The Role of Concentration Ratio of TTiP : AcAc on the Photocatalytic Activity of TiO 2 Thin Film in Reducing Degradation Products of Used Frying Oil

The TiO2 thin film has been applied to reduce degradation products (free fatty acid/FFA and peroxide value/PV) in used frying oil under ultraviolet (UV) light irradiation. FFA and PV are degradation products in used frying oil that can cause various diseases in human. In this study, the TiO2 thin films were made from precursor solution with concentration ratio of titanium tetraisopropoxide (TTiP) and acetylacetone (AcAc) of 1:1, 1:2, 2:1, 2:3, and 3:2. The aim of this study is to investigate the effect of concentration ratio of TTiP and AcAc on the photocatalytic activity of TiO2 thin film in reducing FFA and PV of used frying oil. The spray coating method was used to deposit precursor solution of TiO2 onto glass substrate at 450 oC. All TiO2 thin films consist of spherical-like grain with dominant structure of TiO2 rutile. The band gap energy of TiO2 thin films was in the range 3.11-3.16 eV. Concentration ratio of TTiP and AcAc of 2:3 results in TiO2 thin film with highest photocatalytic activity in reducing FFA and PV of used frying oil. Copyright © 2017 BCREC Group. All rights reserved


Introduction
Titanium dioxide (TiO2) has interesting properties, i.e. high brightness, very high refractive index, and strong UV light absorption [1].Therefore, the TiO2 has been widely used as white pigment, sunscreen, and photocatalyst.Under UV light irradiation, TiO2 has strong photocatalytic activity.As a photocatalyst, TiO2 has been applied in many fields, especially for pollutant photodegradation.Ioana and Moldovan reported the photodegradation of the indoor organic pollutant (toluene and chlorobenzene compounds) in the air [2].Photodegradation of nitrogen oxides, odorous compounds, fungi, etc. in the air was studied by Miller and Fox [3].In addition, photodegradation of pathogenic microorganisms [4] and different dyes (methylene blue and congo red) in the water were also reported [5].
Free fatty acid (FFA) and peroxide value (PV) are degradation products in frying oil pro-duced during frying process.FFA and PV are degradation products that produced by frying process and used to monitor quality of frying oil [6][7][8][9].These molecules cause extreme human health problems, cancer and heart coronary disease [10][11][12][13].The common method in reducing FFA and PV is filtration using adsorbent [14][15][16].However, this method need long process and a lot of equipment.The other methods to reduce FFA and PV are the using of acid catalyst [17][18][19] and adding antioxidant [20,21].
Recently, TiO2 in the form of thin film has been used as an alternative technology in reducing free fatty acid (FFA) and peroxide value in used frying oil as a simple process and equipment [22,23].Kaltsum et al. [22] used TiO2 thin film to reduce FFA and PV in used frying oil under sunlight and UV light irradiation for 5 hours.The precursor solution of TiO2 thin film was mixture of Titanium Tetraisopropoxide (TTiP) 0.2 M, Acetylacetone (AcAc) 0.4 M, and ethanol was deposited by spray coating method.The result showed that the TiO2 thin film could reduce FFA and PV up to 67.10 % and 79.15 % in used frying oil, respectively .
Comparison of reduction FFA and PV using sunlight and UV light irradiation was also analyzed by Kaltsum et al. [23].Under sunlight irradiation, the TiO2 thin film was able to reduce FFA and PV up to 49.09 % and 20.96 %, respectively.On the other hand, under UV light irradiation, TiO2 thin film reduced FFA and PV up to 61.81 % and 53.22 %, respectively.Therefore, UV light irradiation was more effective in reducing FFA and PV than sunlight irradiation.Oja et al. [24] used TiO2 thin film made of TTiP and AcAc with concentration ratio of 2:1 for structural and electrical characterization.
In this study, concentration ratio of TTiP and AcAc in precursor solution of TiO2 thin film is varied into five ratios and the thin film used to purify used frying oil.The aim of this study is to investigate the effect of concentration ratio of TTiP and AcAc on the photocatalytic activity of TiO2 thin film to purify used frying oil under UV light irradiation.The parameter of percent FFA and PV are used to monitor the quality of used frying oil.

