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Adsorbed Thallium(I) Ions as a Probe of Surface Charge in UV-Irradiated Titania/Aqueous Solution Interfaces Puangrat Kajitvichyanukul,†,‡ C. R. Chenthamarakshan,§ Syed R. Qasim,† and Krishnan Rajeshwar*,§ Department of Civil and Environmental Engineering and Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received February 12, 2001. In Final Form: April 19, 2001 Thallium(I) is shown to be a sensitive probe of titania particle/aqueous solution interfaces in the dark and under ultraviolet (UV) illumination. We show that experimental conditions can be chosen such that photocatalytic reduction or oxidation of this probe is avoided. Thus, any changes in the measured solution levels of this probe can be simply attributed to corresponding variations in its adsorption tendency on the TiO2 particle surface. Data are presented on how this interfacial probe tracks modulations in the oxide surface charge induced either by variations in solution pH or by UV irradiation.
Introduction It is now well established that a variety of substances, organic, inorganic, or biological, can be photooxidized or photoreduced at titania particle/aqueous solution interfaces.1-3 However, the mechanistic details associated with such interfacial processes are less clear. The influence of initial substrate adsorption (on the oxide surface) on the rate/efficiency of photocatalytic conversion is also not conclusively established although studies are emerging in this direction.4,5 Particularly useful in this regard would be a solution probe that can report on the state of charge of the oxide surface and respond to any changes induced in it, for example, by modulation of solution pH or via oxide illumination.6,7 Ideally, this probe should be “inert” toward photooxidation (or photoreduction) so that the measured concentration changes undergone by it can be solely and straightforwardly attributed to the adsorption/ desorption event. We show herein that Tl(I) exhibits these prerequisites as a surface charge probe at the TiO2/aqueous solution interface. The data presented below illustrate its sensitivity to the surface charge of the oxide particles and modulations induced in it. Experimental Section The TiO2 (Degussa P-25) sample used was predominantly anatase and had a specific surface area of ∼55 m2/g. The TiO2 suspension dose was 2 g/L. Tl2SO4 (99.9%) was from Aldrich and was used without further purification. CAUTION: Extreme care is to be exercised in the preparation, handling, and disposal of thallium solutions because of the toxicity of this element. * Author for correspondence:
[email protected]. † Department of Civil and Environmental Engineering. ‡ On leave from King’s Mongkut University of Technology, Thonburi, Thailand. § Department of Chemistry and Biochemistry. (1) Blake, D. M. Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds from Water and Air, NREL/TP-340-22197; National Renewable Energy Laboratory: Golden, CO, 1997. (2) Rajeshwar, K.; Ibanez, J. Environmental Electrochemistry; Academic Press: San Diego, CA, 1997. (3) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis; BKC: Tokyo, Japan, 1999. (4) Dobson, K. D.; McQuillan, A. J. Langmuir 1997, 13, 3392. (5) Ekstrom, G. N.; McQuillan, A. J. J. Phys. Chem. B 1999, 103, 10562. See also references therein. (6) Boxall, C.; Kelsall, G. H. J. Chem. Soc., Faraday Trans. 1991, 87, 3537. (7) Dobson, K. D.; Connor, P. A.; McQuillan, A. J. Langmuir 1997, 13, 2614.
Deionized water (Corning Megapure) was used in all the cases for preparing solutions or suspensions. All other chemicals were of reagent grade and used without further purification. The photoreactor used has been described elsewhere.8,9 The UV light source was a medium-pressure Hg arc lamp with 100-, 250-, or 400-W output, with the 400-W lamp nominally used unless specified otherwise. The TiO2 suspensions (containing 200 µM Tl(I)) were agitated by sparging with ultrapure Ar in the dark for 30 min. Then the UV light was turned on and aliquots were syringed out periodically. The inert gas sparge was maintained throughout the experiment. The TiO2 particles were removed by 0.45 µm poly(tetrafluoroethylene) (PTFE) syringe filters, and the solutions were analyzed for Tl(I) levels by a colorimetric procedure.10,11 UV-visible spectra for this purpose were recorded on a HewlettPackard model HP8452 diode array spectrometer. Calibrations with standard Tl(I) solutions were done using Beer’s law plots constructed from the measured absorbance of the thalliumBrilliant green complex at 640 nm.10,11 The solution pH was adjusted from ∼7 to the targeted value by adding either H2SO4 (acidic range) or ammonia (basic range).
