Xu Tin,Xiuxiu Cui,Tingrun Li,Jie Ren,Zhicho Yng,Mingjing Xio,Bingsen Wng,Xuechun Xio, Yude Wng

a School of Materials and Energy, Yunnan University, 650091, Kunming, PR China

b National Center for International Research on Photoelectric and Energy Materials, Yunnan University, 650091, Kunming, PR China

c Key Lab of Quantum Information of Yunnan Province, Yunnan University, 650091, Kunming, PR China

Keywords:Gas sensor TiO2 nanomaterials TiO2-Based composites Gas sensing mechanism

ABSTRACT Hazardous gases have been strongly associated with being a detriment to human life within the environment.The development of a reliable gas sensor with high response and selectivity is of great significance for detecting different hazardous gases. TiO2 nanomaterials are promising candidates with great potential and excellent performance in gas sensor applications, such as hydrogen, acetone, ammonia, and ethanol detection. This review begins with a detailed discussion of the different dimensional morphologies of TiO2,which affect the gas sensing performance of TiO2 sensors. The diverse morphologies of TiO2 can easily be tuned by regulating the manufacturing conditions. Meanwhile, they exhibit unique characteristics for detecting gases, including large specific surface area, superior electron transport rates, extraordinary permeability, and active reaction sites,which offer new opportunities to improve the gas sensing properties. In addition, a variety of efforts have been made to functional TiO2 nanomaterials to further enhance sensing properties, including TiO2-based composites and light-assisted gas sensors. The enhanced gas sensing mechanisms of multi-component composite nanomaterials based on TiO2 include loaded noble metals, doped elements, constructed heterojunctions, and compounded with other functional materials. Finally, several studies have been summarized to demonstrate the comparative sensing properties of TiO2-based gas sensors.

1. Introduction

With society's progression and the development of science and technology, tremendous environmental air pollution problems revolve around human life. Various hazardous gases include volatile organic compounds (VOCs), flammable and explosive gases, nitrogen oxides,sulfur oxides, and carbon oxides. VOCs such as xylene, methanol,ethanol,acetone,formaldehyde,toluene,and benzene are widely utilized in industrial processes,transportation,and domestic activities[1].VOCs that exceed a tolerable health standard within the environment of human activities cause severe damage to the human sense of smell, vision, mucosa, lung function, liver function, and central nervous system [2-5].Flammable and explosive gases include hydrogen, methane, carbon monoxide, and hydrogen sulfide exist within chemical industries and coal mining. If they exceed a certain limit in a specific space, safety accidents such as explosions and combustion will occur, which bring significant hazards to human lives and industrial productions. Also, industrial and transportation emissions such as nitrogen oxides, sulfur oxides, and carbon oxides tend to cause acid rain and photochemical smog.

Given the above situations,tremendous efforts have been put towards developing gas sensors to detect toxic, flammable, and exhaust gases in the fields of disease diagnosis, military security, environmental and industrial production [6]. Indeed, the low-cost, low power-consuming,reliable, portable, real-time efficient, extremely sensitive, and selective gas sensors are in great demand due to a wide range of applications. In the past few years,many materials have been investigated as promising sensing materials,including two-dimensional materials such as graphene oxide, transition metal dichalcogenides, transition metal carbides, nitrides, and carbonitrides due to their high surface area and excellent electron transport capability. However, semiconductor metal oxide(SMO) gas sensors still play a dominant role in their application due to their simple synthesis,high response, and low-cost that fulfill ideal sensors’ requirements.

Titanium dioxide (TiO2) is a high resistance n-type semiconductor material with a band gap around 3 eV,has attracted significant attention for its applications in photocatalysis [7], solar cells [8], and gas sensors[9] due to its environmental friendliness, chemical stability, catalytic properties, and the modulation in its structural, optical, and transport properties[10].Additionally,TiO2has a three crystal structure in nature:anatase,brookite,and rutile.Rutile is the most stable while the anatase and brookite phases are metastable and can be converted irreversibly to the rutile phase when heated under high temperature in the range of 600-800°C [11]. Anatase is the most widely gas sensor due to its prominent gas reaction capacity and high oxygen vacancies [12]. However, the pristine TiO2gas sensors still have limitations, such as high operating temperatures, low selectivity, unstable repeatability, and stability when exposed to reducing or oxidizing gases[13].

Recently,various novel TiO2nanomaterials with new structures and composites have been used as gas sensors.In this review,three sections relate to TiO2gas sensors. First, we focus on the effect of different dimensional morphologies of TiO2on their gas sensitivity. Second, we review the different TiO2-based nanocomposites on their sensing properties and summarize their sensing mechanisms with respect to loaded noble metals, doped elements, constructed heterojunctions, and compounded with various other functional materials.Finally,we will discuss the sensing properties of TiO2-based light-assisted gas sensors. Furthermore, conclusions in current reports of TiO2-based gas sensors will be discussed.

