Spatially Tailored Asymmetric Oxygen Vacancies Induce Nonradiative Recombination for Ultrafast and Stable NO2 Sensing

Spatially Tailored Asymmetric Oxygen Vacancies Induce Nonradiative Recombination for Ultrafast and Stable NO2 Sensing | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Spatially Tailored Asymmetric Oxygen Vacancies Induce Nonradiative Recombination for Ultrafast and Stable NO 2 Sensing Wei Wang, Yucheng Ou, Nana Xu, Haiyang Song, Fuwen Wang, Tao Liu, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8016517/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Extending the lifetime of electrons while enhancing surface activity is crucial for improving charge transfer efficiency and reaction activity in photoactivated gas sensors. However, the inherent radiative charge recombination process significantly restricts the simultaneous enhancement of both kinetic and thermodynamic performance. In this study, leveraging the differences in surface oxygen vacancy migration barriers, we precisely regulated the spatial distribution of oxygen vacancies in CeO 2 by controlling the vacuum heat treatment temperature, successfully constructing a vertically asymmetric oxygen vacancy structure. This asymmetric configuration induces localized electric dipoles and strong electron-phonon coupling, enabling rapid transfer and highly efficient non-radiative recombination of photogenerated carriers, thereby triggering a pronounced photothermal effect. Leveraging this strategy, the CeO 2 -based sensor sets a new benchmark for response speed, achieving an ultra-rapid and stable detection of 5 ppm NO 2 in merely 4 s at room temperature. Our work introduces a general strategy of inducing non-radiative recombination that can be extended to photoactivated gas sensor, creating more tunability and variability in gas sensor. Physical sciences/Materials science/Materials for devices/Sensors and biosensors Physical sciences/Materials science/Nanoscale materials/Electronic properties and materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Effective real-time monitoring of NO 2 in chemical plant environments is crucial for preventing large-scale gas leakage and dispersion 1 . However, operational challenges in industrial settings, such as air turbulence and acoustic interference, can compromise the accuracy of NO 2 gas sensors 2 . Consequently, the development of gas sensors capable of rapid NO 2 response is of paramount importance for precisely identifying pollution sources and mapping the spatial distribution of this hazardous gas. Conventional metal oxide semiconductors (MOS) gas sensors, while offering advantages such as low cost and compact size, generally suffer from slow response speed and insufficient stability at room temperature 3-5 . The photoactivated gas sensing strategy addresses these limitations by utilizing photogenerated charge carriers to participate in surface reactions, significantly enhancing gas detection performance under ambient conditions 6-8 . However, the rapid recombination of photogenerated electron-hole pairs and low-concentration reaction site severely restricts carriers lifetime and surface reaction efficiency, ultimately impeding further improvement of photoactivated gas sensor performance 9 . In recent years, defect engineering has become a crucial strategy for modulating the electronic structure and surface chemical activity of MOS 10-12 . Studies have demonstrated that constructing surface oxygen vacancies (V o S ) in materials such as TiO 2 , CeO 2 , SnO 2 and In 2 O 3 can effectively enhance their gas adsorption capacity and light absorption performance 13-15 . However, conventional methods primarily rely on creating symmetrically distributed V o S to improve surface reactivity and these symmetric configurations possess inherent limitations 16 . Their highly coordinated nature favors gas adsorption at the cost of intensified radiative recombination and inherent instability in oxidizing environments 17-19 . Therefore, the substantial radiative dissipation of photogenerated electron-hole pairs severely impairs carrier utilization and surface reaction activity. Ultimately, the accumulation of intermediate products and compromised long-term stability inevitably occur in MOS 20 . Previous research has reported that converting radiative recombination into non-radiative recombination can transform carriers recombination energy into lattice vibrational energy 21-25 . This approach effectively suppresses energy dissipation through photon emission while simultaneously enhancing surface reaction kinetics via the generated localized photothermal effect. The underlying energy conversion mechanism significantly prolongs carrier lifetime, permitting increased electron participation in surface gas reactions 26 . Concurrently, the accompanying thermal effect activates and converts reactant molecules more efficiently, thereby reducing intermediate product accumulation. As a non-radiative recombination-dominated process, it circumvents the energy loss characteristic of radiative recombination and ensures continuous energy supply for surface reactions, ultimately achieving substantial improvements in both stability and reaction activity 27-30 . However, achieving efficient and controllable induction of non-radiative recombination processes still lacks a universal strategy. In particular, precise regulation of vacancy configurations at the atomic scale to balance recombination pathways and surface activity remains challenging. Additionally, although the lattice vibrational energy generated during non-radiative recombination can promote localized photothermal effects, the energy transfer efficiency and spatial distribution are difficult to precisely control, which may lead to uneven thermal gradients or localized energy dissipation, thereby limiting the overall reaction efficiency. Current research predominantly focuses on single types of vacancy defects, and there is a lack of in-depth analysis of the coupling mechanism between vacancy spatial distribution and electronic structure. As a result, the synergistic relationship between non-radiative recombination and surface reaction sites has not been fully elucidated, hindering the further application of non-radiative recombination in gas sensor and catalysis. Hence, this study proposes a method to customize the spatial distribution of V o through thermally driven migration under vacuum, successfully achieving the construction of asymmetric V o . During the migration process, V o spontaneously form electron transport channels connecting the bulk to surface regions, while generating an defect level structure with gradient. This unique spatial configuration enhances electron transfer efficiency and induces photothermal effect. The experimental results indicate that the synergistic enhancement of electron lifetime and reactivity significantly improves the sensing performance of CeO 2 , enabling an ultra-fast and stable response within 4 s to 5 ppm NO 2 at room temperature. DFT calculations further confirm that the performance enhancement stems from the synergistic optimization of the carrier transport mechanism and surface active sites, both of which alter the adsorption behavior of gas molecules and significantly reduce the reaction activation energy barrier. This study provides an in-depth understanding, from atomic and electronic perspectives, of the influence of spatially tailored V o on the response mechanism of photoactivated gas sensors, and lays the foundation for the application of non-radiative recombination in the field of gas sensor. Results and Discussion Structural Characterization of Various Samples From the perspectives of energy minimization and defect diffusion kinetics, low-temperature vacuum heat treatment induces the formation of a high concentration of V o on the CeO 2 surface, thereby establishing a significant chemical potential gradient between the surface and the bulk 31 . As the treatment temperature increases, the system acquires sufficient energy to activate oxygen ions, enabling their migration via a lattice diffusion mechanism. This kinetic process drives the V o to move from the surface toward the interior of the material along the direction of decreasing chemical potential. Firstly, to investigate V o migration path with the change of temperature, we employed a combined approach using molecular dynamics (MD) and density functional theory (DFT). As shown in Fig. 1a, the evolution of oxygen atoms occurs in distinct stages with increasing temperature. Initially, as the temperature rises to 473K, the surface oxygen atom is escaped. Upon further heating to 560K, cleavage of Ce-O bonds takes place in the bulk region. When the temperature reaches 673K, a bulk oxygen atom migrates to the surface and bonds with a surface Ce atom. This is followed by the rupture of another bulk Ce-O bond at 780K. Ultimately, An additional oxygen atom from the bulk region migrates to the surface and bonds again with a surface Ce atom at 873K, completing a second migration cycle (Fig. S1). The energy barriers changes during V o migration is further explored via DFT. The reaction barrier for the breaking of surface Ce-O bonds and the escape of an oxygen atom is 3.59 eV. The subsequent cleavage of Ce-O bonds in the bulk region has a reaction barrier of 0.34 eV. The migration of a bulk oxygen atom to the surface and the formation of a Ce-O bond exhibit a reaction barrier of -0.52 eV. The rupture of another bulk Ce-O bond has a barrier of 1.44 eV, while the migration and bonding of an additional bulk oxygen atom to the surface show a reaction barrier of -1.81 eV. Both MD and DFT results indicate that as temperature increases, internal Ce-O bonds begin to break, while the migration of bulk oxygen atoms to the surface and subsequent Ce-O bond formation occur spontaneously without requiring external energy input. Therefore, by controlling the vacuum heat treatment temperature, the spatial distribution of V o in CeO 2 can be tuned to achieve a balance between V o S and bulk oxygen vacancies (V o B ), thereby simultaneously optimizing both charge carrier transport efficiency and surface reactivity. To track the oxygen migration in real time and unravel its correlation with the structural evolution of CeO 2 , we utilized a combined in situ approach to probe the dynamic evolution of the crystal structure and surface chemistry. X-Ray Diffraction (XRD) patterns and Raman spectra confirm that the cubic phase structure remains intact after annealing treatments in vacuum atmospheres (Fig. S2). In situ transmission electron microscopy (TEM) is employed to investigate the structural evolution of CeO 2 during vacuum heat treatment. As shown in Fig. 1b and Fig. S3, the interplanar spacing of the (200) planes in CeO 2 is measured to be 0.271 nm. When the temperature reached 200 o C, the interplanar spacings in the edge and center regions of CeO 2 are 0.311 nm and 0.294 nm, respectively. Under low-temperature vacuum annealing, oxygen desorption from the CeO 2 surface generates positively charged V o . To compensate the charge imbalance, adjacent Ce 4+ ions are reduced to larger-radius Ce 3+ ions. The enhanced interionic repulsion induces local lattice expansion, leading to an increase in interplanar spacing. When the temperature is increased to 400 o C, the interplanar spacing in the edge region decreased from 0.311 nm to 0.301 nm, while that in the center region increased from 0.294 nm to 0.302 nm. This indicates that Ce-O bonds reorganize in the bulk region, generating additional V o B , while oxygen atoms migrate toward the surface and bond with surface Ce atoms, thereby reducing V o S . At 600 o C, the interplanar spacing in the edge region further decreased, whereas that in the center region continued to increase. The dynamic evolution of V o S during the heating process is further investigated using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). As the temperature increased from room temperature to 200 o C under vacuum, surface oxygen atoms are released from CeO 2 , causing the V o S concentration to rise from 4.4% to 7.2%. Subsequently, when the temperature is elevated from 200 o C to 400 o C, the V o S concentration decreased from 7.2% to 5.2%. Finally, a further increase in temperature from 400 o C to 600 o C lead to a reduction in V o S concentration from 5.2% to 4.6% (Fig.1c and Fig. S4). The spatial distribution of V o in CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C are further investigated by hard X-ray photoelectron spectroscopy (HAXPES). As shown in Fig.1d-e, the concentration of V o S and V o B in CeO 2 -B are 5.6% and 4.2%, respectively. More notably, the variation in V o S and V o B concentrations across CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C provides direct evidence that elevating the vacuum annealing temperature drives the inward migration and conversion of V o S to V o B (Fig. S5-S6). In conclusion, the spatial distribution of V o in CeO 2 can be precisely controlled by adjusting the vacuum heat treatment temperature. This provides a foundation for constructing asymmetric V o , thereby inducing a non-radiative recombination-mediated photothermal effect (Fig.1f). The migration of V o causes changes in the CeO 2 crystal structure and surface chemistry, resulting in significant electronic structure modifications. We further utilized atomic-resolution monochromated EELS to probe the impact of V o migration on the electronic structure across the outermost 6 atomic layers at the CeO 2 particle edge. (Fig. 