Preparation of TiO 2 / SiO 2 Nanocomposite with Non-ionic Surfactants via Sol-gel Process and their Photocatalytic Study

TiO2/SiO2 nanocomposite particles were prepared from TTIP precursor for titania and TEOS precursor for silica, by using a sol-gel method at low temperature in acidic pH with various templates: poly(ethylene glycol) (PEG) and Triton X-100 (TX-100). The properties of resulting Ti/Si nanocomposites materials were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV-Vis absorption spectroscopy; Photo-Luminescence spectra and Fourier transform infrared (FTIR) techniques. The results show that the TS cores were uniformly coated by Triton X-100 having spherical nanoparticles. XRD result showed that TS:TX smaller crystallite size than the TS:TX nanocomposite particles and both samples include anatase phase. The UV– Vis spectrum of as-synthesized samples shows absorption in the visible range. The PL emission intensity of TS:TX nanocomposite was slightly decreased compared with TS:PEG nanocomposite due to decrease in recombination rate. From FTIR results metal oxide stretching was confirmed in both the samples. The photocatalytic activity of samples was tested for degradation of methylene blue (MB) solutions. The results clearly showed that Triton X-100 coated TS sample exhibited higher activity than that of TS:PEG under UV irradiation, due to the low recombination rate of photogenerated electrons and holes.


INTRODUCTION
Titanium dioxide nanomaterials have been intensively studied because of their unique properties, various uses 1,2 and versatile controlled nucleation and growth processes in dilute Ti (IV) solutions [5][6][7] .Among them the sol-gel process is one of the most convenient ways to synthesize various metal oxides due to some advantages, such as homogeneity at molecular level, a wide range of a precursor selection, control over microstructure, low demands upon reagent purity, possibility of a fine adjustment of the end product properties and low processing temperature 8 .TiO 2 crystallizes in three types: anatase, rutile and brookite.Among the three, crystalline titania, especially its anatase phase is an n-type semiconductor 9,10 has most stable modification 11 .Most researchers have paid greater attention to TiO 2 anatase phase in photocatalytic reactions and photoelectrochemical cell its potential application in degradation of various environmental pollutants under ultraviolet (UV) irradiation 12 in both gaseous and liquid phases because of its strong oxidizing power, non-toxicity and long-term photo-stability 13,14 .For more than a decade, studies have mainly concentrated on the suspension of TiO 2 fine powder because of its higher photocatalytic activity photodegrade pollutant molecules when radiated with UV radiation compared to TiO 2 thin films 15 .TiO 2 powders possess interesting optical, dielectrical, and catalytical properties, which results in industrial applications 16,18 .In recent years great efforts have been devoted to the study of TiO 2 materials for photocatalytic degradation of organic and inorganic contaminants of water and air [19][20] .The synthesis of TiO 2 by the sol-gel method has proven to be a very useful tool for photo-induced molecular reactions to take place on a titanium dioxide surface 21 .Titania has band gap energy of around 3.2 eV and is therefore a UV absorber.When TiO 2 is subjected to UV light ( > 385 nm), an electron/hole pair (e CB -/ h VB + ) is generated.Photo-catalytic reactions take place primarily on the surface of TiO 2 , where the photo-generated electrons and holes are trapped.In general, good crystalline materials with optimum surface area are ideal candidates as photocatalysts.
Recently, titania-silica composite materials have been obtained by sol-gel method due to its possible capability in controlling the textural and surface properties of the composite, to offer unique advantages for the preparation of such highly dispersed tetrahedrally coordinated and transparent photocatalysts material 22 .This sol-gel process provides to synthesis mesoporous TiO 2 / SiO 2 composite materials to enhanced stability, textural properties and surface area useful for applications as catalysts and solar-cells 23,24 .SiO 2 has high thermal stability, good carriers properties and excellent mechanical strength widely used in organics compounds to active sites on TiO 2 particles [24][25][26][27] .The addition of SiO 2 on TiO 2 matrix suppresses the crystal growth of TiO 2 and enhanced the photocatalytic performance 28 .Abdolreza Nilchi 29 et.al reported that the sol-gel produced TiO 2 /SiO 2 nanocomposite has good photocatalytic properties due to its anatase phase, existence of tetrahedral coordination of TiO 2 in the SiO 2 matrix and very large surface area.Kim et.al, 30 proposed two models for titanium dioxide activation in the presence of silicon dioxide.There are two types of interaction between TiO 2 and SiO 2 : physically mixed and chemically bonded 31 .Shaozheng Hu et.al studied the SiO 2coated TiO 2 composite materials with enhanced photocatalytic activity under UV light was prepared by a simple catalytic hydrolysis method 32 .
There is a growing interest in the development of nanocomposites consisting of organic polymers and titania or amorphous silica nanoparticles.Mosurkal et.al view nanocomposites of siloxane copolymers and nano-TiO 2 as 'environmentally safe' 33 .The textural properties of TiO 2 -SiO 2 composites synthesized by sol-gel process were successively enhanced using surfactant to enhance the photocatalytic properties 34 .The TiO 2 -SiO 2 coatings were also prepared with poly-ethylene-glycol (PEG) to lower the sol-gel processing temperature to examine biocompatibility of new biomaterials and derived implant coatings 35 .The formation of nanocomposites by the sol-gel method has been reported to be strongly dependent on the choice of a solvent and a foreign stabilizing surfactant.To synthesize TiO 2 -SiO 2 nanocomposites at room temperature with the addition of polymer (PVA) could effectively suppress the particle growth and improve the stability of TS hydrosols 36 .Kimura Isao et.al reported the preparation of composite microspheres suspended in dye solution under ultraviolet irradiation and examined the possibility of applying to wastewater treatment 37 .
In this work, a novel sol-gel method was employed for the synthesis of TiO 2 -SiO 2 nanocomposite particles with non-ionic surfactants polyethylene glycol (PEG) and polyethylene glycol tert-octylphenyl ether (Triton X-100) at low temperature.Photocatalytic degradation activities of the synthesized particles were studied using Methylene Blue (MB) as a model organic compound under UV light irradiation.

