In Situ Imaging of Catalytic Reactions on Tungsten Oxide Nanowires Connects Surface–Ligand Redox Chemistry with Photocatalytic Activity

Abstract

Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis. Understanding the role of ligands used to prepare colloidal nanocrystals in catalysis is challenging due to the complexity and heterogeneity of nanocrystal surfaces. We use in situ singlemolecule fluorescence imaging to map the spatial distribution of active regions along individual tungsten oxide nanowires before and after functionalizing them with ascorbic acid. Rather than blocking active sites, we observed a significant enhancement in activity for photocatalytic water oxidation after treatment with ascorbic acid. While the initial nanowires contain inactive regions dispersed along their length, the functionalized nanowires show high uniformity in their photocatalytic activity. Spatial colocalization of the active regions with their surface chemical properties shows that oxidation of ascorbic acid during photocatalysis generates new oxygen vacancies along the nanowire surface. We demonstrate that controlling surface−ligand redox chemistry during photocatalysis can enhance the active site concentration on nanocrystal catalysts.

Getting the Most Out of Fluorogenic Probes: Challenges and Opportunities in Using Single-Molecule Fluorescence to Image Electro- and Photocatalysis

“…Pioneering studies in SMF imaging of heterogeneous catalysis focused on model systems including zeolite microcrystals, − gold and platinum nanoparticles, ,− and the photocatalytic generation of reactive oxygen species by TiO 2 microcrystals. − Over the last 18 years, this technique has expanded to more complex catalytic systems and has enabled visualization of nanoconfinement in porous catalysts, − photoexcited charge flow in semiconductor–semiconductor and semiconductor–metal heterostructures, − and nonuniform activity in metal and semiconductor catalysts due to nanoscale spatial variations in their defect concentration. ,,− Many of these papers use the fluorogenic probes shown in Figure . Resazurin (Figure a) ,,− ,,,,,,− ,− ,,, and 8-(3,4-dinitrophenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza- s -indacene (DN-BODIPY) (Figure b) ,, can be used to image reduction reactions, while Amplex red (Figure c) ,,− ,− ,,,,,,, and 3′-( p -aminophenyl) fluorescein (APF) (Figure d) ,,,…”

“…Similar to Amplex red, APF is typically first dissolved in DMF (rather than DMSO) and then diluted with water. For SMF imaging of semiconductor photocatalysts including TiO 2 , tungsten oxide, and indium selenide, APF has been used in aqueous phosphate buffer solutions at pH 7.4. ,,,, …”

“…We have used the spatial correlation between different fluorogenic probes to understand how the nonuniform distribution of oxygen vacancies, a common defect in metal oxide semiconductors, controls spatial variations in their photocatalytic activity. ,, In collaboration with Lew and co-workers, we developed a coordinate-based colocalization algorithm to quantify the spatial correlation between two data sets of single-molecule localizations . For example, tungsten oxide with the W 18 O 49 monoclinic phase is a semiconductor that is active for the photocatalytic oxidation of water to oxygen.…”

“…The nanowires were synthesized via a hydrothermal method that did not use ligands that bind to their surface. We then used ascorbic acid as a ligand to functionalize the W 18 O 49 nanowires and incorporate more oxygen vacancies through a photochemical process . Under photoexcitation, the functionalized W 18 O 49 nanowires extract electrons from surface-bound ascorbic acid molecules, which reduces tungsten ions and introduces additional oxygen vacancies for charge balance.…”

Understanding Photo(electro)catalysts for Energy Conversion via Operando Functional Imaging

“…The two fluorogenic probe colocalization method provides a promising approach to elucidate the relationship between surface defects and reactivity on a single photocatalyst. , By employing hydroxyl radical-selective probe (3′-( p -aminophenyl)­fluorescein, APF) and furfuryl alcohol probe (FA) to detect surface defects such as oxygen vacancies, it becomes feasible to establish a clear structure–activity relationship at the single-molecule level. Remarkably, a strong spatial correlation is observed between hydroxyl radical and oxygen vacancy-induced reactions on each nanorod.…”