Enhanced acetone gas sensing performance of ZnO polyhedrons decorated with LaFeO3 nanoparticles

Since acetone is potentially harmful to humans, it is necessary to develop a high-performance acetone gas sensor. In this study, ZnO polyhedrons decorated with LaFeO3 (LFO) nanoparticles with high acetone-sensing performances were prepared by a facile microwave-assisted hydrolytic reaction method, and the p-n heterojunction was successfully constructed. The crystal structure, surface morphology, and internal composition of the LaFeO3/ZnO composites were analyzed by various characterization methods. The results showed that LaFeO3 nanoparticles were successfully composited with ZnO polyhedra. Compared with the pure ZnO sensor, the LaFeO3/ZnO sensor showed a significant improvement in sensitivity, recovery time, and selectivity. For example, at the optimal operating temperature of 340 °C, the response of the LaFeO3/ZnO sensor to 100 ppm acetone could reach ∼208.7, which was 39 times higher than that of the pure ZnO sensor. And the recovery time of the LaFeO3/ZnO sensor was reduced to 15.4 s. Meanwhile, the LaFeO3/ZnO sensor had the highest selectivity for acetone. The significant improvement of the sensing performance of the LaFeO3/ZnO sensor might be attributed to the formation of p-n heterojunctions and the good catalytic effect of LaFeO3.


Introduction
Volatile organic compounds (VOCs), which mainly originate from motor vehicle emissions and industrial emissions, are a class of toxic and hazardous substances that can cause very serious harm to human health and the environment [1][2][3][4].In recent years, how to detect such VOCs in a timely and effective manner has attracted extensive attention from researchers.Acetone, as a common VOCs, plays an important role in laboratories [5], home improvement [6], medicine [7] and industrial production [8].However, prolonged exposure to an acetone atmosphere can cause health problems such as headaches [9], breathing difficulties and neurological damage [10].Therefore, the detection of acetone in the environment is particularly important.Among several acetone detection methods, semiconductor metal oxide gas sensors are widely used due to their simple preparation, high sensitivity and stability [11][12][13][14][15][16].
ZnO, a typical n-type semiconductor material, has a wide band gap of 3.37 eV [17].Because of its unique crystal structure, high carrier mobility, and high stability, it has become a current research hotspot for acetone gas detection.However, the pure ZnO sensor will have unnegligible demerits, such as poor sensitivity, slow response recovery time and poor selectivity [18].Therefore, there is a need to find a suitable method to modify zinc oxide to obtain superior acetone sensor performance.In recent years, many studies have demonstrated that the acetone-sensing performances of ZnO could be improved by modulating the morphology of ZnO, 2.2.Synthesis of LaFeO 3 LaFeO 3 perovskite-type oxides were synthesized by the sol-gel method.4.33 g 10 mmol La(NO 3 ) 3 •6H 2 O and 4.04 g 10 mmol Fe(NO 3 ) 3 •9H 2 O were dissolved in 75 ml of deionized water, and citric acid monohydrate was added to the above solution as a complexing agent and stirred to form a clear mixed solution with a molar ratio of nitrate to citric acid monohydrate of 1:4.The solution was heated and stirred in a constant temperature water bath at 70 °C until a wet gel was obtained, and then dried in an oven at 100 °C to obtain a dry del.The dry gels were calcined in a muffle furnace at 500 °C for 3 h, and then the temperature was increased to 700 °C for 3 h.Perovskite-type LaFeO 3 nanoparticles were synthesized.

