CuFe₂O₄/MoS₂ Mixed-Dimensional Heterostructures with Improved Gas Sensing Response

Abstract

Mixed-dimensional (2D + nD, n = 0, 1, and 3) heterostructures opened up a new avenue for fundamental physics studies and applied nanodevice designs. Herein, a novel type-II staggered band alignment CuFe₂O₄/MoS₂ mixed-dimensional heterostructures (MHs) that present a distinct enhanced (20–28%) acetone gas sensing response compared with pure CuFe₂O₄ nanotubes are reported. Based on the structural characterizations and DFT calculation results, the tentative mechanism for the improvement of gas sensing performance of the CuFe₂O₄/MoS₂ MHs can be attributed to the synergic effect of type-II band alignment and the MoS₂ active sites.

Introduction

Integration of nanostructured materials with dissimilar physical properties is essential for creating multifunctional devices and it has long been a pursuit of nanomaterials science community [1,2,3,4,5]. Two-dimensional (2D) layered materials, such as graphene, g-C₃N₄, and MoS₂, have received broad interdisciplinary attention [6,7,8,9,10,11,12,13], owing to their potential in diverse technologies, including sensors, electronics, optoelectronics, and so on [14,15,16,17,18,19,20]. In particular, 2D layered materials provide a new platform for building mixed-dimensional heterostructures (MHs) efficiently with 0D and 1D nanostructures (including quantum dots, nanowires, and nanotubes) [21,22,23,24,25,26,27,28,29]. According to previous reports, the electrical conductivity, surface activity, and sensing response of MHs can be efficiently tailored by choosing the suitable candidate materials [30,31,32,33,34,35]. Although most research has been focused on the novel physical properties of MHs based on 2D layered materials, more efforts are still needed to develop the 0D/1D MH-based nanodevices. CuFe₂O₄ is an important n-type metal oxide semiconductor with an indirect bandgap in the range of 1.3–1.95 eV [36, 37], which has been considered a promising material for gas sensors because of its naturally abundance, low-cost, environmental friendliness, simple electronic interface, low maintenance, ease of use, and fabrication [38,39,40]. It is worth noting that the CuFe₂O₄-based gas sensors exhibited relatively low responses toward some target gasses (such as ethanol and acetone) [37]. Therefore, it is significant to improve the sensitivity performance of CuFe₂O₄-based gas sensors by the reasonable design of MHs. MoS₂ is one of the most prominent 2D materials possessing a bandgap of 1.2–1.8 eV, because of high surface to volume ratio and highly sensitive to oxygen adsorption allowing their exploration in chemical sensing applications [41].

In this paper, we report a CuFe₂O₄/MoS₂ MHs (1D/2D) for the first time synthesized by two-step method using electrospinning followed by a hydrothermal process. The morphologies, crystal structures, and compositions of the CuFe₂O₄/MoS₂ MHs have been confirmed, and the density function theory (DFT) results further indicate the formation of type-II band alignment in the MHs. The CuFe₂O₄/MoS₂ MHs have obvious advantages for gas sensing, which benefits from the type-II band alignment and active sites in MoS₂ ultrathin nanosheets. Gas sensing properties of the CuFe₂O₄/MoS₂ MHs are studied in both ethanol and acetone gasses. As was expected, the MHs-based sensor shows substantial improved gas sensing performance compared with pure CuFe₂O₄ nanotubes therefore suggesting potential applications of CuFe₂O₄/MoS₂ MHs in highly sensitive gas sensors.

Method Section

Synthesis of CuFe₂O₄/MoS₂ MHs

The detailed preparation processes of CuFe₂O₄/MoS₂ MHs are shown in Fig. 1. Firstly, the pure CuFe₂O₄ nanotubes were pre-synthesized by electrospinning method. Firstly, 0.5 mmol of Cu(NO₃)₂·3H₂O, 1.0 mmol of Fe(NO₃)₃·9H₂O, and 0.68 g of polyvinylpyrrolidone (PVP) were dissolved in 5 mL of ethanol and 5 mL of N,N-Dimethylformamide(DMF). After stirring for 6 h, the above solution was placed in a syringe and injected with a feeding rate of 0.4 mL h⁻¹. A DC voltage of 15 kV was applied between the needle tip and stainless-steel mesh with a distance of 18 cm. The as-spun precursor fibers were collected in a tube furnace and maintained at 500 °C for 2 h in air.

