Komai Yosuke

Masahashi Naoya

Mizukoshi Yoshiteru

Nishimura Rokuro

Okitsu Kenji

MATERIALS TRANSACTIONS

English

Komai, Yosuke

Mizukoshi, Yoshiteru

Okitsu, Kenji

Nishimura, Rokuro

Masahashi, Naoya

Institutional Repositories DataBase (IRDB)

Effects of Ultrasonic Irradiation on Preparation of Titanium Dioxide Photocatalyst by Anodic Oxidation Method

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Effects of Ultrasonic Irradiation onPreparation of Titanium Dioxide Photocatalystby Anodic Oxidation Method著者 Komai  Yosuke, Mizukoshi  Yoshiteru, Okitsu Kenji, Nishimura  Rokuro, Masahashi  Naoyajournal orpublication titleMaterials Transactionsvolume 50number 9page range 2182-2186year 2009URL http://hdl.handle.net/10097/52226Eﬀects of Ultrasonic Irradiation on Preparation of TitaniumDioxide Photocatalyst by Anodic Oxidation MethodYosuke Komai1;*, Yoshiteru Mizukoshi2, Kenji Okitsu1,Rokuro Nishimura1 and Naoya Masahashi21Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan2Institute for Materials Research, Tohoku University, Sendai 980-8577, JapanTitanium dioxide (TiO2) photocatalyst was fabricated by anodic oxidation on pure titanium in an electrolyte of a sulfuric acid, and eﬀect ofultrasonic irradiation during anodic oxidation was explored. It was found that ultrasonic irradiation increased crystallinity of the anodic oxide.The hydrophilicity of the as-anodized oxide was improved by applying ultrasonic irradiation, and it is similar to that of the annealed oxidewithout applying ultrasonic irradiation. In contrast, the photoactivity of the as-anodized oxide was not altered by applying ultrasonic irradiation.[doi:10.2320/matertrans.MAW200912](Received April 24, 2009; Accepted July 3, 2009; Published August 25, 2009)Keywords: photocatalysis, anodic oxidation, titanium oxide, ultrasonic irradiation1. IntroductionTitanium dioxide (TiO2) has attracted considerable inter-ests due to its photocatalytic activity and hydrophilicity, andextensive studies have been carried out to elucidate themechanism of the characteristics and expand its practicalapplications.1)Ultrasound has been applied to fabricate novel materialsfor more than decades.2,3) Chemical eﬀects of ultrasound areoriginated from cavitation phenomena, namely adiabaticalcollapse of cavitation bubbles, which are generated in themedium under high power ultrasonic irradiation.4) Throughthe collapse of bubbles, generation of hot reaction sites5)accompanied by formation of radicals such as hydroxylradicals6) has been proposed. Further, shock wave,7) and jetstreams on the heterogeneous surface8,9) have been reportedas other eﬀects. Sonochemical eﬀects on electrochemicalreaction have been widely explored, because ultrasoundprovides degassing, promotion of mass transfer, and destruc-tion of electrical double layer.10) However, sonochemicaleﬀects on anodic oxide during anodic oxidation have notbeen investigated except a few reports.11,12)The authors studied an anodic oxidation to form photo-catalyst TiO2 on titanium and its alloys,13–16) because itprovides adhesive strength at the interface with a lowinterfacial strain due to a small diﬀerence in the thermalexpansion coeﬃcient between titanium and oxide. In thisstudy, we have investigated the photocatalytic characteristicsof the oxide prepared by anodic oxidation in an electrolyteof a sulfuric acid expecting an enhancement of oxideformation by applying ultrasonic irradiation.2. ExperimentalPure titanium plates (cp-Ti, grade I) with dimensions of1:0 2:0 0:1 cm3 were prepared for use as the anode inanodic oxidation. A constant current of 50mA/cm2 wasapplied to the Ti plate in a sulfuric acid electrolyte of 0.02and 0.79M, and a Pt mesh electrode was used as the cathode.The anodic oxidation was controlled galvanostatically withapplying the conversion voltage ranging from 40 to 210V,and the time duration of oxidation was ﬁxed at 0.5 h. Theanodized electrode was rinsed with distilled water and driedat room temperature, followed by annealing at 723K for5 h in air. The electrolyte