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Transition metal (Co, Mn) co-doped ZnO nanoparticles: Effect on structural and optical properties
Accepted Manuscript Transition metal (Co, Mn) co-doped ZnO nanoparticles: Effect on structural and optical properties Darshan Sharma, Ranjana Jha PII:
S0925-8388(16)34148-2
DOI:
10.1016/j.jallcom.2016.12.227
Reference:
JALCOM 40149
To appear in:
Journal of Alloys and Compounds
Received Date: 8 November 2016 Revised Date:
29 November 2016
Accepted Date: 17 December 2016
Please cite this article as: D. Sharma, R. Jha, Transition metal (Co, Mn) co-doped ZnO nanoparticles: Effect on structural and optical properties, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2016.12.227. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Graphical Abstract
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Transition metal (Co, Mn) co-doped ZnO Nanoparticles: Effect on structural and optical properties
a
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Darshan Sharma *a, and Ranjana Jha a Department of Physics, Netaji Subhas Institute of Technology, University of Delhi, Sector-3, Dwarka, New Delhi-110078, India.
*Corresponding author, Email: [email protected]
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Contact No.: +91-9990135625
Abstract
Pure ZnO and Co/Mn co-doped ZnO {Zn0.98-xCo0.02MnxO (0≤x≤0.06)} nanoparticles were synthesized by co-precipitation method. The structural, morphological and optical properties of prepared samples were explored in detail. Rietveld refinement of x-ray diffraction (XRD) data revealed the single phase,
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hexagonal wurtzite structure without any impurity phase. XRD and Fourier transform infra-red (FTIR) analysis confirmed the incorporation of Co/Mn ions at Zn site into host lattice structure. The morphology of samples was examined using scanning electron microscopy (SEM) and transmission
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electron microscopy (TEM) analysis. The particle size from TEM results was corroborated well with XRD
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data. The absorption spectra showed the initial decrease in optical energy band gap for low Mn concentration. The optical energy band gap further increased with a higher Mn concentration in codoped ZnO samples. Photoluminescence (PL) spectra showed five emission peaks due to different defect states. The paper enhances the understanding of structural, optical properties of Co/Mn co-doped nanocrystals. This paves the path for its potential application in the optoelectronic devices e.g. solar cell.
Keywords ZnO; Doping; Co-precipitation; XRD; Optical properties
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1.
Introduction
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Spintronics is the most enticing field which has potential future prospective in spin-based devices, data storage, and optoelectronic devices[1]. In spintronics, both spin state and charge can be utilized. Diluted magnetic semiconductors (DMSs) have been intensely studied to achieve spin-polarized charge carriers
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for spintronic device operations[2]. Physical properties of metal oxide DMSs depend on the size and surface functionality of material. In addition, structural and optical properties of metal oxides are
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modified by doping transition metal (TM) ion concentrations.
Zinc oxide (ZnO) is a most popular host material for TM ions doping. It is an optically transparent II-VI semiconductor having wide direct band gap, large exciton binding energy and wurtzite lattice structure. Theoretical studies have predicted ferromagnetism above room temperature for TM element doped ZnO for spintronic applications[1,3]. Different research groups investigated the role of TM ions in ZnO
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nanocrystals[4–14]. Despite numerous reports, development of DMSs with desired optical and magnetic properties remains a controversial issue. The presence of two TM ions in ZnO is considered as a promising alternative technique to achieve stable optical and magnetic properties[15]. Therefore,
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researchers have attempted simultaneous doping of two TM ions in ZnO[16–24]. Co-dopant ions are
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selected depending on the ionic radius, valence states, coordination number and individual magnetic contribution. In our study, we investigated the role of cobalt (Co) and manganese (Mn) ions in ZnO. Magnetic properties of Co/Mn co-doped ZnO nanocrystals have been explored by different research groups[20,25–29]. But, development of Co/Mn co-doped ZnO, its magnetic behavior, and origin of room temperature ferromagnetism are still controversial. In addition, optical properties in Co/Mn co-doped ZnO nanocrystals are found contradictory in scantily reported literature. Less explored optical properties of Co/Mn co-doped ZnO nanocrystals limit its potential application in optoelectronic devices.
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An attempt has been made to well explore the optical properties and its correlation with structural properties through this experimental investigation. In this study, pure ZnO and Co/Mn co-doped ZnO nanocrystals were synthesized by co-precipitation method. Structural, morphological, and optical
devices (e.g. thin film solar cells) and ceramics.
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2. Experimental section:
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properties were studied. It paves the path for further promising application in spintronic, optoelectronic
2.1Chemicals used
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To synthesize the pure and Co/Mn co-doped ZnO nanocrystals, the Zinc Nitrate hexahydrate Zn(NO3)2·6H2O (LOBA, 98% purity), Cobalt (II) Nitrate hexahydrate Co(NO3)2·6H2O (LOBA, 99% purity), Manganese(II) Nitrate tetrahydrate Mn(NO3)2·4H2O (Sigma-Aldrich, 99.9% purity), Lithium hydroxide monohydrate LiOH·H2O (LOBA, 99.5% purity), Ethanol (Merck) and Acetone (LOBA, 99.5% purity) were
2.2 Synthesis process
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used. All the chemicals were used of analytical reagent grade without any prior treatment.
