Envlron. Sci. Technol. 1992, 26, 1963-1966
Environmental Photochemistry on Surfaces. Charge Injection from Excited Fulvic Acid into Semiconductor Colloids K. Vlnodgopal'
Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408 Prashant V. Kamat'
Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 The ability of naturally occurring fulvic acid to sensitize a large band-gap semiconductor, colloidal ZnO, has been investigated by fluorescence emission and transient absorption measurements in a mixed alcohol-water system. The fulvic acid strongly adsorbs on the semiconductor particles, with an apparent association constant of 12000 f 500 M-l. The net charge-transfer efficiency as determined by the fluorescence quenching of reference Suwanee River fulvic acid (SFA) by ZnO was 73%. The laser flash photolysis experiments that elucidate the mechanistic details of the charge injection from excited fulvic acid into the conduction band of ZnO (ket = 6.8 X lo8 s-l) are described.
Introduction The photocatalyzed degradation of organic environmental pollutants in the presence of a semiconductor such as TiOz or ZnO has become a subject of increasing study over the last decade (1-8) and shows promise in becoming a viable commercial technology (9). In most cases, these studies have involved photoredox reactions induced by charge separation (electrons and holes) which are created as a result of illuminating the semiconductor particles. Since most of the semiconductors used in these studies have large band gaps, they require excitation in the ultraviolet region of the spectrum. An important way to extend the response of the semiconductor is by photosensitization. Photosensitization of a stable large band-gap semiconductor is an interesting and useful phenomenon which extends the semiconductor's absorptive range and thus enables photoelectrochemical reactions under visible light irradiation (10). An obvious choice as photosensitizers would be humic substances. Humic substances (HS) are polymeric oxidation products that result from the decomposition of plant and animal residues. One important characteristic of humic substances lies in their ability to initiate the photochemical transformation of organic compounds in natural water and their eventual degradation (11-17). Although the use of humic substances as sensitizers for extending the photoresponse of a large band-gap semiconductor has been suggested in the past (16),there is no direct experimental evidence in the literature that demonstrates the feasibility of such an approach. A considerable volume of literature (see, for example, refs 13 and 14) exists on the photochemistry of humic substances although knowledge of the primary photochemical processes is limited. Laser flash photolysis studies (24-17) have indicated the formation of three transients during photolysis, viz., the aqueous hydrated electron and transients with radical and triplet properties, respectively. The hydrated electron is believed to result from photoejection of an electron from the excited state of humic substances. The electron can back-react to form a neutral HS molecule or electron transfer is now possible to a suitable electron acceptor. 0013-936X/92/0926-1963$03.00/0
For the first time we have obtained evidence for a direct charge-transfer interaction between a semiconductor ZnO and a humic substance by using the fluorescence emission of the latter as a probe. The results that describe the charge injection mechanism from fulvic acid to ZnO are presented here.
Experimental Section Suwanee River fulvic acid (reference) (SFA) was obtained from the International Humic Substances Society (Colorado School of Mines, Golden, CO) (18). A 0.02 M ZnO colloidal suspension in ethanol was prepared by the method described by Spanhel and Anderson (19) with stoichiometric addition of LiOH to the organometallic zinc precursor solution. Since the colloidal ZnO suspensions tend to precipitate in aqueous solution, solutions of SFA were prepared in ethanol in the following manner. A concentrated solution of SFA was prepared in doubly distilled water. A 50-pL aliquot of this stock solution was then diluted with 3 mL of alcohol such that the optical density of the resulting solution (1.6-98.4% ethanol-water) was -0.2 (path length 1cm) at 400 nm. AU solutions were deoxygenated by bubbling nitrogen gas through them for a period of 10-15 min. Background-corrected emission and excitation spectra of the SFA samples were measured