Room Temperature Flexible Ammonia Sensor Based on Sb‐Doped SnO2/Polypyrrole Nanohybrid

In this study, Sb‐doped SnO2 nanosphere composite polypyrrole nanohybrid with different doping ratios (0–5 mol%, Sb:Sn) and composite ratios (0–30 mol%, Sb‐doped SnO2:polypyrrole) is synthesized by hydrothermal method and in situ chemical oxidation method. The flexible sensors are fabricated by drop‐casting the materials on polyamide substrate and gas sensing performances are investigated systematically at room temperature. The results show that the 3 at% Sb‐doped 20 mol% SnO2/polypyrrole nanohybrid exhibits excellent sensitivity (≈213% toward 100 ppm NH3) at room temperature, which are about 3 times as much as those of polypyrrole, as well as excellent selectivity and humidity resistance, reliable repeatability, and good robustness. The enhance sensing performance can be attributed to the formation of p‐n junction between conducting polymers and metal oxide semiconductor materials and the doping of Sb elements allows more electrons to transfer to polypyrrole, which further thickens the depletion layer and decreases hole concentrations in air. Therefore, Sb‐doped SnO2/polypyrrole nanohybrid may be a promising sensitive material for the design and manufacture of room temperature flexible ammonia gas sensors.


Introduction
As an important chemical raw material of nitrogen fertilizer, refrigerant, textiles, and so on, the detection of NH 3 gas is of great significance in agriculture, industry, and human daily life. [1]OI: 10.1002/adsr.202300020 Under certain conditions, a large amount of NH 3 emissions form secondary aerosols, reducing visibility and having a highly toxic effect on the ecological environment. [2]According to the United States Conference of Governmental Industrial Hygienists, the threshold for exposure to NH 3 is 25 ppm for 8 h.If the human body is exposed to large amounts of NH 3 , it can affect the skin, eyes or respiratory system, causing a negative impact on human health. [3]s a potential biomarker of kidney disease and hepatitis, NH 3 has become increasingly important in the diagnosis of breath. [4]Therefore, effective monitoring of NH 3 is an urgent need and necessary for environmental monitoring and chemical process control, and has a wide application prospect in disease diagnosis and human health monitoring.
Rigid ammonia sensors based on metal oxide semiconductors like SnO 2 , WO 3 , TiO 2 , and MoO 3 are the most widely utilized in NH 3 detection and the sensing mechanism were known based on the double-Schottky barrier model. [5,6]They tend to have high sensitivity and stability, but they usually need to work at high temperatures, with poor selectivity and mechanical properties, which makes the application prospect more limited. [3,7]On the other hand, conductive polymers (CPs) have shown good potential in the field of ammonia sensing. [5,6,8]10][11] However, it still has the disadvantages of slow response-recovery time and low sensitivity.16][17] SnO 2 is an n-type semiconductor metal oxide with wide-band gap.[23] Among them, antimony is a common n-type dopant.The ionic radius of antimony is close to that of Sn, which is easy to form substitutional doping.Otherwise, antimony-doped tin oxide material is easy to synthesize, and its morphology is controllable.Compared with pure tin oxide, the addition of antimony can improve the sensing performance of SnO 2 , because the Fermi energy of Sb-doped SnO 2 is higher than pure SnO 2 which increase the carrier concentration and thus promote electron transfer when interacting with the target gas. [22,24,25]From above, we can predicted that the heterostructures between SnO 2 and polypyrrole could improve the sensing performances and the doping of Sb allows more electrons to transfer to polypyrrole, which further thickens the depletion layer and decreases hole concentrations in air, so that enhances the response.
Herein, for the first time, Sb-doped SnO 2 nanosphere composite polypyrrole nanohybrid with different doping ratio (0-5 mol%, Sb:Sn) and composite ratio (0-30 mol%, Sb-doped SnO 2 :polypyrrole) were synthesized by hydrothermal method and in situ growth method, and were drop cast on the polyimide substrate.As the results shown, the 3 at% Sb-doped 20 mol% SnO 2 /PPy nanohybrid sensor exhibits high sensitivity (≈213% toward 100 ppm NH 3 ), and has excellent humidity resistance and selectivity, reliable repeatability and good robustness at room temperature.Compared with the initial state, the response value of PyS3 sensor decreases by 2.6%, 6.1%, and 12.0% after bent for 100, 300, or even 500 times toward 50 ppm NH 3 , respectively.The enhancement of sensing performance owes to the formation of p-n junction between conducting polymers and metal oxide semiconductor materials and the doping of Sb elements allows more electrons to transfer to polypyrrole, which further thickens the depletion layer and decreases hole concentrations in air.Therefore, Sb-doped SnO 2 /polypyrrole nanohybrid may be a promising sensitive material for the design and manufacture of room temperature flexible NH 3 gas sensors.

