Investigation on structural, thermal, optical and
sensing properties of meta-stable hexagonal
MoO3 nanocrystals of one dimensional structure
Angamuthuraj Chithambararaj and Arumugam Chandra Bose*§
Full Research Paper
Address:
Nanomaterials Laboratory, Department of Physics, National Institute
of Technology, Tiruchirappalli – 620 015, India
Email:
Angamuthuraj Chithambararaj - a.chithambararaj@gmail.com;
Arumugam Chandra Bose* - acbose@nitt.edu
Open Access
Beilstein J. Nanotechnol. 2011, 2, 585–592.
doi:10.3762/bjnano.2.62
Received: 01 April 2011
Accepted: 02 August 2011
Published: 14 September 2011
Associate Editor: J. J. Schneider
* Corresponding author
§ Tel: +91-9444065746; Fax: 91-431-2500133
© 2011 Chithambararaj and Bose; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
fiber optic sensor; hexagonal phase; molybdenum oxide; one
dimensional rod; phase transition
Abstract
Hexagonal molybdenum oxide (h-MoO3) was synthesized by a solution based chemical precipitation technique. Analysis by X-ray
diffraction (XRD) confirmed that the as-synthesized powder had a metastable hexagonal structure. The characteristic vibrational
band of Mo–O was identified from Fourier transform infrared spectroscopy (FT-IR). Scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images clearly depicted the morphology and size of h-MoO3. The morphology study
showed that the product comprises one-dimensional (1D) hexagonal rods. From the electron energy loss spectroscopy (EELS)
measurement, the elemental composition was investigated and confirmed from the characteristic peaks of molybdenum and oxygen.
Thermogravimetric (TG) analysis on metastable MoO3 revealed that the hexagonal phase was stable up to 430 °C and above this
temperature complete transformation into a highly stable orthorhombic phase was achieved. The optical band gap energy was estimated from the Kubelka–Munk (K–M) function and was found to be 2.99 eV. Finally, the ethanol vapor-sensing behavior was
investigated and the sensing response was found to vary linearly as a function of ethanol concentration in the parts per million
(ppm) range.
Introduction
Considerable research interest has been focused on metastable
nanocrystalline materials due to their unusual and enhanced
properties as compared to their bulk counterparts. Synthesis of
metastable nanocrystals with controlled size and shape has
always been one of the most fascinating challenges as the properties of nanomaterials are essentially determined by their
phase, shape, size and chemical composition [1]. Recently,
molybdenum oxide and its compounds have aroused interest for
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various potential applications, such as gas sensors, highly reactive catalysts, high optical contrast electrochromic devices and
cathodic electrodes for lithium batteries, due to their unique
structural, optical and electrical properties [2-5]. Several synthetic techniques have been developed for controlled synthesis
of nanocrystalline MoO3 materials [6-11]. Recently, Dhage et
al. [12], Ramana et al. [13] and Song et al. [14] have prepared
h-MoO3 and studied its structural and thermal properties. Zheng
et al. prepared highly dispersed h-MoO3 through a solutionbased technique and investigated its photochromic and
electrochromic properties [4]. By adjusting the hydrothermal
reaction temperature, metastable (h-MoO 3 ) and stable
(α-MoO3) MoO3 nanoparticles were successfully synthesized
by Chithambararaj et al. [15].
Thus, many research articles are focused on the synthesis of
h-MoO3 as opposed to the stable α-MoO3, due to the reduced
crystallite size and one dimensional growth. Usually, the
α-MoO3 is a stable structure formed at a higher temperature
relative to the metastable h-MoO3. Thus, there is a phase transformation from h-MoO3 to α-MoO3 at 400 °C, which is inferred
from TGA/DTA studies [14,16]. With an increase in temperature, the crystallites aggregate and promote rapid growth of the
particle. Hence, crystallite size in the nanometer range is difficult to control in the case of α-MoO3.
The present work demonstrates the synthesis of metastable
h-MoO3 material through a solution-based chemical precipitation technique. The structure, shape and size of the as-synthesized sample were characterized by XRD, SEM and TEM
analysis. The effects of the crystallite size and strain, on the
diffraction peak broadening, were deconvoluted and their values
were estimated using Scherrer and Wagner–Aqua methods. The
phase stability and its transformation into a highly stable
orthorhombic structure were confirmed by thermal studies. The
optical band structure and ethanol vapor-sensing behavior were
studied by means of diffuse reflectance spectroscopy (DRS) and
fiber optics spectroscopy, respectively. To the best of our
knowledge, this paper reports for the first time the ethanol
vapor-sensing mechanism with h-MoO3 by fiber optics sensor.