2a). Fig. 2b displays the ratios and positions of the Ce-M 5 and Ce-M 4 edges across atomic layers from region-1 to region-3 in CeO 2 -B, confirming that the valence state of Ce is predominantly Ce 3+ in regions-1 to regions-2, while it is mainly Ce 4+ in region-3. EDS mapping of Ce also confirmed that Ce 3+ is dominant in the edge region. Furthermore, the position of the M 5 edge in region-2 shifts toward higher energy and the ratio of the Ce-M 5 to Ce-M 4 edges is lower than that in region-1, indicating a higher concentration of Ce 3+ in region-1 compared to region-2. The Aberration Corrected Transmission Electron Microscope (AC-TEM) also confirm the existence of vacancies in CeO 2 -B surface (Fig. 2c). EELS analysis of CeO 2 and CeO 2 -A confirmed that the concentration of V o increases when calcined in a vacuum environment at 200 o C. However, as the heat treatment temperature rises to 400 o C, the V o concentration in regions-2 is continue decreased with heat treatment temperature further increases to 600 o C. The differences in plasmon signal intensity further demonstrate that heat treatment in a vacuum environment disrupts the Ce-O structure, which enhance the free electron concentration on the CeO 2 surface. Nevertheless, the plasmon signal intensity of CeO 2 -A surface is stronger than that of CeO 2 -B and CeO 2 -C, indicating that during the increase in vacuum heat treatment temperature from 200 o C to 400 o C, a small number of lattice oxygen atoms within CeO 2 break away to form V o B , which then migrate to the surface and partially fill the V o , thereby restoring the Ce-O structure (Fig. S7-S9). When the vacuum heat treatment temperature reaches 600 o C, this phenomenon becomes more pronounced. EELS results confirm that V o S in region-1 exhibit high stability and do not migrate under vacuum thermal driving, thereby preserving the electron concentration and reaction activity. In contrast, V o S in regions-2 and regions-3 are less stable, which migrate into bulk to form V o B . Electron paramagnetic resonance (EPR) analysis confirms that the heat treatment promotes the conversion of high-coordinated, low-activity Ce 3+ -g1 on the CeO 2 surface into low-coordinated, highly active Ce 3+ -g2 (Fig. 2e). The introduction of heat treatment at vacuum environment enhance the specific surface area of CeO 2 , which promote the adsorption gas ability (Fig. S10). X-ray absorption near-edge structure (XANES) measurements reveals a red shift in the absorption edge of CeO 2 -A compared to pristine CeO 2 , indicating the detachment of surface oxygen atoms from CeO 2 after vacuum heat treatment at 200 o C. In contrast, CeO 2 -B exhibits a blue shift relative to CeO 2 -A, suggesting that V o on the surface are filled by oxygen atoms migrating from the sub-surface layer, thereby reducing the concentration of V o S . Furthermore, CeO 2 -C showed a red shift compared to CeO 2 -A, which can be attributed to the formation of extensive V o B resulting from the increased annealing temperature of 600 o C (Fig. 2f). X-ray absorption fine structure (EXAFS) fitting curves and wavelet-transform pattern-EXAFS signals is shown in Fig. 2g-h, which also confirm the dynamic reduction mechanism in which V o initially form on the surface, migrate through the sub-surface region and ultimately propagate into the bulk phase as the vacuum heat treatment temperature increases from 200 o C to 600 o C (Fig. S11-S12). Therefore, an appropriate vacuum heat treatment temperature will drive the migration of V o , leading to a balanced concentration of V o on the surface and inside CeO 2 , thereby forming stable asymmetric V o . The density of states (DOS) analysis confirms that the generation of V o S in CeO 2 -A introduces defect levels within the band gap and leads to the formation of a hybridized state between Ce4f and O2p orbitals, which enhances oxygen mobility and free electron transfer efficiency. As V o S become filled and V o B emerge, the defect states of these two spatially distinct V o are located at different positions near the Fermi level. This results in minimal overlap and hybridization between their wavefunctions. Consequently, no hybridized state is observed within the bandgap of either CeO 2 -B and CeO 2 -C. Nevertheless, the presence of V o B still reduces the band gap, which facilitates electron transfer (Fig. 2i). 2.2. Excited State Dynamics Consequently, the altered electronic structure further modifies the lifetime and transfer mechanisms of photogenerated charge carriers. To probe this effect, we employed femtosecond transient absorption (TA) spectroscopy to investigate how the spatially tailored behavior of V o influences the charge excitation dynamics in CeO 2 . The change from photo-absorption to photo-bleaching reflects the cooling process of the carriers to the band edge. Fig. 3a shows that the 2D pseudo-color TA map of CeO 2 -B exhibits a broader range of photoinduced absorption compared to CeO 2 , CeO 2 -A and CeO 2 -C (Fig. S13). This result indicates the V o B and V o S form a complementary energy-level relationship at equilibrium. Specifically, surface vacancies primarily form shallow defect states due to their unsaturated coordination environment, while bulk vacancies generate deeper and more broadly distributed localized states. Together, they constitute a quasi-continuous defect-state band, significantly promoting multi-path photoinduced electron transitions from the valence band to defect states and among different defect states. Time-resolved TA spectra shows that the increase in signal intensity within the 1~8 ps time range corresponds to the trapping of photogenerated electrons from the conduction band by deep-level V o B defect states, indicating an increase in defect state population. The subsequent signal decay between 8~120 ps reflects the non-radiative recombination between trapped electrons and holes, leading to a gradual reduction in the number of excited electrons. This two-stage kinetics, characterized by an initial rise followed by a decay, demonstrates that V o act as effective electron traps, delaying recombination and prolonging charge carrier lifetime. The attenuation dynamics of ground state bleach signals in CeO 2 -B at 350 and 390 nm are fitted using a multi-exponential model. Fig. 3c displays the transient absorption kinetics of CeO 2 -B probed at 350 nm, covering both long-term (0~150 ps) and short-term (0~10 ps) timescales. The long-term kinetics reflect the carrier thermalization and relaxation processes, while the short-term dynamics represent rapid carrier trapping or initial recombination. Oscillatory signals emerging after 10 ps suggest that carrier thermalization and relaxation are dominated by electron-acoustic phonon interactions 32 . Furthermore, the decay lifetime of CeO 2 -B is significantly longer than those of CeO 2 , CeO 2 -A and CeO 2 -C. The results of TA confirms that the formation of a vertically asymmetric V o structure enables the rapid transfer and efficient non-radiative recombination of photogenerated carriers. This process induces a photothermal effect and prolongs the charge carrier lifetime. The enhancement of lifetime and transfer efficiency of photogenerated carriers can increase the surface free electron concentration of CeO 2 -B. Therefore, the light-assisted Kelvin probe force microscopy (KPFM) is employed to characterize the contact potential difference (CPD) of various samples. As shown in Fig. 3d, the CPD values of CeO 2 -B measure 87 mV in the dark and 125 mV under light illumination, respectively. This increase indicates a significant rise in surface electron concentration upon light irradiation. Moreover, both the surface potential and its light-induced change are substantially greater in CeO 2 -B compared to the other samples, suggesting that CeO 2 -B exhibits enhanced photo-induced electron generation (Fig. S14). Additionally, the photocurrent signal, EIS and PL spectra result are confirming the CeO 2 -Bhave stronger carries transfer ability than that of CeO 2 , CeO 2 -A and CeO 2 -C(Fig. S15). Photoluminescence spectroscopy confirms that radiative recombination in CeO 2 is strongly enhanced by V o S due to their coordinatively unsaturated structure and weak electron-phonon coupling. On the contrary, a part of V o S convert to V o B can produce the stronger electron-phonon coupling, which promotes efficient multiphonon relaxation. This process dissipates excited-state energy non-radiatively as heat, leading to significant fluorescence quenching (Fig.3e). The carrier lifetime for various samples is further explored via time-resolved photo-luminescence spectroscopy. The raw data and fitted data of ns-level timeresolved fluorescence spectra of various samples also confirm the electron lifetime of CeO 2 -B is longer than that of CeO 2 , CeO 2 -A and CeO 2 -C (Fig.3f). Under identical illumination conditions, the temperature distribution of CeO 2 -B confirms the presence of photothermal effect (Fig. 3g). Upon light irradiation, the surface temperature of CeO 2 -B reaches 68.3 o C. After the light source is removed, the surface temperature returns to 26.1 o C. Electron spin resonance (ESR) spectroscopy is further employed to investigate the ability of different samples to generate reactive free radicals on their surfaces. Under light irradiation, photogenerated holes in the valence band of CeO 2 can react with surface-adsorbed water to produce •OH radicals. The generation of •OH not only consumes surface-adsorbed water, thereby reducing the influence of humidity, but also promotes the deep oxidation of NO 2 to NO 3 - , which suppresses secondary reaction processes and contributes to a shorter response and recovery time. As shown in Fig. 3a, the •OH signal intensity of CeO 2 -B is stronger than that of CeO 2 , CeO 2 -A and CeO 2 -C, indicating that the introduction of asymmetric V o enhances hole generation and increases the concentration of surface •OH active species. The carrier transfer mechanism is schematically illustrated in Fig. 3i. V o B function as efficient electron trapping and transport channels, suppressing bulk recombination and facilitating directional electron migration to the surface. Meanwhile, V o S serve as energy release terminals, strongly attracting electrons and holes to undergo highly efficient non-radiative recombination at localized sites. This synergistic effect collectively converts carrier energy into lattice vibrational energy via multi-phonon emission, thereby inducing a pronounced localized photothermal effect on the material surface. 2.3. Gas Performance of Detecting NO 2 at Room Temperature the simultaneous enhancement of carrier lifetime and transfer efficiency modifies the surface reactivity of CeO 2 -B. To further assess its practical applicability, we systematically evaluated its response-recovery time, dynamic response characteristics, and long-term stability. The sensing layers of CeO 2 -B is authenticated to be 142 μm thick (Fig. S16). The response values of CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C to various concentrations of NO 2 under both light and dark conditions are presented in Fig. 4a. In the dark, the response values of CeO 2 -A to NO 2 is higher than that of CeO 2 , which can be attributed to the variation in V o S . CeO 2 -C exhibits a slightly improved response compared to CeO 2 -A due to the formation of V o B , which enhance the migration efficiency of O - species and facilitate electron transfer. In contrast, when the concentrations of V o S and V o B reach equilibrium, the highly efficient bulk oxygen migration and charge transport channels provided by V o B achieve optimal kinetic matching with the gas adsorption and reaction sites dominated by V o S . As a result, CeO 2 -B demonstrates a higher response value than CeO 2 -C (Fig. S17-S18). Under light illumination, the response values of all samples are enhanced owing to the generation of photoinduced charge carriers. Notably, the response of CeO 2 -B increases by a factor of three (Fig. S19-S20). The baseline resistance of the sensing layers for all samples is shown in Fig. 4b. Under dark conditions, the baseline resistance increases significantly with rising NO 2 concentration, thereby reducing the detection accuracy of the sensor. In contrast, illumination suppresses the baseline resistance drift. However, the rapid recombination of electron hole pairs in CeO 2 , which occurs on a submicrosecond timescale, still limits the performance of the photoactivated sensor. While the introduction of V o S or V o B mitigates baseline resistance drift in CeO 2 -A and CeO 2 -C, the baseline resistance of CeO 2 -B remains particularly stable throughout the detection of 0.5 to 10 ppm NO 2 . Additionally, CeO 2 -B exhibits a positive linear response to NO 2 within this concentration range. Using the Root Mean Square Deviation (RMSD) method 33 , the theoretical limit of detection (LOD) for CeO 2 -B is determined to be 7.24 ppb in the dark and 0.002 ppb under light, both of which are superior to CeO 2 , CeO 2 -A and CeO 2 -C (Fig. 4c). The influence of asymmetric V o on response time is also illustrated in Fig. 4c. Under dark conditions, the response times of CeO 2 -A and CeO 2 -C are shorter than that of CeO 2 . The balance between bulk oxygen replenishment and surface activity oxygen consumption modifies surface properties and enhances adsorption activation, thereby further shortening the response time of CeO 2 -B. Under light conditions, the response times of CeO 2 -A and CeO 2 -C are significantly reduced by approximately