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\begin{document}

\title{Cadmium oxide: green synthesis, characterization and H 2 storage
performance}



\author[1]{Mustafa A. Alheety}%
\author[2]{Shaymaa N. Ismail}%
\author[3]{Ehab M. Ali}%
\author[2]{Adi M. Abdul Hussien}%
\affil[1]{Al-Hadi University College}%
\affil[2]{University of Technology}%
\affil[3]{University of Samarra}%


\vspace{-1em}



  
  \date{May 27, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
This study aims to green synthesize and full characterize CdO
nanoparticles by means of an environmentally friendly synthesis using
Crocus sativus in its reaction with cadmium ions. The CdO nanoparticles
were characterized and their purity was confirmed by XRD and UV.-Vis.
spectroscopy. Moreover, scanning electron microscopy (SEM) showed that
the prepared cadmium oxide nanoparticles were polymorphic. Furthermore,
cadmium oxide nanoparticles were cast in the gas storage (H
\textsubscript{2}) study. The hydrogen storage results prove that the
maximum H \textsubscript{2} uptake was equal to 2.85 Wt.\%H
\textsubscript{2} at a pressure of 69 bar at 77 K with [?]H=0.62607 KJ/mol
H \textsubscript{2} and [?]S=3.35697 J/mol H \textsubscript{2}. K.
Moreover, the thermodynamic studies under four different temperatures
proves that the maximum H \textsubscript{2} uptake could be recorded at
a pressure limit of 69-86.2 bar.%
\end{abstract}%



\sloppy


\textbf{1-Introduction}

Nanotechnology encompasses processes that manipulate matter to achieve
atomic-scale sizes, leading to materials with novel properties. Advanced
techniques have emerged for manufacturing highly efficient
nanomaterials. Green synthesis, a simpler alternative to traditional
methods, aims to reduce harsh processing conditions. This approach
involves the use of bacteria, fungi, and human cells as viable systems
for green synthesis {[}1-6{]}. Metal oxides are highly significant
nanomaterials with versatile applications in various fields such as
environment, health, and industry. Cadmium oxide, in particular, has
been extensively studied and utilized. However, the toxicity of cadmium
salts restricts its widespread use, despite its excellent reactivity and
effectiveness in many applications. To address this issue, researchers
have explored eco-friendly approaches for synthesizing cadmium oxide
nanoparticles using aqueous plant extracts, providing a more sustainable
and environmentally conscious method of preparation {[}1-3{]}. From
plant extracts for this purpose, for example, citrus limitta peels were
used to prepare cadmium oxide nanoparticles of approximately 51
nanometers and irregular shapes {[}7{]}. Agathosma betulina was also
used in the preparation of this oxide in the form of irregular
sphere-like structures with a size not exceeding 50 nanometers {[}8{]}.
Studies remained directed for this purpose in order to obtain cadmium
oxide in a regular nanoscale form, and indeed most studies have achieved
this. For example, andrographis paniculata{[}9{]}, green tea{[}10{]},
and leucaena leucocephala{[}11{]} were used, in which the particle size
ranged between 5 and 57 nm. In the pursuit of environmentally friendly
alternatives and to mitigate pollution risks, our focus shifted towards
utilizing cadmium oxide in hydrogen storage applications. As hydrogen
plays a vital role in the new era of clean science, extensive research
has been dedicated to finding cost-effective materials with superior
hydrogen storage capabilities. Cadmium oxide emerges as a favorable
option due to its potential. Several oxides, such as nickel oxide with
added metals, have demonstrated hydrogen storage capacities of
approximately 0.7 wt.\%{[}12{]}. In contrast, zinc oxide exhibited a
lower hydrogen storage capacity of around 1 wt.\%{[}13{]}. These studies
contribute to the exploration of optimal materials for hydrogen storage
purposes. The pure cadmium oxide was not used until now, as only the
composite Cd/CdO was used. However, the storage value reached 1.3 Wt.\%
{[}14{]}. Therefore, this study aims to prepare and characterize cadmium
oxide nanoparticles and then complete the lack of information about the
ability to store hydrogen in metal oxides.

