In this work we have sought to demonstrate the possibility o f modulating the gas sensing characteristics o f semiconducting oxides by functionalizing the surface with particular reactive centres which, being equivalent to surface electronic states positioned within the band gap, communicated electronically with the oxide so that variations in their chemical state due to gas chemisorption could be transduced into an electrical conductivity change at ambient temperature. Surface grafting techniques were used to provide reactive Ru and Ti surface centres connected electronically with the semiconducting oxide supports. Surface modification with Pt was performed by a simple wetness impregnation procedure. In general, surface engineering o f SnO2-0.2%Sb with Ru, Pt, Ti; o f TiO2-10%Nb with Pt, Ti; and o f BaSno.9 7Sbo.0 3O3 with Pt in each case induced a room temperature gas sensitivity o f the electrical conductivity with particular differences specific to each system. Grafting o f Ti onto n-type SnO2-0.2%Sb induced a p-type resistance response o f the material to CO at room temperature. The Ti modified TiO2-10%Nb showed room temperature gas response stable to variations in the water vapor pressure. The effect o f the room temperature gas sensitive electrical conductivity from surface grafting o f specific reactive centres onto oxide surface was demonstrated for the first time.
In particular Ru centres were anchored onto SnO2-0.2%Sb surface
via
an interaction o f organometallic Ru precursor with the surface functional groups on the tin dioxide support, followed by decomposition o f the grafted complex to drive off the organic component. Decomposition in hydrogen produced reactive surface bound Ru centres in electronic contact with the tin dioxide support. Formation o f surface bound Ru centres was confirmed by EXAFS and the electronic interaction was revealed by XPS, where Ru was shown to introduce states within the tin dioxide band gap lying 1.3eV above the valence band edge, and also by resistivity measurements, since the effect o f SnO2-0.2%Sb surface modification with Ru was to increase the room temperature resistivity o f the material.The atmosphere induced variations in the electrical conductivity o f the Ru modified Sn02- 0.2%Sb were correlated with variations in the chemical state o f the attached Ru, caused by
gas chemisorption. TPD studies indicated gas interactions with the attached unsaturated Ru centres at room temperature: adsorption o f CO, NO and oxygen was demonstrated.
Adsorption o f CO and NO onto surface bound Ru centres resulted in a resistance decrease, whereas atmospheric oxygen interaction with the reactive Ru centres caused a positive baseline resistance drift induced by partial oxidation o f the Ru. XPS studies revealed partial oxidation o f the surface Ru on interaction with atmospheric oxygen. Exposure o f the Ru modified sample to moisture in air resulted in a further resistance increase induced by further oxidation o f the Ru. The effect o f moisture on the electrical conductivity was shown to be irreversible. XPS analysis indicated formation o f the hydroxylated Ru^^ surface species and the EXAFS revealed the presence o f four-coordinated Ru"*^ centres on the surface o f the hydroxylated sample. Strongly bound OH groups inhibited subsequent gas interactions with the Ru surface centres and as a result the room temperature gas response was lost in
ambient air. We have further demonstrated by EXAFS that decomposition in air o f the grafted Ru organometallic complex resulted in the formation o f the bulk RUO2 particles on the SnO2-0.2%Sb and in this case the gas sensitivity o f the electrical conductivity was lost. W e indicated that if the unsaturated reactive surface centres are destroyed by thermal oxidation, the electrical effects are also destroyed.
DFT molecular cluster calculations on SnnOmHx clusters, chosen to model the Ru modified Sn02 (110) surface, showed that electronic interaction between the attached Ru and the model cluster support occurred through surface oxygen atoms due to a direct Ru-O(-Sn) chemical bonding. Formation o f R u -0 trap orbitals, positioned in the energy gap above the cluster HOMO were identified. These molecular orbitals were largely localized on the attached Ru atom with partial surface oxygen character. Interaction o f gaseous molecules with the attached Ru resulted in variations in the R u -0 trap orbital composition. Changes in the degree o f localization o f the R u -0 orbital on the surface Ru atom were related to experimentally observed changes in electrical conductivity in response to gas adsorption onto grafted Ru centres.
For the Pt decorated SnO2-0.2%Sb the electrical conductivity at room temperature was demonstrated to be controlled by the chemical state o f the supported Pt. Exposure o f the sample which exhibited only metallic Pt species to moisture in air caused partial oxidation o f the surface Pt° to Pt^^, which was detected by XPS. Partial oxidation o f the supported Pt® on moisture exposure was reflected in the resistance increase o f the material. The effect o f the room temperature gas sensitivity o f the electrical conductivity was demonstrated to be specific to the presence o f Pt^^ surface species. Supported metallic Pt was shown to be a good room temperature CO oxidation catalyst, but the material decorated with Pt® showed no CO resistance response. The effect o f the room temperature CO response was assigned to specific CO chemisorption onto Pt^^ sites, resulting in partial charge transfer and
consequently reduction o f the supported Pt, measured as a resistance decrease.
For the Pt decorated BaFeOs-x the conductance response mechanism was dominated by electronic properties o f the support, rather than the surface Pt, as was indicated by XPS. In this case the gas sensitivity o f the electrical conductivity at room temperature was lost.
In conclusion we have shown that surface tailoring o f semiconducting oxides with organometallic precursors can be employed to achieve gas sensitivity o f the electrical
conductivity at room temperature. In this case the electrical conductivity can conceivably be used to monitor changes in the chemical state o f the grafted centre caused by gas