Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 544–550
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Synthesis and spectral characterizations of trivalent ions (Cr3+, Fe3+) doped CdO nanopowders T. Aswani, B. Babu, V. Pushpa Manjari, R. Joyce Stella, G. Thirumala Rao, Ch. Rama Krishna, R.V.S.S.N. Ravikumar ⇑ Department of Physics, University College of Sciences, Acharya Nagarjuna University, Nagarjuna Nagar-522510, AP, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
From XRD measurements average
crystallite sizes of Cr3+, Fe3+ doped CdO nanopowders were 30, 32 nm. 3+ 3+ The g = 1.973 and 2 for Cr and Fe doped CdO nanopowder indicate distorted octahedral site. Photoluminescence spectra show different emissions both dopant ions.
a r t i c l e
i n f o
Article history: Received 17 September 2013 Received in revised form 4 November 2013 Accepted 5 November 2013 Available online 13 November 2013 Keywords: Cadmium oxide Sonication Nanopowder X-ray diffraction Electron microscopy CIE diagram
a b s t r a c t Trivalent transition metal ions (Cr3+, Fe3+) doped CdO nanopowders via sonication in the presence of Sodium lauryl sulfate as stabilizing agent were synthesized and characterized. Powder XRD studies indicate that the obtained CdO has a cubic phase and concluded that the trivalent ions doping induced the lattice constants to change some extent. Optical absorption spectra exhibited the characteristic bands of Cr3+ and Fe3+ ions in octahedral site symmetry. Crystal field (Dq) and inter-electronic repulsion (B and C) parameters are evaluated for Cr3+ doped CdO nanopowders as Dq = 1540, B = 619 and C = 3327 cm1 and for Fe3+ doped CdO nanopowders Dq = 920, B = 690, C = 2750 cm1. EPR spectra of the Cr3+ and Fe3+ doped CdO nanopowders exhibited resonances at g = 1.973 and g = 2 respectively which indicate distorted octahedral site for both ions with the host. Photoluminescence spectra shows the emission bands in violet and bluish green regions for Cr3+ doped CdO, ultraviolet and blue emissions for Fe3+ doped CdO nanopowders. The CIE chromaticity coordinates were also evaluated from the emission spectrum. FT-IR spectra indicate the presence of various functional groups of host lattice. Ó 2013 Elsevier B.V. All rights reserved.
Introduction In recent years, nanomaterials have shown keen interest because of their unusual chemical and physical properties, which are significantly different from the bulk materials [1,2]. Nanocrystalline cadmium oxide (CdO) is an important n-type semiconductor ⇑ Corresponding author. Tel.: +91 863 2346381 (Lab.)/+91 863 2263458 (Res.), mobile: +91 9490114276; fax: +91 8632293378. E-mail address: [email protected] (R.V.S.S.N. Ravikumar). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.018
metal oxide with a rock-salt crystal structure (FCC) with a direct band gap of 2.2–2.7 eV and an indirect band gap of 1.36–1.98 eV [3]. Different values for band gap have been reported in the literatures that can be originated from different preparation conditions [4]. It has high optical transmittance in the visible region of the solar spectrum along with a moderate refractive index make it useful for various applications such as solar cells, transparent electrodes, phototransistors, photodiodes and gas sensors [5–7]. Because of these interesting applications, efforts to prepare
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nanoparticles of CdO using a variety of methods have been reported in the literature [8–11]. Among these methods utilization of sonochemical method for production of nanomaterials has been a research topic of great interest. This is a consequence of the simplicity, inexpensive of equipment and in many cases the as- prepared materials obtainable in the crystalline phase. Recently Vahid Safarifard et al. [12] and Maryam Ramazani et al. [13] successfully prepared CdO nanoparticles by sonochemical method. Application of powerful ultrasound radiation (20 kHz–10 MHz) [14] induces physical or chemical changes during cavitation (the formation, growth, and instantaneously implosive collapse of bubbles in a liquid) which can generate local hot spots having temperatures of roughly 5000 K, pressures of about 500 atm and a lifetime of a few microseconds. These extreme conditions not only can propel chemical reactions, but also promote the formation of nanosized structures, mostly by the instantaneous formation of a plethora of crystallization nuclei [15–17]. In the preparation of nanomaterials, the stabilizers are used to modify the surface of nanoparticles to improve the stability of their suspension against flocculation [18]. Stabilizers play a major role in deciding the surface properties of the nanomaterials. Stabilizers significantly reduced the particle size without changing the shape and size reducing ability depending on the type of stabilizer [19]. Generally, various stabilizers have been used in the synthesis and characterization of different nanomaterials [20,21]. Recently, the research has been focused on modifying the surface of the semiconductor by adding transition metal impurities, which give rise to the mixed oxide semiconductors formation [22]. The considerable attention has been devoted to the possibility of tailoring the structural and optical properties of these materials by using donor impurity. This purpose has been mainly achieved by the doping with different dopants. In the present study, we investigate the influence of trivalent ions (Cr3+, Fe3+) doped CdO nanopowders by sonication in the presence of suitable stabilizer sodium lauryl sulfate (SLS). Experimental section Samples preparation
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(SEM) and Energy Dispersive X-ray Spectroscope (EDS/EDX) images were taken from JEOLJSM 6610 LV. Powder sample mixed with liquid paraffin was used for optical absorption studies and recorded on JASCO V-670 Spectrophotometer in the region of 200–1400 nm. The Fourier-Transformed Infra Red (FT-IR) spectra were recorded using KBR accessories on Bruker’s Alfa Spectrophotometer in the region of 500–4000 cm1. Photoluminescence (PL) spectra were taken from Horiba Jobin–Yvon Fluorolog-3 Spectrofluorimeter with Xe continuous (450 W) and pulsed (35 W) lamps as excitation sources.
