MIXED METAL OXIDE MESOPOROUS NANOPARTICLES FOR ENVIRONMENTAL REMEDIATION

Mesoporous mixed metal oxides (SnO2(x) -TiO2 (1-x) , x= 0.75,0.50 and 0.25) were synthesized by evaporation induced self assembly using cationic surfactant, Cetyl Trimethyl Ammonium Bromide (CTAB) as the structure directing agent. The small angle X-ray diffraction pattern of mesoporous SnO2 and SnO2-TiO2 mixed metal oxides revealed the presence of well defined mesostructure in the metal oxides. The mixed metal oxide system has crystallized in orthorhombic structure, resembling the host lattice. Mesopore channels were collapsed upon calcinations at 550°C. The optical absorption of the SnO2 has been extended into the visible region upon incorporation of “Ti”. A remarkable enhancement of the photocatalytic degradation efficiency (60% ) of (SnO2(0.5) -TiO2 (0.5) was observed against aqueous solution of methylene blue dye.


INTRODUCTION
Metal oxides are prospective materials for applications in various fields such as solar energy conversion, photocatalysis, electrochemical catalysis, lithium/sodium ion batteries, field effect transistors and super capacitors [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and have been intensively studied due to their inherent chemical stability, abundance, low cost and environmental friendliness. Metal oxide nanostructures are being widely used in place of bulk counterparts as the unique morphology, surface structure and optoelectronic characteristics associated with the nanostructures are uniquely enhancing the performance devices. Rational design and reproducible synthesis of stable nanomaterials of particular shape, size and microstructure is highly desirable. In particular, synthesis of porous metal oxides with ordered pore structures, as required for photocatalytic applications is remaining a challenging task. Notably, tin oxide (SnO2), a wide band gap semiconductor, has appropriate optoelectronic characteristics suitable for photocatalytic applications but SnO2 nanostructures produced by solvothermal / hydrothermal methods have always exhibited a fairly low specific surface area(<50 m 2 g -1 ) [18]. Hence it is imperative to improve the synthesis strategies to produce mesoporous SnO2 with improved specific surface area. Surfactant templating strategy for the synthesis of non-silica based mesostructures, mainly metal oxides in which both positively and negatively charged low molecular weight surfactants are widely being used for the synthesis of mesoporous metal oxide nanoparticles. It was found that charge density matching between the surfactant and the inorganic species is important for the formation of the organicinorganic mesophases. In the recent past efforts have been made to employ the potential of mesoporous metal oxides/metal oxide nanocomposites for environmental remediation [19][20][21][22][23][24].
In the present work, an attempt has been made to synthesize Mesoporous Tin Oxide (SnO2) by Evaporation Induced Self Assembly and to extend/optimise the synthesis procedure to synthesize SnO2 -TiO2 mixed metal oxide system. Attempts have been made to analyse the photocatalytic activity of the mesoporous metal oxides for the degradation of methylene blue.

MATERIALS AND METHODS
In the present work following methodology adopted for the synthesis of mesoporous SnO2 and SnO2(x) -TiO2 (1-x) mixed metal oxides by Evaporation-Induced Self-Assembly.

Synthesis of Ordered Mesoporous Titania
Mesoporous tin oxide (SnO2) is synthesized using cationic surfactant, Cetyl Trimethyl Ammonium Bromide (CTAB) as the structure directing agent and tin tetrachloride (1.0 M in methylene chloride, Sigma Aldrich) and titanium tetrachloride (1.0 M in methylene chloride, Sigma Aldrich) as the source for tin and titanium respectively. The surfactant solution is obtained by dissolving 2.5 g of CTAB in 50 ml of cyclohexanol (Sigma Aldrich) and the solution (Sol A) is continuously stirred for 2 h during which 3.5 ml of concentrated HCl is added drop wise. To the resulting solution A, 10 ml of tin tetrachloride is added drop wise and stirred for 4 hours. The resulting solution thus obtained is made as a thin layer and kept in hot air oven maintained at 60°C for 4 days. The solid product obtained is calcined in a tubular furnace at a temperature of 550°C for 6 hours at a heating rate of 1°C / minute with air flow. The sample is coded as MSNO-43. Similarly mixed tin -titanium metal oxides (SnO2(x) -TiO2 (

Material characterization
The characteristics of materials prepared in present work were systematically analyzed using X-Ray Diffractometer (Rigaku Miniflex II), High Resolution Transmission Electron Microscope (HRTEM, JEOL JEM 2100, operated at an accelerating voltage of 120 kV), UV-Vis. Spectrophotometer (JASCO, V-650).

Photocatalytic activity
The synthesized SnO2 and metal oxide were tested for photocatalytic degradation of methylene blue. Around 0.2g of the catalyst was suspended in quartz cell along with 200ppm, 5ml aqueous solution of the dye. Prior to light irradiation, the suspension was stirred for 30 minutes in dark to attain the absorption-desorption equilibrium. The sample was irradiated using natural sunlight. At periodic intervals, 5ml aliquots were taken from the system and analysed using UV-Vis spectrophotometer.

RESULTS AND DISCUSSION
The small angle X-ray diffraction pattern of mesoporous SnO2 and SnO2-TiO2 mixed metal oxides are shown in figures 1-5. The presence of well defined diffraction peak centered at 2θ of 0.7° (Fig.1) is indicative of the formation of long range ordered pore structure and the peaks are arising from (100) reflections associated with 2D hexagonal (P6mm) arrays of uniform mesopores [25].
The X-ray diffraction pattern of mesoporous SnO2 prepared in the present work is shown in figure  6. The samples were found to have crystallized in orthorhombic structure, the formation of which is favored at higher temperatures [26]. XRD pattern of TiO2 reveals the formation of a mixed phase containing anatase, rutile and brookite.The XRD pattern of mixed metal oxides prominently featured characteristic features of orthorhombic SnO2 (JCPDS Card No. 78-1063).     enhances the surface area of the semiconductor  widely,  remarkable  enhancement  in  the  photocatalytic efficiency of the mesoporous  photocatalyst  was observed. Photocatalytic efficiency of mesoporous SnO2(0.5) -TiO2 (0.5) (MSOTO-43) nanoparticles was the highest (60%) and further doping has been found to decrease the photocatalytic efficiency. The creation of defect level in the host metal oxide due to formation of mixed metal oxide system plays a pivotal role in enhancing the visible light absorption and in increasing the lifetime of photogenerated charge carriers.

CONCLUSION
Mesoporous tin oxide and SnO2(x) -TiO2 (1-x) mixed metal oxides were synthesized by evaporation induced self assembly method. Systematic analysis on the characteristics of the material revealed the formation of crystalline and mesoporous nanoparticles. The SnO2(x) -TiO2 (1-x) exhibited visible light activity which originates from the creation of electronic states in the band gap of the material.
The enhanced optoelectronic characteristics of the system SnO2(0.5) -TiO2 (0.5) extends the potential of material for environmental remediation through the treatment of organic pollutants such as 4-Chlorophenol and synthetic dyes.