Synthesis of metal oxide composite nanosheets and their pressure sensing properties

  • 1. Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah 21589, Saudi Arabia
  • 2. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
  • 3. Interdisciplinary Research Center, COMSATS Institute of Information Technology, Wah Cantt, Pakistan
  • 4. Physical Technical Institute of Academy of Sciences, Dushanbe 734025, Tajikistan
  • 5. Division of Nano Sciences and Department of Chemistry, Ewha Womans University, Seoul, Korea

Key words: nanostructuresoxideschemical synthesisoptical propertieselectron microscopy

Abstract: This study presents the synthesis of metal oxides composite nanosheets (oxides of cobalt, zinc and iron) and their pressure sensing properties. A transducer has been fabricated to directly measure the resistance—pressure and impedance—pressure relationships of pristine nanopowder. At the initial stage, a nanopowder sample of 10 mm diameter and 1 mm height was placed in the transducer and by applying pressure of up to 8.15 kN/m2; the DC resistance and the impedance are reduced by 44% on average. It can be explained by the densification of the samples and a decrease in porosity due to the effect of pressure. It was also observed that the DC resistance increases with time and saturated within 8 min. It is considered that this phenomenon is based on the effect of displacement currents of bound charges. The dependences of the impedance phase (θ) on frequency and pressure have also been investigated.

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1.   Introduction
  • For the understanding of the basic properties of materials, the investigation of the effect of pressure on the properties of semiconductors is very important[1, 2, 3]. The investigation of the conductivity and thermoelectric power of quasi-one and quasi-two dimensional organic semiconductors and conductor crystals showed that, as a rule, the conductivity increases and the thermoelectric coefficient decreases for semiconductors, while for conductors, these properties remain approximately constant[4]. The study of the influence of pressure on metal-insulator transitions in tetrathio-fulvalenium tetracyanoquinodimethane is also presented by Chu et al.[4]. On the other hand, investigations of the electric properties of materials, in particular semiconductors, can result in the fabrication of different kinds of sensors[5]. Pressure transducers are mostly used with resistance, capacitance, inductance and piezoelectric sensors and also with devices such as diaphragms and bellows[5, 6]. The network of pressure sensors with pentacene-based organic transistors was fabricated and investigated under an applied pressure of 30 kPa[7]. Unusual electromechanical effects (piezoelectricity and electrostriction) were also observed in organic semiconductor Schottky junctions due to the presence of the non-uniform spatial electric field distribution in the junction and the softness of organic semiconductors; these effects can potentially be used for the fabrication of electromechanical transducers[8].

    The piezoresistance of CNTs grown on deformable thin-film membranes of silicon nitride were investigated[9] and it was found that the gauge factor ($\Delta R$/$R \varepsilon$) was 400 and 850 for the semiconducting and small-gap semiconducting (SGS) tubes, respectively, whereas the maximum value of gauge factor in silicon is 200. The small band-gap semiconducting (or quasimetallic) nanotubes were investigated and it was found that SGS nanotubes exhibit a piezoresistive gauge factor from 600 to 1000 under axial strains, which is much larger than that of metallic nanotubes[9]. The electromechanical properties of the CNTs were also investigated, and the piezo-resistors were fabricated that are used in mechanical sensors such as strain gauges, pressure sensors and accelerometers[9, 10, 11, 12, 13, 14]. The fabrication of SWNT thin-film transistors on plastic substrates was described by Xue et al.[14]. They found that the resistance of the 14 and 16 layers SWNT thin film decreases by 38.2 % and 47.1 %, respectively with an increase in the bending of the elastic substrate, which is more than ten times higher than silicon. The piezoresistive effect in the pristine CNTs films is demonstrated in Reference [11] and it is shown that at room temperature, the gauge factor is 65 under 500 microstrains. The gauge factor also increases with temperature. The mechanical deformation-conductivity relationship of free-standing membranes of SWNTs has been investigated[10] and it is shown that the gauge factor of piezo-resistive nanotubes is 2.3-2.5 times larger than that of the silicon substrate. The nano-electromechanical piezoresistance transducers based on SWNTs have been investigated and it is described theoretically and experimentally that ballistically conducting SWNTs show nonlinear piezoresistive gauge factors of up to 1500 (under an applied strain of 1 %)[13].

    In the majority of these experiments and sensors, crystals, thin films or press-tablets were used. It would be reasonable to investigate the resistance-pressure relationships directly on pristine powders. In this paper, we have synthesized the powder of metal oxides composite nanosheets and, designed and fabricated a transducer for the measurement of the impedance of pristine composite nanosheets under pressure, which has been used for the investigation of impedance-pressure relationships.

2.   Experimental

    2.1.   Synthesis of nanosheets

  • For the synthesis of composite nanosheets, all the reagents and precursors were bought from Sigma Aldrich. In the typical process, 0.1 M solutions of cobalt chloride, zinc chloride and iron chloride were mixed in a ratio of 3 : 2 : 1, respectively. By adding a 0.5 M NaOH solution, the pH of the mixture was raised above 10[15, 16]. The solution was then heated overnight and the resulting precipitate was washed several times. The product was dried, ground and then used for detailed characterization and application.

