Langmuir 1998, 14, 1725-1727
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Effect of the TiO2 Surface on the Reactivity of Photocatalytically Generated (SCN)2•- and I2•John L. Ferry and Marye Anne Fox* Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712 Received October 27, 1997. In Final Form: December 19, 1997 The radical ions (SCN)2•- and I2•- were produced by photocatalytic flash excitation (355 nm) of oxygenated TiO2 sols suspended in 1.00 M aqueous solutions of KSCN or KI. To determine the effect of surface heterogeneity of the photocatalyst on the secondary reactions of these transients, these radical ions were reacted with a series of electron donors. The resulting heterogeneous rate constants are compared to those known for the same reactions in homogeneous solution, where (SCN)2•- and I2•- were generated by pulse radiolysis. Relative to the solution phase, the reaction between (SCN)2•- and ascorbic acid on a TiO2 sol was accelerated by approximately a factor of 40, that between (SCN)2•- and hydroquinone was essentially unchanged, and that between (SCN)2•- and cysteine was slowed by approximately an order of magnitude. The dismutation of (SCN)2•- on TiO2 was accelerated by a factor of 3. The reaction of I2•- with cysteine on a TiO2 sol was approximately 25 times slower than that in solution, and its dismutation was accelerated by a factor of 4. These changes in kinetic selectivity are explained by the association of the photocatalytically generated radical ion with the TiO2 surface.
Introduction Titanium dioxide, upon excitation, is capable of photocatalytically initiating a wide variety of one-electrontransfer reactions.1 Absorption of a photon of an energy greater than the band gap creates an electron (e-)/hole (h+) pair that may either recombine or react with some redox-active substrate adsorbed on the semiconductor surface. To achieve high quantum efficiency, the latter step must be maximized relative to the former. The mechanism of the electron transfer induced by photoexcitation of TiO2, the application of such photocatalysis for the functionalization of organic compounds, and its utility in the degradation and mineralization of organic wastes have been extensively reviewed.1-3 Although a great deal of research has been conducted on the mechanism of interfacial electron transfer between the electron-hole pair and the adsorbed substrate,2 little attention has been focused on the effect of the photocatalyst surface on the subsequent reactivity of the transient species thus produced.4,5 For many transients, this distinction is difficult because the absorption spectra are often virtually identical when adsorbed onto the TiO2 surface or when homogeneously dispersed in solution. However, there is strong evidence that photocatalytically generated radical ions differ in their behavior from these same species produced in solution by pulse radiolysis, although their absorption spectra appear unchanged. For example, Draper and Fox found that the bimolecular rate constant for the dismutation of photocatalytically generated (SCN)2•- on a TiO2 sol increased to 6.0 × 109 M-1 s-1 from 1.3 × 109 M-1 s-1 in homogeneous aqueous solution.4 A similar effect was observed for I2•-, with an increase in the rate constant being observed for dismutation from 3.2 (1) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (2) Kamat, P. Chem. Rev. 1993, 93, 358. (3) Legrini, O.; Oliveros, E.; Braun, A. M. Chem. Rev. 1993, 93, 671. (4) Draper, R. B.; Fox, M. A. J. Phys. Chem. 1990, 94, 4628. (5) Draper, R. B.; Fox, M. A. Langmuir 1990, 6, 1396.
