# Effect of Ammonium Nitrate on Nanoparticle Size Reduction

Authors: Kalyana C. Pingali; Shuguang Deng; David A. Rockstraw
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

Ammonium nitrate was added to the spraying solution as a foaming agent to reduce the particle size of nanoparticles synthesized in the spray-pyrolysis process. Ammonium nitrate was effective in breaking the aerosol droplet size and generating nanoparticles that were of approximately one order-of-magnitude (from 200 to 20 nm) smaller diameter than those created in the absence of ammonium nitrate in the feed solution. This technique makes it possible to control the particle diameter of metallic nanoparticles below 20 nm.


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## Body

## 1. Introduction

Size control of nanoparticle diameter in the range of 10–20 nm during the synthesis
remains a technological challenge of current relevance. Spray pyrolysis was
used to synthesize silver nanoparticles from aqueous silver nitrate solution
[1]. To get the desired particle size, a concentration in the range of 10−7 M of the feed
solution was necessary. This will be problematic for the synthesis of
commercial quantities of nanoparticles; the production rate of nanoparticles
will be extremely low. It was observed that it becomes difficult to further
reduce particle size by reducing concentration when the subnanometer range is
approached. There is a need for a high-rate process that produces a particle size below 5 nm [2].
In this communication, the production of small nanoparticles at moderate
feedstock precursor concentration is presented.Various particle reduction techniques are described in the literature
[3–9]. Several
mechanisms  and methods have been used to study the effect of ammonia on
particle size. Arriagada and Osseo-Asare [3] observed that  the presence of ammonia decreases
the size of a microemulsion and final particle size. Ammonia was applied to
synthesize nanocrystalline aluminum nitride powders with particle size of 8 nm
[4]. Borgna et al. [5] observed a narrow particle size
distribution with the introduction of ammonia gas in the feed stream. Ammonia
gas was also used to synthesize fine metallic tin particles [6], where particle
size was found to decrease as the ammonia gas flow rate increased. Thomas [7]
employed ammonia catalyzed hydrolysis and observed that particle size was
significantly affected. The study done by Park et al. [8] showed that the
concentration of ammonia was one of the parameters affecting the particle size
of silica nanoparticles.The use of ammonia in spray
pyrolysis has seen little attention in the literature. In this study, ammonia
is generated in the aqueous phase of the droplet to reduce the aerosol size,
thereby leading to the production of smaller particle size using a moderate to high
precursor concentration in the feed solution.


## 2. Experiments

Nanoparticles were generated by spray pyrolysis of aqueous feed solution which was gravity
fed from the burette to an atomizer. The atomized spray was discharged into the
reaction chamber that was maintained at a controlled temperature. Detail
descriptions of the experimental apparatus and operation procedures can be
found in our previous work [1]. Experiments were run with and without the
addition of ammonium nitrate (2.5×10−4 M) to the precursor feed
solutions. Samples of metallic nanoparticles were collected on separate quartz
plates at the bottom of the reactor kept perpendicular to the process flow path.
Particle sizes of a sample were manually counted from the images obtained from
TEM with a JEOL 200CX or SEM using a Hitachi
S-3200 N. Samples to be analyzed by TEM were collected on 200-mesh nickel grids
with formvar coating.


