Colloidal Synthesis and Characterization of ZnO and ZnS Nanoparticles

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Colloidal Synthesis and Characterization of ZnO and ZnS Nanoparticles Powered By Docstoc
					              Colloidal Synthesis and Characterization of Nanoparticles
A. Sugunan, H.C. Warad, C. Thanachayanont* and J. Dutta

Microelectronics, School of Advanced Technologies, Asian Institute of Technology,
P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand

*National Metal and Materials Technology Center, 114 Thailand Science Park, Phaholyothin Road,
Klong 1, Klong Luang, Phathumthani 12120, Thailand

1. Introduction
        The progress of technology and quality of life of mankind has always been closely knit with
the progress in material science and material processing technology. Most material processing
techniques are based on breaking up large chunk of a material into desired shapes and sizes,
inducing strain, lattice defects and other deformations in the processed material. Recent
developments in nanotechnology and the demonstration of various quantum size effects in nano-
scale particles [1], implies that most of the novel devices of the future will be based on properties of
nanomaterials [2]. Each nanoparticle contains only about 3-107 atoms/molecules. Lattice defects and
other imperfections induced by the traditional material processing techniques will no longer be
diluted by sheer number of atoms, when used for synthesizing nanoparticles. Furthermore, it is
difficult to achieve size selective synthesis of such small particles, by using the traditional
approach [3,4].
        Alternative synthetic technique for nanoparticles involves controlled precipitation of
nanoparticles from precursors dissolved in a solution. A micro emulsion can also be formed
between two immiscible liquids, using surfactants, with the reactants isolated inside a micelle,
through hydrophobic versus hydrophilic forces. The resultant nanoparticles form a colloidal
suspension. Various thermodynamic factors as well as van der Waal’s forces induce particle growth
and agglomeration [5], resulting in bigger particles that settle down over time. A prerequisite in
utilizing colloidal nanoparticles is that they remain stable in colloidal suspension. Stabilization
mechanism of nanoparticles can be categorized as a) electrostatic stabilization: involving the
creation of a double layer of adsorbed ions over the nanoparticles resulting in a coulombic repulsion
between approaching nanoparticles; or b) Steric hindrance: achieved by adsorption of polymer
molecules over the nanoparticles. Osmotic repulsion felt by the polymer molecules due to localized
increase in their concentration when polymer coated nanoparticles approach each other, keeps them
(along with the nanoparticles) well separated [5].
        Chemical methods of synthesis have a further advantage of tunable surface properties of the
synthesized nanoparticles, offered by the adsorbed ions (for electrostatic stabilization) or the
passivating polymer (for steric hindrance). Stable colloidal nanoparticles find many futuristic
applications, for example semiconductor and metallic nanoparticles can be used to make futuristic
electronic and optoelectronic devices [3]. Tunable photoluminescence displayed by II-VI
semiconductors nanoparticles like CdSe [6], and ZnS [7,8], amongst others, finds applications as bio-
labels [9,10]. ZnO nanoparticles, being piezoelectric as well as semiconducting material, find
potential applications in electronics [11], optoelectronics [12,13], SAW devices [14], gas sensors [15,16],
amongst others. Quantum confinement effects shown by metal nanoparticles [17], as well as surface
plasmon resonance [18], makes them suitable for applications in electronics [19], optoelectronics, bio-
sensors [20,21], amongst others.
        Here we will discuss chemical synthetic techniques for the synthesis of II-VI
semiconductors; namely doped ZnS and ZnO, as well as metal nanoparticles (Au and Ag).
        Synthesis of doped ZnS is based on a co-precipitation reaction, in an aqueous solution of
precursors. The dopant used in our experiments is manganese. In a typical synthesis, zinc acetate

and manganese acetate of varying molar ratio are mixed in de-ionized water along with 0.1 %
solution of chitosan in acetic acid. This mixture is rapidly stirred while sodium sulfide solution is
added to it. The reaction proceeds rapidly with the formation of white precipitates of ZnS. Upon
illumination with a low power UV lamp a bright orange red glow is observed due to the presence of
Mn2+ trap centers slightly below the Fermi level of the band-gap of ZnS. This confirms that
                           P   P

manganese ions have been incorporated into the crystal structure of ZnS and is not present as
amorphous MnS mixed with ZnS.
        In the precipitation of multi-component material, special attention is given to control co-
precipitation conditions in order to achieve chemical homogeneity of the final product. This is due
to the fact that different ions often precipitate under different conditions of pH and temperature
having different solubility product constants. Hence a coarse control of the doping concentration
can be achieved by tuning the reaction conditions. These nanoparticles are sterically stabilized by
the bio-polymer chitosan present in the reaction medium. XRD analysis shows that the
nanoparticles exhibit the typical zinc-blende structure (Fig. 1a). SEM images show that the
nanoparticles are ~30 nm in diameter (Fig 1b & c). HRTEM analysis reveals the individual
crystallite sizes to be 2-4 nm (Fig 1d), which is consistent with the theoretical calculations from
XRD data.




