Polyaniline Supported Ag-Doped ZnONanocomposite: Synthesis, Characterization, and Kinetics Study for Photocatalytic Degradation of Malachite Green

Ag-ZnO/PANI nanocomposite was prepared via the sol-gel technique following in situ oxidative polymerization of polyaniline (PANI). XRD, UV-Vis, and FT-IR spectroscopy were employed to study the crystal size, bandgap energy, and bond structure of assynthesized nanocomposites. *e mean crystallite size of the nanocomposite determined from XRD was 35.68 nm. Photocatalytic degradation of malachite green (MG) dye using as-synthesized photocatalysts was studied under visible light irradiation. *e highest degradation efficiency was recorded for Ag-ZnO/PANI nanocomposites (98.58%) than Ag-ZnO nanoparticles (88.23%) in 120min. *e kinetics of photocatalytic degradation of MG follows pseudo-first-order reaction with rate order of 1.16 10min. Moreover, the photocatalytic activity of Ag-ZnO/PANI nanocomposites was evaluated and compared with Ce-Cd oxide, electrospun P(3HB)-TiO2, and with other catalysts in the literature. *e optimal conditions for photocatalytic degradation are as follows: the concentration of malachite green (0.2 g/l), pH (8), and the concentration of catalyst load (0.2 g/l) under visible light with an irradiation time of 120min.


Introduction
Malachite green (MG) is an organic compound that has arisen as a dubious agent in hydroponics in aquaculture, and it is utilized as a dyestuff for coloring materials such as cotton silk, paper, wool, and leather [1]. It is strongly dissolvable in water and ethanol to form blue-green solutions. Dye-contaminated wastewaters mostly enter the environment as discharges, and their release into the environment in processing wastewater poses a serious risk to both human health and the ecosystem [2][3][4][5][6][7][8]. MG has strong effects on the immune and reproductive systems and exhibits potential carcinogenic and genotoxic effects [9]. ere are a range of methodologies, such as physical (adsorption), biodegradation, chemical, and electrochemical techniques, that have been developed to eliminate these pollutants from wastewater. However, they are nondestructive, since they just transfer organic compounds from water to another phase, thus causing secondary pollution.
Heterogeneous semiconductor photocatalysis is the most extensively read technique for the degradation and decolorization of numerous wastes in the watery medium under UV-Visible light [10,11]. It takes out defilements as opposed to just moving them to another stage without the usage of possibly hazardous oxidants [12]. Photocatalysis is started by photons from UV light, which cause the electrons on the superficial of the photocatalyst to become excited in the valance band; these cause the electrons to go up into the conduction band and then leave positive openings [13]. e produced electron/hole pair incites a complex arrangement of reactions that can bring about the total degradation of organic contaminants, for example, a dye adsorbed on the semiconductor surface [14,15].
ZnO is a significant semiconductor material with a wide bandgap (3.37 eV) and enormous excitation binding energy (60 eV), effective nonlinear resistance, and better thermal conductivity [19,20]. ZnO has a few limitations including the quick recombination rate of photogenerated electronhole pairs, low quantum yield in the photocatalytic reaction in aqueous solution, and photocorrosion which impede the commercialization of the photocatalytic degradation process.
Even if various modification techniques are reported in the literature, neither metal nor nonmetal doping alone can solve the above problems; there is still a dearth of knowledge on metal-nonmetal codoping. Reports related to Cr-N codoped ZnO, Cr-doped ZnO, or N-doped ZnO are well documented [21,22].
Conducting polymers (CPs) are appreciably used as adsorbents for the removal of heavy metal ions or dyes from waste water and have attracted great attention due to facile synthesis, electrical conductivity, porosity and low fabrication cost, and environmental stability. Among the conductive polymers (CPs), polyaniline (PANI) was picked as one of the promising conductive polymers to tune composites' optical, electrical, and photocatalyst properties [23]. However, it possessed poor life span owing to the fragile backbone chain.
ere for synthesizing PANI/ZnO composite photocatalysts has benefits of the capacity to prevent corrosion dissolution of ZnO during photocatalysis as well as the capacity to empower photocatalysis under solar irradiation due to a decreased bandgap [24].
ere are earlier reports on the improvement of the electronic properties and catalytic potential of ZnO by introducing noble metals such as silver, gold, platinum, and palladium [25,26]. Among these, silver merits exceptional consideration due to its stability, conductivity, nontoxic nature, and comparatively less expensive. e incorporation of Ag-doped ZnO nanoparticles in the polymer matrix upgrades the mechanical properties and conductivity of polymer composite [27,28]. Doping factor is another parameter to amplify the conducting properties of the inorganic filler. Subsequently, in the present study, Ag-ZnO/ PANI composites have synthesized via the chemical oxidative in situ polymerization process of metal nanoparticles with the monomer unit.
To the best of our knowledge, there is no study in the literature that investigates the reaction kinetics of photocatalytic degradation of malachite green using polyaniline/ Ag-ZnO nanocomposite to improve optical and photocatalytic performance. us, this research was deliberated to synthesize the photocatalysts by incorporating Ag and PANI comodified ZnO nanoparticles and discovering the optical properties and kinetics of the photocatalytic activity under visible light irradiation for the degradation and decolorization of MG dye.  Figure 1).

