Nanorods Assembled Hierarchical Bi2S3 for Highly Sensitive Detection of Trace NO2 at Room Temperature

Abstract

The bismuth sulfide nanostructure has become a promising gas sensing material thanks to its exceptional intrinsic properties. However, pristine Bi₂S₃ as a room-temperature sensing material cannot achieve the highly sensitive detection of ppb-level NO₂ gas. Herein, 1D nanorods with self-assembled hierarchical Bi₂S₃ nanostructures were obtained via a simple hydrothermal process. The as-prepared hierarchical Bi₂S₃ nanostructures exhibited outstanding NO₂ sensing behaviors, such as a high response value (R_g/R_a = 5.8) and a short response/recovery time (τ₉₀ = 28/116 s) upon exposure to 1 ppm NO₂. The limit of detection of hierarchical Bi₂S₃ was down to 50 ppb. Meanwhile, the sensor exhibited excellent selectivity and humidity tolerance. The improved NO₂ sensing properties were associated with the self-assembled hierarchical nanostructures, which provided a rich sensing active surface and accelerated the diffusion and adsorption/desorption processes between NO₂ molecules and Bi₂S₃ materials. Additionally, the sensing response of hierarchical Bi₂S₃ nanostructures is much higher at 100% N₂ atmosphere, which is different from the chemisorption oxygen model.

1. Introduction

Nitrogen dioxide (NO₂), as a typically hazardous gas released from the burning of fossil fuels, can trigger serious environmental and health problems [1,2]. As reported, continuous exposure to trace NO₂ gas may cause respiratory diseases such as chronic bronchitis, asthma, and emphysema [3,4,5,6,7,8,9]. The U.S. Environmental Protection Agency sets 53 ppb NO₂ as the ambient exposure concentration standard. Meanwhile, ppb-level NO₂ gas detection can contribute to auxiliary diagnoses of humans’ physical health, such as lung disease and gastrointestinal disorder symptoms [10,11,12]. Chemiresistive gas sensors based on metal oxides have shown superb NO₂ sensing properties, such as high sensitivity and a short response time [13]. However, such NO₂ sensors generally need a high operating temperature to obtain high sensitivity, fast response/recovery speed, and a low limit of detection (LOD). The high working temperature brings about extra power consumption and triggers safety problems, which seriously restrict their potential application in medical treatment and environmental monitoring [14,15,16]. With the rapid development of the emerging 5G network and the Internet of Things, it is very important to research new room-temperature (RT) sensing materials for highly sensitive trace NO₂ detection.

The layer metal chalcogenides as sensing nanomaterials were extensively researched in the gas sensor field for recognizing low-concentration NO₂ molecules at RT due to their unique physicochemical properties. Among them, Bi₂S₃ nanostructured materials have a tunable bandgap of 1.3–1.7 eV [17,18,19,20] and high carrier mobility of about 10³ cm² V⁻¹ s⁻¹ [21,22], which can facilitate the electron transfer process between Bi₂S₃-based sensing materials and target gas molecules at RT. Bi₂S₃-based gas sensors have been used for various gas detections, such as H₂ [23], NH₃ [24], H₂S [25], etc. In recent years, Bi₂S₃ nanostructures as RT NO₂ sensing materials have gained widespread attention due to their excellent affinity for NO₂ molecules [26]. For example, Liu et al. reported that synthesized Bi₂S₃ nanobelts showed superior RT NO₂ sensing performance, such as a short response time and superb selectivity [21]. Yang et al. reported that the Au/Bi₂S₃ heterojunction nanosheets exhibited rapid sensing response and outstanding selectivity toward NO₂ gas at RT, which was associated with the increasing active sites and accelerated electron transfer arising from sulfur vacancies [27]. Zhang et al. constructed a CuS/Bi₂S₃ nanosheet sensor, which exhibited excellent NO₂ sensing response at RT due to a mass of sensing active sites and quantum size effect [28]. Unfortunately, Bi₂S₃-based gas sensors display some poor RT NO₂ sensing properties, such as a long recovery time and a high LOD, which severely impede their spread and applications [21,29].

