Selective CO₂ Detection at Room Temperature with Polyaniline/SnO₂ Nanowire Composites

Abstract

In this study, tin oxide (SnO₂)/polyaniline (PANI) composite nanowires (NWs) with varying amounts of PANI were synthesized for carbon dioxide (CO₂) gas sensing at room temperature (RT, 25 °C). SnO₂ NWs were fabricated via the vapor–liquid–solid (VLS) method, followed by coating with PANI. CO₂ sensing investigations revealed that the sensor with 186 μL PANI exhibited the highest response to CO₂ at RT. Additionally, the optimized sensor demonstrated excellent selectivity for CO₂, long-term stability, and reliable performance across different humidity levels. The enhanced sensing performance of the optimized sensor was attributed to the formation of SnO₂-PANI heterojunctions and the optimal PANI concentration. This study underscores the potential of SnO₂-PANI composites for CO₂ detection at RT.

1. Introduction

The greenhouse effect, driven by industrial and human activities, contributes to global warming, altering climatic patterns and negatively impacting ecosystems, agriculture, and human health [1,2]. CO₂, primarily produced through fossil fuel consumption, is a major greenhouse gas responsible for about 76% of the effects of global warming. Elevated CO₂ levels absorb reflected short-wavelength radiation, raising Earth’s temperature and disrupting climate stability, leading to events like droughts, floods, and polar ice melting [3,4]. High indoor CO₂ concentrations, as per ASHRAE standards, should not exceed 1000 ppm to avoid adverse health effects such as headaches, dizziness, and respiratory issues [5]. Additionally, CO₂ monitoring is essential across industries, including healthcare, biotechnology, and food production, where even slight concentration changes impact health and process outcomes [6,7]. In healthcare, for instance, sensitive CO₂ sensors are used in capnography and astronaut safety on space missions, while the food industry relies on CO₂ detection for quality control in modified atmosphere packaging [8,9]. Traditional CO₂ detection methods, such as infrared spectrometry and gas chromatography, though effective, face limitations like high costs, bulkiness, and power consumption, making them less suitable for new applications that demand compact, low-cost, low-power sensors [10]. Consequently, recent developments focus on enhancing CO₂ sensors by using nanostructured materials, metal-oxide composites, and polymeric materials that operate effectively at room temperature. These advancements address challenges in response speed, sensitivity, and power efficiency, which are essential for widespread CO₂ monitoring applications [11,12].

Single-component materials in gas sensing often face limitations in sensitivity, selectivity, stability, and energy efficiency, particularly when operating at room temperature. These limitations arise because individual materials may lack sufficient charge transfer properties or effective adsorption sites for diverse gas molecules [13,14]. For instance, traditional metal oxides often require high operational temperatures to achieve adequate gas response, increasing power consumption and reducing device lifespan [15]. To overcome these shortcomings, recent research has focused on creating heterojunction-based composite materials, which combine distinct materials with complementary properties to enhance gas sensing performance [16]. Among various composite configurations, organic–inorganic hybrids—especially those combining metal oxide semiconductors with conducting polymers—have emerged as a prominent approach. These hybrids utilize the benefits of both organic and inorganic components, providing a more robust sensing platform with improved sensitivity, selectivity, and stability in ambient conditions [17,18].

Polyaniline (PANI) is a widely studied conducting polymer in gas sensing due to its unique electrical, redox, and doping properties, which allow for high conductivity modulation upon exposure to gases. PANI operates effectively at room temperature, making it energy efficient and practical for applications that demand real-time sensing [19]. The polymer’s conductivity is easily tuned through protonic acid doping, enabling rapid responses to gas molecules. PANI’s low synthesis cost, environmental stability, and ease of deposition on various substrates add to its appeal for scalable sensor production [20]. These attributes have established PANI as a preferred material for gas sensors, especially for detecting gases like ammonia (NH₃) and nitrogen dioxide (NO₂). However, despite its advantages, PANI alone may struggle with achieving high selectivity and long-term stability under diverse environmental conditions [21].