## 3. Results and Discussion

### 3.1. Silver

Nanoparticle samples were created both with and without ammonium nitrate in the feedstock
solution for three sets of experimental conditions. Table1 compares the size
of the silver nanoparticles created both with and without ammonium nitrate in
the process feed solution. A reduction in average particle size was observed when
ammonium nitrate was present. Figure 1 shows the SEM and TEM images of silver
nanoparticles without ammonium nitrate in the feed solution. When ammonium
nitrate was added to the same solution feedstock, average particle size was
reduced from 302 to 16 nm. Such particle reduction in the presence of ammonia
was described in previous studies [3–6]. In this work, it is hypothesized that ammonia
gas generation leads to a physical destruction of the aerosol droplet as well
as potentially affecting the surface tension of water. This can be viewed as a
rapid phase transition created by chemical reaction (decomposition of ammonium
nitrate), causing a violent mass transfer effect in the existing liquid phase.
Ammonia gas formed in the pyrolysis process significantly reduces the size of aerosol
droplets, resulting in a smaller nanoparticle size deposits.Table 1
Comparison of average silver particle size with and without ammonium nitrate.
Experimental conditionsResultsWithout NH4NO3With NH4NO3Concentration (N)Power (W)Temperature (°C)Size (nm)Size (nm)10−188002941410−187003011710−1480031019Size comparison of silver nanoparticles produced in the absence (a) and presence (b) of ammonium nitrate. Image (a) presents an SEM image of silver nanoparticle (301 nm) without ammonium nitrate; and Image (b) is a TEM image of nanoparticles (17 nm) produced with ammonium nitrate present. Both the experiments were run at10−1 M (precursor concentration), 700°C (operating temperature), and 8 W (atomizer power).
(a)(b)


### 3.2. Ruthenium and Nickel

Experiments were performed using other metal chloride systems, such as
ruthenium and nickel, to synthesize metal nanoparticles of these metals.
Ammonium nitrate was again used in these feed solutions. Table2 compares the results
of ruthenium nanoparticles synthesized from ruthenium chloride with ammonium
nitrate present in the feed solution. SEM and TEM images as shown in Figure 2 were obtained for ruthenium nanoparticles and
used to determine the average diameter of the particles. The effect of ammonium
nitrate was expected to reduce the particle size, therefore a high feed
solution concentration was employed, whereas atomizer power and reactor
temperature were kept at lower limits in the operating range. This afforded the
flexibility to operate the experiment at optimum reactor conditions.
Nanoparticles of larger average size were synthesized from a higher feed
solution concentration; yet an average particle size of less than 50 nm was
obtained with a relatively monodisperse
size distribution. A similar distribution was observed for particles of less
than 20 nm diameter. The average particle size may also depend on the density
of solution, which varies with the nature of the solution, though this effect
would be expected to be minor.Table 2
Experimental results for ruthenium nanoparticles synthesized from ruthenium chloride using
ammonium nitrate.
Experimental conditionsResultsWithout NH4NO3With NH4NO3Concentration (N)Power (W)Temperature (°C)Size (nm)Size (nm)10−257003503110−4570031010Size comparison of ruthenium nanoparticles produced in the absence (a) and presence (b) of ammonium nitrate.  Image (a) is a SEM of ruthenium nanoparticles (310 nm) produced without ammonium nitrate; while Image (b) is a TEM of ruthenium nanoparticles (10 nm) produced with ammonium nitrate present. Both the experiments were run at10−4 M (precursor concentration), 700°C (operating temperature) and 5 W (atomizer power).
(a)(b)Table3 shows the results of nickel nanoparticles synthesized from nickel
chloride with ammonium nitrate present in the feed solution. Similar to silver
and nickel nanoparticles, SEM and TEM images were also obtained for nickel
nanoparticles as seen in Figure 3. The operating
conditions were maintained similar to those of the experiments performed using
ruthenium chloride. The nature of the metal salt  had the least impact on the
distribution and average particle size. Ruthenium and nickel chloride salts
provided similar results as seen from the similar average particle size
distributions. Another significant finding in this work was that the average
nanoparticle size decreased as the precursor concentration decreased. The
effect of concentration of feed solution was seen both with and without the
presence of ammonium nitrate. It is evident from Table 2 that the particle
decreased from 350 nm to 310 nm (without ammonium nitrate) and from 31 nm to 10 nm
(with ammonium nitrate) for ruthenium nanoparticles when the precursor
concentration was decreased from 10−2 M to 10−4 M. Table 3
shows a similar trend for nickel nanoparticles. A detailed discussion of the
effect of precursor concentration on the particle size was discussed in our
previous work [1]. Operating parameters and the effect of ammonium nitrate in
the feedstock were the dominant factors determining size distribution. The particle size distributions for silver, ruthenium, and nickel
particles obtained with and without ammonia nitrate are shown in
Figure 4. As shown in this figure, the addition of ammonia nitrate
to the feed solutions not only reduced the average particle
diameter, but also made the nanoparticles more uniformly dispersed.Table 3
Experimental results for nickel nanoparticles synthesized from
nickel chloride using ammonium nitrate.
Experimental conditionsResultsWithout NH4NO3With NH4NO3Concentration (N)Power (W)Temperature (°C)Size (nm)Size (nm)10−257002902410−457002469Size comparison of nickel nanoparticles produced in the absence (a) and presence (b) of ammonium nitrate.
Image (a) is a SEM image of nickel nanoparticles (246 nm) produced without ammonium nitrate, while image  (b) is a TEM image of particle
size distribution of nickel  nanoparticles (9 nm) produced with ammonium nitrate. Both experiments were run at10−4 M (precursor concentration), 700°C (operating temperature), and 5 W (atomizer power).
(a)(b)Particle size distribution (PSD) of nanoparticles created in this study.  Images (a), (b) and (c) present the PSDs of silver, ruthenium and nickel nanoparticles, respectively, in the absence of ammonium nitrate; while (d), (e) and (f) are PSDs of silver, ruthenium and nickel nanoparticles in the presence of ammonium nitrate, respectively.