As we know, the morphology and structure characteristics of gas sensing materials play a key role in improving sensing behaviors because the sensing properties depend on the diffusion and adsorption/desorption processes between the sensing materials and the target gas. Low-dimension nanostructures (nanobelts, nanorods, and nanosheets) easily stack and aggregate together, which would reduce the active surface to adsorb gas molecules and obstruct gas diffusion and adsorption/desorption processes, weakening the sensing materials intrinsic sensing properties. Low-dimensional nanostructures can effectively avoid restacking together and obviously increase active surface via the constructed hierarchical nanostructures method, which can provide rich sensing active sites and promote detection of gas diffusion and adsorption/desorption on the surface of nanomaterials. For example, Liu et al. prepared hierarchical SnS₂ nanoflowers, which showed excellent low NO₂ LOD due to the mass of the available surface-active sites [30]. Wang et al. synthesized the nanoplate-assembled SnSe₂ nanoflowers, which exhibited a highly sensitive response to ppb-level NO₂ at RT [31]. Zhang et al. reported that hierarchical MoS₂ nanospheres improved the sensing behaviors toward NO₂ gas due to the open 3D nanostructure and abundant reaction active sites [32]. The hierarchical nanostructured Bi₂S₃ (flower-like, urchin-like, sheaf-like, etc.) have been successfully prepared through various means in order to elevate their functional properties [24,33,34,35,36]. For example, Fu prepared a 3D Bi₂S₃ nanowire network, which showed excellent sensing properties for NH₃ at RT due to its large surface area [24]. Therefore, constructed hierarchical Bi₂S₃ nanomaterials are an effective approach to enhance their NO₂-sensitive properties, which may provide a new route for optimizing nanostructures to enhance the sensing behaviors of low-dimensional nanomaterials.

Herein, 1D nanorods with self-assembled Bi₂S₃ hierarchical nanostructures were prepared using a facile hydrothermal procedure. As-prepared Bi₂S₃ nanomaterials showed hierarchical morphology, which was conducive to a mass of NO₂ molecules rapidly diffused inward and adsorbed on the surface of the hierarchical Bi₂S₃ nanostructure materials to enhance their gas sensing performance. The NO₂ sensors based on 1D nanorods and self-assembled Bi₂S₃ sensing materials showed high sensitivity, rapid response/recovery speed, and a low LOD for NO₂ at RT. These findings demonstrated that the constructed hierarchical Bi₂S₃ nanostructures serve as promising candidates for hypersensitive NO₂ detection devices.

2. Materials and Methods

2.1. Synthesis of Hierarchical Bi₂S₃ Nanomaterials

Hierarchical Bi₂S₃ nanostructures assembled from 1D nanorods were obtained via facile hydrothermal means inspired by the previous studies [37,38]. Typically, 303 mg of Bi(NO₃)₃·5H₂O powder was dispersed in 30 mL of deionized water with vigorous mixing for 30 min, forming a milky suspension. Then, 476 mg of CH₄N₂S powder was added to the above suspension, followed by continuous mixing for 60 min to form a yellow reaction solution. The above mixture was sealed in a 50 mL hydrothermal reactor. Subsequently, the above solution was heated to 140 °C and kept for 12 h. After naturally cooling down to ambient temperature, the black powder was collected via centrifugation and washed with ethanol and deionized water 3 times. Finally, the obtained product was dried at 60 °C for 12 h.

To study the formation mechanism of the 1D nanorods self-assembled in Bi₂S₃ hierarchical structures, a series of samples were prepared by adjusting the hydrothermal reaction time from 0 h to 24 h. The corresponding samples were named BS-X, where X stands for the hydrothermal reaction time in an hour.

