Next Article in Journal
Hardness Distribution and Growth Behavior of Micro-Arc Oxide Ceramic Film with Positive and Negative Pulse Coordination
Previous Article in Journal
Encapsulation of Tenebrio molitor Hydrolysate with DPP-IV Inhibitory Activity by Electrospraying and Spray-Drying
Previous Article in Special Issue
Hierarchical Nanoheterostructure of HFIP-Grafted α-Fe2O3@Multiwall Carbon Nanotubes as High-Performance Chemiresistive Sensors for Nerve Agents
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Nanomaterials
Volume 14
Issue 10
10.3390/nano14100841
nanomaterials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Synergistic Effect of ZIF-8 and Pt-Functionalized NiO/In2O3 Hollow Nanofibers for Highly Sensitive Detection of Formaldehyde

by
Lei Zhu
1,2,†,
Ze Wang
1,†,
Jianan Wang
1,*,
Jianwei Liu
3,4,
Wei Zhao
2,
Jiaxin Zhang
1 and
Wei Yan
1,*
1
Xi’an Key Laboratory of Solid Waste Resource Regeneration and Recycling, State Key Laboratory of Multiphase Flow Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
2
School of Physics and Electrical Engineering, Weinan Normal University, Chaoyang Street, Weinan 714099, China
3
Xianggui Manganese Industry Co., Ltd., Ziyang, Ankang 725300, China
4
School of Chemistry and Chemical Engineering, Xi’an University of Science & Technology, Xi’an 710054, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Nanomaterials 2024, 14(10), 841; https://doi.org/10.3390/nano14100841
Submission received: 9 April 2024 / Revised: 26 April 2024 / Accepted: 4 May 2024 / Published: 10 May 2024
(This article belongs to the Special Issue Nanoscale Material-Based Gas Sensors)
Browse Figures
Versions Notes

Abstract

:
A rapid and accurate monitoring of hazardous formaldehyde (HCHO) gas is extremely essential for health protection. However, the high-power consumption and humidity interference still hinder the application of HCHO gas sensors. Hence, zeolitic imidazolate framework-8 (ZIF-8)-loaded Pt-NiO/In2O3 hollow nanofibers (ZPNiIn HNFs) were designed via the electrospinning technique followed by hydrothermal treatment, aiming to enable a synergistic advantage of the surface modification and the construction of a p-n heterostructure to improve the sensing performance of the HCHO gas sensor. The ZPNiIn HNF sensor has a response value of 52.8 to 100 ppm HCHO, a nearly 4-fold enhancement over a pristine In2O3 sensor, at a moderately low temperature of 180 °C, along with rapid response/recovery speed (8/17 s) and excellent humidity tolerance. These enhanced sensing properties can be attributed to the Pt catalysts boosting the catalytic activity, the p-n heterojunctions facilitating the chemical reaction, and the appropriate ZIF-8 loading providing a hydrophobic surface. Our research presents an effective sensing material design strategy for inspiring the development of cost-effective sensors for the accurate detection of indoor HCHO hazardous gas.

1. Introduction

Formaldehyde (HCHO) is regarded as a common indoor pollutant. Prolonged exposure to high levels of HCHO can lead to a variety of health problems [1,2,3]. The World Health Organization (WHO) guidelines have set an exposure threshold value of 82 ppb for HCHO in indoor living areas [4]. At present, metal oxide semiconductor (MOS)-based sensors are one of the effective methods for monitoring indoor HCHO with higher sensitivity and shorter reaction times [5,6]. However, the high-power consumption of MOS sensors and the susceptibility of their sensitivity to ambient humidity are the main drawbacks that affect the reliability of gas sensors in practical applications [7]. Thus, it is necessary to develop an MOS-based sensor with a lower operating temperature (<200 °C) and good anti-humidity for high-sensitivity HCHO detection.
The surface modification of MOS structures by noble metals (Pt, Ag, and Pd) with unique electronic states and chemical properties reduces the adsorption activation energy and promotes the electron transfer efficiency, thus enhancing the sensing capabilities of MOS [8]. For instance, Liu et al. [9] synthesized Ag-SnO2 nanoparticles, and the sensitivity reached 14.4 towards 10 ppm HCHO at 125 °C. In addition, p-n heterojunctions can integrate the physical and chemical properties of p- and n-type MOS into one system [2,10,11]. Nickel oxide (NiO), as a typical p-type metal oxide, can accelerate the catalytic decomposition of the target gas and promote the gas-sensing reaction on the surface [12,13], further improving the response/recovery characteristics and reducing the operating temperature of the gas sensor. Kumar et al. [14] prepared SnO2-decorated NiO nanoparticles and investigated the catalytic kinetics during the HCHO adsorption–desorption process. As mentioned above, the construction of hybrid materials combining noble metals and p-n heterojunctions is expected to realize the highly sensitive detection of HCHO gas at low temperatures.
It has been suggested that the surface modification of MOS materials can improve the water-resistance performance of gas sensors by ensuring minimal effects from humidity [15,16,17]. Metal–organic frameworks (MOFs) featuring a high specific surface area and abundant porous structure can effectively avoid contact between water molecules and MOS-sensing materials [18]. Meanwhile, gas molecules can diffuse through the pores of MOFs [19]. Hence, MOF-based hydrophobic surface modifications have been found to enhance the anti-humidity performance of MOS-based gas sensors without reducing their sensitivity.
Zeolitic imidazolate framework-8 (ZIF-8), a well-studied MOF with a robust structure and highly porous properties, is composed of Zn2+ ions coordinated with 2-methylimidazolate ligands, which has a wide range of applications in gas sensors. In addition, the hydrothermal technique is a facile and effective method for the synthesis of gas-sensing materials. Ferlazzo et al. [20] prepared samarium oxide (Sm2O3) nanorods by a facile hydrothermal route for efficient detection of volatile organic compounds gas in the indoor environment. Jin et al. [21] constructed SnO2/SnS2 n-n heterojunction anchored on rGO using a hydrothermal route for detection HCHO.
Herein, ZIF-8 loaded on the surface of Pt-NiO/In2O3 hollow nanofibers (ZPNiIn HNFs) were synthesized by facile electrospinning technique combined with the subsequent hydrothermal method to serve as the high-performance HCHO gas-sensing materials. To evaluate the effect of p-n NiO/In2O3 heterojunctions, ZIF-8, and noble metal Pt nanoparticles on sensor performance, HCHO-sensing measurements based on pristine In2O3, NiO/In2O3, ZIF-8@NiO/In2O3, and ZPNiIn sensors were carried out. The ZPNiIn sensor presents excellent HCHO-sensing properties with a high response value, high rapid response/recovery speed, and excellent anti-humidity compared with the other three sensors. The enhanced sensing mechanism of the ZPNiIn HNF-based sensor was further analyzed, which is related to the high catalytic activity of Pt, the formation of interfacial heterojunction, and the appropriate loading of ZIF-8 nanoparticles.

