Chemistry Reference
In-Depth Information
(0001)
(1102)
(1102)
(1102)
(1102)
(0001)
FIGURE 7.3
Bulk.unit.cell.of.hematite.(α-Fe
2
O
3
),.along.with.its.dominant.cleavage.planes,.
(0001). and. (1
‾
02).. The. oxygen. positions. are. shown. in. red. and. the. metal. ion. positions. are.
shown.in.blue.
pressure. and. temperature.. For. instance,. Sanchez. et. al.. found. that. α-Fe
2
O
3
. (0001).
exhibits.strong.temperature-dependent.surface.reconstructions,.with.(1.×.1).patterns.
observed.only.at.annealing.temperatures.of.1173.K.or.above,.and.the.formation.of.
a.Fe
3
O
4
.(111).layer.on.the.α-Fe
2
O
3
.(0001).surface.at.lower.annealing.temperatures..
In. contrast,. α-Fe
2
O
3
. (11
_
02). exhibits. strong. O
2
. partial. pressure-dependent. surface.
reconstructions,.with.(1.×.1).patterns.observed.at.high.O
2
.partial.pressures.above.10
−6
.
Torr,.and.(2.×.1).patterns.observed.at.low.O
2
.partial.pressures.down.to.10
−10
.Torr,
76
.
as.depicted.in.Figure.7.4..Unfortunately,.for.many.years,.no.theoretical.calculations.
were.available.to.verify.these.surface.reconstructions.
77
Clean.α-Fe
2
O
3
.(0001).contains.three.chemically.distinct.surface.terminations,.
depending.on.location.of.the.cleavage.plane:.a.single.iron.layer.(Fe-O
3
-Fe-R),.a.
double. iron. layer. (Fe-Fe-O
3
-R),. and. an. oxygen. layer. (O
3
-Fe-Fe-R).
78,79
. While.
the. highly. relaxed. single-layer. Fe-termination. (Fe-O
3
-Fe-R). is. the. most. stable.
surface. coniguration. under. ultrahigh. vacuum. conditions,
80
. ab. initio. DFT. calcu-
lations. helped. identify. a. ferryl. (Fe. double-bonded. to. oxygen,. Fe
‾
O). terminated.
surface.as.more.favorable.under.higher.oxygen.pressure
81,82
.as.well.as.a.possible.
pathway.for.creating.this.surface
83
.that.had.been.observed.via.scanning.tunneling.
(121)
(A)
(B)
(100)
FIGURE 7.4
. Schematics. of. the. LEED. patterns. obtained. on. the. α-Fe
2
O
3
. (1
‾
02). (1.×.1).
(A). and. (2.×.1). (B). surface. reconstructions.. (Adapted. from. Henderson,. M.A.,.
Surf. Sci
.,.
515,.253,.2002.)
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