Thioalkanoates as site-directing nucleating centers for the preparation of patterns of CdS nanoparticles within 3-D crystals and LB films of Cd alkanoates

Article abstract of DOI:10.1021/ja991354x

A method is described for the preparation of hybrid organic/inorganic structures where the inorganic component comprises semiconductor nanoparticles aligned in periodic layers within three-dimensional (3-D) crystalline powders and Langmuir-Blodgett (LB) films. The preparation process comprises the organization of metal ions in the form of periodic arrays within 3-D crystals or the LB films, followed by a topotactic gas/solid reaction. The method is illustrated for the organization of CdS nanoparticles within alkanoic acids. The order of the nanoparticles is achieved by introducing site directing nucleation centers of Cd thioalkanoates within Cd alkanoates, in the form of solid solutions. The formed particles are attached to the organic matrix via -C(O)S-Cd-S- bonds. The structure of those supramolecular architectures has been characterized by a variety of complementary methods, including transmission electron microscopy (TEM) and electron diffraction (ED), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other spectroscopic measurements.

Full text of DOI:10.1021/ja991354x

J. Am. Chem. Soc. 1999, 121, 9589-9598
9589
Thioalkanoates as Site-Directing Nucleating Centers for the
Preparation of Patterns of CdS Nanoparticles within 3-D Crystals and
LB Films of Cd Alkanoates
Shouwu Guo,‡ Leandro Konopny,‡ Ronit Popovitz-Biro,‡ Hagai Cohen, Horia Porteanu,§
Efrat Lifshitz,§ and Meir Lahav*,‡
Contribution from the Department of Materials and Interfaces, Chemical SerVice Unit,
Weizmann Institute of Science, 76100 RehoVot, Israel, and Department of Chemistry and
Solid State Institute, Technion, 32000 Haifa, Israel
ReceiVed April 26, 1999
Abstract: A method is described for the preparation of hybrid organic/inorganic structures where the inorganic
component comprises semiconductor nanoparticles aligned in periodic layers within three-dimensional (3-D)
crystalline powders and Langmuir-Blodgett (LB) films. The preparation process comprises the organization
of metal ions in the form of periodic arrays within 3-D crystals or the LB films, followed by a topotactic
gas/solid reaction. The method is illustrated for the organization of CdS nanoparticles within alkanoic acids.
The order of the nanoparticles is achieved by introducing site directing nucleation centers of Cd thioalkanoates
within Cd alkanoates, in the form of solid solutions. The formed particles are attached to the organic matrix
via -C(O)S-Cd-S- bonds. The structure of those supramolecular architectures has been characterized by a
variety of complementary methods, including transmission electron microscopy (TEM) and electron diffraction
(ED), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other spectroscopic
measurements.
Introduction
Scheme 1. Schematic Packing Arrangement of 3-D Crystals
and LB Films of Cadmium Alkanoates
The design of novel inorganic/organic composite materials,
where the inorganic component forms periodic patterns within
the organic host matrix, is a major objective in the materials
sciences. Of particular interest is the development of new
synthetic methodologies for the preparation of systems where
the inorganic particles are in the quantum regime with a high
degree of monodispersity. As a result of the organization, such
materials might display variable optoelectronic properties that
differ from those of the isolated particles.1 In recent years,
various strategies have been followed including reactions in
reversed micelles,2 organization of monodispersed quantum
particles in organic and biological polymeric templates,3-8 or
specific interactions of the inorganic particles with bolaam-
phiphilic aromatic molecules.9
Here we describe a synthetic strategy that comprises two
major steps: selection of 3-D crystals or crystalline ultrathin
films of organometallic systems of desired architectures, fol-
lowed by a topotactic reaction of these solids with a gas.10,11
Such reactions might yield quantum size particles arranged in
patterns, provided the inorganic particles preserve partially the
periodic order of the reactant matrix. Furthermore, by selecting
the alkanoic acid with the appropriate chain length, one can
determine the spacing between the layers of the quantum
particles. The preparation of such patterns requires a control
over the chemical reactivity of the ions with the gas, a mass
transport of the inorganic molecules via anisotropic diffusion
within the organic matrix, and finally, the nucleation of the
particles at desired sites within the reactant phase. This method
was successfully applied for the preparation of quantum size
disks of lead sulfide arranged in layers within the organic matrix
‡ Department of Materials and Interfaces, Weizmann Institute of Science.
Chemical Service Unit, Weizmann Institute of Science.
§ Department of Chemistry and Solid State Institute, Technion.
(1) (a) Heitmann, D.; Kottaus, J. P. Phys. Today 1993, 46 (6), 56. (b)
Fendler, J. H. Nanoparticles and nanostructured films: preparation,
characterization and applications; Wiley-VCH: Weinheim, 1998.
(2) (a) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99,
16425. (b) Motte, L.; Billoudet, F.; Douin, J.; Pileni, M. P. J. Phys. Chem.
B 1997, 101, 138.
(3) Wang, Z. L. AdV. Mater. 1998, 10, 13.
(4) (a) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (b)
Osenar, P.; Braun, P. V.; Stupp, S. I. AdV. Mater. 1996, 8, 1022.