Results and discussion XRD studies Fig. 1 represents the X-ray diffraction pattern obtained for Cr3+ and Fe3+ doped CdO nanopowders after annealing at 400 °C for 4 h. The diffraction peaks of Cr3+ doped CdO powder sample observed at 2h = 33.38°, 38.67°, 55.64°, 66.25° and 69.57° whereas for Fe3+ doped CdO powder sample, 2h = 33.39°, 38.68°, 55.66°, 66.26°and 69.60° which were corresponding to the diffraction planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2). These results are compared with the standard pattern (JCPDS 75-0592). The standard inter-planar spacing matches well with the calculated and observed, which is the confirmation of CdO formation. The prominent peaks were used to estimate the grain size of sample with the help of Scherrer’s equation d = (0.9 k/bCos h) where k is the wavelength (k = 1.5406 Å) (Cu Ka), b is the full width at half maximum (FWHM) of the line, and h is the diffraction angle. The grain sizes estimated using the relative intensity peak (1 1 1) for Cr3+ and Fe3+ doped CdO powders was found to be 30 and 32 nm respectively. The XRD shows that the prepared CdO powders are polycrystalline in nature with a cubic crystal structure. The lattice cell parameters of Cr3+ and Fe3+ ions doped CdO nanopowders were calculated as a = 0.4669 and 0.4668 nm respectively. The incorporation of Cr3+ and Fe3+ has reduced the size of lattice parameter compared to the standard lattice cell parameter. The shrinkage of lattice constant after doping is due to the substitution of Cd ions
Chemicals used for the synthesis are of analytical grade from Merck and used without further purification. A typical one step solid state reaction method using sonication is employed [23] for the preparation of Cr3+, Fe3+ doped CdO nanopowders. Cadmium acetate (2.6653 g, 0.01 mol%) was ground for 5 min and SLS (2.018 g, 0.007 mol%) was added to cadmium acetate. After the mixture was ground for 5 min, it was allowed to stand for 2 h. Sodium hydroxide pellets (0.89 g, 0.02 mol%) were added to the above mixture and grounded again for 30 min. After that 0.1 mol% of Cr(NO3)2 (0.4005 g) and Fe(NO3)2 (0.404 g) were added separately into above mixture and ground for 15 min. The products were washed several times (till the pH of solution becomes neutral) in an ultrasonic bath with deionized water and alcohol, alternatively to remove any by-product and excess of SLS. After washing, the solutions were centrifuged at 10,000 rpm and the products were dried at 80 °C for 2 h and the resulting solids were subsequently annealed at 400 °C for 4 h to get Cr3+, Fe3+ doped CdO nanopowders. Characterization techniques The sonication process was carried out by using Oscar Ultrasonic Sonapros PR-250instrument. Powder X-ray diffraction patterns were recorded on PANalytical XPert Pro-diffractometer with Cu Ka radiation (1.5406 Å). Scanning Electron Microscope
Fig. 1. Powder X-ray diffraction pattern of (a) Cr3+ and (b) Fe3+ doped CdO nanopowders.
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by smaller Cr3+ and Fe3+ ions. Usually lattice strain can arise due to segregation of a large amount of dislocations such as crystal imperfections. The X-ray line broadening is used for the investigation of dislocation distribution. To get the more information about the Cr3+ and Fe3+ doped CdO nanopowders, strain (e) and dislocation density (d) are calculated from the XRD data. The strain induced in Cr3+ and Fe3+ doped CdO nanopowders was calculated by Stokes–Wilson equation [24–26], e = (bCos h)/4. The dislocation density was also calculated from the relation, d = 15e/aD. The average lattice strain and the dislocation density of Cr3+ and Fe3+ were estimated as 1.1863 103, 1.2636 1015 lines/m and 1.0685 103, 1.067 1015 lines/m respectively.
segregation among the crystal nucleus. The EDX spectrum of Cr3+ and Fe3+ doped CdO nanopowders confirms the presence of cadmium and oxygen. The stoichiometry of the samples was examined by EDX spectrum and is shown in Figs. 2(b) and 3(b). Electron paramagnetic resonance studies EPR spectrum of Cr3+ doped CdO nanopowder is shown in Fig. 4. From the spectrum, the g value is found to be 1.973, which is similar to previous reports [27]. The g value obtained from EPR results of Cr3+ doped CdO nanopowder indicate distorted octahedral site for the ions in the host. The bonding can be predicted using the expression [28]
SEM and EDX studies
h ¼ ððBfree BÞ=Bfree Þ=kcr
The SEM micrographs of Cr3+ and Fe3+ doped CdO nanopowders shown in Figs. 2(a) and 3(a) respectively. The particle morphology of samples was stone like structure. The grain size determined by XRD is different from the grain sizes observed from SEM due to
where h and k are nephelauxetic parameters of the ligands and the central metal ion, respectively. For Cr3+ ion, the value of k = 0.21 [29]. The calculated value of h is 1.566. The value of h indicates increased delocalization of the d-electrons suggesting covalent nature
3þ
Fig. 2. (a) SEM and (b) EDX images of Cr3+ doped CdO nanopowder.
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547
Fig. 3. (a) SEM and (b) EDX images of Fe3+ doped CdO nanopowder.
