Environmental Engineering Reference
In-Depth Information
18.3
Microwave-assisted nanoparticle synthesis
Microwaves are radio waves with wavelengths ranging from as long as 1 m to as short as 1 mm, or equivalently, with
frequencies between 300 Mhz (0.3 Ghz) and 300 Ghz. In addition to applications in communication, radar, radio
astronomy, and navigation, microwave-assisted heating has attracted significant attention in organic synthesis and inor-
ganic material preparation. The fast reaction times, high-throughput capabilities, and beneficial crystallization effects
induced by “hotspot” heating helps in the preparation of nanoparticles. specifically, microwave chemistry has been
employed recently in the synthesis of semiconducting nanoparticles, f-block oxide nanostructures, and some interesting
hetero-bimetallic d-metal nanoparticles. In this regard, Peng et al. recently reported synthesis of silver nanoparticles
using bamboo hemicelluloses as stabilizer and glucose as reducing agent under microwave heating in aqueous medium.
The results specified the synthesis of spherical, nanometer-sized particles in the range of 8.3-14.8 nm [39]. Using
microwave irradiation Raghunandan et al. also reported silver nanoparticles of 26 ± 5 nm from guava (
Psidium guajava
)
leaf extract. The reaction occurs very rapidly within 90 s and results in spherical nanoparticle formation [40]. Recently,
eluri and Paul applied the microwave irradiation method for the synthesis of nickel nanoparticles (Ni NP) from the nickel
acetate tetrahydrate [Ni(ch
3
co
2
)
2
.4h
2
o] precursor, using sodium hypophosphite monohydrate (NaPh
2
o
2
.h
2
o) reagent.
The effect of additive concentration on particle size and morphology was studied under microwave heating. The isolated
spherical particles (7.2 ± 1.5 nm) changed to agglomerated nano flowers (72 ± 14 nm) by increasing Naoh concentration,
whereas addition of small amounts of cetyltrimethyl ammonium bromide (cTAB) or polyvinylpyrolidone (PVP) resulted
in slight increase in particle size [41].
Galletti et al. reported an efficient and highly reproducible process for palladium nanoparticle synthesis using the micro-
wave-assisted solvothermal technique. Nanoparticle synthesis has been carried using ethanol solution of Pd(oAc)
2
in the
presence of PVP as capping agent and by adopting mild reaction conditions and very short irradiation times. The preparation
of γ-Al
2
o
3
-supported catalysts has been suitably carried out in absence of PVP, leading to supported palladium nanoparticles
with an average particle size of 5-8 nm [42].
yu et al. reported shape-controlled synthesis of palladium nanoparticles using the microwave irradiation technique. Palladium
nanocubes and nanobars with a mean size of approximately 23.8 nm were readily prepared with h
2
Pdcl
4
as a precursor and
tetraethylene glycol (TeG) as both a solvent and a reducing agent in the presence of PVP and cTAB under microwave irradia-
tion for 80 s. The PdBr
4
2-
formation due to the coordination replacement of the ligand cl
-
ions by Br
-
ions in the presence of
bromide was responsible for the synthesis of Pd nanocubes and nanobars. A milder reducing power and higher viscosity with
stronger affinity of TeG were helpful in producing larger-sized Pd nanocubes and nanobars [43].
dahal et al. [44] conducted a widespread comparative study on the effects of microwave versus conventional heating on the
nucleation and growth of monodispersed Rh, Pd, and Pt nanoparticles. The obtained results reveal that microwave-assisted
heating allows the convenient preparation of nanoparticles with improved morphological control, monodispersity, and higher
crystallinity, compared with the conventional heating method under identical conditions. This work indicates a variety of ben-
eficial effects of the microwave heating method such as (i) under identical reaction conditions, morphological control and crys-
tallinity of NPs are significantly improved due to microwave heating; (ii) the microwave-induced nucleation process is faster
and more reproducible compared to conventional heating methods; (iii) microwave synthesis results in cubic RhNPs that are
more highly active catalysts for hydrogenation reactions.
In addition to metal nanoparticle synthesis, this technique was also explored for the synthesis of various nanocrystalline metal
oxides; for example, Bhosale et al. reported synthesis of cu
2
o nanoparticles using benzyl alcohol under microwave irradiation.
Benzyl alcohol as solvent, stabilizer, and reagent helps in the synthesis of spherical nanoparticles up to size of 15 nm [45].
however, Bhatte et al. reported additive-free synthesis of nanocrystalline magnesium hydroxide (Mh) and magnesium oxide
(Mgo) using the microwave technique. Use of 1,3-propanediol as a solvent played multiple roles in circumventing the addi-
tional requirement of any extraneous species such as base and other capping agents [46].
In continuation with this work, the same group reported an additive-free synthesis of nanocrystalline zinc oxide using
the microwave technique. The microwave irradiation technique was found to be faster, cleaner, and more cost-effective for the
synthesis of zinc oxide nanocrystalline materials than the conventional method. The results prove that microwave heating can
produce polygonal zinc oxide within a short span of time [47].
Idalia et al. showed a fast synthesis route to a variety of binary and ternary metal oxide nanoparticles with high crystallinity
and proposed a way to control the reaction rate by applying microwave irradiation to accelerate the organic reaction pathway
occurring in a parallel way to nanoparticle formation. In contrast to the parallel synthesis procedures performed in the autoclave,
the reactions were much faster and the crystallite size was tuneable [48]. The x-ray diffraction (xRd) pattern of the synthesized
metal oxide nanoparticles helped confirm the crystalline nature of the synthesized nanoparticles (Fig. 18.4); however, transmission
electron microscopy (TeM) images provide a morphological image of the nanomaterials (Fig. 18.5).
