Modified Sol-Gel Synthesis of Vanadium Oxide Nanocomposites ...

Mat. Res. Soc. Symp. Vol. 658 © 2001 Materials Research Society
Modified Sol-Gel Synthesis of Vanadium Oxide Nanocomposites Containing Surfactant
Ions, and Their Partial Removal
Arthur Dobley, Peter Y. Zavalij, Jürgen Schulte, and M. Stanley Whittingham
Chemistry Department and the Institute for Materials Research,
State University of New York at Binghamton,
Binghamton, New York 13902-6016, U.S.A.
ABSTRACT
Recently, there has been much interest in creating new layered transition metal oxides.
Vanadium oxides may be used as sorbents, catalysts, and cathodes in lithium batteries. The
modified sol-gel technique allows for some control towards the final structure of the compound.
Using this technique, a new layered vanadium oxide phosphate material, containing the
surfactant dodecylphosphate, has been synthesized. The compound was analyzed using powder
XRD, TGA, SEM, FTIR, TEM, and solid state NMR for both 51 V and 31 P.
V 2 O 3 (PO 4 C 12 H 25 ) 3 Na 2-x K x (H 2 O) 3.2 is the general formula of the layered product with an
interlayer spacing of 36.6 Å. The initial compound is composed of a vanadium oxide phosphate
layer sandwiched between two hydrocarbon layers. The synthesis, composition, and structure of
the initial compound will be discussed. Interestingly, when this compound is calcined to 400°C,
the structure changes and is possibly hexagonal. Preliminary results are presented on this
calcined material.
INTRODUCTION
Since the discovery at Mobil [1, 2] of MCM-41, a mesoporous structured aluminum
silicate, there has been great interest in extending the research to include transition metals [3, 4].
However, very little of this work has targeted vanadium oxides. This is surprising as many
vanadium oxide compounds are of particular interest as they have many potential commercial
applications such as molecular sieves, sorbents, catalysts, and energy storage devices.
Vanadium oxides have a particularly rich structural chemistry [5] and have also been found to
form a wide range of inorganic/organic materials [6]. Along with the quest of discovering new
materials analogous to MCM-41, new techniques are being used to synthesize these compounds.
One of the techniques is the modified sol-gel method. Sol-gel synthesis involves a sol (a fluid
colloidal system) turning into a gel (jelly-like mass with 3D system). This occurs by a metal
alkoxide undergoing hydrolysis and polymerization. Further drying yields a xerogel. Organics
present in the product make it a ‘modified’ sol-gel. Templating, done by surfactants, forms
micelles (shapes) in solution. Metal oxides can be fashioned to a particular shape (i.e. tunneled,
layered structures) around these micelles. The surfactants can then be removed to leave behind
the metal oxide with the desired nano-structure. Earlier work in our laboratory showed that
vanadium oxide surfactant materials were not mesoporous, despite TEM indications of a 40 Å
lattice, but rather were composed of Keggin-like vanadium oxide clusters [7, 8]. Single crystal
X-ray diffraction confirmed the presence of clusters and of organic/inorganic systems swellable
by a wide range of solvents [9].
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EXPERIMENTAL
A modified sol-gel method [10] was used to react a vanadium alkoxide with the surfactant
dodecylphosphate. The air sensitive reagents required the use of a double manifold system with
vacuum and argon gas lines, together with ground joint glassware. Also a means of cooling the
violent or exothermic reactions is needed, and water or slush baths were used. A reflux set up
was used with a cooling bath of ice water to react 5 grams of sodium metal (Aldrich) with 300
ml of isopropanol (Aldrich) in 300 ml of diethyl ether (Aldrich). After completion, the alcohol
and ether were removed by vacuum to leave the white needle like crystals of NaOCH(CH 3 ) 2 .
The following synthesis, of the vanadium alkoxide, was modified from that of Turevskaya [11].
About 250 ml of dry diethyl ether was added to the flask with the NaOCH(CH 3 ) 2 , and a dry ice
acetone bath was applied. Then 3.4 ml of VCl 4 (Aldrich) was added slowly via a syringe to the
reaction vessel. The resulting NaCl was removed by a fritted filter, and the ether was vacuumed
off to leave V[OCH(CH 3 ) 2 ] 4 . Vanadium (IV) isopropoxide was then reacted with acetylacetone
(also known as 2,4-pentanedione from Aldrich) with a 1:1 molar ratio in isopropanol.
