Vol. 27, No. 11 (2017)
577
Self-Cleaning and Photocatalytic Performance of TiO
2Coating Films Prepared by Peroxo Titanic Acid
Hemraj M. Yadav
1,2and Jung-Sik Kim
1†1Department of Materials Science and Engineering, University of Seoul, Seoul 02504, Republic of Korea
2Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea
(Received September 27, 2017 : Revised September 27, 2017 : Accepted October 2, 2017)
Abstract
Self-cleaning and photocatalytic TiO2 thin films were prepared by a facile sol-gel method followed by spin coating using peroxo titanic acid as a precursor. The as-prepared thin films were heated at low temperature(110oC) and high temperature (400oC). Thin films were characterized by X-ray diffraction(XRD), Field-emission scanning electron microscopy(FESEM), UV- Visible spectroscopy and water contact angle measurement. XRD analysis confirms the low crystallinity of thin films prepared at low temperature, while crystalline anatase phase was found the for high temperature thin film. The photocatalytic activity of thin films was studied by the photocatalytic degradation of methylene blue dye solution. Self-cleaning and photocatalytic performance of both low and high temperature thin films were compared.Key words
TiO2, thin films, sol-gel, self-cleaning, photocatalysis, methylene blue.1. Introduction
The existence of several hazardous organic pollutants in water wastes and air results in a serious environmental pollution. Photocatalytic oxidation using semiconducting materials is one of the advanced oxidation processes for the removal of such organic pollutants. However, numerous semiconducting materials such as TiO
2, ZnO, WO
3, and Fe
2O
3have been used for environmental remediation.
1)Semiconducting titanium dioxide (TiO
2) is a promising candidate for photocatalysis due to its high stability, nontoxicity, low cost, super hydrophilicity, long durability, and excellent photocatalytic activity towards removal of pollutants in water and air.
2-5)During the last decades, numerous researchers have focused on the development of TiO
2photocatalyst in the form of thin films as well as powders.
6)The photocatalytic performance of TiO
2nanoparticles has mostly been studied by its immobilization. The application of TiO
2powder is restricted due to incomplete recovery of the photocatalyst during experiments.
7,8)The preparation of thin film photo- catalyst will overcome this obstacle and also extensively applicable in water and air-cleaning technologies, self- cleaning materials, photocatalytic inhibition of microor-
ganisms, cancer cells and viruses.
9)TiO
2thin films are fabricated by different techniques such as sol-gel,
10,11)chemical vapor deposition,
12)hydro- thermal and sputtering.
14)Among these techniques, sol-gel method plays a key role in the stability and photo- catalytic activity of the immobilized TiO
2. The sol-gel method mostly includes costly and highly reactive metal- organic precursors that quickly reacts with water in atmosphere.
15)To overcome these limitations, the peroxo route has been used to prepare TiO
2thin films. Several investigations have been carried out in order to prepare TiO
2thin films by peroxo titanic acid modified sol-gel method.
8,15-18)However, in most of the cases, polymeric agents, and complicated synthesis procedures were adopted to prepare peroxotitanic acid sol-based TiO
2thin films.
In this paper, a facile and low cost sol-gel method is demonstrated to prepare TiO
2thin films for coating layer.
The peroxo titanic acid sol was prepared using amorphous TiO
2fine particles and hydrogen peroxide solution. The structural and optical properties, self-cleaning performance, and photocatalytic reaction of TiO
2coating films heat treated at low and high temperature were investigated. The main focus of this investigation is to demonstrate self- cleaning and photocatalytic properties of TiO
2coatings
†Corresponding author
E-Mail : [email protected] (J.-S. Kim, Univ. of Seoul)
©Materials Research Society of Korea, All rights reserved.
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creative- commons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
ture below 120
oC.
2. Experimental
The TiO
2coating thin films were prepared by a sol-gel method using peroxotitanic acid solution. First, a solution mixture of titanium tetra isopropoxide and isopropyl alcohol was hydrolyzed to get TiO
2fine particles. The molar ratio of TTIP:IPA:H
2O was 1:10:4. The obtained TiO
2particles were filtered, washed using deionized water and oven dried at 110
oC for 4 h. Then, 0.2 g of amorph- ous TiO
2powder was magnetically stirred with 30 % aqueous hydrogen peroxide solution (4 mL) to get a transparent orange gel of peroxotitanic acid. Finally, the gel was diluted with 1 mL aqueous hydrogen peroxide solution followed by 5 mL deionized water to prepare peroxotitanic acid sol. Before spin coating, the glass slides were washed using acetone and ethanol in a sonicator and then rinsed in deionized water. Then, the peroxo titanic acid sol was spin-coated at 2,000 rpm for 10 s. After spin coating, the glass slides were heated at 110
oC for 2 minutes on a preheated hot plate and 400
oC for 1 h in an electric furnace. The TiO
2thin films prepared at low temperature at 110
oC and high temperature at 400
oC were denoted as TiO
2-110 and TiO
2-400, respectively.
