Facile Fabrication of Novel Hetero-Structured OrganicâInorganic High

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Facile Fabrication of Novel Hetero-Structured Organic–Inorganic HighPerformance Nanocatalyst: A Smart System for Enhanced Catalytic Activity towards Ciprofloxacin Degradation and Oxygen Reduction Papri Mondal, Jit Satra, Uday Kumar Ghorui, Namrata Saha, Divesh Narayan Srivastava, and Bibhutosh Adhikary ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00937 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Nano Materials

Facile

Fabrication

of

Novel

Hetero-Structured

Organic–Inorganic

High-

Performance Nanocatalyst: A Smart System for Enhanced Catalytic Activity towards Ciprofloxacin Degradation and Oxygen Reduction Papri Mondal,† Jit Satra,† Uday Kumar Ghorui,† Namrata Saha,† Divesh N. Srivastava, ‡ and Bibhutosh Adhikary, *† †Department

of Chemistry, Indian Institute of Engineering Science and Technology,

Shibpur, Howrah 711 103, West Bengal, India ‡Department

of Analytical Science, Central Salt and Marine Chemicals Research

Institute, Gijubhai, Badheka Marg, Bhavnagar 364002, Gujarat, India *Corresponding author Tel: +91-3326684561 Ext. 512, Fax: +91-3326682916, E-mail: [email protected]

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: In this study, a novel organic-inorganic polyaniline/silver/silver molybdate (PANI/Ag/Ag2MoO4) heterojunction nanocatalyst has been fabricated by in situ depositing Ag2MoO4 on p-type PANI to explore two different catalytic properties, e.g., photocatalytic degradation of antibiotic ciprofloxacin (CIP) and electrocatalytic reduction of oxygen. After hybridization of Ag2MoO4 with PANI, the p-n heterojunction developed and consequently z-scheme induced efficient photogenerated charge carrier separation and migration occur due to the internal electric field developed at the heterojunction

interface.

Compared

with

pure

Ag2MoO4,

the

heterojunction

nanaocomposite possessed significantly enhanced photocatalytic activity as evidenced by the degradation of Congo red (CR) dye under UV light irradiation. The optimum composite with 20 wt % PANI nanorods exhibited paramount photocatalytic activity over all the composites. Subsequently, 99.99% of CIP removal was achieved with this optimal composite within 40 minutes. On the other hand, it displays admirable electrocatalytic activity towards oxygen reduction reaction (ORR) compared to Ag2MoO4 with high ORR onset potential. It is worth mentioning that PANI was beneficial towards both improved photocatalytic and electrocatalytic activity. Our study provides a simple method to design and fabricate a semiconductor composite material with staggered band structure which exhibits enhanced photocatalytic and ORR performance.

KEYWORDS: polyaniline/silver/silver molybdate, nanoheterojunction, photocatalysis, ciprofloxacin, oxygen reduction reaction.


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1. INTRODUCTION Over the last few decades, the rising concerns about energy crisis and environmental pollution have stimulated a significant research on heterogeneous photocatalysis.1-5 Of late, the sustained discharge of pharmaceutical residues through waste water and their environmental accumulation seriously pose a threat to the living system. This harmful outcome has aroused a worldwide concern. Antibiotic, a significant drug in livestock medicines, is one of the pharmaceutical contaminant that has a high consumption rate both in humans and aquatic life. Unfortunately, it possesses some negative impacts on living organism as well as in water-soil system because of the universal requirements of these antibiotics, mainly in antibacterial resistance. Therefore, in order to remove these antibiotic residues, several techniques such as photocatalytic degradation, adsorption, ion-exchange, biodegradation and membrane filtration have been utilized. But, due to being green, cost effective and an efficient process, semiconductor (SC) photocatalysis has become the main spotlight of present research for antibiotic treatment.6-9 Not only the environmental pollution, but also the rapid depletion of fossil fuel is also a worrisome threat to the global life. The rapid increase of global energy demand and fast consumption of non-renewable energy resources have already compelled human society to give global thrust towards the utilization of renewable energy sources. The energy conversion and storage through electrochemical oxidation or reduction reaction such as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are supposed to be most promising methods for producing renewable energy, as they are very efficient and economically favorable processes. The ORR has spurred significant attention because of its key role in energy conversion, as well as in electrochemical devices, such



as fuel cells and metal ion batteries.10,11 Till date, Pt-based catalysts have been recognized as the most effective catalyst for ORR because of its highest activity in catalyzing the oxygen reduction via desirable 4-electron pathway. But prohibitive cost, limited supply and even its high tendency to aggregate in nanoscale structure restrict its widespread practical application12 and necessitates development of non-precious active catalysts which possess superior durability along with excellent 4-electron selectivity towards ORR.13-22 In this regard, we have turned our attention to design a material with appreciable photocatalytic and electrocatalytic performance. More recently, oxide SCs have received appreciable attention because of their high stability, non-toxicity and cost effectiveness for application in both oxygen reduction and environmental remediation. Interestingly, Ag-based oxides have become a good candidate for these applications because of its unique photosensitivity as well as its high ORR activity, reasonable electrochemical and thermodynamic stability, and also being low cost among the potential alternatives to Pt.23-26 Ag2MoO4 has been demonstrated as a promising n-type semiconductor catalyst for its unique properties such as photoluminescence, good catalytic activity, high electrical conductivity, environmental friendliness and also due to its electrochemical energy storage performance.27,28 To the best of our knowledge, Ag2MoO4 has very rare application in photocatalysis due to its poor photostability and undesirable quick recombination of photogenerated electron-hole pair that limit the improvement of its photocatalytic activity. Not only that, the high peroxide yield also limit the ORR activity of Ag2MoO4.29 As a result, several attempts have been urgently made to get rid off from these disadvantages, including composite and heterojunction formation,30-34 controlling the morphologies35-37 and also doping or tuning


