1 Introduction

The synthesis of binary II–IV semiconductors gained immense concern due to their distinctive optical, electric, magnetic, and physical properties. CdO is an II–IV semiconductor that belongs to the metal oxides family that has been under study lately due to its enormous range of applications such as photovoltaic cells [1], solar cells [2], and gas sensors [3]. The alterable oxygen deficiency and the switchable cation vacancies are two characteristics that make such oxides unique. However, using their nanoparticles has revolutionized the era, enhancing the multiple properties to a great extent, especially after being nanoscaled into nanowires, nanoneedles, nanobelts, and nanospheres.

CdO is an inorganic compound having a face-centered cubic rock salt structure similar to NaCl, having Cd2+ ions octahedrally coordinated to its neighbors O2− ions. Studying the optical properties of pure bulk CdO revealed that it is a vital n-type semiconductor. This means that it has an excess of negatively charged carriers with a direct bandgap of 2.3 eV and an indirect bandgap of 1.98 eV [4]. However, CdO nanoparticles have a direct band gap energy of 3.4 eV [5] and indirect band gap energy of 2.04 eV [6]. The band gap structure in CdO nanoparticles showed metallic nature with high electrical conductivity (103 Ω−1 cm−1) and high carrier mobility (142 cm2/Vs) due to native defects of oxygen vacancies and cadmium interstitials. It is also reported that among the different metal oxide nanoparticles, CdO nanoparticles have 70% transparency in the visible region. Moreover, oxygen vacancies in the CdO nanoparticles, have a great contribution to the electrical conductivity as they capture electrons forming \(V_{{\text{o}}}^{ + }\) centers.

Bulk CdO is diamagnetic; however, the existence of O-vacancies offers a medium for spin–spin interactions, thus causing ferromagnetism in nanoparticles [7]. This is due to the co-existence of oxygen and cadmium vacancies. Ab initio calculations done on the CdO nanoparticles showed that these vacancies induce spin-polarization of O-2p and Cd-4d orbitals near the Fermi level, leading to ferromagnetically coupled vacancies [7]. Moreover, semiconductors prepared at the nanoscale are usually associated with grain boundaries [8], having a larger surface area with a nonstoichiometric proportion. Consequently, local moments are produced due to the large concentration defects formed, which in turn produce ferromagnetism at room temperature.

Single doping has been attention-grabbing since early times; nowadays a promising study related to co-doping has been inspected in which two elements are incorporated into CdO concurrently to improve its optical, electrical, and magnetic properties. Manjula et al. [9] studied the co-doping of CdO with Mn and Cl, where an increase in the optical transparency for the co-doped compared to the pure CdO sample, a blue shift of the optical bandgap of pure CdO with co-doping and a decrease in the electrical resistivity were witnessed. Noorunnisha et al. [10] investigated the effect of Zn-Co co-doping and found that the crystallite size of pure CdO increased from 29 to 35 nm with co-doping, the optical band gap of CdO increased from 2.47 to 2.5 eV upon co-doping, the electrical resistivity decreased with co-doping, and the diamagnetic behavior switched to ferromagnetic behavior for co-doped. Manjula et al. [11] studied, in another study, the co-doping of CdO with Mn and F and discovered that the crystalline quality improved with double doping, the optical transparency increased with the co-doping of Mn and F, the electrical resistivity decreased from 3.78 × 10–4 Ω cm to 0.93 × 10–4 Ω cm and the co-doped CdO exhibited ferromagnetic ordering.

No previous studies shed the light on the effect of simultaneously doping CdO nanostructures by the transition metal (Mn) and rare-earth element (Sm). The transition metal manganese (Mn) has an ionic radius of 80 pm less than that of Cd2+ ions. When Mn2+ ions are doped in CdO, they alter the morphological, electrical, optical, and structural properties. Manjula et al. [12] reported the effects of Mn doping by which increasing the Mn dopants caused an increase in transmittance and electrical resistivity without altering its crystal structure and a decrease in the energy band gap as well as the crystallite size. Mn is considered among an important class of dopants called the luminescence activators, a property when manipulated gives rise to the expansion of the range of useful spectroscopic properties. On the other hand, the rare-earth element samarium (Sm) has an Sm3+ ion with an ionic radius of 95.8 pm, which is slightly greater than Cd2+ ions. When CdO is doped by Sm, a significant improvement in the electrical conductivity was observed compared to pure CdO, with an increase in the doping level of Sm, meanwhile, a decrease in size, and an increase in the antibacterial activity, as reported by Xavier et al. [13].

Hydrophobicity refers to the property of a surface being water repellent due to the difference of surface energies between the solid and the liquid. Nanoparticle’s hydrophobicity favors the targeting of the drug delivery associated with the interactions with the phospholipid layer of the bacterial membrane [14]. Quite the reverse, hydrophilic nanoparticles have reduced interaction with opsonins triggering extended blood circulation [15]. CdO nanoparticles have shown good hydrophobic properties [16, 17] causing better bactericidal activity.

Cd-based materials can show some hazardous side effects. Cadmium is a toxic heavy metal categorized as human carcinogen [18]. Its toxicity has been investigated in both prokaryotes and eukaryotes [19, 20]. Cd toxicity is accredited to the Cd2+ ion discharge, that is absorbed by biological tissues [21, 22]. The last 2 decades perceived broad studies related to semiconductor nanostructured materials particularly on Cd based CdO, CdS and CdSe because of their exceptional size and shape dependent properties nominating them as prospective candidates for optical, electronics, optoelectronic and biomedical applications. Nonetheless, CdO nanoparticles have been revealed as toxic; CdO nanoparticles have both cytotoxic and genotoxic effects. In some cells, the treatment with the CdO nanoparticles led to a decline in cell viability, a decrease in ATP content, and an increase in LDH leakage [23]. CdO nanoparticles also resulted in DNA breaks and chromosome damage [24] studies suggest that the toxicity of CdO NPs was caused by the nanoparticles themselves and not by the minimally released Cd ions [23, 24]. However, there are only few studies available on CdO. The bulk studies have dealt with inhalation toxicity retaining rodent models [25,26,27]. Another research discovered the influence of CdO nanoparticles on E. coli division by improving the oxidative stress in the bacteria being a mechanism in CdO toxicity [28]. Accordingly, the mechanism of toxicity is less clear. But the use of organic molecules like surfactants and ligands during the manufacturing of CdO nanoparticles can reduce CdO toxicity in biological systems. These organic molecules forms a carbon surface protective layer that reduce the release of toxic Cd2+ ion into water or in biological systems such as glucose encapsulated cadmium oxide nanoparticle that showed no observable toxicity [29].

