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J. Phys. Chem. 1983, 87, 5498-5503
fraction of reactive ions, e.g., OH-, are in the diffuse layer. The postulated reaction of ions in the diffuse layer may also be important for small cationic aggregates which do not bind hydrophilic anions specifically in a Stern layer.
Acknowledgment. Support of this work by the National Science Foundation (Chemical Dynamics Program), the
Army Office of Research, CNR Rome, and the North Atlantic Treaty Organization is gratefully acknowledged. Registry No. 2, 87696-35-3; 2a, 54385-45-4; 3, 61095-52-1; 4, 87696-36-4; 6, 44519-34-8; Br-, 24959-67-9; Me3N+CH2CH20H, 62-49-7; OH-, 14280-30-9; p-nitrophenyl diphenyl phosphate, 10359-36-1;2,4-dinitrochlorobenzene, 97-00-1; 2,4-dinitrochloronaphthalene, 2401-85-6.
Surface Effects in the Photochemistry of Colloldal Cadmium Sulfide
J. Kuczynski and J. K. Thomas"' Department of Chemistry, Universiw of Notre Dame, Notre Dame, Indiana 46556 (Received: February 3, 1983; In Final Form: April 18, 1983)
Both steady-state and pulsed-laser photoexcitation techniques have been used to investigate photoinduced processes at the surface of CdS colloids. The colloid surfaces have been modified by use of adsorbed reactants and surfactants, the latter conveniently changing the charge of the colloid and lending greater stability to the system. Luminescence of CdS is observed, the spectral quality of which depends on the excitation light intensity, and the nature of the adsorbed species. The CdS luminescence is quenched rapidly (7 600 nm, while recombination of bulk e--h+ pairs give rise to emission at X = 525 nm. It is argued that any available surface states are of lower energy than bulk states. A similar blue shift in the fluorescence was observed for a CdS colloid upon irradiation with high intensity light. Parts A and B of Figure 2 show the effect of various additives on the emission intensity of colloidal CdS samples. As indicated earlier, O2pressures over the samples up to 1atm did not affect the fluorescence yields. For the postively charged CdS-CDBAC colloid, I- did not affect the fluorescence intensity. The same result was obtained
for I- quenching of the negatively charged CdS-SDS colloid. Both CdS colloids exhibit a strong pH dependence with respect to fluorescence intensity. The emission intensity peaks at a pH between 8 and 9 and moderately decreases at either more acidic or more basic pH values. The fluorescence spectral maxima for both colloids, SDS or CDBAC, were unaffected by pH. In the presence of Cu2+the absorption spectra for both CdS-SDS and CdS-CDBAC colloids are increased at all wavelengths. The CdS luminescence was effectively quenched upon addition of small concentrations of Cu2+ ion. Since Cu2+forms an insoluble sulfide (ksp = it is attracted to the CdS surface where it is strongly adsorbed. The emission peak is red shifted some 70 nm in both colloidal systems, and subsequently quenched as the concentration of Cu2+is increased. Note, however, that in the CdS-SDS colloid Cu2+quenches the luminescence approximately ten times more effectively than in the CdS-CDBAC colloid. This is attributed to an electrostatic repulsion of Cu2+away from the positively charged CdSCDBAC particle surface and, possibly, to competition between CDBAC and Cu2+for the negative surface sites on the CdS colloid. The Stern-Volmer plot for Cu2+ quenching of the fluorescence shows upward curvature. Part C of Figure 2 shows a Perrin plot if In Io/I vs. [Cu2+]. This plot is linear and indicates that Cu2+quenching of CdS luminescence is static, a fact which accounts for the upward curvature of the Stern-Volmer plot for Cu2+in Figure 2A. The Stern-Volmer plot for MV2+quenching of CdS fluorescence shows reasonable linearity in Figure 2A. However, it is important to consider the amount of MV2+adsorbed onto the colloid. These data are shown in Figure 3, both as a plot of MV2+adsorbed vs. [MV2+] in solution (Figure 3A), and as a Langmuir plot (Figure 3B). The latter plot shows that the adsorption of MV2+ in CdS-SDS colloidal particles approximates to a Langmuir process. When these data are taken in conjunction with the quenching data of Figure 2A, then it is again observed that the kinetics best fit a Stern-Volmer plot. MV2+ adsorbed at the CdS surface competes with the
Photochemistry of Colloidal Cadmium Sulfide