EXPERIMENTAL Materials
In this synthesis procedure, we use tetraethoxysilane (TEOS) (99.9 %, Sigma Aldrich) as a precursor material of SiO 2 and titanium tetraisopropoxide (TTIP) (99.9 %, Sigma Aldrich) as a precursor material of TiO 2 .Concentrated hydrochloric-acid (HCl) (99.9 %, Merck) as a catalyst and solvents absolute ethanol (99.9 %, Merck), polyethylene glycol (PEG) (M w = 570 gram/mole), and polyethylene glycol tert-octylphenyl ether (Triton X-100) (M w = 646.87gram/mole) (99.9 %, Merck) were used without any further purification.Methylene Blue (MB) (Sigma Aldrich) was used a model organic compound for Photocatalytic study.Deionised water was used throughout the experiments.The synthesis was carried out in the continuous stirring at the temperature of 60 o C. The stirring was conducted using an electromagnetic stirrer with a stirring speed of ca.1100 rpm.

Synthesis of titania/silica with surfactants composite
In the synthesis of TiO 2 /SiO 2 composite particles were prepared according.Titanium tetraisopropoxide (TTIP) (1 mole) prepared by dissolving in solvent ethanol (10 ml), and deionized water (35 ml) mixture was stirred for 10 min, pH range of 2.5.And, then added, catalyst hydrochloric-acid (HCl) (1 mole) drop-wise into the above mixture and magnetically stirred for 30 min.at temperature 60 o C, in the pH range 1, to become a yellowish homogeneous transparent solution.Then, 1 mole of silica precursor material tetraethoxysilane (TEOS) was slowly added in the titania sol phase and the mixture was stirred for 1 hour at 60 °C.The surfactant (0.1 mole) was added to the above mixture was stirred for 1 hour at temperature 60 o C. A white lump free paste was formed.The resulting pastes were dried at room temperature during 1 month.Finally, the product was grinded into a powder to obtain fine white powder and used for further material characterization.For convenience, the abbreviations TiO 2 /SiO 2 prepared with surfactant PEG is denoted as TS:P EG and with surfactant Triton X-100 is denoted as TS:T X respectively.

Characterization
The prepared titania-silica composite nanoparticles were characterized for surface morphology was studied using scanning electron microscopy (SEM) (Model JEOL-JSM-6390).Powder X-ray diffraction measurement was carried out using Bruker D8 Advance X-ray diffract-metre.The X-rays were produced using a sealed tube and the wavelength X-ray was 0.154 nm (CuK-alpha).The X-rays were detected using a fast counting detector based on Silicon strip technology (Bruker LynxEye detector).The optical properties studied using UV-Visible absorption spectra were recorded with a Carry 5000 UV-Vis-NIR spectrometer, and emission spectra were observed by photoluminescence spectroscopy (F-7000, Hitachi spectrophotometer) with excitation wavelength ( ex ) = 350nm using xenon lamp as source at room temperature..The FTIR spectra were measured on a Bruker Model-Vertex 70-Spectroscopy with a wave number range of 1000 to 4000 cm -1 .