Synthesis of Sb-Doped SnO 2 Nanospheres/Ppy Nanohybrid
In a typical preparation process, [23] SnCl 2 •2H 2 O (240 mg) was dissolved in ethanol (40 mL) with HCL (1 mL), and stirred for 15 min (solution A).Then, SbCl 3 (100 mg) were dissolved in ethanol (20 mL) with HCL (0.5 mL), and stirred for 15 min (solution B).Drop an appropriate amount of solution B into solution A and magnetic stirred for 1 h.Transferred the obtained solution into a 50 mL Teflon autoclave and heated at 180 °C for 24 h.After cooling to room temperature naturally, the obtained precipitates were centrifuged and washed with deionized water and absolute ethanol for several times, dried at 60 °C overnight, and annealed at 400 °C for 2 h in air.
Finally, the resulting solution was cooled to 15 °C in an ice bath.The polymerization was performed for 1 h.The obtained precipitate was collected by centrifugation and washed successively with deionized water and ethanol and then dried at 60 °C for 12 h to obtain a black powder.

Fabrication of the Sensor
In this work, Sb-doped SnO 2 nanosphere composite polypyrrole nanohybrid with different doping ratio (0-5 mol%, Sb:Sn) and composite ratio (0-30 mol%, Sb-doped SnO 2 :polypyrrole) were synthesized by hydrothermal method and in situ growth method.As it is shown in Scheme 1, after the hydrothermal reaction and the in situ polymerization, the obtained composite material was assembled on a pre-hydrophilic treated PI substrate by drop casting.First of all, the obtained black powder was added into DI water to form a turbid liquid.Then, drop cast the turbid liquid onto the interdigital electrodes of polyimide film.At last, dry the sensor overnight at room temperature to get a stable device.

Characterization Results
For convenience, we named n at% Sb-doped SnO 2 as SBn and n at% Sb-doped SnO 2 /PPy as PySn (select the best composite ratio material by default).The morphology and structure of 3 at% Sb-doped SnO 2 nanospheres, tartaric acid-doped polypyrrole (TAdoped PPy) and 3 at% Sb-doped 20 mol% SnO 2 /PPy nanohybrid (PyS3) were characterized by FESEM and TEM.In Figure 1a, 3 at% Sb-SnO 2 nanospheres with a diameter of 150-200 nm were synthesized by hydrothermal method, showing the growth structure of small particles agglomeration as shown in Figure S1 (Supporting Information).As shown in Figure 1b, TA-doped PPy nanowires were formed by chemical oxidation synthesis, it has better flexibility compared with PPy nanoparticles.In addition, we performed HRTEM image studies on PyS3. Figure 1c showed that the PPy wraps on the surface of Sb-doped SnO 2 to form a network, which is conducive to gas permeation and diffusion.Figure 1d shows the crystal plane (110) and (101) of the cubic phase SnO 2 (JCPDS Card No. 41-1445), and the corresponding lattice distances are 0.340 and 0.265 nm, respectively.EDS elemental spectroscopy was used to observe element distribution of PyS3.As shown by the results, Sb was uniformly doped on SnO 2 nanospheres (Figure 1j), and PPy successfully assemble on the surface of SnO 2 nanospheres (Figure 1f,g).
The phase and crystallinity of TA-doped PPy, SB0, SB3, SB5, PyS0, PyS3, and PyS5 were studied by XRD (Figure 2a).We can observe a wide diffraction peak at about 21-25°of PPy, indicating the amorphous behavior of polypyrrole prepared by chemical oxidative polymerization method. [26]All typical characteristic peaks of SnO 2 correspond to the rutile structure can be clearly seen (JCPDS: 41-1445).In addition, the microcrystal sizes of SnO 2 , SB3, and SB5 were calculated as 12.8, 12.7, and 11.7 nm, respectively, according to Scherrer equation.Since the doping of antimony elements produced lattice defects, resulting in lower crystallinity and hindered the growth of lattice, casing microcrystal    sizes to decrease with the increase of doping amount. [25]Moreover, the XRD peaks of PS0, PS3, and PS5 are slightly weaker than those of SnO 2 , SB3, and SB5, since that Sb-doped SnO 2 are covered by PPy which has low crystallinity.The results showed that PPy and Sb-doped SnO 2 nanospheres co-existed through in situ chemical oxidation polymerization.
Besides, the chemical state of PyS3 were further investigated by X-ray photoelectron spectroscopy analysis.The survey spectra displayed in Figure 2b-e 2c, there are two peaks at 399.7 and 401.0 eV in N 1s spectrum, respectively come from amine nitrogen (-NH-) and positively charged nitrogen (-NH + -). [27]As shown in C 1s spectra and N 1s spectra, polypyrrole is successful synthesized.The Sn 3d spectrum (Figure 2d) exhibits two peaks at 495.7 and 487.1 eV, respectively come from Sn 3d 3/2 and Sn 3d 5/2, which are identified to be the typical characteristic peaks of SnO 2 . [13]An Sb 3d peak was observed in Figure 2e to match the standard value of Sb (3d 3/2 at 540.2 eV and 3d 5/2 at 539.1 eV), demonstrating successful doping of Sb 5+ and Sb 3+ .When the doping amount is small, antimony mainly exists in the form of Sb 5+ , and it replaces Sn 4+ into the tin oxide lattice, [22,25] which can be proved by the comparison of peak area.

