4026
J. Phys. Chem. 1981, 85, 4026-4029
Effect of Platinization on the Photoproperties of CdS Pigments in Disperslon. Determinatlon by H2 Evolution, O2 Uptake, and Electron Spin Resonance Spectroscopy John R. Harbour,. Robert Woikow, and Michael L. Hair Xerox Research Centre of Canada, Mississauga, Ontario. L5L 1J9, Canada (Recelved: June 17, 1981; In Final Form: September 14, 198 1)
Platinization of CdS was accomplished and differences between the photochemistry of the Pt-CdS particles and normal CdS particles in dispersion were determined. Oxygen uptake, hydrogen evolution, and ESR measurements were used to monitor these differences. The pH and temperature dependence of H2 evolution with Pt-CdS and EDTA as sacrificial donor were determined. The light intensity dependence of both CdS and Pt-CdS were monitored with O2uptake and significant differences emerged. ESR studies detecting both methyl viologen cation radical and superoxide through spin trapping were consistent with the O2uptake and H2evolution findings. Use of Pt on Kaowool in a CdS dispersion revealed that this system was also effective in generating H2 both with and without methyl viologen. Transmission electron microscopy of the Pt-CdS showed that Pt is deposited on the surface of CdS as ultrafine particles of 20 to 50 A. The consequences of this type of deposition are discussed.
Introduction Solar energy utilization can be accomplished through the use of pigment dispersions.'" The pigment powder must absorb a fraction of the solar radiation creating states within these photoconducting particles which can eventuate in redox reactions with species in s o l ~ t i o n These .~ states (holes and electrons) must be either created very near the surface or be capable of migrating to the surface in order to undergo interfacial electron transfer reactions. Hence, reduction and oxidation must occur at the same interface for typical pigment dispersions (as contrasted to PEC cells, where the semiconductor electrode is either a photocathode or photoanode).6 Recently, Krauetler and Bard' have shown that pigments such as Ti02 can be surface modified by the photochemical deposition of platinum and other metals. Platinization creates a heterogeneous surface which potentially can offer different sites for oxidation and reduction. For n-type materials, light is absorbed near the exposed surface creating a hole-electron pair. The hole can undergo a reaction at this site (oxidation) while the mobile electron can migrate to the platinum surface and subsequently undergo a reaction with a solution species (reduction). The Pt-semiconductor junctions in these cases are ohmic. The creation of this catalytic platinum surface can affect both the type of reaction and the interfacial charge-transfer rate and, consequently, the overall e f f i c i e n ~ y . ~This - ~ ~ ~is obviously important for solar energy utilization where a particular product is desired (e.g., hydrogen) and the highest efficiency possible is required. This report details our earlier publication on the photochemistry of platinized CdS.1° H2 evolution, oxygen (1)Rubin, T.R.; Calvert, J. G.; Rankin, G. T.; MacNevin, W. J.Am. Chem. SOC.1963,75,2850. (2)Frank, S. N.; Bard, A. J. J. Am. Chem. SOC.1977,99,303. (3)Harbour, J. R.;Hair, M. L. J.Phys. Chem. 1977,81,1791. (4)Bard, A. J. J.Photochem. 1979,10,59. (5)Harbour, J. R.; Tromp, J.; Hair, M. L. J.Am. Chem. SOC.1980,102, 1874. (6) Gerischer, H. In "Solar Power and Fuels"; Bolton, J. R., Ed., Academic Press: New York, 1977. (7)Kraeutler, B.; Bard, A. J. J. Am. Chern. SOC.1977,99,303. (8)Kogo, K, Yoneyama,H.; Tamura, H. J.Phys. Chem. 1980,84,1705. (9)Izumi, I.; Dunn,W. W.; Wilbourn, K. 0.;Fan, F. F.; Bard, A. J. J. Phys. Chem. 1980,84,3207. 0022-3654/81/2085-4026$01.25/0
uptake, and radical production were monitored for these aqueous pigment dispersions. In particular, these results are contrasted to those obtained with normal CdS. An article has recently appearedx1which also discusses the effect of platinization as well as Ru02 deposits on CdS in a light mediated water cleavage reaction.
