Synthesis and Characterisation of Mesoporous TiO 2 Nanoparticles by Novel Surfactant Assisted Sol-gel Method for the Degradation of Organic Compounds

A sol-gel method was employed to synthesise pure titanium dioxide (TiO2) and surfactant assisted TiO2 nanoparticles (NPs). The effect of novel surfactant viz., Lauryl lactyl lactate on photocatalytic properties of TiO2 was studied. TiO2 NPs were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-Vis Diffuse Reflectance spectra (DRS), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM), Thermo gravimetric analysis (TGA), and Brunauer – Emmet Teller (BET) surface area. Anatase phase of TiO2 was confirmed by X-Ray diffraction pattern and the crystallite size was between 9–19 nm. Addition of surfactant improved the BET surface area, surface defects, while the agglomeration of particles was reduced. DRS results revealed that the addition of surfactant to TiO2 sol induced a red shift of the absorption edge which resulted in the reduction of band gap from 3.23 to 3.21 eV. These physicochemical properties of TiO2 NPs were correlated with photocatalytic degradation of phenol. About 92% of phenol degradation was observed for surfactant assisted TiO2 NPs (SA-TiO2). Salicylic acid and caffeine were also degraded using SA-TiO2 NPs.


Introduction
A major environmental problem is the waste water containing undesirable organic pollutants that are released to the aquatic system.Among various methods available to remove the organic pollutants from aquatic system, photocatalytic technique is one of the most advanced oxidation method [1].TiO 2 is one of the most studied photocatalytic material in this field [2].TiO 2 exists in three different crystalline forms.Among them, anatase and rutile phases find many applications.A higher photocatalytic activity has been proved by anatase phase in degradation of organic compounds.TiO 2 NPs can be synthesised by various methods like sol-gel, hydrothermal, emulsion, anodization, vapour deposition, sonication, and microwave [3][4][5][6][7][8][9][10]. Reddy et al. reported the synthesis of TiO 2 NPs by molten salt method [11][12][13][14].Zhu et al. prepared rice grain shaped TiO 2 NPs by electro spinning [15].Cherian et al. fabricated (N, F) co doped TiO 2 NPs by pyroammonolysis of TiF 3 [16].The properties of TiO 2 NPs depend on various factors such as the method of synthesis, the experimental conditions and the polymorph structure [17,18].Li and Wang synthesised TiO 2 NPs by micro emulsion method and the particle size was 10 to 20 nm.This method is too expensive, as only a small quantity of TiO 2 NPs can be obtained by using a large quantity of surfactant and oil [19].The sol-gel is the most extensively used method for the synthesis of TiO 2 , because it can be carried out at room temperature and cost effective [20].The advantages of molten salt method are high diffusivity due to the molten reactants and do not require mechanical mixing [11].Crystal structure, particle size, and surface porosity of TiO 2 are important factors for photocatalytic activity.However, TiO 2 has large band gap (3.2 eV) and high recombination rate of charge carriers, various modification methods are adopted to improve its activity by reducing the rate of recombination of charge carriers [21,22].The recombination of photo generated electron-hole pairs can be retarded by doping with metal or non-metal.The charge carrier's lifetime can be extended and therefore photocatalytic activity of the material is enhanced [23][24][25].
The advanced materials chemistry goal is to synthesise TiO 2 nanocrystals with required shape, which results in potential materials with shape dependent properties.Crystal surface energy is responsible for crystal growth rate.The surface energy of the crystals can be enhanced or reduced by surfactant adhesion which results in anisotropic nanocrystal growth [26].In this article, we have reported the novel surfactant viz., Lauryl lactyl lactate assisted sol-gel method for the synthesis of TiO 2 NPs and examined its photocatalytic activity for the degradation of phenol, salicylic acid and caffeine.
Phenol and its derivatives are major water pollutants in chemical industries viz., textile, petrochemical, paint and are carcinogenic and mutagenic, poses a risk to mammalian and aquatic life [27].When waste water containing phenol is chlorinated, polychlorinated phenols are formed, which are toxic in nature.Such waste water must be treated and then released to the environment.Various parameters like catalyst dosage (0.25 to 1.5 g/L) and phenol concentration (0.1 to 0.43 mmol/L) have been examined to know the effect of photocatalytic degradation.
Wastewater coming from industries like cosmetic, paper mill, human and veterinary drugs contains salicylic acid, which is a hazardous substance.The higher concentration of salicylic acid in water leads to lung and kidney damage in human beings.Advanced treatment technologies are needed to remove salicylic acid from waste / industrial water [28].
Pharmaceutical drugs are used to treat diseases in human beings and animals and keep healthy.However, the active pharmaceutical ingredients in these medicines can enter the environment either as drug or as its metabolites [29].High dosage of caffeine can cause nervousness and insomnia.Hence, there is a requirement of processes which can remove such ingredients from the environment.Caffeine is one such pharmaceutical ingredient.Chuang et al. synthesised TiO 2 NPs by modified homogeneous precipitation process and employed it for the degradation of caffeine [30].Arfanis et al. prepared TiO 2 nanotubes by one step electrochemical anodization technique for the degradation of salicylic acid and caffeine [31].In this research, an attempt has been made to degrade the pollutants in particular various concentrations of phenol.Also salicylic acid and caffeine were considered for the degradation studies.
To the best of our knowledge, no research work has been reported on synthesis of TiO 2 NPs assisted by lauryl lactyl lactate (Koplactylate) surfactant and used for the photocatalytic degradation of phenol, salicylic acid and caffeine.
2 Materials and method 2.1 Chemicals Titanium (IV) butoxide (Ti (OC 4 H 9 ) 4 , 97%) was obtained from Sigma-Aldrich.Ethyl alcohol and Hydrochloric acid were procured from S. D. Fine chemicals.Salicylic acid and caffeine were obtained from Merck.Lauryl lactyl lactate (C 18 H 34 O 5 ) was procured from Kumar organic products Pvt.Ltd., Bangalore.Coumarin (99%) was obtained from Loba Chemie.All the chemical reagents were used without further purification.Double distilled water was used for all the experiments.

