English

• 0.   Introduction

  Triethylamine (TEA), which is also referred to as N, N-diethylamine, belongs to a category of volatile organic compounds (VOCs) that are noted for their strong and distinctive smell. TEA is utilized in numerous domains. For example, it acts as a fundamental substance, a solvent, and a catalyst in the process of industrial synthesis. Additionally, it is employed in the preparation of rubber vulcanization accelerators, enamel hardeners, liquid rocket propellants, and acid collectors^[1-2]. In the pharmaceutical industry, TEA is utilized in the synthesis of drugs such as vitamin B₆, cefradine, berberine hydrochloride, pioneer Ⅳ, and verapamil. However, TEA is known to cause significant irritation to the mucous membranes and skin. Inhalation of its vapors can lead to damage to the central nervous system, mucosal tissues, and liver, and may even result in death^[3-5]. Consequently, it is crucial to create an efficient gas sensor for identifying TEA gas.

  So far, gas sensors that utilize metal oxide semiconductors have been widely applied in gas detection tasks owing to their affordability, straightforward operation and consistent performance^[6-8]. In response to the growing complexity of gas monitoring demands, it is acknowledged that there is a necessity to create advanced gas sensors, where the fundamental challenge is significantly tied to the innovation and production of novel sensitive materials. In recent years, numerous ternary metal oxides have been synthesized through advancements in nanotechnology and chemical synthesis techniques, offering advantages such as unique crystal structures, abundant defect states, and controllable chemical compositions^[9-11]. These materials have emerged as highly promising candidates for gas sensing. Among them, ZnSnO₃, recognized as a common n-type semiconductor, has been utilized for the identification of VOCs^[12-15]. However, low response values and slow response/recovery speeds significantly hinder their further research and application.

  To address these issues, it is essential to design and modify the microstructure of ZnSnO₃ materials. Hollow structures are recognized for creating swift channels for gas diffusion and offering a substantial specific surface area^[16-17], thereby enhancing the surface interaction between oxygen species and TEA molecules. For instance, Liu et al. synthesized Al/Mo co-doped porous Co₃O₄ hollow tetrahedrons using a simple solvothermal method, achieving a response of 132 to 100 μL·L^-1 TEA^[18]. Similarly, Yang et al. prepared hollow SnO₂/Zn₂SnO₄ composites, which exhibited a response value of 48 to 100 μL·L^-1 TEA^[19]. Therefore, the development of ZnSnO₃ materials with a hollow structure is imperative.

  The construction of heterostructures is a crucial approach to enhancing the sensing properties of gas sensors, primarily due to their highly efficient charge transfer channels. To date, numerous heterostructures have been synthesized, among which p-n heterostructures are particularly common and frequently demonstrate excellent gas-sensing properties. For example, Yu et al. prepared CuO/ZnSnO₃ hollow microspheres using an in-situ precipitation method, finding that the CuO/ZnSnO₃ samples exhibited high response values to ethanol^[20]. Similarly, Jin et al. reported the synthesis of CuO-coated ZnSnO₃ nanorods via thermal evaporation, with these samples showing significant response values to H₂S at 100 ℃^[21]. Furthermore, Xu et al. fabricated Co₃O₄-ZnSnO₃ arrays through a hydrothermal method, which also displayed high response values to ethanol^[22]. Thus, it is anticipated that identifying a suitable p-type semiconductor material in conjunction with ZnSnO₃ will yield a high-performance gas sensor. NiO, recognized as a common p-type semiconductor material, has been widely employed in gas sensing because of its advantageous electrical and chemical characteristics^[23-24]. As a result, the creation of ZnSnO₃-NiO heterostructures is expected to enhance the advancement of a TEA sensor noted for its strong response values and brief response/recovery durations. Furthermore, as far as we are aware, there have been no existing studies on ZnSnO₃/NiO heterostructures aimed explicitly at TEA detection.

