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Structural, optical and antibacterial investigation of La, Cu dual doped ZnO nanoparticles prepared by co-precipitation method
Journal Pre-proof Structural, optical and antibacterial investigation of La, Cu dual doped ZnO nanoparticles prepared by co-precipitation method S. Anitha, S. Muthukumaran PII:
S0928-4931(18)32831-5
DOI:
https://doi.org/10.1016/j.msec.2019.110387
Reference:
MSC 110387
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Materials Science & Engineering C
Received Date: 18 September 2018 Revised Date:
30 October 2019
Accepted Date: 30 October 2019
Please cite this article as: S. Anitha, S. Muthukumaran, Structural, optical and antibacterial investigation of La, Cu dual doped ZnO nanoparticles prepared by co-precipitation method, Materials Science & Engineering C (2019), doi: https://doi.org/10.1016/j.msec.2019.110387. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Structural, optical and antibacterial investigation of La, Cu dual doped ZnO nanoparticles prepared by co-precipitation method S. Anithaa, S. Muthukumaranb,* a
Department of Physics, Arulmigu Palaniandavar College of Arts and Culture, Palani – 624 601, Tamilnadu, India b
PG and Research Department of Physics, Government Arts & Science College, Melur, Madurai- 625106, Tamilnadu, India
ABSTRACT La, Cu dual doped ZnO with Cu = 0% to 4% were prepared by co-precipitation route. The XRD pattern optimized the Cu content as 2% which restrict the secondary phase formation. The new phase at 38.7° corresponding to CuO (111) and the another phase at 42.3° related to Zn (101) came from un-reacted Zn2+ ions appeared in Cu = 4% doped sample. Zeta potential measurements confirms the stability of the particles. The blue shift of absorption edge and the energy gap from 3.66 eV to 3.99 eV by Cu doping was discussed by the shape of the particles, the distortion of the host lattice and generation of defect concentrations. The characteristic IR peaks around 470-489 cm−1 was related to the octahedral sites of Zn-O for Cu = 0 and 2% which is shifted to 616 cm-1 corresponding to the tetrahedral site at Cu = 4%. The shift in frequency was originated from the dissimilarity in the volume and bond lengths by the substitution of La and Cu in Zn-O. Based on the antibacterial report, it can be concluded that the Cu-doped Zn-La-O solid solution compose an effectual antimicrobial agent against pathogenic microorganisms. Cu = 4% doped sample possessed highest bacterial killing capacity because of the enhanced crystallite size and high density of oxygen vacancies which led the higher ROS values. Tuning of crystallite size and energy gap and the enhanced bactericial killing capacity by Cu addition is useful for opto-electronic device and medical applications. Keywords: La, Cu dual doped ZnO; crystallite size; oxygen vacancies; antibacterial activities; ROS mechanism 1
______________________ * Corresponding author. Tel.: Tel.: +91 0452 2415467; fax: +91 0452 2415467 E-mail address: [email protected] (S. Muthukumaran) 1. Introduction Nano-structured oxide semiconductors are gaining more interest nowadays owing to their wide band gap and its associated properties. Among the different metal oxide semiconductors, zinc oxide (ZnO) received much awareness within the systematic researchers due to its photocatalytic action, tuning of band-gap [1-3], unique optical, magnetic and electronic properties, etc. [4, 5]. Now, rare earth (RE) ion doped ZnO nanoparticles obtained much consideration since such doping can alter and develop the better optical properties of the semiconductor nanoparticles [6-8]. RE metals are important because of their capability to trap the electrons, which can be successfully dropping the recombination of photo-generated electron–hole pairs [9]. The modification in lattice parameter of metal-doped ZnO is depending on the radius of doping ions, which can replace with the Zn ion in the lattice [10]. The radius of the dopant ion is significant factor, which can powerfully influence the capability of the dopant to go into oxides crystal lattice and creates distortion [11]. In the doping reactive procedure it can either iso-morphously substituted or interstitially establish into the matrix of ZnO to generate oxygen vacancies which speed up the nanocrystallite growth of wurtzite ZnO [12]. The luminescence characters of RE doped ZnO have stimulated great attention to many researchers [13–16]. There are only a small number of research woks were taken out on dyes [17] using ZnO doped with RE metal ions. Anandan et al. demonstrated the n the degradation of mono-crotophos [18] in La substituted ZnO. La substituted ZnO demonstrates the admirable gas sensitivity and photocatalytic action which comprise the well-organized modulation of the emission in the visible range due to their exclusive optical properties.
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La/Ce substituted ZnO possess enhanced photocatalytic activities than pure ZnO [19]. The defect states increased with increase of La3+ ions in ZnO which results reducing UV emission and narrow energy gap [20] was experienced by Manikandan et al. In the present study, La is chosen as first element doped into ZnO due to its excellent optical properties and also it is restricted to 3% to avoid the metallic cluster creation. Cu-ZnO received immense curiosity for generating a broad scope of highly developed applications like FET [21], effective diodes [22], biosensors [23], catalyst [24], solar cells [25]. Magnetic property and energy gap of can be modified with either modifying by oxygen or Zn vacancies by Cu1+ and/or Cu2+ in ZnO [26]. All said problems can be modified by altering method of preparations and conditions. Since Cu can modify the microstructure of ZnO [28] and induce defects states, Cu is chosen as second doping element with ZnO. Even though, many techniques like hydrothermal, chemical co-precipitation, sol-gel and microwave technique [29-32] have been used to prepare La, Cu dual doped ZnO, chemical precipitation is chosen to prepare the nanoparticles because it has several significance like better quality, low-processing cost, relatively low temperature and superior yield etc. in comparison with other methods. The detailed discussion about the antibacterial performance on La, Cu dual doped ZnO nanoparticles is still scanty. Therefore, the present research wok focused on examining the consequence of Cu doping concentrations on the structure, optical properties and antibacterial evaluation of La, Cu dual doped ZnO nanoparticles. Further, the size of the nanocrystals are interrelated with band gap and antibacterial activities. 2. Materials and experimental procedure 2.1. Synthesis of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles Undoped Zn0.97La0.03O and Cu-Zn0.97La0.03O with Cu = 0% to 4% have been prepared by co-precipitation method using Zn acetate dihydrate [Zn(CH3CO2)2.2H2O], La chloride 3
