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\hline
\multicolumn{5}{c|}{Pure ZnONPs} & \multicolumn{7}{c}{Ag doped ZnONPs} \\
\hline
\parbox[t]{3pc}{\centering Plasmonic signature (UV--vis) and band gap}\raisebox{-45pt}{} & 
\parbox[t]{3pc}{\centering Excitonic effects (PL)} &
\parbox[t]{3pc}{\centering Structural refinement average crystallite} & 
\parbox[t]{3pc}{\centering Electrical properties} & 
\parbox[t]{3pc}{\centering Photo\-catalytic performance} & Doped \% & 
\parbox[t]{3pc}{\centering Plasmonic signature (nm) and band gap (eV)} & 
\parbox[t]{3pc}{\centering Excitonic effects (PL)} & 
\parbox[t]{3pc}{\centering Structural refinement---average crystallite size (nm)} & 
\parbox[t]{3pc}{\centering Electrical properties} & 
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\DOI{10.5802/crphys.286}
\datereceived{2026-01-21}
\daterevised{2026-05-13}
\datererevised{2026-07-12}
\dateaccepted{2026-07-28}
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\foreignlanguage{english}{\gdef\deuxpoints{:}}

\begin{DefTralics}
{\catcode`\:=11\xdef\deuxpoints{:}}
\end{DefTralics}

\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliation other than their research organizations.}

\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Green synthesis of Ag:ZnO nanoparticles via solvothermal method:
characterization, photocatalytic and dielectric properties}

\alttitle{Synth\`{e}se \'{e}cologique de nanoparticules d'Ag\deuxpoints{}ZnO par la
m\'{e}thode solvothermique : caract\'{e}risation, propri\'{e}t\'{e}s
photocatalytiques et di\'{e}lectriques}

\author{\firstname{Manal Ahmed} \lastname{Awad}\CDRorcid{0000-0002-5294-1721}\IsCorresp}
\address{King Abdullah Institute for Nanotechnology, King Saud
University, Riyadh 11451, Saudi Arabia}
\email[M. A. Awad]{mawad@ksu.edu.sa}

\author{\firstname{Khalid Mustafa} \lastname{Ortashi}\CDRorcid{0000-0002-4030-1583}}
\address{Department of Chemical Engineering, King Saud University,
Riyadh, 11421, Saudi Arabia}

\author{\firstname{Hayat} \lastname{Althobaiti}}
\address{Department of Physics, College of Science, King Saud
University, Riyadh 11451, Saudi Arabia}

\author{\firstname{Nada\nobreakauthor Ali}\nobreakauthor\lastname{Almohawis}}
\address{Department of Chemistry, College of Science, King Saud
University, P.O. 2455, Riyadh 11451, Saudi Arabia}

\keywords{\kwd{Ag:ZnONPs}\kwd{Photocatalytic}\kwd{Electrical properties}}

\altkeywords{\kwd{Ag\deuxpoints{}ZnONPs}\kwd{Photocatalyse}\kwd{Propri\'{e}t\'{e}s \'{e}lectriques}}

\begin{abstract}
This study investigates the photocatalytic and dielectric
properties of green-synthesized silver-doped zinc oxide nanoparticles
(Ag:ZnONPs). FTIR analysis confirmed the presence of functional groups
derived from the plant extract, which played a key role in the capping,
and stabilization of the nanoparticles. SEM observations revealed
rod-like and quasi-spherical nanostructures, while EDX analysis
verified the successful incorporation of Ag into the ZnO matrix. XRD
results confirmed the hexagonal wurtzite structure of synthesized
sample, with an average crystallite size of approximately 20 nm.
Photoluminescence studies showed near-band-edge emission with
suppressed defect-related recombination, indicating improved charge
carrier separation due to Ag doping. The Ag:ZnONPs demonstrated
excellent visible-light photocatalytic performance, achieving nearly
99\% degradation of crystal violet within 24 h and methyl orange within
72 h. Dielectric investigations revealed reduced impedance, and enhanced
AC conductivity at room temperature, reflecting improved charge
transport. The results highlight the multifunctional potential of
green-synthesized Ag:ZnONPs for environmental remediation and advanced
electronic applications.
\end{abstract}