2. Materials and Methods

2.1. Chemicals

All the chemicals and reagents used in this work can be found in the Supporting File (Text S1).

2.2. Synthesis of Pristine In2O3 and NiO/In2O3 Hollow Nanofibers (NiIn HNFs)

NiIn HNFs were prepared with the one-step electrospinning method. In detail, 1 g of In(NO3)3·4.5H2O and 0.2 g of Ni(NO3)2·6H2O were dissolved in N,N-dimethylformamide (8 mL) and ethanol (5 mL) under vigorous stirring. Then, an electrospun precursor was formed by adding 1 g of PVP to the mixture and stirring for 6 h at room temperature. An electrostatic voltage of 14 kV was applied between the needle tip of a 10 mL syringe containing the above precursor. Finally, further calcination at 600 °C for 3 h in air with a heating rate of 5 °C per min−1 formed NiIn HNFs from the precursor fibers. This same process was used to prepare the pristine In2O3 HNFs without Ni(NO3)2·6H2O.

2.3. Synthesis of ZIF-8@NiO/In2O3 (ZNiIn) and ZIF-8@Pt-NiO/In2O3 (ZPNiIn) HNFs

In the ZIF-8@Pt-NiO/In2O3 (ZPNiIn) HNF synthesis process, 0.4 g of zinc oxide and 0.6 g of 2-methylimidazole were dissolved in methanol (30 mL), forming mixture A. Then, 0.05 g of as-prepared NiIn HNFs was suspended in methanol with 0.15 mL of 10 mg mL−1 H2PtCl6·6H2O solution. The prepared suspension was ultrasonically treated for 20 min; then, a reducing agent of 0.008 g of NaBH4 was added dropwise, and the mixture was stirred to form mixture B. The above two solutions were then mixed and continually stirred to form a homogenous solution. Then, the solution was transferred to a Teflon reactor and heated to 120 °C for 3 h. We collected the precipitate and washed it with methanol after centrifugation. Finally, the obtained ZPNiIn HNFs were dried at 60 °C overnight. The ZNiIn HNFs were obtained with the same process without the addition of H2PtCl6·6H2O and NaBH4.

2.4. Characterizations

SEM (GeminiSEM 500, China) and TEM (JEOL JEM2100, Japan) were used to characterize the samples’ morphology. X-ray diffraction (XRD) patterns were acquired on a PANalytical X’pert MPDPro (The Netherlands) using a Cu Kα radiation source (40 kV, 40 mA). The AXIS ULtrabld (UK) instrument was used to obtain X-ray photoelectron spectroscopy (XPS) using a monochromatic Al Kα radiation source (15 kV, 1486.6 eV). Fourier-transform infrared spectroscopy (Bruker Tensor 37 spectrometer, China) was used to determine surface functional groups and chemical bonds of samples. An HR800 Raman spectrometer (France) was used for Raman scattering, and excitation was carried out with 532 nm laser light. The specific surface area and pore sizes of the samples were detected with N2 adsorption/desorption using an SSA-4300 (China) instrument. Sessile drops were measured using the optical contact angle meter (OCA25HTV, DataPhysics, China), where distilled water drops were placed on solid surfaces using a dispenser.

2.5. Gas Sensor Fabrication and Sensing Tests

Using drops of deionized water, the prepared materials were mix into a homogeneous paste. Then, this paste was coated uniformly on the ceramic tube with Au electrodes, further annealed in a muffle furnace, and annealed at 150 °C for 4 h. As shown in Figure S1, gas-sensing performance was evaluated using a WS-30A measurement system after aging the sensors for seven days in air [22]. A static liquid–gas distribution method was used to prepare the different concentrations of tested gases. First, reference air from the cylinder was continuously introduced into the gas distribution chamber at a rate of 50 mL/min to replace the air in the gas distribution chamber for 10 min. After stopping the introduction of reference air, a certain volume Q of HCHO solution (37%) was collected using a microsyringe, injected into the evaporator in the gas distribution chamber, and heated to evaporation. Finally, the HCHO vapor is completely mixed with the reference air using a built-in fan. Specifically, acetone, benzene, toluene, methanol, ethanol, and formaldehyde vapors were obtained by evaporating acetone (≥99.5%), benzene (≥99.5%), toluene (≥99.5%), methanol (≥99.5%), ethanol (≥99.7%), and formaldehyde (37%) solution, respectively, and the methane gas was the standard gas. The volume Q can be obtained with the Equation (S1). The specific description is shown in Text S2.

