study of blueshift of optical band gap in zinc oxide (zno) nanoparticles

Study of blueshift of optical band gap in zinc oxide ZnO nanoparticlesprepared by low-temperature wet chemical method Department of Instrumentation & USIC, Gauhati University, Guwahati 7

Study of blueshift of optical band gap in zinc oxide (ZnO) nanoparticles

prepared by low-temperature wet chemical method

Department of Instrumentation & USIC, Gauhati University, Guwahati 781014, India

a r t i c l e i n f o

Article history:

Received 29 July 2013

Accepted 16 August 2013

Available online 24 August 2013

Keywords:

ZnO nanoparticles

Semiconductors

UV–vis absorption

Blueshift

Optical band gap

a b s t r a c t

We report synthesis of zinc oxide (ZnO) nanoparticles via wet chemical method using molar solutions of zinc nitrate hexahydrate [Zn(NO3)2
6H2O] and ammonium hydroxide (NH4OH) with polyvinylpyrolli-done (PVP) as capping agent The synthesized ZnO nanoparticles are characterized by X-ray diffraction (XRD) technique and electron microscopy (TEM and SEM with EDX) for compositional analysis and surface morphology The average crystallite size calculated from XRD pattern has been found to be on the order of 8.5 nm In this work, the hyperbolic band model (HBM) and UV–vis absorption spectra have been utilized to calculate the band gap and nanoparticle size which was proposed by Meulenkamp

It was Meulenkamp who correlated the particle sizes with the wavelength at which the absorption becomes half of that at the shoulder (λ1/2) The optical band gap value of the prepared ZnO nanoparticle is found to be on the order of 3.63 eV which indicates the presence of blueshift In order to compare the size and distribution of ZnO nanoparticles, TEM has been utilized

& 2013 Elsevier B.V All rights reserved

1 Introduction

Nano-semiconductor materials that exhibit peculiar

proper-ties which are not shown by their bulk counterparts have

attracted much interest from both fundamental and

technologi-cal researchers ZnO is technologitechnologi-cally an important material due

to its wide range of optical and electrical properties; also it is a

semiconductor crystal with a large binding energy (60 meV) and

wide band gap (3.3 eV at 300 K) ZnO nanoparticles are used in a

variety of applications such as UV absorption, antibacterial

treatment[1], UV light emitters[2], photocatalyst[3]and as an

additive in many industrial products It is also used in the

fabrication of solar cells[4], gas sensors[5], luminescent

materi-als[6], transparent conductor, heat mirrors and coatings

Differ-ent synthesis methods have been developed for the preparation

of ZnO nanoparticles and among these, the wet chemical method

is the most attractive one due to its perfect control of

morphol-ogy, purity, crystallinity, composition and low cost for large-scale

production This work reports the synthesis of ZnO nanoparticle

for investigation of the blueshift of the optical band gap of

nanocrystals at pH¼7.5

2 Experimental Materials: All materials were purchased from the commercial market with highest purity (99.99%) Zinc nitrate hexahydrate [Zn (NO3)2
6H2O] and ammonium hydroxide (NH4OH) as the starting materials, polyvinylpyrrolidone (PVP) as capping agent and double-distilled water as dispersing solvent were used to prepare ZnO nanoparticle

Preparation of ZnO nanoparticles: 100 ml of 0.1 mol solution of Zn(NO3)2
6H2O was stirred constantly for 30 min at 601C (solu-tion A) 3 wt% of PVP was stirred constantly at 601C for 30 min (solution B) Now NH4OH was slowly added drop by drop into solution A and stirred at room temperature for 15 min and the pH

of the solution was continuously measured When the pH of the solution is 7.5 and a white solution (solution C) of ZnO is formed the addition of NH4OH is stopped Thefinal mixture (solution B and solution C) is stirred constantly for 1 h at 601C and allowed to cool down at room temperature till the white precipitate of ZnO is formed The whole solution is allowed to settle overnight in a dark chamber Finally, the precipitate isfiltrated which is washed with distilled water to dissolve the impurities and dried at 601C in an oven for 12 h The basic chemical reaction governing the formation

of ZnO has been reported in our earlier work[7] Characterization methods: Powder X-ray diffraction(XRD) pat-tern of prepared ZnO nanoparticle is recorded by a Philips X-ray Diffractrometer (X'Pert Pro) with Cu Kα1radiation (λ¼1.5406 Å) A scanning electron microscope (SEM with EDX, JEOL JSM Model 6390LV) has been used for surface morphology and compositional

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Materials Letters

0167-577X/$ - see front matter & 2013 Elsevier B.V All rights reserved.

n Corresponding author Tel.: þ91 985 998 3361.