SnO₂ is an n-type semiconductor with a wide band gap, known for its high chemical stability, electron mobility, and effective gas adsorption capacity, especially for reducing gases [22,23]. SnO₂ has been extensively used in gas sensing applications due to its robust response characteristics; however, its performance is generally optimal at elevated temperatures (300–450 °C), which increases energy demands and shortens device lifespan [24]. High-temperature operation, while effective in enhancing sensitivity, limits the applicability of SnO₂ sensors in energy-sensitive and room-temperature applications. Nonetheless, SnO₂’s inherent properties make it a strong candidate for hybridization with conducting polymers, where it can contribute enhanced gas-sensing responsiveness and durability [25].

In this work, the PANI/SnO₂ composite leverages the ambient-temperature operability and tunable conductivity of PANI along with the stability and high electron mobility of SnO₂. This combination enables the formation of a pn heterojunction, which improves charge transfer and response sensitivity to CO₂ gases at RT. Consequently, the PANI/SnO₂ hybrid structure provides a promising solution to the limitations of single-material sensors, offering enhanced performance for advanced, efficient, and versatile gas sensing applications.

2. Materials and Methods

2.1. Materials

Tin oxide (SnO₂, ≥99%), aniline (≥99.5%), ammonium persulfate (APS, ≥98%), and hydrochloric acid (HCl, 36.5–38%) were procured from Sigma-Aldrich (Saint Louis, MO, USA). High-purity gases (99.99%) used to assess sensing performance were supplied by Samjung Special Gas (Incheon, Republic of Korea), while ethanol (99.5%) was obtained from Deajung Chemical (Siheung, Republic of Korea). All other chemicals and reagents were used as received without further purification unless otherwise noted.

2.2. Synthesis of SnO₂ NWs

SnO₂ NWs were synthesized using a vapor–liquid–solid (VLS) mechanism. This process began with the deposition of a 3 nm Au interdigital electrode (IDE) onto a SiO₂-coated wafer, as shown in Figure 1. For the synthesis, 1 g of high-purity Sn powder was placed in a 25 mL alumina crucible along with the IDE chip, which was positioned carefully inside. The crucible was sealed and situated at the center of a horizontal tube furnace, where the internal pressure was maintained at a low 1 mTorr. The temperature was then gradually increased to 950 °C without the use of any additional gases. Once the target temperature was reached, a controlled gas flow of 2 sccm O₂ and 48 sccm N₂ was introduced, and this environment was maintained for 15 min to ensure thermal stability. After this period, the gases were turned off, and the chamber was allowed to cool naturally to room temperature (RT). A unique white, velvet-like layer was subsequently observed on the Au-coated substrate surface.

2.3. Preparation of PANI/SnO₂ Composites

Figure 1 illustrates the fabrication process of PANI/SnO₂ composites. PANI was synthesized using an in situ oxidative polymerization method in an ice-water bath maintained at 0−5 °C. For a typical synthesis, an IDE chip with white SnO₂ NWs was immersed in 50 mL of 1 M HCl solution, followed by the addition of 186 μL of aniline. Subsequently, 10 mL of 1 M HCl containing APS was added to the solution. The reaction mixture was kept in an ice-water bath, and the final products were collected by centrifugation. The resulting dark green precipitate was washed several times with deionized water and ethanol to remove any unreacted Cl⁻ ions. The product was then filtered and dried at 80 °C for 5 h, yielding the PANI/SnO₂ nanocomposites. To prepare nanocomposites with varying concentrations of aniline, including 0, 20.3 mM, 40.7 mM, and 61.0 mM, the above procedure was repeated. Four nanocomposites were thus synthesized and labeled SP0, SP1, SP2, and SP3, according to the different aniline concentrations in the solutions.

2.4. Characterization

The sample morphologies were characterized using field emission scanning electron microscopy (FE-SEM, Hitachi S8010, HITACHI, Tokyo, Japan). Crystal structures were examined using X-ray diffraction (D/Max-2500/PC, Rigaku, Tokyo, Japan) with a 5° incident angle and CuKα radiation (λ = 1.5418 Å), spanning a 0–80° scattering angle range. Further detailed analysis of morphology and crystal structure was conducted via field emission transmission electron microscopy (FE-TEM, Jeol 2100F, JEOL, Tokyo, Japan). Additionally, X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe II, ULVAC-PHI, Chigasaki, Japan) was employed to assess sample binding energies.

