Structural, electrochemical sensor and photocatalytic activity of combustion synthesized of novel ZnO doped CuO NPs

CuO nanoparticles doped with various concentrations of ZnO (5, 10, and 15 mol%) were synthesized by using the solution combustion method. The as-synthesized nanoparticles were characterized by x-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and UV–Vis spectroscope. The XRD analysis revealed that the physical parameters such as crystallite size and lattice parameters of CuO nanoparticles were affected after the doping of ZnO. The UV–Vis spectrum analysis showed an enhanced absorption spectrum and narrowed down the bandgap of CuO from 2.6 eV to 2.16 eV with ZnO doping and resulted in an increasing optical activity. The photocatalytic activities of the as-synthesized sample were investigated by the photocatalytic degradation of organic dyes such as direct green (DG) and fast blue (FB) under UV light irradiation. The highest photocatalytic efficiency is obtained with ZnO (10 mol%) doped CuO at 95.15% and 76.4% for DG and FB dyes. The electrochemical properties of CuO and Zn-CuO nanoparticles were performed using cyclic voltammetry and the results confirmed the enhancement of the redox potential output. These CuO@ZnO electrodes also displayed an enhanced capacity to detect an extremely dangerous chemical like arsenic.


Introduction
Copper oxide, also known as cupric oxide (CuO) has received greatest research attention as Cu 2 O and Cu 2 O 3 are not stable copper sub oxides [1]. The cupric oxide (CuO) has been extensively used for a variety of uses, including high-temperature superconductors, lithium ion batteries, solar cells, antibacterial agents, and catalysis. The P-type copper oxide is a semiconductor with a number of noteworthy qualities, including the ability to resist germs, high stability, and use. It has been used in a number of applications, including gas sensors, infrared photo detectors, supercapacitors, lithium-ion batteries, and photocatalysis [2][3][4][5]. Given that its optical absorption edge sits in the range between 1.2 eV and 1.9 eV [6,7], it is a perfect absorbent material for solar cells. It is a top-notch sensing substance and it has been synthesized by utilizing wide variety of methods [8,9]. The combustion approach employing the water bath technique stands out among them because it offers the advantages of low-temperature development, cheap cost, and a straightforward procedure [6]. It is possible to improve electrical, optical, and functional properties by doping transition metals into the semiconductor matrix [7,[10][11][12]. Although Zn 2+ and Cu 2+ have similar ionic radii and share the same oxidation states, it is the transition metal that produces the most effective doping overall [13][14][15][16]. One of the most dangerous gases and a significant air contaminant in the environment is nitrogen dioxide. It is often produced by burning fossil fuels, industrial waste, automotive byproducts, cigarette smoke, and power plants. Because of how it interacts with atmospheric water molecules, it causes acid rain [17,18]. As a result, ZnO-doped CuO octahedral crystals were synthesized in the current study employing the water bath method. In-depth research was done on the ZnOdoping effects on the structural, optical, and electrical characteristics of CuO octahedral crystals. The sensor films showed a selective response to low NO 2 concentrations at lower working temperatures of 150°C. On the basis of investigations on gas sensing and work functions, a sensing mechanism is presented. Due to their great chemical and physical stability, broad optical absorption spectrum, and non-toxic makeup. It is possible to use CuO nanostructures as a photocatalyst. Several CuO nanostructures, including spherical, one-dimensional, two-dimensional, and hierarchical nanostructures, have been developed and exploited for the photocatalytic eradication of organic contaminants and multidrug-resistant bacteria. Furthermore, CuO nanoparticles blended with other textiles have also been shown to exhibit strong antibacterial action in the past [12][13][14]. These textiles made from nanomaterials have been suggested for usage in medical apparel [13,14]. According to reports, CuO nanoparticles' physio-chemical behaviour can be modified by doping. Ni, Ce, and Zn, among other dopants, have recently been employed to increase the photocatalytic activity of CuO nanostructures. There are not many studies on ZnO-doped CuO nanoparticles' bactericidal effectiveness. Yet the concentration of ZnO dopant in those investigations is only one particular percentage. This is the first investigation emphasizing on effect of different doping concentrations of ZnO on CuO nanostructures are in killing bacteria. In the current study, CuO hierarchical nanostructures doped with varied Zn molar percentages were made using soft chemical techniques. The structural, morphological, optical, and antibacterial characteristics of the synthesized nanostructures have been thoroughly investigated [16,17].
Many chemical pollutants in the wastewater have the potential to harm the ecosystem. The most dangerous pollutant produced by the textile industry is chemical dye, whereas other synthetic dyes, organic dye components, and mineral dye components are derived from agricultural, urban, and industrial resources [18,19]. Many businesses run water purification facilities to remove toxins from wastewater at a low environmental cost in order to preserve and improve the quality of water for drinking and other applications [20,21]. Industries use a variety of treatments (recycling processes) to clean up tainted water. Heterogeneous photocatalysis is one of the popular and simple methods. A capable option for the removal of organic pollutants from water is the heterogeneous photocatalytic system [22,23]. The photocatalytic activity of the oxides increases with the surface area and fault density. The present photo-degradation approach makes use of the transition metal oxides as catalysts. By changing the absorbance to a longer wavelength, transition metal doping in oxides tunes the optical band and increases the crystal defect [24]. With a band gap of 3.4 eV, zinc oxide (ZnO) stands out among these oxides as a potential material for the photocatalytic applications [25,26]. ZnO nanoparticles are regarded as catalysts for the breakdown of organic pollutants due to their higher surface activity, high crystallinity, and distinctive microstructural features [27]. Due to its environmental stability, affordability compared to other nano size oxides, and greater surface area, which demonstrates enhanced photocatalytic activity, ZnO is recognized as an endowed semiconductor. ZnO is incredibly efficient when exposed to UV light, but it is quite inefficient when exposed to visible light. CuO-doped ZnO NPs are produced utilizing physical and chemical synthesis techniques to control the optical band gap of CuO and enhance photocatalytic efficiency [28]. The combustion approach of lowering band gap with CuO resulted in CuO@ZnO NPs with particle sizes of 25 nm. To examine the microstructure characteristics of CuO@ZnO NPs for photocatalytic application, we synthesized the varied concentrations (5, 10, and 15 wt.%) of doped CuO@ZnO nanoparticles by the flash combustion technique.

