1 Introduction

The micro-/nanostructure studies demand a better understanding of crystal facet engineering with tailored architecture that can be attained by the new design and facile synthesis methods [1,2,3]. In the past few decades, cupric oxide (CuO) is intensively studied binary transition metal oxide [4]. CuO nanostructures with large surface areas and potential size-effects possess superior physical and chemical properties that remarkably differ from those of their micro- or bulk counterparts [5]. It has excellent architectures with different shapes and dimensions, such as zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanotubes, 1D nanowires/rods, two-dimensional (2D) nanoplates, 2D nanolayers as well as several complex three-dimensional (3D) nanoflowers, urchin-like and spherical-like nanostructures [6, 7]. These nanostructures have been extensively used in various applications such as solar cells [8], photodetectors [9], field emissions [10], lithium-ion batteries (LIBs) [11], magnetic storage media [12], energetic materials [12], electrochemical sensors/bio-sensors [13], supercapacitors [14], nanofluid [15], removal of inorganic pollutants [16], photocatalysis [17], and so on. In addition, the complex geometry of ordered self-assembly of CuO nano/microscale building blocks is a hot topic in recent materials research [4]. Several important innovations have been directed toward the production of CuO, out of which many of them involve complexity of chemical reactions and problems associated with the reproducibility [1].

Thus, an alternative, environmentally approachable method is required. Green route-assisted CuO nanostructures have been recognized as a technologically imperative material with its several applications in the fields of cutting-edge science and technology [18]. The consumption of plants in the biosynthesis of CuO-NPs involves the content of secondary metabolites as reducing agents [19]. Apparently, biological agents act as reducers, stabilizers, or both in the process of making nanoparticles [20]. Several approaches for CuO synthesis and surface modification have been proposed through utilizing various parts of plants such as leaves, fruit, and flowers [21,22,23,24]. Several microorganisms, plants, and plant extracts have been extensively used to synthesize CuO nanoparticles (Table 1) to avoid the consumption of toxic chemicals [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. The O. sanctum (Tulsi) is supposed to contain oleanolic acid, rosmarinic acid, eugenol, carvacrol, Linalool, β-caryophyllene, and ursolic acid [39,40,41,42]. The oil extracted from O. sanctum leaves contains a higher amount of eugenol with the balance presence of numerous trace compounds, typically terpenes [43]. O. sanctum is a small herb that is seen all over India and extremely used in medicinal purpose. It is also known as phytomedicine plant and has been recognized as owning antioxidant, antimicrobial properties and non-toxicity [44], which has encouraged us to perform the current investigations.

Table 1 Copper oxide nanoparticles prepared in Plant Extracts by chemical reduction methods [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]
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CuO nanoparticles synthesized using leaf extracts had shown good photocatalytic efficiency against methylene blue (MB) dye [45,46,47]. Moreover, Sreeju [48] had reported that the CuO-NPs are effective against bacteria killing. Biosynthesized CuO nanoparticles exhibit good antibacterial property for both gram-positive and gram-negative microbes [35]. These reports reveal that the green chemistry-assisted CuO nanoparticles are highly promising candidates for photocatalytic as well as antimicrobial activity. However, to the best of author’s knowledge, there have been no reports on a complete investigation of the photocatalytic and antibacterial properties of O. sanctum (Tulsi)-extracted Eugenol (4-allyl-2-methoxyphenol)-assisted CuO-NPs. Thus, the aim of the present work is to synthesize CuO nanostructures using eugenol extracted from O. sanctum leaves (the detailed eugenol isolation procedures are shown in Electronic Supporting Information (ESI)), and the obtained product was evaluated for the photocatalytic activity against the organic dye (methylene blue) for water rectification and bacteria killing.

2 Experimental Details

All the details such as the extraction of eugenol from O. sanctum leaves, synthesis of copper oxide nanostructures, characterizations, and photocatalytic and antibacterial measurements are reported in ESI.

