Remember me
Herein, for that matter, lead-free stacked layered Cs3Sb2Br9 perovskites were successfully synthesized via a modified ligand-assisted reprecipitation (m-LARP) approach and explored their humidity sensing properties relevant for humidity sensor applications. The humidity sensors of the as-fabricated Cs3Sb2Br9 exhibit a very high impedance change scope of 5.6 E5 in the surrounding with relative humidity (RH) changing from 11% to 95% and a short recovery/response time below 1 s, which could be employed for the real-time exploration of moisture. More exciting, the testing range can be extended to a very low humidity (2% RH). The excellent performances of such Cs3Sb2Br9-based perovskite humidity sensors illustrate more possible usages of lead-free perovskites in the domain of sensors.
The crystal structure of Cs3Sb2Br9 originates from the conventional structure of the perovskite CsSbBr3 [Fig. 1(a)] through moving each third Sb layer along the ⟨111⟩ plane so as to gain the right charge equilibrium. The unit cell of Cs3Sb2Br9 perovskite is graphically depicted, as seen in Fig. 1(b). It contains bioctahedral (Sb2Br9)3− clusters that are enclosed by cations of the cesium. Figure 1(c) shows XRD (x-ray diffraction) patterns of as-prepared Cs3Sb2Br9 perovskites. Diffraction peaks consist with the standard triclinic P63mmc Cs3Sb2Br9 phase (ICSD #77–1055) without any impurity phases. For the purpose of probing the steadiness of our Cs3Sb2Br9 perovskites, the sample was conserved under the air condition (30%–60% RH, 298 K) for one month and in 95% RH bottle for one day. XRD patterns of the conserved sample are analogous to the fresh one and maintain main crystal planes, such as (201), (112) coupled with (022), demonstrating the stability of these Cs3Sb2Br9 samples.The morphology of the as-fabricated Cs3Sb2Br9 perovskites could be seen through scanning electronic microscopy (SEM) [Figs. 1(d) and 1(e)]. The diameters of 0.5–1 μm are found with a quasi-hexagonal block in shape. The surface is not smooth but many gullies on the surface, which will benefit humidity sensing performance because of the increase in the contact area with water molecules. Moreover, XPS was employed in order to probing chemical components of Cs3Sb2Br9. As shown in Fig. 1(f), Cs 3d, Sb 4d, and Br 3d were detected. As to the Cs element, two strong peaks are noticed at approximately 724.2 and 738.5 eV, corresponding to Cs 3d5/2 and Cs 3d3/2, separately. As to Br 3d5/2 and Br 3d3/2 of Br 3d, eminent peaks are located at around 68.9 and 69.7 eV, respectively, when the couple peaks located at 530.1 and 539.4 eV are ascribed to Sb 3d5/2 and Sb 3d3/2,2727. Z. Zheng, Q. Hu, H. Zhou, P. Luo, A. Nie, H. Zhu, L. Gan, F. Zhuge, Y. Ma, H. Song et al., Nanoscale Horiz. 4, 1372–1379 (2019). https://doi.org/10.1039/C9NH00426B respectively, indicating its predominant +3 state.27,2827. Z. Zheng, Q. Hu, H. Zhou, P. Luo, A. Nie, H. Zhu, L. Gan, F. Zhuge, Y. Ma, H. Song et al., Nanoscale Horiz. 