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Structural, optical, FTIR and photoluminescence properties of Zn0.96−xCo0.04CuxO (x=0.03, 0.04 and 0.05) nanopowders
Physica B 407 (2012) 3448–3456
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Structural, optical, FTIR and photoluminescence properties of Zn0.96 xCo0.04CuxO (x ¼ 0.03, 0.04 and 0.05) nanopowders S. Muthukumaran a,n, R. Gopalakrishnan b a b
PG & Research Department of Physics, H.H. The Rajah’s College, Pudukkottai—622 001 Tamilnadu, India Department of Physics, Oxford Engineering College, Tiruchirapalli—620 009 Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2012 Received in revised form 20 April 2012 Accepted 24 April 2012 Available online 11 May 2012
Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO, x ¼ 0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The synthesized samples have been characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–Visible spectrophotometer and Fourier transform infrared spectroscopy. The calculated average crystalline size increases from 37.3 to 50.6 nm for un-hydrogenated samples from x ¼ 0.03 to 0.05 and it changes from 29.4 to 34.9 nm for hydrogenated samples. The change in lattice parameters, micro-strain, a small shift of X-ray diffraction peaks towards lower angles and reduction in energy gap reveal the substitution of Cu2 þ ions into Zn–Co–O lattice. The hydrogenation effect reduces the particle size and induces the more uniform distribution of particles than the un-hydrogenated samples which is confirmed by SEM micrographs. Photoluminescence spectra of Zn0.96 xCo0.04CuxO system shows that red shift in near band edge ultraviolet emission from 393 to 403 nm with suppressing intensity and a blue shift in green band emission from 537 to 529 nm with enhancing intensity confirms the substitution of Cu into the Zn–Co–O lattice. & 2012 Elsevier B.V. All rights reserved.
Keywords: Zn0.96 xCo0.04CuxO nanopowder Optical properties SEM FTIR Photoluminescence
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted a great attention due to its enormous applications in optoelectronics and spintronics [1–3]. Particularly, ZnO based DMSs have considerable attraction due to its large band gap, high refractive index and large exciton binding energy [4,5]. It has very attractive intensive research effort due to its unique properties and versatile applications in transparent electronics, hi-tech applications, ultraviolet (UV) light emitters, piezoelectric devices and chemical sensors [6,7]. Doping ZnO with magnetic ions such as Fe, Co, Ni induces magnetic properties due to their possible applications in the field of spintronics [8]. Among these different metallic doping elements Co and Cu are important because, (i) it is a prominent luminescence activator, which can modify the luminescence of ZnO crystals by creating localized impurity levels [9], (ii) it has many physical and chemical properties similar to those of Zn. (iii) It can change microstructure and optical properties of ZnO system [10]. Different physical or chemical methods have been used to prepare ZnO nanoparticles such as thermal decomposition, thermolysis [11], chemical vapour deposition, sol–gel [12], spray
n
Corresponding author. Tel.: þ91 4322 221558; fax: þ91 4322 230490. E-mail address: [email protected] (S. Muthukumaran).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.04.057
pyrolysis, precipitation [13] and co-precipitation [14]. Among these different methods, the co-precipitation is one of the most important methods to prepare nanopowder. Co-precipitation is the name given by analytical chemists to a phenomenon whereby the fractional precipitation of a specified ion in a solution results in precipitation not only of target ion but also of other ions existing side by side in the solution. The enhanced ferromagnetic order was observed in Co and Cu doped ZnO powder where Cu acted as an acceptor [15]. Chakraborti et al. have studied structural and magnetic properties of Zn0.90Co0.10O prepared by the co-precipitation technique [16]. He indicated that itinerant electrons were responsible for the ferromagnetism in Co, Cu co-doped ZnO films. Sato et al. predicted that 3d transition metal atoms of Mn, Fe, Co, and Ni show ferromagnetic ordering in ZnO [17]. Hou et al. observed transition temperature of Zn0.98Cu0.02O was to be about 350 K but was decreased to 320 K with nitrogen doping [18]. The influence of shape and hydrogenation on ferromagnetic properties of Zn0.93Co0.05Cu0.02O nanoparticles at room temperature was demonstrated by Xu et al. [19]. Even though some of the research works have been carried out on Cu and Co co-doped ZnO system [15,19–21], most of the works are on the thin films and comprehensive study of the structural and optical properties of Cu and Co co-doped ZnO nanopowders is still scanty. Therefore, in the present investigation, hydrogenated and unhydrogenated Cu and Co co-doped ZnO (Zn0.96 xCo0.04CuxO, x¼0.03, 0.04 and 0.05) nanopowders have been synthesized by
S. Muthukumaran, R. Gopalakrishnan / Physica B 407 (2012) 3448–3456
co-precipitation method. The effect of Cu substitution and the hydrogenation on its structural, optical, morphological and photoluminescence properties has been studied extensively. Further, the size-dependent properties of the nanoparticles are correlated with band gap.
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2.5. Fourier transform infrared spectroscopy (FTIR) studies The presence of chemical bonding in Cu and Co co-doped ZnO nanopowders was studied by FTIR spectrometer (Model: Perkin Elmer, Make: Spectrum RX I) in the range of 400 to 4000 cm 1. 2.6. Photoluminescence (PL) studies
2. Experimental procedure 2.1. Preparation of Zn0.96 xCo0.04CuxO nanopowders The high purity chemicals (4 99% purity) such as zinc(II)nitrate hexahydrate (Zn(NO3)2 6H2O), copper(II)nitrate trihydrate (Cu(NO3)2 3H2O), cobalt(II)nitrate hexahydrate (Co(NO3)2 6H2O) were used as precursors without further purification. Initially, the appropriate amounts of ethanol and water (1:4 ratio) were mixed systematically using magnetic stirrer. Then the appropriate amounts of zinc nitrate, cobalt nitrate and copper nitrate were dissolved in ethanol solution and kept in magnetic stirrer for 2 h under constant stirring. A separate NaOH solution was prepared by dissolving appropriate amounts of sodium hydroxide in the double distilled water. The pH value of the solution was selected as 4.6 for better precipitation reaction. The prepared NaOH solution was then added drop wise to the initial solution under constant stirring for 2 h at room temperature to produce a white, gelatinous precipitate. The white precipitates were filtered and washed with distilled water many times. The final precipitates were dried in oven at 80 1C for 2 h. The dried precipitates were collected and ground in an agate mortar. Finally, the collected nanopowder was annealed at 500 1C for 2 h under air atmosphere followed by furnace cooling. The same procedure is repeated to the remaining samples synthesized with nominal compositions of Zn0.96 xCo0.04CuxO (x¼0.03, 0.04 and 0.05). Again, a set of three samples (as prepared and without annealing) for x¼0.03, 0.04 and 0.05 were annealed at 500 1C for 2 h under H2 gas atmosphere (represented by (H)) to study the hydrogenation effect.
