Zinc sulphide and Cadmium sulphide core shell nanoparticles: The influence of Cadmium content on bandgap, structural, optical and

electrical properties.

Uzma Jabeen, Syed Mujtaba Shah*,

Department Of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan.

Corresponding author :

Email: smschem69@yahoo.com

Phone: 0092-51-90642205

Fax: 0092-51-90642241

Postal Address: Dr. Syed Mujtaba Shah, Assistant Professor of Chemistry

Department of Chemistry, Quaid-i-Azam University, Islamabad, 45320, Pakistan.

1

Abstract

Zinc sulphide, cadmium sulphide and their coreshell nanoparticals, (ZnS/CdS/ZnS) were

synthesized by wet chemical method. The structural, optical and electrical properties of the

nanoparticles were investigated using X-ray diffraction (XRD), energy dispersive X-ray

spectroscopy (EDX), scanning electron microscopy (SEM), UV-visible and photoluminescence

spectroscopy. Micro-structural analysis using Scanning Electron Microscope (SEM)

supplemented with EDAX were carried out for the samples to find grain size and chemical

composition. SEM study revealed that the nanoparticles were aggregated to form clusters. The

variation in band gap of ZnS by CdS doping was investigated using optical spectroscopy. A

significant bathochromic shift of absorption band was observed by increasing the cadmium

concentration in core shell assembly. In a similar fashion emission peak of ZnS nano composite

was also tuned in a visible region by increasing the concentration of cadmium. The specific

surface area of ZnS, CdS and core shell nanoparticles was calculated at different temperature

using BET method. Finally, electrical conductivity was worked out by measuring the resistance

of the nanomaterial (in pallets) pasted on microscopic glass. The conductivity of the material was

enhanced by increasing cadmium concentration.

Key words: ZnS nanoparticles, CdS nanoparticles, core shell, surface modification, lattice strain,

conductivity.

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1. Introduction

Semiconducting nanoparticles such as metal sulphide and metal oxides are well known for their

outstanding physical and chemical properties, owing to their use in photo-optics, optoelectronics,

electroluminescence devices, bio imaging technologies, light emitting devices, catalysis, sensors,

and solar energy cells. [1-7].

Size, shape, surface area and band gap energy are the important parameters which decide the

fate of nanoparticles for a specific application. Decrease in size leads to the increase of specific

surface area and bandgap energy. This is related to the quantum size effect or quantum

confinement [8]. Large surface area of nanoparticles results in their high adsorption capacity and

catalytic activity.

Doping impurities into nanoparticles is an effective approach for tuning their electronic,

magnetic, structural and optical properties for various desired applications [9 – 18]. ZnS is an

important II-VI semiconductor with a direct and wide band gap of 3.66 eV. It is widely used in

photovoltaic devices, solar cells and field emission devices (FED) [19 - 22]. Cubic ZnS is an

attractive host semiconductor for doping on account of its stability, low cost, and low toxicity

[23- 26]. A number of metal ions, such as Mn2+, Cu+, Pb2+, Cd2+ , Ag+, and Eu2+, have been

successfully doped into ZnS to produce photoluminescence or electroluminescence in different

regions of the visible spectrum[27- 33].

Surface coatings not only alter charge and reactivity of the materials but also enhance their

functional properties by the localization of the electron-hole pairs [34]. Recently the synthesis of

composite nanomaterials is one of the hottest research domain particularly core-shell

semiconducting nanoparticles wherein a low bandgap semiconductor is sandwitched between

two layers of large bandgap semiconductor [35-37]. Core-shell semiconductors are typically

classified as type-I or type-II, depending on the relative placement of conduction and valence

band edges. In the type-I, both the conduction and the valence band edges of one semiconductor

are positioned within the energy gap of the other semiconductor. In this case, an electron-hole

pair tends to confine in semiconductor with lower band gap, which provides the lowest energy

states for both electrons and holes. In the type-II, the lowest energy states for electrons and holes

are in different semiconductors [38]. These nanocomposite materials have widespread

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application in solar photovoltaic devices, chemical and biological sensors, and optoelectronics

such as light-emitting devices and optical switches [39]. Though much work has been done on

the synthesis of such nanocomposite materials but detailed investigation of their properties is yet

to be explored. Owing to its low bandgap, stability and photo-sensitizibility [40] we chose CdS

as a core and ZnS as shell material.

