Electrospinning and hydrothermal synthesis of recyclable MoS₂/CNFs hybrid with enhanced visible-light photocatalytic performance

Abstract

In this work, a two-step synthesis route combining an electrospinning method and a hydrothermal process was used to prepare MoS₂/CNFs hybrid. CNFs was applied as the matrix for the nucleation and growth of MoS₂ nanosheets. In this hybrid, the crisscrossed MoS₂ nanosheets were randomly aligned and densely packed over the surface of CNFs. We probed the photocatalytic activity of MoS₂/CNFs hybrid to degrade rhodamine B (Rh B) in an aqueous solution under visible light irradiation. The hybrid displayed higher photodegradation performance relative to MoS₂ and mechanical mixture of MoS₂ with CNFs, with 67% Rh B completely degraded over 5 h-period. We attributed such enhancement in photocatalytic activity to the enhanced absorption property and electrical conductivity due to the synergy between MoS₂ and CNFs. The hybrid can furthermore be easily separated from the solution and reused for the subsequent photodegradation cycles. We verified the negligible loss in the photodegradation activity of MoS₂/CNFs hybrid towards Rh B during the three subsequent cycles. The high photocatalytic activity and recyclability of the hybrid render its practical application to degrade organic pollutants (i.e., dye compounds) in industrial wastewater.

Introduction

The development of industries brings along organic pollutants emission issue in industries’ wastewaters. These pollutants are harmful to the environment and human health and are difficult to degrade naturally [1], [2], [3]. Their removal from wastewater or remediation thus becomes imperative to minimize their negative effects [1]. Semiconductor-based photocatalysis is one of the most promising technologies to degrade organic pollutants in wastewater at ambient temperature and pressure [4], [5], [6]. TiO₂, a typical n-type semiconductor, in particular, has been widely used as a photocatalyst given its non-toxicity, photochemical stability, and high photocatalytic activity [7], [8], [9]. Nevertheless, the relatively large 3.2 eV band gap for TiO₂ renders it an inefficient catalyst since its response is limited onto ultraviolet light range that accounts for only 4% of the solar spectrum [10], [11]. Despite the on-going improvements in TiO₂–based photocatalyst performance, it is still desirable to develop an alternative photocatalyst that can harness visible light so that a more efficient use of solar energy can be achieved [12].

Given its narrow bandgap of 1.8 eV, molybdenum disulphide (MoS₂) can absorb light in the visible light range [13], [14]. This characteristic makes it suitable for photocatalytic application under visible light. In 1999, Thurston et al. studied the photooxidation performance of MoS₂ nanocluster for the first time [15]. He confirmed that MoS₂ nanocluster exhibits photodegradation activity for organic molecules under visible light. So, the higher utilization of solar radiation for nano MoS₂ makes it a possible alternative to TiO₂. Still, the drawbacks of MoS₂ lie on the quick recombination of the photogenerated electrons and holes under visible light excitation, the corrosion during reaction, and the particle aggregation induced by their small size. To address these drawbacks, support materials have often been added to suppress the nanoparticle aggregation and the charge recombination process [16].

As support materials, carbon nanofibers (CNFs), which have been utilized as electrode materials, adsorbents, and catalyst substrates, become an attractive candidate given their large surface areas, high electrical conductivity, corrosion resistance, and flexibility [17]. Instead of using the conventional chemical vapor deposition (CVD) method that involves complicated procedure, a lower-cost and simpler electrospinning technique can be used to fabricate continuous nanofibers from solutions of polymers or polymer blends [18]. As a result, numerous studies in electrochemistry, adsorption, and catalysis topics have employed electrospun fibers [19], [20], [21].

To harness simultaneously the high photocatalytic activity of MoS₂ and the high electrical conductivity and the supporting matrix function of CNFs, here we reported the synthesis of MoS₂/CNFs hybrid via electrospinning technique followed by hydrothermal process. The photocatalytic activity of the hybrid photocatalyst was evaluated in the context of degradation of Rhodamine B (Rh B). Moreover, recycling tests were performed to evaluate the photochemical stability of MoS₂/CNFs hybrid. We additionally proposed the photocatalytic mechanism in MoS₂/CNFs to explain the observed enhancement for the hybrid relative to MoS₂.