Metal oxide semiconductors-based chemiresistive gas sensors have been extensively researched and applied to detect gas molecules due to their advantages of high sensitivity, long service life, and low cost. However, the interactions between the sensing layer and water molecules cause variations of the baseline resistance and surface state, and thus the gas sensing performance is significantly affected by ambient water molecules, resulting in a bottleneck in applications. To solve this problem, researchers have developed many strategies to improve the anti-humidity performance of metal oxides based on these concepts of inhibiting the surface adsorption of water, suppressing the competitive adsorption between water molecules and oxygen species, or regulating the chemical reactions between water molecules and adsorbed oxygen. The specific approaches of the mentioned strategies are summarized as follows. (1) Introducing the hydrophobic and breathable coating on the surface of the gas sensing layer, which can physically prevent water molecules from touching the surface of sensing layers and directly eliminate the damage. (2) By doping the gas-sensitive material with hydroxyl absorbents, the water molecules will preferentially adsorb on the surface of hydroxyl absorbents, thereby inhibiting the competition adsorption between vapor and oxygen at active reaction sites. (3) Adjust the chemical adsorption characteristics of oxygen anions on the surface, change the chemical and electronic effects of the surface, or reduce the adsorbed oxygen that has formed hydroxyl groups with water, inhibit the reaction process of water and oxygen, and maintain the concentration of adsorbed oxygen, thereby improving the anti-humidity performance of the semiconductor gas sensors. Here, the influence mechanism of the water molecule on gas sensing performance is analyzed and the future development of the mentioned anti-humidity promotion strategies has been prospected. This article is expected to provide solutions and method guidance for the improvement in the anti-humidity performance of gas-sensitive materials based on metal oxide semiconductors.
Contents
1 Introduction
2 Mechanisms by which humidity affects the sensing property of metal oxide semiconductor gas sensitive materials
Yiling Tan, Shichun Li, Xi Yang, Bo Jin, Jie Sun.
Strategies of Improving Anti-Humidity Performance for Metal Oxide Semiconductors Gas-Sensitive Materials.
Progress in Chemistry, 2022, 34(8): 1784-1795 DOI:10.7536/PC211033
Gas sensors have important application prospects in air pollution monitoring, breath health diagnosis, industrial safety, production quality control, national defense and military fields[1,2]. In recent years, driven by the application scenarios of the Internet of Things, the demand for gas sensors in smart home, wearable devices, smart mobile terminals and other fields has increased rapidly, which puts forward higher and higher requirements for the comprehensive performance of gas sensors[3]. Because the environmental humidity may affect the detection accuracy of the gas sensor, the environmental adaptability of the gas sensor to humidity must be considered in engineering applications. For example, in 2015, the U.S. Department of Energy issued a performance index for hydrogen sensors used in hydrogen electric vehicles, which stipulates that the sensors should work normally in the relative humidity range of 10% to 98%[4,5].
According to the sensing principle, gas sensors can be divided into three types: optical type, electrochemical type and electrical type. Electrical gas sensors are subdivided into resistive and non-resistive gas sensors. Among them, the resistive metal oxide semiconductor gas sensor is the most widely used gas sensor because of its high sensitivity, long service life and low manufacturing cost[6⇓⇓~9].
Metal oxide semiconductor gas sensor is based on the gas-sensitive reaction (chemical reaction/chemical adsorption) of the target gas on the surface of the gas-sensitive material, resulting in charge transfer, which changes the resistance of the gas-sensitive material, thus realizing the sensing and detection of the target gas. Sensitivity, selectivity and stability are the core indexes to evaluate the sensing performance of metal oxide semiconductor gas sensors. As the functional core of gas sensor, gas sensing material can identify the target gas and convert the chemical signal into electrical signal, and its physical and chemical characteristics determine the sensing characteristics of gas sensor. The gas sensing reaction mechanism of metal oxide semiconductor gas sensor is the redox reaction of the target gas with the gas sensing material or the high temperature chemisorbed oxygen (O2-, O- and O2-) on the surface of the gas sensing material. In this process, the ambient humidity will affect the reactivity of the gas sensing material or change the reaction process of the target gas on the surface of the gas sensing material, thus affecting the baseline resistance, sensitivity and performance stability of the metal oxide semiconductor gas sensor[10,11]. Therefore, improving the humidity resistance of metal oxide semiconductor gas sensing materials, eliminating the interference of environmental humidity on the sensing performance of the sensor, and realizing the effective monitoring of trace target gas have become the bottleneck problems to be solved in the engineering application of metal oxide semiconductor gas sensors.
Understanding the mechanism of humidity affecting the sensing performance of metal oxide semiconductor gas sensing materials and developing strategies and methods to solve the problem of humidity interference are the research difficulties in the field of metal oxide semiconductor gas sensors. With "gas sensor", "humidity" and "stability" as the key words, bibliometric analysis was carried out based on the Web of Science core database, and the results are shown in Figure 1. In recent years, the number of literatures in this research direction has increased year by year, among which the largest number of published papers are from research institutions in China, India and the United States, but the number of literatures in the whole field of gas sensor research is still not high. The bibliometric results show that the humidity stability of gas sensing materials for gas sensors has attracted more and more attention, but the research is difficult and the research progress is slow.
