Memoria Investigaciones en Ingeniería, núm. 30 (2026). pp. 145-163
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145
Comparative Evaluation of Chemically and Green-Synthesized Silica-Modified
CeO₂ Nanostructures for Time-Dependent Room-Temperature Ammonia
Sensing
Evaluación comparativa de nanoestructuras de CeO₂ modificadas con sílice,
sintetizadas química y ecológicamente, para la detección de amoníaco a
temperatura ambiente en función del tiempo
Avaliação comparativa de nanoestruturas de CeO₂ modificadas com sílica,
sintetizadas quimicamente e por métodos ecológicos, para detecção de amônia em
função do tempo e à temperatura ambiente.
Danish Majeed
1
, Syeda Sarah Zehra Zaidi
2
, Syed Muhammad Mohsin
3
,
Muhammad Sajid Ali Asghar
4
, Asad A. Zaidi
5
(*)
Recibido: 12/01/2026 Aceptado: 27/03/2026
Summary. - Silica nanoparticles were synthesized via two distinct routes – a conventional chemical process and a
sustainable green approach using sugarcane bagasse – and incorporated into cerium oxide (CeO₂) nanostructures for
comparative evaluation as room-temperature ammonia (NH₃) gas sensors. The chemical route yielded silica by
precipitating sodium silicate, whereas the green route extracted bio-silica from agricultural waste (sugarcane bagasse).
Both silica types were integrated with CeO₂ through a precipitation/coating method to form silica–modified CeO₂
composite nanoparticles, which were fabricated into chemiresistive sensor devices. Structural characterization by
scanning electron microscopy (SEM) revealed an elongated, rod-like CeO₂ morphology distributed in a silica-rich
matrix, and energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of Si, Ce, and O, indicating successful
composite formation. Gas sensing tests demonstrated that all sensors responded to NH₃ at room temperature, with an
initial rapid decrease in resistance upon NH₃ exposure. The gas response (defined as change in resistance ratio) reached
over 600% within seconds of exposure for fresh sensors and progressively increased with continued exposure up to 10
min. After 15 min of continuous NH₃, however, the sensor response became negative (~–11%), suggesting surface
saturation or irreversible adsorption of NH₃ on the active sites. These results suggest that sugarcane bagasse-derived
silica can produce NH₃ response trends broadly comparable to chemically synthesized silica under the present
experimental conditions. However, a full statistical comparison using multiple devices is still required to confirm
equivalent performance. The incorporation of green-sourced silica thus provides an environmentally friendly pathway
to high-performance, room-temperature gas sensors, though calibration and long-term stability tests are needed for
further development.
Keywords: Silica nanoparticles; Green synthesis; Cerium oxide; Chemiresistive ammonia sensor; Room-temperature
gas sensing
(*) Corresponding author.
1
PhD Scholar, Department of Materials Engineering, NED University of Engineering and Technology (Pakistan),
Danish.majeed@neduet.edu.pk, ORCID iD: https://orcid.org/0009-0006-1005-8443
2
Bachelor of Engineering (Materials), Department of Materials Engineering, NED University of Engineering and Technology (Pakistan),
sara.zaidi@gmail.com. ORCID iD: https://orcid.org/0009-0005-1762-6530
3
Bachelor of Engineering (Materials), Department of Materials Engineering, NED University of Engineering and Technology (Pakistan);
mohsim.m@gmail.com, ORCID iD: https://orcid.org/0009-0006-0799-7925
4
Associate Professor, Department of Materials Engineering, NED University of Engineering and Technology (Pakistan), smsajid@neduet.edu.pk,
ORCID iD: https://orcid.org/0000-0002-4551-1299
5
Professor, Department of Mechanical Engineering, Faculty of Engineering, Islamic University of Madinah (Saudi Arabia),
ORCID iD: https://orcid.org/0000-0001-5457-5684
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Resumen. - Se sintetizaron nanopartículas de sílice mediante dos rutas distintas: un proceso químico convencional y
un enfoque verde sostenible utilizando bagazo de caña de azúcar. Estas nanopartículas se incorporaron a
nanoestructuras de óxido de cerio (CeO₂) para su evaluación comparativa como sensores de gas de amoníaco (NH₃)
a temperatura ambiente. La ruta química produjo sílice mediante la precipitación de silicato de sodio, mientras que la
ruta verde extrajo sílice biológica de residuos agrícolas (bagazo de caña de azúcar). Ambos tipos de sílice se
integraron con CeO₂ mediante un método de precipitación/recubrimiento para formar nanopartículas compuestas de
CeO₂ modificadas con sílice, las cuales se utilizaron para fabricar dispositivos sensores quimiorresistivos. La
caracterización estructural mediante microscopía electrónica de barrido (SEM) reveló una morfología de CeO₂
alargada, en forma de varilla, distribuida en una matriz rica en sílice. La espectroscopia de rayos X de energía
dispersiva (EDS) confirmó la presencia de Si, Ce y O, lo que indica la formación exitosa del compuesto. Las pruebas
de detección de gases demostraron que todos los sensores respondieron al NH₃ a temperatura ambiente, con una
rápida disminución inicial de la resistencia tras la exposición al NH₃. La respuesta al gas (definida como el cambio
en la relación de resistencia) alcanzó más del 600 % en segundos para los sensores nuevos y aumentó progresivamente
con la exposición continua hasta los 10 minutos. Sin embargo, después de 15 minutos de exposición continua al NH₃,
la respuesta del sensor se volvió negativa (~–11 %), lo que sugiere saturación de la superficie o adsorción irreversible
de NH₃ en los sitios activos. Estos resultados sugieren que la sílice derivada del bagazo de caña de azúcar puede
producir tendencias de respuesta al NH₃ ampliamente comparables a las de la sílice sintetizada químicamente en las
presentes condiciones experimentales. No obstante, aún se requiere una comparación estadística completa con
múltiples dispositivos para confirmar un rendimiento equivalente. La incorporación de sílice de origen ecológico
proporciona, por lo tanto, una vía respetuosa con el medio ambiente para obtener sensores de gas de alto rendimiento
a temperatura ambiente, aunque se necesitan pruebas de calibración y estabilidad a largo plazo para su posterior
desarrollo.
Palabras clave: Nanopartículas de sílice; Síntesis verde; Óxido de cerio; Sensor quimiorresistivo de amoníaco;
Detección de gases a temperatura ambiente.
Resumo. - Nanopartículas de sílica foram sintetizadas por duas rotas distintas – um processo químico convencional e
uma abordagem verde sustentável utilizando bagaço de cana-de-açúcar – e incorporadas em nanoestruturas de óxido
de cério (CeO₂) para avaliação comparativa como sensores de gás amônia (NH₃) à temperatura ambiente. A rota
química produziu sílica por precipitação de silicato de sódio, enquanto a rota verde extraiu biossílica de resíduos
agrícolas (bagaço de cana-de-açúcar). Ambos os tipos de sílica foram integrados ao CeO₂ por meio de um método de
precipitação/revestimento para formar nanopartículas compósitas de CeO₂ modificadas com sílica, que foram
utilizadas na fabricação de dispositivos sensores quimiorresistivos. A caracterização estrutural por microscopia
eletrônica de varredura (MEV) revelou uma morfologia alongada, em forma de bastonete, do CeO₂ distribuída em
uma matriz rica em sílica, e a espectroscopia de raios X por dispersão de energia (EDS) confirmou a presença de Si,
Ce e O, indicando a formação bem-sucedida do compósito. Os testes de detecção de gás demonstraram que todos os
sensores responderam ao NH₃ à temperatura ambiente, com uma rápida diminuição inicial na resistência após a
exposição ao NH₃. A resposta ao gás (definida como a variação na razão de resistência) atingiu mais de 600% em
segundos após a exposição para sensores novos e aumentou progressivamente com a exposição contínua por até 10
minutos. Após 15 minutos de exposição contínua ao NH₃, no entanto, a resposta do sensor tornou-se negativa
(aproximadamente -11%), sugerindo saturação da superfície ou adsorção irreversível de NH₃ nos sítios ativos. Esses
resultados sugerem que a sílica derivada do bagaço de cana-de-açúcar pode produzir tendências de resposta ao NH₃
amplamente comparáveis à sílica sintetizada quimicamente nas condições experimentais atuais. No entanto, uma
comparação estatística completa utilizando múltiplos dispositivos ainda é necessária para confirmar o desempenho
equivalente. A incorporação de sílica de origem verde proporciona, portanto, um caminho ecologicamente correto
para sensores de gás de alto desempenho que operam em temperatura ambiente, embora sejam necessários testes de
calibração e estabilidade a longo prazo para o desenvolvimento futuro.
