CHAPTER 3. EFFECT OF PRASEODYMIUM DOPING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ZnO NANORODS

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1 46 CHAPTER 3 EFFECT OF PRASEODYMIUM DOPING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ZnO NANORODS 3.1 INTRODUCTION Zinc oxide, one of the most promising materials, has been demonstrated to be applicable in solar cells (Rensmo et al 1997, Law et al 2005), light-emitting diodes (Hwang et al 2005, He et al 2007 a), gas sensors (Kind et al 2002), field-effect transistors (Arnold et al 2003) and piezoelectric-gated diode (He et al 2007b). It has attracted increasing interest in fabricating ZnO structures with designed morphology and properties while the desired optical and electrical properties could be achieved by doping ZnO with various elements. Introduction of impurity atoms is the most widely adopted method to tune the magnetic, electrical and optical properties of materials. But doping becomes difficult in aqueous systems because the doping species would form metal-aquo complex easily and not merge into the crystal lattice (Greene et al 2006). Recently, studies on doping of transition metals and rare earth materials have received considerable interests for their possible applications in visible light emitting phosphors (for displays) and other optoelectronics devices (Ashtaputre et al 2008, Yang et al 2008). In the case of transition

2 47 metals or rare earth elements, the radiative efficiency of the impurity-induced emission increases significantly. It is possible to get more interesting Photoluminescence (PL) properties when rare earth metals are introduced into the lattice of semiconductor nanorods. Considering the unique optical properties, along with the tunable emission wavelengths ranging from blue to infrared region, Pr 3+ ion, a well-known activator dopant has been chosen for the current study (Suyama et al 1982). It is believed that the sharp and narrow intense emissions (in the visible range) of this rare earth metal in its trivalent form could support the aforementioned application aspect. Zinc oxide is chosen to be the host semiconductor (Mais et al 1999, Ishii et al 2001), not only due to the wide band gap, which can be applicable to the excitation of Pr, but also the controllable electrical conductivity. Up to now, physical doping methods, such as ion implantation (Wang et al 2006), laser ablation (Komuro et al 2000), thermal evaporation (Liu et al 2004), hydrothermal method (Liu and Zeng 2003) and solution synthesis method (Wahab et al 2007), have been reported for the fabrication of ZnO nanostructures. Recently Inoue et al (2006) prepared Pr doped ZnO phosphor powders by dry reaction and studied the green emission. Herein, we report a facile method for the fabrication of Pr-doped ZnO nanorods in large scale based on simple solution process to introduce Pr into ZnO nanorods. Compared to physical doping methods, solution method are advantageous for low reaction temperature, low cost, minimum equipment requirement, atmospheric pressure and product homogeneity (Wahab et al 2007). In addition, doping species will fill into the whole structure and not only on the surface. The proper reaction conditions to grow Pr-doped ZnO nanorods were

3 48 found to be very constricted by varying the concentration of reactants, additive amount of base and Pr salt, species of mineralizers as well as reaction temperature and time. The grown structures have been studied for their structural and optical properties. 3.2 MATERIAL SYNTHESIS Zinc acetate (Merck %) and praseodymium nitrate (Aldrich %) were used as the sources for zinc and praseodymium respectively, for the preparation of Pr-doped ZnO nanorods. Ethylenediamine (EDA) along with sodium hydroxide (Merck) has been used as the structure-directing agent. Deionized water (Milli-Q 18.2M ) was used as the solvent in all cases. The alkali solution of zinc was prepared from zinc acetate (10 g) and ethylenediamine (EDA) (10 g), made up to 100 ml in a standard flask. One normality (1N) sodium hydroxide solution was added in drops to the above solution until white precipitation resulted. The solution mixture was then pretreated in an ultrasonic water bath for 1 h prior to refluxing. The refluxing process was then carried out for 24 h at 95 C in a round bottom flask under constant stirring, using an oil bath set up. After which, the white crystalline products were harvested through centrifugation and thorough washing with deionized water and ethanol to remove the possible ions remaining in the products. The obtained products were initially dried at 60 C for 10 h and then subsequently calcined at 500 C for 2 h in a muffle furnace (in free supply of air to remove residual nitrate and organic material). Prdoped ZnO nanorods were prepared through the same methodology by adding praseodymium nitrate (0.2g) to the zinc stock solution prior to precipitation reactions. The flowchart for the preparation of Pr doped ZnO nanorods are shown in Figure 3.1.

