Structural, optical, dielectric and antibacterial studies of Mn doped Zn0.96Cu0.04O nanoparticles

Accepted Manuscript Structural, optical, dielectric and antibacterial studies of Mn doped Zn0.96Cu0.04O nanoparticles R. Sangeetha, S. Muthukumaran, M. Ashokkumar PII: DOI: Reference:

S1386-1425(15)00220-6 http://dx.doi.org/10.1016/j.saa.2015.02.056 SAA 13358

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

11 July 2014 14 February 2015 16 February 2015

Please cite this article as: R. Sangeetha, S. Muthukumaran, M. Ashokkumar, Structural, optical, dielectric and antibacterial studies of Mn doped Zn0.96Cu0.04O nanoparticles, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.02.056

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Structural, optical, dielectric and antibacterial studies of Mn doped Zn0.96Cu0.04O nanoparticles R. Sangeetha†, S. Muthukumaran*, M. Ashokkumar †

N.P.R. College of Engineering and Technology, Natham - 624 401, Dindigul, Tamilnadu, India PG and Research Department of Physics, Government Arts College, Melur -622 001, Madurai, Tamilnadu, India Abstract Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles were synthesized by sol-gel method. The

X-ray diffraction pattern indicated that doping of Mn and Cu did not change the ZnO hexagonal wurtzite structure. The Mn doped nanoparticles had smaller average crystallite size than undoped Zn0.96Cu0.04O nanoparticles due to the distortion in the host ZnO lattice. This distortion prevented the subsequent growth and hence the size reduced by Mn doping. The changes in lattice parameters, average crystallite size, peak position and peak intensity confirmed the Mn substitution in Zn-Cu-O lattice. The Mn and Cu co-doping increased the charge carrier density in ZnO nanoparticles which led to increase the dielectric constant. The dielectric constant also varied by depend the size of the nanoparticles. The change in morphology by Mn-doping was studied by transmission electron microscope. The optical absorption and band gap were changed with respect to both compositional and size effects. The band gap was initially increased from 3.65 to 3.73 eV at 1% of Mn doping, while decreasing trend in band gap was noticed for further increase of Mn. The band gap was decreased from 3.73 to 3.48 eV when Mn concentration was increased from 2 to 4%. Presence of chemical bonding and purity of the nanoparticles were confirmed by FTIR spectra. The antibacterial study revealed that that the antibacterial activity of Zn0.96Cu0.04O is enhanced by Mn doping.

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Keywords: Mn and Cu co-doped ZnO; TEM; FTIR, Optical property; Antibacterial study __________________________________

* Corresponding author. Tel.: + 91 0452-2415467, Fax.: +91 0452-2415467 E-mail address: [email protected] (S. Muthukumaran) 1. Introduction ZnO semiconductor gets much attention from researchers due to their extraordinary properties and potential applications. The wide band gap (3.4 eV), high excitonic binding energy (60 meV), high piezo-electric constant and environmental friendly characteristics of ZnO [1, 2] makes its to have large number of applications such as solar energy conversion, storage device, spintronic devices, gas sensing, antibiotic, etc [3-5]. The doping of transition metals (TM) offers feasible means of tuning physical properties, especially optical, electrical and magnetic properties to suit specific needs and applications [6]. The most commonly used metallic dopants in ZnO based systems are Cr, Co, Cu, Ni, Mn, etc. [7–11]. Ferromagnetic (FM) ground state at 25 at% of 3d transition metal (TM) doping was reported by Sato and Katayama-Yoshida using density functional theory [12]. But, the dopant related clusters were formed in the host lattice at higher doping concentration (≥ 5 at%) [13, 14]. The formation of metallic cluster decreases the charge density of material [15]. To avoid the formation of metallic cluster, simultaneous doping of two or more transition metals is used to increases the doping percentage [16, 17]. Substitution of Cu into ZnO lattice is used to tune its properties such as photocatalytic activity, gas sensitivity, room temperature ferromagnetism, etc. [18-20]. Similarly, the incorporation Mn2+ ions into ZnO lattice leads to very interesting and novel magnetic, electrical and optical properties, owing to its half-filled 3d shell having the largest ionic moment (5µB) [21]. In addition, the solubility of Mn metallic ion in ZnO lattice is 2