3. Results and Discussion

3.1. Morphology Characterization and Phase Composition

The schematic illustration of the ZIF-8@Pt-NiO/In2O3 HNFs is illustrated in Figure 1 [22]. SEM images, as illustrated in Figure 2a–c, present that the obtained nanofibers of pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNFs consist of continuous nanofibers that form web-like network structures (Figure 2a–c and Figure S2). A clear hollow structure of NiIn nanofibers can be observed in Figure 2b (red circle in inset Figure 2b). Unlike the smooth surfaces of In2O3 and NiIn HNFs, the ZPNiIn surface was uniformly loaded with ZIF-8 nanoparticles. The sharp contrast occurred between the dark edge and the pale interval in TEM images (Figure 2d,e) of ZIF-8@Pt-NiO/In2O3, confirming the hollow structure. Lattice fringes of 0.23, 0.29, and 0.21 nm correspond to the crystal planes of Pt (111) [23], In2O3 (222) [24], and NiO (200) [10], respectively (Figure 2f). The EDS elemental mappings in Figure 2g reveal that In, Ni, O, Pt, Zn, and N elements evenly distribute in ZPNiIn HNFs, proving the formation of a heterojunction. The EDS spectrum shown in Figure S3 further presents that the percentages of In, Ni, O, Pt, Zn, and N are 22.68%, 6.54%, 68.22%, 0.48%, 1.39%, and 0.69%, respectively.
The phases and crystallinities of pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNFs are examined using XRD patterns, as illustrated in Figure 3a. Pristine In2O3 exhibits a cubic-phase In2O3 (JCPDS No. 06-0416) [25]. The as-synthesized NiIn nanocomposites are composed of the two phases of In2O3 and NiO. The diffraction peaks of the XRD spectra of ZNiIn and ZPNiIn HNFs correspond to cubic In2O3 (JCPDS No. 06-0416), ZIF-8, and NiO (JCPDS No. 47-1049), respectively [26]. No obvious peak belonging to Pt is observed in ZPNiIn HNFs, indicating that adding Pt will not change the crystal structure of NiO/In2O3 and that the Pt content is too low to detect the XRD signal [27]. The FT-IR spectra are shown in Figure 3b. The spectrum of pristine In2O3 with distinctive intense bands at 428, 538, 565, and 602 cm−1 could be ascribed to the stretching vibrations of the In-O [28]. For the FT-IR curve of ZPNiIn, some new absorption peaks belonging to the stretching vibrations of Ni-O and ZIF-8 are observed [29], suggesting the successful formation of the ZPNiIn HNFs.
To further investigate the crystal structure of the as-prepared samples, the Raman spectra were investigated. As shown in Figure 3c, the Raman spectrum of pristine In2O3 presents peaks located at 306, 496, and 629 cm−1, which are ascribed to the δ (InO6), In-O-In, and υ(InO6), respectively [25]. The peak of NiO at 495 cm−1 belongs to the optical mode of longitudinal vibration (LO). The peak located at 1074 cm−1 indicates the 2LO phonon vibration of NiO [30]. Compared with the In2O3 and NiO, the Raman peaks of NiIn, ZNiIn, and ZPNiIn HNFs exhibit a slight blue shift due to the electron–phonon interaction generated by heterojunction formation, indicating that heterojunctions can modulate the chemical bonds and electrical properties [31].
The nitrogen adsorption–desorption measurements of pristine In2O3 and ZPNiIn HNFs were carried out in Figure 3d,e. For the pristine In2O3 and ZPNiIn HNFs, the nitrogen adsorption/desorption isotherms show the typical IV isotherm with an H3 hysteretic loop, indicating the coexistence of micropores and mesopores (Figure 3d). Furthermore, the filling of ZIF-8 micropores increased ZPNiIn HNF sorption under low relative pressure [32]. The BET surface area of the ZPNiIn HNFs is 760.6 m2 g−1, which is a nearly 40-fold increase compared to that of pristine In2O3 HNFs (19.1 m2 g−1), demonstrating the enhanced gas adsorption performance of ZPNiIn HNFs. The average pore diameter of pristine In2O3 and ZPNiIn HNFs are 7.9 nm and 23.9 nm, respectively.
The XPS spectra of samples were obtained to analyze the elemental composition and valence states. The peaks at 443.9–444.3 eV and 451.5–451.9 eV are found in all the samples corresponding to In 3d5/2 and In 3d3/2, respectively, verifying the presence of In3+ [33] (Figure 3f). For Zn 2p spectra, peaks corresponding to Zn 2p3/2 and Zn 2p1/2 of Zn2+ at 1021.8 eV and 1044.7 eV are observed in ZNiIn and ZPNiIn HNFs due to the ZIF-8 loaded on the surface of NiO/In2O3 HNFs (Figure 3g). The spectra of Ni 2p (Figure 3h) present two peaks of Ni 2p1/2 (871.6 eV) and Ni 2p3/2 (853.1 eV) for NiIn, ZNiIn, and ZPNiIn samples, indicating the existence of Ni2+ [34]. Figure 3i shows the O 1s spectra of pristine In2O3 and ZPNiIn HNFs. The spectra of O 1s can be categorized as lattice oxygen (OL), oxygen vacancies (OV), and chemisorbed oxygen (OC) [35]. The area ratios of the oxygen fractions of samples are further listed in Table S1. The adsorption of air oxygen on the surface of the sensing materials results in the formation of OC, which can react with the analyzed gas [36]. The contents of OC in pristine In2O3 and ZPNiIn HNFs are 23.3% and 36.9%, demonstrating the ZIF-8 loaded and the heterojunction formation contribute to increasing the surface OC content. The significantly increased proportion of OC can facilitate the gas-sensing reaction and enhance the sensitivity.

