Investigations on Impact of YSZ Nanoparticles on Morphology & Electrical conductivity of PEDOT: PSS

Conductive polymer nanocomposites, which combine the flexibility and conductivity of polymers with the unique properties of nanofillers, have generated interest in various fields such as energy storage, sensors, coatings, and corrosion protection. This study discusses the effect of Yttria-stabilized zirconium nanoparticles (YSZ NPs) on the morphology and electrical conductivity of poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS). Thin films of PEDOT: PSS and PEDOT: PSS: YSZ were fabricated using spin coating technique. FTIR and XRD characterizations confirmed the interaction between YSZ nanoparticles and the PEDOT: PSS matrix, leading to changes in chemical morphology and film crystallinity. The investigation of the current-voltage (I-V) relationship showed improved electrical conductivity in PEDOT: PSS films with the addition of YSZ nanoparticles, with respective conductivities of 0.028×10−6 S cm−1 and 0.0885×10−4 S cm−1 for pristine and YSZ-containing films. Additionally, the sensing properties of the PEDOT: PSS: YSZ film in detecting organic vapours were studied. These findings suggest that these conducting polymer nanocomposite thin films could potentially be used as electrolyte components in battery cells, supercapacitors, and fuel cells, as well as serve as sensors for certain organic vapours.


Introduction
In the field of polymeric materials research, conducting polymers have attracted a lot of attention since their discovery in 1978.[1].This is mostly justified by their numerous uses, including in secondary batteries and biosensors.Nevertheless, conducting polymers' restricted processibility prevents their prospective applications from getting completely fulfilled.Because of their favourable processability, polymers that can dissolve in water or other solvents that are polar are therefore widely known and highly desirable.Among the wide range of conductive polymers that are readily accessible for purchase, because of its exceptional ability to dissolve in water, poly (3,4-ethylenedioxythiophene): poly(4styrenesulfonate), or PEDOT: PSS, is frequently referred to as the most efficient conducting polymer in the field of practical applications.[2].In addition, it should be emphasized that PEDOT: PSS exhibits notable characteristics such as high transparency, favourable mechanical flexibility, and exceptional thermal stability [3][4][5].It's also important to note that popular solution-based techniques like spin coating, spray pyrolysis, solution casting, and printing may be employed to create high-quality PEDOT: PSS films on a variety of substrates.It is crucial to emphasize that most other conducting polymers cannot be effectively treated with these methods.There are several uses for poly (3,4ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS), ranging from its use as an electrode constituent to its use as a photographic substance.Its intrinsic flexibility, transparency, and oxidative stability are responsible for its adaptability It is widely used in many different sectors, including transistors, biosensors, electrochemical displays, and flexible electrodes.However, a noteworthy disadvantage of PEDOT: PSS is its comparatively low electrical conductivity, with a value below 1 Scm -1 .1300 (2024) 012023 IOP Publishing doi:10.1088/1757-899X/1300/1/012023 2 PEDOT: PSS consists of two components: Poly(3,4-ethylenedioxythiophene) (PEDOT), which is insoluble in water, and poly(4-styrenesulfonate) (PSS), which is soluble in water.The insulating feature shown by the PSS component is responsible for the low conductivity seen in virgin PEDOT: PSS.Therefore, it is essential to enhance the conductivity of PEDOT: PSS films.To enhance the connection between PEDOT: PEDOT chains and facilitate the efficient passage of electric charges, it is necessary to mitigate the columbic interaction between the positively charged PEDOT polyions and the negatively charged PSS polyions [6][7][8].Numerous studies have been conducted on the improvement of conductivity in PEDOT: PSS using various ways and procedures.Among these approaches, the inclusion of different additives, particularly metallic nanoparticles in the PEDOT: PSS matrix, has shown superior outcomes.In a study conducted by Bhowal et al. (2019), gold (Au) and silver (Ag) nanoparticles were introduced as dopants into a matrix of PEDOT: PSS.The researchers observed changes in the morphology of the PEDOT: PSS and an improvement in its electrical conductivity [6].In a study conducted by Vidyashree Hebbar et al. (2018), the authors investigated the impact of incorporating graphene oxide particles on the conductivity of PEDOT: PSS films [9].In a study conducted by Zhuang Zhang et al. (2015), a coating of multi-walled carbon nanotubes (CNTs) was applied over PEDOT: PSS in order to investigate the relationship between the concentration of CNTs and the resulting increase in conductivity [10].According to the study conducted by Melendez et al. (2010), the authors observed the impact of silver nanoparticles on the electrical conductivity of PEDOT: PSS [11].Yttrium-stabilized zirconia's remarkable thermal expansion coefficient, low thermal conductivity, high ionic conductivity, and desirable thermal and chemical stability make it a valuable material for a wide range of applications, such as high-temperature structural materials [12], thermal barrier coatings [13,14], catalysis [15], and the electrolyte of solid oxide fuel cells [16].Furthermore, compared to bulk materials, nanostructured materials have better or distinctive characteristics and a wide range of capabilities.As an illustration, consider nanocrystalline rare-earth-stabilized zirconia, which, in comparison to its traditional microcrystalline form, has shown improved radiation resistance, decreased thermal conductivity, and increased ionic conductivity [17,18].Taking into account all of these aspects, we looked at how YSZ nanoparticles, a flexible metal ceramic material, affected the electrical conductivity and morphology of pure PEDOT: PSS in the current study.Additional research was done on the sensing capabilities of the PEDOT: PSS: YSZ thin film in the detection of various organic vapours.

