UV- or Visible-Light-Induced Degradation of X3B on TiO2

Crystal Growth & Design 0 (proofing),. Abstract | Full ..... Nanoscale Research Letters 2016 11 (1), .... Design and preparation of bio-based dielectr...
1 downloads 0 Views 84KB Size
Download PDF
Langmuir 2001, 17, 897-902

897

UV- or Visible-Light-Induced Degradation of X3B on TiO2 Nanoparticles: The Influence of Adsorption Yiming Xu*,† and Cooper H. Langford‡ Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, China, and Department of Chemistry, Calgary University, Calgary, Alberta T2N 1N4, Canada Received August 2, 2000. In Final Form: November 1, 2000

Photodegradation of a textile dye X3B using either UV (λ g 320 nm) or visible light (λ g 450 nm) over three catalysts of highly adsorptive TiO2 nanoparticles in water has been examined. All the adsorption isotherms demonstrated the Langmuir type behavior. The common observation was confirmed that for all the reactions induced by UV or visible light, the apparent initial rate of X3B loss in the aqueous phase increased with the initial equilibrated concentration of X3B. However, this correlation was changed when the rate was determined by the decreased concentration both in the aqueous phase and on the catalyst surface. This increase of real initial rate with the initial equilibrated concentration was observed only in the visible-light-induced reaction over TiO2 of Degussa P25. For all the other reactions, especially under UV irradiation, the real initial rate was found to increase only initially and then decrease with the initial equilibrated concentration. The result suggests that there is a screening effect by the adsorbed dye in the TiO2 photocatalytic reaction and a solution filter effect in the photosensitized reaction. Moreover, the photosensitized photodegradation of X3B was found to be also dependent on the physical properties of TiO2, but interestingly the relative activity among the catalysts was similar to that demonstrated in the photocatalytic reaction.

1. Introduction Photodegradation of organic compounds in aerated aqueous suspensions of titanium dioxide has been studied extensively as an alternative method for wastewater treatment.1,2 In these reactions, the organic pollutants are oxidized by the photogenerated holes or by reactive oxygen species such as OH• and O2•- radicals formed on the UV-irradiated TiO2 surface. For the colored pollutant, the degradation can also be initiated using visible light. In this case, the dye is only the light absorbing species. From the dye excited state, an electron is injected into the conduction band of TiO2 where it is captured by the surfaceadsorbed O2 to produce O2•- radicals, and the dye cation radicals are decomposed subsequently via attack by oxygen or O2•- radicals.3 Both the reaction mechanisms suggest that preliminary adsorption of substrate on the catalyst surface is a prerequisite for highly efficient oxidation. Much effort has been addressed to the effect of adsorption.3-9 A rate equation of the Langmuir-Hinshelwood type is often observed for the photodegradation † ‡

Zhejiang University. Calgary University.

(1) Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier Science Publishers B.V: Amsterdam, 1993. (2) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 1999, 103, 4862. (4) Cunningham, J.; Al-Sayyed, G. J. Chem. Soc., Faraday Trans. 1990, 86, 3935. (5) Cunningham, J.; Srijaranai, S. J. Photochem. Photobiol., A 1991, 58, 361. (6) Cunningham, J.; Al-Sayyed, G.; Srijaranai, S. In Aquatic and Surface Photochemistry; Helz, G. R., Zepp, R. G., Crosby, D. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; Chapter 22, pp 317-348. (7) Chen, H. Y.; Zahraa, O.; Bouchy, M.; Thomas, F.; Bottero, J. Y. J. Photochem. Photobiol., A 1995, 85, 179. (8) Dagan, G.; Tomkiewicz, M. J. Phys. Chem. 1993, 97, 12651. (9) Zhang, F.; Zhao, J.; Shen, T.; Hidaka, H.; Pelizzetti, E.; Serpone, N. Appl. Catal., B 1998, 15, 147.

