A CH₃NH₃PbI₃ film for a room-temperature NO₂ gas sensor with quick response and high selectivity

Abstract

A room-temperature NO₂ gas sensor with excellent performances is fabricated using an MAPbI₃ (MA = CH₃NH₃⁺) thin film. It presents a high response even under extremely low NO₂ concentrations. Its average response and recovery times are only ∼5 s and ∼25 s at room temperature, respectively, exhibiting its quick-responsive character. In addition, the MAPbI₃-based NO₂ sensor exhibits good selectivity. Interestingly, the sensitivity of the MAPbI₃-based NO₂ sensor is strikingly improved under high-pressure gas conditions. This phenomenon can be used for the online monitoring of chemical reaction processes and in situ detection of trace-level gas impurities under high-pressure conditions. Furthermore, based on theoretical calculations, a simple model is proposed to illustrate the corresponding gas-sensing mechanism.

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1. Introduction

In modern industrialized society, the emission of harmful gases has increased substantially. Toxic gases are hazardous to human health and extremely dangerous air pollutants, among which NO₂ is one of the most common harmful gases. The increasing concern for protecting the environment and biological systems has resulted in continuous endeavors in improving gas sensors.^1–4 To date, many types of inorganic and organic compounds have been utilized as sensing materials.^5,6 Although traditional metal oxides generally present high responses, they have an obvious disadvantage, which is the high working temperatures. For example, although CuO plates showed high sensitivity to NO₂, the optimum operating temperature was 200 °C.⁷ A ZnO nanowire sensor exhibited high sensitivity and fast response to ethanol gas at a temperature as high as 300 °C.⁸ WO₃ was believed to be a promising material for NO₂ gas sensor, but it was typically used in the temperature range from 200 °C to 350 °C.^9,10 This disadvantage severely limits their further applications in the field of gas sensors. To achieve the requirement of a low optimum working temperature, some organic compounds have been used as low-temperature gas sensing materials. However, quick response and recovery speed are other challenging issues for organic gas sensors. For instance, the response and recovery time of a lead phthalocyanine (PbPc) film sensor were about hundreds of seconds for NO₂.¹¹ The gas sensing behaviors of tetra-(tert-butyl) phthalocyanine copper (CuPc(t-Bu)₄) films were investigated at room temperature, but their response and recovery speed were very slow.¹² Therefore, based on the advantage of the low working temperature of organic gas sensors, how to greatly improve the response and recovery speed has been a great challenge to date.

Organometallic halide perovskites MAPbX₃ (X = Cl, Br, I) have attracted much attention due to their interesting and intriguing properties, including moderate bandgaps, large optical absorption coefficients,^13,14 extraordinarily long carrier diffusion lengths and high carrier mobilities.^15,16 They present some extensive applications in solar cells, light emitting diodes (LEDs),¹⁷ photodetectors,¹⁸ sensors,¹⁹ field effect transistors (FETs),²⁰ lasers²¹ and so on. As a typical bipolar charge transport semiconductor,²² the electrical conductivity of MAPbX₃ is greatly influenced by some oxidizing and reducing gases. For instance, ammonium gas could induce a quick reversible phase transformation in a perovskite film, resulting in an obvious decrease in its resistance.^23,24 MAPbI₃ was also sensitive to some polar organic compounds, such as 4-tert-butylpyridine²⁵ and acetonitrile.²⁶ MAPbI_3−xCl_x films could absorb many types of gases, e.g. NO₂, SO₂, alcohol and acetone, which caused obvious changes in their conductivity.²⁷ In addition, the resistance of an MAPbI_3−xCl_x thin film also exhibited a quick reversible decrease in the presence of moisture. These quick room-temperature reversible behaviors of perovskite films indicate their potential application in the field of gas sensors. Thus, MAPbX₃-based gas sensors with low working temperatures, and quick responses and recovery speeds can be expected. However, most research groups have focused on the photoelectric fields of MAPbI_3−xCl_x and a few investigations have been reported on MAPbX₃-based gas sensors. Furthermore, because the stability of organic–inorganic hybrid semiconductors is strikingly improved under a high-pressure atmosphere,²⁸ it would be quite interesting to investigate the response performance of stabilized perovskite films to target gases under high pressure.

