Enhanced formaldehyde gas-sensing response based on indium oxide nanowires doped with same-valence metal cations

Abstract

Mesoporous indium oxides nanowires (In₂O₃ NWs) doped with Al, Sb and La were synthesized using a nanocasting method, and then the components, microstructures and morphology were characterized. All results indicate that the doped metals are well dispersed in the In₂O₃ NWs and hardly affect the NW microstructure and morphology. While the cations-doped concentration decreases greatly with the increasing radii of Al, Sb and La cations. The cation-doping not only causes the lattice distortion to improve the adsorbed oxygen on the surface, but also increases the ground-state resistance of the In₂O₃ NWs. In this way, the cation-doping greatly affects the gas-sensing behavior of formaldehyde gas. Considering the resistance in air and formaldehyde gas, the In_1.984La_0.016O₃ NW sensor exhibits the best gas-sensing performance of 10 ppm formaldehyde gas with a response of 39.51 at 210 °C owing to the largest radius and lowest doping content of La³⁺.

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1. Introduction

Formaldehyde (HCHO) is the simplest aldehyde, and is a colorless irritating gas.¹ Long-term exposure to HCHO gas at a concentration higher than 0.1 ppm may cause serious eye irritation, a burning sensation in the throat, dyspnea, asthma-related symptoms and cancer.² Considering the health hazards to humans and animals, it is very important to monitor the concentration of HCHO gas both indoors and outdoors.³ Therefore, test instruments and techniques should be well developed to detect and monitor HCHO gas in real-time.⁴ Nowadays, gas chromatography,⁵ high performance liquid chromatography,⁶ colorimetry-based derivatization,⁷ protein-based biosensing systems⁸ and gas sensors^9,10 have been developed and used in formaldehyde monitoring.

Gas sensors based on metal oxide semiconductors (MOS) have been well studied and developed to detect and monitor poisonous and harmful gases.¹¹ The gas response of a MOS sensor is defined as the ratio of the resistance in target gas to that in air.¹² For gas sensors it is important to control the microstructure of the material, to increase the specific surface area of MOS nanostructure. Nowadays, one-dimensional MOS nanostructures are widely studied as gas-sensing mediums due to their very large surface-to-volume ratio, high specific surface area and high porosity.^13–15 In recent years, nanowires (NWs) have been used widely in gas sensors. For example, Wang et al. synthesized mesoporous Co₃O₄ NWs using a nanocasting method, and these NWs indicated an excellent gas-sensing performance for toluene gas.¹⁶ Hassen et al. fabricated rGO-W₁₈O₄₉ NWs, improving the gas-sensing response to toluene gas.¹⁷ However, microstructural regulation only improves the surface area to some extent.

As is known, heterogeneous doping can effectively adjust the carrier concentration of one-dimensional MOS nanostructures.¹⁸ Heterogeneous doping not only adjusts the resistance in the ground state but also causes lattice distortion of the MOS nanostructure. This lattice distortion results in more oxygen vacancies, which leads to more adsorbed oxygen on MOS nanostructures. Chen et al. synthesized high-valence-cation-doped NiO NWs, and the responses of the NiO NW sensor was greatly increased with doping of the high-valence donors.¹⁹ Mokoena et al. studied a Eu³⁺-doped NiO sensor for toluene gas detection, and the response was higher than for the NiO sensor.²¹

Due to its broad bandgap, low cost, low resistivity and high catalytic activity, In₂O₃ nanostructures as n-type MOS have been widely investigated all over the world to improve the gas-sensing performance.²² Jin et al. synthesized hierarchical porous In₂O₃ nanospheres, the sensor for which showed better selectivity for TEA and ethanol.²³ Liu et al. prepared Sb-doped In₂O₃ microstructures for acetone detection, which showed a higher response compares with In₂O₃.²⁴ Liang et al. synthesized Ca-doped In₂O₃ nanotubes, which showed excellent selectivity and a high response toward formaldehyde.²⁵

In order to more accurately reveal the influence of lattice distortion from cation-doping, the interference of the carrier concentration, morphology, average particle size and specific surface area on the gas-sensing behavior of In₂O₃ nanostructures should be excluded.²⁶ In this paper, a nanocasting method and the same process conditions were used to synthesize In₂O₃ NWs and cation-doped In₂O₃ NWs with a similar diameter, microstructure and specific surface area. The cations Al³⁺, Sb³⁺ and La³⁺ were doped into the In₂O₃ NWs using SBA-15 silica as a hard template. The influence of the doping content and cation radius on the microstructure and gas-sensing properties toward HCHO was investigated in detail. After optimizing the operating conditions of the In₂O₃ NW gas sensors, the physical mechanism was analyzed theoretically based on the doped semiconductor theory.

