Electrospinning Hetero-Nanofibers In2O3/SnO2 of Homotype Heterojunction with High Gas Sensing Activity

In2O3/SnO2 composite hetero-nanofibers were synthesized by an electrospinning technique for detecting indoor volatile organic gases. The physical and chemical properties of In2O3/SnO2 hetero-nanofibers were characterized and analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Energy Dispersive X-Ray Spectroscopy (EDX), specific surface Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy (XPS). Gas sensing properties of In2O3/SnO2 composite hetero-nanofibers were measured with six kinds of indoor volatile organic gases in concentration range of 0.5~50 ppm at the operating temperature of 275 °C. The In2O3/SnO2 composite hetero-nanofibers sensor exhibited good formaldehyde sensing properties, which would be attributed to the formation of n-n homotype heterojunction in the In2O3/SnO2 composite hetero-nanofibers. Finally, the sensing mechanism of the In2O3/SnO2 composite hetero-nanofibers was analyzed based on the energy-band principle.


Introduction
With the aggravation of environmental pollution and the deterioration of the climate, human beings have put forward higher requirements on the gas detection of living environment. Semiconductor metal oxides have been investigated extensively and applied widely in the various fields of gas detection technology for its low cost, high accuracy, small size and easy to carry [1][2][3][4]. There are many parameters and factors that affect and limit gas sensor applications, such as adsorption ability, catalytic activity, sensitivity, selectivity, thermodynamic stability, and so on [5].
Among the various semiconductor gas sensitive materials, SnO 2 is used popularly as gas sensors because of excellent adsorption ability, high sensitivity at relatively low operating temperatures, but there is a challenge to improve the sensing performance due to the cross-sensitivity of SnO 2 gas sensors [6,7]. To enhance the gas sensing properties of SnO 2 , doping or surface modification methods have been adopted [8,9]. In addition, the preparation of composite semiconductor metal oxides attracts more attention [10,11]. The physical interface between two dissimilar materials is often referred to as a heterojunction. The new composite incorporates two components forming a heterostructure. Gas sensors based on the composite are more sensitive than the individual components due to the interface heterostructure and a synergistic effect between the two components [5,12]. Some researchers found that the heterojunction acts as a lever in electron transfer, through which electron transport is facilitated or restrained resulting in superior sensing performance [13][14][15][16][17][18]. Recently, many heterostructure composites such as SnO 2 /ZnO [19,20], TiO 2 /SnO 2 [21], SnO 2 /Fe 2 O 3 [22] have been prepared to be applied as gas sensing materials to obtain the sensitive and selective gas sensors. In 2 O 3 and SnO 2 heterostructure composites have also been synthesized to enhance the properties of sensors [23][24][25]. In 2 O 3 /SnO 2 heterojunction microstructures synthesized by a facile room temperature solid-state reaction route exhibited high response to chlorine [26]. A rapidly responding methanol sensor based on electrospinning In 2 O 3 /SnO 2 nanofibers was reported [27]. Many researchers have found that In 2 O 3 can be used to effectively limit grain growth of the SnO 2 host nanoparticles in the high temperature synthesis process to affect gas sensitivity and selectivity [28]. However, to the best of our knowledge, few reports have shown that In 2 O 3 -SnO 2 heterostructure composites have an excellent response and selectivity for formaldehyde.
In this manuscript, In 2 O 3 /SnO 2 composite hetero-nanofibers with n-n-type heterojunction were synthesized successfully by a mixed electrospinning method. Gas sensors based on In 2 O 3 /SnO 2 composite hetero-nanofibers were fabricated for detecting low concentration indoor volatile organic gas. Six kinds of indoor volatile organic gases (including formaldehyde, ethanol, acetone, methanol, toluene and ammonia) were tested based on In 2 O 3 /SnO 2 composite hetero-nanofibers sensors by a static gas test method. It was found that In 2 O 3 /SnO 2 composite hetero-nanofibers sensors show excellent formaldehyde sensing properties superior to the pure SnO 2 or In 2 O 3 nanofibers sensors at a low operating temperature of 275 • C, which could be attributed to the formation of n-n homotype heterojunction in the In 2 O 3 /SnO 2 composite hetero-nanofibers.

Preparation and Characterization of In 2 O 3 /SnO 2 Composite Hetero-Nanofibers
All chemical reagents used were of the analytical grade in the experiments; 0.76 g indium nitrate (In(NO 3 ) 3 ·4.5H 2 O), 0.45 g stannous chloride (SnCl 2 ·2H 2 O) and 10 ml ethanol were mixed and stirred vigorously for 1 h. The molar ratio of In to Sn in the composites is 1:1. Then, 1 g Polyvinyl pyrrolidone (PVP, M w = 1,300,000) and 8 ml N, N Dimethylformamide (DMF) were added to the mixture solution and stirred for 3 h until dissolved evenly at the room temperature. The colorless and transparent mixed electrospinning precursor solution was formed.
