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Module 1: Solar Energy and Photocatalysis

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Approach to Tackling an Electron-Hole

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Now, when this has access to water, this H plus which is you know hole and not to be confused with the proton, the H plus the that’s why have you small h. So, small h is the hole. So, h plus hole capital H plus is a proton okay, hydrogen atom without electron. So, that capital H plus is a proton which is a hydrogen atom without its electron right. So, now, this h plus is available this hole is available it’s a positive charge. So, if it reacts with the species and pulls off an electron, it can essentially do oxidation. So, that’s what it does. So, it reacts with water and creates 4 protons plus oxygen ok. So, it has now created 4 protons of the 4 holes reacted with water and created 4 protons and released the oxygen. So, the 2HO we gave you 4 H plus and O2. So, this happens at one location, some location on that particle. Now the electron has been put into the conduction band it may go to some other location, that location it will if it finds protons in that location 4 H plus these electrons that you brought here will now react with that proton and get you hydrogen okay. So, this process you have now taken you to know a situation where at several locations on the particle, the holes are reacting with water generating protons and generating oxygen. This proton is moving around or is in a position to move on and if it happens to also see electrons coming there from the conduction band these protons react with those electrons and generate hydrogen. So, you can generate oxygen on at some sites and you can generate hydrogen at some sites. So, this is the overall reaction, you have water and in the presence of sunlight and the presence of this photocatalyst, you are generating hydrogen and you are generating oxygen okay. But the photocatalysis is important; if you just add sunlight this is not going to happen. The sunlight acts on the photocatalyst and does this transition, only because of this transition now you have an electron that is available at some location which can do the reduction and a hole that is available at some other location which can do the oxidation. And since you can switch these two reactions I mean locate these two reactions and specific sites, you can get the hydrogen off the location where reduction is happening and you get oxygen at the location where the oxidation is happening okay. So, this is how it is happening, now I must also point out that you know you had the first step of this before this is the first step of this you know incoming radiation that came in and did this transition. So, you can consider this is step 2 and so, you created in step 2 you created these electron-hole pairs right. So, as step 2 was electron-hole pair creation. So, in step 2 after that is done, the electron and holes have two options available to them. They can migrate to the surface which is what you are seeing here. So, after step 2 they could either migrate to the surface or alternately they can simply recombine which is what you see here. So, we can consider this as 2 a for example, and this is 2 b for example, two possible ways in which this can happen right. So, or we can 3 a and 3 b if you want to look at it that way. So, two you have done you know the transition from the valence band to the conduction band, and then a step 3 you can do something with those that electron-hole pair. So, one possibility is that it goes to the surface. So, that is 3 a, and the other possibility it is that it simply recombine. So, this is also 3 a. So, 3 a is that this electron-hole pair moved to surface sites, where they can do you know any reaction or 3 b is simply they recombine. So, this recombination 3 b is the recombination and that is a loss in the sense that you use the photon to create this electron-hole pair, but you could not use that electron-hole pair to do anything useful you lost the electron-hole pair right. So, that 3 b is a loss and then the final thing that happens is 4 which is the reaction right. So, these are the various steps. So, if you want to put it down we have one incoming photon, 2 electron-hole pair generation, 3 a moving to the surface site, 3 b recombination and 4 surface reaction. So, these are the various processes we want to do 4. So, we want to in the scheme of what I have just described here we want to avoid 3 b. So, this is something we want to avoid, 3 b is the recombination is something that we want to avoid 4 is what we want to enable. So, that is a general idea. So, this is what is happening in photocatalysis, this is photocatalysis. So, this article would be present. So, essentially all you need is beaker in which you have water and you have this Potocatalyst material is in powder form available within that beaker. Once that is there and then you put it out in sunlight you would start seeing bubbles coming out, the bubbles should be both hydrogens as well as oxygen okay because in this beaker both are being generated. So, this is what is happening. So, that also causes a challenge because if you have hydrogen and oxygen present in the same beaker, they can react and again reform water. So, that is the reverse reaction is also possible here. So, that is also something that we don’t want. We don’t want recombination; we do not want a reverse reaction. So, this is something that we will keep in mind ok. (Refer Slide Time: 23:53) So, what are some considerations relative to this? So, now, if we see here we have if you go to the electrochemical series you will have here a potential for the hydrogen you know redox reaction, and that is usually set at zero volts, and concerning that, you have the oxygen reduction potential and so that, that will come to your 1.23 volts okay. So, this potential is a difference in potential is what you have to provide water to start