MWCNT (1D-1D) composite for photocatalyst and supercapacitor applications

Author’s Accepted Manuscript Solvothermal synthesis of BiPO4 nanorods/MWCNT (1d-1D) composite for photocatalyst and supercapacitor applications S. Vadivel, A.N. Naveen, J. Theerthagiri, J. Madhavan, T. Santhoshini Priya, N. Balasubramanian www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30687-3 http://dx.doi.org/10.1016/j.ceramint.2016.05.080 CERI12885

To appear in: Ceramics International Received date: 20 April 2016 Revised date: 9 May 2016 Accepted date: 13 May 2016 Cite this article as: S. Vadivel, A.N. Naveen, J. Theerthagiri, J. Madhavan, T. Santhoshini Priya and N. Balasubramanian, Solvothermal synthesis of BiPO nanorods/MWCNT (1d-1D) composite for photocatalyst and supercapacitor a p p l i c a t i o n s , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.05.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Solvothermal synthesis of BiPO4 nanorods/MWCNT (1D-1D) composite for photocatalyst and supercapacitor applications S. Vadivela, A. N. Naveenb, J. Theerthagiric, J. Madhavanc, T. Santhoshini Priyad, N. Balasubramaniand* a Department of Chemistry, NGM College, Pollachi-642001, India b Department of Physics, St. Joseph’s College of Engineering, OMR, Chennai-119, India c Solar Energy Lab, Department of Chemistry, Thiruvalluvar University, Vellore-632115, India d Department of Chemical Engineering, A. C. Tech Campus, Anna University, Chennai-600 025, India [email protected]

[email protected] *

Corresponding author. Tel.: +91 44 22359190.

Abstract A novel BiPO4/MWCNT (1D-1D) composite was synthesized by a simple one step solvothermal approach. The crystallanity, morphology, and photophysical properties of the samples were characterized by XRD, Raman, SEM, TEM, XPS, UV–Vis spectroscopic techniques. The nanostructured BiPO4/MWCNT composite showed large surface area and the incorporation of MWCNT caused a red-shift of BiPO4 in (ultraviolet) UV region. A maximum specific capacitance of 504 F g-1 at a scan rate of 5 mV s-1 was obtained for BiPO4/MWCNT composite. BiPO4/MWCNT composite shows good capacity retention (94%) upon cycling over 1000 cycles. The BiPO4/MWCNT composite exhibits better photocatalytic activity than pure BiPO4 under UV light irradiation in view of degrading methyl orange (MO) as target pollutant. The degradation of MO could get 95% in BiPO4/MWCNT photocatalysts under optimum reaction conditions. The improved photoactivity of BiPO4/MWCNT could be attributed to effective separation of photoinduced hole-electron pairs between host BiPO4 and MWCNT. This study offers a new fabrication strategy to prepare BiPO4 based materials that can be used in energy storage devices and environmental applications.

Keywords: Composite materials, Semiconductors, Electron microscopy, XPS.

1. Introduction With the increasing demands for energy problem, there is a need for society to develop some novel technological systems applicable to conversion of alternative energy sources and fabrication of new energy storage devices [1]. As an intermediate system between dielectric capacitors and batteries, supercapacitors have attracted a huge interest among researchers owing to strong demand for flexible portable energy management in the rapidly growing world economy [2]. Notably RuO2 have been intensively studied and recognized as excellent supercapacitor electrode material [3]. Despite the high capacitance value the toxicity and high cost exclude it from the commercial applications [4]. Therefore, researchers have shifted their attention towards low cost and abundant metal oxides, phosphates and metal hydroxides [5]. Therefore it is very important to develop alternative electrode with combination of improved performance and cost viability. Recently, BiPO4 paid significant importance in the field of heterogeneous catalyst, luminescence materials and humidity sensor materials [6]. To date increasing interests have been concentrated on BiPO4 material in supercapacitor electrode and some major progress has been made. For instance Nithya et.al for the first time reported the pseudocapacitor property of monoclinic phase of BiPO4 nanostructure by sonochemical approach and identified the BiPO4 as a new competitive material for supercapacitor [7]. However there are fewer reports about the electrochemical property of BiPO4 material. Meanwhile, water pollution has become a major issue threatening human health and aquatic life. Commercially used pesticides, herbicides and dyes usually account for a major portion of the water pollutants [8]. Therefore, efficient techniques that can degrade those