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their composition.38,39 Generally, constructing heterostructured SCs are advantageous compared to single-component SCs to accelerate the photocatalytic reaction due to enhanced light-harvesting capacity40,41 and delayed charge recombination exerted by the synergistic effect42,43 that prolongs the life time of charge carriers. More specifically, the use of organic polymer as a co-catalyst has attracted much attention in the fields of both photocatalysis and electrocatalysis. Recently, π-conjugated conductive heteroatom polymer, polyaniline (PANI) was recognized as an efficient nitrogen-carbon template towards ORR44-49 and photodegradation with high catalytic performance due to its high absorption coefficient, ultrafast response, high charge carrier mobility and admirable environmental stability. The existence of PANI in various oxidation states and the degree of protonation range from the fully reduced leucoemeraldine base form, through halfreduced emeraldine base form, to the completely oxidized pernigraniline base form, make it an interesting and very ideal polymeric material. PANI has been also used as a good substrate for photocatalysts where it acts as electron donor and hole acceptor under illumination. Additionally, it also acts as a binding agent as well as a reducing agent during nanoparticles (NPs) formation over it.50,51 Based on these properties, many PANI based organic-inorganic heterojunction SCs have been fabricated with enhanced photocatalytic

performances.52,53

Una

Bogdanovic

prepared

gold

polyaniline

nanocomposite and found that this composite shows rapid charge transfer kinetics with highly selective dioxygen reduction to water.54 Liang Zhang investigated ORR with Ag dendrites (AgDS), PANI and PANI/AgDS.55 Zhao et al. synthesized PANI decorated Bi2MoO6 nanosheets for efficient interfacial charge separation during photocatalysis.56 In



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various SCs PANI has been used as a photosensitizer to increase the charge separation efficiency, thereby enhancing photocatalytic efficiency of the catalyst. In this article, we have focused our attention on fabrication of a SC device in such a way where the hole and electron work separately in two different catalytic reactions. Therefore, in this regard, novel PANI/Ag/Ag2MoO4 was commendably fabricated via in situ deposition of n-type Ag2MoO4 on p-type PANI (Scheme 1). During this in situ growth, little amount of metallic silver was formed on PANI surface. Herein, prepared PANI may exist as conducting leucoemeraldine base form which possesses weak reduction capability due to presence of –NH- bonds. During composite synthesis, some of the Ag+ ions easily get adsorbed on PANI surface because of the lone pair over N atoms. These adsorbed Ag+ ions oxidize the –C-NH- bond to –C=N- bond and itself gets reduced to Ag0. This nanocomposite promotes the photocatalytic CIP degradation during photooxidation process. At the same time, PANI also enhances the ORR activity with 4 eselectivity. Currently, synthesis and catalytic activity of PANI/Ag/Ag3PO4 has been reported.57 However, their catalytic activity is much less than our material. Interestingly, our composite possesses much higher catalytic activity in both photooxidation and electroreduction. Furthermore, very little amount (7mg) of catalyst has been required during

photo-oxidation

over

PANI/Ag/Ag3PO4

(100mg).

Therefore,

the

PANI/Ag/Ag2MoO4 nanocomposite is supposed to exhibit improved catalytic performance that provides considerable advantages with respect to PANI/Ag/Ag3PO4. Our work may provide useful insights into the strong interfacial interaction between PANI and Ag2MoO4 that have a remarkable influence on their band structure and also the electron-hole separation.


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STEP I

STEP II

NH2

Ag2MoO4 Ag Polyaniline

Addition on N

N

N

H

H

H

Polyaniline/Ag/Ag2MoO4 n

Scheme 1. Synthetic representation of Polyaniline and Polyaniline/Ag/Ag2MoO4.

2. EXPERIMENTAL SECTION 2.1. Synthesis. PANI was synthesized according to the literature method [supporting information (S.I.)].57 Preparation of Ag2MoO4 Nanoparticles and PANI/Ag/Ag2MoO4 Nanocomposites. For the synthesis of Ag2MoO4 NPs, 2.0 mmol of AgNO3 was dissolved in 5 mL of ethylene glycol and stirred for 30 min. Then 1 mmol Na2MoO4 in 30 mL H2O was added to the AgNO3 solution under continuous stirring and continued stirring for another 30 min at room temperature. The obtained solid was centrifuged, rinsed repeatedly with H2O and ethanol and finally calcined at 400 °C for 4 h. The different wt % of PANI/Ag/Ag2MoO4 nanocomposites was synthesized through in situ growth of Ag2MoO4 NPs on the PANI surface by following similar method. Typically, 20 wt % PANI/Ag/Ag2MoO4 nanocomposite was prepared by using 0.1 g of PANI, dispersed into 30 mL of H2O through ultrasonic vibration for 10 min. Then 0.5 g (2.94 mmol) of AgNO3 was added into the dispersion and stirred for 1 h. A 30 mL



