Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Estimation of lattice strain in nanocrystalline RuO2 by Williamson–Hall and size–strain plot methods
Graphical abstract
Introduction
Ruthenium is a rare transition metal belonging to the platinum group and shows very interesting properties. Among the various transition metal oxides RuO2 is one of the most versatile materials due to its unique characteristics, such as wide bandgap (3.37 eV), metallic conductivity, high chemical stability, catalytic activities, electrochemical and redox properties [1], [2], [3]. RuO2 possesses a tetragonal rutile structure. In nanomaterial, both crystallite size and lattice strain due to large volume of grain boundaries have their own contribution to X-ray diffraction peak broadening [4]. Scherrer’s method is a well-known technique to predict the size of crystallites based on the width of the diffraction peak. But Scherrer’s formula provides only lower bound on the crystallite size because it has not taken into account the peak broadening resulting from other important factors such as inhomogeneous strain and instrumental effects. Although W–H analysis is an average method, it still holds an unavoidable position for crystallite size determination and used for dislocation distribution apart from TEM micrographs. The two main parameters extracted from peak width analysis are the crystallite size and lattice strain. Both these effects increase the peak width, intensity and shift the 2θ peak position accordingly. The lattice strains in nanomaterials are of two types. The first kind extends over the entire crystal while the other is localized over a few lattice spacing [5]. Such analysis results in a more realistic estimation of the crystallite size and could give information regarding micro strain. In order to measure the crystallite size correctly and to study the modifications introduced by strain in the properties of nanomaterials, strain calculations are required [6]. Hence the methods such as Warren–Averbach (W–A) analysis, Williamson–Hall (W–H) analysis, Fourier technique and Rietveld refinement could be used for separating the crystallite size and lattice strain contributions to the line broadening [7], [8], [9]. Warren–Averbach analysis requires at least two reflections along the same crystallographic direction in the higher angle reflections and Fourier technique involves convolution of size and strain broadening, and these methods are very difficult to carry out. W–H analysis is a simplified integral breadth method that clearly differentiates size induced and strain induced peak broadening by considering the peak width as a function of 2θ. There are few reports on the W–H analysis of nanostructured samples such as silver, alloy powder (AuxCu1−x), hydroxyapatite [Ca10(PO4)6(OH)2], zinc oxide, etc., in the literature [10], [11], [12], [13]. There is no report on W–H analysis of nanostructured RuO2 so far. Most of the works based on RuO2 nanoparticles were focused on the development of super capacitor and catalytic systems. This article reports microstructural parameters such as lattice strain and stress present in the as prepared and annealed RuO2 nanoparticles calculated from the X-ray diffraction pattern using W–H analysis namely uniform deformation model (UDM), uniform stress deformation model (USDM), uniform deformation energy density model (UDEDM) and size strain plot method (SSP). In addition the crystallite size values obtained from Scherrer and W–H methods are compared and further confirmed by direct TEM measurements.
Section snippets
Experimental section
All the chemicals (AR grade) were purchased from Sigma–Aldrich and used without further purification. RuO2 nanoparticles were synthesized by the hydrothermal method [14]. Ru(acac)3 (1.99 g) was dissolved in ethanol (20 mL) and poly ethylene glycol (5 mL) was added under vigorous stirring. Then NaOH solution (1 M) was added drop wise to adjust the pH to 12. After refluxing for 3 h, the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 100 °C for 4 h and then
FT-IR
FT-IR spectra of the as prepared and annealed samples are shown in Fig. S1 (see Supplementary material). A broad peak at 3452 cm−1 is attributed to the symmetric stretching vibration of O−H bond while the band at 1623/1655 cm−1 is due to O–H bending mode of the adsorbed water. Peaks at 1103/1105 and 1374/1386 cm−1 attributed to the vibration of C–OH are clearly visible in the FT-IR spectra of RuO2 nanoparticles. A strong peak at 620/625 cm−1 due to the vibrational frequency of Ru–O indicates the
Conclusions
In conclusion RuO2 nanoparticles were synthesized by the hydrothermal process. The line broadening has been analyzed by the Scherrer formula and modified forms of W−H analysis. From the results, it is observed that the strain decreases with increase in the particle size in annealed sample. Annealing caused changes in particle size, surface area, and porosity of RuO2 nanoparticles. The TEM result is in good agreement with the W–H and the SSP methods. BET analysis showed that the specific surface
Acknowledgement
One of the authors R. S. is thankful to the Department of Science and Technology, Government of India, New Delhi for the award of research fellowship under the DST PURSE programme.
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