Structure controlled enhanced photocatalytic activity of polyaniline (PANI)

Abstract

Here authors report the preparation of polyaniline (PANI) with crystalline structures in the form of Emeraldine salt (ES) following a simple wet chemical route by chemical oxidative polymerization method. Crystalline PANI is used for various applications such as photocatalysis, solar cells, supercapacitors, electromagnetic shielding, and anticorrosive protection. Enhanced crystallinity with modulated band structure is highly desirable for the effective use of this functional material. Hence, we attempted to synthesize polyaniline with enhanced crystallinity by controlling various process parameters. As process parameters, the molarity of HCl viz. 0.1M, 0.5M, 1.0M, 2.0M, and 6.0M; and precipitation (polymerization) temperature viz. 25 °C (room temperature, RT) and 0 °C (using ice) were investigated to synthesize PANI with maximum crystallinity to find optimum synthesis conditions. The structure of the polyanilines was investigated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) ultraviolet–visible (UV–vis) spectroscopy, differential scanning calorimetry (DSC) and zeta potential. Analyzing experimental data in addition to percent crystallinity, bond structure, particle morphology and optical band gap were studied. Finally, photocatalytic activity was investigated using Rh B dye. PANI synthesized using 2.0 M HCl and polymerization temperature 25 °C showed maximum photocatalytic activity which is attributed to its highest percent crystallinity with maximum I_Q to I_B ratio and largest band gap.

1. Introduction

Polyaniline (PANI), an intrinsically conducting semicrystalline polymer (ICP), has drawn immense interest and has been reported to be thoroughly studied in recent times because of its unique opto-electronic and magnetic properties [1]. In general, polymers are regarded as a class of materials having semi-crystalline structure, heat sensitivity, and electrically insulating properties [2]. However, unlike these common polymers, PANI offers its potential for many advanced applications due to its conducting semicrystalline nature [3]. Conducting polymers have the unique advantages of having flexible mechanics and polymer processability in addition to the electrical and optical characteristics of metal and inorganic semiconductors. Therefore, this finds numerous applications such as modified electrodes, electrode buffer layers, transparent conducting electrodes, and electrochemical supercapacitors, etc [4]. Besides these applications, PANI has been reported to show promising candidacy as a photocatalyst due to its direct wide band gap. Like other wide-band-gap semiconductor metal oxides, the wide band gap of PANI inhibits early recombination of free exciton pairs leading to enhanced photocatalytic reaction.

Rapid urbanization and industrialization are the causes of various environmental pollution including severe water contamination. Such pollution severely impairs human health due to the poisonous and carcinogenic nature of the contaminants [5]. The purification of wastewater is crucial for maintaining human health and a clean environment. Most of the means of water purification are expensive while photocatalysis can be preferentially chosen due to its low cost and simple working mechanism. Usually, metal oxide semiconductors are used as photocatalysts [6]; however, considering the cost and ease of synthesis, conducting polymers like PANI seem to be potential photocatalysts [7]. Though in recent times, many studies have been reported on photocatalysis performance of PANI-metal oxide nanoparticles (NPs) composites, the scope of enhancing photocatalytic activity of pure PANI is yet to be investigated [8]. Especially due to its three isolable oxidation states (fully reduced, half-oxidized, and fully oxidized state namely Leuco-emeraldine, Emeraldine, and Pernigraniline, respectively), structure control can be a key factor in modulating its photocatalytic activity of PANI [9]. Again, one of the oxidative states, namely Emeraldine, shows differential catalytic performance based on its base or salt structure. Synthesis parameters can control such structure of the polymer and subsequently enhance its photocatalytic activity.

Polyaniline was chosen due to its high stability, less expensive, facile synthesis route, and the switching ability between conductive and resistive states [10]. Here, authors have investigated the effect of molarity of HCl and precipitation (polymerization) temperature on the structure of PANI and finally studied the effect of structural modification on its photocatalytic performance. The nanoparticles (NPs) were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy, differential scanning calorimetry (DSC) and zeta potential. Acid doping was chosen to control the oxidative state of the polymer and also as doping improves the spatial isolation of charge carriers and accelerates the degradation of impurities [11]. Polymerization temperature was also believed to control the overall crystallinity due to diffusion-controlled nucleation and growth mechanism of the polymerization process. Finally, it was concluded that the increased percent crystallinity, maximum I_Q to I_B ratio with Emeraldine salt structure, and wide band gap of PANI resulted in maximum photocatalytic activity of PANI synthesized using 2 M HCl and polymerization temperature 25 °C.

2. Experimental

2.1. Materials

Chemicals of analytical grade were used in as received condition. The following materials were used to synthesize PANI: aniline (ANI) from Sigma–Aldrich; ammonium persulfate, ${\left(\mathrm{N}{\mathrm{H}}_4\right)}_2{\mathrm{S}}_2{\mathrm{O}}_8$ (APS) from Vetec, Brazil; 37 % hydrochloric acid, HCl from Merck Germany; and ethanol and acetone from Nuclear, Brazil.