2. Experimental

2.1 Synthesis of In₂O₃ and cation-doped In₂O₃ NWs

All chemicals and reagents in this study were of analytical grade and used directly without further purification. The In₂O₃ NWs and Al-, Sb- and La-doped In₂O₃ NWs were synthesized using mesoporous SBA-15 silica as a hard template.19 Indium nitrates, aluminum chloride, antimony chloride, lanthanum nitrate and the SBA-15 powder were uniformly dissolved and dispersed in ethanol solution according to the atomic ratio of Si, i.e., (In + Al, Sb and La) : (Al, Sb and La) = 2 : 1 : 0.05, and then n-hexane was added. After evaporating the above solution, the obtained powder samples were calcined at 260 °C using a muffle furnace. After washing to remove undecomposed soluble salts, the powder samples were sintered in air at 600 °C for 6 h and the SBA-15 silica was removed using a 2 M NaOH aqueous solution. Compared with the In₂O₃ molecular formula, the cation-doped In₂O₃ NWs were denoted as In_2−xM_xO₃ (M = Al, Sb or La), in which x is the doping content calculated via EDS.

2.2 Characterization of microstructures and gas-sensing properties

The phases, morphology and microstructures of undoped and cation-doped In₂O₃ NWs were characterized via XRD (SmartLabSE, 3 kW, Cu target, wavelength 0.154 nm, step 0.02°), TEM (JEOL, JEM-1400Flash, 120 kV), and EDS (HITACH, SU8010). Moreover, the bandgap and specific surface area were measured and deduced the from UV-visible adsorption spectra (UV3600 Spectrophotometer) and N₂ adsorption–desorption isotherms (ASAP2020 Surface Area and Porosity Analyzer, −196 °C, relative pressure of 0.06 to 0.2), respectively.

The gas-sensing tests were performed via static distribution using a CGS-4TPs gas-sensing measurement system with atmospheric air as the interference gas, where the relative humidity was held at about 30%. First, 2 mg In₂O₃ NWs or cation-doped In₂O₃ NWs and 1 ml deionized water were mixed together to form a homogeneous paste, which was brush-coated onto an Al₂O₃ ceramic substrate with interdigitated Ag–Pd electrodes. The as-prepared simulated sensors were dried in air at room temperature and then aged at 350 °C for 2 h in air to improve their stability. Considering the volume of the test chamber, the volume of HCHO liquid (37–40%), ammonia water (25–28%), water, ethanol, xylene, toluene and methanol were calculated, and then the above liquid was injected into the chamber and evaporated to a gas with a concentration from 1 ppm to 15 ppm. The response for n-type In₂O₃ NW sensors is defined as the ratio of the resistance in air (R_air) to that in HCHO gas (R_gas).¹³

3. Results and discussion

3.1 Microstructures and morphology of cation-doped In₂O₃ NWs

The phase analysis of In₂O₃ and cation-doped In₂O₃ NWs was performed using XRD, as shown in Fig. 1, in which all peaks are normalized according to the strongest (222) peak. The diffraction peaks of all the samples are consistent with the characteristic peaks of cubic In₂O₃ (PDF#71-2194). The diffraction peaks at 2θ values of 21.65, 30.76, 35.618, 51.188 and 60.85° correspond to the (211), (222), (400), (440) and (622) facets according to the standard card. The characteristic peaks belonging to Al-, Sb- and La-based compounds are not observed. The average crystallite sizes calculated using the Scherrer equation are listed in Table 1. In order to reveal the influence of doping with different cations on the lattice distortion of the In₂O₃ NWs, the (222) peaks of all the samples are enlarged and shown in Fig. 1b. Clearly, the (222) peak of the Al-doped In₂O₃ NWs is shifted to a higher angle and the La-doped sample shifted to a lower angle, while the Sb-doped In₂O₃ NWs are the same the In₂O₃ NWs due to the similar radius. According to Bragg's law:

2dsin θ = λ (1)

where d is the distance between parallel atomic planes, λ is the incident wavelength and θ is the angle between the incident light and the crystal plane. In this way, La-doped In₂O₃ NWs should present a higher d for the larger radius of La³⁺ while Al-doped In₂O₃ NWs should possess a lower d for the lower Al³⁺ radius compared with In³⁺ according to eqn (1). Therefore, it is concluded that Al, Sb and La were successfully doped into the In₂O₃ lattice, which leads to the lattice distortion of the In₂O₃ NWs.²⁷

Fig. 1 Full-angle range of XRD patterns (a) and shift of the (222) peak (b) for undoped In₂O₃ and cation-doped In₂O₃ NWs.

