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One-electron oxidation of aromatic sulfides adsorbed on the surface of TiO2 particles studied by time-resolved diffuse reflectance spectroscopy
Chemical Physics Letters 382 (2003) 618–625 www.elsevier.com/locate/cplett
One-electron oxidation of aromatic sulfides adsorbed on the surface of TiO2 particles studied by time-resolved diffuse reflectance spectroscopy Takashi Tachikawa, Sachiko Tojo, Mamoru Fujitsuka, Tetsuro Majima
*
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 565-0047, Japan Received 22 July 2003; in final form 13 September 2003 Published online:
Abstracts One-electron oxidation of 4-methyl-p-tolylsulfide (MTS) and 4-methylthiophenylmethanol (MTPM) adsorbed on the surface of TiO2 powder slurried in acetonitrile has been investigated by the time-resolved diffuse reflectance spectroscopy. Relatively high concentration of the adsorbed MTPM determined by UV absorption spectral measurements clearly indicates that the –OH group of MTPM plays an important role in adsorbing on the surface of TiO2 particles. The initial concentration of the radical cations of substrates generated from one-electron oxidation with valence band holes or trapped hydroxyl radical significantly depended on the substrate concentration adsorbed on the TiO2 surface. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction The photo-oxidation of organic compounds on TiO2 particle surfaces has been examined extensively and is being applied widely to decompose a variety of environmentally harmful organic compounds and toxic compounds [1–5]. Typically, the processes are initiated by the band-gap excitation of the TiO2 particles with ultraviolet irradiation to generate OH radicals derived from the oxidation of OH from water adsorbed on the particle sur-
*
Corresponding author. Fax: +81-6-6879-8499. E-mail address: [email protected] (T. Majima).
face. These OH radicals have been considered to play an important role in control the overall kinetics of the oxidative process. Although a great deal of research has been conducted on the oxidation reactions between the OH radicals and variable substrates, little attention has been focused on the origin of the OH radicals and the subsequent reactivity of the transient species concerned with the oxidation processes. The timeresolved diffuse reflectance method is a powerful tool for investigations of photocatalysis in various conditions [6–11]. TiO2 nanoparticles are only soluble over a limited pH range and generally soluble only in the solvent in which they are synthesized. In a nonaqueous medium, it is expected
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.10.110
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
that the TiO2 particle surfaces are main reaction fields of the direct oxidation. A time-resolved diffuse reflectance spectroscopy has been used to study the interfacial electron transfer between molecular adsorbate and wet TiO2 powders. Fox and co-workers concluded that many oxidation reactions appear to occur by the direct electron transfer from a variety of substrates to the photoexcited TiO2 powder in acetonitrile [7,8]. However, only a few quantitative studies have been reported on the oxidation processes of organic compounds on the TiO2 particle surface and the interactions between the substrate and TiO2 surface. The adsorption and desorption processes for aromatic compounds are also unclear. In the present Letter, we have investigated the one-electron oxidation kinetics of 4-methyl-ptolylsulfide (MTS) and 4-methylthiophenylmethanol (MTPM) during the photoexcitation of the TiO2 powder slurried in acetonitrile using time-resolved diffuse reflectance spectroscopy. Photocatalytic degradation of aromatic sulfides is among possible methods for destruction of chemical warfare agents [12]. We discussed the relationship between the substrate concentrations adsorbed on the TiO2 surface and radical cations of substrates generated by the laser flash photolysis of TiO2 powder.
2. Experimental and analysis 2.1. Materials Titanium dioxide (TiO2 ) powder (P25, Japan Aerosil) was a mixture of rutile (20%) and anatase (80%) with a BET surface area of 50 m2 g1 , average primary particle size of 21 nm, and an isoelectric point at pH 6.6. MTS (Tokyo Kasei) was SCH3
SCH3
CH3
CH2 OH
619
used without further purification and MTPM (Aldrich) was purified by vacuum sublimation before use. The structures of MTS and MTPM are shown in Fig. 1. Fresh acetonitrile (CH3 CN, spectral grade) was used as the solvent without further purification. 2.2. Steady-state UV absorption measurements Steady-state absorption spectra were measured by UV–VIS–NIR spectrophotometer (Shimadzu, UV-3100) at room temperature. The sample solutions containing TiO2 powder (20 g dm3 ) were sonicated for 10 min and TiO2 particles in solution were then completely removed by the centrifugation (10,000 rpm, 10 min) for UV absorption measurements. All the sample preparations were performed with shielding the UV light. 2.3. Time-resolved diffuse reflectance measurements Time-resolved diffuse reflectance measurements were performed using the third harmonic generation (355 nm, 5 ns FWHM) from a Q-switched Nd:YAG laser (Continuum, Surelite II-10) for the excitation operated with a temporal control by a delay generator (Stanford Research Systems, DG535). In the experiments, the spot irradiated on the sample cell with a thickness of 2 mm was approximately 1 cm2 . The analyzing light from a pulsed 450 W Xe-arc lamp (Osram, XBO-450) was collected by a focusing lens and directed through a grating monochromator (Nikon, G250) to a photomultiplier tube (Hamamatsu, Photonics R928). The transient signals were recorded with a digitizer (Tektronix, TDS 580D). TiO2 powder was suspended in CH3 CN by sonicating 40 mg of TiO2 in 2 103 dm3 of solvent for 5 min. The suspension was maintained at room temperature, and a new aliquot of the suspension was used after approximately 128 laser pulses. 2.4. Kinetic analysis based on Albery model
4-methyl-p-tolylsulfide (MTS)
4-methylthiophenylmethanol (MTPM)
Fig. 1. Structures of MTS and MTPM.
