Introduction {#Sec1}
============

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 \[[@CR1]--[@CR5]\].  Two-dimensional (2D) layered materials, such as graphene, g-C~3~N~4~, and MoS~2~, have received broad interdisciplinary attention \[[@CR6]--[@CR13]\], owing to their potential in diverse technologies, including sensors, electronics, optoelectronics, and so on \[[@CR14]--[@CR20]\].  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) \[[@CR21]--[@CR29]\].  According to previous reports, the electrical conductivity, surface activity, and sensing response of MHs can be efficiently tailored by choosing the suitable candidate materials \[[@CR30]--[@CR35]\].  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.  CuFe~2~O~4~ is an important n-type metal oxide semiconductor with an indirect bandgap in the range of 1.3--1.95 eV \[[@CR36], [@CR37]\], 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 \[[@CR38]--[@CR40]\].  It is worth noting that the CuFe~2~O~4~-based gas sensors exhibited relatively low responses toward some target gasses (such as ethanol and acetone) \[[@CR37]\].  Therefore, it is significant to improve the sensitivity performance of CuFe~2~O~4~-based gas sensors by the reasonable design of MHs.  MoS~2~ 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 \[[@CR41]\]. 

In this paper, we report a CuFe~2~O~4~/MoS~2~ 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 CuFe~2~O~4~/MoS~2~ MHs have been confirmed, and the density function theory (DFT) results further indicate the formation of type-II band alignment in the MHs.  The CuFe~2~O~4~/MoS~2~ MHs have obvious advantages for gas sensing, which benefits from the type-II band alignment and active sites in MoS~2~ ultrathin nanosheets.  Gas sensing properties of the CuFe~2~O~4~/MoS~2~ 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 CuFe~2~O~4~ nanotubes therefore suggesting potential applications of CuFe~2~O~4~/MoS~2~ MHs in highly sensitive gas sensors. 

Method Section {#Sec2}
==============

Synthesis of CuFe~2~O~4~/MoS~2~ MHs {#Sec3}
-----------------------------------

The detailed preparation processes of CuFe~2~O~4~/MoS~2~ MHs are shown in Fig. [ 1](#Fig1){ref-type="fig"}.  Firstly, the pure CuFe~2~O~4~ nanotubes were pre-synthesized by electrospinning method.  Firstly, 0.5 mmol of Cu(NO~3~)~2~·3H~2~O, 1.0 mmol of Fe(NO~3~)~3~·9H~2~O, 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.  1Schematic illustration of the preparation processes of CuFe~2~O~4~/MoS~2~ MHs

The CuFe~2~O~4~/MoS~2~ MHs were synthesized by hydrothermal method in the second step.  CuFe~2~O~4~ nanotubes were dispersed in deionized (DI) water (15 mL) via sonication.  The (NH~4~)~6~Mo~7~O~24~·4H~2~O and CN~2~H~4~S 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 {#Sec4}
--------------------------------

The morphology and structure of pure CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ 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 {#Sec5}
------------------------------------------

Gas sensors were fabricated by coating the mixture of the tested materials (pure CuFe~2~O~4~ or CuFe~2~O~4~/MoS~2~ MHs) and DI water onto the interdigitated Au electrode arrays (gap and width are 200 μm) on the SiO~2~/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 {#Sec6}
======================

The morphologies of pure CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs are shown in Fig. [ 2](#Fig2){ref-type="fig"} and Additional file [1](#MOESM1){ref-type="media"}: 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](#MOESM1){ref-type="media"}: Figure S1b).  The SEM images (Fig. [ 2](#Fig2){ref-type="fig"}a, b) show CuFe~2~O~4~/MoS~2~ MHs still maintains the original tubular structure after the hydrothermal process.  And we can see that the CuFe~2~O~4~ nanotubes have a relative smooth surface before compositing with tiny MoS~2~, while the rough surfaces appear in the CuFe~2~O~4~/MoS~2~ MHs.  Moreover, Raman spectroscopies were performed to verify the presence of MoS~2~ in the CuFe~2~O~4~/MoS~2~ MHs.  The strong vibrational modes of CuFe~2~O~4~ (T~2g~ − 477 cm^−1^, A~1g~ − 685 cm^−1^) and MoS~2~ ($\documentclass[12pt]{minimal}
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                \begin{document}$$ {\mathrm{E}}_{2\mathrm{g}}^1 $$\end{document}$ − 382 cm^−1^, A~1g~ − 409 cm^−1^) can be found in pure CuFe~2~O~4~ nanotube or MoS~2~ nanosheet samples (Fig. [ 2](#Fig2){ref-type="fig"}c).  By comparing with the pure CuFe~2~O~4~ nanotubes and MoS~2~ nanosheets (Additional file [1](#MOESM1){ref-type="media"}: Figure S2), the Raman vibrational mode of CuFe~2~O~4~ (T~2g~, A~1g~), and MoS~2~ ($\documentclass[12pt]{minimal}
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                \begin{document}$$ {\mathrm{E}}_{2\mathrm{g}}^1 $$\end{document}$, A~1g~) all appeared in the Raman spectrum of CuFe~2~O~4~/MoS~2~ MHs.  The position of these four peaks is unchanged, indicating the formation of the composite structure of CuFe~2~O~4~ and MoS~2~ in the CuFe~2~O~4~/MoS~2~ MHs.  Meanwhile, the XRD results of pure CuFe~2~O~4~ and CuFe~2~O~4~/MoS~2~ MHs are shows in Additional file [1](#MOESM1){ref-type="media"}: Figure S3.  It can be seen that the diffraction peaks of CuFe~2~O~4~ are well indexed to the standard JCPDS card (34-0425), revealing that the CuFe~2~O~4~ belongs to a body-centered tetragonal structure.  The XRD pattern of the CuFe~2~O~4~/MoS~2~ is superimposed by the diffraction peaks of CuFe~2~O~4~ and MoS~2~, respectively (the standard JCPDS card of CuFe~2~O~4~ (34-0425) and MoS~2~ (06-0097)), and there is no characteristic peak for impurity in the XRD pattern, indicating that the composite is consisted by the CuFe~2~O~4~ and MoS~2~ only.  Fig.  2SEM and Raman characterization of CuFe~2~O~4~ and CuFe~2~O~4~/MoS~2~ MHs.  FE-SEM images of **a** pure CuFe~2~O~4~ nanotubes and **b** CuFe~2~O~4~/MoS~2~ MHs. ** c** Raman spectra of pure CuFe~2~O~4~ nanotubes, pure MoS~2~ nanosheets, and CuFe~2~O~4~/MoS~2~ MHs

