In-situ investigations of the photoluminescence properties of SiO 2 / TiO 2 binary and Boron-SiO 2 / TiO 2 ternary oxides prepared by the sol-gel method and their photocatalytic reactivity for the oxidative decomposition of trichloroethylene

Photoluminescence behavior of TiO2, SiO2/TiO2 binary and Boron-SiO2/TiO2 ternary oxides prepared by the sol-gel method was investigated. The differences in their photocatalytic reactivities of TiO2based photocatalysts were interpreted in terms of the relationship of the difference in their photoluminescence characteristics. The addition of SiO2 into TiO2 matrix induced new photoluminescence sites, which were 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 photoluminescence completely disappeared by the addition of boron into SiO2/TiO2 binary oxide, since the emitting sites having a Ti−O−Si bond were destroyed and the new sites having B−O−Ti or Si−O−B bonds were constructed on the surface, being in agreement with the results obtained by FT-IR measurements. For all TiO2-based photocatalysts, a significant quenching of photoluminescence was observed by the addition of oxygen. It was found that the photocatalytic reactivity of TiO2-based photocatalysts for the decomposition of trichloroethylene was clearly associated with their relative quenching efficiencies of photoluminescence; photocatalyst showing high quenching efficiency exhibited a high photocatalytic reactivity.


INTRODUCTION
TiO 2 nano-particles are the best candidate as a photocatalyst having high thermal stability and high activity for the decomposition of various toxic organic materials in aqueous solutions [1][2][3][4][5][6].The enhancement of photoreactivity of TiO 2 photocatalysts has been a big objective in the research field of photocatalysis.The following factors can be considered to play vital roles in controlling the photocatalytic reactivity of TiO 2 ; particle size, crystal phase, temperature of heat treatment, surface area, surface-bounded species, pH of solution, and the kinds of additives [2,4,[7][8][9][10][11].In recent years, the second metal oxide such as SiO 2 , AlO 3 , ZrO 2 , and WO 3 are frequently used as additives to modify the surface or bulk properties of TiO 2 photocatalysts [12][13][14][15][16]. Photocatalytic reactions proceed on the surface of TiO 2 catalysts, therefore, it is important to know the detailed morphology of the surface active sites which interact with photogenerated electrons and/or holes as well as the reactant molecules.
The measurement of photoluminescence of the photocatalysts is one of the most important and useful † E-mail: sbpark@lamp.kaist.ac.kr ‡ E-mail: anpo@ok.chem.osakafu-u.ac.jp ways to elucidate the surface properties related to adsorption, catalysis, and photocatalysis [17].For example, the photoluminescence spectrum of TiO 2 nano-particles is efficiently quenched by the addition of oxygen onto the surface through an increase in the extent of the band bending of TiO 2 photocatalyst due to the adsorption of O 2 − species [18][19][20][21][22]. Thus, it is possible to monitor the changes in the reactivity of the photocatalyst in various atmosphere by measuring the photoluminescence properties of the photocatalysts.In this work, three kinds of TiO 2 -based photocatalysts (TiO 2 , SiO 2 /TiO 2 , Boron-SiO 2 /TiO 2 ) were prepared by the sol-gel method and their characteristics of photoluminescence properties were investigated to elucidate the factors which control the photoactivity of these photocatalysts.

EXPERIMENTAL
The conventional sol-gel technique was used to prepare TiO 2 -based photocatalyst.Titanium ethoxide (TEOT, Aldrich, ∼ 20% Ti in excess ethanol) and Tetraethylorthosilicate (TEOS, Aldrich, 98%) were used as TiO 2 and SiO 2 precursor, respectively.Boric acid (Aldrich, 99.99%) was used as the precursor of boron.Pure TiO 2 The major crystal phase of all TiO 2 -based photocatalysts was determined from the X-ray diffraction patterns obtained by using a Rigaku D/MAX-III(3 kW) diffractormeter.Surface areas of the prepared Ti-based particles were determined by nitrogen physisorption data at 77 K using a Micrometritics ASAP 2400.FT-IR spectra of the catalysts were obtained by a Bomem MB-100 spectrometer.UV/visible spectra were measured by an UV/visible spectrophotometer (UV-250 1PC, Shimadzu).
Semi-circulation batch reactor of annular shape was used to test the photoreactivity of these TiO 2 -based photocatalysts for the decomposition of trichloroethylene (TCE).The photocatalyst used and the initial concentration of TCE were fixed to be 1 g/l and 37 ppm, respectively.The reaction solution suspending photocatalyst particles and solving reactant molecules was irradiated by ultraviolet light (15 W, black light).The change in the TCE concentration was monitored by Cl − electrode (Orion, model 96-17B) as a function of reaction time.
The photoluminescence spectra of these photocatalysts were measured at 77 K using a Shimadzu RF-5000 spectrofluorophotometer.A quarts cell with a window and furnace section connected to a vacuum system (10 −6 Torr) was used for the in situ measurements of the photoluminescence spectra before and after various thermal pretreatments.Prior to spectroscopic measurements, each photocatalyst sample (TiO 2 , SiO 2 /TiO 2 , and 5% Boron-SiO 2 /TiO 2 ) was evacuated at 573 K for 3 hrs then calcined at 473 K with oxygen of 200 Torr for 2 hrs followed by the evacuation at the same temperature. Intensity/a.u.

