Klaus Rademann

Nucleation and Growth of Gold Nanoparticles Studied via in situ Small Angle X-ray Scattering at Millisecond Time Resolution

Gold nanoparticles (AuNP) were prepared by the homogeneous mixing of continuous flows of an aqueous tetrachloroauric acid solution and a sodium borohydride solution applying a microstructured static mixer. The online characterization and screening of this fast process ( approximately 2 s) was enabled by coupling a micromixer operating in continuous-flow mode with a conventional in-house small angle X-ray scattering (SAXS) setup. This online characterization technique enables the time-resolved investigation of the growth process of the nanoparticles from an average radius of ca. 0.8 nm to about 2 nm. To the best of our knowledge, this is the first demonstration of a continuous-flow SAXS setup for time-resolved studies of nanoparticle formation mechanisms that does not require the use of synchrotron facilities. In combination with X-ray absorption near edge structure microscopy, scanning electron microscopy, and UV-vis spectroscopy the obtained data allow the deduction of a two-step mechanism of gold nanoparticle formation. The first step is a rapid conversion of the ionic gold precursor into metallic gold nuclei, followed by particle growth via coalescence of smaller entities. Consequently it could be shown that the studied synthesis serves as a model system for growth driven only by coalescence processes.

Size dependent catalysis with CTAB-stabilized gold nanoparticles.

CTAB-stabilized gold nanoparticles were synthesized by applying the seeding-growth approach in order to gain information about the size dependence of the catalytic reduction of p-nitrophenol to p-aminophenol with sodium borohydride. Five different colloidal solutions of stabilized gold nanoparticles have been characterized by TEM, AFM, UV-Vis, SAXS, and DLS for their particle size distributions. Gold nanoparticles (mean sizes: 3.5, 10, 13, 28, 56 nm diameter) were tested for their catalytic efficiency. Kinetic data were acquired by UV-Vis spectroscopy at different temperatures between 25 and 45 °C. By studying the p-nitrophenol to p-aminophenol reaction kinetics we determined the nanoparticle size which is needed to gain the fastest conversion under ambient conditions in the liquid phase. Unexpectedly, CTAB-stabilized gold nanoparticles with a diameter of 13 nm are most efficient.

Formation Mechanism of Colloidal Silver Nanoparticles: Analogies and Differences to the Growth of Gold Nanoparticles

The formation mechanisms of silver nanoparticles using aqueous silver perchlorate solutions as precursors and sodium borohydride as reducing agent were investigated based on time-resolved in situ experiments. This contribution addresses two important issues in colloidal science: (i) differences and analogies between growth processes of different metals such as gold and silver and (ii) the influence of a steric stabilizing agent on the growth process. The results reveal that a growth due to coalescence is a fundamental growth principle if the monomer-supplying chemical reaction is faster than the actual particle formation.

Formation Mechanism of Silver Nanoparticles Stabilized in Glassy Matrices

In any given matrix control over the final particle size distribution requires a constitutive understanding of the mechanisms and kinetics of the particle evolution. In this contribution we report on the formation mechanism of silver nanoparticles embedded in a soda-lime silicate glass matrix. For the silver ion-exchanged glass it is shown that at temperatures below 410 °C only molecular clusters (diameter <1 nm) are forming which are most likely silver dimers. These clusters grow to nanoparticles (diameter >1 nm) by annealing above this threshold temperature of 410 °C. It is evidenced that the growth and thus the final silver nanoparticle size are determined by matrix-assisted reduction mechanisms. As a consequence, particle growth proceeds after the initial formation of stable clusters by addition of silver monomers which diffuse from the glass matrix. This is in contrast to the widely accepted concept of particle growth in metal-glass systems, in which it is assumed that the nanoparticle formation is predominantly governed by Ostwald ripening processes.

Photoluminescence of atomic gold and silver particles in soda-lime silicate glasses

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.

