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16 Surfactant-Free Synthesis of Metal and Metal Oxide Nanomateria ls:… 407
that MIBK may be made more effectively in a single step with a metal-supported
catalyst. Acetone condensation can occur via either an enol or an enolate process,
depending on the acidity/basicity of the support. The intensity of the acid/base sites
also influences the pace of conversion (Sifniades et al.
2010; Diallo-Garcia et al.
2011; Shylesh et al. 2015).
In this investigation, Esumi and associates investigated the production of palladium nanoparticles by the thermal breakdown of bis(2,4-pentanedionato) palladium
(II) Pd(acac) in a range of organic solvents without the use of additives, including
DMSO, p-xylene, oxylene, bromobenzene, and chloroform. Pd(acac) had to be
dissolved in an organic solvent and refluxed at boiling point for a duration of
20–24 h as part of the preparation process. A stable dispersion of Pd nanoparticles
was initially generated by MIBK, which served as simultaneously the solvent and
the stabilizing ingredient (Esumi et al. 1989). The mean diameter of the as-prepared
Pd nanoparticles was 8–10 nm, and their size distributions were mostly unaffected
by the quantity of precursor Pd ions. Solvents that display high dipole moments have
the tendency to dissolve the palladium particles, stabilizing the Pd nanoparticles in
MIBK. Chen and colleagues employed microwave irradiation to break down Pd
acetate in a MIBK solution with KOH, resulting in the synthesis of solvent-stabilized
Pd nanoparticles with approximate diameters of 33 nm. The solvent-stabilized Pd
nanoparticles were shown by TEM and X-ray diffraction studies to be composed of
clusters formed from numerous tiny 3–4 nm palladium nanocrystals.
16.2.2 Simple Ion Assisted Synthesis
16.2.2.1 Citrate Assisted Synthesis
A typical stabilizing compound for metal nanoparticles is citrate, which offers a
negatively charged surface that may be moved by ligands or other molecules
(Fig. 16.3). Due to their lack of steric stability, noble metal nanoparticles are
prone to aggregation in non-aqueous solvents and elevated salt solutions despite
having good stability in water and weakly-buffered solutions. Its tiny molecule
forms weak associations with the surface of the particles due to the numerous
carboxylic acid groups. Because of its great movability, the surface can interchange
ligands with other ligands or proteins. Molecules containing amines or thiols can
Fig. 16.3 Citrate surface
408 S. Churi et al.
readily replace citrate and form a strong bond with surfaces that are gold or silver.
The hydrodynamic diameter and the TEM measured diameter are almost same.
Citrate ions undergo interactions with the surfaces of gold or silver nanoparticles,
as reported by Turkevich et al., when utilized as a reductant in the formation of metal
colloidal materials (John Turkevich et al.
to produce go
ld colloids, citrate ions serve as simultaneously a reducing agent and a
). In the citrate reduction method used
1941
stabilizing factor. These ions form a charged coating on the nanoparticle surfaces
that generates electrical repulsive forces in an aqueous environment. The interaction
of the amino or thiol group with the gold nanoparticles displaces poorer bound
citrate ions. The size of the gold nanoparticles is dependent on the ratio of gold ions
to the reduci
alter the sizes
ng agent. Turkevich et al.-based approaches have been developed to
and shapes of the nanoparticles. A variety of sizes, from 9 to 120 nm,
may be produced in Au nanoparticles using the citrate-mediated synthesis method.
For stable nanoparticles, the lowest possible actual amount of the gold ions is under
two millimeters. Elevated concentrations cause precipitation on vessel walls, but
suspensions at concentrations below 1 mm remain for several months. U
revised Turkevich
strategies, monodispersed sub-10 nanometer silver and gold
nanoparticles have been generated subsequently (Pillai and Kamat
tilizing
2004; Uppal
et al. 2010).
