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Synthesis and characterization of VO2+ doped ZnO–CdS composite nanopowder
Accepted Manuscript Synthesis and characterization of VO2+ doped ZnO-CdS composite nanopowder G. Thirumala Rao, B. Babu, R. Joyce Stella, V. Pushpa Manjari, Ch. Venkata Reddy, Jaesool Shim, R.V.S.S.N. Ravikumar PII: DOI: Reference:
S0022-2860(14)01057-6 http://dx.doi.org/10.1016/j.molstruc.2014.10.044 MOLSTR 21040
To appear in:
Journal of Molecular Structure
Received Date: Revised Date: Accepted Date:
24 September 2014 18 October 2014 20 October 2014
Please cite this article as: G. Thirumala Rao, B. Babu, R. Joyce Stella, V. Pushpa Manjari, Ch. Venkata Reddy, J. Shim, R.V.S.S.N. Ravikumar, Synthesis and characterization of VO2+ doped ZnO-CdS composite nanopowder, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.10.044
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Synthesis and characterization of VO2+ doped ZnO-CdS composite nanopowder G. Thirumala Raoa, B. Babua, R. Joyce Stellaa, V. Pushpa Manjaria, Ch. Venkata Reddyb, Jaesool Shimb, R.V.S.S.N. Ravikumara*. a
Department of Physics, University College of Sciences, Acharya Nagarjuna University, Nagarjuna Nagar-522510, India.
b
School of Mechanical Engineering, Yeungnam University, Gyeongsan-712-749, Republic of Korea. Email: [email protected]
Corresponding Author’s Address: Dr. R.V.S.S.N. Ravikumar Assistant Professor Department of Physics Acharya Nagarjuna University Nagarjuna Nagar-522510 A.P., India. Phone No: +91-863-2346381 (Lab) +91-863-2263458 (Resi.) Mobile No: +91-9490114276 Fax: +91-863-2293378 Email: [email protected]
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Abstract VO2+ doped ZnO-CdS composite nanopowder has been synthesized by chemical precipitation method. The prepared sample has been characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), FT-IR, photoluminescence (PL), optical absorption and EPR spectroscopy. From XRD pattern, average crystallite size is about 18 nm. SEM and TEM images showed sphere like structures. FT-IR spectrum indicates the presence of fundamental modes of ZnO, CdS and other functional groups. The PL spectrum of VO2+ doped ZnO-CdS composite nanopowder exhibits UV, blue and green emissions. Optical and EPR studies revealed the tetragonal compressed octahedral site symmetry for VO2+ ions. The bonding between VO2+ and its ligands is ionic. Keywords: Photocatalyst, Chemical precipitation method, Surface defects, Vanadyl ions, Electron paramagnetic resonance, CIE diagram.
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1. Introduction Various fabrications of semiconducting materials like cadmium sulfide (CdS) and zinc oxide (ZnO) have attracted much attention to researchers over the past decades because of their synthesis procedure and functional behavior having unusual optical, electric properties and potential applications in nanodevices [1]. ZnO based nanostructures received significant attention in the recent development of science and technology because of their potential applications in optics, catalysts and solar cells [2-4]. These semiconducting nanostructures revealed appreciable structural, optical, electronic, luminescence and photocatalytic properties. ZnO is a direct band gap semiconductor, which has great application potential for photocatalysts and photovoltaic cells, due to its wide band gap of 3.37 eV, low cost, superior carrier mobility and large excitation binding energy of 60 meV at room temperature. As well as cadmium sulfate (CdS) has received considerable attention due to a large number of technical applications like photocatalysis, sensors and solar cells. Due to its narrow band gap of 2.4 eV, CdS acts as visible-light responsive photocatalyst and a photoanode material. The combined ZnO and CdS system in nanosize, such as nanocomposites is an emerging research area [5-7]. ZnO-CdS nanocomposites have attracted a great deal of attention in recent years because of their electronic and optical properties that makes them suitable for use as light emitting devices, optoelectronics, photoconductive devices, photocatalysts, photovoltaic solar cells and fluorescence probes for biomedical applications [8-14]. Various physical and chemical synthesis techniques have been used to prepare ZnO-CdS nanocomposite, such as electrochemical deposition [15], facile chemical route [7], colloidal chemical synthesis [16], chemical bath deposition technique [17], combined sol-gel/hydrothermal/SILAR method [13], sonochemical synthesis [18] and chemical precipitation method [19]. The results show a significant improvement in their photocatalytic activity and optical properties. Among these techniques chemical
