J . Phys. Chem. 1990, 94, 6197-6804 decreases for most of the pentane isomers. A simple qualitative explanation using transition-state theory can explain this trend in the surface desorption preexponentials. In the minimum-energy binding configuration, the rotations of the molecule will be severely hindered. These rotations will become unhindered in the transition or gas-phase state. Consequently, the differences between the rotational partition functions in the transition states for the various pentane isomers will affect their relative desorption preexponentials. The rotational partition function, qr,is proportional to (IAIBIC)’/’ where I,, Ig, and ICare the principal moments of inertia of the molecule. As the molecular branching increases, the moment of inertia for the pentane isomers will decrease and the rotational frequency will increase. Likewise, the rotational partition function will decrease as the molecular branching increases. The rotational partition functions in the transition state, q:, will decrease with increased molecular branching. The ratio of the rotational partition functions q?/qr, where qr designates the hindered binding state, will also decrease as the molecular branching increases. Thus, smaller surface desorption preexponentials would be expected for the more branched pentane isomers. Assuming a rigid molecule, the principal rotational moments of inertia for the pentane isomers have been estimated by finding the eigenvectors for the moment of inertia tensor.44 These moments of inertia and appropriate symmetry numbersu predict the relative rotational partition functions of 19:ll: 1 1 :1 for isopentane:cyclopentane:n-pentanemeopentane. This explanation does not quantitatively predict the effect of branching on the desorption preexponential. Note that the estimated ratios for the desorption preexponentials span a range of X19, whereas the observed range is X75. The diffusion preexponentials for the pentane isomers on Ru(001) follow the same general trend as that observed for the surface desorption preexponentials. The diffusion preexponential tends to decrease as the pentane isomer becomes more branched. This trend can be explained by similar arguments based on (44) McQuarrie, D. A. Starisrical Mechanics; Harper & Row: New York, 1976.
6797
transition-state theory and rotational partition functions. V. Conclusions The surface diffusion and desorption of n-pentane, isopentane, cyclopentane, and neopentane was examined on Ru(001), using laser-induced thermal desorption (LITD) and temperature-programmed desorption (TPD) methods. The surface diffusion activation energies varied from Edir= 3.0 f 0.3 kcal/mol for neopentane to &if = 4.4 f 0.2 kcal/mol for n-pentane. Similarly, the desorption activation energies ranged from Eds = 10.7 f 0.2 kcal/mol for neopentane to Ed- = 13.8 f 0.9 kcal/mol for npentane. The surface corrugation ratio, Q Edir/,!?+, was determined to be constant at Q Z= 0.30 for all the pentane isomers on Ru(001). In agreement with the earlier results for n-alkanes on Ru(001), this constancy indicates a linear scaling between the diffusion activation energy and the desorption activation energy. These results suggest that the pentane isomers have similar binding configurations and move on the Ru(001) surface in a concerted process. Conformational degrees of freedom do not appear to influence the surface diffusion mechanism. The surface diffusion and desorption activation energies scaled inversely with the degree of branching for the pentane isomers. The Wiener index, W(G), was used to characterize the branching and was found to correlate with the diffusion and desorption activation energies. Likewise, a simple physisorption model using a Lennard-Jones (6-12) pair potential was employed to determine the predicted surface binding energies and configurations. In agreement with the measurements, these physisorption calculations also displayed an inverse scaling between the adsorption energies and the degree of branching. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-8908087. Some of the equipment used in this work was provided by the NSF-MRL program through the Center for Materials Research at Stanford University. M.V.A. thanks the Eastman Kodak Co. for a graduate fellowship. S.M.G. acknowledgesthe National Science Foundation for a Presidential Young Investigator Award and the A.P. Sloan Foundation for a Sloan Research Fellowship.
Importance of Surface Reactions in the Photochemistry of ZnS Colloids Dave E. Dunstan,*qt Anders Hagfeldt,t*tMats Almgren,+ Hans 0. G. Siegbahn,*and Emad Mukhtart Departments of Physical Chemistry and Physics, Uppsala University, Uppsala, Sweden (Received: October 30, 1989; In Final Form: February 22, 1990)
The photochemistry of colloidal ZnS has been studied under a variety of experimental conditions. The time dependence of the photocorrosion has been examined by using time-resolved fluorescence,static fluorescence, and electron spectroscopy for chemical analysis (ESCA) techniques. A qualitative model for the semiconductor and its electrolyte interface is developed to explain the experimental observations where elemental Zn, S,and SO,” formed on the surface act as e-/h+ recombination centers. The photocorrosion products have been identified as SO4’-, SO,2-, and S. The importance of the surface chemistry in the photochemistry is shown via the correlation between the growth in the emission intensities and the formation of the photocorrosion products on the surface of the dried colloids.
Introduction Much has been written about semiconductor colloids and their photochemistry due to the recent explosion in interest in them as potential solar energy converters. A general review is that by
* Authdr to whom correspondence should be addressed. Present address: Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, CA 93106. ‘Department of Physical Chemistry. *Department of Physics. 0022-3654/90/2094-6797$02.50/0
Henglein’ while an extensive treatise on the photochemistry of ZnS is found in the book by Leverentz.’ However, because of the difficulty in understanding the complicated experimental results, no complete theory or physical explanation for their absorption and emission properties has been developed to date.3*4 ( I ) Henglein, A. Top. Curr. Chem. 1988, 143, 1 1 3 . (2) Leverenz, H. W. Luminescence of Solids; Wiley: New York, 1950. (3) Uchida, I. J . Pfiys. SOC.Jpn. 1964, 19, 670. (4) Samelson, H.;Lempicki, A. Pfiys. Reu. 1962, 125, 901.
