 Welcome to the course on nanostructured materials, synthesis properties, self assembly and applications. We are in the module 4 of this course and we are working on photo catalysis right now and we had two lectures on photo catalysis and today is the third lecture on photo catalysis and this is also the lecture 3 of module 4. So, in photo catalysis in the previous lectures we looked at the basic ideas of catalysis in the presence of light and how a UV light and visible light catalysts, catalyzed reactions through the generation of electrons and holes and two important criteria are the electron and hole should be generated efficiently and the second thing is the electron and hole should be separated from each other, so that they do not recombine and release of photon. So, this recombination has to be prevented and this charge separation is an important part in the design of new materials which will allow for enhanced efficiency of the photo catalyst. Now, the photo catalysis as we see you can have high surface area, small crystalline size, the especially we are discussing for titanium dioxide and in this case the anatase form of titanium dioxide is very important. There are three forms of titanium dioxide the rutile form is the high temperature form whereas, the anatase form is the low temperature form and we require the anatase form for high photo catalytic activity. So, the anatase form is very important then T I O 2 photo catalysts can be prepared with high crystallinity it can be prepared with high porosity. So, porous structures of titanium can be prevented and the activation through a light source is possible in T I O 2. Let us look at the conditions affecting the photo catalytic activity of T I O 2. Now, T I O 2 is normally prepared through an alkoxide through solution methods we start from a titanium alkoxide and typically we take titanium isopropoxide and when you hydrolyze it with water you get this kind of hydrates at low temperature and then when you heat it further you calcine it you get the anatase form of T I O 2. If you heat it further that is you further calcine you get the rutile form of T I O 2. Now, if you look at the properties if you have at low temperature you normally have the amorphous form and at low temperatures you have low very low surface area in this case you have a large when you have large crystallinity especially like in rutile. So, if you have the surface area which is low and you may have very large surface area both are possible the activity the activity goes down if the surface area goes down. So, the activity will be large when the surface area is very high and the activity will become small when the surface area is small. In crystallinity if you have very high crystallinity then which you get at high temperatures and in the rutile form you will get this high crystallinity. In the anatase form somewhere in the middle you have reasonable crystallinity and reasonable surface area whereas, in the rutile form you have high crystallinity and small surface area. And the difference between rutile and anatase in their band gap anatase has a band gap of 3.2 electron volts and rutile has a band gap of 3 electron volts. So, if you decrease the surface area the number of active sites also decrease. So, what you want is very a balance between crystallinity and surface area and that you achieve in the middle because if you have very amorphous type of Ti O 2 which you will get in the initial stages of the hydrate formation then your active sites will be low although you are having a high surface area because of poor crystallinity. So, it is good to work in this region where you have the anatase form of Ti O 2. Now, Ti O 2 is a very efficient photo catalyst, but you need ultraviolet radiation UV light normally we would like to use visible light and to use the solar spectrum we would like to use the visible light and the material should be active under visible light. Visible light is in the lower energy region compared to UV light. So, visible light active photo catalyst is really needed for practical applications and will be economical if we have efficient visible light catalyst. So, how do we modify the normal Ti O 2 which is a UV light photo catalyst to make it into a visible light photo catalyst and that can be done by doping the Ti O 2 with different materials like non metals, transition metals or organic dyes. All of them can modify the behavior of Ti O 2 with respect to the incoming radiation and all of them you have to choose appropriately the type of non metal or transition metal or organic dye to control the band gap of Ti O 2 or effectively generate electron and hole pairs by using visible light instead of ultraviolet light. You can also increase the activity of the catalyst by loading a metal oxide and we will see examples of these. The metal oxide typically reduces the chance of recombination of the electrons and holes which are produced during the photo catalytic reactions. So, hence there are several ways by which people are trying to use dopants to modify the band gap or to effectively create electrons and holes in the using visible light because Ti O 2 is available in large amounts in the earth and Ti O 2 is environmental friendly. So, it is a very challenging problem and important problem to convert Ti O 2 to a visible light photo catalyst instead of a UV photo catalyst. Now, how do you make titanium dioxide if you have to make it in the lab? Titanium dioxide is of course, available in nature also, but you want to make titanium dioxide in the lab or in industry in bulk quantities. So, how do you make titanium dioxide which is active and hence you want to make anatase form of titanium dioxide. So, people have found several methods like the