Synthesis, characterization, and growth rates of aluminum- and Ge,Al-substituted silicalite-1 materials grown from clear solutions

The synthesis, characterization, and growth rates of aluminum- and germanium,aluminum-substituted silicalite-1 (Al-silicalite-1, Ge,Al-silicalite-1) materials grown from clear solutions are reported. In the case of aluminum substitution, the crystallinity of the materials as determined by powder X-ray diffraction (PXRD) decreases with increasing aluminum content, as does the micropore volume determined by nitrogen adsorption and the growth rate determined by in situ small-angle X-ray scattering (SAXS). The final materials possess slightly lower Si/Al ratios than the initial synthesis mixtures based on X-ray fluorescence analysis. In the case of simultaneous incorporation of germanium and aluminum, the final materials have a slightly lower Si/Al ratio than the synthesis mixture but a much higher Si/Ge ratio, indicating the aluminum is more readily incorporated in the zeolite as compared to germanium. This result is consistent with studies of individual heteroatom substitution behavior. Germanium incorporation in the final material increases at higher heteroatom contents (Si/(Ge+Al) = 50 and 25). The promoting effect of germanium on the growth rate of silicalite-1 dominates at low heteroatom content (Si/(Ge+Al) = 100), leading to enhanced zeolite growth rates as compared to pure silicalite-1. This promoting effect is insensitive to the Ge/Al ratio at a Si/(Ge+Al) = 100. The influence of aluminum on the growth rate, as well as the crystallinity of final materials, becomes observable when the heteroatom content is increased (Si/(Ge+Al) = 50 and 25). This is the first study we are aware of that reports the synthesis of Ge,Al-substituted silicalite-1 phases formed in hydroxide media or from clear solutions and has implications for the synthesis of nanoparticulate zeolitic materials for catalysis.     

Silicalite-1 growth from clear solution: Effect of alcohol identity and content on growth kinetics

In situ small-angle X-ray scattering (SAXS) is used to investigate the influence of alcohol identity and content on silicalite-1 growth from clear solutions at 368 K. Several tetraalkyl orthosilicates (Si(OR)4, R = Me, Pr, and Bu) are used to synthesize silicalite-1 from clear solution mixtures comparable to those previously investigated (i.e. 1:0.36:20 TEOS:TPAOH:H2O (TEOS = tetraethyl orthosilicate; TPAOH = tetrapropylammonium hydroxide), 368 K). All TPAOH-organosiloxane mixtures studied form silica nanoparticles after aging at room temperature for 24 h. Full-profile fitting analysis of the SAXS data indicates the particles are ellipsoidal and is inconsistent with the presence of "nanoslabs" or "nanoblocks". Synthesis using TEOS as the silica source have an induction period of approximately 7.5 h and a growth rate of 1.90 +/- 0.10 nm/h at 368 K. Changing the silica source to tetramethyl orthosilicate (TMOS) does not change the induction period; however the particle growth rate is decreased to 1.65 +/- 0.09 nm/h at 368 K. Variable-temperature SAXS measurements for syntheses with TEOS and TMOS show the activation energy for silicalite-1 growth is 60.0 +/- 2.9 and 73.9 +/- 2.8 kJ/mol, respectively, indicating the alcohol identity does influence the growth rate. By mixing tetrapropyl orthosilicate (TPOS) with TEOS (1.6:1.0 molar ratio) as the silica source, the precursor solution shows a shorter induction period (6.0 h) and a faster particle growth rate (2.16 +/- 0.06 nm/h). The alcohol identity effect is more pronounced when other organocations (e.g. alkyltripropylammonium cations) are used to make silicalite-1 at 368 K. Removing ethanol from the precursor solution decreases the induction period to approximately 4.5 h and increases the particle growth rate to 2.99 +/- 0.13 nm/h. Mixtures with 2 equiv of ethanol have an induction period and particle growth rate of 6.0 h and 2.04 +/- 0.03 nm/h, respectively. The results demonstrate the alcohol identity and content influence silicalite-1 growth kinetics. One possible explanation is varying the alcohol identity and content changes the strength of the hydrophobic hydration of the structure-directing agent and the water-alcohol interaction, resulting in less efficient interchange between clathrated water molecules and solvated silicate species.     