2.5. Fabrication and Measurement of Gas Sensor

Gas-sensing performance was assessed using a custom-built setup, featuring a quartz tube (50 mm in diameter and 500 mm in length) as the sensor chamber. The IDE chip was placed centrally and linked to a Keithley 2450 source meter. Positioned 10 mm from the chip, a 365 nm UV LED on a PCB provided UV illumination. To maintain measurement accuracy, the chamber was kept at RT to minimize temperature-dependent effects. Prior to testing, the gas was dried using a DRIERITE unit. The gas stream was then split, with each line controlled by a mass flow controller (MFC). One portion was directed into a heated flask containing DI water to produce humid air, achieving relative humidity (RH) levels between 0.7 and 98.5% based on the temperature. This humidified stream was mixed with dry air, allowing precise RH control between 0% and 80%. RH values were measured using a digital humidity sensor (SK-140TRH, SATO, Tokyo, Japan) before each test. To evaluate humidity impact on sensor behavior, RH was varied at intervals of 0%, 20%, 40%, 60%, and 80%, while target gas concentration was adjusted by blending it with synthetic air. During tests, target gas concentrations were set at six levels: 100 ppm, 200 ppm, 400 ppm, 600 ppm, and 800 ppm, and 1000 ppm achieved by alternating target gas injection with synthetic air. Sensor response was calculated using (R_g − R_a) × 100/R_a (%), where R_a is the resistance in air and R_g is the resistance in the target gas. Response and recovery times were noted when the sensor resistance reached 90% of the total change during adsorption and desorption

3. Results

3.1. Characterization of PANI/SnO₂ Composites

The surface morphology of the PANI/SnO₂ composites was examined via SEM, as shown in Figure 2. Figure 2a–d present the overall images of the samples SP0, SP1, SP2, and SP3, respectively. All samples are composed of a large number of NWs, which exhibit uniform diameters and considerable lengths, demonstrating the consistency of the SnO₂ NW synthesis process. Magnified SEM images, provided in Figure 2e–h, reveal the detailed surface structure of individual NWs. In the case of SP0, which consists of pure SnO₂ without PANI, the NW surfaces appear smooth, with an average diameter of approximately 200 nm. This smooth surface confirms the absence of any surface modifications or coatings. In contrast, SP1, SP2, and SP3 exhibit increasingly rough surfaces due to the incorporation of PANI. For SP1, the surface features slight protrusions, indicating the presence of a small amount of PANI. However, most of the SnO₂ NW surface remains exposed, with only partial coverage by PANI. The NWs in SP2 are coated with a thin, uniform layer of PANI, resulting in moderately rough surfaces. This indicates a more significant presence of PANI compared to SP1, though the SnO₂ NWs are still discernible beneath the coating. The morphology of SP3 demonstrates the most pronounced surface modifications. The NWs are extensively coated with a thick layer of PANI, to the extent that the underlying SnO₂ NWs are almost entirely obscured. The excessive PANI deposition increases the NW diameters compared to SP0 and creates a highly irregular surface with abundant protrusions. This extensive roughness reflects the excessive amount of PANI in the SP3 sample. Figure 2i,j present the TEM images of a single SP2 nanowire and its corresponding magnified view, respectively. The TEM analysis provides further insights into the surface structure and crystallinity of the composite material. As shown in Figure 2i, the surface of the SP2 nanowire is uniformly coated with a thin layer of PANI, confirming the SEM observations. The even coverage of PANI indicates a successful integration of the polymer onto the SnO₂ NW surface without significant agglomeration or uneven deposition. The magnified TEM image in Figure 2j reveals the lattice fringe patterns of the SnO₂ core beneath the PANI layer. The observed lattice fringes exhibit a spacing of 3.35 Å, which corresponds to the (110) lattice planes of SnO₂. This clearly demonstrates the high crystallinity of the SnO₂ NWs and confirms that the structural integrity of the SnO₂ core is preserved even after PANI coating. The TEM results further validate the formation of a core-shell structure in SP2, where the SnO₂ NWs act as the core and the PANI layer acts as the shell. This nanoscale architecture is expected to play a critical role in enhancing the material’s gas sensing properties by combining the high conductivity of PANI with the surface activity of SnO₂.