pollutants in the contaminated water are highly desired [9]. Among various physical and chemical techniques, photocatalytic degradation is most impressive and effective technique, since it can degrade dye molecules using visible or natural sun light, which have great potential in real time applications [10]. However, very similar to TiO2, BiPO4 is also a wide band gap semiconductor and the quantum efficiency is not high enough to meet the requirement of industrial purposes [11]. Thus, it still needs to improve the performances of BiPO4 semiconductor. To improve the specific capacitance value and photocatalytic property of the semiconductor material many researchers focused on to incorporate the carbon materials as they contribute to enhance the conductivity to enhance the specific capacitance values and degradation rate [12]. Among various carbon materials such as activated carbon, graphene, carbon fibers, and carbon nanotubes (CNT), have been investigated as electrode material for supercapacitor and photocatalysts [13]. CNT have outstanding electrical properties apart from its high aspect ratio, chemical stability and surface area that make it for an ideal candidate in energy and environment problems [14]. Both SWCNT and MWCNT have been studied for supercapacitor electrodes and catalytic due to their remarkable properties [15, 16]. However research on the potential of (1-D-1D) architectures for supercapacitor and photocatalyst have so far been limited. Herein we synthesized highly dispersed BiPO4 nanorods embedded in MWCNT as (1D1D) composite as an electrode material for supercapacitor and photocatalyst material for degradation of MO. The BiPO4 nanorods are in situ grown on MWCNT’s to ensure the strong interfacial bonding between the BiPO4 nanorods and MWCNT matrix. In the constructed 1D-1D architecture the MWCNT effectively anchored with BiPO4 nanorods provide the pore channel

for electrolyte ions between the electro active materials during charge discharge process and afforded proficient electron transfer during photodegradation process. Such elevated properties provide important prospects for BiPO4/MWCNT composite to be widely used as alternative electrode material for supercapacitor and excellent photocatalyst material for environmental remediation.

2. Experimental: 2.1.Chemicals and Reagents Bismuth Nitrate (BiNO3)3.5H2O), Polyvinylpyrrolidone (PVP), Ammonium phosphate (NH4H2PO4) ethylene glycol, ethanol and acetone were purchased from Aldrich. MWCNT powder (purity 95%) with a diameter of >50 nm and a length of 10-30µm was purchased from SISCO research laboratories, India and purified according to the previous report [17].

2.2. Synthesis of BiPO4/MWCNT composite Thus synthesis of BiPO4 nanorods/MWCNT composite as follows 1.5 g Bi(NO3)3·5H2O, 0.85 g NH4H2PO4 and 50 mg of MWCNT were dispersed in 25 mL of ethylene glycol by ultrasonication for 30 min and vigorously stirred for 30 min. Then, 0.75 g polyvinyl pyrolidone (PVP) was added into above prepared solution and was stirred for another 60 min; subsequently, the mixture was transferred into a 100 mL Teflon-lined autoclave. The autoclave was sealed and heated at 160° C for 12 h; afterwards it was left to cool down to room temperature. Next, a black colored precipitate was collected from the autoclave and washed thrice with DI water and

acetone to remove the excess reactants and surfactants. The collected BiPO4/MWCNT composite was dried in an oven at 60° C for overnight. The synthesis process for BiPO4/MWCNT composite was illustrated in Scheme 1

3. Materials Characterizations The crystallanity of the as-prepared samples was analyzed using X-ray diffraction (XRD) patterns measured by an Xpert Pro PAN analytical X-ray diffractometer using a Cu target radiation source operated at (0.1543 nm, 40 kV, 150 mA). Raman spectroscopy was analyzed using Nanophoton confocal Raman system in the visible light range of 532 nm. The UV–vis diffuse reflectance spectra (UV–vis DRS) were obtained using the dry-pressed disk samples with a VARIAN spectrometer using BaSO4 as the reflectance sample. The surface area of samples was measured with a Micromeritics ASAP 2020 Porosimeter. The photo catalytic efficiency, and decrease in the concentration of the dye solutions was analyzed with respective time using the UV-1800 Shimadzu Corp, UV-vis spectrophotometer, by checking their absorbance values. The X-ray Photoelectron spectroscopy (XPS) spectrum was recorded using Ms Omicron Nanotechnology, GmbH, (Germany) XM 1000-AlK monochromator at 1486 eV energy radiation, operated at 300W.

