Electrically conductive polymers are grouped together under the general term, "conducting polymers" (CP) [1]. For CPs, the presence of non-bonding electrons or delocalized electron clouds throughout the chain of molecules creates a conduction channel. Due to the delocalization of electrons in a continually overlapping pi (π) orbital along the polymer backbone, they exhibit several remarkable electrical and optical features. The attractive and distinctive optoelectronic features of CPs may have applications beyond only protecting metals from corrosion, including non-linear optical devices, artificial actuators, sensor devices, all-plastic transistors, and light-emitting screens [2-4]. π-conjugated alternate single and double bonds of polymer backbones provide conducting polymers their conductivity by allowing the overlapping of π-bonded electrons along the polymeric chain, making the polymer chain itself conductive [5]. By incorporating different electron-withdrawing functional groups into the polymeric backbone, the HOMO-LUMO bandgap can be efficiently raised or lowered to achieve emission in the desired luminance range, allowing the synthesised conducting polymer to be used effectively in optoelectronics [6,7]. As a result of charge carrier generation, conductivity improves. Bandgap theory and a quasi-one-dimensional system describe how current flows through these polymers. Conducting polymers are very sensitive to gases as a result of the remarkable electrochemical and electrical characteristics that they possess. It is simple to synthesise, has a long-term stability, and is very inexpensive [8]. These are some of its benefits. However, its performance is hindered by a number of drawbacks, including a low sensitivity, a long response and recovery process, poor thermal stability and selectivity. Doping metal/metal oxides/carbon materials into conducting polymers, which makes them composite conducting polymer, will modify these limitations as advantages and enables them for room temperature applications. Amongst various kind of conducting polymers, polyanilines are studied widely due to its easier synthesisation in aqueous media and are environmentally stable. Polyaniline (PANI) and its derivatives have wide technological applications because of their high electrical conductivity and reversible proton doping. It is also easier to prepare it in bulk [9]. In recent years PANI derivatives have engaged the increasing attention of many researchers due to its wide applications in making humidity sensors [10]. Its electrical conductivity is reported to be strongly dependent on the doping level, redox state and moisture content [11].Vanadium is a transition metal with atomic number 23 and possess many oxide forms like VO, VO₂, V₂O₃, V₂O₅, V₃O₇ etc. Amongst inorganic dopant, the vanadium pentoxide (V₂O₅) is prominently used because of its many applications in electronic devices [10]. The bandgap of V₂O₅ is 2.3 eV. V₂O₅ can exhibit both photovoltaic and photoconductive nature or it can convert light energy to electrical energy directly and decreases electrical resistance with increase in radiance. Commercially, V₂O₅ is used as industrial catalyst due to its catalytic properties. Moreover, V₂O₅ is highly stable in nature and has many applications in electro chromic devices, sensors, catalysts, batteries etc. In this study, PANI and V₂O₅/PANI synthesized through in-situ chemical oxidation polymerization method. The XRD, UV, SEM and FT-IR techniques were used to study the conducting polymer and its nanocomposites The IV characteristics have been studied for electrical properties of polyaniline/silver nanocomposites.

Materials and Methods

Chemicals are used to prepare polyaniline and V₂O₅/polyanilinecomposite are aniline (M.W.93.13, SDFCL), hydrochloric acid (HCl), ammonium persulfate (NH₄)₂ S₂O₈ (M.W.228.20, Sigma-Aldrich), vanadium oxide (V₂O₅) (M.W.181.88, Sigma-Aldrich) of analytical grade. And synthesized by in-situ chemical oxidation polymerization method by using HCl as catalyst and ammonium persulfate (NH₄)₂S₂O₈ as oxidiser.

Synthesis of PANI

PANI was synthesised by in situ polymerization through the oxidation of 0.56M aniline monomer (M.W.93.13, SDFCL) with initiator oxidant 0.5M ammoniumperoxy-di-sulphate (APS) (M.W.228.20, Sigma-Aldrich) in 50mL water. The above two solutions were prepared independently and later mixed by stirring at 400rpm. The blue solution was kept overnight for polymerization. The green polymerized solution is filtered using vacuum filtration. The collected residue was dried in open air and powdered to get PANI powder. The overall synthesis of PANI polymer is shown in figure [11].

Fig 1: Synthesis of PANI by In-situ polymerization.