Fig. 4. EPR spectrum of Cr3+ doped CdO nanopowders. Fig. 5. EPR spectrum of Fe3+ doped CdO nanopowders.
of bonding between Cr3+ and the ligands. By correlating optical and EPR data, the chemical bonding parameter is evaluated using the formula [30],
g 0 ¼ g e 8ak=D where ge represents the free-electron g value (2.0023), k is the spinorbit coupling constant (91 cm1), D is the gap between the excited and ground levels. From experimental values g0 = 1.9630 and
D = 14,594 cm1, the value of a is evaluated to be 0.44. The evaluated value of a = 0.44 suggests that the Cr3+ ion exhibits the covalent nature in the CdO nanopowder. The EPR spectrum of Fe3+ doped CdO nanopowder is shown in Fig. 5. The spectrum exhibits a signal at g = 2.0, which can be attributed to iron into the host lattice at distorted octahedral site symmetry [31,32]. These results indicate that Fe3+ was successfully
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incorporated into the host. In as synthesized crystalline samples, all iron ions are octahedrally coordinated.
Table 1 Optical absorption band head data of Cr3+ and Fe3+ doped CdO nanopowders. Transitions
Observed band positions Wavelength Wavenumber (nm) (cm1)
Calculated Wavenumber (cm1)
For Cr3+ A2g(F) ? 2A1g (G) 4 A2g(F) ? 4T1g(F) 4 A2g(F) ? 4T2g(F) 4 A2g(F) ? 2Eg(G)
359 463 649 685
27847 21592 15404 14595
27857 21582 15400 14574
For Fe3+ A1g(S) ? 4Eg(D) 6 A1g(S) ? 4T2g(D) 6 A1g(S) ? 4A1g(G) + 4Eg(G) 6 A1g(S) ? 4T2g(G) 6 A1g(S) ? 4T1g(G)
382 410 447 533 685
26170 24383 22365 18756 14594
26054 24395 22432 18815 14617
Optical absorption studies Optical absorption spectrum of Cr3+ doped CdO nanopowder at the room temperature (RT) and is shown in Fig. 6. The RT spectrum exhibits two strong bands at 649, 463 nm and two weak bands at 359, 685 nm. The broad bands 649 and 463 nm are observed and attributed to the spin allowed transitions 4A2g(F) ? 4T2g(F) and 4A2g(F) ? 4T1g(F) respectively. The band position (m1 = 15404 cm1) corresponding to the transition 4A2g(F) ? 4T2g(F) gives the 10 Dq value. The wavenumber of the band corresponding to 4A2g(F) ? 4T1g(F) is 21592 cm1(m2) [33]. Using the following formula [34]
4
6
B ¼ ð2m21 þ m22 3m1 m2 Þ=ð15m2 27m1 Þ; the value of B is evaluated and found to be 619 cm1. The value of C is evaluated using the relation given by [35]
C=B ¼ 1=3:05 Eð2 EÞ=B 7:9 þ 1:8ðB=DqÞ
Fig. 7. Optical absorption spectrum of Fe3+ doped CdO nanopowders.
448 472
400000
419
300000
Intensity (CPS)
The calculated value of C is 3327 cm1. The other weak bands at 685 and 359 nm are attributed, with the help of Tanabe-Sugano diagram [36,37] to the spin forbidden transitions from 4A2g(F) to 2Eg(G) and 2 A1g(G) respectively. Based on the above assignments the energy matrices for different values of Dq, and C are solved and the following values give good agreement between the calculated and observed band positions: Dq = 1540, B = 619 and C = 3327 cm1. The band head data along with their assignments are presented in Table 1. Optical absorption spectrum of Fe3+ doped CdO nanopowder in the region of 350–800 nm at room temperature is shown in Fig. 7. UV–Visible spectrum of the Fe3+ doped CdO nanopowder shows several bands corresponding to d5 high spin Fe3+ cation. The bands around 685 (14,594 cm1) and 533 (18756 cm1) nm were assigned to the transitions 6A1g(S) ? 4T1g(G) and 6A1g(S) ? 4T2g(G) respectively. The other bands at 447 (22365 cm1) and 410 (24,383 cm1) nm correspond to 6A1g(S) ? 4A1g(G) + 4Eg(G) and 6 A1g(G) ? 4T2g(D) transitions. The band at 382 (26170 cm1) correspond to 6A1g(S) ? 4Eg(D). The assignment of these bands is shown in Table 1. The values of crystal field (Dq) and Racah parameters (B and C) with the Tree’s correction factor (a = 90 cm1) for the Fe3+ cation are calculated as 920, 690 and 2750 cm1 respectively. From the optical absorption spectrum it is concluded that the site symmetry for Fe3+ ion in host CdO material is identified as distorted octahedral.
200000
100000
0 400
450
500
550
600
Wavelength(nm) Fig. 8. Emission spectrum of Cr3+ doped CdO nanopowders.
Photoluminescence (PL) studies
Fig. 6. Optical absorption spectrum of Cr3+ doped CdO nanopowders.