3.2. Gas-Sensing Performance

To evaluate the effect of p-n heterojunctions, ZIF-8, and noble metal Pt nanoparticles on the sensor performance, HCHO-sensing measurements based on pristine In2O3, NiIn, ZNiIn, and ZPNiIn sensors were carried out. As depicted in Figure 4a, the responses of pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNF sensors to HCHO (100 ppm) were investigated at the operating temperatures of 140–220 °C. The results of the four sensors indicate that response values are trending upward until they reach the maximum of 13.5 (In2O3), 20.5 (NiIn), 32.6 (ZNiIn), and 52.8 (ZPNiIn) at 180 °C. Then, the responses decreased as the operating temperature increased from 180 to 220 °C. Among them, the ZPNiIn sensor exhibits an excellent response value, 3.9-fold higher than the In2O3 sensor. The substantial enhancement in the HCHO response value can be ascribed to the synergistic impact of ZIF-8 and Pt modification and the construction of a heterojunction. Hence, the optimal operating temperature for the sensors is 180 °C. Figure S4 displays the baseline sensor resistance (Ra) in air at the operating temperatures of 140–220 °C. The results reveal that the Ra of the four sensors decreases with increasing temperature, which is due to the characteristics of metal oxide semiconductors [37]. Notably, the resistance of ZNiIn and ZPNiIn sensors dramatically increased, which was caused by the poor electric properties of ZIF-8 [38].
Figure 4b and Figure S5 depict the real-time response and recovery curves of four sensors toward 100 ppm HCHO at 180 °C. The response/recovery time of pristine In2O3, NiIn, ZNiIn, and ZPNiIn sensors are 15/34 s, 10/12 s, 10/14 s, and 8/17 s, respectively. Compared to pristine In2O3, the ZPNiIn sensor offers a 2-fold increase in response/recovery speed. Figure 4c reveals the gas response curves of sensors toward different HCHO concentrations (1–100 ppm) at 180 °C. It can be seen from the curve that the gas response values of sensors increase as the HCHO concentration increased. Among these four sensors, the ZPNiIn sensor presents the highest response values toward different concentrations of HCHO. Moreover, all sensors present good linearity relationship between HCHO concentrations and the response within the given concentration range (Figure 4d). The theoretic limits of detection (LODs) of the sensor can be calculated based on Equation (1) [39,40]:
L O D = 3 × R M S n o i s e s l o p e
where RMSnoise denotes the root mean square noise based on 50 experimental baselines in the air. The slope can be obtained in the concentration–response curve. After calculation, the LOD of the ZPNiIn sensor to HCHO gas is calculated as low as 46.7 ppb, while the LODs of pristine In2O3, NiIn, and ZNiIn are 258.2, 122.3, and 63.8 ppb, respectively, implying the ZPNiIn sensor can be used to detect HCHO at lower ppb concentrations.
Furthermore, the responses of four sensors toward different interfered volatile organic compound (VOC) gases (100 ppm) were also investigated at 180 °C (Figure 4e). The response of the ZPNiIn sensor to HCHO is significantly higher than to other interfering gases. Meanwhile, the sensitivity of the ZPNiIn sensor surpasses that of other sensors, demonstrating its excellent selectivity. The five consecutive cyclic response/recovery curves of the ZPNiIn sensor toward 100 ppm HCHO at 180 °C are shown in Figure 4f and Figure S7, and the response value of the sensors only fluctuates slightly with unchanged response/recovery times, suggesting the excellent reproducibility of the ZPNiIn sensor.
The interference of humidity on pristine In2O3 and ZPNiIn sensors’ sensitivity toward 100 ppm HCHO was also evaluated as displayed in Figure 4g. With the increase in humidity from 27% to 70% RH, the HCHO response value of the ZPNiIn sensor is reduced by 14.2%, which is significantly superior to that of the pristine In2O3 sensor (the response decreased by 58.7%). Meanwhile, the baseline resistances of both sensors are decreased with increasing humidity (Figure S6). Notably, the ZPNiIn sensor can still achieve a high response value to HCHO under 70% RH (Ra/Rg = 44.4), indicating the improved humidity tolerance of the sensor. The water contact angles between sensing materials and drops of water are shown in Figure 4h. The ZPNiIn has stronger hydrophobicity with the water contact angle of 34.2°, which is bigger than pristine In2O3 (19.9°). The improved anti-humidity of the ZPNiIn sensor is mainly attributed to the surface modifications with ZIF-8 coating [41] and the functionalization of Pt noble metals [15,42]. Based on Figure 4i, the sensitivity of the ZPNiIn sensor toward HCHO has barely fluctuated over 30 days, implying the significant long-term stability of the ZPNiIn sensor. To further estimate the sensing properties of the ZPNiIn sensor, some similar works on HCHO gas-sensing are summarized as listed in Table S2. The ZPNiIn sensor also exhibits a more outstanding sensitivity toward HCHO gas compared with those reported HCHO sensors.

3.3. Gas-Sensing Mechanism

During the adsorption and desorption of target gas molecules on the surface of the sensing material, MOS changes its electrical resistance to detect the gas [43,44]. The obtained ZIF-8@Pt/NiO-In2O3 presented n-type semiconducting gas-sensing characteristics. In the ambient atmosphere, the adsorbed oxygen on the sensing material surface is then ionized into O 2 − , O−, or O2−, depending on when the operating temperature is 150 °C or less, between 150 and 400 °C, or more than 400 °C, respectively [45]. The optimum temperature in our work is 180 °C. Thus, the adsorbed oxygen ions of this work are O− (Equation (3)). Once the HCHO gas has been introduced, it reacts with O−, resulting in the generation of CO2 and electrons (Equation (4)); electrons are released again into the sensing material’s conduction band. The process is expressed by Equations (2)–(4) as follows:
O2 (gas) → O2 (ads)
O2 (ads) + e− → 2O− (ads)
HCHO (gas) +2O− (ads) → CO2 (gas) + H2O (gas) + 2e−
Figure 5 displays the schematic diagram of ZPNiIn for HCHO gas sensing. Sensors based on ZPNiIn are more sensitive to HCHO than those based on other materials. This could be explained as follows:
On the one hand, heterojunctions increase oxygen molecule absorption and target gas detection. When these three components come in contact, since the work function of NiO (9.4 eV) [37] is greater than that of In2O3 and Pt, electrons will migrate from In2O3 and Pt to NiO until their Fermi energy levels reach equilibrium, and the Schottky barrier formed at the contact interfaces, as shown in Figure 5a,b. The aggregation of electrons on NiO increases its concentration and improves its ability to capture oxygen, thereby promoting oxygen adsorption. This results in the superior performance of the isopropanol sensor since the resistance changes considerably during the sensing process.
On the other hand, the chemical sensitization of Pt NPs on the surface of NiO/In2O3 composites is the main reason for the improvement of the gas-sensing capability [46,47]. The Pt NPs facilitate the dissociation of O2 molecules into adsorbed oxygen (O−) through the “spillover effect”, increasing the concentration of adsorbed oxygen (Figure 5c). Therefore, ZPNiIn possessed more surface-adsorbed oxygen, which is favorable to promote the active sites for enhancing the gas-sensing properties [48]. The characterization results of XPS also confirmed that the adsorbed oxygen content of ZPNiIn increased (Table S1).
Moreover, the coated ZIF-8 NPs also play a significant role in the improvement of the sensing capabilities. From the results of the N2 adsorption–desorption measurement, the specific surface area of the ZPNiIn HNFs is increased 40-fold due to the load of ZIF-8. This can increase the number of active sites available for the adsorption of the target HCHO gas during sensing [38,49]. For practical applications, water vapor is partially repellent from the hydrophobic ZIF-8 and does not interfere with sensing performance under humidity conditions [50]. In addition, gas adsorption and diffusion are facilitated by the inner and outer surfaces of ZPNiIn hollow structures, which provide ample active sites for gas reactions [51].

4. Conclusions

In order to develop a high-performance HCHO sensor for indoor pollutant detection, the ZIF-8 loaded Pt/NiO-In2O3 HNFs were synthesized using the electrospinning method combined with hydrothermal techniques. The ZPNiIn sensor exhibits an enhanced response (Ra/Rg) of 52.8 towards 100 ppm HCHO with a fast response (8 s) and recovery (17 s) time at 180 °C. Moreover, the sensor shows excellent selectivity and good anti-humidity. The improved gas-sensing properties can be attributed to the heterojunctions between components, the catalytic and sensitization effects of Pt NPs, and the coated ZIF-8. This study presents an effective strategy to design and fabricate high-performance HCHO sensors.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano14100841/s1, Figure S1. Photographic images and schematic diagram of a fabricated gas sensor and WS-30A measurement system; Figure S2: SEM image of as-prepared ZNiIn HNFs; Figure S3: EDS spectrum for ZPNiIn HNFs; Figure S4: Base resistance in air of all sensors under different operating temperatures; Figure S5: Response–recovery curves of four sensors toward 100 ppm HCHO at 180 °C; Figure S6: The baseline resistances of pristine In2O3 and ZPNiIn sensors in the air under different relative humidity conditions; Figure S7: The repeatability of four parallel sensors based on ZPNiIn HNFs to HCHO (100 ppm); Table S1: The relative percentages of three different oxygen species for pristine In2O3, and ZPNiIn HNFs; Table S2: Comparison of HCHO gas-sensing performance with other gas sensors. References [39,52,53,54,55] are cited in the Supplementary Materials.

