Introduction

Wastewater combined with a variety of organic compounds especially dyestuffs is the source of environmental contamination [1]. The primary part of organic compounds is aromatic compounds, particularly sulfo, nitro, amidocyanogen, or chloro series [2]. Methylene blue (MB) is a heterocyclic aromatic chemical compound used in industry for dying, e.g. cotton, wool, acrylic, silk [3, 4]. Photocatalytic degradation is a much more efficient way to treat wastewater. Therefore, photocatalysts play an important role in the degradation of organic compounds. Various types of photocatalysts, including WO3 [5], TiO2 [6–8], ZnO [9], ZnS [10], CuS [11], and composited semiconductors [12–27], were prepared to catalyze the degradation of organic contaminations.

Copper oxide (CuO) is a significant p-type semiconductor with a narrow bandgap, which has been widely applied in many potential fields, such as super-capacitors, sensors, solar cells, photodetectors, catalysis, enhancement of thermal conductivity of nano-fluids, field emission displays (FED), superhydrophobic surfaces, photocatalysis [28–31]. CuO has some features that make it suitable to be a photocatalyst, such as low cost, photocatalytic activity, reusability, nontoxicity, and abundant resources etc. [32, 33].

Because of the wide applications of CuO, many researchers have focused on discovering new methods of synthesising CuO. Several chemical synthesis methods of CuO were reported, including wet chemistry [34], electrochemical [35], microwave irradiation [36], ultrasonic waves [37], precipitation [38], and hydrothermal [39–42]. Most of these methods effectively control the synthesis of prepared CuO powders. However, some methods sometimes need multistep reactions and high temperature. Therefore, it is a challenge to design a low temperature and simple route for the synthesis of CuO.

Different copper sources, including Cu(OH)2 [43], Cu(CH3COO)2·2H2O [44], Cu(NO3)2·3H2O [34], CuCl2·2H2O [45], Cu2(OH)3NO3 [46], Cu2(OH)3Cl [47, 48], Cu4SO4(OH)6 [49, 50] CuCO3·Cu(OH)2 [51–53], have been used as precursors or intermediates for the preparation of CuO. Among the copper sources, CuCO3·Cu(OH)2 widely exists in natural ores, so it is plentiful, cheap, and easier to be achieved. Zhang et al. [51] and Zhu et al. [52] used CuCO3·Cu(OH)2 as a precursor to fabricate CuO via calcination. Wang et al. [53] used CuCO3·Cu(OH)2 as precursor and H2O2 aqueous solution as an additive for the preparation of CuO via hydrothermal method.

In this paper, a mild alkaline condition was adopted to produce CuO, which was obtained by the dehydration of Cu(OH)2 intermediate from the precursor of CuCO3·Cu(OH)2. A series of control experiments were carried out to investigate the influencing factors and the possible formation processes of as-obtained CuO particles. The catalytic activity of CuO was tested by the degradation of MB.

Experimental

Materials

Chemicals including CuCO3·Cu(OH)2, NaOH, MB, H2O2 aqueous solution, anhydrous ethanol were all of analytical grade and used in this work without any further purification. Distilled water was used throughout the experiments.

Synthesis of CuO

A quantity of 2.21 g CuCO3·Cu(OH)2 powder was added into NaOH aqueous solution (40 mL 0.5 M) at a stoichiometric ratio of 1:2 in a beaker under constant stirring. The beaker was sealed and heated at 80 °C for 6 h. After the reaction was finished, the black precipitate was collected by centrifugation and washed with distilled water and anhydrous ethanol several times. Finally, the precipitate was dried in an infrared drying oven. By using single factor experiment, synthesis condition was optimized. The details of reactants and reaction conditions are shown in Table 1. The schematic of synthesis processes was shown in Fig. 1.