(5) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389,
585.
(6) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270,
1335.
(7) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature
1996, 382, 607.
(8) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth,
C. J.; Bruchez, M. P., Jr.; Schultz, P. G. Nature 1996, 382, 609.
(9) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.;
Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science
1996, 273, 1690.
(10) For reactions of layered metal phosphonates in the solid phase with
gaseous H2S and H2Se see: Cao, G.; Rabenberg, L. K.; Nunn, C. M.;
Mallouk, T. E. Chem. Mater. 1991, 3, 149.
(11) (a) Guo, S.; Popovitz-Biro, R.; Weissbuch, I.; Cohen, H.; Hodes,
G.; Lahav, M. AdV. Mater. 1998, 10, 121. (b) Guo, S.; Popovitz-Biro, R.;
Arad, T.; Hodes, G.; Leiserowitz, L.; Lahav, M. AdV. Mater. 1998, 10,
657.
10.1021/ja991354x CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/04/1999

9590 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Guo et al.
Scheme 2. Schematic Packing Arrangement of Mixed
Crystals of Cadmium Alkanoates with Thioalkanoates and
the Proposed Nucleation Centers for CdS during the
Reactions of These Mixed Crystals with Dry H2S
Table 1. Layer Spacings of Cd Ions (Å) within Crystals of
Cadmium Alkanoates and Thioalkanoates As Determined by XRD
Cn
C16
C18
C22
C26
C30
alkanoic
thioalkanoic
45.32
42.43
50.48
46.50
59.53
56.35
69.50
65.40
79.51
76.45
Figure 2. (A) UV-vis absorption spectra of CdS particles generated
within (a) C16, (b) C18, (c) C22, (d) C26, and (e) C30 cadmium alkanoate
matrices after completion the reaction with dry H2S and extracted in
DMSO. (B) Photoluminescence spectra of CdS particles generated in
the (a) C16, (b) C18, (c) C22, and (d) C30 cadmium alkanoate matrices,
measured directly on the reacted powders.
Selection of the Model. Schematic packing arrangements of
the homologous series of Cd alkanoates crystals or LB films
are shown in Scheme 1. The Cd ions within these crystals or
LB films form 2-D layers that are separated from one another
by a spacing that is twice that of the length of the hydrocarbon
chains. Consequently, the spacing between these layers can be
altered just by selecting the acid with the appropriate hydro-
carbon chain length. Upon reaction of the crystallites with H2S,
CdS molecules are generated, which have a strong tendency to
crystallize as particles. The diffusion of the CdS molecules prior
to crystallization within the organic matrix is expected to be
anisotropic. One may anticipate the formation of CdS particles
aligned in layers provided the CdS molecules nucleate prefer-
entially within the hydrophilic layers where the Cd ions reside.
Results on the Cd salts of the common fatty acids, such as stearic
(C18), behenic (C22), and hexacosanoic (C26) acid, were disap-
pointing since the CdS particles were either substantially large
or randomly distributed within the organic matrix. Enhanced
nucleation of the particles along the ionic rows can be achieved
by introducing, regioselectively, nucleating centers along the
hydrophilic layers of the reactant. Thus, for example, Cd
thioalkanoates of the same chain lengths should be easily
dissolved within the corresponding Cd alkanoates to form
Figure 1. XRD patterns of (a) cadmium pure C22 alkanoate, mixed
crystals (solid solutions) containing (b) 10%, (c) 20%, (d) 30%, (f)
50%, (g) 70%, and (h) 90% of the corresponding C22 thioalkanoate,
(e) 1:1 solid mixture, and (i) pure thioalkanoate.
of the fatty acids by reacting powders of lead alkanoates [CH3-
(CH2)nCOO]2Pb (n ) 16-28) with H2S.11b On the other hand,
cadmium, zinc, and manganese alkanoates with similar layer
structures yield particles randomly arranged or do not react upon
treatment with H2S. Therefore, the formation of superlattices
in systems undergoing reaction depends on the ability to control
the size of the particles and the sites where they nucleate. This
goal can be achieved by introducing efficient site directing
nucleation centers for the inorganic particles at desired loci
within the reactant matrix. The approach is illustrated here for
the preparation of superlattices of CdS quantum particles within
3-D crystals and Langmuir-Blodgett (LB) films of Cd al-
kanoates upon their reaction with H2S, using the thioalkanoates
as the site directing nucleating species.

Preparation of Hybrid Organic/Inorganic Structures
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9591
Figure 3. (a-d) TEM images of CdS particles generated within pure C16, C22, C26, and C30 cadmium alkanoates matrices, respectively. (e-g) TEM
images of CdS particles generated in the mixed crystals of Cd C16, Cd C22, and Cd C26 alkanoates containing 10% of the corresponding Cd thioalkanoate.
Insert in panel f shows selected area electron diffraction of CdS particles formed within the Cd C22 alkanoate/10% Cd C22 thioalkanoate mixed
crystal. (h) High-resolution TEM images of CdS particles shown in panel g. Images a-g were taken at the same magnification.
random solid solutions and thus assuming the conformation and
tilt angle of the chains of the molecules in the host crystal.