of many organic pollutants over TiO2 using either UV or visible light. This similarity has been taken as evidence of a quasi-bimolecular reaction between organic substrate adsorbed and charge carriers on the catalyst surface. Because most organic pollutants have weak adsorption on the TiO2 particles, inert adsorbents such as activated carbon,10 silica gel,11,12 and zeolites10,13 have been used as a support for TiO2 loading so as to concentrate the target molecules around TiO2 particles and to enhance the photodegradation rate relative to the bare TiO2 catalyst. In all these studies, the reaction rate was determined only from the decreased concentration of the substrate in the aqueous phase. However, for a target substrate that has high adsorption on the TiO2 or supported TiO2 catalyst, this kind of rate might not be taken as the real rate of photodegradation. There is no guarantee that the total amount of the substrate degraded after irradiation is exactly equal to the amount of the substrate lost in the aqueous phase. In this case, the concentration change of the adsorbed substrate on the catalyst surface should be also considered in the rate determination. Clarification of this question would be significant for the future development of a highly adsorptive (photo)catalyst. For this purpose, a textile dye of reactive brilliant red X3B has been chosen as the model compound in the present study. This dye exhibits a relatively strong adsorption on TiO2 and resists UV irradiation (λ g 320 nm) in a homogeneous solution but is easily degradable in the presence of TiO2 under either UV or visible light (λ g 450 nm) irradiation.14 Different conclusions about the adsorption effect have been obtained from the reaction rates discussed above. (10) Torimoto, T.; Ito, S.; Kuwabata, S.; Yoneyama, H. Environ. Sci. Technol. 1996, 30, 1275. (11) Tada, H.; Akazawa, M.; Kubo, Y.; Ito, S. J. Phys. Chem. B 1998, 102, 6360. (12) Xu, Y.; Zheng, W.; Liu, W. J. Photochem. Photobiol., A 1999, 122, 57. (13) Xu, Y.; Langford, C. H. J. Phys. Chem. B 1997, 101, 3115. (14) Xu, Y. Chemosphere, in press.

10.1021/la001110m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/03/2001

898

Langmuir, Vol. 17, No. 3, 2001

Xu and Langford

Figure 1. The structure and the electronic absorption spectrum of reactive brilliant red X3B in water at pH 2.2.

2. Experimental Section Materials. Three commercial TiO2 samples were employed in this study, named here as PT (Degussa P25), AT, and RT-TiO2 (both from Taixing Nanomaterials, China). The X-ray diffraction analysis (X’ Pertmpd X-ray diffractometer) showed that all the samples were a mixture of anatase and rutile. The percentages of anatase phase and the particle sizes, estimated from the diffraction intensity and the peak width, were 97% and 13 nm for AT, 84% and 24 nm for PT, and 92% and 28 nm for RT. The nitrogen adsorption on an ASDI RXM-400 apparatus by the four points method gave the Brunauer-Emmett-Teller (BET) surface area, which was 124 m2 g-1 for AT, 50 m2 g-1 for PT, and 41 m2 g-1 for RT. Reactive brilliant red X3B (Color Index: C. I. Reactive Orange 86) from Jining dye manufacture of China was chosen as a target substrate in this study and used as received (98%) without any purification. The molecular structure is displayed in Figure 1, together with its electron absorption spectrum in water at pH 2.2. All the X3B aqueous solutions were prepared in a medium of perchloric acid at pH 2.2. Adsorption. The X3B adsorption on TiO2 from aqueous solution was determined in the dark by mixing 50.0 cm3 of X3B solution at various initial concentrations C0 with a fixed weight (0.050 g) of the TiO2 catalyst. The suspension was shaken in the dark at a constant rate overnight and then filtered through a membrane filter (pore size 0.45 µm, Shanghai Xingya). The absorbance of the filtrate at 540 nm was measured to determine the equilibrium concentration C2b, and then the decreased concentration (∆C2b ) C0 - C2b) was used to calculate the amount of equilibrium adsorption, n2s, in the number of dye moles adsorbed per gram of TiO2. Irradiation. The photodegradation rate was measured in a Pyrex reactor using a 75-W Xenon lamp (USHIO) as the irradiation source. The lamp was enclosed in an A1010 lamp housing (P.T.I) and powered at 5.00 A by a LPS-200 lamp power supply. Before entrance into the Pyrex reactor, the light beam passed through a water cell of 10-cm path length to remove the infrared or through a dichromate solution (1 × 10-3 M K2CrO4 in 23 g L-1 of Na2CO3) to perform the visible light (λ g 450 nm) irradiation. Prior to the illumination, the suspension containing 50.0 cm3 of X3B solution and 50.0 mg of TiO2 was shaken at a constant rate overnight so as to achieve the equilibrium of dark adsorption. After irradiation using UV light for 15 min or using visible light for 120 min, the suspension was filtered and analyzed spectrophotometrically at 540 nm to obtain the substrate concentration *C2b in the filtrate. The C0, *C2b, or C2b was then employed as the base for the reaction rate calculation (note that in either the UV or visible light reaction the *C2b decreased linearly with the irradiation time, up to at least 50 min in the former and at least 4 h in the latter reaction). All the experiments were carried out at the same conditions (concentration, pH, temperature 25 °C) as the adsorption studies.

