U.S. patent application number 10/693988 was filed with the patent office on 2004-05-13 for bulk synthesis of long nanotubes of transition metal chalcogenides.
This patent application is currently assigned to Yeda Research & Development Co., Ltd.. Invention is credited to Homyonfer, Moshe, Rothschild, Aude, Tenne, Reshef.
Application Number | 20040089410 10/693988 |
Document ID | / |
Family ID | 11072753 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089410 |
Kind Code |
A1 |
Tenne, Reshef ; et
al. |
May 13, 2004 |
Bulk synthesis of long nanotubes of transition metal
chalcogenides
Abstract
Nanotubes of transition metal chalcogenides as long as 0.2-20
microns or more, perfect in shape and of high crystallinity, are
synthesized from a transition metal material, e.g. the transition
metal itself or a substance comprising a transition metal such as
an oxide, water vapor and a H.sub.2X gas or H.sub.2 gas and X
vapor, wherein X is S, Se or Te, by a two-step or three-step
method. The transition metal chalcogenide is preferably WS.sub.2 or
WSe.sub.2. Tips for scanning probe microscopy can be prepared from
said long transition metal chalcogenide nanotubes.
Inventors: |
Tenne, Reshef; (Rehovot,
IL) ; Rothschild, Aude; (Chevreuse, FR) ;
Homyonfer, Moshe; (New York, NY) |
Correspondence
Address: |
BROWDY AND NEIMARK, P.L.L.C.
624 NINTH STREET, NW
SUITE 300
WASHINGTON
DC
20001-5303
US
|
Assignee: |
Yeda Research & Development
Co., Ltd.
Rehovot
IL
|
Family ID: |
11072753 |
Appl. No.: |
10/693988 |
Filed: |
October 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10693988 |
Oct 27, 2003 |
|
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09959664 |
Jan 28, 2002 |
|
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09959664 |
Jan 28, 2002 |
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PCT/IL00/00251 |
May 2, 2000 |
|
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Current U.S.
Class: |
156/230 ;
156/247; 156/272.2; 156/296 |
Current CPC
Class: |
C01B 19/02 20130101;
C01P 2006/60 20130101; C01B 17/20 20130101; C01P 2004/64 20130101;
C01G 41/00 20130101; C30B 29/46 20130101; C01P 2002/72 20130101;
Y10S 977/844 20130101; C01P 2002/82 20130101; G01Q 70/12 20130101;
C01P 2002/74 20130101; C30B 25/005 20130101; C01P 2004/13 20130101;
C01P 2004/62 20130101; C01P 2004/03 20130101; C01P 2006/10
20130101; C01G 41/02 20130101; C30B 29/605 20130101; C01P 2004/04
20130101 |
Class at
Publication: |
156/230 ;
156/272.2; 156/247; 156/296 |
International
Class: |
B44C 001/165; B32B
031/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 2, 1999 |
IL |
129718 |
Claims
What is claimed is:
1. A method for preparation of tips for scanning probe microscopy
which comprises: a) transferring adhesive from carbon tape to a
microfabricated Si tip; and b) pulling off bundles of a transition
metal chalcogenide nanotubes with said tip from a mat of long
nanotubes prepared on a different area of the tape, wherein said
long nanotubes have a size of 0.2-20 .mu.m or greater, and are
obtained by bulk synthesis of long nanotubes of transition metal
chalcogenides from a transition metal material, water vapor and
H.sub.2X gas or H.sub.2 gas and X vapor, wherein X is S, Se or Te
and synthesis, comprising: a) either heating a transition metal
material in the presence of water vapor in a vacuum apparatus or
electron beam evaporating a transition metal material in the
presence of water vapor, at a preselected pressure, to obtain
nanoparticles of the transition metal oxide as long as 0.3 microns;
and b) annealing the transition metal oxide nanoparticles obtained
in step (a) in a mild reducing atmosphere with a H.sub.2X gas or
H.sub.2 gas and X vapor, wherein X is S, Se or Te, at a suitable
temperature, thus obtaining said long nanotubes of the transition
metal chalcogenide, said nanotubes.
2. A method according to claim 1, wherein said transition metal
chalcogenide is WS.sub.2 and/or WSe.sub.2.
Description
[0001] The present application is a division of copending parent
application Ser. No. 09/959,664 filed Jan. 28, 2002, which itself
is the national stage under 35 U.S.C. 371 of the international
application PCT/IL00/00251, filed May 2, 2000, which designated the
United States, and which international application was published
under PCT Article 21(2) in the English language.
FIELD OF THE INVENTION
[0002] The present invention relates to methods for the bulk
synthesis of long nanotubes of transition metal chalcogenides and
to methods for preparation of tips for scanning probe microscopy
from said long nanotubes.
BACKGROUND OF THE INVENTION
[0003] The discovery of carbon nanotubes in 1991 (Iijima, 1991) has
generated intense experimental and theoretical interest over the
last few years because of their unusual geometry and physical
properties. Besides the original carbon structure, similar
inorganic structures have also emerged: BN (Chopra et al., 1995),
V.sub.2O.sub.5 (Ajayan et al., 1995), MoS.sub.2 (Feldman et al.,
1995; Remskar et al., 1998 and 1999a; Zelensky et al., 1998); and
WS.sub.2 (Tenne et al., 1992; Remskar et al., 1998, 1999a and
1999b). The reason for such an analogy between the pure carbon and
inorganic structures is based on the fact that they all stem from
lamellar (2D) compounds.
[0004] The case of the layered transition-metal dichal-cogenides
(WS.sub.2 and MoS.sub.2) was the first example of such an analogy.
Indeed, in 1992, IF (inorganic fullerene-like) structures and
nanotubes of WS.sub.2 were reported by the laboratory of the
present inventors (Tenne et al., 1992; EP 0580019; U.S. Pat. No.
5,958,358), followed shortly by similar results on MoS.sub.2
(Margulis et al., 1993) and the respective selenides (Hershfinkel
et al., 1994). However, it is noteworthy to underline that the
samples contained minute amounts of IF particles. Instead, most of
the samples consisted of WS.sub.2 platelets (2H-WS.sub.2). The
nanotubes were relatively rare and constituted even a smaller
fraction of the total composition. Besides this statistical fact,
the reproducibility of the nanotubes growth was rather poor.
Consequently, a lot of effort has been recently devoted to the
study of nanotubes from new other related materials.
[0005] None of the methods described recently for the synthesis of
WS.sub.2 and MoS.sub.2 nanotubes mentioned above permit synthesis
of bulk quantities of a single phase of inorganic nanotubes and
mostly perfect inorganic nanotubes to be obtained.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide methods
for the bulk synthesis of inorganic nanotubes, particularly of long
nanotubes of transition metal chalcogenides.
[0007] In one aspect, the invention relates to a two-step method
for bulk synthesis of long nanotubes of transition metal
chalcogenides from a transition metal material, water vapor and a
H.sub.2X gas or H.sub.2 gas and X vapor, wherein X is S, Se or Te,
said method comprising:
[0008] a) either heating a transition metal material in the
presence of water vapor in a vacuum apparatus or electron beam
evaporating a transition metal material in the presence of water
vapor, at a suitable pressure, to obtain nanoparticles of the
transition metal oxide as long as 0.3 microns; and
[0009] b) annealing the transition metal oxide nanoparticles
obtained in step (a) in a mild reducing atmosphere with a H.sub.2X
gas or H.sub.2 gas and X vapor, wherein X is S, Se or Te, at a
suitable temperature, in order to obtain long nanotubes of the
transition metal chalcogenide.
[0010] In alternative routes, in order to obtain larger nanotubes,
either a foil of the transition metal is heated in poor vacuum
conditions (e.g. 1 Torr) or nanoparticles of the transition metal
oxide as large as 0.3 microns of step (a) are further elongated, to
obtain transition metal oxide whiskers/nanoparticles as long as
10-20 microns or more, which are then annealed with the H.sub.2X
gas or with H.sub.2 gas and X vapor.