2.2. Synthesis of nanoparticles

The ratio of aniline to APS was 1:1. The volume percentage of aniline was 1.824 % (v/v), and the concentration of APS was 0.0567 g/mL. After preparing both ANI and APS solutions, APS was slowly added to the precursor solution to initiate polymerization via oxidizing monomers. Deep green color precipitates were settled down after 24-h of polymerization at 25 °C (room temperature, RT) and 0 °C (using ice). Precipitates were then settled down on centrifugation after a number of times washing with deionized water (DI). After drying at room temperature for 7 days, the samples (Fig. 1a) were crushed to get fine particles (Fig. 1b). Nanoparticles synthesized using HCl of different molarity viz. 0.1M, 0.5M, 1.0M, 2.0M, and 6.0M for polymerization temperature viz. 25 °C (room temperature, RT) are designated as PANI-0.1H, PANI-0.5H, PANI-1.0H, PANI-2.0H and PANI-6.0H, respectively. To describe the effect of polymerization temperature, nanoparticles synthesized using HCl of strength 2.0M for polymerization temperature viz. 25 °C (room temperature, RT) and 0 °C (using ice) are designated as PANI-2.0H RT-RT and PANI-2.0H Ice-RT, respectively.

Fig. 1.

Synthesis of PANI NPs (a) after drying and (b) after crushing.

2.3. Characterization

XRD of the NPs was performed on an X-ray diffractometer (PANALYTICAL EMP 3) using Cu-K_α radiation of wavelength 0.15406 nm for Bragg's angle (2θ) range from 20° to 80° (40 mA, 40 kV, step size 0.02, scanning rate 10°/min). The value of $({\mathrm{I}}_{\mathrm{Q}}$/ ${\mathrm{I}}_{\mathrm{B}})\text{,}$ intensity ratio of the quinonoid to benzenoid, was calculated from Fourier transform infrared spectroscopy (Nicolet™ iS20 FTIR Spectrometer) data recorded over 1000–4000 cm range. The morphology of the nanoparticles was studied using a TESCAN VEGA Scanning electron microscope. Diffuse reflectance spectra (DRS) along with absorbance were measured with PerkinElmer Lambda 365 spectrophotometer. The direct optical band gap of the nanoparticles was measured using the Kubelka-Munk formula from the DRS. A Hitachi DSC 200 was used to measure the ES powder's DSC spectra using an argon purge at 10 °C/min being heated to 300 °C. Zeta potential and particle size were measured using HORIBA Nano Particle Analyzer SZ-100V2.

2.4. Photocatalytic activity measurement

Rh B dye was used for photocatalytic performance evaluation in a water-based solution under ultraviolet (UV) irradiation in order to examine the photocatalytic activities of pure PANI NPs. pH of the aqueous solution was constantly controlled using sodium hydroxide solution at 10. To achieve the absorption desorption equilibrium, Rh B dye with nanoparticles was magnetically stirred for half an hour in the dark to the UV irradiation. To determine the degradation under UV light, 4 mL solutions were taken out at regular intervals to begin the analysis. The separated samples were immediately centrifuged to eliminate any leftover particles from the solution, and the UV–vis spectrometer was used to determine the absorbance. Finally, the degradation efficiency and kinetics of the reactions were calculated.

3. Results and discussion

3.1. Structural investigation

The combined XRD pattern exhibits diffraction peaks at 2θ = 15.3°, 20.3°, and 25.5° for (011), (020), and (200) planes of PANI NPs, respectively (Fig. 2a). The peak at 15.3° indicates the periodicity of the repeating unit of polyaniline chain [12]. Two peaks, centered at 20.3° and 25.5°, belong to the periodic arrangement in parallel and perpendicular directions with respect to the polymer chain, correspondingly [13]. PANI synthesized with lower acidity (0.1 mol/L and 0.5 moL/l, and 1.0 mol/L HCl) shows minor peaks at 23.2° due to π–π periodic stacking of the rigid phenazine‐like structures, which progressively disappear with increasing acidity [14]. Intensities of the primary crystalline peaks at 20.3° and 25.3° mitigate as the acidity increased, which is attributed to the reduction in phenazine ring oligomers and less PANI chain ordering [13]. Considering as a whole, the observed peaks verify that the polymer structure has both short- and long-range order. The XRD diffraction analysis reveals a notable alteration in peak sharpness related to crystallinity. Sharp, high-intensity peaks in the XRD pattern generally signify well-ordered crystalline materials.

Fig. 2.

(a) XRD patterns and (b) crystallinity of PANI NPs for different molarity of HCl.

Polymers are mostly semicrystalline in nature composed of crystalline lamellae embedded into an amorphous phase, constructing a highly internetwork [15]. XRD patterns are used to determine the crystallinity of polyaniline. Crystallinity of the nanoparticles was estimated using the following formula:

$$\text{Crystallinity}(\%)=\frac{\text{Area}\text{of}\text{Crystalline}\text{Peaks}}{\text{Area}\text{of}\text{all}\text{peaks}\left(\text{Crystalline}+\text{Amorphous}\right)}$$

Usually, conjugation length increases with increasing molarity of acid, which is proportional to the molecular chain length [13]. Increasing acidity causes an increased amount of HCl doping that can result in increased crystallinity by increasing active nucleation site density (Fig. 2b). Experimental findings also reveal that crystallinity rises with increasing acid concentration up to 2 mol/L and no significant change in crystalline structure is observed for further increase in molarity of the acid (Table 1). This can be attributed to the transformation from linear to fibrous molecular structure of PANI NPs, where bulky molecular structure inhibits the overall crystallization process.

