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Article

NO2-Sensitive SnO2 Nanoparticles Prepared Using a Freeze-Drying Method

by
Lin Liu
1,
Jinbo Zhao
2,
Zhidong Jin
1,
Fei Liu
1,
Dewen Zhao
1,
Zhengyang Liu
3,
Fenglong Wang
1,
Zhou Wang
1,
Jiurong Liu
1 and
Lili Wu
1,*
1
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education and School of Materials Science and Engineering, Shandong University, Jinan 250061, China
2
School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China
3
School of Mechanical Engineering, Shandong University of Technology, Zibo 255000, China
*
Author to whom correspondence should be addressed.
Materials 2024, 17(15), 3714; https://doi.org/10.3390/ma17153714 (registering DOI)
Submission received: 29 June 2024 / Revised: 19 July 2024 / Accepted: 22 July 2024 / Published: 27 July 2024
Browse Figures
Versions Notes

Abstract

:
The n-type semiconductor SnO2 with a wide band gap (3.6 eV) is massively used in gas-sensitive materials, but pure SnO2 still suffers from a high operating temperature, low response, and tardy responding speed. To solve these problems, we prepared small-sized pure SnO2 using hydrothermal and freeze-drying methods (SnO2-FD) and compared it with SnO2 prepared using a normal drying method (SnO2-AD). The sensor of SnO2-FD had an ultra-high sensitivity to NO2 at 100 °C with excellent selectivity and humidity stability. The outstanding gas sensing properties are attributed to the modulation of energy band structure and the increased carrier concentration, making it more accessible for electron exchange with NO2. The excellent gas sensing properties of SnO2-FD indicate its tremendous potential as a NO2 sensor.

Graphical Abstract

1. Introduction

With the development of industry, global environmental pollution has become increasingly serious, and the World Health Organization (WHO) considers nitrogen dioxide (NO2) to be a serious pollutant [1]. NO2 is a significant source of global warming, haze, acid rain, and photochemical smog [2]. Moreover, NO2 has an impact on vegetation and crops by affecting plant growth efficiency and reducing crop yields [3]. On the other hand, NO2 is hazardous to human health, and high levels of NO2 inhalation can cause severe health risks such as pulmonary edema, breathing difficulties, and bronchospasm [4]. Long-term exposure to NO2 increases the risk of high blood pressure [4]. According to statistical analyses, each 10 μg/m3 increase in NO2 exposure increases all-cause mortality by 2%, acute lower respiratory disease by 6%, and chronic obstructive pulmonary illness by 3% [1,5,6]. Therefore, the development of sensors responding to low concentrations of NO2 is urgently demanded for improving the air environment and protecting human health.
Currently, the most common gas sensors are electrochemical sensors [7,8], solid electrolyte sensors [9], optical sensors [10,11], and semiconductor sensors [12,13]. Semiconductor sensors are widely used in the detection of toxic and hazardous gases owing to their low cost, high sensitivity, and good stability [14]. However, semiconductor sensors still have problems such as poor selectivity, high operating temperature, etc., which hampers their actual applications.
As a typical n-type metal oxide, SnO2 has excellent physical and chemical stability, with a low cost and non-toxic characteristics, which makes it widely used in gas sensors [15]. In recent decades, researchers have been devoted to tackling the aforementioned problems via multiple approaches for metal oxide semiconductor (MOS) sensors, including geometric structure modification [16], elemental doping [17], heterostructure construction [18,19], and noble metal loading [20]. Huang et al. prepared nanoflower-like Au/SnS2/SnO2 heterojunctions using a solvothermal method and in situ decoration. The response value to 8 ppm NO2 was 22.3 at 80 °C. These good gas-sensitizing properties were attributed to the formation of heterojunctions and the formation of more S vacancies, promoting more gas adsorption on the material surface [21]. Mnrugesh et al. synthesized p-Co3O4/n-SnO2 heterojunctions using a hydrothermal method. The prepared 10% Co3O4/SnO2 had a response of 88% at 150 °C for 100 ppm NO2 with good selectivity. The enhancement of the sensing properties was attributed to the formation of a potential barrier at the Co3O4/SnO2 heterointerface, the high specific surface area, and the increase in oxygen vacancy content [22].
Unfortunately, disadvantages still exist with the above modification strategies. Doping inhomogeneous elements into the MOS matrix will change the original crystal structure and increase surface defects [23]. The construction of heterojunctions via exogenous MOS or noble metals will increase the interfacial potential barrier, thereby increasing the baseline resistance and power consumption as well [24,25,26]. All of the above methods require the introduction of other elements into the MOS matrix, which increases the preparation cost and makes the production process more cumbersome. Moreover, freeze-drying treatment does not form a gas–liquid interface during the whole process, and the capillary force does not cause structural collapse. During the freeze-drying process, the material is first cooled below its freezing point, where the moisture in it freezes to form ice crystals. The formation and growth of ice crystals exert physical stresses on the surrounding material [27,28]. For semiconductor materials, this stress can lead to lattice distortions, which can introduce defects such as point defects, dislocations, and other defects, which, in turn, affect the electronic properties of the material. And the introduction of these defects can introduce new energy levels in the forbidden bands of semiconductors as trap energy levels or composite centers [29,30]. Hitherto, fewer studies have been reported on pristine MOS-based material sensors through freeze-drying treatments.
In this work, SnO2 nanoparticles were prepared using both a hydrothermal method and the following freeze-drying treatments. The results showed that the response value of SnO2-FD (886.2) to 10 ppm NO2 at 100 °C was 17 times higher than that of SnO2-AD (52.5), with a shorter response recovery time (74/27 s) and a low detection limit (1.69 ppb). The effect of the drying method on their gas-sensitizing properties was systematically investigated. The small particle size of the nanoparticles allowed a larger area to be in full contact with the target gas, which provided more active sites for gas adsorption. The enhanced performance is also attributed to the increase in adsorbed oxygen and the improvement of electronic structure. Therefore, this study paves novel ways for developing high-performance MOS-based sensors.

