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Article

CO2 Interaction Mechanism of SnO2-Based Sensors with Respect to the Pt Interdigital Electrodes Gap

by
Adelina Stanoiu
1,
Alexandra Corina Iacoban
1,
Catalina Gabriela Mihalcea
1,2,
Ion Viorel Dinu
1,
Ovidiu Gabriel Florea
1,
Ioana Dorina Vlaicu
1 and
Cristian Eugen Simion
1,*
1
National Institute of Materials Physics, Atomistilor 405A, 077125 Magurele, Romania
2
Faculty of Physics, University of Bucharest, Atomistilor 405, 077125 Magurele, Romania
*
Author to whom correspondence should be addressed.
Chemosensors 2024, 12(11), 238; https://doi.org/10.3390/chemosensors12110238
Submission received: 24 September 2024 / Revised: 8 November 2024 / Accepted: 14 November 2024 / Published: 16 November 2024
(This article belongs to the Special Issue Advanced Chemical Sensors for Gas Detection)
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Abstract

:
The tuning sensitivity towards CO2 detection under in-field-like conditions was investigated using SnO2-sensitive material deposited onto Al2O3 substrates provided with platinum electrodes with interdigital gaps of 100 µm and 30 µm. X-ray diffraction, low-magnification and high-resolution transmission electron microscopy, and electrical and contact potential difference investigations were employed to understand the sensing mechanism involved in CO2 detection. The morpho-structural analysis revealed that the SnO2 nanoparticles exhibit well-defined facets along the (110) and (101) crystallographic planes. Complex phenomenological investigations showed that moisture significantly affects the gas sensing performance. The experimental results corroborated the literature evidence, highlighting the importance of Pt within the interdigital electrodes subsequently reflected in the increase in the CO2 sensing performance with the decrease in the interdigital gap. The catalytic efficiency is explained by the distribution of platinum at the gas-Pt-SnO2 three-phase boundary, which is critical for enhancing the sensor performance.