Powders of the pure Cd thioalkanoates, with related crystal
structures to those of the alkanoate salts, react very slowly, if
at all, with dry H2S and so are designated as an “inert pair”.12
Within the mixed crystals, one may foresee the formation of a
Cd carboxylates pair and a mixed pair of two components. For
the low concentrations, the number of inert thiocarboxylates
pairs would be small, as shown in Scheme 2. Consequently, in
the mixed crystals the Cd carboxylates pairs are expected to
react as in their pure salts to yield the CdS molecules, while
the Cd ions of the mixed pairs react with H2S at the -COOCd
only, but not at the -C(O)SCd sites. Upon such reaction the
CdS molecules formed at the carboxylates sites diffuse and
nucleate at the nearest -C(O)SCd sites that are bound to the
organic matrix, resulting in the formation of layers of CdS
nanoparticles, within the hydrophilic layers of the reactant
crystals. These patterns should be preserved provided that the
fatty acid formed after reaction does not recrystallize. Further-
more, by adjusting the concentration of the thioalkanoates in
the mixed crystals one should be able to control, albeit within
limits, the number of nucleating centers and consequently the
size of the inorganic crystallites.
Experimental Section
I. Materials. Cadmium chloride (CdCl2), sodium sulfide (Na2S),
sulfuric acid (H2SO4), potassium hydroxide (KOH), phosphorus pen-
(12) The reactivity with wet H2S occurs at a moderate rate due to some
hydrolysis.13

9592 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Guo et al.
percent of the corresponding potassium thioalkanoates on the aqueous
solution of CdCl2 at pH 7.2. The films were transferred onto solid
supports by the vertical dipping method on a Lauda FW-2 trough.
II. Characterization Methods. FT-IR spectra of the 3-D crystal
powders were measured in KBr pellets on a Nicolet-460 spectrometer.
LB films were deposited on CaF2 slides and the FT-IR spectra were
measured.
UV-vis absorption spectra of the CdS nanoparticles after extraction
from the reacted powders were recorded on a Beckman DU-7500
spectrometer. The spectra of the LB films were measured after
deposition on glass slides.
TEM images and ED patterns were taken on a Philips CM12
transmission electron microscope operated at 100 kV or a Philips
CM120 transmission electron microscope operated at 120 kV. The
composite 3-D crystallites, after completion of the reaction with H2S,
were placed on collodion/carbon 400 mesh electron microscope grids
from sonicated 1:1 water-ethanol suspension, which were blotted after
30 s. LB films for TEM experiments were deposited on collodion/
carbon coated nylon grids (inert to H2S gas) which were attached to
glass slides for the dipping procedure and then reacted with H2S on
the grids. Images of these films were taken at -170 °C (using a Gatan
cold stage) under low-dose conditions.
The X-ray powder diffraction patterns were recorded on a Rigaku
RU-200B rotaflex diffractometer using Cu KR λ ) 1.54 Å.
XPS measurements were performed on a Kratos AXIS HS instru-
ment, using a monochromatized Al (KR) source and a 20 eV pass
energy. Though energy resolution was limited to about 0.5 eV, the
accuracy (and stability) of the energy scale enabled determination of
peak relative positions within (0.05 eV. For final calibration of the
energy scale, the main C (1s) peak was set to 284.8 eV. Special care
has been taken to diminish the beam-induced damage, found to be
nonnegligible on time scales of about 1 h. To obtain nondamaged
reliable spectra, low-power radiation was used (5 mA emission at 15
kV). Evolution of spectra measured at a fixed spot was first studied,
yielding the relevant time scale for “fresh area” data recording, and
providing the information required for extrapolations (to zero radiation
time) in the analysis of element concentrations and line shapes. The
present results account for 5-10 min exposure time or 20 min at most.
Curve-fitting analysis of line shapes was performed with Vision
software, using Shirley background subtraction and Gaussian-Loren-
zian line shapes. The spectra of the LB films were taken on glass slides.
The photoluminescence spectra were recorded by immersing the
sample in a variable-temperature 12CNDT Janis cryogenic Dewar. The
spectra were obtained by exciting the sample with a continuous 457.9
nm Ar+ laser (Coherent, Innova 90). The transmitted or emitted light
passed through a holographic grating monochromator (Acton Model
SpectraPro 300) and was detected by an ICCD camera (PI model 576).
Figure 4. (a) UV-vis absorption spectra of CdS particles generated
within Cd C18 mixed crystals containing 1%, 2.5%, 5%, and 10%
corresponding thioalkanoates that were measured after extracted in
DMSO suspensions. (b) Photoluminescence spectra of the corresponding
CdS particles, measured directly in the solid state at 77 K within reacted
powders.
toxide (P2O5), methanol, ethanol, dimethyl sulfoxide (DMSO) (Merck),
and alkanoic acids CH3(CH2)nCOOH (n ) 14-28) (Sigma) were used
without additional purification.
