The present work derives its motivation and approach from the reported modified gas sensing behavior o f finely porous sintered SnOz decorated with sufficiently small particles o f Pt, equivalent to surface trap states controlling the conductivity. In the first part o f this work which is outlined in Chapter III, the room temperature conductance response mechanism in this system is examined in detail.
Although supported platinum has been widely used as a promoter o f the gas sensing
characteristics little is known about the role o f oxidized platinum surface species in the gas sensing action. Henshaw, Ridley and Williams [109] reported the study o f the effects o f Pt particle size, dispersion and the surface state o f the Pt on the room temperature gas sensing behavior o f Pt decorated SnO:. In the final stage o f the sensor preparation, the
decomposition temperature o f the precursor Pt complex was varied between 300 and 800°C. In this way variation in Pt particle size and chemical state was obtained simultaneously, the latter being determined by XPS. The effect o f Pt surface additives was to increase the base line resistance o f Sn0 2 in air at room temperature. P t was shown to introduce states in the band gap o f SnOz above the valence band edge and induce an increase in the band bending. Hence an electronic interaction between Pt additives and SnOi support was proposed. The authors showed that CO sensitivity at room temperature decreased with an increase in particle size o f the Pt crystallites and the effect was lost when the latter exceeded 8nm. Increase in the Pt particle size with decomposition temperature coincided with variation in the Pt spéciation. In the active sensor the chemical state o f the surface Pt was shown to be a mixture o f Pt® and P t(0 H )2.. The relative amounts o f both species varied with
decomposition temperature, as did the Pt particle size. In the developed model the variations in the room temperature CO sensitivity o f the electrical conductivity was assigned to the increase in the Pt particle size. The effect o f variation in the Pt spéciation, therefore,
remained unclear. Catalyzed combustion was recorded for CO over the active sensor. Since both Pt® and Pt(0 H) 2 were found on the surface o f the active sensor both species were proposed to induce the combustion o f CO at room temperature.
In the work reported in this thesis we obtain variation in the P t spéciation independently o f the particle size and show the sensitivity o f the electrical conductivity to the surface state o f Pt. We demonstrate that the materials which exhibit only Pt° species catalyze CO oxidation at room temperature, but show no CO sensitivity o f the electrical conductivity at room temperature. We show that the room temperature CO response is due to CO adsorption onto specific Pt(II) surface sites.
The second part o f this work, described in Chapter IV, is concerned with surface modification o f SnOi
via
surface-grafting o f Ru active centers. We give a method for surface grafting o f Ru onto SnOz, characterize the grafted centers, demonstrate electrical conductivity changes at room temperature caused by gas adsorption and correlate this with changes in the chemical state o f the surface grafted Ru determined by X-ray photoelectron spectroscopy.The structure and reactivity for CO hydrogenation o f the Ru centers derived from Ru3(CO)i2 supported on various inorganic oxides was described by Asakura
et al
[122]. They showed that in the case ofR u/SiO i following decomposition in H2 at 500®C small supported metallic Ru particles were formed and no apparent Ru - O(surface) bonding was found. An average particle size was determined as Inm with nuclearity o f 12. In the case o f AI2O3/RU, however, supported Ru clusters were formed which were characterized byRu-O-Al bonding and low coordination number o f 3 .1. Hence Ru6-fi"amework surface bound clusters were deduced. They showed that chemical interaction between metal species and support through the R u -0 - support bonding was an essential factor in the activity and selectivity o f Ru catalysts and proposed that such interactions controlled local structure and electronic states o f metal sites. The Ru-O-support interaction was taken to control the oxidation state o f Ru atoms and it was pointed out that all Ru atoms in small clusters constitute highly unsaturated Ru sites. The preferential formation o f higher
hydrocarbons (> € 5 ) over the sample was attributed to the fact that CO can multiply adsorb on such unsaturated Ru sites.
Resent work by Chaudhary a / [123] described the performance o f Ru modified tin dioxide sensors. The surface o f tin dioxide was functionalized with ruthenium oxide via immersion o f sintered SnOi pellets in a dilute solution o f ruthenium chloride in isopropanol/water, drying and sintering for 2 hours at 400-650®C in air. On the basis o f TGA analysis they claimed formation o f strongly anchored Ru species on the surface o f tin dioxide. The
formation o f well dispersed particles o f RuOi was, claimed on the basis o f SEM images and the oxidation state o f Ru was identified by XPS, corresponding to Ru02 with some Ru metal. The gas sensitivity o f the samples to LPG (liquid petroleum gas),H2, NH 3, H2S and CO was examined at 300®C. The surface-ruthenated Sn02 showed high sensitivity and selectivity to LPG at this temperature. RUO2 on the surface was taken to act as a catalyst to enhance the oxidation rate o f a combustible gas.