2.2. Material Characterizations

The crystal structure information of the as-prepared materials was acquired using a Bruker D8 Advance X-ray diffractometer (Bruker Technology Co., LTD, Saarbrucken, Germany). The scanning electron microscopy images of the as-prepared powder were acquired with a Zeiss Sigma 300 (Carl Zeiss AG, Oberkochen, Germany). The high-resolution transmission electron microscopy was carried out on a FEI Talos F200S (FEI Company, Hillsboro, OR, USA). The information on the surface chemical states of the obtained product was studied via X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha spectrometer, Thermo Fisher Scientific, Waltham, MA, USA). The Brunauer–Emmett–Teller (BET)-specific surface area was measured with a Micromeritics ASAP 2460 apparatus (Micromeritics Instruments Corporation, Norcross, GA, USA).

2.3. Gas Sensing Measurements

First, the mixture containing the as-prepared Bi₂S₃ powder (10 mg) and ethanol (1 mL) was prepared. Then, the 20 μL above mixture was coated on an Al₂O₃ substrate with Ag-Pd interdigitated electrodes to form a sensing film. Finally, the thin-film gas sensor was dried in a vacuum chamber at 60 °C for 3 h. The detailed parameters of the sensing device are shown in Figure S1.

Gas sensing properties were obtained using a homemade gas sensor test system, as in our previous work [39]. The dynamic gas sensor test system was employed to investigate the sensing mechanism (Figure S2). As-prepared sensors based on different sensing materials were aged for 24 h under a 5 V voltage at RT before the sensing test. The relative humidity (RH) of the testing chamber was controlled with the humidity generator. The real-time resistance information of the sensors was recorded using the electrochemical workstation. The known concentration of detection gas rapidly flowed through the testing chamber. The sensing response value (S) was calculated using the equation: S = R_a/R_g (when R_a > R_g) and R_g/R_a (when R_a < R_g), where R_g and R_a were the RT resistance values recorded in detection gas and clean air, respectively. Herein, the response and recovery times (τ₉₀) were defined as the time to reach 90% of the total resistance variation during gas-in and gas-out, respectively.

3. Results

3.1. Morphology and Structure

The hierarchical Bi₂S₃ nanostructures are obtained through a one-step hydrothermal process, and their structures are highly dependent on synthesized time (as shown in Figure 1a). The as-prepared hydrolysis product shows 1D rod-like structures, as shown in the SEM image (Figure 1b). According to the calculated method of standard deviation by measuring 20 rod-like structures, the diameter of rod-like structures is about 1.2 μm. Subsequently, many nanowire-assembled microsphere structures appear after a hydrothermal reaction for 1 h (Figure 1c). With increasing hydrothermal reaction for 12 h, the as-prepared sample shows 1D nanorods of self-assembled hierarchical Bi₂S₃ structures with a diameter of about 4.0 μm (Figure 1d). The length of self-assembled nanorods is approximately 2.0 μm, with a diameter of about 150 nm. The TEM and HRTEM images further show the hierarchical nanostructures of the synthesized Bi₂S₃ sample (Figure 1e,f). These hierarchical Bi₂S₃ nanostructures can efficaciously prevent the 1D nanorods from restacking together, which may dramatically add exposure sensing sites and facilitate the diffusion and adsorption/desorption processes of gas molecules. The 1D nanorods assembled into hierarchical nanostructures would show superior sensing behaviors than those of only 1D nanorods [40,41]. The EDS mapping confirms the uniform distribution of Bi and S elementals along the full hierarchical Bi₂S₃ (Figure S3). The EDS spectrum of the as-prepared Bi₂S₃ sample further demonstrates the sample consists of Bi and S without other elementals, and the atomic ratio of Bi to S of the sample almost meets the stoichiometry ratio.

