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\begin{document}

\title{Influence of fuel ratio on synthesis and characterization of LaCoO3
perovskite nanoparticles by sol-gel auto-combustion method}



\author[1]{Hamed Sadabadi}%
\author[1]{Saeed Allahkaram}%
\author[2]{Amir  Kordijazi}%
\author[3]{Omid Akbarzadeh}%
\author[2]{Pradeep Rohatgi}%
\affil[1]{University of Tehran College of Engineering}%
\affil[2]{University of Wisconsin Milwaukee}%
\affil[3]{University of Malaya}%


\vspace{-1em}



  
  \date{May 18, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Recently, nanostructure perovskite oxides such as LaMO3 (M=Co, Ni, Fe,
\ldots{}) received attention in academic researches due to its catalytic
properties. In this research, the LaCoO3 perovskite nanoparticles have
been synthesized by single-step route via the sol-gel auto-combustion
method. The precursors used in this method were lanthanum nitrate and
cobalt nitrate, as metals sources dissolved in distilled water and also
using PVP as a surfactant, Urea and glycine as an oxidizer. The sol
formed at stirring stage at 60 \selectlanguage{ngerman}°C continued by gelation through the
water evaporation at 90 °C and then auto-combustion occurred. Product of
combustion step was washed and centrifuged three times and later
calcined at 600 °C for 2 h. As synthesized nanoparticles are
characterized by scanning electron microscopy (SEM), X-ray powder
diffraction (XRD), and particle size analyzer (PSA). The
characterization results proved the synthesis of nanoparticles below 100
nm with perovskite structure in narrow size distribution by using this
method. The purity and size of nanoparticles vary depending on the fuel
and fuel to oxidizer ratio.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\emph{Keywords: Sol-gel auto-combustion, Nanoparticles, Perovskite,
characterization, LaCoO\textsubscript{3}}

\textbf{Introduction:}

Metal oxide Nanoparticles have certain industrial and technological
properties that include optical, magnetic, electrical, catalytic,
mechanical strength, thermal stability and chemical properties. The
production of metal oxide nanoparticles is carried out using a number of
chemical methods, which include sol-gel, reduction-oxidation,
hydrothermal hydroxide oxidation, and metal salts decomposition methods.
Time efficiency, production cost, ability to operate continuously, and
production rate are the parameters that have been considered in order to
choose the best synthesis method for a specific application. Solution
combustion synthesis (SCS) or sol-gel auto-combustion method appears to
be a promising method due to low production cost, because it requires
low-cost precursor materials and equipment. It's feasible to fabricate a
wide range of nanoparticles and metal oxides, and more efficiently
compared to some other methods {[}1{]}--{[}5{]}.

The perovskite oxides are mixed metal oxides with the general formula of
ABO\textsubscript{3}, where A is larger cation usually an alkaline-earth
or rare-earth metal cation and B is the smaller cation, usually, a
transition-metal cation (e.g., Mn, Co, Fe, Ni,) surrounded by six oxygen
atoms in an octahedral coordination {[}6{]}. However, perovskite oxides,
LaMO\textsubscript{3}, in which M is a transition-metal containing Co,
Fe, Ni, have been used in a variety of applications, including gas
sensors (CO, propane and ethanol sensing) as nanostructures' film,
hollow spheres and mesoporous nanowires, nanoparticles, porous and
fibrous structures {[}7{]}--{[}11{]}, photocatalytic reduction of
CO\textsubscript{2} with visible light, electronic devices,
electrochemical systems, solid oxide fuel cell, solar cells, and
catalysts {[}11{]}--{[}17{]}. Different methods have been uses to
synthesize and develop perovskite phase of LaCoO\textsubscript{3} as
nanoparticles and nanowires, over the years; these include thermal
decomposition, hydrothermal, microwave-assisted coprecipitation,
co-precipitation, sol-gel, and combustion methods {[}10{]},
{[}17{]}--{[}22{]}.

The purpose of this research is to synthesize nanoparticles of
LaCoO\textsubscript{3} perovskite in a single step by combining the
sol-gel and auto-combustion steps. Investigation of the influence of
fuel ratio and different fuel on nanoparticle characteristics has been
done. These particles can be used as a catalyst in the decomposition of
nitrogen oxide gases (NOx).

\textbf{Experimental:}

The following materials were used as precursors for synthesis of
perovskite particles. Lanthanum Nitrate
(La(NO\textsubscript{3})\textsubscript{3}-6H\textsubscript{2}O) and
Cobalt Nitrate
(Co(NO\textsubscript{3})\textsubscript{2}-6H\textsubscript{2}O) as a
supplier of metal elements, glycine
(C\textsubscript{2}H\textsubscript{5}NO\textsubscript{2}) and urea
(CH\textsubscript{4}N\textsubscript{2}O) as fuel, and PVP as a
surfactant were used in these experiments. All of the precursor
materials were purchased from Merck-Germany and used in the experiments
without further purification.

