Journal of the Korean Physical Society, Vol. 51, No. 3, September 2007, pp. 1105∼1108

Surface Properties of Photocatalytic TiO2 Thin Films with MicrowaveDischarged Plasma Surface Treatment

C.-K. Jung, S.-J. Cho, I.-S. Bae, S. H. Jeong, Y.-H. Song and J.-H. Boo∗

Department of Chemistry and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746

T. K. Kim

RIAN and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746

S. Kim

Center for Materials and Processes of Self-Assembly, Kookmin University, Seoul 136-702

(Received 13 November 2006)

We have deposited titanium dioxide (TiO2) thin films on glass using a single molecular precursorsuch as titanium (IV) iso-propoxide (Ti[OCH(CH3)2] 4, 97 %) by sol-gel process. In order toelevate photocatalytic activity of the as-grown TiO2 films, argon and oxygen plasmas ignited bymicrowave (MW) discharge at 300 W under vacuum condition were also used within 5 min. atroom temperature. Photocatalytic activity was evaluated by the measurements of the contactangle, FT-IR, XPS, and AFM analysis. In this work, the effect of the plasma on the improvementof hydrophilic property of TiO2 photocatalyst has mainly been investigated. Superhydrophilicproperty and smooth surface morphology appeared in the UV light irradiation with O2 plasmatreatment more than Argon plasma treatment. Based on this work, we confirmed that the oxygenMW plasma treatment method was very reliable method for the synthesis of TiO2 thin films withhigh catalytic performance.

PACS numbers: 74.77.Jh, 35.11.BoKeywords: MW plasma treatment, Photocatlytic TiO2, Superhydrophilicity, Aging effect

I. INTRODUCTION

Photo-catalytic technology is becoming more andmore attractive to industry today because global envi-ronmental pollution has come to be recognized as a se-rious problem that needs to be addressed immediately.TiO2 is an inexpensive, non-toxic, and biocompatiblematerial that shows high photo-efficiency and activity.As a result, TiO2-based photo-catalytic process provedto be very effective in removing many air and water pol-lutants [1]. TiO2 is a goal optical material with a trans-mittance from ultraviolet to visible light. This propertycan be used for a variety of uses, for example, the dis-solution of hazardous VOCs, the removal of endocrinedisrupters, the recovery of heavy metal, anti-fogging, de-contamination and self-cleaning, etc. [2]. Up to now,the TiO2 photocatalyst thin films have been preparedby a number of methods such as dip-coating, and sol-gel method, the thermal oxidation of metal or reactive

∗E-mail: jhboo@skku.edu

magnetron sputtering [3–10]. Among them sol-gel pro-cessing is one of the most common methods to producephotocatalyst TiO2. Recently, considerable research isfocused on plasma treatment of TiO2 thin films to im-prove photocatalytic activity towards to the decomposi-tion of organic and inorganic pollutants under the ultra-violet (UV) light [11]. Moreover, plasma treatment forsurface modification is also used to produce hydrophobicor hydrophilic surfaces on metals, plastics or polymers[12]. The photocatalytic efficiency seems to depend onmany parameters. Concerning the morphology, crystalphase, and composition of the film, it is highly desir-able to prepare more effective photocatalytic films forsol-gel and MW plasma treatment [13,14]. In this paperwe thus made the TiO2 photocatalytic thin films usingsol-gel method to get the smooth morphology as well ashomogeneous composition. In addition, a plasma sur-face treatment with either argon or oxygen plasma wasalso utilized for obtaining highly efficiency TiO2 photo-catalysts with superhydrophilicity and very smooth sur-face. The effect of the plasma on the improvement ofhydrophilic property has mainly been investigated.

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II. EXPERIMENT

Deposition of TiO2 thin films was carried out by adip-coating method with the titanium isopropoxide sol(TiO2 coating sol 3 %), which was used as solvent precur-sor. A coating sol for TiO2-based photocatalyst was alsoprepared by the sol-gel method using titanium tetraiso-propoxide, acid (HCl, HNO3, HF etc.) and water. Ti-tanium tetraisopropoxide was slowly dropped into the0.4 % nitric acid solution that was stirred vigorously for2 hours at room temperature, and heated at 80 ◦C for24 hours. During reaction the isopropanol was removedby distillation and the milky-bulk solution was graduallychanged to a blue fine-milky solution. Prior to the depo-sition of TiO2 photocatalyst, the glass substrates were infirst degreased, thoroughly cleaned and dried. Then thesubstrate was dipped into the viscous TiO2-precursor solpulled out at a uniform pulling rate of 5 mm/s, and driedat room temperature for 24 hours. Optical properties ofas-deposited TiO2 films were characterized with contactangle and infrared spectroscopy. The surface morphologyof TiO2 films was characterized using atomic force mi-croscopy (AFM) analysis. Moreover, for durability test,we have investigated changes of composition with X-rayphotoelectron spectroscopy (XPS).