The mixed precursor solution was loaded into the glass syringe with a metal jet. A high DC voltage was applied between the jet and the collector. The precursor was ejected from the jet and formed a wet nanofibers net under the electric force. Then, the net was heated at 600 • C for 120 min in the air using a muffle furnace with temperature control system. PVP and water in the polymer nanofibers volatilized during the annealing process. Finally, the light yellow In 2 O 3 /SnO 2 composite hetero-nanofibers were obtained. Pure SnO 2 and In 2 O 3 electrospinning nanofibers were synthesized by using a traditional electrospinning device, respectively. The annealing process of SnO 2 and In 2 O 3 electrospinning nanofibers were the same as that of In 2 O 3 /SnO 2 composite hetero-nanofibers.
The phase structure and morphology of the In 2 O 3 /SnO 2 composite hetero-nanofibers were characterized by an X-ray diffraction instrument (XRD: D/Max 2400, Rigaku, Japan) in 2θ region of 20~80 • with Cu Kα1 radiation and a field emission scanning electron microscope (FE-SEM: Hitachi S-4800, Japan), respectively. The composition and contents were analyzed by using an Energy Dispersive X-Ray Spectroscopy (FE-SEM: Hitachi S-4800, Japan). The specific surface was measured by a Brunauer-Emmett-Teller (BET: Quanta AUTOSORB-1-MP, Boynton Beach, FL, USA) method. The chemical composition and surface state were examined using high-resolution X-ray photoelectron spectroscopy (XPS) on a spectrometer (XPS: Thermo ESCALAB 250Xi, Westmont, IL, USA). The AC impedance spectroscopy of electrochemical properties was carried out and analyzed on an impedance analyzer (B1500A, Agilent, Santa Clara, CA, USA).

Sensors Fabrication and Measurements
In 2 O 3 /SnO 2 composite hetero-nanofibers were mixed with deionized water to form a paste. The paste was coated onto the surface of a ceramic tube with a pair of gold electrodes to form a sensitive film (about 300 µm thick) and dried in hot air, then, annealed at 500 • C for 2 h in open air. Finally, a Ni-Cr heating wire used as a heating electrode was inserted into the ceramic tube to form an inside-heated gas sensor. The inside-heated gas sensor picture is shown in Figure 1a. Indoor volatile organic gases were measured based on In 2 O 3 /SnO 2 composite hetero-nanofibers sensor using a static-state gas-sensing test method [17]. In the detection process of volatile organic gases, a given amount of target gas was injected into a hermetic chamber (50 L in volume) by a syringe through a rubber plug. For a required concentration, the volume of the injected gas (V) can be calculated by theEquation (1) [29]: where C is the concentration of the target gas (ppm), and v% is the volume fraction of bottled gas injected from the gas inlet hole. The gas response (S) was defined as a ratio of the electrical resistance of the sensor in the air (R a ) to that in target gas (R g ): S = R a /R g [17]. A structure schematic diagram of the static gas testing system was shown in Figure 1b.

Sensors Fabrication and Measurements
In2O3/SnO2 composite hetero-nanofibers were mixed with deionized water to form a paste. The paste was coated onto the surface of a ceramic tube with a pair of gold electrodes to form a sensitive film (about 300 μm thick) and dried in hot air, then, annealed at 500 °C for 2 h in open air. Finally, a Ni-Cr heating wire used as a heating electrode was inserted into the ceramic tube to form an inside-heated gas sensor. The inside-heated gas sensor picture is shown in Figure 1a. Indoor volatile organic gases were measured based on In2O3/SnO2 composite hetero-nanofibers sensor using a static-state gas-sensing test method [17]. In the detection process of volatile organic gases, a given amount of target gas was injected into a hermetic chamber (50 L in volume) by a syringe through a rubber plug. For a required concentration, the volume of the injected gas (V) can be calculated by theEquation (1) [29]: where C is the concentration of the target gas (ppm), and v% is the volume fraction of bottled gas injected from the gas inlet hole. The gas response (S) was defined as a ratio of the electrical resistance of the sensor in the air (Ra) to that in target gas (Rg): S = Ra/Rg [17]. A structure schematic diagram of the static gas testing system was shown in Figure 1b.