RESULTS AND DISCUSSION
The photocatalytic reactivity of these binary and ternary oxide photocatalysts is strongly dependent on the crystal phase of the moiety of TiO 2 .Figure 1 shows the XRD patterns of the TiO 2 -based photocatalysts prepared by the sol-gel method.All samples exhibit a pure anatase phase even after the calcination of SiO  having energies larger than the bandgap energy of the catalyst.These observed photoluminescence spectra can be attributed to the radiative decay processes from the photoformed electron and hole pair states at the specific surface sites.The difference in the photoluminescence yields of these catalysts can be ascribed to the differences in numbers of the surface sites responsible for the photoluminescence or the different efficiency in the rates of the thermal deactivation process of the photoformed electron and hole pair states.It is notable that the SiO 2 /TiO 2 binary oxide calcined at 1073 K shows a new photoluminescence peak at around 400-480 nm.This peak, however, could not be observed with the Boron-SiO 2 /TiO 2 ternary oxide or pure TiO 2 powder.In general, the photoluminescence peak with bulk TiO 2 is observed at around 450-550 nm [20,23].The peak at around 400-480 nm is not due to bulk TiO 2 but highly dispersed TiO 2 moiety anchored onto the SiO 2 surfaces or into zeolite framework [24][25][26].
Photocatalysis is a surface phenomenon initiated by the irradiation of UV light of higher energy than the bandgap of the photocatalyst used.It can be seen that the surface properties involving the nature of active sites and its numbers play a key role in determining the photocatalytic reactivity of the catalysts.It is, therefore, important to characterize the surface active sites and their reactivity for various photocatalysts prepared by different preparation methods.The quenching of the photoluminescence of the catalyst can be related to the changes in the surface properties which result from the interaction between the surface and the electron acceptor molecules.In the present work, therefore, the relative reactivity of photocatalysts toward oxygen molecule was estimated by the quenching degree of the photoluminescence by the addition of O 2 .
For all TiO 2 -based photocatalysts, the addition of 20 Torr of O 2 led to a considerable quenching of the photoluminescence.As shown in Figure 5, in the presence of 20 Torr of O 2 , the intensity of the photoluminescence decreased to 89% of its original intensity for SiO 2 /TiO 2 , 37% for TiO 2 and 26% for Boron-SiO 2 /TiO 2 , respectively.It has been reported that the addition of O  It is clear that there is a close relationship between quenching efficiency and photocatalytic activity, i.e., the higher the quenching efficiency, the higher is the photocatalytic activity.Here, the effect of the crystal structure on the photocatalytic activity can be excluded since all of the prepared TiO 2 -based photocatalysts have an anatase phase structure.Therefore, it can be considered that O 2 − anion radicals are efficiently formed on the TiO 2 -based photocatalyst which exhibits the high quenching efficiency, leading to the high decomposition rate of TCE through the efficient reaction of O 2 − anion radicals with TCE.Thus, it is demonstrated that the photoluminescence investigations of the photocatalysts can be applied to estimate the activity of the photocatalysts for the various oxidative reactions, especially by monitoring the quenching efficiency of the photoluminescence in the presence of gaseous oxygen [17].

CONCLUSIONS
Three kinds of different photocatalysts such as TiO 2 , SiO 2 /TiO 2 binary and Boron-SiO 2 /TiO 2 ternary oxides were prepared by the sol-gel method.The photoluminescence properties of these TiO 2 -based photocatalysts were investigated and applied to interpret the difference in the photocatalytic reactivity of these three types of catalysts for the decomposition of TCE.For all prepared TiO 2 -based photocatalysts, a considerable quenching of photoluminescence was observed in the presence of oxygen, which leads to an efficient electron scavenging to form O 2 − anion radicals on the surfaces.It was found that the photocatalytic reactivity of TiO 2 -based photocatalysts for the decomposition of trichloroethylene is related to their relative quenching efficiency of photoluminescence, i.e., photocatalyst showing larger quenching efficiency in photoluminescence exhibit higher photocatalytic reactivity.