Synthesis of Ag-Doped ZnO Nanoparticle.
Silver-doped zinc oxide (Ag-doped ZnO) nanoparticles were prepared by a sol-gel method [29]. About 50 mmol of Zn(II) acetate dehydrate was dissolved in ethanol (100 ml), and the solution was stirred for 30 min.
Oxalic acid dehydrate (2.51 g) was dissolved in ethanol (40 ml), and the solution was added slowly with constant stirring to the above Zn(II) acetate dehydrate solution. After the addition of oxalic acid, a white sol was formed, and the stirring was continued for three hours. To this, 2 wt% of silver nitrate (AgNO 3 ) was added and stirred for a further three hours. Sol was dried on a waterbath to form xerogel. e xerogel was then calcined at 500°C in a muffle furnace at a heating rate of 5°C/min and held at this temperature for 120 min. en, it was grinded by using the mortar and pestle. e powder was kept in the desiccator at room temperature.

Characterization of As-Synthesized Photocatalysts.
X-ray diffraction patterns were obtained using a BRUKER D8 (West Germany and equipped with Cu Kα radiation (λ �1.5405Å) at room temperature in the scan range 2θ between 10 and 90 o . Accelerating voltage and the applied current were 40 kV and 30 mA, respectively. e absorbance of the photocatalysts was recorded by the Sanyo UV-Vis spectrophotometer model (SP65, UK). 0.2 g of the photocatalyst was dissolved in 100 ml of deionized water. e absorbance was measured using a quartz tube with a scanning range of 400-800 nm. Fourier transform infrared (FT-IR) spectroscopy was used in the region between 4000 and 400 cm −1 to determine the functional groups and surface structure of the samples using a model of Shimadzu 8400S (German). About 5-10 mg of photocatalyst powder was mixed with a drop of paraffin and sandwiched between two KBr plates to measure the wave number.

Photocatalytic Activity.
Catalytic activities of the synthesized photocatalysts were studied for the degradation of malachite green (MG) under dark and visible light. 0.02 g of MG was dissolved in 500 ml (at pH � 8 and T � 25°c) of deionized water and MG solution was prepared. 0.2 g of Agdoped ZnO nanoparticles and Ag-ZnO/PANI nanocomposite samples were dispersed in 100 ml of MG solution separately and stirred for 30 min in the dark to establish the adsorption-desorption equilibrium of dye with the catalyst. NaOH was used to adjust the pH value of the solution. en, the reaction was carried out for 120 min because no further degradation was observed after that. e photocatalytic degradation was assessed by recording the absorbance values at definite time intervals. Percentage degradation of MG dye was calculated using the following formula: where C t is the concentration of dye at time t and C o is the concentration of dye at initial stage.

Initial Concentration of Malachite Green and pH.
Effect of initial concentration of MG on its degradation was observed by taking the difference dye initial concentrations 0.1-0.5 g/L and fixing other parameters constant. And the effect of pH was also investigated by taking the pH range from 6 to 10 keeping other parameters constant (photocatalyst load � 0.2 g, pH � 8).