heptahydrate (LaCl3·7H2O) and Cu chloride dihydrate (CuCl2.2H2O) as metal precursors (Zn, La and Cu, respectively) and NaOH. The prepared NaOH solution was allowed as drop wise into the initial solution to increase the pH to 10. The preparative technique is described in our previous paper [33]. Exact amount of Zn acetate, La chloride and Cu chloride in the precipitation solution and percentage of Cu obtained from EDX for Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) is presented in Table 1. 2.2. Characterization Techniques XRD spectra have been carried out using RigaKu C/max-2500 diffractometer with Cu Kα radiation from 30˚ to 70˚. The content in the samples like Zn, O, La and Cu were obtained by energy dispersive X-ray (EDX) spectrometer. The surface image was obtained by scanning electron microscope (SEM, JEOLJSM 6390) and TEM. Malvern Zetasizer ZS (Malvern Instruments, Malvern, UK) is employed to analyse the size distribution and zeta potential of the synthesized samples. It is measured via analyzing 0.1 g of La/Cu doped ZnO in 10 ml of water where water is used as dispersant. The optical characters were taken out by UV–Visible spectrometer (Model: lambda 35, Make: Perkin Elmer) from 300 to 600 nm. The chemical bonding was studied by FTIR spectrometer (Model: Perkin Elmer, Make: Spectrum RX I) from 400 to 4000 cm-1. Antibacterial activity of the prepared nanoparticle was determined using well diffusion method using Mueller Hinton agar media. Mueller Hinton agar media was purchased from Millipore Sigma (catalog no. 70191). After sterilization and solidification process, wells have been cut on the Mueller Hinton agar. Two bacteria Staphylococcus aureus (gram positive, ATCC 25923 (MSSA)) and Pseudomonas aeruginosa (gram negative, ATCC 9027) were taken for the study. Norfloxin 10µg was used as the standard. 0.2 ml of nanoparticle solution with 50mg/ml concentration was added into the well. The plates were incubated at 37oC for 24 hours, and then ZOI was measured in mm. 4
3. Results and Discussion 3.1. X - ray diffraction (XRD): Structural studies X-ray profile examination is a potent tool to compute the structural parameters [34]. The typical XRD patterns of La/Cu doped ZnO with different Cu content from 0% to 4% attained using co-precipitation technique are presented in Fig. 1. XRD pattern of Zn0.97La0.03O nanoparticles exhibit strong crystalline character of the La doped ZnO. The well-pronounced diffraction peaks are matching to the hexagonal structure. During the initial doping of Cu (Cu = 2%), the wurtzite structure of host ZnO was not changed. No trace of secondary/impurity phases indicate the phase clarity of the sample. The enhanced XRD peak intensity with Cu = 2% doping indicate the better crystal quality. It is understood from Fig. 1 that at Cu (4%), the new phases comes into view at 38.7° and 42.3°. The initial new secondary phase appeared at 38.7° related to CuO (111) (represented by the symbol
,
JCPDS 05-0661) due to the stimulation of CuO phase from un-reacted Cu2+ ions in the solution. Another peak at 42.3° related to Zn (101) (represented by the symbol , JCPDS 04 0831) comes from excess Zn2+. Fig. 2a and b illustrates the peak position and peak intensity of Zn0.97-xLa0.03CuxO with Cu = 0 to 4% along (101) and (100) planes, respectively. Both Fig. 2a and b showed the higher intensity at Cu = 2% which indicate the better crystal quality of the sample. In both cases, diffraction peaks shifted towards higher angles as Cu level increases; this is owing to the lesser radius of Cu (0.057 nm) than that of Zn (0.060 nm). The similar upward transition of peak position was described by Liu et al. [35] due to lattice constant contraction. Fig. 2c shows the intensity ratio between (100) and (101) planes from Cu = 0 to 4%. Only slight increase is noticed between 0% and 2% which is shifted to higher value for Cu = 4%. The
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enhanced intensity ratio at higher Cu content signifies that the better orientation of (100) plane by Cu-doping. The change in 2θ, FWHM, intensity along (101) plane, average crystallite size (D) and microstrain (ε) of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) are listed in Table 2. The crystallite size is carried out by XRD broadening along (101) plane using Scherrer’s formula [33] 0.9λ/ β cosθ, where, λ = 1.5406 Å. The micro-strain (ε) can be derived by the relation [36] ε = β cosθ/4. Fig. 3 shows the modification of peak intensity and average crystallite size. The elevated intensity at the initial substitution of Cu (Cu = 2%) recommended that the crystal quality is improved by Cu doping. The noticed increment in size at Cu = 2% supports the slight enhancement in intensity. Yet, the situation at higher Cu doping is different, where the XRD peak intensity declined but size gets elevated because of the formation of high density secondary phases. Table 3 illustrates the variation of peak position (2θ), d-value, cell parameters ‘a’ and c, c/a ratio and volume (V) of Zn0.97-xLa0.03CuxO with Cu = 0% to 4%. The lattice constant obtained from 2d sin θ = λ [37] and 1/d2 = 4/3 ([h2+hk+k2]/a2) +l2/c2. The volume of unit cell is obtained by V = 0.866 x a2 x c. The alteration in size and lattice parameters can be described with lattice strain and stress due to defect or vacancies provoked by the replacement of Cu ion into the ZnO lattice [38]. The replacement of Cu2+ ions into the Zn2+ sites guide to lattice parameter lesser but the replacement of Cu1+ ions into Zn2+ ions bring about the lattice constant improved [26]. The outcome concludes that only Cu2+ ion replacement for Zn2+ ions in Zn-La-O lattice without varying ZnO wurtzite structure. The presence of blended Zn-O and Cu-O composites at higher Cu concentrations is exposed in XRD spectra (Fig. 1). Since the local structure, the coordination number and the bond property of Cu-O is close to Zn-O, it is possible to attain the hexagonal wurtzite structure at 6
higher Cu concentrations. 3.2. Microstructure and compositional studies Surface morphology of Zn0.97-xLa0.03CuxO extracted from SEM for various Cu content is illustrated in Fig. 4. Noticeable morphology alteration in shape and particle size was observed. Almost spherical-like and some hexagonal-like morphology was shown in Fig. 4a for un-doped Zn0.97La0.03O. When Cu is bring in to Zn-La-O lattice (Fig. 4b and c), the morphology and size were altered. To achieve high precision of particle size and shape pattern, TEM study was carried out. As no dopant was introduced (Zn0.97La0.03O), the size varied as 10 - 30 nm (Fig. 5a). During the substitution of Cu (Fig. 5b and c), the morphology was considerably altered and the rod shape structure with size from 10 to 40 nm was noticed. EDX spectra of Zn0.97-xLa0.03CuxO illustrated in Fig. 6a-c obviously support that the prepared samples are made of Zn, O, La and Cu. No other peaks associated to impurities have been identified from Fig. 6 that authenticates the transparency of the compounds. The atomic % of the constituents in the sample is given as inset of Fig. 6. Fig. 6 reflects the steady increment of Cu% which depicts the well incorporation of Cu in Zn-La-O. The lowest presents of O in Fig. 6c at Cu = 4% evidenced the existence of oxygen vacancies. 3.3. Particle size and Zeta potential measurements The common particle size was investigated by DLS method using Malvern Zetasizer ZS. Fig. 7a illustrates the particle size of La/Cu doped ZnO which varies from 20-35 nm for different Cu content. The increasing trend of particle size with increase of Cu concentration confirms the results obtained from XRD and SEM analysis. Fig. 7b represents the value of measured Zeta potential of the synthesized nanostructures in the colloidal solution. The change in Zeta potential is associated to the movement of nanoparticles under the control of an applied electric field. The movement of nanoparticles correlates with both surface charge and the local environment of the particles [39]. Here, Zeta potential varied between 23.8 mV 7