Palavras-chave: Nanopartículas de sílica; Síntese verde; Óxido de cério; Sensor quimiorresistivo de amônia;
Detecção de gases em temperatura ambiente
D. Majeed, S. S. Zehra Zaidi, S. M. Mohsin, M. S. Ali Asghar, A. A. Zaidi
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1. Introduction. - Silica nanoparticles (SiO₂ NPs) are widely regarded for their high surface area, tunable size, thermal
stability, and biocompatibility, making them versatile for applications in sensors, drug delivery, pollutant remediation,
and catalysis [1], [2]. Traditionally, silica NPs are produced via chemical methods (e.g. sol–gel or Stöber processes)
which offer controlled size and morphology but often involve harsh chemicals and energy-intensive steps [3]. These
conventional processes raise environmental and safety concerns. In recent years, green synthesis approaches have
gained momentum as sustainable alternatives [4], [5], [6]. Agricultural wastes such as rice husk, corn cob, and
sugarcane bagasse have been identified as rich silica sources that can be processed into nanosilica, reducing waste and
chemical usage [7], [8]. For instance, sugarcane bagasse ash can contain over 70 wt% silica [9], making it an attractive
precursor for silica extraction. Utilizing such bio-derived silica not only diminishes the need for toxic reagents but also
adds value to agro-industrial byproducts [10]. Recent studies have demonstrated feasible routes to synthesize silica
nanoparticles from sugarcane bagasse and other plant wastes, underscoring the potential of green nanotechnology in
materials engineering [11], [12], [13].
Cerium oxide (CeO₂) is a prominent metal oxide semiconductor with excellent redox activity and a high density of
oxygen vacancies, properties that are highly beneficial for gas sensing applications [14]. These oxygen vacancies
facilitate the adsorption and reaction of gas molecules on the surface, while CeO₂ has good thermal stability and
environmental durability which are advantageous for sensor longevity [15]. CeO₂ is an n-type semiconductor that
typically requires elevated operating temperatures to achieve sufficient surface reactivity; however, various strategies
such as nanostructuring and doping have been employed to overcome its limitation of low room-temperature
conductivity [16], [17]. In particular, modifying CeO₂ with secondary phases or dopants can dramatically enhance its
gas response at lower temperatures by increasing surface area, creating heterojunctions, or introducing catalytic sites
[18]. For example, incorporating noble metals or other oxides into CeO₂ has been shown to improve sensitivity and
selectivity to target gases [19]. In one recent report, Ti-doped CeO₂ thin films exhibited a gas response of ~5910% to
250 ppm NH₃ at room temperature (compared to pristine CeO₂’s significantly lower response), with rapid
response/recovery times of ~15 s [20]. These improvements were attributed to increased oxygen vacancy concentration
and electronic modulation by the Ti dopant.
Another effective modification of CeO₂ is the addition of silica. Silica itself is electrically insulating, but when
combined with metal oxides it can act as a high-surface-area framework that promotes dispersion of the active oxide
phase and enhances gas diffusion through a porous network [21], [22]. Wang et al. demonstrated that a sensor using
CeO₂ nanostructures modified with ~8 wt% SiO₂ achieved a remarkably high NH₃ response (~3244% to 80 ppm) at
room temperature, far superior to pure CeO₂, along with a lower detection limit of 0.5 ppm [23]. The enhanced
performance was attributed to the silica providing a larger effective surface and improved hydroxyl-mediated
conductivity on the composite surface [24]. Silica additives can also stabilize the nanostructure and prevent particle
agglomeration, thereby preserving active surface sites. Furthermore, silica’s acid/base surface groups may interact with
ammonia, influencing the sensor’s electron transfer processes [25]. By fine-tuning the SiO₂ content, researchers have
achieved sensors operable at room temperature with fast response and recovery and extended stability [26].
Accordingly, the objective of this study is to comparatively evaluate silica obtained via (i) a conventional chemical
route and (ii) an agricultural waste–derived green route (sugarcane bagasse), after integration into silica-modified CeO₂
nanostructures for room-temperature NH₃ sensing. The work aims to (a) document the synthesis and fabrication
workflow, (b) examine morphology and elemental composition using SEM/EDS, and (c) assess the time-dependent
evolution of chemiresistive response under qualitative NH₃ exposure over 0–15 min. The emphasis is placed on
comparative response trends and exposure-time effects relevant to practical room-temperature operation rather than
ppm-calibrated sensitivity.
2. Materials and Methods. -
2.1 Materials and Reagents. - The silica sources and chemicals used in this work include: sodium silicate solution
(Na₂SiO₃, for chemical silica synthesis), sugarcane bagasse (agricultural waste sourced locally, for green silica
synthesis), hydrochloric acid (HCl, 2.5% aqueous solution), silver nitrate (AgNO₃), ethanol (C₂H₅OH), nitric acid
(HNO₃), sodium hydroxide (NaOH), ammonium hydroxide (NH₄OH), and cerium(III) nitrate hexahydrate
(Ce(NO₃)₃•6H₂O). Distilled water was used in all synthesis and washing steps. All chemicals were of analytical grade
and used as received.
2.2 Silica Nanoparticle Synthesis by Chemical Route. - Silica nanoparticles were synthesized using both a
conventional chemical route and a green route based on agricultural waste in order to enable a comparative evaluation
of their suitability for gas sensing applications. In the chemical synthesis approach, diluted sodium silicate solution
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was added dropwise to 2.5% hydrochloric acid under continuous magnetic stirring at approximately 60 °C. The
controlled neutralization process resulted in the formation of a cloudy, viscous silica gel due to the condensation of
silicate species. The obtained gel was repeatedly washed with distilled water to remove residual chloride ions, which
was confirmed by the absence of white precipitate upon addition of silver nitrate to the filtrate. The washed gel was
then dried at 100 °C for 24 h, followed by calcination in air at 1000 °C for 1 h to obtain silica nanoparticles with
enhanced thermal stability and structural integrity. The overall workflow of the chemical synthesis route is
schematically shown in Figure I.
Figure I.- Schematic illustration of the chemical-route synthesis of silica nanoparticles via acid precipitation, washing, drying,
and calcination.
The green synthesis route, composite preparation, and sensor fabrication steps are described separately in the following
subsections to avoid repetition and improve clarity.
