Experimental and Density Functional Theory Simulation Research on PdO–SnO2 Nanosheet Ethanol Gas Sensors

Pure SnO2 and 1 at.% PdO–SnO2 materials were prepared using a simple hydrothermal method. The micromorphology and element valence state of the material were characterized using XRD, SEM, TEM, and XPS methods. The SEM results showed that the prepared material had a two-dimensional nanosheet morphology, and the formation of PdO and SnO2 heterostructures was validated through TEM. Due to the influence of the heterojunction, in the XPS test, the energy spectrum peaks of Sn and O in PdO–SnO2 were shifted by 0.2 eV compared with SnO2. The PdO–SnO2 sensor showed improved ethanol sensing performance compared to the pure SnO2 sensor, since it benefited from the large specific surface area of the nanosheet structure, the modulation effect of the PdO–SnO2 heterojunction on resistance, and the catalyst effect of PdO on the adsorption of oxygen. A DFT calculation study of the ethanol adsorption characteristics of the PdO–SnO2 surface was conducted to provide a detailed explanation of the gas-sensing mechanism. PdO was found to improve the reducibility of ethanol, enhance the adsorption of ethanol’s methyl group, and increase the number of adsorption sites. A synergistic effect based on the continuous adsorption sites was also deduced.


Introduction
Ethanol is a common and valuable volatile organic solvent widely used in the chemical, manufacturing, food, environment, health care, and pharmaceutical industries [1].Ethanol can quickly evaporate and diffuse in the air, forming ethanol gas.Ethanol gas irritates the human respiratory system and central nervous system.Long-term exposure to a certain amount of ethanol gas may cause symptoms such as breathing difficulties, drowsiness, dizziness, nausea, and headaches [2].The threshold limit value (time-weighted average value) of the ethanol vapor proposed by the Occupational Safety and Health Administration (OSHA) and the American Conference of Governmental Industrial Hygienists (ACGIH) is 1000 ppm [3].High concentrations of ethanol gas (3.3%~19%) may also cause severe fire and explosion accidents when exposed to fire.For drunk driving screening and alcohol dependence patients, the ethanol content in exhaled breath is the crucial detection indicator [4].In China, the ethanol detection threshold for exhaled breath for determining a driving under the influence (DUI) charge is 0.086 mg/L (~42 ppm).Similarly, European countries have set similar threshold values for DUI [5].Overall, ethanol gas sensors with diverse detection ranges have broad application prospects in air quality detection, explosive alarms, and exhaled breath analysis.
Sensors 2024, 24, 4970 2 of 19 Semiconductor gas sensors utilizing SnO 2 as the primary gas-sensitive material have been demonstrated to exhibit outstanding performance in detecting ethanol gas [6][7][8].These SnO 2 sensors offer numerous advantages, including high sensitivity, rapid response time, exceptional stability, long lifespan, and affordability, making them extensively utilized in various everyday applications.Researchers consistently strive to enhance gas-sensing capabilities by exploring novel synthesis approaches, modulating the materials' morphology, and incorporating precise quantities of sensitizers such as metal oxides and noble metals.Additionally, in some work, density functional theory (DFT) calculations were employed to investigate the interaction between gases and materials at the atomic level [9,10], offering a fresh perspective on comprehending the gas adsorption process preceding the gas-sensitive oxidation reaction.
For instance, Li et al. manipulated the parameters of the hydrothermal method to synthesize SnO 2 with diverse morphologies, demonstrating that the hollow-spherestructured SnO 2 displayed the lowest operational temperature, highest response, and superior selectivity.Through DFT calculations, an adsorption configuration of ethanol on the surface of SnO 2 , along with the electrical properties of SnO 2 , were elucidated, aiding in explaining the gas-sensing mechanism [11].Li et al. prepared LaCoO 3 /SnO 2 nanoflowers using a hydrothermal method, which showed a lower operating temperature and a higher response than pure SnO 2 .DFT calculations revealed stronger ethanol adsorption and a more extensive charge transfer on the LaCoO 3 /SnO 2 surface than that on pure SnO 2 and pure LaCoO 3 surfaces, which can explain the mechanism of response enhancement [12].Xiao et al. synthesized PdO-SnO 2 hollow microcubes through precipitation annealing and chemical etching, which showed excellent sensitivity and selectivity to ethanol gas at 300 • C. The hollow microsphere structure provides a large specific surface area and adsorption sites, and the catalytic properties of PdO promote the chemical adsorption and dissociation of target gases [13].Inderan et al. also synthesized Pd/PdO-SnO 2 nanorods by hydrothermal method, exhibiting better gas-sensing performance than pure SnO 2 and Ni-SnO 2 nanorods [14].It can be seen that the PdO-SnO 2 material system shows excellent potential in ethanol detection because PdO has a high catalytic performance in ethanol [15].Moreover, DFT calculations are also used to explore the sensing mechanism.Pan et al. studied the O 2 adsorption on the PdO(101) surface by DFT method with both PBE and HSE exchange-correlation functional.Three adsorption configurations were obtained, which were in agreement with the results of the temperature programmed desorption experiment.The adsorption was highly related to the gap between the PdO d-band center and the lowest unoccupied molecular orbital (LUMO) of O 2 .This work proved that the PdO could facilitate the O 2 adsorption process, which was a critical step for the oxidation reaction [16].
Despite some studies on the PdO-SnO 2 ethanol gas sensors having been conducted, further exploration of the synthesis methods that produce PdO-SnO 2 materials with diverse morphologies is still needed.In addition, DFT simulation calculations of ethanol adsorption behavior on the PdO-SnO 2 surface have not yet been reported.
In this paper, 1 at.%PdO-SnO 2 and SnO 2 nanosheets were prepared using a simple hydrothermal reaction.Their structure and composition were analyzed by XRD, SEM, TEM, and XPS.The formation of PdO-SnO 2 heterojunctions was demonstrated through TEM and XPS.Compared with pure SnO 2 nanosheets, PdO-SnO 2 nanosheets showed better ethanol-sensitive properties.The adsorption characteristics of ethanol on SnO 2 and PdO(101)-SnO 2 (110) surfaces were studied using the DFT method.The enhancement effect of PdO was obtained, and a possible synergistic effect was proposed.The gas-sensing mechanism was analyzed through microstructures, catalytic effects, heterostructures, and DFT results.