Table 1 Structural and gas-sensing parameters of In₂O₃ and cation-doped In₂O₃ NWs

Sample	Crystallite size (nm)	Average pore size (nm)	S BET (m^2 g^−1)	E g (eV)	Response, S
In2O3	17.9	6.29520	57.37	3.34	26.16
In1.838Al0.162O3	38.8	8.29376	53.55	3.39	28.73
In1.816Sb0.184O3	45.8	7.82711	49.21	3.40	19.78
In1.984La0.016O3	23.0	6.37254	57.29	3.36	39.51

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The components of In₂O₃ and cation-doped In₂O₃ NWs are characterized via EDS in Fig. 2. Considering the atomic percentage of all samples, cation-doped In₂O₃ NWs can be expressed as In_1.838Al_0.162O₃, In_1.816Sb_0.184O₃ and In_1.984La_0.016O₃ NWs, respectively. Owing to the different cation radius values of In, Al, Sb and La in the initial solution, the probability of these cations entering the mesochannels of SBA-15 is different.¹⁹ The probability of entering the mesochannels decreases greatly with the increasing radius of the doped cation. As a result, the In_1.838Al_0.162O₃ and In_1.816Sb_0.184O₃ NWs present the highest doping concentration while the In_1.984La_0.016O₃ NWs show the lowest doping concentration. Moreover, Fig. 3 shows the EDS spectra of In_1.838Al_0.162O₃, In_1.816Sb_0.184O₃ and In_1.984La_0.016O₃ NWs, indicating that Al, Sb and La are well-dispersed in the In_1.838Al_0.162O₃, In_1.826Sb_0.184O₃ and In_1.984La_0.016O₃ NWs. It can be seen easily from the number of bright spots that the La content is the lowest while the separate Al and Sb contents are much higher.

Fig. 2 EDS images of In₂O₃ and cation-doped In₂O₃ NWs.

Fig. 3 Al, Sb and La EDS spectra of cation-doped In₂O₃ NWs.

The morphology and microstructure of In₂O₃ and the cation-doped In₂O₃ NWs are observed using TEM in Fig. 4. It is shown that all samples exhibit mesoporous structures and that cation doping hardly affects the microstructure of the In₂O₃ NWs. The diameter of the nanowires is measured to be 8–9 nm and mesochannel size is about 2–3 nm.²⁸ The N₂ adsorption–desorption isotherms of In₂O₃ and the cation-doped In₂O₃ NWs are shown in Fig. 5. The curves of all samples exhibit type-IV adsorption isotherms, which are characteristic of mesoporous structures according to IUPAC. The BET surface area (S_BET) and average pore size of all the samples calculated from the N₂ adsorption–desorption isotherm data are given in Table 1. The specific surface area changes a little from 49.2 m² g⁻¹ to 57.4 m² g⁻¹. The pore size distribution diagrams of all the samples are shown in the insets of Fig. 5, in which the average pore size is mainly focused around 4–5 nm.

Fig. 4 TEM images of (a) In₂O₃, and (b) Al-, (c) Sb- and (d) La-doped In₂O₃ NWs.

Fig. 5 N₂ adsorption–desorption isotherms and pore size distribution plots (inset) of (a) In₂O₃, and (b) Al-, (c) Sb- and (d) La-doped In₂O₃ NWs.

The UV-visible absorption spectra of In₂O₃ and the cation-doped In₂O₃ NWs are used to calculate the bandgap (E_g) in Fig. 6. E_g is the basis of the optical absorption spectra according to eqn (2):²⁹

(αhν)^1/n = A(hν − E_g) (2)

where α is the absorption coefficient, h is the Planck constant, ν is the photon frequency, A is constant and n is the electronic transition parameter (which is equal to 1/2 for a direct bandgap semiconductor). Since α is proportional to the absorbance (A) according to the Beer–Lambert law, the energy intercept of the curve of (αhν)²versus hν gives E_g when the linear region is extrapolated to the zero ordinate.²⁰ The E_g values of all the samples are given in Table 1, which shows that there is a small increase from 3.34 eV for the In₂O₃ NWs to about 3.40 eV for the cation-doped In₂O₃ NWs owing to lattice distortion.