4 Figure 3.1 Flow Chart for the preparation of Pr doped ZnO nanorods 49

5 MECHANISM OF FORMATION OF ZnO NANORODS ZnO is a polar crystal exhibiting positive and negative polar planes, rich in Zn and O, respectively. Generally, it is believed that the morphology of ZnO crystals is related to both their intrinsic crystal structure and external factors (Zhang et al 2002, Zhang et al 2004). The possible mechanism for the formation of single crystalline ZnO nanorod like structures could be understood from the scheme given below. Zn H 2 NCH 2 CH 2 NH 2 [Zn (H 2 NCH 2 CH 2 NH 2 ) 3 ] Ethylenediamine is a bidentate ligand, which reacts with the zinc ions to give the zinc- ethylenediamine complex in the solution as shown in equation 3.1. (Masuda and Kato 2008). In the next stage, zincethylenediamine complex decomposes to increase the concentration of Zn 2+ ions at elevated temperature (generally ethylenediamine reacts with water to form OH ions) NH 2 CH 2 CH 2 NH 2 + 2H 2 O NH 3 + CH 2 CH 2 NH OH 3.2 Also, during the synthesis process, a part of the Zn (OH) 2 colloids dissolves into Zn 2+ and OH - ions accordingly, increasing their saturation limit, Zn OH Zn (OH) Zn 2+ (aq) + 2OH ZnO(s) + H 2 O (nucleation) 3.4

6 51 When the concentration of Zn 2+ and OH - reaches the supersaturation, ZnO nuclei are formed according to reaction 3.4. Hence, in the presence of EDA the concentrations of the OH ions are increased to a certain level, where some part of the Zn(OH) 2 reacts with excess OH to form [Zn(OH) 4 ] 2- following Zn(OH) 2 + 2OH - [Zn(OH) 4 ] The adsorbed EDA molecules could provide electrons to the Zn atoms, and hence modifying the distribution of the charges between Zn and O atoms, thereby enhancing the Zn-O bond. On the other hand, the zinc complex species in the solution phase are more attracted to adsorption sites with high surface energy (0002) plane of ZnO in the subsequent redeposition, resulting in the smoother and larger ZnO nanorods to be formed (Liu and Zeng 2004). 3.4 CHARACTERIZATION STUDIES X-ray diffraction Analysis The prepared ZnO nanorods were subjected to powder X-ray diffraction analysis. Figure 3.2 (a) shows the XRD patterns of the pure and Pr-doped ZnO nanorods prepared at 95 C. XRD patterns reveals that the Pr-doped ZnO samples possess hexagonal wurtzite crystal structure (JCPDS card no ).

7 52 Figure 3.2 (a) Powder XRD patterns for undoped and Pr-doped ZnO nanorods, (b) Magnified region of (002) peak

8 53 The sharp and narrow peaks indicate the products obtained are to be well in crystalline form. The absence of peaks related to praseodymium oxides reveal the possibility for Pr 3+ ion substitution at the Zn 2+ sites or in the ZnO lattice. A significant shift in the (002) planes were observed towards the lower angle on Pr substitution, resulting from the increase in lattice parameter c values (5.193 Å to Å for Pr-doped ZnO). This might be due to the larger ionic radius of Pr 3+ (0. 99 Å) than that of Zn 2+ (0.74 Å).Thus the peak shift and increase in lattice constant values provide conclusive evidence for the substitution of Pr 3+ partly into the ZnO lattice. In order to confirm the possibility of Pr substitution replacing Zn ions in the Pr doped ZnO nanorods, the angle shift of 2 ( (2 )) for the peak of ZnO (002) reflection as a function of doping of Pr (%) has been calculated and illustrated in Figure 3.2(b) (shows the magnified region of (002) peak). (2 ) increases up to 0.21º for Pr doped ZnO compared to undoped ZnO, demonstrating the presence of an effective substitution of Pr 3+ for Zn 2+ ions in the nanorods Scanning electron microscopic Studies Figures 3.3 (a) and (b) show the FE-SEM images of undoped and Pr-doped ZnO nanorods prepared at 95 C. From the images it is clear that the undoped and Pr-doped ZnO specimens are composed of a number of nonuniform nanorod like structures. Their average diameters were evaluated to be about nm, with length even upto 1-3 µm. The Pr compositions in the nanorods were estimated using the energy dispersive X-ray analysis as shown in Figure 3.4. The EDX results were collected from several parts of the doped nanorods, which reveal them to be composed of Zn and Pr trace elements. In order to determine the exact composition of Pr in the doped ZnO nanorods, ICP-OES analyses were carried out and the Pr content was determined to be 1.88% (for 2% Pr substituted samples).