solution of deionized water containing 0.36 g (1.47 mmol) Na2MoO4 was then added drop-wise to the AgNO3 and PANI mixture under stirring and further stirred again for 4 h. Later, the obtained particles were collected through centrifugation and washed with H2O and EtOH. Finally, it was kept under vacuum at 55°C for 24 h. A series of x (5, 10, 15, 25) wt % PANI/Ag/Ag2MoO4 nanocomposites were prepared in a similar manner. 2.2. Photocatalytic Degradation of Congo Red (CR) and Ciprofloxacin (CIP). To assay the photocatalytic potential of the all the materials, CR was tested as a model system. In this typical experiment, the photocatalyst (7 mg) was suspended into 10-5 M aqueous solution (40 mL) of CR. Before illumination, this suspension was kept under dark for 30 min to achieve the adsorption-desorption equilibrium and then irradiated by using a 40 W UV tube (Phillips) under stirring, keeping 15 cm distance from dye solution. During this experiment, the solution was maintained in a thermostated bath at 250 C. During irradiation, samples were withdrawn from the system at an interval of 2 min and then centrifuged. This process was continued for 40 min. The clear dye solutions were analyzed by measuring their variable absorption maxima on a UV-Vis spectrophotometer. It has been observed that the rate of degradation is maximum for 20 wt % PANI Ag/Ag2MoO4 nanocomposite and to support further the activity of this composite, we have run the photocatalytic degradation of hazardous antibiotic CIP. Finally, to investigate the role of active species in reaction kinetics, different scavengers were used (K2Cr2O7 as an electron scavenger, ammonium oxalate as a hole scavenger and pbenzoquinone as a superoxide radical scavenger).57


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2.3. Electrochemical ORR. Electrocatalytic activity of Ag2MoO4 and 20 wt % PANI Ag/Ag2MoO4 nanocomposite towards oxygen reduction reaction (ORR) were measured with a conventional three-electrode system cell connected to a CHI7014E Electrochemical workstation including a rotating disk electrode (RDE, disk diameter 3 mm), a Pt electrode as counter electrode, a Ag/AgCl electrode (with saturated KCl solution) as reference electrode and an aqueous solution of KOH (0.1 M) was used as electrolyte. In a typical measurement, 7 mg of catalyst was ultrasonically vibrated into the mixture containing 30 μL of nafion and 570 μL of H2O. After that, 22 μL of this solution was loaded onto the pre-polished glassy carbon disk surface. Herein, Linear Sweep voltammetry of the modified electrodes were recorded in O2-saturated KOH solution under different rotation speeds at ambient temperature. 3. RESULT AND DISCUSSION 3.1. Materials Characterization. Structure and Morphology. The crystallinity and phase structure of PANI, Ag2MoO4 and a series of PANI/Ag/Ag2MoO4 nanocomposites were characterized by XRD (Figure 1). It can be noticed that all diffraction peaks can be perfectly indexed to the facecentered cubic structure of Ag2MoO4 (JCPDS No.76-1747). The XRD patterns for PANI/Ag/Ag2MoO4 nanocomposites having different wt % of PANI are shown in the Figure 1 [(5-25%), which correspond to plots (i-v) respectively]. As can be seen, there is no extra characteristic peak in case of plots (i-iii) with respect to the diffraction pattern of Ag2MoO4, indicating that little amount of PANI can not affect the composition and crystal form of Ag2MoO4. But surprisingly, as we increase the amount of PANI to 2025%, four additional peaks at 38.40, 44.270, 64.440 and 77.440 were observed which are



none other than the (111), (200), (220) and (311) planes of Ag0, respectively (JCPDS No. 04-0783). Additionally, the peak intensities got gradually increased with increasing PANI percentage (iv-v), illustrating the reducing property of PANI. Due to the presence of −NH− bonds in PANI, some Ag+ ions were easily adsorbed on the outer layer of PANI

which oxidizes the –NH- bonds to –N= by reducing itself to Ag0.58 The crystalline size of Ag2MoO4, (5-25) wt % PANI composites have been calculated by using Debye Scherrer formula and the calculated sizes are 10.3, 9.8, 7.4, 3.0, 5.2 and 2.9 nm respectively59 and these results are also consistent with the surface area values of composites (Table 1).

Figure 1. XRD of PANI, Ag2MoO4 and PANI/Ag/Ag2MoO4 nanocomposites. Herein, we have characterized and examined the pure Ag2MoO4 and all the composites to investigate the highest photocatalytic activity and observed that the catalytic performance of 20 wt % PANI/Ag/Ag2MoO4 nanocomposite exceeded all samples by far. Therefore, we have studied with pure Ag2MoO4 and 20 wt % PANI/Ag/Ag2MoO4