Synthesis of CuO-ZnO hybrid nanocomposite
High purity Zn(NO 3 ) 2 .6H 2 O, C 6 H 8 O 7 , and (Cu(NO 3 ) 2 ) were used for the synthesis of CuO-ZnO Hybrid Nanocomposite materials. The materials were purchased from Sigma Aldrich. Flash combustion approach was followed to synthesize the nanocomposites. The experiment involved grinding together 0.5 g of (Cu(NO 3 ) 2 ) and 0.5 g of C 6 H 8 O 7 in four separate porcelain crucibles. One of the four crucibles was handled as pure, while the other three were doped with Zn by properly grinding the calculated quantities of Zn(NO 3 ) 2 .6H 2 O (5, 10, and 15 wt%). All the four crucibles were later put into a highly stable furnace and heated for 3 h to 550°C. The crucibles were left undisturbed overnight and were allowed to cool down to room temperature [29,30]. All of the synthesized nanocomposites were taken out of the crucibles. The CuO@ZnO nanocomposites exhibited deeper colour than pure CuO, suggesting that there had been ZnO doping. As the end products were similar to fine materials, no additional grinding was required before any measurement. The samples were labeled as A, B, C, and D, respectively, for undoped CuO, 5% ZnO-doped CuO, 10% ZnO-doped CuO, and 15% ZnOdoped CuO.

Photocatalytic experimental procedure
Direct Green (DG) and Fast Blue (FB) dyes were used as model dyes in order to study photocatalytic activity of various CuO@ZnO nanocomposites. For this purpose, a 20 ppm or 250 ml aqueous dye solution along with 0.06 g of photocatalyst (nanocomposite) were taken in a glass crucible and irradiated with UV light source (Mercury vapour lamp, 370 nm) while being continuously stirred with a magnetic stirrer [31]. The UV light was focused directly onto the reaction mixture at a distance of 23 cm from the rim during the irradiation, which was done outside. A Shimadzu UV-vis spectrophotometer model 2600 was used to measure the absorbance between 200 and 800 nm [32].