hydrolysis of chlorides or sulfates of titanium salts and they have used the sol gel method which is a very popular method and we have discussed in our earlier lectures or they have used the hydrothermal method where you use a steplon or steel containers to create a pressure in either aqueous medium or using a solvent and some pressure is created and reaction occurs under pressure at temperatures around 100 to 200 250 degree centigrade or may be sometimes up to 300 350 degree centigrade. And there is another method which is the micro emulsion method where you use titanium salts and appropriate surfactants to make micro emulsions where you can make nano reactors using these micro emulsions which are present in solution depending on the type of surfactant and solvent you can control the size of the nano reactors. And then you can do reactions using the titanium salts and hydrolyze them within the nano reactors to give you nano particles of T i O 2 which are of the anatase form after certain calcination step. So, people can have used all these methods to make titanium oxide nano powders in large amounts and in the anatase form. Now, this is another technique by which T i O 2 can be synthesized this is the metal organic chemical vapor deposition or M O C V D in general as it is told where you have your material which can be titanium isopropoxide as discussed earlier. So, it is an alkoxide of titanium and that passes through a heater and there are several other heaters and temperature sensors in the furnace. So, there is a furnace through which this titanium isopropoxide is passed in the presence of oxygen and argon and then at this temperature which you can control the titanium isopropoxide can be calcined over a substrate and T i O 2 is generated on top of a substrate and if you want some metals to be doped or metal oxides to be doped then you put this metal oxide or metal as a dopant here and the temperature should be sufficient to volatilize this dopant and then the dopant mixes with the incoming stream of the titanium isopropoxide and forms T i O 2 on this substrate along with the metal or metal oxide dopant. And this kind of system can generate large amount of very high quality films of T i O 2 on the surface of appropriately chosen substrates. Substrates can vary from silicon to glass to quartz and variety of substrates you can choose depending on what is your application of this photo catalyst. Now, so how do we modify T i O 2 because we just discussed that we want to modify T i O 2 which is a visible light UV light catalyst in general, but we want to use visible light. So, how do we modify this T i O 2 by doping metals metal oxides and what happens when you dope metal and metal oxides to the band gap or to the valence band levels or to the conduction band levels. So, this is a kind of a graphic to show you that when the photons fall on the photo catalyst which has got some metal particles on top. So, it is a metal doped T i O 2. So, we have modified T i O 2 with a metal the metal particles are here and this is the titanium dioxide particle and when light falls on this particle T i O 2 then electrons and holes are generated. Now, the electrons if they can migrate to the surface and meets the metal the metal is a efficient conductor. So, this will take away this electron quickly and so the electron this electron which is produced here is separated from the hole and that is what is one of the important points how to separate electron and hole and because of this metal dopant the electron has this tendency to go to the metal and the metal it is a good conductor and then it removes the electron. So, the electron and hole are separated. So, both the properties generation of electron and hole pair and transport of electron and hole away from each other such that they do not recombine both then can be met which makes then this material this metal doped T i O 2 will be good for photo catalyst. Now, you can also doped a semiconductor. So, cadmium sulphide is a semiconductor and this is your original T i O 2 particle. So, assume that you have cadmium sulphide particle doped doped in T i O 2. Now, T i O 2 has a band gap of 3.2 electron volts and this is the valence band and this is the conduction band level of T i O 2 whereas, cadmium sulphide has a smaller band gap of 2.5 electron volt. So, in it can take lower energy and create electron hole pair. So, cadmium sulphide can be triggered to generate electrons and holes with lower energy of 2.5 electron volt whereas, T i O 2 will require 3.2 electron volt. So, lower energy if you give you can still generate electron and hole because of the presence of cadmium sulphide. Then this electron can migrate to the conduction band of the T i O 2 and the hole this hole is generated in the cadmium sulphide. So, when the light falls on cadmium sulphide the electron is goes to the conduction band and the electron is transferred to the T i O 2. So, the electron and hole are separated now on 2 different particles. The hole is retained on the cadmium sulphide and the electron goes to the T i O 2. Now, this is so this was a metal doped T i O 2 this was semiconductor doped T i O 2 and then you can have an organic dye doped T i O 2. So, if a organic dye is doped in T i O 2 then what happens is this energy level of the energy gap of the organic dye to create an electron when light falls is suppose it is in the visible then this electron goes to the excited state of the dye the dye is now in an excited state and then the electron from the conduction levels or the higher orbital is transferred to the conduction band of T i O 2 and then you can have some acceptor molecules which will take over this electron. So, the from the conduction band of T i O 2 the