  Inspired by the aforementioned analysis, ZnSnO₃ hollow cubes have been prepared using a co-precipitation way followed by an annealing process. Subsequently, varying amounts of NiO nanosheets were attached to the ZnSnO₃ surface, resulting in the creation of ZnSnO₃/NiO heterojunctions. The gas-sensing properties of the sensors, derived from these samples, have been assessed. Findings show that the ZnSnO₃/NiO-2 sensor revealed high response values along with a short response and recovery time to TEA. Furthermore, it displayed excellent selectivity, repeatability and stability over extended periods. A thorough investigation into the improved gas-sensing mechanism has been conducted.

  1.   Materials and methods

  1.1   Synthesis of ZnSnO₃/NiO heterostructures

  All the chemicals (AR) were purchased from Shanghai Aladdin Industrial Corporation and used without further purification. The ZnSnO₃ samples were prepared based on the reported method with several modifications^[25-26]. In general, 2 mmol of sodium citrate and 2 mmol of ZnCl₂ were dissolved into 20 mL of deionized (DI) water. Then a solution of 10 mL of SnCl₄·5H₂O (0.1 mol·L^-1) was mixed with the above solution. After 15 min, 50 mL of NaOH solution (0.41 mol·L^-1) was gradually added to this mixture and stirred for 30 min. Subsequently, an additional 30 mL of NaOH solution (2 mol·L^-1) was incorporated. The resulting white precipitate was then washed three times with anhydrous ethanol followed by DI water and dried at a temperature of 60 ℃. Finally, the dried products underwent annealing at 450 ℃ for 2 h, resulting in the formation of ZnSnO₃ samples.

  Secondly, 0.04 g of the as-obtained ZnSnO₃ samples, different amounts of Ni(NO₃)₂·6H₂O, 0.04 g of urea, and 0.04 g of polyvinyl pyrrolidone were added into the mixed solution of 70 mL ethanol and 10 mL DI water for stirring. After 10 min, the solution was heated to 70 ℃ and stirred continuously for 10 h. Then the precipitate was rinsed with ethanol and DI water three times and dried at 60 ℃. Finally, the dried powder was annealed at 450 ℃ for 2 h at a heating rate of 1 ℃·min^-1. The molar ratios of Ni to Zn in the samples were 1∶4, 1∶2, and 1∶1, respectively, and they were named ZnSnO₃/NiO-1, ZnSnO₃/NiO-2, and ZnSnO₃/NiO-3, respectively.

  1.2   Characterization

  The crystalline phase was examined using powder X-ray diffraction (PXRD) on a Rigaku D/max-2500 diffractometer (40 kV, 30 mA) employing Cu Kα radiation (λ=0.154 18 nm) in a range of 20°-80°. To analyze the morphology and microstructure, field-emission scanning electron microscopy (FESEM, Magellan 400, 2 kV) and transmission electron microscopy (TEM, JEOL, JEM-2200FS, 200 kV) were utilized. Additionally, the chemical composition and elemental valence were investigated through X-ray photoelectron spectroscopy (XPS, ESCALAB MK Ⅱ).

  1.3   Fabrication and measurement of sensors

  The gas sensor was prepared according to the previous way^[27-28]. In brief, small quantities of ZnSnO₃/NiO-based samples were combined with ethanol to create a homogeneous slurry, which was subsequently applied to the alumina tube′s surface. Before this, two Au electrodes with four Pt wires were secured to the alumina tube. The gas sensor then underwent an aging process at 300 ℃ for 24 h to improve its stability. Following this, a Ni-Cr alloy was placed inside the alumina tube to manage the sensor′s temperature by adjusting the current. The gas sensing capabilities were evaluated using a CGS-8 intelligent gas sensing analysis system (Beijing Elite Tech. Co., Ltd., China) under controlled laboratory conditions [(25±5) ℃, (30±5)% RH (relative humidity)]. The response values were calculated as R_a/R_g, with R_g and R_a representing the sensor resistance in the test gas and air, respectively. The response and recovery times (t_res and t_rec) were defined as the duration required to reach 90% of the total change in sensor resistance.

