Photocatalytic degradation of methyl orange dye by pristine titanium dioxide, zinc oxide, and graphene oxide nanostructures and their composites under visible light irradiation
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Discharge of azo dyes by textile and allied industries to the environment is a growing problem. Degradation of an azo dye, methyl orange (MO), was tested in simulated wastewater with different oxide nanomaterials acting as photocatalysts under visible light. Titanium dioxide (TiO2), zinc oxide (ZnO), and graphene oxide (GO) were synthesized, characterized, and applied for adsorptive and photocatalytic removal of the dye. Factors such as initial concentration of MO and size of nanoparticle photocatalyst were varied to determine the optimum conditions for dye removal. Finally, nanocomposites of the three materials (GO–TiO2–ZnO) were synthesized and tested for its photocatalytic performance. The composition of the individual oxide in the nanocomposite was then varied to achieve the best photocatalytic performance.
KeywordsNanoparticles Photocatalysis ZnO TiO2 Graphene oxide
Textile and dye-manufacturing industries are discharging toxic and non-biodegradable azo dyes, such as methyl orange, into the environment (Daneshvar et al. 2003). Of the total world production of dyes, up to 20% is lost during industrial processing, causing environmental pollution and contributing to eutrophication that affects aquatic life (Konstantinou and Albanis 2004). Current methods used to treat these effluents are mainly physicochemical, causing a disposal issue for the sludge residue. For example, chemical precipitation and separation of pollutants, electrocoagulation and elimination by adsorption do not destroy the contaminants: they only transfer them to solids which are primarily disposed of into landfills (Daneshvar et al. 2003). Applying nanotechnology to dye degradation shows great potential, as nanoparticles can chemically react with the dyes to form non-toxic products that may require no removal (Biswas and Wu 2005). Photocatalysis is a technique utilizing nanotechnology under thorough study now (Ahmad et al. 2015; Meng et al. 2014; Meng and Ugaz 2015).
The process of photocatalysis is powered by photons that match or exceed the band gap energy of a given semiconductor (Saleh and Gupta 2012). An electron in its valence band (VB) is excited to the conduction band (CB), leaving a positive hole in the VB that forms a hydroxyl radical with the hydroxyl ion in water, which is then available for oxidation. Meanwhile, the excited electron reduces oxygen in the CB, which can also act as an oxidizing agent (Saleh and Gupta 2012). However, the photo-generated electrons are unstable in the excited state, thus can easily recombine to their respective holes. This process dissipates the input light energy and results in low-efficiency photocatalysis. Therefore, the development of a more efficient photocatalyst is an important consideration (Saleh and Gupta 2012; Dai et al. 2014; Sakthivel et al. 2003; Ahmad et al. 2015).
Titanium dioxide has been extensively studied as a photocatalyst due to its physical and chemical stability, non-toxicity, low cost, and insolubility under various conditions (Saleh and Gupta 2012). TiO2 is a semiconductor oxide with a high band gap energy of 3.2 eV, allowing pollutant degradation when exposed to high energy light. Similarly, zinc oxide also possesses wider band gap (3.37 eV) than TiO2, as well as higher electron mobility (Dai et al. 2014). While both materials show great potential, they alone do not operate with high photocatalytic efficiency due to electron/hole pair recombination (Dai et al. 2014; Liu et al. 2016).
Strategies to improve the photocatalytic performance of these metal oxides have been widely tested over the past decade (Dai et al. 2014; Konstantinou and Albanis 2004; Saleh and Gupta 2012). For example, altering the textural design, doping, and forming semiconductor composites have been examined. One particular focus of research is the utilization of graphene in combination with semiconductor photocatalysts to act as an electron-transfer medium to reduce recombination (Dai et al. 2014; Nguyen-Phan et al. 2011). There are reports on the performance of nanocomposites of graphene oxide (GO) and TiO2; however, little work has been done with GO–ZnO, GO–TiO2 at different concentrations, composite ratios, and size. In the present study, photocatalytic degradation efficiencies of nanomaterials in pristine (TiO2, ZnO) and composite (ZnO–TiO2, GO–TiO2, GO–ZnO, GO–ZnO–TiO2) form were investigated for the degradation of methyl orange dye under visible light irradiation. Further, the effect of nanoparticle loading, size, and composition were also studied.
Materials and methods
The primary objective was to study the photocatalytic activity of engineered TiO2, ZnO and GO nanoparticle/composite. Details of the experimental plan are described in the following sections.
Synthesis and characterization of nanomaterials
In the present study, we tested ZnO, TiO2 and graphene oxide nanomaterials.