Turkevich strategy is used to study the processes of nucleation of nanoparticles,
conversion of silve r ions into atoms, and citrate oxidation. Citrate is not as powerful
a reductant as the oxidized radical Cit(-H)•. The generation ratios of CO
and
2
dicarboxy acetone (DCA) during Cit(H)• oxidation validate the decarboxylation
mechanism (Al Gharib et al. 2019). Pulse radiolysis of solutions containing silver
perchlorate and sodium citrate from 20 ps to 800 ms exhibits transient spectra and
kinetics. The mechanism initiates the nucleation and growth of reduced silver
oligomers by transferring one electron from citrate radicals to silver ions. OH•
radicals sequentially abstract H from citrate after this. The reduction potential of
citrate and Cit(-H)• can account for a portion of the anti oxidant properties of the
compound.
16.2.2.2 Amino Acid Assisted Synthesis
Due to their zwitterionic behavior, lack of a tightly linked bond, and structural
chirality, amino acids are significant compounds (Nidya et al. 2015). Because they
may be utilized as capping agents for the formation of nanoparticles (Vemula et al.
2007
despite having any negative effects, amino acids and vitamins are environ-
)
mentally safe substances. Protein building components, amino acids, are the perfect
biodegradable capping agents for fabricating metal nanoparticles without the need
for an extra surfactant. Amino acids are used in this approach as a reductant and
stabilizer, much like citrate is. For the formation of metal nanoparticles, a variety of
amino acids can be employed, including glutamic acid, histidine, aspartic acid,
tryptophan, phenylalanine, cysteine, and methionine. These amino acids have
stabilizing and reductant properties (Shao et al.
Ma and Han 2008; Liu et al.
2004;
2010). The quickest reducing agent among the twenty amino acids is tryptophan.
Using histidine as a reducing agent and a protective ligand, Yang et al. produced
16 Surfactant-Free Synthesis of Metal and Metal Oxide Nanomateria ls:… 409
water-soluble Au 10 NCs that fluoresced high-intensity bluish-green light, as
demonstrated by a straightforward process (Yang et al.
2011).
Reducing stability and managing the size and shape of nanoparticles (NPs) are
problems for green chemical techniques. One way to address this issue is to use
eco-friendly reagents such as α-amino acids (α-AA) as capping and reducing agents.
In order to conjugate importan t biomolecules, improve biocompatibility, and minimize problems in biomedical applications, α-AA provides amino and carboxy
groups. It has been demonstrated to be effective in producing homogeneous,
quasi-spherical gold nanoparticles by reducing gold salts. The benefits of α-AA
are its availability, low cost, and nontoxicity. Numerous research works have
documented the use of one, several, or all 20 proteinogenic α-AA in the synthesis
of AuNPs. Nonetheless, the majority of research uses distinct reaction parameters for
various amino acids. For contrast, only three papers employed all 20 proteinogenic
α-AA for AuNPs production under comparable or identical reaction circumstances.
Gold complexes can be reduced by α-AA in a concentration-dependent way, where
smaller particles result from greater concentrations and vice versa. A reaction
technique for converting gold complexes with amino acids has been suggested in a
short number of works (Berghian-Grosan et al.
2014; Chakraborty et al. 2018; Sun
et al. 2018).
The study used various reagents, including l-glycine, l-alanine, l-valine, l-leucine,
l-isoleucine, dl-phenylalanine, l-tryptophan, l-methionine, l-proline, l-cysteine,
l-glutamine, l-asparagine, l-tyrosine, l-serine, dl-threonine, l-(4)-hydroxyproline,
l-glutamic acid, l-aspartic acid, l-histidine, l-lysine, and l-arginine, to investigate
the physicochemical properties of α-amino acids (Figat et al. 2023). These
properties, including solubility, stability, pK, and pI, were expected to influence
the reactions’ outcomes. The pK is particularly important as it is known that the form
of gold salts and reducing agents affects the reactions’ results. The study aimed to
understand the unique physicochemical properties of these amino acids (Das and Raj
2011).