Synthesis of TiO 2 NPs by sol-gel method
SA-TiO 2 nanostructured photocatalysts were prepared by modified sol-gel method using titanium (IV) butoxide (Ti (OC 4 H 9 ) 4 ,) as precursor.Two solutions A and B were prepared as follows.The solution A was composed of 4 mL ethanol and 0.9 mL distilled water.The solution B was composed of known quantity of lauryl lactyl lactate (0.25 mL, 0.5 mL, 0.75 mL and 1 mL), 8 mL ethanol, 2 mL titanium butoxide and 0.1 mL concentrated hydrochloric acid.Compared to other toxic additives (i.e.(,) ammonium salts with long chain), lauryl lactyl lactate is biodegradable, inexpensive and also can control large inorganic clusters.
The solutions A and B were mixed together and stirred for about 8 min until the gel formed.After 12 h aging at room temperature, the gel was dried at 80 °C in hot air oven for 3 h.The solid was ground and subjected to calcination using muffle furnace from room temperature to 475 °C with a heating rate of 5 °C / min and the same temperature was maintained for 3 h.The TiO 2 NPs were cooled naturally to room temperature.The SA-TiO 2 NPs were marked as R [molar ratio of surfactant to titanium butoxide] equal to 0.125, 0.25, and 0.375 and 0.5.Pure TiO 2 NPs were prepared using the same method in absence of surfactant [32].

Characterisation
XRD studies of the TiO 2 materials were performed using Rigaku Smart Lab with Cu Kα radiation.Bond stretching frequencies of TiO 2 were analysed by FTIR (Bruker-alpha).Diffusion Reflectance Spectra (DRS) of TiO 2 samples were analysed using a Varian Cary 5 UV-Vis-NIR spectroscope fitted with a 11 cm diameter integrating sphere coated with poly tetra fluoro ethylene.The surface morphologies of the prepared TiO 2 samples were obtained using Field emission scanning electron microscopy (GEMINI-ULTRA 55).The shape and size of the TiO 2 crystallites were determined using Transmission electron microscope (PHILIPS CM 2000).Thermal analysis of the as prepared TiO 2 sample was carried out using PERKIN ELMER, USA thermal analyzer.The specific surface area of TiO 2 samples were measured by Brunauer-Emmett-Teller analysis (Quantachrome NOVA-1000, USA) using N 2 adsorptiondesorption isotherms at 77 K.The UV absorbance of the pollutants was measured using UV-Visible spectrophotometer (Agilent technology-Cary-60).Photoluminescence studies were recorded using fluorescence spectrophotometer (Agilent technology-Cary Eclipse).

