 So you've you've heard for the last several days about all kinds of sustainable energy technologies, and I want to give you maybe a somewhat related but different take on it So we like to think about all of the various devices we can use and sustainable energy These are some of the common ones. So we have photovoltaics things like super capacitors batteries If you want to take sunlight and directly make fuels we might use something like a photo electrochemical cell We can generate things like hydrogen which we can store we might later want to turn that back into electricity So we would use something like a fuel cell The reason I put this slide up is if you look at all of these technologies today They already involve many many nanomaterials nanoscale materials So all of these components of things like photovoltaics or super capacitors if you look at a lot of the materials They're at a very small scale and I think going forward We're going to be able to do more and more powerful things get better energy conversion efficiencies by Exploiting some of the properties at the nanoscale. So that's why We look at nanoscale materials. You've been hearing a lot about energy I told you at the Tomcat Center to tie back into that for a minute We're interested in interconnections and other global challenges. So besides energy there are challenges in human health water food Etc and these are some opportunities that are coming up the pike These also will benefit in many ways Most of them from materials at the nanoscale So I like to think about those global problems having very small solutions And of course, you're probably all familiar with what nanoscale materials mean But we're basically talking about things that are 10 to the minus 9 meters in Size on average one thing that's interesting about working with nanomaterials is it often takes really big macro devices Or equipment to monitor them. So we have lots of big machinery The smaller the material the bigger and more complicated the machinery the apparatus usually is so these are just some Chambers and experimental apparatus in my own lab some of my students So we often have lots of big Devices to both make the materials and I'll tell you about one of the techniques we use to make nanoscale materials And then to monitor what we have we have to use a lot of spectroscopies involving photons and electrons to be able to see And microscopes to be able to see what we've made So I'm going to actually talk about two vignettes or two examples of applying nanoscale materials to energy conversion Systems that my lab works on I'm going to start off with solar cells And then I'm going to give you an example of using nanoscale materials and catalysis So I know you heard I think from Professor Mike McGee yesterday or the day before Talking about solar cells we work with Mike And I'm going to tell you about one of the things we've done looking at perovskite solar cells So it's hard to be Involved in any research at all in solar cells without having heard about perovskite So we like to show various efficiency plots when you look at photovoltaics versus year And if you look at most technologies, it's maybe a little hard to read this But these black dots are silicon crystalline silicon technologies. They're all going up That's the first thing to notice, but generally over the last couple of decades. They've gone up pretty slowly So here's silicon. It's creeping up one or two percentage over those decades here other thin film materials And then there's this red curve here, which is a new material or relatively new. It's just over the last say seven years Has gone up from just sort of a niche material with a few percent efficiency To being competitive with almost with crystal and silicon here above 22% so the current record using these Lead halide perovskites is over 22% Silicon still is great It's really really hard to compete with silicon because it's also efficient and it's relatively inexpensive But what's nice about these perovskite materials is you can make them from solution You can spin them on to some Substrate and anneal them and you can get good solar cell materials So they're solution processable and you can tune the band gap over a wide range and that makes it useful for things like tandem solar cells Which I'm going to mention So tandem solar cells are a good way to raise the efficiency compared to just your standard single junction solar cell which involves typically a PN junction of semiconductor materials and You can get really high efficiency So if you use things that are extremely high quality materials like gallium arsenide, for example And you stack a bunch of these together They can be about 46% efficient, but they're really really expensive So a square meter of this kind of stack of a solar cell costs about $40,000 okay, so this is really useful for things like space application satellites But not going to be on our rooftops anytime soon on the other hand You can calculate that if you take a basic silicon solar cell, which is the predominant technology In in the world right now and you put on top of that one of these perovskites Remember I said you can kind of spin those on so you might be able to do that pretty cheaply on top of an existing solar cell You can get not as high an efficiency as something like a Compound semiconductor like this one, but still quite a good boost and you can do it really cheaply Okay, so that's kind of the the goal of what we're looking at it turns out though to stack materials like this requires lots of control over every layer in in a device so the challenges with making solar cell Tandems using this new perovskite material is that you need to have various layers including this one here Which has to be transparent because you're gonna let the light all the way through to get to both reach both cells in here The silicon and the perovskite cell And it needs to be very much protective. It has to have good electrical properties But also in parts stability because these perovskites the dirty little secret about perovskites is they have great efficiency But they're horribly