16.2.2.3 Iodide Assisted Synthesis
When forming metal nanoparticles, such anisotropic Au nanoparticles, iodide is a
commonly employed surfactant. Raj et al. showed that single-crystalline polyhedral
Au nanostructures may be synthesized via iodide-mediated reduction of AuCl
-ions.
4
This is accomplished by disproportioning Au (I) to metallic Au (0) and reducing an
Au (III) complex to a metastable Au (I) complex. The quantity of iodide ions and the
existence of Ag ions determine the form and surface structure of Au nanoparticles.
Higher concentrations of KI (10 μ m) lead to uncertainty, whereas lower quantities
protect the Au nanoparticles. Dendritic Au nanostructures develop more quickly
when silver ions are present (Rashid et al. 2006)
.
Metal nanocrystals (NCs) based on rhodium have been shown to be efficient
catalysts for a number of processes, such as hydroformylation, CO oxidation, NO
reduction, hydrogenations, and electro-oxidations. However, in comparison with
palladium, platinum, silver, and gold, their molecular-level control has received less
research attention. A research employing foreign metal crystal surfaces as substrates
410 S. Churi et al.
found Rh epitaxial overgrowth initiated by iodide. Different metal substrates can
support epigrowth, and their catalytic characteristics were studied in the electrooxidation of formic acid and CO stripping. In the ov ergrowth process, iodide is
essential, even on gold surfaces with a significant lattice mismatch. Selected etching
of core substrates has also been used to create hollow Rh nanostructures. This new
role of iodide in the overgrowth and high control for Rh
nanoscale control of
(Sneed et al.
2012).
this important metal’s architecture for heterogeneous catalysis
could be crucial for future
16.2.2.4 Buffer Assisted Synthesis
Sodium borohydride (NaBH4) and/or sodium citrate are used as reducing agents in
the chemical transformation of chloroauric acid (HAuCl
) to create gold
4
nanoparticles (AuNPs). To prevent therapeutic side effects, safe, biocompatible
NPs with improved furtivity and biocompatibility are needed for AuNP uses in
medicine. Natural macromolecules have garnered attention as an eco-friendly
method of producing AuNPs has emerged in recent years. There have been reports
of natural substances such as chitosan, cellulose, and spider-silk fiber acting as Au
3+
reducing agents. For highly biocompatible AuNPs, proteins which are currently
utilized in nanoparticle synthesis make excellent candidates. Based on drug-gold
complexes, Moustaoui et al. suggested a novel technique for producing AuNPs as
nanotherapeutic agents (Arib et al.
2021).
Xie et al. used 2-[4-(2-Hydroxyethyl)-1-piperazinyl] ethanesulfonic acid
(HEPES) as a relatively mild reducing, protecting, and shape-directing agent for
the beneficial surfactant-free manufacture of Au nanoflowers. The process involved
mixing 100 mM HEPES with 9 mL of deionized water, then adding 250 μlof20mM
HAuCl
solution. The resulting Au nanoflowers, also called gold nanoflowers, had
4
more than ten points and were almost spherical in shape. They also included threedimensionally expanding nanomaterials. Three stages are involved in producing
gold nanoflowers: reducing Au(III) ions to basic Au nanocrystals; agglomerating
initial Au nanocrystals into intermediary agglomerates; and developing
agglomerates anisotropically into flower-like nanostructures (Xie et al.
2008).
16.2.2.5 Tetrakis (Hydroxymethyl)Phosphonium Chloride (THPC)
Assisted Synthesis
THPC is used in the production of silver, gold, platinum, and palladium metal
nanoparticles as a reducing and stabilizing agent. It makes it easier for metal ions
to be reduced under control to create nanoparticles with the correct sizes and
characteristics. THPC was employed in the study as a reducing agent to create
silver, platinum, and palladium seed nanoparticles, which had a diameter of around
4 nm. Afterwards, on silica cores, these nanoshells were utilized to create pure silver,
platinum, and palladium nanoshells as well as hybrid platinum/silver nanoshells.