electron can be accepted by say an acceptor and that acceptor will get reduced. So, a will get reduced to a minus because the electron will be transferred from the conduction band of T i O 2 to the acceptor. So, 3 particular cases we looked at modification of T i O 2 metal doped modification of T i O 2 semiconductor doped modification of T i O 2 and then organic dye a doped T i O 2 which we call dye sensitized solar cells is a very important area in which you are using a dye to start this mechanism of electron being generated and then electron whole being separated. So, if you look at the band diagrams you can create different types of levels. So, suppose this is the original levels this is the conduction band of T i O 2 and this is the valence band of T i O 2 and when you are doping transition metals it is possible to have choose a transition metal such that you now have levels which are lower than the conduction band of T i O 2. So, the electron will be here because lower in energy than this and you can have a hole in the lower energy level or you can have a non metal doping T i O 2 then you have a energy level which is higher than the valence band of T i O 2. So, this is the valence band of T i O 2 and so it is higher than the valence band of T i O 2 and the electron this energy may be close to the conduction band of T i O 2. So, either you can lower the conduction band or you can increase the energy of the valence band electron or hole valence band hole. So, the hole will be here and the electron is here. So, these are two modifications to the bare T i O 2 which is shown here. So, this is also possible and this is also possible. So, this kind of changes in the band diagram by doping is called generally band gap engineering and you can also sensitize or increase the efficiency of titanium dioxide nano particles by using metal particles through their plasmon resonance. Now, we all know when you have metal nanoparticles the conduction electrons of the metal nanoparticles can have collective oscillations. Now, these collective oscillations of the conduction electrons can sensitize the T i O 2 nanoparticles and this has been observed for silver gold copper nanoparticles where they exhibit plasmon resonance in the visible range. So, you shine light in the visible and the conduction electrons of the metal nanoparticles like silver or gold then get excited and you have this collective oscillations and then this excitation acts like an antenna the metal particle acts as an antenna because this collective oscillation is generated by trapping the visible light energy or the solar energy and then it can enhance the catalysis of T i O 2. Once the silver nanoparticles which is doped on top of T i O 2 acting as an antenna catches the visible light radiation and then it can create electrons and holes in the T i O 2 to which it is connected. So, this is what is shown here. So, you have this metal particle and you have the T i O 2. So, this is the conduction band of T i O 2 this is a valence band of T i O 2. Now, what can happen is the if you use UV light then you can excite this. So, you can create electron here and hole here in the T i O 2 and when you create electrons here then another property of the metal is that it can quickly take away the electron and then it can reduce any species for example, oxide can oxygen gas can be reduced to this kind of super oxide ion. Now, this reduction is possible because the electron generated in the conduction band of titanium or T i O 2 is quickly transferred to the metal and the metal then the electron in the conduction band of the metal can get will will reduce this oxygen and the hole which is in the valence band can be used to oxidize some species. So, this is another possibility where you are using UV light, but the metal is enhancing the application of the electron and the hole that is the reducing and the oxidizing capabilities are enhanced by the presence of the metal. In the previous case these surface plasmon are basically acting as an antenna to trap energy in the visible spectral range and then it helps in the sensitization of the titanium nanoparticles. In this case you are not using the visible light you are using UV light, but you are efficiently removing the electron and hole because of the conduction properties of the metal and the metal then can quickly reduce some molecule which may be oxygen and give you this super oxide ion. Now you can have so here the metal nanoparticles act as an electron sink as I mentioned earlier and it promotes interfacial charge transfer and basically it reduces the probability that the electron created in the conduction band of T I O 2 will interact with the hole on T I O 2 because the electron is being removed from there. So, the metal is acting as a electron sink now you can also have more complex nanostructures where you can have metal decorated on core shell nanoparticles. So, core shell nanoparticles we know that there is one particle and it is covered by something else. So, the particle inside is the core and the material which is covered outside is the shell. So, here you can have metals like silver gold or alloys like silver gold or copper nickel or you can make a large number of alloys of various sizes say between 5 to 150 nanometers. Then you take this metal particle inside and on the outside this gray part you put your titanium oxide and there are methods by which you can make this shell out of pure titanium oxide. So, the smaller particles the better you will have higher catalytic property the efficiency will be high. So, if you have small particles of T I O 2 and you make a shell around the metal nanoparticles then you will get a structure of this. So, that will be core shell nanoparticle, but in