1 Introduction

As one of the most important rare earth oxides, cerium dioxide (CeO2) has been extensively studied over the past decades [1]. CeO2 has been used prolifically in various applications, such as photocatalysis [2], water–gas shift [3], automobile exhaust-gas treatments [4], photocatalysts [5], adsorbents [6], and potential sensing applications [7, 8]. This widespread applicability mainly originates from the outstanding properties of CeO2 [9], i.e., high oxygen storage capacity [10], quick and expedient mutation of the oxidation state between Ce4+ and Ce3+ [11], plentiful defects and high ionic mobility [12]. Therefore, based on the excellent performance of CeO2 itself, researchers have started in-depth research on the synthesis and morphology regulation of cerium oxides [13]. A large number of studies showed that the morphology of CeO2 has a great impact on its performance [14, 15]. The amelioration of CeO2 performances was mainly focused on the preparation of nanoscale material or morphological change [16,17,18], such as nanotubes, nanowires, nanorods, nanocubes, nanopolyhedra, nanoplates, and nanosheets. Among them, nanosheets have attracted extensive attention in recent years due to their special morphology, larger specific surface area and more direct conduction pathways [19, 20]. Murray et al. reported a simple solution-phase synthetic method to prepare ultrathin CeO2 nanosheets in the presence of mineralizers [13]. Xie et al. synthesized ultrathin CeO2 nanosheets using an intermediate precursor of CeCO3OH sheets [21]. However, the controllable synthesis of ultrathin CeO2 nanosheets is still a challenge process, since the synthesis conditions are relatively complex [22], such as mineralizers required, surfactants needed, and high reaction temperature. Therefore, the development of simple and cost-effective processes for the synthesis of CeO2 nanosheets is important to further explore their size- and shape-dependent properties.

In recent years, various CeO2 nanomaterials have been utilized in highly efficient humidity sensors, such as nanoparticles [23], nanobelts [24], and aero-gel [25]. Additionally, CeO2 nanostructures have been fully proved as active materials and additives/catalysts [26] for high-performance gas sensing applications, including nanospheres [7], nanopolyhedra [8], and nanoparticles [27]. Recently, research community focuses on metal oxide-based gas sensors operated at low temperature [28,29,30] or even room temperature [7, 27, 31] to reduce power consumption, simplify device architecture, etc. However, compared with that worked at conventional high temperatures (typically 200 °C–400 °C) [8, 26, 32], the room temperature gas sensors are commonly suffered from far lower response [7, 27] and much longer recovery time [7] for volatile organic compounds (VOCs) detection, especially formaldehyde [27]. For example, CeO2 nanopolyhedra worked at 220 °C [8] exhibited much higher response (Sr = 12) toward 1 × 10−6 formaldehyde than that of CeO2 nanoparticles operated at room temperature (Sr = 1.56) [27]. Therefore, it seems to be of significance to detect formaldehyde at high temperature for high and reversible response. Although great progresses have been made on CeO2 synthesis and humidity/gas sensing properties, yet the bifunctional gas-humidity sensory applications of nano-CeO2 have not been reported so far, the relative exploration of which can remarkably expand the application scope of CeO2 materials.

Herein, ultrathin CeO2 nanosheets were synthesized using a simple, one-step, inexpensive, low temperature and efficient hydrothermal method. Based on characterization and measurement, it was found that CeO2 nanosheets can be used as potential bifunctional materials for humidity and formaldehyde detection. Finally, the excellent humidity and formaldehyde sensing mechanism were discussed as well.

2 Experimental

2.1 Synthesis of CeO2 nanosheets

All reagents are analytical grade and used without further purification. In a typical synthesis, 2.5 mmol Ce(NO3)3·6H2O was dissolved into 50 ml NaOH aqueous solution (6 mol·L−1), followed by stirring for 30 min. Then the mixed solution was moved into a 50 ml Teflon bottle and then sealed tightly in a stainless-steel autoclave. Hydrothermal treatment was carried out at 100 °C for 24 h. After the autoclave was cooled down to room temperature, the white precipitate was centrifugated, washed with distilled water several times, dried at 70 °C overnight. Finally, the dried yellow powders were annealed in air at 400 °C for 2 h.

2.2 Characterization

The structure was characterized by X-ray diffraction (XRD, D/Max 2500 pc) with a Cu Kα1 radiation (λ = 0.15406 nm) operating at 40 kV. The morphology was conducted by a field emission scanning electron microscope (FESEM, FEI Company, QUANTA FEG 250). The specific surface area was measured via the Brunauer-Emmett-Teller (BET) method using a N2 adsorption at 77 K using a BELSORP-max apparatus.

