Langmuir 1992,8, 539-545
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Studies of Surface Properties of Clay Laponite Using Pyrene as a Photophysical Probe Molecule. 2. Photoinduced Electron Transfer Xinsheng Liu, Kai-Kong Iu, and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received August 27,1991. I n Final Form: November 15,1991 Photoinduced ionization of pyrene on clay Na+-laponitesactivated at different temperatures was examined by using photophysicaland electron spin resonance techniques. The results reveal that irradiation of the samples with UV light (370-300 nm) dramatically increases the yield of the pyrene cation radicals. The formation of pyrene cation radicals is shown to be a single photon ionization process. This was done by varying the intensity of UV light and by irradiation under the presence of 02.The mean lifetime of the pyrene cation radical created on the surface of Na+-laponite (tip) is 74 h. There is a distribution of electron-accepting sites on the surface which ionize the adsorbed pyrene molecules. Most of the active sites (82-86571)are located on the lateral surface, while others (14-18%) are located elsewhere in the regions that are unaffected by adsorbed water, polymetaphosphate, and basic molecules such as NH3. Introduction Clays have long been used as catalysts, adsorbents, and hosts in many aspects of industrial processes.'+ Studies of the surface properties of clays are essential for an understanding of the functions that the clay surface plays in catalysis. Previous studieslJ-9 have shown that active sites are present on clay surfaces which can accept electrons from organic molecules such as aromatic amines, the amine cations being formed.'t8 These sites are attributed to lewis acid sites created via valence unsaturation of framework constituent and to transition metal cations8with strong redox properties (either as charge balancing cations or as impurities). However, the exact nature of these active sites and the mechanism of electron transfer from the aromatic molecules to the surface are still not fully understood. Our previous studies'O on the surface properties of Na+-laponite using pyrene as a photophysical probe molecule have revealed that the adsorptive uptake of the laponite surface for pyrene is dramatically increased as the activation temperature for the laponite increases above 100"Cand that the surface active sites (which accept electrons from the adsorbed pyrene molecules) only exist in the temperature range of 100-400 OC. At higher temperatures, the electron affinity of the surface or its ability to remove e- from pyrene molecules is no longer significant. The relationship between the formation of pyrene cation radicals and the preactivation temperature indicates that only certain kinds of active sites, created during prethermal activation, are capable of ionizing pyrene molecules. In the present study, we concentrate on photoinduced (1) Theng, B. K. G. The Chemistry of Cloy-OrganicReactions; Adam Higler: London, 1978. (2) Thomas, J. K. Ace. Chem.Res. 1988,21,275-280, and the references therein. (3) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Moleculor Sieves; Academic Preee: London, 1978. (4) Thomas, J. M. Intercalation Chemistry;Academic Press: LondonNew York, 1982; p 55. (5) Rozengart, M. I., V'yunova, G. M.; Isagulyanta, G. V. Rws. Chem. Reo. (Engl. Trarurl.) 1988,57 (2), 115, and the references therein. (6) Brindley, G. W.; Sempels, R. E. Clay Miner. 1977,12,229. (7) Mortlaud, M. M.; Ramau, K. W. Cloys Clay Miner. 1968,16,393. (8) (a) Solomon, D. H.; Swiff, J. D.; Murphy, A. J. J. Macromol. Sci. Chem. 1971,5, 587. (b) Solomon, D. H.; Loft, B. C.; Swiff, J. D. Clay Miner. 1988, 7,387. (c) Clay Miner. 1968, 7,389. (9) Labbe, P.; Reverdy, G. Langmurr 1988,4, 41+425. (10) Liu, X.; Thomas, J. K. Langmuir 1991, 7 , 2808-2816.
