Haizheng Tao

Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF‐4

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K.

Second-harmonic generation in IR-transparent β-GeS 2 crystallized glasses

IR-transparent β-GeS2 crystallized glasses were fabricated by crystallization of the glass with a stoichiometric composition corresponding to target crystal and only β-GeS2 crystallites were precipitated. A clear second-harmonic generation was observed in the crystallized glasses by the Maker fringe technique. The optical nonlinear susceptibility, χ(2), of β-GeS2 crystallized glasses was found to be ~7.3 pm/V, being comparable to that created by thermal/electric or other poling methods, which opens a new way to an optical converter operating in the mid-IR spectral region.

Melt-Quenched Glasses of Metal-Organic Frameworks

Crystalline solids dominate the field of metal-organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal-ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.

Study on the third and second-order nonlinear optical properties of GeS(2)-Ga(2)S 3 -AgCl chalcohalide glasses.

Third-order optical nonlinearities, chi((3)) of GeS(2)-Ga(2)S(3)-AgCl chalcohalide glasses have been studied systematically utilizing the femtosecond time-resolved optical Kerr effect (OKE) technique at 820nm, showing that the value of chi((3)) enhances with increasing atomic ratio of (S+Cl/2)/(Ge+Ga). From the compositional dependence of glass structure by Raman spectra, a strong dependence of chi;(3) upon glass structure has been found, i.e. compared with [Cl(x)S(3-x)Ge(Ga)-Ge(Ga)S(3-x)Cl(x)] ethane-like s.u. as the structural defectiveness, [Ge(Ga)S(4-x)Cl(x)] mixed tetrahedra make greater contribution to the enhancement of chi((3)). The maximum chi(3) among the present glasses is as large as 5.26x10(-13)esu (A1 (80GeS(2)-10Ga(2)S(3-) 10AgCl)), and the nonlinear refractive index (n2) of A1 glass is also up to 4.60x10(-15) cm(2)/W. In addition, using Maker fringe technique, SHG was observed in the representative A1 glass poled by electron beam (25 kV, 25 nA, 15 min), and the second-order optical nonlinear susceptibility is estimated to be greater than 6.1 pm/V. There was no evident structural change detected in the as-prepared and after irradiated A1 glass by the Raman spectra, and maybe only electronic transition and distortion of electron cloud occurred in the glasses. The large third/second-order optical nonlinearities have made these GeS(2)-Ga(2)S(3)-AgCl chalcohalide glasses as promising materials applied in photoelectric fields.

Optical second-order nonlinearity of the infrared transmitting 82GeS2∙18CdGa2S4 nanocrystallized chalcogenide glass

The infrared (IR) transmitting nanocrystallized chalcogenide glass only containing α-CdGa2S4 IR nonlinear optical crystallites, which were confirmed by x-ray diffraction and Raman scattering measurements, was fabricated through the controllable nucleation and crystallization of the glass with the composition 82GeS2∙18CdGa2S4 at a temperature of 325°C far below the glass transition temperature (Tg:415°C) for a long duration. A clear second harmonic generation (green light, 532nm) could be observed in the transparent 82GeS2∙18CdGa2S4 nanocrystallized chalcogenide glass, demonstrating the IR frequency-doubling function of the prepared nanocrystallized composite material.

A new method of fabricating internally sol-gel coated capillary tubes

Abstract A new method for fabricating internally sol–gel coated long capillary tubes was developed to solve the problems of the capillary vibration and the difficulty in gelation reaction inner the capillary during sol–gel processing. Firstly, a sol was coated on the inner wall of a standing long silica glass capillary filled with the sol solution by lowering the sol. Then the air was very slowly pumped into the capillary to create an air condition similar to the outside space. By this way the gelation reaction went as well as it usually goes in the dip-coating process. The multilayer or thick coating could easily be fabricated by repeating the sol up-and-down and air flowing process. Subsequently, the capillary was subjected to heat treatment in a special stove while oxygen was kept flowing through the capillary. In this way the organic groups in the gel burned as completely as possible, and the burned gases could also be easily removed out of the capillary. The equipment specially designed for the fabrication of this coating and such processing parameters as the sol-dropping speed, the fluxes of the air and oxygen and temperature programs are also discussed in the paper. The results show that this method is not only an effective method for fabricating inner sol–gel coating in a capillary but also a good supplement for the dip-coating technique under some circumstances where a dip-coating method is limited to be used.

