Polyaniline–single walled carbon nanotube composite – a photocatalyst to degrade rose bengal and methyl orange dyes under visible-light illumination

Abstract

The polyaniline/single walled carbon nanotube (PANI–SWCNT) composites were prepared by the polymerization of an aniline monomer in the presence of sulfosalicylic acid with SWCNT under in situ conditions. PANI and PANI–SWCNT composites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultra violet-visible spectroscopy (UV-Vis), Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), photoluminescence spectroscopy (PL) and Brunauer–Emmett–Teller (BET) analysis. The application of the composites as a recyclable photocatalysts with visible-light-driven photocatalytic activity and photostability for the degradation of organic dyes – Rose Bengal (RB) and Methyl Orange (MO) – is demonstrated. PANI–SWCNT composite, with 2 weight% SWCNT content, has emerged as the best with degradation efficiencies of 95.91% and 90.34% against the performance of PANI with degradation efficiencies of 85.2% and 75.9% for RB and MO within 10 and 30 minutes, respectively. The synergistic effect between PANI and SWCNT is found to impart improved photogenerated carrier separation in the composite. A possible photocatalytic mechanism on the enhancement of the visible light performance of the composite over pure PANI is discussed based on the results of an active species trapping experiment.

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1. Introduction

Hazardous dyes are used worldwide with ever growing demand in sectors like printing, textiles, automobiles and other industries. These non-biodegradable organic dyes that are discharged from industries are the main contaminants in wastewater and are potentially harmful to mankind and other living animals. So it is a challenge to develop efficient techniques for dye removal from wastewater to maintain the ecosystem and environment.¹

Recently combinations of conjugated polymers with inorganic or carbon based materials for enhanced visible light responsive photocatalysis have emerged as a significant area of research. Some such photocatalysts used for degradation of various dyes in recent past are as follows: graphene–polyaniline (Gr–PANI),² and aluminium doped zinc oxide polyaniline hybrids (Al doped ZnO–PANI)³ for Rose Bengal dye (RB), Al doped ZnO–PANI hybrids,³ titanium dioxide–polyaniline (TiO₂–PANI),⁴ PANI-modified TiO₂ composite,⁵ polypyrrole (PPY)–TiO₂,⁶ polythiophene/titanium dioxide,⁷ polythiophene/TiO₂,⁸ PANI–Cr₃O₄ ⁹ for methyl orange dye (MO), PANI and ZnO nanocomposite,¹⁰ PANI–ZnO,¹¹ zinc oxide via hybridization with monolayer polyaniline,¹² TiO₂ modified by PANI,¹³ polyaniline modified TiO₂,¹⁴ TiO₂/polyaniline composites,¹⁵ silver (Ag)–TiO₂–PANI,¹⁶ Ag@TiO₂/PANI,¹⁷ Ag/AgCl–polyaniline,¹⁸ PANI–nickel ferrite composite,¹⁹ PANI-g-C₃N₄,²⁰ PPY–TiO₂,²¹ ZnO₂/polypyrrole²² and poly(3,4-ethylenedioxythiophene) (PEDOT)–ZnO²³ for methylene blue dye (MB), Ag@TiO₂/PANI¹⁷ and polyaniline–silver–silver phosphate (PANI–Ag–AgPO₄)²⁴ for brilliant blue dye, PANI–ZnO¹¹ for malachite green dye, ZnO₂/polypyrrole,²² (PANI–Ag–AgPO₄),²⁴ spindle-like PANI/BiVO₄,²⁵ TiO₂ modified by PANI,¹³ polyaniline modified TiO₂ ¹⁴ for rhodamine B, poly(pyrrole-co-aniline)-coated TiO₂/nanocellulose²⁶ for eosin yellow, spindle-like PANI/BiVO₄ ²⁵ for phenol.