PL spectroscopy is an important tool to characterize the optical properties of a semiconductor. The photoluminescence (PL) spectrum of Cr3+ doped CdO nanopowder excited at 360 nm is shown in Fig. 8. The intense peak at 448 nm arises from the combination of the electrons from the conduction band and holes from the valence band. It was reported that depending on the particle size and exciting wavelength, the different emission peaks could be seen. The peak at 448 nm is corresponding to the band edge emission for Cr3+ doped CdO nanopowder [38]. Photoluminescence spectrum of Cr3+ doped CdO at 472 nm arises from the
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transition between the conduction and valence bands. The suggested mechanism of the bluish green emission is mainly due to the concentration of free electrons, and the existence of various point defects due to the heat treatments and/or oxidation associated with the process which helps form the recombination centers [39]. The peak near 419 nm arises due to violet emission. It is well known that Cr3+ ions in ruby produce red emission. However, we found no red luminescence band in our chromium doped CdO nanopowder samples. The effect of chromium doping is to enhance and introduce splitting of the bluish green emission band. We can only tentatively assign the peak at 472 nm to the T2 ? E transition [40]. The room temperature PL spectrum of Fe3+ doped CdO nanopowder excited with UV light source (272 nm) is shown in Fig. 9. PL intensity in visible region may be directly correlated with the defect density in a nanomaterial. For Fe3+ doped CdO sample, four emission peaks appears at 372, 406, 452 and 484 nm. The peak near 406 nm arises due to violet emission whereas other two peaks belong to blue emission. The peak observed at 452 nm can be assigned to the intrinsic defects particularly interstitial defect [41]. For the CdO nanopowders, majority donors for luminescence in the visible region can be attributed to structural defects such as vacancies and surface traps [42,43]. As can be seen from the figure, Fe3+ doped CdO nanopowder show a dominant emission peak around 372 nm. This peak has been assigned to the near band edge emission. The enhancement in the UV emission is observed for the nanopowders with better crystal quality [44,45]. If the concentration of oxygen vacancies is reduced in the synthesized products then it results in the appearance of a sharp and strong intensity near band edge (NBE) [46]. In present synthesized sample strong and sharp emission NBE confirmed the good optical properties with less structural defects. To understand further luminescent properties of the prepared materials, CIE 1931 Chromaticity coordinates were also calculated from the emission spectrum. The CIE system characterizes the colors by a two color coordinates x and y which specify the point on the chromaticity diagram. This system offers more precision in color measurement because the parameters are based on the spectral power distribution (SPD) of the light emitted from a colored object. CIE chromaticity coordinates of Cr3+ and Fe3+ doped CdO nanopowders were calculated from the emission spectrum and is shown in the Fig. 10. The location of the color coordinates of Cr3+, Fe3+ doped CdO nanopowders are represented in the CIE chromaticity diagram by signs (H), () indicates the color of the nanopowders respectively. From this figure, one can see that the color of the Cr3+ and
Fig. 10. CIE diagram of Cr3+ and Fe3+ doped CdO nanopowders.
Fe3+ doped CdO nanopowders are located in the bluish green region and blue region and the CIE coordinates are (x = 0.147, y = 0.140) and (x = 0.138, y = 0.058) respectively. FT-IR studies The FT-IR spectra of Cr3+, Fe3+ doped CdO nanopowders were shown in Fig. 11. The vibrational modes appear at 620, 621, 697 cm1 was assigned for CdAO stretching mode of vibration [47,48]. The bands observed at 857, 858 cm1 are due to the presence of CdAOH metallic bonds. The bands observed at 1121, 1199 cm1 indicates the presence of SO4 anti symmetric stretching. The bands observed at 1385, 1418 cm1 corresponds to the ACH3
372 3000
Intensity (CPS)
2500
2000
1500
452 484 406
1000
500 350
400
450
500
Wavelength(nm) Fig. 9. Emission spectrum of Fe3+ doped CdO nanopowders.
549
Fig. 11. FT-IR spectrum of (a) Cr3+ and (b) Fe3+ doped CdO nanopowders.
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Table 2 Vibrational assignments of Cr3+ and Fe3+ doped CdO nanopowders. Wavenumbers (cm1) Cr3+ Fe3+
Band assignment
620 857 1121 1385, 1417 1631 1746 2854
CdAO (stretching) CdAOH metallic bond SO4 antisymmetric stretching ACH3 (bending) CAC (stretching) C@O (stretching) ACH2 (stretching)
621, 697 858 1105, 1199 1418 1638 1747 2925
bending mode. The observed bands at 1631, 1638 cm1 and 1746, 1747 cm1 were attributed to CAC and C@O stretching of carbonyl groups [49]. The IR band positions and their assignments are presented in Table 2. Conclusions In this study, Cr3+, Fe3+ doped CdO nanopowders were synthesized successfully via sonication followed by annealing at 400 °C for 4 h. The effect of trivalent ions doping on the structural and optical properties of CdO nanopowders were investigated. XRD measurements revealed that prepared samples possess cubic structure and polycrystalline in nature. The average crystallite sizes of Cr3+, Fe3+ doped CdO nanopowders were 30, 32 nm. SEM micrograph revealed stone like structure for Cr3+ doped and sphere like structure for Fe3+ doped CdO nanopowders. For Cr3+ and Fe3+ doped CdO nanopowder g = 1.973 and 2 respectively indicate distorted octahedral site for the ions in the host. The value a = 0.44 suggests that the Cr3+ ion exhibits the covalent nature in the CdO nanopowder. From the optical absorption spectrum it is also concluded that the site symmetry for Cr3+, Fe3+ in host CdO material are identified as distorted octahedral. Photoluminescence spectrum showed the enhanced bluish green emission of Cr3+ doped CdO nanopowder and a sharp and strong UV emission of Fe3+ doped CdO nanopowder at room temperature. FT-IR confirms the presence of CdO. Acknowledgments Authors are thankful to UGC–DRS and DST–FIST, New Delhi for sanctioning financial assistance to the Department of Physics, Acharya Nagarjuna University to carry out the present research work. Authors thankful to the Dr. P. Sudhakar, Coordinator, Department of Bio-Technology for providing Sonication facility and Director, Centralized Laboratory, ANU for providing Ultracentrifuge. References [1] [2] [3] [4]
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Synthesis and spectral characterizations of trivalent ions (Cr3+, Fe3+) doped CdO nanopowders T. Aswani, B. Babu, V. Pushpa Manjari, R. Joyce Stella, G. Thirumala Rao, Ch. Rama Krishna, R.V.S.S.N. Ravikumar ⇑ Department of Physics, University College of Sciences, Acharya Nagarjuna University, Nagarjuna Nagar-522510, AP, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
From XRD measurements average
crystallite sizes of Cr3+, Fe3+ doped CdO nanopowders were 30, 32 nm. 3+ 3+ The g = 1.973 and 2 for Cr and Fe doped CdO nanopowder indicate distorted octahedral site. Photoluminescence spectra show different emissions both dopant ions.