larger than 10 mol% [22]. Therefore, in the present investigation Cu and Mn are co-doped with ZnO to ensure higher doping concentration without metallic cluster. The doping percentage of Cu and Mn is limited to 4 at% to avoid the metallic cluster formation. Among the ddifferent physical or chemical synthetic methods used to prepare the doped ZnO nanoparticles [17, 23-25], the sol-gel is used in the present investigation because it is one of the most important methods to prepare the nanoparticles in large scale with low cost. Even though some of the research works have been carried out on Cu or Mn doped ZnO system separately [18-22], comprehensive study of the structural and optical properties of Cu and Mn co-doped ZnO nanoparticles is still scanty. Therefore, in present investigation, Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles were synthesized by sol-gel method and the effect of Mn dopant on its structural, dielectric, optical and morphological properties has been studied and discussed in detail. 2. Experimental procedure 2.1. Preparation of Zn0.96-xCu0.04MnxO nanoparticles For the synthesis of Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles, the analytical grade (AR), high purity chemicals (Merc, > 99% purity) such as Zinc acetate dihydrate ((CH3COO)2Zn.2H2O), Cupric acetate monohydrate ((CH3COO)2Cu.H2O) and Maganese (II) acetate tetrahydrate ((CH3COO)2Mn.4H2O) were used as precursors. Initially, appropriate amount of Zinc acetate dihydrate, Cupric acetate monohydrate and Maganese (II) acetate tetrahydrate were dissolved in N, N-Dimethyl formamide (C3H7NO) one by one with 20 mins interval. The temperature of the sol was gradually raised to 80˚C and kept for 6h to get gel. The gel was transferred into 150˚C pre heated hot air furnace and kept in furnace for 12h. The final products were collected and grounded using an agate mortar. Finally, the synthesized 3

nanoparticles were annealed at 450˚C in air atmosphere for 2h followed by furnace cooling. The same procedure was repeated to remaining Mn concentrations. 2.2. Characterization techniques The crystal structure of Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles was determined by powder X-ray diffraction. XRD patterns were recorded by RigaKu C/max-2500 diffractometer using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA from 2θ = 30˚ to 70˚. Dielectric measurements were carried out in the frequency range from 50 Hz to 200 KHz using LCR meter at room temperature. The samples used for this measurement was in the pellet form and the pellets were coated with silver paste to form parallel plate capacitor geometry. The morphology of Zn0.96Cu0.04O, Zn0.94Cu0.04Mn0.02O and Zn0.92Cu0.04Mn0.04O nanoparticles was studied using a transmission electron microscope (TEM, JEOL JEM 200CX) at an acceleration voltage of 200 kV). The UV–Visible optical absorption study was carried out to explore their optical properties. The spectral absorption was determined using UV–Visible spectrometer (Model: lambda 35, Make: Perkin Elmer) in the wavelength ranging from 300 to 600 nm at room temperature. The presence of chemical bonding in Zn0.96-xCu0.04MnxO nanoparticles was studied by FTIR spectrometer (Model: Perkin Elmer, Make: Spectrum RX I) from 400 to 4000 cm-1. The samples used for this measurement is in the form of pellets prepared by mixing the nanoparticles with KBr at 1 weight %. The antibacterial activity of Zn0.96-xCu0.04MnxO nanoparticle on Escherishia coli and Staphylococcus aureus was studied through disk diffusion method. 3. Results and discussion 3.1. X-ray diffraction (XRD) – Structural studies The X-ray diffraction patterns of Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles are shown in Fig. 1a. All the diffraction peaks are very close to the standard data of pure ZnO (a = 4