TiO2 nanoparticles were synthesized via hydrothermal reaction of titanium alkoxide stabilized in acidic ethanol–water mixture (Chae et al. 2003). A precursor, titanium isopropoxide (0.02 M; 97% Sigma-Aldrich, MO, USA) was added to ethanol/water (1:8) solution and pH was adjusted to 0.7 using nitric acid. Then the solution was kept on stirring for 4 h at room temperature followed by hydrothermal reaction at 240 °C. The crystallized TiO2 nanoparticles were then obtained as a colloidal suspension.
ZnO nanoparticles were prepared by the sol–gel method (Zak et al. 2016). In brief, 5 g of starch (capping agent) was dissolved in 75 mL of deionized (DI) water at 75 °C and stirred for 30 min. Then, 11.2 g of Zn(NO3)2·6H2O was dissolved in DI water, stirred for 30 min, and added to the starch solution. The mixture was then incubated and stirred for 10 h at 80 °C. ZnO nanoparticles were obtained after the powder product was calcined for 5 h at 500 °C.
A modified Hummer’s method (Wang et al. 2012; Song et al. 2014) was used to synthesize GO, and functional groups (e.g., epoxy, hydroxyl, carboxyl) on the surface allowed for its dispersion in polar solvents including water. In brief, 50 mL of concentrated sulfuric acid (H2SO4) was added into a beaker containing 2 g of graphite (45 µm, Sigma-Aldrich, MO, USA) at room temperature. The beaker was cooled to nearly 0 °C using an ice bath. Six grams of potassium permanganate (KMnO4) was then slowly added to the above mixture while it was allowed to warm to room temperature. The suspension was stirred for 2 h at 35 °C. After the suspension was cooled in an ice bath, it was diluted by 350 mL of deionized (DI) water. Then, hydrogen peroxide aqueous solution (H2O2, 30%) was added until the gas evolution ceased to reduce residual permanganate. The suspension was then filtered, washed by DI water, and dried at room temperature for 24 h to obtain brownish graphite oxide powder. The dry powder was dispersed in DI water and sonicated for 3 h to get exfoliated single layer graphene oxide nanosheets. The suspension was then centrifuged at 10,000 rpm for 30 min, and the supernatant was used for the experiment.
The synthesized nanomaterials were characterized for the size and morphology. Transmission electron microscopy (TEM, FEI Technai G2 Spirit) was used to determine the diameters and structure of the synthesized nanoparticles.
Preparation of nanomaterial solutions and dye suspensions
Nanomaterial/composite solutions were prepared at different concentration ratios to test their effects on photocatalytic performance. The first nanocomposite material was prepared by mixing 2 mL each of the 1000 ppm TiO2, ZnO, and GO suspensions. The solution was then sonicated for 30 min before use. The TiO2/ZnO, TiO2/GO, and ZnO/GO nanocomposites were prepared by mixing 5 mL of each component suspension, and again, sonicating for 30 min before use. Finally, the ZnO/GO composite with a 5:1 and 10:1 ratio was prepared by mixing 5 or 10 mL of ZnO suspension to the GO suspension. All the nanomaterial/nanocomposite solution were sonicated before used to ensure the uniform dispersion of the material in the solvent (water).
To perform the photocatalytic activity, methyl orange dye was used as a base in this study. The dye was obtained from commercial sources (Sigma-Aldrich, MO, USA; MW: 327.33) as analytical reagent grade. Dye solutions of requisite concentration have been prepared by proper dilution from a stock solution of 1000 mg/L concentration.
Photocatalytic performance tests and its efficiency
Results and discussions
Titanium dioxide (TiO2), zinc oxide (ZnO), and graphene oxide (GO) were synthesized, characterized, and applied for adsorptive and photocatalytic removal of methyl orange (MO) dye. It was found that TiO2 better removes dye through adsorption at higher initial concentrations of MO, but better removes the dye through photocatalysis at lower initial concentrations. With this, the removal of MO by TiO2 was compared to that by ZnO and GO, all at a lower initial concentration of dye. ZnO degraded the most dye, followed by TiO2 then GO. The effect of the size of a particle on photocatalytic dye degradation was tested by varying the size of ZnO, and the smaller 25 nm diameter particles were found to be most efficient. Finally, TiO2/ZnO/GO, TiO2/ZnO, TiO2/GO, and ZnO/GO nanocomposites were synthesized at 1:1 ratio via sonication and characterized via analytical methods. TiO2/ZnO/GO was the most effective composite for degrading the dye. The degradation increased when the ratio of ZnO to GO was increased in the nanocomposite.
This work was performed in part at the Nano Research Facility of Washington University in St. Louis. Partial support by the Lopata Endowment is gratefully acknowledged. CA and SC acknowledge Yao Nie, Washington University in St. Louis for providing training on Xe-lamp. SC thanks the Department of Energy, Environmental and Chemical Engineering Department, Washington University in St. Louis for the travel Grant as a visiting scientist during July–August 2015. CA acknowledges support by a MAGEEP undergraduate summer internship research fellowship from Washington University in St. Louis.
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