Photocatalytic degradation of methyl orange dye by pristine titanium dioxide, zinc oxide, and graphene oxide nanostructures and their composites under visible light irradiation
Appl Nanosci
Photocatalytic degradation of methyl orange dye by pristine titanium dioxide, zinc oxide, and graphene oxide nanostructures and their composites under visible light irradiation
Ramesh Raliya 0 1 2
Caroline Avery 0 1 2
Sampa Chakrabarti 0 1 2
Pratim Biswas 0 1 2
0 & Pratim Biswas
1 Department of Chemical Engineering, University of Calcutta , Calcutta, West Bengal 700073 , India
2 Department of Energy, Environmental and Chemical Engineering , Washington University in St. Louis, St. Louis, MO 63130 , USA
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.
Nanoparticles; Photocatalysis; ZnO; Graphene oxide
-
TiO2
Introduction
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
photogenerated 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 oran (...truncated)