Fig. 6 UV-vis spectrum of (a) In₂O₃ and the cation-doped In₂O₃ NWs; and (b–e) plots of (Ahν)²versus hν for (b) In₂O₃, and the (c) Al-, (d) Sb- and (e) La-doped In₂O₃ NWs.

3.2 Gas-sensing performance of cation-doped In₂O₃ NWs sensors

The gas-sensing behavior for 10 ppm HCHO gas of the In₂O₃ and cation-doped In₂O₃ NW sensors is characterized at operating temperatures from 190 to 240 °C in Fig. 7(a). For all the sensors, the response (S = R_air/R_gas) to HCHO gas increases initially up to a maximum value at 210 °C with increasing temperature, and then decreases to some extent. This phenomenon could be attributed to the balance of the surface chemical reaction and the HCHO gas diffusion. In this way, the operating temperature was confirmed to be 210 °C for all sensors in the subsequent experiments. Comparing all sensors, the response of the In_1.984La_0.016O₃ and In_1.838Al_0.162O₃ NWs sensors is higher than that of the In₂O₃ NW sensor, while the In_1.186Sb_0.184O₃ NWs sensor shows the lowest response to HCHO gas at 210 °C. The change in resistance for In₂O₃ and the cation-doped In₂O₃ NW sensors to 10 ppm HCHO gas at 210 °C is given in Fig. 7(b), for which the resistance values in air and HCHO gas are also listed in Table 2. Owing to the electron-depletion layer at the shell of In₂O₃ NWs in air from adsorbed oxygen, R_air for all the sensors is much larger than R_gas. Furthermore, the differences in the resistance are relatively large due to the thickness differences of all the sensors. The R_air values of all the sensors at the different operating temperatures are given in Fig. 7(c), in which R_air decreases with the increasing temperature. The response and recovery time are defined as the time required for a change in the resistance to reach 90% of the equilibrium value after HCHO is injected and then removed. The response time for In₂O₃ and the cation-doped In₂O₃ NWs sensors is 80 s, 33 s, 77 s and 50 s, respectively. The response time of the cation-doped In₂O₃ NWs sensors is shorter than for the In₂O₃ NW sensor. This should be attributed to the destruction of the periodic potential field for cation doping, which facilitates the diffusion of molecular oxygen.⁴⁰

Fig. 7 (a) Response of In₂O₃, and Al-, Sb- and La-doped In₂O₃ NW sensors to 10 ppm HCHO gas at different operating temperatures; (b) response and recovery curves of all the sensors to 10 ppm HCHO gas at 210 °C; and (cs) R_air values for all sensors at the different operating temperatures.

Table 2 Resistance values of all sensors in air and 10 ppm HCHO gas at 210 °C

Sample	R air (Ω)	R gas (Ω)
In2O3 NWs	153 644	6650
In1.838Al0.162O3 NWs	267 635	10 389
In1.816Sb0.184O3 NWs	47943	2424
In1.984La0.016O3 NWs	954 557	24 202

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The influence of the HCHO gas concentration from 1 ppm to 15 ppm on the response of In₂O₃ and the cation-doped In₂O₃ NW sensors at 210 °C is also discussed in Fig. 8(a). It was seen that the response of all the sensors increases with the HCHO gas concentration. HCHO gas first reacts with the adsorbed oxygen, and re-injects the extracted electrons back to the shell of In₂O₃ and the cation-doped In₂O₃ NWs. As the result, the resistance decreases greatly when in HCHO gas. The higher the HCHO gas concentration, the lower is the resistance value of all the sensors. Therefore, the response to HCHO gas increases greatly with the HCHO gas concentration for all sensors. Moreover, the relationship between the response (S) and the HCHO gas concentration (C) can be expressed in eqn (3) as:

S = a(C)^b + 1 (3)

where a and b are constants.²⁹ Based on the data in Fig. 8(a), a and b can be calculated using eqn (3). The above equation can therefore be rewritten as:

log (S − 1) = blog (C) + log (a). (4)

In this way, the relationship between log (S − 1) and log (C) is plotted in Fig. 8(b) for In₂O₃ and the cation-doped In₂O₃ NW sensors, in which the response exhibits a good linear relationship with the HCHO gas concentration. Therefore, the HCHO gas concentration can be easily predicted according to the response value, indicating an excellent application in practice.