Author Contributions

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

Funding

This study was funded by the National Natural Science Foundation of China (51978569, and 52302198), the Natural Science Foundation of Shaanxi Province (2023-JC-QN-0494), the Young Talent Research Project of Shaanxi Science and Technology Association (20220401), China Postdoctoral Science Foundation (2023M742788), Research Fund for Young Star of Science and Technology in Shaanxi Province (2023KJXX-125), Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 23JK0445), and Shaanxi Province Postdoctoral Science Foundation (2023BSHEDZZ30).

Data Availability Statement

The data that have been used are confidential.

Conflicts of Interest

Author Jianwei Liu was employed by Xianggui Manganese Industry Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Lee, J.; Kim, Y.J.; Lee, W. Synergetic crystal phases of SnO2/NiO heterostructure in an interconnected morphology for chemiresistive formaldehyde sensors. Sens. Actuators B 2024, 404, 135257. [Google Scholar] [CrossRef]
  2. Meng, D.; Xie, Z.; Wang, M.; Xu, J.; San, X.; Qi, J.; Zhang, Y.; Wang, G.; Jin, Q. In Situ Fabrication of SnS2/SnO2 Heterostructures for Boosting Formaldehyde−Sensing Properties at Room Temperature. Nanomaterials 2023, 13, 2493. [Google Scholar] [CrossRef]
  3. Min, Y.; Yuan, C.; Fu, D.; Liu, J. Formaldehyde Gas Sensors Fabricated with Polymer-Based Materials: A Review. Chemosensors 2023, 11, 134. [Google Scholar] [CrossRef]
  4. Souri, M.; Salar Amoli, H.; Yamini, Y. Three-dimensionally ordered porous In-doped SmFeO3 perovskite gas sensor for highly sensitive and selective detection of formaldehyde. Sens. Actuators B 2024, 404, 135213. [Google Scholar] [CrossRef]
  5. Wu, K.; Kong, X.Y.; Xiao, K.; Wei, Y.; Zhu, C.; Zhou, R.; Si, M.; Wang, J.; Zhang, Y.; Wen, L. Engineered Smart Gating Nanochannels for High Performance in Formaldehyde Detection and Removal. Adv. Funct. Mater. 2019, 29, 1807953. [Google Scholar] [CrossRef]
  6. Kong, D.L.; Wu, W.J.; Hong, B.; Xu, J.C.; Peng, X.L.; Ge, H.L.; Li, J.; Zeng, Y.X.; Wang, X.Q. MIL-68 derived In2O3 microtubes and Co3O4/In2O3 heterostructures for high sensitive formaldehyde gas sensors. Ceram. Int. 2024, 50, 6995–7005. [Google Scholar] [CrossRef]
  7. Suematsu, K.; Sasaki, M.; Ma, N.; Yuasa, M.; Shimanoe, K. Antimony-Doped Tin Dioxide Gas Sensors Exhibiting High Stability in the Sensitivity to Humidity Changes. ACS Sens. 2016, 1, 913–920. [Google Scholar] [CrossRef]
  8. Zhou, L.; Chang, X.; Zheng, W.; Liu, X.; Zhang, J. Single atom Rh-sensitized SnO2 via atomic layer deposition for efficient formaldehyde detection. Chem. Eng. J. 2023, 475, 146300. [Google Scholar] [CrossRef]
  9. Liu, D.; Pan, J.; Tang, J.; Liu, W.; Bai, S.; Luo, R. Ag decorated SnO2 nanoparticles to enhance formaldehyde sensing properties. J. Phys. Chem. Solids 2019, 124, 36–43. [Google Scholar] [CrossRef]
  10. Meng, D.; Liu, D.Y.; Wang, G.S.; Shen, Y.B.; San, X.G.; Li, M.; Meng, F.L. Low-temperature formaldehyde gas sensors based on NiO-SnO2 heterojunction microflowers assembled by thin porous nanosheets. Sens. Actuators B 2018, 273, 418–428. [Google Scholar] [CrossRef]
  11. Lou, C.; Pan, H.; Mei, H.; Lu, G.; Liu, X.; Zhang, J. Low coordination states in Co3O4/NiOx heterostructures by atomic layer deposition for enhanced gas detection. Chem. Eng. J. 2022, 448, 137641. [Google Scholar] [CrossRef]
  12. San, X.; Zhao, G.; Wang, G.; Shen, Y.; Meng, D.; Zhang, Y.; Meng, F. Assembly of 3D flower-like NiO hierarchical architectures by 2D nanosheets: Synthesis and their sensing properties to formaldehyde. RSC Adv. 2017, 7, 3540–3549. [Google Scholar] [CrossRef]
  13. Li, C.; Choi, P.G.; Masuda, Y. Highly Sensitive and Selective Gas Sensors Based on NiO/MnO2@NiO Nanosheets to Detect Allyl Mercaptan Gas Released by Humans under Psychological Stress. Adv. Sci. 2022, 9, 2202442. [Google Scholar] [CrossRef]
  14. Das, S.; Kumar, A.; Singh, J.; Kumar, M. Fabrication and modeling of laser ablated NiO nanoparticles decorated SnO2 based formaldehyde sensor. Sens. Actuators B 2023, 387, 133824. [Google Scholar] [CrossRef]
  15. Wang, Y.; Zhou, Y. Recent Progress on Anti-Humidity Strategies of Chemiresistive Gas Sensors. Materials 2022, 15, 8728. [Google Scholar] [CrossRef] [PubMed]
  16. Li, L.; Zhou, L.; Hu, Z.; Li, T.; Chen, B.; Li, H.-Y.; Liu, H. Hollow-Out Fe2O3-Loaded NiO Heterojunction Nanorods Enable Real-Time Exhaled Ethanol Monitoring under High Humidity. ACS Appl. Mater. Interfaces 2023, 15, 15707–15720. [Google Scholar] [CrossRef]
  17. Kim, K.; Park, J.K.; Lee, J.; Kwon, Y.J.; Choi, H.; Yang, S.-M.; Lee, J.-H.; Jeong, Y.K. Synergistic approach to simultaneously improve response and humidity-independence of metal-oxide gas sensors. J. Hazard. Mater. 2022, 424, 127524. [Google Scholar] [CrossRef]
