Electrospun SnO2/WO3 Heterostructure Nanocomposite Fiber for Enhanced Acetone Vapor Detection

Volatile organic compounds (VOCs), often invisible but potentially harmful, are prevalent in industrial and laboratory settings, posing health risks. Detecting VOCs in real-time with high sensitivity and low detection limits is crucial for human health and safety. The optical sensor, utilizing the gasochromic properties of sensing materials, offers a promising way of achieving rapid responses in ambient environments. In this study, we investigated the heterostructure of SnO2/WO3 nanoparticles and employed it as the primary detection component. Using the electrospinning technique, we fabricated a sensing fiber containing Ag NPs, poly(methyl methacrylate) (PMMA), and SnO2/WO3 (PMMA-Ag-SnO2/WO3) for acetone vapor detection. Following activation via UV/ozone treatment, we observed charge migration between WO3 and SnO2, resulting in a substantial generation of superoxide radicals on SnO2 nanoparticles. This phenomenon facilitates structural deformation of the fiber and alters the oxidation state of tungsten ions, ultimately leading to a significant change in extinction when exposed to acetone vapor. As a result, PMMA-Ag-SnO2/WO3 fiber achieves a detection limit of 100 ppm and a response time of 1.0 min for acetone detection. These findings represent an advancement in the development of sensitive and selective VOC sensing devices.


Introduction
Volatile organic compounds (VOCs), predominantly colorless, can cause severe harm to the human body.VOCs are widely detected in industrial production and laboratory environments.The danger they pose is noteworthy, as their invisibility can lead to unintentional overexposure.Humans subjected to excessive VOCs can experience symptoms such as fatigue, headaches, and nervous system damage, thereby casting a shadow on overall health and well-being [1][2][3].Acetone, a highly irritating compound, could pose significant health risks with excessive inhalation.Prolonged or high-dose exposure can irritate the eyes, nose, throat, and lungs, and cause symptoms such as dizziness, confusion, increased heart rate, unconsciousness, and even coma due to its depressant effect on the central nervous system.In addition, acetone can be found in the exhaled breath of those who suffer from diabetes.In light of this, the development of precise and responsive acetone sensors is of great significance to human health and industrial safety.
A large fraction of gas-sensing techniques is based on resistive gas sensors [4][5][6][7][8], the infrared absorption method [9][10][11][12], optical sensors [13][14][15], acoustic sensors [16][17][18][19], etc. Resistive gas sensors are highly praised for their high sensitivity, which can effectively convert changes in the gas sensing environment into unique resistance signals, thus enabling accurate detection of various gases.So far, many of them do operate effectively at room Polymers 2023, 15, 4318 2 of 11 temperature, which goes against the common belief that resistive gas sensors generally require high operating temperatures.However, despite these advances, the manufacturing and operating costs associated with resistive gas sensors remain relatively high compared to other sensing technologies, primarily due to the complexity of their material synthesis, the need for specialized equipment, or, in more demanding cases, the requirement of high temperatures, increasing energy consumption [20,21].To date, optical gas sensors have become widespread in recent years due to their unique properties.Optical VOC sensors can operate at room temperature, are highly resistant to electromagnetic radiation, and are portable and simple to use compared to traditional electrical resistive gas sensors, which require a heating component for high-temperature functioning [22][23][24][25].Given the advances in nanomaterial technology combined with innovative synthesis methods, it has been possible to create affordable and small sensing devices with a higher sensitivity and selectivity for acetone.Nanoscale metal oxides have been widely applied as susceptible sensing materials owing to their outstanding optical and electrical properties.SnO 2 has been widely applied in VOC sensors, which can respond to acetone, ethanol, formaldehyde, etc. [26][27][28].Porous nanospheres of SnO 2 , catalyzed by Co 3 O 4 , have been showcased as an effective material for acetone sensing.The presence of Co 3 O 4 introduces additional active sites that facilitate sensing reactions.Furthermore, the p-n heterointerface created between Co 3 O 4 and SnO 2 promotes rapid sensing reactions on the surface of the material [29].WS 2 /WO 3 p-n heterojunctions were constructed, exhibiting a high sensitivity to acetone owing to their increased carrier density and rapid charge transfer, able to detect acetone in a wide range from 20 to 500 ppm [30].