Synthesis of LaFeO 3 /ZnO Composites
A series of p-LaFeO 3 /n-ZnO composites were synthesized using a simple liquid-phase reaction with microwave heating.Specifically, a certain amount of Zn(CH 3 COO) 2 •2H 2 O and Na 3 C 6 H 5 O 7 were dissolved in deionized water and stirred vigorously.After thorough mixing, a certain amount of prepared LaFeO 3 was added.The mixed solution was sonicated for 30 min and stirred for 2 h.Then a certain amount of ammonia was added drop by drop into the above-mixed solution to make the pH value ∼10.After thorough mixing, the obtained homogeneous solution was transferred to a beaker and kept at 90 °C for 40 min in a microwave oven.After the reaction, the products were collected by centrifugation, washed several times with deionized water and anhydrous ethanol, and then dried at 80 °C for 12 h.Finally, they were annealed in a muffle furnace at 500 °C for 2 h.The mass ratios of the prepared p-LaFeO 3 /n-ZnO composite oxides were X-LaFeO 3 /ZnO (X is the LaFeO 3 mass fraction: 5%, 10%, 15%, 20%, 25%).Also, for subsequent comparison, ZnO powders were prepared using the above method.

Characterization
X-ray diffraction (XRD) patterns were adopted on a Rigaku Ultima IV(Japan) with Cu Kα radiation source (λ = 0.1541 Å) at a scan rate of 4.8°min −1 ranging from 20°to 80°in the step of 0.04°in order to characterize the crystal structure and phase purity of the products.The morphologies of the sample were observed by Scanning electron microscopy (SEM, ZEISS Sigma 300, German).Transmission electron microscopy (TEM, JEOL JEM-2100F, Japan) and High-resolution transmission electron microscopy (HRTEM) images were used to observe both the microstructure and crystalline features of the prepared sample.The surface chemical compositions and chemical states of the sample were investigated using X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, America) with Al Kα source (1486 eV).Moreover, XPS results were calibrated by adventitious carbon C 1 s peak at 285 eV.

Fabrication and performance test of gas sensor
In this paper, a para thermal resistance sensor is used.The sensor is mainly composed of three parts, Al 2 O 3 ceramic tube, Ni-Cr alloy wire, and ceramic tube base, and its preparation process is as follows.Take ZnO as an example, take an appropriate amount of ZnO powder dispersed in anhydrous ethanol and coat it on the surface of the Al 2 O 3 ceramic tube with a paint pen dipped in the above slurry, thus forming a sensing film.The four electrode wires of the ceramic tube and the Ni-Cr heating wire are welded together to the base of the sensor to form a par thermal resistance gas sensor.At this point, the prepared gas-sensitive sensor is inserted into the gassensitive test plate and aged for about 2 h.The gas-sensitive sensor performance can then be tested.The preparation process of the gas sensor is shown in figure 1.
The gas sensitivity test was performed on a WS-30A gas sensitivity measurement system (Zhengzhou Weisheng Electronic Technology Co., Ltd.).First, the welded sensor to be tested is placed in the test system exposed to air, and the baseline resistance position is determined by selecting a suitable resistance card.This data shows the resistance of the sensor when exposed to air.And then the lid of the test system is covered, and an equivalent volume of liquid for the concentration of the gas to be tested is calculated by equation (1) [31], and the required amount of liquid is injected into the heated platform of the gas test box using a micro syringe, and is made to become a gas by heating, and a fan system ensures that the gas is uniformly distributed inside the test box.The target gas comes into contact with the sensor and the element responds and is characterized by a change in resistance measured in the test system.After the resistance value reaches a steady state, the cap is then opened to release the gas, at which point the element resistance returns to its initial resistance.And the specific liquid calculation formulas are as follows: Where C (ppm) , ρ (g•ml −1 ), d, V x (μl), M (g•mol −1 ) and V (L) are the concentration of the target gas, the density of the liquid to be measured, the purity of the liquid, the volume of the test liquid, the molecular weight of the liquid, and the volume of the gas test chamber, respectively.For n-type semiconductors, for reducing gases, the gas sensing response of the sensor is defined as ΔR/R g , where ΔR = R a -R g [32], and When the gas-sensitive semiconductor sensor begins to come into contact with the detected gas, the element begins to respond and this is indicated by a change in resistance measured in the test system.It takes time from the beginning of the element's response to the detected gas until the resistance has changed to a steady state value, which is defined as the time it takes for the resistance to reach 90% of its steady state value [31].Similarly, the recovery time is the time it takes for the resistance of the element to recover to 90% of its original value after the measured gas has been released [31], and the shorter the response and recovery time, the better the performance of the semiconductor element.