Table 1.

Crystallinity of PANI NPs at room temperature polymerization and drying.

PANI Samples (RT-RT)	Crystallinity (%)
PANI-0.1H	21.1
PANI-0.5H	25.6
PANI-1.0H	27.8
PANI-2.0H	34.5
PANI-6.0H	33.3

Fig. 3 shows the normalized FTIR spectra of PANI NPs for polymerization at room temperature. C=C stretching vibrations of quinonoid, Q (–N = Q = N–); C=C stretching vibrations of benzenoid (B) (–B–NH–); C–N stretching vibrations of secondary aromatic amines and BBB units; C=N stretching and N = Q = N vibration mode resulted in the absorption bands roughly at 1584, 1500, 1308, 1242 and 1172 ${\text{cm}}^{-1}$, respectively for all PANIs synthesized with different HCl concentrations at room temperature polymerization and drying [16]. The other absorption bands at 1448, 1420, and 1342 ${\text{cm}}^{-1}$ show the formation of aniline oligomers, which are typically formed by the ortho coupling [12]. These aniline oligomer-representing characteristic bands are identified for PANI-0.1H, which is totally absent for higher HCl concentration [17]. For weakly acidic conditions, polymerization takes place specifically in the ortho coupling route, neutral aniline molecules can be oxidized, leading to the formation of phenazine structures through intramolecular cyclization; this complicates the directed self-assembly of PANI, whereas for strong acidic conditions, polymerization takes place in the para coupling route. In para coupling mode, out‐of‐plane C–H bending of 1,2‐disubstituted ring structure promotes head‐to‐tail coupling of aniline, resulting in linear and fibrous structures causing the disappearance of the oligomer-representing characteristic bands like at 1420 cm⁻¹ [18].

Fig. 3.

Normalized FTIR spectra of PANI NPs for polymerization at room temperature.

With increasing molarity of acid, Q and B bands shifted from 1584 ${\text{cm}}^{-1}$ to 1567 ${\text{cm}}^{-1}$ and from 1500 ${\text{cm}}^{-1}$ to 1489 ${\text{cm}}^{-1}$, respectively. The increased conjugation interaction and HCl doping cause these noticeable alterations. The proportion conjugation is ascertained using ${\mathrm{I}}_{\mathrm{Q}}$/ ${\mathrm{I}}_{\mathrm{B}}$. FTIR data analysis shows that the ratio increases with increasing HCl concentration, referring to the improved conjugation interactions. The values of ${\mathrm{I}}_{\mathrm{Q}}$/ ${\mathrm{I}}_{\mathrm{B}}$ for PANI‐0.1H, PANI‐0.5H, PANI‐1.0H, PANI‐2.0H and PANI‐6.0H are 0.83, 0.86, 0.84, 0.93 and 0.93, respectively. The largest value of ${\mathrm{I}}_{\mathrm{Q}}$/ ${\mathrm{I}}_{\mathrm{B}}$ for PANI‐2.0H and ANI‐6.0H confirms maximum conjugation attained for HCl of strength 2.0M or higher. Increasing the molarity of acid results in increased protonation and subsequently increases the pH of the reaction media leading to a greater extent of conjugation [18]. XRD study has revealed enhanced crystallinity as the rate of conjugation reactions rises. The increased crystallinity associated with high I_Q/I_B ratios of PANI contributes to higher electron transfer and lower recombination rate which results in enhanced photocatalytic performance.

3.2. Morphology study

Synthesis of different polyaniline molecular structures for various acid molarities leads to different morphologies and nanostructures (Fig. 4). For low acidic conditions, granular textures are evident, where the morphology is attributed to the stacking of plate shapes of the oligomers. With increasing molarity of HCl, a heterostructure comprising oligomeric cores with polyaniline fibers is observed.

Fig. 4.

SEM images of (a) PANI-0.1H, (b) PANI-0.5H, (c) PANI-1.0H, (d) PANI-2.0H and PANI-6.0H NPs.

In the PANI-0.1H sample (Fig. 4a), multiple rod- and plate- or petal-like shapes are prominently evident. Stacking of the rod and plate shapes is observed, particularly in smaller regions. In the PANI-0.5H sample (Fig. 4b), an increased number of rod-like shapes are seen, along with some overlapping structures. The PANI-1.0H sample (Fig. 4c) features needle-like structures and overlapping structures are more prevalent in this sample. The PANI-2.0H sample (Fig. 4d) exhibits needle-like structures along with numerous nano-rod and plate-type structures arranged in stacked layers over a larger area. Additionally, the growth of agglomeration can be seen on some surfaces. The needle-like structures are more prominent, and larger areas show stacking in many layers on the PANI-6.0H sample (Fig. 4e).

Agglomeration is increased with increasing overlapping of the particles to reduce the overall system energy, where oligomeric cores act as templates. As explained earlier, the fibrous structure was developed due to para coupling for higher acid molarity. The hydrophobic oligomeric structures serve as the core, whereas PANI exhibits some hydrophilic properties. The interplay between minimizing face-to-face repulsion between adjacent aromatic rings and PANI's low solubility leads to PANI chains' precipitation into primary granular particles [19].

