in-situ investigations of the photoluminescence properties ...wavenumber/cm−1 si−o ti−o−si...

Vol. 05 INTERNATIONAL JOURNAL OF PHOTOENERGY 2003

In-situ investigations of the photoluminescenceproperties of SiO2/TiO2 binary and Boron-SiO2/TiO2ternary oxides prepared by the sol-gel method and

their photocatalytic reactivity for the oxidativedecomposition of trichloroethylene

Kyeong Youl Jung,1 Seung Bin Park,1,† Masaya Matsuoka,2

and Masakazu Anpo2,‡

1 Department of Chemical Engineering, Korea Advanced Institute of Science and Technology

373-1, Kusong-dong Yusong-gu, Taejon 305-701, Korea2 Department of Applied Chemistry, Graduate School of Engineering, Osaka Perfecture University

Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan

Abstract. Photoluminescence behavior of TiO2, SiO2/TiO2 binary and Boron-SiO2/TiO2 ternary oxidesprepared by the sol-gel method was investigated. The differences in their photocatalytic reactivities of TiO2-based photocatalysts were interpreted in terms of the relationship of the difference in their photolumines-cence characteristics. The addition of SiO2 into TiO2 matrix induced new photoluminescence sites, whichwere due to anchored titanium oxide species (i.e., the formation of Ti−O−Si bonds) located on the surface.The photoluminescence was found to be very sensitive to the presence of oxygen. These new photolumi-nescence completely disappeared by the addition of boron into SiO2/TiO2 binary oxide, since the emittingsites having a Ti−O−Si bond were destroyed and the new sites having B−O−Ti or Si−O−B bonds wereconstructed on the surface, being in agreement with the results obtained by FT-IR measurements. For allTiO2-based photocatalysts, a significant quenching of photoluminescence was observed by the addition ofoxygen. It was found that the photocatalytic reactivity of TiO2-based photocatalysts for the decompositionof trichloroethylene was clearly associated with their relative quenching efficiencies of photoluminescence;photocatalyst showing high quenching efficiency exhibited a high photocatalytic reactivity.

1. INTRODUCTION

TiO2 nano-particles are the best candidate as a photo-catalyst having high thermal stability and high activityfor the decomposition of various toxic organic mate-rials in aqueous solutions [1–6]. The enhancement ofphotoreactivity of TiO2 photocatalysts has been a bigobjective in the research field of photocatalysis. The fol-lowing factors can be considered to play vital roles incontrolling the photocatalytic reactivity of TiO2; parti-cle size, crystal phase, temperature of heat treatment,surface area, surface-bounded species, pH of solution,and the kinds of additives [2, 4, 7–11]. In recent years,the second metal oxide such as SiO2, AlO3, ZrO2, andWO3 are frequently used as additives to modify the sur-face or bulk properties of TiO2 photocatalysts [12–16].Photocatalytic reactions proceed on the surface of TiO2catalysts, therefore, it is important to know the detailedmorphology of the surface active sites which interactwith photogenerated electrons and/or holes as well asthe reactant molecules.

The measurement of photoluminescence of thephotocatalysts is one of the most important and useful

† E-mail: [email protected]‡ E-mail: [email protected]

ways to elucidate the surface properties related toadsorption, catalysis, and photocatalysis [17]. Forexample, the photoluminescence spectrum of TiO2nano-particles is efficiently quenched by the additionof oxygen onto the surface through an increase in theextent of the band bending of TiO2 photocatalyst dueto the adsorption of O2

− species [18–22]. Thus, it ispossible to monitor the changes in the reactivity of thephotocatalyst in various atmosphere by measuring thephotoluminescence properties of the photocatalysts.

In this work, three kinds of TiO2-based photocata-lysts (TiO2, SiO2/TiO2, Boron-SiO2/TiO2) were preparedby the sol-gel method and their characteristics of pho-toluminescence properties were investigated to eluci-date the factors which control the photoactivity ofthese photocatalysts.