Manufacture of thin film
The material and procedure used in this research was based on the material and procedure as describe in the previous research [1,2].The TiO2 thin film was made by spraying precursor solution onto glass substrate at temperature 450 o C. The precursor solution was made by mixing TTiP (97 %, Sigma-Aldrich, AcAc (99 %, Merck KgaA, and absolute ethanol (Merck KgaA).The concentration ratio of TTiP:AcAc was varied into 5 conditions.They were 1:1, 1:2, 2:1, 2:3, and 3:2, respectively.

TiO2 thin film characterization
The properties of TiO2 thin films were characterized using JEOL JSM-6510 LA Scanning Electron Microscope (SEM), Shimadzu 1240 UV-visible spectrophotometer, and MAXima_X XRD-7000X-ray diffractometer.The thickness of TiO2 thin films was estimated using SEM image and the result as shown in Table 1.The XRD pattern was used to analyze structure and crystallite size of TiO2 thin films.The optical properties of TiO2 thin films were analyzed through absorbance spectra in the range 200-800 nm.

Determination of FFA and PV
Reduction of FFA and PV in used frying oil was carried out by putting TiO2 thin in used frying oil and irradiated by UV light for 5 hours.The FFA and PV of used frying oil before and after irradiation were determined by titration method of AOCS [25,26].Ten gram used frying oil was added to 50 mL hot ethanol and 5 drops of phenolphthalein.The mixture was titrated with 0.1 N KOH by shaking until a light pink color appeared.The value of FFA was determined using equation (1).(1) where, N is the normality of KOH, V is the volume of KOH (mL), and W is the mass of used frying oil (g).
Five gram used frying oil was added to 50 mL acetate acid and shaked until the mixture homogeneous.0.5 mL KI and 30 mL distilled water were added to that the mixture and shaked for 1 minute, respectively.The mixture was titrated by 0.1 N Na2O3S2 until the yellow color disappear.0.5 mL amilum liquid was added to mixture.The titration was continued until the blue color disappear.The peroxide value was determined using equation ( 2). ( where N is the normality of Na2O3S2, V is the volume of Na2O3S2 (in mL), and W is the mass of used frying oil (in grams).310) plane of rutile TiO2, and (024) plane of brookite TiO2, respectively.However, a small peak was also observed at 2θ = 37 o for thin film with TTip:AcAc ratio of 1:1, 1:2, 2:1, and 2:3 which correspond to the (004) plane of anatase (JCPDS No. .This result indicates that TiO2 thin film was formed by a mixture of rutile, brookite, and anatase structure which rutile as predominant structure.The crystallite size of TiO2 thin films was calculated using equation ( 3) for strongest diffraction peak of rutile (210) [27]. ( where D is crystallite size, λ is X-ray wavelength, β is full width at half of maximum, and θ is Bragg's angle.Table 1 shows the crystallite size of TiO2 thin film.It can be found that the crystallite size of TiO2 thin film with ratio 2:3 was the largest, 59.83 nm.
The surface morphology of TiO2 thin films was characterized by SEM as shown in Figure 2. The surfaces of all TiO2 thin films consist of spherical like-grains.

Optical properties
The absorbance spectrums of all TiO2 thin films were investigated by UV VIS spectrophotometer as depicted in Figure 3.All TiO2 thin films show strong absorption at ultra violet range of 290-400 nm and low absorption at visible light range of 400-800 nm.Therefore, the high photocatalytic activity of TiO2 thin films can be obtained by using UV light as a radiation source.
In spite of being the same material, the absorbance of each thin film in Figure 3 is different each other.It is due to each thin film has a different rutile, brookite, and anatase phase composition and different thickness.The high thickness leads more light absorbed, so it increases absorbance.The same result also was reported by Samoom et al. [28] that increasing of thickness increased absorbance.The strong UV absorbtion obtained for TiO2 thin film deposited at TTiP:AcAc ratio of 1:2 and 2:3.
The absorbance data that can be used to determine energy gap of thin film by Tauc plot method based on the Equation for indirect transition (Equation 4) [29] and the result is listed in Table 2.The indirect band gap of rutile TiO2 was 3.10 eV as reported by Welte and Monllor [30,31].(4) where α is absorption coefficient, hυ is photon energy, A is constant, and Eg is band gap energy.
The band gap energy of TiO2 thin films are in the range 3.11-3.16eV.The highest band gap energy is shown by TiO2 thin film with ratio 1:2.On the contrary, the lowest band gap energy was shown by TiO2 thin film with ratio 1:1.

