J Sol-Gel Sci Technol (2015) 74:756–764 DOI 10.1007/s10971-015-3660-1
ORIGINAL PAPER
Experimental and theoretical investigations of structural and optical properties of copper doped ZnO nanorods Mohua Chakraborty • Anima Ghosh R. Thangavel
•
Received: 8 September 2014 / Accepted: 9 February 2015 / Published online: 21 February 2015 Springer Science+Business Media New York 2015
Abstract We report on structural and optical properties of undoped and Cu-doped ZnO nanorods on glass substrates by sol–gel and hydrothermal method. The grown thin film nanorods samples were characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM) and UV visible spectroscopy. The XRD and FESEM reveal that grown nanorods have hexagonal wurtzite structure along the c-axis. The UV–Vis absorption spectra for all samples displayed a band gap absorption peak at about 365 nm. The band gap value has decreased since 5 and 10 % doped ZnO indicates a red shift in comparison with undoped ZnO and 15 and 20 % doping concentration shows blue shift compared with lower doping. However, with doping, a decrease in the band gap value of undoped sample was observed by indicating a red shift of fundamental absorption edge. Photoluminescence spectra of the nanorod arrays samples show that a higher concentration of Cu exhibited a 61.82 % increased IUV/ IDLE ratio compared with undoped samples. Oxygen insufficiencies were reduced with doped samples. Cu doping diminishes the recombination of electrons and holes by bonding with electrons in singly ionized oxygen vacancies. As a result, green emission was abridged, and the optical properties have improved with Cu incorporated
in host lattice. The observed experimental band gap value was tuned from 3.21 eV (ZnO) to 3.07 eV (Zn0.9Cu0.1O), which was closer to the theoretical band gap values confirmed by first principle calculations that combined generalized gradient approximation with included scissor operator. Graphical abstract • • •
•
The undoped and doped ZnO nanorods were grown by sol–gel and hydrothermal synthesis method. The grown nanorods were vertically well aligned and very dense hexagonal array. Our experimental and simulation studies revealed red shift in absorption edge and the band gap narrowing with Cu doping. PL spectra depicted ZnO nanorods would be reasonably useful for UV emission device and PEC devices.
M. Chakraborty A. Ghosh R. Thangavel (&) Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India e-mail:
[email protected];
[email protected] A. Ghosh Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, USA
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Keywords Sol–gel Hydrothermal Wurtzite structure X-ray diffraction Optical properties
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1 Introduction
2 Experimental and computational details
In recent years, a new-generation multifunctional semiconductor material, zinc oxide (ZnO) thin film and micro-/ nanostructures, has received extensive attention. ZnO is II– VI semiconductor with wide band gap (3.37 eV) [1] and has a large exciton binding energy of 60 meV at room temperature [1]. The tunable optical and electronic properties of ZnO provide different potential applications in various fields [2–6]. Among a wide variety of nanostructures, such as nanoflowers, nanobelts, nanorods, nanodots, nanocages and nanoribbons, the rods show unique advantage because of having a high surface-to-volume ratio and, consequently, endows with a large surface area [7, 8]. Various methods have been employed for growing ZnO nanorods as electrodeposition [9], pulsed laser deposition [10], chemical vapor deposition [11], spray pyrolysis [12] and hydrothermal deposition [13, 14]. Among all these methods hydrothermal method is the simplest, most inexpensive and most efficient method for the synthesis of ZnO nanorods. In order to enhance the function of ZnO nanorod-based optoelectronic device, the performance of the device should be adjusted by controlling the optical and electronic properties of ZnO nanorods. In search of improved optical and electronic properties, transition material (TM)-doped ZnO nanorods have emerged as an attractive candidate from both a theoretical [15] and experimental point of view [16–20]. The doping of metal ions in ZnO nanorods can lead to the effects such as an enhancement/decrease in fluorescence and influence the concentration of surface defects. Iqbal et al. [21] reported Ni-doped ZnO nanorod powders exhibited strong green emission in visible luminescence, which is closely associated with oxygen-vacancy defects over undoped ZnO nanorods. Rajamanickam et al. [22] reported a red shift in the UV emission and introduced a high concentration of oxygen defect in Mn-doped ZnO nanorods over undoped ZnO nanorods. One particular TM dopant that can encourage several interests in ZnO nanorods is copper (Cu) because of the different structure of the electronic shell and the similar size of Cu and Zn [23, 24]. Recently, Iqbal et al. [25] reported the hydrothermal synthesis of Cu-doped ZnO nanorod powder samples and characterized intensity ratio of UV to visible peak (IUV/IVL) for Cu-doped ZnO nanorods is higher than that of undoped ZnO nanowires, which shows an improvement in the crystal band and luminescence property. In this work, we synthesize Cu-doped ZnO nanorod thin films on glass substrates by a hydrothermal method. The properties of nanorods are investigated by XRD, FE-SEM and UV–Visible-NIR spectroscopy. In addition to that, the optical properties are calculated by using FP-LAPW (Wien2 K).