The thioalkanoic acids were synthesized through the reactions of
KSH with acyl chlorides of the corresponding alkanoic acids.14 The
acyl chlorides were prepared by the reaction of the fatty acids with
POCl3. KSH was prepared by bubbling H2S gas through an ethanolic
solution of KOH. The purity of the thioalkanoic acids was demonstrated,
selectively shown here for CH3(CH2)20COSH (molecular weight M )
356), by mass spectrometry (m/e 357); NMR (CDCl3) δ 0.96 (t, 3H),
1.33 (m, 38H), 2.68 (t, 3H); IR (KBr) 2929, 2851, 2557, 1676, and
1472 cm-1. The thioalkanoic acids were converted into the more stable
corresponding potassium salts and stored as such at -5 °C. The Cd
salts of the acids and the mixed crystals were prepared by mixing
ethanol solutions of the corresponding potassium salts with aqueous
solution of CdCl2.
H2S (g) was either prepared through the reaction of Na2S with 20%
H2SO4 and used without drying (hereafter wet H2S) or taken from
commercial cylinders (Merck) and used after drying by passing it
through a tube of P2O5 (hereafter dry H2S). The gas-solid reactions
were performed at room temperature (25 °C).
The Langmuir films were prepared by spreading 10-3 M chloroform
solutions of the alkanoic acids or chloroform (mixed with 5% of
ethanol) solutions of the mixtures of alkanoic acids with a certain
Results and Discussions
I. Topotactic Gas/Solid Reactions in 3-D Crystalline
Powders. The homologous series of cadmium C16, C18, C22,
C26, and C30 alkanoates, thioalkanoates, and their mixtures were
prepared and characterized by XRD and FT-IR spectra. The
spacings between the Cd ion layers in the different crystals are
listed in Table 1.
XRD patterns of Cd C22 alkanoate and thioalkanoate are
shown in Figure 1a, i. These patterns indicate that the Cd ions
within these powders are arranged in layers that are separated
by a spacing that is twice the length of the corresponding
hydrocarbon chain in the extended conformation (see Scheme
1). The packing arrangements of the thioalkanoates are very
similar to those of the corresponding alkanoates, with the
exception that the spacing between the Cd ion layers is
somewhat shorter. It implies that the hydrocarbon chains in these
crystals are tilted about 20° off the normal to the plane. The
difference in organization arises from the binding of the Cd
ions in the two systems. In the alkanoates, the Cd ions are bound
equally to two oxygen atoms, as deduced from the XPS data,
as well as from the asymmetric carboxylate stretching vibration
(13) Tao, Y.; Pandiaraju, S.; Lin, W.; Chen, L. Langmuir 1998, 14, 145.
(14) (a) Noble, P., Jr.; Tarbell, D. S. Organic Syntheses; Wiley: New
York, 1963; Collect. Vol. 4, p 924. (b) Shin, H. C.; Quinn, D. M. Lipids
1993, 28, 73.

Preparation of Hybrid Organic/Inorganic Structures
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9593
Figure 5. TEM images of CdS particles formed in cadmium (a) C18 alkanoate and mixed crystals containing (b) 1%, (c) 2.5%, (d) 5%, and (e) 10%
of C18 thioalkanoate. (f) High-resolution TEM images of the cubic CdS particles shown in panel d. Images a-e were taken at the same magnification.
band at 1542 cm-1 in the FT-IR spectra. On the other hand, in
the thioalkanoate, the Cd ions are bound preferentially to the
sulfur atoms. The latter was concluded from XPS data and the
appearance of the IR stretching band at 1656 cm-1, assigned to
the absorption edge by about 25 nm. Moreover, those bands
show a radiative lifetime of a microsecond and a multiexpo-
nential decay process (not shown). Bawendi et al.19 showed that
interparticle interactions within a self-assembled periodic struc-
ture cause a red shift of an exciton of about 2 nm, with respect
to that of an individual particle, and also shorten the exciton
lifetime in the nanosecond time range. Thus, interparticle
interactions alone will not explain the substantially larger Stokes
shift and the long lifetime in our case. Instead, it may indicate
that this band is not excitonic, and rather corresponds to a defect
recombination process. Despite the deepness of the luminescence
band, it shows a pronounced blue shift with a decrease in particle
size. This indicates that at least one of the recombining defect
sites has a shallow state that manages to follow the quantization
effect of the adjacent band edge.20,21 Figures 3a-d and 5a show
the bright field TEM images of the Cd alkanoate powders after
completion of the reaction with H2S. In agreement with the
UV-vis measurements, the particles generated within C16 and
C18 are substantially large, over 100 Å in diameter (Figures 3a
and 5a). The particles formed within the C22 and C26 (Figure
3b,c) are smaller, 20-40 Å in diameter, but still randomly
distributed in the matrices. The particles generated within
crystals of C30 are not only small, with a size range of 20-30
Å in diameter, but also arranged in discrete layers within the
crystals (Figure 3d).
a carbonyl bond, and from the absence of a band at 1150 cm-1
,
due to the CdS bond15.