Pure ZnO and Co/Mn co-doped ZnO {Zn0.98-xCo0.02MnxO (0≤x≤0.06)} samples were prepared by co-
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precipitation route. In synthesis process, metal nitrate powders were mixed thoroughly in an
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appropriate proportion and dissolved in ethanol to obtain 0.1 M homogeneous precursor solution (Sol A). Another 0.1 M homogeneous solution (Sol B) of LiOH·H2O was prepared by dissolving it in ethanol. The Sol B was added drop-wise into Sol A in the proportion of 1:2 under vigorous stirring. The drop rate was set at 20-25 drops per minute. The obtained solution was allowed to relax for next two hours. Then, the solution was centrifuged at 6000 rpm at 20˚C. The precipitate was washed multiple times with deionised water, absolute ethanol and acetone. Afterward, the precipitate was dried at 70˚C for overnight in hot air oven. Obtained product was ground and further annealed at temperature 300 ˚C for
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3 hours followed by grinding to get Co/Mn co-doped ZnO nanocrystalline powder. The whole synthesis process was repeated without adding cobalt and Manganese precursors for the synthesis of pure ZnO
C2M4 (for Zn0.94Co0.02Mn0.04O), and C2M6 (for Zn0.92Co0.02Mn0.06O). 2.3 Characterization
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nanoparticles. Prepared samples were termed as C0M0 (for pure ZnO), C2M2 (for Zn0.96Co0.02Mn0.02O),
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XRD patterns were recorded using X-ray diffractometer (Miniflex-II, Riguku, Japan) with Cu Kα radiation source (λ = 1.54056 Å) operated at 40 kV and 40 mA. The angle (2θ) step size was set at 0.02˚ for the
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range of 20-80˚. The scan rate in XRD was kept at 0.02˚ per second. Scanning electron microscope (JEOL Japan JSM 6610 LV) was used to study the morphologies of samples. TEM images were obtained using transmission electron microscope (Technai G2 S-Twin FEI). FTIR spectrometer (Spectrum one, Perkin Elmer instrument, USA) was used for FTIR spectra in the range of 4000 – 400 cm-1. Optical properties of
(LS-45, Perkin Elmer, USA).
3.1 X-ray diffraction
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3. Result and Discussions
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samples were analyzed using UV-Vis spectrometer (Perkin Elmer, lamda 25, USA) and PL spectrometer
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The Rietveld refined XRD profiles of pure ZnO and co-doped ZnO nanocrystals are shown in figure 1. In the refinement models, hexagonal wurtzite structure (C46v, space group P63mc) was considered. Bragg peaks in XRD pattern were indexed using JCPDS file 36-1451. In XRD profiles, there are no extra peaks correspond to Co, Mn or Co/Mn cluster, any other impurity or secondary phase. It confirms single phase, hexagonal wurtzite structure of synthesized samples. The lattice parameters (a, c) were obtained from Rietveld refinement of XRD for pure and co-doped ZnO samples (Table 1). The interplanar spacing (dhkl value) was calculated using the relation[30,31]; 4|P age
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1
=
4 3
+
+
+
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Where h, k, and l are Miller’s indices. Volume of hexagonal close packed unit cell was estimated using following equation[30,31];
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= 0.866
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Figure 1 Rietveld refinement profiles of XRD data of the Pure and Co/Mn co-doped ZnO nanocrystals. The red circles represent the observed data while the solid black line through the circles shows the calculated profile, vertical green tics below curves indicate allowed Bragg-reflections for the wurtzite phase. The difference pattern of the observed data and calculated profile (blue line) is provided below Bragg reflections.
The values of different structural parameters such as c/a, internal parameter u, degree of distortion R, bond lengths (b, b1), bond angles (α, β) etc. were calculated following Morkoç and Özgür [32,33]. These calculated values have been provided in Table 1.
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The obtained data revealed a slight increasing pattern in lattice parameters a, c with doping of Co/Mn concentrations. The values of c/a, u and R remain almost constant with doping of Co/Mn ions. In tetrahedral arrangement, the ionic radii of Zn2+, Co2+ and Mn+2 are 0.60, 0.58, 0.66 Å,
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respectively[34,35]. The increase in lattice parameters with doping is attributed to the linear mismatch in ionic radius of dopant Co+2, Mn+2, and host Zn+2 atoms. The unit cell volume is also increased with doping concentration because of increased lattice parameters. Co/Mn ions have been successfully
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substituted at Zn ion sites within host lattice.
Table 1 Calculated values of lattice parameters, interplanar spacing, bond lengths, bond angles, density,
C2M2 3.2494 5.2052 1.6019 0.3799 1.0194 2.8142 2.6026 2.4755 1.9775 1.9775 108.4290 110.4931 5.961 47.4894
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C0M0 3.2496 5.2050 1.6018 0.3799 1.0195 2.8144 2.6025 2.4755 1.9775 1.9775 108.4249 110.4969 5.799 47.4856
C2M4 3.2491 5.2067 1.6025 0.3798 1.0190 2.8148 2.6034 2.4754 1.9775 1.9775 108.4509 110.4720 6.073 47.4859
C2M6 3.2497 5.2060 1.6020 0.3799 1.0194 2.8149 2.6030 2.4757 1.9777 1.9777 108.4328 110.4893 6.188 47.4920
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Parameters a (Å) c (Å) c/a U R d (100) (Å) d (002) (Å) d (101) (Å) “b” dZn-O (Å) “b1” α (Oa-Zn-Ob) (o) β (Ob-Zn-Ob) (o) density (gm-cm-3) V (Å3)
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and volume of unit cell for pure and Co/Mn co-doped ZnO nanocrystals
The d-values of (100), (002) and (101) planes are increased with doping of Co/Mn concentration which is due to change in bond length and bond angles. Due to the substitution of Zn ions by Co/Mn dopant ions, the average base-apex angles (Ob-Zn-Oa) increase and average basal bond angles (Ob-Zn-Ob) decrease, where Oa and Ob are oxygen atoms at the apex and base, respectively.
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The lattice strain produced is responsible for the change in interplanar spacing[30]. As d value of (002) plane increase, it is concluded that the uniform tensile strain has been produced in the perpendicular
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direction of the plane (002). The average crystallite size D of the nanocrystal was obtained from Debye-Scherrer’s formula[31,36];
cos
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=
Where K is Scherrer constant (K=0.9 for spherical symmetry), λ is the wavelength of XRD radiation used
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(λ=1.5406 Å), θ represents the Bragg angle and β is the full width at half maximum (FWHM). Average crystallite size D and lattice strain ε were also calculated from Williamson-Hall uniform deformation model (WH-UDM) plot by using equation[30];
= 4 sin +
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cos
In WH-UDM, (βcosθ) was plotted on the y-axis with respect to (4sinθ) on the x-axis. D and ε were calculated from intercept and slope of the linear fit of data, respectively (Table 2).
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The values of D and ε were also estimated from ‘size-strain plot’ using the following relation[37,38]; !