Among various techniques for nanomaterial synthesis, electrospinning offers significant advantages in the fabrication of gas sensors.The primary benefit is the high surface-to-volume ratio of electrospun nanofibers, which increases the exposure of reactive sites, thereby enhancing the sensitivity and response time in gas detection.This structural property facilitates a more efficient and effective adsorption process for gas molecules, crucial for achieving lower detection limits.Additionally, the porosity and interconnectedness of electrospun networks promote diffusion pathways for gas molecules, ensuring rapid sensor responses and recovery times.Moreover, electrospinning allows for the incorporation of various functional materials into the nanofibers, such as polymers, composites, and nanoparticles, which can be precisely engineered to detect specific gases with high selectivity [31].Our group has developed a series of electrospun-fiber-based materials for VOC sensing [32][33][34][35][36][37].The mechanism of our sensing material is based on detecting the change in extinction that contributes to the collapse of the 3D fiber structure after VOC exposure.The P3HT/PMMA fibrous film can achieve the detection limits of acetone, toluene, and o-xylene at 500 ppm [35].Ag/CdSe-CdS/PMMA freestanding film with double-slit UV/ozone etching enabled an advanced VOC-sensing efficiency and reached the detection limit for 100 ppm of butanol within 1 min [36].WO 3 /SnO 2 /Ag/PMMA can detect 20 ppm of acetone vapor within 10 min under room temperature, which shows potential for exhaled breath detection.The interfaces between SnO 2 and WO 3 might construct n-n junctions, which could help to improve the sensitivity of sensing materials when they are subjected to acetone surroundings [37].In this regard, we would like to more thoroughly investigate the mechanism underlying acetone detection.
In this study, we synthesized a SnO 2 -decorated WO 3 heterostructure nanocomposite using the solvothermal method to design a heterostructure for constructing n-n junctions.The in-depth characteristics of the heterostructure of SnO 2 /WO 3 NPs were investigated.Moreover, the PMMA-Ag-SnO 2 /WO 3 was then electrospun with Ag NPs and poly (methyl methacrylate) (PMMA) to form a sensing fiber film for acetone detection.Contributing to the gasochromic property of WO 3 , the optical properties showed an obvious difference after exposure to acetone.Based on contact potential difference measurement, we revealed a possible charge migration between WO 3 and SnO 2 under UV LED illumination.Based on the correlation between the optical properties and photo-assisted contact potential Polymers 2023, 15, 4318 3 of 11 differences when exposed to acetone, a detection mechanism is proposed.The PMMA-Ag-SnO 2 /WO 3 fibers successfully achieved a detection limit of 100 ppm for acetone.

Preparation of Sensing Materials
Silver nanoparticles (Ag NPs) were prepared through a solution-based chemical reduction reaction [38].Amounts of 0.6 mmol of silver nitrate (AgNO 3 , >99.5%, Sigma-Aldrich, St. Louis, MO, USA) and 0.6 mmol of oleylamine (C 18 H 37 N, >98%, Sigma-Aldrich, St. Louis, MO, USA) were dissolved in 50.0 mL of chlorobenzene (C 6 H 5 Cl, >99%, ACROS, Geel, Belgium) in a four-necked flask.The mixture underwent a heating process at 120 • C for 1 h with vigorous stirring under a nitrogen flow.After cooling to room temperature, the Ag NP colloid solution was obtained.For the WO 3 @SnO 2 NP preparation, commercial tungsten oxide (WO 3 , 99.9%, US Research Nanomaterials Inc., Houston, TX, USA) powder with an average size of 60 nm was used as the template and was decorated with SnO 2 using the solvothermal method.Amounts of 1.0 g of SnCl 4 •5H 2 O (>98.0%,Nacalai Tesque, Kyoto, Japan) and 1.0 g of WO 3 NPs were dispersed in 80.0 mL of benzyl alcohol (C 7 H 8 O, 99%, ACROS, Geel, Belgium) with stirring for 10 min.Subsequently, the mixture solution was transferred into a 100.0 mL Teflon-lined autoclave and heated at 120 • C for 12 h.The resulting powders were subjected to three wash cycles using diethyl ether and then collected through centrifugation at 4500 rpm for 5.0 min.