Figure 2. Adsorption isotherm (A) and the corresponding Langmuir plots (B) of X3B on TiO2 samples AT (a), PT (b), and RT (c), from aqueous solution (pH 2.2) at 25 °C.

3. Results and Discussion 3.1. Adsorption Isotherm. The dye X3B has a relatively strong adsorption on the TiO2 catalyst, especially in an acidic medium. For the purpose of the present study, all the experiments were thus performed in the acidic medium of HClO4 at pH 2.2. Figure 2A shows the adsorption isotherm of X3B from aqueous solution at pH 2.2 on the TiO2 of AT (a), PT (b), and RT (c), where the amount of equilibrium adsorption, n2s, is plotted as a function of equilibrium concentration in the bulk solution, C2b. We see that each curve demonstrates the Langmuir type behavior, and the adsorption of X3B on AT-TiO2 is remarkably higher than those on the other two samples (PT and RT). According to the Langmuir adsorption model in solution, ns2 can be expressed by the equation of n2s ) nsK C2b/(1+ K C2b), where ns is the total amount of adsorption sites and K is the adsorption constant.15 Shown in Figure 2B, these adsorption parameters can be obtained from the slope and intercept of the plot C2b/n2s versus C2b at C2b ) 0 (the linear regression coefficients for curves a, b, and c are 0.997, 0.998, and 0.993, respectively), and the results are summarized in Table 1. We see that both the adsorption parameters ns and K become larger in the order of AT > PT > RT. This difference can be attributed to the particle size of the TiO2 adsorbent. By the powder X-ray diffraction analysis, the TiO2 particle size was found to decrease in the order of AT (13 nm) < PT (24 nm) < RT (28 nm). The smaller particles of TiO2 (15) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986; Chapter 7.

Degradation of X3B on TiO2 Nanoparticles

Langmuir, Vol. 17, No. 3, 2001 899

Figure 3. The apparent initial rate of the UV-induced degradation of X3B measured in the aqueous phase as a function of the initial equilibrium concentration over TiO2 samples AT (a), PT (b), and RT (c). Table 1. Adsorption Parameters and the Langmuir-Hinshelwood Parameters, Measured in the Dark and under UV Irradiation, Respectively, for X3B in an Aqueous Suspension of TiO2 at 25 °C (pH 2.2) TiO2 samples