twofold and are much faster than that of CeO 2 . Moreover, the photothermal effect associated with nonradiative recombination further decreases the response and recovery time of CeO 2 -B, which reaches 4 s for 5 ppm NO 2 detection (Fig. 4d). The cycle stability performance of various samples is evaluated toward 2 ppm NO 2 at room temperature over five response-recovery cycles (Fig. 4e). Under dark conditions, the low surface activity of CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C leads to the accumulation of intermediate products on the surface during reactions, resulting in a gradual decrease in response values as the number of cycles increases. After the introduction of light illumination, the cycling stability of all samples improves. However, V o S and V o B , which act as recombination centers for electron-hole pairs, still partially reduce the cycling stability of CeO 2 -A and CeO 2 -C. In contrast, the photothermal effect generates hot electrons that significantly enhance the surface activity of CeO 2 -B, promoting the formation of stable reaction products on its surface. As a result, CeO 2 -B exhibits exceptional cycling stability under light conditions (Fig. S21-S22). The XPS spectra of CeO 2 -Bafter reaction confirm the stability of Ce 3+ and V o (Fig. S23). The reaction mechanism between active species and O 2 is further investigated by testing the gas-sensing performance at different oxygen concentrations. As shown in Fig. 4f, the response values of all samples toward NO 2 detection increase with rising oxygen concentrations under dark conditions. The variation in response values of CeO 2 -A and CeO 2 -C is significantly greater than that of pristine CeO 2 , indicating that the introduction of V o S or V o B enhances the adsorption activity or oxygen transfer capability of CeO 2 , thereby facilitating the generation of more active oxygen. Notably, the variation in response value of CeO 2 -B is more pronounced than that of CeO 2 -A and CeO 2 -C, confirming that the equilibrium between V o S and V o B enables optimal kinetic matching between the reaction rate of surface-active oxygen and the migration rate of internal oxygen. Under light conditions, the generation of photoinduced carriers further enhances the surface reactivity of all samples. In particular, the high-energy hot electrons generated via the photothermal effect significantly improve the oxygen adsorption and activation capabilities of CeO 2 -B. Consequently, the response values of all samples toward NO 2 detection increase further across varying oxygen concentrations, with CeO 2 -B exhibiting a more substantial change in response compared to CeO 2 -A and CeO 2 -C. In summary, holes play an important role in photoactivated gas sensors. Therefore, we further explored the influence of holes on response-recovery time by introducing TEOA into the reaction system to capture holes during the surface reaction process. As shown in Fig. 4g, the recovery time of CeO 2 -B toward NO 2 detection increases significantly when holes are eliminated. This suggests that the removal of holes leads to an accumulation of NO 2 - in the intermediate products on the surface of CeO 2 -B. Additionally, the response value of CeO 2 -B toward NO 2 detection decreases. The response dynamic curve of CeO 2 -B toward low-concentration NO 2 is shown in Fig. 4h. The response value of CeO 2 -B toward 5 ppb NO 2 is 1.22, indicating that the sensor is capable of detecting concentrations below the EPA limit (53 ppb). Furthermore, long-term stability monitoring over 40 days of NO 2 detection under light conditions is shown in Fig. 4i. The response value and response time of CeO 2 -B show only minor changes compared to the first day, demonstrating excellent long-term stability. Compared with previously reported semiconductor-based NO 2 gas sensors, CeO 2 -B exhibits a faster response time (Fig. 4j). Additionally, the baseline resistance of CeO 2 -B remains stable at room temperature. Therefore, CeO 2 -B shows great potential for practical applications. Theoretical results To understand the influence of the vertically asymmetric V o on the adsorption and activation of molecules on the CeO 2 surface, DFT calculations are employed to analyze the differential charge density of various samples in response to O 2 and NO 2 . The optimized adsorption models and adsorption sites are shown in Fig. 5a. The adsorption energies of CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C toward O 2 are -1.32, -1.37, -1.83 and -1.38 eV, respectively. The variation in electron accumulation and depletion is more pronounced in CeO 2 -B than in CeO 2 , CeO 2 -A and CeO 2 -C, indicating that the formation of V o B and V o S enhances carrier transfer during the generation of active oxygen. When active oxygen is already present on the surface, the adsorption energies of CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C toward NO 2 are -0.41, -1.02, -1.36 and -0.97 eV, suggesting that the formation of V o B and V o S also improves the reaction efficiency between active oxygen and NO 2 . The projected density of states (PDOS) for O 2 adsorption on CeO 2 -B further confirms that the formation of V o B enhances electron accumulation at V o S sites, thereby accelerating the conversion of O 2 into active oxygen. The amount of charge transferred between O 2 and V o S is considerably higher than that in CeO 2 , CeO 2 -A and CeO 2 -C (Fig. S24). Additionally, the formation of V o B and V o S in CeO 2 increases electron concentration and reduces the energy required to excite electrons from the O 2p state to the Ce 4f state. The PDOS of NO 2 adsorption on CeO 2 -B also confirms significant electron transfer between NO 2 and the V o S sites (Fig. 5b). The Bader charge transfer between O 2 and CeO 2 -B is 0.314 e and the O-O atomic distance is 1.234 Å. For NO 2 and CeO 2 -B, the Bader charge transfer is 0.244 e and the N-O atomic distance is 1.225 Å. These results confirm that the formation of asymmetric V o facilitates the adsorption and activation of O 2 and NO 2 (Fig. 5c). Consequently, the detection efficiency of CeO 2 -B toward NO 2 is significantly superior to that of CeO 2 , CeO 2 -A and CeO 2 -C (Fig. S25). The reaction process between NO 2 and O 2 in CeO 2 -Bis explored via in situ NAP-XPS (Fig. 5d). Under dark environment, the reaction of electrons with O 2 in CeO 2 -Bto produce activity oxygen causes the peaks of V o S shift from 528.81 eV to 529.24 eV and the concentration of V o S is decrease from 5.6% to 3.3%. Then, the peaks of V o S in CeO 2 -Bshift from 529.24 eV to 529.06 eV when NO 2 enters the reaction chamber and the concentration of V o S is increased from 3.3% to 4.1%. Under light environment, the introduction of light illumination enables CeO 2 to generate more photogenerated carriers, which enhance the concentration of V o S . Similarly, the reaction of electrons with O 2 in CeO 2 -Bto produce activity oxygen causes the peaksshift to higher binding energy. However, the peak associated with V o S is disappeared. This indicates that the photothermal effect promotes the transformation of unpaired electrons at V o S sites into high-energy hot electrons. The high reactivity of these hot electrons subsequently lowers the energy barrier for the conversion of O 2 to activity oxygen, resulting in the complete occupancy of the V o S sites by activity oxygen. Then, the peaks of V o S is appearedin CeO 2 -Bandshift to lower binding energy when NO 2 enters the reaction chamber. Similarly, under both dark and illuminated conditions, the introduction of O 2 and NO 2 also induces peak shifts of V o S in CeO 2 , CeO 2 -A and CeO 2 -C. However, unlike the case of CeO 2 -B, the conversion of O 2 to activity oxygen does not lead to the disappearance of V o S in CeO 2 , CeO 2 -A and CeO 2 -C. This suggests that the formation of photothermal effect enhances the reactivity of CeO 2 -B and reduces the energy barrier for the conversion of O 2 to ROS. Furthermore, during the reaction between activity oxygen and NO 2 , the V o S concentration in CeO 2 , CeO 2 -A and CeO 2 -C does not recover to near its initial level. This indicates that the formation of nonradiative recombination in CeO 2 -B not only accelerates electron transfer efficiency but also supplies sufficient lattice oxygen to the surface to participate in the reaction, thereby improving the long-term stability of V o S and ultimately enabling rapid and stable detection of NO 2 (Fig. S26-S28). In-situ FTIR spectroscopy is conducted to analyze the reaction of NO 2 on the surface of various samples under both dark and light conditions. As shown in Fig. 5e, the surface reaction process of CeO 2 -B during NO 2 detection can be divided into three stages. Under dark conditions, the peaks observed at 1484 cm -1 and 2406 cm -1 correspond to the formation of NO 2 - and bi-NO 3 - , respectively 34 . The absorbance intensities of NO 2 - and bi-NO 3 - indicate that the surface products of CeO 2 -B are primarily composed of unstable NO 2 - . During the equilibrium stage, a peak appears at 1253 cm -1 corresponding to bi-NO 3 - , suggesting that unstable NO 2 - can react with activity oxygen to form bi-NO 3 - , which reduces the timeliness and accuracy of the sensor response 35 . In the recovery stage, the absorbance intensity of bi-NO 3 - continues to increase initially, indicating that CeO 2 -B exhibits poor desorption capacity due to its low activity in the dark, thereby prolonging the recovery time. In contrast, the surface reaction process under light irradiation differs significantly across the samples. During the response stage, peaks appear at 1423 cm -1 and 1612 cm -1 , corresponding to br-NO 3 - and mon-NO 3 - , respectively 36 . This demonstrates that the photothermal effect enhances the activity of the CeO 2 -B surface, facilitating the conversion of NO 2 into stable NO 3 - and effectively suppressing subsequent secondary reactions (Fig. S29). The peak intensities of bi-NO 3 - is stronger than that of mon-NO 3 - and br-NO 3 - . Moreover, the peak intensity of NO 2 - under light irradiation disappears almost completely compared to that in the dark. Therefore, the photothermal effect promotes the conversion of NO 2 - to NO 3 - , further reducing the NO 2 detection time. Based on the in-situ FTIR spectral analysis, the intermediate and final products of CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C during NO 2 detection are identified. The activation barriers of the reaction processes are further investigated using DFT calculations (Fig. 5f). Initially, adsorbed O 2 molecules react with electrons to spontaneously generate activity oxygen species on the surfaces of CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C. The lower energy barriers of CeO 2 -A, CeO 2 -B and CeO 2 -C compared to CeO 2 imply that the formation of V o B and V o S promotes the generation of more activity oxygen species. The reaction barriers for CeO 2 , CeO 2 -A and CeO 2 -C are 2.37 eV, 1.48 eV and 1.15 eV, while that of CeO 2 -B is 1.07 eV. These results indicate that the introduction of asymmetric V o enhances charge transfer capability at the reaction interface and reduces the reaction barrier. Subsequently, the generated activity oxygen species spontaneously react with NO 2 to form NO 3 - . The reaction barriers for CeO 2 , CeO 2 -A, CeO 2 -B and CeO 2 -C are 0.01 eV, -0.93 eV, -2.08 eV and -0.58 eV, demonstrating that asymmetric V o also strengthen the interaction between NO 2 and CeO 2 -B. Therefore, the significantly accelerated detection of NO 2 can be attributed to the asymmetric V o , which enhance carrier transport and induce a photothermal effect. Therefore, the surface reaction mechanism of CeO 2 -B toward NO 2 under light illumination is illustrated in Fig. 5g. Under light irradiation, the generated holes exhibit strong oxidation capability, enabling the deep oxidation of NO 2 to NO 3 - . Moreover, the photothermal effect original from phonon scattering can produce hot electrons, which further mitigates the accumulation of the intermediate reactive species NO 2 - on the surface. Theoretically, suppressing the formation of secondary reaction products can help regulate the baseline resistance of the sensing layer. Discussion This study demonstrates that tailoring the spatial distribution of V o in CeO 2 can overcome the fundamental limitation of high electron-hole recombination rates in photoactivated gas sensor. In this system, V o B functions as an electron reservoir and transport channel, effectively suppressing bulk recombination while directing electrons to surface reaction sites. Simultaneously, V o S acts as a non-radiative recombination center, converting carrier energy into lattice vibrations via multi-phonon scattering and generating a localized photothermal effect. This unique mechanism transforms the typically detrimental recombination process into a beneficial one and producing hot electrons and providing thermal energy to lower reaction barriers. The enhanced charge activity and prolonged carrier lifetime increase the concentration of surface active species, which effectively suppresses the formation of intermediate products and prevents secondary reactions, thereby enabling CeO 2 to achieve stable detection of 5 ppm NO 2 within 4 s at room temperature. DFT confirm that the formation of non-radiative recombination promotes the adsorption of both O 2 and NO 2 on CeO 2 , while reducing the reaction energy barrier for the conversion of NO 2 to NO 3 - . By revealing the performance enhancement mechanism through the perspective of photothermal effects induced by non-radiative recombination regulating surface chemical properties, this research provides novel design principles for developing high-efficiency, highly stable gas sensors. Methods CeO 2 was synthesized by hydrothermal method as well. 0.868 g Ce(NO 3 ) 3 ·6H 2 O and 9.6 g NaOH are dissolved in 35 mL and 5 ml of distilled water, respectively. As the clear solution formed, NaOH aqueous solution is slowly added to the Ce(NO 3 ) 3 ·6H 2 O with vigorous stirring. After stirring for about 30 mins, and mixture solution was subjected to hydrothermal treatment at 180 o C for 24 h. The obtained powders were washed with water and ethanol for three cycles, dried in vacuum over a night, and further subjected to calcination at 550 o C for 4 h. CeO 2 -A, CeO 2 -B and CeO 2 -C are synthesized by the high temperature heat treatment at 200 o C, 400 o C and 600 o C for 1 h under vacuum. Fabrication and testing of a gas sensor and experimental detail 5 mg of samplesare mixed with 50 μL of deionized water to obtain the corresponding slurry. 