Effect of Mn Doping on The Structural, Optical, Magnetic Properties and Antibacterial Activity of ZnO Nanospheres


 In this work, a systematic study of structural, optical, magnetic and antibacterial properties of Mn doped ZnO has been investigated. Zinc oxide (ZnO) and Mn2+ doped zinc oxide (ZnMnO) nanoparticles (NPs) were prepared through co-precipitation method. The X-ray diffraction studies confirmed that the synthesize nanoparticles did not modify the crystal structure upon Mn doping, but the microstructural parameters were changed considerably while increasing the concentration of Mn dopant. The HRTEM images showed that the ZnO NPs were exhibited nanospheres like morphology and a reduction in the average particle size from 41 nm to 33 nm were observed upon Mn2+ doping. The elemental composition of Zn, Mn and O atoms were identified by EDAX spectra. The Zn-O stretching bands were observed at 539 and 525cm-1 in the FTIR spectra and, the zinc and oxygen vacancies defects were confirmed by PL spectra. From the UV-Vis spectra, the band gap was estimated as 2.7 eV for pure and 2.9 eV for Mn doped ZnO NPs. The Mn doped ZnO NPs showed greater antibacterial effect than the pure ZnO NPs. The magnetization measurements for Mn doped ZnO samples under room temperature ferromagnetism (RTFM) showed the ferromagnetic phase that could originated from the interactions between Mn2+ ions and oxygen vacancies and the defects incorporated in the ZnO matrix.


Introduction
With the rapid development of nanotechnology, a diverse range of nanomaterials and nanoproducts are emerging [1]. ZnO nanoparticles, as one common engineered nanomaterial, have used in various technological field such as sunscreen products, textiles, paintings, industrial coatings, and antimicrobial agents [2,3]. The II-VI semiconductor ZnO NPs possess the large exciton binding energy of 60 meV and direct band gap (3.36 eV) at room temperature, which have the promising applications in the optoelectronics and photonics [4][5][6][7]. The synthesis of the size and shape of the metal oxide nanostructures is significant role in controlling their physical and chemical properties for their potential application. Size of the materials becomes smaller; the band gap becomes larger, thereby changing the optical and electrical properties of the material. As a result of these changes, it is now possible to develop newer applications and devices using these materials. The modification of electrical and optical properties of a semiconductor is the generally accepted methods and it's the addition of impurity atoms or doping [8][9][10]. In early literature, the synthesis of ZnO and doped ZnO nanoparticles, this can be characterized into either chemical or physical method [11,12], hydrothermal process [13], sol-gel method [14] and co-precipitation method [15]. Among these method of co-precipitation is suitable and preferred methods to prepare the nanoparticles. Cetin et al., (2012) reported that magnesium doped ZnO nanofibers has increased the band gap [16] and calcium doping in ZnO NPs has relaxed strain in the unit cell [17].
The antibacterial activity of ZnO NPs can be widely studied [18][19][20][21]. Commonly accepted mechanism of antibacterial action is the material production of reactive oxygen species (ROS) [22] on the surface of these NPs in the light causes oxidative stresses in bacterial cells and leads to the cell death. ROS contain the most reactive hydroxyl radical 4 (OH), less toxic superoxide anion radical (O 2-) and hydrogen peroxide with a weaker oxidizer (H2O2). This can damage DNA, cell membranes etc., leading to cell death [23]. It is suggested that both ZnO NPs and Zn 2+ are toxic but have different modes of action taking place in the antibacterial cell death. In the present investigation, pure and Mn 2+ doped ZnO NPs are synthesized by co-precipitation method and prepared nanoparticles characterized by structural, optical, magnetic and antibacterial properties.

Material synthesis
All chemicals were analytically pure and used without additional purification. Mn doped ZnO nanoparticles were prepared as follows: 0.024 g (1%) 0.049 g (2%) and 0.073 g (3%) of manganese acetate was added to 100 ml of distilled water to get the Mn solution separately, and then each solution was added to zinc acetate solution of 2.17 g, 2.15 g and 5 2.12 g respectively. Then NaOH solution of 3.2 g was added drop wise in the above each solution; it formed a dark brown precipitate. The procedure for the precipitation of undoped ZnO NPs was adopted for the preparation of Mn doped ZnO nanoparticles. The precipitates appeared as black coloured Mn doped ZnO samples. Finally, obtained samples were annealed at 700 °C for 2 hrs for the energy from the heat can enhance the vibration and diffusion of lattice atoms for crystallization. The annealed nanopowder of ZnO will be used for further studies. The prepared pure and Mn doped ZnO samples were code named as ZnO, ZnMnO1, ZnMnO2 and ZnMnO for 0%, 1%, 2% and 3% of Mn concentrations, respectively.

