Reduced Graphene Oxide Composite-Enhanced Visible

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Article Cite This: ACS Omega 2019, 4, 1623−1635

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Polyaniline/Reduced Graphene Oxide Composite-Enhanced VisibleLight-Driven Photocatalytic Activity for the Degradation of Organic Dyes Mousumi Mitra,† Sk. Taheruddin Ahamed,‡ Amrita Ghosh,‡ Anup Mondal,*,‡ Kajari Kargupta,§ Saibal Ganguly,∥ and Dipali Banerjee*,† Department of Physics and ‡Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, West Bengal, India § Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India ∥ Department of Chemical Engineering, BITS Pilani, K K Birla Goa Campus, Zuarinagar, Sancoale 403726, Goa, India ACS Omega 2019.4:1623-1635. Downloaded from pubs.acs.org by 91.200.82.108 on 01/21/19. For personal use only.

†

S Supporting Information *

ABSTRACT: 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.

1. 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 TiO2,8−108−10 CdS,11,12 CdSe,13 BiOBr,14 Cu2O,15 WO3,16 Fe2O3,1717 ZnO,18 and SnO219 are extensively used nowadays. © 2019 American Chemical Society

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 Received: October 25, 2018 Accepted: January 9, 2019 Published: January 18, 2019 1623

DOI: 10.1021/acsomega.8b02941 ACS Omega 2019, 4, 1623−1635

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ion exchange, reverse osmosis, evaporation, direct precipitation, etc. Hence, the progress of cost-saving and less-energyconsuming 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 3 (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-lightdependent 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.

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 dyes30−34 and anionic dyes31 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 1. Table 1. Photocatalysts Used for Various Dye Degradations24,41−52 photocatalyst reduced graphene oxide/CuI/PANI polyaniline-conjugated graphene

polyaniline/graphite oxide ZnO/rGO/polyaniline ternary nanocomposite reduced graphene oxide/ZnFe2O4/PANI carbon nitride/PANI/ZnO ternary heterostructure graphene/PANI graphene/PANI/Cu2O SWCNT/PANI PANI/gC3N4 PANI/TiO2/graphene hydrogel gC3N4/rGO g-C3N4/RGO/TiO2

dye

2. RESULTS AND DISCUSSION 2.1. Characterization of the As-Prepared Photocatalysts. 2.1.1. Morphological Characterizations. Figure 1A−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 1A,D, pristine PANI is interconnected, consisting of agglomerated nanoparticles. Figure 1B,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 1C,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. 2.1.2. Spectral Characterizations. Figure 2 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 3 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

reference

RhB MO MO MB RhB MB MO RhB MB

24

RB CR RB, MO MB BPA 2,4-DCP, RhB MB

46 47 48 49 50 51 52

41

42 43 44 45

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 1624


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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.

Figure 2. XRD spectra of (a) PANI, (b) RGO, and (c) PG2 composite. Figure 3. UV−vis spectra of (a) PANI and (b) PG2 composite.

The inset of Figure 3 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 4a) 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 4b 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 4c) 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 4d, 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 2.2. Photocatalytic Activity. 2.2.1. 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 1625


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Figure 4. (a) Survey and deconvoluted (b) C 1s, (c) O 1s, and (d) N 1s XPS spectra of the PG2 composite.

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.

(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 1626


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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.

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.

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.

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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 5A−C. 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 6A−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. 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 7A−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. 2.2.2. 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 8A−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 2.

Figure 9. Degradation percentages with RGO (wt %) for composites (PG1, PG2, and PG3) under visible light irradiation for MG, CR, and RhB dyes.