  2.   Results and discussion

  2.1   Characterization

  Fig.S1 (Supporting information) illustrates the PXRD pattern for the ZnSn(OH)₆ precursors. The observed diffraction peaks align with the standard peaks associated with the cubic phase of ZnSn(OH)₆ (PDF No.74-1825), with no additional peaks detected, indicating that the prepared samples are pristine ZnSn(OH)₆. Furthermore, the presence of strong and sharp peaks for ZnSn(OH)₆ demonstrates its good crystallinity. Fig. 1 displays the PXRD patterns of ZnSnO₃ and ZnSnO₃/NiO composites. The diffraction peaks can be indexed to the standard ZnSnO₃ phase. The absence of sharp diffraction peaks with high intensity in this pattern suggests that the resulting samples exhibit poor crystallinity.

  Figure 1

  

  Figure 1.  PXRD pattern of ZnSnO₃ and ZnSnO₃/NiO composites

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  Fig.S2 presents the FESEM image of ZnSn(OH)₆ precursors. The image displayed that ZnSn(OH)₆ precursors consisted of numerous uniform and well-formed hollow cubes, with a diameter of approximately 2 μm. Fig. 2 displays the FESEM images of both pure ZnSnO₃ and ZnSnO₃/NiO samples. It is evident that many hollow cubes were present, and their size was comparable to that of the ZnSn(OH)₆ precursors. Furthermore, as the NiO content increased, the number of nanosheets attached to the surface of the ZnSnO₃ cubes progressively rose.

  Figure 2

  

  Figure 2.  FESEM images: (a, e) pure ZnSnO₃, (b, f) ZnSnO₃/NiO-1, (c, g) ZnSnO₃/NiO-2, and (d, h) ZnSnO₃/NiO-3

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  The elemental mapping image for ZnSnO₃/NiO-2 is presented in Fig. 3, showing a uniform distribution of the elements Zn, Sn, O, and Ni across the sample. The content of NiO in ZnSnO₃/NiO-1, ZnSnO₃/NiO-2, and ZnSnO₃/NiO-3 samples was investigated by X-ray energy dispersive spectrum (EDS) analysis, as shown in Fig.S3a-S3c. As illustrated in the figure, the quantity of NiO corresponds to the amount of the initial reagent introduced.

  Figure 3

  

  Figure 3.  Elemental mappings of ZnSnO₃/NiO-2

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  Fig. 4a presents the TEM image of a single ZnSnO₃/NiO-2 sample. The figure clearly illustrated a distinct contrast between the inner region of the cube and its outer shell, confirming that the synthesized sample was a hollow cube with a side length of approximately 2 μm, consistent with the results obtained from FESEM. The enlarged TEM image of ZnSnO₃/NiO-2 is depicted in Fig. 4b. Notably, numerous NiO nanosheets were observed to be attached to the surface of the ZnSnO₃ cube, indicating the formation of ZnSnO₃/NiO heterostructures. The selected area electron diffraction (SAED) characterization was conducted on the portion of ZnSnO₃/NiO-2 corresponding to the NiO nanosheets, as illustrated in Fig. 4c. The diffraction rings were distinctly visible in the figure, which indicates a polycrystalline structure. Furthermore, these diffraction rings, from innermost to outermost, correspond to the (111), (200), (220), (222), (422), and (511) crystal planes of NiO, respectively. Fig. 4d shows the high-resolution transmission electron microscopy (HRTEM) image of ZnSnO₃/NiO-2. The measured lattice spacing in the lamellar region was 0.211 nm, corresponding to the (200) plane of NiO, which confirms that NiO has successfully adhered to the surface of the ZnSnO₃ cube.

