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The Morphology and Microstructure of Colloidal Silver and Gold.

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[ I ] W. Petz. Chem. Rep. 86 (1986) 1019.
[Z] J. J. Zuckerman. X l t h In/. Cot$. Organomet. Chem. 1993 (Callaway Gardens, GA. USA), Abstr. p. 167.
[3] a ) M. Veith, W. Frank, Angew. Clwm. 97 (1985) 213: Angew. Chem. I n / .
Ed. Engl. 24 (1985) 223; h) S. 1. Archer, K. R. Koch, S. Schmidt. Inorg.
Chim Acra 126 (1987) 209. and references cited therein.
141 H. H. Karsch. B. Deubelly, J. Riede, G. Muller, Angew. Chem. 99 (1987)
703: Angew Cliem. In,. Ed. Engl. 26 (1987) 673.
[ 5 ] H. H. Karsch, B. Deubelly, G. Hanika, J. Riede, G. Miiller, unpublished
results.
[6] Crystal structure determination: Syntex P2, diffractometer, Mona rddiagraphite monochromator, T = -40°C; monoclinic,
tion, I =0.71069
P2,/n. a=21.886(3), b = I1.977(1), r=22.339(3)
8=96.47(1)",
V=5818.4 A', &,,,<,,= 1.442 g cm.-', Z = 4 , p(Mo,,)=29.3 c m - ' . 8058
unique reflections, 6222 with I > 2.0u(1) "observed" (a-scan, Ao=O.8",
+ h . i k , +1. (sin9/l),n.,\ =0.549). Solution by automated Patterson methods (SHELXS-86). R=0.044, R,, =0.045, w = I/u'(F,J for 460 refined parameters (anisotropic, H constant, GFMLX). &,,,, =0.60 e k ' . Further
details of the crystal structure investigation may be obtained from the
Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-7514
Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number
CSD-52416, the names of the authors, and the journal citation.
[7] Cf.. e.g., d(Ge-Ge)=2.541
in [(ChH,(CH3)&Ge],: S. Masamune, Y.
Hanzawa, J . A m . Chem. Sor. 104 (1982) 6136.
181 V. 1. Kulishov, N. G . Bokii, Yu. T. Struchkov, 0. M. Nefedov, S. P. Kolesnikov, B. L. Perl'mutter, Zh. Slrukt. Khim. I ! (1970) 71.
A,
A,
A
The Morphology and Microstructure
of Colloidal Silver and Gold""
By D. G. Duff, A . C . Curtis. Peter P. Edwards,*
D . A . Jefferson, Brian F. G . Johnson, A . I . Kirkland,
and D. E. Logan
The brilliant colors characteristic of colloidal gold and
silver have been known for over a millennium.['-31 AIthough the technique of electron microscopy has been applied previously to study the nucleation, growth, and topology of colloidal
it is only recently that the
resolution of the microscopes has been improved to such
an
as to yield reliable images which may reflect
the atomic microstructure of these divided
Here
we demonstrate how direct structural imaging by high-resolution electron microscopy (HREM), coupled with appropriate chemical routes to ultrafine particles, can indeed
yield important information on the morphology and
atomic structure of individual colloidal metal particles.
Thus, the atomic imaging of a 35-A-diameter silver particle
by HREM enables a direct observation of the packing of
individual silver atoms around a dodecahedra1 core. This
observation is supported by computer-simulated images
for an entire particle; the silver particle exhibits a perfect
pentagonal symmetry in its atomic arrangement around the
central C, axis.
All HREM examinations were carried out in a modified
JEOL-2OOCX operating at 200 kV, with a new type of sideentry specimen stage utilizing an objective lens with characteristics C, = 0.6 mm, C,= 1.05 mm, and focal length
v)=1.6 mm. Full details of this modification are given
[*] Dr. P. P. Edwards, D. G. Duff, A. C. Curtis,
[**I
D. A. Jefferson, Dr. B. F. G. Johnson, A. 1. Kirkland
University Chemical Laboratory
Lensfield Road, Cambridge CB2 IEW (England)
D. E. Logan
Physical Chemistry Laboratory, University of Oxford
South Parks Road, Oxford OX1 342 (England)
We thank the SERC. Johnson Matthey, and ICl for support. P. P. E.
also thanks the Nuffield Foundation for the award of a One Year Research Fellowship.