Figure 3 presents the XRD patterns of SP0, SP1, SP2, and SP3. For all samples, the characteristic peaks of SnO₂ are located at 26.5° (110), 33.8° (101), 37.9° (200), 38.98° (111), 51.6° (211), 54.7° (220), and 57.8° (002) (JCPDS 72-1147) [26]. In samples SP1, SP2, and SP3, additional peaks at 15.5°, 20.4°, and 25.5° were observed, corresponding to the (011), (020), and (200) crystal planes of PANI (JCPDS 53-1891) [27]. However, in SP0, no characteristic peaks of PANI were detected. As the PANI content increased in SP1, SP2, and SP3, the intensity of PANI’s characteristic peaks at these positions became progressively stronger. Notably, in SP1, the intensity of PANI’s peaks was significantly weaker than that of SnO₂, whereas in SP3, the intensity of PANI’s peaks greatly exceeded that of SnO₂. This trend is consistent with the SEM images, which reveal a corresponding increase in PANI content across the samples.

The XPS survey scans for the Sn 3d, C 1s, N 1s, and O 1s photoelectron peaks for SP0, SP1, SP2, and SP3 are presented in Figure 4. In Figure 4a, the Sn 3d peaks are prominently observed at approximately 486 eV and 494 eV, corresponding to the Sn 3d5/2 and Sn 3d3/2 energy levels, respectively. The consistent energy separation of 8.41 eV between these two levels is a key observation. This result aligns closely with established values for Sn⁴⁺ in SnO₂, as extensively reported in the literature. This strong correlation confirms the complete oxidation state of Sn⁴⁺ in all samples [28,29].

Figure 4b–d present the C 1s spectrum of PANI, highlighting a characteristic peak at approximately 285 eV, which is attributed to carbon bonding within the benzenoid ring structure of PANI. This peak is further deconvoluted into three distinct components, corresponding to C–H, C–N, and C=N bonds, respectively. The presence of these bonds reflects the unique chemical environment of carbon atoms in PANI, where C=N represents the protonated imine groups formed during acid doping. A slight shift in the positions of these peaks is observed, which can be attributed to the interaction between the positively charged C=N groups in PANI and the oxygen atoms of SnO₂. This interaction occurs via hydrogen bonding, which modifies the electronic environment around the carbon atoms and is indicative of the strong coupling between PANI and SnO₂ [30,31]. These interactions play a crucial role in influencing the material’s electronic properties, which are critical for its gas-sensing performance.

The N 1s spectrum of PANI, shown in Figure 4e–g, reveals three distinct deconvoluted peaks, each corresponding to different nitrogen species. The peak at lower binding energy is attributed to uncharged amine groups (–N=), which represent the neutral state of nitrogen in the polymer backbone. The intermediate binding energy peak corresponds to radical cationic nitrogen atoms (–N–), indicating the presence of oxidized nitrogen species within the conjugated polymer. The peak at higher binding energy is associated with protonated nitrogen (N⁺), specifically arising from interactions with protons introduced during the acid doping process, forming species such as –NH₂⁺– [32,33]. These features are consistently observed across all three samples, though slight shifts in peak positions are evident. These shifts are attributed to interactions between the nitrogen atoms in PANI and the oxygen atoms in SnO₂, facilitated by hydrogen bonding or charge transfer. Such interactions alter the electronic environment of the nitrogen atoms, further confirming the strong coupling between PANI and SnO₂. This coupling is significant as it affects the material’s charge transport properties and enhances its potential for gas sensing applications.

The O 1s spectra, shown in Figure 4h–k, provide crucial insights into the oxygen states in the samples, which significantly influence gas sensing performance. Three peaks were identified, corresponding to lattice oxygen (O_lattice), oxygen vacancies (O_vac), and surface-adsorbed oxygen (O_ads). For SP2, these peaks were located at 530.28 eV (83.10%), 531.88 eV (11.74%), and 532.68 eV (5.15%). In contrast, SP0, SP2, and SP3 showed additional shifts and variations, reflecting changes in oxygen chemistry due to sample modifications. O_lattice is chemically stable and typically inert in gas sensing, while O_vac plays a pivotal role by providing active sites for gas adsorption and dissociation. These vacancies enhance free electron concentration, boosting sensitivity, but excessive O_vac can form defect clusters, reducing carrier mobility and sensor performance. Thus, controlled O_vac concentration is essential for optimal response. O_ads further contributes to gas sensing by participating in surface redox reactions, enabling effective charge transfer during gas interactions [34,35]. Together, the balance between O_vac and O_ads determines the gas-sensing efficiency of metal oxide materials, emphasizing the need for precise control of oxygen states in sensor design.