Powders of the pure Cd alkanoates were exposed to dry H2S
at atmospheric pressure, yielding CdS particles and centrosym-
metric pairs of the alkanoic acids, as followed from the
replacement of the carboxylate stretching band at 1542 cm-1
by that of the carbonyl at 1705 cm-1 in the FT-IR spectrum.16-17
After completion of the reaction, the CdS particles were
extracted in DMSO using 3-mercapto-1,2-propanediol as a
stabilizer. The UV-vis absorption spectra of the CdS particles,
generated in the different salts, are shown in Figure 2A. The
absorption onsets of the CdS particles generated within C16 and
C18 are at 520 nm or above, and correspond to relatively large
CdS particles. On the other hand, the absorption onsets of the
particles formed within C22, C26, and C30 are blue shifted to
470, 460, and 455 nm, respectively, indicating that the particles
are quantum sized.18 A pronounced blue shift is also recognized
in the photoluminescence (PL) spectra of CdS particles within
the alkanoates, as presented in Figure 2B. Those spectra consist
of a relatively broad emission band that is Stokes shifted from
(15) In the FT-IR spectra of the corresponding potassium thioalkanecar-
boxylates, the CdS stretching band at 1150 cm-1 was observed implying
that the K+ ion is attached to the oxygen.
(16) Holland, R. F.; Nielsen, J. R. J. Mol. Spectrosc. 1962, 9, 436.
(17) Dote, J. L.; Mowery, R. L. J. Phys. Chem. 1988, 92, 1571.
(18) Wang, Y.; Suna, A.; Mahler, W.; Kasowski, R. J. Chem. Phys. 1987,
87, 7315.
Summarizing the reactivity in these related systems, one finds
a correlation between the spatial separation of the ion layers in
the reactant phase (as determined by the hydrocarbon chain
(20) Lifshitz, E.; Dag, T.; Litvin, I. D.; Hodes, G.; Roisfeld, R.; Zelner,
M.; Minti, H. Chem. Phys. Lett. 1998, 288, 188.
(21) Lifshitz, E.; Litvin, I. D.; Porteanu, H.; Lipovskii, A. A. Chem. Phys.
Lett. 1998, 295, 249.
(19) Kagan, C. R.; Murry, C. B.; Bawendi, M. G. Phys. ReV. B 1996,
54, 8633.

9594 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Guo et al.
Figure 6. (a, b) TEM images CdS particles formed within cadmium C18 alkanoates containing 50% and 70% of the corresponding thioalkanoates
after exposure to dry H2S for 7 h, respectively. Insert in panel a shows selected area electron diffraction of the CdS particles. (c) XRD patterns of
Cd C18 alkanoate containing 50% of the corresponding thioalkanoate (A) before and (B) after exposure to dry H2S for 7 h.
length) and the size and organization of the resultant CdS
particles. When the ion layers are separated by a distance of
40-50 Å, the CdS particles are large and randomly distributed;
when the layer spacing is 60-70 Å, the particles are small, but
still randomly distributed. Finally, the particles are small and
arranged in layers when a spacing of 80 Å or more separates
the ion layers.
Since Cd alkanoate and thioalkanoate have similar structures,
solid solutions of the two could be formed, as suggested in
Scheme 2. Three different regimes were observed in the solid
solutions as shown by the XRD patterns of the C22 (Figure 1b-
d,f-h). In regime I, with nominal concentration of the thioal-
kanoate lower than 20%, the mixed crystals assume the same
packing arrangement as that of the pure alkanoate. In regime
II, where the nominal concentration of the thioalkanoates is
above 50%, the mixtures precipitate in the same phase as that
of the pure thioalkanoate and in this phase the hydrocarbon
chains of the alkanoate assume the structure and tilt angle of
the thioalkanoate. In regime III, when the concentration of
thioalkanoate is 20-50%, two phases are formed.
as periodic layers. For C22 and C26, the particles are 20-30 Å
in diameter, with a high degree of alignment. Their TEM images
are shown in Figures 3e, 5e, and 3f,g, respectively.
The effect of thioalkanoate concentration within the solid
solution on the size and arrangement of the CdS particles was
systematically investigated on the C18 system. Mixed crystals
were prepared from solutions containing an increasing percent-
age of the thioalkanoate. The concentrations of the thioal-
kanoates in the mixed crystals were determined by elemental
analysis, shown in parentheses: 1% (0.67%), 2.5% (2.56%),
and 10% (8.83%). Figure 4 shows UV-vis and PL spectra of
CdS particles, formed in mixed crystals precipitated from
solutions bearing a given percentage of the thioalkanoates. The
presence of 1% thioalkanoate does not change the absorption
onset of the particles relative to that of the CdS bulk. However,
a blue shift is already observed upon increase to 2.5% or above,
reaching a plateau with 5% of the thioalkanoate. The absorption
onset with 10% thioalkanoate is almost identical with that of
the 5% mixture. These conclusions were independently con-
firmed by PL spectra of the CdS particles recorded within the
reacted powders.