a r t i c l e
i n f o
Article history: Received 17 September 2013 Received in revised form 4 November 2013 Accepted 5 November 2013 Available online 13 November 2013 Keywords: Cadmium oxide Sonication Nanopowder X-ray diffraction Electron microscopy CIE diagram
a b s t r a c t Trivalent transition metal ions (Cr3+, Fe3+) doped CdO nanopowders via sonication in the presence of Sodium lauryl sulfate as stabilizing agent were synthesized and characterized. Powder XRD studies indicate that the obtained CdO has a cubic phase and concluded that the trivalent ions doping induced the lattice constants to change some extent. Optical absorption spectra exhibited the characteristic bands of Cr3+ and Fe3+ ions in octahedral site symmetry. Crystal field (Dq) and inter-electronic repulsion (B and C) parameters are evaluated for Cr3+ doped CdO nanopowders as Dq = 1540, B = 619 and C = 3327 cm1 and for Fe3+ doped CdO nanopowders Dq = 920, B = 690, C = 2750 cm1. EPR spectra of the Cr3+ and Fe3+ doped CdO nanopowders exhibited resonances at g = 1.973 and g = 2 respectively which indicate distorted octahedral site for both ions with the host. Photoluminescence spectra shows the emission bands in violet and bluish green regions for Cr3+ doped CdO, ultraviolet and blue emissions for Fe3+ doped CdO nanopowders. The CIE chromaticity coordinates were also evaluated from the emission spectrum. FT-IR spectra indicate the presence of various functional groups of host lattice. Ó 2013 Elsevier B.V. All rights reserved.
Introduction In recent years, nanomaterials have shown keen interest because of their unusual chemical and physical properties, which are significantly different from the bulk materials [1,2]. Nanocrystalline cadmium oxide (CdO) is an important n-type semiconductor ⇑ Corresponding author. Tel.: +91 863 2346381 (Lab.)/+91 863 2263458 (Res.), mobile: +91 9490114276; fax: +91 8632293378. E-mail address: [email protected] (R.V.S.S.N. Ravikumar). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.11.018
metal oxide with a rock-salt crystal structure (FCC) with a direct band gap of 2.2–2.7 eV and an indirect band gap of 1.36–1.98 eV [3]. Different values for band gap have been reported in the literatures that can be originated from different preparation conditions [4]. It has high optical transmittance in the visible region of the solar spectrum along with a moderate refractive index make it useful for various applications such as solar cells, transparent electrodes, phototransistors, photodiodes and gas sensors [5–7]. Because of these interesting applications, efforts to prepare
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nanoparticles of CdO using a variety of methods have been reported in the literature [8–11]. Among these methods utilization of sonochemical method for production of nanomaterials has been a research topic of great interest. This is a consequence of the simplicity, inexpensive of equipment and in many cases the as- prepared materials obtainable in the crystalline phase. Recently Vahid Safarifard et al. [12] and Maryam Ramazani et al. [13] successfully prepared CdO nanoparticles by sonochemical method. Application of powerful ultrasound radiation (20 kHz–10 MHz) [14] induces physical or chemical changes during cavitation (the formation, growth, and instantaneously implosive collapse of bubbles in a liquid) which can generate local hot spots having temperatures of roughly 5000 K, pressures of about 500 atm and a lifetime of a few microseconds. These extreme conditions not only can propel chemical reactions, but also promote the formation of nanosized structures, mostly by the instantaneous formation of a plethora of crystallization nuclei [15–17]. In the preparation of nanomaterials, the stabilizers are used to modify the surface of nanoparticles to improve the stability of their suspension against flocculation [18]. Stabilizers play a major role in deciding the surface properties of the nanomaterials. Stabilizers significantly reduced the particle size without changing the shape and size reducing ability depending on the type of stabilizer [19]. Generally, various stabilizers have been used in the synthesis and characterization of different nanomaterials [20,21]. Recently, the research has been focused on modifying the surface of the semiconductor by adding transition metal impurities, which give rise to the mixed oxide semiconductors formation [22]. The considerable attention has been devoted to the possibility of tailoring the structural and optical properties of these materials by using donor impurity. This purpose has been mainly achieved by the doping with different dopants. In the present study, we investigate the influence of trivalent ions (Cr3+, Fe3+) doped CdO nanopowders by sonication in the presence of suitable stabilizer sodium lauryl sulfate (SLS). Experimental section Samples preparation
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(SEM) and Energy Dispersive X-ray Spectroscope (EDS/EDX) images were taken from JEOLJSM 6610 LV. Powder sample mixed with liquid paraffin was used for optical absorption studies and recorded on JASCO V-670 Spectrophotometer in the region of 200–1400 nm. The Fourier-Transformed Infra Red (FT-IR) spectra were recorded using KBR accessories on Bruker’s Alfa Spectrophotometer in the region of 500–4000 cm1. Photoluminescence (PL) spectra were taken from Horiba Jobin–Yvon Fluorolog-3 Spectrofluorimeter with Xe continuous (450 W) and pulsed (35 W) lamps as excitation sources.
