Synthesis, characterization and degradation of organic dye over Co₃O₄ nanoparticles prepared from new binuclear complex precursors

Abstract

The synthesis and characterization of Co₃O₄ nanoparticles via solid state thermal decomposition of a binuclear cobalt(III) complex, as a new precursor is reported. Solid state thermal decomposition of the complex at 450 °C for 3 h produces Co₃O₄ nanoparticles which are characterized by FT-IR spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). SEM and TEM images exhibit the quasi-spherical shape of the Co₃O₄ nanoparticles of between 20 and 30 nm. Moreover, adsorption of methylene orange dye on Co₃O₄ nanoparticles was investigated and the uptake% was determined to be >45% in 200 min.

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1. Introduction

In recent years, the preparation of transition metal oxide nanoparticles has been widely studied due to their versatile applications, such as chemical sensors, catalysts, antistatic coating, radioactive waste management, etc.^1,2 The synthesis and characterization of different transition metal oxide nanoparticles with specific sizes and morphology have been reported.^3–8

Co₃O₄ is a significant antiferromagnetic p-type semiconductor with outstanding properties such as gas-sensing, catalytic and electrochemical properties, and has been investigated greatly for applications in solid-state sensors, electrochromic devices and heterogeneous catalysts as well as lithium batteries.^9,10 When decreased down to the nanometer scale, Co₃O₄ were found to have interesting magnetic, optical, field emission and electrochemical properties that are interesting in device applications.^11,12 Thus enormous attempts have been guided in recent years to the synthesis and investigation of properties of Co₃O₄ nanostructures.

To the present time, various approaches have been presented in order to prepare Co₃O₄ nanoparticles, involving sol–gel approach,¹³ hydrothermal approach,^14,15 combustion approach,¹⁶ microemulsion approach,¹⁷ chemical spray pyrolysis,¹⁸ chemical vapor deposition,^19,20 thermal decomposition of cobalt precursors,^21–27 sonochemical approach,^28,29 co-precipitation,³⁰ microwave irradiation,³¹ and mechanochemical processing.³²

Between the numerous procedures developed for preparing metal oxides nanoparticles, the thermal decomposition molecular precursor method has been considered as one of the most useful and applicable techniques, because it not only capable to prevent special instruments and complicated procedures and severe preparation conditions, but also supplies good control over purity, composition, homogeneity, phase and microstructure of the resultant products.^33,34

In our group, we have been fascinated for a few years in the synthesis of metal, metal oxide and magnetic nanoparticles, utilizing new inorganic precursor, taking profit of the tools of organometallic chemistry.^35–38 A main fascinate at this time is in the evolvement of organometallic or inorganic compound for preparation of nanoparticles. Utilizing of the novel compound can be useful and open a new method for synthesizing nanoparticles to control nanoparticle size, shape and distribution size.

Herein, we report on the preparation of Co₃O₄ nanoparticles by direct solid-state thermal decomposition of the solid molecular precursor; [(NH₃)₅Co(O₂)Co(NH₃)₅](NO₃)₄; complex as a new precursor for the first time.

2. Experimental

2.1. Materials and characterization

All the chemical reagents used in our experiments were of analytical grade and were used as received without further purification. Cobalt(II) nitrate hexahydrate, ammonia and sodium nitrate were obtained from Merck Co. XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Kα radiation. Scanning electron microscopy (SEM) images were obtained on Philips XL-30ESEM. Transmission electron microscopy (TEM) images were obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 100 kV. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. The magnetic measurement was carried out in a vibrating sample magnetometer (VSM) (BHV-55, Riken, Japan) at room temperature.

2.2. Synthesis of binuclear precursor; [(NH₃)₅Co(O₂)Co (NH₃)₅](NO₃)₄ (ref. 39)

5 g quantity of cobalt(II) nitrate hexahydrate is dissolved in 100 ml of water and filtered. Aqueous ammonia (15 M, 25 ml) is added, and after the mixture has been cooled to 5–15 °C, a current of oxygen (1000 ml min⁻¹, i.e., a brisk stream) is passed through the cooled solution for an hour, the mixture being stirred with a magnetic stirrer. Oxygen may be replaced by air, in which case longer bubbling (2–3 h) at 0–5 °C is recommended. Sodium nitrate (2 g) dissolved in 5 ml of water is then added; oxygen (or air) is passed through the solution for another hour, with the mixture cooled in ice toward the end of this period. The dark brown crystals are gathered on a glass filter and washed with a small quantity of 15 M aqueous ammonia and then with ethanol.

2.3. Synthesis of Co₃O₄ nanoparticles

Black Co₃O₄-based nanocomposites were produced by subjecting 0.01 mol of the as-prepared binuclear complexes Co(III); [(NH₃)₅Co(O₂)Co(NH₃)₅](NO₃)₄; powders to heat treatment at a relatively low temperature (773 K) in air. An average temperature increase of 303 K every minute was selected before the temperature reached 773 K, and after keeping the thermal treatment at 773 K for 3 h, it was allowed to cool to the room temperature naturally.