To fabricate electrospun PMMA-Ag-SnO 2 /WO 3 fibers, a homogeneous precursor solution containing 1.0 wt.% Ag and 5.0 wt.% poly (methyl methacrylate) (PMMA, MW∼120,000 Da, Sigma-Aldrich, St. Louis, MO, USA) in dimethylformamide (DMF, C 3 H 7 NO, >99.5%, Fisher Scientific, Fair Lawn, NJ, USA) was prepared at first.Then, 20.0 mg of SnO 2 /WO 3 NPs was added to the precursor solution.The PMMA-Ag-SnO 2 /WO 3 precursor solution was subjected to continuous stirring at 80 • C for 3 h, ensuring complete dissolution of the PMMA.Next, the PMMA-Ag-SnO 2 /WO 3 fibers were fabricated via the electrospinning technique.The electrospinning apparatus comprised a syringe pump (KDS−100, KD Scientific Inc., Holliston, MA, USA), a high-voltage power supply (SC−PME50, Cosmi Global Co,. Ltd., New Taipei City, Taiwan), and a grounded rotary collector with dimensions measuring 15.0 cm in diameter and 15.0 cm in width (FES−COS, Falco Tech Enterprise Co., Ltd., New Taipei City, Taiwan).An applied voltage of 10.0 kV, a working distance of 10.0 cm, a flow rate of 0.5 mL/h, a solution volume of 10 mL, and a rotation rate of the collector plate of 500 rpm were employed.To activate the surface of sensing fibers, PMMA-Ag-SnO 2 /WO 3 fiber was subjected to a 10 min dry etching through UV/ozone treatment (UV/Ozone, IAST0001−020, STAREK Scientific Co., Taipei City, Taiwan).The UV/ozone apparatus was equipped with four UVC lamps (PL−L, 36W, λ max = 254 nm, Philips, Amsterdam, The Netherlands).

Material Characterization
The crystal structure analysis of WO 3 and SnO 2 /WO 3 NPs was conducted using X-ray diffractometer to record the diffraction patterns of as-prepared nanomaterials.Sphericalaberration-corrected field emission transmission electron microscope (ULTRA-HRTEM, JEM-ARM200FTH, JEOL, Tokyo, Japan) equipped with Si drift detector for elemental analysis was employed to observe the morphology and elemental distribution.The size distribution of particles was recorded and calculated from individual measurements of at least 100 particles.The contact potential difference was measured using a Kelvin probe analyzer (SKP 5050, KP technology, Wick, Caithnes, UK).The morphology of the fibers was observed using field-emission scanning electron microscope (FESEM, model SU8010, Hitachi, Tokyo, Japan).UV−VIS spectrophotometer (UV−1900i, Shimadzu, Kyoto, Japan) was used to examine the optical properties of the fibers.

Detection of Acetone Vapor
To examine the sensitivity of PMMA-Ag-SnO 2 /WO 3 sensing fiber to acetone vapor, the sensing chip was placed in the middle of the container for the acquisition of extinction measurement.The specific volume of acetone solution was injected into a 100.0 mL glass liner.The volume for various concentrations of acetone vapor production was calculated using the ideal gas equation of state (PV = nRT).Following evaporation, we sampled the concentrated acetone vapor and transferred it into a 4.5 cm by 4.0 cm by 4.0 cm quartz glass container.At the same time, we calculated the volume needed to dilute the concentrated vapor.Subsequently, upon injecting the acetone vapor, we initiated measurements of the extinction spectra.The extinction spectrum was immediately recorded using a UV-VIS spectrometer, scanning wavelengths from 400 nm to 900 nm, with measurements taken every 30 s.The examination of acetone detection was conducted at room temperature (25 • C) within a relative humidity range of 60%.In general, the transmittance of the materials results in various extinction intensities.PMMA-Ag-SnO 2 /WO 3 fiber exhibits a broad extinction shoulder at around 410 nm.Hence, we defined the change in extinction as the difference in extinction peak observed before and after exposure to VOC vapor.The equation of extinction change (∆E t ) is described as follows: where E 0 represents the extinction before exposure to VOCs and E t represents the extinction intensity after exposure to VOC vapor at a specific time point, t mins.

