Yulin Kong,Yuxiu Li,Xiuxiu Cui,Linfeng Su,Din M,Tingrun Li,Liji Yo,Xuechun Xio,Yude Wng

a School of Materials and Energy,Yunnan University,650091,Kunming,People's Republic of China

b State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals,Kunming Institute of Precious Metals,650106,Kunming,People's Republic of China

c National Center for International Research on Photoelectric and Energy Materials,Yunnan University,650091,Kunming,People's Republic of China

d Key Lab of Quantum Information of Yunnan Province,Yunnan University,650091,Kunming,People's Republic of China

Keywords:SnO2 nanostructured materials Gas sensor Preparation methods Gas-sensing performances

ABSTRACT SnO2 has been extensively used in the detection of various gases.As a gas sensing material,SnO2 has excellent physical-chemical properties,high reliability,and short adsorption-desorption time.The application of the traditional SnO2 gas sensor is limited due to its higher work-temperature,low gas response,and poor selectivity.Nanomaterials can significantly impact gas-sensitive properties due to the quantum size,surface,and small size effects of nanomaterials.By applying nanotechnology to the preparation of SnO2,the SnO2 nanomaterial-based sensors could show better performance,which greatly expands the application of SnO2 gas sensors.In this review,the preparation method of the SnO2 nanostructure,the types of gas detected,and the improvements of SnO2 gas-sensing performances via elemental modification are introduced as well as the future development of SnO2 is discussed.

1.Introduction

In recent years,with the development of industry and the increasing number of automobile production and population,the air pollution problem is seriously aggravated and threatens public health.Many harmful gases are discharged into the atmosphere with automobile or industrial exhaust,causing environmental pollution and air quality problems.Numerous kinds of toxic and harmful gases,such as carbon oxide,nitrogen oxides,and volatile organic compounds,are discharged through various means.On one hand,they may bring potential threats to the environment and produce toxic pollutants.After being absorbed by the human body,these harmful gases will cause irreversible damage to the tissues of the body[1,2].Simultaneously,indoor air pollution has become a“hidden killer”that harms people's health.New building and organic synthetic materials used in indoor decorations all contain harmful gases such as formaldehyde,benzene,and ammonia have serious carcinogenic effects on the human body,seriously affecting people's health[3].The concentration of these compounds in the air is very low parts per billion(ppb)range.The gas-surface-interaction is weak;consequently,only minor chemisorption,mainly physisorption,is taking place and therefore electronically almost impossible to be detected.Therefore,it is very urgent and essential to develop a sensitive and efficient gas sensor to detect harmful gases.

Gas sensors are a device that converts gas concentration into electrical signals[4].There are many types of gas sensors,such as electrochemical gas sensors[5],metal oxide semiconductor(MOS)gas sensors[6],and infrared gas sensors[7].Among them,metal oxide semiconductor gas sensors have quickly become a hotspot in current gas sensor research due to their advantages such as low cost,fast response speed,simple development,and long service life[8].Metal oxide semiconductor(MOS)gas sensors can be classified into resistive and non-resistive types.Resistance-type MOS gas sensors mainly include surface and volume-controlled models[9].Surface controlled models refer to the surface of the gas sensor when the gas chemical adsorption on the surface of the components in the near-surface charge position,creating a charge layer formation,makes close contact to the surface of the semiconductorenergy band bending.The bending degree is related to the carrier concentration on the surface layer,thus causing a change in conductivity[10];surface-controlled model MOS gas sensors include SnO2,ZnO,and In2O3[9].Volume controlled models refer to the reducing gas adsorption on the surface of the metal oxide semiconductor,part of the surface of the metal ions valence becomes low,and spreads to the crystal.This adsorption results in the valence changes that lead to conductivity changes.However,when reducing gas desorption,the reduction of metal ions via oxygen in the air causes a change back to the metal's original valency[10].Volume controlled model MOS gas sensors include TiO2and CoO[9].Non-resistive MOS gas sensors include gas sensors with a diode rectifier function,gas sensors with field-effect transistor characteristics,and capacitive gas sensors[9].

Fig.1.Crystal structures of the SnO2 polymorphs.(a)Rutile(P42/mnm)and CaCl2 type(Pnnm),(b)a-PbO2-type(Pbcn),(c)pyrite-type(Pa3（—）),(d)ZrO2-type(Pbca),(e)fluorite-type(Fm3（—）m),and(f)cotunnite-type(Pnam)[16].