ionization of pyrene molecules adsorbed on the surface of Na+-laponih by using photophysical and ESR techniques. The results showed that under UV irradiation (370-300 nm) the yield of pyrene cation radicals was dramatically increased compared to that produced thermally and, most significantly, that the process is verified to be a single photon ionization event. The results also showed that back electron transfer between the escaped electron and the pyrene cation radicals took place slowly and that the cations had a lifetime of tens of hours. In addition, we examine and discuss the possible location of electronaccepting sites on the laponite surface. Experimental Section Chemicals. Laponite RD (Laporte Industries) was used as received. The composition of the laponite is Si02 55.6% ,MgO 25.1%, Liz0 0.7%, NazO 3.6%, K2O 0.2%, and Fez03 0.O4%.l1 n-Pentane (AldrichProduct HPLC grade)was dried and purified by using molecular sieve 3A pellets. Pyrene crystals (Aldrich Product, 99%) were purified by column chromatography (silica gel and cyclohexane were used as adsorbent and eluant respectively),followed by recrystallization from cyclohexane solution. Preparation of Pyrene/Laponite Samples. One gram of laponite powder was heated to temperatures ranging from 20 to 650O C for 24 h, transferred into a desiccator, and allowed to cool. The pyrene-n-pentane solution with a concentration of 2 X 10-3 moVL was prepared by dissolving purified pyrene crystale in n-pentane. The solution (0.15mL) was mixed with the activated laponite present in a glass vial immediately after cooling and capped. The suspension was shaken for several minutes and left in the dark for about 12 h. The solid was then separated from its supernatant by filtration and washed with n-pentane. The supernatant was collected for UV-visible spectrophotometric measurement to estimate the amount of pyrene adsorbed on the clay. The final solid-pyrene samples were used for the photophysical measurements. Instruments. The electron spin resonance (ESR) spectra were recorded on a Varian E-lines Century Series electron spin resonance spectrometer operated in the X-band frequencyregion. The intensities of the ESR signals were measured by following the method described by Wertz and Bolton.'* Quartz tubes were used for samples under vacuum, which were connected to a vacuum line. Diffuse reflectance measurementswere carried out (11) Laporte Industries Technical Brochure L64. (12) Wertz, J. E.;Bolton, J. R.Electron Spin Resononce, Elementary Theory and Practical Applications; Capman and Hall: New York, 1986; p 21.
0743-746319212408-0539$03.00/0 0 1992 American Chemical Society
Liu et al.
540 Langmuir, Vol. 8, No. 2, 1992 on a Cary 3 UV-visible double beam spectrophotometer. The machine is equipped with an integrating sphere reflectance unit for measurement of solid opaque samples. For UV irradiation experiments, a photochemical reactor (Rayonet Model RPR100)equipped with 300-nmfluorescenttubes was used. A detailed description of the time-resolved diffuse reflectance spectrometer has been given in elsewhere.13 The Kubelka-Munk function was used to calculate the radical concentration from the reflectance data.14 A more detailed description of the nature of the photophysical-photochemical processes is given in ref 15.
Results and Discussion Laponite is a synthetic counterpart of natural hectorite clay with a layered structure. The layer is composed of twoSiO4 tetrahedralsheets and one Mg06 octahedral sheet arranged in a TOT sandwich (T = Si04 tetrahedral sheet and 0 = MgO6 octahedral sheet). The Mg2+ions in the octahedra are partially substituted by Li+ ions, which provides the source of negative charge of the sheet. The negative charge produced by the substitution is compensated by Na+ cations located in the interlayer space. The interlayer space can also accept water molecules and, as a consequence, is expanded to an extent, depending on the actual conditions of hydration. In an aqueous solution, laponite layers may separate completely and are present in the form of individual layers. On drying, the layers aggregate together and form particles with three or four layers in a stack." The actual structure of Na+-laponite may not be regular and may form a so-called "house of cards" structure owing to the ease of delamination. The surface of a dried laponite particle is built up of an outer surface of the stack which involves siloxane bonds and a lateral surface of the layers which include broken and terminated Si-0 and Mg-0 and/or Li-0 bonds as well as an interlayer surface partially covered by the Na+cations. The hydrated form of laponite exhibits surfaces covered by water. Fluorescence Decay Kinetics. It was found that the fluorescence decay of pyrene on clay@ and other solids such as Si0217 and A120a1* did not follow a single exponential decay process and that the decay could be treated by using the Gaussian distribution model, first developed by Albery et al.19 The model describes the natural logarithm of observed rate constants as a Gaussian distribution with two parameters: K, a mean rate constant, and y, adistribution parameter. The dispersion in the first-order rate constants for -m Ix Im becomes In (k)= In (K) + yx
(1) The observed decay profile is composed of the summation of the contributions from each microscopic species. A numerical solution of the integration (eq 2) over the distribution, exp(-x2)
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Figure 1. (a)Time-resolvedfluorescencedecayof excited pyrene on laponite preactivated at different temperatures. For clarity, three temperatures are shown: dashed line, 22 "C; dotted line, 105"C;solid line, 205 "C. Pyrene loading is about 1X lO-'mol/g. (b) Change of fluorescence decay parameters as a function of preactivation temperature. Samples are the same as in part a.
0.2 377
= 7 ( 2 [ g ( O . l ) + g(0.3)
+ g(0.5) + g(0.7) +
g(0.9)l + g(0.2) + g(0.4) + g(0.6) + g(0.8) + exp(-kt)) (3)
where, g(A) = X-l exd-[ln (X)]2J{exp(-ktA~) + exp(-KtXr)J. Because of the heterogeneity and the Gaussian distribution of the system, the observed fluorescencedecay rates in the presence of quenchers cannot be properly described by equations suitable for homogeneous solutions kobs = 12,
+ k,[quencherl
(4)
+ k,'[quencherl
(5)
but by the equation
I(')
J-Iexp(x2) exp[-kt exp(yx)l dx
I(0)
J-Iexp(-.z2) dx
-=
(2)
where J-1exp(-x2) dx = 1r1/2and x is the integration range, after transformation of the variable, x = In (A) for x < 0 and x = -In (A) for x > 0, gives equation (13) Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1991,95,506. (14) Kubelka, P.; Munk, F. Z . Tech. Phys. 1931, 12, 593. (15) Thomas, J. K. The Chemistry of Excitation at Interfaces; ACS Monograph 182; American Chemical Society: Washington, DC, 1984. (16) Kuykendall, V. G. Ph.D. Thesis, University of Notre Dame, 1989. (17) Krasnansky, R.; Koike, K.; Thomas, J. K. J. Phys. Chem. 1990, 94,4521-4528. (18) Pankasem, S.; Thomas, J. K. J.Phys. Chem. 1991,95,7385-7393. (19) Albery, W. J., Bartlett, P. N., Wilde. C. P. and Darwent. J. R.. J. Am. Chem. SOC.1985,107, 1854.
hobs
= KO
Lobs and KO are the average decay rate constants with and without quenchers in the conceptual framework of Gaussian distribution. After curve fitting with a single exponential, a double exponential, and a Gaussian distribution model, the best fit of the fluorescence decay of pyrene on laponite is via the Gaussian distribution model. The time-resolved fluorescence decays of pyrene on the laponites preactivated at different temperatures are shown in Figure la. Figure l b shows the changes of fluorescence decay parameters, KO and y, as a function of the preactivation temperature. The changes of the parameters with the preactivation temperature reflect the changes of the
Langmuir, Vol.8, No. 2, 1992 541
Surface Properties of Clay Laponite a
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Table I. Bimolecular Rate Constants for Oxygen Quenching of Excited Pyrene on Laponites preactivation temp, concn of pyrene, O C mol/g x 107 K,' X lO+/Torr.s 22 3.57 0.021 f 0.03 105 9.08 1.522 f 0.001 205 9.58 1.474f 0.001 325 9.91 1.478f 0.001 500 9.16 1.669i 0.001
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Figure 2. (a) A typical change of fluorescence decay of pyrene on the surface of laponite as a function of 02 pressure. The pressure of 02 increases from top to bottom in the order 0,17, 52,116,and 438 Torr. The-laponite was activated at 325 O C for 12 h. (b) A typical plot of kob as a function of 02 pressure. The sample is the same as in part a. surface properties of laponite on heating. It is seen from Fibure l b that the average decay rate constant (KO) increases for laponite preheated at 100 and 200 "C and then decreases as the temperature increases. This indicates that interactions between the adsorbed pyrene molecules and the laponite surface are stronger on increasing the preactivation temperature and then become weaker at higher temperatures (>300"C). The changes of ho with the preactivation temperature is consistent with the change of the IIVI intensity ratio of the fluorescence vibronic bands with the temperature, as shown in the previous studies.1° The change in the surface polarities on preactivation temperature is reflected by changes in the average decay rate constants. The difference of the mean lifetimes of the excited pyrene on the surfaces preactivated at different temperatures is about 35 ns. The change of y with temperature shows that the surface becomes more heterogeneous at high preactivation temperatures compared to that preactivated at low temperatures (see Figure lb). Figure 2a shows a typical quenching of pyrene fluorescence on the surface of laponite as a function of 02pressure. Attempts to fit the curves in Figure 2a also suggest that the decay under 02 does not follow a single exponential. However the curves are fitted with the Gaussian distribution function given above and the quenching parameters of the pyrene fluorescence obtained. A typical plot Of hobs as a function of pressure of 0 2 is given in Figure 2b. The linear change of hobs with the 0 2 pressure implies that the concentration of 02 on the laponite surface is proportional to that in the gas phase. Using the pseudo-first-order bimolecular quenching reaction kinetics (equation (5))