Graphical Abstract

1. Introduction

The origin of metal oxide semiconductor (MOS)-based gas sensors may be traced back to the early 1970s, when Taguchi introduced the first gas sensing device utilising tin dioxide (SnO2) [1]. In addition to its use as a transparent coating element for electrode material in solar cells or displays, SnO2 is primarily involved in gas sensing and catalysis. The SnO2-based sensors have been utilised in various industrial sectors, including automotive, chemical, environmental detection and control, food, medicine, military, and safety. The initial requirement involves monitoring the emissions from the vehicle, which mostly consist of carbon in the form of small particles, unburned hydrocarbons, carbon dioxide, carbon monoxide, nitrogen oxides, sulphur oxides, water vapour, and various other low-level chemicals. The need to monitor the levels of different gases has led to extensive research and development efforts to create sensors based on SnO2 that are widely applicable in various fields [2]. Moreover, there are plenty of studies related to doped or un-doped SnO2 specific to the aforementioned target sectors. When it comes to the detection of carbon dioxide (CO2), MOS-based gas sensors are almost insensitive under normal working in-field conditions (e.g., the presence of oxygen and relative humidity—RH).
Carbon dioxide is an inert gas that absorbs short-wavelength light, contributing significantly to the greenhouse effect. It is commonly found in the atmosphere at concentrations between 300 and 400 parts per million (ppm). As the concentration of CO2 in the environment rises, the human body will progressively encounter discomfort, difficulty breathing, impaired sensory function, dizziness, shock, and perhaps death [3]. Monitoring the concentration of CO2 in respiration can provide valuable insights into respiratory status and circulation disorders [4]. It can also help in the timely detection of respiratory dysfunction and other abnormal situations during general anaesthesia, making it highly valuable in both the operating room and the intensive care department [5]. From an industrial perspective, the extensive utilisation of fossil fuels leads to an increase in the concentration of CO2 in the atmosphere. Hence, the monitoring of CO2 is interconnected with the well-being, health, and safety of individuals, as well as the government’s oversight of carbon emissions and precise decision-making regarding energy structure regulation [6].
Currently, CO2 detection is performed with optical sensors [7], resistive sensors [8], potentiometric sensors [9], amperometric sensors [10], capacitive sensors [11], surface acoustic wave sensors [12], and quartz crystal microbalance sensors [13], among others. Chemical gas sensors, which rely on the changes in the electrical resistance (conductance) of MOS under variable surrounding conditions, are the simplest, cheapest, and most user-friendly alternative [14]. In this respect, binary and ternary MOSs with high porosity, a large specific surface area, and defect structures have attracted the attention of researchers. For instance, ternary metal oxides exhibit Schottky, Frenkel, and interstitial atom defects, due to variations in crystal structure, cation species, and valence states. In addition, the presence of doped cations occupying the A/B sites will result in the formation of certain non-intrinsic defects, which in turn contribute to the absorption of the target gas. The primary ternary metal oxide materials utilised for CO2 sensing are the perovskite structure (ABO3) and spinel structure (AB2O4) oxides. These include barium titanate (BaTiO3), lanthanum ferrite (LaFeO3), nickel ferrite (NiFe2O4), and magnesium ferrite (MgFe2O4). Typically, they are manufactured to create integrated structures or heterostructures by combining them with metal oxides to improve their gas-sensing abilities [15,16]. In the work of Chavali et al., metal oxide nanoparticles are described by their gas-sensing performance concerning hierarchical structure, shape, and size. Not only the gas-sensing applications are discussed, but also their complex applications in nanotechnologies [17]. Unfortunately, the atmospheric relative humidity (RH) significantly affects the absorption and release of carbon dioxide. The impact of RH on the sensing mechanism was investigated using density functional theory (DFT). Under conditions of low humidity (RH < 35%), the interaction between carbon dioxide (CO2) and tin dioxide (SnO2) with pre-adsorbed oxygen ions (O2− and O−) from the (110) crystal surface was minimal. At a higher RH, CO2 molecules were adsorbed onto the (110) crystal surface of SnO2. During this process, CO2 reacts with OH− ions (from the dissociative adsorption of water) and leads to the formation of carbonate. Additionally, an exchange of electrons between CO2 and SnO2 occurs. The results suggest that the pre-adsorbed OH− on the (110) crystal facet enhances the CO2 sensing properties [18]. This indicates that certain crystal surfaces have a strong ability to adsorb or react with various atomic, ionic, and molecular groups. Xiong et al. investigated the sensing properties of SnO2 films doped with La, Gd, and Lu, as well as pure SnO2 films, for the detection of CO2 in an environment devoid of oxygen [19]. La significantly enhanced the performance of CO2 sensing, with the highest performance observed when the La doping content reached 4%. The rate at which carbonates develop and the amount of oxygen vacancies have a significant impact on the features of gas sensing. Consequently, the researchers analysed the XPS spectra of the O 1 s and C 1 s samples. They then determined the ratio of peak areas for residual carbon (organic carbon pollution) and carbonates, as well as the ratio of peak areas for oxygen (O− and O2−). The optimal doping ratio was determined to be 8 at. % when the concentrations of O2 in the carrier gas reached 21% (in-field oxygen level), and it should increase to 16 at.% in the presence of free oxygen.
In the present study, we aimed to address a realistic scenario of the gas-surface interaction mechanisms involved in CO2 detection under in-field-like conditions characterised by the presence of oxygen and variable relative humidity. Therefore, we performed DC electrical investigations and simultaneous electrical resistance measurements assisted by the contact potential differences on planar sensors, which were obtained by screen-printed SnO2 thick layers on substrates with different interdigital gaps of 100 and 30 µm, respectively.

2. Materials and Methods

2.1. Powders Synthesis

As previously reported [20], a sodium hydroxide solution was added dropwise over a solution of tin (IV) chloride containing CTAB (hexadecyltrimethylammonium bromide). The mixture was stirred for half an hour at room temperature, was afterward sealed in a Hydrothermal Synthesis Autoclave Reactor with a PTFE Lined Vessel of 100 mL, and was left to react at 160 °C for 18 h. SnO2 powder was isolated by centrifugation, washed repeatedly, and air-dried at 120 °C. To complete the synthesis process, the dried powder was thermally treated at 550 °C. The chemicals were used as purchased, without any further purification: tin (IV) chloride (SnCl4, 99.999%, Acros Organics, Geel, Belgium), sodium hydroxide (NaOH, >98%, Honeywell Fluka, Seelze, Germany), and hexadecyltrimethylammonium bromide (CTAB, 99%, Acros Organics, Geel, Belgium). A final thermal treatment at 550 °C has completed the preparation processes.