5 μL of slurry was then dripped on an Pt interdigitated electrodes (10 mm × 5 mm × 0.25 mm, AURORA technologies, China) to form a resistance-type sensor. All the fabricated sensors were aged in air at 80 °C for 2 h. The gas-sensing performance of the fabricated sensors was evaluated using an intelligent gas-sensing analysis system (CGS-4TPs, Beijing Elite Tech Co., Ltd.). To produce test gases with the necessary concentrations, a dynamic gas and liquid distribution system (DGL-III, Beijing Elite Tech Co., Ltd, China) having three mass flow controllers was used. We use nitrogen as the carrier gas, and control the gas flow of oxygen and nitrogen to control the different oxygen concentrations in the reaction chamber. Characterization and measurements The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and Xray energy dispersive spectroscopy (EDS) mapping was recorded on aberration-corrected TEM (FEI Titan Cubed Themis G2 300) at an accelerating voltage of 300 kV. X-ray absorption fine spectra (XAFS) measurements were measured on the B11 station in Shanghai Synchrotron Radiation Facility (SSRF). The KPFM (Bruker MultiMode-8 surface potential mode) was used to characterize the surface potential. The SCM-PIT-V2 model tip and AS-130VLR (“J” vertical) scanner model was used, and the radius and elastic coefficient of tip were 35 nm and 3 N/m, respectively. The In situ Fourier-transform infrared (In situ FT-IR) spectra were recorded on a Bruker Vertex 70 FTIR spectrometer equipped with in situ reaction chamber. The X-ray photoelectron spectroscopy (Axis Supra) measurements were operated with Al Ka radiation (1486.6 eV). 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08:55:37","extension":"html","order_by":15,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":118903,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/a4754b493efa24f67998bbfe.html"},{"id":97667249,"identity":"dcd03f9a-29a7-4f74-8025-6ad6d8df97ab","added_by":"auto","created_at":"2025-12-08 09:23:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1576555,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStructural characterization of CeO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e a MD analysis of CeO\u003csub\u003e2\u003c/sub\u003e. b \u003cem\u003eIn-situ \u003c/em\u003eTEM image of CeO\u003csub\u003e2\u003c/sub\u003e. c \u003cem\u003eIn-situ \u003c/em\u003eNAP-XPS of O2p high-resolution. d-e HAXPES of O2p high-resolution. f Diagram of V\u003csub\u003eo\u003c/sub\u003e transport.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/652a42d35b9f35decdb5968b.png"},{"id":97424336,"identity":"acb7dac1-2d7d-4d36-88a9-f28883bdc8ae","added_by":"auto","created_at":"2025-12-04 08:55:36","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1745422,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectronic characterization of CeO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e a-b EELS analysis of CeO\u003csub\u003e2\u003c/sub\u003e-B. c AC-TEM image of CeO\u003csub\u003e2\u003c/sub\u003e-B. d Valence electron energy loss spectra of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C. e EPR analysis. f Ce L3-edge XANES spectra. g Ce L3-edge Fourier-transformed EXAFS spectra. h WT-EXAFS signals at Ce L3-edge. i DOS analysis.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/9a8f757a6013e73f5e5a308e.png"},{"id":97424338,"identity":"b8465160-c621-4ee0-9324-15a359c08189","added_by":"auto","created_at":"2025-12-04 08:55:36","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1887317,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCarrier dynamics analysis of CeO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e a 2D pseudo-color TA maps of CeO\u003csub\u003e2\u003c/sub\u003e-B. b TA spectra of the CeO\u003csub\u003e2\u003c/sub\u003e-B at different time delays. c corresponding normalized exciton relaxation dynamics. d KPFM analysis. e PL spectra. f TRPL spectra. g temperature distribution toward CeO\u003csub\u003e2\u003c/sub\u003e-B under the same light irradiation conditions. h DMPO•OH spin[1]trapping ESR spectra. i Module functionalization diagram of asymmetric V\u003csub\u003eo\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/81b37b025832e174ff78e1d7.png"},{"id":97424337,"identity":"0925a524-3ce2-4b79-adc6-a3ae10d84aba","added_by":"auto","created_at":"2025-12-04 08:55:36","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1617131,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGas sensing performance of CeO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e a Response values. b Baseline resistance. c Response-concentration correlation. d Response time. e 5-cycle dynamic sensing response. g Trapping experiment of CeO\u003csub\u003e2\u003c/sub\u003e-B toward holes. h Response values of CeO\u003csub\u003e2\u003c/sub\u003e-B toward low concentration NO\u003csub\u003e2\u003c/sub\u003e. i Long-term stability test. j response-recovery time (operating temperature: 25\u003csup\u003eo\u003c/sup\u003eC) of this work compared with previous works under light. (Date adapted with permission from Springer Nature, John Wiley and Sons, American Chemistry Society and Elsevier).\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/1e3b9b09015b43651f8c6c05.png"},{"id":97667336,"identity":"b21db000-ac34-4c77-992f-f8edce225a62","added_by":"auto","created_at":"2025-12-08 09:23:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1333756,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eReaction mechanism of CeO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e.\u003c/strong\u003e a Adsorption energy of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C toward O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e. b DOS analysis of CeO\u003csub\u003e2\u003c/sub\u003e-B toward O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e. c Charge difference distributions of CeO\u003csub\u003e2\u003c/sub\u003e-B toward O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e. d \u003cem\u003eIn-situ\u003c/em\u003e NAP-XPS analysis of CeO\u003csub\u003e2\u003c/sub\u003e-B under O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e. e \u003cem\u003eIn-situ\u003c/em\u003e FTIR spectra. f Reaction energy (Ea) and reaction model for the reaction of NO\u003csub\u003e2\u003c/sub\u003e with activity oxygen. g Diagram of reaction process.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/b0de25af459c864c3b12a4a1.png"},{"id":97677536,"identity":"eece920d-aa5b-4953-b104-c72963f70328","added_by":"auto","created_at":"2025-12-08 09:53:24","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":9139317,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/75cdab67-e778-4a0d-ab21-b2b3923030ab.pdf"},{"id":97667497,"identity":"5b9ca390-86c0-49c0-9fe8-2a04b78208b0","added_by":"auto","created_at":"2025-12-08 09:23:39","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":13151721,"visible":true,"origin":"","legend":"supporting imformation","description":"","filename":"supportingimformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8016517/v1/cc0cf70a73fd80576ea4ad17.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eSpatially Tailored Asymmetric Oxygen Vacancies Induce Nonradiative Recombination for Ultrafast and Stable NO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e Sensing\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eEffective real-time monitoring of NO\u003csub\u003e2\u003c/sub\u003e in chemical plant environments is crucial for preventing large-scale gas leakage and dispersion\u003csup\u003e1\u003c/sup\u003e. However, operational challenges in industrial settings, such as air turbulence and acoustic interference, can compromise the accuracy of NO\u003csub\u003e2\u003c/sub\u003e gas sensors\u003csup\u003e2\u003c/sup\u003e. Consequently, the development of gas sensors capable of rapid NO\u003csub\u003e2\u003c/sub\u003e response is of paramount importance for precisely identifying pollution sources and mapping the spatial distribution of this hazardous gas. Conventional metal oxide semiconductors (MOS) gas sensors, while offering advantages such as low cost and compact size, generally suffer from slow response speed and insufficient stability at room temperature\u003csup\u003e3-5\u003c/sup\u003e. The photoactivated gas sensing strategy addresses these limitations by utilizing photogenerated charge carriers to participate in surface reactions, significantly enhancing gas detection performance under ambient conditions\u003csup\u003e6-8\u003c/sup\u003e. However, the rapid recombination of photogenerated electron-hole pairs and low-concentration reaction site severely restricts carriers lifetime and surface reaction efficiency, ultimately impeding further improvement of photoactivated gas sensor performance\u003csup\u003e9\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eIn recent years, defect engineering has become a crucial strategy for modulating the electronic structure and surface chemical activity of MOS\u003csup\u003e10-12\u003c/sup\u003e. Studies have demonstrated that constructing surface oxygen vacancies (V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e) in materials such as TiO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e, SnO\u003csub\u003e2\u003c/sub\u003e and In\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e can effectively enhance their gas adsorption capacity and light absorption performance\u003csup\u003e13-15\u003c/sup\u003e. However, conventional methods primarily rely on creating symmetrically distributed V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e to improve surface reactivity and these symmetric configurations possess inherent limitations\u003csup\u003e16\u003c/sup\u003e. Their highly coordinated nature favors gas adsorption at the cost of intensified radiative recombination and inherent instability in oxidizing environments\u003csup\u003e17-19\u003c/sup\u003e. Therefore, the substantial radiative dissipation of photogenerated electron-hole pairs severely impairs carrier utilization and surface reaction activity. Ultimately, the accumulation of intermediate products and compromised long-term stability inevitably occur in MOS\u003csup\u003e20\u003c/sup\u003e. Previous research has reported that converting radiative recombination into non-radiative recombination can transform carriers recombination energy into lattice vibrational energy\u003csup\u003e21-25\u003c/sup\u003e. This approach effectively suppresses energy dissipation through photon emission while simultaneously enhancing surface reaction kinetics via the generated localized photothermal effect. The underlying energy conversion mechanism significantly prolongs carrier lifetime, permitting increased electron participation in surface gas reactions\u003csup\u003e26\u003c/sup\u003e. Concurrently, the accompanying thermal effect activates and converts reactant molecules more efficiently, thereby reducing intermediate product accumulation. As a non-radiative recombination-dominated process, it circumvents the energy loss characteristic of radiative recombination and ensures continuous energy supply for surface reactions, ultimately achieving substantial improvements in both stability and reaction activity\u003csup\u003e27-30\u003c/sup\u003e. However, achieving efficient and controllable induction of non-radiative recombination processes still lacks a universal strategy. In particular, precise regulation of vacancy configurations at the atomic scale to balance recombination pathways and surface activity remains challenging. Additionally, although the lattice vibrational energy generated during non-radiative recombination can promote localized photothermal effects, the energy transfer efficiency and spatial distribution are difficult to precisely control, which may lead to uneven thermal gradients or localized energy dissipation, thereby limiting the overall reaction efficiency. Current research predominantly focuses on single types of vacancy defects, and there is a lack of in-depth analysis of the coupling mechanism between vacancy spatial distribution and electronic structure. As a result, the synergistic relationship between non-radiative recombination and surface reaction sites has not been fully elucidated, hindering the further application of non-radiative recombination in gas sensor and catalysis.