Study on the Properties and Photoactivity of TiO 2 ( nanorod )-SiO 2 Synthesized by Sonication Technique

A series of TiO2 (nanorod)-SiO2nanocomposite (TS) with different mole ratios was prepared by sonication technique. Nanorod TiO2 was prepared by solvothermal assisted by EDTA, while SiO2 was prepared by sol-gel method. Various amount of SiO2 powder was combined with nanorod TiO2 to form TiO2 (nanorod)-SiO2nanocomposite. Based on XRD and TEM results, the TiO2 was rod-like anatase crystalline phase with the rod length of about 20-50 nm, while spherical SiO2 was a large particle with a diameter of about 200 nm. The deposition of nanorod TiO2 on the surface of spherical SiO2 resulted in composites with slightly smaller crystallite size than that of the pristine nanorod TiO2 of about 21-27 nm. The BET surface area of the TS composites was variable, of about 61.8–67.6 m2 g-1. The photoactivity of the composites was examined by measuring the zone inhibition of the composites to the growth of E. coli and B. subtillis, as a model of gram-negative and gram-positive bacteria, respectively. The composites have shown significant antimicrobial activity over both bacteria, which are slightly higher than that of pristine nanorod TiO2. keywords: nanorod, TiO2, SiO2, nanocomposite, sonication.

T i O 2 h a s b e e n w i d e l y u s e d a s a photocatalyst in various applications from cleaning the environmental to generating electricity.These include applications in the air and water purification systems 1,2 , products of self-cleaning surfaces 1,3 ,sterilization 1,3 , hydrogen evolution 1,2 , and photoelectrochemical conversion 4 .TiO 2 is considered an ideal semiconductor for photocatalysis because of its high stability, low cost and safety toward both humans and the environment 5 .The development of new materials, however, is strongly required to provide enhanced performances with respect to the photocatalytic properties and to find new uses for TiO 2 photocatalysis.TiO 2 or titania is also used in many applications like in pigments, ceramics, sun lotions, etc. Titania decomposes organic material when illuminated with ultraviolet (UV) rays.This trait is utilised in the treatment of the waste material, air pollutant, drinking water, and in the development of self-cleaning surfaces [6][7][8] .
Recently, titania suppor ted on silica (SiO 2 ) have attracted much attention as advanced materials.A more uniform titania distribution was obtained by doping TiO 2 with SiO 2 , ZrO 2 , and other metal oxides [9][10][11] .The addition of SiO 2 , or other oxides into anatase TiO 2 can improve the thermal stability and specific surface area [12][13] .An increase in the photocatalytic activity is an added advantage to the self-cleaning and antibacterial properties [14][15] .TiO 2 supported on silica was reported to exhibit a better photocatalytic performance than TiO 2 itself.This improved performance was believed due to the interaction between TiO 2 and SiO 2 , and improved adsorption of the pollutant on the silica over pure TiO 2

16
. Various type of mixed oxide TiO 2 -SiO 2 have been synthesised and applied to reduce organic dye pollutant 16 .TiO 2 and SiO 2 nanospheres were commonly used to prepare TiO 2 -SiO 2 mixed oxide.
Recently, various types of TiO 2 such as nanorods, nanotubes, nanocrystals and mesoporous materials have been synthesized 17 .One-dimensional (1-D) TiO 2 nanostructures such as ellipsoidal nanoparticles 18 , nanowires 19 , and nanotubes 20 , have been investigated extensively due to their special properties, which are attributed to dimensional anisotropy.Nanorod TiO 2 has been used to produce a superhydrophobic surface in a coating 19 .In 1-D nanostructured crystals of TiO 2 , the space charge region is well constructed along the longitudinal direction of TiO 2 nanocr ystal, meaning that photogenerated electrons can flow in the direction of the crystal length.Increased delocalization of electrons at 1-D nanostructured crystals can lead to a remarkable decrease in e H = e -recombination probability 21 .
In this study, the nanorod titania decorated on silica were synthesised by mixing silica sol and titania sol through sonication technique 16,23 .Titania nanorod was prepared by solvothermal techniques 23 .On the other hand, SiO 2 nanoparticle was prepared by sol-gel methods 24 .If the surface of a larger particle is covered by a layer of fine particles, a core-shell system might be resulted 22 .The properties of nanorod TiO 2 decorated on SiO 2 at several compositions will be studied, including the surface area, crystallinity, crystallite size, and the effect of the composition of the mixed oxide on the photocatalytic activity.The photocatalytic performance of these materials was investigated by testing their ability to inhibit the growth of bacteria.E. coli was used to represent gram-negative bacteria and B. subtillis to represent gram-positive bacteria.

