# Synthesis of Ag or Pt Nanoparticles by Hydrolysis of EitherAg2Na or PtNa

Authors: Huabin Wang; Derek O. Northwood
Journal: Research Letters in Materials Science
 (2008)

---

## Abstract

Ag and Pt nanoparticles have successfully been synthesized by hydrolysis of eitherAg2Na or PtNa at room temperature. The oxidation of sodium in the Pt-Na pellets was much faster than that in the Ag-Na pellets since Pt is a catalyst for H2O formation reaction from hydrogen and oxygen at room temperature. The hydrolysis byproduct, NaOH, has a high solubility and easily is removed. This method offers a simple method of preparing transition metal nanoparticles. The Ag and Pt nanoparticles prepared by this method were crystalline in nature,
and spherical in shape with a mean size of around 10 nm.


---

## Body

## 1. Introduction

Transition metal nanoparticles have
attracted considerable attention since they have unique catalysis, magnetism,
and optical properties, and have great potential for use in chemical
synthesis [1] and fuel cells [2]. Although a variety of preparation methods have
been developed to synthesize transition metal nanoparticles [3–5], and some of
these synthesis methods are, in certain ways, reaching maturity, an economical
synthesis method that is capable of being scaled up to large volume production
is still required.Recently, we have found that Mg2Ni
spontaneously undergo hydrolysis in water and forms Ni nanoparticles, Mg(OH)2,
and hydrogen. After the hydrolysis product, Mg(OH)2 was carefully
removed using an acid, pure Ni nanoparticles were obtained [6]. Cu, Ag, and Au
nanoparticles have also been produced by hydrolysis of their magnides [7, 8].  On further
investigation, certain aluminides such as Al3Ni were shown to
exhibit a similar hydrolysis behavior in water at room temperature [9].
However, due to the chemical characteristics of Al(OH)3, including its
low acidity,  chemically active
transition metal nanoparticles, such as Fe, Co, and Ni, cannot be produced by
using dilute hydrochloric acid  to remove
Al(OH)3.In this study, the hydrolysis behavior of
Ag-Na or Pt-Na intermetallics in water at room temperature was investigated.
The Ag and Pt nanoparticles prepared by hydrolysis of the Ag-Na or Pt-Na
intermetallics were characterized in detail.


## 2. Experimental

Ag2Na or PtNa were synthesized by
in situ reaction of mixtures of
Ag or Pt powders with a weight ratio (Ag or Pt to Na) of about 1 : 4 at 600°C for 2
hours.  Small Ag2Na and PtNa
pellets (about 0.05 g each) were placed, one by one, into 500 mL distilled water
in a matrass. After about 12 hours, the Ag and Pt nanoparticles started to
deposit on the bottom of the matrass. The solid products were rinsed three
times using distilled water, and then three times using ethanol.The phase compositions of the small Ag-Na
and Pt-Na pellets were characterized using a Philips X-ray powder
diffractometer. The morphology of Ag and Pt particles and the chemical
composition were characterized by a TEM. The size of the Ag and Pt particles
(no drying) was analyzed using a Zetasizer 3000 HS instrument.


