Sinclair- The Case of TiO and BaTiO3 Flashcards
Useful properties of TiO2
Photo-catalysis, electrolysis of H2O, pigment, dielectric
Why should TiO2 be written as TiO2+-x?
Because of the existence of both n and p type behaviour observed under 1000°C.
What influences the value and distribution of x?
Purity, sintering atmosphere and cooling conditions
Graph of resistance vs -log(pO2)
Resistance on a log scale, -log(pO2) goes from 30 to 0 left to right. Shape is a dome. The dome moves lower and the peak moves right for increasing temperature. Left of the peak is O loss so n-type TiO2-x and right of peak is O gain so p-type TiO2+x. At the peaks of resistance there is stoichiometric TiO2
What are the intrinsic point defects in rutile (TiO2)?
Oxygen vacancies, Ti vacancies, interstitial Ti ions
What are trivalent Ti and monovalent O ions equivalent to
Trivalent Ri (Ti3+, d1) equivalent to quasi free electron
Ti(Ti)’=e’
Monovalent O ions (O-, p5) equivalent to quasi free hole
Oo•=h•
The 5 main defect equations (descriptions) that dominate the defect chemistry
Conventional O loss with partial reduction of lattice Ti to Ti3+.
Interstitial Ti4+ ions with O loss resulting in partial reduction of lattice Ti to Ti3+.
Interstitial Ti3+ ions with O loss resulting in further reduction of lattice Ti to Ti3+.
Intrinsic generation of charge carriers and no change in O content.
Oxygen gain on surface creating metal vacancies with partial oxidation of some lattice O2- to O-
Conventional O loss with partial reduction of lattice Ti to Ti3+.
Oo^x -> 1/2O2(g) + Vo•• + 2e’
2e’ + 2Ti(Ti)^x -> 2Ti(Ti)’
Slope of -1/6
Interstitial Ti4+ ions with O loss resulting in partial reduction of lattice Ti to Ti3+.
Ti(Ti)^x + 2Oo^x -> VTi”” + Ti(i)•••• + 2Vo•• + O2(g) + 4e’
4e’ + 4Ti(Ti)^x -> 4Ti(Ti)’
Slope of -1/5
Interstitial Ti3+ ions with O loss resulting in further reduction of lattice Ti to Ti3+.
Ti(Ti)^x + 2Oo^x -> VTi”” + Ti(i)••• + 2Vo•• + O2(g) + 3e’
3e’ + 3Ti(Ti)^x -> 3Ti(Ti)’
Slope of -1/4
Intrinsic generation of charge carriers and no change in O content.
nil -> e’ + h•
pO2 independent
Oxygen gain on surface creating metal vacancies with partial oxidation of some lattice O2- to O-
O2(g) -> 2Oo^x + VTi”” + 4h•
4h• + 4Oo^x -> 4Oo•
Slope of +1/5
The real formula of Ti2+-x with 8 species
(Ti(Ti)^x)a(Ti(Ti)’)b(VTi””)c(Ti(i)••••)d(Ti(i)•••)e(Oo^x)f(Oo•)g(Vo••)h
Which defect is useful for electrolysis of H2O?
The VTi”” has been identified as a surface active site able to provide holes to adsorb H2O and enable splitting into H2 and O2
3 main conditions for predominant defects
x<0: O vacancies, Ti interstitials, electrons, n-type TiO2-x.
x=0: intrinsic TiO2 (Eg roughly 3.2eV)
x>0: excess O, Ti vacancies, holes, p-type TiO2+x