676
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mbH. 0-6940 Weinheim. 1987
elsewhere.[x1The interpretable point resolution of the modified instrument at optimum defocus, as defined by the
first zero of the phase-contrast transfer function['I was
2.03
as compared with 2.41 A for the standard instrument. For gold, therefore, both { 1 1 I ) and (200) diffracted
beams (corresponding to spacings of 2.35 and 2.04 A, respectively) could be recombined into the image with the
same relative phase shift at optimum defocus, and the
atoms appeared as black dots in images viewed down the
( 1 lo} directions. With silver, however, the most useful
images were recorded at a defocus setting considerably
greater than optimum with the atoms being resolved as
white dots.
Owing to the extremely small size of the colloidal metal
particles, no attempt was made to orient them with respect
to the electron beam and particles were examined by systematic screening of the grid. Micrographs were recorded at
ca. 490000 x , a series of pictures for each group of particles being taken, with a variation of objective lens defocus of ca. 300 A between successive exposures. Astigmatism was corrected by observing the granularity of the
amorphous support film, and diffraction patterns were obtained when required by means of optical diffraction from
the recorded images.
The gold particles could be classified into two categories:
A,
1. The first type includes multiply twinned particles
(MTPs) similar to those previously described for gold samples prepared by evaporation of the bulk metal.["'. ' 'I These
particles gave very similar HREM images and varied in
size from ca. 100 to 400 A in diameter. A typical example
of a decahedral particle being illustrated in Figure la.
Strong diffraction contrast at the twin boundaries between
the individual regions within these MTPs suggested that
either considerable strain or defects were present at the
boundaries. In all cases. however, the individual sub-
Ftg.
a) Typ,cal decahe.
dral particle in colloidal
gold, with strong contrast
observable at the boundaries. b) Pseudo-trigonal particle of colloidal gold. The
inset shows the atomic detail at the truncated corners. c) Small, apparently
flat single-crystal gold particle, believed to be a precursor of the larger flat particles.
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Angew. Chem. In?. Ed. Engl. 26 (1987) No. 7
units within the MTPs examined were of roughly equal
size.
2. The second type of gold particle is shown in Figure Ib.
Tilting experiments on a lower-resolution instrument confirmed the flat nature of these particles. Although some
were of irregular outline, most were clearly triangular, generally with truncated corners (Fig. lb). Surface steps were
visible at the corners, as shown in the inset. The lateral dimensions of these particles-showed a wide variation, but
were rarely less than 200A and frequently extended to
greater than 1000 A. Smaller particles without twinning
were observed (Fig. Ic). Although not of triangular outline,
they were definitely not of the icosahedral or decahedral
type, and were possibly representative of the first stages of
triangular particle formation, before the characteristic
shape had developed.
Colloidal silver was prepared by the reduction of a solution of silver nitrate in acetonitrile by poly(ethy1ene imine)
t H 2 C C H 2 N H j , (PEI). The use of PEI as a protecting
agent for stabilizing gold sols against particle aggregation
was first investigated by Thiele and uon Levern.'''] Later
work by Thiele ahd Kowallik~"l demonstrated that PEI
could be used as a reducing agent in the preparation of
colloidal gold from chloroauric acid, this method producing stabilized sols of extremely small particle size. In these
reactions the polymer has the twin function of reducing
and protecting agent.
In the present study we adapted this method for the production of colloidal silver in acetonitrile. This reaction
mixture was treated in two ways. Immediate preparation of
a sample for electron microscopy (involving dessication of
the polymer-metal ion adduct onto a carbon film)* produced a myriad of particles of diameter less than 50 A. Allowing the mixture to stand in daylight caused the development, over the course of several weeks, of a rather unstable silver sol and the appearance of larger particles (ca.
