Journal of Materials Chemistry A
Communication
spectra were recorded using a Thermo Nicolet 8700 (USA)
3.2 Properties of the silver–tungsten nanostructure material
instrument in the frequency range 4000–400 cmꢄ1
.
XRD patterns of the three samples are shown in Fig. 1. All the
three samples exhibit the characteristic peaks at 2q values of
38.1ꢁ, 44.3ꢁ, 64.4ꢁ and 77.4ꢁ, and are indexed to the diffractions
of the (111), (200), (220) and (311) crystal planes of metallic Ag
(JCPDS no. 04-0783), respectively. The peaks at 2q values of
23.2ꢁ, 23.5ꢁ, 24.3ꢁ, 33.2ꢁ and 34.2ꢁ correspond to the monoclinic
WO3 (JCPDS no. 43-1035). SEM images of the AgW-N sample
exhibit a nearly uniform rod-like morphology (Fig. 4a and b)
and the HRTEM images show a high dispersion of ꢀ7 nm Ag
nanoparticles on the WO3 nanorod support with diameter ꢀ60
nm (Fig. 4d). The particle size distribution of silver nano-
particles is shown in the inset (Fig. 4d). The lattice fringes in the
HRTEM image with d-spacings of 0.23 nm and 0.38 nm corre-
spond to the (111) and (020) crystal face of Ag and WO3,
respectively, and can also be seen in Fig. 4e. The energy
dispersive X-ray analysis also conrms the presence of Ag, W
and O species in the sample and no impurities could be
observed in the spectra (Fig. 4c). The mean diameter of AgNPs
remained ꢀ7 nm aer the reaction conrms the stability of the
small metallic AgNPs on WO3 nanorods, presumably resulting
from the strong metal support interaction, which also helps to
resist the sintering of AgNPs (Fig. S3†). Fig. 5a shows that the
aloevera like microstructure (AgW-A) is composed of one
dimensional nanorods extended outward from the centre. The
average diameter of individual nanorod as conrmed from the
TEM image is approximately ꢀ400 nm (Fig. 5b). The lattice
fringes in the HRTEM image with d-spacings of 0.20 nm and
0.43 nm, correspond to the (200) and (111) crystal face of Ag and
WO3, respectively, and can also be seen in Fig. 5c. The hexag-
onal morphology (AgW-H) is also composed of nanorods which
could be observed from the corresponding SEM and TEM
images (Fig. 5e and f). The lattice fringes in the HRTEM image
with d-spacings of 0.15 nm and 0.23 nm correspond to the (220)
and (111) crystal face of Ag and d spacings of 0.36 nm and 0.38
nm correspond to the (200) and (020) crystal plane of WO3,
respectively, can also be observed (Fig. 5g). The samples
prepared by the conventional impregnation method show
irregular and larger particles (Fig. S2d†). The association of the
3. Results and discussion
3.1 Generation of the silver–tungsten nanostructure
It is well known that Ag, W and CTAB exist in the form of Ag+,
WO4 , and CTA+ in aqueous alkaline solution (pH > 7)24 and
2ꢄ
cooperative self assembly between cationic part of the surfac-
tant, CTAB and anionic species can be formed via electrostatic
interaction.25 When the CTAB concentration is high (more
than the critical micelle concentration), cationic CTAB mole-
2ꢄ
cules adsorb onto the surface of the WO4 ions and form
spherical micelles.26 The curvature of an ionic micelle can be
tuned from spherical to rod-like micelle by adding certain
additives.27 Here, in our preparation method, during addition
of Ag+ ions, the spherical micelles of CTA–WO4 were trans-
1ꢄ
formed into rod shaped micelles as a result of attractive elec-
trostatic interaction between the CTA-stabilized negatively
charged tungstate ions and positively charged silver ions,
where Ag+ ions induce symmetry-breaking anisotropic growth
on selective adsorption onto particular crystal planes of tung-
sten. Thus, the decrease in surface energy of the WO3 seed
crystals in one direction results in the formation of the one
dimensional rod-like micellar structure and this acts as a
nucleating agent for the growth of WO3 nanorods. Ag+ adsorbs
at the specic surface on WO42ꢄ, leading to preferential growth
of WO3 along one direction and we could observe the Ag sup-
ported WO3 nanorod structure (designated as AgW-N). In our
case, WO3 nanoparticles were formed instead of a rod-like
morphology in the absence of Ag+ ions (Fig. S2b†). In the
absence of CTAB, agglomerated Ag/WO3 was formed
(Fig. S2a†). Therefore, the amphiphilic CTAB is not only acting
as a ligand for the WO42ꢄ ions, but also as a capping agent for
both the silver and WO3 and stabilize them against aggrega-
tion. When the synthesis was performed with a high concen-
tration of CTAB, with a [CTAB] to [Ag] molar ratio of 5, an
aloevera-like structure was formed (designated as AgW-A).
Here, hydrazine also plays a crucial role in the synthesis
procedure. Generally, hydrazine reduces Ag+ through the
formation of a complex Ag(N2H4)+.28 So, hydrazine acts not only
as a reducing agent but also as a chelating agent. In the
absence of hydrazine, we could observe only agglomeration,
suggesting that hydrazine also act as a stabilizer (Fig. S2c†).
Being a reducing agent hydrazine reduces Ag+ to Ag onto the
surface of WO42ꢄ, thus controls the growth of tungsten and
silver nanoparticles and we observed only silver deposition on
the WO3 surface. On prolonging stirring from 7 h to 48 h, the
nanorods nucleate and aggregate to grow into the hexagonal
structure (designated as AgW-H). This can be attributed to the
fact that the diameter of the nanorods being small, their
surface energy is comparatively higher, the nanorods assem-
bled together in order to reduce the surface energy, and form
the hexagonal structure which is clear from SEM and TEM
images discussed later. So, well-dened Ag/WO3 structures can
be prepared by tuning the synthesis parameters.
Fig. 1 XRD patterns of (a) Ag(0), (b) Ag(I) oxide, (c) W(VI) oxide, (d) AgW-
N, (e) AgW-A and (f) AgW-H.
15728 | J. Mater. Chem. A, 2014, 2, 15726–15733
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