Experimental Section The platinization12 of CdS was accomplished as follows: 200 mg of CdS was added to a solution of 50 mL of methanol containing 2 mL of triethylamine (Eastman) and 0.32 g of hydrogen hexachloroplatinate(1V)hydrate (Aldrich). The mixture was ultrasonically irradiated and transferred to a 250-mL round-bottom flask. The flask was purged with N2, sealed, and placed in a water bath maintained at 10 OC. The contents were stirred using a magnetic stirrer, and illumination was accomplished using a 300-W Quartzline lamp at 90 V for 24 h. The product was thoroughly washed in methanol and dried under N2. O2 uptake measurements were accomplished with a YSI Model 53 oxygen monitor as described previ~usly.~ ESR spectra were obtained using a Varian E-109E series spectrometer. H2 detection was accomplished with a Varian 2800 gas chromatograph with a thermal conductivity detector. Argon was used as the carrier gas. For the H2evolution experiments, the YSI Model 53 oxygen monitor was used except that the glass test tube was sealed with a septum rather than the O2 electrode casing. This system maintains the desired temperature and vigorously stirs the dispersion. The solution was thoroughly purged with N2 and sealed with the septum. A gas-tight syringe was used to periodically remove 100 pL of the air above the liquid in the cell. A 300-W quartzline lamp, operating at 100 V and filtered with a Corning 3-72 filter, was used as the light source. No H2 was evolved for the following systems in H20: Pt on Kaowool; MV2+EDTA; Pt on Kaowool EDTA. (10)Harbour, J. R.; Wolkow, Hair, M. L. Book of Abstracts, Third International Conference on Photochemical Conversion and Storage of Solar Energy, Boulder, Colo., 1980. (11)Kalyanasundaram, K.; Borgarell, E.; Gratzel, M. Helu. Chirn. Acta 1981,64,362. (12)This method is a modification of the procedure presented in ref. 7.
0 198 1 American Chemical Society
The Journal of Physical Chemistty, Vol. 85,
Photoproperties of CdS Pigments
TABLE I: Relative
H, Production time of illumination
system in H, 0
10
20
min
min
30 min
3
22
-
20
-
18
-
No. 26, 198 1 4027
16-
0
CdS? EDTAb
2
CdS, Pt o n Kaowool, EDTA CdS, Pt o n Kaowool, EDTA, M V ' Pt-CdS Pt-CdS, EDTA Pt-CdS, EDTA, MV" TiO,. EDTA TiO;; Pt o n Kaowool, EDTA Pt-TiO,, EDTA
4 4
7 7
10 10
7 3
14 6
20 9
14-
0 n. L
12-
uJ 1 0 -
0.5
0.5
1 1.5
1.5
3
Pigment concentration was 1 m g / l mL. EDTA throughout was 5 X l o - , M. MV2+was present a t 2
2 4
li
6 4 -
a
2 -
m g / l mL. "
CdS powder was obtained from Fisher and methyl viologen and platinum on Kaowool from BDH. EDTA (the disodium salt) and 5,5-dimethyl-l-pyrrolinyl-l-oxy (DMPO) were purchased from Aldrich.
Results and Discussion Characterization of the Surface. Platinization of CdS resulted in a powder which was considerably darkened in appearance. Krauetler and Bard' have used ESCA analysis to show that Pt metal is produced during the platinization of Ti02. Previous ESCA work on a different pigment in our laboratory demonstrated that the method of platinization used in this report also produces P t O . A TEM analysis revealed that CdS particles are coated with extremely small particles of Pt ranging in diameter from 20 to 50 A. This demonstrates that platinization proceeds in a nonhomogeneous fashion. This deposition leads to a "checkering" effect which may be fundamental to metallization reactions used for solar energy utilization systems. A certain fraction of the surface remains exposed and will account for the absorption of most of the photons. Light incident on the metal surface (Pt) will be partially absorbed by the metal and converted to heat. Therefore, an incoming photon absorbed in an exposed region will generate, with a certain probability, an electron-hole pair. Since CdS is an n-type semiconductor, the electron can migrate to a platinum site while the hole can react at the surface of CdS. The fact that Pt particles are very near by creates a situation where the distance travelled by the electron can be small. This in turn limits the possibility of electron-hole recombination within CdS by reducing the lifetime of the electron in the semiconductor. Hence, "checkering" may be fundamental in increasing the quantum efficiency. Hydrogen Production. Methyl viologen (MV2+)has been used extensively as an oxidant in solar energy studies since ita singly reduced form (MV+)is fairly stable and will, in the presence of a catalytic surface, generate H2 (2MV+ + H+ 2MV2+ H,?). We have selected MV2+along with the sacrificial donor EDTA to compare the photochemistry of CdS with Pt-CdS. Table I reveals that H2 is produced under these conditions with Pt-CdS-EDTA but is not generated with CdS-EDTA. If, however, Pt on Kaowool is added to the CdS-EDTA dispersion, H2 is produced. These results suggest that H2 will be evolved as long as a platinum surface is available. By depositing the platinum directly onto the CdS, a composite, bifunctional particle is created, thereby avoiding the necessity of two particle systems (pigment and dispersed catalyst). It is interesting to note that both systems produce H2 roughly at the same rate.