Photocatalytic properties of TiO2 thin films
In this research, photocatalytic properties of TiO2 thin films is applied to reduce FFA and PV in used frying oil.The photocatalytic process in TiO2 thin film can be explained in three steps.First, when the UV light (photon) irradiates thin film, electron in valence band excites to conduction band and produce hole and conduction electron as shown in Equation ( 5) [32].Second, hole and conduction electron react with carboxylic acid (main component of frying oil) produce hydrocarbon radical and hydrogen radical as shown in Equations (5-7), respectively [33].The last, both radical react with degradation products (FFA and oxidation products), so the percentage of FFA and PV reduced.
The photocatalytic activities of the thin film in used frying oil were evaluated via the reduction of degradation products (FFA and PV) under UV light irradiation.Reduction of FFA and PV can be determined using Equation ( 8) and ( 9), respectively.
where R is reduction (in %), FFAi is initial value of FFA (in %), FFAf is final value of FFA (in %), PVi is initial value of PV (in meq/kg), and PVf is final value of PV (in meq/kg).The results were shown in Figures 4-5.
The photocatalytic activity of thin film is affected by the absorbance, band gap energy, surface area, thickness [14,34], and annealing temperature [35,36].The large absorbance means the large intensity of light adsorbed by thin film, so the high photocatalytic process occured.The smaller band gap energy, the more electron can be excited by photon to be conduction electron.The larger surface area, the more interaction between thin film and molecule of sample.Increasing thickness film leads increasing in surface area, so increasing contact between thin film and molecule of sample [34].Increasing the number of conduction electron and interaction or contact between thin film and molecule increase the photocatalytic activities.In this research, the molecule meant is carboxylic acid as the main component of frying oil.The high photocatalytic activities refer to the high FFA and PVreduction in used frying oil.
The high reduction both of FFA and PV in used frying oil was shown by TiO2 thin film with ratio 2:3.This might be mainly affected by the high thickness, large absorbance and small band gap energy of TiO2 thin film with ratio 2:3.Effective reduction of FFA and PV is also observed in TiO2 thin film with ratio 1:2 and 2:1, respectively.However, thin film with ratio 1:1 and 3:2 reduced small FFA and PV because of big band gap energy, low absorbance, and small thickness.
Addamo et al. [37] used three TiO2 thin films (pure rutile, pure anatase, and pure brookite) to reduce 2-propanol.The result showed that pure brookite and pure anatase had better photocatalytic activity than purerutile.In this study, all the TiO2 thin films have predominant rutile structure.However, they have high photocatalytic activity.It might be due to affected by the thickness, absorbance, and small band gap of thin film.In addition, rutile phase has the highest ultraviolet absortivity among the titanium dioxide phases, so it has high photocatalytic activity [38].

Conclusions
The TiO2 thin films deposited at various concentration ratio of TTiP and AcAc have TiO2 rutile predominant structure.The surface morphology of thin films was spherical-like grain.The band gap energy was around 3.11-3.16eV.The TiO2 thin films reduced FFA and PV up to 70.90 % and 58.54 % under UV light irradiation for 5 hours.The TiO2 thin film with TTiP:AcAc concentration ratio of 2:3 posses the highest photocatalytic activity and the most effective in reducing FFA and PV of used frying oil.

1
Properties of TiO2 thin film 3.1.1Structure and morphology The structure of TiO2 thin films was analyzed by XRD patterns as shown in Figure 1.It can be seen that all TiO2 thin films have the same pattern with three strong peaks at 2θ of 44 o , 64 o , and 77 o , respectively.By comparing with JCPDS No. 21-1276 and No. 16-0617, the three strong peaks corresponded to the (210) and (

Figure 1 .
Figure 1.The XRD pattern of five TiO2 thin films

Figure 4 .Figure 5 .
Figure 4.The percentage of FFA before and after photocatalysis process using TiO2 thin film under UV light irradiation

Table 1 .
The mean crystallite size and the thick-