Download chapter 9

CHAPTER 9
CONCLUSION AND FUTURE WORK
9.1
CONCLUSION
Pure and doped Zinc oxide (ZnO) nanostructures were synthesized
using simple sol–gel chemical route and were annealed at 500C for 2h. The
effect of doping on the structural, morphological and optical properties was
studied for an optimum concentration. Structural and morphological analysis
were carried out using XRD, EDX, SEM, TEM and AFM, while optical
properties were explored using UV-Visible, Photoluminescence, FTIR and μRaman spectroscopy. The antibacterial activities of the as-synthesized pure
and doped ZnO nanostructures were carried out against both gram positive
and gram negative bacteria by using well diffusion method. Furthermore, the
photocatalytic activities of these samples are also studied by degrading
methylene blue (MB) in water under the irradiation of UV light. On the basis
of the results obtained during the course of this study, the following key
conclusions are summarized as:
The influences of annealing effects have been explored on the
crystallinity, morphology, optical and magnetic properties of Ag-ZnO
nanorods. The synthesized Ag-ZnO nanorods are found to have hexagonal
wurtzite crystal structures and their grain size increases while lattice strain
decreases on annealing. Due to annealing effect, Ag doped ZnO shows
higher saturation magnetization at room temperature. Photoluminescence
emission spectra pointed up additional peaks due to annealing effect of AgZnO. These supplementary emission bands point towards promising
applications of the annealed Ag doped ZnO samples, especially bearing in
mind their low cost and availability. UV-driven white light emission generation
in luminescent lamps, flexible displays, and down-shifting of solar spectrum
145
for enhancement of efficiency of solar cells are some of their device
applications. The magnetization increases with annealing
indicating that
annealed Ag doping ZnO might be an effective way to obtain more prominent
RT-FM in ZnO-based DMS nanoparticles.
The Ag-ZnO nanocomposites were synthesized to investigate the
bacterial sensitivity against Gram positive and Gram negative bacteria,
comparatively by using well diffusion method.
Ag/ZnO nanorods were
synthesized by the simple wet chemical sol-gel method. The grain size was
controlled by using polyvinyl pyridine as capping agent. Nanoparticle
crystallinity, quality of the samples, chemical composition, and the optical
properties were investigated by XRD, μ-RS, FTIR, PL spectrometer and HRTEM. The enhanced bioactivity was demonstrated by studying the
antibacterial activity of ZnO and Ag/ZnO samples. These improved
bioactivities of smaller particles were attributed to the higher surface to
volume ratio. The smaller particles need more particles to cover a bacterial
colony, which results in the generation of active oxygen species, which will
kill bacteria more effectively. Therefore, Ag/ZnO nanorods were found to be
more effective for killing the bacteria and thus, contribute to the greater
mechanical damage for all the functions of bacteria. Hence, synthesized
Ag/ZnO nanorods showed potential applications in photodegradation of
organic dye pollutants and destruction bacteria.
XRD analysis revealed that the as-synthesized Cr doped ZnO
nanostructures are well-crystalline and possessing hexagonal wurtzite
structure in nano regime. SEM images confirmed that the synthesized
nanostrucures were grown in high density with less agglomeration. The
different types of morphologies were also observed after doping in ZnO
nanostructures. Optical absorption measurements confirmed that doping
enhances the excitonic oscillator strength and showed the blue shift in the
band-gap which is also supported by Raman effect and PL. Doped
nanostructures were found to be more effective than undoped ZnO and thus
146
contribute to the greater mechanical damage to all functions of bacteria and
enhanced the bactericidal impact of Cr doped ZnO nanostructures.
ZnO
and
Al-doped
ZnO
nanopowders
were
successfully
synthesized via simple sol-gel chemical technique. The influences of Al
doping on the microstructure, morphology, optical and antibacterial properties
of ZnO and its photocatalytic activity are investigated. It is interesting to
observe Al doped ZnO nanoparticles proved to be more lethal to the bacteria
than Cr-doped ZnO nanostructures. Also same scenario was observed in the
degradation efficiency of the organic pollutant, M.B dye. The reason may be
due to the excellent substitution of Al nanoparticles in ZnO which is also
confirmed by EDX. This enhancement is possible because ionic radius of Al
(0.54nm) is less than that of Cr (0.63nm).
Finally, the investigations of optical, structural, antibacterial and
photocatalytic properties of sol-gel synthesized dual doped (Al-Cr) - ZnO
nanostructures were presented. XRD results showed that undoped and dualdoped ZnO nanostructures have crystalline nature. The morphology was
found as honey-comb like structure for dual-doping sample. Optical
absorption measurements showed that when the dopants Al and Cr were
incorporated into ZnO, there is a difference in the optical absorption towards
the edge which is blue shifted from 3.32 to 3.41 eV after dual-doping. The
enhanced bioactivity was demonstrated by studying the antibacterial activity
of undoped and dual doped ZnO samples against two bacteria Microccus
leutus (M. leutus) and Vibrio cholera (V. cholera). The enhanced bioactivity of
smaller particles is attributed to the higher surface area to volume ratio. The
smaller particles need more particles to cover a bacterial colony which results
in the generation of active oxygen species which will kill bacteria more
effectively. After doping the experimental diameter values of inhibition zones
for M. leutus and V. cholera bacteria estimated are 20 and 24 mm. It
indicates doped ZnO were found to be more effective towards V. cholera
than M. leutus, thus contribute to the greater mechanical damage to all
147
functions of bacteria and enhanced bactericidal impact of dual doped ZnO
nanoparticles.
9.2
FUTURE WORK
ZnO is a very promising candidate for a number of applications like
sensing,
photovoltaic,
antibacterial
and
biomedical,
gas
sensing,
optoelectronic device and dilute magnetic semiconductors when doped with
transition metals. Therefore, in future this work can be extended in the
following different ways as given below:
1.
ZnO nanostructures showed enhanced antibacterial activities
after doping against both gram positive and gram negative
bacteria. The next step is to check their toxic behavior for
biomedical applications.
2.
In the present investigations, doped ZnO nanostructures
showed good luminescence properties. Attempts can be
made to understand the non-linear optical (NLO) property for
optoelectronics applications.
3.
Thin films can also be prepared and the properties can be
compared. The film parameters are to be optimized suitable
for biomedical and photocatalytic applications.
148