2.2. Structural and Morphological Investigations

Structural investigations were performed using powder X-ray diffraction with a D8 ADVANCE diffractometer (BRUKER-57AXS GmbH, Karlsruhe, Germany) with Ni-filtered Cu radiation (λ = 1.54184 Å) at room temperature in Bragg–Brentano geometry in the range of 2θ from 20° to 95°. The XRD data were analysed using the Rietveld refinement with MAUD version 2.99 software to determine the lattice parameters and average crystallite size.
The morphology of the SnO2 sample was investigated using a JEOL JEM-ARM200F transmission electron microscope (JEOL LTT., Japan, Tokyo), operated at 200 kV. To prepare the TEM sample, a small quantity of the powder was mixed with ethanol, then it was ultra-sonicated for 10 min, and a small droplet from this mix was drop-cast on a TEM copper grid provided with a carbon membrane.

2.3. Layer Deposition and Sensing Investigations

The layer deposition process is the subject of a national patent application, OSIM/Nr. A 00110/18 March 2024. In brief, the as-prepared sensitive powders were combined with 1,2 Propanediol and grounded to produce a paste with a medium viscosity. Subsequently, the paste was applied onto commercial alumina substrates using the screen-printing technique to form thick layers. The sensors obtained were gradually dehydrated and thermally treated at 500 °C in air. This procedure enables the thorough elimination of the organic solvent, guaranteeing the porosity of the layer and enhancing its adhesion to the substrate. The alumina substrates are made using planar technology and are provided with platinum electrodes and a heater on the backside. The interdigital electrodes on the substrate have gaps measuring 100 μm and 30 μm, respectively. The obtained samples were labelled “SnO2 100 μm” and “SnO2 30 μm”. The electrical power through the heater is adjusted to control the temperature of the sensor layer, which in turn modulates the chemical interaction between the MOS layer and the test gases. The substrates were passivated with glass on both sides to avoid parasitic catalytic effects, thus highlighting the catalytic processes that are taking place solely on the sensitive layer or at the three-phase boundary (e.g., gas–electrodes–SnO2).
A Gas Mixing System (GMS) was used to simulate the in-field atmosphere (Figure 1). The GMS is managed by specialised software and comprises eleven gas channels fitted with mass flow controllers, solenoid valves, and high-purity gas bottles. The system functions in a dynamic state, maintaining a total flow rate of 200 mL/min. A relative humidity (RH) is accomplished using a distinct pathway involving a gas-washing bottle containing moisture. The real-time acquisition of the electrical resistance changes in sensors positioned inside the sensor chamber was performed using a dedicated electrometer Keithley 6517A (Tektronix, OHIO, Solon, OH, USA) operated under constant 3V bias, while the contact potential differences (CPD) were recorded using a McAllister 6500 Kelvin Probe (McAllister Technical Services, CALIFORNIA, Berkeley, CA, USA).

3. Results and Discussions

3.1. XRD Characterisation

The powder X-ray diffraction (XRD) of the SnO2 sample prepared by the hydrothermal method at a temperature of 160 °C presents the characteristic diffraction lines of the SnO2 (Figure 2). It exhibits a tetragonal structure with a symmetry space group of P42/mnm, according to CIF no. 1526637 from the COD database.
Through Rietveld refinement, using the MAUD version 2.99 software, the lattice parameters and the average crystallite size of 21.47 ± 0.19 nm were determined. The crystallographic parameters for the lattice are slightly different than the reference data.

3.2. Analytical TEM Characterisation

The low-magnification TEM images were used to characterise the sample’s morphology. Figure 3a shows that the sample consists of nanoparticles with the following different dimensions: smaller quasi-spherical nanoparticles (having ~9 nm) and larger elongated nanoparticles (having ~35 nm).
The structure of the sample was determined using selected area electron diffraction (SAED). The SAED pattern shown in Figure 3b is typical for a polycrystalline material, and it reveals the tetragonal structure of SnO2, with the space group P42/mnm. The white arrows are pointing towards crystallographic planes (110), (101), and (211), as indexed according to cif no. 2104754 from COD database.
The high-resolution TEM (HRTEM) investigations revealed the presence of (110) and (101) crystallographic facets (Figure 4). Kuncser et al. [20], using VESTA modelling, showed that (110) and (101) facets mostly have oxygen terminations.
Figure 4a shows two nanoparticles having the edges along (110) crystallographic planes. The double white lines in Figure 4b indicate the (110), (10-1), and (011) crystallographic planes of a nanoparticle oriented along the [1,2,3,4,5,6,7,8,9,10,11] axis.
The average dimension was determined by measuring ~400 nanoparticles using low-magnification TEM images similar to the TEM image shown in Figure 3a. The size distribution is shown in Figure 5. After fitting the as-obtained histogram with a log-normal function, we obtained a mean value of 15.4 nm and a standard deviation of 6.9 nm. Within the error limits, the mean value of the particle size obtained from TEM is in accordance with the value of the crystallite size obtained from Rietveld refinement.
In conclusion, the morpho-structural TEM investigations have shown that the sample consists of nanoparticles with different dimensions, having well-defined facets along (110) and (101) crystallographic planes [21,22,23].