3.1. Photocatalytic study The photocatalytic experiments were carried out under UV irradiation using Heber scientific multilamp photoreactor. It consists of 8 W medium pressure mercury vapour lamps set

in parallel with wavelength of 365 nm. It is equipped with a magnetic stirrer and reaction glass tubes of 50 ml capacity. The aqueous solution of dye with catalyst is continuously aerated by a pump for proper mixing during photocatalyst process. At each run the 0.1 g catalyst is dispersed in 50 ml of 10 mgL-1 MO solution (pH at 7). On prior to UV irradiation, the suspension was stirred under dark for 1h to achieve the adsorption-desorption equilibrium between the catalyst and dye solution. At different time intervals, 2 ml of sample is withdrawn and it is then centrifuged with 5000rpm for 10 min to remove the catalyst. The supernatant are analyzed using UV–visible spectrometer to measure the absorbance of the dye solution.

3.2. Electrochemical studies The electrochemical characterization of synthesized materials was carried out using an electrochemical workstation (CHI 660) with three electrode system in a 2 M KOH solution. The working electrodes were prepared by coating homogeneous slurry containing 70 wt% of electro active materials, 20 wt% of carbon black, and 10 wt% of polyvinylidene difluoride (PVDF) were coated in glassy carbon electrode and then dried at 100 °C for 5 h in a vacuum. Platinum electrode was used as counter electrode and saturated calomel electrode (SCE) was used as reference electrode. The working electrodes were investigated by cyclic voltammetry (CV) studies with the potential range between 0.0 and -1.0 V at various rates between 5-100 mV s-1. Galvanostatic charge-discharge studies were performed with current densities ranged from 1 to 5 Ag-1 at the potential of 0.0 to -1.0 V. The electrochemical impedance spectroscopy (EIS) was performed in the frequency range between 100 kHz and 0.01 Hz [18, 2].

4. Results and discussion 4.1. XRD Spectroscopy X-ray diffraction (XRD) measurements were employed for the analysis of phase and structure of the synthesized materials. As shown in Fig. 1 inset the XRD patterns of the MWCNT (curve 2. inset ) show a broad peak at 2θ =25.32°, 42.71° corresponding to the (002) and (100) plane reflection of MWCNT [19, 20]. In the XRD pattern of BiPO4 nanorods (curve 1) the peaks at 2θ values of indicated that all the diffraction peaks match well with that of monoclinic BiPO4 phase (JCPDS No.89-0287) [21,22]. XRD patterns of BiPO4 /MWCNT composite (curve 2) was in accordance to that of BiPO4 nanorods (curve b) implying the monoclinic phase was still maintained after incorporation of MWCNT respectively. Furthermore, no diffraction peaks of MWCNT could be observed implies that the lower amount and relatively low diffraction intensity of MWCNT [23]. No characteristics peak is observed for other impurities such as surfactants confirming that the composite exist in the pure state. The existence of MWCNT was further investigated by Raman, SEM and TEM analysis.

4.2.Raman Spectroscopy The effective incorporation of MWCNT in BiPO4 matrix was confirmed by Raman spectrum. As shown in Fig. 2 the Raman spectrum of MWCNT displays two major peaks at 1339 cm-1 and 1563 cm-1 respectively (inset). The peak at 1563 cm-1 corresponding with G band indicates that good arrangement of the hexagonal lattice of graphite. The G band is common to all graphite-like materials and is attributed to the in-plane vibration of the C-C bond, whereas the peak at 1339 cm-1 corresponding with D band which is associated with the disorders or defects in

the graphitic carbon in CNT matrix [24]. While in Raman spectrum of BiPO4/ MWCNT composite, besides two peaks in MWCNT (1344 and 1569 cm-1) the peaks at 550, 596, 961 and 1038 cm-1 was observed. The Raman bands at 550 and 596 cm-1 were attributed to the bending modes of PO4 units, and two intense bands at 961 and 1038 cm-1 were ascribed to the ν1 symmetric and ν3 antisymmetric stretching modes of the PO4 tetrahedron respectively [25,26]. The ID/IG band intensity ratio was linearly related to the inverse of the in-plane crystallite dimensions. The ratio of the intensities (ID/IG) was 1.08 for the pristine MWCNT and 1.14 for the BiPO4/ MWCNT composite. These results suggested that the deposition of BiPO4 onto the MWCNT decreased the particle crystallanity [27].