Material characterization
The crystal structure and purity of the synthesized materials were tested with the help of XRD, and three materials, ZnO, LaFeO 3 , and LaFeO 3 /ZnO with different masses of LaFeO 3 , were analyzed in detail.As shown in figure 2, the pure ZnO material has a hexagonal wurtzite structure, and each of its diffraction peaks corresponds to the data on the standard card JCPDS No. 36-1451, and no other spurious peaks were observed, indicating that the material has high purity.Secondly, by analyzing the spectrum of the LaFeO 3 sample, it can be observed that the sample conforms to the ABO 3 -type perovskite structure, and each of its diffraction peaks corresponds to the data on the standard card JCPDS No. 37-1493, and no diffraction peaks of other impurities are detected, indicating a high purity of the material.Finally, the spectra of LaFeO 3 /ZnO with different masses of LaFeO 3 were analyzed, and it can be observed that most of the diffraction peaks in the figure are characteristic peaks of ZnO, while the peaks of LaFeO 3 can be seen at some angles such as 2θ = 22.5°, 32.2°, 39.6°, 46.1°, 57.4°, and no  peaks related to other materials were found in the XRD plots, which indicates that no reaction between LaFeO 3 and ZnO.The appearance of ZnO and LaFeO 3 peaks in the diffractograms of LaFeO 3 /ZnO composites without other spurious peaks in the products confirms the successful synthesis of ZnO and LaFeO 3 .Looking at the diffraction peaks of LaFeO 3 /ZnO, it can be found that the intensity of the ZnO diffraction peak decreases with the increase of the amount of LaFeO 3 in the product.When LaFeO 3 is integrated into the ZnO lattice, the amplitude of the diffraction peaks decreases, which can be attributed to the small lattice microstrain and distortion induced during the preparation process of loading LaFeO 3 onto ZnO, resulting in grain rotation [33].
In addition, from the diffraction plane of (101) and (202) in LaFeO 3 , the intensities of the diffraction peaks of LaFeO 3 increase with its increasing mass fraction, which indicate that the successful synthesis of LaFeO 3 onto ZnO [34].
The SEM images (figure 3(a)) show the microscopic morphology of the 10% LaFeO 3 /ZnO composites.In the low magnification images, it can be observed that the composites are in the shape of polyhedra with a relatively uniform distribution.There are nanoparticle aggregations around some of the polyhedra.There are many small holes on the surface of the composite material with an obvious porous surface structure (figure 3(b)), which is favourable for the diffusion of gases into the whole sensing material, thus improving the sensing performance.The elemental distributions on the surface of the 10% LaFeO 3 /ZnO composites are presented in figure 3(c).It can be seen that in addition to the two elements Zn and O, there are also La and Fe elements.Combined with the above XRD results, it can be confirmed that the successful composite of LaFeO 3 and ZnO.In addition, the interface between ZnO and LaFeO 3 can be clearly observed, so it can be inferred that a heterogeneous structure is formed between the two phases [35].
The elemental composition and chemical oxidation states of samples were determined by XPS.The XPS full spectrum of the 10% LaFeO 3 /ZnO composite is shown in figure 5(a), from which it can be seen that the XPS full spectrum of the composite includes five elements of Zn, O, La, Fe, and C. The above is consistent with the XRD analysis results containing ZnO and LaFeO 3 substances, indicating that the material is a LaFeO 3 /ZnO composite [36,37].Element C was present as an additive and was used to calibrate the XPS measurement spectra.In order to investigate the chemical states of elements other than element C, the XPS spectral peaks of each element were    and 3d 3/2 , respectively, indicating that La ions are present in the form of La 3+ [39,40].The asymmetric XPS spectrum of Fe 2p (figure 5(d)) can be decomposed into four components.710.3 eV and 723.5 eV binding energy peaks are Fe 3+ and 712.9 eV and 726.1 eV binding energy peaks are Fe 4+ [40][41][42].The presence of Fe 4+ proves that the element Fe is in the mixed-valence state in the chalcogenide LaFeO 3 , and oxygen vacancies must occur in order to ensure the electroneutrality of the compound, which is consistent with previous studies [43,44].Therefore, to investigate the variation of the oxygen vacancy content, the O 1 s XPS energy spectra of ZnO and 10% LaFeO 3 /ZnO materials were analyzed and the results are shown in figures 5(e) and (f).In figures 5(c) and (d), the three peaks at the binding energy positions of 532.5 eV, 531.8 eV, and 530.4 eV in the O 1 s peak spectra correspond to adsorption oxygen (O C ) peak, oxygen vacancy (O V ) peak and lattice oxygen (O L ) peak, respectively [45,46].Table 1 lists the contents of the three oxygen states before and after compounding.The oxygen vacancy content in the 10% LaFeO 3 /ZnO composites (24.25%) is significantly higher than that in ZnO (16.12%).Oxygen vacancies are a good adsorption site [47], which can increase the adsorbed oxygen content on the surface and accelerate the migration of oxygen.In general, the higher the level of Oxygen vacancies, the more it contributes to the gas sensing performance.In summary, XRD, SEM, EDS, and XPS analyses show that we successfully synthesized LaFeO 3 /ZnO composites.