Mixed crystals of C16, C18, C22, and C26 containing 10% of
the corresponding thioalkanoate, in the alkanoate phase, were
exposed to H2S to generate CdS particles. The CdS particles in
the C16 mixture are substantially smaller as compared to those
generated in the pure alkanoate, but still randomly arranged in
the matrix. Within the mixed crystals of C18, the average size
of the CdS particles is smaller as compared to those generated
in the pure alkanoate and, furthermore, the particles are arranged
Figure 5 shows TEM images of reacted salts of pure Cd C18
alkanoate and mixtures of the latter with varying amounts (1-
10%) of Cd C18 thioalkanoates. Cubic CdS particles (Figure
5f) of 30-35 Å in diameter are formed. Within powders that
contain 5% or more of the thioalkanoate the CdS particles are
arranged as periodic layers. The size of the particles, as
determined by the TEM measurements, is in agreement with

Preparation of Hybrid Organic/Inorganic Structures
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9595
Table 3. The Nominal and XPS Determined Thioalkanoate
Concentrations in the Mixed Crystals, and the XPS Determined
Core and Surface Ratios of Cd and S of the CdS Particles Formed
within Different Mixed Crystals
concn of
thioalkanecarboxylate (%)
XPS determined core and surface ratios
of Cd and S of CdS particles (%)
nominal XPS determined Cd + S in core Cd on surface S on surface
80
50
10
0
67
33
10
0
12
40
70
90
40
26
20
10
48
34
8
<5
II. Organization of the CdS Nanoparticles in Superlattices
within LB Films. The reactivity of LB films of Cd alkanoates
(stearate, arachidate, and behenate) with H2S has been over-
whelmingly studied over the years.22-24 Yet an issue that still
remains under debate is whether the CdS particles generated
within these films are periodically aligned or randomly distrib-
uted therein. In some investigations,25,26,27 researchers had
deduced that superlattices of the CdS particles were probably
generated. Ordering of the CdS particles within these LB films
is difficult to detect by TEM because of an improper orientation
of the alkanoic acids layers vis-a-vis the probing electron beam.
Recent studies applying Rutherford backscattering measurements
on cadmium arachidate LB films composed of 700 layers have
demonstrated that the CdS particles were not organized in layers,
but are rather distributed randomly within the organic film.28
These observations are in agreement with the present studies
on the 3-D powders of the corresponding Cd alkanoates that
assume packing arrangements that are analogous to the LB films.
Due to the strong hydrophobic interactions between the hydro-
carbon chains, crystalline domains are formed at the water
surface, thus limiting the preparation of LB films of Cd
alkanoates to hydrocarbon chains up to C22.29 Hence, the method
of site directing nucleation should be most appropriate for the
preparation of nanoparticle superlattices in such systems.
LB films of Cd alkanoate and thioalkanoate mixtures with
different compositions were prepared and deposited on hydro-
philic glass slides. Since the thioalkanoates undergo partial
hydrolysis on the water surface prior to transfer to the glass,
the relative ratios of the alkanoate to thioalkanoate had been
predetermined by XPS for each specimen film before reaction.
The progress of reaction was monitored by FT-IR and XPS, as
for the 3-D crystalline powders. The UV-vis absorption spectra
of the LB films of pure Cd C22 (behenate) and mixtures
containing 10%, 50%, and 80% of thiobehenate, after reaction
with H2S at 25 °C and at atmospheric pressure, are shown in
Figure 7. From these spectra, one deduces that by increasing
the concentration of the thiobehenate, the absorption onset is
blue shifted, implying a decrease in size of the particles. The
Figure 7. UV-vis absorption spectra of LB films of (a) pure Cd C22
alkanoate and the mixtures containing (b) 10%, (c) 50%, and (d) 80%
of the corresponding thioalkanoates after completing the reactions with
H2S.
Table 2. Binding Energies (eV) as Determined by XPS for
Oxygen (1s), Carbon (1s), Cadmium (3d5/2), and Sulfur (2p3/2),
Measured in 3-D Crystals and LB Films of Pure Cadmium C22
Alkanoate and Mixtures with the Corresponding Thioalkanoates
thioalkanoate related peaksa
pure alkanoatec
before after
low concnd
high concne
before after
before after
reaction reaction reaction reaction reaction reaction
O (1s)
531.45 532.00 532.00 532.00 532.00 532.30
533.30
C (1s)
288.50 289.15 287.00 287.00 287.50 287.50
288.0b
Cd (3d5/2
)
405.45 405.05
405.55 405.40
S (2p3/2
)
161.40 162.00 162.40 162.50 162.40
163.80
a In the mixed crystals, the carboxylate related peaks appear in
addition to the thiocarboxylate related ones. b Associated with -C(O)SH.
c
d
e
(22) Geddes, N. J.; Urquhart, R. S.; Furlong, D. N.; Lawrence, C. R.;
Tanaka, K.; Okahata, Y. J. Phys. Chem. 1993, 97, 13767.
(23) Moriguichi, I.; Hosoi, K.; Nagaoka, H.; Tanaka, I.; Teraoka, Y.;
Kagawa, S. J. Chem. Soc., Fraaday Trans. 1994, 90 (2), 349.
(24) Urquhart, R. S.; Hoffmann, C. L.; Furlong, D. N.; Geddes, N. J.;
Rabolt, J. F.; Grieser, F. J. Phys. Chem. 1995, 99, 15987.
(25) Smotkin, E. S.; Lee, C.; Bard, A. J.; Campion, A.; Fox, M. A.;
Mallouk, T. E.; Webber, S. E.; White, J. M. Chem. Phys. Lett. 1988, 152,
265.
(26) Pan, Z.; Liu, J.; Peng, X.; Li, T.; Wu, Z.; Zhu, M. Langmuir 1996,
12, 851.
