Catalyzed growth of a metastable InS crystal structure as colloidal crystals [21]

Full text of DOI:10.1021/ja000106u

3562
J. Am. Chem. Soc. 2000, 122, 3562-3563
Catalyzed Growth of a Metastable InS Crystal
Structure as Colloidal Crystals
Jennifer A. Hollingsworth,† Damadora M. Poojary,‡
Abraham Clearfield,*,§ and William E. Buhro*,†
Department of Chemistry, Washington UniVersity
St. Louis, Missouri 63130-4899
Symyx Technologies, 3100 Central Expressway
Santa Clara, California 95051
Department of Chemistry, Texas A&M UniVersity
College Station, Texas 77843
ReceiVed January 10, 2000
Figure 1. Crystal structures of III-VI compounds. In atoms are dark
gray; Se and S atoms are light gray:25 (a) InSe;11-13 (b) InS, lowest-
energy network form;14 (c) InS, metastable layered form from eq 1a.16
Biosynthesis and organic synthesis, adept at controlling mo-
lecular structure, produce selectively a rich variety of lowest-
energy and higher-energy (metastable) structures. However,
materials synthesis, which affords nonmolecular substances such
as ceramics and semiconductors, generally produces only the
lowest-energy crystal structure.1,2 Molecular synthesis achieves
the structural diversity lacking in nonmolecular synthesis through
kinetic control, wherein the structure obtained is the one that forms
the fastest under conditions that prevent its equilibration to the
lowest-energy, most-stable structure. Kinetic control is often
achieved by chemical catalysis, but the absence of catalytic
strategies in materials synthesis has historically precluded ap-
plication of kinetic control to the formation of metastable
nonmolecular solids.1 Recently, Johnson and co-workers have
achieved kinetic control in noncatalyzed syntheses of metastable
solids by using thin-film multilayer reactants.3-5 Metastable solids
have also been recently grown by chemical vapor deposition6,7
and electrodeposition.8 We now report the catalyzed synthesis of
a metastable InS crystal structure under kinetic control.
of g400 °C to be surmounted.9,10 The results established that the
benzenethiol catalyst activated low-barrier crystallization pathways
in the InS and InSe syntheses.
The XRD pattern of the InSe product from reaction 1b (eq 1)
corresponded to that of the known, standard-pressure (1 atm)
structure (Figure 1a).11,12 The structure contains planar, covalently
bonded sheets that are 4 atomic layers thick and are separated by
van der Waals gaps. This layered, pseudographitic structural form
is also exhibited by GaS and several other III-VI members of
the GaS family.13
However, the known structure of InS (Figure 1b) at standard
pressure differs from that of the GaS family.14 The greater ionic
character of InS relative to InSe, GaS, and the other family
members destabilizes the layered structure relative to the 3D
network structure shown in Figure 1b.15 Until the present work,
a layered form of InS was unknown.
The XRD pattern of the InS product from reaction 1a (eq 1)
did not match any known InS phase. Consequently, the crystal
structure was solved from XRD data collected on an as-
precipitated, unannealed powder sample, and refined by the
Rietveld method.16 As shown in Figure 1c, the new InS structure
is a close variant of the layered, pseudographitic GaS (InSe)
structure (Figure 1a). Thus, InS exists in both layered and network
structural forms; we show below that the layered structure is a
higher-energy, metastable form.
Catalyzed syntheses of InS and InSe were conducted under
solution-phase reaction conditions analogous to those used in
molecular synthesis (eq 1). When benzenethiol (C6H5SH) was
The InS and InSe materials from eq 1 precipitated with
colloidal-crystal nanostructures. Both materials were found
primarily in platelet morphologies. The InSe platelets ap-
proximated hexagonal shapes (Figure 2a,b), whereas the InS
platelets exhibited triangular features (Figure 2d). Electron-
diffraction patterns collected perpendicular to the large-area
platelet surfaces and over large areas (collection radii ≈ 150 nm)
established that the crystallographic c axes were perpendicular
to these platelet surfaces and that the platelets diffracted coher-
ently, as single crystals would. However, examination of TEM
employed as a catalyst (in 10 mol %), the InS and InSe products
precipitated in 85-95% yields as polycrystalline powders having
sharp X-ray powder diffraction (XRD) lines. Elemental analyses
of the bulk InS powder established a 1:1 In:S ratio, and low levels
of residual carbon (<2%) and hydrogen (<0.5%). When the
reactions were conducted without the benzenethiol catalyst, the
product powders were crystallographically amorphous, as evi-
denced by broad, featureless XRD patterns. Amorphous products
are expected from low synthesis temperatures such as the 203
°C employed here. Crystal-growth barriers for covalent nonmo-
lecular solids are typically high and require synthesis temperatures
(9) Stein, A.; Keller, S. W.; Mallouk, T. E. Science 1993, 259, 1558-
1564.
(10) Buhro, W. E.; Hickman, K. M.; Trentler, T. J. AdV. Mater. 1996, 8,
685-688.
† Washington University.
(11) Likforman, A.; Carre´, D.; Etienne, J.; Bachet, B. Acta Crystallogr.
1975, B31, 1252-1254.
‡ Symyx Technologies.
§ Texas A&M University.
