Langmuir 1990,6, 1519-1521
1519
Letters Template-Directed Semiconductor Size Quantization at Monolayer-Water Interfaces and between the Headgroups of Langmuir-Blodgett Films Kyunghee C. Yi and Janos H. Fendler' Department of Chemistry, Syracuse University, Syracuse, New York 13244-4100 Received February 20, 1990. I n Final Form: July 2, 1990 Surface pressure-surface area isotherms of monolayers formed from n-C16H33C(H)[CON(H) (CH2)2NH&, 1, over 2.5 X 104 M aqueous CdCl2 showed an appreciable dependence on the pH of the subphase. The headgroup area of 1 increased from 24 A2/molecule at pH 6.1 to 44 A2/molecule at pH 9.1. Slow infusion of HIS into monolayers of 1 floating on CdCl2 resulted in the formation of CdS particulate films which could be transferred to quartz substrates. Absorption spectrophotometry indicated size quantization of the CdS particles. No CdS formation could be observed at pH values lower than 7.0, and the formation of CdS particles of 150- and 35-A mean diameters at pH 8.6 and 9.1 was indicated by absorption spectrophotometry. Electron microscopy showed the presence of appreciably larger clusters of CdS particles at pH 8.6 than at pH 9.1. Steady-state illumination of quartz-supported CdS particles in an aqueous methylviologen, MW+,solution in the presence of benzyl alcohol resulted in photoelectrontransfer. The smaller cluster of particles showed a more efficient photoelectron transfer than the larger ones. CdS particles of 22-A mean diameter were also in situ generated between the headgroups of a 20-layer LangmuirBlodgett (LB) film prepared from 1. Inherent theoretical interest and potential applicability together as possible appeared to reduce the headgroup area have prompted the investigations of sizequantized colloidal to 24 A2/molecule. semiconductor particles.ltz Their preparation and Slow infusion of hydrogen sulfide (Matheson Gas maintenance require highly selective and carefully Products) into CdC12 solutions coated with a monolayer controlled experimental conditions. Recently, we reported of 1resulted in the formation of CdS particles at the monothe in situ generation of semiconductor particles at monolayer-water interface. Particle formation and thickness layer interface^.^ Careful infusion of hydrogen sulfide were monitored by reflectivity measurements? The optical resulted in nucleation a t several sites, slow growth, and thickness of the growing semiconductor particles increased the eventual formation of a semiconductorparticulate fii. steadily to a plateau value. The rate of growth of the CdS particulate film depended markedly on the rate of HzS Extension of this methodology to the formation of infusion and on the pH of the subphase. semiconductor films composed of controllable diameter uniform particles is the subject of the present comCdS particulate films, formed at the interface of monomunication. Advantage has been taken of pH-dependent layers prepared from I, were transferred, by horizontal metal ion complexation to control the sizes of semilifting, to quartz substrates (well cleaned by chromic acid conductor particles, in situ formed at amine-functionaland dust-free water) at different stages of their growth. ized monolayer interfaces. Absorption spectra of CdS particulate films, in situ generated at the interfaces of cadmium ion containing 1 Surface pressure-surface area isotherms of monolaymonolayersby 3,5, and 10 min of HzS exposure, are shown ers formed from ~-C~~H~~C(H)[CON(H)(CHZ)~NHZ]Z, 1: in Figure 2. Plots of the data according to over 2.5 X M aqueous CdClz (Baker Analyzed Reagents) at different pH values are shown in Figure 1. (aha)' = hoC - E,C (1) The headgroup area of 1 is seen to increase from 24 Az/ molecule at pH 6.1 to 44 A2/molecule at pH 9.1. Increasing (where a is the absorption coefficient; A = ad, with A the pH values increased the Cd2+binding in the cavity of 1. absorbance and d the optical thickness of the semiThis caused greater separation of the amino groups, which conductor particulate film determined form reflectivity manifested in increased headgroup areas of 1. Projections measurements; and hw is the photon energy) led to direct of CPK space-filling models (Ealing Corp.) led to the band gaps (E,) of 2.82,2.78,and 2.74 eV for the 3-, 5-, and assessment of 40 A2/molecule for the headgroup area of 10-min HzS-exposed samples, respectively (Figure 2). The fully stretched 1. Tightening the amine groups as close observed range of E, values (2.82-2.74 eV) corresponded to 30-50-A-diameter CdS particles on Henglein's published (1) Henglein, A. Top. Curr. Chem. 1988,143,113. Brus, L. A. J . Phys. E, vs particle size curve.6 Longer HPS exposures led to Chem. 1986,90,2656. Andrea, R. P.; Averback, R. S.;Brown, W. L.; Brus, larger interconnected arrays of particles which continued L. E.; Goddard, W. A.; Kaldor, A.; Louie, S. G.; Moskovita, M.; Percy, P. S.; Riley, S. J.; Siege], R. W.; Spaepen, F.; Wang, Y. J.Muter. Res. to grow in thickness until a contiguous film, composed of 1989,4, 704. porous 20-30-A-thick, 75-100-A-diameter, disk-shaped (2) Fendler, J. H. Chem. Rev. 1987,87, 877. semiconductor particles, was formed. (3) Zhao, X.K.; Xu, S.;Fendler, J. H. Langmuir, in press. (4) We thank Professor P. Tundo for preparing 1. Changes in the pH of the subphase profoundly affected ( 5 ) 100 rL (4 X 10'' molecules/mL) of 1 in HPLC grade CHC13: the morphology of the CdS particles grown at the interfaces ethanol = 9 1 (v/v) were spread on a Lauda Model P film balance. Water was purified by using a Millipore Milli-Q filter system provided with a 0.2-rm Millistack filter at the outlet.
0743-7463/90/2406-1519$02.50/0
(6)Henglein, A. Chem. Reu. 1989,89, 1861-1873.
0 1990 American Chemical Society
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1520 Langmuir, Vol. 6, No. 9,1990
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Letters W
SURFACE AREA. AVmolcculc
Figure 1. Surface pressure-surface area isotherms of monolayers of 1 on aqueous 2.5 X 10.' M C ~ C I PpH . ~ of the subphase was adjusted to 6.1 (a), 7.4 (b), 8.4 (c), 8.8 (d), and 9.1 (e) by the addition of 0.10 M NaOH. Headgroup areas, indicated on the figure by HA, were calculated to be 24 (a), 27 (b), 35 (c). 38 (d), and 44 Az/molecule(e). 70000
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