III. RESULTS AND DISCUSSION

Figure 1 shows optical images of contact angles ob-tained from the TiO2 thin films which deposited onglass substrates using TiO2 sol without (a) and withAr (b) and O2 (c) plasma treatments. Comparing theFigure 1(a) with the Figures 1(b) and (c), we can re-alize that the contact angles obtained from TiO2 layerswith plasma treatment is very low rather than that layerwithout plasma treatment, indicating that relatively hy-drophilic surface with high surface energy was obtainedafter plasma surface treatment. This means that we cancontrol the surface energy as well as contact angle withplasma treatment. Noticeable thing is that we have ob-tained superhydrophilic surface with O2 plasma surfacetreatment as shown in the Figures 1(c) and 3(b). It isinitially obtained a contact angle value of 33◦ from TiO2

sol coating layer only while that of TiO2 layer treatedwith Ar plasma further exhibited 10◦(see Figure 1(b)),suggesting more hydrophilic nature with plasma surfacetreatment. However, after 300 W MW Ar and O2 plasmasurface treatments for less than 5 minutes, the values ofcontact angle were decreased as much as 2◦ as shownin Figure 1(c), signifying a superhydrophilic surface for-mation. It means that the contact angle of TiO2 layertreated with O2 plasma decreased more rapidly than thatwith Ar plasma under the same plasma condition.

Figure 2 shows AFM images of TiO2 films obtainedboth before plasma treatment (a) and after MW plasmatreatments with (b) Ar and (c) O2. Based on this AFM

Fig. 1. Contact angle of water droplet on TiO2 films pro-duced by sol-gel method. Photos of water droplets on theTiO2 surfaces of (a) before plasma treatment and after MWplasma treatments with (b) Ar and (c) O2.

analysis, we realized that the as-deposited TiO2 thin filmon glass substrate has originally high root mean squared(RMS) roughness of 4.3 nm (see Figure 2(a)), while thosefilms with Ar and O2 plasma treatments observed rela-tively low RMS values of 3.3 and 2.2 nm, as shown inthe Figures 2(b) and 2(c) respectively, indicating verysmooth surfaces. No evidence of different morphologyof the TiO2 film was obtained from these images. How-ever, the rms roughenss of TiO2 film with O2 plasmatreatment was much decreased than that with Ar plasmatreatment. It means that the effect of plasma treatmentis likely to increase in the uniformity of the surface with-out distinct change of surface morphology, and the oxy-gen MW plasma treatment is very reliable method forthe synthesis of TiO2 thin film photocatalysts with highcatalytic performance.

Infrared (IR) spectroscopy is a promising methodfor the kinetic analysis of the photocatalytic reactions.Therefore, it has an advantage of observing molecular vi-brations. Figure 3(a) shows high-resolution FT-IR spec-tra of the TiO2 thin film photocatalysts synthesized us-

Surface Properties of Photocatalytic TiO2 Thin Films· · · – C.-K. Jung et al. -1107-

Fig. 2. AFM images of the TiO2 surfaces of (a) beforeplasma treatment and after MW plasma treatments with (b)Ar and (c) O2.

ing TiO2 sol both without plasma treatment and withAr or O2 plasma treatment under the same condition.Considering on the FT-IR handbook and IR data by theprevious researches [4], we interpreted our observed IRspectra as following. The peak appeared at 3300 cm−1

is attributed to the O-H stretching mode of differenthydroxyl species. A reaction with a hydroxyl anion isthought as the initial step of the photo-oxidation of wa-ter. Since our TiO2 sol layer has relatively small hard-ness (about 6 Gpa) and soft surface, it seems to be easyto dissociate the water or oxygen molecules on the solsurface compared with other photocatalysts that syn-thesized by sputtering or other methods. In addition,we could see the increase of O-H absorption peak inten-sities with Ar and O2 plasma treatment. Especially, wehave obtained higher intensities with O2 plasma treat-ment. This indicates that more high amounts of hydroxylgroups can exhibit in the photocatalytic TiO2 thin films.This phenomenon can be explained with two things bycombination of XPS data. First, since the Ar+ ions inthe plasma can make many defect sites on the surface,there are numerous chemisorbed oxygen and/or hydroxylspecies on the film surface, resulting in increasing of dis-sociation rate (in other words, decreasing dissociationbarrier) for water and oxygen molecules. Second, oxy-gen plasma can give many radicals and atoms as well assome ions into the surface, and then excess oxygen layerswith non-stoichiometric ratio would form on the stoichio-metric TiO2 layer, suggesting further dissociated oxygen