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precipitation method is most suitable, simple and cost effective for the fabrication of ZnO-CdS nanocomposites. To the best of our knowledge there are no reports about the effect of transition metal (TM) ions on structural and spectroscopic properties of ZnO-CdS nanocomposites. The optical properties greatly influenced by doping of TM ions into the host lattice. However, vanadium doped systems have attracted much attention because of their interesting catalytic properties [20, 21]. The results indicate that the catalytic activity of vanadium is strongly influenced by its local environment, dispersion and stability of the vanadium species present in the solid matrix. In the present investigation, V2O5 doped ZnO-CdS composite nanopowder was successfully prepared by the simple chemical precipitation method. Structural, morphological and optical properties were studied to identify the coordination site symmetry of doped ions in the host material and as well as its bonding nature with the ligands. 2. Experimental 2.1. Materials To synthesize V2O5 doped ZnO-CdS composite nanopowder, the following materials were used. All the chemical reagents were analytical grade without further purification. Zinc acetate, cadmium acetate, sodium hydroxide, sodium sulfide, vanadium pentoxide and ethanol were used as precursors. Deionized water was used for all dilution and sample preparation. All the chemicals are above 99% in purity. All the glassware used in this experimental work was acid washed. 2.2. Synthesis of VO2+ doped ZnO-CdS composite nanopowder In a typical procedure, 2.2 g (0.2 mol) of zinc acetate [Zn(CH3COO)2·2H2O] in 50 mL of deionized water-ethanol matrix (equal volumes) and an equal molar amount of sodium hydroxide [NaOH] in another deionized water-ethanol matrix were mixed drop 4
by drop. The mixture was stirred magnetically at 80 oC until a homogeneous white solution was obtained. Then, 50 mL deionized water-ethanol matrix of cadmium acetate [Cd(CH3COO)2·4H2O] (0.1 mol) solution was added to the above solution. After 10 min, 50 mL of sodium sulfide [Na2S·xH2O] (0.1 mol) solution (prepared in a deionized waterethanol matrix) was added to the above colloidal solution drop wise with continuous stirring. Subsequently, the white solution turned light yellow, indicating the formation of ZnO-CdS nanocomposite. Later 0.01 mol% of vanadium pentoxide [V2O5] dissolved in 20 mL of water-ethanol matrix was added to the above solution and then stirred for 4 h. The obtained dispersions were washed with deionized water and ethanol several times to remove impurities. After washing, the solution was centrifuged at 10,000 rpm about 30 minutes. The settled powder was collected and dried in a hot air oven at 120 oC for 2 h. The synthesized VO2+ doped ZnO-CdS composite nanopowder was characterized using different techniques. 2.3. Characterization Powder X-ray diffraction was done on PANalytical XPert Pro-diffractometer with CuKα radiation (1.5406 Å). Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscope (EDS) images were taken from ZEISS EVO 18. Transmission electron microscope (TEM) images are recorded on HITACHI H-7600 and CCD CAMERA system AMTV600 by dispersing the samples in ethanol. Fourier Transformed Infra Red (FT-IR) spectrum was recorded using KBr pallets on Thermo Nicolet 6700 spectrometer in the range of 4000–400 cm–1. Photoluminescence (PL) spectrum was obtained from Horiba Jobin-Yvon Fluorolog-3 Spectrofluorimeter with Xe continuous (450 W) and pulsed (35 W) lamps as excitation sources. Optical absorption (UV-vis) spectrum was taken from JASCO V-670 Spectrophotometer in the wavelength region of 200–1400 nm. The EPR spectrum was obtained from JEOL JES-TE 100 EPR spectrometer operating at X-band frequencies and having a 100-kHz field modulation.
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3. Results and Discussion 3.1. Structural characterization The phase identification, structural analysis and crystallite size evaluation of prepared sample were performed by powder X-ray diffraction study. XRD pattern of VO2+ doped ZnO-CdS composite nanopowder is shown in Fig. 1. All the diffraction peaks can be indexed to wurtzite hexagonal ZnO structure and hexagonal CdS structure, which is well consistence with standard JCPDS No: 36-1451 and 65-3414 respectively. Broad and strong intensity peaks corresponding to the (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 1 2) planes of CdS with lattice cell parameters a = 0.4125, c = 0.6726 nm. Sharp and high intensity peaks corresponding to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) (1 0 3), (2 0 0), (1 1 2), (2 0 1), (2 0 2) planes of ZnO with lattice cell parameters a = 0.3250, c = 0.5207 nm. No diffraction peaks from other crystalline forms are detected, which demonstrates that the sample having high purity and well crystallinity. The average crystallite size was calculated using Debye-Scherrer’s formula. D = 0.9 λ / β cosθ
(1)
where λ is wavelength (CuKα), β is full width at half maximum (FWHM), θ is diffraction angle. The average crystallite size of sample was found to be 17.7 nm. The dislocation density (δ) and micro strain (ε) were calculated using equations, δ = 1/D2 and ε = β cosθ/4 respectively. Evaluated values of dislocation density and micro strain are 3.1919 x 1015 lines/m2 and 1.9578 x 10-3 respectively. Williamson and Hall (W-H) suggested a method to calculate both crystallite size and micro strain of the sample. W-H equation is expressed as β cosθ = (0.9 λ / D) + 4ε sinθ
(2)
W-H equation represents a straight line between 4 sinθ (X-axis) and β cosθ (Yaxis). Slop of the line gives micro strain (ε) and intercept (kλ/D) of the line represents 6
