Development and Performance of ZnO/MoS₂ Gas Sensors for NO₂ Monitoring and Protection in Library Environments

Abstract

The presence of harmful oxidizing gases accelerates the oxidation of cellulose fibers in paper, resulting in reduced strength and fading ink. Therefore, the development of highly sensitive NO₂ gas sensors for monitoring and protecting books holds significant practical value. In this manuscript, ZnO/MoS₂ composites were synthesized using sodium molybdate and thiourea as raw materials through a hydrothermal method. The morphology and microstructure were characterized by X-ray diffraction analysis (XRD), energy dispersive spectroscopy (EDS), field emission scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The ZnO/MoS₂ composite exhibited a flower-like structure, with ZnO nanoparticles uniformly attached to the surface of MoS₂, demonstrating advantages such as high specific surface area and good uniformity. The gas sensitivity of the ZnO/MoS₂ nanocomposites reached its peak at 260 °C, with a sensitivity value around 3.5, which represents an improvement compared to pure ZnO, while also enhancing sensitivity. The resistance of the ZnO/MoS₂ gas sensor remained relatively stable in air, exhibiting short response times during transitions between air and NO₂ environments while consistently returning to a stable state. In addition to increasing adsorption capacity and improving light utilization efficiency, the formation of hetero-junctions at the ZnO-MoS₂ interface creates an internal electric field that effectively promotes the rapid separation of photo-generated charge carriers within ZnO, thereby extending carrier lifetime.

1. Introduction

As society develops and the extensive use of non-renewable energy sources continues, coupled with industrial emissions, human activities have exacerbated atmospheric pollution. This has led to a gradual increase in both the types and concentrations of harmful gases present in the air. To ensure human safety and respond to international calls for environmental protection, there is an urgent need to establish highly sensitive methods for detecting toxic and hazardous gases [1,2,3,4,5]. Nitrogen dioxide (NO₂) is a reddish-brown gas that possesses a pungent odor and is toxic, exhibiting strong oxidative properties. Its oxidizing nature can lead to the degradation of cellulose fibers in paper materials, resulting in fading ink. When NO₂ reacts with water, it forms nitric acid, which is characterized by its strong acidity and high oxidation potential. Thus, NO₂ serves as both an acidic harmful gas and an oxidative harmful gas. Research has demonstrated that acids are significant contributors to the aging process of paper materials. Specifically, acid hydrolysis primarily damages the cellulose fibers within paper. Additionally, oxidative harmful gases further promote cellulose oxidation in paper fibers, leading to decreased tensile strength and faded ink. Therefore, developing highly sensitive NO₂ gas sensors for monitoring and protecting books holds substantial practical value.

The sensing layer of gas-sensitive sensors typically consists of semiconductor metal oxides, such as ZnO [6], CuO [7], and TiO₂ [8]. Semiconductor metal oxide gas-sensitive thin films function as impedance devices, where ion exchange occurs between gas molecules and the sensitive film, leading to reduction reactions that cause changes in the resistance of the sensitive film [9,10,11]. ZnO is a representative wide-bandgap metal semiconductor characterized by advantages such as low cost, high sensitivity, good recovery properties, rapid response times, and long lifespan. It has been widely applied across various scientific research fields [12,13,14,15], including the detection of gases like formaldehyde [16], acetone [17], ethanol [18], carbon dioxide [19], and nitrogen dioxide [20]. With advancements in materials science and industrial automation applications, there is an increasing demand for novel gas sensors [11]. The modification of ZnO gas-sensitive materials remains a prominent area of current research.

There are numerous synthesis methods for ZnO nanomaterials, including hydrothermal synthesis, the sol–gel method, the template method, the electrochemical deposition method, and electrospinning. Among these methods, hydrothermal synthesis offers environmental friendliness, economic viability, simplicity, and low energy consumption while allowing for control over nanoparticle shape and size through parameter adjustments [21,22]. Graphene-like molybdenum disulfide (MoS₂) is a two-dimensional layered semiconductor material belonging to transition metal dichalcogenides (TMDCS), classified as MX₂-type compounds [23]. MoS₂ exhibits advantages such as large specific surface area, high electrical conductivity, and elevated electron mobility [24]. However, the relatively high recombination rate of photogenerated carriers in pure MoS₂ limits its application due to lower sensitivity and longer response times. Consequently, MoS₂ is often combined with other materials to enhance its gas-sensing performance. Song et al. [25] successfully prepared three-dimensional flower-like ZnO/MoS₂ micro-nanosphere arrays using a two-step approach, and the resulting gas sensor effectively detected ethanol at a concentration of 938.776 mg/m³ at 200 °C. Wang et al. [26] further synthesized particulate ZnO via hydrothermal growth following the initial hydrothermal synthesis of MoS₂, resulting in ZnO-MoS₂ nanocomposites that demonstrated long-term stable detection capabilities for acetylene concentrations ranging from 0.531 to 53.061 mg/m³ at temperatures around 70 °C.