To further characterize the microstructure of CuFe~2~O~4~/MoS~2~ MHs, TEM observations were carried out, as shown in Fig. [ 3](#Fig3){ref-type="fig"} a. The low-resolution TEM images (Fig. [ 3](#Fig3){ref-type="fig"}b) show that the surfaces of CuFe~2~O~4~ nanotubes are uniformly covered with many hexagonal nanosheets 15--20 nm in diameter.  Figure [3](#Fig3){ref-type="fig"} c gives the high-resolution TEM (HRTEM) images of tiny nanosheets marked in Fig. [ 3](#Fig3){ref-type="fig"}b.  The lattice fringes spacing of 0.27 nm can be corresponded to the (100) plane of MoS~2~. In addition, the morphology and size of MoS~2~ can be tailored by adjusting the hydrothermal reaction conditions (Additional file [1](#MOESM1){ref-type="media"}: Figure S2).  Selected area electron diffraction (SAED) pattern also reveals the hexagonal symmetry for the layered MoS~2~ (Additional file [1](#MOESM1){ref-type="media"}: Figure S4).  To demonstrate the distribution of MoS~2~ nanosheets on the surface of CuFe~2~O~4~ nanotubes, the in situ EDS elemental mapping images of CuFe~2~O~4~/MoS~2~ MHs (marked in Fig. [ 3](#Fig3){ref-type="fig"}b) are performed as shown in Fig. [ 4](#Fig4){ref-type="fig"}.  The homogeneous distribution of Mo, S, Cu, Fe, and O elements indicates that a large number of MoS~2~ nanosheets are uniformly dispersed in CuFe~2~O~4~/MoS~2~ MHs.  Fig.  3TEM characterization of CuFe~2~O~4~/MoS~2~ MHs.  Low-resolution TEM image of **a** CuFe~2~O~4~/MoS~2~ 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.  4EDS result of CuFe~2~O~4~/MoS~2~ MHs. ** a** SEM image of sample in dotted line of Fig. [ 3](#Fig3){ref-type="fig"}a. ** 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 CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs gas sensors were fabricated as shown in Fig. [ 5](#Fig5){ref-type="fig"} a and Additional file [1](#MOESM1){ref-type="media"}: Figure S5.  Figure [5](#Fig5){ref-type="fig"}b and c preset the response-recovery curves of pure CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs gas sensors toward 100 ppm ethanol and acetone (6 cycles), respectively.  After compositing with the MoS~2~ nanosheets, it can be seen that the CuFe~2~O~4~/MoS~2~ MHs sensor shows positive responses on exposure to both ethanol and acetone, which are about 18--20% higher than those of pure CuFe~2~O~4~ nanotubes.  Evidently, the CuFe~2~O~4~/MoS~2~ MHs sensor exhibits consistent sensing responses even after 6 cycles, indicating the good reversibility and repeatability.  Figure [5](#Fig5){ref-type="fig"}d and e give the dynamic transient response curves of pure CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs gas sensors to various acetone concentrations (0.5--1000 ppm).  The CuFe~2~O~4~/MoS~2~ MHs sensor exhibits improved response to each acetone concentration (Fig. [ 5](#Fig5){ref-type="fig"}f).  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 CuFe~2~O~4~/MoS~2~ MHs are more sensitive to acetone in contrast with pure CuFe~2~O~4~. Fig.  5Sensing measurements of CuFe~2~O~4~/MoS~2~ MHs. ** a** Fabricated diagram of gas sensor and photos of fabricated gas sensor (CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs).  Sensing reproducibility of the CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs gas sensor to 100 ppm **b** ethanol and **c** acetone. ** d**, **e** Dynamic response-recovery curves of CuFe~2~O~4~ nanotubes and CuFe~2~O~4~/MoS~2~ MHs gas sensors at different acetone concentrations. ** f** The response increment rate of CuFe~2~O~4~/MoS~2~ MHs device relative to pure CuFe~2~O~4~ nanotube device at different acetone concentrations