Results and Discussion
bulk compound. The observed broadening is a convolution of
the instrumental error, the reduced crystallite size and the existence of microstrain in the synthesized sample product.
Figure 1: XRD pattern of as-synthesized h-MoO3.
The overall contribution to the broadening is expressed as
βhkl = βins + βsize + βstrain, where βins is the full width at half
maximum (FWHM) due to instrumental error, and βsize and
βstrain are the FWHM due to the crystallite size and strain, respectively. The broadening due to instrumental error is eliminated by the Rachinger method [17]
(1)
where β obs and β hkl refer to the FWHM of observed and
measured sample profiles, respectively. In order to study the
crystallite size and strain effect on the peak broadening,
Scherrer [18] and Wagner–Aqua (W–A) [17] methods were
considered. The Scherrer method is used to calculate the crystallite size (Dhkl) with the assumption of negligible strain,
(2)
Crystal phase analysis
The XRD pattern of the as-synthesized MoO3 powder is shown
in Figure 1. The sample product crystallizes in the hexagonal
phase of MoO3, and the diffraction peaks are indexed with
reference to a standard (JCPDS-21-0569: a = 10.522 Å and
c = 14.888 Å) data file. No secondary impurity peaks corresponding to other polymorphs of MoO3, namely orthorhombic or
monoclinic, are present. From the XRD pattern, the high intensity peak observed at 26.5° is broadened in comparison to the
where K is the shape factor (0.9), λ is the wavelength of Cu Kα1
radiation (1.5406 Å), and θhkl is the Bragg diffraction angle.
The crystallite size was estimated and found to be 51 nm.
Although the size of the crystallite is in the nanometer range,
significant structural defects such as dislocations, staking faults,
twin boundaries and intergrowth, etc., induce strain in the material, which results in considerable broadening of the diffrac-
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tion peak profile. In order to distinguish and deconvolute the
crystallite size and strain effects on the peak broadening, the
W–A method is adopted and the equation is
(3)
Figure 2 shows the W–A plot for the as-synthesized sample. To
determine the crystallite size and strain, a graph was plotted
with (4sinθhkl)2 along the X-axis and (βhklcosθhkl)2 along the
Y-axis. From the linear fit, the crystallite size (D hkl ) was
extracted from the Y-intercept and the microstrain (εhkl) from
the slope of the fit.
Figure 3: FT-IR spectrum of as-synthesized h-MoO3.
phase. A broad and complex band peaked at 600 cm−1 corresponds to the Mo–O vibration [14].
Surface morphology and particle size
analysis
Figure 2: W–A plot of h-MoO3 for crystallite size and microstrain estimation.
The crystallite size and strain were estimated from the graph
and the values are 77 nm and 3.509 × 10−6, respectively. From
these results, it is clearly confirmed that the observed broadening in the XRD peaks is mainly due to the smaller crystallite
size.
Figure 4 displays the low and high magnification SEM images
of as-synthesized h-MoO3 powder. From Figure 4a, the particles are widely distributed and are found to be aggregated in a
spherical structure whose diameter is in the micron range and
comprises a bunch of nanorods. The corresponding high magnification images (Figure 4b–d) show that the particles are in the
form of one-dimensional hexagonal rods. A typical TEM image
of the as-synthesized particle is shown in Figure 5. It clearly
illustrates that the as-synthesized powder comprises 1D structures of ~200 nm diameter and length of about 800 nm.
Functional group analysis
The functional groups present in MoO3 are identified through
FT-IR analysis and are shown in Figure 3. A small band at
3434 cm−1 and a sharp band at 1616 cm−1 correspond to the
stretching and bending vibrations, respectively, of the hydrogen
bonded –OH group in water molecules.
Distinct peaks at 3219 cm−1 and at 1404 cm−1 are attributed to
the stretching and bending vibrations of N–H in NH4+. These
results are consistent with the previously reported results [19].
The peaks between 1000 cm−1 and 900 cm−1 are ascribed to the
Mo=O characteristic stretching vibration of the hexagonal
Figure 4: Low and high magnification SEM micrographs of h-MoO3.
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Figure 5: TEM micrograph of the one-dimensional structure of
h-MoO3.
Figure 6 shows the HRTEM image and the inset is the corresponding SEAD pattern of h-MoO3. From the lattice resolved
HRTEM image, the distance between the two planes was found
to be 0.345 nm, belonging to the (210) plane of h-MoO3. The
electron diffraction, with a highly intense dotted pattern, reveals
the single crystalline nature of h-MoO 3 . Furthermore, the
elemental composition and chemical bonding information were
confirmed by EELS investigation.