3 Results and Discussion

3.1 Synthesis Mechanism and Morphological Analysis

The plant extracts derived from various plants as shown in Table 1 have been reported for CuO-NSs synthesis by the green approach. It is well known that the most preferred green approach method is bio-reduction that includes the reaction between the biologically active produces isolated from plants with CuO in the reduced state [49]. In view of those ideas, we have chosen O. sanctum (Tulsi) leaf for the extraction of eugenol as a capping agent well as a stabilizing agent. In the beginning stage experiment, we have used the steam distillation setup to isolate eugenol oil from O. sanctum leaf extract, the mass of product isolated from O. sanctum leaf extract was examined through gas chromatography–mass spectrometry and confirmed the isolated product is 4-allyl-2-methoxyphenol (eugenol) (Fig. S1). The eugenol has a phenylpropene and an allyl chain-substituted guaiacol [40] and six reaction sites (acts as a hexadentate ligand) to form Cu2+ ion complex [50]. Based on the above assumptions and using the Job’s method, we have explained the possible growth mechanism schematically as shown in Fig. 1a. The OH− ions coordinate with Cu2+ ions and control the reaction process under alkaline conditions, leading to nucleation and hence the growth of CuO micellar structures [51]. These structures form a network with each other through van der Waals forces and hydrogen bonding resulting in the formation of observed geometry. From the examination of eugenol structure we have found, it had replaceable hydrogen and a neighboring donor in the oxygen of the o-methoxy group [52] and generally shares two eugenol molecules to one copper in the formation of \({\text{Cu}}^{{2 + \left( {\text{eugenol}} \right)_{2}^{ - } }}\) complex [53]. This process is led by the active reduction of Cu2+ ions through acid–base reactions and followed by nanoparticle formation, presented as Eqs. 1 and 2:

$${\text{Cu}}^{{2 + \left( {\text{eugenol}} \right)_{2}^{ - } }} + {\text{H}}_{2} {\text{O}} \to {\text{Cu}}\left( {\text{OH}} \right)_{2} + 2\left( {{\text{H}} - {\text{eugenol}}} \right)$$
(1)
$${\text{Cu}}\left( {\text{OH}} \right)_{2} + 2\left( {{\text{H}} - {\text{eugenol}}} \right)\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\rightarrow}$}}{\Delta } {\text{CuO}} + 2\left( {{\text{H}} - {\text{eugenol}}} \right)$$
(2)
Fig. 1
figure 1

a, b Tentative mechanism of crystal growth through copper–eugenol complex process. c FESEM images of eugenol-assisted CuO nanoflowers with different magnifications. d EDX spectrum. e TEM image, and f SAED pattern of eugenol-assisted CuO nanoflowers. (Color figure online)

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As the time elapses, few free molecules in the reaction mixture start to redeposit on the faintly larger particles to attain a thermodynamically stable state [4]. This condition leads to the complete exhaustion of the smaller particulates, further resulting in a large flower-like shape. The evolution of the flower-shaped CuO-NSs is believed to be the result of eugenol capping, and the growth mechanism can also be understood through the microstructural investigation. The FESEM image for surface morphology of CuO flower-shaped structures is shown in Fig. 1a (magnification 10.0 k×, scale 2 μm). It was clearly seen that the flower-shaped branches of the single product grow in different directions and are formed in large quantity with almost uniform sizes. The rich assessment of the single flower-shaped structure is illustrated in the inset of Fig. 1c, which exposes that the flowers comprise several triangular-shaped petals. The diameters of the petals are different from the roots to the tips (i.e., display sharpened tips with the broader roots). The broader roots of the petals are associated with each other, fixed in one center and in conclusion built a lovely flower-like morphology. The single petal length is approximately 150–200 nm with a diameter of around 50–30 nm at their roots, and tips are about 20–30 nm. A complete one-flower-shaped structure is ~ 250 nm in range, and had spectral signal of elemental oxygen and copper ions only in EDX analysis (Fig. 1d). The petal of the flower-shaped structures is a buildup of some thousands of tiny particles as displayed by transmission electron microscopy (TEM) images (Fig. 1e) which validate the results observed in the FESEM. Figure 1f shows the SAED pattern of the circled portion of single petal shown in Fig. 1e. The bright spots reveal that the made petals have crystalline features [54, 55].

