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Communication

Yttrium Doping of Perovskite Oxide La2Ti2O7 Nanosheets for Enhanced Proton Conduction and Gas Sensing Under HighHumidity Levels

by
Jian Wang
,
Caicai Sun
,
Jusheng Bao
,
Zhiwei Yang
,
Jian Zhang
* and
Xiao Huang
*
Institute of Advanced Materials (IAM), School of Flexible Electronics (SoFE), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China
*
Authors to whom correspondence should be addressed.
Sensors 2025, 25(3), 901; https://doi.org/10.3390/s25030901
Submission received: 16 December 2024 / Revised: 15 January 2025 / Accepted: 30 January 2025 / Published: 2 February 2025
(This article belongs to the Special Issue Advanced Sensors in Atomic Level)
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Simple Summary

In this work, Y-doped LTO nanosheets were synthesized for NO2 gas sensing under different humidity conditions. These materials showed increased responses at higher humidity levels due to enhanced proton conduction and NO2-assisted proton generation. Notably, Y-doped LTO nanosheets exhibited higher responses than pure LTO, as Y-doping created more oxygen vacancies, improving water adsorption and proton dissociation.

Abstract

Water molecules from the environment or human breath are one of the main factors affecting the accuracy, efficiency, and long-term stability of electronic gas sensors. In this contribution, yttrium (Y)-doped La2Ti2O7 (LTO) nanosheets were synthesized by a hydrothermal reaction, demonstrating improved proton conductivity compared to their non-doped counterparts. The response of Y-doped LTO with the optimal doping concentration to 100 ppm NO2 at 43% relative humidity (RH) was −21%, which is four times higher than that of bare La2Ti2O7. As the humidity level increased to 75%, the response of Y-doped LTO further increased to −64%. Unlike the gas doping effect observed in previous studies of semiconducting metal oxides, the sensing mechanism of Y-doped LTO nanosheets is based on the enhanced dissociation of H2O in the presence of target NO2 molecules, leading to the generation of more protons for ion conduction. This also resulted in a greater resistance drop and thus a larger sensing response at elevated humidity levels. Our work demonstrates that proton-conductive oxide materials are promising gas-sensing materials under humid conditions.

1. Introduction

Nitrogen dioxide (NO2) is a type of atmospheric pollutant released from fuel combustion and industry exhausts [1,2] which can cause environmental issues such as dense fog and acid rain [3,4]. Long-term exposure to parts per billion (ppb) levels and above of NO2 can cause diseases such as pulmonary edema, tracheitis, and pneumothorax [5,6]. Exhaled NO2 in one’s breath is also a gas-phase biomarker related to diseases such as nasal polyposis and chronic obstructive pulmonary disease [7,8]. Therefore, there is a great need to develop NO2 sensing materials and devices [6,9]. Under ambient conditions, the detection of NO2 is often interfered by H2O molecules in humid air or from human breath [10,11,12]. Many gas-sensing materials show decreased responses at high humidity levels due to competitive adsorption between NO2 and H2O molecules [13,14]. Various methods have thus been developed to address this issue, including increasing the surface roughness of materials to decrease surface energies and thus reduce hydrophilicity [15,16] and coating the sensing materials with hydrophobic layers [17,18] or metal nanoparticles [19]. For instance, Pd/TiO2 NTs coated with a hydrophobic polydimethylsiloxane (PDMS) layer was prepared by a thermal evaporating method, which can enhance the humidity resistance and improve the long-term stability of Pd/TiO2 NT-based sensors [20]. In another example, combining electrospinning with a thermal sulfidation approach, a hydrophobic all-inorganic self-supporting SnO2-SnS2-SiO2/SiO2 sensor was fabricated and enhanced moisture resistance was achieved due to the formation of Sn-S bonds during the sulfidation of SnO2 nanofibers [21]. However, the complicated preparation process and poor conductivity of sensing materials in low-humidity environments still hinder the application of these materials. Therefore, other material systems capable of delivering a high gas-sensing performance at high humidity levels require continuous exploration.
La2Ti2O7 (LTO) is an oxide with a layered perovskite structure. It has shown good catalytic activity, stability, and proton transport properties and has been widely applied in electronics, catalysis, and energy storage [22,23,24]. This type of oxide has a general formula of AnBnO3n+2, characterized by corner-shared BO6 octahedra and A cations filling in the dodecahedral holes formed by the BO6 octahedra [25]. Using elements with a lower valence, such as Y3+, Yb3+, and Bi3+, to replace Ti4+ can make LTO lose electrical neutrality and generate oxygen vacancies [26,27,28,29], which can further improve its surface activity and proton transport ability [30].
In this study, Y-doped LTO nanosheets (with doping concentrations of 0.3%, 0.7%, and 1.2% in weight) were synthesized using a hydrothermal method. Among them, 0.7% Y-doped LTO nanosheets exhibited superior proton conductivity over a wide humidity range and demonstrated the best gas-sensing performance. The sensors also exhibited enhanced performance with increasing humidity levels, which is different from conventional gas sensors. The Y-doping-induced formation of oxygen vacancies likely enhances water adsorption, which then reacts with the exposed NO2 to generate more protons, leading to an improved sensing response of the Y-doped LTO nanosheets. Additionally, these doped nanosheets displayed remarkable selectivity towards NO2, underscoring their potential in practical gas-sensing applications such as breath analysis.