unstable. They degrade and moisture with temperature and any sort of Active reactive species so we want to be able to protect them So we came into this problem Saying well We think we have a technique for putting down a material here Which will be gentle and protect the perovskite have very good electrical properties and allow us to then do other layers on top of it with more reactive Deposition techniques and we use this with it We use a technique called atomic layer deposition which we can do it relatively low Temperature and put down really interesting electronic materials So if you look at the stack here lots of different materials These are all relatively thin the perovskite itself is 500 nanometers roughly we have these various layers here And we're looking here a few nanometers of this material Which we're going to put down by atomic layer deposition and then continue the rest of the stack with more conventional techniques So atomic layer deposition is called ALD for short. It's basically Technique that's was developed 30 or 40 years ago for use in large-scale displays flat panel displays And over the last decade or two has been widely adopted by the semiconductor industry So it's used for lots of microelectronics probably every chip in every computer now has ALD in some part of its fabrication probably multiple parts But what the technique is is it uses a vacuum system and you pulse in various reactants in the gas phase So you pulse in reactant gas and it'll react at your surface It undergoes a thermal reaction, but it's self-limiting. So it won't react to more than one layer Okay, this is the key part of this technique Then you come in with another precursor that also will react just to one layer it saturates And if you put both of these precursors in you get one atomic layer of whatever material you're interested in Ideally and then you repeat this multiple times and you can grow a film as thick as you want You're typically are growing on the order of one angstrom or tenth of a nanometer per cycle So it's a very slow process not at all useful if you're trying to grow thick films But when you're trying to grow films that are only a few nanometers It's actually a great technique and that's why it's used so widely in microelectronics where everything is really small Also very thin in these solar cells. So these are just some examples of materials that have been grown by ALD This is an example a few years ago My group took a silicon nanowire from each ways group and we coated it with platinum by ALD And you could see by this image the platinum is uniformly coating everywhere around it So it has this nice feature of being able to make thin films that conformally coat all kinds of complex structures Okay, so we We have techniques for growing all kinds of materials this is just to show you the types of gases that we have to flow in there these metal organic precursors they react with things like water and We found that zinc oxide and tin oxide together and make a really good electrical layer for these perovskite solar cells So we we can grow them using these precursors at temperatures around 150 degrees C. So relatively low I Skip that so I mentioned that the perovskites are very sensitive to degradation and one of the very first things we had to check is How we can run this process to grow tin oxide or zinc dope tin oxide on top of the perovskite without degrading the perovskite So my student Axel Palmstrom who ran most of these experiments tried all kinds of conditions and all kinds of temperatures And one of the things about the perovskites where these are x-ray diffraction images They are crystalline. So you see these peaks here with the dash lines that come from the perovskite and As it starts to degrade you get a very clear signature an x-ray diffraction of lead iodide Which is one of the degradation products. So it's kind of hard to miss. So when your perovskite starts to degrade You start to see this very strong perov a lead iodide peak So you can see that depending on the conditions of growing this al d film on top of the perovskite Some of them are great They maintain the perovskite and other ones we start to degrade it into lead iodide So we modified the process to be able to gently deposit the material And then what we did is we put this on a stack So we had a underlying silicon solar cell that actually was Contributed by a group part of this big collaboration at Arizona State Zach Holman's group and then the McGee group helped put perovskite down and we did some of these other layers And so this many many complex layers all went together and right in here is this al d layer That gently goes down on the perovskite and allows us to do the next steps and what we found is it worked really really well So we were able to make this tandem silicon perovskite tandem with efficiencies of about 23.6 percent and very stable. Okay, that's the key thing. Can you keep the efficiency? For long periods of time and we showed that these were very stable because this very thin film helps encapsulate the perovskite and protect it We also looked at Making perovskite tandems just out of perovskite. So this is a perovskite perovskite tandem Where we stack one perovskite of one bandgap and another perovskite of another bandgap together and again Many layers have to go into this and I'll just point out a lot of them involve ALD So that's where we put our ALD layers We choose these of different bandgaps so we can collect more of the solar spectrum and depending on how you wire this up You can do a two-terminal or four-terminal Structure we can get efficiencies above 20% and the nice thing about being able to make a pure perovskite tandem Is that these can now be flexible because silicon solar cells are typically crystalline or polycrystalline and they're not Particularly flexible unless you make them ultra thin like Daniel was referring to Okay, so this is just an example of taking a thin film of about a few nanometers and Completely changing and improving the electrical properties of a solar cell that was very effective. I Want to turn to the second