Physica B 407 (2012) 1223–1226

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Physica B

0921-45

doi:10.1

n Corr

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E-m

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Cu-doped ZnO nanoparticles: Synthesis, structural and electrical properties

Sonal Singhal n, Japinder Kaur, Tsering Namgyal, Rimi Sharma

Department of Chemistry, Panjab University, Chandigarh 160014, India

a r t i c l e i n f o

Article history:

Received 30 November 2011

Received in revised form

21 December 2011

Accepted 17 January 2012Available online 2 February 2012

Keywords:

Transmission Electron Microscopy

X-ray diffraction

Electrical properties

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016/j.physb.2012.01.103

esponding author. Tel.: þ91 172 2534421 (o)

1 172 2545074.

ail address: sonal1174@gmail.com (S. Singhal

a b s t r a c t

Pure and Cu doped ZnO nanopowders (5, 10, 15, 20, 25 and 30 at% Cu) have been synthesized using co-

precipitation method. Transmission Electron Microscopic analysis has shown the morphology of ZnO

nanopowders to be quasi-spherical. Powder X-ray Diffraction studies have revealed the systematic

doping of Cu into the ZnO lattice up to 10% Cu, though the peaks corresponding to CuO in 10% Cu are

negligibly very small. Beyond this level, there was segregation of a secondary phase corresponding to

the formation of CuO. Fourier Transform Infrared spectra have shown a broad absorption band at

�490 cm�1 for all the samples, which corresponds to the stretching vibration of Zn–O bond. DC

electrical resistivity has been found to decrease with increasing Cu content. The activation energy has

also been observed to decrease with copper doping i.e. from �0.67 eV for pure ZnO to �0.41 eV for

30 at% Cu doped ZnO.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

ZnO is considered to be one of the most important oxide materialsowing to its unique features and wide range of technologicallyimportant applications. ZnO is an n-type semiconductor with a widebandgap of 3.37 eV, a large exciton binding energy of 60 meV and ischemically and thermally stable. Moreover, it is cheap and envir-onmentally friendly as compared to other metal oxides. Due to theseproperties, it has found potential applications in fields, such as gassensors [1], solar cells [2], varistors [3], light emitting devices [4],photocatalyst [5], antibacterial activity [6] and cancer treatment [7].ZnO lacks center of symmetry, which makes it beneficial for its use inactuators and piezoelectronic transducers. A number of methods havebeen devoted for the fabrication of transition metal doped ZnOnanoparticles, such as auto-combustion method [8], ball-millingmethod [9], co-precipitation method [1], sol–gel process [10], hydro-thermal route [11,12] and so on. Among these methods, co-precipita-tion method is of great interest because of its simplicity, lowequipment cost, relatively lower processing temperature and isenvironmentally benign. This method is advantageous over othermethods because the reagents are mixed at molecular level andtherefore, there is good control of stoichiometry, morphology, purityand homogeneity.

Properties of ZnO can be tuned according to the research interest,by doping with various metal atoms to suit specific needs andapplications. The metal doping induces drastic changes in optical,electrical and magnetic properties of ZnO by altering its electronic

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, þ91 9872118810 (m);

).

structure. Many authors have reported the changes induced byincorporation of transition metal ions into ZnO lattice [13–15].

Albeit the large number of reports on transition metal doped ZnOsystem, very less work is done on Cu doped ZnO. Substitution ofcopper into the ZnO lattice has shown to improve properties such asphotocatalytic activity, gas sensitivity and magnetic semiconductivity[16–18]. Copper doped zinc oxide Zn0.95Fe0.03Cu0.02O was foundto exhibit ferromagnetic performance at room temperature [19].But this Cu incorporation reduced the saturation magnetization ofFe-doped ZnO magnetic semiconductors. Photoluminescence (PL) ofCu doped ZnO nanocrystals were found to show pronounced UVemission and negligible visible emission with peak positions coincid-ing with that of undoped ZnO [20].

Literature shows the substitution limit of Cu in ZnO to be low(around 5 at%) [21]. So, this work is a strategy to synthesizehigher compositional level of copper doped ZnO lattice. Thepresent investigation deals with the synthesis of Cu doped ZnOnanopowders with Cu content varying from 5 to 30 at% via co-precipitation method, followed by the characterization of samplesusing Transmission Electron Microscopy (TEM), Powder X-rayDiffraction (XRD) and Fourier Transform Infrared Spectroscopy(FT-IR) techniques. Influence of Cu content on the structural andelectrical properties has also been investigated.