100-500 A diameter), as determined by transmission electron microscopy (TEM). If the sol was kept in the dark,
this nucleation process was slowed down considerably. A
partially reacted sol yielded a bimodal size distribution of
particles on the electron microscope grid; the number of
smaller particles decreased and the number of larger particles increased as the reaction proceeded. The samples under examination here were exposed to daylight for three
days; this procedure yielded a good number of large and
small particles for examination.
For the silver samples, no flat plates were observed, but
again the particles could be classified into two categories:
1. MTPs were the predominant type, generally of lateral
dimensions less than 500 A and, by contrast with those observed in the gold samples, typically showed an arrangement of individual subunits that was far from perfect. A
typical particle is shown in Figure 2a, where, although at
first sight the particle appears t o be of the decahedral type,
closer examination reveals the existence of six component
units. At least one of the boundaries within this particle is
not along a crystallographic direction, and this may possibly be the method by which the metal adapts the bulk facecentered cubic structure to the developed morphology of
the MTP. Also in contrast to the gold samples, no major
strain contrast was observed at the boundaries within the
MTPs, indicating that the manner in which the lattice adapts to strain might differ between these two colloidal samples.
Angew. Chem.
In1
Ed. Engl. 26 (1987) No. 7
Fig. 2. a) Typical MTP in
colloidal silver, viewed
down {Ilo}. The positions
of the boundaries are indicaled. b) Almost perfect decahedral particle of silver,
with no visible strain at the
boundaries.
The
inset
shows the structural model
deduced from the image
and the simulated image at
an objective lens defocus of
ca. 650A. c) Aggiomeratelike particle of silver, with
no clear indication of distinct subunits o r boundaries between them. The detail of the amorphous support film suggests that the
objective lens defocus of
this image is similar to that
of Figure 2b.
When the particle dimensions become very small
(<40 A), they appeared to adopt only the decahedral form
(Fig. 2b). Although faceting leading to truncation of the
edges of the particles was noted, the atomic arrangement
appeared to be a perfect decahedral one, and no strain
contrast or zigzagging of boundaries was noted. Computer-simulated images of the particle of Figure 2b were calculated using the multislice method"" as previously applied to image calculations of very small particles by Gai et
al.,[Is1 although in the present investigation a simplified
version was adopted; i.e., no account was taken of the
atomic structure of the amorphous carbon background.
This simplification is appropriate since the particles studied here are much larger, and the corresponding percentage contrast is much greater, than those in the studies carried out by Gai et al.""
The structural model employed and a simulated image
are shown in the inset in Figure 2b. The model used contained 1427 silver atoms and, if a homogeneous strain"".'71
was employed, necessitated a maximum expansion of
2.75% in the metal-metal bond length parallel to the edges
of the decahedron. At the position where the notches at the
boundaries occurred, the metal-metal distance in this direction was allowed to relax back to its ideal position, and
this appeared to correspond to the actual arrangement
within the particle. Given the small size of particle, it was
found to be impossible to incorporate any type of dislocation to eliminate this strain without it being immediately
visible in the simulated image; this would have not been in
agreement with the experimental image (Fig. 2b). In addition, the image simulation indicated that the particle was
elongated parallel to the fivefold symmetry axis, this again
being as predicted by the theory of Marks."'' Consequently, the homogeneous strain appeared to be most appro-
0 VCH Verlag.~ge.~ellsclia~
mhH. 0-6940 Weinheim, 1987
0570-0833i87/0707-0677 $ 02.50/0
677
priate in these very small particles and their real structure
did not therefore correspond exactly to the bulk face-centered-cubic structure.
