Nature of gas-sensing n-n heterojunctions designed in α-Fe2O3-CuFe2O4 composite nanofibers: A key role of oxygen vacancies

Heterojunction (HJ) nanofibers (NFs) incorporated with different semiconductor metal oxides significantly affect gas-sensing performance because of the formation of intimate electrical contact between n-n HJs. However, the nature of such n-n HJs in NFs has not been experimentally explored. In this paper, to better understand the influence of the n-n HJs formed by alpha-Fe2O3 and CuFe2O4 on the gas-sensing performance, we designed the alpha-Fe2O3-CuFe2O4 composite NFs and alpha-Fe2O3/CuFe2O4 mixed-NFs via the electrospinning technique. The gassensing characteristics of these materials were thoroughly evaluated to various reducing and oxidizing gases such as 0.1-1 ppm H2S, 1-10 ppm NO2, 25-250 ppm (CH3)2CO, and CH3CH2OH at 250-450 degrees C. The results indicated that establishing internal HJs in alpha-Fe2O3-CuFe2O4 composite NFs showed higher gas sensitivity than external HJs formed by the alpha-Fe2O3/CuFe2O4 mixed-NFs. In particular, by using the positron annihilation measurements, we explored the interfaces of the external HJs in alpha-Fe2O3/CuFe2O4 mixed-NFs formed by the weak interactions due to the Van de Waals force, whereas the strong interactions between ions/defects were determined at the interfaces of the internal alpha-Fe2O3-CuFe2O4 composite NFs. Furthermore, the alpha-Fe2O3-CuFe2O4 composite NFs contained the highest number of oxygen vacancies (VO) on the alpha-Fe2O3 surface, which led to the redistribution of electron configurations of Fe and Cu at the HJs. These factors may play key roles in the enhanced gas-sensitive properties of the alpha-Fe2O3-CuFe2O4 composite compared to the alpha-Fe2O3/CuFe2O4 mixedNFs. Furthermore, the classification of tested gases using these NF-based sensors was drastically improved with artificial intelligence algorithms.