Sub-particle reaction and photocurrent mapping to optimize catalyst-modified photoanodes

Abstract

The splitting of water photoelectrochemically into hydrogen and oxygen represents a promising technology for converting solar energy to fuel. The main challenge is to ensure that photogenerated holes efficiently oxidize water, which generally requires modification of the photoanode with an oxygen evolution catalyst (OEC) to increase the photocurrent and reduce the onset potential. However, because excess OEC material can hinder light absorption and decrease photoanode performance, its deposition needs to be carefully controlled--yet it is unclear which semiconductor surface sites give optimal improvement if targeted for OEC deposition, and whether sites catalysing water oxidation also contribute to competing charge-carrier recombination with photogenerated electrons. Surface heterogeneity exacerbates these uncertainties, especially for nanostructured photoanodes benefiting from small charge-carrier transport distances. Here we use super-resolution imaging, operated in a charge-carrier-selective manner and with a spatiotemporal resolution of approximately 30 nanometres and 15 milliseconds, to map both the electron- and hole-driven photoelectrocatalytic activities on single titanium oxide nanorods. We then map, with sub-particle resolution (about 390 nanometres), the photocurrent associated with water oxidation, and find that the most active sites for water oxidation are also the most important sites for charge-carrier recombination. Site-selective deposition of an OEC, guided by the activity maps, improves the overall performance of a given nanorod--even though more improvement in photocurrent efficiency correlates with less reduction in onset potential (and vice versa) at the sub-particle level. Moreover, the optimal catalyst deposition sites for photocurrent enhancement are the lower-activity sites, and for onset potential reduction the optimal sites are the sites with more positive onset potential, contrary to what is obtainable under typical deposition conditions. These findings allow us to suggest an activity-based strategy for rationally engineering catalyst-improved photoelectrodes, which should be widely applicable because our measurements can be performed for many different semiconductor and catalyst materials.

Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

“…Much effort has been directed towards searching efficient and stable semiconductor materials for PEC cells, including silicon (Si) 5–10 , III–V compounds 11,12 , and various oxides 13,14 . In comparison to oxides and III–V semiconductors, Si is more attractive because of its low cost, low bandgap (~1.1 eV), and applicable conduction band edge position for hydrogen evolution reaction (HER) 15–18 .…”

Intermittent photocatalytic activity of single CdS nanoparticles

“…In recent years, single-nanoparticle catalysis, which studies the catalytic activity at single-nanocatalyst level, has received great attention due to its unique strengths to understand the microscopic catalytic kinetics and mechanism (4)(5)(6). For semiconductor photocatalysts, single-nanoparticle photocatalysis is particularly interesting because of their comprehensive photophysical processes including photoinduced carrier generation, migration, and recombination.…”

“…Although powerful, fluorescence microscopy requires a fluorogenic model reaction and might compromise the nature of many important reactions where products are nonfluorescent (for example, photocatalytic H 2 production reactions). Besides, because it often takes tens of minutes to accumulate enough counts for statistical analysis, it has been mostly used to study the spatial heterogeneity at a cost of temporal resolution (4)(5)(6)16). A previous study reported the inhibition and reappearance of photocatalytic activity of single Sb-doped TiO 2 nanorods as a result of the absorption and desorption of surface adsorbates (17).…”

Electrochemical Production of Oxalate and Formate from CO2by Solvated Electrons Produced Using an Atmospheric-Pressure Plasma

“…Scientists have approached the challenge of CO 2 reformation with a variety of tools and techniques including catalytic reduction, 3 photoelectrochemical reduction, 4,5 and electrochemical reduction 6 to produce chemicals including carbon monoxide (CO), formic acid (HCOOH), oxalic acid (C 2 H 2 O 4 ), formaldehyde (CH 2 O), and methanol (CH 3 OH) among others. In catalysis, large molecules or rare earth metals have sites with low electron affinity 7 onto which the CO 2 adsorbs and is reduced. In photoelectrochemical reduction, a photon (usually in UV range) promotes an electron in the cathode to an excited state with enough kinetic energy to reduce the CO 2(ads) .…”