In order to obtain the formation mechanism of the hierarchical Bi₂S₃ nanostructures, serious samples with different reaction times were prepared and analyzed, combined with XRD patterns and SEM images. Firstly, Bi(NO₃)₃·5H₂O powder was dispersed in deionized water and generated hydrolysis, which resulted in the formation of a milky solution. The obtained white hydrolysate shows 1D nanorod structures (Figure 1b). As shown in Figure 2a, the appearing XRD spectrum peaks of the hydrolysis product conform to the bismuth oxide hydroxide nitrate hydrate (JCPDF no. 70-1226). Subsequently, with CH₄N₂S added to the homogeneous solution, the color of the above solution became yellow due to [Bi(CH₄N₂S)n]³⁺ chelates from the strong reaction between Bi³⁺ and CH₄N₂S [34,42]. Secondly, the CH₄N₂S of the reaction mixture could decompose to generate H₂S under 140 °C, which would react with the Bi³⁺ to form Bi₂S₃ nanocrystals. At the early stage of the hydrothermal reaction for 1 h, the Bi₂S₃ nanostructures were confirmed with the XRD pattern. With increasing the reaction time from 2 h to 8 h, the microspheres gradually transfer to 1D nanorods assembled into hierarchical nanostructures (Figure S4a–d) [43]. The microsphere structures of the BS-12h sample almost disappear, and the assembled 1D nanorods of hierarchical nanostructures become sturdy and short, with the reaction process lasting 24 h (Figure S4e–f). The diffraction peaks of as-prepared Bi₂S₃ samples at 2θ = 15.8°, 17.6°, 22.4°, 24.9°, 28.6°, 31.8°, 39.0°, 46.5°, and 52.6° match well with (020), (120), (220), (130), (211), (221), (041), (431), and (351) planes of orthorhombic Bi₂S₃ (JCPDF no. 17-0320). The intensity of diffraction peaks in the prepared sample shows inconspicuous variation after hydrothermal reaction time over 4 h. The sharp diffraction peaks without any other impurity phase confirm the high purity of the prepared Bi₂S₃.

The information on the surface chemical states of the as-prepared hierarchical Bi₂S₃ sample was obtained via XPS. The XPS survey spectrum of the BS-12h sample is shown in Figure 2b. The highly pure Bi₂S₃ is further confirmed. The high-resolution XPS spectra of the Bi 4f and the S 2p are shown in Figure 2c. The Bi 4f core level spectrum of the hierarchical Bi₂S₃ shows the Bi 4f_7/2 and Bi 4f_5/2 strong peaks at 158.6 and 163.9 eV, respectively [44,45]. The peaks located at 161.3 and 162.5 eV can be assigned to S 2p_3/2 and S 2p_1/2, respectively, confirming the existence of S²⁻ [44,45]. All the above results demonstrate that the hierarchical Bi₂S₃ was prepared successfully.

3.2. Characterization of Gas Sensing Performance

In order to demonstrate the internal linkage between morphologies and structures with sensing characteristics, the RT sensing properties of the as-prepared BS-8h, BS-12h, and BS-24h samples were measured using a dynamic gas sensor analysis system. The RT dynamic sensing response curves of the above sensors (BS-8h, BS-12h, and BS-24h) for low concentration NO₂ ranging from 0.1 to 1.0 ppm are shown in Figure 3a. All sensors show typical n-type semiconductor sensing behaviors, and the sensing response resistance gradually increases with increasing exposure to NO₂ concentration. Figure 3b displays the linear relationship of all prepared sensors between sensitivity and NO₂ concentration. The BS-12h sensor shows high sensitivity and a good linear relationship to trace NO₂ gas at RT. The RT response/recovery curves of the BS-8h, BS-12h, and BS-24h sensors upon exposure to 1 ppm NO₂ are shown in Figure S5. The sensors based on the hierarchical Bi₂S₃ display superb response/recovery properties. The response and recovery time of the sensor based on the BS-12h sample toward 1 ppm NO₂ were 28 and 116 s, respectively. These results show that the 1D nanorod self-assembled Bi₂S₃ structures have superior RT NO₂ sensing responses, which could be ascribed to the special hierarchical nanostructures. According to the test results, the BS-12h sample was chosen as a promising sensing material for sensors after deeply appraising its NO₂ sensing properties.