× 109 M-1 s-1 in solution to 3.1 × 1010 M-1 s-1 on the photocatalyst.5,6 This increase in rate constant was attributed to the reduced dimensionality of the heterogeneous TiO2 surface, i.e., the radical ions generated on a surface can more easily encounter each other in their diffusional movement than those generated in solution, so that apparent diffusion controlled solution phase reactions appear to proceed with even higher rate constants on the photocatalyst. This effect has also been observed for radical reactions on the surfaces of micelles.7 These studies showed that the subsequent reactivity of singly oxidized reactive intermediates produced on a photocatalyst surface may be significantly affected by their environment. Moreover, applied photocatalysis often requires efficient reaction between a photocatalytically generated radical ion and some neutral or charged electron donor present in solution or coadsorbed with the reactant of interest. This study addresses that situation by determining the effects of the heterogeneous environment on the kinetics of the reactions between (SCN)2•- and cysteine, ascorbic acid, or hydroquinone and of the reaction between I2•- and cysteine, as determined by the use of competitive kinetics techniques. Experimental Section Materials. Titanium tetraisopropoxide (98%), KSCN (99%), KI (99+%), hydroquinone (99%), ascorbic acid (99.5%), and HClO4 (concentrated) were obtained from Aldrich; cysteine (99%) was from Sigma; 2-propanol (GC2 grade) was from Baxter; and water (ASTM grade) was from a Milli-Q filtration system. All reagents were used without further purification. Sol Preparation. Transparent TiO2 sols were prepared by acid hydrolysis of titanium tetraisopropoxide.4,5 A 100 mL solution of 5% titanium tetraisopropoxide was slowly added dropwise to 900 mL of aqueous HClO4 (0.067 M) at 3 °C over 2 h with rapid stirring. The solution was allowed to warm to room temperature overnight with constant stirring and was stored in (6) Baxendale, J. H.; Bevan, P. L. T.; Stott, D. A. Trans. Faraday Soc. 1968, 643, 2389. (7) Frank, A. J.; Gra¨tzel, M.; Kozak, J. J. J. Am. Chem. Soc. 1976, 98, 3317.
S0743-7463(97)01160-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/04/1998
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Figure 1. Bimolecular decay of photocatalytically generated (SCN)2•- produced by flash excitation (355 nm) on an aqueous TiO2 sol (0.050 M KSCN, pH 1.66) (9, lower curve) in the presence of cysteine, 6.85 × 10-3 M, and ([, upper curve) without added cysteine. The transient was monitored at 480 nm. the dark until use. The optical densities of the sols at 355 nm were 0.65 ( 0.05. Sample Preparation. Volumetric flasks were charged with aqueous (1.00 M) KSCN or KI as needed to achieve the desired substrate concentration. The resulting solution was then diluted to 10 mL with the TiO2 sol. The total volume of the stock solution never exceeded 10% of the total volume of the sample. Samples were prepared immediately before use and were bubbled with a slow stream of oxygen for at least 5 min before irradiation. The final sample pH was adjusted to 1.66 with concentrated HClO4. Photolysis. Flash photolyses were conducted with a Qswitched Nd:YAG laser (Quantel) at 355 nm. The laser pulse width was 10 ns, and the pulse energy was 15 mJ. Probe light was supplied by a 150 W Xe arc lamp (Oriel). Flash photolysis of the TiO2 sols with hydroquinone, ascorbic acid, or cysteine alone did not produce any observable signals at 480 or 400 nm.
Results and Discussion (SCN)2•- Reactions. Dismutation. One-electron oxidation of thiocyanate ion, by either h+ or by an adsorbed hydroxyl radical, produces the thiocyanogen radical SCN•.8,9 This radical is rapidly trapped by unreacted SCNto produce the anion radical dimer ((SCN)2•-), which is easily observed in transient absorbance studies because of its high extinction coefficient at 480 nm (480 ) 7600 M-1 cm-1).6 When generated photocatalytically under our experimental conditions, the decay of this radical was best fit as a second-order process (at 480 nm), with an apparent bimolecular rate constant for dismutation of 3.51 × 109 M-1 s-1, an increase of 270% relative to the solution phase rate constant.6 Cysteine. The addition of cysteine increased the apparent decay rate of (SCN)2•- relative to its dismutation, Figure 1. At sufficiently high cysteine concentrations ([cysSH]o g 6.85 × 10-3 M), the reaction of (SCN)2•- with cysteine completely dominated over dismutation and the bimolecular reaction, monitored by (SCN)2•- decay, became pseudo first order. The bimolecular rate constant for the reaction between (SCN)2•- and cysteine was determined to be (4.50 ( 0.06) × 106 M-1 s-1. In contrast, the bimolecular rate constant for the reaction of cysteine with (SCN)2•- produced by pulse radiolysis in water is 5.0 × 107 M-1 s-1, approximately an order of magnitude higher.9 The observation that different rate constants are observed for the reaction of cysteine and (SCN)2•- with and without a TiO2 sol requires that these reactions must take place in different environments. That is, desorption (8) Colombo, D. P.; Bowman, R. M. J. Phys. Chem. 1996, 100, 18445. (9) Neta, P.; Huie, B.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 1170.
