Catalytic thin film coatings

Catalytic thin film coatings David Cameron Professor of Material Technology Advanced Surface technology Research Laboratory (ASTRaL) University of Lappeenranta Finland Part-financed by the European Union (European Regional Development Fund Outline Introduction to catalysis Basic principles Photocatalysis Measurement of photocatalytic activity Thin film production processes for catalysts 1

Uses of catalysts Widely used in industry Polymer industry - olefin polymerization Energy industry cracking of crude oil (zeolite catalyst) Chemical industry - selective synthesis and process efficiency o e.g. Haber process for ammonia production (Fe or Ru catalyst) Effluents By-product and waste minimization o e.g. Vehicle exhaust catalysers (precious metal catalyst) Self cleaning surfaces, glass, concrete, etc. (titanium oxide) Hydrogenation of vegetable oil to margarine (Ni catalyst) 2 Examples of photocatalysts Lamp glasses Self cleaning concrete Tent fabric with and without photocatalyst 3

Examples of other catalyst uses bacteriocidal surfaces 4 Basic principles 5

Basics of catalysis Heterogeneous - catalyst and medium are in different phases Homogeneous - catalyst and medium are in same phase 6 Basics of catalysis Catalysts change the speed of a reaction without being themselves changed. Change the activation energy of a reaction initial state final state Energy barrier to forming reaction product Probability of crossing the energy barrier E kt 7

Basics of catalysis Catalysts change the speed of a reaction without being themselves changed. Change the activation energy of a reaction Easier for reaction to occur Reduced energy barriers 8 Basics of catalysis Intermediate steps: Adsorption of gas or liquid on the solid catalyst surface Reactants are loosely bound to the catalyst surface and so brought into proximity so facilitate the reaction Desorption of reacted product Note: No change in initial and final energies No change in chemical equilibrium Only a change in the energy barrier 9

Catalyst materials Very diverse Multifunctional solids Zeolites, alumina, higher-order oxides, graphitic carbon nanoparticles, nanodots, and facets of bulk materials Transition metals often used to catalyse redox reactions palladium, platinum, gold, ruthenium, rhodium, and iridium 10 Photocatalyst action Light stimulates the process Removal of organic pollutants Typically requires oxidation of organic compounds products H2O, CO2, mineral acids, etc. Reduction-oxidation (redox) reaction Typically needs presence of moisture 11

Redox reaction Oxidation is the loss of electrons or an increase in oxidation state of a molecule, atom, or ion. Reduction is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion. A catalyst which promotes a redox reaction must transfer an electron into the compound and also accept an electron from it. 12 Photocatalysts 13

Example: Photocatalysis of organic compounds in water. Typically uses a semiconductor as the catalyst Solid material Need to understand the interactions between the electrons in the semiconductor and the electrons in the organic compounds and water. Need to consider the energy states in the semiconductor 14 Semiconductors Bonding between atoms in the semiconductor produces certain allowed energy states for electrons chemical potential electron energy almost empty Valence Band: states are almost fully occupied by electrons CB Conduction band Conduction Band: states are almost empty of electrons Forbidden gap (or Energy Gap): no allowed electron states Fermi level: potential where energy states have a 50% probability of being occupied by electrons ( chemical potential of semiconductor) VB Forbidden gap Fermi level Valence band almost full 15

Formation of energy bands Single molecule Two molecules Cluster Crystal Lowest unoccupied molecular orbital LUMO CB Energy gap HOMO Highest occupied molecular orbital VB 16 Illumination of semiconductors When a semiconductor is illuminated by light whose energy is greater than the energy gap, electrons are excited from the valence band to the conduction band. CB - This gives extra electrons in the CB and missing electron (holes) in the VB which are free to move. These can then move to the surface and take part in redox reactions if the semiconductor is in contact with a reactive material h VB Forbidden gap + 17