  18. Zhang, Y.; Wang, M.; San, X.; Zhang, L.; Wang, N.; Wang, G.; Meng, D.; Shen, Y. Highly selective gas sensors for formaldehyde detection based on ZnO@ZIF-8 core-shell heterostructures. Sens. Actuators B 2024, 398, 134689. [Google Scholar] [CrossRef]
  19. Wang, X.; Wang, Y.; Qi, H.; Chen, Y.; Guo, W.; Yu, H.; Chen, H.; Ying, Y. Humidity-Independent Artificial Olfactory Array Enabled by Hydrophobic Core–Shell Dye/MOFs@COFs Composites for Plant Disease Diagnosis. ACS Nano 2022, 16, 14297–14307. [Google Scholar] [CrossRef]
  20. Jamnani, S.R.; Moghaddam, H.M.; Leonardi, S.G.; Neri, G.; Ferlazzo, A. VOCs sensing properties of samarium oxide nanorods. Ceram. Int. 2024, 50, 403–411. [Google Scholar] [CrossRef]
  21. Meng, D.; Zou, X.; San, X.; Ji, Y.; Zhang, Y.; Wang, G.; Qi, J.; Jin, Q. Construction of SnO2/SnS2 n-n heterojunction anchored on rGO for synergistically enhanced low temperature formaldehyde sensing performance. Sens. Actuators B 2024, 406, 135359. [Google Scholar] [CrossRef]
  22. Zhu, L.; Wang, J.; Liu, J.; Xu, Z.; Nasir, M.S.; Chen, X.; Wang, Z.; Sun, S.; Ma, Q.; Liu, J.; et al. In situ enrichment amplification strategy enabling highly sensitive formaldehyde gas sensor. Sens. Actuators B 2022, 354, 131206. [Google Scholar] [CrossRef]
  23. Han, B.; Wang, H.; Yang, W.; Wang, J.; Wei, X. Hierarchical Pt-decorated In2O3 microspheres with highly enhanced isoprene sensing properties. Ceram. Int. 2021, 47, 9477–9485. [Google Scholar] [CrossRef]
  24. Ma, J.; Fan, H.; Zhang, W.; Sui, J.; Wang, C.; Zhang, M.; Zhao, N.; Kumar Yadav, A.; Wang, W.; Dong, W.; et al. High sensitivity and ultra-low detection limit of chlorine gas sensor based on In2O3 nanosheets by a simple template method. Sens. Actuators B 2020, 305, 127456. [Google Scholar] [CrossRef]
  25. Zhu, L.; Wang, J.; Liu, J.; Wang, Z.; Sun, S.; Li, M.; Yan, W. ZIF-L(Co) derived cobalt doped In2O3 hollow nanofibers with high surface activity for efficient formaldehyde gas sensing. Sens. Actuators B 2024, 403, 135129. [Google Scholar] [CrossRef]
  26. Yang, J.; Han, W.; Jiang, B.; Wang, C.; Sun, Y.; Zhang, H.; Shimanoe, K.; Sun, P.; Lu, G. Sn2+ doped NiO hollow nanofibers to improve triethylamine sensing characteristics through tuning oxygen defects. Sens. Actuators B 2023, 387, 133801. [Google Scholar] [CrossRef]
  27. Yang, J.; Han, W.; Jiang, B.; Wang, C.; Shimanoe, K.; Sun, Y.; Cheng, P.; Wang, Y.; Liu, F.; Sun, P.; et al. On the beneficial effect of Rh2O3 modification of Sn doped NiO nanofibers for conductometric triethylamine gas sensing. Sens. Actuators B 2023, 382, 133481. [Google Scholar] [CrossRef]
  28. Alaizeri, Z.M.; Alhadlaq, H.A.; Aldawood, S.; Akhtar, M.J.; Ahamed, M. Photodeposition mediated synthesis of silver-doped indium oxide nanoparticles for improved photocatalytic and anticancer performance. Environ. Sci. Pollut. Res. 2022, 30, 6055–6067. [Google Scholar] [CrossRef]
  29. Sharma, S.K.; Sudarshan, K.; Yadav, A.K.; Jha, S.N.; Bhattacharyya, D.; Pujari, P.K. Investigation of compression-induced deformations in local structure and pore architecture of ZIF-8 using FTIR, X-ray absorption, and positron annihilation spectroscopy. J Phys. Chem. C 2019, 123, 22273–22280. [Google Scholar] [CrossRef]
  30. Vivek, P.; Sivakumar, R.; Selva Esakki, E.; Deivanayaki, S. Fabrication of NiO/RGO nanocomposite for enhancing photocatalytic performance through degradation of RhB. J. Phys. Chem. Solids 2023, 176, 111255. [Google Scholar] [CrossRef]
  31. Wu, R.; Mao, J.; Li, H.; Yang, Y.; Hao, W.; Wang, Y.; Hao, J. Revealing the relationship of NO2 sensing with energy level in 2D van der Waals SnS1−xSex alloys. Chem. Eng. J. 2023, 469, 144018. [Google Scholar] [CrossRef]
  32. Wu, X.N.; Xiong, S.S.; Gong, Y.; Gong, Y.J.; Wu, W.W.; Mao, Z.H.; Liu, Q.; Hu, S.; Long, X.G. MOF-SMO hybrids as a H2S sensor with superior sensitivity and selectivity. Sens. Actuators B 2019, 292, 32–39. [Google Scholar] [CrossRef]
  33. Sui, N.; Xu, Y.; Zhang, P.; Cao, S.; Zhou, T.; Zhang, T. MIL-68 (In) and ZIF-8 assisted construction of n-n heterostructure for the effective sensing of trace-level ozone. Sens. Actuators B 2023, 380, 133312. [Google Scholar] [CrossRef]
  34. Shi, C.; Yu, L.; He, X.; Zhang, Y.; Liu, J.; Li, S.; Zhang, C.; Cao, L.; Nan, N.; Du, H.; et al. Vertically aligned mesoporous Ce doped NiO nanowalls with multilevel gas channels for high-performance acetone gas sensors. Sens. Actuators B 2024, 401, 134888. [Google Scholar] [CrossRef]
  35. Wang, X.; Zhang, W.; Wang, X.; Li, X.; Sui, X.; Liu, G.; Li, B.; Zhou, J.; Xie, E.; Zhang, Z. Facile fabrication of NiO foam@Sn-doped In2O3 nanowire heterosturctures for highly sensitive ethylene glycol gas sensors at low temperatures. Sens. Actuators B 2023, 377, 132991. [Google Scholar] [CrossRef]