[0011] Thus, according to another embodiment, the invention relates
to a three-step method for bulk synthesis of long nanotubes of a
transition metal chalcogenide from a transition metal material,
water vapor and a H.sub.2X gas or with H.sub.2 gas and X vapor,
wherein X is S, Se or Te, said method comprising:
[0012] a) either heating a transition metal material in the
presence of water vapor in a vacuum apparatus or electron beam
evaporating a transition metal material in the presence of water
vapor, at a suitable pressure, to obtain nanoparticles of the
transition metal oxide as large as 0.3 microns;
[0013] b) elongating the transition metal oxide nanoparticles as
large as 0.3 microns of step (a) to obtain nanoparticles as long as
20 microns or more; and
[0014] c) annealing the elongated transition metal oxide
nanoparticles obtained in step (b) in a mild reducing atmosphere
with a H.sub.2X gas or with H.sub.2 gas and X vapor, wherein X is
S, Se or Te, at a suitable temperature, in order to obtain long
nanotubes of the transition metal chalcogenide.
[0015] The elongation of the transition metal oxide nanoparticles
in step (b) can be carried out by any known and suitable method,
for example by heating the oxide under mild reducing conditions for
a few minutes such as for 5-30, preferably, 10 minutes, or by
electron beam irradiation of the oxide in high vacuum
conditions.
[0016] When a mixture of nanotubes of two different metal
chalcogenides is desired, for example metal sulfide and metal
selenide, the annealing step is carried out by alternating the
annealing atmosphere, for example, by alternating H.sub.2S and
H.sub.2Se gas or by alternating the S and Se vapors in the presence
of H.sub.2.
[0017] The nanotubes obtained by the methods of the invention are
perfect in shape and of high cristallinity and may be 0.2-20 micron
long or more. For the sake of convenience, the nanotubes of the
invention shorter than 0.5 microns are sometimes herein in the
specification referred to as "short" nanotubes to distinguish them
from the longer nanotubes.
[0018] The metal material may be the transition metal itself, a
mixture of or an alloy of two or more transition metals, a
substance comprising a transition metal, e.g. an oxide, and a
mixture of substances comprising two or more transition metals.
Examples of transition metals include, but are not limited to, Mo,
W, V, Zr, Hf, Pt, Re, Nb, Ta, Ti, and Ru. The electron beam
evaporation embodiment is more suitable for refractory transition
metals, e.g. Nb, V, Ta, Ti.
[0019] In one preferred embodiment, the invention relates to a
two-step method for bulk synthesis of long nanotubes of WS.sub.2
and/or WSe.sub.2 which comprises:
[0020] a) either heating W in the presence of water vapor in a
vacuum apparatus, or electron beam evaporating W or WO.sub.3 in the
presence of water vapor, at a pressure of 1-20, preferably 8-12,
Torr, thus obtaining WO.sub.3 nanoparticles as large as 0.3
microns; and
[0021] b) annealing the WO.sub.3 nanoparticles obtained in step (a)
in a mild reducing atmosphere with H.sub.2S or H.sub.2Se gas or
with H.sub.2 and S or Se vapor, or by alternating the annealing
atmosphere with H.sub.2S and H.sub.2Se or with H.sub.2 and S or Se
vapor, at 800-850.degree. C., preferably at 835-840.degree. C.,
thus obtaining relatively long and hollow WS.sub.2 and/or WSe.sub.2
nanotubes as long as 10 microns or more.
[0022] Longer WS.sub.2 and WSe.sub.2 nanotubes can be obtained when
in step (a) a W foil is heated in poor vacuum conditions, e.g. of 1
Torr, and the long tungsten oxide whiskers obtained are then
annealed sulfidized or selenized.
[0023] In another preferred embodiment, the invention relates to a
three-step method for bulk synthesis of long nanotubes of WS.sub.2
and/or WSe.sub.2 which comprises:
[0024] a) either heating W in the presence of water vapor in a
vacuum apparatus or electron beam evaporating W or WO.sub.3 in the
presence of water vapor, at a pressure of 1-20, preferably 8-12,
Torr, thus obtaining WO.sub.3 nanoparticles as large as 0.3
microns;
[0025] b) heating the WO.sub.3 nanoparticles as long as 0.3 microns
under mild reducing conditions at 800-850.degree. C., preferably at
835-840.degree. C., for about 10 minutes to obtain WO.sub.3
nanowhiskers as long as 10 microns; and
[0026] c) annealing the WO.sub.3 nanoparticles obtained in step (b)
in a mild reducing atmosphere with H.sub.2S or H.sub.2Se gas or
with H.sub.2 and S or Se vapor, or by alternating the annealing
atmosphere with H.sub.2S and H.sub.2Se or with H.sub.2 and S or Se
vapor, at 800-850.degree. C., preferably at 835-840.degree. C.,
thus obtaining relatively long and hollow WS.sub.2 and/or WSe.sub.2
nanotubes as long as 10 microns or more.
[0027] The mild reducing conditions for elongation of the oxide
nanoparticles in step (b) of the three-step method include, for
example, heating the oxide nanoparticles under the flow of H.sub.2
(0.05-1.0%)/N.sub.2 (99.95-99%)-110 ml/min gas stream for up to 10
minutes. Under these conditions, elongation of the oxide
nanoparticles is achieved. With higher amounts of H.sub.2, e.g.
above 5% H.sub.2, no elongation is obtained.
[0028] The mild reducing atmosphere for annealing the oxide
nanoparticles includes, for example, sulfidization or selenization
under the flow of H.sub.2 (1%)/N.sub.2 (99%)-110 ml/min and
H.sub.2S-1 ml/min. If the flow of H.sub.2S is lower than 1 ml/min,
then longer transition metal chalcogenide nanotubes are
obtained.
[0029] The invention further relates to long transition metal
chalcogenide nanotubes as long as 20 microns or more obtained by a
method of the invention. In one embodiment, said transition metal
chalcogenide is WS.sub.2 and/or WSe.sub.2.
[0030] The invention additionally relates to tips for scanning
probe microscopy (both STM and FTM) and methods for preparation of
such tips from the long transition metal chalcogenide nanotubes
obtained by the methods of the invention, comprising:
[0031] a) transferring adhesive from carbon tape to a
microfabricated Si tip; and
[0032] b) pulling off bundles of said transition metal chalcogenide
long nanotubes with this tip from a mat of nanotubes prepared on a
different area of the tape.
BRIEF DESCRIPTION OF THE FIGURES
[0033] FIG. 1a is a typical TEM image of WO.sub.3-x needles 40 nm
in length (the scale bar represents 100 nm); FIG. 1b shows hollow
WS.sub.2 nanotubes obtained from the asymmetric oxide nanoparticles
shown in FIG. 1a (the scale bar represents 20 nm).
[0034] FIGS. 2A-2B are SEM micrographs of a mat of WS.sub.2
nanotubes (6-7 sulfide layers) with an oxide core at 2 different
magnifications (2A--50 .mu.m; 2B--10 .mu.m). The inset in FIG. 2A
shows a typical hollow nanotube obtained after completion of the
sulfidization process (the scale bar represents 20 nm). Distance
between each two WS.sub.2 layers (fringes) is 0.62 nm.
[0035] FIG. 3 shows Raman spectra of: curve A. WO.sub.3; curve B.
WO.sub.2.9 long nanowhiskers; curve C. WO.sub.2 powder; curve D.
WS.sub.2 nanotubes. Excitation source used: 1 mW 632.8 nm laser
beam.
[0036] FIGS. 4a-c show TEM micrographs and the corresponding ED
patterns of tungsten oxide (WO.sub.3) particles synthesized at
different water vapor pressures: (a) P.sub.H2O=5 Torr (scale
bar--50 nm); (b) P.sub.H2O=12 Torr (scale bar--50 nm); (c)
P.sub.H2O20 Torr (scale bar-20 nm).
[0037] FIG. 5 shows TEM micrograph of "short" WS.sub.2 nanotubes
with oxide in the core.
[0038] FIG. 6a shows SEM micrographs of long hollow or oxide-free
WS.sub.2 nanotubes at two different magnifications (upper figure--2
.mu.m; lower figure--500 nm). FIG. 6b shows TEM micrograph of long
WS.sub.2 nanotubes (scale bar--1 .mu.m).
