Original synthesis of chromium (III) oxide nanoparticles

https://doi.org/10.1016/j.jeurceramsoc.2009.09.019Get rights and content

Abstract

Nanosized chromium (Cr2O3) oxide was prepared by the common thermal decomposition of Cr(NO3)3·9H2O chromium (III) nitrate nonahydrate. Prior to the heat treatment at 550 °C, the commercial reagent was first dissolved in a colloidal silica solution and then dried at a low temperature to slowly evaporate the aqueous solvent. The SiO2/Cr(NO3)3·9H2O weight ratio (R) was changed from 0 to 2. The various Cr2O3 powders were characterized by XRD, FTIR, nitrogen adsorption, SEM and TEM techniques. A maximum specific surface area of 113 m2 g−1, associated with a pore volume of 0.72 cm3 g−1, was obtained for the Cr2O3 powder prepared with R = 2. These pristine chromium oxide nanoparticles, with a slightly sintered sphere-shaped morphology, exhibited a 10 nm particle size with a monocrystalline character as demonstrated by the TEM and XRD correlation.

Introduction

The chromium (III) oxide (Cr2O3) material is being widely investigated nowadays due to its numerous application domains, including green pigments,1 heterogeneous catalysts,2 coating materials for thermal protection3 and wear resistance.4 It is considered to be an important refractory material with its high melting point temperature (2435 °C) and oxidation resistance, although its sintering ability is very poor and requires special sintering conditions so that a high density can be achieved. To overcome these hindrances, the preparation of nanosized Cr2O3 particles is currently the subject of a growing concern.

The basic and traditional process used by the manufacturers consists of reducing an alkali dichromate by sulphur, carbon, wood or ammonium chloride,5 as described by the following chemical reaction:Na2Cr2O7 + S  Cr2O3 + Na2SO4

Although simple and cheap, this process leads to the synthesis of large agglomerated particles.

In the literature many preparative techniques for Cr2O3 nanoparticles are described, including a hard-template pathway with mesoporous silica,6 hydrothermal reduction,7, 8 solution-combustion synthesis,9 hydrazine reduction and thermal treatments,10 urea-assisted homogeneous precipitation,11, 12 mechanochemical processing,13 supercritical alcohol,14 laser-induced pyrolysis,15 sonochemical reaction,16 microwave plasma,17 condensation–polymerization process,18 precipitation–gelation reaction19 and gas condensation.20 For most of them a large particle size distribution, low yields or important costs are the major disadvantages.

In the present study, a simple and original method for the synthesis of nanosized Cr2O3 chromium oxide is described. This novel process involves the use of chromium (III) nitrate nonahydrate and nanometric silica spheres as chromium precursor and template agent, respectively. The surface properties, size, morphology and crystallographic structure of the as-made Cr2O3 particles are investigated and discussed in the following sections.

Section snippets

Experimental

The synthesis was achieved by means of commercially available reagents without further purification. Chromium (III) nitrate nonahydrate (Cr(NO3)3·9H2O—99%) and Ludox® HS-40 were purchased from Sigma–Aldrich. Hydrofluoric acid (40%) was obtained from Fischer Scientific.

The preparation of the chromium oxide was performed as follows: the Cr(NO3)3·9H2O reagent was dissolved in a 40% dispersion of 12-nm silica particles in water. The weight ratio of the SiO2/Cr-based precursor (R) was fixed at 0,

Results and discussion

The process developed in the present study is based in the utilization of the interstitial spaces of a silica building as nano-reactors for the synthesis of inorganic nanoparticles. In our case, the elaboration of a building template is based on the capacity of silica nanospheres, dispersed in an aqueous solution, to self-assemble into different packing domains (simple cubic packing, face-centred cubic or hexagonal close packing) from the liquid phase removal by evaporation. The most probable

Conclusion

Chromium trioxide Cr2O3 nanoparticles were successfully synthesized via an easy and fast route by using an aqueous dispersion of nanosized silica spheres as template agent. With a [silica/chromium precursor] weight ratio equal to 2, the chromium (III) oxide powder with a specific surface area and a pore volume of 113 m2 g−1 and 0.72 cm3 g−1, respectively, was synthesized. This sample is composed of highly agglomerated nanoparticles with an average diameter of 10 nm. To our knowledge, this is the

Acknowledgments

The authors are grateful to Dr. Denis Spitzer for fruitful discussions.

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