Characterization
The XRD patterns of the pure SnO2, In2O3 nanofibers and In2O3/SnO2 composite hetero-nanofibers are shown in Figure 2a Figure 2c, both SnO2 and In2O3 exist in the In2O3/SnO2 composite hetero-nanofibers. According to the results calculating by the Debye-Scherrer equation [30]: where τ is the size of the crystalline domains, which may be smaller or equal to the grain size; K is the dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9; λ is the X-ray wavelength; β is the line broadening at half the maximum intensity, after

Characterization
The XRD patterns of the pure SnO 2 , In 2 O 3 nanofibers and In 2 O 3 /SnO 2 composite hetero-nanofibers are shown in Figure 2a According to the results calculating by the Debye-Scherrer equation [30]: where τ is the size of the crystalline domains, which may be smaller or equal to the grain size; K is the dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about crystallite size of In2O3/SnO2 composite hetero-nanofibers is much smaller than that of pure In2O3 nanofibers and the same as that of the pure SnO2 nanofibers.  There is a big difference among the morphologies and structures of the three kinds of nanofibers. In the process of nanofibers formation, the forces of each nanoparticle will be influenced by many factors, such as the surface tension, solubility, electric field force and so on. Of the factors  crystallite size of In2O3/SnO2 composite hetero-nanofibers is much smaller than that of pure In2O3 nanofibers and the same as that of the pure SnO2 nanofibers.  There is a big difference among the morphologies and structures of the three kinds of nanofibers. In the process of nanofibers formation, the forces of each nanoparticle will be influenced by many factors, such as the surface tension, solubility, electric field force and so on. Of the factors that affect the morphology and structures, the conductivity of salt solutions must be considered. The conductivities of SnCl2 solution, In(NO3)3 solution and the mixed solution are different, which will result in the great difference of the electric field force and the Coulomb force of the nanofibers ejected from the spinning jet. These will have an effect on the diameters and morphologies of the nanofibers. At the same time, SnCl2 and In(NO3)3 are mixed sufficiently, the distribution of Sn and In ions in the solution is relative uniformity. The arrangement and distribution of two kinds of There is a big difference among the morphologies and structures of the three kinds of nanofibers. In the process of nanofibers formation, the forces of each nanoparticle will be influenced by many factors, such as the surface tension, solubility, electric field force and so on. Of the factors that affect the morphology and structures, the conductivity of salt solutions must be considered. The conductivities of SnCl 2 solution, In(NO 3 ) 3 solution and the mixed solution are different, which will result in the great difference of the electric field force and the Coulomb force of the nanofibers ejected from the spinning jet. These will have an effect on the diameters and morphologies of the nanofibers. At the same time, SnCl 2 and In(NO 3 ) 3 are mixed sufficiently, the distribution of Sn and In ions in the solution is relative uniformity. The arrangement and distribution of two kinds of nanoparticles in the nanofibers are uniform. Figure 4 gives the Energy Dispersive X-ray (EDX) spectra of the In 2 O 3 /SnO 2 composite hetero-nanofibers. It can be observed in Figure 4 that O, In, and Sn elements are present in the spectra of the In 2 O 3 /SnO 2 composite hetero-nanofibers.    Figure 5 gives the N2 isotherms adsorption-desorption curves of In2O3/SnO2 composite hetero-nanofibers. As shown in Figure 5, the N2 isotherms adsorption-desorption curves of In2O3/SnO2 composite hetero-nanofibers belong to the IV-type isotherms, which demonstrates that In2O3/SnO2 composite nanofibers are the mesoporous solid materials. The adsorption curve and desorption curve are different. H3-type hysteresis loop is shown in Figure 5, which means that the whole adsorption process can be divided into the monolayer adsorption process of the first zone, the multilayer adsorption process of the second zone, and the capillary condensation process of the third zone. The adsorption process belongs to the weak saturation adsorption [31]. According to the inset in Figure 5, the specific surface (BET) of In2O3/SnO2 composite hetero-nanofibers is 16.67 m 2 /g, and the pore distribution is in the range of 2.3-9.5 nm. The pore distribution of In2O3/SnO2 composite hetero-nanofibers spreads out widely and disorderly. The saturated zone is not obvious. The results show that the In2O3/SnO2 composite hetero-nanofibers adsorption mechanism is disorder mesoporous adsorption. This kind of hierarchically composite fibers composed of small nanoparticles does not show significant absorption limit in the relatively high-pressure region.   Figure 5 gives the N 2 isotherms adsorption-desorption curves of In 2 O 3 /SnO 2 composite hetero-nanofibers. As shown in Figure 5, the N 2 isotherms adsorption-desorption curves of In 2 O 3 /SnO 2 composite hetero-nanofibers belong to the IV-type isotherms, which demonstrates that In 2 O 3 /SnO 2 composite nanofibers are the mesoporous solid materials. The adsorption curve and desorption curve are different. H3-type hysteresis loop is shown in Figure 5, which means that the whole adsorption process can be divided into the monolayer adsorption process of the first zone, the multilayer adsorption process of the second zone, and the capillary condensation process of the third zone. The adsorption process belongs to the weak saturation adsorption [31]. According to the inset in Figure 5, the specific surface (BET) of In 2 O 3 /SnO 2 composite hetero-nanofibers is 16.67 m 2 /g, and the pore distribution is in the range of 2.3-9.5 nm. The pore distribution of In 2 O 3 /SnO 2 composite hetero-nanofibers spreads out widely and disorderly. The saturated zone is not obvious. The results show that the In 2 O 3 /SnO 2 composite hetero-nanofibers adsorption mechanism is disorder mesoporous adsorption. This kind of hierarchically composite fibers composed of small nanoparticles does not show significant absorption limit in the relatively high-pressure region.