Modification of carbon paste electrode (CPE) with CuO@ZnO
In an agate mortar, the prepared samples (A, B, C, and D), graphite powder (20 mg, 98% purity), and silicon oil were manually mixed for around 20 min As previously mentioned, [33,34], the produced carbon paste was next gently pressed into a handmade Teflon hollow tube with a surface area of 0.3 mm. Using Debye-Scherrer's formula for CuO-ZnO hybrid NCs, the particle size (D) of the calcined samples were deduced,   Where, λ = wavelength of the x-ray (λ = 1.541 A°), θ = angle of the Bragg, k = constant depending on the shape of the grain (0.94) [37]. The samples' typical particle size ranged between 25 to 30 nm. The Zn 2+ insertion strained the Cu 2+ sites of the CuO host lattice, which led to the observed increase in crystallite size. This behavior may be attributed to the slightly larger ionic radii of dopant Zn 2+ (0.074 nm) as compared to host ions Cu 2+ (0.073 nm) [38]. Also, with increasing ZnO content, there is a modest shift of the primary diffraction peak (111) towards a lower (2) angle, which might be attributed to the replacement of Cu 2+ ions in the CuO host lattice structure by Zn 2+ ions. SEM images show that the ZnO concentration has a substantial impact on the form and particle size of the synthesized nanocomposites. The fluctuation in structural disorder and micro-strain brought on by ZnO doping in the CuO host matrix may be related to this phenomenon. Estimated structural parameters of CuO-ZnO Hybrid Nanocomposites using XRD is tabulated in table 1.  15-25 nm, 25-35 nm and 35-50 nm for pure and 5, 10, and 15% of ZnO respectively. The average calculated particle size were 20 ± 2 nm, 30 ± 2 nm, 40 ± 2 nm and 50 ± 2 nm for pure, 5, 10, and 15% of ZnO doped CuO NCs respectively [38,40]. The surface area of the tiny particles is large, as a result, there are more atoms available on the particle's surface than those within. These surface atoms are more active in their interactions with other surface atoms nearby or in the adsorption of other species, which results in the creation of particle clusters [41,42]. Figures 4 (a)-(d) presents the diffused reflectance spectra of CuO and hybrid NC's of CuO-ZnO with different composition of zinc oxide. All the CuO-ZnO NCs exhibited maximum reflectance with visible sharp absorption edges. CuO-ZnO hybrid NC's has a higher absorption band located in the UV range with main absorption band at 320 nm [43]. The Kubelka-Munk function F(R) is calculated from equation (2) [44]. It is mostly used to evaluate powders and convert diffused reflectance into an equivalent absorption coefficient.

Diffuse reflectance spectroscopy (DRS)
Using equation (3), optical energy gap was calculated employing the Tauc relation.  utilized in the current study. The energy band gap was demonstrated to decrease from 2.6 eV to 2.16 eV by increasing the ZnO concentration. The degree of structural order or disorder in the lattice may affect the distribution of the intermediate energy level within the band gap, which may affect the lowering of Eg for doped samples [47,48]. Surface trap or point defects may also play a role in the reduction of Eg for doped NCs.