2. The second type of silver particle (Fig. 2c) observed
was much more enigmatic. These particles were approximately spherical, and initially were assumed to consist of
small icosahedral units. However, the number of distinct
individuals was in many cases much too large for a single
icosahedral MTP; it is possible, however, that these particles are “polyparticles” as observed by Smith and Marks
for evaporated samples. These particles varied considerably in size, although most were less thap 100 A. When they
were of very small dimensions ( < 25 A), they gave every
indication of being quasiamorphous, in that no lattice rows
were discernable, and it was often quite difficult to distinguish them from the background of the amorphous support film. Although it is tempting to consider such poorly
structured particles as a type of large cluster, the number
of atoms contained is still relatively high (from density calculations, 479 atoms in a 25-A sphere) and little can be
said about their structure until further image simulations
are performed. It is possible, however, to consider that this
second type of particle may actually form at a later stage
than the MTPs, possibly in the final stages of evaporation
of the sol. Although this would in turn imply that they
were an essentially nonequilibrium phenomena, continued
annealing in the electron beam did not appear to produce
atomic rearrangement, unlike the case of gold particles on
a silicon support, as observed recently.Ih1
In this work we hope to have demonstrated that, in conjunction with appropriate chemical synthetic routes, direct
imaging by HREM provides much detailed information
about the atomic structure of colloidal metal particles on a
nanometer scale. Perhaps the most striking results are the
observations that certain small particles of colloidal silver
exhibit a fivefold symmetry axis (Fig. 2b) and that metal
particles grown from colloidal solutions show marked similarities to those produced by evaporation methods. The
relative ease of experimental control on both the particle
dimensions and morphology is considerably more straightforward in the case of colloidal preparations.
Experimental Proeedure
Stringent conditions of experimental cleanliness were followed throughout
for both the gold and silver preparations-glassware was washed variously
with aqua regia, nitric acid, or chromic acid and then rinsed copiously with
water, the last few times with doubly distilled water. The sols were initially
characterized by low-resolution TEM (JEOL ZOO-CX) with scanning electron
microscope facihty. Optical spectra were obtained using a Pye-Unicam
SP8800 UV-VIS spectrophotometer, the samples being contained in quartz
cells.
Colloidal gold was prepared by citric acid reduction of chloroauric acid using the method of Turkevich et al. [4]: 5 mL of a I % aqueous solution of citric
acid (AR grade, monohydrated salt) was added to 95 mL of a refluxing 0.01%
aqueous solution of chloroauric acid (49.42% Au) with stirring and the mixture was boiled for one hour (all water was doubly distilled before use).
Silver sol: The silver nitrate used was in the form of analytical reagent grade
crystals (99.99% pure). Acetonitrile (HPLC grade) was distilled over calcium
hydride before use. PEI was used as received as a very viscous liquid
( M = 1800 g/mol). The concentrations of reagents used were so as to yield a
sol containing lo-’ M silver nitrate. The starting mixture consisted of a 5.4: 1
molar ratio of imine residues to silver ions: 0.120g of PEI was dissolved in
100 mL of freshly distilled acetonitrile. A 20 mL aliquot of this solution was
added to a further 70 mL of acetonitrile and then 10 mL of a 0.01 M solution
of silver nitrate in acetonitrile was added with stirring. For subsequent procedure, see text.
[ I f See, for example, M. Kerker, J Colloid Interface Sci. IUS (1985) 297.
[2] M. Faraday, Philos. Trans. R . Soc. London 147 (1857) 145.
131 M. Kerker: The Scattering of Lrght and other Electromagnetic Radiation.
Academic Press, New York 1969.
[4] B. von Borries, G. A. Kausche, Kolloid-Z. 90 (1940) 132; J. S. Turkevich,
P. S. Stevenson, J. Hillier, Discuss. Faraday Soc. I 1 (1951) 55; W. 0.
Milligan, R. H. Morriss, J . Am Chem. Soc. 86 (1964) 3461; N. Uyeda,
M. Nishino, E. Suito, J . Colloid Interface Sci. 43 (1973) 264.
151 J:O. Bovin, L. R. Wallenberg, D. J. Smith, Nature (London) 317 (1985)
47; D. J. Smith, A. K. Petford-Long, L. R. Wallenberg, J.-0. Bovin,
Science (Washington) 233 (1986) 872.