0 1990 American Chemical Society
6798 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 It has been found in this study that the precise experimental conditions of preparation of the colloids must be carefully controlled to obtain reproducible results. For the colloidal dimension, the surface-to-bulk ratio is large and therefore any surface effects are enhanced. While current understanding of semiconductor properties incorporates an understanding of charge transfer across the semiconductor/electrolyte i n t e r f a ~ e the , ~ importance of the interface has not, we believe, been properly recognized in the photochemistry of such systems. Some recent theoretical attempts to predict semiconductor properties from a quantum mechanical basis have however recognized the importance of the surface. In these calculations a surface charge is required to localize the earound the nuclei in the crystal lattice.6 A generally accepted view of the absorption and emission processes in semiconductors is that of an initial excitation of an electron into the conduction band of the semiconductor by ultrabandgap radiation. Emission then occurs via a radiationless relaxation of the electron or hole to a recombination center followed by radiative recombination. The recombination centers may be due to surface states or crystal defects which lie at energy levels within the bandgap. The energy loss occurs via the creation of phonons in the crystal lattice and enables the e- or h+ to be localized at the recombination centers. The recombination of the e-/h+ pair from these or other centers may also be nonradiative. For many semiconductors, a bandgap emission is observed although this is not seen for ZnS colloids. A description of the absorption and emission processes in colloidal semiconductors. with specific reference to CdS, using a quantum mcchanical basis is developed by Chestnoy et al.' A more precise physical description of the absorption and emission mechanisms requires more experimental evidence to be developed further, in particular with regard to the coupling to photochemical transformations. A general chemical reaction scheme which involves the proposed absorption and emission processes combined with some of the proposed photochemical reactions for ZnS under illumination is as follows: ZnS
ZnS(e-/h+)
-
h%b
ZnS(e-/h+)
ZnS
-+
ZnS
huemission
(nonradiative recombination) (3)
-
+ 2eS2- + 2h+ -* S032-+ 2 0 H - + 2h+ Zn2+
-
-+
-
-+
Zn
+ 2H+
-
-
Zn2+
(4) Zn
(5)
S
(6)
S042-+ H 2 0
(7)
+ H,
(9)
The photogenerated e-/h+ pairs are of sufficient energy to drive reactions 4-9 which compete with radiative recombination (2) and hence emission. The emission may also occur by the recombination occurring from recombination centers such as the products of reactions 4-7, viz., Zn, S, and S042-at the surface of the colloid. The photocorrosion products of both CdS and ZnS have been postulated for some time; however, the photocorrosion products of CdS have only recently been verified by using ESCA on electrodes.* In their study, Meissner et al. identify the surface corrosion products as Sod2-, S, Cd2+,and Cd. Earlier studies on ZnS fluorescence have attributed the emission to recombination centers within the crystal lattice created by impurities in the crystal ( 5 ) Morrison, S . R. Electrochemistry at Semiconductor and Oxidised Metal Electrodes; Plenum: New York, 1980. (6) Harker, A. H . J . Chem. Soc., Faraday Trans. 2 1989, 85, 471. (7) Chestnoy, N.: Harris, T. D.; Hull, R.; Brus, L. E. J . Phys. Chem. 1986, 90,3393. (8) Meissner. D.; Benndorf, C.; Memming. R. Appl. Surf. Sci. 1987, 27, 423.
Dunstan et al. such as Ag+, Cu2+,Mn2+,or S2-/Zn2+vacancies within the crystal4 or possible interstitial Zn or S as postulated by Leverew2 who comments on the lack of good experimental evidence for such an hypothesis. The ZnS unit cell in the wurtzite structure obviously has room to fit either ions or atoms between the lattice ions. The above imperfections in the perfect crystal are thought to create energy levels within the bandgap of the semiconductor which act as centers for the recombination of the e-/h+ pair. While this view is typically held for macroscopic semiconductor physics, our results show the importance of the surface states in the photochemistry for particles of small dimension.