Preparation of the Gas-Sensing Materials
The SnO 2 and PdO-SnO 2 nanosheets were synthesized using a simple and efficient hydrothermal method.The experimental procedure followed a series of steps.Firstly, 100 mL of a white suspension containing 0.01 M SnCl 2 , 0.01 M CTAB, and 0.2 M NaOH was prepared.For the PdO-SnO 2 material, 0.01 mmol PdCl 2 (1 at.%) was added.The suspension was stirred for 60 min and transferred into a hydrothermal reaction vessel.The vessel was positioned in a drying oven and heated to 160 • C for 24 h to activate the hydrothermal reaction.After the reaction, the vessel was allowed to cool naturally to room temperature, and a suspension was obtained.The suspension was centrifuged to obtain the precipitate, which was then cleaned with water and ethanol.The centrifugation and cleaning process was repeated three times.The product was dried at 90 • C overnight and then annealed at 600 • C for two hours at a pressure of one atmosphere, which not only removed any organic matter and residual hydroxides but also promoted the crystallization of the oxides.Finally, the SnO 2 and PdO-SnO 2 nanosheet materials were obtained.

Characterization
The microstructure of the samples was investigated using a D/MAX 2400 X-ray powder diffractometer Rigaku, Japan (CuKα 1, λ = 0.154056 nm) to perform X-ray diffraction (XRD) analysis.The microscopic morphology of the materials was characterized using a ZEISS MERLIN field-emission scanning electron microscope (FESEM, Carl Zeiss, Oberkochen, Germany) and a Thermo Talos F200X transmission electron microscope (TEM, Thermo Fisher Scientific, Waltham, MA, USA).The composition and chemical states of the elements in the materials were analyzed using a Thermo Scientific Escalab 250XI Xray photoelectron spectrometer (XPS, Thermo Fisher Scientific, Waltham, MA, USA).In the XPS test, the monochromator light source was Al Kα X-rays (1486.6 eV), the spot size was 400 µm, the base pressure was about 5 × 10 −9 mbar, and a charge neutralizer was used.The charge correction was performed with the C 1s peak (284.8 eV) as a reference.

Fabrication and Measurement of Gas Sensors
The prepared material was fully ground and mixed with ethanol to form a slurry, which was then coated on a commercial ceramic sheet (3 mm × 3 mm × 0.25 mm) that was designed and fabricated for gas sensor applications.On the front of the ceramic sheet, a pair of Au electrodes was fabricated to test the resistance of the gas-sensitive material.On the back, a heating resistance wire was fabricated to heat the device.By applying different voltages to the heating resistance wire, the temperature of the ceramic sheet can be controlled from room temperature to 400 • C. The ceramic sheet coated with the gas-sensing materials was dried at 60 • C for 2 h and then annealed at 400 • C for 2 h.Finally, the gas sensor was prepared by mounting the ceramic sheet on a metal base in a suspended state through four leads.
The gas sensors' properties were measured on a homemade gas-sensing test system described elsewhere [17].The gas sources (Dalian Special Gas Products Co., Ltd., Dalian, China) included dry air and a standard gas containing the target gas at a specific concentration.Mass flow controllers were used to control the concentration of the target gas, which was introduced into a chamber containing the gas sensors.The resistances of the gas sensors were measured using a multimeter (Keithley DAQ6510, Tektronix, Beaverton, OR, USA).The resistances of a gas sensor in air and the target gas were marked R a and R g , respectively.The response was defined as S = R a /R g .The response/recovery time was defined as the time corresponding to a 90% resistance change during the response/recovery stage [18,19].

Density Functional Theory Calculations
The DFT calculations were performed using the Dmol3 module in Materials Studio 2019 software.The Perdew-Burke-Ernzerhof (PBE) method within the framework of Generalized Gradient Approximation (GGA) was employed to describe the exchangecorrelation energy.The DNP-4.4 basis set was chosen for the functional basis.This basis set was developed by Delley in 2006, which can give energy differences close to the DFT basis set limit [20,21].Monireh et al. proved its high accuracy compared to the DND-4.4 and DNP-3.5 basis sets [22].Meanwhile, employing the DNP-4.4basis set in the Dmol3 module provides reasonable time and cost savings compared to the plane-wave method in the CASTEP module.A k-point mesh with a sampling size of 3 × 1 × 1 was utilized.During geometry optimization, the convergence criteria for energy, forces, and displacements were set to 1.0 × 10 −5 Ha, 2 × 10 −3 Ha•Å −1 , and 5.0 × 10 −3 Å, respectively.
The SnO 2 (110) and PdO(101)-SnO 2 (110) surfaces were constructed and are described in the Supplementary File.These surfaces were selected according to the TEM and XRD test results.The adsorption configurations of an ethanol molecule on the SnO 2 (110) and PdO(101)-SnO 2 (110) surfaces were computed.The adsorption energy was calculated using Equation (1) [23,24] where E ads represents the adsorption energy, E m+gas represents the energy of the system after the gas is adsorbed on the nanomaterial, E m is the energy of the gas-sensing material, and E gas is the energy of the gas molecule.A negative adsorption energy indicates a decrease in the system's energy after gas adsorption, signifying a stable adsorption configuration where the gas molecule can spontaneously adsorb on the nanomaterial's surface.Furthermore, the more negative the adsorption energy, the stronger the adsorption interaction is [25].