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. Photooxidation 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 1−11. The proposed photocatalytic degradation mechanism is given below: (a) For the photocatalysts, light absorption causes the following reactions:

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)

malachite green (MG)

PANI RGO PG1 PG2 PG3 PANI RGO PG1 PG2 PG3 PANI RGO PG1 PG2 PG3

61.07 91.54 80.06 99.68 85.04 70.46 89.45 86.68 99.35 90.58 73.66 86.07 82.22 98.73 87.86

15

rhodamine B (RhB)

congo red (CR)

30

40

Kapp (min−1) 6.30 1.65 1.07 3.84 1.27 4.07 7.50 6.72 1.68 7.87 3.33 4.93 4.32 1.09 5.27

× × × × × × × × × × × × × × ×

10−2 10−1 10−1 10−1 10−1 10−2 10−2 10−2 10−1 10−2 10−2 10−2 10−2 10−1 10−2

hυ

Figure 9 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 3 using carbon-based composites given in refs 24, 41−52 for decolorization of numerous cationic and anionic 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 10, while the PANI/RGO photocatalyst is being irradiated in visible light, PANI gets

PANI → (h+ VB + e−CB)PANI

(1)

(e−CB)PANI → RGO(FL)(e−)

(2)

(e

−

FL )RGO

H 2O + •

+ O2 → O2

• − O2

−

•

→ OOH + OH

(3) −

(4)

•

OOH + H 2O → OH + H 2O2

H 2O2 + (e

1628

•

−

FL )RGO

•

→ OH + OH

(5) −

(6)

H 2O2 + (h+ VB)PANI → •OOH + H+

(7)

H 2O2 + •OOH → •OH + H 2O + O2

(8)


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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 PANI/RGO

reduced graphene oxide/CuI/PANI PANI conjugated graphene

PANI/graphite oxide ZnO/rGO/PANI reduced graphene oxide/ZnFe2O4/PANI carbon nitride/PANI/ZnO ternary heterostructure graphene/PANI graphene/PANI/Cu2O PANI/SWCNT PANI/g-C3N4 PANI/TiO2/graphene hydrogel

dye

DTa (min)

DPb (%)

MG RhB CR RhB MO MO MB RhB MB MO RhB MB RB CR RB MO MB BPA

15 30 40 50 70 70 150 120 180 60 60 80 180 20 10 30 120 40

99.68 99.35 98.73 100 96

irradiation source visible

visible natural sunlight

89 100

56 97.91 98.6 94.35 92.8 80

visible UV visible visible visible UV visible visible visible UV

Kappc (min−1) (calculated) 3.84 1.68 1.09 9.21 4.60 2.59 7.17 3.52 1.12

× × × × × × × × ×

10−1 10−1 10−1 10−2 10−2 10−2 10−2 10−2 10−2

6.06 × 10−2 2.6 × 10−2 4.56 × 10−3 1.93 × 10−1 4.24 × 10−1 9.51 × 10−2 2.19 × 10−2 4.02 × 10−2

reference this work

24 41

42 43 44 45 46 47 48 49 50

a

Degradation time (DT). bDegradation percentage (DP). cRate constant (Kapp).

8A−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. Two methods, including the nitroblue tetrazolium (NBT) probe technique67 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 11a, 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 8) for the PG2 sample. The kinetic plots of intensity versus irradiation time at 259 nm are shown in Figure 11b, which are well fitted by the pseudo-first-order reaction equation for (i) PANI, (ii) RGO, and (iii) PG2 samples, respectively. Figure 11c 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 8). The PL intensity versus irradiation time plots for all of the prepared materials shown in Figure 11d 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. 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%

Figure 10. Mechanism of the visible-light-driven charge transfer process of the photogenerated electrons and holes in the PANI/RGO composite.

(b) For dyes, light absorption causes the following reactions hυ

dye → dye*(e−LUMO)

(9)

For MG dye* + (h+ VB)PANI → dye+ + PANIVB

(10a)

H 2O + (h+HOMO)dye* → •OH

(10b)

For RhB and CR dye* → dye+ + PANI/RGO(e−FL)

(10c)

(h+ VB)PANI + dye → dye+ + PANIVB

(10d)

•

OH + •O2− + dye+ → colorless degraded products (11)

The involvement of the active species of the visible-lightinduced 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 1629


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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.