and 18.9 mV by Cu addition which signified that even after the storage of few months at room temperature the nanoparticles are stable. Kavitha et al. [40] demonstrated that simillar zeta potential of ZnO was 17.6 mV. 3.4. Optical studies The evaluation of optical characters is extremely essential to decide on the materials for its specific applications. The optical characteristics of Zn0.97-xLa0.03CuxO with different doping concentrations were examined by UV–Vis spectra (Fig. 8a). Fig. 8b demonstrates the enlarged absorption spectra between 300 nm and 366 nm which visualize the significant change in absorption in near UV region. It was observed from Fig. 8b that the cut-off bandedge absorption of the entire samples situated at just about 310-350 nm. The blue shift of absorption edge by Cu doping is accountable for the generation of defect states and secondary phase formation like CuO. Fig. 9 stands for the transmittance spectra of Zn0.97-xLa0.03CuxO from Cu = 0 to 4% between 300 nm and 600 nm. All the samples display improved transmittance in the high wavelength region than UV region due to its fundamental absorption characteristic. During the initial doping of Cu, the penetration of Cu through the Zn-La-O lattice form the complex centers (CuZn, Cui) [41]. The solid solution of Cu- Zn-La-O exists by a dissociative mechanism that is a mixture of the interstitial and vacancy mechanism where Cu can restore either substitutional or interstitial. The noticed lower transmittance for Cu = 2% is because of appropriate replacement of Cu and also by the absorption losses. The higher transmittance after Cu = 2% is accountable for both Cu interstitials and high density defect states. The band gap is estimated with the help of the relation [42], αhʋ = A (hʋ - Eg)n where, Eg is energy gap and n = ½. The Eg is achieved from (αhʋ)2 verses hʋ curves as shown in Fig. 10. The energy gap of Zn0.97La0.03O obtained from Tauc formula is 3.66 eV that is superior than bulk ZnO (3.37 8
eV) [43] due to quantum confinement effect (QCE) [44]. Fig. 11 illustrates the alteration of crystallite size and band gap for different Cu concentrations. The size and shape of the particles and density of defect levels [45] are the main factors to vary Eg of materials. QSE i.e., Eg is inversely proportional to the size is not suitable in the present case in view of the fact that the size of the particles are higher than excitonic Bohr radius. As a result, size effect is uncertain to modify the Eg. Therefore, it could be decided that the shape of the particles, distortion of the host lattice and generation of defects influenced a considerable role in the modification of Eg. The increase of Eg with Cu concentrations is supported the results reported by Sakai et al. [46]. 3.5. Nature of chemical bonding Fig. 12 illustrates the FTIR spectra of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) from 400– 4000 cm−1. The band at around 3400 cm−1 is due to the O-H stretching mode which [47]. The noticed peak at ∼ 1580 cm-1 is attributed to H–O–H bending vibration of H2O present in the nano-crystalline ZnO [48]. Another strong absorption peaks noticed around 1394 cm-1 are related to the carboxyl assembly (C=O) [49]. The little and feeble absorption bands around 855 cm−1 stands for the defect states adjacent to La ions in Zn-O lattice [50]. The strong absorption bands around 725-748 cm−1 originated from the vibration of La-Zn-O-Cu local bond [51]. The alteration in intensity, band position and FWHM authenticate the subsistence of defect states. The characteristic peaks around 470-489 cm−1 related to the occurrence of octahedral sites in Zn-O which is absent at higher Cu. The peak position corresponding to octahedral site is shifted to 616 cm−1 resulting from tetrahedral site at Cu = 4%. The shift in frequency is originated from the dissimilarity in the volume and bond lengths by the substitution of La and Cu in Zn-O. 3.6. Antibacterial studies In the current work the antibacterial activity was carried out for Zn0.97-xLa0.03CuxO 9
against both gram positive (S.aureus) and gram negative (P.aeruginosa) bacteria to examine the effect on killing efficiency. Fig. 13 a and b illustrates the zone of inhibition (ZOI) in mm for different Cu concentrations against S.aureus and P.aeruginosa bacteria, where 1, 2, 3 and S represents Cu = 0%, 2%, 4% and standard, respectively. Fig. 13 c shows the graphical representation of ZOI for different micro-organisms such as S.aureus and P.aeruginosa bacteria. Cu doping in Zn0.97La0.03O reveals the enhanced antibacterial activity against the target cultures as shown in Fig. 13c. The dissimilarity in antibacterial reaction of nanoparticles against the two different bacterial strains is probably due to differences in the cell walls of the considered bacteria. Even though the correct mechanism of its antibacterial action is not obviously recognized, different mechanisms have been projected: photocatalyst activities, electrostatic interactions [52], metal ion release, ROS (Reactive Oxygen Species) [53] and membrane damage. The first and foremost mechanism is decomposition of ZnO and formation of oxygen reactive species. The graphical representation of mechanism to explore the antibacterial activity of La, Cu co-doped ZnO nanoparticles is shown in Fig. 14. ROS such as OH, O2• −, and H2O2 [54-56] are generated by the transfer of an electrons from VB to CB.
•
The most active hydroxyl radicals (•OH) is obtained from an oxidative process. H2O2 and the hydroxyl radical (•OH) being powerful oxidizing agents to prevent the growth of bacterial cells and introducing heavy toxicity to bacteria. Padmavathy and Vijayaraghavan [57] described that hydroxyl radicals and superoxide anion radical influence the outer portion of the cellular membrane, but H2O2 can travel in the interior of bacterial cells. The generation of H2O2 is as follows: Zn-La-Cu-O + hυ
e- + h+
h+ + H2O
H+ + •OH 10
e-
+ O2.
O2• -
O2. + H+
HO2•
HO2.
HO2• -
+ e-
HO2• - + H+
H2O2
Among the different Cu doped samples Cu = 4% doped sample has highest bacterial killing capacity becase of the enhanced crystallite size and high density of oxygen vacancies which show the way to higher ROS values [58]. ROS accelerates the disintegration of the cell wall and consequently the leakage of cell elements, and ultimately cell death [59]. Moreover, the escalation of bacteria is slowed down by the liberation of heavy metal ions like Zn2+, La3+ and Cu2+ . The negative charge of the cell and La/ Cu/Zn ions possessing positive charge commonly attract, and these metal ions penetrate inside the cell and react with the – SH groups in the cell surface. Thus, the inactivation of the proteins leads to death of the microbe [60].
