Investigation On Synthesis, Phase Transformation And Catalytic Property Of Ag-Cu And TiO₂ Catalytic Materials

The hydrogen fuel cell which can directly convert the chemical energy to electrical energy through electrochemical reaction, is an advanced utilization method for the energies. However, the low photocatalytic hydrogen production efficiency and the high costs of hydrogen storage and transportation hinder the mass production of hydrogen fuel cell in the practical applications. Under this condition, zinc-air battery which use the abundant zinc to replace hydrogen as fuel, has become the research focus due to its advantages such as high energy density and green environmental protection. The key point of the research on zinc-air batteries is focused on the seeking of effective and low-cost oxygen reduction reaction electrocatalyst to replace the precious metal nanoparticles, such as Pt/C, Pd etc. O n the other hand, the researchers are also keep seeking for the hydrogen production method with low costs. As a representative of the photocatalysts, Ti O2 has wide application prospect in the field of photocatalytic hydrogen production and environmental pollution control due to its various advantages such as high specific surface area, excellent photocatalytic property and absorption performance, high surface activity and non-toxic features. Previous researches indicate that mixed-phase Ti O2 nanostructures are shown to be more efficient than single Ti O2 polymorph. Therefore, the synthesis of mixed-phase Ti O2 is a feasible way to achieve high efficient photocatalytic hydrogen production and effective control of environmental pollution.In this paper, the pulsed laser deposition(PLD) method was first employed to prepare the bimetallic Ag-Cu nanoalloy thin film catalysts. Through the electrochemical measurement and transmission electron microscope(TEM) observation, it is approved that the existence of Ag-Cu alloy nanoparticles in the film is the mainly attributed to the high catalytic properties. The capacity and energy density of zinc-air primary cell assembled by using the thin film catalysts increased to 678 m Ah g-1 and 725.5 m Wh g-1 under the current density of 20 m A cm-2. While the charge and discharge voltage of the rechargeable battery assembled by using the thin film catalysts almost remained the same after 400 cycles. The energy efficiency only decreased from 55% to 52.7%, indicating excellent stability of the batteries. On the other hand, Electrochemcial deposition method(EDM) was employed to synthesize the bimetallic Ag-Cu nanodendrites. The crystallographic features and growth mechanism of such nanodendrites were obtained by TEM investigations. It is noticed that the bimetallic Ag-Cu nanodendrites are Ag3Cu(L12) single crystals, the growth directions of main and lateral branches are all along ?110 ?, which is also the close packed directions of FCC structure. Based on the experimental results, the oriented attachment and Oswald riping mechanisms during different growth stage of Ag-Cu nanodendrite were also put forward.The investigations on the mixed-phase Ti O2 photocatalysts can be divided into 3 parts. Firstly, the morphology evolutions and interface microstructures during the phase transformation from hydrothermal prepared H2Ti3O7(HT) to Ti O2(B)(TB) and TB to anatase(TA) were investigated by means of TEM in-situ heating method. The Invariant Deformation Element(IDE) model was employed to calculate the crystallographic features and atomic mechanisms of the two phase transformation process. The results indicated that the crystallographic orientation relationship between HT/TB and TB/TA are HT TB[001] / /[001], HT TB(020) / /(020) and HT TB(200) / /(200) as well asTB TA[001] / /[100], TB TA(200) / /(002) and TB TA(020) / /(020) respectively. Due to the similar structure between HT and TB, the interface between the two phases are fully coherent. While one coherent and two other incoherent interfaces are coexisted in TB/TA phase transformation system. The interface types and morphologies are closely related to the calcination temperature. In addition, the atomic mechanism of the two types of phase transformations are proposed by using the fundamental building block(FBB) model. Based on the discussions, the IDE model was employed to calculate the crystallographic features of the two phase transformation process. The calculation results are fully consistent with the experimental observations.By calcinating TB nanofibers in air and vacuum, the influence of different calcination atmospheres on the phase transformation products can be obtained. Based on the TEM results, the atomic mechanism of phase transformation from TB to TinO2n-1. A new method to prepare new Ti O2 multi-phase nanomaterials are proposed at the same time. The experimental results indicated that two intermediate phases Ti3O5(T-I) and Ti6O11(T-II) which have the same space group with TB are coexisted in the TB nanofibers calcinated in air under 650 °C for 1 hour. While in vacuum, only T-I phase can be found when the temperature reaches 650 °C. This may due to the high surface tension of the nanofibers under high vacuum condition which hinders the transformation from T-I to T-II phase. According to the TEM results of the samples calcinated under different atmospheres, the transformation sequence and corresponding chemical reaction formulas of TB?T-I?T-II?TA process has been proposed. In addition, the crystallographic orientation relationships of the four types of coherent phase interfaces between TB/T-I, TB/T-II, T-II/TA and TB/TA are also revealed to be TB T-I[100] / /[001] 、TB T-II[100] / /[100] 、T-II TA[100] / /[100] 和TB TA[100] / /[100] through the HRTEM characterization. A new synthesis method of Ti3O5 and Ti6O11 is also put forward based on the results obtained above.During the phase transformation of TB nanofibers, TEM observations were carried out to monitor the morphology evolution and distribution of nanocavities at the surface of nanofibers under different calcination atmospheres. The influence mechanism of nanocavities on the phase transformation process from TB to TA and TA to TR were discussed at the same time. TEM results suggest that the nanocavities formed during the phase transformation from TB to TA. Different growth morphology and distribution of such nanocavities are found in the air and vacuum calcination respectively. In the atmospheric environment, the amount of nanocavities is larger than that in vacuum condition. The morphology exhibits hexagonal prism state with the bottom and prism plane parallel to TA{100} and TA{011}, TA{010} respectively. While the nanocavities generated in vacuum are few in number with irregular morphologies. In addition, the nanocavities which tend to change to spherical shape drastically reduced in number on the surface of the nanofibers. Comparing the phase transformation process under two different atmospheres, the existence of nanocavities will all obstacle the transformation. The reason for this may be due to the surface reconstruction process which consume the energy and lead to the vanish of nanocavities.