example, which is catalyst so very different type of system But also takes advantage of nanoscale material. So I want to give a little bit of background a catalysis I think you probably also heard from Tom professor Tom Haramio about catalysis and electric catalysis on an earlier day We collaborate with his group as well Incredibly important so more than 80% of all industrial chemical processes use some catalytic step probably multiple catalytic steps But I mean by industrial processes is everything from pharmaceuticals to plastics to petroleum. So it's huge Catalyst themselves are big business. So the sales of catalyst is about 15 billion dollars a year But if you project out the annual value of products that were manufactured with catalysts, it's about a trillion dollars Okay, so incredibly important set of materials Just so that everybody remembers their high school chemistry Catalyst is something that alters the rate of reaction without itself being consumed If you went on and learned more about that later, you probably learned that that's an oversimplification as with most things but the way it actually works is it takes a You know set of reactants and set of products and it comes up with an alternative mechanism Molecular mechanism for the reactants to get to the products that basically takes less energy has a lower barrier So it's faster. So that's basically what a catalyst is doing on the Molecular level and we're very interested in catalyst So I started off the title of my talk is nano Materials or nanotech for sustainable energy or something like that Nanotechnology and nanomaterials have been around for a really long time. Catalyst is an old field the Haber-Bosch catalyst which converts Nitrogen into ammonia incredibly industrial important process Celebrated its hundred year anniversary, you know, a few years ago and that is made up of nanoscale materials Okay, so this is nothing new We've just come up with really interesting ways of making nanoscale materials and characterizing them That we didn't have many decades ago, but but it's been around for a while, especially in catalysis So catalysts tend to be really complicated structures generally especially commercial catalysts in heterogeneous catalysts of the type I'm talking about Chemistry the way that the catalyst speeds up the reaction it only takes place on the outer surface of it So you can have a great catalyst and make a big sphere of it and the whole inner part of that's useless So what people do with catalysts is they try to break it up into lots of little particles You know size particles so that they have a lot of surface area because that's the part that does the catalytic conversion So they tend to have all kinds of complex structures So what catalytic system am I going to tell you about there are lots of many many many important catalytic reactions I'm going to talk about a Cycle here that is a carbon neutral fuel production. Okay, so if you imagine a car you have your internal combustion engine It's it's Combusting something like ethanol or some other fuel it generates carbon dioxide Okay, plants can take carbon dioxide and turn that into biomass biomass Can be turned into something called synthesis gas through a gasification process, which is well known So this is carbon monoxide and hydrogen and it's called synthesis gas Syn gas because it can be used to then synthesize lots of other interesting materials or Chemicals so if we had a catalyst that can take this and convert it back into say ethanol or the fuel Then we would have this carbon neutral fuel production The problem is we certainly know how to do this step and this step happens naturally This one we know how to do but there really aren't great catalysts for going from synthesis gas to good Fuels and so that's what the focus is of the project. I'm going to tell you about What we want to do is take the carbon and waste in the form of co2 and convert it to valuable fuels And also chemicals and the product or type of products that's useful for this or what we call higher oxygenate So ethanol is one of these higher oxygenates It means it has more than two carbons in the molecule and it has some form of oxygen in there Okay, so butanol or acid aldehyde. There are lots of Different molecules. They're nice because they're liquids They can be used as clean transportation fuels and they're very useful as a feedstock for manufacturing other important chemicals But as I mentioned, we don't really have a good effect of catalyst for this and here's one of the problems We take something simple like carbon monoxide and hydrogen just these two small molecules and we want to go this way We want to form these higher oxygenates, but you can see by the arrows here. There are a lot of competing reactions This is always the problem with catalysis. You have to have selectivity for the product you want and Not make all the byproducts that you're not interested in so we can get just hydrocarbons through a well-known process called Fisher tropes We could hydrogenate too early and then we get methanol which is too short Or we can do what I think is the worst possibility go back to carbon dioxide, which we don't want to do Okay, so this is what we're trying to generate Catalyst for and we're we benefit greatly from the fact that here at Stanford We have a big theoretical effort directed by Jens Norskov who can take reactions like this and they can calculate for lots of potential metals or Metal alloys or other systems what might lead to the product we want Okay, so they come up with plots like this and I can see it's too hard to see it here You they're they have different metals listed on here So as a starting point we can go into one of those metals and say let's start with that and try to synthesize that catalyst And see if we can make a lot of things like ethanol Okay, so if you're able to see that plot you would have seen that rhodium was one of the interesting materials on it And it's also been shown in the literature that rhodium is a pretty good catalyst for Converting sin gas to higher Alcohols or other oxygenates and