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

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.

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. Water Quality Parameter UnitsTypical Municipal Water Supply Typical Ultrapure Water Product ResistivityM ohms-cm0.004>18 pHUnits86 TOCppb3500<10 Ammoniumppb300<1 Calciumppb22000<1 Magnesiumppb4000<1 Potassiumppb4500<10 Silicappb4780<1 Sodiumppb29000<1 Chlorideppb15000<1 Fluorideppb740<1 Sulfateppb42000<1

Overview Goals and objectives Introduction/ background –TiO 2 as a photocatalyst/photocatalytic process –Role of promoters in catalytic oxidation (Ag) –Effects of nitrogen doping in TiO 2 Experimental Results/Highlights Future goals

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.

Introduction: Photocatalytic Process Photo-generation electron/hole pairs Formation of radicals Radical oxidation of Organic compound.

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

Conduction Band e - e - e - e - e - e - Role of Promoters in TiO 2 Photocatalytic process Valence Band h + h + h + h + h + Electron/hole pair recombination Electron/hole pair generation Metallic promoter attracts electrons from TiO 2 conduction band and slows recombination reaction e - (M) <-- M+e - EgEg

Effects of Nitrogen Doping in TiO 2 TiO 2 Bond Orbitals TiO 2-x N x Bond Orbitals Conduction Band Ti d + (O2p) Ti d + O2p +N2p) Valence Band N2p + O2p O2P + (Ti d)+ (Ti d) Energy Ti d O2p Ti d N2p O2p Addition of nitrogen increases the size of the bond orbitals, decreasing the energy bandgap E g = 3.2 eV E g = 2.5 eV

Experimental -Sol-gel method #1 3-TiO 2 layers 3-bakes -Sol-gel method #2 3-TiO 2 layers 2-extra TiO 2 coats Ag doped before 3rth bake -CVD method N 2 doped -Ethylene glycol -urea -Triton X-100 surfactant contaminants

Preparation of Supported Catalyst by Chemical Vapor Deposition Method (CVD) Experimental Setup 1 4 Impregnation chamber 2 3 HP nitrogen cylinder Stripper TiCl 4 reservoir

Experimental Setup for Batch Reactivity Testing UV lamp 254 nm Water bath/ shaker/ lamp holder Coated screens

Results and Highlights

Sol-gel method #2 used

Results and Highlights Sol-gel method#2 used

Results and Highlights

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.

Photocatalytic Model TiO 2 #1 S = 3.5 CVD S = 10 TiO 2 #2 S = 14 cm 2 S = active surface area

Photocatalytic Model OHCOTOCOH 22 k 3 +¾®¾+· Triton X100 k3 = 0.6 ethylene glycol k3 = 0.4 Urea k3 = 0.05

Future Goals Find new substrates for better deposition of TiO 2. Investigate new ways that would improve our TiO 2 loading method. Improve CVD method. Improve nitridation method.