Photoelectrochemical cells have the potential to convert solar energy to liquid fuels. Nanostructured electrode materials are attractive light-harvesting materials for this purpose because their nanoscale dimensions minimize charge carrier transport distances to the solid/liquid interface. However, nanoscale materials are heterogeneous, and the influence of impurities, defects, and particle size/shape on the rates and efficiencies of the fundamental processes in photoelectrochemical cells is not entirely understood. This Perspective highlights molecular reaction imaging methods that spatially resolve principal processes in photoelectrochemical cells: (1) reaction intermediate imaging, (2) surface recombination imaging, and (3) charge transport imaging. We discuss opportunities to couple optical microscopy techniques with electron or X-ray microscopy characterization tools to reveal atomic-level structure–property relationships of single-nanoentity photoelectrodes. The atomistic detail of the semiconductor/electrolyte interface may guide the design and synthesis of nanostructured electrodes for direct solar fuels production.

Transition metal dichalcogenide (TMD) nanoflake thin films are attractive electrode materials for photoelectrochemical (PEC) solar energy conversion and sensing applications, but their photocurrent quantum yields are generally lower than those of bulk TMD electrodes. The poor PEC performance has been primarily attributed to enhanced charge carrier recombination at exposed defect and edge sites introduced by the exfoliation process. Here, a single nanoflake PEC approach reveals how an alternative effect, doping heterogeneity, limits ensemble-level PEC performance. Photocurrent mapping and local photocurrent–potential (i–E) measurements of MoS2 nanoflakes exfoliated from naturally occurring bulk crystals revealed the presence of n- and p-type domains within the same nanoflake. Interestingly, the n- and p-type domains in the natural MoS2 nanoflakes were equally efficient for iodide oxidation and tri-iodide reduction (IQE values exceed 80%). At the single domain-level, the natural MoS2 nanoflakes were nearly as efficient as nanoflakes exfoliated from synthetic n-type MoS2 crystals. Single domain-level i–E measurements explain why natural MoS2 nanoflakes exhibit an n-type to p-type photocurrent switching effect in ensemble-level measurements: the n- and p-type diode currents from individual domains oppose each other upon illuminating the entire nanoflake, resulting in zero photocurrent at the switching potential. The doping heterogeneity effect is likely due to nonideal stoichiometry, where p-type domains are S-rich according to XPS measurements. Although this doping heterogeneity effect limits photoanode or photocathode performance, these findings open the possibility to synthesize efficient TMD nanoflake photocatalysts with well-defined lateral p- and n-type domains for enhanced charge separation.

Imaging tools advance nanoscience education by enabling students to see and engage with matter that is not visible to the human eye. However, undergraduate students generally lack access to expensive research-grade electron and optical microscopes, especially at primarily undergraduate institutions. New, inexpensive, and hands-on microscopy activities are needed to advance nanoscience education. Here we demonstrate a microscopy activity in which students assemble a total internal reflection-based fluorescence microscope using interlocking building bricks and image a series of micrometer- and nanometer-sized fluorescent beads. The hands-on experiments are accompanied by fluorescence image analysis, single-particle detection, and particle size measurements. The activity enables students to explore the optical diffraction limit effect that was the subject of the 2014 Nobel Prize in Chemistry. This activity introduces undergraduate students to underlying principles of optical microscopy, optical components and light sources, Snell’s law, and image processing and analysis procedures while reinforcing major concepts like data analysis and statistics, fluorescence, emission, and absorption.

Understanding reaction processes at the semiconductor/electrolyte interface is critical to developing (photo)electrochemical energy conversion systems. For the water oxidation reaction at a photoanode in a solar water splitting cell, there are multiple reaction steps that produce surface adsorbed reaction intermediates, such as OH • and H 2 O 2. Little is known about how the reaction intermediate surface concentrations evolve during the water oxidation reaction. Here we use single-molecule fluorescence microscopy to track the time-dependent surface concentration of Ti-OH • species on TiO 2 photoanodes under chopped light illumination. We correlate the reaction intermediate surface concentration with the net rate of photoelectrochemical water oxidation (i.e., the photocurrent). The reaction intermediate dynamics follow different time scales in its temporal evolution from the photocurrent dynamics. By fitting the temporal evolutions of Ti-OH • species and photocurrent over a range of light on/off conditions and applied potentials, we observed that the rate constants for interfacial hole transfer and O-O bond formation depend on the applied potential under illumination. The variation in the rate constants could be attributed to the presence of surface states and/or a change in the chemical nature of Ti-OH • species on the photoanode surface. Our findings provide insight into water oxidation kinetics under intermittent solar irradiation conditions.

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