1958
J . Phys. Chem. 1990, 94, 1958-1962
Mossbauer Spectroscopy Study of the Kinetics of Photoreduction of Fe3+ on CdS Semiconductor Powders T. A. Gessert,*,+D. L. Williamson,*%$ and A. J. Nozik*,' Solar Energy Research Institute, 1617 Cole Blud., and Department of Physics, Colorado School of Mines, Golden, Colorado 80401 (Receiued: May 5, 1989)
Photoreduction of Fe3+to Fez* via illumination of CdS particles suspended in ferric chloride electrolyte has been monitored by "Fe Mossbauer spectroscopy. The conversion rate was studied as a function of the relative amounts of CdS and Fe3+ in the semiconductor powder-electrolyte slurry. Systematic variations in the rate constant with [CdS]/[Fe3+]are attributed to the optical density change associated with the I-fim CdS particle size and to a high efficiency for photocorrosion of the CdS by the photogenerated holes. Surface reduction of Fe3+to Fe2+also occurs in the dark. One experiment was performed w i t h a colloidal solution of 10-nm CdS particles, and very different behavior was observed compared to the powders.
Introduction The study of light-induced electron-transfer reactions at semiconductor-electrolyte interfaces has become an active area of research because of the fundamental role of such reactions in photoconversion processes. Of particular recent interest is the use of semiconductor powders and colloidal dispersions for photocatalysis and Various experimental techniques have been developed to monitor the redox reactions that occur in these systems,'-3 including our use of 57Fe Mossbauer spect r o ~ c o p y . ~This latter technique is an electrodeless, in situ, quantitative method for the direct observation of changes in the ionic state of iron ions. For the case of TiOz dispersions in a photoelectrochemical cell, Ward and Bard5 showed that addition of the metal ions Cu2+or Fe3+to the solution dramatically increased the photocurrent output of the cell. A fast photoreduction, or electron-trapping reaction, Fe3+ + e-(Ti02) Fe2+,was proposed5 to account for reduced electron-hole recombination and therefore for the enhanced photocurrent. The Mossbauer study of TiO, dispersions4 directly verified this reaction and additionally demonstrated that the back-oxidation reaction, Fe2+ h+(Ti02) Fe3+,also occurred such that the steady-state concentration of Fe2+ was clearly less than that of Fe3+for several types of Ti02dispersions. Attainment of the steady-state condition for Ti02 powders required a few hours of i l l ~ m i n a t i o n ,corresponding ~~~ to an overall first-order rate constant for the reduction reaction of about min-1.4 Here, the ability of CdS dispersions to photoreduce Fe3+ is investigated by Mossbauer spectroscopy. In contrast to Ti02. 100% conversion to Fe2+ is observed with photoreduction rate constants that are about a factor of 100 larger. Back-oxidation of the Fe2+ is found to not play a significant role because of preferential photocorrosion of the CdS by the photogenerated holes. In addition, a dark redox reaction of Fe3+ with the CdS surface is demonstrated which leads to the formation of Fez+ and a sulfur layer on the CdS particle surfaces.
-
-
+
Experimental Procedures The procedures used here are similar to those reported earlier,4 and we give additional details only where appropriate. One major change is the repeated use of the same sample for many illumination/Mossbauer measurement cycles. Sample Preparalion and Illumination. CdS powder la beled as 99.99% pure was obtained from Cerac, Inc. The average particle size obtained by optical microscopy was about 1 .O pm; the width of the particle size distribution at half-height was about 0.5 pm. X-ray diffraction of the powder revealed the hexagonal (wurtzite) structure and the presence of a minor contaminating phase that could not be identified. The ferric chloride solution (95% enriched in 57Fe)was initially 28 mM in Fe3+and was diluted with deionized H 2 0 to prepare 'Solar Energy Research Institute.
*
Colorado School of Mines.
