Enhanced Photoelectrochemical Water Oxidation via TiO2-Coated Fluorine-Doped Tin Oxide Nanoparticle

The direct generation of hydrogen fuel from solar energy is considered to be the “holy grail” of hydrogen production. Photoelectrochemical water-splitting devices offer an opportunity to reach this goal if their efficiencies are improved. TiO₂ is an attractive semiconductor anode material for PEC water-splitting electrodes because of its functionality, long-term stability in corrosive environments, nontoxicity, and low cost.  Despite the extensive efforts devoted to optimizing the TiO2-based photoelectrodes, their solar-to-fuel conversion efficiencies still fall well short of their theoretical values. In this study, we report on the enhancement observed when thin TiO2 films are uniformly synthesized by atomic layer deposition (ALD) over a conductive scaffold composed of fluorine-doped tin oxide nanoparticles (nanoFTO). The conductive network of nanoFTO facilitates the efficient transport and collection of photogenerated electrons, while their intrinsic porosity increases the interfacial surface area with the electrolyte. Moreover, the decoupling of carrier diffusion and optical penetration length have the synergistic effects of maximizing photon absorption while also minimizing the diffusion length required for the photogenerated hole to reach the electrolyte/TiO₂ interface. Using an effective nanoFTO scaffold thickness of 2 µm, nanostructured TiO₂ samples can achieve a photocurrent density enhancement of more than 425% over TiO₂ films coated over conventional planar FTO scaffolds while operating at ~60 mW/cm² under AM 1.5 G simulated sunlight. In summary, by noting the effects of air annealing treatments on the electrochemical impedance spectroscopy (EIS) response and the incident photon to current efficiency (IPCE) spectra, we will shed light on the nature of this photoelectrochemical enhancement. These findings contribute to our understanding of the effects of nanostructuring TiO2 materials on conductive nanoparticle networks, and can aid in the development of more viable PEC water-splitting devices.