\u003c/p\u003e\n\u003cp\u003eHence, this study proposes a method to customize the spatial distribution of V\u003csub\u003eo\u003c/sub\u003e through thermally driven migration under vacuum, successfully achieving the construction of asymmetric V\u003csub\u003eo\u003c/sub\u003e. During the migration process, V\u003csub\u003eo\u003c/sub\u003e spontaneously form electron transport channels connecting the bulk to surface regions, while generating an defect level structure with gradient. This unique spatial configuration enhances electron transfer efficiency and induces photothermal effect. The experimental results indicate that the synergistic enhancement of electron lifetime and reactivity significantly improves the sensing performance of CeO\u003csub\u003e2\u003c/sub\u003e, enabling an ultra-fast and stable response within 4 s to 5 ppm NO\u003csub\u003e2\u003c/sub\u003e at room temperature. DFT calculations further confirm that the performance enhancement stems from the synergistic optimization of the carrier transport mechanism and surface active sites, both of which alter the adsorption behavior of gas molecules and significantly reduce the reaction activation energy barrier. This study provides an in-depth understanding, from atomic and electronic perspectives, of the influence of spatially tailored V\u003csub\u003eo\u003c/sub\u003e on the response mechanism of photoactivated gas sensors, and lays the foundation for the application of non-radiative recombination in the field of gas sensor.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003eStructural Characterization of Various Samples\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFrom the perspectives of energy minimization and defect diffusion kinetics, low-temperature vacuum heat treatment induces the formation of a high concentration of V\u003csub\u003eo\u003c/sub\u003e on the CeO\u003csub\u003e2\u003c/sub\u003e surface, thereby establishing a significant chemical potential gradient between the surface and the bulk\u003csup\u003e31\u003c/sup\u003e. As the treatment temperature increases, the system acquires sufficient energy to activate oxygen ions, enabling their migration via a lattice diffusion mechanism. This kinetic process drives the V\u003csub\u003eo\u003c/sub\u003e to move from the surface toward the interior of the material along the direction of decreasing chemical potential. Firstly, to investigate V\u003csub\u003eo\u003c/sub\u003e migration path with the change of temperature, we employed a combined approach using molecular dynamics (MD) and density functional theory (DFT). As shown in Fig. 1a, the evolution of oxygen atoms occurs in distinct stages with increasing temperature. Initially, as the temperature rises to 473K, the surface oxygen atom is escaped. Upon further heating to 560K, cleavage of Ce-O bonds takes place in the bulk region. When the temperature reaches 673K, a bulk oxygen atom migrates to the surface and bonds with a surface Ce atom. This is followed by the rupture of another bulk Ce-O bond at 780K. Ultimately, An additional oxygen atom from the bulk region migrates to the surface and bonds again with a surface Ce atom at 873K, completing a second migration cycle (Fig. S1). The energy barriers changes during V\u003csub\u003eo\u003c/sub\u003e migration is further explored via DFT. The reaction barrier for the breaking of surface Ce-O bonds and the escape of an oxygen atom is 3.59 eV. The subsequent cleavage of Ce-O bonds in the bulk region has a reaction barrier of 0.34 eV. The migration of a bulk oxygen atom to the surface and the formation of a Ce-O bond exhibit a reaction barrier of -0.52 eV. The rupture of another bulk Ce-O bond has a barrier of 1.44 eV, while the migration and bonding of an additional bulk oxygen atom to the surface show a reaction barrier of -1.81 eV. Both MD and DFT results indicate that as temperature increases, internal Ce-O bonds begin to break, while the migration of bulk oxygen atoms to the surface and subsequent Ce-O bond formation occur spontaneously without requiring external energy input. Therefore, by controlling the vacuum heat treatment temperature, the spatial distribution of V\u003csub\u003eo\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e can be tuned to achieve a balance between V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and bulk oxygen vacancies (V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e), thereby simultaneously optimizing both charge carrier transport efficiency and surface reactivity.\u003c/p\u003e\n\u003cp\u003eTo track the oxygen migration in real time and unravel its correlation with the structural evolution of CeO\u003csub\u003e2\u003c/sub\u003e, we utilized a combined in situ approach to probe the dynamic evolution of the crystal structure and surface chemistry. X-Ray Diffraction (XRD) patterns and Raman spectra confirm that the cubic phase structure remains intact after annealing treatments in vacuum atmospheres (Fig. S2). In situ transmission electron microscopy (TEM) is employed to investigate the structural evolution of CeO\u003csub\u003e2\u003c/sub\u003e during vacuum heat treatment. As shown in Fig. 1b and Fig. S3, the interplanar spacing of the (200) planes in CeO\u003csub\u003e2\u003c/sub\u003e is measured to be 0.271 nm. When the temperature reached 200\u003csup\u003eo\u003c/sup\u003eC, the interplanar spacings in the edge and center regions of CeO\u003csub\u003e2\u003c/sub\u003e are 0.311 nm and 0.294 nm, respectively. Under low-temperature vacuum annealing, oxygen desorption from the CeO\u003csub\u003e2\u003c/sub\u003e surface generates positively charged V\u003csub\u003eo\u003c/sub\u003e. To compensate the charge imbalance, adjacent Ce\u003csup\u003e4+\u003c/sup\u003e ions are reduced to larger-radius Ce\u003csup\u003e3+\u003c/sup\u003e ions. The enhanced interionic repulsion induces local lattice expansion, leading to an increase in interplanar spacing. When the temperature is increased to 400\u003csup\u003eo\u003c/sup\u003eC, the interplanar spacing in the edge region decreased from 0.311 nm to 0.301 nm, while that in the center region increased from 0.294 nm to 0.302 nm. This indicates that Ce-O bonds reorganize in the bulk region, generating additional V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e, while oxygen atoms migrate toward the surface and bond with surface Ce atoms, thereby reducing V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e. At 600\u003csup\u003eo\u003c/sup\u003eC, the interplanar spacing in the edge region further decreased, whereas that in the center region continued to increase. The dynamic evolution of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e during the heating process is further investigated using in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). As the temperature increased from room temperature to 200\u003csup\u003eo\u003c/sup\u003eC under vacuum, surface oxygen atoms are released from CeO\u003csub\u003e2\u003c/sub\u003e, causing the V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e concentration to rise from 4.4% to 7.2%. Subsequently, when the temperature is elevated from 200\u003csup\u003eo\u003c/sup\u003eC to 400 \u003csup\u003eo\u003c/sup\u003eC, the V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e concentration decreased from 7.2% to 5.2%. Finally, a further increase in temperature from 400 \u003csup\u003eo\u003c/sup\u003eC to 600 \u003csup\u003eo\u003c/sup\u003eC lead to a reduction in V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e concentration from 5.2% to 4.6% (Fig.1c and Fig. S4). The spatial distribution of V\u003csub\u003eo\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C are further investigated by hard X-ray photoelectron spectroscopy (HAXPES). As shown in Fig.1d-e, the concentration of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e in CeO\u003csub\u003e2\u003c/sub\u003e-B are 5.6% and 4.2%, respectively. More notably, the variation in V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e concentrations across CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C provides direct evidence that elevating the vacuum annealing temperature drives the inward migration and conversion of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e to V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u0026nbsp;\u003c/sup\u003e(Fig. S5-S6). In conclusion,\u0026nbsp;the spatial distribution of V\u003csub\u003eo\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e can be precisely controlled by adjusting the vacuum heat treatment temperature. This provides a foundation for constructing asymmetric V\u003csub\u003eo\u003c/sub\u003e, thereby inducing a non-radiative recombination-mediated photothermal effect (Fig.1f).\u003c/p\u003e\n\u003cp\u003eThe migration of V\u003csub\u003eo\u003c/sub\u003e causes changes in the CeO\u003csub\u003e2\u003c/sub\u003e crystal structure and surface chemistry, resulting in significant electronic structure modifications. We further utilized atomic-resolution monochromated EELS to probe the impact of V\u003csub\u003eo\u003c/sub\u003e migration on the electronic structure across the outermost 6 atomic layers at the CeO\u003csub\u003e2\u003c/sub\u003e particle edge. (Fig. 2a). Fig. 2b displays the ratios and positions of the Ce-M\u003csub\u003e5\u003c/sub\u003e and Ce-M\u003csub\u003e4\u003c/sub\u003e edges across atomic layers from region-1 to region-3 in CeO\u003csub\u003e2\u003c/sub\u003e-B, confirming that the valence state of Ce is predominantly Ce\u003csup\u003e3+\u003c/sup\u003e in regions-1 to regions-2, while it is mainly Ce\u003csup\u003e4+\u003c/sup\u003e in region-3. EDS mapping of Ce also confirmed that Ce\u003csup\u003e3+\u003c/sup\u003e is dominant in the edge region. Furthermore, the position of the M\u003csub\u003e5\u003c/sub\u003e edge in region-2 shifts toward higher energy and the ratio of the Ce-M\u003csub\u003e5\u003c/sub\u003e to Ce-M\u003csub\u003e4\u003c/sub\u003e edges is lower than that in region-1, indicating a higher concentration of Ce\u003csup\u003e3+\u003c/sup\u003e in region-1 compared to region-2. The Aberration Corrected Transmission Electron Microscope (AC-TEM) also confirm the existence of vacancies in CeO\u003csub\u003e2\u003c/sub\u003e-B surface (Fig. 2c). EELS analysis of CeO\u003csub\u003e2\u003c/sub\u003e and CeO\u003csub\u003e2\u003c/sub\u003e-A confirmed that the concentration of V\u003csub\u003eo\u003c/sub\u003e increases when calcined in a vacuum environment at 200\u003csup\u003eo\u003c/sup\u003eC. However, as the heat treatment temperature rises to 400\u003csup\u003eo\u003c/sup\u003eC, the V\u003csub\u003eo\u003c/sub\u003e concentration in regions-2 is continue decreased with heat treatment temperature further increases to 600\u003csup\u003eo\u003c/sup\u003eC. The differences in plasmon signal intensity further demonstrate that heat treatment in a vacuum environment disrupts the Ce-O structure, which enhance the free electron concentration on the CeO\u003csub\u003e2\u003c/sub\u003e surface. Nevertheless, the plasmon signal intensity of CeO\u003csub\u003e2\u003c/sub\u003e-A surface is stronger than that of CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C, indicating that during the increase in vacuum heat treatment temperature from 200\u003csup\u003eo\u003c/sup\u003eC to 400\u003csup\u003eo\u003c/sup\u003eC, a small number of lattice oxygen atoms within CeO\u003csub\u003e2\u003c/sub\u003e break away to form V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e, which then migrate to the surface and partially fill the V\u003csub\u003eo\u003c/sub\u003e, thereby restoring the Ce-O structure (Fig. S7-S9). When the vacuum heat treatment temperature reaches 600\u003csup\u003eo\u003c/sup\u003eC, this phenomenon becomes more pronounced. EELS results confirm that V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e in region-1 exhibit high stability and do not migrate under vacuum thermal driving, thereby preserving the electron concentration and reaction activity. In contrast, V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e in regions-2 and regions-3 are less stable, which migrate into bulk to form V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eElectron paramagnetic resonance (EPR) analysis confirms that the heat treatment promotes the conversion of high-coordinated, low-activity Ce\u003csup\u003e3+\u003c/sup\u003e-g1 on the CeO\u003csub\u003e2\u003c/sub\u003e surface into low-coordinated, highly active Ce\u003csup\u003e3+\u003c/sup\u003e-g2 (Fig. 2e).