Firstly, 0.1M solutions of lanthanum nitrate and cobalt nitrate was
prepared in 100 ml of distilled water in a 250 ml beaker and stirred on
a magnetic stirrer for 15 minutes until the material was completely
dissolved and a uniform pink solution was obtained. In the second step,
four different samples were prepared, as shown in Table 1. The
stoichiometric amount of glycine and urea were added slowly to the
solution. The glycine/metal and urea/ metal ratio were kept 1 and 2 in
this experiment (F/O=1 \& 2). The solution temperature was increased to
60 \selectlanguage{ngerman}°C and PVP by 5\% by weight of the available metals was added. PVP
played the role of surfactant in the experiment, to help obtain a
uniform shape and narrow size distribution. Solutions were mixed using
vigorous stirring on a magnetic stirrer for half an hour. The next step
was combustion. At this stage, the solution temperature was increased up
to 90 °C in order to evaporate water slowly, followed by gelation and
combustion. The solution was stirred using a magnetic stirrer, until
combustion occurred.

The powder was washed with distilled water and centrifuged at 2000 RPM
for 15 minutes for three times, and collected nanoparticles were
calcined at 600 °C for 2 hours in a furnace.

Table 1: experimental conditions.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Sample No. & Fuel & F/O ratio & Metal solution Molarity &
PVP\tabularnewline
\midrule
\endhead
LCG 1 & Glycine & 1 & 0.1M & 5\%wt\tabularnewline
LCG 2 & Glycine & 2 & 0.1M & 5\%wt\tabularnewline
LCU 1 & Urea & 1 & 0.1M & 5\%wt\tabularnewline
LCU 2 & Urea & 2 & 0.1M & 5\%wt\tabularnewline
\bottomrule
\end{longtable}

During the experiment, all parameters that influence the properties of
nanoparticles were kept constant, just fuel and fuel to oxidizer ratio
used as variable parameters, and influence of these two parameters was
studied.

\textbf{Results}

X-ray powder diffraction (XRD) patterns were recorded by a D8 Advance
(Bruker) with Cu K\selectlanguage{greek}α \selectlanguage{english}(wavelength 1.54 \selectlanguage{ngerman}Å) after nanoparticles were
calcined at 600 °C for 2 hours. SEM (FEI Company, USA-Quanta 200) was
used to investigate the morphology, size, and uniformity of the
products. A particle size analyzer was used to illustrates the size
distribution. During the PSA test, the ethanol was used as a dispersant,
temperature was set at 25 °C, and the test duration was 50 s.

The powder XRD data are shown in Fig.1(a-b) and Fig.2(a-b), indicates
the crystalline structure of the as-synthesized perovskite-type oxide
nanoparticles obtained using the sol-gel auto-combustion method. The
ratio of fuel to oxidizer (F/O), and using different fuel in this
method, causes the changes crystalline properties and purity of the
final product. However, XRD patterns of all samples exhibited
diffraction peaks of LaCoO\textsubscript{3} (JCPDS No.-01-084-0848),
irrespective of the F/O ratio and fuel. But the existence of impurity
was observed and discussed as follows. Fig.1(a-b) shows the XRD patterns
of sample LCG1 and LCG2 respectively. As illustrated in Fig.1-a, by
using glycine in the ratio of F/O=1, the mixture of the cobalt oxide
(CoO, Co\textsubscript{3}O\textsubscript{4}), lanthanum oxide
(La\textsubscript{2}O\textsubscript{3}), La(OH)\textsubscript{3} and
perovskite phase obtained, but Fig1-b shows the purity of perovskite
phase. It was shown strong peaks at 32.9° (110) and 33.3° (104), and the
peaks observed around 32°, 60° and 70° are bifurcated which is the
characteristic peak for the rhombohedral structure and also indicated no
trace of CoO, Co\textsubscript{3}O\textsubscript{4},
La(OH)\textsubscript{3} or La\textsubscript{2}O\textsubscript{3}, which
corresponded to the high degree of La and Co oxide incorporation into
the perovskite structure achieved by glycine as fuel and F/O=2.
Furthermore, Fig.2 (a-b) shows that using urea as fuel can cause changes
in purity and crystalline properties also. The production of
LaCoO\textsubscript{3} nanoparticles, with
Co\textsubscript{3}O\textsubscript{4} and
La\textsubscript{2}O\textsubscript{3} as impurities by using both F/O=1
and 2 was achieved. However, it can be concluded that higher ratio of
glycine causes complete oxidation while urea in low ratio and high ratio
cause partial oxidation and lower degree of La and Co oxide
incorporation into the perovskite structure.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig.1: XRD patterns of LaCoO\textsubscript{3} nanoparticles, using
glycine as fuel and calcined at 600\selectlanguage{ngerman}°C in air a) sample LCG1 F/O=1, b)
sample LCG2 F/O=2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Fig.2: XRD patterns of LaCoO\textsubscript{3} nanoparticles, using urea
as fuel and calcined at 600\selectlanguage{ngerman}°C in air a) sample LCU1 F/O=1, b) sample
LCU2 F/O=2.

Finally, crystallite size and the average crystallite size was estimated
according to the Debye and Scherrer formula from the (110), (024) and
(214) reflection of the LaCoO\textsubscript{3} phase at 2q\selectlanguage{english}[?] 32.9deg, 47.5deg
and 58.9deg respectively, and is represented in Table 2.

