78 · CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 1: GENERAL APPROACHES Table 2.6. Average work functions of some metal oxides ORK FUNCTION W O XIDE BaO CaO ZnO SnO 2 TiO 2 ZrO 2 SrTiO 3 V 2 O 3 Fe 2 O 3 NiO (eV) 1.6­2.7 2.5­2.8 4.2­4.9 4.3­4.7 5.1­5.9 5.0­6.6 4.2­5.2 4.9 5.4 4.3­4.65 Concentration of native surface states is another metal-oxide surface parameter that is important for chemical sensors, especially solid-state gas sensors. To achieve effective operation of solid-state gas sensors, the concentration of these states must be minimized, because in this case the surface Fermi level will not be pinned. A low concentration of surface states creates a possibility for modulating the surface