Characterization
The crystal structures of the prepared SnO 2 and PdO-SnO 2 materials were studied using XRD, and their XRD diffraction patterns are shown in Figure 1A.The diffraction peaks of the two materials can both be indexed to tetragonal SnO 2 .For SnO 2 , the highest diffraction peak at around 26.61 • was derived from the SnO 2 (110) crystal plane, which is consistent with the standard pattern (JCPDS No. 41-1445).However, for PdO-SnO 2 , the highest diffraction peak appeared at around 33.86 • , which is near the SnO 2 (101) peak and the PdO(101) peak (JCPDS No. .It is unreasonable to infer that the highest diffraction peak comes only from the SnO 2 (101) crystal plane, because in previous studies, PdO did not have the function of regulating the growth orientation of SnO 2 crystals [14,26].In addition, the SEM results in Figure 2 do not show obvious changes in morphology or crystal orientation, while TEM observation in Figure 3F confirms the formation of PdO.Therefore, we infer that the PdO(101) diffraction peak exists and forms the highest diffraction peak after superposition with the SnO 2 (101) peak.Similar XRD results and explanations have also been reported by Amit [27].The peak fitting results are shown in Figure 1B. Figure 1C shows the SnO 2 (110) diffraction peak patterns of the two materials.The SnO 2 (110) diffraction peak of PdO-SnO 2 shifted by 0.04 • to a low angle, indicating that the lattice of SnO 2 increased after doping with PdO.Microstructural characterization of the SnO2 and PdO-SnO2 materials was conducted using SEM.In Figure 2, a nanosheet structure is observed.These nanosheets were inter twined with each other.Some areas of the nanosheets assembled into a porous morphol ogy, as shown in the high magnification image.These pores are beneficial in increasing the specific surface area of the material and promoting the diffusion and adsorption of ga within the material.The morphologies of the SnO2 and PdO-SnO2 materials were rela Microstructural characterization of the SnO 2 and PdO-SnO 2 materials was conducted using SEM.In Figure 2, a nanosheet structure is observed.These nanosheets were intertwined with each other.Some areas of the nanosheets assembled into a porous morphology, as shown in the high magnification image.These pores are beneficial in increasing the specific surface area of the material and promoting the diffusion and adsorption of gas within the material.The morphologies of the SnO 2 and PdO-SnO 2 materials were relatively similar, except that more small-sized nanoparticles were observed on the surface of PdO-SnO 2 .
The structural properties of the PdO-SnO 2 material were analyzed more deeply using TEM. Figure 3A shows a low magnification HAADF (high angle annular dark field) image of the PdO-SnO 2 material.Nanosheets loaded with or surrounded by nanoparticles were observed.The enlarged image of the red rectangle area is shown in Figure 3B.There were pores among the nanoparticles, facilitating the diffusion of gas molecules.Meanwhile, the nanosheets and nanoparticles with high specific areas provide abundant adsorption sites for gas molecules.The EDS mapping images in Figure 3B for Pd, Sn, and O are displayed in Figure 3C-E.Two nanoparticles exhibited a high content of Pd while other nanoparticles had a high content of Sn.O was evenly distributed in all nanoparticles.Therefore, two PdO nanoparticles and several SnO 2 nanoparticles were found in this area.Figure 3F shows a high-resolution TEM image of the PdO-SnO 2 material.The visible 0.263 nm and 0.324 nm lattice fringes were ascribed to the PdO(101) and the SnO 2 (110) crystal planes, respectively.The PdO nanoparticle is in contact with the edge of the SnO 2 nanosheet, implying the formation of a PdO(101)-SnO 2 (110) heterojunction.
XPS was employed to characterize the elemental composition and chemical states of the materials.Figure 4A presents the full-survey spectra of the SnO 2 and PdO-SnO 2 samples.The results indicated the presence of Sn, O, and C in both samples and Pd in the PdO-SnO 2 sample.Figure 4B displays the high-resolution XPS spectra for Sn 3d.Doublets with a peak separation of 8.4 eV were observed for both samples, which can be assigned to tetravalent Sn [28,29].The binding energies of the PdO-SnO 2 for Sn 3d were 0.2 eV lower than those of the SnO 2 , which can be attributed to the PdO doping [30].Specifically, forming a heterojunction between PdO and SnO 2 results in charge transfer, leading to changes in the density and energy of electrons in SnO 2 .Figure 4C shows the high-resolution XPS spectra of the PdO-SnO 2 sample for Pd 3d.The Pd 3d 3/2 and Pd 3d 5/2 peaks were at 341.8 eV and 336.4 eV, indicating that Pd was divalent or there was PdO in the sample [13].The Pd to Sn content measured using XPS was 1.43 at.%, close to the ratio of the raw materials.Figure 4D presents the high-resolution O 1s spectra.For the SnO 2 sample, the O 1s peak can be deconvoluted (using Tougaard background) into three peaks located at 532.2 eV, 531.3 eV, and 530.5 eV, corresponding to chemisorbed O (O C ), vacancy O (O V ), and lattice O (O L ), respectively [19,31,32].The O C , O V , and O L proportions were 7.5%, 23.2%, and 69.3%, respectively.For the PdO-SnO 2 sample, the peaks for the O C , O V , and O L were at 532.0 eV, 531.1 eV, and 530.3 eV, with proportions of 16.4%, 19.1%, and 64.5%, respectively.Similar to the Sn 3d spectra, the binding energies of the PdO-SnO 2 material for O 1s also decreased due to the PdO doping.The PdO-SnO 2 sample showed a higher O C proportion than the SnO 2 sample, which may have resulted from the PdO's spillover effect.Meanwhile, The PdO-SnO 2 sample had a lower O V proportion.We inferred that this was also because of the PdO's spillover effect.In the annealing process, the PdO could provide active oxygen species to the SnO 2 surface, promoting surface oxidation and crystallization, thereby reducing oxygen vacancies.The O V + O C proportion is considered to be highly correlated with the responses of the gas sensors [31,32].The O V , which represents the oxygen vacancies, can act as the oxygen adsorption sites.The O C includes chemisorbed oxygen and other oxygen containing species such as OH − , H 2 O, etc.The chemisorbed oxygen can directly participate in the gas-sensing reaction.The amount of oxygen-containing species also reveals the material's oxygen adsorption ability since these sites may transform into oxygen adsorption sites after desorption at a high working temperature.Moreover, Pd(OH) 2 can react with reducing substances to generate CO 2 and H 2 O [33].Therefore, it is reasonable that the PdO-SnO 2 gas sensor exhibited superior performance since its O V + O C proportion was 4.8% higher than that of the SnO 2 sensor.
be highly correlated with the responses of the gas sensors [31,32].The OV, which represents the oxygen vacancies, can act as the oxygen adsorption sites.The OC includes chemisorbed oxygen and other oxygen containing species such as OH − , H2O, etc.The chemisorbed oxygen can directly participate in the gas-sensing reaction.The amount of oxygencontaining species also reveals the material's oxygen adsorption ability since these sites may transform into oxygen adsorption sites after desorption at a high working temperature.Moreover, Pd(OH)2 can react with reducing substances to generate CO2 and H2O [33].Therefore, it is reasonable that the PdO-SnO2 gas sensor exhibited superior performance since its OV + OC proportion was 4.8% higher than that of the SnO2 sensor.