Transmission electron microscopy (TEM), selected area electron diffraction, X-ray
photoelectron spectroscopy, FTIR, and ultraviolet-visible spectroscopy were among
the methods used to describe the nanoshells. A variety of metal nanoshells, including
very unifo rm platinum and palladium nanoshells, were quickly seeded to develop
16 Surfactant-Free Synthesis of Metal and Metal Oxide Nanomateria ls:… 411
using the THPC-mNPs. The researchers also optimized the nanoshell syntheses for
generating metallic shells wi th continuous, smooth surfaces (Bryan et al.
2016).
With potential uses in catalysis (Hueso et al. 2013), tetrakis-(hydroxymethyl)-phosphonium chloride (THPC) has been employed as a stabilizing ligand and reducing
agent in the one-step production of monometallic nanoparticles and bi -/tri-metallic
nanoalloys comprising noble metals. These colloidal suspensions exhibit excellent
stability in aqueous solutions and mean sizes of less than 4 nm.
16.2.3 Physical Process-Mediated Synthesis
16.2.3.1 Photochemically-Mediated Synthesis
Recent advances in photoredox catalysis have led to a significant rise in the use of
synthetic processes mediated by photochemistry. But problems with photochemical
reaction scaling, mainly with light entering bigger reaction vessels, secondary
photoreactions at longer reaction times, and thermal effect control, have forced
continuous flow photochemical reactors to be widely used in labs and development
settings. Segmented flow is frequently needed for multiphasic systems (liquid/liquid,
gas liquid), yet the majority of reactor designs are straightforward tubular reactors
that manage homogenous reactions. Using patterns or oscillations, mixing is frequently utilized to increase mass transfer rates. According to recent reports, there is a
need for photochemical reactors that can handle solid/liquid biphasic systems. These
reactors can be continuous stirred tank reactors (CSTRs), plate flow reactors that
combine oscillatory flow and static mixers, or immersion well reactors operating in
semi-continuous recirculatory mode. The ‘fReactor’ is a compact, easily
reconfigurable, modular CSTR platform with strong mass transfer characteristics
that can handle multi-phasic mixes of gas, liquid, and solid. This technology, which
combines strong mass transport with a high surface area to volume ratio for good
light penetration, has demonstrated first uses as a continuous flow photochemical
reactor (Grimm et al. 2020; Bonfield et al. 2020).
McGilvray
et al. described the photochemical synthesis of stable, unprotected
gold nanoparticles without the need for conventional stabilizing compounds. They
subjected it to radiation at a wavelength of 350 nm using 1-[4-(2-hydroxyethoxy)
phenyl]-2-hydroxy-2-methyl-1-propane-1-one (Irgacure-2959). The reducing agents
called ketyl radicals changed Au
3+
into Au 0 to develop Au nanoparticles. Over
several months, the synthesis process proceeds quickly and steadily. To modify the
particle size, the illuminating radiation’s intensity can be adjusted. Particles with a
diameter of 150–300 nm were formed after 3 days of exposure to room lighting,
whereas Au nanoparticles with sizes of 40 ± 10, 12 ± 3, and 8 ± 2 nm were created
by irradiation with light intensities of 7, 40, and 100 W/m
2
. Water-soluble thiols
such as 3-mercapto-1-propane sulfonic acid (MPSA) can be used to functionalize
these Au nanoparticles (McGilvray et al.
2006).
