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

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 [1-5]. Two-dimensional (2D) layered materials, such as graphene, g-C3N4, and MoS2, have received broad interdisciplinary attention [6-13], owing to their potential in diverse technologies, including sensors, electronics, optoelectronics, and so on [14-20]. 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) [21-29]. According to previous reports, the electrical conductivity, surface activity, and sensing response of MHs can be efficiently tailored by choosing the suitable candidate materials [30-35]. 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 [38-40]. 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 Ra/Rg, where Ra is the resistance in atmospheric air and Rg 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 (T2g − 477 cm−1, A1g − 685 cm−1) and MoS2 ( − 382 cm−1, A1g − 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 (T2g, A1g), and MoS2 (, A1g) 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 (Ra/Rg) 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). 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 (Rg) decreases in target gas. It is noteworthy that the MoS2 edges offer high density of potential active sites for reduction reaction [42-44]. 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.

Additional file 1: Figures S1–S5. Additional experimental details, SEM, TEM, SAED and XRD results.