Synthesis Of Boron Nitride, Boron Carbide And Rare Earth (Alkaline Earth) Borides Nanomaterials In Mild Temperatures

In this thesis, the h-BN micromesh, ultrathin boron nitride nanosheets, hollow structured boron carbide as well as 11 kinds of rare earth/alkaine earth hexaborides have been prepared in mild temperatures. And several progresses was listed below:(1) Porous materials, might be valuable for a range of potential applications, including catalysis and catalyst supports, chemical filtration, separations and optoelectronics. Among porous materials two dimensional (2D) mesh-like materials, which syncretize the merits of hollow materials and the plate-like materials, are emerging fields of research intrest in chemistry and materials science. However, the h-BN mesh, especially h-BN micromesh with the diameter of tens of micrometers and pore size above 100 nm, which might be used as catalysts support under severe operating conditions, has not been well studied owing to a lack of knowledge for the preparation.In this study, hexagonal boron nitride (h-BN) micromeshes with high crystallinity and yield have been prepared by using B2O3, Mg and NaN3 in a stainless steel autoclave at 450℃, which have diameters of up to 100μm, thickness of～210 nm and pore size of 2.8μm in average. A near-band edge emission at 217 nm of these micromeshes have been observed through the photoluminescence (PL) excited spectra. Their formation mechanism was considered through an oriented aggregation process, in which the in-situ produced MgO microparticles served as template. As B2O3 was substituted by other boron sources(such as H3BO3, Na2B4O7, NH4HB4O7 or Mg(BO2)2·H2O), h-BN micromesh with different pore sizes could also be produced, therefore, it is a general way for the convenient synthesis of crystalline h-BN micromesh. Moreover, the hexagenal pore of the h-BN can be obtained by thermal treatment at 1550℃. The product is considered as promising catalyst support and other applications are also foreseen.(2) In recent years, graphene has been regarded as a rising star in materials science and it has been attracted great research interest in physics and chemistry due to its potential application. Though the same crystal structure with nearly the same cell parameters h-BN and graphite have, the electronic properties of them are distinct. The h-BN is typically an insulator or the wide-band semiconductor. On the other hand, BN possesses much higher chemical and thermal stability than carbon. Those attractive properties make BN an excellent complementary material to carbon nanosheets and graphene. Exfoliated in organic solvents, micromechanical cleavage and the chemical vapor deposition were used to obtain monolayer and few layed BN materials. However, the yields of the product were scarce due to limitations of fabrication methods adopted and it is difficult to obtain useful amounts of few layered BNNSs or ribbons. Thus, an even higher yield of suspended ultrathin BNNSs will facilitate the further studies, especially on applications.In this study, a convenient synthetic route for the gram scale ultrathin aligned boron nitride nanosheets (BNNSs) was reported. The ultrathin (2-6 nm) BNNSs were prepared by using common chemicals (boron oxide, zinc powders and N2H4·2HCl) in autoclave at mild temperature (500℃). Its near band edge at 5.71 eV obtained by photoluminescence excited spectrum as well as the optoelectronic properties recorded by cathodoluminescence emission might be essential for the optoelectronic applications in the ultraviolet region. The high thermal stability (below 850℃) in the ambient atmosphere, high specific surface area (226 m2/g) and total pore value (0.405 cm3/g) of the BNNSs could make it a potential candidate for the high efficiency catalyst support. The Pt/BNNSs catalyst towards CO conversion was taken as an example, which shows relatively lower full conversion temperature and good stability.(3) Boron carbide is an effective catalyst for the alkylation and acylation of aromatics to produce polybenzyls, ketones, esters and alkyl benzenes. Macroporous or hollow structured materials might be valuable for a range of potential applications, including catalysis, catalyst supports and chemical filtration. Though boron carbide with hollow structure or macropores is usually avoided in some technical areas, the boron carbide with those structures might be valuable in catalysis and catalyst supports, chemical filtration at extreme conditions.In this study, boron carbide particles with macropores and hollow nanocage structure were prepared by the HClO4 method. First, the crude boron carbide powders with carbon impurities were prepared by using MgB2 and CCl4 at 350℃in an autoclave. As the raw products were purified by HClO4 refluxing, the boron carbide with macrospores and hollow nanocage structures were produced.(4) Due to their striking properties, RB6 and AB6 are of special research interest. Up to now, various routes have been employed for their preparation, such as, direct combination of the corresponding elements, carbon-thermal reduction, boron reduction and aluminum flux reduction. Among those traditional routes, the temperature involved is usually high (1100～1800℃); Electrochemical synthesis of LaB6, CeB6, NdB6 EuB6 and GdB6 in the molten salt around 800℃were developed. Up to now, to the best of our knowledge, there are few general routs have been reported for the preparation of RB6 and AB6 below 400℃.In this study, a versatile route for the convenient synthesis series of rare earth hexaborides (RB6, R=La, Ce, Pr, Nd, Sm, Eu) and alkaline earth hexaborides (AB6, A=Ca, Sr, Ba) is reported. The single crystalline cube-like RB6 (R=La, Ce, Pr, Sm, Eu) with mean sizes ranging from 450 to 700 nm were prepared through the co-reduction of rare earth oxides and boric acid by Mg powder in the presence of I2 in 170～250℃. Similarly, the NdB6 as well as AB6 (A=Ca, Sr, Ba) can be produced at 350℃. It is found that CaB6 nanorods with a proportion of above 20% and lengths up to several micrometers were produced by using CaO as calcium source; When CaCO3 was used instead of CaO, single crystalline hollow CaB6 cages were obtained at 400℃. The as-obtained products have high purity and high thermal stabilities below 630℃. And the cube-like LaB6 was taken as an example for the synthesis of series of hexaborides by adjusting the experimental parameters. And it is believed that its formation was considered through an oriented attachment mechanism.