Fig. 13 c shows the enhanced ZOI for gram-negative bacteria. Even though all
bacteria possesses the inner cell wall, gram-negative bacteria holds a distinctive outer cell which prohibits some drugs and antibiotics from cell damage. This special outer cell moderately responsible for why gram-negative bacteria are normally more opposing to antibiotics than gram-positive bacteria. 4. Conclusions La, Cu dual doped ZnO with Cu = 0% to 4% have been synthesized by coprecipitation route. The XRD pattern optimized the Cu content as 2% which restrict the secondary phase formation. The new phase at 38.7° corresponding to CuO (111) and the another phase at 42.3° related to Zn (101) came from un-reacted Zn2+ ions appeared in Cu = 4% doped sample. Zeta potential measurements confirms the stability of the particles. The blue shift of absorption edge and the energy gap from 3.66 eV to 3.99 eV by Cu doping was 11
discussed by the shape of the particles, the distortion of the host lattice and generation of defect concentrations. The characteristic IR peaks around 470-489 cm−1 was related to the octahedral sites of Zn-O for Cu = 0 and 2% which is shifted to 616 cm-1 corresponding to the tetrahedral site at Cu = 4%. Based on the antibacterial report, it can be concluded that Cu = 4% doped sample possessed highest bacterial killing capacity because of the enhanced crystallite size and high density of oxygen vacancies which led the higher ROS values. Tuning of crystallite size and energy gap and the enhanced bactericial killing capacity by Cu addition is useful for opto-electronic device and medical applications.
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Figure captions Figure 1 X-ray diffraction pattern of Zn0.97La0.030 and Cu-doped Zn0.97La0.030 nanoparticles with Cu = 0, 2% and 4% between 30° and 70°. Figure 2 Enlarged view of X-ray diffraction pattern (a) from 35° to 37° along (101) plane, (b) 31.1° to 32.3° along (100) plane and (c) variation of I100/I101 ratio as a function of Cu concentrations Figure 3. The variation of XRD peak intensity and average crystallite size along (101) plane for different Cu concentrations from 0% to 4%. Figure 4. SEM images of (a) Zn0.97La0.030, (b) Zn0.95La0.03Cu0.02O and (c) Zn0.93La0.03Cu0.04O nanoparticles. Figure 5. TEM images of (a) Zn0.97La0.030, (b) Zn0.95La0.03Cu0.02O and (c) Zn0.93La0.03Cu0.04O nanoparticles. Figure 6. Energy dispersive X-ray (EDX) spectra of (a) Zn0.97La0.030, (b) Zn0.95La0.03Cu0.02O and (c) - Zn0.93La0.03Cu0.04O nanoparticles. Figure 7. (a) Particle size distribution of the synthesized nanoparticles by DLS method and (b) Zeta potential measurements of La/Cu doped ZnO nanoparticles Figure 8. (a) UV-Visible absorption spectra of Zn0.97-xLa0.03CuxO (x = 0, 0.02 and 0.04) nanoparticles from 300 nm to 600 nm, (b) high resolution absorption spectra from 300 nm to 366 nm. Figure 9. Transmittance spectra of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% Figure 10. The (αhυ)2 versus hυ curves of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% for the optical energy gap calculation. 18
Figure 11. The variation of crystallite size and energy gap of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% Figure 12. (a) FTIR spectra of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% at room temperature in the wave number from 400 cm-1 to 4000 cm-1 at room temperature. Figure 13. The zone of inhibition (ZOI) in mm for different Cu concentrations against (a) S.aureus and (b) P.aeruginosa bacteria, where 1, 2, 3 and S represents Cu = 0%, 2%, 4% and standard, respectively, (c) the graphical representation of ZOI for different micro-organisms such as S.aureus and P.aeruginosa bacteria. Figure 14. The graphical representation of mechanism behind the antibacterial activity of La, Cu co-doped ZnO nanoparticles.
19
Figure 1
20
Figure 2a
Figure 2b
21
Figure 2c
Figure 3
22
Figure 4
23
Figure 5
24
Figure 6
25
Figure 7a
Figure 7b
26
Figure 8a
Figure 8b
27
Figure 9
28
Figure 10
29
Figure 11
30
Figure 12
31
Figure 13
32
Figure 14
33
Table 1 Exact amount of zinc acetate dihydrate, lanthanum chloride heptahydrate and copper chloride dihydrate in the precipitation solution and percentage of Cu obtained from EDX for Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles
Samples
Cu content
Amount of powders in 50 ml precipitation solution (grams)
(%)
% of Cu obtained
Zinc acetate Lanthanum chloride Copper chloride
from EDX
Zn0.97La0.03O
0
10.65
0.56
-
Zn0.95La0.03Cu0.02O
2
10.43
0.56
0.17
2.04
Zn0.93La0.03Cu0.04O
4
10.21
0.56
0.34
3.73
34
-
Table 2 The variation of peak position (2θ), full width at half maximum (FWHM) value, intensity along (101) plane, average crystallite size (D) and microstrain (ε) of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles
Samples
Peak position 2θ(º)
FWHM β (º)
Intensity
Average
Micro-strain
(counts)
crystallite size
ε (10-3)
D (nm)
Zn0.97La0.03O
36.12
0.352
3040
23.7
1.46
Zn0.95La0.03Cu0.02O
36.24
0.334
3384
25.0
1.39
Zn0.93La0.03Cu0.04O
36.31
0.302
1260
27.7
1.25
35
Table 3 The variation of peak position (2θ), d-value, cell parameters ‘a’ and c, c/a ratio and volume (V) of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles Samples
Peak position,
d-value
Cell parameters (Ǻ) c/a
Volume, V
(Ǻ)
a=b
c
ratio
(Ǻ)3
2θ (˚)
Zn0.97La0.03O
36.12
2.484
3.264
4.968
1.52
45.8353
Zn0.95La0.03Cu0.02O
36.24
2.477
3.255
4.954
1.52
45.4544
Zn0.93La0.03Cu0.04O
36.31
2.472
3.257
4.944
1.52
45.4184
36
• Cu-doped Zn0.97La 0.03O nanoparticles were prepared by co-precipitation method. • Cu content optimized at Cu = 2% after which impurity phase formed • The enhanced Eg by Cu doping is accountable for the generation of defect states • The IR peaks confirm the octahedral Zn-O at Cu = 0 and tetrahedral at Cu = 4%. • Highest bacterial capacity at Cu = 4% is due to the oxygen vacancies and higher ROS.
