Xianwei Fua,
Shilong Jiaoa,
Ning Donga,
Gang Lian*a,
Tianyu Zhaoa,
Song Lva,
Qilong Wangb and
Deliang Cui*a
aState Key Lab of Crystal Materials, Shandong University, Jinan 250100, P.R. China. E-mail: liangang@sdu.edu.cn; cuidl@sdu.edu.cn
bKey Laboratory for Special Functional Aggregated Materials of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, P.R. China
First published on 2nd January 2018
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.
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 MAPbI3−xClx 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 MAPbI3−xClx 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 MAPbI3−xClx 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.
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:
(1) |
(2) |
For both oxidizing and reducing gases, the sensitivity S is defined as:
(3) |
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 MAPbI3−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.
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 |
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.
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–HMA+ on the typical (110) facet of perovskite, where, HMA+ 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 nt is much less than n (Fig. S10†), thus the molecule number concentration of target gases at high pressure ct can be expressed as:
(4) |
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.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra11149e |
This journal is © The Royal Society of Chemistry 2018 |