Fig. 3. (a) High resolution FT-IR spectra obtained fromthe synthesized TiO2 films with/without plasma treatment.(b) shows variation of contact angles as a function of UVirradiation time obtained after 2 months.

species. That’s why we obtained very strong O-H absorp-tion peak from the photocatalytic TiO2 thin film withO2 surface treatment under the same condition. The ev-idence of hydroxyl radical formation is already describedpreviously based on the ESR (electron spin resonance)spectra measurements [15,16]. Moreover, we measuredtheir contact angels again after let the samples in a darkroom for 2 months to check a durability of the photo-catalytic TiO2 films using UV irradiation. Figure 3(b)shows variation of contact angles as a function of UVirradiation time. Before UV irradiation, we found thatthe value of contact angle for the as-deposited TiO2 filmis increased up to 33◦ from 10◦ after two months, whilethose of TiO2 films with Ar and O2 plasmas are slightlychanged to 25◦ and 14◦, respectively. However, after UVirradiation for less than 1 hour, we again obtained verysimilar values of contact angle in the both cases as shownin the results of Figures 1 and 3(b). It means that ourTiO2 thin films can maintain hydrophilic property (inother words, photocatalytic effect) for a long time (atleast 2 months).

To explain why our films have so high photocatalyticeffect and superhydrophilic nature in details, we havecarried out X-ray photoelectron spectroscopy (XPS)analysis. Figures 4(a) and 4(b) show typical highly res-

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Fig. 4. High-resolution XP spectra of TiO2 films syn-thesized with O2 plasma treatment taken after 1 day and2 months; (a) O1s and (b) Ti2p.

olution XP spectra of O1s (a) and Ti2p (b) measured byregional scans. Different XPS profiles were obtained fromthe O2 plasma treated TiO2 samples that let in the darkroom for 1 day and 2 months, respectively. As shownin the Figure 4(a), for the 1 day sample, O1s bindingenergy of the main peak appeared at 530.0 eV togetherwith the shoulder peak of 532.0 eV. We interpreted thatthe O1s peak of 530.0 eV as the typical stoichiometricoxide species like TiO2 while the O1s peak of 532.0 eVis attributed to non-stoichiometric oxide species such asTiOx, Ti(OH)x, TiO(OH) etc. Typical thing was thatthe shoulder peak is decreased after 2 months. It meansthat photocatalytic effect for TiO2 samples depend oncomposition of OH species according to durability. Inconclusion, in order to maintain photocatalytic property,we have used MW power of 300 W with O2 plasma treat-ment under UV irradiation within 1 hour. It has beensuggested that these surface hydroxyls contribute to thephotocatalytic property [17].

IV. CONCLUSIONS

Photocatalytic TiO2 films have been deposited onglass substrates using titanium isopropoxide sol. After

thin film deposition by sol-gel processing, surface treat-ments with Ar and O2 MW plasmas carried out at 300W for comparative study. The results showed that MWplasma treated TiO2 photocatalyst has more catalyticefficient than that of TiO2 coating layer without plasmatreatment. The contact angle of water decreases as ar-gon and oxygen plasma treatment since hydrophilic func-tional groups such as non-stoichiometric TiOx were in-troduced on the TiO2 surface. Moreover, photocatalyticTiO2 films with MW plasma treatment have maintainedlow contact angle even after 2 months. We have also ob-tained a very smooth surface with 2.2 nm rms roughnessby O2 plasma treatment.

ACKNOWLEDGMENTS

Support of this research by the BK 21 project ofthe Ministry of Education, Korea is gratefully acknowl-edged. This work was supported by the Korea ResearchFoundation Grant funded by the Korean Government(MOEHRD, KRF-2005-005-J11902) and by the FacultyResearch Fund 2005 of the Sungkyunkwan University.

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