Gas Sensing Properties
Gas sensing performance was measured in a dynamic test system which was showed in Scheme 2 at 25 ± 2 °C, 70% RH.The gas sensing performance was evaluated using a self-made gas mixing resistance measurement system.The sensor is placed in a 150 mL gas chamber.The standard gases of 500 ppm ammonia (balanced by nitrogen and oxygen mixture with 99.99% purity, V nitrogen :V oxygen = 8:2) was diluted with gas mixture of nitrogen and oxygen to prepare different concentrations of ammonia (1-100 ppm) using digital mass flow controllers.The electrical resistance was monitored for every 1 s by measuring the voltage across a standard resistor (R 0 ) under an applied voltage of DC 0.08 V.The resistance of sensor is calculated by Equation ( 1), The response of sensor is defined as Equation ( 2), Among them, R a and R g are the resistance in air and target gas, respectively.
To select the best sensitive material, the doping ratio and composite ratio of composite products were discussed.As shown in Figure 3 with error bars (calculating the average of the repeatability test results obtained from two dynamic gas sensitivity tests as the central point, with the upper and lower limits being the true values of the two tests), each PySn has four different composite ratios.The gas sensing performance showed that the best composite ratio materials were selected as PyS0 with composite ratio of 10%, PyS3 with composite ratio of 10% and PyS5 of composite ratio of 20% for subsequent gas-sensitive performance detection.Obviously, the composite with metal oxide can greatly improve the performance of TA-doped PPy, while the appropriate amount of element doping can further improve the performance of the composite.With the increase of the composite ratio, the response of PyS0, PyS3, and PyS5 showed a tendency to increase and then decrease, and the response values of the optimum composite ratio material were 128%, 167%, and 213% in 100 ppm NH 3 , respectively.As shown by the results, 3 at% Sb-doped 20 mol% SnO 2 /PPy sensor exhibits high sensitivity NH 3 (≈213% toward 100 ppm NH 3 ), which were about 3 times as much as those of polypyrrole.The resistance of the composite material is significantly higher than that of PPy.As shown in Figure 4b, the response of the prepared device gradually increases with the increase of ammonia concentration.Among them, the enhancement of the response of PyS3-based sensor to NH 3 is most obvious.As for Figure 4c with error bars (calculating the average of the repeatability test results obtained from two dynamic gas sensitivity tests as the central point, with the upper and lower limits being the true values of the two tests), the response of TA-PPy, PyS0, PyS3, and PyS5 sensors toward NH 3 with different concentrations in the range of 1-100 ppm at room temperature were measured.Obviously, the responses of TA-doped PPy, PyS0, PyS3, and PyS5 showed an approximate linear relationship with the increase of NH 3 concentration.Since selectivity is an indispensable indicator to measure the gas sensing performance, eight typical interfering gases are selected to evaluate the selectivity at room temperature, including 10 ppm of ammonia, nitrogen dioxide, hydrogen sulfide, acetone and 100 ppm of carbon monoxide, methane and ethylene as shown in Figure 4d.The composite materials enhanced the selectivity of PPy to ammonia gas, especially PyS3.
According to above results, PyS3 based sensor is particularly outstanding in the enhancement of sensitivity and selectivity, so this material is selected for the following stability test.In addition, after repeating the response-recovery test toward 50 ppm for 6 times, the PyS3 based sensors showed good reproducibility at room temperature (Figure 5a).As shown in Figure 5b, the PyS3 sensor exhibits excellent long-time stability by testing diurnal variations of sensitivity over a week, and only 8% reduction in response was observed, which provides a good application prospect in wearable sensor devices.This is because there is no significant decrease in the number of active adsorption sites within 7 days, so that the adsorption degree of ammonia at the same concentration changes slightly, resulting in the response value doesn't change much.
To explore the effect of humidity on gas sensing performance, the response of the sensor to 50 ppm NH 3 was tested under different relative humidity (10-70% RH) at room temperature.As shown by Figure 6b with error bars (calculating the average of the repeatability test results obtained from two dynamic gas sensitivity tests as the central point, with the upper and lower limits being the true values of the two tests), with the increase of humidity, the sensitivity gradually increases, and the resistance decreases at the same time, probably due to the addition of high concentration of water promote the reaction between acidified polypyrrole and ammonia.The results show that the PyS3 sensor can still operate under high humidity conditions.
In addition, the flexible characteristics of the PyS3 sensor are further explored.Figure 7a-h shows the continuous resistance curves of the PyS3 sensors to 10 ppm NH 3 after bent for 0-500 times.As for Figure 7i, with the increase of bending times, the limiting resistance of the device increases continuously, and the attenuation of the response value is not obvious.This is because micro-cracks form between sensitive materials during multiple bending cycles at angles close to 180°, resulting in an increase in resistance (Figure S3c, Supporting Information).However, since the increase in resistance is caused by physical factors, active adsorption sites will not decrease, leading to an insignificant decrease in sensitivity.Compared with the initial state, the sensitivity of PyS3 sensor to 50 ppm NH 3 decreases by 2.6%, 6.1%, and 12.0% after bent for 100, 300, or even 500 times, respectively, indicating that PyS3 has excellent robustness.These results show that PyS3 sensor has satisfactory flexibility and has a promising application prospect in wearable gas sensing.
Table 1 presents the comparison of the Sb-doped SnO 2 /PPy sensor with previous reports in terms of sensing material, type of sensor and sensor response.The comparative results show that the Sb-doped SnO 2 /PPy sensor has superior ammonia-sensing performances.

