J. Phys. Chem. 1992,96,6377-6381 and the initial flux of carriers toward the surface opposes the flow of carriers toward the bulk. Because carriers in the bulk are more likely to undergo radiative relaxation, a lower luminescence intensity can be anticipated under picosecond illumination than for cw illumination, given that all other parameters are unchanged. Examination of eq 6 indicates that a lower measured intensity associated with this non-steady-state condition will result in calculated surface minority trapping velocities which err to larger values. The degree to which this condition influences the results is unknown and will require a modification of the dead layer model which includes dynamic effects. However, a simple correction to the model can be made by using a small value for D. This has the effect of holding the steady-state model carrier distribution closer to the surface, as is the case in the experiment under picosecond pulse excitation. (A similar approach has been applied to diffusion length measurements in order to account for selfabsorption. Self-absorption of luminescence photons has been shown to increase the effective diffusion length in GaAs and requires the use of a large diffusion coefficient in order to model experimental resultss.) In fact when we use a value of 3.5 cm2/s for the diffusion coefficient in our calculations, the extracted STV's are in agreement with the values obtained from the luminescence decays. The assumption that the surface minority trapping velocity is infinite at +0.5 V may not hold under the higher power illumination. The dead layer model as it is presented here provides no means for correcting for this. Qualitatively, however, we expect the luminescence intensity at +0.5 V to increase as the STV at +0.5 V becomes smaller, and the entire scan should then be scaled to greater intensity. Normalizing the data to the model at +0.5 V therefore also introduces a condition which may result in STV's which err toward values which are too large.
6377
Conclusions The present study provides further independent evidence that saturation of surface traps can occur under modest picosecond pulse excitation. Comparison of results under cw illumination of the same average power also indicate that it is the peak power of the picosecond pulses which saturates the traps, and we reemphasize the cautions stated in the study of luminescencedecay profiles: STV's measured under high injection conditions (or even under milder excitation) may not represent the true rate of surface minority trapping under normal operating conditions for surface barrier devices. Finally, comparison of the STV's recovered from luminescence decay profiles and the dead layer model suggest that the dead layer model alone is inadequate for determination of the dependence of the surface minority trapping velocity on photon flux under picosecond pulse excitation. Acknowledgment. This work was supported by DOE Grant DEF G06-86ER45273. Registry No. GaAs, 1303-00-0; Na2S, 13 13-82-2. References and Notes (1) (a) Evenor, M.; Huppert, D.; Gottesfeld, S.J. Electrochem. Soc. 1986, 133,296. (b) Benjamin, D.; Huppert, D. J. Phys. Chem. 1988,92,4676. (c) Bessler-Podorowski,P.; Huppert, D.; Rosenwaks, Y.; Shapira, Y. J. Phys. Chem. 1991, 95, 4370. (2) Kauffman, J. F.; Balko, B. A,; Richmond, G. L. J. Phys. Chem., preceding paper in this issue. (3) Smandek, B.; Chmiel, G.;Gerischer, H. Ber. Bunsen-Ges.Phys. Chem. 1989, 93, 1094. (4) Chmiel, G.;Gerischer, H. J. Phys. Chem. 1990, 94, 1612. (5) Hobson, W. S.;Ellis, A. B. J . Appl. Phys. 1983, 54, 5956. (6) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (7) Li, J.; Peter, L. M. J. Electroanal. Chem. 1986, 199, 1 . (8) Ehenberg, M. Appl. Phys. Lett. 1977, 30, 207.