 Intensity (a. u.)


                     370                                                                                          [311]






                       20.00        25.00   30.00           35.00   40.00        45.00         50.00      55.00   60.00   65.00   70.00
                                                                                Angle 2θ



                                                                                         (c)                                                    (d)
                                   Figure 1 Analysis of Mn doped ZnS nanoparticles (a) XRD analysis showing zinc-blende structure with
                                   characteristic peaks at [111], [220] and [311]. (b & c) SEM images of the primary particles showing an
                                   average diameter of ~ 30 nm. (d) HRTEM image of a single primary particle revealing the individual
                                   crystallites, which are 2-4 nm in size.

       Nanoparticles obtained by this method show some degree of selectivity in attachment to the
cell-walls of bacteria [10], demonstrating potential applications in biological labeling akin to CdSe
                                                                      P     P

nanocrystals [9]. In contrast to the tuning mechanism of the emission wavelength in case of CdSe
                                               P      P

nanocrystals, based on the quantum size induced variation of band-gaps, control in the emission

wavelength of ZnS is afforded by the choice of the dopant material. It is reported that doping with
manganese, copper, and silver results in red, green and blue colored emission respectively. Other
potential applications of doped ZnS is as phosphor layer in flat panel display devices
        Aqueous solution based synthesis of ZnO nanoparticles follows a similar procedure as in the
case of ZnS. However in this case, the initial precipitates of the reaction is Zn(OH)2, which by                  B   B

subsequent hydrothermal treatment gives ZnO nanoparticles. Synthesis usually involves dissolution
of zinc acetate in a mixture of water and alcohol (9:1 ratio by volume). To this mixture, glacial
acetic acid is added and the reaction flask is placed in a water bath at 65 0C for 12 hours. Upon
                                                                                              P       P

addition of glacial acetic acid, the solution turns milky white due to precipitates of Zn(OH)2. After a                   B   B

period of time the hydroxide forms ZnO, which is colorless. The resultant nanoparticles range from
30-50 nm (Fig 2) in size and fairly spherical. ZnO nanoparticles obtained by this process have been
successfully used as seeds to mediate the self assembled growth of nanowire structures [15].                                      P   P

Uniformity in size and shape of the obtained nanoparticles is essential for the growth of uniform
nanowires, which have potential applications as field emitters in futuristic display devices like FED
P   . Currently studied material for field emitters are carbon nanotubes [22]. The relative ease in
     P                                                                                    P       P

obtaining aligned nanowires of ZnO and also the availability of a simple synthetic technique for a
high yield of such aligned nanowires makes them a promising alternative for carbon nanotubes, as
field emitters. Other potential applications of ZnO nanowires, which can be obtained from these
nanoparticles, are as SPM probes [23], gas sensors [16], amongst others.
                                              P   P          P   P

    (a)                                                              (b)

          Figure 2 SEM micrograph of a mono-dispersed layer of ZnO over a glass substrate. (a) Image showing a
          large number of nanoparticles with an average diameter of ~ 30 nm. (b) A few isolated nanoparticles with
          diameters ~ 50 nm.

        Synthesis of gold and silver nanoparticles by wet chemical synthesis proceeds by a
reductions reaction followed by subsequent particle growth by ostwald ripening and finally stop in
the growth process when the metal surfaces are sufficiently passivated by adsorbed anions.
Typically, the reducing agent used is tri-sodium citrate. In case of gold nanoparticles, the
nanoparticle formation is based on the well-documented Turkevitch process [17] in which the               P   P

particle size is controlled by the ratio of concentration of the gold salts and tri-sodium citrate.
        A solution of HAuCl4 (100 µM) in de-ionized water is heated until it is brought to boil.
                                      B   B

