# Fabrication of multifunctional TANI/Cu2O/Ag nanocomposite for environmental abatement

Aug 21, 2020

### Formation mechanism of TANI/Cu2O/Ag NC

Initially, the monomer NPPD undergoes protonation by react with HNO3 to obtain the protonated ions in toluene medium and then the protonated ions undergo polymerization in presence of aqueous solution of APS. Here, the interjunction is formed between the toluene and aqueous media. The ions maces together and extend themselves into straight chain (1, 4 joining of phenyl groups) but the side products didn’t occur due to the fast reaction of anilinium ions. The protonation occurs on imine unit of quninonoid part of EB (TANI) and removes lone pair of electrons on nitrogen so that gets positive charge. The free NO3 accumulates onto the EB (green colour) because of electrostatic attraction between NO3 and TANI. The copper ions (Cu+) of Cu2O and Ag+ ions from AgNO3 which accommodate onto TANI as presented in Fig. 1.

### Characterisation study of prepared samples

#### XRD study

The pure phase formation of TANI, TANI/Cu2O and TCA composite was confirmed by powder XRD and the diffraction peaks of prepared materials are presented in Fig. 2. The broad peak at 2θ = 25° confirmed the presence of TANI29 along with other two major peaks at 20.6° and 27.5° due to the periodicity of parallel and perpendicular to the phenyl ring of TANI which authorizes the (110) plane and the peak was presented at 14.6° declared that formed TANI is amorphous in nature, thus XRD peaks are not sharp. Further, TANI implies four peaks have been observed in TCA NC with 2θ values of 38.4°, 44.9°, 65°and 78° which are consistent to their respective planes (111), (200), (220) and (311). The presence of sharp peak at 39°confirmed that Ag NPs are embedded in TANI/Cu2O/Ag matrix. The XRD pattern of Cu2O NPs has sharp peak at 2θ = 44° which corresponds to the (111) planes (standard JCPDS file No. 05-661) along with certain other peaks of low intensity30. However, these peaks show slightly shifted from their respective standard positions which may be due to the presence of Cu2O and Ag into TANI matrix. These results are further compiled with FTIR.

#### FTIR analysis

The FTIR spectra of synthesized Cu2O, TANI, TANI/Cu2O and TCA NCs are depicted in Fig. 3. The broad absorption peak in the range of 750–550 cm−1 can be attributed to C–NO3 bonding is observed in TANI. The sharp peak appeared at 834 cm−1 due to N–H bending vibrations. Two peaks were observed at 1,163 cm−1 and 1,307 cm−1 could be assigned to the presence of quinonoid ring (N = Q = N) and C–N stretching mode vibrations respectively. The peaks were centred at 1581 cm−1 and 1,489 cm−1 are attributed to C–N and C–C stretching mode for quinonoid and benzenoid rings31. The intensities of these two peaks are almost confirms that the prepared TANI is conductive with emeraldine base (EB) form. Another peak was presented at 2,919 cm−1 is due to the C–H stretching vibrations whereas the peak at 3,382 cm−1 for the N–H stretching vibrations. These peaks were slightly shifted from their normal positions of pure TANI32 owing to the existence of Cu2O, Ag into TANI matrix.

#### FESEM–EDS analysis

The morphology and structural elemental composition of prepared Cu2O, TANI, TANI/Cu2O and TCA NCs were investigated by FESEM and EDS respectively. Figure 4a,b shows the FESEM images of Cu2O clearly exhibited the octahedral like shape with smooth surface and an average edge length of about 478 nm. Figure 4c,d describes about the TANI, a clear and transparent sheet like structure was obtained. Figure 4e,f implies the FESEM images of TANI/Cu2O, in which the Cu2O are embedded in the matrix of TANI. It is clearly found that the surfaces of the prepared TCA NC were rough and uneven because of the presence of Ag and Cu2O nanoparticles are decorated on the surface of TANI as shown in Fig. 4g,h.