The morphology for lower acidic conditions, the granular structure resulted in oligomers with flat phenazine-like oligomer structures stacked together on π- π interactions, which was also confirmed earlier from XRD analysis. The flat molecular structure of the phenazine rings at terminals of the molecular chains could produce strong intermolecular attractions. Thus, PANI sheets of the granular structure were formed by self-organization of the short oligomers. The increase in acid concentration accelerated the accessible conformations due to the growth of a few para-coupled oligomer chains. Therefore, PANI sheets acted as templates for the fibrous molecules for higher acidic conditions. Initially, homogeneous nucleation controlled the synthesis of the linear oligomers. Then, linear oligomers acted as nucleation centers for the subsequent chain growth. Partial anilines were rapidly oxidized in the highly acidic systems to form radical centers. With the increasing extent of HCl doping, the formation of active sites consumed plenty of aniline molecules on nucleation sites, resulting in the fibrous molecular structure [20]. The balance between interchain spacing, along with the formation of random coils, and the low solubility of polyaniline (PANI) to preserve its granular morphology can lead to increased agglomeration. The size and development of this agglomeration can vary depending on the pH conditions during the synthesis process. Specifically, lowering the pH increases agglomeration as the aniline is in its predominantly protonated form, resulting in the reduction of phenazine units [21].

3.3. Optical properties

As Fig. 5a shows, emeraldine salt exhibits significant absorption across the wide spectra. The π–π∗ peak around 290 nm appeared for aniline oligomers, whereas the peak around 370–390 nm is attributed to the phenazine or leuco-emeraldine‐type oligomers [13]. These bands grew weaker and finally disappeared with increasing acidic conditions. The peak around 600–660 nm is ascribed to p–π∗ excitation from B to Q rings [17]. The mitigated peak intensity of π–π∗ excitation and intensified peak intensity of p–π∗ excitation reveal the reduced extent of oligomers and increased conjugation length with increasing acid molarity. The precise position of the peak around 600 nm is determined by the oligomer chain's length as well as the quantity of reduced and oxidized rings [22].

Fig. 5.

(a) UV–vis spectra and (b) Kubelka-Munk plot of PANI NPs synthesized with different HCl concentrations.

Diffuse reflectance spectra (DRS) recorded with a UV–vis spectrophotometer were analyzed to study the optical properties of the nanoparticles. The direct band gap was estimated from ${\left[\mathrm{F}\left(\mathrm{R}\right)\mathrm{h}\upsilon \right]}^{\mathrm{n}}$ vs $\mathrm{h}\upsilon$ plot, where the power coefficient, or parameter n, has a value of 2.

$${\left[\mathrm{F}\left(\mathrm{R}\right)\mathrm{h}\upsilon \right]}^{\mathrm{n}}=\mathrm{A}\left(\mathrm{h}\mathrm{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\mathrm{h}\mathrm{\upsilon }-{\mathrm{E}}_{\mathrm{g}}\right)$$

Where A is constant, F(R) is the Kubelka-Munk function, h $\mathrm{\upsilon }$ is the energy of the incidence photon and ${\mathrm{E}}_{\mathrm{g}}$ is the band gap [23]. The calculated band gap energy of PANI‐0.1H, PANI‐0.5H, PANI‐1.0H, PANI‐2.0H, and PANI‐6.0H were 3.139, 3.142, 3.147, 3.156, and 3.164, respectively (Fig. 5b). In most cases they use lower acidic concentration for polyaniline synthesis. With increasing acidity, conjugation reaction increases which results in increasing band gaps. In our case due to using high HCl concentration, we found a band gap in the upper portion in the band gap range.

In polyaniline, a conjugated system is created when carbon atoms form covalent bonds with alternating single and double bonds. In these hydrocarbons, the electron p_z orbitals become delocalized, resulting in a delocalized bonding π orbital and π∗ antibonding orbital. The delocalized π orbital acts as the highest occupied molecular orbital (HOMO), while the π∗ orbital serves as the lowest unoccupied molecular orbital (LUMO). In the context of organic semiconductor physics, the HOMO corresponds to the valence band, and the LUMO represents the conduction band. The energy difference between the HOMO and LUMO levels is referred to as the band gap in organic electronic materials. Nitrogen atoms of PANI are combined with aromatic rings to form a system of conjugated single and double bonds [24]. The quinonoid form has a lower energy gap but higher energy than the aromatic benzenoid form, which is energetically more stable [25]. However, delocalization along the conjugated bond can result in an increased band gap. The energy required to change from the aromatic to the quinonoid state is determined by the aromatic stabilization resonance. The π-electrons in the aromatic form are trapped inside the aromatic ring and are unable to delocalize along the polymer backbone. In addition, the bandgap of the material gets affected by rotational disorder along the conjugated backbone. Large angles between successive units increase the bandgap and restrict the π-electrons' ability to delocalize along the conjugated backbone [31]. As a result, bandgap rises with increasing acidity because, as previously demonstrated, the I_Q to I_B ratio increased with increasing HCl concentration. A photon with energy higher than the band gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) excites an electron from HOMO into an empty LUMO. This creates a photoexcited electron (e⁻) in the LUMO and a hole (h⁺) in the HOMO. These charge carriers can either recombine, resulting in their annihilation without initiating a chemical reaction, or they can engage in oxidation and reduction reactions with substrates that are adsorbed on the surface of the photocatalyst, using the HOMO and LUMO, respectively. With increasing acid concentration, the distance between HOMO and LUMO increases, so this recombination rate decreases, which results in enhanced photodegradation.