2.2. X-ray diffraction (XRD) and energy dispersive X-ray (EDX) studies The crystal structure of Zn0.96 xCo0.04CuxO nanopowders were determined by powder X-ray diffraction. XRD patterns were recorded on a RigaKuC/max-2500 diffractometer using CuKa ˚ at 30 kV and 100 mA from 2y ¼ 101 to radiation (l ¼1.5408 A) 801. The topological features and composition of Zn, O, Co and Cu were determined by energy dispersive X-ray spectrometer on K and L lines.
2.3. Scanning electron microscope (SEM) studies The surface morphology of Zn0.96 xCo0.04CuxO nanopowders were studied using a scanning electron microscope (SEM, JEOL JSM 6390).
The photoluminescence (PL) spectrum within the wavelength ranges from 350 to 600 nm were obtained under 325 nm line of Xe laser excitation using a fluorescence spectrophotometer (F-2500).
3. Results and discussion 3.1. XRD—Structural studies The typical XRD diffraction peaks of un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04, 0.05 are shown in Fig. 1a and b, respectively. The pronounced diffraction peaks in the XRD pattern clearly shows the crystalline nature with sharp peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) planes. The standard diffraction peaks shows crystal structure of Zn0.96 xCo0.04CuxO is hexagonal wurtzite structure (space group P63mc, 186, JCPDS data card No. 36-1451) [22] with preferred orientation along (1 0 1) plane in all the samples. This indicates that the simultaneous substitution of Co and Cu cannot disturb the structure of ZnO. No additional peaks (such as Co, CoO, Cu, CuO, Cu2O) were observed in Zn0.96 xCo0.04CuxO, which indicates no impurity exist in the samples. Even though, there are no secondary phases detected by XRD analysis, the existence of secondary phases cannot be completely excluded due to limitation of this characterization technique [23]. It is noticed from Fig. 1a and b that the peak intensity increases with Cu concentration, which means that the crystalline quality is improved with Cu doping and also Cu2 þ ion is understood to have occupied Zn2 þ without changing crystal structure. Wei et al. reported the same trend, the increase of diffraction peak intensity with Cu doping [15]. Furthermore, the inset of Fig. 1a and b clearly shows diffraction peak along (1 0 1) plane is shifted towards the lower diffraction angle gradually with addition of Cu concentrations. The small shift of diffraction peak (D2y ¼0.0331 for un-hydrogenated samples and D2y ¼0.0341 for hydrogenated samples) towards the lower angle and increase of peak intensity indicates that Cu2 þ ions are doped into Zn–Co–O crystal lattice successfully in the position of Zn2 þ ions and also the crystal lattice has no obvious change by Cu doping. The average crystal size of the samples is calculated from the broadening of diffraction peaks of (1 0 1) plane using Debye Scherrer’s formula, Average crystal size ðDÞ ¼
ð1Þ
˚ b is the where, l is the wave length of X-ray used (1.5408 A), angular peak width at half maximum in radian along (1 0 1) plane, and y is the Bragg0 s diffraction angle. The micro-strain (e) can be calculated using the formula [24],
2.4. UV–Visible optical absorption studies Micro-strain ðeÞ ¼ The UV–Visible optical absorption study of samples was carried out with a view to explore their optical properties. The spectral absorption was determined using UV–Visible spectrometer (Model: lambda 35, Make: Perkin Elmer) in the wavelength ranging from 350 to 650 nm at room temperature.
0:9 l b cos y
b cos y 4
ð2Þ
Table 1 shows 2y value, full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D) and micro-strain (e) of un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders.
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Fig. 1. Powder X-ray diffraction pattern of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders at room temperature. The inset shows the shift of diffraction peaks along (1 0 1) plane towards the lower diffraction angle.
The resultant compound maintains a hexagonal wurtzite structure, but the d-value and the lattice parameters a and c are slightly increased with Cu doping. The increase of lattice constants is consistent with the earlier literature [15]. The change in lattice parameters could be well understood by the substitution of Cu2 þ ˚ which are larger than that of Zn2 þ ions ions (ionic radius¼0.73 A) ˚ (ionic radius¼0.60 A) [15] in their tetrahedral coordinates. The b values constantly decreases from 0.1721 to 0.1251 for un-hydrogenated samples and 0.2841 to 0.2391 for hydrogenated samples from x¼ 0.03 to 0.05. Mean while, the ‘2y’ values gradually decreases from 36.2461 to 36.1961 for un-hydrogenated samples and 36.231 to 36.1961 for hydrogenated samples from x¼0.03 to 0.5. The change in b values inferred that the average crystal size is increased from x¼0.03 to 0.05 and the hydrogenated samples have lower crystal size than the un-hydrogenated samples. The calculated D value from Eq. (1) ascribed the increase of average crystal size from 37.3 to 50.6 nm (37.3 nm for x¼0.03; 38.7 nm for x¼0.04 and 50.6 nm for x¼ 0.05) for un-hydrogenated samples, 0.03rxZ0.05. For hydrogenated samples, it is increased from 29.4 to 34.9 nm which is lower than the un-hydrogenated samples. It clearly shows the existence of nano-sized particles in the samples. It is observed from Table 1 that micro-strain (e) of un-hydrogenated samples are decreased from x¼ 0.03 (0.562 10 3) to 0.05 (0.401 10 3). The similar decreasing trend was noticed for the hydrogenated samples. The variation in micro-strain is due to the change in microstructure, size and shape of the particles. The atoms trapped in the non-equilibrium position could shift to a more equilibrium position, and it could release the strain. Hence, it decreases from x¼0.03 to 0.05. The decrease of micro-strain is caused by enhancing crystal size and reducing FWHM value as noticed in Table 1. The observed decrease in line broadening (b) for both hydrogenated and un-hydrogenated samples may be due to the size or micro-strain or both [25]. Meanwhile, the crystal quality
