Silicon Nanowires Show Improved Performance as Photocathode for Catalyzed Carbon Dioxide Photofixation

摘要

The ability to use sunlight, the most abundant form of energy on earth's surface, to power chemical reactions is a unique feature of natural photosynthesis.The process enables the storage of solar energy that is intermittent in nature. With carbon dioxide (CO2) as a feedstock, it also produces highly specific organic chemicals that are the essential energy suppliers or building blocks for a wide range of important natural processes. Significant research efforts have been devoted to mimicking the process in artificial systems, the major focus being on how to improve solar energy conversion efficiencies. Relatively underwhelming attention is paid to issues related to low product specificities when CO2 is reduced. Inspired by the detailed mechanisms of the dark reactions in the Calvin cycle,we recently reported a strategy to combat this problem. Briefly, the key was to avoid direct reduction of CO2, which is prone to produce carbon atoms of varying oxidation states. Instead, we rely on the creation of an intermediate that subsequently reacts with CO2 selectively. The idea is similar to the approach of Bocarsly et al. using pyridinium for the production of methanol, although we seek to produce more complex and hence synthetically relevant organic molecules. In principle, the scope of reactions can be significantly broadened if the intermediate produced by photoreduction is a catalyst that can be used to react with CO2 to yield the desired product. Similar to photosynthesis, the carbon-carbon bond-forming reactions are independent of photons (e.g. the dark reactions), allowing for an improved control in selectivity. To test this hypothesis, here we report our success in performing CO2 photofixation with the help of the [Ni(bpy)2] catalyst. To our surprise and delight, we observed that the Si nanowire (SiNW) photo-electrode exhibited more than 300 mV turn-on potential increase when compared with planar Si. We attributed this to the multifaceted nature of the nanowires.