4.3. Morphological studies Fig.3 displays the typical FESEM images of MWCNT, BiPO4 nanorods, and BiPO4/ MWCNT composite. As seen in Fig.3a the morphology of bare BiPO4 is typically composed of 1D nanorods within the range of 75-150 nm. The dimensions are not uniform among the individual BiPO4. As we can see, the pure MWCNTs form clew-like aggregates with dimensions ranging from several hundreds of micrometers (Fig.3b). Furthermore, the SEM image of BiPO4/ MWCNT composite (Fig.3 c) shows that the effective interfacial contact between the BiPO4 and MWCNT was observed which is beneficial for the improvement of desired properties in BiPO4/ MWCNT composite. This consequently may to improve the electron conductivity in photocatalyst pathway and specific capacitance values towards supercapacitors applications [28]. Fig.3 (d) showed typical TEM images of the MWCNT and asprepared BiPO4/ MWCNT sample. It can be observed that the MWCNTs are randomly twisted

and intertwined with each other. From Fig.3 (e) it could be seen that the tubular materials was CNT, and the dark regions were due to the presence of BiPO4 nanoparticles. Apparently, CNT were evenly and densely decorated with BiPO4 nanoparticles. The BiPO4 rods without any defects which confirm there is no structural disturbance during the sonication process. The EDAX in figure shows the presence of only Bi, P, C and O in sample which confirms the purity of BiPO4/ MWCNT composite from Fig. 3 (f)

4.4. XPS Spectroscopy The X-ray photoelectron spectrum (XPS) was further employed to elucidate the surface elemental composition of the BiPO4/ MWCNT composite. In XPS survey spectrum of BiPO4/ MWCNT composite from Fig. 4 (a) the signals of Bi 4p, Bi 4d, Bi 4f, Bi 5d, O KLL, O1s, P2p, and C1s can be clearly observed [29]. The XPS spectrum confirms that definite existence of only Bi, O, P, and C elements in the BiPO4/ MWCNT composite. In Fig. 4 (b) the peaks centered at 164.7 and 159.7eV were ascribed to Bi4f

5/2

and Bi4f

7/2

spins suggesting that the presence of

Bi3+ in the sample [30]. The C1s spectrum can be split into three peaks, (Fig. 4 c) containing the carbon in C-C at 284.5 eV, carbon singly bound to oxygen (i.e., C-O) at 286.2 eV, carbon doubly bound to oxygen (i.e., C=O) at 287.6 eV. These groups on the surface of MWCNT play a crucial role in the fabrication of BiPO4/ MWCNT composite [31] Furthermore, the O 1s spectrum shows that lattice oxygen (530.5 eV) of the PO43− in BiPO4 and surface adsorbed oxygen species at 532.3 eV and oxygen singly bound to carbon (i.e., O-C) at ~533.1 eV coexisted in the BiPO4/ MWCNT composite as shown in Fig. 4 (d) [32]. The peak for P2p at 132.6eV was due to the PO43-, in BiPO4 as shown in [33] Fig. 4 (e).

4.5. BET Surface area The BET surface area analysis (Fig.5) was carried out to investigate the specific surface area and pore-size distribution of the synthesized BiPO4/ MWCNT composite. From the inset figure pore distribution is relatively narrow and mainly centered in the range of 18.1 nm. It is well known that mesoporous materials with high surface area and suitable mesoporous structure can provide large electroactive sites and short diffusion paths for ion transport, which are required in the Faradic process and higher photocatalytic activity [34]. Moreover, the BET specific surface area of the BiPO4/ MWCNT composite is calculated to be 38.25 m2 g-1 from the nitrogen adsorption desorption isotherm which is quite higher than pure BiPO4 (25.10 m2 g-1). The higher specific surface area can enhance the exposure of active sites available for reaction on the surface [35]. Therefore, BiPO4/ MWCNT composite with high specific surface area and suitable mesoporous distribution is believed to be extremely beneficial for the supercapacitor and photocatalytic applications.

4.6. UV –DRS spectrum The UV-DRS spectra of BiPO4 and BiPO4/ MWCNT composite were shown in Fig.6. It can be clearly seen that the absorption edge of BiPO4 occurs at about 265 nm which was in agreement with the previous reports [36]. However the absorption edge of BiPO4/ MWCNT composite is quite enhanced at range of 290 nm in comparison with pure BiPO4 due to effective incorporation of MWCNT suggesting that BiPO4/ MWCNT composite may absorb UV light more efficiently and would probably have higher photocatalytic activity.