of both samples was constantly increased by Cu doping. It is evident that the decrease of strain causes the increase of crystal size and reduction in the broadening whereas increase of strain causes reduction of crystal size and increase of broadening. 3.2. Energy dispersive X-ray (EDX) studies The precise composition of Cu and Co co-doped ZnO nanopowders was determined by an EDX study, and it revealed the presence of Zn, Co, Cu, and O as elementary components. The typical EDX spectra of un-hydrogenated and hydrogenated samples for Zn0.96 xCo0.04CuxO, x ¼0.03 and 0.05 are as shown in Fig. 2a and b, respectively. The inset of Fig. 2a and b shows the weight percentage of Zn, O, Co and Cu elements. It clearly shows that the intensity of Cu increases with the addition of Cu concentrations. Therefore, the addition of Cu induces a dominant effect on the optical, structural and morphological properties of ZnO. The EDX analysis confirms presence of Cu in ZnO system and wt% is very nearly equal to their nominal stoichiometry within the experimental error. It is observed from Fig. 2a (un-hydrogenated samples) that the Cu/(Zn þCu) ratio is derived to be 3.03% and 5.09% for 3% and 5% of Cu doping. For hydrogenated samples (Fig. 2b), the Cu/(ZnþCu) ratio is found to be 2.98% and 5.05% for 3% and 5% of Cu doping. Therefore, the EDX spectra show a well agreement with the experimental concentration used for Zn0.96 xCo0.04CuxO system. 3.3. Scanning electron microscope (SEM)—Microstructural studies SEM is one of the promising techniques for the topography study of the samples and it gives important information regarding shape and size of the particles. The surface morphology of the unhydrogenated and hydrogenated samples for Zn0.96 xCo0.04CuxO,
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Table 1 The 2y value, FWHM value, d-value, cell parameters a and c, c/a ratio, average crystal size (D) and micro-strain (e) of un-hydrogenated and hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders. Samples
2y value (1)
FWHM (b) (1)
˚ d-value (A)
˚ Cell parameter (A)
c/a ratio
Average crystal size (D) (nm)
Micro-strain (e) (10 3)
–
–
–
–
a
c
–
–
–
Zn0.93Co0.04Cu0.03O Zn0.93Co0.04Cu0.03O (H) Zn0.92Co0.04Cu0.04O Zn0.92Co0.04Cu0.04 (H) Zn0.91Co0.04Cu0.05O Zn0.91Co0.04Cu0.05O (H)
36.246 36.230 36.238 36.203 36.213 36.196
0.172 0.284 0.152 0.251 0.125 0.239
2.4766 2.4778 2.4772 2.4795 2.4788 2.4800
3.2513 3.2519 3.2518 3.2524 3.2528 3.2531
5.2081 5.2085 5.2088 5.2087 5.2096 5.2102
1.6018 1.6017 1.6018 1.6015 1.6015 1.6016
37.3 29.4 38.7 33.3 50.6 34.9
0.731 0.920 0.494 0.803 0.401 0.761
Fig. 2. Energy dispersive X-ray (EDX) spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03 and 0.05 nanopowders at room temperature.
x ¼0.03 and 0.05 are as shown in Fig. 3a and b, respectively. The SEM micrographs clearly show the average size of nanoparticles is in the order of nanometer size. The SEM images from Fig. 3a and b clearly show that the particles are uniformly distributed over the surface with good connectivity containing dominant deformed spheroid-like particles. The particle size was slightly increased with Cu concentrations. The average particle size of Zn0.93Co0.04Cu0.03O sample varies between 15 and 40 nm whereas Zn0.91Co0.04Cu0.05O sample has slightly larger particle size around 20 to 50 nm. The increase of Cu doped concentrations causes more defects and deformed lattice structures. The distribution of particles in the hydrogenated samples (Fig. 3b) is more uniform than the un-hydrogenated
samples. It consists of more dense and tightly aggregated spheroidlike particles with the average particle size from 15 to 35 nm. The observed improved crystalline structure and the reduction in the particle size in Fig. 3b are due to the hydrogenation effect which can induce ferromagnetic spin–spin interaction between neighbouring magnetic ions through the formation of a bridge bond [26,5]. A good correlation is found to exist between mathematical calculations from XRD analysis and the SEM studies. 3.4. Optical study UV–Visible absorption spectroscopy is a powerful technique to explore optical properties of semiconducting nanoparticle. The
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Fig. 3. Scanning electron microscope (SEM) images of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03 and 0.05 nanopowders at room temperature.
Fig. 4. UV–Visible absorption spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders.
UV–Visible optical absorption spectrum of un-hydrogenated and the hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04 and 0.05 have been carried out at room temperature using UV–Visible spectrometer (Model: Lambda 35, Make: Perkin Elmer) from 350 to 650 nm. Fig. 4a and b shows the optical absorption spectra of un-hydrogenated and the hydrogenated Zn0.96 xCo0.04CuxO samples, respectively. The absorption spectra show that the absorption of the un-hydrogenated and hydrogenated samples is increased with increasing Cu concentrations. The inset of Fig. 4a and b shows the clear picture of absorption changes around 472 nm. The absorption edge of Zn0.93Co0.04Cu0.03O sample is found to be 393 nm which is shifted towards the higher wavelengths (red shift) with increasing Cu concentrations as Zn0.92Co0.04Cu0.04O sample has the absorption edge around 398 nm; Zn0.91Co0.04Cu0.05O
sample has the edge around 401 nm. In the case of hydrogenated samples, the absorption edge is shifted from 396 nm (x¼0.03) to 403 nm (x¼0.05) which is slightly higher than the un-hydrogenated samples. The change in absorption edge in both cases can be attributed to the photo-excitation of electrons from valence band to conduction band. The same red shift by Cu doping was noticed in the literature [25]. The red shift phenomenon can be explained by the Burstein–Moss band gap widening and gap narrowing due to electron–electron and electron-impurity scattering [27]. The typical room temperature transmittance spectra of the un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04 and 0.05 are shown in Fig. 5a and b, respectively. The transmission spectra of Zn0.96 xCo0.04CuxO nanopowders show just opposite trend of the optical absorption spectra.
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Fig. 5. Transmittance spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders.
(DEg ¼0.07 eV) during Cu is increased from 0.03 to 0.05. The noticed slight red shift in the band gap is due to Cu doping and hydrogenation effect in Zn–Co–O lattice. The similar narrowing of band gap was observed by Diouri et al. in Cu doped ZnO. It is explained by p–d spin-exchange interactions between the band electrons and localized d electrons of the transition-metal ion substituting Cu2 þ ion [29]. The reduction of band gap by Cu doping is also due to strong p–d mixing of O and Cu [30]. This is also in good agreement to the quantum confinement effect of the nanoparticles [31]. 3.5. FTIR studies
Fig. 6. A plot between hg and (ahg)2 of un-hydrogenated and hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders.