Materials and Methods Ventron high-purity Zn(C104)2, Merck pro analyse ZnCI,, Zn(N03)2, Na2S, and Fluorometric grade MeOH were used. Milli-Q Ultrapure water was used in all experiments. The ZnS colloids were prepared by a simple rapid injection t e ~ h n i q u e . ~A solution containing the ion in excess was rapidly injected into the other solution to yield a transparent suspension. The concentration of the ions, either Zn2+ or Sz-, not in excess was held constant at 2.6 X M, and the volume of the excess ion to be injected was adjusted according to the required excess concentration. This ensured the same effective particle concentration in each preparation. A fresh Na2S solution was prepared immediately before the colloid was made. Analysis of the absorption at 250 nm showed the colloids to be formed in less than half a second. It should be noted that reproducible preparation of the colloids is quite difficult. Care must be taken to maintain the same rate of mixing of the solutions for each preparation to ensure the same particle size is produced. Slow injection of either of the solutions into the other yielded a cloudy colloid, indicative of a larger particle size and therefore smaller surface-to-bulk ratio. Colloids prepared with high excess concentration (CO.1 M) of either Zn2+or S2-were more stable than those which were nearly stoichiometric, Le., closer to the point of zero charge (pzc). When the injection time and mixing conditions were reproduced as accurately as possible, reproducible photochemical behavior, Le., the same growth rates in emission and changes in the absorption spectra after the same period of illumination, were obtained. Steady-state fluorescence and absorption spectra were measured with a Spex Fluorolog 1680 0.22-111 double spectrometer and a Cary 2400 spectrophotometer, respectively. Fluorescence decay measurements were made using a singlephoton counting technique which has been described elsewhere.i0 The time-resolved spectra were obtained with this spectrometer by measurement of the decay curves at I O wavelengths over the observed wavelength range. These curves were then normalized to yield an intensity at each time and wavelength. Electron microscopy was carried out using a Siemens Model 1A microscope. ESCA measurements were made on samples which were prepared by dipping a gold disk into the colloidal suspension and subsequently allowing it to dry in ambient atmosphere. No additional steps were undertaken to remove the solvent before transfer into the spectrometer vacuum. The measurements were performed with the samples exposed to vacuum only through a small slit (the spectrometer slit) via a differential pumping stage. The typical residual pressure over the sample was thus mbar. This resulted in the presence of substantial amounts of residual solvent being present on the sample surface (verified by the O( Is) spectra). We thus believe that sample surfaces during ESCA measurements at least partially emulate solution conditions. Indeed, significant and systematic differences between the measurements for different solvents ( H 2 0 and MeOH) were observed. The samples were subjected to an initial ESCA measurement, after which they were taken out for a 45-min illumination using a Hg lamp and then reintroduced, etc., up to a total illumination time of 120 min. The (9) Weller, H.; Koch, U.; Gutierrez. M.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 649. (IO) Almgren, M.; Alsins, J.; van Stam, J.; Mukhtar, E. Prog. Colloid Polym. Sci. 1988, 76, 68.
Photochemistry of ZnS Colloids
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6799 !63.8 162.1
ESCA S2p
I
!69.!
I
175
Figure I . ESCA S(2p) spectra obtained from ZnS colloidal particles before illumination (10% excess ion concentration). The top spectrum is S” excess in H20, the middle S2- excess in MeOH, and the bottom Zn2+excess in MeOH. For identification of the peaks see text.
systematic changes observed in the spectra give us confidence in the procedure outlined.
16?
163.8 l Q , !
ESCA S2p
170
Binding Bergy levj
1w
165
Figure 2. S(2p) spectra obtained from ZnS colloids prepared from 10% S2-excessin H 2 0 after illumination with 253-nm radiation for (top) 45 min, (middle) 90 min, and (bottom) 120 min. For identification of the peaks see text.
I
!69.1
163.8 162.1
Experimental Results A. Electron Microscopy.
I. Mobility Measurement. Examination of the particles using electron microscopy showed primary particles of size approximately 4-nm diameter, often clustered into larger aggregates. The aggregates were of size large enough to suggest that the aggregation occurred during grid preparation as they should have scattered light noticeably or precipitated very rapidly if they had been present in suspension. Aging of the colloids under illumination often caused aggregation, depending on the concentration of the potential-determining ions S2-and ZnZ+ and the solution pH which determines the surface potential of the particles via the S2-/HS- equilibrium.” At high pH the { potential is observed to be less sensitive to the S2-concentration. The mobilities of the colloids and the solution pHs were measured before and after illumination for both Zn2+ and S2- excess suspensions. These colloids were prepared with a 3:1 excess such that the volume of excess ion is 3 times that of the other. The Zn2+ excess colloids showed a decrease in mobility from +1.5 to approximately 0 pm cm-l s-I V-I and an increase in pH from 4.5 colloids had a constant to 4.7 upon illumination while the S‘ exmobility of -2.5 pm cm-l s-l V-I and a decrease in pH from 1 1.4 to 1 1.2. These changes were observed for suspensions that were illuminated for 1 h in the fluorimeter at 300 nm with constant stirring. The total photon dose was 5.0 X 10’’ photons. (See fluorescence spectra results.) The Zn2+ excess colloids were seen to be generally less stable suspensionsunder illumination than those with S2-excess, although at low excess concentrations of either Zn2+ or S2-,near the pzc, both types of colloid were unstable. 2. ESCA Measurement. Figures 1-3 show the S(2p) ESCA spectra of the samples before (Figure 1) and after illumination (Figures 2 and 3) with 253-nm radiation from a Hg lamp. The spectra of the unilluminated samples show a low-energy peak due to S2-at 162.1 eV. Also present with varying intensity is a peak at 163.8 eV, which is identified as due to elemental S species. The intensity of the S peak is most pronounced for the colloids prepared ( I I ) Williams, R.; Labib, M.E. J . Colloid InterJuce Sci. 1985, 106, 25 I .
Figure 3. S(2p) spectra obtained from ZnS colloids prepared from 10% Zn2+excess in H 2 0 . Details are as for Figure 2.
with S2-excess. Further experiments must be conducted to elucidate the dependence of the intensity of this peak on the S” excess concentration. The dried colloids measured here were of 10% excess ion concentration and were dried from both water and MeOH. The spectra on samples dried from MeOH are not presented here but show the same salient features. For the illuminated samples (Figures 2 and 3), the gradual appearance of a high binding energy peak at 169.1 eV is the most prominent feature. This peak is due to the photocorrosion product SO4”. The growth rate of the relative intensity of this peak was found to be approximately the same and constant up to 45-min illumination time for all samples except the S2-excess in H 2 0 (see
6800 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 2
,
,
,
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Dunstan et al.
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Fluorescence spectra for 200% Zn2+ excess ZnS colloidal suspension i n H20under N, atmosphere before and after illumination N i t h 300-nm radiation for 1 h i n the Spex fluorimeter Figure 5
I 0
200
250
300
350
400
Wavelength (nm)
Figure 4. (a) Absorption spectra for 200% Zn2+ excess colloid at various illumination times. (b) As for (a) but 200% S2- excess.