Measurement of photocatalytic activity
The photocatalytic performances of the synthesized composite particles were studied by the degradation experiments using Methylene Blue (MB) dye as a model compound under ultraviolet light irradiation.An UV Xenon lamp with 125 W was used as UV source.Photocatalysis experiments were performed in a self designed photo-oxidation reactor which is a dark black box.The test materials was kept in a glass beaker covered with aluminum foil to prevent light from other sources from entering the beaker.This ensured that all light came from one direct source.The lamp was fitted on the top of the reactor.The distance between lamp and photocatalysis system was about 1 cm.Prior to illumination, a suspension containing 0.05g of powder catalyst was added 1 ml L -1 MB (10 -4 ) aqueous solution was mixed in a glass beaker and magnetically stirred in dark for 30 min., to attain adsorption equilibrium and the solution was exposed to UV-visible irradiation.The experiment was performed at room temperature.At regular intervals, 5 ml solution were taken from the reactor and centrifuged to remove the photocatalyst before analysis by UV-visible spectrometer at 664 nm corresponding to maximum absorption wavelength ( max ) of MB.

SEM characterization
The study of morphologies of the powder samples of Ti/Si nano composite with surfactants PEG and Tritonx-100 was observed with SEM as shown in Fig. 1 (a) (b).
As it can be seen in Fig. 1 (a), the TS:P EG nanocomposite consists of irregular morphology due to the agglomeration of primary particles, which may cause the surface to be non-uniform and illdefined.In Fig. 1 (b), the accumulation of particle indicates packed and dense structure.Whereas TS:T X nanocomposite particles exhibited regular morphology since the TS cores were uniformly coated by Triton X-100 and having well-structured granular nano-surface with clear interstices between the particles/aggregates.The TS:T X nanocomposite powder particles show the formation of uniform spherical and narrowly dispersed particles.

X-ray diffraction analysis
The XRD diffraction patterns of sol-gelderived nano TS:P EG and TS:T X composites were carried out on powder form are shown in Fig. 2. The crystallite type of the TS nanocomposite particles was the pure anatase.In the diffraction pattern, it is clearly assigned that the peaks of SiO 2 are not observed because they are mainly amorphous in nature, and the diffractogram consist strongest diffraction peaks of TiO 2 in anatase phase (with the reference pattern (JCPDS 21-1272) of TiO 2 ).
The powder samples showed the crystalline pattern and the observed d 101 lines match the reported values for the anatase phase.The average crystallite size of the samples was determined from Scherrer's equation 38 using the (101) reflections of the anatase phase assuming spherical particles.Using the Scherrer's formula, D = K / ² cos¸, an estimate of the grain size (D) from the broadening of the main (101) anatase peak can be done, K is a constant depend on the crystallite shape (0.9),  is the X-ray wavelength (in this case,  = 1.5418Å for Cu-K  radiation),  is the full width at half maxima (FWHM) of the diffraction peaks (radian) and ¸ is the Bragg's angle and also the unit cell volume as correlated to reference pattern as shown in Table 1.
The observed d 101 anatase line for TS:T X sample is very broader and weaker than the TS:P EG sample.The broadening of the XRD peaks indicates that the particles are of very small crystallite size in nature 39 .The introduction of SiO 2 can reduce the growth rate of anatase titania nanoparticle.The TS:T X nanoparticles growth is retarded due to the Si atom with Triton X-100, this agree with the small value of grain size and effectively increase the stability of anatase TiO 2 .The result illustrates that Triton X-100 template induces the formation of small and uniform anatase crystallite sizes.The diffraction peak intensity of the TS:T X composite material shows the high crystallinity than nano TS:P EG composite nanoparticle, resulting in production of more charge carriers and high surface area for photocatalytic activity.It is obviously shown that the TS:T X prepared powder sample is completely crystalline than TS:P EG and entirely consists of anatase phase.

UV-visible absorption spectra
The UV-Visible absorption spectra of assynthesized samples show in Fig. 3.It can be seen that the TS:P EG nanocomposites exhibited an absorption edge at 352 nm in Fig. 3a.The absorption edge of TS:T X nanocomposite slightly falls into the visible region at the wavelength of 400 nm as shown in Fig. 3b.The optical band gap can be calculated by the equation: E g =1239.8/» where E g is the band gap (eV) and  (nm) is the wavelength of the absorption edge in the spectrum.The optical band gaps E g for both the synthesized samples were estimated and tabulated in Table 1.
Both the as-synthesized samples show slightly similar absorption pattern, in which the TS:T X nanocomposite shows better absorbance in visible range compared to TS:P EG nano composite particles.Furthermore, due to the higher dispersity and the coverage of SiO 2 on TiO 2 surface restrained the absorption of UV light irradiation.In addition, the absorption spectra of the TS:T X sample shows stronger absorbance than TS:P EG nanocomposite particle under UV irradiation, indicating that TS:T X could be a promising approach for increasing the catalytic activity.