3.2488 Å, c = 5.2061 Å, space group P63mc, 186, JCPDS data card No. 36-1451). The welldeveloped diffraction peaks of Zn0.96-xCu0.04MnxO nanoparticles show the ZnO hexagonal wurtzite structure. There are no extra peaks corresponding to metallic Cu/Mn or secondary and impurity phases of Cu/Mn which may be attributed to the incorporation of Cu2+ and Mn2+ into Zn2+ lattice site. The enlarged image of the diffraction peak corresponding to (101) plane of Zn0.96-xCu0.04MnxO nanoparticles is shown in Fig. 2 which is used to study the Mn doping effect on peak position and peak intensity. The intensity of the peaks gradually decreased with the increase of Mn doping percentage from 0 to 4% and it reveals that the crystal quality is affected by Mn doping. Even though, there is no similar gradual shift noticed in peak position with Mn doping, the peak position of Mn doped nanoparticles are shifted to higher 2θ side (except Mn = 4%) compare to undoped Zn0.96Cu0.04O nanoparticles. The initial Mn doping (1%) shifts the peak position towards higher 2θ side by ~ 0.086˚ but the subsequent Mn doping from 2 to 4% gradually shifts towards lower 2θ side by very small amount (~ 0.03˚). The peak position shift depend several factors such as inter-planer distance (d), average crystallite size (D) and strain inside the lattice. Even though, the average crystallite size and strain are calculated as a function of both angular peak width at half maxima (β) and peak position (2θ), the role of 2θ in determining these values is very small compare to β. But, the inter-planer distance have inversely proportional relation with peak position. Therefore, the inter-planer distance (d-value) plays major role in peak position shift. Decrease of d-value at 1% Mn doping shifts the peak position to higher 2θ side and the subsequent increase in d-value shifts to lower 2θ side. The average crystallite size of the nanoparticles is calculated after appropriate background correction from Xray line broadening of the diffraction peaks of (101) plane using Debye Scherrer’s formula [14], 5

Average crystal size (D) =

(1)

where, λ is the wavelength of X-ray used (1.5406 Å), β is the angular peak width at half maximum in radian along (101) plane and θ is Bragg’s diffraction angle. The micro-strain (ε) can be calculated using the formula [14], Micro-strain (ε) =

(2)

Table 1 shows the variation of full width at half maximum (FWHM) value, average crystallite size (D) and micro-strain (ε) of Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles. The average crystallite size of Mn doped Zn0.96Cu0.04O nanoparticles (16.6 - 21 nm) are smaller than Zn0.96Cu0.04O nanoparticles (23.1 nm). The initial doping of Mn (1%) increases the micro strain from 1.50 x 10-3 to 2.09 x 10-3, whereas the subsequent Mn doping from 2% to 4% decreases the micro strain to 1.65 x 10-3. The substitution of Mn produces distortion in the host ZnO lattice due to mismatch between ionic radii of Zn2+ (0.60 Å) and Mn2+ (0.66 Å). The distortion produced by the foreign impurity reduces the subsequent growth of the grain surface [11]. Hence, the average crystallite size of the Mn doped nanoparticles gets reduced than Zn0.96Cu0.04O nanoparticles. Creation of newer nucleation centers by Mn2+ dopant ion may also responsible for such a reduced average crystallite size of Mn doped Zn0.96Cu0.04O nanoparticles [26]. The increase in bond length due to the substitution of larger ionic radii Mn2+ (0.66 Å) into the position of smaller ionic radii Zn2+ (0.60 Å) leads to expands the volume of the lattice [27]. Therefore, the observed small gradual increase in average crystallite size with the increase of Mn concentration is due to the expansion of lattice volume. The change of FWHM, peak intensity may be due to the size or micro-strain or size and micro-strain [28]. Bond length (l) and volume (V) of the Zn0.96Cu0.04O nanoparticles are calculated 6

manually from cell parameters by using equations (3) and (4) and tabulated in Table 2. The Zn– O bond length has been calculated using the relationship [29], Bond length (l) = where,