"#
$ =
"#
+% ' 2
Where, dhkl is interplanar spacing for (h, k, and l) plane. D and ε were calculated from slope and intercept of the linear fit of the plot between (dhkl β cosθ / λ)2 and (dhkl2 β cosθ / λ2), respectively (Table 2).
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Table 2 Calculated crystallite size and average lattice strain for pure ZnO and Zn0.98-xCo0.02MnxO samples
C0M0 C2M2 C2M4 C2M6
Scherrer Formula 26.82 27.78 28.04 25.98
Crystallite Size (nm) W-H Plot Size-Strain Plot 29.70 27.89 33.55 29.66 31.57 30.35 28.60 27.96
W-H Plot 4.060×10-4 6.202×10-4 4.333×10-4 3.821×10-4
Strain Size-Strain Plot 3.367×10-3 3.798×10-3 3.907×10-3 3.924×10-3
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Sample
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Average crystallite size initially increased for low Co/Mn doping concentration. Subsequently, crystallite size decreased for higher Co/Mn concentrations in ZnO nanocrystals. Such change in crystallite size is
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because of the mismatched ionic radius of Mn2+ (0.66 Å), Co2+ (0.58 Å) and Zn2+ (0.60 Å). The average lattice strain increased in Co/Mn co-doped ZnO samples than that of pure ZnO nanocrystals. 3.2 Morphological studies
The morphology of pure and Co/Mn co-doped ZnO nanocrystals were examined using SEM and TEM.
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Figure 2 represents the SEM images of synthesized samples (C0M0, C2M2, C2M4, and C2M6). SEM images showed the more or less spherical shape of prepared nanocrystals. In all the samples, the particle size was found less than 50 nm. Careful analysis of SEM images revealed almost uniform size
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distribution and grain boundaries with small agglomeration.
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TEM was used to further study the morphology of C2M2 ZnO sample. Figure 3 shows the typical TEM images and selected area electron diffraction (SAED) pattern of C2M2 nanoparticles. It was observed that most of the nanoparticles were spherical in shape with smooth surfaces. TEM images showed the clear and well-defined grain boundaries for synthesized nanoparticles. According to Straumal et al.[39], these grain boundaries affect the physical characteristics of the sample. The particle size of C2M2 sample estimated from histogram using TEM images corroborates with the size calculated from XRD analysis (Figure 3 c).
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C2M2
C2M4
C2M6
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C0M0
Figure 2 SEM images of pure and Co/Mn co-doped ZnO nanocrystals. SAED pattern of the C2M2 sample was indexed (Figure 3 d) which revealed the polycrystalline nature of
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prepared sample. The d-values obtained from SAED pattern further confirmed the wurtzite structure of
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the sample. Detailed analysis of XRD data and TEM images suggested the incorporation of Co/Mn ions into the host lattice of ZnO.
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(b)
(d)
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(a)
Figure 3 (a), (b) TEM images of the C2M2 sample (Zn0.96Co0.02Mn0.02O), (c) histogram for calculating
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average particle size for the C2M2, and (d) SAED pattern of the C2M2 sample
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3.3 FTIR spectroscopy
FTIR was used to study the vibrational mode of the C0M0, C2M2, C2M4 and C2M6 samples. Figure S1 shows FTIR spectra of samples. The band frequencies below 800 cm-1 are generally because of the bond between inorganic elements. All observed vibrational band frequencies are provided in Table 3. The prominent bands are observed at around 424 - 428 cm-1 due to the stretching vibration of Zn-O bond in tetrahedral coordination. The bands at around 671 - 672 cm-1 are very weak bands originated because of the stretching vibrations of Zn-O bonds in octahedral arrangements. The Zn-O bonds in tetrahedral co10 | P a g e
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ordinations are much stronger than the octahedral coordinations in the prepared samples. Therefore, analysis of FTIR spectra further confirms the wurtzite structure of samples[40,41]. The weak bands at around 671 cm-1 suggest that Co/Mn ion co-doping do not affect band related to octahedral
octahedral coordination in ZnO lattice structure.
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coordinations. It is inferred that Co/Mn ions are substituted only at tetrahedral arrangement instead of
Table 3 FTIR data of different vibrational modes of the pure and Co/Mn co-doped ZnO samples C2M6 426 671 872 1068 1385 1617 2346 2855 2926 3450
Modes (cm-1) Zn–O bond (tetrahedral) Zn–O bond (octahedral) ethanol precursor ethanol precursor Asymmetric stretching of C=O Symmetric stretching of C=O CO2 molecules in air C–H bond bending C–H bond stretching O–H bond
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C2M4 426 671 876 1060 1385 1615 2346 2855 2925 3451
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C2M2 424 671 880 1063 1385 1615 2346 2855 2925 3451
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C0M0 428 672 876 1058 1385 1616 2346 2854 2925 3452
Band frequencies at around 1385 cm-1 are ascribed to asymmetric stretching vibrations of C=O group due to Lewis acidity. Vibrational bands at around 1615-1617 cm-1 are assigned to the symmetric
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stretching vibrations of C=O group due to Bronsted acidity. Band frequencies at around 2346 cm-1 are originated due to CO2 molecules present in alcohol and in the
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air. The band around 2854 - 2855 cm-1 and 2925 - 2926 cm-1 are reflected due to C-H bond bending and C-H bond stretching, respectively. These bands show the presence of absorbed groups on the surface of nanocrystals. Broad absorption peaks at around 3450 - 3452 cm-1 are attributed to hydroxyl (–OH) group due to H2O. It represents the existence of water molecules adsorbed on the surface of nanocrystals. The Co/Mn co-doped ZnO nanocrystals can be easily dispersed into nonpolar and polar solvents because of surface hydroxyl groups. These surface hydroxyl groups assist in providing functional groups which can
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react with functional organic molecules e.g. dye or quantum dots for fabrication of effective junction devices (e.g. dye-sensitized solar cells, quantum dot sensitized solar cells etc)[41,42].
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3.4 UV-Visible spectroscopy Room temperature UV-Visible spectra were recorded by homogeneously dispersing the nanocrystals in DI water and used DI water as the reference. The absorption spectra of samples and corresponding
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peaks are observed at around 376 – 379 nm.