The RT dynamic response curve of the BS-12h sensor with varied NO₂ concentrations is shown in Figure 4a. The recorded real-time response resistance value exhibits an increasing trend when the concentration of NO₂ increases from 50 ppb to 10 ppm. The main reason may be associated with more NO₂ molecules trapped on the surface of hierarchical nanomaterials and obtaining more electrons from the surface of the Bi₂S₃ nanostructure. Meanwhile, the BS-12h sensor displays fast response speed and full-recovery sensing properties, which could be ascribed to the hierarchical nanostructures of the Bi₂S₃ materials. These results confirm the BS-12h sensor can effectively realize low-concentration NO₂ detection from 50 ppb to 10 ppm at RT. The sensor based on hierarchical Bi₂S₃ nanostructures displays two sectional linear relations between sensitivity and NO₂ concentration from 0.05 to 2 ppm and 2 to 10 ppm (Figure 4b). These phenomena were associated with decreasing carrier mobility due to the scattering effects of adsorbed gas molecules on the surface of sensing materials [46,47].

The humidity tolerance ability of the BS-12h sensor was investigated considering the practical application of the gas sensor. The response resistance curves of the Bi₂S₃-based sensors to 1 ppm NO₂ under different relative humidity levels from 15% to 85% are shown in Figure 4c. The sensor based on hierarchical Bi₂S₃ nanostructures displays excellent response/recovery properties under different relative humidity conditions. The RT resistance value of the BS-12h sensor sustainably declines as the relative humidity gradually increases (Figure 4d), which may be attributed to the increased electron density of Bi₂S₃ sensing materials. As ambient relative humidity increases, abundant H₂O molecules adsorbed on the surface of the Bi₂S₃ nanostructures act as electron donors [48,49,50]. The electrons will transfer from H₂O molecules to the surface of Bi₂S₃ nanostructures, which leads to increased conductivity in the Bi₂S₃ sensor. The sensitivity of the BS-12h sensor exhibits a downward trend from 5.8 to 4.1 as ambient humidity increases from 15% RH to 85% RH. The calculated sensitivity deviation of the BS-12h sensor is less than 30%, which could be ascribed to the surface-adsorbed H₂O molecules occupying NO₂ sensing sites, which will decrease the adsorbed amount of NO₂ gas molecules [31]. The relatively small attenuation of the sensing response under a high concentration of H₂O molecules in the environment proves that the hierarchical Bi₂S₃ nanostructures have good anti-humidity performance, which could be associated with the improved adsorption of NO₂ molecules with the incorporation of H₂O molecules in Bi₂S₃ [26]. Combined RT resistance variation with sensitivity deviation, the hierarchical Bi₂S₃ sensor could be used to obtain NO₂ concentration information under different humidity atmospheres using the humidity compensation method.

The selectivity of the sensor based on the BS-12h sample was investigated to further recognize the specific interaction between hierarchical Bi₂S₃ nanostructures and NO₂ molecules. Figure 5a shows the sensing response of several kinds of interfered gases, including the main hazardous gases (H₂S, CO, SO₂, and NH₃) and green energy gas (H₂) existing in the atmosphere. The sensitivity of the BS-12h sensor at RT to 100 ppm H₂S, CO, SO₂, NH₃, and H₂ gas is 1.64, 1.07, 1.02, 1.07, and 1.07, respectively. Compared with the sensitivity of 5.8 to 1 ppm NO₂, these results confirm the optimal RT NO₂ detection performance of Bi₂S₃ sensing materials. The intrinsic mechanism of the selectivity of pristine Bi₂S₃ to NO₂ has been investigated using first-principle calculations [26]. The calculated adsorption energy between Bi₂S₃ nanomaterials and NO₂ molecules is much larger than that of the other interfered gases, which results in the excellent detection ability of NO₂ gas. Meanwhile, the newly formed O-S chemical bonds between NO₂ molecules and Bi₂S₃ further enhance the selectivity.