The presence of elements such as copper (Cu), oxygen (O), Silver (Ag), Carbon (C) and Nitrogen (N) in the EDS spectrum was confirmed the formation of pure TCA via a simple, facile insitu polymerization method (Fig S1).

#### Transmission emission microscopy (TEM)

Further morphology of prepared TCA NC was studied using HRTEM and Selected area electron diffraction (SAED); the images (Fig. 5a,b) indicated a poor crystalline nature of NC, supported by XRD results. It is in well agreement with the diffraction planes of TCA NC. The average particle size of TCA NC was 8.26 nm calculated from histogram (Fig. 5c) using imageJ software.

#### X-ray photoelectron spectroscopy (XPS)

Figure 6 displayed the electron spectroscopy of synthesized TCA NC was examined by XPS analysis. A polymer TANI contains the carbon and nitrogen shown the respective peaks of phenyl C1s, C–C (Fig. 6b) and nitrogen containing aromatic polymers N1s (Fig. 6c) were centred at 284.8 and 399.7 eV. Besides that, the Cu and O in Cu2O were observed at binding energy of 934.6 eV (Fig. 6d) for Cu 2p3/2, 955.02 eV for Cu 2p1/2 and 532.2 eV (Fig. 6e) respectively33. When compared to Ag°, a slight peak lower shift was observed in prepared Ag which is located at 367 and 372 eV are ascribed to Ag3d5/2 and Ag3d3/2 respectively, this may be due to disturb the chemical environment around Ag NPs in synthesized composite34.

#### UV–visible/DRS spectroscopy

The UV–Visible absorption spectra of Cu2O, TANI, TANI/Cu2O and TCA NCs are shown in Fig. 7a. The UV–Visible absorption spectra of TANI, TANI/Cu2O and TCA NCs show a peak at 326 nm which corresponds to π–π* transition of benzenoid ring of TANI35. Figure 7b (a–c) shows the optical bandgap energies of TANI, TANI/Cu2O and TCA NCs obtained from the Eq. (1).

$${text{E}}_{{text{g}}} = {text{h}}upnu = {text{hc/}}uplambda = {124}0{/}uplambda$$

(1)

where ‘Eg’ represents bandgap/forbidden energy, ‘h’ is plank’s constant, ‘ʋ’ is wave frequency, ‘c’ is the speed of light and ‘λ’ is the wavelength.

Table S1 in ESI clearly stated that the somewhat decrease in bandgap energy of TCA NC (Eg = 2.82 eV) from pure TANI (Eg = 2.93 eV) due to embedded the p-type semiconductor, Cu2O (Eg = 1.98 eV) into TANI matrix. The wider bandgap value originates from the quantum confinement effect which is caused by the nanodimensional state of materials. Lesser the bandgap materials possess excellence in catalytic activity, thus TCA NC has potent applications than other materials prepared.

#### Thermogravimetric (TG–DTA) analysis

Thermogram of the prepared NCs in an argon atmosphere as a carrier at a flow rate of 10 °C/min with a temperature range of 30–1,000 °C is depicted in Fig. 8a. The resultant graph implies the change in mass of the sample with increase in temperature. It was observed that pure TCA NC undergo three major steps of weight loss patterns but TANI and TANI/Cu2O shows two major steps.

1. 1.

The first small fraction of weight loss in the temperature range from 30–270 °C is due to the loss of hydrate molecules from the surface of TANI and TCA NC held through weak ionic interactions36.

2. 2.

The second major step of continuous weight loss occurs due to vanish the oligomers in TANI, TANI/Cu2O and TCA NCs at the temperature range of 270–540°C37.

3. 3.

Third step is decomposition process, shows a gradual weight loss occurs in the temperature range from 600–850 °C was observed due to thermal oxidative decomposition and degradation of the polymer fraction of TCA NC38. From the graph, it was seen that the thermal stability of TCA NC is considerably less than that of pure TANI and TANI/Cu2O NC which shows high thermal stability. This may be due to decreased in chain length and the presence of bulky groups in the composites. This thermal stability was decreased in TCA NC is due to addition of Ag, may cause a decrease in the tension induced by thermal excitation in the C–C bond39.