2.3 Silica Nanoparticle Synthesis by Green Route. - For the green synthesis route, silica was extracted from sugarcane
bagasse, an agricultural by-product high in silica content [27]. The bagasse (fibrous sugarcane waste) was first sun-dried
and cut into small pieces. It was then thoroughly washed with distilled water and soaked for 24 h to remove dirt and soluble
impurities. After oven drying at 90 °C, the cleaned bagasse was subjected to acid leaching: the fibers were immersed in a
1 N HCl solution and heated in a water bath at ~75 °C for several hours. This step dissolves many metal impurities from
the biomass [3]. The material was filtered, washed to neutrality, and dried again (90 °C). Next, the pretreated bagasse was
transferred to a basic extraction step to dissolve its silica content. The dried fibers were boiled in 1 M NaOH solution
(liquid-to-solid ratio ~10 mL g⁻¹) at 90 °C for 1 h, which converted the bio-silica into soluble sodium silicate. The hot
mixture was filtered to remove residual biomass, yielding a sodium silicate solution. Silica was then precipitated from this
solution by the addition of acid: concentrated HNO₃ was added dropwise to the filtrate under stirring until the pH lowered
to ~8. At this point, the silicate condenses into silica particles, forming a suspension. A small amount of ethanol (20 mL)
was also added to promote particle formation. The suspension was continuously stirred, then left overnight. The silica
product was collected by centrifugation at ~4000 rpm for 45 min, washed repeatedly with distilled water, and dried at 90
°C. A final calcination at 600 °C for 30 min was applied to obtain purified silica nanoparticles. Figure II summarizes the
green synthesis steps.
Figure II.- Schematic workflow for green synthesis of SiO₂ nanoparticles from sugarcane bagasse via acid leaching, alkaline
extraction, and acid precipitation.
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Both synthesis methods yielded fine white silica powders. The chemically synthesized silica had a yield of approximately
1.5–2 g from the starting solution volumes, whereas the green route yield depended on ash content (bagasse ash after acid
treatment yielded ~20–25% silica by weight). The green synthesis is notably less toxic and aligns with sustainable practices
by utilizing a renewable waste resource.
2.4 Preparation of Silica–Modified CeO₂ Nanostructures. - Silica-modified ceria nanostructures were prepared via
an in situ precipitation of CeO₂ onto the silica nanoparticles. In a typical procedure, 6 g of the prepared silica
nanoparticles (from either route) were dispersed in 100 mL of distilled water. The dispersion was ultrasonicated for 30
minutes to ensure a uniform suspension of silica particles (which serve as a substrate or template for CeO₂ growth). In
parallel, 2.17 g of cerium (III) nitrate hexahydrate (Ce(NO₃)₃•6H₂O) was dissolved in 50 mL of distilled water to form
a Ce³⁺ precursor solution. This cerium nitrate solution was then slowly added to the silica suspension under constant
magnetic stirring. After 15 minutes of mixing, ammonium hydroxide (NH₄OH, ~28% solution) was added dropwise
to the mixture until the pH reached approximately 10–11. The addition of NH₄OH precipitates cerium as cerium
hydroxide, which subsequently forms cerium oxide (ceria) in the presence of OH⁻. The reaction was maintained at ~80
°C with continuous stirring for 2 hours to allow CeO₂ nanoparticles to nucleate and coat onto the silica surfaces. The
appearance of a homogenous light yellow or off-white suspension indicated the formation of CeO₂ on SiO₂. The
composite product was collected by centrifugation at 10,000 rpm for 10 minutes, then washed thoroughly with distilled
water to remove any residual ions (e.g., NH₄⁺, NO₃⁻). The collected silica–ceria composite was dried in an oven at 80
°C for 12 hours. This drying step yields a powder comprising silica nanoparticles decorated with cerium oxide (denoted
CeO₂–SiO₂ nanostructures). Figure III provides a stepwise illustration of the composite synthesis.
Figure III.- Schematic of CeO₂–SiO₂ composite formation via precipitation/coating of ceria onto dispersed silica
nanoparticles.
In this synthesis, silica (from either source) plays the role of a high-surface-area scaffold. The precipitation of CeO₂ in the
presence of dispersed silica promotes the formation of a composite rather than separate CeO₂ particles. Prior studies on
similar systems have reported core–shell type structures where silica cores are coated with ceria shells [27], [28]. Our
approach is expected to produce a network of CeO₂ nanocrystallites attached to silica surfaces, which can increase the
effective interface with gas molecules during sensing [29].
2.5 Sensor Fabrication and Electrical Measurements. - A chemiresistive gas sensor device was fabricated using the
silica–modified CeO₂ composite as the active material. The sensing element was prepared on a small glass substrate
(approximately 2 × 2 cm). Prior to coating, the glass substrate was cleaned ultrasonically in ethanol and acetone (10 min
each) and dried at 90 °C. Interdigitated electrodes were then formed on the glass surface to allow electrical contact to the
sensor material. To do this, a thin conductive layer was applied using silver and gold pastes: first, a silver paste was painted
as a seed layer for good adhesion, defining two opposite electrode pads. Then a layer of gold paste was applied over the
silver tracks to provide a chemically inert, highly conductive electrode surface. The electrode pattern was dried and cured
by heating at 90 °C for several hours, ensuring the metal contacts were well-adhered. Fine platinum wires were attached to
the electrode pads (using additional silver paste) to serve as lead-out connections.
To prepare the sensing film, the dried CeO₂–SiO₂ composite powder was blended with a small amount of poly(vinyl
alcohol) (PVA) as a binder. Specifically, PVA was dissolved in ethanol to create a viscous solution, and the silica–
ceria powder was gradually added to form a slurry. Additional PVA or ethanol was mixed in as needed to achieve a
spreadable paste with uniform consistency. The PVA acts as a temporary binder to hold the nanoparticles together and
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adhere them to the substrate; it will evaporate or decompose upon subsequent heating, leaving a porous ceramic film.
The composite paste was then coated onto the area between the gold contact pads on the glass substrate, covering the
electrode gap. This coating was done carefully to create a thin, even film. After air drying, the coated sensor was cured
by heating at 400 °C for 2 hours in air. Although the general fabrication procedure was described, some device-level
parameters were not measured, including exact electrode gap width, finger length/number, film thickness, active coated
area, curing ramp rate, and contact-resistance stability. These parameters may affect the absolute resistance and
response values, so the device data were interpreted as proof-of-concept results. This thermal treatment burns off the
PVA binder and improves adhesion of the composite to the substrate, as well as potentially enhancing particle
connectivity by slight sintering. The result was a robust, porous sensing layer of silica–modified CeO₂ bridging the
gold electrodes. Figure IV shows the fabrication steps and assembled sensor device.
For gas sensing tests, the fabricated sensor was placed inside an airtight chamber of approximately 1 L volume. Figure
V shows the measurement setup. To introduce ammonia, a known volume of aqueous ammonia solution (~25% NH
in water) was placed in a shallow dish inside the sealed chamber. The volatile NH evaporates from the solution,
quickly enriching the chamber atmosphere with ammonia vapor. (Note: this method provides an uncalibrated
concentration of NH; the exposure in this study is qualitative, since no mass flow controller or precise ppm control
was used – see Limitations). Upon introducing NH, the sensor’s resistance in the ammonia environment (Rgas) was
recorded continuously. The sensor was exposed to ammonia for a total of 15 minutes, and the response was monitored
over this duration. At 5-minute intervals (0, 5, 10, 15 min), the chamber was briefly opened and then resealed (for
instance, to refresh the vapor or simulate consecutive exposures), and the subsequent resistance behavior was
measured. After 15 min, the chamber was opened to allow the ammonia to dissipate and the sensor to recover in fresh
air. Because the chamber was opened and resealed at 5-minute intervals, the ammonia concentration and oxygen
availability inside the chamber were not constant throughout the experiment. Each opening allowed partial gas
exchange with ambient air, which may have changed the NH₃ level and refreshed oxygen on the sensor surface.