In this work, pure and (Mn, Sm) co-doped CdO nanostructures are studied according to the formula Cd1–2xMnxSmxO, with x = 0.00, 0.005, 0.01, 0.02, 0.03, and 0.04. Pure and (Mn, Sm) co-doped CdO nanostructures are prepared by the co-precipitation method. The crystal structure, morphology, and vibrational bands of the prepared nanoparticles are studied. Then, the optical, electrical, magnetic, and antibacterial properties were investigated.

2 Experimental techniques

Cd1–2xMnxSmxO nanostructures with x = 0.000, 0.005, 0.010, 0.02, 0.03, and 0.040 were prepared via the co-precipitation technique, using the precursors cadmium chloride anhydrous CdCl2, manganese(II) chloride MnCl2, and samarium(III) chloride hexahydrate SmCl3·6H2O according to stoichiometric ratios in addition to Ethylenediamine tetra-acetic acid (EDTA). The chemicals were procured from Sigma Aldrich and directly used without further refinement. First, the powder salt was weighed and dissolved in distilled water to prepare 1 M chloride solutions. EDTA solutions of 0.1 M were added to each solution. The obtained solutions were stirred using a magnetic stirrer for 30 min. Then, they were titrated with 4 M of sodium hydroxide (NaOH) until the pH reached 12 [30]. Then, the solutions were continuously stirred and heated at 353 K for 2 h. The final solutions were washed with deionized water until the pH reached about 7, to get rid of the residuals. Consequently, the obtained products were dried at 373 K for 16 h, followed by grinding and calcination at 923 K for 3 h. Finally, the obtained powders were ball milled, for 10 min at 400 rpm speed, with a 5:1 balls-to-powder mass ratio, to obtain a homogeneous grinding of the powder.

After the preparation, the crystallinity and phase formation of the prepared samples were investigated by powder X-ray diffraction (XRD), using the Bruker D8 Focus diffractometer with Cu-Kα radiation (λ = 1.54056 Å) in the range of 25° ≤ 2θ ≤ 75°, at 40 mA and 40 kV operating current and voltage, respectively. Transmission electron microscopy images were obtained by using the Jeol transmission electron microscope JEM 100CX, with 0.1 nm resolution and 80 kV operating voltage, to study the morphology and particle size of CdO. The optical studies were characterized by using the ultraviolet–visible near-infrared spectrophotometer JASCO V-670 which measure the absorption spectra at a wavelength of 200–700 nm, at room temperature. Photoluminescence spectra were recorded, at room temperature, using the JASCO FP-8600 spectrofluorometer, equipped with a Xenon lamp, at an excitation wavelength of 260 nm. The samples were dispersed in deionized water for the UV and PL optical investigations, where the prepared solutions were placed in the 1 × 1 cm cuvette for the measurements. The DC Conductivity measurements were performed using the four-probe method. This technique measures the voltage and the current at room temperature and then at different temperatures dependence by heating the sample to 300 °C and calculating, that during slow cooling, the conductivity at every 10 °C decreases in the temperature range from 300 to 30 °C. To examine the magnetic properties of CdO nanoparticles, a Lakeshore 7410 vibrating sample magnetometer (VSM) was used, up to a field of 20 kG. About 0.10 g powder was mounted in an insulated tube constructed of nonmagnetic material, a quartz tube, in the gap between the electromagnets of the magnetometer. To study the antibacterial activity, the prepared pure and co-doped CdO were transferred using autoclaved tips into the wells in Muller Hinton Agar (MHA) 14 cm diameter plates. The media was punched to form 6 mm diameter wells on the Muller–Hinton agar plates and synthesized pure or co-doped CdO was poured into these wells. Antibiotic and autoclaved distilled water were added in separate wells and used as positive and negative controls (labeled as + ve and − ve) against bacterial growth. The Petri dishes were stored for 2 h in a fridge at a temperature of 10 °C. Finally, the Petri dishes were incubated for 24 h at 37 °C. After incubation, plates were observed for the presence of a clear zone of inhibition.

3 Results and discussions

3.1 XRD investigation

The room temperature XRD patterns of the prepared samples of CdO, co-0.5, co-1, co-2, co-3 and co-4 are displayed in Fig. 1a. All diffraction peaks show a clear match with the cubic crystal structure of CdO. This indicates a good crystalline nature of the face-centered-cubic phase with space group Fm-3m for all prepared samples [4]. Consequently, there are no characteristic peaks arising from Mn, Sm, and their oxides are present, revealing that the Mn2+ and the Sm3+ ions are successfully incorporated in the CdO host crystal lattice [12, 13]. This was confirmed by the similarity in the XRD patterns and by executing the Rietveld refinement, for all the samples, using the MAUD program [31]. The goodness-of-fit parameter, denoted by \(\chi^{2}\), is shown in Fig. 1a and it indicates a good match between the experimental and refined patterns (\(\chi^{2} \sim 1\)).

Fig. 1
figure 1

a XRD patterns. b Raman spectra of pure and (Mn, Sm) co-doped CdO nanostructures

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The lattice parameters were calculated using all the obtained reflection peaks following this equation [32].

$$d_{hkl} = \frac{a}{{\sqrt {(h^{2} + k^{2} + l^{2} )} }},$$
(1)

where \(d_{hkl}\) is the interplanar distance and h, k, and l refer to the Miller indices. The volume strain due to doping was calculated relative to the pure CdO sample by Eq. (2):

$$\varepsilon_{{{\text{vol}}}} = \Delta V_{{{\text{cell}}}} /V_{{{\text{cell}}}} ,$$
(2)