Photocatalytic studies
During photocatalytic experiment, the selected dyes Direct Green-23 (DG-23) and Fast Blue (FB) were degraded in the presence of UV radiation and solar illumination using the synthesized CuO, CuO+5% ZnO, CuO+10% ZnO, and CuO+15% ZnO NCs as photocatalyst.
CuO doped ZnO NCs were successfully applied towards the photocatalytic degradation of the model dyes Direct Green-23 (DG-23) and Fast Blue (FB). The photodegradation of dyes was carried out under UV light irradiation. 250 ml of 20 ppm dye solution and 40 mg of the photocatalyst (CuO, CuO+5%ZnO, CuO+10% ZnO, and CuO+15%ZnO NCs) were taken in a cylindrical glass reactor having 169.8 cm 2 surface area and exposed to UV light for 90 min The dye solution along with photocatalyst was continuously stirred while it was exposed to UV light, and 5 ml of the solution was pipetted out every 15 min to measure for absorbance using UV-Visible spectrometer [49][50][51]. To preserve the balance between adsorption and desorption, the experiment was first conducted in complete darkness. The dye solution appears to degrade at a negligibly low rate (4%) when it's in dark. In order to carry out photocatalysis, the dye was then exposed to UV for 120 min [52][53][54].
The photocatalytic activity of the CuO, CuO+5% ZnO, CuO+10% ZnO, and CuO+15% ZnO Nc's was measured using Direct Green (DG), a cationic dye with maximum absorption at 613.7 under UV light  The photodegradation percentage for DG dye was calculated as 32%, 65.4%, 95.15%, and 93.85% under UV light for CuO, CuO + 5% ZnO, CuO + 10% ZnO, and CuO + 15% ZnO, respectively, after 120 min Whereas, the percentage photodegradation values for the FB dye was calculated as 53.75%, 61.5%, 76.4%, and 71.3%  under UV for various CuO@ZnO NC's ( figure 7(a) and (b)). This clearly shows that when exposed to UV radiation, CuO + 10% ZnO exhibited higher photocatalytic behaviour as a catalyst for the degradation of DG and FB dye. Our findings show the importance of the synthesis technique, crystal size, shape, and electron-hole recombination in the photocatalytic degradation of DG and FB dyes [57,58]. Equation (4) was used to deduce the percentage degradation of the dye,  8(a) and (b)).
The mechanism of photodegradation of dyes using as synthesized CuO@ZnO NC's can be described as follows. As figure 9 suggest, doping CuO with ZnO has increased the number of electron hole formation and in turn improved the degradation of dyes under illumination of UV light. CuO NPs were prompted to excite  electrons from the valence band to the conduction band when exposed to UV light, leaving behind the same number of holes in the valence band. The electrons from the conduction band might react with H 2 O and O 2 molecules preasent in the vicinity of the NC's during this reaction to form superoxide radicals (O 2
As a result, as seen in figure 9, the photo degradation of dye is substantially determined by the amount of CuO@ZnO photocatalyst used. In addition to degradation of dyes, the synthesized NC's must be stable and reusable. For instance, the stability of a CuO+10%ZnO sample under UV light for the degradation of DG and FB dyes for 5 cycles was studied and the sample exhibited high photocatalytic activity and reliability [62]. As shown in figures 10(a) and (b), the degradation percentage has only slightly reduced after each run which can be attributed to the reduction of number of active sites available on the surface of the photocatalyst. Because nanoparticles were used for this study and due to their highly magnetic nature, the photocatalyst was readily removed using a magnet after each cycle [63,64]. In order to verify the role of free radicals in the degradation of DG and FB dyes in the presence of CuO+10%ZnO photocatalyst under UV light, scavenging experiments were also carried out, and the results are displayed in figures 11(a) and (b).  For scavenging experiment, three different scavengers including AgNO 3 , ethanol, and benzoic acid were used in accordance with the experimental methods. Under UV light, in the presence of these three scavengers the degradation values for DG were 87.4%, 80.14%, and 76.4%, respectively, and for FB it was 57.4%, 50.14%, and 46.4%. The experimental photo-degradation results therefore showed that holes (h + ) are primarily responsible for photo degradation, since scavengers inhibited electrons, OH• radicals, and holes after 120 min [65]. CuO +10%ZnO NPs are hence useful for treating sewage water.