(3) is the potential parameter of the hexagonal structure. The volume of

unit cell of hexagonal system has been calculated from the equation [30], Volume (V) = 0.866 x a2 x c

(4)

The peak position (2θ), d-value, cell parameters ‘a’ and ‘c’, c/a ratio, stress (σ), bond length (Ɩ) and volume (V) of Zn0.96-xCu0.04MnxO nanoparticles are tabulated in Table 2. The dvalue, cell parameters, bond length and volume of Mn doped Zn0.96Cu0.04O nanoparticles are less than undoped Zn0.96Cu0.04O nanoparticles and increases slightly with the increases of Mn concentration from 1% to 4%. The lattice parameters of semiconductors depend on the concentration of foreign atoms, defects, external strain and the difference of their ionic radii with respect to the substituted matrix ion [31]. In addition, many theoretical and experimental studies have demonstrated size dependent changes in lattice parameters [32, 33]. Therefore, the size effect should be the possible reason for smaller d-value, cell parameters, bond length and volume of Mn doped Zn0.96Cu0.04O nanoparticles. Further, the small increase in lattice parameters with Mn doping concentration is due to the substitution of larger ionic radii Mn2+ instead of smaller ionic radii Zn2+ in Zn-Cu-O lattice [27]. Mera et al. already reported the similar expansion of lattice parameters when Mn ion is doped into ZnO [27]. Interestingly, there is no solubility limit found due to the high solubility of Mn in ZnO lattice. The changes in d-value, cell parameters, volume, bond length, average crystallite size, peak position shift and peak intensity confirms the Mn substitution in Zn-Cu-O lattice. The constant c/a ratio reveals that the hexagonal wurtzite structure of ZnO does not affect by Mn doping. 7

The stress (σ) in the ZnO plans can be determined using the following expression [34], σ = -233×109 ((Cbulk – C)/ Cbulk)

(5)

where, C is the lattice constant of ZnO planes calculated from x-ray diffraction data, Cbulk is the strain-free lattice parameter of ZnO (5.2061 Å). The compressive stress (0.434 GPa) of Zn0.96Cu0.04O nanoparticles gets increase to 0.989 GPa at initial Mn introduction in Zn-Cu-O lattice and the subsequent Mn concentration decreases the compression along c-axis. The decreased compression along c-axis is due to the increase of lattice volume and bond length. 3.2. Dielectric properties Fig. 3a shows frequency dependence of real part of dielectric constant for Zn0.96-xCu0.04MnxO (x= 0, 0.02 and 0.04) nanoparticles at room temperature. All the samples exhibit high dielectric constant (ε') at lower frequency and low ε' at higher frequency. The rapid decrease of ε' at lower frequency is due the quick polarization occurring in the samples. At the same time, the electric dipoles fail to follow external applied field at higher frequencies and hence the ε' decreases with the increase of frequency and show almost frequency independent behaviour at higher frequencies. The Mn and Cu co-doped samples have high ε' than Cu doped samples. The initial substitution of Ni increase the ε’. According to Maxwell-Wagner interfacial (MWI) model, the well conducting grains are separated by poorly conducting (or resistive) grain boundaries [35]. When external field is applied, the charges are accumulated at the grain boundaries and induced the space charge polarization which acts as an electric dipole in the presence of external field. Increase of charge carrier intensity by Mn doping increases the dipole moment of the electric dipole, which lead to increase the dielectric constant. Since, the decrease in crystallite size increases the number of electric dipole per unit volume the reduced crystallite size by Mn doping 8