Herein, we prepare MAPbI₃ thin film gas sensors and investigate their gas-sensing properties under both ambient and high pressure. Interestingly, the gas sensors exhibit rather quick responses and recovery speeds, excellent selectivity and high sensitivity to NO₂ gas even at room temperature. Furthermore, the sensitivity of the prepared sensor is strikingly improved under high-pressure gas. Additionally, theoretical simulation of the adsorption of NO₂ molecules on the surface of perovskite is conducted to illustrate the response mechanism of the sensor.

2. Experimental

2.1 Preparation of perovskite thin film and gas sensor devices

In a typical process, 1 mmol of methylammonium iodide (MAI, AR, Tokyo Chemical Industry Co., Ltd, Tokyo, Japan) and 1 mmol of PbI₂ (AR, Aladdin Industrial Corporation, Shanghai, China) were successively dissolved in 1 mL N,N-dimethyl formamide (DMF, AR, Sinopharm Chemical Reagent Co., Ltd, China) at 65 °C under stirring for 12 h.^29,30 Afterwards, the precursor solution was spin-coated on an alumina (Al₂O₃) ceramic substrate (10 × 14 mm) at 1500 rpm for 9 s, followed by spinning at 2000 rpm for another 10 s. The film was then dried under high purity argon gas at 100 °C for 30 min. Finally, gold interdigital electrodes were sputter-coated on the perovskite film (Fig. S1†). Then, the gas sensor was prepared for further characterization.

2.2 Characterization of samples

Fourier transformation infrared (FTIR) absorption spectra were collected on a Thermo-Nicolet NEXUS 670 infrared spectrometer using the attenuated total reflection (ATR) mode, and the wavenumber resolution was 4 cm⁻¹ (range 4000–650 cm⁻¹). X-ray diffraction (XRD) patterns were recorded on a Bruker-AXS D8 Advance X-ray diffractometer with CuKα radiation (λ = 1.54186 Å) in the 2θ range of 10–60°. The morphology was observed via scanning electron microscopy (SEM, Hitachi S-4800) and scanning probe microscopy (SPM, Veeco Instruments Inc, Dimension Icon). The transient gas response characteristics (I–t curves) of the gas sensors were examined using a Keithley 4200-SCS semiconductor parameter analyzer at room temperature (25 ± 3 °C) in a specially designed gas cabinet with a volume of 180 mL, and the relative humidity was 25–30%.

The high-pressure and atmosphere pressure gas-sensing performances of the sensors were examined using a specially designed chamber with a capacity of 180 mL (Fig. S2†), which was connected to the Keithley 4200-SCS semiconductor parameter analyzer. Before characterization, the air in the chamber was completely expelled with high-purity argon gas. Afterwards, a specific volume of target gas was stored in the target gas cylinder which was connected to the chamber. When the pressure inside the chamber was increased to designated pressures (1, 2, 3, 4, 5, 6, 7 and 8 MPa) by introducing high-pressure argon gas, the target gas stored in the cylinder was injected into the testing chamber. The variation in the resistance of the sensors was recorded simultaneously. When the measuring process was finished, the high-pressure gas was released from the chamber, which carries away the target gases. Then the resistance of the sensor recovered to its original value. Details of the operation process are presented in ESI Fig. S3.†

For oxidizing gases, we define the response R of a gas sensor as:

whereas, for reducing gases, the sensor response R is defined as:

For both oxidizing and reducing gases, the sensitivity S is defined as:

where, R_g and R_a are the resistances of the sensor with and without target gases, respectively. c_t is the concentration of target gases. Besides, the response (recovery) time of the sensors is defined as the time interval in which the variation in resistance reaches 90% of the total change upon introducing (evacuating) the target gases, respectively.

2.3 Theoretical calculation

To explain the gas-sensing experimental results, molecular dynamics simulation was conducted using the FORCITE package in the Materials Studio. The COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies)^31,32 force-field was performed to describe the interaction between NO₂ and the surface atoms of perovskite. For the case in which NO₂ interacted with perovskite, the (110) surface was created by periodical replication of an elementary cell in both x and y directions, where the periodical condition was applied. The thickness of the vacuum slab was set to be 15 Å in order to avoid unnecessary interactions between adjacent layers. The NVT ensemble with a simulation time of 5 ns was used to run the test, and the step was maintained at 1 fs.