Fig. 1

Schematic illustration of the preparation processes of CuFe₂O₄/MoS₂ MHs

The CuFe₂O₄/MoS₂ MHs were synthesized by hydrothermal method in the second step. CuFe₂O₄ nanotubes were dispersed in deionized (DI) water (15 mL) via sonication. The (NH₄)₆Mo₇O₂₄·4H₂O and CN₂H₄S were then added into the mixture. After stirring for 30 min, the solution was transferred into a 25-mL polytetrafluoroethylene (PTFE) autoclave and kept at 200 °C for 10 h. Finally, the MHs were collected in a centrifuge, washed with DI water and dried at 60 °C.

Microstructural Characterization

The morphology and structure of pure CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs were characterized by field emission scanning electron microscopy (FE-SEM, FEI NanoSEM200). X-ray diffraction (XRD) patterns were recorded on a Rigaku Smartlab with Cu Kα radiation operating at 45 kV and 200 mA. Transmission electron microscopy (TEM) measurements were conducted on the JEOL 2100F. The energy dispersive X-ray spectrometer (EDS) was introduced to identify the chemical composition. Raman measurements were performed using a Renishaw inVia at room temperature with a 532-nm excitation laser (2 mW).

Fabrication and Measurement of Gas Sensors

Gas sensors were fabricated by coating the mixture of the tested materials (pure CuFe₂O₄ or CuFe₂O₄/MoS₂ MHs) and DI water onto the interdigitated Au electrode arrays (gap and width are 200 μm) on the SiO₂/Si substrate. Gas sensing properties of the sensors were measured by using a commercial CGS-4TPs system (Beijing Elite Tech Co., Ltd., China). The response is defined as R_a/R_g, where R_a is the resistance in atmospheric air and R_g is the resistance in the tested gas, respectively.

Results and Discussion

The morphologies of pure CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs are shown in Fig. 2 and Additional file 1: Figure S1. Both of the samples are well-defined tubular nanostructures with several tens of micrometers in length, and 70–150 nm in diameter, which can be confirmed by the cross-section of broken nanotubes (Additional file 1: Figure S1b). The SEM images (Fig. 2a, b) show CuFe₂O₄/MoS₂ MHs still maintains the original tubular structure after the hydrothermal process. And we can see that the CuFe₂O₄ nanotubes have a relative smooth surface before compositing with tiny MoS₂, while the rough surfaces appear in the CuFe₂O₄/MoS₂ MHs. Moreover, Raman spectroscopies were performed to verify the presence of MoS₂ in the CuFe₂O₄/MoS₂ MHs. The strong vibrational modes of CuFe₂O₄ (T_2g − 477 cm⁻¹, A_1g − 685 cm⁻¹) and MoS₂ (${\mathrm{E}}_{2\mathrm{g}}^1$ − 382 cm⁻¹, A_1g − 409 cm⁻¹) can be found in pure CuFe₂O₄ nanotube or MoS₂ nanosheet samples (Fig. 2c). By comparing with the pure CuFe₂O₄ nanotubes and MoS₂ nanosheets (Additional file 1: Figure S2), the Raman vibrational mode of CuFe₂O₄ (T_2g, A_1g), and MoS₂ (${\mathrm{E}}_{2\mathrm{g}}^1$, A_1g) all appeared in the Raman spectrum of CuFe₂O₄/MoS₂ MHs. The position of these four peaks is unchanged, indicating the formation of the composite structure of CuFe₂O₄ and MoS₂ in the CuFe₂O₄/MoS₂ MHs. Meanwhile, the XRD results of pure CuFe₂O₄ and CuFe₂O₄/MoS₂ MHs are shows in Additional file 1: Figure S3. It can be seen that the diffraction peaks of CuFe₂O₄ are well indexed to the standard JCPDS card (34-0425), revealing that the CuFe₂O₄ belongs to a body-centered tetragonal structure. The XRD pattern of the CuFe₂O₄/MoS₂ is superimposed by the diffraction peaks of CuFe₂O₄ and MoS₂, respectively (the standard JCPDS card of CuFe₂O₄ (34-0425) and MoS₂ (06-0097)), and there is no characteristic peak for impurity in the XRD pattern, indicating that the composite is consisted by the CuFe₂O₄ and MoS₂ only.