The percent absorption (100 DJ ) was calculated by the following equation: DJ ¼ 1 J =J0 ;
ð1Þ
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
where c is the parameter describing the width of the distribution of the average deactivation rate constant kd and x is the distribution variable, being allowed to take values between plus and minus infinity [13]. The basic concept of this treatment is the use of a Gaussian distribution of the free energy change as a suitable description of the heterogeneity of the system. The observed decay profile is a sum of the different contributions to the decay. When DJ and DJ0 are proportional to the transient concentration and initial concentration, respectively, and when x is integrated over a Gaussian distribution, one obtains Z þ1 DJ expðx2 Þ exp½kd t expðcxÞdx : ð3Þ ¼ DJ0 expðx2 Þdx 1 In the limit of c ¼ 0, this expression reduces to an ordinary first-order rate law. Since the integral has no analytical solution, this model has been successfully applied to heterogeneous powders and powder suspensions [7–9,13,14].
The concentrations of the adsorbates in CH3 CN containing TiO2 powder (20 g dm3 ) after reaching the adsorption equilibrium ([S]eq ) were determined from the absorbance observed by UV absorption experiments. For example, Fig. 2A depicted the UV absorption spectra observed for MTS and MTPM in CH3 CN at room temperature. The absorbance of absorption spectrum of MTPM ([MTPM] ¼ 2 103 M, M mol dm3 ) in CH3 CN after reaching the adsorption equilibrium (d) indicated the reduction of 9%, compared to that of MTPM in CH3 CN (b). This result clearly suggests the adsorption of MTPM molecules on TiO2 particles in solution. On the other hand, in the case of MTS ((a) and (c)), the reduction of absorbance was relatively small. The results indicate that the OH group plays an important role in adsorbing on the surface of TiO2
3
3 MTS
Absorbance
ð2Þ
3.1. Langmuir adsorption isotherm
2
1
(d)
0 240
280
320
240
280
320
Wavelength / nm
(A) 0.5
2.5. Cyclic voltammetry measurements
0.4
MTPM
0.3 0.2 MTS
0.1 0.0 0
Cyclic voltammograms were obtained by using a conventional three-electrode system (BAS CV50W) in CH3 CN solution at 298 K. A platinum electrode was used as the working electrode and an Ag/AgNO3 electrode was used as the reference electrode.
2
(c)
1
MTPM
(b)
(a)
0
-1
kobs ¼ kd expðcxÞ;
3. Results and discussion
-4
where J0 and J are the intensities of the diffuse reflected light without and with the excitation laser light, respectively [7–9]. The DJ values obtained in the diffuse reflectance experiments have no convenient connection with the transient concentrations and absorptivity in contrast to the transmission transient absorption experiments as presented by BeerÕs law, and no standard aqueous and nonaqueous suspensions are available for a diffuse reflector. However, the signal intensity was found to be proportional to transient concentrations at a low laser power [7]. The distribution of the particle size was estimated by the Albery model and the dispersion in the observed first-order rate constants (kobs ) is given by
nad / 10 mol g
620
(B)
2
4
6
8
10
[S]eq / mM
Fig. 2. Steady-state UV absorption spectra of MTS (2 mM) (a) and MTPM (2 mM) (b) in CH3 CN, and MTS (c) and MTPM (d) after reaching the adsorption equilibrium in CH3 CN (A). Langmuir type plots for MTS and MTPM in CH3 CN (5 103 dm3 ) containing TiO2 powder (100 mg) (B).
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
particles in CH3 CN. The determined concentrations are summarized in Table 1. Fig. 2B shows the Langmuir-type adsorption isotherms of MTS and MTPM on the TiO2 surface, where the amount of equilibrium adsorption, nad , in the number of adsorbed substrates per gram of TiO2 was plotted as a function of the equilibrium concentration in the bulk solution, [S]eq . The nad values were calculated from the difference in the absorbance observed for solutions with and without TiO2 powder. According to the Langmuir adsorption model for the solid–liquid interface, nad can be expressed by the following equation, nad ¼
Kad ½Seq ns ; 1 þ Kad ½Seq
ð4Þ
where ns is the total amount of adsorption sites and Kad is the adsorption constants defined as Kad ¼ kad =kad , where kad and kad are adsorption and desorption rate constants, respectively [15–17]. We obtained the Kad values of 20 and 60 M1 for MTS and MTPM adsorbed on the surface of TiO2 in CH3 CN, respectively. The Kad value determined for MTPM is three times larger than that for MTS, indicating that MTPM molecules bind to the TiO2 surface by –OH group. The determined Kad values, however, were considerably small, compared to those of some molecules, e.g.