4, 1372–1379 (2019). https://doi.org/10.1039/C9NH00426B28. Z. Imran, S. S. Batool, H. Jamil, M. Usman, M. Israr-Qadir, S. H. Shah, S. Jamil-Rana, M. A. Rafiq, M. M. Hasan, and M. Willander, Ceram. Int. 39, 457–462 (2013). https://doi.org/10.1016/j.ceramint.2012.06.048 On the basis of the analytical results of XPS, SEM, and XRD, large dimension and high crystalline Cs3Sb2Br9 perovskites were successfully synthesized in our work.The experimental setup as shown in Fig. S2 was utilized to investigate the moisture sensing performance of Cs3Sb2Br9 sensors in disparate RH followed Ref. 2929. W. D. Lin, C. T. Liao, T. C. Chang, S. H. Chen, and R. J. Wu, Sens. Actuators, B: Chem. 209, 555–561 (2015). https://doi.org/10.1016/j.snb.2014.12.013. The target humidity can be obtained by adjusting the ratio of dry air to wet air, where a humidity sensor is used to monitor and calibration the actual humidity. The flow rate is a fixed value of 4.3 l min−1, which is the optimized value of flow rate before leaving the factory. With a view to determine the best frequency, a relation picture of the humidity and impedance at disparate frequencies was attained, as exhibited in Fig. 2(a). At a voltage of 1 V, the impedance reduces as RH grows from 11% to 95% for each test frequencies. The impedance variation on one logarithmic scope is hard to differentiate at low humidity but reduces linearly at high humidity, providing a feasible moisture sensing scope. For our humidity experiment setup, the impedance under 11% RH for 10 and 50 Hz is too large, which exceeds the measuring range and cannot be obtained by our humidity experiment setup, so there are no data of the impedance under 11% RH for 10 and 50 Hz. The largest impedance change in magnitude of over five orders is obtained at 100 Hz. Therefore, the optimal frequency of 100 Hz is adopted for succedent moisture-sensing attributes test of the Cs3Sb2Br9 sensor. The humidity sensitive response (S value) is able to be confirmed by means of formula below: S=ZL/ZH, where ZL is the impedance at 11% RH and ZH for impedance at other level of RH. For our Cs3Sb2Br9 humidity sensor, impedance worth for 11%–95% RH is consecutively found to be 621 223 552, 253 466 896, 5 600 522, 2683, and 1117 Ω. Therefore, the maximum S value can reach 5.6E5 from 11% to 95% RH at 100 Hz.During the real-time humidity exploration, the sensor is requested to response and recover rapidly. The real-time impedance of the Cs3Sb2Br9 perovskite humidity sensor at RH from 11% to 95% for eight cycles is shown in Fig. 2(b), demonstrating the outstanding dynamic steadiness and repeatability. Synchronously, the response and recovery time of the sensor, estimated in the surrounding with humidity shift from 11% to 95%, reach 0.9 and 3.0 s [Fig. 2(c)], separately. A high-sensitive response and fast response speed are comparable with armies of humidity sensing materials, as shown in Table I. The consequences boost the possible application in rapid response humidity sensors with the super-high sensitivity. The long-run humidity monitoring test is implemented with disparate RH worth, which is shown in Fig. 2(d). Having been conserved under the ambient air without any protection, lasting for fifty days, the Cs3Sb2Br9 humidity sensor exhibits negligible changes in the impedance, indicating excellent stability against aging.
TABLE I. Comparison of humidity sensing properties of Cs3Sb2Br9 perovskite obtained in the present work with other perovskite materials reported in recent studies.