splitting the water okay. Now you have used some photocatalysts right. So, you are generating an electron and you are generating a hole. Where should those electron and hole be in terms of energy for them to enable this reaction to happen? So, the way that is matched is that whatever is the conduction band level, that should be higher than this okay and whatever is the valence band level should be here. Only if you have this situation the electrons from here that electrons that are sitting up in this conduction band, will go and do this reduction process and the here you will also have the oxidation and therefore, I mean the holes from here will be in a position to participate in the oxidation process okay. So, that is how the energy of this system is set up, and that is what you see here this hole reacts with the water and generates the protons and that electron which is sitting here reacts with the protons and generates hydrogen. So, that is how these relative energy levels are set up. So, you have to have a bandgap which is higher than that that corresponds to this 1.23 volts, and you have to have it’s not just the value that is important, the position of the band is also very important. Because only then you will have oxidation occurring at one location and reduction occurring at the other location, otherwise only one reaction will be favoured, the other reaction will not be favoured and since you have to have continuity the reaction will stop. Otherwise you little it will not proceed so, because you are not completing the process. Now incidentally there is something that we have to pay attention to here when you look at the band energy levels. So so, for example, this energy level and this energy level here, these are defined relative to the vacuum level. So so, there is some vacuum level sitting there which is set at 0. So, this is a vacuum, this is set at 0 and relative to that you have all these energy levels. This on the other hand is you know standard electrochemical series based set up, where arbitrarily this reaction has been set at zero volts, and relative to that you have all the other voltages. So, on the one side, you have an absolute scale, on the other side, you have a relative scale okay. So, we have to at least be alert to that. So, they on the one side you have an absolute scale and the other side you have a relative to the scale. So, then how do you match them up? So, sometime in the seventies, people have done a lot of work on this. So, they have looked at what happens when metal gets oxidized. So, oxidation of different systems they have looked at especially metallic systems getting oxidized, and then and looked at how you know oxidation occurs in different systems. The oxidation of many solids is directly related to the location of their bands, because that is how again this is exactly what is happening what I have shown you on the picture here is exactly happening when you oxidize a metal also when you create a metal oxide things like that. So, by comparing those band energies they have been able to figure out how to line up those band energies relative to the electrochemical series because that potential is required and that position is required for the process to occur. So, by understanding that carefully, they have understood how to line these two up and that is how you know that the TiO2 which is a material that I have mentioning here, titanium dioxide is usable in this condition. Incidentally, the material that has been most used for doing this photocatalysis is titanium dioxide, that’s why it is shown here as well it is the I mean it is one of the earliest materials studied for this process, but there is more to it is also very stable material, and although they have looked at various other materials it’s also very I mean it’s not toxic in any way, it is a relatively safe material and therefore, it is you know preferred over many of the other systems that have been studied. People are still it still has some drawbacks. So, they have not completely sorted out all the issues with titanium dioxide, but it is still the preferred material for use. So, so that’s how you know this process is happening and that is how the band structure of the material which is your photocatalyst relates to the kinds of potential is required for water electrolysis and this is how the process works okay. So, this is what we are looking at. (Refer Slide Time: 29:03) Now, just to you know convey this idea of this band position versus you know the water electrolysis process, I have just shown you here not just the titanium dioxide which is what we first looked at, but other materials you have zinc oxide, you have silicon carbide, and you have cadmium sulfide. So, 3 different additional materials I am showing you, in all of these materials have enough window between enough bandgap that and the position of their bands is correct, because all of them have their valence bands sitting below this value, and all of them have conduction bands sitting above this value. And therefore, they can support reduction processes the conduction bands can support reduction processes of them given that they have access to water, and the valence bands can support the oxidation processes given that they have access to water right. So, this is how it is set up, and the band gaps are also very important because they decide; what is the energy of the incoming photon that will be required to do the transition right. So, it is not it’s not something that we can ignore. So, for example, I mean we if you have a bandgap that is too small, that will that’s a material that will not work. If you have material which has this position for bandgap that will also not work if you have this position for bandgap also will not work relative to these dotted lines okay. So, the only ones that will work are the ones that you are seeing on your image. So, that’s the way you want to look at. But I mean even though you have no choice of materials that are listed here, as