0022-3654 J9012094- 1958$02.50/0
TABLE I: Samples and Characteristics R M," me IFe3+l.6mM 0.15 0.9 34 0.4 0.7 4 16 25 0.2'
1.9 1.4
IO 40
35 0.2
S.d cm2
( t ) . Cum
1.o
12 24 31 125 500 440 280
2.0 2.5
28 20 14 14 8 6
11 42 37 0.2
Mass of CdS. Molarity of Fe3+. 'Average thickness of CdS calculated as mass of CdS divided by the product of sample area and CdS density. dTotal surface area of CdS in dispersion assuming 1.0-pmdiameter spheres for powder and IO-nm-diameter spheres for colloid. e Colloidal dispersion.
the desired dispersions. A slightly higher concentration of 34 mM was obtained for one sample due to evaporation of H 2 0 from the solution. Care was taken to mix the CdS and electrolyte in the dark and to keep it shielded from any room light for Mossbauer measurements prior to illumination. The sample holder used for both the illumination and Mijssbauer measurements was an annular Plexiglass disk (2.2 cm 0.d. X 1.6 cm i.d. X 0.64 cm thick), sealed with an acetate film (0.01 cm thick) on either side. The optical absorption edge of the acetate was measured to be at 320 nm. The allowable thickness of the sample holder is constrained by electronic absorption of the 14.4-keV Mossbauer y-rays by the sample. The acetate film windows, and the use of silicone glue to attach them to the Plexiglass, allowed the repeated freezing and thawing cycles required for the repeated Mossbauer/illumination operations for a single sample. Infrared filtering of the 450-W xenon arc lamp was accomplished via a IO-cm column of H 2 0 and a Melles Griot KG4 filter. In this configuration the output of the lamp was calibrated to 100 mW/cm2 using an Ortec thermopiIe placed in the same location as the sample. To keep the CdS particles suspended during illumination, the sample disks were mechanically agitated. One sample of colloidal CdS was prepared by the technique given in ref 6. The solution was 2.5 mM in CdS, and the CdS particles were about 10 nm in size as estimated by optical techniques. Samples were prepared with different amounts of CdS powder and Fe3+ ions in solution. The ratio R E number of Cd at~~~
~
~~
~
____~
Ed. Energy Resources through Photochemistry and Catalysis; Academic Press: New York, 1983. (1) Gratzel, M . ,
(2) Pelizzetti, E.; Serpone, N., Eds. Homogeneous and Heterogeneous Photocatalysis; Nato AS1 Series C , Vol. 174; Reidel: Boston, 1986. (3) White, J. R.: Bard, A. J. J . Phys. Chem. 1985, 89, 1947. (4) Brown, J. D.: Williamson, D. L.; Nozik, A. J. J . Phys. Chem. 1985, 89, 3076. ( 5 ) Ward, M. D.; Bard, A. J . J . Phys. Chem. 1982, 86, 3599. (6) Nozik, A. J.; Williams, F.; Nenadovic, M. T.; Rajh, T.: Micic, 0. I. J . Phys. Chem. 1985, 89. 397.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 1959
Photoreduction of Fe3+ on CdS n Fez' I Fe3'
7
1.000
iI
1.00
1
R:0.15
~
'V
0.795 0.89
1.000
1.00
0.873
J
0.85
tl
1.00 1.000
3 min 0.82
1
J
1.00
0.78
I
0 6 Velocity Imm/sl Figure 1. Mossbauer spectra versus illumination time from the dispersion with R = 16. The resonances due to Fe3+ and Fe2+are indicated. A solid line passes through the data points and is a computer fit of a superposition of the individual Lorentzian lines shown. -6
oms/number of Fe atoms is used to identify each sample. These R values and other characteristics of the samples used are summarized in Table I. Mossbauer Measurements. Temperature control, spectrum accumulation, and fitting procedures were the same as those used earlier.4 The significantly improved resonance signals observed in this study required that thickness corrections7~*be made to the resonance areas in some cases. Such corrections are important at the extremes of either Fe3+or Fe2+ dominance where thickness saturation affects primarily the stronger resonance. Our prior assumption that the recoilless fractions of the Fe3+ and Fe2+ sites in the frozen solution are the same4 was verified here. The total resonance signal from a given sample prior to iilumination ( 100% Fe3+)was the same after illumination (100% Fe2+) within about 5%. Several control experiments were made to ensure that other effects were not important: (1) y-irradiation from the 57C0 Mossbauer source (-20 mCi) causes no measurable photoreduction of the Fe3+; (2) no photoreduction occurs in the absence of CdS; (3) no thermal changes occur for samples heated several degrees above ambient.