composite hetero-nanofibers spreads out widely and disorderly. The saturated zone is not obvious. The results show that the In2O3/SnO2 composite hetero-nanofibers adsorption mechanism is disorder mesoporous adsorption. This kind of hierarchically composite fibers composed of small nanoparticles does not show significant absorption limit in the relatively high-pressure region. To further confirm the chemical composition and surface oxidation state, the X-ray photoelectron spectroscopy (XPS) of In2O3/SnO2 composite hetero-nanofibers is performed, and the To further confirm the chemical composition and surface oxidation state, the X-ray photoelectron spectroscopy (XPS) of In 2 O 3 /SnO 2 composite hetero-nanofibers is performed, and the XPS survey spectrum of In 2 O 3 /SnO 2 composite hetero-nanofibers is shown in Figure 6. As expected, the XPS spectrum of In 2 O 3 /SnO 2 is dominated by the lines of In, Sn, O and C. The C element is attributed to adventitious carbon-based additives and the C1s, whose banding energy peak locating at 283.08 eV, is used as reference for calibration. There are nine main peaks in the spectrum of In 2 O 3 /SnO 2 composite hetero-nanofibers, and they are Sn3p 1/2 , Sn3p 3/2 , In3p 1/2 , In3p 3/2 , O1s, Sn3d 3/2 , Sn3d 5/2 , In3d 3/2 and In3d 5/2 , respectively. XPS survey spectrum of In2O3/SnO2 composite hetero-nanofibers is shown in Figure 6. As expected, the XPS spectrum of In2O3/SnO2 is dominated by the lines of In, Sn, O and C. The C element is attributed to adventitious carbon-based additives and the C1s, whose banding energy peak locating at 283.08 eV, is used as reference for calibration. There are nine main peaks in the spectrum of In2O3/SnO2 composite hetero-nanofibers, and they are Sn3p1/2, Sn3p3/2, In3p1/2, In3p3/2, O1s, Sn3d3/2, Sn3d5/2, In3d3/2 and In3d5/2, respectively.  As can be seen in In3d core level XPS spectrum of In2O3/SnO2 composite hetero-nanofibers (Figure 7), two binding energy peaks centered at 443.33 eV and 450.93 eV are ascribed to In3d5/2 and In3d3/2, respectively, which originate from In-O in In2O3 lattice [32]. Figure 8 shows Sn3d core level XPS spectrum of In2O3/SnO2 composite hetero-nanofibers. As shown in Figure 8, the binding energy of Sn3d3/2 and Sn3d5/2 obtained from the XPS measurements are 484.88 eV and 493.28 eV, respectively, which indicates that Sn in the composites has s valence of +4 [33]. Compared with In3d5/2 (452.09 eV) and In3d3/2 (444.54 eV) of pure In2O3, the binding energy of both In3d5/2 and In3d3/2 of In2O3/SnO2 composite hetero-nanofibers moves to higher binding energy direction. On the contrary, the binding energy of both Sn3d3/2 and Sn3d5/2 of In2O3/SnO2 composite hetero-nanofibers moves to lower binding energy direction compared with Sn3d3/2 (493.43 eV) and Sn3d5/2 (485.03 eV) of pure SnO2 nanofibers. This phenomenon, due to electron transfer, will occur from In2O3 to SnO2 until the energy-band diagram of the n-n heterojunction of In2O3/SnO2 comes to equilibrium [14,34,35]. This phenomenon can also be explained by the strong interaction between In2O3 and SnO2, which will lead to the decreased surface activity of SnO2 and the increased surface activity of In2O3 [36]. As can be seen in In3d core level XPS spectrum of In 2 O 3 /SnO 2 composite hetero-nanofibers (Figure 7), two binding energy peaks centered at 443.33 eV and 450.93 eV are ascribed to In3d 5/2 and In3d 3/2 , respectively, which originate from In-O in In 2 O 3 lattice [32]. Figure 8 shows Sn3d core level XPS spectrum of In 2 O 3 /SnO 2 composite hetero-nanofibers. As shown in Figure 8, the binding energy of Sn3d 3/2 and Sn3d 5/2 obtained from the XPS measurements are 484.88 