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Video 1: Photocatalysis
Welcome back to the lecture series in application of nanotechnology in agriculture.So today we are talking about one of the case studies in last couple of classes of titaniumdi oxide, we have already seen it has agricultural applications, now you are observing that thesame material is having applications in diverse areas including electronics, photo electrochemicalcell, splitting of water in a photo electrochemical cell by virtue of which it can be used forpurification of water.Production of hydrogen as a fuel it is having amazing anti-fogging and self-cleaning propertybecause when for excited its hydrophilicity become marvellous it can self-clean itselfand the light falls on it.So it could be used as the coating material for vehicles and then we observe that thiscan be used as desensitized solar cells.So let us look at little bit now about the electronics which are involved in it.We will be talking about now electronic processes of titanium dioxide photo catalysis, electronicthis will be brief electronic process of titanium dioxide, photo catalysis.Now we need to talk about this photo catalysis is widely used to describe the process inwhich acceleration of a reaction occurs there is an acceleration, so there is a reactionhappening and this reaction is getting accelerated when a material usually a semiconductor interactswith light of sufficient energy of a certain wavelength.So basically there is a semiconductor, this was your semiconductor, so the semiconductoris interacting with light of certain wavelength or multiple wavelength, is interacting withmultiple or specific wavelength, while it attract with the light, interact with lightof sufficient energy to produce reactive oxygen species are ROS, so this semiconductor isproducing ROS reactive oxygen.And you all are well versed with ROS, we have talked about CeO2 cerium oxide with scavengerROS.Now we are talking about a material which generates ROS, having just give you littlebit of overview of this concept, when you talk about ROS here, so as I told you ROSis a double edged sword, at one hand it could lead to inflammation and diseases and otherhand in human body itself ROS could be used by the immune cells to fight against microbes.But then there is another aspect of ROS, so what we have learnt in biology is it couldhave detrimental effect and or it could have it could have negative, it could have positive,but in industry section ROD could be used for destroying deadly organic waste.So this ROS on a different fragment could be used for a totally different purpose.Now coming back to the mechanism what you are talking about which can led to the photocatalytic transformation of a pollutant.So this ROS is basically here we are talk about that whole pollutant disposal aspectsof it, it must be noted that during the photo catalytic reaction at least 2 even must occur,and this part is the extraction important there need for 2 events which must occur simultaneouslyin order for a successful production of ROS species to occur.What are those events E1 and E2, for successful production of ROS which is reactive oxygenspecies.So that 2 critical events involves where water which is absorbed in the material has to getoxidized and the event 2 which will I be talking now.So the first event which needed to happen out here is this is the event 1 where wateris coming and what it is doing is that oxidation of the dissociative adsorb H2O.This is the dissociative adsorb H2O by photo generated holes.So these are the photo generated holes, so these photo generated holes exactly leadsto oxidation of the water and the second even involve reduction of an electron acceptorwhich is typically dissolved oxygen, reduction of an electronic acceptor and within thiscase is oxygen which is the dissolved oxygen.So it is the dissolved oxygen by photo excited electrons.These reaction led to the formation of hydroxyl this whole thing leads to the formation ofhydroxyl and superoxide radical anion hydroxyl and superoxide, so hydroxyl and superoxideradical anions which is essential is your reactive oxygen species, it is clear thatphoto catalysis implies photon assisted generation of catalytically active species rather thanthe action of light as a catalyst in a reaction.So this part is important to understand it is clear that when you talk about photo catalysisimplies photon assistant, this whole thing is photon assisted, so photon assisted generationof catalytically active species, so you are generating catalytically active species.So this catalytically active species are basically photon generate, so rather than action oflight as catalyst in a reaction.So essentially what is to be highlighted is that light is only providing sufficient energyone should realise light is just that mild energy which is needed.So you have this light coming on a semiconductor material and semiconductor material is ejectingout an electron.So light is just providing that assistance, so it could be provided by something elsealso, in order for that it may be heat also.It could be something else also, but the semiconductor materials means that small amount of energyto ensure that it generates an electron, why this is important if you look at it and ifyou go back to when I was talking about photosynthesis exactly the same thing, is visible light whichis providing the energy to happen and why it is call photosynthesis is that.This is driven in terms of it is assisted by light.And (10:47) it could be any light photosynthesis, it is the life which is helping for providingthe sufficient energy for the synthesis of the carbohydrate molecules exactly same wayjust imagine in the centre instead of chlorophyll which is nature’s molecule for generationof photo excited electrons.So these are those photo excited electron.Now just equated with TiO2.So in other word then chlorophyll is also a semiconductor material.And it has certain light where it generates an electron.So this is that similarity I am trying to draw and in result is fairly the same, soand that reactive oxygen species are the reactive electron is out here, that electron whichis photo excited.Now if the initial photo excitation process occurs in an adsorbent molecule which theninteracts with the ground state of the catalyst substrate.The process is referred to as catalyst photo reaction, if on the other hand the initialphotos excitation takes place in the catalyst substrate and the photo excited catalyst thatinteracts with the ground state adsorbent molecule.The process is called sensitized photo reaction.So there are 2 concept catalyse photo reaction and sensitized photo reaction.This catalyse if you wanted to going depth these are the terminologies you have to understand.Catalysed photo reaction and sensitized photo reaction.These are the 2 concept when we talk about these kind of materials, catalyse photo reaction,if on the other hand the initial photo excitation takes place in the catalyst substrate andthe photo excited catalyse then interacts with the ground state adsorbent molecule theprocesses a sensitize photo reaction and in most cases heterogeneous photo catalysis refersto semiconductor photo catalysis or semiconductor sensitize photo reaction.

