Synthesis and Electrical Properties of Well-Ordered Layered α-MoO3 Nanosheets

Abstractα-MoO3 ordered nanosheets have been synthesized under hydrothermal conditions using commercial MoO3 and hydroquinone as structuring agent. X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to analyse the obtained material. The conductivity mechanism of the Molybdenum ordered nanosheets has been investigated using combined complex impedance and modulus formalism.The temperature dependence of the conductivity, which was between 473 and 573 K, is very close to the Arrhenius’ law, with an activation energy of 0.76 eV. However, the conductivity of the material increases with temperature. It shows a typical negative temperature coefficient resistance (NTCR) similar to that of a semiconductor. The dielectric properties of the MoO3 compound have been studied in the temperature range of 473–573 K as well as the frequency range of 10 Hz to 13 MHz. The ac-conductivity for high frequency σac (ω) obeys the universal power law.


Introduction
The control of the morphology and size of nanomaterials has become a dominant theme of study in the material physics field since these parameters are the key elements that determine the physical properties of nanomaterials. Low dimensional nanostructures, such as nanowires, nanorods, nanotubes, nanobelts, and nanosheets are increasingly being given more attention due to their novel electrical, magnetic and optical properties [1][2][3][4][5][6][7][8]. Nanosheet materials have been synthesized using various methods: the vapor-liquid-solid (VLS) process [9], chemical vapor deposition (CVD [10], the solution based method [11] and the Langmuir-Blodgett (LB) technique [12].
It must be noted that the layered transition ox-ides have been given considerable attention in different applications such as in the electrosynthesis of electrochromic molybdenum oxide [13], in gas sensing [14], in catalysis [15], the pillaring method [16] and inactive electrode materials [17]. Among the layered transition metal oxides, orthorhombic MoO 3 has been receiving considerable attention within the field of solid state materials chemistry owing to its fascinating properties and its great potential to be used in many fields such as: in ammonia sensing [18], in alkane dehydrogenation catalysts [19], in supercapacitors [20] and in active anode materials for batteries [21].
In this paper we study, on the one hand, the synthesis and the characterization of the molybdenum oxide nanosheets; and on the other hand, the electrical properties of this nanomaterial. The details concern-ing the dielectric response of α-MoO 3 as a function of frequency and temperature has been investigated. Due to its layered structure being favorable to lithium insertion, α-MoO 3 nanomaterial appears promising for lithium battery applications.

Sample preparation
The synthesis was carried out using the hydrothermal process. Sigma-Aldrich molybdenum trioxide (1313-27-5) (0.262 g) and hydroquinone (HDQ) HO-C 6 H 4 -OH (0.2 g) were introduced in 10 ml of distilled water. The molar ratio of these three components is (1:1:300). This mixture was treated in a Parr bomb at 180 ℃. The reaction time is 96 h. The cooling time of this system was fixed at 24 h. The pH of the reaction mixture was kept at 7. The resulting black powder was washed with ethanol in order to remove the residual hydroquinone, and then dried at 80 ℃ for 8 h.

Characterization techniques
X-ray diffraction patterns were recorded for all samples using the Panalytical "X'Pert Pro" diffractometer, using Cu K α radiation (λ=1.54056Å). The texture of the prepared sample was examined using a Scanning Electron Microscope (SEM) Quanta 200. Transmission electron microscopy (TEM) results were recorded on a JEOL 100 CX II electron microscope operated at 200 kV.
A polycrystalline sample of α-MoO 3 was crushed and pressed at room temperature into a tablet of 13.06 mm in diameter and 1.01 mm in thickness. Dense pellets suitable for electro-physical measurements were heated at 100 ℃ for 24 h. Metallic silver was deposited on both sides and served as electrodes. The pellet was placed between two blocking electrodes in a tubular furnace and subjected to a temperature regulator. Conductivity measurements were carried out from 473 K to 573 K in 5∼20 ℃ intervals by checking the complex impedance spectroscopy with a Hewlett Packard 4129A impedance analyzer. The signal frequency ranged from 10 Hz to 13 MHz.

