Microwave-assisted hydrothermal synthesis of porous SnO₂ nanotubes and their lithium ion storage properties

Abstract

Porous SnO₂ nanotubes have been synthesized by a rapid microwave-assisted hydrothermal process followed by annealing in air. The detailed morphological and structural studies indicate that the SnO₂ tubes typically have diameters from 200 to 400 nm, lengths from 0.5 to 1.5 μm and wall thicknesses from 50 to 100 nm. The SnO₂ nanotubes are self-assembled by interconnected nanocrystals with sizes ∼8 nm resulting in a specific surface area of ∼54 m² g⁻¹. The pristine SnO₂ nanotubes are used to fabricate lithium half cells to evaluate their lithium ion storage properties. The porous SnO₂ nanotubes are characteristic with high lithium ion storage capacity, that is found to be 1258, 951, 757, 603, 458, and 288 mAh g⁻¹, at 0.1, 0.2, 0.5, 1, 2, and 4C, respectively. The enhanced electrochemical properties of the SnO₂ nanotubes can be ascribed to their unique geometry and porous structures.

Introduction

SnO₂ is an important wide bandgap semiconductor with extensive applications in gas sensing [1], [2], [3], photocatalysis [4], water treatment [5], dye-/quantum dot-sensitized solar cells [6], [7], [8], [9] and hybrid solar cells [10]. SnO₂ has also been considered as a promising anode material for lithium ion batteries (LIBs) due to its reported high specific capacity (>600 mAh g⁻¹), low lithium ion (Li⁺) intercalation potential, non-toxicity, and low production cost [11]. However, the practical application of SnO₂ as anode materials in LIBs has largely been hampered by its poor cycle performance and rate capability, which are caused by the huge volume change (>300%) [12] during charge and discharge, giving rise to pulverization of electrode materials and loss of electrical contact with current collector. As a result, much effort has been directed to improve the electrochemical performance of SnO₂ materials. An available route is the conformal coating of a thin carbon layer on the SnO₂ surfaces to stabilize the electrode structures during charge–discharge processes [13], [14], [15], [16], [17], [18]. Other approaches include in-situ deposition or growth of SnO₂ on carbon nanotubes [19], [20], [21], [22], [23] or grapheme [24], [25], [26] to enhance the electronic conductivity of the electrodes and alleviate their volumetric variation during electrochemical cycling. In addition to these strategies, it is well recognized that the electrochemical properties of materials strongly depend on their structural parameters, such as crystallinity, morphology, particle size, specific surface area and pore structure. For instance, porous spherical TiO₂ [27] and LiMn₂O₄ [28] structures can exhibit much better cycle performance and rate-capability than their dense counterparts when used as LIB electrodes. Therefore, rational design and optimization of the pristine SnO₂ microstructures play an important role in improving their electrochemical performance. In this regard, various SnO₂ nanostructures, such as nanocrystals [29], [30], nanowire/nanorods [31], [32], [33], nanotubes [34], [35], [36], nanosheets [37], [38], porous and hollow structures [39], [40], [41], [42], [43], [44], [45], have been investigated as anode materials for LIBs. These nanostructures usually possess much higher specific surface areas than the bulk material, which can considerably elevate the reaction kinetics at surface and interface. Porous and hollow structures can also effectively accommodate the volume variation of SnO₂ during electrochemical charge–discharge processes and improve the cycling performance.

Various synthetic routes have been developed to prepare SnO₂ nanostructures, including chemical vapor deposition [8], sol–gel techniques [31], [39], micro-emulsion processes [4], template-assisted etching [36], [45], and hydrothermal/solvothermal methods [32], [40]. Among these methods, hydrothermal synthesis provides a facile route for the preparation of SnO₂ nanostructures due to its easiness, mild reaction conditions, and low-cost. However, conventional hydrothermal methods usually take a long reaction period [41], [42]. Recently, microwave-assisted hydrothermal synthesis has increasingly attracted scientific attention due to its distinct heating means and significantly reduced reaction time [29], [46], [47], [48], [49]. In this work, we report the fast synthesis of SnO₂ nanotubes with highly porous wall structures and evaluation of their electrochemical properties. The synthesized porous SnO₂ nanotube structures are suitable for construction of anodes in LIBs because their ability to accomodate volume variation at charging and discharging cycles.