Photochemistry of Azo Compounds on Silver Island Films Studied by Surface Enhanced Raman Spectroscopy D. Franzket and A. Wokam* Physical Chemistry II, University of Bayreuth, D- W-8580 Bayreuth, Germany (Received: February 5, 1992; In Final Form: April 14, 1992)
Photochemical decomposition reactions of (methoxyphenyl)azosulfonates,of (3-vinylphenyl)azosulfonate,and of the model compound 4-nitrobenzoic acid, are studied by surface enhanced Raman scattering (SERS). The reactants are deposited onto silver island films by spin coating. Subsequent to pulsed excimer laser irradiation at 308 nm,Raman spectra are excited using a continuous wave visible laser. In the case of 4nitrobenzoic acid, the formation of an azodibenzoateradical recombination product is confirmed. For (2-methoxyphenyl)azosulfonate,the photolysis yield has been determined by monitoring the decrease in Raman intensities of the -N=N- stretching mode at 1486 cm-I and of the 1064-cm-I skeletal vibration. When referred to the azosulfonate absorption, the photochemical quantum yield is found to be enhanced by 1 order of magnitude, as compared to the decomposition reaction in aqueous solution.
Introduction
In suitable systems, surface enhanced Raman scattering (SERS)l4 has been established as a sensitive spectroscopic technique for detecting and characterizing adsorbed layers on group IB metal surfaces. Raman signals can be enhanced by up to 6 orders of magnitude for molecules which are close to a rough surface of certain noble metals, in particular Ag, Cu,and Au. On these surfaces, the incident laser field excites localized surface plasmons which give rise to strong local electromagnetic fields Author to whom correspondence should be addressed. Resent addreas: Physical Chemistry Laboratory,Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Ztirich, Switzerland.
0022-3654/92/2096-6377$03.00/0
close to the ~ u r f a c e . ~ . ~Together .~ with the "chemical contribution": which is due to resonant electron-hole pair creation followed by vibrational excitation of the adsorbate, the local fields give rise to the observed enhancement of the Raman scattering cross section. It has been proposed earlier that the surface fields could also be used to induce photochemical reactions in suitable adsorbates>*6 Nitzan and B m 5have drawn attention to the radiationless energy transfer to the metal substrate, which depopulatesthe excited state of the molecule, and thereby competes with the enhanced absorption. The distance between metal surface and adsorbate is crucial for the radiationless transfer. These concepts have been verified experimentally by Wokaun et al.' and by Leitner et al.,* 0 1992 American Chemical Society
Franzke and Wokaun
6378 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 SCHEME I
'SO,-
/
R
R
using fluorescence as a probe for the excited-state population. In the present communication, SERS is used to study the mechanism of photochemical decomposition of azosulfonate compounds. The substances are adsorbed on a silver island film, which serves the dual purpose of enhancing the rate of the photochemical reaction and of monitoring the time-dependent concentrations of adsorbed reactants and products. Phenylazosulfonates,with the common structure I, were first synthesized by Schmidt in 18699and have become the subject of several investigations in the recent years.'*I6 These compounds are stable at temperatures up to 150 OC, whereas they can be easily decomposed by photochemical reactions upon irradiation at wavelengths 1400 nm. The basic mechanism of the decomposition depends on reaction conditions and is especially influenced by the solvent. In polar solvents like water, an ionic mechanism is prevailing;" there is evidence for a radical pathway in solvents like alcohol. Radical intermediates have recently been detected by ESR studies.I2 The overall reaction proceeds as shown in Scheme I: The polar azosulfonate group is replaced by an unpolar -H or -OH group; thereby the solubility in water is drastically decreased. This change has recently been used" for developing water-soluble photoresist systems based on azosulfonate monomers. In earlier investigations,"-I6 the decomposition of several phenylazosulfonates has been analyzed by ESR and UV spectroscopy. Here we are monitoring the reaction pathway for (vinylpheny1)- and (methoxypheny1)azosulfonates adsorbed on silver island films by SERS. For testing our setup and experimental procedures, some experiments have been performed with 4-nitrobenzoic acid, which is a well-characterized probe molecule that has often been used in SERS.2*7*18