\u0026nbsp;The introduction of heat treatment at vacuum environment enhance the specific surface area of CeO\u003csub\u003e2\u003c/sub\u003e, which promote the adsorption gas ability (Fig. S10). X-ray absorption near-edge structure (XANES) measurements reveals a red shift in the absorption edge of CeO\u003csub\u003e2\u003c/sub\u003e-A compared to pristine CeO\u003csub\u003e2\u003c/sub\u003e, indicating the detachment of surface oxygen atoms from CeO\u003csub\u003e2\u003c/sub\u003e after vacuum heat treatment at 200\u003csup\u003eo\u003c/sup\u003eC. In contrast, CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits a blue shift relative to CeO\u003csub\u003e2\u003c/sub\u003e-A, suggesting that V\u003csub\u003eo\u003c/sub\u003e on the surface are filled by oxygen atoms migrating from the sub-surface layer, thereby reducing the concentration of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e. Furthermore, CeO\u003csub\u003e2\u003c/sub\u003e-C showed a red shift compared to CeO\u003csub\u003e2\u003c/sub\u003e-A, which can be attributed to the formation of extensive V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e resulting from the increased annealing temperature of 600\u003csup\u003eo\u003c/sup\u003eC (Fig. 2f). X-ray absorption fine structure (EXAFS) fitting curves and wavelet-transform pattern-EXAFS signals is shown in Fig. 2g-h, which also confirm the dynamic reduction mechanism in which\u0026nbsp;V\u003csub\u003eo\u003c/sub\u003e initially form on the surface, migrate through the sub-surface region and ultimately propagate into the bulk phase as the vacuum heat treatment temperature increases from 200\u003csup\u003eo\u003c/sup\u003eC to 600\u003csup\u003eo\u003c/sup\u003eC (Fig. S11-S12). Therefore, an appropriate vacuum heat treatment temperature will drive the migration of\u0026nbsp;V\u003csub\u003eo\u003c/sub\u003e, leading to a balanced concentration of\u0026nbsp;V\u003csub\u003eo\u003c/sub\u003e on the surface and inside CeO\u003csub\u003e2\u003c/sub\u003e, thereby forming stable asymmetric\u0026nbsp;V\u003csub\u003eo\u003c/sub\u003e. The density of states (DOS) analysis confirms that the generation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e in CeO\u003csub\u003e2\u003c/sub\u003e-A introduces defect levels within the band gap and leads to the formation of a hybridized state between Ce4f and O2p orbitals, which enhances oxygen mobility and free electron transfer efficiency. As V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e become filled and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e emerge, the defect states of these two spatially distinct\u0026nbsp;V\u003csub\u003eo\u003c/sub\u003e are located at different positions near the Fermi level. This results in minimal overlap and hybridization between their wavefunctions. Consequently, no hybridized state is observed within the bandgap of either CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C. Nevertheless, the presence of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e still reduces the band gap, which facilitates electron transfer (Fig. 2i).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2. Excited State Dynamics\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConsequently, the altered electronic structure further modifies the lifetime and transfer mechanisms of photogenerated charge carriers. To probe this effect, we employed femtosecond transient absorption (TA) spectroscopy to investigate how the spatially tailored behavior of V\u003csub\u003eo\u003c/sub\u003e influences the charge excitation dynamics in CeO\u003csub\u003e2\u003c/sub\u003e. The change from photo-absorption to photo-bleaching reflects the cooling process of the carriers to the band edge. Fig. 3a shows that the 2D pseudo-color TA map of CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits a broader range of photoinduced absorption compared to CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C (Fig. S13). This result indicates the V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e form a complementary energy-level relationship at equilibrium. Specifically, surface vacancies primarily form shallow defect states due to their unsaturated coordination environment, while bulk vacancies generate deeper and more broadly distributed localized states. Together, they constitute a quasi-continuous defect-state band, significantly promoting multi-path photoinduced electron transitions from the valence band to defect states and among different defect states. Time-resolved TA spectra shows that the increase in signal intensity within the 1~8 ps time range corresponds to the trapping of photogenerated electrons from the conduction band by deep-level V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e defect states, indicating an increase in defect state population. The subsequent signal decay between 8~120 ps reflects the non-radiative recombination between trapped electrons and holes, leading to a gradual reduction in the number of excited electrons. This two-stage kinetics, characterized by an initial rise followed by a decay, demonstrates that V\u003csub\u003eo\u003c/sub\u003e act as effective electron traps, delaying recombination and prolonging charge carrier lifetime. The attenuation dynamics of ground state bleach signals in CeO\u003csub\u003e2\u003c/sub\u003e-B at 350 and 390 nm are fitted using a multi-exponential model. Fig. 3c displays the transient absorption kinetics of CeO\u003csub\u003e2\u003c/sub\u003e-B probed at 350 nm, covering both long-term (0~150 ps) and short-term (0~10 ps) timescales. The long-term kinetics reflect the carrier thermalization and relaxation processes, while the short-term dynamics represent rapid carrier trapping or initial recombination. Oscillatory signals emerging after 10 ps suggest that carrier thermalization and relaxation are dominated by electron-acoustic phonon interactions\u003csup\u003e32\u003c/sup\u003e. Furthermore, the decay lifetime of CeO\u003csub\u003e2\u003c/sub\u003e-B is significantly longer than those of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C. The results of TA confirms that the formation of a vertically asymmetric V\u003csub\u003eo\u003c/sub\u003e structure enables the rapid transfer and efficient non-radiative recombination of photogenerated carriers. This process induces a photothermal effect and prolongs the charge carrier lifetime. The enhancement of lifetime and transfer efficiency of photogenerated carriers can increase the surface free electron concentration of CeO\u003csub\u003e2\u003c/sub\u003e-B. Therefore, the light-assisted Kelvin probe force microscopy (KPFM) is employed to characterize the contact potential difference (CPD) of various samples. As shown in Fig. 3d, the CPD values of CeO\u003csub\u003e2\u003c/sub\u003e-B measure 87 mV in the dark and 125 mV under light illumination, respectively. This increase indicates a significant rise in surface electron concentration upon light irradiation. Moreover, both the surface potential and its light-induced change are substantially greater in CeO\u003csub\u003e2\u003c/sub\u003e-B compared to the other samples, suggesting that CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits enhanced photo-induced electron generation (Fig. S14).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAdditionally, the photocurrent signal, EIS and PL spectra result are confirming the CeO\u003csub\u003e2\u003c/sub\u003e-Bhave stronger carries transfer ability than that of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C(Fig. S15). Photoluminescence spectroscopy confirms that radiative recombination in CeO\u003csub\u003e2\u003c/sub\u003e is strongly enhanced by V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e due to their coordinatively unsaturated structure and weak electron-phonon coupling. On the contrary, a part of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e convert to V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e can produce the stronger electron-phonon coupling, which promotes efficient multiphonon relaxation. This process dissipates excited-state energy non-radiatively as heat, leading to significant fluorescence quenching (Fig.3e). The carrier lifetime for various samples is further explored via time-resolved photo-luminescence spectroscopy. The raw data and fitted data of ns-level timeresolved fluorescence spectra of various samples also confirm the electron lifetime of CeO\u003csub\u003e2\u003c/sub\u003e-B is longer than that of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C (Fig.3f). Under identical illumination conditions, the temperature distribution of CeO\u003csub\u003e2\u003c/sub\u003e-B confirms the presence of photothermal effect (Fig. 3g). Upon light irradiation, the surface temperature of CeO\u003csub\u003e2\u003c/sub\u003e-B reaches 68.3\u003csup\u003eo\u003c/sup\u003eC. After the light source is removed, the surface temperature returns to 26.1\u003csup\u003eo\u003c/sup\u003eC. Electron spin resonance (ESR) spectroscopy is further employed to investigate the ability of different samples to generate reactive free radicals on their surfaces. Under light irradiation, photogenerated holes in the valence band of CeO\u003csub\u003e2\u003c/sub\u003e can react with surface-adsorbed water to produce •OH radicals. The generation of •OH not only consumes surface-adsorbed water, thereby reducing the influence of humidity, but also promotes the deep oxidation of NO\u003csub\u003e2\u003c/sub\u003e to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, which suppresses secondary reaction processes and contributes to a shorter response and recovery time. As shown in Fig. 3a, the •OH signal intensity of CeO\u003csub\u003e2\u003c/sub\u003e-B is stronger than that of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C, indicating that the introduction of asymmetric V\u003csub\u003eo\u003c/sub\u003e enhances hole generation and increases the concentration of surface •OH active species. The carrier transfer mechanism is schematically illustrated in Fig. 3i. V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e function as efficient electron trapping and transport channels, suppressing bulk recombination and facilitating directional electron migration to the surface. Meanwhile, V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e serve as energy release terminals, strongly attracting electrons and holes to undergo highly efficient non-radiative recombination at localized sites. This synergistic effect collectively converts carrier energy into lattice vibrational energy via multi-phonon emission, thereby inducing a pronounced localized photothermal effect on the material surface.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3. Gas Performance of Detecting NO\u003csub\u003e2\u003c/sub\u003e at Room Temperature\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ethe simultaneous enhancement of carrier lifetime and transfer efficiency modifies the surface reactivity of CeO\u003csub\u003e2\u003c/sub\u003e-B. To further assess its practical applicability, we systematically evaluated its response-recovery time, dynamic response characteristics, and long-term stability. The\u0026nbsp;sensing layers of CeO\u003csub\u003e2\u003c/sub\u003e-B is authenticated to be 142 μm\u0026nbsp;thick (Fig. S16). The response values of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C to various concentrations of NO\u003csub\u003e2\u003c/sub\u003e under both light and dark conditions are presented in Fig. 4a. In the dark, the response values of CeO\u003csub\u003e2\u003c/sub\u003e-A to NO\u003csub\u003e2\u003c/sub\u003e is higher than that of CeO\u003csub\u003e2\u003c/sub\u003e, which can be attributed to the variation in V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e. CeO\u003csub\u003e2\u003c/sub\u003e-C exhibits a slightly improved response compared to CeO\u003csub\u003e2\u003c/sub\u003e-A due to the formation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e, which enhance the migration efficiency of O\u003csup\u003e-\u003c/sup\u003e species and facilitate electron transfer. In contrast, when the concentrations of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e reach equilibrium, the highly efficient bulk oxygen migration and charge transport channels provided by V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e achieve optimal kinetic matching with the gas adsorption and reaction sites dominated by V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e. As a result, CeO\u003csub\u003e2\u003c/sub\u003e-B demonstrates a higher response value than CeO\u003csub\u003e2\u003c/sub\u003e-C (Fig. S17-S18). Under light illumination, the response values of all samples are enhanced owing to the generation of photoinduced charge carriers. Notably, the response of CeO\u003csub\u003e2\u003c/sub\u003e-B increases by a factor of three (Fig. S19-S20). The baseline resistance of the sensing layers for all samples is shown in Fig. 4b. Under dark conditions, the baseline resistance increases significantly with rising NO\u003csub\u003e2\u003c/sub\u003e concentration, thereby reducing the detection accuracy of the sensor. In contrast, illumination suppresses the baseline resistance drift. However, the rapid recombination of electron hole pairs in CeO\u003csub\u003e2\u003c/sub\u003e, which occurs on a submicrosecond timescale, still limits the performance of the photoactivated sensor. While the introduction of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e or V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e mitigates baseline resistance drift in CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C, the baseline resistance of CeO\u003csub\u003e2\u003c/sub\u003e-B remains particularly stable throughout the detection of 0.5 to 10 ppm NO\u003csub\u003e2\u003c/sub\u003e. Additionally, CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits a positive linear response to NO\u003csub\u003e2\u003c/sub\u003e within this concentration range. Using the Root Mean Square Deviation (RMSD) method\u003csup\u003e33\u003c/sup\u003e, the theoretical limit of detection (LOD) for CeO\u003csub\u003e2\u003c/sub\u003e-B is determined to be 7.24 ppb in the dark and 0.002 ppb under light, both of which are superior to CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C (Fig. 4c). The influence of asymmetric V\u003csub\u003eo\u003c/sub\u003e on response time is also illustrated in Fig. 4c. Under dark conditions, the response times of CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C are shorter than that of CeO\u003csub\u003e2\u003c/sub\u003e. The balance between bulk oxygen replenishment and surface activity oxygen consumption modifies surface properties and enhances adsorption activation, thereby further shortening the response time of CeO\u003csub\u003e2\u003c/sub\u003e-B. Under light conditions, the response times of CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C are significantly reduced by approximately twofold and are much faster than that of CeO\u003csub\u003e2\u003c/sub\u003e. Moreover, the photothermal effect associated with nonradiative recombination further decreases the response and recovery time of CeO\u003csub\u003e2\u003c/sub\u003e-B, which reaches 4 s for 5 ppm NO\u003csub\u003e2\u003c/sub\u003e detection (Fig. 4d). The cycle stability performance of various samples is evaluated toward 2 ppm NO\u003csub\u003e2\u003c/sub\u003e at room temperature over five response-recovery cycles (Fig. 4e). Under dark conditions, the low surface activity of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C leads to the accumulation of intermediate products on the surface during reactions, resulting in a gradual decrease in response values as the number of cycles increases. After the introduction of light illumination, the cycling stability of all samples improves. However, V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e, which act as recombination centers for electron-hole pairs, still partially reduce the cycling stability of CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C. In contrast, the photothermal effect generates hot electrons that significantly enhance the surface activity of CeO\u003csub\u003e2\u003c/sub\u003e-B, promoting the formation of stable reaction products on its surface. As a result, CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits exceptional cycling stability under light conditions (Fig. S21-S22). The XPS spectra of CeO\u003csub\u003e2\u003c/sub\u003e-Bafter reaction confirm the stability of Ce\u003csup\u003e3+\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e (Fig. S23).