621
6000 M1 for anthracene-9-carboxylic acid, chemically bonded on the TiO2 surface [17]. It is possible to estimate the total amount of adsorption sites (ns ) on the surface of TiO2 from the Langmuir plot as shown in Fig. 2B. We obtained the ns of 1.2 104 mol g1 for MTPM adsorbed on the TiO2 surface. If the adsorbed MTPM is approximated as a cylinder with a radius of 0.3 nm, derived from a space-filling model, the binding area of adsorbed MTPM is calculated to be approximately 20 m2 g1 . Thus, we estimated the occupied sites of 40%, since the TiO2 (P25) particle has the surface area of 50 m2 g1 . The binding area of MTPM adsorbed on the surface of TiO2 estimated from ns was actually lower than the BET surface area, suggesting that the TiO2 particles aggregate due to its high concentration. 3.2. Time-resolved diffuse reflectance absorption It is well established that, under the band gap excitation, TiO2 particles can initiate the oxidation and reduction processes of the adsorbed substrate. In the present study, TiO2 was used as a reactive surface for the one-electron oxidation of aromatic sulfides. In TiO2 particles, excitation with light energy greater than its band gap (3.2 eV) generates electron–hole pairs (Eq. (5)),
Table 1 Initial concentration ([S]) dependences of the equilibrium concentration ([S]eq ), the amount of adsorbed substrate (nad ), DJ0 , and kinetic parameters as given by Eq. (3) for MTS and MTPM [MTS] (mM)
½MTSeq (mM)
nMTS (104 mol g1 ) ad
100J0
kd (106 s1 )
c
0.5 1 2 5 10
0.49 0.97 1.9 4.8 9.5
0.01 0.02 0.04 0.12 0.24
– – 0.17 0.34 0.48
– – 0.22 0.24 0.20
– – 1.0 1.0 0.8
[MTPM] (mM)
½MTPMeq (mM)
nMTPM (104 mol g1 ) ad
100DJ0
kd (106 s1 )
c
0.25 0.5 1 2 5 7.5 10
0.22 0.45 0.88 1.8 4.5 6.8 9.2
0.02 0.03 0.06 0.12 0.23 0.33 0.41
– 0.25 0.50 1.0 1.8 2.0 2.65
– 0.6 0.8 1.0 1.3 1.5 1.7
– 1.0 1.2 2.0 2.3 2.3 2.7
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
Conduction band electrons are trapped at Ti4þ centers of the colloid within sub or a few picoseconds after excitation and generate Ti3þ species and holes [18–20]. These photogenerated carriers migrate to the particle surface and participate in redox processes at the surface. Rothenberger et al. observed the broad absorption spectra with maxima around 620 nm using pico- and nanosecond transient absorption experiments for the colloidal particles of TiO2 in aqueous solution and assigned this absorption band to trapped electrons [18]. In the present system, we observed the similar absorption band (DJ0 0:01 at 545 nm) in the absence of substrates. As assumed that the long-lived transient species is uninvolved in the one-electron oxidation process of substrates, we can subtract the DJ values observed in the absence of substrates (DJtr ) from that in the presence of substrates (DJobs ) as follows DJ ¼ DJobs DJtr :
3
ð5Þ
ð6Þ
Transient diffuse reflectance absorption spectra were measured for MTPM in TiO2 slurried CH3 CN solution. Fig. 3A shows the transient diffuse reflectance absorption spectra obtained during the laser photolysis of TiO2 with 355 nm light (1.5 mJ pulse1 ) in the presence of MTPM (1 102 mol dm3 ) in CH3 CN at room temperature. The transient absorption band at 500–700 nm was appeared immediately after a laser flash and assigned to MTPM radical cation (MTPMþ ), although the spectral shape was approximately 1.5 times broader than in the absence of TiO2 [21]. According to their chemical nature, the trapped charges react with various dissolved and adsorbed components. The trapped positive hole, like surface bound OH radical, typically oxidizes an organic molecule and thus induces its further oxidative degradation while the trapped electron, like surface bound Ti3þ , typically reduces dioxygen to superoxide radical anion. The surface-trapped hole is chemically equivalent to a surface-bound OH radical, which can initiate primary oxidation reaction of substrates adsorbed on the TiO2 surface before diffusing into the bulk solution. The
100 ∆J
hm
þ TiO2 ! TiO2 ðe cb þ hvb Þ:
a b c d
2
1
0 450
500
550
600
650
700
Wavelength / nm
(A) 3
100 ∆J at 545 nm
622
2
b
1 a
0 0
(B)
1
2
3
4
5
6
Time / µs
Fig. 3. Transient diffuse reflectance absorption spectra attributed to MTPMþ observed at 50 (a), 100 (b), 300 (c), and 1000 (d) ns after a laser flash during the 355-nm laser photolysis of TiO2 powder in the presence of MTPM in CH3 CN (A). DJ at 545 nm vs. time after a laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of MTS (10 mM) (a) and MTPM (10 mM) (b) in CH3 CN. Solid lines indicate simulation results fitted by the Albery model, kd ¼ 0:2 106 s1 , c ¼ 0:8 and kd ¼ 1:7 106 s1 , c ¼ 2:7 for MTS and MTPM, respectively (B).
MTPMþ transient absorption signal was generated immediately after the excitation pulse, clearly suggesting that the one-electron oxidation of MTPM occurred by the valence band hole (Eq. (7)) or surface-bound OH radicals (Eqs. (8) and (9)) at the surface of TiO2 powder in CH3 CN solution within a laser pulse duration, þ TiO2 ðhþ vb Þ þ MTPM ! MTPM ;
ð7Þ
TiO2 ðhþ vb Þ þ OHsurf ! OHsurf ;
ð8Þ
OHsurf þ MTPM ! MTPMþ þ OH :
ð9Þ
Fig. 3B shows the decay of the 545-nm absorption obtained by the laser photolysis of TiO2 with 355 nm light (1.5 mJ pulse1 ) in the presence of MTS (1 102 mol dm3 ) and MTPM (1 102
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
mol dm3 ) in CH3 CN. The absorption at 545 nm decays according to the Albery model with kd ¼ 0:2 106 s1 , c ¼ 0:8 and kd ¼ 1:7 106 s1 , c ¼ 2:7 for MTS ([MTS] ¼ 10 mM) and MTPM ([MTPM] ¼ 10 mM), respectively. Remarkably small DJ0 and slow charge recombination rate obtained for MTSþ in spite of the relatively large molar extinction coefficient (e550 (MTSþ )/e545 (MTPMþ ) ¼ 1.2) and low oxidation potential (1.57 and 1.59 V vs. NHE for MTS and MTPM, respectively) [21–23]. Significant substrate concentration dependences of the signal intensity and the decay kinetics were observed for MTPM in the TiO2 slurry. Fig. 4A depicted the transient signals observed for the MTPM concentrations of 1 (a), 5 (b), and 10 mM (c). The intensities of transient signals obtained at 545 nm remarkably increases with increasing in the MTPM concentrations. Fig. 4B indicates the plots of the DJ0 values listed in Table 1 against the MTPM concentration. The significant increase of