Photo-reactor set up
The photo-reactor was procured from "HEBER" scientific, Chennai.The photocatalytic reactor consists of a medium pressure mercury vapour lamp (125 W, λmax = 365 nm) in a jacketed quartz tube.The heat generated from the lamp was removed by continuous circulation of cold water through the jacket.A set of 100 mL quartz tubes of 2.3 cm i.d., 2.7 cm o.d., and 37 cm length containing pollutant were placed surrounding the lamp.The distance between the mercury lamp and the quartz tube containing pollutant was 6 cm.The uniform distribution of catalyst particles throughout the solution was achieved using an air pump.

Degradation of pollutants
100 mL phenol of known concentration was taken in a quartz tube.To this, known quantity of catalyst was added.The quartz tube was kept in the dark for 30 min to attain adsorption-desorption equilibrium.Then, the UV light was switched on. 2 mL of aliquots were withdrawn for every 30 min interval.The samples were centrifuged at 10000 rpm using micro centrifuge (Torson, spinwin) for 10 min in order to separate TiO 2 NPs.The concentration of phenol was measured quantitatively using UV-Vis spectrophotometer at 270 nm.Spectrophotometric measurements were carried out using a quartz cell having a path length of 1 cm.Degradation efficiency was calculated using Eq.(1).

DE Ao At Ao
where, A o and A t are the absorbance at time zero and time t, respectively.Salicylic acid and caffeine were also analysed by the same method.
The hydroxyl radicals play a major role in the degradation of pollutants.The formation of hydroxyl radicals was detected by photoluminescence spectrophotometer using coumarin as probe molecule.
3 Results and discussion 3.1 X-Ray Diffraction patterns Fig. 1 shows the XRD patterns of TiO 2 and SA-TiO 2 NPs calcined at 475 °C.XRD patterns confirm that all the TiO 2 NPs were in anatase phase (Peaks at 2θ = 25.25°,37.78°, 48.32°, 54°, 55° and 62.7°) with a typical anisotropic growth along (001) direction.XRD pattern well matches with the JCPDS number 21-1272.Lattice parameters of SA-TiO 2 (R = 0.25) NPs were found to be a, b = 3.786 Å, c = 9.502 Å (Space group: I 41/a m d).XRD patterns confirm that all the TiO 2 NPs were crystalline.The calcination temperature determines the phase formation of NPs [33].Debye-Scherrer's equation was used to estimate the mean crystallite size of TiO 2 NPs.
where, D is average crystallite size (nm), λ is wavelength of Cu Kα (0.154 nm), β is full width at half maximum (FWHM) in radian and θ is Bragg angle.The calculated crystallite sizes are given in Table 1.The crystallite size was varying between 9 to 19 nm.The average crystallite size of SA-TiO 2 (R = 0.25) NPs after refinement was found to be 11 nm.calcined at 475 °C for 3 h.A comparison between these two spectra shows that organic functional groups present in surfactant (peaks between 1000 and 1600 cm -1 ) were decomposed completely after calcination.Fig. 2 (a) represents TiO 2 NPs calcined at 475 °C for 3 h.A comparison between SA-TiO 2 (R = 0.25) with pure TiO 2 NPs after calcination suggests that there were no organic molecules associated with SA-TiO 2 NPs (R = 0.25) after calcination at 475 °C.The stretching peaks at 3400 cm -1 and 1630 cm -1 are characteristic of O-H bending of adsorbed water.

UV-Vis diffuse reflectance spectra
The UV-Vis diffuse reflectance spectra of TiO 2 and SA-TiO 2 NPs are shown in Fig. 3 (a).A red shift of SA-TiO 2 NPs was observed compared to pure TiO 2 NPs.The SA-TiO 2 NPs absorbed UV light around 396 nm, while the pure TiO 2 NPs absorbed the UV light with a lower wavelength (391 nm).The red shift of SA-TiO 2 NPs could be due to the large surface defects on particle surfaces [34].The band gap energy was determined using Kubelka -Munk model [35].
( ) where R ∞ = R sample / R reference , R sample and R reference are reflectance corresponding to sample and BaSO 4 respectively.The plots of modified Kubelka-Munk function against band energy are shown in Fig. 3(b).The indirect band gap energies of synthesised TiO 2 and SA-TiO 2 (R = 0.25) NPs are 2.97 and 2.93 eV respectively.The band gap energies were much lower than the actual (3.2 eV).Hence, the direct band gap energies are estimated and found to be 3.23 and 3.21 eV respectively.

