effect of pt loading on the photocatalytic reactivity of titanium oxide thin films prepared by ion...

Res. Chem. Intermed., Vol. 29, No. 6, pp. 619–629 (2003)Ó VSP 2003.Also available online - www.vsppub.com

Effect of Pt loading on the photocatalytic reactivityof titanium oxide thin � lms preparedby ion engineering techniques

MASATO TAKEUCHI, KOUICHIROU TSUJIMARU, KENJI SAKAMOTO,MASAYA MATSUOKA, HIROMI YAMASHITA and MASAKAZU ANPO ¤

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan

Received 20 March 2003; accepted 2 July 2003

Abstract—Platinum-loaded titanium oxide thin-� lm photocatalysts were prepared by using anionized cluster beam (ICB) deposition method and a RF magnetron sputtering (RF-MS) depositionmethod as dry processes. From the results of the photocatalytic oxidation of acetaldehyde with O2under UV light irradiation,small amounts of Pt loading (less than 10 nm � lm thickness)were found todramatically enhance the photocatalytic reactivity. However, when TiO2 thin � lms were loaded withrelatively larger amounts of Pt (more than 30 nm as the � lm thickness), the photocatalytic reactivitybecame lower than for the pure TiO2 thin � lms. Moreover, investigations of the ratio of Pt loadedonto the surface of the thin � lm catalysts by XPS measurements revealed that the small amounts ofPt loaded exist as very small clusters working to ef� ciently enhance the charge separation, whereas,large amounts of Pt covers the entire surface of the TiO2 thin � lms, resulting in a decrease of thephotocatalytic reactivity.

Keywords: Titanium oxide thin � lm; photocatalytic oxidation; Pt loading effect; ion cluster beamdeposition method

INTRODUCTION

Since the discovery of the Honda and Fujishima effect [1], various TiO2 photocat-alytic systems combined with noble metals such as Pt and Pd have been investigated[2–9]. Among these various noble metals, the loading of small amounts of Pt ontothe TiO2 photocatalysts is well known to dramatically enhance the photocatalyticperformance. This may be due to the fact that the electrons that are generated bylight irradiation in the TiO2 photocatalysts transfer to the Pt particles immediately,

¤To whom correspondence should be addressed. E-mail: [email protected]

620 M. Takeuchi et al.

resulting in a decrease in the electron-hole recombination as well as the catalyticeffects of the Pt for the reduction reactions. In other words, small amounts of Ptloaded onto TiO2 may work as a promoter to enhance the charge separation of theelectrons and holes. In order to con� rm these experimental results, the dynamicsof the charge carrier trapping and recombination in the TiO2 semiconductor havebeen intensively investigated by the transient absorption spectra using picosecondor nanosecond laser � ash photolysis [10–19].

TiO2 thin � lms are one of the most interesting photo-functional materials dueto their high transparency, high photocatalytic performance and super-hydrophilicproperties induced by UV light irradiation of the TiO2 surface [20–23]. Variousmethods, such as the sol–gel method [24–27], MOCVD [28, 29], an electrochem-ical technique [30, 31], ion-engineering techniques such as metal ion-implantationmethod [32], ion cluster beam deposition [33] and a RF magnetron sputtering de-position method [34] have been widely studied to prepare highly active TiO2 thin� lm photocatalysts, and many of these studies concern the photocatalytic decompo-sition of pollutants and toxic compounds. In an antibacterial system, the additionof copper or silver � ne particles onto TiO2 thin � lms is known to improve its pho-tocatalytic performance, since the added copper or silver works as an antibacterialagent under dark conditions while TiO2 works as a photocatalyst to kill bacteria bylight irradiation.

In this paper, we will deal with the preparation of the TiO2 thin � lms loadedwith small amounts of Pt by using ion-engineering techniques such as an ionizedcluster beam (ICB) deposition method [33] and a RF magnetron sputtering (RF-MS)deposition method [34] in order to improve their photocatalytic reactivities underUV light irradiation. The interactions between the states of the Pt deposited ontothe TiO2 surface and their photocatalytic performance for the oxidation reaction ofacetaldehyde with O2 will be also discussed.

EXPERIMENTAL

TiO2 thin � lms were prepared by an ionized cluster beam (ICB) deposition methodin accordance with previous literatures [33]. Prior to the deposition process for theTiO2 thin � lms, the quartz substrates were ultrasonically cleansed in acetone for15 min, dried at 373 K for half a day, and then calcined in air at 723 K for 5 h inorder to obtain a clean surface. A schematic diagram of the ICB deposition methodis shown in Fig. 1.