## 3.1. Silver

Nanoparticle samples were created both with and without ammonium nitrate in the feedstock
solution for three sets of experimental conditions. Table1 compares the size
of the silver nanoparticles created both with and without ammonium nitrate in
the process feed solution. A reduction in average particle size was observed when
ammonium nitrate was present. Figure 1 shows the SEM and TEM images of silver
nanoparticles without ammonium nitrate in the feed solution. When ammonium
nitrate was added to the same solution feedstock, average particle size was
reduced from 302 to 16 nm. Such particle reduction in the presence of ammonia
was described in previous studies [3–6]. In this work, it is hypothesized that ammonia
gas generation leads to a physical destruction of the aerosol droplet as well
as potentially affecting the surface tension of water. This can be viewed as a
rapid phase transition created by chemical reaction (decomposition of ammonium
nitrate), causing a violent mass transfer effect in the existing liquid phase.
Ammonia gas formed in the pyrolysis process significantly reduces the size of aerosol
droplets, resulting in a smaller nanoparticle size deposits.Table 1
Comparison of average silver particle size with and without ammonium nitrate.
Experimental conditionsResultsWithout NH4NO3With NH4NO3Concentration (N)Power (W)Temperature (°C)Size (nm)Size (nm)10−188002941410−187003011710−1480031019Size comparison of silver nanoparticles produced in the absence (a) and presence (b) of ammonium nitrate. Image (a) presents an SEM image of silver nanoparticle (301 nm) without ammonium nitrate; and Image (b) is a TEM image of nanoparticles (17 nm) produced with ammonium nitrate present. Both the experiments were run at10−1 M (precursor concentration), 700°C (operating temperature), and 8 W (atomizer power).
(a)(b)