100 ∆J at 545 nm
3 c b a
2
1 0 0
1
2
3
4
5
6
Time / µs
(A) 4
100 ∆J0
3 2
623
DJ0 , particularly, was observed at low concentrations, suggesting that the DJ0 values depend on the concentration of MTPM adsorbed on the TiO2 surface as discussed below. We also observed the remarkable acceleration in the decay kinetics (kd ) of MTPMþ generated from the one-electron oxidation and the increase of the heterogeneously parameter c at high concentrations as listed in Table 1. There is worth to clarify the sequential reactions caused by Sþ , because the efficiency of photocatalytic reaction would be significantly dependent on the interfacial charge recombination rate which competes with many others. The charge recombination reaction between the trapped electrons and MTPMþ was influenced by numerous factors, for example, a relaxation time from shallow to deep trap states, a redox potential of substrates, a distance between the electron acceptor and donor [23,24]. In the present system, the signal intensity of MTPMþ increased with increasing laser intensity, while kd remained relatively constant, suggesting that disappearance of MTPMþ on the surface of TiO2 powder suspended in CH3 CN followed first-order kinetics with a distribution of reaction rate constants. These results are well consistent with that obtained for (SCN) 2 on the surface of TiO2 powder suspended in water reported by Fox et al. [7]. We also observed the almost same signal traces for the oxygen and argon gas saturated sample. These results clearly suggest the deactivation of MTPMþ is mainly attributable to the bimolecular reaction with adsorbates, although the reaction schemes are unclear. It seems that remarkable c values observed at the high MTPM concentrations are attributable to the differences in the reactivity of MTPM adsorbed on the heterogeneous surface of TiO2 powder. 3.3. Effects of adsorption on the one-electron oxidation of MTS and MTPM
1 0 0
(B)
5
10
15
20
[MTPM] / mM
Fig. 4. DJ at 545 nm vs. time after a laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of MTPM of 1 (a), 5 (b) and 10 (c) mM (A). MTPM concentration dependence of DJ0 (B).
In the case of the present experiments, it is possible to extract the relative change of the MTPMþ concentration from the initial signal intensity due to the fact that the signal intensity was found to be approximately proportional to transient concentrations, although we do not know the
624
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
3
100 ∆J0
MTPM
2
1
0 0.0
MTS
0.1
0.2
0.3 -4
0.4 -1
nad / 10 mol g
Fig. 5. The relationship between the DJ0 at 545 nm and the amount of absorbed substrates (nad ). The lines indicate guides for eyes.
absolute concentration of MTPMþ . Fig. 5 shows the relationship between nad and DJ0 . The DJ0 values were found to be approximately proportional to the nad values in the present concentration range for both systems. As an electron transfer reaction rate (ket ) from the substrate to holes competes with the fast recombination reaction rate (kr ), the observed quantum yield of radical cation generated by the electron transfer reaction (/et ) is tentatively given by /et ¼
ket nad : kr þ ket nad
ð10Þ
As kr > ket nad , the /et values would be approximately proportional to the nad values. On the other hand, the efficiency of one-electron oxidation, that is DJ0 =nad , is MTPMMTS. For example, at nad 0:12 104 mol g1 , the DJ0 value obtained for MTPM is about three times larger than that for MTS. These results suggest that interaction between MTPM and TiO2 surface, such as a hydrogen bonding, plays an important role in the oxidation kinetics. It seems that such interaction induces the large electronic coupling between the electron acceptor and donor, resulting in an acceleration of the electron transfer reaction rate.
4. Conclusions We obtained the Kad values of 20 and 60 M1 for MTS and MTPM adsorbed on the TiO2 sur-
face in CH3 CN from the Langmuir type model, respectively. The result suggests that the –OH group of MTPM plays an important role in adsorbing to the surface of TiO2 particles. The oneelectron oxidation of MTS and MTPM by photoexcited TiO2 was observed by the timeresolved diffuse reflectance spectroscopy. The decay kinetics of MTSþ and MTPMþ in CH3 CN was interpreted by the Albery model for the dispersed first-order kinetics. The almost linear relationship between nad and DJ0 was obtained for MTS and MTPM, clearly indicating that the efficiency of the one-electron oxidation mainly depends on the amount of adsorbate on the TiO2 surface. The relatively high efficiency of one-electron oxidation also was obtained for MTPM, suggesting that the interactions between the substrates and the TiO2 surface is a key factor in the one-electron oxidation reaction on the TiO2 surface.
Acknowledgements This work has been partly supported by a grant-in-aid for Scientific Research on Priority Area (417), 21st Century COE Research, and others from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese Government.
References [1] A. Fujishima, K. Hashimoto, T. Watanabe, TiO2 Photocatalysis: Fundamentals and Applications, BKC, Tokyo, 1999. [2] P.V. Kamat, Chem. Rev. 93 (1993) 267. [3] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341. [4] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemannt, Chem. Rev. 95 (1995) 69. [5] A. Mills, S.L. Hunte, J. Photochem. Photobiol. A 108 (1997) 1. [6] R.W. Kessler, F.J. Wilkinson, Chem. Soc., Faraday Trans. 1 (77) (1981) 309. [7] R.B. Draper, M.A. Fox, J. Phys. Chem. 94 (1990) 4628. [8] R.B. Draper, M.A. Fox, Langmuir 6 (1990) 1396. [9] M.A. Fox, M.T. Dulay, J. Photochem. Photobiol. A: Chem. 98 (1996) 91. [10] D.P. Colombo Jr., R.M. Bowman, J. Phys. Chem. 100 (1996) 18445.
