Synthesis and structural investigation of polyoxovanadate systems for semiconductor nox sensing.

摘要

Nitrogen oxides (NOx) present in the atmosphere can cause several detrimental effects to the environment and health. Due to the advantages of semiconducting NOx sensors like low cost, easy to make, in-line operation and greater sensitivity, even though the higher operating temperatures and unable to customize the sensor limits its use, the study focused on semiconducting systems for the detection of NOx. To circumvent these limitations, a new class of metal oxide systems called polyoxovanadates, [CdH3 (H20)12VIv16V v2O36(OH)6(A04)]·24H 20, (A= V, S) (1), (NH4)8[V Iv12Vv6O42 (SO 4)]· 10H2O (2a) and (NH4) 6[V10O28·8H20 (3) were chosen. Considering the advantages (able to form variety of shapes, sizes, dimensions) of these polyoxovanadates, a series of experiments were designed to identify the suitability for the semiconductor NOx sensors that can be operated under ambient conditions. These experiments includes several molecular as well as atomic spectroscopy techniques and the state-of-art facilities provided by Argonne National Laboratory.;To identify the suitable polyoxovanadate material for semiconductor NOx sensing, various materials were screened by measuring the band gap, resistance and the sensing properties. As the interactions between the polyoxovanadate surface and oxygen during gas sensing are unavoidable, its effect on structure and property of the polyoxovanadate systems were studied. In relation to the structural changes, both 1 and 2, interacts with the oxygen probably due to the coordinative unsaturation of vanadium and results oxidation state increase from +4 to +5 and the coordination number change from 5 to 6. In property study, the decrease in band gap and resistance were observed with both the samples. The decrease in resistance and band gap with time could not be explained when oxidizing species interact with n-type semiconductor metal oxide surfaces. This anomaly was explained for vanadates as the increase in coordination number, due to oxidation, helps overlapping more O 2p and V 3d orbitals and resulting the low band gap. This result was validated by synthesizing the vanadate and measuring the resistance of (NH4) 2[Ni(H2O)5(NH3)]2[V 10028]·4H2O (4) which contains {V06} units. In NOx sensing studies, it was concluded that 1 shows better sensitivity, response time and reversibility than all other materials under ambient conditions.