Table 1 The details of reactants and reaction conditions
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Fig. 1
figure 1

Schematic of the experimental procedure

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Characterization

The crystallographic structure of the products prepared was characterized by X-ray diffraction (XRD, D/max-RB 12 kW) with CuKα radiation (λ = 0.15418 nm), operating at 40 kV and 100 mA, and the diffraction patterns were carried out in the range of 10° < 2θ < 70°. The Debye–Scherrer equation is used to calculate the particle size of CuO powder:

$$D = \frac{k\lambda }{\beta \cos \theta }$$
(1)

where D is the mean size of the particles, k is a dimensionless shape factor (0.89) with a value close to unity, and λ is the X-ray wavelength (0.15418 nm), and β is the line broadening at half the maximum (FWHM) of the diffraction peak, and θ is the Bragg angle.

The varieties and contents of elements in samples were analyzed by JSM-7500F energy-dispersive X-ray spectroscopy (EDX, JEOL, Japan). The surface morphology of the as-prepared samples was analyzed by a SU800 scanning electron microscope (SEM, Hitachi, Japan). Transmission electron microscopy (TEM) images were performed on a JEOL 2200FS (200 kV) transmission electron microscope.

UV–Vis diffuse reflectance spectra (DRS) were performed by Agilent Cary 5000 UV–Vis spectrophotometer at room temperature.

Photocatalytic experiment

In the photocatalytic experiment, MB was used to assess the photocatalytic performance of the as-prepared CuO samples. An aqueous solution of 0.140 mol L−1 H2O2 and 0.01 g of CuO samples were added to 70 mL of original dye (10 mg L−1). The solution was stirred at room temperature. In the first 10 min, the solution was kept in dark conditions to achieve adsorption equilibrium. After that, the solution was continuously stirred and irradiated with a 125 W Hg lamp at irradiation wavelength 365 nm at room temperature. The lamp was 20 cm above the surface of the solution. During the whole experimental process, the solution was sampled and centrifuged to separate the catalyst at given time. The supernatants were analyzed by UV–Vis spectrophotometer (UNICO UV-2000) to obtain the absorbance value of different reaction time at 664 nm. Meanwhile, the degradation efficiency formula is given by Eq. (2),

$$\eta = \frac{{(A_{0} - A)}}{{A_{0} }} \times 100\,\%,$$
(2)

where η is the degradation efficiency (%), A 0 is the initial absorbance of dye, A is the absorbance of dye after the irradiation.

Results and discussion

Nanoparticle characterization

The crystallographic structures of the as-prepared CuO particles with different temperatures, reaction time, and alkali concentrations were characterized by XRD in Figs. 2, 3, and 4, respectively. In Fig. 2, CuCO3·Cu(OH)2 and NaOH solution (0.5 M) were heated at different temperatures (25, 40, 50, 60, 70, and 80 °C) for 6 h. When the temperature is below 40 °C, the diffraction peaks contain impure peaks. Cu(OH)2 was not converted into CuO completely. When the temperature goes up, all of the diffraction peaks in the XRD patterns of the as-obtained samples agree with the standard CuO XRD pattern, for which the 2θ values of 32.488, 35.550, 38.667, 48.840, 53.398, 58.204, 61.578 could be indexed to the (h k l) indices (1 1 0), (−1 1 1), (1 1 1), (−2 0 2), (0 2 0), (2 0 2), and (−1 1 3), respectively. Therefore, the minimum transition temperature of CuCO3·Cu(OH)2 to CuO is 50 °C, which is in good agreement with that reported in the literature [54].

Fig. 2
figure 2

XRD patterns of CuO produced with different temperatures

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Fig. 3
figure 3

XRD patterns of CuO produced with different reaction time

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Fig. 4
figure 4

XRD patterns of CuO produced with different concentrations of NaOH

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In Fig. 3, CuCO3·Cu(OH)2 and NaOH solution (0.5 M) were heated at 50 °C for different reaction times (3, 4, 5, and 6 h). The diffraction peaks of samples that were synthesized at 5 and 6 h can be indexed to the standard monoclinic structures of CuO crystal in great agreement with JCPDS No. (89-2529) without any characteristic peaks of impurities, such as Cu(OH)2 and CuCO3·Cu(OH)2. When the reaction time is <5 h, impure peaks exist in the diffraction pattern.