(12) Rigoult, J.; Rimsky, A.; Kuhn, A. Acta Crystallogr. 1980, B36, 916-
918.
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(16) Crystallographic and refinement data: space group R3m (no. 160);
hexagonal axes, a ) b ) 3.86714(75) Å, c ) 24.2580(48) Å; RF ) 0.053 for
60 reflections, Rwp ) 0.067, Rp ) 0.052. Atomic fractional coordinates: In-
(1), [0, 0, 0]; In(2), [0, 0, 0.11487(11)]; S(1), [0, 0, 0.8297(4)]; S(2), [0, 0,
0.2964(6)].
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10.1021/ja000106u CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/28/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 14, 2000 3563
Figure 2. Transmission electron micrographs of InSe and InS platelets: (a) InSe platelets approximating hexagonal shapes; inset, electron-diffraction
pattern collected along the [0001] zone axis of a platelet; (b) InSe platelet revealing texture; (c) InSe platelet edge revealing constituent hexagonal
nanocrystallites; (d) InS platelets with triangular features; inset, electron-diffraction pattern collected along the [0001] zone axis of a platelet; and (e)
InS platelet center revealing internal structure.
to be incompletely closed trough, ribbon, and bowl morphologies.
All of the InS morphologies possessed the papier-maˆche´-like
texture observed in the platelets, establishing similar colloidal-
sheet nanostructures. Because of the nanosheet texturing, the InS
nanotube and nanosphere walls lacked the parallel-fringe patterns
characteristic of carbon and pseudographitic fullerene nanotubes
and nano-onions grown at conventionally high temperatures of
g1000 °C.18-21
The conversion of layered to network InS (Figure 1) under
equilibration conditions strongly suggested a greater relative
stability for the network structure. Layered InS from reaction 1a
(eq 1) was suspended in the reaction solvent (1,3-diisopropyl-
benzene) in the presence of indium-metal powder (30 mol %),
Figure 3. Transmission electron micrographs of fullerene-like morphol-
ogies of InS from eq 1a: (a) nanotubes with TEM aperture in; note the
grainy texture, thickened walls, and closed ends; (b) nanotubes with TEM
aperture out; and (c) nanospheres; note some hollow, incomplete shells.
and the mixtures were heated to reflux at 203 °C. The resulting
molten-indium recrystallization catalyst23 effected equilibration
images revealed texture and ragged edges in the platelets
of the layered and network crystalline forms, yielding complete
consistent with internal structure (Figure 2c,e). Close inspection
conversion to the network crystal structure, in support of a greater
of the hexagonal InSe platelet edges showed that the platelets
stability for the network structure. When conducted in the solid
were constructed of hexagonal nanocrystallites having ca. 10-
state, the same layered-to-network structural transformation
nm lateral dimensions (Figure 2c). Similarly, the InS platelets
required temperatures of g400 °C. The results indicated the
were constructed of irregularly shaped nanocrystallites having 10-
layered InS crystal structure to be metastable under the low-
20-nm lateral dimensions (Figure 2e). In both cases the nano-
temperature conditions of its formation, and to have therefore
crystallites were assembled in a crystallographically coherent
been produced by a kinetically controlled synthetic pathway.
manner to build papier-maˆche´-like colloidal crystals. We surmise
The mechanism of the benzenethiol-catalyzed crystal growth
that the nanocrystallites consist of one or a few tetra-atomic-layer
in 1 is presently unknown. The catalyst apparently promotes
sheets, the basic structural units of the InSe and InS crystal
condensation of an isolable [InS(t-Bu)]x polymer intermediate;24
structures (Figure 1a,c), which grew in solution to the 10-20-
a proposed pathway will be described subsequently in a full report.
nm lateral dimension, and concurrently or subsequently stacked
This work demonstrates that the syntheses of nonmolecular solids
into colloidal-crystal assemblies.
having metastable crystal structures, which are generally inac-
The morphological distribution found in the InS precipitates
cessible by conventional crystal-growth methods, may be cata-
was ca. 80-85% platelets (Figure 2d), 13-19% nanotubes and
lyzed.
nanorods (Figure 3a,b), and 1-2% nanospheres (Figure 3c).17 The
nanotube and nanosphere morphologies appeared to be hollow
Acknowledgment. This work was supported by the NSF in separate
structures reminiscent of those found in carbon and pseudogra-
grants to W.E.B. (CHE-9709104) and A.C. (DMR-9707151) and by a
phitic (e.g. MoS2) fullerene samples,18-21 and in clays.22 At higher
NASA graduate research fellowship (NGT3-52313) to J.A.H.
magnifications, some of the nanotubes and nanospheres appeared
JA000106U
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(24) The intermediate [InS(t-Bu)]x was isolated as a white, insoluble solid
having a chemical analysis corresponding to the formula C3.78H8.77InS.
Calculated weight percentages for C4H9InS: C, 23.55; H, 4.45; S, 15.72.
Found: C, 22.34; H, 4.32; S, 15.79.
(25) The ball-and-stick structures were generated with PowderCell freeware
(www.bam.de/a•v/v•1/powder/e•cell.html) and rendered with Pov-Ray
freeware (www.povray.org).
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(22) Sudo, T.; Shimoda, S.; Yotsumoto, H.; Aita, S. Electron Micrographs
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133.