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nanocomposite in detail and explained the ground of enhanced photocatalytic activity of composite. ICP measurements were carried out to monitor the leaching of Ag in the reaction system and it was found that the Ag/Mo atomic concentration ratio increases with increasing the PANI weight percentage. The experimental value of Ag/Mo ratio was 2.12, 2.22, 2.35, 2.74 and 3.11 for PANI (5-25 wt %)/Ag/Ag2MoO4 respectively which all are greater than the Ag/Mo ratio of 2.01 in Ag2MoO4 (Figure S2b). The typical Figure S2a shows the calculated amount of Ag leaching in all composites and this Ag leaching increases with PANI weight percentage. This result further indicates that the adsorbed Ag+ ions can be reduced to Ag0 on PANI surface which can promote the formation of PANI/Ag/Ag2MoO4 composites. In order to confirm the existence of PANI in nanocomposite, FT-IR spectra (Figure S1) of PANI and 20 wt % nanocomposite were taken. The peaks of nanocomposite at 1574 cm-1 and 1493 cm-1 can be attributed to the C=C stretching of quinonoid and benzenoid rings respectively55 while the peak centered at 1303 cm-1 corresponds to C-N stretching mode for benzenoid ring.60 The characteristic peak located at 1146 cm-1 is associated with the C-H aromatic in-plane bending vibration.55 Compared to pure PANI, the relative peak intensity of C=C of quinonoid to benzenoid in nanocomposite is increased and also slightly red shifted that indicates the partial oxidation of benzenoid to quinonoid form. Therefore, FT-IR spectrum indicates the good interaction between PANI and Ag2MoO4, further supporting the formation of Ag0.



X-Ray photoelectron spectroscopy (XPS) was used to evaluate the chemical composition and the chemical states of silver, molybdenum, oxygen, nitrogen and carbon in the as-prepared 20 wt % PANI/Ag/Ag2MoO4 nanocomposite as shown in Figure 2. The typical Figure 2a shows the total survey spectrum that confirms the existence of Ag, Mo, O, C and N and Figure 2b shows the XPS core level spectrum of Ag 3d. The two binding energy peaks located at 367.5 eV and 373.5 eV are ascribed to Ag 3d5/2 and Ag 3d3/2 electron orbitals, respectively. These two peaks decompose into four peaks by the XPS peak fitting programme and it has been found that the strong binding peaks at 367.5 eV and 373.5 eV are derived from Ag+, whereas another two weak binding energy peaks at 368.0 eV and 374.2 eV are attributed to Ag0.57 The typical Figure 2c displays the Mo 3d XPS core level spectrum and the two binding energy peaks at 232.0 eV and 235.1 eV are ascribed to the electron orbit of Mo 3d5/2 and Mo 3d3/2 .61 N 1s core level spectrum is presented in Figure 2d. The binding energy peak located at 398.9 eV is ascribed to the C=N group62 indicating the oxidation of C-NH groups of PANI to C=N groups by Ag+ and this Ag+ is reduced to Ag0 accordingly during the 20 wt % PANI/Ag/Ag2MoO4 nanocomposite synthesis. Thus, the XPS result further confirms the successful formation of Ag0 as an intermediate layer during nanocomposite formation with 20 wt % PANI.


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(a)

(b)

(c)

(d)

Figure 2. XPS spectra of the 20 wt % PANI/Ag/Ag2MoO4 nanocomposite. (a) total survey spectrum, (b) the Ag 3d, (c) the Mo 3d, and (d) the N 1s XPS core level spectrum. To obtain a better perception about the elemental distribution of Ag2MoO4 and 20 wt % nanocomposite, an EDS study was carried out. The typical EDS pattern (Figure 3c, Table S1) shows that the atomic ratio of Ag to Mo is 2:1, which meets the required stoichiometric ratio of Ag2MoO4. The EDS result (Figure 3d, Table S2) shows the presence of C and N besides Ag, Mo and O, indicating the formation of nanocomposite. The detailed structure, size and morphology of the resulting Ag2MoO4 NPs and 20 wt %



nanocomposite were investigated by FESEM and TEM analysis. The FESEM image of the Ag2MoO4 shows typical pebble stone like morphology (Figure 3a) whereas, the composite shows the growth of pebble stone on the surface of rod-like structures of PANI (Figure 3b).

(a)

(b)

(c)

(d)

Figure 3. FESEM of (a) Ag2MoO4 and (b) 20 wt % PANI/Ag/Ag2MoO4 nanocomposite; the EDS results of (c) Ag2MoO4 and (d) 20 wt % PANI/Ag/ Ag2MoO4 nanocomposite.


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(a)

(b)

( 220) Ag2MoO4 [311]

( 311) d311 =0. 28 nm

( 422)

(c)

(d)

(e)

(f) d111 = 0.24 nm Ag [111] Ag [200] Ag2MoO4 {

d220 = 0.33 nm

[220] Ag2MoO4 [220]

[220]

Figure 4. (a) SAED and (b) Fringe pattern of Ag2MoO4 ; (c) TEM image, (d) Particle distribution of Ag2MoO4 on PANI surface, (e) SAED pattern, and (f) Fringe pattern of 20 wt % PANI/Ag/ Ag2MoO4 nanocomposite.