Results and Discussion It is important to emphasize at the outset that all the changes in the Tl(I) solution levels described below arise from adsorption (or desorption) of this probe on the TiO2 particle surface. Under the conditions extant in this study, Tl(I) does not undergo photocatalytic reduction (to Tl0) or oxidation (to Tl(III)) in the irradiated TiO2 suspensions. This is because direct reduction of Tl(I) by the photogenerated electrons is prohibited by its rather negative standard reduction potential (-0.336 V versus the standard hydrogen electrode12). Similarly, photocatalytic oxidation of Tl(I) (to TlO or Tl2O3)13 is inhibited by the lack of an electron acceptor (e.g., O2) in our Ar-purged solutions. Support for the above assertions comes from our related studies (to be published separately),14 from solution and oxide analyses (for Tl(III) and Tl0, respectively) after prolonged (several hours) UV irradiation, and (8) Lin, W.-Y.; Wei, C.; Rajeshwar, K. J. Electrochem. Soc. 1993, 140, 2477. (9) Lin, W.-Y.; Rajeshwar, K. J. Electrochem. Soc. 1997, 144, 2751. (10) Ariel, M.; Bach, D. Analyst 1963, 88, 30. (11) Fogg, A. G.; Burgess, C. Analyst 1973, 98, 347. (12) Handbook of Chemistry and Physics, 79th ed.; CRC Press: Boca Raton, FL, 1998-1999. (13) For example: Inoue, T.; Fujishima, A.; Honda, K. Chem. Lett. 1978, 1197. (14) Kajitvichyanukul, P.; Chenthamarakshan, C. R.; Qasim, S. R.; Rajeshwar, K. To be published.
10.1021/la010228f CCC: $20.00 © 2001 American Chemical Society Published on Web 06/02/2001
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Figure 1. Change in the concentration of Tl(I) adsorbed on the TiO2 particle surface in the dark, as a function of solution pH. The surface concentration values shown on the right axis were computed from TiO2 particle surface area and solution dose. The concentration of the adsorbed Tl(I) species was determined from the change in the corresponding solution levels before and after the 30 min equilibration period (see Experimental Section).
even from visual observations. For example, photocatalytic reduction of Tl(I) to Tl0 brings about a noticeable change in the color of the TiO2 suspension from white to gray.14 No such color changes were noted in the experiments described below. Figure 1 maps the change in the concentration of Tl(I), adsorbed on the TiO2 surface in the dark, as a function of solution pH. The adsorption is minimal in strongly acidic media and then steadily increases leveling off to a plateau between pH 6 and pH 8. Beyond pH 8, the adsorption strongly increases in basic solutions. At a pH of 11, for example, almost all the Tl(I) species initially present in the solution (∼99% out of 200 µM initially present) are adsorbed on the oxide surface. Thus, it appears from the data in Figure 1 that there is a strong electrostatic component to the adsorption of Tl(I) ions on the TiO2 surface. Recall that the oxide surface is positively charged below the point of zero charge (PZC) which, for TiO2, lies in the 6-8 range.2,15,16 Like-charge repulsions will become less predominant as the solution pH approaches the PZC regime explaining the systematic increase observed in the amount of Tl(I) adsorbed. Beyond the plateau region, the increased negative surface charge borne by the TiO2 surface and a possible switch to a multilayer adsorption mode both contribute to the much steeper increase in [Tl+]ads noted in the pH 9-11 range. Blank experiments omitting TiO2 did not reveal any precipitation of Tl(I) in the pH range of interest here. Figure 2 contains data for Tl(I) levels in solution as a function of UV irradiation time for both acidic (Figure 2a) and basic (Figure 2b) oxide suspensions. In interpreting these data, it is noted that the Tl(I) solutions were preequilibrated with the TiO2 particle surfaces in all the cases prior to irradiation. In all the cases in Figure 2, an initial concentration of Tl(I) of 200 µM was employed. Thus the ordinate value at time zero in each experiment reflects the amount of Tl(I) initially adsorbed on the TiO2 surface in the dark. Interestingly, the changes in [Tl+]ads, when the UV light is turned on, are opposite in sign for acidic vs basic media. That is, more Tl(I) is adsorbed (relative to the dark adsorption level) in the presence of UV irradiation of the (15) Parfitt, G. D. Prog. Surf. Membr. Sci. 1976, 11, 198. (16) Noh, J. S.; Schwarz, J. A. J. Colloid Interface Sci. 1989, 130, 157.
