4-1

Chapter 4

Rare Earth Elements Doped ZnO Nanoparticles

This chapter presents the effect of the rare earth elements (Nd and

Gd) doping on the structural, morphological, optical, magnetic and

photocatalytic properties of ZnO nanoparticles. This chapter

consist two sections A and B, effect of Nd doping and Gd doping on

the various properties of ZnO nanoparticles, respectively. The

broadness in the XRD peaks has been analysed in the light of short

range ordering and this mechanism is correlated with decreased

average particle size with an increase in doping concentration.

Origin of enhanced broad visible emission has been discussed

briefly in the light of enhanced oxygen vacancies due to increased

surface to volume ratio of doped ZnO nanoparticles. The

photocatalytic experiment is also conducted in order to observe the

enhanced degradation rate of organic dyes under UV light

irradiation.

Chapter 4

4-2

4.1 Introduction

Nanometer-scale materials and semiconductors in particular seem to be

important and promising in the development of next-generation electronic and

optoelectronic devices [Nirmal and Brus (1999), Alivisatos (1996), Muuray et al.

(1995)]. In recent years, semiconductor hybrid materials (SHMs) become the centre

of attention due to their enhanced optical, photocatalytic and magnetic properties.

Especially, synthesis and properties of semiconductor–metal heterostructures such as

TiO2, SnO2, ZnO etc. (doped with Au, Ag, Nd and Gd) have been investigated in

recent years due to their potential and important applications in catalysis, cellular

imaging, immunoassay, luminescence tagging, spintronics and drug delivery [Lee et

al. (2010), Nayral et al. (1999), Ozgur et al. (2005), Chao et al. 2009), Wang et al.

(2004), Dietl et al. (2000)]. For example, if made magnetic on doping, SHMs will

become a kind of multifunctional material with semiconducting, optical,

photocatalytic and magnetic properties.

Recent experimental results have shown that the introduction of rare-earth

metals (REMs) such as Gd ion in wide band-gap semiconductor, GaN, results in

ferromagnetic property [Dhar et al. (2005), Dhar et al. (2005)]. This has motivated the

researchers working in this field on REM ion(s) doping in ZnO for spintronics

application. In recent years, different types of SHMs studied such as ZnO-Ag, ZnO-

Au, ZnO-Gd, ZnO-Nd etc. [Li et al. (2010), Wang et al. (2009), Ungureanu et al.

(2007), Zhou et al. (2009)]. Doping method has been extensively used for the

modification the electronic structure of ZnO nanoparticles to achieve new or

improved optical, magnetic, catalytic properties. Dopants can segregate on the ZnO

nanostructure surfaces or they can incorporate into the lattice or both [Li et al.

(2005)]. Doping in ZnO with a suitable dopant can make it more or less efficient in

photodegradation of organic and toxic pollutants. When the Nd3+

ions occupy the

substitutional position of Zn2+

ions then cationic vacancies are created locally due to

their different ionic states (ionic radii of Zn2+

& Nd3+

are 0.083 & 0.108 nm

respectively). Thus induced cationic vacancies created by Nd3+

doping in the ZnO

new kinds of defects [Shahmoradi et al. (2010), Cao et al. (2009)]. Additionally, for

Rare earth elements doped ZnO ….

4-3

device applications, high efficiency UV light emitting devices is required, it is

important to suppress the visible emission.

Nd is one of the most widely used elements for high power laser applications

and recently these lasers have shown their usefulness in inertia confined fusion

experiments [Yu et al. (2007)]. Furthermore, Nd3+

doping reduces the band gap

energy and enhances the possibility of the photodegradation of dyes under visible

light [Shahmoradi et al. (2010)], also shown by us in the present study under UV

light. Previous reports on Nd doped ZnO focused on the luminescence, magnetic,

electrical and photocatalytic properties in ZnO or TiO2 based materials [Liu et al.

(2010), Cao et al. (2009), Xie and Yuan (2004)]. On the other hand, Gd3+

is also of

great interest due to its scintillation properties [Cozzoli et al. (2003)] and used as a

phosphor films in optical devices [Xie (2006), Fu et al. (2001), Seo et al. (2002), Bae

et al. (2003)] in place of other luminescent materials. Moreover, Gd3+

doped SiO2 is

used as a good UV light emitter whereas Gd3+

doped ZnO nanomaterials have shown

improved room temperature ferromagnetic properties [Garcia-Murilla et al. (2002),

Garcia-Murilla et al. (2002), Potzger et al. (2006)] due to its (Gd) presence in 4f state

which makes ZnO/Gd hybrid structure as a potential candidate for spintronics

applications. Furthermore, these 4f states of Gd responsible for improved hole

conductivity because the holes in 4f states are more active than electrons [Liu et al.