2. Materials and Methods

Materials: La(NO3)3·5H2O and Y(NO3)3·6H2O were purchased from Shanghai Alfa Aesar Chemical Co., Ltd., Shanghai, China. Ti(SO4)2 was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. Absolute ethanol (99.7%) was purchased from Wuxi Yasheng Chemical Co., Ltd., Wuxi, China. NaOH and HCl were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All gasses were purchased from Nanjing Special Gas Co., Ltd., Nanjing, China. Interdigitated Au electrodes (0.1 mm spacing over a 1 × 2 cm2 area, substrate: Al2O3) were purchased from Huizhou Xinwenxiong Commerce & Trade Co., Ltd., Huizhou, China.
Preparation of pure La2Ti2O7: Typically, 0.4 mmol La(NO3)3·5H2O and 0.4 mmol Ti(SO4)2 were dispersed in 2 mL deionized water and ultra-sonicated for 5 min to make the solution uniform. Then, 2 mL NaOH (2 mol/L) was slowly added into the mixture. After stirring for 4 h, the whole mixture was added to a 5 mL Teflon-lined stainless steel autoclave and then heated at 240 °C for 10 h. After being naturally cooled to room temperature, the solution was centrifuged and washed with HCl (2 mol/L), deionized water (two times), and ethanol, respectively, and finally re-dispersed in 10 mL ethanol for further use. By following the same procedure, except for partially replacing Ti(SO4)2 with Y(NO3)3·6H2O, La2Ti2O7 doped with different concentrations of Y was prepared.
Characterizations: Transmission electron microscope (TEM, JEOL, 2100Plus, Tokyo, Japan) and high resolution transmission electron microscope (HRTEM, JEOL, 2100F, Tokyo, Japan) were used to investigate the morphologies and microstructures of the samples. X-ray diffraction (XRD, Rigaku, XtaLAB mini II, Tokyo, Japan) was performed with Cu Ka radiation (λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS, Versa Probe, PHI 5000, Chigasaki, Japan) was utilized to analyze the bonding environment of each element and all the binding energies were calibrated to the C 1s band at 284.6 eV. The sample powder was pressed into a pellet and then fixed between two copper electrodes to measure its impedance spectra at an electrochemical station (Autolab 86,567, Englewood, CO, USA). Inductively coupled plasma-atomic emission spectrometry (ICP-AES, Perkin Elmer, Avio200, Waltham, MA, USA) was applied to measure the Y-doped contents of YLTO nanosheets. Dynamic vapor sorption (DVS, SMS-DVS, Intrisic, London, UK) was used to test the water adsorption of the nanosheets.
Gas sensing measurement: Fabrication of the gas sensor typically began with 50 μL ethanoic solution of the sensing material (30 mg/mL) being dropped onto a Au interdigitated electrode (with 0.1 mm spacing over a 2 × 1 cm2 area, Changchun Mega Borui Technology Co., Ltd., Changchun, China). Then, the whole electrode was dried in air at room temperature (RT). Gas sensing tests were conducted at RT in a sealed chamber. Constant currents were applied to the sensor electrodes and the variation in resistance was recorded using a data acquisition system (Agilent 34972A, Santa Clara, CA, USA). A gas diluter (Nanjing Dibaisi Technology Co., Ltd., Nanjing, China) was employed to aerate different concentration target gasses within the test chamber.
Measurement of proton conductivities: The as-prepared samples were cold-pressed into pellets with a diameter of 10 mm under a pressure of 6 MPa. The thickness of the pellets was measured using a Vernier caliper. Then, the pellet was fixed between two copper electrodes to measure its impedance spectra in a home-made chamber, where temperature and humidity were controlled by an oven and saturated salt solutions, respectively. The ionic conductivity (σ) was calculated using σ = L/(R × A), where L is the thickness of the measured pellet, A is the contact area between the sample and the electrode, and R is the resistance obtained by fitting the semicircle.