The nature of transition, direct or indirect, is determined by the relation [28], Absorption co-efficient ðpÞ ¼
AðhgEg Þn hg
ð3Þ
where, a is the absorption co-efficient, hg is the incident photon energy, A is a constant, Eg is the optical band gap of the material and the exponent n depends upon the type of transition. The values of n for direct allowed, indirect allowed and direct forbidden are 1/2, 2, 3/2, respectively. The energy band gap of the un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x ¼0.03, 0.04 and 0.05 can be obtained by plotting (ahg)2 versus hg and extrapolating the linear portion of the absorption edge to find the intercept with energy axis as shown in Fig. 6. The overall band gap is gradually decreased from 3.17 to 3.08 eV (DEg ¼0.09 eV) when Cu is increased from 0.03 to 0.05 in the Zn0.96 xCo0.04CuxO system. For un-hydrogenated Zn0.96 xCo0.04CuxO, Eg decreased from 3.17 to 3.105 (DEg ¼0.065 eV) meanwhile in the hydrogenated samples, it is decreased from 3.15 to 3.08 eV
FTIR is a technique used to obtain information about the chemical bonding in a material. It is used to identify the elemental constituents of a material. The characteristic peaks exhibited by FTIR spectra of un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04 and 0.05 are as shown in Fig. 7a and b, respectively. The IR frequencies along with vibrational assignments of the unhydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x ¼0.03, 0.04 and 0.05 assigned at room temperature are listed in Table 2. The broad absorption peaks centered at 3467 cm 1 for Zn0.93Co0.04Cu0.03O is attributed to normal polymeric O–H stretching vibration of H2O in Zn–O lattice [32] which is shifted to 3469 cm 1 for Zn0.92Co0.04Cu0.04O and Zn0.91Co0.04Cu0.05O; and 3473 cm 1 for Zn0.91Co0.04Cu0.05O (H). A small absorption peak observed around 2442–2447 cm 1 is due to the existence of CO2 molecule in air [33]. Two principal absorption peaks are observed around 1645–1722 cm 1 and 1387–1384 cm 1, corresponding to the asymmetric and symmetric stretching of carboxyl group (C¼O) [33]. The absorption band at 449 cm 1 represents the stretching mode of Zn–O [34] which is shifted to higher frequency as 452 cm 1 for x¼0.04; 459 cm 1 for x ¼0.05; 457 cm 1 for x¼0.04 (H) and 464 cm 1 for x ¼0.05 (H). No sign of CoO (857, 713 cm 1) [35] or Co3O4 (598 cm 1) [36] cluster formation is visible. The shift of absorption peak corresponding to Zn–O reflects the Zn–O–Co network is perturbed by the existence of Cu in its environment. Therefore, the FTIR results also indicate that Cu is occupying Zn position in Zn–O–Co matrix as observed in XRD measurements.
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Fig. 7. FTIR spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders.
Table 2 IR peaks and their assignments of un-hydrogenated and hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders. Assignments
O–H stretching vibration of H2O CO2 molecule in air Asymmetric stretching of the carboxyl group (C¼ O) Symmetric stretching of the carboxyl group (C¼ O) Stretching mode of Zn–O
Wave number (cm 1) Zn0.93Co0.04Cu0.03O Zn0.93Co0.04Cu0.03O (H)
Zn0.92Co0.04Cu0.04O Zn0.91Co0.04Cu0.05O (H)
Zn0.91Co0.04Cu0.05O Zn0.92Co0.04Cu0.04O (H)
3467 2442 1645
3468 2445 1672
3469 2447 1720
3472 2447 1720
3469 2447 1720
3473 2447 1722
1387
1387
1393
1394
1394
1394
449
452
452
457
459
464
3.6. Photoluminescence (PL) studies Generally, the densities of defects and oxygen vacancies affect significantly the optical properties of oxide nanostructures. ZnO is a well-known optoelectronic material. However, the luminescence properties of Cu and Co co-doped Zn–O have rarely been studied. The correlation between structure and property is investigated by PL spectra. The room temperature photoluminescence spectra of the un-hydrogenated and hydrogenated Cu, Co codoped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x ¼0.03, 0.04 and 0.05 are as shown in Fig. 8a and b, respectively. It is noticed from Fig. 8a that Zn0.93Co0.04Cu0.03O sample exhibits three strong bands. At first, a strong ultra violet (UV) near band edge (NBE) emission centered at 393 nm, second, a weak blue emission band centered at 472 nm, and third, relatively weak green emission band centered at 536 nm. The strong UV band is assigned to the occurrence of free excitons recombination through an exciton–exciton collision process [37]. The intensity of UV band is more or less same but exhibits small red shift from 393 to 403 nm for x ¼0.03 to 0.05 (H). This could mainly due to the s–d and p–d exchange interactions between the band electrons and the localized ‘d’ electrons of Cu2 þ ions substituting Zn2 þ ions [38] which causes a negative and a positive correction to the conduction and valence band, and hence leads to band gap narrowing [39] as noticed in Fig. 6. There is no appreciable band shift for blue band (BB) emission centered at 472 nm, the only difference is the luminescence intensity. The intensity is gradually
increased with Cu doping and reaches maximum for x¼0.05 (H). The origin of BB emission is due to the interstitial position of Zn2 þ ion [40]. Zeng et al. [41] explained this emission by the transition from extended Zni states, which are slightly below the Zni band and valence band. The excited electrons relax to extended Zni states and transit to the valence band with the emission of blue emission. The defect related green band (GB) emission exhibits blue shift from 536 to 529 nm with increase of Cu concentrations. The GB emission for x¼ 0.03 is 536 nm; for x¼0.04 is 533 nm and for x¼0.05 is 529 nm. The luminescence intensity is gradually increased with Cu doping in addition to broadening effect. The origin of green band may be ascribed to the oxygen vacancies and intrinsic defects [42,43]. Vanheusden et al. described this green emission by the transition between photo-excited holes and singly ionized oxygen vacancies [44]. The increased intensity with Cu coping is due to (i) the introduction of Cu ion and (ii) to create some defects in the host ZnO lattice [45]. The increase in particle size and the reduction in micro-strain increase the PL intensity and broadening. Even though the PL intensity is increased, the width of the band is reduced for hydrogenated samples compared with un-hydrogenated samples which are due to the reduction in particle size and the increase of micro-strain. The doping of Cu and hydrogenation effect in the present system is useful to tune the emission wavelength and hence acting as the important candidates for the optoelectronic applications. The change in intensity, the shift in near band edge
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Fig. 8. Room temperature photoluminescence spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders.
(NBE) emission from 393 to 403 nm and a shift in green band (GB) emission from 536 to 529 nm confirm the substitution of Cu into the Zn–O–Co lattice.