Sensing Mechanism
The sensing mechanism of polypyrrole to NH 3 is wildly known as a process of deprotonation-protonation through the adsorptiondesorption process of NH 3 on surface of PPy.The deprotonation and protonation process of NH 3 is followed by the Equations ( 3) and ( 4). [32]y + + NH 3 (g) ↔ PPy 0 +NH 3 (ads) (3) When the composite materials exposed to ammonia, the nitrogen complex in ammonia loses an electron to the nitrogen of the polymer skeleton, thus forming an ammonium ion.The decrease in the number of carriers leads to an increase in the resistance of PPy.When the compound is recovered in the air, reaction occurs in the opposite direction, thus the resistance of sensor can be gradually recovered, which leads to the sensing response to NH 3 .
In addition, the above experimental results clearly show that the response value of PyS3 to NH 3 is much higher than that of polypyrrole.Considering that the sensitization mechanism     comes from the following two aspects, namely, composite sensitization and doping sensitization.According to previous research, the gas sensitive response can be written as Equation ( 5). [33] where R a and R g are the sensor resistances in air and target gas, and p a and p g are the hole concentrations in air and target gas, Δ p = p a − p g is the change in hole concentration during exposure to the target gas.Therefore, when the hole concentration of the p-type oxide semiconductor in the air decreases, that is, p a decreases, the gas sensitivity increases accordingly.
On the one hand, the significant increase in the NH 3 gas response of nanocomposites is mainly due to the formation of p-n heterojunctions at the interface between polypyrrole and SnO 2 .It is well known that PPy is a typical p-type semiconductor.At the same time, SnO 2 and are n-type semiconductors.When SnO 2 is coated with polypyrrole, electrons flow from tin oxide to polypyrrole, resulting in the combination of the two to form a depletion layer, making p a smaller and R a larger.When exposed to NH 3 , the nitrogen complex in ammonia loses an electron to the nitrogen in the polymer skeleton, forming an ammonium ion.A decrease in the number of carriers leads to a thickening of the depletion layer, narrowing of the conductive channel, and an increase in R g , resulting in a significant increase in sensitivity.
On the other hand, the significant increase in the performance of ammonia sensing is due to antimony doping.As shown in Figure 8, the work function of pure SnO 2 and 3 at% Sb-doped SnO 2 were 5.2 and 4.3 eV, respectively.Due to the fact that the work function of antimony doped tin oxide is smaller than that of pure tin oxide, the former will provide more electrons for polypyrrole to achieve equilibrium in the Fermi energy level, resulting in further thickening of the depletion layer and further lowering of p a .When exposed to NH 3 , the nitrogen complex in ammonia loses an electron to the nitrogen in the polymer skeleton, forming an ammonium ion.The decrease in the number of carriers leads to the thickening of the depletion layer, extreme narrowing of the conductive channel, and a sharp increase in R g , resulting in a further increase in sensitivity (Figure 9).

