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Ever since increasing a reaction’s yield while shortening the reaction time is the main objective in synthesis optimization. Microwave reactors meet these demands. In literature however their usage is under discussion due to claims of the existence of non-thermal effects resulting from the microwave radiation. Especially for nano-material syntheses it is of crucial importance to be aware of influences on the reaction pathway. Therefore, we compare ultra-small silver nanoparticles with mean radii of 3 nm, synthesized via conventional and microwave heating. We employed a versatile one-pot polyol synthesis of poly(acrylic acid) (PAA) stabilized silver nanoparticles, which display superior catalytic properties. No microwave specific effects in terms of particle size distribution characteristics, as derived by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS), are revealed. Due to the microwave reactor’s characteristics of a closed system, syntheses can be carried out at temperatures beyond the solvent’s boiling point. Particle formation was accelerated by a factor of 30 by increasing the reaction temperature from 200 °C to 250 °C. The particle growth process follows a cluster coalescence mechanism. A post-synthetic incubation step at 250 °C induces a further growth of the particles while the size distribution broadens. Thus, utilization of microwave reactors enables an enormous decrease of the reaction time as well as the opportunity of tuning the particles’ size. Possibly, decomposition of the stabilizing ligand at elevated temperatures results in reduced yields. A temperature of 250 °C and a corresponding reaction time of 30 s represent a compromise between short reaction times and high yields.
Zinc oxide nanostructures possess optical properties that are dependent on particle shape and size. Here, we report on the synthesis of elongated zinc oxide tubes via the in-situ aggregation of spherical particles. Using a custom-built, lab-based instrument, x-ray scattering can be investigated over more than three decades in scattering vector q, allowing for a complete investigation from atomic distances up to larger-than-nano structures.
For this study, the hydrolytic synthesis of stearate stabilized zinc oxide nanostructures in tetrahydrofuran was performed and the reaction mixture was continuously fed through the SAXS/WAXS apparatus by means of a peristaltic pump. For comparison to nanospheres, oleate-functionalized zinc oxide particles were synthesized in a microwave-assisted fashion, and depending on reaction temperature, sphere radii could be adjusted between 2.6 and 3.8 nm, changing the optical and crystal lattice properties.
The evaluation of the in-situ measurements showed that at the beginning of the synthesis of the stearate-stabilized zinc oxide, similar, spherical particles are formed as in the oleate-based synthesis. In contrast, as the reaction progresses, the stearate-capped particles aggregate into elongated rods with radii of a few nanometres, which eventually form the nanotubes. These have radii of 30-50 nm and lengths of several hundred nanometres. Nevertheless, these structures still possess optical properties like the ultra-small zinc oxide spheres, i.e. a bright, yellow fluorescence. Therefore, we assume that the originally formed, ultra-small spheres, are still present within the tube structure, but separated by stearate, and thus determine their fluorescence properties.
Zinc is an essential trace element and is ingested daily by humans, partly in dissolved form. The first contact is with saliva, in which many ions are dissolved whose solubility product with zinc can be low. This could result in compounds forming, possibly in nanoparticular form, which could have different effects on the organism than pure zinc ions. Available biopolymers, e.g. α-amylase, can in turn stabilise as-formed nanostructures.
In this study, we report on the saliva stage of the artificial digestion of zinc chloride as a model substance for zinc ions. To facilitate in situ measurements, the sample is continuously passed through a small-angle x-ray scattering (SAXS) system. This custom-made machine is capable of measuring over a wide q-range and thereby able to resolve structures from around 250 nm down to the crystal structure. It is thus an excellent tool for investigating both the particle size distribution and the atomic structure of the sample.
By curve fitting (see Figure 1), we found that shortly after addition of zinc chloride to saliva, small particles with a mean radius of 1.9 ± 0.1 nm and a distribution width of 0.6 ± 0.1 nm formed. These particles are aggregated to compact mass fractals with a fractal aggregate size of 14.7 ± 0.1 nm and a fractal dimension of 2.96 ± 0.02. Approximately 7200 single particles stick together by protein and form mass fractals, whose radii of gyration were found to be 36 ± 1 nm. To determine the compound that was formed, infrared spectroscopy was used in addition to the SAXS measurements, and zinc phosphate was identified as the product.
Ever since increasing a reaction’s yield while shortening the reaction time is the main objective in synthesis optimization. Microwave reactors meet these demands. In literature however their usage is under discussion due to claims of the existence of non-thermal effects resulting from the microwave radiation. Especially for nano-reference-material syntheses it is of crucial importance to be aware of influences on the reaction pathway. Therefore, we compare ultra-small silver nanoparticles with mean radii of 3 nm, synthesized via conventional and microwave heating.