Due to the advantages of high gas response,stable performance,simple preparation process,and compatibility with modern electronic equipment.Resistance-type MOS gas sensors are widely used,with a wide range of applications,including air quality monitoring,detection of toxic,flammable,and explosive gases,as well as emission control[11–14].As a typical N-type metal-oxide semiconductor material with a band gap width of 3.6 eV,SnO2is used in numerous semiconductor gas sensors.SnO2is composed of tetrode and orthomorphic systems,and its crystal structure can be changed with pressure.As in Fig.1,the transformation sequence is rutile-type (P42/mnm)→CaCl2-type(Pnnm)→α-PbO2type(Pbcn)→pyrite-type(Pa3（—）)→ZrO2-type(Pbca).At higher pressures and temperatures,it transforms into fluorite(Fm3（—）m)and cotunnite-type(Pnam)[15,16].At room temperature,the most common and widely used SnO2sensor is the rutile structure with a tetragonal structure[17].In the tetragonal rutile structure,the SnO2cell,Sn4+occupies the top and center of the tetrahedron,and O2-occupies specific positions within the structure.Each SnO2crystal cell contains 2 Sn atoms and 4 O atoms.The three crystal axes form a 90°angle with each other,that is,α=β=γ=90°with the lattice parameters of a=b=4.737?,c=3.186?[18].

The gas sensing mechanism of SnO2is based on the change of resistance caused by a change of electron concentration that is caused by gas adsorption or desorption:SnO2adsorbs oxygen in the air and dissociates it into oxygen ions,causing a change of the potential barrier between the SnO2grains,resulting in a change of resistance(conductivity)of the gas sensor.When the sensor comes in contact with the gas under detection,the gas molecules react with the adsorbed oxygen anions.The oxygen trapped electrons are released back to the conduction band of SnO2,which makes the resistance of the SnO2gas sensor decrease.The change of resistance of the gas sensor is converted into an electrical signal,which can detect gas[10].Therefore,the conductivity of SnO2depends on the density of oxygen ions adsorbed on the surface.The adsorption performance and chemical stability make SnO2a suitable material for gas sensors[19].However,the sensing performances of traditional SnO2based gas sensors still have some issues,including high working temperatures and poor selectivity.The less active sites of traditional SnO2,limits the adhesion of oxygen atoms to its surface,resulting in unsatisfactory sensing performance.Therefore,the main research focus is to improve the active sites of SnO2through surface modifications,morphology,size control,and nanostructure,further improving the gas sensing performance of the gas sensors via elemental or metal oxide doping.

In recent years,SnO2gas sensors with micro-nano structures have been proven to have superior gas-sensitive properties.Therefore,SnO2nanostructured materials have attracted more and more researchers'attention.So far,SnO2nanomaterials with various structures have been reported,including zero-dimension[20,21],one-dimension(nanowires[22],nanorods[23],nanobards[24]),two-dimensional[25,26],and three-dimensional materials[27,28]as well as hierarchical structures[29,30].There have been many reviews on the nanostructure of SnO2,so herein,we discuss the preparation methods of nanostructured SnO2,detection of gas types and element doping.We will review the work of both our group and other groups in the field of SnO2sensors,the current research progress,and the future of SnO2gas sensors.

2.The preparation methods of nanostructured SnO2

The gas sensing properties of SnO2gas sensors with varying morphologies are very different;these properties have been of great scientific interest[31].Various preparation methods can be synthesize SnO2with different morphologies.The dispersion and morphology of SnO2products are closely related to the gas sensing characteristics of the sensors.Therefore,the preparation methods for SnO2with various morphologieshave been developed to exhibit specific properties yielding expected results.Different preparation methods,such as hydrothermal[32,33],evaporation[34,35],sol-gel[36],chemical vapor deposition(CVD)[37],and electrospinning[38,39],have been used to prepare SnO2materials with the different dimensional nanostructures.In the following review,we will summarize the common preparation methods of nanostructure SnO2based on research results.The processing routes developed for the growth of SnO2nanostructures can be divided into four categories:(I)wet processing,(II)molten processing,(III)solid processing,and(IV)vapor processing routes.Wet processing routes include hydrothermal[40]and electrospinning[46],while molten processing routes include the use of molten salt solutions.Nano-engraving and direct oxidation represent solid-state processes,while thermal evaporation[49]is used in gas-phase processes.

Fig.2.(a–d)Typical FESEM and TEM images of SnO2 spheres,(e.f)FESEM images of the hydrothermal as-synthesized hierarchical α-Fe2O3/SnO2 composites[45].

2.1.Hydrothermal method

The hydrothermal method is one of the most commonly used methods to prepare SnO2nanomaterials due to its controllable conditions,low cost,good dispersion,and high purity.The hydrothermal method provides a high-temperature and high-pressure condition using aqueous solutions as a reaction medium in a closed system so that insoluble substances can be dissolved and recrystallized.Therefore,it is widely used in the preparation of functional nanomaterials.Low dimensional SnO2nanomaterials prepared by the hydrothermal method usually have good crystal orientation,crystallinity,and controllable morphology.However,some aspects,such as the nucleation process and crystal growth control,still need in-depth study;satisfactory conclusions have not been obtained,affecting further analysis of the experimental mechanisms.