Results
The heterostructure of WO 3 and SnO 2 was confirmed using X-ray diffraction analysis and via observation using an electron microscope.Figure 1a shows the X-ray diffraction patterns of pristine WO 3 NPs and SnO 2 /WO 3 NPs.The typical diffraction of WO 3 is observed in both nanoparticles, which is located at 2θ of 23.15  , and 61.75 • indexed to the lattice planes of (001), (020), ( 200), (120), (111), (021), (220), ( 221), (104), (402), and (340) [JCPDS 00-005-0363], respectively.A weak diffraction of SnO 2 in SnO 2 /WO 3 NPs can be indexed to (200) and (211) at 2θ of 38.96 • and 51.90 • [JCPDS 00-021-1250], respectively.With the incorporation of SnO 2 and undergoing the solvothermal reaction, the diffraction intensity of WO 3 decreased dramatically, attributed to the disintegration of the aggregated WO 3 NPs and slight doping by Sn [39]. Figure 1b,c show the morphology of WO 3 NPs and SnO 2 /WO 3 NPs, and ultra-small particles are especially decorated on WO 3 NPs.To confirm the presence and composition of SnO 2 , we further investigated their microstructure and detailed surface information via a transmission electron microscope.The aggregated WO 3 showed a bare surface (Figure 1d) with a diameter of around a hundred nanometers, and plenty of nanoparticles were covered when incorporated with SnO 2 (Figure 1e).In the magnified image of Figure 1f, we were able to index the lattice fringe for WO 3 with a d-spacing of 3.61 Å for (220) in the core template and SnO 2 with a d-spacing of 3.36 Å for (110) on the surface as a shell.It depicts the successful incorporation of SnO 2 onto WO 3 NPs and establishes the heterostructure.The elemental mapping and estimated composition of SnO 2 /WO 3 are shown in Figure 1g-l.The good distribution of Sn covering the WO 3 NPs is realized and the composition is 59.0% ± 0.8, 34.4% ± 0.7, and 6.7% ± 0.5 for tungsten, oxygen, and tin, respectively.In brief summary, the nanoscale heterostructure of nano-sized SnO 2 decorated on WO 3 NPs can be achieved via hydrothermal reaction.
We then incorporated the as-prepared SnO 2 /WO 3 NPs into the PMMA and Ag NP precursor solution and proceeded to fabricate a sensing fiber using the electrospinning technique.A photograph of the PMMA-Ag-SnO 2 /WO 3 sensing chip is illustrated in Figure S1a.Pure PMMA-Ag fibers present a regular and uniform fiber morphology with an average diameter of 198 ± 37 nm, and a smooth surface (Figure 2a,b).However, with the incorporation of SnO 2 /WO 3 NPs, the interconnected network structure is reduced, and some beads become embedded into the PMMA-Ag fibers.It leads to the average diameter, and the relevant deviation in the calculated diameter increases to 204 ± 88 nm (Figure 2c,d).Through EDS mapping observation, we confirmed the presence of embedded particles composed of WO 3 and SnO 2 , as well as the random distribution of SnO 2 /WO 3 NPs in the fibers (Figure 2e-i).In addition, UV/ozone treatment was employed to activate the surface of composite fibers before the acetone detection.The outward appearance of the sensing chip after UV/ozone treatment shows an insignificant change (Figure S1b).To confirm the preservation of the three-dimensional network structure of composite fibers after 10 min of UV/ozone treatment, observation of the morphology was carried out, which is shown in Figures S2 and S3.The fibers demonstrated slight shrinkage and their surfaces became rough.Importantly, the integrity of the three-dimensional network structure remained unaltered.This suggests that ozone molecules and oxygen radicals engaged in a mild reaction with the surface of composite fibers.We then incorporated the as-prepared SnO2/WO3 NPs into the PMMA and Ag NP precursor solution and proceeded to fabricate a sensing fiber using the electrospinning technique.A photograph of the PMMA-Ag-SnO2/WO3 sensing chip is illustrated in Figure S1a.Pure PMMA-Ag fibers present a regular and uniform fiber morphology with an av-  To evaluate the VOC detection of PMMA-Ag-SnO2/WO3 sensing fibers, we measured the transmittance using a UV-Vis spectrometer and converted it into extinction spectra.The change in maximum extinction over time corresponds to the detection response.Figure 3a presents the extinction spectra of PMMA-Ag-SnO2/WO3 fibers exposed to 10,000 ppm of acetone.A significant downshift in extinction appears at wavelengths ranging from 400 to 500 nm, inferring the fibers' structure deformation and WO3 gasochromic properties when exposed to acetone vapor.The Ag-induced SPR effect magnified the trend of decreased extinction.The outward appearance also exhibits a slight