2. EXPERIMENTAL

The conventional sol-gel technique was used to prepareTiO2-based photocatalyst. Titanium ethoxide (TEOT,Aldrich, ∼ 20% Ti in excess ethanol) and Tetraethyl-orthosilicate (TEOS, Aldrich, 98%) were used as TiO2and SiO2 precursor, respectively. Boric acid (Aldrich,99.99%) was used as the precursor of boron. Pure TiO2

32 Kyeong Youl Jung et al. Vol. 05

Table 1. Physicochemical properties of as-prepared TiO2 based oxides.

Catalyst Calcination temperature (K) Crystal phase Dcc) (nm) BET (m2/g)

TiO2 773 Anatase 23.1 45.9

SiO2/TiO2a) 1073 Anatase 14.1 189.9

Boron-SiO2/TiO2b) 1173 Anatase 14.5 33.5

a) Ti/Si = 2.3 (30 at.% of Si),b) Ti/B = 19(5% of Boron) and Ti/Si = 2.3,c) crystallite size calculated by the Scherrer equation at 2θ = 23.3◦.

particles were prepared in an excess water solution(H2O/M+ = 150 mole %, where M+ indicates the totalmole of alkoxide used) containing HCl (H+/M+ = 0.2)and ethyl alcohol (Et−OH/M+ = 1). For SiO2/TiO2binary oxide, TEOS was hydrolyzed in the first stepwith the pre-mixed water solution because the hydrol-ysis rate of TEOS was much slower than that of TEOT.The hydrolyzed precursor solution was mixed for24 hrs at room temperature and for 5 hrs at 348–353 Kto remove the added and produced alcohol. Finally,the sol solution was dried in a dry oven at 373 K for12 hrs and then the obtained xerogel was calcinedat 773 to 973 K. For Boron-SiO2/TiO2 ternary oxide,boric acid was added to the silica/titania sol solutionwhich was prepared by hydrolyzing both TiO2 andSiO2 precursors in the same ways as the preparationof SiO2/TiO2 binary oxide. The percentage of siliconand boron with respect to titanium atom in SiO2/TiO2binary or Boron-SiO2/TiO2 ternary oxides was fixed tobe 30 atm. % and 5%, respectively.

The major crystal phase of all TiO2-based photo-catalysts was determined from the X-ray diffractionpatterns obtained by using a Rigaku D/MAX-III(3 kW)diffractormeter. Surface areas of the prepared Ti-basedparticles were determined by nitrogen physisorptiondata at 77 K using a Micrometritics ASAP 2400. FT-IRspectra of the catalysts were obtained by a BomemMB-100 spectrometer. UV/visible spectra were mea-sured by an UV/visible spectrophotometer (UV-2501PC, Shimadzu).

Semi-circulation batch reactor of annular shape wasused to test the photoreactivity of these TiO2-basedphotocatalysts for the decomposition of trichloroethy-lene (TCE). The photocatalyst used and the initial con-centration of TCE were fixed to be 1 g/l and 37 ppm,respectively. The reaction solution suspending photo-catalyst particles and solving reactant molecules wasirradiated by ultraviolet light (15 W, black light). Thechange in the TCE concentration was monitored by Cl−

electrode (Orion, model 96-17B) as a function of reac-tion time.

The photoluminescence spectra of these photocat-alysts were measured at 77 K using a Shimadzu RF-5000 spectrofluorophotometer. A quarts cell with awindow and furnace section connected to a vacuumsystem (10−6 Torr) was used for the in situ measure-ments of the photoluminescence spectra before and

after various thermal pretreatments. Prior to spec-troscopic measurements, each photocatalyst sample(TiO2, SiO2/TiO2, and 5% Boron-SiO2/TiO2) was evacu-ated at 573 K for 3 hrs then calcined at 473 K with oxy-gen of 200 Torr for 2 hrs followed by the evacuation atthe same temperature.

Inte

nsi

ty/a

.u.

20 30 40 50 60

2θ

SiO2/TiO2

Boron-SiO2/TiO2

TiO2

Figure 1. XRD patterns of TiO2 (calcined at 773 K),

SiO2/TiO2 (calcined at 1073 K), and Boron-SiO2/TiO2(calcined at 1173 K) powder prepared by the sol-gel method.