2. Experimental Section

2.1. Chemicals

Tin tetrachloride pentahydrate (SnCl4·5H2O, 99.0%), urea (CO(NH2)2, 99.5%), and ammonia solution (NH3, 25.0 ~ 28.0%) were purchased from Sinopharm Chemical Co., Ltd (Shanghai, China) and were used without further purification. Deionized water (DI) and absolute ethanol (C2H5OH, 99.7%) were also used in this work.

2.2. Synthesis of SnO2 Nanoparticles

SnO2 nanoparticles were synthesized using a facile hydrothermal method. In total, 2.35 mmol SnCl4·5H2O and 10.8 mmol urea were dissolved in a mixed solvent with a volume of 17.2 mL deionized water and 2 mL absolute ethanol with 15 min magnetic stirring. Then, 2 mL ammonia was added to the above solution. After another 15 min of magnetic stirring, the mixture was transferred into a 100 mL Teflon-lined stainless-steel autoclave and was maintained at 200 °C for 14 h. The white products were collected and washed with deionized water and absolute ethanol. Two drying methods were employed to remove solvents. One involved drying the products obtained via centrifugation at 80 °C. The other involved rapidly pre-freezing the products in liquid nitrogen after aging them in deionized water for 1 day to improve the stability of the samples and to form a more homogeneous ice crystal structure. And then, the samples were further freeze-dried at −50 °C for 2 days. The white powders obtained using the two methods were calcined at 500 °C for 2 h and were, respectively, named SnO2-AD and SnO2-FD.

2.3. Material Characterizations

The crystal structure of the samples was analyzed by X-ray diffraction analysis (XRD, DMAX-2500 PC, Tokyo, Japan) with Cu-Kα (λ = 1.5418 Å) from 10° to 90° with a scanning speed of 10°/min. The chemical compositions and the valence state of elements were characterized via an X-ray photoelectron spectrometer (XPS, AXIS Supra, Manchester, UK) with Al-Kα (hν = 1486.6 eV). The binding energy was calibrated using C 1s peaks at 284.8 eV. The morphology and microstructure of the samples were investigated by scanning electron microscope (SEM SU-70, Tokyo, Japan). The specific surface areas and pore size assignment of the samples were tested by a full-automatic specific surface and porosity analyzer (TriStar II 3flex, Micromeritics, Norcross, GA, USA) and separately calculated through Brunauer–Emmett–Teller (BET) and Barrett–Joiner–Halenda (BJH) methods. The electrical properties and carrier concentrations of the samples were measured by Hall Effect Measurement (HSM-5000, Seoul, Republic of Korea). The UV-vis spectra and band gaps of the samples were characterized via UV-vis diffuse reflection spectrum (Uv3600plus Shimadzu, Kyoto, Japan). The molecular structure of samples was analyzed by Raman spectroscopy (Thermo DXR2xi, Waltham, MA, USA) with a 1064 nm laser excitation.