3.2 Structural and Optical Analysis

The crystallographic phase of the as-prepared flower-shaped CuO-NSs was investigated via powder (D8 advance) X-ray diffraction pattern (XRD) technique. Rietveld analysis of XRD pattern is shown in Fig. 2a. Refinement was undertaken in space group C 62h , C2/C for monoclinic CuO with all atoms in general positions [57, 58]. The marked (110), (002), (\(\bar{1}\) 11), (111), (200), (11 \(\bar{2}\)), (20 \(\bar{2}\)), (112), (020), (021), (022), (11 \(\bar{3}\)), (113), (310), (113), and (220) hkl diffraction planes (│standing line for Bragg position θ) are well indexed to standard CuO (JCPDS card No. 48-1548). The three-dimensional view of the flower-shaped CuO-NSs crystal is built with the help of VESTA software as a depicted inset in Fig. 2a.

Fig. 2
figure 2

a Rietveld refinement of the XRD pattern and b Raman spectrum of eugenol-assisted CuO nanoflowers with the high-resolution fitted XPS spectrum of c Cu 2p and d O 1s. (Color figure online)

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After numerous recursive refinements, the possible best-refined lattice parameters obtained are as follows (weighted profile factor (Rwp) = 12.3, profile factor (Rp) = 11.9, expected R-factor (Rexp) = 7.8, Bragg R-factor (RBragg) = 7.02, goodness of fit (GOF) = 1.03 and χ2 = 1.48) with unit cell parameters a = 4.6878 Ǻ, b = 3.4269 Ǻ, and c = 5.14567 Ǻ, and crystallite size ~ 15.7 nm, by using Scherrer’s formula [56]. Additionally, refinement data (solid line) are in good agreement with experimental (● circle) data as the difference between these two is very less without any variations (solid line). Thus, the formation of CuO phase is predominant in the prepared sample.

To further support and clarify the crystallographic information, Raman spectroscopy was performed on the prepared sample (Fig. 2b). The spectrum was taken at 533 nm excitation wavelength with He–Ne laser at room temperature (RT). The peak located at 277.3 cm−1 is assigned to be Ag mode at high frequency, which corresponds to the in-phase/out rotation of the Cu and O atoms in the monoclinic phase [44]. The occurrence of the B 1g and B 2g modes discloses the bending and the symmetric oxygen stretching of the Cu–O assigned to the monoclinic crystal structure of CuO that is consistent with the XRD result [59].

Further, in-depth analysis of chemical compositions and X-ray spectroscopy was performed. No impurity was observed for the prepared sample through the XPS survey spectrum (Fig. S1). The high-resolution core-level spectrum of Cu 2p and O 1s is schematically shown in Fig. 2c, d. Conferring to Fig. 2c, the Cu 2p peak of CuO was fitted into four peaks, consisting of two kinds of spin–orbit lines, named as SP-1 and SP-2 which were located at higher binding energies as compared to the main peaks which infer the occurrence of an empty Cu-3d9 shell and consequently approve the existence of Cu2+ in the sample [60]. The characteristic peaks located at 934.27 and 954.26 eV were assigned to the Cu 2p3/2 and Cu 2p1/2 peaks with the binding energy difference between ~ 19.9 eV which further confirms the formation of CuO [61]. Figure 1d shows a high-resolution O 1s spectrum of flower-shaped CuO-NSs. Broad asymmetric curves were fitted to three sub-peaks named as Oa, Ob, and Oc for binding energies between 529–530, 530–531, and 532–533 eV, respectively [62]. There co-existed lattice oxygen (Oa ~ 529.98 eV), Cu(OH)2 (Ob ~ 531.4 eV) and adsorbed oxygen from hydroxyl groups (Ob ~ 532.2 eV) of CuO-NFs formation via green route synthesis method. The UV–vis-NIR absorption spectrum of the flower-shaped CuO-NSs evaluated optical properties (Fig. S2). The absorption edge of the flower-shaped CuO-NSs is ≈ 560 nm. Inset of Fig. S2 shows that the Eg of the as-prepared flower-shaped CuO-NSs is ≈ 2.31 eV, as projected by applying Kubelka–Munk theory to the absorption spectrum [63].