was irradiated with ultrasound of6W/cm2, and 200 kHz (TA-4021, KAIJO). Figure 1 showsthe experimental setup for anodic oxidation with ultrasonicirradiation.The crystallographic structure of the anodized oxide wasdetermined by X-ray diﬀraction (XRD, PANalytical X’Pertdiﬀractometer, Phillips). Microstructure observations wereconducted with a scanning electron microscopy (SEM, VE-8800, Keyence) at an operating voltage of 20 kV.The photocatalytic activity under UV light illuminationwith an intensity of 1.5mW/cm2 at the surface of the reactioncell was investigated using methylene blue (MB) bleachingtests. The MB bleaching test was conducted by measuring theabsorbance of MB at 664 nm using a UV-vis spectropho-tometer (V-550DS, Jasco) after UV illumination and theDC Power Source +ve-veH2SO4 electrolyteTo Power SourceWaterTi PtUltrasonic WaveUltrasonicOscillatorFig. 1 Experimental setup for anodic oxidation with ultrasonic irradiation.*Graduate Student, Osaka Prefecture UniversityMaterials Transactions, Vol. 50, No. 9 (2009) pp. 2182 to 2186#2009 The Japan Institute of Metalsdetail was ascribed in elsewhere.13–15) Degradation rate ofMB was calculated according to the following equation,DMB ¼ fðAblank  AmeasuredÞ=Ablank  100g, wherein Ablankand Ameasured are absorbance at 664 nm without and withTiO2 photocatalyst, respectively. In order to evaluate hydro-philicity of the anodic oxides, the contact angle of distilledwater (0.1mL) was measured by using a goniometer system(Drop Master 500, Kyowa Interface Science), and the contactangle was measured every half hour during UV illuminationwith an intensity of 0.2mW/cm2 at the surface of the sample.3. Results and Discussion3.1 MicrostructuresFigure 2 shows the microstructures of the oxides anodizedin an electrolyte of sulfuric acid concentrations of 0.02 (a),(b) and 0.79M (c), (d) with ultrasonic irradiation (b), (d) andwithout ultrasonic irradiation (a), (c). When the sulfuric acidconcentration is 0.02M, the thickness of the anodized oxideis thin. This is supported by charge-up distribution observa-tion in SEM images, where the substrate is observed ascharge-up free as circled in (a). As reference, the micro-structures of the titanium substrate are shown in (e), (f). Withultrasonic irradiation during anodic oxidation, glassy ano-dized oxide with submicron sized pores is observed as shownin (b). The observed pores are distributed inhomogeneouslyin shape and size, and their frequency is quite low. With anincrease in the concentration of sulfuric acid of 0.79M,round-shaped pores appear homogeneously in the oxide asshown in (c). However, ultrasonic irradiation did not alter themicrostructure of the anodic oxides as shown in (d). Theseresults imply that the eﬀect of ultrasonic irradiation onmicrostructure evolution is recognized when the sulfuricacid concentration in the electrolyte is low. Detail will bediscussed in the next section.3.2 Crystal structureFigure 3 shows the XRD proﬁles of the oxides anodizedin an electrolyte with sulfuric acid concentrations of 0.02M(a) and 0.79M (b). The relative peak intensity ratio ofthe predominant oxide peak to hcp Ti (101) (2 ¼ 40:1)increases with an increase in the sulfuric acid concentrationand the conversion voltage, indicating that the thickness ofthe oxide layer increases with the sulfuric acid concentrationand the conversion voltage. When the sulfuric acid concen-tration is 0.02M, the intensity of anatase (101) (2 ¼ 25:4)increases with the conversion voltage and ultrasonic irradi-ation as compared to that without ultrasonic irradiation asshown in (a). In contrast, similar variation is not recognizedwhen the sulfuric acid concentration is 0.79M. The predom-inant oxide phase converts from anatase to rutile when thesulfuric acid concentration is 0.79M and the conversionvoltage is 160V as shown in (b). In order to evaluate thecrystallinity of the oxides, the full width at half maximum(FWHM) of anatase (101) was calculated and the result isshown in Fig. 4. When the sulfuric acid concentration