3. Results and discussion

Fig. 1a–c reveal a typical XRD pattern of the sample obtained by thermal decomposition of binuclear complexes Co(III). All the reflection peaks in this pattern could be readily indexed to crystalline Co₃O₄ nanoparticles (JCPDS Card file no. 78-1970). All of the reflection peaks could be readily indexed to crystalline cubic phase Co₃O₄ with a lattice constant of a = 8.065 Å. The crystallite sizes of the as-synthesized Co₃O₄ nanoparticles, D_c, was calculated from the major diffraction peaks of the base of (1 1 1) using the Scherrer formula (eqn (1)),⁴⁰

D_c = Kλ/(β cos θ) (1)

where K is a constant (ca. 0.9); λ is the X-ray wavelength used in XRD (1.5418 Å); θ is the Bragg angle; β is the pure diffraction broadening of a peak at half-height that is, broadening due to the crystallite dimensions. The diameter of the Co₃O₄ nanoparticles calculated by the Scherrer formula is 25–30 nm for Fig. 1a–c, respectively.

Fig. 1 XRD patterns of Co₃O₄ nanoparticles obtained by the thermal decomposition of the precursor at (a) 250, (b) 300, (c) 400 °C for 5 h.

FT-IR spectroscopy is an applicable device to understand the functional group of any organic molecule (Fig. 2). For the complex, the characteristic bands of NH₃ are observed at approximately 3438, 1638, 1054 cm⁻¹ and band of the NO₃ appeared at 790 cm⁻¹.²⁷ Metal oxides; Co₃O₄; generally give absorption bands below 1000 cm⁻¹ resulting from interatomic vibrations. The spectra (Fig. 2) also included two strong absorption bands at ∼790 and 470 cm⁻¹ which verify the spinel structure of Co₃O₄. The former peak at ∼790 cm⁻¹ is ascribed to the stretching vibration mode of M–O in which M is Co⁺² and is tetrahedrally coordinated. The band at ∼470 cm⁻¹ can be assigned to the M–O in which M is Co⁺³ and so coordinates octahedrally.⁴¹ This result further approved the formation of Co₃O₄ nanoparticles by the above-mentioned thermal decomposition.

Fig. 2 IR spectra of the product obtained on thermal decomposition of the precursor at (a) 400, (b) 500, (c) 700 and (d) 900 °C for 5 h.

The morphology of the Co₃O₄ nanoparticles is investigated by SEM (Fig. 3a–c). The SEM micrograph of the Co₃O₄ nanoparticles treated at 400 °C exhibits that the sample includes of agglomerated particles and the grains are approximately spherical with nearly uniform particle-size (Fig. 3a). By enhancing the treatment temperature to 600 °C, the primary particles and the aggregates tend to become larger, as can be seen from the SEM image (Fig. 3b and c).

Fig. 3 SEM images of the Co₃O₄ nanoparticles at (a) 400, (b) 500 and (c) 600 °C for 5 h.

The TEM images (Fig. 4) exhibit the presence of dense agglomerates. The particles have a spherical shape, and their distribution, likewise, is not uniformed (10–15 nm).

Fig. 4 TEM images of the Co₃O₄ nanoparticles at 400 °C for 5 h.

Fig. 5 exhibits the magnetic properties of the Co₃O₄ nanoparticles. The fine shape of the hysteresis loops is attributing to a weak ferromagnetic behavior, although bulk Co₃O₄ is antiferromagnetic.⁴² From the inset, the coercive field (H_c) and the remanent magnetization (M_r) are estimated to be 0.010 kOe and 0.001 emu g⁻¹, respectively. The low coercive fields and remanent magnetizations approve that the Co₃O₄ nanoparticles have weak ferromagnetic properties. Co₃O₄ nanoparticles comprise of small magnetic domains, each characterized by its own randomly oriented magnetic moment. The total magnetic moment of the nanoparticles is the sum of these magnetic domains coupled by dipolar interactions. The change from an antiferromagnetic state for bulk Co₃O₄ to a weakly ferromagnetic state for Co₃O₄ nanoparticles can be assigned to uncompensated surface spins and/or finite size effects.^43,44

Fig. 5 Magnetization versus applied at 300 K for Co₃O₄ nanoparticles.

The photocatalytic activity of the Co₃O₄ nanoparticles was evaluated by monitoring the degradation of methyl orange (MO) in an aqueous solution, under irradiation with visible light (Fig. 6, Scheme 1). Without light or nanoparticles, nearly no MO was break down after 200 min, revealing that the contribution of self-degradation was insignificant. However, Co₃O₄ nanoparticles exposed high photocatalytic activity. The heterogeneous photocatalytic processes include many steps, such as diffusion, adsorption and reaction, appropriate distribution of the pore is advantageous to diffusion of reactants and products, which prefer the photocatalytic reaction. In this paper, the enhanced photocatalytic activity may be ascribed to suitable distribution of the pore, high hydroxyl content and high separation rate of photo induced charge carriers.⁴⁵

Fig. 6 Decolorization of MO dye of Co₃O₄ nanoparticles.

Scheme 1 Reaction mechanism of MO photodegradation over Co₃O₄ nanoparticles under visible light irradiation.

4. Conclusion

Adopting the self-prepared binuclear precursor Co(III) as precursor, Co₃O₄ nanoparticles have been synthesized by thermal decomposition method. The proposed methods to Co₃O₄ nanoparticles were simple, mild and cheap, which makes them very suitable for scale-up production. We have presented a simple method to synthesize Co₃O₄ nanoparticles through simple thermal decomposition of the [(NH₃)₅Co(O₂)Co(NH₃)₅](NO₃)₄ complex as a new precursor for the first time. The synthesized Co₃O₄ nanoparticles can be used in some analytical applications such as removal of methylene orange dye, since the %uptake was found to be >45% within 200 min.

Acknowledgements

The authors are grateful to University of Kashan for supporting this work by Grant no. (159271/195).

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