with an SLM 5-8000 spectrofluorometer. Unless otherwise stated, emission spectra were corrected for the wavelength dependence of the detection system. Rhodamine 6G solution was used as a reference counter for recording fluorescence excitation spectra. The absorption spectra were recorded with a Perkin-Elmer 3840 diode-array spectrophotometer. Time-resolved laser flash photolysis experiments were carried out in a rectangular quartz cell with a 6-mm path length. The 532-nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta Ray DCR-1 NdYAG laser system was used for the excitation of the sample. A 1000-W xenon lamp was used as the monitoring source. The details of the flash photolysis setup are described elsewhere (18). A typical experiment consisted of a series of 5-10 replicate shots per sample, and the average signal was processed with an LSI-11 microprocessor interfaced to a VAX 11/70 computer. All experiments were carried out at room temperature (296 K). Results and Discussion The absorption, emission, and excitation spectra of SFA in water are shown in Figure 1. As is well-known (21),the absorption spectrum of fulvic acid in water is featureless, showing no maximum or minimum. The fluorescence spectrum is broad with a maximum around 475 nm, consistent with earlier reports in literature (22,23).Changing the excitation wavelength did not show any effect on the emission spectrum. In contrast to the featureless absorption spectrum,the excitation spectrum of SFA displays three bands: two in the W, one below 320 and the second centered at -340 nm, and a visible band with a maximum
@ 1992 American Chemical Society
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Wavelength, nm Figure 1. (a) Absorption, (b) corrected fluorescence emission, and (c) fluorescence excitation spectrum of SFA in water. The excitation wavelength for spectrum b was 370 nm, and the emission wavelength for spectrum c was 500 nm. The emission spectrum was corrected for backgroup using water as a blank and for the wavelength dependence of the detection system. I .o
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The excitation spectrum of the SFA-ZnO system shows a decrease in the emission intensity consistent with the quenching of the fluorescence but otherwise exhibits no change in spectral features as compared to SFA alone. The marked decrease in the fluorescence yield was due to the quenching of the excited singlet state of SFA by ZnO colloid and indicates an electron transfer from SFA to the ZnO semiconductor. The broad nature of the absorption and emission spectra of SFA does not permit resolution of any spectral changes that might occur as a result of this charge-transfer interaction with the ZnO surface. An efficient quenching of the excited singlet state is seen as a result of the strong adsorption of the sensitizer on the semiconductor particles. The participation of ZnO colloid in the quenching of HS emission can be analyzed by considering an equilibrium between adsorbed and unadsorbed molecules of the sensitizer (HS) with an apparent association constant, Kapp(equilibrium 1). K w
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Wavelength ( n m ) Figure 2. Fluorescence emission spectra of SFA in ethanol at various concentrations of colloidal ZnO: (a) 0, (b) 0.13,(c) 0.16,(d) 0.19,(e) 0.32,and (f) 0.64 mM. Excitation wavelength was 370 nm. (The spectra were not corrected for the photomultiplier response, but were corrected for the background scatter.)
at -420 nm. Previous studies (21)on SFA have suggested that the band observed in the visible region of the excitation spectrum at 420 nm is possibly an artifact with its origin in the Raman scattering of water. Such an interpretation can be ruled out, however, since the fluorescence studies displayed have their background subtraded in each case. Also the excitation maximum at 420 nm was independent of the monitoring wavelength of emission. The fluorescence quantum yield of SFA was calculated to be 0.011, using the emission from a solution of quinine sulfate in 0.5 M H$04 as the reference (& = 0.55 f 0.03). Colloidal ZnO (particle diameter 20 A) is a large bandgap semiconductor (e.g., >3.2 eV), which absorbs below 330 nm. The fluorescence quantum yield of the SFA decreased upon successive addition of colloidal ZnO, as is shown in Figure 2. The blue shift in the residual emission is probably an artifact which arises as a result of scattering. 1964 Environ. Sci. Technol., Vol.