2.1 Characterization
The structure of coated films was analyzed by a Rigaku Dmax 2500/PC X-ray diffractometer with Cu-K
α. Photoinduced hydrophilicity of the samples was analyzed by a X-ray diffractometer(Dmax 2500/PC, Rigaku, Japan) under UV light illumination(Daytime CFL 20W Black- light, Korea). The observation of surface morphology was carried out with a field-emission scanning electron microscope(FE-SEM, Hitachi S-4300). The energy dis- persive spectroscopy(EDS) analysis was carried out by SEM(JSM-6010, JEOL, Japan). Optical transmittance of the samples was obtained by a UV-Vis(S-4100 PDA, Scinco, Korea).
2.2 Photocatalytic activity
The photocatalytic performance of the samples was evaluated by photodegradation of methylene blue(MB) dye solution. In a glass vessel, 5 mL of aqueous methylene blue solution (5.34 × 10
−6M) was added and the sample of TiO
2coated glass plate (2.5 × 2.5 cm) was immersed completely. To attain the adsorption-desorption equilibrium of MB dye solution, the solution was kept in the dark for 30 min before UV-light illumination. The solution was then exposed to UV light lamp from a distance of 5 cm.
At regular interval of time, 1 mL of MB dye solution was withdrawn and analyzed using a UV-Vis spectrophotometer.
3. Results and Discussion 3.1 X-ray diffraction analysis
Fig. 1 shows XRD patterns of TiO
2-110 and TiO
2-400 thin films. The sample prepared heat treated at low temperature exhibit no detectable diffraction peaks. This reveals that the TiO
2-110 sample is amorphous. However, TiO
2-400 thin film exhibits diffraction peaks at 2 θ = 25.39
o, 38.1
o, 48.04
o, 54.17
o, 55.10
o, 62.77
o, 68.95
o, 70.34
o, 75.04
ocorresponds to the crystal facets of (1 0 1), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0), and (2 1 5) anatase phase TiO
2(JCPDS No. 04- 014-5764), respectively. The crystallite sizes of TiO
2-400 thin films was calculated using Debye Scherer’s equation.
The calculated crystallite size of TiO
2-400 thin film was about 12 nm.
3.2 Surface morphology and elemental analysis Fig. 2(a-b) shows FE-SEM images of TiO
2-110 and TiO
2-400 thin films. From the FE-SEM image of TiO
2- 110 thin film(Fig. 2(a)), amorphous nature of TiO
2nanoparticles has been clearly observed. The FE-SEM image of TiO
2-400 thin film reveals slight agglomerations on the surface of thin films. Almost spherical particles were detected from FE-SEM analysis of both thin film samples.
Fig. 3(a-b) shows EDS spectra of TiO
2-110 and TiO
2- 400 thin films. The EDS spectra of both thin film samples reveal the existence of titanium(Ti) and oxygen(O) on the surface of thin films while remaining elements such as calcium(Ca), magnesium(Mg), and silicon(Si) are from glass substrate. The EDS analysis confirms the formation of TiO
2without any elemental impurities.
Fig. 1. XRD patterns of TiO2-110 and TiO2-400 thin films.
3.3 Optical properties
The optical transmittance in the wavelength range of 250-1000 nm was measured for TiO
2-110 and TiO
2-400 thin films as shown in Fig. 4. The average transmittance of TiO
2-110 and TiO
2-400 thin films in the visible region was 89 % and 78 %, respectively. The transmittance of the TiO
2-400 thin film was lower than that of TiO
2-110.
The increase in light scattering due to the microstructure change, the transmittance decreased in the case of TiO
2- 400 thin films.
19)The optical properties of thin films are mainly influenced by thickness and morphology of the thin films.