3. RESULTS AND DISCUSSION

The photocatalytic reactivity of these binary andternary oxide photocatalysts is strongly dependent onthe crystal phase of the moiety of TiO2. Figure 1 showsthe XRD patterns of the TiO2-based photocatalysts pre-pared by the sol-gel method. All samples exhibit a pureanatase phase even after the calcination of SiO2/TiO2binary and Boron-SiO2/TiO2 at 1073 and 1173 K, re-spectively. The physicochemical properties of theseTiO2-based photocatalysts are summarized in Table 1.It should be noted that the crystallite sizes of TiO2which were calculated by the Scherrer equation at 2θ =23.3◦ increase in the order of SiO2/TiO2 (14.1 nm) <Boron-SiO2/TiO2 (14.5 nm) < TiO2(23.1 nm).

For the SiO2/TiO2 and Boron-SiO2/TiO2 catalysts,SiO2 moiety is embedded into the TiO2 moiety to formSiO2/TiO2 binary oxide and may exist as a segregatedamorphous phase of SiO2 which prevents TiO2 moi-ety to form large crystals. In such cases, it is expected

Vol. 05 In-situ investigations of the photoluminescence properties of SiO2/TiO2 binary … 33

Tra

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1.2

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2000 1750 1500 1250 1000 750 500

Wavenumber/cm−1

Si−OTi−O−Si

TricoordinatedBorn

Boron-SiO2/TiO2

SiO2/TiO2

TiO2

SiO2

Figure 2. FT-IR spectra of TiO2, SiO2, SiO2/TiO2, and Boron-

SiO2/TiO2.

that the Ti−O−Si bonds are formed in some part ofthe SiO2/TiO2 matrix. Figure 2 shows the FT-IR spec-tra of TiO2, SiO2/TiO2, and Boron-SiO2/TiO2 catalystsas well as SiO2 as a reference. The peak observed ataround 1095 cm−1 corresponds to the asymmetric vi-bration of a Si−O−Si bond [27, 28]. The peak at around940 cm−1 attributed to the linkage of the Ti−O−Si bondare observed with both SiO2/TiO2 binary and Boron-SiO2/TiO2 ternary oxides, indicating that the Ti−O−Sibonds are formed by mixing SiO2 with TiO2 to formbinary oxides [27]. An intrinsic peak at 1393 cm−1 canbe seen only with Boron-SiO2/TiO2 ternary oxide. Thischaracteristic peak can be associated with the forma-tion of tri-coordinated boron in the SiO2/TiO2 binaryoxide frameworks [29, 30]. These results clearly suggestthat Si−O−Ti bonds are successfully produced by mix-ing TiO2 phase and amorphous SiO2 phase, it resultingin the prevention of the formation of large TiO2 crys-tals, while boron exists in a tri-coordinated state in theSiO2/TiO2 binary oxides.

Figure 3 shows the UV/Vis reflectance spectra ofTiO2, SiO2/TiO2, Boron-SiO2/TiO2 catalysts. TiO2 ex-hibits a typical absorption band due to the bandgaptransition of TiO2 having an anatase phase (λ <390 nm), while the absorption spectra of SiO2/TiO2binary and Boron-SiO2/TiO2 ternary oxides were foundto shift toward shorter wavelength regions. Since theabsorption bands were found to shift toward shorterwavelength regions with a decrease in the TiO2 crystalsize of the catalysts (TiO2(23.1 nm) > Boron-SiO2/TiO2(14.5 nm) > SiO2/TiO2(14.1 nm)), such a spectral shifttoward shorter wavelength can be attributed to the sizequantization effect due to the presence of TiO2 par-ticles of particularly small size of TiO2 moiety in theSiO2/TiO2 binary oxide.

As shown in Figure 4, TiO2-based photocatalystsshows the photoluminescence spectra at around 450–550 nm upon the excitation when excited with light

Ab

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ance

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.