Gas-Sensing Properties
A series of gas-sensing tests were conducted to gain an in-depth understanding of the PdO-SnO2 and SnO2 nanosheet gas sensors.Temperature is an important parameter that affects the performance of gas sensors.In order to determine the appropriate temperature for sensor operation, the responses of the two sensors towards 50 ppm ethanol gas at 200-350 °C were tested.The results are shown in Figure 5A.As the temperature increased, the PdO-SnO2 and SnO2 gas sensors exhibited an "increase-decrease" trend.The trend can be explained as follows.The redox reaction at the materials' surface, which determines the responses of the sensors, highly depends on the temperature.As the temperature increases from 200 °C to 300 °C, the adsorbed oxygen ions' activity and the redox

Gas-Sensing Properties
A series of gas-sensing tests were conducted to gain an in-depth understanding of the PdO-SnO 2 and SnO 2 nanosheet gas sensors.Temperature is an important parameter that affects the performance of gas sensors.In order to determine the appropriate temperature for sensor operation, the responses of the two sensors towards 50 ppm ethanol gas at 200-350 • C were tested.The results are shown in Figure 5A.As the temperature increased, the PdO-SnO 2 and SnO 2 gas sensors exhibited an "increase-decrease" trend.The trend can be explained as follows.The redox reaction at the materials' surface, which determines the responses of the sensors, highly depends on the temperature.As the temperature increases from 200 • C to 300 • C, the adsorbed oxygen ions' activity and the redox reaction are enhanced, increasing the response.When the temperature is above 300 • C, high temperature may suppress the adsorption of ethanol and oxygen, and carriers are also activated, which weakens the sensor's sensitivity by decreasing R a [17].The highest response of PdO-SnO 2 and SnO 2 gas sensors were 45.0 and 8.5, respectively, when working at 300 • C, which was determined as the optimal operating temperature for the sensors.In addition, the PdO-SnO 2 sensor, whose response was 5.3 times higher than that of the SnO 2 sensor, exhibited a significant advantage.The reasons are analyzed in detail in Section 3.4.A comparative analysis of the ethanol-sensing performances of other materials is depicted in Table 1.As a result, the PdO-SnO2 nanosheet sensor in this work achieved superior ethanol-sensing performance, particularly a strong response and a low limit of detection.These properties make this sensor a promising candidate for DUI detection and air quality monitoring.

DFT Calculation Results
The experiment results showed a significant increase in the response after doping PdO into SnO2.DFT calculations were also employed to investigate the adsorption characteristics of ethanol on the SnO2(110) and PdO(101)-SnO2(110) surfaces.The atom symbols used in the discussion are listed in Table 2. Further investigation on the responses of PdO-SnO 2 and SnO 2 gas sensors to 1-70 ppm ethanol at 300 • C was carried out, and the result is depicted in Figure 5B.The responses of the two sensors had a positive correlation with the gas concentration.As the ethanol concentration rose from 1 ppm to 70 ppm, the PdO-SnO 2 gas sensor's response increased from 4.6 to 52.7, and the SnO 2 gas sensor's response grew from 1.3 to 11.1.In addition, the response time and recovery time of the two gas sensors to 50 ppm ethanol were also measured (Figure 5B).The response time and recovery time of the PdO-SnO 2 gas sensor were 103 s and 149 s, respectively, and those of the SnO 2 gas sensor were 120 s and 135 s.
The relationships between the sensors' responses (S) and ethanol gas concentration (C) in the linear and logarithmic coordinate system are shown in Figures 5C and 5D, respectively.Linear fits were performed in both coordinate systems.In the linear coordinate system, the relationships between the responses and the ethanol concentrations for the SnO 2 and PdO-SnO 2 gas sensors were fitted as S = 0.149 × C + 1 and S = 0.825 × C + 1, respectively, with R 2 values of 0.997 and 0.973.Similarly, in the logarithmic coordinate system, the relationships between the responses and the ethanol concentrations for the SnO 2 and PdO-SnO 2 gas sensors were fitted as lg(S − 1) = 0.856 × lgC − 0.596 and lg(S − 1) = 0.637 × lgC + 0.543, respectively, with R 2 values of 0.987 and 0.998.The closer to 1 the R 2 value is, the smaller the fitting error is.Therefore, the SnO 2 and PdO-SnO 2 gas sensors showed good linear relationships between the responses and the ethanol concentrations in the linear and the logarithmic coordinate systems, respectively.Such a difference may be ascribed to the differences in the electronic characteristics and catalytic effects induced by PdO [15,17].
Selectivity is the ability of a gas sensor to detect target gases and eliminate the influence of interfering gases, which is very important for actual use. Figure 5E shows the responses of the PdO-SnO 2 gas sensor to ethanol and other interfering gases, including formaldehyde, methane, ethylene, carbon monoxide, and hydrogen.The PdO-SnO 2 gas sensor showed a response of 45.0 and 7.4 to 50 ppm ethanol and 50 ppm HCHO, respectively.The response ratio reached 6, indicating an excellent selectivity for ethanol and a poor selectivity for HCHO.The other interfering gases were tested at a concentration of 1000 ppm, and the responses were still far lower than that of 50 ppm ethanol, indicating an even poorer selectivity for these interfering gases.The results demonstrated the excellent selectivity of the PdO-SnO 2 gas sensor to ethanol.This may be attributed to the catalyst effect of PdO and the strong adsorption of ethanol on the material's surface [15,34].
Stability refers to the ability of the sensor to maintain sensitivity to the measured gas over a period of time.Figure 5F displays the responses of the two sensors to 50 ppm ethanol over 15 days.Small amplitude oscillations in the responses were observed, and no performance degradation or failure occurred, indicating that both sensors had remarkable stability.The repeatability of the sensors was also validated by cyclically measuring the responses to 50 ppm ethanol.As shown in Figure 5G, both the PdO-SnO 2 and SnO 2 sensors consistently exhibited stable gas-sensing responses across four cycles, demonstrating the excellent repeatability of the sensors.
A comparative analysis of the ethanol-sensing performances of other materials is depicted in Table 1.As a result, the PdO-SnO 2 nanosheet sensor in this work achieved superior ethanol-sensing performance, particularly a strong response and a low limit of detection.These properties make this sensor a promising candidate for DUI detection and air quality monitoring.

