Fabrication of highly photocatalytic active Anatase TiO2-Graphene oxide heterostructures via solid phase ball milling for the degradation of organic pollutants
Abstract: In this work, a series of TiO2/GO composites have been synthesised using TiO2 and graphene oxide as starting material. The composites were synthesised by simple solid-phase ball milling process without using any solvent or high temperature treatment. The structure of the heterophotocatalyst was characterised by X-ray diffraction, fourier transform infrared, raman, UV-Vis, scanning electron microscopy and transmission electron microscopy methods. The results showed that TiO2 particles are anchored firmly and are decorated on graphene oxide sheets. The photocatalytic performance of TiO2/GO composites for degradation of organic pollutants was investigated against methylene blue (MB) dye as test pollutants. The results showed the much higher photocatalytic activity of anatase TiO2/GO nanocomposites than that of anatase TiO2 under light irridation with TiO2/GO0.4 composite possessing the highest photocatalytic activity. These photocatalytic reactions followed the pseudo first order kinetics from which rate of reaction was also determined. The possible photocatalytic mechanism of MB degradation by TiO2/GO composite is discussed.
Keywords: photodegradation; nanocomposites; Graphene oxide; ball mill; methylene blue.
1. Introduction
In the recent years, the fast pace of industrialisation, population growth, urbanisation, depletion of environmental resources etc has lead to the increased contamination of potable water [1]. Textile and other dye manufacturing industries discharge toxic and non biodegradable dyes such as methylene blue (MB) into the environment. Almost 20% of the total world dye production is lost during industrial processing leading to environmental pollution particularly water pollution [2]. Methylene blue, an organic dye, has wide variety of applications including temporary hair colorant, paper colouring, dyeing various textile fabrics etc [3]. MB has some serious ill effects on living organisms as it causes nausea, vomiting, diarrhoea, dyspnea, tachycardia, cyanosis, methemoglobin, convulsions etc [4]. MB also has adverse effects on central nervous system as it exerts neurotoxic effects on the central nervous system [5]. In view of these serious harmful effects of dyes, proper treatment is essential before discharging these wastes to environment.
In this direction, a number of methods have been designed for the elimination of these toxic dyes from contaminated water. These methods include adsorption, reverse osmosis, precipitation, chemical oxidation, ion exchange and photocatalytic degredation [6]. Among these methods, heterogenous photocatalysis had remained an area of huge interest because of its great potential for the degredation of organic dyes from polluted water. The fabrication of heterocatalyst with a high energy ball mill in absence of any solvent or high temperature has great potential in catalyst fabrication for degredation of organic pollutants from waste water. In semi-conductor photocatalysis, organic dyes are decomposed under light illumination and thus provide an economical, low cost and easy way of water purification from organic pollutants [7]. During semi conductor photocatalysis, electrons get excited from valance band of semi-conductor to conduction band of semi-conductor and as a result electron-hole pairs are created at the surface of photocatalyst. These electron-hole pairs generate reactive oxygen species that oxidise the organic dyes.
Graphene, a two dimensional carbon material, has created a great deal of interest in recent years due to its excellent electronic properties and other possible applications in various fields [8]. However, large scale production of perfect graphene is still a big challenge. Also, strong Vander walls attraction results in aggregation of graphene sheets and is poorly soluble in water and polar organic solvents [9]. Graphene oxide, being a perfect functionalised graphene, contains numerous reactive oxygen functional groups on its surface. These reactive oxygen functionalities are mostly present in the form of hydroxyl and epoxy groups on the basal plane and a few as carbonyl and carboxyl at the sheet edge. These functional groups on GO provides anchoring sites for the homogenous growth of a number of metals, metal oxides, bio molecules, drugs and inorganic particles [10,11]. Thus in comparision with graphene, graphene oxide has attracted huge attention because of its easy availability in bulk quantity, readily undergoes chemical functionalization, good dispersion in water and high bio compatibility [8].