Therefore, the response curves at 0, 5, 10, and 15 min should be interpreted as successive chamber-exposure intervals
rather than a strictly controlled continuous NH₃ exposure history.
The gas response of the sensor is defined in percentage terms using the standard chemiresistive formula:
GasResponse(%) air gas
air
Where, Rair is the sensor resistance in air (baseline) and Rgas is the resistance in the target gas (here, NH₃) [9].
According to this definition, a decrease in resistance upon gas exposure (typical for an n-type oxide like CeO₂ in the
presence of a reducing gas) yields a positive response percentage. Since this response calculation depends on the
selected air baseline resistance, any baseline drift during prolonged exposure or incomplete recovery can strongly affect
the calculated response value. Therefore, a change in response sign may occur if the baseline shifts, and it should not
automatically be interpreted as a true inversion of the sensing mechanism. All response values reported were calculated
with this formula. In our measurements, the sensor’s real-time resistance changes were captured and later converted to
response (%) versus time for analysis.
No external heating was applied to the sensor – all tests were conducted at room temperature (~25 °C). The humidity
level was ambient (~50% RH) and was not specifically controlled. After each test, the chamber was purged and the
sensor was allowed to recover in air; however, as discussed later, the 15-minute prolonged exposure led to behavior
suggesting incomplete recovery.
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Figure IV. Schematic representation of gas sensor fabrication using silica-modified CeO₂ nanostructures on a glass
substrate.
It should be noted that because the ammonia concentration in the chamber was not quantitatively measured (relying
on evaporation from solution), the response data are interpreted qualitatively. The emphasis of this study is on
comparing the relative sensor behavior (especially time-dependent characteristics and the effect of prolonged
exposure) rather than on an exact sensitivity value to a known ppm. The performance of our sensor is also compared
with literature reports to contextualize its sensitivity.
Figure V. Schematic of the sealed-chamber setup used for room-temperature ammonia sensing measurements.
3. Results and Discussion. -
3.1 SEM Analysis of Silica–Ceria Nanostructures. - The morphology of the silica–modified CeO₂ nanostructures
was examined by SEM. Representative SEM micrographs at different magnifications are shown in Figure VI.
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Figure VI. SEM micrographs of silica-modified CeO₂ nanostructures at different magnifications.
Comparing our SEM observations with literature, we note that Kitsou et al. (2019) achieved core–shell ZnO@SiO₂
particles where silica served as a smooth nanoscopic template for uniform oxide deposition [14]. In our case, the ceria
does not form discrete core–shell particles but rather continuous ceria-rich rods embedded in silica. This structure
resembles findings by Vaizoğullar et al. (2016), who prepared SiO₂@CeO₂ core–shell nanoparticles and observed a
silica-rich composition with ceria coating the silica surfaces [30]. Our sample’s rod-like, intergrown morphology may
result from the high loading of CeO₂ relative to silica (6 g silica vs ~2 g Ce precursor used). As ceria nucleates and
grows, particles coalesce into larger rods, possibly with silica particles decorating or interspersed within these rods.
Some cracks or irregular voids are visible in the SEM images, which could be attributed to stresses during the high-
temperature curing or calcination process. Thermal history (e.g. rapid drying or the 400 °C curing step) can induce
fracturing in the deposited film, as noted in other nanoparticle-based coatings [31]. Despite these fractures, the SEM
images show an interconnected composite morphology. However, based on the EDS results, the material should be
described more carefully as a silica-rich structure containing dispersed Ce-containing oxide regions, rather than as a
fully ceria-rich network.
The elongated ceria structures observed here are known to enhance gas sensing performance by offering a high surface-
to-volume ratio and facilitating charge transport along the length of the rods [10]. One study on porous ceria nanorods
(Tian et al., 2015) noted their high catalytic activity and robust performance, attributing it to the rod morphology
providing abundant active sites and pathways for reactant diffusion [20]. In our composite, the silica component further
contributes by increasing porosity and stability. Thus, the SEM results confirm that we have successfully created a
nanostructured CeO₂–SiO₂ material with characteristics favorable for gas sensing: nanoscale rods, extensive surface
area, and a percolating network structure. It should be noted that the rod-like features observed at the applied SEM
magnifications correspond to micron-scale elongated structures, likely formed by the aggregation of nanoscale CeO₂
crystallites.
3.2 EDS Analysis and Composition. - Energy-dispersive X-ray spectroscopy was used to verify the elemental
composition of the silica–ceria composite. Figure VII shows the EDS spectrum of a representative sample region.
Figure VII. EDS spectrum of silica-modified CeO₂ nanostructures showing the presence of Si, Ce, and O.
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The EDS results should be interpreted as semi-quantitative local compositional data from the analyzed surface region.
The measurement was performed at 20.0 kV with a probe current of 1 nA, and the calculated weight percentages may
vary with selected area, surface roughness, sample thickness, and local particle distribution. Therefore, the EDS values
were used mainly to confirm the presence of Si, Ce, and O, rather than to define the exact bulk composition of the
composite. The silica-rich composition has important implications for gas sensing. A high SiO₂ content contributes a
large surface area and porous structure, as silica generally forms porous networks. The Ce-containing oxide phase was
present in a lower amount than silica and was likely distributed as dispersed regions, clusters, or partial coating on the
silica-rich matrix. Therefore, the role of CeO₂ in sensing was discussed as the contribution of accessible Ce-containing
active sites, while silica was considered mainly as the supporting matrix. Vaizoğullar et al. (2016) observed that in
SiO₂@CeO₂ core–shell nanoparticles with similarly low Ce content, the configuration enhanced adsorption capabilities
– in their case for Hg(II) removal – due to the silica providing a high-surface-area support for the active CeO₂ phase.
By analogy, our composite’s elemental makeup (low Ce, high SiO₂) points to a design wherein SiO₂ is the structural
scaffold and CeO₂ provides the reactive sites for ammonia sensing. This division of roles can be advantageous: silica
ensures a stable, porous film, and ceria, despite its smaller fraction, can strongly influence sensor response due to its
catalytic redox interactions with gases [24].
Furthermore, the presence of oxygen (50+ wt%) in the EDS confirms that the material consists of oxides (as expected:
SiO₂ and CeO₂). No metallic Ce or unoxidized silicon is detected, implying that during synthesis and curing, cerium
precipitated as cerium oxide and the silica remained in oxide form. The EDS result thus corroborates that the intended
composite (SiO₂ and CeO₂) was obtained. We acknowledge that a precise phase identification (e.g. crystalline phase
of CeO₂) would require X-ray diffraction (XRD). In this study, XRD was not performed; however, literature and the
synthesis conditions strongly suggest that the cerium is present as CeO₂ (fluorite structure), likely nanocrystalline or
possibly amorphous if insufficiently crystallized at 80 °C. The decision to focus on SEM/EDS for composition was
made to prioritize understanding the sensor’s functional performance. Despite the lack of XRD confirmation, the
effective gas sensing behavior (discussed below) provides indirect evidence of active CeO₂ presence, since pure silica
is inert to ammonia under these conditions. This revised interpretation reconciles the SEM and EDS observations: the
composite was not treated as a ceria-rich percolating network, but as a silica-rich material containing Ce-containing
oxide sites that may participate in NH₃ sensing.