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estimated optical energy band gaps are shown in figure 5 (a) and 5 (b), respectively. The absorption
(a)
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(b)
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Figure 5 (a) Absorption spectra (UV-Visible), and (b) plot of estimated optical energy band gap of the pure and Co/Mn co-doped ZnO sample.
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The estimated band gap for pristine ZnO is 3.295 eV which is slightly less than the band gap of bulk ZnO (3.31 eV). Such insignificant change in the optical band gap of pure ZnO is because of the oxygen vacancies or/and defects at the surface of ZnO nanocrystals. In FTIR analysis of pure ZnO, the –OH surface groups are found which are effective n-type defects in ZnO. The absorption edge is red shifted for lower Co/Mn concentration (for C2M2) whereas absorption edge is blue shifted with higher Co/Mn co-doping in ZnO lattice (for C2M4 and C2M6). The initial decrease in
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optical band gap is interpreted as stronger sp-d exchange interaction between the band electrons of ZnO (in valence and conduction band) and the localized d electrons of the Co/Mn ions substituting for Zn2+ ions[43]. The strong interaction (s-d and p-d) result into a positive and negative correction to the
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valence and conduction band edges, respectively[44]. This is a possible cause of shrinkage of the band gap in the co-doped sample.
Further increase in optical band gap for Co/Mn co-doped ZnO samples is interpreted as 4s-3d and 2p-3d
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exchange interactions which are responsible for the decrease of Zn 3d electron density and increase of Mn 3d electron density below the valence band maxima[45]. Broadening of optical band gap with
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Co/Mn co-doping in ZnO sample can also be due to Burstein-Moss band filling effect[38,46,47]. Because of Co/Mn doping in ZnO sample, Fermi level shifts inside the conduction band. As the states below such shifting in the conduction band are filled, the absorption edge shifts to higher energy, resulting in larger optical band gap with Co/Mn co-doping in ZnO sample[47]. In our study, the crystallite sizes are much
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larger than Bohr exciton radius of ZnO. Therefore, we believe that the quantum confinement effect cannot be responsible for the change in the band gap of samples. The structural changes play a
concentrations.
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3.5 PL spectroscopy
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significant role for change in the optical energy band gap for ZnO nanocrystals due to the Co/Mn ions
The de-convolution of room temperature PL spectra is shown in figure 6. The room temperature PL spectra were measured by exciting the nanocrystalline samples at 320 nm. PL emission peaks are broad possibly due to the presence of defects and several recombination sites. The asymmetric nature of PL spectra is imputed to the presence of other inherent emission peaks due to distributed defect states in the interior and on the surface of nanocrystals.
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In PL spectra, the defects related emissions dominate over the near band edge (NBE) emission of ZnO. Therefore, the NBE emission of ZnO is weakly resolved. The PL spectra have five peaks originating
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approximately around 386, 412, 436, 474 and 515 nm. The first emission peak corresponds to the ultraviolet (UV) region. Other four peaks are related to violet, violet-blue, blue-green and green in the visible region. The intensity of emission peaks is varied with Co/Mn ions doping concentration. A slight shift in emission peaks towards higher wavelength is
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observed with doping of Co/Mn ions in ZnO. The peak in UV region is assigned to the near band edge
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exciton-exciton collision process[48,49].
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emission. UV peak is possible reflected due to radiative recombination of free excitons through an
Figure 6 De-convoluted PL spectra of the pure and Co/Mn co-doped ZnO sample. The violet emission around approximately 412 nm corresponds to Zn vacancies. The violet-blue emission around 436 nm is related to the interstitial Zn vacancies (Zni). The violet-blue emission is possibly due to radiative defects related to trapping states existing at grain boundaries. The presence of emission peak corresponds to the blue-green band (~474 nm) is attributed to the surface defects[50]. The green band emission (~515 nm) is reflected due to recombination of electrons with holes trapped in singly ionized 14 | P a g e
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oxygen vacancies[51]. The green emission intensities in PL spectra decrease with increasing Co/Mn ions concentrations which signify the fewer oxygen vacancies. Since these defects serve as recombination centers in dye or quantum dot sensitized solar cells, Co/Mn co-doped ZnO nanocrystals are potential
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wide band gap semiconducting materials for fabrication of efficient solar cells[42].
4. Conclusions
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Co-precipitation method was used to synthesize pure and Co/Mn co-doped ZnO nanocrystals. Structural, morphological and optical properties were studied using XRD, SEM, TEM, UV-Vis and PL spectroscopic
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measurements. Rietveld refinement of XRD profiles and FTIR confirmed the hexagonal wurtzite structure of prepared samples. Co/Mn ions are successfully substituted the Zn sites in ZnO lattice structure. Lattice parameters, bond angles, bond length, crystallite size etc were calculated through XRD analysis. FTIR confirms the tetrahedral coordination of the oxygen ions surrounding the Zn ions and vice versa. Prepared nanocrystals are more or less spherical in nature which is confirmed by SEM and TEM
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techniques. The particle size estimated from histogram using TEM images is corroborated well with the size calculated from XRD analysis. Absorption spectra showed an initial decrease in optical band gap energy for C2M2 sample and further increase for a higher concentration of Co/Mn ions in ZnO (for C2M4
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and C2M6). PL spectra show the UV emission (NBE), violet, violet-blue, blue-green and green emissions
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in the visible region. Co/Mn co-doped ZnO nanocrystalline samples with tuned optical properties are promising candidates for applications in spintronic, optoelectronic devices (e.g. dye/quantum dot sensitized solar cells) and ceramics.
Acknowledgement
The authors gratefully acknowledge the Research Lab for Energy Systems, N.S.I.T., New Delhi for providing the necessary support and infrastructure for experimental work. The authors also
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acknowledge Prof. K. Sreenivas and U.S.I.C., University of Delhi, New Delhi for providing the characterization facilities.
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ACCEPTED MANUSCRIPT Highlights Pure and Co-Mn doped ZnO nanocrystals were synthesized by co-precipitation route.