Lupan et al.[40]reported a cheap and rapid preparation technique for rutile SnO2nanofibers using the hydrothermal method at low temperatures without using any templates or surfactants.Lin et al.[41]prepared SnO2/graphene oxide(GO)3D nanocomposites via the hydrothermal method using GO and SnCl2as precursors.Flower-like microspheres composed of SnO2nanosheets are distributed between GO layers decorated with tiny SnO2nanoparticles and have a high surface area(94.9 m2/g).GO acts as a template in the hydrothermal process,promoting the preferential growth of SnO2nanocrystals and preventing the agglomeration of SnO2nanoparticles.The material has a fast response and good reversibility to NH3.

The hierarchical structure of SnO2has a large specific surface area,low density,and unique spatial structure that is conducive to gas diffusion and electron transmission,thus improving gas sensing performance,so it plays a crucial role in the research of gas sensing materials[42].The hierarchical structure of SnO2materials with unique morphology can be obtained through the hydrothermal method[43,44].Sun et al.[45]developed the hierarchical assembly of SnO2hollow spheres with α-Fe2O3nanosheets.The materials were prepared by combining hydrothermal,and microwave-assisted hydrolysis used for the synthesis of hollow SnO2nanospheres and α-Fe2O3nanosheets,respectively.In Fig.2,the obtained nanocomposite consists of a hollow inner sphere and a double-shell structure.The composite material with the novel heterostructure shows excellent sensing performance to ethanol.

2.2.Electrospinning

Electrospinning is an intensely facile methodology for the precise manufacturing of nanofibers through the manipulation of electrostatic forces.When the precursor solution is placed in a strong electrostatic field,the voltage rises,the droplets then begin to stretch on the spinning nozzle and form a very fine jet that is sprayed on to the collector at high speed.After solvent evaporation,ultrafine fibers are collected.Onedimensional SnO2composites prepared via electrospinning have a high specific surface area and porosity,which are beneficial to the adsorption of gas molecules on the surface of SnO2.The SnO2gas sensors manufactured this way usually show excellent gas sensing properties[46].Li et al.[47]prepared SnO2hollow nanofibers by single capillary electrospinning and calcination,which show excellent ethanol sensing performance at the optimal temperature of 300°C.They studied therelationship between time,temperature,and concentration response/recovery time.The results show that the high response and relatively short response/recovery time are related to the high surface area and oxygen vacancy of the SnO2fiber.Wang et al.[48]synthesized SnO2hollow nanofibers via single capillary electrospinning and calcination at 600°C for 2 h.The results show that the hollow nanofibers have perfect sensing performances for ethanol,with excellent selectivity,high response(170 at 500 ppm),and rapid recovery(7 s for adsorption and 8 s for desorption).

Fig.3.Schematic of the sol–gel applied to the silicon-oxide layer[53].

Though electrospun nanofibers show excellent efficiency,getting uniform nanofibers with a smaller diameter is an existing challenge.With further future research,these limitations will be overcome,and electrospinning can be used as a preparation method for SnO2sensors with different sensing properties.

2.3.Thermal evaporation

The thermal evaporation method vaporizes SnO2or Sn powder at a high temperature in a vacuum;then inert gas is injected,cooled,and deposited on a Si/SiO2substrate to form uniform one-dimensional and two-dimensional SnO2nanomaterials.Thermal evaporation is usually used to prepare high-purity SnO2nanorods,nanowires,and thin films.This method can be classified as a form of physical vapor deposition.Wang et al.[49]prepared SnO2microrods on SiO2/Si substrates by thermal evaporation and fabricated gas sensors based on SnO2microrods.The gas response and stability of the sensor were tested at the optimum temperature of 200°C.When the sensor is exposed to 100 ppm of TEA gas,the response time is 4 s with a detection limit below 1 ppm.Ying et al.[50]developed a process to synthesize SnO2nanowhiskers via thermal evaporation on Au coated Si substrate.When the high-purity tin powder is heated in a constant current with atmospheric N2and O2,the nanowhiskers cross-section is rectangular and has a diameter between 50 and 200 nm.The length is greater than 10 μm,and the SnO2nanowhiskers exhibited a gas response value ranging from 23 to 50 ppm of ethanol gas at 300°C,which is much higher than that of regular SnO2films.

Although SnO2nanomaterials with high quality can be prepared by thermal evaporation the degree of vacuum inside the coating chamber must be kept high during deposition.The vacuum is often compromised by gas being emitted from the evaporation material,substrate,or other equipment inside the chamber.Also,any remaining moisture inside the chamber,which leads to the experimental variables,is difficult to control.