  • 2.2.   Characterization of nanosheets

  • The morphology of composite nanosheets was examined by field emission scanning electron microscopy (JEOL FE-SEM, Japan), while the elemental analysis was carried out using an energy dispersive spectrometer (EDS). The prepared composite nanosheets were also analyzed by an ARLTM ARLX'TRA Powder Diffractometer with Cu K$\alpha $ radiation ($\lambda$ $=$ 1.54056 ). The structural characterization was carried out by using Fourier Transform Infrared (FTIR) spectroscopy (Perkin Elmer, spectrum 100) and the optical properties of the synthesized composite nanosheets were studied by using ultraviolet and visible spectrophotometry (UV-vis) (UV-2960, LABOMED Inc.).

  • 2.3.   Fabrication of transducer and electrical characterization of nanopowder

  • Figure 1 shows the transducer for investigation of the impedance of the powder under pressure. The transducer consists of the following elements: a cylinder-piston system, substrate, terminals and load. The cylinder is made from glass, while the piston and substrate were made from metal. Under the applied load, powder is squeezed between the piston and the substrate. The diameter of the cylinder and piston was equal to 10 mm. The initial powder's height was equal to 1 mm. The DC resistance and impedance at 100 Hz, 1 kHz, 10 kHz, 100~kHz and 200 kHz was measured by an MT 4090 LCR meter. The load (Figure 1) was variable up to 0.64 N, which allowed making the pressure up to 8.15 kN/m$^{2}$. The pressure ($p$) was measured as a ratio of the weight ($F$) to the cross-section area ($A$) of the piston in the cylinder:

    During the experiments, weights of different capacity were applied on to the piston. The weights are shown in Figure 1 as a load.

    The voltage and current were not measured, while the impedance, resistance (in DC) and the phase angle were measured by using a standard LCR meter (MT4090).

3.   Results and discussion
  • Figures 2(a) and 2(b) show FESEM images at low and high magnification, which clearly show that the synthesized composite is grown in the form of nanosheets. These nanosheets are grown in a large quantity. The composition of nanosheets was evaluated by the EDS spectrum, which is depicted in Figure 3. This spectrum shows that only oxygen, Co, Zn and Fe are present, which also confirms that synthesized nanosheets are a composite of cobalt, zinc and iron oxides without any impurity. The XRD pattern shown in Figure 4(a) displays the good crystalline structure of the composite nanosheets. The XRD pattern of the composite nanosheets shows the existence of three oxide phases (i.e. Co$_{3}$O$_{4}$, ZnO and FeO). These phases are similar to that shown in Reference ~[17] for the Co$_{3}$O$_{4}$ phase, in Reference [18] for the ZnO phase and in Reference [19] for the FeO phase.

    The chemical structure of the composite nanosheets is also examined by FTIR analysis, which is recorded in the range of 400-4000 cm$^{-1}$ and shown in Figure 4(b). The intensive bands are observed at 549 and 652 cm$^{-1}$, which are attributed to the M-O and M-O-M (M $=$ Co, Zn, Fe) bonds, respectively. Supplementary peaks centered at 840, 1361, 1642 and 3370 cm$^{-1}$ are assigned to CO$_{2}$ and H$_{2}$O captivated from the atmosphere[20].

    The optical properties of the composite nanosheets are scrutinized by using a UV-vis spectrophotometer, and the spectrum is shown in Figure 5(a). In the UV/visible absorption method, the energy band gap of nanomaterials can be acquired by analyzing their optical absorption. The UV-vis absorption spectrum exhibits an absorption peak at 450 nm and shows a band gap energy equal to 2.8 eV, which is calculated by Tauc's formula. The relation between the absorption coefficient ($\alpha )$ and the incident photon energy ($h \upsilon)$ is given by the equation[21].
    $\alpha h\upsilon =A(h\upsilon -E_{\rm g})^{n}, $(2)
    where $A$ is a constant, $E_{\rm g}$ is the band gap of the material, and the exponent $n$ depends on the type of transition, like $n$ $=$ 1/2, 2, 3/2 and 3, which correspond to allowed direct, allowed indirect, forbidden direct and forbidden indirect, respectively. Taking $n$ $=$ 1/2, we have calculated the direct energy band gap from the ($\alpha h \upsilon)^{1/n}$ versus $h \upsilon$ plots (Figure 5(b)). The band energy was calculated and compared with the band gap energy reported for manganese and zinc oxides[16]. The band gap energy of manganese and zinc oxides is slightly higher than that of composite nanosheets.

    Figure 6 shows the DC resistance-pressure and impedance-pressure relationships for the pristine nanopowder of metal oxides composite nanosheets (oxides of cobalt, zinc and iron) at different frequencies. It is found that under the effect of pressure, the magnitude of DC resistance and the impedance of nanosheets decreases on average by 44 %. Particularly, this decrease at DC, 100 Hz, 1 kHz, 10 kHz, 100~kHz and 200 kHz is 58 %, 66 %, 67 %, 47 %, 15 % and 9 %, respectively. The observed behavior can be explained by the densification of the powder samples and a decrease of their porosity under the effect of pressure. All relationships are quasi linear.