photogenerated holes for e-, a process which leads to a decrease in the luminescence yield. The mobility of M V , and hence probably MV2+(see later section), on the particle surface is low, and little diffusion takes place in a time s). compared to the e--h+ recombination time However, the mobility of e- on the particle surface is high, and during ita short lifetime e- encounters adsorbed MV+. The competing processes approximate to Stern-Volmer kinetics as observed. In the case of Cu2+quenching, it is suggested that adsorbed Cu2+,as CuS, serves as a holetrapping site that ultimately leads to a decrease in the luminescence. The mobility of CuS on the CdS surface is very low and it is suggested that hole trapping by CuS is a static process in direct competition with e--hole recombination at the CdS surface. That is, the CdS surface may trap the hole via recombination with e- at the surface and it is this process which gives rise to the observed luminescence at 620 nm. The wavelength red shift of the luminescence at low [Cu2+]may be due to a specific hole trapping site which consists of several CdS and one or more CuS molecules. The energy level of this trap is located within the band gap of the semiconductor below the conduction band edge. Recombination at this hole site with e- gives rise to the luminescence at 680 nm. As the CuS content increases the trapped hole possesses more CuS character and luminescence is lost. Figure 2 also shows the effect of methyl viologen, MV2+, and propyl viologen sulfonate, PVS, on the CdS emission. The standard reduction potentials of MV2+and PVS are -0.446 and -0.41 V vs. NHE, respectively, and could therefore quench the fluorescence by e- transfer from the conduction band electrons to the adsorbed viologen. The observed differences in the quenching ability of the two viologens is controlled, in part, by electrostatic interactions between the CdS colloid and the viologen molecules. PVS is a zwitterionic molecule and would not be expected to be strongly influenced by the surface charge on the colloids. This is borne out by the data in Figure 2 which show that PVS is capable of quenching the emission of both the negatively charged CdS-SDS colloid and the positively charged CdS-CDBAC colloid. Note that PVS quenches the CdS-SDS emission with approximately 15 times greater efficiency than the CdS-CDBAC emission. Methyl viologen, on the other hand, is positively charged and presumably experiences strong electrostatic interactions with the CdS colloids. Repulsion of MV2+away from the CdS-CDBAC surface could explain the lack of quenching exhibited by MV2+ in this system. Conversely, strong electrostatic attraction of MV2+to the CdS-SDS surface is likely to occur in this sample resulting in a strong reduction of the CdS fluorescence intensity. This is also shown in Figure 2. Figure 4A shows the relative intensity of fluorescence and the growth in the optical density of the radical cation MV+' observed at the end of a 120-11s pulse as a function of MV2+concentration. The radical cation is produced by e- transfer from CdS to absorbed MV2+. The reduction in the emission intensity mirrors the growth in the optical density of the radical cation indicating that the quenching process occurs via e- transfer. Figure 4B shows the effect of [MV2+]on MV+ steady-state yields, i.e., yields of MV+ observed at long periods of time (minutes) after either low-intensity steady-state irradiation or laser-pulsed irradiation of CdS-SDS-MV2+ colloids. The data at both low and high irradiation intensities are identical. This figure also shows the decrease in fluorescence intensity on addition of MV2+. It is apparent that a considerable yield of MV+ is produced at M P concentrations which are well
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5501 11.0
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beyond the state where the luminescence is quenched. These data suggest that one source of e-, which produces MV+ via electron transfer, is derived from the e--h+ recombination process which gives rise to luminescence, while yet another source of e- is available to produce MV+, at higher [MV2+]. This latter source of e- does not participate in processes which give rise to luminescence. Parts A-D of Figure 5 show the effects of various additives or conditions on the yield of MV+ produced by irradiation of CdS-MV2+ colloids. All conditions lead to a decrease in the yield of MV+ on continued irradiation, an effect which is at least partly due to the absorption of exciting light by the MV+ product. Increasing [MV2+], Figure 5A, leads to an increase in the MV+ yield, the initial portions of these data are also illustrated in Figure 4B. Figure 5B shows that increasing the pH to 11.8 leads to increasing