Figure 2 ) . In this case the rate was slower by approximately 20%. The growth rate of SO,2- showed a gradual decrease for illumination times longer than 45 min. This is probably an effect of the diffusion limitation of 0, through the growing layer although the absorption of the radiation may be less efficient with SO-: on the surface. In solution, S042-should partially dissolve away from the surface and the kinetics of photocorrosion will be different where the 0, must diffuse through the solution. During illumination, the relative intensity of the elemental S peak was also found to vary, although not in any clearly systematic way. Another interesting feature, most obvious in the spectra obtained for S2-excess samples prepared in H 2 0 (see Figure 2), was the occurrence of a small peak at 167 eV. We interpret this peak as due to the presence of SO3,- species on the surface. This peak seems to grow systematically with illumination time. Thus, the ESCA results confirm the presence of the species expected from and thus further validate the proposed reactions 4 and 7. Unfortunately, the Zn peaks are not separated well enough to enable a precise determination of the oxidation state of the Zn species at the surface. ESCA measurements on solutions that had been illuminated and then dried gave no evidence for the presence of SO4,-. This is thought to be due to the solubilizing to a large extent and drying as ZnSO, crystals. These are not necessarily evenly distributed over the sample surface, and since only a minor portion of the sample was studied in the ESCA measurement this may explain the lack of S042-signal in this case. The main photocorrosion product for samples illuminated on the disk was For the dried S2-colloids there were still large S2-and S peaks present after 2-h illumination. B. Absorption Spectra. The absorption spectra of the colloidal suspensions change significantly upon illumination. The spectra for both 200% Zn2+ and S2-excess colloids are shown in Figure 4 at varying times of illumination. At 300 nm, the excitation wavelength for the fluorescence measurements, the absorbance increases from 1 .O to 1.2 for the S2-excess colloids and decreases from 0.6 to 0.3 for the Zn2+ excess colloids after 1-h illumination with the Hg lamp. For 10% S2-excess colloids the absorbance decreased from 0.8 to 0.35 under the same conditions while the Zn2+ excess colloids absorbance decreased from 1 . 1 to 0.4.
u 1
______
400
500
600
700
Wavelength (nm) Figure 6. Fluorescence spectra for 200% S2- excess ZnS colloidal suspension in H,O in air before and after illumination with 300-nm radiation for 1 h in the Spex fluorimeter.
C. Fhorescence Spectra. Fluorescence spectra were recorded under a wide variety of chemical conditions in both H 2 0 and organic solvents. All spectra were recorded over the wavelength range from 350 to 750 nm by using 300-nm excitation radiation. The same conditions of aperture width and excitation intensity were used to record all the steady-state spectra to enable comparison between them to be made. The Spex excitation source was calibrated by using an actinometer solution as described by Hatchard et aI.l2 and found to deliver at 300 nm a photon flux of 1.4 X I O l 4 photons s-' to the sample chamber. For the standard particle concentration used, there are on average one photon per particle every 50 s. For typical absorbances of the suspensions this corresponds to one e-/h+ pair being created in each particle every 50-100 s. The Hg lamp gave a photon flux of 7.2 X I O I 5 photons s-l to the sample at the standard separation distance used. The quantum efficiencies observed for the suspension emissions for the Zn2+excess are in the range 4 = 2 X lo4 to C$ = 2 X colloids before and after illumination for 1 h. respectively. The similar 4 values for S2- excess colloids are I$ = 4 X to 4 = I X For the dried samples 4 is an order of magnitude greater for both types of colloid. However, the C$ values for the dried colloids have little meaning as the exact drying procedure was difficult to control. All the 4 values were evaluated only approximately with pyrene as a standard. D. H,O Suspensions. It was observed that three peaks of varying intensities are generally present in the emission spectra, namely, at 420, 480-500, and 550-570 nm,as shown in Figures (12) Hatchard, C. G.;Parker, C. A . Proc. R . SOC.London, A 1956, 235, 518
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6801
Photochemistry of ZnS Colloids 16000 1 I
I
I
400
500
600
700
Wavelength (nm) Figure 7. Fluorescence spectra for 10% Zn2+and S" excess ZnS colloids dried onto a quartz support before and after illumination with 300-nm radiation for 1 h in the Spex fluorimeter.