## 3. Results and Discussion

Ag2Na, Na, and NaOH were observed in the XRD patterns for the
Ag-Na pellets prepared at 600°C for 2 hours (see Figure 1(a)).
No Ag peaks were observed, which shows that all the Ag powder in the initial
materials was totally converted to Ag2Na after the conventional
melting and casting processing. The presence of NaOH suggests that Na in the
pellets reacted with moisture, rather than oxygen in air, during in situ synthesis, storage, and
testing.Figure 1
XRD powder diffraction patterns for (a)  the Ag2Na pellets and  (b) the PtNa pellets.Na, PtNa, NaOH, and Pt diffraction
peaks were observed in the XRD patterns for the Pt-Na pellets prepared at 600°C for 2 hours (see Figure 1(b)).
The Pt peaks in Figure 1(b) were fairly broad, which
reflects the fact that the crystallite size of Pt particles was extremely small
(much smaller than the size of the initial Pt particles (500 nm)). The XRD result (see Figure 2(a)) indicates that
the solid product in the Ag-Na sample, after rinsing in distilled water and ethanol,
only consisted of Ag particles. Discrete Ag nanoparticles were observed
(see Figure 3(a)) by TEM.Figure 2
XRD powder diffraction patterns for (a) the Ag nanoparticles and (b) the Pt
nanoparticles.Morphology of the Ag nanoparticles (a) TEM image and (b) HRTEM image, and the Pt
nanoparticles (c) TEM image and (d) HRTEM image.
(a)(b)(c)(d)There are two intermetallic compounds in the Na-Pt binary system [10]. It has been
reported that Pt-Na with an atomic ratio of 1 : 1 is totally converted into PtNa
at 350–400°C, and a lower transformation temperature
can be achieved when the Pt concentration is less than 50 atomic
percent [11]  According to the
compositions of the initial materials in the Na-Pt sample, a homogeneous liquid alloy melt would be
generated at 600°C. In other words, Pt should be
completely converted into PtNa after cooling down. Therefore, the Pt particles
in the Pt-Na pellets should not come from the initial Pt
powders. In addition, the Pt diffraction peaks in Figure 1(b) were fairly broad, which reflects the fact that the crystallite size of the Pt
particles was extremely small. Hence, the Pt particles in the Pt-Na ingot pellets
must result from the dissociation of PtNa.Only a part of the Na was oxidized in the Ag-Na pellets during storage. In
comparison with the Ag-Na pellets, the Pt-Na ingot pellets exhibited a more
severe oxidation behavior in air with a rapid change in color from metallic to gray,
and then to black.The oxidation product of sodium in both the Ag-Na and Pt-Na samples was NaOH. A possible oxidation
reaction for Na in moist air is as follows:(1)2Na(s)+2H2O(g)→2NaOH(s)+H2(g), where s and g in brackets denote the solid and gas states,
respectively.Pt is a catalyst for
the formation reaction of H2O from hydrogen and oxygen at room
temperature [12]. The hydrogen formed in Reaction 1 could react with oxygen in
air and form water again. Thus, Pt is able to accelerate the oxidation of
sodium in the Pt-Na pellets.When the Ag-Na or the Pt-Na pellets were dropped into a matrass,
which contained 500 mL distilled water, the reactions were very fast, and
sometimes were accompanied by slight explosions or combustion. Water in the matrass
instantly became a colloid.The reactions of Na2Ag and PtNa
in water can be written as follows:(2)2Ag2Na(s)+2H2O(l)→2NaOH(l)+4Ag(s)+H2(g),2PtNa(s)+2H2O(l)→2NaOH(l)+2Pt(s)+H2(g), where l in brackets
denotes the liquid state.After about 12 hours, the colloids started
to deposit on the bottom of the matrass. The solid products were rinsed three
times using distilled water, and then three times using ethanol. Ag and Pt
nanoparticles were obtained.The Ag nanoparticles were predominantly
spherical in shape and had a size of around 10 nm, which agrees with the
results of the particle size analysis (see Figure4(a)).  The size of the Ag nanoparticles ranged from 5.4 nm
to 17.2 nm with a mean size of 8.3 nm. The high-resolution TEM image for
individual nanoparticles (see Figure 3(b)) shows that the spacing for the
lattice fringe of the indicated nanoparticles is 0.230 nm. This is close to the
interplanar spacing for the (111) planes of Ag (0.2259 nm). In addition, the
fringes for individual particles indicate that some of the small Ag
nanoparticles are single crystals. No significant oxygen and Na was detected by
EDS. This result suggests that the Ag nanoparticles are resistant to oxidation
from both the dissolved oxygen in an aqueous solution and the oxygen in flowing air.Particle size distribution of (a) the Ag nanoparticles and (b) the Pt
nanoparticles.
(a)(b)XRD (Figure2(b))
shows that the solid final product in the Pt-Na sample consisted only of Pt
particles. The morphology of the Pt nanoparticles (see Figure 3(c)) was similar to that of the Ag
nanoparticles (spherical in shape and a size of around
10 nm). The high-resolution TEM image for individual nanoparticles (see Figure 3(d))
shows that the spacings for the lattice fringes of the indicated nanoparticles
are 0.235 nm. This is close to 0.2265 nm, the interplanar spacing for the {111}
planes in Pt. The results for the particle size analysis (see Figure 4(b))
shows that Pt nanoparticles ranged in size from 5.6 nm to 17.9 nm with a mean
size of 8.7 nm. The size of the initial Pt particles for preparing the Pt-Na
pellets ranged from 150 nm to 500 nm, much larger than above results, which
confirmed that these Pt nanoparticles resulted from the dissociation of the
Pt-Na intermetallic compounds.No significant oxygen and Na was detected by EDS, which indicates that Pt nanoparticles are resistant
to oxidation from both the dissolved oxygen in an aqueous solution and the
oxygen in flowing air.Compared with the hydrolysis of magnides, the reaction byproduct, NaOH, has a high
solubility and is easily removed. This method offers a simple method of
preparing transition metal nanoparticles. In addition, the shape and size of both Ag and
Pt nanoparticles could be easily modified by chemical methods since these
nanoparticles were prepared in an aqueous solution. The
drawback of this method is the difficulty in controlling the reaction rate.Besides the sodium intermetallic compounds,
there are many other alkali metal or alkali earth metal intermetallic
compounds. Due to their high chemical activity, they also could exhibit a
similar hydrolysis behavior in water. Hence, they could also have the potential
to produce transition metal and semiconductor element nanoparticles.


## 4. Summary

The oxidation of sodium in the Pt-Na pellets in air was much faster than that in
the Ag-Na pellets since Pt is a catalyst for formation reaction of H2O
from hydrogen and oxygen at room temperature.
The hydrolysis byproduct, NaOH, has a high solubility and is easily removed. This
method offers a simple method of preparing transition metal nanoparticles.  The Ag and Pt nanoparticles prepared by the
hydrolysis of either Ag2Na or PtNa were crystalline in nature, had a
spherical shape with a mean size of around 10 nm.



---

*Source: 619032-2008-04-03.xml*