2.4.Sol-gel method

Sol-gel is a wet treatment process;it takes inorganic salts as precursors and forms a stable,transparent sol system in a solution through a series of chemical reactions.After heating at low temperature,the sol turns into the gel,and the metal oxides or hydroxides precipitate slowly in the sol.Finally,the gel is heated at a higher temperature to obtain the required materials.According to the sol-gel method rheological properties,it can be used to prepare thin films from nano units by spraying,dipping,and impregnating.Quantitative doping can be achieved by mixing various reactant solutions to effectively control the film composition[51].Shukla et al.[52]synthesized nanocrystalline(6–8 nm)SnO2thin films(100–150 nm)via the sol-gel dipping method.The typical reduction gases such as hydrogen can be detected at room temperature by using nanocrystalline SnO2films.Ebrahimi et al.[53]prepared SnO2/CuO through a sol-gel spin coating process and deposited it on a porous SiO2substrate.As shown in Fig.3,the patterned layer is exposed in the texturing step.The exposed areas on the silicon-oxide layer are treated with the reducing gases.Finally,after applying the sol-gel solution,the remaining layer is washed away.It is found that the highly selective deposition of the sol-gel layer on porous SiO2can form a stable composite film.Combined with micromachining of the silicon substrate,the sensor shows high sensitivity to various H2S concentrations.However,the uniformity of SnO2films prepared through spin coating is still lacking consistency.

2.5.Chemical vapor deposition(CVD)

CVD directly utilizes gas raw materials or other means to gasify the raw materials so that one or more kinds of gas can undergo thermal decomposition,oxidation,reduction,and various other reactions through the effects of heat,light,electricity,magnetism,and chemistry.Nanoparticles are precipitated from the gas phase and deposited on the substrate to form a film.By adjusting the substrates precursors and temperature,SnO2thin films with different orientations can be prepared by the CVD method.Stoycheva et al.[54]deposited SnO2from tin complexes via aerosol-assisted CVD.They used different precursors to prepare the SnO2thin films and found that the structure of SnO2thin films depends on the precursors’composition.By testing the gas sensing characteristics of the deposition layer,it was found that the precursor without water molecules in the inner sphere had an excellent response to10 ppm NO2in the air at 300°C,which showed a shorter recovery time and a higher response than the precursors with the water molecules in the inner sphere.Liu et al.[55]deposited SnO2thin films on alumina substrates with pre-patterned electrodes by an aerosol-assisted CVD process.Comparing and analyzing the gas sensing properties of the films prepared under different deposition conditions,the gas response of 50 ppm H2S at room temperature was 98.4 when SnCl2?2H2O was used as the precursor.

Fig.4.Schematically illustrated two-step process of ZnO-doped porous SnO2 nanospheres[58].

In some cases,the CVD reaction source and residual gas are toxic and flammable,so it is necessary to take measures to prevent environmental pollution.In addition,the operating temperature of CVD is higher than that of PVD,which limits its application.

2.6.Other methods

Except for the above common methods,nanostructured materials such as nanospheres and hollow spheres can be prepared by spray pyrolysis[56,57].This is a two-step method utilizing a combination of various methods,as in Fig.4[58,59],sputtering[60],and solution combustion synthesis[61]used to manufacture sensors with different properties.There are many methods to prepare SnO2nanostructured materials,and the technology is evolving becoming more and more reproducible and efficient.The constant maturity and emergence of new and old preparation technologies make SnO2nanostructured materials used as gas sensing materials both a powerful and novel technology for in-depth research and widely distributed applications.Combined with doping and other means,it will further promote the multi-functional and intelligent development of SnO2gas sensors.

Researchers have prepared nanostructured SnO2gas sensing materials with different morphologies and doped them with the different metals or metal oxides using various methods.SnO2gas sensing performances have been significantly improved compared to traditional SnO2gas sensors.However,there are still problems,such as the complex preparation process of gas sensing materials,limiting the application of this technology.The morphologies of the nanostructured SnO2are related to the preparation methods.The development of new and novel preparation methods may allow the preparation of more SnO2gas sensors with different morphologies,therefore better able to obtain SnO2gas sensing materials.However,there is a long way to go as well as the need for many researchers to work together to overcome these difficulties.

3.Elements modification

SnO2has become a novel material for gas sensors because its resistance changes due to the barrier changes before and after the gas molecules adsorption.Therefore,the performance of the materials used as gas sensors can be changed using doping.For example,doping with the different elements or oxides can change the materials‘energy band characteristics and improve the detection performance of the materials used as gas sensors.By doping with specific elements or oxides,the materials can only react with a particular gas or preferentially react with a gas to enhance its detection selectivity.

