixture was taken after 60 min for analysis.
Characterisation of xerogel samples
The structure of the catalyst was examined by X-ray diffraction (XRD) on a Rigaku X-ray diffractometer system equipped with as RINT 2000 wide angle Joniometer using Cu Kα radiation and a power of 40 kV × 30 mA. The intensity data were collected at 25°C over a 2θ range of 10°–80°. N2-adsorption measurement was carried out at 77 K using Nova 2000 series, Chromatech. Prior to analysis, the samples were outgased at 250°C for 4 h. The amount of ZnO and SiO2 in the crystalline samples were obtained by Energy Dispersive X-ray technique (Oxford) that combined with Scanning Electron Microscope (JEOL-JSM-5410). Particle sizes of the produced samples were recorded by Horiba Dynamic light Scattering Particle Size Analyzer LB-500.
Results and discussion
Synthesis of ZnO–SiO2 xerogel
Controlling the homogeneity of ZnO in the SiO2 matrix is very important for improving the surface area and photoactivity of the catalyst. This depends on the microstructure of ZnO–SiO2 binary xerogel, which is changed by the ZnO:SiO2 molar ratio, the TEOS:C2H5OH:H2O:HNO3 molar ratio, the reaction time, the calcination temperature, and the calcination time.
Effect of the ZnO:SiO2 molar ratio
Zn/Si atomic % ratio of the optimised sample measured by EDX Technique at three areas
ZnO/SiO2 molar ratio
Zn/Si atomic % ratio
Average Zn/Si atomic % ratio
To study the effect of the ZnO:SiO2 molar ratio on the surface area and the photodegradation efficiency of methylene blue, a series of experiments was carried out by changing ZnO:SiO2 molar ratio from 0.05 to 0.66. We found that as the ZnO:SiO2 molar ratio changed from 0.05 to 0.42, the photodegradation efficiency increased from 74.16 to 81% while surface area decreased from 400 to 320 m2/g due to the decrease in SiO2 concentration. As the ZnO:SiO2 molar ratio increased to 0.66, the surface area still decreased until it reached 290 m2/g where the photodegradation efficiency decreased to 78%, as shown in Fig. 3. These results indicated that as the amount of ZnO increased the photodegradation efficiency increased, even though the surface area is decreased. However, the decreasing efficiency at high amounts of ZnO could be attributed to the strong decrease in surface area. It should be mentioned here that the effective material for photocatalytic degradation is ZnO not SiO2 (Ismail et al. 2004). The optimal condition of the ZnO:SiO2 molar ratio was 0.42.
Effect of C2H5OH:TEOS molar ratio
Effect of the H2O:TEOS molar ratio
Effect of reaction time
Effect of the calcination temperature
Effect of calcination time
Effect of acid catalysts
ZnO–SiO2 xerogel loading
Influence of ZnO–SiO2 xerogel loading on % of photocatalytic degradation of methylene blue dye
ZnO–SiO2 xerogel loading (wt%)
Methylene blue dye removal efficiency (%)
ZnO–SiO2 mixed oxides have been prepared via a sol–gel approach using TEOS and Zn (NO3)2·6H2O. The best conditions to obtain xerogel material for photocatalytic degradation of MB are 30:70 of the ZnO:SiO2 molar ratio and 1:16:12:0.04 of the TEOS:C2H5OH:H2O:HNO3 molar ratio at room temperature for 60 min. Furthermore, the sample was calcinated at 550°C for 5 h. Under these conditions, the photoactivity and surface area of ZnO–SiO2 were 99% and 500 m2/g, respectively, using 0.033 wt% of catalyst and after 60 min of reaction time. Increasing xerogel loading led to decrease of reaction time from 60 to 40 min. Therefore, 0.050 wt% of ZnO–SiO2 xerogel is enough for degradation about 99% after 40 min and the overall kinetic for degradation of MB dye was found to be of the first order.
The authors wish to express their sincere thanks to the King Abdulaziz City for Science and Technology (KACST), Saudi Arabia, for providing financial support (Grant number: P–S-10-0028).
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