Synthesis of sodium vanadate nanosized materials for electrochemical applications

Article abstract of DOI:10.1016/j.jpcs.2007.10.072

Sodium vanadate nanomaterials with the shape of belts and flakes were prepared by simply controlling the pH from the alkaline solution of V₂O₅. Nanostructured materials have grown from the sol evaporated from the pH-controlled soluti

Full text of DOI:10.1016/j.jpcs.2007.10.072

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 1261–1264
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Synthesis of sodium vanadate nanosized materials
for electrochemical applications
Ã
Ch.V. Subba Reddya, In-Hyeong Yeob, Sun-il Mhoa,
aDivision of Energy Systems Research, Ajou University, Suwon 443-749, Republic of Korea
bDepartment of Chemistry, Dongguk University, Seoul 100-715, Republic of Korea
Received 28 June 2007; received in revised form 25 October 2007; accepted 30 October 2007
Abstract
Sodium vanadate nanomaterials with the shape of belts and flakes were prepared by simply controlling the pH from the alkaline
solution of V2O5. Nanostructured materials have grown from the sol evaporated from the pH-controlled solutions. The dimensions of
the nanobelts are about 50–500 nm in width and 1–6 mm in length with rectangular flat tips with four sharp corners. Smooth facets
enclose the surfaces of the nanostructures and form the sharp edges. The structures were identified as Na1.1V3O7.9 and NaVO3 by an
XRD analysis. The cyclic voltammograms of the electrodes composed of the mixture of Na1.1V3O7.9 and NaVO3 has higher current
density than the electrodes made of the Na1.1V3O7.9 nanostructures.
r 2007 Elsevier Ltd. All rights reserved.
PACS: 82.45.Yz; 81.05.Ys
1. Introduction
complicated. Therefore, a simple and effective methodol-
ogy for the synthesis of low-dimensional nanomaterials
would be much preferred by nanoscience researchers.
Recently, low-temperature solution-phase synthesis has
been proposed as an attractive approach toward low-
dimensional nanosized materials of high crystallinity [8,9].
Vanadium pentoxide and vanadium pentoxide-based
compounds allow a wide range of practical applications
such as humidity sensors, electrochromic devices, and
cathodic electrodes for lithium batteries, due to their
outstanding structural flexibility combined with chemical
and physical properties [9–13]. The attention in sodium
vanatates such as NaVO3 and Na1.1V3O7.9 is strongly
increased due to the charge ordering and magnetic ordering
occurring with mixed-valence states V5+and V4.9+ [11].
These compounds are usually prepared by means of
Low-dimensional nanostructures, such as nanobelts,
nanorods, nanowires, and nanotubes have attracted
significant attention due to their novel size- and morphol-
ogy-dependent properties [1–9]. It has also been noted that
some low-dimensional nanostructured materials were used
as active components or interconnects in fabricating
nanoscale electronic, optical, optoelectronic, electrochemi-
cal, and electromechanical devices. Outstanding nanode-
vices include field-effect transistors (FETs), light-emitting
diodes (LEDs), and biological and chemical sensors [2–4].
For low-dimensional nanostructures, atoms or other
building blocks are rationally assembled into a structure
with nanometer size but much larger length [5]. Various
strategies developed for the synthesis of nanomaterials
typically suffer from the limits of high temperature, special
conditions, and templates or catalysts [6,7]. In particular,
the introduction of templates or catalysts to the reaction
system tends to make the synthetic process much more
high-temperature solid-state reactions. Recently, CaV6O16
ꢀ
3H2O and Na2V6O16 ꢀ 3H2O nanobelts have been synthe-
sized by hydrothermal methods [12,13]. In this work, we
describe a facile low-temperature method to selectively
grow NaVO3 or Na1.1V3O7.9 nanosized materials from the
alkaline solutions of V2O5 dissolved in NaOH without
using any catalysts or templates.
Ã
Corresponding author. Tel.: +82 31 219 2599; fax: +82 31 219 1615.
E-mail address: mho@ajou.ac.kr (S.-i. Mho).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.10.072