3.2. Gas Sensing Properties

The experimental capacitance curves of the sensors, tested with CO₂ concentrations ranging from 100 ppm to 1000 ppm at RT under UV irradiation, are presented in Figure 5a. Upon CO₂ exposure, the resistances of all devices increased, as CO₂ molecules act as electron acceptors from the n-type SnO₂, leading to a rise in sensor response with increasing CO₂ concentration. Additionally, it can be observed that the baseline resistance of the four sensors varies, with SP0 displaying the lowest baseline resistance and SP3 the highest. This difference is attributed to the formation of a pn junction structure between n-type SnO₂ and p-type PANI, causing an increase in the sensor’s baseline resistance. Furthermore, as the PANI content increases, the baseline resistance of the sensors correspondingly increases.

Figure 5b illustrates the response–recovery curves of the four sensors to varying CO₂ concentrations, based on the resistance changes shown in Figure 5a. For the SP0, SP1, and SP2 sensors, the response curves gradually increase when CO₂ is introduced, and then sharply decrease upon CO₂ removal. The response magnitude also increases with rising CO₂ concentration. Notably, however, the response of SP3 deviates from this trend. At the initial injection of CO₂ gas, SP3’s resistance initially decreases and then rises as CO₂ continues to flow. Additionally, after air is reintroduced, the recovery of the SP3 sensor does not reach saturation. This phenomenon is likely due to an excess of PANI encapsulating the surface of the SnO₂ nanowires, blocking initial gas access to SnO₂. Because PANI is a p-type semiconductor, it interacts with CO₂ first, causing the initial decrease in resistance. As the gas subsequently diffuses through the PANI and contacts the SnO₂, a further reaction with SnO₂ leads to a resistance increase. When air is reintroduced, the excess PANI again obstructs the adsorption and desorption of gas molecules, preventing complete recovery, highlighting how excessive PANI content can impair sensor response performance.

Figure 5c shows the response of the four sensors as a function of CO₂ concentration under UV irradiation at RT. The sensor responses increase with rising CO₂ concentration, with SP2 exhibiting a notably higher response rate due to the presence of a sufficient number of heterojunctions. The maximum response of SP2 at 1000 ppm CO₂ is 26, which is 14.4 times greater than that of SP0 (1.8). The SP2 sensor demonstrates a detection limit as low as 46 ppm, indicating its high sensitivity and selectivity for CO₂. Moreover, as seen in Figure 5b,c, the response of the SP2 sensor shows a strong linear relationship with increasing CO₂ concentration, with a linearity ranking of R²(SP2) > R²(SP0) > R²(SP3) > R²(SP1). This linearity suggests that SP2 is suitable for quantitative CO₂ detection.

In Figure 5d, the response and recovery times were calculated for varying CO₂ concentrations. Sensors SP0, SP1, and SP2 all show a decreasing response time with increasing gas concentration. In contrast, recovery times for these sensors tend to lengthen with increasing CO₂ concentration. Notably, both the response and recovery times of SP2 are shorter than those of the other sensors. This improvement is likely due to the optimized interaction between SnO₂ and PANI, where the pn junction formed by the suitable PANI-SnO₂ combination enhances the adsorption and desorption kinetics of gas molecules.

Figure 5e presents the selectivity curves of the SP2 sensor for various gases at RT under UV irradiation. Among the tested gases, only CO₂ and NO₂ are oxidizing gases, while the others (H₂S, ethanol, acetone, and methanol) are reducing gases. The optimized sensor shows distinct selectivity towards CO₂, with the highest response observed among all tested gases. This enhanced selectivity for CO₂ highlights the sensor’s practical applicability, as it demonstrates a strong preference for CO₂ detection in mixed gas environments, which is highly beneficial for real-world sensing applications.

Repeatability is a critical performance metric. Figure 5f displays the response–recovery curves of the SP2 sensor exposed to 1000 ppm CO₂ over 10 cycles, demonstrating excellent reversibility and reproducibility in repeated response cycles. This indicates that the SP2 sensor maintains consistent performance under cyclic CO₂ exposure.

Figure 5g illustrates the long-term stability of the SP2 sensor over a 30-day period. Although minor fluctuations are observed in the response curve, the sensor’s overall response remains consistent, indicating reliable stability over time. This stable performance under continuous operation reinforces the suitability of the SP2 sensor for practical CO₂ sensing applications.