5 and 6. Spectra were measured for both 200% and 10% excess colloids. Both show the same general behavior with subtle differences as discussed below. The emission at these wavelengths increases in intensity upon illumination of the colloids either in suspension or as dried powders. See Figure 7. The relative growth rates of the emission intensities at these wavelengths depend on the specific chemistry of the suspension and to a lesser extent for the dried powders, the conditions under which it was prepared. The fluorescence spectra for Zn2+ excess under N 2 atmosphere and S2-excess in air are shown in Figures 5 and 6, respectively. The important features of all the spectra measured are summarized as follows. For the Zn2+ excess suspensions, (i) the overall growth rates in emission intensities are higher than those for the Sz- excess colloids, (ii) the presence of O2 decreases the overall growth in intensity of the emission and increases the relatiue intensity of the 420-nm emission, and (iii) the higher the ZnZ+excess concentration, the higher the overall growth rates in emission intensity and the higher the relative intensity of the 490-nm emission. For the S2- excess suspensions, (i) the overall growth rates are lower than for the Zn2+ suspensions, (ii) the presence of O2 increases the overall growth rates and decreases the relative intensity of both the 420- and 560-nm emissions, (iii) the higher the S2concentration, the lower the overall growth rates and the higher the relatiue growth of the 560-nm emission (the 490-nm emission grows relatively less for higher S2-excess concentrations also), and (iv) the 420-nm emission is relatively higher in the S2-excess colloids. The 420-nm peak was initially thought to be due to impurities in the crystals such as Mn2+, Ag', or Cu2+ which act in much the same way as crystal defects (i.e., interstitial Zn2+ or S2vacancies). The cations are substituted for Zn2+ in the crystal lattice and act as recombination centers for the e-/h+ pair.3,4J3 This hypothesis is disproved by the following observations. The ZnZ+salts were purified by running them through a Zn columnk4 to remove these impurities, but no changes in the spectra were observed. The Na,S was assumed to be free of these impurities as the sulfides of these ions are insoluble. Preparation of colloids in the presence of IO4 M Cu2+or Ag+ ions did not enhance the 420-nm peak. I n the presence of Cu2+, the emission over the measured wavelength range was quenched dramatically although the general shape of the curves remained unchanged. The presence of Ag+ only reduced the intensities slightly. Preparation of colloids in either K2S or Na2S or with CI-, CI04-, or NO3- counterions did not change the position or the relative growth in the emission intensity of the 420-nm peak, indicating that it is not associated with these ions placed interstitially within (13) Tamaka, S . ; Kobayashi, H.; Sasakura, H. J . Appl. Phys. 1976, 47, 5391. (14) Nydahl, F. Talonfa 1974, 2 1 , 1259.
the crystal lattice or adsorbed at the surface of the colloid. In suspensions prepared in 0.1 M N a 2 S 0 4the measured intensities were higher after I -h illumination than those prepared in H,O. These colloids flocculated upon illumination but show slightly greater emission intensity at 420 nm than those prepared in pure H 2 0 for both the Zn2+ and S2- colloids. When the pH was increased in Zn2+excess colloids before illumination, the growth of the 490-nm emission was reduced. This is thought due to the stabilization of the Zn2+at the surface as Zn(OH),2' which slows the formation of Zn. Illumination of suspensions with the same UV source as that used during the ESCA measurements yielded spectra with a smaller difference between the Zn2+ and S2-excess colloids. The emission intensities reached a maximum after approximately 15 min and then decreased. Illumination with this source, of higher intensity and broader spectral width than the fluorimeter excitation source, causes significant photocorrosion of the surface and thus of the colloids. The result is a decrease in the emission intensities due to either a decrease in the number of colloids present or an increase in the number of nonradiative pathways. Solutions that were illuminated for 2 h by the Hg lamp showed very low emission intensities. In this case the photocorrosion of the colloids to ZnZ+ and S042-must be almost complete. Illumination in this manner caused significant heating of the sample. The S2-excess colloid turned yellowish probably from the presence of elemental S. To maintain charge neutrality, elemental Zn must also be produced concomitantly as has been proposed e1~ewhere.l~The conductivity of the suspensions increased with increasing illumination time. This is thought due to the formation of soluble SO4,- and Zn2+. Addition of Ba2+ to illuminated suspensions showed a slightly opaque appearance, indicating the presence of small amounts of Sod2in solution. The spectra measured for colloids dried onto quartz plates are shown in Figure 7. Here the 420-nm emission is more intense than the other emissions while the overall growth in emission intensities is higher than for the suspensions. The 490-nm emission is slightly higher for the colloid dried from Zn2+ suspension compared to the emission for the dried S2-excess colloid. Exact comparison of the intensities of the peaks with the suspension spectra is not possible because of the nature of the drying process, although the 420-nm emission is relatively higher for the dried colloids after illumination for the same time period as the suspension. Spectra measured on dried powders at 80 K showed an extremely enhanced 420-nm peak relative to the 490-560-nm Torr, region. When the sample was pumped down to 5 X the emission intensities increased. This is most likely due to O2 desorption. Thawing the sample to 25 "C increased the 500-nm emission relative to the 420-nm emission, although the overall intensities decreased by a factor of IO. The emission spectra also showed some fine structure at wavelengths less than 420 nm at 80 K. For Zn plates dipped in 0.1 M Na2S the initial spectra showed a peak at around 380 nm which reduces in intensity as the other peaks grow. This peak is at an energy consistent with edge emission in ZnS which has not been previously reported. Upon illumination of the plate, the 380-nm peak decreases in intensity and the other peaks grow less rapidly than for the colloids. Quenching experiments using MV2+ (methylviologen) and NFA- (5-nitro-2-furoic acid) (Na' salt), on S2-and Zn2+colloids, respectively, which had been illuminated, quenched the 480- and 560-nm emission effectively. When the quenchers were added to fresh colloids, no increase in emission intensities was observed over the period of several hours. When the quenchers were added to the samples that had been illuminated, both the 480- and 560-nm emissions were quenched more effectively than the 420-nm emission. Zn2+ excess colloids were stabilized by the addition of a negatively charged surfactant, 4,4'-azobis(cyanovaleric acid), which was polymerized on the surface of the particles. These colloids showed a reasonably intense 420-nm emission which was constant (15) Hayes, R.; Freeman, P. A.; Mulvaney, P.; Grieser, F.; Healy, T. W. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 231.