\begin{altabstract}
Cette \'{e}tude examine les propri\'{e}t\'{e}s photocatalytiques et
di\'{e}lectriques de nanoparticules d'oxyde de zinc dop\'{e}es \`{a}
l'argent (Ag\deuxpoints{}ZnONPs) obtenues par synth\`{e}se \og verte \fg.
L'analyse FTIR a confirm\'{e} la pr\'{e}sence de groupes fonctionnels
issus de l'extrait v\'{e}g\'{e}tal, qui ont jou\'{e} un r\^{o}le
cl\'{e} dans le rev\^{e}tement et la stabilisation des nanoparticules.
Les observations au MEB ont r\'{e}v\'{e}l\'{e} des nanostructures en
forme de b\^{a}tonnets et quasi-sph\'{e}riques, tandis que l'analyse
EDX a confirm\'{e} l'incorporation r\'{e}ussie de l'argent dans la
matrice de ZnO. Les r\'{e}sultats de la diffraction des rayons X (DRX)
ont confirm\'{e} la structure hexagonale de type wurtzite de
l'\'{e}chantillon synth\'{e}tis\'{e}, avec une taille moyenne des
cristallites d'environ 20 nm. Les \'{e}tudes de photoluminescence ont
montr\'{e} une \'{e}mission en bord de bande avec une recombinaison
li\'{e}e aux d\'{e}fauts r\'{e}duite, indiquant une meilleure
s\'{e}paration des porteurs de charge due au dopage \`{a} l'argent. Les
nanoparticules Ag\deuxpoints{}ZnO ont d\'{e}montr\'{e} une excellente
caract\'{e}ristique photocatalytique en lumi\`{e}re visible, atteignant
une d\'{e}gradation de pr\`{e}s de 99 \% du violet cristallis\'{e} en
24 h et du m\'{e}thylorange en 72 h. Les analyses di\'{e}lectriques ont
r\'{e}v\'{e}l\'{e} une imp\'{e}dance r\'{e}duite et une
conductivit\'{e} en courant alternatif am\'{e}lior\'{e}e \`{a}
temp\'{e}rature ambiante, refl\'{e}tant un meilleur transport de
charge. Ces r\'{e}sultats mettent en \'{e}vidence le potentiel
multifonctionnel des nanoparticules Ag\deuxpoints{}ZnO synth\'{e}tis\'{e}es en
chimie douce pour la d\'{e}pollution environnementale et les
applications \'{e}lectroniques de pointe.
\end{altabstract}

\shortrunauthors

\thanks{Research Institute/Centre supporting program (ORF-RICSP-2026-1BT)}

%\input{CR-pagedemetas}

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\section{Introduction}\label{sec1}

Nanomaterials have attracted significant scientific interest due to
their unique physical and chemical properties that differ markedly from
those of bulk materials. In particular, nanoparticles (NPs) exhibit
high surface area, tunable band gaps, and enhanced catalytic activity,
enabling their application in gas sensing, photocatalysis, energy
storage, electronics, antibacterial treatments, and environmental
remediation~\cite{1}. As a result, increasing research efforts are
focused on developing sustainable and eco-friendly nanomaterials,
especially through green synthesis approaches, to overcome the
limitations associated with conventional bulk materials and
energy-intensive fabrication methods~\cite{2}. Among inorganic
nanomaterials, zinc oxide (ZnO) nanoparticles have received
considerable attention owing to their low cost, wide availability, and
excellent physicochemical properties, including high surface area,
optical transparency, wide band gap, and favorable electrochemical
behavior. ZnO nanoparticles are widely used in photocatalysis, sensors,
optoelectronic devices, pharmaceuticals, cosmetics, and ultraviolet
light-emitting applications~\cite{3}. However, the synthesis route
plays a critical role in determining the morphology, crystallinity, and
functional performance of ZnO nanostructures. Conventional physical and
chemical synthesis methods often involve high temperatures, toxic
chemicals, hazardous solvents, and long reaction times, raising serious
environmental and biological concerns. Green synthesis strategies using
plant extracts have emerged as promising alternatives, offering
environmentally benign, cost-effective, and biocompatible routes for
nanoparticle fabrication. In plant-mediated synthesis, naturally
occurring biomolecules act as reducing, capping, and stabilizing
agents, minimizing toxicity while improving nanoparticle
stability~\cite{4}. Various plant extracts have been successfully
employed to synthesize metallic nanoparticles with applications in
antibacterial, anticancer, antidiabetic, and tissue engineering
fields~\cite{5}. The functional properties of ZnO nanoparticles can be
further enhanced through elemental doping. In particular, silver (Ag)
doping has been shown to improve optical, structural, antibacterial,
and electrical properties by modifying defect states, reducing band gap
energy, and enhancing charge transport~\cite{6}. Dielectric properties
such as electric modulus, impedance, and AC conductivity are especially
important for electronic and energy-storage applications~\cite{7}. This
study is a continuation of our previous work~\cite{8}. In the present
investigation, Trigonella extract was employed as a reducing and
stabilizing agent for the green solvothermal synthesis of silver-doped
zinc oxide nanoparticles (Ag:ZnONPs). The influence of key reaction
parameters on nanoparticle formation was systematically examined.
Comprehensive characterization using X-ray diffraction (XRD),
UV--visible spectroscopy (UV--Vis), photoluminescence (PL)
spectroscopy, Fourier transform infrared (FTIR) spectroscopy, scanning
electron microscopy (SEM), and energy-dispersive X-ray spectroscopy
(EDS) confirmed the successful synthesis, stabilization, and effective
Ag incorporation. Furthermore, the photocatalytic activity and
frequency-dependent electrical properties of the green-solvothermally
synthesized Ag:ZnONPs were evaluated, demonstrating their strong
potential for multifunctional applications.