Titanium metal (purity: 99.99%) as the source material was heated up to about2200 K in a crucible and titanium vapor was introduced into the high vacuumchamber (pressure ranges: lower than 10¡7 Torr) to produce titanium metal clusters.At this time, the titanium clusters reacted immediately with the O2 moleculesin the vacuum chamber to form stoichiometric TiO2 clusters (oxygen pressure:2 £ 10¡4 Torr). The TiO2 clusters were ionized by the electron beams and theseionized clusters were then accelerated by an electric � eld (acceleration voltage

Effect of Pt loading on the photocatalytic reactivity of TiO2 thin � lms 621

Figure 1. Schematic diagram of the ICB deposition method.

500 V) and bombarded onto the substrate. The substrate temperature was heatedup to 623 K to produce TiO2 thin � lms having good crystallinity. The TiO2 � lmthickness (500 nm as the thin � lm) and the deposition rate (0.1 nm/s) could beeasily controlled by using a quartz � lm thickness meter (In� con).

Pt loading onto these TiO2 thin � lms was carried out by a RF magnetron sputtering(RF-MS) deposition method using a Pt metal target (purity 99.99%). It is generallydif� cult to use Pt, W and Mo as the source materials in such an ICB depositionmethod due to their high melting points. However, since the RF-MS depositionmethod does not necessitate the melting of the target materials by heating athigh temperatures, materials having high melting points can be applied as the ionsources. Pt loading on the TiO2 thin � lms was carried out at room temperaturewithout heating the TiO2 thin � lms. The amount of Pt loaded was controlled bychanging the deposition time and was calculated by the deposition rate (1.0 nm/s).Pt(5) /TiO2 /Quartz refers to the TiO2 thin � lm photocatalyst with a Pt loading of5 nm as the � lm thickness.

The resulting transparent TiO2 thin � lms were then characterized by variousspectroscopic means such as XRD (Rigaku, RINT-1200) and UV-VIS absorption(Shimadzu, UV-2200A) investigations at room temperature. The chemical states ofthe deposited Pt on the TiO2 thin � lms were also investigated by XPS (Shimadzu,ESCA-3200) measurements at room temperature. The Pt LIII-edge XANES spectrawere obtained in the � uorescence mode at 295 K under vacuum at the BL01B1facility of SPring-8. A Si(111) monochromator was used to monochromatize the


X-ray from the 8 GeV electron storage ring. The normalized spectra were obtainedby a procedure described in previous papers [35, 36].

The photocatalytic properties of these Pt-loaded TiO2 thin � lms were alsoinvestigated by analysis of the photocatalytic oxidation of acetaldehyde with O2

under UV light irradiation (¸ > 270 nm) at room temperature. A thin-� lm catalystwas placed on the � at bottom of the quartz reaction cell (about 60 ml) connected toa conventional vacuum system (10¡6 Torr range). Prior to the photoreactions, thecatalysts were degassed at 723 K for 2 h, heated in O2 at the same temperature for2 h and � nally evacuated to 10¡6 Torr at 473 K. UV light (¸ > 270 nm) irradiation ofthese thin � lms in the presence of acetaldehyde (14.6 ¹mol) and O2 (24.3 ¹mol) wascarried out using a 100 W high-pressure Mercury lamp (Toshiba SHL-100UVQ-2)through a cut-off � lter (Toshiba Glass, UV-27) at 275 K. The reaction products werethen analyzed by a gas chromatography equipped with both TCD and FID detectors.

RESULTS AND DISCUSSION

The XRD patterns of the prepared thin � lms are shown in Fig. 2 and it can beseen that the TiO2 thin � lms prepared by the ICB deposition method are mixturesof anatase and rutile structures while the ratio of anatase to rutile was found to bealmost the same as that of P-25 (about 70 to 80%), a reference TiO2 photocatalyst.These results indicate that the TiO2 thin � lms prepared by this method can be

Figure 2. XRD patterns of TiO2 thin � lm (a) and TiO2 thin � lms loaded with the different amount ofPt (b–e). The amount of Pt loaded (as the � lm thickness): (a) 0, (b) 5, (c) 10, (d) 30, (e) 60 nm.


Figure 3. UV-VIS absorption (transmittance)spectra of TiO2 thin � lm (a) and TiO2 thin � lms loadedwith the different amount of Pt (b–e). The amount of Pt loaded (as the � lm thickness): (a) 0, (b) 5,(c) 10, (d) 30, (e) 60 nm.

expected to show high photocatalytic properties. In the case of TiO2 thin � lmsloaded with Pt (Fig. 2b–e), new peaks attributed to the Pt metal and/or PtOx couldnot be observed, indicating that the loaded Pt may exist as small sized clusters or asa much thinner layer on the TiO2 thin � lms.