## 3.2. Ruthenium and Nickel

Experiments were performed using other metal chloride systems, such as
ruthenium and nickel, to synthesize metal nanoparticles of these metals.
Ammonium nitrate was again used in these feed solutions. Table2 compares the results
of ruthenium nanoparticles synthesized from ruthenium chloride with ammonium
nitrate present in the feed solution. SEM and TEM images as shown in Figure 2 were obtained for ruthenium nanoparticles and
used to determine the average diameter of the particles. The effect of ammonium
nitrate was expected to reduce the particle size, therefore a high feed
solution concentration was employed, whereas atomizer power and reactor
temperature were kept at lower limits in the operating range. This afforded the
flexibility to operate the experiment at optimum reactor conditions.
Nanoparticles of larger average size were synthesized from a higher feed
solution concentration; yet an average particle size of less than 50 nm was
obtained with a relatively monodisperse
size distribution. A similar distribution was observed for particles of less
than 20 nm diameter. The average particle size may also depend on the density
of solution, which varies with the nature of the solution, though this effect
would be expected to be minor.Table 2
Experimental results for ruthenium nanoparticles synthesized from ruthenium chloride using
ammonium nitrate.
Experimental conditionsResultsWithout NH4NO3With NH4NO3Concentration (N)Power (W)Temperature (°C)Size (nm)Size (nm)10−257003503110−4570031010Size comparison of ruthenium nanoparticles produced in the absence (a) and presence (b) of ammonium nitrate.  Image (a) is a SEM of ruthenium nanoparticles (310 nm) produced without ammonium nitrate; while Image (b) is a TEM of ruthenium nanoparticles (10 nm) produced with ammonium nitrate present. Both the experiments were run at10−4 M (precursor concentration), 700°C (operating temperature) and 5 W (atomizer power).
(a)(b)Table3 shows the results of nickel nanoparticles synthesized from nickel
chloride with ammonium nitrate present in the feed solution. Similar to silver
and nickel nanoparticles, SEM and TEM images were also obtained for nickel
nanoparticles as seen in Figure 3. The operating
conditions were maintained similar to those of the experiments performed using
ruthenium chloride. The nature of the metal salt  had the least impact on the
distribution and average particle size. Ruthenium and nickel chloride salts
provided similar results as seen from the similar average particle size
distributions. Another significant finding in this work was that the average
nanoparticle size decreased as the precursor concentration decreased. The
effect of concentration of feed solution was seen both with and without the
presence of ammonium nitrate. It is evident from Table 2 that the particle
decreased from 350 nm to 310 nm (without ammonium nitrate) and from 31 nm to 10 nm
(with ammonium nitrate) for ruthenium nanoparticles when the precursor
concentration was decreased from 10−2 M to 10−4 M. Table 3
shows a similar trend for nickel nanoparticles. A detailed discussion of the
effect of precursor concentration on the particle size was discussed in our
previous work [1]. Operating parameters and the effect of ammonium nitrate in
the feedstock were the dominant factors determining size distribution. The particle size distributions for silver, ruthenium, and nickel
particles obtained with and without ammonia nitrate are shown in
Figure 4. As shown in this figure, the addition of ammonia nitrate
to the feed solutions not only reduced the average particle
diameter, but also made the nanoparticles more uniformly dispersed.Table 3
Experimental results for nickel nanoparticles synthesized from
nickel chloride using ammonium nitrate.
Experimental conditionsResultsWithout NH4NO3With NH4NO3Concentration (N)Power (W)Temperature (°C)Size (nm)Size (nm)10−257002902410−457002469Size comparison of nickel nanoparticles produced in the absence (a) and presence (b) of ammonium nitrate.
Image (a) is a SEM image of nickel nanoparticles (246 nm) produced without ammonium nitrate, while image  (b) is a TEM image of particle
size distribution of nickel  nanoparticles (9 nm) produced with ammonium nitrate. Both experiments were run at10−4 M (precursor concentration), 700°C (operating temperature), and 5 W (atomizer power).
(a)(b)Particle size distribution (PSD) of nanoparticles created in this study.  Images (a), (b) and (c) present the PSDs of silver, ruthenium and nickel nanoparticles, respectively, in the absence of ammonium nitrate; while (d), (e) and (f) are PSDs of silver, ruthenium and nickel nanoparticles in the presence of ammonium nitrate, respectively.


## 4. Conclusions

It has demonstrated in experiments the feasibility of adding ammonium nitrate to
the precursor solution as a foaming agent to reduce the size of nanoparticles
synthesized in the spray-pyrolysis process. The average diameter of silver,
ruthenium, and nickel nanoparticles synthesized in spray-pyrolysis was reduced
from 200–300 nm to 10–20 nm
after  adding ammonium nitrate to the spraying feed solution. It is
believed that the ammonia gas generated from the decomposition of ammonium
nitrate has significantly reduced the aerosol droplet size and the final size
of the metallic  nanoparticles.



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*Source: 756843-2008-04-09.xml*