(2008)].

In this chapter emphasis on the variation of optical and magnetic and

photocatalytic properties with the Nd3+

and Gd3+

doping is presented. These rare earth

doped ZnO nanoparticles are further used as a photocatalyst for the degradation of

aqueous Rh B dye and realised to be highly efficient catalyst.

Chapter 4

4-4

Section A

Nd3+

doped ZnO nanoparticles

4.2 Sample Preparation

4.2.1 Wet Chemical Method

ZnO nanoparticles doped with Nd (0.01, 0.02, 0.03, 0.04 and 0.05 mol%) are

synthesized via wet chemical method using neodymium chloride (NdCl3.6H2O), zinc

acetate (Zn(CH3COO)2.2H2O), sodium hydroxide (NaOH) as a starting materials

without any further purification. Stoichiometric composition is chosen for zinc acetate

and sodium hydroxide. This technique is based on the hydrolysis of the precursor used

to prepare ZnO nanoparticles. 0.1 M solution of zinc acetate is prepared in ethanol

and refluxed at 80 oC for 6 h which results Zn

2+ ions. Separately, 0.2 M solution of

sodium hydroxide is prepared in ethanol and added dropwise to Zn2+

ions solution

under constant magnetic stirring at 40 oC and kept the reaction for 5 h under continues

stirring. The obtained white precipitates are separate out using centrifugation at 6000

rpm and washed several times with ethanol and dried at 100 oC for 12 h. For doping,

the appropriate amount of NdCl3.6H2O is added to zinc acetate solution keeping all

parameters the same and used for further characterization. However, the

concentrations of the impurity stated here refer to the amount added to the solution

during this process and not the actual amount that would have incorporated into the

host of ZnO matrix. But it could be presumed that the concentrations of Nd inside the

matrix remain the same.

4.2.2 Photocatalytic Experiments

Photocatalytic studies under UV (λ < 400 nm) light are carried out in an

immersion-type, in-house fabricated photochemical reactor. A double-lined quartz

tube (with dimensions of 3.5 cm inner diameter, 4.5 cm outer diameter, and 20 cm

length) is placed in an outer Pyrex glass reactor of 14 cm inner diameter and 20 cm

length. A high-pressure mercury vapor lamp of 125 W (Philips, India) is placed inside

the quartz tube. Here also water circulation is carried out to avoid any thermal effects.

Rare earth elements doped ZnO ….

4-5

The appropriate amount of dye solution, to be decomposed, is taken along with the

required amount of catalyst in the outer Pyrex container and is constantly stirred to

maintain a homogeneous suspension. The dye is dissolved in doubly distilled water. A

typical experiment of degradation is carried out as follows: 0.03 g of the catalyst is

added to 100 mL of aqueous solution of Rh B with an initial concentration of 5x10-6

mol/L for irradiation experiments. Prior to irradiation, the suspension of the catalyst

and dye solution is stirred in the dark for 1 h to reach the equilibrium adsorption. Five

millilitre aliquots are pipetted out periodically from the reaction mixture. The

solutions are centrifuged, and the concentration of the solutions is determined by

monitoring the intensity of 552 nm absorption peak.

4.3 Structural and morphological investigations

Fig. 4.1: XRD patterns of undoped and Nd doped ZnO nanoparticles at different

doping levels, (a) 0.00, (b) 0.01, (c) 0.02, (d) 0.03, (e) 0.04 and (f) 0.05 mol%.

The XRD patterns of undoped and Nd-doped ZnO with different Nd doping

contents are shown in Fig. 4.1. The undoped ZnO powders are identified as a wurtzite

structure ZnO. (ICSD card No. 06-2151, space group: P63mc) with lattice constants a

Chapter 4

4-6

= 3.2568, c = 5.2125 Å. For all the Nd-doped ZnO materials, the diffraction peaks are

almost similar to that of pure ZnO. It is possible for Nd3+

ions cooperate with the

matrix of ZnO particles to form Nd-Zn-O solid solutions since the radius of Nd3+

is

bigger than Zn2+

. Increasing dopant concentration in a host matrix by different ions

may change the lattice parameters because of the ionic radius difference between the

dopant and host atoms [Maris et al. (2004)]. It may also generate stress due to this

mismatch. XRD data shows with increase the doping concentration of Nd3+

ion,

however, the intensity of the diffraction peaks decreased and full width at half

maximum (FWHM) has gradually increased. This suggests that ions Nd3+

ions replace

the Zn2+

lattice sites or interstitial sites in the sample. Similar, type of behaviour of

ZnO is also observed [Dakhel and El-Hilo (2010)] with Gd3+

doping, however, the

changes observed by them in lattice parameter are lesser than observed by us with

Nd3+

. This, further, confirms the doping of ZnO with Nd3+

. The increase of doping

concentration results the decrease of crystallite size and increase of disorder effect

which resulted in the broadening and decrease of intensity of the XRD peaks.