S0928-4931(18)32831-5
DOI:
https://doi.org/10.1016/j.msec.2019.110387
Reference:
MSC 110387
To appear in:
Materials Science & Engineering C
Received Date: 18 September 2018 Revised Date:
30 October 2019
Accepted Date: 30 October 2019
Please cite this article as: S. Anitha, S. Muthukumaran, Structural, optical and antibacterial investigation of La, Cu dual doped ZnO nanoparticles prepared by co-precipitation method, Materials Science & Engineering C (2019), doi: https://doi.org/10.1016/j.msec.2019.110387. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Structural, optical and antibacterial investigation of La, Cu dual doped ZnO nanoparticles prepared by co-precipitation method S. Anithaa, S. Muthukumaranb,* a
Department of Physics, Arulmigu Palaniandavar College of Arts and Culture, Palani – 624 601, Tamilnadu, India b
PG and Research Department of Physics, Government Arts & Science College, Melur, Madurai- 625106, Tamilnadu, India
ABSTRACT La, Cu dual doped ZnO with Cu = 0% to 4% were prepared by co-precipitation route. The XRD pattern optimized the Cu content as 2% which restrict the secondary phase formation. The new phase at 38.7° corresponding to CuO (111) and the another phase at 42.3° related to Zn (101) came from un-reacted Zn2+ ions appeared in Cu = 4% doped sample. Zeta potential measurements confirms the stability of the particles. The blue shift of absorption edge and the energy gap from 3.66 eV to 3.99 eV by Cu doping was discussed by the shape of the particles, the distortion of the host lattice and generation of defect concentrations. The characteristic IR peaks around 470-489 cm−1 was related to the octahedral sites of Zn-O for Cu = 0 and 2% which is shifted to 616 cm-1 corresponding to the tetrahedral site at Cu = 4%. The shift in frequency was originated from the dissimilarity in the volume and bond lengths by the substitution of La and Cu in Zn-O. Based on the antibacterial report, it can be concluded that the Cu-doped Zn-La-O solid solution compose an effectual antimicrobial agent against pathogenic microorganisms. Cu = 4% doped sample possessed highest bacterial killing capacity because of the enhanced crystallite size and high density of oxygen vacancies which led the higher ROS values. Tuning of crystallite size and energy gap and the enhanced bactericial killing capacity by Cu addition is useful for opto-electronic device and medical applications. Keywords: La, Cu dual doped ZnO; crystallite size; oxygen vacancies; antibacterial activities; ROS mechanism 1
______________________ * Corresponding author. Tel.: Tel.: +91 0452 2415467; fax: +91 0452 2415467 E-mail address: [email protected] (S. Muthukumaran) 1. Introduction Nano-structured oxide semiconductors are gaining more interest nowadays owing to their wide band gap and its associated properties. Among the different metal oxide semiconductors, zinc oxide (ZnO) received much awareness within the systematic researchers due to its photocatalytic action, tuning of band-gap [1-3], unique optical, magnetic and electronic properties, etc. [4, 5]. Now, rare earth (RE) ion doped ZnO nanoparticles obtained much consideration since such doping can alter and develop the better optical properties of the semiconductor nanoparticles [6-8]. RE metals are important because of their capability to trap the electrons, which can be successfully dropping the recombination of photo-generated electron–hole pairs [9]. The modification in lattice parameter of metal-doped ZnO is depending on the radius of doping ions, which can replace with the Zn ion in the lattice [10]. The radius of the dopant ion is significant factor, which can powerfully influence the capability of the dopant to go into oxides crystal lattice and creates distortion [11]. In the doping reactive procedure it can either iso-morphously substituted or interstitially establish into the matrix of ZnO to generate oxygen vacancies which speed up the nanocrystallite growth of wurtzite ZnO [12]. The luminescence characters of RE doped ZnO have stimulated great attention to many researchers [13–16]. There are only a small number of research woks were taken out on dyes [17] using ZnO doped with RE metal ions. Anandan et al. demonstrated the n the degradation of mono-crotophos [18] in La substituted ZnO. La substituted ZnO demonstrates the admirable gas sensitivity and photocatalytic action which comprise the well-organized modulation of the emission in the visible range due to their exclusive optical properties.
2
La/Ce substituted ZnO possess enhanced photocatalytic activities than pure ZnO [19]. The defect states increased with increase of La3+ ions in ZnO which results reducing UV emission and narrow energy gap [20] was experienced by Manikandan et al. In the present study, La is chosen as first element doped into ZnO due to its excellent optical properties and also it is restricted to 3% to avoid the metallic cluster creation. Cu-ZnO received immense curiosity for generating a broad scope of highly developed applications like FET [21], effective diodes [22], biosensors [23], catalyst [24], solar cells [25]. Magnetic property and energy gap of can be modified with either modifying by oxygen or Zn vacancies by Cu1+ and/or Cu2+ in ZnO [26]. All said problems can be modified by altering method of preparations and conditions. Since Cu can modify the microstructure of ZnO [28] and induce defects states, Cu is chosen as second doping element with ZnO. Even though, many techniques like hydrothermal, chemical co-precipitation, sol-gel and microwave technique [29-32] have been used to prepare La, Cu dual doped ZnO, chemical precipitation is chosen to prepare the nanoparticles because it has several significance like better quality, low-processing cost, relatively low temperature and superior yield etc. in comparison with other methods. The detailed discussion about the antibacterial performance on La, Cu dual doped ZnO nanoparticles is still scanty. Therefore, the present research wok focused on examining the consequence of Cu doping concentrations on the structure, optical properties and antibacterial evaluation of La, Cu dual doped ZnO nanoparticles. Further, the size of the nanocrystals are interrelated with band gap and antibacterial activities. 2. Materials and experimental procedure 2.1. Synthesis of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles Undoped Zn0.97La0.03O and Cu-Zn0.97La0.03O with Cu = 0% to 4% have been prepared by co-precipitation method using Zn acetate dihydrate [Zn(CH3CO2)2.2H2O], La chloride 3