Melt-Quenched Hybrid Glasses from Metal-organic Frameworks

While glasses formed by quenching the molten states of inorganic non-metallic, organic, and metallic species are known, those containing both inorganic and organic moieties are far less prevalent. Network materials consisting of inorganic nodes linked by organic ligands do however exist in the crystalline or amorphous domain. This large family of open framework compounds, called metal-organic frameworks (MOFs) or coordination polymers, has been investigated intensively in the past two decades for a variety of applications, almost all of which stem from their high internal surface areas and chemical versatility. Recently, a selection of MOFs has been demonstrated to undergo melting and vitrification upon cooling. Here, these recent discoveries and the connections between the fields of MOF chemistry and glass science are summarized. Possible advantages and applications for MOF glasses produced by utilizing the tunable chemistry of the crystalline state are also highlighted.

Study of the synthesis of SiO2–TiO2–GeO2 gel glass for hollow waveguide application in CO2 laser delivery

The SiO2–TiO2–GeO2 sol–gel glass system was firstly ascertained to be the material to use in the fabrication of a hollow waveguide for application in CO2 laser delivery. SiO2–TiO2–GeO2 sols of various compositions were prepared by using Si(OC2H5)4, Ti(OC4H9)4 and Cl3GeCH2CH2COOH as the precursors. It was found by viscosity and TEM analysis that the stability of the sol was weakened greatly when the content of germanium dioxide reached 35% because of self-polycondensation of hydrolyzed Cl3GeCH2CH2COOH. The sol with a composition of 40SiO2·30TiO2·30GeO2 was chosen as the coating solution because of its relatively high content of Ti and Ge atoms and its long lifetime of 30 days. It was confirmed by XRD analysis that no crystal TiO2 and GeO2 phases separated from SiO2 and that the material remained glassy. Compared with the IR reflectivity peaks of silica glass, the IR reflectivity peaks of the SiO2–TiO2–GeO2 gel glass are greatly broadened due to the formation of various mixed bonds between Ti, Si and Ge through bridging O atoms. The wavelength of the CO2 laser is included in this peak and a refractive index below one is obtained. The use of this material in a hollow waveguide structure for the delivery CO2 laser is discussed on the basis of the calculated complex refractive index and transmission loss. The results show that a SiO2–TiO2–GeO2 sol with a composition of 40SiO2·30TiO2·30GeO2 is a good candidate material for this application.

CdS quantum dots-sensitized TiO2 nanotube arrays for solar cells

CdS quantum dots(QDs) sensitized TiO2 nanotube arrays photoelectrodes were investigated for their photovoltaic performance of quantum dots-sensitized solar cells. The highly ordered TiO2 nanotube arrays(TNAs) were synthesized on Ti foils by anodic oxidation method. Then CdS quantum dots were deposited onto the TiO2 nanotube arrays by successive ionic layer absorption and reaction(SILAR) method to serve as the sensitizers. Cd(NO3)2 and Na2S were used as the precursor materials of Cd+ and S2− ions, respectively. It is found that the CdS QDs sensitizer may significantly increase the light response of TiO2 nanotube arrays. With increasing CdS QDs deposition cycles, the visible light response increases. Maximum photocurrent was obtained for the QDs that have an absorption peak at about 500 nm. Under AM 1.5 G illuminations(100 mW cm−2), a 4.85 mA/cm2 short circuit current density was achieved, and the maximium energy conversion efficiency of the asprepared CdS QDs-sensitized TNAs solar cells was obtained as high as 0.81 % at five SILAR cycles.

Thermal behavior and lithium ion conductivity of L2O-Al2O3-TiO2-SiO2-P2O5 glass-ceramics

A lithium ion conductive solid electrolyte, L2O-Al2O3-TiO2-SiO2-P2O5 glass with NASICONtype structure have been synthesized and transformed into glass-ceramic through thermal-treatment at various temperatures from 700 to 1 000 °C for 12 h. The differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance techniques were employed to characterize the samples. The experimental results indicated that the capability of glass forming in this system is superior to that of L2O-Al2O3-TiO2-P2O5. The glass has an amorphous structure and resultant glass-ceramic mainly consisting of LiTi2(PO4)3 phases. Impurity phases AlPO4, TiO2, TiP2O7 and unidentified phase were observed. With the enhanced heat-treatment temperature, grain grew gradually and lithium ion conductivity of glass-ceramics increased accordingly, the related impedance semicircles were depressed gradually and even disappeared, which could be analytically explained by the coordinate action of the ‘Constant phase element’ (CPE) model and the ‘Concept of Mismatch and Relaxation’ model (CMR). When the sample is devitrified at 1 000 °C, the maximum room temperature lithium ion conductivity comes up to 4.1×10−4 S/cm, which is suitable for the application as an electrolyte of all-solid-state lithium batteries.