  Figure 4

  

  Figure 4.  (a, b) TEM, (c) SAED, and (d) HRTEM images of ZnSnO₃/NiO-2

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  The XPS analysis, depicted in Fig. 5, was conducted to examine the elemental composition and valence states of both pure ZnSnO₃ and ZnSnO₃/NiO-2. Fig. 5a presents the full spectrum of ZnSnO₃/NiO-2, which predominantly consists of Zn, Sn, and O elements. Additionally, peaks corresponding to the Ni element are also observed in the spectrum. The Zn2p spectrum of ZnSnO₃/NiO-2 is illustrated in Fig. 5b. This figure shows that the peaks observed at 1 022.2 and 1 045.4 eV are associated with Zn2p_3/2 and Zn2p_1/2, respectively, suggesting that the oxidation state of Zn ions in the sample is +2^[29]. Fig. 5c displays the Sn3d spectrum of ZnSnO₃/NiO-2, with peaks at 486.7 and 495.3 eV attributed to Sn3d_5/2 and Sn3d_3/2, respectively, confirming that the valence state of Sn ions in the sample is +4^[30]. Fig. 5d illustrates the Ni2p spectrum of ZnSnO₃/NiO-2, where the peaks at 854.9 and 861.1 eV correspond to Ni2p_3/2, while those at 872.9 and 879.4 eV correspond to Ni2p_1/2, thus demonstrating the presence of NiO in the sample^[31]. Fig. 5e and 5f present the O1s spectra for both pure ZnSnO₃ and ZnSnO₃/NiO-2. The findings suggest that the two asymmetrical O1s peaks can be categorized into three separate peaks. The peaks located around 530.1, 530.9, and 531.9 eV are associated with lattice oxygen (O_L), oxygen vacancies (O_V), and surface-adsorbed oxygen (O_C), respectively^[32]. Furthermore, the proportions of O_L, O_V, and O_C in pure ZnSnO₃ samples were 52.3%, 18.3%, and 29.5%, respectively. In contrast, the proportions of O_L, O_V, and O_C in ZnSnO₃/NiO-2 were 37.7%, 26.5%, and 35.8%, respectively. Notably, the attachment of NiO nanosheets to the surface of ZnSnO₃ results in a significant increase in the proportion of O_V, which is expected to substantially enhance the gas sensitivity of the sensitive material.

  Figure 5

  

  Figure 5.  XPS analysis: (a) survey, (b) Zn2p, (c) Sn3d, and (d) Ni2p spectra of ZnSnO₃/NiO-2; O1s spectra of (e) pure ZnSnO₃ and (f) ZnSnO₃/NiO-2

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  2.2   Growth mechanism

  Fig. 6 shows the detailed formation of ZnSnO₃/NiO heterostructures. The growth process of ZnSn(OH)₆ hollow cubes involves a rapid co-precipitation reaction among Zn²⁺, Sn⁴⁺, OH^-, and citrate ions, resulting in the swift formation of solid ZnSn(OH)₆ cubes due to its cubic crystal structure. With the introduction of an excess amount of OH^- ions, these solid cubes engage with OH^- ions, resulting in the creation of soluble [Sn(OH)₆]^2- and [Zn(OH)₄]^{2- [33]}. Furthermore, because the external structure of the ZnSn(OH)₆ cube is more stable than its internal structure, the solid cube ultimately transforms into a hollow cube^[34]. The growth process of ZnSnO₃/NiO heterostructures begins with the high-temperature treatment of the previously generated ZnSn(OH)₆ sample, which removes water from the structure and fully converts it to ZnSnO₃. When varying amounts of Ni(NO₃)₂ are added to the ethanol solution containing ZnSnO₃, Ni²⁺ ions adsorb onto the surface of the ZnSnO₃ sample. Concurrently, urea in the solution decomposes to release NH₃ and CO₂, resulting in an alkaline environment. Consequently, Ni²⁺ ions react with OH^- to form Ni(OH)₂. After stirring the solution at 70 ℃ for 6 h, Ni(OH)₂ uniformly adheres to the surface of the ZnSnO₃ cube. Ultimately, ZnSnO₃/NiO composites are successfully obtained following an annealing treatment. The relevant chemical reaction equations are as follows:

  $ \mathrm{Zn}^{2+}+\mathrm{Sn}^{4+}+6 \mathrm{OH}^{-} \rightarrow \mathrm{ZnSn}(\mathrm{OH})_6 $

  (1)

  $ \mathrm{ZnSn}(\mathrm{OH})_6+4 \mathrm{OH}^{-} \rightarrow\left[\mathrm{Zn}(\mathrm{OH})_4\right]^{2-}+\left[\mathrm{Sn}(\mathrm{OH})_6\right]^{2-} $