161 S. lijima, T. Ichihashi, Phys. Reu. Lett 56 (1986) 616.
171 P. P. Edwards in C . N. R. Rao (Ed.): Aduonces in Solrd State Chemistry
IProc. 1NSA Gold Jubilee Symposium. New Delhi 1984), Indian National
Science Academy, New Delhi 1986, p. 265.
[81 D. A. Jefferson, J. M. Thomas, G. R. Millward, K. Tsuno, A. Harriman,
R. D. Brydson, Nature (London) 323 (1986) 428.
191 H. P. Erickson, A. Clug, Philos. Trans. R . SOC.London Sect. 8 2 6 1 (1971)
105.
[lo] L. D. Marks, A. Howie, Nature (London) 282 (1979) 196; L. D. Marks, A.
Howie, D. J. Smith, Con6 Ser. Inst. Phys. (Bristol) 52 (1980) 397.
[ I l l L. D. Marks, D. J. Smith, J. Cryst. Growth 54 (1981) 425; J. Microsc.
(Oxford) 130 (1983) 249.
1121 H. Thiele, H. Schroder von Levern, J . Colloid Sci. 20 (1965) 679.
[I31 H. Thiele, J. Kowallik, Kolloid-Z. Z . Polym. 234 (1969) 1017.
[I41 J. M. Cowley, A. F. Moodie, Acta Crystallogr. I0 (1957) 609; P. Goodman, A. F. Moodie, ibrd. Sect. A 30 (1974) 280.
[I51 P. L. Gai, M. J. Goringe, J. C. Barry, J . Microsc. (Oxford) 142 (1986) 9.
1161 S. Ino, J . Phys. Soc. Jpn. 27(1969) 941.
1171 K. Heinemann, C. Y . Yang, M. J. Yacaman, H. Poppa, 7iiin Solid Films
58 (1979) 163.
[I81 L. D. Marks, Philos. Mag. Part A 49 (1984) 81.
The Synthesis of 1,3- and 1,4-Phenylene-Linked
Bisquinone-Substituted Porphyrin Dimers**
By Jonathan L. Sessler* and Martin R. Johnson
A key feature of photosynthesis in both green plants and
bacteria is light-induced electron transfer from a “special
pair” dimeric chlorophyll (or bacteriochlorophyll) donor
(P), via a series of intermediate prosthetic groups, to a quinone acceptor (Q). Recent X-ray structural studies of the
bacterial photosynthetic reaction center (RC) from Rhodopseudomonas viridis have revealed a well-defined orientation of six tetrapyrrolic macrocycles at the active site:
The dimeric bacteriochlorophyll special pair (P) is flanked
on two sides, at a distance of 13 A and interplane angle of
70°, by two monomeric bacteriochlorophylls (Bchls), each
of which is in turn in contact with a bacteriopheophytin
(Bph). The Bchl and Bph are separated by an interplane
angle of 65” and a center-to-center distance of 1 1
These structural results suggest that both donor-acceptor
distances and intermacrocycle orientations play a crucial
role in mediating the photosynthetic charge separation
process. In recent years a number of photosynthetic model
systems have been synthesized and studied in an effort to
understand the first of these factors.”I Intermacrocycle
orientation effects, however, have not been thoroughly addressed in model systems.[31We report here the synthesis of
the quinone-substituted 1,3-phenylene-linked “gable”typei4] porphyrin dimer 1 and its more open 1,CphenyIene-bridged analogue 2. System 1 was specifically designed so as to constrain the two porphyrin subunits to a
[*I Prof. J . L. Sessler, M. R. Johnson
[**I
Received: December 4. 1986;
revised: March 24, 1987 [ Z 1999 IE]
German version: Angew. Chem. 99 (1987) 688
678
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Department of Chemistry, University of Texas
Austin, TX 78712 (USA)
This work was supported by the Robert A. Welch Foundation, by the
Camille and Henry Dreyfus Foundation (Distinguished New Faculty
Grant 1984). and by a National Science Foundation (USA) Presidential
Young Investigator Award.
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Angew. Chem. Int. Ed. Engl. 26 (1987) No. 7
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