3.3. Sensing Characterisation

The response of MOS sensors depends on the operating temperature of the gas-sensitive material, which determines the adsorption of atmospheric oxygen in its molecular or ionic forms. The response capacity decreases at higher working temperatures due to the different rates of the adsorption/desorption processes [24]. Consequently, we conducted an initial experiment to observe the changes in the sensor signal as the operating temperature varied in the range of 100–400 °C for sensors based on SnO2 thick layers deposited onto interdigital Pt electrodes with alternative gaps of 100 and 30 μm. The sensor signal was determined by calculating the ratio of the electrical resistance measured under a reference atmosphere (e.g., a synthetic air 5.0 background with 50% relative humidity) to the electrical resistance measured while exposed to 3000 ppm of CO2 under the same RH conditions. The highest level of sensitivity was achieved at an operating temperature of 350 °C (Figure 6a).
When the sensors operate at moderate temperatures, oxygen physisorption on the MOS surface is generally accepted. This process does not entail any transfer of electrical charge from the sensitive material to the adsorbed molecular oxygen. When the temperature rises, oxygen chemisorption occurs at the surface of the SnO2 grains. Consequently, electron-depleted layers are formed near the surface of the grains due to the electron transfer from the sensitive material to the adsorbed O− oxygen species. In terms of energy, the depleted layers act as potential barriers, impeding the flow of electrons between the boundaries of the grains [25].
Considering the interdigital gap as d ≈ N × g (where N is the number of grains and g is the average grain size), the larger d is, the more grains we have and thus the more intergranular barriers. From the electrical point of view, this situation is associated with electrical resistance in dry air conditions, explaining the higher resistance in the case of SnO2 deposited on the substrate with an interdigital gap of 100 μm (Figure 6b).
In humid conditions, the electrical resistance decreases due to the known reducing effect of RH [26].
2Sn + O− + H2O → 2(Snδ+ − OHδ−) + e−
A similar behaviour is related to the presence of CO2, although it is known as a rather inert gas [27]. Herein, the resistance of SnO2 decreases under CO2 exposure, suggesting interactions with previously chemisorbed O− (Equation (2)) or OH− (Equation (3)) species, leading to the release of electrons in CB.
Sn + O− + CO2 → Sn − CO3− + VO+ + e−
(Snδ+ − OHδ−) + CO2 → Sn + HCO3− + VO2+ + 2e−
Translating the electrical resistance changes in the sensor signals for both sensitive structures (Figure 6c,d), it can be observed that the smaller interdigital gap (30 µm), the higher the sensor signal is. Moreover, one can see that both sensors exhibit a special relation with water, as long as the CO2 sensitivity increases in the presence of RH compared to dry air. Complex phenomenological investigations (e.g., simultaneous electrical resistance and contact potential differences) provide the perspective of an explanation regarding this behaviour (Figure 7).
The CPD measurements provide insights into the changes in work function (ΔΦ), allowing us to discriminate between physisorption (associated with changes in the electronic affinity, Δχ) and ionosorption (associated with changes in the surface potential barrier, qΔVs). Considering the electrochemical potential (μ) of the SnO2 constant, we can write
q × ΔCPD = ΔΦ = Δχ + qΔVs = Δχ − kBTlnRair/Rgas
where q is the electron charge, ΔCPD is the contact potential difference measured with the Kelvin Probe, kB is the Boltzmann constant, T is the operating temperature, Rair is the electrical resistance measured under humid synthetic air, and Rgas is the electrical resistance measured under CO2 exposure.
As shown in Figure 7a,b, ΔΦ does not vary because it is the same SnO2 material deposited onto substrates with different interdigital gaps. On the other hand, the greater the water effect (Δχ), the greater the potential barrier variation (qΔVs).
At this point, it is important to understand whether the differences induced by the interdigital gap are determined by their electrical contribution to the total resistance of the sensors or by the catalytic activity of the platinum electrodes.
The first hypothesis is supported by the geometrical approach of a SnO2 polycrystalline sensitive material, considering incompletely depleted grains, arranged between Pt electrodes with interdigital gaps (L) of 100 and 30 µm (Figure 8a). The so-called surface state model [28] was used to represent the surface and bulk energetic dependencies with respect to the percolation path phase of the electric current (Figure 8b).
For gas sensors consisting of a thick polycrystalline-sensitive MOS layer, the conduction mechanism is dominated by surface phenomena. Exposure to an in-field-like atmosphere and operating temperature favours oxygen and water vapour chemisorption, determining the covering of the surface with the negative charge. This causes the appearance of a depletion layer near the surface due to the Coulomb repulsion towards the electrons in the bulk. Thus, the conduction is limited by the Schottky barriers (qVs) formed at the grain boundaries. In terms of the electrical equivalent circuit (EEC), the surface is associated with a resistance Rs in parallel with a capacity Cs (Figure 8c). These are controlled by the charging/discharging of the surface traps which are directly modulated by the surface chemical gas interactions [29].