eV and 493.28 eV, respectively, which indicates that Sn in the composites has s valence of +4 [33]. Compared with In3d 5/2 (452.09 eV) and In3d 3/2 (444.54 eV) of pure In 2 O 3 , the binding energy of both In3d 5/2 and In3d 3/2 of In 2 O 3 /SnO 2 composite hetero-nanofibers moves to higher binding energy direction. On the contrary, the binding energy of both Sn3d 3/2 and Sn3d 5/2 of In 2 O 3 /SnO 2 composite hetero-nanofibers moves to lower binding energy direction compared with Sn3d 3/2 (493.43 eV) and Sn3d 5/2 (485.03 eV) of pure SnO 2 nanofibers. This phenomenon, due to electron transfer, will occur from In 2 O 3 to SnO 2 until the energy-band diagram of the n-n heterojunction of In 2 O 3 /SnO 2 comes to equilibrium [14,34,35]. This phenomenon can also be explained by the strong interaction between In 2 O 3 and SnO 2 , which will lead to the decreased surface activity of SnO 2 and the increased surface activity of In 2 O 3 [36].
of Sn3d3/2 and Sn3d5/2 obtained from the XPS measurements are 484.88 eV and 493.28 eV, respectively, which indicates that Sn in the composites has s valence of +4 [33]. Compared with In3d5/2 (452.09 eV) and In3d3/2 (444.54 eV) of pure In2O3, the binding energy of both In3d5/2 and In3d3/2 of In2O3/SnO2 composite hetero-nanofibers moves to higher binding energy direction. On the contrary, the binding energy of both Sn3d3/2 and Sn3d5/2 of In2O3/SnO2 composite hetero-nanofibers moves to lower binding energy direction compared with Sn3d3/2 (493.43 eV) and Sn3d5/2 (485.03 eV) of pure SnO2 nanofibers. This phenomenon, due to electron transfer, will occur from In2O3 to SnO2 until the energy-band diagram of the n-n heterojunction of In2O3/SnO2 comes to equilibrium [14,34,35]. This phenomenon can also be explained by the strong interaction between In2O3 and SnO2, which will lead to the decreased surface activity of SnO2 and the increased surface activity of In2O3 [36].      Figure 9 gives O1s core level XPS spectrums of In2O3/SnO2 composite hetero-nanofibers, In2O3 nanofibers and SnO2 nanofibers, respectively. As can be seen in Figure 9a, an asymmetric peak at 528.73 eV is shown on O1s core level XPS spectrum of In2O3/SnO2 composite nanofibers. The peak of O1s can be separated into two peaks at Olat (528.73 eV) and Oads (530.28). The Olat (528.73 eV) can be assigned to the lattice oxygen on the surface of In2O3/SnO2 composite hetero-nanofibers. It will not participate in the chemical reaction in the process of gas adsorption below 300 °C. The Oads is attributed to the adsorbed oxygen of In2O3/SnO2 composite hetero-nanofibers. Adsorbed oxygen participates in the reaction with activity in the process of gas adsorption. The relative intensities of Olat and Oads contributions are 70.5% and 29.5%, respectively. Similarly, the relative intensities of Olat and Oads contributions in SnO2 nanofibers are 82.6% and 17.4%, respectively (shown in Figure 9b). The relative intensities of Olat and Oads contributions in In2O3 nanofibers are 72.7% and 27.3%, respectively (shown in Figure 9c). The proportion of adsorbed oxygen in In2O3/SnO2 composite hetero-nanofibers is higher than that in pure SnO2 nanofibers or pure In2O3 nanofibers. It may improve adsorption capacity of In2O3/SnO2 composite hetero-nanofibers, and then, increase the response sensitivity of gas sensors [37][38][39].    