In order to summarize the research progress of humidity stability of metal oxide semiconductor gas sensing materials, the mechanism of humidity affecting the baseline resistance, reactivity and performance stability of gas sensing materials is introduced.Then, the anti-humidity strategies and technical approaches based on the humidity sensing mechanism of gas sensing materials are discussed with examples. Finally, these anti-humidity strategies are summarized, and the development direction of anti-humidity strategies is prospected. This paper is expected to provide solutions and direction guidance for the improvement of moisture resistance of metal oxide semiconductor gas sensing materials.
2 Influence Mechanism of Humidity on Sensing Properties of Metal Oxide Semiconductor Gas Sensing Materials
The sensing of metal oxide semiconductor gas sensing materials is mainly based on the change of physical properties such as conductivity or resistance of materials. The n-type metal oxide semiconductor gas sensing material forms an electron depletion layer on the surface of the material through the desorption of oxygen in the air on the surface of the material, so that the resistance of the gas sensing material is increased. Negatively charged adsorbed oxygen ions react with reducing gases to release electrons, which is manifested by a decrease in resistance[12]. On the contrary, the adsorbed oxygen reacts with the oxidizing gas to capture the surface electrons of the gas sensing material, and the resistance of the metal oxide semiconductor gas sensing material increases; P-type metal oxide semiconductor gas sensing materials are just the opposite[13,14].
The gas sensing mechanism of metal oxide semiconductor gas sensing materials is introduced by taking the response of the most widely used n-type metal oxide semiconductor SnO2 gas sensing film to the reducing gas CO as an example, as shown in Figure 2. When the gas-sensitive material is in pure air, at a certain working temperature, because the electron affinity potential energy of oxygen is greater than the work function of SnO2, the oxygen molecules physically adsorbed on the surface of the gas-sensitive material will capture electrons from the conduction band of the SnO2 gas-sensitive material.So as to change into chemisorbed oxygen ions (O2-, O- and O2-, the ion species is determined by the working temperature), and at the same time, a space electron depletion layer is formed in the SnO2 gas sensitive film. The formation path of chemisorbed oxygen ions is shown in (1) ~ (4):
O2 (gas)→O2 (ads)
O2(ads)+e-→ (T ≤100 ℃)
+e-→2O-(100≤ T < 300 ℃)
O-+e-→O2-(300 ℃≤ T)
Among them, adsorbed oxygen ions O2-, O- and O2- are stable below 100 ° C, between 100 and 300 ° C and above 300 ° C, respectively[12,18].
According to the conduction band theory of metal oxide semiconductor, the space electron depletion layer is characterized by the bending of the conduction band and the formation of Schottky barrier at the grain contact interface of the gas sensitive film, as shown in Figure 2a. At this time, when the carrier (electron) circulates in the SnO2 gas sensitive film, it must overcome not only the resistance Rg generated by the depletion layer, but also the resistance Rb of the grain itself. Because the Rg is far greater than the Rb, the resistance of the SnO2 gas sensing film material is mainly determined by the Rg of the depletion layer resistance.The depletion layer resistance Rg can be characterized by the depletion layer thickness (XD), and its value can be calculated from equations (5) and (6):
Where :Vs is the surface potential, qVs is the band bending produced by the depletion layer, kB is the Boltzmann constant, Ld is the Debye length, ε is the dielectric constant, and Nd is the donor concentration[11,17].
When the SnO2 gas-sensitive material is contacted with the target gas CO, the chemisorbed oxygen ions on the surface of the SnO2 are subjected to oxidation-reduction reaction with CO, and electrons are released back to a conduction band of the SnO2 gas-sensitive film, so that the thickness of an electron depletion layer is thinned; According to the band theory of metal oxide semiconductor, the thinner thickness of the electron depletion layer means that the height of the conduction band bending is reduced, the height of the grain boundary barrier is reduced, and the resistance Rg generated by the depletion layer becomes smaller, so that the SnO2 gas sensing material enters a low resistance state, showing a decrease in resistance, as shown in Figure 2B.
However, in the practical application of sensors, gas sensing materials are easy to adsorb gases due to their high surface state, including not only chemisorbed oxygen and target gas, but also gaseous water with high polarity in the environment. The effect of water vapor on the baseline resistance and chemical activity of metal oxide gas sensing materials is complex, which can be divided into physical adsorption and chemical adsorption.
The physical adsorption of water will mainly change the dielectric constant ε of the metal oxide gas sensing material, and according to Formula 5 and Formula 6, the change of ε will lead to the change of the thickness of the depletion layer, thus affecting the resistance of the gas sensing material[11,17]. In addition, when the particle size of the gas sensing material is small, the conduction path of the carrier is also small, and the scattering probability of the carrier will be increased after the surface adsorbs water vapor, which will also affect the resistance of the gas sensing material[19].
