AUSTRALIAN FRONTIERS OF SCIENCE, 2008
The Shine Dome, Canberra, 21-22 February
Metal oxide network structures for environmental applications
by Dr Rachel Caruso
 |
Rachel Caruso obtained her degree and PhD from The University of Melbourne before taking up postdoctoral fellowships in prestigious institutes in Germany. In Germany she established her own research group and conducted research in the area of materials chemistry. Her novel templating techniques to produce inorganic porous structures have gained her an international reputation. In 2003 Rachel received the inaugural Centenary Research Fellowship from the Science Faculty at the University of Melbourne, and was awarded an ARC Australian Research Fellowship. She has built a strong research group in the School of Chemistry at The University of Melbourne, focusing on the fabrication of porous structures with application in solar energy, photodecomposition of pollutants and radionuclide sequestration. Rachel is a Victorian Young Tall Poppy Science Award recipient and has recently been presented a COSMOS Bright Sparks Award. |
My research group is looking at metal oxides and the structuring of metal oxides. Structure is what is really important. We are trying to control the structure of our materials and, by controlling that structure, enhance the performance of the materials when we put them to application.
(Click on image for a larger version)
As a materials chemist, what I am interested in is: how do we actually control the synthesis process to allow us to get a very well-structured material? So, doing the chemistry, how do we then process the material do we heat it to high temperatures, do we allow it to dry out, what are we doing to our material? And can we change the composition of our material? This also allows us to change certain properties within the material we are making. These properties influence the performance of the material.
My research group looks at a lot of different applications. Today I will tell you a little about our environmental clean-ups, looking at photocatalysis, using the titanium dioxide materials we are preparing, and also energy devices, where we are looking at photovoltaics with the materials.
(Click on image for a larger version)
So why is structure important? Here you see a picture of a gecko. One thing about the gecko that amazes us is that it can walk up walls, it can walk across a ceiling, it can stick to surfaces, be they wet or dry. The question is: how does it do this?
There have been some engineers studying the gecko to determine what it is about the gecko's foot that allows it to have this ability to stick to surfaces. (This is a natural adhesion: you can stick it on, take it off and do it again, repeatedly. Our sticky tapes that we are making at the moment can't do that.) So they had a look at the foot of a gecko, and they found that on the foot of a gecko there are about 500,000 tiny stalks, as shown here. These are about 130 micrometres in length, and their diameter is less than about a tenth of the diameter of your hair. So they are very small.
Then when the engineers looked at the ends of the stalks they found tiny spatulae, about 200 nm in diameter, so very, very small. And when these stalks and then the spatulae come into contact with a surface, there are intermolecular interactions that allow all those spatulae to adhere to the surface. From studying this they have determined how it is that geckos have the ability to run up walls and across ceilings.
Then some materials physicists at the University of Manchester said, 'Well, if the gecko can do this, can we produce a material that can work in a similar manner?'
(Click on image for a larger version)
They used a lithographic technique and some etching. They took a polymer and then they etched away until they were left with small polymer columns, and they were able to use this like a sticky tape.
(Click on image for a larger version)
The example they showed in their publications and their publicity was of Spider Man. When they put the 'gecko' sticky onto the toy Spider Man hand, it could hold the weight of three kilograms for a 1cm-square block of their polyamide sticky.
That is a bit different from the gecko, which can actually hold many times its mass, and this synthetic 'gecko' sticky was able to be reapplied a number of times. So we are getting close to what nature can do, but there is still a lot that we can learn from nature. In this example it is the structure of the material that is really important.
(Click on image for a larger version)
To move to another example, which Andreas Stein has mentioned in a review article that he wrote just recently, shown here is the inverse opal. This is made up of lots of empty spheres which are open to air or solvent, and then you have the material that makes up the structure, going around those actual spheres, whether it is titanium dioxide, zirconia or silica.
Here we see changes in the optical properties of the material.
By changing the sphere or pore size you get a change in the actual colour of the materials, or by changing the solvent that is placed in the open spheres you can change the colour and properties of the material. Such materials have lots of applications as sensors, in catalysis, in fuel cells, and so they have lots of potential.
The reason I like this example is that it is made using a templating process, and it is templating that I am going to be talking about today. So how do we template, and why is templating important?
(Click on image for a larger version)
Templating is a very powerful technique, and if we have a look at the latest edition of Chemistry of Materials (2008, 20(3)) we find that the whole issue is dedicated to templating processes. So there are a lot of people working in the area. There are many, many different types of templates that can be used, the different templates inducing different properties in the final materials that are being formed.
Using a template, you can regulate the final form of your material, and in the examples I will be showing you, you will see that we can make films or pellets, or we can make fibres or spheres of material, just by changing the initial template that we are working with.
The other thing that is important is the actual porous structure. If we are looking at materials where we want to conduct reactions at the surface of the material say we want catalytic reactions to occur at the surface the more surface we have, the more reactions are going to occur. And so if we can control the porosity of our material how many pores are there, how big the pores are and also the total surface area of the material, as we saw with Cameron's talk before, if we can get really good surface areas there are lots of applications for these materials.
Another parameter that we can change is the composition. If we have, say, a titanium dioxide material, we can add a small amount of a second metal, or a metal oxide, to that and it changes the temperature at which crystallisation occurs, it can change the total surface area of the material, and it can also change other properties like surface acidity. These can all be important when you are looking at the application of the material.
In a nutshell, what we are trying to do is to take a template and use that to form a very structured inorganic material, and in controlling the actual structure of the material we want to increase its ability to work in certain applications.
(Click on image for a larger version)
How do we do the templating process? What I have got here is a schematic where the orange depicts my 'template'. Often the template is an organic material. So I have here the template, and shown in white is the pore structure within the template.
(Click on image for a larger version)
We often use a chemical process called 'sol-gel' chemistry. Basically, we take a liquid, which is our precursor material, and it infiltrates the porous structure of the template.
(Click on image for a larger version)
We then do our chemistry: we add some water to this, reactions occur we have hydrolysis and condensation reactions occurring and we end up with an amorphous inorganic coating on the original template. So we have got a very thin coating on the actual template that we started with.
(Click on image for a larger version)
Then we heat our sample. This burns out the organic material that is there. We heat to about 450°C, which removes the organic material that was present and also crystallises the inorganic material. So you end up with the inorganic structure shown at the bottom right of the slide, which has morphological properties very similar to those of the original template (bottom left of the slide) that we were using.
(Click on image for a larger version)
Let's have a look at some of the structures. The first I am talking about is a very simple one, cellulose acetate. This is often used as a filter paper.
(Click on image for a larger version)
We take this filter paper, which is about 2½ cm in diameter, and if we have a look at this under a scanning electron microscope we see it is about 120 micrometres in thickness.
If we do our chemistry inside this so, we put in our titania precursor and we do the hydrolysis/condensation reactions we can produce a titanium dioxide film. Again it is very porous, as I will show you in a minute, and it is also a film. There is slight shrinkage during the process, but you get a membrane of titanium dioxide that has a very porous structure throughout it.
(Click on image for a larger version)
To look at the inner structure of the actual cellulose acetate membrane so I have actually broken it apart, and we are looking inside the film here we see it is a very porous structure: if you are looking at it as a fluid it would flow through the membrane, it is a very open, porous structure. If you are looking across the edge of the actual membrane, it is much denser but still porous.
(Click on image for a larger version)
You see that pore structure in the final titanium dioxide that is produced as well. So here we have a very dense structure when you are looking at the edge of the film, and then a much more open, porous structure.
Simply by changing the pore size of that filter paper, you can change the surface area of your final titanium dioxide material. So if you have a pore size of 450 nm, you get a surface area of around 20 m2 per gram. If you decrease that pore size to 200 nm, then the surface area increases to above 70 m2 per gram.