3. Results

La2Ti2O7 (LTO) nanosheets were prepared by reacting La(NO3)3·5H2O and Ti(SO4)2 hydrothermally [31], and Y-doped LTO (YLTO) nanosheets were synthesized by partially replacing Ti(SO4)2 with Y(NO3)3·6H2O. The Y doping concentrations were 0.3%, 0.7%, and 1.2% in weight, respectively, as measured by ICP-AES. Both LTO and YLTO were 200~800 nm in lateral size, as shown in the TEM images in Figure 1a,b and Figure S1. Therefore, Y doping did not alter the sheet-like morphology of the product. As shown in Figure 1c, lattice spacings of 2.8 and 3.0 Å can be observed in a typical 0.7% Y-doped LTO, assignable to the (410) and (−212) planes of LTO. As shown Figure S2, the selected area electron diffraction (SAED) pattern revealed a pattern along the [02-1] zone axis and exhibited diffraction plots belonging to the (−112) and (212) crystal faces of LTO. Energy-dispersive X-ray (EDX) mapping of a typical nanosheet (Figure 1d) revealed a uniform distribution of La, Ti, O, and Y elements, indicating successful Y doping. The XRD patterns of the Y-doped LTO nanosheets with different doping concentrations (Figure 1e) were compared with the non-doped one (PDF#280517) and showed a slight shift to lower angles. This might be because Ti atoms were partially replaced by Y atoms, which have a larger ionic radius, resulting in lattice expansion and thus a shift in the XRD peaks to smaller angles.
An XPS analysis was performed to investigate the valence states of the elements in the LTO and YLTO nanosheets (Figure 2 and Figures S3–S6). For 0.7% doped LTO, peaks for La3+ and Ti4+ species were clearly observed in the La 3d and Ti 2p spectra, respectively (Figure 2a,b). The high-resolution Y 3d spectrum shows a doublet at 157.4 and 159.5 eV, corresponding to the 3d3/2 and 3d5/2 bands of Y3+, respectively (Figure 2c), further proving the successful doping of Y. In the high-resolution O 1s spectrum, peaks at 529.7, 531.5 eV, and 532.6 eV were observed, corresponding to Ti-O, oxygen vacancies, and C-O, respectively [32]. By calculating the areas under the fitted peaks, we also found that the concentration of oxygen vacancies increased with increasing Y-doping concentrations, from 0 to 0.7% (Figure S4). This is likely because replacing Ti4+ with Y3+ disrupted the charge neutrality, and thus a higher Y doping level favored the loss of more oxygen atoms [33]. However, further increasing the doping concentration from 0.7% to 1.2% did not create more oxygen vacancies, but generated more defects, as evidenced by the increased full-width-at-half-maximum (FWHM) of the XRD peaks (Figure 1e) [34,35].
The sensing performance of the LTO and YLTO nanosheets towards NO2 was first conducted under 43% RH and RT. Their sensing response is defined by ΔR/R0, where ΔR= Rg − R0 and R0 and Rg are the resistances of the sensor before and after exposure to NO2, respectively. As shown in Figure 3a and Figure S7, both LTO and YLTO nanosheets showed decreased resistance upon NO2 exposure. Among the YLTO with different doping levels, 0.7% Y-doped LTO showed the highest sensing responses (Figure 3b). Next, the sensing performance of YLTO nanosheets towards NO2 under different RH levels at RT was further investigated. All of the Y-doped LTO nanosheets exhibited increased sensing responses with increasing RH levels and outperformed non-doped LTO nanosheets (Figure 3c,d, Figures S7 and S8). In addition, the 0.7% Y-doped LTO nanosheets demonstrated a fast response–recovery rate, with a response time of 30 s and a recovery time of 204 s when detecting 10 ppm NO2 at 75% RH, as shown in Figure S9. The 0.7% Y-doped LTO nanosheets had an outstanding performance under high-humidity conditions compared to previously reported single-component metal oxide sensors (Table S1). The sensing response of the 0.7% Y-doped LTO nanosheets toward 5 ppm NO2 under 75% RH was repeatedly tested (Figure 3e), showing their good stability. The sensing response of the 0.7% Y-doped LTO nanosheet remained nearly unchanged even after one week, further demonstrating its excellent durability. (Figure S10). In addition, its sensing response towards NO2 was found to be much higher than that towards other gasses, including SO2, CO, C3H8, CH2O, C3H6O, and C2H6O (Figure 3f), suggesting outstanding selectivity.

4. Discussion

Note that LTO and Y-doped LTO showed a poor/no sensing response under dry or low-humidity (23%) conditions (Figure S11); LTO and YLTO are not typical p-type semiconductors. This suggests that the sensing response observed in Figure 3 was not likely due to the p-doping effect of NO2. Therefore, we suspected that the increased gas sensing performance of LTO and YLTO under higher humidity conditions was related to their ionic conductive nature. We thus measured the impedance spectra of LTO and YLTO nanosheets in H2O and D2O vapor environments to determine their ionic carriers (Figure 4a and Figure S12). The results showed that their ionic conductivities were at least three times higher in an H2O environment than in D2O, suggesting that the main charge carriers were protons rather than hydroxyl ions [36]. We also measured the activation energy (Ea) required for proton conduction in the 0.7% Y-doped nanosheets and found that Ea decreased from above 0.4 eV (0.44 eV) to below 0.4 eV (0.32 eV) as the environment humidity increased from 43% to 75% (Figure 4b and Figure S13), suggesting a transition from the vehicle mechanism to a more effective hopping mechanism for proton conduction [37]. This further agrees with the enhanced sensing performance seen under higher humidity levels.
Based on the above observations, we propose the following sensing mechanism: NO2 gas molecules can interact with H2O molecules adsorbed on the surface of YLTO nanosheets and promote the dissociation of H2O molecules to generate protons [38]. The protons thus increase the ionic conduction of the YLTO nanosheets, resulting in a resistance drop for the sensor (Figure 4c).
The reason why 0.7% Y-doped LTO showed the best gas-sensing performance can be attributed to its superior water adsorption and proton conduction ability. These abilities mainly come from its high concentration of surface oxygen vacancies, based on the above XPS analysis (Figure 2 and Figure S4). These oxygen vacancies can assist in water adsorption (Figure S14) and promote the dissociation of water molecules to yield protons [30].
To further prove our conjecture, the proton conductivities of YLTO nanosheets with different doping concentrations were obtained, based on their Nyquist plots, under 43% RH, 60% RH, and 75% RH (Figure 5). Their proton conductivities were calculated and are summarized in Figure 5d. All the nanosheets exhibited increasing proton conductivities with increasing RH levels. The 0.7% Y-doped LTO nanosheets showed the highest proton conductivity compared with the others, which is in line with their superior sensing performance.
Note that despite the similar oxygen vacancy content in 0.7% Y-doped and 1.2% Y-doped LTO, the proton conductivity of the 1.2% Y-doped LTO was lower than that of the 0.7% Y-doped one. This can be attributed to the interplay between the effects of oxygen vacancies and crystallinity on the proton conductivity of these materials. At relatively low Y contents (≤0.7%), the number of oxygen vacancies increased with the Y content (Figure S4), resulting in improved proton conductivity. At higher Y contents, e.g., 1.2%, an increased defect level or reduced crystallinity was observed, as mentioned above (Figure 1e), which might block the proton transport pathways, resulting in a poorer sensing performance.