where \(\Delta V_{{{\text{cell}}}}\) and Vcell is the change due to the doping of the unit-cell volume and the volume of the pure sample, respectively. Table 1 depicts the findings of the calculated lattice parameters and the volume strain due to doping. The lattice parameter and the unit cell volume display a continuous and systematic increase with (Mn, Sm) double doping level as shown in Table 1, this indicates that both Mn and Sm ions were successfully incorporated into the crystalline lattice of CdO. This can be explained based on the difference in ionic radii between Mn2+ (0.80 Å), Sm3+ (0.958 Å), and Cd2+ ions (0.95 Å) and the possible formation of a substitutional solid solution (SSS) with Cd2+ ions, according to Hume–Rothery rules [33]. The possible formation of (SSS) is stipulated by the fact that the ionic radii difference between the host and dopant must be less than 15%. The Mn2+ and Sm3+ ions might be incorporated into the CdO lattice by (1) filling interstitial sites in the CdO crystalline lattice, (2) filling oxygen vacancies, and (3) substituting Cd2+ ions depending on the formation (SSS). Mn2+ ions cannot form (SSS) because their radii are 15% smaller than that of Cd2+ ions. As a result, rather than Cd ion substitution, Mn2+ ions are more likely to be incorporated into the CdO crystalline structure by occupying interstitial positions. Additionally, more Cd2+ vacancies would be created in the CdO structure as a result of this incorporation with Mn2+ ions, allowing the CdO unit cell to maintain its charge balance. It is also plausible that thermal motion could cause some Mn2+ dopant ions to fall into Cd2+ ion vacancies. Because the radius of Sm3+ ions is slightly larger than that of Cd2+ ions by only 0.8 pm, the incorporation of Sm3+ ions is preferred by the substitution for Cd2+ ions forming (SSS). This means that if Sm3+ ions occupy interstitial sites, the crystalline structure of the host CdO will be strongly distorted. Thus, doping of Mn2+ and Sm3+ ions disrupt the charge balance between Cd2+ and O2− ions (Cd2+/O2−) of the CdO unit cell. As a result, to keep this charge balance, the creation and/or annihilation of an equivalent number of Cd and/or O vacancies must occur, depending on the level of doping. Therefore, the above mechanism of incorporation of Mn2+ and Sm3+ ions into the CdO lattice unit cell causes an increase in lattice parameter, unit cell volume, and the structure volume microstrain (εvol). Similar results were reported by Bououdina etc. al [34] for CdO dilute magnetic nanocrystalline semiconductor co-doped with Cu and Ni ions, where the Vcell increased with increasing Ni content.

Table 1 Calculated parameters for pure CdO and (Mn, Sm) co-doped nanostructures
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The average crystallite sizes (D) of pure CdO and (Mn, Sm) co-doped are calculated by using the well-known Scherrer’s equation for the main peak (111) using:

$$D = \frac{k\lambda }{{\beta_{hkl} \cos \theta_{hkl} }},$$
(3)

where k is a constant, λ is the wavelength of the incident X-rays, \(\beta_{hkl}\) is the full width at half maximum, and θhkl is the relative angle of incident radiation.

The obtained average crystallite sizes of the samples were listed in Table 1. The average crystallite size values ranged between 34.75 and 43.47 nm for pure and co-1 nanostructure, respectively. This change in the crystallite size is due to the CdO lattice being distorted by the doping of Mn2+ and Sm3+ ions. Consequently, this affects the rate of growth of the CdO [35,36,37]. The major characteristics of nanostructured metal oxides are affected by the lattice micro-strain (\(\varepsilon\)), which is calculated using:

$$\varepsilon = \frac{\beta \cos \theta }{4}.$$
(4)

Table 1 lists the calculated values of ε, which has an inverse relationship with the crystallite size. The variation of the micro-strain with Mn2+ and Sm3+ doping indicates a change in the microstructure due to the change in size and shape of the particles, as can be seen from the variation of reflection peaks as discussed in XRD analysis and the TEM analysis section, as discussed later [38, 39]. Additionally, the variation of the lattice micro-strain can be explained on the base of the trapping of the atoms according to the equilibrium positions [40].

3.2 Raman spectroscopy

Phonon modes can be examined by studying the lattice dynamics and optical modes through the Raman spectrum, which is a priceless tool for the investigation and characterization of nanostructures. Figure 1b reveals the room temperature spectra collected for pure and co-doped CdO nanostructures. The first-order scattering is not allowed in CdO due to the symmetry group according to the Raman selection rules since CdO has a rock salt structure with space group Fm-3m, and both transverse optical (TO) and longitudinal optical (LO) modes are dipole forbidden [4, 41, 42]. Consequently, all features in the spectra can be attributed to second-order Raman scattering processes in CdO. Raman modes are detected at (255, 333, 470, 606, and 950 cm−1). The broad peak observed at 255 cm−1 for pure and co-doped samples is considered one of the important features in the Raman spectra of CdO assigned as second-order transverse acoustic (2TA) mode [42, 43]. The peak observed at 333 cm−1 is assigned to the second-order longitudinal acoustic (2LA) at different points of the Brillouin zone [42, 43]. The peaks that appeared at 470 and 606 cm−1 are due to the second-order vibration of 2TO and 2LO, respectively. It is attention-grabbing to note here that on Mn2+ interstitials and Sm3+ substitution, all the modes become less sharp and clear, which indicates the defects of the crystal symmetry to give first-order Raman active modes. The variation in the peak intensities reflected the change in the concentration of the co-dopants within the CdO crystal structure, which affected the Raman vibrational modes of the (Mn, Sm)–Cd–O bonds during Raman analysis. Thus, the changes in Raman modes observed due to structural modification in the CdO lattice were associated with crystal defects or micro-strain [44,45,46]. Besides, as the concentration of the co-dopants increases, the discrete peaks change to a broad peak straddling from 255 to 606 cm−1, with the least intensity for the co-4. A similar character for pure CdO prepared by the co-precipitation method was also reported by Reddy et al. [47]. The last peak observed at 950 cm−1 refers to the 2LO mode arising from intraionic anharmonicity of polarizable ion O− [48]. The intensity of this peak decreases as the co-doping concentration increases. Lastly, the Raman spectra disclose the major phonon modes of CdO without any indicator from the Mn and Sm dopants [41]. This shows that the low concentration of Mn2+ and Sm3+ ions are well substituted into the CdO lattice not altering its structure, supporting the XRD analysis.