Fig. 8 Concentration-dependent response curves (a) and the predicted concentration-response curves (b) for In₂O₃ and the cation-doped In₂O₃ NW sensors to HCHO gas at 210 °C.

Furthermore, the selectivity of all the sensors towards 10 ppm ethanol, ammonia gas (ammonia water changed to ammonia gas after evaporation), water vapor, xylene, toluene, methanol and HCHO gases at 210 °C are given in Fig. 9(a). All sensors exhibit a much smaller response to water vapor than that of HCHO gas, indicating that the influence of humidity on the gas-sensing performance can be ignored. Clearly, the response to HCHO gas presents the highest value of all the gases, which indicates the excellent selectivity of the sensor to HCHO gas. In order to investigate the stability of In₂O₃ and the cation-doped In₂O₃ NW sensors, the response to 10 ppm HCHO gas was tested 5 times at 210 °C, as shown in Fig. 9(b). The response of all the sensors hardly changes, indicating the long-term stability of In₂O₃ and the cation-doped In₂O₃ NWs. Compared with the work of other researchers, the response to HCHO gas using the different gas-sensing media is listed in Table 3, in which the In_1.984La_0.016O₃ NW sensor presents one of the highest responses to 10 ppm HCHO gas with an S value of about 40.

Fig. 9 Selectivity (a) and long-term stability (b) of In₂O₃ and the cation-doped In₂O₃ NW sensors.

Table 3 HCHO gas-sensing properties of the different sensing media

Sensing material	T sens (°C)	HCHO concentration (ppm)	S	Ref.
α-Fe2O3 particles	270	500	23	30
ZnSnO3 cubic	180	30	25	31
rGO/ZnSnO3 microspheres	110	10	12	32
In2O3@rGO nanocubes	225	25	88	33
WOx/In2O3 nanosheets	170	100	25	34
SnO2 nanowires	270	10	2.45	35
Er-doped porous In2O3 nanotubes	260	20	12	36
2 at% Al-doped ZnO	320	50	6.8	37
In1.984La0.016O3 NWs	210	10	39.51	This work

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3.3 Gas-sensing mechanism of cation-doped In₂O₃ NW sensors

For In₂O₃ and the cation-doped In₂O₃ NWs in air, the higher surface area adsorbed more oxygen molecules on the surface. The adsorbed oxygen molecules extract electrons from the surface shell and become O⁻ at 210 °C to form an electron-depletion layer.³⁸ As a result, the shell resistance in air (R_shell(air)) increases dramatically owing to the high surface area of the mesoporous structures. The more adsorptive O⁻ that are present, the thicker electron-depletion layer, as shown in Fig. 10. When HCHO gas is injected, the HCHO gas molecules prefer to react with O⁻ at the surface. The adsorptive O⁻ decreases with the increasing HCHO gas concentration and more and more electrons are re-injected back to the shell of In₂O₃ and the cation-doped In₂O₃ NWs. R_gas hardly changes without the adsorbed O⁻ at the extreme high concentration of HCHO gas. As a result, R_gas decreases and the response increases with the HCHO gas concentration.

Fig. 10 Theoretical schematic of the electron-depletion layer for In₂O₃ and the cation-doped In₂O₃ NWs in air and formaldehyde.

The conduction pathway for n-type In₂O₃ NWs is equivalent to a series circuit of the core resistance (R_core) and shell resistance (R_shell).²⁹ In this way, the response can be rewritten as follows in eqn (5):