Experimental Section Phykmulfonates. Sodium salts of the substituted phenylazosulfonates have been synthesized from the corresponding amines via the diazonium salts, as described in detail elsewhere."J3 Silver Island Fii. As a substrate for the evaporation of the Ag island films, standard microscope slides were cut into square pieces, of 2.5 cm side length. These plates were carefully cleaned by rinsing with methanol and acetone (Merck, p.a. quality), dried in a nitrogen stream, and transferred into the chamber of the evaporator. After a vacuum of lod mbar had been established, silver was evaporated at a rate of 1 nm/min. The mass thickr~m'~ of the silver layer, as controlled by a quartz crystal monitor, was held in the range between 4 and 5.5 nm; such films are known to give the best local field enhancement.20q21 Absorption spectra of the films were recorded to control their quality and properties; the maximum position and the width of the obtained absorption band are indicators for the size, shape, and uniformity of the produced silver islands.L8Jo.21 Furthermore, a SERS spectrum from the surface of the as-prepared island film was recorded to check for the presence of impurities. (As a consequence of the high sensitivity of SERS, the adsorption of
1500
1200
900
600
Raman shift / cm-' Figure 1. Photolysis of a 4-nitrobenzoic acid monolayer deposited onto a silver island film by spin coating. Raman spectra were excited at 514.5 nm; intensities are given in counts s-] mW-', i.e. they are normalized with respect to laser power and integration time. Spectra a through e were recorded after 0, 200, 400, 1000, and 5000 excimer laser pulses, of 90 mJ energy and 308 nm wavelength, had been delivered to the sample at a rate of 1 Hz.
molecules from the laboratory atmosphere can give rise to sizable signals, which could lead to a misinterpretation of the spectra recorded in the subsequent experiments.) Photolysis Experiments. As a light source for the irradiations, an excimer laser (Lambda Physik, Model LPX 120) operated with XeCl as the lasing medium has been used. At a wavelength of 308 nm, pulses of 70-150 mJ energy and 15 ns duration were delivered to the sample in a beam of 2 cmz area. These specifications correspond to a peak intensity of -5 MW cm-2. Repetition rates were kept low (1-10 Hz) to avoid heating of the sample. Furthermore the island films were mounted on a rotating sample holder to enlarge the irradiated area and, thus, to exclude thermal decomposition of the adsorbates. The azosulfonates were deposited onto the Ag islands by mounting the films on a rotor, applying 1 or 2 drops of an 0.1 M solution of the compound in water, and rotating the substrate at -2000 rpm. Raman Spectroscopy. For exciting the Raman spectra, the 488.9- or 514.5-nm lines of an Ar+ laser have been used. The laser was focused to an area 0.1 mm X 8 mm in size by a cylindrical lens. Laser power was adjusted in the range between 10 and 50 mW, to avoid heating, the sample holder was rotated during the Raman measurements as well. Two detection systems have alternatively been used. The first consisted of a double monochromator (SPEX, Model 14018) and a cooled GaAs photomultiplier (RCA, C31034). In the second system, parallel detection is realized on a triple monochromator (SPEX, Model 1877) equipped with a 1200 lines/" grating and a cooled intensified photodiode which results in a resolution of -5 cm-'. Results and Discussion QNitrobenzoic Acid. A 0.01 M solution of pnitrobenzoic acid (PNBA) in methanol was deposited on a silver island film by spin coating. The SERS spectrum recorded showed good agreement with the published reference spectra.I7J8 Subsequently, the coated silver island film was irradiated by a series of excimer laser pulses at 308 nm, of 85 mJ energy. In the Raman spectra shown in Figure 1 the peaks at 1355 and 1099 cm-I are decreasing significantly as a result of irradiation. On the low wavenumber side of the decreasing peak at 1355 cm-I, an appearing band first rises in intensity and then is broadened
Photochemistry of Azo Compounds on Silver Island Films
The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 6379
TABLE I: SERS Frequencies of Phenylazosulfonateson Silver I S M Filmsa
phenylazosulfona tes 2-methoxy- 3-methoxy- 4-methoxy(11)
(111) 1603
(IV)
1588 1486 1450
1472 1428
1460
1252 1195
1267 1212
1276
1114 1064
1097 1047
894 750
1598 1501
1052 882
806 741 624 531
anisoleb
assignment v(C=C)
ar.