again like I mentioned the way these work is we try to take rhodium Which is very expensive precious metal and we try to disperse it in very small fine particles Inside a powder so that we maximize the surface area So it's sitting on a powder usually made up of something like silicon dioxide sometimes alumina or titanium And so this is called the support We pack a bunch of that powder into a reactor and you flow the gas through and out comes your products in an ideal system one thing we know about What are called supported metal catalysts like this is that the Metal oxide that they're sitting on plays a huge role in what products you make Okay, so it's not just some inert things sitting around that doesn't have an effect and everything's happening on the rhodium It turns out this is really important too This material sometimes Sits on top of the rhodium and in that case we call it a promoter. It's something that promotes Production of the molecule of interest Okay, so for example We can take the standard rhodium on silica or we can put the rhodium on some other metal oxide And we would find probably that the reacted completely differently Or we could take that same metal oxide and put it all around the rhodium And this we would call a promoter and we might get a completely different product so we were very interested in using some of our synthesis strategies to be able to Put down small amounts of material to see whether One metal oxide had an effect on the rhodium different from another and why it Controlled the products the way it did So we can use our technique of atomic layer deposition Which I like to say we've stolen from the semiconductor industry and applied to lots of other problems to take Catalysts that we synthesize through conventional methods These are usually just solution methods where you take a powder and you shove a lot of metal salt in it and it dries And leaves you the metal particles But we could also try to change out the metal oxide support and the promoter and we do that by Coming in and taking our support and adding some other metal oxide on it a very very thin layer Maybe one or two atomic layers thick and then putting down the catalyst or doing it on top of the catalyst So we can get all of these different types of structures to try to understand the interplay between the metal oxide and the catalyst Okay, so I mentioned we use our atomic layer deposition method again The first metal oxide we looked at for this rhodium system was manganese oxide so here's our molecule that we use with water to grow it and And we looked at three configurations one is the standard catalyst One is the catalyst where we actually took the silica We grew a very thin layer of manganese oxide and then we put down the rhodium and One where we first put down the rhodium and then we grew a little layer of manganese oxide on top of that So this is like an overcoat and that's part of the support We can look at this all kinds of characterization of the catalyst you can see that these are all very small So the catalyst themselves, this is a particle size distribution. They're on the order of two or three nanometers So the rhodium is a really teeny particle here, but it's being strongly influenced by this metal oxide that's sitting next to it and We take all these catalysts that we synthesize with slightly different structure We stick them into this packed bed reactor and we look at the product distribution that we get and that's what I'm showing here So this is the standard catalyst rhodium on silica and I'm showing Several pieces of information here. So these are the seal activity So here in gray the size of this bar is how much methane out of a hundred percent that we're forming the green is higher Hydrocarbons, so we don't really care about either of those This little blue line is methanol and the orange is what we want. Okay, those are the higher oxygenate So it does okay the standard rhodium catalyst does okay It makes a fair amount of these by maybe 20% and this number on top is the overall activity So how much of the carbon monoxide and hydrogen do you put in gets converted to any sort of product? That's this number up here These three bars on the right are when we add some manganese oxide to it And the first thing you could see is the number goes up here So we're making more product of all types And happily we're getting more of the product we want which is the orange bar these higher oxygenate But it actually matters where we put down the manganese and we put it on the bottom versus the top These are the two middle bars. There's a different seal activity pattern and that's interesting to us on a fundamental level So we do things like spectroscopy Where we go in with infrared light and we can actually detect How carbon monoxide is bonding to the rhodium we can see this by a spectroscopic signature and we can correlate the products with How many of the higher so we can correlate the species here at the surface with how much higher oxygenate Production we get and then we can connect that with theory which is again is done by the Norsegov group Which can calculate why manganese would have the effect that it does on the product distribution that we see So this all tells us that manganese oxide is a pretty good promoter It helps for more of the product that we want and makes the catalyst more active Um, we can also extend extend this and we did to a bunch of different metal oxide So now that we have this technique of saying we can we can tweak in a very controlled way a supported metal catalyst We can start to look at other metal support interaction So we looked at this comparison of titanium alumina and silica if you look in the literature There have been all kinds of reports that you get different activity and different seal activity with rhodium depending on the different supports But we suspected that what was happening is when people do standard synthetic techniques with these different supports They change a lot about the system They change the size of the particles the size of the pores and they're not having a controlled experiment