Figure 3 shows the UV-VIS absorption (transmittance) spectra of the unloadedTiO2 thin � lm and the TiO2 thin � lms loaded with different amounts of Pt. TheTiO2 thin � lm without Pt loading showed much higher transmittance (about 80 to90%) and speci� c interference fringes in the visible light region, indicating thathighly transparent and uniform TiO2 thin � lms could be easily prepared by the ICBdeposition method. The loading of small amounts of Pt (Fig. 3b, c) does not changethe high transparency of the original TiO2 thin � lms. On the other hand, as theamount of Pt loaded on the thin � lms was increased (Fig. 3d, e), the transmittanceof these thin � lms decreases (about 50 to 60%) and the clear interference fringesin the visible light region disappeared. These results clearly indicate that the smallamounts of Pt loaded exist as very small sized clusters and do not interrupt thepassage of the incident light into the TiO2 thin � lm, while large amounts of Ptdeposited exist as a thin layer which works as a half mirror to re� ect a part of theincident light.

In order to investigate the chemical states of the Pt loaded on the thin � lms, XPSmeasurements were carried out. Figure 4A and 4B show the Ti2p and Pt4f XPSspectra, respectively. As shown in Fig. 4A, the intensity of the Ti2p peak becomessmaller and the peak position is found to shift towards higher binding energy as the


(A)

(B)

Figure 4. XPS spectra (A, Ti2p; B, Pt4f) of TiO2 thin � lm (a) and TiO2 thin � lms loaded withdifferent amounts of Pt (b–e). The amount of Pt loaded (as the � lm thickness): (a) 0, (b) 5, (c) 10,(d) 30, (e) 60 nm.


amount of Pt is increased. The binding energy of the Ti2p 3=2 peak in pure TiO2

powder is known to be 458.5 eV [35]. The binding energy of the Ti2p 3=2 in theTiO2 thin � lms prepared by this method was found to be slightly higher (459.3 eV)compared with the pure TiO2 powder. This is because the thin � lms were strainedby the stress on the interface between the substrate and thin � lm. The Ti2p peakshifts towards higher binding energy regions can be explained by the coverage ofthe Pt thin layers on the TiO2 surface which makes it dif� cult to emit photoelectronsduring XPS measurements. On the other hand, as shown in Fig. 4B, the intensityof the Pt4f peak becomes larger and the peak position shifts towards higher bindingenergy as the amount of Pt loaded is increased. The binding energies of the Pt4f7=2 peak for the Pt metal and PtO are well known to be 70.9 eV and 73.6 eV,respectively [35]. The binding energy of the Pt4f 7=2 in the thin � lm loaded as the5 nm Pt � lm thickness (Fig. 4B, spectrum b) was slightly higher, i.e. 72.0 eV, thanfor the Pt metal, but smaller than for PtO. These results show that a small part ofthe Pt loaded on this thin � lm is slightly oxidized to PtO, although most exist as Ptmetal. Moreover, the binding energy of the Pt4f 7=2 peak in the thin � lm loadedwith Pt as the 60 nm � lm thickness (Fig. 4B, spectrum e) was 73.5 eV, indicating thatthe surface of the Pt loaded on this thin � lm was completely oxidized to form PtO.

Figure 5. Pt LIII-edge XANES spectra of TiO2 thin � lms loaded with different amounts of Pt (a,b), Pt foil (c) and PtO2 (as reference). The amount of Pt loaded (as the � lm thickness): (a) 10 nm,(b) 30 nm.


Figure 5 shows the Pt LIII-edge XANES spectra of Pt loaded on TiO2 thin � lmsas well as for the Pt foil and PtO2 as references. In these spectra, a sharp absorptionpeak, the so called ‘white line’, at 11563 eV can be observed. This peak is attributedto the electron transfer from 2p3=2 of the Pt LIII-edge to the vacant d orbital of theabsorbing atom [36]. In the Pt LIII-edge XANES measurements, the intensity of thewhite line in the Pt metal is generally much smaller than that of PtO2 and is in goodagreement with the chemical state of the Pt species. The intensity of the white linein both Pt(10)/TiO2 /Quartz and Pt(30)/TiO2 /Quartz is larger than that for the Ptfoil but smaller than that for PtO2. Also, the shape of the EXAFS oscillation in thesesamples was found to be more similar to that of Pt foil than to PtO2. These resultsclearly indicate that small amounts of Pt loaded onto the TiO2 thin � lms exist as Ptmetal rather than Pt oxide species. Further analysis of the Fourier transforms of theEXAFS oscillations need to be carried out in order to investigate the coordinationnumbers and bond lengths, however, results from the XAFS measurements were ingood correspondence with those of the XPS measurements.