  36. Chen, K.; Jiang, Y.; Tao, W.; Wang, T.; Liu, F.; Wang, C.; Yan, X.; Lu, G.; Sun, P. MOF Structure engineering to synthesize core-shell heterostructures with controllable shell layer thickness: Regulating gas selectivity and sensitivity. Sens. Actuators B 2023, 378, 133117. [Google Scholar] [CrossRef]
  37. Sui, C.; Zhang, M.; Li, Y.; Wang, Y.; Liu, Y.; Liu, Z.; Bai, J.; Liu, F.; Lu, G. Pd@Pt Core–Shell Nanocrystal-Decorated ZnO Nanosheets for ppt-Level NO2 Detection. ACS Sens. 2024, 9, 1967–1977. [Google Scholar] [CrossRef]
  38. Liu, Y.S.; Wang, R.; Zhang, T.; Liu, S.; Fei, T. Zeolitic imidazolate framework-8 (ZIF-8)-coated In2O3 nanofibers as an efficient sensing material for ppb-level NO2 detection. J. Colloid Interface Sci. 2019, 541, 249–257. [Google Scholar] [CrossRef]
  39. Lou, C.; Huang, Q.; Li, Z.; Lei, G.; Liu, X.; Zhang, J. Fe2O3-sensitized SnO2 nanosheets via atomic layer deposition for sensitive formaldehyde detection. Sens. Actuators B 2021, 345, 130429. [Google Scholar] [CrossRef]
  40. Lou, C.; Yang, C.; Zheng, W.; Liu, X.; Zhang, J. Atomic layer deposition of ZnO on SnO2 nanospheres for enhanced formaldehyde detection. Sens. Actuators B 2021, 329, 129218. [Google Scholar] [CrossRef]
  41. Yao, M.S.; Tang, W.X.; Wang, G.E.; Nath, B.; Xu, G. MOF thin film-coated metal oxide nanowire array: Significantly improved chemiresistor sensor performance. Adv. Mater. 2016, 28, 5229–5234. [Google Scholar] [CrossRef] [PubMed]
  42. Wei, C.; Zhu, M.; Zhou, Z.; Zhao, S.; Mao, J.; Yin, D.; Li, J.; Wang, Y.; Hao, J. Two-dimensional Bi2O2S based high-sensitivity and rapid-response humidity sensor for respiratory monitoring and Human-Machine Interaction. Chem. Eng. J. 2024, 485, 149805. [Google Scholar] [CrossRef]
  43. Meng, F.-J.; Guo, X.-M. Tuning the oxygen defects and Fermi levels via In3+ doping in SnO2-In2O3 nanocomposite for efficient CO detection. Sens. Actuators B 2022, 357, 131412. [Google Scholar] [CrossRef]
  44. Zhou, S.; Wang, H.; Hu, J.; Lv, T.; Rong, Q.; Zhang, Y.; Zi, B.; Chen, M.; Zhang, D.; Wei, J.; et al. Formaldehyde gas sensor with extremely high response employing cobalt-doped SnO2 ultrafine nanoparticles. Nanoscale Adv. 2022, 4, 824–836. [Google Scholar] [CrossRef] [PubMed]
  45. Thamaga, B.R.J.; Theka, T.J.; Motsoeneng, R.G.; Coetsee-Hugo, L.; Swart, H.C.; Motaung, D.E. Remarkable surface area engineering of nanosheet-assembled hierarchical p-n Ag-loaded NiO-CeO2 heterostructure for superior ethanol sensing performance. J. Alloys Compd. 2024, 976, 173110. [Google Scholar] [CrossRef]
  46. Sun, C.; Liu, H.; Shao, J.; Pan, G.; Yang, X.; Wang, M.; Dong, J.; Zhu, M.; Qi, Y. Au-loaded Zn2SnO4/SnO2/ZnO nanosheets for fast response and highly sensitive TEA gas sensors. Sens. Actuators B 2023, 376, 132951. [Google Scholar] [CrossRef]
  47. Liu, Y.; Gao, X.; Li, F.; Lu, G.; Zhang, T.; Barsan, N. Pt-In2O3 mesoporous nanofibers with enhanced gas sensing performance towards ppb-level NO2 at room temperature. Sens. Actuators B 2018, 260, 927–936. [Google Scholar] [CrossRef]
  48. Sun, Y.; Wang, B.; Wang, B.; Zhao, Z.; Zhang, W.; Zhang, W.; Suematsu, K.; Hu, J. Construction of Flower-like PtOx@ZnO/In2O3 Hollow Microspheres for Ultrasensitive and Rapid Trace Detection of Isopropanol. ACS Appl. Mater. Interfaces 2023, 15, 12041–12051. [Google Scholar] [CrossRef] [PubMed]
  49. Ren, G.; Li, Z.; Yang, W.; Faheem, M.; Xing, J.; Zou, X.; Pan, Q.; Zhu, G.; Du, Y. ZnO@ZIF-8 core-shell microspheres for improved ethanol gas sensing. Sens. Actuators B 2019, 284, 421–427. [Google Scholar] [CrossRef]
  50. Lei, Z.; Deng, Y.; Wang, C. Multiphase surface growth of hydrophobic ZIF-8 on melamine sponge for excellent oil/water separation and effective catalysis in a Knoevenagel reaction. J. Mater. Chem. A 2018, 6, 3258–3263. [Google Scholar] [CrossRef]
  51. Zhu, L.; Wang, J.; Liu, J.; Chen, X.; Xu, Z.; Ma, Q.; Wang, Z.; Liang, J.; Li, S.; Yan, W. Designing highly sensitive formaldehyde sensors via A-site cation deficiency in LaFeO3 hollow nanofibers. Appl. Surf. Sci. 2022, 590, 153085. [Google Scholar] [CrossRef]
  52. Cao, J.; Zhang, N.R.; Wang, S.M.; Zhang, H.M. Electronic structure-dependent formaldehyde gas sensing performance of the In2O3/Co3O4 core/shell hierarchical heterostructure sensors. J. Colloid Interface Sci. 2020, 577, 19–28. [Google Scholar] [CrossRef] [PubMed]
  53. Zhang, S.; Song, P.; Li, J.; Zhang, J.; Yang, Z.; Wang, Q. Facile approach to prepare hierarchical Au-loaded In2O3 porous nanocubes and their enhanced sensing performance towards formaldehyde. Sens. Actuators B 2017, 241, 1130–1138. [Google Scholar] [CrossRef]
  54. Li, Y.; Chen, N.; Deng, D.; Xing, X.; Xiao, X.; Wang, Y. Formaldehyde detection: SnO2 microspheres for formaldehyde gas sensor with high sensitivity, fast response/recovery and good selectivity. Sens. Actuators B 2017, 238, 264–273. [Google Scholar] [CrossRef]