(27) (a) Basu, J. K.; Sanyal, M. K. Phys. ReV. Lett. 1997, 79, 4617. (b)
Vitta, S.; Metzger, T. H.; Major, S. S.; Dhanabalan, A.; Talwar, S. S.
Langmuir, 1998, 14, 1799.
the size deduced from the UV-vis absorption and photolumi-
nescence measurements.
Superlattices of CdS nanoparticles within the reactant matrices
were also formed within the thioalkanoate-enriched phases of
regime II (Figure 6). The XRD patterns of the crystalline
powders after completion of the reaction with H2S showed a
few new broad peaks (Figure 6c). The low-angle (001) diffrac-
tion peak in the product could not be observed on our
instrument. The spacings between the layers of particles are
larger than those between the ion layers measured in the reactant
prior to reaction. This observation implies that the particles are
formed within the matrix itself releasing the stress built up
during reaction by an expansion of the crystal lattice.
(28) Yamaki, T.; Asai, K.; Ishigure, K. Chem. Phys. Lett. 1997, 273,
376.
(29) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.;
Schwartz, D. K. Science 1994, 263, 1726.

9596 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Guo et al.
Figure 8. TEM images of CdS particles formed in LB films of C22 alkanoate containing 50% of the corresponding thioalkanoate (a) and pure Cd
C22 alkanoate (b). Part a shows a folding in the LB film, revealing both top and side views. The inset shows FFT from the side view region. All
images were taken at the same magnification.
Figure 9. XPS spectra of O (1s), C (1s) of carbonyl, Cd (3d5/2), and S (2p) in the 3-D crystals and LB films of pure Cd C22 alkanoate (a) before
and (b) after completion of the reaction with H2S.
XPS results are practically identical with those obtained from
3-D crystalline powders as summarized in Table 2 and Figures
9 and 10 (see next paragraph), revealing that part of the CdS
nanoparticles are linked directly to the thioalkanoates in the LB
films, in agreement with the proposed mechanism. In fact, even
narrower XPS lines of the films indicate an overall better
homogeneity as compared with the 3-D analogues (Scheme 2).
TEM measurements of the LB films, deposited on nylon grids
and reacted with H2S, demonstrate that some of the latter had
folded on the grid, thus exposing regions in an appropriate
orientation vis-a-vis the electron beam. Consequently, periodic
order of the CdS nanoparticles could be detected in these
regions, as shown in Figure 8. Stripes of disklike CdS particles
were observed, 10-20 Å in width and 30-45 Å in length, in
agreement with the results obtained in the 3-D crystallites. In
contrast, the CdS particles formed in the LB films of the pure

Preparation of Hybrid Organic/Inorganic Structures
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9597
sample is attributed to that of the pure carboxylic acid (a value
verified by a reference sample). Correspondingly, before reaction
the O (1s) line shows a dominant signal at 531.45 eV, reflecting
the equal role of the four oxygen atoms in the carboxylates Cd
complex. After a reaction, the O (1S) peaks at 532.0 and 533.3
eV are assigned to the oxygen of carbonyl (CdO) and the
oxygen of the hydroxide in the [-C(O)-OH], respectively. (It
is noted that at low intensity, additional oxygen signals are
always detected, as discussed below.)
Analysis of the mixed crystals comprising the thioalkanoates
demonstrates evolvement of new peaks (see Table 2 and Figure
10). The carbon signal of the thiocarboxylate group at 287.5
eV is found to increase with an increase of the concentration
of the thioalkanoates (before reaction); however, not in a simple
linear manner. We also noticed a shift from 287.0 eV (low
concentrations of thioalkanoate, namely thio-diluted) to 287.5
eV (high concentrations of thioalkanoate, namely thio-rich). This
difference in energies (see Table 2), is attributed to the formation
of different pairs around the Cd ions: thiocarboxylate/carboxy-
late or thiocarboxylate/thiocarboxylate. After sulfidization, these
lines are slightly modified, depending on the concentration of
the thioalkanoate, unfolding into three overlapping contributions
determined at 287.0, 287.5, and 288.0 eV. The latter component
is associated with “free” -C(O)S-H groups as verifed from
the appearance of the corresponding S (2p3/2) signal at 163.8
eV.
Accordingly, before reaction with H2S, the S (2p3/2) line from
the thio-diluted samples appears at 161.9 eV (associated with
thiocarboxylate/carboxylate pairs), while in the thio-riched
samples it is at 162.5 eV (attributed to the statistically favored
pairs thiocarboxylate/thiocarboxylate). After sulfidization, two
sulfur lines at 161.4 (CdS particle associated) and 162.5 eV
[-C(O)SCd- bonds], are detected from both thio-diluted and
thio-rich samples; however, with varying relative intensities.
This is an important proof for the thioalkanoates as site-directing
nucleation centers for CdS. Finally, as already mentioned above,
a signal at 163.8 eV was observed from the thio-rich sample,
assigned to the -C(O)S-H bonds created during the sulfidiza-
tion.
Before reaction with H2S, the Cd (3d5/2) line of the thio-rich
samples appears around 405.6 eV, slightly higher than the Cd
of pure carboxylates (405.45 eV). It is noted that a similar
analysis of the thio-diluted Cd signal is meaningless, as the
unresolved thiocarboxylate related signal is too small. After
sulfidization, the line of the thio-rich samples is slightly shifted
to 405.4 eV, a value differing more distinctly from that of the
pure carboxylates after reaction (405.05 eV).
Finally, the O (1s) signal of thiocarboxylate [-C(O)-S-]
appears at 532.0 eV before reaction and at 531.5 and 532.0 eV
after reaction. The first value agrees with the expected chemical
CdO bonds, indicating that the Cd ions bind preferentially to
the sulfur atoms, in full agreement with the FT-IR measure-
ments. The 531.5 eV signal is related to the intermediate bonding
states, such as -C-O-Cd-S, at the surfaces of the CdS
particles formed within the organic matrices.
Figure 10. (A) XPS spectra of the carbonyl carbon (1s) in (a) pure
C22 Cd alkanoate and within mixtures containing (b) 10%, (c) 50%,
and (d) 80% of the corresponding thioalkanoate; (B) XPS spectra of S
in the mixtures of Cd C22 alkanoate containing (a) 50% of the
thioalkanoate before reaction with H2S and (b) 80% of thioalkanoate
after reaction with H2S and (c) XPS spectrum of S in the pure Cd C22
alkanoate after reaction with H2S.
Cd behenate are randomly distributed with an average diameter
of 40 Å, consistent with the Rutherford backscattering observa-
tions.28
III. XPS of Cd C22 Matrices. Binding energies and the lines
of the various elements, C (1s), O (1s), Cd (3d), and S (2p),
provided a rather complete picture of the headgroups structures,
both in the powders and in the LB films. The binding energies,
listed in Table 2, were calibrated for a backbone carbon peak
at 284.8 eV. As shown in Figure 9, assignment of the pure
alkanoates XPS lines is relatively straightforward. Before
reaction, a single chemical state for the Cd (3d5/2) (405.45 eV)
is suggested by the relatively narrow lines, while the carbon
signal of the carboxylate at 288.5 eV reflects high homogeneity
of the studied material. Upon sulfidation, these lines exhibit a
clear chemical shift (to 405.05 and 289.15 eV, respectively).
The small shift of the Cd line (accompanied by the appearance
of a sulfur line) is in full agreement with the formation of CdS
particles. The observed carbon signal at 289.15 eV in the reacted
The above results can also be interpreted in terms of “core”
and “surface” signals corresponding to the various types of CdS
particles. “Core” signals are extracted from the Cd and S lines
via comparison with reference CdS samples. The ratios of Cd
and S ions residing within the cores vs those located at the
surfaces of the CdS particles, for C22 alkanoate and thioalkanoate
with different compositions, are given in Table 3. The surface-
related Cd and S signals are found at positions consistent with
Cd-O and/or S-C bonds, where their relative magnitude varies

9598 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
Guo et al.
considerably with increasing concentration of the thioalkanoate,
in close agreement with the corresponding changes of the CdS
particle sizes deduced independently by other techniques. In
fact, by increasing the thioalkanoate concentrations in the mixed
crystals, the particles are found to be so small that almost 90%
of their atoms are located at the surface. These results are in
agreement with the spectroscopic measurements.
tion of the CdS quantum particles within LB films of cadmium
alkanoates.
TEM studies have also shown that in samples of micron size
dimensions, these topotactic reactions occur via single-crystal
to single-crystal transformations. In principle, if this tendency
could be expressed in systems that display related chemical
behaviors, but in addition yield large single crystals, the present
method is expected to be useful for the preparation of single
crystalline composites that might display interesting anisotropic
optoelectronic properties.
Conclusions
In the present study we demonstrate a successful application
of topotactic gas/solid reactions for the preparation of CdS
quantum particles arranged in patterns within organic matrices.
The key step in such transformations is the ability to control
the diffusion of the inorganic particles and sites where they
nucleate and grow. Diffusion of the product molecules within
the crystallites is anisotropic. In the present class of materials,
control on the spacing that separates the ion layers within the
reactant materials is achieved by selecting alkanoic acids with
appropriate chain lengths. With the assistance of the Cd
thioalkanoates as site-directing nucleation centers, it became
possible to improve the organization of the nanoparticles even
in crystals with shorter hydrocarbon chains, as well as to reduce
their overall sizes. The homodispersity of the particles is
moderate, but there is still room for further improvements. The
present method was also successfully applied for the organiza-
Site directing nucleation should be general and applicable
for the preparation of other composites with other patterns. The
exploitations of unique optoelectronic properties of such systems
are currently being studied, and the results will be presented in
following publications.
Acknowledgment. We thank the Petroleum Research Funds
of the American Chemical Society, the Levine funds at the
Weizmann Institute of Science, and the Israeli Ministry of
Science for Financial support. L.K. thanks the Campomar
Foundation and the G.M.J. Schmidt Minerva Center for a
fellowship. We also acknowledge Ms. Edna Shavit for synthesis
of the thioalkanoic acids and Prof. Leslie Leiserowitz and Dr.
Isabelle Weissbuch for fruitful discussions.
JA991354X