I said you know other considerations are there such as you know toxicity and so on. So, cadmium is for example, not at all favourable from that perspective, but you should also understand that the if you have a very large bandgap, then you are expecting very high energy photon to enable the transition and therefore, we need to compare this set of band gaps that you see here, to what is coming in your our solar radiation. (Refer Slide Time: 31:15) So, if you look at it the visible spectrum has wavelengths from 400 nanometers to 700 nanometers, and this corresponds to band gaps of 3.1 electron volts to 1.8 electron volts. So, 1.8 would be your red end of the spectrum going up to 3.1 which is the violet end of the spectrum. So, if you go back here, you find that titanium dioxide which is extensively used is 3.2, which is even more than 3.1 right. And so, these are all significantly large band gaps that you are seeing here. So, if you compare 3.2 which is as I said the titanium dioxide is the most commonly used material that is a value that is greater than this, 3.2 electron volt Eg for Ti right. So, Eg for Ti is 3.2 if you want I put it there TiO2, 3.2 electron volts. So, it means you have to have a photon of energy greater than 3.2 electron volts, to do the transition which means it is of energy level higher than this and wavelength the values lower than this, which means you are looking at not the visible spectrum. So, the visible spectrum cannot do a transition in the titanium dioxide sample okay. So, the visible spectrum cannot participate in photocatalysis. If you are using titanium dioxide as the catalyst, you cannot use infrared also because infrared is even lower energy than the visible spectrum. You can you only use ultraviolet radiation that too with a small I mean not all of it, but little past 3.2 from 3.2 electron volts onwards. So, you can use the ultraviolet part of the radiation. So, now, if you look at the spectrum, which is what you see here, you are looking at 97 per cent is visible plus IR infrared; visible plus IR is 97 per cent of the spectrum, only 3 per cent is ultraviolet. So, by using titanium dioxide even though it is the most popular photocatalysts and so a lot of people work with it, you are working only with 3 per cent of the incoming radiation you are missing out 97 per cent of the radiation and therefore, from a scientific perspective it is interesting to try and see if we can do things, too I know to alter the situation right. So, this situation is not necessarily acceptable to us right. (Refer Slide Time: 33:49) So, as I said here the limitations are only the ultraviolet part of the spectrum is utilized, we also said that you know that you can have some amount of electron-hole recombination, because you have put you know electron in the conduction band well in the hole in the valence band, and if you just give it some time it will recombine. I mean it it’s just a, you know there is a probability associated with it if you the longer the time will give, it will recombine. And also given that hydrogen and oxygen are getting generated in the same chamber essentially, you have a great possibility that after doing all this you are still not able to use the hydrogen, because before you use the hydrogen it is already reacted back with water we are reacted back with oxygen and generated backwater. So, that’s the reverse reaction. So, you have 3 challenges there, you have to deal with these challenges to know more effectively utilize this idea of photocatalysis. (Refer Slide Time: 34:39) So, there are some approaches. So, the approaches are you have to do some possible band gap tuning. So, there are ways in which you can change the bandgap or at least provide a different kind of band gaps for the reaction to occur, which will then help you create a situation where you are not restricted only to the ultraviolet part of the spectrum that you can use the rest of the spectrum as well. We could also look at adding something called a sacrificial agent or reducing the particle size, both of this help prevent the recombination process. So, for example, this particle size reduction is an interesting way to go because, when you reduce the particle size there is less distance for the electron or hole to travel to reach the surface site. And the lower the distance that is there for it to reach the surface site, you are reducing the chance that it will participate in any other reaction that it will recombine itself, it will reach the surface and do the reaction that you want. So, that’s one thing that you can do sacrificial agents are another way in which you can you know prevent the electron-hole from recombining, we look at it and of course, co-catalysts can be used to you know which slow down the reverse reaction ok. So, that is something. So, I will particularly more actively look at the bandgap tuning and the use of the sacrificial agent. (Refer Slide Time: 35:53) So, the bandgap tuning can be done something like this. So, this is band gap tuning. So, let us say you have CdS in addition to TiO2. So, now, this is a little bit tricky because there are several things that we have to consider, we have to consider what is the kind of contact between them, how do those bandgaps line up because typically Fermi energy will be used to figure out, how they line up. So, those there are issues associated with this which we have to look at a little bit more carefully, but assuming we have looked at all that and this is the situation we have, then when you do the transition, from you know TiO2 and you get. So, this is CdS here, you do the transition then basically the electron will stay the conduction band level of TiO2 is lower than the conduction band level of the CDs. So, the electron continues to stay on the TiO2 and then helps do the; this reduction process. So, some species it takes and then does the reduction process. So, you get hydrogen here right. The holes that you generate the hole that you generate here are