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Structural, morphological and optical properties of samples have been studied.
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Oxygen related defect states are reduced with increasing doping concentrations.
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Co-Mn doped ZnO samples are promising material for application in solar cells.
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S0925-8388(16)34148-2
DOI:
10.1016/j.jallcom.2016.12.227
Reference:
JALCOM 40149
To appear in:
Journal of Alloys and Compounds
Received Date: 8 November 2016 Revised Date:
29 November 2016
Accepted Date: 17 December 2016
Please cite this article as: D. Sharma, R. Jha, Transition metal (Co, Mn) co-doped ZnO nanoparticles: Effect on structural and optical properties, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2016.12.227. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Graphical Abstract
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Transition metal (Co, Mn) co-doped ZnO Nanoparticles: Effect on structural and optical properties
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Darshan Sharma *a, and Ranjana Jha a Department of Physics, Netaji Subhas Institute of Technology, University of Delhi, Sector-3, Dwarka, New Delhi-110078, India.
*Corresponding author, Email: [email protected]
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Contact No.: +91-9990135625
Abstract
Pure ZnO and Co/Mn co-doped ZnO {Zn0.98-xCo0.02MnxO (0≤x≤0.06)} nanoparticles were synthesized by co-precipitation method. The structural, morphological and optical properties of prepared samples were explored in detail. Rietveld refinement of x-ray diffraction (XRD) data revealed the single phase,
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hexagonal wurtzite structure without any impurity phase. XRD and Fourier transform infra-red (FTIR) analysis confirmed the incorporation of Co/Mn ions at Zn site into host lattice structure. The morphology of samples was examined using scanning electron microscopy (SEM) and transmission
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electron microscopy (TEM) analysis. The particle size from TEM results was corroborated well with XRD
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data. The absorption spectra showed the initial decrease in optical energy band gap for low Mn concentration. The optical energy band gap further increased with a higher Mn concentration in codoped ZnO samples. Photoluminescence (PL) spectra showed five emission peaks due to different defect states. The paper enhances the understanding of structural, optical properties of Co/Mn co-doped nanocrystals. This paves the path for its potential application in the optoelectronic devices e.g. solar cell.
Keywords ZnO; Doping; Co-precipitation; XRD; Optical properties
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1.
Introduction
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Spintronics is the most enticing field which has potential future prospective in spin-based devices, data storage, and optoelectronic devices[1]. In spintronics, both spin state and charge can be utilized. Diluted magnetic semiconductors (DMSs) have been intensely studied to achieve spin-polarized charge carriers
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for spintronic device operations[2]. Physical properties of metal oxide DMSs depend on the size and surface functionality of material. In addition, structural and optical properties of metal oxides are
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modified by doping transition metal (TM) ion concentrations.
Zinc oxide (ZnO) is a most popular host material for TM ions doping. It is an optically transparent II-VI semiconductor having wide direct band gap, large exciton binding energy and wurtzite lattice structure. Theoretical studies have predicted ferromagnetism above room temperature for TM element doped ZnO for spintronic applications[1,3]. Different research groups investigated the role of TM ions in ZnO
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nanocrystals[4–14]. Despite numerous reports, development of DMSs with desired optical and magnetic properties remains a controversial issue. The presence of two TM ions in ZnO is considered as a promising alternative technique to achieve stable optical and magnetic properties[15]. Therefore,
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researchers have attempted simultaneous doping of two TM ions in ZnO[16–24]. Co-dopant ions are
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selected depending on the ionic radius, valence states, coordination number and individual magnetic contribution. In our study, we investigated the role of cobalt (Co) and manganese (Mn) ions in ZnO. Magnetic properties of Co/Mn co-doped ZnO nanocrystals have been explored by different research groups[20,25–29]. But, development of Co/Mn co-doped ZnO, its magnetic behavior, and origin of room temperature ferromagnetism are still controversial. In addition, optical properties in Co/Mn co-doped ZnO nanocrystals are found contradictory in scantily reported literature. Less explored optical properties of Co/Mn co-doped ZnO nanocrystals limit its potential application in optoelectronic devices.
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An attempt has been made to well explore the optical properties and its correlation with structural properties through this experimental investigation. In this study, pure ZnO and Co/Mn co-doped ZnO nanocrystals were synthesized by co-precipitation method. Structural, morphological, and optical
devices (e.g. thin film solar cells) and ceramics.
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2. Experimental section:
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properties were studied. It paves the path for further promising application in spintronic, optoelectronic
2.1Chemicals used
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To synthesize the pure and Co/Mn co-doped ZnO nanocrystals, the Zinc Nitrate hexahydrate Zn(NO3)2·6H2O (LOBA, 98% purity), Cobalt (II) Nitrate hexahydrate Co(NO3)2·6H2O (LOBA, 99% purity), Manganese(II) Nitrate tetrahydrate Mn(NO3)2·4H2O (Sigma-Aldrich, 99.9% purity), Lithium hydroxide monohydrate LiOH·H2O (LOBA, 99.5% purity), Ethanol (Merck) and Acetone (LOBA, 99.5% purity) were
2.2 Synthesis process
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used. All the chemicals were used of analytical reagent grade without any prior treatment.