-
+
4
7
10
13
PH
Figure 1. The pH dependence of H, evolution for Pt-CdS (1 mg/ 1 mL) containing 5 X lo-* M EDTA. Each point was taken after 30 min of illumination.
20 TEMPERATURE
40
60
(OC)
Figure 2. The temperature dependence of H, evolution for Pt-CdS (1 mg/l mL) containing 5 X lo-' M EDTA. Each point was taken after 30 min of illumination.
H2 was still produced with Pt-CdS and EDTA in the absence of MV2+. In fact, the rate of production was double that with MV2+ present. For the CdS-EDTA system, no H2 was observed. However, addition of Pt on Kaowool to this CdS-EDTA system resulted in H2 production at half the rate of the Pt-CdS-EDTA system. These results are important for two reasons. First, it is possible with Pt-CdS to generate H2 without MV2+and at an increased rate. Secondly, it is not necessary to have platinum directly on the surface of the CdS for H2production. This second point is unexpected, since the transfer of an hydrogen atom from CdS to Pt would not appear to occur at a high efficiency. The continual mixing of this system during photolysis may, however, enhance contact between CdS and Pt or lead eventually to adherence of some of the Pt to the CdS. The fact that the H2 rate is doubled without MV2+suggests that MV+ or MV2+may be irreversibly reacting with He. The pH dependence of the production of H2 was determined for the Pt-CdS-EDTA system (Figure l). This dependence on pH most likely reflects the efficiency of oxidation of the EDTA and the availability of protons. It has been a r g ~ e d ' ~that J ~ Y3- and Y4-are the more readily
The Journal of Physical Chemistty, Vol. 85,No. 26, 1981
4028
ld
L
Harbour et al.
30 min 140
-
0
20
30
10
mg Pt on Kaowool
Figure 3. The relative H, evolution for a system of CdS (1 mg/l mL) containing 5 X lo-, M EDTA as a function of amount of 5 % Pt on Kaowool. Each point was taken after 30 min of illumination.
oxidized forms of EDTA which predominate at pH levels 19. However, the proton concentration at these levels drops. The temperature dependence of the Pt-CdS-EDTA system is shown in Figure 2. The efficiency of Hz production increased as the temperature of the bath increased. The quantum efficiency, defined as 2. (number of Hz produced)/input number of photons, was determined to be 2 X (0.2%) for the Pt-CdS-EDTA system. The factor of 2 enters into the definition since the reduction of H+ to Hz is a 2 electron, 2 photon event. This quantum efficiency is an apparent value since light is both reflected and absorbed by the Pt particles. Photons absorbed by the Pt will be converted to heat due to the extremely short lifetimes of the excited states of metals. Hence, an indeterminate number of photons are lost by reflection and absorption by the Pt. Finally, no effect was expended for this report on the effect of degree of coverage on the photochemistry. For the system CdS-EDTA-Pt on Kaowool, the rate of Hzproduction was determined to be a function of the amount of Pt used. Figure 3 shows that a plateau is reached after 10 mg of 5% Pt on Kaowool/5 mL solution. Table I reveals that platinized TiOz will generated H2 from EDTA. In addition, TiO, in the presence of Pt on Kaowool with EDTA also will produce Hz upon illumination. Oxygen Uptake. The photoactivity of pigments can be determined by monitoring the oxygen uptake in a dispersion upon illumination. For either CdS or Pt-CdS without an added donor, the rates of oxygen uptake are considerably different. The rate is at least five times faster with Pt-CdS over CdS. H20z is the usual product of this photoreaction with CdS, yet no HzOz is observed with Pt-CdS under illumination. A rate difference (up to 12) was observed in the presence of the sacrificial donor EDTA. Again, no HzOzwas observed for Pt-CdS while for CdS-EDTA, HzOZwas generated with a conversion ratio close to one. No increase in the rate of 0, uptake was observed when Pt on Kaowool was added to a CdSEDTA solution and HzOzwas still photogenerated. Addition of Pt-CdS to a solution of HzOzresulted in a significant decrease in H202 concentration. This destruction of HzOzwas not observed with Pt on Kaowool. Hence, if H202is generated with Pt-CdS-EDTA, then it would be destroyed by the Pt surface and could account for the lack of detection of H20zwith Pt-CdS. (13)Bonneau, R.J.; Joussot-Dubien, J.; Faure, J. Photochem. Photobiol. 1975, 21, 173. (14)Fife, D.J.; Moore, W. M.Photochem. Photobiol. 1979,29,43.
c .