[0039] FIG. 7 shows TEM micrographs of: (7a) apex of one WS.sub.2
nanotube synthesized by the "three-step method"; (7b) nanotube
walls, which contain several defects.
[0040] FIGS. 8A-8C depict a schematic illustration of the growth
process of the encapsulated sulfide/oxide nanowhisker. 8A.
Initialization of the sulfidization process of the asymmetric oxide
nanoparticle; 8B. Growth of a long sulfide/oxide encapsulated
nanowhisker. 8C. [010] is the growth axis of the oxide
whiskers.
[0041] FIGS. 9A-9D show a comparison of microfabricated sharp Si
tip (NT-MDT) and WS.sub.2 nanotube tip for measuring deep
structures (nominal 670 nm) of varying linewidth. 9A and 9B
microfabricated S.sub.1 and WS.sub.2 nanotube tips, respectively,
on 350 nm linewidth structure; 9C and 9D--600 nm linewidth
structure. Note that in case of FIG. 9A, the Si tip cannot reach
the bottom of the trench, while the nanotube in FIG. 9B is able to
follow the trench contour faithfully.
DETAILED DESCRIPTION OF THE INVENTION
[0042] According to the present invention, the synthesis of a pure
phase of very long and hollow WS.sub.2 nanotubes from short but
asymmetric oxide nanoparticles was achieved. In this process, the
oxide nanoparticle grows along its longest axis; and subsequently
its outermost layer is being sulfidized, while the growing oxide
tip remains uncoated as long as the nanowhisker continues to grow.
Thereafter, a superlattice of {001}R crystal shear is formed in the
oxide core, and the diffusion controlled sulfidization of the oxide
core is completed within 60-120 min.
[0043] In one embodiment, the synthesis of WS.sub.2 nanotubes
involves two steps, each one carried-out in a separate reactor:
first, W is heated in the presence of water vapor in a vacuum
apparatus or W or WO.sub.3 is electron beam evaporated in the
presence of water vapor, at a pressure of 1-20 Torr, and then the
thus obtained WO.sub.3 nanoparticles as large as 0.3 microns are
then reacted with H.sub.2S gas under mild reducing conditions.
[0044] In order to obtain longer WS.sub.2 nanotubes, a three-step
can be carried out in which an intermediate step is added for the
elongation of the WO.sub.3 nanoparticles as large as 0.3 microns
before they are reacted with H.sub.2S gas under mild reducing
conditions.
[0045] The simultaneous reduction and sulfidization reactions were
found to be essential for the encapsulation process, which is the
key step in the formation of nested fullerene-like WS.sub.2
structures from oxide nanoparticles (Feldman et al., 1998). While
sulfidization of the sulfide/oxide composite nanoparticle proceeds,
more sulfide layers are being added from the outside inwards.
Concomitantly, the remaining oxide core is further reduced and
gradually transforms into an ordered superlattice of {001}R CS
planes. These planes, which stretch along the whisker's growth's
axis can be easily observed by TEM since they present strong
contrast modulation. This microscopic structure is a direct
manifestation of the reduction process, which affects the
homologous series of tungsten suboxides phases--W.sub.nO.sub.3n-1
(Miyano et al., 1983).
[0046] A great advantage of the process of the present invention is
the absence of almost any contaminant or byproduct. It is also
remarkable that no catalyst, which must be separated and dislodged
from the nanotube mat at the end of the growth, is necessary in the
current process. Therefore, tedious purification steps to isolate
the nanotubes, which are time consuming and expensive, are not
required once the process is completed.
[0047] Similar conditions as described herein for the bulk
synthesis of long WS.sub.2 nanotubes can be used for carrying out
the bulk synthesis of other transition metal chalcogenide
nanotubes.
[0048] The transition metal chalcogenide nanotubes of the invention
can be used for the preparation of tips for scanning probe
microscopy by methods well-known in the art such as the procedure
of Dai et al., 1996. Using these tips, images of high aspect ratio
replica and evaporated Ti films with sharp asperities became
feasible, which could not be achieved with commercially available
sharp Si tips.
[0049] The invention will now be illustrated by the following
non-limiting Examples.
EXAMPLES
Example 1
Synthesis of WS.sub.2 Nanotubes (Experiment A)
[0050] 1a. Synthesis of Precursor Nanoparticles of WO.sub.2.9
[0051] In the first step (stage I), a powder consisting of
asymmetric oxide nanoparticles of ca. 10-30 nm in diameter and a
length of 40-300 nm is produced in a high vacuum evaporator. After
pumping to .about.10.sup.-4 Torr, water vapor from an external
reservoir is introduced through a needle valve, while pumping with
a rotary vane pump so that the pressure can be regulated to any
desired value up to the vapor pressure of water at room
temperature, ca. 20 Torr. A tungsten filament is heated to
1600.+-.20.degree. C. The water molecules react with the hot
tungsten filament and produce (WO.sub.3).sub.n clusters which
condense on the walls of the bell jar or, alternatively, onto a
water-cooled copper surface. If the water vapor pressure is
maintained in the pressure range 8-12 Torr, crystalline oxide
nanoparticles with an asymmetric shape are produced. A transmission
electron microscope (TEM) image of a typical batch of nanoparticles
produced at 12 Torr, is shown in FIG. 1a. Lower vapor pressures
(<5 Torr) yield amorphous nanoparticles of a non-defined
(spaghetti-like) or spherical shape. It is also noted that the
powder color varies with the water vapor pressure and is an
excellent indicator for the deviation from stoichiometric
yellow-green WO.sub.3 phase. Whereas at high vapor pressure
(.about.20 Torr) a powder in light blue color, which is identified
by electron diffraction (ED) as a mixture of WO.sub.3 and
WO.sub.2.9 is obtained, a deep blue WO.sub.2.9 phase is accrued
under lower water vapor pressure.
[0052] 1b. Synthesis of Long Nanotubes of WS.sub.2 (Two-step
Method)
[0053] The tungsten oxide powder of Example 1a was collected and 50
mg thereof were transferred to another reactor, in which
sulfidization under controlled temperature (835-840.degree. C.),
and N.sub.2/H.sub.2+H.sub.2S gas flow takes place. Sulfidization of
oxide nanocigars ca. 40 nm long under mild reducing conditions,
i.e. N.sub.2(99%)/H.sub.2(1%)-110 ml/min and H.sub.2S-1 ml/min,
lead to the formation (ca. 40 mg) of relatively long or "short"
nanotubes of WS.sub.2 (.about.0.2-0.5 .mu.m), as shown in FIG.
1b.
[0054] When the precursor oxide consisted of nanoparticles 100-300
nm in length (FIG. 1a), nanotubes as long as 10 microns were
obtained. FIG. 2 shows typical scanning electron microscope (SEM)
images of this phase at two different magnifications, showing a mat
of nanotubes consiting of 6 sulfide layers with an oxide core. The
formation of hollow WS.sub.2 nanotubes from oxide nanowhiskers was
followed by both SEM and TEM imaging. While no discrenible changes
in the overall shape of the nanoparticles could be observed by the
SEM, the detailed microscopic changes, during the oxide to sulfide
conversion, could be easily followed by the TEM work.
[0055] TEM micrograph of the apex of a long and hollow WS.sub.2
nanotube, obtained by this method, is shown in the inset of FIG.
2A. While the cross section of the nanotubes is quite similar to
that of the precursor nanoparticles, their length increases by a
factor of 20-40 during the sulfidization step. Note, however, that
about 10% of the nanotubes have an enlarged cross-section diameter
of app. 100 nm.
[0056] 1c. Elongation of Precursor Nanoparticles of WO.sub.2.9
(Stage II of Three-step Method)
[0057] In another series of experiments (stage II), app. 5 mg of
the amorphous spaghetti-like oxide nanoparticles, were heated to
835-840.degree. C. under the flow of H.sub.2(1%)/N.sub.2(99%)-110
ml/min gas stream for 10 min. This process yielded a mat of oxide
nanowhiskers, typically 10 .mu.m long and 20-50 nm thick. About 80%
of the oxide nanowhiskers, obtained in this way, were thin (ca. 30
nm) and cylindrical in shape. The rest, did not have a circular but
rather a rectangular cross section (ca. 10.times.100 nm).