Field Emission Scanning Electron Microscopy
The information about the distribution of NPs was obtained by referring to FESEM images.The images show that the TiO 2 NPs (Fig. 4 (a), (b)) were agglomerated and formed large particle blocks, whereas the SA-TiO 2 NPs (Fig. 4 (c)-(f)) showed a lower degree of agglomeration.The addition of surfactant reduced the degree of agglomeration.

Thermo Gravimetric Analysis (TGA)
TGA analysis of TiO 2 and SA-TiO 2 (R = 0.25) NPs dried at 80 °C is shown in Fig. 6.For TiO 2 NPs (Fig. 6 a), an initial weight loss of 34 % was observed when the temperature was increased to 250 °C, which was attributed to the loss of   physically absorbed water, ethanol and hydrochloric acid.
For SA-TiO 2 NPs (R = 0.25), a further weight loss (14 %) was observed from 250 to 370 °C, which is attributed to the loss of surfactant.There after no further significant thermal effects were detected even up to 800 °C, shows the stability of the materials at higher temperatures.[12] and the surface area was 80 m 2 /g.

Evaluation of photocatalytic activity of phenol
The photocatalytic activity of pure TiO 2 and SA-TiO 2 NPs (R = 0.125, 0.25, 0.375 and 0.5) was studied by observing the degradation of phenol.From Fig. 8, it confirms that the maximum degradation of phenol was observed for SA-TiO 2 NPs (R = 0.25).The degradation process depends on the surface area of the catalyst.As the surface area increases, the number of active sites increases, hence more phenol molecules are adsorbed and degradation increases.
These results were in good agreement with the BET surface area results also i.e., the maximum specific surface area was obtained for SA-TiO 2 NPs (R = 0.25).Further, photocatalytic degradation of phenol was studied using SA-TiO 2 NPs (R = 0.25) by varying the catalyst dose and initial concentration of phenol.

Effect of catalyst dosage
The catalyst dosage was varied from 0.25 g/L to 1.5 g/L at a fixed phenol concentration of 0.21 mmol/L and the experimental results are presented in Fig. 9.It reveals that the phenol degradation efficiency increases with increase in catalyst concentration up to 1 g/L; while the rate decreases with further increase in catalyst concentration.The phenol degradation was maximum at 1 g/L dose of SA-TiO 2 NPs.With increase in catalyst dose the number of active sites increases, which causes more hydroxyl radicals generation on the surface of catalyst.At higher catalyst dose above 1 g/L, the system opacity increases; causing decrease in the penetration of UV light [36].Thus a higher dose above 1 g/L results in scattering of UV light and aggregation.
The degradation of phenol leads to the formation of intermediates such as catechol and hydroquinone.These aromatic compounds undergo oxidation to yield aldehydes

Effect of phenol concentration
The initial concentration of phenol was varied from 0.10 -0.43 mmol/L (10-40 ppm) keeping the catalyst dose 1 g/L and the results are shown in Fig. 10.As the concentration of phenol increases, the degradation rate decreased.Maximum degradation rate was observed for 0.10 mmol/L phenol concentration.As the concentration of phenol increases, light absorbed by phenol is more compared to that of SA-TiO 2 NPs.The light absorbed by phenol is not available to carry out the phenol degradation.The equilibrium adsorption of phenol on active sites increases, more and more phenol molecules get adsorbed on the catalyst surface.Competitive adsorption of OH -on the same site decreases and hence the amount of hydroxyl radicals (OH · ) on the catalyst surface decreases.Hence the possibility of phenol molecules to react with OH · decreases resulting in lower degradation efficiency [38].About 92 % of phenol was degraded within 4 h at natural pH.Phenol degradation results obtained were better compared to the earlier work [38].