Chemisorbed water affects the sensing properties of metal oxide semiconductor gas sensing materials mainly through competitive adsorption with chemisorbed oxygen on the surface of metal oxide or reaction with chemisorbed oxygen[17]. The competitive adsorption is mainly manifested by the fact that the water molecules adsorbed on the surface of the metal oxide are decomposed into hydroxyl groups and hydrogen atoms with the characteristics of acid bases, which occupy the surface adsorption sites and lead to the poisoning of the hydroxyl groups of the metal oxide. The reaction equations are shown in Equations (7) and (8) (taking SnO2 as an example):[20]
H2Ogas +SnSn +OO =(S -OH-)+(OH +e-
H2Ogas +2SnSn +OO=2(S -OH-)+ +2e-
If (S -OH-) is regarded as a hydroxyl group, formula (7) shows that the water in this reaction undergoes a separation reaction, and the hydrogen atom produced combines with the oxygen in tin dioxide to form (OH)O, while ((OH)O has a relatively low electron affinity, so it is ionized to produce an electron. The formula (8) considers that the hydroxyl generated by the combination of hydrogen atoms and oxygen in tin dioxide combines with tin atoms to ionize electrons and oxygen vacancies () to improve the surface conductivity of gas sensing materials and reduce the resistance of materials[20].
On the other hand, the reaction between water and adsorbed oxygen on the surface of the gas sensitive material will compete with the reaction between the target gas and adsorbed oxygen, thus generating hydroxyl on the surface of the metal oxide semiconductor gas sensitive material and releasing electrons to the material, affecting the gas sensitive response of the sensor. The reaction equations are shown in Equations (9) and (10):[17]
H2Ogas + → 2OH + e - (100≤ T < 300 ℃)
H2Ogas+ →2OH+2e-(T≥300 ℃)
In addition, Tang et al., Selyama et al. And Tokuda et al. Used TGA to show that water vapor can not be completely desorbed even at a high temperature of 526 ℃, while the working temperature of metal oxide semiconductor gas sensors is generally lower than this temperature, so the adsorption of humidity is difficult to avoid by preheating and other temperature control means[21⇓~23].
To sum up, water, whether physically adsorbed or chemically adsorbed on the surface of metal oxide semiconductor gas sensing materials, will affect the physical and chemical characteristics of metal oxide semiconductor gas sensing materials, thus changing the baseline resistance and gas sensing reactivity of the sensor, and affecting the gas sensing performance of the sensor.
3 Research Progress on Moisture Resistance Strategy of Metal Oxide Semiconductor Gas Sensing Material
It can be seen from the above discussion that the water adsorbed on the surface of the metal oxide semiconductor gas sensitive material will change the baseline resistance and gas sensitive reactivity of the sensor by changing the physical properties of the gas sensitive material, competing with chemisorbed oxygen, and reacting with chemisorbed oxygen, thus affecting the gas sensitive performance of the sensor. Based on the understanding of the influence mechanism of humidity on the performance of metal oxide semiconductor gas sensing materials, the core idea of reducing the interference of water vapor on the gas sensing performance of metal oxide semiconductor sensors is to inhibit the influence of water vapor on the performance of three ways, including (1) inhibiting the surface adsorption of water; (2) inhibiting the competitive adsorption of water and oxygen; And (3) regulating the reaction between water and adsorbed oxygen.
The schematic diagram of the strategy for improving the moisture resistance of the sensor and inhibiting the influence of water on the gas sensitive material is shown in Figure 3. (1) coating the surface of the gas-sensitive material with hydrophobic and breathable materials to physically block the adsorption of water on the surface of the gas-sensitive material; (2) a hydroxyl absorbent is doped in the gas sensitive material, and the absorbent preferentially adsorbs water vapor, thereby inhibiting the competitive adsorption of water and oxygen at the chemical adsorption site of the gas sensitive material; (3) regulating and controlling the chemical adsorption characteristics of oxygen anions on the surface of the gas-sensitive material, and changing the chemical and electronic effects of the surface of the gas-sensitive material; Or reducing the adsorbed oxygen which has formed hydroxyl with water, inhibiting the reaction process of water and adsorbed oxygen, and maintaining the concentration of adsorbed oxygen on the gas sensitive material, thereby improving the moisture resistance of the metal oxide semiconductor gas sensor.
3.1 Hydrophobic and Permeable Coating of Gas Sensing Material
It is a direct and effective means to inhibit the influence of water vapor molecules on the performance of gas sensing membrane by coating hydrophobic layers, anchoring hydrophobic groups or constructing hydrophobic structures on the surface of gas sensing materials to physically block the contact between water vapor and gas sensing membrane. In order to reduce the influence of the coating material itself on the gas sensing properties, the coating material is also required to have high gas permeability. At present, the materials commonly used for hydrophobic modification of gas sensing films are mainly fluoropolymers, silanes, MOF materials and so on[24⇓⇓⇓⇓⇓~30].