Figure 7: EELS spectrum of resultant h-MoO3.
Growth mechanism and formation of 1D
h-MoO3
On the basis of experimental results, the possible mechanism
for the formation of h-MoO 3 is outlined as follows: In the
present work, (NH4)6Mo7O24·4 H2O and HNO3 are used as
reactant precursors for the synthesis of h-MoO3 nanocrystals.
As per the following reaction Equation, MoO6 is considered as
an initial seed nucleus for the formation of MoO3.
(NH4)6Mo7O24·4 H2O (aq) + 6 HNO3 →
7 MoO3 (s) + 6 NH4NO3 (aq) + 7 H2O
Figure 6: HRTEM image of an as-synthesized h-MoO3 nanorod and
their electron diffraction pattern (inset).
Figure 7 depicts the typical EELS profile of an individual
hexagonal rod. It shows two distinct edges at 532 eV (oxygen K
edge) and 382 eV (molybdenum M edge), confirming the presence of only O and Mo with a covalent interaction [20]. The
observed O-K edge peak reflects the electronic transitions from
the oxygen 1 s core level to the unoccupied final state of O 2p
hybridized with the Mo 4d state [21], and the Mo-M2,3 peak is
due to excitation from 3p1/2 and 3p3/2 core level electrons to
unoccupied states (4d and 5s) of molybdenum [22].
Here, molybdenum is located at the center and six oxygen
atoms are coordinated in octahedral sites. During the nucleation,
these octahedral structures interact with each other through
corners (a-axis) and edges (c-axis) by means of electrostatic
interactions between NH4+ and OH− ions. The stacking and
assembling of these octahedral structures result in the formation of a stable hexagonal structure. Thus, the NH4+ functional
group present in the reaction condition acts indirectly as a structure directing agent [4,23]. Moreover, the initial seed nuclei
formed during the nucleation stage are composed of hexagonal
unit cells which can induce anisotropic growth along the c-axis,
and thus hexagonal shaped nanocrystals with 1D structure are
favored [24,25].
Thermal analysis
The thermal behavior was studied in detail and is depicted in
Figure 8. From the TG and DTG graph, the first weight loss
(2.33 wt %) in the 70–190 °C region corresponds to vaporization of the hydrogen bonded water molecules that are physically adsorbed on the surface of the MoO3. The second weight
loss (1.13 wt %) takes place between 190 and 270 °C, due to
vaporization of gaseous nitrates and ammonia compounds from
the material.
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Figure 8: TG/DTG and DTA curve of h-MoO3.
The same observation is made from the DTA curve, which
exhibits two broad exothermic peaks well below 270 °C. After
dehydration and removal of ammonia compounds, a sharp
exothermic peak at 430 °C with weight loss of 1.35 wt % is
attributed to liberation of coordinated water and ammonia molecules from the internal structure of the MoO3 material, which
promotes an irreversible phase transformation from the
hexagonal to the orthorhombic structure [14,16]. The powder
subjected to TGA measurements was subjected to XRD
analysis, and the result confirmed that above 450 °C the product is in a stable orthorhombic structure (Figure not included).
Above 450 °C no changes in the TG/DTA curve are seen
revealing that the thermodynamically stable α-MoO 3 was
achieved.
Optical absorption studies
The optical absorption behavior and band gap energy of
h-MoO3 was studied by means of DRS, as shown in Figure 9.
The spectrum shows a maximum reflectance in the region
between 600 nm and 450 nm corresponding to lower absorption. At 420 nm, a decrease in reflectance is seen due to fundamental absorption (valance band to conduction band) by the
material [26]. Further, the optical band gap is evaluated using
K–M function as follows [27]:
(4)
where F(R∞) is the K–M function or re-emission function, R∞ is
the diffuse reflectance of an infinitely thick sample, K(λ) is the
absorption coefficient, s(λ) is the scattering coefficient, hν is the
photon energy and Eg is the band gap energy for indirect transition.
Figure 9: DRS spectrum of h-MoO3 (Inset: optical band gap energy of
h-MoO3).
The indirect optical band gap energy is determined by extrapolating the linear portion of the plots of (F(R∞)hν)1/2 versus (hν)
shown as an inset in Figure 9. The estimated energy band gap
value is about 2.99 eV and is considerably higher than that of
the bulk (2.95 eV). This increase in E g is attributed to the
reduced particle size.