The combination of ZnO with graphene-like MoS₂ is beneficial for enhancing its gas-sensing performance. The large specific surface area and high conductivity of graphene-like MoS₂ provide an excellent conductive layer for ZnO, thereby improving the gas-sensing characteristics of the composite material [27,28]. Based on this, our study aims to utilize the advantages of ZnO and MoS₂ in the field of gas sensing. We plan to employ a relatively simple hydrothermal method to prepare ZnO-MoS₂ nanocomposites and explore effective ways to enhance the gas-sensing performance of ZnO/MoS₂-based materials, with an application focus on monitoring and protecting library environments.

2. Experimental Methods

2.1. Preparation of ZnO/MoS₂ Nanocomposites

In the preliminary experiments, we prepared TiO₂ NT/MoS₂ nanocomposites and investigated its gas-sensing properties [29]. We further optimized the experimental process and, in conjunction with gas-sensing property testing, established the experimental conditions for this manuscript. A total of 0.075 g of pre-synthesized molybdenum disulfide (MoS₂) powder was weighed using an electronic balance and then dispersed in 60 mL of deionized water, followed by ultrasonic treatment for 10 min (Nanjing Spico Testing Instruments Co., Ltd., Nanjing, China, SB3200). Subsequently, 0.0320 g of zinc chloride (ZnCl₂) and 0.0563 g of sodium hydroxide (NaOH) were sequentially added to the aforementioned solution, which was stirred for 30 min until all reagents were completely dissolved. The resulting solution was transferred into a 100 mL reaction vessel, which was then placed in an electric blast drying oven set at 180 °C for a duration of 16 h.

After removing the reaction vessel and allowing it to cool to room temperature, the obtained gray precipitate was collected into a centrifuge tube and subjected to centrifugation (Tuohe Electromechanical Technology Co., Ltd., Shanghai, China, 80-2). The centrifuge washing was performed alternately with deionized water and anhydrous ethanol for three cycles, each lasting 10 min at a speed of 15,000 rpm. The precipitate underwent alternating washing with deionized water and anhydrous ethanol three times via centrifugation. Finally, the washed precipitate was transferred to a dish and dried in an electric blast drying oven at 60 °C for 10 h, yielding a gray solid product. Finally, the washed precipitate was transferred to a dish and dried in an electric blast drying oven at 60 °C for 10 h, yielding a gray solid product. The prepared ZnO/MoS₂ was mixed with conductive graphite and the binder polytetrafluoroethylene (PTFE) in a ratio of 8:1:1 to form a gel-like substance.

2.2. Characterization of ZnO/MoS₂

The phase analysis of the products was conducted using an X-ray diffractometer (XRD, DX-2000, Hitachi High-Technologies Corporation, Tokyo, Japan) with Cu K_α radiation (λ = 0.154184 nm). The operating conditions were set at a tube voltage of 30 kV and a current of 25 mA, with a scanning speed of 0.03°/s over a range from 5° to 65°. The microstructure and morphology of the materials were examined using a scanning electron microscope (SEM, S-4800 model, Hitachi High-Technologies Corporation, Tokyo, Japan) and a transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN model, FEI Company, Hillsboro, OR, USA). Energy dispersive spectroscopy (EDS) was employed to characterize the surface elements and their chemical states in the materials.

2.3. Fabrication and Testing of ZnO/MoS₂ Sensors

The ZnO/MoS₂ nanocomposites prepared via the hydrothermal method are in powder form [29,30]. The fabrication process for the gas-sensitive element using the powdered sample is illustrated in Figure 1. First, we disperse the finely ground sample in anhydrous ethanol. Next, we allow it to settle through a standing method, enabling the powder to deposit onto the surface of the interdigitated electrode. Finally, we proceed with drying and measure its resistance, and only those elements exhibiting resistances on the order of MΩ are considered gas-sensitive components.