4.7.Photocatalytic studies The photocatalytic activity of BiPO4 and BiPO4/ MWCNT composite was evaluated using the photocatalytic degradation of MO aqueous solution under UV light irradiation. The photocatalytic degradation process was investigated by examining the absorption peak of MO at 464 nm. Fig. 7 (a) shows the variation in MO relative concentration C/C0 with the irradiation time over samples under UV light irradiation (where C is the concentration at different time interval and the C0 is the initial concentration of MO dye). For comparison the direct photolysis of MO (blank) was also tested under the identical condition without the photocatalyst. It is observed that photolysis of MO dye degradation was negligible without catalyst under the UV light irradiation. As shown in Fig. 7 (a) all of the BiPO4/ MWCNT composite exhibits higher photocatalytic activity than the bare BiPO4 under the UV illumination. After 150 min nearly 95% of MO dye was degraded by BiPO4/ MWCNT composite catalyst. In order to verify the intimate contact between BiPO4 and MWCNT on the photocatalytic pathway, a reference experiment on the ex-situ mixing of BiPO4 and MWCNT with the same MWCNT content was also conducted and shown in Fig.7 (b) The photocatalytic activity of the solvothermal synthesized BiPO4/ MWCNT composite was higher than that of ex-situ mixed BiPO4/ MWCNT composite. The result indicates that solvothermal route is much beneficial for the formation of intimate contact between BiPO4 and MWCNT which is much important for the enhancement of photocatalytic activity for MO dye degradation [37-38]. To evaluate stability of photocatalyst cycling photodegradation process was also conducted for BiPO4/ MWCNT composite. Fig. 8 (a) shows the data of cycling experiments of degradation of MO over the BiPO4/ MWCNT composite under UV light irradiation. The result

proves that lesser photocatalytic activity loss is observed during three successive recycles only 11% of catalytic activity was lost during three cycles, suggesting that the synthesized BiPO4/ MWCNT composite is stable during the photodegradation. Additionally, TEM image from Fig. 8 (b) clearly reveals that the morphology of the nanocomposites (rod like structure) is still maintained after degrading MO molecules further confirms their outstanding durability in the photodegradation process. From our experimental data the possible mechanism for the photocatalytic enhancement in the BiPO4/ MWCNT composite was represented in Scheme.2 The irradiation of BiPO4 nanorods with UV light excited the electrons from valence band (VB) to conduction (CB) and created holes in VB. In the absence of MWCNT only a small portion of electrons and holes participated in the photocatalytic process due to their high recombination rate, resulting in a low photocatalytic activity. The MWCNT incorporated into the BiPO4/ MWCNT composite may support an efficient photoinduced electron transfer from BiPO4 to MWCNT matrix. Recombination rate between the photoinduced electron–hole pairs on the surfaces of the BiPO4 particles in BiPO4/ MWCNT composite was effectively suppressed. The longer-lived electrons and holes migrated to the surfaces of BiPO4 and MWCNT where, after generating species such as hydroxyl radicals and superoxide radicals via a reaction with H2O and O2, they degraded the adsorbed MO dye molecules. The BiPO4/ MWCNT composite were found to be more efficient in photodegradation of MO dye compared to BiPO4 alone as a photocatalyst [39].

4.8.Electrochemical Studies

In order to study the electrochemical properties cyclic voltametry (CV), galvanostatic charge discharge and electrochemical impedance studies were performed using 2M KOH aqueous electrolyte. The CV studies of BiPO4 and BiPO4/ MWCNT composite was recorded in the potential range of 0 to -1 V vs. Ag/AgCl at different scan rate of 5 mVs-1, 10 mVs-1, 25 mVs1

, 50 mVs-1, 75 mVs-1 and 100 mVs-1 respectively and shown in Fig. 9 (a, b). From CV curves a

pair of redox peaks appears around -0.6 and -0.85 V for all the samples due to the redox reaction of the BiPO4. The results confirms that all the electrodes respond towards KOH electrolyte stores energy mainly through the redox reaction between the host BiPO4 and OH- ions on the electrode surface [40]. This mechanism is quite different from the traditional electrical double layer capacitors (EDLC). The shapes of CV curves confirm that capacitance characteristics provided by BiPO4/ MWCNT composite material was mainly governed by Faradic process [41]. The current densities as well as area surround by CV curves for BiPO4/ MWCNT composite are apparently larger than the BiPO4 at the same scan rate and current density indicating that BiPO4/ MWCNT composite exhibits higher capacitance than bare BiPO4 [42]. The improvement in specific capacitance for BiPO4/ MWCNT composite is mainly due to the introduction of MWCNT that enhances the conductivity and specific surface area of the electro active material. The promotion of capacitance in BiPO4/ MWCNT composite is due to the high porosity which facilitates more electrons and electrolyte to interact with the composite materials [43]. From CV results the excellent electrochemical properties of the BiPO4/ MWCNT composite was ascribed to the integral composite structure and synergistic effect between 1D BiPO4 nanorods and 1 D MWCNT substrates [44].

