Photovoltaic and photocatalytic performance of electrospun Zn₂SnO₄ hollow fibers

Highlights

• Phase pure Zn₂SnO₄ and ZnO-SnO₂-Zn₂SnO₄ composite fibers were prepared by electrospin technique.

• The samples exhibited distinct difference in photovoltaic performance and photocatalytic activity.

• The composite fiber of ZnO-SnO₂-Zn₂SnO₄ performs better as a photocatalyst.

• 1.93% efficiency has been achieved for the pure Zn₂SnO₄ porous fiber with N719 dye.

Abstract

The phase pure hollow Zn₂SnO₄ and green emitting ZnO-SnO₂-Zn₂SnO₄ composite fiber have been prepared by post calcining the as formed fiber by electrospin technique. Depending upon the calcination temperature, the as prepared fiber exhibited a striking variation in composition, microstructure, optical and photo-electrochemical properties. The composition dependent dissimilarity in photovoltaic performance and photocatalytic activity has been established in this work. A relatively enhanced open circuit voltage (Voc) of 0.76 V, fill factor (FF) of 59.78%, short circuit current (Jsc) of 4.2 mA/cm² and an overall conversion efficiency (ɳ) of 1.93% have been achieved for the phase pure Zn₂SnO₄ porous fiber obtained at the elevated calcination temperature of 1000 °C. On the contrary, a relatively reduced Voc, FF, J_SC and ɳ of 0.70 V, 42.54%, 3.8 mA/cm² and 1.17%, respectively, have been achieved for the 800 °C calcined dense fiber consisting of a mixture of three distinct phases ZnO, SnO₂ and Zn₂SnO₄. Unlike photovoltaic behaviour the trend in photocatalytic performance interestingly got reversed for the ZnO-SnO₂-Zn₂SnO₄ composite fiber owing to its superior photo-induced charge separation ability followed by generation of larger amount of active hydroxyl radicals (OH^.). Our results establish the composite fiber as a preferred photocatalyst in comparison to phase pure Zn₂SnO₄ towards the textile dyes Methylene blue and Congo red and non absorbing organic pollutants such as Phenol and Bisphenol A under UV illumination.

Introduction

One dimensional (1D) semiconducting oxides of various morphologies such as rods, wires, tubes, belts, fibers etc. are exceptionally impressive towards diverse applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. For instance, the emerging photovoltaic device dye sensitized solar cells (DSSCs) and the green approach of removing toxic waste water pollutant through photocatalysis are noteworthy. The metal oxide fiber has been exploited to a lesser extent as the photoanode-photocatalyst materials in the above mentioned areas compared to other 1D morphologies where 1D fibers with controllable length to diameter ratio, porous fibers, hollow fibers, cable like multi-core fibers and so forth could be effective enough for the same [11], [12], [13], [14], [15], [16]. In this regard, the composition of the materials also plays a significant role along with the morphology. Apart from the mostly studied simple binary oxides such as TiO₂, ZnO, SnO₂, Nb₂O₅, In₂O₃, Bi₂O₃ and Fe₂O₃ the ternary oxides by coupling of metal oxides or the semiconductor–semiconductor heterojuction are of equally fascinating owing to their modified electronic and optical properties [17], [18], [19]. Interesting reports are also available on various Bi based ternary oxides and hypohalides exhibiting quite appreciable inorganic-organic pollutant removal capabilities [20], [21], [22], [23], [24]. The performances of these ternary oxide based systems regarding photodegradation of waste water pollutant turned out more impressive upon modifying with quantum dots, p-type materials, metal ion doping etc. where the possibility of photo-induced charge separation has been reported to suppress the recombination reaction. For example, Bi₂WO₆ modified with carbon quantum dot has been introduced for the removal of gaseous volatile organic compounds (VOC) [20]. Liang et. al., elsewhere reported the Ag treated Bi₂WO₆ and its enhanced photocatalytic activity towards RhB dye [21]. BiPO₄ has also been claimed to be effective for organic contaminanta degradation with the help of p-type Cu₂O nanoparticles [22]. In addition, Cui et. al., highlighted Cu₂O quantum dot embedded flower-like BiOBr for enhanced photocatalytic activity [23]. Zhua et. al., reported MB degradation with improved photocatalytic performance by BiPO₄/Bi₂WO₆ dual ternary oxides composite [24]. Other than hazardous Bi, Ag based semiconducting oxides for instance Ag₃PO₄ has also been reported to be effective for the same [25]. Reports on Ag modified K₄Nb₆O₁₇, another interesting ternary oxide as an efficient photocatalyst owing to its plasmonic behavior is noteworthy [26]. Very recently, Wang et. al., reported the hydrothermally synthesized SnO_x/Zn₂SnO₄ composite having wide heterojunction separation of electrons and holes for improved photocatalytic activity [27]. Taking into account all the feasibilities, it could be interesting to study the performance of the ternary oxide Zn₂SnO₄ having high electron mobility (10–15 cm² V⁻¹s⁻¹), high electrical conductivity (∼10⁴ S cm⁻¹), sufficient thermodynamic stability and fascinating optical properties with typical fiber morphology in photovoltaic and photocatalytic application [28], [29]. In addition, the approach in utilising the ZnO-SnO₂-Zn₂SnO₄ composite oxide fiber having a combination of the unique features of both the binary and ternary oxides is also significant. Among some common synthetic methods such as chemical vapour deposition, hydrothermal and template assisted technique etc. the electrospun has been characterised to be an effective and versatile technique to fabricate the 1D fibers [11]. In this work, we have reported a successful preparation of dense and fluorescent ZnO-SnO₂-Zn₂SnO₄ composite fiber and phase pure Zn₂SnO₄ porous fiber by calcining the as prepared fiber at two different temperatures of 800 and 1000 °C respectively, where the as prepared fiber have been synthesized by the electrospin technique. The transformation of the green emitting binary-ternary oxide composite fiber to the phase pure hollow ternary oxide fiber as a function of temperature has been systematically monitored. The combined effect of the composition, microstructure and optical properties of the two distinct kinds of fibers on the DSSCs performances has been evaluated and thereafter compared their efficiency towards the degradation of textile dyes Methylene blue (MB) and Congo red (CR). Finally, the photocatalytic activities of the electrospun fiber samples were further verified for Phenol and Bisphenol A, as representative non-absorbing organic pollutants.