  (2)

  $ \mathrm{ZnSn}(\mathrm{OH})_6 \rightarrow \mathrm{ZnSnO}_3+3 \mathrm{H}_2 \mathrm{O} $

  (3)

  $ \mathrm{CO}\left(\mathrm{NH}_2\right)_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CO}_2+2 \mathrm{NH}_3 $

  (4)

  $ \mathrm{NH}_3+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{NH}_3 \cdot \mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-} $

  (5)

  $ \mathrm{Ni}^{2+}+2 \mathrm{OH}^{-} \rightarrow \mathrm{Ni}(\mathrm{OH})_2 $

  (6)

  $ \mathrm{Ni}(\mathrm{OH})_2 \rightarrow \mathrm{NiO}+\mathrm{H}_2 \mathrm{O} $

  (7)

  Figure 6

  

  Figure 6.  Detailed formation of ZnSnO₃/NiO heterostructures

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  2.3   Gas sensing properties

  The properties of a gas sensor are significantly linked to its operating temperature. As a result, the response measurements of pure ZnSnO₃ and NiO sensors, and those based on ZnSnO₃/NiO to 100 μL·L^-1 TEA were first evaluated at temperatures varying from 180 to 260 ℃, as shown in Fig. 7a. The response values of all five sensors demonstrated a trend of initial increase followed by a decrease, with maximum response values occurring at 220 ℃, indicating that this was their optimal working temperature. At this temperature, a sufficient quantity of energy is generated, leading to a heightened concentration of chemisorbed oxygen species on the surface of the sensor, which improves the sensor′s response values^[35]. Nevertheless, as the operating temperature keeps increasing, gas molecules that are adsorbed earlier might volatilize from the material surface before the chemical reaction occurs, resulting in lower response values^[36]. Moreover, the response values recorded in sensors enhanced by ZnSnO₃/NiO were significantly greater than those of pure ZnSnO₃ and NiO sensors, suggesting that incorporating NiO substantially boosted the sensors′ performance. Among these, the ZnSnO₃/NiO-2-based sensor exhibited the highest gas response values.

  Figure 7

  

  Figure 7.  (a) Response values of pure ZnSnO₃-, NiO-, and ZnSnO₃/NiO-based sensors to 100 μL·L^-1 TEA at different operating temperatures; (b) Gas selectivity of pure ZnSnO₃- and ZnSnO₃/NiO-based sensors at 220 ℃

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  Gas selectivity is a crucial metric for evaluating sensor performance. Fig. 7b demonstrates the response values of four sensors when exposed to 100 μL·L^-1 of VOCs, which comprise ethanol, acetone, methanol, benzene, formaldehyde, and TEA at 220 ℃. Notably, all four sensors demonstrated the highest response values to TEA gas. The response value of the pure ZnSnO₃-based sensor to TEA was comparable to its response values to other test gases, indicating a lack of selectivity. In comparison, the response values of the three ZnSnO₃/NiO sensors to TEA were considerably greater than their response values to the other tested gases, indicating strong selectivity. This suggests that the incorporation of NiO not only enhances the response values but also improves selectivity.

