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Doping of Hematite Nanomaterials Based on Laser Ablation in Liquids and Their Physicochemical Properties

Author LiuJun
Tutor LiangChangHao
School University of Science and Technology of China
Course Materials Physics and Chemistry
Keywords doping of semiconductors laser ablation in liquids hematite colloidalsolution nanostructuring photoelectrochemical performance
Type PhD thesis
Year 2013
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Doping plays an important role in improving the physicochemical properties of semiconductor nanomaterials, and it has always been the significant challenge to accomplish the controllable doping, for which the selection of dopant precursors and the control of nuclei and growth for semiconductor nanocrystals are the critical points. Owing to the unique non-equilibrium thermodynamics characteristic and rapid quenching kinetics characteristic, laser ablation in liquids (LAL) technology can provide metastable nanomaterials colloidal solutions. In general, these metastable nanomaterials are highly active and dispersed in solution with uniform size, as well as clean surface without capping any reagents for preventing aggregation and sedimentation to some extent. These colloidal solutions can be used as desired dopant precursors and expected to achieve controllable doping at atomic-scale, which obviously provide new approach for adjusting the physicochemical properties and exploring novel functional nanomaterials.In this thesis, we firstly studied the growth, phase transition and unique properties of LAL-induced metastable nanomaterials. Subsequently, by utilizing the LAL-induced metastable nanomaterials as dopant precursors, and selecting the hematite (α-Fe2O3) nanomaterials as target semiconductors, we put forward a novel doping strategy for the controllable doping of α-Fe2O3nanomaterials in hydrothermal environments. In addition, we further explored how the doping process affected the physicochemical properties of α-Fe2O3nanomaterials. The detailed investigation and innovative results are listed as following:1) Studying of spontaneous growth and chemical reduction properties of Ge nanoparticles (NPs) with high activityHighly active and metastable Ge NPs were obtained by using LAL technology. These Ge NPs dispersed in deionized water and placed in a sealed chamber under ambient temperature. By increasing the aging time, the initial Ge NPs in amorphous structure showed spontaneous growth behavior, which gradually evolved into a metastable tetragonal structure as an intermediate phase and then transformed into a stable cubic structure, being consistent with the Ostwald’s rule of stages for the growth in a metastable system. The laser-induced initial Ge NPs demonstrate a unique and prominent size-dependent chemical reductive ability, which is evidenced by the rapid degradation of organic molecules such as chlorinated aromatic compounds, organic dyes, and reduction of heavy metal Cr(VI) ions.2) Exploration of novel LAL-based doping strategy for α-Fe2O3nanocrystalsBy using the LAL-induced metastable nanomaterials as dopant precursors and combination with hydrothermal reactions, we established a novel strategy for successful incorporation of various impurities (Ge, Si, Mn, Sn, Ti) inside the regular crystal lattice of a-Fe2O3and modulation of the microstructures of a-Fe2O3nanocrystals. Detailed characterizations exhibited that the dopant atoms either form superlattice structures (Ge and Si) or distribute as disordered solid solutions (Mn, Sn, Ti) inside the crystal lattice of hematite. The doping level, morphology, and structure of the hematite nanocrystals are remarkabally affected by the type and amount of the used colloidal precursors. In addition, we investigated the modification of band gap structures induced by various atoms doped a-Fe2O3nanocrystals and proposed the possible growth and doping mechanism of a-Fe2O3nanocrystals.3) Controllable synthesis, characterizations and applications of Ge-doped a-Fe2O3nanocrystalsDoping impurities and nanostructuring are two effective ways to modulate the photoelectrochemical (PEC) properties of hematite materials. We obtained Ge-doped a-Fe2O3nanocrystals with different doping level and microstructures through the aforementioned doping strategy. The doping level of Ge, morphology and structure of a-Fe2O3nanocrystals could be readily tuned by adjusting the amounts of Ge colloidal solution. Hematite nanocrystals doped with2at.%Ge presented as ultra-thin circular nanosheets, and Ge presented randomly at the Fe sites in the a-Fe2O3lattice of the2at.%doped a-Fe2O3nanocrystals. While doping level of Ge increased to5at.%, the α-Fe2O3nanocrystals assembled by several thicker nanosheets with hexagonal profile and Ge distributed orderly in the host lattice to form superlattice structures. The band gap of α-Fe2O3nanocrystals can be reduced by Ge doping, which also evidently increased the photocurrent density and improve the PEC performance of a-Fe2O3nanocrystals.4) Preparation of Ge-doped a-Fe2O3nanoarrays and study of their physicochemical propertiesTo further improve the PEC performance and explore the potential applications, preparation of a-Fe2O3nanoarrays structures, and simultaneously treating by doping and nanostructuring should be ideal approaches. We combined LAL technology with hydrothermal method:the LAL technology provided dopant precursors and the β-FeOOH nanorods array were used as template, the Ge-doped a-Fe2O3nanosheets array can be assembled through the hydrothermal reactions. The detailed characterization of the phase, components and morphology of prepared Ge-doped a-Fe2O3nanosheets array indicated that, by modulation the amounts of Ge colloidal solution, not only determined the doping level of Ge, but also inducedthe preferred growth along the [110] lattice direction of a-Fe2O3nanosheets array. The photocurrent density measurements demonstrated that the Ge-doped a-Fe2O3nanosheets array with higher doping level and preferential growth orientation exhibited more excellent PEC performance, which adequately verified the doping and nanostructuring can evidently improve the PEC performance of a-Fe2O3nanomaterials.

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