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.

for RhB), and (98.73−92.78% for CR), as depicted in the photodegradation plots in Figure 12. 2.2.3. 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 13A 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 13A. 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 follows

Cr2O7 2 − + 6e− + 14H+ = 2Cr 3 + + 7H 2O (E 0 red = 1.33 V)

To 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 13, 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 13B represents the reusability of photocatalytic reduction of Cr(VI) for five cycles. 2.3. Photoluminescence (PL) Property. In the present study, the photoluminescence (PL) spectra in Figure 14 clearly demonstrate the following: (i) a broad peak at 443 nm for neat PANI and the PANI/RGO (PG2) composite, caused by the 1630


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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).

pore volume distributions with respect to the pore width of the RGO and PG2 composite are shown in Figure 15b,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. 2.5. 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. 2.6. 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].

Figure 14. Room temperature PL spectra of (a) PANI and (b) PG2 composite.

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 2.4. 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 15. It is the type IV isotherm (Figure 15a), 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

3. 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 1631


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Figure 15. (a) Isotherms, (b) cumulative and (c) differential pore volumes for the RGO and PG2 composite.

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. 4.3. 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 filter80 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

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.

4. EXPERIMENTAL SECTION 4.1. 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), orthophosphoric 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. 4.2. 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 1632


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(6) Bhattacharyya, R.; Ray, S. K. Micro- and nano-sized bentonite filled composite superabsorbents of chitosan and acrylic copolymer for removal of synthetic dyes from water. Appl. Clay Sci. 2014, 101, 510−520. (7) Mondal, P.; Satra, J.; Ghorui, U. K.; Saha, N.; Srivastava, D. N.; Adhikary, B. Facile fabrication of novel hetero-structured organicinorganichigh-performance nanocatalyst: A smart system for enhanced catalytic activity towards ciprofloxacin degradation and oxygen reduction. ACS Appl. Nano Mater. 2018, 1, 6015−6026. (8) Basha, M. H.; Gopal, N. O.; Nimbalkar, D. B.; Ke, S. C. Phosphorus and boron co-doping into TiO2 nanoparticles: an avenue for enhancing the visible light photocatalytic activity. J. Mater. Sci.: Mater. Electron. 2017, 28, 987−993. (9) Vandarkuzhali, S. A. A.; Pugazhenthiran, N.; Mangalaraja, R. V.; Sathishkumar, P.; Viswanathan, B.; Anandan, S. Ultrasmall plasmonic nanoparticles decorated hierarchical mesoporous TiO2 as an efficient photocatalyst for photocatalytic degradation of textile dyes. ACS Omega 2018, 3, 9834−9845. (10) Ali, T.; Tripathi, P.; Azam, A.; Raza, W.; Ahmed, A. S.; Ahmed, A.; Muneer, M. Photocatalytic performance of Fe-doped TiO2 nanoparticles under visible-light irradiation. Mater. Res. Express 2017, 4, No. 015022. (11) Chen, F.; Cao, Y.; Jia, D.; Niu, X. Facile synthesis of CdS nanoparticles photocatalyst with high performance. Ceram. Int. 2013, 39, 1511−1517. (12) Li, Q.; Li, X.; Wageh, S.; Al-Ghamdi, A. A.; Yu, J. CdS/ graphene nanocomposite photocatalysts. Adv. Energy Mater. 