Gas sensing performance
It is well known that the operating temperature affects the sensing process of gas sensors, so the optimal operating temperature of gas sensors is one of the key factors to be considered in practical applications.Based on this, the temperature-dependent sensing characteristics of different mass fraction LaFeO 3 /ZnO gas sensors for 100 ppm acetone in the range of 250 °C-370 °C were investigated (figure 6(a)).The results show that the response values of the five composite gas sensors for acetone increase gradually with the increase of the operating temperature, reaching the maximum response value at the optimum operating temperature of 340 °C, and then decreasing with the further increase of the operating temperature.The reason for this situation is that the acetone molecules are less active at lower operating temperatures and cannot obtain enough energy to react with the adsorbed oxygen on the surface of the composite material, thus producing a relatively low response.When the operating temperature increases, more and more acetone molecules react with the adsorbed oxygen, and electrons are transferred, so the response value increases.However, when the operating temperature is too high, the sensor response value decreases again because the acetone molecules gain too much power at a high temperature and will keep vibrating, which is difficult to stabilize the adsorption on the surface of the composite material.In addition, it can be visualized from the figure that among the five composite gas sensors, the 10% LaFeO 3 /ZnO gas sensor has the highest response value at the optimum operating temperature of 340 °C, with a response value of ∼208.7 for 100 ppm acetone, which is higher than the response values of the other four LaFeO 3 /ZnO composite sensors.The responses of LaFeO 3 /ZnO gas sensors with different LaFeO 3 composite ratios exposed to 100 ppm acetone at an optimum operating temperature of 340 °C are shown in figure 6(b).It is found that the gas response of the LaFeO 3 /ZnO gas sensor is not monotonically enhanced with increasing  amounts of LaFeO 3 composite.Figure 6(c) shows the temperature response curves of the ZnO and 10% LaFeO 3 /ZnO gas sensors for 100 ppm acetone.It can be seen that the response value of ZnO for 100 ppm acetone is significantly lower than that of the 10% LaFeO 3 /ZnO gas sensor.Specifically, the maximum response value of the ZnO gas sensor is 5.3 and the optimum operating temperature is 370 °C.The maximum response value of the 10% LaFeO 3 /ZnO gas sensor is 208.7 and the optimum operating temperature is 340 °C.It can be seen that the response value of the 10% LaFeO 3 /ZnO composite gas sensor is much higher than that of pure ZnO, about 39 times higher than that of ZnO, and the operating temperature is also lower than that of ZnO.Meanwhile, combined with figure 6(a), it is not difficult to find that pure ZnO is not as good as the other five composite ratios of gas sensors in terms of both sensitivity and optimal operating temperature, indicating that the performance of LaFeO 3 /ZnO gas sensors is still significantly better than that of pure ZnO gas sensors to some extent.The response/recovery characteristics of the ZnO and 10% LaFeO 3 /ZnO gas sensors for 100 ppm acetone at an optimum operating temperature of 340 °C are depicted in figure 7. The response time and recovery time for the ZnO gas sensor were 11 s and 86.9 s, respectively, while the response time and recovery time for the 10% LaFeO 3 /ZnO gas sensor were 16 s and 15.4 s, respectively.It can be seen that the response times of these two sensors for acetone are comparable, but the recovery time of the 10% LaFeO 3 /ZnO gas sensor is 71.5 s shorter than that of ZnO, which proves that the introduction of LaFeO 3 has greatly contributed to the improvement of the performance of the ZnO sensor.In terms of practical applications, the 10% LaFeO 3 /ZnO gas sensor is more suitable for acetone gas detection because of its faster response and recovery characteristics.