3.1.Metal modification

On the basis of not changing the original characteristic structure of SnO2,metal modification can change the energy band structures of the materials and improve the gas sensing performances of the materials.The conductivity of gas sensing material is related to its band-gap,appropriate amount of metal modification can narrow the band-gap of SnO2,so as to improve the conductivity of SnO2and reduce its working temperature,At present,the most common metal modification includes noble metals and rare earth elements.Among them,noble metals mainly include Pd[62,63],Pt[64,65],Au[66],etc.,rare earth elements mainly include Ce[67],Pr[68],Er[69],and other metals such as Cu[70],and so on.

Fig.5.Methanol gas response of as-synthesized SnO2 microspheres composited with the different contents of Pt nanoparticles towards a gas concentration of 100 ppm at the different operating temperatures[79].

3.1.1.Pd modified SnO2

Palladium(Pd)is a common material used to modify the gas sensing properties of SnO2.Pd doping can effectively improve the sensitivity of SnO2and reduce the response temperature to the gas.The improved sensing characteristics can be attributed to the chemical and electronic sensitization of the noble metals.Bulk Pd can absorb a significant amount of hydrogen.Shen et al.[71]successfully prepared Pd doped SnO2nanowires using the thermal evaporation method.Compared with the original SnO2material,the Pd doped SnO2material has a high response to H2(1000 ppm)at the same working temperature,and its response increases with the Pd concentration.At 100°C,the highest response reaches 253.The results demonstrate that Pd modification improves the sensor response and lowers the operating temperature to maximize the senor response.Chen et al.[72]successfully prepared pure and Pd-doped SnO2nanoparticles and measured their gas sensing properties for CH4,C2H6,C2H4,and C2H2.Pd2+ions improve the conductivity of the SnO2sensor;and shows high conductivity and gas sensing properties for characteristic hydrocarbon,which improves its gas sensing performance for hydrocarbon gas and has a quick response recovery performance.Our research group[73]successfully prepared Pd–SnO2materials through the solvent thermal method and compared the degree of response of different materials to 3000 ppm H2at 200°C.It was found that the addition of Pd improves the gas-sensitive performance of the material.The gas-sensitive response of a 10% mol Pd–SnO2material reaches 315.34,which is 8 times that of the unmodified SnO2material.We also prepared SnO2nanofibers using a simple hydrothermal method.The SnO2nanofibers are a fibrous structure composed of many nanoparticles with large specific surface areas.The sensor showed a high gas response,high detection range(10 ppm–1000 ppm),and a fast response to various volatile organic compounds(VOCs)and gases(methanol,ethanol,isopropanol,acetone,formaldehyde,and n-butanol)at the optimal operating temperature of 260°C,[74].Also,we also prepared Pdfunctionalized SnO2nanofibers and SnO2/In2O3nanocomposite sensors,which showed a fast response(<3.6 s)/recovery time(<7.9 s),excellent stability and selectivity to 3000 ppm butane at 260°C and 320°C[75,76].However,the addition of excessive Pd may increase the resistance of SnO2and decrease the response of SnO2;therefore,the optimal concentration of Pd doped SnO2has always been a concern of researchers.

Fig.6.Gas responses of Cd-doped SnO2 nanofiber sensors[82].

3.1.2.Pt modified SnO2

Platinum(Pt)doping can signifigantly improve the gas sensitivity of SnO2to H2S and CO,which is related to the spatial distribution of Pt in SnO2.The presence of Pt facilitates the surface reaction,which improves the gas sensing properties.Dong et al.[77]prepared Pt–SnO2nanofibers via electrospinning,and it was found that the Pt–SnO2gas sensor had an increased gas sensing performance for H2S at 300°C.Through a further study on the structure-activity relationship of the Pt–SnO2material,it was found that the reason for the improved gas sensing performance of the material was due to the addition of Pt.Pt modified SnO2has a higher specific surface area and a lower agglomeration probability.Our research group prepared Pt decorated SnO2using the traditional hydrothermal method[78].Through further comparison,it was found that in terms of the reaction conditions of CO at 80°C,the Pt–SnO2material with 1.5%Pt molar content had the best gas response,reaching 610.45,which is 3 times that of pure SnO2.In addition,the SnO2nanocomposites modified with the Pt nanoparticles were successfully prepared via a solvothermal method.The experimental results showed that the Pt–SnO2nanocomposites are composed of SnO2microspheres and Pt nanoparticles,containing many small spheres,each of which is composed of numerous primary nanocrystals.As in Fig.5,the gas sensing response of 5.0%mol Pt–SnO2is 190.88 to 100 ppm methanol,which is 10 times that of pure SnO2[79].

