# Green Synthesis of Ag and Pd Nanospheres, Nanowires, and Nanorods Using VitaminB2: Catalytic Polymerisation of Aniline and Pyrrole

Authors: Mallikarjuna N. Nadagouda; Rajender S. Varma
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

For the first time, we report green chemistry approach using vitaminB2 in the synthesis of silver (Ag) and palladium (Pd), nanospheres, nanowires, and nanorods at room temperature without using any harmful reducing agents, such as sodium borohydride (NaBH4) or hydroxylamine hydrochloride and any special capping or dispersing agent. Vitamin B2 was used as reducing agent as well as capping agent due to its high-water solubility, biodegradability, and low-toxicity compared with other reducing agents. The average particle size of nanoprticle was found to be Ag (average size 6.1±0.1 nm) and  Pd (average size 4.1±0.1 nm) nanoparticles in ethylene glycol and  Ag (average size 5.9±0.1 nm, and average size 6.1±0.1) nanoparticles in acetic acid and NMP, respectively. The formation of noble multiple shape nanostructures and their self assembly were dependent on the solvent employed for the preparation. When water was used as solvent media, Ag and Pd nanoparticles started to self-assemble into rod-like structures and in isopropanol Ag and Pd nanoparticles yielded wire-like structures with a thickness in the range of 10 to 20 nm and several hundred microns in length. In acetone and acetonitrile medium, the Ag and Pd nanoparticles are self-assembled into a regular pattern making nanorod structures with thicknesses ranging from 100 to 200 nm and lengths of a few microns. The so-synthesized nanostructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive
X-ray (EDX) analysis, and UV spectroscopy. The ensuing Ag and Pd nanoparticles catalyzed the reactions of aniline and pyrrole to generate polyaniline and polypyrrole nanofibers and may find various technological and biological applications. This single-step greener approach is general and can be extended to other noble metals and transition metal oxides.


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## Body

## 1. Introduction

Recently,
much effort has been devoted to the controlled synthesis of nanostructured
materials because of their unique chemical and  physical properties that are
different from those of the bulk materials [1]. Particularly, metal
nanomaterials have attracted considerable attention because of their unique
magnetic, optical, electrical, and catalytic properties and their potential applications
in nanoelectronics [2]. Hierarchical
assembly of solution-based nanocrystals as building blocks is of great interest
because of their potential in controlling morphologies of nanostructures, and
hence their properties [3–6] wherein structured nanoparticle assemblies such as
wires, rings, and superlattices, can be prepared [7–13]. However, the challenge
of synthetically controlling particle shape had limited success. Nevertheless,
some physical and solid state chemical methods have been developed for making
semiconductor, metal nanowires, nanobelts, and nanodots [14–18] in addition to
wet-chemical methods [19–26]. Presently
there are some methods available  for making rods with controllable aspect ratio
using seeding approaches [27], electrochemical and membrane-templated syntheses [28–30]. In solution phase, pH-dependent
assembly of gold nanorods [31] and streptavidin-linked nanorods [32] is
possible but requires elevated  temperatures and, in most
cases, [33] they afford
poor yield of desired particle shape. Thus, the developments of bulk solution
synthetic methods that offer shape control are very desirable to realize the
full potential of these materials.
Herein, we report a simple strategy using different solvents under which
noble metal nanoparticles effectively self-assemble into spheres, nanowires,
and nanorods in presence of vitamin B2 (Riboflavin) and the ensuing
self-assembled Ag and Pd nanoparticles can catalyze monomers such as pyrrole
and aniline to afford conducting polymer nanocomposites. The study also serves
the need for a greener protocol as there is a  renewed interest in using green
chemistry to synthesize metal nanoparticles
[6, 33] Green chemistry is the
design, development, and implementation of chemical products and the process to
reduce or eliminate the use and generation of substances hazardous to human
health and the environment [34–37]. Strategies to address mounting
environmental concerns with current approaches include the use of
environmentally benign  solvents, biodegradable polymers, and nontoxic
chemicals [34–37]. In the synthesis of metal nanoparticles by reduction of the
corresponding metal ion salt solutions, there are three areas of opportunity to
engage in green chemistry: (i) choice of solvent, (ii) the reducing agent
employed, and (iii) the capping agent (or dispersing agent). In this area,
there has also been increasing interest in identifying environmentally friendly
materials that are multifunctional. For example, the vitamin B2 used
in this study functions both as a reducing and capping agent for Ag and palladium nanostructures, in addition to its high-water solubility, low toxicity and biodegradability; and water serves as a benign media. Herein,  we describe a general method to
prepare noble nanostructures such as Ag and Pd and their catalytic application
to make polyaniline and polypyrrole nanofiber nanocomposites. This greener and general approach used to
produce multiple shaped nanostructures such as Ag and Pd and their nanocomposites
could find newer technological and medicinal applications.