Figure 8b shows the DTA curves of pure TANI, TANI/Cu2O and TCA NCs. This depict explored about thermal behaviour of composites from pure TANI. In Fig. 8b(C), the two exothermic peaks were observed at 280 and 420 °C, corresponding to their respective temperatures of polymer degradation and oxidation of NC. The oxidation at higher temperature means increase the content of Cu2O in TCA composite and is evident from the Thermogram (Fig. 8a(C)). Figure 8b(B) shows the broad endothermic peak around the temperature of 440 °C, which corresponds to the decrease in degradation temperature of TANI/Cu2O. Figure 8b(A) confirms the presence of exothermic peak at the temperature of 240 °C, corresponding to decomposed the TANI. These results tells about the interaction of Cu2O and Ag NPs with polymers surfaces lead to formation of new compound of polymers which do not decompose into simple molecules such as carbon dioxide. The decrease in the degradation temperature of polymer nanocomposite to 280 °C, as compared to 240 °C of pure TANI, indicate that the polymers in the nanocomposite is different in its chemical nature compared to that in the pure state. However, the interpretation of the result needs further investigation.

#### Photoluminescence (PL) study

The photoluminescence spectra used not only to characterize the defects of a semiconductor but also to study the exciton spectra in TANI and its nanocomposites. Figure 9 illustrates the emission spectra of TANI and TCA NC at the excitation wavelength of 280 nm. In order to evidence the contribution of pure TANI and TCA NC shifted to 466 nm due to transitions from the polaronic band to the -band (HOMO) structures of TANI and the emission wavelength is shifted towards the blue when compared to the bulk TANI. This emission is due to the recombination of electron and hole pairs.

### Catalytic applications of prepared nanomaterials

#### Photocatalytic activity

In addition, the rate order kinetics of prepared composites calculated using Eq. (2), according to the Langmuir–Hinshelwood kinetics model, the function of ln(C/C0) versus reaction time (t) displayed a linear relationship as show in ESI (Fig S2b).

$$ln {text{C/C}}_{0} = {text{kt}}$$

(2)

where ‘C/C0′ is normalized initial concentration, ‘t’ is the reaction time, and ‘k’ is the reaction rate constant (min−1). The corresponding pseudo first order kinetic rate constant (k) and regression coefficient (R2) of PRA were calculated and are listed in the Table S2 (ESI). These results confirmed that all are follows the pseudo first order rate.

### Proposed mechanism

Proposed a plausible mechanism for improved the photocatalytic degradation efficiency of TANI/Cu2O/Ag ternary nanocomposite in the presence of visible light irradiation was displayed in schematic diagram Fig. 11. Initially, TANI was kept in visible light illumination, π → π * transition takes place, which release the electrons Eq. (5). In similar way, Cu2O undergoes charge separation process that leads to the promotion of electrons from valence band (VB) to conduction band (CB) and leaving a hole in the VB and releasing of electrons Eqs. (6, 7) due to the formation of heterojunction. Ag reduced and resulting in release of electrons Eq. (8). Finally, the photogenerated electrons are captured by the TANI through semiconductor Cu2O due to heterojunction which helps in enhancing the photogenerated charge separation efficiency in ternary nanocomposite. Simultaneously an equal amount of holes have been formed in semiconductor nanocomposite. These separated electrons and holes directly reacts with oxygen and water to generate highly reactive superoxide (O2) and hydroxyl (OH) radicals Eq. (9). These energetic reactive radicals react with surface adsorbed PRA molecules and degraded them into CO2 and H2O Eq. (10).