The resistance Rb corresponds to the electrons in the bulk and is weakly influenced [30]. The SnO2-Pt electrode contact is associated with the RcCc parallel circuit.
The electrons will flow from the semiconductor towards the metal because the SnO2 work function (4.53 eV) is lower than the Pt work function (5.64 eV) [31,32]. Accordingly, the energy bands bend up due to the depletion of electrons in the semiconductor, and the electrons flowing towards the Pt electrode must cross over the potential barrier qVc, as in the case of the grain-to-grain Schottky element. But, taking into account that the Pt resistance is much lower than the SnO2 surface resistance (Rc « Rs), its contribution can be neglected, whether the interdigital gap L is 100 or 30 µm.
By close observation and through the aforementioned reasons, the electrical influence of the interdigital electrodes on a thick film gas sensor can be considered negligible.
The second hypothesis is supported by the known spill-over effect of Pt [33,34]. In brief, when a reducing gas is adsorbed on Pt, its dissociation is activated, followed by migration (spill-over) on the MOS surface where it interacts with oxygen, increasing the conductivity in the case of an n-type semiconductor [35].
In addition to the effects induced by impregnation/doping with noble metals, the role of Pt electrodes must be considered in terms of the geometric and catalytic effects. Several authors [36,37] have pointed out the significant role of the geometry influence of the electrodes placed at the bottom region of the sensitive MOS layer. When a certain gas molecule hits the MOS surface, it diffuses through the porous film and induces an electrical effect at the bottom electrodes (so-called “three-phase boundary regions”). Starting from the empiric demonstration of the differences in sensitivity induced by the variation in the electrode gaps, Hoefer et al. [38,39] propose a rigorous work aiming to establish precisely the role of interdigital geometry on the sensing performance. As such, a sensor array with a highly asymmetric electrode configuration could develop a low-cost sensor system with improved selectivity (e.g., the selective detection of NO2 versus CO by varying the interdigital electrode gaps).
On the other hand, the Pt electrode shows increased activity in the direct conversion of CO2, with implications on the electrical properties of the SnO2 sensitive layer [40]. Besides direct conversion, the spillover effect activates the gas adsorption on the Pt, which subsequently leads to the diffusion or migration of the adsorbed species on the neighbouring gains [41]. By spanning the literature, one can find that the chemical activation of CO2 as a quite inert gas is still a matter of debate, especially with MOS-based gas sensors. The complex work of Wang et al. [42] stated that Pd and Pt represent the most favourable metals for CO2 conversion. The role of Pt as a single-atom catalyst has attracted extensive attention due to the superior catalytic performance towards CO2 conversion via a reverse water gas shift reaction (RWGS). Thus, the work of He et al. [43] presented a simple route to load 6.4 wt %Pt on silicon carbide with superior catalytic properties. Mondoza-Núñez et al. [44] investigated the effect of Pt addition on CO2 methanation catalysed by ZrO2-supported Rh. Quite similar results have been obtained by Ramos Gonzalez et al. [40] on pristine Pt-catalysed SnO2 hybrid nanostructures when operated at low temperatures (100 °C), envisaging a gas sensing mechanism based on the adsorption of OH− ions onto the SnO2 surface. The sensing mechanism was attributed to the electron transfer from the porous silicon-based band to the conduction band of the SnO2. In the case of ZnO nano-ribbons [45], the role of Pt was used alongside Pd, Fe, Ag, and Au as simple adatom catalysts towards sensing H2, H2S, and CO2.
With the aforementioned review of the literature and considering the technological details of our samples (Figure 9), it is beyond doubt that the noble parts of the sensor (e.g., at the three-phase boundary between gas–Pt interdigital electrodes and SnO2) have a great share in the catalytic activity of the CO2 gas.
Specifically, the thickness of the SnO2 layer (2.42 μm) and the Pt electrodes (3.51 μm) was determined with the Ambios XP-100 Profilometer, and their ratio suggests that the dominating amount of Pt controls the overall sensing effect.
It is worth mentioning that besides the Pt interdigital electrodes (e.g., where solely catalytic effects take place), other Pt regions are covered with glass to avoid parasitic CO2 catalytic effects.
In this context, we strongly believe that the sensitivity to CO2 increases with the proportion of Pt in the sensitive structure. This explains the higher sensor signal when SnO2 is deposited over electrodes with an interdigital distance of 30 µm, compared to 100 µm.
These results are consistent with the results from the literature which highlights the role of Pt in CO2 reactivity; although, in the field of chemical sensors, the findings are less true for MOS-sensitive materials.