2.4. Construction of the Gas-Sensitive Testing System

In this experiment, we utilized a self-assembled gas-sensitive testing platform to evaluate the gas sensitivity of composite materials. Figure 2 illustrates the components of our gas-sensitive system: (1) composite vacuum gauge; (2) double-stage rotary vane vacuum pump; (3) mass flow meter; (4) flow integrator; (5) vacuum gas-sensitive testing chamber; and (6) Model 2450 Interactive SourceMETE^® Instrument (resistance tester). The interior of the testing chamber primarily consists of four tungsten steel needle electrodes and a heating platform for the gas-sensitive electrode. During operation, we first place the prepared gas-sensitive electrode onto the heating platform within the vacuum testing chamber. The four tungsten steel electrodes are arranged in pairs with alternating positions—two on top and two below—ensuring contact between them. Subsequently, we connect two of these electrodes to measure resistance. Initially, we use the resistance tester to determine the resistance value of the gas-sensitive material in air. Once this measurement is complete, we open the valve on a gas cylinder and set parameters via a flow integrator to send signals for predetermined flow control through a mass flow meter. Finally, using our resistance tester again, we measure the resistance value at that specific concentration of gas.

3. Results and Discussion

3.1. Surface Morphology of ZnO/MoS₂

Figure 3a,b present high-resolution SEM images of the surface morphology of MoS₂ nanoflower spheres obtained through hydrothermal synthesis. The resulting MoS₂ exhibits a flower-like spherical morphology with an approximate diameter of 2 μm, characterized by distinct petal growth that is uniformly distributed, forming a multi-layered and porous structure. This architecture serves as a gas-sensitive material, offering a substantial specific surface area conducive to the absorption of NO₂ and O₂ gases, thereby enhancing the effective reaction area. Figure 3c,d depict the surface morphology of the ZnO/MoS₂ nanocomposites. It is evident that the ZnO/MoS₂ nanocomposites retains a flower-like morphology, with ZnO uniformly distributed on the surface of MoS₂ in the form of nanoparticles. Due to the layered structure and porous nature of the nanoflower shaped MoS₂, this composite will exhibit an increased specific surface area which significantly enhances the capacity for oxygen ion adsorption. Furthermore, MoS₂ will form a conductive layer on the sphere’s surface, thereby improving electron mobility in ZnO.

3.2. XRD Analysis of ZnO/MoS₂

XRD was employed to characterize the synthesized MoS₂, ZnO, and ZnO/MoS₂ composites, with results presented in Figure 4. The peaks at 28.4°, 33.7°, and 58.7° correspond to the (004), (101), and (110) crystal planes of MoS₂, respectively. Meanwhile, the peaks at 31.7°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, 67.9°, and 77.0° are attributed to the (100), (002), (101), (102), (110), (103), (112), and (202) crystal planes of ZnO. All diffraction peaks align well with standard diffraction patterns from JCPDS No. 36-1451, indicating that the products belong to a hexagonal wurtzite structure with lattice constants a = 0.3249 nm and c = 0.5206 nm. As shown in Figure 4, the sharpness of the peaks suggests that the obtained ZnO/MoS₂ composite exhibits excellent crystallinity. Furthermore, no additional impurity peaks are present in the spectrum, confirming that the synthesized samples are pure without any other phases.

3.3. Elemental Composition of ZnO/MoS₂

The elemental composition of the ZnO/MoS₂ was analyzed using energy dispersive spectroscopy, with the results presented in Figure 5. The micro-area elemental scanning was conducted under an acceleration voltage of 10 kV and a magnification of 30,000. It can be observed that the sample contains the elements Mo, S, Zn, O, and C, and the presence of carbon is primarily attributed to the conductive adhesive used during preparation. The ZnO/MoS₂ composites consists predominantly of four elements: Zn, O, Mo, and S. According to the data presented in the table, the atomic ratio of Mo to S is approximately 1:1.7, indicating that MoS₂ serves as the primary component within this grain structure. No additional impurity peaks were detected in the EDS spectrum analysis, suggesting high purity of the sample. The EDS results indirectly confirm the formation of a heterostructure between these two materials of ZnO and MoS₂.