\textbf{2. Experimental}

\textbf{2.1. Synthesis of crocus sativus extract}

1 g of crocus sativus plant was immersed in 100 ml of deionized water
and refluxed at f 75\textsuperscript{o}C for 2 minutes. The mixture was
then passed from cotton filter to remove the big cellulose particles and
the then centrifuged to collect the extract.

\textbf{2.2. Synthesis of CdO NPs}

Cadmium nitrate (100 ml, 0.5 M) was heated under stirring at a
temperature 80\textsuperscript{o}C. Thereafter 100 ml of freshly
prepared extract was added drop by drop and left on a hot plate for an
hour at a temperature of 160\textsuperscript{o}C under continuous
stirring and finally off-white precipitate was formed which was then
collected by centrifuge, washed thoroughly with hot deionized water and
calcined at 500\textsuperscript{o}C for 2 hour.

\textbf{3. Results and Discussion}

\textbf{3.1. Characterization of the synthesized CdO NPs}

\textbf{3.1.1. XRD pattern}

Scherrer equation that illustrated in equation 1, was used to determine
the average particle size of cadmium oxide nanoparticles.

D = (094 \selectlanguage{greek}λ\selectlanguage{english}) / (\selectlanguage{greek}β \selectlanguage{english}cos\selectlanguage{greek}θ\selectlanguage{english})\ldots{}\ldots{}\ldots{}\ldots{}\ldots{}. (1)

Where \selectlanguage{greek}λ \selectlanguage{english}= 1.542 \selectlanguage{ngerman}Å, \selectlanguage{greek}β \selectlanguage{english}FWHM in radians and \selectlanguage{greek}θ \selectlanguage{english}is the diffraction position.
The (111), (200), (220), (311) and (222) reflections were determined in
the XRD measurement and the result are in agreement with reference
patterns of CdO (JCPDS file No. 05-0640) {[}15{]}. See Figure 1.

Figure 1: X-ray diffraction of the as-prepared CdO

\textbf{3.1.2. Scanning electron microscope (SEM) and transmission
electron microscopy (TEM)}

SEM measurement which was shown in Figure 2 shows a homogeneous
distribution of the prepared CdO polymorphic, with a median particle
size of 64.58 nm. The TEM measurement showed very well separated
particles with irregular sphere like structure of 45-67 nm. Furthermore,
the measurement showed a rhombic shape-like structure of 50 nm, as shown
in Figure 3. However, the particle size distribution in Figure 3
confirmed that the mean particle size was 55 nm which is in consistence
with XRD and SEM.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 2: SEM image of CdO nanoparticles and particle size distribution

\textbf{Hosted file}

\verb`image3.emf`  available at \url{https://authorea.com/users/226665/articles/660565-cadmium-oxide-green-synthesis-characterization-and-h-2-storage-performance}

Figure 3: TEM image of CdO nanoparticles and particle size distribution

\textbf{3.1.1.3. Ultra Violet-Visible (UV-Vis) spectrum}

The UV-Vis spectrum was illustrated in Figure 4 which representing the
absorbance spectrum of as-prepared cadmium oxide at the wavelength range
of 190 -- 600 nm. The measurement proves that the absorption peak was
found at 302 nm. This peak proves the formation of CdO nanparticles
{[}14{]}.

Figure 4: Wavelength vs Absorption of CdO nanoparticles

The energy band gap is measured with the help of absorption spectra and
a graph of \textbf{(} ?h?)\textsuperscript{\textbf{2}} versus energy is
plotted. The energy band gap of cadmium oxide nanoparticle is 4.65 eV
that the value of band gaps increased due to quantum size effect.