3.1.3.Other element modification

In addition to Pd,Pt,and other noble metal elements,some rare earth,and transition metals are often used to modify SnO2.Song et al.[80]prepared Ce doped SnO2hollow balls by introducing Ce with polystyrene as the templating agent.They found that the Ce–SnO2material had an excellent gas-sensitive response to 500 ppm acetone at 250°C.Jin et al.[81]prepared a Cu-doped SnO2material via the traditional hydrothermal method.Through experimental tests,they found that the Cu–SnO2material had a flower-like hierarchical structure,and its response to acetone is 11.5 times that of ammonia water at 260°C,exhibiting a significant increase in gas-sensitive performance.Our research group[82]prepared one-dimensional(1D)Cd-doped SnO2nanofibers by a simple hydrothermal method using grapefruit peel as a biological template.We observed a unique one-dimensional fibrous structure with many nanoparticles through the materials characterization,and the Cd-doped SnO2nanofibers had a larger specific surface area.The results show that the highest gas response value of 51.11 towards 100 ppm formaldehyde was nearly 4 times higher than pristine SnO2at a lower operating temperature of 160°C,Fig.6.This is because the unique one-dimensional fibrous structure with a high specific surface area and Cd additive can provide more active sites for the oxygen ions and formaldehyde molecules to be adsorbed on the surface.

Fig.7.Schematic plot illustrating the possible transition from coupled TiO2/SnO2 to TiO2–SnO2 composites and its electron transfer process[92].

3.2.Metal oxide modification

In addition to the elemental modification of SnO2,metal oxide modification is also a primary method to improve the gas sensing performance of SnO2gas sensors.Metal oxide and SnO2can form n-p or n-n heterojunctions,which can transfer electrons between the grain interfaces between them.It makes it easier for electrons to enter the conduction band,thus improving the gas sensing performance.Metal oxides commonly used for modifying SnO2are NiO[83,84],ZnO[85,86],CeO2[87,88],and TiO2[89].Pourfayaz et al.[90,91]prepared CeO2doped SnO2materials synthesized via the sol-gel method.They found that a 2%wt CeO2–SnO2had an excellent gas response to ethanol gas under the conditions of co-existence of ethanol,CO,and CH4gases at 300°C.The influence of gas humidity on the gas sensing properties of CeO2–SnO2was further studied.It is found that 2% wt CeO2–SnO2shows good selectivity for ethanol gas under 80%humidity and a temperature of 50°C.CeO2–SnO2gas-sensing response reaches a maximum at 300°C,but decreases with an increase of relative humidity.Zeng et al.[92]successfully prepared TiO2doped SnO2materials by simply mixing TiO2with SnO2,the TiO2–SnO2mixed system schematic plot is shown in Fig.7.Chemical reactions take place between the VOC molecules and the pre-adsorbed oxygen on the surface of the composite material.It was found that TiO2–SnO2has excellent gas-sensing performance for VOCs;the improvement of sensitivity to VOCs is attributed to the noticeable grain refinement via doping and the potential facilitation of oxygen adsorption on the sensor surface.

Fig.8.Schematic illustration of the tentative mechanism for the template-directed synthesis of the Ag2O-loaded SnO2 nanotubes.The amount of Ag2O in SnO2 nanotube is controlled by the initial Ag core[102].

3.3.Other compounds modification

In addition to the common mono-metal oxides,many mixed metal oxides have been successfully applied in the modification of SnO2.Zhang et al.[93]prepared ZnSnO3by fabricating SnO2materials through the traditional hydrothermal method.It was found that ZnSnO3/10% wt SnO2materials show good gas sensing performance in the reaction against acetone due to its high specific surface area,a large number of oxygen vacancies,and the interaction between ZnSnO3and SnO2,after changing the additive ratio.

Sun et al.[94]prepared Ag3PO4doped SnO2nanospheres via hydrothermal and chemical deposition methods.At the operating temperature of 100°C,the material has a favorable gas sensing performance for H2S,and its gas sensing response value reaches 118.Liu et al.[95]prepared a novel mesoporous ZnSe core/SnO2shell microsphere chemical sensor for ppb level NO2detection.Our team[96]introduced the ternary oxide Zn2SnO4into the rod-like nanostructured SnO2gas sensor to detect formaldehyde by a simple one-step hydrothermal synthesis method.Controlling the amount of Zn in the precursor solution can effectively achieve one-and two-dimensional coexistence of structured SnO2–Zn2SnO4(SnZn)nanocomposites.The sensor-based on an SnZn composite showed an excellent response and selectivity to HCHO gas at a low operating temperature of 162°C,indicating that the presence of Zn2SnO4species in SnO2powder could effectively improve the conductivity,reduce the optimal operating temperature,and improve the gas response of the sensor.