3.3 TEM measurements

Figure 2a–g show the TEM micrograph images with the corresponding particle size histograms, of pure CdO and (Mn, Sm) co-doped nanostructure, respectively. It is seen that all the prepared samples are crystalline in nature with particles of different shapes and sizes, depending on the (Mn, Sm) co-doping. Figure 2a reveals that CdO nanostructures are straight nanowires with smooth surfaces of different dimensions and diameters. The average diameters estimated from the histogram data of the pure sample were found to be 17.01 nm, in agreement with Zhang et al. [49] and Liu et al. [50]. In the co-0.5 sample, there are two sets of nanowires, one with a majority average diameter of 13 nm and another one is 23 nm. With the further addition of (Mn, Sm) co-doping to co-1, the nanowires appeared with a flaky stage shape [51], accompanied by an increase in the average diameter (40.69 nm). For the co-1 sample, it attained the largest size relative to other co-dopants, as listed in Table 1, and this is consistent with the average crystallite size calculated from XRD analysis. Upon increasing the co-dopant concentration to co-2 (Fig. 2e), the nanowires with a flaky stage shape begin to decompose and convert to nanowires having a thinner diameter (8.64 nm) and more elongated length with the appearance of a nanocluster. These nanoclusters are almost spherical in shape, with an average size of 25.74 nm. For the co-3 and co-4 samples, the nanoclusters formation increases with the increase in the average size of the particles (29.67 and 35.78 nm), accompanied by a reduction in the average diameter of the nanowires (12.29 and 11.31 nm). This shows that a change in the shape and size of the nanoparticles in the CdO nanostructure depends on the concentration of the co-dopants (Mn and Sm). These alterations can be explained based on the difference in the ionic radii between the host Cd2+ ion and dopants (Mn2+ and Sm3+ ions), leading to a deformation of the CdO lattice [9, 11]. Such deformation was observed for the co-doping of CdO as reported by Gurbuz et al. [35] for CdO co-doped with Mn and Ni transition metals and Aydin and Sahin [36] for (Li, Ce) co-doped CdO films. Additionally, these changes can be attributed to the adsorption effect of Mn2+ and Sm3+ ions on the surface of the CdO lattice throughout the chemical reaction and thermal treatment [9, 52, 53].

Fig. 2
figure 2figure 2

A TEM images, with a 100 nm scale, for pure and (Mn, Sm) co-doped CdO nanostructures. B Particle size histograms for pure and (Mn, Sm) co-doped CdO nanostructures

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There is a mismatch between the average crystallite size calculated from XRD (DXRD) and the dimensions of nanowires and nanoclusters (spherical and hexagonal) as calculated from TEM histograms. The particle size measured from XRD is larger than the size obtained from TEM analysis since the X-ray analysis is related to the average particle volume, while the particle size calculated from TEM images corresponds to the average diameters. Hence, the size measured from TEM analysis is considered to be more reliable for samples with mixed morphologies. In particular, the TEM measurements enabled the separation between the nanowires and the nanoclusters' morphologies; however, the XRD analysis based on the Debye Scherrer formula assumes that the particles are of spherical morphologies.

3.4 UV–vis spectroscopy

Figure 3 shows the optical spectra of the pure and (Mn, Sm) co-doped CdO nanostructures studied in the UV–visible region. As seen, all the samples demonstrate better absorption in the UV range and less absorption in the visible range except co-3, mainly due to the least energy gap it has among the samples. This energy gap refers to the intrinsic bandgap absorption due to the electron transition from the valence band to the conduction band (O2p–Cd3d). The direct bandgap was calculated for the pure and co-doped samples listed in Table 1 by means of the linear fit in the Tauc plot shown in Fig. 3b using:

$$\left( {\alpha h\nu } \right)^{1/n} = B\left( {h\nu - E_{{\text{g}}} } \right),$$
(5)

where α is the absorption coefficient, B is a constant independent of wavelength, hv is the energy of the absorbed photon, and the exponent n depends on the nature of the transition whether direct or indirect where: n = 1/2 for direct transitions and n = 2 for indirect transitions. The direct bandgap energy for pure CdO is 2.7 eV in agreement with Abbas et al. [30] for pure CdO nanoparticles prepared by the co-precipitation method. This energy increases as the concentration of the co-dopants increases to reach 2.92 eV for co-2, then it decreases for further increase in the co-dopants to 2.28 eV at co-4. For doped transparent conductive oxides, the bandgap variation is a consequence of two competing effects: bandgap widening related to the Burstein–Moss effect or renormalization due to many-body interactions [54, 55], as reported earlier by Xavier et al. [13] and Manjula et al. [9, 11, 56]. Both Mn and Sm dopants act as effective donors either to be substituted into the Cd sites or shifted as interstitial atoms. The interstitials atom caused the broadening of the band gap due to an increase in carrier concentrations in the conduction band. Consequently, the Fermi levels shifted into the conduction band with an increasing concentration of the co-dopants. The carrier concentrations in the conduction band increase as the band gap increases. This type of change in the band gap was observed due to the carrier concentration, leading to the Burstein–Moss effect [54, 55]. Similar results were reported by Sahin et al. [57] where the optical band gap of the CdO films was first decreased and then increased with the increase in barium doping concentration.

Fig. 3
figure 3

a Absorption spectra. b Tauc plot for pure and (Mn, Sm) co-doped CdO nanostructures for the direct bandgap

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3.5 PL spectroscopy

Figure 4 shows the PL spectra of the pure and (Mn, Sm) co-doped CdO nanostructures, measured at room temperature, at an excitation wavelength of 260 nm. These spectra were deconvoluted, using the Fit-y-k program to subdivide the spectra into discrete peaks. The deconvolutions, shown in Figs. 5a–f, demonstrate six peaks. Three emission peaks appeared around 359 nm (intense and sharp), 377, and 400 nm which are slightly variant for all the prepared samples. These peaks in the violet range, between 360 and 400 nm are dominant, indicating the abundance of Cd interstitials [13]. They correspond to the near band edge emission of CdO, originating from exciton transitions between the electrons in the conduction bands and the holes in the valence bands. As the concentration of co-dopants increases to x = 0.01, the intensity of these peaks increases, then a subsequent decrease occurs as the concentration is further increased to reach a minimum at x = 0.04, as shown in Fig. 5. The increase in the intensity is attributed to the increase in the Cd interstitials as an effect of the Mn dopant and vice versa [13]. Due to these strong emission peaks, the CdO nanoparticles can be utilized in the industry of high-quality monochromatic lasers [58]. A fourth peak appears in the range of 430–450 nm referring to the blue emission. For the co-3 and co-4, the peak at 430 nm may be related to an exciton bound to a donor-level impurity emission [59]. This peak is blue-shifted for the pure CdO, co-0.5, co-1, and co-2 which is due to the combination of an electron from the conduction band and holes from the valence band [13]. A fifth peak in the range of 490–498 nm is due to the singly ionized oxygen vacancy which is the main factor in the production of hydroxyl radicals on the surface of CdO samples [13]. Thus affecting the efficiency of the antibacterial activity of the prepared sample. This peak is blue-shifted in the sample co-4, explaining the least effect of this sample on the bacteria tested. Another emission peak in the range of 600–610 nm was attributed to indirect near-band edge emission from CdO nanoparticles [60]. The sample co-4 showed the least intensity among the sample. This minimization in intensity shows a decrease in the electron–hole recombination rate, which is preferred for photocatalytic activity [61].