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe reaction mechanism between active species and O\u003csub\u003e2\u003c/sub\u003e is further investigated by testing the gas-sensing performance at different oxygen concentrations. As shown in Fig. 4f, the response values of all samples toward NO\u003csub\u003e2\u003c/sub\u003e detection increase with rising oxygen concentrations under dark conditions. The variation in response values of CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C is significantly greater than that of pristine CeO\u003csub\u003e2\u003c/sub\u003e, indicating that the introduction of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e or V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e enhances the adsorption activity or oxygen transfer capability of CeO\u003csub\u003e2\u003c/sub\u003e, thereby facilitating the generation of more active oxygen. Notably, the variation in response value of CeO\u003csub\u003e2\u003c/sub\u003e-B is more pronounced than that of CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C, confirming that the equilibrium between V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e enables optimal kinetic matching between the reaction rate of surface-active oxygen and the migration rate of internal oxygen. Under light conditions, the generation of photoinduced carriers further enhances the surface reactivity of all samples. In particular, the high-energy hot electrons generated via the photothermal effect significantly improve the oxygen adsorption and activation capabilities of CeO\u003csub\u003e2\u003c/sub\u003e-B. Consequently, the response values of all samples toward NO\u003csub\u003e2\u003c/sub\u003e detection increase further across varying oxygen concentrations, with CeO\u003csub\u003e2\u003c/sub\u003e-B exhibiting a more substantial change in response compared to CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C. In summary, holes play an important role in photoactivated gas sensors. Therefore, we further explored the influence of holes on response-recovery time by introducing TEOA into the reaction system to capture holes during the surface reaction process. As shown in Fig. 4g, the recovery time of CeO\u003csub\u003e2\u003c/sub\u003e-B toward NO\u003csub\u003e2\u003c/sub\u003e detection increases significantly when holes are eliminated. This suggests that the removal of holes leads to an accumulation of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003ein the intermediate products on the surface of CeO\u003csub\u003e2\u003c/sub\u003e-B. Additionally, the response value of CeO\u003csub\u003e2\u003c/sub\u003e-B toward NO\u003csub\u003e2\u003c/sub\u003e detection decreases. The response dynamic curve of CeO\u003csub\u003e2\u003c/sub\u003e-B toward low-concentration NO\u003csub\u003e2\u003c/sub\u003e is shown in Fig. 4h. The response value of CeO\u003csub\u003e2\u003c/sub\u003e-B toward 5 ppb NO\u003csub\u003e2\u003c/sub\u003e is 1.22, indicating that the sensor is capable of detecting concentrations below the EPA limit (53 ppb). Furthermore, long-term stability monitoring over 40 days of NO\u003csub\u003e2\u003c/sub\u003e detection under light conditions is shown in Fig. 4i. The response value and response time of CeO\u003csub\u003e2\u003c/sub\u003e-B show only minor changes compared to the first day, demonstrating excellent long-term stability. Compared with previously reported semiconductor-based NO\u003csub\u003e2\u003c/sub\u003e gas sensors, CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits a faster response time (Fig. 4j). Additionally, the baseline resistance of CeO\u003csub\u003e2\u003c/sub\u003e-B remains stable at room temperature. Therefore, CeO\u003csub\u003e2\u003c/sub\u003e-B shows great potential for practical applications.\u003cbr\u003e\u003cstrong\u003eTheoretical results\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo understand the influence of the vertically asymmetric V\u003csub\u003eo\u003c/sub\u003e on the adsorption and activation of molecules on the CeO\u003csub\u003e2\u003c/sub\u003e surface, DFT calculations are employed to analyze the differential charge density of various samples in response to O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e. The optimized adsorption models and adsorption sites are shown in Fig. 5a. The adsorption energies of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C toward O\u003csub\u003e2\u003c/sub\u003e are -1.32, -1.37, -1.83 and -1.38 eV, respectively. The variation in electron accumulation and depletion is more pronounced in CeO\u003csub\u003e2\u003c/sub\u003e-B than in CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C, indicating that the formation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e enhances carrier transfer during the generation of active oxygen. When active oxygen is already present on the surface, the adsorption energies of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C toward NO\u003csub\u003e2\u003c/sub\u003e are -0.41, -1.02, -1.36 and -0.97 eV, suggesting that the formation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e also improves the reaction efficiency between active oxygen and NO\u003csub\u003e2\u003c/sub\u003e. The projected density of states (PDOS) for O\u003csub\u003e2\u003c/sub\u003e adsorption on CeO\u003csub\u003e2\u003c/sub\u003e-B further confirms that the formation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e enhances electron accumulation at V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e sites, thereby accelerating the conversion of O\u003csub\u003e2\u003c/sub\u003e into active oxygen. The amount of charge transferred between O\u003csub\u003e2\u003c/sub\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e is considerably higher than that in CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C (Fig. S24). Additionally, the formation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e in CeO\u003csub\u003e2\u003c/sub\u003e increases electron concentration and reduces the energy required to excite electrons from the O 2p state to the Ce 4f state. The PDOS of NO\u003csub\u003e2\u003c/sub\u003e adsorption on CeO\u003csub\u003e2\u003c/sub\u003e-B also confirms significant electron transfer between NO\u003csub\u003e2\u003c/sub\u003e and the V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e sites (Fig. 5b). The Bader charge transfer between O\u003csub\u003e2\u003c/sub\u003e and CeO\u003csub\u003e2\u003c/sub\u003e-B is 0.314 e and the O-O atomic distance is 1.234 Å. For NO\u003csub\u003e2\u003c/sub\u003e and CeO\u003csub\u003e2\u003c/sub\u003e-B, the Bader charge transfer is 0.244 e and the N-O atomic distance is 1.225 Å. These results confirm that the formation of asymmetric V\u003csub\u003eo\u003c/sub\u003e facilitates the adsorption and activation of O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e (Fig. 5c). Consequently, the detection efficiency of CeO\u003csub\u003e2\u003c/sub\u003e-B toward NO\u003csub\u003e2\u003c/sub\u003e is significantly superior to that of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C (Fig. S25). The reaction process between NO\u003csub\u003e2\u003c/sub\u003e and O\u003csub\u003e2\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e-Bis explored via \u003cem\u003ein situ\u003c/em\u003e NAP-XPS (Fig. 5d). Under dark environment, the reaction of electrons with O\u003csub\u003e2\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e-Bto produce activity oxygen causes the peaks of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u0026nbsp;\u003c/sup\u003eshift from 528.81 eV to 529.24 eV and the concentration of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e is decrease from 5.6% to 3.3%. Then, the peaks of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u0026nbsp;\u003c/sup\u003ein CeO\u003csub\u003e2\u003c/sub\u003e-Bshift from 529.24 eV to 529.06 eV when NO\u003csub\u003e2\u003c/sub\u003e enters the reaction chamber and the concentration of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e is increased from 3.3% to 4.1%. Under light environment, the introduction of light illumination enables CeO\u003csub\u003e2\u003c/sub\u003e to generate more photogenerated carriers, which enhance the concentration of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e. Similarly, the reaction of electrons with O\u003csub\u003e2\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e-Bto produce activity oxygen causes the peaksshift to higher binding energy. However, the peak associated with V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e is disappeared. This indicates that the photothermal effect promotes the transformation of unpaired electrons at V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e sites into high-energy hot electrons. The high reactivity of these hot electrons subsequently lowers the energy barrier for the conversion of O\u003csub\u003e2\u003c/sub\u003e to activity oxygen, resulting in the complete occupancy of the V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e sites by activity oxygen. Then, the peaks of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e is appearedin CeO\u003csub\u003e2\u003c/sub\u003e-Bandshift to lower binding energy when NO\u003csub\u003e2\u003c/sub\u003e enters the reaction chamber. Similarly, under both dark and illuminated conditions, the introduction of O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e also induces peak shifts of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e in CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C. However, unlike the case of CeO\u003csub\u003e2\u003c/sub\u003e-B, the conversion of O\u003csub\u003e2\u003c/sub\u003e to activity oxygen does not lead to the disappearance of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e in CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C. This suggests that the formation of photothermal effect enhances the reactivity of CeO\u003csub\u003e2\u003c/sub\u003e-B and reduces the energy barrier for the conversion of O\u003csub\u003e2\u003c/sub\u003e to ROS. Furthermore, during the reaction between activity oxygen and NO\u003csub\u003e2\u003c/sub\u003e, the V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e concentration in CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C does not recover to near its initial level. This indicates that the formation of nonradiative recombination in CeO\u003csub\u003e2\u003c/sub\u003e-B not only accelerates electron transfer efficiency but also supplies sufficient lattice oxygen to the surface to participate in the reaction, thereby improving the long-term stability of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e and ultimately enabling rapid and stable detection of NO\u003csub\u003e2\u003c/sub\u003e (Fig. S26-S28).