## 2. Experimental Procedure

All  reagents were used as received
without further purification. Vitamin B2 (Riboflavin), silver nitrate (99.99%), and palladium chloride (99.99%) were
purchased from Aldrich. In all
experiments, Milli-Q water was used. The
synthesis and self-assembly were accomplished by reacting respective metal salts with vitamin B2 dissolved in solvents of different chemical nature. The reaction mixture was
allowed to stand for 24 hours at room temperature and then analyzed. In
particularly, 50 mg of Riboflavin was dissolved in 20 mL (glass vial) of
solvents such as ethylene glycol, acetic acid, N-methylpyrrolidinone (NMP), water, isopropanol, acetone, and acetonitrile
in separate experiments. To this, 2 mL
of aqueous solution containing 1×10−1 M AgNO3 were added
at room temperature, hand-shaken for a minute and allowed to settle. The solution turned orange immediately after
the  addition of AgNO3 and the formation of silver nanoparticles was
confirmed using UV spectroscopy and energy dispersive X-ray analysis
(EDX). Similarly, experiments were
conducted using 1×10−1 M PdCl2. TEM specimens were
prepared by placing 1 μL of the particle solution on a carbon-coated copper grid
and dried at room
temperature. Transmission electron microscopy (TEM) was performed with a
JEOL-1200 EX II microscope operated at 120 kV.