  Fig. 8a illustrates the response values of ZnSnO₃- and ZnSnO₃/NiO-based sensors to TEA concentrations ranging from 5 to 200 μL·L^-1 at 220 ℃. As the concentration of TEA increased, the response values of all four sensors also rose correspondingly. Specifically, the response values of pure ZnSnO₃-based sensor at 10, 20, 50, 100, and 200 μL·L^-1 of TEA were 1.2, 3.4, 7.2, 11.5, and 16.1, respectively. The response values increased gradually with rising TEA concentrations, and the sensor reached saturation quickly. In contrast, the ZnSnO₃/NiO-based sensors exhibited a rapid increase in response to increasing TEA concentration, indicating that these sensors were more sensitive to TEA compared to the pure ZnSnO₃ sensor. For the ZnSnO₃/NiO-2 sensor, the response values at 5, 10, 20, 50, 100, and 200 μL·L^-1 of TEA were 3.1, 4.2, 10.7, 28.8, 70.6, and 104.6, respectively. It is evident that the ZnSnO₃/NiO-2 sensor demonstrated higher response values than both the ZnSnO₃/NiO-1 and ZnSnO₃/NiO-3 sensors, regardless of whether the TEA concentration was low or high. Fig. 8b illustrates the correlation between the response and the concentration of TEA for sensors based on pure ZnSnO₃ as well as those incorporating ZnSnO₃/NiO. The response values for all sensors rose as the concentration of TEA was elevated. Additionally, the response values of the three ZnSnO₃/NiO-based sensors to TEA were uniformly greater than those observed in the pure ZnSnO₃ sensor, with the ZnSnO₃/NiO-2 sensor exhibiting the highest response values. This suggests that the addition of NiO greatly improves gas response values, with an ideal molar ratio of Ni to Zn at 1∶2.

  Figure 8

  

  Figure 8.  (a) Response curves of pure ZnSnO₃- and ZnSnO₃/NiO-based sensors to 5-200 μL·L^-1 TEA at 220 ℃; (b) Relationship between the response value and TEA concentration for pure ZnSnO₃- and ZnSnO₃/NiO-based sensors; (c-e) Response/recovery curves of ZnSnO3/NiO-based sensors to 100 μL·L^-1 TEA at 220 ℃; (f) Repeatability of ZnSnO₃/NiO-2 based sensor

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  Fig. 8c-8e illustrate the response and recovery curves of ZnSnO₃/NiO-based sensors when exposed to 100 μL·L^-1 TEA at 220 ℃. The response and recovery times for the ZnSnO₃/NiO-1, ZnSnO₃/NiO-2, and ZnSnO₃/NiO-3 sensors were 1 s/31 s, 1 s/18 s and 1 s/23 s, respectively. Fig. 8f illustrates the repeatability of the ZnSnO₃/NiO-2 sensor. The results indicate that both the response values and the rate of response/recovery for the sensor remained consistently stable, suggesting that the sensor exhibited dependable repeatability. The long-term stability of the ZnSnO₃/NiO-2-based sensor is presented in Fig.S4. Over the 30-day testing period, the sensor response values showed minimal variation, demonstrating its excellent long-term stability.

  Furthermore, the gas sensing properties of the ZnSnO₃/NiO-2-based sensor were compared with TEA sensors reported in the literature, as summarized in Table 1^{[19, 37-44]}. The sensor composed of ZnSnO₃ and NiO exhibited elevated response values and swift response and recovery rates when exposed to TEA gas, highlighting the potential of ZnSnO₃/NiO heterostructures as promising sensitive materials.

  Table 1

  

  Table 1.  Comparison of TEA sensing performance of different gas sensors*

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  Material	Tow / ℃	Con. / (μL·L-1)	Res. (Ra/Rg)	tres / s	trec / s	Ref.
  SnO2/Zn2SnO4 spheres	250	100	48.2	2	184	[19]
  ZnSnO3/ZnO heterostructures	160	50	101	32	41	[37]
  Polyhedral Zn2SnO4	200	100	37	2	17	[38]
  Co-doped SnO2	300	100	40.1			[39]
  ZnO rods	280	50	60	11	40	[40]
  Hollow SnO2 microfiber	270	100	49.5	14	12	[41]
  Porous ZnSnO3 nanocubes	350	100	57.5	4	1 040	[42]
  ZnSnO3/Zn2SnO4	190	100	179.7	19	37	[43]
  V2O5 hollow sphere	370	100	7.3	20	96	[44]
  ZnSnO3/NiO	220	100	70.6	1	18	This work
  * Tow=optimal working temperature, Con.=TEA concentration, Res.=response value.

