Metals possess many electrons that can easily absorb energy from the environment — for instance, in form of light or heat — and use that extra energy to become more mobile. This is why metals are good conductors. Metal nanoparticles also possess a high number of electrons, which can also interact with light and heat.
Uniquely to their composition and their extremely small dimensions (a nanoparticle of 50 nm is 1000 times smaller than the average hair), gold nanoparticles very efficiently absorb light at specific frequencies, which are determined by the nanoparticle’s composition and shape and the properties of the environment surrounding it. When a gold nanoparticle absorbs light at these particular frequencies, its electrons can capture a large fraction of the energy carried by the light wave and become excited. Electrons are defined as hot when they have absorbed sufficient energy from light to escape their fundamental state, move to the surrounding environment, and interact with it.
When an electron reacts with a molecule present in the environment, it reduces it. The more electrons are produced by a nanoparticle, the more efficient the nanoparticle will be at reducing molecules in the surrounding environment. The generation of electrons by a nanoparticle can be enhanced if it is synthesized in the shape of a star. Because of the presence of the star spikes, which act as tiny antennas, the electric field carried by the light gets concentrated at the tips of the spikes, becoming extremely intense. As a result, many hot electrons are produced at these locations.
When a gold nanoparticle is coupled with a catalytic material, such as titania, the reduction ability of electrons can be further increased. Titania is well known to be an excellent catalyst for several reactions, among which the reduction of water to produce hydrogen. When illuminated by UV light, electrons in titania can absorb the energy carried by light too, and become excited, similarly to what happens for electrons in metal nanoparticles. As a consequence, one can imagine that when water absorbs sunlight in the presence of titania, the two hydrogen atoms contained in the water molecules can react with the electrons generated into the catalyst and produce molecular hydrogen, H2.
Hydrogen production is technologically important as this is considered a type of clean of fuel. However, there are two limitations in this process: 1) titania is not as efficient as gold nanoparticles at absorbing light, even though it is a better catalyst, and 2) titania can absorb only UV light, which represents only 5% of the solar spectrum. If you couple gold nanoparticles and titania, more light can be absorbed, as the nanoparticles can absorb light in the visible and near infrared, while titania absorbs in the UV. Furthermore, because gold nanoparticles absorb light more efficiently, they can produce more hot electrons, which can then be transferred to titania, where the reduction of water is more efficient.
Until recently, to increase the catalytic ability of titania, one would decorate it with gold nanoparticles. However, this does not fully optimize the process, as the particles and the titania substrate would only have one point of contact through which the electrons could be transferred from the nanoparticle to the catalyst. This traditional design has however been flipped upside down in a work appeared on the journal Chem (Chem 2018, 4, 2140). In this work, star-shaped gold nanoparticles were instead coated with a thin layer of titania. When immersed in water and illuminated with broad-spectrum solar light, they produced molecular hydrogen with yields seven times higher than titania alone and four times better than ever possible.
To understand why this design worked better, let’s think about the reaction steps that need to occur for hot electrons to reduce water and produce hydrogen. First, the nanoparticle needs to absorb light to produce hot electrons. Then the electrons need to migrate from where they have been generated to the gold-titania interface. They then have to tunnel into the titania shell, to quickly cross it and reach the surrounding water. The star-shaped nanoparticle is ideal in this construct for three reasons: 1) it absorbs both visible and near-infrared light, thus increasing the utilization of the solar radiation, 2) its spikes concentrate the field, which leads to the production of more hot electrons, and 3) because the electrons are generated primarily at the tips of the spikes, they do not need to migrate very far before encountering the gold-titania interface, which optimizes the number of electrons that can be used. When the electron reaches the interface, it is important for it to not encounter any obstacles in order to be able to cross it.
Think of a highway: The fewer cars are on it and the better the quality of the roadway (i.e. obstacles are minimized), the faster you can reach your destination. For electrons, the obstacles are represented by impurities in the titania and by the misalignment between the gold, the titanium, and the oxygen atoms at the interface. In order to optimize atom alignment, a process called epitaxial growth needs to be employed to coat the gold nanoparticle with titania. This is, however, very difficult and was only achieved on nanoparticles for the first time in this work.
If we now assume that the electron has reached the titania, in order for it to migrate quickly into the surrounding water, it should again not find any obstacles. In order to achieve this goal, titanium and gold atoms in titania need to be highly organized, that is the material needs to be crystalline. However, to synthesize crystalline titania one should heat it up at least 200°C. At this temperature, however, the nanoparticles reshape into spheres, losing their initial advantage of being star-shaped. Therefore, the Rutgers University team has devised a unique low-temperature synthesis that has for the first time produced crystalline titania on star-shaped gold nanoparticles. Another challenge that they overcame was the need to synthesize the shell to be very thin, 4 nm at most (remember that 50 nm is one thousand times thinner than your hair?). A thin titania shell ensures that the hot electrons do not get stuck in the catalyst, but efficiently reach their intended target, water. Only when all these conditions are met at the same time, hydrogen evolution can reach the levels achieved in this work.
In the future, the authors plan on looking at different nanoparticle shapes and compositions and are curious about the possibility of absorbing the entire solar spectrum if nanoparticles of different shapes are mixed together. Importantly, they plan to research whether other chemical reactions, in particular those that produce important polymer precursors from CO2, could benefit from this catalyst design. One challenge could be determining how to scale up the nanoparticle synthesis to prepare amounts usable at the industrial scale, but this discovery will be left for the next episode.
These findings are described in the article entitled TiO2 on Gold Nanostars Enhances Photocatalytic Water Reduction in the Near-Infrared Regime, recently published in the journal Chem. This work was conducted by Supriya Atta, Ashley M. Pennington, Fuat E. Celik, and Laura Fabris from Rutgers University.