Based on this mechanism, explained the photocatalytic degradation of PRA dye using TCA nanocomposite under visible light irradiation as shown by the following Eqs. (38).

$${text{TANI}}left( pi right) + {text{h}}v to {text{TANI}}left( {pi^{*} } right) + {text{TANI}}left( {{text{e}}^{ – } } right)$$

(3)

$${text{Cu}}_{{2}} {text{O}}left( {{text{h}}^{ + } } right) + {text{H}}_{{2}} {text{O }} to {text{Cu}}_{{2}} {text{O}} +^{ cdot } {text{OH}} + {text{H}}^{ + }$$

(4)

$${text{Cu}}_{{2}} {text{O}}left( {{text{e}}^{ – } } right) + {text{O}}_{{2}} to {text{Cu}}_{{2}} {text{O}} +^{ cdot } {text{O}}_{{2}}^{ – }$$

(5)

$${text{Ag}} to {text{Ag}}^{ + } + {text{e}}^{ – }$$

(6)

$${text{TANI/Cu}}_{{2}} {text{O/Ag}}left( {{text{e}}^{ – } } right) + {text{O}}_{{2}} to {text{TANI/Cu}}_{{2}} {text{O/Ag}} +^{ cdot } {text{O}}_{{2}}^{ – }$$

(7)

$$^{ cdot } {text{OH/}}^{ cdot } {text{O}}_{{2}}^{ – } {text{/HO}}_{{2}}^{ cdot } + {text{PRA}};{text{dye}}; to {text{Degradation}};{text{products}}$$

(8)

Based on experimental conditions, TCA ternary nanocomposite possesses high photocatalytic activity towards the degradation of PRA dye solution under visible light irradiation than the binary composite and pure TANI.

### Radical affirmation by Scavenger test

In order to endorse the radicals scavenging, initially the 10 mg/L PRA dye solution was taken along with 30 mg TCA NC allowed to irradiation for 70 min in the presence of scavengers such as tertiary butyl alcohol (t-BuOH), Benzoquinone (BQ) for OH, O2, and potassium iodide (KI) for hole and OH radical. Remarkably, the absence of scavenger has shown better results almost 96.3% of PRA dye suspension yet, when used scavengers declined the removal percentage like KI-40.9%, BQ-44.5%, t-BuOH-42.7% as presented in ESI (Fig S3).

### Stability and recycling of TCA composite

In favour of investigate the reusability of synthesized TCA composite, the synthesized TCA was cleaned with milli Q water followed by ethanol and centrifuged to execute the later reaction. Significantly, the photodegradation of PRA dye is declined gradually up to five successive runs as 97, 96.1, 95.2, 94 and 92.7% for the successive five cycles respectively. This may be due to the loss of TCA during washing for recyclability. Fig S4(a) in ESI stated that prepared TCA composite acted as very dynamic photocatalyst up to five successive runs, thus the TCA photocatalyst remains stable and potent because of the presence of TANI surface to Cu2Oand Ag in synthesized composite that leads to mobilize the photogenerated electron and holes, so enhance the photocatalytic efficacy41. Fig S4(b) in ESI implies the XRD patterns of TCA which confirmed that no phase change was noticed in recycled TCA composite after reusable for successive five runs. From the obtained results, we strongly declared that polymerization synthesized TCA nanocomposite can be probably used as a stable photocatalyst for the dye (PRA) degradation in large scale.

### Removal of cadmium ion (Cd2+) by adsorption

The prepared pure TANI and their composites (TANI/Cu2O and TCA) were studied the adsorption efficiency by the removal of cadmium (Cd2+) from contaminated water under the conditions of catalyst dose 30 mg and Cd2+ solution 10 mg/L in alkaline medium. The obtained results declared the ternary composite shows better adsorption (7.5 mg/L) of Cd2+ than binary TANI/Cu2O (6 mg/L) and TANI (4.5 mg/L) owing to the presence of Cu2O and Ag NPs into TANI matrix are displayed in Fig. 12. However, the polymer based composite exhibits more adsorption capacity due to the presence of more surface groups, greater adsorption capacity, good stability and mechanic viability42. In this study, three most common isotherms such as Freundlich, Langmuir and Temkin models are used to interpret the experimental adsorption data. The equilibrium adsorption isotherms were determined using batch studies.

