Formation of self-assembled hierarchical structure on Zn doped in CuO nanoparticle using a microwave-assisted chemical precipitation approach

In the present work, CuO and Cu1-xZnxO were synthesized with the function of the Zn doping ratio by microwave-assisted chemical precipitation approach. The X-ray diffraction pattern shows that the mono-phase CuO with a mono-clinical structure and no other secondary phase has been observed for the Cu1-xZnxO with different Zn ratio and confirms CuO lattice does not get destroyed by the addition of Zn. The Raman spectra and HR-TEM analysis support the XRD results. The self-assembled hierarchical flower morphology was obtained for the higher doping ratio of Zn. The energy dispersive analysis of X-ray spectrum confirms the presence of Zn in the CuO lattice and the stoichiometry obtained. The optical band gap was found to be 1.78 eV for CuO nanoparticles, and the values are between 1.80 and 2.29 eV for Zn-doped CuO. For higher Zn-doped CuO, optical band splitting was observed due to flower-like morphology. The recombination of an electron–hole was reduced for higher doping ratio nanoparticles. These properties are needed for photocatalytic application.


Introduction
In recent years, photocatalytic activity using semiconducting nanoparticles is one of the important processes for the degradation of the organic pollutants. In particularly, metal oxides have greater attention in potential applications mainly due to unique characteristics like structural, electronic, and light absorption properties. Recent days, photocatalytic degradation has been driven by metal oxides. The photocatalytic degradation was investigated with several metal oxides such as SnO 2 , CeO 2 , Fe 2 O 3 , TiO 2 , MgO, MnO, and ZnO etc. [1,2]. Also, several hybrid structured metal oxides such as ZnO-SnO 2 , TiO-SnO 2 , Fe 2 O 3 -ZnO, a-Fe 2 O 3 -MnO 2 [3], CdO-ZnO [4], CuO/MnO 2 [5], and CuO-ZnO have been reported [6,7]. Among these, the wide bandgap metal oxides provide excellent response under ultraviolet irradiation. It gives growing attraction towards the choice of dopant and metal oxide-based composites formation. In addition, the photo-catalysts bandgap can be tunable through heterostructure formation. It can help to utilize the entire solar spectrum regions for photo-activation process. In order to enhance the photocatalytic activity, the catalyst ability has to be improved to absorb the organic pollutant and charge carrier recombination should be minimized [8,9].
Copper oxide, the typical p-type multifunctional material, has gained attraction in visible region photo-catalyst. The value of the energy bandgap ranges between 1.2 and 1.90 eV for pure CuO [10,11]. The crystal structure and dimension play a very important role in tuning the optical response and in enhancing the catalytic properties. In addition, it has unique advantages like abundance in nature, costeffectiveness, thermal stability, and excellent optical response. Several synthesis methods have been reported for CuO nanostructures such as co-precipitation [12] hydrothermal [13], solvothermal [10], microwave approach [14], sonochemical [15] and solgel methods [16]. Among these, the co-precipitation approach is a promising method for CuO nanostructure due to simple, minimal cost, and low temperature [17]. Also, microwave treatment is an effective process to attain nanostructures. It can provide sufficient penetrating strength into the materials and enhance the reaction during the synthesizing process. These conditions are much needed to boost the reaction process, enhance yield, and increase material purity, good stoichiometry control, and well-defined size distribution [18].
CuO-based metal oxides have a tunable bandgap in the lattice by host impurity ions. The optical structural, morphological, and electrical properties of the host material can be tailored by adding a metal [19,20]. CuO-based heterojunction catalysts have been reported and investigated due to its extraordinary photon absorption and charge carrier separation across the interface leading to reduce the electronhole recombination. Besides, the degradation performance of ZnO was found to be more active in ultra violet region due to wide bandgap.
On other hand, research has been performed by several investigators to understand the doping effects of metals such as Ni, Co, Zn, Cr, and Fe in CuO lattice. Among all transition metals, Zn causes more effective doping and possesses the same oxidation states [14,16,21]. Due to comparable ionic radii Cu2 ? , zinc ion causes more efficient doping and exhibits the same oxidation state. The zinc ionic radius (0.074 nm) is nearly equivalent to the copper ionic radius (0.072 nm). It is very easy to substitute the Zn ions in the CuO lattice without changing crystal orientation [18,22]. Besides, the Zn 2? doping can effectively produce defects in CuO nanostructures.
The photocatalytic activity of the catalyst also strongly relies on the surface and structural properties, such as surface area, crystal composition, bandgap, and particle size [23]. The average grain size is also important and directly related to its specific surface area, which improves the efficiency of the catalyst. The large surface area, unique morphology, and crystallinity play a vital role to enhance photocatalytic performance. CuO nanoparticle can almost satisfy the condition by adding Zn into CuO lattice and causes tailoring the properties such as structural, optical, and morphological for better photocatalytic performance. To the best of our knowledge, very few papers on CuO-and Zn-doped CuO nanoparticles are available to synthesize using a microwave-assisted chemical precipitation approach.
In the present study, single-phase CuO and Zn x-Cu 1-x O nanoparticles (x = 0, 0.03, 0.06, and 0.09 M) have been synthesized via a microwave-assisted precipitation method. The current work aims to investigate the effect of Zn doping on the structural, optical, and morphological properties of CuO. We investigated the impact of Zn doping, which could significantly boost photocatalytic activity.