crystallite size (D). Fig. 2 shows the W-H plot of VO2+ doped ZnO-CdS composite nanopowder. From the W-H method crystallite size and micro strain were found to be 19.5 nm and 3.1219 x 10-3 respectively. These values are in good agreement with that obtained from Debye-Scherrer’s equation. 3.2. Microstructure and morphological studies Scanning electron microscopy is widely used to obtaining information about the surface morphology of as-synthesized nanopowder. Fig. 3 presents the SEM images of VO2+ doped ZnO-CdS composite nanopowder. From the SEM micrographs, the formation of non-uniformly distributed spherical like structures with size below 100 nm were clearly observed. Fig. 4 illustrates the EDS spectrum of VO2+ doped ZnO-CdS composite nanopowder. The result shows independent peaks for zinc (Zn), oxygen (O), cadmium (Cd), sulphur (S) and Vanadium (V) species, which was consistent with ZnO and CdS being present in the composite. The microstructures of the prepared sample were further studied using TEM. The TEM images of VO2+ doped ZnO-CdS composite nanopowder were shown in Fig. 5, which reveal that the particles are spherical in shape. The average particle size of VO2+ doped ZnO-CdS composite nanopowder is around 20 nm. These results are good agreement with XRD data. 3.3. FT-IR analysis FT-IR spectrum of VO2+ doped ZnO-CdS composite nanopowder is shown in Fig. 6, which exhibits the characteristic bands of ZnO and CdS. In addition to this, the spectrum exhibits fundamental vibrations of O-H groups. The band observed at 459 cm–1 is corresponds to metal oxide bond (Zn–O), which confirm the formation of ZnO [22]. The band appeared at 612 cm–1 is attributed to the stretching vibrational mode of Cd-S [23]. The band observed at 1115 cm–1 is assigned to O-H stretching mode of H2O molecule [24]. The band at 1384 cm–1 corresponds to the C=O stretching vibration [25]. 7
The vibrational band observed at 1631 cm–1 is a characteristic of symmetric bending vibration of H2O molecule. The band located near 2359 cm–1 can be attributed to the existence of atmospheric CO2 in the sample [26]. The absorption bands centered at 2847 and 2925 cm–1 are assigned to the stretching vibrations of –CH2– group. A broad peak in the range of 3200 cm–1 to 3500 cm–1 corresponds to the vibrational mode of O-H bond [27]. 3.4. Photoluminescence study To investigate effect of impurity doping and structural defects in the prepared sample PL study was performed. Fig. 7 shows the room temperature PL spectrum of VO2+ doped ZnO-CdS composite nanopowder under the excitation of 325 nm. PL spectrum exhibits five emission peaks in UV and visible regions. The UV emission band centered at 368 nm (3.375 eV) is attributed to the near band edge (NBE) emission of ZnO arising from arising from energy loss due to strong electron–phonon interactions at room temperature [28]. Visible emission peaks are assigned to the defects present in the sample such as Oxygen vacancies (VO), Oxygen interstitials (Oi), Zinc interstitials (Zni), Zinc vacancies (VZn), sulfur vacancies (VS), cadmium vacancies (VCd) and surface dangling bonds. Sharp blue emission band centered at 426 nm (2.915 eV) is associated with Zn vacancies [29]. The bluish green emission centered at 469 nm (2.648 eV) is probably caused by electron transition from shallow donor levels, created by the oxygen vacancy to valence band [30]. The green emission peaks at 510 (2.435 eV) and 536 nm (2.317 eV) may have been either due to sulfur/cadmium vacancies or interstitial defects [12]. Fig. 8 illustrates the CIE 1931 chromaticity diagram of VO2+ doped ZnO-CdS composite
nanopowder.
Chromaticity
coordinates
were
calculated
from
the
corresponding PL spectrum as (x, y) = (0.202, 0.344), which indicates colour vision of the sample. The dark spot on the Chromaticity diagram represents the bluish green region. From CIE 1931 chromaticity coordinates (x, y), correlated colour temperature (CCT) is calculated using McCamy equation [31]. 8
CCT = – 449 n3 + 3525 n2 – 6823.3 n + 5520.33
(3)
Where n = (x–xe)/(y–ye) is inverse slope line with xe = 0.3320, ye = 0.1858. CCT is an indication of colour appearance of the light emitted by the light source. Under the long-wavelength UV excitation, a set of fluorescence with colour temperatures varying from 5000 to 20000 K can be applied to circadian lights. In the present study, CCT is evaluated as 13715 K, which represents bluish green light emission. 3.5. Optical absorption study Optical absorption spectrum of VO2+ doped ZnO-CdS composite nanopowder is shown in Fig. 9. The spectrum exhibited three characteristic absorption bands centered at 816 nm (12,251 cm–1), 611 nm (16,362 cm–1) and 378 nm (26,448 cm–1). In an octahedral crystal field, the d1 electron occupies lowest lying dxy orbital (2B2g). On the basis of the molecular orbital theory, the observed bands are assigned to the transitions, 2B2g → 2Eg (dxy → dxz, dyz), 2B2g → 2
B1g (dxy → dx2 – y2) and 2B2g → 2A1g (dxy → dz2) in increasing order of energy [32]. The crystal
field parameter (Dq) and tetragonal field parameters (Ds and Dt) are evaluated from the following expressions: 2
B2g → 2Eg
: –3Ds + 5Dt
(4)
2
B2g → 2B1g
: 10Dq
(5)
B2g → 2A1g
: 10Dq – 4Ds – 5Dt
(6)
2
The evaluated values are: Dq = 1636 cm–1, Ds = –3191 cm–1 and Dt = 535 cm–1.