To probe the important role of MoS~2~ nanosheets in the gas sensing reaction, the electronic band structures of CuFe~2~O~4~ and multilayer MoS~2~ were calculated respectively by using DFT (Fig. [ 6](#Fig6){ref-type="fig"}a, b).  The indirect bandgap of CuFe~2~O~4~ and multilayer MoS~2~ is about 1.3 eV and 1.2 eV, respectively.  According to the results, the band alignment of CuFe~2~O~4~/MoS~2~ MHs is drawn in Fig. [ 6](#Fig6){ref-type="fig"}c, 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 CuFe~2~O~4~ nanotubes, while most electrons will be injected into MoS~2~ layers.  When the pure CuFe~2~O~4~ or CuFe~2~O~4~/MoS~2~ MHs sensors are exposed to air, oxygen molecules will adsorb on the surface of sensors to generate oxygen species (O~2~^−^, O^−^, and O^2−^).  Meanwhile, the free electrons transfer from CuFe~2~O~4~ or CuFe~2~O~4~/MoS~2~ 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., CH~3~COCH~3~ + 8O^−^ → 3CO~2~ + 3H~2~O + 8e^−^) and release free electrons to the CuFe~2~O~4~ or CuFe~2~O~4~/MoS~2~ MHs.  Thus, the sensor resistance (*R*~g~) decreases in target gas.  It is noteworthy that the MoS~2~ edges offer high density of potential active sites for reduction reaction \[[@CR42]--[@CR44]\].  Figure [6](#Fig6){ref-type="fig"} d shows the calculated adsorption energy of CH~3~COCH~3~ on CuFe~2~O~4~/MoS~2~ MHs by using the DFT method.  The adsorption energy for CH~3~COCH~3~ molecules over the edge of CuFe~2~O~4~/MoS~2~ MHs is − 30.07 eV (very small).  That means the edge of CuFe~2~O~4~/MoS~2~ MHs are active sites for CH~3~COCH~3~ molecules.  Benefiting from the active sites in MoS~2~ nanosheets, the CuFe~2~O~4~/MoS~2~ MHs obtained free electrons more efficiently compared with pure CuFe~2~O~4~ (Fig. [ 6](#Fig6){ref-type="fig"}e).  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.  6DFT results of CuFe~2~O~4~/MoS~2~ MHs.  Electronic structures of **a** CuFe~2~O~4~ nanotubes and **b** multilayer MoS~2~. **c** Schematic illustrations of the type-II band alignment in CuFe~2~O~4~/MoS~2~ MHs. ** d** The edge adsorption energy for CH~3~COCH~3~ molecules on CuFe~2~O~4~/MoS~2~ MHs. ** e** Model for the CuFe~2~O~4~/MoS~2~ MHs in acetone vapor

Conclusions {#Sec7}
===========

We report a novel CuFe~2~O~4~/MoS~2~ MHs and the obvious improvement of sensing performance for acetone.  The CuFe~2~O~4~/MoS~2~ MHs are confirmed by Raman, SEM, XRD, TEM, and EDS results.  The coupling interactions between CuFe~2~O~4~ and MoS~2~ lead to the formation of type-II heterostructures, which is verified by DFT results.  The practical gas sensor devices were fabricated based on CuFe~2~O~4~/MoS~2~ 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 CuFe~2~O~4~/MoS~2~ MHs can be attributed to the effect of type-II band alignment and the MoS~2~ active sites.  We believe that our studies will be valuable for the various applications of mixed-dimensional heterostructures. 

Supplementary information
=========================

 {#Sec8}

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

2D

:   Two-dimensional

DFT

:   Density function theory

EDS

:   Energy dispersive X-ray spectrometer

MHs

:   Mixed-dimensional heterostructures

SEM

:   Scanning electron microscope

TEM

:   Transmission electron microscopy

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Supplementary information
=========================

**Supplementary information** accompanies this paper at 10.1186/s11671-020-3268-4. 

KNZ, CHZ, CCD, and QCF performed the experimental design and analysis and wrote the manuscript.  CCD, ZW, and BJP contributed to the preparation of devices and TEM and SEM measurements.  YHS, LJZ, and WZ contributed to the Raman spectroscopy and sensing measurements.  KNZ and CCD contributed equally to this work.  All authors read and approved the final manuscript. 

This work was funded by the Department of Science and Technology of Sichuan Province (2019YFG0446) and Sichuan Distinguished Scientists of China (grant no.  2019JDJQ0050). 

All data are fully available without restriction. 

The authors declare that they have no competing interests.