Study on the Properties and Photoactivity of TiO₂(nanorod)-SiO₂ Synthesized by Sonication Technique

Introduction

TiO₂has been widely used as a photocatalystin 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 conversion⁴. TiO₂ is considered an ideal semiconductor for photocatalysisbecause of its high stability, low cost and safety toward bothhumans and the environment⁵. The development of new materials, however, isstrongly required to provide enhanced performances with respect to the photocatalytic properties andto find new uses for TiO₂photocatalysis. TiO₂ or titaniais also usedin 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 thewaste material, air pollutant, drinking water, and in the development of self-cleaningsurfaces^6-8.

Recently, titania supported on silica (SiO₂) have attracted much attention as advanced materials. A more uniform titania distribution was obtainedby doping TiO₂ with SiO₂, ZrO₂, and other metal oxides^9-11. Theaddition of SiO₂, or other oxides intoanatase TiO₂ can improve the thermal stability and specific surface area^12-13. An increase inthe photocatalytic activity is an added advantage to the self-cleaning and antibacterialproperties^14-15.TiO₂ supported on silica was reported to exhibit a better photocatalytic performance than TiO₂ itself. This improved performance was believed due to the interaction between TiO₂ and SiO₂, and improved adsorption of the pollutant on the silica overpure TiO₂¹⁶. Various type of mixed oxide TiO₂-SiO₂ have been synthesised and applied to reduce organic dye pollutant¹⁶. TiO₂ and SiO₂nanospheres were commonly used to prepare TiO₂-SiO₂ mixed oxide.

Recently, various types of TiO₂ such as nanorods, nanotubes, nanocrystals and mesoporous materials have been synthesized¹⁷. One-dimensional (1-D) TiO₂ nanostructures such as ellipsoidal nanoparticles¹⁸, nanowires¹⁹, and nanotubes²⁰, havebeen investigated extensively due to their special properties, which are attributed to dimensional anisotropy. Nanorod TiO₂ has been used to produce a superhydrophobic surface in a coating¹⁹. In 1-D nanostructured crystals of TiO₂, the space charge region is wellconstructed along the longitudinal direction of TiO₂nanocrystal,meaning that photogenerated electrons can flow in the directionof the crystal length. Increased delocalization of electrons at1-D nanostructured crystals can lead to a remarkable decreaseine⁻/h⁺recombination probability²¹.

In this study, the nanorod titania decorated on silica were synthesised by mixing silica sol and titania sol through sonication technique^{16, 23}. Titaniananorod was prepared by solvothermal techniques²³. On the other hand, SiO₂ nanoparticle was prepared by sol-gel methods²⁴. If the surface of a larger particle is covered by a layer of fine particles, a core–shell system might be resulted²². The properties of nanorod TiO₂decorated on SiO₂at 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. coliwas used to represent gram-negative bacteria and B. subtillisto representgram-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 diaminetetra-acetic acid disodium salt (Na₂EDTA, Sigma-Aldrich), tetraethylorthosilicate (TEOS, Merck Germany) and ethanol (Merck Germany) were used as starting materials.

Synthesis of spherical SiO

The SiO₂ nanoparticles were synthesised using modified Stober process. The starting materials used for SiO₂ synthesis were TEOS, NH₃, ethanol, and H₂O with 9:7:7: 27 (v/v). TEOS and ethanol were mixed and stirred for 15 min. Then, NH₃ and H₂O 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 mixture²⁴. Sonication process took place in an Ultrasonic Homogenizer (Krisbow model 5510, 40 kHz).

Synthesis of nanorod TiO₂

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 ata 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 h²³.

Synthesis of TiO₂(nanorod)-SiO₂

Onegram of TiO₂ resulted from solvothermal method were dissolved in 50 mL ultrapure water, 1 mL HNO₃ 0.1 M and 10 mL acetic acidswere added in order to get the pH of 2 of the sol while sonicated for 10 min.  The as-synthesized SiO₂ powder in the various mole percentages was dispersed in TiO₂ sol carefully. The mixture was dispersed in an ultrasonic bath for 30 min and kept for 24 h to form TiO₂deposited on SiO₂ sphere¹⁶. The suspension mixture then was filtered, dried, and calcined in air at 450°C for 3 h. The product was TiO₂-SiO₂ composite with 10%, 20%, and 30% mole content of SiO₂and 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 CuKa (0.15418 nm) radiation at 45 kV and 20 mA. The crystallite size was calculated using the Scherer formula.