\(D=\frac{0.94\lambda}{\beta\cos\theta}\) (1)

Where:

D is the crystallite size

\selectlanguage{greek}λ \selectlanguage{english}is the X-ray wavelength (1.5418 \selectlanguage{ngerman}Å)

\selectlanguage{greek}β \selectlanguage{english}is the full width of the half maximum (FWHM) of the diffraction peak

\selectlanguage{greek}θ \selectlanguage{english}is the Bragg diffraction angle

It was evident from the calculation that
LaCoO\textsubscript{3}synthesized using glycine exhibited the lowest
crystallite size (\textasciitilde{}14.9 and 28.1 nm) compared to when
urea was used (\textasciitilde{}41.6 and 25.7 nm). The larger
crystallite size of the LaCoO\textsubscript{3} phase observed in the
LCU1 sample was indicative of the greater growth of its crystal domains.
This result reveals that glycine and urea act in opposite manner, while
increasing fuel ratio in glycine increase the crystallite size and
purity, increasing urea decreases crystallite size and improves purity.

Table 2. Crystallite size, average crystallite size and Z-average size
of four samples synthesized with different fuel and fuel to oxidizer
ratio (F/O).\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
\begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
LCG1\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
LCG2\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
LCU1\strut
\end{minipage} & \begin{minipage}[b]{0.17\columnwidth}\raggedright\strut
LCU2\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Crystallite size D\textsubscript{110} (nm)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
15.9\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
18\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
36.7\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
31.4\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Crystallite size D\textsubscript{024} (nm)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
18.9\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
37.8\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
57.7\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
25.6\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Crystallite size D\textsubscript{214} (nm)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
9.9\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
28.4\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
30.3\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
20.2\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
Crystallite size D\textsubscript{Ave.} (nm)\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
14.9\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
28.1\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
41.6\strut
\end{minipage} & \begin{minipage}[t]{0.19\columnwidth}\raggedright\strut
25.7\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
PSA-average (d.nm)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
72.2\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
70.2\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
72.0\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
72.8\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
Z-average (d.nm)\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
104.1\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
97.9\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
101.9\strut
\end{minipage} & \begin{minipage}[t]{0.17\columnwidth}\raggedright\strut
106.5\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

Also, Results from particle size analyses shows the narrow size
distribution as illustrated by Fig.3-4 and the SEM micrograph proves
that.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Fig.3: PSA graph of sample LCG1(---) and LCG2 ( ).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Fig.4: PSA graph of a) sample LCU1(---) and sample LCU2( ).

Morphologies of LaCoO\textsubscript{3} synthesized with the sol-gel
Auto-combustion method by glycine and urea as fuel are presented in
Fig.5 (a-b) and Fig.6 (a-b), respectively. The SEM images show nonporous
nature of the synthesized perovskite particles with spherical shape in
all samples. The porous nature can be due to the nitrate precursors
because burning of the nitrates at higher temperature may lead to the
porous structures.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

A comparison of these micrographs reveals that the glycine in ratio of 2
resulted in a lower particle size and homogeneity, while the powder
prepared by the urea showed a relatively larger particle size, some
agglomeration and inhomogeneity in shape and size is observed. The
average particle size of LaCoO\textsubscript{3} synthesized by glycine
is below 70 nm with a spherical shape, whereas the powder made of urea
process has a broader particle size distribution and inhomogeneity in
shape. However, SEM results are contradictory to the results of
crystallite size estimation (see Table 2). Particle size is different
from crystallite size suggesting that the particles have
multi-crystalline structure. A particle may be made up of several
different crystallites.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Fig.5: SEM micrograph of LaCoO\textsubscript{3} nanoparticles, using
glycine as fuel and calcined at 600\selectlanguage{ngerman}°C in air a) sample LCG1 F/O=1, b)
sample LCG2 F/O=2.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image11/image11}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image12/image12}

\end{center}
\end{figure}

Fig.6: SEM micrograph of LaCoO\textsubscript{3} nanoparticles, using
urea as fuel and calcined at 600\selectlanguage{ngerman}°C in air a) sample LCU1 F/O=1, b)
sample LCU2 F/O=2.

Also, Results from particle size analyses shows the narrow size
distribution as illustrated by Fig.5 and 6 and the SEM micrograph proves
that.

\textbf{Conclusion:}

LaCoO\textsubscript{3} perovskite nanoparticles can be produced by
sol-gel auto-combustion, using a single step method. This method is
low-cost, time-efficient and does not require advanced equipment for the
production process. Investigation of the lattice structure, morphology,
particle size by XRD, SEM and PSA analysis methods show that particles
are uniform in shape and size and these characteristics play important
roles in the application of nanoparticles as the catalyst and cause
uniformity in the properties of the catalyst. The production of uniform
size nanoparticles using this method is the result of the use of
surfactants in the synthesis process. The results confirm the successful
synthesis of LaCoO\textsubscript{3} nanoparticles which are in a
spherical shape and below 100 nm in size.

\textbf{Conflict of Interest:}

Authors have no conflict of interest relevant to this article.

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