4.Gas category detected by SnO2 based gas sensors

Toxic and harmful gases cause irritation,poisoning,and suffocation to the human body.When the human body is exposed to a specific concentration of toxic and harmful gases,it causes damage to the respiratory tract,mucosa,skin and even has the potential to cause death.Common toxic and harmful gases include SO2,H2S,CO,and formaldehyde.Flammable gas leakage is a common problem in petrochemical,mining,public safety,and other industries.These kinds of gases will not cause direct harm to the body,but it does present a fire and explosion risk if it is not prevented or detected in time;they pose a signifigant threat to public safety.Common flammable gases include methane,ethanol,and H2and can be classified into organic and inorganic gases.In the following paragraphs,we will briefly review the research progress of SnO2based gas sensors in detecting these harmful and flammable gases based on our and other groups‘work.

4.1.Organic harmful flammable gases

Organic gases are mainly VOCs and other flammable gases containing organic substances,including alcohols(methanol,ethanol),and aldehydes(formaldehyde,acetaldehyde).There are many different definitions of volatile organic compounds;the US Environmental Protection Agency defined VOCs as:“any compound of carbon,excluding CO,CO2,carbonic acid,metallic carbides or carbonates,and ammonium carbonate which participate in atmospheric photochemical reactions”[97].China describes volatile organic compounds as“any organic compound that is volatile under normal pressure and temperature.”[98].The World Health Organisation(WHO)Air Quality Guideline for Europe establishes guideline values for toluene,260 μg/m3over 1 week,formaldehyde,100 μg/m3over 30 min,and tetrachloroethylene,250 μg/m3over 1 year[99].Therefore,VOC detection is of great significance to environmental protection and people's health.SnO2-based VOC sensors continue to dominate the market due to their easy production,low cost,and high portability[100].Meanwhile,new SnO2sensors are still being reported.Bahuguna et al.[101]proposed a novel VOC sensor,which uses a fluorinated SnO2thin film to make a transparent thin film display.The doped fluorine leads to a significant increase in conductivity and a decrease in the persistent photoconductivity,accompanied by a faster decay of photogenerated charge carriers.The sensor shows a stable ppm level response to different VOCs and recovers quickly in a few minutes at 150°C.Also,SnO2nanotubes modified with metal oxides are also used to prepare gas sensors for toluene and ethanol detection.The addition of metal oxides improves the surface reaction of adsorbing,dissociating,and ionizing oxygen,which significantly improves the response and recovery speed of SnO2nanotube sensors.In Fig.8,this modification concept also provides a feasible way to improve the gas sensing performance of SnO2nanotubes[102,103].Li et al.[104]reported a hierarchical SnO2nanostructure for detecting various typical VOCs such asacetone, isopropyl alcohol, methanol, and ethanol.The three-dimensional cone-shaped hierarchical structure of SnO2shows a relatively high response of 175 to 100 ppm acetone,and its detection of various VOCs has good stability and repeatability.

In recent years,our research group has also carried out much research detecting VOCs and other organic gases.We reported a sensor-doped nanoporous structure nanoporous with 2.5% mol Pd–SnO2composite and showing an ultra-fast response of 17.60 at 340°C in 3000 ppm CH4,reaching a stable state and rapid recovery in 5 s.The sensor based on CH4shows excellent performance and has not been reported until now[105].The detection of formaldehyde gas using SnO2microsphere gas sensors is also reported.The synthesized tin dioxide microspheres consist of many microspheres with an average diameter of 250 nm.The resulting product is used as a sensing material for the detection of formaldehyde gas.At the operating temperature of 200°C,the sensor's response value to 100 ppm formaldehyde is 38.3[106].Our research group has also done research on the detection of methanol[79,107],isopropanol[108–110],and n-butanol[111].Table 1 shows a list of the above mentioned SnO2nanostructured materials used for the detection of organic gases,including their operating temperature,gas sensing properties,and preparation methods.

Table 1List of SnO2 nanostructured materials towards organic gases detection.

Table 2List of SnO2 nanostructured materials towards inorganic gases detection.

4.2.Inorganic harmful flammable gas

Inorganic harmful flammable gases are common in factories and laboratories,usually consisting of decomposed forms of various organic compounds or by-products.Usually,trace amounts of inorganic gases will pose a threat to the health of workers.Long-term exposure may be a carcinogenic risk to the human body.Therefore,the sensitive detection of these inorganic harmful and flammable gases in the workplace is of great significance.Much research has been done on SnO2gas sensors used to detect various common inorganic harmful flammable gases,such as H2S[112,113],CO[114,115],H2[116,117],and Cl2[118].H2S is one of the most toxic and flammable gases used commercially in the laboratory.SnO2synthesized with various structures have been shown to be highly selective for H2S,including CuO/n-SnO2heterostructure[119]and the copper doped SnO2nanocrystalline film[120].Zhang et al.[121]prepared the ordered copper-doped SnO2porous films with large specific surface areas on substrates using self-assembled soft templates combined with simple physical co-deposition.The gas response,selectivity,response,and recovery time of the copper-doped tin dioxide porous film gas sensors are excellent.