3. Results and discussion

Initially, the phase composition of the film was identified via XRD. In Fig. 1a, the peaks at 14.1, 28.4, 31.9 and 43.2° can be indexed to the (110), (220), (310) and (314) planes of tetragonal MAPbI₃, respectively,³³ while the other peaks originate from the Al₂O₃ substrate (JCPDS no. 46-1212). It was reported that the porous morphology plays an important role in the gas-sensing performance of a gas sensor.³⁴ Fig. 1b presents the surface morphology of the MAPbI₃ film, which was constructed by interconnected MAPbI₃ nanoplates. The film has a large number of pores, which is also supported by the cross-section image (Fig. S4†). The MAPbI₃ nanoplates were also measured via TEM (Fig. S5†). Furthermore, the height profile of the film recorded with AFM (within an area of 20 × 20 μm², Fig. 1c and d) reveals a rough surface with a root mean square roughness (MRS) of ∼125 nm. This structural characteristic provides plentiful diffusion channels and active surface sites for the target gas molecules. Thus, a good gas-sensing performance can be anticipated.

Fig. 1 Characterization of the prepared MAPbI₃ film. (a) XRD pattern, (b) SEM image and (c and d) AFM morphology profiles.

Furthermore, the gas-sensing performance of the MAPbI₃ film to NO₂ gas at different concentrations was tested. The sensor response was then calculated using eqn (1), as plotted in Fig. 2. When NO₂ gas was injected into the testing chamber, the resistance of the film sensor with NO₂ adsorbed obviously decreased. This tendency was more prominent at higher concentrations of NO₂ (Fig. 2a). The inset reveals an almost linear relationship between the sensor response and the concentration of NO₂, and the average sensitivity was as high as 0.62 ppm⁻¹ (Fig. 2a). Besides, the dynamic response curves and sensor response vs. NO₂ concentration under different bias voltages are shown in Fig. S6.† The signal-to-noise ratio slightly increased with the improvement in bias voltage. More importantly, the detection concentration of NO₂ gas was as low as 1 ppm, and a clear response was still presented (Fig. 2a and S6†). An extremely low detection limit of the MAPbI₃ film sensor, which indicates rather high sensitivity at room temperature, could be undoubtedly expected. More interestingly, the room-temperature sensing performance of the MAPbI₃ film sensor for NO₂ strikingly surpassed the high-temperature performance of most conventional metal oxide sensors (Table 1).^19,35–40 Compared to the room-temperature CuTAP(t-Bu)₄ film sensors, the gas-sensing performance of the prepared MAPbI₃ film is also superior (Table 1). In addition, the response–recovery curve of the film (Fig. 2b) exhibits a much quicker response and recovery speed than MAPbI_3−x(SCN)_x film sensors at room temperature. The average response and recovery times were ∼5 s and ∼25 s, respectively, which are preponderant for a room-temperature gas sensor.

Fig. 2 (a) Dynamic response curves of the MAPbI₃ film sensor recorded in different concentrations NO₂ (1–60 ppm), inset shows the response of the MAPbI₃ film sensor vs. concentration of NO₂. (b) Response and recovery processes in 2 ppm NO₂. All the curves were recorded at room-temperature under a bias-voltage of 10 V.

Table 1 Comparison of the performances of NO₂ gas sensors fabricated from different semiconductor materials

Structure	Operating temperature	NO2 concentration (ppm)	Sensitivity	Response time (s)	Recovery time (s)	Detection of target gases	Ref
In2O3 nanowires	250 °C	1	2.57	20	40	NO2	35
SnO2–ZnO/polyaniline	180 °C	35	368.9	9	27	NO2	36
P3HT–SnO2 composite	100 °C	30	55	50	30	NO2	37
CuTAP(t-Bu)4 films	Room temperature	50	5	270	540	NO2	38
ZnO-nanowire	225 °C	0.5	15	24	12	NO2	39
WO3 nanoplates	100 °C	5	10	50	500	NO2	40
MAPbI3−x(SCN)x	Room temperature	0.2	3	222	360	NO2	19
Our work	Room temperature	1	3.3	22	13	NO2