v(N=N) 1457 1300 1248
6,(CH3) 6,(CHJ B(CH) def. in plane
1176 1073
v(Ph-N) p(CH) p(CH) v,(s03)
995 891 820 785 759
ring breathing -y(CH)
zi:}phenyl substituents
"Table entries are in wavenumbers (cm-I). bReference data from Green, J. H . S . Spectrochim. Acta 1962, 18, 39.
in the further course of irradiation. This observation indicates the generation of more than one product. The signal at 1604 cm-l is shifted toward higher wavenumbers, whereas the line at 858 cm-I is shifted toward 845 cm-I and becomes more intense. New bands appear at 1380, 1175,and 1150 cm-'. Several of these bands can be interpreted in terms of the generation of azodibenzoate; the SERS spectrum of the latter compound'* is characterized by peaks at 1608 cm-I (ring stretching vibration) and 1460 cm-' (-N=Nstretch) and a doublet at 1150 cm-l split by a few wavenumbers. The remaining peaks may be assigned to nitrobenzene produced via decarboxylationof PNBA and further decomposition products of PNBA down to adsorbed graphitic carbon. These results are similar to the ones of Roth et a1.I8 who photolyzed PNBA on silver island films by an Ar+ laser at 514.5 nm, using a power of 5 mW. From this agreement, the present setup is found suitable for monitoring photochemical reactions by means of SERS. Pbenylazosulfonates. Raman frequencies of the investigated compounds are summarized in Table I. Earlier attempts to monitor the progress of photolysis in solution had not been successful: Upon irradiation with the 308-nmexcimer laser, a rapidly increasing fluorescence background is observed. Before any relevant changes can be detected in the Raman spectra, the fluorescence signal becomes so strong that the Raman peaks can no longer be discerned (Figure 2). No significant improvements were obtained by shifting the excitation wavelength toward the red (647 nm line of a Kr+ laser). It is well-known that the formation of small amounts of fluorescing products can completely overshadow the much weaker Raman signals. Surface enhanced Raman spectroscopy may provide a solution in this situation.23 The presence of a metal island substrate not only enhances the Raman signals but at the same time quenches the fluorescence by radiationless energy transfer from the molecules to the metal surface. For these reasons, the photolysis of phenylazosulfonates was carried out in the adsorbed state on a silver island film. A 0.1 M solution of (2-methoxyphenyl)azosulfonate (11) was prepared and deposited onto a 5 nm thickness silver island film by spin coating. The frequencies observed in the SERS spectra were in good agreement with those of an aqueous solution, although the SERS peaks are slightly broadened as a consequence of the inhomogeneous environment. If the island film is irradiated at 308 nm, the photochemical reaction can be monitored in the SEIS spectra presented in Figure 3. The peak at 1487 cm-I, which was assigned to the -N=Nstretching mode, is clearly observed to decrease. Furthermore, the peaks at 1063 and 1197 cm-' are losing intensity and finally vanish.
,600
1400
1200
1000
800
600
Raman shift / cm-'
Figure 2. Photolysis of (2-methoxypheny1)azosulfonate (11) in aqueous solution. Raman spectra have been excited at 514.5 nm using a power of 40 mW. The lower trace (b) represents the spectrum of a saturated solution of I1 prior to irradiation. The upper trace (a) was recorded after a series of 2000 excimer laser pulses, of 100 mJ energy at 308 nm,had been delivered to the sample. Note the strong fluorescent background.
.
1600
,
1400
.