Sensing materialsImpedance variation (Ω)tres (s)trec (s)Measure range (%)ReferenceTiO2/SrTiO3104–1083.17611–753030. M. Zhang, S. Wei, W. Ren, and R. Wu, Sensors 17, 1310 (2017). https://doi.org/10.3390/s17061310CdTiO3107–1084640–902828. Z. Imran, S. S. Batool, H. Jamil, M. Usman, M. Israr-Qadir, S. H. Shah, S. Jamil-Rana, M. A. Rafiq, M. M. Hasan, and M. Willander, Ceram. Int. 39, 457–462 (2013). https://doi.org/10.1016/j.ceramint.2012.06.048NaNbO3106–101133–2320–803131. Y. Zhang, X. Pan, Z. Wang, Y. Hu, X. Zhou, Z. Hu, and H. Gu, RSC Adv. 5, 20453–20458 (2015). https://doi.org/10.1039/C5RA00205BLa0.7Sr0.3MnO310–1050.84.911–953232. Z. Duan, M. Xu, T. Li, Y. Zhang, and H. Zou, Sens. Actuators, B: Chem. 258, 527–534 (2018). https://doi.org/10.1016/j.snb.2017.11.169K0.5Na0.5NbO310–10781811–953333. M. Yuan, Y. Zhang, X. Zheng, B. Jiang, P. Li, and S. Deng, Sens. Actuators, B: Chem. 209, 252–257 (2015). https://doi.org/10.1016/j.snb.2014.11.118Cs3Cu2Br5103–109221033–953434. Y. Huang, C. Liang, D. Wu, Q. Chang, L. Liu, H. Liu, X. Tang, Y. He, and J. Qiu, J. Phys. Chem. Lett. 12, 3401–3409 (2021). https://doi.org/10.1021/acs.jpclett.1c00559Cs2BiAgBr6108–10111.780.455–753535. Z. Weng, J. Qin, A. A. Umar, J. Wang, X. Zhang, H. Wang, X. Cui, X. Li, L. Zheng, and Y. Zhan, Adv. Funct. Mater. 29, 1902234 (2019). https://doi.org/10.1002/adfm.201902234CsPbBr3104–1052.89.711–853636. Z. Wu, J. Yang, X. Sun, Y. Wu, L. Wang, G. Meng, D. Kuang, X. Guo, W. Qu, B. Du et al., Sens. Actuators, B: Chem. 337, 129772 (2021). https://doi.org/10.1016/j.snb.2021.129772Cs2PdBr6104–1090.71.711–953737. W. Ye, Q. Cao, X. Cheng, C. Yu, J. He, and J. Lu, J. Mater. Chem. A 8, 17675–17682 (2020). https://doi.org/10.1039/D0TA05193DCH3NH3PbI0.2Cl2.8104–108242430–903838. K. Ren, L. Huang, S. Yue, S. Lu, K. Liu, M. Azam, Z. Wang, Z. Wei, S. Qu, and Z. Wang, J. Mater. Chem. C 5, 2504–2508 (2017). https://doi.org/10.1039/C6TC05165KCs3Bi2Br9108–10112.566.2375–901818. C. Pi, W. Chen, W. Zou, S. Yan, Z. Liu, C. Wang, Q. Guo, J. Qiu, X. Yu, B. Liu, and X. Xu, J. Mater. Chem. C 9, 11299–11305 (2021). https://doi.org/10.1039/D1TC02339JCs3Sb2Br9103–1080.932–95This workIn the absorption and desorption courses, humidity dependence of the impedance curve is expressed in Fig. 3(a), which gives the hysteretic impact of the as-fabricated Cs3Sb2Br9 humidity sensor. It is evident that the desorption and adsorption curves are greatly matched, which shows extremely tiny hysteresis in all the humidity scope. The hysteresis value is calculated by the formula2929. W. D. Lin, C. T. Liao, T. C. Chang, S. H. Chen, and R. J. Wu, Sens. Actuators, B: Chem. 209, 555–561 (2015). https://doi.org/10.1016/j.snb.2014.12.013 of log(Zads)−log(Zdes)log(Zads)×100% in which Zads is the impedance value of the adsorption course, whereas Zdes for the desorption. The max hysteretic value of 2.9% is obtained at 73% RH, which is shown in Fig. 3(b). Such a good reversibility illustrates that the sensor is able to absorb and desorb the water molecules freely, depending on the outside humidity.The detection in ultra-low humidity environment is a higher requirement for the humidity sensor application, such as glove boxes, cultural relics preservation, and medical processing. For the Cs3Sb2Br9 humidity sensor, we further carried out tests under lower humidity conditions. Figure 3(c) shows the change in the intensity peak with an increasing and decreasing cycle from 2% RH to 11% RH. Notably, the Cs3Sb2Br9 humidity sensor still shows excellent impedance variation in the ultra-low humidity range (as low as to 2% RH), and the moisture adsorption and desorption are almost symmetrical. In this work, the humidity testing range can be extended to a very low humidity (2% RH) such exploration has never been reported in other perovskite-based humidity sensors. The lower humidity tests have not been done as the setup in our lab cannot achieve humidity lower than 2%. Notably, this sensor is