initial and plateau yields of MV+. This could be attributed to an increase in the amount of adsorbed MV2+at higher OH- concentrations or due to an increased escape of MV+ from the CdS surface. Figure 5C shows that the yield of MV+ in the CdS-SDS-MV2+ colloid is smaller than in the case of the CdS-SDS-PVS system. As will be seen later, the initial yield of reduced viologen is similar in both systems when observed at the end of a 120-11s pulse of light. The larger yield in the PVS system under steady-state conditions is due to a greater extent of escape of reduced PVS from the CdS particle surface. Excess sulfide and polysulfide decrease the yield of reduced viologen in both systems. Figure 5D shows that sodium oleate increases the yield of MV+; other long-chain fatty acids such as sodium decanoate behave in a similar fashion. However, small molecular weight acids, such as acetate, have no effect. It is suggested that the -COO- group of the adsorbed fatty acid repairs the positive hole, and leads
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The Journal of Physical Chemistty, Vol. 87, No. 26, 1983
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Figure 5. Yields of MV+' upon low-intensiy (steady-state) irradiation of CdS-surfactant colloids. (A) Effect of [MV"] in a CdS-SDS coliold (V, M MV2+;a, 7 X lo4 M MV2+;A,4 X lo4 M MV2+;0 , lo4 M MV2+). (b) Effect of pH in a CdS-Brij 35 colloid (0,pH 11.8;V,pH 10.8;D, pH 9.2). (C) Effect of polysulfide on MV+ and PVS- yields in CdS-SDS colloids (A, M PVS; D, M MV2+; V, M PVS polysulfide; 0 , M MV2+ and polysulfide). (D) Effect of oleate in M MV2+). M MV2+ and oleate; D, a CdS-SDS colloid (0,
to larger yield of e- for reduction of MV2+. Smaller acids are not adsorbed at the particle surface and cannot undergo this e- transfer process. Pulsed studies confirm that oleate ion increases the yield of MV+ that does not undergo recombination with the hole on the CdS surface. Pulsed Kinetics. It is important to confirm the suggestions made from steady-state data by using pulsed-laser excitation in order to ascertain the kinetic nature of the events already described. A disturbing feature in such a
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Flgure 6. Pulsed-laser studies of MV" formation and decay in CdSSDS colloids. (A) High intenslty laser pulse (0.1J/puise, A = 490 nm): transient absorption observed at A = 540 nm (1)and A = 610 nm (2). (6)Measurement as in (a) carried out at low laser intensity ((0.01 J/puise): transient absorption observed at A = 540 nm (1)and A = 610 nm (2). (C) Effect of Cu2+ on the MV" decay (1,no Cu2+; 2, M cu2+; 3, 2 x 1 0 - ~ M cu2+; 4, 1 0 - ~M CU*+; 5, 2 x M CU"; 6,5 X M Cu"). (D) Effect of O2 (1)and pH on MV" decay (2, pH 8.2;3, pH 11.2).
procedure is the change in the A, of the CdS fluorescence with laser intensity. The data in Figure 4 present some justification for utilizing both steady-state data at low light intensities and pulsed-laser studies at extremely high light levels. These data show that the yield of MV+ formed by excitation of CdS-SDS-MV2+ colloids is identical under steady-state and under pulsed-laser conditions. Pulsed-laser excitation of a CdS-SDS-MV+ colloid with light of wavelength 337 or 490 nm leads to the prompt formation of reduced MV2+,i.e., MV+ ( T < 5 ns).9J2 The MV+ was identified by its characteristic absorption spectrum. Irradiation at high light levels and high [MV2+] leads to two species with, X at 610 and at 540 nm, which exhibit different kinetics. Parts A and B of Figure 6 show
Photochemistry of Colloidal Cadmium Sulfide
transient absorption decay curves of MV+' at two different wavelengths and at high and low laser intensity, following excitation of a CdS-SDS colloid with a laser pulse of 490-nm light. The absorption due to the radical cation was monitored by fast kinetic spectroscopy. The dramatic difference in the shape of the decay curves is indicative of two distinct species formed at the surface of the colloidal particles, one of which decreases at X = 610 nm, while the other grows at 540 nm. The decay at 610 nm is characteristic of the MV+' monomer whereas the absorption decay at 540 nm is attributed to dimer formation on the particle surface as this species has a ,A, at 540 nm.14 As the laser intensity decreases the decay of the 610-nm species decreases and the two decay curves, at X = 610 nm aned 540 nm, become identical. This behavior is typical of the aforementioned scheme, the formation of MV+ at the particle surface followed by dimerization of 2MV+. This process occurs efficiently