CHAPTER- 3

Transition metal doping effects: Influence of Ni(II)/Pd(II)

doping effects on tris(thiourea)zinc(II) sulphate crystals

ABSTRACT

Single crystals of Ni(II)/Pd(II) doped tris(thiourea)zinc(II)-

sulphate (ZTS) are grown from an aqueous solution by conventional

slow evaporation solution growth technique. The characteristic

functional groups are identified by FT-IR analysis. Crystal stress is

indicated by powder XRD patterns and FT-IR analysis. Incorporation

of dopant into the crystalline matrix during crystallization process is

evidenced by energy dispersive X-ray spectroscopy and quantified by

inductively coupled plasma technique. The surface morphological

changes are observed in the doped specimen. Mechanical stability of

the as-grown specimen is analyzed by Vickers microhardness

analysis. Thermal studies reveal no decomposition upto the melting

point. Lattice parameters determined by single crystal XRD analysis

reveal only minor variations as a result of low doping levels.

Ni(II)/Pd(II) doping has a catalytic effect on the second harmonic

generation efficiency of metal thiourea complex.

3.1. INTRODUCTION

Tris(thiourea)zinc(II) sulphate is a semi-organic nonlinear

optical material which finds applications in the area of laser

Published in Optik, 2014, 125, 710–713.

35

technology, optical communication, data storage and optical

computing because it has a high resistance to laser induced damage,

high nonlinearity, wide transparency, low angular sensitivity and good

mechanical hardness compared to many organic NLO crystals1-4. ZTS

is a good engineering material for second harmonic generation (SHG)

device applications. It is a novel metal organic crystal with potential

application in electo-optic modulation5. It crystallizes in orthorhombic

system with noncentrosymmetric space group Pca21 (point group

mm2). Metal ion doped materials are currently receiving a great deal

of attention due to the rapid development of laser diodes6, 7. Transition

metal impurity in the crystalline matrix generally influences the

physical properties of the crystal8-13. Palladium doping enhances the

gas sensing properties of nanoparticles14-19. Incorporation of

palladium strongly affects the manganese reducibility and shifts the

reduction process to lower temperature20. Also, palladium doping

enhances the stability of gold cluster21. Hexacoordinated bivalent

nickel complexes usually show three spin-allowed absorption bands in

the near infrared, visible and near ultraviolet regions22. The dielectric

constant of the base compositions increased with the addition of NiO

upto 2 mol %23. Widening of the band gap with increased Ni doping

was also discussed with Burstein-mass effect and the Ni doped ZnO

can be considered a suitable candidate for DSSC applications24.

Recently, we have investigated the effect of doping organic

additives25-27, Mn(II)28, Ce(IV)29, Cs(I)30, Mg(II)31 and benzene32 on ZTS

crystals. In the present investigation, the effect of Ni(II)/Pd(II) doping

36

ZnSO47H2O + 3(CS(NH2)2) Zn(CS(NH2)2)3SO4

on ZTS crystals has been studied using FT-IR, XRD, SEM and EDS,

microhardness thermal, dielectric studies and Kurtz powder SHG

measurements.

3.2. EXPERIMENTAL

3.2.1. Synthesis and crystal growth

ZTS was synthesized33 using zinc sulphate heptahydrate

(EM) and thiourea (SQ) in a stoichiometric ratio of 1:3. To avoid

decomposition, low temperature (<70°C) was maintained during the

preparation of solution in deionized water.

After successive recrystallization processes, crystals were grown by

slow evaporation solution growth technique26, 29. Doping of nickel

(5 mol %) in the form of nickel sulphate and palladium (5 mol %) in

the form of palladium chloride was done during the crystallization

process. The crystallization took place in 20-23 days and the

macroscopic defect-free crystals were harvested. Photographs of

as-grown nickel(II)/palladium(II) doped ZTS crystals are shown in

Fig. 3.2.1.

37

Fig. 3.2.1. Photographs of (a) pure (b) Ni(II) doped and

(c) Pd(II) doped ZTS crystals

(a)

(b)

(c)

38

3.3. RESULTS AND DISCUSSION

3.3.1. FT-IR

A close observation of FT-IR spectra (Fig. 3.3.1) of the

doped specimens reveals that the doping results in small shifts in

some of the characteristic vibrational frequencies (Table 3.3.1). The

CN stretching frequencies of thiourea (1089 cm-1 and 1472 cm-1) are

shifted to higher frequencies for Ni(II)/Pd(II) doped ZTS (1152 cm-1 and

1514 cm-1/1157 cm-1 and 1505 cm-1). The CS stretching frequencies

(1417 cm-1 and 740 cm-1) are shifted to lower frequencies (1403 cm-1

and 716 cm-1/ 1430 cm-1 and 713 cm-1) for Ni(II)/Pd(II) doped

specimens.

Table 3.3.1. FT-IR frequencies (cm-1)

S.