\section{Experimental}\label{sec2}

Ag-doped ZnO nanoparticles were synthesized using a green solvothermal
approach following a previously reported method with slight
modifications~\cite{8}. Trigonella seeds were washed, shade-dried, and
ground into a fine powder, which was then extracted using a mixed
ethanol--deionized water solvent system (11:5 v/v) under continuous
stirring for 48~h. All reagents used were of analytical grade, and
deionized water was employed throughout the synthesis. Three precursor
solutions were prepared: 0.75~M NaOH, 1~mM AgNO\tsub{3} in ethanol, and
0.3~M zinc acetate dihydrate in ethanol. The zinc acetate solution was
added to the Trigonella extract under stirring, followed by the gradual
addition of AgNO\tsub{3} and NaOH until the pH reached 12. The reaction
mixture was maintained at 80~\textdegree C for 2~h, leading to the
formation of a brick-colored precipitate. The mixture was then
transferred to a Teflon-lined autoclave and subjected to solvothermal
treatment at 190~\textdegree C for 10~h. After cooling, the product was
washed repeatedly with ethanol and deionized water, centrifuged, dried
at 75~\textdegree C, and calcined at 400~\textdegree C to obtain Ag
doped ZnO nanoparticles.

The synthesized nanoparticles were comprehensively characterized to
evaluate their structural, morphological, chemical, and electrical
properties. Fourier Transform Infrared Spectroscopy 
(FTIR---Perkin-Elmer FTIR-spectrum BX, USA) was used to identify functional
groups involved in nanoparticle stabilization, while scanning electron
microscope (SEM, JEOL JSM-6360A instrument, Japan) analysis provided
information on surface morphology and particle size distribution.
Elemental composition was confirmed using energy dispersive
spectroscopy (EDS, SEM, JEOL JSM-6400, Japan). Crystallographic
structure and average crystallite size were determined by X-ray
diffraction (XRD, D8 Advance X-ray diffractometer, Bruker, USA)
analysis using Cu K${\upalpha}$ radiation and the Scherrer equation. For
dielectric studies, the Ag:ZnO nanopowder was fabricated into
disc-shaped pellets using a PVA binder. Frequency-dependent electrical
properties, including impedance, electric modulus, and AC conductivity,
were measured at room temperature using a precision component analyzer,
demonstrating the suitability of the synthesized Ag:ZnO nanoparticles
for advanced functional applications.

\section{Results and discussions}\label{sec3}

Figure~\ref{fig1} presents the FTIR spectrum of the green-synthesized
Ag-doped ZnO nanoparticles recorded in the range of 400--4000
cm$^{-1}$, revealing the functional groups involved in nanoparticle
formation and stabilization. A broad band at approximately
3425~cm$^{-1}$ corresponds to O--H stretching vibrations, indicating
the presence of surface hydroxyl groups and adsorbed water molecules.
The absorption peak near 1643 cm$^{-1}$ is attributed to N--H bending
or C${=}$C stretching vibrations of amine or aromatic groups originating
from residual phytochemicals in the \textit{Trigonella}
extract~\cite{9}. Weak bands observed at 2376 and 2499 cm$^{-1}$ are
associated with organic residues, confirming the role of plant-derived
biomolecules in the synthesis process. In the low wavenumber region,
characteristic peaks between 645 and 416 cm$^{-1}$ are assigned to
Zn--O stretching vibrations, confirming the formation of the ZnO
lattice~\cite{8}. Additional bands at around 1435 and 879 cm$^{-1}$
correspond to C--H rocking and C--N stretching vibrations,
respectively. The presence of carboxyl and methoxy groups highlights
their role in the reduction, capping, and stabilization of Ag:ZnONPs
during green synthesis~\cite{10}.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}FTIR spectrum of Ag--ZnO nanoparticles~\cite{8}.}
\end{figure}

Figure~\ref{fig2}A shows the SEM micrograph of the green-synthesized
Ag-doped ZnO nanoparticles, revealing a densely packed assembly of
rod-like and quasi-spherical nanostructures with nanoscale dimensions.
The particles exhibit a relatively uniform distribution with noticeable
agglomeration, which is commonly observed in metal-oxide nanoparticles
synthesized via green routes due to surface energy effects and
phytochemical capping~\cite{11}. Figure~\ref{fig2}B presents the EDS
spectrum of the Ag:ZnO nanoparticles, confirming the presence of Zn, O,
and Ag as the principal elements, with no detectable impurity peaks.
The characteristic Zn--K and O--K peaks confirm the presence of Zn and
O elements associated with ZnO formation, whereas the appearance of
Ag--L peaks indicates the presence of Ag in the synthesized
sample~\cite{8}. The elemental composition obtained from EDS analysis,
showing Zn and Oxygen as the dominant elements with a small but
distinct silver (Ag) content, confirming effective Ag doping. The
slight enrichment of Ag relative to theoretical values may be
attributed to surface localization, highlighting the efficiency of the
green synthesis approach~\cite{12}.