Preparation of pristine PEDOT: PSS & PEDOT: PSS-YSZ nanocomposite thin films
By employing the spin coating technique, PEDOT: PSS and PEDOT: PSS: YSZ nanocomposite thin films were fabricated through the combination of Pure Yttria Stabilized Zirconium oxide (3YSZ) with liquid PEDOT: PSS.The process involved taking 5ml of Pristine PEDOT: PSS and another mixture of 5wt% YSZ nanoparticles dispersed in 5ml of PEDOT: PSS, which were separately subjected to stirring for an hour at 500 RPM and subsequent ultrasonication to ensure even distribution of the components.The resulting samples were then spin coated onto dry and pristine glass plates to produce transparent thin films suitable for further analysis and measurements.FTIR spectra was recorded between 400 to 4000 cm -1 (wavelength) in transmittance mode using Shimadzu IR Prestige-21 ATR-FTIR with resolution of 4 cm -1 .

X-ray diffraction
To check the effect of additive & to know the variation of dopant induced crystallinity, X-ray diffraction measurements were done.A powder X-ray diffractometer (model Miniflex-II; M/S Japan) was used to assess the samples utilizing Cukα X-ray radiation (λ=1.5418Å).To get the pattern the scanning range was set to a diffraction (2Ɵ) angle of 10̊ to 80.

Electrical measurements
I-V graph yields valuable information about resistance and conductivity.I-V curves were plotted to know the electrical conductivities of both the samples, by the four-probe method.I-V characteristics was measured using 4-point probe (Napson-3000).

X-ray Diffraction Studies
X-ray diffraction (XRD) investigation was conducted in order to examine the influence of an additive and examine the modifications to crystallinity accomplished by the dopant.The X-ray diffraction patterns acquired for the pristine PEDOT: PSS and YSZ-doped PEDOT: PSS samples are shown in Figure 4, with the diffraction angle ranging from 10 to 80 degrees.A prominent diffraction peak was seen at around 22.61˚ in the X-ray diffraction pattern of virgin PEDOT: PSS.This peak corresponds to the alternating lamella stacking arrangement of PEDOT and PSS, characterized by a lattice spacing of approximately 2.55Å.In the case of PEDOT: PSS, a wider peak was seen at an angle of about 21.86˚ (2θ), indicating a lattice spacing of around 2.63 Å for YSZ.The presence of faint crystallinity in the materials is indicated by the Border peaks seen in the X-ray diffraction (XRD) patterns.The X-ray diffraction (XRD) examines reveals corroboration of the amorphous characteristics of both virgin PEDOT: PSS and PEDOT: PSS: YSZ thin films.YSZ nanoparticles treatment of PEDOT: PSS results in observed peak shifting and the appearance of new peaks.The C-O-C bond stretching peak at 1051 cm -1 shifts to 1066 cm -1 .The C-S stretching peaks at 785 cm -1 and 830 cm -1 shift to 683 cm-1 and 812 cm -1 , respectively.The shifts in these peaks indicate specific molecular vibrations.The stretching of C-C bonds causes the initial peak at 1385 cm -1 to shift to 1378 cm -1 , while stretching of C=C bonds causes the initial peak at 1648 cm −1 to shift to 1652 cm -1 .The stretching of C=S bonds leads to the shift of the initial peak at 1969 cm -1 to 1913 cm -1 .The presence of a C-N bond causes the initial peak at 2213 cm -1 to shift to 2910 cm -1 , and stretching of C=O bonds causes the initial peak at 2976 cm -1 to shift to 2970 cm -1 .The stretching of C-H bonds results in the change of the absorption peak from 3431 cm -1 to 3411 cm -1 .Finally, the stretching of O-H bonds causes the initial peak at 3753 cm -1 to shift to 3739 cm -1 .These observed shifts indicate conformational changes in PEDOT and PSS chains, resulting from electrostatic interactions between YSZ nanoparticles and PSS.FTIR measurement confirms the alteration in the chemical structure of PEDOT: PSS chains with the inclusion of YSZ nanoparticles.

