Bimetallic MOF-Derived NiO/In₂O₃ Heterojunctions for NO₂ Sensing

Abstract

Low-temperature (including room-temperature) gas sensors are crucial for energy-efficient and safe detection applications. In this study, we report the synthesis of In₂O₃-sensitized NiO nanoparticles (NPs) for NO₂ detection. The NiO/In₂O₃ hybrid materials were obtained by pyrolysis of Ni/In bimetallic metal–organic framework (MOF) nanosheets (NSs) fabricated through ultrasonic synthesis and cation exchange. Gas sensing tests revealed that the In₂O₃ sensitization significantly enhances the NO₂ sensing performance of NiO, enabling a response of 1.5 at room temperature (RT) and an optimal response at 100 °C. The NiO/In₂O₃ sensor demonstrates enhanced selectivity toward NO₂, an ultra-low detection limit (41 ppb), and long-term stability. This study presents an effective MOF-derived route for developing high-performance low-power gas sensors.

1. Introduction

As a product of automobile and industrial exhaust, nitrogen dioxide (NO₂) is the main component of air pollutants [1]. Prolonged exposure to elevated NO₂ concentrations poses significant risks to human health, and even a few ppm NO₂ can cause serious damage to the respiratory system. The U.S. Environmental Protection Agency (EPA) mandates that ambient NO₂ must not exceed 53 ppb [2]. Thus, there is an urgent demand for high-performance gas sensors capable of real-time monitoring of NO₂ at ppb-level concentrations, featuring high sensitivity, superior selectivity, and low-power operation at room temperature (RT). This need arises from the practical limitations of conventional metal oxide sensors, which typically require high operating temperatures (often 200–400 °C) that entail substantial power consumption and potential safety risks [3].

Nickel oxide (NiO) is a p-type metal oxide semiconductor with promising applications in gas sensing [4,5,6] and other fields [7,8]. Its relatively wide band gap enables a pronounced resistance change upon interaction with gas molecules, which is a fundamental requirement for sensing [9]. The high operating temperatures of NiO-based gas sensors lead to material degradation, reduced stability and lifespan, increased power consumption, and safety concerns for flammable gas detection [10]. These issues collectively hinder their practical deployment. Therefore, achieving satisfactory sensing performance at lower or even RT is crucial for advancing the practical utility of NiO gas sensors [3].

Metal–organic frameworks (MOFs), characterized by their high specific surface area and surface-to-volume ratio, coordinated electronic structure, and ordered pore structure, have received extensive attention in many fields [11], such as gas storage [12], catalysis [13], and gas sensors [14,15]. Since MOF materials possess nanoscale cavities and open channels that provide influx and escape pathways for small molecules, they can be used as starting materials or templates for the manufacture of highly efficient gas sensors. For instance, MOF-derived materials have been employed in detecting various gases, such as acetone (Sun et al.) [16], NO₂ (Hassan et al. using NiO/CdS-CdO composites) [17], and xylene (Hu et al. using a SnO₂/NiO heterostructure) [18]. A critical analysis of these and other recent studies reveals a common, significant challenge: many high-performance MOF-derived sensors, particularly those based on Ni, still require elevated operating temperatures [19], which undermines their practicality. Therefore, it remains a research priority to develop high-sensitivity Ni-based gas sensors that can operate at significantly lower temperatures [20].

In this study, we aim to design a NO₂ gas sensor based on a NiO-based heterojunction derived from a MOF precursor. A two-dimensional layered Ni/In MOF was first synthesized via an ultrasonic-assisted method combined with a cation exchange strategy, which was subsequently pyrolyzed to form NiO/In₂O₃ nanoparticles. The primary objective was to investigate the effect of introducing In₂O₃ on the gas-sensing performance of the resulting heterojunction, achieving enhanced gas-sensing performance of the material [21].