fade when the sensing chip is exposed to acetone (Figure S1c).As for the control sample of PMMA-Ag-WO3 fibers (Figure 3b), the maximum ∆E when exposed to air over time is approxi- To evaluate the VOC detection of PMMA-Ag-SnO 2 /WO 3 sensing fibers, we measured the transmittance using a UV-Vis spectrometer and converted it into extinction spectra.The change in maximum extinction over time corresponds to the detection response.Figure 3a presents the extinction spectra of PMMA-Ag-SnO 2 /WO 3 fibers exposed to 10,000 ppm of acetone.A significant downshift in extinction appears at wavelengths ranging from 400 to 500 nm, inferring the fibers' structure deformation and WO 3 gasochromic properties when exposed to acetone vapor.The Ag-induced SPR effect magnified the trend of decreased extinction.The outward appearance also exhibits a slight fade when the sensing chip is exposed to acetone (Figure S1c).As for the control sample of PMMA-Ag-WO 3 fibers (Figure 3b), the maximum ∆E when exposed to air over time is approximately −0.008.This value serves as the threshold for acetone detection to mitigate the influence of environmental factors.The response time is identified when the change in extinction (∆E) at a given exposure time (t) exceeds the threshold.Furthermore, both PMMA-Ag-WO 3 fibers and PMMA-Ag-SnO 2 /WO 3 fibers (Figure 3b,c) exhibit a noticeable ∆E when exposed to acetone vapor.The change in ∆E over time for PMMA-Ag-SnO 2 /WO 3 fibers, when exposed to acetone concentrations of 10,000, 1000, and 100 ppm, is higher compared to that of PMMA-Ag-WO 3 fibers.This indicates that the incorporation of SnO 2 NPs to fabricate the heterostructured SnO 2 /WO 3 NPs enhances the extinction response and sensitivity of acetone vapor.The reproducibility of the sensing fiber was further validated.The sensing fibers displayed highly consistent changes in extinction, as shown in Figure S4.To further realize the lowest concentration of detection, we collected the several changes in extinction of sensing fibers exposed to acetone vapor with various concentrations and depicted the calibration curve (Figure S5).A positive correlation was observed between the vapor concentration and the change in extinction (Figure S5a), indicating a strong dependence of the change in extinction on the concentration of acetone vapor.Additionally, through the formulation of vapor concentration and the corresponding change in average extinction at t = 30 min, the calibration curve can be defined as follows: where ∆E represents the change in extinction at a given exposure time (t), and C denotes the vapor concentration in ppm.Considering the threshold (−0.008) of the change in extinction in ambient air, the sensing fiber is anticipated to exhibit sensitivity down to a concentration of 0.212 ppm.Consequently, the expected detection limit and response time for acetone detection using PMMA-Ag-SnO 2 /WO 3 fibers are notably below 100 ppm and within 1.0 min, respectively.
mately −0.008.This value serves as the threshold for acetone detection to mitigate the influence of environmental factors.The response time is identified when the change in extinction (∆E) at a given exposure time (t) exceeds the threshold.Furthermore, both PMMA-Ag-WO3 fibers and PMMA-Ag-SnO2/WO3 fibers (Figure 3b,c) exhibit a noticeable ∆E when exposed to acetone vapor.The change in ∆E over time for PMMA-Ag-SnO2/WO3 fibers, when exposed to acetone concentrations of 10,000, 1000, and 100 ppm, is higher compared to that of PMMA-Ag-WO3 fibers.This indicates that the incorporation of SnO2 NPs to fabricate the heterostructured SnO2/WO3 NPs enhances the extinction response and sensitivity of acetone vapor.The reproducibility of the sensing fiber was further validated.The sensing fibers displayed highly consistent changes in extinction, as shown in Figure S4.To further realize the lowest concentration of detection, we collected the several changes in extinction of sensing fibers exposed to acetone vapor with various concentrations and depicted the calibration curve (Figure S5).A positive correlation was observed between the vapor concentration and the change in extinction (Figure S5a), indicating a strong dependence of the change in extinction on the concentration of acetone vapor.Additionally, through the formulation of vapor concentration and the corresponding change in average extinction at t = 30 min, the calibration curve can be defined as follows: where ΔE represents the change in extinction at a given exposure time (t), and C denotes the vapor concentration in ppm.Considering the threshold (−0.008) of the change in extinction