Results and discussion XRD studies Fig. 1 represents the X-ray diffraction pattern obtained for Cr3+ and Fe3+ doped CdO nanopowders after annealing at 400 °C for 4 h. The diffraction peaks of Cr3+ doped CdO powder sample observed at 2h = 33.38°, 38.67°, 55.64°, 66.25° and 69.57° whereas for Fe3+ doped CdO powder sample, 2h = 33.39°, 38.68°, 55.66°, 66.26°and 69.60° which were corresponding to the diffraction planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2). These results are compared with the standard pattern (JCPDS 75-0592). The standard inter-planar spacing matches well with the calculated and observed, which is the confirmation of CdO formation. The prominent peaks were used to estimate the grain size of sample with the help of Scherrer’s equation d = (0.9 k/bCos h) where k is the wavelength (k = 1.5406 Å) (Cu Ka), b is the full width at half maximum (FWHM) of the line, and h is the diffraction angle. The grain sizes estimated using the relative intensity peak (1 1 1) for Cr3+ and Fe3+ doped CdO powders was found to be 30 and 32 nm respectively. The XRD shows that the prepared CdO powders are polycrystalline in nature with a cubic crystal structure. The lattice cell parameters of Cr3+ and Fe3+ ions doped CdO nanopowders were calculated as a = 0.4669 and 0.4668 nm respectively. The incorporation of Cr3+ and Fe3+ has reduced the size of lattice parameter compared to the standard lattice cell parameter. The shrinkage of lattice constant after doping is due to the substitution of Cd ions
Chemicals used for the synthesis are of analytical grade from Merck and used without further purification. A typical one step solid state reaction method using sonication is employed [23] for the preparation of Cr3+, Fe3+ doped CdO nanopowders. Cadmium acetate (2.6653 g, 0.01 mol%) was ground for 5 min and SLS (2.018 g, 0.007 mol%) was added to cadmium acetate. After the mixture was ground for 5 min, it was allowed to stand for 2 h. Sodium hydroxide pellets (0.89 g, 0.02 mol%) were added to the above mixture and grounded again for 30 min. After that 0.1 mol% of Cr(NO3)2 (0.4005 g) and Fe(NO3)2 (0.404 g) were added separately into above mixture and ground for 15 min. The products were washed several times (till the pH of solution becomes neutral) in an ultrasonic bath with deionized water and alcohol, alternatively to remove any by-product and excess of SLS. After washing, the solutions were centrifuged at 10,000 rpm and the products were dried at 80 °C for 2 h and the resulting solids were subsequently annealed at 400 °C for 4 h to get Cr3+, Fe3+ doped CdO nanopowders. Characterization techniques The sonication process was carried out by using Oscar Ultrasonic Sonapros PR-250instrument. Powder X-ray diffraction patterns were recorded on PANalytical XPert Pro-diffractometer with Cu Ka radiation (1.5406 Å). Scanning Electron Microscope
Fig. 1. Powder X-ray diffraction pattern of (a) Cr3+ and (b) Fe3+ doped CdO nanopowders.
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by smaller Cr3+ and Fe3+ ions. Usually lattice strain can arise due to segregation of a large amount of dislocations such as crystal imperfections. The X-ray line broadening is used for the investigation of dislocation distribution. To get the more information about the Cr3+ and Fe3+ doped CdO nanopowders, strain (e) and dislocation density (d) are calculated from the XRD data. The strain induced in Cr3+ and Fe3+ doped CdO nanopowders was calculated by Stokes–Wilson equation [24–26], e = (bCos h)/4. The dislocation density was also calculated from the relation, d = 15e/aD. The average lattice strain and the dislocation density of Cr3+ and Fe3+ were estimated as 1.1863 103, 1.2636 1015 lines/m and 1.0685 103, 1.067 1015 lines/m respectively.
segregation among the crystal nucleus. The EDX spectrum of Cr3+ and Fe3+ doped CdO nanopowders confirms the presence of cadmium and oxygen. The stoichiometry of the samples was examined by EDX spectrum and is shown in Figs. 2(b) and 3(b). Electron paramagnetic resonance studies EPR spectrum of Cr3+ doped CdO nanopowder is shown in Fig. 4. From the spectrum, the g value is found to be 1.973, which is similar to previous reports [27]. The g value obtained from EPR results of Cr3+ doped CdO nanopowder indicate distorted octahedral site for the ions in the host. The bonding can be predicted using the expression [28]
SEM and EDX studies
h ¼ ððBfree BÞ=Bfree Þ=kcr
The SEM micrographs of Cr3+ and Fe3+ doped CdO nanopowders shown in Figs. 2(a) and 3(a) respectively. The particle morphology of samples was stone like structure. The grain size determined by XRD is different from the grain sizes observed from SEM due to
where h and k are nephelauxetic parameters of the ligands and the central metal ion, respectively. For Cr3+ ion, the value of k = 0.21 [29]. The calculated value of h is 1.566. The value of h indicates increased delocalization of the d-electrons suggesting covalent nature
3þ
Fig. 2. (a) SEM and (b) EDX images of Cr3+ doped CdO nanopowder.
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547
Fig. 3. (a) SEM and (b) EDX images of Fe3+ doped CdO nanopowder.