I//.
v-
0
20
40
60
80
100
RELATIVE LIGHT INTENSITY
Figure 4. The dependence of the oxygen uptake rate on the relative light intensity. The actual rate of O2 uptake can be obtained by multiplying the number of (lo-’ mol of OJmin: ( 0 )CdS and 5 X lo-’ EDTA; (W) R-CdS and 5 X IO-, EDTA.
The quantum efficiency for O2 uptake, defined as number of 0, molecules consumed/the number of incident photons, was determined for both systems. The values were obtained 9(CdS-EDTA) = 0.026 and Wt-CdSEDTA) = 0.067. However, this quantum efficiency was dependent upon the light intensity as shown in Figure 4. For higher light intensities, the ratio of the oxygen uptake rates for Pt-CdS-EDTA vs. CdS-EDTA approaches a value of 12. At lower light intensities, the value for this ratio drops to 1.9. This demonstrates that the Ws for CdS vs. Pt-CdS have markedly different intensity dependencies. The a’s quoted above have a ratio of 2.5 which suggests an intensity of light somewhat higher than that used for the minimum light intensity shown in Figure 4. This was confirmed by experiment. This difference in light intensity dependence for CdS and Pt-CdS can be accounted for by consideration of the “checkering“ effect. As explained earlier, the photoproduced electron has a shorter lifetime within the semiconductor in the Pt-CdS since it has a high probability of transferring through an ohmic contact to a Pt particle. It is therefore separated from the “hole” and undergoes an interfacial electron transfer into the solution. For CdS, the electron must reside within the semiconductor while waiting for its chance to redox react with a solution species. As the light intensity increases, the population density of holes and electrons increases and the rate of hole/electron recombination increases. In fact, this rate is faster for CdS than Pt-CdS since the electrons are available for reaction within CdS but are isolated when they are transferred to the Pt. The rate of oxygen uptake was relatively constant with temperature. The pH dependence revealed that the rate peaked at pH of 6 and decreased as the pH increased. These results are different than those of H, production. Finally, the rate of O2uptake had doubled as the dissolved [O,]increased from 22% to 88%. The fact that Pt-CdS so readily reduces molecular oxygen suggests that systems designed to generate both H2 and O2will have a reduced efficiency. This results since the evolved oxygen will compete with protons for the photoproduced electrons. When the donor sodium formate was added to the PtCdS dispersion, a fast uptake in the dark occurred. A similar uptake was observed with Pt on Kaowool and formate. These results suggest that a reaction between oxygen and formate can be catalyzed by the platinum surface. No analysis was carried out on the products of this reaction. Although the catalytic decomposition of formic acid is a well studied reaction, it has been looked
J. Phys. Chem. 1981, 85,4029-4033
at mainly in gas-solid systems.15 A much slower rate of oxygen uptake was also observed with EDTA and Pt-CdS. In addition, a dark oxygen uptake in methanol with PtCdS occurred. Addition of the nitrone, DMPO at a concentration of -2 X M did not increase the rate of O2 uptake in either the Pt-CdS or Pt-CdS-EDTA systems. This suggests that singlet oxygen is not generated upon photolysis of the platinized pigment.16 ESR Measurements. The reduced form of MV2+is the cation radical (MV') which is readily detected by ESR. Upon illumination of a N2purged CdS-EDTA dispersion, a large ESR signal due to MV+ is observed. This is consistent with the fact that no catalytic surface is available to convert MV+ to H2 Addition of Pt on Kaowool(1 mg/l mg of solution) reduced the ESR signal intensity by a factor of 5 . Doubling the Pt concentration reduced the intensity of the MV+ ESR signal by an additional factor of 6. It should be noted that these ESR experiments were carried out in an aqueous ESR cell with no stirring which can be contrasted to both the O2uptake and Hz production experiments where vigorous stirring of the dispersion occurred. Finally, with N2purged Pt-CdS-EDTA no MV+ was detedable upon irradiation. This suggests that if MV+ is being generated, it is efficiently being converted to Hz and MV2+. When MV2+ is excluded and O2 allowed in the CdSEDTA system, the generation of 0, can be detected upon photolysis using the spin trap, DMPO." Addition of Pt on Kaowool to this dispersion does not prevent the gen(15) Mars, P.; Scholten, J. J. F.; Zwietering, P., Adu. Catal. 1963, 14, 35.