Repeatability and long-term stability are also key properties of gas sensors. The five consecutive cyclic response/recovery curves of the BS-12h sensor are shown in Figure 5b. Obviously, the fluctuation of the sensing response resistance of the BS-12h sensor is negligible, indicating the optimal NO₂ sensing repeatability of the hierarchical Bi₂S₃ nanomaterials. The long-term stability of the sensor based on the BS-12h sample was also acquired by measuring the sensing behaviors toward 1 ppm NO₂ at RT after aging for 60 days (Figure 5c). The sensing response values are in the range of (5.6 ± 0.3) during 60 days of aging, which shows the hierarchical Bi₂S₃ sensors can acquire stable NO₂ concentration data. The insert of Figure 5c shows the dynamic response data of the sensor based on the BS-12h sample to 1 ppm NO₂ at RT after aging for 30 and 60 days. The BS-12h sensor, after different aging times, displays fast response speed and full-recovery sensing properties at RT. In comparison to the dynamic resistance curve of the BS-12h sensor after aging for 60 days (Figure S6), the response/recovery time and RT resistance value display a negligible deviation. These results demonstrate its superior stability.

The NO₂ sensing properties of typical metal sulfide-based gas sensors in the literature and our prepared 1D nanorods with self-assembled Bi₂S₃ hierarchical structures are summarized in Table 1. The Bi₂S₃ nanostructures display obviously competitive NO₂ sensing properties, such as high sensitivity, fast response, recovery speed, and low LOD at RT. Combined with the simple prepared means, the synthesized hierarchical Bi₂S₃ nanostructures as NO₂ sensing materials are suitable for practical production.

3.3. Gas Sensing Mechanism

The sensing mechanism of metal sulfide nanomaterials relies on the electron-migration case between the surface of nanostructures and adsorbed target gas molecules [58,59,60]. The Bi₂S₃ nanomaterials as n-type semiconductors exhibit increased resistance during the electrophilic NO₂ sensing process. In order to confirm the role of O₂ molecules during the NO₂ sensing process of Bi₂S₃ nanostructures, a dynamic gas dilution system was adopted to control the O₂ concentration during the NO₂ sensing test process. The dynamic NO₂ sensing curve with different O₂ concentrations and N₂ as a balance gas is shown in Figure 6. The BS-12h sensor based on hierarchical Bi₂S₃ nanostructures shows an optimum dynamic response/recovery process to 1 ppm NO₂ in different O₂ concentrations. The sensitivity of the BS-12h sensor displays a decline trend, which could be associated with more O₂ molecules trapped on the surface of Bi₂S₃ nanostructures with the increasing O₂ concentration [30]. The RT resistances of the prepared Bi₂S₃ nanostructures under different O₂ concentrations (N₂ as a balance gas) are shown in Figure S7. The resistance value of Bi₂S₃ nanostructures in a 100% N₂ atmosphere is the smallest and exhibits a gradually increasing trend with increasing O₂ concentration. The phenomenon confirms that O₂ molecules can be trapped on the surface of Bi₂S₃ nanostructures and capture electrons from their conduction band to form O₂⁻. Upon exposure to NO₂, these NO₂ molecules competitively adsorb on the sensing active sites and trap the electron to form NO₂⁻. The little fluctuation in the sensing response of the Bi₂S₃ nanostructures can be associated with the much bigger adsorption energy of NO₂ molecules than that of O₂ molecules [26]. The result confirms that ambient O₂ molecules are not directly involved in NO₂ sensitive processes in Bi₂S₃ nanomaterials. The electron transfer process between the NO₂ molecules and Bi₂S₃ nanostructures could be depicted using the following equation:

NO₂ (gas) + e⁻ → NO₂⁻ (ads)(1)

The schematic diagram of the NO₂ sensing mechanism of hierarchical Bi₂S₃ nanostructures is shown in Figure 7. The NO₂ molecules adsorb on the surface of the Bi₂S₃ nanostructure and trap electrons, which leads to decreased electron density in Bi₂S₃ and the formation of an electron depletion layer. The formed electron depletion layer on the surface of the sensing materials will cause the resistance of the Bi₂S₃ nanomaterials to increase. When the Bi₂S₃ nanostructure is re-exposed to air, NO₂ molecules gradually finish the desorption process and release the electron to the surface of Bi₂S₃, which results in the resistance value of the sensor recovering up to its baseline resistance.