is also another possible reason for the enhancement of dielectric constant. Gupta et al. also found that size depending dielectric properties [36]. As discussed above, the decreased ε' from Ni = 2 to 4% is due to the increase of crystallite size. The frequency dependence of dielectric loss for Zn0.96-xCu0.04MnxO (x = 0, 0.02, 0.04) samples at room temperature is shown in Fig. 3b. Dielectric loss is represented as dissipated energy in a dielectric system. The less dielectric loss at high frequencies makes it to be suitable material for high frequency application. 3.3. Transmission electron microscope (TEM) The morphologies of Zn0.96-xCu0.04MnxO (x = 0, 0.02, 0.04) nanoparticles are shown in Figs. 4a-c. Fig. 4a shows the TEM image of Zn0.96Cu0.04O nanoparticles. It has agglomerated spherical and hexagonal structure with un-even grain size around 25 nm. Fig. 4b shows the TEM image of Zn0.94Cu0.04Mn0.02O nanoparticles and it shows more agglomeration and reduced grain size

than

Zn0.96Cu0.04O

nanoparticles.

The

well

agglomerated

grain

structure

for

Zn0.92Cu0.04Mn0.04O nanoparticles is shown in Fig. 4c. All images show densely packed grain structure and are homogeneous and uniformly distributed throughout the structure with poor crystallinity. 3.4. UV-Visible - Optical studies Fig. 5 shows the UV–visible optical absorption spectra of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0 to 4% between 300 and 600 nm at room temperature. The absorption in UV region arises due to the transition of valance band electron into conduction band by absorbs the energy from optical radiation. The Mn doped Zn0.96Cu0.04O nanoparticles show less absorption (except 4% of Mn) than undoped Zn0.96Cu0.04O nanoparticles, whereas the absorption gets increase with the increase of Mn doping concentration. The absorptions of the nanoparticles are expected to depend on several factors, such as size of the 9

particle, band gap, defects and impurity centers. Therefore, it is concluded that the less absorption of Mn doped Zn0.96Cu0.04O nanoparticles is due to its low average crystallite size (size effect). The gradual increase with Mn doping percentage is due to the increase of average crystallite size with Mn concentration (size effect) and Mn concentration in nanoparticles (compositional effect). The shift in peak position in UV region is responsible for the band gap variation by Mn doping. The band gap of the nanoparticles change with respect to several factors such as dopant atom, dopant concentration (compositional effect) and size of the nanoparticles (size effect). The room temperature transmittance spectra of Zn0.96-xCu0.04MnxO nanoparticles from 300 to 600 nm are shown in Fig. 6. The transmittance spectra show just opposite trend of the optical absorption spectra. The optical band gap is evaluated using the Tauc relation [37]: hυ = A(hυ - Eg)n

(6)

where, A as a constant, Eg is optical band gap of the material and the exponent n depends upon the type of transition. The values of n for direct allowed, indirect allowed and direct forbidden are ½, 2, 3/2. In the present case, n is taken as 1/2. The Tauc plot is used to calculate the band gap of Zn0.96-xCu0.04MnxO nanoparticles as shown in Fig. 7. The extrapolation of the straight line to the energy (hυ) axis gives the band gap of the material. The initial doping of 1% Mn increases (blue shift) the energy gap from 3.65 to 3.73 eV (∆Eg ≈ 0.08 eV) due to the decreased average crystallite size. Whereas, the band gap gradually decreases (red shift) from 3.73 eV to 3.48 eV when Mn doping increases till 4% due to the increase of average crystallite size with Mn doping. Similar size dependent band gap tailoring was reported in the previous literature [37]. The observed narrowing of band gap (red shift) is also originated from sp–d exchange interactions between the band electrons and the localized d10

