Synthesis of Cu_xCo_3−xO₄ nanoparticles by a sonochemical method and characterization of structural and optical properties and photocatalytic activity for the degradation of methylene blue

Abstract

In the present investigation, Cu_xCo_3−xO₄ (x = 0, 0.02, 0.04, 0.06, 0.1) nanoparticles have been synthesized by the sonochemical method and subsequent calcination at high temperature. The synthesized nanoparticles were further characterized using X-ray diffraction, UV-vis spectroscopy, FT-IR spectroscopy, Raman spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The analysis results showed that nanoparticle size, band gap and photocatalytic activity of Cu_xCo_3−xO₄ compounds change by altering the Cu doping content. According to the XRD study, no impurity peaks associated with copper or copper oxide phases were observed in the diffractograms of doped samples. The highest degradation of methylene blue dye under visible light was observed in the presence of Cu_0.02Co_2.98O₄ nanoparticles and found to be 87.51% after 330 minutes. This value increased with increasing the amount of photocatalyst and with decreasing dye concentration.

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1 Introduction

Cobalt oxide (Co₃O₄) nanoparticles are one of the interesting metal oxides because they have unique physical and chemical properties. They are widely used in gas sensors, pH sensors, catalysts, anode materials in Li-ion rechargeable batteries, supercapacitors, electrochemical sensors, photocatalysts, solar absorbers and electrochromic materials.^1–9 Co₃O₄ is a p-type antiferromagnetic semiconductor and mixed valence compound (CoO·Co₂O₃). It adopts the normal spinel structure. So far, several synthesis methods have been employed to produce Co₃O₄ nanoparticles including hydrothermal/solvothermal,^10,11 precipitation,¹² sonochemical,¹³ sol–gel,¹⁴ solution combustion,¹⁵ successive ionic layer adsorption and reaction (SILAR),¹⁶ chemical spray pyrolysis¹⁷ and thermal decomposition.¹⁸ According to reports, the incorporation of metal dopants is an effective method for improving electrical conductivity and catalytic activity of oxide nanoparticles. Rehman et al.¹⁹ prepared Ce and Zn doped CuO nanoparticles by co-precipitation method and a remarkable increment in degradation efficiency for methylene blue (81.64%) under visible light was observed in the presence of these nanocatalysts compared to undoped samples (38.77%). Degradation efficiency was found to be 78% for methylene blue dye under UV-A light using 3% Cu-doped NiO nanoparticles obtained by green synthesis combined with a sol–gel method.²⁰ Several metals are used to modify the Co₃O₄ nanoparticles including Fe, Mn, Cu, Ni, Mo,^20,21 Ag, and Pd.^22,23 Jincy and Meena synthesized copper-doped cobalt oxide nanoparticles by a hydrothermal method and they observed that these nanoparticles exhibited extreme sensitivity towards NH₃ gas at room temperature even at a lower concentration of 5 ppm in comparison to undoped Co₃O₄.²⁴ In another work, Cu-doped Co₃O₄ nanosheets obtained by a solvothermal method show enhanced electrochemical performance due to the reduced particle size and larger surface area.²⁵ Dong and co-workers fabricated mesoporous Ag-doped Co₃O₄ nanowire arrays supported on an FTO substrate by an electrodeposition-hydrothermal process for water oxidation in 0.5 M H₂SO₄ and it has been determined that the addition of silver as a dopant significantly improves both the conductivity and stability of the electrocatalyst in an acidic environment.²⁶

As far as we know, there have been no previous reports in the literature regarding the synthesis of Cu-doped Co₃O₄ nanoparticles using the sonochemical method. In this study, we carried out the synthesis of undoped Co₃O₄ nanoparticles and various concentrations of Cu-doped Co₃O₄ nanoparticles using the sonochemical method, followed by thermal treatment at high temperature. For synthesis, we chose the sonochemical method due to its simplicity and low cost. We selected copper as a dopant for our study because the ionic radius of Cu²⁺ is close to the ionic radius of Co²⁺ and therefore Cu²⁺ ions can easily enter into the Co₃O₄ matrix. The influence of dopant concentration on the structural, optical properties and photocatalytic activity of Co₃O₄ nanoparticles was examined in detail.

2 Experimental

2.1 Materials and instrumentation

All chemicals [cobalt nitrate (Co(NO₃)₂·7H₂O), copper(II) chloride(CuCl₂·2H₂O), sodium hydroxide (NaOH), polyvinyl alcohol (PVA), hydrogen peroxide (H₂O₂) and methylene blue (MB) (C₁₆H₁₈ClN₃S) were of analytical grade and were used without any further purification. Bath type ultrasonicator (VCX500) was used for the synthesis of Co₃O₄ nanoparticles.

The structural analysis of the synthesized oxide powders was analyzed using Rigaku Mini Flex 600 X-ray diffractometer (λ = 1.54060 Å) using Ni-filtered Cu Kα radiation with a step width of 0.1° and a scan speed/duration time of 5° per minute over a range of 2θ from 10° to 90°. Optical properties of the samples were assessed using Specord 250 Plus UV-vis Spectrophotometer. Fourier transform infrared (FTIR) measurement has been carried out within 400–4000 cm⁻¹ region on a Varian 3600 FTIR spectrometer. Transmission electron microscopy (TEM) measurements were conducted on the JEOL JEM-1400 transmission electron microscope. EDX study was carried out on X-Max Energy dispersive spectrometer. Raman spectroscopy measurements were recorded on an EnSpectr R532 spectrometer in the spectral range of 100–800 cm⁻¹.

2.2 Preparation of undoped and Cu-doped Co₃O₄ nanoparticles

Undoped Co₃O₄ nanoparticles were synthesized by the sonochemical method and subsequent calcination process with some modifications reported in our previous research.²⁷ In this experiment, sodium hydroxide was used instead of sodium borohydride as a precipitating agent. For the synthesis, 100 ml of 0.1 M Co(NO₃)₂·7H₂O aqueous solution was prepared and stirred with PVA solution as a stabilizer agent (2.0 wt%) for 15 min to obtain a homogenous solution. Then, as-prepared 100 ml of 0.2 M NaOH solution was added drop by drop into the above solution under the sonication process using an ultrasonic probe. This solution was irradiated with 20 kHs ultrasound wave for an hour. The pink precipitate was obtained at the end of the reaction. Finally, the precipitate was centrifuged at 6000 rpm, washed three times with distilled water and ethanol in the end, and dried at room temperature. Thereafter, the product was annealed at 500 °C for 4 h in a Muffle furnace and the color of it turned black powder. For preparing the different concentrations of Cu-doped Co₃O₄ nanoparticles (Cu_xCo_3−xO₄, x = 0.02, 0.04, 0.06, 0.1), a stoichiometric amount of CuCl₂·2H₂O was dissolved in 100 ml distilled water and added to cobalt nitrate solution. The rest of the experiments is the same as described above. The schematic representation of the synthesis process of Cu_xCo_3−xO₄ nanoparticles is given in Fig. 1.

Fig. 1 Schematic representation of the experimental work.

2.3 Photocatalytic activity measurement

The photocatalytic activity of undoped Co₃O₄ and Cu-doped Co₃O₄ nanoparticles was investigated on methylene blue (MB) dye under visible light irradiation. For experiments, initially, 10 mg catalyst was added to the 10 ppm dye solution. Before irradiation, the stirring was performed in the dark for 45 min to get an adsorption–desorption equilibrium between the dye and catalyst. Then 0.4 ml of 450 mM hydrogen peroxide (H₂O₂) solution as an oxidant was added to the solution and the solution was exposed to visible light irradiation on a magnetic stirrer. 116 W halogen lamp was used as a source of visible light. The experiments were performed at room temperature. At pre-determined time intervals, 3 ml of dye solution was taken out from the beaker and centrifuged at 6000 rpm for 5 min to completely precipitate particles. Then UV-vis adsorption spectra were measured in the range of 400–800 nm. The experiments were also performed by varying the initial dye concentration (5 ppm and 20 ppm) and amount of catalyst (20 mg and 30 mg). Degradation efficiency was calculated by Beer–Lambert law using the following equation:^22,28where C₀ and A₀-initial concentration and absorbance of dye solution, C and A-concentration and absorbance of dye solution after the time interval, respectively.

3 Results and discussion

3.1 Structural analysis

To elucidate the formation mechanism of Cu_xCo_3−xO₄ nanoparticles, structural analysis was conducted before thermal annealing. Fig. 2a shows the XRD patterns of undoped and different concentrations (2%, 4%, 6% and 10%) of Cu-doped samples before thermal annealing. The low-intensity peaks occurred at 2θ = 19.23°, 38.27° corresponding to (001) and (101) planes in the diffractogram of the undoped sample. These results match the JCPDS-30-0443 standard data, confirming the formation of a brucite-like phase of β-Co(OH)₂ with a hexagonal crystal structure. It should be noted, there were no diffraction peaks attributed to α-Co(OH)₂, Cu(OH)₂, or impurities, signifying a high-purity final product. The size and interplanar spacing of the β-Co(OH)₂ nanoparticles were determined as 5.17 nm and 4.61183 Å, respectively, based on the most intense peak corresponding to the (001) plane. The lattice parameters were calculated as follows: a = b = 3.17 Å, c = 4.61 Å, and α = β = 90°, γ = 120°. The Debye–Scherrer formula was employed to calculate the crystallite size:^29,30

D = 0.89λ/β cos θ (2)

where θ is Bragg's angle, λ and β are the X-ray wavelength and the full width at half maximum intensity, respectively.

Fig. 2 XRD patterns of undoped and 2%, 4%, 6%, 10% Cu doped (a) β-Co(OH)₂ and (b) Co₃O₄ nanoparticles.

Interplanar spacing (d) is evaluated by Bragg's Law equation:³⁰

where n is an integer, θ-is a diffraction angle and λ is the wavelength of X-rays.

The lattice constants of β-Co(OH)₂ nanoparticles with a hexagonal crystal structure are calculated by the following equation:³¹

The values of lattice parameters “a” and “c” were calculated from (101) and (001) reflections, respectively, and the obtained values are consistent with the standard data.

One diffraction peak appeared at 2θ = 38.36° corresponding to the (101) plane in the XRD pattern of 2% Cu doped sample before thermal annealing. In the XRD pattern of 4% Cu doped sample, the small intensity peaks appearing at 2θ = 19.36°, 38.52° and 51.68° values are ascribed to the (001), (101) and (102) crystal planes, respectively. One diffraction peak at 2θ = 38.64° corresponding to (101) plane appeared in the diffractogram of the 6% Cu doped sample, two diffraction peaks at 2θ = 19.65° and 38.06° values corresponding to (001) and (101) planes were observed in the diffractogram of the 10% Cu doped sample. Before thermal annealing, the XRD patterns of Cu-doped samples show peaks associated with β-Co(OH)₂, with no observable peaks related to Cu(OH)₂. The results reveal that as the concentration of copper increases in doped β-Co(OH)₂, the diffraction peak corresponding to the (001) plane shifts to a higher angle. The size and interplanar spacing of 4% Cu doped β-Co(OH)₂ nanoparticles were estimated as 5.26 nm and 4.57881 Å, respectively. However, the peak corresponding to the (001) plane was not detected in 2% and 6% Cu-doped Co(OH)₂ nanoparticles, and the intensity of this and other peaks in 10% Cu-doped Co(OH)₂ nanoparticles was minimal, preventing an accurate size determination.

Fig. 2b illustrates the XRD patterns of undoped and various concentrations (2%, 4%, 6% and 10%) of Cu-doped samples after thermal annealing at 500 °C for 4 h. (111), (220), (311), (222) and (440) reflection planes were observed at 2θ = 18.91°, 31.25°, 36.84°, 38.56°, 65.19° diffraction angles in the XRD pattern of the undoped sample. This result aligns well with JCPDS-00-009-0418 standard data, confirming the formation of Co₃O₄ with a cubic crystal structure. The space group is identified as Fd-3m (227), and the lattice parameters are a = b = c = 8.0849 Å; α = β = γ = 90°. The lattice constant (a) was calculated using the interplanar spacing value with the following equation,³² yielding a value close to the standard (a = 8.084 Å):

The lattice constant and nanoparticle size were determined using the most intense peak in the diffractogram, which corresponds to the (311) plane. The crystallite size of undoped, 2%, 4%, 6% and 10% Cu-doped Co₃O₄ nanoparticles is 22.55 nm, 29.17 nm, 21.87 nm, 34.19 nm and 31.26 nm, respectively. It can be observed that the size of Co₃O₄ nanoparticles undergoes irregular changes with increasing Cu concentration.

No additional peaks related to the copper or copper oxide were detected in the diffractograms of Cu-doped Co₃O₄ nanoparticles. This observation suggests that Cu atoms effectively replaced Co atoms, leading to a well-dispersed distribution of Cu within the Co₃O₄ lattice.^33,34 The similarity in ionic radii between Co²⁺ (0.72 Å) and Cu²⁺ (0.71 Å) in tetrahedral coordination facilitates the easy incorporation of Cu²⁺ ions into the Co₃O₄ matrix, substituting the Co²⁺ ions. As a result of Cu substitution, the lattice constant of the Co₃O₄ nanoparticles decreased compared to the undoped samples. Furthermore, the peaks in the diffractograms of Cu-doped Co₃O₄ nanoparticles exhibited a shift towards higher angles compared to the undoped sample. To further investigate the impact of Cu doping on the Co₃O₄ nanoparticles, lattice strain (ε), dislocation density (δ), and unit cell volume were calculated from the XRD data using the equations provided, and the corresponding values are presented in Table 1.

V = a³ (8)

Table 1 The values of crystal structure parameters of Cu_xCo_3−xO₄ (x = 0, 0.02, 0.04, 0.06, 0.1) nanoparticles

Sample	2θ (°)	Crystallite size (nm)	Interplanar distance (Å)	Lattice strain	Dislocation density (10^−3)	Lattice constant (a = b = c) (Å)	Unit cell volume (Å)^3
Undoped Co3O4	36.84	22.55	2.43768	0.0051	1.967	8.0849	528.47
Cu0.02Co2.98O4	36.94	29.17	2.43131	0.0039	1.175	8.0637	524.33
Cu0.04Co2.96O4	36.86	21.87	2.43653	0.0052	2.091	8.0810	527.71
Cu0.06Co2.94O4	36.94	34.19	2.43131	0.0033	0.855	8.0637	524.33
Cu0.1Co2.9O4	36.95	31.26	2.43093	0.0037	1.023	8.0625	524.09

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As the nanoparticle size increases, the lattice strain value decreases, and conversely, as the nanoparticle size decreases, the lattice strain value increases. Below are the possible chemical reactions that could occur:

Co(NO₃)₂·7H₂O → Co²⁺ + 2NO₃⁻ + 7H₂O

NaOH → Na⁺ + OH⁻

Co²⁺ + 2OH⁻ + xH₂O → Co(OH)₂·xH₂O

CuCl₂·2H₂O → Cu²⁺ + 2Cl⁻ + 2H₂O

Co²⁺ + Cu²⁺ + 2OH⁻ + xH₂O → Cu_xCo_1−x(OH)₂·xH₂O

3.2 UV-vis spectroscopy

The optical absorption spectra of undoped and different concentrations of Cu-doped Co₃O₄ nanoparticles were recorded at room temperature in the wavelength range of 250 nm and 900 nm. To perform UV-vis spectroscopy measurements, polyvinyl alcohol (PVA) films were prepared based on all synthesized samples by mechanical stirring. Fig. 3a shows the optical absorption spectra of all samples thermal annealed at 500 °C. Three absorption peaks in the ultraviolet, visible and near-infrared regions were observed for all samples. These peaks were found at 272.2 nm, 469.2 nm, and 773.8 nm in the spectrum of undoped Co₃O₄ nanoparticles. The peak at 272.2 nm is attributed to the PVA. As for the Cu-doped Co₃O₄ nanoparticles, the absorption peaks were detected at the following wavelengths: 449.4 nm and 762 nm for 2% Cu-doped Co₃O₄, 485.6 nm and 777.8 nm for 4% Cu-doped Co₃O₄, 491.2 nm and 778.2 nm for 6% Cu-doped Co₃O₄, and 534 nm and 826.4 nm for 10% Cu-doped Co₃O₄ nanoparticles. The results indicate that a blue shift, or a shift towards a lower value of wavelength, was observed in the absorption curve of 2% Cu-doped Co₃O₄ nanoparticles compared to undoped Co₃O₄ nanoparticles. Conversely, the absorption curves of 4%, 6%, and 10% Cu-doped Co₃O₄ nanoparticles showed a red shift, or a shift towards a higher value of wavelength, compared to undoped Co₃O₄ nanoparticles. The optical band gap values (E_g) of samples have been determined using the following Tauc's equation:

α = [B(hν − E_g)ⁿ]/hν (9)

where α is the absorption coefficient, hν is the photon energy, B is a constant, and the value of n depends on the nature of transition: n = 2 for indirect allowed transition and n = 1/2 for direct allowed transition, n = 3/2 for direct forbidden transition, n = 3 for indirect forbidden transition. The (αhν)² versus hν curves for samples annealed at 500 °C are shown in Fig. 3b. It reveals two band gaps associated with the electronic band structure of cobalt oxide. The band gap energy of samples was estimated by extrapolating the linear part of the curves to zero absorption (α = 0).³⁵ The direct E_g values are given in Table 2. The presence of two band gaps is ascribed to the electronic band structure of cobalt oxide. The first band gap (E_g1) is attributed to the O²⁻ Co³⁺ charge transfer (with the Co³⁺ level located below the conduction band), whereas the higher band gap (E_g2) is assigned to the O²⁻ Co²⁺ charge transfer process.^35,36

Fig. 3 (a) The absorption spectrums and (b) the band gap energy plots for Cu_xCo_3−xO₄ nanoparticles; (c) FT-IR spectra of undoped (2) and 10% Cu doped Co₃O₄ nanoparticles (1).

Table 2 The band gap energy values and degradation efficiency in 330 min for Cu_xCo_3−xO₄ nanoparticles

Sample	The band gap (eV) (for samples annealed at 500 °C)	Degradation efficiency (%)
Undoped Co3O4	1.58 and 3.33	79.20
Cu0.02Co2.98O4	1.70 and 3.43	87.51
Cu0.04Co2.96O4	1.45 and 3.26	74.66
Cu0.06Co2.94O4	1.44 and 3.21	85
Cu0.1Co2.9O4	1.26 and 3.08	65.60

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The E_g value of pure Co₃O₄ nanoparticles increased from 1.58 eV and 3.33 eV to 1.70 eV and 3.43 eV when doped with 2% Cu. However, with higher Cu content, the band gap decreased. The increase in the band gap (E_g) of 2% Cu-doped Co₃O₄ nanoparticles can be attributed to the Moss–Burstein effect. The energy band broadening occurs when the Fermi level lifts into the conduction band according to Moss–Burstein effect. The decrease in band gap at higher Cu dopant concentrations is associated with crystal defects. The incorporation of more Cu²⁺ ions into the Co₃O₄ lattice leads to an increase in defects.³⁷ When Cu²⁺ ions enter the crystal lattice of cobalt oxide in large quantities, the obtained mixed oxide shows combined physicochemical characteristics of both copper oxide and cobalt oxide.

3.3 FT-IR spectroscopy

IR spectra of undoped and 10% Cu-doped Co₃O₄ nanoparticles are given in Fig. 3c and d, respectively. Two sharp absorption peaks appeared at 661 cm⁻¹ (υ₁) and 568 cm⁻¹ (υ₂) in the spectrum of undoped Co₃O₄ nanoparticles. The υ₁ band is attributed to the tetrahedral coordinated Co²⁺ ions, while the υ₂ band is assigned to the octahedral coordinated Co³⁺ ions.³⁸ These characteristic peaks were observed at 662 cm⁻¹ and 572 cm⁻¹ in the spectrum of 10% Cu-doped Co₃O₄ nanoparticles. So, this result confirms the formation of Co₃O₄ with spinel structure. The absence of a peak related to Cu–O bond in the spectrum of Cu-doped Co₃O₄ nanoparticles can be explained by the overlapping of Cu–O peak with Co–O peak due to their closely similar vibration wavelengths.^25,39

3.4 TEM and EDX studies

TEM measurements were conducted to determine the size and morphology of obtained samples. The TEM images of pure Co₃O₄ nanoparticles are presented in Fig. 4A. For TEM measurements, the sample was dispersed in ethanol by ultrasonic vibration and was dropped onto the surface of carbon-coated copper grids. It can be seen that nanoparticles show spherical-like, hexagonal-like and irregularly structured morphology and the average diameter of Co₃O₄ nanoparticles is in the range of 5.57–79.15 nm. Fig. 4B displays TEM images of 10% Cu-doped Co₃O₄ nanoparticles. These nanoparticles exhibit various morphologies, including spherical-like, hexagonal-like, rhombohedral-like, and irregular structures. The size of nanoparticles is in the range of 15.78–78.31 nm.

Fig. 4 TEM images of (A) undoped Co₃O₄ and (B) 10% Cu-doped Co₃O₄ nanoparticles.

To determine the elemental composition, stoichiometry and purity of samples EDX analysis was carried out. EDX analysis results of undoped Co₃O₄ nanoparticles (Fig. 5A) indicate the presence of cobalt and oxygen elements, with each element constituting 50% of the composition. The EDX spectrum depicted in Fig. 5B confirms the presence of cobalt, copper and oxygen elements in the synthesized 2% Cu-doped Co₃O₄ nanoparticles and atomic percentages of these elements are 48.43%, 0.31% and 50.32%, respectively. Based on this result, the percentage ratio of Cu/Co atoms is found to be 0.006, which this value is close to the theoretical ratio of Cu/Co atoms (0.007) in the Cu_0.02Co_2.98O₄ sample. For 4% Cu-doped Co₃O₄ nanoparticles (Fig. 6A), atomic percentages of cobalt, copper and oxygen elements are 49.47%, 0.53% and 50%, respectively. The percentage ratio of Cu/Co atoms is equal to 0.01 and this value coincides with the theoretical ratio of Cu/Co atoms (0.01) in the Cu_0.04Co_2.96O₄ compound. For 6% Cu-doped Co₃O₄ nanoparticles (Fig. 6B), atomic percentages of cobalt, copper and oxygen elements are 48.98%, 1.02% and 50%, respectively. According to the result, the percentage ratio of Cu/Co atoms is equal to 0.02 and this value coincides with the theoretical ratio of Cu/Co atoms (0.02) in the Cu_0.06Co_2.94O₄ compound.

Fig. 5 Element mapping and EDX spectrum of (A) undoped Co₃O₄ and (B) 2% Cu-doped Co₃O₄ nanoparticles.

Fig. 6 Element mapping and EDX spectrum of (A) 4% Cu-doped, (B) 6% Cu-doped and (C) 10% Cu-doped Co₃O₄ nanoparticles.

EDX analysis results of 10% Cu-doped Co₃O₄ nanoparticles (Fig. 6C) reveal the presence of cobalt, copper, oxygen, sodium, chlorine elements and the atomic percentages of these elements are 43.97%, 2.90%, 48.57%, 3.39% and 1.17%, respectively. The occurrence of trace amounts of sodium and chlorine elements can be attributed to the initial reagents utilized in the synthesis process. Based on the results, the percentage ratio of Cu/Co atoms is equal to 0.06, which is close to the theoretical ratio of Cu/Co atoms (0.03) in the Cu_0.1Co_2.9O₄ sample. When examining the element mapping of the Cu-doped Co₃O₄ nanoparticles, it becomes evident that the distribution of Co, Cu, and O elements coincides.

3.5 Raman spectroscopy

The Raman spectra of pure, 2% Cu-doped Co₃O₄ and 10% Cu-doped Co₃O₄ nanoparticles are presented in Fig. 7. Three Raman peaks at 184 cm⁻¹, 464 cm⁻¹ and 666 cm⁻¹ are observed for pure Co₃O₄ nanoparticles. These peaks are correspond to the F_2g, E_g and A_1g modes, respectively, which they are in good agreement with the reported literatures.^40–42 The strong band at 666 cm⁻¹ (A_1g mode) is assigned to the characteristics of the octahedral sites, while F_2g and E_g modes are attributed to the combined vibrations of the tetrahedral site and octahedral oxygen motions.⁴² As can be seen from the figure, the intensity of Raman peaks decreased after doping of Co₃O₄ with Cu. The Raman peaks at 514 cm⁻¹ and 674 cm⁻¹ are observed for 2% Cu-doped Co₃O₄ nanoparticles and the Raman peaks at 512 cm⁻¹ and 674 cm⁻¹ are observed for 10% Cu-doped Co₃O₄ nanoparticles. A slight shift of the peak observed at A_1g mode to higher wavenumbers occurred after doping.

Fig. 7 Raman spectra of (a) undoped, (b) 2% Cu-doped Co₃O₄ and (c) 10% Cu-doped Co₃O₄ nanoparticles.

3.6 Photocatalytic activity

Fig. 8 illustrates the photocatalytic degradation of methylene blue in the presence of Cu_xCo_3−xO₄ (x = 0, 0.02, 0.04, 0.06, 0.1) nanoparticles. Initially, we examined the dye's degradation efficiency under visible light over time in the presence of H₂O₂ without a catalyst, for comparative purposes (Fig. 8b). The MB dye exhibits its highest absorption peak at a wavelength of 663.4 nm. The figure clearly indicates a time-dependent reduction in the intensity of the absorption curve. Based on the experimental findings, the degradation efficiency for MB dye in the presence of undoped Co₃O₄ nanoparticles and H₂O₂ (Fig. 8a) was determined using the eqn (1) and was 79.20% after 330 min. Conversely, in the absence of a catalyst, the dye's degradation was minimal, with an efficiency of only 2.72% after 330 minutes. According to reports, the decomposition of MB dye without H₂O₂ proceeds with difficulty when Co₃O₄ is used as a photocatalyst. The degradation efficiency improves significantly by introducing an oxidant and carefully choosing its concentration. While a higher concentration of the oxidizing agent enhances the degradation rate by increasing hydroxyl radicals (OH˙), there are instances where this process may lead to the counterproductive scavenging of valuable OH˙ radicals, resulting in the formation of weaker hyperoxyl radicals ().⁴³ The pH of the dye solution plays a critical role in the degradation. Researchers found that using Co₃O₄ nanoparticles as a catalyst, an alkaline medium lead to the decomposition of H₂O₂ into water and oxygen, while in an acidic medium, the number of OH˙ radicals decreases with the formation of positively charged H⁺ ions, resulting in decreased degradation efficiency.⁴³ Thus, for our experiments, a neutral pH environment was chosen. The mechanism of the photocatalytic degradation process is as follows: when the light energy is equal to or exceeds the band gap energy of the catalyst, electrons are excited from the valence zone to the conduction zone. This generates positively charged holes in the valence zone and negatively charged electrons in the conduction zone. The positively charged holes produce OH˙ radicals through water oxidation or by being captured by OH⁻. Electrons in the conduction zone react with oxygen adsorbed on the catalyst's surface and form superoxide anion radicals (˙O₂⁻). Consequently, these radicals decompose the dye solution.^28,43 The generation of hydroxyl radicals occurs according to the Photo-Fenton reaction mechanism. Based on this complex reaction mechanism, Co²⁺ ions react with H₂O₂ to form OH˙ radicals, and Co²⁺ ions are regenerated as a consequence of the reduction of Co³⁺ ions together with the ligand present in the solution. The detailed reaction mechanism is as follows:⁴³

Co₃O₄ + hυ → h_vb⁺ + e_cb⁻

H₂O + h_vb⁺ → OH˙ + H⁺

O₂ + e_cb⁻ → ˙O₂⁻

˙O₂⁻ + H₂O → H₂O₂ + e_cb⁻ → OH˙

Co²⁺_(ads) + H₂O₂ → Co³⁺_(ads) + OH⁻ + OH˙

Co³⁺-L + hυ → Co²⁺ + L˙

Methylene blue dye + h_vb⁺ → oxidation product

Methylene blue dye + OH˙ → degradation product

Fig. 8 Time-dependent photocatalytic degradation of MB dye (a) in the presence of undoped Co₃O₄ nanoparticles and H₂O₂; (b) in the presence of H₂O₂ without catalyst under visible light irradiation; in the presence of (c) 2%, (d) 4%, (e) 6%, (f) 10% Cu-doped Co₃O₄ nanoparticles and H₂O₂ under visible light irradiation.

The degradation efficiency of MB dye in the presence of Cu_0.02Co_2.98O₄ nanoparticles improved. In the presence of Cu_0.02Co_2.98O₄, Cu_0.04Co_2.96O₄, Cu_0.06Co_2.94O₄, Cu_0.1Co_2.9O₄ nanoparticles, the degradation efficiency of MB dye increased from 46.77% to 87.51%, 38.76% to 74.66%, 44.95% to 85% and 31.47% to 65.60% for 90–330 min irradiation time, respectively. As can be seen from the results, the highest degradation was observed in the presence of Cu_0.02Co_2.98O₄ nanoparticles and the weak degradation was observed in the presence of Cu_0.1Co_2.9O₄ nanoparticles. When Cu_0.04Co_2.96O₄ and Cu_0.1Co_2.9O₄ nanoparticles are used, the degradation efficiency decreases in comparison to undoped Co₃O₄ nanoparticles. Cu_0.02Co_2.98O₄ nanoparticles have a wide band gap and as a result of their use as a catalyst, an increase in the degradation efficiency occurs with decreasing electron–hole pair recombination and the generation of free electrons and holes for the formation of radicals. However, when the concentration of copper is high, the photocatalytic activity diminishes. This can be ascribed to excessive oxygen vacancies and the high amount of the dopant, which acts as recombination centers for photoinduced electrons and holes.³⁴ The experiments were also performed for the degradation of MB in the presence of Cu_0.02Co_2.98O₄ nanoparticles by varying the initial dye concentration (5 ppm and 20 ppm) and amount of photocatalyst (20 mg and 30 mg). When using 20 mg and 30 mg Cu_0.02Co_2.98O₄ nanoparticles, the degradation efficiency of MB dye was found to be 90.94% and 93.19% in 330 min, respectively. For these experiments, the concentration of the dye was taken as 10 ppm. When using 5 ppm and 20 ppm dye concentrations, the value of the degradation efficiency of MB dye was 90.48% and 86.44% in 330 min, respectively. For these experiments, the amount of photocatalyst was taken as 10 mg (Fig. 9).

Fig. 9 Time-dependent photocatalytic degradation of MB dye in the presence of Cu_0.02Co_2.98O₄ nanoparticles: (a) amount of photocatalyst is 20 mg, (b) amount of photocatalyst is 30 mg, (c) the concentration of the dye is 5 ppm and (d) the concentration of the dye is 20 ppm.

4 Conclusion

In the present study, Cu_xCo_3−xO₄ (x = 0, 0.02, 0.04, 0.06, 0.1) nanoparticles were synthesized by sonochemical method and subsequent calcination at high temperature. Based on the XRD analysis, the size of Co₃O₄ nanoparticles exhibits irregular changes as the concentration of Cu increases. The band gap value of undoped Co₃O₄ nanoparticles initially rises from 1.58 eV and 3.33 eV to 1.70 eV and 3.43 eV with a 2% Cu doping content, but subsequently decreases with higher Cu content. The increase in the band gap of Cu_0.02Co_2.98O₄ nanoparticles can be attributed to the Moss-Burstein effect and the decrease of band gap at higher dopant concentrations can be explained with crystal defect. According to TEM measurements, Cu_0.1Co_2.9O₄ nanoparticles exhibit spherical-like, hexagonal-like, rhombohedral-like and irregularly structured morphology and the size of nanoparticles is in the range of 15.78–78.31 nm. EDX results reveal that the percentage ratio of Cu/Co atoms is near to the theoretical ratio of Cu/Co atoms in the Cu_xCo_3−xO₄ compound and the distribution of Co, Cu and O elements coincide. FT-IR and Raman results also confirm the formation of Co₃O₄. The highest degradation efficiency for MB dye (87.51%) was observed in the presence of Cu_0.02Co_2.98O₄ nanoparticles and this value increased with increasing the amount of photocatalyst and with decreasing of dye concentration to 93.19% and 90.48%, respectively.

Conflicts of interest

There are no conflicts to declare.

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