In order to get further insight into the morphology of these materials, TEM analysis was performed. The selected area diffraction (SAED) pattern of the Ag2MoO4 (Figure 4a) shows several concentric rings that signify the diffraction from (220), (311) and (422) lattice planes and are consistent with the values of XRD pattern of Ag2MoO4. Figure 4b shows the lattice fringes with d spacing of 0.28 nm that corresponds to (311) plane of Ag2MoO4. The typical TEM image (Figure 4c) of 20 wt % nanocomposite indicates that the surface of rod like PANI is wrapped with spherical nano particles of Ag2MoO4. The average diameter of the Ag2MoO4 NPs is 7.6 nm, with the size distribution exhibiting a range between 3 to 10 nm (Figure 4d) over PANI. From the fringe pattern (Figure 4f), two crystalline phases with different d spacings noticeably appeared on the material surface that support the in situ formation of Ag0 along with Ag2MoO4 as both Ag0 (111) and Ag2MoO4 (220) planes are identified. The corresponding SAED pattern (Figure 4e) also confirms the coexistence of both Ag and Ag2MoO4. Optical Absorption Properties. UV-Vis absorption spectra of Ag2MoO4 and 20 wt % nanocomposite were obtained by dispersing these solid materials in H2O, CHCl3 and MeOH separately. After an in-depth investigation of the optical properties, it can be concluded that the potent interfacial interactions between PANI and Ag2MoO4 can play an important role in tailoring their band structures. The UV-Vis spectra (Figure S3) indicate that the additional absorption band ranging from 300 to 500 nm in 20 wt % nanocomposite compared to Ag2MoO4 is originated from the co-existence of PANI. The typical band gaps of Ag2MoO4 and PANI were around 3.33 eV and 2.89 eV respectively, calculated from the band edge absorption using the Tauc's relation, as shown in Figure S4. [by the equation : αhѵ=A(hѵ-Eg)1/2 where, α, h, ѵ, Eg and A represent the


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proportionality constant, Plank constant, light frequency, band gap energy and absorption coefficient, respectively]. 3.2. Evaluation of Photocatalytic Activity. Incipiently, to predict the highest photocatalytic activity of the materials, we have studied the photocatalytic degradation of CR for 40 min. As depicted in Figure 5a, pure Ag2MoO4 exhibits very poor photocatalytic degradation capability with only 2.91 %, which is presented as bar diagram [Figure S5a]. The introduction of PANI in Ag2MoO4 appreciably improves the dye adsorption and degradation capability of Ag2MoO4. The adsorption capacity as well as degradation performance increases with increasing PANI percentage in composites up to 10%, however, the degradation capacity decreases in case of 15% along with further increase of its adsorption property. Interestingly, the degradation efficiency of 20 wt % nanocomposite increases further thereby exceeding all other composites whereas the adsorption efficiency was highest for 25%. The highest catalytic performance of 20 wt % nanocomposite due to its exceptional charge transfer rate, lower charge recombination, moderate PANI concentration and optimal amount of produced Ag0. The z- scheme heterojunction system significantly enhances the charge transfer rate and reduces the charge recombination. The band structure of nanocomposite may change with changing the weight percentage of PANI that can affect on separation and migration of charge carriers and it is also reflected from ESI and chronoamperometric measurements. The abnormal trend of adsorption property of 15 %, 20% and 25 % is closely related to their specific surface (Table 1). The large amount of adsorbed dye in case of 15 % and 25 % nanocomposites may hinder the light absorption by Ag2MoO4 that reflects in the drop of their degradation efficiency. Not only that, the excess PANI will also decrease the light



absorption intensity of Ag2MoO4. Alternatively, little amount of PANI is not conducive for promotion of the photocatalytic degradation efficiency of Ag2MoO4. The overall adsorption-degradation results demonstrate that PANI can intensify the adsorption ability of CR and with increasing the PANI wt %, large amount of CR can be adsorbed on catalyst surface. PANI and these adsorbed CR will reduce the photocatalytic degradation activity of composite inhibiting the light absorption of Ag2MoO4 due to their strong light absorption ability. Therefore, on the basis of these results, the moderate PANI concentration (20%) is recommended as an optimal composite for further applications. Additionally, the formation of Ag0 in nanocomposites also has an effect on the photocatalytic degradation process. Because of the existence of Ag0 along with PANI, the photoinduced holes in Ag2MoO4 can be rapidly moved to the PANI surface to cause reduced photocorrosion of Ag2MoO4. But the amount of formed Ag0 is a crucial factor for separation of photogenerated charge carriers and a favourable charge separation occurs up to a certain amount of Ag0. After exceeding this amount, Ag0 act as a recombination centre of photoinduced electrons and holes, thereby decreasing the photocatalytic activity of the nanocomposite.57 Not only that, excess formation of Ag0 also darken the colour of the nanocomposite that may inhibit the light absorption by Ag2MoO4. Here, amount of produced Ag0 in 20 wt % nanocmposite is optimum and helps in rapid charge transfer which has also been reflected from ESI and chronoamperometric measurements but in case of 25 wt % PANI nanocomposites, Ag0 may act as a recombination centre thereby reducing catalytic performance.


