Page 295 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 5 CHARACTERIZATION METHODS FOR NANOSTRUCTURE OF MATERIALS
References Since the most stable structure of a pure material is
crystal, where the atoms are periodically arranged, the
[1] T. Adschiri: Chem. Eng. Jpn., 66, 554 (2002). pattern of the positions and intensities of XRD peaks
[2] I.L. Medintz, H.T. Uyeda, E.R. Goldman and can be uniquely assigned to the material. Therefore,
H. Mattoussi: Nat. Mater., 4, 435–446 (2005). XRD measurement is important to identify the main
[3] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, component of materials. The most commonly used
A. Ohtomo, H. Koinuma and Y. Segawa: Appl. Phys. database for the identification is called JCPDS-
Lett., 72, 3270–3272 (1998). ICDD. JCPDS database consists of sets of data about
interplanar distances and intensities of the significant
[4] M. Kawasaki, A. Ohtomo, I. Ohkubo, H. Koinuma,
diffraction peaks. Since the diffraction angle 2 the
Z.K. Tang, P. Yu, G.K.L. Wong, B.P. Zhang and
interplanar distance d and X-ray wavelength are
Y. Segawa: Mater. Sci. Eng. B, 56, 239–245 (1998).
connected with each other by the Bragg’s law:
[5] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind,
E. Weber, R. Russo and P. Yang: Science, 292,
2d sin , (5.2.1)
1897–1899 (2001).
[6] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani,
the observed diffraction angle 2 can straightly be
T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke,
related to the interplanar distance d, applying the X-
Y. Segawa, H. Ohno, H. Koinuma and M. Kawasaki: Nat. ray wavelength used for the measurement.
Mater., 4, 42–46 (2005). A conventional powder X-ray diffractometer com-
monly used in laboratories is enough for the purpose
of evaluating the structures in most cases. Since the
5.2 Crystal structure
characteristics of the diffractometer with the conven-
tional design (Bragg–Brentano geometry) is well
5.2.1 X-ray diffraction method known, it is often more reliable than diffractometers
with more advanced designs. The CuK characteristic

X-ray diffraction (XRD) method is the most basic X-ray (mean wavelength: 0.15418 nm) is usually used
method for characterizing the crystal structures. as the X-ray source for the powder XRD measure-
Evaluation of crystal structure is generally important ments. Since the CuK characteristic X-ray (wave-

even for the nanoscale materials, the properties of length: 0.13922 nm) is also radiated from the X-ray
which might be affected by the structures of several tube with a Cu target, a Ni filter or curved graphite
to several hundreds nanometer scale, while the crys- monochromator is used to eliminate the XRD peak
tal structures are specified by the arrangement of caused by the K X-ray. CuK X-ray is a 2:1 mixture
atoms separated by about 0.1 nm. In some cases, the of K (0.15405 nm) and K (0.15443 nm) doublet.
2
1
crystal structure of each nanoparticle should be more It is recommended to use the value of K wavelength
1
appropriately characterized by electron beam diffrac- and the data obtained by eliminating the effect of K 2
tion (ED) method using a transmission electron subpeak to precisely determine the interplanar dis-
microscope (TEM) rather than XRD. However, XRD tance. The software for the Rachinger method [1],
method certainly has the following advantages: which is well known as a conventional method to
(i) measurements in the air or necessary atmosphere eliminate the K subpeak, is often attached as stan-
2
are allowed, (ii) the preparation of sample is easier, dard support software to a commercial powder X-ray
and (iii) the average crystal structures can be quanti- diffractometer. It should be noted that one should be
tatively evaluated. careful on interpretation of the data after the elimina-
The XRD and small-angle X-ray scattering (SAXS) tion of K subpeaks, because the application of the
2
methods are based on the measurements of X-ray method generally introduces additional errors into the
intensities scattered by the statistically distributed data. In the Rietveld method, which will be described
electrons belonging to the atoms in the material. The later, the raw data without applying elimination of K 2
arrangement of atoms or the population of electrons is subpeaks are usually used for the analysis.
determined by analysis of angular dependence of Combination of a scintillation counter and a receiv-
scattered X-ray in both the methods. The XRD ing slit has been conventionally used to detect the dif-
method is suitable to determine the crystal structures fracted X-ray, but an advanced type of diffractometers
by analyzing the positions and intensities of diffrac- attaching position-sensitive detectors (PSD) are cur-
tion peaks typically observed for the well-crystallized rently commercially available, which can virtually
material in the range of diffraction angle from 10 to improve the efficiency of detection. It is worth taking
150°. The method is also used for evaluating the into consideration the use of those new-type detection
microstructures (crystallite size and microstrain) by systems, especially for such cases where rapid meas-
analyzing the width and the shape of the peak pro- urement or reduction of statistical uncertainty is
files. The XRD method is sometimes called as wide- strongly required.
angle X-ray diffraction (WAXD) method, to be Evaluation of crystallite size by peak profile analy-
distinguished from the SAXS method. sis is one of the important applications of powder
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