Polyaniline/Reduced Graphene Oxide Composite-Enhanced Visible-Light-Driven Photocatalytic Activity for the Degradation of Organic Dyes.

Creation of an innovative composite photocatalyst, to advance its performance, has attracted researchers to the field of photocatalysis. In this article, a new photocatalyst based on polyaniline/reduced graphene oxide (PANI/RGO) composites has been prepared via the in situ oxidative polymerization method employing RGO as a template. For thermoelectric applications, though a higher percentage (50 wt %) of RGO has been used, for photocatalytic activity, lesser percentages (2, 5, and 8 wt %) of RGO in the composite have given a significant outcome. Furthermore, photoluminescence (PL) spectra, time-resolved fluorescence spectra, and Brunauer-Emmett-Teller surface area analyses confirmed the improved photocatalytic mechanism. PANI/RGO composites under visible light irradiation exhibit amazingly improved activity toward the degradation of cationic and anionic dyes in comparison with pristine PANI or RGO. Here, a PANI/RGO composite, with 5 wt % RGO(PG2), has emerged as the best combination with the degradation percentages of 99.68, 99.35, and 98.73 for malachite green, rhodamine B, and congo red within 15, 30, and 40 min, respectively. Experimental findings show that the introduction of RGO can relieve the agglomeration of PANI nanoparticles and enhance the light absorption of the materials due to an increased surface area. Moreover, the PG2 composite also showed excellent photocatalytic activity to reduce noxious Cr(VI). The effective removal of Cr(VI) up to 94.7% at pH 2 was observed within only 15 min. With the help of the active species trapping experiment, a plausible mechanism for the photocatalytic degradation has been proposed. The heightened activity of the as-synthesized composite compared to that of neat PANI or RGO was generally because of high concentrations of •OH radicals and partly of •O2 - and holes (h+) as concluded from the nitroblue tetrazolium probe test and photoluminescence experiment. It is hoped that the exceptional photocatalytic performance of our work makes the conducting polymer-based composite an effective alternative in wastewater treatment for industrial applications.

Introduction

The extensive use of various toxic elements by several manufacturing companies (such as textile and paper printing industries, etc.) pollutes water and affects the ecosystem.[1−5] Decolorization of these dyes is an important process of wastewater management before being discharged to the environment because of their synthetic origin, complex chemical structure, toxicity, and carcinogenic or explosive nature.[6,7] To make use of the maximum of the solar spectrum (43% of the visible light), new visible-light-sensitive semiconductor photocatalysts are required for the degradation of toxic dyes in wastewater management. Sunlight-driven photocatalysts such as doped TiO2,[8−10][8−10] CdS,[11,12] CdSe,[13] BiOBr,[14] Cu2O,[15] WO3,[16] Fe2O3,[17][17] ZnO,[18] and SnO2[19] are extensively used nowadays.

It is interesting that the conducting polymer polyaniline (PANI) doped with different dopants has been broadly used as an adsorbent,[18,20−26] being a p-type semiconductor. Other advantages of PANI are its high hole transport ability, slow charge recombination rate in electron transfer processes, fast charge carrier separation, and stability.[18,23] PANI has been employed as a visible-light-driven photocatalyst for the degradation of cationic dyes such as rhodamine B (RhB),[23] malachite green (MG),[24] methylene blue (MB),[26] and various anionic dyes.[18,23,25] The ease of synthesis, environmentally friendly nature, and modulating properties of PANI have made it an attractive candidate for degradation of organic dyes. Due to these inherent advantages, more research should be carried out to develop PANI-based composites for better photocatalytic materials.