Gao et al. Used polydimethylsiloxane (PMDS) to coat TiO2 nanotube arrays, which greatly improved the hydrophobic properties of TiO2 nanotube arrays and effectively blocked the interference of water vapor on the response performance of TiO2 gas sensors in the process of hydrogen detection. It can be seen from Fig. 4 that the gas sensing response of Pd-TiO2 nanotubes to hydrogen decreases significantly with the increase of humidity (Fig. 4B); After a layer of PDMS was modified on the surface of the Pd-TiO2 nanotube, the surface of the gas sensing membrane became hydrophobic, and its response to hydrogen was almost unaffected by humidity (Figure 4C)[24]. Based on a similar packaging strategy, Du et al. Modified a fluorocarbon film on the surface of In2O3 gas sensing film by magnetron sputtering, and also significantly suppressed the influence of humidity on its nitrogen dioxide gas sensing response performance by using the gas permeable and hydrophobic properties of fluorocarbon film[28]. In addition, Liu et al. Covered a hydrophobic PVDF porous membrane on the surface of Pd-SnO2 gas sensing membrane by spin-coating impregnation method, which also improved the moisture resistance of the device[29].
However, the coating of superhydrophobic materials on the surface of gas sensing membrane materials is not a sufficient condition to improve the moisture stability of gas sensors. Although the superhydrophobic surface can block condensed water droplets well, it can not completely block the water vapor molecules in the environment. Therefore, the number and thickness of the hydrophobic gas-permeable layer should be considered when the hydrophobic gas-permeable membrane with large pore size (such as PMDS material) is used to modify the sensitive material of the gas sensor. Only when the hydrophobic gas permeable layer is completely covered on the surface of the gas sensitive material and reaches a certain thickness, the influence of humidity on the gas sensor can be effectively eliminated[24]. That is to say, it is particularly important for hydrophobic and breathable membrane materials to have the characteristic of adjustable pore size. Metal-organic framework materials (MOFs), which are popular and used in the field of gas sensors, have this characteristic.
Metal-organic frameworks (MOFs) are crystalline porous materials with periodic network structure formed by self-assembly of transition metal ions and organic ligands, which have the advantages of high porosity, low density, large specific surface area, regular pore channels and adjustable pore size, and have become an important research direction in many branches of chemistry, such as inorganic chemistry and organic chemistry[30]. Due to the strong selective gas adsorption behavior of MOFs, they have great attraction in the selectivity of gas sensors. MOFs are also used as gas sensing materials for gas sensors by taking advantage of their three-dimensional regular porous structure and gas permselectivity. Gas sensors based on MOFs have selective response to VOC gases, but they are highly disturbed by humidity and have low sensitivity to analytes. Therefore, it is proposed to use hydrophobic MOFs to coat gas sensing materials with hydrophobic and breathable properties, so as to improve the gas selectivity and moisture resistance of gas sensors[31⇓~33].
The hydrophobic gas permeation strategy based on hydrophobic MOFs combines the high sensitivity of metal oxide semiconductor gas sensing materials with the hydrophobicity, high gas selectivity and catalytic activity of MOFs to enhance the moisture stability of gas sensors. For example, Yao et al. Coated a layer of MOF film on the surface of ZnO nanowires by in-situ growth to obtain a ZnO @ ZIF-CoZn core-sheath nanowire material[34]. The material realizes high selectivity response to acetone in a large relative humidity range (0 to 90 RH%). Compared with the ZnO nanowire material without MOF coating, the ZnO @ ZIF-CoZn-based gas sensor has significantly improved sensing performance, and more importantly, it has strong moisture resistance, and its humidity change coefficient in the range of 0 RH% ~ 90 RH% is only 1/5 of that of the ZnO nanowire material without MOF coating (Fig. 5). In addition, Nair et al. Also obtained ZnO @ ZIF-8 gas sensing film by in-situ growth of a layer of ZIF-8 on the surface of ZnO nanorod vertical array film[35]. It was found that compared with ZnO, the response of ZIF-8 modified ZnO to ethylene and propylene was almost unchanged above 25 RH%. In addition, after ZIF-8 modification, its gas sensing performance has been improved to a certain extent, which they attributed to the interface between ZIF-8 and ZnO has a certain enhancement effect on the gas sensing reaction process[35].
Hydrophobic and gas-permeable coating strategy In addition to the assembly and coating of single-layer hydrophobic and gas-permeable materials with gas sensing materials, multi-layer modification and coating of gas sensing materials can also be used, such as Qu et al. By using the double coating strategy of superhydrophobic materials PDMS and MOF materials, CoSnO3@MOF@PDMS gas sensing materials were prepared by in-situ growth of MOFs and CVD coating[36]. The results of gas sensing properties show that the response change rate of the double-coated CoSnO3@MOF@PDMS material to different target gases is only 1. 3% in the humidity range of 0 ~ 90 RH%, which is 96% lower than that of the uncoated gas sensing material, and the influence of water on the gas sensing properties of the CoSnO3 sensing material is eliminated. In addition, the PDMS/MOF double coating strategy can also enhance the moisture stability of other traditional gas sensing materials (such as ZnO and SnO2), and this strategy has certain generality[36].