E-mail addresses: mrinaldebanath@rediffmail.com (M.K Debanath) ,

sanjibkab@rediffmail.com (S Karmakar)

analysis of the prepared ZnO nanoparticles The optical absorption

spectra of ZnO dispersed in water are recorded by a UV–vis

spectrometer (HITACHI model U-3210) Transmission electron

microscopy (TEM) observations are carried out on a JEM-100 CX

II electron microscope The band gap of the ZnO nanoparticle is

calculated from the UV absorption result

3 Results and discussion

X-ray diffraction analyses: In order to identify the phase and to

know the structure of ZnO nanoparticles, X-ray diffraction pattern is

recorded in the range of 25–751 (2θ) at a scanning rate of 0.021/s and

1 s/step.Fig 1(a) shows the presence of only ZnO phase in the sample

which corresponds to (100), (002), (101), (102), (110), (103), (200),

(112), (201), and (004) planes in the hexagonal phase of ZnO with

space group P63mc and unit cell parameters are a¼b¼3.257 Å and

c¼5.213 Å (PDF No 79-0207) The broadening of peaks is observed

which is mainly due to thefinite size (D) of the crystallite and some

contribution of strain The following procedure is adopted prior to XRD observation as reported earlier[7]:

(1) The diffractometer is calibrated by a standard silicon sample (2) Correction of instrumental broadening ofβ2θarising due to slit width of the Kα1 and Kα2 lines is also made The diffraction broadening only due to grain sizeβgis given by the Warren rule[8]

β2

g¼ β2

θβ2

where β is FWHM of a line produced under similar geometrical conditions by the standard material (silicon) with crystallite size

54 nm The crystallite size (D) for the chemically prepared ZnO nanoparticles is then evaluated for the preferred planes (hkl) using Debye Scherrer's formula[9]

D¼ Kλ=βg cosθ whereλ is the wavelength of radiation used, θ is the Bragg angle and K¼0.9 for spherical shape The average crystallite size is found

to be 8.5 nm in the directions perpendicular to (101) and (102) planes

XRD is widely used to determine the particle size of nanopar-ticles but TEM is the best way for the measurement of nanoparticle size The Scherrer method for calculating particle size gives an average value of the entire particle responsible for diffraction However, by TEM, besides directly measuring particle size, the morphology of the particles can also be observed The typical HRTEM result in Fig 1(b) shows almost spherical shapes with smooth surface Almost non-dispersed, individual nanoparticles are identified as bulk dots in the HRTEM micrograph having average diameter of nanoparticle is6.5 nm It is to be noted that the HRTEM photograph pattern is recorded using selected area observations and the particle size is determined in higher magni-fication (2  105) The grain boundaries (GBs) of nanograined ZnO can drastically change the physical properties [10–12] It is observed that the ferromagnetism (FM) at room temperature (RT)

of the pure and doped ZnO nanoparticles is because of GBs and related oxygen vacancies or defects arranged in GBs and the RT ferromagnetism of the microstructure in ZnO and doped ZnO depends on the specific grain boundary (GB) area, i.e the ratio of

GB area to grain volume SGB[13] FM of ZnO is Mn-doped ZnO are observed only if the grain boundary area in the unit volume of the material is greater than a certain threshold value Sth(Sth¼(773) 

107m2/m3for pure ZnO and Sth¼(274)  105m2/m3for Mn-doped ZnO)[14] Thus nanograined ZnO is considered to be an important material for applications in solar cells, sensors, photovoltaics, photo-catalysis and sprintonics[15]

UV–vis characterization: UV–vis spectra are taken from the colloidal solution of ZnO The UV absorption spectra in Fig 2(a) show broad peak atλ298.8 nm (4.16 eV) which indicates that the presence of blueshift is observed with decrease in particle size with respect to bulk ZnO (λ376 nm; 3.3 eV) and this could be attributed to the confinement effects [16] Fig 1(a) and Fig 3

indicate that ZnO nanoparticles are successfully synthesized via the wet chemical method The direct band gap of ZnO colloid is estimated from the graph of hν versus (αhν)2through the absorp-tion coefficient α which is related to the band gap Egas (αhν)2