Among various conjugated polymers, PANI is a p-type material with high hole transporting ability^5,14 together with fast charge separation capability, slow charge recombination rate in electron transfer processes and stability.^3,12,15 Due to that PANI has many applications in catalytic and photocatalytic field.^{13,25,27–32}

On the other hand, due to superior electronic and transport properties,³³ CNT and CNT based composites have also been explored as visible light photocatalysts for degradation of dyes like MO,³⁴ MB,³⁵ indigo carmine, congo red and orange G dyes,³⁶ alizarin yellow R,³⁷ eosin yellow³⁸ and for removal of Cr(VI) from aqueous solution.³⁹ In the formation of the composite, SWCNTs have particular advantage over MWCNTs due to the greater degree of interphase contact that can be accessed by the surface of the second component of the composite with the bundle of small individual SWCNTs.

Based on that, PANI–SWCNT composite is expected to have exclusive properties such as enhanced photocatalytic activity, high stability because of the synergism between the constituents. Accordingly this paper reports the comparative photocatalytic activity of PANI and PANI–SWCNT composites with varying percentages under visible sunlight irradiation for the degradation of RB and MO. Toxic to human skin and eyes, RB is known to be a heavily used dye in textile and printing industries,² having IUPAC name 4,5,6,7-tetrachloro-2′,4′,5′,7′-tetraiodofluorescein disodium salt (C₂₀H₂Cl₄I₄Na₂O₅), molecular weight 1017.64.

Another dye MO, having IUPAC name sodium 4-[[4-(dimethylamino)phenyl]diazenyl]benzenesulfonate (C₁₄H₁₄N₃NaO₃S), molecular weight 327.33, has mutagenic properties and hence very much toxic for living animals. Apart from conjugated polymer and carbon based composites which we have already mentioned, some other materials reported to have been explored for the degradation of RB under visible light illumination are – ZnO in the presence of metal ions,⁴⁰ Cu₇S₄ thin films,⁴¹ Cu₇Te₄ thin films,⁴² p-CuO/n-ZnO thin film heterojunction,⁴³ manganese dioxide (MnO₂).⁴⁴

Similarly materials like, p-CuO/n-ZnO thin film heterojunction,⁴³ Cu(II) in presence of tartaric acid,⁴⁵ fluidized bed reactor loaded with cross-linked chitosan embedded nano-CdS,⁴⁶ carbonaceous adsorbent prepared from waste rubber tires (WRT),⁴⁷ gold (Au) nanoparticles loaded graphitic carbon nitride nanosheets⁴⁸ and poly(3,4-ethylenedioxythiophene) (PEDOT)⁴⁹ have been used as photocatalysts for degradation of MO under visible light illumination. As a ready reference, irradiation time, degradation efficiency, rate constant and nature of the irradiated light for the degradation of RB and MO using different photocatalysts are tabulated in Table 1 for a clear comparison.

Table 1 Comparison of kinetic parameters using different photocatalysts under visible and ultraviolet (UV) light irradiation for the degradation of RB and MO dyes