412 S. Churi et al.
16.2.3.2 Sonochemically Assisted Synthesis
For the synthesis of noble metal nanoparticles (NPs) like Au, Ag, Pt, and Pb, the
sonochemical process is an intriguing approach (Bang and Suslick
2010; Kumar
et al. 2020). The underlying process is called cavitation, which produces gaseous
bubbles of μm sizes that burst and collapse inside the medium. As a result, there is
sufficient chemical susceptibility to decrease noble metals, including silver, since
there is enough energy, temperature, and pressure to change solvent molecules into
highly reactive free radicals. Sonochemical processes provide several benefits,
including as spherical and uniform variations, symmetric distributions, and superior
purity of nanomaterials, rapid reaction speeds, adjustable settings, clarity, and
stability. Sonochemical techniques are not dependent on high temperatures,
extended reaction durations, pH regulation, shape control agents, or steric capping
agents like NP wet synthesis techniques are. Sonochemistry is a promising method
for producing AgNPs in the sub-15 nm range with specified dimensional features
and possibly stable metallic nuclei in aqueous condit ions for an extended length of
time since it also provides the ability to produce AgNPs with particular particle sizes
(Bang and Suslick 2010; Xu and Suslick 2010).
The primary process for the sono-chemical production of metallic nanoparticles is
the cavitation phenomena. It produces gaseous bubble s with μm diameters that burst
and collapse inside the medium, producing enough heat, pressure, and energy to
change solvent molecules into reactive species’ free radicals, which may lower noble
metals like silver (Restrepo and Villa 2021; Calderón-Jiménez et al. 2022).
Utilizing ultrasonic irradiation as a green reducing agent, a green sono-chemical
process was devised to manufacture silver nanoparticles (Ag-NPs) at various
concentrations of kappa carrageenan (κ-carrageenan). As the concentration of
κ-carrageenan increased, so did the amount of Ag-NPs . Ag/κ-carrageenan was
formed, as demonstrated by UV-visible spectroscopy, and X-ray diffraction showed
a face-centered cubic structure. The TEM and SEM pictures revealed a spherical
form, and the green and economical characteristic was provided by light irradiation
(Elsupikhe et al. 2015).
16.2.3.3 Laser Ablation-Mediated Synthesis
Colloidal metallic nanoparticles such as silver and gold are produced by laser
ablation of a solid noble metal target in a liquid media. There are several solvents
utilized, including n-hexane, ethanol, and water. Larger clusters of metal are formed
when smaller metal clusters are ablated from the solid metal. Even in the absence of
surfactants, these metal nanoparticles remain stable against aggregation in water. It
was discovered that the partly oxidized surfaces of Au nanoparticles created by laser
ablation in water were negatively charged. The polarity of the solvent, processing
duration, and laser intensity may all be adjusted to regulate the formation of
nanoparticles in the absence of surfactants (Sylvestre et al. 2005; Muto et al. 2007).
Wet chemical
techniques provide higher productivity and more control over
laser-ablated nanoparticles. Colloid concentration has grown along with productivity
thanks to recent developments in dynamic flow-based laser ablation technology. By
adjusting solvent flow and flow conditions, Freeland et al. (2021) were able to
16 Surfactant-Free Synthesis of Metal and Metal Oxide Nanomateria ls:… 413
accomplish effective silicon nanoparticle ablation (Freeland et al. 2021;
Balachandran et al.
by Aneta Plaza et
fiber laser. Marc Labusch et al. achieved steady ablation processes by use of acoustic
emission monitoring. The colloid quality and environmental friendliness of LASiS
are making them increasingly appealing for the production of conductive inks and
nanoparticles. This overview covers the variables that influence the characteri
and yield of LASiS nanop
problems that need to be solved to enhance performance (Labusch et al.
2022). Chemically pure silver-109 nanoparticles were obtained
al. using a 2D galvanometer scanner in conjunction with a pulsed
stics
articles, how printed electronics use them, and the
2019).
16.3 Application of Metallic Nanoparticles Synthesized Using
Surfactant Free Solutions
Ultrahigh cooling performance is constrained by a multitude of industrial
technologies to maintain the instrumentation cool for efficient operation. Heattransfer efficiency of the base fluids is improved by the formation of nanomaterial
suspension (Wan et al.
using various nanomaterials, such as metal and metal oxide, and they have
demonstrated improved thermal conductivity (Baby and Ramapr abhu 2011). For
appropriate dispersion, nanoparticles are typically either dispersed in surfactants or
functionalized by a capping agent. The thermal conductivity of nanofluids may be
reduced by the addition of surfactants and/or capping agents.
