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
Location of ASTRaL Mikkeli R U S S I A Lappeenranta Lappeenranta University of Technology 1
Mikkeli and surroundings 2
Outline Introduction to catalysis Basic principles Photocatalysis Measurement of photocatalytic activity Thin film production processes for catalysts 3
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 (Fe 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) 4
Examples of photocatalysts Lamp glasses Tent fabric with and without photocatalyst Self cleaning concrete 5
Examples of other catalyst uses bacteriocidal surfaces 6
Basic principles 7
Basics of catalysis Heterogeneous - catalyst and medium are in different phases Homogeneous - catalyst and medium are in same phase 8
Basics of catalysis Catalysts change the speed of a reaction without being themselves changed. Change the activation energy of a reaction Energy barrier to forming reaction product Probability of crossing the energy barrier E kt 9
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 10
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 11
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 12
Photocatalyst action Light stimulates the process Removal of organic pollutants Typically requires oxidation of organic products H 2 O, CO 2, mineral acids, etc. Reduction-oxidation (redox) reaction Typically needs presence of moisture 13
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. 14
Photocatalysts 15
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 16
Semiconductors Bonding between atoms in the semiconductor produces certain allowed energy states for electrons Valence Band: states are almost fully occupied by electrons 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) chemical potential CB VB almost empty Conduction band Forbidden gap Fermi level Valence band almost full electron energy 17
Formation of energy bands Single molecule Two molecules Cluster Crystal Lowest unoccupied molecular orbital LUMO CB Energy gap HOMO Highest occupied molecular orbital VB 18
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 hν VB Forbidden gap + 19
Redox reaction CB semiconductor - adsorbed species A + e - A - (reduction) VB Forbidden gap + B - e - B + (oxidation) or B + h + B + 20
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 E CB < Red 1 holes must be able to oxidise the reduced part, E VB > Ox 2 Compatible CB Ox 2 CB Ox 2 CB Ox 2 CB Ox 2 VB Red 1 VB Red 1 VB Red 1 VB Red 1 21
Potential (vnhe) (V) Energy level of various semiconductors -2-1,5 GaAs CdS most effective and most commonly used -1 3.0-0,5 0 1.4 TiO 2 ZnO WO 3 CdSe H 2 /H + 0,5 1 1,5 3.0 3.2 1.7 OH - /O 2 H 2 O 2 /OH - 2 3.2 2,5 3 3,5 22
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 23
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? 24
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 B B + VB + recombination Electrons and holes can recombine before they reach the surface and cause a reaction Recombination reduces efficiency 25
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 26
Surface trapping Enhance trapping with small particles of noble metals (Au, Pt, Pd, Ag, etc) - E F A - A Thin films are not perfectly smooth many faces of the small crystallites are exposed to the adsorbed species D + represents a TiO 2 crystal 200 nm D + 27
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) 28
Composite semiconductors Mixtures of crystals with different energy levels for the band edges Electrons and holes separated Electron energy - - A - A D + + D + TiO 2 SnO 2 Electrons find their lowest energy position 29
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 D + + rutile anatase Stabilises charge separation. Hurum et al., J Phys Chem. B 107 (2003) 4545 30
Solar spectrum Usable part of the spectrum for TiO 2 Absorption edge of anatase is 382 nm, rutile 416 nm 31
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 32
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. 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. Enhanced photogeneration More e-h pairs available - + CB VB - + Enhanced recombination fewer e-h pairs available - + CB VB 33
Anion doping Substitute N, C or S for O TiO x D 2-x, (D is dopant) Increases absorption of light in the visible region What mechanisms are the reason? Does this improve photocatalytic performance? 34
