Artificial photosynthesis, the biomimetic approach to converting sunlight's energy directly into chemical fuels, offers an attractive way to address the need for a clean, renewable source of energy. In plants, chloroplasts store the sun's energy using a system of integrated photosynthetic nanostructures including light-absorbing pigments, electron-transport chains, and chemical catalysts. However, neither such integration of nanostructures nor energy conversion efficiency suitable for practical applications has been achieved in artificial photosynthesis. In this context, the subject of my graduate research is to develop an integrated system using nanowire-based nanostructures to imitate natural photosynthesis. This centers on two themes: (1) constructing novel integrated nanostructures for solar-to-fuel conversion, and (2) developing next-generation materials and catalysts for improved photoelectrochemical (PEC) performance.
Although natural photosynthesis organizes its active components at the nanometer scale to better control the process of energy conversion, this level of integration had not been realized until recently for artificial photosynthesis. Here the construction of a nanowire-based integrated system to realize such a nanoscopic control is demonstrated.
Since all of the processes in PEC relate to the interfaces among semiconductor light-absorbers, electrocatalysts, and the electrolyte, the first to realize an integrated nanosystem was to under how photo-excited carriers would transfer within these interfaces. By using kelvin probe force microscopy (KPFM), the local electrostatic potential of an asymmetric nanowire composed of Si and a TiO2 shell, which was covered in a layer of water. Different local potentials were observed in dark and under illumination, which provides the knowledge that the heterojunction of Si and TiO2 could function as a Z-scheme system for solar water splitting.
After obtaining this piece of information, we moved forward to develop an integrated nanosystem for artificial photosynthesis. Taking the concept of Z-scheme, we used Si and TiO2 nanowires as building blocks to construct a tree-shaped heterostructure. In this structure, the positions of the reduction and oxidation components were pre-defined to mimic the spatial control found in chloroplasts. The integrated standalone device splits H2O into H2 and O2 under simulated sunlight, with an efficiency of solar-to-fuel conversion comparable to that of natural photosynthesis. This first demonstration paves the way for using nano-sized building blocks to achieve efficient solar-to-fuel conversion.
The nanowire-based integration allows individual building blocks to be replaced with newly developed ones. In this dissertation advanced building blocks for artificial photosynthesis is also demonstrated.
We have explored new materials and methods to improve the energy conversion efficiency of semiconductor light-absorbers. Solution-phase synthesis of III-V semiconductor nanowires was successfully demonstrated for photocatalytic reactions, and the nanowires' electronic properties could be fine-tuned to fit the needs of device integration. Also we demonstrated enhancement of the photoanodic activity of hematite (Fe2O3) using the surface plasmon resonance of exquisitely controlled Au nanostructures.
Additionally, new electrocatalysts suitable for practical applications are developed. We first looked into the lower limit of platinum (Pt) loading as a catalyst for the H2 evolution reaction (HER). Using the atomic layer deposition (ALD) technique, it is possible to quantitatively controlled the Pt loading down to about 0.2% of a monolayer (~10 ng/cm2), which is sufficient for some PEC applications. Cobalt sulfide, an earth-abundant catalyst, was also synthesized by electrodeposition. It acted as a HER catalyst in water at neutral pH and could be coupled with a Si photocathode for solar H2 production. Moreover we are developing an effective CO2 reduction catalyst of near unity selectivity for acetate production, which could be added into the integrated nanostructure.¬¬¬
In conclusion, my graduate research focuses on the integration of nanowire-based structures to achieve more efficient artificial photosynthesis. This is demonstrated in this dissertation not only at system level for an intergrated nanostructure, but also at component level for advanced building blocks. This research can serve as a foundation for the efforts of other researches in the field of artificial photosynthesis.