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Table 1. Values of surface area of the materials. Materials

Surface area(m²/g)

Ag2MoO4

3.5

15 wt % PANI/Ag/ Ag2MoO4

11.0

20 wt % PANI/Ag/ Ag2MoO4 25 wt % PANI/Ag/ Ag2MoO4

8.0 11.1

Therefore, to confirm further the appreciable photocatalytic activity of the 20 wt % nanocomposite, we studied the photocatalytic degradation of the antibiotic CIP (Figure 5b). The photocatalytic reaction reveals that 99.99 % of CIP was degraded within 38 min under UV irradiation. The rate of degradation was significantly faster (~ 4.5 times) compared to TiO2-P25. The typical Figure 5d shows the recycling experiments of the catalyst for degradation of CIP to evaluate the stability and sustainability of the 20 wt % nanocomposite. This experiment did not show any significant loss and it attained 85% degradation even after five consecutive cycles, indicating superior stability and reusability of the catalyst. All above obtained results illustrate that the incorporation of PANI over Ag2MoO4 appreciably enhances the photocatalytic degradation efficiency as well as the photostability of Ag2MoO4. In addition, the stability of the composite was further confirmed by analyzing XRD and TEM (Figure S6) results which remain similar even after 5 cycles of photocatalytic degradation studies.



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In order to investigate the dramatic active enhancement in depth and clarify the reaction mechanism, the primary active species formed during reaction were confirmed by adding different quenchers and results are shown in Figure 5c. As h+ scavengers (ammonium

oxalate)

was

added

to

capture

the

photogenerated

holes,

the

photodegradation ratio appreciably declined compared to the CIP solution without ammonium oxalate, obviously indicating the major dynamic role of photogenerated h+ in the degradation process. The typical degradation curve of 20 wt % nanocomposite in the CIP solution containing 1mM p-benzoquinone and 1mM K2Cr2O7 reveals that they do not affect the photocatalytic CIP degradation process, implying that superoxide radical and electron do not play any important role for the effective degradation of CIP. These results noticeably evidenced that photo-induced h+ are the principal active species in the photocatalytic degradation of CIP.


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(a)

(b)

(c)

(d)

.

Figure 5 (a) The photocatalytic CR degradation efficiency, (b) the comparable study of photocatalytic degradation of CIP under different conditions, (c) the photocatalytic CIP degradation efficiency of the 20 wt % PANI/Ag/Ag2MoO4 nanocomposite under UV light irradiation in the absence and presence of scavengers: K2Cr2O7, p-benzoquinone, ammonium oxalate, and (d) the reusability of the 20 wt % PANI/Ag/Ag2MoO4 photocatalyst. To

confirm

the

enhanced

photocatalytic

activity

of

the

nanocomposite,

photoluminescence study was conducted and it (Figure 6a) reflects the lower electronhole recombination rate for 20 wt % nanocomposite as compared to pure Ag2MoO4. After introduction of PANI, the strong emission of Ag2MoO4 was markedly weakened,



Page 22 of 43

illustrating that PANI can enormously inhibit the recombination rate of the photogenerated charge carriers due to development of interface junction between PANI and Ag2MoO4. To gain better understanding about the existence of the electron-hole pairs, nanosecond time-resolved emission decay of materials were also conducted and Figure 6b demonstrates that 20 wt % nanocomposite shows slower decay ( 6.07 ns) than the pure Ag2MoO4 ( 3.51 ns), further confirming the high charge separation efficiency of the nanocomposite that leads to longer lifetime of photogenerated charge carriers.63,64

(b)

(a)

Figure 6. (a) Photoluminescence spectra, and (b) Nanosecond time-resolved transient fluorescence decay of Ag2MoO4 and 20 wt % PANI/Ag/Ag2MoO4 nanocomposite. In addition to the optical properties, the charge separation and transfer properties also play a strong role in photocatalytic process. ESI experiment (Figure 7a) was conducted to get deeper insight into these above characteristics and also to confirm the highest ability of the materials used for hindering the electron-hole recombination. The arc radius of ESI


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nyquist plot reflects the charge transfer rate on the electrode surface; smaller arc indicating more effective electron-hole pair separation. The relative arc sizes (Figure 7a) are represented in the following order: Ag2MoO4> (5 wt % > 10 wt % > 15 wt % > 25 wt % > 20 wt %) nanocomposite. Obviously, 20 wt % nanocomposite possesses the highest charge transfer rate, showing just a straight line in the low frequency region instead of a semicircle.65 Beyond the optimal amount of PANI (above 20 wt%), an excess amount of PANI increase the surface thickness that hinder the light absorption by the materials, as well as the effective migration of photogenerated electrons. In addition, the photocurrent response of the photocatalysts as shown in Figure 7b

further correlates with the

generation and separation of the photogenerated electron-hole pairs during photocatalytic process, which suggests that the annihilation of photoexcited carriers were greatly retarded in case of 20 wt % nanocomposite, which agrees well with the ESI measurement. (a)

Figure 7. (a) The Nyquist plot and (b) the Chronoamperometric response of Ag2MoO4 and the different wt % PANI/Ag/Ag2MoO4 nanocomposite.


