Path Way For Electrical Conduction In ZnO Basedternary Nanocomposite Room Temperature Ethanol Sensors

In the present investigations, the samples such as zinc oxide, tin oxide, vanadium oxide and their ternary combinations prepared by hydrothermal route were characterised using the state-of-the-art facilities and were systematically analysed. The interface properties between the grain and grain boundaries of these oxides were studied using Conductive atomic force microscopic (CAFM) studies. From the detailed investigations on topography and I-V characteristics, it is revealed that, zinc-tin-vanadium oxide nanocomposite with smaller barrier height of 0.189 eV exhibited pronounced response magnitude of 98.96 % at a faster rate of 32 s at room temperature. The outstanding ethanol sensing property of the ternary nanocomposite is attributed to the hierarchical morphology with large surface area, the formation of heterojunction at the interface, tuning the schottky barrier height, depletion layer manipulation and the electronic effects.

Research on MOS is also highly inspired by various desirable traits such as improved sensitivity [8], enhanced adsorption ability [9], extensive catalytic activity [10] and high thermodynamic stability [11] towards the analyte gases. Yet obtaining all of these characteristics using a single metal oxide has proved to be difficult. However, reports are available on coupling these different MOS in appropriate proportion to form heterostructures. These heterostucture nanocomposites favours enhanced functional properties compared to their individual metal oxide counterparts in sensing gases like LPG, NO2, H2, NH3 and so on [12]. This might be due to the formation of junctions at the interface of the heterostructure and thereby shortening the electron transport distance [13][14][15][16][17]. The synergistic effect of these nanocomposites has paved the platform to explore them as novel materials as high performance gas sensors.
Among various volatile gases, ethanol is used in various fields such as chemical industry, medicine and food industry [11,18]. Ethanol being a hypnotic gas causes severe harmful effect on environment due to its toxic nature. Hence, monitoring and controlling the production of ethanol and their usage in industries and laboratories, there is a need to deploy sensors integrated with electronic systems to avoid accidents as well as health risks [19]. Among various MOS, loosely arranged zinc oxide (ZnO)nanorods of flat end shape [20], tin oxide (SnO2) nanoparticles [21] and elongated nanostructured vanadium oxide (V2O5) [22] have been identified as room temperature ethanol sensors. ZnO and SnO2with reduced dimensions provide more active sites for the adsorption of gases [23] and V2O5, due to its high work function with semiconducting properties shows charge recombination [24], generation and transportation in many device applications. Stable sensing performance towards ethanol at room temperature could be improved by hybrid nanostructures [25]. There occurs Fermi-level mediated charge transfer effects and synergistic behavior of different components which could contribute for the enhanced ethanol sensing at room temperature. The mechanism behind the sensing of ethanol by heterojunction nanocomposites is: the depletion layer could be further depleted by the adsorption of oxygen gas onto the surface, which contracts the conduction channel by extracting electrons from the conduction band of the semiconductor and enhances the response towards the analytes [26]. The semiconductor with a lower Fermi energy acts as a metal in a Schottky junction. At that point, introducing foreign analytes at the heterointerface could alter the conductivity in various ways depending on the types of analyte and heterointerface [15]. The addition of a second constituent will undoubtedly change the resistance just by a rule-of-mixtures such that adding a more conductive material will increase the conductivity. Literature reports on, binary and ternary nanocomposite using these mentioned individual oxides and their investigations on band bending are available [15,27].
To explore further, the interface properties between zinc oxide, tin oxide and vanadium oxide and to analyse the path way conduction mechanism in the ternary nanocomposite, conductive atomic force microscopy (CAFM) has been employed. Therefore, in this present work, for the first time, ZnO, SnO2 and V2O5 were investigated for the topography and local conducting property using conducting atomic force microscopy. The conduction mechanism in ethanol sensing with respect to the potential barrier height established between the interface layers of the prepared samples are thoroughly investigated.

Experimental details
Analytical grade (AR) chemicals were used for the experiment and were used as such without further purification. Initially, 0.1 M aqueous solution of zinc chloride (ZnCl2) was taken and stirred well. During constant stirring, 0.1 M glyoxalic acid was added. Then, ammonium hydroxide was added dropwise and the pH was adjusted to 9 under stirring. The gel was obtained and it has been transferred into an autoclave and was kept in an oven at 160ºC for 3 h. Aerogel obtained was collected and it is washed with absolute ethanol and deionized water. The powders obtained were dried in air and calcined at 600 ºC and named as ZnO. Similarly, SnO2 and V2O5 were obtained by changing the suitable precursors as tin chloride (SnCl2.2H2O) and vanadium chloride (VCl3) respectively. The same protocol is followed to obtain the binary and ternary combination of these individual oxides by taking equimolar mixture of the corresponding precursors and the final samples obtained were named as ZT (zinc -tin nanocomposite), ZV (zinc -vanadium nanocomposite), TV (tin -vanadium nanocomposite) and ZTV (zinc -tinvanadium nanocomposite) and the stepwise procedure is given in Fig. 1.