Fig. 4
figure 4

PL spectra of pure and (Mn, Sm) co-doped CdO nanostructures

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Fig. 5
figure 5

Deconvolution of the PL spectra for pure and (Mn, Sm) co-doped CdO nanostructures

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3.6 Electrical conductivity

Figure 6a shows the temperature dependence of the electrical conductivity (σ), measured from room temperature up to 573 K, for pure and (Mn, Sm) co-doped CdO. The electrical conductivity decreases as temperature increases, indicating metallic behavior for the samples at the studied temperature range, except for co-1 and co-2. The metallic behavior may be due to excess cadmium atoms which determines the metallic characteristics of the electrical conductivity. This is consistent with the excess of cadmium interstitials detected by PL studies [62]. The increment of the temperature yielded a significant decrease in the conductivity, a fact that can be explained by the oxidation of Cd atoms from intercrystallite domains [62]. However, the conductivity decreases with the increase in temperature up to 523 and 503 K for the co-1 and co-2 samples, respectively, and thereafter increases. Hence, the samples co-1 and co-2 showed combined electrical behavior, where the co-1 sample attained metallic behavior in the range of (303–523 K) and semiconducting nature above 523 K and the co-2 sample attained metallic behavior in the range of (303–503 K) and semiconducting nature above 503 K. Therefore, it is assumed that a phase change occurs at this critical temperature [63]. Below this critical temperature, the above nanostructures show metallic behavior and above this temperature, they behave as a semiconductor. A similar type of phase change occurred at some critical temperature of around 370 K for the as-deposited CdO thin films prepared by Rahman et al. [64], at 365 K for CdO prepared by Dantus et al. [62], at 374 K for pure CdO shifted to higher value upon doping with Ag except at 1% concentration showing the minimal value (358 k) [63]. The temperature dependence of the electrical conductivity of a semiconductor is typically expressed by the Arrhenius equation [63]:

$$\sigma = \sigma_{0} e^{{ - \frac{{E_{{\text{a}}} }}{kT}}} ,$$
(6)

where σ0 is a constant, Ea is the activation energy, k is the Boltzmann constant and T is the absolute temperature [19]. Figure 6b shows a plot of ln σ versus 1/T. The variation of ln σ versus 1/T is a straight line with a slope equal to Ea/k. All the samples are fitted with one straight line giving one activation energy E1 except co-1 and co-2, which are fitted by two straight lines. The straight line fitting, give the value of the activation energy for the lower temperature region (303–523 K) denoted by E1 and E2 for the higher temperature region (above 523 K) for co-1 and co-2 shown in Fig. 6b. The activation energies, given in Table 2, vary between 0.035 and 0.075 eV and are consistent with the values obtained by Alam et al. [63]. The presence of two different activation energies, in the samples co-1 and co-2, indicates that two conduction channels are present. The channels are due to the hopping of charge carriers between ions of the same elements [65]. Possible conduction can occur through the hopping of holes for different Cd oxidation states or through the hopping of electrons. One conduction channel is dominant in a lower temperature range whereas the second conduction channel is dominant in a higher temperature range. The presence of two activation energies as observed in the ln σ versus 1/T curve may correspond to these two different conduction channels.

Fig. 6
figure 6

a The variation of the conductivity. b Ln σ versus 1000/T for pure and (Mn, Sm) co-doped CdO nanostructures

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Table 2 The activation energy for the pure and the (Mn, Sm) co-doped nanostructures
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Electrical conductivity strongly depends on dopant size, preparation condition, and temperature. Depending upon preparation or processing condition, it is either a non-degenerate or degenerate n-type semiconductor rising from cation interstitials or anion vacancies, resulting in free electrons [66]. The ionized donors give rise to an impurity level below the conduction band, the impurity band may overlap with the conduction band giving rise to quasi-metallic or metallic properties as observed in this study [63]. When the ionic radii of the dopants are different from the ionic radius of Cd2+ ions, cation interstitials and anion vacancies are favored, thus affecting this overlap, creating conducting channels and causing the transition from metallic to semiconducting behavior having a critical value that is strongly temperature dependent. For co-1 and co-2 the critical temperature was determined within the given studied range, however, it is assumed that a critical temperature exists for the other samples but within a range higher than the studied one.

3.7 Magnetic investigation

Figure 7a–f illustrates the room temperature M–H loops for pure and (Mn, Sm) co-doped CdO nanostructures with an applied magnetic field strength up to ± 20 kG. The magnetic parameters are depicted from the M–H loop and listed in Table 3. The pure CdO (x = 0.0) and co-0.5 nanoparticles divulge two magnetic contributions, the ferromagnetic contribution, represented by (\(M_{{{\text{FM}}}}\)) and the diamagnetic contribution, represented by the linear component (\(\chi H\)). These contributions were separated using the equation:

$$M = M_{{{\text{FM}}}} + \chi H.$$
(7)
Fig. 7
figure 7

a–f The magnetic hysteresis for pure and (Mn, Sm) co-doped CdO nanostructures

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Table 3 Magnetic parameters for pure and (Mn, Sm) co-doped CdO nanostructures
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The χ value for pure CdO was obtained as χ = − 8.154 × 10–7 from the linear fitting, however, for co-0.5, it attained a value of χ = − 7.573 × 10–7. It was found that the χ value is negative, assuring the diamagnetic (DM) nature. Hence, there is an indication of a weak ferromagnetic order in the low field and diamagnetic order in the high field (H > 8 kG). This goes well with the early testimony by ferromagnetic behavior of pure CdO though it is diamagnetic in nature [7]. The presented O-vacancies favor spin–spin interactions, causing ferromagnetism. This ferromagnetic behavior was also explained by Bououdoni et al. [7] in pure CdO nanoparticles based on the co-existence of oxygen and cadmium vacancies. These vacancies provoke spin-polarization of O-2p and Cd-4d orbitals near the Fermi level, leading to ferromagnetically coupled vacancies. For co-1, pure ferromagnetic behavior was detected compared to the pure CdO sample. Based on the TEM results, the nanoparticles appear to have two different shapes; nanowires and nanoclusters. This observation leads to the assumption of the contribution of shape anisotropy to the total one. For co-1 the transformation occurs from nanowires to nanoclusters creating a pinning layer preventing demagnetization. Accordingly, this is evidence of large coercivity for this sample as a special case. The existence of some defects that could result from the difference in the ionic radii of dopants as compared to the host caused the development of magnetic polarons. Upon increasing the co-doping concentration to x = 0.02, the M–H loop depicted a superparamagnetic (SPM) contribution, as shown in Fig. 7d. This SPM behavior is favored in samples with smaller sizes, as reported by Badreddine et al. [67]. They justified the presence of SPM in samples of Sm-doped ZnO nanoparticles, with sizes less than 35 nm, where the thermal fluctuations affect the system and cannot withstand stable bulk magnetization. This aligns well with the sample co-0.2, where the TEM image and particle size distribution showed the presence of combined morphologies from nanowires and nanoclusters, having dimensions with a majority less than 35 nm. For the higher co-doped concentration (x = 0.03 and 0.04) evidence of antiferromagnetism (AFM) behavior arose accompanied by a ferromagnetic contribution. The saturation magnetization was found to be 8.925 memu/g for the pure CdO sample. This magnetization varied with every co-doped concentration, it decreased till it reached the least value (3.213 memu/g) for co-1 and then increased with further increase in concentration to reach its highest value (76.040 memu/g) for co-4.

Different magnetic contributions may be attributed to the particle size of the prepared samples. This variation affects tremendously the coercivity [34, 37, 68, 69]. The coercivity (Hc) values are extracted from the M–H loops and listed in Table 3. Changing the particle size drastically changed the magnetic properties. As the crystallite size decreased the saturation magnetization increased and vice versa.

To explain further the above discrepancy, a critical value of the grain size should be taken into consideration in the range of 20–30 nm. The development of a single ferromagnetic domain is perceived when particles have a grain size less than this critical value. However, superparamagnetism is governed by a further decrease in particle size, where coercivity values decrease to zero. A conversion from a single to a multi-domain state and reduction in coercivity is sensed for particles of grain size greater than the critical value [30, 70]. So the values of coercivity are directly related to shape and size anisotropy. Larger particles of grain size much larger than the critical grain size develop low coercivity, while smaller particles of grain size less than the critical grain size develop high coercivity. This supports the highest coercivity of the co-1 sample and the low coercivity of the co-2 sample. Hence, co-doping is an effective way of spin injection into CdO to achieve diluted magnetic semiconductors of small size with amazing magnetic properties. The immense variation of Hc, as reflected from the anisotropy with the large value of room temperature coercivity, recommends the use of this type of magnetic nanoparticles as a pinning layer in spin valves demanding a difficult and large demagnetizing field [71]. Furthermore, the squareness ratio (Sr = Mr/Ms) reflects the nature of the structure of the magnetic domain [38]. For a single-domain structure, this ratio is greater than 0.5 and for a multi-domain structure, it is less than 0.5 [39]. This is evidence of the FM nature of the CdO and co-1 sample having a ratio greater than 0.5 and the AFM for co-2, co-3, and co-4 samples with a ratio less than 0.5.

3.8 Antibacterial activity

The antibacterial activity of pure and co-doped CdO, having different doping levels (0.005, 0.010, 0.020, 0.030, and 0.040) was investigated using the well diffusion method. In this work, the synthesized nanomaterials were subjected to three different grams positive (Staphylococcus aureus, Staphylococcus epidermidis, and Streptococcus intermedius) and three different grams negative (Escherichia coli, Citrobacter braakii, and Klebsiella pneumoniae) bacterial strains respectively. From Fig. 11 and Figure S1 (supplementary material), it is evident that all the samples effectively prevented the growth of bacteria and produced well-defined zones around the samples. For the co-doped samples, variations occur with the different co-doping levels.

For the nanoparticles, pure CdO revealed maximum activity against E-coli with a ZOI equivalent to 3.22 cm. For the co-0.5, co-1, co-2, co-3, and co-4, the highest value of ZOI (3.49, 3.94, 3.36, 3.16, and 3.12 cm) was observed for the Citrobacter braakii showing maximum bactericidal activity compared to other bacteria as shown in Fig. 8.

Fig. 8
figure 8

Bar graphs showing the zone of inhibition for the prepared nanostructures with different concentrations for the studied bacteria

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For K. pneumoniae maximum ZOI was witnessed for the pure CdO (3.09 cm), this zone decreased with the increase in the co-dopants concentration, which signifies that co-doping is ineffective against K. pneumoniae. However, for C. braakii, S. epidermidis, and E. coli, the bacterial activity of the co-doped samples showed an inhibition zone (3.94, 2.54 and 3.26 cm, respectively) greater than that for the pure CdO, especially for the 0.01 concentration with maximum efficacy. This can be explained by the difference in morphology and crystallite size of the samples since co-1 has solely nanowire shape as observed in the TEM results with the highest crystallite size, from the XRD results, the crystallite size was found to be 43 nm which guides to the antibacterial activity [72]. Yang et al. reported the same results for ZnO nanostructures where rods and wires showed higher penetration into the cell walls of bacteria when compared to ZnO nanospheres [73]. Additionally, Gupta et al. verified higher ZOI for higher crystallite size for CdO-doped nanostructures [5]. Cheon et al. evaluated the antibacterial effect of silver nanoparticles, having different morphologies (Ag nanospheres, Ag nanorods, and Ag nanotriangles) against E. coli. Among these, the Ag nanospheres showed the widest inhibition region of 4.8 mm [74]. For S. aureus, the bacterial activity of the co-doped samples showed an inhibition zone (3 cm) greater than that for the pure CdO, especially for the 0.005 concentration with maximum efficacy. Lastly, for the S. intermedius, the diameter of the ZOI increased as the co-dopant concentration increased revealing maximum activity (2.7 cm) at the highest concentration (0.04 mg/ml). Consequently, this shows that for S. intermedius, C. braakii, S. epidermidis, S. aureus, and E. coli better antibacterial effect is achieved when co-doping CdO nanostructure.

Among the pure and the co-doped samples, as the concentration of the sample increases, the ZOI increases as shown in all the bar graphs in Fig. 8. This shows that the antibacterial activity is concentration dependent as reported in many previous studies. Yamamoto et al. [75] found that the antibacterial activity of ZnO for S. aureus was similar to those for E Coli and increased with increasing powder concentration. Zhang et al. [76] showed that the bacteriostatic action of ZnO towards E. coli increased at higher ZnO-NPs concentrations.