Figure 9c). The proportion of adsorbed oxygen in In 2 O 3 /SnO 2 composite hetero-nanofibers is higher than that in pure SnO 2 nanofibers or pure In 2 O 3 nanofibers. It may improve adsorption capacity of In 2 O 3 /SnO 2 composite hetero-nanofibers, and then, increase the response sensitivity of gas sensors [37][38][39]. and Oads contributions in SnO2 nanofibers are 82.6% and 17.4%, respectively (shown in Figure 9b). The relative intensities of Olat and Oads contributions in In2O3 nanofibers are 72.7% and 27.3%, respectively (shown in Figure 9c). The proportion of adsorbed oxygen in In2O3/SnO2 composite hetero-nanofibers is higher than that in pure SnO2 nanofibers or pure In2O3 nanofibers. It may improve adsorption capacity of In2O3/SnO2 composite hetero-nanofibers, and then, increase the response sensitivity of gas sensors [37][38][39].   Figure 10 gives the response curves of the gas sensors based on SnO2, In2O3 and In2O3/SnO2 composite hetero-nanofibers versus operating temperatures to 10 ppm formaldehyde, respectively. It can be seen that the response of In2O3/SnO2 sensor increases as the operating temperature increases from 100 °C to 275 °C. The response of the gas sensor decreases when the operating temperature increases from 275 °C to 450 °C. The response reaches the maximum value (8.7) in 10 ppm formaldehyde at the operating temperature of 275 °C. Therefore, 275 °C is chosen as the optimum operating temperature of the In2O3/SnO2 composite hetero-nanofibers sensors. Similarly, 300 °C and 350 °C are selected as the optimum operating temperatures of the pure SnO2 and In2O3 nanofibers sensors, respectively.

Gas Sensing Properties
The optimum operating temperature of the In2O3/SnO2 composite hetero-nanofibers sensors is lower than that of pure SnO2 or In2O3 nanofibers sensors, which may be explained as follows. Comparing with the pristine In2O3 and SnO2 nanofibers, In2O3/SnO2 composite hetero-nanofibers have a different crystallographic structure, which is decided by their own combination of crystallographic planes, nanocrystal framing and structure, which is decided by its own total combination of surface electron parameters. It includes surface state density; energetic position of the levels, induced by adsorbed species, adsorption-desorption energies of interacted gas molecules, concentration of adsorption surface states, position of surface Fermi level, activation energy of native point defects, and so on. Also, the gas sensitivity, operating temperature and rate of response and recovery time are magnitude by combination of surface electron parameters according to the chemisorption theory of Volkenshtein [40][41][42][43], another reason maybe arises by transport characteristics of In2O3/SnO2 hetero-nanostructures. The electron transport characteristics of In2O3/SnO2 composite hetero-nanofibers will be significantly strongly tuned by the heterojunction barrier comparing the pristine In2O3 or SnO2 due to the formation of many semiconductor heterojunctions at interfaces. The conductivity of the heterojunctions will consequently be greatly increased. Therefore, the barrier with adjustable height controls the transport of electrons in the  Figure 10 gives the response curves of the gas sensors based on SnO 2 , In 2 O 3 and In 2 O 3 /SnO 2 composite hetero-nanofibers versus operating temperatures to 10 ppm formaldehyde, respectively. It can be seen that the response of In 2 O 3 /SnO 2 sensor increases as the operating temperature increases from 100 • C to 275 • C. The response of the gas sensor decreases when the operating temperature increases from 275 • C to 450 • C. The response reaches the maximum value (8.7) in 10 ppm formaldehyde at the operating temperature of 275 • C. Therefore, 275 • C is chosen as the optimum operating temperature of the In 2 O 3 /SnO 2 composite hetero-nanofibers sensors. Similarly, 300 • C and 350 • C are selected as the optimum operating temperatures of the pure SnO 2 and In 2 O 3 nanofibers sensors, respectively.