3.2 Hydroxyl absorbent
The hydroxyl absorbent is loaded in the gas sensitive material, and water is directly absorbed by the hydroxyl absorbent by utilizing the high-capacity adsorption performance of the hydroxyl absorbent on water, so that the competitive adsorption of water and a target gas on an active site on the surface of the gas sensitive material is inhibited. The hydroxyl absorber and the gas sensing material can also form a heterojunction structure with excellent catalytic effect, which can inhibit the poisoning effect of hydroxyl and improve the performance of the sensor. At present, the commonly used hydroxyl absorber materials are mainly copper oxide, nickel oxide and so on[14,37⇓⇓⇓⇓⇓~43].
Choi et al. Prepared hollow microspheres with CuO loaded onto the surface of SnO2, as shown in Fig. 6[44]. High humidity (80 RH%) can significantly affect the response of SnO2 hollow microspheres to hydrogen sulfide, especially at low operating temperature. When loaded with CuO, the effect of high humidity environment (80 RH%) on the response performance of SnO2 hollow microspheres is limited in a wide temperature range[44]. As a hydroxyl absorbent, the loaded CuO will preferentially form Cu-OH with water vapor, thus absorbing most of the water to inhibit the influence of water on the gas sensing properties of SnO2. For another example, Sun et al. Prepared CuO/In2O3 nanosphere gas sensing materials with different CuO contents[45]. Compared with the pure In2O3 nanosphere gas sensing material (InCu0), the CO gas sensor made of In2O3/CuO nanosphere gas sensing material (InCu2) with 2 mol% CuO loading not only has a high response speed, but also has excellent humidity-independent characteristics in a wide relative humidity range (25 ~ 95 RH%). The heterojunction formed by CuO and In2O3, and the high chemical affinity of CuO for water are the main reasons for the high humidity stability of the composite gas sensing materials[45].
In addition, Qiu et al. Prepared the NiO/ZrO2 composite, and NiO and ZrO2 formed a p-p heterojunction[43]. Since both materials have excellent catalytic oxidation, the heterojunction formed by the two catalysts produces a synergistic effect, which significantly improves the gas-sensitive response characteristics of the NiO/ZrO2 composite material to xylene. The response value of the NiO/ZrO2 composite to xylene under different humidity conditions is only reduced from 22.5 (11 RH%) to 19.4 (95 RH%), and NiO has excellent adsorption characteristics to water, so it can inhibit the hydroxyl poisoning effect of water on the sensitive material, thus achieving the purpose of enhancing the moisture resistance stability, and the formed heterojunction also has a certain sensing response enhancement effect[43].
Hydroxyl absorbents can not only be used as the loading material of gas sensing materials, but also be directly used as gas sensing materials to improve the moisture resistance of sensors, but they have the disadvantage of low gas sensing response. Therefore, metal elements are often used to dope and modify the gas sensor, so as to achieve the purpose of improving the gas sensing response of the gas sensor and enhance its moisture resistance stability. For example, Yang et al. Synthesized Ru-doped NiO nanoflowers by hydrothermal method[46]. It was found that Ru doping could significantly improve the response of Ru-NiO nanoflowers to acetone, and more importantly, Ru doping could make NiO nanoflowers have excellent moisture stability, and the baseline resistance and its response to acetone were almost unchanged under the condition of 15-90 RH%. On the one hand, because Ru is used as a catalyst, the concentration of oxygen ions adsorbed on the surface and the gas-sensitive reaction activity of the gas-sensitive material are increased, thereby improving the moisture resistance stability of the gas-sensitive material. On the other hand, the defects introduced by Ru doping will make water molecules preferentially adsorbed by defect sites, thus reducing the ability of water to compete with the target gas for chemical adsorption of oxygen, and improving the moisture resistance stability of gas sensing materials, which is also the main reason for the improvement of moisture resistance stability of Ru-NiO nanoflower materials[46]. For example, Gao et al. Synthesized a series of Sn-doped nanosheet-assembled NiO microspheres by hydrothermal method. The gas sensing response performance of Sn-NiO microspheres is almost not affected by humidity in low concentration xylene (1. 1 ~ 500 ppb) atmosphere, and it still has excellent gas sensing response performance in 90 RH% atmosphere. For NiO, water will react with the chemisorbed oxygen on the surface of NiO and produce a competitive gas-sensitive reaction with xylene, thus reducing the gas-sensitive performance of the material. The reaction equation is shown in formula (11):[47]
+ H2O (gas) + 2NiNi+h+→2(N -OH-)+S
For Sn-NiO materials, the doping of Sn will produce a large number of defects based on Sn atoms on the surface of NiO, which is the main reason for improving the moisture resistance stability of Sn-NiO materials. When water molecules exist in the environment, these water molecules will selectively adsorb on the defects of Sn-NiO materials without reacting with the adsorbed oxygen in the materials, thus improving the moisture stability of the materials[47]. The similar moisture resistance is also reflected in the response of Sn-doped NiO hollow fiber material to TEA gas, and it also has excellent gas sensing properties and moisture resistance[48].