2.4. Gas Sensing Performance Test

The gas sensors were fabricated using the prepared SnO2 materials. First, the prepared samples were dispersed in deionized water with a mass of 1:5 and thoroughly ground in a mortar to form a homogeneous paste. The paste was applied to an Al2O3 substrate with four electrodes printed on it and dried at 80 °C. This process was repeated five times to form a homogeneous sensitive film and heated in air for 10 h at 80 °C. Then, the substrates coated with the sensing layer were soldered to the pedestal and aged for one week at 3 V to ensure their stability. The gas sensing properties were measured with a WS-30B gas sensitivity instrument (Zhengzhou Winsen Electronics Co., Ltd., Zhengzhou, China). The target gases were injected into the test chamber via a syringe. Built-in fans in the test chamber rotated to bring the target gas into rapid and full contact with the sensor. Ra and Rg represent the stable resistance of the sensing material in air and after exposure to the target gas, respectively. The response value (S) is denoted by S = Rg/Ra for oxidizing gases and S = Ra/Rg for reducing gases. Response and recovery time are recorded as 90% time of total resistance changes in responding/recovering processes.

3. Results and Discussion

3.1. Characterizations

The crystal structure of SnO2 was measured by XRD as shown in Figure 1a. All diffraction peaks of SnO2-AD and SnO2-FD are in accordance with the tetragonal structure of SnO2 (JCPDS 41-1445). No other diffraction peaks appeared in the pattern, proving that the synthesized samples did not contain any other material phases. It can be observed that the SnO2-FD diffraction peaks are of higher intensity, indicating superior crystallinity compared to SnO2-AD [31]. Increased crystallinity means fewer grain boundaries, which are the scattering centers of carriers since the arrangement of atoms on the grain boundaries is different from that inside the grains [32]. On the other hand, grain boundaries are commonly accompanied by localized stresses and strains [33]. Therefore, the reduction in grain boundaries reduces carrier trapping and scattering at grain boundaries, thus improving carrier mobility of SnO2-FD [34]. The prepared SnO2 grain sizes can be approximately calculated using the Debye–Scherrer equation as indicated in Equation (1) [35]:
D = 0.9 λ β cos θ
where λ is the wavelength of the radiation (1.5418 Å), β is the half-height width of the peak, and θ is the Bragg diffraction angle. The average grain sizes are 10.3 and 9.0 nm, corresponding to SnO2-AD and SnO2-FD samples. SnO2-FD has a smaller grain size, and its grain size is close to twice the Debye length of SnO2 (3 nm) [36]. As we know, when the grain size of aerogels is nearly twice the Debye length, the size of the grains affects their electrical conductivity, that is, they are more likely to be activated for some nanometer effects [37]. Therefore, the depletion layer accounts for a large proportion of the particle volume, which is more favorable for exposing the SnO2 active surface and thus exchanging electrons with the target gas. Thereby, the response value and the response/recovery speed of SnO2-FD can be improved [38].
The chemical compositions and the valence state of elements were characterized via XPS. As shown in Figure 1b, the Sn and O elements are identified in the wide spectrum. The Sn 3d XPS spectrum of SnO2 is shown in Figure 1c, the two peaks at 486.52 eV and 495.03 eV corresponding to SnO2-AD are Sn 3d5/2 and Sn 3d3/2, respectively [39]. It can be observed that the Sn 3d5/2 and Sn 3d3/2 peaks of SnO2-FD are, respectively, shifted by 0.28 eV and 0.27 eV toward the high binding energy. Previous studies have shown that the total charge of an atom has a close influence on the chemical shifts of the peaks of the energy spectrum [40]. The SnO2-FD binding energy displays a redshift, indicating that more electrons are captured by the