3.6. Electron paramagnetic resonance study Electron paramagnetic resonance (EPR) spectroscopy offers detailed information of the coordination site symmetry as well as bonding nature produced by the ligands around the paramagnetic ions. Fig. 10 represents the room temperature EPR spectrum of VO2+ doped ZnO-CdS composite nanopowder. In the present case, poorly resolved eight line hyperfine patterns are observed. It has been reported that the most common 9
coordination of vanadium is octahedral with tetragonal distortions [33]. The low resolution and unresolved of the hyperfine couplings confirms the presence of different V4+ species in an axially symmetrical environment. The extra ligands may come from the solvent molecules, which can yield the distorted octahedral complex. The spinHamiltonian parameters are evaluated as g║ = 1.947, g┴ = 1.996 and A║ = 150.9 x 10–4 cm–1, A┴ = 58.8 x 10–4 cm–1. An octahedral site with a tetragonal compression would give g║ < g┴ < ge and A║ > A┴. The Δ║/Δ┴ = (ge – g║)/(ge – g┴) ratio that measures the tetragonality of the VO2+ site and the ratio is greater than unity, the VO2+ ions are tetragonally distorted [34, 35]. The present values of the spin-Hamiltonian parameters agree with the above conditions. From these observations it is suggested that the paramagnetic V4+ ion in the complex exists as vanadyl ion, VO2+, in tetragonally distorted octahedral site (C4v). By correlating EPR and optical parameters, the molecular bonding coefficients β12, β22 and γ2, the Fermi contact term κ and the dipolar hyperfine coupling constant P can be determined from the following expressions [36, 37]: g║ = ge [1 – (4λβ12β22)/ Δ║]
(7)
g┴ = ge [1 – (λγ2β22)/Δ┴]
(8)
and A║ = –P [κ + (4/7) β22 +(ge – g║) + 3/7(ge – g┴)] A┴ = P[2/7 – κ + 11/4 (g┴ – ge)]
(9) (10)
where g║ and g┴ are related to the bonding parameters and ge (2.0023) is the free electron g value. λ is the free ion value of spin orbit coupling constant for VO2+ ion and is equal to 170 cm–1. Δ║ and Δ┴ are energy separations from the ground state 2B2g to the two nearest higher states 2Eg and 2B1g respectively. β12, β22 and γ2 are the molecular bonding coefficients of the d1 electron. β12 and γ2 are measures of the degrees of σ and π bonding 10
with equatorial ligands respectively. β22 is the covalency ratio of V=O bonds. The degree of distortion can be estimated from Fermi contact terms κ and the P parameter. P is related to the radial distribution of the wave function of the ions and is defined as P = gegN βe βN 〈r−3〉. The dipolar coupling constant (P) is evaluated by ignoring the second order effects and taking negative values for A║ and A┴ [38]. P = 7(A║ – A┴)/(6+(3λ/2Δ║))
(11)
The isotropic and anisotropic (g and A) parameters are determined using the formulae, giso = (2g║ + g┴)/3
(12)
Aiso = (2A║ + A┴)/3
(13)
Using the above expressions with Eq. (9) and (10) one gets κ = – (Aiso/P) – (ge – giso)
(14)
and the Fermi contact parameter is evaluated. Using P and κ in Eq. (9) and (10), β22, which is the covalancy ratio of the V=O bonds, is calculated. Using these values in (7) and (8), β12 and γ2 are evaluated. The evaluated values of P, κ are 108 x 10–4 cm–1, 0.81 respectively, and β12, β22, γ2 are 0.71, 0.93, 0.23 respectively. The deviation of β22 from unity usually represents the degree of admixture of the ligand orbitals and increase in the degree of the covalancy. In the present study, β22 is clearly indicates that the bonding is nearly ionic and represents poor π bonding of the ligands. However, κ value is lower than β22 and nearly equal to unity. The deviation of κ from unity indicates the admixture of the 4s orbital into the dxy orbital. It may be due to a low symmetry ligand field. If β12 = 1, the bond would be completely ionic. If β12 = 0.5, the bond would be completely covalent. The parameters (1 − β12) and (1 − γ2) are the measures of the 11
covalency. The first term gives an indication of the influence of σ bonding between the vanadium atom and equatorial ligands, while the second indicates the influence of π bonding between the vanadium ion and the vanadyl oxygen. The bonding coefficients β12, β22 and γ2 characterize in plane σ bonding, in plane π bonding and out of plane π bonding respectively. In the present investigation, β12, γ2 are lower than β22, which indicating that the in plane σ bonding, out of plane π bonding are more covalent than the in plane π bonding. 4. Conclusions In summary, VO2+ doped ZnO-CdS composite nanopowder was prepared by using chemical precipitation method. Powder X-ray diffraction pattern revealed the hexagonal phase of ZnO and CdS. SEM and TEM images showed uniformly distributed sphere like structures. FT-IR spectrum reveals the presence of fundamental modes of ZnO, CdS and other functional groups. The PL spectrum of VO2+ doped ZnO-CdS composite nanopowder exhibited strong emission bands in visible region. Optical and EPR studies revealed the tetragonal compressed distorted octahedral site symmetry for VO2+ ions. The evaluated bonding parameters suggest that VO2+ ions exhibits ionic nature with its ligands. These blue-green emission nanomaterials are promising for applications in light emitting nanodevices. Acknowledgments This work was supported by UGC, New Delhi, India under the scheme of UGCBSR Meritorious fellowship. Authors would like to thank the Director, Centralized Laboratory, ANU and the Head, SAIF, IIT Madras for providing Ultracentrifuge and EPR facility respectively.
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Figure Captions
Fig. 1
XRD pattern of VO2+ doped ZnO-CdS composite nanopowder
Fig. 2
W-H plot of VO2+ doped ZnO-CdS composite nanopowder
Fig. 3
SEM images of VO2+ doped ZnO-CdS composite nanopowder
Fig. 4
EDS pattern of VO2+ doped ZnO-CdS composite nanopowder
Fig. 5
TEM images of VO2+ doped ZnO-CdS composite nanopowder
Fig. 6
FT-IR spectrum of VO2+ doped ZnO-CdS composite nanopowder
Fig. 7
Room temperature PL spectrum of VO2+ doped ZnO-CdS composite nanopowder
Fig. 8
CIE 1931 Chromaticity diagram of VO2+ doped ZnO-CdS composite nanopowder
Fig. 9
Optical absorption spectrum of VO2+ doped ZnO-CdS composite nanopowder
Fig. 10 Room temperature EPR spectrum of VO2+ doped ZnO-CdS composite nanopowder
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Fig. 1
16
Fig. 2
17
Fig. 3
Fig. 4
18
Fig. 5
19
Fig. 6
20
Fig. 7
21
Fig. 8
22
Fig. 9
23
Fig. 10
24
Graphical abstract
TEM images of VO2+ doped ZnO-CdS composite nanopowder reveal that the sample contains spherical like structures. The average particle size of VO2+ doped ZnOCdS composite nanopowder is around 20 nm. These results are in good agreement with X-ray diffraction data.