on the foundation of the variances in electric properties, which has far more factual significance than that on the foundation of the variances in optical properties, because electrical properties are easier and more accurate to detect than optical properties.Table I compares the sensing capabilities for Cs3Sb2Br9 humidity sensor to those of other impedance-type humidity sensors that have been documented in the published literature. The comparison shows that our Cs3Sb2Br9 humidity sensors have been assured to have superior humidity detecting qualities since they have the greatest sensitivity and equivalent recovery and response time performances. The comparison attaches importance to the supreme sensitivity and comparable implementation of response and recovery time, illuminating that the super humidity sensing attributes from our Cs3Sb2Br9 humidity sensors have been secured.The surface water absorption of impedance-type humidity sensors has a significant impact on their performance. In general, chemisorption and physisorption are involved in the surface adsorption of H2O molecules.3939. J. H. Anderson and G. A. Parks, J. Phys. Chem. 72, 3662–3668 (1968). https://doi.org/10.1021/j100856a051 The water molecules adsorption procedure can be introduced to give an explanation for the humidity sensing mechanism of the fabricated sensors at room temperature. As shown in Fig. 4 (stage I), the pre-adsorbed oxygen species on the surface of the sensing material are likely to trap electrons near the surface (O2(ads) + e− → O2(ads)−).4040. N. Zhang, Y. L. Deng, Q. D. Tai, B. R. Cheng, L. B. Zhao, Q. L. Shen, R. X. He, L. Hong, W. Liu, S. Guo et al., Adv. Mater. 24, 2756–2760 (2012). https://doi.org/10.1002/adma.201200155 Under low humidity conditions, H2O molecules are going to take place of the pre-adsorbed oxygen and then release electrons back (stage II). Then, H+ and the nearby surface O2− ion combine to generate the second OH− group, which follows the dissociation of water molecules into OH− and H+.4141. M. Zhuo, T. Yang, T. Fu, and Q. Li, RSC Adv. 5, 68299–68304 (2015). https://doi.org/10.1039/C5RA09903J Therefore, the formation of chemically absorbed OH− can be described to be H2O + O2− → 2OH−.4242. M. Gong, Y. Li, Y. Guo, X. Lv, and X. Dou, Sens. Actuators, B: Chem. 262, 350–358 (2018). https://doi.org/10.1016/j.snb.2018.01.187 The high impedance of the sensor in this process can be explained using the proton conductivity model of Anderson and Parks.3939. J. H. Anderson and G. A. Parks, J. Phys. Chem. 72, 3662–3668 (1968). https://doi.org/10.1021/j100856a051 The discomposed bare proton H+ jumps to one neighboring H2O molecule in the domain of an external electricity (stage III). At this stage, there are only a few H2O molecules absorbed on the materials surface, and because of the patchy surface covering by water, ion transport is challenging to achieve.With the increase in RH, the chemical adsorption is completed and physisorption starts. As shown in stage IV of Fig. 4, OH− groups offer the nucleation sites to the other water molecules through hydrogen bond physical adsorption. The first physisorbed water layer is fixed due to the restriction of neighboring two hydrogen bonds and will not affect the impedance of the sensors. However, through hydrogen bonds, more water molecules would be physisorbed on the first water layer. According to the mechanism of Sheng et al.,4343. M. Sheng, L. L. Gu, R. Kontic, Y. Zhou, K. B. Zheng, G. R. Chen, X. L. Mo, and G. R. Patzke, Sens. Actuators, B: Chem. 166–167, 642–649 (2012). https://doi.org/10.1016/j.snb.2012.03.030 as the water layer builds up, protons can tunnel via hydrogen bonds from one water molecule to an adjacent one, where the latter receives the proton and then releases another, and the like. The dissociation reaction of physically adsorbed water is 2H2O → H3O+ + OH−. The beginning state along with the ending state is the identical: H2O + H3O+ → H3O+ + H2O. In this process, these H2O molecules could shape one liquid-like net by hydrogen-bonding. In liquid water, the hydration of protons (H3O+) is energetically advantageous. As a result of physisorption, the free movement of hydrated proton can significantly reduce the resistance of the Cs3Sb2Br9-based humidity sensor. According to the above mechanism analysis, we believe that it is easier for Cs3Sb2Br9 perovskites to adsorb water molecules and complete the process from chemical adsorption to physical adsorption. Additionally, the as-fabricated Cs3Sb2Br9 perovskite exhibits quasi-hexagonal block with the size of 0.5–1 μm, and many gullies appear on the surface, which increase the contact area with water molecules.In order to further confirm humidity sensing mechanism of the Cs3Sb2Br9 humidity sensor, CIPs (complex impedance plots) have been investigated. Figure 5(a) shows the Nyquist plot measured by Cs3Sb2Br9 humidity sensors at different RH, with ImZ and ReZ corresponding to the imaginary and real parts of the complex impedance, respectively. The Nyquist plot is almost straight at the lower RH (11%), indicating that there are little intrinsic dipoles of Cs3Sb2Br9 in an AC electric field. The Nyquist curve is a complete semicircle at 33% relative humidity, indicating that conductivity increases with RH increasing. This is consistent with the above-mentioned proton conductivity model. Only a few water molecules are adsorbed on the Cs3Sb2Br9 surface, and because the water is not evenly distributed and proton migration only occurs via hopping, the Cs3Sb2Br9 humidity sensor exhibits high impedance at low humidity levels. Excluding the semicircle, there is a linear tail at low frequencies at 53% RH, which denotes the start of physisorption. When humidity further increases to 73%–95%RH, the semicircle thoroughly disappears. In addition, the impedance reduced rapidly. Water molecules constitute a liquid network by means of hydrogen-bonding, where hydrated protons transfer between near H2O molecules, to further enhance the conductivity.The CIPs were used to obtain the capacitance-frequency curves for the sensor under various RH, as shown in Fig. 5(b). The inset of Fig. 5(b) displays the equivalent circuit. The resistor Rs is in series with the paralleled capacitor C and resistor R. Alternating voltage (AV) frequency only slightly affects the capacitance when it is high. The capacitance, however, starts to rapidly decrease with frequency and increase with RH when the AV frequency falls below 1 × 104 Hz. The aforesaid findings are explained by the relaxation of H2O molecule polarization and proton transfer at low AV frequencies,37,3837. W. Ye, Q. Cao, X. Cheng, C. Yu, J. He, and J. Lu, J. Mater. Chem. A 8, 17675–17682 (2020). https://doi.org/10.1039/D0TA05193D38. K. Ren, L. Huang, S. Yue, S. Lu, K. Liu, M. Azam, Z. Wang, Z. Wei, S. Qu, and Z. Wang, J. Mater. Chem. C 5, 2504–2508 (2017). https://doi.org/10.1039/C6TC05165K which can enhance the dielectric permittivity and cause an increase in capacitance when the RH rises.To sum up, environmental-friendly lead-free Cs3Sb2Br9 perovskite is used to prepare humidity sensors. For humidity ranging of 11%–95%, the sensitivity of the Cs3Sb2Br9 humidity sensor reaches 5.6E5. More exciting, the testing range can be extended to a very low humidity (2% RH). This Cs3Sb2Br9 humidity sensor has a quick adsorption and desorption speed to water molecules, as evidenced by its short reaction time and recovery time of 0.9 s and 3 s, respectively. Additionally, the Cs3Sb2Br9 has a good humidity detecting performance as well as long-term stability for at least 50 days at various RH values. The humidity sensing properties of Cs3Sb2Br9 are better than those of most impedance-type humidity-sensitive materials. This work will help us to understand the Cs3Sb2Br9 humidity sensing mechanism better and is helpful for improving the performance of various applications.