2.1 Preparation of ZnO nanorods Hydrothermal route was used to synthesize ZnO nanorod arrays as reported by several researchers. Before hydrothermal deposition, the normal glass substrates were cleaned by an ultrasonic cleaner with aqua regia (HNO3: HCl: 1:3), Acetone, Ethanol for each 20 min, then cleaned with deionized water and dried. The precursor solution was synthesized according to literature procedure [26, 27]. The starting material, zinc acetate dihydrate, was first dissolved in the solvent 2-methoxyethanol with stirring for complete dissolution. Then, monoethanolamine (MEA) was dropped into the mixture solution as a stabilizer. The molar ratio of MEA to zinc acetate was 1:1, and the concentration of zinc acetate was 0.5 M. Then, the resulting mixture was stirred at 60 C for 2 h until a clear and transparent homogeneous solution was formed. Then, the solution was spin coated at a rate of 3000 rpm for 20 s using a spin coater. The as-deposited thin film was heated in an oven at 120 C for 10 min to remove the solvent. After repeating the spin coating and drying procedures for five times to yield the required thickness, the resulting thin film was annealed at 400 C in the furnace for 1 h to obtain the ZnO seed layer. After the formation of seed layer, ZnO nanorod arrays were formed of them through hydrothermal method. The substrates with a seed layer were put upside down in a glass beaker filled with the aqueous solution of 50 mM zinc nitrate hexahydrate and 50 mM hexamethylenetetramine (HMT), sealed, heated at 90 C for 5 h, then cleaned with distilled water and dried. 2.2 Preparation of Cu-doped ZnO nanorods Cu-doped ZnO nanorods were prepared by dissolving copper nitrate trihydrate in aqueous solution of zinc nitrate hexahydrate and HMT at room temperature. The amount of Cu agent was varied from 2.5 to 10 mM, which corresponds to 5 to 20 % in molarity maintained in the solution. After the preparation of the four group solutions with different Cu content, the substrates with the seed layers were put into the above solution, sealed, then heated at 90 C for 5 h, cleaned with distilled water and dried. 2.3 Characterization The grown samples were investigated by X-ray diffractometer (XRD) (Bruker D8 Advance Diffractometer) ˚ ). using monochromatic Cu-Ka1radiation (k = 1.5406 A The surface morphology of the sample was observed using a ZEISS Supra 55 field emission scanning electron microscope (FESEM). Elemental analysis was observed by EDAX. The
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optical band gap of the sample was characterized from their absorption spectra using Agilent Cary 5000 UV-Vis-NIR double beam spectrophotometer. Photoluminescence is carried out by using Hitachi f-2500 spectrophotometer. 2.4 Computational details In addition to above experimental investigations, first principle calculations were also performed to evaluate and compare the optical properties of Cu-doped ZnO. The calculations have been performed within the framework of density functional theory (DFT) with the generalized gradient approximation GGA and GGA ? U methods using WIEN2 k package [28]. Exchange and correlation effects were treated within Perdew-Bruke-Ernzerhoff (PBE) [29] of the generalized gradient approximation (GGA) and the local density approximation LDA [30]. The values of Kmax 9 Rmax = 7.0 and lmax = 10 were kept constant throughout the calculations. The charge density was Fourier expanded with Gmax = 12. A 3 9 3 9 1 division for k-point sampling was used, and the tetrahedral method [31] was employed for the Brillouin zone integrations. The calculations were iterated until the total energies were converged below 10-5 Ry with respect to Brillouin zone integration. A supercell of 40 atoms was generated for performing this Cu-doped ZnO.
