In recent years, nanomaterials with heterojunction structures which is conductive to the enhancement of sensing performance of a single component, have attracted wide attention in the field of gas sensors due to the synergistic effects induced by the coupling and the heterointerface between two different classes of nanomaterials . As heterostructured nanomaterials have strong heterointeraction between the closely packed interface nanounits, their properties cannot be considered as a simple contact of the individual components, but more complex and more superior . As a p-type semiconductor with an energy gap of 4.2 eV, there have been a large number of researches focusing on the gas sensing characteristics of different NiO-SnO2 nanostructures, and results have exhibited enhanced sensitivities compared to the single component [3-5]. However, the discussion about the interface bonds at the p-n heterointerface that can facilitate electron transfer is not detailed.
In this report, an acetone gas sensor based on hierarchical NiO-SnO2 nanofibers was fabricated by electrospinning. XRD results showed that NiO-SnO2 was formed without any other impurity peaks. Hierarchical nanofibers composed of tiny nanocrystals were clearly observed from the SEM images. Meanwhile, the sensing response of the NiO-SnO2 heterostructure manifested as NiO do, namely p-type response. In addition, electrochemical impedance spectroscopy was also examined to demonstrate the differentiation in the interface resistance of the NiO-SnO2 nanofibers.
2. Experimental Details
In a typical procedure for preparing NiO-SnO2 composite nanofibers, precursor solution with a molar rate of 1:1, 0.36 g SnCl2·2H2O and 0.464 g Ni(NO3)2·6H2O were dissolved in 4.7 mL of EtOH solvent at room temperature with magnetic stirring for 30 minutes. Subsequently, 0.824 g PVP and 3.9 mL of DMF were added into above solution, agitating for 24 h until a viscous emerald green precursor solution completely formed. The electrospinning parameters were as follows: the voltage and the distance between the needle (positive pole) and the collector (negative pole) were 20 kV and 15 cm, respectively. The ambient temperature and relative humidity were 14.5∘C and 33 %RH. The diameter of the needle was 0.7 mm. In order to decompose PVP completely, the as-synthesized nanofibers were sintered at 600∘C for 3 h in air with a slow heating rate of 1∘C/min. Fig. 1 illustrates the complete schematic of NiO-SnO2 composite nanofibers based gas sensors by simple electrospinning.
3. Characterization Results
Fig. 2 (a) and (b) display the SEM images of NiO-SnO2 nanofibers with low magnification and high magnification, respectively. It can be seen that after calcination, the surface of the nanofibers becomes rough and is self-assembled by numerous graded nanoparticles.
XRD patterns of NiO-SnO2 nanofibers are shown in Fig. 3. It can be seen that the NiO-SnO2 composite nanofibers have two crystalline phases of tetragonal SnO2(JCPDF#99-0024) and cubic NiO (JCPDF#73-1523) simultaneously, indicating that the calcination temperature of 600∘C is sufficient to remove PVP completely and form the two oxides.
4. Gas Sensing Properties and Mechanism
Fig. 4 (a) shows that the gas sensor based on NiO-SnO2 nanofibers has a maximum gas response at the operating temperature of 275∘C, while the sensor based on NiO shows highest responses at 325∘C. As a result, 275 o C and 325 o C were respectively selected as the operating temperature for NiO-SnO2 and NiO sensors in the following gas testing process. From Fig. 4 (b) it can be clearly seen that NiO-SnO2 exhibits a better selectivity than NiO, having a preferential response to acetone. It implies that NiO-SnO2 could be used as a very promising candidate for selective acetone detection. The transient acetone sensing characteristics of NiO and NiO-SnO2 in a range of 20-75 ppm at their own optimal operating temperatures are shown in Fig. 4 (c) and (d), respectively. Furthermore, the long-time stability of NiO and NiO-SnO2 are also measured. Both sensors exhibit good stability towards 20 ppm acetone in 60 days, as shown in Fig. 4 (e).
The enhanced acetone sensing properties of NiO-SnO2 nanofibers may be ascribed to the formation of p-n junction between p-type NiO and n-type SnO2 nanograins. The energy band structure of the NiO-SnO2 heterojunction is shown schematically in Fig. 5 (b), without taking into account the interface states. In order to obtain equalization of Fermi levels, a relative motion of carriers, namely electronics flowing from SnO2 to NiO, while holes in the opposite direction occur in the physical interface between p-type NiO and n-type SnO2, resulting in band bending. At the same time, an electronic depletion layer has been formed on the surface of SnO2 while an electronic accumulation layer on the side of NiO, as shown in Fig. 5 (b). Before introducing acetone, the oxygen molecules in air will capture electrons from the conduction bands of both SnO2 and NiO. The adsorption of oxygen results in the further widening of electron depletion layer and hole depletion layer on the surface of SnO2 and NiO, respectively. Therefore, compared with bare NiO, the NiO-SnO2 nanofibers perform with a much higher initial resistance due to the p-n heterojunction effect. Upon exposure to acetone gas, the acetone molecules react with chemically adsorbed oxygen ions and release the captured electrons back to Ni vacancies; thereby the resistance of the NiO-SnO2 nanofibers increases. Herein, according to the significantly increased initial resistance and the degenerated in the equivalent hole concentration of the NiO-SnO2 nanofibers due to the p-n heterojunction effect, the sensitivity of the NiO-SnO2 nanofibers toward acetone will obviously improve.
In order to obtain as more p-n heterojunction as possible, heterojunction nanofibers with 1:1 molar ratio of NiO to SnO2 were fabricated via simple electrospinning technique. It was found that the NiO-SnO2 nanofibers exhibited an enhanced p-type response to acetone compared to bare NiO. The p-type sensitivity of NiO-SnO2 nanofibers may be ascribed to the more NiO content than SnO2. Basing on the heterojunction theory, the initial resistance of NiO-SnO2 nanofibers is higher than that of bare NiO due to the equalization of different Feimi levels. Meanwhile, the built-in electrical field at the heterujunction can effectively block the acetone adsorption-induced local electrons in the SnO2 to NiO, which contributes to the increasing of equivalent hole concentration in the NiO-SnO2 nanofibers and then leads to the improvement of sensitivities.