2. Different TiO2 nanostructures for gas sensing

The gas sensing properties of sensing materials are greatly dependent on their morphologies.Nanostructured materials offer new opportunities for improving gas sensors' properties due to their advantages of larger surface areas and better electron transitions. Meanwhile, different morphologies can easily change the electronic properties, surface activities,as well as exhibiting unique characteristics.At present,various strategies have been employed to control the synthesis of TiO2nanostructured materials with different shapes and sizes. The geometry of TiO2nanomaterials can be varied from zero, one, two, or even three-dimensional nanostructures. This review, will briefly discuss the research developments of different morphologies of TiO2based nanomaterials used in gas sensor applications.

2.1. Zero-dimensional TiO2 based nanomaterials

The zero-dimensional (0D) TiO2nanostructures include quantum dots,nanospheres,and nanoparticles,that can be prepared using various techniques (Fig. 1) such as hydrothermal [14], sol-gel method [15],metal-organic decomposition [16], and laser ablation in liquid [17,18].The 0D nanostructures are the best candidates for ultrasensitive and highly miniaturized chemical reactions because of their large specific surface area provide numerous sites for the adsorption of gases[19].

Navale et al. [14] reported the anatase TiO2nanoparticles to have a parts per billion(ppb)level of detection and a high selectivity to acetone at an operating temperature of 270°C.The high selectivity is attributed to the higher adsorption activities of the TiO2surfaces. Meanwhile, the sensor responds rapidly to acetone and quickly attains a nearly stable state. When the target gas is exposed to the sensor, the same response value is achieved,demonstrating excellent reproducibility.

Fig. 1. 0D TiO2 nanostructures reported by various methods. (a) TiO2 nanoparticles were prepared by laser ablation in liquid [17]; (b) Pecan-kernel-like TiO2 was synthesized via hydrothermal method[20];(c)TiO2 nanoparticles were developed by laser irradiation and thermal annealing[18];(d)-SEM and(e)-TEM TiO2 hollow microspheres were synthesized via the carbon sacrificial template [21]; (e) TiO2 nanoparticles were prepared by metal-organic decomposition method [16]. Images(a)-(e) adopted with the permission from Elsevier. Image (f) adopted with the permission from American Chemical Society.

Sugahara et al. [16] synthesized spherelike anatase TiO2nanostructures on silica glass and plastic substrates via the metal-organic decomposition (MOD) method, using them as VOCs gas sensors. The sensor detects methanol,ethanol,and 1-propanol;the responses increase in the order of 1-propanol ＞ethanol ＞methanol at concentrations of 50-350 ppm,which involves the reductive nature of the gas species due to their dependence upon the number of CH chains. Indeed, the sensor shows an excellent high-speed gas sensing response and recovery time(1 s/1 s) towards propylene glycol at 350°C because of the nanospheres having a large surface area and a highly crystalline structure.

2.2. One-dimensional TiO2 based nanomaterials

The one-dimensional (1D) TiO2nanostructures include nanotubes,nanorods, nanobelts, and nanofibers/nanowires. 1D TiO2could be the ideal nanostructure for gas detection due to their inherent qualities of a large specific surface area,strong adsorption capacity,superior electron transport rate[22],and numerous porous sites for the faster diffusion of test gases [23]. Various methods (Fig. 2) of synthesizing 1D TiO2have been reported including electrochemical anodization[24],hydrothermal[25,26], template-assisted synthesis [27], electrospinning [28], and matrix-assisted pulsed laser evaporation[29].Among them,vertical TiO2nanotube arrays prepared using electrochemical anodization has numerous oxygen vacancies that provide effective gas diffusion and more active sites. They are considered to be an ideal platform for gas sensing due to their fast response time [30,31]. Nevertheless, the hydrothermal method for preparing TiO2nanotubes has the advantages of simple operation and cost-effectiveness [32]. The diameters and walls of the TiO2nanotubes are smaller and thinner than those prepared by electrochemical anodization [33]. To date, pristine 1D TiO2gas sensors have been successfully used to detect acetone [34], carbonic oxide [35], isopropanol [36], formaldehyde [37], ethanol [38], trimethylamine [39],hydrogen,isopropanol[40], methane[41],and oxygen[42].

Fig. 2. 1D TiO2 nanostructures reported by various methods. (a)-(c) TiO2 NTs synthesized by electrochemical anodization [47], hydrothermal method [32], and template-assisted synthesis [27], respectively; (d) TiO2 nanorods in situ growing on the FTO glass substrate by hydrothermal method [45]; (e) TiO2 nanowires synthesized by hydrothermal method using C12H28O4Ti in alkaline solution [48]; (f) TiO2 nanorods fabricated by electrospinning technique [49]; (g) Schematic of Au/TiO2 NTs/Ti sandwich-structured sensor [43]. Images (a) and (b), (e)-(g) adopted with the permission from Elsevier. Images (c) and (d) adopted with the permission from American Chemical Society.