Serial no.	Ref. no.	Name of the photocatalyst	Name of the dye	Irradiation source	Irradiation time	Degradation efficiency (%) and/or rate constant (min^−1)
1	2	Graphene/polyaniline	RB	Visible	3 h	56%
2	3	Aluminium doped zinc oxide–polyaniline hybrids	RB	Visible	150 min	2.61 × 10^−2 min^−1
3	40	ZnO (pH 8)	RB	Visible	180 min	5.53 × 10^−3 min^−1
4	41	Cu7S4 thin films (in the presence of H2O2)	RB	Visible	3 h	81%
						1 × 10^−2 min^−1
5	42	Cu7Te4 thin films (in the presence of H2O2)	RB	Visible	90 min	92%
6	43	p-CuO/n-ZnO thin film heterojunction	RB	Visible	150 min	92%
						1.40 × 10^−2 min^−1
7	44	Manganese dioxide (MnO2)	RB	Visible	150 min	2.39 × 10^−2 min^−1
8	3	Aluminium doped zinc oxide–polyaniline hybrids	MO	Visible	150 min	1.77 × 10^−2 min^−1
9	50	Polyaniline-hybrid defective ZnO	MO	UV	120 min	94%
						2.33 × 10^−2 min^−1
10	4	TiO2/polyaniline	MO	Visible	120 min	49.9%
						3.7 × 10^−3 min^−1
				UV	120 min	92.9%
						2.35 × 10^−2 min^−1
11	5	PANI-modified TiO2 composite	MO	UV	125 min	81.3%
						4.14 × 10^−2 min^−1
				Visible	6 h	21.5%
						8.7 × 10^−3 min^−1
12	51	Carbon nanotubes doped TiO2	MO	UV	80 min	4.93 × 10^−2 min^−1
13	6	Polypyrrole–TiO2	MO	Visible	160 min	9.31 × 10^−3 min^−1
14	52	Poly(3-hexylthiophene)/TiO2	MO	UV	150 min	92.7%
						1.86 × 10^−2 min^−1
15	53	TiO2 nanoparticles using a triboelectric nanogenerator	MO	UV	120 min	76%
16	54	Nanostructured TiO2/ZnO heterojunctions	MO	UV	30 min	97%
17	55	Aqueous TiO2 suspensions (at pH 2)	MO	UV	40 min	98.31%
18	56	TiO2-coated activated carbon	MO	UV	100 min	89%
19	46	Fluidized bed reactor loaded with cross-linked chitosan embedded nano-CdS (pH 4)	MO	Visible	80 min	99%
						2.6 × 10^−2 min^−1
20	49	PEDOT nanospindles	MO	UV	15 min	100%
				Visible	180 min	100%
21	43	p-CuO/n-ZnO thin film heterojunction	MO	Visible	150 min	81%
						1 × 10^−2 min^−1
22	48	Au nanoparticles loaded graphitic carbon nitride nanosheets	MO	Visible	2.5 h	92.6%
23	45	Cu(II) in presence of tartaric acid (for pH 3)	MO	Visible	70 min	92%
24	57	TiO2/graphene	MO	UV	60 min	85%

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For the present work, the composites were prepared by in situ oxidative polymerization of aniline in presence of SWCNT. The degradation efficiency and the rate constant of PANI and various composites were evaluated and the later showed higher efficiency and rate constant than PANI. The PANI–SWCNT composite exhibits excellent photocatalytic performance through the charge carrier separation compared to other photocatalysts. A possible photocatalytic mechanism was proposed and the improved photocatalytic activity is attributed to the electron–hole separation caused by the synergistic effect between PANI and SWCNT.

2. Experimental

2.1. Materials used

Single-walled carbon nanotubes (SWCNTs) were procured from US Research Nanomaterials, Inc. General grade aniline monomer was purchased from Fisher Scientific, India. Ethanol, 5-sulfosalicylic acid (SSA) and ammonium persulfate (APS) were bought from Merck Chemicals. De-ionised water was obtained from Hydrolab, India. Methyl Orange (MO) and Rose Bengal (RB) dyes were acquired from Himedia, India. Sodium oxalate (SO) and p-benzoquinone (BQ) were procured from Fisher Scientific, India. Potassium persulfate (K₂S₂O₈) and tert-butanol (TBA) were obtained from Sigma Aldrich and SD Fine-Chem Limited (SDFCL), respectively.

2.2. Synthesis of PANI and PANI–SWCNT nanocomposite via in situ chemical polymerization technique

PANI–SWCNT nanocomposite was synthesized by in situ chemical oxidative polymerization process as described in our earlier work.⁵⁸ Aniline was added to an aqueous solution of SSA. Depending on the desired weight percentage content of SWCNTs in the composite, certain amounts of SWCNTs were added to the SSA mixed aniline solution and stirred for half an hour. Aqueous solution of APS was added drop by drop and after leaving for overnight, the blackish green precipitate was centrifuged and filtered out after rinsing with water and ethanol repeatedly. The samples were dried properly, powdered and used for the experiments.

