Author: Akshay M Achari, Research Scholar, Centre for Nanotechnology Research (CNR), Vellore Institute of Technology (VIT), Vellore - 632014.
From Sunlight to Solutions - A Tale of Photosynthesis
In the movie, "Cloudy with the chance of meatballs", Flynt Lockwood, a scientist creates food with the help of a machine using air, moisture, and lightning from the sky. Seemingly, a solution to world hunger, but still, it will be a very long time before we will be able to do it ourselves. This is something where mankind, or some of them, looks towards Mother nature for answers. She has been doing this since time memorial in the form of autotrophs (organisms that can produce their own food). Centre of which, lies photosynthesis – the ability to create food from air and water, much like Flynt in the movie. On the other hand, the destruction of organic pollutants in the environment has become a major concern due to the vast spread of awareness of its hazardous effect on health and our lifestyle.
Photocatalysis is a process where a catalyst, when exposed to light, triggers a chemical reaction without being consumed in the process. In simple terms, it's like using light to speed up a chemical reaction without using it up. This process has applications in various fields, including environmental remediation, energy production, and chemical synthesis.
The Enigmatic Dance of Light and Chemistry
Most of it can be removed by adsorption techniques that are sticking harmful substances like a sponge separating clean water from pollutants or breaking it down into simple harmless compounds with the help of biological and chemical means through redox reactions. Now, a question may arise. What is common between these two processes? Even though it seems diametrically opposite in nature – one being the making of the organic compounds and the other being the breaking down of another set of organic compounds. Both the biochemical processes are driven by the photo-electrochemistry within the leaf – in the case of photosynthesis, and chemical reagents – in the case of pollutant degradation. When these redox reactions are carried out via the energy of ambient photon energy, these reactions are called photocatalysis. As the name suggests, photo-catalysis can also be understood as a photon-driven catalysis reaction where it facilitates or accelerates a particular reaction with help of absorbed light energy.
The Essence of Photocatalysis
Photocatalysis can be classified into two categories – i.e. homogenous and heterogenous photocatalysis. Where homogenous catalysis is said to occur when the photocatalyst and the medium of photocatalysis are of the same phase. Implying that the photocatalysts are soluble in the medium. On the other hand, heterogeneous photocatalysis utilizes photocatalysts majority exhibiting semiconducting properties, which are of different phases from that of the medium. Since the photocatalyst is not soluble in nature it can be separated from the medium of photocatalysis. The gap between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy states is another problem of homogenous photocatalysis which is addressed by heterogeneous photocatalysts due to their semiconducting behavior. Depending on the bandgap, the photocatalyst can absorb a wide range of wavelengths of solar energy which makes it versatile in a plethora of chemical redox reactions.
Figure. Mechanism of Photocatalysis
A typical photocatalytic mechanism follows as such: the photocatalyst absorbs the ambient photonic energy which, if equal to or more than the energy of the band gap, can excite electrons to the conduction band or LUMO of the catalyst. The absence of the electrons creates holes in the valence band/HOMO which has an oxidative tendency depending on how low the position of the bandgap on the NHE (Normal Hydrogen Electrode) scale or SHE (Standard Hydrogen Electrode) scale is. Similarly, the higher the position of the conduction band, the greater the reductive capabilities of the separated electrons. The electrons reduce the adsorbed oxygen in the medium and result in the formation of superoxide radical (•O2-) which is a highly reactive reducing species. Whereas holes created at the valence band oxidizes the hydroxyl ion to create hydroxyl radicals, another useful reactive species. Now, let’s take the example of photosynthesis and pollutant degradation to understand this further. In the case of photosynthesis, the reduction process can help transform molecules like ADP and NADP into ATP and NAPDH which can store energy for further conversion of CO2 and H2O into glucose molecules via the necessary redox reaction. Similarly, in the case of a pollutant, its structural bonds are attacked by the superoxide and hydroxyl radicals in order to break down the polluting chemical in CO2 and H2O or a less harmful form of material.
"In light's graceful dance, molecules awaken, catalysts ignite, a symphony unshaken."
Exploring Challenges and Illuminating New Horizons: The Path of Photocatalysis
Even with promising research in the applications such as hydrogen generation, nitrogen fixation, CO2 sequestration, plastic degradation, and wastewater management to mention a few, photocatalysis has many practical challenges to be addressed. One among many such challenges might be to utilize the full spectrum of solar energy available on the earth’s surface. A material with a suitable bandgap is required to use the full solar spectra. The commercial materials have a very high band gap of 3.2 eV which requires ultraviolet energy (4-5%) to excite the electrons into the conduction band. After a lot of research, the band gap of the material has been reduced to such an extent, it will use either visible (40%) or near-infrared (55%) spectra of ambient light. This was achieved through band gap engineering which involved doping of various metal or non-metallic elements, formation of heterojunctions, fabrication of composites with low bandgap semiconductors, etc.,
In conclusion, the enthralling saga of photocatalysis unfolds as a testament to the timeless harmony between nature and innovation, weaving together the captivating narratives of photosynthesis and environmental renaissance. As we step into the luminous embrace of light-driven transformations, the journey of photocatalysis emerges as a beacon of hope, casting its radiant glow upon the pathways to a sustainable and vibrant future.
Reference
[1] Serpone, N. (2000). Photocatalysis. Kirk‐Othmer Encyclopedia of Chemical Technology.
[2] Schneider, J., Bahnemann, D., Ye, J., Li Puma, G., Dionysiou, D. D., Schneider, J., ... & Dionysiou, D. D. (Eds.). (2016). Photocatalysis: fundamentals and perspectives. The Royal Society of Chemistry.