Fig. 4.2: TEM images of (a): undoped, (b): 0.03 mol% doped, (c): 0.05mol% doped

ZnO nanoparticles (scale bar = 20 nm).

TEM images of the ZnO nanoparticles are shown in Fig. 4.2. Nearly circular

shapes for the dark spots in the images indicate that the ZnO nanoparticles are almost

spherical. The estimated average particle size corresponding to (a), (b) and (c) are 16,

12 and 9 nm. When the concentration of Nd is increased the particle size decreased

and this is clearly seen in the TEM images. These results are also consistent with

other rare earth metal ions doped like Gd-doped ZnO and also in Nd doped ZnO

[Subramaniam et al. (2010), Huang et al. (2011), Zhou et al. (2009)].

Rare earth elements doped ZnO ….

4-7

4.4 Optical studies

The presence of Nd3+

in the ZnO lattice deforms the structure and can be

detected by Raman analysis. Fig. 4.3 shows the measured Raman spectra for the Nd

doped ZnO samples with different concentration of the Nd3+

. The peaks at 102 cm-1

(E2low

) and 436 cm-1

(E2high)

are attributed to the nonpolar E2 vibrational modes due to

the vibration of Zn and O lattice in Wurtzite ZnO. The intensity of both the E2low

and

E2high

modes decreased gradually with Nd3+

concentration, which can be attributed to

the distortion in the crystal.

Fig. 4.3: Raman spectra of ZnO samples prepared with different Nd amount, (a) 0.00,

(b) 0.01, (c) 0.02, (d) 0.03, (e) 0.04 and (f) 0.05 mol%.

The peak at 1080 cm-1

are attributed to the TO plus LO mode. The peak at

661, 931, 1345, 1417 and 2934 cm-1

are associated to the OCO symmetric bending, C-

C vibration, CH3 symmetric bending, CO symmetric stretching and CH3 symmetric

stretching present in the radicals (CH3COO-) of the zinc acetate dehydrate

respectively [Xing et al. (2003), Umar et al. (2005), Jeong et al. (2003), Ben et al.

(2008), Scepanovic et al. (2010), Yang et al. (2005)]. The distortion in the crystal is

also found out from the XRD results.

Chapter 4

4-8

Obtained ZnO precipitates are already washed several times with ethanol to

remove the by-products. The prepared ZnO nanoparticles have residual intermediate

compound on the surface in the form of an acetate group, which acts as defect centers

for the emission of green luminescence [Kumar and Sahare (2011)]. The observed

acetate radicals are loosely bound on the surface of ZnO by dangling bond

[Raghvendra et al. (2010)].

Fig. 4.4: PL spectra of the undoped and Nd doped ZnO, (a) 0.00, (b) 0.01, (c) 0.02,

(d) 0.03, (e) 0.04 and (f) 0.05 mol% of Nd, measured at room temperature with

excitation wavelength 280 nm.

Fig. 4.4 shows the emission spectra of undoped ZnO and Nd doped ZnO

nanoparticles. ZnO nanoparticles exhibited a strong visible emission centered at about

at 565 nm excited at a wavelength of 280 nm and a sharp near band edge emission at

around 384 nm. Peak at 384 nm is associated with the near band edge transition.

Broad band is due to the defects at the surface of ZnO such as oxygen vacancies and

hydroxyl group [Wong et al. (1999), Monticone et al. (1998), Li et al. (2004), Tam et

al. (2006)]. PL properties of Nd3+

doped ZnO with doping concentration varied from

0 to 0.05 mol%. As the concentration of Nd3+

is increased, visible emission enhanced

Rare earth elements doped ZnO ….

4-9

greatly (about 10 times for 0.04 mol %). However, excessive Nd3+

ions consumed the

ZnO nanoparticles, decreasing the PL intensity, reversely. According to previous

report, the reduction of particle’s size usually induced the increase in the content of

oxygen vacancies [Xiong et al. (2009)]. Nd doping decreased the size of ZnO, which

might increase the content of oxygen vacancies. However, excessive Nd3+

ions

consumed the ZnO nanoparticles, decreasing the fluorescence intensity, reversely. To

complete the valence site of one Zn2+

, one O2-

will attached to form a ZnO compound

while in case of Nd3+

, two Nd3+

will attached to the three O2-

to forms a Nd2O3

compound. In this process two Nd3+

ions replace the three Zn2+

ions and oxygen

concentration remains constant. Thus, the reason on of the change in oxygen

vacancies (defect sites such as oxygen vacancies) is not the replacement Zn2+

by Nd3+

but the increase of surface to volume with decrease in size and well reported in

literature [Xiong et al. (2009), Haase et al. (1988), Antony et al. (2006)].