Fig.10 show the variation in the specific capacitance of BiPO4/ MWCNT composite and BiPO4 as a function of scan rates, respectively. Specific capacitance values of the materials were calculated from the following equation.

(

)

∫

The SC values were calculated graphically by integrating the area under the I-V curves and then dividing by the sweep rate (mVs−1), the mass of the material (m), and the potential window (Va to Vc) [45]. In comparison with BiPO4 the capacitance performances of BiPO4/ MWCNT composite are significantly improved. It can also be found that the specific capacitance decreases with the increase of scan rates from 5 to 100 mV s-1. For BiPO4/ MWCNT composite the specific capacitance was 504 F g-1 at a scan rate of 5 mV s-1. For BiPO4, the specific capacitance reaches 296 F g-1 at a scan rate of 5 mV s-1. The high specific capacitance at high scan rates shows that BiPO4/ MWCNT composite as a promising electrode material for supercapacitors applications. A galvanostatic charge discharge study is reliable for measuring the specific capacitance of supercapacitor electrode materials at constant current [46]. Fig. 11 (a, b) shows the galvanostatic charge discharge curves for BiPO4 and BiPO4/ MWCNT composite at different current densities. The specific potential window of -1 to 0 V was adopted to avoid the hydrogen evolution reaction during the charge-discharge process. The galvanostatic charge discharge curves of BiPO4 and BiPO4/ MWCNT composite at different current densities which are symmetrical indicating the good pseudocapacitive behavior of electro active material. The

symmetric galvanostatic charge discharge curves pointed out its high reversibility of electro active material [47]. The specific capacitance is calculated by the equation.

Where, SC is the specific capacitance, Δt is the discharging current, I is current density, ΔV is potential window, m is the mass of the electro active material. A specific capacitance of 368 F g-1for BiPO4/ MWCNT composite is obtained at current density 1 A g-1which is higher than that of bare BiPO4 (292 F g-1). Even at high current density of 5A g-1, the specific capacitance value of the composite is 260 F g-1. In contrast the bare BiPO4 delivers a much lower capacitance of 150 F g-1 at 5A g-1 (pure MWCNT exhibits only a specific capacitance of 45 F g-1 at current density 1 A g-1). The favorable specific capacitance of BiPO4/ MWCNT composite could be attributed to the benign electrical conductivity of interconnected MWCNT and higher surface area which facilitated the transfer of electrons and ions effectively. When the current density increased the capacitance value of both BiPO4 and BiPO4/ MWCNT composite were reduced significantly as shown in Fig 12 [48]. This trend of the specific capacitance indicates that the partial surface of the electrode is accessible at high current density. Finally, the specific capacitance values decrease with the increase in current densities, which is attributed to the fact that the redox reaction and the charge diffusion rate cannot match the rapid increase in current density values. The improved electrochemical performances of BiPO4/ MWCNT composite electrodes can be attributed to that the MWCNTs with high conductivity were introduced into the composites, and formed a 1D-1D architectures. This unique structure can significantly improve the charge transfer and reduce the distance for ions diffusion, meanwhile adequately take