Synthesis of V₂O₅/PANI Composite

The V₂O₅/PANI composites were prepared by in situ chemical oxidization polymerization method. To the mixture of double distilled 0.01molaniline and 1M hydrochloric acid was added and stirred for few minutes to form aniline hydrochloride. V₂O₅ was added in different weight percentages (10, 20, 30wt %) with vigorous stirring to keep V₂O₅ suspended in the solution. 1.12g of Ammonium persulfate was added slowly with constant stirring at 0–5^oC. The reaction mixture was kept for stirring about 24h. The greenish precipitate was formed which was recovered by vacuum filtration and washed several times with distilled water. The obtained composite V₂O₅/PANI is kept in oven for drying 24h to get a constant weight [12].

Fig 2: Synthesis of PANI/V₂O₅ composite.

Polyaniline was prepared by in-situ polymerization method and transition metal oxide, i.e., V₂O₅ doped polyaniline was also prepared with different weight percentages (10%, 20%, 30%). In order to verify the applicability of synthesized composites characterizations of powdered samples took place using XRD, UV-Vis spectroscopy, FTIR spectroscopy and SEM images for surface morphology. For measuring the conductivity of the composite IV characteristic study was done with pellets of sample made with the help of hydraulic press and silver paste was coated for better measurements. The following findings have been made as a result of this study.

X-Ray Diffraction Analysis

XRD patterns of undoped PANI exhibits the semicrystalline nature, whereas the XRD patterns of V₂O₅ doped polyaniline confirms the semicrystalline to amorphous nature of the composite made and that why sharp peaks are less, as shown in figure. In undoped PANI, i.e., 0% V₂O₅ sample, an amorphous hump is found in between 23⁰ and 26⁰ and it is displaced to 15-18^o as mentioned in the JCPDS file no-00-024-1544. Up to 20% of V₂O₅ doped PANI, three small peaks continuously appear near 17^o (200), 21^o (001), and 26^o (110) which confirms the presence of V₂O₅ which is coinciding with the JCPDS file no 01-077-2418. In 30%, peak at 17^o is found to be vanishing thus found that V₂O₅ dopants interact very well with PANI which results in continuous changes in conductivity as well as crystallinity of the composite, i.e., V₂O₅/PANI. The polymer chain alignment or single or multiple helices may be to blame for the variations in crystallinity of the composites. Generally, the presence of wide peak in the pattern shows the amorphous behavior of the given material and it is known as amorphous peak. If there is a presence of sharp peak throughout the pattern indicates polymer has crystallinity [9]. Crystallite size of PANI and V₂O₅/PANI with different dopant concentrations have been calculated from Debye-Sherrer formula and values are given in the table 1.

The small decrease in crystallite size indicates that, according to the increase of doping concentrations; disruptions given by the V₂O₅ molecules in polymer chain increases and prevents the formations of larger crystallites or V₂O₅ will act as an obstacle to PANI matrix in crystal growth process and it will hinder the movements and alignments of the chains, inhibiting the growth of large crystals.

Fig 3: XRD graph of PANI and V₂O₅/PANI with different weight percentage.

Table 1: Crystalline size of PANI and V₂O₅/PANI.

Fourier Transform Infrared (Ftir) Spectroscopy

In FTIR spectrum, two types regions are there. One is called fingerprint region (0-1000cm^-1) and other is called functional group region (1000-4500cm^-1). Through the FTIR analysis of pristine PANI and V₂O₅ doped PANI, some changes in functional groups are observed as shown in figure. The comparison between doped V₂O₅/PANI and undoped PANI spectrum exhibits shifting in peaks. For example, Quinoid peaks from 1579cm⁻¹ to 1559cm⁻¹, bonding between Ar. Carbon and nitrogen at 1296cm^-1 is shifted to 1290cm^-1 and stretching peak of carbon and N⁺ ion at 1243cm^-1 is shifted to 1236cm^-1.The peaks occurring in PANI also can be found in V₂O₅/PANI composite, but due to the doping this was affected by a decrease in transmittance. The shift in peaks of doped V₂O₅/PANI in FTIR study indicates the formation of more ions during doping. The alkyne formation started taking place beyond doping of 20% and above. This shows that the triple bonds formations affect the semi-conducting properties of the doped V₂O₅/PANI polymer. Hence the semi-conducting nature of the doped V₂O₅/PANI polymers increases despite alkyne formation [13].

Fig 4: FTIR spectra of PANI and V₂O₅/PANI with different weight percentage.

Table 2: FTIR table of PANI.