3.3 Photocatalytic and Antibacterial Activities

Methylene blue (MB, C16H18N3SCl) [64] dye degradation was carried using the as-prepared flower-shaped CuO-NSs. The setup and testing are provided in ESI. MB is a thiazine cationic dye which has an absorption peak at λmax ≈ 663 nm (π → π*) (Fig. 3a). Additionally, the absorption spectra of an MB solution photocatalyzed through H2O2 (alone) and flower-shaped CuO (alone) are shown in Fig. S3. The H2O2 was used to improve the degradation rate of the MB dye [65]. The absorbance depends on the number of molecules reacted with it. The photocatalytic activity (absorption spectra) of the flower-shaped CuO + peroxide (H2O2) was observed when it is used as a photo-catalyzer of the methylene blue dye (MB) solution under UV light. It is seen from Fig. 3b that the intensity of absorption peak at λmax decreases from 0.66 to 0.04 a.u. as reaction time increases from 0 min to 120 min and had no new absorption peak during the entire reaction process. This exhibits the comprehensive photodegradation of MB. Also, the histogram (Fig. 3c) shows that around 90% degradation was reached after 120 min of exposure of light which have a strong proof that the flower-shaped CuO effectively degraded the MB dye molecules. The graph of radiation time against ln(C0/C) (Fig. 3d) shows kinetics [64,65,66] of green synthesized flower-shaped CuO-NSs photocatalyst based on the model reaction. It follows pseudo-first-order kinetics. (A straight line in the slope is equal to the rate of degradation.) The rate constant of MB dye degradation by the photocatalyst flower-shaped CuO + peroxide (H2O2) is 0.05321 min−1. The possible proposed main reaction involved in photocatalytic degradation can be simply described as Eqs. 3–10:

$${\text{CuO}} + hv \to e^{ - } \left( {{\text{CB}} - {\text{CuO}}} \right) + h^{ + } \left( {{\text{VB}} - {\text{CuO}}} \right)$$
(3)
$$e^{ - } \left( {{\text{CB}} - {\text{CuO}}} \right) + {\text{O}}_{2} \to {\text{O}}_{2}^{ \cdot - } + {\text{H}}^{ + } \to {\text{HO}}_{2}^{*}$$
(4)
$${\text{HO}}_{2}^{*} + {\text{O}}_{2}^{ \cdot - } + {\text{H}}^{ + } \to {\text{H}}_{2} {\text{O}}_{2} + {\text{O}}_{2}$$
(5)
$${\text{HO}}_{2}^{*} + e^{ - } \left( {{\text{CB}} - {\text{CuO}}} \right) \to {\text{HO}}^{ - } + {\text{H}}^{*}$$
(6)
$${\text{H}}_{2} {\text{O}}_{2} + hv \to 2{\text{OH}}^{* \cdot }$$
(7)
$$h^{ + } \left( {{\text{VB}} - {\text{CuO}}} \right) + {\text{H}}_{2} {\text{O}} \to {\text{H}}^{ + } + {\text{OH}}^{*}$$
(8)
$$h^{ + } \left( {{\text{VB}} - {\text{CuO}}} \right) + {\text{HO}}^{ - } \to {\text{OH}}^{*}$$
(9)
$${\text{HO}}^{*} + {\text{OM}} \to {\text{Degradation}}\;{\text{intermediates}} \to {\text{CO}}_{2} + {\text{H}}_{2} {\text{O}} + {\text{salt}}$$
(10)

When the light (photon) strikes the surface of flower-shaped CuO-NSs, it gets absorbed.