5. Conclusions

In summary, Y-doped LTO nanosheets were synthesized and utilized for NO2 gas sensing under different humidity conditions. These sensing materials all showed an increased response under higher humidity levels due to increased proton conduction and NO2-assisted proton generation. In particular, Y-doped LTO nanosheets showed higher responses compared to those based on pure LTO. This is because the Y doping induced the formation of oxygen vacancies, which helped improve the adsorption of water and its dissociation into protons. Our work demonstrates that proton-conductive oxide materials are promising gas-sensing materials for practical use under high humidity levels.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/s25030901/s1, Figure S1. TEM images of (a) 0.3%, (b) 0.7% and (c) 1.2% Y-doped LTO nanosheets; Figure S2. SAED pattern of 0.7% Y doped LTO nanosheet; Figure S3. High-resolution Y 3d spectra of (a) 0.3% and (b) 1.2% Y-doped LTO nanosheets; Figure S4. High-resolution O 1s spectra of (a) pure LTO, (b) 0.3% and (c) 1.2% Y-doped LTO nanosheets. (d) Oxygen vacancy ratio table of YLTO nanosheets; Figure S5. High-resolution La 3d spectra of (a) pure LTO, (b) 0.3% and (c) 1.2% Y-doped LTO nanosheets; Figure S6. High-resolution Ti 2p spectra of (a) pure LTO, (b) 0.3% and (c) 1.2% Y-doped LTO nanosheets; Figure S7. Sensing responses of LTO nanosheets to NO2 under (a) 43% RH, (b) 60% RH and (c) 75% RH; Figure S8. Sensing responses of 0.7% Y-doped LTO nanosheets to NO2 under 60% RH; Figure S9. Dynamic response-recovery curves of 0.7% Y doped LTO to 10 ppm NO2 at 75% RH; Figure S10. Dynamic response-recovery curves of 0.7% Y doped LTO after one week standing; Figure S11. Sensing responses of LTO and 0.7% Y-doped LTO nanosheets to 100 ppm NO2 under 23 % RH; Figure S12. Nyquist plot of LTO nanosheet in H2O and D2O environments under 100% RH; Figure S13. Nyquist plots of 0.7% Y-doped LTO nanosheets under (a) 43% RH and (b) 75% RH; Figure S14. Water adsorption curves of LTO and 0.7% Y-doped LTO nanosheets; Table S1. Comparison of the sensing performance of the recently reported NO2 sensors based single-component metal oxides. References [38,39,40,41,42,43,44,45] are cited in Supplementary Materials.

