AUSTRALIAN FRONTIERS OF SCIENCE, 2008

The Shine Dome, Canberra, 21-22 February

Session 5: Discussion

Question: In your last slide but one, headed 'Photocatalysis Results', why did the efficiency go down again? You had several improvements in efficiency but then a drop.

Rachel Caruso: When you are adding the gold nanoparticles, what is important is not only how much gold is going in to the structure, but also the size of the gold nanoparticles that are in there. Initially as we added the gold to the network structure, the particles were remaining quite small, and at 2 wt % we reached a limit where we had as the maximum amount of gold in the structure that retained a small particle size. Once we went to higher concentrations, greater than 2 wt %, the particles became larger, and we lost the advantage of small gold particles, decreasing the ability to separate the electron/hole pairs.

Question: As someone who is making a career out of charge separation, I am always interested to see another mechanism. What drives the charge separation of the electron-hole pairs in the titanium dioxide? Obviously, that must be a significant efficiency-limiting step in the organic solar panels.

Rachel Caruso: Are you are asking about what is driving that separation when you are shining a light on the sample?

Question (cont.): Not the formation of the electron-hole pair, but what causes the electron to go one way and the hole to go the other way. If they don't do that, they just fall back again, as you pointed out, and you lose them.

Rachel Caruso: To my knowledge, recombination events actually occur in about 90 per cent of instances. The majority of electron-holes that are formed recombine. So charge separation is a difficult issue. The electron and hole can become trapped within the titania with the electrons being trapped at Ti centres and the holes being trapped at the oxygen. Studies have shown that the electrons and holes that don't recombine are found at defect sites on the surface of the particles and in the bulk. In the dye-sensitised solar cell, the charge separation occurs on injection of the electron from the excited dye molecule into the conduction band of the titania. Hence a selection criteria of the dye is that the energy level of the excited dye is slightly higher than the conduction band of the titania to allow this electron 'injection'.

Question: The pore sizes that you are getting here are of the order of a little larger than cells. Has anyone tried using these things as a bone scaffold, or a scaffold for bioengineering or tissue engineering?

Rachel Caruso: A lot of the pores that we are working with are too small for such applications. I have a PhD student who is looking at porous hydroxyapatite structures, using this type of technique. He is using much larger templates to start off with. According to our reading of the literature, you need pore sizes in the hundreds of microns, and so that is what we are working with to build up such materials as bone scaffolds.

Question: With your gecko, how the pads of the feet stick sort of looks intuitive. What is less intuitive to me is how they unstick so well. Anyone who has watched a gecko climb a wall knows they can do it pretty fast. Do you want to comment on that? Is there any implication for the sorts of things you are doing?

Rachel Caruso: No, I can't really go into the details. It is intermolecular forces that are interacting, and the gecko has its mechanisms for actually pulling that off the surface again when it wants to move on. Studies have shown this is dependent on the orientation of the spatulae relative to the surface, with detachment occurring at a particular angle.

Question: Cameron, I understand that you can store oxygen gas by making a copper complex. But how do you release it, or how stable is that storage?

Cameron Kepert: That is a good question. I didn't mention that. It is actually an extremely reversible process. We have cycled that system through multiple cycles of oxygen gas uptake and release, and it seems to be entirely reversible. We have done it about 10 times. Building this property into a porous framework host is really what is giving you that reversibility in the process.

Question: Rachel, does the wavelength of the incident light have any effect on the efficiency in titanium dioxide of the electron-hole production?

Rachel Caruso: Sure. Titanium dioxide only absorbs light of wavelength below about 380 nm, and so you do have to be shining light of that wavelength or less to get any photoactivity in the titania. This is the reason why, if you put titanium dioxide into a photovoltaic cell without the dye covering to inject the electron into the titania, there isn't sufficient photon to electron conversion efficiency there. The titanium dioxide itself is only absorbing about 5 per cent of the solar spectrum.

Question: Cameron, what is the role of the lower temperature in hydrogen storage? Why does it have to be lowered to 70 Kelvin?

Cameron Kepert: That arises purely from the fact that hydrogen, being so small, has only got two electrons, and it has a very low quadrupole moment. It is a molecule that has very weak intermolecular forces, so if you want to attach a hydrogen molecule to something you really need to find surfaces that are very well suited for that binding or be obliged to go to extreme conditions. In fact, most of the research in the last few decades has focused on a very different approach to hydrogen storage, where you react the hydrogen with a material and form a hydride, which can then be released at much higher temperature.

There are really these two approaches for tackling this problem. In a way, there are problems with the hydride side of things, because those are often very reactive systems – things like magnesium metal – and so they can be oxidised in air, whereas our systems are right down the other end of the spectrum, where we have a very, very weak interaction of the hydrogen molecule. Anyone on the technological side of things really doesn't want to be anywhere down near those sorts of temperatures. I believe there are some car manufacturers who are quite happy to go down to liquid nitrogen temperature to store hydrogen, but I am sure that if they didn't have to, they would be happier.

Really, it is purely a result of hydrogen being so small. It has a very high energy density, but is problematic in that it is a very non-sticky molecule. So that is the problem we are really trying to focus on solving.

Question: Cameron, you showed the switching between the ferromagnetic and anti-ferromagnetic states, which is below 10 Kelvin. What is the physical mechanism for why it appears only at very low temperature?

Cameron Kepert: The reason it is at low temperature is that we don't have very much magnetic exchange coupling between our cobalt(II) cations in that system. We have hydroxide bridges between our cobalt atoms, and so in that particular system the exchange coupling is weak. We have other systems where we are getting magnetic ordering at temperatures up at around above 60 K and, in fact, there are some cyanide systems that order magnetically above room temperature. So there is excellent prospect for getting magnetic ordering at some useful temperatures. I guess that is answering half your question about why it is such a low temperature.

As to the question of why the guest molecule is causing such a large effect, we don't know whether it is magnetic exchange coupling through the guest, or whether it is purely that the structural perturbation you get as the guest molecule goes into the host is enough to distort your exchange coupling pathways and change the sign of that exchange coupling.

Question: Cameron, I am just wondering whether these structures of yours have, essentially – by accident or by intention – designed flexibility in them, so that when you put gas in and take it out and so on, which must distort things eventually, that is compensated for.

Cameron Kepert: That is a very interesting aspect of these materials, in that they are now well regarded as not just being like zeolites but actually being unlike zeolites as well. Many of them are extremely flexible. We have some that expand and contract by 30, 40, 50 per cent when you put gas molecules in and out; they can hinge and do all sorts of things. That itself is actually a very interesting property, because you start to look at selectivity based not just on size and shape of the guest molecule but actually on whether the guest molecule can interact with the host sufficiently well to allow those sorts of structural distortions to occur. So it is really an entirely new mechanism, in a sense, for selectivity for these types of materials. It is something I didn't talk about, but it is a very interesting aspect of them.

Question: Cameron, I have a question about biology and the link here. There are a number of enzymes which can make hydrogen. I am just wondering whether you have any information about the stickiness of hydrogen in the active sites of some of these enzymes versus the stickiness you have in your materials, and whether there are any tricks that biology has already developed for making hydrogen sticky.

Cameron Kepert: That's a great question, and I confess to not knowing the answer to it. From the little I know about these sorts of hydrogen-producing enzymes – and there is some work being done at the University of Queensland, for example, where Ben Hankamer is making a green slime that gives off hydrogen gas – my understanding is that as soon as the hydrogen gas has been produced at the active sites in the enzyme it is just volatilised and turns into a gas straight away. I don't think there are any biological precedents for binding of H2 in any active sites. So we really have to explore new territory there, to find ways to do that.