(
nm
Figure 3. Time-resolveddiffusereflectance absorption spectra of pyrene and anthracene on laponite taken 1 pa after the laser flash: dotted line with 0 symbol, pyrene on laponite under vacuum; dashed line with * symbol, pyrene on laponite under 400Torr of 02;solid line with X symbol, anthracene on laponite under vacuum. gives k,' for all samples, as shown in Table I. The most significant change in the 0 2 quenching rate is observed for samples preactivated at temperatures between 20 and 100 "C. The heterogeneity of the system causes the distribution parameter, y, to increase, a phenomenon which has already been observed in the pyrene-silica systems.17 Spectral Absorption Studies. Figure 3 shows the time-resolveddiffuse reflectance spectra of pyrene on l a p nite with and without 02 taken a t 1 ps after the laser flash, together with a similar spectrum for anthracene. Under vacuum, the adsorption bands of pyrene triplet at 410 nm and pyrene cation at 450nm are observed. The formation of the pyrene cation is confirmed by retaking the spectrum under 400 Torr 02 which eliminates interference due to the spectral overlap by the pyrene triplet (see Figure 3). The spectrum of anthracene will be discussed later. Figure 4shows a typical steady-state diffuse reflectance spectrum of a pyrene/laponite sample before and after photoreactor irradiation with UV light of wavelength of 300 nm. A dramatic increase in the yield of pyrene cation radicals, 24 times larger than that produced thermally, is observed. The dramatic increase in the yield of py-rene cation radicals on UV irradiation is also evident by a color change of the sample from white to yellow-green. The photoinduced formation of pyrene cation radicals on the surface of laponite indicates that an electron transfer reaction between the adsorbed pyrene molecules and the laponite surface has taken place and that the electrons have been captured by the electron accepting sites on the surface. Figure 5 shows changes in the yield of pyrene cation radicals as a function of preactivation temperature. In the figure, the general trend is that more cations are created at high preactivation temperatures (>300 "C). The
542 Langmuir, Vol. 8, No. 2,1992
Liu et al. 1
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Wavelength
+
( nm
Figure 4. Typical steady-statediffusereflectance transmittance spectra of pyrene/laponik dotted line, before irradiation; solid line, after irradiation with UV light of wavelength 300 nm. The laponite was activated at 325 O C for 12 h. 4
P
0
100
200
Pre-activation
475T
1
300 400 500 600 Temperature ("C)
Figure 6. Changes in yield of pyrene cation radicals as a function of preactivation temperature.
increase in the yield of pyrene cation radicals at high temperatures (>300"C)exhibits different characteristicsfrom the thermal case where the yield of pyrene cation radicals decreases.1° Irradiation of pyrene/air-dried laponite samples with UV light under vacuum also creates pyrene cation radicals, which was not observed in the thermal case. A comparison between the thermal and photoinduced cases for formation of pyrene cation radicals suggests that the electron-accepting sites responsible for the formation of pyrene cation radicals in the photoinduced case are not only located on the lateral surface but also on the other surfaces. In the photoinduced case, the electron-accepting sites on the surface are not destroyed by adsorbed water or by high preactivation temperature. ESR Studies. ESR studies of the samples exhibit results which are consistent with those seen in the UVvisible diffuse reflectance studies (Figure 6a). In order to avoid saturation of the ESR signals,low microwave power (