Instrumentation
The ZnO NPs were characterized by using X-ray diffractometer (XRD model:

Testing of antibacterial activity
The bacterial activity is frequently tested using a disk diffusion test, by using antibiotic filled disks (Kirby-Bauer). A related test with nanoparticle laden disks was used in

Particle size determination:
The particle size of the pure and Mn doped ZnO can be calculated using Debey-Scherer's formula: where the constant k is the shape factor = 0.94, λ is the wavelength of X-rays (1.5418 for Cu Kα), θ is Bragg's angle, β is the full width at the half-maximum (FWHM). The particle size of the pure ZnO was calculated as 41.6 nm and when the dopant manganese was added to pure ZnO, the crystallite size of the NPs gets decreased to 40.1 and 33.8 nm for ZnMnO2 and ZnMnO3, respectively. Staumal et al. [24] investigated that the decrease of crystallite size is due to increase of Mn solubility in ZnO. However, in all the Mn doped samples, the particle size gets reduced gradually with increasing of Mn concentration [ Table 2].
The lattice parameters of ZnO nanostructures are highly depend on the doping concentrations since there is difference of dopant ionic radius with respect to the ZnO matrix. . The dopant induced change in the bond length was deduced from the relation where, 'a', and 'c' are lattice parameters and 'up' is the positional parameter which can be calculated by the formula 25 .
The dopant induced lattice strain can be extracted from the peak broadening of diffraction line. The peak broadening of diffraction line is due to the combination of lattice strain and the particle size with respect to diffraction angle and it can be written as where t  is line broadening due to the particle size and e  is due to the strain induced by dopant ions. The particle size could be evaluated by using the Scherrer formula and peak broadening due to strain can be calculated where  is the microstrain. The peak broadening due to the combination of particle size as well as microstrain can be written mathematically [29]     (Fig. 3a). The variation in the microstarin upon Mn concentration is plotted and shown in Figure 3b. The results revealed that the up to 2% of Mn concentration, the microstrain is decreased with respect to the pure ZnO and increases for 3% of Mn concentration. This is due to lattice mismatch when the concentration of Mn is large [30]. 9

HRTEM studies
The ordered lattice fringes in the HRTEM image further confirmed the single crystalline nature of pure ZnO and Mn doped ZnO NPs. Fig. 4.1 (a-d) shows the distribution of spherical shaped pure ZnO NPs. Fig. 4.2 (a-d) it's seen that images found from the portion of an individual ZnMnO particle confirms the better crystalline nature of ZnMnO NPs.

EDAX analysis
The EDAX spectra of the pure ZnO, ZnMnO1, ZnMnO2 and ZnMnO3 NPs are shown in Fig. 5 (a-d). From the EDAX spectra, the amounts of Zn, Mn, O atoms that present in the samples are given in

UV spectroscopy analysis
The UV-Vis absorption spectra of the pure and Mn doped ZnO NPs samples were done by dispersing 3 mg of powder in 10 mL water and the spectrum is shown in Fig. 6. The absorbance is expected to depend on some factors like band gap, oxygen deficiency, surface roughness and impurity centres [27]. The absorbance spectra exhibit an absorption edges  [30]. The blue shift may be attributed due to agglomeration in the sample [28][29].

Estimation of optical bandgap
The optical bandgap of the synthesized ZnO and Mn doped ZnO nanoparticles can be calculated using from this relation,

A t
Where A is the absorbance and t is the thickness of the cuvette. The optical band gap of the NPs was determined by applying the Tauc relationship given by, The bandgap of undoped and Mn doped ZnO samples is measured by extrapolation of linear portion of the graph between the kublka-munk function (hυα) 2 Vs (hυ) [27]. Fig. 7   11 shows the band gap energy diagram for pure and Mn doped ZnO nanoparticles. The calculated bandgap of pure ZnO nanoparticles was found to be 2.7 eV while in the case of Mn doped ZnO it was at 2.9 eV. The band gap energy of ZnO increases with the addition of Mn (except ZnMnO2). This is attributed to the increase in particle size of doped ZnO. It's clear that more level of doping (except ZnMnO2) causes an increase in the exitonic bandgap, proving the Burstein-Moss effect [26].