2.3 Sensor fabrication and measurement

The assembly process of a planar humidity device based on CeO2 nanosheets is introduced as follows. As-prepared CeO2 nanosheets were mixed with water and ground into slurry, which was uniformly coated on an Al2O3 substrate (8 mm × 4 mm × 0.5 mm) with 5 pairs of Ag-Pd interdigitated electrodes (IDEs, electrode width and gap: 0.15 mm) to form a humidity sensor [33]. The humidity characteristic curves were measured on a CHS-1 Humidity Sensing Analysis System (Beijing Elite Tech. Co. Ltd., China) at room temperature (20 °C). The humidity sensing performance was examined at different RH conditions by adopting the method of saturated salt solutions. The humidity gradient was controlled by 6 jars (1 L) with rubber stoppers filled with different saturated salt solutions. The sensor in a shielded socket is sealed in a rubber stopper which is the same as that used for other jars. The tested ambient RH was achieved by saturated salt solutions in closed vessels, including LiCl (11%RH), MgCl2 (33%RH), Mg(NO3)2 (54%RH), NaCl (75%RH), KCl (85%RH), and K2SO4 (97%RH). Before humidity sensing tests, 8 h stabilization of the jars was necessary to get fully liquid–gas equilibrium to ensure the RH accuracy. In a typical test cycle, the sensor was put in one jar until the impedance value achieved balance then it was switched to another one. After reaching steady, the sensor was taken back to the former jar waiting the next impedance stability, finally finishing a cyclic test. The humidity sensor response is defined as SRH = Z11%/ZRH, where Z11% and ZRH represent the impedance values of the humidity sensor taken at 11%RH and a detecting RH. The response time (τres) or recovery time (τrec) is defined as the time duration needed for the sensor achieving 90% of total impedance change in the case of moisture adsorption or desorption, respectively.

The fabrication procedure of a CeO2 based gas sensor is similar to our previous reports [20, 34]. The gas sensing properties of CeO2 nanosheets were performed on a NS-4000 Smart Sensor Analyzer (Beijing Zhongke Micro-Nano Networking Technology Co. Ltd., China) with formaldehyde as the probe molecule. The gas sensor response is defined as Sr = Ra/Rg, where Ra and Rg are the resistances of the gas sensor in dry air and the target gas, respectively. The response time (τres) or recovery time (τrec) is defined as the time taken for the sensor output reaching 90% of its saturation after applying or switching off the gas in a step function.

3 Results and discussion

3.1 Material characterization

The XRD pattern (Fig. 1a) provides crystallinity and phase composition of as-prepared powders. All diffraction peaks can be well indexed to the standard diffraction data of the fluorite crystal structure CeO2 (PDF No. 81-0792). No peak of any other impurities was detected, indicating the high purity of CeO2. In addition, the broadening diffraction peaks indicate the nanoscale characteristic [20] of the CeO2 sample. As shown in Fig. 1b, the internal structure of CeO2 nanosheets was determined by N2 adsorption–desorption isotherms. The curve shows a type-IV isotherm with an obvious hysteresis loop. This fact describes typical adsorption behavior of mesoporous materials via capillary condensation, which occurs between the desorption and the adsorption branch [35]. The BET specific surface area was calculated to be as high as 98.39 m2·g−1. As displayed in the inset of Fig. 1b, the pore size distribution using the BJH (Barrett-Joyner-Halenda) method indicates that the average mesopore diameter is 2.44 nm. The morphology of CeO2 was recorded by FESEM. The low-magnification FESEM image (Fig. 1c) shows that CeO2 powders are evenly distributed nanosheets with uniform size. As presented in the high-magnification FESEM image (Fig. 1d), the nanosheets are ~500 nm in length, ~150 nm in width, and ~10 nm in thickness. The unique morphology of ultrathin CeO2 nanosheets is responsible for broadening diffraction peaks (Fig. 1a) and large specific surface area (Fig. 1b), these advantages of which imply a good candidate material for sensing applications.