Fig. 6 presents the DSC plots of HCl-doped PANI samples with different acid concentrations. Two endothermic peaks were observed for all the samples, one at about 90–150 °C and the other at about 200–300 °C. The first endothermic peak can be attributed to the loss of loosely bonded water molecules, i.e., moisture, especially for the peaks at less than 100 °C. At slightly higher temperatures, negative heat flow results from the loss of comparatively strongly bonded water molecules, often called bound water, which serves as a secondary dopant [26]. Therefore, both loss of moisture and dopant decomposition contribute to the first broad endothermic peak. The first endothermic peaks for PANI‐0.1H, PANI‐0.5H, PANI‐1.0H, PANI‐2.0H, and PANI‐6.0H were found to be 90.5 °C, 95 °C, 101 °C, 109 °C, and 135 °C, respectively. Thus, the redshift of this peak position indicates lower moisture content and a greater extent of dopant degradation for moderate to highly doped PANI samples.

Fig. 6.

DSC plot of PANI NPs synthesized with different HCl concentrations.

For moderate to higher doping concentrations, dopant degradation was more prominent since excessive doping causes structural instability of the doped ES structures. Again, higher dopant concentration usually reduces the hydrophilicity of organic molecules. Thus, for the broad first endothermic peak, with increasing dopant concentration, the contribution from loss of moisture was continuously mitigated, and the contribution of the dopant degradation was continuously increased. This primarily shifts the first endothermic peak position to a slightly higher temperature. Overall, as dopant concentration increased, the interaction between organic molecules and the dopant ions increased, and the mobility of the macromolecules got restricted, leading to higher thermal stability. The second endothermic peak indicates the glass transition temperature (T_g). T_g appears as a slight step in the heat flow graph, which is usually not associated with weight loss. Here, no significant change in glass transition temperature was evident with increasing acid doping. However, the step in the heat flow curve was less prominent for higher doping concentrations. Usually, conjugation length increases with increasing acid concentration, proportional to the molecular chain length [13]. As the length of the molecular chain increases, its mobility decreases, leading to such changes in glass transition behavior. Also, as acidity increases, the rate of conjugation reactions increases, which enhances crystallinity and further contributes to the observed phenomena.

Zeta potential is measured to relate it to the particles’ dispersion stability. The positive zeta potential value signifies moderate photodegradation properties of PANI for the selected cationic dye, Rh B dye (Fig. 7) [27]. Due to the lower zeta potential value of the PANI-0.1H sample, it showed maximum agglomeration and was highly unstable, whereas the remaining samples were at the threshold of delicate dispersion (Table 2). PANI synthesized under highly acidic conditions results in increased protonation, leading to a higher zeta potential value. On the contrary, for PANI-0.1H, insufficient protonation results in lower zeta potential. According to the zeta potential values, the PANI-0.5H and PANI-1.0H nanoparticle dispersions were relatively stable, while PANI-2.0H and PANI-6.0H were moderately stable [28,29].

Fig. 7.

Zeta potential plot of PANI NPs synthesized with different HCl concentrations.

Table 2.

Zeta potential and Conductivity of PANI NPs synthesized with different HCl concentrations.

Sample	Zeta potential (mV)	Conductivity (mS/cm)
PANI‐0.1H	+7.80	0.224
PANI‐0.5H	+14.35	0.230
PANI‐1.0H	+16.97	0.232
PANI‐2.0 H	+21.00	0.236
PANI‐6.0H	+22.03	0.241

These stability levels correlate with their respective degradation efficiencies. Higher zeta potential leads to less agglomeration, ensuring a more stable solution. This results in a higher effective surface area, increasing free ion concentration, leading to more excellent conductivity and improved photocatalytic activity [30].

For PANI-0.1H, lower acidity leads to insufficient protonation, causing more agglomeration, which prevents accurate measurement of particle size and rather measures the aggregation size (Fig. 8). As the molarity of the remaining samples increases, protonation also increases (Table 3). PANI forms granular structures during protonation, which may later agglomerate, resulting in a larger particle size [21].

Fig. 8.

Particle size plot of PANI NPs synthesized with different HCl concentrations.

Table 3.

Particle size of PANI NPs synthesized with different HCl concentrations.

Sample	Particle Size (nm)
PANI‐0.1H	841.00a
PANI‐0.5H	205.00
PANI‐1.0H	445.00
PANI‐2.0 H	458.84
PANI‐6.0H	508.00

^aAgglomeration size.

3.4. Photocatalysis

3.4.1. Degradation efficiency and kinetics

The photocatalytic performance was studied by the decomposition of Rh B using UV light (Fig. 9a), and the degradation rate for PANI‐0.1H, PANI‐0.5H, PANI‐1.0H, PANI‐2.0H and PANI‐6.0H was determined. With degradation under UV irradiation, the characteristic peak intensity of Rh B at 558 nm steadily dropped. PANI NPs produce free excitons by absorbing radiation with an energy equal to or greater than its band gap. The reduced form of the cationic dye is created when it combines with photo-generated electrons and protons. The rate at which Rh B is broken down by photocatalysis is determined by this step. Ultimately, the dye's Leuco form breaks down into compounds containing C ${\mathrm{O}}_2$ and ${\mathrm{H}}_2\mathrm{O}$ [32] Degradation efficiency was calculated using the following formula:

$$\text{Degradation}\text{efficiency}(\%)=\frac{C_o-C}{C_o}\times 100$$

Fig. 9.