Sensor study
The ethanol vapor-sensing behavior was examined by means of
an optical fiber sensor setup and analyzed on the basis of simultaneous changes in the refractive index and evanescent wave
absorption. In our experiment, a portion of the fiber cladding
(~3 cm) was partially removed and coated with as-synthesized
h-MoO3; this is considered as the sensing area. In the sensor
study, the characteristic spectrum as a function of wavelength in
the range of 200–1100 nm was recorded for different ethanol
concentrations. The recorded spectrum is shown in Figure 10
and the inset represents the magnified view of the spectra
recorded in the range between 600 and 800 nm. In this plot, the
Y-axis denotes the intensity, measured in counts, for each
concentration of ethanol. The ethanol concentration was
increased from 0 to 500 ppm in increments of 50 ppm. From
Figure 10, it is clearly seen that the intensity peaks are a
maximum at 684, 764 and 935 nm. The magnified spectrum,
shown as an inset, clearly shows the gradual increase and variation in intensity with increase in ethanol concentration. The gas
sensitivity is calculated using the formula
(5)
The sensitivity is estimated by plotting a separate graph of
ethanol concentration (in ppm) versus intensity (in counts) for
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each value of λmax (684, 764 and 935 nm respectively). The plot
is depicted in Figure 11: The slope of the curve determines the
sensitivity and is found to be 74, 67 and 26 counts/ppm for
λmax = 684, 764 and 935 nm, respectively. From the plot, the
maximum sensitivity is seen for λmax = 684 nm.
For h-MoO 3 , the oxygen vacancies and interstitial molybdenum atoms play a major role in gas sensing operation. The
reaction between ethanol gas and the chemisorbed oxygen (O2−,
O− and O2−) at the surface of the MoO3 material changes the
refractive index of the clad structure. Due to this change, the
light wave travelling through the removed and coated portion of
the cladding experiences a change in the evanescent wave
absorption [28-32]. Thus, the resultant intensity recorded in the
spectrum is a sum of changes due to refractive index and
evanescent wave absorptions.
Conclusions
Figure 10: Spectral response of h-MoO3 for varying concentration of
ethanol in ppm.
In summary, metastable h-MoO3 was prepared by a solutionbased chemical precipitation technique. The metastable
hexagonal phase was confirmed by XRD. The crystallite size of
the h-MoO3 was in nanometer range and the corresponding
strain was in the order of 10−6. The metal–oxygen vibrational
states (M=O and M–O) were identified from the vibrational
spectrum. The one-dimensional hexagonal rod structure, with
regular arrangement and preferred orientation, was observed in
SEM, TEM and HRTEM images. From EELS, the O-K edge
and Mo-M2,3 peaks confirmed the existence of molybdenum
and oxygen. From thermal analysis, the structural transformation from hexagonal to stable orthorhombic was observed at
430 °C. A formation and growth mechanism for the h-MoO3
nanocrystals with 1D structure was proposed. The estimated
band gap was 2.99 eV from the optical absorption studies.
Finally, the ethanol vapor-sensing behavior was investigated
and the sensor response increased linearly as a function of
ethanol concentration in the range of 0 to 500 ppm. In the
present study, changes in the refractive index and the evanescent wave absorption phenomenon were proposed to explain the
gas sensing mechanism. The reaction between ethanol gas and
the chemisorbed oxygen (O2−, O− and O2−) at the surface of the
MoO3 material simultaneously changes the refractive index and
evanescent wave absorption thereby varying the intensity at the
detector.
Experimental
Figure 11: Plot of peak intensity versus ethanol concentration for
values λmax = 684, 764 and 935 nm.
Here, it is proposed that the gas sensing mechanism follows the
changes in the refractive index and evanescent wave absorption.
All the chemicals were purchased from Merck and were used
without any further purification. Ammonium heptamolybdate
tetrahydrate (AHM) was dissolved in distilled water (10 mL), to
give a concentration of 0.2 M. A homogeneous solution was
obtained after stirring for 15 minutes and was then mixed with
concentric nitric acid (5 mL). The mixture was then heated at
85 °C for 1 h and the resulting precipitate was subsequently
washed and centrifuged with distilled water several times. The
obtained powder was dried at 70 °C in vacuum for 6 h. Thus, by
a chemical precipitation technique, the h-MoO3 nanocrystal was
synthesized by acid decomposition of ammonium molybdate
according to the following reaction equation,
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(NH4)6Mo7O24·4 H2O (aq) + 6 HNO3 →
7 MoO3 (s) + 6 NH4NO3 (aq) + 7 H2O.
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