Modulating The Electron And Phonon Structure Of Low-Dimensional Solids For Smart Characteristics

As a type of important functional materials, inorganic solids with smart characteristic are able to sense and response to the stimuli of environment, such as light, electricity, magnetic field and etc., which plays a key role in the realization of future intelligent materials. Furthermore, the conversion of different energy forms and changes of physical properties accompanied with smart behavior have huge applicable potentials in the field of energy storage and utilization&information written and read. Designing smart materials with high performance to promote sensitivity, responsive speed and energy conversion efficiency has been a long standing target of scientific research. Along with the rapid development of nanotechnology, low-dimensional solids show promising sign in enhancing smart properties of materials. Due to the quantum confinement effect and surface effect brought by size reduction, low-dimensional nanomaterials have distinctive electron and phonon structure, which result in special physical properties compared to macroscopic bulk. In this regard, low-dimensional nanomaterials provide an ideal platform to optimize smart performance and develop new smart materials.This dissertation aims at realizing the function-oriented regulation of electron/phonon structure based on the particular electron and phonon structure required by specific smart characteristic, in order to achieve optimized smart performance and explore novel smart behavior in low-dimensional solid nanomaterials. Taking advantages of various chemical tuning methods, i.e. elemental doping, hydrogen modification, dimension control and vacancy engineering, the electron and phonon structure of low-dimensional nanomaterials have been tuned effectively, which leads to the enhanced smart performance on responsive to magnetic field, heat and light. The modulation strategy on electron and phonon structure of low-dimensional nanomaterials in this dissertation will shed new light on designing smart materials with high performance. The main content of this dissertation is summarized as following:1. The authors first synthesized new-phased anatase VTiO4nanocrystals through controllable solution chemistry method. The as-prepared anatase VTiO4realized large magnetodielectric effect at room temperature. On the basis of the similarity of atomic radius between V4+and Ti4+ion, large amount of V4+ions have been introduced into anatase TiO2matrix to form a new catalogue of ionic type solid solution with unprecedentedly high V/Ti ratio of1/1, which also expanded anatase structure into the field of vanadium oxides. Benefited from plentiful doped V4+ions with unique3d1electrons, synergetic advantages of magnetic and polarization ordering have been brought into anatase VTiO4. From one aspect, room temperature ferromagnetism has been induced through the superexchange interaction among V4+-O2-V4+. From another aspect, V4+ions with Jahn-Teller effect brought lattice distortion to lower the symmetry of the structure, which enhanced ability of ionic polarization, leading to giant dielectricity. The coexistence of ferromagnetism and phonon structure in distorted VTiO4triggered spin-phonon coupling effect, which induced large magnetic-dielectric response (△ε/ε0=7.2%) at room temperature. The strategy of introducing large amount of magnetic Jahn-Teller ions into simple oxides would provide new insight into designing and exploring new magnetodielectric materials.2. Hydrogen modification on monoclinic V02(M) realized effective regulation of electron-electron correlation effect in one-dimensional (1D) vanadium-vanadium atomic chain, which achieved the decoupling modulation on electron and phonon structure, leading to optimized thermoelectric property. In this experiment, the non-ambient rutile VO2(R) has been stabilized into room temperature due to the enhanced e-e correlation effect brought by electron injection via hydrogen incorporation. In the meantime, the carrier concentration could be tuned by hydrogen content, which provided a series samples with carrier concentration gradients, i.e. metallic hydric V0O(R), intermediate hydric VO2(M-R) and semiconducting V02(M) at room temperature. The metallic hydric V02(R) has larger than three orders of magnitude carrier concentration than that of V02(M). Moreover, with the smallest atomic radius, hydrogen modification could maintain the V-O framework to obtain unchanged phonon structure. Therefore, this strategy could regulate the electron-electron correlation effect without changing crystallographic structure to achieve decoupling modulation of electron and phonon structure, giving excellent thermoelectric performance with wider working temperature range in or below room temperature. It’s worthy of noting that there is few report on simple oxides reaching high ZT value below room temperature. Our finding revealed that regulation on electron-electron correlation could be a powerful tool to selectively controlling carrier concentration towards thermoelectric materials with high performance.3. We first realized UV photothermal effect in2D ZrNCl nanosheets via quantum confinement effect. Through the combination of chemical lithium intercalation and liquid ultrasonication exfoliation,2D ZrNCl nanosheets with less than4monolayers thickness have been fabricated. The synergetic advantages of enhanced relaxation energy brought by atomic thickness confinement and strengthened bond vibration originated from unique four-atom [Cl-Zr-N-Cl] monolayer structure, achieved maximized electron-phonon interaction to cause high-efficient energy conversion from UV photons to heat. Under illumination of UV light at intensity of200mW/cm2, the heat flow generated from2D ZrNCl nanosheets could achieve5.25W/g, which was24times higher than that of bulk material and even dozens times of that of conventional wide band gap semiconductors. The conversion efficiency of UV photothermal effect of ZrNCl nanosheets accomplished72%. The performance of ZrNCl nanosheets in UV photothermal energy conversion recorded the best among inorganic solids nowadays. This work not only realized a new energy conversion form from UV light, but also guided a new way for developing new photothermal material from the concept of enhancing electron-phonon coupling by confinement effect.4. Through vacancy engineering in two-dimensional ZrNCl nanosheets, the resistivity and energy band gap have been decreased compared with pristine ZrNCl nanosheets, which resulted in enhanced performance in photoelectrochemical (PEC) water splitting. This work first introduced transition metal nitride chloride into the field of photoelectrochemical (PEC) catalysis. Under irradiation of visible light (λ≥400nm), vacancy-ZrNCl nanosheets generated photocurrent of3.3mA/cm2(1.6V vs RHE), which was7.3times higher than that of vacancy-free ZrNCl nanosheets and22times higher than that of vacancy-ZrNCl bulk. Our experiments revealed that the optimized PEC performance is originated from the combined effect of Cl-vacancies and2D morphology. From one aspect, the improved conductivity of vacancy-ZrNCl nanosheets was in favor of the charge transfer process in PEC. Moreover, the vacancy state also brought the absorption edge shifted into visible range, which lead to efficiency of visible light usage increasing. From the other aspect,2D nanosheets not only provided more contact area to electrolyte to prompt surface reaction, but also help to contact tighter with electrode to facilitate electron conducting. This strategy of modulating electron structure via introducing vacancies in2D nanosheets to achieve optimization of PEC process paved a new way to optimize and develop new PEC catalysts.