Acetylacetone acts as a chelating agent to slow down the hydrolysis reactions of the metal
alkoxide. If hydrolysis occurs too fast, then amorphous phases will be obtained. The
isopropanol (iPrOH) was removed by vacuum. A 3.4 wt % aqueous solution of the surfactant
was prepared by dissolving dodecylphosphate (from Lancaster Synthesis Inc.) into pH 5 adjusted
water (with concentrated HCl). The 3.4 wt % value was due to the addition of extra solvent in
order to increase solubility. The reactants, in the molar ratio 1 V[OCH(CH 3 ) 2 ] 4 : 1 acetylacetone
(acac) : 1 surfactant :1 KCl were mixed in a flask for several hours. The contents of the flask
were then transferred to a beaker, oven heated at 80 o C for 5 days, washed with water, and then
oven dried at 120 o C for 1day. After drying, the solid crystalline product was black in color
suggesting partial reduction of the vanadium. This synthesis scheme is shown in Figure 1.
The material, upon drying, was a black hardened gel with small variations in color. After
grinding it became gray. Initial results from XRD indicated the presence of NaCl and KCl salts,
byproducts of the synthesis, which were removed by washing with reverse osmosis water.
Samples were characterized via X-ray powder diffraction on a Phillips PW3040-MPD
powder diffractometer using CuKα radiation, fitted with an Anton Parr heating stage model
HTK-10 calibrated to the m.p. of lead. Thermalgravimetric analysis, to determine water and
organic content as well as overall stability, was done on a Perkin Elmer TGA 7. Electron
Microprobe analyses and SEM images were obtained using a JEOL 2000 Electron Microprobe.
NMR analysis was performed on a Bruker AVANCE 600 MHz instrument set up for solids.
Concentrated phosphoric acid (aq) was used as a reference for 31 P NMR, and V 2 O 5 was used as
a reference for 51 V (which has a –310 ppm shift in relation to VOCl 3 which is set at 0 ppm) [12].
The FTIR instrument used was a Perkin Elmer 1600 with all of the samples being pressed into
KBr pellets. TEM was done on a Hitachi H-7000 Electron Microscope at 125,kV with samples
mounted on a lacey carbon coated copper grid after suspension in an ethanol mixture.
RESULTS
Initial Compound
The powder XRD pattern of the initial compound, shown in Figure 2(a) is consistent with a
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layered structure with approximate d values of 37Å, 18Å, 12Å. and 9.7Å. These observed peaks
are in the 00l direction only. This lattice spacing of 37Å is consistent with a bilayer of
surfactant molecules sandwiched between two sheets of a vanadium oxide phosphate. No
evidence for residual NaCl or KCl was observed after the washing.
Na(s) + isopropanol
0°C
NaOCH(CH 3 ) 2 + H 2 (g) + heat
vacuum
4 NaOCH(CH 3 ) 2 + VCl 4 + Et 2 O
-78 o C
V[OCH(CH 3 ) 2 ] 4 + 4 NaCl (s)
Filter salt, vacuum
V[OCH(CH 3 ) 2 ] 4 (s)
1 V[OCH(CH 3 ) 2 ] 4 (iPrOH) + 1 acetylacetone (acac)
V[OCH(CH 3 ) 2 ] 3 (acac) + HOCH(CH 3 ) 2
Remove iPrOH under vacuum
V[OCH(CH 3 ) 2 ] 3 (acac) + surfactant (aq) + KCl Å Product
(Heat 80 o C for 5 days, wash with water, 120 o C for 1day)
Figure 1. Synthesis scheme.
Relative Intensity
b
a
2 6 10 14 18
2 Theta
Figure 2. The XRD of the initial sample at (a) 25°C and (b) 400°C [13].
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100
90
Weight %
80
70
60
50
40
100 200 300 400 500 600
Temperature o C
Figure 3. TGA of initial sample undergoing calcination in oxygen [13].
Thermal gravimetric analysis (TGA) in oxygen (Figure 3) or nitrogen gave a similar curve.
This graph revealed a weight loss of around 10% for H 2 O, and about 45% for the hydrocarbon
chain of the surfactant. The TGA of the surfactant itself showed a steep weight loss around
200°C, which is also seen in the TGA of the sample.
The electron microprobe images showed a layered morphology for the material. For use as a
reference in the elemental analysis for the electron microprobe, VOPO 4 was synthesized [14]
and then furnace dried at 500°C for several days. VOPO 4 exhibited a ratio of 1.01 P to 1.00 V.