9 54 Figure.3.3 SEM images of (a) pure and (b) Pr-doped ZnO nanorods Figure 3.4 EDX spectrum confirm the presence of Pr without any impurities peaks

10 High resolution transmission electron microscopy Figure 3.5 shows the transmission electron microscopy analysis of Pr-doped ZnO nanorods. TEM image shown in Figure 3.5 (a) reveals the 1D ZnO nanorods. The structure and crystallinity of the Pr-doped ZnO nanorods were examined by HRTEM studies, where the lattice fringes were clearly noticed to be separated by a distance of 0.52nm, corresponding to the (001) crystal planes. (a) (b) (c) Figure 3.5 TEM images of (a) Pr-doped ZnO nanorods and (b) HRTEM image and (c) SAED pattern of the Pr-doped ZnO nanorods

11 56 The SAED pattern clearly indicates the crystalline nature of the nanorods, which corresponds to wurtzite ZnO, and is also in good agreement with the XRD results Fourier Transform Infrared spectroscopy FT-IR spectroscopic studies were carried out to analyze the influence of Pr ions on the formation of wurtzite structured ZnO nanorods as shown in Figure 3.6. The band at 524 cm -1 that could be correlated with the Zn-O bonding, which has been observed to shift towards the lower wavenumbers on Pr incorporation (Music et al 2002). Figure 3.6 FTIR spectra of undoped and Pr doped ZnO nanorods The peak at 892 cm -1 corresponds to the CH 2 rocking. The vibrational modes at 1600 cm -1 could be attributed to the NH 2 bending mode that gets shifted slightly in their band positions on Pr substitution. Also the

12 57 absence of Pr-O related bonds confirm the phase purity of the prepared specimens. Hence, FT-IR spectroscopy provides conclusive evidence for the substitution of Pr ions in the host lattice Raman Spectroscopy Raman scattering is a versatile technique that is used to study the nature of dopant incorporation and the defects and lattice disorder produced in-turn. The wurtzite ZnO belongs to C6v symmetry group with two molecules in a Bravais unit cell. By using the correlation between the site group and factor group for each site, and eliminating the acoustic modes, the vibrational modes of wurtzite structured ZnO could be expressed as: (k=0) = 1A 1 + 2B 1 + 1E 1 + 2E where A 1 and E 1 symmetry belongs to polar phonons and is in-turn split into transverse-optical (TO) and longitudinal-optical (LO) phonons. E 2, corresponding to the non-polar phonon mode, has been associated with the two wavenumbers: E 2 (high) and E 2 (low), which have been assigned with the oxygen atoms and Zn sublattice, respectively. In this investigation, Raman spectra of the undoped and Pr doped ZnO nanorods have been studied over the range of cm 1 as shown in Figure 3.7. Four distinct Raman modes have been observed at 331, 383, 438, and 583cm -1, respectively. The strong and sharp peak observed at 438cm 1 corresponds to the non polar optical phonons E 2 (high) mode of ZnO, which indicates the Pr doped ZnO nanorods to be of highly crystalline nature. The features located at 331 and 383 cm 1 correspond to the multi-phonon scattering process E 2H E 2L and A 1 (TO) phonons of ZnO crystal, respectively