(16) Harbour, J. R.; M e r , S. L.; Hair, M. L. J. Am. Chem. SOC.1980, 102,7770. (17) Harbour, J. R.; Hair, M. L. J. Phys. Chem. 1978, 82, 1397.
4029
eration of 02-.However, with Pt-CdS-EDTA no 0; adduct was observed upon irradiation. In fact, no adducts were detected with this sytem (e.g., no H.adduct).'* These ESR results are consistent with the O2uptake and H2 evolution data. In both the CdS-EDTA and CdSEDTA-Pt on kaowool systems, 02-was detectable and H202produced. Since 02-is an intermediate in the two electron reduction of O2 to H202,these results are consistent. Similarly,02-is not observed with Pt-CdS-EDTA and no Hz02was detected. For the MV+ system, addition of Pt on Kaowool reduced the MV+ signal which is consistent with the observation of H2 evolution.
Conclusions Platinization of CdS results in the deposition of ultrafiie particles (20 to 50 A) of Pt onto the surface of the larger CdS particles. These Pt particles appear to be evenly distributed over the surface creating a checkered appearance. This checkering quality explains the marked light intensity dependence differences between Pt-CdS and CdS in the O2 uptake experiments. The fact that Pt-CdS has a rate 12 times that of CdS at high light intensities is important in solar energy systems where solar concentrators may be employed. H2 evolution was accomplished under illumination of the Pt-CdS EDTA system with a @J of 0.2%. Addition of MV2+actually halves the efficiency. Addition of Pt on Kaowool to a CdS-EDTA system also resulted in the photoproduction of H2. It is particularly significant that this latter system also works without MV2+. Acknowledgment. We thank Alexandra Perovic for carrying out the TEM analysis and Denis Brillon for measuring the intensity dependence of the O2 uptake. (18) Jaeger, C. D.; Bard, A. J. J. Phys. Chem. 1979,8, 3746.
Measurement of the Rates of Detergent Exchange between Micelles and the Aqueous Phase Using Phosphorescent Labeled Detergents John D. Boltt and Nicholas J. Turro" Chemistry Department, Columbia UniversitysNew York, New York 10027 (Received: June IS, 1981)
Phosphorescence quenching is used to measure micelle-probedetergent dynamics. For phosphorescent detergent probes with varying hydrocarbon length the rate constants for escape (k-) from cationic host micelles are measured using cobalt(II1) hexamine as an aqueous soluble triplet quencher. For 10-(4-bromo-l-naphthoyl)decyltrimethylammonium bromide (BND-lo),k- is 3.2 X lo3 s-l, and k+, the reentry rate constant, is 5.7 X 10' M-l s-l for hexadecyltrimethylammonium chloride (HDTCl) host micelles at 25 OC. The log of k- is a linear function of the number of methylenes in the probe alkyl chain, in agreement with rates determined previously with relaxation methods. The apparent activation energy for escape of BND-10 from HDTCl micelles is 9 kcal/mol. Escape rates are measured for several host micelles and for micelles composed of probe detergents-self-micelles.
nk.
Introduction Solutions of detereent molecules exist as a dvnamic equilibrium between monomer detergents and micellar aggregates.' A stepwise equilibrium can be formulated to include entrance and exit rate constants, k+ and k-, for the detergent/micelle exchange
D, k,Dn-' + D1 + ... nD1 where D, represents a micelle as an aggregate of n monomer detergents, D,. Typical mean aggregation numbers, ( n ) , are of the order of 50-100 for ionic micelles. For micelles of aggregation number close to ( n )it is assumed
Textile Fibers Pioneering Research Laboratory, E. I. du Pont de Nemours and Company, Wilmington, DE 19898.
(1) M. Kahlweit and M. Teubner, Adu. Colloid Znterjace Sci., 13, 1 (1980).
Y
0022-3654/81/2085-4029$01.25/0
0 1981 American Chemical Society