2. Experimental

2.1. Synthesis

Nanocrystalline powders of Cu doped ZnO with 0, 5, 10, 15, 20,25 and 30 at% Cu, have been synthesized via co-precipitationmethod using ZnCl2 and CuCl2 �2H2O as starting materials. The

0

1000

2000

3000

4000

5000

6000

7000

25 30 40 50 60 70

(201

) (2

00) (1

12)

(103

)

(110

)

(102

)

**

(002

) (100

)

Angle (2θ)

Rel

ativ

e In

tens

ity

(g)

(101

)

(f)(e)(d)(c)(b)(a)

* CuO

Fig. 2. X-ray diffraction patterns of ZnO doped with (a) 0.0 at% Cu, (b) 5 at% Cu,

(c) 10 at% Cu, (d) 15 at% Cu, (e) 20 at% Cu, (f) 25 at% Cu and (g) 30 at% Cu, annealed

at 600 1C.

S. Singhal et al. / Physica B 407 (2012) 1223–12261224

reagents were of analytical grade and were used without furtherpurification. The constituents, in desired proportion, have beendissolved in distilled water and carefully mixed. Aq. ammonia hasbeen added slowly to the solution, controlling pH of the solutionwithin range 7–8. The resulting precipitates have been collected,thoroughly washed with distilled water, and dried in oven for3–4 h. The samples were then annealed in Muffle furnace at600 1C for 2 h under normal atmospheric conditions, to improvethe crystallinity of samples.

2.2. Physical characterization

The morphological studies have been carried out using TEM(H-7500 Instrument, HITACHI, operated at 120 kV). The crystalstructure of samples has been investigated using Bruker-AXS D8ADVANCE X-ray Diffractometer (XRD) with CuKa radiation ofwavelength 1.541 A and scanning angle 2y ranging from 101 to1001. The crystallite size has been determined using Debye–Scherrer formula. The FT-IR spectra of the samples have beenrecorded using Perkin Elmer IR Spectrophotometer with KBrplates over the range 4000–400 cm�1. For investigating electricalproperties, conductivity measurements have been carried outusing the two probe method in the temperature range 300–400 K.The powder samples have been consolidated by applying pressure of5ton to make a pellet of thickness 1–2 mm and diameter 12 mm.Silver paint was applied on both sides of the pellet and the pelletwas inserted between probe and base plate and a constant voltagewas applied and corresponding current values were noted atdifferent temperatures.

3. Results and discussion

3.1. Transmission Electron Microscopy (TEM)

Fig. 1 shows the typical TEM micrograph of ZnO. TEM micro-graph has revealed the formation of quasi-spherical nanoparticleswith non-uniform particle size distribution. This in-homogeneityof particle size can be due to the aggregation of nanoparticles.

100 nm

Fig. 1. TEM Micrograph of ZnO nanopowder annealed at 600 1C.

Large surface area to volume ratio and high surface energy areresponsible for this aggregation.

3.2. Powder X-ray Diffraction (XRD) analysis

The structural parameters and phase purity have been studiedusing the powder X-ray diffraction patterns. Fig. 2 shows the XRDpattern of undoped and doped powders with varying concentra-tions of copper. Peaks have been found to be quite sharp andintense which implies high crystallinity of samples. The analysisof diffraction peaks has revealed the presence of wurtzite struc-tural phase in all the compositions and no trace of copper relatedphase (metallic copper, oxides of copper or any binary zinc copperphase) has been detected for 5% and 10% Cu doped sample,though the CuO peaks for the 10% Cu-doped are negligibly verysmall. This indicates that the Cu ions have substituted Zn siteswithout affecting the crystal structure of ZnO much. This is due tothe fact that ionic radius of Cu2þ (0.73 A) is very close to that ofZn2þ (0.74 A), due to which Cu can easily penetrate into ZnOcrystal lattice. However, with the increase in doping percentage ofCu to 15%, very weak peaks corresponding to CuO appeared, andthese have been found to grow in intensity with further increasein Cu doping. So for 15% Cu doping, CuO started segregating. So, atsmaller doping concentrations of Cu, its ions can very wellsubstitute Zn ions, but with increasing Cu concentration, CuOstarts to form cluster and is isolated as impurity phase.