Furthermore, they were completely crystalline and the prevailing
oxide phase was identified by ED as WO.sub.2.9. TEM of the
nanowhiskers revealed {10.infin.}={001}R crystal shear (CS) planes
along the [010] growth axis of the nanowhiskers, but the CS planes
were not equally spaced occasionally however, needles with an
ordered CS superlattice were obtained. If the process was
overextended, complete reduction of the oxide into tungsten metal
nanorods was observed.
[0058] 1d. Synthesis of Long Nanotubes of WS.sub.2 (Stage III of
the Three-step Method)
[0059] In the next step (stage III), the elongated oxide
nanowhiskers of 1c above were heated at 835-840.degree. C. under
the flow of a gas mixture consisting of H.sub.2S (2 ml/min) and
N.sub.2 (110 ml/min) for 120 min yielding about 4 mg of WS.sub.2
nanotubes with characteristics very similar to the one presented in
FIG. 2. Nonetheless, a substantial fraction (ca. 20%) of the
material, obtained in this way, formed non-perfectly closed
nanotubes. Selenization of the same oxide nanowhiskers lead to the
formation of very long WSe.sub.2 nanotubes, and mixed
WS.sub.2/WSe.sub.2 nanotubes were also prepared by alternating the
annealing atmosphere with H.sub.2S and H.sub.2Se.
[0060] 1e. Characterization of Precursor WO.sub.2.9 and of WS.sub.2
by Raman Spectra
[0061] The Raman spectra at different stages of the oxide
nanowhisker growth and reduction were measured and are shown in
FIG. 3. The spectrum of a pure WO.sub.3 powder (curve A) is in good
agreement with previous studies (Horsley et al., 1987). The signal
of the WO.sub.2.9 precursors, which are X-ray amorphous, consisted
of an intense background, indicating that these oxide nanoparticles
are indeed amorphous. Unfortunately, the strong absorption and
Raman scattering of the sulfide envelope hides the Raman pattern of
the oxide core, in the partially sulfidized nanowhiskers. In order
to gain some insight into the structure of the reduced oxide
nanowhiskers, its Raman spectrum was measured. Curve B shows a
spectrum of the 3-10 .mu.m long oxide nanowhiskers (prepared in
stage II), which have partially ordered shear structure {001}R. The
Raman spectrum of WO.sub.2, which was not reported hitherto, is
shown in curve C. Finally, the Raman spectrum of the WS.sub.2
nanotubes (prepared in stage III) coincides with that of
crystalline 2H--WS.sub.2 (curve D) (Frey et al., 1998).
[0062] The absence of data in the literature for the Raman of the
reduced WO.sub.3-x (0<x.ltoreq.1) phases probably reflects the
difficulty to prepare the pure phases of the different suboxides
(however vide infra). It is important to note, that although the
suboxide has a non-stoichiometric composition, it produces a
distinct Raman spectrum. The appearance of new peaks in the Raman
spectrum of the suboxide (curve B) reflects the appreciable
distortion in the WO.sub.6 octahedra in this phase. Using an
empirical formula described by Hardcastle et al., 1995, it is
possible to associate the 870 cm.sup.-1 mode in curve b with the
stretch mode of a 1.78 .ANG. W--O bond. This value compares
favorably with the calculated 1.77 .ANG. for one of W--O
bondlengths in the W.sub.3O.sub.8 structure. The new bands in the
W--O bending region (200-400 cm.sup.-1) are attributed to the fact
that each nanowhisker contains at least one of several possible
members of the W.sub.nO.sub.3n-1 homologous series, with different
CS distance. The most intense peak in the Raman spectra of WO.sub.2
(curve C) appears at 285 cm.sup.-1, and is assigned to the W--O--W
bending mode, which appears in 275 cm.sup.-1 for WO.sub.3 (curve
A). Thus, the shift of this band to a higher frequency is
attributed to the constrained W--O--W bending in the more compact
distorted-rutile structure of WO.sub.2. It is also important to
note that no Raman bands around 950 cm.sup.-1, indicative of
hydrated clusters, could be discerned.
[0063] Hence, the present Raman measurements strongly indicate the
formation of partially ordered CS planes in the reduced oxide
nanowhiskers (stage II). This superstructure is likely to be an
important intermediate stage in the formation of WS.sub.2
nanotubes.
Example 2
Synthesis of WS.sub.2 Nanotubes (Experiment B) 2a. Experimental
Section
[0064] (i) Synthesis of the WO.sub.3-x Particles
[0065] Tungsten suboxide particles (WO.sub.3-x) were produced by
heating a tungsten filament (model ME11 from the R. D. Mathis
company) in the presence of water vapor inside a vacuum chamber, by
the following procedure: once the vacuum in the bell-jar had
reached a value of 10.sup.-4 Torr, the filament was heated for a
few minutes in order to remove the superficial oxide layer. Water
vapor was then allowed to diffuse into the vacuum chamber through
an inlet, until the desired pressure was reached. The filament was
heated to around 1600.+-.20.degree. C., while the pressure in the
chamber was maintained constant during the evaporation process (a
few Torr). After a few minutes of evaporation, a blue powder
condensed on the bell-jar walls. The accrued powder consisted of
needle-like WO.sub.3-x particles (ca. 50 nm in length and 15 nm in
diameter) under a specific water vapor pressure.
[0066] NiCl.sub.2 or CoCl.sub.2 (2.times.10.sup.-3 M) salts were
dissolved in the water reservoir before each evaporation. The
nanoparticles produced in the presence of the transition-metal salt
appeared to be more crystalline than those obtained without the
addition of a salt, as shown by ED (electron diffraction).
[0067] (ii) Synthesis of the WS.sub.2 Nanotubes Starting From the
WO.sub.3-x Nanoparticles
[0068] The synthesis of the WS.sub.2 nanotubes starting from the
needle-like WO.sub.3-x particles was done in a reactor similar to
the one used for the synthesis of IF-WS.sub.2 particles (Feldman et
a., 1996, 1998). The principle of the synthesis is based on a
solid-gas reaction, where a small quantity (5 mg) of WO.sub.3-x
particles (solid) is heated to 840.degree. C. under the flow of
H.sub.2/N.sub.2 (forming gas)+H.sub.2S gas mixture. In order to
avoid cross-contamination between the different runs and minimize
memory effects, which can be attributed to the decomposition of
H.sub.2S and deposition of sulfur on the cold walls of the reactor,
flushing of the reactor (10 min) with N.sub.2 gas flow was
performed after each synthesis.
[0069] Samples were studied using a scanning electron microscope
(SEM) (Philips XL30-ESEM FEG), a transmission electron microscope
(TEM) (Philips CM 120 (120 keV)) and X-ray diffraction (XRD)
(Rigaku Rotaflex RU-200B) having Cu--K.alpha. anode. The electron
diffraction (ED) patterns were obtained on a high resolution
transmission electron microscope (HRTEM) (JEM-4000EX) operated at
400 kV. Ring patterns from TiCl were used as a calibration
reference standard for the ED patterns. The accuracy of the
d-spacings was estimated at .+-.0.005 nm.
[0070] 2b. Synthesis of Tungsten Oxide Needle-like Nanoparticles
(Stage I)
[0071] Three different values of water vapor pressure were
selected: P.sub.H2O=5, 12 and 20 Torr, the latter corresponding to
the thermodynamic limit of the water vapor pressure at room
temperature (22.degree. C.). The texture of all the batches
appeared to be more or less the same after a few minutes of
evaporation. However, a variation in the color of the powder, which
was collected on the walls of the bell-jar, was noticed. A color
range, which goes from dark blue for P.sub.H2O=5 Torr to light blue
for P.sub.H2O=20 Torr, was observed.