Catalyst
Load. e effect of catalyst load was observed by taking the difference amount of Ag-ZnO and PANI-Ag-ZnO nanocomposites (0.1-0.4 g) at constant dye concentration (0.2 g/L) and constant pH 8. e relation between the percent degradation of MG with irradiation time was studied over the reaction time 20-120 min, using a fixed concentration of dye (50 mg/L), catalyst load (0.2 g/L), and pH 8.

Kinetic Studies of Photocatalytic Degradation of MG.
e kinetics of the photocatalytic degradation of MG solutions was investigated using optimized photocatalyst load, dye initial concentration, and pH at ultraviolet and visible irradiations.

X-Ray Diffraction Analysis.
e average crystallite size (d) was calculated from the XRD pattern according to the Scherrer equation [31]: where k is a constant (about 0.9), λ is the wavelength . It was observed that PANI has a broad amorphous nature, and the diffraction peak is located at 2θ � 25.42°. e average crystal grain size of the composite materials calculated at 2θ � 38.06, 44.23, and 64.36°is chosen to determine the average diameter; the average size of the nanoparticle is 31.52 nm. e change in the intensity or broadness of the nanocomposite is due to the strong interfacial interaction between nanoparticles and the polymer matrix.

UV-Vis Absorption
Spectra. UV/Vis absorption spectra of the as-synthesized photocatalysts Ag-ZnO, PANI, and Ag-ZnO/PANI are shown in Figure 3. e bandgap of the catalyst directly influenced the photocatalytic activity in this way that the direct absorption of bandgap photons would lead to the generation of electron-hole pairs within the catalysts; subsequently; the charge carriers started to difuse to the surface of the catalysts. Journal of Chemistry is could be due to the bandgap narrowing resulting from the creation of dopant energy levels below the conduction band [33]. e wavelengths of the absorption edges in the UV-Vis spectra were determined by plotting a vertical line from the apex of the curve which is given in Table 1, and the energies are calculated using the following Debye-Scherer's equation e delocalized metal electron of Ag 3d state accounts for narrowing bandgap energy of Ag-ZnO nanoparticles from 3.2 eV to 2.87 eV. Ag-doped ZnO has also showed a red shift compared to the bare ZnO nanoparticles by transferring electrons from the conduction band of ZnO to the conduction band of metal 4d states. Addition codoping of PANI enhances further shift in wavelength to large wavelength of 474 nm and narrowing the bandgap to 2.61 eV of former Ag-ZnO/PANI photocatalysts.
However, some holes are also created in the VB of PANI due to electron transfer from VB of PANI to VB of Ag/ZnO, as the VB of PANI (3.25 eV) is at higher energy than the VB of Ag/ZnO (2.87 eV).
e transferred electrons will neutralize some holes in the VB of Ag/ZnO. In this way, a certain fraction of the holes in the VB and electrons in the CB of PANI get separated, reducing the chance of recombination and thereby enhancing the chance of photocatalytic activity.
is is the advantage of using a coupled system [33,34]. Further reduction in the bandgap of the Ag-ZnO/PANI might be due to the synergetic effect of the two dopants Ag and PANI that control the crystal size and enhance the photo harvesting of nanocomposites.

FT-IR Analysis
. Ag-doped ZnO shows a peak at the wavenumber from 3400 to 3452 cm −1 . e peak at 3442 cm −1 is attributable to the HO − stretching vibration of water and an O-H group on the surface of the photocatalyst. e strong absorption peak centered at 508 cm −1 is the metallic stretch assigned to the Ag-doped ZnO. e symmetric and asymmetric bending modes of C-O bonds were in 1636 cm −1 .
ere were some bands that originated from the presence of water moisture and carbon dioxide in the air in the process of making the pellet (Figure 4(a)). e FT-IR spectrum of Ag-ZnO/PANI shows a strong peak of Ag-doped ZnO at 609 cm −1 . e absorption band of PANI has occurred at 1126 cm −1 . e peak associated with the N-H, C-H, and C-N stretching vibrations is located at 3442, 2932, and 1470 cm −1 , respectively. e spectrum at 1293 and 801 cm −1 corresponds to the C-N in plane deformation and � CH in plane vibration (Figure 4(b)) of PANI. us, from the FT-IR spectra, it is confirmed that the nanoparticles are well inserted into the macromolecular chain of PANI, and aniline monomers are successfully polymerized on the surface of Ag-doped ZnO nanoparticles.