(a) Degradation curves and (b) kinetics of photocatalytic degradation using PANI NPs synthesized with different HCl concentrations under UV light irradiation.

Where C_o and C represent the initial and final concentration of dye, respectively. Degradation efficiency for PANI‐0.1H, PANI‐0.5H, PANI‐1.0H, PANI‐2.0H and PANI‐6.0H are 33 %, 41 %, 43 %, 47 % and 47.1 % respectively. The faster charge carrier recombination, which limits the mobility of the free charge carrier and reduces degradation efficiency, is the cause of the lower degradation of pure PANI.

First-order kinetic parameters were estimated using the following formula (Fig. 9b):

$$\mathit{ln}\frac{C_o}{C}=\mathrm{K}t$$

Where, $\mathrm{K}$ is the first-order rate constant.

$K$ values were determined from slope of the lines and the estimated values were 0.0022 min⁻¹, 0.00303 min⁻¹, 0.00312 min⁻¹, 0.00343 min⁻¹ and 0.00343 min⁻¹ for PANI‐0.1H, PANI‐0.5H, PANI‐1.0H, PANI‐2.0H and PANI‐6.0H, respectively (Table 4). The photocatalytic activity was enhanced gradually with increasing the HCL concentration. The regression correlation coefficient (R²) was used to validate the credibility of the calculated rate constants.

Table 4.

First-order rate constant and regression coefficient calculation.

Sample	K (min−1)	R2
PANI‐0.1H	0.0022	0.9816
PANI‐0.5H	0.00303	0.9934
PANI‐1.0H	0.00312	0.9935
PANI‐2.0 H	0.00343	0.9936
PANI‐6.0H	0.00343	0.9906

3.4.2. Mechanism

In general, there are three main steps in the photocatalysis process: absorption of light, generation of free excitons, and the interfacial catalytic redox reactions after interfacial charge transfer (Fig. 10) [33]. The π-π∗ electronic transition takes place from the valence band (VB) to the conduction band (CB) on ultraviolet light irradiation [34]. Photogenerated charges migrate to the free surface of conducting polymer (CP) and activate the adsorbed oxygen molecules to form superoxide radicals (O₂·^-), which in turn produce hydroxyl radicals (OH·). Moreover, the hydroperoxyl radical (HO₂·) is produced when H⁺ and superoxide species (O₂·^-) react. Furthermore, HCl doping caused the introduction of a side-way band above the valence band of PANI NPs. The produced electrons and holes are trapped at this sub-energy level, which ultimately prevents electron-hole recombination. The entire degradation mechanism can be depicted as follows.

$$\text{PANI}+\mathrm{h}\mathrm{v}\to \text{PANI}\left({\mathrm{e}}_{\left(\text{cb}\right)}^{‐}+{\mathrm{h}}_{\left(\text{vb}\right)}^{+}\right)$$

$${\mathrm{O}}_2+{\mathrm{e}}_{\left(\text{cb}\right)}^{‐}\to {\mathrm{O}}_2{·}^{‐}$$

$${\mathrm{O}}_2{·}^{‐}+{\mathrm{H}}^{+}\to {\text{HO}}_2·$$

$${\text{HO}}_2·+{\mathrm{H}}^{+}+{\mathrm{e}}_{\left(\text{cb}\right)}^{‐}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{e}}_{\left(\text{cb}\right)}^{‐}\to \text{OH}·+{\text{OH}}^{‐}$$

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}_{\left(\text{vb}\right)}^{+}\to {\mathrm{H}}^{+}+\text{OH}·$$

$${\text{OH}}^{‐}+{\mathrm{h}}_{\left(\text{vb}\right)}^{+}\to \text{OH}·$$

$$\text{OH}·+\text{Rh}\mathrm{B}\to \text{Degradation}\text{product}$$

Fig. 10.

Photocatalytic degradation mechanism.

However, PANI is a conductive polymer, and the conductivity of polyaniline is correlated to I_Q to I_B, i.e., the high to I_Q to I_B ratio directly contributes to the enhanced conductivity of polyaniline. However, the crystalline structure and degree of doping of polyaniline influence its conductivity [18]. Higher electrical conductivity of the materials results in enhanced photocatalytic properties. Effective electron-hole separation, triggered electron mobility, and increased photogeneration quantum efficiency are the reasons behind the observed impact. The acceleration of electron mobility in the photocatalyst's surface layers is caused by an increase in conductivity, which also increases quantum efficiency and photoactivity. Both the number of ${\mathrm{H}}^{•}$ radicals and the rate of photodegradation rise in this scenario [35]. As a result, PANI NPs synthesized with higher HCl concentration (PANI‐2.0H and PANI‐6.0H) having higher I_Q to I_B ratio shows better photocatalytic activity.