electrons of the Mn2+ ions [37]. Band gap narrowing by sp-d exchange interactions was theoretically explained by Bylsma et al. [38] using second order perturbation theory. Thus, both the compositional and size effect involves the band gap tailoring in Zn0.96-xCu0.04MnxO nanoparticles. 3.5. Fourier Transform Infrared (FTIR) studies FTIR is a technique used to obtain information regarding chemical bonding in a material. The band positions and numbers of absorption peaks are depending on crystalline structure, chemical composition and also on morphology [39]. The characteristic peaks exhibited by FTIR spectra of Zn0.96-xCu0.04MnxO nanoparticles are shown in Fig. 8a. The broad absorption bands around 3400 and 1600 cm-1 attributed to O-H vibration of H2O in Cu-Zn-O lattice [40]. The absorption around 3400 cm-1 arises due to the normal stretching vibration of O-H and the bending vibration creates the band around 1600 cm-1. Very week absorption at 2924 cm−1 is due to C-H stretching vibration of residual acetate and a sharp absorption peak between at 2336 cm−1 is because of an existence of CO2 molecule in air [41]. The absorption peaks around 1380 cm−1 are due to the stretching vibration of carboxyl group (C=O). The characteristic IR peaks below 1000 cm-1 is very important to study the presence or absence of Zn- O / Cu-O / Mn-O bonds and the functional groups. The enlarged spectrum in the wave number range below 1000 cm-1 is shown in Fig. 8b. The very strong absorption bands observed in the range of 410-550 cm-1 are attributed to the stretching modes of Zn-Cu-Mn-O. Even though the absorption band is observed in all nanoparticles, the absorption peak positions are changed with respect to Mn doping concentration. A single absorption peak is observed in un-doped Zn0.96Cu0.04O nanoparticles at 441 cm-1. The absorption band becomes broader and the peak position is shifted to 425 cm-1 for 1% Mn doping. The absorption band start splits into two 11

peaks into 499 cm-1 and 426 cm-1 when Mn doping is increased to 2%. Two well distinct peaks centered at 538 cm-1 and 424 cm-1 are appeared for 3% of Mn doping. The peaks around 425 cm-1 become week and a dominated peak at 535 cm-1 is noticed for 4% of Mn doping. The absorption at 425 cm-1 is assigned to stretching mode of ZnO [42]. Pandiyarajan et al. experimentally shows the linear relationship between bond length and IR absorption frequencies [43]. Doping of larger ionic radii of Mn2+ (0.66 Å) and Cu (0.73 Å) instead of smaller ionic radii of Zn (0.60 Å) increases the bond length, considerably. Therefore, these enlarged bands absorb the IR frequencies at higher side than pure ZnO. From the above discussion, it is concluded that the noticed absorption band from 410 to 550 cm-1 rather than single absorption peak at 410 cm-1 is due to the presence of Mn and Cu in the ZnO lattice and the increasing trend in absorption at 550 cm-1 by Mn doping confirms the above result. The absorption centered at 425 cm-1 become week at 4% of Mn doping and it suggests that the Zn-O network is perturbed at higher level doping. Week absorption at 873 cm-1 is due to the presence of defects states around the Cu and Mn ions in the Zn-O lattice. The observed results are well supported with XRD and optical studies. 3.6. Antibacterial activity studies Growing resistance of microorganisms to potent antibiotics has renewed a great interest towards investigating bactericidal properties of nanoparticles. ZnO is an inorganic compound which strongly resists microorganisms. The advantage of using these oxides as antimicrobial agents is that they contain mineral elements essential to human and exhibit strong activity even when administrated in small amounts. To study the antibacterial activity of the prepared nanoparticles was performed according to the disc diffusion method using Muller Hinton agar. In this work, bacterial strains of E. coli and S. aureus were employed during the antibacterial test. The above mentioned bacteria were 12

grown individually on Wattman no.1 filter discs at 37°C for 24 h. Erythromycin was used as reference drug. Fig. 9 shows the photograph of antibacterial activity of Zn0.96-xCu0.4MnxO

nanoparticles with different Mn concentrations from 0% to 4% on E.coli and S.aureus organisms. It is noticed from Fig. 9 that zone of inhibition (ZOI) formation i.e., the antibacterial efficiency increased with decreasing crystallite size. This may be attributed to increase in the surface area [44] as particle size reduced. Table 3 shows the atibacterial activity of