Finally, it can be noted that all the prepared samples have a stronger antibacterial activity on gram-negative than gram-positive bacteria. Though the bacteria cell wall is made up of a large number of peptidoglycans, lipopolysaccharides, mucopeptides, murein, lipoteichoic acids, etc., these are penetrated by a greater number of negatively charged free radicals such as superoxide anions and hydroxyl radicals, into the cell membrane, causing severe damage and kill the cells. In the same manner, Somasundaram et al. [77] proved the effectiveness of CdO nanoparticles against gram-negative more than the gram-positive, and Xavier et al. [13] showed high activity of the Sm doped CdO towards gram-negative bacteria than the gram-positive.

The antibacterial activity of CdO nanostructures has been attributed to numerous issues, but the meticulous toxicity mechanism is not utterly well-lit and still debatable, as there are some interrogations within the range of antibacterial activity lacking profound clarifications [74, 75], the possible mechanisms revealed from recent studies are listed as follows:

  1. 1.

    Direct contact of nanoparticles with cell walls, leads to the destruction of bacterial cell integrity [78].

Direct interaction of nanoparticles with the bacterial cell wall is tailed by entrance into the cytoplasm. Nanoparticles accumulate on the bacterial cell wall and membrane. Consequently, triggering morphological alterations such as construction of multiple electron-dense pits, reduction of the cytoplasm, prevention of biofilm formation, detachment of membrane, finally membrane disruption [79, 80].

  1. 2.

    Liberation of antimicrobial ions mainly Cd2+ [77].

Nanoparticles with positive charge exhibit elevated toxicity due to their electrostatic interaction with the negative charge of the bacterial cell wall [81]. Abbaszadegan et al. studied the antibacterial activity of three different silver nanoparticles that were positively, negatively, and neutrally charged. The study showed the highest bactericidal activity for the positively charged nanoparticles, yet the least for the negative ones [82].

  1. 3.

    Reactive oxygen species ROS formation in light and even in dark [83, 84].

ROS can disturb bacterial cells whether produced outside or inside the cell [85]. Metal nanoparticles increase ROS production in bacterial cells that are capable of penetrating into the cells, thus killing or inhibiting microorganisms. Wang et al. established that the bactericidal activity of Ag/CeO2 nanoparticles against E. coli is owed to intracellular ROS production and not to the ion silver release [85].

  1. 4.

    Shape-dependent interaction leading to different effective surface areas in terms of active facets [86].

Nanoparticles can have different shapes from 3D like spherical, rods, flowers and others to 2D, having an evident impact on their antibacterial activity. Hong et al. showed that Ag nanowires displayed an inferior antibacterial activity compared to Ag nanocubes and nanospheres [87]. While nanocubes had superior antimicrobial properties than nanospheres, due to higher reactive facets. The above is due to the presence of edges, corners, or defects proliferating toxicity [88].

  1. 5.

    Trapping the bacteria within the aggregated nanomaterials [89].

Recognition elements like antibodies favors the specific binding of nanoparticles to the bacteria forming a nanoparticle-bacteria complex. This complex is mediated by the change in color of the solution. Thus aggregation occurs followed by discrimination [90].

  1. 6.

    DNA damaging [91].

Nanoparticles have a profound effect on DNA damage. In the comet and micronucleus assays, the nanoparticles significantly increase the levels of DNA damage and chromosome breaks compared to their concurrent controls [23].

  1. 7.

    Oxidative stress [29].

In bacterial cells, the production and removal of ROS are typically in balance. On the other hand, when ROS are produced in excess as a results of nanoparticles, the cell's redox balance mediates oxidation. This out-of-balance condition causes oxidative stress, which harms the bacterial cells' individual parts [92, 93].

  1. 8.

    Contribution in generation/explosion of nanobubbles [94].

On the bacterial membranes, there exist electronegative chemical groups of the polymers that are sites of metal cation attraction. The carboxylic acid groups in the proteins grant the negative charge on the surface both of bacteria and spores at biological pH. The difference in charge between the bacterial membranes and the CdO nanostructures leads to electrostatic attraction and hence accumulation of Cd2+ on the cell surface allowing entry into the bacteria, leading to toxicity to micro-organisms. Gram-negative bacteria have a greater negative charge than Gram-positive bacteria and hence the electrostatic interaction will be stronger in Gram-negative strains [76]. Lipopolysaccharide (LPS) in the outer leaflet of the lipid bilayer has more charge per unit surface than other phospholipids in gram-negative bacteria, thus rendering them highly negative in charge [95]. Cd2+ ions offer a strong bond with membranes, causing the disorder of cell walls. The disturbance of the membrane escalates permeability. The pores on the membranes are in the order of nanometres, whereas the bacterial size is in the micrometer range, which gives the possibility of entry into bacterial cells. binding with mesosomes and hereafter modify cellular respiration, cell division, and DNA replication [76].

The photocatalytic generation of ROS has been considered the foremost cause of the antibacterial activities of several metal oxides [75, 78]. The production of ROS (OH−, H2O2, and O2−) on the CdO surface forms an association between photon reactions and antibacterial activity as follows.

When CdO nanostructures interact with light, the electrons (e−) are raised to the conduction band, thus creating a hole h+ in the valence band. The electron and hole interact with water (H2O) to produce \({\text{H}}^{ - }\) and \({\text{H}}^{ + }\). Besides, \({\text{O}}_{2}^{ - }\) molecules harvest superoxide anion (\({\text{O}}_{2}^{ - }\)), which reacts with H+ to produce \({\text{HO}}_{2}^{ - }\). Subsequently, \({\text{HO}}_{2}^{ - }\) interferes with electrons and combines with \({\text{H}}^{ + }\) giving hydrogen peroxide (H2O2) molecules. The latter can enter the membrane either damaging or killing the bacteria.

$${\text{CdO }} + \, h\nu \, \to {\text{ e}}^{ - } + {\text{h}}^{ + }$$
$$h^{ + } + {\text{H}}_{2} {\text{O}} \to {\text{OH}}^{ - } + {\text{H}}^{ + } ,$$
$$e^{ - } + {\text{O}}_{2} \to {\text{O}}_{2}^{ - } ,$$
$${\text{O}}_{2}^{ - } + {\text{H}}^{ + } \to {\text{HO}}_{2}^{ - } ,$$
$${\text{HO}}_{2}^{ - } + {\text{H}}^{ + } + {\text{e}}^{ - } \to {\text{H}}_{2} {\text{O}}_{2} .$$

The negative charges of the superoxides and hydroxyl radicals prevent them from penetrating the membrane [13]. Thus, these species reside on the outer surface of the bacteria, by contrast, H2O2 molecules can pass through the bacterial cell wall, consequently causing injuries, destruction, and finally triggering cell death [96, 97]. When CdO kills or interacts with the cell membrane, the particles possibly are adsorbed at the surface hindering further antibacterial activity. Once CdO is in the growth media, they continue releasing peroxides wrapping the whole surfaces of the dead bacteria. Consequently, this uninterrupted peroxide discharge leads to greater bactericidal efficacy.