3.3 Control of Oxygen Adsorption Characteristics on Gas Sensitive Material Surface
Noble metal (Pd, Pt, Ru, etc.) Doping/modification is often used to regulate the surface oxygen adsorption characteristics of gas sensing materials, so as to change the surface chemical and electronic effects of gas sensing materials. The chemical effect mainly means that the noble metal activates the target gas, and the activated gas overflows onto the surface of the metal oxide semiconductor and reacts with the adsorbed oxygen; The electronic effect mainly refers to the bending of the energy band of the gas sensing material and the change of the width of the electron depletion layer, which makes the oxidized noble metal act as a strong electron receiver from the oxide and induces a surface space charge layer with strong electron depletion in the oxide near the interface. A noble metal, when in contact with a target gas, can cause a significant increase in response to that target gas by relax the space charge layer by return electrons to the oxide. By regulate that oxygen adsorbed on the surface of the gas sensitive material, on the one hand, the gas sensitive reactivity of the gas sensitive material can be regulate; And on that other hand, the chemical adsorption characteristic of the surface of the gas sensitive material can be regulated and control, and the influence of water molecules on the sensing performance of the gas sensitive material is inhibit or eliminated.
In dry air, the type of oxygen adsorbed on the surface of the metal oxide material is mainly O2-, while in wet air, the adsorption of the metal oxide material on the O2- is affected by -OH to form O-, so the type of oxygen adsorbed on the surface of metal oxide material in wet air is mainly O-, which also leads to the change of the response of metal oxide materials to the target gas in wet air. When the noble metal (such as Pd) is modified, in humid air, PdO can provide sites for the preferential adsorption of O2-, which is not affected by -OH, and overflow to the surface of metal oxide materials, so that the concentration of O2- on the surface remains stable. In addition, the adsorption of O2- on the PdO surface interface broadens the width of the electron depletion layer, which can inhibit the adsorption of -OH on the surface of metal oxides, and the gas sensing performance of gas sensing materials is not affected by water vapor, and the moisture resistance stability of gas sensing materials is improved[49].
For example, Suematsu et al. Studied the effect of antimony (Sb) doping on the humidity sensitivity of SnO2 nanoparticles. It is found (as shown in Fig. 7 a) that the response value of Sb-SnO2 nanoparticles to hydrogen does not decrease significantly in the humidity range of 16 – 96% RH, while the hydrogen-sensitive response of undoped SnO2 nanoparticles decreases gradually with the increase of humidity. In addition, when the SnO2 nanoparticles were co-doped with Pd and Sb, their gas sensing response to hydrogen was greatly improved, and they had strong moisture stability in the humidity range of 16% – 96% RH. The study of oxygen species adsorbed on the surface of gas sensing film showed that the chemisorbed oxygen ions on the surface of pure SnO2 nanoparticles changed from O2- to O- when the humidity increased from 16% to 96%. However, for Sb-SnO2 nanoparticles, the chemisorbed oxygen ion O2- does not change with humidity on the surface of Sb-SnO2. The schematic diagram of the mechanism of Pd/Sb-SnO2 improving its hydrogen sensitivity and moisture resistance is shown in Fig. 7B. Water molecules react with chemisorbed oxygen ion O2- on the surface of the SnO2 nanoparticles to generate O-, and when Pd is loaded on the SnO2, oxygen is chemisorbed on the Pd surface to form the oxygen ion O2-,And then overflow to that surface of the SnO2 nano particle, and the gas sensitive response of the SnO2 gas sensitive material to hydrogen in a humid environment is further improved by Chan the chemisorption state of adsorbed oxygen[50]. Therefore, the use of surface oxygen adsorption characteristics to regulate gas sensing materials is expected to improve the sensing characteristics of gas sensing materials, such as the response to macromolecular gases, the enhancement of gas selectivity and the rapid response recovery speed, while maintaining the humidity stability.
Yoon et al. Fabricated CeO2-In2O3 hollow sphere composites. The gas sensing properties showed that the CeO2 nanocluster-supported In2O3 hollow spheres exhibited excellent selective response to acetone, and the baseline resistance, response value, detection limit, and response/recovery speed were not affected by humidity. While pure In2O3 hollow spheres in humid atmosphere showed that gas response, selectivity and baseline resistance all deteriorated significantly with increasing humidity. This is because the CeO2 nanoclusters can dynamically regenerate the surface of In2O3 hollow spheres, that is, the CeO2 nanoclusters can reduce the chemisorbed hydroxyl groups on the surface of In2O3 and provide chemisorbed oxygen ions to them.O that the sensor can dynamically and effectively update the sensing surface of the In2O3 in a regeneration mode in a humid atmosphere environment, and the sensor has humidity-independent gas sensing characteristics[51]. This dynamic regeneration mechanism is based on the analogy of Prabhakaran et al. And Wang et al. In the study of proton exchange membrane fuel cell (PEMFC) that CeO2 nanoparticles can minimize the free radicals (· OH and · OOH) produced by PEM fuel cell during long-term operation, thus causing the degradation of proton exchange membrane (PEM)[52][53]. CeO2 nanoparticles can scavenge hydroxyl radicals by regenerating two reactions, (12), (13), caused by oxidation/reduction of Ce4+ and Ce3+.