O2 molecules in air, resulting in a lower density of nearby electron clouds and an increase in the binding energy [41]. Figure 1d, e shows the O 1s XPS spectra. The peaks of SnO2-AD located at ca. 530.4, 531.8, and 533.4 eV correspond to lattice oxygen (OL), oxygen vacancy (Ov), and adsorbed oxygen (Oc), respectively [24,42]. It can be noted that the Oc and Ov contents of SnO2-FD are higher than those of SnO2-AD. The presence of Ov can supply more electrons and promote the formation of adsorbed oxygen ions [43,44]. On the other hand, Ov disrupts the metal oxide integrity and provides more active sites for target gas adsorptions and gas-sensitization reactions [45,46,47]. In particular, the increase in Oc may promote an alternative gas-sensitive reaction pathway for NO2 at the material surface [48,49,50].
The crystallography and structural features of SnO2-AD and SnO2-FD were investigated via a Raman system as shown in Figure 1f. The SnO2 lattice typically generates the following major vibrational modes [51]:
Γ = A1g + A2g + B1g + B2g + Eg + A2u + 2B1u + 3Eu
where A1g, B1g, B2g, Eg are Raman active modes, A2u and Eu are infrared active modes, A2g and B1u are inactive modes. The peak at around 633 cm−1 is assigned to the symmetric O-Sn-O vibration (A1g). The broadening of the A1g peak of SnO2-FD indicates a reduction in its grain size [52]. The Raman peak at around 484 cm−1 corresponds to the shear vibration of the oxide (Eg) [53]. And the Raman peak at around 776 cm−1 is due to the asymmetric O-Sn-O stretching (B2g) [54]. The Raman peaks of SnO2-FD all showed different degrees of blue shift, which might be caused by the increased content of oxygen vacancies [52]. The appearance of these Raman peaks indicates the tetragonal structure of SnO2. The peaks near 249 and 306 cm−1 are inactive Raman modes, which can be attributed to localized structural disturbances [55]. The enhancement of their strength is possibly due to structural defects introduced during the freeze-drying process.
The morphology and microstructure of the samples were investigated by SEM as shown in Figure 2. It can be seen that both SnO2-AD and SnO2-FD are homogeneous nanospheres. It is indicated that the two drying methods have no significant effect on their morphology. The diameters of the SnO2 nanospheres are all approximately 10 nm, corresponding to the XRD results. This suggests that each SnO2 nanosphere is composed of a single crystal [56]. Moreover, such a small particle size gives them a larger specific surface area for full contact with the target gas [57]. The presence of abundant pore structures between the nanospheres further facilitates target gas diffusion.
To further analyze the specific surface area and pore size distribution of SnO2, N2 adsorption–desorption tests were performed as shown in Figure 3. The N2 adsorption–desorption isotherms of both SnO2-FD and SnO2-AD are of type IV mode with the type H2(b) hysteresis loop, indicating that both of them have mesoporous structures with similar hierarchical structures [58]. The specific surface areas of SnO2-AD and SnO2-FD are ca. 55.63 and 52.90 m2/g, respectively. The larger specific surface areas are attributed to the small particle size of SnO2 nanoparticles. This large specific surface area supplies more active sites for the adsorption of the target gas, which is positive for the surface of the gas-sensitive reaction, thus shortening the response/recovery time of the sensors and enhancing the response value [59,60]. As displayed in BJH measurement, the average pore sizes of SnO2-AD and SnO2-FD were calculated as ca. 9.71 and 6.95 nm, respectively. The smaller pore size of SnO2-FD indicates the presence of smaller primary particles formed, tightly aggregating to form smaller mesopores [61]. The pore size of the mesopore facilitates the adsorption and desorption of the target gas, thus effectively enhancing the gas sensing performance of SnO2 [62,63].

