Using Atomic Force Microscopy to Map Nanoscale Interfaces in Photoelectrochemical Devices

Photoelectrochemical devices capture light energy and generate alternative energy storage compounds. These devices rely on semiconductors to capture light energy, and nanocatalysts to drive chemical reactions. However, the way in which the two components work together is not well understood.

In a recent study, scientists at the University of Oregon presented a way to map the electrochemical properties of interfaces between semiconductor surfaces and single nanocatalysts particles in an operational system using operando atomic force microscopy (AFM).

Solar panels are photoelectrochemical devices that capture light energy and generate alternative energy for use or storage. (Image Credit: Gencho Petkov / Shutterstock.com).

Since the first oil crisis in the 1970s, scientists have been preoccupied with finding alternative sources of energy. Furthermore, increasing concern about climate change has intensified the search for carbon-neutral fuels over the past few decades. Capturing light energy to produce electricity and energy storage compounds, such as hydrogen and hydrocarbons, is an attractive green energy source and, unsurprisingly, has been an intense area of study in recent years.

Photoelectrochemical devices harness the energy from light to catalyze chemical reactions and produce alternative fuel molecules. They rely on semiconductors to capture light energy and produce charge, and nanocatalysts to transfer charge and enable chemical reactions in the electrolyte solutions.

Understanding how the materials in a photoelectrochemical device function together is critical for improving efficiency and designing better technologies for capturing and storing light energy but requires advanced characterization techniques that work in situ while the cell is operating.¹

Photoelectrochemical devices rely on a combination of materials

Semiconductors usually have a small band-gap (around 1 eV) between the filled valence band and the empty conduction band. In photoelectrochemical devices, light energy excites the electrons so they can jump to the conduction band, which generates electron pairs in the conduction band and holes (positive charges) in the valence band. Catalyst nanoparticles on the semiconductor surface collect the electrons or holes, which are then used in chemical reactions to form fuel molecules (e.g. 2H⁺ + 2e^- → H₂).

Understanding interfaces is critical for designing photoelectrochemical cells

Catalysts are often deposited onto photoactive semiconductor surfaces in the form of nanoparticles so they can collect the electrons or holes they need to catalyze reactions, but they don’t block light from hitting the semiconductor. The interface between the catalyst and the semiconductor is one of the most critical parts of a photoelectrochemical device, but also one that isn’t completely understood.

What is known is that the properties of these interfaces vary widely depending on the materials used, deposition technique, surface treatments, and particle sizes. What’s more, the features of the interfaces can change as the device operates, so a full understanding of how the materials in electrochemical cells operate requires in-situ, operando characterization techniques.¹

Studying nanoscale interfaces requires cutting-edge techniques

Scientists who are designing photoelectrochemical devices must be able to understand and control the flow of electrons and holes between semiconductors and nanocatalysts. However, it can be very challenging to characterize nanoscale semiconductor/catalyst interfaces in situ while the device is operating.

Atomic force microscopy (AFM) is one of the best candidates for characterizing interfaces in photoelectrochemical devices because it provides nanoscale surface mapping in a variety of environments.

AFM involves scanning the surface of a material using a mechanical probe called a cantilever. The use of a mechanical probe means atomic force microscopy can achieve much higher resolutions (down to fractions of a nanometer) than optical microscopy techniques, so it is well suited for nanoscale surface characterization.²

AFM can measure electronic properties, which are important in photoelectrochemical cells. However, most AFM techniques can only measure average currents and average voltages across a surface. Greater spatial resolution is required to really understand what is happening at interfaces in photoelectrochemical devices.²

Kelvin-probe force microscopy (KPFM), a noncontact form of atomic force microscopy, is able to provide nanoscale mapping of the composition and electronic properties of a surface. While KPFM works in some liquids, it cannot cope well with mobile ions, such as those that are present in electrolyte solutions, so it is not well suited to in situ characterization of materials in photoelectrochemical cells.^2,3

The latest operando AFM technology for mapping the properties of nanoscale interfaces

A team of researchers from the University of Oregon, funded by the Department of Energy, has found a new way to study nanoscale semiconductor/catalyst interfaces in situ in operational photoelectrochemical systems.