\begin{figure}
\includegraphics{fig02}
\caption{\label{fig2}(A) SEM image, and (B) EDX analysis of
Ag:ZnONPs~\cite{8}.}
\end{figure}

Figure~\ref{fig3} illustrates the characteristic hexagonal wurtzite
structure with P63mc symmetry of ZnO nanoparticles, exhibiting peaks at
2${\uptheta}$ values of 31.64\textdegree, 34.45\textdegree,
36.23\textdegree, 47.61\textdegree, 56.62\textdegree, 62.97\textdegree,
66.45\textdegree, 67.85\textdegree, and 68.97\textdegree. These peaks
correspond to the (100), (002), (101), (102), (110), (103), (200),
(112), and (201) planes, respectively, and are in accordance with
JCPDS, File No.\ 036-145145. Furthermore, two distinct peaks at
64.53\textdegree\ and 77.39\textdegree\ were found to correspond to the
(202) and (004) planes of silver (Ag), respectively. The existence of
Ag in the sample is confirmed by the fact that all the XRD peaks can be
easily indexed to a face-centered cubic structure of Ag according to
the literature (JCPDS, File No.\ 4-078346). Because the peak intensity
of the Ag phase for Ag--ZnO nanoparticles became sharper and more
intense as the Ag contents increased, it may be inferred that the Ag
metallic phase has been generated on the surface of the ZnO-NPs instead
of being integrated into the ZnO complex. The crystallite size is 20
nm, calculated from the Scherrer formula. One possible explanation is
that the synthesis of metallic Ag occurred due to the difference in
ionic radius between Ag$^+$ (126~pm) and Zn$^{2+}$ (74~pm). Additionally,
the fact that the peak locations of Ag--ZnO nanoparticles have not
changed suggests that the Ag particles are located on the surfaces of
the well-crystalline ZnO-NP~\cite{13}.

\begin{figure}
\includegraphics{fig03}
\caption{\label{fig3}The XRD patterns of Ag:ZnONPs~\cite{8}.}
\end{figure}

The photoluminescence (PL) spectrum of the synthesized 1\% Ag-doped ZnO
nanoparticles was recorded in the wavelength range of 350--700 nm to
investigate the optical emission behavior, defect structure, and
charge-carrier recombination characteristics of the material. The PL
spectrum exhibited three distinct emission peaks centered at
approximately 397, 414, and 433~nm, along with a broad and weak visible
emission band extending from approximately 470--600~nm
(Figure~\ref{fig4}). The emission
peak observed at approximately 397~nm corresponds to the near-band-edge
(NBE) ultraviolet emission of ZnO and is attributed to the radiative
recombination of free excitons between the conduction and valence 
bands~\cite{14}. The intense emission peak centered at approximately
414 nm is associated with defect-related electronic transitions and may
originate from shallow donor levels associated with zinc interstitials
(Zn$_{\mathrm{i}}$), oxygen antisite defects, or Ag-induced defect
states within the ZnO lattice. The enhancement of this emission
indicates that Ag doping modifies the electronic structure of ZnO by
introducing localized energy states within the band gap. The relatively
high intensity of this peak further suggests strong interaction between
Ag dopants and the ZnO host lattice, as reported by~\cite{15}. The
shoulder peak located at approximately 433~nm is attributed to blue
emission associated with singly ionized oxygen vacancies 
({V}\tsub{{o}}\tsup{\tplus}),
surface defects, or defect-assisted recombination processes. The
presence of this peak further confirms the formation of intrinsic and
extrinsic defect states resulting from Ag doping and nanoparticle
synthesis. In addition, the broad weak visible emission band observed
in the green region (approximately 470--600 nm) is commonly attributed
to deep-level emissions originating from oxygen vacancies, lattice
imperfections, and surface-related defects. These defect states play a
significant role in influencing the photocatalytic, dielectric, and
electrochemical properties of ZnO-based nanomaterials. The
incorporation of Ag significantly affects the PL behavior of ZnO
nanoparticles. Meng et al.; repotted that, Ag ions can act as
electron-trapping centers, thereby suppressing the rapid recombination
of photogenerated electron--hole pairs and promoting efficient charge
separation~\cite{16}. This behavior is advantageous for photocatalytic
and electrochemical applications, as it enhances charge-transfer
efficiency and prolongs carrier lifetime. 

\begin{figure}
\includegraphics{fig04}
\caption{\label{fig4}PL spectrum of synthesized Ag:ZnONPs.}
\end{figure}