Turkevich in New Robes: Key Questions Answered for the Most Common Gold Nanoparticle Synthesis

This contribution provides a comprehensive mechanistic picture of the gold nanoparticle synthesis by citrate reduction of HAuCl4, known as Turkevich method, by addressing five key questions. The synthesis leads to monodisperse final particles as a result of a seed-mediated growth mechanism. In the initial phase of the synthesis, seed particles are formed onto which the residual gold is distributed during the course of reaction. It is shown that this mechanism is a fortunate coincidence created by a favorable interplay of several chemical and physicochemical processes which initiate but also terminate the formation of seed particles and prevent the formation of further particles at later stages of reaction. Since no further particles are formed after seed particle formation, the number of seeds defines the final total particle number and therefore the final size. The gained understanding allows illustrating the influence of reaction conditions on the growth process and thus the final size distribution.

Dynamic plowing nanolithography on polymethylmethacrylate using an atomic force microscope

We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.

Electronic spectroscopy of fluorobenzene Van der Waals molecules by resonant two-photon ionization

Abstract Van der Waals (vdW) clusters of fluorobenzene (FB), synthesized in a seeded supersonic rare-gas expansion were studied by laser-induced, resonant two-photon ionization (R2PI) combined with TOF-mass spectrometry. The molecules were excited near the FB monomers vibronic origin of the S 1 (ππ*) ← S 0 transition (λ 00 = 2644 A). The heterogeneous clusters FB·Ar n ) ( n 00 induced by vdW interaction (FB·Ar: −23 cm −1 : FB·Ar 2 : −46 cm −1 : FB·Ar 3 : +4.6 cm −1 ). Additional satellite bands appeared due to intermolecular photofragmentation. A second band found for FB·Ar at 20 cm −1 was assigned to a vdW vibration (ν vdW = 43 cm −1 ). Similar results were obtained for FB·Kl n ??? ( n 2 and FB 3 were more complex. The dimer spectrum showed two broad spectral features, one blue- the other red-shifted relative to λ 00 . Each one is probably due to a different isomer. The blue-shifted contained progressions, which were tentatively assigned to a vdW vibration with 20 cm −1 in the ground and 15 cm −1 in the excited state. The trimer spectrum showed a broad blue-shifted absorption maximum with prominent bands at −2.6, −20, −29 and −50 cm −1 . From the observed spectra the feasibility of cluster-specific spectroscopy is discussed.

Mass-selected infrared photodissociation spectroscopy of V4O10+

The gas-phase infrared spectroscopy of V4O10+ produced by laser vaporization has been studied in the spectral region from 7 to 16 µm. Mass-selected V4O10+ cations were stored in a helium filled radio frequency hexadecapole ion trap and excited using tunable infrared radiation from a free electron laser. The photodissociation spectrum was recorded by monitoring the V4O8+ yield (O2 loss) as a function of the excitation wavelength. Two absorption bands at 842 and 1032 cm−1 are observed, which are assigned to resonant excitation of the antisymmetric V–O–V stretching and VO stretching vibrations, respectively. Comparison to recent theoretical and experimental studies indicate that the absorbing species consists of a V4O8+ ionic core weakly bound to an oxygen molecule.

Three-photon-induced luminescence of gold nanoparticles embedded in and located on the surface of glassy nanolayers

We report on the multiphoton-induced luminescence of gold nanoparticles embedded in thin glassy silicate–titanate films. The glassy layers doped with gold(III) chloride are synthesized by a sol–gel coating process. Gold nanoparticles are generated by subsequent annealing of the thin films at 300 ◦ C. Intensive near-infrared femtosecond laser irradiation also initiates the formation of gold particles, providing the possibility of spatially resolved photoactivation of the film. The reduction of gold ions to gold nanoparticles is monitored by Au L3-edge x-ray absorption near edge spectroscopy (XANES), UV–vis absorption spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particle sizes and shapes can be tuned by changing the metal concentration in the matrix. We demonstrate that the particles exhibit an efficient, long time stable, white luminescence during near-infrared Ti:sapphire femtosecond laser excitation. The laser power-emission intensity law indicates that the luminescence is induced by the absorption of three laser photons. Cross-sectional TEM images show that gold nanoparticles are both embedded in the glassy matrix and located on the film surface. Hence, the particles should be accessible for viable applications, for example as sensor materials, and could therefore become a powerful alternative to organic and semiconducting fluorophores in biological imaging. (Some figures in this article are in colour only in the electronic version)