The enhanced RT NO₂ sensing properties of as-prepared hierarchical Bi₂S₃ nanostructures assembled from 1D nanorods can be illustrated by the following factors. Firstly, the 1D nanorods self-assembled Bi₂S₃ nanostructure can adequately prevent the 1D nanorods from restacking together, which will markedly increase the exposure-sensing active surface of Bi₂S₃ materials for NO₂ adsorption. The N₂ adsorption/desorption cycles of as-prepared hierarchical Bi₂S₃ nanostructures (BS-8h, BS-12h, and BS-24h samples) were used to calculate the BET surface areas (Figure S8). The specific surface area of the above three samples is 3.6, 4.2, and 4.0 m² g⁻¹, respectively. The BS-12h sample shows a much larger active surface than that of other samples. According to the process of growth of the hierarchical Bi₂S₃ nanostructures, the BS-12h sample displays more 1D nanorod-assembled Bi₂S₃ hierarchical nanostructures than the BS-8h sample (Figure S4), which leads to much larger specific surface areas than that of the BS-8h sample. With increasing the reaction time for 24 h, the nanorods of assembled hierarchical Bi₂S₃ structures become short and thick due to their continuous growth, which decreases the exposed active surface of the as-prepared BS-24h sample. The BS-12h sensor shows superior RT NO₂ sensing properties than that of the other sample, in accordance with the specific surface area test value. The large specific surface areas of the BS-12h sensor will provide a mound of NO₂ sensing active sites, which enhance the NO₂ sensing response due to massive NO₂ molecules adsorbing on the surface of hierarchical Bi₂S₃ nanostructures and accelerating the electron transfer from the surface of Bi₂S₃ materials to NO₂ molecules. In addition, the 1D nanorod-assembled Bi₂S₃ as NO₂ sensing materials exhibit fast response and recovery speed and full-recovery properties, which could be associated with a special hierarchical nanostructure and intrinsic physicochemical characteristics. As the mechanism diagram shows in Figure 7, these adjacent 1D nanorods form a mass of open voids, which, as inner flow passages, will dramatically accelerate the diffusion and adsorption/desorption processes between NO₂ molecules and hierarchical Bi₂S₃ nanostructures. When NO₂ molecules adsorb on the 1D nanorods assembled in Bi₂S₃ hierarchically, the NO₂ molecules rapidly diffuse inward through the inner spread path. On the contrary, upon re-exposure to air, the adsorbed NO₂ molecules rapidly and fully desorb from the surface of hierarchical Bi₂S₃ nanostructures and diffuse into the air. Therefore, the sensor based on the 1D nanorods assembled in Bi₂S₃ hierarchical structures shows short response/recovery times and full recovery performance. Additionally, the Bi₂S₃ materials have high carrier mobility and a narrow bandgap [17,18,19,20,21,22]. These superb physical properties endow Bi₂S₃ with fast electron transfer speed, which will further shorten the response/recovery time. Above all, the special hierarchical nanostructures and unique intrinsic properties of Bi₂S₃ are advantageous in obtaining high sensitivity and a short response/recovery time at RT.

4. Conclusions

In conclusion, 1D nanorods with a self-assembled Bi₂S₃ hierarchical nanostructure were successfully prepared via a simple hydrothermal process and showed improved RT NO₂ sensing properties. The hierarchical Bi₂S₃-based gas sensor showed a high sensitivity (R_g/R_a = 5.8 at 1 ppm), a low LOD (50 ppb), optimum selectivity, and anti-humidity. The outstanding RT sensing properties were associated with the special hierarchical nanostructures and unique intrinsic properties of Bi₂S₃ materials, which accelerate diffusion, adsorption/desorption, and electron transfer processes between NO₂ molecules and hierarchical Bi₂S₃ nanostructures. Additionally, hierarchical Bi₂S₃ exhibited almost the same sensing response in the absence of O₂ gas. The phenomenon confirms that the sensing mechanism of Bi₂S₃ is different from the chemisorption oxygen model. This study focuses on the fundamental sensing mechanism of the hierarchical Bi₂S₃ and provides a kind of novel nanostructure material for low-concentration NO₂ detection at RT.