Matsushima
et al
[84] described the use o f the fixation method to achieve highly dispersed nanometer Pd particles on Sn02. The Pd chloro-complex [PdCU]^' ( prepared by dissolving PdC b and NH4CI at 1:2 in water) was anchored on the surface o f Sn02 via reaction with thesurface hydroxyl groups:
2-
[P d C U f + 20 H s 2NH >► (2NH4O + 2HCL
The fixed Pd species were subsequently converted to metallic Pd by heating at 300®C in hydrogen. Final pelletized samples were calcined at 600°C for 5h to convert supported Pd to PdO. The mean particle size, identified by TEM was found to be 2nm with a very narrow
size distribution. Sensors showed high sensitivity to hydrogen with maximum at lOO^C. An increase in the base fine resistance was also recorded for Pd loaded samples. An electronic interaction between Pd and SnÛ2 support was proposed and the gas sensitivity at 100®C was assigned to variations in the oxidation state o f the supported Pd caused by interaction with the target gas.
Attached metal centers prepared by using the reaction between an organometalhc complex and the support surface, often exhibit specific reactivities different from those o f the corresponding supported metallic particles or oxides [124]. In this work we have used a monomeric Ru aryl precursor in acetone solution to functionalize the SnOi surface. We show that as a result o f decomposition o f the SnOi- anchored Ru aryl complex in H2 at 300°C surface bound unsaturated Ru clusters form. Grafting o f unsaturated Ru clusters onto SnOi induced room temperature gas sensitivity o f electrical conductivity. We also show that on formation o f supported RuOi particles the effect is lost.
To complement the study o f the gas sensing behavior o f surface-ruthenated SnO:, molecular cluster modeling, based on density functional theory (DFT) [125,126], was performed. This is outlined in Chapter V o f this thesis. Trends in the behavior o f molecular clusters, chosen to model the Ru modified SnOi (110) surface, are employed to shed some light on the origin o f the conductance response in the Ru/SnOi system. In the simulations we did not attempt to calculate the actual electronic structure o f SnOi, but rather to probe trends in electrical behavior o f the material induced by Ru surface grafting.
Over the last four years first principles quantum techniques based on density functional theory have been apphed to solid-state problems for oxides and have made important contributions to understanding o f oxide surfaces including M 0O3[127,128], V2O5
[127,129], Ti02 [130,131] and Sn02 [131,132]. The approach is known to give accurate results for the energetics o f lattice defects, surfaces and molecular adsorption [126,131]. Application o f cluster model studies based on DFT to modeling o f the surface defect sites [127-129] and bonding o f molecules on to surface sites [129,133,134] has been reported. The study o f electronic structure and bonding o f non-stoichiometric lithium oxide clusters Lin02 (n < 10) by means o f DFT molecular cluster modeling was reported by Finocchi
et al
[135]. DFT is a theory o f the ground state and is designed to give accurate results for the valence electron distribution [125]. On the other hand it is known that DFT generally underestimates band gaps, as it is not designed to describe the excited-state properties [136]. However, as was pointed out by Finocchi
et
or/. [135] it gives information onthe nature o f the conduction states and on the overall stability o f the cluster models. In this work the stoichiometric hydroxylated Sn0 2(l 10) surface was modeled by molecular clusters o f increasing size, SnnOmHx, and the variations in the electronic structure induced by the attached Ru were ascertained qualitatively by means o f density functional methods. Finally the problem o f adsorption o f gaseous molecules onto attached Ru has been
addressed.
In the final part o f this work, outlined in Chapter VI, we set about to test the generality o f the effect o f modified gas sensing behavior in semiconducting oxides induced by surface modification. The argument developed in this work suggests that modification o f an oxide surface with reactive species, alternative to electronic trap states positioned in the gap, induce the modified gas sensitivity o f electrical conductivity. It then follows that such effect would be observed for any alternative semiconducting oxide, providing the electronic properties o f the supported species exercise dominating control over the charge carrier density.