(b)


ORR: Taking the advantage of the highest activity of 20 wt % nanocomposite, rotating disk electrode experiment was conducted for the reduction of O2. In order to investigate the kinetics of ORR on the electrode surface, linear sweep voltametry (LSV) of Ag2MoO4 and the composite were performed and with increasing the rotation speed, the limiting current density was observed to be increased for both the materials as expected. Figure 8a represents the LSV plots of Ag2MoO4 in O2 saturated 1M KOH solution with onset potential 0.60 V and cathodic current density of 3.7988 mA cm-2 at a potential 0.2 V vs. RHE at 1925 rpm whereas, the composite exhibits a positive shift of 90 mV in onset potential having onset potential 0.69 V and corresponding current density 4.8789 mAcm-2 at potential of 0.2 V (Figure 8c) which is better than the reported literature value.66 Moreover, for the composite, the limiting current density was found to be higher than pure Ag2MoO4 for each rotational speed. These results demonstrate that PANI greatly enhances the ORR charge transfer kinetics thereby, promoting the bonding to the intermediates of ORR that enhances its redox ability to adsorb O2 readily on the electrode surface. The N-atom in the C-N bond of PANI being more electro-negative than the C atom, it causes the adjacent C atom to be positively charged and promotes the O2 adsorption on the C nanostructure of PANI.67 Figure 8b represents the K-L plots of Ag2MoO4 that illustrates J-1 vs. ω -1/2 plots were not parallel enough at different potentials and undergo a various number of electron transfer pathways ranging from 1.29 to 1.42 per O2 molecule in the potential range of 0.20 V to 0.35 V, suggesting that the oxygen reduction is not a single step reaction. On the other hand, for the composite, the nice coincidence and linearity of the K-L plots demonstrate first order reaction kinetics with similar number of electron transfer per oxygen molecule and it is calculated to be 4


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within the potential 0.20 V to 0.35 V. It should be noted that PANI is advantageous to overcome the poor O2 adsorption by Ag2MoO4 and also helps to improve the charge transfer kinetics between electrode surface and catalyst due to its extreme conductivity. The synergistic effect between Ag2MoO4 and PANI effectively improve the electrocatalytic efficiency of the Ag2MoO4. Additionally, the little amount of metallic Ag produced on PANI surface provides the high electron transfer conductivity.

(a)

(b)

(c)

(d)

Figure 8. LSV curves of (a) Ag2MoO4 and (c) 20 wt % PANI/Ag/Ag2MoO4 nanocomposite at various rotation speeds.; The corresponding K-L plots at different electrode potentials: (b) Ag2MoO4 (d) 20 wt % PANI/Ag/Ag2MoO4 nanocomposite.



Band Diagram: In the light of above aforementioned analysis, the improved photocatalytic activity of the composite can be described by assuming the development of a p-n heterojunction, as p-type PANI composites with n-type Ag2MoO4 and the possible energy band structure is illustrated schematically (Figure 9). Based on Mott-Schottky measurements it is worth mentioning that the positive slope of this plot confirms Ag2MoO4 as an n-type semiconductor with flat band potential -0.11 V vs. NHE at pH 4.7 (Figure S8a) that can be attributed to the conduction band minimum of Ag2MoO4 and in conjugation with band gap energy, the valence band potential can be estimated to be 3.22 V. The HOMO onset of pure PANI was confirmed to be -5.76 eV (see S.I., eq. 9) below fermi level (EF) via cyclic voltammetric method and combined with optical absorption spectra, the LUMO level can be estimated to be -2.87 eV (see S.I., eq. 10). Based on these above results, the energy band structure of PANI, Ag and Ag2MoO4 before contact was constructed in Figure 9. As the EF of PANI differs from Ag2MoO4, it is obliged to cause movements of fermi levels of these materials to attain their fermi levels align in order to reach the system in thermal equilibrium state. After eventually reaching these alignment, the flat band potential was estimated to be 0.81 V (Figure S8b) and subsequently the band bending would be expected due to the carrier concentration gradient that leads to diffusion of electrons from n to p-type and holes from p to n-type region.


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2.89 eV

EF

E (eV) E (V)/NHE

0.5 -0.5 -1.5 -2.5 -3.5 -4.5 -5.5 -6.5 -7.5 -8.5 -9.5

E (eV) E (V)/NHE

-5 -4 -3 -2 -1 EF 0 2.89 eV Ag 1E F 3.33 eV 2 3 PANI 4 Ag2MoO4 5

Before interface equilibrium (BIE)

0.5 -0.5 -1.5 -2.5 -3.5 -4.5 -5.5 -6.5 -7.5 -8.5 -9.5

e-

EF

Ag PANI 3.33 eV

2.89 eV

EF PANI

Ag2MoO4

-5 (BIE) -4 -3 -2 -1 2.89 eV 0 EF 1 Ag 2 PANI 3.33 eV 3 4 Ag2MoO4 5

After interface equilibrium (AIE)

Figure 9. Schematic illustration of energy band diagram of PANI, Ag and Ag2MoO4 before and after heterojunction formation. With the help of electrochemical study, the HOMO and LUMO level of CIP were found to be -5.23 eV and -1.03 eV respectively (see S.I.). On the ground of above experimental results, a reasonable mechanism for CIP degradation under UV irradiation is drawn in Figure 10 that shows a staggered band position which is more desirable for the charge separation.