Powder X-ray diffraction
The XRD pattern of the resulting product ( Fig. 1(b)) differs from that measured on the commercial molybdenum oxide precursor ( Fig. 1(a)). This means that the structure of the obtained material is different from that of the precursor, indicating that the intercalation of HDQ is taking place during the present process. The obtained material is well defined as a pure phase of MoO 3 that crystallizes in the orthorhombic system with Pnma space group symmetry (JCPDS data, card number 65-2421). The lattice parameters were refined using the WinCELL program with a=3.9728Å, b=13.8023Å and c=3.6710Å. It is to be noted that three intense reflections (020), (040) and (060) were observed in the XRD pattern, indicating the formation of a layered structure of MoO 3 . Figure 2 shows the projection of the layered structure of molybdenum oxide along the (001) direction.

SEM and TEM
The SEM micrograph of the obtained material ( Fig. 3) proves the presence of a stratified structure.  This morphology is confirmed by the TEM micrographs which show the presence of well-ordered regular sheets composed of a series of flake sheets. The layered structure can be clearly seen in ( Fig. 4(a)) and ( Fig. 4(b)). This result is corroborated by the increase of the 0k0 peaks observed in the XRD pattern of the synthesized phase, indicating the well-orientated structure of the material. Yi Jing et al. [22] have synthesized a similar phase by intercalating a long chain of alkylamine in the pre-treated molybdenum trioxide with p-(N-ethyl-Nethylamino trimethylquaternary ammonium methylsulfate salt)-p'-(nitro)-o'-(chloro) azobenzene (GTL). Taking into account the absence of polarity and the strong electronegativity of the oxygen in the hydroxyl group in the organic precursor used, we can explain the instability of the layer.
Complex impedance analysis Figure 5 shows the results of the research performed on the electrical properties of MoO 3 , using impedance analysis. Figure 5(a) shows the complex impedance spectra (Nyquist plots) at different temperatures for the MoO 3 nanomaterials. The impedance spectrum is characterized by the appearance of a single parabola with a radius that decreases with increasing temperature. The intercept of the parabola with the real axis (Z ) gives us an estimated resistance (R b ) for the material. It has been observed that the resistance decreases with increasing temperature for MoO 3 nanomaterials. The result may be interpreted as a negative temperature coefficient resistance (NTCR) similar to that observed in a semiconductor. The electrical behavior observed in the material may be represented by an equivalent electrical circuit as shown in Fig. 5(a) (Inset). Figure 5(b) shows the variation of the real part of impedance (Z ) as a function of frequency at different temperatures. It has been observed that the impedance value is higher at low temperatures in the low frequency domain; but gradually decreases with increasing temperature. The decrease in Z with a rise in temperature and frequency indicates a possible increase of the ac-conductivity with the increase in temperature and frequency. Figure 5(c) represents the variation of the imaginary part of Z (impedance loss spectrum) with frequency. It is characterized by the appearance of peaks at a particular frequency in the temperature range [473∼573 K]. The appearance of a peak in the loss spectrum is an indication of the beginning of electrical relaxation in the material at different temperatures. The peaks that appear are asymmetric and their positions appear to be shifting towards the higher frequency side with increasing temperature. The variation of the conductivity, calculated from the complex impedance plots, with reciprocal temperature for MoO 3 is shown in Fig. 5(d). The conductivity obeys the Arrhenius' law (σ=Cexp(-E a /kT), where E a is the activation energy and k is the Boltzmann constant. The corresponding activation energy E a =0.76 eV.
Electrical conductivity analysis Figure 6 shows ac-conductivity σ ac (ω) vs. frequency plots for different temperatures. Inspection of these plots over the measured temperature range reveals the presence of a low frequency conductivity plateau, followed by high frequency conductivity dispersion with a change in gradient. The temperature at which grain resistance dominates over grain boundary resistance is marked by a change in gradient of ac-conductivity with frequency. The frequency at which the change in gradient takes place is known as the critical or hopping frequency. The hopping frequency shifts to higher frequencies with increasing temperature, suggesting a possible enhancement in the carrier-hopping rate of the mobile charge carriers with a rise in temperature. It is seen from Fig. 6 that σ ac increases with increasing frequency and becomes independent of frequency at higher values where the ac electrical conductivity (σ ac ) values are very close to each other. These results suggest that the nature and mechanism of the conductivity    [23][24][25], where σ dc is the dcconductivity in a particular range of temperatures, n is the temperature-dependent exponent in the range 0≤ n ≤1 of the interionic coupling strength and ω H is the hopping frequency at which a change in gradi-ent of the conductivity occurs. The required σ dc values were obtained from the σ ac (ω) spectrum in equation (1) by using σ ac (ω H ) = 2σ dc (2). Figure 7 shows the variation of the dielectric constant (ε and ε ) as a function of frequency at selected temperatures (473∼573 ℃) for MoO 3 nanosheets. Figure  7(a) shows that the dielectric constant (ε ) decreases at lower frequencies and remains constant at higher frequencies. The constant decrease of the value of the dielectric constant with increasing frequency may be attributed to electronic, ionic and interfacial polarization at low frequencies [26]. Generally, the observed dielectric dispersion at low frequencies can be explained based on the Maxwell-Wagner model of interfacial polarization [27]. According to the Maxwell-Wagner model, the dielectric structure of ferrites consists of two layers: the first layer consists of a large number of grains and acts as a conducting layer at higher frequencies, while the second layer consists of grain boundaries that act as a highly resistive medium at lower frequencies. At low frequencies, the polarization process in MoO 3 nanoparticles can be explained  by the dipole orientation in the direction of the applied field. Whereas, at high frequencies, the dipole orientation cannot follow the fast alternating field and therefore its contribution to polarization ceases.