actually in a position to move up here, and they become more stable in the CdS setting and therefore, they can do the reverse process. So, they are in generating oxygen. So, the h plus moves off to the CDs, thee minus remains on titanium dioxide, and in that process and even if you generate any electron-hole pairs on the CDs, that will also do the same thing it will the electron will come off here the holes will remain here. So, the holes will preferentially collect on the cadmium sulphide, electrons will preferentially collect on the titanium dioxide. It is sort of like a p-n junction kind of it’s although you are not looking at I mean not necessarily looking at you know it is at least a junction between two materials, and then you may not have been doped it in any way, but you are creating a situation where electrons will preferentially stay on one the material, the holes will preferentially stay on the other material. And therefore, you generate oxygen at a slightly different location you generate hydrogen at a slightly different location. And more specifically the electron holes have a less of a tendency to now recombine because you have not allowed them to stabilize themselves later at different locations. So, this is called bandgap tuning, and more specifically it also helps you create a situation where you access a little bit more of the visible spectrum because you also now have the bandgap associated with CdS right, which is now smaller than the bandgap associated with the titanium dioxide right. So, therefore, photons of even lower energy can participate in this process. So, this is called band gap tuning. (Refer Slide Time: 38:31) Now, having created the electron-hole pair, we if we want to first of all again stabilize electron-hole pair, we want to not have a recombination process, we also want to you know to prevent the reverse reaction. So, as I said you know making the particle size smaller, reduces the chances of either of these electrons or holes encountering a defect in route and therefore, they don’t recombine they sustainably reach the surface and enable the reaction. But what do we mean by a sacrificial agent? So, for example, if you had glycerol, then what happens is after you have generated the electron and hole pair, the glycerol is what preferentially gets oxidized. So, glycerol gets preferentially oxidized generating all of these protons and generate CO2 okay. So, oxygen is not sitting freely now. Previously you had oxygen being generated and oxygen was available there that oxygen can re react with the hydrogen that you generated, and then essentially you know to do the reverse reaction of what you had originally set out to do. Therefore, the availability of free oxygen in the same location as hydrogen is not at all a preferred situation. It’s a safety hazard you do not want to do that. So, by having another agent there will some kind of alcohol you can create a situation where the holes that you generate will preferentially react with thus that alcohol oxidized alcohol generates CO2 and the protons? So, the same protons can additionally now go back here, in addition to the protons that are coming I mean. So, you get a lot of protons in the process and those protons can react with the electrons that are generated, and generate the H2. So, in this case for example, if you have 14 hydrogens. So, you will have 14 protons here, and that will react with the 14 electrons and you will get 7 H2. So, 14 protons and 14 electrons will react and get you your 7 H2, and that way this is you know balanced concerning the reaction that you see below here. So, that is how you can use and this agent that you are using this glycerol that you are using is what is being referred to as a sacrificial agent, and it is something that we do in an experimental setup we do it, but this opens up very interesting possibilities. So, in the realm of photocatalysis people look at photocatalysis for multiple purposes. So, we have spoken about it from the perspective of hydrogen generation right. So, that is the perspective in which we have spoken about it. We can also look at photocatalysis as a means of cleaning impurities that may be present in water, okay, and that process would be essentially similar to what I have just described, the process is exactly similar except that your impurity would be to take the place of this glycerol to take the place mean. So, you have water with impurity and you put photocatalysts catalyst in it, and you expose it to sunlight. When you do that you have this electron-hole pair is generated, the electron goes ahead and creates your hydrogen which is what you see here. So, your hydrogen gets generated; the hole goes and oxidizes some species in the water present in the water that species could be the impurity that you are trying to get rid of. So, it cleans up the impurities destroys, the impurity and converts them to carbon dioxide etcetera and generates more protons for you to convert to hydrogen. So, this is a nice way of combining two activities; you can do water purification you can also do hydrogen generation. And then the hydrogen generation the generated hydrogen you can use for running some you know activity that requires energy and when once that is completed, that hydrogen will again end up as water and so, you get back clean water right. So, this is a nice combination of activities that can be done, if you look at the possibility that your water might contain some contaminant, which you are sort of you know doing dual duty with this activity. So, this is one possibility. So, as you can see there are some challenges associated with the use of photocatalyst, and at least I have described you two different ways in which you can handle those challenges. So, you can use this sacrificial agent, you can also do band gap tuning. A bandgap tuning can also be done in different ways which is something we have not actively looked at through this course, basically again if you take a semiconductor which