It is essential and urgent to develop an efficient yet cost-effective approach to convert solar energy into useful energy sources to sustain our society in the near future. TiO2 supported nanostructures have the potential to tackle this challenge due to its low-cost nature and flexibility in device design. The light absorbers that are anchored onto TiO2 surface play important roles on determining the device performance, however, the current systems provide low efficiencies even after decades of studies. In this dissertation, I have investigated two emerging nanomaterials, glutathione-capped gold nanoclusters and organometal halide perovskites as light absorbers, respectively. Their performances in TiO2 supported systems for solar to electricity and solar to fuel conversion applications are examined and discussed.
Glutathione-capped gold nanoclusters which exhibit molecule-like properties are first investigated for solar to electricity conversion applications. The excited state behavior of the gold nanoclusters elucidated from time-resolved spectroscopy shows favorite long-lived excited state lifetime. These long-lived excited states are found to inject energetic electrons into the conduction band of TiO2 upon light excitation, confirming the role of gold nanoclusters as the new class of photosensitizer alternative to dye molecules and quantum dots. We fabricated a metal cluster-sensitized solar cell (MCSCs) using these clusters as the photosensitizers and demonstrated its ability of delivering stable power conversion efficiency of 2 %. This new class of photosensitizer is further explored as a co-sensitizer in a dye-sensitized solar cell (DSSC). The metallic core of the gold nanoclusters is found to accumulate electrons and raise the quasi-fermi level of TiO2 under illumination when two of them are in contact. The resulting co-sensitized solar cells show greater energy conversion efficiency compared to the solar cells composed of single photosensitizer.
The promising gold nanoclusters-sensitized TiO2 nanostructures are further studied for light-driven hydrogen production application in two electrode photoelectrochemical cells (PECs) and photocatalytic slurry systems. The glutathione-capped gold nanoclusters characterized using cyclic voltammogram show suitable energy levels for hydrogen and oxygen evolution reaction at neutral pH. The gold nanoclusters-sensitized TiO2 nanostructures are capable of producing hydrogen gas under visible light illumination at neutral pH without applying external bias or sacrificial reagents. These exceptional performances are superior to the pre-existing systems using dye molecules and quantum dots as the photosensitizers.
Lastly, organometal halide perovskite is explored for light-driven hydrogen production. This emerging light absorber composed of inexpensive, earth-abundant elements and can be prepared using low-cost solution processes, has already demonstrated its ability of delivering power conversion efficiency of 20 % in TiO2 supported nanostructures. The uniqueness of perovskite solar cells in delivering high open circuit voltage is designed to supply additional photovoltage to a n-type semiconductor photoanode to drive the overall water splitting reaction in a photoanode-photovoltaic tandem device. The solar to hydrogen conversion efficiency of this device is 2.5 % under simulated sunlight illumination. Further improvement on the device efficiency and stability are expected via optimizing the electrochemical activity of the front photoelectrode.