Mesoporous Tin Oxide Nanomaterials Enabled by Anodization Methods and Their Energy- and Environment- related Applications

Abstract

Tin dioxide (SnO2), an important n-type semiconductor (Eg = 3.6 eV), has been regarded as one of the most promising materials used in energy-and environment-related areas, due to its various advantages, such as its chemical stability, abundance, low cost, facile fabrication, environmental friendliness and high theoretical energy storage capacity (790 mA h g-1 for Li-ion batteries and 667 mA h g-1 for Na-ion batteries). Compared to bulk materials, mesoprous nanostructured materials with large surface area, high pore volume ratio and accessibilities are extremely attractive for various applications. The introduction of large number of mesopores can provide more access for the electrolyte and intermediates/products as well as more active sites for efficient photocatalytic or catalytic reactions. Moreover, the large surface area and short diffusion lengths, make the mesoporous material moreattractivefor high-rate electron orLi-ion/Na-ionstorage applications.

In this thesis, we focused on a facile, low-cost anodization method to fabricate hierarchical mesoprous tin oxide nanostructures, which exhibited higher surface areas than tin oxide nanomaterials synthesized by traditional hydrothermal/solvothermal methods. The fabricated mesoporous tin oxide nanomaterials were applied as electrodes in supercapacitors and Li-ion/Na-ionbatteries.

The opening chapter briefly introduces different methods to fabricate mesoporous tin oxide nanostructures and their applications in different research areas including Li-ion/Na-ion batteries, supercapacitors, sensors, fieldeffect transistorsandphotocatalysts.

Chapter 2 presents a convenient, low-cost strategy to fabricate one-dimensional, vertically oriented nanoporous assembly of SnO2 upon a Cu substrate as a potentially promising anode system for Na-ion batteries application. The major novelty of the fabrication stage resides in anodizing a Sn/Cu bilayer film that is created by a facile cold-rolling procedure amenable to large-scale production. The open, nanoporous morphology of SnO2 facilitates the diffusion of electrolytes to access the SnO2surface. The high porosity of the SnO2 phase also provides large void space to effectively accommodate the volume expansion/contraction during sodiation/desodiation. As a result, the 1-D nanoporous SnO2 thus assembled on the Cu substrate can be directly used as an effective electrode system for Na-ion storage--without the need for additives, dielivering a remarkable capacityof326mAhg-1 over200cycles at acurrentrate of 0.2C.

The as-anodized nanoporous SnO2is amorphous in nature, which limits the direct usage in many areas, including gas sensing, photocatalysis and energy storage/conversion applications, where crystalline SnO2 is desirable. Chapter 3 reports a facile, room-temperature strategy to crystallize anodic one-dimensional mesoporous SnO2 nanostructures. The as-anodized amorphous SnO2 nanochannels could be directly transformed into rutile phase after soaking in water for a while, without any traditional high temperature treatment. The obtained mesoporous rutile SnO2 (60 oC 2 h) possess a remarkable surface area improvement, which is nearly two times higher than that of as-anodized samples and there times higher than high-temperature annealed SnO 2. Detailed observation shows that the 1-D nanochannels are consisted of very tiny nanoparticles (~ 3.8 nm) as the building blocks. The water-soaking crystallized mesoporous SnO2 on the copper substrate exhibits a superior reversible capacity of 514 mAh g-1 after 100 cycles at a current rate of 0.1 C, implying a potential application as a binder-free electrodefor sodiumionstorage.

Chapter 4 moves on to further report a novel type of hierarchical mesoporous SnO2 nanostructures fabricated by a facile anodization method in a novel electrolyte system (an ethylene glycol solution of H2C2O4/NH4F) followed by thermal annealing at a low temperature. The SnO2 nanostructures thus obtained feature highly porous nanosheets with mesoporous pores well below 10 nm, enabling a remarkably high surface area of 202.8 m2/g which represents one of the highest values reported to date on SnO2 nanostructures. The formation of this novel type of SnO2 nanostructures is ascribed to an interesting self-assembly mechanism of the anodic tin oxalate, which was found to be heavily impacted by the anodization voltage and water content in the electrolyte. The electrochemical measurements of the mesoporous SnO2 nanostructures indicate their promising applications as lithium-ion battery and supercapacitorelectrode materials.

Chapter 5 bases on the fabrication of a kind of tin organic nanosheets by anodizing in a glycerol solution of H2C2O4/NH4F. After thermal treatment in Ar, mesoporous SnO/SnO2 nanosheets were obtained. In order to improve the stability, carbon-coated binder-free flexible porous SnOx nanosheets (SnO/SnO2 heterogeneous structure) were fabricated and tested as anode materials for Na-ion batteries (NIBs). The novel free-standing and binder-free porous C@SnOx nanosheets were first self-assembled on the Cu substrate via a facile, low-cost anodization method followed by the carbonization treatment. Instrument analyses show that the porous C@SnOx nanosheets exhibit a remarkably large surface area of 221 m2 g-1, delivering a reversible discharge capacity of 510 mA h g-1 after 100 cycles at 100 mA g-1, demonstrating great potential for Na+ storage applications. The superior electrochemical performance is ascribed to the unique hierarchical porous architecture which greatly facilitates electrolyte penetration and ion transportation with the carbon coating further increasing the electrode conductivity and alleviating strains generated by volume change upon Na+ ionsinsertion/extraction.

Date of Award	5 Jun 2017
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhengtao XU (Supervisor)