Plasmonic photochemistry has a large potential to replace energy-intensive chemical processes with low-temperature, low-pressure light-driven chemical reactions. Plasmonic nanostructures have emerged as promising photocatalysts with exceptional and tunable light absorption across the entire solar spectrum. Further, upon photon absorption they generate energetic charge carriers (hot electrons and hot holes) that can selectively drive chemical reactions. Despite intense research, major questions remain on the microscopic nature of hot-carrier-driven chemical transformations. Indeed, realizing precisely engineered systems where photonic, electronic, and chemical processes can be quantified is challenging. This thesis explores various aspects of plasmonic photocatalysis: (i) synthesis of high-quality single-crystalline gold microflakes (SC Au MFs) compatible with nanofabrication techniques, (ii) microscale characterization of hot-carrier-driven processes in precisely engineered photocatalytic systems, and (iii) realization of scalable perfect light absorbers for upscaling of efficient light-harvesting photochemical devices. Single-crystalline metal films can be used to obtain high-quality plasmonic nanoantennas with unique optical properties and well-defined crystallographic surfaces, which exhibit distinct catalytic and hot-carrier transport properties. In this thesis, we first present a facile gap-assisted synthesis strategy that overcomes the challenges related to the epitaxial growth and wet-chemical colloidal approaches, resulting in on-substrate growth of ultra-thin and extra-large SC Au MFs. Practical realizations of plasmonic hot carrier devices require a full understanding of hot-carrier-driven processes, from plasmon excitation to hot-carrier generation, transport and interfacial collection. To date, the two latter aspects have remained elusive. In the second part of this thesis, we investigate these aspects in micro-scale plasmonic photocatalytic devices consisting of Au nanodisk arrays fabricated from SC Au MFs on semiconducting substrates with different polarities (TiO2 and p-GaN). Using light-assisted scanning electrochemical microscopy, we quantify the wavelength-dependent photochemical response of these systems by locally probing hot-hole-driven oxidation of ferrocyanide redox molecule on Au/TiO2 photoanodes and hot-electron-driven reduction of ferricyanide on Au/p-GaN photocathodes. We reveal different injection probabilities and distinguish inner/outer sphere hot carrier transfer in our devices. Light-conversion photocatalytic devices would highly benefit from perfect light harvesting. However, simultaneously satisfying the requirements of broadband spectrum, omnidirectionality, polarization insensitivity, and scalability is very challenging. In the last part of this thesis, we propose a facile and scalable fabrication of a perfect light absorber consisting of Cu nanowires (NWs) coated with radially grown carbon nanotubes (CNTs). Our absorber harvests on average 99% of light over the broad 400-1000 nm spectral range, is polarization insensitive, and preserves its performance for incident angles up to 60°. Our modeling shows that Cu NW and CNT components synergistically suppress reflection while maximizing light absorption. Overall, this thesis provides an in-depth understanding of hot-carrier-driven processes in plasmonic photochemistry and establishes new guidelines for developing optimized plasmonic catalytic systems.