Author Contributions

Conceptualization, X.H.; methodology, X.H. and J.W.; validation, J.B., C.S. and J.W.; formal analysis, C.S., Z.Y. and J.W.; investigation, X.H. and J.W.; resources, X.H.; data curation, J.W.; writing—original draft preparation, J.W.; writing—review and editing, J.Z. and X.H.; visualization, J.Z. and X.H.; supervision, X.H.; project administration, X.H.; funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (grant nos.: 62374086 and 62288102).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Bai, H.; Guo, H.; Wang, J.; Dong, Y.; Liu, B.; Xie, Z.; Guo, F.; Chen, D.; Zhang, R.; Zheng, Y. A room-temperature NO2 gas sensor based on CuO nanoflakes modified with rGO nanosheets. Sens. Actuators B 2021, 337, 129783–129795. [Google Scholar] [CrossRef]
  2. Dai, J.; Ogbeide, O.; Macadam, N.; Sun, Q.; Yu, W.; Li, Y.; Su, B.-L.; Hasan, T.; Huang, X.; Huang, W. Printed gas sensors. Chem. Soc. Rev. 2020, 49, 1756–1789. [Google Scholar] [CrossRef]
  3. Liang, T.; Dai, Z.; Liu, Y.; Zhang, X.; Zeng, H. Suppression of Sn2+ and Lewis acidity in SnS2/black phosphorus heterostructure for ppb-level room temperature NO2 gas sensor. Sci. Bull. 2021, 66, 2471–2478. [Google Scholar] [CrossRef] [PubMed]
  4. Li, Z.; Zhang, Y.; Zhang, H.; Jiang, Y.; Yi, J. Superior NO2 sensing of MOF-derived indium-doped ZnO porous hollow cages. ACS Appl. Mater. Interfaces 2020, 12, 37489–37498. [Google Scholar] [CrossRef] [PubMed]
  5. Wang, J.; Fatima-Ezzahra, E.; Dai, J.; Liu, Y.; Pei, C.; Li, H.; Wang, Z.; Huang, X. Ligand-assisted deposition of ultra-small Au nanodots on Fe2O3/reduced graphene oxide for flexible gas sensors. Nanoscale Adv. 2022, 4, 1345–1350. [Google Scholar] [CrossRef]
  6. Wei, S.; Li, Z.; Murugappan, K.; Li, Z.; Zhang, F.; Saraswathyvilasam, A.G.; Lysevych, M.; Tan, H.H.; Jagadish, C.; Tricoli, A.; et al. A Self-Powered Portable Nanowire Array Gas Sensor for Dynamic NO2 Monitoring at Room Temperature. Adv. Mater. 2023, 35, e2207199. [Google Scholar] [CrossRef] [PubMed]
  7. Zhang, L.; Li, Z.; Liu, J.; Peng, Z.; Zhou, J.; Zhang, H.; Li, Y. Optoelectronic gas sensor based on few-layered InSe nanosheets for NO2 detection with ultrahigh antihumidity ability. Anal. Chem. 2020, 92, 11277–11287. [Google Scholar] [CrossRef]
  8. Broza, Y.Y.; Vishinkin, R.; Barash, O.; Nakhleh, M.K.; Haick, H. Synergy between nanomaterials and volatile organic compounds for non-invasive medical evaluation. Chem. Soc. Rev. 2018, 47, 4781–4859. [Google Scholar] [CrossRef]
  9. Huang, W.; Zhuang, X.; Melkonyan, F.S.; Wang, B.; Zeng, L.; Wang, G.; Han, S.; Bedzyk, M.J.; Yu, J.; Marks, T.J.; et al. UV–Ozone Interfacial Modification in Organic Transistors for High-Sensitivity NO2 Detection. Adv. Mater. 2017, 29. [Google Scholar] [CrossRef]
  10. Yu, J.; Wang, D.; Tipparaju, V.V.; Tsow, F.; Xian, X. Mitigation of Humidity Interference in Colorimetric Sensing of Gases. ACS Sens. 2020, 6, 303–320. [Google Scholar] [CrossRef]
  11. Nasriddinov, A.; Rumyantseva, M.; Konstantinova, E.; Marikutsa, A.; Tokarev, S.; Yaltseva, P.; Fedorova, O.; Gaskov, A. Effect of Humidity on Light-Activated NO and NO2 Gas Sensing by Hybrid Materials. Nanomaterials 2020, 10, 915. [Google Scholar] [CrossRef]
  12. Lin, H.; Wang, J.; Xu, S.; Zhang, Q.; Cheng, Y.; Han, D.; Wang, H.; Zhuo, K. Au-WO3 Nanowire-Based Electrodes for NO2 Sensing. ACS Appl. Nano Mater. 2022, 5, 14311–14319. [Google Scholar] [CrossRef]
  13. Mohanta, D. Ahmaruzzaman Novel Ag-SnO2-βC3N4 ternary nanocomposite based gas sensor for enhanced low-concentration NO2 sensing at room temperature. Sens. Actuators B Chem. 2020, 326, 128910. [Google Scholar] [CrossRef]
  14. Xu, Y.; Xie, J.; Zhang, Y.; Tian, F.; Yang, C.; Zheng, W.; Liu, X.; Zhang, J.; Pinna, N. Edge-enriched WS2 nanosheets on carbon nanofibers boosts NO2 detection at room temperature. J. Hazard. Mater. 2021, 411, 125120. [Google Scholar] [CrossRef] [PubMed]
  15. Kumar, A.; Gupta, G.; Bapna, K.; Shivagan, D. Semiconductor-metal-oxide-based nano-composites for humidity sensing applications. Mater. Res. Bull. 2022, 158. [Google Scholar] [CrossRef]
  16. Qin, Y.; Jiang, Y.; Zhao, L. Enhanced humidity resistance of porous SiNWs via OTS functionalization for rarefied NO2 detection. Sens. Actuators B Chem. 2018, 283, 61–68. [Google Scholar] [CrossRef]
  17. Mandić, V.; Bafti, A.; Pavić, L.; Panžić, I.; Kurajica, S.; Pavelić, J.-S.; Shi, Z.; Mužina, K.; Ivković, I.K. Humidity Sensing Ceria Thin-Films. Nanomaterials 2022, 12, 521. [Google Scholar] [CrossRef]
  18. Zhu, Y.; Zhang, Y.; Yu, J.; Zhou, C.; Yang, C.; Wang, L.; Ma, L.; Wang, L.J. Highly-sensitive organic field effect transistor sensors for dual detection of humidity and NO2. Sens. Actuators B Chem. 2022, 374. [Google Scholar] [CrossRef]
  19. Yu, S.; Zhang, H.; Lin, C.; Bian, M. The enhancement of humidity sensing performance based on Eu-doped ZnO. Curr. Appl. Phys. 2018, 19, 82–88. [Google Scholar] [CrossRef]
