Langmuir 1985, I , 158-161
158
Table IV. Fraction in Each Charge Class ( N t = 1.2 ion s/cm3: D = 32 nm) exptl Gentry Fuchs Hoppel Laframboise
0 charges 4.9 1.4 10.7 6.5 47.0
1 61 57
54 67 33
2+ 16 21 16 7
0.3
X
lo'
charge/no. 0.95 1.00 0.86 0.80 0.34
number of elementary charges penetrating the analyzer can be measured with the electrometer. The change in the penetration can be measured with the CNC which thus gives the number of particles in the charge class. The ratio of these two measurements is a small integer giving the number of charges per particle. This method is usable only for small particles where the net charge is small. With larger charges the fractional change in mobility caused by adding or deleting one charge is too small to cause a sharp change in the CNC and electrometer outputs. The results of the measurements are given in Table IV. Again the
Gentry and Fuchs theories agree well with the experimental measurements.
Conclusion The importance of accounting for particle loss in the charger was indicated by both experiment and simulation. This loss occurs primarily as a result of the electrostatic precipitation of charged particles caused by the field produced by the corona discharge. In order to simulate this it is necessary to model the aerosol's parabolic flow and the charger field. A simplified model with one adjustable parameter was used. The parameter was fixed at the value that gave the best agreement between measured and calculated aerosol loss rates. Of the four models used those of Fuchs, Gentry, and Hoppel gave results that were qualitatively consistent with the data. The theory of Laframboise and Chang did not, perhaps due to the fact that this theory does not include the effects of the image potential.
Photochemical Behavior of Cetyltrimethylammonium Bromide Stabilized Colloidal Cadmium Sulfide: Effects of Surface Charge on Electron Transfer across the Colloid-Water Interface J. Kuczynski and J. K. Thomas* Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 Received September 18, 1984. In Final Form: November 15, 1984 It is shown that ethylenediaminetetraacetate(EDTA)greatly increases the efficiency of photoinduced electron transfer from aqueous colloidal cadmium sulfate to methylviologen (MV2+).This effect is very apparent in colloids stabilized by cetyltrimethylammoniumbromide (CTAB), a cationic surfactant that imparts a positive change to the CdS surface. Experiments are reported that show that the crucial event in the system is the formation of a complex of EDTA and MV2+with a resultant negative charge. This complex is electrostaticallybound to the cationic CdS surface where photoinduced electron transfer occurs. Subsequentbreak up of the EDTA-reduced methylviologen complex leads to MV+, which is repelled away from the cationic CdS surface. The above effects are reversed on preparing a CdS colloid with a negatively charged surface by using sodium lauryl sulfate as a stabilizer. A mechanism for e- transfer is discussed which highlights the effect of surface type on the efficiency of e- transfer.
Introduction Photoredox reactions a t micellar and semiconductorelectrolyte interfaces have received considerable attention within the past d e ~ a d e . l - ~Colloidal semiconductor systems are of particular interest for several reasons. These systems exhibit rapid carrier mobility and efficient electron/hole separation, the ability to simultaneously carry out photooxidation and photoreduction reactions at their interfaces while also exhibiting large surface areas essential for favorable reaction yields. Furthermore, specific adsorption of ions and/or charged surfactants provides (1) Fender, J. H. "Membrane Mimetic Chemistry"; Wiley: New York, 1983. Turro, N.; Braum, A.; Gratzel, M. Angew. Chem., Int. Ed. Engl. 1980,19,675.Thomas, J. K. ACS Monogr. 1984,No. 181. Harbour, J. R.; Wolkow, R.; Hair, M. L. J. Phys. Chem. 1981,85,4026. (2)Kraeulter, B.; Bard, A. J. Am. Chem. SOC.1978,100,4317.Izumi, I.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1981,85,218. Bard, A. J. J. Phys. Chem. 1982,86,172 and references therein. (3)Kalyanasundarum, K.; Borgarello, E.; Griitzel, M. Helu. Chem. Acta 1981,64,362. Gratzel, M.Acc. Chem. Res. 1981,14, 376 and references therein. (4)Henglein, A. J. Phys. Chem. 1982,86,2291. (5) Nakato, Y.;Tsumura, A.; Tsubomura, M. Chem. Phys. Lett. 1982, 85. 387.