26,No. 10, 1992
(1) As shown earlier (10,241, the observed quantum yield, cbem(obsd)of the ZnO-HS system can be related to the emission yields of unassociated (@Oem) and associated HS by the equation
+em(obsd) = (1- a ) @ O e r n + a@'em (2) where a is the degree of association between the sensitizer HS and the semiconductor colloid. At relatively high ZnO concentrations, a can be equated to K,,,[ZnO]/(l + K,,,[ZnO]) and eq 2 could be simplified to the following expression (IO) 1 @'ern
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If the observed fluorescence quenching is entirely due to the associated complex of colloidal ZnO and the sensitizer, then a plot of l / ( @ O e r n - @,,(obsd)) versus the reciprocal concentration of ZnO colloid is expected to be linear with an intercept equal to l / ( @ O e m - @'em) and a slope equal to l/Ka Jdoern- @'em). Such a straight-line plot (Figure 3) is in fact observed for the SFA-ZnO system, giving a Kap, value of 12000 f 500 M-l. The large value of K,, that is obtained with SFA indicates a strong association between it and the zinc oxide colloids, which is an essential prerequisite €or observing the heterogeneous charge-
transfer process at the semiconductor sensitizer interface. If the decrease observed in the emission yield of the humic substance is entirely due to a charge injection process, it is possible to determine the rate constant for the charge injection (kdfrom excited humic substance into ZnO colloid. In the absence of a quencher, the deactivation of excited humic substance occurs by radiative (k,)and nonradiative (k,) processes. When HS is associated with ZnO, electron transfer (keJ from the excited state of the humic substances can also contribute to the emission decay. If we consider that the decrease observed in the emission yield is entirely due to the charge injection process, then we can express the fluorescence quantum yields, $‘em and 4’em as kr
(4)
dJ0m=
The rate constant for the charge injection process (k,) can be expressed as (5) ket = (4’em - 4’ern)(l/74’em) where and +’em are the emission yields of unassociated and associated HS and 7 is the emission lifetime of HS, which is expressed as T = l/(kr + knr). and T are experimentally measurable paSince rameters and the emission yield of the semiconductorhumic substance complex (#’em) can be determined from the intercept of the plot in Figure 3, it is possible to calculate k,, from expression 5. For example, 40emfor SFA and dJLm for SFA-ZnO are 0.011 and 0.003, respectively. If we substitute these values and the value of the average emission lifetime of SFA in ethanol [ (7)= 3.9 ns (25,2611 in eq 5, we obtain the value of k,, as 6.84 X lo85-l. This rate constant for the heterogeneous electron transfer between HS and semiconductor colloid is comparable to the rate constants obtained in other dye sensitization experiments, indicating that SFA could serve as an efficient photosensitizer. If we express the net charge injection efficiency as 71 (%) = (4’em - 4’em)/(4’em) X 100 (6) one can obtain an upper limit for the sensitization efficiency. An upper limit of 73% was obtained for the sensitization of ZnO colloids with excited SFA. The quenching of the fulvic acid fluorescence is a consequence of the charge injection into the semiconductor according to SFA SFA* (7)
-
+
+
SFA* ZnO SFA’+ ZnO(e-) (8) If indeed such a process should occur in the presence of ZnO, we should be able to characterize the products by laser flash photolysis measurements. Laser Flash Photolysis of SFA-ZnO System. The transient absorption spectrum of SFA recorded following the excitation of SFA in the absence of ZnO colloids is shown in Figure 4 (spectrum a). As shown earlier (14-17), the spectrum exhibits absorption bands in the region 400-500 nm and at wavelengths greater than 600 nm. The transient absorption at 480 nm is attributed to the cation radical SFA’+, while the absorption at longer wavelengths (A > 600 nm) is due to the solvated electron which is formed as a result of photoionization of SFA
nhv
+
SFA* SFA’+ e-(solv) (9) The dependence of relative yield of solvated electrons on the 532-nm laser dose indicates this photoionization process in ethanol to be a biphotonic process.
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The transient spectrum recorded in the presence of ZnO colloids (Figure 4 (spectrum b)) exhibits similar features, but the yield of the product is considerably higher. In this case, reaction 8 dominates so that the injected electron gets trapped at the ZnO surface. Zepp et al. (17) have studied the photoionization of SFA in aqueous media using 355-nm laser excitation. They reported the absorption maximum of the solvated electron to be in the region of 700-750 nm. In the present study, the absorption maximum is blue shifted as a result of the change in the medium as well as the trapping effects on the ZnO surface. It had been shown earlier (27)that trapped electrons in ZnO colloids also exhibit broad absorption in the red region. Further support for this mechanism is obtained from the linear dependence of the transient absorbance at 640 nm on the laser intensity, suggesting thereby that this process is monophotonic. The trapping of electrons at the semiconductor surface also reduces the recombination between injected charge and the cation radical. Such a long-lived charge at the semiconductor surface is responsible for improving the photocatalytic properties of the semiconductor. Further support for the charge injection from excited SFA into ZnO colloids was obtained by varying the concentration of ZnO colloids. The maximum absorption at 620 nm was taken as a measure of the net electron-transfer yield in the sensitization of the ZnO colloids. The dependence of AA (620 nm) on the ZnO concentration is shown in Figure 5. At low ZnO concentrations, an increase in the electron-transfer yield was observed as an increasing amount of excited SFA interacted with ZnO colloids. However at concentrations greater than 1 . 0 mM, the yield reached a plateau. At these higher ZnO concentrations, the charge injection process is limited by the availability of SFA adsorbed onto the ZnO surface. Similar dependence of coverage had been demonstrated in earlier charge-transfer studies of semiconductor colloids (28). The key question that remains to be answered is how one can utilize such trapped charge carriers for the reduction of another substrate, such as an environmental pollutant. Detailed studies to answer this question as well as to elucidate the mechanistic and kinetic details of the sensitization of colloidal TiOz and ZnO by humic and fulvic acids are currently being carried out.