The lattice parameters have been calculated using Le-Bail andPawley refinement method. The unit cell parameters ‘a’ and ‘c’obtained from the XRD data for all the samples are tabulated inTable 1. For Cu doped samples, lattice parameters ‘a’ and ‘c’ havebeen found to be smaller as compared to those of undoped ZnO.This is due to a very small mismatch in ionic radius between Zn2þ

and Cu2þ . However, there is no systematic variation in latticeparameters with increasing Cu content. The c/a parameter hasalso been found to show a good match with the value 1.633 forideally close packed hexagonal structure. The volume of unit cellfor hexagonal system has been calculated from the followingequation [22]:

V ¼ 0:866� a2 � c ð1Þ

All the calculated parameters are presented in Table 1.

Table 1Lattice parameters, crystallite size, volume and bond lengths of ZnO doped with

varying copper content annealed at 600 1C.

Coppercontent(at%)

Crystallitesize (nm)

Lattice parameter Volume

(A3)

Zn–O bond

length (A)a (A) c (A) c/a

0 48.64 3.2496 5.2058 1.6019 47.6058 1.9774

5 45.20 3.2494 5.2054 1.6019 47.5977 1.9773

10 46.45 3.2483 5.2026 1.6016 47.5393 1.9766

15 44.69 3.2480 5.2023 1.6017 47.5279 1.9764

20 46.43 3.2492 5.2040 1.6016 47.5772 1.9771

25 33.18 3.2497 5.2051 1.6017 47.6039 1.9774

30 36.03 3.2492 5.2037 1.6015 47.5767 1.9772

Wavenumber

Rel

ativ

e %

Tra

nsm

issi

on

Fig. 3. FT-IR spectra of (a) pure and (b) 10 at% Cu, (c) 20 at% Cu and (d) 30 at% Cu

doped ZnO nanopowders annealed at 600 1C.

S. Singhal et al. / Physica B 407 (2012) 1223–1226 1225

The Zn–O bond length L is given by [23]

L¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2

3þ

1

2�u

� �2

c2

!vuut ð2Þ

where the u parameter is given by (in the wurtzite structure)

u¼a2

3c2þ0:25 ð3Þ

The Zn–O bond lengths of undoped and Cu doped ZnO aregiven in Table 1. It has been observed that with the increase in Cucontent, bond length decreases up to 15% doping. This can be dueto the fact that once Cu2þ ions replace Zn2þ ions, Cu–O bonds arealso formed in ZnO lattice, whose bond length is smaller than Zn–O bond length. However, with further increase in Cu doping, thereis an increase in bond length values. This could be due to thesegregation of CuO.

The average crystallite size has been estimated by measuringthe full-width at the half-maximum of the most intense diffrac-tion peak (1 0 1) using Debye–Scherrer equation [22]:

d¼0:9lbcosy

ð4Þ

where d is the average crystallite size, l is the wavelength of theincident X-ray beam (1.541 A), y is the Bragg’s diffraction angleand b is the angular width of the diffraction peak at the half-maximum in radians on 2y scale. The average crystallite sizes ofthe samples have been found to be in 33–49 nm range and arepresented in Table 1. The average crystallite size for Cu doped ZnOhas been observed to be smaller than that of undoped ZnO. Toprevent a particle growth, the motion of grain boundary must beimpeded. Resistance of the motion of grain boundaries can beexplained by Zener-Pinning Effect [24]. This motion is preventedby either precipitation of secondary phase or contamination atthe surface of ZnO. When the moving boundaries attach the zincinterstitial and the substituted copper ions, moving boundariesare obstructed by the generation of retarding force. If the retard-ing force is more than the driving force for the grain growth, theparticles cannot grow any longer. So the presence of Cu2þ ions inthe ZnO prohibited the growth of crystal grains. It is noticeablethat the variation of the crystallite size with the copper concen-tration is not monotonic.