n s/ mol g-1

K/ dm3 mol-1

kL-H/ M min-1

*K/ dm3 mol-1

AT PT RT

2.28 × 10-4 1.05 × 10-4 9.81 × 10-5

1.02 × 105 7.85 × 104 3.85 × 104

9.11× 10-7 4.54 × 10-6 1.57 × 10-6

4.65 × 104 1.42 × 104 5.28 × 104

are expected to possess a higher surface area per unit of mass, thus having more sites for X3B to be adsorbed. This tendency is consistent with the nitrogen adsorption, from which the BET area for AT, PT, and RT has been determined to be 124, 50, and 41 m2 g-1, respectively. As demonstrated recently by Zhang et al.,16 the adsorption constant K increases with the decrease of the particle size, because of a higher driving force for adsorption on the finer particles. The result indicates that in the concentration range studied the Langmuir model can be used to describe the adsorption process of X3B on TiO2 in the acidic aqueous medium. Because the TiO2 has different adsorption strengths for the substrate of X3B from the aqueous solution, these samples would be good candidates as a (photo)catalyst for the study of adsorption effects on the heterogeneous photoreactions. 3.2. Effect of Adsorption on the Photodegradation of X3B Using UV Light. The photodegradation experiment under UV irradiation (λ g 320 nm) was carried out after the dark adsorption equilibrium was achieved. Because no photodegradation of X3B was observed in the absence of TiO2, any observed decrease of X3B concentration in the aqueous phase was attributed to the dye photodegradation by TiO2 photocatalysis. As a first step, the apparent initial rate of X3B photodegradation (Rapp) was determined directly by using the decreased concentration of X3B in solution, that is, ∆*C2b ) C2b -*C2b, where C2b and *C2b are the concentrations of X3B before and after irradiation, respectively, both detected in the aqueous phase. This kind of rate has been often used in the literature for kinetic studies of TiO2 photocatalysis3-7,9 or as a measure of photoactivity among different catalysts.8,10-13 Figure 3 shows the apparent initial rate as a function of increased initial equilibrium concentration of X3B (16) Zhang, H. Z.; Lee Penn, R.; Hamers, R. J. J. Phys. Chem. B 1999, 103, 4656.

for three TiO2 photocatalysts. We see that the apparent initial rate increases with the initial equilibrium concentration (the rate over RT begins to decrease somewhat at high C2b). The linear plot of 1/Rapp versus 1/C2b (not shown here) has been also obtained for each case (the linear regression coefficient is 0.995 for AT, 0.998 for PT, and 0.875 for RT). This result is similar to the common observation made in the literature for the photooxidation of many organic compounds over TiO2, for which the Langmuir-Hinshelwood (L-H) mechanism has been postulated.1,2 From the slope and the intercept of this linear plot, the corresponding rate constant, kL-H, and the “adsorption” constant, *K, can be determined, which are summarized together in Table 1. However, this apparent initial rate cannot reflect the real initial rate of the reaction, which will be demonstrated below. It would then be improper to make any comments here on the apparent difference between the K measured in the dark and the *K obtained under irradiation or comments on the difference in kL-H among the photocatalysts. According to the adsorption isotherm shown in Figure 2, any change of the dye concentration in bulk solution must be followed by a change of the substrate concentration on the catalyst surface so as to reach a new equilibrium. Because of that, the total amount of the X3B photodegraded should include not only the decreased amount of X3B in the aqueous phase but also the differential amount of the surface-adsorbed X3B before and after photoreaction. From this analysis, the real initial rate of X3B photodegradation is expected to be larger than the apparent initial rate determined above from the aqueous phase. The question now becomes how to evaluate the amount of the undegraded X3B left on the catalyst surface. The dye X3B has high solubility in water (80 g L-1 at 20 °C), and it was difficult to extract the dye from the aqueous solution and off the catalyst surface by organic solvents such as chloroform. Assuming that after photoreaction the adsorption/desorption equilibrium is reached immediately between the adsorbed X3B and the X3B in the bulk solution and that the adsorption parameters of ns and K are the same as those determined in the dark (Figure 2 and Table 1), the amount of X3B left on the surface after reaction, *n2s, could be simply evaluated by the Langmuir adsorption equation of *n2s ) nsK C2b/(1 + K C2b), where *C2b is the concentration of X3B detected in the aqueous phase after photoreaction (the interference from the intermediates is negligible because the photodegradation here was performed at low substrate conversion). Then the real initial rate (R) of X3B photodegradation could be calculated using the decreased concentration of ∆*C2 ) ∆*C2b + ∆*C2s, where ∆*C2b ) C2b - *C2b in the aqueous phase and ∆*C2s ) (n2s -*n2s)w/V on the catalyst surface (n2s is the adsorption amount before irradiation, w is the mass of the TiO2 catalyst, and V is the volume of the irradiated suspension). Alternatively, the ∆*C2 can be calculated by an equation of ∆*C2 ) C0 - *C2b - *C2s, where C0 is the initial concentration of X3B before addition of the catalyst and *C2s ) *n2s w/V. It was confirmed that both the ∆*C2 calculations gave the same value for the photodegradation of X3B over all three TiO2 catalysts. Figure 4A shows the plot of the (calibrated) real initial rate R as a function of increased C2b for three TiO2 photocatalysts. Surprisingly, all the curves are much different from those shown in Figure 3. The rate does not always increase with the initial equilibrium concentration. Instead, it increases only initially and then decreases sharply with the increased C2b. It was further found that the decay rate of R with the initial surface concentration

