X-ray absorption fine structure spectroscopy in nanomaterials

REVIEWS SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 10 April 2015 | doi: 10.1007/s40843-015-0043-4 Sci China Mater 2015, 58: 313–341 X-ray absorption fine structure spectroscopy in nanomaterials Zhihu Sun, Qinghua Liu, Tao Yao, Wensheng Yan and Shiqiang Wei X-ray absorption fine structure (XAFS) spectroscopy has been widely used for decades in a wide range of scientific fields, including physics, chemistry, biology, materials sciences, environmental sciences, etc. This review article is devoted to the applications of XAFS in nanomaterials. The basic principles of XAFS are briefly described from the view point of practical application, including its theory, data analysis and experiments. Using selected examples from recent literatures, the power of XAFS in determination of local atomic/electronic structures is illustrated for various nanomaterials, covering metal and semiconductor nanoparticles, catalysts, core/shell structures, ultrathin nanosheets, and so on. The utilization of time-resolved XAFS technique is also briefly introduced, for in-situ probing the nucleation/growth processes of nanomaterials and identifying reaction intermediates of nanostructured catalysts under operando conditions. INTRODUCTION With the emergence of nanoscience and nanotechnology in the late 1980s [1], they have risen dramatically to be one of the most important research areas in the 21st century. The nanoscience and nanotechnology include the study of objects and systems with at least one dimension in the nanometer scale (typically 1−100 nm) [2]. The objects studied in such a size range are larger than atoms and small molecules but smaller than macroscopic bulk structures. The dimensions of these systems are comparable to the characteristic length scales that define the overall properties of materials. As a result, many of the physical and chemical properties of the nanometer-scaled materials (nanomaterials) are changed relatively to their bulks, and numerous unique behaviors of the nanomaterials emerge, like the quantum size-effect, quantum confinement effect, near-field optical effects, electron tunneling, and so on. Further development of new and improved nanomaterials requires the ability to control their structure at smaller and smaller scales and complete understanding of their behaviors at nanoscale. With these improved abilities, there will be great potential to create a rich diversity of materials with novel characteristics, functions and applications. To synthesize nanomaterials in a controllable manner and understand their unique and interesting properties, thorough characterizations of these materials in either static or transient states are essential. Inspired by these motivations, many new techniques (for instance, the atomic force microscope [3] and the scanning tunneling microscope [4]) have been developed and many traditional techniques (like X-ray absorption fine structure (XAFS) reviewed here) have been extensively applied to this rapidly developing field. Many other spectroscopic techniques have been found wide applications in nanomaterials. For example, optical absorption, photoluminescence and Raman spectroscopy have shown their power in investigating the elemental excitations in nanomaterials, but these are beyond the scope of this article, which aims at reviewing the application of XAFS in the determination of atomic structure and electronic structures in nanomaterials. The last four decades has witnessed the great development of XAFS spectroscopy, as well as its wide applications in nanomaterials. XAFS refers to the oscillatory fine structure of the X-ray absorption coefficient μ(E) that changes as a function of the incident X-ray photons with energy E beyond the absorption edge E0 of a specific atom. Its most outstanding features are the sensitivity to short-range order and element-specificity, which enable them to selectively probe the environments surrounding a specific element in solids, liquids, and even gases [5,6]. The development of XAFS as a powerful tool of structure characterization is associated with the development of an effective scattering theory to express the essential physics, and with the availability of synchrotron radiation (SR) light sources that emit tunable and brilliant X-ray photons. Since 1970’s, the number of XAFS experiments performed has grown exponentially and XAFS has been routinely used as a local structure probe nowadays. Owing to the building of SR light sources with high-brilliance, numerous measurement methods have been developed to obtain high quality XAFS data under various conditions, such as transmission XAFS for usual bulk material, fluorescence XAFS for trace element, grazing-incidence XAFS for surface research, mag- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China * Corresponding author (email: ) 313 April 2015 | Vol.58 No.4 © Science China Press and Springer-Verlag Berlin Heidelberg 2015 REVIEWS SCIENCE CHINA Materials BASIC PRINCIPLES OF XAFS In the X-ray spectral region, the impinging photons interact with matter through the dominant photoelectric absorption. The basic physical quantity measured in XAFS is the X-ray absorption coefficient μ(E), which describes how the X-ray absorption of a sample changes as a function of the incident X-ray photons energy E. At specific energies, the X-ray photon has sufficient energy to liberate electrons from the low-energy bound states in the absorbing atoms, and causes a sudden increase in the absorption coefficient μ(E). These energies are called element-specific X-ray absorption edges. Excitation of electrons from 1s, 2s, 2p1/2, and 2p3/2 corresponds to the K, L1, L2, and L3 edges, respectively. Across the absorption edge, the X-ray absorption coefficient μ(E) exhibits a jump, followed by oscillatory structure at higher energies [5]; XAFS then describes the details of X-ray absorption at energies near and above the absorption edge. Fig. 1 shows the Cu K-edge spectrum of copper foil, demonstrating the features of a typical XAFS spectrum. The oscillatory structure observed over a wide energy range above the edge, roughly covering typically 30–50 eV until 1000 eV or more, is generally called extended X-ray absorption fine structure (EXAFS) spectroscopy. The spectral region extending the energy range from the absorption edge (or a little below) to 30–50 eV above it, is commonly referred to as X-ray absorption near-edge structure (XANES). The distinction between XANES and EXAFS is somewhat arbitrary and there is no fundamental difference in the physics giving rise to the fine structures. However, their complexity and analysis methods are quite different. In the EXAFS region, the excited photoelectrons have high kinetic energy and are weakly affected by the neighboring atoms’ potential; therefore, the single-scattering processes of the photoelectrons by the neighbors are domina (...truncated)