4. Conclusions Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The XRD and SEM measurements revealed the substitution of Cu in the Zn–O–Co lattice without changing the hexagonal wurtzite structure. The increase of lattice constants, the average particle size, the slight shift of XRD peaks and the reduction in the band gap indicated that Cu had really doped into the Zn–O–Co lattice. The presence of functional groups and the chemical bonding with Cu and Co are confirmed by FTIR spectra. PL spectra of the Zn0.96 xCo0.04CuxO system described the shift in near band edge (NBE) UV emission from 393 to 403 nm and a shift in green band (GB) emission from 536 to 529 nm which confirms the substitution of Cu and Co into the Zn–O lattice. These results showed that Cu and Co co-doped ZnO nanoparticles are a great candidate for the optoelectronic devices. Hydrogenation effect induces the uniformity in distribution of the particles with reduced size and it improves the crystal quality and optical properties of the samples. References [1] [2] [3] [4] [5] [6] [7] [8]
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Structural, optical, FTIR and photoluminescence properties of Zn0.96 xCo0.04CuxO (x ¼ 0.03, 0.04 and 0.05) nanopowders S. Muthukumaran a,n, R. Gopalakrishnan b a b
PG & Research Department of Physics, H.H. The Rajah’s College, Pudukkottai—622 001 Tamilnadu, India Department of Physics, Oxford Engineering College, Tiruchirapalli—620 009 Tamilnadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2012 Received in revised form 20 April 2012 Accepted 24 April 2012 Available online 11 May 2012
Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO, x ¼ 0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The synthesized samples have been characterized by powder X-ray diffraction, energy dispersive X-ray spectra, UV–Visible spectrophotometer and Fourier transform infrared spectroscopy. The calculated average crystalline size increases from 37.3 to 50.6 nm for un-hydrogenated samples from x ¼ 0.03 to 0.05 and it changes from 29.4 to 34.9 nm for hydrogenated samples. The change in lattice parameters, micro-strain, a small shift of X-ray diffraction peaks towards lower angles and reduction in energy gap reveal the substitution of Cu2 þ ions into Zn–Co–O lattice. The hydrogenation effect reduces the particle size and induces the more uniform distribution of particles than the un-hydrogenated samples which is confirmed by SEM micrographs. Photoluminescence spectra of Zn0.96 xCo0.04CuxO system shows that red shift in near band edge ultraviolet emission from 393 to 403 nm with suppressing intensity and a blue shift in green band emission from 537 to 529 nm with enhancing intensity confirms the substitution of Cu into the Zn–Co–O lattice. & 2012 Elsevier B.V. All rights reserved.
Keywords: Zn0.96 xCo0.04CuxO nanopowder Optical properties SEM FTIR Photoluminescence
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted a great attention due to its enormous applications in optoelectronics and spintronics [1–3]. Particularly, ZnO based DMSs have considerable attraction due to its large band gap, high refractive index and large exciton binding energy [4,5]. It has very attractive intensive research effort due to its unique properties and versatile applications in transparent electronics, hi-tech applications, ultraviolet (UV) light emitters, piezoelectric devices and chemical sensors [6,7]. Doping ZnO with magnetic ions such as Fe, Co, Ni induces magnetic properties due to their possible applications in the field of spintronics [8]. Among these different metallic doping elements Co and Cu are important because, (i) it is a prominent luminescence activator, which can modify the luminescence of ZnO crystals by creating localized impurity levels [9], (ii) it has many physical and chemical properties similar to those of Zn. (iii) It can change microstructure and optical properties of ZnO system [10]. Different physical or chemical methods have been used to prepare ZnO nanoparticles such as thermal decomposition, thermolysis [11], chemical vapour deposition, sol–gel [12], spray
n
Corresponding author. Tel.: þ91 4322 221558; fax: þ91 4322 230490. E-mail address: [email protected] (S. Muthukumaran).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.04.057
pyrolysis, precipitation [13] and co-precipitation [14]. Among these different methods, the co-precipitation is one of the most important methods to prepare nanopowder. Co-precipitation is the name given by analytical chemists to a phenomenon whereby the fractional precipitation of a specified ion in a solution results in precipitation not only of target ion but also of other ions existing side by side in the solution. The enhanced ferromagnetic order was observed in Co and Cu doped ZnO powder where Cu acted as an acceptor [15]. Chakraborti et al. have studied structural and magnetic properties of Zn0.90Co0.10O prepared by the co-precipitation technique [16]. He indicated that itinerant electrons were responsible for the ferromagnetism in Co, Cu co-doped ZnO films. Sato et al. predicted that 3d transition metal atoms of Mn, Fe, Co, and Ni show ferromagnetic ordering in ZnO [17]. Hou et al. observed transition temperature of Zn0.98Cu0.02O was to be about 350 K but was decreased to 320 K with nitrogen doping [18]. The influence of shape and hydrogenation on ferromagnetic properties of Zn0.93Co0.05Cu0.02O nanoparticles at room temperature was demonstrated by Xu et al. [19]. Even though some of the research works have been carried out on Cu and Co co-doped ZnO system [15,19–21], most of the works are on the thin films and comprehensive study of the structural and optical properties of Cu and Co co-doped ZnO nanopowders is still scanty. Therefore, in the present investigation, hydrogenated and unhydrogenated Cu and Co co-doped ZnO (Zn0.96 xCo0.04CuxO, x¼0.03, 0.04 and 0.05) nanopowders have been synthesized by
S. Muthukumaran, R. Gopalakrishnan / Physica B 407 (2012) 3448–3456
co-precipitation method. The effect of Cu substitution and the hydrogenation on its structural, optical, morphological and photoluminescence properties has been studied extensively. Further, the size-dependent properties of the nanoparticles are correlated with band gap.
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2.5. Fourier transform infrared spectroscopy (FTIR) studies The presence of chemical bonding in Cu and Co co-doped ZnO nanopowders was studied by FTIR spectrometer (Model: Perkin Elmer, Make: Spectrum RX I) in the range of 400 to 4000 cm 1. 2.6. Photoluminescence (PL) studies
2. Experimental procedure 2.1. Preparation of Zn0.96 xCo0.04CuxO nanopowders The high purity chemicals (4 99% purity) such as zinc(II)nitrate hexahydrate (Zn(NO3)2 6H2O), copper(II)nitrate trihydrate (Cu(NO3)2 3H2O), cobalt(II)nitrate hexahydrate (Co(NO3)2 6H2O) were used as precursors without further purification. Initially, the appropriate amounts of ethanol and water (1:4 ratio) were mixed systematically using magnetic stirrer. Then the appropriate amounts of zinc nitrate, cobalt nitrate and copper nitrate were dissolved in ethanol solution and kept in magnetic stirrer for 2 h under constant stirring. A separate NaOH solution was prepared by dissolving appropriate amounts of sodium hydroxide in the double distilled water. The pH value of the solution was selected as 4.6 for better precipitation reaction. The prepared NaOH solution was then added drop wise to the initial solution under constant stirring for 2 h at room temperature to produce a white, gelatinous precipitate. The white precipitates were filtered and washed with distilled water many times. The final precipitates were dried in oven at 80 1C for 2 h. The dried precipitates were collected and ground in an agate mortar. Finally, the collected nanopowder was annealed at 500 1C for 2 h under air atmosphere followed by furnace cooling. The same procedure is repeated to the remaining samples synthesized with nominal compositions of Zn0.96 xCo0.04CuxO (x¼0.03, 0.04 and 0.05). Again, a set of three samples (as prepared and without annealing) for x¼0.03, 0.04 and 0.05 were annealed at 500 1C for 2 h under H2 gas atmosphere (represented by (H)) to study the hydrogenation effect.
2.2. X-ray diffraction (XRD) and energy dispersive X-ray (EDX) studies The crystal structure of Zn0.96 xCo0.04CuxO nanopowders were determined by powder X-ray diffraction. XRD patterns were recorded on a RigaKuC/max-2500 diffractometer using CuKa ˚ at 30 kV and 100 mA from 2y ¼ 101 to radiation (l ¼1.5408 A) 801. The topological features and composition of Zn, O, Co and Cu were determined by energy dispersive X-ray spectrometer on K and L lines.