Silicalite-1 growth from clear solution: Effect of the structure-directing agent on growth kinetics

Small-angle X-ray scattering (SAXS) has been used to quantify how perturbations of the tetrapropylammonium (TPA) cation structure affect the growth of silicalite-1 from clear solutions at 368 K. Alkyltripropylammonium (RN(C3H7)3 +OH-, R = Me, Et, Bu, and Pe), dialkyldipropylammonium (R2N(C3H7)2 +OH-, R = Et and Bu), and bis-1,6-(tripropylammonium)hexamethylene dihydroxide (TPA-dimer) cations are used as structure-directing agents (SDAs) to synthesize silicalite-1 from clear solution mixtures comparable to those that have been previously investigated for the TPAOH mediated synthesis (i.e., 1 TEOS:0.36 TPAOH:20 H2O, 368 K). All mixtures studied except those employing dialkyldipropylammonium cations lead to the formation of silicalite-1. The in-situ SAXS investigations show that TPA cations lead to the shortest reaction time as indicated by the observance of Bragg diffraction peaks (15 approximately 16.5 h) and the largest particle growth rate (1.9 +/- 0.1 nm/h). Substituting a propyl group of the TPA moiety with a different alkyl group significantly affects silicalite-1 nucleation and growth with the trend Bu > Et > Pe > Me. Synthesis mixtures containing the TPA-dimer also show a slower growth rate. All the solutions show a bimodal particle distribution throughout zeolite growth with the primary particle size being approximately 5 nm in all cases, independent of the SDA identity. Syntheses using diethyldipropylammonium hydroxide, dibutyldipropylammonium hydroxide, and 4,4'-trimethylenebis(1-methyl-1-hexyl-piperidinium) dihydroxide as the SDA do not result in silicalite-1 formation, showing that the nucleation of silicalite-1 from clear solution at 368 K is sensitive to the SDA geometry.     

Behavior of tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]silicate in aqueous solution: analysis of a sol-gel process by small-angle neutron scattering

The racemic hexacoordinate silicon(IV) complex tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]silicate (1) was synthesized by treatment of Si(OMe)4 with 2 molar equiv of citric acid and 2 molar equiv of N(n-Bu)3. The corresponding germanium analogue, tri(n-butyl)ammonium bis[citrato(3-)-O1,O3,O6]germanate (5; structurally characterized by single-crystal X-ray diffraction), was obtained analogously, starting from Ge(OMe)4. Upon dissolution in water, the lambda6Si-silicate dianion of 1 hydrolyzes spontaneously (formation of Si(OH)4 and citric acid), whereas the lambda6Ge-germanate dianion of 5 was found to be stable in water. Aqueous "solutions" of 1, with concentrations that are significantly higher than the saturation concentration of Si(OH)4, look absolutely clear over a period of several weeks; however, in reality, these solutions are sols with very small particles that slowly grow with time and finally form a gel that precipitates. This sol-gel process was monitored by small-angle neutron scattering (SANS). For reasons of comparison, an aqueous solution of the hydrolytically stable germanium compound 5 was also studied by the SANS technique.     

Self-assembly and phase behavior of germanium oxide nanoparticles in basic aqueous solutions

We show that germania nanoparticle self-assembly in basic aqueous solutions occurs at a critical aggregation concentration (CAC) corresponding to a 1:1 GeO2/OH- molar ratio. A combination of pH, conductivity, and small-angle X-ray scattering (SAXS) measurements was used to monitor the effect of incremental additions of germanium (IV) ethoxide to basic solutions of sodium hydroxide or tetraalkylammonium cations. Plots of pH versus total germania concentration at varying alkalinities generated a phase diagram with three distinct regions. The diagram was analyzed with a thermodynamic model based on the chemical equilibria of germania speciation and dissociation. The model, which uses the GeO-H dissociation constant (pK = 7.1) as the single fitting parameter, quantitatively captures trends in the CAC and pH. SAXS patterns reveal that the germania nanoparticles have either a cubic or a spherical geometry of dimension approximately 1 nm that is independent of solution pH and cation. On the basis of these and other literature findings, we propose that the germania nanoparticle structure is that of the cubic octamer (double four-membered ring, Ge8O12(OH)8), which is common among condensed GeO2 materials and building units in [Ge,Si]-zeolites. Comparisons between germania and silica solutions show distinct differences in their phase behavior and nanoparticle structure. The results presented here, in combination with previous studies of siliceous solutions, provide a framework for ongoing studies of combined germania-silica phase behavior, which is part of an overarching effort to understand the influence of heteroatoms in the growth and structure direction of zeolites.     