Highlights
VO2+ doped ZnO-CdS composite synthesized by simple chemical precipitation method. Average particle size is found to be 17.7 nm. Spherical like structures were observed in SEM and TEM micrographs. Ionic bonding is observed between vanadyl ions and its ligands Vanadyl ions entered into the host lattice as tetragonally distorted octahedral sites. 25
S0022-2860(14)01057-6 http://dx.doi.org/10.1016/j.molstruc.2014.10.044 MOLSTR 21040
To appear in:
Journal of Molecular Structure
Received Date: Revised Date: Accepted Date:
24 September 2014 18 October 2014 20 October 2014
Please cite this article as: G. Thirumala Rao, B. Babu, R. Joyce Stella, V. Pushpa Manjari, Ch. Venkata Reddy, J. Shim, R.V.S.S.N. Ravikumar, Synthesis and characterization of VO2+ doped ZnO-CdS composite nanopowder, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.10.044
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.
Synthesis and characterization of VO2+ doped ZnO-CdS composite nanopowder G. Thirumala Raoa, B. Babua, R. Joyce Stellaa, V. Pushpa Manjaria, Ch. Venkata Reddyb, Jaesool Shimb, R.V.S.S.N. Ravikumara*. a
Department of Physics, University College of Sciences, Acharya Nagarjuna University, Nagarjuna Nagar-522510, India.
b
School of Mechanical Engineering, Yeungnam University, Gyeongsan-712-749, Republic of Korea. Email: [email protected]
Corresponding Author’s Address: Dr. R.V.S.S.N. Ravikumar Assistant Professor Department of Physics Acharya Nagarjuna University Nagarjuna Nagar-522510 A.P., India. Phone No: +91-863-2346381 (Lab) +91-863-2263458 (Resi.) Mobile No: +91-9490114276 Fax: +91-863-2293378 Email: [email protected]
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Abstract VO2+ doped ZnO-CdS composite nanopowder has been synthesized by chemical precipitation method. The prepared sample has been characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), FT-IR, photoluminescence (PL), optical absorption and EPR spectroscopy. From XRD pattern, average crystallite size is about 18 nm. SEM and TEM images showed sphere like structures. FT-IR spectrum indicates the presence of fundamental modes of ZnO, CdS and other functional groups. The PL spectrum of VO2+ doped ZnO-CdS composite nanopowder exhibits UV, blue and green emissions. Optical and EPR studies revealed the tetragonal compressed octahedral site symmetry for VO2+ ions. The bonding between VO2+ and its ligands is ionic. Keywords: Photocatalyst, Chemical precipitation method, Surface defects, Vanadyl ions, Electron paramagnetic resonance, CIE diagram.
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1. Introduction Various fabrications of semiconducting materials like cadmium sulfide (CdS) and zinc oxide (ZnO) have attracted much attention to researchers over the past decades because of their synthesis procedure and functional behavior having unusual optical, electric properties and potential applications in nanodevices [1]. ZnO based nanostructures received significant attention in the recent development of science and technology because of their potential applications in optics, catalysts and solar cells [2-4]. These semiconducting nanostructures revealed appreciable structural, optical, electronic, luminescence and photocatalytic properties. ZnO is a direct band gap semiconductor, which has great application potential for photocatalysts and photovoltaic cells, due to its wide band gap of 3.37 eV, low cost, superior carrier mobility and large excitation binding energy of 60 meV at room temperature. As well as cadmium sulfate (CdS) has received considerable attention due to a large number of technical applications like photocatalysis, sensors and solar cells. Due to its narrow band gap of 2.4 eV, CdS acts as visible-light responsive photocatalyst and a photoanode material. The combined ZnO and CdS system in nanosize, such as nanocomposites is an emerging research area [5-7]. ZnO-CdS nanocomposites have attracted a great deal of attention in recent years because of their electronic and optical properties that makes them suitable for use as light emitting devices, optoelectronics, photoconductive devices, photocatalysts, photovoltaic solar cells and fluorescence probes for biomedical applications [8-14]. Various physical and chemical synthesis techniques have been used to prepare ZnO-CdS nanocomposite, such as electrochemical deposition [15], facile chemical route [7], colloidal chemical synthesis [16], chemical bath deposition technique [17], combined sol-gel/hydrothermal/SILAR method [13], sonochemical synthesis [18] and chemical precipitation method [19]. The results show a significant improvement in their photocatalytic activity and optical properties. Among these techniques chemical
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precipitation method is most suitable, simple and cost effective for the fabrication of ZnO-CdS nanocomposites. To the best of our knowledge there are no reports about the effect of transition metal (TM) ions on structural and spectroscopic properties of ZnO-CdS nanocomposites. The optical properties greatly influenced by doping of TM ions into the host lattice. However, vanadium doped systems have attracted much attention because of their interesting catalytic properties [20, 21]. The results indicate that the catalytic activity of vanadium is strongly influenced by its local environment, dispersion and stability of the vanadium species present in the solid matrix. In the present investigation, V2O5 doped ZnO-CdS composite nanopowder was successfully prepared by the simple chemical precipitation method. Structural, morphological and optical properties were studied to identify the coordination site symmetry of doped ions in the host material and as well as its bonding nature with the ligands. 