In Science, it is imperative to connect the dots when trying to solve the world’s greatest challenges. One of these challenges, and where the pieces need to be marred together, is stipulated to be in the conversion of solar energy into chemical energy and the production of solar hydrogen energy by artificial photosynthesis, also known as solar water splitting (SWS). Research has found that solar energy harvesting technologies require a net positive energy balance, which has been achieved by a team of Researchers from India who has developed an artificial leaf platform for SWS processes.
Photocatalytic water splitting, or solar water splitting (SWS), is a form of artificial photosynthesis for the dissociation of water into its molecular constituents (hydrogen and oxygen) using either natural or artificial light.
Many materials have been tried and tested for the production of solar hydrogen through artificial photosynthesis methods, but research has yet to produce a single photocatalyst which possesses high activity, scalability and sustainability.
Much of this has been due to the photocatalyst having to balance and integrate a number of factors, including the amount of potential light absorption, charge separation, charge migration, charge utilization at redox sites, width of the bandgap and the ability to absorb light in the ultraviolet (UV) range.
The team of Researchers have created a wireless composite device based on quasi-artificial leaf concept (QuAL), which is composed of gold nanoparticles integrated into a porous Titania electrode sensitized by chalcogenide (PbS and CdS) quantum dots (QDs).
The structure of the device was meticulously designed so that the gold nanoparticles are in close physical proximity to the Titania electrode and solar harvesting QDs, which took advantage of the so-called PIRET enhancement mechanism.
The resulting device was a wireless photochemical cell or quasi-artificial leaf (QuAL).
The plasmon induced resonance energy transfer (PIRET) enhancement mechanism generally occurs between metallic and semiconducting materials and then is transfer of energy, by a non-radiative energy transfer process, that occurs from the dipole of the plasmonic metal nanoparticle to the dipole of the excited semiconductor within a limited area.
To characterize the QuAl device, the Researchers employed a combination of X-ray diffraction (XRD, PANalytical X’pert Pro), scanning electron microscopy (SEM, Leica, Model Stereoscan-440) energy dispersive X-ray spectroscopy (EDX, Bruker D451-10C Quantax 200 with X-flash detector), high-resolution transmission electron microscopy (HRTEM, FEI TECNAI 3010), diffuse reflectance UV–Vis spectroscopy (Shimadzu, UV-2550), Raman spectroscopy (Horiba JY LabRAM HR 800), gas chromatography (GC, Agilent 7890A), chronoamperometry and potentiometry (Gamry Reference 3000) and solar simulation (Newport UUX 1404565) methods.
The carefully designed structure allowed for the production of an enhanced electric field around the gold nanoparticles, which ultimately led to an increase in both the photocurrent and the solar harvesting capabilities of the device. Solar hydrogen was produced using the device due to a larger number of electron-hole pairs being formed in the QDs from the influence of the localized electric field.
The research produced a QuAL which could generate hydrogen without applying any potential to the device. The QuAl was found to harvest the inbound solar light and spontaneously convert it into moist hydrogen gas (H2) with a high efficiency; much higher than that of a wired device.
The device showed a sustainable solar hydrogen production of 490 ± 25 µmol/h, which is 12 ml of H2 gas every hour, and arose from just 2 mg of photoanode material coated over an area of 1 cm2. Using a mathematical extrapolation approach, the Researchers have deduced that 6 L of hydrogen gas could be produced by only using an area of 23 x 23 cm2, with a 4.3 mA/cm2 photocurrent generation and a power conversion efficiency of 5.6%. Thus, showcasing its potential for scale-up and as a commercially driven process.
The hydrogen produced by the QuAl can be used and directly fed into many applications, including one of the most common applications - fuel cells.
The integration and structural design of the device has opened a new pathway for more efficient solar light harvesting. The Researchers have also proposed many areas of improvement and given examples of how it could be taken to a commercially viable standpoint if optimized.
It is thought that the light adsorption capacity of the photoanode and cost-effectiveness of the device, could be improved by using a naturally abundant co-catalyst. By increasing the concentration of gold nanoparticles, without a change in their size, it has been stipulated that the photocurrent generation could be improved, but by replacing the gold with cheaper SPR metals (such as silver), the device (and process) could be made more economical.
The efficiency of the SWS method is thought to have the potential for improvement by tuning the porosity of the Titania electrode and through an even distribution of light absorption components which are integrated within it. Efforts would also be required to scale up the size of the photoanode and test for long periods of time if they are to be used for real-world applications.
One area which is set to require a lot of future work is in the production of a counterpart for the current photoanode system. Such developments would allow for a system which efficiently utilizes the holes for oxygen generation and would make the system whole for water splitting applications without the need for sacrificial agents.
Despite needing some improvements for them to be commercially viable, the Researchers have designed and fabricated an efficient QuAl which can be used to design future light harvesting synthetic architectures for the efficient production of solar fuels.
“Possibly scalable solar hydrogen generation with quasi-artificial leaf approach”- Patra K. K., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-06849-x