Author Contributions

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

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Table

Figure 1. (a) Schematic illustration of the formation process of hierarchical Bi2S3 nanomaterials. SEM images of the hydrolysis product (b), the as-prepared sample with a 1 h hydrothermal reaction time (c), and the sample with a 12 h reaction (d); TEM (e) and (f) HRTEM images; and the EDS spectrum of the BS-12h sample (g).

Figure 2. (a) XRD patterns of prepared samples with different reaction times. (b) XPS survey; (c) Bi4f and S2p core level spectrum of the BS-12h sample.

Figure 3. RT NO2 sensing performance of BS-8h, BS-12h, and BS-24h samples: (a) dynamic sensing response curves and (b) the corresponding linear relationship between sensitivity and NO2 concentration ranging from 0.1 to 1.0 ppm.

Figure 4. The RT NO2 sensing properties test of the BS-12h sensor: (a) Dynamic NO2 sensing response curves with different concentrations. (b) Linear relationship fitting curve. (c) Repeatability toward 1 ppm NO2. (d) Selective response toward interfering gases.

Figure 5. The RT NO2 sensing properties test of the BS-12h sensor: (a) Anti-humidity ability to 1 ppm NO2 under different relative humidity. (b) Baseline resistance and sensitivity to 1 ppm NO2 under varied humidity environments. (c) Long-term stability.

Figure 6. Dynamic RT response/recovery curve of the hierarchical Bi2S3 sensor based on the BS-12h sample to 1 ppm NO2 in 100% N2, 100% O2, and different O2 concentration with N2 as a balance gas.

Figure 7. Schematic illustration of the NO2 sensing mechanism of the 1D nanorod-assembled Bi2S3 nanostructures.

Table 1

Comparison of NO₂ sensing behaviors of reported metal sulfide nanomaterials and our prepared hierarchical Bi₂S₃ nanostructures.

Materials	NO2 Conc.(ppm)	Response	τres/τrec(s/s)	LOD(ppb)	Reference
WS2 nanosheets	10	1.4	45/60	2000	[51]
WSe2 nanosheets	1	1.3	66/1020	100	[52]
SnS2 nanograins	10	7.0	272/3800	1000	[53]
SnSe2 nanosheets	1	1.6	142/935	100	[54]
MoS2 nanograins	500	3.5	~180/~480	25,000	[55]
MoSe2 nanosheets	5	1.4	450/600	5000	[56]
NbS2 nanosheets	5	1.2	~3000/~9000	241	[57]
Bi2S3 nanobelts	1	6.9	72/400	500	[21]
CuS/Bi2S3 nanosheets	10	3.4	18/388	500	[27]
Au/Bi2S3 nanosheets	5	5.6	18/338	250	[28]
Bi2S3 hierarchical nanostructures	1	5.8	28/116	50	This study

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/chemosensors12010008/s1, Figure S1: Schematic diagram of the interdigital electrodes, sensors based on different sensing materials, and fixture of sensors; Figure S2: Schematic diagram of the gas sensor dynamic analysis system; Figure S3: The elemental mapping of the BS-12h sample; Figure S4: SEM images of as-prepared samples with different reaction times; Figure S5: RT dynamic response/recovery curves of BS-8h, BS-12h, and BS-24h sensors toward 1 ppm NO₂; Figure S6: The dynamic resistance curve of the BS-12h sensor after aging 60 days toward 1.0 ppm NO₂; Figure S7: The RT resistance of the BS-12h sensor with different O₂ concentration atmosphere; and Figure S8: Nitrogen adsorption/desorption isotherms of as-prepared samples with different reaction times.

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