Ag

EF 3.33 eV

Ag2MoO4

(AIE)


e-

E (eV) E (V)/NHE

0.5 -0.5 -1.5 -2.5 -3.5 -4.5 -5.5 -6.5 -7.5 -8.5 -9.5

e-

-5 CIP* -4 LUMO e- e- ee -3 LUMO -2 4.2 eV -1 e-e- e- e2.89 eV 0 CB h+ h+ EF 1 HOMO HOMO 2 CIP 3.33 eV 3 h+ h+ 4 h+ h+ VB 5 Ag2MoO4 h+

Figure 10. Schematic diagram of migration paths of photo-excitons under UV irradiation along with CIP degradation and ORR. Under UV irradiation, both PANI and Ag2MoO4 absorbed photons and the photoexcited electrons in the LUMO of PANI can easily move to the CB of Ag2MoO4. As expected, the holes that are photo generated in the VB of Ag2MoO4 can also be transferred to the HOMO of PANI which effectively hindered the recombination and enhanced the lifetime of photoexcitons. These are in accordance with the photocurrent and PL results. The photogenerated holes in PANI and those obtained from Ag2MoO4 would oxidize CIP to its non toxic fragments rather than generation of OH• radical because EHOMO of PANI (0.81 V vs. NHE) is lower than the standard potential of OH•/H2O (+2.27 V vs. NHE).


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e-CB(Ag2MoO4 ) +h+VB(Ag2MoO4)

Ag2MoO4 + hν PANI + hν

e-LUMO(PANI) + h+HOMO(PANI)

e-LUMO(PANI) + Ag2MoO4

PANI + e-CB(Ag2MoO4 )

h+VB(Ag2MoO4) + PANI

h+HOMO(PANI) + Ag2MoO4

h+HOMO(PANI) + CIP

degradation product.

On the other hand, the larger work function () of Ag2MoO4 (4.6 eV) than Ag (4.26 eV) favours the electron flow from Ag to Ag2MoO4 and makes the Ag2MoO4 centre as an electron reservoir. Additionally, z- scheme heterojunction system formed between p-type PANI and n-type Ag2MoO4 significantly enhances the transmission of electron from PANI to Ag2MoO4. These charge transfer influence the O2 adsorption energy on Ag2MoO4. During ORR process, the O2 adsorption on catalyst is a vital factor and it depends on the electronic coupling between the oxygen 2p orbital and the eg orbital of bulk Mo ions, assuming that O2 preferably adsorbs on the surface of Mo sites. The bonding and antibonding states are formed as a result of this coupling and coupling becomes weaker after the filling of the antibonding states by the electrons transferred from both PANI and Ag. Consequently, it lowers the O2 adsorption energy on Ag2MoO4. This reduced adsorption energy makes our material a more proper ORR catalyst. As a result, it can be concluded that a z- scheme heterojunction system significantly enhanced the transmission and separation of electron-hole pairs, resulting in enormous enhancement of photocatalytic and electrocatalytic activity.



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4. CONCLUSIONS This present work manifested a successful synthetic procedure to obtain PANI/Ag/ Ag2MoO4 as well as its usefulness as an efficient single catalyst towards photocatalytic degradation of neurotoxic fluoroquinolone drug CIP and electrocatalytic reduction of oxygen. We have indentified and optimized that 20 wt % PANI/Ag/Ag2MoO4 nanocomposite possesses the highest photocatalytic performance and it efficiently degrades CIP with appreciable rate. In addition, this composite also acts as an efficient electrocatalyst for ORR. The remarkable catalytic activity can be explained by the improved electron-hole pair separation led by p-n heterojunction formation. Most importantly, our catalyst exhibits superior stability for the photodegradation because hole transporting polymer PANI hinders self oxidation of Ag2MoO4. Additionally, PANI also improved the electrocatalytic oxygen reduction activity of Ag2MoO4. In our work, this Pt free ORR catalyst has not only the rapid charge transfer ability but also high selectivity towards oxygen reduction to water (via 4 e- transfer) and not peroxide. Therefore, it can be concluded that this PANI/Ag/Ag2MoO4 nanocomposite can be efficiently used as a single catalyst towards photodegradation as well as energy conversion.

Our work

undoubtedly provides an inspiration for designing various organic-inorganic admirable catalytic

systems

which

exhibit

excellent

photocatalytic

and

electrocatalytic

performances. We hope such materials would response to a worthy progression in near future.


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ASSOCIATED CONTENT Supporting Information Materials, synthesis and elemental analysis of PANI, sample characterization, photoelectrochemical measurements, FTIR spectra, table of EDS analysis, ICP measurements, UV-Vis spectra, Tauc plots, adsorption-degradation yield of CR, UV-Vis kinetics plot of CIP degradation, reusability test of photocatalyst, XRD of photocatalyst before and after the photocatalytic degradation and fringe pattern of photocatalyst after the photocatalytic degradation, BET analysis, Mott−Schottky measurement, ORR and calculation of molecular orbital energy levels. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel.: +91-033-2668-4561-64 ext: 512; Fax: +91033-2668-2916 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Authors are greatly thankful to Prof. A. Mondal, Department of Chemistry, IIEST, Shibpur, India, for helping valuable discussion and necessary corrections. P. Mondal is indebted to IIESTS, J. Satra is thankful to UGC-RGNF, U.K. Ghorui is thankful to IIESTS, and N. Saha is thankful to DST-Inspire for providing fellowship. The authors acknowledge MHRD (India) for providing instrumental facilities to the Department of



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Table of Content Graphics e- e-

e-

LUMO e- e- e- e- e-

Ag0 h+

ORR

e- e- e- e- e-

CB

EF

h+ h+ h+ h+ HOMO

PANI Ciprofloxacin Degradation

h+ h+ + h

h+ h+ h+ h+ h+ VB

Ag2MoO4

PANI/Ag/Ag2MoO4


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