\textbf{3.1.1.4. Fourier transformation infrared spectroscopy (FTIR)}

The measurement showed low-intensity broad band centered at 3533
cm\textsuperscript{-1}, which usually appears in nanomaterials due to
water adsorption on their surfaces. This band is due to the stretching
vibration of hydroxyl in water which is proven by the presence of the
bending stretching band of hydroxyl in water at 1620
cm\textsuperscript{-1} {[}16{]}.

The band at 1448 cm\textsuperscript{-1} indicates the presence of the
C=C bond, which is present after the combustion of organic compounds and
the transformation into an skeletal carbonic structure only {[}17{]}.
The distinctive bands of cadmium oxide appeared in the spectrum as
follows: a band at 1049 cm\textsuperscript{-1} and a band at 540
cm\textsuperscript{- 1} and a band at 1311 cm\textsuperscript{-1}, which
are attributed to the stretching vibration of Cd-O {[}18-20{]}. See
Figure 5.

Figure 5: FTIR of the CdO nanoparticles

\textbf{3.2. Hydrogen storage of CdO nanoperticles}

As shown in Figure 6, the measurement proved that cadmium oxide has the
ability to store hydrogen at a pressure of 20 bar, with a value of 0.132
Wt.\%H\textsubscript{2}. After that, the storage continued to increase
by increasing the pressure until it reached the highest storage value,
which amounted to 2.85 Wt.\%H\textsubscript{2} at a pressure of 80 bar,
after which the increase in pressure did not have any effect on the
ability to store {[}21{]}.

The Van't Hoff equation proves that the storage process has a regression
coefficient value of 0.97. Through the slope and intercept, the enthalpy
value reached 0.62607 KJ/mol H\textsubscript{2}, while the entropy value
reached 3.35697 J/mol H\textsubscript{2}, which means that the
adsorption is of the physical type {[}21-24{]}. In any case, the
temperature change did not have an effect on the weight value, because
the pressure change was relatively small, which means that it is
possible to reach the highest storage without the need for a low
temperature. See Figure 7 and Table 1.

Figure 6: Hydrogen storage isotherm in CdO nanoparticles at 223 K

Figure 7: Vant Hoff plot (1/T Vs. Ln P) of equilibrium pressures and the
linear fit to the data

Table 1: Thermodynamic properties of H\textsubscript{2} storage in CdO
NPs\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
T (K) & 1/T & P (bar) & ln (P) & [?]H & [?]S\tabularnewline
\midrule
\endhead
& (K\textsuperscript{-1}) & & & (KJ/mol H\textsubscript{2}) & (J/mol
H\textsubscript{2}. K)\tabularnewline
77 & 12.987 & 69 & 4.343805 & 0.62607 &\tabularnewline
173 & 5.78 & 77 & 5.153292 & & 3.35697\tabularnewline
223 & 4.4843 & 80 & 5.407172 & &\tabularnewline
273 & 3.663 & 86.2 & 5.609472 & &\tabularnewline
\bottomrule
\end{longtable}

\textbf{Conclusions}

From this study, we concluded the successful synthesis of CdO using the
green method which is easy, cost-effective, and short technique.
Furthermore, the study proved the formation of CdO nanoparticles with
irregular sphere like structures of 55 nm particle size. Regarding the
hydrogen storage, the results prove that the maximum
H\textsubscript{2}uptake was equal to 2.85 Wt.\%H\textsubscript{2} at a
pressure of 69 bar at 77 K with [?]H=0.62607 KJ/mol H\textsubscript{2} and
[?]S=3.35697 J/mol H\textsubscript{2}. K. Moreover, the thermodynamic
studies at four different temperatures proves that the maximum
H\textsubscript{2} uptake could be recorded at a pressure limit of
69-86.2 bar. Meaning that, the temperature change did not have an effect
on the weight value, because the pressure change was relatively small,
which means that it is possible to reach the highest storage without the
need for a low temperature.

\textbf{Acknowledgment}

None

\textbf{Conflict of interest statement}

The authors declare no financial conflict of interest statement

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