Nanoscaled materials are becoming increasingly important in various applications such as coatings, paints, cosmetics, ceramics, polymers, catalysis and drug delivery. Even at relatively low concentrations such additives may be highly effective in delivering significant improvements in the desired field of application. The properties and performance characteristics of nanoparticles are to a large extent determined by their size distribution, specific surface area, and degree of dispersion. Similarly, with porous materials containing nanosized pores, as used e.g. in delivery systems, separation processes or catalysis, the pore size distribution and specific surface area are key quality parameters. It is thus essential to control and quantify these aforementioned properties.
With most available experimental methods this task becomes increasingly challenging with decreasing sizes below 100 nm. X-rays having a wavelength of the order of 1 Å are widely used in diffraction experiments for the analysis of crystalline materials with atomic resolution. Powder diffraction allows for qualitative and quantitative analysis of crystalline phases, as well as for the determination of crystallite size.
When extending such wide-angle X-ray diffraction (XRD, WAXS) measurements down to very small angles, one is probing the electron density distribution on increasingly larger length scales in the range of nanometers. The small-angle X-ray scattering (SAXS) technique [1-2] is thus ideally suited for the structural characterization of nanoscaled materials, and among others, allows for nanoparticle and pore size analysis and specific surface area determination. It is important to note that crystallite size and particle size are usually not the same. XRD and SAXS thus yield complementary structural information.
In this paper we present how SAXS measurements can be performed on a multi-purpose X-ray diffractometer platform. Application examples for nanopowders, colloids, nanocomposites and porous materials will be given. The advantages of SAXS as compared to other available measurement techniques will also be discussed.
A multi-purpose X-ray diffractometer platform was configured for small-angle X-ray scattering (SAXS) measurements. This technique was applied to nanoparticle and pore size distribution determination in nanopowders, colloidal dispersions, nanocomposites and porous materials in a range of 1-100 nm. SAXS is extremely versatile and yields truly ensemble-averaged results from a large sample volume, does not require knowledge about the refractive index or any other physical properties, and it is applicable to crystalline and amorphous materials alike.
Furthermore sample preparation is minimal. Advanced data analysis software allows to reveal complex multimodal size distributions with unrivaled resolution. Data acquisition and analysis can be automated for routine production control. The instrument platform used can be reconfigured within minutes for a multitude of complementary X-ray analytical techniques, such as powder diffraction, thin film analysis, and computed tomography.
Setups for SAXS measurements require a narrow, highly collimated and intense X-ray beam, the effective suppression of any parasitic scattering, and a detector with a high linearity range. The objective is to measure the scattered intensities in the immediate vicinity of the direct beam, typically down to 0.1° and below. The smallest accessible scattering angle determines the upper limit of the structural feature size (e.g. particle diameter) that can be studied.
A multi-purpose X-ray diffractometer platform (Empyrean, PANalytical), which can be configured for a variety of applications by choosing the optimal combination of optical modules, sample stages and detectors, was used. Typical applications on this platform range from the identification of unknown crystalline phases and quantification of mixtures, to the determination of residual stress and preferred orientation of crystallites in bulk materials as well as in thin films. It may also be configured for computed X-ray tomography. The system makes use of pre-aligned fast interchangeable X-ray (PreFIX) modules. Thus there is no need for realignment when changing between different setups.
Using this platform, SAXS measurements were done in a transmission geometry (see Fig. 1) using Cu Kα radiation (wavelength λ = 0.154 nm) from a long fine focus X-ray tube powered by a high voltage X-ray generator at 45 kV and 40 mA. A well-collimated, intense, monochromatic X-ray beam was obtained by using an X-ray mirror. The size of the incident beam at the sample position was set to approx. 20 mm x 50 µm by using a combination of low-angle slits and a beam mask.
Figure 1: Schematics and photograph of experimental SAXS setup: 1: X-ray tube, 2: beam defining slits and X-ray mirror, 3: sample stage, 4: anti-scatter slit and detector.