In Fig. 4, CuCO3·Cu(OH)2 and NaOH solution with different concentration (0.025, 0.05, 0.1, and 0.5 M) were heated at 50 °C for 5 h. The stronger diffraction peaks of the three samples agree well with the standard CuO XRD pattern. Among the investigated alkali concentration, CuO could be synthesized as long as the molar ratio of CuCO3·Cu(OH)2 and NaOH is 1:2.

Considering the volume of solution, 0.1 M NaOH solution is more appropriate. The optimal condition of CuO synthesis is as follows: the molar ratio of CuCO3·Cu(OH)2 and NaOH is 1:2 with 0.1 M of NaOH at 50 °C for 5 h. The grain size of CuO in the optimal condition is 13.6 nm which is calculated by Debye–Scherrer equation. According to the investigation of reaction conditions, the formation process of CuO could be inferred as follows:

$${\text{CuCO}}_{3} \cdot {\text{Cu}}({\text{OH}})_{2} + 2{\text{NaOH}} \to 2{\text{Cu}}({\text{OH}})_{2} + {\text{Na}}_{2} {\text{CO}}_{3}$$
(3)
$$2{\text{Cu}}({\text{OH}})_{2} \to 2{\text{CuO}} + 2{\text{H}}_{2} {\text{O}}$$
(4)

After CuCO3·Cu(OH)2 and NaOH aqueous solution mixed, Cu(OH)2 and Na2CO3 was produced at the beginning. Then Cu(OH)2 was dehydrated into CuO and H2O. The overall reaction was shown as follows:

$${\text{CuCO}}_{3} \cdot {\text{Cu}}({\text{OH}})_{2} + 2{\text{NaOH}} \to 2{\text{CuO}} + {\text{Na}}_{2} {\text{CO}}_{3} + 2{\text{H}}_{2} {\text{O}}$$
(5)

Besides CuO, Na2CO3, an important raw material in chemical manufacture, can be obtained by recrystallization.

The EDX spectra of the elemental compositions of CuO nanoparticles is shown in Fig. 5. The atomic ratio of Cu to O is 1.04:1, which is close to the theoretical stoichiometry of CuO (Cu:O = 1:1). The existence of C originates from conductive tape.

Fig. 5
figure 5

EDX spectra of the CuO samples

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Figure 6 shows the scanning electron microscopy (SEM) of as-prepared CuO. Figure 6a, the low-magnification SEM image, indicates that the as-prepared CuO nanostructures are sphere-like. To further examine the microstructure of the microspheres, the high-magnification SEM (Fig. 6b) image was recorded. The sphere-like nanostructures were actually constituted by numerous small nanoplates. The diameter range of microsphere is about 11.75–12.68 μm.

Fig. 6
figure 6

SEM images of CuO sample prepared at optimal synthesis condition

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The morphology and crystallographic characterization of the as-prepared CuO microsphere were further characterized by TEM (Fig. 7). It can be seen that the sphere-like products are composed of nanosheets, and some of these nanosheets have joined together and disorderly attached to form hierarchical features. It can be seen that the nanoplates which compose CuO microsphere is about 10 nm in thickness.

Fig. 7
figure 7

TEM image of CuO sample prepared at optimal synthesis condition

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The UV–Vis DRS of the as-prepared CuO samples was shown in Fig. 8. It has a broad absorption peak at about 394 nm. In addition, the optically wide direct band gap of the CuO particles from the spectrum can be evaluated by the following equation:

$$\left( {\alpha h\upsilon } \right)^{n} = {\rm A}\left( {h\upsilon - E_{\text{g}} } \right)$$
(6)

where hυ is the photon energy, α is the absorption coefficient, A is a constant relative to the material and the matrix element of the transition and n is either 2 for a direct electronic transition or 1/2 for an indirect electronic transition. CuO is direct electronic transition, so n is equal to 2. The inserted spectrum was the best linear relationship of plotting (αhυ)2 and hυ. The straight line to the x axis (α = 0) is the optical absorption edge energy of the as-obtained CuO. The band gap (E g) value is about 1.92 eV which is larger than the normal value of bulk CuO (E g = 1.85 eV). The result shows a blue shift in band gap energy with respect to the corresponding bulk CuO. The increase of the band gap is ascribed to the quantum size confinement effects for semiconductors [55, 56]. Similar results have been obtained from CuO nanoflakes (2.15 eV) [53], CuO nanoparticles (2.43 eV) [55], and CuO nanowires (3.48 eV) [57].