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 CuKa = 0.15418 nm), b_hkl is the line width at half-maximum height in radians, and θ 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 TiO₂ and TiO₂-SiO₂ 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 TiO₂. ScanningElectron Microscopy (SEM) observations were carried out using a JEOL JSM-6510LA electron microscope.

Antimicrobial activity study

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 10⁴ cells/mL were spread on nutrient agar plates. 50 µL nanomaterial (TiO₂) suspension or 50 mL nanocomposite (TiO₂-SiO₂) 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 cultures²⁵.

Results and Discussion

X-ray Diffraction

The powder XRD pattern of TiO₂synthesized via Ti-EDTA chelated complex was presented in Fig. 1. The diffraction peaks are analogous to anatasecrystal structure. For eachsample, allpeaks correspond to anatase phase at 2θ = 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 SiO₂ the lower the intensity of the peaks. The peak associated with SiO₂confirms an amorphous nature of silica. The increasing of SiO₂ particle in the formation of TiO₂-SiO₂ 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 SiO₂ in the composite also affect the crystallite size and surface area. This effectmay be due to the SiO₂ limiting the agglomeration of nanorodTiO₂. Commonly, all crystallite size of the TS nanocomposite was not significantly different. Sirimahachaiet al.¹⁶ obtained a similar tendency about the influence of the amount of SiO₂ against the crystallinity and crystallite size of the composites.

Fig. 2 is proposed to explain the arrested growth of nanorod TiO₂ by spherical SiO₂. 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.

Figure 1: XRD patterns of TiO2, SiO2, and different ratio of TS composite Click here to View figure

 

Fig. 2a and 2b demonstrate the attachment model of the spherical TiO₂ on the surface of spherical SiO₂. These can affect the crystallite size of the resulted composites, resulting in small crystallites of TiO₂¹⁶. Spherical morphology of TiO₂ allows its deposition evenly over the surface of SiO₂. In another way, TiO₂ can be deposited in several layers, such as forming an aggregation as suggested by Fig. 2b. The ideal model of nanorod TiO₂ attachment on the surface of silica is presented in Fig. 2(c), the rod-shaped TiO₂are aligned vertically on the surface of SiO₂. Fig. 2d and 2e displayed other possible alignments of nanorod deposition on spherical surface. The nanorod may also attach vertically and horizontallyon the surface of SiO₂.

Table 1: The crystallite size and surface properties of TiO₂ and TS composite

Figure 2: Some composite model of TiO2 deposited on SiO2: a) and b) spherical TiO2 coated SiO2 c), d), and e) rod-shaped TiO2 coated SiO2 Click here to View figure

 

TEM image

The TEM images ofTiO₂nanopowder(Fig. 3), shows the morphology of nanorods with the length size of about 50 nm. The TEM image endorsesthe formation of TiO₂nanorods and their presence is quite consistent. Toconfirm the degree of orientation, the texture coefficient (TC) for (004) and (200) peaks of theoriented nanocomposite powder were determined and compared with the TC value of the standard TiO₂powder²⁶. TC of the synthesised TiO₂ 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 ofTC denotes preferred orientation of grains²⁶. The calculated TC (004) and (200) values of as-synthesizednanocomposite 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 synthesis²³. Christy et al.has been succeeding to synthesisenanorodsTiO₂with a simple EDTA modifier²³.