3.4. TEM Morphology of ZnO/MoS₂

Figure 6a,b display low-magnification TEM images, while Figure 6c shows a high-resolution TEM image, and Figure 6d illustrates the selected area electron diffraction (SAED) pattern. From Figure 6a,b, it is evident that the sample is composed of flower-like structures assembled from layered materials, with uniform size and thickness across these layers. Notably, we can clearly observe stress contrast images characterized by black-and-white stripes on the surface of ZnO nanoparticles, which indirectly suggests that a thin layer of nano-MoS₂ covers the surface of these zinc oxide particles. In Figure 6c, it is observed that the interplanar spacing of approximately 0.24 nm corresponds to the (101) plane of ZnO. Additionally, local distortions in the crystal lattice are noted alongside a reduced layered structure for MoS₂ due to interaction with ZnO nanoparticles. The measured interplanar spacing is 0.27 nm, which can be confirmed as the (100) crystal plane of MoS₂. This further substantiates the formation of a heterojunction between MoS₂ and ZnO. The diffraction rings observed in Figure 6d suggest hexagonal wurtzite structure for ZnO along with hexagonal phase for 2H-MoS₂. No other phases were detected, and these findings align well with our XRD results.

3.5. Gas-Sensing Characteristics of ZnO/MoS₂

The temperature significantly influences the gas-sensing performance of semiconductor sensors. The reaction between gas molecules and oxygen molecules adsorbed on the sample surface requires an elevated temperature to enhance the activation energy for the reaction. At low temperatures, gas molecules may not overcome the activation energy barrier necessary for oxidation reactions on the material’s surface, resulting in increased sensitivity with rising temperature. Conversely, at excessively high temperatures, gas adsorption on the surface of material cannot provide sufficient active sites to meet increasing demands. Therefore, there exists an optimal operating temperature for semiconductor gas-sensing materials like ZnO, where sensitivity reaches its maximum value. At a specific temperature, we denote the resistance of the gas sensor in stable air conditions as R_a, and that when exposed to NO₂ at equilibrium as R_g. The sensitivity (S) of a gas sensor is defined by S = R_g/R_a. The time required for achieving 90% of total resistance during adsorption and desorption processes is referred to as response time and recovery time.

The gas sensitivity of ZnO and ZnO/MoS₂ at different temperatures is illustrated in Figure 7, with the NO₂ concentration set at 4 ppm during testing. It is observed that ZnO exhibits the highest gas sensitivity of 3.0 at a temperature of 300 °C. In contrast, the ZnO/MoS₂ nanocomposite demonstrates its peak gas sensitivity of 3.5 at a lower temperature of 260 °C, which indicates a decrease in optimal operating temperature compared to pure ZnO while simultaneously enhancing sensitivity. The incorporation of MoS₂ into ZnO not only enhances adsorption capacity but also facilitates the formation of heterojunctions that establish an internal electric field at the ZnO-MoS₂ interface. This effectively promotes the rapid separation of photogenerated carriers within ZnO and extends their lifetime. In addition, due to the high charge carrier mobility, MoS₂ provides direct conduction paths for carriers to be transported from the junction to the external electrode, and thus the electrical signals link closely and propagate rapidly. This enables the MoS₂/ZnO nanocomposite to achieve its maximum gas sensitivity response at lower temperatures [31].

As shown in Figure 8, gas-sensing tests were conducted on samples with varying MoS₂ content at different temperatures by introducing 3 ppm NO₂. This investigation aimed to explore the response time, sensitivity, and recovery time of ZnO/MoS₂ towards NO₂ under various temperature conditions. Ultimately, a working temperature of 260 °C was determined for the ZnO/MoS₂ composite containing 0.75 g of MoS₂, which exhibited high sensitivity. Under these conditions, the gas-sensing properties of ZnO/MoS₂ towards NO₂ were further examined. In this composite material, the ZnO nanoparticles serve as active catalytic centers for gas-sensing responses, while MoS₂ primarily acts as a co-catalyst. Among the four groups of ZnO/MoS₂ sensors tested, the one with 0.75 g MoS₂ demonstrated optimal gas-sensing performance. This indicates that an excess amount of MoS₂ can inhibit the light utilization efficiency of ZnO and consequently reduce its photoquantum efficiency, thereby diminishing the overall gas-sensing performance of the material.