-
Results Figure 1 shows a series of Mossbauer spectra for the sample with R= 16. The conversion to Fe2+ is nearly complete after 5 min of illumination. This is in sharp contrast to our earlier results for T i 0 2 suspensions with similar R values and illumination condition^.^ Note that a small amount of Fe2+is readily apparent before illumination. This dark conversion of Fe3+to Fe2+ varies systematically with R value as illustrated in Figure 2. The photoreduction rate of Fe3+for R < 1 is greatly reduced as shown for R = 0.4 in Figure 3. Also shown in the figure is the very different nature of the Fe3+ Mossbauer resonance from the colloidal dispersion: a well-resolved doublet is obtained rather than the broad Fe3+ resonance observed from all the powder dispersions. Table I1 summarizes typical spectral parameters of (7) Abe, N.; Schwartz, L. H. Mater. Sci. Eng. 1974, 14, 239. (8) Williamson, D. L.; Guettinger, T. W.; Dickerhoof, D. W. Mossbauer Spectroscopy and Its Chemical Applications; Stevens, J. G . , Shenoy, G. K., Eds.; American Chemical Society: Washington, DC, 1981; p 179.
v
0.915L 1
1
I
-6
0
6
Velocity Imm/sl Figure 2. Mossbauer spectra from dispersions with different R values kept in the dark for approximately equal lengths of time (- 1 h). 7
1
Powder - .. 1.000
w
0.767 L
m t r-
.-c f
Colloid 1.000
-m ~
I
0.866 1.000
0.861
1
!
-6
- 1
0
6
Velocity Imm/s] Figure 3. Comparison of Mossbauer spectra from powder and colloidal dispersions. TABLE 11: Mossbauer Spectral Parameters of Fe3+ and Fez+ in Frozen Aqueous Solutions with CIS Particles at 130 K (h5K)" spectral component 6, mm/s A, mm/s r, mm/s 0.50 (3) 0 1.9-1.1b'C Fe3+ (sharp)b Fe3+ (broad)b 0.5 (1) 0 12-8'*' Fe3+ (colloid) 0.51 (1) 0.45 (2) 0.45 (2) Fe2+ (light or dark)d 1.35 (2) 3.15 (5) 0.52 (3) Fe2+ (colloid) 1.36 (1) 3.16 (5) 0.51 (3) " 6 is the isomer shift relative to a-Fe at room temperature. A is the quadrupole splitting. r is the full width at half-maximum. Numbers in parentheses give the range of values observed for the last significant figure (e.g., 0.50 (3) 0.50 f 0.03). bSee ref 4 for discussion of origin of the broadened Fe3+ resonance. Values decrease systematically with decreasing Fe3+ upon illumination and the broad component disappears. dNo differences are found for Fe2+ species produced in the dark (36% Fe2+ for R = 25) and those produced upon illumination.
1960 The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
,
I
1
TABLE III: Results of First-Order Rate Eguation Fits to Fez+ Concentration versus Illumination Time" R % % k k', min-' 47 (2) 0.017 (2) (k' = 0.009)b 0.15 0 (I) 0.4 1 (I) 100 0.023 (4) 0.7 1 (I) 100 0.11 (1) 0.37 (5) 4 6 (2) 100 0.45 ( 5 ) 16 6 (2), 17 ( 3 ) d 100 25 36 (2) 100 0.55 ( 5 ) 0.2c 5 (1) 11 (2) 21 ( k ' > 0.9)*
loo[ ao
c+,,
R
P
Gessert et al.
401Y
25 m 4 0 0.2 (Colloid)
i
+
e+,,
OUncertainties are given in parentheses. bFrom eq 2. 'Colloid. dSecond sample checked for C2+,. 0.61
a
L .
I
I
- 0.31 ,/ Illumination time (min)
Figure 4. Fez+ fractions obtained from Mossbauer spectra versus illumination time for several of the dispersions. Solid lines are least-square fits to eq 2.
the Fe2+and Fe3+resonances obtained for the various experimental conditions. The slightly larger value of the quadrupole splitting of Fe2+ compared to the Ti02 study4 is attributed to the lower temperature of 130 K (1 50 K used in TiOz study4). The Fe2+ produced in the dark (Le., before illumination) has spectral parameters identical with those of Fe2+ produced upon illumination. The kinetics of the Fe3+ photoreduction are shown in Figure 4 for several of the samples. The solid lines are model fits to the data as described in the next section.