2. Experiment

2.1. Materials

All chemical reagents used in this work are analytical grade and without further purification. Reagents for synthesis such as Nickel (II) chloride hexahydrate (NiCl₂·6H₂O, ≥98%), Indium (III) chloride tetrahydrate (InCl₃·4H₂O, 36.1–37%), terephthalic acid (1,4-BDC, ≥99%), triethylamine (TEA, ≥99%), N,N-dimethylformamide (DMF, ≥99.5%), and ethanol (C₂H₅OH, ≥99.7%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Synthesis of Ni MOF and Ni/In MOF

The Ni MOF was synthesized based on previous studies [22,23,24], with some modifications. 1,4-BDC (125 mg) was first dissolved in a mixed solution of DMF (30 mL), absolute ethanol (2 mL), and deionized water (2 mL) under vigorous stirring, and then 475 mg NiCl₂·6H₂O and TEA (1 mL) were further added under stirring for 10 min to form a uniform colloidal suspension. The mixture was ultrasonicated continuously for 2 h at RT using a KQ3200B ultrasonic instrument (Hangzhou Chuanheng Experimental Instrument Co., Ltd., Suzhou, China) to promote the growth of Ni MOF [25], thereby facilitating the subsequent cation exchange between In³⁺ and the preformed Ni MOF framework [26]. The precipitate was collected via centrifugation using a centrifuge (Hangzhou Chuanheng Experimental Instrument Co., Ltd., Hangzhou, Zhejiang, China) at 7000 r/min for 6 min, washed with ethanol for three times, and dried at 60 °C for 12 h.

The preparation process of Ni/In MOF was performed in accordance with Ni MOF. While NiCl₂·6H₂O and TEA were being added, InCl₃·4H₂O was additionally introduced at dosages of 73 mg and 146 mg, which corresponded to Ni:In molar ratios of 8:1 and 4:1, respectively. Resulting products were designated as Ni/In MOF-1 and Ni/In MOF-2.

2.3. Synthesis of NiO and NiO/In₂O₃ NPs Derived from MOF Precursors

The obtained Ni/In MOF-1 and Ni/In MOF-2 were calcined in a tubular furnace (OTF-1200 X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China) in air. The heating rate was 5 °C min⁻¹ from RT to 500 °C and the samples were calcined at 500 °C for 3 h. After cooling to RT, the powders were collected. The resulting NiO/In₂O₃ nanoparticles (NPs) were denoted as NiO/In₂O₃-1 and NiO/In₂O₃-2. Figure 1a shows the schematic illustration of the formation of NiO/In₂O₃ NPs.

2.4. Gas Sensor Fabrication and Test

A total of 10 mg of NiO or NiO/In₂O₃ composite powder was dispersed in 0.5 mL of deionized water and thoroughly mixed to form a homogeneous slurry. Subsequently, 2 μL of the slurry was pipetted and drop-cast onto the surface of clean interdigital electrodes (IDEs) to construct the sensitive functional layer of the sensor. It is worth noting that the fabrication process of the IDEs used in this experiment has been detailed in our previous research [27]. After slurry deposition, the electrodes loaded with the sensitive layer were air-dried naturally at room temperature.

Gas-sensing performance of the sensors was evaluated via a static gas-sensing test method under ambient atmospheric pressure, following the specific procedure below: fabricated sensor devices were fixed on the test platform, exposed to air, and connected to the test circuit, which consisted of the test chamber connected to a source meter (Keithley 2400, Keithley Instruments, Inc., Cleveland, OH, USA); after baseline resistance of the sensors stabilized, the gas chamber lid was closed, and target gas was injected into the chamber to record the real-time resistance variation trend of the sensors; when resistance variation tended to stabilize, the chamber lid was opened, allowing the sensors to be re-expose to air and enter the resistance recovery stage.

Concentration gradient of the target gas was regulated via the quantitative syringe injection method: different volumes of the target gas were injected into the test chamber to create corresponding concentration atmospheres, thus enabling systematic characterization of sensor performance under varied concentrations.