,
1200
,
I
1000
I
800
,
,
800
Raman shift / cm-l Figure 3. Photolysis of (2-methoxypheny1)azosulfonatedeposited onto a silver island film by spin coating. Experimental conditions are the same as in Figure 1. Spectra a through d were recorded after 0, 10,20, and 4 0 excimer laser pulses, of 85 mJ energy at 308 nm, had been delivered to the sample.
If the photolysis is carried out in water, guaiacol (methoxyphenol or 2-hydroxyanisole) has been identified as the main product by GC/MSanalysis. Therefore, the spectrum of the latter compound in aqueous solution, as well as on a silver island film, has been recorded. The Raman frequencies of guaiacol do not match those of the end product(s) obtained by irradiation of (2-methoxyphenyl)azosulfonateson the silver island film at 308 nm. As a reason for this lack of agreement, one might argue that guaiacol, once formed by photolysis of phenylazosulfonate,would be further photolytically decompsed by the strong electromagnetic fields on the silver surface. To test this hypothesis, pure guaiacol was deposited on a silver island film, and exposed to 308-nm irradiation. However, the photolysis products were not the same as observed before with compound 11. We note that good SERS spectra of guaiacol could only be obtained when the substance was deposited on the surface as the neat liquid, i.e. undiluted. In contrast, the expected surface concentration of the end product obtained by the photolysis of 11, under the conditions described above, is much lower. From this observation one may conclude that guaiacol, if formed in the
6380 The Journal of Physical Chemistry, Vol. 96,No. 15, 19'92
photolysis, was probably not detected as a consequence of a low scattering cross section in the SERS experiment. Photolysis Yield. Surface Enhancement. In this section we would like to investigate whether photochemistry is enhanced when the reaction is carried out on a silver island surface, i.e. whether the quantum yield is higher as compared to the reaction carried out in solution. For this purpose, initial quantum yields have been determined for both cases. The photolysis quantum efficiency was defined as the ratio of the number of decomposed molecules of phenylazosulfonate and the number of the photons absorbed by the chromophor. When compound I1 is irradiated in solution at a wavelength of 308 nm, formation of a diazonium ion intermediate is observed first, which is subsequently decomposed into final product(^).^^ To determine the amount of starting material decomposed, UV spectra were sequentially recorded after delivering defined sets of laser pulses to the solutions. The spectra were deconvoluted to determine the mole fractions of starting material, intermediate, and product; the Gaussian deconvolution procedure used is described in detail elsewhere.24 For calculating the quantum efficiency of the surface reaction, the following assumptions have been used. (i) The flux of photons per pulse (Z,) incident on the relevant part of the island film, which was evaluated by the recording the Raman spectra, corresponded to 1.5 X mol of photons per cm2. (ii) The coverage B of molecules deposited onto the silver surface by spin coating was estimated to correspond to 1 X lOI4molecules cm-2,22which is equivalent to 1.6 X lo-', mol cmW2. (iii) For an order of magnitude estimate, the value for the effective molar absorption coefficient, e*, was set to t* (308 nm) = 1 X lo6 mol-' cm2. (The corresponding absorption coefficient in solution was determined as t = 6.35 X lo3 mol-' dm3 cm-I.) The resulting absorption, A , due to the monolayer is given by A = -log
[(zo - q / z 0 ]
=
€*e =
1.6 x 10-4
(iv) The number photons absorbed per cm2 in the monolayer is then calculated to be
AZ = 2.3e*8Z0 = 5.5 X 10-l2 mol of photons cm-2 (v) The number of the decomposed phenylazosulfonate molecules was determined from the relative decrease in the Raman intensity of the 1486-cm-' band, which was assigned to the -N=N- stretching mode. The background was subtracted from each spectrum. To check the results thereby obtained, the band at 1064 cm-I was used in addition to determine the residual concentration of the starting material. Both methods gave nearly identical results for the relative concentration changes. In the described way, the number of molecules decomposed per laser pulse at the beginning of the reaction was determined as 3.3 X 10-l2 mol cm-2. Dividing by the number of absorbed photons indicated above, an apparent quantum efficiency of 0.6 was obtained, which is =30 times higher than the one determined in solution.24In view of the uncertainty involved in the determination of the absorption coefficient E* of the adsorbed dye layer, we conclude that the apparent quantum efficiency of photochemical decomposition is enhanced by about 1 order of magnitude. For clarity, it should be noted that this estimate of the quantum yield refers to the number of photons that would be absorbed by a monolayer of azosulfonate molecules on a transparent substrate. For identical incident laser pulses and the same surface coverage of chromophores, =10 times more molecules are decomposed on an island film,as compared to an inert substrate. The island film itself is, of course, strongly absorbing and is characterized by absorbance values of A = 0.5 at the resonance, X = 500 nm. Further SERS Experiments. In addition to compound 11, we have investigated the SERS spectra and photolysis of several other compounds (e.g., (3- and 4-methoxyphenyl)azosulfonate,and (3-vinylpheny1)azosulfonate). The observed Raman bands, together with their assignment, are summarized in Table I. Raman data of anisole (methoxybenzene) have been included to support the assignment of the vibrations. While the SERS spectra re-
Franzke and Wokaun corded during photolysis do give evidence for the decomposition of the respective starting materials, interpretation is less straightforward than in the cases of PNBA and of 11; the spectra are complicated by a large number of impurity bands. Likely, the Raman cross sections of the respective compounds on the silver surfaces are lower than those of 11. As a consequence, signals due to pollution of the island films by molecules from the laboratory atmosphere were too strong under our experimental conditions, such that a quantitative interpretation of the photolysis results was not feasible. In the case of the vinyl-substituted azosulfonate, two bands at 615 and 925 cm-I were observed to appear in the course of the photolysis. After reaching a transient maximum, the intensity of these bands decreased again, until they disappeared in the further course of the reaction. The two bands are tentatively assigned to a surface-adsorbed sulfite ion.2s,26 The observed time-dependent behavior indicates that the sulfite group is first split off from the azosulfonate and is adsorbed on the silver surface. Subsequently, laser induced desorption is taking place, and the signals due to sulfite are vanishing again.
Conclusions The present experiments show that SERS on silver island films is a suitable technique for monitoring the course of selected photochemical reactions. A necessary prerequisite is the requirement that the enhanced Raman scattering signal of the reactant must exceed those of passible contaminants adsorbed on the Ag island film. Perturbations due to impurities can be minimized by protecting the films between evaporation and spin coating, e.g. by immersion in liquid nitr~gen.