Fig. 8
figure 8

UV–Vis DRS of CuO sample prepared at optimal synthesis condition

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Photocatalytic activity

The catalytic activity of the as-obtained CuO samples was estimated by the degradation of MB at room temperature. In Fig. 9, the degradation efficiency of MB is only 1.78 % in the absence of H2O2. While in the presence of H2O2, the degradation efficiency is much higher (98.78 %). Therefore, H2O2 is a necessary factor for the photocatalysis experiment.

Fig. 9
figure 9

Effect of the presence of H2O2; [MB] = 10 mg L−1

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The degradation efficiencies of MB versus concentration of H2O2 (Fig. 10) with 0.01 g CuO are almost the same in 100 min, but there is much difference in the degradation rate. The degradation of MB dye follows a first-order kinetic process, which declines linearly in the reaction. The kinetic equation is given by Eq. (7) [58, 59],

$$\ln \left( {\frac{C}{{C_{0} }}} \right) = - kt,$$
(7)

where C 0 is the initial concentration of reactants, C is the concentration of reactants, k is the first-order reaction rate constant, t is the reaction time. Because the concentration of reactants is proportional to the absorbance, C/C 0 is equivalent to A/A 0. The variation trend of kinetic slope is equal to the variation trend of reaction rate constant. Increasing the concentration of H2O2, more ·OH radicals were produced. However, when the concentration of H2O2 exceeds optimal concentration, the degradation rate will decrease. Because H2O2 react with ·OH radicals to scavenge ·OH radicals at higher concentration of H2O2, which is shown in Eqs. (8) and (9) [58, 60].

$${{\text{HO}}^{ \cdot }} + {\text{H}}_{2} {\text{O}}_{2} \to {\text{H}}_{2} {\text{O}} + {{\text{HO}}_{2}^{ \cdot }}$$
(8)
$${{\text{HO}}_{2}^{ \cdot }} + {{\text{HO}}^{ \cdot} } \to {\text{H}}_{2} {\text{O}} + {\text{O}}_{2} .$$
(9)
Fig. 10
figure 10

Degradation of MB versus concentration of H2O2 (a) and photodegradation kinetics of MB (b); [MB] = 10 mg L−1; m CuO = 0.01 g

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Figure 11 illustrates the degradation efficiency and the degradation kinetics of MB versus different CuO dosage. Compared with the absence of CuO, the degradation efficiency increased obviously with the presence of CuO. So CuO is an effective catalyst in the photodegradation of MB. With the increase of CuO dosage, the slope of straight line goes up gradually. Because of the increased CuO dosage, both the adsorption of UV-light and the formation of active species were improved. When the dosage of CuO is excessive, the degradation efficiency is decreased because of the aggregation of solid particles [61–64].

Fig. 11
figure 11

Degradation of MB versus CuO dosage (a) and photodegradation kinetics of MB (b); [MB] = 10 mg L−1; H2O2 = 0.140 mol L−1

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The effect of initial dye concentration on the degradation of MB was investigated under irradiation. The range of MB concentration was changed from 5 to 15 ppm. The degradation efficiency and photodegradation kinetics were shown in Fig. 12. When the MB concentration is increased, the degradation efficiency was reduced. With the increase of dye concentration, much more dye molecules were adsorbed on the surface of CuO. The dye molecules which adsorbed on the catalyst surface hinder the adsorption of OH− ions and decrease the active sites for the adsorption of OH− ions. It leads to the reduction of the formation of ·OH radicals. Also, when the UV-light penetrates the solution, redundant dye molecules prevent the photos from reaching the surface of CuO. It brings on a decrease amount of photons which were adsorbed by CuO and a reduction of degradation efficiency [32, 62–64]. It can be seen that the photodegradation efficiency decreases with the increase of dye concentration when the dosage of CuO remains constant.