We employed a versatile one-pot polyol synthesis of poly(acrylic acid) (PAA) stabilized silver nanoparticles, which display superior catalytic properties. No microwave specific effects in terms of particle size distribution characteristics, as derived by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS), are revealed. Due to the microwave reactor’s characteristics of a closed system, syntheses can be carried out at temperatures beyond the solvent’s boiling point. Particle formation was accelerated by a factor of 30 by increasing the reaction temperature from 200 °C to 250 °C. The particle growth process follows a cluster coalescence mechanism.
A post-synthetic incubation step at 250 °C induces a further growth of the particles while the size distribution broadens. Thus, utilization of microwave reactors enables an enormous decrease of the reaction time as well as the opportunity of tuning the particles’ size. Possibly, decomposition of the stabilizing ligand at elevated temperatures results in reduced yields. A temperature of 250 °C and a corresponding reaction time of 30 s represent a compromise between short reaction times and high yields.
Zinc oxide (ZnO) as a wide-bandgap II-VI semiconductor finds application in areas like optoelectronics, photocatalysis as well as in detection systems. While band-gap engineering in macroscopic ZnO can be performed by alloying, the band-gap of ZnO nanoparticles is also dependent on their size. Since small-angle X-ray scattering (SAXS) provides a much higher resolution in terms of ultra-small nanoparticle size analysis compared to other techniques, it allows for a careful examination of the correlation between particle size and band-gap.
We report on the microwave-assisted synthesis of oleate-capped, photoluminescent zinc oxide nanoparticles with adjustable size as dispersions in organic solvents. The spherical particles were obtained by hydrolysis of the metal precursor in presence of a strong base at temperatures above the solvent’s boiling point. Hence, the reaction is dramatically accelerated and within seconds – instead of hours at lower temperatures –, narrowly dispersed particle systems are yielded. The particles’ sizes as derived from SAXS strongly depend on the reaction temperature and time. Choosing the right reaction conditions, the particle size and thus their band gap can be finely tuned. A size increase can be achieved both by increasing the reaction temperature and the reaction time.
See Figure 1 for an exemplary comparison of five-minute syntheses at different temperatures. Here, the yielded particles display diameters between 5.0 and 7.6 nm and corresponding band-gaps of 3.32 up to 3.41 eV. The size increase is accompanied by a red-shift of the UV/Vis absorption edges and fluorescence emission.
Furthermore, these particles can be transferred into water by coating with polysorbates.
Zinc oxide (ZnO) nanoparticles find manifold applications, most prominently in photovoltaics, where their unique optical properties are exploited. Particularly important are their band gap energy of 3.37 eV, which can be widely tuned through doping, and a large exciton binding energy of 60 mV. As a wide-bandgap II-VI semiconductor, the optical band gap energy and fluorescence energy become size-dependent when moving to particle radii of a few nanometers.
To gain a deeper insight into this issue, we report on a microwave-assisted, size-selective synthesis of pure ZnO nanoparticles. By hydrolysis of the metal precursor in presence of a strong base at temperatures exceeding the solvent’s boiling point, the reaction is dramatically accelerated, and narrowly dispersed, spherical particles are yielded within seconds – instead of hours at lower temperatures. The determination of their size distributions in high resolution using small-angle x-ray scattering (SAXS) allows for a precise mapping of the optical properties (UV/Vis absorption and fluorescence) to particle size.
We observed that the mean particle radii increase from 2.6 ± 0.1 nm with increasing synthesis temperature from 125 °C to 200 °C. This is accompanied by a red shift of the optical band gap and the fluorescence energies, the latter of which can be seen in Figure 1. Thus, undoped ZnO nanoparticles with narrow size distributions and pre-defined size as well as optical properties can be obtained through a microwave-assisted synthesis.
Zinc is an essential trace element and is ingested daily by humans, partly in dissolved form. The first contact is with saliva, in which many ions are dissolved whose solubility product with zinc can be low. This could result in compounds forming, possibly in nanoparticular form, which could have different effects on the organism than pure zinc ions.
In this study, we report on the saliva stage of the artificial digestion of zinc chloride as a model substance for zinc ions. To facilitate in situ measurements, the sample is continuously passed through a small-angle x-ray scattering (SAXS) system. This custom-made machine is capable of measuring over a wide q-range and thereby able to resolve structures from around 250 nm down to the crystal structure. It is thus an excellent tool for investigating both the particle size distribution and the atomic structure of the sample.
By curve fitting, we found that shortly after addition of zinc chloride to saliva, small particles with a mean radius of 1.9 ± 0.1 nm and a distribution width of 0.6 ± 0.1 nm formed. These particles are aggregated to compact mass fractals with a fractal aggregate size of 14.7 ± 0.1 nm and a fractal dimension of 2.96 ± 0.02. Approximately 7200 single particles form each mass fractal, whose radius of gyration was found to be 36 ± 1 nm. The growth of these structures continues over the course of several weeks. To determine the compound that was formed, infrared spectroscopy was used in addition to the SAXS measurements, and zinc phosphate was identified as the product.