2.3. Scanning electron microscope (SEM) studies The surface morphology of Zn0.96 xCo0.04CuxO nanopowders were studied using a scanning electron microscope (SEM, JEOL JSM 6390).
The photoluminescence (PL) spectrum within the wavelength ranges from 350 to 600 nm were obtained under 325 nm line of Xe laser excitation using a fluorescence spectrophotometer (F-2500).
3. Results and discussion 3.1. XRD—Structural studies The typical XRD diffraction peaks of un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04, 0.05 are shown in Fig. 1a and b, respectively. The pronounced diffraction peaks in the XRD pattern clearly shows the crystalline nature with sharp peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) planes. The standard diffraction peaks shows crystal structure of Zn0.96 xCo0.04CuxO is hexagonal wurtzite structure (space group P63mc, 186, JCPDS data card No. 36-1451) [22] with preferred orientation along (1 0 1) plane in all the samples. This indicates that the simultaneous substitution of Co and Cu cannot disturb the structure of ZnO. No additional peaks (such as Co, CoO, Cu, CuO, Cu2O) were observed in Zn0.96 xCo0.04CuxO, which indicates no impurity exist in the samples. Even though, there are no secondary phases detected by XRD analysis, the existence of secondary phases cannot be completely excluded due to limitation of this characterization technique [23]. It is noticed from Fig. 1a and b that the peak intensity increases with Cu concentration, which means that the crystalline quality is improved with Cu doping and also Cu2 þ ion is understood to have occupied Zn2 þ without changing crystal structure. Wei et al. reported the same trend, the increase of diffraction peak intensity with Cu doping [15]. Furthermore, the inset of Fig. 1a and b clearly shows diffraction peak along (1 0 1) plane is shifted towards the lower diffraction angle gradually with addition of Cu concentrations. The small shift of diffraction peak (D2y ¼0.0331 for un-hydrogenated samples and D2y ¼0.0341 for hydrogenated samples) towards the lower angle and increase of peak intensity indicates that Cu2 þ ions are doped into Zn–Co–O crystal lattice successfully in the position of Zn2 þ ions and also the crystal lattice has no obvious change by Cu doping. The average crystal size of the samples is calculated from the broadening of diffraction peaks of (1 0 1) plane using Debye Scherrer’s formula, Average crystal size ðDÞ ¼
ð1Þ
˚ b is the where, l is the wave length of X-ray used (1.5408 A), angular peak width at half maximum in radian along (1 0 1) plane, and y is the Bragg0 s diffraction angle. The micro-strain (e) can be calculated using the formula [24],
2.4. UV–Visible optical absorption studies Micro-strain ðeÞ ¼ The UV–Visible optical absorption study of samples was carried out with a view to explore their optical properties. The spectral absorption was determined using UV–Visible spectrometer (Model: lambda 35, Make: Perkin Elmer) in the wavelength ranging from 350 to 650 nm at room temperature.
0:9 l b cos y
b cos y 4
ð2Þ
Table 1 shows 2y value, full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D) and micro-strain (e) of un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders.
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Fig. 1. Powder X-ray diffraction pattern of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders at room temperature. The inset shows the shift of diffraction peaks along (1 0 1) plane towards the lower diffraction angle.
The resultant compound maintains a hexagonal wurtzite structure, but the d-value and the lattice parameters a and c are slightly increased with Cu doping. The increase of lattice constants is consistent with the earlier literature [15]. The change in lattice parameters could be well understood by the substitution of Cu2 þ ˚ which are larger than that of Zn2 þ ions ions (ionic radius¼0.73 A) ˚ (ionic radius¼0.60 A) [15] in their tetrahedral coordinates. The b values constantly decreases from 0.1721 to 0.1251 for un-hydrogenated samples and 0.2841 to 0.2391 for hydrogenated samples from x¼ 0.03 to 0.05. Mean while, the ‘2y’ values gradually decreases from 36.2461 to 36.1961 for un-hydrogenated samples and 36.231 to 36.1961 for hydrogenated samples from x¼0.03 to 0.5. The change in b values inferred that the average crystal size is increased from x¼0.03 to 0.05 and the hydrogenated samples have lower crystal size than the un-hydrogenated samples. The calculated D value from Eq. (1) ascribed the increase of average crystal size from 37.3 to 50.6 nm (37.3 nm for x¼0.03; 38.7 nm for x¼0.04 and 50.6 nm for x¼ 0.05) for un-hydrogenated samples, 0.03rxZ0.05. For hydrogenated samples, it is increased from 29.4 to 34.9 nm which is lower than the un-hydrogenated samples. It clearly shows the existence of nano-sized particles in the samples. It is observed from Table 1 that micro-strain (e) of un-hydrogenated samples are decreased from x¼ 0.03 (0.562 10 3) to 0.05 (0.401 10 3). The similar decreasing trend was noticed for the hydrogenated samples. The variation in micro-strain is due to the change in microstructure, size and shape of the particles. The atoms trapped in the non-equilibrium position could shift to a more equilibrium position, and it could release the strain. Hence, it decreases from x¼0.03 to 0.05. The decrease of micro-strain is caused by enhancing crystal size and reducing FWHM value as noticed in Table 1. The observed decrease in line broadening (b) for both hydrogenated and un-hydrogenated samples may be due to the size or micro-strain or both [25]. Meanwhile, the crystal quality
of both samples was constantly increased by Cu doping. It is evident that the decrease of strain causes the increase of crystal size and reduction in the broadening whereas increase of strain causes reduction of crystal size and increase of broadening. 3.2. Energy dispersive X-ray (EDX) studies The precise composition of Cu and Co co-doped ZnO nanopowders was determined by an EDX study, and it revealed the presence of Zn, Co, Cu, and O as elementary components. The typical EDX spectra of un-hydrogenated and hydrogenated samples for Zn0.96 xCo0.04CuxO, x ¼0.03 and 0.05 are as shown in Fig. 2a and b, respectively. The inset of Fig. 2a and b shows the weight percentage of Zn, O, Co and Cu elements. It clearly shows that the intensity of Cu increases with the addition of Cu concentrations. Therefore, the addition of Cu induces a dominant effect on the optical, structural and morphological properties of ZnO. The EDX analysis confirms presence of Cu in ZnO system and wt% is very nearly equal to their nominal stoichiometry within the experimental error. It is observed from Fig. 2a (un-hydrogenated samples) that the Cu/(Zn þCu) ratio is derived to be 3.03% and 5.09% for 3% and 5% of Cu doping. For hydrogenated samples (Fig. 2b), the Cu/(ZnþCu) ratio is found to be 2.98% and 5.05% for 3% and 5% of Cu doping. Therefore, the EDX spectra show a well agreement with the experimental concentration used for Zn0.96 xCo0.04CuxO system. 3.3. Scanning electron microscope (SEM)—Microstructural studies SEM is one of the promising techniques for the topography study of the samples and it gives important information regarding shape and size of the particles. The surface morphology of the unhydrogenated and hydrogenated samples for Zn0.96 xCo0.04CuxO,
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Table 1 The 2y value, FWHM value, d-value, cell parameters a and c, c/a ratio, average crystal size (D) and micro-strain (e) of un-hydrogenated and hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders. Samples
2y value (1)
FWHM (b) (1)
˚ d-value (A)
˚ Cell parameter (A)
c/a ratio
Average crystal size (D) (nm)
Micro-strain (e) (10 3)
–
–
–
–
a
c
–
–
–
Zn0.93Co0.04Cu0.03O Zn0.93Co0.04Cu0.03O (H) Zn0.92Co0.04Cu0.04O Zn0.92Co0.04Cu0.04 (H) Zn0.91Co0.04Cu0.05O Zn0.91Co0.04Cu0.05O (H)
36.246 36.230 36.238 36.203 36.213 36.196
0.172 0.284 0.152 0.251 0.125 0.239
2.4766 2.4778 2.4772 2.4795 2.4788 2.4800
3.2513 3.2519 3.2518 3.2524 3.2528 3.2531
5.2081 5.2085 5.2088 5.2087 5.2096 5.2102
1.6018 1.6017 1.6018 1.6015 1.6015 1.6016
37.3 29.4 38.7 33.3 50.6 34.9
0.731 0.920 0.494 0.803 0.401 0.761
Fig. 2. Energy dispersive X-ray (EDX) spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03 and 0.05 nanopowders at room temperature.
