Enhancing photoelectrochemical performance using embedded nanoparticles, and core shell nanowires

Abstract

The demand for sustainable and renewable energy has increased recently due to the emerging concerns over the global climate change caused by traditional fossil fuel usage. As a result, over the past few decades, many researchers have focused on developing renewable and sustainable sources of energy, such as solar cells, fuel cells, and water splitting, as potential alternatives to fossil fuels. Hydrogen, with its high energy density and clean properties, has the potential to fulfill the world's energy needs. Photoelectrochemical (PEC) water splitting is one method for generating hydrogen from water utilizing sunlight. Its applications, despite its advantages, are limited by the poor efficiency caused by the recombination of charge carriers, high overpotential, and sluggish kinetics of the reaction. Increasing efficiency and stability can be achieved through the synergistic effect of co-catalyst decoration. The utilization of nanoparticles as co-catalysts has traditional chemical or physical deposition methods, and recently, exsolution method is gaining attention. However, the exsolution method is not being effectively utilized in PEC water splitting, despite its potential to enhance efficiency and stability through the uniform and high density of the embedded nanoparticles.

In the first part of the dissertation, exsolution generates metal nanoparticles anchored within crystalline oxide supports, ensuring efficient exposure, uniform dispersion, and strong nanoparticle–perovskite interactions. Increased doping level in the perovskite is essential for further enhancing performance in renewable energy applications; however, this is constrained by limited surface exsolution, structural instability, and sluggish charge transfer. Here, hybrid composites are fabricated by vacuum-annealing a solution containing SrTiO3 photoanode and Co cocatalyst precursors for photoelectrochemical water-splitting. In situ transmission electron microscopy identifies uniform, high-density Co particles exsolving from amorphous SrTiO3 films, followed by film-crystallization at elevated temperatures. This unique process extracts entire Co dopants with complete structural stability, even at Co doping levels exceeding 30%, and upon air exposure, the Co particles embedded in the film oxidize to CoO, forming a Schottky junction at the interface. These conditions maximize photoelectrochemical activity and stability, surpassing those achieved by Co post-deposition and Co exsolution from crystalline oxides. Theoretical calculations demonstrate in the amorphous state, dopant─O bonds become weaker while Ti─O bonds remain strong, promoting selective exsolution. As expected from the calculations, nearly all of the 30% Fe dopants exsolve from SrTiO3 in an H2 environment, despite the strong Fe─O bond’s low exsolution tendency. These analyses unravel the mechanisms driving the amorphous exsolution.

In the second part of dissertation, CoO loading on the SrTiO3 thin film improves PEC water splitting efficiency by enhancing photogenerated carrier separation and facilitating hole transportation into the electrolyte through the CoO co-catalyst. In this study, Co nanoparticles (NPs) were embedded on the surface of SrTiO3 thin films to form type II heterostructures to promote charge separation. This structure forms through eutectic melting in the thermal treatment, involving high-temperature annealing of spin-coated Co, followed by phase separation between Co and SrTiO3 at room temperature, presenting notable advantages and enhanced adaptability compared to traditional exsolution methods. The loading of CoO NPs on SrTiO3 thin film exhibited the most significant improvement in photocurrent, reaching approximately 1.12 mA/cm at 2 V (vs. RHE), surpassing both pristine SrTiO3 films and CoO films/SrTiO3 films. Furthermore, in the long-term stability test conducted over a period of 24 hours, embedded CoO NP/SrTiO3 exhibited the highest level of stability among all the tested samples. The notable enhancement in PEC water splitting performance achieved through CoO NP loading can be attributed to the efficient separation and transportation of photogenerated charges. The eutectic melting method employed to embed a co-catalyst onto the photocatalyst's surface enables the formation of a robust heterostructure with exceptional durability, offering a simpler approach compared to conventional exsolution processes.

In the last part of dissertation, Core-shell nanowire (C/S NW) photoanodes for photoelectrochemical water splitting (PEC-WS) is an effective and promising strategy to enhance the light harvesting. Here, ZnO/Ta3N5 C/S NWs passivated by pyridine are fabricated for the first time for use as photoanodes in PEC-WS. The ZnO/Ta3N5/Pyridine C/S NW with an optimal Ta3N5 shell coating, 10 nm thick, exhibit a maximum photocurrent density of 4.46 mA/cm2 at 1.23 V vs. RHE and a photoconversion efficiency of 2.21% at 0.35 V vs. RHE, representing 12.53-fold and 36.83-fold enhancements, respectively, compared to ZnO NWs. This optimized photoanode displays remarkable stability over 24 hours with 98% retention, in contrast to the ZnO/Ta3N5 C/S NW, which fails after 19 hours. Furthermore, we confirm that the ZnO/Ta3N5 C/S NW outperforms the ZnO/TaON and ZnO/Ta2O5 C/S NWs in PEC-WS due to the extensively staggered type-II band alignment for efficient charge separation and the smaller bandgap for efficient light absorption. The pyridine grafting produces a surface dipole, creating a built-in electric field at the surface, and passivates the surface defects of Ta3N5, impeding the surface charge recombination. Detailed analyses clarifying the band alignments and charge transport/transfer mechanisms of aforementioned C/S NW photoanodes are provided.