Zn0.96-xCu0.4MnxO nanoparticles with different Mn concentrations from 0% to 4% on E.coli and S.aureus bacteria where the concentration of the nanoparticles is 50 mg/L. Mn = 2% doped sample showed maximum activity (ZOI ∼ 29mm) against S. aureus whereas ZOI is 26 mm against E. coli. The study revealed that the Cu, Mn co-doped ZnO showed an increased antibacterial activity against the bacteria than Cu-doped ZnO. In this paper it is shown that the antibacterial activity of Zn0.96-xCu0.4MnxO nanoparticles is enhanced by Mn doping. Moreover, the concentration of bacteria decreased gradually and the rate of decrease is higher for low crystallite size. 4. Conclusions The following are the conclusions drawn from the present investigation: •

Zn0.96-xCu0.04MnxO (0 ≤ x ≤ 0.04) nanoparticles were synthesized by sol-gel method.

•

The X-ray diffraction pattern indicated that doping of Mn and Cu did not change the ZnO hexagonal wurtzite structure.

•

The Mn doped nanoparticles had smaller average crystallite size than un-doped Zn0.96Cu0.04O nanoparticles due the distortion produced in the host ZnO lattice by Mn doping.

•

The changes in d-value, cell parameters, volume, bond length, average crystallite size, peak 13

position shift and peak intensity confirms the Mn substitution in Zn-Cu-O lattice. •

Increase of charge carrier density by Mn doping and decrease of average crystallite size enhanced the dielectric properties of Mn and Cu co-doped samples.

•

The change in morphology by Mn-doping was studied by scanning electron microscope and the presence of compositional elements such as Mn, Cu and Zn with their nominal stoichiometry was confirmed by energy dispersive X-ray spectra within its experimental error.

•

The optical absorption and the band gap were changed with respect to both compositional and size effects. The band gap was initially increased from 3.65 to 3.73 eV at 1% of Mn doping, while decreasing trend in band gap was noticed for further increase of Mn.

•

The band gap was decreased from 3.73 to 3.48 eV when Mn concentration was increased from 2 to 4%.

•

Presence of chemical bonding and purity of the nanoparticles were confirmed by FTIR spectra.

•

The antibacterial study revealed that that the antibacterial activity of Zn0.96Cu0.04O is enhanced by Mn doping.

Acknowledgments: The authors are thankful to the University Grant Commission (UGC), New Delhi, India, for financial support under the project [File no.: 41-968/2012 (SR)].

14

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17

Figure captions Figure 1. Powder X-ray diffraction pattern of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0% to 4% at room temperature Figure 2. The enlarged diffraction spectra of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0% to 4% from 35.8° to 37° along (101) plane. Figure 3. (a) Dielectric constant of Zn0.96-xCu0.04MnxO (x=0, 0.02 and 0.04) nanoparticles as a function of frequency at room temperature. (b) Dielectric loss of Zn0.96-xCu0.04MnxO (x = 0, 0.02 and 0.04) nanoparticles as a function of frequency at room temperature. Figure 4.

Transmission

electron

microscope (TEM)

images

of (a)

Zn0.96Cu0.04O,

(b) Zn0.94Cu0.04Mn0.02O and (c) Zn0.92Cu0.04Mn0.04 O nanoparticles at room temperature. Figure 5. UV-Visible absorption spectra of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0% to 4% between 300 and 600 nm at room temperature. Figure 6. Transmittance spectra of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0% to 4% between 300 and 600 nm at room temperature. Figure 7. The (αhυ)2 versus hυ curves of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0% to 4% for the optical energy gap calculation. Figure 8. (a) FTIR spectra of Zn0.96-xCu0.04MnxO nanoparticles for different Mn concentrations from 0% to 4% at room temperature, (b) The enlarged FTIR spectra in the wave number range <1000 cm-1 at room temperature. Figure 9. The antibacterial activities of Zn0.96-xCu0.04MnxO nanoparticles against bacterial strains of Escherichia coli and Staphylococcus aureus.