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C.V. Subba Reddy et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1261–1264
1262
2. Experimental
(a)
V2O5 powder (99.5%) was dissolved in 1 M NaOH
solution (0.33 M) at room temperature. To investigate the
influence of the pH values, a set of experiments were
designed. The pH of the solution was adjusted to 2, 4, 7,
and 10 by adding a HNO3 solution. The pH-adjusted
solution was placed in an oven for ageing at 90 1C for 5
days. The clear solution turned orange and red upon
ageing, while its viscosity progressively increased. A dense
colloid suspension with dark red color was formed. The
dense colloid suspension thus obtained was dried at 60 1C
for 48 h under static-air conditions. The products were
filtered, washed with distilled water, and dried in air.
Calcination was performed at 250 1C. This synthesis is
advantageous because it is simple and easy to control
besides being time-saving and cost-effective.
(b)
(c)
(d)
Na1.1V3O7.9(JCPDS # 45-0498)
NaVO3(JCPDS # 32-1198)
Crystallographic information of the samples was ob-
tained using a Rigaku D-max-gA X-ray powder diffract-
ometer equipped with graphite monochromatized Cu Ka
10
20
30
40
50
60
70
2θ (degree)
˚
radiation (l=1.54187 A). Diffraction data were collected
Fig. 1. XRD patterns of sodium vanadate nanostructures obtained from
solutions of various pH values: (a) Na1.1V3O7.9 (at pH ¼ 2);
(b) Na1.1V3O7.9 (at pH ¼ 4); (c) a mixture of Na1.1V3O7.9 and NaVO3
(at pH ¼ 7); (d) NaVO3 (at pH ¼ 10).
over the 2y range from 2 to 701. Fourier transform infrared
(FTIR) absorption spectra of the nanostructures were
recorded using a 60-SXB IR spectrometer of 4 cmꢁ1
resolution, over a wave number range of 400–4000 cmꢁ1
.
Raman spectra were taken under ambient condition by
using Renishaw inVia Raman microscope excited with the
514-nm light of an Ar+ laser. The morphologies of the
resulting products were characterized by field-emission
scanning electron microscopy (FESEM, JSM 6700F). The
electrochemical properties of the nanostructured sodium
vanadates were examined by cyclic voltammetric (CV)
measurements using a three-electrode cell with a platinum
counter electrode and a silver wire as a pseudo-reference
electrode. The working electrode, prepared by mixing
75 wt% of active material, 20 wt% of carbon black and
5 wt% of ethylene cellulose, was then coated on a 1.5 cm2
ITO glass. A solution of 1 M lithium perchlorate (99.99%,
Aldrich) in propylene carbonate (99.7%, Aldrich) was used
as the electrolyte. CV measurements were carried out
between the potential limits of ꢁ1.0 and 1.0 V against the
pseudo-reference Ag electrode using a potentiostat/galva-
nostat (Zahner IM6). The CV curves were recorded at a
scan rate of 1 mV/s.
˚
32-1198) with lattice parameters of a ¼ 5.36 A,
˚
˚
b ¼ 14.15 A, and c ¼ 3.65 A. The diffraction peaks of the
sample prepared from the pH ¼ 7 solution (Fig. 1c) can be
indexed as mixture of Na1.1V3O7. 9 and NaVO3.
Vanadium in its higher oxidation state is known to give
various isopolyvanadates at different pH values in aqueous
solution [14]. We were able to obtain meta-NaVO3
(at pH ¼ 10), the mixture of meta-NaVO3 and poly-
Na1.1V3O7.9 (at pH ¼ 7), and poly-Na1.1V3O7.9 vanadate
salts from the solutions of low pHs.
The size and morphology of the sodium vanadates were
examined by FESEM. Uniformly distributed nanostruc-
tures have grown from the sol evaporated from the pH-
controlled solutions as shown in (Fig. 2). The nanobelts or
flakes with the dimension of 50–500 nm in width and
1–6 mm in length are straight and have rectangular flat tips
with somewhat sharp corners. Sodium vanadates prepared
from the solutions of low pH seem to crystallize in smaller
sizes compared with that from the high pH solutions.
Raman scattering spectrum of sodium vanadate
(NaVO3) is shown in Fig. 3. The Raman peaks at 950,
915, and 892 cmꢁ1 are assigned to the stretching vibrations
(ns) of the V–O (1) and V–O (2), which are well matched
with the published data [9,15]. The bands at 737 and
562 cmꢁ1 are assigned to the asymmetric stretching (nas) of
the O–V–O, while the band at 435 cmꢁ1 is assigned to the
symmetric stretching (ns) of the O–V–O. The Raman peaks
of the low-frequency region (lower than 300 cmꢁ1) may be
related to the external vibrations, i.e. crystal lattice
translational (T) and rotational (R) modes.
3. Results and discussion
The crystal structure and phase purity of the samples
have been examined by powder XRD, which is shown in
Fig. 1. The samples are identified as Na1.1V3O7.9 and
NaVO3. The samples prepared from the solutions of
pH ¼ 2 and 4 are pure Na1.1V3O7.9, all of the reflection
peaks are indexed as monoclinic Na1.1V3O7.9 (JCPDS No:
˚
˚
45-0498) with lattice parameters of a ¼ 13 A, b ¼ 8.388 A
˚
and c ¼ 14.102 A, b ¼ 101.71. All the diffraction peaks of
the sample prepared from the solution of pH ¼ 10 (Fig. 1d)
are indexed with the orthorhombic NaVO3 (JCPDS No:

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1263
Fig. 2. FESEM photographs of sodium vanadate nano-materials obtained under different pH conditions from the NaVO3 solution: (a) Na1.1V3O7.9
(at pH ¼ 2); (b) Na1.1V3O7.9 (at pH ¼ 4); (c) a mixture of Na1.1V3O7.9 and NaVO3 (at pH ¼ 7); (d) NaVO3 (at pH ¼ 10).
2.0
(a) NaVO3
(a) NaVO3
1.8
1.6
1.4
(b) Na1.1V3O7.9
1.2
1.0
0
200
400
600
800
1000
1200
Raman Shift (cm-1)
Fig. 3. Raman spectra of the NaVO3 nanobelts prepared from the
solution of pH ¼ 10.
(c) NaVO3 (from the bottle)
The FTIR spectra of the sodium vanadates are shown in
Fig. 4. For NaVO3 powder sample, the IR absorption
bands at low wavenumber region are well matched with the
Raman scattering data; the V–O stretching vibration bands
at the 950–880 cmꢁ1 region and the O–V–O asymmetric
vibration band at about 560 cmꢁ1. For Na1.1V3O7.9, the IR
absorption bands at the 1012–953 cmꢁ1 region are assigned
to V–O stretching vibration which are somewhat shifted to
higher wavenumber compared with those of NaVO3, while
the bands at 755 and 540 cmꢁ1 are attributed to asymmetric
stretching vibrations of V–O–V bonds. The FTIR results of
the NaVO3 nanobelts are in good agreement with sodium
vanadate (NaVO3) from the reagent bottle.
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. IR patterns of (a) the NaVO3 nanobelts prepared from the solution
of pH ¼ 10, (b) Na1.1V3O7.9 nanoflakes prepared from the solution of
pH ¼ 2, and (c) NaVO3 from the reagent bottle.
of the electrode made of the mixture of Na1.1V3O7.9 and
NaVO3 has higher current densities than the CV curve of
the electrode made of Na1.1V3O7.9, which may be the cause
of multi-oxidation states.
4. Conclusions
The cyclic voltammograms of the electrodes made of
Na1.1V3O7.9 and of the mixtures of Na1.1V3O7.9 and
NaVO3 nanomaterials are shown in Fig. 5. The CVs of
sodium vanadium oxide nanomaterials show one broad
cathodic reduction peak at ꢁ0.44 V, and a broad anodic
reduction peak is observed at about 0.14 V. The CV curve
Sodium vanadate, NaVO3 and Na1.1V3O7.9 nanobelts
and nanoflakes were successfully prepared by simply
controlling the pH of the initial alkaline solution at
room temperature. The structural properties of sodium
vanadates are confirmed by X-ray diffraction, FTIR and

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C.V. Subba Reddy et al. / Journal of Physics and Chemistry of Solids 69 (2008) 1261–1264
1264
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0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
-0.08
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The authors wish to acknowledge the support from the
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