The optical transmittance spectrum of the Ag-doped ZnO nanoparticles
exhibits a pronounced absorption in the ultraviolet region followed by
a sharp increase in transmittance at around 376~nm
(see Figure~\ref{fig5}), which corresponds
to the fundamental absorption edge of ZnO. This absorption edge is
associated with direct band-to-band electronic transitions. The low
transmittance observed in the UV region is attributed to strong
intrinsic ZnO absorption as well as the presence of defect-related
states and Ag-induced electronic levels, which enhance light--matter
interaction. Beyond the absorption edge, the transmittance gradually
increases throughout the visible and near-infrared regions, reaching
higher values at longer wavelengths due to the reduced photon energy
being insufficient to promote electronic transitions. The moderate
reduction in visible-light transmittance compared to pristine ZnO can
be linked to Ag incorporation~\cite{17}, which may introduce localized
states and increase scattering at the nanoscale. Findings
from~\cite{18}  are in agreement with the effects of surface diffusion
and agglomeration on the size and shape of Ag particles. According to
reference~\cite{19}, there is a surface plasmon resonance (SPR)
absorption band in the 400--500 nm region in the optical transmission
spectra of Ag:ZnO NPs. Reduced UV transmittance arises from intrinsic
ZnO absorption and Ag-induced defects, while increased visible--NIR
transmittance reflects modified optical response, enhancing
photocatalytic and optoelectronic performance~\cite{20}.

\begin{figure}
\includegraphics{fig05}
\caption{\label{fig5}Transmittance spectrum of Ag:ZnONPs.}
\end{figure}

Ag-doped ZnO nanoparticles (Ag:ZnONPs) exhibited markedly enhanced
visible-light photocatalytic activity compared with undoped ZnO. Nearly
complete degradation (${\sim}$99\%) of crystal violet (CV) was achieved
within 24~h, while the degradation of methyl orange reached ${\sim}$99\%
within 72~h (Figure~\ref{fig6}). 
The superior photocatalytic performance of Ag:ZnONPs is
primarily attributed to Ag-induced surface plasmon resonance (SPR),
which enhances visible-light absorption and facilitates efficient
charge carrier separation~\cite{21}. Kinetic analysis further supports
this enhancement, showing a higher apparent rate constant ($k\approx
6.2$~day$^{-1}$) and a shorter half-life ($t_{1/2}\approx 2.7$~h) for
CV degradation. In contrast, methyl orange exhibited slower kinetics
($k\approx 2.3$~day$^{-1}$; $t_{1/2}\approx 7.2$~h), which can be
attributed to the higher stability of its azo bonds; nevertheless, Ag
doping significantly improved its degradation efficiency. The enhanced
activity arises from the formation of a Schottky barrier at the Ag:ZnO
interface, where Ag nanoparticles act as effective electron traps,
suppressing electron--hole recombination. Upon light irradiation,
photogenerated electrons are transferred from the ZnO conduction band
to Ag, while holes remain in the valence band, promoting the generation
of reactive oxygen species (ROS). Superoxide 
(${\bullet}$O\tsub{2}\tsup{\tminus}) and
hydroxyl ($\bullet$OH) radicals, along with intermediate
H\tsub{2}O\tsub{2}, actively attack and mineralize organic dye
molecules into CO\tsub{2} and H\tsub{2}O. These synergistic effects
confirm that Ag incorporation significantly improves charge separation,
ROS generation, and visible-light photocatalytic efficiency. The
present results are in good agreement with earlier reports,
demonstrating that Ag doping reduces recombination losses and SPR
enhances visible-light-driven photocatalysis, making Ag:ZnONPs highly
promising for environmental remediation applications~\cite{22,23,24}.

\begin{figure}
\includegraphics{fig06}
\caption{\label{fig6}Photocatalytic performance of Ag:ZnONPs toward the
degradation of CV and orange dyes.}
\end{figure}

The graph shows how the electric modulus changes as a function of
frequency. The electric modulus spectra of green-synthesized Ag-doped
ZnO nanoparticles reveal detailed information about bulk relaxation and
charge transport mechanisms. In Figure~\ref{fig7}A, the real part of
the electric modulus ($M^{\prime}$) exhibits very low values at low
frequencies ($M^{\prime}\approx 0.00419$ at $\log f=2$), indicating
strong electrode polarization and high effective permittivity due to
long-range mobility of charge carriers. With increasing frequency,
$M^{\prime}$ gradually increases and reaches a maximum value of
approximately 0.0376 at log $f\approx 6.70$, signifying the suppression
of electrode effects and the dominance of bulk (grain interior)
response at higher frequencies. This monotonic increase in $M^{\prime}$
reflects a transition from long-range charge migration to localized
hopping conduction~\cite{25}. The imaginary part of the electric
modulus ($M^{\prime\prime}$) shows a clear relaxation behavior. At low
frequencies, $M^{\prime\prime}$ remains small (${\approx}$0.00374 at
$\log f=2$), then increases with frequency and reaches a maximum
relaxation peak value of approximately $M^{\prime\prime}\max\approx
0.0160$ at $\log f\approx 6.70$. This peak corresponds to the
characteristic relaxation frequency of the system and indicates
conductivity-related relaxation associated with hopping of charge
carriers and defect dipoles, particularly oxygen-vacancy-related
centers introduced by Ag doping. The corresponding relaxation time,
calculated using  $\tau = 1/(2 \uppi f_{\max}$), is in the order of
${\sim}10^{-7}$ s, confirming fast relaxation dynamics. The broad
nature of the $M^{\prime\prime}$ peak suggests non-Debye relaxation
behavior, indicating a distribution of relaxation times due to
structural disorder, nanowire morphology, and heterogeneous conduction
\mbox{pathways.} The combined $M^{\prime}$ and $M^{\prime\prime}$ behavior
demonstrates that Ag incorporation enhances charge carrier density and
defect-assisted transport, leading to improved electrical relaxation
and bulk conductivity. These dielectric relaxation characteristics are
highly favorable for electrochemical and energy storage applications,
supporting the suitability of Ag--ZnO nanowires as advanced functional
electrode materials~\cite{26,27}.

