Layer-structured indium selenides have attracted extensive attention in energy applications, including high-efficiency thermoelectrics for waste heat energy conversion and high-performance Li-ion batteries for energy storage, due to their extraordinary and tunable electrical and thermal properties. Recently, it has been found that nanostructuring can enhance thermoelectric and electrochemical properties of various inorganic phases. However, there lacks a controllable solution-phase method to synthesize layer-structured indium selenide nanostructures with controllable crystal structure, morphology, and size.
Accordingly, this thesis focuses on the development of a controllable solution-phase method to synthesize new-phase layer-structured indium selenide nanostructures, the determination of the crystal structure by advanced characterization techniques, the investigation of their structural stability under various environments, and the exploration of their energy applications. The objectives of this thesis are achieved through the following aspects:
(1) Structure determination of new-phase layer-structured In3Se4
A new-phase layer-structured In3Se4 crystal was synthesized by a facile and mild solvothermal method. Detailed structural and chemical characterizations using transmission electron microscopy, coupled with synchrotron X-ray diffraction analysis and Rietveld refinement, indicate that In3Se4 crystallizes in a layered rhombohedral structure with lattice parameters of a = 3.964 ± 0.002 Å and c = 39.59 ± 0.02 Å and a space group of R3 ̅m, and with a layer composition of Se-In-Se-In-Se-In-Se.
(2) Controllable synthesis and formation mechanism of In3Se4 nanostructures
Layer-structured In3Se4 hierarchical nanostructures, assembled by thin nanosheets with a thickness of ~20 nm, are controllably synthesized by an ethylenediaminetetraacetic acid (EDTA) and ascorbic acid (AA) assisted solvothermal method in a solvent of deionized water (DIW) and ethylene glycol (EG). Through systematic investigation, the key synthesis parameters, namely the EDTA/In mole ratio, DIW/EG volume ratio, AA dosage, and reaction temperature, are found to play vital roles in the formation of such hierarchical nanostructures. The investigation of intermediate products shows that the In3Se4 hierarchical nanostructures are formed by the reaction between Se microspheres and In precursors.
(3) Physical properties and structural stability of In3Se4 nanostructures
Physical properties: The band structure of In3Se4 crystal has been calculated by theoretical modelling, from which n-type semiconducting behaviour is expected. Electrical property measurement reveals that In3Se4 nanostructures are n-type semiconductors with room temperature carrier density of 1 x 1017 cm-3. The optical absorption measurement shows that In3Se4 nanostructures have a low optical gap of ~0.55 eV.
Thermal stability under different environments: In-situ synchrotron X-ray diffraction in a sealed system reveals that In3Se4 has good thermal stability up to 900 °C. In contrast, In3Se4 has lower thermal stability up to 550 or 200 °C when heated in an atmosphere flushed with Ar or in air, respectively. The degradation mechanism was determined to be the oxidation of In3Se4 by O2 in the heating environment.
Thermal stability under air: In3Se4 nanostructures are not stable and will transform into cubic-structured In2O3 when heated at a temperature higher than 500 °C. This thermal instability can be used for the synthesis of Se4+-doped In2O3 nanostructures. Se4+-doped In2O3 hierarchical nanostructures, consisting of nanoparticle-assembled nanosheets, have been synthesized by thermal oxidation from their template of In3Se4 nanostructures. The photoluminescence property measurements show that such doped nanostructures have red light emissions centered at 630, 670, and 770 nm and near infrared emissions centered at 910 nm, which is ascribed to the Se4+ doping.
Structural stability in solution: In3Se4 nanostructures are not stable in BiCl3-containing ethylene glycol solution, and will transform into Bi2Se3 structure. This structural instability can be used for the synthesis of In-doped Bi2Se3 nanostructures. In-doped Bi2Se3 hierarchical nanostructures have been synthesized by an in-situ cation exchange route from their template of In3Se4 nanostructures. The electrochemical measurements reveal that the synthesized In-doped Bi2Se3 hierarchical nanostructures show much better discharge capacity, improved cycle stability, and rate performance as an anode material for Li-ion batteries, compared to its undoped counterparts.
(4) Energy applications of In3Se4 hierarchical nanostructures
In3Se4 hierarchical nanostructures assembled by thin nanosheets have been developed as new anode materials for high-performance Li-ion batteries. Electrochemical performance measurement shows that these hierarchical nanostructures are featured with superior discharge capacity, excellent long-term cycling stability, and high rate performance, which demonstrate their great potential as high-performance anode materials for next generation Li-ion batteries.
In summary, this thesis develops a controllable solvothermal method to synthesize new-phase layer-structured In3Se4 hierarchical nanostructures, systematically demonstrates their structural stability under different environments, as well as explores their intrinsic physical properties and energy applications. This thesis should not only build up the foundation for the potential energy applications of this new layered In3Se4 phase, but also pave a new way in the synthesis of layer-structured semiconducting compounds with advanced properties.