Characterization Techniques
The structural property of all the samples was analysed by recording X-ray Diffraction pattern using PANanalytical X'Pert PRO diffractometer. The morphology of the samples was obtained from ZEISS ultra-field emission scanning electron microscopy (FESEM). Brunauer-Emmett-Teller (BET) based on the nitrogen adsorption-desorption isotherm is used for determining the specific surface area and size of the pores for all the samples. To analyse the compositional differences in the electrically conducting and non-conducting oxides, imaging of all the samples were performed using Scanning Probe Microscopy (SPM, NTEGRA NT-MDT Russia).
Topography and current maps were obtained in contact mode using a conducting cantilever (force constant of 2.5 N/m and radius of curvature 25 nm with a PtIr coated tip).

Structural studies
All the samples were subjected to X-ray Diffraction and the patterns have already been reported [13]. The peaks obtained in the X-ray diffraction pattern of the individual oxides were indexed The peaks obtained for TV nanocomposite matches with tetragonal rutile structured SnO2 and orthorhombic structured V2O5. In a similar fashion, we observe distinct peaks of SnO2, ZnV2O6, ZnV3O8 and Zn2SnO4from our ZTV sample. particles. The nanorods, nanoparticles and flakes with surface area of 123.9 m 2 /g, 118.8 m 2 /g and 116.5 m 2 /g respectively [13] could have the ability to provide more surface active sites to enhance the adsorption of ethanol molecules over the surface so as to increase the sensitivity in our earlier studies. The nitrogen adsorption-desorption isotherm obtained for all the samples exhibited type IV with type H3 hysteresis loop for the relative pressure P/Po in the range of 0.1 -1 which indicated the presence of mesoporous structure [13]. Among the three samples, ZnO nanorods possessed large surface area compared to SnO2 and V2O5 and the pore size of about 11.8 nm confirms the mesoporous nature of these samples [13]. attached over the nanorods resulting in surface irregularity leading to the specific surface area of (151.6 m 2 /g). In the vanadium loaded tin oxide as in Fig. 2(f), the reduction in the grain size of SnO2 may be due to the presence of V2O5 and resulted in surface area of (128.5 m 2 /g). EDAX spectra of the three binary oxides are available in supplementary. The adsorptiondesorption isotherm of ZT, ZV and TV exhibited the type IV -H3 hysteresis loop and from the pore size distribution plot the existence of mesoporous structure was identified in all the samples. From the Table 1, it was noted that ZT possessed larger surface area compared to ZV and TV nanocomposite.  . 3 (a and b)). The flower like microstructure is composed of nanoflakes extending axially from the center. The respective EDAX analysis (Table. 2) on the individual flake indicated the presence of excess amount of Zn than Sn and V [13]. In addition, the composite also exhibited spherical nanoparticles with 11 nm diameter in which excess Sn is identified from its elemental composition. The ternary oxide also exhibited type IV-H3 hysteresis loop as in Fig.3(c). Further, the pore size distribution plot (Fig. 3 (d)) revealed the existence of maximum number of pores with less than 10 nm diameter. The surface area and average pore size determined from BET and BJH for ZTV were 165.7 m²/g and 8.3 nm respectively Among the detailed analysis, it is revealed that the elongated nanostructures [15] especially ZTV exhibits the largest surface area with pore size of 8.3 nm. This could attribute to the best sensing performance experienced by ZTV towards ethanol at a faster rate even at room temperature.

Gas sensing analysis
The prepared samples were tested for ethanol sensing property at room temperature using the gas sensing apparatus [13]. The sensitivity was calculated for all the samples using the formula in equation (1)which is described as the ratio of the modulus of magnitude of change in resistance upon exposure to ethanol vapour to that of in air without vapour. The response time and recovery time was is defined as the time needed to reach 90% and 10% of the base line resistance after the injection and removal of ethanol respectively.
From the detailed analysis, it has been observed that the sensitivity of ZTV is considerably better than the individual oxides at a faster adsorption -desorption rate. This might be due to the large surface area and the synergistic effect. Moreover, the gas sensing mechanism [3] for n-type semiconductor nanocomposites can be explained based on the change in resistance which is Later, when the nanocomposite ZTV is subjected to the exposure of ethanol at room temperature, the vapor molecules will react with the adsorbed oxygen species and form CO2 and H2O. This in turn promotes the re -injection of the trapped electrons back to the depletion layer. Therefore, the width of the depletion layer gets decreased and so the sensor resistance gets decreased further. Fig. 4

AFM analysis and Electrical Conductivity Studies
The information about the topographical and the nano scale properties of all the samples were provided by the C-AFM technique [17]. The current flow between the conductive tip and sample is carried out initially. The cantilever deflection is monitored further as the tip scans over the surface. Topography images of all the samples were recorded to provide more information about the structural peculiarities. The conductive tip of a nanoprobe is used to provide the I-V measurements of the selected region of the samples.