2. Experimental 2.1. Materials To synthesize V2O5 doped ZnO-CdS composite nanopowder, the following materials were used. All the chemical reagents were analytical grade without further purification. Zinc acetate, cadmium acetate, sodium hydroxide, sodium sulfide, vanadium pentoxide and ethanol were used as precursors. Deionized water was used for all dilution and sample preparation. All the chemicals are above 99% in purity. All the glassware used in this experimental work was acid washed. 2.2. Synthesis of VO2+ doped ZnO-CdS composite nanopowder In a typical procedure, 2.2 g (0.2 mol) of zinc acetate [Zn(CH3COO)2·2H2O] in 50 mL of deionized water-ethanol matrix (equal volumes) and an equal molar amount of sodium hydroxide [NaOH] in another deionized water-ethanol matrix were mixed drop 4
by drop. The mixture was stirred magnetically at 80 oC until a homogeneous white solution was obtained. Then, 50 mL deionized water-ethanol matrix of cadmium acetate [Cd(CH3COO)2·4H2O] (0.1 mol) solution was added to the above solution. After 10 min, 50 mL of sodium sulfide [Na2S·xH2O] (0.1 mol) solution (prepared in a deionized waterethanol matrix) was added to the above colloidal solution drop wise with continuous stirring. Subsequently, the white solution turned light yellow, indicating the formation of ZnO-CdS nanocomposite. Later 0.01 mol% of vanadium pentoxide [V2O5] dissolved in 20 mL of water-ethanol matrix was added to the above solution and then stirred for 4 h. The obtained dispersions were washed with deionized water and ethanol several times to remove impurities. After washing, the solution was centrifuged at 10,000 rpm about 30 minutes. The settled powder was collected and dried in a hot air oven at 120 oC for 2 h. The synthesized VO2+ doped ZnO-CdS composite nanopowder was characterized using different techniques. 2.3. Characterization Powder X-ray diffraction was done on PANalytical XPert Pro-diffractometer with CuKα radiation (1.5406 Å). Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscope (EDS) images were taken from ZEISS EVO 18. Transmission electron microscope (TEM) images are recorded on HITACHI H-7600 and CCD CAMERA system AMTV600 by dispersing the samples in ethanol. Fourier Transformed Infra Red (FT-IR) spectrum was recorded using KBr pallets on Thermo Nicolet 6700 spectrometer in the range of 4000–400 cm–1. Photoluminescence (PL) spectrum was obtained from Horiba Jobin-Yvon Fluorolog-3 Spectrofluorimeter with Xe continuous (450 W) and pulsed (35 W) lamps as excitation sources. Optical absorption (UV-vis) spectrum was taken from JASCO V-670 Spectrophotometer in the wavelength region of 200–1400 nm. The EPR spectrum was obtained from JEOL JES-TE 100 EPR spectrometer operating at X-band frequencies and having a 100-kHz field modulation.
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3. Results and Discussion 3.1. Structural characterization The phase identification, structural analysis and crystallite size evaluation of prepared sample were performed by powder X-ray diffraction study. XRD pattern of VO2+ doped ZnO-CdS composite nanopowder is shown in Fig. 1. All the diffraction peaks can be indexed to wurtzite hexagonal ZnO structure and hexagonal CdS structure, which is well consistence with standard JCPDS No: 36-1451 and 65-3414 respectively. Broad and strong intensity peaks corresponding to the (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 1 2) planes of CdS with lattice cell parameters a = 0.4125, c = 0.6726 nm. Sharp and high intensity peaks corresponding to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0) (1 0 3), (2 0 0), (1 1 2), (2 0 1), (2 0 2) planes of ZnO with lattice cell parameters a = 0.3250, c = 0.5207 nm. No diffraction peaks from other crystalline forms are detected, which demonstrates that the sample having high purity and well crystallinity. The average crystallite size was calculated using Debye-Scherrer’s formula. D = 0.9 λ / β cosθ
(1)
where λ is wavelength (CuKα), β is full width at half maximum (FWHM), θ is diffraction angle. The average crystallite size of sample was found to be 17.7 nm. The dislocation density (δ) and micro strain (ε) were calculated using equations, δ = 1/D2 and ε = β cosθ/4 respectively. Evaluated values of dislocation density and micro strain are 3.1919 x 1015 lines/m2 and 1.9578 x 10-3 respectively. Williamson and Hall (W-H) suggested a method to calculate both crystallite size and micro strain of the sample. W-H equation is expressed as β cosθ = (0.9 λ / D) + 4ε sinθ
(2)
W-H equation represents a straight line between 4 sinθ (X-axis) and β cosθ (Yaxis). Slop of the line gives micro strain (ε) and intercept (kλ/D) of the line represents 6
crystallite size (D). Fig. 2 shows the W-H plot of VO2+ doped ZnO-CdS composite nanopowder. From the W-H method crystallite size and micro strain were found to be 19.5 nm and 3.1219 x 10-3 respectively. These values are in good agreement with that obtained from Debye-Scherrer’s equation. 