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625 [11] A. Furube, T. Asahi, H. Masuhara, H. Yamashita, M. Anpo, Res. Chem. Intermed. 27 (2001) 177. [12] A.V. Vorontsov, A.A. Panchenko, E.N. Savinov, C. Lion, P.G. Smirniotis, Environ. Sci. Technol. 36 (2002) 5261. [13] W.J. Albery, P.N. Bartlett, C.P. Wilde, J.R. Darwent, J. Am. Chem. Soc. 107 (1985) 1854. [14] J.K. Thomas, Chem. Rev. 93 (1993) 301. [15] J. Moser, S. Punchihewa, P.P. Infelta, M. Gr€atzel, Langmuir 7 (1991) 3012. [16] Y. Xu, C.H. Langford, Langmuir 17 (2001) 897. [17] P.V. Kamat, J. Phys. Chem. 93 (1989) 859. [18] G. Rothenberger, J. Moser, M. Gr€atzel, N. Serpone, D.K. Sharma, J. Am. Chem. Soc. 107 (1985) 8054.
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[19] N. Serpone, D. Lawless, R. Khairutdinov, E. Pelizzetti, J. Phys. Chem. 99 (1995) 16655. [20] R.D. Miller, G.L. McLendon, A.J. Nozik, W. Schmickler, F. Willig (Eds.), Surface Electron Transfer Processes, VCH, New York, 1995. [21] H. Mohan, J.P. Mittal, Bull. Chem. Soc. Jpn. 74 (2001) 1649. [22] M. Ioele, S. Steenken, E. Baciocchi, J. Phys. Chem. A 101 (1997) 2979. [23] Y.-X. Weng, Y.-Q. Wang, J.B. Asbury, H.N. Ghosh, T. Lian, J. Phys. Chem. B 104 (2000) 93. [24] J.N. Clifford, G. Yahioglu, L.R. Milgrom, J.R. Durrant, Chem. Comm. (2002) 1260.
One-electron oxidation of aromatic sulfides adsorbed on the surface of TiO2 particles studied by time-resolved diffuse reflectance spectroscopy Takashi Tachikawa, Sachiko Tojo, Mamoru Fujitsuka, Tetsuro Majima
*
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 565-0047, Japan Received 22 July 2003; in final form 13 September 2003 Published online:
Abstracts One-electron oxidation of 4-methyl-p-tolylsulfide (MTS) and 4-methylthiophenylmethanol (MTPM) adsorbed on the surface of TiO2 powder slurried in acetonitrile has been investigated by the time-resolved diffuse reflectance spectroscopy. Relatively high concentration of the adsorbed MTPM determined by UV absorption spectral measurements clearly indicates that the –OH group of MTPM plays an important role in adsorbing on the surface of TiO2 particles. The initial concentration of the radical cations of substrates generated from one-electron oxidation with valence band holes or trapped hydroxyl radical significantly depended on the substrate concentration adsorbed on the TiO2 surface. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction The photo-oxidation of organic compounds on TiO2 particle surfaces has been examined extensively and is being applied widely to decompose a variety of environmentally harmful organic compounds and toxic compounds [1–5]. Typically, the processes are initiated by the band-gap excitation of the TiO2 particles with ultraviolet irradiation to generate OH radicals derived from the oxidation of OH from water adsorbed on the particle sur-
*
Corresponding author. Fax: +81-6-6879-8499. E-mail address: [email protected] (T. Majima).
face. These OH radicals have been considered to play an important role in control the overall kinetics of the oxidative process. Although a great deal of research has been conducted on the oxidation reactions between the OH radicals and variable substrates, little attention has been focused on the origin of the OH radicals and the subsequent reactivity of the transient species concerned with the oxidation processes. The timeresolved diffuse reflectance method is a powerful tool for investigations of photocatalysis in various conditions [6–11]. TiO2 nanoparticles are only soluble over a limited pH range and generally soluble only in the solvent in which they are synthesized. In a nonaqueous medium, it is expected
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.10.110
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
that the TiO2 particle surfaces are main reaction fields of the direct oxidation. A time-resolved diffuse reflectance spectroscopy has been used to study the interfacial electron transfer between molecular adsorbate and wet TiO2 powders. Fox and co-workers concluded that many oxidation reactions appear to occur by the direct electron transfer from a variety of substrates to the photoexcited TiO2 powder in acetonitrile [7,8]. However, only a few quantitative studies have been reported on the oxidation processes of organic compounds on the TiO2 particle surface and the interactions between the substrate and TiO2 surface. The adsorption and desorption processes for aromatic compounds are also unclear. In the present Letter, we have investigated the one-electron oxidation kinetics of 4-methyl-ptolylsulfide (MTS) and 4-methylthiophenylmethanol (MTPM) during the photoexcitation of the TiO2 powder slurried in acetonitrile using time-resolved diffuse reflectance spectroscopy. Photocatalytic degradation of aromatic sulfides is among possible methods for destruction of chemical warfare agents [12]. We discussed the relationship between the substrate concentrations adsorbed on the TiO2 surface and radical cations of substrates generated by the laser flash photolysis of TiO2 powder.
2. Experimental and analysis 2.1. Materials Titanium dioxide (TiO2 ) powder (P25, Japan Aerosil) was a mixture of rutile (20%) and anatase (80%) with a BET surface area of 50 m2 g1 , average primary particle size of 21 nm, and an isoelectric point at pH 6.6. MTS (Tokyo Kasei) was SCH3
SCH3
CH3
CH2 OH
619
used without further purification and MTPM (Aldrich) was purified by vacuum sublimation before use. The structures of MTS and MTPM are shown in Fig. 1. Fresh acetonitrile (CH3 CN, spectral grade) was used as the solvent without further purification. 2.2. Steady-state UV absorption measurements Steady-state absorption spectra were measured by UV–VIS–NIR spectrophotometer (Shimadzu, UV-3100) at room temperature. The sample solutions containing TiO2 powder (20 g dm3 ) were sonicated for 10 min and TiO2 particles in solution were then completely removed by the centrifugation (10,000 rpm, 10 min) for UV absorption measurements. All the sample preparations were performed with shielding the UV light. 2.3. Time-resolved diffuse reflectance measurements Time-resolved diffuse reflectance measurements were performed using the third harmonic generation (355 nm, 5 ns FWHM) from a Q-switched Nd:YAG laser (Continuum, Surelite II-10) for the excitation operated with a temporal control by a delay generator (Stanford Research Systems, DG535). In the experiments, the spot irradiated on the sample cell with a thickness of 2 mm was approximately 1 cm2 . The analyzing light from a pulsed 450 W Xe-arc lamp (Osram, XBO-450) was collected by a focusing lens and directed through a grating monochromator (Nikon, G250) to a photomultiplier tube (Hamamatsu, Photonics R928). The transient signals were recorded with a digitizer (Tektronix, TDS 580D). TiO2 powder was suspended in CH3 CN by sonicating 40 mg of TiO2 in 2 103 dm3 of solvent for 5 min. The suspension was maintained at room temperature, and a new aliquot of the suspension was used after approximately 128 laser pulses. 2.4. Kinetic analysis based on Albery model
4-methyl-p-tolylsulfide (MTS)
4-methylthiophenylmethanol (MTPM)
Fig. 1. Structures of MTS and MTPM.