Microscopic Parameters of Heterostructures with Ge Nanoclusters and Thin Films in a Si Matrix

Microstructural parameters (interatomic distances, coordination numbers, and their anisotropy) of heterostructures with Ge nanoclusters and thin films were determined by EXAFS spectroscopy. The variation of these parameters is correlated to the morphology of the germanium quantum points on Si(001), and adequate structural models are suggested. It is found that the pseudomorphic four-monolayer germanium films buried in a Si matrix contain up to 50% Si atoms. The pyramidal germanium islands (lateral size 15 nm, height 1.5 nm) formed during further Stranski–Krastanov heteroepitaxial growth have a thinner Ge–Si intermediate boundary layer near two monolayers and Ge–Ge interatomic distances 0.04 shorter than those in pure germanium in agreement with the results of bond length calculations in the framework of the valence force field (VFF) model.     

Solution synthesis of germanium nanowires using a Ge2+ alkoxide precursor

A simple solution synthesis of germanium (Ge0) nanowires under mild conditions (<400 degrees C and 1 atm) was demonstrated using germanium 2,6-dibutylphenoxide, Ge(DBP)2 (1), as the precursor where DBP = 2,6-OC6H3(C(CH3)3)2. Compound 1, synthesized from Ge(NR2)2 where R = SiMe3 and 2 equiv of DBP-H, was characterized as a mononuclear species by single-crystal X-ray diffraction. Dissolution of 1 in oleylamine, followed by rapid injection into a 1-octadecene solution heated to 300 degrees C under an atmosphere of Ar, led to the formation of Ge0 nanowires. The Ge0 nanowires were characterized by transmission electron microscopy (TEM), X-ray diffraction analysis, and Fourier transform infrared spectroscopy. These characterizations revealed that the nanowires are single crystalline in the cubic phase and coated with oleylamine surfactant. We also observed that the nanowire length (0.1-10 microm) increases with increasing temperature (285-315 degrees C) and time (5-60 min). Two growth mechanisms are proposed based on the TEM images intermittently taken during the growth process as a function of time: (1) self-seeding mechanism where one of two overlapping nanowires serves as a seed, while the other continues to grow as a wire; and (2) self-assembly mechanism where an aggregate of small rods (<50 nm in diameter) recrystallizes on the tip of a longer wire, extending its length.     

O,O-Monochelate complexes of silicon and germanium halides: The derivatives of l-mandelic N,N-dimethylamide

Reactions of O-trimethylsilyl-l-mandelic N,N-dimethylamide (1) with tetrachlorosilane and tetrachlorogermane lead to O,O-monochelate complexes, [1-(dimethylcarbamoyl)-1-phenylmethoxy]trichlorosilane (2) and [1-(dimethylcarbamoyl)-1-phenylmethoxy]trichlorogermane (3). Pentacoordination of silicon and germanium in these complexes was confirmed by X-ray studies.X-ray data show that the Si and Ge atoms in 2 and 3 have TBP environments where the ether oxygen and two halogens are equatorial while the third halogen and the amide oxygen occupy axial positions. The axial O–M and Cl–M (M = Si, Ge) distances are somewhat longer than those in similar compounds of tetracoordinate silicon and germanium.Intramolecular coordination in compounds 1–3 and relative stabilities of different conformations of their molecules were studied by quantum-chemical calculations.     