No. Thiourea ZTS

Ni(II) doped

ZTS

Pd(II) doped

ZTS

Assignment

of vibrations

1 3376 3399 3370 3406 as(NH2)

2 3167 3195 3177 3196 s(NH2)

3 1627 1624 1631 1626 (NH2)

4 1089 1122 1152 1157 s(C-N)

5 1472 1502 1514 1505 (N-C-N)

6 1417 1398 1446 1430 as(C=S)

7 740 712 716 713 s(C=S)

8 648 619 --- 619 as(N-C-S)

9 492 471 533 476 as(N-C-N)

39

Fig. 3.3.1. FT-IR spectra of (a) pure (b) Ni(II) doped and

(c) Pd(II) doped ZTS

(a)

(b)

(c)

3567

3196

3406

1626

1505

1430

1400

713

619

476

1157

3399

3307

3195 1624

1502

1398

1122

951

712

619

471

953

533 7

16

1032

1152

1631

1514

1446

3177

3370

3446

4000 3000 2000 1000

Wavenumbers (cm-1)

% T

ran

sm

itta

nce

An absorption band in the region 2750 to 3400 cm-1

corresponds to the symmetric and asymmetric stretching frequencies

of NH2 group of zinc(II) coordinated thiourea. Spectral studies suggest

that the metal coordinate with thiourea through sulfur atom.

40

3.3.2. XRD analysis

A comparison of powder XRD patterns of pure and doped

Ni(II)/Pd(II) specimens clearly reveal the intensity variations

(Fig. 3.3.2). Diffraction patterns indicate no phase changes and

narrow peaks indicate good crystallinity. However, a slight variation in

intensity is observed as a result of doping. The most prominent peaks

with maximum intensity of the XRD patterns of pure and doped

specimens are quite different. These observations could be attributed

to the strains in the lattice. The grown crystal belongs to the

monoclinic system and the cell parameters of pure and doped

specimens are given in the Table 3.3.2. Minor variations in cell

parameters are justified because of low doping levels.

Table 3.3.2. Lattice parameters

Crystal a/(Å) b/(Å) c/(Å) Volume/(Å3)

Pure ZTS* 11.21 7.72 15.60 1350.23

Ni(II) doped ZTS 11.08 7.75 15.42 1323.00

Pd(II) doped ZTS 11.32 7.80 15.51 1369.00

*JCPDS No. 76-0778

41

Fig. 3.3.2. Powder XRD patterns of (a) pure

(b) Ni(II) doped and (c) Pd(II) doped ZTS

(a)

(b)

(c)

Position [°2 Theta]

Cou

nts

42

(a) (b) (c)

Fig. 3.3.3.1. SEM micrographs of (a) pure (b) Ni(II) doped and

(c) Pd(II) doped ZTS

3.3.3. SEM and EDS

The effect of the influence of dopant on the surface

morphology of ZTS crystal faces reveals structure defect centers and

voids as seen in the SEM micrographs (Fig. 3.3.3.1). SEM

micrographs of the doped specimen show more scatter centers than

that of the undoped specimen. A small quantity incorporation of

Ni(II)/Pd(II) in the doped specimen was confirmed by EDS as clearly

seen in Fig. 3.3.3.2.

Inductively coupled plasma (ICP) analysis indicates that the

palladium incorporation (1.2 ppm) is small but significant. The dopant

concentration in the host lattice is not proportional to the

concentration taken during the crystallization process since the host

crystal can accommodate the dopant only to a limited extent. Ease of

incorporation even by small quantity doping is justified by comparing

the ionic radii of Zn2+ (74 ppm) with that of Pd2+ (78 ppm).

43

3.3.4. Thermal studies

The simultaneous TG/DTA curves in nitrogen recorded for

doped ZTS specimens are given in the Fig. 3.3.4. The absence of

water of crystallization in the molecular structure is indicated by the

absence of weight loss around 100º C. The melting point of sample is

Fig. 3.3.3.2. EDS spectra of (a) pure

(b) Ni(II) doped and (c) Pd(II) doped ZTS

(a)

(b)

(c)

44

Fig. 3.3.4. TG/DTA curves of (a) Ni(II) doped and (b) Pd(II) doped ZTS

Temperature (°C)

DTA

/(m

W/m

g)

TG

/%

(a)

(b)

slightly increased in the case of Ni(II) doping whereas it is slightly

decreased in the case of Pd(II) doped ZTS suggesting the

incorporation. No decomposition upto the melting point ensures the

suitability of the material for the application in lasers where the

crystals are required to withstand high temperatures.