The percent absorption (100 DJ ) was calculated by the following equation: DJ ¼ 1 J =J0 ;
ð1Þ
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
where c is the parameter describing the width of the distribution of the average deactivation rate constant kd and x is the distribution variable, being allowed to take values between plus and minus infinity [13]. The basic concept of this treatment is the use of a Gaussian distribution of the free energy change as a suitable description of the heterogeneity of the system. The observed decay profile is a sum of the different contributions to the decay. When DJ and DJ0 are proportional to the transient concentration and initial concentration, respectively, and when x is integrated over a Gaussian distribution, one obtains Z þ1 DJ expðx2 Þ exp½kd t expðcxÞdx : ð3Þ ¼ DJ0 expðx2 Þdx 1 In the limit of c ¼ 0, this expression reduces to an ordinary first-order rate law. Since the integral has no analytical solution, this model has been successfully applied to heterogeneous powders and powder suspensions [7–9,13,14].
The concentrations of the adsorbates in CH3 CN containing TiO2 powder (20 g dm3 ) after reaching the adsorption equilibrium ([S]eq ) were determined from the absorbance observed by UV absorption experiments. For example, Fig. 2A depicted the UV absorption spectra observed for MTS and MTPM in CH3 CN at room temperature. The absorbance of absorption spectrum of MTPM ([MTPM] ¼ 2 103 M, M mol dm3 ) in CH3 CN after reaching the adsorption equilibrium (d) indicated the reduction of 9%, compared to that of MTPM in CH3 CN (b). This result clearly suggests the adsorption of MTPM molecules on TiO2 particles in solution. On the other hand, in the case of MTS ((a) and (c)), the reduction of absorbance was relatively small. The results indicate that the OH group plays an important role in adsorbing on the surface of TiO2
3
3 MTS
Absorbance
ð2Þ
3.1. Langmuir adsorption isotherm
2
1
(d)
0 240
280
320
240
280
320
Wavelength / nm
(A) 0.5
2.5. Cyclic voltammetry measurements
0.4
MTPM
0.3 0.2 MTS
0.1 0.0 0
Cyclic voltammograms were obtained by using a conventional three-electrode system (BAS CV50W) in CH3 CN solution at 298 K. A platinum electrode was used as the working electrode and an Ag/AgNO3 electrode was used as the reference electrode.
2
(c)
1
MTPM
(b)
(a)
0
-1
kobs ¼ kd expðcxÞ;
3. Results and discussion
-4
where J0 and J are the intensities of the diffuse reflected light without and with the excitation laser light, respectively [7–9]. The DJ values obtained in the diffuse reflectance experiments have no convenient connection with the transient concentrations and absorptivity in contrast to the transmission transient absorption experiments as presented by BeerÕs law, and no standard aqueous and nonaqueous suspensions are available for a diffuse reflector. However, the signal intensity was found to be proportional to transient concentrations at a low laser power [7]. The distribution of the particle size was estimated by the Albery model and the dispersion in the observed first-order rate constants (kobs ) is given by
nad / 10 mol g
620
(B)
2
4
6
8
10
[S]eq / mM
Fig. 2. Steady-state UV absorption spectra of MTS (2 mM) (a) and MTPM (2 mM) (b) in CH3 CN, and MTS (c) and MTPM (d) after reaching the adsorption equilibrium in CH3 CN (A). Langmuir type plots for MTS and MTPM in CH3 CN (5 103 dm3 ) containing TiO2 powder (100 mg) (B).
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
particles in CH3 CN. The determined concentrations are summarized in Table 1. Fig. 2B shows the Langmuir-type adsorption isotherms of MTS and MTPM on the TiO2 surface, where the amount of equilibrium adsorption, nad , in the number of adsorbed substrates per gram of TiO2 was plotted as a function of the equilibrium concentration in the bulk solution, [S]eq . The nad values were calculated from the difference in the absorbance observed for solutions with and without TiO2 powder. According to the Langmuir adsorption model for the solid–liquid interface, nad can be expressed by the following equation, nad ¼
Kad ½Seq ns ; 1 þ Kad ½Seq
ð4Þ
where ns is the total amount of adsorption sites and Kad is the adsorption constants defined as Kad ¼ kad =kad , where kad and kad are adsorption and desorption rate constants, respectively [15–17]. We obtained the Kad values of 20 and 60 M1 for MTS and MTPM adsorbed on the surface of TiO2 in CH3 CN, respectively. The Kad value determined for MTPM is three times larger than that for MTS, indicating that MTPM molecules bind to the TiO2 surface by –OH group. The determined Kad values, however, were considerably small, compared to those of some molecules, e.g.