2015, 5, No. 1500010. (13) Thirugnanam, N.; Song, H.; Wu, Y. Photocatalytic degradation of brilliant green dye using CdSe quantum dots hybridized with graphene oxide under sunlight irradiation. Chin. J. Catal. 2017, 38, 2150−2159. (14) Liu, W.; Gao, Y.; Yang, Y.; Zou, Q.; Yang, G.; Zhang, Z.; Li, H.; Miao, Y.; Li, H.; Huo, Y. Photocatalytic composite of a floating BiOBr@graphene oxide@melamine foam for efficient removal of organics. ChemCatChem 2018, 10, 2394−2400. (15) Su, Y.; Li, H.; Ma, H.; Robertson, J.; Nathan, A. Controlling surface termination and facet orientation in Cu2O nanoparticles for high photocatalytic activity: A combined experimental and density functional theory study. ACS Appl. Mater. Interfaces 2017, 9, 8100− 8106. (16) Dong, P.; Hou, G.; Xi, X.; Shao, R.; Dong, F. WO3-based photocatalysts: morphology control, activity enhancement and multifunctional applications. Environ. Sci. Nano 2017, 4, 539−557. (17) Mishra, M.; Chun, D. M. α-Fe2O3 as a photocatalytic material: A review. Appl. Catal., A 2015, 498, 126−141. (18) Mitra, M.; Ghosh, A.; Mondal, A.; Kargupta, K.; Ganguly, S.; Banerjee, D. Facile synthesis of aluminium doped zinc oxidepolyaniline hybrids for photoluminescence and enhanced visiblelight assisted photo-degradation of organic contaminants. Appl. Surf. Sci. 2017, 402, 418−428. (19) Bui, P. D.; Tran, H. H.; Kang, F.; Wang, Y. F.; Cao, T. M.; You, S. J.; Vu, N. H.; Pham, V. V. Insight into the photocatalytic mechanism of tin dioxide/polyaniline nanocomposites for NO degradation under solar light. ACS Appl. Nano Mater. 2018, 1, 5786−5794. (20) Zhang, J.; Han, J.; Wang, M.; Guo, R. Fe3O4/PANI/MnO2 core−shell hybrids as advanced adsorbents for heavy metal ions. J. Mater. Chem. A 2017, 5, 4058−4066. (21) Medina-Llamas, J. C.; Chávez-Guajardo, A. E.; Andrade, C. A. S.; Alves, K. G. B.; de Melo, C. P. Use of magnetic polyaniline/ maghemite nanocomposite for DNA retrieval from aqueous solutions. J. Colloid Interface Sci. 2014, 434, 167−174. (22) Wang, L. P.; Wang, W.; Di, L.; Lu, Y. N.; Wang, J. Y. Protein adsorption under electrical stimulation of neural probe coated with polyaniline. Colloids Surf., B 2010, 80, 72−78. (23) Eskizeybek, V.; Sarı, F.; Gülce, H.; Gülce, A.; Avcı, A. Preparation of the new polyaniline/ZnO nanocomposite and its photocatalytic activity for degradation of methylene blue and

spectrophotometer. The effectiveness of photocatalytic degradation (η) was evaluated using the following equation η = (C0 − C t)/C t × 100%

where 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.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02941. Dark adsorption results of PANI, RGO, and the composite (Figure S1); the calculation of rate constants for the other two composites PG1 and PG3 (Figures S2 and S3); UV−vis spectral changes of PG1 and PG3 composites (Figure S4); time-resolved fluorescence decay spectra of the samples (Figure S5), EPR spectra of the composites (Figure S6); transient photocurrent density vs time (Figure S7); HRTEM images of the materials (Figure S8) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +91 7044369052 (A.M.). *E-mail: [email protected]. Phone: +91 9830299253 (D.B.). ORCID

Dipali Banerjee: 0000-0002-6815-6138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors M. Mitra and A. Ghosh thank the DST-INSPIRE (IF 130168), India, and University Grant Commission (UGC), India, for providing their research fellowships, respectively. Authors acknowledge CEGESS, IIEST, Shibpur, for providing FESEM images. Prof. Nilmoni Sarkar, Department of Chemistry, IIT, Kharagpur, is duly acknowledged for the measurement of time-resolved emission spectra of the materials.

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REFERENCES

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