Fig. 4.5: Changes in the PL emission spectra of the Nd-doped ZnO (0.04 mole %)

sample under the UV irradiation. The arrow pointing downwards indicates the

decrease in the ‘visible’ emission band with time.

Chapter 4

4-10

The thermoluminescence glow curves of the undoped and Nd doped ZnO

(0.04 mole %) nanoparticles irradiated at room temperature in air with increasing

exposure of the UV-irradiation with rate of 5 K/s are shown in Fig. 4.6. The peak

intensity goes on increasing with the exposure time of the UV-irradiation. There is not

much change in the glow curves of the pure ZnO samples except the intensity is very

low as compared to the Nd-doped samples.

Fig. 4.6: Thermoluminescence (TL) glow curves for the Nd-doped ZnO (0.04 mole

%): (a) 4h, (b) 2h, (c) 1h, (g) unirradiated samples. The same for the undoped ZnO

material are also shown as (d) 4h, (e) 2h, (f) 1h, (h) unirradiated. The emission

(black-body radiation) for the blank plate is also shown as curve (i). The TL intensity

is corrected for the plate emission. The variation of TL intensity with UV-irradiation

is as shown in the inset.

This shows that the number of traps developed in case of the pure samples are

much less than the doped samples. The exact mechanism of the recombination of

traps in ZnO is not yet completely understood [Vanheusden et al. (1996), Liu et al.

(1992)]. The TL, in this case, is mainly due to the recombination of charge carriers

released from the surface states associated to the singly occupied oxygen vacancy

Rare earth elements doped ZnO ….

4-11

centres [Secu and Sima (2009)]. TL in ZnO associated with the interaction of the

vacancies with close neighbour defect sites [Wang et al. (2011)]. This is possible due

to the local charge mismatch in valency and size on trivalent Nd3+

doping. This could

also be correlated with the PL visible emission band which increases with the Nd3+

doping but decreases steadily with the exposure of the UV irradiation. This could be

understood easily by considering the increasing number of defects on Nd3+

doping

due to the local charge imbalance and also due to the stress (because mismatch in

ionic size), the different kinds of defects are generated.

Fig. 4.7: Fitted glow curve for 2h irradiated Nd (0.04 mol %) doped ZnO

nanoparticles.

A single broad peak could be observed at around 640 K. The glow peak is also

theoretically fitted (Fig. 4.7) using the Glow Fit computerized glow curve

deconvolution program CGCD software code [Puchalska and Bilski (2006)]. The

trapping parameters for Nd doped ZnO are found from the curve fitting are, peak

temperature (Tm) 640 K, order of kinetics 1, (b) frequency factor (s) 2.21x108 and

activation energy (Ea) as 1.129 eV. These values are found to be the same for all the

glow curves. It could be seen from the TL response that the ZnO: Nd phosphor could

Chapter 4

4-12

be used for the UV-radiation dosimetry. It could also be seen that the Nd-doped

phosphor is around 3 times more sensitive than the undoped ZnO. It also has added

advantage that the fading would be low (around 10% in one month) as the TL peaks

appear at temperature (~640 K).

4.5 Magnetic studies

Fig. 4.8: Room-temperature magnetic hysteresis loop of undoped and Nd doped ZnO,

(a) 0.00, (b) 0.01, (c) 0.02, (d) 0.03, (e) 0.04 and (f) 0.05 mol %.

Fig. 4.8 shows the room temperature magnetization effect for the undoped and

Nd doped ZnO nanostructures. It could be clearly seen from the Fig. that saturation

magnetization increases as Nd concentration increases from 0.01 to 0.04 mol%. The

saturation magnetization is found to be 0.013, 0.018, 0.025, 0.028, 0.033 and 0.025

emu/g for the samples 0.00, 0.01, 0.02, 0.03, 0.04 and 0.05 mol% of Nd respectively.

It is observed that the saturation magnetization decreases as Nd concentration

increases beyond 0.04 mol%. The concentration of oxygen vacancies played an

important role in mediating the ferromagnetism exchange between Nd3+

ions. For low

concentration (0.01 mol% doped Nd sample), an appreciable ferromagnetic

Rare earth elements doped ZnO ….

4-13

component is developed despite the fact that this sample contains less Nd ions as

compared to other samples having more Nd ions. For the samples containing more Nd

ions have more oxygen vacancies might have been generated which are responsible

for the observed long-range ferromagnetism order. The corecivity is also increased

with Nd concentration and maximum for 0.04 mole %.