·OH + Ce3+ +H+→Ce4+ + H2O
4Ce4+ + 2H2O → 4Ce3+ + 4H+ +O2
Although the dynamic regeneration mechanism can effectively regulate the concentration level of oxygen ions adsorbed by gas sensing materials to achieve the purpose of improving the moisture resistance stability of gas sensors, in the composite of two metal oxide semiconductor materials,It is still a challenge to uniformly distribute the support material on the gas sensing material, so the preparation process of the gas sensing composite with dynamic regeneration mechanism still needs to be further improved.
In addition, Tricoli et al. Prepared a series of Ti-SnO2 solid solution gas sensing materials with different Ti doping amounts, and the gas sensing property study showed that the Ti-SnO2(3.0%~4.6%) with the optimal doping amount showed a higher response value to ethanol gas than pure SnO2[54]. More importantly, the humidity stability of the sensor is significantly improved by Ti doping, which is attributed to the fact that Ti doping reduces the band gap of SnO2 crystal and produces more defects on its surface, so that its reactivity to ethanol becomes higher and the influence of humidity on the sensing characteristics is alleviated. On the other hand, the doping of Ti changes the surface adsorption characteristics of the SnO2, and the width of the electron depletion layer is changed, so that the adsorption amount of hydroxyl and water groups adsorbed on the surface of the SnO2 is sharply reduced, thereby reducing the hydroxyl poisoning effect of humidity on the gas sensing material, and achieving the purpose of improving the moisture resistance stability of the sensor[54].
In order to further explain the influence of surface point defects on the sensing performance of target gas sensing materials in the presence of water vapor, Kim et al. Proposed a method based on proton beam irradiation to increase the surface point defects on the surface of gas sensing materials (ZnO)[55]. And based on surface analysis and density functional theory (DFT) calculations, it was found that the proton beam induced the formation of three types of point defects, which are oxygen defects, zinc defects and zinc-oxygen dimer defects (VO, VZn and VZn-O),The binding energy (BE) of three types of point defects on ZnO (100) surface for target gas (NO2) and water molecules was calculated, and the competitive adsorption of NO2 and water molecules on the surface of gas sensing materials was explained from the point of view of molecular adsorption dynamics. The BE of NO2 and water molecules on the defect-free ZnO gas sensing material surface are − 0.527 eV and − 1.146 eV, respectively, indicating that water molecules are more easily adsorbed on the defect-free ZnO surface than NO2. Therefore, defect-free ZnO has a strong environmental humidity dependence. In contrast, when defects are formed on the surface of ZnO (100) (VO, VZn and VZn-O),NO2 are more easily adsorbed on the surface of ZnO (100) material than water molecules, thus reducing the combination of water molecules and adsorbed oxygen, which plays a major sensing role on the gas sensing material, and weakening the influence of medium and low humidity on NO2 detection[55]. However, the sensitivity of the gas sensing material is still significantly affected by water molecules in the environment with high relative humidity. To solve this problem, researchers have significantly reduced the effect of high humidity levels on gas sensing by adding various multivalent lanthanide compounds to suppress the effect of humidity on the sensing performance of gas sensing materials[56⇓~58]. Although this method is effective in stabilizing the sensor against humidity changes, it will lead to a significant decline in gas sensing response, which hinders the accurate detection of target gases at low concentrations.
Considering the balance between humidity independence and gas response, a new method that can simultaneously improve the humidity stability and gas sensing response of gas sensing materials is essential for the development of reliable gas sensors. In order to achieve this goal, the introduction of rare earth elements into metal oxide semiconductor gas sensing materials is a new method in recent years. The method can stabilize the moisture resistance of the gas sensitive material and maintain the gas sensitive responsiveness of the gas sensitive material. On the one hand, due to the unique atomic structure of rare earth elements, the unfilled 4F orbital is completely shielded by the electrons in the external 5s2p6 orbital, so that the rare earth atoms cannot form hydrogen bonds with the hydroxyl groups in the water molecules, which prevents the water molecules from occupying the oxygen adsorption on the surface of the gas sensing material[59]. On the other hand, in addition to the inherent crystal defects in the sensing layer, rare earth atoms are doped into the metal oxide semiconductor gas sensing material, thereby changing the electron distribution on the surface of the gas sensing material and reducing the adsorption energy of the target gas at the active site[60,61]. For example, Young et al. Doped yttrium atoms into tin oxide nanofibers to improve the humidity independence and gas sensitivity of gas sensing materials[62]. Based on the experimental results and van der Waals (VdW) modified DFT calculations, it is found that the doping of tin oxide with equivalent yttrium atoms will produce oxygen vacancies, which makes the surface of gas sensing materials more reactive with NO2 than water molecules. In addition, the yttrium atoms of the unsubstituted tin oxide lattice form hydrophobic yttrium oxide nanoparticles on the entire nanofiber surface, which act to synergistically repel water vapor[62].