  55. Wang, X.S.; Zhang, J.B.; Wang, L.Y.; Li, S.C.; Liu, L.; Su, C.; Liu, L.L. High response gas sensors for formaldehyde based on Er-doped In2O3 nanotubes. J. Mater. Sci. Technol. 2015, 31, 1175–1180. [Google Scholar] [CrossRef]
Nanomaterials 14 00841 g001
Figure 1. Schematic illustration of the fabrication of ZIF-8@Pt-NiO/In2O3 hollow nanofibers.
Figure 1. Schematic illustration of the fabrication of ZIF-8@Pt-NiO/In2O3 hollow nanofibers.
Nanomaterials 14 00841 g001
Nanomaterials 14 00841 g002
Figure 2. SEM images of as-prepared (a) pristine In2O3 HNFs, (b) NiO/In2O3 HNFs (red circle is the hollow structure), (c) ZIF-8@Pt-NiO/In2O3 HNFs; (d,e) TEM images, (f) HRTEM, and (g) EDS element mapping of ZIF-8@Pt-NiO/In2O3 HNFs.
Figure 2. SEM images of as-prepared (a) pristine In2O3 HNFs, (b) NiO/In2O3 HNFs (red circle is the hollow structure), (c) ZIF-8@Pt-NiO/In2O3 HNFs; (d,e) TEM images, (f) HRTEM, and (g) EDS element mapping of ZIF-8@Pt-NiO/In2O3 HNFs.
Nanomaterials 14 00841 g002
Nanomaterials 14 00841 g003
Figure 3. Structural characterizations for pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNFs. (a) XRD patterns; (b) FT-IR spectra and (c) Raman spectra of various samples. (d) N2 adsorption–desorption isotherms and (e) pore-size distributions of pristine In2O3 and ZPNiIn HNFs; XPS spectra of (f) In 3d, (g) Zn 2p and (h) Ni 2p of various samples, and (i) O 1s of pristine In2O3 and ZPNiIn HNFs.
Figure 3. Structural characterizations for pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNFs. (a) XRD patterns; (b) FT-IR spectra and (c) Raman spectra of various samples. (d) N2 adsorption–desorption isotherms and (e) pore-size distributions of pristine In2O3 and ZPNiIn HNFs; XPS spectra of (f) In 3d, (g) Zn 2p and (h) Ni 2p of various samples, and (i) O 1s of pristine In2O3 and ZPNiIn HNFs.
Nanomaterials 14 00841 g003
Nanomaterials 14 00841 g004
Figure 4. Sensing property measurements for pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNFs at 180 °C. (a) Response toward HCHO (100 ppm) under different operating temperatures; (b) response and recovery characteristics of sensor exposure to HCHO (100 ppm); (c) response curves of sensors to HCHO from 1 to 100 ppm concentrations; (d) linearity between the concentration of sensors and the response value; (e) selectivity of sensors toward 100 ppm of various VOC gas; (f) repeatability of the ZPNiIn sensor to HCHO (100 ppm); (g) dynamic response curves of the pristine In2O3 and ZPNiIn sensors to HCHO (100 ppm) under different relative humidities (RHs); (h) water contact angles of the pristine In2O3 and ZPNiIn HNF-sensing materials’ surface; (i) long-term stability of ZPNiIn sensor towards HCHO (100 ppm).
Figure 4. Sensing property measurements for pristine In2O3, NiIn, ZNiIn, and ZPNiIn HNFs at 180 °C. (a) Response toward HCHO (100 ppm) under different operating temperatures; (b) response and recovery characteristics of sensor exposure to HCHO (100 ppm); (c) response curves of sensors to HCHO from 1 to 100 ppm concentrations; (d) linearity between the concentration of sensors and the response value; (e) selectivity of sensors toward 100 ppm of various VOC gas; (f) repeatability of the ZPNiIn sensor to HCHO (100 ppm); (g) dynamic response curves of the pristine In2O3 and ZPNiIn sensors to HCHO (100 ppm) under different relative humidities (RHs); (h) water contact angles of the pristine In2O3 and ZPNiIn HNF-sensing materials’ surface; (i) long-term stability of ZPNiIn sensor towards HCHO (100 ppm).
Nanomaterials 14 00841 g004
Nanomaterials 14 00841 g005
Figure 5. (a,b) The energy level diagram and (c) the schematic sensing mechanism of ZPNiIn HNFs.
Figure 5. (a,b) The energy level diagram and (c) the schematic sensing mechanism of ZPNiIn HNFs.
Nanomaterials 14 00841 g005
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zhu, L.; Wang, Z.; Wang, J.; Liu, J.; Zhao, W.; Zhang, J.; Yan, W. Synergistic Effect of ZIF-8 and Pt-Functionalized NiO/In2O3 Hollow Nanofibers for Highly Sensitive Detection of Formaldehyde. Nanomaterials 2024, 14, 841. https://doi.org/10.3390/nano14100841

AMA Style

Zhu L, Wang Z, Wang J, Liu J, Zhao W, Zhang J, Yan W. Synergistic Effect of ZIF-8 and Pt-Functionalized NiO/In2O3 Hollow Nanofibers for Highly Sensitive Detection of Formaldehyde. Nanomaterials. 2024; 14(10):841. https://doi.org/10.3390/nano14100841

Chicago/Turabian Style

Zhu, Lei, Ze Wang, Jianan Wang, Jianwei Liu, Wei Zhao, Jiaxin Zhang, and Wei Yan. 2024. "Synergistic Effect of ZIF-8 and Pt-Functionalized NiO/In2O3 Hollow Nanofibers for Highly Sensitive Detection of Formaldehyde" Nanomaterials 14, no. 10: 841. https://doi.org/10.3390/nano14100841

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop