Surface grafting as a route to modifying the gas-sensitive resistor properties of semiconducting oxides: Studies of Ru-grafted SnO2

Abstract

The phenomenon of electrical conductivity of an oxide being controlled by the chemical state of a surface grafted reactive centre, resulting in a room-temperature gas response, is demonstrated. Surface grafting of Ru centres onto SnO₂ by reaction of [(η⁶-C₆H₆)Ru(acetone)₃][BF₄]₂ with surface OH, followed by thermal decomposition in either H₂/N₂ or air resulted in additional electronic states in the SnO₂ band gap associated with surface Ru species, revealed by XPS and correlated with resistance increase of the material. The samples were characterized by EXAFS to confirm the structure of the surface Ru species, TPD, UV–VIS spectroscopy, XPS and electrical measurements. Thermal decomposition in H₂/N₂ resulted in isolated Ru centres bound to SnO₂. The chemical state of these Ru centres varied as a result of gas chemisorption (oxygen, water vapour, carbon monoxide and nitric oxide). An electronic interaction between grafted Ru centres and the SnO₂ support was manifested in room-temperature conductivity being controlled by the surface state of the Ru and by gas adsorption onto these centres. Decomposition in air resulted in small surface-bound clusters of RuO₂ and did not result in a gas-sensitive preparation. DFT molecular cluster calculations successfully illustrated the main features of the experimental results, supporting the interpretation that the conductivity changes were caused by gas interaction with surface Ru centres altering the trapping of conduction electrons on these centres. Oxygen, NO and CO were non-dissociatively adsorbed. The Ru centres slowly oxidised in air and hydroxylated in the presence of water vapour. Carbon monoxide and NO in ppm concentrations in air displaced oxygen from the Ru centres but the electrical response was strongly affected by the presence of water vapour in the atmosphere. In principle, surface-grafted reactive centres can be chosen to be specific to a particular gas, providing a route to new types of gas detector tailored for a particular application.