So we felt we could do a controlled experiment and get rid of the structural effects and just Explore the different chemical properties of these supports by using this fine atomic layer deposition technique And so that's what we did here This is how we grow little layers of titanium This is how we grow little layers of alumina and we can compare that everything looks the same structurally This is the particle size distribution They're spot-on the same rhodium distribution independent of these different metal oxides and Interestingly when we then made all the catalysts and study them we found that contrary to the literature They really didn't vary that much Okay, so the seal activity didn't change that much you could just tell that by sort of the size of the various bars There's some changes, but not anything huge Consistent with our theory that this was really about structure all the literature reports, but chemically they didn't differ that much Let me Sum up here So I have a little bit of time to let you take a breather or answer any questions I try to give you two examples of where nano materials play an important role in energy conversion Or catalytic conversion I like to think atomic layer deposition is a powerful technique for controlled synthesis of nano materials It's one of many interesting techniques for doing that We applied it to solar cells So we were looking at these tandem perovskite solar cells and we also applied it to heterogeneous catalysis Where we can do what we consider sort of a rational design of catalysts to really try to One make better catalysts and to understand fundamentally what's happening in them Lots of Students and collaborators to think so I mentioned Axel Pomstrom Led in my group the perovskite tandem work. We collaborated with Mike McGee's group and Zach Holman's group at ASU with their students and Three of my students worked on the syngas catalysis Noya Yong, Joseph Singh and Aruna Sundi and we collaborated with Jens Norskov Thank you for your attention and I'd be happy to take a couple of questions I think people could probably I mean hundreds of nanometers is pretty reasonable so One thing I didn't mention is people are looking at ways to do rapid processing through ALD So there's a whole push to engineer fast systems And large-scale systems so for example solar applications you might need to do a really large area And you typically don't want to build a vacuum system quite that big although they do for sputter deposition So they're developing things ALD systems that can actually be done roll-to-roll in atmosphere Okay, so as they develop those I think that that number I'm giving you of a few hundred nanometers will increase But for now that's kind of a reasonable now That would take my group forever to do a few hundred nanometers because ours is a slow home-built system. It's pretty slow Yeah Yeah, so I would say that Photable takes in general is more Edisonian I mean if there's such an art to knowing what layer works and what and they're just a ton of people trying tons of Different things to get the right efficient, you know the best efficiency The catalysis has historically been very Edisonian very empirical But the nice thing about having the theory experiment collaboration is that we are working Before we even go into the lab with advanced knowledge of what might work and there's usually quite good Correlation between theory and experiment if there's not then the theorists go back or the experimentalists go back And so that works well, so I would say that's where we're really trying to take kind of a Educated Information before we go and try to make something and hopefully that will translate eventually to things like photovoltaics too But currently that's there's a lot of art It has been it has been but again the Norse scab group and Suncat has been working on now looking at Calculating the effects of various supports, so they are now also able to predict those effects as well. Yeah Yeah They usually they often come about because of students so students will say oh I'm working on something and gee I'd really like to you know go over to use Something in the McGee lab or I really need some theory and so it's often through students that these collaborations occur Sometimes it's kind of top-down because we're writing a grant proposal together or something There's almost never a problem with splitting the work because usually people students will bring different expertise to a collaboration and They will each contribute what their own skill set is and they can work very closely together. So for example axel and Kevin worked Very very closely on those perovskite cells, but they worked on different parts of it Even though they watched each other do you know the full assembly of it? So I think it usually works out quite naturally Yeah, I mean there are different aspects of that There's sort of the bio-inspired design, which I think was the last part of what you asked and a lot of what we are trying to do I didn't mention this, but maybe Tom Haramio did there's like a trying to artificially do nitrogen reduction And we're inspired very much by nitrogen ace and sort of the bio memetic aspect of it. I think there probably are some people doing the direct biological system You know enzymatic catalysis at Stanford, but I'm less familiar With that work. There's probably a bigger effort on the kind of inorganic synthetic That's inspired by biology, but not actually using the biology Okay So you mentioned that perovskite layer is a little fragile to things done. Is it due to Like energy, why can't you use it? Yeah, those even these precursors that we use that aren't plasma Excited they're so reactive that if you don't do it right They'll get in there and just react chemically with the perovskite. The perovskites are pretty complicated materials and they have a lot of Organic parts and organic parts and they'll get in there and react. So the problem with plasmas or other energetic things is Those ions or radicals will just react with them and convert it to something else Which doesn't act like a perovskite