HEOPrAHEHECKEE MATEPHAXbl, 2010, moM 46, № 2, c. 203-207
UDC 546.47
TUNING THE BAND GAP OF ZnO NANOPARTICLES BY ULTRASONIC IRRADIATION
© 2010 R. S. Yidav*, **, P. Mishra*, A. C. Pandey*' **
*Nanophosphor Application Centre, University of Allahabad, India **Physics Department, University of Allahabad, India e-mail: raghvendra_nac@yahoo.co.in Received 02.11.2008
In this present paper, we report the tunability of ZnO nanoparticles by ultrasonic irradiation. Different sized ZnO nanoparticles viz. 2.58—2.97 nm have been synthesized with variation of ultrasonic irradiation time 75— 270 min in presence of Histidine as capping agent. UV and visible spectroscopy study revealed that as ultrasonic irradiation time increases, there is increase in amount of formed ZnO nanoparticles and also there is red shift in absorption edge. This confirms the tunability of bandgap of histidine capped ZnO nanoparticles with ultrasonic irradiation. Growth mechanism for controlling the size of ZnO nanoparticles are also discussed.
INTRODUCTION
Semiconductor nanoparticles have very interesting size-dependent properties such as tuning of band gap with particle size [1—3], reduction in the melting point [4] and luminescence enhancement [5, 6]. ZnO is a wide band gap (3.37 eV) semiconductor material with a large exciton binding energy (60 meV) at room temperature. It is one of the most promising semiconductor material for electronic, photonic applications, in piezoelectric devices, to chemical and biological sensors [7—10]. It is well known that nanosized particles may have superior optical properties over bulk material due to quantum confinement effect [11]. Many synthesis methods for semiconductor nanoparticles have been developed without a suitable support; it is undesirable since semiconductor nanoparticles aggregate due to the high surface energy, reducing surface area and restricting control over particle size [12, 13]. To overcome this problem, semiconductor nanoparticles have been capped by different materials. A variety of capping agent have been employed to stabilize ZnO nanoparticles [14—18].
Recently, sonochemical method has been proven to be a useful technique to generate novel materials with unusual properties [19]. The chemical effects of ultrasound arise from acoustic cavitation: the formation, growth and implosive collapse of bubble in liquid. The implosive collapse of bubble generates localized hot spots through adiabatic compression or shock wave formation within the gas phase of the collapsing bubble. The conditions formed in these hot spots have been experimentally determined, with transient temperature of ~5000 K, pressure of 1800 atm, and cooling rate in excess of 1010 K/s. These extreme conditions attained during bubble collapse have been exploited to prepare nanoparticles of metals [20], metal
carbides [21], metal oxides [22], and metal sulphides [23]. In this paper, ZnO nanoparticles were synthesized by sonochemical method by utilizing histidine as the capping agent. Histidine is a favourable chelator for Zn, Cd etc. [24].
EXPERIMENTAL
The Zinc acetate, sodium hydroxide and histidine were from E. Merck Ltd., Mumbai 400018, India. These chemical were directly used without special treatment. ZnO nanoparticles were synthesized by a sonochemical method using sonochemical bath. Our experiment is simple and aims at realising the effectiveness of sonochemical waves and the role played by Histidine in controlling the size. The procedure to prepare is as follows: 0.1 M Zinc acetate was mixed with 0.1 M Sodium hydroxide and 0.2 M Histidine was used as capping agent. After mixing all the three constituents in the beaker it was kept in sonochemical bath (33 kHz, 350 W) at room temperature for different ultrasonic irradiation time 75, 90, 105, 210 and 270 min. The resulting white-brown precipitate was then centrifuged and washed with alcohol. To obtain the powder of ZnO nanoparticles, the precipitate was then dried at 60°C.
The optical absorption of the ZnO nanoparticles were examined with a Perkin-Elmer Lambda 35 UV-Visible spectrometer.
RESULTS AND DISCUSSION
UV-VIS Absorption spectra study: Figure 1 shows the UV-Visible absorption spectrum of ZnO nanopar-ticles synthesized by sonochemical method by using histidine as stabilizer at different ultrasonic irradiation
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с« Й ft о 0.16 0.14 0.12 -
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200 300 400
Fig. 1. UV and visible absorption spectra of Histidine capped trasonic irradiation time: 75, 90, 105, 210 and 270 min.