Photoluminescence (PL) analysis
Generally, the densities of defects and oxygen vacancies affect significantly the optical properties of oxide nanostructures. The correlation between structure and property is investigated by PL spectra of undoped and Mn doped ZnO nanoparticles excitation wavelength of 325 nm at room temperature is shown in Fig. 8. The PL spectra of the samples ZnO, ZnMnO1, ZnMnO2 and ZnMnO3 NPs demonstration a UV emission peak at ~387 to 397 nm and broad visible emission peaks including violet emission at ~416 to 418 nm, bluegreen emission at ~480 to 479 nm and green emission at ~523 to 524 nm. The UV emission band is associated to near band-edge (NBE) emission of the ZnO NPs, and is due to the recombination of free excitons by exciton-exciton collision process [31,32]. Several authors have investigated the PL properties of ZnO nanostructures [31][32][33][34][35][36][37][38]. Generally, visible emission in ZnO contains of blue, violet, green and yellow emission peaks, this may be ascribed to many intrinsic defects such as oxygen vacancies (Vo), zinc vacancies (VZn), oxygen interstitials (Oi), zinc interstitials (Zni) and oxygen antisites (OZn) [30]. UV peak is shifted from 394 to 387 nm with respect to the Mn concentrations (Fig. 8b), that shows the result is consistent with UV absorption spectra.
The violet emission in synthesized ZnO NPs centered at ~416 to ~418 nm is attributed to an electron transition from a shallow donor level of the neutral Zni to the top level of the valence band [39]. A blue-green emission observed at ~480 to 479 nm is due to a radiative transition of an electron from the shallow donor level of Zni to an acceptor level of neutral Vzn [31]. The green emission at ~523 to 524 nm is attributed to radiative transition from conduction band to the edge of the acceptor levels of Ozn caused by oxygen antisites (Ozn) [31,40]. The UV emission and reduced visible emission shows that the undoped and Mn doped ZnO NPs have a good crystal structure with fewer oxygen vacancies. to the -OH group, may due to the water adsorbed on the surface of nanoparticles.

Antibacterial Properties
At present the nanoparticles are being extensively studied to antibacterial activity.
Several factors such as reduced amount of toxicity level and heat resistance are accountable for the use of NPs in the biological applications [42,43]. The pure and Mn doped ZnO nanoparticles are investigated with respect to potential antimicrobial applications. The microbial sensitivity to the nanoparticles is found to vary depending on the microbial species Normally, bactericidal agents are potential to inhibit in the clinical field because bactericides lead to rapid and better recovery from the bacterial infection and such as minimize the risk of the emergence of drug resistance [45].
Interestingly in the present study the zone of inhibition reflects the degree of susceptibility of the microorganism. The strains incorporated to the disinfectants exhibit larger inhibition zone, whereas resistant strains exhibit smaller inhibition zone (see Fig.11).
All the bacterial strains depict higher sensitivity to the higher concentration (1.2 mg/ml) for both Pure ZnO and Mn doped ZnO NPs. When compared to the positive control Methicillin (10mg/ml), except S. epidermis, it exhibits larger zone of inhibition than the positive control at pure ZnO and Mn doped ZnO NPs.

8 Magnetic properties
The influence of pure and Mn doped ZnO NPs on the magnetic properties are studied.
The undoped ZnO NPs is existence with diamagnetic behaviour at magnetic moment 0.0096 emu/g and also observed that the magnetic moment (0.00809, 0.0304 and 0.0408 emu/g) of Mn doped ZnO NPs increased with the increase of Mn doping concentration (Fig. 12) Hence, the possibility of strong ferromagnetic behaviour of Mn doped ZnO is ruled out, but it results to the weak ferromagnetic or super-paramagnetic behaviour.

Conclusion
The pure ZnO and Mn doped ZnO NPs were synthesized by the co-precipitation method. The X-ray diffraction (XRD) results confirmed that the synthesized NPs formed with hexagonal wurtzite structure. The HRTEM images of the synthesized NPs showed nanospheres morphology and a reduction in the particle size with respect to the Mn concentrations. The bandgap of ZnO NPs was increased from 2.7 to 2.9 eV upon Mn doping.
The antibacterial studies were performed against a set of bacterial strains. Among them, Staphylococcus epidermidis showed the higher sensitivity to Mn doped ZnO nanoparticles which were powerful than the positive control and Pseudomonas aeruginosa strain was less sensitive to NPs.

Conflicts of interest/Competing interests: Authors declared no conflict of interest
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