Precursor method—A nonconventional route for the synthesis of ZnCr2O4 spinel

https://doi.org/10.1016/j.jpcs.2013.04.007Get rights and content

Highlights

  • ZnCr2O4 was synthesized via precursor method.

  • The X-ray diffraction revealed the formation of cubic spinel structure.

  • The crystallite size was about 60 nm.

Abstract

Zinc chromites (ZnCr2O4) have been synthesized via thermal decomposition of (NH4)2[ZnCr2(C4O6H4)4(OH)2]∙9H2O and (NH4)3[ZnCr2(C6O7H11)4(OH)7]∙H2O precursors. The complex precursors have been characterized by infrared spectroscopy (IR), ultraviolet-visible spectroscopy (UV–vis), thermal analysis and scanning electron microscopy (SEM). ZnCr2O4 samples have been investigated and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible (UV–vis) and photolumininescence (PL) spectroscopy and Brunauer–Emmett–Teller (BET) N2 adsorption–desorption analyses. Nanoscaled (∼60 nm) single-phase zinc chromites were obtained by heating the precursors at 900 °C for 1 h. The influence of thermal treatment of tartarate precursor on structural, morphological and textural properties of ZnCr2O4 is discussed.

Introduction

Zinc chromite (ZnCr2O4) is a mixed oxide which crystallizes in the cubic system and has a normal spinel structure. Non-magnetic Zn2+ and magnetic Cr3+ ions have a strong preference for the tetrahedral A- and the octahedral B-sites, respectively. ZnCr2O4 is a geometrically frustrated antiferromagnet with a first order transition at 12.5 K from paramagnetic phase with cubic structure to antiferromagnetic phase with tetragonal structure [1], [2], [3], [4]. ZnCr2O4 is very attractive as air depollution catalytic material, for a variety of reaction like oxidation of hydrocarbons, oxidative dehydrogenation of hydrocarbons, synthesis of methanol [5], [6], [7], [8], as photocatalyst [9], [10], gas sensing [11] and humidity sensing [7], [12], [13], [14].

ZnCr2O4 has been synthesized by various methods including conventional ceramic route [5], [15], [16], [17], [18], [19], mechanical activation [20], [21], microwave process [22], spray pyrolysis [23], hydro/glycothermal route [10], [24]. It is well known that high surface area materials can facilitate the processes taking place at the phase boundary which is very important for their applications as catalysts or sensors [25]. For this reason, in recent years, several new synthesis methods of ZnCr2O4 have been developed: thermal decomposition of layered double hydroxides (LDHs) [25], sol–gel process [26], [27], metal-organic solution evaporation/calcination [28], [29], self-combustion reactions [30], [31].

It is very important to recognize that the proper choice of the precursor and knowledge of its composition and structure is crucial to tailor-make a pure product [22]. To our knowledge, a few studies have been focused on the understanding of the influence of chemical composition/morphology of precursors on the structure and morphology of spinel oxides [14], [23], [27], [28].

For this reason, in order to obtain spinel zinc chromite nanoparticles, we have selected a synthesis method belonging to wet chemistry—precursor method (thermal decomposition of polynuclear multimetallic compounds) [32].

There are many variants of the precursor method which are named according to the polycarboxylic/polyhydroxy carboxylic acids employed as complexation agent (e.g. citrate, tartarate, oxalate, etc.). Tartaric and gluconic acids enjoyed a special attention among the polyhydroxy carboxylic acids that can work as ligand, because of their multiple coordination capacity [33], [34], [35].

It is the aim of this paper to present the preparation and characterization of zinc chromite nanoparticles via the thermal decomposition of gluconate and tartarate multimetallic compounds.

Section snippets

Experimental

Chromium(III) nitrate (Cr(NO3)3∙9H2O), zinc(II) nitrate (Zn(NO3)2∙4H2O), tartaric acid (C4O6H6), δ-gluconolactone (C6H10O6), all of reagent quality, (Merck - Germany) and methanol from Lach-ner (Czech Republic) were used in the synthesis procedure.

In this work, our study was focused on the use of gluconate and tartarate coordination compounds as precursors for ZnCr2O4 spinel.

For this reason, the following systems were investigated:2Cr3+: 1Zn2+: 8C6O7H112Cr3+: 1Zn2+: 4C4O6H42−where C6O7H11

Results and discussion

The successful application of the precursor method for obtaining zinc chromite spinel requires: (i) a detailed study on the formation and characterization of polynuclear multimetallic compounds which function as precursors; (ii) a study of the thermal decompostion processes of precursors accompanied by the structural and morphological characterization of spinel phase.

Conclusion

The following conclusions can be drawn from this study:

  • (i)

    Zinc chromites have been synthesized using precursor method via thermal decomposition of (NH4)2[ZnCr2(C4O6H4)4(OH)2]·9H2O tartarate compound and (NH4)3[ZnCr2(C6O7H11)4(OH)7]·H2O gluconate compound. Both ZnCr2O4 precursors decompose up to 500 ºC with the formation of single-phase zinc chromite. It was shown that the well-crystallized cubic spinel structure is formed after calcination of tartarate/gluconate precursors at 900 °C/1 h. The average

Acknowledgments

Support of the EU (ERDF) and Romanian Government, that allowed for acquisition of the research infrastructure under POS-CCE O 2.2.1 Project INFRANANOCHEM—Nr. 19/01.03.2009, is gratefully acknowledged. The work also benefits from the support of the “Coordination and supramolecular chemistry” Programme of the “Ilie Murgulescu” Institute of Physical Chemistry, financed by the Romanian Academy.

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