Redox reaction semiconductor adsorbed species CB - A + e - A - (reduction) Forbidden gap VB + B - e - B + (oxidation) or B + h + B + 18 Will a redox reaction occur? From a thermodynamic point of view, for redox reactions to occur in the adsorbed species, the redox potentials of the species must be compatible with the CB and VB energies. electrons must be able to reduce the oxidised part, i.e ECB < Red1 holes must be able to oxidise the reduced part, EVB > Ox2 CB VB Ox2 Red1 Re CB VB Ox2 Red1 CB Ox2 VB Red1 Re Compatible CB Ox Ox2 VB Red1 Red 19

-2-1.5-1 -0.5 0.5 1.5 0 1 2 2.5 3 3.5 20 Potential (vnhe) (V) Energy level of various semiconductors most effective and most commonly used GaAs CdS 3.0 1.4 TiO2 ZnO CdSe 3.0 3.2 WO3 1.7 H2/H + OH - /O2 H2O2/OH - 3.2 Titanium dioxide 3 crystal phases anatase energy gap 3.2 ev stable at lower temperature brookite energy gap 3.3 ev rutile energy gap 3.0 ev stable at higher temperature 21

Performance Anatase has been generally found to be more effective However, powder materials (not thin film materials) are usually a mixture of rutile and anatase for best performance (e.g. Degussa P25) What controls the performance? 22 Recombination CB - CB - surface A A - CB - surface hv hv hv VB Electrons and holes generated inside the thin film must get to the surface so that they can cause a reaction. + VB + diffusion Electrons and holes must diffuse to the surface in order to act on adsorbed species Recombination reduces efficiency B B + VB + recombination Electrons and holes can recombine before they reach the surface and cause a reaction 23

How to reduce recombination? Separate the electrons and holes before they recombine. if they are not in the same vicinity, can t easily combine Trap the electrons and holes at different surface locations Make the electrons and holes diffuse away from each other alter the band structure to allow this 24 Surface trapping Enhance trapping with small particles of noble metals (Au, Pt, Pd, Ag, etc) - EF A - A Thin films are not perfectly smooth many faces of the small crystallites are exposed to the adsorbed species D D + + represents a TiO2 crystal 200 nm 25

Surface trapping Add impurities to the titanium oxide to produce trapping levels within the energy gap for example Fe doping creates surface traps on crystal electron and hole traps are at separate locations: less chance of recombination - Fe III/Fe II Fe (III) + e - Fe(II)* (shallow trap) Fe III/Fe IV + Fe (III) + h + Fe(IV)* (shallow trap) 26 Composite semiconductors Mixtures of crystals with different energy levels for the band edges Electrons and holes separated Electron energy - - A - A D D + TiO2 + + SnO2 Electrons find their lowest energy position 27

Multiphase materials Mixture of anatase and rutile titanium dioxide Electron energy - A - A Rutile rapidly transfers electrons to anatase trapping sites, then electrons transfer to anatase surface trapping sites. D + Stabilises charge separation. D + rutile anatase Hurum et al., J Phys Chem. B 107 (2003) 4545 28 Solar spectrum Usable part of the spectrum for TiO2 Absorption edge of anatase is 382 nm, rutile 416 nm 29

Activation by visible spectrum Doping of TiO2 to reduce bandgap. cation doping (replace Ti with another metal) anion doping (replace O with a non-metal) Cation doping Fe, Cr, V, Ni, etc no consistent picture of benefit increases the absorption of visible light but has been found to have contradictory effects on the catalytic activity produces defect levels rather than bandgap narrowing 30 Metal ion doping Forms discrete energy levels in the energy gap Effect depends on whether it is a real doping effect or formation of dopant metal oxide clusters. Enhanced photogeneration More e-h pairs available - CB - Enhanced recombination fewer e-h pairs available - CB Dopant metal oxide can enhance recombination which may reduce effectiveness. Substitutional metal doping can change the electronic state of the TiO2 and may enhance photogeneration. VB + + + VB 31