Electrochemical anodization is the most common method for synthesizing TiO2nanotubes (TiO2NTs) on titanium foil since it results in highly ordered and organized nanostructures. Prateek B et al. [43] reported the selective detection of four different VOCs methanol,ethanol,acetone,and 2-propanol at room temperature using an Au/TiO2NTs/Ti type sandwich-structured sensor (Fig. 2 (g)). The TiO2NT array grown via an electrochemical anodization process exhibited a maximum capacitive response towards methanol compared to the other three gases.However, it cannot completely recover its response to all the gases mentioned above.Researchers have also synthesized single,double,and triple layered TiO2NT arrays via a voltage pulse assisted anodization method[44].The preparation of single layered TiO2NTs uses a constant potential for a specific amount of time, and the multi-layered TiO2NTs were synthesized by modulating anodization voltage with time.The TiO2NT sensors have a maximum response to 160 ppm ethanol at room temperature. The response value to all tested gases increased as the number of layers increased. Zhao et al. [45] fabricated a novel double-layered TiO2nanorod (TiO2NRs) array using etched fluorine-doped tin dioxide (FTO) glass as the in situ growing substrate different growth times,and a new-type gas sensing electrode obtained via the facile droplet-coating and hydrothermal method. The results show that the TiO2NRs have a growth time of 6 h exhibit a high gas sensing response of 100 ppm to NH3with a value of 102%at room temperature.Zhou et al.[46] prepared different rutile TiO2{002},{101},and{110}facet nanorods via a hydrothermal process on an FTO substrate using tetrabutyl titanate as the titanium source.The rutile TiO2with{101}and{002} facets exposed were controllably synthesized by adjusting the hydrothermal solvents’ ethanol content. All the prepared TiO2faceted nanorod arrays based on hydrogen detection performed excellently at room temperature. TiO2hydrogen-based sensors with both {110} and{002} facets exposed gave a faster response, as well as better repeatability and stability than those with only{002}facets.

2.3. Two-dimensional TiO2 based nanomaterials

The two-dimensional (2D) TiO2nanostructures include nanosheets,nanoplates,and thin films(Fig.3). They exhibit many versatile features like high specific area, quantum Hall effect, flexibility, and superior mechanical strength [50]. Furthermore, 2D subunits via crystal facet engineering can effectively improve the surface reactivity and selectivity[51].To date,some strategies have been reported to synthesize 2D TiO2nanomaterials such as a one-step annealing process [52], chemical decomposition method [53], hydrothermal method [54], and reactive evaporations [55]. Among them, the hydrothermal method is the most common method for the synthesis of 2D TiO2nanomaterials. In recent years,pristine 2D TiO2gas sensors have been successfully used to detect acetone [56], methanol [57], carbon monoxide [10], hydrogen [58],hydrogen sulfide [59], and ammonia [60]. Ge et al. [52] synthesized hierarchical porous structured TiO2hexagonal nanosheets derived from layered TiSe2nanosheet templates via a one-step annealing process.The sensor exhibits ultrafast response times(response and recovery times are 0.75 s and 0.5 s, respectively) and high selectivity towards 200 ppm acetone vapor at 400°C.The electron transport between acetone and the TiO2surface achieved both quick and higher responses under the synergistic driving of the unique porous hierarchical structure, strong interface coupling, and crystal facet engineering. Wang et al. [61] prepared the TiO2nanoplates with defective and complete {001} crystal facets by adjusting the concentration ratio between hydrofluoric and hydrochloric acid solutions during the hydrothermal process. They discussed the effect of the defects within the TiO2nanoplates with defective and complete {001} facets on acetone's sensing properties. The results show that the sensing response of TiO2nanoplates with complete{001}facets was 70%higher than that of defective TiO2nanoplates.The poor gas response of defective TiO2can be ascribed to fewer adsorption sites and numerous lattice defects that blocked the electron's transfer.Further confirming the high-energy {001} crystals of complete TiO2nanoplates play an important role in improving gas sensing performances.

2.4. Three-dimensional TiO2 based nanomaterials

Fig. 3. 2D TiO2 nanostructures reported by various methods. (a) TiO2 hexagonal nanosheets prepared via one-step annealing process [62]; (b)-(f) TiO2 nanoplates prepared by hydrothermal process [56,63-66]. Images (b)-(e) adopted with the permission from Elsevier. Images (a) and (f) adopted with the permission from American Chemical Society.