3.2. Gas Sensing Performance

The NO2 sensing characteristics of SnO2 sensors were investigated. The optimal operating temperature is an important indicator for evaluating the performance of gas sensors. Figure 4a shows the response of SnO2 sensors to 10 ppm NO2 under different operation temperatures. The response values of both SnO2-FD and SnO2-AD increase with increasing temperature and decrease after reaching a maximum at 100 °C. The reason is that the lack of thermal energy leads to gas adsorption that is weak or insufficient to overcome the energy barrier for gas-sensitive reactions at low temperatures, while the gas desorption rate is too fast for gas-sensitive reactions to occur at higher temperatures [64,65]. The response value of SnO2-FD (886.2) to 10 ppm NO2 at 100 °C is about 17 times higher than that of SnO2-AD (52.5).
Figure 4b illustrates the baseline resistance variation of SnO2-FD and SnO2-AD at various temperatures, and it can be observed that the baseline resistance of the SnO2 samples decreases with increasing temperature, exhibiting typical semiconductor characteristics [66]. Interestingly, the baseline resistance of SnO2-AD is about two magnitudes higher than that of SnO2-FD at the respective temperatures. This may be due to differences in carrier concentration. SnO2-FD has a higher concentration of carriers and therefore has a higher conductivity leading to a lower baseline resistance [67]. On the other hand, as an n-type semiconductor, the response value (S) of SnO2 to the oxidizing gas NO2 is defined by the ratio of the stabilized resistance (Rg) exposed to NO2 to the baseline resistance (Ra) in air. A small baseline resistance causes a more significant change in resistance, resulting in a larger response value [68,69]. The response/recovery curves of SnO2-AD and SnO2-FD sensors to 10 ppm NO2 at 100 °C are shown in Figure 4c,d. The response/recovery times of SnO2-FD are all shorter than those of SnO2-AD. In particular, the recovery time of SnO2-FD is 27 s profoundly lower than that of SnO2-AD (218 s), which is due to the increased porosity and the small particle size of SnO2-FD that promotes gas diffusion. To avoid errors due to serendipity, we performed two repetitive response recovery tests for SnO2-FD and SnO2-AD, respectively. As shown in Figure S1, the response/recovery times of SnO2-AD were 83/244 s and 96/202 s, whereas the response/recovery times of SnO2-FD were 71/43 s and 85/25 s, respectively. This indicates a significant improvement in the adsorption/desorption kinetics of SnO2-FD.
The dynamic response curves of SnO2 sensors toward 0.1–15 ppm NO2 at 100 °C are shown in Figure 5a. The response values of SnO2-AD and SnO2-FD increase continuously with increasing NO2 concentration, and the response value of SnO2-FD is much higher than that of SnO2-AD over the entire concentration range. It can be observed that there is still a significant response of SnO2-FD to 100 ppb NO2. Figure 5b records the response of SnO2 sensors toward different concentrations of NO2 at 100 °C. The response value of the sensor is basically linear with NO2 concentration, indicating its potential capability of quantitative NO2 detection. Figure 5c is a magnified image of the NO2 concentration in the range of 0.1–2 ppm in Figure 5b. As can be seen from Figure 5c, the SnO2-AD sensor response values are all below 10 when the NO2 concentration is less than 2 ppm, whereas the SnO2-FD sensor still has a high response value toward a low concentration of NO2, which is still as high as 214.9 at 2 ppm. A linear fit is performed for the response values versus the concentration of NO2 in this range. The slope of SnO2-FD (116.7) is 25 times higher than the slope of SnO2-AD (4.7), indicating that the presence of a trace amount of NO2 can cause a variation in the response value. Moreover, the regression value (R2) of SnO2-FD reached 0.975, indicating a favorable linearity, which is capable of providing accurate concentration measurements. Furthermore, the good linearity simplifies data analysis [70]. We can calculate the actual gas concentration from the response value of the sensor output by the known linear equation fitted [59,71]. The detection limit (LOD) of the sensor is predicted by Equation (3) [72]:
LOD = 3 × rms slope
where rms is the root mean square deviation of the baseline resistance and slope is the slope of the fitted line. The LOD of SnO2-AD is 127.53 ppb, and that of SnO2-FD is 1.69 ppb NO2. And the regression value (R2) of SnO2-FD amounts to 0.975, indicating a high reliability in practical applications.
Figure 5d,e show the response/recovery time of SnO2 sensors to NO2 with different concentrations. It can be observed that the recovery time of SnO2-FD exposed to high NO2 concentration is drastically shortened, and the response time is also reduced. The response/recovery time of a gas sensor is related to the diffusion rate of the gas and its surface reaction rate [73]. Its response/recovery at low concentrations is dominated by the effect of the gas diffusion rate [74]. The target gas concentration gradient at the sensor surface is quite low, resulting in a long response/recovery time [75,76]. The responses of SnO2 sensors to 10 ppm NO2, 10 ppm H2S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol at 100 °C are displayed in Figure 5f. The sensor is generally unresponsive to all gases except NO2, indicating that the sensor has excellent selectivity for NO2. The comparison of the performance of the SnO2-FD sensor in this work with the reported NO2 sensor is shown in Table 1. Compared to the reported NO2 sensor, the SnO2-FD sensor exhibits a high NO2 response value (886.2) and a short response recovery time (74/27 s) towards 10 ppm NO2 at 100 °C with an extremely low detection limit (1.69 ppb).
Figure 6a,b show the stability of SnO2-AD and SnO2-FD sensors to 10 ppm NO2 at 100 °C in five cycles. The response values of the sensors remain essentially unchanged over the five cycles, indicating the good reliability of the sensors. Ambient humidity is a factor that must be taken into account in the practical application of gas sensors. The response of SnO2 sensors under different humidity levels to 10 ppm NO2 at 100 °C is shown in Figure 6c. The increased humidity leads to a reduction in the resistance of the material, as shown in Figure S2. It is attributed to the reaction of water molecules with adsorbed oxygen species on the surface of the material to form hydroxyl groups and release electrons into the conduction band of the material [81]. Moreover, the hydroxyl groups formed by water molecules can occupy the active sites on the material surface, which leads to metal oxide hydroxyl poisoning and inhibits gas adsorption [82,83]. On the other hand, the reaction of water molecules with adsorbed oxygen on the surface of the material generates a competitive relationship with the reaction of NO2 and adsorbed oxygen, which affects the gas-sensitive response of the sensor [84]. They stabilize at relative humidity up to 40 RH% and SnO2-FD still has a higher response value (284.29) at 80 RH% compared to SnO2-AD (6.41). Figure 6d shows the response change of SnO2 sensors to 10 ppm NO2 at 100 °C for 30 days. The response values of the SnO2-FD sensors are generally stable over a period of 30 days with an average value of about 871.86, indicating favorable long-term stability.