Dianionic complexes with hexacoordinate silicon(IV) or germanium(IV) and three bidentate ligands of the hydroximato(2-) type: syntheses and structural characterization in the solid state

Reaction of Si(OMe)(4) with acetohydroxamic acid or benzohydroxamic acid and HNMe(2) (molar ratio 1:3:2) in MeCN yielded dimethylammonium fac-tris[acetohydroximato(2-)]silicate (fac-5) and N,N-dimethylacetamidinium fac-tris[benzohydroximato(2-)]silicate (fac-8), respectively. Reaction of Si(OMe)(4) with benzohydroxamic acid and HNMe(2) (molar ratio 1:3:2) or ethane-1,2-diamine (molar ratio 1:3:1) in MeOH gave dimethylammonium fac-tris[benzohydroximato(2-)]silicate-methanol (fac-6.MeOH) and ethane-1,2-diammonium mer-tris[benzohydroximato(2-)]silicate-dimethanol (mer-9.2MeOH), respectively. Reaction of Ge(OMe)(4) with benzohydroxamic acid and HNMe(2) (molar ratio 1:3:2) in MeOH resulted in the formation of dimethylammonium fac-tris[benzohydroximato(2-)]germanate-methanol (fac-7.MeOH). Single-crystal X-ray diffraction studies showed that the Si(Ge)-coordination polyhedra of the racemic hexacoordinate silicon (germanium) compounds fac-5, fac-6.MeOH, fac-7.MeOH, fac-8, and mer-9.2MeOH are distorted octahedra. All compounds were additionally characterized by solid-state VACP/MAS NMR studies ((13)C, (15)N, (29)Si). The structural investigations were complemented by computational studies of the dianions of fac-5 and mer-5.     

New calcium germanium nitrides: Ca2GeN2, Ca4GeN4, and Ca5Ge2N6

We report three new calcium germanium nitrides synthesized as crystals from the elements in sealed niobium tubes at 760 degrees C using liquid sodium as a growth medium. Black Ca2GeN2 is isostructural with the previously reported strontium analogue. It is tetragonal P4(2)/mbc (no. 135) with a = 11.2004(8) A, c = 5.0482(6) A, and Z = 8. It contains GeN2(4-) units which have 18 valence electrons, and consequently are bent, like the isoelectronic molecule SO2. In contrast, clear, orange Ca4GeN4 with fully oxidized germanium contains isolated GeN4(8-) tetrahedra and is monoclinic P2(1)/c (no. 14) with a = 9.2823(8) A, b = 6.0429(5) A, c = 11.1612(9) A, beta = 116.498(6) degrees, and Z = 4. Clear, colorless Ca5Ge2N6, also with fully oxidized germanium, contains infinite chains, 1 infinity[GeN2N2/2(5-)], of corner-sharing tetrahedra similar to those found in pyroxenes. However, the precise structure of this latter phase has not yet been determined because of twinning problems.     

Synthese und Strukturen von Bis(amino)germa- und -stanna-Chalkogeniden

Synthesis and Structures of Bis(amino)germa and -stanna Chalcogenides The cyclic bis(amino)germylene 1 and the -stannylene 2 react with elemental S, Se and Te to yield oxydation products of the general formula Me₂Si(NtBu)₂MEl₂M(NtBu)₂SiMe₂ (M = Ge, El = S ( 4 ), El = Se ( 5 ), El = Te ( 6 ); M = Sn, El = Se ( 9 ), El = Te ( 10 )). As may be deduced from X-ray structures ( 4, 5, 6, 9, 10 ) all compounds show similar central skeletons: the three spirocyclicly connected four-membered rings SiN₂M (2x) and MEl₂M are oriented in an orthogonal way to oneanother. The germanium and the tin atoms thus are in a distorted tetrahedral coordination while the chalcogen atoms only have two neighbours in acute angles. If 1 is allowed to react with trimethylamine-N-oxide, the oxygen is transferred to germanium and [Me₂Si(NtBu)₂GeO]₃ ( 3 ) is formed. Contrarily to the other compounds 3 can be described as a trimer. There is a central almost planar Ge₃O₃ six-membered ring, the germanium atoms serving as spiro-cyclic centres to three GeN₂Si four-membered rings (X-ray structure of 3 ). In the central four-membered rings of 4, 5, 6, 9 and 10 no transanular bonding between the chalcogen atoms have to be considered although these atoms have small distances to oneanother. The mean M-El distances have been found to be: Ge? O 1.762(5), Ge? S 2.226(3), Ge? Se 2.363(3), Ge? Te 2.592(5), Sn? Se 2.536(3), Sn? Te 2.741(3) Å.     