only at high laser intensities. The data are indicative of movement of MV+ on the CdS-SDS particle surface over periods of several microseconds. Further evidence of movement of MV+ on the CdS-SDS surface is obtained from the increased rate of decay of MVt ion the presence of Cu2+,or CuS, as shown in Figure 6C. The MV' decay is complex and not simple first-order kinetics. However, the enhanced rate of decay of MV+ does follow the [Cu2+],and a rate constant of reaction of k1 = 5 X lo4 L mol-l is obtained by taking the rate of decay and the local [Cu2+](calculated as 0.8 M for 10" M Cu2+bulk solution concentration) at the particle surface. This is calculated from the radius of the particle, from the number of Cu2+/particle,and by assuming that Cu2+exists in a lo-A layer around the particle. This rate constant may be compared to that measured for MV+ and Cu2+ in aqueous solution where k = 1.4 X lo6 Lmol-l s-l. A strict comparison is not valid since Cu2+in the particle exists in the form CuS. The data indicate a movement of MV+ and/or Cu2+on the CdS surface. As Cu2+is in the form CuS, it is unlikely to move at any rapid rate on the particle surface. Hence, the rate constant k1 defines the movement of MV+ on the surface, the inidications being that its movement is only slightly restricted as compared to homogeneous aqueous solution. The reaction of Cu2+and MV+ occurs on the CdS-SDS surface, rather than in bulk solution, and evidence to support this claim is forthcoming from the 0, data presented below. Figure 6D also shows the transient decay of MV+' in the presence and absence of O2 and at alkaline pH in a CdSSDS colloid. In the aerated sample, the photoproduced MV+' is rapidly consumed by reaction with dissolved 02. Removing O2 from the sample by bubbling with nitrogen produced a dramatic change in the decay curve. There is initially a very rapid reduction in the optical density due to MV+' followed by a comparatively long-lived signal decay which lasts for steady-state detection. The very rapid decay is due to the back-reaction of MV+' with valence band holes. Addition of various hole traps, such as ethylenediaminetetraacetate, EDTA-, I-, OH-, and triethanolamine, had not effect on the rapid decay. Figure (14)Kosower, E. M.; Cotter, J. L. J.Am. Chern. SOC. 1964, 86, 5524.
The Journal of Physical Chemistry, Vol. 87, No. 26, 1983 5503
6D shows the trace for OH- addition to the CdS colloid. The long-lived decay is greater in the presence of OHwhich is in agreement with the increased MV+ yield at higher pH in steady-state irradiation results. 0, reacts rapidly with MV+ in homogeneous aqueous solution, k being 3 X lo8 L mol-l s-l.15 The rate is much decreased in less polar solvents being 3.3 x lo6 L mor1 s-l in methanol.16 The reactivity of MV+ with 0, on the CdS-SDS colloid is also quite low, and data such as those in Figure 6D show that the rate constant for the reaction is 1.0 X lo7 L mol-' s-l in aerated and fully oxygenated solution. This is indicative of the fact that MV+ is not in a completely aqueous environment during the course of these reactions. In aerated solution [O,] = 2.7 X W4mol/L and 7 1 / 2 for the reaction of MV+ with 0, is 260 ps. Hence, it can be stated that the exit of MV+ from the colloid surface takes much longer than 260 ps. Conclusion The present data confirm earlier suggestions that photoexcitation of CdS colloids leads to e- transfer to electron acceptors such as MV+. The present work draws attention to the importance of the colloid surface on such reactions. Initial excitation of CdS leads to an electron-hole pair, e--h+, some of which may reach the surface of the particle, and give rise to luminescence upon recombination. This process is rapid as determined by the luminescence lifetime which is 260 ps).
As indicated, MV' remains at the CdS surface for a considerable time thereby increasing the probability of back-reaction with the hole. Further work is needed to develop techniques to interrupt the progress and chemistry of the photoformed hole in order to increase the long-term yield of reduced product. Such studies are presently under way.
Acknowledgment. The authors thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. Registry No. CdS, 1306-23-6; Cu, 7440-50-8;sodium dodecyl sulfate, 151-21-3; cetyldimethylbenzylammonium chloride, 12218-9; methyl viologen, 1910-42-5; propyl viologen sulfonate, 77951-49-6. (15) Farrington, J. A.; Ebert, M.; Land, E. J.; Fletcher, K. Biochem. Biophys. Acta 1973,314, 372. (16) Patterson, L. K.; Small R. D.; Scaiano, J. C. Radiat. Res. 1977, 72, 218.