[0072] The water vapor pressure in the chamber apparently
influences the morphology and the stoichiometry of the
nanoparticles obtained by evaporation. For a low value (P.sub.H2O=5
Torr), the oxide nanoparticles did not have a well-defined
morphology (FIG. 4a). The ED pattern confirms that the powder is
completely amorphous (not shown). When the pressure was increased
(P.sub.H2O=12 Torr) the nanoparticles presented a cylindrical shape
and were crystalline. A typical batch is shown in FIG. 4b, where
the dimensions of the whiskers are typically around 50 nm in length
and 15 nm in diameter. For the thermodynamic limit of the water
pressure at room temperature (P.sub.H2O=20 Torr), a growth in both
directions (along the nanoparticle long axis and perpendicular to
it) led to the formation of needle-like particles with much smaller
aspect ratio and steps perpendicular to the long axis. The whiskers
are crystalline as could be evidenced from the ED pattern, which is
similar to the one observed for the particles produced at 12 Torr
(FIG. 4c).
[0073] The stoichiometry of the particles could not be easily
assigned by XRD for several reasons. First, most of the samples
were not sufficiently crystalline for generating well defined peaks
in the spectrum. Moreover, several nonstoichiometric tungsten oxide
phases have been reported in the literature and all of them exhibit
very similar patterns. Consequently, assigning the stoichiometry of
the concerned phase accurately from the XRD data, was rather
difficult. The measurement by electron diffraction of a bundle of
individual needle-like crystals was more informative in this case.
The values of the d.sub.hkl spacings were calculated for the
crystalline whiskers synthesized at P.sub.H2O=12 Torr and at
P.sub.H2O=20 Torr. Both sets of whiskers can be interpreted as
having an average substructure similar to that of the reported
tetragonal phase W.sub.20O.sub.58 (WO.sub.2.9) originally described
by Glemser et al. (1964) and the results are shown in Table 1. The
needles can be described according to a substructure of WO.sub.3
interspersed with defects attributable to random crystallographic
shear planes occurring either parallel to the needle axis or,
alternatively, at some angle to the beam direction as the needles
are viewed in the HRTEM. Further evidence of the randomness of the
defects occurring in the needles is given by the prominent diffuse
streaking that is often observed in ED patterns obtained from these
needles (Sloan et al., 1999). It is noteworthy to underline that,
whatever the pressure inside the chamber, the batches appeared to
be homogeneous in their morphology, providing needle-like particles
of relatively constant oxide stoichiometry for a given
preparation.
[0074] A detailed study of the conditions required for the
whisker's growth was then undertaken, the role of the water in this
process being examined first.
[0075] 2c. The role of Water in the Tungsten Oxide Whisker's
Growth
[0076] To study the role of water in the oxidation of the tungsten
filament, evaporations were performed with oxygen instead of water
vapor in the chamber. Indeed, oxidation of the tungsten filament
could be performed either with water vapor according to equation 1
below or with pure oxygen according to equation 2, both reactions
being exothermic in the conditions of the present measurements
(temperature of the filament: 1600.+-.20.degree. C., and pressure
in the chamber maintained at 12 Torr). The free energies of the
reactions were calculated using the data described in Horsley et
al., 1987, for STP (standard) conditions. 1 W ( s ) + 3 H 2 O ( g )
WO 3 ( s ) + 3 H 2 ( g ) , G ( 1873 K and P = 12 T ) = - 21 k J mol
- 1 [ 1 ] W ( s ) + 3 / 2 O 2 ( g ) WO 3 ( s ) , G ( 1873 K and P =
12 T ) = - 150.5 kJ mol - 1 [ 2 ]
[0077] To perform the evaporation with the same quantity of oxygen
as for the one performed in the presence of water vapor, the oxygen
pressure was maintained at P.sub.O2=6 Torr compared to P.sub.H2O=12
Torr (n.sub.O2=1/2n.sub.H2O). The resultant particles were 100%
spherical or faceted, typically 5 to 30 nm in diameter. The color
of the powder was light blue, which can be ascribed to a slight
reduction of the powder by traces of water still present in the
vacuum chamber. When the oxygen pressure was decreased, light blue
phases of spherical or faceted nanoparticles were observed as
well.
[0078] The absence of needle-like particles in presence of oxygen
in the chamber is indicative of the role played by hydrogen in
generating an asymmetric growth of the nanoparticles (see equations
1 and 2).
[0079] These findings allude to the fact that the needle's growth
consists of a two-step process occurring simultaneously on the hot
filament surface. The first step is the oxidation of the tungsten
filament, which leads to the formation of WO.sub.3 particles. In
the next step, reduction of these particles results in the
formation of WO.sub.3-x needle-like particles (eq 3).
WO.sub.3 (s)+H.sub.2 (g).fwdarw.WO.sub.3-x (g)+x H.sub.2O
(g)+(1-x)H.sub.2 (g) [3]
[0080] It is important to note that the direct reaction between
water vapor and the W filament is not the only plausible oxidation
route. Indeed two pathways could be contemplated for the oxidation
of W with water. The first one corresponds to the direct reaction
of water molecules with W atoms (eq 1). Alternatively, partial
water decomposition (see eq 4) leads to the oxidation of the hot
tungsten filament by liberated oxygen.
H.sub.2O (g).fwdarw.1/2O.sub.2 (g)+H.sub.2 (g), .DELTA.G.sub.(1873
K and P=12 T)+33.9 kJ mole.sup.-1 [4]
[0081] Regardless of whether the direct or indirect mechanism is
correct, H.sub.2 is a resultant product of both reactions. It is
therefore believed that hydrogen is involved in the production of
needle-like particles as opposed to the spherical ones, which are
obtained in the absence of hydrogen in the chamber.
[0082] 2d. Increasing the Needle-like Tungsten Oxide Particle
Length via High Temperature Reaction (Stage II)
[0083] Since hydrogen was found to be indispensable for the growth
of the needles (see 2c above), an alternative procedure for
promoting their growth under more controllable conditions was
pursued. The basic idea was to promote the uniaxial growth of the
short tungsten suboxide needles obtained in stage I under very low
hydrogen gas concentration. For that purpose, the needles were
placed in a reactor operating at around 840.degree. C. in a flow of
(H.sub.2/N.sub.2) gas mixture where the concentration of hydrogen
was progressively increased to 1% (stage II). It was believed that
the separation between the two reactions, i.e. formation of the
needle-like germs in the first place and their subsequent growth,
would afford a better control of the process, enabling more uniform
whiskers to be derived.
[0084] Experiments were performed with the three types of particles
synthesized in Example 2b above in stage I by evaporation at:
P.sub.H2O=5, 12 and 20 Torr. The results are summarized in Table
2a. The first point to emphasize is that the two sets of short
needle-like particles (evaporated at P.sub.H2O=12 and 20 Torr) in
contact with the gas mixture (1% H.sub.2/99% N.sub.2) are
transformed into long tungsten oxide nanowhiskers (several microns
in length) at 840.degree. C. This is quite a large elongation
considering the fact that the starting material consists of oxide
needles that are usually no longer than 50 nm. Moreover, the
amorphous oxide nanoparticles, which are shapeless (P.sub.H2O=5
Torr), are also converted into very long whiskers. Consequently,
the starting needle-like morphology is apparently not relevant for
inducing the growth of very long oxide whiskers during the
annealing (stage II). Indeed, it can be summarized that the lesser
the crystallinity of the precursor (tungsten suboxide) particles,
the thinner and longer are the microlength oxide nanowhiskers
obtained after stage II annealing. The most likely explanation for
this observation would be that a sublimation of a part of the
tungsten suboxide particles is followed by a transport of the
clusters in the gas and their condensation on some other tungsten
suboxide particles, which did not sublime. For example, amorphous
nanoparticles smaller than say 5 nm are likely to be more volatile
than the larger nanoparticles (Ostwald ripening), a point discussed
in greater detail hereinbelow.