The impact of RH is critical for RT operation, as water molecules in the air can easily adsorb onto the sensor surface, potentially hindering gas adsorption and thereby reducing sensor response [36]. Figure 5h displays the CO₂ response curves and dynamic resistance of the SP2 sensor under varying RH levels (0%, 20%, 40%, 60%, and 80%) at RT with UV illumination. As shown in Figure 5h, both the CO₂ response and baseline resistance decrease as RH increases from 0% to 80%. A slight decrease in baseline resistance is observed with rising RH. Despite this, the response remains within an acceptable range, indicating that the SP2 sensor demonstrates relatively good humidity tolerance. The decline in sensor response with increasing RH can be attributed to competition between water vapor and CO₂ for adsorption sites on the sensor surface. The presence of physically adsorbed water interferes with the direct interaction between chemisorbed oxygen and CO₂ molecules, thus affecting the ionic conductivity of the hybrid sensing material [37].

A comparison between various nanocomposites from the literature for CO₂ sensing and the present work is represented in

Table 1. Comparison of gas sensing performance for various materials toward CO₂ detection.

Material	Sensor Response	Response Time (s)	Recovery Time (s)	Concentration (ppm)	Operating Temperature (°C)	Concentration (ppm)
LaOCl/SnO2	3.7 1	24	92	1000	300	[38]
CeO2/CdS	3.62 1	12	20	1000	RT	[39]
La/ZnO	65 2	90	38	5000	400	[40]
CdO	1.7 2	200	300	5000	250	[41]
La2O3	1.92 1	50	73	350	250	[42]
SnO2/PANI	47.4 2	35.1	43.2	5000	RT	[43]
ZnO/SnO2	19 2	65	90	150,000	300	[44]
La2O3@Pd	2.8 1	80	50	400	250	[45]
SnO2	1.24 1	250	4	2000	240	[46]
SnO2/LaFeO3	2.72 1	20	-	4000	250	[47]
La2O3/SnO2	1.52 1	-	-	1000	400	[48]
La2O3/SnO2	1.75 1	-	-	500	300	[49]
SnO2@ZIF-67	16.5 2	25	96	5000	205	[50]
La2O3/SnO2	1.59 1	-	-	2000	400	[51]
SnO2/PANI	26 2	293	1510	1000	RT	This work

Sensor response: ¹ R_g/R_a, ² (R_a − R_g)/R_a × 100.

.

3.3. Discussion and Gas Sensing Mechanism

As demonstrated in the above analyses, the PANI-SnO₂ composite shows outstanding performance for CO₂ detection at RT, characterized by a high response magnitude, an ultralow LOD, reliable repeatability, and excellent selectivity. Notably, in this study, SnO₂ alone does not exhibit a significant response to CO₂ at RT, indicating that the gas response of the composite primarily depends on the combined sensing behavior of both PANI and SnO₂. Furthermore, the response magnitude of PANI is influenced by variations in preparation parameters, with the SP2 sensor displaying a markedly higher response compared to the other three sensors. In general, nanostructured materials are recognized to enhance the sensing properties of both organic–inorganic and inorganic–inorganic nanocomposites. The significantly higher response observed in MOS–organic composites, such as PANI-SnO₂, compared to either component alone, can be attributed to the formation of a pn junction [52]. Observations revealed that SnO₂ alone exhibits very high resistance and shows no significant gas response at RT. Evidently, the strong contact between PANI and SnO₂ results in the formation of a pn junction, driven by the diffusion of holes and electrons across the interface, as illustrated in Figure 6. This interaction creates a hole depletion region on the PANI side and an electron depletion region on the SnO₂ side [53]. Additionally, the interaction between CO₂ and the PANI layer plays a critical role in modifying the electronic state of PANI. CO₂ molecules can interact with the protonated imine groups in PANI, leading to changes in the polymer’s charge carrier density. These modifications dynamically alter the depletion region on the PANI side of the pn junction, thereby influencing charge transfer and the overall resistance of the sensor. This synergistic effect between PANI’s electronic state and the pn junction is pivotal in achieving the observed high sensitivity and selectivity of the composite sensor. However, when the thickness of the PANI layer exceeds a certain threshold (e.g., SP3), the adsorbed CO₂ molecules have restricted access to the PANI/SnO₂ interface. This results in a diminished pn junction effect and, consequently, a reduced response magnitude. Thus, the sensor’s response in SP3 closely resembles that of PANI alone, with only a slight enhancement due to the nanostructured configuration, as shown in Figure 5a. Conversely, if the PANI layer is relatively thin (e.g., SP1), the pn junction formed between PANI and SnO₂ has limited influence. In the case of SP2, an optimal PANI thickness allows CO₂ molecules to penetrate and reach the PANI/SnO₂ interface, enabling effective gas interactions. CO₂ molecules are known to withdraw electrons from SnO₂, increasing the hole concentration and, subsequently, increasing the overall resistance. Given the substantial resistance difference between the depletion region and the SnO₂ surface layer, when the depletion region expands to occupy the entire SnO₂ nanowire, the composite exhibits a significantly higher resistance after exposure to CO₂. As a result, the composite’s response magnitude can far exceed that of SnO₂ alone, underscoring the beneficial impact of the pn junction structure.