(Click on image for a larger version)
To look at another system that we worked with: these are chromatography beads. Obviously, for chromatography uses the beads have to be very porous. It is also important that the beads are fairly monodisperse, that is they are all of similar diameter.
(Click on image for a larger version)
If we have a look at the beads up close again this is a scanning electron microscopy image we see that the bead surface is very porous.
(Click on image for a larger version)
And then, if we break one of those beads apart, we can see that the porosity continues through the actual bead structure.
(Click on image for a larger version)
By adapting our templating process here we are now working with very small, individual entities where we are wanting to produce small, individual identities, not agglomerated systems we could produce our titanium dioxide spheres again, infiltrating with our precursor, doing our chemistry and then burning out the organic template that was there.
(Click on image for a larger version)
If you look at the surface of these, you see again the surfaces are very, very porous.
(Click on image for a larger version)
And if you were to break one of these titania beads apart, you would see that the porosity is maintained throughout the actual sphere of the inorganic material.
(Click on image for a larger version)
We have also looked at fibres as templates. These fibres are very interesting, in that their surface properties are quite unique.
(Click on image for a larger version)
These fibres had small indentations along their surfaces.
(Click on image for a larger version)
So we templated these, having to adapt our process to quite different conditions. If you are coating fibres you get tubular structures as your final material, so the tubes shown here were formed. The diameter varied throughout the final material as the initial templates also had varied diameter.
(Click on image for a larger version)
But what we found when we looked, using transmission electron microscopy, at the final structures on the left here is our original polymer fibre, and on the right is the final titanium dioxide structure was that we had nodules on the inside of the tubular structure. This showed that we were able to mimic, down to the nanoscale, the original templates that we were working with. This was an exciting achievement.
(Click on image for a larger version)
Looking again at the inner porosity: we worked with a lot of different polymer gels. Here is the polymer gel on the left; it has quite a globular structure. Our final inorganic material, on the right, is also very globular in its structure. So this is looking on the inside of our materials.
(Click on image for a larger version)
This polymer was quite different. It had very large pores and quite fine fibre structures holding the polymer together. Again when we looked at our inorganic structure, on the right, we found those large pores present in that inorganic structure.
(Click on image for a larger version)
Then, if you look closely at the inorganic structure, you see the very fine tubes that are produced due to the templating of those fine fibres in the template.
(Click on image for a larger version)
So just by altering the initial polymer gel, the actual template that you are working with, you get a lot of control over the surface area. Looking at a few of the different polymer gels we worked with, we could change the surface area from 10 m2 per gram to 100 m2 per gram.
Let's have a look at how we can actually apply some of these materials.
(Click on image for a larger version)
One area that I am interested in is solar energy, and in particular the dye-sensitised solar cell. Shown here are commercial products of the dye-sensitised solar cell that are actually produced in Queanbeyan, which is very close to Canberra, and holds IP rights to the cell and cell production processes.
(Click on image for a larger version)
How this cell works depends on an electrode in the actual cell. This is often a nanoporous structure composed of titanium dioxide particles. So you have this nanoporous titanium dioxide structure, it is coated with a monolayer of dye, it is placed on a conducting glass substrate, and there is another glass substrate closing the cell which is filled with an electrolyte solution.
(Click on image for a larger version)
If you shine light onto the cell, you get excitation of the dye which is adsorbed on the titanium dioxide.
(Click on image for a larger version)
On excitation of the dye you produce an electron which can then be injected into the titanium dioxide, which is a semiconductor.
(Click on image for a larger version)
Once the electron is injected into the titanium dioxide, it can then flow through the titanium dioxide network.
(Click on image for a larger version)
You can harness the energy from the electron as it moves to the other side of the cell, and it interacts in redox reactions.
(Click on image for a larger version)
That electron, in effect, is given back to the dye. So it is a completely cyclic process.
What we are trying to do is to control the porosity of this material (the electrode), and make slight changes to the composition of the titania to enhance the efficiency of this cell.