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Besides the sensor response, reproducibility and selectivity are also crucial for gas sensors. In order to investigate the reproducibility of the MAPbI₃ film gas sensor, NO₂ was repeatedly injected into and evacuated from the testing chamber. The sensor response of the film remained stable after more than 12 cycles (Fig. 3a). For comparison, the reproducibility of the MAPbI₃ film sensor in ambient environment with a relative humidity of 35% was also examined (Fig. S7†). Compared with the situation in Ar atmosphere, although the current of this sensor slightly decreased, it still retained good stability after four cycles. To examine the selectivity of the sensor, NO₂ (1% NO₂ in Ar), SO₂ (1% SO₂ in Ar), HCHO (1% HCHO in Ar), CH₄, CO, NH₃ (1% NH₃ in Ar), (CH₃)₃N, O₂ (21% O₂ in Ar) and H₂O were selected as reference gases. According to the definition of sensitivity (eqn (3)), we calculated the sensitivities of the MAPbI₃ film sensor to the abovementioned gases, which are presented in Fig. 3b. The sensitivity of the MAPbI₃ film sensor to NO₂ was 0.67 ppm⁻¹, whereas that for the reference gases were rather low, such as 0.016 ppm⁻¹ for SO₂, 0.011 ppm⁻¹ for NH₃, 0.034 ppm⁻¹ for HCHO, 0.0001 ppm⁻¹ for CH₄, 0.0001 ppm⁻¹ for CO, 0.00005 ppm⁻¹ for (CH₃)₃N, 0.0042 ppm⁻¹ for H₂O and 0.0001 ppm⁻¹ for O₂. The dynamic response and recovery curves are shown Fig. S8.† The MAPbI₃ film sensor response to the reference gases was in striking contrast with its response to NO₂, except for H₂O and O₂. The resistance increased when the reference gases were injected into the testing chamber. This characteristic caused the MAPbI₃ film sensor to quite easily distinguish NO₂ from the other gases.

Fig. 3 (a) Reproducibility of the MAPbI₃ sensor exposed to 30 ppm NO₂ and (b) sensitivity to a series reference gases.

In order to analyze the gas-sensing mechanism, in situ FTIR measurements of the MAPbI₃ film exposed to NO₂ gas for different durations were conducted. The obtained spectrum (Fig. 4a) is in good accordance with the reported result.⁴¹ The bands at 3091, 2819, 1659, 1487, 1416, 1244 and 902 cm⁻¹ are attributed to the N–H stretch, C–H stretch, N–H bend, C–H bend, C–N stretch, C–H twist and C–H rock vibration modes, respectively.⁴² It was worth noting that a new band appeared at 1325 cm⁻¹ (Fig. 4b), which can be assigned to the surface bound NO₂⁻ ions formed by charge transfer chemisorption.^43,44 When the film was exposed to NO₂ for a longer time, more gas molecules were adsorbed on the active sites in the porous film. Therefore, this new band became stronger (Fig. 4c). The electron transfer process is also supported by the red-shift of the N–H stretching vibration band from 3091 cm⁻¹ to 3076 cm⁻¹. The corresponding increase in hole concentration of the P-type MAPbI₃ film led to the resistance decreasing in the electrical field.

Fig. 4 FTIR spectra of the MAPbI3 film sensor exposed to 1% NO₂ for different durations.

The interaction between the adsorbed NO₂ molecules and MAPbI₃ film was further investigated via computational simulations. Fig. 5a–c show the radial distribution functions (RDFs) of NO₂–H_MA⁺ on the typical (110) facet of perovskite, where, H_MA⁺ includes two types of –CH₃ and –NH₃. The peaks at 2.50 Å and 2.63 Å (Fig. 5a) reveal a stronger hydrogen-bond interaction between –CH₃ and NO₂, compared to that between –NH₃ and NO₂ (the peaks at 3.53 Å and 2.94 Å, respectively (Fig. 5b)). This result is consistent with that of FTIR spectra quite well. Subsequently, a schematic illustrating the electron transfer process from perovskite to the NO₂ molecule via the hydrogen-bond channel was proposed (Fig. 5c). According to the FTIR spectra and theoretical simulation results, the adsorbed NO₂ molecules first interact with the –CH₃ groups and then attract some electrons from the organic components. In fact, the inorganic Pb–I skeleton in the perovskite semiconductor serves as the major transportation path for the charge carriers.⁴⁵ Thus some electrons on the Pb–I skeleton subsequently transfer to MA⁺ ions via the NH⋯I bonds (Fig. 5c). As a result, the hole concentration of the perovskite film is enhanced, leading to a striking decrease in the resistance of the film.