Fig. 12
figure 12

Degradation of MB versus concentration of MB (a) and photodegradation kinetics of MB (b); H2O2 = 0.140 mol L−1; m CuO = 0.01 g

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In the degradation processes, addition of H2O2 is good for the formation of ·OH, so ·OH could probably be the main active species. In order to confirm the conjecture above, tert-butyl alcohol (TBA) was used as the hydroxyl radical scavenger [65]. The reaction between the hydroxyl radical and TBA is shown in Eq. (10),

$$\left( {{\text{CH}}_{3} } \right)_{3} {\text{COH}} + {{\text{HO}}^{ \cdot }} \to \left( {{\text{CH}}_{3} } \right)_{3} {{\text{CO}}^{ \cdot} } + {\text{H}}_{2} {\text{O}}_{2}$$
(10)

Figure 13 illustrates the degradation efficiency with the presence of TBA as radical scavenger at 0.01 g CuO, 0.140 mol L−1 H2O2 , and 10 mg L−1 MB. The degradation efficiency of MB is decreased with the increasing quantity of TBA. Therefore, the results indicate that the hydroxyl radical is an oxidative intermediate and the main active specie in the catalytic degradation of MB. The possible chemical reactions of photodegradation are given as follows [32, 33]:

$${\text{CuO}} + h\nu \to h_{{\nu {\text{B}}}}^{ + } + e_{\text{CB}}^{ - }$$
(11)
$${\text{H}}_{2} {\text{O}} + h\nu \to 2{{\text{HO}}^{ \cdot }}$$
(12)
$${\text{H}}_{2} {\text{O}}_{2} + e_{\text{CB}}^{ - } \to {{\text{HO}}^{ \cdot} } + {\text{OH}}^{ - }$$
(13)
$${\text{H}}_{2} {\text{O}} + h_{{\nu {\text{B}}}}^{ + } \to {{\text{HO}}^{ \cdot} } + {\text{H}}^{ + }$$
(14)
$${\text{OH}}^{ - } + h_{{\nu {\text{B}}}}^{ + } \to {{\text{HO}}^{ \cdot} }$$
(15)
$${\text{Dye}} + {{\text{HO}}^{ \cdot}} \to {{\text{Dye}}^{ \cdot}} \to {\text{degradation}}\,{\text{products}}$$
(16)
$${\text{Dye}} + h_{{\nu {\text{B}}}}^{ + } \to {\text{Dye}}^{ + } \to {\text{degradation}}\,{\text{products}}$$
(17)

In order to test the photocatalyst stability of CuO nanoparticles, the catalyst was collected after the degradation process. Then CuO nanoparticles were washed with distilled water and dried at ambient temperature. The collected CuO was reused as the catalyst for the degradation of MB. The data are collected in Table 2. The photocatalyst stability was tested during five runs of photocatalytic reaction and the degradation efficiencies were 98.63, 97.67, 96.42, 95.27, and 93.98 %, respectively. The degradation efficiency is only slightly decreased with the increase of running times. Therefore, CuO nanoparticles are relatively stable photocatalysts.

Fig. 13
figure 13

Effect of radical scavenger (TBA) on the degradation of MB; [MB] = 10 mg L−1; H2O2- = 0.140 mol L−1; m CuO = 0.01 g

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Table 2 Effect of cycle number on the photocatalyst stability of CuO nanoparticles
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Conclusion

In this paper, CuO samples have been fabricated by using CuCO3·Cu(OH)2 as starting material. In the reaction process, Cu(OH)2 occurred as an intermediate which was converted into CuO at 50 °C for 5 h. The CuO microsphere with about 11.75–12.68 μm in diameter was made up of numerous nanoplates with about 10 nm in thickness. CuO nanoparticles show a good photocatalytic property and photocatalyst stability in the degradation of MB dye under UV-light irradiation.