6802 The Journal of Physical Chemistry, Vol. 94, No. 17, 1990
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1000 1500 Time (ns) Figure 8. Fluorescence decay spectra for 200%S2-excess ZnS colloidal
1000 1500 Time (ns) Figure 9. Fluorescencedecay curves for 200%S2-and Zn2+excess ZnS
suspension in HzO at 420 and 560 nm before and after illumination for 1 h in the Spex fluorimeter at 300 nm. The curves are as labeled, and the total intensities are normalized.
colloidal suspensions in H 2 0 at 480 nm after illumination for 1 h in the Spex fluorimeter at 300 nm. The curves are as labeled, and the total intensities are normalized.
0
500
with time and showed almost no 490-560-nm emission. Suspensions prepared in AOT reversed micelles in hexane show only 420-nm emission in Zn2+ excess and only a distinct 560-nm peak in the S2- excess preparation. E. Organic Solvents (MeOH, CH3NH2). The emission intensities did not increase to the same extent for colloids prepared in the organic solvents, for either solutions or dried colloids, although slight increases were observed at 560 nm for colloids prepared in MeOH. The emission intensities were of the same magnitude as the initial ones in H20 and remained approximately the same under illumination. This is thought to be due to the stabilization of the surface by coordination of the solvent to the ions at the surface. The organic solvents may also act as h+ traps or be oxidized and therefore quench the emission. The high concentration of O2in MeOH was not responsible for quenching the emission as removal of the O2by N2 purging gave no increase in the emission intensity in the MeOH suspensions. In formamide it was observed that bubbling O2through the colloidal suspension increased the 420-nm emission dramatically. In formamide only a slight 480-nm peak is observed in both Zn2+and S2-suspensions. In the presence of O2 in MeOH, however, small peaks were observed at approximately 560 nm. These were not present in 02-free solutions. Colloids prepared in formamide showed higher intensities but similar growth rates to those in MeOH. The spectra observed in MeOH showed better resolution of the 480- and 560-nm peaks despite their lower intensities. F. Fluorescence Decay Measurements. The determination of the decay curves was complicated by the illumination time dependence of the intensities; however, the changes in the lifetimes were real. Exacting analysis is difficult under these circumstances, but overall trends are observable. The lifetimes at the three measured wavelengths, 420,480, and 560 nm, are all similar with a larger component of the fastest lifetime present at the shorter wavelengths. See Figure 8. The fluorescence lifetimes in general show the same increasing trend upon illumination for both the S2- and Zn2+ colloids. Shorter lifetimes at the measured wavelengths are observed for the S2-excess colloids. See Figure 9. The decay curves are fitted reasonably well with a three-exponential fit at each wavelength although the curves may well be fitted with a multiexponential decay, which suggests a complicated electron-hole recombination mechanism. It also appears that there may occur very fast initial decays on time scales less than the laser pulse width. Average lifetime vs intensity at 420 nm is shown for both Zn2+ and S2- excess colloids in Figure 10. The average lifetime is the average over their measured lifetimes weighted by their relative amplitudes. The decay curves were fitted to four exponentials for this analysis. G . Time-Resolved Spectra. The time-resolved spectra for an S2-excess colloid is shown in Figure 1 1. This illustrates the time
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m m L
01
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4
100
50
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Intensity (arb. units] Figure 10. Emission intensity vs average lifetime at 420 nm for both 200%Zn2+and S2-excess colloids as labeled. The emission intensity was varied by different times of illumination.
L
a
'. . . . 350
400
450
500
550
Wavelength Iiinl
Figure 11. Time-resolved spectra for 200%S2-excess suspension. Times are as follows: (a) t = 0.04 ns, (b) t = 20 ns, (c) t = 200 ns, and (d) t = 780 ns. Curve d also shows the total intensity (dashed line).
decay behavior of the ZnS emission at the characteristic wavelengths, Le., 420, 490, and 560 nm immediately after illumination. For times up to 8 ns the dominant emission occurs at 420 nm. See Figure 1 la. The 490-nm emission increases after 20 ns and
Photochemistry of ZnS Colloids reaches the same intensity as the 420-nm emission after -40 ns. See Figure 1 1 b. For times greater than 200 ns the emission is dominated by the 490- and 560-nm emission. See Figure 1 Ic,d. In general, the shorter wavelength, 420-nm emission, is more rapid than at the longer wavelengths.