Fig. 4. Images of different shapes of the TiO2 NFs synthesized by hydrothermal method. (a)TiO2 NFs synthesized in a mixed solution of deionized water and liquid ammonia[70];(b)TiO2 NFs synthesized in an acetic acid solution,the hydrothermal time was 8 h[77];(c)TiO2 NFs synthesized in HCl/H2O solution(volume ratio ＝1:1)with 1.50 mL titanium butoxide;(d)TiO2 NFs synthesized in HCl/H2O solution(volume ratio ＝1:1)with 1.0 mL titanium butoxide the reaction time was 5 h;(e)TiO2 NFs synthesized in HCl/H2O solution(volume ratio ＝1:1)by adding the surfactant ethylene oxide/propylene oxide block copolymer;(f)TiO2 NFs synthesized in HCl/H2O solution(volume ratio ＝1:1) by adding the surfactant sodium dodecyl sulfate [75].Image (a)adopted with the permission from MDPI. Image(b)adopted with the permission from Elsevier. Images (c)-(f) adopted with the permission from Editorial Board of Chinese Journal of Structural Chemistry.

Recently, the three-dimensional (3D) TiO2nanoflower (TiO2NFs)structure (Fig. 4) has attracted the attention of researchers. The nanoflower structure possesses novel properties that promote the adsorption and diffusion of target gas molecules on its surface.It even allows them to penetrate the interior part of the flower [67]. Moreover, some reports indicate that nanoflowers exhibit superior electronic properties compared to nanosheets, nanoparticles, or other corresponding nanostructures. This is due to the structural defects acting as active reaction sites between the material and gas[68].The TiO2NFs are synthesized via a facile hydrothermal route using TiCl3[69],TiCl4[70],Ti powders[71],TiOSO4[72], C12H28O4Ti [73], or Ti foil [74] as raw material. The nanoflowers are composed of hundreds of one-dimensional TiO2nanorods, nanosheets, nanowires, and nanospheres, Fig. 4. The one-dimensional structures are attributed to the different shapes of the nanoflowers as well as controlling the reaction time, reaction temperature, reaction solution, solution acidity, amount of raw material, and addition agents [75]. Bhowmik et al. [76] synthesized the 3D TiO2NFs consisting of 2D nanosheets using a low-temperature hydrothermal process.The sensor revealed high selectivity and fast response/recovery times towards acetone at a low operating temperature of 60°C. The authors consider the effect of the bond dissociative activation energy(BDAE),vapor pressure of the individual VOCs, acetone selectivity,and the diffusivity of the gas species at the nanoflower surface to describe the low operating temperature.Gases with lower BDAE actively react with Oions and reduce the operating temperature.The higher vapor pressure of the gases accelerates the adsorption/attachment process at the nanoflower's surface.They conclude that the dual effect of the low BDAE and the higher vapor pressure of the acetone molecules compared to the other VOCs (butanone, propanol, methanol, and toluene) led to a maximum response towards acetone. Wang et al. [71] reported a 3D hierarchical TiO2NF gas sensor and used it to detect ethanol with high selectivity,good stability, and fast response/recovery times at room temperature.One of the reasons for the exceptional gas sensing performance could be ascribed to the flower-structures nanopetals and spacing intervals,which provide more adsorption sites for the oxygen species and the target gases and facilitates the interactions between the oxide surfaces and the gas molecules.

2.5. Other TiO2 nanostructures

Various reports have developed various morphologies of TiO2nanomaterials to use as gas sensors, such as the amorphous butterfly wing structure,bowl-like structure,spongy layers,and other novel hierarchical architectures obtained using various preparation methods. It is worth mentioning that the hierarchical architecture is a high-dimensional complex structure that is self-assembled from several basic subunits.The hierarchical architectures can contribute to the gas molecules'diffusion and promote sensing performance due to their unique structures, high surface area, coupling effect, extraordinary shell permeability,and well-defined interior voids.Meanwhile,this unique structure can efficiently prevent the agglomeration of basic units and exert the utility of the active sites [78,79]. Fig. 5 presents the different morphologies of TiO2nanostructures. Abundant nanostructures can exhibit unique response characteristics to different types of gases. Zhang et al.[80],Fig.5b,reported a novel bowl-like TiO2submicron particle via an electrospray technique combined with high-temperature calcination.The pristine TiO2has a response to xylene at the optimal operating temperature of 302°C.The fast response and recovery times are approximately 12 and 2 s, respectively. The fast response/recovery times benefit from the bowl-like particle's numerous macropores allowing the target gas to quickly diffuse in and out of the particles by molecular diffusion. Mei et al.[81]Fig.5c designed a pine-branch-like TiO2nanostructure with a core-shell hierarchical structure via electrospinning and a hydrothermal reaction for the detection of ethanol at 375°C. The pine-branch-like morphology consists of numerous 3D hollow holes. They can provide a large number of channels for molecular diffusion and adsorption in both the inner and surface regions. Furthermore, the separately dispersed fibers perform charge conduction between other fibers along the boundary of the fiber; thus the sensor's response to ethanol affected the overall resistance. Yang et al. [82] Fig. 5d shows the amorphous/nanocrystal hybrid TiO2with a butterfly wing structure via a controlled self-deposition sintering method.This unique structured sensor provides sufficient and effective gas diffusion channels and gas molecule reaction sites,exhibits better repeatability and long-term stability to trace acetone at room temperature.