Furthermore, to fabricate a gas sensor with enhanced performance at RT, UV light was applied to the SnO₂/PANI sensor as an activation source. In the case of the SnO₂/PANI pn heterojunction, UV irradiation promotes enhanced gas sensing performance by generating electron-hole pairs (EHPs) [54]. Given the p-type nature of PANI and the n-type nature of SnO₂, UV excitation facilitates charge separation and transfer at the junction interface. Specifically, due to the higher work function of SnO₂, photogenerated holes in the valence band of SnO₂ migrate to PANI, while electrons accumulate in SnO₂. This results in an electron depletion layer in PANI and an electron accumulation layer in SnO₂. The sensing enhancement mechanism under UV irradiation lies in the interaction between these accumulated charge carriers and the gas molecules, as well as the generation of reactive oxygen species (ROS) on the sensor surface [55]. In the presence of UV light, SnO₂ becomes increasingly electron-rich. When CO₂ molecules interact with the sensor, they withdraw electrons from the SnO₂ conduction band, leading to an expansion of the depletion region at the pn junction. This de-doping effect reduces the number of mobile charges in the PANI layer, increasing the overall resistance of the device. The increased depletion region and reduced carrier mobility collectively explain the rise in resistance. These accumulated electrons in SnO₂ readily transfer to the gas molecules, producing a measurable electrical signal that correlates with the gas concentration. Moreover, the UV light-induced ROS formation on the SnO₂/PANI surface adds to the sensor’s reactivity. These ROS, that are highly reactive under UV exposure, interact with the target gas molecules, triggering further electron transfer reactions that amplify the sensor’s response. The combined effects of photogenerated electrons, accumulated charge carriers, and ROS formation at the SnO₂/PANI interface greatly enhance the sensor’s sensitivity, response time, and selectivity at RT, offering a promising pathway for effective gas detection in ambient conditions.

In summary, the results of this study demonstrate the significant potential of the PANI/SnO₂ composite sensor for selective and sensitive CO₂ detection at room temperature. The optimized SP2 sensor exhibited superior performance due to the formation of a well-defined pn heterojunction, which enhanced charge transfer and response sensitivity. The sensor’s excellent selectivity for CO₂ over other gases, coupled with its stability, reproducibility, and low detection limit under varying environmental conditions, underscores its practical applicability. The incorporation of UV illumination further improved the sensor’s performance by enhancing the surface reactivity and adsorption–desorption kinetics. These findings highlight the importance of material design and structural optimization in achieving high-performance gas sensors. Future studies will aim to explore advanced thermodynamic analyses and alternative composite configurations to further enhance the sensing capabilities of such materials.

4. Conclusions

PANI/SnO₂ nanowire composites were successfully synthesized and optimized for efficient CO₂ sensing at room temperature. Among the samples, the SP2 sensor demonstrated the best performance due to the formation of a pn heterojunction, achieving a high CO₂ response (26 at 1000 ppm), a low detection limit (46 ppm), excellent selectivity, and reliable stability over 30 days. The role of UV illumination in enhancing the sensor’s response and recovery times was also emphasized, attributed to photogenerated electron-hole pairs and increased surface reactivity. These findings position the PANI/SnO₂ composite as a promising candidate for low-power, room-temperature CO₂ sensing with applications in environmental monitoring and industrial safety.