Gas Sensing Properties
The optimum operating temperature of the In 2 O 3 /SnO 2 composite hetero-nanofibers sensors is lower than that of pure SnO 2 or In 2 O 3 nanofibers sensors, which may be explained as follows. Comparing with the pristine In 2 O 3 and SnO 2 nanofibers, In 2 O 3 /SnO 2 composite hetero-nanofibers have a different crystallographic structure, which is decided by their own combination of crystallographic planes, nanocrystal framing and structure, which is decided by its own total combination of surface electron parameters. It includes surface state density; energetic position of the levels, induced by adsorbed species, adsorption-desorption energies of interacted gas molecules, concentration of adsorption surface states, position of surface Fermi level, activation energy of native point defects, and so on. Also, the gas sensitivity, operating temperature and rate of response and recovery time are magnitude by combination of surface electron parameters according to the chemisorption theory of Volkenshtein [40][41][42][43], another reason maybe arises by transport characteristics of In 2 O 3 /SnO 2 hetero-nanostructures. The electron transport characteristics of In 2 O 3 /SnO 2 composite hetero-nanofibers will be significantly strongly tuned by the heterojunction barrier comparing the pristine In 2 O 3 or SnO 2 due to the formation of many semiconductor heterojunctions at interfaces. The conductivity of the heterojunctions will consequently be greatly increased. Therefore, the barrier with adjustable height controls the transport of electrons in the heterostructures, and accordingly controls the sensing characteristics of the nanocomposites, including the lower operating temperature [22].   Figure 11a shows the relationship of the response of In2O3/SnO2 sensor versus the concentration of formaldehyde, methanol, acetone, toluene, ammonia, ethanol vapor. The In2O3/SnO2 sensor shows the highest response to the formaldehyde vapor in the whole concentration range. The response to ethanol is lower than that to formaldehyde, and the response to ammonia is the lowest.   Figure 11a shows the relationship of the response of In 2 O 3 /SnO 2 sensor versus the concentration of formaldehyde, methanol, acetone, toluene, ammonia, ethanol vapor. The In 2 O 3 /SnO 2 sensor shows the highest response to the formaldehyde vapor in the whole concentration range. The response to ethanol is lower than that to formaldehyde, and the response to ammonia is the lowest. Figure 11b gives the transient response curves of the In 2 O 3 /SnO 2 sensor to formaldehyde in the concentration of 0.5-50 ppm. In the experiment, ten cycles were recorded. The In 2 O 3 /SnO 2 sensor shows excellent sensitivities to formaldehyde vapor. The inset shows the response and recovery time for 10 ppm formaldehyde vapor at an operating temperature of 275 • C with the relative humidity of 40% RH. It can be seen that the response and recovery time are 37 s and 42 s to 10 ppm formaldehyde vapor, respectively.
In order to compare the gas sensing properties of In 2 O 3 /SnO 2 composite hetero-nanofibers sensors with that of the SnO 2 or In 2 O 3 sensors, Figure 11c  °C with relative humidity of 40% RH. Figure 11a shows the relationship of the response of In2O3/SnO2 sensor versus the concentration of formaldehyde, methanol, acetone, toluene, ammonia, ethanol vapor. The In2O3/SnO2 sensor shows the highest response to the formaldehyde vapor in the whole concentration range. The response to ethanol is lower than that to formaldehyde, and the response to ammonia is the lowest.

The Gas Sensing Mechanism of the In 2 O 3 /SnO 2 Composite Hetero-Nanofibers Sensor
The experiment results demonstrate that the In 2 O 3 /SnO 2 sensors show the excellent formaldehyde sensing properties compared to those of SnO 2 or In 2 O 3 sensors, which means that the adsorption capability of In 2 O 3 /SnO 2 composite hetero-nanofibers sensor to formaldehyde is greatly enhanced. The reason may be explained by the mechanism of oxygen adsorption. It is well known that the reducing gas will interact with the adsorbed oxygen on the surface of the semiconductor, which will lead to the change of sensor resistance. The more surface-adsorbed oxygen exists on the gas sensing material, the greater the sensor's resistance changes when meeting the reducing volatile organic gases. Figure 12 illustrates the energy-band diagram of the In 2 O 3 /SnO 2 composite hetero-nanofibers system. As seen in Figure 12a-c, the pre-adsorbed oxygen could give rise to a depletion layer near to the surface of n-type semiconducting oxides [44], which results in a band bending around the surface. It is well known that SnO 2 and In 2 O 3 are the prototypical n-type transparent conducting oxide semiconductor. The band gap and the work function of SnO 2 are 3.59 eV and 4.9 eV, respectively [45], while the band gap and the work function of In 2 O 3 are 2.8 eV and 5.28 eV, respectively. SnO 2 has higher electron affinity (4.5 eV) than In 2 O 3 (4.45 eV). Both the bottom of the conduction band and the top of the valence band of In 2 O 3 are higher than those of SnO 2 (Figure 12d,e), since the SnO 2 has a