3.3. Fourier Transform Infrared Spectroscopy (FT-IR)

FT-IR Spectroscopy has been employed to study the influenceof Cu doping on Zn–O bonding. FT-IR spectra for varying Cucontent have been recorded in the range 4000–400 cm�1 and theresults are shown in Fig. 3. A highly intense broad band has beenobserved at around 490 cm�1 for the pure ZnO corresponding tothe formation of Zn–O bond [25]. Bands observed in the ranges

3050–2820, 1540–1340 and 760–720 cm�1 are due to nujol,which has been used as mulling agent. A broad peak observedat �3505 cm�1 has been attributed to –OH group of H2O, whichindicates the existence of water adsorbed on the surface ofnanocrystalline powder. An absorption band has been observedat �2400 cm�1 which arises from the absorption of atmosphericCO2 on the metallic cations. Similar features have been observedfor all the samples. However a slight shift in the position ofabsorption band has been observed with increase in Cu content.The shift in band position can be related to the change in the bondlength that takes place upon substitution of Zn with Cu. More-over, with increase in doping percentage, additional peaks startappearing at 620 and 430 cm�1 which are related to the vibrationof Cu(II)–O bond. Thus the formation of ZnO wurtzite structurehas been further corroborated by FT-IR spectra.

3.4. DC electrical characterization

ZnO is a well-known n-type semiconductor. Its electrical con-ductivity at room temperature is associated with intrinsic defects(zinc interstitials and oxygen vacancy), which can lead to shallowdonor in ZnO. However, there is contradictory opinion amongstdifferent authors for room temperature intrinsic conduction ofZnO. Simpson and Cordero [26] have reported oxygen vacancy tobe responsible for room temperature electrical conductivity. On theother hand, Sukker and Tuller [27] have proposed Zinc interstitialsto be the main cause of electrical conductivity of ZnO.

The DC electrical conductivity measurements have been car-ried out in the temperature range 300–400 K. The electricalresistivity (r) has been calculated by the formula:

r¼ VA

Itð5Þ

where V is the applied voltage, I is the measured current, A is the areaof the pellet and t is the thickness of the pellet. The electricalresistivity has been found to decrease with the increase in Cuconcentration. This can be explained on the basis of smaller resistivityof Cu (1.67�10�6 O cm) than that of Zn (5.92�10�6 O cm).

The temperature dependency of the DC resistivity can beshown by the well-known Arrhenius equation, given by

r¼ ro expEa

KBT

� �ð6Þ

where ro is the pre-exponential factor, Ea is the activation energy,KB is the Boltzmann constant and T is the temperature (in Kelvin).

Res

istiv

ity ×

107

(ΩΩ m

)

Temperature (K)

Fig. 4. Typical plot of dc electrical resistivity with temperature for ZnO doped

with 5 at% Cu nanopowder annealed at 600 1C.

1000/T (Kelvin-1)

log

ρ

(a)

(b)

(c)(d)(e)

Fig. 5. Arrhenius plots of ZnO doped with (a) 0.0 at% Cu, (b) 5 at% Cu, (c) 15 at% Cu,

(d) 25 at% Cu and (e) 30 at% Cu, annealed at 600 1C.

Table 2Activation energies of ZnO doped with varying

copper content annealed at 600 1C.

Copper content (at%) Activation energy (eV)

0 0.668

5 0.534

15 0.518

25 0.459

30 0.407

S. Singhal et al. / Physica B 407 (2012) 1223–12261226

The variation of electrical resistivity with temperature forZn0.95Cu0.05O is shown in Fig. 4. A decrease in electrical resistivitywith increase in temperature has been observed, suggesting thesemiconducting behavior of sample. Similar results have beenobtained for all the samples.

Fig. 5 shows the plots of the log of resistivity versus inversetemperature (log r versus 1/T) for undoped and copper dopedzinc oxide. The activation energy has been calculated using slopeof this figure for each composition. Activation energy has beenfound to decrease with increase of copper content. The calculatedvalues of activation energy are presented in Table 2. When ZnO isdoped with Cu, then Cu atoms replace Zn atoms which can beeasily ionized because of smaller ionization potential of Cuthan that of ZnO. Therefore upon Cu doping, there is an increasein donor concentration and hence a consequent decrease inelectrical resistivity. Thus there is a corresponding decrease inactivation energy.

4. Conclusion

The present study demonstrates the synthesis of undoped andCu doped ZnO nanoparticles via co-precipitation method. Mor-phological studies have revealed the formation of quasi-sphericalnanoparticles. Systematic investigation on the effect of Cu dopingon the structural and electrical properties of ZnO lattice has beenpresented. The results have shown that the solubility limit of Cuin ZnO lattice is 10 at%. Beyond this doping level, there has beenevolution of a very small peak corresponding to the formationof a secondary phase of CuO. Decrease in lattice parameters andcrystallite size has been observed for Cu doped samples. DCresistivity as well as the activation energy has been found todecrease with Cu doping.

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