  20. Gao, Z.; Song, G.; Zhang, X.; Li, Q.; Yang, S.; Wang, T.; Li, Y.; Zhang, L.; Guo, L.; Fu, Y. A facile PDMS coating approach to room-temperature gas sensors with high humidity resistance and long-term stability. Sens. Actuators B Chem. 2020, 325, 128810. [Google Scholar] [CrossRef]
  21. Liu, J.; Zhang, J.; Yu, Q.; Liu, Y.; Zhang, X.; Zhu, G.; Jia, Y.; Lu, H.; Gao, J.; Wang, H.; et al. Flexible, Breathable and Hydrophobic SnO2–SnS2–SiO2/SiO2 All-Inorganic Self-Supporting Nanofiber Membrane for Ultralow-Concentration NO2 Sensing Under High Humidity. Adv. Funct. Mater. 2024, 35. [Google Scholar] [CrossRef]
  22. Ma, Z.; Li, Y.; Lv, Y.; Sa, R.; Li, Q.; Wu, K. Synergistic Effect of Doping and Compositing on Photocatalytic Efficiency: A Case Study of La2Ti2O7. ACS Appl. Mater. Interfaces 2018, 10, 39327–39335. [Google Scholar] [CrossRef] [PubMed]
  23. Wang, J.; Asakura, Y.; Hasegawa, T.; Yin, S. High-concentration N-doped La2Ti2O7 nanocrystals: Effects of nano-structuration and doping sites on enhancing the photocatalytic activity. Chem. Eng. J. 2021, 423. [Google Scholar] [CrossRef]
  24. Cai, X.; Mao, L.; Fujitsuka, M.; Majima, T.; Kasani, S.; Wu, N.; Zhang, J. Effects of Bi-dopant and co-catalysts upon hole surface trapping on La2Ti2O7 nanosheet photocatalysts in overall solar water splitting. Nano Res. 2021, 15, 438–445. [Google Scholar] [CrossRef]
  25. Gao, Z.; Wu, L.; Lu, C.; Gu, W.; Zhang, T.; Liu, G.; Xie, Q.; Li, M. The anisotropic conductivity of ferroelectric La2Ti2O7 ceramics. J. Eur. Ceram. Soc. 2017, 37, 137–143. [Google Scholar] [CrossRef]
  26. Gilardi, E.; Fabbri, E.; Bi, L.; Rupp, J.L.M.; Lippert, T.; Pergolesi, D.; Traversa, E. Effect of Dopant–Host Ionic Radii Mismatch on Acceptor-Doped Barium Zirconate Microstructure and Proton Conductivity. J. Phys. Chem. C 2017, 121, 9739–9747. [Google Scholar] [CrossRef]
  27. Han, D.; Shinoda, K.; Sato, S.; Majima, M.; Uda, T. Correlation between electroconductive and structural properties of proton conductive acceptor-doped barium zirconate. J. Mater. Chem. A 2014, 3, 1243–1250. [Google Scholar] [CrossRef]
  28. Wang, H.; Li, Q.; Wang, C.; He, H.; Yu, J.; Xu, J. Growth and Dielectric Properties of Ta-Doped La2Ti2O7 Single Crystals. Crystals 2018, 8, 113. [Google Scholar] [CrossRef]
  29. Lv, L.; Lei, L.; Chen, Q.-W.; Yin, C.-L.; Fan, H.; Zhou, J.-P. Oxygen vacancies-modified S-scheme heterojunction of Bi-doped La2Ti2O7 and La-doped Bi4Ti3O12 to improve the NO gas removal avoiding NO2 product. Appl. Catal. B Environ. 2023, 343. [Google Scholar] [CrossRef]
  30. Zhu, K.; Shi, N.; Zhang, L.; Huan, D.; Li, X.; Zhang, X.; Song, R.; Xia, C.; Peng, R.; Lu, Y. Engineering oxygen vacancy to accelerate proton conduction in Y-doped BaZrO3. Ceram. Int. 2022, 49, 13321–13329. [Google Scholar] [CrossRef]
  31. Li, K.; Wang, Y.; Wang, H.; Zhu, M.; Yan, H. Hydrothermal synthesis and photocatalytic properties of layered La2Ti2O7 nanosheets. Nanotechnology 2006, 17, 4863–4867. [Google Scholar] [CrossRef]
  32. Cai, X.; Zhu, M.; Elbanna, O.A.; Fujitsuka, M.; Kim, S.; Mao, L.; Zhang, J.; Majima, T. Au Nanorod Photosensitized La2Ti2O7 Nanosteps: Successive Surface Heterojunctions Boosting Visible to Near-Infrared Photocatalytic H2 Evolution. ACS Catal. 2017, 8, 122–131. [Google Scholar] [CrossRef]
  33. Cao, Y.; Tang, P.; Qiu, W.; Zhao, T. Preparation of Y-Doped La2Ti2O7 Flexible Self-Supporting Films and Their Application in High-Performance Flexible All-Solid-State Supercapacitor Devices. ACS Omega 2020, 5, 29722–29732. [Google Scholar] [CrossRef] [PubMed]
  34. Zhu, W.; Kochanski, G.P.; Jin, S.; Seibles, L. Defect-enhanced electron field emission from chemical vapor deposited diamond. J. Appl. Phys. 1995, 78, 2707–2711. [Google Scholar] [CrossRef]
  35. Djurišić, A.B.; Leung, Y.H.; Tam, K.H.; Hsu, Y.F.; Ding, L.; Ge, W.K.; Zhong, Y.C.; Wong, K.S.; Chan, W.K.; Tam, H.L.; et al. Defect emissions in ZnO nanostructures. Nanotechnology 2007, 18. [Google Scholar] [CrossRef]
  36. Z Zhang, X.; Jiang, L.; Guo, S.; Han, D. (La1-xMx)2(Nb0.45Yb0.55)2O7-δ (M=Ca, Sr, Ba) Ionic Conductors Promoted by Foreign/Domestic Dual Acceptor-Doping Strategies. ChemSusChem 2022, 15, e202201879. [Google Scholar] [CrossRef] [PubMed]
  37. Agmon, N. The Grotthuss mechanism. Chem. Phys. Lett. 1995, 244, 456–462. [Google Scholar] [CrossRef]
  38. Mertes, S.; Wahner, A. Uptake of Nitrogen Dioxide and Nitrous Acid on Aqueous Surfaces. J. Phys. Chem. 1995, 99, 14000–14006. [Google Scholar] [CrossRef]
  39. Chen, X.; Zhao, S.; Zhou, P.; Cui, B.; Liu, W.; Wei, D.; Shen, Y. Room-temperature NO2 sensing properties and mechanism of CuO nanorods with Au functionalization. Sens. Actuators B 2021, 328, 129070. [Google Scholar] [CrossRef]
  40. Ganesan, M.; Jayaraman, V.; Selvaraj, P.; Mani, K.M.; Kim, D.-H. Pyrochlore cerium stannate (Ce2Sn2O7) for highly sensitive NO2 gas sensing at room temperature. Appl. Surf. Sci. 2023, 624, 157135. [Google Scholar] [CrossRef]
  41. Dwivedi, C.; Srivastava, S.; Singh, P. Highly selective dual gas (NO & NO2) sensing depended on the operating temperature of WO3 thin films sputtered at room temperature. Curr. Appl. Phy. 2025, 69, 70–80. [Google Scholar] [CrossRef]