~' SERS characterization should be particularly valuable for cases in which Raman spectra cannot be recorded in aqueous solution, as a consequence of strong fluorescence. This aspect is particularly relevant in photochemistry, where the formation of small quantities of fluorescing side products is frequently observed. For the photolysis of (2-methoxypheny1)azosulfonate(11), we have shown that the quantum efficiency of photochemical decomposition on silver island surfaces is enhanced by l order of magnitude, as compared to the one determined in aqueous solution. In the present experiment, the molecular absorption was excited at 308 nm, i.e. at a wavelength far off the surface plasmon resonance of the silver islands. The fact that enhanced decomposition is observed even under nonresonant conditions demonstrates the feasibility of surface enhanced photochemistry. Further studies of photoreactions using resonant excitation are in progress in our laboratories. Acknowledgment. Financial support of this work by grants of the Deutsche Forschungsgemeinschaft (SFB 213) is gratefully acknowledged. References and Notes (1) Chang, R. K., Furtak, T. E., Eds. Surface Enhanced Raman Scattering, Plenum Press: New York, 198 1. (2) A r m , R.; Kovacs, G. J. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1991; Vol. 19, p 5 5 . (3) Otto, A. In Light Scattering in Solids; Cardona, M., Giintherodt, G., Eds.; Springer: Berlin, 1983; Vol. 4, p 289. ( 4 ) Wokaun, A. Mol. Phys. 1985,56, 1. Wokaun, A. Solid Stare Phys. 1984, 38, 217. ( 5 ) Nitzan, A.; Brus, L. E. J . Chem. Phys. 1981, 74, 5321. ( 6 ) Nitzan, A.; Brus, L. E. J . Chem. Phys. 1981, 75, 2205. (7) Wokaun, A.; Lutz, H. P.; King, A. P.; Wild, U.P.; Ernst, R. R. J . Chem. Phys. 1983, 79, 509. (8) Leitner, A.; Lippitsch, M. E.; Draxler, A.; Riegler, M.;Aussenegg, F. R. Appl. Phys. B 1985,36,105. Aussenegg, F. R.; Leitner, A.; Lippitsch, M. E.; Reinisch, H.;Riegler, M.Sur$ Sci. 1987, 1891190, 935. (9) Schmidt, R.; Lutz, L. Ber. Dtsch. Chem. Ges. 1869, 2, 51. (10) Bock, H. Angew. Chem. 1965, 77,469. (11) Nuyken, 0.;Knepper, T.; Voit, B. Makromol. Chem. 1989, 190, 1015. (12) Stasko, A,; Nuyken, 0.; Voit, B.; Biskupic, S.Tetrahedron Lett. 1990, 31, 5737. Cholvad, V.; Szabova, K.; Stasko, A,; Nuyken, 0.; Voit, B. Magn. Reson. Chem. 1992, 29,402. (13) Nuyken, 0.; Voit, B. Makromol. Chem. 1989, 190, 1325. Voit, B. Ph.D. Thesis, University of Bayreuth, 1990. (14) Engel, P. S. Chem. Reu. 1980, 80, 99.
J. Phys. Chem. 1992, 96,6381-6388
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F.; Bergmann, J. G.; Olson, D. H. Opt. Lett. 1980, 5, 368. (21) Weitz, D. A.; Garoff, S.;Gramila, T. J. Opt. Letr. 1982, 7, 168. Garoff, S.;Weitz, D. A.; Alvarez, M. S . J. Lumin. 1984, 31/32, 930. (22) Meier, M.; Carron, K.T.; Fluhr, W.; Wokaun, A. Appl. Spectrosc. 1988, 42, 1066. (23) Weitz, D. A.; Garoff, S.;Gersten, J.; Nitzan, A. J . Chem. Phys. 1983, 78, 5324. (24) Franzke, D. Ph.D. Thesis, University of Bayreuth, 1991. Franzke, D.; Voit, B.; Nuyken, 0.;Wokaun, A. Mol. Phys., in press. (25) Bates, J. L.; Dorain, P. B. J . Chem. Phys. 1989, 90, 7478. (26) Tsai, W. H.; Boerio, F. J.; Clarson, S.J.; Montaudo, G. J. Raman Spectrosc. 1990, 3 11. (27) Otto, A. Private communication.
(15) van Beck, L. K. H.; Helfferich, J.; Houtmann, H. J.; Jonker, H. Receuil 1967, 86, 975. (16) van Beck, L. K. H.; Helfferich, J.; Houtmann, H. J.; Jonker, H. Receuil 1967, 86, 981. (17) Murray, C. A.; Allara, D. L. J . Chem. Phys. 1982, 76, 1290. (18) Roth, P. G.; Venkatachalam,R.S.;Boerio, F. J. J. Chem. Phys. 1986, 85, 1150. Venkatachalam, R. S.;Boerio, F. J.; Roth, P. G. J. Raman Spectrosc. 1988, 19, 28 1. (19) The mass thickness d , is defined as the mass of silver deposited per unit area, as monitored by a quartz crystal microbalance, divided by the density. (20) Bergmann, J. G.; Chemla, D. S.;Liao, P. F.; Glass, A. M.; Pinczuk, A.; Hart, R. M.; Olson,D. H. Opt. Lett. 1981,6, 33. Glass, A. M.; Liao, P.