\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eIn-situ\u003c/em\u003e FTIR spectroscopy is conducted to analyze the reaction of NO\u003csub\u003e2\u003c/sub\u003e on the surface of various samples under both dark and light conditions. As shown in Fig. 5e, the surface reaction process of CeO\u003csub\u003e2\u003c/sub\u003e-B during NO\u003csub\u003e2\u003c/sub\u003e detection can be divided into three stages. Under dark conditions, the peaks observed at 1484 cm\u003csup\u003e-1\u003c/sup\u003e and 2406 cm\u003csup\u003e-1\u003c/sup\u003e correspond to the formation of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and bi-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, respectively\u003csup\u003e34\u003c/sup\u003e. The absorbance intensities of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and bi-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e indicate that the surface products of CeO\u003csub\u003e2\u003c/sub\u003e-B are primarily composed of unstable NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. During the equilibrium stage, a peak appears at 1253 cm\u003csup\u003e-1\u003c/sup\u003e corresponding to bi-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, suggesting that unstable NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e can react with activity oxygen to form bi-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, which reduces the timeliness and accuracy of the sensor response\u003csup\u003e35\u003c/sup\u003e. In the recovery stage, the absorbance intensity of bi-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e continues to increase initially, indicating that CeO\u003csub\u003e2\u003c/sub\u003e-B exhibits poor desorption capacity due to its low activity in the dark, thereby prolonging the recovery time. In contrast, the surface reaction process under light irradiation differs significantly across the samples. During the response stage, peaks appear at 1423 cm\u003csup\u003e-1\u003c/sup\u003e and 1612 cm\u003csup\u003e-1\u003c/sup\u003e, corresponding to br-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and mon-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, respectively\u003csup\u003e36\u003c/sup\u003e. This demonstrates that the photothermal effect enhances the activity of the CeO\u003csub\u003e2\u003c/sub\u003e-B surface, facilitating the conversion of NO\u003csub\u003e2\u003c/sub\u003e into stable NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e and effectively suppressing subsequent secondary reactions (Fig. S29). The peak intensities of bi-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e is stronger than that of mon-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u0026nbsp;\u003c/sup\u003eand br-NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. Moreover, the peak intensity of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e under light irradiation disappears almost completely compared to that in the dark. Therefore, the photothermal effect promotes the conversion of NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, further reducing the NO\u003csub\u003e2\u003c/sub\u003e detection time. Based on the in-situ FTIR spectral analysis, the intermediate and final products of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C during NO\u003csub\u003e2\u003c/sub\u003e detection are identified. The activation barriers of the reaction processes are further investigated using DFT calculations (Fig. 5f). Initially, adsorbed O\u003csub\u003e2\u003c/sub\u003e molecules react with electrons to spontaneously generate activity oxygen species on the surfaces of CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C. The lower energy barriers of CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C compared to CeO\u003csub\u003e2\u003c/sub\u003e imply that the formation of V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e and V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e promotes the generation of more activity oxygen species. The reaction barriers for CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A and CeO\u003csub\u003e2\u003c/sub\u003e-C are 2.37 eV, 1.48 eV and 1.15 eV, while that of CeO\u003csub\u003e2\u003c/sub\u003e-B is 1.07 eV. These results indicate that the introduction of asymmetric V\u003csub\u003eo\u003c/sub\u003e enhances charge transfer capability at the reaction interface and reduces the reaction barrier. Subsequently, the generated activity oxygen species spontaneously react with NO\u003csub\u003e2\u003c/sub\u003e to form NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. The reaction barriers for CeO\u003csub\u003e2\u003c/sub\u003e, CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C are 0.01 eV, -0.93 eV, -2.08 eV and -0.58 eV, demonstrating that asymmetric V\u003csub\u003eo\u003c/sub\u003e also strengthen the interaction between NO\u003csub\u003e2\u003c/sub\u003e and CeO\u003csub\u003e2\u003c/sub\u003e-B. Therefore, the significantly accelerated detection of NO\u003csub\u003e2\u003c/sub\u003e can be attributed to the asymmetric V\u003csub\u003eo\u003c/sub\u003e, which enhance carrier transport and induce a photothermal effect. Therefore, the surface reaction mechanism of CeO\u003csub\u003e2\u003c/sub\u003e-B toward NO\u003csub\u003e2\u003c/sub\u003e under light illumination is illustrated in Fig. 5g. Under light irradiation, the generated holes exhibit strong oxidation capability, enabling the deep oxidation of NO\u003csub\u003e2\u003c/sub\u003e to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. Moreover, the photothermal effect original from phonon scattering can produce hot electrons, which further mitigates the accumulation of the intermediate reactive species NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e on the surface. Theoretically, suppressing the formation of secondary reaction products can help regulate the baseline resistance of the sensing layer.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study demonstrates that tailoring the spatial distribution of V\u003csub\u003eo\u003c/sub\u003e in CeO\u003csub\u003e2\u003c/sub\u003e can overcome the fundamental limitation of high electron-hole recombination rates in photoactivated gas sensor. In this system, V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eB\u003c/sup\u003e functions as an electron reservoir and transport channel, effectively suppressing bulk recombination while directing electrons to surface reaction sites. Simultaneously, V\u003csub\u003eo\u003c/sub\u003e\u003csup\u003eS\u003c/sup\u003e acts as a non-radiative recombination center, converting carrier energy into lattice vibrations via multi-phonon scattering and generating a localized photothermal effect. This unique mechanism transforms the typically detrimental recombination process into a beneficial one and producing hot electrons and providing thermal energy to lower reaction barriers. The enhanced charge activity and prolonged carrier lifetime increase the concentration of surface active species, which effectively suppresses the formation of intermediate products and prevents secondary reactions, thereby enabling CeO\u003csub\u003e2\u003c/sub\u003e to achieve stable detection of 5 ppm NO\u003csub\u003e2\u003c/sub\u003e within 4 s at room temperature. DFT confirm that the formation of non-radiative recombination promotes the adsorption of both O\u003csub\u003e2\u003c/sub\u003e and NO\u003csub\u003e2\u003c/sub\u003e on CeO\u003csub\u003e2\u003c/sub\u003e, while reducing the reaction energy barrier for the conversion of NO\u003csub\u003e2\u003c/sub\u003e to NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e. By revealing the performance enhancement mechanism through the perspective of photothermal effects induced by non-radiative recombination regulating surface chemical properties, this research provides novel design principles for developing high-efficiency, highly stable gas sensors.\u003c/p\u003e\n\n\n\n\n\n"},{"header":"Methods","content":"\u003cp\u003eCeO\u003csub\u003e2\u003c/sub\u003e was synthesized by hydrothermal method as well. 0.868 g Ce(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO and 9.6 g NaOH are dissolved in 35 mL and 5 ml of distilled water, respectively. As the clear solution formed, NaOH aqueous solution is slowly added to the Ce(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e·6H\u003csub\u003e2\u003c/sub\u003eO with vigorous stirring. After stirring for about 30 mins, and mixture solution was subjected to hydrothermal treatment at 180 \u003csup\u003eo\u003c/sup\u003eC for 24 h. The obtained powders were washed with water and ethanol for three cycles, dried in vacuum over a night, and further subjected to calcination at 550 \u003csup\u003eo\u003c/sup\u003eC for 4 h. CeO\u003csub\u003e2\u003c/sub\u003e-A, CeO\u003csub\u003e2\u003c/sub\u003e-B and CeO\u003csub\u003e2\u003c/sub\u003e-C are synthesized by the high temperature heat treatment at 200\u003csup\u003eo\u003c/sup\u003eC, 400\u003csup\u003eo\u003c/sup\u003eC and 600\u003csup\u003eo\u003c/sup\u003eC for 1 h under vacuum.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eFabrication and testing of a gas sensor and experimental detail\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\u003cp\u003e5 mg of samplesare mixed with 50 μL of deionized water to obtain the corresponding slurry. 5 μL of slurry was then dripped on an Pt interdigitated electrodes (10 mm × 5 mm × 0.25 mm, AURORA technologies, China) to form a resistance-type sensor. All the fabricated sensors were aged in air at 80 °C for 2 h.\u0026nbsp;The gas-sensing performance of the fabricated sensors was evaluated using an intelligent gas-sensing analysis system (CGS-4TPs, Beijing Elite Tech Co., Ltd.). To produce test gases with the necessary concentrations, a dynamic gas and liquid distribution system (DGL-III, Beijing Elite Tech Co., Ltd, China) having three mass flow controllers was used. We use nitrogen as the carrier gas, and control the gas flow of oxygen and nitrogen to control the different oxygen concentrations in the reaction chamber.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eCharacterization and measurements\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eThe high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and Xray energy dispersive spectroscopy (EDS) mapping was recorded on aberration-corrected TEM (FEI Titan Cubed Themis G2 300) at an accelerating voltage of 300 kV. X-ray absorption fine spectra (XAFS) measurements were measured on the B11 station in Shanghai Synchrotron Radiation Facility (SSRF). The KPFM (Bruker MultiMode-8 surface potential mode) was used to characterize the surface potential. The SCM-PIT-V2 model tip and AS-130VLR (“J” vertical) scanner model was used, and the radius and elastic coefficient of tip were 35 nm and 3 N/m, respectively. The In situ Fourier-transform infrared (In situ FT-IR) spectra were recorded on a Bruker Vertex 70 FTIR spectrometer equipped with in situ reaction chamber. The X-ray photoelectron spectroscopy (Axis Supra) measurements were operated with Al Ka radiation (1486.6 eV). Binding energies (BE) were calibrated by setting the measured BE of C 1s to 284.8 eV. DFT calculations are carried out using the VASP code. The projector augmented-wave (PAW) method and Perdew-Burke-Ernzerhof generalized gradient approximation (GGA-PBE) ae used for the exchange correlation functionals. The energy cutoff of 400 eV is used. The molecular dynamics simulations are carried out in the canonical ensemble (NVT) with the Nose-Hoover thermostat.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data supporting the findings of this study are reported in the main text or the Supplementary Information.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eCooper, M. J. et al. Global fine-scale changes in ambient NO\u003csub\u003e2\u003c/sub\u003e during during COVID-19 lockdowns. \u003cem\u003eNature\u003c/em\u003e. \u003cstrong\u003e601\u003c/strong\u003e, 380-387 (2022).\u003c/li\u003e\n\u003cli\u003eR\u0026uuml;ck, T. Bierl, R. Matysik, F.M. 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Chem\u003c/em\u003e. \u003cstrong\u003e95\u003c/strong\u003e, 1057-1064 (2023)\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8016517/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8016517/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eExtending the lifetime of electrons while enhancing surface activity is crucial for improving charge transfer efficiency and reaction activity in photoactivated gas sensors. However, the inherent radiative charge recombination process significantly restricts the simultaneous enhancement of both kinetic and thermodynamic performance. In this study, leveraging the differences in surface oxygen vacancy migration barriers, we precisely regulated the spatial distribution of oxygen vacancies in CeO\u003csub\u003e2\u003c/sub\u003e by controlling the vacuum heat treatment temperature, successfully constructing a vertically asymmetric oxygen vacancy structure. This asymmetric configuration induces localized electric dipoles and strong electron-phonon coupling, enabling rapid transfer and highly efficient non-radiative recombination of photogenerated carriers, thereby triggering a pronounced photothermal effect. Leveraging this strategy, the CeO\u003csub\u003e2\u003c/sub\u003e-based sensor sets a new benchmark for response speed, achieving an ultra-rapid and stable detection of 5 ppm NO\u003csub\u003e2\u003c/sub\u003e in merely 4 s at room temperature. Our work introduces a general strategy of inducing non-radiative recombination that can be extended to photoactivated gas sensor, creating more tunability and variability in gas sensor.\u003c/p\u003e","manuscriptTitle":"Spatially Tailored Asymmetric Oxygen Vacancies Induce Nonradiative Recombination for Ultrafast and Stable NO2 Sensing","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-04 08:55:32","doi":"10.21203/rs.3.rs-8016517/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"509aded5-48fe-4ad1-995c-d57ebc6ac584","owner":[],"postedDate":"December 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":59074561,"name":"Physical sciences/Materials science/Materials for devices/Sensors and biosensors"},{"id":59074562,"name":"Physical sciences/Materials science/Nanoscale materials/Electronic properties and materials"}],"tags":[],"updatedAt":"2026-04-16T13:39:51+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-04 08:55:32","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8016517","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8016517","identity":"rs-8016517","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}