UV-Vis Spectroscopy

According to the figure, the UV-Vis absorption spectra of PANI and V₂O₅/PANI composites is displayed. The UV spectrum, which extends from 200 to 250nm, as well as around 400nm, is absorbed by the PANI. When this occurs, the value of absorbance in the visible region (400-600nm) decreases at an exponential curve. The amount of V₂O₅ that is doped into the UV spectrum, specifically between 200 and 250nm, causes a decrease in absorption. Similar to the absorption peak that occurs at 400 nm as well. As the level of doping increases, a blue shift of some kind can be observed happening in the peak about 400nm. From the absorption spectrum, the values of band gap (E_g) were calculated by linearly extrapolating the Tauc’s relation given as;

Where h is Plank’s constant, ν is the photon’s frequency, Eg is the band gap energy, and A is a constant. γ is associated with the type of transition which has values 1/2 and 2 for indirect and direct transitions respectively. Here, it is indirect plot, i.e., (αhn)^1/2 vs hn. The reason is that, in an indirect band gap material, the maximum energy point of the valence band and the minimum energy point of the conduction band occur at different momenta (k). As a result, for an electron to transition from the valence band to the conduction band, it must change its momentum by interacting with lattice vibrations (phonons) or other electrons. According to increase in doping concentration, the increase in lattice distortions can be found as well. Thus, structural changes can affect electronic and optical properties including band gap. It can cause the formation of localized states within the band gap limit and leads to small increase in band gap energy.

Fig 5: UV-Vis spectra of PANI and V₂O₅/PANI with different weight percentage.

Fig 6: Tauc plot of PANI and V₂O₅/PANI with different weight percentage.

Table 3: Band gap with different weight percentage of doping.

Scanning Electron Microscope (SEM)

The Scattering Electron Microscope (SEM) study of V₂O₅/PANI helps us to analyse the effect on morphology of PANI when it is doping with V₂O₅. In the current analysis it is found that doping of V₂O₅ in PANI exhibits the change in surface morphology of composite as increased cluster formation represented in figure. Further V₂O₅ appears to be dispersed in composite polymer V₂O₅/PANI as shown in Figure. These clusters formed due to the doping of V₂O₅ results in higher value of conductivity.

Fig 7: SEM images of PANI and V₂O₅/PANI composite respectively.

IV Characteristics

IV characteristics study was done on pellets made by V₂O₅/PANI composite with the help of Hydraulic press. Approximately 3-ton pressure is applied to make the pellets. For making contacts silver pasting was applied on pellets and characterized. I-V characterization of PANI and V₂O₅/PANI was performed to obtain the behaviour and durability of the composite. From Fig. 8, the input bias of doped samples is -4 V to 4 V and -6 V to 6V in PANI. All the samples exhibit Ohmic behaviour at this range. The graphs of pure- PANI shows a linear response while V₂O₅ doped samples show a non-linear response. This is due the Schottky contact cause by the contact of silver (metal) and V₂O₅ (semiconductor). It is also showing that conductance of PANI sample is micro range which increases with doping of V₂O₅ or with increase in weight percentage of V₂O₅. Basically, the conductivity of polyaniline comes under semiconductor range. But, doping of V₂O₅ can improve mobility of charge carriers (electrons or holes) within the composite material. The enhanced mobility allows the charges to move more freely through the material, reducing the resistance to the flow of current. Or the introduction of n-type V₂O₅ into p-type PANI which makes the composite, will decrease its resistance or increase its conductance.

Fig 8: IV characteristics of PANI and V₂O₅/PANI with different weight percentage.

In our study, we described the comparative study of polyaniline (PANI) and vanadium pentoxide polyaniline composites (V₂O₅/PANI) in different weight percentages (10%, 20% & 30%) synthesized by chemical oxidation polymerization. From the study, it was observed that incorporating V₂O₅ into polyaniline matrix will influence the structural, optical, electrical properties of PANI. From XRD analysis, doping of V₂O₅ will disrupt the polymer chains of PANI and prevents the formation of larger chains. FTIR analysis confirms structural changes in PANI due to the doping of V₂O₅. UV-Vis spectroscopy analysis conveys the presence of blue shift according to the increase in dopant concentration. Furthermore, validates the existence of indirect band gap within the composite. From the IV studies, one can see that, according the increase of dopant concentration decreasing of resistance also occurs. This means that incorporation of V₂O₅ n PANI is influencing the conductance of the conducting polymer.

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