DFT Calculation Results
The experiment results showed a significant increase in the response after doping PdO into SnO 2 .DFT calculations were also employed to investigate the adsorption characteristics of ethanol on the SnO 2 (110) and PdO(101)-SnO 2 (110) surfaces.The atom symbols used in the discussion are listed in Table 2.The ethanol adsorption configurations on the SnO 2 (110) surface are illustrated in Figure 6, and the corresponding images of the charge density distribution are depicted in Figure 7.In this study, we first placed the ethanol molecule (with the −OH group pointing downward) above the Sn 5c and O 2c (SnO 2 ) atoms, and Figure 6A,C were obtained after geometry optimization.Then, we reversed the orientation of the ethanol molecule (with the -CH 3 group pointing downward), and Figure 6B,D were obtained.The adsorption configurations in Figure 6 are sorted in descending order according to the absolute value of the adsorption energy.The optimal configuration had the most negative adsorption energy, indicating that its structure was the most stable.The charge density distribution images can be used to analyze the adsorption more intuitively.Adsorption occurs when the charge clouds of two atoms overlap, and a large charge density at the overlapping region indicates a strong adsorption.
Sensors 2024, 24, x FOR PEER REVIEW 11 of 20 optimal adsorption configuration with the most robust interaction had few transferred charges (0.151 e) due to the adsorption of the O-OH atom with a high affinity for electrons.
The O-OH atom received about 0.17 e more electrons than those in the other three configurations.Therefore, the total transferred charge was less.
To summarize the four configurations, the primary adsorption sites on the SnO2(110) surface were Sn5c and O2c(SnO2).The adsorption strength was strongly related to the -OH group of the ethanol molecule: (a) a strong adsorption indicated that the H-OH and O-OH atoms were both adsorbed; (b) a medium adsorption indicated that the H-OH atom was adsorbed, and the O-OH atom was not; and (c) a weak adsorption indicated that the H-OH and O-OH atoms were not adsorbed, and only the H-CH3 atom was adsorbed.Figure 6A corresponds to the optimal adsorption configuration with the most negative adsorption energy.In Figure 7A, it can be observed that the charge cloud of the H -OH atom overlapped with that of the O 2c (SnO 2 ) atom on the SnO 2 (110) surface.Similarly, the charge cloud of the O -OH atom overlapped with that of the Sn 5c atom.These overlapped clouds indicate the hybridization of the electronic orbitals between the ethanol molecules and the SnO 2 (110) surface.The substantial charge density (red color) between the adsorbed atoms reveals a robust adsorption procedure.The adsorption distance between the H -OH atom and the O 2c (SnO 2 ) atom was 1.011 Å, and the adsorption distance between the O -OH atom and the Sn 5c atom was 2.068 Å.The bond length between the O -OH and H -OH atoms significantly increased from 1.064 Å (in the free state) to 1.789 Å, suggesting that this adsorption configuration is conducive to the dissociation of the -OH group.Additionally, one H -CH2 atom also underwent charge exchange with the O 2c (SnO 2 ) atom on the right side.However, the charge density at the overlapping region was relatively low (green color), indicating a weaker adsorption effect.Therefore, the ethanol molecule primarily adsorbed onto the SnO 2 (110) surface through the -OH group.The adsorption energy for this configuration was −2.05 eV, and 0.151 e electrons were transferred from the ethanol molecular to the SnO 2 (110) surface.On the PdO(101)-SnO2(110) surface, nine adsorption configurations were obtained through DFT calculations, as depicted in Figure 8.In the calculations, we first placed the ethanol molecule (with the -OH group pointing downward) above the Pd3c and Pd4c atoms, and obtained Figure 8D and 8G, respectively.When the ethanol molecule was placed above the oxygen atoms, an adsorption configuration similar to Figure 8D could be obtained due to the strong interaction between the O-OH and Pd3c atoms.Then, we placed the ethanol molecule with the -OH group pointing downward and approached the Pd3c, O3c, and O2c atoms from the side, and obtained Figure 8B, 8A, and 8F, respectively.Afterward, we placed the ethanol molecule (with the -CH3 group pointing downward) above the Pd atoms (each H atom above one Pd atom) and obtained Figure 8H.Then, we pointed the -CH3 group downward and approached the O2c atom from the side to obtain Figure 8I.When the -CH3 group of the ethanol molecule was pointed downward and to the side of the O3c atom, the -OH group would be adsorbed because of the low position of the O3c atom, and Figure 8C,E were obtained depending on the adsorption of the -OH group.The adsorption configurations in Figure 8 are sorted in descending order according to the absolute value of the adsorption energy.We believe these typical configurations cover the majority of the adsorption situations and can provide us with sufficient information to analyze the adsorption characteristics.Figure 9 shows the charge density distribution of the configurations in Figure 8, allowing us to analyze the adsorption more intuitively and accurately.
Figure 8A shows the optimal adsorption configuration with the most negative adsorption energy.In Figure 9A, the O-OH atom was strongly adsorbed on the Sn5c atom due to the large charge density between the two atoms.Meanwhile, the H-OH atom was adsorbed on the O3c atom, and their charge density was smaller, indicating a weaker interaction.In this configuration, the ethanol molecule was adsorbed on both the PdO and The three additional ethanol adsorption configurations on the SnO 2 (110) surface are illustrated in Figure 6B-D, with their corresponding charge density distributions shown in Figure 7B-D.The adsorption energies for these three configurations were −0.90 eV, −0.75 eV, and −0.34 eV, respectively.The electrons transferred from the ethanol molecular to the SnO 2 (110) surface for the three configurations were 0.253 e, 0.227 e, and 0.146 e, respectively, which was positively related to the adsorption strength.However, the optimal adsorption configuration with the most robust interaction had few transferred charges (0.151 e) due to the adsorption of the O -OH atom with a high affinity for electrons.The O -OH atom received about 0.17 e more electrons than those in the other three configurations.Therefore, the total transferred charge was less.
To summarize the four configurations, the primary adsorption sites on the SnO 2 (110) surface were Sn The adsorption configurations in Figure 8 are sorted in descending order according to the absolute value of the adsorption energy.We believe these typical configurations cover the majority of the adsorption situations and can provide us with sufficient information to analyze the adsorption characteristics.Figure 9 shows the charge density distribution of the configurations in Figure 8, allowing us to analyze the adsorption more intuitively and accurately.