Fig.9.Schematic diagrams of the hydrogen mechanism of pristine SnO2 and Pd–SnO2 composite microspheres[73].

Under the optimal operating temperature of 180°C,the average response and recovery time of H2S at 100 ppm are 10.1 s and 42.4 s.CO is the product of incomplete combustion of organic materials.It is a colorless and odorless gas,which is difficult to detect by the human body,but it can cause significant harm to human health.Therefore,the detection of CO is also of great significance.Tischnere et al.[122]manufactured a SnO2sensing film with a thickness ranging from 50 to 100 nm on an oxidized silicon substrate using a spray pyrolysis process,creating a newly developed gas sensor based on an ultra-thin SnO2film.The sensor is highly sensitive to humidity and carbon monoxide and can detect carbon monoxide concentrations below 5 ppm at operating temperatures of 250–400°C.Wang et al.[123]studied cassiterite as a sensing element in carbon monoxide gas sensors.The results show that the V–O–Sn structure provides redox activity and helps oxygen activation respond to the CO gas sensing reaction.H2is a highly flammable gas,and its leakage poses a threat to environmental safety.Our group[73]synthesized SnO2–Pd composite nanoparticles via a solvothermal method and calcined them to prepare an H2sensor.The sensor has excellent selectivity and stability for H2.The sensor's maximum sensing response is3000 ppm H2at 315.34 a fast response recovery time of 4 s–10 s,and a minimum detection limit of 10 ppm.The sensing mechanism of hydrogen is shown in Fig.9.

In addition to the above gases,nanostructured SnO2gas sensors can detect many other gases[124–128].In general,gas sensors based on nanostructured SnO2have a low cost,but the gas-sensitive performances need to be improved,and the operating temperatures are still too high.By adjusting the morphology and adding different substances,the selectivity and response of SnO2gas sensors to various gases can be altered,and SnO2gas sensors can be widely applied to numerous fields[129–132].SnO2nanocrystallization can improve gas sensors‘performance and reduce the size and power consumption of the gas sensor elements[133].Therefore,nanostructured SnO2gas sensors will continue to be at the forefront of research in the future,which requires multidisciplinary cooperation.Table 2 shows the list of the above mentioned SnO2nanostructured materials used to detect inorganic gases,including their operating temperature,gas sensing properties,and preparation methods.

5.Conclusions and outlooks

Nanostructured SnO2and its composites have been the quintessence of resistance-type semiconductor gas sensors used to detect toxic,flammable,or explosive gases for years due to their low operating temperature,high response,stability,and simple preparation.Hundreds of studies have established that three factors influence the properties of SnO2gas sensors:1)elemental modification,2)methods of preparation,and 3)the microstructure of the sensing materials body.The composites usually exhibit improved performances compared to pure SnO2in terms of gas response,reversibility,response time,and detection limit.Therefore,exploring the design and fabrication of SnO2composites is an essential and crucial way to obtain high-performance gas sensors.

This review has highlighted the recent progress of gas sensing studies on nanostructured SnO2composites.The review includes the different elemental modifications and the methods used to prepare the nano microstructure of gas sensors that have been used to detect toxic,flammable,or explosive gases.Hydrothermal,electrospinning,and thermal evaporation methods were used to form various microstructures,like nanotube,nanoball,and nano-porous membranes used to respond to the corresponding detected gases.The gas response has been improved through addition of one or more various elements or different metallic oxides.

Although significant progress has been made in the design and fabrication of nanostructured SnO2composites for gas sensors,further work is still required to understand better the synergistic interactions between each component in the composite and the possible sensing mechanism for each sensor.These questions play a significant role in understanding high-performance gas sensors.In the meantime,we find that SnO2gas sensors with composites that are binary are better than those of pure SnO2.A few studies were conducted to prove that using ternary composites for gas sensors usually showed better performance than single or binary systems.This will be an easy and effective way to synthesize superior SnO2gas sensors.The research on gas sensors is a multidisciplinary endeavor related to many fields including,physics,chemistry,electronics,and mathematics.Solving these problems will come with great challenges,but it is necessary in order to enhance interdisciplinary collaboration.

Declaration of competing interest

There is no conflict of business interests in this paper.

Acknowledgments

This work was supported by National Natural Science Foundation of China(No.61761047 and 41876055),the Department of Science and Technology of Yunnan Province via the Key Project for the Science and Technology(Grant No.2017FA025)and Program for Innovative Research Team(in Science and Technology)in University of Yunnan Province.