Video 2: Role of Titanium Dioxide in Agriculture
So in photo catalysis if you look about it light of energy greater than the band gapof semiconductor.This part is extremely important light having an energy greater than the band gap of a semiconductor.So in titanium oxide if you remember when we talk about retile, we talk about it isa band gap of 3 to 3.2, it has 3 polymorphs and they vary from 3 to 3.2 electron volts.So the light energy what has to be provided in photo catalysis light of energy greaterthan the band gap of a semiconductor.So the light energy h mu has to be greater has been light energy has to be greater than3 to 3.2 electron volt.In case of anatase Tio2 the band gap is 3.2 electron volt, therefore UV light of wavelengtharound 387 nanometer is required.This has more energy than 3.2, the adsorption of the photon excites an electron to the conductionband, the adsorption of the photon, so you have the semiconductor material which is gettingwavelength from in that case that case what will happen to repeat it.In the case of anatase TiO2 the band gap is 3.2 therefore you needed a UV wavelength of387 nanometer, the absorption of a photon excites an electron to the conduction bandwhich is e CB which is the conduction band electron now generating a positive hole inthe valence band, in the valance band hvB because valence band is losing an electrontherefore this becomes a positive.So now we have TiO2+h mu is generating a hVB, this is the valance band, this is the oneand + an electron eCB.So this is in the conduction band and this in the valence band.The charge carriers can be trapped, now this part is extremely important to understandthe charge carriers can be trapped as Ti3+ and O defects site in TiO2 lattice.So titanium has this defect site.So as remember when we talked about iron pyrite I told you that iron pyrite has been stoichiometricdefects.So much of these molecules come with a series of isometric defects such a defects whichcould be identified by using x-ray photoelectron spectroscopy and very interestingly thesedefects could be utilised for its application.So defect is in a sense give the material positive layer in terms of its utility, usabilityand translatability.So coming back where I was so I told you that TiO2+ water+ sorry + light is generating combinationelectron and hole.And the charge carriers can be trapped as Ti3+ and O- defect site in the TiO2 latticeor they can recombine dissipating the energy and this is another problem that they couldeven recombine electron and whole can recombine and they can dissipate the energy.So you have to prevent the recombination alternatively the charge carriers can migrate to the catalystsurface and initiate redox reaction with adsorbent.Now this is what you are asking for alternative labour charge carriers can migrate so youhave the charge carriers.So these are the charge carriers what you are having, so these charge carriers are adsorb,charge carriers can be trapped into Ti3+ or O- sites or charge carrier may recombine anddissipate the energy.Now what will happen alternatively the charge carriers can migrate to the catalyst surface.And initiate redox reaction with adsorbent and adsorb has water, positive holes can oxidisedOH- or water at the surface.So positive holes can oxidise OH- of water or water because when we talk about waterwe are talking about H+ and OH-.So now this OH- is transform to produce OH radical which is hydroxyl radical, we talkedabout it just few minutes back which are extremely powerful oxidant.So this was one powerful oxidant because we are generating ROS reactive oxygen species.Similarly the hydroxyl radical can subsequently be oxidized OH could subsequently be oxidized,because can subsequently oxidized the organic species.So this is when you talk about the organic pollutants or any other stuff oxidized organicspecies with mineralization producing mineral salt like Ceo2 and water.So now if I so this is that reactive oxygen species whatwe are creating eCB- in the conduction band +H vB+ which is in the valence band youhave the energy and H2O+h in the valence bandis generating OH.+ a proton which you could see as a H+ are there.Now on the other hand the next set of reaction varyof oxygen+eCB conduction band generate another free radical or another reactive oxygen species.Now here is next reaction where you are having OH dot+the pollutant is generating H2O+CO2and the other reaction could be O2.- +H+ generating OOH or OOH+another OOH it could create hydrogenperoxide.This will remind you ofthe role of pyrite while generates trace amount ofthis O2.- + a pollutant.This is where you are destroying theextremely harmful organic waste H+pollutant generating CO2+H2O.And if you look atit there is series of reactions which are happening here and these are and these arejust a handful of them.So electron in the conduction band can be rapidly trap by the molecular oxygen, so electronin the conduction band could be rapidly trap by the molecular oxygen adsorb on the titaniumparticles which is reduced to form superoxide radicalwhich is O2.-.And that may further react with H+ to generate hydroperoxyl radical.This is called hydroperoxyl radical and further electrochemical reduction yields H2O2 as Ishowed you inthe reaction.These reactive oxygen species may also contribute tothe oxidative pathway such as degradation of pollutant.So to realise that same titanium dioxide whichhas agricultural role has now showing tremendous promise in photo electrochemical cell.So I am closing here, inthe next class will little bitfurther before closing down, thank you.