This work was financially supported by General Projects of Chongqing Natural Science Foundation (Nos. cstc2020jcyj-msxmX0619, cstc2021jcyj-msxmX0736, and cstc2019jcyj-msxmX0522); the Scientific and Technological Innovation Project of “double city economic circle construction in Chengdu and Chongqing area” (No. KJCX2020026); Science and Technology Research Program of Chongqing Municipal Education Commission (Nos. KJQN201900515, KJQN202100510, and KJQN202100507); the National Key Research and Development Program of China (No. 2018YFB2200500); the National Natural Science Foundation of China (Nos. 61975023, 51775070, 22072010, and 61875211); Guangdong Province International Scientific and Technological Cooperation Projects (No. 2020A0505100011); State Key laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, No. 2021-KF-19); and Innovation and Entrepreneurship Training Program for College Students (Nos. S202210637012 and S202110637126).
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Han You and Daofu Wu contributed equally to this paper.
Han You: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Xiaosheng Tang: Funding acquisition (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Daofu Wu: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Jian Wang: Data curation (equal); Formal analysis (equal); Project administration (equal); Resources (equal); Validation (equal); Writing – original draft (equal). Jiao He: Data curation (equal); Investigation (equal); Methodology (equal); Resources (equal); Supervision (equal); Validation (equal); Writing – original draft (equal). Xinyi Kuang: Data curation (equal); Formal analysis (equal); Investigation (equal); Resources (equal); Visualization (equal); Writing – original draft (equal). Chenlu Li: Data curation (equal); Formal analysis (equal); Resources (equal); Writing – original draft (equal). Fawen Guo: Data curation (equal); Formal analysis (equal); Supervision (equal); Validation (equal). Dingke Zhang: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Qi Qi: Investigation (equal); Project administration (equal); Software (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).
The data that support the findings of this study are available from the corresponding author upon reasonable request.
REFERENCES
1. W. Xu, F. Li, Z. Cai, Y. Wang, F. Luo, and X. Chen, J. Mater. Chem. C 4, 9651–9655 (2016). https://doi.org/10.1039/C6TC01075J, Google ScholarCrossref2. R. Liang, A. Luo, Z. Zhang, Z. Li, C. Han, and W. Wu, Sensors 20, 5601 (2020). https://doi.org/10.3390/s20195601, Google ScholarCrossref3. Z. Duan, Y. Jiang, and H. Tai, J. Phys. Chem. C 9, 14963–14980 (2021). https://doi.org/10.1039/D1TC04180K, Google ScholarCrossref4. J. Yang, R. Shi, Z. Lou, R. Chai, K. Jiang, and G. Shen, Small 15, 1902801 (2019). https://doi.org/10.1002/smll.201902801, Google ScholarCrossref5. Z. Li, J. Wang, Y. Xu, M. Shen, C. Duan, L. Dai, and Y. Ni, Carbohydr. Polym. 270, 118385 (2021). https://doi.org/10.1016/j.carbpol.2021.118385, Google ScholarCrossref6. M. A. Haque, A. Syed, F. H. Akhtar, R. Shevate, S. Singh, K.-V. Peinemann, D. Baran, and T. Wu, ACS Appl. Mater. Interfaces 11, 29821–29829 (2019). https://doi.org/10.1021/acsami.9b07751, Google ScholarCrossref7. P. Zhang, L. X. Zhang, H. Xu, Y. Xing, J.-J. Chen, and L.-J. Bie, Rare Met. 40, 1614–1621 (2021). https://doi.org/10.1007/s12598-020-01619-7, Google ScholarCrossref8. X. Wang, J. G. Liang, J. K. Wu, X. F. Gu, and N. Y. Kim, Sens. Actuators, B: Chem. 351, 130935 (2022). https://doi.org/10.1016/j.snb.2021.130935, Google ScholarCrossref
Comments (0)