time: 75, 90, 105, 210 and 270 min. ZnO nanoparticles synthesized at ultrasonic irradiation time 75 min shows well defined exciton absorption peak at ~293 nm with a significant blue-shift compared to bulk ZnO. After ultrasonic irradiation time 75 min, absorption edge progressively red shift towards 384 nm, corresponding to the bulk band gap for ZnO of 3.2 eV [25]. From absorption spectra in UV and visible ranges, it is also clear that as ultrasonic irradiation time increases, absorption intensity also increases. This implies that number of formed ZnO nanoparticles, i.e., yield increases with ultrasonic irradiation time. The absorption band edge is also very narrow, which confirms the formation of monodisperse ZnO nanoparticles synthesized by sonochemical method utilizing histidine as a stabilizer. The red-shift in the absorption edge with ultrasonic irradiation time implies growth of ZnO nanoparticles with ultrasonic irradiation time. This
Variation of bandgap and particle size with ultrasonic irradiation time utilizing Histidine as capping agent
Ultrasonic irradiation time, min Absorption edge, nm Band gap energy, eV Particle diameter, nm
75 293.339 4.2277 2.580
90 297.696 4.1658 2.660
105 298.399 4.1560 2.674
210 311.051 3.9869 2.942
270 312.316 3.9708 2.972
500 600 700 800 Wavelength, nm
ZnO nanoparticles synthesized by sonochemical method with ul-
implies that ultrasonic irradiation strongly affect the particle size and hence band gap of the ZnO nanoparticles. Table and Figure 2 show the variation of band gap and nanoparticle size with ultrasonic irradiation time. For convenience, we use the particle size inferred from the absorption edge. The average particle size of ZnO nanoparticle as a function of ultrasonic irradiation time was determined from the absorption spectra using the effective mass model derived by Brus [26]. In the strong confinement regime, the confinement energy of the first excited electronic state can be approximated by
E _ e +
E(gap, nanocrystal) E(gap, bulk) +
+ (rt2h2/R2)(1/m*m0 + 1/m*m0) - 0.248 E*Y, where E(gap, bulk) is the bulk bandgap, h is Planks constant divided by 2n, R is the particle radius, m* is the effective mass of the electrons, m* is the effective mass
of the holes, m0 is the free electron mass, E*Y is the exciton binding energy at room temperature (~60 meV).
It is reported that growth of semiconductor nanoc-rystals occurs via a diffusion limited "Ostwald ripening" mechanism [27]. For a system of highly dispersed particles where the growth is controlled by diffusion, the rate law is given by
r3 - r0 _ Kt, (1)
where r is the mean particle radius and r0 is the initial particle radius. The constant K is given by
K _ (8yD Vm CJ/9RT. (2)
TUNING THE BAND GAP OF ZnO NANOPARTICLES
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The above equation (1) implies the dependence of cube of radius of nanoparticle with time. To study the growth kinetics of histidine capped ZnO nanoparticles in sonochemical process, a plot of the cube of particle diameter as a function of increase of the ultrasonic irradiation time has been plotted. Figure 3 shows the variation of cube of average diameter of ZnO nanoparticles synthesized by sonochemical method as a function of ultrasonic irradiation time.
It is found that the growth of any nanoparticles via a solution route must be controlled essentially by two processes. One is the diffusion process of the reactants to the surface of the growing crystallite, while the second one is the reaction at the surface of the crystallite to incorporate the reactant as a part of the growth process. From Fig. 3, it is clear that cube of radius of histidine capped ZnO nanoparticles follow linear relation upto some instant of ultrasonic irradiation time, beyond it, it deviates from linear relation. This confirms the diffusion limiting growth in the formation of histidine capped ZnO nanoparticles upto 210 min of ultrasonic irradiation time. After 210 min of ultrasonic irradiation time, bigger sized nanoparticles are formed in expense of smaller nanoparticles due to higher surface to volume ratio of smaller nanoparticles. Because of this there is deviation in the linear relation between cube of diameter of histidine stabilized ZnO nanoparticles.
Growth mechanism: It has been found that three different regions are formed during the sonochemical process [28]:
I. The inner environment (gas phase) of the collapsing bubble, where elevated temperature (several thousands of degrees) and pressure (hundreds of atmospheres) are produced;
II. The interfacial region where the temperature is lower than that in the gas-phase region but still high enough to induce a sonochemical reaction;
III. The bulk solution, which is at ambient temperature.
Among the above-mentioned three regions, it seems that the current sonochemical reaction occurs within the interfacial region. If the reaction takes place inside the collapsing bubble, the product is amorphous as a result of the cooling rates (>1010 K/s) [29], whereas if the reaction takes place at the interface, one expects to obtain crystalline product. It appears that the current sonochemical reaction occurs within the interfacial region, yielding nanoparticles, because of the high quenching rate experienced by the product. This is because zinc acetate is better described as a somewhat ionic compound; it is certainly involati
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