4 Conclusion and prospect
A summary of the three moisture resistance enhancement strategies is shown in Table 1:
As shown in Table 1, the effects of three strategies, namely, hydrophobic and breathable coating of gas sensing materials, hydroxyl absorber, and regulation of oxygen adsorption characteristics on the surface of gas sensing film, on the moisture stability of metal oxide semiconductors are summarized. Although these strategies have made important progress in improving the moisture stability of metal oxide semiconductors, they still have their own shortcomings in real-time gas detection applications. Hydrophobic and breathable coating of gas sensitive materials is a relatively common modification strategy, which can directly isolate the contact between water molecules and sensitive materials.It can maintain the baseline and response stability of the sensor in a humid environment, but the hydrophobic and gas-permeable coating also inhibits the contact between the target gas and the sensitive material to some extent, sacrificing its sensitivity and increasing the response time. In addition, organic polymers and porous organometallic framework materials are mostly used in the selection of hydrophobic and breathable coating materials, which mostly show low chemical and thermal stability, which will limit their practical application. Hydroxyl absorbers can improve the moisture stability of the sensor and improve the gas sensing response by building heterostructures or doping defects, but their moisture resistance is relatively poor, the baseline resistance is easily affected by the change of humidity, and the selectivity of the gas sensing material is easily affected by the addition of absorbers. In addition, there are few kinds of hydroxyl absorbers available, which makes it difficult to diversify the structure and design of gas sensing materials to achieve better gas sensing response stability in humid environment. The regulation of oxygen adsorption characteristics on the surface of the gas sensing film can also improve the gas sensing response and the moisture resistance stability of the sensor. Due to the different moisture resistance mechanism and hydroxyl absorber loading, it can show excellent response and baseline resistance stability in humidity environment. However, due to the limitation of its control methods (loading, doping, etc.), its selectivity and gas sensitivity will be affected. In addition, most of the research mechanisms only describe the difference of Fermi energy and the formation of dielectric layers, and there is no clear definition of the electronic and chemical mechanisms from which the performance improvement comes. Therefore, there is still a long way to go to improve the gas sensing and moisture resistance and put it into practice and application.
In order to further eliminate the influence of environmental humidity on gas sensing performance, future research can also try to carry out from the device structure and post-signal processing[71,72]. In terms of device structure, on the one hand, the packaging structure of the sensor can be designed, for example, a moisture absorption layer is introduced above the gas sensitive material to absorb water molecules, or a molecular sieve layer is introduced to block water molecules or other interfering gases.Or a catalytic decomposition layer is introduced to decompose and convert water molecules and other interference gases so as to eliminate the influence of the interference of the water molecules and other gases and improve the selectivity of the device; On the other hand, the metal oxide sensors can be integrated to form a sensor array, and the purpose of identifying the gas qualitatively and quantitatively can be achieved by extracting the high-recognition characteristic parameters of the response signal. In the later signal processing, the response signals (temperature, humidity, response value, response resistance, response time and recovery time, etc.) Can be used as input parameters, and the temperature and humidity compensation model can be established through principal component analysis, linear discriminant analysis or neural network algorithms to eliminate the impact of humidity[73,4].
So far, various metal oxide semiconductors have been studied for sensing and monitoring of gas sensors in high humidity environment, but these studies have not systematically solved the problem of poor moisture stability of metal oxide semiconductors. At present, the hydrophobic and breathable coating strategy of gas sensing materials is mainly used to physically block water to the outside of the gas sensing materials, so that it can not contact with the main gas sensing materials. By dope that hydroxyl absorbent, the water and the hydroxyl absorbent are combine to generate a compound to avoid the formation of hydroxyl or remove the generated hydroxyl, thereby reducing the contact between the water and the gas sensitive material to achieve the purpose of inhibit the poisoning of the hydroxyl of the gas sensitive material; By adjust and controlling that oxygen adsorption characteristic on the surface of the gas sensitive film, the chemical and electronic effect on the surface of the gas sensitive material are changed, the reactivity of target gas and adsorbed oxygen is improved, or the energy band Ben degree and the width of an electronic depletion layer of the gas-sensitive material are changed,And that concentration of oxygen adsorb on the surface of the gas sensitive material is maintained, and the hydroxyl poison reaction process is inhibited, so that the moisture resistance of the material is improved. Through these three methods, the humidity dependence of gas sensing characteristics can be suppressed, but the design and research of metal oxide semiconductor gas sensors that are not affected by humidity at all is still in the exploratory stage, and it is proposed that assembly advantages and cross-innovation may be new growth points in the future.
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