The formation and consumption of nanometer scale precursor particles during the hydrothermal synthesis of Si-TPA-MFI from a clear solution has been studied in situ using a combination of X-ray scattering techniques and with electron microscpy. The combination of wide-, small-, and ultra-small-angle X-ray scattering allowed us to obtain information on a continuous range of length scales spanning over four orders of magnitude (0.17−6000 nm), covering all particle populations present during the complete course of the crystallization process. The use of high-brilliance synchrotron radiation allows us to perform time-resolved experiments. Two types of precursor particles were observed:  2.8 nm sized primary units and aggregates (≈10 nm). Variation of the alkalinity of the synthesis mixture revealed a strong correlation between the concentration of the aggregates and the rate of the crystal nucleation. The presence of the 2.8 nm sized primary units appears to be independent on the alkalinity. The addition of seed crystals to a synthesis mixture that does not show spontaneous nucleation (no aggregates observed) resulted in normal crystal growth. The size distribution of the growing crystals could be followed in situ by fitting calculated scattering patterns to experimental curves and showed good agreement with electron microscopy results. The apparent activation energy for crystal growth is determined to be 83 kJ/mol by following in situ the crystal growth process at various reaction temperatures. These data show that the formation of aggregates of primary units is an essential step in the nucleation process and suggest that the crystal growth step is the reaction-controlled inclusion of the 2.8 nm sized primary units at the crystal surface.

Aggregates of fractal dimension were found in the intermediate gel phases that organize prior to nucleation and crystallization (shown right) of silicalite from a homogeneous reaction mixture. Small‐ and wide‐angle X‐ray scattering studies prove that for zeolites nucleation may be homogeneous or heterogeneous.

The formation of precursors and the growth and aggregation of silicalite-1 crystals using tetrapropylammonium as a template (Si-TPA-MFI) has been studied in situ using X-ray scattering techniques. Simultaneous smallangle and wide-angle X-ray scattering (respectively SAXS and WAXS) experiments using synchrotron light showed that the formation of amorphous colloidal aggregates in water clear synthesis mixtures was dependent on the alkalinity of the solutions. In situ time-resolved ultra-small-angle X-ray scattering (USAXS) showed the form factor oscillations of the growing crystals. Fitting of the USAXS patterns to the scattering pattern of spherical particles having a normal particle size distribution showed a linear growth of the average crystal diameter, which was approximately the same for both alkalinities studied. The final size of the crystals was highest for the synthesis mixture having the highest alkalinity, which can be explained in terms of number of viable nuclei formed. At the end of the linear growth the crystals form aggregates corresponding with a diffusion limited aggregation process (mass fractal dimension of 1.8).

determined to be 50 and 2000 , respectively, using a calibrated oscillating quartz crystal thickness monitor. After deposition, the devices were encapsulated with epoxy (Loctite quick-set epoxy) under an argon atmosphere in order to minimize exposure to oxygen and moisture. All device measurements were made at room temperature.Photoluminescence and electroluminescence spectra were measured on a SPEX Fluorolog-2 equipped with a liquid N 2 cooled InGaAs detector (800± 1600 nm), or on a spectrometer consisting of an ISA-SPEX Triax 180 spectrograph equipped with a liquid N 2 cooled CCD detector (Hamamatsu CCD, 1024 64 pixel, 400±1100 nm). Emission quantum yields were measured by relative actinometry with H 2 TPP (u = 0.11) or ZnTPP (u = 0.033). Near-IR quantum yields were determined for Yb(TPP)L(OEt) on the CCD fluorescence system using the visible H 2 TPP and ZnTPP actinometers. Then the emission quantum yields for the Nd and Er complexes were determined relative to the Yb(TPP)L(OEt) using the SPEX Fluorolog-2 with the InGaAs detector.Power for electroluminescence (EL) measurements was supplied using a Keithley 228 voltage/current source. A 100 W primary standard quartz halogen lamp was used to calibrate the Triax 180 spectrograph/CCD detector system in irradiance units (lW cm 2 nm ±1 ). Measurements were made normal to the surface of the devices, and in the computation of the EL quantum efficiencies it was assumed that the spatial distribution of the emission was Lambertian [23]. External device quantum efficiencies were calculated as described in the literature [27].

Combined small and wide angle X-ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO 2 matrix of diatom frustules. One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra. These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance ( d -value ഠ 0.06 nm) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of Ϫ 4.0 to Ϫ 2.5, with two clear common regions among species: d Ͻ 10 nm (slopes -4, denoted as region I and also called the Porod region), and 10.0 Ͻ d Ͻ 40.0 nm (slopes Ϫ 2.9 to Ϫ 3.8, denoted as region II). The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO 2 matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl 3 to the growth medium. The results indicate that diatoms deposit silica with pores Ͻ 50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennate or centric, the formation of silica at a nanoscale level is a uniform process.

Poly(ethylene glycol) (PEG) was employed as templating agent for the synthesis of porous silica. The effect of PEG chain length and of the PEG/silica ratio on textural properties (fractality, pore size, and pore distribution) were investigated by monitoring the development of silica-PEG intermediates using ultrasmall and smallangle X-ray scattering analysis with high-brilliance synchrotron radiation to obtain sufficient radiation intensity for dynamic results at the subminute scale. We show that even a simple structure-directing polymer, such as PEG, results in silicas with pores of diameters spanning a range of less than 2 nm up to 20 nm, depending on polymer molecule length and polymer/silica ratio. Flocculation may well be the most important distinction between silicas prepared with small and large PEG. In this view, small PEG600 gets encapsulated by silica and forms pools within the silica framework, whereas large PEG20,000 is entangled in a mass of silica spheres, making enclosure by silica or phase separation impossible. Both polymer chain length and the polymer/silica ratio govern the relative importance of flocculation, phase separation, and hydrophobic silica-PEG interactions steering the silica polymerization. Increase in hydrophobicity results in a larger surface area and a more uniform pore size distribution, an effect confirmed by scanning electron microscopy (SEM) and observations of physical adsorption of nitrogen gas (BET). Polymers, such as PEG, may well be an inexpensive and versatile substitute and model for polypeptides known as structure-directing agents in biomineralization if silicas resembling natural ones, notably the ones present in such a huge diversity in algae of the group of diatoms, are the focus of scientific attention, e.g., for biomimicking with a view on industrial applications.

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The gel transformations and subsequent crystallization that occur in the precursor reaction mixture of silicalite were investigated using simultaneous small‐ and wide‐angle X‐ray scattering (SAXS–WAXS). The SAXS–WAXS measurements, together with the use of a high flux of synchrotron radiation and a newly developed high‐pressure reaction cell, provide the possibility of in situ hydrothermal and time‐resolved monitoring of amorphous gel transformations and crystallization.