Fig. 1
figure 1

a XRD pattern of CeO2 nanosheets; b typical N2 adsorption–desorption isotherms and pore size distribution plot (inset) of CeO2 nanosheets; c low-magnification image and d enlarged FESEM image of CeO2 nanosheets

Full size image

3.2 Humidity sensing performance

In order to determine the optimum operating frequency (OOF), 10 Hz–100 kHz were selected to measure the frequency-dependent impedance of the humidity sensor at different RH levels at room temperature. As shown in Fig. 2a, it is obvious that the results of 100 Hz in the whole RH range have the highest response (S97% = 3.1 × 104) and good linearity, so 100 Hz is adopted as the testing frequency. The response value of CeO2 nanosheets is a lot higher than that of CeO2 nanoparticles [23] and CeO2 nanobelts [24], while nearly equal to that of CeO2 aero-gel [25]. Figure 2b shows the impedance of the sensor alternate switching between 11%RH and 97%RH at 100 Hz. The back-and-forth exposure process between low RH and high RH can effectively demonstrate the excellent repeatability of the CeO2 nanosheets sensor. The response time and recovery time have a great influence on the application of sensors. Figure 2c indicates the humidity sensor based on CeO2 nanosheets enjoys a fast response time (11 s) and an acceptable recovery time (183 s). The response time of CeO2 nanosheets is comparable to that of reported CeO2 nanostructures [23,24,25], nevertheless the relatively longer recovery time needs further improvement. During the moisture adsorption–desorption process at 100 Hz (Fig. 2d), the maximum humidity hysteresis (ΔHmax) of the CeO2 nanosheet sensor is only 2.1%RH. Interestingly to find that this value is smaller than or equal to that of some reported results of CeO2 nanostructures [23,24,25]. This fact means that CeO2 nanosheets possess excellent reversible sensing characteristics, which has the potential to be used as a high-performance material for humidity detection.

Fig. 2
figure 2

a Impedance versus RH of CeO2 nanosheet sensor at various frequencies; b repeatability of CeO2 nanosheet sensor; c response and recovery time of humidity sensor; d hysteresis of humidity sensor at 100 Hz

Full size image

The conduction mechanism of humidity sensors can be effectively studied by complex impedance spectroscopy (CIS) and the corresponding equivalent circuit (EC). In Fig. 3a, at low humidity (11%RH), the impedance of CeO2 nanosheets is very high, and the CIS basically seems like a straight line, which is actually an arc of a semicircle. The arc is related to the impedance of the electrode–electrolyte interface (Warburg impedance), because the intrinsic electrons of CeO2 are the main carriers conducting electricity at relative low humidity. Since the relaxation time of ion diffusion is longer than that of charge transfer, the straight line can only be observed at high frequency [36, 37]. When RH gradually increases from 11% to 33% and then 54%, water molecules are physically absorbed on the surface of CeO2 nanosheets, forming a discontinuous water layer and accelerating the transfer between H2O and H3O+. At this time, the CIS presents a semicircle, and the impedance is still large, because CeO2 nanosheets absorb only a few water molecules. Therefore, the EC at low RH, as illustrated in the inset of Fig. 3a, is connected in series by two parts: one is the resistance (Rs), and the other part is the resistance (R) in parallel with the constant phase element (CPE1). Rs is the electrode resistance, which is independent of RH. R and CPE1 represent the resistances of the CeO2 film and the role of H3O+ in the CeO2 sensing layer.

Fig. 3
figure 3

Complex impedance plots and equivalent circuits (inset) of humidity sensor at a low RH range of 11%–54% and b high RH range of 75%–97%

Full size image

When high RH is reached (75%, 85%, and 97%), the impedance of the sensor decreases with the increase of RH. As shown in Fig. 3b, the impedance diagram shows a semicircle and an oblique line, where the semicircle represents the inherent impedance of the sensor and the oblique line reveals the Warburg impedance [38]. In this process, water physically adsorbs on the surface of CeO2, forming a continuous layer of water. The water layer acts as a channel for charge transfer and accelerates the transfer between H2O and H3O+ [36], improving the conductivity of the CeO2 nanosheet sensor. As shown in the inset of Fig. 3b, the EC is similar to that of the sensor at low RH (11%, 33%, and 54%). The difference is that the resistance of the sensing film is connected in series by the resistance (R) and the constant phase element (CPE2). The straight line at the low frequency of CIS can also be represented by the constant phase element (CPE1), which describes that H3O+ influences the physisorbed water layer through the jumping of H+ between adjacent water molecules.