MATERIAL ANd METHOdS
All chemicals used in this experiment were of high purity and used as received.Titanium tetra-isopropoxide (TTIP, Sigma-Aldrich), anhydrous toluene (Mallinckrodt), ethylene diamine tetra-acetic acid disodium salt (Na 2 EDTA, Sigma-Aldrich), tetraethyl orthosilicate (TEOS, Merck Germany) and ethanol (Merck Germany) were used as starting materials.

Synthesis of spherical SiO 2
The SiO 2 nanoparticles were synthesised using modified Stober process.The starting materials used for SiO 2 synthesis were TEOS, NH 3 , ethanol, and H 2 O with 9: 7: 7: 27 (v/v).TEOS and ethanol were mixed and stirred for 15 min.Then, NH 3 and H 2 O were added dropwise slowly and carefully while sonication was performed for 2 h.All concentrations of the reactant were calculated based on final volume in the reaction mixture 24 .Sonication process took place in an Ultrasonic Homogenizer (Krisbow model 5510, 40 kHz).

Synthesis of nanorod TiO 2
Initially, 5 mmol of diethylene tetraacetic acid disodium salt (EDTA) complexing agent was added to 25 mL of millipore water and stirred until the dissolutions complete.The pH was maintained at around 8 by adding ammonia solution, and then 50 mL of toluene was slowly added and stirred for about 30 min.To this mixture, 5 ml of TTIP was added at a constant rate during the course of the reaction.The toluene to water ratio was set to 2:1, and the water to TTIP ratio was kept as 5:1.The solution turned into the slurry as a result of slow hydrolysis of TTIP in water.The final mixture was transferred into an autoclave (80% filling) sealed properly and heated at a constant temperature of 180ºC for 3 h.The product was collected after centrifuge and washed repeatedly with millipore water and dried at 120ºC for 6 h 23 .

Synthesis of TiO 2 (nanorod)-SiO 2
One gram of TiO 2 resulted from solvothermal method were dissolved in 50 mL ultrapure water, 1 mL HNO 3 0.1 M and 10 mL acetic acids were added in order to get the pH of 2 of the sol while sonicated for 10 min.The as-synthesized SiO 2 powder in the various mole percentages was dispersed in TiO 2 sol carefully.The mixture was dispersed in an ultrasonic bath for 30 min and kept for 24 h to form TiO 2 deposited on SiO 2 sphere 16 .The suspension mixture then was filtered, dried, and calcined in air at 450°C for 3 h.The product was TiO 2 -SiO 2 composite with 10%, 20%, and 30% mole content of SiO 2 and named as TS-10, TS-20, and TS-30, respectively.

Characterization
The crystal structure of powder was analysed by X-ray diffraction (XRD) using a Rigaku Miniflex 600, X-ray powder diffraction (Rigaku Corporation, Tokyo, Japan) with Ni-filtered Cu Ka (0.15418 nm) radiation at 45 kV and 20 mA.The crystallite size was calculated using the Scherer formula.

L=
where L is the average crystallite size in nm, K is a constant usually taken as 0.9, l is the wavelength of the X-ray radiation (using Cu Ka = 0.15418 nm), b hkl is the line width at half-maximum height in radians, and q is the diffracting angle 18,23 .The functional group of the compound was identified by Shimadzu FTIR-820 IPC.The Brunauer-Emmett-Teller (BET) surface area of TiO 2 and TiO 2 -SiO 2 powders were determined using Surface Area Analyzer (Nova 3200e Quantachrome).The transmissions electron microscope (TEM) images were obtained using a JEOL JEM-1400 electron microscope to identify the morphology of TiO 2 .Scanning Electron Microscopy (SEM) observations were carried out using a JEOL JSM-6510LA electron microscope.

Antimicrobial activity studies
The antimicrobial activity of the synthesised nanocomposites was investigated by well diffusion method against two bacterial strains, Escherichia coli (FN CC0051) as gram-negative bacteria, and Bacillus subtilis (ATCC 6633) as gram-positive   bacteria.The obtained bacterial cultures were maintained on nutrient agar slants.The test bacterial suspensions (50 µL) containing 10 4 cells/mL were spread on nutrient agar plates.50 µL nanomaterial (TiO 2 ) suspension or 50 mL nanocomposite (TiO 2 -SiO 2 ) suspension (10 mg/mL) was added in the tested wells.The samples were initially incubated for 15 min at 4°C to allow diffusion and later illuminated with UV lamp (365 nm) for 40 min and then were incubated at 37°C for 12-18 h for the bacterial cultures 25 .