Using no capping agent, nickeloxide (NiO) and cobaltoxide (CoO) nano-particles
were created in the first stage. The resulting nanoparticles were then disseminated in
base-fluid using gentle ultrasonic mixing technique in the second stage. In reality
metal salts are precipitated with hydroxide ions and then dehydrated to create metal
oxides. Particle sizes can vary when water is utilized, and this might be challenging
to regulate the process when using organic solvents. Ethanol, serves as a solvent
along with source of OH ions. At elevated temperatures, a slow release of OH ions
can be achieved. This method gives you control over particle size while also
reducing the quantity of additional additives you employ in the reaction (Parashar
et al. 2010).
Rashmi P
ing scattered metal oxide nanoparticles without the use of capping ligands or
inorganic bases. A separate 0.2 M solution of Cobalt (II) chloride hexahydrate and
Nickel (II) acetate tetrahydrate was produced in 25 ml of HPLC grade water for the
production of CoO and NiO nanoparticles. After adding 1.5 ml of 0.2 M Cobalt
(II) chloride hexahydrate aqueous solution and 1.5 ml HPLC grade water, 24 ml of
room-temperature ethanol solvent was added. For 20 h, the reaction was stirred at
80 °C while the container was maintained tightly closed. After that, the first step’s
reaction-mix was transferred to a 25 ml autoclave along with a teflon lining so that it
could undergo a hydrothermal reaction for 3 h at 160 °C.
The identical
prepare nickel oxide nanoparticles. The thermal conductivity of nanofluids was
arashar a
2012). Numerous research teams have created nanofluids
nd colleagues presented a novel and simple method for creat-
procedures used to make cobalt oxide nanoparticles were used to
414 S. Churi et al.
determined using the Hot Disk Thermal Constants Analyzer. The device measures
the thermal conductivities of nanofluids using the transient plane source method.
Since the current synthesis technique does not allow for the filtering or separation of
the nanoparticles of extremely small sizes from the suspension product, they utilized
the nanoparticles as liquid suspensions whose portion added to base fluids, such as
ethylene glycol. Heat conduction observed in NO-EG na
et al.
CO-EG (Parashar
2014).
nofluids is lower than in
16.3.1 Synthetic Catalysis
The existence of strongly bound organic layers which is protective on action and
surrounding metal nanoparticles might render it unfit for catalytic applications, since
this process occurs on surfaces of metal. Hence for catalysis, the metal nanoparticles
produced by manufacturing techniques which devoid use of surfactants will be
appropriate. Studies on supported metal nanoparticles, or those utilized in heterogeneous catalysis, done extensively (Zhang et al. 2012; Wu et al. 2012) however they
are not within the purview of this review. Only unsupported metal nanoparticles, or
colloidal metal nanoparticles, that are produced as synthetic catalysts without the use
of surfactants are covered in this review. The section on electrocatalysis will cover
heterogeneous catalysis employing supported metal nanoparticles made without the
need of surfactants.
urfac
The s
coupling reactions was described by Obora and colleague (Hyotanishi et al. 2011).
he D
T
were 1–1.5 nm in size. Because the DMF solvents stabilized the Pd-nanoclusters,
they displayed good dispersion in a variety of polar solvents. During organic
synthesis, the Suzuki Miyaura and Mizoroki Heck cross-coupling reactions are
two very efficient ways to create carbon-carbon bonds. The DMF-stabilized
Pd-nanoclusters investigated as catalysts in these processes. In Suzuki-Miyaura
and Mizoroki-Heck cross-coupling reactions, the Pd-nanoclusters exhibited strong
catalysis capability. In the presence of 10
number (TN) was as high as 6.0 × 10
catalytic activity compared to the subnanometer Pd-nanoclusters (about 0.7 nm)
supported on polymer micelles, demonstrating a high TN of around 2.8 × 10
(Okamoto et al. 2005). Additionally, by employing a liquid-liquid extraction technique, the researchers devised a way to recycle this catalyst during Suzuki-Miyaura
cross coupling reaction at least five times.