Fig. 3
figure 3

a UV–vis absorption spectrum of MB dye, b spectral variation of MB dye in different time intervals, c photocatalytic degradation of MB dye under the irradiation of light over eugenol-assisted CuO nanoflowers. d First-order kinetic plot of ln(C0/C) versus time for the degradation of MB dye. (Color figure online)

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The photon (hv) with energy greater than or equal to the band-gap energy (Eg) of flower-shaped CuO creates an electron–hole (e− ↔ h+) pair, and both the valence band (VB) and conduction band (CB) receive equal amounts of photon generating h+ and e−, respectively, as shown in Eq. 3 [66]. These photoexcited carriers move to the surface of the flower-shaped CuO and react with oxidants such as O2 and reductants such as OH−, respectively [67]. Generally, the dissolved pollutants and O2 will be more prone to being adsorbed on the surface of the flower-shaped CuO in the mixed solution due to its larger specific surface area calculated through N2 adsorption–desorption analysis (Fig. S4). In the presence of photocatalyst, H2O2 oxidizes the CB and condenses itself to be extremely reactive •OH oxidizing potential. However, when it reacts with water molecules, which further oxidizes the stable MB into reactive intermediates, it stops the recombination process of electron-hole pairs [64]. Thus, the intermediate species (OH radicals, O2−, H2O2, and O2) interacted by surface charges of photocatalyst and caused a speed-up in the mineralization of dye molecules (OM) into the end-product carbon dioxide (CO2) and water (H2O) with less toxic inorganic acids [65]. Also to achieve our basic objective, we have utilized the as-prepared flower-shaped CuO-NSs as an antibacterial agent and tested their antibacterial efficiency using agar well diffusion process report by Naika et al. [26] and Sharma [68] against E. coli, S. aureus, and P. fluorescens bacterial strains.

Figure 4 illustrates the inhibition tendency of varied concentrations CuO-NFs. The O. sanctum leaves-extracted eugenol mediated synthesized flower-shaped CuO-NSs that played like a potential inhibitor at 100 µL concentration for all examined bacterial organisms, which is exposed from the inhibition zone [69]. Moreover, plant extract also shows noteworthy results (zone of inhibition) in contrast to the tested pathogenic organisms due to the existence of antibacterial efficiency in O. sanctum leaves extract [70, 71]. Also, due to the size of the as-synthesized flower-shaped CuO-NSs, strong electrostatic interaction between bacterial organisms could have been developed which oxidized the bacterial cell wall to destruct leading to immediate death [72,73,74]. The as-synthesized CuO-NFs show strong barrier against E. coli (29 ± 2 mm) at the concentration of 100 µg mL−1, while at concentration of 25 µg mL−1 weak barrier was found in all examined bacterial organisms. The obtained results confirm that the prepared flower-shaped CuO-NSs showed good antibacterial activity.

Fig. 4
figure 4

Comparison of antibacterial activity of eugenol-assisted CuO nanoflowers against Pseudomonas fluorescens, E. coli, and S. aureus using well agar diffusion process. (Color figure online)

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4 Conclusion

On the basis of the results and discussion of the present study, we can summarize that the flower-shaped CuO-NSs can successfully be synthesized via green route using Ocimum sanctum (Tulsi) leaves-extracted Eugenol (4-allyl-2-methoxyphenol) as a capping agent as well as the stabilizing agent. The results obtained from XPS analysis corroborated with the crystallographic (XRD, Raman) results, revealing the formation of pure monoclinic CuO nanostructure. The detailed morphological characterizations revealed that the Eugenol created OH− ions which lead to a high percentage exposure of active planes that encourage the formation of flower-shaped CuO nanostructures with high precision. The synthesized flower-shaped CuO-NSs possess photocatalytic activity with H2O2 oxidant against degradation of methylene blue. Moreover, the antibacterial activity of flower-shaped CuO-NSs has proven the biological importance in ecological and antimicrobial applications. The present work highlights the attractive benefits of O. sanctum-extracted Eugenol (4-allyl-2-methoxyphenol), e.g., high yield, less time, and an inexpensive and nontoxic route to synthesize flower-shaped nanostructures with excellent ecological properties.