#### Freundlich model

According to Freundlich equation, the amount adsorbed increases infinitely with increasing concentration or pressure. The Freundlich Eq. (9) isotherm can be linearized into the following form:

$${log};{text{Q}}_{{text{e}}} = {log};{text{k}}_{{text{F}}} + {text{1/n}};{log};{text{C}}_{{text{e}}}$$

(9)

The regression coefficient (R2), slope and intercept were 0.94, 1.26 and 1.77 respectively. From these results, n and kF values were calculated to be 0.78 and 58.41 respectively (Fig. 12a).

#### Langmuir model

Langmuir isotherm Eq. (10) is derived from simple mass kinetics, assuming chemisorption. This model was depicted in Fig. 12b. From this graph, calculated the regression coefficient (R2), slope and intercept were 0.93, 0.36 and − 1.13 respectively. From these results, Qmax and kL are 2.73 and 0.32 respectively, desorption of the Cd2+ is very low.

$${text{C}}_{{text{e}}} {text{/Q}}_{{text{e}}} = {text{1/k}}_{{text{L}}} {text{Q}}_{{max}} + {text{C}}_{{text{e}}} {text{/Q}}_{{max}}$$

(10)

#### Temkin model

The linear relationship can be calculated from Eq. (11) and the result was depicted in Fig. 12c. The regression coefficient (R2), slope and intercept were calculated to be 0.98, 5.35 and − 3.44 respectively. The adsorption capacity and adsorption intensities, Qe and ln kT; 5.9 and 0.63 respectively obtained.

$${text{Q}}_{{text{e}}} = {text{B}}_{{1}} {ln};{text{k}}_{{text{T}}} + {text{B}}_{{1}} {ln},{text{C}}_{{text{e}}}$$

(11)

From these results, the Freundlich model was fitted for the adsorption of Cd2+ onto ternary nanocomposite (Table 1).

### Corrosion inhibition

The corrosion resistance behaviour of synthesized pure TANI, TANI/Cu2O and TANI/Cu2O/Ag composite coatings were evaluated by sweeping the potential from equilibrium potential towards negative and positive potentials against Ag/AgCl reference electrode in H2SO4 (1 M), HCl (1 M) and NaCl (3.5%, w/w) electrolytes using Tafel polarisation and electrochemical impedance spectroscopy techniques (Fig. 13). The trends of open-circuit potential curves provided clear evidence that the incorporation of Cu2O into TANI nanostructures is beneficial, as it introduces potential shift towards the noble positive potential for both binary and ternary composite coatings. But the ternary composite doesn’t show the good corrosive resistance behaviour than binary composite because of low percent of Ag content in composite, which doesn’t shows the adverse effect on corrosion inhibition. Also, the corrosion resistance was enhanced with increase in the Cu2O content in TANI supported nanocomposite coatings. Table 2 displays the values related to the corrosion current (Icorr), corrosion potential (Ecorr), cathodic (βa), anodic (βc) slopes and corrosion rate (CR) were calculated from tafel plots for pure TANI, TANI/Cu2O and TANI/Cu2O/Ag nanocomposites, also their polarization resistance (Rp) was calculated from kinetic parameters obtained from Tafel plots as Stern–Geary Eq. 43.

$${text{R}}_{{text{p}}} = frac{{upbeta _{{text{a}}} timesupbeta _{{text{c}}} }}{2.303} times {text{I}}_{{{text{corr}}}} left( {upbeta _{{text{a}}} +upbeta _{{text{c}}} } right)$$

(12)

where βa and βc are the respective slopes of cathodic and anodic tafel plots. Fig S5 (ESI) represents the bar graph which declared that TANI/Cu2O binary composite has high intensity in corrosion inhibition than TANI/Cu2O/Ag ternary composite and pure TANI was confirmed by the values of binary composite goes towards positive of Ecorr, with the comparison of mild steel (MS) as reference. The Tafel polarization studies showed the decreased corrosion current for all the TANI/Cu2O composite coatings than TANI/Cu2O/Ag and TANI. Hence, the corrosion inhibition rate of TANI/Cu2O nanocomposite coatings is less than that of TANI/Cu2O/Ag. It is revealed that TANI/Cu2O nanocomposite had more corrosion resistant in 3.5% NaCl solution.