this case it is metal decorated core shell that means on top of the T I O 2 you again have either some metal or metal oxide. So, either metal or metal oxide you can choose one of them and depending on that you will have now plasmon enhanced separation of electron whole pair. So, this metal nanoparticle will be activated by the light and you will have this plasmon surface plasmon because of the oscillations of the conduction electrons and that will couple to that T I O 2 and then you will get electron and holes and if you have a particle like a metal like platinum then the electron will quickly get separated on to the platinum and then the hole will reside in the core and the electron will move to the surface. So, you have effectively separated the hole and the electron if you use an oxide like ruthenium oxide then it does not take up the electron like platinum does, but it will remove the hole. So, then the electron will remain in the core and the hole will go to the surface. So, depending on whether you have put platinum or you have put ruthenium dioxide on the surface of the T I O 2 you can have the hole on the T I O 2 on the R U O 2 surface. So, hole will be removed from the metal or if you put metal particles like platinum on the surface of the T I O 2 then electrons can be removed more effectively and then the hole will remain inside the core of the core shell nanoparticles. So, this is a very efficient and interesting design of core shell metal oxide nanostructures where you have a three materials designed you have a inner core which is made of a metal nanoparticle like silver gold platinum or something and then you have a shell of titanium dioxide on top of that you have some decoration of small particles of either metal and metal or metal oxide and this particles on the surface which decorate the surface of T I O 2 act as a sink for either the electron or the hole. So, as we discussed if it is platinum particles on the surface it will act as a sink for electrons and then the hole are inside the core and the electron is on the surface. So, they are charge separated. So, the efficiency of the photo catalysis is enhanced the opposite is the hole goes to the surface and the electron stays in the core that is possible if you use ruthenium dioxide as particles decorating on the surface. Now, if you have metal particles covered with T I O 2 covered with S I O 2 or T I O 2 then the visible light is not active because T I O 2 will not activate the will not generate electron or holes because the band gap of T I O 2 is 3.2 electron volts and the visible light is insufficient in energy to create electrons and holes. So, this is not possible. However, if you shine UV light and you have the metal and you have T I O 2 then you can have electrons and holes, but the metal core shortens the electron hole pairs. So, they recombine and hence you do not get charge separation easily and so this is not going to be a very efficient photo catalyst. Although you will generate electron and hole, but the lifetime of the electron and hole will be very small and they will quickly recombine. So, this is also not a good situation. Now, the third situation is that you have a metal particle inside then you put a layer of something like silica this brown part and then on top of that you put titanium and titanium is a efficient UV photo catalyst and this titanium which creates the electron and holes the because of the silica coating the electron which goes towards the metal will not be recombining with the hole. So, electron hole recombination will be prevented by the presence of an intermediate thin layer of silica. So, this is another design of core shell structures where you have made titanium to be active in the UV efficiently. Of course, still it is not active in the visible with this configuration, but it is active in the UV and the efficiency is high by putting a silica layer in between the metal core and the outside titanium which will actually create electron hole pair when UV light shines on it. So, these are three models one model not effective in visible light, second model not effective in visible light and also not effective efficiently in UV light and this is a model which is not effective in visible light, but very good catalyst in UV light. Now, if what is the role of the metal oxide you can dope metals we studied have different role and we looked at several cases of metal doping how it can remove the electron if it is outside and what it does if it is in the core. Now, if look at metal oxide and choose a metal which can have variable oxidation states. Now, these can lot of all transition metal oxide show variable oxidation states for example, iron shows oxidation states of 2 plus 3 plus sometime 4 plus and in extreme cases even 6 plus. So, you can have iron at with various oxidation states similarly, you can also have cobalt with 2 plus 3 plus or vanadium 4 plus 5 plus etcetera. So, if you choose a metal oxide which can show variable oxidation states and dope it with a titanium Ti O 2 then it can be a very good photo catalyst in the visible range. So, how does that happen you have again the large sphere showing you Ti O 2 particle and the band gap of Ti O 2 this is the conduction band and valence band and now you have doped this titanium dioxide with metal oxide. The metal oxide that you have chosen is iron oxide. So, iron oxide will have possibilities of being in iron 2 plus 3 plus 3 plus 4 plus and. So, if you have this kind of oxides on top of the surface. So, what can happen. So, if you have electron and hole generation even with less energy you can create these levels where electron in the conduction band can go to the iron 3 plus and iron 3 plus will get converted to iron 2 plus. So, you