Doinita Neiner

A new solution route to hydrogen-terminated silicon nanoparticles: synthesis, functionalization and water stability

Hydrogen capped silicon nanoparticles with strong blue photoluminescence were synthesized by the metathesis reaction of sodium silicide, NaSi, with NH4Br. The hydrogen capped Si nanoparticles were further terminated with octyl groups and then coated with a polymer to render them water soluble. The nanoparticles were characterized by TEM, FT-IR, UV-VIS absorption, and photoluminescence. The Si nanoparticles were shown to have an average diameter of 3.9 ±1.3 nm and exhibited room-temperature photoluminescence with a peak maximum at 438 nm with a quantum efficiency of 32% in hexane and 18% in water; the emission was stable in ambient air for up to 2 months. These nanoparticles could hold great potential as a non-heavy element containing quantum dot for applications in biology.

Resonance stabilization energy of 1,2-azaborines: a quantitative experimental study by reaction calorimetry.

Aromatic and single-olefin six-membered BN heterocycles were synthesized, and the heats of hydrogenation were measured calorimetrically. A comparison of the hydrogenation enthalpies of these compounds revealed that 1,2-azaborines have a resonance stabilization energy of 16.6 ± 1.3 kcal/mol, in good agreement with calculated values.

Kinetic and thermodynamic investigation of hydrogen release from ethane 1,2-di-amineborane

The thermodynamics and kinetics of hydrogen (H2) release from ethane 1,2-di-amineborane (EDAB, BH3NH2CH2CH2NH2BH3) were measured using Calvet and differential scanning calorimetry (DSC), pressure-composition isotherms, and volumetric gas-burette experiments. The results presented here indicate that EDAB releases ∼ 10 wt.% H2 at temperatures ranging from 100 °C to 200 °C in two moderately exothermic steps, approximately −10 ± 1 kJ mol−1 H2 and −3.8 ± 1 kJ mol−1 H2. Isothermal kinetic analysis shows that EDAB is more stable than ammonia borane (AB) at temperatures lower than 100 °C; however, the rates of hydrogen release are faster for EDAB than for AB at temperatures higher than 120 °C. In addition, no volatile impurities in the H2 released by EDAB were detected by mass spectrometry upon heating with 1 °C min−1 to 200 °C in a calorimeter.

A versatile low temperature synthetic route to Zintl phase precursors: Na4Si4, Na4Ge4 and K4Ge4 as examples

Na(4)Si(4) and Na(4)Ge(4) are ideal chemical precursors for inorganic clathrate structures, clusters, and nanocrystals. The monoclinic Zintl phases, Na(4)Si(4) and Na(4)Ge(4), contain isolated homo-tetrahedranide [Si(4)](4-) and [Ge(4)](4-) clusters surrounded by alkali metal cations. In this study, a simple scalable route has been applied to prepare Zintl phases of composition Na(4)Si(4) and Na(4)Ge(4) using the reaction between NaH and Si or Ge at low temperature (420 degrees C for Na(4)Si(4) and 270 degrees C for Na(4)Ge(4)). The method was also applied to K(4)Ge(4), using KH and Ge as raw materials, to show the versatility of this approach. The influence of specific reaction conditions on the purity of these Zintl phases has been studied by controlling five factors: the method of reagent mixing (manual or ball milled), the stoichiometry between raw materials, the reaction temperature, the heating time and the gas flow rate. Moderate ball-milling and excess NaH or KH facilitate the formation of pure Na(4)Si(4), Na(4)Ge(4) or K(4)Ge(4) at 420 degrees C (Na(4)Si(4)) or 270 degrees C (both M(4)Ge(4) compounds, M = Na, K). TG/DSC analysis of the reaction of NaH and Ge indicates that ball milling reduces the temperature for reaction and confirms the formation temperature. This method provides large quantities of high quality Na(4)Si(4) and Na(4)Ge(4) without the need for specialized laboratory equipment, such as Schlenk lines, niobium/tantalum containers, or an arc welder, thereby expanding the accessibility and chemical utility of these phases by making them more convenient to prepare. This new synthetic method may also be extended to lithium-containing Zintl phases (LiH is commercially available) as well as to alkali metal-tetrel Zintl compounds of other compositions, e.g. K(4)Ge(9).

Synthesis and Characterization of K8−x(H2)ySi46

A hydrogen-containing inorganic clathrate with the nominal composition, K(7)(H(2))(3)Si(46), has been prepared in 98% yield by the reaction of K(4)Si(4) with NH(4)Br. Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. Elemental analysis and (1)H MAS NMR confirmed the presence of hydrogen in this material. Type I clathrate structure is built up from a Si framework with two types of cages where the guest species, in this case K and H(2), can reside: a large cage composed of 24 Si, in which the guest resides in the 6d position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position (cubic space group Pm3n). Potassium occupancy was examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the K is deficient in both the 2a and the 6d sites. (1)H and (29)Si MAS NMR are consistent with the presence of H(2) in a restricted environment and the clathrate I structure, respectively. FTIR and (29)Si{(1)H} CP MAS NMR results show no evidence for a Si-H bond, suggesting that hydrogen is present as H(2) in interstitial sites. Thermal gravimetry (TG) mass spectrometry (MS) provide additional confirmation of H(2) with hydrogen loss at approximately 400 degrees C.