Fig. 4. EPR spectrum of Cr3+ doped CdO nanopowders. Fig. 5. EPR spectrum of Fe3+ doped CdO nanopowders.
of bonding between Cr3+ and the ligands. By correlating optical and EPR data, the chemical bonding parameter is evaluated using the formula [30],
g 0 ¼ g e 8ak=D where ge represents the free-electron g value (2.0023), k is the spinorbit coupling constant (91 cm1), D is the gap between the excited and ground levels. From experimental values g0 = 1.9630 and
D = 14,594 cm1, the value of a is evaluated to be 0.44. The evaluated value of a = 0.44 suggests that the Cr3+ ion exhibits the covalent nature in the CdO nanopowder. The EPR spectrum of Fe3+ doped CdO nanopowder is shown in Fig. 5. The spectrum exhibits a signal at g = 2.0, which can be attributed to iron into the host lattice at distorted octahedral site symmetry [31,32]. These results indicate that Fe3+ was successfully
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incorporated into the host. In as synthesized crystalline samples, all iron ions are octahedrally coordinated.
Table 1 Optical absorption band head data of Cr3+ and Fe3+ doped CdO nanopowders. Transitions
Observed band positions Wavelength Wavenumber (nm) (cm1)
Calculated Wavenumber (cm1)
For Cr3+ A2g(F) ? 2A1g (G) 4 A2g(F) ? 4T1g(F) 4 A2g(F) ? 4T2g(F) 4 A2g(F) ? 2Eg(G)
359 463 649 685
27847 21592 15404 14595
27857 21582 15400 14574
For Fe3+ A1g(S) ? 4Eg(D) 6 A1g(S) ? 4T2g(D) 6 A1g(S) ? 4A1g(G) + 4Eg(G) 6 A1g(S) ? 4T2g(G) 6 A1g(S) ? 4T1g(G)
382 410 447 533 685
26170 24383 22365 18756 14594
26054 24395 22432 18815 14617
Optical absorption studies Optical absorption spectrum of Cr3+ doped CdO nanopowder at the room temperature (RT) and is shown in Fig. 6. The RT spectrum exhibits two strong bands at 649, 463 nm and two weak bands at 359, 685 nm. The broad bands 649 and 463 nm are observed and attributed to the spin allowed transitions 4A2g(F) ? 4T2g(F) and 4A2g(F) ? 4T1g(F) respectively. The band position (m1 = 15404 cm1) corresponding to the transition 4A2g(F) ? 4T2g(F) gives the 10 Dq value. The wavenumber of the band corresponding to 4A2g(F) ? 4T1g(F) is 21592 cm1(m2) [33]. Using the following formula [34]
4
6
B ¼ ð2m21 þ m22 3m1 m2 Þ=ð15m2 27m1 Þ; the value of B is evaluated and found to be 619 cm1. The value of C is evaluated using the relation given by [35]
C=B ¼ 1=3:05 Eð2 EÞ=B 7:9 þ 1:8ðB=DqÞ
Fig. 7. Optical absorption spectrum of Fe3+ doped CdO nanopowders.
448 472
400000
419
300000
Intensity (CPS)
The calculated value of C is 3327 cm1. The other weak bands at 685 and 359 nm are attributed, with the help of Tanabe-Sugano diagram [36,37] to the spin forbidden transitions from 4A2g(F) to 2Eg(G) and 2 A1g(G) respectively. Based on the above assignments the energy matrices for different values of Dq, and C are solved and the following values give good agreement between the calculated and observed band positions: Dq = 1540, B = 619 and C = 3327 cm1. The band head data along with their assignments are presented in Table 1. Optical absorption spectrum of Fe3+ doped CdO nanopowder in the region of 350–800 nm at room temperature is shown in Fig. 7. UV–Visible spectrum of the Fe3+ doped CdO nanopowder shows several bands corresponding to d5 high spin Fe3+ cation. The bands around 685 (14,594 cm1) and 533 (18756 cm1) nm were assigned to the transitions 6A1g(S) ? 4T1g(G) and 6A1g(S) ? 4T2g(G) respectively. The other bands at 447 (22365 cm1) and 410 (24,383 cm1) nm correspond to 6A1g(S) ? 4A1g(G) + 4Eg(G) and 6 A1g(G) ? 4T2g(D) transitions. The band at 382 (26170 cm1) correspond to 6A1g(S) ? 4Eg(D). The assignment of these bands is shown in Table 1. The values of crystal field (Dq) and Racah parameters (B and C) with the Tree’s correction factor (a = 90 cm1) for the Fe3+ cation are calculated as 920, 690 and 2750 cm1 respectively. From the optical absorption spectrum it is concluded that the site symmetry for Fe3+ ion in host CdO material is identified as distorted octahedral.
200000
100000
0 400
450
500
550
600
Wavelength(nm) Fig. 8. Emission spectrum of Cr3+ doped CdO nanopowders.
Photoluminescence (PL) studies
Fig. 6. Optical absorption spectrum of Cr3+ doped CdO nanopowders.