\begin{figure}
\includegraphics{fig07}
\caption{\label{fig7}(A,B) Electric properties of Ag:ZnONPs.}
\end{figure}

The impedance behavior of the Ag:ZnONPs was analyzed using the
frequency dependence of the real ($Z^{\prime}$) and imaginary
($Z^{\prime\prime}$) components of impedance (Figure~\ref{fig7}B). At
low frequencies (${\approx}$20--100 Hz), the real part of impedance
($Z^{\prime}$) exhibits a high value on the order of
${\sim}$10$^{6}$--10$^{7}~\Omega$, indicating significant resistance
arising from grain boundaries, defect states, and space-charge
polarization effects. As the frequency increases, $Z^{\prime}$
decreases sharply and reaches a much lower, nearly
frequency-independent value of ${\sim}$10$^{4}~\Omega$ in the
high-frequency region (${\approx}$10$^{5}$--10$^{6}$~Hz). This pronounced
reduction in $Z^{\prime}$ reflects enhanced charge carrier mobility and
a decrease in grain boundary resistance, which can be attributed to the
presence of Ag dopants that facilitate electron hopping and improve
interfacial conductivity within the ZnO matrix.

Similarly, the imaginary part of impedance ($Z^{\prime\prime}$) shows a
high magnitude at low frequencies, with values approaching
${\sim}$10$^{6}~\Omega$, and decreases continuously with increasing
frequency, tending toward nearly zero at high frequencies. The absence
of a distinct $Z^{\prime\prime}$ relaxation peak within the measured
frequency range suggests a wide distribution of relaxation times and
indicates non-Debye type relaxation behavior. The continuous
suppression of $Z^{\prime\prime}$ with frequency implies reduced energy
dissipation and faster polarization response of charge carriers under
an alternating electric field. The simultaneous decrease in both
$Z^{\prime}$ and $Z^{\prime\prime}$ with increasing frequency confirms
that 1\% Ag doping effectively lowers impedance, reduces resistive
losses, and enhances charge transport in ZnO nanoparticles, making them
promising candidates for applications in electronic devices, sensors,
and photocatalytic systems where efficient charge transfer is
required~\cite{28}.

The AC conductivity ($\sigma_{\mathrm{ac}}$) behavior of the 1\%
Ag-doped ZnO nanoparticles exhibits a strong dependence on frequency
(see Figure~\ref{fig8}), reflecting the underlying charge transport
mechanisms in the material. At low frequencies (${\approx}$20--100 Hz),
the AC conductivity shows relatively low values on the order of
${\sim}$10$^{-8}$--10$^{-7}$~S${\cdot}$cm$^{-1}$, which can be attributed to
limited charge carrier mobility and the dominance of grain boundary
resistance and space-charge polarization. As the frequency increases,
$\sigma_{\mathrm{ac}}$ rises steadily and reaches values of
approximately ${\sim}$10$^{-5}$--10$^{-4}$~S${\cdot}$cm$^{-1}$ in the
high-frequency region (${\approx}$10$^{5}$--10$^{6}$ Hz). This pronounced
increase in conductivity with frequency indicates enhanced hopping of
charge carriers between localized states and defect sites, consistent
with a hopping conduction mechanism commonly observed in doped
semiconducting oxides. The incorporation of Ag$^{+}$ ions into the ZnO
lattice introduces additional localized energy levels and defect
states, which facilitate charge carrier hopping and reduce potential
\mbox{barriers} at grain boundaries~\cite{27}. Consequently, the improved AC
conductivity at higher frequencies suggests more efficient charge
transport and reduced resistive losses. The enhanced
$\upsigma_{\mathrm{ac}}$ values observed for the 1\% Ag:ZnONPs confirm
the beneficial role of Ag doping in improving electrical performance,
making these materials promising for applications in electronic
devices, sensors, and photocatalytic systems where frequency-dependent
charge transport is critical~\cite{28}.

\begin{figure}
\includegraphics{fig08}
\caption{\label{fig8}Variation of ac conductivity as a function of
frequency for Ag:ZnONPs.}
\end{figure}

The comparative Table~\ref{tab1} presents a literature-based overview
of the physicochemical and functional properties of pure ZnONPs and
Ag-doped ZnONPs synthesized using different preparation routes and
doping concentrations. The comparison highlights the effect of Ag
incorporation on the structural, optical, dielectric, electrical, and
photocatalytic characteristics of ZnO nanomaterials. The reported
studies indicate that Ag doping generally reduces the crystallite size
and modifies the crystal structure through lattice distortion and
defect formation. Optical investigations reveal changes in band gap
energy and enhanced visible-light absorption due to Ag-induced
localized surface plasmon resonance effects and defect-related energy
states. PL analysis demonstrates that Ag incorporation suppresses
electron--hole recombination by introducing defect levels and
electron-trapping centers, leading to modified excitonic and
defect-related emissions. Furthermore, Ag doping significantly improves
electrical conductivity and dielectric properties by enhancing charge
transport and interfacial polarization mechanisms. The photocatalytic
performance of Ag-doped ZnONPs is also notably enhanced compared with
pure ZnO due to improved charge separation efficiency and increased
generation of reactive oxygen species~\cite{29,30}.