Cyclic voltammetric studies
The reversibility of the electrode reaction and the material's charge transfer effectiveness were examined at room temperature using cyclic Voltammetric tests. For the investigations, a three-electrode setup with a working electrode made of prepared graphite, a reference electrode made of Ag/AgCl, and a counter electrode made of platinum was employed. Figure 12 displays the CuO and ZnO-doped CuO samples in 0.1 N HCl electrolyte (a)-(d) for the conduction of electrochemical experiments. A pair of peaks in the CV is seen at the undoped CuO electrode as a result of the oxidation and reduction of Zn and Cu, respectively. This resulted in the peaks of oxidation and reduction in samples of 5% doped ZnO. The increased peak current in 10% ZnO-doped CuO was explained by the zinc doping's better conductivity and electrocatalytic characteristics [66].
It's interesting that this sample had a significantly higher electrocatalytic activity than the others. It had a few distinct and powerful redox peaks. The charge carriers' proximity to CuO's conduction band, which may be implicated in charge transfer at the metal oxide/electrolyte interface, caused the current density to rise [67]. Based on this finding, the concentration of the dopant was increased by 10%, which increased the electrical conductivity, speed up the electron transfer kinetics, and produced outstanding electroactive surfaces, all of which improved the electrochemical response [68].
As evidence for this, Nyquist plots, like the one in figure 13, illustrate how the resistance is directly correlated with the semicircle's width: the wider the semicircle, the greater the resistance. In figure 13, electrode B has the smallest semicircle, and the low-frequency imaginary line is travelling in the direction of the Y-axis. Moreover, it demonstrates that electrode C has higher capacitance and lower resistance than other electrodes [69].
An analogous circuit model was used to compare their mathematical EIS evaluations for each electrode, as shown in the inset of figure 13 and validated by table 2. All of the prepared electrodes were found to have generally vertical linearity, especially for pure CuO electrodes beyond 100 ohms with the real axis but for doped electrodes beyond 50 ohms.
By fitting the experimental data to the equivalent circuit, table 2 provides the RS, RCt, Cdl, and Zw parameters for the produced CuO carbon paste electrodes with various zinc concentrations. The electrode's electrochemical activity is being improved as RCt decreases and Cdl rises. The information at hand made it very evident that CuO+ 10% ZnO electrode had higher electrochemical activity.
The CV findings after the produced electrode was tested with arsenic to ascertain its sensing capacity for CuO and a hybrid nanocomposite of CuO-ZnO with various compositions are shown in figures 14(a)-(d). By monitoring the change in analyte concentration at the created CPE, the electrochemical mechanism of the arsenic analyte was thoroughly understood. Both the cathodic peak current and the anodic peak current  increased linearly with respect to arsenic content (figures 15(a)-(d)). Even though the cyclic voltammogram's shifting oxidation and reduction peak locations show that the constructed electrodes detect arsenic metal electrochemically.
The lowest limit of detection for the created electrode was established to be 1×10 −3 mol l. The diffusioncontrolled activity that was seen within the concentration range is supported by the linear fitting reported in the calibration curve [53].

Conclusion
In summary, CuO nanoparticles doped with various concentrations of ZnO (5, 10 and 15 mol%) were successfully synthesized by the solution combustion method. The detailed characterization showed the influence of ZnO dopants in the crystal structure, morphology, optical, photocatalytic properties and electrochemical properties of CuO nanoparticles. It was found that with the increase in ZnO concentration, the crystallite size increased. Moreover, the bandgap of CuO nanoparticles decreases from 2.6 eV to 2.16 eV after ZnO dopants concentration increase from 5 mol% to 15 mol%. The best photodegradation efficiency was obtained with 10 wt % CuO/ZnO. The values of DG degrading with un-doped CuO and CuO/ZnO were measured to be 32% and 95.15% respectively, after 2 h of UV light irradiation and for FB photodegradation the values were 53.75% and 76.4%. The CuO and CuO/ZnO graphite modified electrode offered an appreciable repeatability and reproducibility. The fabricated electrode selectively detected the arsenic in the presence of  intrusive ions with lowest limit of detection of x10 −3 mol l −1 . Moreover, the developed sensor electrode showed good stability. Therefore, the present low-cost CuO/ZnO (5%, 10% and 15%) system can be used as an excellent nano electrode-based system for the electrochemical detection of arsenic. This sensor electrode can be used potentially for the precise detection of arsenic into food and industrial samples.

Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
No funding received.