From these results, the observed ferromagnetic behaviour in the samples may

be attributed to defects like oxygen vacancies, which is consistent with the bound

magnetic polarons (BMP) model [Coey et al. (2005)]. According to the BMP model,

bound electrons in defects like oxygen vacancies can couple the Nd3+

ions and cause

the ferromagnetic regions to overlap giving rise to long-range ferromagnetic order in

the sample. In accordance with the BMP model, the magnetization of the system is

assumed to originate from regions of correlated and isolated spins. Similar results also

have been observed in Co doped ZnO [Pal and Giri (2010)].

4.6 Photocatalytic Studies

The photocatalytic activity of Nd3+

(0.04 mol%) doped ZnO are investigated

by photocatalytic decomposition of aqueous solution of the dye Rh B under UV light

irradiation as shown in Fig. 4.9 along with the catalysis of Rh B in the dark without

any irradiation. The maximum absorbance for the aqueous RhB dye is observed at

around 552 nm. In the presence of Nd3+

doped ZnO as the catalyst, the absorbance

decreased initially indicating adsorption of the dye Rh B (in the dark). Further, a

substantial decrease in the absorbance of Rh B is observed after conducting the

reaction under UV light irradiation with time increasing. The solution turned colorless

within 60 min of irradiation. Similar experiments are carried out for the pure ZnO

nanocrystals as well for the compare study. From these experiments, the variation in

the concentrations of the Rh B solutions is plotted against the time (Fig. 9). These

results clearly demonstrated that Nd doped ZnO samples decolorizes Rh B faster than

synthesized ZnO under similar experimental conditions. This is attributed to higher

oxygen vacancies in Nd doped ZnO nanoparticles. The presence of a higher

concentration of oxygen vacancies is further supported by the enhancement in the PL

Chapter 4

4-14

intensity under excitation with λ = 280 nm (Fig. 4.4). Though one could not directly

derive any correlation between the intensity of the emission in the PL spectrum and

the photocatalytic activity in excitonic oxide semiconductors [Jing et al. (2006)] in

which a possible intense PL emission could possibly result in higher photocatalytic

activity due to the higher concentration of oxygen vacancies offered a satisfying

explanation for our observation.

Fig. 4.9: Photodegradation of Rh B solution with time: (a) Adsorption of Rh B in the

presence of undoped ZnO, (b) adsorption of Rh B in the presence of Nd doped ZnO,

(c) Rh B in the presence of undoped ZnO under UV irradiation and (d) Rh B in the

presence of Nd doped ZnO under UV irradiation.

Rare earth elements doped ZnO ….

4-15

Section B

Gd3+

doped ZnO nanoparticles

4.7 Sample preparation

For Gd (0.01, 0.03 and 0.05 mol%) doping, the appropriate amount of

GdCl3.6H2O is added to zinc acetate solution keeping all parameters the same and is

used for further characterization. For photocatalytic experiment details are given in

section A.

4.8 Structural and morphological investigations

Fig. 4.10: XRD patterns of undoped and Gd3+

doped ZnO nanoparticles at different

doping levels, (a) 0.00, (b) 0.01, (c) 0.03 and (d) 0.05 mol%.

The XRD patterns of undoped and Gd3+

doped ZnO with different Gd3+

doping contents are shown in Fig. 4.10. For all the Gd3+

-doped ZnO materials, the

diffraction peaks are almost similar to that of pure ZnO. XRD data shows that with an

increase in the doping concentration of Gd3+

, the intensity of the diffraction peaks

decreases and full width half maximum (FWHM) gradually increases and this implies

a decrease in the crystalline quality of the ZnO nanoparticles. The grain size were

Chapter 4

4-16

calculated using Scherrer’s formula and were found to be 15, 12, 9 and 7 nm for 0.00,

0.01, 0.03 and 0.05 mol% of Gd3+

, respectively.

Fig. 4.11 shows the shift in the (101) peak position, in order to confirm the

possibility of the substitution of Gd3+

ions for Zn2+

ions in the ZnO matrix. The angle

shift of 2θ (δ(2θ)) for the strongest peak of ZnO (101) reflection as a function of the

doping molar percentage of Gd (mol%) is calculated and illustrated in Fig. 2 (inset

shows the magnified region of (101) peak). δ(2θ) increases little as Gd3+

concentration increases from 0 to 0.05 mol%, up to 0.21°, demonstrating the presence

of effective incorporation of Gd3+

for Zn2+

ions in the nanoparticles.

Fig. 4.11: Peak shift for ZnO (101) as a function of the doping molar percentage of

Gd3+

.

TEM images of the undoped and Gd3+

doped (0.05 mol%) ZnO nanoparticles

are shown in Fig. 4.12 (a) and 4.12 (a’), respectively. Nearly circular shapes for the

dark spots in the images indicate that the ZnO nanoparticles are almost spherical.