The aim of this work is to synthesize ZnS, CdS, and ZnS/CdS/ZnS core shell nanoparticles and

investigate their optical and structural properties by sandwiching CdS nanoparticles between two

ZnS layers. The influence of varying the concentration of shell precursor, CdS between two ZnS

particles layers on the variation of bandgap, structural, optical and electrical properties is

thoroughly investigated in this report in order to check its feasibility in opto-electronics and

photocatalysis.

2. Experimental

2.1 Chemicals

Zinc acetate (Zn (CH3COO)2•2H2O) and sodium sulfide, analytical grade were purchased from

Sigma Aldrich and were used as received. Deionized water and absolute alcohol (Sigma Aldrich)

were used as medium for the synthesis.

2.2 Synthesis of ZnS nanoparticles: ZnS nanoparticles were synthesized by wet chemical

method described elsewhere with a slight modification [41]. The synthesis was carried out in

water/ethanol mixed solvent (50:50). Zinc acetate (Zn (CH3COO)2•2H2O) and sodium sulfide

(Na2S•3H2O) were used as a precursors . For the synthesis of ZnS nanoparticles, 1 M of zinc

acetate was dissolved in 100 ml of solvent (50 mL deionized water and 50 mL ethanol). An

equimolar solution of Na2S was added drop by drop to the solution of zinc acetate under

continuous magnetic stirring at 60 ◦C until a homogenous solution was obtained. Thereafter the

solution was cooled to room temperature. After 60 min, a white precipitate was appeared. The

precipitatae was carefully settled down and the supernatant solvent was discarded. The

precipitate was washed with deionized water and ethanol several times to remove impurities.

Finally it was dried in oven at 120 ◦C for 2 h before subjecting them to structural and optical

characterization.

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2.3 Synthesis of CdS nanoparticles: CdS nanoparticles were also synthesized in water/ethanol

medium by wet chemical method [41]. Cadmium acetate (Cd (CH3COO) 2•4H2O) and sodium

sulfide (Na2S•3H2O) were used as precursors. In a typical experiment 0.2 M of cadmium acetate

was dissolved in 100 ml of solvent (water + ethanol, 50:50). An equimolar solution of Na2S was

added drop wise to the above solution under constant magnetic stirring at 60 ◦C. After the

addition of Na2S solution, a yellow precipitate was appeared. The supernatant solvent was

discarded and the precipitate was washed thrice with deionized water and ethanol to remove

impurities. The nanoparticles were dried in oven at 120 ◦C for 2 h and properly characterized.

2.4 Synthesis of ZnS/CdS/ZnS core-shell nanoparticles: In a typical experiment, 1 M of Zn

(CH3COO)2•2H2O was dissolved in 100 ml of distilled water–ethanol matrix (equal volume) and

an equimolar solution of sodium sulfide in the same solvent was slowly added to it drop wise.

The mixture was continuously stirred at 60 ◦C until a homogeneous white solution was obtained.

After 30 min, different concentrations of cadmium acetate solution (0.2–0.5 M in 100 ml

distilled water–ethanol matrix) were added to the above solution drop by drop. Consequently the

white color was tuned into greenish yellow. It could be due to the formation of the ZnS/CdS in

the colloidal solution. In the subsequent step, 0.5 M solution of zinc acetate and equimolar

solution of sodium sulfide solution (prepared in the same matrix) was added to the above

colloidal solution drop by drop with continuous stirring until the solution was turned yellow.

This indicated the formation of ZnS/CdS/ZnS core–shell nanoparticles. The reaction was stopped

and the yellow precipitate was carefully settled at room temperature. The precipitate was washed

thrice with deionized water and freshly distilled ethanol. Thereafter the core-shell nanoparticles

were dried in oven at 120 ◦C for 2 h [41] and subjected to different characterization techniques.

