might not be marked or might be absent, as observed on the Ti0 2 branch.
Tn our previous discussion o f the Sn0 2 (n-type)-CrNb0 4(p-type)-Ti0 2(n-type) series (Chapter 7. and [7]), we interpreted the p-n transition on the Ti0 2 branch to the energy o f the oxygen surface state moving closer to the conduction band edge as the Ti content increased. For the discussion o f the behaviour o f the Sn0 2-Ti0 2 series in Chapter 7., again, it was necessary to postulate that the energy o f the oxygen surface state moved closer to the conduction band edge as the Ti content increased. These statements are equivalent to postulating a marked change o f K3 with Ti0 2 content,
and so are consistent with the interpretation presented here for the difference in behavior between Ti0 2 &"d Sn0 2 branches. The interpretation o f the variation in conductance with composition presented above is also consistent with the postulate
that K3 decreased (that is, the energy difference between conduction band edge and
surface acceptor state energy increased) with increase o f wolframite concentration.
It remains to discuss the relative selectivity Sco/Sch4 , Sco/Snh3 observed, and the different effects o f water vapour along the different series in comparison to the effects o f the reactive gases. The notable effects on selectivity are the maximum in Scq/Sch4
for the Fe and Zn series along the SnOi branch, the uniformly higher selectivity Scq/Sch4 for the Cu series across the whole range and the maximum in Scq/Snhs for the Zn series along the TiOi branch. Previous work has shown that the selectivity o f porous pellets can be altered as a consequence o f changes in the rate o f surface catalysed combustion o f the different gases. To explain the observed maximum in this way would require that the combustion rate o f CO was drastically decreased for these particular compositions. Another explanation might be that there are two different types o f oxygen surface state mediating the response, and that, for the series showing the maxima, the relative proportions o f these two states change. This last interpretation is supported by comparing surface concentration o f the Co and Fe series along the Sn0 2 branch which show different behaviour in the way surface composition changes with bulk composition.
As stated earlier, all the reactive gases were consistent in their classification o f the materials as n- or p-type. However, the sign o f response to water vapour was not always consistent. Overall, the Co-, Cu- and Zn- based materials response to water behaved as expected as n-type or p-type materials. Clearly, there were some differences between water and the other gases for Mn-, Fe- and Ni-based compounds which is certainly a result o f technological importance. The sign o f the resistance change associated with adsorption o f water by the oxides can be discussed [6, Chapterô] in terms o f the surface hydroxide trap energy; in particular its position relative to the Gads' trap state. For the n-type oxides which exhibit a resistance decrease upon exposure to water vapour or the p-type oxide which exhibit a resistance increase, the effect could be explained by assuming dissociative adsorption o f water with the OHads trap state produced lying higher in energy than the Gads.
The opposite behaviour, where materials which otherwise behave as p-type give a resistance decrease with water n-type materials give a resistance increase means that the OHads state must lie lower in energy than Oads The fact that the introduction o f some transition metal ions causes a change in sign o f response to water vapour without causing a change in sign o f response to other gases implies that there are surface binding sites for water vapour which are specifically associated with these substituents. Fig 8.9 shows that the behaviour may be associated with the surface excess o f tungsten over tin, and there is the possibility that tungsten could be present on the surface in a lower oxidation state. In this sense, the results parallel those found for the effects o f Sb(IIT) segregation, in Chapter 3.
Since the compounds that contain the wolframites exhibit smaller sensitivity to water than Sn0 2 (typically Sh2o ^ 0.5), the energies o f the acceptor states Oads* and OHads on the substituted compounds must be closer together than for pure SnOi.
8.5. Conclusion
The response o f gas-sensitive resistors fabricated from solid solutions compounds o f W olffamites-based [(MW0 4)x([Sn-Ti]0 2)i.x> 0<x<l and M; Mn, Fe, Co, Ni, Cu, Zn] to carbon monoxide, methane, ammonia and water have been studied. The conductance and the activation energy for conductance variation with composition associated with the energy and number density o f 3d-transition metal states depend upon the nature o f the transition metal ion and its concentration. The resistivity behaviours o f all the compounds are believed to be due to eg states acting as donors leading to the oxidation o f cations. The activation energy for conduction gives an indication on where the localised d-states lie with respect to the conduction or valence band. Difficulties in interpreting the results arise upon surface segregation o f one o f the constituent The surface trap-limited conductivity model can explain, at least partially, the switch from n- to p- type response to reducing gases if assumptions are made about the local state occupancy and the energy o f these states with respect to the energy associated with O adsorbed on the surface. The sign o f response to water was not always consistent with that expected with the other reactive gases, which clearly represents a result o f technological importance. The sign of
resistance change associated with adsorption o f water can be explained in terms o f the surface OH trap energy relative to the 0^^* trap state. This observation implies that there are surface binding sites for water vapour which are specifically associated with particular transition metal ions.