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.

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.

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.

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.