Synthesis of CuO nanoparticle
The CuO and Cu 1-x Zn x O nanoparticles are synthesized by microwave-assisted chemical precipitation. In a systematic synthesizing process, The precursor of CuCl 2 is stirred in magnetic stirrer for 30 min. Now the precursor is turned into a transparent blue solution. Next, the KOH solution is added to the CuCl 2 precursor drop by drop. Then, the intermediate mixture is allowed to stir overnight at room temperature and light blue precipitation has been obtained.

Microwave treatment
After the process, the precursor mixture is irradiated with microwave for 180 s with the time interval of 10 s. The mixture is allowed to cool at room temperature, and now the blue color has turned into black precipitation. Ethanol is added to the final product. The solution is further centrifuged for about 30 min and then filtered, washed with distilled water and acetone several times. The obtained product is dried in a hot air oven about 80°C for 8 h and calcined at 600°C for 3 h in a box furnace. The same procedure was adopted for the synthesis of Cu 1-x-Zn x O nanoparticles with the addition of ZnCl 2 at various molar ratios. All the CuO and Cu 1-x Zn x O nanoparticles are characterized using powder X-ray diffraction, electron microscopy scanning, photoluminescence spectroscopy, and UV-Vis spectroscopy.

Result and discussion
3.1 X-ray diffraction  (044) correspond to the monoclinic structure of the CuO and are well-matched with the standard data (JCPDS card No. 27-0159). The same diffraction planes were obtained for all Cu 1-x Zn x O nanoparticles. No other significant peak is found and indicates a single monoclinic phase [24,25]. The well-resolved peak found at angles 35.888 and 38.358 belongs to (002), (111) planes, respectively. The decrease in intensity is directly related to the incorporation of Zn in the CuO lattice, which reduces the intensity. Changes in intensity and peak changes were observed due to the addition of the Zn 2? ions to the Cu 2? ions lattice. The structural properties have changed significantly due to a larger ion radius of Zn. Figure 2 shows the variation of (111) and (002) with a doping ratio of Zn. This indicates the position of the peaks slightly shifted toward the lower angle of Cu 0.97 Zn 0.03 O. In contrast, the peaks for Cu 0.94 Zn 0.06 O and Cu 0.91 Zn 0.09 O were dramatically shifted to higher angles than Cu 0.97 Zn 0.03 O. However, the peak position is much closer to the CuO nanoparticle for (Cu 0.91 Zn 0.09 O). The higher angle shifts might be pressure stress, while tensile stress contributes to lower angle shifts [24][25][26].
For CuO-and Zn-doped CuO nanoparticles with different Zn doping ratios, the average grain size (D), dislocation density (d), and micro-strain (e) were estimated and it is shown in Table 1 The FWHM values are decreased for low doping of Zn-doped CuO (Cu 0.97 Zn 0.03 O and Cu 0.94 Zn 0.06 O) and increased for higher doping of Zn (Cu 0.91 Zn 0.09 O) as seen in Table 1. The average grain size was estimated using Scherrer's relations [24,25]. The grain size was estimated to be between 22 and 27 nm for CuO and Zndoped CuO nanoparticles. The size of the grain is reduced by increasing the doping ratio compared to the CuO nanoparticles. The grain size is inversely proportional to the microstrain and dislocation density. This change is mainly due to the CuO lattice strain-induced mechanism [27]. The variation in the diffraction plane position has shown that However, the low doping ratios of Zn induce a slight increase in the grain size of Cu 0.97 Zn 0.03 O nanoparticles, but on a further increase in Zn doping decreases the grain size. To prevent the growth of CuO nanocrystals with Zn doping, the Zener Pinning effect is explained. Typically, the Cu-O-Zn bond is formed at the grain boundary due to the existence of a retarding force. In this case, the retarding force is greater than the driving force for the grain growth, the grain does not grow bigger and thus the size of the grain reduces.
The lattice constants of monoclinic CuO and Cu 1- x Zn x O nanoparticles were estimated using the following relation, and it can help to understand the substitution of radius Zn ions into Cu ions [28].
where a, b, and c are the lattice constants, d is the interplanar distance, a is the interfacial angle, and h, k, l are the Miller indices. The calculated lattice constant values are almost identical to the standard values and are in good agreement with the previous report as shown in Table 2. In comparison with CuO, the interfacial angle, unit cell volume and lattice constant values are slightly reduced by adding Zn content in CuO lattice. These values are in good agreement with the standard data values. This indicates that no significant changes have been made due to the addition of Zn in CuO lattice. It is an interesting consequence to note that the addition of Zn will not disrupt the monoclinic crystal structure of CuO. Figure 3 shows the CuO and Cu 1-x Zn x O nanoparticles SEM images with a different Zn ratio. The CuO nanoparticle image shows that randomly aligned rods and cubes were identified with irregular distribution of the particles as seen in Fig. 3a. This is mainly due to the anisotropic growth of the CuO monoclinic crystal and O 2and Cu 2? ions arranged  Table 1 The structural parameters of (0 0 2), (1 1 1) and (2 0 2) planes alternately in particular directions [29]. Figure 3b shows spindle-like particles for Cu 0.97 Zn 0.03 O. The hierarchical shape with random size spherical particle is obtained for Cu 0.94 Zn 0.06 O as seen in Fig. 3c. For the Cu 0.91 Zn 0.09 O, the petals are self-assembled and cause to form the three-dimensional hierarchical flower structures as seen in Fig. 3d. This may be due to the strong binding energy between individual petals due to the attractive electrostatic force. This force might be a key factor in the construction of a flower-like morphology due to self-assembled petals [29,30]. Similarly, a type of morphology has been reported for ZnO, CZTS in previous papers [31]. From the SEM study, the Zn doping ratio has a significant influence on the shape and particle size of the nanoparticle.   Figure 4 shows the optical absorption spectra of CuO and Cu 1-x Zn x O nanoparticles with different Zn doping ratios. It shows that the absorption edges for all samples were identified in the UV-Visible region. The absorbance edge of the Cu 1-x Zn x O nanoparticles was slightly shifted to a shorter wavelength (blue shift) than the CuO nanoparticles. The bandgap energy was determined using the following equation for CuO-and Zn-doped CuO nanoparticles [31].