0.0

0.2

0.4

0.6

0.8

1.0

250 300 350 400 500

Wavelength/nm

(a)

(b) (c)

Figure 3. UV/Vis reflectance spectra of SiO2/TiO2 (a),

Boron-SiO2/TiO2 (b) and TiO2 (c).

Inte

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ty/a

.u.

0

10

20

30

40

50

60

70

Wavelength/nm

400 500 600 700 800

(a)

(b)

(c)

Figure 4. Photoluminescence spectra of TiO2 (a), Boron-

SiO2/TiO2 (b) and SiO2/TiO2 (c) measured at 77 K under

a vacuum condition.

having energies larger than the bandgap energy of thecatalyst. These observed photoluminescence spectracan be attributed to the radiative decay processes fromthe photoformed electron and hole pair states at thespecific surface sites. The difference in the photolumi-nescence yields of these catalysts can be ascribed to thedifferences in numbers of the surface sites responsiblefor the photoluminescence or the different efficiencyin the rates of the thermal deactivation process of thephotoformed electron and hole pair states. It is no-table that the SiO2/TiO2 binary oxide calcined at 1073 Kshows a new photoluminescence peak at around 400–480 nm. This peak, however, could not be observed withthe Boron-SiO2/TiO2 ternary oxide or pure TiO2 pow-der. In general, the photoluminescence peak with bulkTiO2 is observed at around 450–550 nm [20, 23]. Thepeak at around 400–480 nm is not due to bulk TiO2 buthighly dispersed TiO2 moiety anchored onto the SiO2surfaces or into zeolite framework [24–26].


Photocatalysis is a surface phenomenon initiated bythe irradiation of UV light of higher energy than thebandgap of the photocatalyst used. It can be seen thatthe surface properties involving the nature of activesites and its numbers play a key role in determiningthe photocatalytic reactivity of the catalysts. It is, there-fore, important to characterize the surface active sitesand their reactivity for various photocatalysts preparedby different preparation methods. The quenching ofthe photoluminescence of the catalyst can be relatedto the changes in the surface properties which resultfrom the interaction between the surface and the elec-tron acceptor molecules. In the present work, therefore,the relative reactivity of photocatalysts toward oxygenmolecule was estimated by the quenching degree of thephotoluminescence by the addition of O2.

For all TiO2-based photocatalysts, the addition of20 Torr of O2 led to a considerable quenching of thephotoluminescence. As shown in Figure 5, in the pres-ence of 20 Torr of O2, the intensity of the photolumi-nescence decreased to 89% of its original intensity forSiO2/TiO2, 37% for TiO2 and 26% for Boron-SiO2/TiO2,respectively. It has been reported that the addition ofO2 at 298 K onto TiO2 photocatalyst leads to the forma-tion of O2

− anion radicals stabilized on Ti4+ sites. Theformation of such negatively charged adducts (O2

−) re-sults in an increase in the surface band bending of theTiO2, leading to a quenching of the photoluminescencethrough a suppression of the efficiency of the radia-tive recombination of the photoformed electrons andholes at the surface [20]. The photoluminescence canbe easily quenched by the addition of O2 in the caseof TiO2-based catalysts with high photocatalytic reac-tivity since an efficient formation of surface O2

− anionradicals which play a important role in oxidation re-action can be expected. In line with these arguments,the reactivity of the photocatalysts toward oxygen wasfound to increase in the order of SiO2/TiO2, TiO2 andBoron-SiO2/TiO2. It is notable that the peak at around400–480 nm, attributed to the photoluminescence fromTiO2 moiety having Ti−O−Si bonds is quenched com-pletely by the addition of oxygen. These results clearlyshow that the peak at around 400–480 nm is attributedto the photoluminescence from the TiO2 moiety havingTi−O−Si bonds and these TiO2 moieties play an impor-tant role as the active sites leading to the formation ofO2

− anion radicals. On the other hand, with the Boron-SiO2/TiO2 ternary oxides, the photoluminescence peakat around 400–480 nm could not be observed. Theseresults showed a good agreement with that the TiO2moieties having Ti−O−Si bonds are not formed on theBoron-SiO2/TiO2 oxides but the TiO2 moieties havingTi−O−B or Si−O−B bonds are formed at the surface.However, the FT-IR peak which could be observed ataround 940 cm−1, as shown in Figure 2, suggest that theTi−O−Si bonds still remain in the bulk of the Boron-SiO2/TiO2 matrix.