start with Fe 3 situation. So, iron is 3 plus, but iron 3 plus when it gains an electron becomes iron 2 plus and if you have a electron which is coming you can do this reduction. Now, if you have iron 3 plus here. So, if you have a hole iron 3 plus will get converted to iron 4 plus. So, in the presence of electron iron 2 plus iron 3 plus will get converted to iron 2 plus in the presence of hole iron 3 plus will convert to iron 4 plus. So, both are possible because of the variable oxidation state of iron and the dopant also creates addition level. So, these levels are created additional levels. So, although C I O 2 has this band gap, but the actual band gap will be due to the smaller the intermediate levels which are introduced by the metal oxide dopant. And then this gap is lower in energy than this gap and if this falls in the visible light region then it will become a visible light photo catalyst. So, this kind of band gap engineering then creates the dopants create additional levels and lowers band gap from the ultraviolet to the visible region and hence this catalyst becomes active in the visible region. So, here the important thing is you have chosen a metal oxide with variable oxidation states. Now, the dopant also acts as electron hole trapping center because here you can see this acts as a electron trap because it is removing electrons from here. And this way you if you can choose some other material it can act as a hole trap. So, you can have dopants which not only lowers the band gap from UV to visible, but they are also acting as electron hole trapping center. Finally, they also can be used as carrier agents to facilitate migration of electrons and holes to the reaction sites. So, wherever reaction is occurring because of the presence of these metal oxides on the surface they act as good carrier agents because they help in the migration of the photo generated electron and holes. So, two things are important as usual charge transfer events how the electron and hole are created and then migration of these charges to the surface both these effects enhance the efficiency of the photo catalyst. Now, this is a case where it is photosensitization using you visible light. So, how you are going to use visible light using a dye which is acts as a photosensitizer. So, in the previous case you brought down the band gap by adding metal oxide as dopants and then visible light became useful to make it act as a photo catalyst. So, T I O 2 with the iron oxide dopant became a photo catalyst in the UV, but here you are not using a metal oxide you are using a dye which is organic compound. So, you see this large molecule with the aromatic rings and some pendant substituents you can see that this dye which is sulphurodamine B this particular dye can be used to sensitize T I O 2 because the energy levels of this sulphurodamine dye is in the region of the visible light. So, using the energy levels of the dye of the sulphurodamine B you can excite the sulphurodamine B using visible light such that the electron goes to a excited state and this S R B excited is the excited molecule and then the electron in this state can be transferred to the conduction band of T I O 2. So, the visible light is basically taken up by the dye the sulphurodamine B dye here and electron gets excited and in the excited state the sulphurodamine B then transfers an electron to the conduction band with which it has an interface. So, the dye is doped on to the T I O 2. So, this electron is transferred to the conduction band and then that electron can be further used for reduction. So, it is then acting as a catalyst because it will do this reduction reaction and if some molecule is there which can pick up an electron then that will get reduced. So, what happens to this molecule after it loses this electron the sulphurodamine B excited molecule after it loses an electron it becomes a radical cation. So, after removal of the electron the electron goes to the titanium dioxide and then the sulphurodamine B becomes a radical cation and then it reacts with oxygen further and then that forms another radical cation and which finally decomposes to form a smaller molecule. So, this organic structure gets decomposed. So, that is also a catalytic reaction that means you are trying to remove an organic dye you are broken the organic dye by exciting the organic dye with visible light and this photo degradation of the organic dye through a radical cation in the presence of oxygen can be seen and the final degradation products of this chromophore. Chromophore is something which can give out light. So, because it flourishes sulphurodamine B is has a property of flourishing and so it is called a chromophore and is organic compound. So, here we are more interested in seeing that how titanium was photosensitized using visible light and the dye breaks down and gives rise to some products plus may be some ions like sulphate ion ammonium ion and gases like carbon dioxide or water vapor and this electron of titanium is also used for reduction. So, this is a case of how organic dyes having a particular difference in the energy levels in the ground state and excited state can be used in conjunction with T I O 2 to act as visible light photo catalyst. So, important thing very important thing is that this energy gap should be such that it can be this molecule can be excited with visible radiation and then only you can get this S R B star which means the excited sulphurodamine B is the ground state of sulphurodamine B is the excited state of sulphurodamine B and from the excited state electron transfer to conduction band of T I O 2 and then it becomes radical cation and further undergoes degradation in the presence of oxygen and this electron