Upon boiling, 3 cc of 25 mM tri-sodium citrate is added, and heating is continued until the color of
the solution changes from the characteristic golden yellow to pale red. Prolonged heating causes the
color to intensify to a deep red color. During the process, Tri-sodium citrate reduces the gold salt to
metallic gold particles, which acts as seeds for subsequent growth. At this stage, the solution
changes from yellow to pale red. The exceedingly small size of the nanoparticles results in a very
high surface energy, which is thermodynamically unstable. In response to this instability, gold ions

from the solution progressively get consumed allowing the seeds to grow by the process called
ostwald ripening [24]. This continues until the surface energy is low enough to ensure
thermodynamic stability. Citrate ions in the solution get adsorbed on the nanoparticle surface to
provide ionic stabilization. Adjusting the concentration of the citrate ions facilitate size selection by
arresting the ostwald ripening at appropriate stages. Nanoparticles of 10 nm diameter with good
uniformity in size and shape have been achieved (Fig. 3). Silver nanoparticles have also been
synthesized by the reduction of aqueous solution of silver salts with tri-sodium citrate. The optical
absorption spectrum of gold and silver nanoparticles is a fairly good indicator of their morphology.
Conduction electrons of metals show strong absorption of electromagnetic waves of the visible
wavelength, which induces a polarization in the electrons with respect to the immobile nucleus.

                      3                                                                               0.6

                     2.5                                                                              0.5
 Absorption (a.u.)

                                                                                  Absorption (a.u.)
                      2                                                                               0.4

                     1.5                                                                              0.3

                      1                                                                               0.2

                     0.5                                                                              0.1

                      0                                                                                0
                       420     470    520   570       620       670   720   770                         320   370   420   470         520         570   620   670   720

                                              Wavelength (nm)                                                                   Wavelength (nm)

                                                     (a)                                                                        (b)

                                                   (c)                                                                          (d)
                             Figure 3 (a) Absorption spectrum of gold nanoparticles obtained by reduction of gold salts by tri-
                             sodium citrate, (b) Absorption spectrum of silver nanoparticles obtained by similar methods, (c) TEM
                             image of an ensemble of spherical gold nanoparticles, (d) Isolated cluster of gold nanoparticles, clearly
                             indicating the average diameter of 10 nm

        This polarization is compensated in the form of a dipole oscillation. When the size of a
nanoparticle is comparable to the skin depth for a particular wavelength, all the electrons in the
nanoparticle resonates, resulting in strong absorption. This phenomenon is called surface plasmon
resonance. The shape of the absorption spectrum gives information about the shape of the
nanoparticles, and the polydispersity in size. Figures 3a and 3b shows the measured UV-Vis
absorption spectra of gold and silver nanoparticles. Absorption spectrum of gold, when compared to
reports from the literature [25,17], suggests that the constituent nanoparticles are 10-20 nm in
diameter. TEM analysis reveals that the particles are very spherical and about 10 nm in diameter
(Fig. 3c & d).

       Uniform sized gold nanoparticles find applications in drug delivery, bio-sensors, and DNA
recognition [26], amongst many others.


        Colloidal nanoparticles can be synthesized by simple precipitation routes. Optimum
condition and mechanisms required to synthesize monodisperse nanoparticle of ZnS:Mn2+ were
explored. Synthesis of ZnS nanoparticles doped with manganese was successfully carried out.
Efforts are currently underway to dope the ZnS nanoparticles with other impurities to obtain all
colors of the phosphors that would be necessary to fabricate a full-color FED.
        Nanoparticles of ZnO have been successfully synthesized by using an aqueous chemical
reaction, followed by hydrothermal treatment. The obtained nanoparticles show uniformity in size.
These obtained nanoparticles were successfully used as seeds for a hydrothermal growth process to
obtain nanowires of ZnO. The nanowires obtained displayed very high uniformity in diameter, due
to the uniformity in the sizes of the seeds used [15]. Efforts are underway to obtain a mechanism for
a patterned fixation of these nanoparticles on conducting substrates, in order to study the field
emission characteristic of nanowires grown from these seeds.
        Metallic nanoparticles have been synthesized by a reduction reaction with tri-sodium citrate
and a protocol for the synthesis of gold nanoparticles of 10 nm in diameter has been developed.
Future research efforts will be oriented towards synthesizing nanocomposites of gold nanoparticles,
which have potential applications as sensors.
        In summary, we have demonstrated that nanoparticles of various materials can be
synthesized by simple wet-chemistry. By controlling the reaction conditions, nanoparticles of
various sizes and shapes can be synthesized. As an extension of the synthetic technique, various
mechanisms for self organization of the obtained nanoparticles into 1D or 2D structures are being
currently pursued, that have wide ranging applications in novel devices of the future.


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