The discharge of immensely colossal quantity of colored dyes from various industriessuch as textile, plastic, paper, printing, pharmaceutical and food industry into waterbodies are life-threatening health hazard to aquatic as well as human life. These dyesbeing toxic nature are imposing a serious threat to environment. Rhodamine B (RhB)is one of the very common water-soluble organic dye, and is extensively used as acolorant in the textile and food industries as well as a biological stain in biomedicallaboratories. RhB has been banished to use in food industry for many years due to itssuspected carcinogenic nature. However, with the development of industry and theillegal discharge, RhB still has the chances to enter the aliment chain to hazard humanhealth.
It is therefore essential to remove these dyes from dye-effluents prior to theirdischarge into the receiving water bodies. To date, several treatment methods such aschemical oxidation, coagulation, biological treatment, ozonolysis and adsorption havebeen extensively explored for the removal of organic pollutants from industrialeffluents, but all such methods suffer from various drawbacks. Photocatalysis usingsemiconductor nanocomposites is involved in a group of waste treatment methodscalled advanced Oxidation Processes (AOPs).
AOPs are recommended whenwastewater components have a high chemical stability and/or low biodegradability. Achemical wastewater treatment using AOPs can produce the complete mineralizationof pollutants to CO2, water, and inorganic compounds.Titanium dioxide (TiO2) is one of the most effective photocatalysts due to its strongoxidizing power, abundant existence in nature, non-toxicity and long-term physicaland chemical stability. Its structure-property relationships have been extensivelyinvestigated, and these reveal that phase type, particle size, surface hydrophilicity,crystallinity, morphology and oxygen vacancy concentration have a great influence inits photocatalytic activity (Shao et al., 2013). However, the wide band gap energy(3.2eV) and the fast electron–hole pair recombination of TiO2 limit the efficiency ofthe photocatalytic reaction significantly.
Thus, inhibiting the recombination ofelectron–hole pairs and extending the light absorption to the visible light region arethe key factors to improve the photocatalytic activity of TiO2. Many efforts have beenmade by forming composites and modifying TiO2 with other materials to overcomethese drawbacks, such as combining the photocatalytic activity of TiO2 with theadsorptivity of porous carbons, reducing electron-hole recombination rate and doping3to achieve a narrow band gap. TiO2-carbon hybrids are some of the most extensivelyinvestigated and most promising materials to improve the photocatalytic performanceof TiO2 because a variety of carbon materials can be developed to meet the demandsof TiO2 as a photocatalyst.
In addition, the lightweight, nonpolar, nonreactive andnontoxic nature of carbon materials and the easy separation of the materials fromwater are attractive in waste water treatment. There are three types of TiO2-C hybrids,carbon-supported, carbon doped and carbon coated TiO2, from which many beneficialfeatures have been obtained.The detailed mechanism of the photocatalytic process on the TiO2 surface is still notcompletely clear, particularly that concerning the initial steps involved in the reactionof reactive oxygen species and organic molecules. A reasonable assumption is thatboth photocatalytic oxidative and reductive reactions occur simultaneously on theTiO2 particle. When the electrons in TiO2 (anatase phase) are irradiated by UV raysthey can be excited from the valence band to the conduction band to generateelectron-hole pairs.
The holes created in the valence band can react with watermolecules to give hydroxide radicals ·OH and the photogenerated electrons aresufficiently reduced to produce superoxide (O2-). The redox potential of the electronholepair permits H2O2 formation. Depending on the reaction conditions, the holes,·OH radicals, O2-, H2O2, and O2 can play important roles in the photocatalytic reactionmechanism. There are several issues that are important in this process. First, exposingthe external surface of TiO2 particles to light is a prerequisite to make such anexcitement happen. Second, the light energy (E=hv) must exceed the band gap (3.20eV) of the anatase-type TiO2, therefore lowering the band gap of TiO2 or using lowwave length light is needed to increase the light utilization efficiency.
Third, theoxidizing species cannot migrate for a long distance and stay near the active centers inthe TiO2 particles. Therefore, polluting molecules have to diffuse to the photo-excitedactive centers. Fourth, the recombination of positive holes with excited electronsbefore they react to create active species and centers has to be avoided. Moreover, theTiO2 loaded in the composites should not decrease under repeated cycles and must beeasy to separate from water from a commercial application point of view.
Graphene oxide (GO), a two dimensional carbonaceous material, has attracted muchattention and has been extensively investigated because of its unique structure andphysico-chemical properties. Benefiting from large surface area, superior electronic,4thermal and optoelectronic properties, GO has aroused enormous interest in variousfields such as catalysts, sensors, supercapacitors, and energy devices. The synthesis ofGO-TiO2 composite is regarded as a significantly important approach for improvingthe performance of photocatalysts. The hybridization of GO-TiO2 can reduce therecombination of photogenerated electron–hole pairs, extend the light absorption tothe visible light region and accelerate the transfer rate of charge carriers.