Designing Electrochemical Photoreactors from Single and Ensemble Multijunction Si Particles

“Photoelectrochemistry” describes the process of taking sunlight and converting it, first, into electrical energy and, second, into chemicals. The chemicals produced could be a fuel like hydrogen, or they could be other desirable products with other commercial uses. Usually, the energy from sunlight is collected using photovoltaic solar cells made from silicon that generate electrical energy.

These solar cells are typically seen on rooftops. However, these systems are not currently used to produce chemicals because it is not cost-effective. This project uses the same semiconductor material silicon to make millions of microscopic particles that each behave as a miniature solar cell.

The project will design the particles to produce high voltages that make them capable of doing chemistry. Unlike a panel on a rooftop, these particles can simply be suspended in water to do the chemistry, providing a new method to perform photoelectrochemistry. The project will develop the fundamental principles to make the particles, stabilize them in water, and make them perform chemistry efficiently.

It will involve undergraduate and graduate students who will gain experience in nanomaterials synthesis, microfabrication, electrochemistry, solid-state catalysis, optical measurements, and optoelectronic modeling. A broad set of outreach efforts will be pursued, including programs and demonstrations in elementary schools and local libraries and an annual public science exposition.

Multijunction Si nanowire photocatalysts blur the distinction between, on the one hand, a photovoltaic structure connected to an electrolyzer, and on the other, a semiconductor photoelectrode performing catalysis. The structures are created by a bottom-up vapor-liquid-solid growth process that permits the synthesis of a precise multijunction structure containing both p-i-n solar cells and n-p tunnel junctions in a high aspect ratio particulate form factor hundreds of nanometers in diameters and microns in length.

They offer unique advantages for particle suspension reactors, including (1) usage of earth-abundant and non-toxic Si, (2) broadband light absorption into the near infrared, (3) photovoltage tunable for specific reaction chemistry by number of p-i-n junctions, and (4) spatial separation of anodic and cathodic reactions due to the high aspect ratio and axial orientation of the photovoltaic structure. Because the nanostructures uniquely contain both photovoltaic and photoelectrode elements, numerous fundamental scientific questions arise about the single-particle properties, ensemble interactions, and co-catalyst functionality.

Thus, the overarching goals of this project are to understand (1) the chemical effects of liquid solution on individual nanowire photovoltaic and photocatalyst properties, (2) the electrochemical and photonic interactions of nanowires in suspension, and (3) co-catalyst design principles that enable well-controlled and efficient photoelectrochemical reactions. The project combines measurements on single nanowires and ensembles of nanowires in suspension with simulation and modeling efforts to predict and interpret their properties.

The project will provide fundamental insights into the operation of particle suspension reactors that are applicable not just to Si nanowires but also to a range of photocatalyst material systems.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.