### Antibacterial assay

The antibacterial activity of as-prepared nanocomposites was tested on four different micro-organisms. For that, initially we examined with the all prepared compounds, of them, only binary and ternary nanocomposites showed better antimicrobial efficacy. Hence based on the results, further studies were on TANI/Cu2O and TANI/Cu2O/Ag NCs against the both gram negative (Shigella flexneri, Salmonella typhimurium) and gram positive (Bacillus coagulans, Staphylococcus aureus) bacterial strains at three consecutive concentrations of 40, 60 and 80 µg/mL.

#### Antimicrobial assay of TANI/Cu2O nanocomposite

Finally, the antimicrobial activity of TANI/Cu2ONC was executed against the four different pathogens including gram positive and negative bacteria. Figure S6 displays the photographic images of an inhibition zone was rendered by TANI/Cu2ONC against four different bacterial strains. The zone of inhibition of TANI/Cu2ONC for various microorganisms is shown in Table S3 (ESI). Among all the strains selected for this study, S.aureus is the most active strain (21 µg/mL) at the concentration of 80 µg/mL. The synthesized TANI/Cu2ONC is effectively inhibited the bacterial growth and gram positive (S.aureus) is observed to be so effective than gram negative (S. typhimurium) bacteria.

#### Antimicrobial activity of TANI/Cu2O/Ag nanocomposite

The TANI/Cu2O/Ag NC was tested against the both gram positive and gram negative bacterial strains. Figure 14 shows the photographic images of an inhibition zone produced by TCA NC against four different bacterial strains. The zone of inhibition of TCA NC for various microorganisms was shown in Table 3. The influence TCA NC on S. aureus (25 µg/mL) and S.typhimurium (22 µg/mL) exhibited the best results at the examined concentration of 80 µg/mL. The prepared TCA NC effectively killing the bacterial growth and gram positive is observed to be so might effective than gram negative bacteria.

The significant antimicrobial activity was not found by pure TANI and Cu2O. The binary and ternary composites show antimicrobial efficiency against both Gram-positive and negative bacteria. These results proposed two different mechanisms of ternary composite with target microorganism interaction, the first: adsorption of ternary composite to cell walls, membrane disruption and cell leakage which is mainly connected with high molecular weight ternary composite, the second: penetration of ternary composite into living cells leading to inhibition of various enzymes and interference with synthesis of mRNA and proteins. However, the actual mechanism of inhibition activity of TANI/Cu2O/Ag composite is not fully understood yet.

In present study, the antimicrobial performance confirmed that gram positive bacterial strains are more affected than gram negative (Fig S7) because Gram positive bacteria have thin peptidoglycan layer with abundant pores that permits foreign molecules to penetrate into cell and damage or malfunctioning the cell organisms. Gram positive and gram negative bacteria have differences in their membrane structure, the most distinctive of which is the thickness of the peptidoglycan layer. The high concentration of Cu0 affected bactericidal effect. Therefore, copper nanoparticles are chosen as a carrier in this work44. A very few reports on Ag ions effects on microorganisms in which Ag+ attracts the negatively charged cell membrane of microorganism through electrostatic interactions45. Hence, Ag+ plays a vital role in the antibacterial activity of Ag in solutions with salts, zeolites and polymers46. For instance, the AgNP-polymeric nanoparticles showed increased antimicrobial properties towards E.coli and S.aureus47. The Ag NPs induced oxidative stress on bacteria and also damaged the cell membrane, protein, and DNA48. However, the Ag NPs can be easily penetrate to cell wall of microorganisms and also their greater surface area enhance the antimicrobial performance by bring a huge atoms into intact with the parts of bacteria47. Hence, the Ag NPs were used in the study, however, they received negative charge from TANI. The biological activity of Ag and Cu nanoparticles is also dependent on their size, causing penetration and damage of Gram-positive and negative bacteria cells49. The mechanism of the inhibitory effects of ternary composite on microorganisms is not yet known clearly. Further investigation was required.

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