The dynamic response curves between responses and concentrations of the 10% LaFeO 3 /ZnO gas sensor toward 5-100 ppm acetone at 340 °C are shown in figure 8(a).It can be seen from the figure that the response of the 10% LaFeO 3 /ZnO gas sensor to acetone increases with the increase in acetone concentration.In addition, it can be seen from the figure that the 10% LaFeO 3 /ZnO gas sensor has a more obvious response/recovery curve at 5 ppm acetone concentration with a response value of ∼4.03, indicating that the 10% LaFeO 3 /ZnO gas sensor has a lower detection limit.Usually, the standard for an effective response of gas sensors: response value is greater than 1.2, so the response value of 10% LaFeO 3 /ZnO gas sensor at 5 ppm acetone concentration in this study is  fully in line with this standard.Figure 8(b) shows the variation of resistance curves of the 10% LaFeO 3 /ZnO gas sensor for different concentrations of acetone at the optimum operating temperature of 340 °C.It was found that with the entry of acetone, regardless of the acetone concentration, the sensor resistance showed a first decrease, then stabilization, and finally, when the acetone vapors were eliminated, the sensor resistance began to gradually increase again until it returned to the initial resistance.Linear fitting was performed for the response values of 10% LaFeO 3 /ZnO gas sensors in the concentration range of 5-100 ppm (figure 8(b)).The results show that the fitted equation of the response of the 10% LaFeO 3 /ZnO gas sensor with the acetone gas concentration is y = −4.9864+2.2645x,which is linearly correlated 10% LaFeO 3 /ZnO gas sensor had good linear fit with the linear fitting coefficient R 2 = 0.9996 (figure 8(c)).
Figure 9(a) detects the response of the 10% LaFeO 3 /ZnO gas sensor at 340 °C for different gases (ammonia, benzene, DMF, methanol, acetonitrile, and acetone) at 100 ppm.According to the results, the 10% LaFeO 3 /ZnO gas sensor showed the highest response value of ∼208.7 for acetone gas, which is 3.6-28.2times higher than the other gases, which indicates that the LaFeO 3 perovskite material reduces the cross-sensitivity of the pure phase ZnO sensor, which may be related to the different adsorption properties of the LaFeO 3 material for the different gases to be measured.The response of the 10% LaFeO 3 /ZnO gas sensor to different target gases was superior to both pure-phase ZnO and LaFeO 3 gas sensors, and the response of the LaFeO 3 /ZnO gas sensor to acetone was calculated to be 63 times higher than that of pure-phase ZnO gas sensor.The five response recovery cycles of the 10% LaFeO 3 /ZnO gas sensor at 340 °C for 100 ppm acetone are shown in figure 9(b).From the figure, it can be found that the response recovery curves for five cycles can be well reproduced.There is no significant change in the response values of the 10% LaFeO 3 /ZnO gas sensor after five cycles of alternate exposure to air and acetone atmosphere, indicating that the 10% LaFeO 3 /ZnO gas sensor has good repeatability and stability.In summary, compared with the pure ZnO material, the LaFeO 3 /ZnO composite has the advantages of fast response recovery, high sensitivity, and good selectivity.Table 2 compares the acetone gas-sensitive properties of LaFeO 3 /ZnO composites with those of ZnO-based acetone sensors in recent years.The influences of relative humidities on the baseline resistance and response to 10% LaFeO 3 /ZnO sensor were studied and shown in figure 10.Observation of the graphs reveals that the performance of the 10% LaFeO 3 /ZnO sensor in detecting acetone vapor is greatly affected in the humidity range of 23%-88% RH.The baseline resistance and response decreases continuously with increasing ambient humidity and is maximum at a humidity of 23% RH.This is due to the fact that water are adsorbed more on the surface of the material, occupying the active sites on the surface of the material, leading to a decrease in the amount of adsorbed oxygen involved in the redox reaction, which results in an increasingly lower response of the sensor at high relative humidity [48,49].Additionally, water molecules can hinder the adsorption of acetone, and at high relative humidity, surface adsorbed oxygen and water molecules react to form surface hydroxyl groups, leading to a decrease in the baseline resistance of the sensor, and thus a decrease in response [50].