Inte

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.u.

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15

20

25

Wavelength/nm

300 400 500 600 700 800

without O2[PL1] with O2-20 Torr

[PL2]

Anchored TiTi−O−Si

PL1−PL2

(c)

Inte

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.u.

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without O2

with O2-20 Torr

(b)

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ty/a

.u.

0

10

20

30

40

50

Wavelength/nm

300 400 500 600 700 800 900

without O2

with O2-20 Torr

(a)

Figure 5. Photoluminescence spectra of Boron-SiO2/TiO2(a), TiO2 (b) and SiO2/TiO2 (c) and measured at 77 K with

and without O2. Added O2 : 20 Torr.

It has been reported that O2− as well as OH radicals

formed on the TiO2 surface under UV irradiation act asimportant oxidants in the photodegradation of organiccompounds in aqueous solution. As mentioned above,the quenching of the photoluminescence in the pres-ence of O2 are closely related to the formation of the

Vol. 05 In-situ investigations of the photoluminescence properties of SiO2/TiO2 binary … 35

0

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40

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80

Qu

ench

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effici

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/%

SiO2/TiO2 TiO2 Boron-SiO2/TiO20.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

activty

Init

ial

rate

/10−6

mol·m

in−1

m−2

Figure 6. Relationship between the quenching efficiency

and the intrinsic photoactivity (TCE decomposition) of TiO2,

SiO2/TiO2, and Boron-SiO2/TiO2.

O2− anion radicals on the TiO2 surface. Figure 6 shows

the relationship between the quenching efficiency ofphotoluminescence and the photocatalytic reactivity ofTiO2-based photocatalysts for the decomposition ofTCE. The quenching efficiency (QE) is defined as follows:

QE(%) = 100∗(PLvacuum − PLO2)/PLvacuum,PLvacuum : Intensity of photoluminescence under

vacuum,

PLO2 : Intensity of photoluminescence in the presence

of 20 Torr of O2.

It is clear that there is a close relationship betweenquenching efficiency and photocatalytic activity, i.e.,the higher the quenching efficiency, the higher is thephotocatalytic activity. Here, the effect of the crys-tal structure on the photocatalytic activity can beexcluded since all of the prepared TiO2-based photo-catalysts have an anatase phase structure. Therefore,it can be considered that O2

− anion radicals are effi-ciently formed on the TiO2-based photocatalyst whichexhibits the high quenching efficiency, leading to thehigh decomposition rate of TCE through the efficientreaction of O2

− anion radicals with TCE. Thus, it isdemonstrated that the photoluminescence investiga-tions of the photocatalysts can be applied to estimatethe activity of the photocatalysts for the various oxida-tive reactions, especially by monitoring the quenchingefficiency of the photoluminescence in the presence ofgaseous oxygen [17].

4. CONCLUSIONS

Three kinds of different photocatalysts such as TiO2,SiO2/TiO2 binary and Boron-SiO2/TiO2 ternary oxideswere prepared by the sol-gel method. The photolumi-nescence properties of these TiO2-based photocatalysts

were investigated and applied to interpret the differ-ence in the photocatalytic reactivity of these three typesof catalysts for the decomposition of TCE.

For all prepared TiO2-based photocatalysts, a con-siderable quenching of photoluminescence was ob-served in the presence of oxygen, which leads to an ef-ficient electron scavenging to form O2

− anion radicalson the surfaces. It was found that the photocatalyticreactivity of TiO2-based photocatalysts for the decom-position of trichloroethylene is related to their relativequenching efficiency of photoluminescence, i.e., photo-catalyst showing larger quenching efficiency in photo-luminescence exhibit higher photocatalytic reactivity.

ACKNOWLEDGMENTS

This work was financially supported by grant No. R01-2000-00344 from the Basic Research Program of the Ko-rea Science & Engineering Foundation and partially byBrain Korea 21 project

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