Figure 9 illustrates the testing results of ZnO and ZnO/MoS₂ nanocomposites for various concentrations of NO₂ gas, specifically at 300 °C and 260 °C, within a range of 2 ppm to 10 ppm. It is evident that the sensitivity of ZnO/MoS₂ nanocomposites increases as gas concentration rises. In contrast, the sensitivity of pure ZnO does not show significant enhancement with increasing NO₂. Moreover, when there are changes in gas concentration, the dynamic response of the ZnO/MoS₂ sensor remains quite stable. Furthermore, during transitions between air and NO₂ environments, it demonstrates a short reaction time while consistently returning to a stable state [29]. Additionally, as indicated by the dynamic curves, both response time and recovery time for the ZnO/MoS₂ decrease with rising gas concentrations.

The results presented in Figure 10 illustrate the selectivity tests conducted at an optimal temperature of 260 °C for ethanol, H₂, and NO₂ at a concentration of 2 ppm. It is evident from the figure that the ZnO/MoS₂ exhibits the highest sensitivity to NO₂ gas, with a response value of approximately 4.0. In contrast, its sensitivity to the other gases is relatively low, remaining below 2.0. Therefore, it can be concluded that the synthesized ZnO/MoS₂ nanocomposites material demonstrates superior selectivity towards NO₂. Compared to previously reported nanoparticles and nanospheres composites, the nanoflower-like composites developed in this experiment offer advantages such as a simplified preparation process and enhanced selectivity towards NO₂ at low concentrations [30].

When MoS₂ is combined with ZnO, it forms a layer on the surface of ZnO. An appropriate amount of MoS₂ will create a heterojunction, and the built-in electric field at the junction interface facilitates the separation of photogenerated charge carriers [31]. This effectively alters the thickness of the depletion region and enhances the gas sensitivity of the sample. However, an increase in MoS₂ content can hinder NO₂ from interacting with active sites on the ZnO surface. Consequently, this significantly reduces NO₂ adsorption, leading to decreased gas sensitivity. In the initial stage, photogenerated electrons are captured by oxygen molecules adsorbed on both ZnO and MoS₂ surfaces to form ${\mathrm{O}}_{2\left(\mathrm{ads}\right)}^{-}$ (as shown in Equation (1)), resulting in an electron depletion region. When NO₂ gas is introduced, two processes occur: first, NO₂ adsorbed onto the surface can directly capture electrons and form as ${\mathrm{NO}}_2^{-}$ (as indicated in Equation (2)); second, due to its stronger electron trapping ability compared to O₂, NO₂ can also capture additional electrons from ${\mathrm{O}}_2^{-}$ to form ${\mathrm{NO}}_2^{-}$ (as described in Equation (3)). These interactions lead to changes in both the width of the depletion region at the surface and carrier concentration, ultimately causing variations in resistance and generating a gas-sensitive response. Once NO₂ flow ceases, any adsorbed species will recombine with photogenerated holes (as represented by Equation (4)), allowing for the recovery of sensor resistance back to its initial state [32].

$${\mathrm{O}}_{2(gas)}+e^{-}\to {\mathrm{O}}_{2(ads)}^{-}(h\nu )$$

(1)

$${\mathrm{NO}}_{2(gas)}+{\mathrm{O}}_{2(ads)}^{-}(h\nu )\to \mathrm{N}{O}_{2(ads)}^{-}(h\nu )+{\mathrm{O}}_{2(gas)}$$

(2)

$${\mathrm{NO}}_{2\left(\mathrm{gas}\right)}+e^{-}\to {\mathrm{NO}}_{2(\mathrm{ads})}^{-}(h\nu )$$

(3)

$${\mathrm{NO}}_{2\left(\mathrm{ads}\right)}^{-}(h\nu )+{\mathrm{h}}^{+}\to {\mathrm{NO}}_{2(gas)}$$

(4)

4. Conclusions

The ZnO/MoS₂ nanocomposites prepared from sodium molybdate solution exhibit favorable gas-sensing properties when subjected to reactions at 260 °C. The ZnO/MoS₂ nanocomposites demonstrates excellent sensitivity towards NO₂ within a concentration range of 2 ppm to 10 ppm, with sensitivity increasing as the gas concentration rises. Furthermore, the dynamic response of the ZnO/MoS₂ gas sensor remains stable despite variations in gas concentrations. The flower-like morphology of MoS₂ contributes to a high specific surface area, enhancing contact between ZnO and oxygen ions. Additionally, MoS₂ forms a conductive layer on its surface that improves electron mobility in ZnO and generates an internal electric field conducive to facilitating photogenerated charge carrier separation. The high sensitivity and stable dynamic response of the ZnO/MoS₂ hold significant potential for applications in NO₂ monitoring sensors used in library protection systems.