Structure of Ce1–xSnxO2 and its relations to oxygen storage property from first-principles analysis

Supplementary Information

Asha Gupta1, Anil Kumar2, M. S. Hegde3 and U. V. Waghmare2*

1 Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

2 Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,

Bangalore 560064, India

3 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.

* Corresponding author’s email:

Experimental Techniques

CeO2, SnO2, Ce0.75Sn0.25O2and Ce0.5Sn0.5O2 have been prepared by solution combustion method.1 SnC2O4 is used as a precursor for SnO2; and is synthesized by dissolving SnCl2 in acidic water giving clear solution followed by precipitation of SnC2O4 crystals by slowing adding saturated oxalic acid solution. A typical combustion reaction for preparation of SnO2, CeO2 and Ce1–xSnxO2 can be written as follows:

9(1–x)(NH4)2Ce(NO3)6 + 9xSn(NO3)4 + 4(6–x)C2H5NO2 9Ce1–xSnxO2 + 8(6–x)CO2 +

(96–46x)H2O + (48–26x) N2

For example, preparation of Ce0.5Sn0.5O2 was carried out by taking 5.48 g (0.01 mol) of (NH4)2Ce(NO3)6 (Loba Chemie, 99%), 2.07 g (0.01 mol) of SnC2O4 (precipitated from SnCl2, Sigma Aldrich, 99.9%), and 3.67 g (0.049 mol) of glycine (C2H5O2N, Merck, 99%) in a 300 mL crystallizing dish, and dissolved in minimum volume of HNO3 and 20 mL of water to form a clear solution. The dish was then kept in a pre-heated furnace at 320 oC. The combustion started after dehydration, and the product was obtained within 60 seconds.

The powder XRD studies of the product were recorded on Philips X’Pert diffractometer at a scan rate of 0.12o min–1 with 0.02o step size in the 2θ range between 20 and 100 degrees. The Rietveld refinement of the powder XRD patterns were carried out using FullProf–fp2k program2 by simultaneously varying 18 parameters that include the overall scale factor, background parameters, unit cell, half width, shape, and isotropic thermal parameters along with the oxygen occupancy.

Results and Discussions

XRD: The Rietveld refinement of the powder XRD patterns of CeO2, SnO2 and Ce0.5Sn0.5O2 shows that the profile fitting is good (Figure S1). All the diffraction lines in the X–ray diffraction pattern of SnO2 could be indexed to rutile structure (JCPDS No. 770449), and those of CeO2 and Ce0.5Sn0.5O2 could be indexed to fluorite structure (JCPDS No. 340394). No diffraction lines corresponding to SnO or SnO2 were observed in the powder diffraction pattern of Ce1–xSnxO2. Our estimates of the lattice parameter, Rf, RBragg and χ2 values after complete refinement (see Table 1), show that the lattice parameter of the solid solution decreases monotonously with Sn concentration (x), which is expected from smaller ionic radius of Sn4+ ion (r = 0.81 Å) than of Ce4+ ion (r = 0.97 Å).

TEM: High-resolution TEM (HRTEM) image of SnO2 and Ce0.75Sn0.25O2(Figure S2), and the corresponding selected area electron diffraction (SAED) pattern were recorded. Crystallite sizes for SnO2 are in the range of 15–20 nm. HRTEM image shows lattice fringes of 3.35 Å corresponding to (110) planes of rutile-SnO2, and distinct lattice fringes and ring pattern indicate that the particles are highly crystalline in nature, and amorphous phase was not detected. HRTEM image for Ce0.75Sn0.25O2 shows lattice fringes of 3.07 Å corresponding to (111) planes of the solid solution. Crystallites sizes are within the range SAED of the solid solution show distinct ring patterns for the fluorite structure and ring pattern corresponding to rutile phase were not observed. Well defined lattice fringes and diffraction pattern indicates that the particles are crystalline in nature. Therefore absence of SnO2 phase, within our detection limits, and significant decrease in lattice parameter (from Rietveld analysis) indeed show that Sn4+ ion are substituted for Ce4+ forming Ce0.75Sn0.25O2 solid solution.

Figure S1. Reitveld refined XRD patterns of (a) CeO2, (b) SnO2 and (c) Ce0.5Sn0.5O2.

Figure S2. HRTEM of (a) SnO2, (b) Ce0.75Sn0.25O2, the inset shows the selected area diffraction (SAED) pattern.

Reference

1T. Baidya, A. Gupta, P. A. Deshpandey, G. Madras, and M. S. Hegde, J. Phys. Chem. C 113 (10), 4059 (2009).

2J. Rodriguez-Carvajal, Multi-pattern Rietveld Refinement program Fullprof. 2k, version 3.30 June 2005-LLB

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