621
6000 M1 for anthracene-9-carboxylic acid, chemically bonded on the TiO2 surface [17]. It is possible to estimate the total amount of adsorption sites (ns ) on the surface of TiO2 from the Langmuir plot as shown in Fig. 2B. We obtained the ns of 1.2 104 mol g1 for MTPM adsorbed on the TiO2 surface. If the adsorbed MTPM is approximated as a cylinder with a radius of 0.3 nm, derived from a space-filling model, the binding area of adsorbed MTPM is calculated to be approximately 20 m2 g1 . Thus, we estimated the occupied sites of 40%, since the TiO2 (P25) particle has the surface area of 50 m2 g1 . The binding area of MTPM adsorbed on the surface of TiO2 estimated from ns was actually lower than the BET surface area, suggesting that the TiO2 particles aggregate due to its high concentration. 3.2. Time-resolved diffuse reflectance absorption It is well established that, under the band gap excitation, TiO2 particles can initiate the oxidation and reduction processes of the adsorbed substrate. In the present study, TiO2 was used as a reactive surface for the one-electron oxidation of aromatic sulfides. In TiO2 particles, excitation with light energy greater than its band gap (3.2 eV) generates electron–hole pairs (Eq. (5)),
Table 1 Initial concentration ([S]) dependences of the equilibrium concentration ([S]eq ), the amount of adsorbed substrate (nad ), DJ0 , and kinetic parameters as given by Eq. (3) for MTS and MTPM [MTS] (mM)
½MTSeq (mM)
nMTS (104 mol g1 ) ad
100J0
kd (106 s1 )
c
0.5 1 2 5 10
0.49 0.97 1.9 4.8 9.5
0.01 0.02 0.04 0.12 0.24
– – 0.17 0.34 0.48
– – 0.22 0.24 0.20
– – 1.0 1.0 0.8
[MTPM] (mM)
½MTPMeq (mM)
nMTPM (104 mol g1 ) ad
100DJ0
kd (106 s1 )
c
0.25 0.5 1 2 5 7.5 10
0.22 0.45 0.88 1.8 4.5 6.8 9.2
0.02 0.03 0.06 0.12 0.23 0.33 0.41
– 0.25 0.50 1.0 1.8 2.0 2.65
– 0.6 0.8 1.0 1.3 1.5 1.7
– 1.0 1.2 2.0 2.3 2.3 2.7
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
Conduction band electrons are trapped at Ti4þ centers of the colloid within sub or a few picoseconds after excitation and generate Ti3þ species and holes [18–20]. These photogenerated carriers migrate to the particle surface and participate in redox processes at the surface. Rothenberger et al. observed the broad absorption spectra with maxima around 620 nm using pico- and nanosecond transient absorption experiments for the colloidal particles of TiO2 in aqueous solution and assigned this absorption band to trapped electrons [18]. In the present system, we observed the similar absorption band (DJ0 0:01 at 545 nm) in the absence of substrates. As assumed that the long-lived transient species is uninvolved in the one-electron oxidation process of substrates, we can subtract the DJ values observed in the absence of substrates (DJtr ) from that in the presence of substrates (DJobs ) as follows DJ ¼ DJobs DJtr :
3
ð5Þ
ð6Þ
Transient diffuse reflectance absorption spectra were measured for MTPM in TiO2 slurried CH3 CN solution. Fig. 3A shows the transient diffuse reflectance absorption spectra obtained during the laser photolysis of TiO2 with 355 nm light (1.5 mJ pulse1 ) in the presence of MTPM (1 102 mol dm3 ) in CH3 CN at room temperature. The transient absorption band at 500–700 nm was appeared immediately after a laser flash and assigned to MTPM radical cation (MTPMþ ), although the spectral shape was approximately 1.5 times broader than in the absence of TiO2 [21]. According to their chemical nature, the trapped charges react with various dissolved and adsorbed components. The trapped positive hole, like surface bound OH radical, typically oxidizes an organic molecule and thus induces its further oxidative degradation while the trapped electron, like surface bound Ti3þ , typically reduces dioxygen to superoxide radical anion. The surface-trapped hole is chemically equivalent to a surface-bound OH radical, which can initiate primary oxidation reaction of substrates adsorbed on the TiO2 surface before diffusing into the bulk solution. The
100 ∆J
hm
þ TiO2 ! TiO2 ðe cb þ hvb Þ:
a b c d
2
1
0 450
500
550
600
650
700
Wavelength / nm
(A) 3
100 ∆J at 545 nm
622
2
b
1 a
0 0
(B)
1
2
3
4
5
6
Time / µs
Fig. 3. Transient diffuse reflectance absorption spectra attributed to MTPMþ observed at 50 (a), 100 (b), 300 (c), and 1000 (d) ns after a laser flash during the 355-nm laser photolysis of TiO2 powder in the presence of MTPM in CH3 CN (A). DJ at 545 nm vs. time after a laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of MTS (10 mM) (a) and MTPM (10 mM) (b) in CH3 CN. Solid lines indicate simulation results fitted by the Albery model, kd ¼ 0:2 106 s1 , c ¼ 0:8 and kd ¼ 1:7 106 s1 , c ¼ 2:7 for MTS and MTPM, respectively (B).