PL spectroscopy is an important tool to characterize the optical properties of a semiconductor. The photoluminescence (PL) spectrum of Cr3+ doped CdO nanopowder excited at 360 nm is shown in Fig. 8. The intense peak at 448 nm arises from the combination of the electrons from the conduction band and holes from the valence band. It was reported that depending on the particle size and exciting wavelength, the different emission peaks could be seen. The peak at 448 nm is corresponding to the band edge emission for Cr3+ doped CdO nanopowder [38]. Photoluminescence spectrum of Cr3+ doped CdO at 472 nm arises from the
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transition between the conduction and valence bands. The suggested mechanism of the bluish green emission is mainly due to the concentration of free electrons, and the existence of various point defects due to the heat treatments and/or oxidation associated with the process which helps form the recombination centers [39]. The peak near 419 nm arises due to violet emission. It is well known that Cr3+ ions in ruby produce red emission. However, we found no red luminescence band in our chromium doped CdO nanopowder samples. The effect of chromium doping is to enhance and introduce splitting of the bluish green emission band. We can only tentatively assign the peak at 472 nm to the T2 ? E transition [40]. The room temperature PL spectrum of Fe3+ doped CdO nanopowder excited with UV light source (272 nm) is shown in Fig. 9. PL intensity in visible region may be directly correlated with the defect density in a nanomaterial. For Fe3+ doped CdO sample, four emission peaks appears at 372, 406, 452 and 484 nm. The peak near 406 nm arises due to violet emission whereas other two peaks belong to blue emission. The peak observed at 452 nm can be assigned to the intrinsic defects particularly interstitial defect [41]. For the CdO nanopowders, majority donors for luminescence in the visible region can be attributed to structural defects such as vacancies and surface traps [42,43]. As can be seen from the figure, Fe3+ doped CdO nanopowder show a dominant emission peak around 372 nm. This peak has been assigned to the near band edge emission. The enhancement in the UV emission is observed for the nanopowders with better crystal quality [44,45]. If the concentration of oxygen vacancies is reduced in the synthesized products then it results in the appearance of a sharp and strong intensity near band edge (NBE) [46]. In present synthesized sample strong and sharp emission NBE confirmed the good optical properties with less structural defects. To understand further luminescent properties of the prepared materials, CIE 1931 Chromaticity coordinates were also calculated from the emission spectrum. The CIE system characterizes the colors by a two color coordinates x and y which specify the point on the chromaticity diagram. This system offers more precision in color measurement because the parameters are based on the spectral power distribution (SPD) of the light emitted from a colored object. CIE chromaticity coordinates of Cr3+ and Fe3+ doped CdO nanopowders were calculated from the emission spectrum and is shown in the Fig. 10. The location of the color coordinates of Cr3+, Fe3+ doped CdO nanopowders are represented in the CIE chromaticity diagram by signs (H), () indicates the color of the nanopowders respectively. From this figure, one can see that the color of the Cr3+ and
Fig. 10. CIE diagram of Cr3+ and Fe3+ doped CdO nanopowders.
Fe3+ doped CdO nanopowders are located in the bluish green region and blue region and the CIE coordinates are (x = 0.147, y = 0.140) and (x = 0.138, y = 0.058) respectively. FT-IR studies The FT-IR spectra of Cr3+, Fe3+ doped CdO nanopowders were shown in Fig. 11. The vibrational modes appear at 620, 621, 697 cm1 was assigned for CdAO stretching mode of vibration [47,48]. The bands observed at 857, 858 cm1 are due to the presence of CdAOH metallic bonds. The bands observed at 1121, 1199 cm1 indicates the presence of SO4 anti symmetric stretching. The bands observed at 1385, 1418 cm1 corresponds to the ACH3
372 3000
Intensity (CPS)
2500
2000
1500
452 484 406
1000
500 350
400
450
500
Wavelength(nm) Fig. 9. Emission spectrum of Fe3+ doped CdO nanopowders.
549
Fig. 11. FT-IR spectrum of (a) Cr3+ and (b) Fe3+ doped CdO nanopowders.
550
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Table 2 Vibrational assignments of Cr3+ and Fe3+ doped CdO nanopowders. Wavenumbers (cm1) Cr3+ Fe3+
Band assignment
620 857 1121 1385, 1417 1631 1746 2854
CdAO (stretching) CdAOH metallic bond SO4 antisymmetric stretching ACH3 (bending) CAC (stretching) C@O (stretching) ACH2 (stretching)
621, 697 858 1105, 1199 1418 1638 1747 2925
bending mode. The observed bands at 1631, 1638 cm1 and 1746, 1747 cm1 were attributed to CAC and C@O stretching of carbonyl groups [49]. The IR band positions and their assignments are presented in Table 2. Conclusions In this study, Cr3+, Fe3+ doped CdO nanopowders were synthesized successfully via sonication followed by annealing at 400 °C for 4 h. The effect of trivalent ions doping on the structural and optical properties of CdO nanopowders were investigated. XRD measurements revealed that prepared samples possess cubic structure and polycrystalline in nature. The average crystallite sizes of Cr3+, Fe3+ doped CdO nanopowders were 30, 32 nm. SEM micrograph revealed stone like structure for Cr3+ doped and sphere like structure for Fe3+ doped CdO nanopowders. For Cr3+ and Fe3+ doped CdO nanopowder g = 1.973 and 2 respectively indicate distorted octahedral site for the ions in the host. The value a = 0.44 suggests that the Cr3+ ion exhibits the covalent nature in the CdO nanopowder. From the optical absorption spectrum it is also concluded that the site symmetry for Cr3+, Fe3+ in host CdO material are identified as distorted octahedral. Photoluminescence spectrum showed the enhanced bluish green emission of Cr3+ doped CdO nanopowder and a sharp and strong UV emission of Fe3+ doped CdO nanopowder at room temperature. FT-IR confirms the presence of CdO. Acknowledgments Authors are thankful to UGC–DRS and DST–FIST, New Delhi for sanctioning financial assistance to the Department of Physics, Acharya Nagarjuna University to carry out the present research work. Authors thankful to the Dr. P. Sudhakar, Coordinator, Department of Bio-Technology for providing Sonication facility and Director, Centralized Laboratory, ANU for providing Ultracentrifuge. References [1] [2] [3] [4]
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