3.2. Microstructure and morphological studies Scanning electron microscopy is widely used to obtaining information about the surface morphology of as-synthesized nanopowder. Fig. 3 presents the SEM images of VO2+ doped ZnO-CdS composite nanopowder. From the SEM micrographs, the formation of non-uniformly distributed spherical like structures with size below 100 nm were clearly observed. Fig. 4 illustrates the EDS spectrum of VO2+ doped ZnO-CdS composite nanopowder. The result shows independent peaks for zinc (Zn), oxygen (O), cadmium (Cd), sulphur (S) and Vanadium (V) species, which was consistent with ZnO and CdS being present in the composite. The microstructures of the prepared sample were further studied using TEM. The TEM images of VO2+ doped ZnO-CdS composite nanopowder were shown in Fig. 5, which reveal that the particles are spherical in shape. The average particle size of VO2+ doped ZnO-CdS composite nanopowder is around 20 nm. These results are good agreement with XRD data. 3.3. FT-IR analysis FT-IR spectrum of VO2+ doped ZnO-CdS composite nanopowder is shown in Fig. 6, which exhibits the characteristic bands of ZnO and CdS. In addition to this, the spectrum exhibits fundamental vibrations of O-H groups. The band observed at 459 cm–1 is corresponds to metal oxide bond (Zn–O), which confirm the formation of ZnO [22]. The band appeared at 612 cm–1 is attributed to the stretching vibrational mode of Cd-S [23]. The band observed at 1115 cm–1 is assigned to O-H stretching mode of H2O molecule [24]. The band at 1384 cm–1 corresponds to the C=O stretching vibration [25]. 7
The vibrational band observed at 1631 cm–1 is a characteristic of symmetric bending vibration of H2O molecule. The band located near 2359 cm–1 can be attributed to the existence of atmospheric CO2 in the sample [26]. The absorption bands centered at 2847 and 2925 cm–1 are assigned to the stretching vibrations of –CH2– group. A broad peak in the range of 3200 cm–1 to 3500 cm–1 corresponds to the vibrational mode of O-H bond [27]. 3.4. Photoluminescence study To investigate effect of impurity doping and structural defects in the prepared sample PL study was performed. Fig. 7 shows the room temperature PL spectrum of VO2+ doped ZnO-CdS composite nanopowder under the excitation of 325 nm. PL spectrum exhibits five emission peaks in UV and visible regions. The UV emission band centered at 368 nm (3.375 eV) is attributed to the near band edge (NBE) emission of ZnO arising from arising from energy loss due to strong electron–phonon interactions at room temperature [28]. Visible emission peaks are assigned to the defects present in the sample such as Oxygen vacancies (VO), Oxygen interstitials (Oi), Zinc interstitials (Zni), Zinc vacancies (VZn), sulfur vacancies (VS), cadmium vacancies (VCd) and surface dangling bonds. Sharp blue emission band centered at 426 nm (2.915 eV) is associated with Zn vacancies [29]. The bluish green emission centered at 469 nm (2.648 eV) is probably caused by electron transition from shallow donor levels, created by the oxygen vacancy to valence band [30]. The green emission peaks at 510 (2.435 eV) and 536 nm (2.317 eV) may have been either due to sulfur/cadmium vacancies or interstitial defects [12]. Fig. 8 illustrates the CIE 1931 chromaticity diagram of VO2+ doped ZnO-CdS composite
nanopowder.
Chromaticity
coordinates
were
calculated
from
the
corresponding PL spectrum as (x, y) = (0.202, 0.344), which indicates colour vision of the sample. The dark spot on the Chromaticity diagram represents the bluish green region. From CIE 1931 chromaticity coordinates (x, y), correlated colour temperature (CCT) is calculated using McCamy equation [31]. 8
CCT = – 449 n3 + 3525 n2 – 6823.3 n + 5520.33
(3)
Where n = (x–xe)/(y–ye) is inverse slope line with xe = 0.3320, ye = 0.1858. CCT is an indication of colour appearance of the light emitted by the light source. Under the long-wavelength UV excitation, a set of fluorescence with colour temperatures varying from 5000 to 20000 K can be applied to circadian lights. In the present study, CCT is evaluated as 13715 K, which represents bluish green light emission. 3.5. Optical absorption study Optical absorption spectrum of VO2+ doped ZnO-CdS composite nanopowder is shown in Fig. 9. The spectrum exhibited three characteristic absorption bands centered at 816 nm (12,251 cm–1), 611 nm (16,362 cm–1) and 378 nm (26,448 cm–1). In an octahedral crystal field, the d1 electron occupies lowest lying dxy orbital (2B2g). On the basis of the molecular orbital theory, the observed bands are assigned to the transitions, 2B2g → 2Eg (dxy → dxz, dyz), 2B2g → 2
B1g (dxy → dx2 – y2) and 2B2g → 2A1g (dxy → dz2) in increasing order of energy [32]. The crystal
field parameter (Dq) and tetragonal field parameters (Ds and Dt) are evaluated from the following expressions: 2
B2g → 2Eg
: –3Ds + 5Dt
(4)
2
B2g → 2B1g
: 10Dq
(5)
B2g → 2A1g
: 10Dq – 4Ds – 5Dt
(6)
2
The evaluated values are: Dq = 1636 cm–1, Ds = –3191 cm–1 and Dt = 535 cm–1.
3.6. Electron paramagnetic resonance study Electron paramagnetic resonance (EPR) spectroscopy offers detailed information of the coordination site symmetry as well as bonding nature produced by the ligands around the paramagnetic ions. Fig. 10 represents the room temperature EPR spectrum of VO2+ doped ZnO-CdS composite nanopowder. In the present case, poorly resolved eight line hyperfine patterns are observed. It has been reported that the most common 9
coordination of vanadium is octahedral with tetragonal distortions [33]. The low resolution and unresolved of the hyperfine couplings confirms the presence of different V4+ species in an axially symmetrical environment. The extra ligands may come from the solvent molecules, which can yield the distorted octahedral complex. The spinHamiltonian parameters are evaluated as g║ = 1.947, g┴ = 1.996 and A║ = 150.9 x 10–4 cm–1, A┴ = 58.8 x 10–4 cm–1. An octahedral site with a tetragonal compression would give g║ < g┴ < ge and A║ > A┴. The Δ║/Δ┴ = (ge – g║)/(ge – g┴) ratio that measures the tetragonality of the VO2+ site and the ratio is greater than unity, the VO2+ ions are tetragonally distorted [34, 35]. The present values of the spin-Hamiltonian parameters agree with the above conditions. From these observations it is suggested that the paramagnetic V4+ ion in the complex exists as vanadyl ion, VO2+, in tetragonally distorted octahedral site (C4v). By correlating EPR and optical parameters, the molecular bonding coefficients β12, β22 and γ2, the Fermi contact term κ and the dipolar hyperfine coupling constant P can be determined from the following expressions [36, 37]: g║ = ge [1 – (4λβ12β22)/ Δ║]