3.3 Gas Sensing Mechanism (Chemiresistive Response). - Before discussing the experimental gas response results,
we briefly outline the sensing mechanism for n-type semiconducting oxides like CeO₂. In ambient air, oxygen
molecules adsorb onto the surface of CeO₂ and capture electrons from the material’s conduction band, forming
chemisorbed oxygen species (O₂⁻, O⁻, O²⁻) [9]. This creates an electron depletion layer at the surface, raising the
sensor’s resistance in air (Rair). When the sensor is exposed to a reducing gas such as NH₃, the gas molecules react
with these adsorbed oxygen ions. For ammonia, a representative surface reaction can be written as:
which indicates that NH₃ donates electrons back to the oxide by consuming surface oxygen species. The product N₂
and H₂O desorb, and the released electrons return to the CeO₂, reducing the depletion layer and thereby decreasing the
sensor’s resistance. Thus, upon NH₃ exposure, an n-type oxide sensor typically shows a drop in resistance, which we
quantify as a positive gas response (%) as per the earlier formula. When the NH₃ is removed and air is reintroduced,
oxygen readsorption can restore the initial conditions (electrons re-trapped by O₂, resistance rises back to baseline),
ideally regenerating the sensor.
In our silica–CeO₂ composite, the same fundamental mechanism applies, but the silica matrix may influence the
process by affecting how gases diffuse and how electrons percolate. The silica itself is insulating, so the conductive
path is primarily through the ceria domains. However, silica can adsorb moisture and surface hydroxyls which might
interact with NH₃ (for example, NH₃ can temporarily bind to silanol groups). This could introduce additional surface
reactions or delay the recovery as discussed later. Nonetheless, the dominant sensing action is expected from the CeO₂
sites.
We next present the sensor response results at different exposure times. To reiterate, all measurements were done at
room temperature with an unquantified but repeatable NH₃ dose (from evaporating ammonia solution). The baseline
resistance in air for our devices was on the order of a few MΩ. Upon NH₃ introduction, the resistance dropped
markedly. We calculated the gas response (%) over time for each exposure interval (0, 5, 10, 15 min exposures). The
dynamic response curves are shown in Figures VIII–XI, and key features are discussed below. It is important to note
that the 0, 5, 10, and 15 min response curves do not represent a fully controlled continuous exposure experiment. Since
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the chamber was opened and resealed between intervals, gas composition, ammonia vapor concentration, and oxygen
availability may have changed during the test. Therefore, the following discussion focuses on qualitative time-
dependent response behavior under the present chamber protocol, rather than controlled kinetic analysis.
3.3.1 Sensor Response at Initial NH₃ Exposure (0 minutes). - “0 minutes” corresponds to the sensor’s immediate
reaction as ammonia is introduced (the first exposure, effectively within the first few seconds of sensing). Figure VIII
displays the response curve at this initial exposure.
Figure VIII. Dynamic NH₃ response of the sensor at 0 min exposure time.
Following the peak, the sensor’s response begins to decline gradually. In Figure VIII, after the first ~2 s spike, the %
response drops and stabilizes around ~390–400% by 10 s. This decline is attributed to a partial recovery or saturation
effect – as ammonia initially floods the surface, a maximum number of reaction sites are consumed (peak response),
and subsequently the system reaches a balance where some NH₃ may desorb or the reaction by-products (e.g. water)
start to occupy sites, causing a slight rebound in resistance [32]. A minor secondary rise (“bump”) around 5–6 s can
be seen, which could indicate a two-step adsorption process or a delayed reaction on deeper sites. By ~10 s, the curve
has leveled off, implying that most accessible sites have reacted and the sensor is in a quasi-steady state with that
ammonia dose. Importantly, when the chamber was later opened (to simulate a fresh exposure at 5 min, as described
next), the sensor resistance returned close to baseline (with air flush), confirming that the interaction at this stage was
largely reversible.
Overall, the initial exposure demonstrates that both chemically synthesized silica and green-synthesized silica
composites can detect NH₃ strongly at room temperature. The magnitude of the response (hundreds of percent) is on
par with or exceeds many conventional metal-oxide sensors at much higher operating temperatures [33]. The rapid
response (sub-second rise) is a favorable attribute for real-time monitoring. This performance can be attributed to the
composite’s high surface area and the efficient catalytic reaction of NH₃ with chemisorbed oxygen on CeO₂. The
presence of silica likely enhances the dispersion of CeO₂ and keeps the nanostructure porous, allowing NH₃ to penetrate
and reach active sites quickly. We also note that at this initial stage, the sensor had not been “aged” by prior exposure,
which often yields the highest response. Subsequent exposures (without thorough reoxidation in between) can show
modified behavior, as we observed.
3.3.2 Sensor Response after 5 Minutes of NH₃ Exposure. - After the first exposure, the chamber was resealed with
a continued NH₃ source for 5 minutes. We define the “5 min” response as the sensor’s behavior when it is exposed to
NH₃ that has been in the chamber for 5 minutes (a somewhat refreshed NH₃ environment, though not a fully desorbed
baseline). Figure IX shows the dynamic response curve at the 5-minute mark.
Figure IX. Dynamic NH₃ response of the sensor at 5 min exposure time.
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Interestingly, the 5-min response curve shows a more complex transient. Around 10–15 s, there is an abrupt dip: the
response drops from ~450% to ~375%, then partially recovers to ~395%, and then gradually falls to ~370% by ~25 s.
This oscillatory behavior suggests a dynamic competition between adsorption and desorption processes. The
momentary drop at ~15 s could indicate that a fraction of NH₃ desorbed or that a second-stage reaction (perhaps
oxidation of intermediate species) occurred, momentarily increasing resistance. The subsequent partial recovery
implies that NH₃ readsorbed on freed sites, and finally the downward drift toward 25 s signals a slow approach to
equilibrium or slight poisoning. Essentially, by 5 minutes exposure, the sensor is entering a regime of saturation and
partial regeneration simultaneously – ammonia molecules continue to interact, but the surface is not as uniformly
receptive as at time zero. Similar kinetic behaviors (initial spike followed by dip and plateau) have been reported for
other porous sensors under continuous gas flow, where the fast initial reaction is followed by slower diffusion-limited
or desorption-limited stages.
From a sensing standpoint, the 5 min exposure still yields a high response (~370% sustained), which confirms that the
sensor remains highly sensitive even after being subjected to NH₃ for a prolonged period. However, the reduction in
peak and the fluctuations indicate that the sensor’s surface is beginning to saturate. This is an important observation
for practical use: it suggests that while short bursts of NH₃ can be detected with extreme sensitivity, continuous
exposure will reduce the incremental response. The sensor is likely desorbing some ammonia during the exposure
(hence the dip and partial recovery), which is a form of self-purging. This behavior is beneficial in that it prevents total
saturation within minutes, but it also means the sensor response at long times may not remain at the initial high level.
3.3.3 Sensor Response after 10 Minutes of NH₃ Exposure. - Continuing the exposure further, the sensor behavior at
10 minutes in ammonia was recorded. Figure X presents the response curve after 10 min of continuous NH₃
environment.
Figure X. Dynamic NH₃ response of the sensor at 10 min exposure time.
Following the peak, from ~3 s onward, the response does not simply decay; instead, it oscillates between about 660%
and 700% for an extended period (several seconds). This quasi-steady oscillation indicates a dynamic equilibrium
where ammonia adsorption and reaction are balanced by desorption and possibly oxygen re-adsorption in pockets of
the sensor [34]. The porous structure likely allows NH₃ to diffuse in and out at different rates, causing fluctuations.