SSA doped pure PANI was also synthesized in the same way as mentioned above without SWCNT and it is denoted as PSA. PANI–SWCNT nanocomposites containing three different weight percentages of SWCNT have been denoted as PC1 (1% SWCNT content), PC2 (2% SWCNT content) and PC4 (4% SWCNT content).

2.3. Characterization and measurements

The synthesized samples were characterized by powder X-ray diffraction spectroscopy (XRD), field emission scanning electron microscopy (FESEM), ultra violet and visible ray spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), X-ray energy photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area measurement and photoluminescence spectroscopy (PL). Using Cu-Kα radiation (λ = 1.5418 Å) with scan range 10–70°, XRD patterns were obtained from a X-ray diffractometer (XRD, Bruker, D8 Advance). Morphology of the synthesized materials were analysed by a field emission scanning electron microscope (FESEM, Carl-Zeiss-SIGMA). UV-Vis absorbance spectra in the range of 328 nm to 1000 nm were obtained using a UV-Vis spectrophotometer (JASCO V-530). FTIR spectra of the samples were recorded in the range 400 cm⁻¹ to 4000 cm⁻¹ by a spectrophotometer (JASCO FT/IR-460-Plus). Al Kα monochromatic X-ray source (VG SCIENTA) with energy 1486.6 eV was used for X-ray energy photoelectron spectroscopy (XPS) measurements along with electron energy analyzer (VG SCIENTAR4000WAL), with total experimental energy resolution better than 0.6 eV at room temperature. The Brunauer–Emmett–Teller (BET) surface area measurements were done by nitrogen adsorption at temperature 77 K using automated gas sorption data acquisition and reduction quantachrome instrument (Quantachrome® ASiQwin™). Room temperature photoluminescence spectra of the samples were recorded using a spectro-fluorometer (Quanta Master 40).

2.4. Photocatalytic experimental set up

The degradation of RB and MO were studied under visible light illumination, using the synthesized samples as photocatalysts. Light from a tungsten lamp (Philips, 200 W) was made to pass through 10 mm deep 1 M solution of NaNO₂. The NaNO₂ solution was used as UV cut-off filter to get a source of light with wavelength greater than 410 nm, resembling visible light.⁵⁹ For each case, a certain amount of dye was added to de-ionised water to get 1 × 10⁻⁵ M dye solution. UV-Vis absorption spectrum of the dye solution was taken and in all cases denoted as the UV-Vis spectrum of the dye at zero time (t₀). Adsorption–desorption equilibrium was attained for each sample and each dye by adding 20 mg of sample as photocatalyst into 50 ml dye solution and kept under magnetic stirring for one hour in dark and under ambient temperature and pressure. Then, the solution was placed 10 cm below the tungsten lamp in the above mentioned way to get an optical irradiance of 70 mW cm⁻². After switching on the tungsten lamp, UV-Vis spectra of the solution were recorded at given time intervals to study the degradation of dyes in the presence of the prepared samples acting as photocatalysts under the visible light illumination. At given time intervals, 3 ml suspension were sampled and centrifuged to remove the photocatalyst powders. The concentration of dye was analyzed through UV-Vis spectrophotometry.

3. Results and discussions

3.1. Structural characterization

XRD spectra analysis. The XRD spectra of the synthesized samples PSA, PC1, PC2 and PC4 are shown in Fig. 1. Peaks around 20° and 25.57° correspond to periodicity perpendicular to and parallel with the polymer backbone chain in PANI respectively.^60,61 XRD peaks of SWCNT occur around 26.2° and 42.7°.⁶² In case of composites, peaks are found around the above mentioned peak simplifying that unit structure of PANI has been retained even in the composites.⁶³

Fig. 1 XRD spectra of (a) PSA, (b) PC1, (c) PC2 and (d) PC4.