18

Figure 1a

19

Figure 2

20

Figure 3a

21

Figure 3b

22

Figure 4

23

Figure 5

24

Figure 6

25

Figure 7

26

Figure 8a

27

Figure 8b

28

Figure 9

29

Table 1 The variation of full width at half maximum (FWHM, β) value, average crystallite size (D) and micro-strain (ε) of Zn0.96-xCu0.04MnxO (x = 0, 0.01, 0.02, 0.03 and 0.04) nanoparticles

FWHM, β

Average crystallite size, D

Micro-strain, ε

(degrees)

(nm)

(10-3)

Zn0.96Cu0.04O

0.36

23.1

1.50

Zn0.95Cu0.04Mn0.01O

0.50

16.6

2.09

Zn0.94Cu0.04Mn0.02O

0.50

16.8

2.00

Zn0.93Cu0.04Mn0.03O

0.48

17.3

2.06

Zn0.92Cu0.04Mn0.04O

0.40

21.0

1.65

Samples

30

Table 2 The variation of peak position (2θ), d-value, cell parameters ‘a’ and c, c/a ratio, stress (σ), bond length (Ɩ) and volume (V) of Zn0.96-xCu0.04MnxO (x = 0, 0.01, 0.02, 0.03 and 0.04) nanoparticles

Samples

Peak

d-

Cell parameters c/a

Stress,

Bond

Volume,

position,

value

(Ǻ)

σ

length,

V (Ǻ)3

2θ

(Ǻ)

ratio

a=b

c

(GPa)

Ɩ (Ǻ)

(˚)

Zn0.96Cu0.04O

36.32

2.472

3.244

5.196

1.602

0.434

1.97

47.35

Zn0.95Cu0.04Mn0.01O

36.40

2.466

3.235

5.184

1.603

0.989

1.99

46.98

Zn0.94Cu0.04Mn0.02O

36.37

2.468

3.238

5.188

1.602

0.819

1.97

47.11

Zn0.93Cu0.04Mn0.03O

36.34

2.470

3.241

5.193

1.602

0.595

1.97

47.23

Zn0.92Cu0.04Mn0.04O

36.29

2.473

3.245

5.200

1.602

0.273

1.98

47.43

31

Table 3 Antibacterial

activity

of

Zn0.96-xCu0.4MnxO

nanoparticles

with

different

concentrations from 0% to 4% on E.coli and S.aureus bacteria

S.No.

Samples

Concentrations (mg/L)

Zone of inhibition (ZOI) (mm) E. coli

S. aureus

1

Zn0.96Cu0.04O

50

20

24

2

Zn0.94Cu0.04Mn0.02O

50

26

29

3

Zn0.92Cu0.04Mn0.04O

50

22

26

32

Mn

Structural, optical, dielectric and antibacterial studies of Mn doped Zn0.96Cu0.04O nanoparticles R. Sangeetha†, S. Muthukumaran*, M. Ashokkumar †

N.P.R. College of Engineering and Technology, Natham - 624 401, Dindigul, Tamilnadu, India PG and Research Department of Physics, Government Arts College, Melur -622 001, Madurai, Tamilnadu, India GRAPHICAL ABSTRACT

34

Highlights

1. Zn0.96-xCu0.04MnxO nanoparticles were successfully synthesized.

2. Mn2+ into Cu-Zn-O lattice was confirmed by EDX, FTIR and optical studies. 3. Size and band gap tailoring by Mn-doping are useful for optoelectronic devices. 4. Antibacterial activity of Cu-doped ZnO is enhanced by Mn doping

33