A syringe was used to inject different volumes of target gas into the test chamber. Different volumes of gas represented different concentrations. For liquid target gas, the required liquid volume (${\mathrm{V}}_{\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}}$) is calculated as Equation (1):

$$\begin{array}{c}{\mathrm{V}}_{\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}}={\displaystyle \frac{{\mathrm{V}}_0·\mathrm{C}·\mathrm{M}·\mathrm{P}}{\mathsf{\rho }·\mathrm{R}·\mathrm{T}·\mathrm{w}}}\end{array}$$

(1)

${\mathrm{V}}_{\mathrm{l}\mathrm{i}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{d}}$ is the volume of the target liquid (mL). ${\mathrm{V}}_0$ is the volume of the test chamber (L). $\mathrm{C}$ is the vapor concentration after liquid evaporation (ppm). $\mathrm{M}$ is the molecular weight of the target gas (g/mol). $\mathrm{P}$ is the atmospheric pressure (atm). $\mathsf{\rho }$ is the density of the liquid (g/mL). $\mathrm{R}$ is the molar gas constant (L·atm/mol·K). $\mathrm{T}$ is the room temperature (K). $\mathrm{w}$ is the purity factor (wt%) [28,29]. The purity value of ethanol is 99.7%; the purity value of triethylamine is 99%; the purity value of acetone is 99%; the purity value of formaldehyde is 89%; the purity value of methanol is 99.9%; and the purity value of N-butanol is 99.5%. All chemical reagents used in this work are analytical grade and without further purification. For the target gas with required gas volume (V), the formula is as shown in Equation (2):

$$\begin{array}{c}\mathrm{V}={\displaystyle \frac{\mathrm{C}·{\mathrm{V}}_0}{{\mathrm{C}}_0}}\end{array}$$

(2)

where ${\mathrm{V}}_0$ is the volume of the test chamber (mL) and ${\mathrm{C}}_0$ is the concentration of the target gas. NO₂ has a ${\mathrm{C}}_0$ of 1000 ppm and was diluted with air.

2.5. Materials Characterization

The products were characterized by X-ray diffraction (XRD, Bruker D2 Phaser, Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation (40 kV, 40 mA). A scanning electron microscope (SEM, Zeiss Sigma 300, Carl Zeiss Microscopy, Oberkochen, Germany) with 10 kV accelerating voltage was used to scan the microstructures. A transmission electron microscope (TEM, JEM2100Plus, JEOL Ltd., Tokyo, Japan) and high-resolution TEM (HRTEM, FEI Tecnai G20, FEI Company, Brno, Czech Repoblic) images were used to observe the structure of the prepared materials. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo ESCALAB 250 (Thermo Fisher Scientific, Waltham, MA, USA) with the Al Kα (1486.6 eV) anode. The peak binding energy at C1s was 284.8 eV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS) were used to obtain the elemental mapping (HAADF-STEM and EDS elemental mapping were performed on the FEI Tecnai G20 instrument, which was also used for HRTEM). Raman spectroscopy was performed on a Raman microscope (HORIBA-IHR550, HORIBA Scientific, Palaiseau/Longjumeau, France) using a 532 nm laser source (Cobolt 0532-04-01-0050-700) with a 50× objective.