x ¼0.03 and 0.05 are as shown in Fig. 3a and b, respectively. The SEM micrographs clearly show the average size of nanoparticles is in the order of nanometer size. The SEM images from Fig. 3a and b clearly show that the particles are uniformly distributed over the surface with good connectivity containing dominant deformed spheroid-like particles. The particle size was slightly increased with Cu concentrations. The average particle size of Zn0.93Co0.04Cu0.03O sample varies between 15 and 40 nm whereas Zn0.91Co0.04Cu0.05O sample has slightly larger particle size around 20 to 50 nm. The increase of Cu doped concentrations causes more defects and deformed lattice structures. The distribution of particles in the hydrogenated samples (Fig. 3b) is more uniform than the un-hydrogenated
samples. It consists of more dense and tightly aggregated spheroidlike particles with the average particle size from 15 to 35 nm. The observed improved crystalline structure and the reduction in the particle size in Fig. 3b are due to the hydrogenation effect which can induce ferromagnetic spin–spin interaction between neighbouring magnetic ions through the formation of a bridge bond [26,5]. A good correlation is found to exist between mathematical calculations from XRD analysis and the SEM studies. 3.4. Optical study UV–Visible absorption spectroscopy is a powerful technique to explore optical properties of semiconducting nanoparticle. The
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Fig. 3. Scanning electron microscope (SEM) images of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03 and 0.05 nanopowders at room temperature.
Fig. 4. UV–Visible absorption spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders.
UV–Visible optical absorption spectrum of un-hydrogenated and the hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04 and 0.05 have been carried out at room temperature using UV–Visible spectrometer (Model: Lambda 35, Make: Perkin Elmer) from 350 to 650 nm. Fig. 4a and b shows the optical absorption spectra of un-hydrogenated and the hydrogenated Zn0.96 xCo0.04CuxO samples, respectively. The absorption spectra show that the absorption of the un-hydrogenated and hydrogenated samples is increased with increasing Cu concentrations. The inset of Fig. 4a and b shows the clear picture of absorption changes around 472 nm. The absorption edge of Zn0.93Co0.04Cu0.03O sample is found to be 393 nm which is shifted towards the higher wavelengths (red shift) with increasing Cu concentrations as Zn0.92Co0.04Cu0.04O sample has the absorption edge around 398 nm; Zn0.91Co0.04Cu0.05O
sample has the edge around 401 nm. In the case of hydrogenated samples, the absorption edge is shifted from 396 nm (x¼0.03) to 403 nm (x¼0.05) which is slightly higher than the un-hydrogenated samples. The change in absorption edge in both cases can be attributed to the photo-excitation of electrons from valence band to conduction band. The same red shift by Cu doping was noticed in the literature [25]. The red shift phenomenon can be explained by the Burstein–Moss band gap widening and gap narrowing due to electron–electron and electron-impurity scattering [27]. The typical room temperature transmittance spectra of the un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04 and 0.05 are shown in Fig. 5a and b, respectively. The transmission spectra of Zn0.96 xCo0.04CuxO nanopowders show just opposite trend of the optical absorption spectra.
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Fig. 5. Transmittance spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders.
(DEg ¼0.07 eV) during Cu is increased from 0.03 to 0.05. The noticed slight red shift in the band gap is due to Cu doping and hydrogenation effect in Zn–Co–O lattice. The similar narrowing of band gap was observed by Diouri et al. in Cu doped ZnO. It is explained by p–d spin-exchange interactions between the band electrons and localized d electrons of the transition-metal ion substituting Cu2 þ ion [29]. The reduction of band gap by Cu doping is also due to strong p–d mixing of O and Cu [30]. This is also in good agreement to the quantum confinement effect of the nanoparticles [31]. 3.5. FTIR studies
Fig. 6. A plot between hg and (ahg)2 of un-hydrogenated and hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders.
