Photocatlaytic water splitting is widely regarded as one of the most promising ways for the generation of hydrogen fuel under solar-light illumination is a sustainable and renewable energy technology to solve environmental issues. Numerous semiconductors with controlled nanostructures have been synthesized and investigated as photoelectrodes to alleviate the limitations that generally face all photoelectrocatalysis, i.e., light harvesting, charge separation, carrier mobility, and photostability. Recently, various modification strategies for metal-oxide photocatalysts, including cation/anion doping for visible-light harvesting, have been suggested. Furthermore, a series of new engineering methods that introduce co-catalysts, dye sensitizers, and quantum dots appear to improve the photocatalytic activity for hydrogen generation.
In this dissertation, I have studied unique nanostructures that range from plasmonic metal-oxide nanocatalyst to porous oxide supported nanoparticle photocatalyst for catalytic activity improvement and diverse surface analysis was performed to investigate its structural, electric and optical properties.
In chapter 1, I give an overview of the molecular-scale factors that influence the efficiency of solar water splitting and outline recent progress of commonly used engineering strategies that search for higher efficiency. In chapter 2, I report the photocatalytic activity of hydrogen generation on Pt nanoparticles deposited on hierarchically porous N-doped $TiO_2$ nanostructures (Pt-NHPT). We found that Pt-NHPT-300 shows a two-fold higher $H_2$ evolution activity than that of Pt-NHPT-800, and 30% higher activity than undoped hierarchical catalysts (Pt-HPT-300). The enhanced photoactivity is attributed to the synergistic effects of N doping, hierarchical porosity, and charge transfer between the $TiO_2$ nanostructures and the Pt co-catalyst.
In chapter 3, a series of photocatalysts were prepared with crystalline macro-mesoporous oxides and Pt nanoparticles (Pt–$TiO_2$, Pt–$Ta_2O_5$, Pt–$Nb_2O_5$, Pt–$ZrO_2$, and Pt–$Al_2O_3$). The high surface area and crystalline walls of the oxides play significant roles in photocatalytic $H_2$ production. Pt–$TiO_2$ catalysts show enhanced photocatalytic water splitting efficiency for $H_2$ generation (solar energy conversion efficiency of 1.06%). The enhanced photocatalytic activity is attributed to correct band alignment of the porous oxides with absorption in the UV-visible range, and ordered macro- and mesoporosity of the crystalline oxides for efficient charge transfer. In
chapter 4, I report the enhancement of photocatalytic activity by the flow of hot electrons on $TiO_2$ nanotube arrays decorated with 5–30 nm Au nanoparticles as photoanodes for photoelectrochemical water splitting. This enhanced photocatalytic activity is correlated to the size of the Au nanoparticles, where higher oxygen evolution was observed on the smaller nanoparticles. Conductive atomic force microscopy and ultraviolet photoelectron spectroscopy were used to characterize the Schottky barrier between the Au and the $TiO_2$, which reveals a reduction in the Schottky barrier with the smaller Au nanoparticles and produces an enhanced transfer of photoinduced hot carriers. This study confirms that the higher photocatalytic activity was indeed driven by the hot electron flux generated from the decay of localized surface plasmon resonance.
Lastly, in Chapter 5, I investigated the quantitative effects of carbon dopants on $TiO_2$ nanotube arrays for photoelectrochemical (PEC) water splitting. There was no change in the morphology of the nanotube arrays, but the dopant (or defect) levels that were introduced into $TiO_2$ band gap absorbed visible light (> 425 nm) to overcome the relatively large band gap of the $TiO_2$, which only reacts under UV light. I also examined interstitial carbon dopants that form oxygen vacancies, which can suppress the recombination of photogenerated charge carriers by temporarily trapping holes. The carbon-doped $TiO_2$ nanotube arrays created via exposure to ethanol for eight hours exhibited maximal efficiency and photocatalytic activity under simulated solar light.