3. Results and Discussion

3.1. Structure and Morphology

The crystallographic structure of the obtained materials is characterized by XRD analysis. Figure 1b,c represent the XRD patterns before and after calcination, respectively. In Figure 1b, the difference between Ni MOF and others is greater with the increase of the In content. In Figure 1c, the diffraction peaks at 2θ of 37.33°, 43.38°, and 63.02° correspond to the (110), (200), and (220) crystal planes of cubic NiO (JCPDS no. 73-1519), respectively. Meanwhile, the peaks at 2θ of 30.59°, 32.90°, 35.46°, 51.02°, and 60.67° are attributed to the (222), (321), (400), (440), and (622) crystal planes of cubic In₂O₃ (JCPDS no. 06-0416), respectively [30]. Moreover, the characteristic peaks of In₂O₃ become more obvious with the increase of In element content. Figure S1a,b show the surface morphologies of Ni MOF and Ni/In MOF-1, whose structures are layered nanosheets. Figure S1c,d and Figure 1d show the structure of NiO and NiO/In₂O₃. NiO is composed of extensive NPs, and there are abundant voids between particles, which helps to improve the gas adsorption of the sensing materials. It is observed that NiO/In₂O₃-1 and NiO/In₂O₃-2 have a surface morphology similar to NiO. In Figure 1d, NiO/In₂O₃-1 shows a porous aggregate of nanoparticles. This unique microstructure contributes to enhanced gas sensing properties by promoting gas adsorption and diffusion [31]. After calcination, MOF structures forms NPs with abundant voids, which can provide a larger specific surface area for target gas molecule adsorption and improves the sensing performance.

Figure 2a,d present the TEM images of NiO and NiO/In₂O₃-1, revealing their NPs morphology. In HRTEM, the high crystallinity of NiO and NiO/In₂O₃-1 is demonstrated. In Figure 2c, the lattice fringe spacing of 0.24 nm is indexed to the (111) crystal plane of NiO. In Figure 2f, the (111) plane of NiO and the (222) plane of In₂O₃ are clearly observed. The corresponding theoretical interplanar spacings for these crystal planes are provided in Table S1 for comparison. Figure S1d–f show the STEM-HAADF and EDS image of NiO NPs. Figure 2g shows the HAADF image of NiO/In₂O₃-1. Elemental mapping (Figure 2h–j) demonstrates the homogeneous distribution of Ni, O, and In elements within the NPs. Compared with Ni, the distribution of In is sparser, confirming the low content of In elements.

To verify the formation of heterostructure, Raman spectra were measured. As shown in Figure 3a, we recorded the Raman spectra of NiO, NiO/In₂O₃-1, and NiO/In₂O₃-2 in the 70–800 cm⁻¹ range. Among them, the peaks at 132 cm⁻¹ and 308 cm⁻¹ are assigned to the In-O vibration of InO₆ structural units and the δ-bending vibration of InO₆ octahedrons, respectively [32]. The broad asymmetric peak at 500 cm⁻¹ corresponds to the (1P + 1M) band of NiO, arising from the cooperative scattering of one-magnon (1M—40 cm⁻¹) and one-phonon (TO—450 cm⁻¹) excitations at the Brillouin-zone center [33,34]. The Raman signals are consistent with both NiO and In₂O₃.

Figure S2 shows the XPS survey spectra of NiO, NiO/In₂O₃-1, and NiO/In₂O₃-2, where characteristic peaks of Ni 2p, O 1s can be observed. At the same time, the XPS peak belonging to In 3d can be observed in NiO/In₂O₃-1 and NiO/In₂O₃-2. Table S2 shows weight percentages (wt%) of Ni, In, and O elements determined by XPS analysis. Figure 3b shows the Ni 2p spectra of the products. The peaks belong to Ni 2p_1/2 and Ni 2p_3/2; two satellite peaks are found in the scans. In addition, with the increase in In₂O₃ content, the binding energy gradually decreases. Table S3 presents the Ni 2p XPS fitting results for the three samples. This is attributed to the strong electronic interaction between NiO and In₂O₃ [35,36]. Figure 3c shows the O 1s XPS spectrum, where the O 1s peaks were deconvoluted into lattice oxygen (O_latt), adsorbed oxygen (O_ads), and surface oxygen (O_surf) through peak fitting. For NiO, the three oxygen components exhibit binding energies of 528.9, 529.7, and 531.0 eV, respectively. For NiO decorated with In₂O₃, they are located at 529.9, 531.6, and 533.3 eV, respectively. The O 1s spectrum fitting results (Table S4) show that the introduction of In₂O₃ altered the relative content characteristics of oxygen species. Among them, O_ads is mainly responsible for the sensing, and the sensing performance is mainly affected by the surface adsorbed oxygen content. The calculated O_ads content of NiO/In₂O₃-1 is 41.7%, which is larger than that of NiO (25.5%) and NiO/In₂O₃-2 (38.3%). In addition, Figure 3d shows that the binding energy of In 3d_3/2 and In 3d_5/2 are positioned at 452.7 and 445.2 eV, respectively [37]. The XPS results indicate that the In₂O₃ content is 24.2 wt% for NiO/In₂O₃-1 and 32 wt% NiO/In₂O₃-2.

3.2. Gas Sensing Performances

To study the effect of other gases on the sensor, the selectivity was tested as shown in Figure 4a. The numbers on the radar chart merely indicate the magnitude of the response values. NO₂, SO_2, and O₃ are oxidizing gases, and the response of the NiO and NiO/In₂O₃ sensors is calculated as $S=R_a/R_g$. For reductive gases such as NH₃, H₂, ethanol, methanol, triethylamine, formaldehyde, acetone, and N-butanol, their responses are calculated as $S=R_g/R_a$, where $R_a$ is the initial resistance of the sensor in air and $R_g$ is the resistance of the sensor in the detected gas. Among the tested gases, the sensing characteristics for NO₂ gas are significantly better than other gases, demonstrating the excellent selectivity of the NiO/In₂O₃. This can be attributed to the lone-pair electrons and high electron affinity of NO₂ molecules, which promote strong interactions between NO₂ and MOF-derived NiO/In₂O₃ [38]. This proves that the sensing materials derived from the MOF structure in this experiment have good selectivity for NO₂. As shown in Figure 4b, the responses of NiO, NiO/In₂O₃-1, and NiO/In₂O₃-2 to 20 ppm NO₂, as well as the resistances of the three sensors in the ambient environment, were measured at various temperatures. As illustrated in the inset of Figure 4b, all sensors exhibit inherent semiconductor behavior, with conductivity increasing with temperature. Moreover, In₂O₃ loading raises $R_a$ due to heterostructure-induced barrier enhancement and majority carrier reduction [39,40]. The NO₂ response of NiO is significantly enhanced after In₂O₃ loading, and NiO/In₂O₃-1 exhibits good NO₂ response at 100 °C. As the operating temperature increases, the response values of the three sensors all exhibit a trend of first increasing and then decreasing. The optimal operating temperature for NiO/In₂O₃-1 is 100 °C, while that for NiO and NiO/In₂O₃-2 is 120 °C. As the temperature rises further, reactive oxygen species readily desorb from the surface and the response decreases. The above results show that the NiO/In₂O₃-1 sensor has the better sensing performance than the NiO and NiO/In₂O₃-2 sensors. As shown in Figure 4c, the resistance curves of the NiO/In₂O₃-1 sensor were measured at 100 °C. After introducing the oxidizing gas (NO₂), the sensor resistance decreases, exhibiting a typical p-type response. Meanwhile, the initial resistance of NiO loaded with In₂O₃ is much greater than the original NiO resistance (Figure 4d). The higher the In₂O₃ content, the greater the initial resistance. This is attributed to the fact that the work function of NiO is higher than that of In₂O₃ [41], leading to electron flow from In₂O₃ to NiO and resulting in increased resistance [42]. The response-recovery time of the NiO/In₂O₃-1 to 20 ppm NO₂ is 23/100 s, and the response and recovery times of the three sensors are statistically analyzed in Figure 4e. According to the results of the three sensors, the NiO/In₂O₃-1 sensor exhibits the fastest response-recovery speed.

To further explore the sensor’s reproducibility, the cycling curve of the NiO/In₂O₃-1 sensor toward 20 ppm NO₂ at 100 °C was investigated (Figure 4f). The six cycle curves show reasonable variation within a small range, demonstrating the excellent repeatability of the sensor. In order to study the low temperature performance, the NiO/In₂O₃-1 sensor was tested at RT, and the response curve is shown in Figure 4g. Even at RT, the response of the NiO/In₂O₃-1 sensor to 20 ppm NO₂ reaches 1.5. The low operating temperature reduces the sensor’s power consumption and provides it with a wide range of practical applications. Additionally, evaluating the impact of ambient humidity on sensor performance is essential. Figure 4h presents the dynamic response curves of the NiO/In₂O₃-1 under various humidity environments. As relative humidity (RH) increases, the response of the sensor decreases significantly. It shows that the environmental humidity has an influence on the sensor. In high-humidity environments, enhanced competitive adsorption of H₂O molecules on the sensitive layer surface occupies abundant active sites, thereby hindering the effective adsorption of surface oxygen species and NO₂ molecules [43]. The reduced amount of adsorbed NO₂ molecules ultimately leads to the decreased sensor response observed under high RH conditions [44]. Meanwhile, the response and baseline resistance data within one month is shown in Figure 4i. The results indicate that the response and resistance changes of NiO/In₂O₃-1 sensor are small, indicating that the sensor has reliable stability.

The sensing performance was tested at the optimal working temperature of the sensors. Figure 5a–c show the response curves of the NiO, NiO/In₂O₃-1, and NiO/In₂O₃-2 to different concentrations of NO₂. In comparison, the response of the NiO loaded with In₂O₃ NPs is higher than that of the NiO sensor, demonstrating that the heterostructure can significantly improve the performance. To further evaluate the sensitivity, the concentration response curves are linearly fitted (Figure 5d), with the calculated LODs being 111, 41, and 62 ppb, respectively. This proves that the NiO/In₂O₃-1 sensor has the lowest LOD (41 ppb). The sensors with low LOD have better results in practical applications [45,46]. Meanwhile, the ability of the NiO/In₂O₃-1 sensor to detect trace NO₂ was tested. Figure S3 shows the detection capability of the NiO/In₂O₃-1 sensor for 0.1 ppm and 0.2 ppm NO₂. Significant response changes can be observed, proving that the sensor can detect very low concentrations of NO₂, which can further justify the calculated LOD.

Table 1. Performance comparison of representative NiO-based, In₂O₃-based, and related heterojunction NO₂ sensors.

Meterials	T (°C)	NO2 (ppm)	Response	LOD (ppb)	Res./Rec. Time (s)	Ref.
Co-NiO	250	10	7.22	-	-	[54]
WO3/NiO	200	10	16.06	20	9/13	[19]
Pd/Pb/NiO	RT	10	19.68	42	49.6/92.4	[47]
NiO-NiV	RT	20	12.42	-	-	[38]
Ce:NiO	150	-	0.719% ppm	-	-	[50]
Ni3N/NiO	RT	1	0.09	300	-	[52]
NiO/Au	600	80	49.1 mV	-	666/-	[53]
NiO	200	200	23.3%	-	20/498	[55]
Pr2Sn2O7/NiO	180	250	27.4	1	2/38	[48]
NiO/CuO	RT	100	77.16%	1 ppm	2/-	[51]
NiO-SE	700	400	19 mV	-	<10/<10	[66]
NiO/Gr/SiC	100	10	33%	-	<60/4~5 h	[49]
RhOX/B-In2O3	RT	5	180	1 ppm	8/17	[58]
In2O3/Ti3C2TX	RT	100	55.9	0.3 ppm	197.6/93.8	[57]
Bi2O3/In2O3	300	5	155.5%	1 ppm	-	[63]
MOF-In2O3	30	200	1210	1	298/18	[61]
Pt/In2O3	40	1	44.9	-	-/7 min	[62]
In2O3/MoS2	RT	200	371.9	8.8	-	[67]
rGO/In2O3	RT	100	393	92.8	27/46	[68]
CuO/NiOOM	110	60	13.9%	5 ppm	18/29	[60]
NiO/Co3O4	RT	100	47.4	10	1.3/9.6	[46]
Ag-In2O3	RT	200	98	0.05	24/40	[56]
In2O3/MXene	RT	4	24.98	3.56	409/-	[65]
In2O3/WO3	200	0.5	182%	500	10/57	[59]
Au/In2O3	RT	10	38.9%			[64]
NiO/In2O3	100	20	12.5	41	23/100	This work