Fig. 5. Diverse morphologies of TiO2 nanostructure. (a) The hierarchical TiO2 particles synthesized by a facile solvothermal reaction [83]; (b) Bowl-like TiO2 submicron scale particles prepared via a simple electrospray technique combined with high-temperature calcination [81]; (c) Pine-branch-like TiO2 synthesized via electrospinning and a hydrothermal reaction [82]; (d) TiO2 with amorphous butterfly wing structure developed using self-deposition sintering method employing princeps paris linnaeus wings as templates [80]; (e) TiO2 nanotubes from the ZnO templates using TiCl4 as a precursor during the pulsed deposition; (f) TiO2 nanoforests formed via 400-cycle surface-reaction-limited pulsed chemical vapor deposition growth [84]. Images (a), (b) and (d) adopted with the permission from Royal Society of Chemistry. Image (c) adopted with the permission from Elsevier. Images (e) and (f) adopted with the permission from American Chemical Society.

Table 1 summarizes the gas sensing properties of different dimensional TiO2nanostructures in recent years. TiO2with numerous nanostructures can exhibit unique response characteristics to different gases. TiO2nanostructures with the same morphology will respond to different gases.

3. TiO2 based nanocomposite gas sensors

The effect of the different morphologies of TiO2nanomaterials on their gas response has been discussed above.However,pristine TiO2still shows a limitation of large band gaps, resulting in a low response and poor selectivity of TiO2based gas sensors towards detecting hazardous gas molecules.Therefore,the multi-component composite nanomaterials based on TiO2have long been considered a feasible method to manipulate gas sensing performances. Multi-component composite nanomaterials possess the properties of the single component and generate new properties due to the synergistic effect and strong interaction at the nanoscale interface between each component. In this way, more active sites can be utilized.The active sites are conducive to the adsorption and diffusion of gas molecules on the composite nanomaterials. In this section, the influences of different TiO2based nanocomposites on the sensing properties and their sensing mechanisms with respect to loaded noble metals, doped elements, construct heterojunctions, and compounds with other functional materials will be discussed.

Table 1 The gas sensing properties of different dimensional TiO2 nanostructures.

3.1. Loading noble metals

It has been experimentally proven that noble metals (NMs) such as Ag, Au, Pd, Pt, and Rh nanoparticles loaded onto the surfaces of TiO2nanostructures could enhance the gas sensing performance [85]. The noble metal's ability to enhance sensing mechanisms is primarily due to the effects of electronic and chemical sensitization[86].

Electronic sensitization is defined as the noble metal nanoparticles which decorate the surface of TiO2nanostructures and change the electron accumulation due to the different work functions between the noble metals and TiO2[87,88]. This results in a transfer of electrons between them until the two systems attain equilibrium forming a new Fermi level[89] and an electron depletion layer on the surface of NMs/TiO2composites. This phenomenon directly contributes to a higher resistance of the NMs/TiO2composites in atmospheric air,becoming more sensitive to a change in resistance. When the NMs/TiO2composites contact the reducing gas molecules,the adsorbed oxygen species(O2-,O-,and O2-)on the surface of NMs/TiO2composites become involved in the reaction with the reducing gas molecules. They then free the electrons back into the conduction band of NMs/TiO2composites, which leads to a significant change in resistance.

Noble metals are often used as effective catalysts,known as chemical sensitization[90].When they are loaded onto the surface of metal oxides,the capacity of adsorbing oxygen molecules is increased, which accelerates the reaction between the adsorbed oxygen species and the reducing gas molecules.Furthermore,the reducing gas molecules will be adsorbed and activated on the noble metals surface, which then undergoes rapid redox reactions with the adsorbed oxygen species promoting the sensing response.

David et al.[91]discussed the effects of Ni,Pd,and Pt nanoparticles on TiO2nanotube thick films used to sense hydrogen.They found that the sensitivity of Pd/TiO2NTs is highest compared to Pt/TiO2NTs and Ni/TiO2NTs when sensing 1000 ppm hydrogen at an operating temperature of 150°C, while Ni/TiO2NTs performed the worst. The XPS study shows that Pd's oxide surface reduces elements and causes the generation of Ti3+ions within the TiO2nanotubes via hydrogen spillover.In the case of Pt/TiO2NTs,only the reduction of the oxide layer of the Pt nanoparticle takes place without any changes in TiO2NTs.Ni/TiO2NTs,reduce the oxide surface over Ni but did not reduce Ti4+to Ti3+within the TiO2nanomaterial. Relative to Ni 3d or Pt 5d, the overlap of Pd 4d orbitals with the conduction band of TiO2along with the spillover effect is responsible for the high sensitivity.

Liu et al.[92]synthesized anatase TiO2quantum dot clusters(QDs)via a hydrolysis method and were further decorated by different amounts(0-5%)of Ag.The 3%Ag-decorated TiO2QDs gas sensor displayed a 25.1 response value to 20 ppm ammonia at room temperature,which is 6 times higher than pure TiO2QDs. The Ag acts as the adsorbent catalyst and promotes the reaction between ammonia and the surface of Ag-TiO2QDs.