Pure ZnO and Co/Mn co-doped ZnO {Zn0.98-xCo0.02MnxO (0≤x≤0.06)} samples were prepared by co-
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precipitation route. In synthesis process, metal nitrate powders were mixed thoroughly in an
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appropriate proportion and dissolved in ethanol to obtain 0.1 M homogeneous precursor solution (Sol A). Another 0.1 M homogeneous solution (Sol B) of LiOH·H2O was prepared by dissolving it in ethanol. The Sol B was added drop-wise into Sol A in the proportion of 1:2 under vigorous stirring. The drop rate was set at 20-25 drops per minute. The obtained solution was allowed to relax for next two hours. Then, the solution was centrifuged at 6000 rpm at 20˚C. The precipitate was washed multiple times with deionised water, absolute ethanol and acetone. Afterward, the precipitate was dried at 70˚C for overnight in hot air oven. Obtained product was ground and further annealed at temperature 300 ˚C for
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3 hours followed by grinding to get Co/Mn co-doped ZnO nanocrystalline powder. The whole synthesis process was repeated without adding cobalt and Manganese precursors for the synthesis of pure ZnO
C2M4 (for Zn0.94Co0.02Mn0.04O), and C2M6 (for Zn0.92Co0.02Mn0.06O). 2.3 Characterization
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nanoparticles. Prepared samples were termed as C0M0 (for pure ZnO), C2M2 (for Zn0.96Co0.02Mn0.02O),
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XRD patterns were recorded using X-ray diffractometer (Miniflex-II, Riguku, Japan) with Cu Kα radiation source (λ = 1.54056 Å) operated at 40 kV and 40 mA. The angle (2θ) step size was set at 0.02˚ for the
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range of 20-80˚. The scan rate in XRD was kept at 0.02˚ per second. Scanning electron microscope (JEOL Japan JSM 6610 LV) was used to study the morphologies of samples. TEM images were obtained using transmission electron microscope (Technai G2 S-Twin FEI). FTIR spectrometer (Spectrum one, Perkin Elmer instrument, USA) was used for FTIR spectra in the range of 4000 – 400 cm-1. Optical properties of
(LS-45, Perkin Elmer, USA).
3.1 X-ray diffraction
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3. Result and Discussions
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samples were analyzed using UV-Vis spectrometer (Perkin Elmer, lamda 25, USA) and PL spectrometer
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The Rietveld refined XRD profiles of pure ZnO and co-doped ZnO nanocrystals are shown in figure 1. In the refinement models, hexagonal wurtzite structure (C46v, space group P63mc) was considered. Bragg peaks in XRD pattern were indexed using JCPDS file 36-1451. In XRD profiles, there are no extra peaks correspond to Co, Mn or Co/Mn cluster, any other impurity or secondary phase. It confirms single phase, hexagonal wurtzite structure of synthesized samples. The lattice parameters (a, c) were obtained from Rietveld refinement of XRD for pure and co-doped ZnO samples (Table 1). The interplanar spacing (dhkl value) was calculated using the relation[30,31]; 4|P age
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=
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+
+
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Where h, k, and l are Miller’s indices. Volume of hexagonal close packed unit cell was estimated using following equation[30,31];
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= 0.866
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Figure 1 Rietveld refinement profiles of XRD data of the Pure and Co/Mn co-doped ZnO nanocrystals. The red circles represent the observed data while the solid black line through the circles shows the calculated profile, vertical green tics below curves indicate allowed Bragg-reflections for the wurtzite phase. The difference pattern of the observed data and calculated profile (blue line) is provided below Bragg reflections.
The values of different structural parameters such as c/a, internal parameter u, degree of distortion R, bond lengths (b, b1), bond angles (α, β) etc. were calculated following Morkoç and Özgür [32,33]. These calculated values have been provided in Table 1.
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The obtained data revealed a slight increasing pattern in lattice parameters a, c with doping of Co/Mn concentrations. The values of c/a, u and R remain almost constant with doping of Co/Mn ions. In tetrahedral arrangement, the ionic radii of Zn2+, Co2+ and Mn+2 are 0.60, 0.58, 0.66 Å,
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respectively[34,35]. The increase in lattice parameters with doping is attributed to the linear mismatch in ionic radius of dopant Co+2, Mn+2, and host Zn+2 atoms. The unit cell volume is also increased with doping concentration because of increased lattice parameters. Co/Mn ions have been successfully
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substituted at Zn ion sites within host lattice.
Table 1 Calculated values of lattice parameters, interplanar spacing, bond lengths, bond angles, density,
C2M2 3.2494 5.2052 1.6019 0.3799 1.0194 2.8142 2.6026 2.4755 1.9775 1.9775 108.4290 110.4931 5.961 47.4894
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C0M0 3.2496 5.2050 1.6018 0.3799 1.0195 2.8144 2.6025 2.4755 1.9775 1.9775 108.4249 110.4969 5.799 47.4856
C2M4 3.2491 5.2067 1.6025 0.3798 1.0190 2.8148 2.6034 2.4754 1.9775 1.9775 108.4509 110.4720 6.073 47.4859
C2M6 3.2497 5.2060 1.6020 0.3799 1.0194 2.8149 2.6030 2.4757 1.9777 1.9777 108.4328 110.4893 6.188 47.4920
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Parameters a (Å) c (Å) c/a U R d (100) (Å) d (002) (Å) d (101) (Å) “b” dZn-O (Å) “b1” α (Oa-Zn-Ob) (o) β (Ob-Zn-Ob) (o) density (gm-cm-3) V (Å3)
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and volume of unit cell for pure and Co/Mn co-doped ZnO nanocrystals
The d-values of (100), (002) and (101) planes are increased with doping of Co/Mn concentration which is due to change in bond length and bond angles. Due to the substitution of Zn ions by Co/Mn dopant ions, the average base-apex angles (Ob-Zn-Oa) increase and average basal bond angles (Ob-Zn-Ob) decrease, where Oa and Ob are oxygen atoms at the apex and base, respectively.
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The lattice strain produced is responsible for the change in interplanar spacing[30]. As d value of (002) plane increase, it is concluded that the uniform tensile strain has been produced in the perpendicular
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direction of the plane (002). The average crystallite size D of the nanocrystal was obtained from Debye-Scherrer’s formula[31,36];
cos
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Where K is Scherrer constant (K=0.9 for spherical symmetry), λ is the wavelength of XRD radiation used
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(λ=1.5406 Å), θ represents the Bragg angle and β is the full width at half maximum (FWHM). Average crystallite size D and lattice strain ε were also calculated from Williamson-Hall uniform deformation model (WH-UDM) plot by using equation[30];
= 4 sin +
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cos
In WH-UDM, (βcosθ) was plotted on the y-axis with respect to (4sinθ) on the x-axis. D and ε were calculated from intercept and slope of the linear fit of data, respectively (Table 2).
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The values of D and ε were also estimated from ‘size-strain plot’ using the following relation[37,38]; !