Possible effects of doping undoped anatase transitions between localised levels localised shallow doping levels near CB and VB wider VB causes bandgap narrowing excitation from localise deep dopant levels to CB N Serpone, J Phys. Chem B 110 (2006) 24287 35
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 TiO 2 and dopant, post deposition doping, etc.) Still needs complete understanding 36
Measurement of photocatalytic activity 37
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 Self cleaning properties Air Water Test compounds Photocatalytic degradation of methylene blue Photocatalytic degradation of (methyl) stearate Batch system, 2-propanol acetone Photocatalytic degradation of simple test molecules, e.g., dichloro acetic acid, methanol Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 38
Methylene blue test Blue organic dye Main absorption at = 665 nm, extinction coefficient 10 5 M -1 cm -1 Becomes colourless under oxidation 39
Methylene blue test Changes from blue to colourless in aerated solution Usually monitored by change in optical absorbance Assumed overall reaction: 40
Bleaching process Mechanism of degradation of MB is complicated 41
MB test: DIN 52980:2007-11 Test conditions 42
DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 43
DIN 52980:2007-11 Substrate may adsorb MB dye from solution without degradation depends on surface area and nature of material 44
DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 45
DIN 52980:2007-11 46
DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 47
MB test Stirring will affect the test www.photocatalysis-federation.eu/fileadmin/documents/m._mills_jep_2009.pdf 48
MB test ph can affect result Mills and Wang, Photochem. Photobiol. A: Chem. 1999, 127, 123. 49
Competing reactions This is the desired reaction which occurs in oxygen-rich conditions 50
Competing reactions This third reaction can also cause bleaching in reducing conditions by formation of LMB Leuco-Methylene Blue LMB - colourless 51
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 52
Bleaching due to non-degradative effect Mills and Wang, Photochem. Photobiol. A: Chem. 1999, 127, 123. 53
DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 54
DIN 52980:2007-11 Ralf Dillert, Gottfried Wilhelm Leibniz Universitat 55
Thin film production processes for catalysts 56
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 57
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 58
Magnetron sputtering Substrate coating Substrate bombarded by neutral and ionised target molecules which form a deposited layer Low pressure inert gas plasma (usually Ar) magnetic field intensifies discharge Target B Target bombarded by high energy ions from the plasma. These eject atoms from the target. N S S N N S -V magnets 25.01.2011 59
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 60
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 61
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 62
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 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. 25.01.2011 64
ALD system cycle Precursor A Purge Substrate is never exposed to both precursors at the same time. Purge Precursor B Reaction only takes place on a monolayer on the surface. A Purge B Purge Extreme conformality. Temperature depends on process. > 250 C for crystalline TiO2. 25.01.2011 65
Vacuum evaporation D M Mattox, Handbook of thin film deposition 66
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 67
Sol-gel process using alkoxides Ti(OR) 4 R=C n H 2n+1 Hydrolysis Ti(OR) 4 + H 2 O (HO)-Ti(OR) 3 + ROH (HO)-Ti(OR) 3 + H 2 O (HO) 2 -Ti(OR) 2 + ROH (HO) 2 -Ti(OR) 2 + H 2 O (HO) 3 -Ti(OR) + ROH (HO) 4 -Ti(OR) + H 2 O Ti(OH) 4 + ROH Condensation (OR) 3 -Ti-OH +HO-Ti-(OR) 3 or (OR) 3 -Ti-OR +HO-Ti-(OR) 3 produces large Ti-O network Gelation [(OR) 3 -Ti-O-Ti-(OR) 3 ] + HOH [(OR) 3 -Ti-O-Ti-(OR) 3 ] + ROH network becomes so large and interconnected that the material no longer behaves like a liquid but elastically deforms 68
Drying Evaporation of excess water and alcohols Densification (by thermal annealing) removal of bound water Ti(OH) 4 TiO 2 + 2H 2 O sintering of nanocrystals removal of porosity 69
Dip coating process http://www.solgel.com/articles/nov00/mennig.htm Multiple layers can be deposited to increase thickness 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 If the individual layers are too thick, cracking will occur on drying 70
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! 71
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. 72