Chemistry Reference
In-Depth Information
high.processing.costs.of.silicon.and.the.low.eficiency.of.non-oxide.materials.
115-118
.
The. main. objection. to. the. use. of. oxide. materials. is. the. large. band. gap. of. these.
materials,.since.the.conversion.eficiency.of.metal.oxide.semiconductors.is.highly.
dependent. on. photon. absorption. and. the. promotion. of. an. electron. from. the. illed.
valence. band. to. the. empty. conduction. band.. Materials. absorb. all. photons. in. the.
solar.spectrum.with.energies.greater.than.or.equal.to.the.band.gap.in.order.to.cre-
ate.excited.electron-hole.pairs.for.initiating.photochemistry..Since.most.metal.oxide.
semiconductors. have. band. gap. values. greater. than. 3.0.eV,. they. can. only. absorb. in.
the.ultraviolet.portion.of.the.spectrum..Thus,.much.attention. has.been.focused. on.
doping.the.metal.oxide.materials.with.both.nonmetal.and.transition.metals,.and.on.
constructing.layered.thin.ilm.devices.with.multiple.junctions.for.capturing.more.of.
the.solar.spectrum.
Titanium. dioxide
119
. has. received. most. of. the. attention. from. researchers. using.
DFT.to.study.the.effect.of.doping.on.the.electronic.band.structure.and.projected.pho-
tovoltaic.performance..For.instance,.both.oxygen.vacancies
120
.and.carbon
121-123
.and.
nitrogen
124-128
.dopants.have.been.shown.to.increase.the.visible.light.photoresponse.
of. TiO
2
.. Transition. metals,. such. as. vanadium,
129,130
. iron,
130-132
. and. chromium
133,134
.
have.also.been.shown.to.enhance.the.photoresponse.of.TiO
2
..DFT.calculations.have.
been.particularly.valuable.for.investigating.changes.in.the.electronic.band.structure.
of. TiO
2
. upon. nonmetal. and. transition. metal. doping.. The. indings. of. these. studies.
and.others.suggest.that.doping.introduces.additional.energy.levels,.or.midgap.states,.
into.the.TiO
2
.band.gap.so.that.electrons.may.be.excited.from.the.valence.band.to.the.
conduction.band.in.a.two-step.process,.thus.lowering.the.energy.required.and.shift-
ing.the.absorption.to.the.visible.spectrum..Much.theoretical.research.is.still.being.
performed.on.doped.oxides.as.well.as.on.mixed.metal.oxides.with.improved.photo-
response.through.the.narrowing.of.the.band.gap,.and.not.just.by.the.introduction.of.
midgap.states..Several.of.these.compounds.include.the.Ti
−
Fe
−
O,
132,135
.Ti
−
Cu-O,
136
.
and. Ti
−
W
−
O
137
. systems. for. improved. photocatalytic. fuel. production. compared. to.
their.basic.compounds.
7.4 SUMMARY AND FUTURE DIRECTIONS
As.mentioned.previously,.the.physics.of.the.system.to.be.studied.often.dictates.the.
level.of.theory.and.model.system.required.to.address.the.problem.of.interest..Within.
DFT.applications,.challenges.remain.in.applications.where.nonbonding.interactions,.
i.e.,.van.der.Waals.forces,.play.a.signiicant.role..Functionals.and.extensions.of.the.
basic.density. functional. approaches. that. attempt. to. address. these. issues. are. active.
areas.of.research..Additionally,.system.sizes.above.several.hundred.atoms.generally.
require.novel.techniques..Time-dependence,.extreme.conditions,.and.high.coverage.
continue. to. be. some. of. the. primary. computational. challenges,. but. are. developing.
areas.of.research.and.application.
Another.problem.in.using.quantum.chemical.methods.to.model.metal.oxide.struc-
ture.and.reactivity.is.the.separation.of.length.and.time.scales..For.instance,.in.cataly-
sis,.reactive.processes.occur.over.time.scales.of.milliseconds.and.up,.while.molecular.
approaches.typically.max.out.at.timescales.of.nanoseconds..Car-Parrinello.molecu-
lar.dynamics,
138,139
.which.is.the.time-integration.of.the.equations.of.motion.with.the.
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