To further reveal the sensing mechanism, density functional theory(DFT) is performed to study the target gas molecules’ adsorption structure and binding energy on NMs/TiO2composites. Sun et al. [93]simulated the adsorption energies of three characteristic decomposition products of SF6: SO2, SOF2, and SO2F2on Pd atom modified TiO2employing DFT. First, the researchers built and optimized various structure models of intrinsic Pd-TiO2and gas adsorbed Pd-TiO2and found the most stable adsorption structure. Second, the band structure,density of state, and the molecular orbits of SO2, SOF2, and SO2F2on Pd-TiO2were further analyzed. The calculation results showed that the adsorption energies of SO2,SOF2,and SO2F2on Pd-TiO2reached-1.921 eV, -2.437 eV, and -2.550 eV, respectively which is attributed to the high chemical activity of the doped Pd atoms.This study can be applied to identify the species and concentrations of SF6decomposed products under electric discharge and provide a theoretical basis to complement the sensing mechanism analysis.

Table 2 summarizes the gas sensing properties of various NMs/TiO2nanocomposites in recent years.

3.2. Doping elements

Elemental doping refers to incorporating elements into the crystal lattice of semiconductor metal oxides in the form of ion substitution.The introduction of impurity levels change the original crystal parameters of semiconductor metal oxides, and at the same time, produces many surface defects (dangling bonds), reducing the grain size, increasing the active surface sites of the sensitive materials, and is more conducive to gas adsorption and reaction [103]. In addition, the doping method can remove the electrons from the top of the valence band or can inject the electrons at the bottom of the conduction band, thus reducing the forbidden bandwidth of the semiconductor material improving the gas sensing performance[104,105].Various elements are used as dopants in TiO2. The recent dopants mainly include two categories: metals doping and non-metals doping.Metal doping(Nb,Co,Zr,Mn,Ni,Cu,Ta,Cr,W,Fe, Al, Y, Hg) can enhance gas response and selectivity of the TiO2hindering, or promoting phase transformation, altering surface potentials,enhancing chemical activity,improving the amount of adsorbed oxygen ions, adjusting the band width, and regulating charge carrier concentration [106-108]. Non-metal doping includes C, N, and F, which can reduce the energy gap of TiO2, promoting the formation of oxygen vacancies and active functional groups,increasing the surface acidity in line with Ti3+ion, enhancing the catalytic activity, and upsurge the charge carrier concentration or defects [109-111], which are beneficial in increasing the gas sensing properties.

Wang et al. [112]reported the La and V co-doped TiO2powders via sol-gel technology. The composites exhibited a better selectivity to methylbenzene at 300°C. There was an apparent response effect for methylbenzene when the La concentration increased. The V-doped composite causes an increase in conductance and the La-doped composite suppresses the transformation from the anatase to the rutile phase of TiO2.Zhao et al.[113]prepared the C-doped TiO2nanoparticles using a solvothermal method and subsequent calcination. Carbon doping significantly enhances the response of TiO2nanoparticles to alcohol vapors when compared to pure TiO2at 170°C. This phenomenon can be ascribed to the appropriate amount of carbon doping, enhancing the sensor's electrical conductivity and accelerating the electron transfer speed on the surface of the material,thus increasing the gas response.

Table 3 summarizes the gas sensing properties of various metal or non-metal doped TiO2sensors.

3.3. Constructing heterojunctions

At present, sensing materials mainly include n-type and p-type materials. The n-type materials possess a relatively higher sensitivity compared with the p-type materials [131]. Simultaneously, the researchers have revealed that p-type materials exhibit better gas selectivity and lower operating temperature over n-type materials.Meanwhile,it has been reported that the response of the p-type materials to a target gas is usually equivalent to the square root of the response of the n-type materials when they have identical morphological structures and size [132,133]. To date, three types of heterojunctions can be constructed,including n-n type,p-n type,and p-p type,when the n-type and p-type materials are compounded. Among them, n-n type and p-n type sensing materials are commonly reported. The heterojunctions can effectively rectify the electron transfer at the two materials contact surface and increase the interface barrier due to their different Fermi levels,which can significantly improve the gas sensitivity of the composite sensing material. Meanwhile, the heterojunctions can produce more O vacancies,and more O2molecules can be absorbed,thus,improving the sensing properties[134].

For the n-n type heterojunction,its sensing mechanism is equivalent to the loaded noble metals’ electronic sensitization. The difference in work function between the TiO2and other n-type sensing materials will induce the directional transfer of electrons from a lower work function to a higher one via the balancing of the Fermi levels when they come into contact with each other [135-137]. As a result, an electron depletion layer on the material with low work function and an electron accumulation layer is formed. The electron accumulation layer promotes the oxygen adsorption on the surface of sensing materials. In contrast, the electron depletion layer increases the potential energy barrier at the interface leading to an increase in the initial resistance of the composites,enhancing the gas response [138,139]. Fig. 6 presents the energy band diagram of TiO2-WO3 n-n type heterojunction at the interface[140].

