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SYNTHESIS AND STRUCTURAL INVESTIGATION OF POLYOXOVANADATE SYSTEMS FOR SEMICONDUCTOR NOx SENSING
Title
SYNTHESIS AND STRUCTURAL INVESTIGATION OF POLYOXOVANADATE SYSTEMS FOR SEMICONDUCTOR NOx SENSING
Creator
Ravikanth Putrevu, Naga
Date
2013, 2013-12
Description
Nitrogen oxides (NO and NO2 collectively termed as NOx) are the common air pollutants produced from various indoor and outdoor activities. It...
Show moreNitrogen oxides (NO and NO2 collectively termed as NOx) are the common air pollutants produced from various indoor and outdoor activities. It can cause several environmental problems like ground-level ozone, acid rain, particulate matter, smog formation and health problems like malfunctioning of lungs, damaging air flow passages and creating respiratory problems. To detect these gases, sensors with greater sensitivity, lower recovery time and longer life time are needed. Despite the advantages, of the most widely used NOx sensing materials, semiconducting metal oxides have the limitations like higher operating temperatures and unable to customize the materials with better sensitivity and selectivity. So the current research discusses about a new class of metal oxide systems called polyoxovanadates (POV) and its suitability for the ambient operating conditions. These POVs are basically the vanadium oxide clusters synthesized in top-down and bottom-up synthesis procedures. The main advantages of these systems are possible to synthesize in various sizes, shapes, geometries and dimensions which can help identifying the suitable material for a specific gas. The experimental part discusses about the synthesis, characterization and semiconducting sensing properties of different POV systems using various atomic and molecular spectroscopic techniques. In Chapter 2, a 3-D open framework polyoxovanadate material, [Cd3(H2O)12V16 IVV2 VO36(OH)6(AO4)]·24H2O, (A=V,S) (1), composed of secondary building blocks ({V18O42(AO4)} (A=V,S) clusters) interconnected by {-O-Cd-O-} bridging groups, was chosen for understanding the effect of atmospheric oxygen on 1 electronic properties like band gap and resistance, considering the oxygen as a common xiv interferent in ambient gas sensing. The results explain that 1 interacts with oxygen during the aging process and results changing the vanadium oxidation state from +4 to +5 and expanding the coordination sphere of vanadium from 5 to 6. This expansion can alter the band gap by overlapping the more number of O 2p orbitals with V 3d orbitals and resulting the decrease in resistance. The experiments conducted on molecular {V18O42(AO4)} (A=V,S) cluster, (NH4)8[VIV 12VV 6O42(SO4)]·10H2O, (2a), in Chapter 3, also shown the similar change in band gap and resistance during aging process. Based on the results obtained in Chapter 2 and Chapter 3, a {VO6} containing vanadium oxide cluster, (NH4)2[Ni(H2O)5(NH3)]2[V10O28]·4H2O (4), was synthesized and proved in Chapter 4, that the higher coordination number help lowering the band gap and the resistance without need of aging. In Chapter 5, the NOx sensing properties of 1, 2a, 3 and 4 were studied and compared with well studied NOx sensing material, WO3. Based on the sensing characteristics, 1 shows good sensitivity and response time towards both NO and NO2 under ambient conditions than all the other materials. The FTIR spectroscopy results indicated that the NO2 sensing mechanism occurs via nitrate formation.
PH.D in Chemistry, December 2013
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