Precious
Because copper is a cheap metal, it is frequently utilized in industry as an alternative.
Significant research has been done to determine whether copper may be used in
catalysis (Evano et al. 2008; Rao and Fu 2011). Obora and associates used the
DFM-based reduction approach to create Cu nanoparticles having size roughly 2 nm,
and they did it without the need for surfactants. In case of Ullmann-coupling
reaction, these Cu nanoparticles served as catalysts (Isomura et al.
tant devoid synthesis of Pd-nanoclusters for use in catalytic cross-
MF-based reduction process was used to create the Pd-nanoclusters, which
-7
mol% of Pd-nanoclusters, the turnover
8.
The Pd-nanoclusters exhibited a greater
metals are expensive, which restricts the industrial uses for them.
As aryl
2012).
5
16 Surfactant-Free Synthesis of Metal and Metal Oxide Nanomateria ls:… 415
halides and phenols were cross-coupled as per Ullmann reaction in the absence of
ligand, Cu nanoparticles showed an elevated catalytic ability with a TN of 2.2 × 10
4
.
In a well-known model reaction that is catalysed by metals (Yamamoto et al.
2012; Hervés et al. 2012) 4-nitrophenol is reduced to 4-aminophenol by NaBH
Kawasaki and colleagues examined catalytic characteristics of DMF-stabilized Au
NCs. Even at low concentrations (10
catalytic ability; at 25 °C, the pseudo-first-order rate constant (k app) was 1.0 × 10
2
for 1.5 μm of the gold catalyst. The idea was that the key to obtaining a high
-7
g), the DMF-stabilized Au NCs had strong
catalytic activity exhibiting DMF-stabilized Au NCs was the reorganization of the
DMF layer, which is really an activation process.
Mandal and colleagues presented a wet-chemical approach of preparing metal
nanoparticles without the use of surfactants. The se nanoparticles were also
employed by them as artificial catalysts. The production of Au nanoparticles ranging
in size from 20 to 140 nm was achieved through utilizing ammonium bismuth citrate
as stabilizing and reducing agent. With a great surface area 42 m
nanoparticles resembled sponges and bearing pores with sizes 2.4–6.0 nm. With a
rate constant k = 2.1 × 10
strong catalysis in the reduction of PNP using NaBH
-3 s-1
, these sponge-like Au nanoparticles demonstrated a
(Rashid et al. 2006). More-
4
2
/g, the Au
over, ferric ammonium citrate utilized as reducing agent to create polygo nal Au
nanoparticles without need for a polymeri c template or surfactant (Rashid and
Mandal 2008). When it came to the reduction of nitrophenol, polygonal Au
nanoparticles’ catalysis outperformed that of the spherical Au nanoparticles made
using the Turkevich approach by a factor of 300–1000. Additionally, an increase in
activity comparable to this was noted in the aerobic oxidation of several d-hexoses.
The polygonal gold nanoparticles enhanced catalytic activity was ascribed to their
increased number of sharp corners and edges.
The a
bility o
f citrate stabilized Au nanoparticles to catalyze the reaction of
thiosulfate ions with ferrercyanide was reported by Freund and colleagues (Freund
and Spiro 1985). The catalytic rate rise as the quantity of Au nanoparticles increased,
indicating the catalytic rate proportional to the surface area of Au nanoparticles. The
reaction was proposed to have happened on the surfaces of the Au nanoparticles and
not been restrained by diffusion. According to research by Rossi and colleagues
(Comotti et al. 2004; Welch and Compton 2006), Under strongly basic conditions,
bare Au nanoparticles with a mean diameter of 3.6 nm were employed to enhance
aerobic oxidation of glucose in an aqueous solution. This process allowed 21%
conversion of glucose within the 200 s. The researchers discovered that the diameter
of Au nanoparticles was inversely correlated with their catalytic activity, utilizing
Au nanoparticles ranging in size from 3 to 6 nm. Au nanoparticles bigger than 6 nm
size, on other hand, didn’t exhibit a linea r rise in catalysis; instead, sudden change
was noted at about 10 nm in diameter. This demonstrated the catalytic activity’s
discontinuity at the nanoscale.