¼k (hνEg), where hν is the incident light energy and k is a constant The extrapolation of the straight line inFig 2(b) to (αhν)2¼0 gives the value of band gap energy Eg The optical band gap (Eg) is found

to be size dependent and there is an increase in the band gap of the semiconductor with a decrease in particle size The optical band gap value obtained for ZnO nanoparticle at pH¼7.5 is 3.63 eV The dependence of the absorption band gap as shown

0

20

40

60

80

100

120

140

160

180

200

220

2θ (degree)

Fig 1 (a) XRD pattern of ZnO nanoparticles prepared at 60 1C and (b) TEM image

the particle size By considering the strong absorption edge in the

absorption spectra of the sample, average particle size has been

estimated by using the following hyperbolic band model[17]:

R¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2π2h2Egb

mnðE2

gnE2

gbÞ

v

u

where R is the radius, m* is the effective mass of the specimen

(m*¼29.15  1031kg for ZnO), Egb is bulk band gap, h is Planck's

constant (6.626 1034J s) and Egnis the band gap at strong

absorp-tion edge Egncan be calculated by the formula Egn¼hc/λgn, where h is

Planck's constant, c is the velocity of light (3 108m s1) andλgnis

the strong absorption edge (298.8 nm) obtained from the absorption

spectra as shown inFig 2(a) The average particle size is calculated and

found to be 6.2 nm

We have calculated particle size of ZnO nanoparticle from

another method which was proposed by Meulenkamp who

introduced the equation below which correlates the particle sizes

toλ1/2[18] The value (2.2470.09) nm for the diameter of the ZnO

nanoparticle is calculated from the following equation:

1240=λ1=2¼ aþb=D2

c=D where a¼3.301, b¼294.0 and c¼ 1.09, λ1/2 (nm) is the

wave-length at which absorption becomes half of that at the shoulder,

and D (Å) is the diameter of the particle It is observed that there are discrepancies among the sizes estimated from above men-tioned methods To obtain the exact size of the particle using hyperbolic band model, the particle should be exactly circular in shape From the HRTEM image it is observed that the particles are not exactly circular; hence discrepancies occur

Particle morphology and elemental study: From the SEM image

prepared ZnO nanoparticles are nearly spherical in shape It is also observed that the surface of the prepared sample is smooth and uniform with no cracks on it The result of EDX analysis of ZnO nanoparticles indicates that the ZnO nanoparticle contains 100% ZnO void of template

4 Conclusions

We have successfully synthesized ZnO nanoparticles by a simple wet chemical method at low temperature The nanostructure of the prepared ZnO nanoparticle has been confirmed using XRD, SEM, UV– vis absorption and TEM micrograph analyses XRD result shows that the obtained ZnO nanoparticles are composed of ZnO phase in the hexagonal system with space group P63mc and unit cell parameters are a¼b¼3.257 Å and c¼5.213 Å with proper crystallinity The average crystallite size obtained by XRD is8.5 nm using the Scherrer formula The estimated optical band gap of ZnO is found to be 3.63 eV which also clearly indicates the presence of blueshift at a growth temperature of 601C The UV absorption result also shows an increase

in optical band gap which exhibits a quantum confinement effect

Acknowledgments The authors thank the SAIF, Department of Instrumentation & USIC, Gauhati University; Tezpur University; SAIF, NEHU Shillong; and Department of Chemistry, Gauhati University; for providing XRD, SEM, TEM and UV–vis spectroscopy measurements respectively

Fig 3 SEM image of ZnO nanoparticles and EDX spectra of the prepared ZnO nanoparticles with composition of elements.

250 260 270 280 290 300 310 320 330 340 350

1.00

1.25

1.50

1.75

2.00

2.25

λ1/2

Wavelength (nm)

298.8

318.9

2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25

0

25

40

35

30

20

15

10

2 x

6 (e

2 m

-2 )

hν (eV)

5

3.63

Fig 2 (a) Absorption spectra of ZnO colloid and (b) optical band gap of ZnO colloid.

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