has a small bandgap and then you make nano-sized particles out of it, it turns out that you know the physics of it is such that the bandgap begins to increase right. So, therefore, you are accomplishing two things, you are also getting the small particle size plus you are also getting a larger bandgap of a say of a system. So, you can have as I have mentioned once before you can have a range of band gaps with the same starting material. So, therefore, you can have a range of band gaps which go through the entire visible spectrum. So, you can have photocatalysts with a wide range of sizes present inside the of the same material, present inside the water and then it will take care of taking the entire sunlight that comes in the spectrum of the sunlight that comes in and a vast majority of the spectrum will now get utilized for generating your hydrogen and oxygen. And then if you have some others you know you know sacrificial agent there that will take care of holding off the oxygen and so, you only get the hydrogen and some you know relatively stable species like carbon dioxide and so, that hydrogen you can then tap independently. So, this is a set of things that we can do and a lot of possibilities are there, extensive research goes on photocatalyst and it is something that you can look at in the literature. (Refer Slide Time: 44:16) So, to conclude photocatalysis can enable direct production of hydrogen from water, and that is the important thing here. We started by looking at electrolysis as a way of generating hydrogen and oxygen, and there are also some positives are there because hydrogen comes in separate chamber oxygen comes in a separate chamber and therefore, it is convenient to handle, but I told you that that’ a two-step process and therefore, there is inefficiency associated with it. So, when you can do a direct process direct production, that is generally more preferred. So, this direct production of hydrogen from water is preferred and therefore, photocatalysis is a preferred route to utilize, and it is also kind of static. You know you just put the catalyst you put the water you show it in sunlight that’s all you have to do and then you have you know one tube, which is pulling off the hydrogen that’s all that you have to do, you do not have to you know to come and check for battery this is that what not once the water runs out you add more water it keeps on generating hydrogen. And by definition, because it is a catalyst, the catalyst is not consumed in any way the catalyst stays there. It is only helping the reaction by enabling this electron-hole pair generation by taking in the photon enabling the electron-hole pair generation. If you set it up correctly the catalyst will not react and only then it is considered as a catalyst, it will continue to be there. So, you can keep on sending in the water you can keep on getting hydrogen and oxygen out. If you just set it up correctly and leave it in the sunlight. So, therefore, that’s a great positive you know all the time we are looking at ways of adding more energy to the system to get our job done here, you don’t have to do that you will get your job done. We also discussed the fact that matching the bandgap of the materials to the voltage window needed for the splitting of water is important. We have to understand what is that band gap, we have to understand those band positions and you have to understand where is this water you know splitting voltage window that is present. And only when you understand those do you will you are in a position to you know properly do this process and effectively generate the hydrogen. Otherwise, you will think it’s supposed to happen, but it is not going to happen. So, there is some you know a fair bit of you know knowledge that you have, to have to figure out which materials will work for this splitting, and that’s not simply based on bandgap it is also required that you match the window correctly window of this of the bandgap with the window for the water electrolysis process. And as I said one major challenge always remains about the separation of the generated hydrogen and oxygen and of course, if you want to use the oxygen itself, then it’s ideal to do it in such a way that you can tap the oxygen independently and tap the hydrogen independently. So, that’s not an easy process you have to think of some nice way of doing it. More likely than you if you can combine it with a process, where you are also doing some cleaning of some impurity in the water stream, then it works out much more conveniently because you are essentially doing two activities at the same time, and you are also not having this oxygen present therein as oxygen molecule which might then end up doing the reverse reaction. So, this separation can be done in multiple ways is the point we have to remember. So, these are our major conclusions for this class, and as I said this is an important research area, lot of people work in this research area for photocatalysis; because as I said this is also this is a lot of emphasis in there in this area for the hydrogen production part, but there is also a lot of emphasis in this area independent of hydrogen production, but focused on you know removal of impurities. So, for both those reasons, this is a very important area of research. And from the perspective of this course, from the perspective of all the discussion that we have done on solar energy, this is one additional way in which you can utilize the solar energy and with this discussion, we will sort of conclude our series of lectures associated with the solar energy. How it is captured, how it is utilized, what are material aspects associated with it, what are scientific aspects associated with it and what are technological aspects associated with it. So, that’s with that we will wrap up our discussion the solar energy usage, we will look at additional technologies and different ways in which we can use renewable energy in the classes going forward. Thank you.