Sn5c.The electrons transferred from ethanol to the PdO-SnO2 surface were 0.252 e, 0.247 e, and 0.229 e in the three configurations.
In Figure 8G, the O-OH atom and the H-OH atom were adsorbed on the Pd4c and the O2c atoms, respectively.Even though both the O-OH atom and the H-OH atom participate in adsorption, the adsorption energy was only −0.77 eV, much smaller than those in Figure 9B-D.This reduced adsorption capability may be because the Pd4c atom had already been coordinated with four surrounding lattice oxygen atoms.This configuration showed a moderate adsorption strength.The electrons transferred from ethanol to the PdO-SnO2 surface in this configuration were 0.180 e.
The configurations in Figure 8E,F had adsorption energies of −1.01 eV and −0.81 eV, respectively, which are moderate adsorption strengths.In both configurations, only the H-OH atom was adsorbed on the O2c atom, while the O-OH atom was not adsorbed.Notably, the adsorption was stronger in the configuration in Figure 8E, which was attributed to the additional adsorption interaction between the H-CH2 atom and the Pd3c atom.The electrons transferred from ethanol to the PdO-SnO2 surface were 0.286 e and 0.229 e in the two configurations.
The adsorption energies for the configurations in Figure 8H,I were around −0.5 eV, indicating weak adsorption strengths.In these configurations, the H-CH3 atoms participated in the adsorption, and the -OH group did not.The results suggest that the H-CH3 atom was far less activated than the O-OH and H-OH atoms.The electrons transferred from ethanol to the PdO-SnO2 surface were 0.186 e and 0.157 e, respectively.Similar to the SnO2(110) surface, the -OH group also had a significant impact on the adsorption on the PdO(101)-SnO2(110) surface.The adsorption characteristics on the SnO2(110) and PdO(101)-SnO2(110) surfaces are summarized in Table 3.The conditions for different adsorption strengths are listed in Table 4.  Figure 8A shows the optimal adsorption configuration with the most negative adsorption energy.In Figure 9A, the O -OH atom was strongly adsorbed on the Sn 5c atom due to the large charge density between the two atoms.Meanwhile, the H -OH atom was adsorbed on the O 3c atom, and their charge density was smaller, indicating a weaker interaction.In this configuration, the ethanol molecule was adsorbed on both the PdO and SnO 2 surfaces.Also, 0.266 e electrons were transferred from ethanol to the PdO-SnO 2 surface.
The adsorption energies for the configurations in Figure 8B-D were all approximately −1.50 eV, exhibiting a strong adsorption.Their distinctive feature was that the O -OH and H -OH atoms participated in adsorption simultaneously.The O -OH atoms were adsorbed on the Pd 3c atom, while the H -OH atom was adsorbed on one O atom.From the charge density distribution images in Figure 9B-D, it is evident that there was a significant overlap between the charge clouds of O -OH and Pd 3c , exhibiting a strong adsorption interaction.Meanwhile, the adsorption of H -OH with the O atom exhibited a weaker interaction due to a lower density of the overlapped charge cloud.Hence, the Pd 3c atom acted as a crucial adsorption site on the PdO surface.However, compared with the optimal adsorption configuration, the ethanol adsorption capability of Pd 3c seemed weaker than that of Sn 5c .The electrons transferred from ethanol to the PdO-SnO 2 surface were 0.252 e, 0.247 e, and 0.229 e in the three configurations.
In Figure 8G, the O -OH atom and the H -OH atom were adsorbed on the Pd 4c and the O 2c atoms, respectively.Even though both the O -OH atom and the H -OH atom participate in adsorption, the adsorption energy was only −0.77 eV, much smaller than those in Figure 9B-D.This reduced adsorption capability may be because the Pd 4c atom had already been coordinated with four surrounding lattice oxygen atoms.This configuration showed a moderate adsorption strength.The electrons transferred from ethanol to the PdO-SnO 2 surface in this configuration were 0.180 e.
The configurations in Figure 8E,F had adsorption energies of −1.01 eV and −0.81 eV, respectively, which are moderate adsorption strengths.In both configurations, only the H -OH atom was adsorbed on the O 2c atom, while the O -OH atom was not adsorbed.Notably, the adsorption was stronger in the configuration in Figure 8E, which was attributed to the additional adsorption interaction between the H -CH2 atom and the Pd 3c atom.The electrons transferred from ethanol to the PdO-SnO 2 surface were 0.286 e and 0.229 e in the two configurations.
The adsorption energies for the configurations in Figure 8H,I were around −0.5 eV, indicating weak adsorption strengths.In these configurations, the H -CH3 atoms participated in the adsorption, and the -OH group did not.The results suggest that the H -CH3 atom was far less activated than the O -OH and H -OH atoms.The electrons transferred from ethanol to the PdO-SnO 2 surface were 0.186 e and 0.157 e, respectively.
Similar to the SnO 2 (110) surface, the -OH group also had a significant impact on the adsorption on the PdO(101)-SnO 2 (110) surface.The adsorption characteristics on the SnO 2 (110) and PdO(101)-SnO 2 (110) surfaces are summarized in Table 3.The conditions for different adsorption strengths are listed in Table 4.After ethanol is adsorbed, electrons are transferred from ethanol molecules to the PdO(101)-SnO 2 (110) surface, so ethanol exhibits a reducing property.However, there is no positive correlation between transferred electrons and adsorption strength because the O -OH atom gains electrons if adsorbed.Considering the strong affinity of the O -OH atom for electrons, we divided the adsorption configurations on the PdO(101)-SnO 2 (110) surface into two categories based on whether the O -OH atom was adsorbed.In Table 3, the corresponding number of transferred electrons is displayed by underlines based on when the O -OH atom is adsorbed.In each category, the transferred electrons and the adsorption strength exhibited a positive correlation.
By comparing the adsorption characteristics on the two surfaces, the following conclusions were drawn: • According to the adsorption energy in Table 3, the ethanol adsorption strength was stronger on the Sn 5c atom (E ads ≈ −2 eV) than on the Pd 3c atom (E ads ≈ −1.5 eV).However, comparing the optimal adsorption configurations on two surfaces, the transferred electrons were increased by 0.115 e on the PdO(101)-SnO 2 (110) surface, indicating a higher reducing property of ethanol with the assistance of PdO.Furthermore, for the weak adsorption configurations, the adsorption energy was more negative on the PdO(101)-SnO 2 (110) surface, revealing the enhancement of the adsorption of the H -CH3 atoms by PdO.