I-V Characteristics
The investigation of the current-voltage characteristics of produced polymer nanocomposite thin films and the subsequent analysis of their corresponding I-V graphs provide significant insights into the resistance and conductivity properties of these materials.The Figures 5 and 6 displays the I-V graphs of acquired for pristine PEDOT: PSS and PEDOT: PSS-YSZ thin films respectively.For PEDOT: PSS: YSZ thin films with a thickness of 96 micron, we observed high electrical conductivity and low electrical resistance.However, while considering the virgin PEDOT: PSS with a thickness of 33µm, it was observed that the electric current flow was significantly reduced and exhibited a high electrical resistance.Therefore, it can be deduced that the electrical resistance of PEDOT: PSS is reduced with the incorporation of YSZ nanoparticles.The observed phenomenon may likely be attributed to the electrostatic contact between YSZ nanoparticles and PSS chains.This interaction facilitates enhanced connectivity between PEDOT chains, hence promoting the unhindered motion of electrons along the PEDOT chain.It was determined that the electrical conductivity of the pure form of PEDOT: PSS is 0.028×10 -6 S cm -1 , whereas the conductivity of PEDOT: PSS: YSZ was found to be 0.0885×10 -4 Scm -1 .The inclusion of YSZ nanoparticles resulted in a threefold improvement in the conductivity of pure PEDOT: PSS.The investigation of current-voltage (I-V) characteristics provides empirical evidence supporting the enhanced electrical conductivity seen in virgin PEDOT: PSS thin films as a result of the incorporation of YSZ nanoparticles.

Sensing characteristics
The sensor has an extensive range of functional features linked to their particularity, reaction time, sensibility, and functional life and may be characterized as a system that can identify, measure, and send a clear output to a physical quantity [19].Percent sensitivity (S), which is shown in the equation below, was used to measure the sensor's performance.
Where, Ra and Rg represent the resistance of the sensor measurements in the presence and absence of an organic solvent gas, accordingly.Experiments were conducted utilizing low-boiling range organic vapor (OV) such as acetone, ethanol, hydrogen peroxide, and tetrahydrofuran (THF) to assess the sensing properties of PEDOT: PSS composite.The % sensing response, response time, and recovery time of PEDOT: PSS sensor thin films were used to evaluate their sensing capabilities.The chosen thin films (prepared composites) were first carefully set into the sample container.A 250 mL beaker containing 3 mL of testing sample OV (acetone, ethanol, hydrogen peroxide, and tetrahydrofuran) was then used to hold it.The thin films were exposed to the test sample's vapor for 250 s.It was then separated off the beaker and left out in the open for a further 250 seconds.Each sensor's resistance was seen to abruptly and steadily drop in the testing sample's vapor until eventually becoming constant.A sensor's resistance increased while it was in the air and finally reached the saturation point, returning to its previous value.

Sensing response
Fundamentally CP based nanoparticles are one of the most significant families of inexpensive sensing materials, outperforming other sensing materials in terms of sensitivity, response time, and reversibility at room temperature.Dopant and its composition may change the resistance of the CPs and their IOP Publishing doi:10.1088/1757-899X/1300/1/0120236 nanocomposites.Using Origin software, the prepared sensor (thin film) PEDOT: PSS collected the data after being exposed to four organic solvents (analytes) using a comparable technique.
It may be expected that PEDOT: PSS and its composite materials exhibit a strong sensitivity response (sensitivity%) to Hydrogen per oxide more than other gasses based on the chemical sensor readings (Table 1) acquired from this study.