2.5 Characterization

UV-visible (Shimadzu 1601) and fluorescence spectrophotometer (Perkin Elmer LS55) were

used to explore the optical properties ofbare and core shell nanoparticles nanoparticles. The

purity and crystalline nature of synthesized nanoparticles were confirmed by X-ray diffraction

measurements. Scanning electron microscope (JSM 6490) supplemented with EDAX

(6490(LA)) was used for knowing the morphology, size and composition of nanocrystals.

5

Surface area analyzer BET was used to calculate surface area of nanoparticles. Conductivity

measurements of the nanocomposite material were conducted using Keithley 2400 Source Meter.

3. Results and discussions

3.1 Optical studies and bandgap tuning

Figure 1A shows the characteristic UV–visible (UV–Vis) absorption spectra of ZnS and CdS

nanoparticles. These spectra have broad absorption bands centered at 316 and 426 nm

respectively.

UV-Vis absorption spectra of core shell nanoparticles (ZnS/CdS/ZnS) with different CdS content

(0.2- 0.5 M) are shown in Fig. 2B. The absorption edge of ZnS/CdS/ZnS core shells shows a red

shift step by step compared to ZnS (316 nm, Fig 1A) with the increasing thickness of CdS. This

is depicted by the bathochromic shift of the broad absorptions centered at 333, 360, 380 and 395

nm. This shift could be associated with the partial leakage of the electronic wave function of the

ZnS into the CdS layer. When a shell is formed on the surface of the core, the total size is larger

than that of the core. As both the conduction and the valence band of CdS are located within the

energy gap of the ZnS nanoparticles, the potential well of ZnS/CdS/ZnS becomes deeper than

that of the pure CdS nanoparticles while decreased with respect to ZnS and consequently the

absorption spectra shows a red-shift.

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(1A) (1B)

Figure 1 (A) UV-vis absorption spectra of (A) bare ZnS and CdS nanoparticles (B) core-shell,

ZnS/CdS/ZnS nanopaticles with different CdS contents.

The UV-visible absorption studies play a vital role in estimating the optical band gaps using

Tauc relation. The optical band gap (Eg), was estimated from the extrapolation of the linear

portion in a plot of hυ versus (Ahυ)2, where A is the absorbance and hυ is the photon energy Ref.

table 1). It is observed that Eg in the bare ZnS and CdS nanomaterial is 3.58 eV and 2.6 eV

respectively. For the core shell nanoparticles ZnS/CdS/ZnS having 0.2 M concentration of

cadmium, Eg is found to decrease to 3 eV. Increasing CdS content beyond 0.2 M, Eg further

decreases to 2.74, 2.55 and 2.44eV, in the case of cadmium concentration of 0.3, 0.4 and 0.5 M

respectively. Figure 2 shows the Tauc plot for the calculation of the band gaps of ZnS, CdS and

core shell nanoparticles having different Cd concentrations.

Figure 2 Tauc plots for ZnS (dash), CdS (dot) and core shell nanoparticles, ZnS/CdS/ZnS, 0.2 M

CdS (dash dot) ZnS/CdS/ZnS, 0.3 M CdS ( dash dot dot) ZnS/CdS/ZnS, 0.4 M (short dash)

ZnS/CdS/ZnS, 0.5 M CdS (short dot).

7

Bandgaps, calculated from the above plots are summarized in table 1. These results show that

increasing CdS content in the core shell nanoparticles leads to decrease in the bandgap energy.

Thus CdS content plays a key role in engineering the bandgap and tuning it to the absorption in

visible region effectively.

Table 1. Optical band gaps of ZnS, CdS and core shell nanoparticles of different compositions.

Column 3 shows the decrease in bandgap with respect to ZnS.