If a higher response is wanted, ΔR_{shell(air–gas)} should be big enough and R_gas should be small as much as possible. The same-valent Al³⁺, Sb³⁺ and La³⁺ ions that are substituted for In³⁺ in the In₂O₃ NWs hardly change the carrier concentration of the In_1.816Sb_0.184O₃, In_1.838Al_0.162O₃ and In_1.984La_0.016O₃ NWs compared with the undoped In₂O₃ NWs. Furthermore, the In₂O₃, In_1.838Al_0.162O₃, In_1.816Sb_0.184O₃ and In_1.984La_0.016O₃ NWs are synthesized using SBA-15 silica as a hard template under the same chemical conditions and calcination temperature, so the microstructures, morphology and defects of all the samples should be similar. Only in this way can the influence of the lattice distortion caused by cation doping on the gas-sensing performance of the In_1.816Sb_0.184O₃, In_1.838Al_0.162O₃ and In_1.984La_0.016O₃ NW sensors be truly revealed. As is known, cation-doping leads to lattice distortion of the In₂O₃ NWs for the different cation radii, resulting in a change in the oxygen vacancy concentration or chemical state of the adsorbed ions on the surface.³⁹ At the same time, cation doping causes destruction of the periodic potential field, making the In₂O₃ lattice more open, which facilitates the diffusion of molecular oxygen through the In₂O₃ lattice.⁴⁰

The radii of Al³⁺ (0.0535 nm) and La³⁺ (0.1032 nm) present a larger difference with that of In³⁺ (0.080 nm), whereas Sb³⁺ (0.076 nm) possesses a similar radius to In³⁺.⁴¹ Doping with the large La³⁺ radius causes the largest lattice distortion for the In_1.984La_0.016O₃ NW lattice whereas doping with Al³⁺ and Sb³⁺, which have a smaller radius than La³⁺, bring about a smaller lattice distortion, as shown in schematic diagram in Fig. 11. Moreover, the cation-doped In₂O₃ NWs present the similar specific surface areas with a value of about 50 m² g⁻¹, which possesses the similar adsorbed oxygen owing to the mesoporous structures.⁴² As shown in Fig. 10, the larger lattice distortion results in more oxygen vacancies, which leads to the more adsorbed oxygen on the surface of the cation-doped In₂O₃ NWs.²² It was reported that rare-earth doping increases the surface alkalinity, which further improves the adsorbed oxygen of the In_1.984La_0.016O₃ NWs.⁴³ The more adsorbed oxygen leads to a larger change of ΔR_{shell(air–gas)} according to eqn (5) for the In_1.984La_0.016O₃ NWs sensor.

Fig. 11 Schematic diagram of lattice distortion for In₂O₃ and the cation-doped In₂O₃ NWs.

Moreover, cation-doping generates the lattice distortion of the In_1.816Sb_0.184O₃, In_1.838Al_0.162O₃ and In_1.984La_0.016O₃ NWs, which scatters the electron transport and increases the R_gas value. The cation-doping content increases from In_1.984La_0.016O₃ to In_1.838Al_0.162O₃ and In_1.816Sb_0.184O₃ NWs, leading to the highest R_gas for the In_1.816Sb_0.184O₃ NWs and the lowest R_gas for the In_1.984La_0.016O₃ NWs. According to eqn (5), the response of all the sensors to HCHO gas increases from 19.78 for In_1.816Sb_0.184O₃ to 26.16 for In₂O₃, 28.73 for In_1.838Al_0.162O₃ and 39.51 for In_1.984La_0.016O₃. La-doping not only improves the adsorbed oxygen on the surface of the In₂O₃ NW sensor but also increases R_gas. Compared with the In₂O₃, In_1.816Sb_0.184O₃ and In_1.838Al_0.162O₃ NW sensors, the In_1.984La_0.016O₃ NW sensor presents the highest response owing to the larger lattice distortion and the lowest doping content.

4. Conclusions

Mesoporous In₂O₃ and cation-doped In₂O₃ NWs have been synthesized using a nanocasting method. Different metals (Al, Sb and La) have been introduced to study the influence on the sensing performance. The characterization results show that both cation-doped In₂O₃ and undoped In₂O₃ NWs have a long-range-order mesoporous structure with a high specific surface area. All sensors present the best gas-sensing performance at the optimal working temperature of 210 °C, and the response of the La-doped In₂O₃ NWs reaches the maximum value of 39.51 for 10 ppm HCHO gas. The cation doping not only brings about lattice distortion to improve the adsorbed oxygen on the surface but also increases the ground-state resistance of the In₂O₃ NWs. Rare-earth doping increases the surface alkalinity, improving the sensor response towards HCHO. La-doping increases both the adsorbed oxygen and the lattice distortion of La-doped In₂O₃ NWs, and is responsible for the highly enhanced HCHO gas-sensing performance of the In_1.984La_0.016O₃ NWs sensor.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the Natural Science Foundation of Zhejiang Province (LY16E030004 and LY20E020011) and the National Natural Science Foundation of China (U1809216).

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