Introduction Ethanol is one of volatile organic compounds. Long-term exposuring ethanol vapors can cause serious human-being health problems, such as headaches, throat irritation, and liver damage dirty. In addition, the human body will have nerve paralysis, slow brain response, and uncontrolled limbs when ethanol in the human blood reaches a certain concentration, resulting in frequent traffic accidents. It is thus necessary to develop gas sensors that can monitor ethanol in the human body. Metal oxide gas sensors have attracted much attention because of their cheap price, convenient portability and stable performance. Zn2SnO4 and CaFe2O4 are novel ternary metal oxide semiconductors with the advantages of high electron migration rate, strong gas adsorption and good thermal stability, which are widely used in gas sensors. However, single Zn2SnO4 or CaFe2O4 generally has the disadvantages of poor selectivity, high operating temperature and long response-recovery time. Some studies indicate that combining two different metal oxides to construct heterojunctions could greatly improve the gas sensing properties. It is prospected that the construction of CaFe2O4/Zn2SnO4 heterojunction is an effective method to enhance the gas sensing performance. However, little work on CaFe2O4/Zn2SnO4 composite for ethanol detection has been reported yet. In this paper, a p-n heterojunction between CaFe2O4 and Zn2SnO4 was constructed by a hydrothermal method to realize the superior gas sensing properties of CaFe2O4/Zn2SnO4 composite to detect ethanol.Methods To prepare Zn2SnO4 octahedral, 2 mmol Zn(CH3COO)2?2H2O and 1 mmol SnCl4?5H2O were dissolved in 70 mL deionized water. Meanwhile, 0.1 g CTAB (cetyltrimethyl ammonium bromide) and 15 mmol NaOH were dissolved in the solution above. Afterwards, the solution was stirred for 40 min, transferred into a 100 mL Teflon-lined autoclave and heated at 180 ℃ for 24 h. After reaction, the powder was washed and calcined at 550 ℃ for 2 h to obtain Zn2SnO4 octahedral.To prepare CaFe2O4 nanorods, 1 mmol CaCl2 and 2 mmol FeCl3 were dissolved into a mixed solution of ethanol (30 mL) and deionized water (10 mL) under stirring at room temperature for 30 min. The mixed solution was transferred into a 50 mL autoclave, heated at 180 ℃ for 12 h. After reaction, the powder was washed and dried at 80 ℃ for 8 h to obtain CaFe2O4 nanorods.To prepare CaFe2O4/Zn2SnO4 composites, 2 mmol Zn(CH3COO)2?2H2O and 2 mmol Zn(CH3COO)2?2H2O were dissolved in 70 mL deionized water. Meanwhile, 0.1 g CTAB (cetyl trimethyl ammonium bromide) and 15 mmol NaOH were dissolved in the solution. The as-prepared CaFe2O4 samples were added into the Zn2SnO4 reaction mixture, and the mixture was stirred for 40 min, transferred into a 100 mL Teflon-lined autoclave and heated at 180 ℃ for 24 h. After reaction, the powder was washed and calcined at 550 ℃ for 2 h to obtain CaFe2O4/Zn2SnO4 composites. At different addition amounts of CaFe2O4 (i.e., 0.006 3, 0.012 5 g and 0.018 8 g), the CaFe2O4/Zn2SnO4 composites with different mole fractions of 2%, 4% and 6% were obtained, and marked as 2%CaFe2O4/Zn2SnO4, 4%CaFe2O4/Zn2SnO4 and 6%CaFe2O4/Zn2SnO4, respectively.Results and discussion The X-ray diffraction patterns show that Zn2SnO4 is a perovskite structure and CaFe2O4 is a clinopyrite structure. The XRD pattern of CaFe2O4/Zn2SnO4 composites is close to that of Zn2SnO4, and no CaFe2O4 diffraction peaks appear, probably due to the small amount of CaFe2O4. For the Fourier transform infrared spectra of CaFe2O4/Zn2SnO4 composites, Ca-O, Fe-O and Sn-O bonds appear at 565, 478 cm-1 and 574 cm-1 in these composites, demonstrating that CaFe2O4/Zn2SnO4 composites are synthesized. In addition, compared with CaFe2O4 and Zn2SnO4, the characteristic absorption peaks of CaFe2O4/Zn2SnO4 composites all shift slightly to the right region, confirming the existence of interfacial contact between CaFe2O4 and Zn2SnO4. For the scanning electron microscopy and transmission electron microscopy images, Zn2SnO4 is an octahedral structure with a uniform size of approximately 350 nm, while CaFe2O4 has a nanorod structure with a length of approximately180 nm. For the X-ray photoelectron spectra, elements Zn, Sn, Fe, Ca and O coexist in CaFe2O4/Zn2SnO4 composites, and Zn2SnO4 incorporated with CaFe2O4 increases the oxygen vacancy defects by the Gauss peak division method. Compared with pure CaFe2O4, Zn2SnO4 and other CaFe2O4/Zn2SnO4 sensors, 4%CaFe2O4/Zn2SnO4 sensor exhibits a prominent gas sensing performance to ethanol (i.e., a favorable selectivity to ethanol, a low detection limit of 0.07 μmol/L, fast response/recovery time of 21 s/63 s, long-term stability, and high gas response (96) toward 40 μmol/L ethanol). The superior ethanol gas sensing performance of 4%CaFe2O4/Zn2SnO4 is attributed to the formation of CaFe2O4-Zn2SnO4 p-n heterojunctions, the high content of oxygen vacancy defects, and the increased surface electron density. Therefore, CaFe2O4-Zn2SnO4 p-n heterojunction composite has a great potential application for detecting ethanol gas. Conclusions Zn2SnO4 octahedral, CaFe2O4 nanorods and CaFe2O4/Zn2SnO4 composites were prepared by a hydrothermal method. 4% CaFe2O4/Zn2SnO4 based sensor exhibited the maximum gas response of 96 μmol/L to 40 μmol/L ethanol at 400 ℃, which was 2.6 times higher than that of Zn2SnO4 and 34 times higher than that of CaFe2O4. Moreover, 4%CaFe2O4/Zn2SnO4 based sensor also achieved a long-term stability, an excellent humidity resistance, fast response-recovery time (21 s/63 s) and a low theoretical detection limit of 0.07 μmol/L for ethanol. The enhanced gas sensing properties of 4%CaFe2O4/ Zn2SnO4 could be attributed to the following factors, i.e., CaFe2O4 coupled with Zn2SnO4 decreased the electron-hole recombination efficiency, and increased the surface electron density; and the formation of p-n heterojunctions between CaFe2O4 and Zn2SnO4 increased thickness of Debye electron layer, resulting in a drastic resistance change. The appropriate amount of CaFe2O4 coupled with Zn2SnO4 could be a promising strategy to enhance the sensing performance of ethanol, having great potentials in manufacturing high response and low detection limit of ethanol sensors.