PANI-2.0 H and PANI-6.0 H show higher and similar IQ to IB ratio, wider band gap, and higher crystallinity than other samples polymerized at room temperature. The wider band gap lowers free excitons recombination rate which results in better photodegradation. Besides, high photocatalytic activity depends on high crystallinity since it prevents electron-hole recombination and the growth of reactant quantity onto the photocatalyst surface [36].Thus, both PANI-2.0 H and PANI-6.0 H exhibit optimal performance. As acidity rises, the synthesis of PANI nanoparticles (NPs) becomes increasingly time-consuming, less sustainable, and more costly. For instance, synthesizing PANI-6.0H required approximately 10 days for drying at room temperature. Additionally, the fumes from the acid pose health risks to humans, making the synthesis process challenging. Therefore, PANI synthesized with 2.0 mol/L HCl appears to be the optimum condition.

The increased crystallinity associated with high I_Q/I_B ratios of PANI contributes to greater stability and enhanced reusability when degrading various environmental pollutants. This higher crystallinity leads to a lower density of defects and fewer grain boundaries, which helps eliminate recombination centers for photogenerated electrons and holes [37].

Polymer photocatalysts are common semiconductors with a larger band-gap energy that can absorb solar light strongly, which excites electrons from the VB to the CB and subsequently form electron-hole pairs. Ultimately, the electrons and holes react to the absorbed pollutants as they reach the photocatalyst surface. Conducting polymers are sophisticated photocatalysts with a wide range of environmental uses, such as solar water splitting, CO₂ reduction, N₂ reduction, and the degradation of organic contaminants [38].

3.5. Effect of polymerization temperature

The combined XRD pattern exhibits identical diffraction peaks at 2θ = 15.3°, 20.3°, and 25.5° corresponding to the (011), (020) and (200) planes of PANI NPs for polymerization temperature 25 °C (room temperature) and 0 °C with 2 mol/L HCl concentration (Fig. 11a). Polymerization at 25 °C resulted in crystallinity of 34.5 %, whereas the polymerized sample at 25 °C has a value of 35.9 %. This suggests that chains made at lower temperatures have fewer structural defects. The super-molecular arrangement of PANI chains is related to the degree of crystallinity. As the reaction temperature decreases, its crystallinity increases [39].

Fig. 11.

(a) XRD patterns and (b) normalized FTIR spectra of PANI NPs for polymerization at room temperature 25 °C and 0 °C (using ice).

The FTIR spectra of the nanoparticles polymerized at 25 °C are identical to the equivalent spectra of nanoparticles polymerized at 0 °C (Fig. 11b). In samples processed in ice, there is a little red shift in the principal peaks that correspond to the ring-stretching deformations of quinone and benzene at 1584 cm⁻¹ and 1497 cm⁻¹, indicating the protonation of the samples [40]. Here, ${\mathrm{I}}_{\mathrm{Q}}$/ ${\mathrm{I}}_{\mathrm{B}}$ ratios for PANI-2.0H synthesized in ice was 0.90, while the ratio was 0.93 for the polymerized sample at the room temperature. The ratio decreases in ice polymerization possibly as a result of fewer conjugation interactions.

Fig. 12 shows polyaniline polymerized at 0 °C that has a relatively compact layered lamellas morphology. The confinement of the polymerizing monomer in an ice environment increases local concentration, which further accelerates the polymerization and causes directional polymerization that results in the lamellar structure [41].

Fig. 12.

SEM images of PANI-2.0H Ice-RT NPs.

According to UV–vis spectra of PANI polymerized at 0 °C, polaronic transitions in protonated conjugated chains are thought to be the source of the bands at 440 nm and the benzenoid at 310 nm (Fig. 13a) [34,42]. In contrast, the π–π∗ peak, which is situated at around 290 nm and 370–390 nm, indicates a very strong absorption in PANI polymerized at room temperature. For both samples, the p-π∗ excitation from benzenoid rings into quinonoid rings was identified as the reason for the peaks at 600–660 nm [17].

Fig. 13.

(a) UV–vis spectra and (b) Kubelka-Munk plot of PANI NPs for polymerization at room temperature 25 °C and 0 °C (using ice).

PANI synthesized with a 2.0 mol/L HCl concentration in ice has an estimated band gap energy of 2.868 eV, whereas PANI synthesized at room temperature with a 2.0 mol/L HCl concentration has a band gap of 3.156 eV (Fig. 13b). Conjugated polymers are large molecules characterized by a backbone chain featuring alternating double and single bonds. These polymers exhibit a higher orbital overlap population, which results in lower bandgaps. The ratio of quinoid to aromatic forms is typically assessed using bond length alternation, defined as the average difference in bond lengths between the single bonds of inter-ring connections and the adjacent double bonds in a conjugated polymer's structure. As the I_Q/I_B ratio decreases due to lower polymerization temperatures, the conjugation reaction intensifies, leading to structural changes that minimize bond length alternation and ultimately produce polymers with smaller band gaps [31].

The DSC graphs of 2 mol/L HCl-doped PANI samples with varying polymerization temperatures are shown in Fig. 14. The first endothermic peaks of PANI‐2.0H polymerized at room temperature 25 °C and 0 °C were 109 °C and 112 °C, respectively. For lower polymerization temperature, the first endothermic peak was slightly moved to a higher temperature; however, the more prominent peak for lower polymerization temperature indicated higher structural instability. Again, for lower polymerization temperature, the second peak was shifted to a higher temperature due to higher crystallinity, as observed earlier from XRD analysis.