can be used for reduction of some other species. So, this was another case. So, we studied cases of metal doped titanium, core shell titanium decorated with metals, then a core shell titanium with an interfacial layer in between how it can become efficient UV photo catalyst. Then we added metal oxides where the metal has variable oxidation states and how that can enable titanium to be a visible light photo catalyst and then this was a dye sensitization. So, the dye an organic dye is used which has particular energy levels which can accept visible light and hence it is a visible light photo catalysis that can be observed using the sulphurodamine B as an example and the electrons in T I O 2 can further reduce other agents. Now, this another property the previous one we photo sensitized using visible light. Now, let us look at another problem where we can do photo catalysis using UV light and with the dye, but this is another mechanism. So, under UV light the energy will be absorbed now with T I O 2. In the presence of visible light T I O 2 cannot accept the radiation because of the lower energy, but the sulphurodamine B accepts the radiation. In this case the UV region the higher energy light is being accepted by the T I O 2 nanoparticles because it matches the energy. So, 3.2 electron volts is in the ultraviolet and so you will have electron generated in the conduction band and holes in the valence band and so this electron can then reduce oxygen and this hole can then act on the dye. So, here we are looking at addition of a hole on the dye to create again a radical cation of sulphurodamine B. This again is simple from here what we studied in the previous slide that the radical cation then acts with oxygen to give you another radical cation and that breaks down to give you small molecules like diethylamine, N N diethylacetamide etcetera and then in the presence of holes or hydroxyl radicals it gets converted to this simple molecules or ions. So, in the previous case the radical cation of sulphurodamine B is formed by the removal of electrons from the excited state of sulphurodamine B and you get the radical cation. The same radical cation in this case you obtain not by removal of electrons, but by addition of hole and so two different processes one acting in the presence of UV light and one acting in the presence of ultraviolet light in the presence of sulphurodamine as a dye or an organic reagent how the same product of the dye occurs can be understood because in this although the mechanism is different in this case the sulphurodamine B degrades through the addition of a hole and in the previous case the sulphurodamine degrades by removal of electron and then goes oxidation and then degradation. So, two different things using similar materials T i O 2 and sulphurodamine B as the organic dye, but the different mechanism of the degradation of the dye is due to the different energy that you are supplying in one case you are supplying visible light in the other case you are supplying ultraviolet light and hence the mechanism changes and the degradation happens due to electron in one case that loss of electrons in one case whereas, in the other case it is addition of holes, but both cases lead to the radical cation of sulphurodamine B and then further reduction further degradation in the presence of oxygen. Now, let us look at few other case studies where we will see what is the role of network of corner shade octahedral units of metal cations. So, we will see when you use metal oxides other than T i O 2 these are not T i O 2 or these are other oxides why they are photo catalyst in certain cases especially when you have corner shade octahedral it shows high photo catalytic efficiency. Then what is the role of the dipole moment and distortion in symmetry how it affects the photo catalytic properties then effect of the electronic band structure that is the overlap of the orbitals of the metal oxides. If there is good overlap the bands will become broad and things will be different and so effect of overlapping of orbitals leading to changes in the electronic band structure then what happens when you have a mixed configuration of metal oxides. So, we will look at some of the case cases which have these different properties. So, in this particular case this is the basically you are looking at how the overlap of the metal particle metal and the oxygen their orbitals overlap and if it is good overlap or bad overlap what happens to the photo catalytic properties. So, these are not T i O 2 now we are discussing other oxides say this is a tantalum based oxide and it can be having a third element also. So, there is some rarer tantalate or barium dependent tantalum oxide etcetera. So, this is rarer tantalate L A T A O 4 in which case you have an orbital of tantalum 5 D or band of tantalum 5 D all the orbitals of 5 D overlap to form a band of tantalum 5 D which looks like this and this is the energy scale and this is an oxide. So, you will have oxygen levels and the oxygen levels are here much below the tantalum 5 D levels which means this band has been formed from the 5 D orbitals of tantalum and this band has been formed from the 2 P orbitals of oxygen. Now, when you have lanthanum also lanthanum appears to have very sharp band or narrow band this is called a narrow band because the width of the band is very broad here in tantalum where compared to that this is very narrow is almost like a discrete energy orbital. But now the energy of the lanthanum 4 F orbital which is a very narrow band is lying somewhere in between within the energy levels possible for tantalum 5 D band when such a thing happens that the orbital energies are similar then there is good overlap of bands. So, there is excellent