Analysis There are two classes of kinetic behavior shown in Figure 4: complete photoreduction and partial photoreduction. In order to describe either complete or partial reduction of Fe3+we assume the simple first-order rate equation4 dC3+/dt = -kC3+
+k ' P
(1)
(e+)
where C3+ is the fractional concentration of Fe3+ (Fe2+) (C3+ c2+ = 1). k and k' are reduction and oxidation rate constants, respectively. This leads to the result
+
where k e+_k + k' and where Po is the concentration of Fe2+ observed prior to illumination (at t = 0 ) . Equation 2 was used to fit the data from all samples with R 2 0.4 where = 1 and k' = 0. was determined by experiment so that k was the only adjustable parameter in the fits. Table 111 lists values of and the fitted rate constants. For the two samples showing < 1 (R = 0.15 and the colloidal sample) eq 2 yields the fits shown in Figure 4 and the rate constants included in Table 111. The values of C?+, were obtained from the experimental data at long illumination time. For the colloidal sample essentially all the change occurred during the first illumination step so that we can only estimate a lower bound on k (and k?.
e+,
e+,
Discussion Photoreduction of Fe3+. The rate constant k characterizes the rate of reduction of Fe3+ on the surface of the CdS particles by photogenerated conduction band electrons: Fe3+ + e-(CdS) Fe2+. The systematic increase in k with the ratio R of CdS to
-
:I 5
OO
I
I
I
C .-
-E
0 '
20
40
(,urn) 20 25
I I
10 15 R ([CdMFeI)
Figure 5. Photoreduction rate constant versus ratio R and versus average CdS thickness of the dispersions (inset).
Fe in each dispersion is shown in Figure 5. There is an apparent saturation behavior that is readily attributed to the increase in optical density as R increases. The inset of Figure 5 shows the data replotted as a function of the average CdS thickness ( t ) (from Table I) and fitted with k = k,( 1 This yields k, = 0.5 min-I and A = lo3 cm-l. For a uniform film of CdS, the fraction of incident photons of a given wavelength (A) that are absorbed varies with film thickness t as (1 - e""), where is the absorption coefficient. For h I 512 nm ( E g = 2.42 eV for CdS at 300 K ) rises sharply to lo5 ~ m - ' . The ~ much lower experimental value of A = IO3 cm-l is due to a quantum efficiency for Fe3+ reduction of less than unity and to the nonuniform nature of the dispersions with small values of ( t ) . If the average particle size is d, one does not expect the dispersion to become optically uniform until ( r ) / d>> 1. Thus the sharp rise in k near A-' = 10 pm is due to the dispersion of CdS becoming sufficiently uniform to absorb essentially all of the incident photons with energy Z E , . The limiting rate constant k, = 0.5 min-' is therefore the appropriate one to estimate the quantum efficiency for Fe3+photoreduction. Before estimating the quantum efficiency, we can first show that k is not limited by diffusion of Fe3+ to the CdS particle. For the I-pm CdS particles and the total masses used in the 1.2-mL samples, one can estimate the average distance between the CdS particles to range from 5 to 22 pm. Taking the diffusion coefficient of Fe3+ in H 2 0 as cm2/s,Ioone estimates the average time to travel such distances as from 0.025 to 0.5 s, respectively. Thus the corresponding rate constants limited by ion diffusion would be of the order of from 2400 to 120 min-', respectively, for the samples used here. These are at least 2 orders of magnitude larger than our experimental values. To estimate the quantum efficiency from k,, the spectral output of the xenon lamp is multiplied by the transmissivity of the Melles Griot KG-4 filter and integrated from X = 320 nm (acetate film cutoff) to X = 512 nm ( E g ) . This integrated intensity corresponds to about 24% of the total 100 mW/cm2 flux used for illumination. Taking an average photon energy of 3.0 eV for the range from 320 to 512 nm yields an average estimated photon flux of 5 X 10I6/(cm2.s). Since the initial density of Fe3+in the optical beam
-
-
(9) Dutton, D. Phys. Rev. 1958, 112(3), 7 8 5 . (10) Bard, A. J.; Faulkner, L. R. Electrochemical MerhodsFundamentals and Applications; W h y : New York, 1980; p 221.
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 1961
Photoreduction of Fe3+ on CdS
2x10'8
t
2H20
+ 4h+
light
4H+
+ O2
(4) Reaction 4 is important in that it produces oxygen which oxidizes the stable sulfur layer into soluble ions as proposed by Meissner (5)
(6) (sulfite)
(7) (8)
(sulfate)
(9)
CdS area (cm2)
Figure 6. Fe2+ atomic concentration in solution versus estimated surface area of the CdS in each dispersion. The open circle is the colloidal sample, and points A and B are discussed in the text.