Fig. 14.

DSC of PANI NPs for polymerization at room temperature 25 °C and 0 °C.

The nanoparticle dispersion of PANI‐2.0H ICE-RT is more stable than that of PANI‐2.0H RT-RT (Fig. 15). In an aqueous system, the dissociation of initiators (APS) is more effective at lower temperatures, allowing for additional polymerization and increasing surface charge and zeta potential (Table 5) [43].

Fig. 15.

Zeta potential plot of PANI NPs for polymerization at room temperature 25 °C and 0 °C (using ice).

Table 5.

Zeta potential of PANI NPs for polymerization at room temperature 25^OC and 0^OC (using ice).

Sample	Zeta Potential (mV)	Conductivity (mS/cm)
PANI‐2.0H ICE-RT	+33.83	0.249
PANI‐2.0H RT-RT	+21.00	0.236

Synthesizing polyaniline (PANI) at lower temperatures increases its molecular weight and crystallinity [44]. Since the polymerization of PANI is an exothermic process, the reaction duration tends to be longer at lower temperatures. Maintaining a lower temperature leads to forming larger particles (Fig. 16) because increased crystallinity results in a larger crystallite size due to enhanced solid-state diffusion, increasing particle size (Table 6) [45].

Fig. 16.

Particle size plot of PANI NPs for polymerization at room temperature 25 °C and 0 °C (using ice).

Table 6.

Particle size of PANI NPs for polymerization at room temperature 25^OC and 0^OC (using ice).

Sample	Particle Size (nm)
PANI‐2.0H ICE-RT	690.83
PANI‐2.0H RT-RT	458.84

3.6. Photocatalysis

Although the polymerization of PANI-2.0H using ice resulted in better crystallinity, better thermal stability, and a higher zeta potential than samples polymerized at room temperature exhibiting lamellae structure, the increased zeta potential caused the particles to become highly dispersed. This dispersion limits charge transfer among the particles. Additionally, a smaller band gap and a lower I_Q to I_B ratio increase the charge carrier recombination rate, reducing photodegradation performance (see Fig. 17a) and lowering the reaction constant. (Fig. 17b). Here, both PANI-2.0H-RT-RT and PANI-2.0H-Ice-RT followed first-order kinetic reaction having K value 0.00343 min⁻¹ with 47 % degradation and 0.00059 min⁻¹ with 12 % degradation, respectively (Table 7).

Fig. 17.

(a) Degradation curves and (b) kinetics of photocatalytic degradation using PANI NPs for polymerization at room temperature 25 °C and 0 °C (using ice).

Table 7.

First-order rate constant and regression coefficient calculation.

Sample	K (min−1)	R2
PANI‐2.0H ICE-RT	0.00059	0.9916
PANI‐2.0H RT-RT	0.00343	0.9936

Though no data is available for the photocatalysis of pure PANI samples using Rh-B dye, Table 8 shows the available data for the Methylene Blue (MB) dye, which is also a cationic dye like the Rh-B dye. We have also included the data for the PANI sample, where the photocatalytic property is evaluated using Methylene Orange (MO) dye, and for the TiO₂/PANI sample, where the photocatalytic property is assessed using Rh-B dye.

Table 8.

Comparison of the photodegradation with the literature survey.

Catalyst	Synthesis Method	Pollutant	Degradation Rate	Reference
PANI	Oxidative polymerization	MB	42 % after 105 min	[46]
TiO2/PANI	In-situ polymerization	Rh-B dye	80 % after 180 min	[47]
PANI	Oxidative polymerization	MO	97.95 % after 120min	[46]

4. Conclusion

Pure polyaniline has been synthesized with varying synthesis process parameters. Structural, morphological, optical, and photocatalytic properties of the nanoparticles were investigated systematically. Due to their distinct morphology relating to the I_Q to I_B ratio, the polymeric materials exhibited structural diversity. Increasing acid concentration resulted in higher agglomeration with an alteration in particle morphology from granular to fibrous structure, whereas polymerization in the ice environment resulted in lamellar structure. PANI-2.0 H and PANI-6.0 H polymerized at room temperature showed better photodegradation due to their higher and similar I_Q to I_B ratio, wider band gap, and higher crystallinity preventing electron-hole recombination rate. 2 mol/L HCl seems to be the optimal concentration for PANI synthesis due to ease of synthesis. It was evident that lower room temperature polymerization results in enhanced photodegradation compared to polymerization using ice. Since PANI-2.0H NPs polymerized at room temperature show better photocatalytic performance, they can be used as an effective photocatalyst in wastewater management and other photocatalytic applications.

CRediT authorship contribution statement

Sadia Rafiq: Writing – original draft, Methodology, Investigation, Formal analysis. Masuma Akther Lovely: Writing – original draft, Methodology, Investigation, Formal analysis. Samiya Rahman Mim: Investigation. Md Shofiqul Islam: Methodology. Mahbub Hasan: Resources. Md Muktadir Billah: Writing – review & editing, Visualization, Supervision, Resources, Conceptualization.

Data availability statement

No data was used for the research described in the article.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.