Dielectric analysis
In Fig. 7(b) we can observe that the MoO 3 nanosheets show a decrease in the complex dielectric constant value of ε with increasing frequency until reaching a constant value. For nanometer-sized particles, the contribution of the interfacial loss and the loss from electrical conductivity is dominant at lower frequencies, however, at high frequencies, these losses are negligible. This may be the reason for the decrease in the dielectric constant value (ε ) at high frequencies. The large value of the dielectric constant ε at lower frequencies is due to the predominance of many species such as oxygen vacancies and grain boundary defects [28].

Modulus formalism
The electrical response of the MoO 3 nanomaterial has also been analyzed via the complex electric modulus M*(ω) formalism [29]. The complex modulus is defined as M*=iωC 0 Z * , where C 0 is the vacuum capacitance of the cell, ω=2πf and i 2 = −1. This formalism was adopted because it ignores the electrode polarization and other interfacial effects in solid electrolytes [30][31][32][33][34].
For a given temperature and frequency, the real part (M ) and the imaginary part (M ) of the M* complex modulus (M*=M +jM ) have been calculated from the complex data (Z*=Z -jZ ) using the relations M = ωC 0 Z and M =ωC 0 Z . A plot of log M and the normalized M /M max imaginary part of the complex modulus versus frequency (in a logarithmic scale) are given in Fig. 8(a) and Fig. 8(b) for the [200∼300 ℃] temperature range. Whatever the temperature, the value of M reaches a maximum (M ∞ =1/ε ∞ ) at high frequencies.
At low frequencies, it decreases sharply and approaches zero, indicating that the electrode polarization phenomena make a negligible contribution to M*(ω) and may be ignored when the electric data is analyzed in this way.
In Fig. 8(b), we introduce the (M /M max ) dependence of log f relative for the MoO 3 nanomaterial at various temperatures. Inspection of these plots over the measured temperature range reveal the presence of a peak which shifts from low to high frequency as the temperature is raised. The modulus peak maxima is defined by τω max =1, where τ is the most probable relaxation time [35][36]. The relaxation process probably originates from the charge carrier hopping mechanism. Observation of the M /M max plots shows the absence of any additional peaks at lower frequencies, which indicates a negligible contribution of grain boundary and electrode effects to the total conductivity. In these figures we observe a maximum in the imaginary part, which corresponds to an inflexion in the curve of the real part of Fig. 8(c). This behavior indicates the presence of a relaxation phenomenon.

Conclusion
In this investigation we have developed a simple method for the synthesis of molybdenum trioxide nanosheets, at a relatively low temperature via a hydrothermal process. It has been outlined in this report that this nanostructure is well-ordered and oriented. We suggest that the role of the organic precursor is fundamental in structuring the molybdenum oxide layers. Due to its structure, the electrical conductivity could be the most important bulk property of the molybdenum trioxide. Indeed, the temperature dependence of the conductivity between 473 and 573 K of the MoO 3 nanomaterial indicates that it obeys the Arrhenius' law, with activation energy of 0.76 eV. The frequency dependence of the ac-conductivity is approximated by a universal power law.