Gas sensing mechanism
ZnO exhibits n-type semiconductor behavior due to the presence of oxygen vacancies [56,57], while LaFeO 3 is a typical p-type semiconductor [58].Since oxygen molecules have a high electron affinity (0.43 eV) [59] when ZnO sensors are exposed to air, oxygen traps electrons from the conduction band of ZnO to adsorb on the surface of the ZnO material to form adsorbed oxygen, and when the ZnO material loses carriers (e − ), an electron depletion layer forms on the surface, leading to an increase in material resistance.Unlike ZnO, the carriers of p-type semiconductors are holes, and when LaFeO 3 materials lose electrons, the concentration of internal holes will increase, forming a hole accumulation layer on the surface of the material, leading to a decrease in semiconductor resistance [22].The adsorbed oxygen mainly consists of three forms, O The optimum operating temperature of the sensor in this study is greater than 300 °C, and thus the negatively charged oxygen ion species should be O 2− .When acetone gas comes in contact with the material, either the n-type semiconductor ZnO or the p-type semiconductor LaFeO 3 , the adsorbed oxygen reacts with the acetone gas, causing electrons to return to the material, as shown in equations (5) and (6).At this point, the electron depletion layer of the ZnO sensor decreases and the hole accumulation layer width of the LaFeO 3 sensor decreases.As a result, the resistance of the n-type ZnO gas sensor decreases in the presence of acetone, while the resistance of the p-type LaFeO The construction of the p-n junction between LaFeO 3 and ZnO is considered to be the key to improving the sensing performance of the sensor.In the air, since the band gap of p-type LaFeO 3 is about 2.02 eV and that of n-type ZnO is 3.37 eV, the band gap of n-type ZnO is higher than that of p-type LaFeO 3 , some of the electrons in the conduction band of ZnO diffuse into LaFeO 3 , while the holes of LaFeO 3 diffuse into ZnO until the Fermi energy level reaches the equilibrium state so that at the LaFeO 3 -ZnO interface will form a p-n heterojunction, as shown in the energy band diagram in figure 11.
In addition to the depletion layer formed on the surface of ZnO, the construction of p-n heterojunction can further extend the depletion layer at the interface between LaFeO 3 and ZnO, so the LaFeO 3 /ZnO composites present a high resistance state in air.When the LaFeO 3 /ZnO composite is exposed to acetone atmosphere, acetone reacts with the adsorbed oxygen on the surface of the composite to produce CO 2 and H 2 O and continuously releases electrons, and the depletion layer on the surface of the heterojunction becomes thinner, leading to a substantial decrease in the resistance of the heterojunction and thus causing an increase in sensitivity, a process shown in figure 12.
In addition to the above reasons, the catalytic activity of LaFeO 3 plays an important role in improving the sensing performance.Observing figures 7(a) and (b), it can be found that the sensitivity of the LaFeO 3 -modified ZnO gas sensor to acetone is greatly improved.At the same operating temperature, not only the sensing performance was significantly improved, but also the recovery time was greatly reduced, and this performance improvement was mainly due to the catalytic effect of LaFeO 3 modification.In the whole composite system,  LaFeO 3 nanoparticles play the role of regulating the electron-withdrawing layer of the main body ZnO.When the LaFeO 3 /ZnO composite is exposed to air, the better catalytic activity of p-type LaFeO 3 attracts a large number of oxygen molecules, which thickens the hole accumulation layer on its surface, so that another electron-withdrawal layer is generated at the interface between LaFeO 3 and ZnO, resulting in an increased sensor surface resistance.In an acetone atmosphere, the electron concentration on the surface of ZnO increases, and the hole concentration on the surface of LaFeO 3 decreases due to the electrons generated by the reaction, making it difficult for electrons to diffuse from ZnO to LaFeO 3 , leading to a substantial decrease in the width of the ZnO depletion layer, which increases the response value.Therefore, this study demonstrates that based on the advantages of LaFeO 3 and ZnO, their combination effectively improves the response to acetone, superior to the sensing performance of pure LaFeO 3 and ZnO.