13 58 (Calleja and Cardona 1997). The peak located at 583 cm 1 could be attributed to the E 1 (LO) peak, associated with the formation of defects such as oxygen vacancy, zinc interstitial, or their complexes (Pradhan et al 2004). Signals corresponding to Pr or its oxide forms have not been observed in the Raman spectrum confirming the purity of Pr doped nanorods, which is consistent with the XRD and FT-IR results. Figure 3.7 Raman spectra of undoped and Pr-doped ZnO nanorods measured at room temperature UV-Visible spectroscopy Figure 3.8 presents the absorption spectra of pure and Pr doped ZnO nanorods. It is clear from the figure that the absorption edge of the Pr doped ZnO nanorods exhibits a red shift compared to pure ZnO. The observed red shift indicates the formation of electronic state impurity band into the band gap of ZnO by Pr (4f) electron localized states, which are

14 59 located closer to the lower edge of the conduction band to form the new lowest unoccupied molecular orbital. The merging of this electronic state with the bottom of the conduction band could be reasoned for the reduction in band gap. The band gap of Pr doped nanorods decreases from 3.27 to 3.22eV. Figure 3.8 Absorption spectra of undoped and Pr doped ZnO nanorods Photoluminescence spectroscopy The room-temperature photoluminescence spectra of undoped and Pr doped ZnO nanorods are shown in Figure 3.9. The spectrum of undoped ZnO consists of a strong UV emission peak centered at 389 nm along with a weak visible emission centered on 530 nm. The UV emissions are commonly attributed to the direct recombination of free excitons (Kong et al 2001), while the weak visible emissions correspond to the singly ionized oxygen vacancy in ZnO that results from the recombination of a photogenerated hole with the singly ionized charge state of the specific defect (Vanheusden et al 1996).

15 60 Figure 3.9 PL spectra of undoped and Pr-doped ZnO nanorods A significant red shift in the UV emission of ZnO nanorods was observed (towards higher wavelength region from 389nm to 395nm) on the introduction of Pr ions into ZnO lattice. Concerning the visible emission, doping of Pr ions into the ZnO lattice results in the increase in number of defects in the samples (Inoue et al 2006). The increase in native defects could be ascribed to the fact that when Pr 3+ replaces Zn 2+, oxygen defects are usually generated to keep the charge neutrality (Yang et al 2008). The increase in the visible emission is therefore correlated with the substitution of Pr atoms into Zn sites in the ZnO lattice. The broad visible emission in Pr doped ZnO was fitted based up on Gaussian fitting, that resulted in two unique peaks shown in Figure 3.10.

16 61 Figure Gaussian fitted region for Pr doped ZnO nanorods over nm The first peak at 535 nm could be correlated with the green luminescence, resulting from the recombination of an electron at the bottom of the conduction band or a shallow donor state such as (V O ) and a hole at an antisite defect (O Zn ) (Lin et al 2001). The next peak centered at 580 nm may be attributed to the yellow emission, resulting from the interstitial oxygen defects (Zeng et al 2008). However, the absence of characteristic emission related to the Pr ions rules out the energy transfer from the ZnO to the rare earth ion, which in-turn could be attributed to the inappropriate energy levels position of rare earth ions relative to the valence and conduction bands of ZnO (Dorenbos and Van der Kolk 2006). This result indicates that the band gap structure of ZnO nanorods can be tailored significantly when Pr 3+ ion is doped into the ZnO lattice, which may change the luminescent process of the host material.

17 CONCLUSION Undoped and Pr-doped ZnO nanorods were synthesized by the aqueous solution route. The XRD measurements revealed that the Pr-doped ZnO nanocrystallites possess wurtzite crystal structure, with increase in lattice parameter c values on Pr substitution into the host system. The crystallinity of the synthesized nanocrystallites and the nature of Zn-O bonding have been studied by Raman spectroscopy and FT-IR spectroscopic measurements. The crystalline nature of the doped nanorods has also been studied by HRTEM analysis. Photoluminescence measurements of Pr doped ZnO shows a significant reduction in the optical band gap of the material, resulting from the newly formed molecular orbital states due to the trapping level and the presence of a large number of defects.