[0085] The second point to underline is the influence of the gas
flow-rate (F) on the morphology of the particles, which is
equivalent to a change in the pressure, (especially the partial
pressure of hydrogen). This is particularly well illustrated in the
series of measurements done with the particles synthesized at
P.sub.H2O=12 Torr. In that case, a low flow-rate (55 cm.sup.3
min.sup.-1) generates spherical particles while a higher one
(.gtoreq.110 cm.sup.3 min.sup.-1) brings about the growth of long
nanowhiskers. The trend is the same whatever the starting tungsten
suboxide precursor. The flow-rate also influences the thickness of
the particles, as shown by the experiment performed with the
precursor synthesized at P.sub.H2O=5 Torr. Indeed, in the
particular case where the limit of the flow-rate allowed by the
equipment was reached (300 cm.sup.3 min.sup.-1), a majority of thin
oxide nanowhiskers (10-20 nm in diameter) were observed instead of
the usual mixture of thin and thick nanowhiskers (diameters up to
100 nm).
[0086] Since the amount of the starting oxide whiskers used for
each experiment was quite similar from one batch to another
(.congruent.5 mg), the differences observed by changing the
flow-rates could be attributed to either of two parameters: the
partial flow-rate of hydrogen in the reactor (partial pressure of
hydrogen) or the total gas flow-rate (total pressure). This point
is particularly well expressed by the experiment performed with the
particles synthesized at P.sub.H2O=12 Torr (stage I) and fired
under a gas flow-rate of 55 cm.sup.3 min.sup.-1 (stage II). In that
case, whatever the fomation mechanism, the flow is so slow that
spherical or faceted nanoparticles are formed. Even the original
needle-like morphology is not preserved in such circumstances. Note
also that the gas flow-rate may influence the apparent temperature
of the gas mixture.
[0087] It can thus be concluded that the higher the flow-rate, the
higher is the driving force to generate long and thin oxide
nanowhiskers.
[0088] The morphology of the oxide whiskers obtained after
annealing the particles evaporated at P.sub.H2O=20 Torr are pretty
different from the previous results, since the length and the
thickness appear to be systematically limited to approximately one
micron and 50-100 nm, respectively. These results indicate that, in
such a case, the initial thickness and perhaps the degree of
cristallinity of the needle-like nanoparticles dictates the final
thickness of the elongated nanowhisker after annealing. Particular
regard should be paid to the apex of these whiskers as they
routinely formed perfect ninety-degree heads following annealing.
This head morphology excludes a vapor-liquid-solid (VLS) growth
mode as a plausible growth mechanism.
[0089] 2e. Influence of the Hydrogen Concentration on the
Elongation Process of the Tungsten Oxide Whiskers
[0090] This point was tested by varying the hydrogen concentration
in the gas mixture. Indeed, by adding extra N.sub.2 gas, the
hydrogen concentration was diluted from 1% to approximately 0.2%,
keeping the total flow-rate constant. The annealing experiments
(stage II) were performed with the precursor synthesized at
P.sub.H2O=12 Torr (stage I). These results are shown in Table
2b.
[0091] The first noticeable observation is that the morphology of
the resultant particles of two different batches annealed at the
same total flow-rate (F.sub.Tot) but at a different partial
flow-rate of H.sub.2 (F.sub.H2), is different. In parallel, for two
experiments, in which annealing was done with the same value of
F.sub.H2 but with two different values of the total flow-rate (i.e.
by varying the nitrogen gas flow-rate), a slight morphological
difference was observed. It is evident that both parameters
(F.sub.H2 and F.sub.Tot) influence the morphology of the particles
(stage II), as it was previously found for the case of the tungsten
filament evaporated in contact with water vapor in the chamber
(stage I).
[0092] Besides this consideration, it is important to note that
this set of experiments was also a useful means of determining the
minimum concentration of hydrogen required for providing the
elongation of the whiskers. Globally, it appears that decreasing
the concentration of hydrogen to 0.2% did not change drastically
the morphology of the particles, which consists of long oxide
whiskers >1 .mu.m (Table 2b). It is noteworthy to underline the
fact that the hydrogen concentration should be adjusted for the
given amount of WO.sub.3-x particles. Indeed, the ratio between the
quantities of hydrogen and the starting WO.sub.3-x powder must be
kept constant in order to get the same kind of morphology during
the annealing (stage II).
[0093] From this last experiment it emerges that a low
concentration of hydrogen (0.2%) is sufficient for inducing the
elongation process of the oxide whiskers. Furthermore, it suggests
that the sublimed phase involved in the process has a stoichiometry
very close to the one of the starting precursor (WO.sub.2.9).
[0094] As a conclusion of these experiments, two key parameters for
inducing the oxide whisker's growth can therefore be discerned
during stage II annealing: the total gas flow (P.sub.Tot) and the
partial flow of hydrogen (P.sub.H2). These two factors are probably
involved in the synthesis of the WS.sub.2 nanotubes as well,
starting from the short WO.sub.3-x whiskers.
[0095] 2f. Synthesis of WS.sub.2 Nanotubes Starting From the Short
Oxide Whisker Precursor (Stages I+III)
[0096] The main process of the WS.sub.2 nanotubes synthesis
consists of sulfidizing the tungsten suboxide powder in a gas
mixture which is composed of H.sub.2/N.sub.2 and H.sub.2S, where
H.sub.2 plays the role of the reducer and H.sub.2S is the
sulfidizing agent according to equation 5 (stage III):
WO.sub.3-x+(1-x)H.sub.2+2H.sub.2S.fwdarw.WS.sub.2+(3-x)H.sub.2O
[5]
[0097] Since the growth process of the sulfide proceeds from
outside in, sulfur atoms have to cross the already existing compact
layers of sulfide and therefore the oxide to sulfide conversion is
diffusion controlled. In this way, after a few hours of reaction,
all the W--O bonds of the starting material are converted into W--S
bonds, leading to hollow structures without a substantial
morphological change. Furthermore, since the density of WO.sub.3
(.rho.=7.16 g. cm.sup.-3) and WS.sub.2 (.rho.=7.5 g. cm.sup.-3) are
quite similar, the original structure of WO.sub.3 (and therefore
WO.sub.3-x) is preserved throughout the reaction as was the case
for the IF nanoparticles starting with quasi-spherical particles of
WO.sub.3.
[0098] Short WO.sub.3-x needle-like particles, produced by
evaporation at P.sub.H2O=12 Torr in stage I, were placed in a
reducing/sulfidizing atmosphere as described previously. In order
to understand which factors are responsible for the morphology of
the converted sulfidized samples, only one parameter amongst three
was changed at a time: the flow-rates of H.sub.2/N.sub.2; H.sub.2S
and the hydrogen concentration in the gas mixture. In all these
experiments the temperature was maintained at 840.degree. C. The
data from these experiments are summarized in Table 3. Each Table
(3a, 3b and 3c) contains experiments in which one parameter was
changed at a time. Comparisons could therefore be done inside each
set of experiments and between them.
[0099] When a gas mixture with 5% hydrogen was used (Table 3a),
most of the needle batches had similar morphologies irrespective of
the flow-rate ratio (F.sub.H2/N2/F.sub.H2S). A TEM picture of a
typical bundle of short nanotubes stemming from those batches is
presented in FIG. 1b and FIG. 5. The particles are hollow (FIG. 1b)
or with some remaining oxide (FIG. 5) and the WS.sub.2 layers
contain very few defects. The apexes of the tubes are quite
perfectly closed. The elongation of the WO.sub.3-x precursors (50
nm in length and 15 nm in diameter) is not very pronounced in this
case. In contrast, when the ratio F.sub.H2/F.sub.H2S was very high,
long nanotubes of several microns in length could be discerned in
the samples amongst bundles of short nanotubes. However, the
formation of long nanotubes was always accompanied by the presence
of metallic tungsten in their cores and, in several cases,
spherical nanoparticles of tungsten were found. Also, the number of
WS.sub.2 layers was rather small in this case. This is attributed
to fast reduction of the tungsten oxide core to the pure metal and
subsequently to the slow diffusion of sulfur through the compact
metallic core (Margulis et al., 1993). Furthermore, a wide
size-distribution amongst the long nanotubes was found. Indeed, in
such conditions, two types of nanotubes were present: "thin
nanotubes", with a typical diameter of about 20 nm and the "thick"
ones, which could reach a diameter up to 100 nm.