(sulfate) I0'*/cm2 (for R = 25 sample), one can suggest k, = p(5 X 10l6cm-2 s-I/3 X 10l8 cm-2), where p represents the overall quantum efficiency for Fe3+ photoreduction. Thus k, = I.Op (min-I) and since k, from the experiments is -0.5 m i d , a very high overall quantum efficiency of p = 50% is obtained. The exact value should not be taken too seriously in view of the aproximations made to estimate the photon flux. Dark Decomposition and Photocorrosion. As mentioned previously, in addition to a higher rate constant, there are two other striking differences between results observed for CdS and TiO,. They are, for CdS, a small but measurable amount of Fe2+ is always observed before illumination and the end-point is dependent upon the ratio of CdS to saturation of Fe2+ Fe3+ atoms ( R ) . Both these observations can be explained by noting that CdS is susceptible to photooxidation in aqueous sol u t i o n ~ . ' ~ In - ~ particular, ~ the following mechanisms are proposed. Dark Decomposition. Since the reaction of CdS with Fe3+to form Fe2+ and So is thermodynamically spontaneous (AG < 0)14 is 3
X
(e+,)
CdS 4- w e 3 +
dark
Cd2+
+ Soad 4- 2Fe2+
(3)
where the Cd2+ goes into solution and the sulfur species are adsorbed (hence the subscript -ad") on to the CdS surface. Notice that these reactions will (1) leave the CdS surface coated with a stable sulfur layer, inhibiting further decomposition, and (2) reduce two Fe3+ atoms per Soadformed. These reactions will proceed until the Soad layer is of sufficient thickness to inhibit additional Cd2+ dissolution. Furthermore, the amount of Fe3+ reduction in the dark should be dependent upon the total surface area of CdS and not necessarily upon the ratio R . In Figure 6 this correlation between Fe2+ ions/cm3 generated in the dark (no) and the surface area of CdS in each sample is demonstrated. The discrepancy of point A ( R = 16) is believed to be due to the sample being frozen relatively soon after sample preparation and thus with insufficient time for the dark reaction to reach equilibrium (although the time left in the dark was similar for each sample, dependent only upon the sealant drying time, the actual time was not monitored). A second sample, prepared with R = 16 and left in the dark for 30 min, did indeed show a much larger no (point B, Figure 6). From the slope of the line in Figure 6, and assuming a surface density of 1 X 10I5Soad/cm2,one can estimate that about 5 monolayers of Soad are sufficient to stop the dark reduction/ decomposition reactions. Light-Znduced Photocorrosion. Once the stable sulfur layer is formed on the CdS particles in the dark, there must be a mechanism by which one can explain the observed photoreduction that commences upon illumination. For this to happen, we propose that, upon illumination, the population of photoactivated electrons and holes is high enough to initiate the surface reaction ( 1 1 ) Williams, R. J . Chem. Phys. 1960, 32, 1505. (12) Ellis, A. B.; Kaiser, S. W.; Wrighton, M . S. J. Am. Chem. SOC.1976, 98, 6855. ( 1 3 ) Wilson, J . R.; Park, S. J . Electrochem. SOC.1982, 129, 149. (14) Bard, A . J.; Faulkner, L. R. Electrochemical MethodsFundamentals and Applications; Wiley: New York, 1980; p 700.