Discussion In this study a wealth of experimental data have been obtained from different experimental techniques. The observed illumination time dependence of the photochemistry and surface chemistry is the essential observation of this work. Below we try to develop an adequate physical model that is consistent with all the experimental observations and gives sound directions for further work. A. Photocorrosion Products. The photocorrosion products of the dried colloids have been identified as S, SO,2-, and S042-by using the ESCA technique. These products are formed under illumination on the surface of colloids dried onto the ESCA disk. In another study using ESCA on CdS electrodes by Meissner et aL8 it is proposed that for CdS in 02-freeconditions the formation of elemental Cd and S on the surface occurs. Unfortunately, in our experiments the Zn peaks are not separated well enough to enable a precise determination of the oxidation state of the Zn species at the surface. However, from the data obtained from CdS it may be inferred that either Zn or some other species (for example, H2) is formed while the elemental S is formed by the oxidation of S2-. The formation of elemental Z n / S and Zn2+/ S042-in the photocorrosion of ZnS has also been proposed by other ~ 0 r k e r s . lThe ~ observed changes in suspension pH upon illumination may be understood in terms of reaction 7, where OHions are being used up in the formation of S042-for the S2-excess colloids and H+ ions are used in reaction 9 for the Zn2+ excess colloids. These reactions have been proposed by Reber et a1.I6 from studies on the effect of SOj2-and S042-on the rate of H 2 formation in ZnS suspensions. B. Fluorescence. I . Quantum Yield. Low quantum efficiencies were measured for both the Zn2+ and S2- excess colloids. The quantum efficiencies increase with illumination time. However, these are still low after the total photon dose used in these experiments. This indicates that the nonradiative recombination is a significant mechanism in the photochemistry of these colloids. 2. Fluorescence Intensities. I n addition to the increase in quantum efficiencies upon illumination, the relative intensities of the peaks increase at different rates in different chemical conditions. The emission intensities for the dried colloids also increase with illumination but are less sensitive to the conditions of preparation. The fluorescence spectra for the dried colloids and the suspensions both show the same characteristic emission peaks. It has been argued elsewhere9 that the changes in the colloid which lead to increased quantum yield are best explained by a decrease in the number of centers at which radiationless recombination occurs and not the formation of more efficient fluorescent centers. This reasoning is difficult to understand where the growth of certain peaks under specific conditions is more pronounced than others as is seen in Figures 5 and 6. We believe that the full picture is a combination of both processes which are interrelated in a complex manner. The formation of the recombination centers occurs at the same time as the reduction in the nonradiative processes. Other workers have also observed time-dependent absorption and fluorescence spectra.]' However, these changes have been mainly attributed to time-dependent changes within the crystal latticeI8 or changes in the size of the colloidal particle^.'^ Ramsden and Gratzel have attributed the luminescence from CdS crystals to recombination of electrons trapped at sulfide vacancies with holes in the valence band.20 That the emission intensities (16) Reber, J.; Meier, K. J . Phys. Chem. 1984, 88, 5903. (17) Rossetti, R.; Hull, R.; Gibson, J. M.; Brus, L. E.J . Chem. Phys. 1985, 82, 552. ( 1 8 ) Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 33. (19) Henglein, A.; Fojtik, A,; Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 441.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6803 also increase for the dried colloids is very strong evidence that the growth in emission intensities does not arise from a decrease in the particle size upon illumination as previously suggested.l If the recombination centers are sulfide vacancies, the question arises as to how the emission increases from these centers in the dried colloids upon illumination. The time dependence in the absorption spectra for colloidal semiconductors under illumination, which are attributed to size quantization effects, has been reported e l ~ e w h e r e . ~ *The ' ~ ~ob'~ served absorption edge for the Zn2+ excess colloids used in this study, without stabilizer present, moves to longer wavelengths upon illumination, suggesting an increase in the particle size. This is in stark contrast to the results of Henglein et aI.l9 Obviously, the presence of the stabilizer used in their study affects the photochemistry of the colloids significantly. Our observations are in agreement with the observations of Rossetti et aLzl 3. Emission Decay Properties. The observed fluorescencedecay behavior is complex. For this reason the average lifetime was used as a quantifiable parameter in comparing the lifetime and intensity changes on irradiation. For both Zn2+and S2-excess colloids both lifetime and intensity increased in parallel on illumination, but the lifetime increase occurred with decreasing rate, in particular for the former. The significance of this will be discussed further below. Four exponential terms were required to fit the fluorescence decay reasonably well, with similar sets of decay times but different amplitudes at the three peaks. This does not imply, however, that the decays necessarily are superpositions of four exponential processes. There is reason to expect that the decay from each site, even if the sites were uncoupled, should show a broad distribution of lifetimes, due both to the particle polydispersity and to the fact that the rate of recombination of trapped electrons and holes depends strongly on the distance between the traps. For some models there will also be a direct coupling between the emission wavelength and the lifetime: longer distances between the traps mean smaller interaction energies, resulting in both longer lifetimes and longer wavelengths. The general trend in the time-resolved emission spectra (Figure 1 1 ) is in agreement with this. The time dependence of the emission from CdS colloids has been studied by several investigator^.^^-^^ They have found the decay to be complex and to depend on the excitation intensity due to the bimolecular nature of the recombination process. In our measurements, however, the single-photon counting technique was utilized, with intensities that are extremely low, so that multiple excitations are completely negligible. Bimolecularity is not a source of nonexponentiality under these conditions. 4 . Assignments. The bewildering complexity of the system and the phenomena observed make it impossible to give precise assignment of the three main emission peaks to definite sites. In all the data the very strong sensitivity to the chemical environment and to the photoinduced surface reactions suggests that the sites responsible for the emission peaks are to be found at the particle surface. There should be at least three different sites, each connected to one of the observed main peaks. The relative peak intensities changed with variations of the chemical environment, with the extent of the photoreactions, and also with the accumulation of photoproducts as monitored in the ESCA experiments. The nature of these changes led us to the following tentative assignments: (i) the 480-500-nm peak is associated with Zn at the surface, (ii) the 550-570-nm peak is associated with S at the surface, and (iii) the 420-nm peak is associated with SO,2- adsorbed at the surface. The arguments are as follows. All the peaks grew under illumination in parallel (20) Ramsden, J . J.; Gratzel, M. J . Chem. SOC.,Faraday Trans. I 1984, 80, 919. (21) Rossetti, R.; Hull, R.; Gibson, J. M.: Brus, L. E. J . Chem. Phys. 1985, 83, 1406. ( 2 2 ) Morgan, J. R.; Natarajan, L. V. J . Phys. Chem. 1989, 93, 5. (23) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1983, 87, 5498. (24) Albery, W. J . ; Brown, G. T.; Darwent, J. R.; Saievar-lranizad, E. J . Chem. SOC.,Faraday Trans. I 1985, 81, 1999.
