The synthesis of flower shaped microstructures of Co₃O₄ by solvothermal approach and investigation of its catalytic activity

Highlights

• Flower like microstructures.

• Prepared by solvothermal method.

• Effect of solvent on morphology.

• Formation mechanism.

• Catalytic activity.

Abstract

A unique flower like microstructures of cobalt oxide (Co₃O₄) are prepared by solvothermal method in two steps: In first step the Co(OH)₂ micro particles are generated and in second step Co₃O₄ flower like microstructures are obtained after annealing for 2 h. The final product of Co₃O₄ is subjected to different instrumental analysis to investigate its morphology and other properties. It is clear from the Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) that the product is flower like in shape with a size of approximately 10–15 μm from sideway view. It is also observed that these structures are composed of nanosheets, which are further organized into layer by layer pattern to form a specific flower like morphology. The experimental data proved that the solvent and temperature played a significant role in the formation of such flowers like morphology under provided reaction conditions. These microstructures have shown remarkable catalytic activity for thermal decomposition of ammonium perchlorate (AP).

Introduction

Nano and micro materials have gained significant importance in recent years. Currently the focus in research is inclined towards these materials because of their novel fundamental physical properties and potential applications into various domains. In particular morphologically controlled synthesis of metal oxides at nano and micro scale by using bottom-up approach has achieved attention as promising method of synthesis [1]. The chemical and physical properties of solid materials dependent upon both size and shape of their constituent microscopic particles. In these materials the ratio of surface area to volume is significantly increased that helps to study quantum mechanical effects such as the quantum size effect [2]. The properties of the metal oxides strongly depend on their morphologies and structures including crystal size, orientation, stacking manner, aspect ratio, and even crystalline density [3]. The structure (microstructure), size, and morphology have significant impact on the material properties and their applications. In this way, the novel properties of nano and micro materials can be obtained by controlling the morphology of inorganic nanomaterials by implementing the proper synthetic strategies [4].

The Co₃O₄ refers to spinel crystal structure taken from a cubic close packing array of oxide ions, in which Co(II) ions hold the tetrahedral 8a sites and Co(III) ions have the octahedral 16d sites and that arrangement make it an outstanding p-type magnetic semiconductor [5], [6]. It is found that the size, crystallinity, and morphology of nanoparticles have different effects on electrochemical performance. Better crystallinity can enhance the initial discharge capacity while porous structure can reduce the irreversible loss [7]. The Co₃O₄ shows high discharge capacity as the anodic material in Li-ion rechargeable batteries and exhibits excellent sensitivity to hydrogen and alcohol because of its higher specific surface area. It has potential applications in solid-state sensors, heterogeneous catalysts, electrochromic devices, photovoltaic [8], [9], homogeneous catalysis [10], [11], pigments [12], [13], [14] and magnetic materials [15], [16]. Currently, the Co₃O₄ with different morphologies including nanocubes [17], nanowire [18], nanospheres [19] and nanotubes [20] have been extensively synthesized by thermal decomposition of intermediate compounds such as carbonates, hydroxides, sub carbonates etc. [21]. Many research groups have demonstrated the application of Co₃O₄ nanofibers in sensitive and selective non-enzymatic glucose detection and the Co₃O₄ nanofibers modified electrode showed excellent performance towards glucose detection in an alkaline medium [22], [23], [24]. Cobalt and its derivatives have also porous and other modified surfaces that are important in adsorption and desorption properties in the field of gas sensors [25].

A significant research effort has been made to design effective methods to synthesize Co₃O₄ nanomaterials with controllable microstructures. Diverse synthetic methods, such as hydrothermal/solvothermal methods, precursor conversion processes, chemical vapor deposition synthesis, and soft-chemical growth routes have been employed to prepare various Co₃O₄ micro/nanostructures with different morphologies including crystalline, spinal, spherical, cuboid, octahedron (diamond shaped), nanosheets, nanocubes, nanotubes, nanoboxes and nanobelts. Fabrication of nano and microstructures of crystalline and porous Co₃O₄ is a milestone achieved by material chemists due to its significant features [26].

The Co₃O₄ is an active material with wide applications and has attracted much attention to the shape-controlled synthesis by various routes such as wet chemical methods, nanocasting, thermal oxidation, mechano-chemical reactions, etc. Among these, the wet chemical methods are the most adapted method for the morphologically controlled synthesis of Co₃O₄ at micro/nanoscale [27]. Yan et al. studied the shape-controlled fabrication of the porous Co₃O₄ nanoflower clusters for the efficient catalytic oxidation of gaseous toluene and proved to be good catalyst [28]. Teng et al. stated the synthesis of porous Co₃O₄ nanorods with a narrow pore-size distribution and the bulk Co₃O₄ by the hydrothermal and the thermal decomposition methods and described the high combustion activity of CH₄ and luminescence properties of CO oxidation [29]. Jiao et al. prepared the hollow Co₃O₄ microspheres by gas–liquid diffusion reaction in the presence of ionic liquid [Bmim][BF₄] in combination with calcination at 300 °C. The hollow Co₃O₄ microspheres showed excellent sensitivity to ethanol vapor at a lower operating temperature [30]. Yang et al. demonstrated the selective and non-selective growth of crystal planes of Co₃O₄ directed by the differently charged surfactants/solvents to control the morphology of Co₃O₄ crystals with spheres and cubes in shape [31].

One of the main objective of this study is to synthesize cobalt oxide microsized flowers composed of nanosheets. This work shows the morphological aspects of the prepared cobalt oxide along with their formation mechanism. During the fabrication process, the main emphasis is to discover the role of solvent, precursor, surfactant and reaction conditions in order to develop the understanding of following factors: 1) The optimal condition to achieve desired morphology and size distribution of particles. 2) The time-dependent complete formation mechanism of cobalt oxide particles from spherical to flower shaped structures. The catalytic activities of the prepared cobalt oxide particles are investigated that endorse this material as a potential catalyst for decomposition of various substances like ammonium perchlorate (AP).