Anion doping Substitute N, C or S for O TiOxD2-x, (D is dopant) Increases absorption of light in the visible region What mechanisms are the reason? Does this improve photocatalytic performance? 32 Possible effects of doping undoped anatase localised shallow doping levels near CB and VB wider VB causes bandgap narrowing excitation from localise deep dopant levels to CB transitions between localised levels N Serpone, J Phys. Chem B 110 (2006) 24287 33

Effect of doping Increases visible light absorption. Probably causes defect levels rather than bandgap narrowing. Effects on photocatalytic performance are contradictory. Behaviour likely to be different depending on crystal phase method of preparation (process, temperature, etc.) method of doping (co-deposition of TiO2 and dopant, post deposition doping, etc.) Still needs complete understanding 34 Measurement of photocatalytic activity 35

How do you measure the effectiveness of the photocatalyst? Need standardised methods which can be reproduced. German DIN working group to formulate standards for removal of organics. (Also in Japan, possible ISO standard) Catalytic property Test compounds Self cleaning properties Photocatalytic degradation of methylene blue Photocatalytic degradation of (methyl) stearate Air Batch system, 2- Water Photocatalytic degradation of simple test molecules, e.g., dichloro acetic acid, methanol Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 36 Methylene blue test Blue organic dye Main absorption at = 665 nm, extinction coefficient 10 5 M -1 cm -1 Becomes colourless under oxidation 37

Methylene blue test Changes from blue to colourless in aerated solution Usually monitored by change in optical absorbance Assumed overall reaction: 38 Bleaching process Mechanism of degradation of MB is complicated 39

MB test: DIN 52980:2007-11 Test conditions 40 DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 41

DIN 52980:2007-11 Substrate may adsorb MB dye from solution without degradation depends on surface area and nature of material 42 DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 43

DIN 52980:2007-11 44 DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 45

MB test Stirring will affect the test www.photocatalysis-federation.eu/fileadmin/documents/m._mills_jep_2009.pdf 46 MB test ph can affect result Mills and Wang, Photochem. Photobiol. A: Chem. 1999, 127, 123. 47

Competing reactions This is the desired reaction which occurs in oxygen-rich conditions 48 Competing reactions This third reaction can also cause bleaching in reducing conditions by formation of LMB Leuco-Methylene Blue LMB - colourless 49

Competing reactions The LMB can be reoxidised to MB thus reversing the effect not a real degradation of the MB. Reaction 4 is slower in acidic solutions 50 Bleaching due to non-degradative effect Mills and Wang, Photochem. Photobiol. A: Chem. 1999, 127, 123. 51

DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 52 DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 53

Thin film production processes for catalysts 54 Deposition processes Deposition processes at atomic or molecular scale Magnetron sputtering MS (Plasma enhanced) chemical vapour deposition (PE)CVD Atomic layer deposition ALD Sol-gel deposition vacuum evaporation More macroscopic deposition processes thermal spraying aerosol processes Chemical processes powders thin films thick films 55

Thin film processes Vapour phase processes Magnetron sputtering MS (Plasma enhanced) chemical vapour deposition (PE)CVD Atomic layer deposition ALD vacuum evaporation Liquid phase processes Sol-gel deposition 56 Magnetron sputtering Substrate coating Substrate bombarded by neutral and ionised target molecules which form a deposited layer magnetic field intensifies discharge Target N S B S N magnets Low pressure inert gas plasma (usually Ar) N S Target bombarded by high energy ions from the plasma. These eject atoms from the target. -V 57

Von nardenne Teer Coating Ltd., UK Von Ardenne Anlagentechnik, Germany 58 Magnetron sputtering Pressure during sputtering ~10-3 to 10-2 mbar Needs high vacuum equipment Substrate temperature can be from close to room temperature upwards Simplest process uses a DC voltage on target This creates a problem for sputtering insulating materials like titanium dioxide insulating TiO2 target +++++++++++++++++++++++++++++++ build up of positive charge repels the bombarding ions -no sputtering -V 59