heptahydrate (LaCl3·7H2O) and Cu chloride dihydrate (CuCl2.2H2O) as metal precursors (Zn, La and Cu, respectively) and NaOH. The prepared NaOH solution was allowed as drop wise into the initial solution to increase the pH to 10. The preparative technique is described in our previous paper [33]. Exact amount of Zn acetate, La chloride and Cu chloride in the precipitation solution and percentage of Cu obtained from EDX for Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) is presented in Table 1. 2.2. Characterization Techniques XRD spectra have been carried out using RigaKu C/max-2500 diffractometer with Cu Kα radiation from 30˚ to 70˚. The content in the samples like Zn, O, La and Cu were obtained by energy dispersive X-ray (EDX) spectrometer. The surface image was obtained by scanning electron microscope (SEM, JEOLJSM 6390) and TEM. Malvern Zetasizer ZS (Malvern Instruments, Malvern, UK) is employed to analyse the size distribution and zeta potential of the synthesized samples. It is measured via analyzing 0.1 g of La/Cu doped ZnO in 10 ml of water where water is used as dispersant. The optical characters were taken out by UV–Visible spectrometer (Model: lambda 35, Make: Perkin Elmer) from 300 to 600 nm. The chemical bonding was studied by FTIR spectrometer (Model: Perkin Elmer, Make: Spectrum RX I) from 400 to 4000 cm-1. Antibacterial activity of the prepared nanoparticle was determined using well diffusion method using Mueller Hinton agar media. Mueller Hinton agar media was purchased from Millipore Sigma (catalog no. 70191). After sterilization and solidification process, wells have been cut on the Mueller Hinton agar. Two bacteria Staphylococcus aureus (gram positive, ATCC 25923 (MSSA)) and Pseudomonas aeruginosa (gram negative, ATCC 9027) were taken for the study. Norfloxin 10µg was used as the standard. 0.2 ml of nanoparticle solution with 50mg/ml concentration was added into the well. The plates were incubated at 37oC for 24 hours, and then ZOI was measured in mm. 4
3. Results and Discussion 3.1. X - ray diffraction (XRD): Structural studies X-ray profile examination is a potent tool to compute the structural parameters [34]. The typical XRD patterns of La/Cu doped ZnO with different Cu content from 0% to 4% attained using co-precipitation technique are presented in Fig. 1. XRD pattern of Zn0.97La0.03O nanoparticles exhibit strong crystalline character of the La doped ZnO. The well-pronounced diffraction peaks are matching to the hexagonal structure. During the initial doping of Cu (Cu = 2%), the wurtzite structure of host ZnO was not changed. No trace of secondary/impurity phases indicate the phase clarity of the sample. The enhanced XRD peak intensity with Cu = 2% doping indicate the better crystal quality. It is understood from Fig. 1 that at Cu (4%), the new phases comes into view at 38.7° and 42.3°. The initial new secondary phase appeared at 38.7° related to CuO (111) (represented by the symbol
,
JCPDS 05-0661) due to the stimulation of CuO phase from un-reacted Cu2+ ions in the solution. Another peak at 42.3° related to Zn (101) (represented by the symbol , JCPDS 04 0831) comes from excess Zn2+. Fig. 2a and b illustrates the peak position and peak intensity of Zn0.97-xLa0.03CuxO with Cu = 0 to 4% along (101) and (100) planes, respectively. Both Fig. 2a and b showed the higher intensity at Cu = 2% which indicate the better crystal quality of the sample. In both cases, diffraction peaks shifted towards higher angles as Cu level increases; this is owing to the lesser radius of Cu (0.057 nm) than that of Zn (0.060 nm). The similar upward transition of peak position was described by Liu et al. [35] due to lattice constant contraction. Fig. 2c shows the intensity ratio between (100) and (101) planes from Cu = 0 to 4%. Only slight increase is noticed between 0% and 2% which is shifted to higher value for Cu = 4%. The
5
enhanced intensity ratio at higher Cu content signifies that the better orientation of (100) plane by Cu-doping. The change in 2θ, FWHM, intensity along (101) plane, average crystallite size (D) and microstrain (ε) of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) are listed in Table 2. The crystallite size is carried out by XRD broadening along (101) plane using Scherrer’s formula [33] 0.9λ/ β cosθ, where, λ = 1.5406 Å. The micro-strain (ε) can be derived by the relation [36] ε = β cosθ/4. Fig. 3 shows the modification of peak intensity and average crystallite size. The elevated intensity at the initial substitution of Cu (Cu = 2%) recommended that the crystal quality is improved by Cu doping. The noticed increment in size at Cu = 2% supports the slight enhancement in intensity. Yet, the situation at higher Cu doping is different, where the XRD peak intensity declined but size gets elevated because of the formation of high density secondary phases. Table 3 illustrates the variation of peak position (2θ), d-value, cell parameters ‘a’ and c, c/a ratio and volume (V) of Zn0.97-xLa0.03CuxO with Cu = 0% to 4%. The lattice constant obtained from 2d sin θ = λ [37] and 1/d2 = 4/3 ([h2+hk+k2]/a2) +l2/c2. The volume of unit cell is obtained by V = 0.866 x a2 x c. The alteration in size and lattice parameters can be described with lattice strain and stress due to defect or vacancies provoked by the replacement of Cu ion into the ZnO lattice [38]. The replacement of Cu2+ ions into the Zn2+ sites guide to lattice parameter lesser but the replacement of Cu1+ ions into Zn2+ ions bring about the lattice constant improved [26]. The outcome concludes that only Cu2+ ion replacement for Zn2+ ions in Zn-La-O lattice without varying ZnO wurtzite structure. The presence of blended Zn-O and Cu-O composites at higher Cu concentrations is exposed in XRD spectra (Fig. 1). Since the local structure, the coordination number and the bond property of Cu-O is close to Zn-O, it is possible to attain the hexagonal wurtzite structure at 6
higher Cu concentrations. 3.2. Microstructure and compositional studies Surface morphology of Zn0.97-xLa0.03CuxO extracted from SEM for various Cu content is illustrated in Fig. 4. Noticeable morphology alteration in shape and particle size was observed. Almost spherical-like and some hexagonal-like morphology was shown in Fig. 4a for un-doped Zn0.97La0.03O. When Cu is bring in to Zn-La-O lattice (Fig. 4b and c), the morphology and size were altered. To achieve high precision of particle size and shape pattern, TEM study was carried out. As no dopant was introduced (Zn0.97La0.03O), the size varied as 10 - 30 nm (Fig. 5a). During the substitution of Cu (Fig. 5b and c), the morphology was considerably altered and the rod shape structure with size from 10 to 40 nm was noticed. EDX spectra of Zn0.97-xLa0.03CuxO illustrated in Fig. 6a-c obviously support that the prepared samples are made of Zn, O, La and Cu. No other peaks associated to impurities have been identified from Fig. 6 that authenticates the transparency of the compounds. The atomic % of the constituents in the sample is given as inset of Fig. 6. Fig. 6 reflects the steady increment of Cu% which depicts the well incorporation of Cu in Zn-La-O. The lowest presents of O in Fig. 6c at Cu = 4% evidenced the existence of oxygen vacancies. 3.3. Particle size and Zeta potential measurements The common particle size was investigated by DLS method using Malvern Zetasizer ZS. Fig. 7a illustrates the particle size of La/Cu doped ZnO which varies from 20-35 nm for different Cu content. The increasing trend of particle size with increase of Cu concentration confirms the results obtained from XRD and SEM analysis. Fig. 7b represents the value of measured Zeta potential of the synthesized nanostructures in the colloidal solution. The change in Zeta potential is associated to the movement of nanoparticles under the control of an applied electric field. The movement of nanoparticles correlates with both surface charge and the local environment of the particles [39]. Here, Zeta potential varied between 23.8 mV 7