The same
quantity of gold was used to examine the differences in the catalysis of
Au nanoparticles supported on carbon versus those that are not. In first 100 s of the
oxidation of glucose, the two types of nanoparticles’ initial response rates were
-
.
4
-
416 S. Churi et al.
rather equal. The inclination of Au nanoparticles to expand and forms agglomerates
in the medium, however, meant that this colloidal Au-nanoparticles were unstable
and eventually experienced deactivation.
16.3.2 Electrocatalysis
Main goal of research is creating an effective metal nanoparticle electrocatalysts that
may transfer electrons between reactants and electrodes. In order to attain a current
density that is near to equilibrium potential, these catalysts transfer electrons.
Electrode-supported metal nanoparticles have been used to exhibit electro-catalytic
reactions for example H
tion. However, the use of surfactants-polymers, which hindered catalytic sites and
reduce catalytic activity, is frequently necessary for the production of materials with
predictable sizes and shapes (Welch and Compton 2006; Hu and Dong 2008).
According to Zhou et al. (2003), a polyol method free of surfactants was used to
create Pt nanoparticles the mean diameter of roughly 2.9 nm. Carbon was used to
support the nanoparticles, and up to 40% weight percentage of Pt was loaded. In
comparison to commercial catalysts, the as-synthesised Pt/C electrocatalyst
demonstrated superior electrocatalysis as cathode in course of reaction for oxygen
reduction in direct methyl fuel cells (DMFC).
A study by Li et al. (Kiadó and Vol 2004) described the creation of Pt/MWNT
nanocomposites, which are supported by multiwalled carbon nanotubes. The Pt salt
was reduced using an aqueous solution (HCHO reduction) and a Ption salt was
reduced in an EG solution. In DMFC, those nanocomposites served as cathode
catalysts. The Pt nanoparticles, ranging in size from 2 to 5 nm, were uniformly
distributed through the utilization of the surfactant-free polyol technique in the
context of Pt/MWNTs. With a Brunauer-Emmett-Teller (B-E-T) having surface
area as 237 m
2
/g, the Pt/MWNT nanocomposites exhibited a much greater catalytic
performance than the commercial catalyst Pt/XC-72. As a result, the surfactant-free
polyol method of producing tiny metal nanoparticles provides an appealing means of
obtaining heterogeneous catalyst (Wang et al.
Using a surfactant-free, microwave-assisted polyol synthesis process, Kundu
et al. (2011) developed Pt nanocomposites supported by reduced graphene oxide
(rGO) with a diameter of 2–3 nm that can serve as electrocatalysts in the oxidation of
methanol and conversion of H
The r
t nanocomposites’ tolerance, methanol oxidation potential, and current
GO/P
were measured; the findings demonstrated that the catalysts had high H
and strong electrocatalytic activity. By combining a HauCl
et al. created a unique method for growing ultrafine Au NCs on rGO sheets without
the need for an extra protective molecule or reductant (Yin et al. 2012). The
graphene oxide sheets had Au NCs with an average diameter of 1.8 nm and a limited
dispersion of 0.2 nm.
This met
hod might also be used to get Pt and Pd agglomerates of other metals on
the rGO sheets. When utilized as the cathode in fuel cells, the synthesised Au
reduction, CO oxidation, oxidation, and oxygen reduc-
2O2
2005).
.
2
conversion
2
solution with rGO, Yin
4