The above samples revealed strong difference in antibacterial activity among the prepared nanoparticles. Considering all the listed mechanisms responsible for antibacterial activity, the shape of the nanomaterial on this activity plays a major role. However, the dominant mechanism is due to the strong electrostatic interaction between the positively charged nanoparticles and the bacteria’s negatively charged membrane [98,99,100]. When CdO nanoparticles interact, Cd2+ ions are released from their surfaces and enter the bacterial cells through the transport systems. In the current study, it was found that growth is inhibited as cadmium oxide nanoparticle concentration rises. This can be understood to mean that the antimicrobial effect is more potent and cell damage occurs more quickly with the increase in the ions released. The formation of biofilms and microbial adhesion may be lessened by nanomaterials. Moreover, the effect on the gram negative bacteria compared to the gram positive bacteria strengthens the foundation of this mechanism showing more interaction due to the opposite charges.

3.9 Zeta potential

Zeta potential, that is, surface charge, governs nanoparticle interaction with the cell membrane of bacteria, which is usually negatively charged [101]. It is also a significant parameter referring to stability in aqueous nanosuspensions. According to some studies, nanoparticles with reduced size and higher zeta potential ensued higher antibacterial activity [102]. On the other hand, other studies attributed the elevated antibacterial activity to the size of nanoparticles [103]. Irrespective of differences in zeta potential values that were not readily noticeable among nanoparticles.

Zeta potential analysis of the nanoparticles was evaluated using Malvern Zetasizer Nano range instrument. The zeta potential distribution of pure, co-1 and co-3 nanoparticles were recorded as shown in Fig. 9.

Fig. 9
figure 9

Zeta potential distribution for a pure, b co-1 and c co-3 CdO nanoparticles

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Zeta potential is adopted to establish cellular interaction with charged ions or molecules. The presence of negatively charged ions decreases the zeta potential whereas positively charged ions increases the zeta potential of the surface [104]. CdO nanoparticles exhibit the average zeta potential value of 8.47 mV because of its poor solubility in aqueous medium [105]. Yet, the zeta potential of the pure CdO was highest than the co-1 (− 4.51 mV) but lowest than co-3 nanoparticles (28.5 mV) being more stable showing a positively charged surface for CdO and co-3 samples. The co-1 nanoparticles had the least zeta potential due to its surface charge, this was reflected by the TEM results that revealed the initiation in formation of nanoclusters. The latter explained knowing that the particles that have low zeta potential values will have no force to prevent the particles coming together and flocculating [106]. The negatively charged bacterial cell membrane had a high affinity for metallic cations released by the nanoparticles, which is apt for their higher antibacterial activity.

The nanoparticles obtained in the present study have small particle size and positive surface charges (pure and co-3), which may improve their stability in the presence of biological cations[107] and improve for their antibacterial activities due to the interaction with negatively charged biological membranes especially for gram-negative bacteria. Another study showed that nanoparticles with higher zeta potential have inferior bactericidal performance [108]. The released cations can attach to the negatively charged cell wall by electrostatic interaction and distress the cell membrane, which leads to the release of intracellular molecules, hence killing the bacteria cells.

4 Conclusion

Cd1–2xMnxSmxO nanostructures were successfully synthesized by the co-precipitation method, with different concentrations of the co-dopants (x = 0.000, 0.005, 0.010, 0.020, 0.030 and 0.040). EDTA was used during the synthesis of these nanoparticles as a capping agent. Then, the pure and (Mn, Sm) co-doped CdO nanoparticles were studied by several techniques. The Rietveld refinements of the XRD patterns verified the pure form of the cubic CdO phase. Hence, the equal and low concentrations of the (Mn, Sm) co-dopants were incorporated successfully into the CdO lattice without developing secondary phases. The face-centered cubic structure of CdO was preserved even after the introduction of the co-dopants, however, the lattice parameters experienced variation, due to the difference in the ionic radii of the co-dopants and the host Cd2+ ion. The pure and (Mn, Sm) co-doped CdO nanostructures conquered different morphologies. For pure and low concentrations (CdO, co-0.5, and co-1), nanowires were detected, however, for higher concentrations (co-2, co-3, and co-4) nanowires besides nanoclusters appeared. The particle size increased with increasing the co-doping concentrations reaching its maximum value of 35.78 nm for co-4. Moreover, the Raman spectra reflected the change in the concentration of the co-dopants within the CdO crystal structure, which affected the Raman vibrational modes of the (Mn, Sm)-Cd–O bonds. The UV–vis absorption showed variation in the bandgap energy as the size of the nanoparticles varied with the incorporation of the co-dopants, following the Burstein–Moss effect. A strong 370 nm emission peak was noticed from PL emission spectra, showing that the CdO nanoparticles can be utilized in the industry of high-quality monochromatic lasers. The electrical conductivity decreased as temperature increased, indicating metallic behavior for the samples at all the studied temperatures. However, co-1 and co-2 samples showed an increase in the electrical conductivity with the increase in the temperature upon starting from 250 °C reflecting a semiconducting behavior at higher temperatures, along with their metallic behavior. The M–H loops of the nanoparticles, at room temperature, depicted a diamagnetic and weak ferromagnetic behavior. The diamagnetic behavior was enhanced with the increasing concentrations of the co-dopants till x = 0.005, then disappeared and an antiferromagnetic behavior aroused instead for further concentrations of the co-dopants. All the prepared samples effectively prevented the growth of bacteria and produced well-defined zones around the samples. For the co-doped samples, variations occur with the different co-doping levels. Finally, it can be noted that all the prepared samples had a stronger antibacterial activity on gram-negative than on gram-positive bacteria.