Figure 2. Tl(I) solution concentration versus irradiation time in Ar-sparged TiO2 aqueous suspensions with pH as a parameter: (a) pH ) 2 (0), 3 (b), 4 (4), 5 (1), 6 (]), 7 (+); (b) pH ) 8 (9), 9 (O), 9.4 ([), 10 (3), 11 (×). Table 1. Change in Tl(I) Photoadsorbed/Photodesorbed on/from the TiO2 Particle Surface as a Function of pHa
pH
change in adsorbed Tl(I) concn upon illuminationb µΜ (µmol/m2)c
3 4 5 6 7
14.7 (0.134) 13.1 (0.119) 7.60 (0.069) 6.70 (0.061) 4.60 (0.042)
pH
change in adsorbed Tl(I) concn upon illuminationb µΜ (µmol/m2)c
8 9 9.4 10
-10.8 (-0.098) -15.4 (-0.140) -22.9 (-0.208) -39.4 (-0.358)
a Determined from data in Figure 2 as described in the text. Data for pH 2 and pH 11 not included for reasons outlined in the text. b Positive sign denotes photoadsorption; negative sign denotes photodesorption. c Surface concentrations computed from TiO2 particle surface area and solution dose.
oxide suspension in acidic solutions (Figure 2a). An exception is the pH 2 case; see above. Conversely, Tl(I) is photodesorbed in basic media (Figure 2b) except in the pH 11 case; see above. It is again emphasized that the observed changes in Figure 2 in Tl(I) solution levels on irradiation of the oxide suspensions are solely due to photoadsorption/photodesorption events and not due to photocatalytic reactions undergone by Tl(I) for reasons mentioned earlier. By the same token, control runs omitting TiO2 show no changes in Tl(I) levels, consistent with the negligible role of any homogeneous photochemical reactions in these cases. Table 1 quantifies the amount of Tl(I) photoadsorbed or photodesorbed as a function of the solution pH. These values were culled in each case from the difference between
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can be expected to be minimal when the TiO2 surface is extensively protonated (as in the pH 2 case) and when like-charge repulsion effects dominate (Figure 2a). In basic solutions, on the other hand, the oxide surface is hydroxylated.2 Further, the hydroxylated surface is known to be reactive toward the photogenerated holes in the oxide.2 Thus, the negative sheath of charge presented by a hydroxylated TiO2 surface (toward the solution phase) can be expected to be affected by the oxidation of some of the surface hydroxyl groups by the holes. This, in turn, would serve to lessen the (electrostatic) binding of the Tl(I) species resulting in their partial desorption from the oxide surface. Once again, in very basic media (e.g., pH 11), this light-induced perturbation of the surface appears superimposed on a large charge envelope and thus exerts a minimal effect on the concentration of the bound Tl(I) species (see Figure 2b). If the above mechanistic picture is correct, then the photon flux would be expected to exert a pronounced effect given that the number of photogenerated electrons and holes is proportional to this variable. The data in Figure 3 illustrate that this is indeed true, for both acidic and basic solutions, as exemplified by the pH 3 and pH 10 cases, respectively. In fact, the amount of Tl(I) species photoadsorbed or photodesorbed in the two cases faithfully tracks the UV lamp output as shown in Figure 3b.
Figure 3. (a) As in Figure 2 but for varying UV lamp output and for two pH values shown. (b) Plots of change in adsorbed Tl(I) probe concentration with lamp output constructed from the data in Figure 3a.
the initial value (after dark adsorption) and the plateau attained after ∼30 min of illumination (Figure 2). It is interesting that the light-induced modulations in [Tl+]ads are more pronounced for basic media vis-a`-vis acidic solutions notwithstanding the opposite polarity of the photoeffects. However, the modulations are minimal at both extrema (2 and 11) of the pH range considered here. How can these light-induced modulations (or the lack thereof, cf. pH 2 and 11) be rationalized? We believe that at least two mechanisms are operative here. Irradiation causes electrons to accumulate on the TiO2 surface.2 The increased negative charge then serves to bind more (positively charged) Tl(I) species. However, this photoeffect
Concluding Remarks Thallium(I) is shown in this study to be a sensitive probe of charge-modulating processes in the TiO2 particle/ aqueous solution interface. Notwithstanding the (as yet unknown) molecular details of the Tl(I) adlayer on the TiO2 surface, variations in the oxide surface charge induced by (a) alterations of solution pH, (b) electron accumulation on the oxide surface, and (c) photooxidation of surface hydroxyls by holes, all bring about corresponding faithful variations in the number of probe species absorbed on the oxide surface. Exceptions to this trend occur in extreme interfacial situations when the probe adsorption is either negligible (pH 2 case) or very appreciable (pH 11 case). Further studies will attempt to characterize the Tl(I) adlayer on the TiO2 surface and to examine whether similar trends apply to other semiconductor oxide/probe combinations as well. Acknowledgment. This research was supported, in part, by the U.S. Department of Energy, Office of Basic Energy Sciences. P.K. wishes to thank the National Energy Policy Office, Government of Thailand for funds to undertake graduate study at the University of Texas at Arlington. We thank the two reviewers for constructive criticisms of an earlier manuscript version. Last, but not least, we thank Ms. Gloria Madden for assistance in manuscript preparation. LA010228F