Photodegradation of Malachite Green under Visible
Irradiation.
e photodegradation of malachite green dye was performed for a total of 2 hours under visible light irradiation as shown in Figure 5. e absorption band intensities of the dyes decrease, and this indicated that the dyes have been degraded completely by the photocatalysts. Ag-ZnO/PANI nanocomposite showed the highest photoactivity (98.58%) compared with Ag-ZnO (88.23%). e pronounced enhancement of the photocatalytic activity of the Ag-ZnO/PANI nanocomposites may be attributed to their having more than one path to form electron-hole pairs because of the existence of different interfaces, and the electron-hole recombination prevented to the maximum extent in such system. e experiments were also realized in dark conditions to understand the effect of the light source when the catalyst material was added into the dyes. As a result, no significant changes were observed in the absorption spectrum of the dyes. Remarkable degradation was observed in visible light.  0.1 g/L to 0.5 g/L and fixing other parameters constant (photocatalyst load 1 mg/L, pH � 8) ( Table 2). e degradation efficiency of MG was increased with an increase in dye concentration up to 0.2 g/L ( Figure 6). Increasing excess dye concentration leads to covering the active sites of the photocatalysts by the dye, and the path length of the photons entering the solution is decreased, resulting in only a few photons reaching the catalyst surface.

Effect of Operational Parameters on Activities of Photocatalyst
Hence, the productions of hydroxyl radicals are reduced. erefore, the degradation efficiency is reduced [35]. It shows that more dye molecules are adsorbed on the active sites of the photocatalysts. e decrease in degradation above 0.2 g/L may be due to the competition of adsorption between dye molecules and dissolved O 2 on the catalyst surface.

Catalyst Load.
e effect of the photocatalyst loading on the decolorization rate of the dyes was examined by varying the photocatalyst concentration from 0.1 g/L to 0.4 g/L of the dye solution as shown in Figure 7 at constant dye concentration and constant pH 8. e degradation of MG initially increases with increase in photocatalyst load from 0.1 g/L to 0.2 g/L. However, further increase of the catalyst load from 0.2 g/L to 0.4 g/L results in decrease of the degradation of dye. e increase in percent degradation at 0.2 g/L is due to the increase in the number of active sites of the photocatalysts. e decrease can be explained by the excess photocatalyst particles that can create a light screening effect that reduces the surface area that is exposed to light illumination and the photocatalytic efficiency [36].   Journal of Chemistry 3.6. pH. pH affects the surface charge properties, size of photocatalyst aggregates, and the position of conductance. e effect of pH on the photocatalytic degradation of MG was investigated by taking the pH range from 6 to 10, keeping other parameters constant. e photocatalyst exhibited a maximum rate of degradation (98.58%) at pH � 8 in 120 min (Figure 8). erefore, at alkaline pH, the number of hydroxyl groups of the photocatalyst was increased, which facilitates the adsorption of MG. e probable reason for the difference in pH can be the adsorption of MG onto the catalyst surface depending on its surface area [37].

Effect of Irradiation Time.
e relation between the percent degradation of MG with irradiation time was studied over reaction time from 20 to 120 min, using a fixed concentration of dye (50 mg/L), catalyst load (1 mg/L), and pH of 8.
It was observed that at 120 min, the dye was completely degraded and becomes colorless. is is due to the fast adsorption rate before the equilibrium is reached; this may be explained by an increased availability in the number of active binding sites on the photocatalyst surface. At the equilibrium stage, the adsorption is likely an attachment-controlled process due to less available sorption sites (Figure 9).