advantage of their synergic effects of the double layer capacitance from MWCNTs and pseudocapacitance nature from BiPO4 [49]. Cyclic stability of electrode material is one of the significant factors for use in practical applications. The long term cyclic stability of the BiPO4/ MWCNT composite electrode in 2M KOH electrolyte was examined by chronopotentiometry at a current density of 1 A g-1 up to 1000 cycles. Fig. 13 shows the cyclic stability of BiPO4/ MWCNT composite with number of cycles. From the graph it is observed that the specific capacitance increases up to 100 cycles from 368 to 380 F g-1 due to the activation process. Then the specific capacitance value increase linearly, with slight decrease 6% in degradation from the maximum value up to 1000 cycles. Besides pure BiPO4 exhibit 12% in degradation for 1000 cycles. EIS analysis is an important tool to investigate the fundamental behavior of the electrode materials for supercapacitor applications. For better understanding, the impedance of BiPO4 and BiPO4/ MWCNT composite were measured in the frequency range of 100-1 kHz at open circuit potential with an ac perturbation of 5 mV before charging process [50]. The Nyquist plots exhibited a semicircle over the higher frequency region followed by linear part in lower frequency region which is mainly due to the internal charge transfer resistance (Rct) and Warburg impedance as shown in Fig.14. The Rct values of BiPO4/ MWCNT composite electrode reduces significantly compared with bare BiPO4 electrode may be attributed to the presence of MWCNT network which facilitates the penetration of electrolyte into the electrode material and greatly enhances the interface between the electrode and electrolyte. These results infer that the formation of BiPO4 nanoparticles on MWCNT can significantly improve the electrical performance due to the intrinsic resistance change in the electrode material [51].

Conclusion In summary, we adopted a simple solvothermal approach to assemble new hybrid BiPO4/ MWCNT composite material for supercapacitors and photocatalyst. The structural features of the BiPO4/ MWCNT composite was described in XRD, Raman, SEM, TEM and XPS spectroscopic analysis. The electrochemical performance has been carried out in 2M KOH electrolyte. The BiPO4/ MWCNT composite exhibited good electrochemical performance with the maximum specific capacitance value of 504 F g-1 at a scan rate of 5 mV s-1. The achieved electrochemical performance of the BiPO4/ MWCNT composite mainly due to the advantages came through their desired (1D-1D) hierarchical hybrid architecture, high specific surface area, excellent electrical conductivity, and well integration of BiPO4 with MWCNT matrix. The photocatalytic results for MO degradation show that BiPO4/ MWCNT composite can effectively degrade 95% MO dye within 150 min under UV light which is greatly superior to that of BiPO4 and physically mixed BiPO4.

Acknowledgement The First author gratefully thanks to Dr B.K. Krishnaraj Vanavarayar, President, NGM College, Pollachi for his assistance in characterization facilities

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Figure: Fig.1 XRD patterns of BiPO4, BiPO4 /MWCNT composite Fig.2 (a) Raman spectra of BiPO4 /MWCNT composite Fig.3 FESEM images of (a) BiPO4, (b) MWCNT and (c) BiPO4 /MWCNT composite (d) TEM images of MWCNT and (e) BiPO4 /MWCNT composite and (f) EDAX spectrum of BiPO4 /MWCNT composite.

Fig.4 (a) XPS spectra BiPO4/ MWCNT composite (b) Bi4f, (c) C1s (d) O1s (e) P2p of BiPO4/ MWCNT composite Fig.5 (a) N2 adsorption-desorption isotherm of BiPO4 /MWCNT composite and Pore size distribution curve (inset) Fig.6 UV-DRS spectra of BiPO4 and BiPO4 /MWCNT composite Fig.7 (a) Photo catalytic degradation of MO dye C/C0 values with respect to time using BiPO4 and BiPO4/MWCNT composite (b) Comparison of photocatalytic efficiency of BiPO4 /MWCNT composite synthesized by solvothermal method and physical method. Fig.8 Photodegradation changes of MO under UV light for three cycles using BiPO4 /MWCNT composite. (b) TEM images of BiPO4 /MWCNT composite after three cycle test Fig. 9 CV curves of (a) BiPO4 (b) BiPO4 /MWCNT composite electrodes at different scan rate Fig. 10 Specific capacitance calculated for BiPO4 and BiPO4 /MWCNT composite at various scan rates from 5-100 mVs-1 in 2 M KOH aqueous electrolyte Fig.11 Galvanostatic charge discharge curves for (a) BiPO4 (b) BiPO4 /MWCNT composite at different current densities. Fig.12 Variation of specific capacitance of (a) pure BiPO4 (b) BiPO4 /MWCNT composite as a function of current density Fig.13 Cyclic stability of BiPO4, BiPO4 /MWCNT composite at 1 A g-1 up to 1000 cycles Fig.14 Impedance plot of (a) pure BiPO4 (b) BiPO4 /MWCNT composite electrodes

Scheme.1 Schematic representation synthesis of BiPO4/MWCNT composite using solvothermal method Scheme.2 Possible photocatalytic mechanism of BiPO4/MWCNT composite under UV light

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Scheme: 2