FESEM images analysis. Fig. 2 shows the FESEM images of PANI and PANI–SWCNT composites. The morphology in Fig. 2(a) reveals polymerization of pure PANI in almost agglomerated form. With the introduction of SWCNTs as in Fig. 2(b–d), some morphological changes take place. FESEM images of the composite show polymerization of PANI over SWCNTs leaving very little PANI in agglomerated form as well as almost no bare SWCNT.

Fig. 2 FESEM images of (a) PSA, (b) PC1, (c) PC2 and (d) PC4.

UV-Vis spectra analysis. The normalized UV-Vis spectra of the samples are given in Fig. 3. It is found that there is an absorbance band around 382–390 nm which corresponds to polaron–π transition,⁶⁴ the shoulder around 600 nm corresponds to exciton absorption of quinoid rings⁶⁵ and a band around 846 nm suggests the presence of polaron due to doping in PANI.⁶⁶ In composites, the band around 846 nm is found to be blue shifted. It is known that pure SWCNTs have peak around 260 nm ⁶⁷ and do not have any absorbance peak in the region 600–1000 nm. So the bands found here are due to the presence of PANI and the blue shift is due to the interaction of SWCNTs with PANI in the composites as observed from FTIR spectra discussed in the next section.

Fig. 3 Normalized UV-Vis spectra of (a) PSA, (b) PC1, (c) PC2 and (d) PC4.

FTIR spectra analysis. Fig. 4 represents the FTIR spectra of PANI and its composites (with SWCNT). The peaks of PANI emeraldine salt are observed at 820 cm⁻¹ associated with the aromatic C–H bending out of the plane for 1,4 di-substituted benzene ring,⁶⁸ 1146 cm⁻¹ due to B–NH⁺=Q stretching, 1348 cm⁻¹ and 1384 cm⁻¹ due to C–N stretching of secondary aromatic amine, at 1506 cm⁻¹ for benzenoid (B) ring stretching, 1591 cm⁻¹ and 1633 cm⁻¹ for C=C quinonoid (Q) ring stretching in emeraldine salt and base respectively.^69,70

Fig. 4 FTIR spectra of (a) PSA, (b) PC1, (c) PC2 and (d) PC4.

The peaks at 597 cm⁻¹ due to out-of-plane bending of SSA ring, 668 cm⁻¹ attributed to in-plane bending and/or out-of-plane bending of SSA ring, 760 cm⁻¹ corresponding to γ_C–H vibration of SSA ring and 1033 cm⁻¹ due to symmetric stretching of SO₃ group confirm that PANI samples are doped with 5-sulfosalicylate anions of the acid (sulfosalicylic acid).^71–73

From the FTIR spectra of the composite, it is found that SWCNT interacts with PANI at different reaction sites due to which PANI peaks (Fig. 4(a)) are shifted as has been observed in PANI/nano-SiO₂ ⁷⁴ and PANI–MWCNT composite.⁷⁵ The stretching modes of C–N at 1384 cm⁻¹ and C=C at 1506 cm⁻¹ and 1633 cm⁻¹ are blue shifted to 1416 cm⁻¹, 1557 cm⁻¹ and 1643 cm⁻¹ respectively. This is an indication of bonds becoming stronger in the composite. Similarly C–C bond at 819 cm⁻¹ and C–H bond at 1033 cm⁻¹ which are red shifted to 780 cm⁻¹ and 1022 cm⁻¹ respectively suggests that these bonds become weaker.

XPS analysis. The valence states and the chemical environment of constituent elements on the surface of the PANI–SWCNT composite is explored by X-ray photoelectron spectroscopy (XPS). Fig. 5(a) shows the spectrum of carbon (C 1s) in PC2 having binding energy of 285.5 eV which is close to that of pure PANI (283.9 eV) and SWNT (283.5 eV).⁷⁰

Fig. 5 XPS (a) C 1s (b) N 1s and (c) O 1s XPS core level spectrum of PC2.

In the spectrum of nitrogen (N 1s), shown in Fig. 5(b), the lowest binding energy at ∼399.2 eV suggests the presence of quinoid amine in the backbone of PANI. The peak at ∼400 eV is due to the benzenoid di-amine nitrogen of PANI while the higher binding energy at ∼401.2 eV represents the positively charged nitrogen (–N˙⁺) and the protonated amine (=N˙⁺).⁷⁶ Fig. 5(c) depicts oxygen (O 1s) spectrum of PC2 with a binding energy peak at ∼530.5 eV usually ascribed to C–O or C–OH group in carbon based nanomaterials.⁷⁷ The higher binding energy peak at ∼532.6 eV could be assigned to bound water molecules.⁷⁸

3.2. Photocatalytic activity study

The characteristic absorptions of RB at λ_max = 550 nm and MO at λ_max = 465 nm were employed to monitor the photocatalytic degradation process.^40,79 In presence of PSA, PC1, PC2 and PC4 as the photocatalysts, Fig. 6(i) and 7(i) are the reaction profiles which show the photodegradation of RB and MO as a function of time, where C_t is the concentration of dye after irradiation and C₀ is the concentration after the adsorption equilibrium on the photocatalyst particles before irradiation. Similarly, in Fig. 6(ii) and 7(ii), the kinetic plots, where the plots of ln(C₀/C_t) vs. irradiation time are shown for RB and MO, respectively. We can see that the degradation process for both the dyes with catalysts follow pseudo first order kinetics. The degradation efficiency and rate constant for RB and MO were calculated from the reaction profiles and kinetic plots in Fig. 6 and 7 using the simple equations as follows:

R% = (1 − C_t/C₀) × 100 (1)

where, R% is degradation efficiency of RB and MO, C₀ is the initial concentration and C_t is the concentration of RB and MO at different irradiation times, and

ln(C₀/C_t) = t × K_app (2)

where, K_app is pseudo-first order rate constant and t is irradiation time.⁸⁰

Fig. 6 (i) Reaction profile of RB degradation in an aqueous solution against specific time intervals under various conditions. (ii) First-order kinetic plot of ln(C₀/C_t) vs. irradiation time of RB degradation for (a) without catalyst in light, (b) with catalyst in dark, (c) PSA, (d) PC1, (e) PC2 and (f) PC4.

Fig. 7 (i) Reaction profile of MO degradation in an aqueous solution against specific time intervals under various conditions. (ii) First-order kinetic plot of ln(C₀/C_t) vs. irradiation time of MO degradation for (a) without catalyst in light, (b) with catalyst in dark, (c) PSA, (d) PC1, (e) PC2 and (f) PC4.

Blank tests (dye solution without any photocatalyst) under visible light exhibited very little photolysis, indicating that dyes are stable under visible light. Under identical experimental conditions, PC2 exhibited much enhanced photocatalytic activity. The photodegradation rate of RB and MO reached 98.6% and 94.35% in presence of the PC2 composite after 10 min and 30 min of visible light irradiation, respectively. Only 85.2% and 75.9% of RB and MO were degraded by PSA, indicating that the PANI–SWCNT composite is a superior photocatalyst than SSA doped PANI. The UV-Vis absorption spectral changes of RB and MO dyes in presence of PC2 due to visible light irradiated photo catalysis is given in Fig. ESI-1.†

The slopes of the kinetic plots give the first order rate constants, estimated to be 0.42394 and 0.09505 min⁻¹ with PC2 for RB and MO dyes, respectively. A comparison of degradation efficiency (R%) and rate constant (min⁻¹) of the samples acting as photocatalysts for the degradation of RB and MO is presented in Table 2 and Fig. 8 depicts the degradation efficiencies of the samples for various SWCNT content for RB and MO dyes.

Table 2 A comparison of rate constant (K_app) and degradation efficiency (R) of the samples acting as photocatalysts for the degradation of RB and MO dyes

Sample name	In case of RB		In case of MO
	Kapp (min^−1)	R (%)	Kapp (min^−1)	R (%)
PSA	0.18719	85.2	0.04661	75.9
PC1	0.27757	93.68	0.06905	87.30
PC2	0.42394	98.6	0.09505	94.35
PC4	0.31782	95.91	0.07772	90.34

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Fig. 8 Degradation efficiency vs. SWCNT content of the samples under visible light irradiation for RB and MO dyes.

To propose the mechanism of the enhanced photocatalytic activity of the PANI–SWCNT composite, the relative band positions of the two semiconductors were compared^81–83 as the band-edge potential levels play a crucial role in determining the flow of photoexcited charge carriers in a coupled heterostructure. The conduction band bottom of PANI is higher than that of SWCNT and the valence band top of PANI is higher than that of SWCNT. The band structure is useful for the separation and transportation of charge carriers. As illustrated in Fig. 9, under visible-light irradiation, both SSA doped PANI and SWCNT are easily excited and electrons and holes are generated. Photogenerated electrons in the SSA doped PANI flows downhill from conduction band of PANI to conduction band of SWCNT because of the intimate contact between the two semiconductors. Simultaneously, holes on the valence band of SWCNT can be transferred to that of PANI under the band energy potential difference. In this way, the photogenerated electrons and holes can be efficiently separated and the recombination of electron–hole pairs can be reduced. Electrons present in conduction bands of both SWCNT and doped PANI can react with dissolved O₂ to form superoxide radical anions (˙O₂⁻) as both of them lies above the reduction potential of O₂/˙O₂⁻.^81–84 Further reduction of superoxide radical anion gives hydroxyl radical.²⁰ On the other hand, dye molecules absorb light and get photoexcited. Photogenerated electrons in dyes can flow downhill from LUMO to the CB of SWCNT and enhance the photodegradation rate.^47,85 The active species like ˙O₂⁻ and ˙OH react with dye cation and forms colorless small degraded products. Direct oxidative attack on dye molecules by holes can also occur.⁸⁶

Fig. 9 Proposed mechanism of PANI–SWCNT structure for charge transfer of the photogenerated electrons and holes under visible light illumination.

To ascertain the role of active species, including hole (h⁺), electron (e⁻), hydroxyl radical (˙OH) and super-oxide radical (˙O₂⁻), corresponding scavengers of them were added in the reaction medium. This is crucial for elucidating the photocatalytic mechanism. As shown in Fig. 10, different scavengers have different effects on the degradation of MO and RB, respectively. The addition of ˙OH, e⁻, h⁺ and ˙O₂⁻ scavengers induce great change in MO and RB photodegradation, indicating role of all of them. The possible mechanism involves the following steps²⁰

Dye* → dye⁺ + (e_CB⁻) SWCNT (4)

Dye* + (h_VB⁺) SWCNT/SSA doped PANI → dye⁺ (5)

(e_CB⁻) SWCNT/SSA doped PANI + O₂ → ˙O₂⁻ (6)

˙O₂⁻ + 2e_CB⁻ + 2H⁺ → ˙OH + OH⁻ (7)

˙OH + ˙O₂⁻ + dye⁺ → colourless degraded products (8)

Dye + (h_VB⁺) SWCNT/SSA doped PANI → colourless degraded products (9)

Fig. 10 Reaction profile of photocatalytic degradation of (i) RB and (ii) MO as a function of irradiation time in presence of PC2 and different scavengers.

The stability of PC2 has been further investigated by recycling the photocatalyst and the relative dye concentration versus irradiation time for four consecutive cycles is shown in Fig. 11(a) for RB and MO dye degradation. The degradation efficiency of RB solution was found to be more than 94% after 4 cycles, while for MO solution it is 92% as shown in Fig. 11(b).