3.4 Photo-induced hydrophilicity
Photo-induced hydrophilicities of TiO
2-110 and TiO
2- 400 samples were studied by water contact angle mea- surement in the presence of UV light illumination. Fig. 5 shows water contact angle measurement of TiO
2-110 and TiO
2-400 samples at different time intervals under UV light illumination. The initial water contact angle of TiO
2-110 thin film was 70
o. After 30 min of UV light illumination, it was decreased to be 31
o. For the TiO
2- 400 thin film, the initial water contact angle was about 17
o. After 5 min UV illumination, the water contact angle
decreased to be 13
oand further to be 5
owith 10 min UV illumination. The TiO
2-400 thin film shows super hydro- philicity under UV light illumination. The decreasing rate of water contact angle for the TiO
2-110 thin film was lower than that of the TiO
2-400 thin film. Thus, the change of the wettability behaviors for coating surface can be attributed to the surface roughness and effect of
Fig. 3. EDS spectra of (a) TiO2-110 and (b) TiO2-400.Fig. 2. FE-SEM images of (a) TiO2-110 and (b) TiO2-400.
Fig. 4. Transmittance spectra of (a) TiO2-110 and (b) TiO2-400.
thermal treatment.
20,21)3.5 Photocatalytic activity
TiO
2has strong oxidizing power which can completely decomposes organic pollutants under UV light illumin- ation.
3,22)TiO
2photocatalyst generates electron-hole pairs consequent to the absorption of UV light equivalent to the energy band gap.
2,23)The photogenerated holes in the valence band migrates towards TiO
2surface and react with adsorbed water molecules, generating hydroxyl radicals (•OH). Similarly, photoexcited electrons in the conduction band generally take part in reduction processes, which are typically combine with molecular oxygen in the air to form superoxide radical anions (O
2• −).
2)These photo- generated radicals easily decompose nearby organic pollu- tants. In this way, TiO
2can keep its surface free from organic contaminations under light illumination.
24-26)Photocatalytic activities of TiO
2-110 and TiO
2-400 coating films were evaluated by measuring the degradation of methylene blue. The photocatalytic activity of TiO
2thin films prepared at low temperature was compared with thin films prepared at high temperature. Fig. 6 shows the photodegradation of methylene blue for TiO
2- 110 and TiO
2-400 films in the presence of UV illumin- ation. The TiO
2-400 thin films represent higher activity than TiO
2-110. The initial concentration of methylene blue was about 5.34 × 10
−6M, upon 90 min UV illumin- ation with thin films, the concentration of methylene blue
dye reached up to 3.139 × 10
−6M and 1.937 × 10
−6M for TiO
2-110 and TiO
2-400 thin films, respectively. The photo- catalytic degradation of methyl blue solution follows a pseudo-first order reaction and its kinetics equation may be expressed as:
ln(C
0/C
t) = kt (1)
where, k is the apparent reaction rate constant, and C
0is initial concentration and C
tis the concentration of methyl
Fig. 5. Water contact angle measurement for TiO2-110 (a-f) and TiO2-400 thin films (g-i).Fig. 6. Photocatalytic degradation of methylene blue dye by TiO2- 110 and TiO2-400 thin films.
blue dye solution at time t, respectively. The photocatalytic reaction rate constants of methyl blue dye solution over TiO
2-110 and TiO
2-400 thin films were calculated from the slope of the curve as shown in Fig. 7. The TiO
2-400 thin film ( k = 0.0112 min
−1) exhibited the highest rate of degradation of methyl blue dye solution compared to TiO
2-110 thin film ( k = 0.00581 min
−1). The lower photo- catalytic activity of the thin film prepared at low tem- perature was attributed to the lower crystallinity of the sample. The photocatalytic TiO
2coating layers annealed at low temperature may provide many useful applications such as a coating of TiO
2films on the polymeric sub- strates for self-cleaning performance which have low melting point.
4. Conclusions
TiO
2thin films were coated on a glass slides using sol- gel method and peroxo titanic acid as a precursor. The self-cleaning and photocatalytic properties of the thin films were changed with heat treatment. TiO
2thin films prepared at low temperature showed lower photocatalytic activity while higher photocatalytic activity was observed for TiO
2thin films prepared at high temperature. The thin films heat treated at low temperature had maintained high transparency, hydrophilicity, and adequate photo- catalytic activity. The coating process of self-cleaning and photocatalytic TiO
2thin films on the glass substrates was quite simple, low-cost, and a low temperature route.
Therefore, this work may provide a promising way to coat self-cleaning and photocatalytic TiO
2thin films for various applications.
Acknowledgement
This research work was supported by a grant(17- CTAP-C133057-01) from Infrastructure and transporta- tion technology promotion research program funded by Ministry of Land, Infrastructure, and Transport of the Korean government.
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