Fig. 2

SEM and Raman characterization of CuFe₂O₄ and CuFe₂O₄/MoS₂ MHs. FE-SEM images of a pure CuFe₂O₄ nanotubes and b CuFe₂O₄/MoS₂ MHs. c Raman spectra of pure CuFe₂O₄ nanotubes, pure MoS₂ nanosheets, and CuFe₂O₄/MoS₂ MHs

To further characterize the microstructure of CuFe₂O₄/MoS₂ MHs, TEM observations were carried out, as shown in Fig. 3 a. The low-resolution TEM images (Fig. 3b) show that the surfaces of CuFe₂O₄ nanotubes are uniformly covered with many hexagonal nanosheets 15–20 nm in diameter. Figure 3 c gives the high-resolution TEM (HRTEM) images of tiny nanosheets marked in Fig. 3b. The lattice fringes spacing of 0.27 nm can be corresponded to the (100) plane of MoS₂. In addition, the morphology and size of MoS₂ can be tailored by adjusting the hydrothermal reaction conditions (Additional file 1: Figure S2). Selected area electron diffraction (SAED) pattern also reveals the hexagonal symmetry for the layered MoS₂ (Additional file 1: Figure S4). To demonstrate the distribution of MoS₂ nanosheets on the surface of CuFe₂O₄ nanotubes, the in situ EDS elemental mapping images of CuFe₂O₄/MoS₂ MHs (marked in Fig. 3b) are performed as shown in Fig. 4. The homogeneous distribution of Mo, S, Cu, Fe, and O elements indicates that a large number of MoS₂ nanosheets are uniformly dispersed in CuFe₂O₄/MoS₂ MHs.

Fig. 3

TEM characterization of CuFe₂O₄/MoS₂ MHs. Low-resolution TEM image of a CuFe₂O₄/MoS₂ MHs and b partial zooming panel a in the dotted line. c HRTEM image of the region in the dotted line in the b

Fig. 4

EDS result of CuFe₂O₄/MoS₂ MHs. a SEM image of sample in dotted line of Fig. 3a. b–f The in-suit EDS intensity map of Mo, S, Cu, Fe, and O, respectively

In order to investigate their gas sensing properties, the pure CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs gas sensors were fabricated as shown in Fig. 5 a and Additional file 1: Figure S5. Figure 5b and c preset the response-recovery curves of pure CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs gas sensors toward 100 ppm ethanol and acetone (6 cycles), respectively. After compositing with the MoS₂ nanosheets, it can be seen that the CuFe₂O₄/MoS₂ MHs sensor shows positive responses on exposure to both ethanol and acetone, which are about 18–20% higher than those of pure CuFe₂O₄ nanotubes. Evidently, the CuFe₂O₄/MoS₂ MHs sensor exhibits consistent sensing responses even after 6 cycles, indicating the good reversibility and repeatability. Figure 5d and e give the dynamic transient response curves of pure CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs gas sensors to various acetone concentrations (0.5–1000 ppm). The CuFe₂O₄/MoS₂ MHs sensor exhibits improved response to each acetone concentration (Fig. 5f). In particular, the percentage of improvement in acetone response exceeds 20% at acetone concentrations not higher than 50 ppm. It is noticeable that the acetone responses improved about 18% even at 0.5 ppm. That means the CuFe₂O₄/MoS₂ MHs are more sensitive to acetone in contrast with pure CuFe₂O₄.