[0100] The variety of morphologies which appear by varying the
flow-rate of forming gas (H.sub.2/N.sub.2) and H.sub.2S shows that
the ratio between the two gases is essential for determining the
final shape of the sulfidized nanotubes.
[0101] More precisely, when the ratio F.sub.H2/F.sub.H2S exceeds
the value of ca. 10, either tungsten particles or nanotubes
containing a tungsten core, start to appear. In that case, the
hydrogen concentration in the reactor is so high compared to that
of sulfur (for the amount of precursor taken), that the tungsten
suboxide particles (WO.sub.3-x) are reduced almost instantaneously
into tungsten. This is another manifestation of the competition
which occurs between the reduction and the sulfidization processes.
To avoid such an unwieldy situation, one has to operate in a
specific ratio with F.sub.H2/F.sub.H2S.ltoreq.10. When the
concentration of H.sub.2 in the forming gas was about 5%, short
nanotubes were produced in the range
1.4.ltoreq.F.sub.H2/F.sub.H2S.ltoreq- .11 (FIG. 5) and long ones
were observed for a ratio F.sub.H2/F.sub.H2S above 11 (not
shown).
[0102] Another aspect for the influence of the flow-rate ratio
F.sub.H2/F.sub.H2S on the nanotubes morphology is illustrated in
the experiments where no H.sub.2S was added to the system at all.
In this case, the reactor was not flushed with N.sub.2 prior to the
experiment. Here traces of sulfur, which remained on the reactor
walls from the previous experiment, led to the formation of long
nanotubes (Table 3a). This point emphasizes the fact that the ratio
F.sub.H2/F.sub.H2S is essential for the final morphology of the
sulfidized particles.
[0103] When the hydrogen concentration in the forming gas was
lowered to 1% instead of 5%, the factor F.sub.H2/F.sub.H2S appeared
not to be the only one responsible for the morphological changes.
Indeed, two different batches, for which the ratio
F.sub.H2/F.sub.H2S was kept constant (Table 3b: batches 3 and 6),
gave two discernable morphologies. Besides that, careful
examination of the data revealed that, decreasing the hydrogen
concentration in the gas mixture leads frequently to the formation
of long nanotubes instead of the usual short ones. This trend was
even more pronounced in experiments performed with an extremely low
hydrogen concentration (less than 1%--see Table 3c). As a matter of
fact, all the batches performed with hydrogen concentration below
1% led to the growth of either a mixture of short and long
nanotubes (not shown) or to almost pure phases of long nanotubes
(see FIGS. 6a, 6b). The results of Table 3 lead to the conclusion
that the appearance of long nanotubes depends on the ratio between
the flow-rate of hydrogen and the total flow-rate of gases
(F.sub.H2/F.sub.Tot).
[0104] It emerges therefore, that in order to achieve the formation
of long nanotubes, two flow-rate ratios have to be carefully
controlled: the ratio F.sub.H2/F.sub.H2S and F.sub.H2/F.sub.Tot.
The conditions required for providing long nanotubes as a majority
phase in a reproducible manner are consequently the following:
0.5.ltoreq.F.sub.H2/F.sub.H2S.ltoreq.4.5 and
0.002.ltoreq.F.sub.H2/F.sub.T- ot.ltoreq.0.007
[0105] To obtain homogeneous phases consisting of purely long
nanotubes without tungsten in their core, the conditions are even
more restrictive:
1.ltoreq.F.sub.H2/F.sub.H2S.ltoreq.2.2 and
0.005.ltoreq.F.sub.H2/F.sub.Tot- .ltoreq.0.007
[0106] It emerges from all these results that a careful control of
the synthesis parameters leads to a specific and desireable
morphology of the nanotubes.
[0107] 2g. Synthesis of WS.sub.2 and WSe.sub.2 Nanotubes Starting
With the Elongated Oxide Whiskers (Stages I+II+III)
[0108] The purpose of this last study was to explore the
possibility to synthesize long WS.sub.2 nanotubes from the already
existing long oxide nanowhiskers obtained in stage II. The long
oxide nanowhiskers synthesized from the short whiskers (see Example
2b above) were placed in a reducing and sulfidizing atmosphere
without taking specific attention to the ratios F.sub.H2/F.sub.H2S
and F.sub.H2/F.sub.Tot. All the attempts led to the formation of
long nanotubes, although the degree of crystallinity of the
nanotubes was not perfect. The WS.sub.2 layers contained plenty of
defects (FIGS. 7a, 7b) and rather quite large proportion of the
nanotubes was not totally closed at their apex (not shown).
[0109] The two-step method of the invention may be more difficult
to control, but it gives very satisfactory results. The three-step
method of the invention lends itself to the synthesis of nanotubes
from related compounds, such as WSe.sub.2 or mixed
WS.sub.2/WSe.sub.2 using preprepared long oxide nanowhiskers as a
precursor.
[0110] Indeed, WSe.sub.2 nanotubes were prepared by heating
selenium ingot at 350.degree. C. downstream of the main reactor,
which was heated to 760.degree. C. Forming gas (1% H.sub.2/99%
N.sub.2-110 cm.sup.3 min.sup.-1) was provided in this case. The
resulting WSe.sub.2 nanotubes were quite perfect in shape.
Example 3
Growth of the WS.sub.2 Nanotubes in the Two-step Process
[0111] From the present measurements, one can visualize the growth
process of the encapsulated nanowhisker as depicted in FIG. 8. At
the first instant of the reaction (FIG. 8A), the asymmetric
tungsten oxide nanoparticle reacts with H.sub.2S and forms a
protective tungsten disulfide monomolecular layer, prohibiting
coalescence of this nanoparticle with neighboring oxide
nanoparticles. Simultaneous condensation of (WO.sub.3).sub.n or
(WO.sub.3-x.H.sub.2O).sub.n clusters on the nanowhisker tip and
reduction by hydrogen gas leads to growth of the sulfide-coated
oxide nanowhisker. This process is schematically illustrated in
FIG. 8B. Note that during the gradual reduction of the oxide core,
the CS planes in the oxide phase rearrange and approach each other
until a stable oxide phase W.sub.3O.sub.8 is reached (Iguchi,
1978). In FIG. 8C, {010} is the growth axis of the W.sub.3O.sub.8
whiskers. This phase provides a sufficiently open structure for the
sulfidization to proceed until the entire oxide core is consumed.
Further reduction of the oxide core would bring the sulfidization
reaction into a halt (Margulis et al., 1993). Therefore, the
encapsulation of the oxide nanowhisker, which tames the reduction
of the core, allows for the gradual conversion of this nanoparticle
into a hollow WS.sub.2 nanotube.
[0112] Naturally, the elongation of the oxide nanowhiskers requires
a reservoir of (WO.sub.3).sub.n clusters in the vapor phase.
Conceivably, the vaporized oxide clusters react only very slowly
with the H.sub.2 and H.sub.2S gases, which would otherwise hamper
the rapid growth of the oxide nanowhisker. The termination of the
nanowhisker growth occurs when the source of tungsten oxide is
depleted and the vapor pressure of the oxide clusters diminishes
below a critical value. In this case the simultaneous reaction of
tip growth/reduction and sulfidization cannot be maintained and the
outer sulfide layer of the encapsulate completely enfolds the oxide
tip. In fact, this is the reason that the WS.sub.2 nanotubes are
almost the sole phase comprising the mat of FIGS. 2 and 6. This
mechanism entails that part of the oxide nanoparticles
predominantly elongate through tip growth, while the rest of the
oxide nanoparticles furnish the required vapor pressure for the tip
growth of the former population, and they slowly diminish in size
(Ostwald ripening). The oxide vapor can not condense and stick onto
the sulfide wall of the encapsulated nanowhiskers and therefore no
thickening of the nanotubes or their bifurcation, is observed.
Example 4
WS.sub.2 Nanotubes as Tips for Scanning Probe Microscopy
[0113] WS.sub.2 nanotubes were attached to microfabricated tips of
an atomic force microscope (AFM) by transferring adhesive from
carbon tape to a Si tip, then pulling off nanotube bundles with
this tip from a mat of nanotubes prepared on a different area of
the tape (Dai et al., 1996). A portion of this mat was glued to the
Si tip, of which the longest nanotube served now as the new tip.