and 18.9 mV by Cu addition which signified that even after the storage of few months at room temperature the nanoparticles are stable. Kavitha et al. [40] demonstrated that simillar zeta potential of ZnO was 17.6 mV. 3.4. Optical studies The evaluation of optical characters is extremely essential to decide on the materials for its specific applications. The optical characteristics of Zn0.97-xLa0.03CuxO with different doping concentrations were examined by UV–Vis spectra (Fig. 8a). Fig. 8b demonstrates the enlarged absorption spectra between 300 nm and 366 nm which visualize the significant change in absorption in near UV region. It was observed from Fig. 8b that the cut-off bandedge absorption of the entire samples situated at just about 310-350 nm. The blue shift of absorption edge by Cu doping is accountable for the generation of defect states and secondary phase formation like CuO. Fig. 9 stands for the transmittance spectra of Zn0.97-xLa0.03CuxO from Cu = 0 to 4% between 300 nm and 600 nm. All the samples display improved transmittance in the high wavelength region than UV region due to its fundamental absorption characteristic. During the initial doping of Cu, the penetration of Cu through the Zn-La-O lattice form the complex centers (CuZn, Cui) [41]. The solid solution of Cu- Zn-La-O exists by a dissociative mechanism that is a mixture of the interstitial and vacancy mechanism where Cu can restore either substitutional or interstitial. The noticed lower transmittance for Cu = 2% is because of appropriate replacement of Cu and also by the absorption losses. The higher transmittance after Cu = 2% is accountable for both Cu interstitials and high density defect states. The band gap is estimated with the help of the relation [42], αhʋ = A (hʋ - Eg)n where, Eg is energy gap and n = ½. The Eg is achieved from (αhʋ)2 verses hʋ curves as shown in Fig. 10. The energy gap of Zn0.97La0.03O obtained from Tauc formula is 3.66 eV that is superior than bulk ZnO (3.37 8
eV) [43] due to quantum confinement effect (QCE) [44]. Fig. 11 illustrates the alteration of crystallite size and band gap for different Cu concentrations. The size and shape of the particles and density of defect levels [45] are the main factors to vary Eg of materials. QSE i.e., Eg is inversely proportional to the size is not suitable in the present case in view of the fact that the size of the particles are higher than excitonic Bohr radius. As a result, size effect is uncertain to modify the Eg. Therefore, it could be decided that the shape of the particles, distortion of the host lattice and generation of defects influenced a considerable role in the modification of Eg. The increase of Eg with Cu concentrations is supported the results reported by Sakai et al. [46]. 3.5. Nature of chemical bonding Fig. 12 illustrates the FTIR spectra of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) from 400– 4000 cm−1. The band at around 3400 cm−1 is due to the O-H stretching mode which [47]. The noticed peak at ∼ 1580 cm-1 is attributed to H–O–H bending vibration of H2O present in the nano-crystalline ZnO [48]. Another strong absorption peaks noticed around 1394 cm-1 are related to the carboxyl assembly (C=O) [49]. The little and feeble absorption bands around 855 cm−1 stands for the defect states adjacent to La ions in Zn-O lattice [50]. The strong absorption bands around 725-748 cm−1 originated from the vibration of La-Zn-O-Cu local bond [51]. The alteration in intensity, band position and FWHM authenticate the subsistence of defect states. The characteristic peaks around 470-489 cm−1 related to the occurrence of octahedral sites in Zn-O which is absent at higher Cu. The peak position corresponding to octahedral site is shifted to 616 cm−1 resulting from tetrahedral site at Cu = 4%. The shift in frequency is originated from the dissimilarity in the volume and bond lengths by the substitution of La and Cu in Zn-O. 3.6. Antibacterial studies In the current work the antibacterial activity was carried out for Zn0.97-xLa0.03CuxO 9
against both gram positive (S.aureus) and gram negative (P.aeruginosa) bacteria to examine the effect on killing efficiency. Fig. 13 a and b illustrates the zone of inhibition (ZOI) in mm for different Cu concentrations against S.aureus and P.aeruginosa bacteria, where 1, 2, 3 and S represents Cu = 0%, 2%, 4% and standard, respectively. Fig. 13 c shows the graphical representation of ZOI for different micro-organisms such as S.aureus and P.aeruginosa bacteria. Cu doping in Zn0.97La0.03O reveals the enhanced antibacterial activity against the target cultures as shown in Fig. 13c. The dissimilarity in antibacterial reaction of nanoparticles against the two different bacterial strains is probably due to differences in the cell walls of the considered bacteria. Even though the correct mechanism of its antibacterial action is not obviously recognized, different mechanisms have been projected: photocatalyst activities, electrostatic interactions [52], metal ion release, ROS (Reactive Oxygen Species) [53] and membrane damage. The first and foremost mechanism is decomposition of ZnO and formation of oxygen reactive species. The graphical representation of mechanism to explore the antibacterial activity of La, Cu co-doped ZnO nanoparticles is shown in Fig. 14. ROS such as OH, O2• −, and H2O2 [54-56] are generated by the transfer of an electrons from VB to CB.
•
The most active hydroxyl radicals (•OH) is obtained from an oxidative process. H2O2 and the hydroxyl radical (•OH) being powerful oxidizing agents to prevent the growth of bacterial cells and introducing heavy toxicity to bacteria. Padmavathy and Vijayaraghavan [57] described that hydroxyl radicals and superoxide anion radical influence the outer portion of the cellular membrane, but H2O2 can travel in the interior of bacterial cells. The generation of H2O2 is as follows: Zn-La-Cu-O + hυ
e- + h+
h+ + H2O
H+ + •OH 10
e-
+ O2.
O2• -
O2. + H+
HO2•
HO2.
HO2• -
+ e-
HO2• - + H+
H2O2
Among the different Cu doped samples Cu = 4% doped sample has highest bacterial killing capacity becase of the enhanced crystallite size and high density of oxygen vacancies which show the way to higher ROS values [58]. ROS accelerates the disintegration of the cell wall and consequently the leakage of cell elements, and ultimately cell death [59]. Moreover, the escalation of bacteria is slowed down by the liberation of heavy metal ions like Zn2+, La3+ and Cu2+ . The negative charge of the cell and La/ Cu/Zn ions possessing positive charge commonly attract, and these metal ions penetrate inside the cell and react with the – SH groups in the cell surface. Thus, the inactivation of the proteins leads to death of the microbe [60].