6804
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with the accumulation of Zn, S, and S042- a t the surface. The 480-nm peak is particularly enhanced in Zn2+ excess colloids and the 550-nm peak in S2-excess under nitrogen atmosphere, whereas the 420-nm peak appears to grow more rapidly when O2is present at low S2-excess and low Zn2+ excess concentrations. This peak is also enhanced when S042-is added to the solution. The 420-nm peak is also the only one observed in 02-purged formamide solution. Having made these tentative assignments, the next step would be to construct a mechanistic scheme against which the mechanistic results, namely, quantum yields, decay curves, intensity vs irradiation dose, etc., could be tested. This is the point, however, where the complexity of the system at present seems to prevent further progress. The colloids are small (and polydisperse), so that the statistics of the distribution over the different particles of various sites for radiative and nonradiative processes must be very important: it is enough with one efficient electron acceptor on a particle to turn off the radiative processes almost completely. One possibility is that the low quantum yields of emission-at best a few percent after illumination, usually 1 or 2 orders of magnitude smaller b e f o r e w e r e due to the presence of an efficient quencher at the surface of most of the particles, which was consumed or made innocent by the photoreactions. However, the irradiation would not affect the emission lifetimes in this case, which it does, and therefore a competition between radiative and nonradiative processes must occur on each of the particles. The change induced by the irradiation toward both longer lifetimes and larger intensities suggests that some efficient nonradiative pathways are closed by the reactions. The quantum yields of the photoreactions were not measured, but the fact that the emission intensities changed dramatically after an irradiation dose corresponding to only a few photons adsorbed per particle suggests that the photochemical paths were very efficient and may be part of
the reason for the low emission quantum yields. If only a few sites on each particle give efficient photochemistry which, at least in part, results in the formation of radiative recombination sites, it could explain the rapid increase in the emission intensity and the increase, at a decreasing rate, of the average lifetime. The three different types of radiative sites are probably present at varying proportions on the separate particles. There must be some type of coupling between the presence of a particular radiative site and the photochemical surface processes (such as the suggested formation of the radiative site in the photochemical process); otherwise, the change of the relative peak heights with irradiation time seems inexplicable.
Conclusions In conclusion, the surface chemistry of the semiconductor colloids has been shown to be important in the photochemistry of these systems. The Zn, and S formed on the surface during photocorrosion may not only act as recombination centers but also reduce the rate of nonradiative decay and quenching of the emission. The coupling of these processes is complex and is not fully elucidated by this work. The exact assignment of the different emission peaks to certain surface states is difficult and conjectural at this stage. The use of other physical techniques to further examine the exact nature of the surface states would be worthwhile. Theoretical calculations would be very useful in comparing the predicted energy levels of elemental Zn, S, and SO,2- at the surface with the experimentally observed ones. Acknowledgment is made to Goran Carlsson for assistance with the electron microscopy. The financial support of this project by The Swedish Board of Technical Development and The Goran Gustafsson Foundation is also gratefully acknowledged.
Determination of Electrostatic Surface Potentials of Oil-in-Water Microemulsion Droplets Using a Lipoidal Acid-Base Spectroscopic Probe Brent S. Murray,+ Calum J. Drummond,* Franz Grieser,*Vt and Lee R. Whites Department of Physical Chemistry, The University of Melbourne, Parkville. Victoria 3052, Australia, CSIRO Division of Chemicals and Polymers, Private Bag IO, Clayton, Victoria 3168, Australia, and Department of Applied Mathematics. The University of Melbourne. Parkville, Victoria 3052, Australia (Received: October 30, 1989; In Final Form: February 23, 1990)
The lipoidal solvatochromic acid-base indicator 1 -hexadecyl-5-hydroxyquinolinehas been used to determine the electrostatic surface potential of the microemulsion droplets in a sodium dodecyl sulfate/pentan-I-ol/n-dodecane/O.1 M NaCl (aq) system as a function of the volume fraction of the nonaqueous phase, 9. Varying C$ (at a constant mass ratio of surfactant + pentanol to oil of 4.56:l) from low values to greater than 0.75 changes the surface potential from around -75 mV to about -50 mV. The electrostatic trend observed experimentally can be modeled by using a cell model, but only if allowance is made for a growth in the radius of the droplets of the order of l0-20%' over the 9 range 0-0.6. The surface potential measurements and modeling are complemented with conductivity measurements which have been examined by using current theories. Our conclusion is that the microemulsion droplets remain as oil-in-water entities at least u p to q5 = 0.6. Beyond this volume fraction we cannot unambiguously establish the form of the emulsion.
Introduction A microemulsion may be defined as a stable, transparent, isotropic mixture of oil, water, and surfactant which forms spontaneously on mixing the components,i As well as requiring a n ionic or surfactant for its formation, a second sur*Author to whom correspondence should be addressed. 'Department of Physical Chemistry, University of Melbourne. *CSIRO. Department of Applied Mathematics, University of Melbourne.
0022-3654/90/2094-6804$02.50/0
factant is freauently reauired, typically a short-chain alcohol. The combined ads'orption oi the suryactants lowers the interfacial free energy to such low values that this is offset by other contributions to the overall free energy of the system so that the structures which form are thermodynamically stable and are too small to scatter visible light. Microemulsions continue to gain commercial importance in areas as wide ranging as detergency, lubrication, and ( I ) Schulman, J. H.; Stoeckenhius, W.; Prince, L. M. J. Phys. Chem. 1959. 63, 1611
0 1990 American Chemical Society