Figure 3 shows the fluorescence emission spectra of bare ZnS and core shell nanoparticles with

different CdS content obtained by exciting the nanomaterial at 316 nm. A characteristic broad

blue emission band at 425 nm further evidenced the synthesis of ZnS nanoparticles. The PL

spectra of ZnS/CdS/ZnS core shell nanoparticles excited at 316 nm show a broad blue-green

emission spreading up to 600 nm which could be ascribed to emergence of multiple emission

bands along with the blue emission of ZnS. These spectra give the impression to have been

deconvoluted into three different bands with peak maxima at ~ 445, 484 and 530 nm

respectively. The first emission is ascribed to the recombination of electron-hole pairs in zinc

and sulphur vacancies of zinc sulphide nanoparticles. The second and third peaks are presumed

to originate from the radiative recombination of carriers at surface trap states resulting from the

effect of cadmium sulphide at ZnS/CdS/ZnS interface. The intense PL emission at 425 nm is

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Sample composition Band gap (eV) Decrease with respect

to ZnS (eV)

ZnS 3.58 _

CdS 2.6 _

ZnS/CdS/ZnS Cd 0.2 M 3.0 0.58

ZnS/CdS/ZnS Cd 0.3 M 2.74 0.88

ZnS/CdS/ZnS Cd 0. 4 M 2.56 1.00

ZnS/CdS/ZnS Cd 0. 5 M 2. 44 1.12

significantly red shifted in the visible region for core shell nano composites [41]. This is in

conformity with the absorbance spectra (Ref. Fig 1 B).

Figure 3 Emission spectra of Zns bare (short dash), core shell nanoparticles with different CdS

content, ZnS/CdS/ZnS, CdS 0.3 M (solid), ZnS/CdS/ZnS, CdS 0.4 M (dot), ZnS/CdS/ZnS, CdS

0.5 M (short dot)

3.1 Morphological studies

The grain size, shape and surface morphology were investigated by Scanning Electron

Microscope (JSM 6490). SEM images of ZnS, CdS , ZnS/CdS/ZnS (Cd 0.2 M) and

9

ZnS/CdS/ZnS (Cd 0.5 M) nanoparticles with different magnifications are depicted in figure 4A,

4B, 4C and 4D respectively. SEM image of ZnS nanoparticles reveals that particle size is not

uniform and the particles form irregular shaped clusters. On the other hand CdS nanoparticles

have regular shapes and uniform sizes. Addition of CdS at low concentration (0.2 M) has a

marked effect on the clusters of ZnS. This makes the particle size uniform. At higher CdS

concentration (0.5 M) the clusters assumes more or less spherical appearance with uniform

particle size approximately.

(4A) (4B)

(4C) (4D)

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Figure 4. SEM Micrographs of, (4A) Zinc sulphide, (4B) Cadmium sulphide, (4C)

ZnS/CdS/ZnS (Cd 0.2M) and (4D) ZnS/CdS/ZnS (Cd 0.5M).

To investigate the chemical composition of ZnS, CdS and their core shell nanoparticles

ZnS/CdS/ZnS , the elemental analyses of samples were performed by Energy Dispersive X-ray

Spectrometer (6490(LA)). Figure 5(A), 5(B), 5(C) and 5(D) shows the EDS spectra of the

synthesized ZnS , CdS and core shell nanoparticles. The spectrum in Figure 8 (A), shows the

EDS spectrum of ZnS, it reveals the presence of Zn and S peaks confirming the formation of zinc

sulphide but some additional peaks of oxygen are also present. The presence of oxygen atom in

nanostructure could be due to the distilled water used in the synthesis. It is also observed that

sample contains higher percentage of Zinc (Zn) and less percentage of sulfur (S). The average

atomic percentage ratio of Zn:S is 31.12:18.36. Figure 5 (B) shows the EDS spectrum of CdS,

reveals the presence of Cd and S peaks confirming the formation of cadmium sulphide. Some

additional peaks of oxygen are also noticed (already interpreted). The average atomic percentage

ratio of Cd:S is 35.68:33.37.

The EDS patterns of ZnS/CdS/ZnS core shell nanoparticles with low and high concentration of

CdS shell thickness are shown in Figure 5 (C) and 5 (D) respectively. Figure 5 (C) shows the

EDS spectrum of ZnS/CdS/ZnS (Cd 0.2M). It shows peaks for Zn and S along with weak peaks

for Cd supporting the formation of CdS shell over ZnS core. The average atomic percentage ratio

of Zn:S:Cd is 28.94:25.53:6.24. Figure 5 (D) shows the EDS spectrum of ZnS/CdS/ZnS (Cd

0.5M). It has peaks for Zn and S along with weak peaks for Cd supporting the formation of CdS

shell over ZnS core. The average atomic percentage ratio of Zn:S:Cd is 19.08:19.15:11.52.