With the increase in the concentration of Gd3+

, particle size decreased and this is

clearly seen in the TEM images. These results are also consistent with other rare earth

metal ions doped like Nd doped ZnO [Huang et al. (2011), Zhou et al. (2009)]. Fig.

4.12 (b) and 4.12 (b’) shows the average particle size, calculated by the Gaussian

Rare earth elements doped ZnO ….

4-17

distribution (solid lines) of the corresponding histograms of undoped and Gd3+

doped

ZnO nanoparticles, were 16.9 and 9.3 nm, respectively. Fig.s 4.12 (c) and 4.12 (c’)

show the HRTEM images of undoped and Gd3+

doped ZnO nanoparticles, where

equally spaced and aligned planes are also clearly seen. HRTEM shows the

interplaner distances (dhkl) is 2.5 Å, which is matching with (101) at 2θ = 36.22 from

XRD pattern of ZnO, corresponding to the maximum intense diffraction peak.

HRTEM investigation of single nanoparticles indicates the increased defects (Oxygen

vacancies, Oxygen interstitial, etc.) with an increase in the concentration of Gd3+

.

There is a clear variation of the lattice fringe contrast inside one particle (Fig. 4.12

(c’)). Such local reduction of contrast in HRTEM micrographs is attributed to the

clusters of lattice distortions inside the ZnO matrix due to the Gd3+

doping, indicated

by the white dotted region.

Fig. 4.12: TEM images of (a): undoped and (a’): 0.05 mol% doped ZnO

nanoparticles and their corresponding ((b) and (b’)) particle size distribution, ((c)

and (c’)) HRTEM and ((d) and (d’)) SEAD patterns.

The induced rings in the SAED of undoped sample (Fig. 4.12 (d)) are indexed

to the corresponding planes and well matched with the XRD pattern which indicates

the long range ordering among the unit cells. The diffused rings in the SAED pattern

(Fig. 4.12 (d’)) of Gd3+

doped sample is attributed to the short range ordering among

the unit cells caused by the doping which indicates the reduced crystallinity of Gd3+

Chapter 4

4-18

doped ZnO nanoparticles. Particle size calculated using Gaussian distribution from

TEM images were higher as compared with that calculated from the XRD results.

This was because of the fact that TEM is likely to contain several primary crystallite

grains leading to a larger TEM particle size than that measured by XRD which gave

the average mean crystallite size. A particle may be made up of several different

crystallites.

4.8 Optical studies

Fig. 4.13: Raman spectra of ZnO samples prepared with different Gd3+

amount, (a)

0.00, (b) 0.01, (c) 0.03, (d) 0.05 mol%.

The presence of Gd3+

in the ZnO lattice deforms the structure and can be

detected by Raman analysis. Fig. 4.13 shows the measured Raman spectra for the

Gd3+

doped ZnO samples with different concentration of the Gd3+

at room

temperature. The peaks at 100 cm-1

(E2low

) and 437 cm-1

(E2high

) are attributed to the

nonpolar E2 vibrational modes due to the vibration of Zn and O lattice in Wurtzite

ZnO. The transition at 200 and 331 cm-1

are attributed to 2E2low

and (multiple-

Rare earth elements doped ZnO ….

4-19

phonon-scattering process) E2high

-E2low

, respectively. The peak at 652 is associated to

the OCO symmetric bending. The band at 581 cm-1

assigned as A1 (LO) mode of ZnO

nanoparticles [Xing et al. (2003), Jeong et al. (2003), Scepanovic et al. (2010), Yang

et al. (2005)]. E2low

and E2high

are representing the wurtzite structure of good crystal

quality. The intensity of both the E2low

and E2high

modes decreased gradually with

Gd3+

concentration, which can be attributed to the lattice distortion in the ZnO matrix.

E2high

mode exhibits a visibly asymmetric line shape with a low frequency tail

and there is a negligible shift with an increase in Gd3+

concentration. FWHM of E2high

mode is also calculated and found that FWHM increased with an increase the Gd3+

concentration and the variation is from 13.51, 20.10, 25.94 to 28.46 cm-1

. The

intensity of E2high

peak decreased along with the broadening of this peak, as the Gd3+

concentration increase in the ZnO matrix which indicates the change in defects states

[Scepanovic et al. (2010)].

Fig. 4.14 shows the Lorentzian curve fitting results of the 1LO (581 cm-1

)

mode of ZnO nanoparticles. The intensity of 1LO mode increases with an increase in

the Gd3+

concentration. The broad asymmetric peak at 581cm-1

is assigned to the 1LO

mode, in agreement with reported values for bulk, thin-film and nanocrystalline ZnO

[Cao et al. (2006), Damen et al. (1966), Yang et al. (2008)]. It can be further

deconvoluted into two contributions positioned at 563 and 581 cm-1

(spectra, a) and

assigned to A1 (LO) and E1 (LO) [Cao et al. (2006)]. The position of A1 (LO) mode

shifts toward the higher wavenumber side from 563, 566, 570 to 577 cm-1

and

variation in the E1 (LO) mode is very slight, from 581, 582, 582 to 583 cm-1

. The

existence of both E1 (LO) and A1 (LO) contributions in the ZnO nanoparticles

indicates a random orientation in conformity with the XRD results [Cao et al. (2006)].

