Characterization of Cobalt Oxide Nanoparticles Prepared by the Thermal Decomposition of [ Co ( NH 3 ) 5 ( H 2 O ) ] ( NO 3 ) 3 Complex and Study of Their Photocatalytic Activity

In this work, thermal decomposition of the [Co(NH3)5(H2O)](NO3)3 precursor complex was investigated under solid state conditions. Thermal analysis (TG/DTA) showed that the complex was easily decomposed into the Co3O4 nanoparticles at low temperature (175 °C) without using any expensive and toxic solvent or a complicated equipment. The obtained product was identified by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). Optical and magnetic properties of the products were studied by UV-visible spectroscopy and a vibrating sample magnetometer (VSM), respectively. FT-IR, XRD and EDX analyses confirmed the formation of highly pure spinel-type Co3O4 phase with cubic structure. SEM and TEM images showed that the Co3O4 nanoparticles have a sphere-like morphology with an average size of 17.5 nm. The optical absorption spectrum of the Co3O4 nanoparticles showed two band gaps of 2.20 and 3.45 eV, which in turn confirmed the semiconducting properties. The magnetic measurement showed a weak ferromagnetic order at room temperature. Photocatalytic degradation of methylene blue (MB) demonstrated that the as-prepared Co3O4 nanoparticles have good photocatalytic activity under visible-light irradiation.

One of the simplest and lowest cost techniques to prepare metal oxide nanostructures is the solid-state thermal decomposition of molecular precursors.44][45][46] 53 have been used to synthesize Co 3 O 4 nanostructures via the thermal decomposition route by us and other research groups.However, some of these precursors still are associated with one or more disadvantages, such as prolonged reaction times (≥ 2 h), high-temperature requirement (≥ 250 °C), use of toxic and expensive organic solvents (e.g.n-hexylamine), and use of surfactants (e.g.oleic acid, SDS and trioctylphosphine oxide).From a practical viewpoint, the development of a simple and new precursor for the synthesis of Co 3 O 4 nanoparticles at lower temperatures and shorter reaction times is still an active area of research.
In the present work, we wish to report on the direct thermolysis of an energetic pentamminecobalt(III) complex, [Co(NH 3 ) 5 (H 2 O)](NO 3 ) 3 , which led to the synthesis of Co 3 O 4 nanoparticles at low temperature (175 °C) without needs expensive and toxic solvents or complicated equipment.The obtained Co 3 O 4 nanoparticles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), UV-visible spectroscopy, and magnetic measurement.The method is a fast, mild, energy-efficient and environmentally friendly route to produce Co 3 O 4 nanoparticles in only one step.

Preparation of Co 3 O 4 Nanoparticles
In order to prepare Co 3 O 4 nanoparticles, 2 g of the [Co(NH 3 ) 5 (H 2 O)](NO 3 ) 3 complex powder in a porcelain crucible was placed in a muffle furnace.The sample was heated at the rate of 10 °C min -1 from room temperature to 150 °C in air and was maintained at this temperature for 1 h.Similar experiment was performed for the sample calcined at the selected temperature of 175 °C.The temperatures for calcining the complex were selected from the TG-DTA data (Figure 1).The black product generated from the complex at each temperature was cooled to room temperature and collected for characterization.

3. Methods of Characterization
Thermal analysis was conducted with a Netzsch STA 409 PC/PG thermal analyzer at a constant heating rate of 10 °C min -1 in air.The composition and phase purity of the products were characterized by a Rigaku D/max C III X-ray diffractometer using Ni-filtered Cu Ka radiation (λ = 1.5406Å).XRD patterns were recorded in the 2θ range of 10°-80° with a scanning step of 0.02°.To investigate chemical bonding of the products, infrared spectra were recorded on the diluted samples in KBr pellets using a Schimadzu 160 FT-IR spectrophotometer within the region of 4000-400 cm -1 .The optical absorption spectrum was recorded on a Shimadzu 1650PC UV-vis spectrophotometer in a wavelength range of 200-700 nm at room temperature.The samples for UV-vis studies were well dispersed in distilled water by sonication for 30 min to form a homogeneous suspension.The morphology of Co 3 O 4 nanoparticles was studied with a Mira3 Tescan field emission scanning electron microscope.The particle sizes of the as-prepared product were observed with a transmission electron microscope (TEM, Philips CM10) and equipped with an energy dispersive X-ray spectroscopy.For the TEM measurements, the powders were ultrasonicated in ethanol and a drop of the suspension was dried on a carbon-coated microgrid.Raman spectrum was obtained using a Thermo Fisher DXR with a laser wavelength of 780 nm and a spot size of 0.5 nm.The magnetic properties of Co 3 O 4 nanoparticles were measured using a vibrating sample magnetometer (VSM, Iran Meghnatis Daghigh Kavir Company).

4. Photocatalytic Tests
50 ml of 25 mg/L methylene blue (MB) aqueous solution was used for the photocatalytic experiment.30 mg of the Co 3 O 4 nanoparticles were added to MB aqueous solution and stirred with a magnetic stirrer in the dark for 30 min to establish adsorption-desorption equilibrium between the solution and catalyst prior to the irradiation from the 400 W high-pressure Mercury lamp (λ ≥ 420 nm).After adding 2 mL of 30% H 2 O 2 to the suspension, the lamp was turned on.Samples were taken out and the changes of MB concentration were monitored using a UV-vis spectrometer.At regular time intervals, 5 ml MB aqueous solution was taken out from the reactor vessel and centrifuged to separate the solution and the suspended catalyst.The , 48 Co(salophen),49Co(C 6 n Acta Chim.Slov.2016,63, 335-343 Farhadi et al.: Characterization of Cobalt Oxide Nanoparticles ...polymer