Optical properties
where F(R) is the function of reflectance, ht is the incident photon energy, a is the absorption coefficient. The optical band gap (Eg) values are estimated from the Tauc plot by extrapolating (ahm) 2 to photon energy. Figure 5 shows that the optical bandgap of CuO and Cu 1-x Zn x O nanoparticles with different Zn doping ratios. The optical band gap was determined using Kubelka-Munk relation, and the estimated value is 1.78 eV for CuO as shown in Fig. 5a. The Eg values are 1.80, and 2.08 and 2.29 eV, respectively, for 3, 6, and 9% of Zn-CuO. The band gap value is slightly lower than the earlier reported values [22]. The optical splitting was observed and the values for 9% of Zn were found to be 1.90 and 2.29 eV. The optical band was tailored due to Zn addition in CuO lattice, and the energy gap values were increased by increasing the Zn percentage. The increase in band gap and optical splitting can be related with the quantum size effect [33,34]. However, the defects in anion oxygen vacancies and presence of anion can be reason for change in bandgap values. However, the oxygen vacancy including the localized electrons helps to improve the adsorption and activate the organic pollutant. Furthermore, these vacancy defects confine the carrier, restrict the recombination and transfer it to organic molecules [35]. The electronhole pair separation is favorable for photo-generation due to combined substitutional and interstitial defects formation in the bulk [36]. It can be correlated with morphological properties, which can also be expressed in the XRD results.

Photocatalytic performance
The photocatalytic activity was examined by methylene blue (MB) degradation under visible light conditions and irradiated by a halogen lamp source. In a systemic process, 20 mg of CuO is blended with 100 ml of MB solution known as a catalyst solution. The solution is being stirred in a dark condition for about 30 min to achieve the adsorption equilibrium. Now, after light irradiation, 3 ml of the catalyst solution was extracted from the beaker. In the same way, the process was performed for different time where A 0 is the dye solution absorbance without catalyst; A t is the dye solution absorbance in with catalyst after t. Figure 6 shows the photocatalytic activity of CuO and Cu 1-x Zn x O nanoparticles with different Zn ratios. In our case, methylene blue dye degradation with an absorption band around 660 nm with visible light irradiation is examined. Degradation of the dye was performed with the function of time and maintained 15 min time interval. The degradation of the dye was observed in a dark and light condition with and without a catalyst. Degradation of MB dye is found to be 45%, 47%, 53% and 64% for CuO, 0.03 M, 0.06 M and 0.09 M ratio of Zn, respectively. The variation in photon harvesting nature among various CuO nanostructures is due to the variation of bandgap with structure. Upon introducing Zn doping in CuO, the band gap values was increased and in addition to that the optical splitting was obtain for higher doped sample. The optical band gap region was tuned in the visible region from red (1.78 eV) to green region (2.29 eV) with function of Zn doping percentages. Besides, the morphological was tailored due to Zn in CuO with the different ratio. In particularly, there is possibility of multiple reflections during light incident on the hierarchical flower. This behavior can lead to enhance the photo-catalytic activity and surface morphology plays crucial role in photocatalytic degradation. In addition, the grain size and band gap are also play key role in the degradation of dyes and degradation efficiency values are in good agreement with the previous study [29]. From the above    [31,37]. It is mostly due to a flower-like morphology that will enhance recombination. The recombination of the electronhole pair with the Zn doping ratio is enhanced from the PL spectra. Figure 8 shows that the schematic representations of photocatalytic activity mechanism. Under UV-light irradiation, the following mechanism can be possible during photocatalytic degradation of MB dyes using our catalyst [37,38].

