by Ajay Manuel (ComSciCon-CanWest 2020)
Human endeavors from the age of the hunter-gatherers to modern civilization have been guided by the allocation and consumption of energy. Light is an integral part of this story. Central to life on Earth is the energy we receive from our Sun: solar energy. In fact, the solar radiation that strikes our planet in just one hour is more than the total annual energy consumption worldwide. Tapping into even 0.02% of this incoming energy would help satisfy all our energy needs.
Nature accomplishes this through photosynthesis where plants capture sunlight along with carbon dioxide (CO2) and water in the atmosphere to produce carbohydrates that we can consume and the oxygen that we breathe. This is a driving factor in the delicate balance of the global ecosystem. However, gradual globalization from agricultural to industrial communities has led to the use and establishment of fossil fuels as the conventional energy resource for sustainable energy and economic development. Little attention has been paid to the detrimental effects of fossil fuels on the planet and our environment. The current green energy movement illustrates the scientific consensus identifying fossil fuels as a major contributor to global warming and climate change. Beyond an increasing need to mitigate this global crisis, fossil fuels are also a limited energy resource.
To address these issues, scientists have taken a leaf out of nature’s book by developing renewable energy-based artificial systems that mimic plants in their ability for photosynthesis, as well as provide for its enhancement via artificial means. An ideal, artificial photocatalyst is a material that can store and convert solar energy to promote chemical reactions for our selective needs. Semiconductor photocatalysts are front runners in this category and have set the current standards for artificial photocatalysts in our efforts to move away from fossil fuels and toward harnessing renewable solar energy.
Semiconductors are materials that fall between insulators and metals in their ability to conduct electricity. This ability is dependent on the density of electrons within the material. Metals contain a sea of electrons that flow freely between the atoms, as opposed to insulators, their mirror opposite. Semiconductors, though, have the unique characteristic where electrons occupy either of two specific bands of energy levels: valence and conduction, which are separated by an energy gap. By providing a semiconductor the energy equivalent to this gap, one can cause electrons initially residing in the valence band to jump to the conduction band while leaving behind positive vacancies called holes at the valence band. Subsequently, the electrons at the conduction band and the holes at the valence band are utilized to promote chemical reactions.
In recent years, titanium dioxide (TiO2), a common ingredient in sunscreens, cosmetics, and coatings (such as non-stick cooking pans), has emerged as the benchmark material for a semiconductor photocatalyst. Under ultraviolet illumination, electrons and holes from the valence and conduction levels of TiO2 can be harvested to initiate and catalyze important chemical reactions that may otherwise be energetically taxing. Thanks to its relatively low cost, high availability, low toxicity, stability in acidic and basic media, and overall resistance to corrosion, TiO2 has been the poster child for a wide range of chemical reactions in artificial photocatalysis, including wastewater treatment, air purification, carbon dioxide reduction, hydrogen fuel generation, etc. Despite the positives, large scale commercialization of semiconductor photocatalytic technology hasn’t taken off due to two major constraints. Most semiconductors, including TiO2, have wide bandgaps and only absorb ultraviolet light, which is only 4% of all incoming solar radiation. In other words, we are not even close to tapping the full bandwidth of solar energy available to us. Furthermore, the excited electrons and holes randomly migrate and recombine leading to low harvesting efficiencies. The scientific community thus faces the challenge of sustaining both the directionality of charge carriers and efficient excitation of their host materials.
Metal nanoparticles to the rescue
Recently, the answer to the challenges just mentioned has been found in metal nanoparticles. By incorporating metal nanoparticles in semiconductors, the resulting composite photocatalytic system can forego the limitations of its individual constituents. In particular, the generation of highly energetic charge carriers or “hot electrons” in these systems bridge the gap in enhancing and supporting photocatalytic activity.
Hot electrons result from taking advantage of the loose electron motion in metals. When light strikes a metal nanoparticle, the electric field component of light pushes the electrons in the metal nanoparticle toward one side. This results in an accumulation of negative charges on one end and positive charges on the other. Much like a spring, these opposite ensembles attract one another, resulting in the oscillatory motion of charges within the metal nanoparticle. By matching the incident frequency of the light to the natural frequency of these particle waves, a resonance can be created that results in a high density of energetic or hot electrons. This coupling of a charged particle soup in the metal, otherwise known as a plasma, with packets of light energy, or photons, is called plasmons.
A particularly exciting prospect involves incorporating gold nanoparticles into TiO2 nanostructures because the resonance for gold nanoparticles occurs in the visible spectrum of light, the predominant light we get from the Sun. In other words, by adding gold nanoparticles in TiO2 nanostructures we have effectively overcome the limited absorption capabilities of TiO2 in the ultraviolet and extended it to accommodate a greater part of the solar spectrum in the visible regime. Thus, a plasmonic photocatalytic system incorporating gold and TiO2 can absorb visible light to generate hot electrons that can populate the conduction band of the semiconductor, while being independent of absorption of UV light by TiO2, and enhance the photocatalytic efficiency of the semiconductor photocatalyst. By sensitizing existing semiconductor photocatalysts for visible light absorption, metal nanoparticles thus open the door toward visible light photocatalysis which is crucial in establishing green, solar-powered photocatalytic systems. It is now only a matter of time before the emergence of plasmonic photocatalysis takes center stage. As a technology, it will be crucial in helping resolve the global energy and environmental crisis while reducing our dependence on fossil fuels and non-renewable resources.
1. Brongersma, M. L., et al., Nat. Nanotechnol. (2015) 10 (1), 25
2. White, T. P., and Catchpole, K. R., Appl. Phys. Lett. (2012) 101 (7), 073905 3. Hartland, G. V., et al., ACS Energy Letters (2017) 2 (7), 1641
4. Linic, S., et al., Nat. Mater. (2011) 10, 911
5. Atwater, H. A., and Polman, A., Nat. Mater. (2010) 9 (3), 205
6. Kale, M. J., et al., ACS Catal. (2013) 4 (1), 116
7. Pelton, Matthew, and Garett W Bryant. Introduction to Metal-nanoparticle Plasmonics. Hoboken, N.J.: John Wiley & Sons Inc., 2013.
Ajay Peter Manuel is a PhD candidate at the University of Alberta, Edmonton, Canada. His research specialization is in plasmonics and involves the fabrication of nanostructured photocatalysts for artificial photocatalysis. Ajay is also an aspiring science writer and artist, and when he’s not doing science, you can find him writing comics, blogging, drawing, and enjoying a good book.