•
On the PdO(101) surface, it was found that the adsorption sites were continuously distributed since every atom of PdO can act as an adsorption site.On the SnO 2 (110) surface, the Sn 5c and O 2c (SnO 2 ) atoms acted as the adsorption sites with a relatively sparse distribution.Therefore, PdO can facilitate ethanol adsorption through numerous adsorption sites.

Gas-Sensing Mechanism
It is generally believed that the oxidation reaction between ethanol and oxygen ions adsorbed on the surface causes a change in SnO 2 -based gas sensors' resistance.In the air, oxygen can be adsorbed on the surface of SnO 2 and capture electrons from SnO 2 , forming oxygen ions such as O 2 − , O − , and O 2− [45].A depletion layer at the surface of SnO 2 is simultaneously formed, which increases the sensor's resistance.When ethanol arrives, it firstly adsorbs on the SnO 2 's surface, and then reacts with the pre-adsorbed oxygen ions, producing CO 2 and H 2 O, as shown in Equations ( 2)-(4) [7,46].Through this reaction, the electrons captured by the oxygen ions escape and flow back into the SnO 2 conduction band, causing a decrease in the thickness of the depletion layer and the sensor's resistance [47].
A heterojunction is formed between the p-type PdO (E g = 3.84 eV) and n-type SnO 2 (E g = 3.6 eV) [48,49].Since the work function of PdO (7.9 eV) is higher than that of SnO 2 (4.5 eV) [30], electrons flow from SnO 2 into PdO and form a depletion layer at the PdO-SnO 2 interface.In the air, oxygen can be adsorbed on p-type PdO, capturing electrons and forming an accumulation layer on the PdO with a high hole concentration.The increase in hole concentration means that the Fermi level of the PdO is reduced, so more electrons flow from the SnO 2 into PdO, increasing the width of the depletion layer at the PdO-SnO 2 interface.When ethanol arrives, it also consumes the adsorbed oxygen ions on the PdO surface.Electrons escape from oxygen and enter the PdO valence band to recombine with holes, causing the hole concentration to decrease and the Fermi level to increase.Therefore, the electrons flow back into SnO 2 , causing the thickness of the depletion layer and the resistance to decrease.The schematic diagrams of the gas-sensing mechanism and the energy band diagram on the SnO 2 surface and at the PdO-SnO 2 interface are shown in Figure 10.In the gas-sensing test, the PdO-SnO2 gas sensor showed excellent gas-sensing performance, which is due to the following four reasons: 1.The PdO-SnO2 material has a two-dimensional nanosheet structure, which gives it a large specific surface area and many adsorption sites [37,50,51].2. PdO has a catalyst effect on ethanol oxidation [15,17].Specifically, PdO can enhance the amount of oxygen adsorbed on the SnO2 surface through the spillover effect [52][53][54], thereby enhancing the gas-sensing reaction on the SnO2 surface.In addition, PdO can also lower the chemical reaction barrier [55].3. The heterostructure can enhance the gas-sensing performance [56,57].Due to the high catalytic performance of PdO, the gas-sensing reaction can cause significant changes in PdO's hole concentration and Fermi level, which can be reflected in the thickness change of the depletion layer and the resistance at the heterojunction.4. According to the DFT calculation results, PdO has the following enhancement effects: First, it enhances the charge transfer amount after adsorption and increases the reducing performance of ethanol.Second, it enhances the adsorption strength of H-CH3 atoms on the material surface.Third, PdO has a large number of continuous adsorption sites, increasing the adsorption probability of ethanol.Furthermore, we propose a possible synergistic effect.Because the adsorption sites are continuous on the PdO surface, the ethanol molecule can move to adjacent adsorption sites through a small amount of energy exchange with the environment.Therefore, ethanol molecules exhibit a certain degree of mobility on the PdO surface.Through this mobility, the ethanol molecule may move to the interface between PdO and SnO2, react with the spillover oxygen, and thereby change the resistance of SnO2.This inferred process can be viewed as a synergistic effect.

Conclusions
Two-dimensional SnO2 and 1 at.%PdO-SnO2 nanosheet materials were synthesized using a facile hydrothermal method.The microscopic morphology and elemental states of the materials were characterized using XRD, SEM, TEM, and XPS.Two-dimensional nanosheets were stacked to form a gas-sensitive thin film, with some regions exhibiting a porous morphology due to the assembly of nanosheets.The TEM observations revealed the heterogeneous junction structure of PdO-SnO2.XPS characterization confirmed the presence of Sn 4+ , O 2− , and Pd 2+ in the PdO-SnO2 material and spectral peak shifts due to the PdO-SnO2 heterojunctions.The gas-sensing tests demonstrated that the PdO-SnO2 In the gas-sensing test, the PdO-SnO 2 gas sensor showed excellent gas-sensing performance, which is due to the following four reasons: 1.
The PdO-SnO 2 material has a two-dimensional nanosheet structure, which gives it a large specific surface area and many adsorption sites [37,50,51].2.
PdO has a catalyst effect on ethanol oxidation [15,17].Specifically, PdO can enhance the amount of oxygen adsorbed on the SnO 2 surface through the spillover effect [52][53][54], thereby enhancing the gas-sensing reaction on the SnO 2 surface.In addition, PdO can also lower the chemical reaction barrier [55].

3.
The heterostructure can enhance the gas-sensing performance [56,57].Due to the high catalytic performance of PdO, the gas-sensing reaction can cause significant changes in PdO's hole concentration and Fermi level, which can be reflected in the thickness change of the depletion layer and the resistance at the heterojunction.4.
According to the DFT calculation results, PdO has the following enhancement effects: First, it enhances the charge transfer amount after adsorption and increases the reducing performance of ethanol.Second, it enhances the adsorption strength of H -CH3 atoms on the material surface.Third, PdO has a large number of continuous adsorption sites, increasing the adsorption probability of ethanol.Furthermore, we propose a possible synergistic effect.Because the adsorption sites are continuous on the PdO surface, the ethanol molecule can move to adjacent adsorption sites through a small amount of energy exchange with the environment.Therefore, ethanol molecules exhibit a certain degree of mobility on the PdO surface.Through this mobility, the ethanol molecule may move to the interface between PdO and SnO 2 , react with the spillover oxygen, and thereby change the resistance of SnO 2 .This inferred process can be viewed as a synergistic effect.

Conclusions
Two-dimensional SnO 2 and 1 at.%PdO-SnO 2 nanosheet materials were synthesized using a facile hydrothermal method.The microscopic morphology and elemental states of the materials were characterized using XRD, SEM, TEM, and XPS.Two-dimensional

Figure 3 .
Figure 3. (A) The HAADF image of the PdO-SnO 2 material, (B) the enlarged image of the red rectangle, and the EDS mapping images for (C) Pd, (D) Sn, and (E) O elements.(F) High-resolution TEM image of the PdO-SnO 2 material.

Figure 4 .
Figure 4. XPS spectra of the SnO2 and the PdO-SnO2 materials: (A) full-survey, (B) Sn 3d, (C) Pd 3d, and (D) O 1s.The black lines in the figures represent the signal background.

Figure 4 .
Figure 4. XPS spectra of the SnO 2 and the PdO-SnO 2 materials: (A) full-survey, (B) Sn 3d, (C) Pd 3d, and (D) O 1s.The black lines in the figures represent the signal background.

Figure 5 .
Figure 5. (A) Responses of PdO−SnO2 and SnO2 gas sensors to 50 ppm ethanol at different operating temperatures.(B) Responses to ethanol gas at different concentrations at an operating temperature of 300 °C.Responses as a function of ethanol concentration in (C) linear and (D) logarithmic coordinate systems.(E) Responses of the PdO−SnO2 gas sensor to ethanol and other interfering gases at different temperatures.(F) The long-term stability and (G) repeatability of the PdO−SnO2 and SnO2 gas sensors.

Figure 5 .
Figure 5. (A) Responses of PdO−SnO 2 and SnO 2 gas sensors to 50 ppm ethanol at different operating temperatures.(B) Responses to ethanol gas at different concentrations at an operating temperature of 300 • C. Responses as a function of ethanol concentration in (C) linear and (D) logarithmic coordinate systems.(E) Responses of the PdO−SnO 2 gas sensor to ethanol and other interfering gases at different temperatures.(F) The long-term stability and (G) repeatability of the PdO−SnO 2 and SnO 2 gas sensors.