(Click on image for a larger version)
Using a control, a commercial material, we can make up our dye-sensitised solar cell. For comparison, I have set the values here all to relative efficiencies. If I use our porous titanium dioxide structures, we can increase the efficiency by 10 per cent. If I then change the composition of the actual titanium dioxide structure with a small amount of silica or zirconia, and depending on what time during the synthesis I am actually adding that silica or zirconia, we can enhance the efficiency even further. So not only are we controlling the actual structure of the material, but by slight changes to the composition we are able to increase the efficiency of this dye-sensitised solar cell.
 |
 |
| (Click on images for larger versions) | |
Titanium dioxide itself is photoactive. That means that when you shine light on titanium dioxide, you can actually produce an electron-hole pair.
(Click on image for a larger version)
This electron-hole pair if they don't immediately recombine, which a lot of them do can move to the surfaces of a titania crystal and you can actually do chemical reactions at the surface. So you have redox reactions occurring.
If you look through the literature, you find that there are a lot of studies looking at decomposing organic molecules in aqueous solutions or even on surfaces coated with titanium dioxide: by shining light on this, you can actually decompose pollutants that adhere to the surfaces.
The problem in aqueous solutions is that, if you are using nanoparticles because you want as much surface as possible, so that you can increase the number of reactions occurring, the particles are very, very small and therefore very difficult to remove from solution once you have finished decontaminating the solution.
So we are trying, firstly, to make a porous structure, so that we still have the very high surface area and secondly, we have a material that is very easy to remove from solution once we have done the chemistry we need to do.
The other thing that I mentioned before was the fact that the electron and the hole often recombine. So what a PhD student in my group is doing is adding a metal to the titanium dioxide structure. The properties of the metal are such that when that electron is produced, it prefers to move to the metal. So the metal is acting as an electron sink. This allows that separation of the electron and the hole, and allows a lot more of the energy that is put in by light to be used to decompose the organic contaminants in solution.
(Click on image for a larger version)
We are looking at water purification using 2-chlorophenol, which is an organic pollutant. In the presence of oxygen, titanium dioxide and light, this is broken down to CO2 and water, removing the toxic 2-chlorophenol.
We have done lots of studies with the 2-chlorophenol, and we have also looked at dye molecules. In the clothing industry dyes are used and the resulting solutions retain dye molecules that need to be broken down before the water can be released back into the environment. My PhD student has been working on the addition of gold particles to the titania to improve the breakdown of the dye.
(Click on image for a larger version)
Here I have a table which shows the amount of gold that has been added to the titanium dioxide structure. You can see that as a weight % quite small quantities of gold are being added. The table also shows how much of the dye a methylene blue dye has decomposed over the 60 minutes that we have shone light on the solution.
What we find with just the titanium dioxide system is that we can break down about 50 per cent of the dye. If we increase the amount of gold present, we get an increase in how much of that dye actually decomposes, to about 70 per cent being decomposed over the same time period. So again the ability to control the structure, to add those metal nanoparticles into the structure, has enhanced the actual efficiency of the materials we are producing.
(Click on image for a larger version)
I would like to conclude by saying that templating is a very diverse area, and it also gives a lot of very interesting structures, so it is very versatile and also powerful for the formation of structured materials.
By using this templating technique and also slight changes in composition to our materials, we have been able to enhance the efficiency of our materials when we are applying them in either photovoltaics or photocatalysis.
(Click on image for a larger version)
I would like to acknowledge the two students whose work I have been talking about. Xingdong Wang has been working on the photocatalysis; and Fuzhi Huang has been working on the photovoltaics work that I have spoken about today.
Funding mainly comes from the Australian Research Council and from sources within the University of Melbourne, the PFPC, and we also get funding from AINSE (Australian Institute of Nuclear Science and Engineering) to allow us to do work at ANSTO (Australian Nuclear Science and Technology Organisation).