Ferry and Fox
Figure 2. Bimolecular decay of photocatalytically generated (SCN)2•- produced by flash excitation (355 nm) on an aqueous TiO2 sol (0.050 M KSCN, pH 1.66) (2, lower curve) in the presence of hydroquinone, 5.30 × 10-4 M, and (b, upper curve) without added hydroquinone. The transient was monitored at 480 nm.
of the adsorbed radical ions produced by photocatalytic interfacial electron transfer to a fully solvated species must be slower than this bimolecular reaction. We can therefore safely infer that the presence of the TiO2 sol affects the observed reactivity. That the reaction was slower in the presence of the sol indicated that the TiO2 surface had an inhibitory effect on the reaction. A possible explanation is the repulsion of like charges: at pH 1.66, TiO2 is positively charged, as is cysteine, and this may depress the adsorbed cysteine concentration present at the surface of the particle. Interestingly, cysteine concentration should be slightly increased relative to the bulk in the particle’s outer Helmholtz plane.10 The fact that the observed rate nonetheless remains low argues that (SCN)2•- must not be diffusing away from the particle to a significant degree. Hydroquinone. Hydroquinone also accelerated the decay of (SCN)2•- relative to its dismutation, Figure 2. The observed decay kinetics became pseudo first order in (SCN)2•- at a much lower concentration (g5.30 × 10-4 M) than with cysteine, indicating a faster bimolecular reaction. The bimolecular rate constant for (SCN)2•- and hydroquinone on a TiO2 sol was determined to be (5.65 ( 0.66) × 107 M-1 s-1. This is essentially the same as the bimolecular rate constant obtained for the reaction in aqueous solution between pulse radiolytically generated (SCN)2•- and hydroquinone (6.0 × 107 M-1 s-1).9 This implies either that hydroquinone, neutral at pH 1.66, is neither repelled by nor associated significantly with the positively charged TiO2 surface or that desorption of the transient took place faster than this reaction. The latter is unlikely, given the differences in homogeneous phase and adsorbed reactivity observed with the slower reacting cysteine and the surface-influenced dismutation. Ascorbic Acid. Ascorbic acid dramatically accelerated the decay of (SCN)2•-, even at very low concentrations, Figure 3. The rate of reaction of (SCN)2•- with ascorbic acid on TiO2 became pseudo first order with a bimolecular rate constant of (4.30 ( 0.02) × 108 M-1 s-1. This is a significant increase over the rate constant observed for the same reaction in aqueous solution, which is only 1.0 × 107 M-1 s-1.9 Since bidentate ligands such as 4-chlorocatechol have been shown to adsorb strongly onto TiO2, ascorbic acid is also expected to bind strongly.11-13 Accordingly, a high local concentration of ascorbic acid (10) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980. (11) Kesselman, J. M.; Lewis, N. S.; Hoffmann, M. R. Environ. Sci. Technol. 1997, 31, 2298.
Effect of TiO2 Surface on Reactivity of Radical Ions
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Several differences are observed between the kinetics of the reaction of I2•- and (SCN)2•- with cysteine. The ratio of the rate constants for the reaction with I2•- with cysteine in solution to that heterogeneously generated on TiO2 suspended in solution containing the same cysteine concentration is 2.59, but the same ratio for (SCN)2•- is 11.1. This indicates that I2•- is less affected by the photocatalyst surface than (SCN)2•-, although both experience a reduction in rate relative to the solution phase. I2•- may be less strongly adsorbed onto the TiO2 surface than are (SCN)2•- radicals under our experimental conditions, although this is speculative. Figure 3. Bimolecular decay of photocatalytically generated (SCN)2•- produced by flash excitation (355 nm) on an aqueous TiO2 sol (0.050 M KSCN, pH 1.66) ([, lower curve) in the presence of ascorbic acid, 7.80 × 10-5 M, and (b, upper curve) without added ascorbic acid. The transient was monitored at 480 nm.