"#
$ =
"#
+% ' 2
Where, dhkl is interplanar spacing for (h, k, and l) plane. D and ε were calculated from slope and intercept of the linear fit of the plot between (dhkl β cosθ / λ)2 and (dhkl2 β cosθ / λ2), respectively (Table 2).
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Table 2 Calculated crystallite size and average lattice strain for pure ZnO and Zn0.98-xCo0.02MnxO samples
C0M0 C2M2 C2M4 C2M6
Scherrer Formula 26.82 27.78 28.04 25.98
Crystallite Size (nm) W-H Plot Size-Strain Plot 29.70 27.89 33.55 29.66 31.57 30.35 28.60 27.96
W-H Plot 4.060×10-4 6.202×10-4 4.333×10-4 3.821×10-4
Strain Size-Strain Plot 3.367×10-3 3.798×10-3 3.907×10-3 3.924×10-3
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Sample
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Average crystallite size initially increased for low Co/Mn doping concentration. Subsequently, crystallite size decreased for higher Co/Mn concentrations in ZnO nanocrystals. Such change in crystallite size is
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because of the mismatched ionic radius of Mn2+ (0.66 Å), Co2+ (0.58 Å) and Zn2+ (0.60 Å). The average lattice strain increased in Co/Mn co-doped ZnO samples than that of pure ZnO nanocrystals. 3.2 Morphological studies
The morphology of pure and Co/Mn co-doped ZnO nanocrystals were examined using SEM and TEM.
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Figure 2 represents the SEM images of synthesized samples (C0M0, C2M2, C2M4, and C2M6). SEM images showed the more or less spherical shape of prepared nanocrystals. In all the samples, the particle size was found less than 50 nm. Careful analysis of SEM images revealed almost uniform size
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distribution and grain boundaries with small agglomeration.
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TEM was used to further study the morphology of C2M2 ZnO sample. Figure 3 shows the typical TEM images and selected area electron diffraction (SAED) pattern of C2M2 nanoparticles. It was observed that most of the nanoparticles were spherical in shape with smooth surfaces. TEM images showed the clear and well-defined grain boundaries for synthesized nanoparticles. According to Straumal et al.[39], these grain boundaries affect the physical characteristics of the sample. The particle size of C2M2 sample estimated from histogram using TEM images corroborates with the size calculated from XRD analysis (Figure 3 c).
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C2M2
C2M4
C2M6
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C0M0
Figure 2 SEM images of pure and Co/Mn co-doped ZnO nanocrystals. SAED pattern of the C2M2 sample was indexed (Figure 3 d) which revealed the polycrystalline nature of
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prepared sample. The d-values obtained from SAED pattern further confirmed the wurtzite structure of
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the sample. Detailed analysis of XRD data and TEM images suggested the incorporation of Co/Mn ions into the host lattice of ZnO.
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(b)
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(a)
Figure 3 (a), (b) TEM images of the C2M2 sample (Zn0.96Co0.02Mn0.02O), (c) histogram for calculating
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average particle size for the C2M2, and (d) SAED pattern of the C2M2 sample
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3.3 FTIR spectroscopy
FTIR was used to study the vibrational mode of the C0M0, C2M2, C2M4 and C2M6 samples. Figure S1 shows FTIR spectra of samples. The band frequencies below 800 cm-1 are generally because of the bond between inorganic elements. All observed vibrational band frequencies are provided in Table 3. The prominent bands are observed at around 424 - 428 cm-1 due to the stretching vibration of Zn-O bond in tetrahedral coordination. The bands at around 671 - 672 cm-1 are very weak bands originated because of the stretching vibrations of Zn-O bonds in octahedral arrangements. The Zn-O bonds in tetrahedral co10 | P a g e
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ordinations are much stronger than the octahedral coordinations in the prepared samples. Therefore, analysis of FTIR spectra further confirms the wurtzite structure of samples[40,41]. The weak bands at around 671 cm-1 suggest that Co/Mn ion co-doping do not affect band related to octahedral
octahedral coordination in ZnO lattice structure.
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coordinations. It is inferred that Co/Mn ions are substituted only at tetrahedral arrangement instead of
Table 3 FTIR data of different vibrational modes of the pure and Co/Mn co-doped ZnO samples C2M6 426 671 872 1068 1385 1617 2346 2855 2926 3450
Modes (cm-1) Zn–O bond (tetrahedral) Zn–O bond (octahedral) ethanol precursor ethanol precursor Asymmetric stretching of C=O Symmetric stretching of C=O CO2 molecules in air C–H bond bending C–H bond stretching O–H bond
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C2M4 426 671 876 1060 1385 1615 2346 2855 2925 3451
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C2M2 424 671 880 1063 1385 1615 2346 2855 2925 3451
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C0M0 428 672 876 1058 1385 1616 2346 2854 2925 3452
Band frequencies at around 1385 cm-1 are ascribed to asymmetric stretching vibrations of C=O group due to Lewis acidity. Vibrational bands at around 1615-1617 cm-1 are assigned to the symmetric
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stretching vibrations of C=O group due to Bronsted acidity. Band frequencies at around 2346 cm-1 are originated due to CO2 molecules present in alcohol and in the
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air. The band around 2854 - 2855 cm-1 and 2925 - 2926 cm-1 are reflected due to C-H bond bending and C-H bond stretching, respectively. These bands show the presence of absorbed groups on the surface of nanocrystals. Broad absorption peaks at around 3450 - 3452 cm-1 are attributed to hydroxyl (–OH) group due to H2O. It represents the existence of water molecules adsorbed on the surface of nanocrystals. The Co/Mn co-doped ZnO nanocrystals can be easily dispersed into nonpolar and polar solvents because of surface hydroxyl groups. These surface hydroxyl groups assist in providing functional groups which can
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react with functional organic molecules e.g. dye or quantum dots for fabrication of effective junction devices (e.g. dye-sensitized solar cells, quantum dot sensitized solar cells etc)[41,42].
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3.4 UV-Visible spectroscopy Room temperature UV-Visible spectra were recorded by homogeneously dispersing the nanocrystals in DI water and used DI water as the reference. The absorption spectra of samples and corresponding
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peaks are observed at around 376 – 379 nm.