Scans on a Ti tip calibrator (Westra et al., 1995) and subsequent
blind reconstruction (using algorithm developed by A. Efimov,
obtainable at http://www.siliconmdt.com) of tip shape gave tip
width of 16 nm for the last 100 nm of the tip length. In order to
demonstrate the capabilities of these tips for investigating deep
and narrow structures, they were used to image a line structure of
depth 670 nm and varying linewidth. As seen in FIG. 9, the nanotube
tips perform significantly better than microfabricated sharp Si
tips. While the WS.sub.2 nanotube tip follows the contour of even
the finest replica and reaches its bottom, the commercial Si tip is
unable to delineate the replica correctly due to its slanted edge.
Also, the Si tip is unable to follow the sample contour very
smoothly (see for example FIG. 9C), because the surface of this tip
is not passivated and therefore strong interaction with the
substrate at close proximity is unavoidable. Thinner or single
walled nanotubes may not be useful for such applications because of
their small spring constant toward bending. Also, due to its
sandwich S--W--S structure, the WS.sub.2 nanotubes are probably
stiffer than their carbon analogs. In contrast to carbon nanotubes,
the present nanotubes can be easily sensitized by visible and
infra-red light and therefore show promise as a selective probe for
nanophotolithography.
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1TABLE 1 Comparative d-spacing data between the needle-like
precursors and the tetragonal WO.sub.2.9 reported by Glemser et
al., 1964. The d.sub.hkl-spacings were obtained from the ED ring
pattern of the oxide particles. A TiCl pattern was used as a
standard reference. Oxide precursors Tetragonal WO.sub.2.9 Irel
d.sub.hkl (.ANG.) Irel d.sub.hkl (.ANG.) hkl 100 3.752 100 3.74 110
20 3.206 20 3.10 101 80 2.640 80 2.65 200 30 2.184 30 2.20 201 --
10 2.02 211 30 1.878 30 1.88 220 10 1.703 10 1.78 300 60 1.558 60
1.67 310 50 1.153 50 1.53 311 -- 10 1.33 222 -- 10 1.25 330 -- 10
1.17 322
[0137]
2TABLE 2a P.sub.H2O Flow 1% H.sub.2/99% N.sub.2 Morphology of the
(Torr) (cm.sup.3 min.sup.-1) particles 5 110 L.sub.ox-t &
L.sub.ox-T (>1 .mu.m) 5 200 L.sub.ox-t & L.sub.ox-T
.multidot. (>>1 .mu.m) 5 300 L.sub.ox-t >> L.sub.ox-T
(>>1 .mu.m) 12 55 S + F 12 110 L.sub.ox-t & L.sub.ox-T
(>>1 .mu.m) 12 200 L.sub.ox-t & L.sub.ox-T (>>1
.mu.m) 20 110 L.sub.ox-T >> L.sub.ox-t (.congruent.1 .mu.m)
20 200 L.sub.ox-T >> L.sub.ox-t (.congruent.1 .mu.m) Table 2.
Influence of the hydrogen concentration on the morphology of the
particles: (a) mixture of 1% H.sub.2/99% N.sub.2. (b) mixture of
gases with less than 1% H.sub.2 in the overall gas mixture.
L.sub.ox-T denotes long oxide whiskers, which are thick (D up to
100 nm). L.sub.ox-t denotes long oxide whiskers, which are thin (D
.congruent. 10-20 nm). S denotes spherical particles. F denotes
faceted particles.
[0138]
3TABLE 2b Flow Flow P.sub.H2O 1% H.sub.2 N.sub.2 F.sub.tot F.sub.H2
% H.sub.2 = Morphology of (Torr) (cm.sup.3 min.sup.-1) (cm.sup.3
min.sup.-1) (cm.sup.3 min.sup.-1) (cm.sup.3 min.sup.-1)
F.sub.H2/F.sub.tot the particles 12 110 100 210 1.1 0.52 L.sub.ox-t
& L.sub.ox-T (>>1 .mu.m) 12 200 100 300 2 0.73 L.sub.ox-T
>> L.sub.ox-t (.congruent.1 .mu.m) 12 200 50 250 2 0.88
L.sub.ox-t & L.sub.ox-T (>>1 .mu.m) 12 200 20 220 2 1
L.sub.ox-t & L.sub.ox-T (>>1 .mu.m) 12 100 200 300 1 0.33
L.sub.ox-t & L.sub.ox-T (>>1 .mu.m) 12 50 200 250 0.5 0.2
L.sub.ox-T >> L.sub.ox-t (.congruent.1 .mu.m)
[0139]
4TABLE 3a Flow of 5% H.sub.2/95% Flow of N.sub.2 H.sub.2S F.sub.H2/
F.sub.H2/N2/ Morphology of the (cm.sup.3 min.sup.-1) (cm.sup.3
min.sup.-1) F.sub.H2S F.sub.H2S samples 110 4 1.375 27.5 Sh 110 2
2.75 55 Sh 110 1 5.5 110 Sh 110 0.5 11 220 Sh with W inside 110 0*
L.sub.T with W inside + W 200 2 5.5 110 Sh 200 1 10 200 Sh with W
inside 200 0.5 20 400 (L.sub.t & L.sub.T + Sh) with W inside 55
2 1.375 27.5 Sh Table 3. Influence of the flow-rate of the gases
and the ratio between them on the morphology of the sulfidized
samples for different concentrations of hydrogen in the gas
mixture: (a) 5% H.sub.2/95% N.sub.2. (b) 1% H.sub.2/99% N.sub.2.
(c) less than 1% H.sub.2/99% N.sub.2. All measurements have been
done with the starting WO.sub.3-x precursor produced at P.sub.H2O =
12 Torr. Sh and L denote short and long nanotubes, respectively.
L.sub.T long and thick nanotubes and L.sub.t long and thin
nanotubes. *synthesis done without addition of H.sub.2S but with
residual sulfur from the previous synthesis.
[0140]
5TABLE 3b Flow of 1% H.sub.2/99% Flow of N.sub.2 H.sub.2S F.sub.H2/
F.sub.H2/N2/ Morphology of the (cm.sup.3 min.sup.-1) (cm.sup.3
min.sup.-1) F.sub.H2S F.sub.H2S samples 110 2 0.55 55 Sh + 2H -
WS.sub.2 110 1 1.1 110 Sh + L.sub.t & L.sub.T + IF + 2H -
WS.sub.2 110 0.5 2.2 220 Sh 110 0.3 3.7 314 Non defined shape + IF
+ Sh 200 2 1 100 Bad encapsulation 200 1 2 200 Sh + very few L + IF
+ 2H - WS2 200 0.5 4 400 (Sh + L.sub.t & L.sub.T + IF) with W
inside 55 2 0.275 27.5 2H - WS.sub.2
[0141]
6TABLE 3c Flow of Flow of Flow of 1% H.sub.2/99% N.sub.2 N.sub.2
H.sub.2S % H.sub.2 = Morphology of (cm.sup.3 min.sup.-1) (cm.sup.3
min.sup.-1) (cm.sup.3 min.sup.-1) F.sub.H2/F.sub.H2S
F.sub.H2/F.sub.tot the samples 200 100 1 2 0.66 Sh + L.sub.t 110
100 1 1.1 0.52 Sh + L.sub.t 100 200 1 1 0.33 Sh with W inside +
L.sub.t & L.sub.T 50 200 1 0.5 0.2 Sh + L.sub.t 100 200 0.5 2
0.33 (Sh + L.sub.t & L.sub.T) with W inside 110 100 0.5 2.2
0.52 L.sub.T & L.sub.t + Sh 200 100 0.5 4 0.66 L.sub.T &
L.sub.t >> Sh with W inside 200 100 2 1 0.66 L.sub.t &
L.sub.T >> Sh 110 100 2 0.55 0.52 Sh >>
2H--WS.sub.2
* * * * *
References