X-ray diffraction
The powder XRD pattern of TiO 2 synthesized via Ti-EDTA chelated complex was presented in Fig. 1.The diffraction peaks are analogous to anatase crystal structure.For each sample, all peaks correspond to anatase phase at 2q = 25.088,37.6400, 47.7900, 53.7000, 54.8300, and 62.5570 (# 01-071-1168).It is found that the higher the percentage of SiO 2 the lower the intensity of the peaks.The peak associated with SiO 2 confirms an amorphous nature of silica.The increasing of SiO 2 particle in the formation of TiO 2 -SiO 2 composites means the increasing of the amorphous particle into the composites, consequently the crystallinity will reduce as confirmed by less intense XRD peaks of the composites.From the data in Table 1, it appears that increasing the amount of SiO 2 in the composite also affect the crystallite size and surface area.This effect may be due to the SiO 2 limiting the agglomeration of nanorodTiO 2 .Commonly, all crystallite size of the TS nanocomposite was not significantly different.Sirimahachai et al. 16 obtained a similar tendency about the influence of the amount of SiO 2 against the crystallinity and crystallite size of the composites.Fig. 2 is proposed to explain the arrested growth of nanorod TiO 2 by spherical SiO 2 .It is assumed that the nanorod attachment on the spherical surface of silica is irregular (Fig. 2d).Thus, the crystallite sizes of the composites are variable.Fig. 2a and 2b demonstrate the attachment model of the spherical TiO 2 on the surface of spherical SiO 2 .These can affect the crystallite size of the resulted composites, resulting in small crystallites of TiO 2 16 .Spherical morphology of TiO 2 allows its deposition evenly over the surface of SiO 2 .In another way, TiO 2 can be deposited in several layers, such

TEM image
The TEM images of TiO 2 nanopowder (Fig. 3), shows the morphology of nanorods with the length size of about 50 nm.The TEM image endorses the formation of TiO 2 nanorods and their presence is quite consistent.To confirm the degree of orientation, the texture coefficient (TC) for ( 004) and ( 200) peaks of the oriented nanocomposite powder were determined and compared with the TC value of the standard TiO 2 powder 26 .TC of the synthesised TiO 2 was calculated from the relative intensity data in the XRD pattern (Fig. 1) and the PDF data file (# 01-071-1168).Deviation of TC values from unity implies preferred growth.A higher value of TC denotes preferred orientation of Fig. 6: The FTIR spectra of TiO 2 , SiO 2 and composites of TiO 2 -SiO 2 (TS) grains 26 .The calculated TC (004) and ( 200) values of as-synthesized nanocomposite powder were 1.17 and 1.02 respectively.This oriented crystal growth obtained because of the act of EDTA as an orienting agent in the synthesis 23 .Christy et al. has been succeeding to synthesise nanorodsTiO 2 with a simple EDTA modifier 23 .
The TEM images of SiO 2 and nanorod TiO 2 deposited on SiO 2 sphere were presented in Fig 4 .From Fig. 4(left) the particle size of SiO 2 is about 100 nm.The spheres of silica are quite homogenous.Sirimahachai et al. 16 obtained spherical SiO 2 with average size of about 350 nm.Wilhelm et al. 22 synthesised SiO 2 through Stober method at the various different size, from 150 nm until 590 nm.In this study, the particle size of SiO 2 was smaller than that of the SiO 2 reported by Sirimahachai et al. 16 .The spherical SiO 2 was then combined with nanorod TiO 2 with the length of about 50 nm.It is assumed that different size and morphology of the source of the composite produces rather a different shape and character of the composite.

BET Surface Area
The BET specific surface area (S BET ) was calculated by using the standard BET method on the basis of the adsorption data 28 .The nitrogen adsorption-desorption isotherms of TiO 2 , SiO 2 , and TiO 2 -SiO 2 (TS) material are shown in Fig. 5. Their different appearances suggest the modifications of the porosity characteristics when the mixed oxides form a new network.The isotherm curve for SiO 2 , shows a type IV isotherm with a broad type H4 hysteresis loop, in the middle range of relatively pressure.This could be associated with the presence of narrow slit pores 29 .For TiO 2 and TS material, type IV isotherms with type H1 hysteresis loop in relatively high pressure are observed.It can be associated with a porous material consisting of well-defined cylindrical-like pore channels 29 .This supports the synthesis design that the nanorod TiO 2 covering the surface of the spherical SiO 2 .
Sirimahachai et al. 16 reported the relatively significant change of surface area against the increasing of SiO 2 , due to the sphere morphology of both SiO 2 and TiO 2 .In this research, the TiO 2 deposited on SiO 2 was rod-shaped.The surface areas of the composites are just slightly changed against the SiO 2 content.It is predicted that nanorod morphology of TiO 2 will provide steric hindrance to covering the SiO 2 particle completely as depicted in Fig. 2d-e.Therefore, the surface areas of the composites are almost the same.