Figure 5 .
Photocatalysis is a surface phenomenon initiated by the irradiation of UV light of higher energy than the bandgap of the photocatalyst used.It can be seen that the surface properties involving the nature of active sites and its numbers play a key role in determining the photocatalytic reactivity of the catalysts.It is, therefore, important to characterize the surface active sites and their reactivity for various photocatalysts prepared by different preparation methods.The quenching of the photoluminescence of the catalyst can be related to the changes in the surface properties which result from the interaction between the surface and the electron acceptor molecules.In the present work, therefore, the relative reactivity of photocatalysts toward oxygen molecule was estimated by the quenching degree of the photoluminescence by the addition of O 2 .For all TiO 2 -based photocatalysts, the addition of 20 Torr of O 2 led to a considerable quenching of the photoluminescence.As shown in Figure5, in the presence of 20 Torr of O 2 , the intensity of the photoluminescence decreased to 89% of its original intensity for SiO 2 /TiO 2 , 37% for TiO 2 and 26% for Boron-SiO 2 /TiO 2 , respectively.It has been reported that the addition of O 2 at 298 K onto TiO 2 photocatalyst leads to the formation of O 2 − anion radicals stabilized on Ti 4+ sites.The formation of such negatively charged adducts (O 2 − ) results in an increase in the surface band bending of the TiO 2 , leading to a quenching of the photoluminescence through a suppression of the efficiency of the radiative recombination of the photoformed electrons and holes at the surface [20].The photoluminescence can be easily quenched by the addition of O 2 in the case of TiO 2 -based catalysts with high photocatalytic reactivity since an efficient formation of surface O 2 − anion radicals which play a important role in oxidation reaction can be expected.In line with these arguments, the reactivity of the photocatalysts toward oxygen was found to increase in the order of SiO 2 /TiO 2 , TiO 2 and Boron-SiO 2 /TiO 2 .It is notable that the peak at around 400-480 nm, attributed to the photoluminescence from TiO 2 moiety having Ti−O−Si bonds is quenched completely by the addition of oxygen.These results clearly show that the peak at around 400-480 nm is attributed to the photoluminescence from the TiO 2 moiety having Ti−O−Si bonds and these TiO 2 moieties play an important role as the active sites leading to the formation of O 2 − anion radicals.On the other hand, with the Boron-SiO 2 /TiO 2 ternary oxides, the photoluminescence peak at around 400-480 nm could not be observed.These results showed a good agreement with that the TiO 2 moieties having Ti−O−Si bonds are not formed on the Boron-SiO 2 /TiO 2 oxides but the TiO 2 moieties having Ti−O−B or Si−O−B bonds are formed at the surface.However, the FT-IR peak which could be observed at around 940 cm −1 , as shown in Figure 2, suggest that the Ti−O−Si bonds still remain in the bulk of the Boron-SiO 2 /TiO 2 matrix.

Figure 6 .
Figure 6.Relationship between the quenching efficiency and the intrinsic photoactivity (TCE decomposition) of TiO 2 , SiO 2 /TiO 2 , and Boron-SiO 2 /TiO 2 .O 2 − anion radicals on the TiO 2 surface.Figure 6 shows the relationship between the quenching efficiency of photoluminescence and the photocatalytic reactivity of TiO 2 -based photocatalysts for the decomposition of TCE.The quenching efficiency (QE) is defined as follows: QE(%) = 100 * (PL vacuum − PL O2 )/PL vacuum , PL vacuum : Intensity of photoluminescence under vacuum, PL O2 : Intensity of photoluminescence in the presence of 20 Torr of O 2 .

Table 1 .
Physicochemical properties of as-prepared TiO 2 based oxides.
2 /TiO 2 binary and Boron-SiO 2 /TiO 2 at 1073 and 1173 K, respectively.The physicochemical properties of these TiO 2 -based photocatalysts are summarized in Table1.It should be noted that the crystallite sizes of TiO 2 which were calculated by the Scherrer equation at 2θ = 23.3•increase in the order of SiO 2 /TiO 2 (14.1 nm) < Boron-SiO 2 /TiO 2 (14.5 nm) < TiO 2 (23.1 nm).For the SiO 2 /TiO 2 and Boron-SiO 2 /TiO 2 catalysts, SiO 2 moiety is embedded into the TiO 2 moiety to form SiO 2 /TiO 2 binary oxide and may exist as a segregated amorphous phase of SiO 2 which prevents TiO 2 moiety to form large crystals.In such cases, it is expected