Plants transform sunlight into energy and this process – photosynthesis – is what inspired solar panels. We have created a man-made system to derive energy from sunlight, just like plants. Furthermore, solar energy is seen as one of the greenest and more renewable energy sources. Sunlight is captured by solar panels to generate energy. When light is absorbed, electrons in the solar panel are excited, ‘move’ around and create an electrical current. This process can be accelerated by photocatalysis, a chemical reaction that speeds up light absorption. Catalysis is provided by a semiconductor.
A semiconductor, as the name indicates, can conduct an electrical current, but only under specific conditions. Materials that are non-conductors can be turned into semiconductors through chemical reactions. For example, copper is always able to conduct electricity. Silicon, on its own, can’t. However, when energy is added to a silicon plate, e.g. heat, electricity flows, making it a semiconductor.
Oxidation-reduction reactions are the most common reactions used in semiconductors. They involve the transfer of electrons between atoms or molecules. The details of this reaction and the functioning of semiconductors is beyond the scope of this article. However, I want to present some of the questions researchers are asking themselves today and give an idea of the conversations happening in this field.
The first question is one that is posed over and over again in chemistry: can the chemical reaction be improved? Can substances be used that aren’t harmful to the environment whilst remaining effective and cost-efficient? Studies show that Cadmium (Cd), for example, is an efficient catalyst, but that it is highly toxic.
Solar power is never an exclusive source of power for homeowners. It can only be applied to certain electrical appliances, so an external system is used for heating or to power vehicles. By using the electricity obtained from solar energy differently, more applications could be utilized.
The electricity derived from solar energy can be converted and stored as hydrogen or carbon fuel. Hydrogen is a zero-emission, clean fuel capable of charging a car or heating a home. A popular method for hydrogen production is splitting water molecules. The energy generated from the solar panels can split H2O molecules to give oxygen (O2) and hydrogen (H2). The aim would be to have a photocatalyst semiconductor system producing hydrogen fuel capable of competing with fossil fuels.
The additional challenge is to engineer a system that absorbs visible light. The light we see is only a section of a spectrum that comprises infrared (IR) and ultraviolet (UV) light. So far, solar panels are very efficient at absorbing IR and UV rays from the sun but less so with visible light. As a large part of solar light is visible light, it is critical to design solar panels capable of absorbing this light to optimize its production.
A recent study released by J-H. Tang and Y. Sun in Material Advances, introduces a visible-light-driven semiconductor able to simultaneously produce hydrogen fuel and a variety of organic reactions. This study explored the questions put forward in this article. In particular, the authors were looking for alternatives for H2 production, such as organic transformations or biomass valorisation, instead of water splitting. As mentioned above, a system capable of absorbing visible-light is highly desirable for hydrogen production.
This field remains new and research is ongoing. Even if photocatalysts for H2 production aren’t yet commercialised, the idea to optimize solar energy and to expand its use beyond electricity is an important one.
J-H. Tang, Y. Sun, Material Advances, 2020, DOI: 10.1039/d0ma00327a.
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