Small angle X-ray scattering measurements probe water nanodroplet evolution under highly non-equilibrium conditions

Our in situ small angle X-ray scattering (SAXS) measurements yield an unprecedented and detailed view of rapidly evolving H(2)O nanodroplets formed in supersonic nozzles. The SAXS experiments produce spectra in a few seconds that are comparable to small angle neutron scattering (SANS) spectra requiring several hours of integration time and the use of deuterated compounds. These measurements now make it possible to quantitatively determine the maximum nucleation and growth rates of small droplets formed under conditions that are far from equilibrium. Particle growth is directly followed from about 10 micros to 100 micros after particle formation with growth rates of approximately 0.2 to 0.02 nm micros(-1). The peak H(2)O nucleation rates lie between 10(17) and 10(18) cm(-3) s(-1).     

Electrodeposition of Ge, Si and Si x Ge 1-x from an air- and water-stable ionic liquid

The electrodeposition of Ge, Si and, for the first time, of Si(x)Ge(1-x) from the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py(1,4)]Tf(2)N) containing GeCl(4) and/or SiCl(4) as precursors is investigated by cyclic voltammetry and high-resolution scanning electron microscopy. GeCl(2) in [Py(1,4)]Tf(2)N is electrochemically prepared in a two-compartment cell to be used as Ge precursor instead of GeCl(4) in order to avoid the chemical attack of Ge(iv) on deposited Ge. Silicon, germanium and Si(x)Ge(1-x) can be deposited reproducibly and easily in this ionic liquid. Interestingly, the Si(x)Ge(1-x) deposit showed a strong colour change (from red to blue) at room temperature during electrodeposition, which is likely to be due to a quantum size effect. The observed colours are indicative of band gaps between at least 1.5 and 3.2 eV. The potential of ionic liquids in Si(x)Ge(1-x) electrodeposition is demonstrated.     

平面五配位硅、 锗XBe5H6(X=Si,Ge)团簇

采用密度泛函理论PBE0方法, 在aug-cc-pVTZ水平上理论预测了含平面五配位硅和锗原子的XBe₅H₆ (X=Si, Ge)团簇. 势能面系统搜索及高精度量化计算表明, 它们均为全局极小结构. XBe₅H₆(X=Si, Ge)团簇整体呈完美的扇形结构: Si/Ge原子被5个金属Be原子配位; 4个H原子以桥基方式与Be原子相键连, 剩余的2个 H原子以端基方式与两端的Be原子成键. 化学键分析表明, XBe₅H₆(X=Si, Ge) 团簇中XBe₅单元具有完全离域的1个π及3个σ键, 外围铍氢间形成4个Be—H—Be 三中心二电子(3c-2e)键及2个定域的Be—H键. XBe₅单元上离域的2π及6σ电子赋予体系π和σ双重芳香性, 并使Si/Ge原子满足八隅律（或八电子规则）. 能量分解-化学价自然轨道分析揭示, Si/Ge和Be₅H₆之间主要为电子共享键.     

Characterization of nanoparticles in diluted clear solutions for Silicalite-1 zeolite synthesis using liquid 29Si NMR, SAXS and DLS

Clear solutions for colloidal Silicalite-1 synthesis were prepared by reacting tetraethylorthosilicate in aqueous tetrapropylammonium hydroxide solution. A dilution series with water resulting in clear solutions with a TEOS ratio TPAOH ratio H2O molar ratio of 25 : 9 : 152 up to 25 : 9 : 15,000 was analysed using liquid 29Si nuclear magnetic resonance (NMR), synchrotron small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Particle sizes were derived independently from DLS and from the combination of SAXS and NMR. NMR allowed quantitative characterization of silicon distributed over nanoparticles and dissolved oligomeric silicate polyanions. In all samples studied, the majority of silicon (78-90%) was incorporated in the nanoparticle fraction. In concentrated suspensions, silicate oligomers were mostly double-ring species (D3R, D4R, D5R, D6R). Dilution with water caused their depolymerisation. Contrarily, the internal condensation and size of nanoparticles increased with increasing dilution. SAXS revealed a decrease of effective nanoparticle surface charge upon dilution, reducing the effective particle interactions. With DLS, the reduction of nanoparticle interactions could be confirmed monitoring the collective diffusion mode. The observed evolution of nanoparticle characteristics provides insight in the acceleration of the Silicalite-1 crystallization upon dilution, in view of different crystallization models proposed in the literature.     