3.3. Gas Sensing Mechanism

The gas sensing mechanism can be explained as the change in resistance of a semiconductor before and after exposure to a target gas, as shown in Figure 7. In air, oxygen molecules are adsorbed on the surface of the SnO2 sensor to capture its conduction band electrons to form reactive adsorbed oxygen species, resulting in an increase in SnO2 resistance [59]. Upon exposure of the sensor to NO2, NO2 further traps electrons in the conduction band of SnO2 due to its higher electron affinity than O2, leading to a further increase in its resistance [85]. On the other hand, NO2 reacts with adsorbed oxygen on the surface to form NO2− resulting in a decrease in the content of O2−, which further robs the electrons in the SnO2 conduction band, leading to an increase in resistance [86]. The SnO2-FD and adsorbed oxygen content is higher than that of SnO2-AD as shown in Figure 1d, which may also be reasonable for why the response value and the response/recovery rate of SnO2-FD are much higher than those of SnO2-AD at a high NO2 concentration.
In order to further understand the electrical properties, VH-I curves were tested using a Hall effect test system, as shown in Figure 8a,b, and the carrier concentrations were then calculated by Equation (4) [87]:
n = IB V H ed
where I is the excitation current, B is the magnetic induction, VH is the Hall voltage, and d is the material thickness. Here, VH/I can be expressed as the slope of the fitted straight line. The deviation of these dispersed points from the fitted straight line may be attributed to the non-uniform thickness of the coated gas-sensitive sensing layer, which results in a different concentration of electrons in each cross-section. During the measurement process, multi-point data were measured and fitted to minimize the error. The calculated carrier concentrations of SnO2-AD and SnO2-FD are 1.903 × 1012 and 7.251 × 1012 cm−3, respectively. According to the XPS results, the content of O/Sn in SnO2-AD (1.76) is higher than that in SnO2-FD (1.60), indicating that the intrinsic defects of n-type SnO2 recombine with oxygen, thus leading to a lower carrier concentration in SnO2-AD [88]. And the higher carrier concentration in SnO2-FD promotes a rapid gas sensing reaction [67]. To have a better understanding of the energy band structure, the UV-vis diffuse reflectance spectra of SnO2-AD and SnO2-FD were tested, as shown in Figure 8c,d. In the visible light wavelength range, the absorbance of SnO2-FD is higher than that of SnO2-AD, indicating that more carriers can be produced in SnO2-FD [89]. The UV absorption edge of SnO2-FD is redshifted; this is due to the straightforward electron transition between the valence bands and conduction bands, suggesting that the decrease in the band gap of SnO2-FD reduces the activation energy of the electron transition [90]. The band gap energies of SnO2-AD and SnO2-FD are ca. 3.15 and 1.94 eV, respectively, indicating that the preparation of SnO2 with the freeze-drying method significantly narrows the band gap. The reduction in the band gap may be due to the introduction of extensive defects [91]. This reduces activation energy for electron migration and allows NO2 to obtain electrons from the SnO2 conduction band more efficiently, thus increasing its response value and response recovery rate [40,72]. On the other hand, more electrons can be excited into the conduction band at a certain temperature, thus increasing the carrier concentration, which in turn promotes the electron transfer between the sensors and NO2 that facilitates the gas-sensitized reaction.
In this work, SnO2-FD has excellent NO2 sensing properties. First, the increase in chemisorbed oxygen content promotes an alternative reaction pathway for NO2 at high concentrations. Second, the SnO2-FD particle size is closer to the Debye length of SnO2, affecting its conductivity and facilitating the target gas contact with SnO2. In addition, SnO2-FD has a higher carrier concentration, which promotes electron exchange between the target gas and the sensing materials. Moreover, the band gap of SnO2-FD is drastically reduced, which lowers the activation energy of electrons transiting from the valence band to the conduction band and promotes the capture of electrons from the conduction band by the target gas, thus improving the response value of the sensor and the response/recovery speed.