(7)
g┴ = ge [1 – (λγ2β22)/Δ┴]
(8)
and A║ = –P [κ + (4/7) β22 +(ge – g║) + 3/7(ge – g┴)] A┴ = P[2/7 – κ + 11/4 (g┴ – ge)]
(9) (10)
where g║ and g┴ are related to the bonding parameters and ge (2.0023) is the free electron g value. λ is the free ion value of spin orbit coupling constant for VO2+ ion and is equal to 170 cm–1. Δ║ and Δ┴ are energy separations from the ground state 2B2g to the two nearest higher states 2Eg and 2B1g respectively. β12, β22 and γ2 are the molecular bonding coefficients of the d1 electron. β12 and γ2 are measures of the degrees of σ and π bonding 10
with equatorial ligands respectively. β22 is the covalency ratio of V=O bonds. The degree of distortion can be estimated from Fermi contact terms κ and the P parameter. P is related to the radial distribution of the wave function of the ions and is defined as P = gegN βe βN 〈r−3〉. The dipolar coupling constant (P) is evaluated by ignoring the second order effects and taking negative values for A║ and A┴ [38]. P = 7(A║ – A┴)/(6+(3λ/2Δ║))
(11)
The isotropic and anisotropic (g and A) parameters are determined using the formulae, giso = (2g║ + g┴)/3
(12)
Aiso = (2A║ + A┴)/3
(13)
Using the above expressions with Eq. (9) and (10) one gets κ = – (Aiso/P) – (ge – giso)
(14)
and the Fermi contact parameter is evaluated. Using P and κ in Eq. (9) and (10), β22, which is the covalancy ratio of the V=O bonds, is calculated. Using these values in (7) and (8), β12 and γ2 are evaluated. The evaluated values of P, κ are 108 x 10–4 cm–1, 0.81 respectively, and β12, β22, γ2 are 0.71, 0.93, 0.23 respectively. The deviation of β22 from unity usually represents the degree of admixture of the ligand orbitals and increase in the degree of the covalancy. In the present study, β22 is clearly indicates that the bonding is nearly ionic and represents poor π bonding of the ligands. However, κ value is lower than β22 and nearly equal to unity. The deviation of κ from unity indicates the admixture of the 4s orbital into the dxy orbital. It may be due to a low symmetry ligand field. If β12 = 1, the bond would be completely ionic. If β12 = 0.5, the bond would be completely covalent. The parameters (1 − β12) and (1 − γ2) are the measures of the 11
covalency. The first term gives an indication of the influence of σ bonding between the vanadium atom and equatorial ligands, while the second indicates the influence of π bonding between the vanadium ion and the vanadyl oxygen. The bonding coefficients β12, β22 and γ2 characterize in plane σ bonding, in plane π bonding and out of plane π bonding respectively. In the present investigation, β12, γ2 are lower than β22, which indicating that the in plane σ bonding, out of plane π bonding are more covalent than the in plane π bonding. 4. Conclusions In summary, VO2+ doped ZnO-CdS composite nanopowder was prepared by using chemical precipitation method. Powder X-ray diffraction pattern revealed the hexagonal phase of ZnO and CdS. SEM and TEM images showed uniformly distributed sphere like structures. FT-IR spectrum reveals the presence of fundamental modes of ZnO, CdS and other functional groups. The PL spectrum of VO2+ doped ZnO-CdS composite nanopowder exhibited strong emission bands in visible region. Optical and EPR studies revealed the tetragonal compressed distorted octahedral site symmetry for VO2+ ions. The evaluated bonding parameters suggest that VO2+ ions exhibits ionic nature with its ligands. These blue-green emission nanomaterials are promising for applications in light emitting nanodevices. Acknowledgments This work was supported by UGC, New Delhi, India under the scheme of UGCBSR Meritorious fellowship. Authors would like to thank the Director, Centralized Laboratory, ANU and the Head, SAIF, IIT Madras for providing Ultracentrifuge and EPR facility respectively.
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Figure Captions
Fig. 1
XRD pattern of VO2+ doped ZnO-CdS composite nanopowder
Fig. 2
W-H plot of VO2+ doped ZnO-CdS composite nanopowder
Fig. 3
SEM images of VO2+ doped ZnO-CdS composite nanopowder
Fig. 4
EDS pattern of VO2+ doped ZnO-CdS composite nanopowder
Fig. 5
TEM images of VO2+ doped ZnO-CdS composite nanopowder
Fig. 6
FT-IR spectrum of VO2+ doped ZnO-CdS composite nanopowder
Fig. 7
Room temperature PL spectrum of VO2+ doped ZnO-CdS composite nanopowder
Fig. 8
CIE 1931 Chromaticity diagram of VO2+ doped ZnO-CdS composite nanopowder
Fig. 9
Optical absorption spectrum of VO2+ doped ZnO-CdS composite nanopowder
Fig. 10 Room temperature EPR spectrum of VO2+ doped ZnO-CdS composite nanopowder
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Fig. 1
16
Fig. 2
17
Fig. 3
Fig. 4
18
Fig. 5
19
Fig. 6
20
Fig. 7
21
Fig. 8
22
Fig. 9
23
Fig. 10
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Graphical abstract
TEM images of VO2+ doped ZnO-CdS composite nanopowder reveal that the sample contains spherical like structures. The average particle size of VO2+ doped ZnOCdS composite nanopowder is around 20 nm. These results are in good agreement with X-ray diffraction data.
Highlights
VO2+ doped ZnO-CdS composite synthesized by simple chemical precipitation method. Average particle size is found to be 17.7 nm. Spherical like structures were observed in SEM and TEM micrographs. Ionic bonding is observed between vanadyl ions and its ligands Vanadyl ions entered into the host lattice as tetragonally distorted octahedral sites. 25