  42. Ghosh, S.; Ilango, M.S.; Prajapati, C.S.; Bhat, N. Reduction of Humidity Effect in WO3 Thin Film-Based NO2 Sensor Using Physiochemical Optimization. Cryst. Res. Technol. 2020, 56, 2000155. [Google Scholar] [CrossRef]
  43. D’Olimpio, G.; Boukhvalov, D.W.; Galstyan, V.; Occhiuzzi, J.; Vorochta, M.; Amati, M.; Milosz, Z.; Gregoratti, L.; Istrate, M.C.; Kuo, C.-N.; et al. Unlocking superior NO2 sensitivity and selectivity: The role of sulfur abstraction in indium sulfide (InS) nanosheet-based sensors. J. Mater. Chem. A 2024, 12, 10329–10340. [Google Scholar] [CrossRef]
  44. Bang, J.H.; Kwon, Y.J.; Lee, J.H.; Mirzaei, A.; Lee, H.Y.; Choi, H.; Kim, S.S.; Jeong, Y.K.; Kim, H.W. Proton-beam engineered surface-point defects for highly sensitive and reliable NO2 sensing under humid environments. J. Hazard. Mater. 2021, 416, 125841. [Google Scholar] [CrossRef]
  45. Ambi, R.R.; Mulla, M.G.; Pittala, R.K. NO2 sensing properties of chemically deposited vertically aligned flowerlike hexagonal ZnO nanorods. Sens. Actuators A 2024, 376, 115621. [Google Scholar] [CrossRef]
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Figure 1. Transmission electron microscope images of (a) LTO and (b) 0.7% Y-doped LTO. (c) High resolution transmission electron microscope image and (d) energy-dispersive X-ray mapping of 0.7% Y-doped LTO nanosheets. (e) X-ray diffraction patterns of Y-doped LTO nanosheets compared to non-doped LTO.
Figure 1. Transmission electron microscope images of (a) LTO and (b) 0.7% Y-doped LTO. (c) High resolution transmission electron microscope image and (d) energy-dispersive X-ray mapping of 0.7% Y-doped LTO nanosheets. (e) X-ray diffraction patterns of Y-doped LTO nanosheets compared to non-doped LTO.
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Figure 2. High-resolution (a) La 3d, (b) Ti 2p, (c) Y 3d, and (d) O 1s X-ray photoelectron spectroscopy spectra of 0.7% Y-doped LTO nanosheets.
Figure 2. High-resolution (a) La 3d, (b) Ti 2p, (c) Y 3d, and (d) O 1s X-ray photoelectron spectroscopy spectra of 0.7% Y-doped LTO nanosheets.
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Figure 3. (a) Response–recovery curves of 0.7% Y-doped LTO nanosheets to NO2 of increasing concentrations under 43% RH. (b) Comparison of responses of Y-doped LTO (doping levels of 0.3%, 0.7%, and 1.2%) toward NO2 of different concentrations at 43% RH. (c) Response–recovery curves of 0.7% Y-doped LTO nanosheets to NO2 of increasing concentrations under 75% RH. (d) Comparison of responses of 0.7% Y-doped LTO nanosheets toward NO2 of different concentrations under 43% RH, 60% RH, and 75% RH. (e) Cyclic stability test of 0.7% Y-doped LTO nanosheets in response to 5 ppm NO2 gas under 75% RH. (f) Sensing responses of 0.7% Y-doped LTO nanosheets towards NO2 and other gasses at a concentration of 10 ppm under 75% RH.
Figure 3. (a) Response–recovery curves of 0.7% Y-doped LTO nanosheets to NO2 of increasing concentrations under 43% RH. (b) Comparison of responses of Y-doped LTO (doping levels of 0.3%, 0.7%, and 1.2%) toward NO2 of different concentrations at 43% RH. (c) Response–recovery curves of 0.7% Y-doped LTO nanosheets to NO2 of increasing concentrations under 75% RH. (d) Comparison of responses of 0.7% Y-doped LTO nanosheets toward NO2 of different concentrations under 43% RH, 60% RH, and 75% RH. (e) Cyclic stability test of 0.7% Y-doped LTO nanosheets in response to 5 ppm NO2 gas under 75% RH. (f) Sensing responses of 0.7% Y-doped LTO nanosheets towards NO2 and other gasses at a concentration of 10 ppm under 75% RH.
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Figure 4. (a) Nyquist plots of 0.7% Y-doped LTO in D2O and H2O atmospheres. (b) Estimation of the proton conduction activation energy of 0.7% Y-doped LTO under 43% and 75% relative humidity. (c) Schematic illustration of the NO2-sensing mechanism of YLTO.
Figure 4. (a) Nyquist plots of 0.7% Y-doped LTO in D2O and H2O atmospheres. (b) Estimation of the proton conduction activation energy of 0.7% Y-doped LTO under 43% and 75% relative humidity. (c) Schematic illustration of the NO2-sensing mechanism of YLTO.
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Figure 5. Nyquist plots of LTO and YLTO nanosheets with different Y doping concentrations under (a) 43% relative humidity, (b) 60% relative humidity, and (c) 75% relative humidity. (d) Comparison of conductivity of various Y-doped LTO nanosheets under 43% relative humidity, 60% relative humidity, and 75% relative humidity conditions.
Figure 5. Nyquist plots of LTO and YLTO nanosheets with different Y doping concentrations under (a) 43% relative humidity, (b) 60% relative humidity, and (c) 75% relative humidity. (d) Comparison of conductivity of various Y-doped LTO nanosheets under 43% relative humidity, 60% relative humidity, and 75% relative humidity conditions.
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MDPI and ACS Style