Kinetics of Photocatalytic Degradation of Malachite
Green. e kinetic study of the degradation of MG was determined by using different initial concentrations of MG from 0.1 to 0.5 g/L. e photocatalytic activity of the synthesized nanocomposites under visible light can be evaluated by comparing the apparent rate constants [17,38] using the following Langmuir-Hinshelwood equation.
where c o and c are the initial and final absorbance of MG, and K app is the apparent rate constant. It can be seen that the photocatalytic activity of Ag-ZnO/ PANI nanocomposites under visible light irradiation is higher than Ag-doped ZnO nanoparticles. Figure 10 shows the relationship between time and the degradation rate (ln(c 0 /c)) of MG for visible light illumination. e regression correlation coefficient (R 2 ) is found to be 0.9972 for Agdoped ZnO nanoparticles and 0.9991 for Ag-ZnO/PANI nanocomposite. From the plot, the K app value for Ag-ZnO nanoparticle and Ag/ZnO/PANI nanocomposite is 1.56 × 10 −2 min −1 and 1.16 × 10 −2 min −1 , respectively. e kinetics curve was pseudo-first-order with respect to Agdoped ZnO and Ag/ZnO/PANI nanocomposites. e proposed mechanism is presented schematically in Figure 11 according to Pham et al., 2020, described in detail as follows [18]. e MG dye adsorbed onto the full photocatalyst. e possible photocatalytic mechanism involves the following steps:   Journal of Chemistry the malachite green into a nontoxic degraded product (such as water and carbon dioxide).
e role of PANI is clearly observed in movement and reduction of the carrier recombination due to the existence of linking doped ZnO particles and successive reduction of the surface resistivity of the entire photocatalyst to the photodegradation of MG. Table 3 provides a short comparison of MG degradation efficiency by different authors using different catalysts through the photocatalytic activity. It could be observed that the degradation efficiency under simulated solar light irradiation by xenon lamp of the PANI/Ag-ZnO was much higher than those of Fe 3+ /H 2 O 2 , electrospun P(3HB)-TiO 2 , Ce-Cd oxide, TiO 2 , and ZnO and CA/TiO 2 bio-nanocomposites, and the degradation rate of MG on PANI/Ag-ZnO nanocomposite was also faster.    Figure 11: Diagrammatic illustration of the mechanism for MG dyes degradation using the optimized photocatalyst composite by attaching PANI to Ag-doped ZnO nanoparticles. erefore, the nanocomposite is likely a decent and environment friendly catalyst in the removal of MG from contaminated (polluted) water.

Conclusions
PANI/Ag-ZnO nanocomposite was successfully synthesized in situ chemical oxidative polymerization.
e FT-IR result proved the strong interfacial interaction between the metal oxide nanoparticles and polar segments of the PANI chain. e XRD results showed that the amorphous nature of PANI got reduced with an increase in the content of metal oxide nanoparticles. e nanoparticle is 35.68 nm, whereas the nanocomposite is 31.52 nm. Ag-ZnO has a bandgap of 2.87 eV (432 nm) and the PANI/Ag-ZnO nanocomposite has 2.61 eV (474 nm). e photocatalytic activity of the MG was investigated, and a photocatalytic oxidation mechanism was proposed. Ag-ZnO and PANI/Ag/ZnO degrade the MG dye 88.23% and 98.58%, respectively. e highest photocatalytic activity of Ag-ZnO/PANI nanocomposites over ZnO photocatalysts was qualified due to dopants, low rate of recombination of the photogenerated electrons-holes, as well as its lower crystal size and bandgap energy.
Adding PANI to the Ag-doped ZnO leads to extra benefits in both surface textures with a high surface area and a simple electron transfer process. e optimal conditions for photocatalytic degradation of malachite green are as follows: the concentration of malachite green is 0.2 g/l, at pH of 8, and the concentration of catalyst load is 0.2 g/l under visible light illumination for 120 min. e kinetics of photocatalysis (PANI/Ag-ZnO) is a pseudo-first-order reaction with a positive slope and rate constant of 1.6 × 10 -2 min -1 . Our PANI-supported Ag-ZnO is predictable to be a promising applicant for environmental claims. e prevalent performance for various dye photodegradations and recycling abilities with a similar presentation is profoundly anticipated.

Data Availability
No data were used to support this study.
98.58 with 120 min is work