(η(2)-(Si/Ge)(4))Zn(η(2)-(Si/Ge)(4))](6-)- novel Zintl clusters with mixed Si/Ge tetrahedra bridged by a Zn atom

The solubility of the ternary Zintl phase K(12)Si(17-x)Ge(x) (x = 5), containing mixed group 14 element clusters, was investigated. Novel dimeric tetrahedral Zintl clusters [(η(2)-E(4))Zn(η(2)-E(4))](6-) with mixed site occupation (E = Si/Ge) were obtained through reaction with (C(6)H(6))(2)Zn in ammonia solutions and investigated by means of X-ray single crystal diffraction.     

Synthesis and characterization of taper- and rodlike si nanowires on Si(x)Ge(1-x) substrate

Taper- and rodlike Si nanowires (SiNWs) are synthesized successfully on Si and Si(0.8)Ge(0.2) substrates. The growth mechanisms of taper- and rodlike SiNWs are proposed to be oxide-assisted growth (OAG) and vapor-liquid-solid (VLS) growth, respectively. For taperlike SiNWs annealed at 1200 degrees C for 3 h, the emission peaks are found at 772, 478, and 413 nm. On the other hand, for rodlike SiNWs annealed at 1200 degrees C for 4 h, emission peaks are found at 783, 516, and 413 nm. From the field-emission measurements, the taperlike Si nanowires exhibit superior field-emission behavior with a turn-on field of 6.3-7.3 V/mum. The field enhancement, beta, has been estimated to be 700 and 1000 at low and high fields, respectively. The excellent field-emission characteristics are attributed to the perfect crystalline structure and the taperlike geometry of the Si nanowires.     

Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 2. Germanium alkoxides

Structural investigations on the hydrolysis and condensation behavior of germanium alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that germanium alkoxides are monomeric in nature and undergo very fast hydrolysis and condensation reactions upon water addition. However, the chelation of germanium alkoxides by acetylacetone does not take place even 48 h after mixing, and any change in hydrolysis and condensation behavior is not observed after acetylacetone addition. When mixed with prehydrolyzed silicon alkoxide, the structures of germanium alkoxides are not modified. Both Si and Ge precursors are insensitive to the presence of each other in the reaction solution even after 48 h of aging. The addition of water to this mixture catalyzes the hydrolysis and condensation reactions very fast and leads to the formation of Ge-O-Ge (and consequently Si-O-Si) homocondensation products.     

Theoretical study on structures and stabilities of [H,Ge,C,N

Theoretical investigations are performed for the first time on the simplest hydrogenated germanium cyanide [H,Ge,C,N], whose analogs [H,C(2),N] and [H,Si,C,N] have been detected in space and laboratory, respectively. The detailed potential energy surfaces in both singlet and triplet states are constructed at the CCSD(T)/6-311+G(3df,2p)//B3LYP/6-31G(d)+ZPVE level, including 18 minimum isomers and 26 interconversion transition states. The former three low-lying and kinetically stabilized isomers are HGeCN (1)1 (0.0 kcal/mol), HGeNC (1)2 (5.1 kcal/mol), and cyclic cCHNGe(1)7 (11.1 kcal/mol). In addition, five isomers HCNGe (1)3 (33.8), HNCGe (1)5 (29.8), cNHCGe (1)8 (37.9), HGeCN (3)1 (30.1), and HNCGe (3)5 (26.5) each have considerable barriers, despite their high energies. Future laboratory characterization and astrophysical detection of the eight [H,Ge,C,N] isomers, especially the former three low-lying species (1)1, (1)2, and (1)7, are highly recommended. The accurate spectroscopic data at the QCISD/6-311G(d,p) level are provided. For some species, the CBS-QB3 calculations are also performed. Wherever possible, comparisons with the analogous [H,C(2),N] and [H,Si,C,N] are made on the structural, energetic, and bonding properties.