lower work function and a strong electron affinity [46]. The transport of electrons should transport form In 2 O 3 to SnO 2 to overcome the heterojunction barriers [22,47]. When the In 2 O 3 /SnO 2 heterostructures are exposed to reducing gases, the reaction between the adsorbed oxygen species and the reducing gases leads to the release of the trapped electrons back simultaneously into the conduction bands of the In 2 O 3 and SnO 2 , resulting in a decrease in the width and height of the barrier potential at their interfaces, as shown in Figure 12e. Therefore, the conductivity of the heterojunction will consequently be greatly increased, the barrier with adjustable height controls the transport of electrons in the heterostructures, which will improve the gas sensing properties of In 2 O 3 /SnO 2 composite hetero-nanofibers sensor [12,13,46,47]. In order to confirm that n-n homotype heterojunctions exist in In2O3/SnO2 composite etero-nanofibers, electrochemical characteristics of SnO2, In2O3 and In2O3/SnO2 sensors were easured at the operating temperature of 275 °C. Figure 13 reports the double-isotype I-V haracteristics of the SnO2, In2O3 and In2O3/SnO2 sensors, respectively. We can see that I-V curves of nO2, In2O3 nanofibers are almost linear, which indicates that there is a good ohmic contact existing side of the n-type SnO2 and In2O3 sensors, respectively. Meanwhile, the I-V curve of In2O3/SnO2 nsor is nonlinear when the applied voltage drops in the reversely biased heterojunction, which is symmetric for positive and negative voltages. The slightly rectifying behavior has to be ascribed to e different shape and crystalline state of the In2O3/SnO2 of the n-n homotype heterojunctions 5,48,49]. Electrochemical characteristics of In2O3/SnO2 sensors indicate the presence of n-n eterojunctions of two-type semiconducting oxides in In2O3/SnO2 composite hetero-nanofibers 8, [50][51][52]. The sensing mechanism of homotype hetero-nanofibers sensing material needs further vestigation. In order to confirm that n-n homotype heterojunctions exist in In 2 O 3 /SnO 2 composite hetero-nanofibers, electrochemical characteristics of SnO 2 , In 2 O 3 and In 2 O 3 /SnO 2 sensors were measured at the operating temperature of 275 • C. Figure 13 reports the double-isotype I-V characteristics of the SnO 2 , In 2 O 3 and In 2 O 3 /SnO 2 sensors, respectively. We can see that I-V curves of SnO 2 , In 2 O 3 nanofibers are almost linear, which indicates that there is a good ohmic contact existing inside of the n-type SnO 2 and In 2 O 3 sensors, respectively. Meanwhile, the I-V curve of In 2 O 3 /SnO 2 sensor is nonlinear when the applied voltage drops in the reversely biased heterojunction, which is asymmetric for positive and negative voltages. The slightly rectifying behavior has to be ascribed to the different shape and crystalline state of the In 2 O 3 /SnO 2 of the n-n homotype heterojunctions [35,48,49]. Electrochemical characteristics of In 2 O 3 /SnO 2 sensors indicate the presence of n-n heterojunctions of two-type semiconducting oxides in In 2 O 3 /SnO 2 composite hetero-nanofibers [48,[50][51][52]. The sensing mechanism of homotype hetero-nanofibers sensing material needs further investigation. the different shape and crystalline state of the In2O3/SnO2 of the n-n homotype heterojunctions [35,48,49]. Electrochemical characteristics of In2O3/SnO2 sensors indicate the presence of n-n heterojunctions of two-type semiconducting oxides in In2O3/SnO2 composite hetero-nanofibers [48,[50][51][52]. The sensing mechanism of homotype hetero-nanofibers sensing material needs further investigation.

Conclusions
In2O3/SnO2 composite hetero-nanofibers were synthesized by a traditional electrospinning. The SnO2 nanofibers and In2O3 nanofibers exist simultaneously in the In2O3/SnO2 composite hetero-nanofibers. Each nanofiber is composed of small uniform nanocrystallites. The morphology and structure of all the nanofibers and SnO2 and In2O3 nanoparticles in same nanofibers are similar. Gas sensors were fabricated based on SnO2 nanofibers, In2O3 nanofibers, and In2O3/SnO2 composite hetero-nanofibers, respectively. The gas sensing properties of In2O3/SnO2 sensor were tested to six kinds of indoor volatile organic gases with gas concentration range of 0.5-50 ppm at the operating temperature of 275 °C. The gas sensor based on In2O3/SnO2 composite hetero-nanofibers exhibits the higher response to formaldehyde. The responses are 24.4 and 2.5 for formaldehyde concentrations of 50 ppm and 0.5 ppm, respectively. The response of In2O3/SnO2 composite hetero-nanofibers sensor was higher than that of SnO2 nanofibers or In2O3 nanofibers sensors, respectively. The gas sensing mechanism of the In2O3/SnO2 composite hetero-nanofibers sensor is analyzed. An n-n homotype heterojunction of In2O3/SnO2 may exist in the composite material, and the nanocrystallite boundary barrier of the homotype heterojunction may decrease when the composite materials meet formaldehyde, which leads to more electrons transfer and the increase of surface adsorption oxygen. As a result, the properties of In2O3/SnO2 composite hetero-nanofibers sensor are improved greatly.