Elemental analysis, by WDS and EDS, of the initial compound showed only 4 elements with the
resulting atomic ratio of V 1 P 1.5 Na 0.4 K 0.3 . The standard deviation is large for Na and K so there is
probably more than what is written for the ratio given.
Solid state NMR analyses were done on the material for both 51 V and 31 P. Lapina et. al.
extensively investigated 51 V solid state NMR of vanadium compounds [12]. They discovered
that the coordination of vanadium oxides may be determined, together with the number of
adjacent vanadium polyhedra from the chemical shift anisotropy (csa) of the static solid state
NMR of 51 V.
Figure 4. The 51 V static
solid state NMR of the
initial sample showing the
chemical shift anisotropy.
The vertical line is caused
by the transmitter offset
[13].
-100
-300
-500
-700
ppm
-900
-1100
-1300
-1500
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The initial compound had a peak with a range of about 350 ppm for static solid state 51 V
NMR. This means a VO 4 tetrahedron has one of the oxygens shared with another vanadium
oxide tetrahedron, forming a V 2 O 7 group. A compound with a known structure containing V 2 O 7
groups was also run, and it had a similar range [15]. The MAS NMR for 51 V gave one peak
indicating 1 type of vanadium nuclei. Also performed on the sample was 31 P MAS NMR, which
indicated 2 types of phosphate nuclei with shifts of 0.6 and 25 ppm. The surfactant
dodecylphosphate was analyzed by 31 P MAS NMR, and showed no shift relative to phosphoric
acid. Phosphates are always tetrahedra, and here they form a chain with the vanadia tetrahedra.
FTIR was also used to help elucidate the structure. Vibrations near 2900 cm –1 for the
aliphatic C-H stretch and a P-O stretch at 1111 cm –1 [16] were seen. No vibrations were
observed at 3000 cm –1 for hydroxyl bonded to a P, nor was a peak seen for the O-H stretch for
a hydrogen bonded hydroxyl (which is also bonded to a P) at 2600 cm –1 [16]. The P=O stretch,
which was seen for the dodecylphosphate at 1239 cm –1 , was not seen in the material. So no
P=O or P-O-H bond vibrations were observed. Unfortunately some of the vanadium oxygen
bonds occur in the region where the P-O exhibits a strong wide band from 900-1200 cm –1 , so
they were not able to be observed.
TEM images showed a layered morphology with a repeat distance of 33 Å. The layers are
stacked on top of one another in what appears to be a random method, like sheets of paper
stacked, with the sheets at different orientations. Thus the crystals have order in one direction
only, as was seen in the XRD pattern. Several crystals were analyzed and all had a similar
layered appearance.
The proposed structure of the initial compound is a bilayer of surfactant molecules aligned
with their hydrophobic hydrocarbon ‘tails’ together, and their phosphate ‘heads’ sticking out
and bonded with a layer of vanadium oxide giving a 37Å interlayer distance. The experimental
evidence is consistent with the formula V 2 O 3 (PO 4 C 12 H 25 ) 3 Na 2-x K x (H 2 O) 3.2 . The hydrocarbon
chains are covalently bonded to the phosphate tetrahedra, and most likely in the ‘plane’ of the
tetrahedra plane (Figure 5). Sodium and potassium cations are bonded to the other oxygens of
the phosphate. The few water molecules can hydrogen bond to the edges of the polyhedra.
Figure 5. The structure of the initial compound with the hydrocarbon chains removed for
clarity; the dark tetrahedra are V, and the light tetrahedra are P [13].
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Calcined Material
Interestingly when the initial compound is heated in air to 400°C, the compound undergoes
a structural change. At this temperature the hydrocarbon portion of the surfactant has
combusted; the phosphate remains with the vanadium oxide. This material has phosphate
tetrahedra that are not bonded to hydrogens. The vanadia are tetrahedra that do not share any
oxygens with other vanadium atoms. The V to P ratio is 1 to 1 from electron microprobe
analyses. From the XRD pattern (Figure 2b) the two peaks can be indexed to a hexagonal unit
cell (37 Å (100), 22 Å (110)) and a = 43 Å. More evidence is needed for the elucidation of the
unit cell for this material. Further TEM analysis and other experiments will help determine the
structure of the calcined material.
ACKNOWLEDGEMENTS
We gratefully acknowledge the National Science Foundation for support of this work
through grant DMR-9810198.
We would like to thank David Kiemle, at the S.U.N.Y. College of Environmental Science
and Forestry, for use of the NMR instrument. Also we thank at Binghamton University, Henry
Eichelberger for use of the TEM, and Professor David Jenkins for use of the x-ray instrument.
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