Table 2 The gas sensing properties of various NMs/TiO2 nanocomposites.

Table 3 The gas sensing properties of various metals or non-metals doped TiO2 sensors.

Fig.6. The formation of an n-n type heterojunction at the interface between TiO2 and WO3.(a)the directional transfer of electrons;(b)the formation of an electron depletion layer in TiO2 and an electron accumulation layer in WO3. Images (a) and (b) adopted with the permission from Elsevier.

For the n-p type heterojunction, the electrons will transfer from ntype TiO2to p-type sensing materials.The holes will flow in the opposite direction due to a more significant concentration gradient of charge carriers between the two materials. Thus, electron and hole recombination will occur at the two materials’ interface to form a depletion layer and resulting in band bending. The height of the potential barrier between the crystal grains hinders the transfer of charge carriers and results in an initial resistance increase [141]. Fig. 7 shows the energy band diagram of TiO2-CuO n-p type heterojunction at the interface[142].Zhang et al. [143] reported the TiO2-ZnCo2O4n-p type heterojunction composite porous nanorods. The sensor could realize selective detection of formaldehyde and triethylamine at 100 ppm by controlling the working temperature at 130°C and 220°C.They indicated that the improved gas sensitivity was also ascribed to the lattice mismatch, and oxygen vacancies formed near the region of n-p heterojunction, forming potential active sites for the gas sensing reaction.

Various TiO2-based heterojunctions sensing materials have been investigated in the last five years, such as n-n type: TiO2-ZnO,TiO2-SnO2,TiO2-WO3,TiO2-InVO4,TiO2-ɑ-Fe2O3,TiO2-Nb2O5and n-p type: TiO2-CuO, TiO2-PANI, TiO2-PPy, TiO2-MgO, TiO2-Co3O4,TiO2-NiO. The general conditions and results are reported in Tables 4 and 5.

3.4. Compounding with other functional materials

The composite of TiO2with other functional materials is another effective way to improve gas sensing properties.The functional materials include carbon-based materials such as reduced graphene oxide (rGO)and multi-walled carbon nanotubes(MWCNT),both having fast electron transport dynamics[175,176],a high surface area ratio,unique electrical and mechanical properties [177], which increase the sensitivity of the TiO2-based gas sensors.Murali et al.[128]reported that nitrogen-doped graphene quantum dots(NGQDs)decorating TiO2nanoplates efficiently detect nitrogen monoxide(NO)at room temperature.The response of the TiO2@NGQDs composite in detecting 100 ppm NO is 4.8 times higher when compared to that of pristine TiO2nanoplates.The reason could be attributed to the conjugated π structure, dangling bond defects at the edge of graphene layers, and the electron-rich nitrogen atoms in the NGQDs upsurged charge carrier concentration and defects,making them more favorable ins detecting NO. Meanwhile, the formation of TiO2@NGQD heterojunctions is another key reason for the superior gas sensing properties. To date, the TiO2/carbon-based composite sensors have been widely used to detect sulfur dioxide[175],carbon monoxide[178,179],carbon dioxide,hydrogen sulfide[180],methanol[181],and ammonia[182].

The transition metal dichalcogenides (TMDs) are emerging 2D nanomaterials owing to their excellent electrical conductivity, high surface area ratio, low operating temperature, and small bandgap [183]making them ideal functional materials. Pan et al. [184] reported the TiO2/WSe2nanocomposite that had a high response to ethanol gas and an extremely fast response/recovery times (2 s/1 s under 30 ppm) at room temperature.WSe2provided a transmission path for charge transfer due to its high carrier mobility and small natural band gap. In contrast,the synergy effect, the large adsorption surface area of the binary nanostructures,and the p-n heterojunction between the TiO2.and WSe2enhanced the ethanol sensing performance.

Besides, MXene Ti3C2Tx[185], porphyrins [186,187], organic [188,189], and metallic compounds [190-192] have also been used to functionalize TiO2in recent years. Researchers have found that these functional materials in TiO2can increase the low conductivity of titanium,improve the surface area,catalytic properties,and the adsorption capacity of the pristine TiO2, which results in high gas sensitivity,selectivity,and low operating temperatures [193]for gas detection.

4. TiO2 based light-assisted gas sensors

Recently,light-assisted gas sensors have shown promise for the activation of TiO2-based gas sensors.The light illumination can increase the conductivity of TiO2by exciting electrons from the valence band into the conductive band,hence promoting the redox reaction between TiO2and gas molecules [194]. Light-assisted gas detection has been extensively researched in order to improve the sensitivity and response/recovery speed of TiO2-based gas sensors at a low operating temperature. In addition,choosing a proper light source is of great importance.TiO2with a bandgap around 3 eV has been reported to be activated under the photon energy of visible-near UV light [195,196].