900

Langmuir, Vol. 17, No. 3, 2001

Xu and Langford

Figure 5. The apparent initial rate of the visible light-induced degradation of X3B measured in the aqueous phase as a function of the initial equilibrium concentration over TiO2 samples AT (a), PT (b), and RT (c).

Figure 4. The real initial rate of the UV-induced degradation of X3B as a function of the initial equilibrium concentration (A) and as a function of the initial surface concentration (B) over TiO2 of AT (a), PT (b), and RT (c).

C2s of the adsorbed X3B was similar for each of the three TiO2 photocatalysts (Figure 4B). Because the dye is red and has an absorption band at 540 nm in the aqueous solution (Figure 1), the sharp decrease of R with C2s might suggest that there is a screening effect by the adsorbed dye at the surface, which cuts off part of the UV light and slows down the generation rate of e-/h+ pairs on the irradiated TiO2 surface and subsequently the photooxidation rate. The validity of directly applying the Langmuir adsorption equation for the real rate calculation might be questioned by the limited number of the adsorbing sites on the monolayer for the coming X3B from the solution. If the intermediate(s) formed from the photodegraded X3B adsorb(s) firmly on the catalyst surface, the maximum sites occupied by the intermediate(s) (*nps) would be expected to equal R ∆t V/w, where ∆t is the reaction time. If the sum (*ns) of the adsorbed intermediate (*nps) plus the adsorbed X3B (*n2s) does not exceed the total number of adsorption sites (ns) on the monolayer surface (Table 1), the real initial rate demonstrated above would be valid. For this concern, the *ns was calculated for all points presented in Figure 4. It was found that all the *ns values for AT and RT were smaller than their corresponding ns (for AT, *ns ) (1.068-2.03) × 10-4 mol g-1; for RT, *ns ) (3.09-7.54) × 10-4 mol g-1), but the *ns values for PT of the last two data points in Figure 4 were larger than its ns. Basically, this estimation has supported the promise of the calculation method for the real initial rate of X3B photodegradation.

3.3. Effect of Adsorption on the Photodegradation of X3B Using Visible Light. The photodegradation of X3B over TiO2 using visible light (λ g 450 nm) was performed under the same conditions as the above. Figure 5 shows the apparent initial rate (Rapp) of X3B photodegradation determined in the aqueous phase for three TiO2 catalysts at different initial equilibrium concentrations (C2b) of X3B. The plots are similar to those observed in the UV-induced photodegradation of X3B (Figure 3) and similar as well to that observed in the visible-lightinduced photodegradation of eosin over TiO2.9 Following the method used above for the photocatalytic reaction, the real initial rate R for the photosensitized photodegradation of X3B was also calculated, and the plot of R versus C2b is shown in Figure 6A. Once again, all the curves are different from those presented by Rapp (Figure 5), but the real initial rate decreases only slowly with the increased C2b for AT and RT and increases continuously for PT. It was confirmed later that the R determination was valid. The sum (*ns) of both the adsorbed X3B (*n2s) and the adsorbed intermediates (*nps) for AT and RT was smaller than the total number of adsorption sites (ns) (*ns ) (1.061-2.11) × 10-4 mol g-1 for AT and *ns ) (4.30-8.76) × 10-4 mol g-1 for RT), and only the last two values of *ns in Figure 6B for PT were larger than its ns. When R is plotted against the initial surface concentration (C2s) of the adsorbed dye (Figure 6B), we see that the plot is also different from the counterpart displayed in the UV-induced reaction (Figure 4B). In the present case, R decays quite slowly rather than sharply with C2s for AT and RT, whereas for PT it increases almost linearly with the initial concentration of the adsorbed X3B on the surface. In our previous study on the homogeneous photobleaching of X3B via Fe(III) photolysis,17 the initial rate was also observed to decrease quite slowly with the increased initial concentration of X3B, which has been attributed to the solution filter effect of the dye. Although the solution filter effect is definitely operative in the present case, the initial rate should still increase with its initial concentration on the catalyst surface because the reaction is initiated from the dye excited state.14 However, it was observed only with PT (Degussa P25, TiO2) and not with both AT and RT (Figure 6). This discrepancy may suggest some other factors that suppress the further (17) Xu, Y.; Lu, Q. J. Photochem. Photobiol., A 2000, 136, 73.