%tab1
\begin{sidewaystable*}[p!]
\caption{\label{tab1}Comparative analysis of the structural, optical,
PL, dielectric, and photocatalytic properties of pure ZnONPs and
Ag-doped ZnONPs reported in previous studies}
\fontsize{8}{10.25}\selectfont
\tabcolsep=7pt
\begin{tabular}{ccccc|ccccccc}
\thead
\multicolumn{5}{c|}{Pure ZnONPs} & \multicolumn{7}{c}{Ag doped ZnONPs} \\
\xxbotline
\parbox[t]{3pc}{\centering Plasmonic signature (UV--vis) and band gap}\raisebox{-45pt}{} & 
\parbox[t]{3pc}{\centering Excitonic effects (PL)} &
\parbox[t]{3pc}{\centering Structural refinement average crystallite} & 
\parbox[t]{3pc}{\centering Electrical properties} & 
\parbox[t]{3pc}{\centering Photo\-catalytic performance} & Doped \% & 
\parbox[t]{3pc}{\centering Plasmonic signature (nm) and band gap (eV)} & 
\parbox[t]{3pc}{\centering Excitonic effects (PL)} & 
\parbox[t]{3pc}{\centering Structural refinement---average crystallite size (nm)} & 
\parbox[t]{3pc}{\centering Electrical properties} & 
\parbox[t]{3.5pc}{\centering Photocatalytic performance} & 
\parbox[t]{2.5pc}{\centering Synthesis route} \\
\endthead

\parbox[t]{3pc}{\centering 360 nm, 3.02 eV} & 
\parbox[t]{3pc}{\centering Large exciton energy accelerates recombination} &
\parbox[t]{3pc}{\centering Hexagonal wurtzite---22.71 nm} & & & 10 wt\% & 
\parbox[t]{3pc}{\centering 450--55~nm, 2.90~eV} &
\parbox[t]{3pc}{\centering Ag traps electrons, reducing recombination} & 
\parbox[t]{3pc}{\centering Hexagonal structure preserved; Ag growth confirmed---19.31 nm} & & 
\parbox[t]{3.5pc}{\centering 98\% methylene blue dye degradation under UV light in 60~min} & 
\parbox[t]{2.5pc}{\centering \textit{Carthamus tinctorius L.} leave \cite{1}} \\

\parbox[t]{3pc}{\centering 372 nm, 3.19 eV} & & 
\parbox[t]{3pc}{\centering Hexagonal wurtzite---23 nm} & 
\parbox[t]{3pc}{\centering Highest resistivity} & & 5 wt\% & 
\parbox[t]{3pc}{\centering 361.5 nm, 3.27 eV} & & 
\parbox[t]{3pc}{\centering Wurtzite ZnO preserved with cubiic Ag---20~nm}\raisebox{-64pt}{} & 
\parbox[t]{3pc}{\centering Increase the conductivity} & & 
Aloe vera \cite{2} \\

\parbox[t]{3pc}{\centering 365 nm, 3.13 eV} & & 
\parbox[t]{3pc}{\centering Hexagonal wurtzite---23 nm} & 
\parbox[t]{3pc}{\centering Supercapacitor performance} & & 1\% & 
\parbox[t]{3pc}{\centering 374 nm, 2.88 eV} & & 
\parbox[t]{3pc}{\centering Improves crystallinity} &
\parbox[t]{3pc}{\centering Improves supercapacitor performance} & & 
\parbox[t]{2.5pc}{\centering Sansevieria trifasciata root \cite{3}} \\

\parbox[t]{3pc}{\centering 374 nm, 3.72 eV} & 525 mm & 
\parbox[t]{3pc}{\centering Hexagonal wurtzite---16.5 nm} & 
\parbox[t]{3pc}{\centering Lower conductivity than composit} & & ${\approx}$50\% & 
\parbox[t]{3pc}{\centering 290 nm \& 404 nm, 3.96 eV} & 
\parbox[t]{3pc}{\centering Green emission band at 533 nm} & 
\parbox[t]{3pc}{\centering Wuritzite ZnO preserved with cubic Ag---32.43~nm}\raisebox{-55pt}{} & & & 
\parbox[t]{2.5pc}{\centering \textit{C.~auriculata} leaf \cite{4}} \\
\tabonesplittabular