Effects of B3+ Content of B-ZSM-11 and B-ZSM-5 on Acidity and Chemical and Thermal Stability Mark W. Simon: Sang Sung Nam,t Wen-qing Xu,+Steven L. Suib,**f*lJohn C. Edwards,s and Chi-Lin O'Young*-l U-60, Departments of Chemistry and Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269-3060, and Texaco, Inc., P.O. Box 509, Beacon, New York 12508 (Received: February 20, 1992; In Final Form: April 21, 1992)
The geometrical environments of B3+ in sol-gel suspensions, dried gels, and resultant crystals of B-ZSM-5 and B-ZSM-11 zeolites have been probed with luminescence, luminescence lifetime, IlB nuclear magnetic resonance (NMR), X-ray powder diffraction (XRD), and thermal methods. The concentration range of B3+ that can be monitored with luminescence is 0.1-0.5 wt % B3+ as evidenced by concentration quenching studies. Both trigonal and tetrahedral B3+ environments are observed by both luminescence and IlB NMR methods. Contraction of unit cell volume has been observed with XRD methods. The thermal stabilities of B-ZSM-5 and B-ZSM-11 have been tracked with XRD and luminescence methods with structural loss occurring near 900 "C with the formation of H3B03 as identified by luminescence emission and an increase in surface [B3+] as detected by X-ray photoelectron spectroscopy (XPS). Treatment of B-ZSM-5 or B-ZSM-11 with HCl also leads to loss of B3+ from the framework. Infrared experiments have shown that both Bransted and Lewis acid sites that are weak exist in these materials. B-ZSM-5 crystals have hexagonal morphologies, whereas B-ZSM-11 crystals are less well-formed. The lateral and depth uniformity of [B3+]in B-ZSM-5 crystals is suggested on the basis of scanning Auger microscopy and XPS. Thermal treatment to 900 OC leads to migration of B3+ to the surface of ZSM-5 as shown by XPS.
Other NMR studies have shown similarities of B environment to that of borosilicate glasses with regard to chemical shifts for TdB3+in B-ZSM-5,4 Y zeolite, and mordenite.s Limited levels of incorporation of B3+in Y and mordenite have been reported.5 Sorption and temperature-programmed desorption (TPD) methods have been used by Kofke et a1.6 and 1:1 complexes of 2-propanamine with B,H-ZSM-5 zeolites (1 sorbent per B atom) were formed, while complexes with NH3 or 2-propanol were less well-defined. Reactivity and TF'D studies6 suggested that hydroxyl groups associated with B3+incorporation led to weak acid sites with respect to incorporation of either Fe3+or A13+. TPD and Fourier transform infrared (FTIR) methods have been used by Chu and Chang' to compare the acidity of substitution of B3+,Fe3+,Ga3+and AI3+in ZSM-5. These studies have shown that the acidity of the B3+materials is considerably weaker than the other systems. Catalytic properties of B-ZSM-5 systems in hexane cracking, ethylbenzene dealkylation, c-C& isomerization, and CH30H-to-hydrocarbon reactions were related to trace amounts of framework AI3+.* Considerable interest in the incorporation of boron into zeolites is due to the modified acidic character of the resultant material. Sayed et aL9 have shown that the incorporation of boron into B-ZSM-5 leads to decreased acidity of the zeolite, which is surprising in light of comparative acidity studies with halides of boron and aluminum. Holderichlo and co-workers1* have found that boron zeolites gave much better selectivity than aluminum zeolites in aldehyde/ketone rearrangements, suggesting boron
I. Introduction The selective placement of ions like Fe3+or B3+substituted into framework sites of zeolites has been the subject of research with molecular sieves over the past 10 years. Extraframework oxidic species are often present in such materials. Spectroscopic methods for confirming the presence of substituted tetrahedral ions at low levels (