Fig. 13 c shows the enhanced ZOI for gram-negative bacteria. Even though all
bacteria possesses the inner cell wall, gram-negative bacteria holds a distinctive outer cell which prohibits some drugs and antibiotics from cell damage. This special outer cell moderately responsible for why gram-negative bacteria are normally more opposing to antibiotics than gram-positive bacteria. 4. Conclusions La, Cu dual doped ZnO with Cu = 0% to 4% have been synthesized by coprecipitation route. The XRD pattern optimized the Cu content as 2% which restrict the secondary phase formation. The new phase at 38.7° corresponding to CuO (111) and the another phase at 42.3° related to Zn (101) came from un-reacted Zn2+ ions appeared in Cu = 4% doped sample. Zeta potential measurements confirms the stability of the particles. The blue shift of absorption edge and the energy gap from 3.66 eV to 3.99 eV by Cu doping was 11
discussed by the shape of the particles, the distortion of the host lattice and generation of defect concentrations. The characteristic IR peaks around 470-489 cm−1 was related to the octahedral sites of Zn-O for Cu = 0 and 2% which is shifted to 616 cm-1 corresponding to the tetrahedral site at Cu = 4%. Based on the antibacterial report, it can be concluded that Cu = 4% doped sample possessed highest bacterial killing capacity because of the enhanced crystallite size and high density of oxygen vacancies which led the higher ROS values. Tuning of crystallite size and energy gap and the enhanced bactericial killing capacity by Cu addition is useful for opto-electronic device and medical applications.
12
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Figure captions Figure 1 X-ray diffraction pattern of Zn0.97La0.030 and Cu-doped Zn0.97La0.030 nanoparticles with Cu = 0, 2% and 4% between 30° and 70°. Figure 2 Enlarged view of X-ray diffraction pattern (a) from 35° to 37° along (101) plane, (b) 31.1° to 32.3° along (100) plane and (c) variation of I100/I101 ratio as a function of Cu concentrations Figure 3. The variation of XRD peak intensity and average crystallite size along (101) plane for different Cu concentrations from 0% to 4%. Figure 4. SEM images of (a) Zn0.97La0.030, (b) Zn0.95La0.03Cu0.02O and (c) Zn0.93La0.03Cu0.04O nanoparticles. Figure 5. TEM images of (a) Zn0.97La0.030, (b) Zn0.95La0.03Cu0.02O and (c) Zn0.93La0.03Cu0.04O nanoparticles. Figure 6. Energy dispersive X-ray (EDX) spectra of (a) Zn0.97La0.030, (b) Zn0.95La0.03Cu0.02O and (c) - Zn0.93La0.03Cu0.04O nanoparticles. Figure 7. (a) Particle size distribution of the synthesized nanoparticles by DLS method and (b) Zeta potential measurements of La/Cu doped ZnO nanoparticles Figure 8. (a) UV-Visible absorption spectra of Zn0.97-xLa0.03CuxO (x = 0, 0.02 and 0.04) nanoparticles from 300 nm to 600 nm, (b) high resolution absorption spectra from 300 nm to 366 nm. Figure 9. Transmittance spectra of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% Figure 10. The (αhυ)2 versus hυ curves of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% for the optical energy gap calculation. 18
Figure 11. The variation of crystallite size and energy gap of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% Figure 12. (a) FTIR spectra of Zn0.97-xLa0.03CuxO nanoparticles with different Cu concentrations from 0% to 4% at room temperature in the wave number from 400 cm-1 to 4000 cm-1 at room temperature. Figure 13. The zone of inhibition (ZOI) in mm for different Cu concentrations against (a) S.aureus and (b) P.aeruginosa bacteria, where 1, 2, 3 and S represents Cu = 0%, 2%, 4% and standard, respectively, (c) the graphical representation of ZOI for different micro-organisms such as S.aureus and P.aeruginosa bacteria. Figure 14. The graphical representation of mechanism behind the antibacterial activity of La, Cu co-doped ZnO nanoparticles.
19
Figure 1
20
Figure 2a
Figure 2b
21
Figure 2c
Figure 3
22
Figure 4
23
Figure 5
24
Figure 6
25
Figure 7a
Figure 7b
26
Figure 8a
Figure 8b
27
Figure 9
28
Figure 10
29
Figure 11
30
Figure 12
31
Figure 13
32
Figure 14
33
Table 1 Exact amount of zinc acetate dihydrate, lanthanum chloride heptahydrate and copper chloride dihydrate in the precipitation solution and percentage of Cu obtained from EDX for Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles
Samples
Cu content
Amount of powders in 50 ml precipitation solution (grams)
(%)
% of Cu obtained
Zinc acetate Lanthanum chloride Copper chloride
from EDX
Zn0.97La0.03O
0
10.65
0.56
-
Zn0.95La0.03Cu0.02O
2
10.43
0.56
0.17
2.04
Zn0.93La0.03Cu0.04O
4
10.21
0.56
0.34
3.73
34
-
Table 2 The variation of peak position (2θ), full width at half maximum (FWHM) value, intensity along (101) plane, average crystallite size (D) and microstrain (ε) of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles
Samples
Peak position 2θ(º)
FWHM β (º)
Intensity
Average
Micro-strain
(counts)
crystallite size
ε (10-3)
D (nm)
Zn0.97La0.03O
36.12
0.352
3040
23.7
1.46
Zn0.95La0.03Cu0.02O
36.24
0.334
3384
25.0
1.39
Zn0.93La0.03Cu0.04O
36.31
0.302
1260
27.7
1.25
35
Table 3 The variation of peak position (2θ), d-value, cell parameters ‘a’ and c, c/a ratio and volume (V) of Zn0.97-xLa0.03CuxO (0 ≤ x ≤ 0.04) nanoparticles Samples
Peak position,
d-value
Cell parameters (Ǻ) c/a
Volume, V
(Ǻ)
a=b
c
ratio
(Ǻ)3
2θ (˚)
Zn0.97La0.03O
36.12
2.484
3.264
4.968
1.52
45.8353
Zn0.95La0.03Cu0.02O
36.24
2.477
3.255
4.954
1.52
45.4544
Zn0.93La0.03Cu0.04O
36.31
2.472
3.257
4.944
1.52
45.4184
36
• Cu-doped Zn0.97La 0.03O nanoparticles were prepared by co-precipitation method. • Cu content optimized at Cu = 2% after which impurity phase formed • The enhanced Eg by Cu doping is accountable for the generation of defect states • The IR peaks confirm the octahedral Zn-O at Cu = 0 and tetrahedral at Cu = 4%. • Highest bacterial capacity at Cu = 4% is due to the oxygen vacancies and higher ROS.