Introduction

Semiconductor-based nanocomposites are remarkable because of their prospective applications [1–10]. Chromite is a main mineral of chromium, and in Iran it is found in Balochestan and Kerman provinces with about 40 % Cr2O3 and a chrome-to-iron ratio of 2.6:1 [11–14]. Spinels with the general formula of AB2O4, where A and B are cations occupying tetrahedral and octahedral sites, respectively, which A is a divalent and B is a trivalent cations [15–18]. Spinel copper chromite (CuCr2O4) is a narrow band semiconductor used as catalysts for oxidation of carbon monoxide. Different methods are reported for the synthesis of chromite spinels including wet chemical process and micro emulsion processes. Among these methods sol–gel process is a cost-effective which operates at low temperatures and more environmental friendly [19–21]. Previously, we have reported the thermal preparation of semiconductor metal oxides [22–24]. Different methods have also been reported to prepare chromites include [25]. Evaluation of cation influence on the formation of M(II)Cr2O4 during the thermal decomposition of mixed carboxylate type precursors have been reported [11]. Nanocomposite of chromites has been synthesized by thermal decomposition of precursors [26–29].

The aim of the present study was to develop an environmental friendly sol–gel route to prepare nanosized copper chromite spinel. Moreover, their structural and physical properties were characterized by TG–DTG, X-ray powder diffractometry (XRD), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray analysis (EDAX) techniques.

Experimental

Materials and methods

Copper nitrate trihydrate, Cu(NO3)2·3H2O (2 mmol) and chromium nitrate nonahydrate, Cr(NO3)3·9H2O (4 mmol) were dissolved in 20 mL distilled water. To the above solution, ethylene glycol (1 mL) and diethyl ether (1 mL) were added and continued stirring. Citric acid (4 mmol) was added to this solution with the molar ratio of citric acid to the Cu of 2:1. After stirring for 30 min, the pH of the solution was adjusted to pH 7.0 by slowly dropping ammonia and continued stirring for 1 h. A homogeneous sol solution was formed which was aged for 1 week at room temperature. The temperature of the obtained stable sol was raised to 70 °C with continued stirring to evaporate the solvent, and the solution turned into high-viscous gel. The gel was then kept at 135 °C for 2 h to allow the Cu–Cr–citric xerogel to form. The xerogel was ignited in air using a few drops of ethanol 96 % as initiating combustion agent. The obtained powder was calcined in air at temperatures ranging from 1,000 °C for 2 h. Figure 1 shows a flow chart of the complete steps sol–gel preparation of nanocomposite copper chromite spinels.

Fig. 1
figure 1

Flow chart for preparation of CuCr2O4 nanocomposites

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Thermal studies

The thermoanalytical measurements (TG–DTG) study for the thermal decomposition of copper nitrate, chromium nitrate, and dried copper chromite sol were carried out with using a Mettler TA4000 system at a heating rate of 10 °C min−1. Air at 20 mL min−1 was used as purge gas.

X-ray diffraction

Copper chromite nanoparticles were characterized by XRD analysis using X-ray diffractometer (D8 Advance, BRUKER) in the diffraction angle range 2θ = 20°–60°, using Cu Kα radiation. The crystallite size D of the sample was estimated using the Scherer’s equation, (0.9λ)/(βcosθ), by measuring the line broadening of main intensity peak, where λ is the wavelength of Cu Kα radiation, β is the full width at half-maximum, and θ is the brag’s angle.

Field emission scanning electron microscopy

Field emission scanning electron microscopy (FE-SEM, Hitachi, model S-4160) was used to observe the surface morphology of the zinc oxide nanoparticles.

Results and discussion

Thermal investigation

The decomposition mechanism of the Cu(NO3)2·3H2O, Cr(NO3)3·9H2O precursors, copper chromite xerogel precursor, and the formation of their oxides was studied by thermal analysis. The TG–DTG of copper nitrate, chromium nitrate precursors, and copper chromite xerogel precursor powders are shown in Figs. 2, 3, and 4. The mass loss from room temperature to about 600 °C was 76, 62, and 84 % in the chromium nitrate nonahydrate, copper nitrate trihydrate, and copper chromites, respectively. As shown in Fig. 2 two distinct mass loss centered on 105 and 140 °C are observed for Cr(NO3)3·9H2O as precursors. Figure 3 shows the TG/DTG curves of Cu(NO3)2·3H2O as precursor. A relative mass loss of 10.0 % is observed at 115 °C attributed to the loss of adsorbed water. At 152 °C a large mass loss of 15.3 % is observed. A final mass loss is observed at 258 °C of 40.2 %. Figure 4 shows the TG/DTG curves of died gel for CuCr2O4 precursor. The DTG peak max at 310 °C can be attributed to the decomposition of nitrates and phase transformation to the complete crystallization of copper chromite (CuCr2O4).

Fig. 2
figure 2

TG and DTG curves of Cu(NO3)2·3H2O as precursor

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Fig. 3
figure 3

TG and DTG curves of Cr(NO3)3·9H2O as precursor

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Fig. 4
figure 4

TG/DTG curves for dried gel of CuCr2O4 nanocomposites using Cu(NO3)2·3H2O and Cr(NO3)3·9H2O as precursors with mol ratio 1:2 dried at 70 °C

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X-ray and structural investigation

Figure 5 shows the XRD patterns for copper chromite precursor annealed at 1,000 °C. The xerogel powder is a mixture of amorphous substances and citrate crystals. The XRD patterns of the as burnt powder and powder calcined at 1,000 °C reveal a single phase spinel CuCr2O4 (PDF No. 34-0424). The XRD pattern shows peaks 18.6 (101), 29.6 (200), 31.1 (112), 35.2 (211), 42.3 (220), 53.4 (312), 56.2 (321), 61.4 (400), 64.8 (411), and 74.4 (422). The diffraction peaks matched the standard data for CuCr2O4 (PDF No. 34-0424).

Fig. 5
figure 5

XRD pattern of CuCr2O4 nanocomposite prepared using Cu(NO3)2·3H2O and Cr(NO3)3·9H2O as precursors with mol ratio 1:2 annealed at 1,000 °C

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FESEM analysis

Figure 6 depicts the FESEM micrographs of the powders calcined at 1,000 °C. The micrograph in Fig. 6 shows the formation of powder consisting of octahedral-like structure particles with an average particle size of 20 nm [30]. EDAX spectrum (Fig. 7) corresponding to the single phase spinel CuCr2O4 calcined at 700 °C indicate the presence of elements Cu, Cr, O in the samples, which further confirms the spinel phase CuCr2O4. The EDAX peaks at 1 and 2 eV are assigned to O and Cu.

Fig. 6
figure 6

FESEM image of CuCr2O4 nanocomposite prepared using Cu(NO3)2·3H2O and Cr(NO3)3·9H2O as precursors with mol ratio 1:2 annealed at 1,000 °C

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Fig. 7
figure 7

EDAX of CuCr2O4 nanocomposite prepared using Cu(NO3)2·3H2O and Cr(NO3)3·9H2O as precursors with mol ratio 1:2 annealed at 1,000 °C

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Conclusions

The nanocrystalline form of copper chromite was fabricated successfully using a simple, sol–gel method. The influence of calcination temperature on properties was investigated by XRD, FESEM, and TG–DTG. The particle size about 20 nm was formed when the calcination temperature was 1,273 K. EDAX was used to characterize the composition of the samples, and it confirmed the presence of Cu, Cr, and O in the sample. For the first time, we fabricated copper chromite nanoparticles by a simple method. The method can be extended to the fabrication of other spinel chromite nanoparticles of interest in nanotechnology.