Individual oxides
Topography image of ZnO in Fig. 5 [18]. The corresponding height profile of ZnO with respect tothe position of the probe on ZnO surface is depicted in Fig. 5(g).

Fig. 5: (a) -(c) AFM image of ZnO, SnO2 and V2O5, (d) -(f) Current maps of ZnO, SnO2 and V2O5, (g) -(i) Height and the corresponding current profile of ZnO, SnO2 and V2O5 respectively.
Conductive Atomic force microscopy was performed meticulously to study the surface topography in a small scale. CAFM topography of SnO2 in Fig. 5(b) shows fine grains with spherical shaped nanoparticles of uniform distribution with grain size around 50 -100 nm.
Slightly bigger grains of 200 nm were also obtained. In the grain boundary region, conductivity is less [19]. In the topographical image, SnO2 nanoparticles seem to be compact, dense withoutmuch vacant sites (gap) in between them. Nanoaggregates of SnO2 nanoparticles are formed upon crystallization at higher calcination temperature [20]. The height and current profile has been depicted in Fig. 5(h). The current map recorded as in Fig. 5(e) in the corresponding scan direction shows maximum current value of 0.5 nA at the curvature of the particles.
C-AFM image in Fig. 5(c) shows flakes like morphology with lateral dimension of 0.5 µ m.
These flakes are stacked diagonally to form bundles with clear lateral view. With respect to the bias voltage of 1 V, maximum current value of the bright zones around 600 pA were observed for the side of the V2O5bundles as in Fig. 5(i).The corresponding line profile for the topography and the current image is also depicted in Fig. 5(f).

Binary oxides and Ternary oxides
C-AFM topography image of ZT nanocomposite shown in Fig. 6(a)   In the case of ZV nanocomposite, the topographic image obtained from CAFM is shown in Fig.   6(b) shows densely packed rough surfaced nanorods. In the current map Fig. 6 (f) In the case of ZTV nanocomposite, clusters of nanoparticles with size of around 25 -50 nm are observed from the topographic image ( Fig. 6(d)). The corresponding current maps of ZTV also reveals the hollow flower like structures with bright edges and dark centersFig. 6(h). Maximum current of 12 nA is observed as in Fig. 6(l).
From the detailed study, among all the samples, ZTV is the most conducting material (Table .3) which is mainly due to the similar topographical image with spherical shaped nanoparticles with smaller grain size compared to pure SnO2 [21]. The synergistic effect and the nanocomposite formation [22] further would have contributed for better conductance of ZTV. Here in ZTV, grain boundary regions are more conducting. In the case of ZTVnanocomposite, the enhanced sensitivity is mainly due to their electrical transport mechanism, which is different from that of the individual metal oxide sensors. It has been analysed that the barriers between the metal oxide grains on the nanocomposites dominate the sensor resistance [23,24,28]. Electrons travel through the metal oxide grains into the nanocomposites and then conduct in between them. The process of adsorption mainly occurs at the surface of the grains. Because the size of the metal oxide grains in the nanocomposites are very small, a large fraction of the atoms are present at the surface and almost all the adsorbed species are active in producing a surface depletion layer for the small grains of nanocomposites (Fig7(a) and (b)). Moreover, the depth of the electrondepletion surface layer [29] due to oxygen ionosorption, has the impact on the particle size and henceforth the sensing performance of ZTV nanocomposte.

I -V Characteristics of individual, binary and ternary systems
Atomic Force Microscopy is a powerful versatile tool for the electrical characterisation of the nanojunctions. The platinum coated silicon tip has been used as a point electrode and the electrical characteristation has been performed in the contact mode while a fixed bias is applied between the conductive probe and the substrate. The electrical contact area between the AFM tip and the sample surface depends on the mechanical properties of the tip and the surface of the sample as well as loading force. In C-AFM, both the surface topography and the load current passing through the sample were measured simultaneously. With this tool, I-V measurements of a selected region can be performed using the conductive tip of a nanoprobe. Though poor mechanical stability of the tip and conductance fluctuation is observed in C-AFM, this tool is still advantageous in nanometer scale compared to conventional electrode evaporation techniques [30].
The thermionic emission current-voltage relation of a Schottky diode [31,32] is given by where Io is the saturation current, q is the electronic charge, V is the applied voltage, n is the ideality factor, k is Boltzmann's constant and T is the temperature in K. The saturation current Io [33,34] is defined by … … … … … … … . (9) where A, n is the theoretical Richardson constant (32 A/m 2 K 2 for ZnO, 120 A/m 2 K 2 for SnO2 and 50.3 A/m 2 K 2 for V2O5), A is the diode area and Φ SB is the zero bias barrier height or schottky barrier height. Φ SB is calculated using the following relation - … … … … … … … … … … … (10)

