Solvothermal synthesis of BiPO₄ nanorods/MWCNT (1D-1D) composite for photocatalyst and supercapacitor applications

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.

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 (1D-1D) 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 electroactive 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.