(10)
The sulfite and the sulfate, being soluble, are quickly removed from the surface during illumination and mechanical agitation of the aqueous CdS sample, eventually cleaning the soad layer and allowing decomposition to proceed following reaction 3, but with light. These reactions will now, however, make use of the high population of photogenerated holes and will continue until either the sample is removed from the illumination source or the CdS particles are completely consumed. The most direct evidence in support of these models is the partial conversion of Fe3+ for the R = 0.15 sample (Figure 4) and the observation that, for this sample, no CdS particles could be visually = 0.47). Finally, observed after saturation was reached (at it should also be noted that if photocorrosion were the only mechanism available for hole scavenging (aside from recombination with electrons), one would expect a maximum possible of 2R (=0.3 or 30% for R = 0.15). Since experimentally we observed that for R = 0.15, C?+, = 0.47, we suggest that this is evidence for competing hole reactions, such as reaction 4. CdS Colloid. The results from the colloidal CdS experiment show several distinct differences from those of the powder dispersions (see Figures 3, 4, and 6 and Tables I1 and 111): (1) a partial conversion of Fe3+ to Fe2+ that is only about one-fourth that of the powder sample with a similar R value (0.15); (2) evidence for a forward rate constant that is much larger than the R = 0.15 powder sample; (3) a dark conversion of Fe3+ to Fe2+ that does not fit the trend with surface area found from the powders; (4)a resolved quadrupole doublet for the Fe3+Mossbauer resonance. This last result suggests that the Fe is complexing with one of the species in the solution used to prepare the colloid (e.g., (NaP03)6). Such complexing could lead to enhanced paramagnetic spin-spin interactions due to smaller Fe3+-Fe3+ distances and therefore to shortened electron spin relaxation times. This would decrease the paramagnetic hyperfine interaction in the Fe3+ system and produce sharper Mossbauer line widths.20 If most of the Fe3+is tied up this way in solution, it may explain results 1, 3, and 4. Result 2 is not unexpected since the optical density of the colloidal dispersion will be much more uniform than the powder dispersion with a similar R . It would be particularly interesting to establish whether a higher rate constant can be achieved with colloids compared to the best powder result (k = 0.5 min-I). This may occur if k is limited by electron-hole recombination in the relatively large powder particles. Obviously, more experiments with colloids are needed to clarify these initial results. Comparison to Related Work. White and Bard3 studied the effects of the addition of a variety of electron acceptors to a photoelectrochemical cell containing CdS powder suspensions. Most of their work was done with methylviologen (MV2+) because of the pH independence of its redox potential and its stability. They observed that additions of either Cuz+ or Fe3+, which had worked well as electron acceptors in Ti02 dispersions, produced
e+,
e+,
(15) Meissner, D.; Memming, R.; Kastening, B.; Bahnemann, D. Chem. Phys. Lett. 1986, 127, 419. (16) Meissner, D.; Benndorf, C.; Memming, R. Appl. Surf. Sci. 1987, 27, 423.
J. Phys. Chem. 1990, 94, 1962-1966
1962
no photocurrent and attributed this inactivity to the "displacement from the lattice" of Cd2+by Fe3+ or Cu2+. The present results clearly show photoactivity, and we suggest that the null result of White and Bard3 was due to the dark decomposition reaction of Fe3+ with CdS (eq 3) which caused complete reduction to Fe2+ in their cell prior to illumination. Their concentrations of Fe3+ and CdS were such that R = 30, and their CdS particle size was -0.1 pm. Note that our sample with R = 25 and I-pm particle size converted 36% of Fe3+to Fe2+via the dark reaction; therefore their particles with about 100 times the surface area should have readily reduced all of the Fe3+ before the critical Soadlayer thickness of about 5 monolayers was formed. The Fe3+ photoreduction rate constant obtained in this study of CdS (-0.5 min-I) is about 2 orders of magnitude faster than those obtained in similar experiments4 with TiO, (-0.004 min-I for anatase and -0.002 min-' for rutile). The CdS photoelectrochemical cell of Bard and White3 yielded a photocurrent rate constant of -0.5 min-l as estimated from their Figure 1 for MV2+ acceptor in solution. In earlier studies of Ti02suspensions under similar conditions, much smaller photocurrent rate constants could be deduced for both Fe3+ and MV2+ additions. In particular, for Fe3+,a photocurrent rate constant of -0.005 min-] can be estimated from Figure 7 of ref 5. The near quantitative agreement of the rate constants estimated from experiments in photoelectrochemical cells with our Mossbauer-determined rate constants is probably fortuitous, but the agreement in relative changes between CdS and Ti02suggests that inherent material properties are controlling the observed rate constants in both types of measurements. For example, the larger band gap of TiO, (3.0 eV) versus CdS (2.4 eV) will directly reduce the effective pseudo-first-order rate constant since a smaller fraction of the incident photons can create electron-hole pairs; however, this cannot explain the factor of 100 difference in rate constants. The much larger electron mobility in CdS (340 cm2/(V.s)18) than in TiO, ( 5 2 cm2/(V-s)19)may also be important. Another possibility is
-
that the very strong tendency for photocorrosion in CdS may enhance the photoreduction since the surface reaction in eq 4 may be very fast and hence effectively reduce the electron-hole recombination rate.