3 Results and discussion 3.1 The structural properties Figure 1 depicts the XRD pattern of Cu-doped ZnO nanorod samples (a) 0 % (b) 5 % (c) 10 % (d) 15 % (e)
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20 % of Cu. The crystallographic characteristics of these nanorods demonstrated the hexagonal wurtzite structures. The highest intensity of the XRD peak is (002) and verifies the nanorods growth preferential orientation on the c-axis (JCPDS 36-1451). The formation of Cu2O, CuO or Cu phases was not detected within the sensitivity of the XRD, which implies that Cu atoms may replace Zn atomic sites substitutionally or incorporate interstitially in the hexagonal lattices. Cu can exist in different valence states with Cu?, Cu2? in Cu ion-doped ZnO nanorod arrays [19, 32– 34]. The full width at half maximum (FWHM) of the (002) plane in all the films was found to increase from 0.346 to 0.529 with the increase in doping concentration. XRD pattern for all the samples was fitted using internal standard program XRDA 3.1. The fitted lattice parameters are given in the Table 1. The lattice parameters ‘a’ and ‘c’ have been calculated using equation as follows [35] Sin2 h ¼
k2 4a2
2 2 4 2 l a h þ hk þ k2 þ 3 c2
where k = wavelength of Cu-Ka1 radiation and ‘h’ is the Bragg diffraction angle of the XRD peak. The calculated lattice parameter, unit cell volume, crystallite size and stress are shown in Table 1. From the table, we found that the variation in lattice parameters with increasing Cu concentration is found to be larger as compared to those of undoped ZnO as shown in Fig. 2. Lattice parameters were increased, which may be due to incorporation of Cu? ion ˚ ) is into ZnO matrix. The ionic radius of Cu? ion (0.96 A 2? ˚ larger compared with Zn ion (0.74 A), and doping concentration is very high; consequently, Cu? ion could not replace by host ion, should be placed at interstitial position of ZnO lattice. The c/a parameter has been found to be close the theoretical value of 1.6 for ideally close-packed hexagonal structure. The volume of unit cell for hexagonal system has been calculated from the following equation [36] pffiffiffi 3 V¼ a2 c 2 The average nanocrystalline size was calculated using Debye–Scherrer’s formula [36, 37], D¼
Fig. 1 XRD patterns of the undoped and Cu-doped ZnO nanorods; inset shows (002) peak in closer view
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kk bhkl Cosh
where D = crystallite size, k = shape factor (0.9), and bhkl is the FWHM. Variation in crystallite size with Cu concentration is shown in Fig. 3, and the values are listed in Table 1. The incorporation of Cu ion into ZnO matrix can induce intrinsic strain and defect in the lattice. The stress of the ZnO thin films was estimated using the following formula [38]
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Table 1 Various parameters calculated from XRD and UV–Vis techniques. Theoretical/simulation results are also summarized here Copper content (M %)
2h at (100) plane ()
0
32.317
5 10
2h at (002) plane ()
Lattice parameter
Crystallite size D (nm)
r (N/m2)
Volume ˚ 3) (A
˚) a (A
c/a
35.066
3.1962
1.5999
24.08
-7.939 9 109
32.305
35.047
3.1972
1.6003
18.72
-7.695 9 109
32.294
35.043
3.1988
1.5997
18.85
Band gap (eV) Experimental
Theoretical with Scissor correction
45.2416
3.216
3.20
45.2947
3.101
3.12
-7.642 9 109
45.3453
3.076
3.10
9
15
32.246
34.990
3.2028
1.6003
15.65
-7.006 9 10
45.5236
3.171
3.13
20
32.184
34.952
3.2089
1.5986
15.74
-6.536 9 109
45.7454
3.191
3.15
where C is the lattice constant obtained from the (002) ˚ for bulk ZnO [38]. This diffraction peak and C0 is 5.205 A lower value of stress followed by increased volume due to the high value of lattice parameters indicates that the nature of the stress is compressive. Here, compressive stress decreases with an increase in Cu concentration in ZnO lattice. The induced stress suppressed the preferential orientation of the nanorods, which can be observed from XRD pattern and FESEM images as well. 3.2 Surface morphology
Fig. 2 Variation in lattice parameter for undoped and Cu-doped ZnO nanorods
Figure 4a–e shows FESEM images that confirmed the growth of highly oriented undoped and Cu-doped ZnO nanorods grown on glass substrates. The inset in Fig. 4a–e represents the top view of ZnO nanorods. The grown nanorods are vertically well aligned and very dense hexagonal array. The average diameter of the nanorods for all samples can be seen from the figure which was approximately 100–200 nm. The nanorods are exhibiting hexagonal surfaces and facets throughout their lengths which confirm that the nanorods are well-crystalline and possessing wurtzite hexagonal phase. FESEM images revealed that increased doping concentration had not affected the shapes of the nanorods, but area of the nanorods could have increased. EDX analysis was performed as shown in Fig. 5a–e. It is confirmed from the EDX analysis that the grown nanorods are composed of copper, zinc and oxygen only. 3.3 Optical properties
Fig. 3 Variation in crystallite size for undoped and Cu-doped ZnO nanorods
N 9 C0 C r 2 ¼ 453:6 10 m C0
3.3.1 UV-Vis Spectroscopy Figure 6 shows optical absorption spectra of undoped and Cu-doped ZnO nanorods. Absorption band edge was observed at 393 nm for undoped ZnO nanorod; Cu doping shows that the absorption edge shifts to a long wavelength.
