Photocatalytic Degradation of Organics

1
Photocatalytic Degradation of Organics

• Elizabeth Buitrago
• University of Arizona
• Department of Chemical and Environmental
  Engineering
• Grad Student Mentor Mike Schmotzer
• Faculty Advisor Dr. Farhang Shadman

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UPW Use

• Wet standard operations account 1/3 of total
  processing steps
• Standard cleans.
• Wet etch processes.
• Chemical mechanical planarization (CMP)
• Wafer is redundantly cleaned to remove
  contaminants and prepare the surfaces between
  processes.

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The Concern

• More than 3000 gallons of UPW can be used to
  process an 8 inch wafer from start to finish.
• Present semiconductor fabrication facilities
  (FABS) typically use 1-3 million gallons of UPW
  per day.
• Final UPW quality highest of any industry.
• Contaminants remaining in water end up in wafer
  surfaces, render a device non-functional.

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Overview

• Goals and objectives
• Introduction/ background
• TiO2 as a photocatalyst/photocatalytic process
• Role of promoters in catalytic oxidation (Ag)
• Effects of nitrogen doping in TiO2
• Experimental
• Results/Highlights
• Future goals

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Goals and Objectives

• Develop new method for photocatalytic oxidation
  of organics
• Lower the energy use through catalytic oxidation
  (UV 185nm used 2003? UV 254 nm used 2004).
• Reduce the use of chemicals.

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Introduction Photocatalytic Process
Photo-generation electron/hole pairs Formation
of radicals Radical oxidation of Organic
compound.
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Role of Promoters in Photocatalytic Process
Photo-generation electron/hole pairs Formation
of radicals (Ox- radical) Radical oxidation of
organic compound. Recombination of electron/hole
pair Metal attracts free electron slows
recombination and promotes radical formation
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Role of Promoters in TiO2 Photocatalytic process
Conduction Band e- e- e- e- e- e- e- e- e- e- e-
e-
e-(M) lt-- Me-
Electron/hole pair recombination
Electron/hole pair generation
Eg
Valence Band h h h h h h h h h h
Metallic promoter attracts electrons from TiO2
conduction band and slows recombination reaction
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Effects of Nitrogen Doping in TiO2
Energy
TiO2 Bond
TiO
N
Bond
2-x
x
Orbitals
Orbitals
Conduction Band
Ti d
Ti d (O2p)
O2p N2p)
Ti d
Ti d
Eg 3.2 eV
Eg 2.5 eV
N2p O2p
O2p
N2p O2p
O2P (
Ti d)
(
Ti d)
Valence Band
Addition of nitrogen increases the size of the
bond orbitals, decreasing the energy bandgap
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Experimental
-Ethylene glycol -urea -Triton X-100 surfactant
contaminants
-Sol-gel method 1
3-TiO2 layers 3-bakes
-Sol-gel method 2 3-TiO2
layers 2-extra TiO2 coats
Ag doped before 3rth bake -CVD method
N2 doped
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Preparation of Supported Catalyst by
ChemicalVapor Deposition Method
(CVD)Experimental Setup
1
4
TiCl4 reservoir
Stripper
HP nitrogen cylinder
3
2
Impregnation chamber
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Experimental Setup for Batch Reactivity Testing
UV lamp 254 nm
Water bath/ shaker/ lamp holder
Coated screens
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Results and Highlights
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Results and Highlights
Sol-gel method 2 used
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Results and Highlights
Sol-gel method2 used
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Results and Highlights
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Model for Photocatalytic Reaction
1. Electron/hole formation 2. Electron/hole
recombination 3. Radical formation 4. Oxidation
of organics 5. Radical combining with X
(anything other than TOC) 6. Metal attracts
electron 0 not metal present.
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Photocatalytic Model
TiO21 S 3.5 CVD S 10 TiO2 2 S
14 cm2 S active surface area
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Photocatalytic Model
k

¾

¾


3
O
H
CO
TOC
OH
2
2
Triton X100 k3 0.6 ethylene glycol k3
0.4 Urea k3 0.05
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Future Goals

• Find new substrates for better deposition of
  TiO2.
• Investigate new ways that would improve our TiO2
  loading method.
• Improve CVD method.
• Improve nitridation method.