The team used a technique called potential-sensing electrochemical AFM (PS-EC-AFM), which allowed them to map the properties of single-particle interfaces on a densely covered substrate surface. Their results were published in Nature Materials earlier this month.⁴

PS-EC-AFM is a contact atomic force microscopy technique that uses a potential sensing probe to measure the surface electrochemical potential directly. When the electrode touches the surface, the system measures the voltage at that point, and the build-up of holes or electrons on the surface can be measured. As a result, the technique provides spatial mapping of the photoelectrochemical properties of a surface.⁵

PS-EC-AFM requires an insulated nanoelectrode, so the scientists utilized a specialized PeakForce SECM probe and a Bruker Dimension Icon AFM in PeakForce Tapping mode. This combination enabled them to quantitatively measure the electronic properties of a model photoelectrochemical device, in particular, the interfaces between single Ni nanocatalyst particles and the densely covered n-Si photoanode surface.

The researchers used point-and-shoot mode to position the tip of the probe on the in situ nanoparticles, allowing them to observe the photovoltage generated during photoelectrochemical oxygen evolution at each individual semiconductor/catalyst particle interface while the system was operating.^4,6

Prior to operation, they measured photovoltages of approximately 300 mV at all the Ni nanoparticles, independent of their size. When the system was operating, they measured much higher photovoltages and the voltage depended on the size of Ni particle.

The team concluded that when the system was operating, the Ni at the surface of the nanoparticles was oxidized to NiOOH, leading to a large depletion region around the semiconductor/catalyst interface, which enhanced the interfacial selectivity for holes, a phenomenon known as a ‘pinch-off’ effect.

Smaller Ni particles are more efficient oxidation catalysts, leading to more NiOOH and increased selectivity for holes, explaining the size dependence of the photovoltage measurements.⁴

Download: AFM-Based Scanning Electrochemical Microscopy (SECM).

AFM opens the door to better photoelectrochemical devices

The University of Oregon study represents the first time that researchers have observed the properties of the interfaces between in situ nanocatalyst particles on a semiconductor photoelectrode surface with nanoscale spatial resolution on a single particle level in an operational photoelectronic device.

The difference in electrochemical properties of the semiconductor/catalyst interface ex situ compared with in situ highlights the importance of operando techniques for characterizing materials for photoelectrochemical devices.

The authors hope that their technique will be used to improve the design and efficiency of a range of photoelectrochemical technologies and other electrochemical devices where interfaces are important, such as batteries and fuel cells.⁷

References

1. ‘Photoelectrochemical Solar Fuel Production: From Basic Principles to Advanced Devices’ —Giménez S, Bisquert J, Technology & Engineering, 2016.
2. ‘Conductive Atomic Force Microscopy: Applications in Nanomaterials’ — Lanza M, John Wiley & Sons, 2017.
3. ‘Probing charge screening dynamics and electrochemical processes at the solid-liquid interface with electrochemical force microscopy’ — Collins L, Jesse S, Kilpatrick KI, et al.,  Nature Communications, 2014.
4. ‘Nanoscale semiconductor/catalyst interfaces in photoelectrochemistry’ — Laskowski FAL, Oener SZ, Nellist MR, et al., Nature Materials, 2019.
5. ‘Potential-sensing electrochemical atomic force microscopy for in operando analysis of water-splitting catalysts and interfaces’ — Nellist MR, Laskowski FAL, Qui J, et al., Nature Energy, 2018.
6. ‘Nanoscale semiconductor/catalyst interfaces in photoelectrochemistry’ — Forrest A. L. Laskowski et al., Nature Materials, 2019.
7. ‘Dimension Icon' - click here for more information.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces. For more information on this source, please visit Bruker Nano Surfaces.