As concerns about energy supply emerge in the mainstream, alternatives to fossil fuels are becoming more and more sought after technologies. One of the consultants that scientists have turned to is nature, whose billions of years of evolution have led to some of the most efficient energy supply methods possible. By attempting to directly harness the power of the sun as bacteria, algae, and plants do through natural photosynthesis, scientists are seeking to produce viable renewable energy resources. While macroscopic solar panels, or silicon photovoltaic cells, have been in use for decades, recent breakthroughs in nanotechnology have led to a more bottom up approach more similar to natural photosynthesis, where engineered nanostructures are used for the capture and conversion of light into usable energy.
The three major components of photosynthesis are the antenna, the reaction center, and the site of recombination. The antenna, which is composed of chlorophyll molecules in plants, collects or “harvests” the light energy from the sun. The energy then goes through a number of stages of energy transfer until it is ready for the reaction center. In the reaction center, the energy is used to drive a charge separation reaction, which produces an electron and a so-called “electron hole” where the electron was removed from. Next, certain structures in the membrane of the cell recombine the charges, a process which creates usable electrochemical energy that is then stored in adenosine triphosphate molecules, also known as ATP.
In artificial photosynthesis, scientists are essentially conducting the same fundamental process that occurs in natural photosynthesis but with simpler nanostructures. The fabrication of these nanostructures has only recently been possible due to breakthroughs in nanotechnology in the areas of imaging and manipulation. With the core processes in photosynthesis being light gathering, charge separation, and recombination, the goal of scientists has been to create efficient synthetic nanostructures that can function as antennae and reaction centers. Devens Gust and fellow researchers at Arizona State University created a hexad, or six-part, nanoparticle made of four zinc tetraarylporphyrin molecules, (PZP)3-PZC, a free-base porphyrin, and a fullerene molecule, P-C60.
Figure adapted from Kodis et al.
When one of the three outer zinc porphyrin is excited by light energy, the energy is transferred through the central zinc porphyrin to the free-base porphyrin, which is connected to the fullerene. The energy causes the free-base porphyrin and fullerene to exist in an excited state where there is electron transfer and charge separation. The free-base porphyrin and fullerene then decay, resulting in recombination and an output of electrochemical energy.
While current artificial photosynthesis methods are far less efficient than the natural process, there has been continual progress in the field. One of the reasons that the technology is being pursued is that, compared to current solar panel technology, molecular nanoparticles are cheaper, lighter, and more environmentally sound. Aside from providing a renewable energy source and eliminating our reliance on rapidly diminishing fossil fuels, it has also been suggested that artificial photosynthesis on a large industrial scale could reverse global warming since the process consumes carbon dioxide and releases oxygen. With the potential of such beneficial impacts on the environment and our energy supply, continued research into combining nanotechnology and natural processes should remain a central goal.