Sputtering insulating targets Radio frequency power when target goes positive, attracts many electrons from plasma because they are light and move easily when target goes negative, attracts fewer ions because they are heavier and move more slowly build up of negative potential on target which allows sputtering to occur However, sputtering is slow, equipment expensive 60 Reactive sputtering Use titanium metal target with DC power Combination of Ar + O2 in sputter gas Ti metal reacts with O to form TiOx on surface of substrate Stoichiometry determined by amount of O2 in gas Needs accurate control of O2 flow Faster sputtering, lower cost Structure depends on Ti, O and Ar flux energy from Ar ion bombardment can affect crystal structure sputtered Ti atoms oxygen from background gas energetic Ar ions 61

Chemical vapour deposition CVD Powered by thermal energy homogeneous reaction generally unwanted produces particulates reactant gas B A exhaust precursor heterogeneous reaction on surface of substrate surface diffusion heated substrate Energy may also be supplied by a plasma produced by electrical energy to minimise temperature 62 Samco international Inc, USA Kurt J. Lesker, USA 63

Atomic Layer Deposition ALD Materials are built up one atomic layer at a time. For example, a compound AB is built up of alternate layers of A and B one atomic layer thick at a time. B A Can make mixed layers, nanolaminates, graded composition structures. 64 ALD system cycle Precursor A Purge A Purge B Purge Purge Precursor B Substrate is never exposed to both precursors at the same time. Reaction only takes place on a monolayer on the surface. Extreme conformality. Temperature depends on process. > 250 C for crystalline TiO2. 65

Beneq, Finland Cambridge Nanotechnology, USA 66 Vacuum evaporation D M Mattox, Handbook of thin film deposition 67

Sol-gel deposition Sol = a stable suspension of colloidal solid particles or polymers in a liquid Gel = porous, three-dimensional, continuous solid network surrounding a continuous liquid phase Sol-gel process Hydrolysis Condensation Gelation Ageing Drying Densification 68 Sol-gel process using alkoxides Ti(OR)4 R=CnH2n+1 Hydrolysis Ti(OR)4 + H2O (HO)-Ti(OR)3 + ROH (HO)-Ti(OR)3 + H2O (HO)2-Ti(OR)2 + ROH (HO)2-Ti(OR)2 + H2O (HO)3-Ti(OR) + ROH (HO)4-Ti(OR) + H2O Ti(OH)4 + ROH Condensation (OR)3-Ti-OH +HO-Ti-(OR)3 [(OR)3-Ti-O-Ti-(OR)3] + HOH or (OR)3-Ti-OR +HO-Ti-(OR)3 [(OR)3-Ti-O-Ti-(OR)3] + ROH produces large Ti-O network Gelation network becomes so large and interconnected that the material no longer behaves like a liquid but elastically deforms 69

Drying Evaporation of excess water and alcohols Densification (by thermal annealing) removal of bound water Ti(OH)4 TiO2 + 2H2O sintering of nanocrystals removal of porosity 70 Dip coating process http://www.solgel.com/articles/nov00/mennig.htm Multiple layers can be deposited to increase thickness If the individual layers are too thick, cracking will occur on drying C. J. Brinker, A. J. Hurd, K. J. Ward in Ultrastructure Processing of Advanced Ceramics, eds. J. D. Mackenzie and D. R. Ulrich, Wiley, New York (1988) 223 71

Characteristics of the thin films Deposition under different conditions affects many film characteristics crystal phase: anatase, rutile, mixture film morphology: rough, smooth, facetted, dense, porous crystallite size: from nm size crystals upwards in size effect of base material: crystallinity, phase Even using the same process a very wide range of film structures can be obtained: good control is important Important to really know what you have! 72 Summary Basic catalysis process Semiconductors as photocatalysts Assessment process for photocatalysts Methods of thin film deposition Still much work to be done on understanding details of why materials behave in a particular way. 73