As the Gd3+

is increased, the separation between A1 (LO) and E1 (LO) mode is

decreased which indicates the reduced crystallinity or increased lattice distortion of

the samples.

Chapter 4

4-20

Fig. 4.14: Deconvoluted 1LO mode from Raman spectra of (a) undoped and Gd3+

doped ZnO nanoparticles (b) 0.01, (c) 0.03 and (d) 0.05 mol%.

To study the influence of Gd3+

doping on the optical properties of the ZnO

nanoparticles, PL measurement was performed at room temperature. Fig. 4.15 shows

the emission spectra of undoped and Gd3+

doped ZnO nanoparticles. ZnO

nanoparticles exhibited a strong visible emission centered at 562 nm and a sharp near

band edge emission at around 384 nm. There is very slight shift in band edge peak,

which indicates the existence of doping. These results are also consistent with other

RE metal’s ions doped like Nd-doped ZnO [Subramanian et al. (2009)], Er-doped

Rare earth elements doped ZnO ….

4-21

ZnO [Choi and Ma (2008)] and Ce-doped ZnO [Sofiani et al. (2006)]. On the other

hand, the broad emission band revealed in the visible region is due to the

superposition of green, yellow-orange and red emissions. It was reported that the

oxygen vacancies responsible for the green emission are mainly located at the surface

[Li et al. (2004)]. The yellow emission is due to the presence of OH groups, while

green emission is likely to originate from surface defects [Kumar and Sahare (2012),

Tam et al. (2006)]. Obtained ZnO precipitates are already washed several times with

ethanol to remove the by-products. But the prepared ZnO nanoparticles have residual

intermediate compounds on the surface in the form of an acetate group, which acts as

defect centers for the emission of green emission [Sakohara et al. (1998)]. These

acetate radicals are loosely bound on the surface of ZnO by dangling bonds

[Raghvendra et al. (2010)].

Fig. 4.15: PL spectra of the undoped and Gd3+

doped ZnO, (a) 0.00, (b) 0.01, (c)

0.03, (d) 0.05 mol% of Gd, measured at room temperature.

Fig. 4.15 also reveals the deconvoluted spectra of undoped and Gd3+

doped

ZnO nanoparticles. As the emission from the defects and impurities present in ZnO

tends to overlap in broad PL spectrum. To resolve these peaks, the visible broad

Chapter 4

4-22

emission band has been deconvoluted into multiple peaks (p1, p2 and p3 from left to

right located at 543, 615 and 708 nm). Peaks located at 2.28, 2.02 and 1.75 eV are

attributed to the Oi (oxygen interstitial), OH group and Vo (oxygen vacancies),

respectively [Tam et al. (2006), Lin and Fu (2001), Vanheusden et al. (1996), Zhou et

al. (2002), Kohan et al. (2000), Vlasenko (2009)]. Peaks p1, p2 and p3 increase with

an increase in the doping concentration. The deconvoluted component of oxygen

interstitial (2.28 eV) and oxygen vacancies (1.75 eV) becoming stronger with an

increase in doping concentration indicates the increased defect states which is also

confirmed by HRTEM and Raman studies. Also, the surface defects are very much

sensitive to the surface to volume ratio of the nanoparticles. Another deconvoluted

component of OH group also increases with an increase in Gd3+

concentration

because OH group depends on the surface to volume ratio of the nanoparticles. The

particle size of the ZnO nanoparticles decreases with Gd3+

concentration, as a result

surface to volume ratio increases which is responsible for the enhancement in the

intensity of peak p2. Also, according to previous reports, the reduction of particle’s

size usually induces the increase in the content of oxygen vacancies [Xiong et al.

(2009)]. In present study, Gd doping decreased the size of ZnO nanoparticles, which

increased the content of oxygen vacancies. Furthermore, to complete the valence site

of one Zn2+,

one O2-

will attach together to form a ZnO compound while in case of

Gd3+

, two Gd3+

will attached to the three O2-

to forms a Gd2O3 compound. In this

process two Gd3+

ions replace the three Zn2+

ions and oxygen concentration remains

constant. Thus, the reason of the change in oxygen vacancies (defect sites such as

oxygen vacancies) is not the replacement Zn2+

by Gd3+

but the increase of surface to

volume with decrease in size and is well reported in literature [Haase et al. (1988),

Antony et al. (2006)].