The nature of transition, direct or indirect, is determined by the relation [28], Absorption co-efficient ðpÞ ¼
AðhgEg Þn hg
ð3Þ
where, a is the absorption co-efficient, hg is the incident photon energy, A is a constant, Eg is the optical band gap of the material and the exponent n depends upon the type of transition. The values of n for direct allowed, indirect allowed and direct forbidden are 1/2, 2, 3/2, respectively. The energy band gap of the un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x ¼0.03, 0.04 and 0.05 can be obtained by plotting (ahg)2 versus hg and extrapolating the linear portion of the absorption edge to find the intercept with energy axis as shown in Fig. 6. The overall band gap is gradually decreased from 3.17 to 3.08 eV (DEg ¼0.09 eV) when Cu is increased from 0.03 to 0.05 in the Zn0.96 xCo0.04CuxO system. For un-hydrogenated Zn0.96 xCo0.04CuxO, Eg decreased from 3.17 to 3.105 (DEg ¼0.065 eV) meanwhile in the hydrogenated samples, it is decreased from 3.15 to 3.08 eV
FTIR is a technique used to obtain information about the chemical bonding in a material. It is used to identify the elemental constituents of a material. The characteristic peaks exhibited by FTIR spectra of un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x¼0.03, 0.04 and 0.05 are as shown in Fig. 7a and b, respectively. The IR frequencies along with vibrational assignments of the unhydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x ¼0.03, 0.04 and 0.05 assigned at room temperature are listed in Table 2. The broad absorption peaks centered at 3467 cm 1 for Zn0.93Co0.04Cu0.03O is attributed to normal polymeric O–H stretching vibration of H2O in Zn–O lattice [32] which is shifted to 3469 cm 1 for Zn0.92Co0.04Cu0.04O and Zn0.91Co0.04Cu0.05O; and 3473 cm 1 for Zn0.91Co0.04Cu0.05O (H). A small absorption peak observed around 2442–2447 cm 1 is due to the existence of CO2 molecule in air [33]. Two principal absorption peaks are observed around 1645–1722 cm 1 and 1387–1384 cm 1, corresponding to the asymmetric and symmetric stretching of carboxyl group (C¼O) [33]. The absorption band at 449 cm 1 represents the stretching mode of Zn–O [34] which is shifted to higher frequency as 452 cm 1 for x¼0.04; 459 cm 1 for x ¼0.05; 457 cm 1 for x¼0.04 (H) and 464 cm 1 for x ¼0.05 (H). No sign of CoO (857, 713 cm 1) [35] or Co3O4 (598 cm 1) [36] cluster formation is visible. The shift of absorption peak corresponding to Zn–O reflects the Zn–O–Co network is perturbed by the existence of Cu in its environment. Therefore, the FTIR results also indicate that Cu is occupying Zn position in Zn–O–Co matrix as observed in XRD measurements.
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Fig. 7. FTIR spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x¼ 0.03, 0.04, 0.05 nanopowders.
Table 2 IR peaks and their assignments of un-hydrogenated and hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders. Assignments
O–H stretching vibration of H2O CO2 molecule in air Asymmetric stretching of the carboxyl group (C¼ O) Symmetric stretching of the carboxyl group (C¼ O) Stretching mode of Zn–O
Wave number (cm 1) Zn0.93Co0.04Cu0.03O Zn0.93Co0.04Cu0.03O (H)
Zn0.92Co0.04Cu0.04O Zn0.91Co0.04Cu0.05O (H)
Zn0.91Co0.04Cu0.05O Zn0.92Co0.04Cu0.04O (H)
3467 2442 1645
3468 2445 1672
3469 2447 1720
3472 2447 1720
3469 2447 1720
3473 2447 1722
1387
1387
1393
1394
1394
1394
449
452
452
457
459
464
3.6. Photoluminescence (PL) studies Generally, the densities of defects and oxygen vacancies affect significantly the optical properties of oxide nanostructures. ZnO is a well-known optoelectronic material. However, the luminescence properties of Cu and Co co-doped Zn–O have rarely been studied. The correlation between structure and property is investigated by PL spectra. The room temperature photoluminescence spectra of the un-hydrogenated and hydrogenated Cu, Co codoped ZnO (Zn0.96 xCo0.04CuxO) nanopowders with x ¼0.03, 0.04 and 0.05 are as shown in Fig. 8a and b, respectively. It is noticed from Fig. 8a that Zn0.93Co0.04Cu0.03O sample exhibits three strong bands. At first, a strong ultra violet (UV) near band edge (NBE) emission centered at 393 nm, second, a weak blue emission band centered at 472 nm, and third, relatively weak green emission band centered at 536 nm. The strong UV band is assigned to the occurrence of free excitons recombination through an exciton–exciton collision process [37]. The intensity of UV band is more or less same but exhibits small red shift from 393 to 403 nm for x ¼0.03 to 0.05 (H). This could mainly due to the s–d and p–d exchange interactions between the band electrons and the localized ‘d’ electrons of Cu2 þ ions substituting Zn2 þ ions [38] which causes a negative and a positive correction to the conduction and valence band, and hence leads to band gap narrowing [39] as noticed in Fig. 6. There is no appreciable band shift for blue band (BB) emission centered at 472 nm, the only difference is the luminescence intensity. The intensity is gradually
increased with Cu doping and reaches maximum for x¼0.05 (H). The origin of BB emission is due to the interstitial position of Zn2 þ ion [40]. Zeng et al. [41] explained this emission by the transition from extended Zni states, which are slightly below the Zni band and valence band. The excited electrons relax to extended Zni states and transit to the valence band with the emission of blue emission. The defect related green band (GB) emission exhibits blue shift from 536 to 529 nm with increase of Cu concentrations. The GB emission for x¼ 0.03 is 536 nm; for x¼0.04 is 533 nm and for x¼0.05 is 529 nm. The luminescence intensity is gradually increased with Cu doping in addition to broadening effect. The origin of green band may be ascribed to the oxygen vacancies and intrinsic defects [42,43]. Vanheusden et al. described this green emission by the transition between photo-excited holes and singly ionized oxygen vacancies [44]. The increased intensity with Cu coping is due to (i) the introduction of Cu ion and (ii) to create some defects in the host ZnO lattice [45]. The increase in particle size and the reduction in micro-strain increase the PL intensity and broadening. Even though the PL intensity is increased, the width of the band is reduced for hydrogenated samples compared with un-hydrogenated samples which are due to the reduction in particle size and the increase of micro-strain. The doping of Cu and hydrogenation effect in the present system is useful to tune the emission wavelength and hence acting as the important candidates for the optoelectronic applications. The change in intensity, the shift in near band edge
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Fig. 8. Room temperature photoluminescence spectra of (a) un-hydrogenated and (b) hydrogenated Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04, 0.05 nanopowders.
(NBE) emission from 393 to 403 nm and a shift in green band (GB) emission from 536 to 529 nm confirm the substitution of Cu into the Zn–O–Co lattice.
4. Conclusions Un-hydrogenated and hydrogenated Cu, Co co-doped ZnO (Zn0.96 xCo0.04CuxO, x ¼0.03, 0.04 and 0.05) nanopowders have been synthesized by co-precipitation method. The XRD and SEM measurements revealed the substitution of Cu in the Zn–O–Co lattice without changing the hexagonal wurtzite structure. The increase of lattice constants, the average particle size, the slight shift of XRD peaks and the reduction in the band gap indicated that Cu had really doped into the Zn–O–Co lattice. The presence of functional groups and the chemical bonding with Cu and Co are confirmed by FTIR spectra. PL spectra of the Zn0.96 xCo0.04CuxO system described the shift in near band edge (NBE) UV emission from 393 to 403 nm and a shift in green band (GB) emission from 536 to 529 nm which confirms the substitution of Cu and Co into the Zn–O lattice. These results showed that Cu and Co co-doped ZnO nanoparticles are a great candidate for the optoelectronic devices. Hydrogenation effect induces the uniformity in distribution of the particles with reduced size and it improves the crystal quality and optical properties of the samples. References [1] [2] [3] [4] [5] [6] [7] [8]
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