overlap of the lanthanum 4 F and tantalum 5 D bands and good overlap of these bands increases the activity of the catalyst. And that is overlap of that is the case shown here where the highest activity is reported because it has excellent overlap of the lanthanum and tantalum orbitals and bands. Now, here you see that this narrow band has shifted down this is the case of cerium and so it will have moderate efficiency whereas in this case where the precedomium compound the narrow band of corresponding to the precedomium 4 F orbitals is much lower and has no overlap between the tantalum 5 D orbitals which are contained in this band with the precedomium 4 F orbitals. So, there is hardly any overlap and if it there is no overlap then this acts as an electron trap. So, once electron comes it stays here and so it can act as a electron trap especially in the 4 F orbitals of precedomium. Since it is a narrow band the more wider the band there is more delocalization the more narrower the band then it is complete localization and very little electronic movements it acts like a electron trap. So, from the band structure this kind of diagrams are called band structure diagrams and give you a lot of idea about the possibility of the movement of electrons the conductivity of electrons through electrons and the band gaps and difference in what is what will be the optical band gap all this you can understand by looking at the detailed band structure for these oxides. So, one thing is clear that better the overlap of the orbitals higher is the activity and poorer is the overlap of orbitals or bands then the activity will be very low and the electron will be trap and the electron will not be able to reduce anything because it is trap there. Now, also possible that you can have a high activity without even a co catalyst co catalyst sometimes is added which helps in reducing the band gap or helps in effective electron transfer or hole transfer. Now, zirconia has a band gap of 5 electron volts and it is a unique photo catalyst that shows a very high activity and the reason of this is it has a high negative flat band potential. So, if your flat band potential is very high then your it should be negative the high negative flat band potential will lead to very good photo catalysis. So, this is an example zirconia is an example of that another very important thing is corner sharing octahedral unit. So, many many structures are known where the metal oxygen bonds are within an octahedral and these octahedral are corner shared or edge shared or phase shared. Now, the most important and a large number of compounds are known where there are corner shared octahedral units of the metal cations. So, if it is a tantalate you have ta osix octahedral. So, niobate you have a niobium osix octahedral and these niobium osix octahedral or tantalum osix octahedral are corner shared. Now, if they are corner shared then it appears that the activity is very high because this corner shared octahedral as shown here will help in migration of electrons and holes and this will be this has been shown to be very good photo catalyst for a water splitting reaction. The reason is the tantalum oxide is highly connected corner shared octahedral units whereas, the rarer the lanthanum also has an octahedra, but it is not connected to the tantalum octahedra in such cases the photo catalysis is very high. There is another example where you have two types of octahedra tantalum oxide and another rarer toxide, but when there are two types of octahedra and they get interconnected then the activity for the photo catalysis decreases. So, what it means is you need one this tantalum oxide or niobium oxide whichever is going to act as the main site for photo catalysis should have this corner connectivity of octahedra and it should not be interfered by some other octahedra say of a rarer. So, if it is unhindered then the activity is very high if it is hindered by other octahedra then it becomes inactive. So, this is very important the role of corner shared octahedral units and the presence of linear chains of these octahedra is very important. If you have distortion in octahedra and if you have dipole moment then what is the effect on the water splitting or photo catalysis activity. Now, the activity for D 10 metal oxides is strongly dependent on the distortion on their structure and you can see that whenever you have a distortion then you will have local internal fields which will contribute to electron hole separation on photo excitation. Now, if you have distorted gallium oxide like this which have a net dipole moment they were found to be photo catalytic activity. So, dipole moment will be generated as a cause of distortion. So, whenever you have a distortion you have high dipole moment this is 0 dipole moment very low activity is high dipole moment and this is high activity. So, distortion high dipole moment high and so it will lead to higher activity. So, there is another case how you can increase the activity by better overlap of metal oxygen orbitals and you can increase the bandwidth by having good hybridization or overlap of metal oxygen orbitals and that will also give you increased mobility of the photo generated electrons in the conduction band and lead to high photo catalytic activity. So, these are some of the examples of oxide photo catalyst based on D 0 metal ions. So, you can see titanium anatase and very good photo catalyst you can see a lanthanum doped sodium tantalite where you have T A O 6 of the hydra and these are linear and shows very high efficiency. So, with these examples I will come to the conclusion of this lecture today and then we will have our continuation of this lecture in our next class. Thank you very much.