Conclusions
In summary, the composite materials of perovskite LaFeO 3 -modified ZnO were successfully synthesized by the liquid-phase reaction method with microwave heating.The modification of LaFeO 3 lowers the optimal operating temperature while also increasing the ZnO sensor's sensitivity for acetone detection.At the optimal working temperature of 340 °C, the sensitivity of 10% LaFeO 3 /ZnO sensor to 100 ppm acetone is 208.7, which is 63 times sensitive than that of pure ZnO.And the response time recovery time of 10% LaFeO 3 /ZnO sensor is 16 s and 15.4 s, respectively, which was 71.5 s shorter than the recovery time of the pure ZnO sensor.And the 10% LaFeO 3 /ZnO sensor also exhibited superior selectivity and repeatability, and wide detection limits.The above-improved performance can be attributed to the formation of the p-n junction, the additional extension of the electron-withdrawing layer in the ZnO matrix, and the catalytic effect of LaFeO 3 .The above findings suggest that the LaFeO 3 /ZnO composite is a strong candidate as a gas-sensitive sensing material for the detection of acetone.

Figure 1 .
Figure 1.Schematic diagram of gas sensor preparation.

Figure 4 (
a) shows the TEM image of the 10% LaFeO 3 /ZnO composite with the same morphology as that of figures 3(a) and (b), which well confirms that the morphology of the composite is a polyhedral structure.The HRTEM image of 10% LaFeO 3 /ZnO composites is shown in figure 4(b), and the results show that ZnO and LaFeO 3 have very obvious lattice stripes, where the stripes of ZnO have a spacing of 0.2823 nm, which matches the (100) crystalline plane of hexagonal wurtzite ZnO structure (JCPDS No. 36-1451), and the stripes of LaFeO 3 have a spacing of 0.2740 nm, which matches the (121) crystallographic surface of the perovskite phase LaFeO 3 (JCPDS No. 37-1493).
Figure 5(b) shows the slow scan spectrum of Zn 2p showing two symmetric binding energy peaks of Zn 2p 3/2 and Zn 2p 1/2 at 1021.7 eV and 1044.7 eV, respectively, indicating the presence of Zn in the form of 2p [38].Figure 5(c) shows that the peak position at 805.6 eV and 833.9 eV are assigned to La 3d 5/2

Figure 6 .
Figure 6.(a) and (b) Curves of the response of series X-LaFeO 3 /ZnO composites with changes in operating temperature for 100 ppm acetone.(c) Responses of the sensors based on the pure ZnO and 10% LaFeO 3 /ZnO composites as a function of operating temperature to 100 ppm acetone.

Figure 7 .
Figure 7. (a) The time-varying curve of the response of ZnO sensors to 100 ppm acetone gas at the optimal operating temperature of 340 °C.(b) Response transient of 10% LaFeO 3 /ZnO composites at the same conditions.

Figure 12 .
Figure 12.A schematic of the acetone sensing mechanism of 10% LaFeO 3 /ZnO: (a) in air and (b) in acetone.

Table 1 .
XPS O 1 s area rations of peaks of various oxygen species.

Table 2 .
Comparison of the acetone-sensing properties of several gas sensors based on ZnO and composites with ZnO.
a /R g ) 3shows the opposite change.