Figure 4. Bimolecular decay of photocatalytically generated I2•- produced by flash excitation (355 nm) on an aqueous TiO2 sol (0.050 M KI, pH 1.66) (9, lower curve) in the presence of cysteine, 6.30 × 10-3 M, and ([, upper curve) without added cysteine. The transient was monitored at 400 nm.
near or on the surface of the particle may account for the apparent increase in the bimolecular rate. I2•- Reactions. The one-electron oxidation of Iproduces an iodine atom, I•, which in the presence of excess iodide, reacts to produce I2•-. This species is easily observed in transient absorbance studies by virtue of its strong absorbance at 400 nm (400 ) 11 700 M-1 cm-1).9 When generated photocatalytically on TiO2, the decay (at 400 nm) of I2•- appeared to follow second-order kinetics, with a rate constant of (1.03 ( 0.05) × 1010 M-1 s-1, an apparent rate increase of 2.5 compared to the reported rate constant in aqueous solution of 3.9 × 109 M-1 s-1.9 Cysteine. As with (SCN)2•-, cysteine accelerated the decay of I2•- relative to its dismutation, Figure 4. The decay of I2•- became pseudo first order in I2•- at a cysteine concentration of 6.85 × 10-3 M. The bimolecular rate constant for the reaction of photocatalytically generated I2•- with cysteine was found to be (4.24 ( 0.05) × 107 M-1 s-1. In contrast, the rate constant for the same reaction in solution is 1.1 × 108 M-1 s-1.9 As with (SCN)2•-, it is possible to explain the difference as a consequence of charge repulsion between the surface and the substrate. (12) Martin, S. T.; Kesselman, J. M.; Park, D.; Lewis, N. S.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 2535. (13) Moser, J.; Punchihewa, S.; Infelta, P.; Gra¨tzel, M. Langmuir 1991, 7, 3012.
Mechanisms There are at least three means by which the photocatalyst surface might affect secondary dark reactions of a photocatalytically produced transient: (1) adsorptive alteration of the transient’s reduction potential, (2) changes in the chemical mechanism of its reaction with the added electron donor, or (3) modification of the local concentration of the electron donor. In the reaction between (SCN)2•- and hydroquinone, the same kinetic profiles are observed in the photocatalytic and homogeneous systems. It is unlikely that a coincidental combination of the three effects would produce a rate constant so close to that reported for the same reaction in homogeneous solution. Furthermore, if the reduction potential of adsorbed (SCN)2•- were unchanged for the reaction with hydroquinone, it would also be unlikely to be changed in any of the other reactions. Thus, the observed changes in reaction kinetics are likely to originate from differences either in the kind of interaction of the substrate with the surface-bound radical or in its local concentration. With cysteine, the repulsion between the like charges of cysteine and TiO2 at pH 1.66 is likely to cause significant depletion of cysteine in the immediate environment of the adsorbed thiocyanate dimer, relative to the concentration of cysteine in the bulk solution. In turn, this nonhomogeneous distribution of reagents would cause a slower apparent rate. With ascorbic acid, the expected strong adsorption of ascorbic acid onto TiO2 would enhance the local ascorbic acid concentration relative to the bulk, hence enabling a faster apparent rate. Conclusions The bimolecular rates of secondary dark reactions of photocatalytically oxidized transients produced on TiO2 sols depend on the local concentration of the coreactant. With a positively charged TiO2 surface, a cationic donor decelerates electron transfer quenching relative to the same reaction in solution, a neutral donor has no effect on the observed kinetics, and a strongly adsorbed electron rich or anionic donor accelerates this quenching. As with previously described dismutation reactions taking place on TiO2, these results illustrate the critical role of the heterogeneous surface, not only in initiating interfacial electron transfer but also on subsequent dark bimolecular chemical transformations. Acknowledgment. This work was suppported by the University Research Initiative of the U.S. Army Research Office and the Texas Advanced Research Program. LA9711600