Res./Rec. Time: response/recovery time; RT: room temperature.

further highlights the performance advantages of the MOF-derived NiO/In₂O₃ sensor in detecting NO₂ at 100 °C by comparing its performance with that of other NiO-based [19,46,47,48,49,50,51,52,53,54,55] and In₂O₃-based [56,57,58,59,60,61,62,63,64,65] NO₂ sensors.

3.3. Gas Sensing Mechanism

Figure 6a depicts a schematic illustration of the sensing mechanism for the NiO/In₂O₃ sensor. When the sensing material is exposed to air, electrons are transferred from its conduction band to oxygen molecules. These molecules adsorb onto the material surface to form chemically adsorbed oxygen species (O²⁻, O⁻ and O₂⁻), concurrently inducing the formation of a hole accumulation layer on the NiO surface [69]. Upon exposure to NO₂ gas, a redox reaction occurs between the surface-adsorbed oxygen and NO₂. This reaction extracts electrons from the conduction band, thereby further increasing the thickness of the hole accumulation layer. Concurrently, the material resistance is reduced, and the reaction mechanism is represented by Equations (3) and (4):

$$\begin{array}{c}{\mathrm{N}\mathrm{O}}_2 \left(\mathrm{g}\mathrm{a}\mathrm{s}\right)+{\mathrm{e}}^{-}={\mathrm{N}\mathrm{O}}_2^{-} \left(\mathrm{a}\mathrm{d}\mathrm{s}\right)\end{array}$$

(3)

$$\begin{array}{c}{\mathrm{N}\mathrm{O}}_2 \left(\mathrm{g}\mathrm{a}\mathrm{s}\right)+{\mathrm{O}}^{\mathsf{\delta }-}={\mathrm{N}\mathrm{O}}_3^{\mathsf{\delta }-} \left(\mathrm{a}\mathrm{d}\mathrm{s}\right)\end{array}$$

(4)

For MOF-derived NiO/In₂O₃ heterostructure, the presence of In₂O₃ will cause more adsorbed oxygen to adhere to the material surface, thereby increasing the number of reactive sites for subsequent reactions. In addition, since the work function of In₂O₃ is smaller than that of NiO (Figure 6b), electrons are transferred from In₂O₃ to NiO until the Fermi level reaches equilibrium (Figure 6c). This process thins the thickness of the hole accumulation layer, leading to the resistance of NiO/In₂O₃ material being larger than that of NiO (Figure 4c,d). Due to the limited number of hole carriers in the material, gas adsorption will be confined to the material surface, which is advantageous for gas sensing [70,71]. The above analysis shows that MOF-derived NiO/In₂O₃ material will greatly improve the sensing ability to detect NO₂.

4. Conclusions

In summary, NiO/In₂O₃ heterostructure NPs were synthesized by pyrolysis of MOF templates. The sensor exhibits a response of 1.5 to low-concentration NO₂ at RT and an enhanced response of 3 at 100 °C. Meanwhile, the effect of In₂O₃ content on the NO₂ gas-sensing performance of the material was investigated. Experiments demonstrated that the introduction of In₂O₃ enhanced the gas-sensing performance of NiO, enabling high-performance detection of NO₂. The sensor exhibits a high response rate and excellent selectivity, as well as low LOD (41 ppb) for NO₂ detection. The enhanced gas-sensing performance of the sensor is attributed to the synergistic effect between NiO and In₂O₃ nanoparticles. This work proposes a novel approach to electronically engineer NiO-based materials for gas sensors, achieving enhanced sensitivity while reducing power consumption.