Alternatively, graphene, owing to its high surface area and exclusive optical, transport, mechanical, and electronic properties, has emerged as a possible contender for other applications such as solar cells,[27] sensors,[28] hydrogen generation,[29] and catalytic activities.[30−35] For visible light degradation of various cationic dyes[30−34] and anionic dyes[31] and for removal of toxic Cr(VI) from aqueous solution,[36,37] reduced graphene oxide (RGO) and its composite photocatalysts have been used. Therefore, a trend has come up for the synthesis of graphene or RGO-based nanocomposite photocatalysts for the degradation of organic dyes in wastewater management.[38−40] We have studied the competence of the PANI/RGO composite as a visible-light-induced photocatalyst for the degradation of cationic MG and RhB dyes and anionic congo red (CR) dye. Various works on heterostructures, combining composite materials for enhanced photodegradation of various dyes,[24,41−52] are presented in Table .

Table 1

Photocatalysts Used for Various Dye Degradations[24,41−52]

photocatalyst	dye	reference
reduced graphene oxide/CuI/PANI	RhB	(24)
	MO
polyaniline-conjugated graphene	MO	(41)
	MB
	RhB
polyaniline/graphite oxide	MB	(42)
ZnO/rGO/polyaniline ternary nanocomposite	MO	(43)
reduced graphene oxide/ZnFe2O4/PANI	RhB	(44)
carbon nitride/PANI/ZnO ternary heterostructure	MB	(45)
graphene/PANI	RB	(46)
graphene/PANI/Cu2O	CR	(47)
SWCNT/PANI	RB, MO	(48)
PANI/gC3N4	MB	(49)
PANI/TiO2/graphene hydrogel	BPA	(50)
gC3N4/rGO	2,4-DCP, RhB	(51)
g-C3N4/RGO/TiO2	MB	(52)

MG, a cationic dye, known to be heavily used in textile and printing industries, is toxic to human skin and eyes. RhB and CR have mutagenic properties and hence noxious for living animals.

A composite structure of PANI with RGO appears to be a potential photocatalyst, owing to its capability (a) to absorb the entire portion of the solar spectrum and (b) for photogenerated electron–hole pair separation. It could also separate the excited charge carrier at a very fast rate to improve the photocatalytic property.

Furthermore, the presence of poisonous metallic elements (for example, chromium) in the marine environment by discharges from industries, such as leather tanning, electroplating, pigment, refractory, and steel production industries,[53] is a great concern nowadays. Safe and efficient degradation of wastewater containing deadly metallic elements is always a challenging job for industrialists and environmentalists because of the fact that there are few cost-effective treatment alternatives. The methods for the disposal of Cr(VI) comprise ion exchange, reverse osmosis, evaporation, direct precipitation, etc. Hence, the progress of cost-saving and less-energy-consuming technology for the elimination of Cr(VI) is highly anticipated.

To the best of our knowledge, visible-light-induced photodegradation by the PANI/RGO (PG) photocatalyst, for the degradation of MG dyes is reported here for the first time. Furthermore, performance-wise, this composite is the best, as given in Table (Results and Discussion section), for degradation of anionic and cationic dyes. In this study, we have shown visible-light-induced photocatalytic performances of PG composites (2, 5, and 8% of RGO) that degrade MG, RhB, and CR dyes within 15, 30, and 40 min with the degradation percentages of 99.68, 99.35, and 98.73, respectively. A plausible photocatalytic mechanism is proposed, and the superior photocatalytic activity is assigned to the electron–hole separation caused by the inclusion of RGO in PANI. It has been observed that thermoelectric (TE) and visible-light-dependent photocatalytic properties of the materials crucially depend on the composition of the constituents PANI and RGO. For TE applications, higher percentages of RGO have been used,[54] and for photocatalytic activity, lesser percentages of RGO in the composite have given the significant outcome.

Table 3

Comparison of First-Order Kinetic Parameters Using Different Polyaniline–Carbon-Based Nanomaterials under Visible and Ultraviolet (UV) Light Irradiation for the Degradation of Various Cationic and Anionic Dyes

photocatalyst	dye	DTa (min)	DPb (%)	irradiation source	Kappc (min–1) (calculated)	reference
PANI/RGO	MG	15	99.68	visible	3.84 × 10–1	this work
	RhB	30	99.35		1.68 × 10–1
	CR	40	98.73		1.09 × 10–1
reduced graphene oxide/CuI/PANI	RhB	50	100	visible	9.21 × 10–2	(24)
	MO	70	96		4.60 × 10–2
PANI conjugated graphene	MO	70		natural	2.59 × 10–2	(41)
	MB	150		sunlight	7.17 × 10–2
	RhB	120			3.52 × 10–2
PANI/graphite oxide	MB	180	89	visible	1.12 × 10–2	(42)
ZnO/rGO/PANI	MO	60	100	UV		(43)
reduced graphene oxide/ZnFe2O4/PANI	RhB	60		visible	6.06 × 10–2	(44)
carbon nitride/PANI/ZnO ternary heterostructure	MB	80		visible	2.6 × 10–2	(45)
graphene/PANI	RB	180	56	visible	4.56 × 10–3	(46)
graphene/PANI/Cu2O	CR	20	97.91	UV	1.93 × 10–1	(47)
PANI/SWCNT	RB	10	98.6	visible	4.24 × 10–1	(48)
	MO	30	94.35	visible	9.51 × 10–2
PANI/g-C3N4	MB	120	92.8	visible	2.19 × 10–2	(49)
PANI/TiO2/graphene hydrogel	BPA	40	80	UV	4.02 × 10–2	(50)

Degradation time (DT).

Degradation percentage (DP).

Rate constant (Kapp).

Results and Discussion

Characterization of the As-Prepared Photocatalysts

Morphological Characterizations

Figure A–F shows transmission electron microscopy (TEM) images and field emission scanning electron microscopy (FESEM) images of PANI, RGO, and the PANI/RGO (PG2) composite, respectively. As shown in Figure A,D, pristine PANI is interconnected, consisting of agglomerated nanoparticles. Figure B,E denotes a uniform, large, creased, and veil-like layered structure of RGO sheets with an average thickness of several hundred nanometers. It can be seen from Figure C,F that RGO is adorned with well-distributed PANI nanoparticles for the PG2 composite, which is beneficial for the photocatalytic activity. No individual PANI agglomerates are observed, which implies that the nucleation and growth processes occur only on the surface of the template RGO. Moreover, the tight attachment between RGO and PANI can advance the performance and reproducibility in photocatalytic degradation, discussed in the previous section.

Figure 1

TEM images of (A) PANI, (B) RGO, and (C) PG2 composite and FESEM images of (D) PANI, (E) RGO, and (F) PG2 composite.

Spectral Characterizations

Figure depicts XRD spectra of the synthesized materials: (a) PANI, (b) RGO and (c) the PANI/RGO (PG2) composite. Peaks at 20.54 and 25.28°, with hkl values (020) and (200), confirm the presence of PANI.[55,56] A broad peak at around 25.44° and a tilt at 43.7° represent the (002) and (100) crystalline planes of the graphite sheet.[54,57] For the PG2 composite, peaks are found at around 20.62, 25.57, and 42.86°, respectively, as mentioned above, confirming that the unit structure of PANI has been maintained even in the composite.[54]

Figure 2

XRD spectra of (a) PANI, (b) RGO, and (c) PG2 composite.

Figure represents UV–vis spectra of the prepared PANI and PANI/RGO (PG2) composite. Both the samples show a major peak at around 400 nm. With the addition of RGO, the peak shifts toward the higher wavelength. The shoulder below 400 nm signifies the π–π* transition of the benzenoid rings. The peak around and above 400 nm is due to localized polarons, indicating protonated PANI.[58] At around 800 nm, the band, in continuation with the 400 nm peak, indicates the extended coil conformation of PANI chains.[59]

Figure 3

UV–vis spectra of (a) PANI and (b) PG2 composite.

The inset of Figure shows the optical band gaps of the samples calculated from the UV–vis spectra using the Tauc equation.[60] A decrease in the band gap is observed after the addition of RGO in the composite. The band gaps of the samples are calculated as 2.95 and 2.74 eV for PANI and PANI/RGO (PG2), respectively. Hence, the energy needed to excite electrons from the valence band to the conduction band is lower in the PANI/RGO (PG2) composite than that for pure PANI.

X-ray photoelectron spectroscopy (XPS) spectrum is mainly used to study the chemical bonding, composition, and surface property of a material. The survey spectrum (Figure a) confirms the presence of three binding energy peaks at ∼284, ∼398, and ∼532 eV corresponding to C 1s, N 1s, and O 1s, respectively. Figure b depicts the spectrum of carbon (C 1s) in the composite having a binding energy of 284.5 eV, which is close to that of pure PANI (283.9 eV) and RGO (284.2 eV for C–C bond).[61] Oxygen (O 1s) spectrum (Figure c) of the composite with a binding energy peak at ∼531.2 eV is due to the C–O or C-OH group in carbon-based nanomaterials.[62] In Figure d, the spectrum of nitrogen (N 1s), at binding energy ∼399.6 eV, is assigned to the presence of the quinoid amine (C=N bond) in the backbone of PANI.[24]

Figure 4

(a) Survey and deconvoluted (b) C 1s, (c) O 1s, and (d) N 1s XPS spectra of the PG2 composite.

Photocatalytic Activity

Photocatalytic Degradation of Organic Dye

We have used a UV cutoff filter to examine the visible-light-driven photocatalytic activity of the samples for the degradation of cationic dyes malachite green (MG), rhodamine B (RhB), and anionic dye congo red (CR) as the probe molecules. In the case of MG, RhB, and CR, the characteristic absorption peaks at 620, 553, and 496 nm, respectively, were used to conduct the degradation study of the dyes. Relative concentrations of MG, RhB, and CR dyes using the PANI/RGO composite and its constituent (PANI and RGO) photocatalysts are shown in Figure A–C.

Figure 5

Plots of the concentration ratios of (A) MG, (B) RhB, and (C) CR in aqueous solutions against specific time intervals under various conditions using the PANI/RGO (PG2) composite.

Rate constants (Kapp) have been calculated from the plots of ln(C0/Ct) versus irradiation time for MG, RhB, and CR, as shown in Figure A–C, respectively. Under visible light irradiation, among the three PANI/RGO composites, PG2 with 5 wt % RGO has come out to be the best with the degradation efficiencies of 99.68, 99.35, and 98.73% for MG, RhB, and CR dyes within 15, 30, and 40 min, respectively.

Figure 6

First-order kinetic plots of ln(C0/Ct) versus time for (A) MG, (B) RhB, and (C) CR discoloration in the presence of all of the photocatalysts under dark and visible light irradiation.

Successive decreases of the absorption intensities of MG at 620 nm, RhB at 553 nm, and CR at 496 nm with light exposure time are shown in Figure A–C, respectively, to explore the photocatalytic activities of the PANI/RGO (PG2) composite. In the presence of pure PANI and RGO, under visible light illumination, the degradation percentages of MG, RhB, and CR dyes are about 61.07, 70.46, and 73.66% and 91.54, 89.45, and 86.07%, respectively. The performance is found to be noticeably enhanced through the presence of the PG2 composite for all of the dyes.

Figure 7

UV–vis absorption spectral changes of aqueous solutions of (A) MG, (B) RhB, and (C) CR in the presence of the PG2 composite under visible light irradiation.

Effect of Scavengers

Reaction profiles of photocatalytic degradation of MG, RhB, and CR dyes as a function of irradiation time in the presence of the PG2 composite using different scavengers are shown in Figure A–C, respectively. Visible-light-induced degradation percentages with time and rate constant (Kapp) values of the various dyes using the prepared photocatalysts have been tabulated in Table .

Figure 8

Reaction profiles of photodegradation of (A) MG, (B) RhB, and (C) CR with irradiation time in the presence of the PG2 composite using different scavengers.

Table 2

Comparison of Degradation Efficiency (R), Degradation Time (T), and Rate Constant (Kapp) for the Degradation of Various Dyes Using Prepared Photocatalysts

dye	photocatalyst	R (%)	T (min)	Kapp (min–1)
malachite green (MG)	PANI	61.07	15	6.30 × 10–2
	RGO	91.54		1.65 × 10–1
	PG1	80.06		1.07 × 10–1
	PG2	99.68		3.84 × 10–1
	PG3	85.04		1.27 × 10–1
rhodamine B (RhB)	PANI	70.46	30	4.07 × 10–2
	RGO	89.45		7.50 × 10–2
	PG1	86.68		6.72 × 10–2
	PG2	99.35		1.68 × 10–1
	PG3	90.58		7.87 × 10–2
congo red (CR)	PANI	73.66	40	3.33 × 10–2
	RGO	86.07		4.93 × 10–2
	PG1	82.22		4.32 × 10–2
	PG2	98.73		1.09 × 10–1
	PG3	87.86		5.27 × 10–2

Figure represents a comparison of degradation percentages of dyes (MG, CR, and RhB) by PANI and PANI/RGO (PG1, PG2, and PG3) composites under visible light illumination. A comparison of first-order rate constants (Kapp) has been presented in Table using carbon-based composites given in refs (24, 41−52) for decolorization of numerous cationic and anionic dyes.

Figure 9

Degradation percentages with RGO (wt %) for composites (PG1, PG2, and PG3) under visible light irradiation for MG, CR, and RhB dyes.

The superior activity of the PANI/RGO architecture is explained by the improvement of charge separation caused by the inclusion of RGO in the PANI matrix. To approach the mechanism of enhanced photocatalytic activity of the PANI/RGO composite, the relative band positions of both were studied.[60,63] As depicted in Figure , while the PANI/RGO photocatalyst is being irradiated in visible light, PANI gets excited and photoinduced electrons and holes are generated. Electrons flow downhill from CB of PANI to the Fermi level (FL) of RGO, leading to suppression in the charge pair recombination process. According to the literature, RGO has a higher reduction potential than O2/•O2– (+0.07 V).[64] Electrons present at the surface of RGO can easily react with dissolved O2 to produce a superoxide radical anion (•O2–), which in turn produces H2O2 in the presence of water. Photo-oxidation and photoreduction of H2O2 occur with electrons and holes at the catalyst surfaces, resulting in the formation of oxidant species •OH radicals, which degrade the dye molecules to colorless products. Under visible light, dyes are excited to dye*. In the case of MG, the LUMO level of MG complements well with VB of PANI.[65] Therefore, photogenerated electrons are transferred from the LUMO level of MG* to VB of PANI. As the HOMO level of MG is situated lower than the oxidation potential of H2O/•OH (+2.32 V),[66] H2O receives hole from the HOMO level of MG* to create •OH directly. In the case of RhB and CR dyes, the excited states RhB* and CR* can inject electrons into the CB of PANI via an electron transfer and these electrons, in turn, flow downhill to RGO, which is scavenged by the O2 on the surface of the catalyst to form a superoxide radical anion. Based on our experimental results and the discussions above, the mechanism of photocatalytic degradation of MG, RhB, and CR dyes on the PANI/RGO catalyst has been proposed, as expressed in eqs –11.

Figure 10

Mechanism of the visible-light-driven charge transfer process of the photogenerated electrons and holes in the PANI/RGO composite.

The proposed photocatalytic degradation mechanism is given below:The involvement of the active species of the visible-light-induced photodegradation was confirmed with scavengers, such as tert-butanol (TBA), sodium oxalate (SO), K2S2O8, and p-benzoquinone (BQ) as •OH radical, hole (h+), electron (e), and •O2 radical scavengers, respectively. As shown in Figure A–C, the addition of scavengers induces an extent of inhibition in the photodegradation of dyes, which is a clear signature of their roles in the degradation process [degradation rate in decreasing order: •OH > e > •O2 > h+]. These results suggest that the photocatalytic degradation of dyes over the PANI/RGO composite is dominated mostly by the •OH radical oxidation process than the generated •O2 radicals and holes of the photocatalyst.

For the photocatalysts, light absorption causes the following reactions:

For dyes, light absorption causes the following reactions

For MG

For RhB and CR

Two methods, including the nitroblue tetrazolium (NBT) probe technique[67] and photoluminescence (PL-OH)[68] experiment, were used to further confirm the presence of •O2 and •OH radicals on the photocatalyst surface. As observed from Figure a, with a longer irradiation time, the maximum absorption peak at 259 nm is gradually decreased. This is due to the reaction between NBT and •O2 radicals. This result fully supports the BQ quenching test (Figure ) for the PG2 sample. The kinetic plots of intensity versus irradiation time at 259 nm are shown in Figure b, which are well fitted by the pseudo-first-order reaction equation for (i) PANI, (ii) RGO, and (iii) PG2 samples, respectively. Figure c represents the PL emission peak at around 426 nm (excited at 312 nm). The formation of •OH radical in the photocatalytic oxidation process is substantiated from the gradual increase in the PL intensity with irradiation time. This is also compatible with the result of the TBA quenching experiment (Figure ). The PL intensity versus irradiation time plots for all of the prepared materials shown in Figure d demonstrate the higher production rate of •OH radicals for PG2 than that for pure PANI or RGO. All of the results are in conformity with the scavenger tests showing that the PG2 composite possesses more visible-light photocatalytic activity than pristine PANI or RGO used in the degradation of both types of dyes.

Figure 11

(a) UV–vis absorption spectra of NBT for PG2, (b) plots of (i) PANI, (ii) RGO, and (iii) PG2 absorbance versus irradiation time at 259 nm, (c) •OH radical-trapping PL spectra in solution of teraphthalic acid, and (d) PL intensity versus irradiation time at 426 nm for the PG2 composite.

The durability of the catalyst is an important criterion for its repetitive use in environmental remediation. It was evaluated by the photocatalytic degradation of MG, RhB, and CR for four cycles using the same catalyst. The PANI/RGO (PG2) composite sample exhibited a remarkably high photostability even after six cycles (99.68–86.63% for MG), (99.35–87.12% for RhB), and (98.73–92.78% for CR), as depicted in the photodegradation plots in Figure .

Figure 12

Relative dye concentrations versus light exposure time for six consecutive runs of the PG2 composite for (a) MG, (b) RhB, and (c) CR dyes.

Photocatalytic Degradation of Toxic Metal Cr(VI)

Apart from organic dye degradation, the as-synthesized PANI/RGO (PG2) composite was found to have an active application in the reduction of other water pollutants, e.g., Cr(VI). Figure A represents the efficient removal of Cr(VI) up to 94.7% at pH 2 in only 15 min. The plot of ln(C0/Ct) vs irradiation time expresses a linear behavior, as shown in the inset of Figure A. From the slope of the linear plot, a high rate constant value of 1.932 × 10–1 min–1 has been computed. In the presence of visible light, the photogenerated electrons on the conduction band of PANI come to the FL of RGO, causing the effective charge separation of the semiconductor. Due to this charge separation, the available electrons can easily reduce Cr2O72– to Cr3+. At pH 2, photocatalytic reduction of Cr(VI) to Cr(III) is described as followsTo monitor the effect of photogenerated electrons on the reduction of Cr(VI) for the RGO-incorporated PANI nanoparticles (PG2), electron scavenger K2S2O8 is added in the medium. As shown in the inset of Figure , the addition of K2S2O8 almost ceases the reduction reaction.[69] H2O2 can take up holes from the VB of PANI to convert •OOH to •OH and to colorless degraded products. Figure B represents the reusability of photocatalytic reduction of Cr(VI) for five cycles.

Figure 13

(A) Reaction profile for the photocatalytic reduction of Cr(VI) in the presence of catalyst materials under visible light irradiation, the inset shows the corresponding linear plot of ln(C0/Ct) vs time in the presence of (a) PG2 + K2S2O8, (b) PANI, and (c) PG2. (B) Relative dye concentrations vs light exposure time for five consecutive runs of the PG2 composite for photocatalytic reduction of Cr(VI).

Photoluminescence (PL) Property

In the present study, the photoluminescence (PL) spectra in Figure clearly demonstrate the following: (i) a broad peak at 443 nm for neat PANI and the PANI/RGO (PG2) composite, caused by the polaronic band of PANI,[70] (ii) decreased emission in PG2 with respect to that in pure PANI, revealing suppression of recombination of carriers. Observation (ii) indicates that RGO serves as an electron acceptor in the composite. These results are in conformity with the experimental observations, that is, the enhanced rate constant (Kapp) and efficiency of photocatalytic degradation of MG, RhB, and CR dyes under visible light irradiation using the PANI/RGO (PG2) composite over pure PANI as a photocatalyst. Because of the separation of photogenerated electrons and holes with higher lifetime, the generation of highly oxidative photoreactive species like •O2 and •OH radicals is promoted in the photocatalytic reaction. Thus, the PANI/RGO composite emerges to be a stronger photocatalyst compared with pure PANI, as these photoreactive species sequentially take part in the degradation processes.[71,72]

Figure 14

Room temperature PL spectra of (a) PANI and (b) PG2 composite.

Brunauer–Emmett–Teller (BET) Surface Area and Pore Size Analyses

BET analysis has been performed to measure the surface area and pore size of RGO and the PANI/RGO (PG2) composite, as shown in Figure . It is the type IV isotherm (Figure a), which indicates the mesoporous structure of the samples.[73,74] The measured surface area of the PG2 composite is about 35.06 m2/g, which is almost 2 times larger compared to that of pristine PANI (15.41 m2/g),[75] as has been observed for other systems such as hollow cobalt ferrite/polyaniline nanofiber photocatalyst,[76] PANI-bismuth selenide photocatalyst,[77] and multifunctional polyacrylonitrile/ZnO/Ag electrospun nanofiber photocatalyst.[78] The cumulative pore volume and differential pore volume distributions with respect to the pore width of the RGO and PG2 composite are shown in Figure b,c, respectively, using Barrett–Joyner–Halenda desorption data. For the PG2 composite, the higher pore volume is observed compared with pure RGO, which is a signature of more porous morphology.

Figure 15

(a) Isotherms, (b) cumulative and (c) differential pore volumes for the RGO and PG2 composite.

Time-Resolved Fluorescence Decay Spectra

Nanosecond time-resolved emission spectra of the PANI/RGO (PG2) composite showing faster decay compared with bare PANI or bare RGO are presented in Supporting Figure S5. Using a triexponential fitting process, average lifetimes of emission decay for all of the materials are tabulated in Supporting Table T1. After decoration of RGO sheets into the PANI matrix, the average time of the PG2 composite decreases to 1.22 ns. The diminished fluorescence lifetime value implies that the PANI/RGO composite can capture electrons, facilitating the electron–hole pair separation.

Electron Paramagnetic Resonance (EPR) Spectra

The presence of PANI in the composite generates semiquinone radical cations (polarons) as charge carriers, which are established from the EPR spectra of the PANI/RGO composites for samples PG1, PG2, and PG3 with a characteristic peak at g ∼2.004.[79] There is an increase in peak intensity from PG1 to PG2 followed by a decrease for PG3, as expected from photocatalytic measurements where the material’s activity shows the same trend [Supporting Figure S6].

Conclusions

PANI and the PANI/RGO composite with different RGO contents have been synthesized via the in situ oxidative polymerization technique. These materials degrade MG, RhB, and CR very efficiently under visible light. The synergistic effect between PANI and RGO (sheetlike structure) in the composite improves visible-light catalytic activity compared to that of PANI and RGO. PG2 (5 wt % RGO) showed the best photocatalytic performance with higher rate constants compared to those of its constituents PANI and RGO, which are ∼6.1 times and ∼2.3 times higher for MG, ∼4.1 times and ∼2.2 times higher for RhB, and ∼3.3 times and ∼2.2 times higher for CR. For TE applications, higher percentages of RGO have been used, and for photocatalytic activity, lower percentages of RGO in the composite have given remarkable results. The mechanism of photodegradation is discussed for the improvement of visible light performance of the composite over PANI through the active species trapping experiment. The composite also showed superior photocatalytic activity toward the reduction of Cr(VI) to Cr(III) under visible light. The diminishing PL intensity and fluorescence lifetime of photoexcited charge carriers are in tune with the improved photodegradation of the dyes by PG2 over PANI. The enhanced activity of the composite was mainly due to the higher production rate of •OH radicals than •O2 and holes (h+), as concluded from the nitroblue tetrazolium (NBT) probe test and photoluminescence (PL-OH) experiments.

Experimental Section

Materials

Pure graphite powder (99%; LOBA Chemie, India, crystalline, 60 mesh), aniline (Fisher Scientific, India), deionized water (Hydrolab, India), 5-sulfosalicylic acid (SSA, Merck), ammonium peroxy-di-sulfate (APS, Merck), sodium nitrate (NaNO3, Merck), potassium permanganate (KMnO4, Merck), sulfuric acid (H2SO4, Merck), ortho-phosphoric acid (H3PO4, Merck), hydrogen peroxide (H2O2, Merck, 30% GR Pro analysis), calcium chloride (CaCl2, Merck), phosphorus pentoxide (P2O5, Merck), and hydrazine hydrate (N2H4H2O, Merck) were used for the material synthesis. Malachite green (MG), congo red (CR), and rhodamine B (RhB) dyes were purchased from Himedia, India. Nitro B.T. AR (nitroblue tetrazolium (NBT)) and benzene-1,4-dicarboxylic acid (teraphthalic acid (TA)) are procured from LOBA Chemie, India.

Synthesis of PANI/RGO (PG) Photocatalysts

Composites of PANI/RGO have been synthesized employing the in situ oxidative polymerization method, as described in our previous work;[54] however, the percentages of compositions are different in the present case. PG composites containing three different weight percentages of reduced graphene oxide sheets have been denoted PG1 (2% RGO), PG2 (5% RGO), and PG3 (8% RGO), respectively.

Evaluation of Photocatalytic Activity

To demonstrate the potential application of neat PANI and RGO and PANI/RGO (PG) composites for the degradation of organic contaminants, we have examined their photocatalytic activities by choosing the photodegradation (at pH = 7) of three model pollutant dyes: cationic malachite green (MG), rhodamine B (RhB), and anionic congo red (CR). Furthermore, the photoreduction of an aqueous solution of 10–5 M Cr(VI) has been studied. To understand the degradation mechanism, the scavenger test and the reusability test were performed for the PG composites. In a usual run of photocatalytic decomposition of MG, RhB, and CR, 10 mg of catalyst was dispersed in 50 mL of dye solution (1.7 × 10–5 M) in a quartz beaker. Then, the solution was magnetically stirred at ambient temperature and pressure in the dark for 1 h to establish adsorption–desorption equilibrium of MG, RhB, and CR molecules on the surface of the catalyst. The changes in the absorbance intensity of the dyes were scrutinized. For visible light, we have used a Phillips 200 W tungsten lamp (total optical irradiance = 70 mW cm–2, the distance between the irradiation source and the top surface of the dye in the container = 10 cm), positioned vertically above the reaction vessel. A solution of NaNO2 was used as a UV cutoff filter[80] to guarantee complete removal of radiation below 420 nm and to ensure that illumination of the photocatalyst system occurred only by visible light wavelengths. After a specific time interval, 5 mL of dye solution was centrifugally collected and then the concentration of the dye solution was investigated through measurement of change in the absorbance using a UV–vis spectrophotometer. The effectiveness of photocatalytic degradation (η) was evaluated using the following equationwhere C0 and Ct are the concentrations of the dyes measured at the time of light-on and after photoexposure to a particular interval of time, respectively.