  2.4   Gas sensing mechanism

  The mechanism by which ZnSnO₃ material senses gas can be explained using the surface control model, which relates to the adsorption and desorption processes of gas molecules occurring on the surface of the material^[45-46]. Upon exposure to air, oxygen species that are chemisorbed appear on the surface of the sensing material in a ZnSnO₃-based sensor, leading to the generation of an electron depletion layer. This phenomenon results in a high resistance state of the sensor in the presence of air. In this study, the optimal operating temperature for the ZnSnO₃-based sensor is identified as 220 ℃, at which point the predominant form of chemisorbed oxygen species is O^-. When the sensor is exposed to TEA gas, the TEA molecules react with O^-, releasing the adsorbed electrons into the conduction band of the material, which results in a decrease in sensor resistance (Fig. 9b). The relevant chemical reaction can be represented by the following equation:

  $ \begin{aligned} & 2\left(\mathrm{C}_2 \mathrm{H}_5\right)_3 \mathrm{~N}(\mathrm{ads})+43 \mathrm{O}^{-}(\mathrm{ads}) \rightarrow \\ & ~~~~~~~~~~~~~~~~~~~~\quad 2 \mathrm{NO}_2+12 \mathrm{CO}_2+15 \mathrm{H}_2 \mathrm{O}+43 \mathrm{e}^{-} \end{aligned} $

  (8)

  Figure 9

  

  Figure 9.  Schematic diagrams of (a) energy band and (b) sensing mechanism of ZnSnO₃/NiO heterostructures

  E_CB: conduction band energy level, E_F: Fermi level, E_VB: valence band energy level.

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  The findings indicate that the sensing capabilities of the sensor based on ZnSnO₃/NiO exceed those of the standalone ZnSnO₃ sensor, attributable to several factors. Initially, since NiO serves as a characteristic p-type semiconductor, the binding of NiO nanosheets to the outer surface of ZnSnO₃ hollow cubes promotes the movement of electrons from ZnSnO₃ to NiO. Simultaneously, holes shift in the reverse direction until the Fermi level of the system attains equilibrium, leading to the creation of a contact barrier at the interface of ZnSnO₃ and NiO, which in turn creates an internal electric field. Consequently, an electron depletion layer forms at the interface adjacent to the ZnSnO₃ material, while a hole depletion layer develops at the interface near the NiO material, leading to an increase in the resistance of the ZnSnO₃/NiO-based sensor in air (Fig. 9a). When the ZnSnO₃/NiO-based sensor is subjected to TEA, it undergoes an electron release, which leads to a reduction in the thickness of depletion layers and a subsequent lowering of the height of the potential barrier. Additionally, the equation can express the connection between the resistance value of heterostructures (R) and the barrier height (Φ): R∝Bexp[qΦ/(kT)], where B is a constant related to ambient temperature, q represents the electronic charge, k is the Boltzmann constant, and T denotes the absolute temperature^[47]. This equation indicates that even a slight alteration in the barrier height can significantly influence the sensor resistance. Therefore, the response values of the sensor can be substantially enhanced by constructing ZnSnO₃/NiO p-n heterostructures.

  Additionally, NiO demonstrates significant catalytic properties, enhancing the uptake of oxygen molecules on the surface of the material. The formation of an increased quantity of chemisorbed oxygen species occurs as a result of this process, significantly improving the surface interaction between adsorbed oxygen ions and TEA molecules^[48]. Moreover, XPS analysis reveals that the ZnSnO₃/NiO-2 material contains a greater proportion of oxygen vacancies compared to pure ZnSnO₃, leading to the production of additional oxygen species on the surface of the ZnSnO₃/NiO-2 material, which aids in enhancing sensor performance^[49-50]. However, an excessive attachment of NiO nanosheets to the surface of ZnSnO₃ can significantly reduce the sensor′s response values, likely due to the excessive NiO covering the active sites on the material′s surface^[51]. The development of a hollow structure additionally creates numerous active sites on the surface of the sensitive material, allowing gas molecules to traverse this configuration rapidly, thereby enhancing responsiveness and shortening the response/recovery time of the sensors^[16-17].