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(a)

(b)

12

(c)

(d)

Figure 6 The EDX Spectra of synthesized (a) ZnS nanoparticles, (b) CdS nanoparticles

(C) ,ZnS/CdS/ZnS (Cd 0.2 M) core shell nanoparticles and (d) ZnS/CdS/ZnS (Cd 0.5 M) core

shell nanoparticles.

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Table 2 (a), (b), (c) and (d) gives the compositions of prepared sample of ZnS, CdS,

ZnS/CdS/ZnS (Cd 0.2M) and ZnS/CdS/ZnS (Cd 0.5M) respectively in weight and atomic

percentage.

3.2 Fourier transform infra-red spectroscopy

14

(a)

(c)

(b)

(d)

Figure 7 a, b and c shows the FTIR spectra of the synthesized ZnS, CdS and core shell

nanoparticles (ZnS/CdS/ZnS, Cd 0.3 M). FTIR spectra confirmed the purity and composition of

the samples. The FTIR spectra could be explained by the various peaks obtained by the samples.

The absorption peak in the range from 3600 to 3200 cm -1 corresponds to the –OH group of water

adsorbed by the samples. The weak absorption band at 1627 to 1636 cm -1was attributed to the

CO2 adsorbed on the surface of the particles, which is a quite common for nanosized powder

with a high surface area [42]. Small peak near 634-638 cm-1 indicated the formation of Zn-S and

Cd-S bonds as this region is assigned to Metal-Sulphur bonds [43].FT-IR spectra of core shell

nanocomposites (figure 7c) is more similar to CdS nanomaterial than ZnS. These results show

that CdS effectively covers the ZnS core.

(a)

15

(b)

(c)

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Figure 7 FT-IR spectra of (a) ZnS, (b) CdS and (c) core shell nanomaterials ZnS/CdS/ZnS

(Cd, 0.3 M)

3.2 Surface area of nanoparticles

Surface area of samples was determined by Brunauer-Emmett- Teller (BET) mehod using

Quantachrome instruments (Model NOVA 2000℮ surface area). Before the specific surface area

of the sample is calculated, it is necessary to remove gases and vapours that may have become

physically adsorbed onto the surface after synthesis and during treatment, handling and storage.

If out gassing is not achieved, the specific surface area may be reduced or may be variable

because an appreciable area of the surface is covered with molecules of the previously adsorbed

gases or vapours. The outgassing conditions are critical for obtaining the required precision and

accuracy of specific surface area measurements.

BET surface area of samples heated at different temperatures starting from 150 RC to 450 RC is

presented in figure 8. All samples have similarly shaped hysteretic behaviors. The value of

surface area first increases (maximum at 250 RC) and then decreases by increasing the heating

temperature. This result reveals that samples heated at 250 0C are more porous or have the

smallest particles size than the samples heated at other temperatures. The decrease in specific

surface area at high temperature may be attributed to the increase in grain size. On heating the

sample at high temperature (4500C), the particles become agglomerated which may lead to

decrease in the specific surface area.

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Fig. 8 surface area (BET) for ZnS (1), CdS (2), ZnS/CdS/ZnS (Cd 0.2 M) (3),ZnS/CdS/ZnS (Cd

0.3 M) (4), ZnS/CdS/ZnS (Cd 0.4 M) (5) and ZnS/CdS/ ZnS (Cd 0.5M) (6) nanoparticles at

different temperatures.

The results of above plots have been summarized in table 3.

Samples BET surface area m2 g-1

1500C 2500C 3500C 4500C

ZnS 36.58 56.82 43.55 22.32

CdS 65.29 79.55 68.903 51.66

ZnS/CdS/ZnS(Cd 0.2 M) 114.36 133.87 123.56 102.82

ZnS/CdS/ZnS (Cd 0.3 M) 110.61 128.22 119.68 96.77

ZnS/CdS/ZnS (Cd 0.4 M) 102.22 117.55 110.22 91.32

ZnS/CdS/ZnS (Cd 0.5 M) 93.97 112.21 104.92 89.301

Table 3. BET surface area of ZnS, CdS and ZnS/CdS/ZnS core shell nanoparticles at various

compositions and heating temperatures.