Degradation of X3B on TiO2 Nanoparticles

Langmuir, Vol. 17, No. 3, 2001 901

Figure 6. The real initial rate of the visible light-induced degradation of X3B as a function of the initial equilibrium concentration (A) and as a function of the initial surface concentration (B) over TiO2 samples AT (a), PT (b), and RT (c).

Figure 7. The real initial rate of the UV-induced (A) and visible light-induced (B) degradation of X3B as a function of the initial surface coverage of X3B on TiO2 samples AT (b), PT (9), and RT (O).

increase in the oxidation rate with the increased concentration of the target substrate on the AT or RT catalyst surface. According to the proposed mechanism of the photosensitized reaction, the adsorption of molecular oxygen on the catalyst surface should be important for the separation of the injected electrons and the dye cation radicals. It is highly possible that the amount of O2 adsorption on each TiO2 is different. In this point of view, the result shown in Figure 6 may indicate that the saturated amount of oxygen adsorption on AT and RT is much less than that on PT. If the adsorption rate of O2 to the surface from the aqueous solution cannot catch up to its consumption rate during the photoreaction, the photodegradation rate of X3B will be limited by the amount of the O2 adsorbed originally on the surface, and consequently the rate will not increase upon the increase of the X3B concentration on the surface. The same activity order of PT > RT g AT is followed in both the UV (Figure 4) and visible-light-induced reactions (Figure 6). This trend becomes much clearer when R is plotted against the initial surface coverage, θ, of X3B on the TiO2 surface (Figure 7), where the curves for AT and RT are almost overlapped and the PT-TiO2 shows a remarkably different activity. For this similarity, one may worry about whether the photooxidation of X3B under visible light irradiation was actually caused by the TiO2 photocatalysis, because the solution filter of dichromate used in the experiment might not be efficient enough to cut off all the UV light from the irradiating beam. With this concern, the photoreduction of dichromate in an acidic suspension of PT using the same visible light

was performed (this reaction has been shown to occur easily under UV irradiation18), but no decrease in the Cr(VI) concentration was observed after 3 h of continuous irradiation with the visible light. This supplementary experiment confirms that the photodegradation of X3B under visible light irradiation proceeded absolutely via the photosensitized pathway. The photodegradation of X3B in the UV-irradiated suspension, on the other hand, involves both the photocatalytic and the photosensitized reactions, but the former is the predominant process, because the reaction rate of X3B photodegradation initiated by the visible-light-excited dye is much slower than that initiated by the UV-excited TiO2. It is generally accepted that in the TiO2 photocatalysis the catalyst activity is mainly determined by the recombination rate of h+/e- pairs, on which the physical properties of the TiO2 particles have great influence. For example, the photocatalytic activity of pure TiO2 has been shown to increase with the surface area19 and with the anatase content in the solid,20 and an optimal particle size of 11 nm for the maximum photocatalytic activity has been evaluated recently by Zhang et al.21 Following this general information, the small difference in structure and (18) Xu, Y.; Chen, X. Chem. Ind. (London) 1990, 6, 497. (19) Pichat, P.; Guillard, C.; Maillard, C.; Amalric, L.; D’Oliveira, J.-C. In Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier Science Publishers B.V: Amsterdam, 1993; pp 207-223. (20) Ohtani, B.; Ogawa, Y.; Nishimoto, S. J. Phys. Chem. B 1997, 101, 3746. (21) Zhang, Z.; Wang, C.-C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B 1998, 102, 10878.