Tong et al. (2017) observed analogous oscillatory response in a highly porous H₂S sensor, attributing it to the interplay
of gas diffusion and reaction kinetics in nanochannels [21]. In our case, the silica–ceria network might have regions
that momentarily saturate and then refresh as NH₃ penetrates deeper. The presence of silica could also buffer moisture
produced by ammonia oxidation, releasing it intermittently and affecting conductivity slightly.
After ~15 s, the sensor’s response in Figure X begins a gradual decline, dropping to around 610% by 25 s. This
downward trend suggests the onset of more significant surface saturation – many of the active sites are occupied by
reaction products or a stable layer of adsorbed ammonia species, and the sensor cannot sustain the earlier high
conductance. Nonetheless, even at 25 s, the response (~610%) is far above the baseline (0%). Compared to the earlier
exposures, the 10-min case shows the sensor reaching its highest sensitivity (initially) and then showing signs of
leveling off at a high response value. The fact that the sensor can still exceed 600% response at 10 min indicates a
time-dependent activation: prolonged exposure appears to have activated additional sites or reduced the material
(increasing Ce³⁺ concentration), thus temporarily boosting sensitivity. Such behavior – an increase in response with
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longer exposure – has been reported for some metal oxide sensors, often due to gradual temperature equilibration or
surface restructuring in continuous gas. In our room-temperature case, it could be chemical restructuring (e.g.,
formation of ammonium species on the surface that facilitate more electron transfer).
This 10-min result highlights that the sensor’s response is not static; instead, it evolves with prolonged gas interaction.
Practically, this could manifest as a drift in sensor readings over time if NH₃ remains present. Initially, the readings
might climb (super-sensitivity), then oscillate, and slowly decline as equilibrium is reached. For a sensing application,
one would ideally calibrate the sensor at a fixed exposure duration or dynamic flow to avoid misinterpreting these
time-dependent changes as concentration changes. In our experiments, since the concentration wasn’t fixed, we
interpret these results qualitatively: the composite sensor retains high responsiveness up to 10 minutes, but the reaction
kinetics and surface coverage become increasingly complex.
3.3.4 Sensor Response after 15 Minutes of NH₃ Exposure. - After a continuous exposure of 15 minutes to ammonia,
a striking change in sensor behavior was observed. Figure XI shows the response curve at the 15-min mark. It should
be noted that the response value was calculated using the air baseline resistance. If this baseline resistance changed
during the prolonged test, or if the sensor did not fully recover before the next measurement interval, the calculated
response could become negative even without a true reversal of the NH₃ sensing mechanism. Therefore, the negative
response at 15 min should be interpreted cautiously.
Figure XI. Dynamic NH₃ response of the sensor at 15 min exposure time showing response inversion.
This anomalous negative response may indicate sensor saturation or overload; however, it cannot be assigned only to
this mechanism because the NH₃ concentration and humidity were not controlled during the test. Baseline drift, changes
in chamber atmosphere, moisture effects from aqueous ammonia, or measurement-related artifacts may also have
contributed to the negative response. By 15 minutes, it appears that the sensor surface has accumulated a high coverage
of adsorbed species (likely NH₃-derived intermediates or reaction products like ammonium salts or strongly bound
oxygen complexes) that fundamentally alter the material’s conduction. One interpretation is that the prolonged NH₃
exposure caused an accumulation of electrons in the material that shifted the baseline (essentially reducing the oxide
significantly). If the sensor baseline resistance had dropped over time (due to partial reduction), then upon final
exposure, introducing NH₃ might no longer produce a drop in resistance – instead, if oxygen was completely depleted
from the surface, NH₃ might actually start donating electrons that overpopulate the conduction band, leading to a
temporary increase in resistance via complex mechanisms (such as formation of surface states that scatter electrons).
Another simpler explanation is surface saturation: all adsorption sites (oxygen vacancies, etc.) are occupied, and
additional NH₃ cannot react with O²⁻ (because few are left); instead, NH₃ may physically adsorb as a neutral or
insulating layer, increasing resistance or blocking current paths [35]. The relatively flat response for the first ~10 s
suggests the sensor was fully saturated – the response hovered near 0% (no change). Then, the continuous decline to –
11% by 25 s could indicate a slow poisoning effect, where the prolonged presence of NH₃ (or perhaps accumulation
of by-products like ammonia-derived surface complexes) inverted the sensor’s response. Essentially, the sensor’s
behavior degraded after too long an exposure, consistent with reversible damage or inhibition of the normal sensing
reaction. Similar findings have been noted by Mei et al. (2024) for prolonged exposure of certain gas sensors, where
signal saturation or even inversion was observed when the target gas was not removed, due to complete consumption
of surface oxygen and accumulation of reducing species [17]. Likewise, Takte et al. (2023) reported that extended
exposure to ammonia could lead to formation of stable surface residues (e.g., ammonium carbonates or amides) on
ceria-based sensors, which alter the material’s electronic structure and impede the usual sensing mechanism [18].
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Therefore, the negative response observed at 15 min should be treated as a possible sign of surface saturation or altered
surface condition, rather than as confirmed evidence of sensor poisoning. Further controlled experiments using fixed
NH₃ concentration, controlled humidity, repeated cycles, and baseline stability checks are required to confirm the cause
of this response inversion.
It is important to clarify that once the sensor is in this state, simply purging with air may not immediately restore the
original baseline. Indeed, after the 15-min test, our sensor showed difficulty returning to the exact initial resistance,
indicating that some NH₃ or reaction product remained bound (irreversible adsorption). In practical terms, this means
the sensor would require re-oxidation (for example, heating or a long ambient recovery) to fully reset after such a
prolonged NH₃ exposure. For shorter exposures (0–10 min), the sensor largely recovered after air purging, but the 15
min case crossed a threshold into saturation. This finding underscores a limitation for using such sensors in high or
continuous ammonia environments: one must either periodically recondition the sensor or interpret the decreasing
signal as an indicator of prolonged exposure (rather than misinterpreting it as lower gas concentration).
In summary, the time-dependent gas response measurements reveal a progression: (i) an initially high and fast response
(0 min), (ii) slight moderation and kinetic complexity at 5 min, (iii) reactivation and oscillatory equilibrium at 10 min,
and (iv) saturation and response inversion at 15 min. This progression can be understood as the sensor material
transitioning from an ideal surface with abundant reactive oxygen (at start) to a surface that is progressively reduced
and saturated by ammonia. The negative response at 15 min is a clear sign of sensor saturation, where additional
reducing gas no longer yields a typical n-type response.
Notably, such behavior is rarely reported in short laboratory tests but is crucial for real-world sensing scenarios. It
suggests that for long-term monitoring, either periodic sensor regeneration (e.g., exposure to clean air or mild heating
to desorb residues) is necessary, or one should limit the sensor’s exposure time within a regime that avoids complete
saturation. The use of silica in our composite might contribute to the saturation effect as well – silica surfaces could
hold NH₃ or moisture strongly over time, hindering the replenishment of oxygen on CeO₂. In future designs, optimizing
the silica content or adding catalytic additives (e.g., Pt, as often done with MOX sensors [9]) could help mitigate such
saturation.