    To explain the transducer's resistance ($R)$, the following expression can be used[22]:
    $R=\frac{d\rho }{A}=\frac{d}{\sigma A}, $(3)
    where $d$ is the length or inter-electrode distance, $A$ is the cross-section area of the sample, and $\rho$ is the resistivity ($\rho =\frac{1}{\sigma }$, where $\sigma$ is conductivity). By using Equation (3), the initial DC conductivity of the pristine doped metal oxide nanopowder can be found as 2.93 $\times$ 10$^{-8}$ $\upOmega $$^{-1}$$\cdot $cm$^{-1}$. The resistance-pressure relationship, shown in Figure 6(a), may be due to the decrease of the thickness ($d)$ and accordingly the porosity of the sample under applied pressure or/and the increase of the conductivity ($\sigma )$ of the nanosheets. Figure 7 shows the dependence of the DC resistance on the pressing time. It can be seen in Figure 7 that the DC resistance increases with time and saturates after 8~min. This phenomenon can be considered as being due to the effect of the displacement currents of bound charges, which is well known[23]. Under various pressures, the increase in resistance with time is 59 % on average.

    The mechanism of conductivity in doped metal oxide nanopowder can be considered as thermally assisted hopping transitions between spatially separated sites or particles that can be attributed to the Percolation theory[24, 25]. According to Percolation Theory, the average conductivity ($\sigma )$ can be calculated by the following expression:
    $\sigma =\frac{1}{LZ}, $(3)
    where $L$ is a characteristic length, depending on the concentration of the sites, and $Z$ is the resistance of the path with the lowest average resistance. With increase in pressure, firstly, the $L$ decreases and secondly the $Z$ decreases. As a result, the conductivity increases and the resistance of the sample decreases, accordingly; it was also observed experimentally (Figure 6).

    Moreover in the nanopowder, usually the sizes of pores are small and comparable with the sizes of nanoparticles, while in micropowders, the pore sizes are much larger accordingly. Therefore, in micropowders, usually it is observed that the repeatability of the results from one experimental cycle to another cycle is a lot less. The advantages of the nanosheet sensing materials are the reliability, high sensitivity and easy fabrication. The disadvantage is that the nanopowder should be fabricated by the use of proper technology.

    The simulation of the experimental results is carried out by using a linear function[26]:

    The above function is modified as follows:
    $R/R_{\rm o} = kp + b, $(6)
    where $R$ and $R_{\rm o}$ are resistances of the sample under pressure and at the initial state, and $k$ is the fitting parameter. For the experimental data presented in Figure 6(a), the value of $k$ is -0.0724 m$^{2}$/kN. The simulated and experimental graphs of the resistance-pressure relationship are shown in Figure 8. It can be seen that the simulated graph in the first approximation satisfies the experimental result. All impedance-pressure relationships can be simulated by the same approach as well.

    The impedance ($Z)$ of the sample is complex impedance and contains magnitude and phase ($Z= $/$Z$/$\angle \theta$). Table 1 represents the values of the phases at the initial state and under the highest pressure. It can be seen from Table 1 that as the frequency increases, the phase also increases at the initial state and under the highest pressure as well. The dependence of the phase on pressure is different at different frequencies; at only 100 Hz, the phase increases under applied pressure, while at all other frequencies, the phase decreases.

    The equivalent electric circuit of the nanopowder samples can be represented by the parallel connected resistor and capacitor (Figure 9). The obtained experimental results and the circuit analysis show that as the frequency ($f)$ increases, the reactance (1/2$\pi fC$) decreases with respect to resistance ($R)$, while at low frequency, as the pressure increases, the reactance decreases relative to resistance, but at higher frequencies, it increases relatively. The conduction current and the displacement currents of bound charges take place in the nanosheets, while the capacitance current takes place through pores (as micro-capacitances). The equivalent circuit presented in Figure 9 seems to be valid for the sample of nanopowder and conductive micro-units comprised of two neighboring nanosheets.

4.   Conclusion
  • The metal oxide composite nanosheets were synthesized successfully and a transducer was fabricated for the direct measurements of the influence of pressure on the impedance of the pristine powder. The transducer allows one to measure the impedance of different kinds of powders without any preliminary processing. As a result of the investigation, it was found that under applied pressure, the DC resistance and impedance (at various frequencies) of doped metal oxide nanopowder decreases by 44 % on average. It was also observed that under the highest applied pressure, the DC resistance increases with time and saturated within 8 min. The impedance-pressure relationships were simulated and the simulated results are in good agreement with the experimental results. As impedance can be represented as a complex number, the dependences of the phase on frequency and pressure was also investigated. The investigated nanopowder samples are represented as parallel connected capacitor and resistor, where conduction takes place through capacitors (micro-pores) and resistors (nanosheets) as well. The change in frequency and/or pressure results in the change in the "resistors" and "capacitors".

Figure (9)  Table (5) Reference (26) Relative (20)

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