Quantum Dots and Artificial Photosynthesis
As we continue our use of convenient and valuable natural gas, coal, and liquid fuels such as oil, achieving carbon dioxide (CO2) reduction and controlling the demand for oil will require that we raise the percentage of renewable liquid fuels manufactured with renewable energy, such as solar energy and natural energy, and that we widen the use of these fuels. To achieve these renewable liquid fuels, a wide range of research and development is addressing future technologies including biofuels and artificial photosynthesis. Around the world, research is particularly focusing on artificial photosynthesis, said to be the ultimate clean energy that makes use of light. Although research has yet to succeed in fully mimicking the photosynthesis of the natural world, artificial photosynthesis technology that directly converts light energy into chemical energy has been partially established. By contrast with conventional solar cells that present problems related to storage of electrical power, artificial photosynthesis eases the storage of energy by generating chemical energy.
Examples of recent full-scale research on artificial photosynthesis include artificial photocatalysis employing semiconductor materials such as titanium oxide and tungsten oxide. After the discovery that manganese clusters function as catalysts when plants decompose water in photosynthesis, attempts have also been made to efficiently achieve artificial photosynthesis by creating similar substances. Many efforts have also been made to use visible light, not only ultraviolet light.
Broadly speaking, photosynthesis comprises of two reactions. The first is a reaction that takes in light and decomposes water into oxygen and hydrogen. The hydrogen is divided into hydrogen ions (hydrogen from which electrons have been stripped) and electrons. Through this, a reaction identical to electrolysis of water is achieved through light energy. The first reaction is called the light reaction of photosynthesis. In plants, the oxygen generated by the reaction is transpired from the surface of the leaves.
The second reaction, called the dark reaction, uses the power of the hydrogen ions and electrons created in the first reaction to create sugars from the carbon contained in carbon dioxide taken in from the air. This reaction harnesses the power of the hydrogen ions and the electrons to link the carbon into chains that form sugars.
Current research into artificial photosynthesis is focusing on materials including metals, alloys, semiconductors, metallic complexes, organic substances, sulfides, and nitrides. Research is also underway to artificially mimic manganese complexes for oxygen generation, as well as zinc chlorine or porphyrin-based materials.
A major premise of artificial photosynthesis and photocatalytic reactions is that photoexcited electron and hole pairs must be used for their respective subsequent reactions before they are recombined. Restated, charge separation and charge transfer become important following photoexcitation. From this viewpoint, the application of quantum dots, particularly in forms such as quantum rods grown in one direction, may be feasible. This is because the spatial existence states of electrons and holes in the conduction band and valence band can be tuned through the modification of size and the chemical composition of the quantum dots, which in turn enables tuning of exciton charge separation and charge transfer. In addition, the tuning of light absorption, charge separation, and catalytic activity in artificial photosynthesis is often performed by binding metals such as platinum to the tips or other selected locations of quantum rods, not only quantum dots. This mechanism using quantum rods is also effective when applied to solar cells. This is because the excited electrons and holes are efficiently separated at the heterostructure interface, which enables control of recombination and prolongs the lifespan of excitons. Stated differently, these charge carriers are able to efficiently move to the exciton acceptors and donors. In this way, potential is expanding for the application of quantum dots and quantum rods to artificial photosynthesis.