3.3.5. Mechanical studies

Transparent crystals free from cracks were selected for

microhardness measurements. Before indentation, the crystals were

45

Fig. 3.3.5. Plot of Vickers hardness number vs Load

Load (g)

Hard

ness n

um

ber

(kg/m

m2)

carefully lapped and washed to avoid surface effects. The Vickers

hardness indentations were made on the as-grown surface of the

Ni(II)/Pd(II) doped crystal at room temperature with the load ranging

from 25 to 100 g, keeping the time of indentations kept as 10 sec for

all trials. The Vickers hardness number Hv was calculated from the

following equation,

Hv =1.8544(P/d2) kg/mm2

where P is the applied load in kg and d is the mean diagonal length of

the indentation impression in micrometer.

A plot of Vickers hardness number (Hv) versus load (P) reveals that the

hardness of the grown crystal increases as the load increases

(Fig. 3.3.5). Cracks started developing around the indentation mark

beyond a load of 100 g. This may be due to the internal stresses

46

released during the indentation.The hardness of the material is found

to increase with the increase of load confirming the prediction of

Onitsch34.

3.3.6. Dielectric properties

Fig. 3.3.6(a-c) is the plot of dielectric constant (r),

dielectric loss (tan δ) and AC conductivity (σac) versus temperature

at different frequencies for both pure and Ni(II) doped specimens. It

can be seen that dielectric parameters are temperature dependent.

It is observed that both r and tan δ are inversely proportional to

frequency. The dielectric constant of a material is generally

composed of four different types of contributions like ionic, electronic,

orientational and space charge polarizations. The large value of

dielectric constant at low frequencies may be due to contribution of all

these polarizations. The decreased dielectric constant at higher

frequencies could be due to the reduction in the space charge

polarization. Space charge polarization is generally active at lower

frequencies and high temperatures indicating the purity and

perfection of the grown crystal35. The increase in conductivity could be

attributed to reduction in the space charge polarization at higher

frequencies36. In the present study, dielectric constant varying

proportionally with temperature is essentially due to the temperature

variation of the polarizability37. The low r value dielectric materials

have potential applications in microelectronic industries.

47

Fig. 3.3.6. Dielectric measurements for pure and Ni(II) doped ZTS

(a) Plot of dielectric constant vs temperature

(b) Plot dielectric loss vs temperature

(c) Plot of AC electrical conductivity vs temperature

Temperature (K)

(a)

(b)

(c)

Ni(II) doped

Ni(II) doped

Ni(II) doped

(a)

(b)

(c)

pure

pure

pure

48

3.3.7. SHG efficiency

The SHG of the specimens were measured by the Kurtz

powder technique38. An Nd:YAG laser with modulated radiation of

1064 nm was the optical source. Uniform particle size (125-150 μm)

specimens were exposed to laser and the output was monochromated

to collect the intensity of the 532 nm component. In order to confirm

the influence of doping on the SHG-activity and as a comparative

measure the pure and Ni(II)/Pd(II) doped specimens were subjected to

SHG test. The output SHG intensities give the relative efficiencies of

the measured specimens (Table 3.3.3). The effect of various metal

dopants on the SHG efficiency of ZTS has been listed in

Table 3.3.4. From this data, it is clear that the metal doping

enhances the SHG efficiency. Metals can behave as electron donors or

acceptors and it appears that they facilitate the charge transfer

thereby nonlinearity. A favorable molecular alignment for nonlinearity

is achieved by incorporation of small quantity of metal into the

crystalline matrix. Small quantity incorporation of Ni(II)/Pd(II) doping

has a catalytic effect on SHG efficiency of ZTS.

Table 3.3.3. SHG outputs

System I2ω (mV) Input energy

(mJ/pulse)

Pure ZTS 2.7 8.8

Ni(II) doped ZTS 3.9 8.8

Pure ZTS 14 2.5

Pd(II) doped ZTS 25 2.5

49

Table 3.3.4. SHG outputs of ZTS doped with metals

System Relative efficiency Reference

ZTS doped with Ni(II) ~1.4 times of ZTS Present work

ZTS doped Pd(II) ~1.7 times of ZTS Present Work

ZTS doped with Mn(II) ~ 2.1 times of ZTS 28

ZTS doped with Ce(IV) ~ 1.6 times of ZTS 29

ZTS doped with Cs(I) ~1.2 times of ZTS 30

ZTS doped with K(I) ~ 1.2 times of KDP 39,40

ZTS doped with Na(I) Slightly greater than ZTS 41

ZTS doped with Li(I) Green light emission 42

ZTS doped with Cd(II) Greater than ZTS 43

3.4. CONCLUSIONS

The influence of Ni(II)/Pd(II) doping on tris(thiourea)zinc(II)-

sulphate has been systematically investigated. A close observation of

XRD and FT-IR profiles of doped specimens reveal some minor

variations, indicating the crystal stress. Doping results in surface

roughness and defects. The transition metal can act as an electron

donor or acceptor and doping is advantageous since it improves the

SHG efficiency of the host crystal to an appreciable extent. Pd(II) is a

more effective dopant than Ni(II) in enhancing the efficiency and it is

quite likely due to the varied electronic configurations.

50

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