Among various semi-conductor photocatalysts, TiO2 is the most frequently used due to its low cost, nontoxicity, long term thermodynamic stability and strong oxidising power. However, the large band gap of TiO2 (3.2eV), its slow reaction rate and very high charge recombination rate has hindered the technology in its practical applications. However, these limitations have been addressed by coupling the metal oxide with noble metal [12], CNTs [13], C60 [14] and graphene/ graphene oxide [15,16]. Among these, surface modification of TiO2 with graphene or graphene oxide are believed to show control on the charge transfer across the TiO2/graphene oxide interface and thus increase the surface area of the photocatalyst. Literature reports shows a number of TiO2/ graphene composites have been synthesised and investigated for photodegredation of organic dyes. MB dye degredation have been studied using TiO2/graphene composites synthesised via facile one step hydrothermal method [17], hydrothermal hydrolysis of Ti(SO4)2 [18] and many other related synthetic procedures[19,20].
Different from the above synthetic procedures, we are interested in synthesising the TiO2/ GO composites via solid phase high energy ball mill, in view of the fact that synthetic procedures are going to affect photocatalytic activity [21,22]. The photodegredation of MB dye over TiO2/ GO composites synthesised via solid phase high energy ball milling process have not been reported so far.
In this work, we report the synthesis of a series of TiO2/ GO composites via solid phase high energy ball milling using TiO2 and GO as starting material. The synthesised materials have been characterised by different spectroscopic and surface analysis and have been studied for photocatalytic efficiency of the composite was evaluated against MB dye under UV-Vis light irridation and the synthesised composites showed improved photodegredation of MB dye.
2. Experimental section
All chemicals used in the present work were analytical reagent grade and were used without any further purification. Materials used in this work include graphite powder, sulphuric acid (H2SO4), phosphoric acid, hydrogen peroxide (H2O2, 30%), potassium permanganate (KMnO4), titanium tetraisopropoxide, ethanol and acetic acid . Distilled deionised water was used throughout this work.
2.1. Synthesis of Graphene Oxide by Improved Hammers method.
GO was synthesized by a reported method [23] with slight modifications from natural graphite powder. Typically, a 9:1 (v/v) mixture of concentrated H2SO4 and H3PO4 was added to a mixture of 0.3 g of graphite powder and 1.8 g of KMnO4. After then reaction mixture was heated to 500C and allowed to stir for 12 h to complete the oxidation process of graphite. The reaction mixture was cooled to room temperature and poured in about 500mL of double distilled ice cold water. To stop the oxidation process, 30% H2O2 solution was added gradually added. For workup, the filtrate was subjected to centrifugation, the solid material was washed several times with water, 30% HCl and finally with ethanol. The final product viz. graphene oxide (GO) obtained as dark brown crystalline powder was vacuum-dried overnight at room temperature.
2.2. Synthesis of Titanium Oxide by sol-gel method.
TiO2 nanoparticles were synthesized by an acid assisted sol-gel method using titanium (IV) tetraisopropoxide (TTIP) , distilled water and acetic acid as starting materials, following the modified method published in the literature [24].
2.3. Synthesis of TiO2/GO Composites via solid phase ball milling.
TiO2/ GO composites were synthesised by solid phase mechano-mixing using high energy ball mill. The composites were synthesised by adding desired amounts of GO and TiO2 powders into a 80 ml stainless steel grinding jar with zirconium balls (0.85 gm in weight) with a ball to weight ratio of 5:1. The solid mixture was then ball milled at a rotation speed of 400rpm for a grinding period of 6 hours at room temperature to produce well crystalline and single phase anatase TiO2/GO composites. For comparision, composites with different weight ratios of CdO/GO were also synthesised by same procedure for same grinding time under similar conditions. These samples were labelled as TiO2-GO0.0, TiO2-GO0.2, TiO2-GO0.3, TiO2-GO0.4.