4.9 Magnetic studies

To investigate the magnetic properties of the Gd3+

doped ZnO nanoparticles,

magnetization hysteresis measurements were performed at room temperature. Fig.

4.16 shows the magnetization versus magnetic field (M-H) curves for ZnO: Gd3+

Rare earth elements doped ZnO ….

4-23

(Gd3+

: 0.00, 0.10, 0.03 and 0.05 mol%) nanoparticles. All the curves exhibited a well-

defined magnetization hysteresis implying ferromagnetic behaviour at room

temperature. All the samples have qualitatively similar hysteresis loops, although the

Gd3+

doped sample shows a remarkable enhancement of the magnetic properties. It

implies that the doping in ZnO nanoparticles increases the magnetic properties of

ZnO.

Fig. 4.16: Room-temperature magnetic hysteresis loop of undoped and Gd3+

doped

ZnO, (a) 0.00, (b) 0.01, (c) 0.03 (d) 0.05 mol %. The inset shows the enlarged view of

hysteresis loops near the centre.

From fig. 4.16, it can be inferred that the magnetization increases with the

increasing Gd3+

concentration. The saturation magnetization is found to be 0.023,

0.037, 0.044 and 0.051 emu/g for the samples 0.00, 0.01, 0.03 and 0.05 mol% of

Gd3+

, respectively. The concentration of oxygen vacancies played an important role in

mediating the ferromagnetism exchange between Gd3+

ions. For low concentration

(0.01 mol% doped Gd3+

sample), an appreciable ferromagnetic component is

developed despite the fact that this sample contains less Gd3+

ions as compared to

other samples. For the samples containing more Gd3+

ions have more oxygen

Chapter 4

4-24

vacancies might have been generated which are responsible for the observed long-

range ferromagnetism order. The corecivity is also increased with Gd3+

concentration

(shown in the inset of fig. 4.16).

4.10 Photocatalytic Studies

Fig. 4.17: Photodegradation of Rh B solution with time: (a) adsorption of Rh B in the

presence of undoped ZnO, (b) adsorption of Rh B in the presence of Gd3+

doped ZnO,

(c) Rh B in the presence of undoped ZnO under UV irradiation and (d) Rh B in the

presence of Gd doped ZnO under UV irradiation.

The photocatalytic activity of ZnO: Gd3+

(0.05 mol%) is investigated by

photocatalytic degradation of aqueous solution of the dye Rh B under UV light

irradiation as shown in Fig. 4.17 along with the catalysis of Rh B in the dark without

any irradiation. In the presence of ZnO: Gd3+

as the catalyst, the absorbance decreased

initially indicating adsorption of the dye Rh B (in the dark). Further, a substantial

decrease in the absorbance of Rh B is observed after conducting the reaction under

UV light irradiation with increasing time. The solution turned colorless within 65 min

of irradiation. Similar experiments were carried out for the pure ZnO nanocrystals as

Rare earth elements doped ZnO ….

4-25

well for the comparative study. From these experiments, the variation in the

concentration of the Rh B solutions is plotted against time (Fig. 4.17). These results

clearly demonstrated that ZnO: Gd3+

decolorizes Rh B faster than synthesized ZnO

under similar experimental conditions. This is attributed to higher oxygen vacancies

in ZnO: Gd3+

, induced by the Gd3+

ion doping. The presence of a higher concentration

of oxygen vacancies is further supported by the enhancement in the PL intensity.

Though one could not directly derive any correlation between the intensity of the

emission in the PL spectrum and the photocatalytic activity in excitonic oxide

semiconductors [Jing et al. (2006)], in which a possible intense PL emission could

possibly result in higher photocatalytic activity due to the higher concentration of

oxygen vacancies, offered a satisfying explanation for our observation.

4.11 Summary

The synthesis of Nd3+

and Gd3+

doped ZnO nanoparticles are successfully

carried out by wet chemical method. Morphology and particle size have been

determined via TEM technique. PL intensity increases prominently with the Nd3+

and

Gd3+

doping which is attributed to the enhanced defects and correlated with XRD and

Raman results. Nd3+

and Gd3+

doped ZnO nanoparticles exhibit room temperature

ferromagnetic properties which support the BMP model. Furthermore, results on

photocatalysis also demonstrate that rare earth elements doped ZnO decolorizes Rh B

organic dye faster than the undoped ZnO under UV light irradiation making it a strong

photocatalyst. Our results shows that doping of Nd3+

and Gd3+

in ZnO matrix reflects

almost similar structural, morphological, optical, magnetic and photocatalytic results.

Thus the rare earth element doped ZnO nanoparticles have many fascinating

properties, giving it the potential to be used for advanced optoelectronic and

spintronic applications.