Photocatalysison TiO2 Surfaces

Photocatalysis onTiO2 Surfaces

KiraWyckoffJune6,2016

Introduction

ElectronicExcitation

TiO2Properties

PhotocatalyticApplications

Light

VB

CBelectron

hole

-

+

Bandgap

Introduction

ElectronicExcitation

TiO2Properties

PhotocatalyticApplications

Light

VB

CBelectron

hole

-

+

Bandgap

Generationofelectron-holepairsinsemiconductorparticles

electronfromdonor cancombinewithholetooxidizedonor

ExcitationofelectronfromVBtoCBbylightwithenergyequaltobandgap

donateelectronstoreduceanelectronacceptor

HoleandelectroncanrecombineonSurfaceorinvolume

Linsebigler,Lu&Yates. Chem.Rev.Vol95,No3,735–58(1995).

QuantumYieldhasidealvalueof1forthephotocatalyticprocess

∅ ∝𝑘$%

𝑘$% + 𝑘'

• Idealization• Recombinationdoesoccur• Concentrationofelectrons(ns)andholes(ps)atthesurfaceisnotequal

• Chargecarriertrapsusedtopromotetrappingofelectronsandholesatsurface

• ForTiO2,ns>psbecauseelectrontransfertoO2 isslow

Chargetransferrate

Electron-holerecombination rate

QuantumYieldorEfficiency

Linsebigler,Lu&Yates. Chem.Rev.Vol95,No3,735–58(1995).

IncreaseinBandGapforDecreasingParticleSize

• Quantumsizeeffectsoccurforsemiconductorparticlesonorderof10-100A

• ArisewhensizeofparticleiscomparabletodeBrogliewave-length

• Confinementproducesquantizationofdiscreteelectronicstates

• Increaseseffectivebandgap

Linsebigler,Lu&Yates. Chem.Rev.Vol95,No3,735–58(1995).

Outline

ElectronicExcitation

TiO2Properties

PhotocatalyticApplications

Light

VB

CBelectron

hole

-

+

Bandgap

TiO2 Anatase showshigherphotocatalyticactivity

• ChainsofTiO6 octahedra• EachTi4+issurroundedbysixO2- ions

• Octahedrondistortedtolowersymmetrythanorthorhombic

• Eachoctahedronincontactwitheightneighbors

• Latticestructurecausesdifferentelectronicbandstructure

Linsebigler,Lu&Yates. Chem.Rev.Vol95,No3,735–58(1995).

DOSshowslargebandgap

O2porbitals

Energy

Ti 3dorbitals

Ti 4s,4porbitals

EFBandGap=3.3eV

Outline

ElectronicExcitation

TiO2Properties

PhotocatalyticApplications

Light

VB

CBelectron

hole

-

+

Bandgap

Application:environmentalcleanup

Light

VB

CBelectron

hole

-

+

Bandgap

Degradationoforganicmolecules:• Depollutionofsmoke• Killbacteria• Decontaminatewater

O2

O2.-

Reduction

Oxidation

P

P+H++OH.-

H2O

Manydifferentnovelcommercialapplications

Clothing

Selfcleaningwindows

Wenxi Guo,etal. JournaloftheAmericanChemicalSociety.

Researchtowardsacatalystsensitivetovisiblelight

• GaN andZnO• Rh-Crmixedoxideco-catalyst• Canabsorbinvisiblelight• Watersplittingtoformhydrogen

andwaterusingsolarenergy

• TiO2 doesnotabsorblightinthevisibleregion

Maeda&Domen. J.Phys.Chem.C.Vol111,No22,2007.Linsebigler,Lu&Yates. Chem.Rev.Vol95,No3,735–58(1995).

Questions?

Extra

Anatase [TiO2]Structure TetragonalSpaceGroup : I 41/amd (No.141)a=3.7845Å, c=9.5143Åa=b=g=90.00Z=4AtomicPositionalParametersTi 4a0.00000.00000.0000O 8e0.00000.00000.2081

Extra