is0.02M (a), the FWHM of anatase (101) of the as-anodizedoxides decreases with the conversion voltage and ultrasonicirradiation, whereas that of the annealed oxides is essentiallyconstant irrespective of the conversion voltage and ultrasonicirradiation. These results suggest that the crystallinity ofanatase phase is improved by ultrasonic irradiation and anapplication of ultrasonic irradiation has the same eﬀect onimproving crystallinity with annealing. It is found that theas-anodized oxides with ultrasonic irradiation exhibit lowFWHM as compared to that without ultrasonic irradiation,suggesting that the crystallinity increases with ultrasonicirradiation. On the other hand, similar result is not recognizedwhen the sulfuric acid concentration is 0.79M (b), which isthe same tendency with the result of microstructure evolutionin the anodic oxides. This is quite interesting in considering(b) (c)1µm 1µm(d)1µm(e) (f)1µm 10µm 1µm(a)Fig. 2 SEM images of the oxides anodized in an electrolyte with sulfuric acid concentrations of 0.02M (a), (b), 0.79M (c), (d) withultrasonic irradiation (b), (d), without ultrasonic irradiation (a), (c) and the titanium substrate (e), (f).Eﬀects of Ultrasonic Irradiation on Preparation of Titanium Dioxide Photocatalyst by Anodic Oxidation Method 2183an eﬀect of ultrasonic irradiation on anodic oxide formation.It is speculated that ultrasonic irradiation promotes move-ments of constituents when the concentration of sulfuric acidis low, whereas it does not aﬀect movements of constituentswhen the concentration of sulfuric acid is high. This meansthat the ultrasonic irradiation has the same eﬀect on oxideformation with increasing sulfuric acid concentration orannealing. Lowering the concentration of the sulfuric acid oromission of high energy processes such as annealing isbeneﬁcial for practical processing due to reduction of theenvironmental burdens. Further studies are necessary toelucidate the mechanism of ultrasonic irradiation on oxideformation.3.3 HydrophilicityFigure 5 shows the plots of the contact angles against theduration of UV light illumination. The contact angle of theoxides anodized in 0.02M sulfuric acid decreases with theduration of UV light illumination when the conversionvoltage is 210V. The contact angle of the annealed oxides islower than that of the as-anodized oxides as shown in (b).Further, the contact angle of the as-anodized oxides isdecreased with ultrasonic irradiation, regardless of theduration of UV light illumination as shown in (a). This isprobably attributed to an improvement of the crystallinitybecause it promotes a generation of oxygen vacancyresponsible for adsorption of hydroxyl groups under UVlight illumination.17) On the other hand, it is not recognizedsimilar characteristics in the oxides when the sulfuric acidconcentration is 0.79M (c), (d). This could be understoodthat the crystallinity is promoted with an increase of thesulfuric acid concentration in the electrolyte regardless ofultrasonic irradiation, thereby the eﬀect of ultrasonic irradi-ation on hydrophilicity is not clear.3.4 Photocatalytic activitiesFigure 6 shows the plots of the degradation rates of MBagainst the conversion voltage. When the sulfuric acidconcentration is 0.02M, the annealed oxides show highactivities as compared to the as-anodized oxides irrespectiveof the conversion voltage as shown in (a). The same char-acteristics are veriﬁed when the sulfuric acid concentration is(a)50000R (101)(b)50000Ti (002)Ti (100)Ti (101)A (101)160V210V3000040000 A (101)R (110)Ti (002)Ti (101)withUS100V160V210V3000040000withUS210V40V100V20000210V40V20000without US40V100V160V010000Intensity / countswithoutUS40V100V160V010000Intensity / counts20 30 40 502theta / degree20 30 40 502theta / degreeFig. 