PL spectroscopy analysis
A photo luminescence spectrum in emission mode is done in the full spectrum range from 200 nm to 700 nm.Fig. 4 shows the PL emission spectrum of TS:P EG and TS:T X nanocomposite samples at the excitation wavelength of 350 nm.
There is a wide photoluminescence band in the wavelength range of 410-600 nm.In whole emission spectra the peak is observed at near 434 nm for TS:P EG composite and 434.6 nm for TS:T X composite.The PL intensity of TS:T X nanocomposite was decreased compared with TS:P EG nanocomposite.The PL intensity of TS:T X was reduced and the shape of the PL spectrum obviously split into many side peaks, which shows the formation of resolved manifold lines were located on well-defined lattice sites.The lower PL intensity indicates the decrease in recombination rate, thus shows the higher photocatalytic activity [40].This results indicated that, the interaction existed between SiO 2 and TiO 2 , which cause the formation of the oxygen vacancy, thus to reduced the recombination rate of excited electrons and holes.

FTIR spectroscopic analysis
The FTIR spectra were measured in the spectral range from 400 to 4000 cm -1 , and measurements were performed at room temperature.The FTIR spectra of the sol-gel derived nano TS:P EG and TS:T X samples of TiO 2 /SiO 2 composite particles are presented in Fig. 5.
The transmittance in the range from 3612 cm -1 to 2862 cm -1 may be related to the presence of -OH stretching vibration.The peaks at 1639 cm "1 in the spectra are due to the bending vibration of the -OH group which is probably because the reabsorption of water from the atmosphere 41 .In TS:P EG , the IR spectrum shows a weak band peak at about 1512 cm -1 corresponding to the bending vibration of -OH groups.In TS:Tx, the IR spectrum shows a strong band peak at about 1512 cm -1 corresponding to the bending vibration of -OH groups.
The FTIR spectrum of the samples shows the strong band in the range at 1462 cm -1 is associated with the characteristic vibration modes of TiO 2 (Ti-O).The band around at 654 cm -1 to 549 cm -1 attributes to the Ti-O-Ti stretching vibration of crystalline TiO 2 phase 42 .The band near 1094 cm -1 corresponds to the asymmetric stretching vibration of Si-O-Si bond 43 .This confirms that the SiO 2 phase is formed.The band observed at 949 cm -1 is associated with Si-O-Ti vibration 28 modes which are due to the overlapping from vibrations of Si-OH and Si-O-Ti bonds.Other peaks appear at 1380 cm -1 and 1353 cm -1 , which may be attributed to organic solvents these peaks become quite weak and even vanish.These results indicate that TS:P EG and TS:T X nanoparticles were prepared by a combination of TiO 2 with SiO 2 nanoparticles which corresponds to the metal oxide bonds in both samples.

Photocatalytic activity
The schematic diagram of setup for photocatalytic activity of nanocomposite samples under UV light source is shown in Fig. 6 and The Photocatalytic degradation study results are given in Fig. 7.The adsorption of MB dye without catalyst remains constant and gets saturated after 60 min., indicating that the contribution of self photolysis degradation was negligible.The TS:P EG composite sample exhibited slower rate of the photo degradation.The sample TS:T X shows the higher photo degradation that must be responsible for the two factors: firstly, surface phenomenon that is being very sensitive to the amount of surface -OH groups which may act as the principal reactive oxidant in the photoreactions 12 .Secondly, the low recombination rate of photo generated electrons and holes which caused the interactions in Ti-Si that improved the separation rate of photo generated electrons and holes, thus leading to the higher photo activity.The TS:T X absorbs the solute better than the TS:P EG catalyst and exibits remarkable improvement in the rate of photocatalytic degradation of MB.With sample TS:T X , only after few minute of exposure to UV light, about 50% of the methylene blue was degraded.These results confirms that the TS:T X nanocomposite crystal exhibit high photocatalytic activity than TS:P EG CONCLUSION TiO 2 /SiO 2 nanocomposite materials have been successfully synthesized via a simple and cheap non-ionic surfactants assisted sol-gel method.In XRD investigation, the anatase phase of TiO 2 with nano-sized particles was obtained for both the samples.The suppressing effect of SiO 2 on crystal growth of TiO 2 is clearly observed with the addition of Triton X-100 could effectively decrease the crystallite size of TS composite particle.The TS cores were uniformly coated by Triton X-100 exhibited regular morphology and having spherical particles.Analysis of FTIR spectra shows that in nanocomposite structure Si-O-Si, Ti-O and Ti-O-Si bonds have been formed.The absorption spectra of the TS:T X nanocomposite sample shows better absorbance in visible region compared to TS:P EG nanocomposite particle.In PL optical properties for TS:T X composite shows lower PL intensity indicates that the decrease in recombination rate, thus higher photocatalytic activity.The TS:T X nanocomposite shows the higher photocatalytic activity in degradation of MB under UV light irradiation.