Response and recovery time
The time it takes for a thin film's resistance to fall by 90% off its starting point following gas contact is referred to as the reaction time.The amount of time required for a thin film's resistance to return to 90% of its original level once the gas (air environment) has been eliminated is determined.Good reaction and recovery times are necessary to enhance sensing capabilities.In this work, liquid reducing analytes at ambient temperature (30 °C) were utilized rather than gas vapor.A liquid can be heated to almost boiling point at PPM value to get the same result.The evaporation rate is thus lower at room temperature.As a result, while the sensor's reaction time is quick, the response and recovery times are longer.Thus, the desirable gas-sensing properties are obtained.The measurements of the sensing response, response time, and recovery time of the PEDOT: PSS: YSZ sensor for various OVs are also shown in Table T1, and the same data is presented in a separate graph in Figure 7. From the reaction and recovery times, it can be seen that the time intervals range widely, from mere seconds to many minutes.PEDOT: PSS: YSZ sensor reaction time for acetone medium was 82 seconds, while tetrahydrofuran medium response time was 210 seconds.Figure 7 illustrates the PEDOT: PSS: YSZ sensor's dynamic recovery behaviour with acetone, ethanol, hydrogen peroxide, and THF OVs

Reversibility test
Superior sensitivity and reversibility, the two features of a commercial gas sensor that must be present for a practical use.The ability of the sensor to switch back and forth between the analyte and the surroundings without losing any detecting effectiveness is known as reversibility.Under various OVs, the reversibility test was run on synthetic thin film.PEDOT: PSS: YSZ were subjected to acetone, ethanol, hydrogen peroxide, and THF OVs for the first 250 s before being exposed to air for the next IOP Publishing doi:10.1088/1757-899X/1300/1/0120237 250 s, for an overall of four cycles, to test for reversibility.Figure 7 depicts the behaviour that was noticed.
For PEDOT: PSS: YSZ, the percentage sensitivity for ethanol, THF, acetone and hydrogen per-oxide OVs was found to be 0.44%, 2.81%, 4.32%, 7.11% respectively.Thus, the prepared PEDOT: PSS: YSZ sensor material shows good sensitivity to hydrogen per oxide compared to other OVs.
Table 1: Sensing characteristics of PEDOT: PSS: YSZ thin films for different Organic Solvents

5.Conclusion
Pure PEDOT: PSS and PEDOT: PSS: YSZ nanocomposite thin films were prepared using a spin coating technique, and the effect of YSZ nanoparticles on pristine PEDOT: PSS was investigated using FTIR, XRD, and I-V characterization techniques.The analysis of FTIR and XRD confirms the interaction between YSZ nanoparticles and the PEDOT: PSS chain.The expansion of XRD peaks and the noted changes in chemical morphology support this finding.The thin films' amorphous structure is shown by the X-ray diffraction (XRD) analysis.The inclusion of YSZ nanoparticles boosted the electrical conductivity of pristine PEDOT: PSS film from 0.028×10 -6 S cm -1 to 0.0885×10 -4 S cm -1 , according to examinations of I-V characteristics.Several metrics, including sensing response, response, and recovery time, were investigated for the produced thin films of PEDOT: PSS: YSZ for various OVs.Of these, PEDOT: PSS: YSZ shown a good sensitivity for hydrogen peroxide gas vapours.

Figure 1 :
Figure 1: Schematic diagram of synthesis of PEDOT: PSS-YSZ thin films by spin coating

4. 2
FTIR Studies FTIR spectroscopy provided molecular-level data to validate changes in the chemical morphology of PEDOT: PSS films when YSZ nanoparticles were added.A comparative analysis was conducted between PEDOT: PSS-YSZ nanocomposite thin films and pristine PEDOT: PSS films using FTIR spectrographs.Wave numbers at 785 and 830 cm -1 indicated the presence of a C-S stretching bond in PEDOT: PSS films, while 1051 cm −1 indicated stretching of C-O-C bonds.The infrared absorption bands at 1385 cm −1 and 1648 cm −1 were due to stretching vibrations of C-C bonds, C=C bonds, and sulfonic acid groups in Poly styrene sulfonate.Other peaks corresponding to the presence of C=S, C-N, C=O, C-H stretching, and O-H stretching were also observed.Additionally, a peak at 3753 cm -1 was attributed to stretching vibrations of O-H bonds in Sulfoxide functional groups.

Figure 7 :
Figure 7: Dynamic response curve of PEDOT PSS YSZ in a) Ethanol b) Tetrahydrofuran c) Acetone d) Hydrogen per Oxide OVs