Fig. 11 (a) Relative dye concentration and (b) degradation efficiencies of PC2 with irradiation time for four consecutive cycles of operation for (i) RB and (ii) MO dye.

3.3. BET surface area and pore size analysis

Fig. 12(a) shows isotherm adsorption/desorption curve with relative pressure for PSA and PC2 and it is seen that adsorption/desorption is greater for composite (PC2) than pure PANI. The surface area for pure PANI (PSA) is 24.9 m² g⁻¹ and for composite (PC2) it is 31 m² g⁻¹ which matches with reported result.⁸⁷ As seen in FESEM images, SWCNTs hinder agglomeration of PANI, which leads to increase the surface area in the composite. Cumulative and differential pore volume distribution with pore width are shown in Fig. 12(b) and (c) respectively by using Barrett–Joyner–Halenda (BJH) desorption data. For composite, higher pore volume is observed which is a signature of more porous morphology.

Fig. 12 (a) Isotherms, (b) cumulative and (c) differential pore volume for PSA and PC2.

3.4. Photoluminescence (PL) property

Fluorescence intensity in the PL spectra provides the information regarding the recombination efficiency of photogenerated electrons and holes of a material. Higher recombination rate of photogenerated electrons and holes produces strong fluorescence intensity. That means these photo generated carriers have lower lifetime. However if the recombination rate is low due to high separation efficiency leading to a longer lifetime of the photogenerated carriers, the fluorescence intensity diminishes. Fig. 13 shows the photoluminescence spectroscopy of PANI and PC2 for excitation wavelength 330 nm. The peaks at 367 nm and 439 nm are clearly due to the π*–π transition and polaronic band in PANI respectively.^88,89

Fig. 13 PL spectra of (a) pure PANI and (b) composite (PC2) at room temperature.

In case of composite with 2% SWCNT (PC2), the intensity of these peaks are lowered suggesting improved separation efficiency and life time of the photogenerated electrons and holes in presence of SWCNT. This result supports the experimental observations – the enhanced rate constant and efficiency of photocatalytic degradation of RB and MO dyes under visible light illumination using PANI–SWCNT composite over pure PANI as photocatalyst. The electron hole pairs with longer lifetime are advantageous for creating more of photoreactive species with strong oxidation capabilities, such as O₂⁻ and ˙OH, which explains for higher photocatalytic activity of PANI–SWCNT composite than pure PANI.⁹⁰

4. Conclusion

SSA doped PANI and PANI–SWCNT composites with different SWCNT content ratio have been synthesized by in situ polymerization technique which exhibit highly efficient visible-light-driven photocatalytic activity for the degradation of RB and MO dyes. The structure, surface morphology, valence states and chemical environment of constituent elements on the surface of the samples have been examined and discussed in detail. SWCNT improves the surface area and increases the visible light absorption ability of the composite photocatalyst. Due to the synergistic effect between PANI and SWCNT, the composite displayed an efficient visible-light-responsive photocatalytic activity as compared to pure PANI through an improved photogenerated carrier separation in the composite. The composite PC2 (2% SWCNT) exhibited the highest photocatalytic performance with a higher rate constant ∼2.3 times for RB and ∼2 times for MO than that of pure PANI. A possible photocatalytic mechanism on the enhancement of visible light performance of composite over pure PANI is discussed based on the active species trapping experiment.

Acknowledgements

M. J. Chatterjee acknowledges Indian Institute of Engineering Science and Technology, Shibpur for Institute Fellowship. A. Ghosh is thankful to University Grants Commission (INDIA) for her SRF-NET fellowship. CEGESS, IIEST, Shibpur and UGC-DAE Consortium for Scientific Research, Kolkata Centre are acknowledged for providing FESEM and XRD spectra measurements facilities, respectively. Dr Krishnakumar S. R. Menon, Surface Physics Division, SINP is acknowledged for providing XPS data. Ms Mousumi Majumder of CGCRI, Kolkata, is acknowledged for providing the data for BET analysis.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra03855k

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