Conclusions The kinetics of the photoreduction of Fe3+ by aqueous dispersions of CdS particles have been studied in a direct manner with 57FeMossbauer spectroscopy. Rate constants are observed that depend on the optical density of the dispersions in an expected fashion. The rate constant observed for the condition of maximum optical density is about 2 orders of magnitude faster than that for similar dispersions of TiO,, and a very high overall quantum efficiency near 50% is estimated for Fe3+to Fe2+conversion. This behavior can be attributed to either the intrinsically better semiconductor properties of CdS and/or the very high efficiency for photocorrosion of the CdS. A dark decomposition reaction occurs on the surface of the CdS particles such that a percentage of Fe3+ is reduced to Fe*+. Although this dark reaction stops before reduction of all the Fe3+, photoreduction will again commence upon illumination. This behavior can be explained by suggesting a dark reaction that occurs until a protective layer of elemental sulfur about 5 monolayers thick is formed. Further, upon illumination, this protective sulfur layer is "cleaned" by a series of reactions that convert the sulfur to sulfate or sulfite. These species then dissolve and allow photoreduction and photocorrosion to continue until either all the Fe3+ ions are reduced or all the CdS is decomposed. Indirect evidence of such reactions is obtained from the quantitative conversion of Fe3+ to Fe2+ relative to the amount of CdS in the dispersion.
Acknowledgment. This work was supported by the US.Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences. Registry No. Fe3+,20074-52-6;Fe2+, 15438-3 1-0; CdS, 1306-23-6; FeCI,, 7705-08-0; S, 7704-34-9.
( 1 7 ) Ward, M. D.; White, J. R.; Bard, A. J. J . Am. Chem. Sot. 1983, 105, 21.
( 1 8) Sze, S. M. Physics of Semiconductor Deuices; Wiley: New York, 1981; p 849.
(19) Jarzebski, Z. M. Oxide Semiconductors; Pergamon: New York, 1983; p 216. (20) Nozik, A. J.; Kaplan, M. J . Chem. Phys. 1968, 49, 4141.
Extraordlnary Viscosity Behavior of Binary Mixtures of Highly Deionized Colloids Tsuneo Okubo Department of Polymer Chemistry, Kyoto University, Kyoto 606, Japan (Received: May 22, 1989; In Final Form: September 11, 1989)
The viscosities of binary mixtures of different sizes of colloidal silica (diameter 8-45 nm) and monodisperse polystyrene spheres (diameter 85-109 nm) are measured in deionized aqueous suspensions. The specific viscosities (v,) of mixtures in the "gaslike" distributions change linearly with the mixing ratio ( x ) . vsp of "liquidlike" mixtures shows a negative deviation from linearity in the qSp-xcurves. This is explained reasonably by the decrease of void space and the increase in mean intersphere distance. Significant positive deviation is observed for mixed suspensions of "crystallike" structures. The sheared flow of binary mixtures forming alloy and superlattice structures is very difficult compared with that of the crystallike structures (face-centered cubic or My-centered cubic lattices) of constituent spheres. These results are consistent with the significant role played by electrical double layers under the influence of purely electrostatic intersphere repulsion in the effective hard-sphere model.
Introduction A study on the formation of ordered structure of monodisperse colloidal spheres in deionized aqueous suspension is helpful in understanding fundamental properties of solid crystals and also electrostatic interactions of macroionic systems. The ordered formation has been ascribed to expanded Debye screening length and electrostatic repulsion between spheres.'-17 ( I ) Luck, W.; Klier, M.; Wesslau, H. Ber. Bunsen-Ges. Phys. Chem. 1963, 67, 1 5 . 8 4 .
0022-3654/90/2094- 1962$02.50/0
In a previous paper,18this author discussed in detail the gaslike, liquidlike, and crystallike distributions of polystyrene spheres at ~~
~~
~~~~
(2) Stone-Masui, J.; Watillon, A. J . Colloid Interface Sci. 1968, 28, 187. (3) Vanderhoff, W.; van de Hul, H. J.; Tausk, R. J. M.; Overbeek, J. Th. G. In Clean Surfaces: Their Preparation and Characterization for Interfacial Studies; Goldfinger, G., Ed.; Dekker: New York, 1970. (4) Hiltner, P. A.; Papir, Y. S.; Krieger, I. M . J . Phys. Chem. 1971, 7 5 , 12, 1881. ( 5 ) Kme, A.; Ozaki, M.; Takano, K.; Kobayashi, Y.; Hachisu, S. J . Colloid Interface Sci. 1973, 44, 330.
0 1990 American Chemical Society