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Fig. 4 FESEM images of a undoped, b 5 %, c 10 %, d 15 %, e 20 % Cu-doped ZnO nanorods; Inset in (a–e) represents top view of undoped and doped ZnO nanorods
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The absorption edge was blue shifted with further increment in Cu concentration to 15 and 20 %. This demonstrates the doping-induced first red shift and then a blue shift in the absorption edge of ZnO host lattice. Such a shift in band edge with increasing copper content is a clear indication for the incorporation of Cu ions into the Zn site of the ZnO matrix. Band gap energy can be calculated by mathematical treatment of the optical absorbance data with Stern equation [39, 40], n k ht Eg 2 A¼ ht Above equation yields 2 ðAhtÞn ¼ k0 ht Eg where A is the absorbance, m is the light constant in every unit of wavelength, h is Planck’s constant, Eg band gap energy and k0 , k, n = constant. The band gap energy (Eg) can be obtained by extrapolating the straight line in the Tauc’s plot of (Ahm)2/n as the function of hm to the baseline where (Ahm)2/n = 0. As the result, the Eg equals to hm. The n value is 1 for direct-gap semiconductors and four for indirect-gap semiconductors. The values of band gap were calculated by extrapolation of the straight line in the plot of (Ahm)2 versus the photon energy for all undoped and doped ZnO nanorods. The experimental and simulated results of the absorption spectra are shown in Fig. 7a, b, respectively. The simulation studies confirm the experimental observations of the optical band gap. The moderate decrease and increase in band gap confirms the Cu ion substitution in the ZnO structure. Here, a remarkable variation in the band gap is observed with lower and higher increment in Cu doping level (Table 1). The observed band gap of the Cu-doped ZnO nanorods was 3.10 eV for 5 %, 3.07 for 10 %, 3.17 for 15 % and 3.19 for 20 % doping, respectively. However, with Cu doping, a decrease in the band gap of pure ZnO is observed, indicating a red shift of fundamental absorption edge. The observed red shift can be attributed to the new ion (Cu?) energy level formed in the band gap, thereby decreasing the ZnO band gap. The blue shift in optical band gap with Cu doping can be described by Burstein– Moss effect. According to the Burstein–Moss effect, with the increase in doping level, the band gap will be blueshifted due to the increase in carrier concentration, which is donated by interstitial zinc atoms or oxygen vacancies at room temperature [41]. The photoluminescence study confirms increase in interstitial Zn atoms with increase in Cu doping level, which can enhance the Burstein–Moss shift. The band gap can be varied back and forth, and the absorption spectra show both red and blue shift, due to the complicated variations in interstitial zinc vacancies and oxygen vacancies with the increase in Cu doping.