Comparatively, the performance of our sensor in the initial phases is highly encouraging. Achieving >600% response
to ammonia at room temperature with a simple composite (and without noble metal catalysts) is a testament to the
synergy of the CeO₂–SiO₂ system. Other recent developments, like Fe/Cd co-doped CeO₂, have reported even higher
responses (e.g. a response ~5004 to 200 ppm NH₃) but required careful doping strategies. Our approach uses a green-
synthesized silica to achieve substantial sensitivity, highlighting that even bio-silica can serve effectively in sensor
composites. Under the present test conditions, the sensors prepared using chemical-route silica and green-route silica
showed broadly similar response patterns. However, because the number of tested devices was limited, these results
should be considered a feasibility demonstration rather than proof of equivalent performance. This is an important
validation of the green silica’s applicability. Any slight variations (such as perhaps surface area differences) could
influence absolute response magnitudes, but within the qualitative scope of our study, both routes produced sensors
that behave similarly. The comparison between chemically synthesized silica and green-synthesized silica should be
considered preliminary. In this study, the response trends were compared under the same general fabrication and testing
procedure, but a rigorous statistical comparison was not performed. Multiple sensors from each group, baseline
resistance distribution, mean response values, standard deviation, and significance testing would be required to confirm
whether both routes truly show equivalent performance.
Overall, the results demonstrate that silica–modified CeO₂ composites are capable of detecting ammonia at room
temperature with high sensitivity. The time-evolution of the sensor response provides insight into the surface chemistry
dynamics, and the eventual saturation warns of the need for calibrated operation. These findings contribute to the
understanding of how integrating a sustainable silica source with a metal oxide can yield a functional sensor, while
also pointing out the practical considerations for deploying such sensors in real conditions.
4. Limitations of the Study. - While the silica–ceria NH₃ sensor demonstrated promising performance, several
limitations must be acknowledged:
• Qualitative Gas Exposure: The ammonia sensing tests in this study were conducted without a calibrated gas
flow or known concentration. NH₃ was introduced by evaporation from a solution in a sealed chamber, which
provides an unquantified concentration of ammonia. Consequently, we report response trends (percentage
change in resistance) rather than sensitivity to a specific ppm level. The lack of precise concentration control
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means we cannot construct a response vs. concentration curve or determine a limit of detection. In practical
sensor development, a controlled test (using mass flow controllers to mix known ppm of NH₃ in air) is
essential for calibration. Our results are thus qualitative – they confirm ammonia detection and relative
response behavior, but do not yield data on minimum detectable concentration or linear range. Future work
should include quantitative gas dosing to fully evaluate the sensor’s sensitivity and selectivity.
• Absence of Structural Phase Confirmation (XRD): We did not perform X-ray diffraction analysis on the
synthesized composites. As a result, the crystallographic structure and phase purity of CeO₂ in the composite
were not directly confirmed. We assumed the formation of CeO₂ based on the precursor and conditions, and
the EDS elemental analysis supports this. However, without XRD we cannot ascertain the crystallite size or
whether any Ce silicate phases formed at the interface. The decision to omit XRD was made to focus on
functional testing and because the silica is largely amorphous (which would produce a broad background in
XRD). This is a limitation because crystallinity can affect gas sensing properties. In future studies, performing
XRD would be useful to correlate the sensor performance with any phase information (e.g., confirming
nanoscale CeO₂ fluorite structure). Nonetheless, the significant sensor response observed suggests that the
active CeO₂ phase was present and operative. In addition, SEM and EDS alone could not confirm the CeO₂
crystalline phase, Ce³⁺/Ce⁴⁺ ratio, oxygen vacancy concentration, or silica–ceria interface chemistry. Further
characterization such as XRD, XPS, BET surface area analysis, and interface-focused analysis is important
to fully confirm the structure and sensing mechanism of the composite. BET surface area and pore-size
analysis were not performed in this study. Therefore, the role of silica in increasing surface area, porosity,
and gas diffusion was not directly confirmed by experimental surface area data. In this manuscript, these
effects were discussed only as possible contributions based on the known behavior of silica-containing oxide
composites and the observed SEM morphology. Future work should include BET and pore-size analysis to
directly relate surface area and porosity to NH₃ sensing performance.
• Surface Saturation and Recovery: As observed in the 15 min exposure test, the sensor can become saturated
by ammonia, leading to an inverted (negative) response and incomplete recovery. This indicates a limitation
in long-term stability under continuous exposure. The irreversible adsorption of NH₃ or its by-products on the
sensor surface can degrade performance and require intervention (e.g., cleaning or reactivation by heating in
air). In a practical scenario, sensors would likely be exposed to fluctuating concentrations rather than constant
high levels of NH₃ for 15 minutes, but the result highlights the need for either periodic regeneration or
operational protocols (like exposure cycles within a safe duration). Our study did not investigate the long-
term repeatability or stability beyond the single sequence of exposures. It is possible that repeated
exposure/recovery cycles could gradually change the baseline or response (sensor drift). This was not
characterized here and remains as future work. In addition, because NH₃ concentration and humidity were not
controlled, the negative response after prolonged exposure may also include the effects of baseline drift,
moisture variation, chamber atmosphere changes, or measurement artifacts. The response calculation is also
sensitive to the selected air baseline resistance; therefore, incomplete recovery or baseline drift during the test
may have affected the calculated response values, including the negative response observed after 15 min.
• Lack of Selectivity Tests: We focused solely on ammonia sensing in this work. The selectivity of the silica–
CeO₂ sensor towards NH₃ over other gases (like humidity, CO₂, ethanol, NO₂, etc.) was not evaluated. CeO₂
is known to also respond to other reducing gases (and to a lesser extent oxidizing gases), and the presence of
silica might introduce sensitivity to moisture (due to hydrophilic silanol groups). Without selectivity data, it
is uncertain how the sensor would perform in complex atmospheres. For instance, a real air environment with
humidity could affect the baseline and response amplitude (water molecules can occupy sites or react with
ammonia to form ammonium hydroxide on the surface). Future investigations should include tests with
common interfering gases and varying humidity to determine the sensor’s selectivity profile. Techniques such
as surface functionalization or operating the sensor in pulsed mode (as some studies do) might be required to
enhance selectivity. For room-temperature NH₃ sensing, humidity effects and cross-sensitivity to common
gases are especially important. In this study, a humidity-response test and selectivity matrix against common
interferents were not performed. Therefore, no claim is made regarding humidity tolerance or selective NH₃
detection in complex gas environments. Future work should include testing at different relative humidity
levels and against common interfering gases such as CO₂, ethanol, NO₂, CO, and other volatile compounds
to confirm practical selectivity.
• Reproducibility and Sample Variability: Due to resource constraints, we tested a limited number of sensor
devices. The data presented are representative of the observed behavior, but we have not performed a
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statistical analysis over many devices or multiple batches of materials. There may be variability in the silica
produced by the green route (depending on bagasse source or processing) which could affect sensor
performance consistency. Ensuring reproducible synthesis, especially for the green-silica, is a challenge –
slight changes in bagasse composition or processing temperature could alter the surface area or impurity
content of the silica. Our results show feasibility, but scalability would require careful standardization of the
green synthesis protocol. In addition, device-to-device variability, repeated exposure–recovery cycles,
recovery time after each NH₃ exposure, and aging stability were not evaluated in this study. Therefore, the
reported sensing results should be considered initial proof-of-concept behavior under the tested conditions.
Further work should include multiple devices, cyclic response testing, recovery-time measurement, and long-
term aging studies to confirm reproducibility and practical stability.
In summary, while the study successfully demonstrates a proof-of-concept sensor using green-sourced silica, the above
limitations suggest caution. The device is at a prototype stage: it works under the conditions tested, but further
refinement and characterization (quantitative calibration, selectivity, stability) are needed before it could be deployed
as a reliable NH₃ sensor. Addressing these limitations will be important for future research. For instance, performing
calibrated tests could reveal the actual sensitivity (e.g., what ppm gives a 100% response, etc.) and whether the green-
silica sensor meets required detection limits (typical safety thresholds for NH₃ are tens of ppm). Despite these
limitations, the study provides valuable insights – particularly the observation of time-dependent response evolution –
that can guide the development of robust gas sensors using sustainable materials.