## 3. Results and Discussion

Vitamin B2 is the most
frequently encountered organic cofactor in nature and it can assume three
different redox states: fully oxidized, one-electron reduced, and fully reduced
[38]. Each of these redox states exists in a cationic, neutral, and anionic
form (see Scheme   1) depending on the pH of the solution. The reduction
potential of vitamin B2 is −0.3 V versus SCE [39] which is
sufficient to reduce palladium (Pd, reduction potential 0.915 V versus SCE) and
silver (Ag, 0.80 V versus SCE).Scheme 1
Structure of anionic (left), neutral (center), and cationic (right) vitamin B2 species in the fully oxidized redox state (R = −CH2(CHOH)3CH2OH).The formation of
noble nanostructures with vitamin B2 probably occurs as follows: (i)
vitamin B complexation with noble metal salts; (ii) simultaneous
reduction of noble metal salts such as Ag (Ag, 0.80 V versus SCE) and Pd (Pd,
reduction potential 0.915 V versus SCE) and formation of capping with oxidized
vitamin B2; (iii) solvent chemical nature with self-assembly
of nanoparticles to form spheres, nanowires, and nanorods. The reaction of respective metal salts with
vitamin B2 in different solvents resulted in the reduction of metal
and simultaneous oxidation and capping of vitamin B2 as confirmed by
UV spectroscopy (see Figure 1 for Ag). The increase in intensity over the time
indicates the completion of the reaction.
Plasmon absorption spectroscopy is usually used to examine the
size- and shape-controlled nanoparticles because of their optical properties in
aqueous suspensions related to these properties [40]. For example,
the UV-vis spectrum for the colloidal solution of spherical Ag nanoparticles
prepared shows a peak at approximately 375 nm.
This broad extinction peak potentially results from inhomogeneity in
morphology for the sample [40]. However, the plasmon absorption peak of the
nanorods showed a more complex absorption pattern due to the absorption of
visible light both along the length of the nanorods (the longitudinal plasmon
band) and along the width of the nanorods (the transverse plasmon band). As the aspect ratio increases, the
longitudinal surface plasmon resonance (SPR)
band (λ1) should be red-shifted significantly, whereas the
transverse SPR(λT)
band should only show a slight blue-shift [40].Figure 1
Reaction of vitamin B2 with silver nitrate over the time in water media. The inset figure shows
control vitamin B2 (from left), reduced silver nanoparticles in
water, and NMP solvent media after 60 minutes.The size of the Ag and Pd
nanoparticles decreased with use of ethylene glycol and acetic acid.
Solvents such as ethylene glycol yielded nanoparticles with average sizes of6.1±0.1 nm for Ag (Figure 2(a)) and 4.1±0.1 nm for Pd (Figure 2(b)). Acetic acid afforded nanoparticles ranging
from 3 to 8 nm with an average of 5.9±0.1 nm for Ag (Figure 2(c))
and 1 to 7 with an average size of 4.1±0.1 nm for Pd (see Supplementary Material  1 Available online at doi:10.1155/2008/782358). Changing
solvent, for example, using N-methylpyrrolidinone
(NMP), yielded nanoparticles ranging from 3 to 9 nm with an average of 6.1±0.1 nm for Ag (Figure 3(d)) and 5 to 13 nm with an average of 9.2±0.2 nm for Pd (see Supplementary Material).TEM image of Ag and Pd
nanoparticles synthesized using vitamin B2. (a) Ag (average size 6.1±0.1 nm), (b) Pd (average size 4.1±0.1 nm) nanoparticles in ethylene glycol, and
(c), (d) Ag (average size 5.9±0.1 nm, and average size 6.1±0.1) nanoparticles in acetic acid and NMP,
respectively. Inset shows corresponding
particle size distribution, electron diffraction, and UV excitation.
(a)(b)(c)(d)TEM micrographs of (a)–(c) Ag
nanoparticles and (e), (f) Pd
nanoparticles synthesized using vitamin B2 in acetic acid, ethylene
glycol, and NMP with sucrose as a capping agent, respectively.
(a)(b)(c)(d)(e)(f)Solvents such as
ethylene glycol, acetic acid, and NMP are not mere solvents but can modify the
reaction conditions by virtue of their reducing character and pH, and so forth, and thereby
change size and shape of the particles. For example, ethylene glycol is essentially
a reducing agent and can help to reduce noble salts to corresponding metals when
compared with other solvents such as NMP, water, and so forth. The more reduction nature
atmosphere, the bigger the particle size formation. In contrast, NMP can form
complexes with noble salts and can slow the metal reduction rate and hence, decrease
in particle size. However, other solvents do not interfere a great deal in the
reduction. In this context, we conducted
control experiments using sucrose as a capping agent wherein we observed only
nanoparticles with higher particle size (see Figure3). This is due to masking
of solvent effect (reducing and complexing behavior) on nanoparticles and hence
the formation of bigger particle size. The TEM micrographs of Ag and Pd
nanoparticles self-assembled in water and isopropanol are shown in Figure 4.TEM
micrographs of (a), (b) Ag and Pd nanostructures synthesized using vitamin B2 in water and (c), (d) in isopropanol, respectively.
(a)(b)(c)(d)In water, Ag,
and Pd nanoparticles started to self-assemble into rod-like structures. In
isopropanol, Ag and Pd nanoparticles yielded wire-like structures with a
thickness in the range of 10 to 20 nm and several hundred microns in length. In acetone and acetonitrile medium,
the Ag and Pd nanoparticles are self-assembled into a regular pattern making
nanorod structures with thicknesses ranging from 100 to 200 nm and lengths of a
few microns (see Figures5(a)–5(d)). The
formation of self-assembly was due to strong inter-particle van-der-Waals
interactions and hydrophobic interactions in aqueous solutions. Irrespective of
solvent used Ag and Pd crystallized in fcc symmetry. To take the advantage of
self-assembly, we were
able to measure two-probe conductivity.
For instance, Ag nanorods prepared from acetonitrile solvent casted on a
gold electrodes 5 μm apart
gave a conductivity of (40 to 80)×106 depending on electrode
contact which is very much comparable to conductivity of the bare Ag reported [41]. It is interesting to note that self-assembled
Pd nanoparticles can catalyze monomers such as pyrrole and aniline to afford
polypyrrole and polyaniline nanocomposites (see Figure 6 for SEM and UV spectra
and Figure 7 for TEM images). In the present study, Pd- and Ag-catalyzed polypyrrole yielded
nanofiber nanocomposite formation in contrast with the literature reports that
deliver spherical nanoparticle composites [42]. The broad absorption spectrum
from 600 to 900 nm is due to the characteristic peak of polypyrrole formation [43].
We believe these one-dimensional fiber nanocomposites filled with
noble
nanoparticles may find applications in sensors, hydrogen-storage material, fuel
cell membranes, catalysis, and other biological applications.TEM
image of self-assembled Ag and Pd nanowires/nanorods synthesized using vitamin
B2. (a), (b) Ag and Pd nanorods in acetone and (c), (d) Ag and Pd
nanowires/nanorods in acetonitrile. Inset shows corresponding selected area electron diffraction patterns.
(a)(b)(c)(d)(a)
SEM image of polypyrrole nanocomposites catalyzed by self-assembled Pd
nanoparticles synthesized in water media using vitamin B2 and (b)
corresponding UV spectra.
(a)(b)TEM
image of (a) polypyrrole nanocomposites and (b) polyaniline nanocomposites
catalyzed by self-assembled Pd nanoparticles synthesized in water media.
(a)(b)In summary, we
report the following.(i)
High yield, room temperature, solvent-assisted
self-assembly to prepare noble nanostructures such as Ag and Pd nanospheres,
nanowires, and nanorods using vitamin B2 in different solvent media
thereby providing new opportunities in a myriad of applications, such as
catalysis, antibacterial coatings, fuel cell membranes, and so forth.(ii)
The method employed no special capping,
dispersing, or polymer as a coating agent.(iii)
Formation of noble
nanostructure multiple shapes and their self-assembly depended on the solvent
used for the preparation.(iv)
This single-step greener approach (when
used in water) is general and may be extended to other transition metal oxides.(v)
Self-assembled Ag and Pd nanoparticles
catalyzed reactions of aniline and pyrrole to generate polyaniline and
polypyrrole nanocomposites.



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*Source: 782358-2008-01-16.xml*