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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

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, Pr3+ 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

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 - 99.9 %) and praseodymium nitrate (Aldrich -

99.9%) 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). Pr-

doped 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.

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

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3.3 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.

Zn2++ 3H2NCH2CH2NH2 [Zn (H2NCH2CH2NH2)3]2+ 3.1

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, zinc-

ethylenediamine complex decomposes to increase the concentration of Zn2+

ions at elevated temperature (generally ethylenediamine reacts with water to

form OH ions)

NH2CH2CH2NH2 + 2H2O NH3+ CH2CH2NH3

+ + 2OH 3.2

Also, during the synthesis process, a part of the Zn (OH)2 colloids

dissolves into Zn2+ and OH- ions accordingly, increasing their saturation limit,

Zn2+ + 2OH Zn (OH)2 3.3

Zn2+ (aq) + 2OH ZnO(s) + H2O (nucleation) 3.4

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When the concentration of Zn2+ 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]2- 3.5

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

3.4.1 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. 36-1451).

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Figure 3.2 (a) Powder XRD patterns for undoped and Pr-doped

ZnO nanorods, (b) Magnified region of (002) peak

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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 Pr3+ ion substitution at the Zn2+ 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 5.212 Å for Pr-doped ZnO). This might be due to the

larger ionic radius of Pr3+ (0. 99 Å) than that of Zn2+ (0.74 Å).Thus the peak

shift and increase in lattice constant values provide conclusive evidence for

the substitution of Pr3+ 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 Pr3+ for Zn2+ ions in the nanorods.

3.4.2 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 non-

uniform nanorod like structures. Their average diameters were evaluated to be

about 150-200 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).

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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

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(b)(a)

(c)

3.4.3 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.

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

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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.

3.4.4 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 CH2 rocking. The

vibrational modes at 1600 cm-1 could be attributed to the NH2 bending mode

that gets shifted slightly in their band positions on Pr substitution. Also the

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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.

3.4.5 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) = 1A1 + 2B1 + 1E1 + 2E2. 3.6

where A1 and E1 symmetry belongs to polar phonons and is in-turn split into

transverse-optical (TO) and longitudinal-optical (LO) phonons. E2,

corresponding to the non-polar phonon mode, has been associated with the

two wavenumbers: E2 (high) and E2 (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 300-700 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 E2 (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 E2H–E2L and A1 (TO) phonons of ZnO crystal, respectively

58

(Calleja and Cardona 1997). The peak located at 583 cm 1 could be attributed

to the E1 (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

3.4.6 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

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

3.4.7 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).

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 Pr3+ replaces Zn2+, 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.

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Figure 3.10. Gaussian fitted region for Pr doped ZnO nanorods over

450 - 600 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 (VO) and a hole at an

antisite defect (OZn) (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 Pr3+ ion is

doped into the ZnO lattice, which may change the luminescent process of the

host material.

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3.5 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.