Fig. 5 EDAX of a undoped, b 5 %, c 10 %, d 15 %, e 20 % Cudoped ZnO nanorods
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3.3.2 Photoluminescence
Fig. 6 Absorption spectra of pure and Cu-doped ZnO nanorods at different concentrations
Fig. 7 a Tauc’s plots of Cu-doped ZnO. b WIEN2 K calculations— Tauc’s plot of Cu-doped ZnO with Scissor correction
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The room temperature PL spectra on both the undoped and Cu-doped ZnO nanorod samples with excitation at a wavelength of 320 nm are depicted in Fig. 8a. The figure shows characteristic UV emission peak at 381, 396 nm, a weak blue band at 452 nm, a strong blue band at 470 nm, a weak blue–green emission at 492 nm and a broad emission of 500–650 nm. The UV emission at 381 nm is attributed to the near band edge emission from the recombination of free excitons through an exciton–exciton collision process [42]. The peak at 396 nm is usually attributed to surface states (VZn) or band tail states in ZnO [43]. The range of wavelength 450–650 nm deep level visible emission region (DLE) is usually recognized to intrinsic defects such as VZn, Zni, and VO, Oi, as well as extrinsic impurities [44]. The surface defect in ZnO nanowire may be one of the causes for weak blue (452 nm) and weak blue–green (492 nm) emission [45]. The strong blue band at 470 nm occurred when the electron transits from the level of
Fig. 8 Change in a PL spectra and b IUV/IDLE ratio of ZnO nanorods with the change in Cu concentration in hydrothermal solution
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interstitial Zni to the VZn [42]. The substitution of Cu ions into a ZnO lattice forms more Zni defects, which in turn increases the intensity of strong blue band peak at 470 nm [42]. The presence of Cu broadens the peak toward the longer wavelength compared with undoped ZnO nanorods. Cu dopant acceptor level below 2.7 eV at the conduction band minimum of ZnO is attributed to blue emission at 480 nm [46, 47]. Furthermore, there is a green emission observed at *543 nm, referred to as a DLE, which was often attributed to singly ionized oxygen vacancies (V? O) [48]. Here, we can observe that higher incorporation of Cu ion on ZnO nanorods shows a progressive increment in UV emission in comparison with visible emission. The change in peak ratio, IUV/IDLE, is plotted in Fig. 8b, when the concentration of Cu is varied in the hydrothermal solution. The results show that an increase in Cu concentration in hydrothermal solution increased the IUV/IDLE ratio. Cudoped ZnO nanorods synthesized in a solution containing 20 M % of Cu exhibited a 67.85 % increase in IUV/IDLE ratio compared with undoped ZnO nanorods. The 20 % Cu-doped ZnO nanorods exhibit the lowest surface defect by reducing the visible emission because weak green emission is an indication that the nanorods have low concentrations of defects [49]. The diminution of surface defects on the Cu-doped ZnO nanorods greatly reduces the recombination of photo-electrons and holes and promotes the charge separation; thereby, it can improve PEC performances [50]. As a result, the PL investigation shows the influence of the doping concentration of the photoluminescence properties of the ZnO nanorods and the potential application in UV emission devices [51] and PEC devices.
4 Conclusions In summary, undoped and doped ZnO nanorods were grown on ZnO seeded glass substrates by sol–gel and hydrothermal synthesis method. The XRD and FESEM analyses show nanorods are highly c-axis oriented and perpendicular to the substrate with high crystalline quality. Our experimental and simulation studies exhibited red shift in absorption edge and the band gap narrowing with Cu doping. PL study reveals strong UV emission and weak broad green emission with high concentration of doping, which indicates the high optical quality and low density of defects. The defect-related emission was studied by varying the doping concentration. The green emission is attributed to the oxygen vacancies (VO) in the doped and undoped ZnO nanorods. Finally, it concluded that with high concentration of doping, the process of recombination of photo-electrons and holes reduced and upheld the charge separation process. These doped ZnO nanorods would be quite useful for UV emission device and PEC devices.
763 Acknowledgments The authors thank the Indian School of Mines, Dhanbad, India, for providing the Junior Research Fellowship and Central Research Facility (CRF). Authors acknowledge Department of Science and Technology (DST) for project with Grant no. SR/FTP/ PS-184/2012, SERB vide Dy.No.SERB/F/5439/2013-14 dated 25.11.2013 and Faculty Research Scheme–FRS(54)/2013-2014/APH. One of the author Anima Ghosh would like to acknowledge Indo-US Science and Technology Forum (IUSSTF) for providing international Bhaskara Advanced Solar Energy (BASE) fellowship.
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