4. Conclusions

SnO2 powder was obtained by hydrothermal growth at a temperature of 160 °C. A final thermal treatment at 550 °C has completed the synthesis processes. X-ray diffraction has shown a tetragonal structure, and the Rietveld refinement revealed a crystallite size of ~21.5 ± 0.19 nm. The morpho-structural TEM/HRTEM investigations highlight nanoparticles with different dimensions, having a mean value of 15.4 nm for a standard deviation of 6.9 nm and well-defined facets along (110) and (101) crystallographic planes. The SnO2 powder was transformed into a paste by mixing it with 1,2 Propanediol, and then it was screen-printed as a thick layer onto commercial Al2O3 substrates having Pt electrodes with alternative interdigital gaps of 100 µm and 30 µm. The obtained sensors labelled “SnO2 100 μm” and “SnO2 30 μm” were thermally treated at 500 °C in air to eliminate the organic solvent and ensure SnO2 adhesion to the substrate. The electrical power through the substrate backside heater controlled the temperature of the SnO2 layer, favouring the chemical interaction with gases. A Gas Mixing System was used to simulate the in-field-like test atmosphere (synthetic air having different relative humidity and CO2 concentration). The contact potential differences (CPDs) induced by the chemical interactions on the SnO2 surface were recorded using a McAllister 6500 Kelvin Probe. Thus, it was possible to differentiate between water physisorption associated with changes in the electronic affinity and oxygen ionosorption associated with changes in the surface potential barrier and, consequently, with the electrical resistance. The differences induced by the interdigital gap over the sensor signal to CO2 were analysed in terms of the electrical equivalent circuit and catalytic activity of the platinum electrodes. The thickness ratio of 2.42 μm/3.51 μm between the SnO2 layer and the Pt electrodes allowed us to conclude that the amount of Pt specific for a 30 μm interdigital gap determines the increase in the sensing response to CO2 in accordance with the catalytic effect reported in the literature.