Conclusions By using a colloidal ZnO system, we have demonstrated the photophysical and photochemical processes that conEnvlron. Sci. Technol., Vol. 26, No. 10, 1992
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Z n O , mM Figure 5. Dependence of the net electron-transfer yield on ZnO concentration. The electron-transfer yield was monitored by the maximum transient absorbance ( A A )at 620 nm, following the 532-nm laser pulse excltatlon of SFA in ethanol. (O.D. at 532 nm 0.0458.)
trol the sensitizing properties of SFA. Both quenching of the SFA emission by ZnO and characterization of the trapped electron at the ZnO surface are consistent with the sensitization of ZnO by SFA.
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1968 Environ. Sci. Technol., Vol. 26, No. 10, 1992
(9) Low, G.K.-C.; Matthews, R. W. Anal. Chim. Acta, in press. (10) Kamat, P. V.; Chauvet, J.; Fessenden, R. W. J. Phys. Chem. 1986,go, 1389. (11) Fischer, A. M. Ph.D. Thesis, University of Califomia-Santa Cruz, 1985. (12) Zepp, R.G.;Schlotzhauer, P. F.; Sink, R. M. Environ. Sci. Technol. 1985,19,74. (13) Zepp, R. G.In Humic Substances and Their Role in the Environment;Frimmel, F. H., Christman, R. F., Eds., John Wiley and Sons: New York, 1988;pp 193-214. (14) Fischer, A. M.; Winterle, J. S.; Mill, T. In Photochemistry of Environmental Aquatic Systems; Zika, R. G., Cooper, W. J., Eds.; ACS Symposium Series 327;American Chemical Society: Washington DC, 1987;pp 141-156. (15) Power, J. F.; Sharma, D. K.; Langford, C.; Bonneau, R.; Joussot-Dubien, J. In ref 14,pp 157-173. (16) Langford, C. H.; Carey, J. H. In ref 14,pp 225-239. (17) Zepp, R. G.;Braun, A. M.; Hoigne, J.; Leenheer, J. A. Enuiron. Sci. Technol. 1987,21, 485. (18) Everett, R. C., Lenheer, J. A., McKnight, D. M., Thorn, K. A., Eds. Humic Substances in the Suwanee River Georgia: Interactions, Properties and Proposed Structures. OpenFile Rep.-US. Geol. Surv. 1989,No. 87-557. (19) Spanhel, L.; Anderson, M. A. J. Am. Chem. SOC.1991,113, 2826. (20) Das,P. K.; Encinas, M. V.; Small, R. D., Jr.; Scaiano, J. C. J. Am. Chem. SOC.1979,101,6965. (21) Donard, 0. F. X.; Belin, C.; Ewald, M. Sci. Total Environ. 1987,62,157. (22) Choudhry, G. G. Toxicol. Environ. Chem. 1981,4, 261. (23) Ewald, M.; Bellin, C.; Berger, P.; Weber, J. H. Environ. Sci. Technol. 1983,17,501. (24) Gopidas, K. R.;Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990,94,6435. (25) Power, J . F.; LeSage, R.; Sharma, D. K.; Langford, C. H. Enuiron. Technol. Lett. 1986,7, 425. (26) Vinodgopal, V.; Kamat, P. V., to be published. (27) Kamat, P. V.; Patrick, B. J , Phys. Chem., in press. (28) Kamat, P. V. In Kinetics and Catalysis in Heterogeneous Systems; Gratzel, M., Kalyanasundaram, K., Eds.; Surfactant Science Series 38;Marcel Dekker, Inc.: New York, 1991;pp 375-436. Received for review January 6,1992. Revised manuscript received May 4,1992.Accepted June 18,1992. P. V.K. acknowledges the support of the Office of Basic Energy Sciences of the Department of Energy, and K.V. acknowledges the support of Indiana University Northwest through a Summer Faculty Fellowship and a Grant-In-Aid. This is ContributionNo. NDRL-3441from the Notre Dame Radiation Laboratory.