Figure 6 shows the results of the photocatalytic oxidation of acetaldehyde withO2 under UV light irradiation as well as the ratio of Pt at the surface determined byXPS measurements. It is well known that most organic compounds can be oxidizedinto CO2 and H2O at room temperature on TiO2 photocatalysts by the irradiation ofUV light in the presence of O2 and H2O or excess amounts of O2 . However, as the

Figure 6. Photocatalytic oxidation of CH3CHO with O2 into CO (left) and CO2 (right) on the TiO2thin � lm photocatalyst under UV light (¸ > 270 nm) irradiation and the ratio of Pt at the surfacedetermined by the XPS measurements (circle plots).


ratio of O2 to CH3CHO was relatively small under these reaction conditions, notonly CO2 as a totally oxidized product but also CO as a partially oxidized productcould be detected in the gas phase. When suf� cient amounts of O2 was introducedinto the reaction system, it was con� rmed that the acetaldehyde was completelyoxidized into CO2 and H2O. As shown in Fig. 6, the loading of small amounts ofPt (less than 10 nm as the � lm thickness) was found to dramatically enhance thephotocatalytic reactivity for the oxidation of acetaldehyde. It is notable that theTiO2 thin � lms loaded with Pt of 10 nm as the � lm thickness showed 3 times higherphotocatalytic reactivity than the original un-loaded TiO2 thin � lm. The effects ofPt loading on TiO2 semiconductor photocatalysts show that the electrons generatedin the TiO2 semiconductor particles transfer to the Pt particles quickly, and theholes generated in the TiO2 particles are able to diffuse to the TiO2 surface withoutrecombination, respectively. Onishi et al. have already reported the results of thetransient absorption spectra using time resolved FT-IR measurements which showthat the possibility for electron-hole recombination decreases when the Pt-depositedTiO2 powder exists together with O2 or methanol [18, 19]. This phenomenon canbe explained by the reasoning why the photo-generated electrons transfer to Ptimmediately and also Pt catalyzes the reaction of electron with O2 to form O¡

2 ,while the photo-generated holes react with the organic compounds (in this case,the methanol molecules) immediately at the TiO2 surface, resulting in the effectiveprevention of the recombination of electrons and holes, thus dramatically improvingthe photocatalytic reactivity [7].

On the other hand, further loading of Pt (more than 30 nm as the � lm thickness)was found to decrease its photocatalytic reactivity. As can be seen in Fig. 6, thecircle plots determined by the results of XPS measurements show the ratio of Pt atthe surface of these thin � lms. As oxygen accounts for about 50% of the thin-� lmsurface, the ratio of Pt of over 40% means that the surface of the TiO2 thin � lm ismostly covered with Pt. On the other hand, when the amount of Pt deposited wasmuch smaller than 10 nm as the � lm thickness, the coverage ratio of Pt was foundto be less than 15%, indicating that the thin � lm is exposed to TiO2 itself. Theseresults clearly suggest that the photocatalytic performance can be strictly controlledby the nano-scale coverage ratio of Pt on the TiO2 surface.

CONCLUSIONS

The loading of small amounts of Pt on the TiO2 thin � lms was found to maintain thehigh transparency of the original TiO2 thin � lms and yet enhance the photocatalyticperformance dramatically, its extent depending on the amounts of Pt loaded. In thecase of pure TiO2 thin-� lm photocatalysts, some parts of the electrons and holesgenerated by UV light irradiation of TiO2 diffuse to the surface of the catalyst toreact with O2 and CH3CHO, respectively, while other parts combine with each otherto dissipate as heat. In the case of TiO2 thin � lms loaded with small amounts of Pt(less than 10 nm as the � lm thickness), the photo-generated electrons transfer to


the Pt particles loaded on the TiO2 surface quickly and the Pt particles catalyze thereaction of electron with O2 to form O¡

2 . In other words, the Pt particles on theTiO2 surface work as promoters to effectively enhance the charge separation of theelectrons and holes generated by UV light irradiation of the TiO2 semiconductor,resulting in an improvement of the photocatalytic performance.

When large amounts of Pt were loaded on the TiO2 thin � lms (more than 30 nmas the � lm thickness), the Pt oxide thin layer completely covered the surface ofthe TiO2 thin � lm, resulting in the diffusion of the photo-generated electrons to thesurface of the catalyst to react with O2 to form O¡

2 , while the photo-generated holescould not diffuse to the surface and remained in the bulk of TiO2 thin � lms.

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effect of pt loading on the photocatalytic reactivity of titanium oxide thin films prepared by ion...

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