5. Conclusions. - In this work, we synthesized silica nanoparticles via two routes – a conventional chemical method
and an agricultural waste-derived green method – and utilized both types of silica to create silica–modified CeO₂
nanostructures for ammonia gas sensing at room temperature. The study was framed to compare the applicability of
chemically produced versus “green” silica in functional sensor devices. Key conclusions are as follows:
• Sustainable Silica Synthesis: Sugarcane bagasse, an abundant agricultural by-product, was successfully
converted into nanosilica using a simple acid/base extraction approach. The green-sourced silica was obtained
after acid leaching and calcination, and its successful incorporation into the sensor demonstrates the feasibility
of using agricultural waste-derived silica in silica-modified CeO₂ gas sensors. However, because only a limited
number of devices were tested, this study does not claim statistical equivalence between the green and chemical
silica routes. This demonstrates the feasibility of a circular economy approach where agricultural waste is
repurposed into value-added nanomaterials for advanced applications.
• Composite Nanostructure Formation: Both types of silica were effectively integrated with cerium oxide to
form CeO₂–SiO₂ composite nanoparticles. SEM characterization revealed an elongated, rod-like morphology
of ceria on a porous silica matrix. The ceria formed elongated, micron-scale rod-like structures, likely
composed of intergrown nanoscale CeO₂ crystallites rather than isolated particles, indicating strong
interactions between CeO₂ and the silica template. EDS analysis confirmed a silica-rich composition with
cerium present (Si ~42 wt%, Ce ~7 wt%), suggesting a structure where CeO₂ is dispersed as a thin layer or
clusters on silica. The absence of any foreign elements in EDS and the known chemistry confirm that the
composite consisted of SiO₂ and CeO₂ (with oxygen from both), fulfilling the material design. Although XRD
was not performed, the successful sensor performance and literature support imply that the CeO₂ is present in
its active oxide form.
• Room-Temperature NH₃ Sensing Performance: The fabricated silica–ceria sensors showed a strong
chemiresistive response to ammonia at room temperature. Upon NH₃ exposure, the n-type composite’s
resistance decreased substantially, yielding a high gas response. Initial exposures produced responses on the
order of 600–650% within seconds, highlighting the sensor’s high sensitivity and fast kinetics at ambient
conditions. This performance is notable since many pure metal oxide sensors require elevated temperatures to
reach similar sensitivity. The enhanced response is attributed to the synergy of CeO₂’s catalytic redox activity
with silica’s high surface area and morphological stabilization, which together facilitate efficient NH₃
adsorption and reaction at low temperature. We found that the sensor’s response was reproducible across both
silica sources – indicating that green silica is a viable alternative to chemically synthesized silica in this context,
with no observed loss of performance. However, this comparison should be considered preliminary because
multiple devices from each synthesis route were not tested statistically. Further work using several sensors,
baseline resistance comparison, mean response values, standard deviation, and significance testing is required
to confirm whether the green-silica and chemical-silica routes give equivalent sensor performance.
D. Majeed, S. S. Zehra Zaidi, S. M. Mohsin, M. S. Ali Asghar, A. A. Zaidi
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• Time-Dependent Response Evolution: A novel observation in this study is the evolution of the sensor’s
response over prolonged NH₃ exposure (0–15 min). Initially, the sensor’s response was positive and large. As
exposure continued, the response dynamics became complex, showing peaks, partial recovery, and oscillations,
and by 15 minutes the sensor exhibited a negative response (resistance higher in NH₃ than in air). We interpret
this behavior as a result of surface saturation and prolonged electron donation: essentially, the sensor surface
became fully saturated with adsorbates (and oxygen vacancies fully filled), so the normal sensing mechanism
(which relies on available O⁻ species) was impaired. The negative response (~–11%) at 15 min suggests that
further NH₃ caused additional electron accumulation or a change in surface conduction (possibly through the
formation of less-conductive surface compounds), which is consistent with a sensor poisoning or flooding
scenario. This finding underscores the importance of considering exposure time in sensor operation – short
exposures can be reliably detected with big signals, but continuous exposure can lead to signal rollover or drift.
In practical use, either sensors should be regenerated periodically or exposure should be cycled to avoid this
saturation. Our work provides a clear example of this phenomenon for ammonia on a ceria-based sensor,
complementing prior reports of long-exposure effects in other systems.
• Environmental and Practical Implications: The successful use of bagasse-derived silica in a functional sensor
highlights the potential of green nanomaterials in electronic applications. We effectively demonstrated that a
waste product can replace a conventionally produced material without sacrificing device performance. This
aligns with sustainable development goals by reducing the need for hazardous chemicals and leveraging
renewable resources. The sensor developed operates at room temperature, meaning it has low power
requirements and is suitable for ambient monitoring (important for safety in agricultural storage, industrial
refrigeration, etc., where NH₃ leaks are a concern). The high sensitivity observed indicates that even trace
levels of NH₃ (certainly in the low ppm range or below) should be detectable, though calibration is needed.
• Future Work: To move toward practical deployment, future studies should calibrate the sensor response to
known NH₃ concentrations and evaluate its selectivity against other gases (such as humidity, which can be a
significant interferent for metal-oxide sensors). Long-term stability tests, including cyclic exposures and
regeneration techniques, will also be important to address the saturation issue observed. Additionally,
incorporating microheaters for periodic high-temperature pulses or UV illumination could help restore the
sensor surface after exposure, if continuous operation in NH₃ is required. From a materials perspective,
exploring different CeO₂:SiO₂ ratios or doping CeO₂ within this composite (e.g., with a catalyst like Pt or a
dopant to increase vacancy concentration) might further enhance performance or mitigate saturation.
Nonetheless, the core finding remains that integrating green-synthesized silica with ceria yields a high-
performance sensor.
In conclusion, this research demonstrates a viable path for green sensor development: using silica from agricultural
waste to fabricate a sensitive room-temperature gas sensor. The silica–modified CeO₂ composites achieved efficient,
rapid detection of ammonia, comparable to sensors made with conventional materials. The study provides insights into
sensor behavior under extended exposure, a factor often overlooked, by revealing how response can diminish or invert
when the sensor surface becomes saturated. By addressing these insights and limitations, the approach outlined here
can be advanced toward robust, eco-friendly gas sensing systems for environmental monitoring and safety applications.
Acknowledgement. - During the preparation of this work, the authors used ChatGPT 4.0 to refine writing and improve
readability. The authors have reviewed and edited the AI-generated content as necessary and take full responsibility
for the contents of this publication.
D. Majeed, S. S. Zehra Zaidi, S. M. Mohsin, M. S. Ali Asghar, A. A. Zaidi
Memoria Investigaciones en Ingeniería, núm. 30 (2026). pp. 145-163
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Author contribution:
1. Conception and design of the study
2. Data acquisition
3. Data analysis
4. Discussion of the results
5. Writing of the manuscript
6. Approval of the last version of the manuscript
DM has contributed to: 1, 2, 3, 4, 5 and 6.
SSZZ has contributed to: 1, 2, 3, 4, 5 and 6.
SMM has contributed to: 1, 2, 3, 4, 5 and 6.
MSAA has contributed to: 1, 2, 3, 4, 5 and 6.
AAZ has contributed to: 1, 2, 3, 4, 5 and 6.
Acceptance Note: This article was approved by the journal editors Dr. Rafael Sotelo and Mag. Ing. Fernando A.
Hernández Gobertti.