4. Conclusions

Small-sized SnO2-FD particles prepared by hydrothermal and freeze-drying methods have good gas-sensitive properties for NO2 at lower temperatures. The SnO2-FD sensor exhibits an ultra-high response (886.2) with a short response recovery time (74/27 s) for 10 ppm NO2 at 100 °C. Moreover, the sensor exhibits an extremely low detection limit, good selectivity, and humidity stability. The SnO2 prepared by the freeze-drying method exhibits a significantly shortened band gap and increased carrier concentration, as well as a reduced particle size of SnO2 particles. This study provides a new idea for research on semiconductor gas-sensitive material preparation methods.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma17153714/s1, Figure S1. The repetitive tests on the response-recovery of SnO2-AD and SnO2-FD to 10 ppm NO2 at 100 °C. Figure S2. The baseline resistance of SnO2-AD and SnO2-FD sensors at different humidity.

Author Contributions

L.L.: methodology, investigation, writing—original draft. J.Z.: formal analysis. Z.J.: data curation, project administration, resources. F.L.: data curation, formal analysis, validation. D.Z.: conceptualization, project administration, resources. Z.L.: resources, data curation. F.W.: writing—review and editing, formal analysis. Z.W.: formal analysis, data curation. J.L.: writing—review and editing, resources. L.W.: writing—review and editing, investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Natural Science Foundation of Shandong Province (ZR2022MF311), The Project of Innovation Team in Jinan City for Universities and institutes (2021GXRC036), the Natural Science and Development Foundation of Shenzhen (JCYJ20190807093205660), and the Young Scholars Program of Shandong University (2018WLJH25).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 17 03714 g001
Figure 1. (a) XRD spectra of SnO2 samples. (b) XPS full-survey spectra and (c) Sn 3d XPS spectra of SnO2 samples. O 1s XPS spectra of (d) SnO2-AD and (e) SnO2-FD. (f) Raman spectra of SnO2 samples.
Figure 1. (a) XRD spectra of SnO2 samples. (b) XPS full-survey spectra and (c) Sn 3d XPS spectra of SnO2 samples. O 1s XPS spectra of (d) SnO2-AD and (e) SnO2-FD. (f) Raman spectra of SnO2 samples.
Materials 17 03714 g001
Materials 17 03714 g002
Figure 2. SEM images of (a,b) SnO2-AD samples and (c,d) SnO2-FD samples.
Figure 2. SEM images of (a,b) SnO2-AD samples and (c,d) SnO2-FD samples.
Materials 17 03714 g002
Materials 17 03714 g003
Figure 3. N2 adsorption–desorption isotherms and BJH pore size distributions (inset) of (a) SnO2-AD and (b) SnO2-FD.
Figure 3. N2 adsorption–desorption isotherms and BJH pore size distributions (inset) of (a) SnO2-AD and (b) SnO2-FD.
Materials 17 03714 g003
Materials 17 03714 g004
Figure 4. (a) The response of SnO2 sensors to 10 ppm NO2 under different operation temperatures. (b) The resistance of SnO2 sensors at various operation temperatures. The response/recovery curves of (c) SnO2-AD and (d) SnO2-FD sensors to 10 ppm NO2 at 100 °C.
Figure 4. (a) The response of SnO2 sensors to 10 ppm NO2 under different operation temperatures. (b) The resistance of SnO2 sensors at various operation temperatures. The response/recovery curves of (c) SnO2-AD and (d) SnO2-FD sensors to 10 ppm NO2 at 100 °C.
Materials 17 03714 g004
Materials 17 03714 g005