Lo et al. [197] compared the gas sensing properties of Ag-TiO2composite film to ozone under UV light and blue light. They found that both lights excite the electrons of the composite film and facilitates gas molecule absorption. However, the gas response of the composite film increases by roughly 6 and 4 times under UV and blue light irradiation.Which indicates that the UV light provides a higher power for detecting ozone.

Sabri et al. [198] reported a fractal like TiO2nanomaterial as a gas sensor for detecting acetone under UV light illumination.The gas sensing properties of TiO2under 365 nm and 60W/m2UV light illumination and dark conditions are compared.The results show that the sensor exhibits a greater than a 3 times response magnitude towards 1.2 ppm acetone when undergoing light-assisted sensing compared to dark conditions at 325°C. In addition, the repeatability and response/recovery speeds are improved significantly under UV exposure.The excellent performance is ascribed to the UV-assisted process leading to a significant increase in the number and density of electrons on the fractal like TiO2layer. Thus,increasing the amount of adsorbed oxygen ions and active sites resulting in higher reaction rates between the sensing material and gas molecules.

Xie et al. [199] investigated the NO2sensing properties of the TiO2films under UV illumination at room temperature. Under UV illumination,the film shows a reversible and intense NO2response compared to the dark conditions. Moreover, the response/recovery speeds are also increased under UV. Meanwhile, the current to NO2remains constant under UV illumination in the two cycles,unlike the linear shifting of the current under dark conditions. These remarkable differences indicate that the UV light enhances the NO2sensing of the TiO2films,which could be ascribed to the UV light enhancing the interaction between NO2and TiO2via abundant photogenerated free electrons.

5. Conclusions and outlook

TiO2has attracted much attention in the field of gas sensors due to its catalytic properties,chemical stability,is non-toxic,and the modulation in its structural and transport properties. This review has presented the gas sensing characteristics of TiO2nanostructured materials. First, the effect of different morphologies of pristine TiO2on their gas sensitivity including zero to three-dimensional nanostructures and other interesting hierarchical structures have been introduced. Second, the sensing mechanisms of TiO2-based composites and the sensing properties of TiO2-based light-assisted gas sensors have also been discussed. Finally,several studies have been summarized to demonstrate the comparative sensing properties of TiO2-based gas sensors.

Fig. 7. The formation of an n-p type heterojunction at the interface between TiO2 and CuO. (a) the transfer between electrons and holes; (b) the formation of a depletion layer. Images (a) and (b) adopted with the permission from Elsevier.

According to this review, various synthesis strategies of TiO2nanostructured materials have been reported. The diverse morphologies of TiO2can be easily tuned by regulating the synthesis/manufacturing conditions. It can be concluded that different dimensional TiO2nanostructured materials exhibit unique sensing characteristics for detecting gases. 0D nanostructured TiO2materials ultrasensitive and highly miniaturized chemical reactions are due to the large specific surface area and can achieve fast response/recovery times for the detection of various gases.The 1D nanostructured TiO2materials are capable of accelerating electron transport rates and help to enhance gas sensitivity.In addition,crystal facet engineering in 2D nanostructured TiO2materials is vital for determining the surface reactivity and selectivity. According to the current reports, the {001} crystal facets of TiO2has a propensity for responding to acetone due to the interface coupling between them.Other 3D and novel hierarchical architectures have extraordinary permeability to gases and numerous adsorption sites for oxygen species.

Some strategies such as loading noble metals,doping elements,constructing heterojunctions,compounding with other functional materials,and light-assisted detection have been utilized to improve the gas sensing properties of TiO2sensors.Generally,noble metal nanoparticles decorate the surface of TiO2nanostructures and change the electron accumulation and enhance the catalytic effect, resulting in the effects of electronic sensitization and chemical sensitization.Elemental doping increases the active surface sites and reduces the forbidden band width of TiO2,enhancing the sensitivity and selectivity. Another effective strategy includes heterojunction constructs and compounds with various other functional materials. The heterojunctions can effectively rectify the electron transfer at the two materials’ contact surface, improving the sensitivity. Meanwhile, the composite of TiO2with other functional materials is expected to achieve a higher gas response at low temperatures due to the synergy effect and material defects. The light-assisted sensors stimulate the electron transport of TiO2, leading to the improvement of its gas sensing properties.

Numerous reports have demonstrated the exceptional gas sensing properties of TiO2-based gas sensors. In most cases, some deficiencies with respect to high working temperatures, poor selectivity, and low sensitivity still limit their practical applications. Hence, for future research, more efforts should be made to coordinate nanostructure and component composition,further developing novel TiO2-based composite gas sensing materials.

Table 5 The gas sensing properties of TiO2-based n-p heterojunctions sensors.

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 Yunnan Provincial Department of Science and Technology through 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.