2.4. Characterizations. X-ray diffraction (XRD) measurements has been done on a D8-Advanced Bruker-AXS diffractometer by using Cu K�� (��=0.1540nm ) irradiation at a rate of 4��/min within the range of 4�����135�� , Bragg���s equation, ��=2d sin��, were used to determine the interlayer spacing of d002 of materials. Fourier transform infrared (FTIR) and Raman spectras were carried out using a FTIR Analyzer (Nicolet/Avatar 370) and laser micro-Raman spectrometer (Renishaw InVia) with an excitation of 532 nm laser light respectively. UV-Vis spectra were analysed on the T-80 UV���Vis double beam spectrophotometer at room temperature. Scanning electron microscopy (SEM) images were collected using field emission scanning electron microscope (JEOL/JSM-6390LV). Transmission electron microscopy (TEM) images were obtained using a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. The optical absorption and emission properties of pure TiO2 and TiO2/GO nanocomposites were investigated using UV���Vis spectrophotometer.
2.5. Photocatalytic Experiments.
Photocatalytic activity of the nanocomposite photocatalyst were confirmed by monitoring the degradation studies of methylene blue (MB) dye as model pollutants from aqueous solution under UV- Vis light irradiation using 500 W mercury-arc lamp. Photocatalytic experiments were done in a dark room and the distance between light source and the photoreaction vessel was about 15 cm. Photocatalytic activity measurements were done by dispersing 25 mg of TiO2/GO nanocomposites in 50 ml of aqueous solution of MB dye with initial concentration of 25 mgL-1. Prior to light irridation, the suspensions were magnetically stirred in dark for 30 min to achieve adsorption-desorption equillibrium of MB dye molecules on the photocatalyst surface. The same was ensured by constancy of percent adsorption of MB dye in dark. The suspension was then exposed to UV- Vis light irradiation with continous stirring. At regular time intervals , 3 mL of the suspension was collected and centrifuged at 10000 rpm for 2 min to eliminate the photocatalyst. The concentration of MB in the supernatant were examined spectrophotometrically by recording variations in the absorption peak maxima in the UV���Vis spectra of the organic dyes using a T-80 UV���Vis double beam spectrophotometer.
The photo-degradation of the dye via the photocatalytic activity of nanocomposite as photocatalyst were obtained using the formula:
Photo-degradation of the dye (%) = (Co ��� Ct)/Co �� 100
Where Co is the initial dye concentration before irradiation and Ct is the concentration of the dye at different irradiation times.
3. Results and Discussion
3.1. Phase Analysis
XRD measurements were done to analyse the crystalline structure of the GO based samples. The XRD diffraction patterns of (a) pure Graphene oxide, (b) pure TiO2 nanoparticles and TiO2/GO nanocomposites (c, d, e) with different weight ratios of graphene oxide are shown in Figure 1. In the figure 1a, an intense peak of GO at about 2�� value of 10.5�� that is attributed to the (001) diffraction of graphene oxide. The increase in interlayer spacing from 0.34 nm in natural graphite to 0.82 nm in graphene oxide confirms the presence of oxygen containing functional groups on the GO sheets [25]. The XRD pattern of pure TiO2 (fig. 1b) were attributed to the anatase phase of TiO2 with no contribution of rutile or brookite phase. The highly crystalline characteristic peaks of TiO2 at 2�� values of 24.7, 37.3, 47.6, 53.5, 54.8, 62.3, 68.4, 70.0, and 74.7�� indexed as (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2 respectively [26]. As observed in figure 1, the XRD patterns obtained for different composites (c, d, e) with varying GO content are identical and exhibit peaks similar to that of anatase phase of pure TiO2. In the diffraction pattern of TiO2/GO composites, the (001) reflections of GO are not observed particularly because the anchored TiO2 nanoparticles destroyed the regular layered and ordered structure of GO after ball milling [25]. The disappearance of the reflection peak of GO in turn confirms that TiO2 nanoparticles have been effectively intercalated in highly exfoliated GO sheets. The sharp intensity of (101) diffraction peak in pure TiO2 and TiO2/GO nanocomposites indicate high crystalline nature of synthesised material. Although the GO content is increased from 0.2wt% to 0.4wt%, it is observed that all the TiO2/GO composites displayed a similar crystal composition compared to pure TiO2. However, the peak intensity of diffraction peaks in composites have decreased with the increase in GO content. Further, the broadened (101) diffraction peak in nanocomposites compared to pure TiO2 is predominately due to the reduction in grain size of TiO2 during ball milling. This reduction in TiO2 grain size could also occur due to incorporation of GO sheets in it during ball milling process.