in ambient air, the sensing fiber is anticipated to exhibit sensitivity down to a concentration of 0.212 ppm.Consequently, the expected detection limit and response time for acetone detection using PMMA-Ag-SnO2/WO3 fibers are notably below 100 ppm and within 1.0 min, respectively.To investigate the enhancement in VOC detection achieved through the incorporation of SnO 2 /WO 3 NPs and its rapid response to acetone vapor, we speculated that the n-n heterojunction of WO 3 and SnO 2 are crucial factors, resulting in the strong change in extinction.Its interfacial charge migration after contact was investigated using a Kelvin probe force analyzer to qualify the change in contact potential difference with UV irradiation.To enhance the response of VOC detection, it is crucial to undergo a UV/ozone pretreatment, which activates both the polymer fibers and SnO 2 /WO 3 NPs.Therefore, 2023, 15, 4318 8 of 11 we employed a UV-LED as a light source to activate the semiconductor when measuring the CPD. Figure 4 shows the equilibrium contact potential difference (CPD) of WO 3 and the SnO 2 /WO 3 heterostructure nanocomposite with and without UV LED irradiation.The average CPDs of WO 3 NPs without and with illumination are 280.39 ± 6.29 mV and 136.75 ± 3.81 mV, respectively (Figure 4a,b).The bare WO 3 NPs exhibit a significant change in CPD, implying the presence of rich electrons on the surface of WO 3 NPs under UV LED irradiation.Through light on/off cycle tests, the dynamic changes in CPD also indicate the photoresponse property of WO 3 NPs, with an average difference of approximately 15.57 mV.Additionally, the growth of SnO 2 NPs onto WO 3 NPs reduces the equilibrium CPD difference, resulting in 315.38 ± 2.99 mV without illumination and 224.35 ± 2.42 mV with illumination (Figure 4d,e).The dynamic changes in CPD show a similar trend, decreasing to 13.26 mV (Figure 4f).We deduced that under UV LED illumination, the photo-induced electrons generated by WO 3 transfer to SnO 2 and may subsequently react with O 2 to promptly generate superoxide radicals, leading to their depletion.The significant depletion of electrons can be attributed to SnO 2 being an intrinsic n-type semiconductor with a high density of oxygen vacancies, which leads to a preference for oxygen adsorption [39].
To investigate the enhancement in VOC detection achieved through the incorporation of SnO2/WO3 NPs and its rapid response to acetone vapor, we speculated that the nn heterojunction of WO3 and SnO2 are crucial factors, resulting in the strong change in extinction.Its interfacial charge migration after contact was investigated using a Kelvin probe force analyzer to qualify the change in contact potential difference with UV irradiation.To enhance the response of VOC detection, it is crucial to undergo a UV/ozone pretreatment, which activates both the polymer fibers and SnO2/WO3 NPs.Therefore, we employed a UV-LED as a light source to activate the semiconductor when measuring the CPD. Figure 4 4d,e).The dynamic changes in CPD show a similar trend, decreasing to 13.26 mV (Figure 4f).We deduced that under UV LED illumination, the photo-induced electrons generated by WO3 can transfer to SnO2 and may subsequently react with O2 to promptly generate superoxide radicals, leading to their depletion.The significant depletion of electrons can be attributed to SnO2 being an intrinsic n-type semiconductor with a high density of oxygen vacancies, which leads to a preference for oxygen adsorption [39].Based on the observation of changes in CPD with and without illumination, we believe that the aforementioned phenomena also occur when the SnO2/WO3 nanoparticles undergo UV/ozone treatment.The possible mechanism is provided as follows.When SnO2/WO3 NPs are exposed to the UV/ozone, photo-induced electrons pass through the interface and transfer from WO3 to SnO2, further reacting with O2 to promptly generate superoxide radicals (O2 − ) (Equation ( 3)).These radicals either adsorb to PMMA or reduce the hexavalent tungsten to pentavalent tungsten (Equations ( 4) and ( 5)).The radicals activate the surface of PMMA but weaken the fibers' structure.When activated PMMA-Ag-SnO2/WO3 fibers are exposed to acetone vapor, the swelling effect diminishes the light Based on the observation of changes in CPD with and without illumination, we believe that the aforementioned phenomena also occur when the SnO 2 /WO 3 nanoparticles undergo UV/ozone treatment.The possible mechanism is provided as follows.When SnO 2 /WO 3 NPs are exposed to the UV/ozone, photo-induced electrons pass through the interface and transfer from WO 3 