Figure 5. (a) Dynamic response curves of SnO2 sensors toward 0.1–15 ppm NO2 at 100 °C. (b) The response of SnO2 sensors toward different concentrations of NO2 at 100 °C. (c) The linear relationship between response value and NO2 concentration from 0.1 ppm to 2 ppm for SnO2 sensors. (d) The response time and (e) the recovery time of SnO2 sensors in response to NO2 with different concentrations. (f) The response of SnO2 sensors to 10 ppm NO2, 10 ppm H2S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol.
Figure 5. (a) Dynamic response curves of SnO2 sensors toward 0.1–15 ppm NO2 at 100 °C. (b) The response of SnO2 sensors toward different concentrations of NO2 at 100 °C. (c) The linear relationship between response value and NO2 concentration from 0.1 ppm to 2 ppm for SnO2 sensors. (d) The response time and (e) the recovery time of SnO2 sensors in response to NO2 with different concentrations. (f) The response of SnO2 sensors to 10 ppm NO2, 10 ppm H2S, 100 ppm CO, 100 ppm HCHO, and 100 ppm ethanol.
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Figure 6. The stability of (a) SnO2-AD and (b) SnO2-FD sensors in response to 10 ppm NO2 at 100 °C in 5 cycles. (c) The response of SnO2 sensors under different humidity levels to 10 ppm NO2 at 100 °C. (d) The response change of SnO2 sensors in response to 10 ppm NO2 at 100 °C for 30 days.
Figure 6. The stability of (a) SnO2-AD and (b) SnO2-FD sensors in response to 10 ppm NO2 at 100 °C in 5 cycles. (c) The response of SnO2 sensors under different humidity levels to 10 ppm NO2 at 100 °C. (d) The response change of SnO2 sensors in response to 10 ppm NO2 at 100 °C for 30 days.
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Figure 7. The NO2 gas sensing mechanism of SnO2-AD and SnO2-FD.
Figure 7. The NO2 gas sensing mechanism of SnO2-AD and SnO2-FD.
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Figure 8. The VH−I curves measured by Hall Effect Measurement of (a) SnO2-AD and (b) SnO2-FD. (c) The UV−vis absorption spectrum of SnO2. (d) T−plots of (αhν)2 versus hν of SnO2.
Figure 8. The VH−I curves measured by Hall Effect Measurement of (a) SnO2-AD and (b) SnO2-FD. (c) The UV−vis absorption spectrum of SnO2. (d) T−plots of (αhν)2 versus hν of SnO2.
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Table 1. The NO2 sensing performance of reported sensors and this work.
Table 1. The NO2 sensing performance of reported sensors and this work.
MaterialsConcentration
(ppm)		S
(Ra/Rg)		T (°C)tres/trec
(s/s)		LOD
(ppb)		Ref.
In2O3/GO4078225106/42-[77]
NiCo2O4/n-WO320153.715013/1652[78]
Ce/ZnO1034.3250168/1981.4[50]
Rh/ZnO1036.1715032/51250[48]
Au/SnS2/SnO2822.380174/359.6-[21]
In2O3/MoS22080.83RT152/1798.8[79]
Sn/In2O3144.690106/85-[80]
SnO2-FD10886.210074/271.69This work
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Liu, L.; Zhao, J.; Jin, Z.; Liu, F.; Zhao, D.; Liu, Z.; Wang, F.; Wang, Z.; Liu, J.; Wu, L. NO2-Sensitive SnO2 Nanoparticles Prepared Using a Freeze-Drying Method. Materials 2024, 17, 3714. https://doi.org/10.3390/ma17153714

AMA Style

Liu L, Zhao J, Jin Z, Liu F, Zhao D, Liu Z, Wang F, Wang Z, Liu J, Wu L. NO2-Sensitive SnO2 Nanoparticles Prepared Using a Freeze-Drying Method. Materials. 2024; 17(15):3714. https://doi.org/10.3390/ma17153714

Chicago/Turabian Style

Liu, Lin, Jinbo Zhao, Zhidong Jin, Fei Liu, Dewen Zhao, Zhengyang Liu, Fenglong Wang, Zhou Wang, Jiurong Liu, and Lili Wu. 2024. "NO2-Sensitive SnO2 Nanoparticles Prepared Using a Freeze-Drying Method" Materials 17, no. 15: 3714. https://doi.org/10.3390/ma17153714

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