Author Contributions

Methodology, writing—review and editing, funding acquisition, A.S.; investigation, writing—original draft, A.C.I.; investigation, writing—original draft, C.G.M.; formal analysis, writing—review and editing, I.V.D.; investigation, validation, O.G.F.; methodology, writing—original draft, I.D.V.; conceptualization, writing—original draft, writing—review and editing, C.E.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Research, Innovation and Digitization CNCS-UEFISCDI through the project PN-III-P4-PCE-2021-0384 within PNCDI III and the Core Program within the National Research Development and Innovation Plan 2022–2027, carried out with the support of the Ministry of Research, Innovation and Digitization, project no. PC1-PN23080101. The contribution of Catalina G. Mihalcea (C.G.M.) to this work is part of the PhD “Nanostructured materials for gas sensing: correlations between functional, electronic and microstructural properties” supported by CERIC-ERIC.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available on request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Gas Mixing System provided with a Kelvin probe involved in DC and contact potential difference measurements (a), a sensor chamber (b), and a PC dedicated to evaluation and control (c).
Figure 1. Gas Mixing System provided with a Kelvin probe involved in DC and contact potential difference measurements (a), a sensor chamber (b), and a PC dedicated to evaluation and control (c).
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Figure 2. XRD pattern of SnO2.
Figure 2. XRD pattern of SnO2.
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Figure 3. TEM image of SnO2 (a) and the corresponding SAED pattern, revealing the tetragonal structure of SnO2 (b).
Figure 3. TEM image of SnO2 (a) and the corresponding SAED pattern, revealing the tetragonal structure of SnO2 (b).
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Figure 4. HRTEM images showing faceted nanoparticles having different dimensions.
Figure 4. HRTEM images showing faceted nanoparticles having different dimensions.
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Figure 5. SnO2 nanoparticles size distribution histogram fitted using a log-normal function.
Figure 5. SnO2 nanoparticles size distribution histogram fitted using a log-normal function.
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Figure 6. The dependence of the sensor signal on the operating temperature (a), the behaviour of the electrical resistance in the atmosphere with variable RH and CO2 concentrations for an operating temperature of 350 °C (b), and the sensor signal to CO2 for SnO2 100 µm versus SnO2 30 µm (c,d).
Figure 6. The dependence of the sensor signal on the operating temperature (a), the behaviour of the electrical resistance in the atmosphere with variable RH and CO2 concentrations for an operating temperature of 350 °C (b), and the sensor signal to CO2 for SnO2 100 µm versus SnO2 30 µm (c,d).
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Figure 7. CO2 influence over the potential changes: SnO2 100 µm (a) and SnO2 30 µm (b) for Top = 350 °C and 50% RH.
Figure 7. CO2 influence over the potential changes: SnO2 100 µm (a) and SnO2 30 µm (b) for Top = 350 °C and 50% RH.
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Figure 8. Schematic representation of the sensitive structure based on SnO2 deposited on a substrate with Pt electrodes with different interdigital gaps L (a), energy band bending (b), and the equivalent electrical circuit (c).
Figure 8. Schematic representation of the sensitive structure based on SnO2 deposited on a substrate with Pt electrodes with different interdigital gaps L (a), energy band bending (b), and the equivalent electrical circuit (c).
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Figure 9. Planar substrate sensor overview. Side a represents the Pt heater meander (a) and sensor components parts (b); Side b1 represents the interdigital electrodes with a 100 μm gap (c); Side b2 represents the interdigital electrodes with a 30 μm gap (d) and a cross-section of the sensor (e).
Figure 9. Planar substrate sensor overview. Side a represents the Pt heater meander (a) and sensor components parts (b); Side b1 represents the interdigital electrodes with a 100 μm gap (c); Side b2 represents the interdigital electrodes with a 30 μm gap (d) and a cross-section of the sensor (e).
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MDPI and ACS Style

Stanoiu, A.; Iacoban, A.C.; Mihalcea, C.G.; Dinu, I.V.; Florea, O.G.; Vlaicu, I.D.; Simion, C.E. CO2 Interaction Mechanism of SnO2-Based Sensors with Respect to the Pt Interdigital Electrodes Gap. Chemosensors 2024, 12, 238. https://doi.org/10.3390/chemosensors12110238

AMA Style

Stanoiu A, Iacoban AC, Mihalcea CG, Dinu IV, Florea OG, Vlaicu ID, Simion CE. CO2 Interaction Mechanism of SnO2-Based Sensors with Respect to the Pt Interdigital Electrodes Gap. Chemosensors. 2024; 12(11):238. https://doi.org/10.3390/chemosensors12110238

Chicago/Turabian Style

Stanoiu, Adelina, Alexandra Corina Iacoban, Catalina Gabriela Mihalcea, Ion Viorel Dinu, Ovidiu Gabriel Florea, Ioana Dorina Vlaicu, and Cristian Eugen Simion. 2024. "CO2 Interaction Mechanism of SnO2-Based Sensors with Respect to the Pt Interdigital Electrodes Gap" Chemosensors 12, no. 11: 238. https://doi.org/10.3390/chemosensors12110238

APA Style

Stanoiu, A., Iacoban, A. C., Mihalcea, C. G., Dinu, I. V., Florea, O. G., Vlaicu, I. D., & Simion, C. E. (2024). CO2 Interaction Mechanism of SnO2-Based Sensors with Respect to the Pt Interdigital Electrodes Gap. Chemosensors, 12(11), 238. https://doi.org/10.3390/chemosensors12110238

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