Solid-State Synthesis and Structure of the Enigmatic Ammonium Octaborate: (NH4)2[B7O9(OH)5]·3/4B(OH)3·5/4H2O

The compound known since the 19th century as ammonium octaborate was structurally characterized revealing the ammonium salt of the ribbon isomer of the heptaborate anion, [B7O9(OH)5](2-), with boric acid and water molecules. Of composition (NH4)2B7.75O12.63·4.88H2O, it approximates the classical ammonium octaborate composition (NH4)2B8O13·6H2O and has the structural formula {(NH4)2[B7O9(OH)5]}4·3B(OH)3·5H2O. It spontaneously forms at room temperature in solid-state mixtures of ammonium tetraborate and ammonium pentaborate. It crystallizes in the monoclinic space group P21/c with a = 11.4137(2) Å, b = 11.8877(2) Å, c = 23.4459(3) Å, β = 90.092(1)°, V = 3181.19(8) Å(3), and Z = 2 and contains well-ordered ammonium cations and [B7O9(OH)5](2-) anions and disordered B(OH)3 and H2O molecules linked by extensive H bonding. Expeditious solid-state formation of the heptaborate anion under ambient conditions has important implications for development of practical syntheses of industrially useful borates.

Electron Microscopy Characterization of Tc-Bearing Metallic Waste Forms- Final Report FY10

The DOE Fuel Cycle Research & Development (FCR&D) Program is developing aqueous and electrochemical approaches to the processing of used nuclear fuel that will generate technetium-bearing waste streams. This final report presents Pacific Northwest National Laboratory (PNNL) research in FY10 to evaluate an iron-based alloy waste form for Tc that provides high waste loading within waste form processing limitations, meets waste form performance requirements for durability and the long-term retention of radionuclides and can be produced with consistent physical, chemical, and radiological properties that meet regulatory acceptance requirements for disposal.

Structure and Properties of Sodium Enneaborate, Na2[B8O11(OH)4]·B(OH)3·2H2O

Millions of tons of sodium borates are used annually by global industries in diverse applications important to modern society. The Na2O-B2O3-H2O phase diagram in the 0-100 °C temperature range contains 13 unique hydrated crystalline sodium borates, including five important industrial products. Structures were previously reported for each of these except for that having the highest boron content, known as sodium enneaborate, Na4B18O29·11H2O or 2Na2O·9B2O3·11H2O (1). Here we report the single-crystal structure of 1, revealing the structural formula Na2[B8O11(OH)4]·B(OH)3·2H2O, and describe some of its properties and relationships to other sodium borates. The structure of 1 features linear polyborate chains composed of the repeating [B8O11(OH)4]2- fundamental building blocks with interstitial water and boric acid molecules integrated by extensive H bonding. Interstitial sodium cations occur in groups of four with interatomic distances of 3.7830(6) and 3.7932(8) Å. Upon heating, 1 initially becomes amorphous and then crystallizes as α-Na2B8O13 along with amorphous B2O3. Notably, α-Na2B8O13 contains octaborate fundamental building blocks that are topologically equivalent to those in 1. Compound 1 crystallizes in the monoclinic space group P21/n with a = 10.2130(8) Å, b = 12.940(1) Å, c = 12.457(1) Å, β = 93.070(2)°, V = 1644.0(2) Å3, and Z = 2.

Structure determination of H-encapsulating clathrate compounds in aberration-corrected STEM

In the quest for alternative energy sources, hydrogen has long been touted as one of the most likely candidates to replace fossil fuels, provided practical solutions for its storage and transport can be found. Clathrate hydrates, amongst other nano-porous materials, have shown remarkable potential for hydrogen storage, adsorbing up to 7.5%wt H, albeit in extreme pressure and temperature conditions [1]. Upon crystallising, these compounds form a three-dimensional host matrix where guest atoms can be accommodated within distinct polyhedral “cages”. Recent work exhibited a sodium silicide clathrate NaxSi46, consisting of a silicon matrix with two distinct Na guest sites, labelled 2a and 6d, stable at room temperature and with promising hydrogen encapsulation properties [2]. Magic angle spinning nuclear magnetic resonance was initially used to show that in the growth conditions specific to this work some of the 6d sites were Na-deficient and H-rich.

Fabrication of silicon-based nanoparticles for biological imaging

Fluorescent silicon nanomaterials have initiated great interest for their potential biological applications. In this paper, we report the synthesis of water-soluble, luminescent silicon nanoparticles with high quantum yield. The surfaces of the Si nanoparticles were capped by hydrophobic or hydrophilic organic molecules that passivate and protect the silicon particles from oxidation. The as-synthesized silicon nanoparticles displayed strong photoluminescence in the blue region of the visible spectrum. The attached organic molecules conveyed both water solubility and high stability, and coating had little adverse effect on the optical properties of nanoparticles. These results have major implications towards using colloidal silicon nanoparticles effectively in biological fluorescence imaging.