Facile Microwave-Assisted Preparation of Hetero-Structured CuCo 2 S 4 /CuCo 2 O 4 Nanoparticles Using Organic Agent of Thiourea

: Ultra-fast one-put microwave assisted strategy was introduced for the synthesis of heterostructured CuCo 2 S 4 /CuCo 2 O 4 nanoparticles into a domestic microwave oven with the power of 900 W for 20 min. Thiourea as an organic agent was used as a sulfur source and driving agent to lead the solvent free combustion reaction to obtain this earth-abundant and low-cost mixed chalco-genide/oxide product. The structural and morphological specifications were studied in details using powder x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT − IR), scanning electron microscopy (SEM), and energy dispersion of x-ray spectrometry (EDX). The employed strategy rel-atively clarifies a new facile, rapid, and eco-friendly route to produce heterostructured nanocompo-sites for different functional purposes.


Introduction
Commonly, hetero-structured materials are defined as sandwich-like building blocks of two or more dissimilar layers with different features to create better functions than each component. One of the interesting heterostructures, is the growth of metal oxides in sulfide-based materials such as chalcogenides. Linnaeite compounds with a layered structure and general formula of AB2X4, where X is sulfur (S) or selenium (Se), A is divalent (Fe, Ni, Co, or Cu), and B is trivalent (Co, Ni), are promising candidates in the formation of the heterostructures' materials. These materials attracted lots of attention after their discovery by the Linnaeite group of the Swedish Carl Linnaeus in 1845, which caused more efforts in identification of more layered transition metal sulfides and also oxides. CuCo2S4 as a member of the carrollite compounds category is a notable material because of having low-cost, nontoxic, abundant, and available constituents [1,2]. On the other hand, metal oxide materials are widely available materials with crystalline structures and different applications in various fields, e.g., electrical/optical industry, redox reactions, catalyst production, agricultural industry, drug delivery/medicine, etc. They have significant properties in terms of efficiency, reactivity, selectivity, and cycle repeatability so that their combination with other materials such as CuCo2S4 can be led to prepare new multifunctional materials.
Various chemical methods were reported to synthesize variety of hetero-structured nanomaterials. The introduction of a simple, inexpensive, high efficiency, and rapid technique is notable to reduce production costs [3][4][5]. Amongst different techniques such as sol-gel, hydrothermal/solvothermal, and precipitation/calcination methods, microwave assisted-method is considered as a facile and rapid strategy to produce nanomaterials.

8].
As a matter of fact, the synthesis of hetero-structured CuCo2S4/CuCo2O4 nanoparticles using the ultra-fast solvent free microwave-assisted procedure has not yet been reported. Thiourea was selected as a suitable sulfur source and also driving agent in process due to its availability, cheapness, and ease of use.

Materials and Method
All chemicals were purchased from Merck Co. and used without further purification. Metal sources with stoichiometric ratio in the presence of thiourea were mixed with each other, put into a microwave oven, and treated by microwave irradiation with a power of 900 W for 20 min. The obtained black powder was rinsed with distilled water and absolute ethanol for several times, dried at 70 °C, and then characterized.

Characterizations
X-ray diffraction (XRD) pattern was recorded by a DRON-8 powder diffractometer using Cu Kα radiation (λ = 1.54060 Å). Fourier transform infrared (FT−IR) spectrum was obtained by a Shimadzu-8400S spectrometer in the range of 400-4000 cm −1 using KBr pellets. Scanning electron microscopy (SEM) images and energy-dispersive x-ray were taken on a VEGA\\TESCAN S360 with gold coating.

Result and Discussion
The observed strong peaks at around 500-700 cm −1 in the recorded FT-IR spectrum of the prepared product shown in Figure 1 can be related to the vibration frequencies of Co-O and Co-S and Cu-S bands of CuCo2S4/CuCo2O4 compound. The peaks at 3423 and 2360 cm −1 can be assigned to the vibrations of the adsorbed H2O and CO2 molecules.   The elemental EDX analysis shown in Figure 3 revealed the presence of Cu, Co, and S elements confirming the mentioned XRD data of the formation of layered building blocks of CuCo2S4/CuCo2O4. SEM images (Figure 4) recorded the resulting product depicted as a spherical particulate morphology with an average particle size of about 40 nm. It was observed that these fine nanoparticles were gathered together and formed a bundle-like texture consisting of particles. It can provide a large surface with more available active sites and efficiently contacts for various targets [7].

Conclusions
In a summary, the hetero-structured CuCo2S4/CuCo2O4 nanoparticles were successfully synthesized by a rapid one-put microwave assisted strategy. This in situ treatment easily produced a suitable medium for the formation of heterostructure nanomaterial with a layered architecture. In fact, the microwave irradiation in the presence of proper chemicals created efficient interactions between reactants in solid state to reach this target. In addition, this procedure was carried out in a solid state without using any organic solvent, which can nominate it as an environment friendly method to prepare various nanomaterials.