FT-IR spectra
The FT-IR spectra of nanorod TiO 2 and TS composite with a various mole ratio of SiO 2 are presented in Figure 6.IR spectra of pure silica were also taken for comparison.The absorption bands at around 3426 and 1635 cm -1 observed in all spectra are attributed to the stretching mode of water and hydroxyl group 14 .The peak around 802 cm -1 and 1103 cm -1 are ascribed to the symmetric vibration of Si-O-Si and the asymmetric stretching vibration of Si-O-Si, respectively 16 .The peak at 950 cm -1 corresponding to the vibration of Si-O-Ti confirmed the formation of Si-O-Ti inorganic network between SiO 2 and TiO 2 in TS-20 and TS-30 samples 30 .In TS-10, the amount of silica is too low for the formation of Si-O-Ti network, hence a peak for Si-O-Ti could not be observed.It is also clear that composites spectra are a combination of the pure SiO 2 spectra and pure TiO 2 spectra 14 .

SEM image
For further characterization of the nanorod TiO 2 deposited on SiO 2 , SEM studies were done.Figure 7 shows SEM images of the SiO 2, and TiO 2 deposited on SiO 2 (TS) particles.The image of SiO 2 illustrates that the particles are spherical and have a smooth surface.After the deposition process, SEM image shows that the coated silica is still spherical, but the surface of the particles is not smooth anymore.This can be ascribed to the presence of nanorod titania deposited on the surface of spherical silica.Similar result was carried out by heterogenic coagulation method to depositing spherical TiO 2 on the surface of SiO 2 23, 31 .It can be concluded that the nanorod attachment may follow Fig. 2e.

Antimicrobial activity
Antimicrobial activity of nanorodTiO 2 and TiO 2 (nanorod)-SiO 2 (TS) composites were investigated by well diffusion method against E. coli and B. subtillis.All tests and inoculation on each plate were run in duplicates.The results then were inspected visually (Fig. 8) and zone inhibition values were recorded (Table 2).Figure 8 (I) and (II) shows inhibition zone for B. subtillis and E. coli respectively.TS-20 composite gives the largest inhibition zone for both B. subtillis and E. coli.This result may have a relationship with the BET surface area, which shows the largest area for this composite.Sirimahachai et al. 32 investigated the performance of various TiO 2 to inhibit bacterial growth and obtained the best result for the TiO 2 nanoparticles with the largest surface area (320.1 m 2 g -1 ).In this study, TS-20 composite inhibits the growth of the bacteria at the same concentration (50 mg/mL) but with the smaller surface area (67.561 m 2 g -1 ).The same result was also obtained by Piskin et al. 25 which investigated the TiO 2 sol synthesised by sonication technique.
All samples also demonstrate higher inhibition toward gram-negative bacteria compare to the gram-positive bacteria.The gram-positive bacteria, B. subtillis, have a relatively thick wall composed of many layers of peptidoglycan polymer and only one layer of membrane.The gramnegative bacteria (E.coli) have only a thin layer of peptidoglycan and a more complex cell wall with two cell membranes, an outer membrane, and a plasma membrane.

CONCLUSIONS
It is concluded that the major phase of the nanorod TiO 2 particle and TiO 2 -SiO 2 composites are the anatase crystalline phase.The presence of silica has retarded the grain growth of nanorod titania so considerably, that the particle size of the composite is slightly smaller than that of pristine TiO 2 .The SEM image of the composite structure confirms the attachment of nanorod titania on the surface of spherical silica.The sonication technique provides a facile technique to produce a composite of titania-silica with a good crystallinity and surface area from TiO 2 and SiO 2 sols as the precursors.TiO 2 (nanorod)-SiO 2 composites have also shown good antimicrobial performance at a small surface area, that might be contributed by the nanorod structure.Study on hydrophobicity properties induced by the nanorod TiO 2 over the SiO 2 is underway.

Fig. 1 :
XRd patterns of TiO 2 , SiO 2 , and different ratio of TS composite

Table 1 : The crystallite size and surface properties of TiO 2 and TS composite Composite Crystallite BET surface Pore volume Pore size(nm)* area (m 2 /g) (cm 3 /g) diameter(nm)
* calculated as average value of crystallite size from d 101 , d 200 , and d 004