\parbox[t]{3pc}{\centering 380 nm, 3.10 eV} & & 
\parbox[t]{3pc}{\centering Hexagonal wurtzite---28746} & 
\parbox[t]{3pc}{\centering Resistance (Rp) ${=}$\  104$\Omega$} & 
\parbox[t]{3pc}{\centering 62\% degradation of Tetracycline after 120 min} & 
\parbox[t]{3pc}{\centering 0.5\%, 1\%, 3\%, 5\%, and 7\%} &
\parbox[t]{4pc}{\centering 355.5 nm--360 nm (Ag at 416 nm)---Ag-0.5
\%/ZnO (3.03 eV) $>$ Ag-1\%/ZnO (2.96 eV) $>$ Ag-3\%/ZnO (2.91 eV) $>$
Ag-5\%/ZnO (2.86 eV) $>$  Ag-7\%/ZnO (2.82 eV)}\raisebox{-123pt}{} & & 
\parbox[t]{4pc}{\centering Ag accumulates at
ZnO boundaries---22.768, 23.181, 26.052, 27.981, and 28.148; 
respectively} & 
\parbox[t]{3pc}{\centering Resistance (Rp) ${=}$\ 46.9$\Omega$} & 
\parbox[t]{4pc}{\centering 93\% within the same 120-min} & 
\parbox[t]{4pc}{\centering Morinda citrifolia fruit \cite{5}} \\

\parbox[t]{3pc}{\centering ${\sim}$375 nm, 3.24 eV} & 
\parbox[t]{3pc}{\centering 546 nm, 414 nm, 442, 464, and 490~nm} &
\parbox[t]{3pc}{\centering Hexagonal wurtzite, 25.945 nm} & 
\parbox[t]{3pc}{\centering Moderate dielecteric constant, low conductivity} & & 
\parbox[t]{3pc}{\centering 0.00, 0.02, 0.06, 0.12} & 
\parbox[t]{4pc}{\centering Red-shifted, 3.21 eV, 3.17 eV, 3.18 eV; respectively} & 
\parbox[t]{4pc}{\centering 386 nm for 12 M\% doping concentration} &
\parbox[t]{3pc}{\centering 27.676, 32.174, 36.455 nm} & 
\parbox[t]{4pc}{\centering Enhances (increases) both the dielectric constant and the AC conductivity}\raisebox{-65pt}{} & & 
\parbox[t]{4pc}{\centering Chemical precipitation \cite{6}} \\

& & & & & 1\% & & 
\parbox[t]{4pc}{\centering Defect-related emissions at 414 and 433 nm with suppressed electron--hole recombination due to Ag doping} & 
\parbox[t]{4pc}{\centering Hexagonal wurtzite ZnO; rod-like and quasi-spherical nanoparticles; average crystallite size ${\approx}$20 nm} & 
\parbox[t]{3pc}{\centering Reduced impedance, and enhanced AC conductivity at room temperature} & 
\parbox[t]{4pc}{\centering ${\approx}$99\% degradation of crystal violet within 24 h and methyl orange within 72 h hunder visible-light irradiation}\raisebox{-93pt}{} & 
\parbox[t]{4pc}{\centering Green solvothermal method using Trigonella foenumgraecum seed extract [current sturdy]}
\botline
\end{tabular}
\vspace*{-33.5pc}
\end{sidewaystable*}

\section{Conclusions}

In this study, green-synthesized silver-doped zinc oxide nanoparticles
(Ag:ZnONPs) were successfully prepared and systematically investigated
for their photocatalytic and dielectric properties. FTIR analysis
confirmed the involvement of phytochemical functional groups from the
Trigonella extract in the reduction, capping, and stabilization of the
nanoparticles. SEM observations revealed rod-like and quasi-spherical
nanostructures with nanoscale dimensions, while EDX analysis verified
the effective incorporation of Ag into the ZnO matrix without
detectable impurities. XRD results confirmed the preservation of the
hexagonal wurtzite structure of ZnO along with the presence of metallic
Ag phases, and the average crystallite size was estimated to be
${\sim}$20 nm. Photoluminescence studies demonstrated near-band-edge
emission with suppressed defect-related recombination, indicating
improved charge carrier separation due to Ag doping. The Ag:ZnONPs
exhibited excellent visible-light photocatalytic performance, achieving
nearly complete degradation of crystal violet and methyl orange dyes.
Furthermore, dielectric investigations revealed non-Debye relaxation
behavior, reduced impedance, and enhanced AC conductivity, reflecting
improved charge transport characteristics. Overall, these findings
demonstrate that green-synthesized Ag:ZnONPs are promising
multifunctional materials for environmental remediation and advanced
electronic applications.

\section*{Acknowledgements}

The authors acknowledge the Research Institute/Centre supporting
program (ORF-RICSP-2026-1BT), King Saud University, Riyadh, Saudi
Arabia.

\CDRGrant[KSU]{ORF-RICSP-2026-1BT}

\printCOI

\section*{Data availability}

All data are available upon request.

\back{}

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\refinput{crphys20260050-reference.tex}

\end{document}