3 XRD proﬁles of the as-anodized oxides prepared in an electrolyte of 0.02M (a), 0.79M (b) sulfuric acid concentration with andwithout ultrasonic irradiation (US).2.53Without ultrasonic irradiation(as-anodized)W(a)2.53(b)2θ/ degree ith ultrasonic irradiation(as-anodized)Without ultrasonic irradiation(annealed)With ultrasonic irradiation(annealed) 2θ/ degree11.511.500.50 50 100 150 200 250FWHM of anatase 101,00.5FWHM of anatase 101, Conversion voltage, V / V 0 50 100 150 200 250Conversion voltage, V / V Fig. 4 FWHM of anatase (101) of the oxides anodized in 0.02M (a) and 0.79M (b) sulfuric acid against the conversion voltage.2184 Y. Komai, Y. Mizukoshi, K. Okitsu, R. Nishimura and N. Masahashi0.79M, and it is found that the photocatalytic activity of theanodic oxide prepared in an electrolyte of 0.79M sulfuricacid is higher than the corresponding of 0.02M irrespectiveof conversion voltage and annealing. In contrast, the photo-catalytic activity was not improved by ultrasonic irradiationirrespective of the sulfuric acid concentration. This result isinconsistent with the fact that an improvement of thecrystallinity leads to an enhancement of the photocatalyticactivity due to the reduction of the recombination sites (e.g.point defects, dislocations etc.) of the photogeneratedelectron and hole. Consequently, it is considered that thephotocatalytic activity in the present study could be aﬀected(a)8090100Without ultrasonic irradiation(160V)With ultrasonic irradiation(160V)Without ultrasonic irradiation(210V)(b)8090100Without ultrasonic irradiation(160V)With ultrasonic irradiation(160V)Without ultrasonic irradiation(210V)40506070 With ultrasonic irradiation(210V)40506070 With ultrasonic irradiation(210V)102030Contact angle / degree 102030Contact angle / degree00 0.5 1 1.5 2.5 3UV illumination time, t / h00 0.5 1.5 2 2.5 3UV illumination time, t / h70(c) 70(d)4050604050602030Contact angle / degree2030Contact angle / degree0100 0.5 1 1.5 2 2.5 3UV illumination time, t / h0100 0.5 1 1.5 2 2.5 3UV illumination time, t / h2 1Fig. 5 Plots of the contact angles against the duration of UV light illumination for the oxides anodized in an electrolyte with sulfuric acidconcentrations of 0.02M; (a) as-anodized and (b) annealed, 0.79M; (c) as-anodized and (d) annealed.90100(b)(a)90100With ultrasonic irradiation(annealed)Without ultrasonic irradiation(annealed)With ultrasonic irradiation(as-anodized)Without ultrasonic irradiation(as-anodized)  607080607080304050D304050Degradation rate of MB /  %010200 50 100 150 200 250Degradation rate of MB /  %010200 50 100 150 200 250Conversion voltage, V / VConversion voltage, V / V Fig. 6 Plots of the degradation rates of MB after UV illumination against the conversion voltage for the oxides anodized in an electrolytewith sulfuric acid concentrations of 0.02M (a) and 0.79M (b).Eﬀects of Ultrasonic Irradiation on Preparation of Titanium Dioxide Photocatalyst by Anodic Oxidation Method 2185by other factors such as the recombination rate of theelectron and hole, the mobility of the electron and hole, andgeometrical factors in the microstructure. Further studieshave been under progress to elucidate it.4. ConclusionsThe photocatalytic characteristics of the anodic oxidefocusing on the eﬀect of ultrasonic irradiation were studied.Ultrasonic irradiation during anodic oxidation improves thecrystallinity of the anodic oxides, and an application ofultrasonic irradiation has the same eﬀect on improvingcrystallinity with annealing. The contact angles of theas-anodized oxides with ultrasonic irradiation were lowerthan those of the as-anodized oxides without ultrasonicirradiation. In contrast, the photocatalytic activity was notimproved by ultrasonic irradiation regardless of annealing.REFERENCES1) A. Fujishima, T. N. Rao and D. A. Tryk: J. Photochem. Photobiol. C1(2000) 1–21.2) A. Gedanken: Ultrason. Sonochem. 11 (2004) 47–55.3) L. H. Thompson and L. K. Doraiswamy: Ind. Eng. Chem. Res. 38(1999) 1215–1249.4) T. J. Masaon (Ed.): Advances in Sonochemistry vol. I, (JAI press,London, 1990) pp. 1–37.5) W. B. McNamara III, Y. T. 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Effects of Ultrasonic Irradiation on Preparation of Titanium Dioxide Photocatalyst by Anodic Oxidation Method

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