Conclusion
The flexible sensors were fabricated by drop-casting the materials on polyamide substrate and investigate the gas sensing performances to ammonia at room temperature.The results show that the 3 at% Sb-doped 20 mol% SnO 2 /PPy nanohybrid (PyS3) showed excellent sensitivity (≈213% toward 100 ppm NH 3 ) at room temperature, which were 3 times as much as those of PPy, as well as excellent selectivity and humidity resistance, reliable repeatability, and good robustness.The enhancement of sensing performance owes to the formation of p-n junction between conducting polymers and metal oxide semiconductor ma-terials and the doping of Sb elements allows more electrons to transfer to polypyrrole, which further thickens the depletion layer and decreases hole concentrations in air.Therefore, Sb-doped SnO 2 /polypyrrole nanohybrid may be a promising flexible sensitive material for room temperature NH 3 sensors and has broad application prospects in the field of wearable chemical sensors.

Scheme 1 .
Scheme 1.The progress of the synthesis of Sb-doped SnO 2 Nanospheres/Ppy Nanohybrid and the fabrication of the sensor.

Scheme 2 .
Scheme 2. The diagram of the self-made dynamic test system.
indicate that PyS3 contains elements C, N, Sn, and Sb. Figure 2b, the C 1s spectrum shows four peaks located at 288.8, 286.4,285.3, and 284.4 eV, respectively attributed to C = O, C-O, C-N, and C-C.As shown in Figure

Figure 3 .
Figure 3.The response of PyS0, PyS3, PyS5 with different composite ratios to 100 ppm NH 3 with error bars.

Figure 4 .
Figure 4. a) Continuous response curves of the sensors based on PPy, PyS0, PyS3, PyS5 to various concentrations of NH 3 at 25 ± 2 °C, 70% RH; b) Continuous resistance curves of the sensors based on PPy, PyS0, PyS3, PyS5 to various concentrations of NH 3 ; c) The spot and line image between the transient response value and the gas concentration with error bars; d) The responses of PyS3 sensor to various testing gases of 10 or 50 ppm.

Figure 5 .
Figure 5. a) Six reversible cycles of PyS3 sensor to NH 3 with concentration of 5 ppm; b) Stability of PyS3 sensor to 50 ppm NH 3.

Figure 6 .
Figure 6.a) Continuous resistance curves of PyS3 sensors to 10 ppm NH 3 ; b) The response values with error bars and the resistance of PyS3 sensor to 10 ppm NH 3.

Figure 7 .
Figure 7. a-h) Continuous resistance curves of the PyS3 sensors to 10 ppm NH 3 after bent for 0-500 times; i) The response transients and transient resistance of PyS3 sensors to 10 ppm NH 3 after bent for 0-500 times.

Figure 8 .
Figure 8. Work function area scan record for SnO 2 and 3 at% Sb-doped SnO 2 via Kelvin probe measurements.

Figure 9 .
Figure 9. Schematic diagram of band variation and carrier transfer.

Table 1 .
Comparison of ammonia sensing ability of different PPy based gas sensor.