to SnO 2 , further reacting with O 2 to promptly generate superoxide radicals (O 2 − ) (Equation ( 3)).These radicals either adsorb to PMMA or reduce the hexavalent tungsten to pentavalent tungsten (Equations ( 4) and ( 5)).The radicals activate the surface of PMMA but weaken the fibers' structure.When activated PMMA-Ag-SnO 2 /WO 3 fibers are exposed to acetone vapor, the swelling effect diminishes the light scattering of the weakened fiber structure, leading to significant structural deformation and a subsequent decrease in extinction.Furthermore, the incorporation of Ag NPs with surface plasmon resonance further magnifies the decreased extinction behavior, thereby enhancing sensitivity (Figure 3a).On the other hand, under the acetone atmosphere, the collapsed PMMA fibers cannot provide sufficient surface-adsorbed O 2 − radicals, and radicals are also exhausted due to acetone degradation (Equations ( 6)-( 8)).These behaviors disrupt the reduction of hexavalent tungsten and cause pentavalent tungsten to release Polymers 2023, 15, 4318 9 of electrons to SnO 2 due to the equilibrium heterojunction, converting back to hexavalent tungsten.Consequently, the extinction at wavelengths around 500-600 nm, attributed to WO 3 , increases, offsetting the overall downward trend.To confirm the mechanism, we further used the chemical inert gas, n-hexane, for sensing the fiber response.The corresponding extinction spectra and change in extinction at varying exposure times are shown in Figure S6.The extinction spectra of sensing fibers remained unaltered upon exposure to n-hexane vapor, suggesting that n-hexane neither deforms the sensing fibers nor influences the light scattering of the sensing fiber.Moreover, it does not trigger any gasochromic behavior in heterostructured SnO 2 /WO 3 NPs.Given that PMMA is insoluble in hexane, swelling and adsorption effects are not anticipated, thereby resulting in no observable changes in light scattering.In addition, the stable alkane structure of n-hexane is not readily influenced by radicals produced from UV/ozone treatment or photo-irradiation of SnO 2 /WO 3 .Thus, no redox reactions with SnO 2 /WO 3 nanoparticles occur, and no gasochromic effects are observed.In summary, these observations confirm an enhanced sensitivity to acetone vapor.(3)

Conclusions
PMMA-Ag incorporated with SnO 2 /WO 3 heterostructure nanocomposite sensing fibers is successfully fabricated using the electrospinning technique.The characteristics of the synthesized SnO 2 /WO 3 confirm the successful growth of SnO 2 on WO 3 nanoparticles via a solvothermal reaction.The morphology and elemental distribution of embedded SnO 2 /WO 3 NPs in PMMA-Ag fibers were demonstrated.After the UV/ozone treatment, the charge migration within the WO 3 and SnO 2 heterojunction, as indicated by changes in the contact potential difference under UV illumination, leads to the substantial generation of superoxide radicals.This phenomenon enhances the change in extinction and detection response over time.As a result, the PMMA-Ag-SnO 2 /WO 3 fibers achieve an acetone detection limit of 100 ppm with a response time of just 1.0 min.This research highlights noteworthy progress in the field of VOC sensing technology.
shows the equilibrium contact potential difference (CPD) of WO3 and the SnO2/WO3 heterostructure nanocomposite with and without UV LED irradiation.The average CPDs of WO3 NPs without and with illumination are 280.39 ± 6.29 mV and 136.75 ± 3.81 mV, respectively (Figure 4a,b).The bare WO3 NPs exhibit a significant change in CPD, implying the presence of rich electrons on the surface of WO3 NPs under UV LED irradiation.Through light on/off cycle tests, the dynamic changes in CPD also indicate the photoresponse property of WO3 NPs, with an average difference of approximately 15.57 mV.Additionally, the growth of SnO2 NPs onto WO3 NPs reduces the equilibrium CPD difference, resulting in 315.38 ± 2.99 mV without illumination and 224.35 ± 2.42 mV with illumination (Figure

Figure 4 .
Figure 4. Contact potential difference (CPD) measurement via Kelvin probe force analyzer.CPD of WO3 NPs (a) without illumination, (b) with illumination, and (c) light-on/off cycles test.And CPD of SnO2/WO3 NPs (d) without illumination, (e) with illumination, and (f) light-on/off cycles test.

Figure 4 .
Figure 4. Contact potential difference (CPD) measurement via Kelvin probe force analyzer.CPD of WO 3 NPs (a) without illumination, (b) with illumination, and (c) light-on/off cycles test.And CPD of SnO 2 /WO 3 NPs (d) without illumination, (e) with illumination, and (f) light-on/off cycles test.