3.3 Gas sensing performance

Gas sensors based on semiconducting metal oxides (SMO) usually exhibit the best response values at a certain temperature, because SMO can be well thermally activated at this temperature. Figure 4a shows the response of the CeO2 based gas sensor to 200 × 10−6 formaldehyde at different operating temperatures. Obviously, 300 °C is selected as the optimal operating temperature (OOT) of the gas sensor, and the response to 200 × 10−6 formaldehyde is up to 30 at this temperature. As represented in Fig. 4a (right Y axis), the resistance of CeO2 nanosheets in air (Ra) decreases with the increase of operating temperature, indicating a characteristic semiconducting behavior. Meanwhile, selectivity is an important parameter to evaluate the potential application of gas sensing materials. As shown in Fig. 4b, various typical VOCs likely to be presented in indoor environments are selected as interfering gases to test the selectivity at a fixed concentration (200 × 10−6) at 300 °C, such as halogenated hydrocarbon (trichloromethane, CHCl3), ketone (acetone, CH3COCH3), aromatic hydrocarbons (benzene, C6H6; toluene, C7H8), and organic amine (methylamine, CH3NH2). Obviously, the gas sensor exhibits the highest response value and thus good selectivity to formaldehyde. The gas sensor presents extremely fast response time (20 s) and recovery time (10 s) toward 200 × 10−6 formaldehyde at 300 °C, as can be seen in Fig. 4c. Meanwhile, the gas sensor performs well in repetitive test cycles (Fig. 4d). Figure 4e shows the response of the gas sensor exposed to different concentrations of formaldehyde in turn at 300 °C exhibited in response (left Y axis) and resistance (right Y axis), indicating good dynamic response-recovery performance. When the formaldehyde concentration reaches 200 × 10−6 and above, the response of the sensor does not change much, implying saturation response. This phenomenon follows the Langmuir–Hinshelwood mechanism that is correspondence with the larger specific surface area (Fig. 1b), and the sensor surface coverage tends to be saturated at higher concentrations than 200 × 10−6. In Fig. 4f, the fitted function interprets the relationship between Sr and concentration (C) of the CeO2 nanosheet sensor, enjoying a good relativity (adjust R2 = 0.9854). Compared with other formaldehyde sensors based on CeO2 nanomaterials [8, 27], as-synthesized CeO2 nanosheet sensor holds favorable gas sensing performance, including high response, fast response-recovery, good repeatability and selectivity, while a higher operating temperature. All these results indicate that the sensor based on ultrathin CeO2 nanosheets is of practical significance to detect formaldehyde vapor.

Fig. 4
figure 4

a Response in 200 × 10−6 formaldehyde (left Y axis) and resistance in air (right Y axis) of CeO2 nanosheets at different operation temperatures; b selectivity of CeO2 nanosheets toward 200 × 10−6 gases at 300 °C; c response-recovery curves and d repeatability of CeO2 nanosheets toward 200 × 10−6 formaldehyde at 300 °C; e dynamic response-recovery curve of CeO2 nanosheets exposed to 5 × 10−6–400 × 10−6 formaldehyde at 300 °C exhibited in response (left Y axis) and resistance (right Y axis), respectively; f fitted curve (solid line) and experimental results (scattered points) of CeO2 nanosheets to 5 × 10−6–400 × 10−6 formaldehyde at 300 °C

Full size image

As depicted in Fig. 5, electron depletion theory is accepted to explain the gas sensing mechanism. This mechanism refers to a drastic resistance change of a SMO based gas sensor (CeO2 here) via the adsorption-oxidization-desorption process toward reducing gas (formaldehyde in this work) [39]. In air, oxygen molecules are absorbed on the surface of CeO2, and capture electrons from the CeO2 conduction band (CB) to form oxygen species (O2−, O− and O2−) at different temperatures [40]. Thus, an electron depletion layer occurs on CeO2 surface, resulting in a high resistance state of CeO2 (Ra). Once exposed to formaldehyde, oxygen species oxidize formaldehyde and electrons trapped in the adsorbed state are released, leading to increased carrier concentration, narrower depletion layer and finally reduced sensor resistance (Rg) [41]. In this work, the superior gas sensing performance of CeO2 nanosheets, especially the high response value and short response-recovery time, is originated from the combined effects of large specific surface area, characteristically small thickness, and unique mesoporous architecture.

Fig. 5
figure 5

Schematic illustration of sensing mechanism of CeO2 nanosheets in air and formaldehyde

Full size image

4 Conclusion

In summary, ultrathin CeO2 nanosheets were synthesized by a facile low-temperature hydrothermal method. These CeO2 nanosheets were investigated as bifunctional sensing materials for humidity and formaldehyde detection for the first time. The humidity sensor based on CeO2 nanosheets possesses impedance change larger than 4 orders of magnitude (S97% = 3.1 × 104), short response-recovery times (20 and 10 s), and narrow hysteresis (2.1%RH). Simultaneously, the gas sensor based on CeO2 nanosheets holds high sensitivity, fast response, good repeatability and selectivity to 5 × 10−6–400 × 10−6 formaldehyde at 300 °C. It is believed that the excellent bifunctional sensing performance can be attributed to the geometric sensitization of ultrathin CeO2 nanosheets. This work provides a good candidate material for both humidity and formaldehyde detection, and furthermore, suggests a new way for the design and synthesis of other rare earth oxides for multifunctional applications.