MTPMþ transient absorption signal was generated immediately after the excitation pulse, clearly suggesting that the one-electron oxidation of MTPM occurred by the valence band hole (Eq. (7)) or surface-bound OH radicals (Eqs. (8) and (9)) at the surface of TiO2 powder in CH3 CN solution within a laser pulse duration, þ TiO2 ðhþ vb Þ þ MTPM ! MTPM ;
ð7Þ
TiO2 ðhþ vb Þ þ OHsurf ! OHsurf ;
ð8Þ
OHsurf þ MTPM ! MTPMþ þ OH :
ð9Þ
Fig. 3B shows the decay of the 545-nm absorption obtained by the laser photolysis of TiO2 with 355 nm light (1.5 mJ pulse1 ) in the presence of MTS (1 102 mol dm3 ) and MTPM (1 102
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
mol dm3 ) in CH3 CN. The absorption at 545 nm decays according to the Albery model with kd ¼ 0:2 106 s1 , c ¼ 0:8 and kd ¼ 1:7 106 s1 , c ¼ 2:7 for MTS ([MTS] ¼ 10 mM) and MTPM ([MTPM] ¼ 10 mM), respectively. Remarkably small DJ0 and slow charge recombination rate obtained for MTSþ in spite of the relatively large molar extinction coefficient (e550 (MTSþ )/e545 (MTPMþ ) ¼ 1.2) and low oxidation potential (1.57 and 1.59 V vs. NHE for MTS and MTPM, respectively) [21–23]. Significant substrate concentration dependences of the signal intensity and the decay kinetics were observed for MTPM in the TiO2 slurry. Fig. 4A depicted the transient signals observed for the MTPM concentrations of 1 (a), 5 (b), and 10 mM (c). The intensities of transient signals obtained at 545 nm remarkably increases with increasing in the MTPM concentrations. Fig. 4B indicates the plots of the DJ0 values listed in Table 1 against the MTPM concentration. The significant increase of
100 ∆J at 545 nm
3 c b a
2
1 0 0
1
2
3
4
5
6
Time / µs
(A) 4
100 ∆J0
3 2
623
DJ0 , particularly, was observed at low concentrations, suggesting that the DJ0 values depend on the concentration of MTPM adsorbed on the TiO2 surface as discussed below. We also observed the remarkable acceleration in the decay kinetics (kd ) of MTPMþ generated from the one-electron oxidation and the increase of the heterogeneously parameter c at high concentrations as listed in Table 1. There is worth to clarify the sequential reactions caused by Sþ , because the efficiency of photocatalytic reaction would be significantly dependent on the interfacial charge recombination rate which competes with many others. The charge recombination reaction between the trapped electrons and MTPMþ was influenced by numerous factors, for example, a relaxation time from shallow to deep trap states, a redox potential of substrates, a distance between the electron acceptor and donor [23,24]. In the present system, the signal intensity of MTPMþ increased with increasing laser intensity, while kd remained relatively constant, suggesting that disappearance of MTPMþ on the surface of TiO2 powder suspended in CH3 CN followed first-order kinetics with a distribution of reaction rate constants. These results are well consistent with that obtained for (SCN) 2 on the surface of TiO2 powder suspended in water reported by Fox et al. [7]. We also observed the almost same signal traces for the oxygen and argon gas saturated sample. These results clearly suggest the deactivation of MTPMþ is mainly attributable to the bimolecular reaction with adsorbates, although the reaction schemes are unclear. It seems that remarkable c values observed at the high MTPM concentrations are attributable to the differences in the reactivity of MTPM adsorbed on the heterogeneous surface of TiO2 powder. 3.3. Effects of adsorption on the one-electron oxidation of MTS and MTPM
1 0 0
(B)
5
10
15
20
[MTPM] / mM
Fig. 4. DJ at 545 nm vs. time after a laser flash during the 355-nm laser flash photolysis of TiO2 powder in the presence of MTPM of 1 (a), 5 (b) and 10 (c) mM (A). MTPM concentration dependence of DJ0 (B).
In the case of the present experiments, it is possible to extract the relative change of the MTPMþ concentration from the initial signal intensity due to the fact that the signal intensity was found to be approximately proportional to transient concentrations, although we do not know the
624
T. Tachikawa et al. / Chemical Physics Letters 382 (2003) 618–625
3
100 ∆J0
MTPM
2
1
0 0.0
MTS
0.1
0.2
0.3 -4
0.4 -1
nad / 10 mol g
Fig. 5. The relationship between the DJ0 at 545 nm and the amount of absorbed substrates (nad ). The lines indicate guides for eyes.
absolute concentration of MTPMþ . Fig. 5 shows the relationship between nad and DJ0 . The DJ0 values were found to be approximately proportional to the nad values in the present concentration range for both systems. As an electron transfer reaction rate (ket ) from the substrate to holes competes with the fast recombination reaction rate (kr ), the observed quantum yield of radical cation generated by the electron transfer reaction (/et ) is tentatively given by /et ¼
ket nad : kr þ ket nad
ð10Þ
As kr > ket nad , the /et values would be approximately proportional to the nad values. On the other hand, the efficiency of one-electron oxidation, that is DJ0 =nad , is MTPMMTS. For example, at nad 0:12 104 mol g1 , the DJ0 value obtained for MTPM is about three times larger than that for MTS. These results suggest that interaction between MTPM and TiO2 surface, such as a hydrogen bonding, plays an important role in the oxidation kinetics. It seems that such interaction induces the large electronic coupling between the electron acceptor and donor, resulting in an acceleration of the electron transfer reaction rate.
4. Conclusions We obtained the Kad values of 20 and 60 M1 for MTS and MTPM adsorbed on the TiO2 sur-
face in CH3 CN from the Langmuir type model, respectively. The result suggests that the –OH group of MTPM plays an important role in adsorbing to the surface of TiO2 particles. The oneelectron oxidation of MTS and MTPM by photoexcited TiO2 was observed by the timeresolved diffuse reflectance spectroscopy. The decay kinetics of MTSþ and MTPMþ in CH3 CN was interpreted by the Albery model for the dispersed first-order kinetics. The almost linear relationship between nad and DJ0 was obtained for MTS and MTPM, clearly indicating that the efficiency of the one-electron oxidation mainly depends on the amount of adsorbate on the TiO2 surface. The relatively high efficiency of one-electron oxidation also was obtained for MTPM, suggesting that the interactions between the substrates and the TiO2 surface is a key factor in the one-electron oxidation reaction on the TiO2 surface.
Acknowledgements This work has been partly supported by a grant-in-aid for Scientific Research on Priority Area (417), 21st Century COE Research, and others from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japanese Government.
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