Wang, J.; Sun, C.; Bao, J.; Yang, Z.; Zhang, J.; Huang, X. Yttrium Doping of Perovskite Oxide La2Ti2O7 Nanosheets for Enhanced Proton Conduction and Gas Sensing Under HighHumidity Levels. Sensors 2025, 25, 901. https://doi.org/10.3390/s25030901

AMA Style

Wang J, Sun C, Bao J, Yang Z, Zhang J, Huang X. Yttrium Doping of Perovskite Oxide La2Ti2O7 Nanosheets for Enhanced Proton Conduction and Gas Sensing Under HighHumidity Levels. Sensors. 2025; 25(3):901. https://doi.org/10.3390/s25030901

Chicago/Turabian Style

Wang, Jian, Caicai Sun, Jusheng Bao, Zhiwei Yang, Jian Zhang, and Xiao Huang. 2025. "Yttrium Doping of Perovskite Oxide La2Ti2O7 Nanosheets for Enhanced Proton Conduction and Gas Sensing Under HighHumidity Levels" Sensors 25, no. 3: 901. https://doi.org/10.3390/s25030901

APA Style

Wang, J., Sun, C., Bao, J., Yang, Z., Zhang, J., & Huang, X. (2025). Yttrium Doping of Perovskite Oxide La2Ti2O7 Nanosheets for Enhanced Proton Conduction and Gas Sensing Under HighHumidity Levels. Sensors, 25(3), 901. https://doi.org/10.3390/s25030901

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