Kinetics of photocatalytic degradation
The photocatalytic degradation of phenol follows first order kinetics as shown in the below equation where Co is the initial concentration, C is the degradation concentration after time t, k is the rate constant.The plots of ln(Co/C) vs time gave straight lines, which confirms the first order degradation as shown in Fig. 11.The slopes of the straight lines are the rate constants for photocatalytic degradation of phenol for TiO 2 and SA-TiO 2 (R = 0.25) NPs.
The rate constant of SA-TiO 2 NPs was 2.6 x 10 -2 min -1 , which was more than double the pure TiO 2 NPs.The rate constant obtained was higher compared to the previous literature values [38].

Photocatalytic degradation of salicylic acid and caffeine
The photocatalytic activity of SA-TiO 2 NPs (R = 0.25) was also measured by degradation of salicylic acid (0.145 mmol/L) and caffeine (0.103 mmol/L) under UV light source.The degradation of these pollutants was observed by the absorption of light at maximum wavelengths, 296 nm and 273 nm for salicylic acid and caffeine respectively.The intensity of absorption peaks decreased with time as shown in Fig. 12 (a), (b).The percentage degradation of salicylic acid was 88 % in 3 h and that of caffeine was 59 % in 4 h duration.

Hydroxyl radical detection using coumarin as probe molecule
100 mg of SA-TiO 2 NPs was added to 50 mL of 1 mM coumarin solution.The solution was irradiated under the UV source in photo reactor. 2 mL of aliquots were withdrawn at every 10 min and the photoluminescence was measured.The coumarin (non-luminescent) reacts with hydroxyl radicals to form 7-hydroxyl coumarin (luminescent), which shows the photoluminescence peak at 454 nm.The intensity of the peak increases with irradiation time indicating the increase in concentration of hydroxyl radicals as shown in Fig. 13.

Conclusions
TiO 2 and SA-TiO 2 NPs were successfully synthesised by modified sol-gel method.The NPs were in anatase form and mesoporous in nature.SA-TiO 2 NPs (R = 0.25) possessed a higher specific surface area and lower band gap energy compared to TiO 2 NPs.SA-TiO 2 NPs (R = 0.25) prepared using surfactant showed a higher photocatalytic activity for the degradation of phenol.The percentage degradation of phenol obtained in our work (92 %) was higher than the value reported in the literature.The rate constant for phenol degradation was found to be 2.6 x 10 -2 min -1 .
The percentage degradation of salicylic acid and caffeine were 88 % and 59 % respectively using SA-TiO 2 NPs (R = 0.25).The SA-TiO 2 NPs can be an effective photocatalyst for the removal of organic contaminants from the industrial waste water under UV light irradiation.

Fig. 5 (
a)-(d) shows the TEM images of TiO 2 and SA-TiO 2 (R = 0.25) NPs.The average size of TiO 2 and SA-TiO 2 NPs were found to be 25-30 nm and were irregular in shape.The Debye rings are from by diffraction points corresponding to the different orientations of a family of crystalline planes.SAED pattern indicates the presence of (101), (112), (200) and (204) planes in both TiO 2 and SA-TiO 2 NPs (Fig. 5 (e), (f)), are in consistent with the XRD results.

Fig. 10 Fig. 9 Fig. 11
Fig. 10 Effect of phenol concentration on photocatalytic degradation of phenol at natural pH.
One of the authors Dr. Nagaraju would like to acknowledge DST Nanomission, Govt. of India, New Delhi (Project No. SR/NM/NS-1262/2013) for the financial support to carry out this research work.Authors would like to thank COE -TEQIP -2, Director and Principal, SIT, Tumakuru for constant support and encouragement of the research activities.

Fig. 13
Fig.13 Photoluminescence spectra for detection of hydroxyl radicals using coumarin as probe molecule.

Fig. 12 (
Fig. 12 (a) UV-visible absorption spectra showing the photocatalytic activity of SA-TiO 2 NPs (R = 0.25) towards photocatalytic degradation of salicylic acid at natural pH.(b) UV-visible absorption spectra showing the photocatalytic activity of SA-TiO 2 NPs (R = 0.25) towards photocatalytic degradation of caffeine at natural pH.

Table 1
XRD and BET results of TiO 2 and SA-TiO 2 NPs

Table 1 .
Zhu et al. fabricated TiO [15] by electro spinning method[15]and the surface area was 34 m 2 /g, lower than the present work.Reddyet al. synthesised TiO 2 NPs by molten salt method