  3.   Conclusions

  In this study, pure ZnSnO₃ and ZnSnO₃/NiO heterostructures were synthesized, characterized, and employed in the preparation of a TEA sensor. The purity, morphology, crystallographic properties, and surface chemical composition of the obtained ZnSnO₃/NiO heterostructure sensors were analyzed using PXRD, FESEM, TEM, element mapping, EDS, and XPS. The findings suggest that an optimal sensor performance was achieved with a molar ratio of Ni to Zn at 1∶2. The response values of the ZnSnO₃/NiO-2 sensor were measured at 70.6 for 100 μL·L^-1 TEA gas at 220 ℃, which were 6.1 times greater than that of the pure ZnSnO₃-based sensors. Furthermore, the sensor showed short response/recovery times (1 s/18 s), strong repeatability, and sustained stability over time. The enhancement in the sensing performance is ascribed to the formation of a hollow architecture along with the ZnSnO₃/NiO p-n heterostructures. In addition, the outcomes of these experiments establish a theoretical basis for additional investigations into sensing materials that incorporate p-n heterostructures, indicating that the unique ZnSnO₃/NiO heterostructure sensors are promising candidates for efficient and reliable detection of TEA.

  Supporting information is available at http://www.wjhxxb.cn

  
  
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  Figure 1  PXRD pattern of ZnSnO₃ and ZnSnO₃/NiO composites

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  Figure 2  FESEM images: (a, e) pure ZnSnO₃, (b, f) ZnSnO₃/NiO-1, (c, g) ZnSnO₃/NiO-2, and (d, h) ZnSnO₃/NiO-3

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  Figure 3  Elemental mappings of ZnSnO₃/NiO-2

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  Figure 4  (a, b) TEM, (c) SAED, and (d) HRTEM images of ZnSnO₃/NiO-2

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  Figure 5  XPS analysis: (a) survey, (b) Zn2p, (c) Sn3d, and (d) Ni2p spectra of ZnSnO₃/NiO-2; O1s spectra of (e) pure ZnSnO₃ and (f) ZnSnO₃/NiO-2

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  Figure 6  Detailed formation of ZnSnO₃/NiO heterostructures

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  Figure 7  (a) Response values of pure ZnSnO₃-, NiO-, and ZnSnO₃/NiO-based sensors to 100 μL·L^-1 TEA at different operating temperatures; (b) Gas selectivity of pure ZnSnO₃- and ZnSnO₃/NiO-based sensors at 220 ℃

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  Figure 8  (a) Response curves of pure ZnSnO₃- and ZnSnO₃/NiO-based sensors to 5-200 μL·L^-1 TEA at 220 ℃; (b) Relationship between the response value and TEA concentration for pure ZnSnO₃- and ZnSnO₃/NiO-based sensors; (c-e) Response/recovery curves of ZnSnO3/NiO-based sensors to 100 μL·L^-1 TEA at 220 ℃; (f) Repeatability of ZnSnO₃/NiO-2 based sensor

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  Figure 9  Schematic diagrams of (a) energy band and (b) sensing mechanism of ZnSnO₃/NiO heterostructures

  E_CB: conduction band energy level, E_F: Fermi level, E_VB: valence band energy level.

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  Table 1.  Comparison of TEA sensing performance of different gas sensors*

  Material	Tow / ℃	Con. / (μL·L-1)	Res. (Ra/Rg)	tres / s	trec / s	Ref.
  SnO2/Zn2SnO4 spheres	250	100	48.2	2	184	[19]
  ZnSnO3/ZnO heterostructures	160	50	101	32	41	[37]
  Polyhedral Zn2SnO4	200	100	37	2	17	[38]
  Co-doped SnO2	300	100	40.1			[39]
  ZnO rods	280	50	60	11	40	[40]
  Hollow SnO2 microfiber	270	100	49.5	14	12	[41]
  Porous ZnSnO3 nanocubes	350	100	57.5	4	1 040	[42]
  ZnSnO3/Zn2SnO4	190	100	179.7	19	37	[43]
  V2O5 hollow sphere	370	100	7.3	20	96	[44]
  ZnSnO3/NiO	220	100	70.6	1	18	This work
  * Tow=optimal working temperature, Con.=TEA concentration, Res.=response value.

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