Photoluminescence spectra
MB dye þ OH ! Degrade products ð10Þ Figure 9a-c shows the high-resolution transmission electron microscopy images of Cu 1-x Zn x O nanoparticles with different ratio of Zn (3%, 6% and 9%). The mixture of spherical and rod-shaped particles was obtained with agglomeration, and it has reconfirmed the similar rod-like morphology from SEM images for 3% sample as seen in Fig. 9a. In Fig. 9b, the spherical particles were found with non-uniform distribution and the average grain size was found in between 22 and 42 nm for 6% sample. In the case of 9% sample, the random orientation of self-assembled layered petals was found with nano-range in dimension as seen in Fig. 9c. It is reassuring the formation of flower morphology in SEM results. All the samples exhibit the multiple rings pattern and reconfirm the poly crystallinity as illustrated in Fig. 9d-f. The multiple rings couldn't have periodicity due to the nano in dimension. Figure 9g confirms  Table 4. The above findings are good agreement with XRD results and SEM analysis. Figure 10 illustrates the Laser-Raman spectra of Cu 1-

Laser Raman spectra
x Zn x O nanoparticles with different ratio of Zn (3%, 6% and 9%). The Raman spectra were recorded at room temperature using a 785 nm excitation wavelength. The three Raman peaks were identified for all the samples as seen in Fig. 10. The well-defined peak at 273 cm -1 is assigned to A g mode of CuO; this characteristic peak confirms the CuO monoclinic structure. The two low intense peaks were found at 316 and 608 cm -1 and assigned to B g (1) and B g (2) modes of the CuO nanoparticles. Upon introducing Zn into CuO lattice, the weak Raman peak obtained at 329 cm -1 and which attributed to the second-order Raman spectrum arising from zone-boundary phonons of ZnO [39]. In comparison with CuO, the Raman active peak was broadened which was slightly shifted towards low frequencies side. The blueshift and peak broadenings are mainly related to quantum confinement characteristic due to the introduction of Zn in CuO. Moreover, there is no significant peak which refers to any other phases and confirms that all the samples have same crystal structure.

Conclusion
In the present work, CuO and Cu 1-x Zn x O nanoparticles were successfully synthesized with the function of the Zn doping ratio via microwave-assisted chemical precipitation approach. The XRD pattern reveals that the mono-phase CuO with a monoclinic structure and no other secondary phase has been observed for the Zn-doped CuO nanoparticles. It also confirms that the doping of the Zn does not destroy the CuO lattice and Raman spectra confirmed the same. The Raman spectra and HR-TEM studies supported the XRD analysis. The SEM images showed the morphological changes caused by the function of Zn doping. The flower-like morphology was obtained for Cu 0.91 Zn 0.09 O nanoparticles. The EDAX spectra indicated the presence of Zn in the CuO lattice, and the stoichiometry corresponds to the experimental ratio. The optical band gap was found to be 1.78 for CuO nanoparticles, and values range between 1.80 and 2.29 eV for Zn-doped CuO. The result showed that the incorporation of Zn increased bandgap values. Optical band splitting was observed for higher Zn-doped CuO due to flower-like morphology. The recombination of the electron-hole pair was estimated, and the recombination was enhanced by Zn doping. Photocatalytic performance efficiency showed higher efficiency of 64% for Cu 0.91 Zn 0.09 O nanoparticles. The crystallinity, grain size, and surface morphology are important factors to be considered in the photocatalytic degradation process. It clearly shows the direct relation between Zn doping and photo-catalyst performance. The result clearly shows that the structural, optical, and morphological and photocatalytic performance of Zn doping has been enhanced. We can tailor the optical band gap by Zn doping, which causes morphological change and leads to improved catalyst performance. From the above finding, the Zn is a more suitable metal in CuO lattice and is evident here.