control of the surface charrge of the semiconductor particles via surface modification. Photochemical redox reactions induced by visible irradiation of colloidal semiconductor solutions have focused on TiOz since this semiconductor is stable with respect to anodic dissolution.6 However, the band gap of TiOz is relatively large (3.2 eV) which results in poor spectral response to visible radiation. For this reason CdS (band gap = 2.4 eV) has been utilized in numerous studies. Laser and luminescence studies"'-l2 have all been used to probe the nature of the interfacial electron-transfer reactions occurring at the semiconductor surface. Par(6)Frank, A. J.; Honda, K. J. Phys. Chem. 1982,86,1933. (7)Kuczynski, J. P.; Thomas, J. K. J. Phys. Chem. 1983,87,5498. (8)Ramsden, J. P.;Gratzel, M. J. Chem. Soc. Faraday Tram. I 1984, 80,919. (9)Rossetti, R.;Nakahara, S.; Porris, E. E. J. Chem. Phys. 1983,79, 1086. (10)Kuczynski, J. P.;Milosavljivic, B.; Thomas, J. K. J. Phys. Chem. 1984,88,980. (11)Henglein, A. Ber. Bumenges Phys. Chem. 1982,86,201. (12)Duonging, D.; Ramsden, J.; Grltzel, M. J. Am. Chem. SOC.1982, 104, 2977.
0743-7463/85/2401-0158$01.50/0 0 1985 American Chemical Society
Langmuir, Vol. 1, No. 1, 1985 159
CTAB-Stabilized Colloidal Cadmium Sulfide ticular emphasis has been placed on methylviologen ( M V ) due to its ability to produce hydrogen from water at a platinum catalyst.13 We have shown that it in the case of colloids of CdS stabilized with sodium dodecyl sulfate (SDS) electron transfer to MV2+ occurred very rapidly (T < lo4 s) and only to surface-adsorbed viologen. The reduced product, MV+, lives on the CdS surface for periods exceeding 260 ps. This results in an increased probability of back reaction with the photogenerated hole resulting in low steady-state quantum yields for the formation of MV+. Addition of various hole traps to the CdS/SDS colloid were ineffective in increasing the steady-state yield of reduced viologen. We report enhanced quantum yields for MV+ production via complex formation of MV2+ with ethylenediaminetetraacetate (EDTA).
Experimental Section Materials. Cadmium chloride (CdC12)(AldrichChemical Co.,
Gold label), sodium sulfide, Na2S.9H20 (Fisher Scientific), ethylenediaminetetraacetate(EDT) (Fisher Scientific),and cetyltrimethylammonium bromide (CTAB) (Sigma Chemical Co.) were used without further purification. Methylviologen, N,N'dimethyl-4,4bipyridiniumdichloride (AldrichChemical Co.), was recrystallized twice from methanol; propylviologensulfonate, 4,4'-bippidinium 1,l'-diylbis(propanesulfonate)sulfonate, was synthesized by refluxing 1,3-propanesultone(Aldrich Chemical Co.) with 4,4'-bipyridine (Eastman Kodak Co.) in dimethylformamide and subsequentlyrecrystallized from methanol 3 times prior to use.14 All other reagents were used as supplied. Preparation of Colloidal CdS. CdS was prepared via slow precipitation by stoichiometric addition of CdC12in a stirring solution of Na2S.9H20plus CTAB. The solution was transparent and colored a brilliant orange. Quasi-elasticlight scatter yielded particle sizes of 150-A radius with a fairly narrow size distribution-negligible particle growth occurred upon standing for several days. Additionally,the CdS colloid exhibited excellent stability with respect to irradiation in air with ultra-band-gaplight. Instrumentation. Colloid particle sizes were measured on a Nicomp HN5-90 dynamic light scatter spectrometer which utilizes the theory of Rayleigh scattering of translational Brownian particles to compute the mean hydrodynamicradius, l?,9ing the Stokes-Einstein relationship for spherical particles: D = kT/ (67rql?). A microcomputer performs rapid quadratic least-SquarEs fit to the temporal decay of the correlation function yielding D, R, u (normalized standard deviation of the intensity weighted distribution of diffusion constants),and (goodnessof fit). The greater the value of u the larger the degree of polydispersity present in the particles sizes-values less than or equal to 0.2 are generally considered to correspondto pure monodisperse systems. A typical result for 4 X M CdS/CTAB is D = 1.41 X lo-' cm2/s,R = 150 A u = 0.6, and X 2 = 0.85. Pulsed absorption and luminescence studies were conducted with either the 337.1-nm beam (8-mJ energy, 6-115 fwhm) from a Lambda Physik XlOO N2 laser or 490-nm light (0.1-J energy, 120-nsfwhm) from a candela SLL-66A dye laser. The transients produced were monitored by fast spectrophotometry (response