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estimated optical energy band gaps are shown in figure 5 (a) and 5 (b), respectively. The absorption
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Figure 5 (a) Absorption spectra (UV-Visible), and (b) plot of estimated optical energy band gap of the pure and Co/Mn co-doped ZnO sample.
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The estimated band gap for pristine ZnO is 3.295 eV which is slightly less than the band gap of bulk ZnO (3.31 eV). Such insignificant change in the optical band gap of pure ZnO is because of the oxygen vacancies or/and defects at the surface of ZnO nanocrystals. In FTIR analysis of pure ZnO, the –OH surface groups are found which are effective n-type defects in ZnO. The absorption edge is red shifted for lower Co/Mn concentration (for C2M2) whereas absorption edge is blue shifted with higher Co/Mn co-doping in ZnO lattice (for C2M4 and C2M6). The initial decrease in
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optical band gap is interpreted as stronger sp-d exchange interaction between the band electrons of ZnO (in valence and conduction band) and the localized d electrons of the Co/Mn ions substituting for Zn2+ ions[43]. The strong interaction (s-d and p-d) result into a positive and negative correction to the
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valence and conduction band edges, respectively[44]. This is a possible cause of shrinkage of the band gap in the co-doped sample.
Further increase in optical band gap for Co/Mn co-doped ZnO samples is interpreted as 4s-3d and 2p-3d
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exchange interactions which are responsible for the decrease of Zn 3d electron density and increase of Mn 3d electron density below the valence band maxima[45]. Broadening of optical band gap with
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Co/Mn co-doping in ZnO sample can also be due to Burstein-Moss band filling effect[38,46,47]. Because of Co/Mn doping in ZnO sample, Fermi level shifts inside the conduction band. As the states below such shifting in the conduction band are filled, the absorption edge shifts to higher energy, resulting in larger optical band gap with Co/Mn co-doping in ZnO sample[47]. In our study, the crystallite sizes are much
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larger than Bohr exciton radius of ZnO. Therefore, we believe that the quantum confinement effect cannot be responsible for the change in the band gap of samples. The structural changes play a
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3.5 PL spectroscopy
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significant role for change in the optical energy band gap for ZnO nanocrystals due to the Co/Mn ions
The de-convolution of room temperature PL spectra is shown in figure 6. The room temperature PL spectra were measured by exciting the nanocrystalline samples at 320 nm. PL emission peaks are broad possibly due to the presence of defects and several recombination sites. The asymmetric nature of PL spectra is imputed to the presence of other inherent emission peaks due to distributed defect states in the interior and on the surface of nanocrystals.
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In PL spectra, the defects related emissions dominate over the near band edge (NBE) emission of ZnO. Therefore, the NBE emission of ZnO is weakly resolved. The PL spectra have five peaks originating
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approximately around 386, 412, 436, 474 and 515 nm. The first emission peak corresponds to the ultraviolet (UV) region. Other four peaks are related to violet, violet-blue, blue-green and green in the visible region. The intensity of emission peaks is varied with Co/Mn ions doping concentration. A slight shift in emission peaks towards higher wavelength is
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observed with doping of Co/Mn ions in ZnO. The peak in UV region is assigned to the near band edge
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exciton-exciton collision process[48,49].
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emission. UV peak is possible reflected due to radiative recombination of free excitons through an
Figure 6 De-convoluted PL spectra of the pure and Co/Mn co-doped ZnO sample. The violet emission around approximately 412 nm corresponds to Zn vacancies. The violet-blue emission around 436 nm is related to the interstitial Zn vacancies (Zni). The violet-blue emission is possibly due to radiative defects related to trapping states existing at grain boundaries. The presence of emission peak corresponds to the blue-green band (~474 nm) is attributed to the surface defects[50]. The green band emission (~515 nm) is reflected due to recombination of electrons with holes trapped in singly ionized 14 | P a g e
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oxygen vacancies[51]. The green emission intensities in PL spectra decrease with increasing Co/Mn ions concentrations which signify the fewer oxygen vacancies. Since these defects serve as recombination centers in dye or quantum dot sensitized solar cells, Co/Mn co-doped ZnO nanocrystals are potential
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wide band gap semiconducting materials for fabrication of efficient solar cells[42].
4. Conclusions
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Co-precipitation method was used to synthesize pure and Co/Mn co-doped ZnO nanocrystals. Structural, morphological and optical properties were studied using XRD, SEM, TEM, UV-Vis and PL spectroscopic
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measurements. Rietveld refinement of XRD profiles and FTIR confirmed the hexagonal wurtzite structure of prepared samples. Co/Mn ions are successfully substituted the Zn sites in ZnO lattice structure. Lattice parameters, bond angles, bond length, crystallite size etc were calculated through XRD analysis. FTIR confirms the tetrahedral coordination of the oxygen ions surrounding the Zn ions and vice versa. Prepared nanocrystals are more or less spherical in nature which is confirmed by SEM and TEM
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techniques. The particle size estimated from histogram using TEM images is corroborated well with the size calculated from XRD analysis. Absorption spectra showed an initial decrease in optical band gap energy for C2M2 sample and further increase for a higher concentration of Co/Mn ions in ZnO (for C2M4
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and C2M6). PL spectra show the UV emission (NBE), violet, violet-blue, blue-green and green emissions
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in the visible region. Co/Mn co-doped ZnO nanocrystalline samples with tuned optical properties are promising candidates for applications in spintronic, optoelectronic devices (e.g. dye/quantum dot sensitized solar cells) and ceramics.
Acknowledgement
The authors gratefully acknowledge the Research Lab for Energy Systems, N.S.I.T., New Delhi for providing the necessary support and infrastructure for experimental work. The authors also
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acknowledge Prof. K. Sreenivas and U.S.I.C., University of Delhi, New Delhi for providing the characterization facilities.
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ACCEPTED MANUSCRIPT Highlights Pure and Co-Mn doped ZnO nanocrystals were synthesized by co-precipitation route.
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Structural, morphological and optical properties of samples have been studied.
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Oxygen related defect states are reduced with increasing doping concentrations.
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Co-Mn doped ZnO samples are promising material for application in solar cells.
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