“Polyaniline nanoparticles: A study on its structural, optical, electrochemical properties along with some possible device applications”

Highlights

• Polyaniline (PANi) nanoparticles (NPs) are prepared by chemical oxidative polymerization method.

• The successful synthesis of PANi NPs was confirmed by XRD, FT-IR, Uv-Vis spectroscopy data.

• For this material, optical band gap was found to be 2.47 eV with indirect allowed transition.

• A diode of this material was also designed and characterized under different photo exposure conditions.

• The designed device show self generation phenomenon and which can be well exploited for light sensing applications.

Abstract

In this report, the Polyaniline (PANi) nanoparticles (NPs) were synthesized by in-situ polymersation method. These PANi NPs were characterized by various standard techniques. The crystal structure study were done by Powder X-Ray diffraction (XRD) technique and confirms its amorphous structure. Scanning Electron Microscopy (SEM) images showed that this material shows micron size microfibers made up of NPs with less porosity. The Fourier Transform Infrared (FT-IR) spectroscopy confirms PANi NPs formation and dominant vibrations corresponding to its Quinone and benzene rings were very much visible in the data. From the UV-Visible spectroscopy data, the absorption data confirms the transitions related to its Quinone ring and pyran rings. The optical band gap was estimated to be around 2.47 eV and this material follows indirect allowed transition. From the Photoluminescence (PL) data, it showed various emission line in visible region after exciting it by various UV photons. PL data also confirms defects and bipolaron formation in it. The Cyclic Voltammetry (CV) of PANi was also studied and the parameters like specific capacitance (C_P), energy density (E_D),and power density (P_D) were estimated. For the current material, the value of C_P, E_D, P_D were calculated to be around 72F/g, 144 J/cm³, and 0.9 W/m³ respectively. Further the energy bands like highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of this material was determined by employing CV and the value obtained goes well as calculated by optical absorption techniques. The Electrochemical Impedance Spectroscopy (EIS) carried on PANi NPs (under different studied conditions) also reveals the formation of defects or the play of existing defects with the incident light photons. From practical application point of view, a diode like structure of this material was also fabricated. To this diode, current-voltage (I-V) and dielectrics spectroscopy measurements was carried out in dark and with photons of various wavelengths. Under different measured conditions, the I-V characteristics showed non-linear behavior. Under above experimental situations, the various parameters related with I-V characteristics were also calculated. Also this material show self-powered phenomenon, one of the essential conditions for photo detector/sensor applications. Further this device was also subjected to transient measurements. The reverse recovery time for this diode was calculated to be around 3.04 μs. The capacitance and loss studied on this device shows well dispersion with applied frequency. The observed capacitance and loss also show some changes in presence of different color of photons. The experimental data and overall properties shown by this material correlate with each other. The various properties shown by this material projects it a good candidate for future flexible optoelectronics applications.

Introduction

The conducting polymers have receive a lot of research interest for last so many decades [1]. These conjugated polymers are highly passionate materials as far as their scientific and practical applications are concerned [2]. Among the various conjugated polymer materials, the polythiophene, polyaniline, [3] and polypyrrole are the highly studied [4]. Nowadays, they find applications in various electronic devices like in organic transistors, organic solar cells, capacitors, and secondary cells [1], [3]. The Polyaniline (PANi) have received a lot of interest by condensed matter research community due to its various physical and chemical properties [5]. The prime feature of PANi is that it’s a conducting polymer and behaves as a P-type semiconductor [6]. Besides electrically conducting, PANi has a number of other distinct properties like cost, preparation, and environmental stability. This polymer can be easily synthesized with low cost and without much complications [7]. PANi shows excellent electrochemical characteristics, and can have interchangeable oxidation states; these properties can be changed by doping/dedoping by acid or base. This doping/dedoping is seen to affect the electric properties of this polymer to large extend [8]. These polymers can be used in various electronic gadgets like in chargeable batteries, photovoltaic, [9] sensing, for chemical separations, anticorrosion, and light emitting diode (LED) [10]. For energy storage, PANi can be employed as anode or cathode composite material and this material can also be used for catalytic (photocatalytic) properties [11].

Additionally, it is advantageous to directly mix with plastic or polymer solar cells or light emitting diodes and hence can create low-cost fully flexible photovoltaic devices [12]. Further research interest in PANi continues to grow due to its oxidative stability and as well as in electrochemical reversibility [13]. The variable redox states of PANi make it further interesting material in different cases [14].

It should be noted that PANi has very fascinating optical properties [9], [10], [11], [12], [13], [14], [15]. These optical properties are much structure dependent (means they depends the final salt like its acid/base) and also vary as per its oxidation state [16]. Also the modification or switching properties of PANi are closely correlated and depend on its various parameters especially when comes the concept of optical switching. Further it has been noticed that PANi has the ability to switch in its various oxidation states directly and then affects its optical characteristics in UV–visible regions [17]. Further it should be noted that the conductive form of the PANi is due to emeraldine salt and further addition of acid / base will protonate and deprotonate the base (-NH-) sites resulting in the switching within its various oxidation states [18].

There are limited reports where PANi is shown to be used for modern devices [19], [20]. However, there are few reports in which hybrid organic-inorganic hetero-junction between PANi (as P-type) and indium tin oxide (ITO)(as N-type) coated on glass substrate are studied [21]. These reports a just pertaining to synthesis, structural, optical, and electrical characterization under normal conditions. Also there is hardly any report related to effect of UV-Visible light on the electrical properties of these PANi/ITO structures or devices.

Therefore, the prime focus of this article was to successfully synthesis PANi (readers should note that here on words PANi and PANi NPs will be called alternatively) and then characterize it with various standard techniques. Further after well characterizations, we developed (by simple technique) a planer diode like structure like ITO/PANi/ITO and studied its various parameters under the exposure of UV-Visible light. A comparative study of these photovoltaic parameters of this proposed device under different light illumination conditions is done and discussed.