Figure 3:  TEM image of nanorods TiO2   Click here to View figure

Figure 4: TEM images for SiO2(left) and TiO2(nanorod)-SiO2(TS) (right) Click here to View figure

 

The TEM images of SiO₂ and nanorod TiO₂deposited on SiO₂ sphere were presented in Fig 4. From Fig. 4(left) the particle size of SiO₂ is about 100 nm. The spheres of silica are quite homogenous. Sirimahachai et al.¹⁶ obtained spherical SiO₂with average size of about 350 nm. Wilhelm et al.²²synthesised SiO₂ through Stober method at the various different size, from 150 nm until 590 nm. In this study, the particle size of SiO₂ was smaller than that of the SiO₂ reported by Sirimahachai et al.¹⁶. The spherical SiO₂was then combined with nanorod TiO₂ 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 data²⁸. The nitrogen adsorption-desorption isotherms of TiO₂, SiO₂, and TiO₂-SiO₂ (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 forSiO₂, 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 pores²⁹.For TiO₂ 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 channels²⁹.This supports the synthesis design that the nanorod TiO₂covering the surface of the spherical SiO₂.

Figure 5: N2 adsorption-desorption isotherms of TiO2, SiO2 and composites of TiO2-SiO2(TS) Click here to View figure

 

Sirimahachai et al.¹⁶ reported the relatively significant change of surface area against the increasing of SiO₂, due to the sphere morphology of both SiO₂ and TiO₂. In this research, the TiO₂ deposited on SiO₂ was rod-shaped. The surface areas of the composites are just slightly changed against the SiO₂ content. It is predicted that nanorod morphology of TiO₂ will provide steric hindrance to covering the SiO₂ 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 TiO₂ and TS composite with avariousmole ratio of SiO₂ 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 group¹⁴. 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, respectively¹⁶. The peak at 950 cm^-1 corresponding to the vibration of Si–O–Ti confirmed the formation of Si–O–Ti inorganic network between SiO₂ and TiO₂in TS-20 and TS-30 samples³⁰. In TS-10, the amount of silica is too lowfor the formation of Si–O–Ti network, hence a peakfor Si–O–Ti could not be observed. It is also clear that composites spectra are a combination of the pure SiO₂ spectra and pure TiO₂ spectra¹⁴.

Figure 6: The FTIR spectra of TiO2, SiO2 and composites of TiO2-SiO2(TS) Click here to View figure

 

SEM image

For further characterization of the nanorod TiO₂ deposited on SiO₂, SEM studies were done. Figure 7 shows SEM images of the SiO_2, and TiO₂ deposited on SiO₂ (TS) particles. The image of SiO₂ 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 TiO₂ on the surface of SiO₂^{23, 31}. It can be concluded that the nanorod attachment may follow Fig. 2e.

Figure 7: SEM images for SiO2(left) and TS-20 (right) Click here to View figure

Antimicrobial activity

Antimicrobial activity of nanorodTiO₂and TiO₂ (nanorod)-SiO₂(TS) composites were investigated by well diffusion method against E. coli and B. subtillis. All tests and inoculation on each plate were run induplicates.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. subtillisand 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 TiO₂to inhibit bacterial growth and obtained the best result for the TiO₂ nanoparticles with the largest surface area (320.1 m² 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 m² g^–1). The same result was also obtained by Piskinet al.²⁵ which investigated the TiO₂sol 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 gram-negative 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.

Figure 8: Zone inhibition of TiO2 and TiO2/SiO2 against B subtilis (I) and E coli (II) a. Blank, b. SiO2, c. TiO2, d. TS-10, e. TS-20, f. TS-30 Click here to View figure

 

Table 2: Mean Diameter of Inhibitory Zone (mm) of TiO₂ and TiO₂–SiO₂ Against Bacterial Strains Bacilus subtillis and Escherichia coli

Conclusion

It is concluded that the major phase of the nanorod TiO₂ particle and TiO₂-SiO₂ 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 TiO₂.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 TiO₂ and SiO₂sols as the precursors. TiO₂(nanorod)-SiO₂composites 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 TiO₂over the SiO₂is underway.

Acknowledgements

We are thankful to the Laboratory of Microbiology, Department of Biology UNNES and LPPT UGM for providing all facilities to direct this work. SW is grateful to the Directorate General of Higher Education for providing adoctoral scholarship.

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