Figure 6 .
Figure 6.The adsorption configuration of a C2H5OH molecule on the SnO2(110) surface.(A) The −OH group is adsorbed on the Sn5c and O2c(SnO2) atoms; (B) the H−OH and the H−CH3 atoms are adsorbed on the O2c(SnO2) and Sn5c atoms, respectively; (C) the H−OH atom is adsorbed on the O2c(SnO2) atom; (D) the H−CH3 atoms are adsorbed on the O2c(SnO2) atom.The red, yellow, and green colors of the rectangle around Eads indicate strong, medium, and weak adsorption strengths, respectively.

Figure 6 .
Figure 6.The adsorption configuration of a C 2 H 5 OH molecule on the SnO 2 (110) surface.(A) The −OH group is adsorbed on the Sn 5c and O 2c (SnO 2 ) atoms; (B) the H −OH and the H −CH3 atoms are adsorbed on the O 2c (SnO 2 ) and Sn 5c atoms, respectively; (C) the H −OH atom is adsorbed on the O 2c (SnO 2 ) atom; (D) the H −CH3 atoms are adsorbed on the O 2c (SnO 2 ) atom.The red, yellow, and green colors of the rectangle around E ads indicate strong, medium, and weak adsorption strengths, respectively.

Sensors 2024 , 20 Figure 7 .
Figure 7.The charge density distribution of the adsorption configurations for an ethanol molecule adsorbed on the SnO2(110) surface, corresponding to Figure 6 (A-D).

Figure 7 .
Figure 7.The charge density distribution of the adsorption configurations for an ethanol molecule adsorbed on the SnO 2 (110) surface, corresponding to Figure 6 (A-D).
5c and O 2c (SnO 2 ).The adsorption strength was strongly related to the -OH group of the ethanol molecule: (a) a strong adsorption indicated that the H -OH and O -OH atoms were both adsorbed; (b) a medium adsorption indicated that the H -OH atom was adsorbed, and the O -OH atom was not; and (c) a weak adsorption indicated that the H -OH and O -OH atoms were not adsorbed, and only the H -CH3 atom was adsorbed.On the PdO(101)-SnO 2 (110) surface, nine adsorption configurations were obtained through DFT calculations, as depicted in Figure 8.In the calculations, we first placed the ethanol molecule (with the -OH group pointing downward) above the Pd 3c and Pd 4c atoms, and obtained Figure 8D and 8G, respectively.When the ethanol molecule was placed above the oxygen atoms, an adsorption configuration similar to Figure 8D could be obtained due to the strong interaction between the O -OH and Pd 3c atoms.Then, we placed the ethanol molecule with the -OH group pointing downward and approached the Pd 3c , O 3c , and O 2c atoms from the side, and obtained Figure 8B, 8A, and 8F, respectively.Afterward, we placed the ethanol molecule (with the -CH 3 group pointing downward) above the Pd atoms (each H atom above one Pd atom) and obtained Figure 8H.Then, we pointed the -CH 3 group downward and approached the O 2c atom from the side to obtain Figure 8I.When the -CH 3 group of the ethanol molecule was pointed downward and to the side of the O 3c atom, the -OH group would be adsorbed because of the low position of the O 3c atom, and Figure 8C,E were obtained depending on the adsorption of the -OH group.

Figure 8 .
Figure 8.The adsorption configuration of a C2H5OH molecule on the PdO(101)−SnO2(110) surface.The −OH group is adsorbed on (A) Sn5c and O3c atoms; (B) Pd3c and O2c(SnO2) atoms; (C) Pd3c and O3c atoms; and (D) Pd3c and O2c atoms.(E) The H−OH and H−CH2 atoms are adsorbed on the O2c and Pd3c atoms, respectively.(F) The H−OH atom is adsorbed on the O2c atom.(G) The O−OH and H−OH atoms are adsorbed on the Pd4c and O2c atoms, respectively.(H) The H−CH3 atoms are adsorbed on the Pd3c and Pd4c atoms.(I) The H−CH3 atoms are adsorbed on the O2c and O2c(SnO2) atoms.The red, yellow, and green colors of the rectangle around Eads indicate strong, medium, and weak adsorption strengths, respectively.

Figure 8 .
Figure 8.The adsorption configuration of a C 2 H 5 OH molecule on the PdO(101)−SnO 2 (110) surface.The −OH group is adsorbed on (A) Sn 5c and O 3c atoms; (B) Pd 3c and O 2c (SnO 2 ) atoms; (C) Pd 3c and O 3c atoms; and (D) Pd 3c and O 2c atoms.(E) The H −OH and H −CH2 atoms are adsorbed on the O 2c and Pd 3c atoms, respectively.(F) The H −OH atom is adsorbed on the O 2c atom.(G) The O −OH and H −OH atoms are adsorbed on the Pd 4c and O 2c atoms, respectively.(H) The H −CH3 atoms are adsorbed on the Pd 3c and Pd 4c atoms.(I) The H −CH3 atoms are adsorbed on the O 2c and O 2c (SnO 2 ) atoms.The red, yellow, and green colors of the rectangle around E ads indicate strong, medium, and weak adsorption strengths, respectively.

20 Figure 9 .
Figure 9.The charge density distribution of the adsorption configurations for an ethanol molecule adsorbed on the PdO(101)-SnO2(110) surface, corresponding to Figure 8 (A-I).

Figure 9 .
Figure 9.The charge density distribution of the adsorption configurations for an ethanol molecule adsorbed on the PdO(101)-SnO 2 (110) surface, corresponding to Figure 8 (A-I).

Sensors 2024 , 20 Figure 10 .
Figure 10.The schematic diagram of the gas-sensing mechanism and energy band diagram of the PdO-SnO2 sensor in (A) air and (B) ethanol gas.

Figure 10 .
Figure 10.The schematic diagram of the gas-sensing mechanism and energy band diagram of the PdO-SnO 2 sensor in (A) air and (B) ethanol gas.

Table 1 .
Ethanol-sensing performances of different gas-sensing materials.

Table 1 .
Ethanol-sensing performances of different gas-sensing materials.

Table 2 .
The atom symbols and their meanings.
3c The O atom coordinated with two Pd atoms and one Sn atom on the PdO(101)-SnO 2 (110) surface

Table 4 .
Adsorption conditions that determine the adsorption strength.
* The underlines indicate the O−OH atom is adsorbed in the configurations.

Table 4 .
Adsorption conditions that determine the adsorption strength.