3.2 Electrical conductivity

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ZnS, CdS and their core shell nanoparticles were also characterized in terms of electrical

conductivity. The data is presented in table 4. Electrical conductivity was tested by measuring

the resistance values using digital multimeter (Keithley 2401). This test is important to know the

movement of electrons in nanocrystallites. According to the band theory of solids, the process of

conduction depends on the number of electrons in the conduction band. [47]. Conductivity is

increased when electrons in valence band are supplied with external energy and excited into the

conduction band. The following two equations were used to calculate the conductivity of ZnS,

CdS and their core shell nanoparticles.

Where R is resistance (Ω), l is length between 2 points (mm), and A is area of nanoparticle paste

onto glass substrate (μm), ρ is resistivity (Ωmm) and σ is conductivity (Ω-1m-1m).

Sample Resistance, Ω Resistivity, Ωm Conductivity, Ω-1m-1

ZnS 1.90E6 35.94E3 2.78E-5

CdS 9.38E6 78.54E3 1.27E-5

ZnS/CdS/ZnS Cd 0.2 M 7.34E6 61. 46E3 1.62E-5

ZnS/CdS/ZnS Cd 0.3 M 6.53E6 54.67E3 1.82E-5

ZnS/CdS/ZnS Cd 0.4 M 4. 43E6 37.09E3 2.69E-5

ZnS/CdS/ZnS Cd 0.5 M 2.99E6 25.036E3 3.99E-5

Table 4 . The average values of resistance, resistivity and conductivity of ZnS, CdS and their

core shell nanoparticles (ZnS/CdS/ZnS) nanoparticles at various Cd content.

The table shows that zinc sulphide has higher conductivity than cadmium sulphide but the the

addition of Cd at various concentrations has an appreciable effect on the conductivity of core

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shell nanoparticles. Core shell nanoparticles at the lowest Cd content (0.2 M), though less

conducting than bare ZnS, has higher conductivity than bare CdS. Further increase of the

cadmium content make the core shell nanoparticles more conducting than both the bare ZnS and

CdS. At cadmium content of 0.5 M, conductivity raises to 3.99 × 10-5 Ω-1m-1 which is higher than

bare ZnS and CdS by 1.21 × 10-5 Ω-1m-1 and 1.72 × 10-5 Ω-1m-1 respectively.

Conclusion: Zinc sulphide , cadmium sulphide and their core shell nanoparticles were

successfully synthesized by chemical route. They were characterized by scanning electron

microscopy, electron diffraction X-ray analysis, fourier transform infra-red spectroscopy, UV-

Vis absorption, photoluminescence spectrophotometry and BET surface area analyzer. The

structural, optical and electrical properties of the bare ZnS, CdS and their core shell nanoparticles

were thoroughly investigated. The effect of cadmium content on the particle size and bandgap

tuning in the core shell nanoparticles was also studied. It was found that the change in the

concentration of Cd had a significant effect on the absorbance. The absorbance peak was

progressively red-shifted by increasing Cd content and consequently the bandgap was reduced

from 3.58 eV (ZnS) to 3 eV for the core shell nanoparticles (Cd, 0.2 M) which was further

reduced to 2.44 eV when the cadmium content was raised to 0.5 M. SEM showed that increasing

the Cd content induced uniformity in the particle size and led to spherical shaped clusters. EDX

pattern gave evidences of the presence of Zn, Cd and S in core shell nanoparticles approximately

in the same ratio in which they were mixed to synthesize the material. Specific surface area of

the core shell nanoparticles was found maximum at 250 RC, by further rising the temperature it

was decreased due the agglomeration of the particles at high temperature. Electrical conductivity

of the core shell nanoparticles was progressively increased by rising Cd content. It attained a

maximum value of 3.99 × 10-5 Ω-1m-1 at cadmium content of 0.5 M which was found higher than

bare ZnS and CdS by 1.21 × 10-5 Ω-1m-1 and 1.72 × 10-5 Ω-1m-1 respectively.

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