902

Langmuir, Vol. 17, No. 3, 2001

Xu and Langford

appears to be of the order of twice the molecular diameter of the dye. Alternatively, there may be a dynamic equilibrium between the screening effect of the adsorbed dye and the reactivity of the trapped carriers for the maximum rate observed.

composition of the sample should be responsible for the photoactivity difference observed among the present TiO2 catalysts of AT, PT, and RT. However, it is difficult to correlate the photoactivity order of PT > RT g AT (Figure 4) with the parameters mentioned above. The surface area and the X3B adsorption (Table 1) increase in the order of AT > PT > RT. The particle size has been estimated to be 13 nm for AT, 24 nm for PT, and 28 nm for RT, and the anatase content in AT, RT, and PT is about 97, 92, and 84%, respectively. Probably, the similarity of the activity order between the UV- and visible-light-induced reactions may suggest that the strength of oxygen adsorption is also important to the photocatalytic degradation of X3B on TiO2. The role of oxygen in TiO2 photocatalysis2,22 is to accept the electron generated on the irradiated TiO2, producing a superoxide radical, and to combine with the organic radical formed from the direct hole oxidation or from the attack of a hydroxyl radical. At present, however, we are not able to provide data for oxygen adsorption on these TiO2 catalysts to confirm this explanation. The two figures (Figure 7A,B) presenting activity of PT-TiO2 (Degussa P25) as a function of the surface coverage fraction, θ, provide an interesting insight into a central issue: what is the “effective radius” over which a trapped carrier is effective? In Figure 7B, we see that the reactivity continues to increase with coverage up to saturation. This is as expected. The excitation process involves a photon absorbed by the dye, and the dye excited state can be expected to transfer an electron to the conduction band from any surface site. In contrast, Figure 7A shows that once coverage by the dye reaches θ ≈ 0.5, there is little further increase in reactivity. Under UV irradiation, hole and electron carriers are generated in TiO2 by the photon and surface trapped. It appears that once substrate covers approximately 50% of the surface sites, all surface-trapped carriers can be harvested for reaction. Thus the reactive diameter of the trapped carrier

It has been shown that the rate of X3B photodegradation determined by the decreased concentration both in the aqueous phase and on the surface is remarkably different from that rate measured only in the aqueous solution. The result would be helpful to the kinetic studies of organic photooxidation over a highly adsorptive TiO2. However, the relative activity of TiO2 among the catalysts is almost unchanged with the rate expression (compare Figure 3 with Figure 4 and Figure 5 with Figure 6), indicating that the rate in the bulk solution can still be used roughly as a measure of the (photo)activity among different (photo)catalysts. The use of the Langmuir adsorption equation is one of the choices to evaluate the amount of the adsorbed substrate on the catalyst surface, but the more reliable way is to extract all by organic solvent. The photosensitized degradation of X3B is also affected by the physical properties of TiO2, similar to those observed in the photocatalytic reactions for the same set of catalysts. To our knowledge, it is the first time this observation has been reported. It would be necessary in the future to make an extensive investigation about the effect of TiO2 morphologies for the ambiguous desire of treating the dye pollutants using either artificial light or sunlight.

(22) Schwitzgebel, J.; Ekerdt, J. G.; Gerischer, H.; Heller, A. J. Phys. Chem. 1995, 99, 5633.

LA001110M

4. Conclusions

Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 29977019 and No. 20010210764) and the Natural Science Foundation of Zhejiang Province, China (No. 299033). Xu thanks Ms. Huiqing Lu and Xiaohan Yu for performing some experiments. The referee’s comment on this manuscript is greatly appreciated.