Fig. 5

Sensing measurements of CuFe₂O₄/MoS₂ MHs. a Fabricated diagram of gas sensor and photos of fabricated gas sensor (CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs). Sensing reproducibility of the CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs gas sensor to 100 ppm b ethanol and c acetone. d, e Dynamic response-recovery curves of CuFe₂O₄ nanotubes and CuFe₂O₄/MoS₂ MHs gas sensors at different acetone concentrations. f The response increment rate of CuFe₂O₄/MoS₂ MHs device relative to pure CuFe₂O₄ nanotube device at different acetone concentrations

To probe the important role of MoS₂ nanosheets in the gas sensing reaction, the electronic band structures of CuFe₂O₄ and multilayer MoS₂ were calculated respectively by using DFT (Fig. 6a, b). The indirect bandgap of CuFe₂O₄ and multilayer MoS₂ is about 1.3 eV and 1.2 eV, respectively. According to the results, the band alignment of CuFe₂O₄/MoS₂ MHs is drawn in Fig. 6c, which forms a type-II band alignment. The improvement of sensor response manifested in changes in the electrical resistance (R_a/R_g) in the presence of air or target gas. Because of the type-II band alignment, the electron-hole pairs can be separated effectively at the heterojunction interface. Holes remain within the CuFe₂O₄ nanotubes, while most electrons will be injected into MoS₂ layers. When the pure CuFe₂O₄ or CuFe₂O₄/MoS₂ MHs sensors are exposed to air, oxygen molecules will adsorb on the surface of sensors to generate oxygen species (O₂⁻, O⁻, and O²⁻). Meanwhile, the free electrons transfer from CuFe₂O₄ or CuFe₂O₄/MoS₂ MHs to oxygen species at sensors surface lead to the decreases of electrical resistance (R_a). In the case of target gas detection, the reaction of adsorbed oxygen species and target molecules will occur on the sensor surface (e.g., CH₃COCH₃ + 8O⁻ → 3CO₂ + 3H₂O + 8e⁻) and release free electrons to the CuFe₂O₄ or CuFe₂O₄/MoS₂ MHs. Thus, the sensor resistance (R_g) decreases in target gas. It is noteworthy that the MoS₂ edges offer high density of potential active sites for reduction reaction [42,43,44]. Figure 6 d shows the calculated adsorption energy of CH₃COCH₃ on CuFe₂O₄/MoS₂ MHs by using the DFT method. The adsorption energy for CH₃COCH₃ molecules over the edge of CuFe₂O₄/MoS₂ MHs is − 30.07 eV (very small). That means the edge of CuFe₂O₄/MoS₂ MHs are active sites for CH₃COCH₃ molecules. Benefiting from the active sites in MoS₂ nanosheets, the CuFe₂O₄/MoS₂ MHs obtained free electrons more efficiently compared with pure CuFe₂O₄ (Fig. 6e). The positive effect is more obvious in low target gas concentration. While the improved gas response performance is limited in the extra-high concentrations due to the limited active sites.

Fig. 6

DFT results of CuFe₂O₄/MoS₂ MHs. Electronic structures of a CuFe₂O₄ nanotubes and b multilayer MoS₂. c Schematic illustrations of the type-II band alignment in CuFe₂O₄/MoS₂ MHs. d The edge adsorption energy for CH₃COCH₃ molecules on CuFe₂O₄/MoS₂ MHs. e Model for the CuFe₂O₄/MoS₂ MHs in acetone vapor

Conclusions

We report a novel CuFe₂O₄/MoS₂ MHs and the obvious improvement of sensing performance for acetone. The CuFe₂O₄/MoS₂ MHs are confirmed by Raman, SEM, XRD, TEM, and EDS results. The coupling interactions between CuFe₂O₄ and MoS₂ lead to the formation of type-II heterostructures, which is verified by DFT results. The practical gas sensor devices were fabricated based on CuFe₂O₄/MoS₂ MHs and shows the high sensitivity and excellent repeatability. A sensing enhancement is also seen with ethanol gas. The enhancement of gas sensing properties of the CuFe₂O₄/MoS₂ MHs can be attributed to the effect of type-II band alignment and the MoS₂ active sites. We believe that our studies will be valuable for the various applications of mixed-dimensional heterostructures.