Individual oxides
The typical I-V characteristic curve for ZnOnanorods has been recorded and is shown in Fig.   8 The I-V characteristic curve for SnO2 nanoparticles observed is shown in Fig. 8  From the I-V plot (Fig. 8(c)) of V2O5, the barrier height has been calculated as 0.439 eV [37].
The barrier height value obtained for the prepared V2O5 flakes lie close to the value obtained for V2O5 nanorods. The I-V characteristics curve shows a turn-on voltage of 0.1 V for the forward bias and a reverse bias breakdown voltage is not observed till -10 V.
Compared to ZnO, SnO2 and V2O5, from CAFM measurements, it is identified that SnO2 is said to be more conducting holding 0.5 nA of current. V2O5 possess slightly more conductive nature when compared to ZnO.

Binary and Ternary oxides
The I-V plot of ZT nanocomposite is depicted in Fig. 9(a). The I-V characteristics curve shows a turn-on voltage of 7.5 V for the forward bias and a reverse bias breakdown voltage is not observed till -1.5 V. The schottky barrier height calculated for ZT nanocomposite is 0.554 eV which is found to be lower than the barrier height of individual zinc oxide. This smaller barrier height might be attributed to the reduction in the growth of the nanorods due to the formation of the secondary phase Zn2SnO4 along with the ZnO and SnO2 phases [37].This decreased barrier height paves the way for the faster adsorption and desorption of ethanol molecules on the surface of ZT which gives better sensitivity of 77.93% while ZnO and SnO2 shows only 51.98 % and 42.92 % respectively.
The I-V plot of ZV nanocomposite is depicted in Fig. 9(b). The I-V characteristics curve shows a turn-on voltage of 3 V for the forward bias and a reverse bias breakdown voltage is not observed till -0.5 V. The schottky barrier height (Table 3) calculated for ZV nanocomposite is 0.389 eV. But for ZnO the barrier height is 0.587 eV and for V2O5the barrier height is 0.489 eV.
The much smaller barrier height observed for ZV nanocomposite might be due to the combination of the elongated nanostructures contributed from both the individual oxides [36].
This decreased barrier height could be the reason for the better sensitivity of 72.97 % with response and recovery time as 98 s and 84 s respectively. The I-V plot of TV nanocomposite is depicted in Fig. 9(c) The I-V characteristics curve shows a turn-on voltage of 5 V for the forward bias and a reverse bias breakdown voltage is not observed till -5 V. The schottky barrier height shown in Table 4 for TV nanocomposite is 0.512 eV. The sensitivity of TV nanocomposite towards sensing of ethanol at room temperature is 63.99 %.
The I-V plot of ZTV nanocomposite is depicted in Fig. 9(d). The I-V characteristics curve shows a turn-on voltage of 1 V for the forward bias and a reverse bias breakdown voltage is not observed till -6 V. The schottky barrier height calculated for ZTV nanocomposite is 0.189 eV (Table 4). ZTV nanocomposite exhibits the smallest barrier height when compared to all the individual and binary oxides. It is understood from the Table 2 that the larger current of 18 nA possess smallest band gap due to the porous regions and hence the barrier height is smaller for ZTV.  [37] with less resistance of ZTV shows the fastest response and recovery of 32 s and 6 s respectively with better sensitivity of 98.93%. The results obtained for ZTV is in accordance with the literature that the metal oxides having smaller barrier height could exhibit good sensitivity at a faster rate towards the test gases.

Conclusion
In this present work, the topography and the electrical property of ZnO, SnO2, V2O5, ZT, ZV, TV and ZTV were systematically analysed. The local conducting studies have been carried out and from the detailed analysis, it is revealed that, among all the samples, ZTV possess the smallest barrier height of 0.189 eV. Also, the smaller resistance offered by the hierarchical architecture of ZTV and its larger surface area incites ZTV to excel with enhanced ethanol sensing even at room temperature. This unique hierarchical porous architecture also provides beneficial structural advantage in gas diffusion and adsorption, surface chemical reactions and electron transfer which might be due to the obtained smallest potential barrier heights between the grain boundaries. The CAFM and valence band characteristics of the hierarchical ZTV further illustrates and confirms that the small energy barrier of ZTV plays an important factor in the fast-response gas detection.
Such local study on the sensor performance of ZTV reported for the first time paves the way for the researchers to fabricate this material as a portable device using MEMS technology.