Fig. 5 (a and b) Radial distribution functions of adsorbed NO₂ molecules on the (110) face of MAPbI3. (c) Proposed model to explain the gas-sensing process of the perovskite film sensor to NO₂ gas.

In addition, based on the theoretical calculation (Fig. S9†), the interaction between the NO₂ molecules and perovskite film is enhanced with the improvement in gas pressure due to the increase in adsorption energy. Consequently, more effective electron transfer can be expected between them under a high-pressure circumstance, which plays an important role in the excellent performance of this sensor. Therefore, the effect of pressure on the gas-sensing performance was investigated. In our experiments, V, R and T are all constants, thus the molar number of argon gas n is simply proportional to the pressure p. According to the ideal gas equation, n = p/p₀ × n₀ (where, n₀ is the molar number of argon gas at atmospheric pressure and p₀ is atmospheric pressure). Besides, the molar number of target gases n_t is much less than n (Fig. S10†), thus the molecule number concentration of target gases at high pressure c_t can be expressed as:

According to the way that we ignore the molar number of target gases, the error was determined to be only 0.005‰ (Fig. S10†), thus this approach is feasible.

Fig. 6a shows the dynamic response of the MAPbI₃ film sensor to NO₂ gas at high pressures, where the molecule number concentration of NO₂ remains constant for all the pressures, namely, 5 ppm. Obviously, the sensor response strikingly increased in a near-linear way with an increase in pressure, and the MAPbI₃ film sensor still exhibited rather quick response and recovery times of 11–25 s and 20–60 s under high pressure. According to eqn (3) and (4), we calculated the sensitivity of the MAPbI₃ film sensor under high pressure, and the results are presented in the inset in Fig. 6a. Interestingly, the sensitivity of the MAPbI₃ film sensor strikingly increased under high pressure, which should be closely related with the fact that the adsorption energy of the NO₂ molecules on the surface of the sensor increased (Fig. S9†) and more NO₂ molecules were strongly absorbed the surface of MAPbI₃ film under higher pressure,⁴⁶ resulting in more effective electron transfer and an increase in hole concentration. Actually, in the ideal gas model, the adsorption of gas molecules is not only determined by the adsorption energy, but also depends on the partial pressure of the target gas. Therefore, the positive effect of the partial pressure of NO₂ cannot be ignored for the enhanced sensitivity. Besides, the MAPbI₃ film sensor still exhibited quite high selectivity towards NO₂ even at high pressure (Fig. S11†). This phenomenon has potential applications in improving the performance of gas sensors when detecting ultra-low concentrations of target gases.

Fig. 6 (a) Dynamic response curve of the MAPbI₃ film sensor recorded in NO₂ with the same molecule number concentration at different pressures, inset shows the sensitivity of the MAPbI₃ film sensor vs. pressure under the same molecule number concentration of NO₂. (b) Transient curve when high-pressure argon gas and NO₂ are successively introduced into the testing chamber at 1 and 7 MPa. All the curves were recorded at room-temperature under a bias-voltage of 10 V.

4. Conclusions

By utilizing an organic–inorganic hybrid semiconductor, a gas sensor with strikingly improved sensitivity and rather quick response and recovery speed was prepared at room temperature. Furthermore, the sensitivity of the gas sensor was greatly increased under high-pressure argon gas, providing the possibility of enabling the sensor to detect target gases with ultra-low concentrations by applying a high-pressure. These findings may provide a new route for the fabrication of ultra-sensitive gas sensors for the on-line monitoring of chemical reaction processes and in situ analyzing impurity gases under high-pressure circumstances.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (NSFC 51372143, 51102151) and Natural Science Foundation of Shandong Province (ZR2011EMQ002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra11149e

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