Structure and mechanism of operation of plasmonic photocathode for plasmon-mediated direct electron injection to drive solar-to-chemical energy conversion. (a) Plasmonic photocathode for solar water splitting, consisting of plasmonic nanoparticles in direct contact with water, p-NiOx as an electron blocking layer, and aluminum as the electrode material. (b) Energy schematic of the structure. Small metal nanoparticles facilitate the efficient decay of plasmons into hot carriers. Here the hot electron is directly injected into water molecules to drive the hydrogen evolution reaction. A NiOx layer blocks electron transport while allowing hole transport to the platinum anode. Note that this novel plasmon-induced hot carrier generation mechanism for water splitting does not rely on a semiconductor/metal Schottky junction to collect hot-electrons. Credit: ACS, Robatjazi et al. Click to enlarge.

Plasmonic metal nanostructures are currently receiving considerable attention in the quest for efficient solar-to-fuel energy conversion, in large part owing to their unique light harnessing capabilities associated with their localized surface plasmon resonance (LSPR) features. LSPR-assisted solar-to-fuel energy conversion has been shown to utilize three primary mechanisms: (I) elastic light scattering by plasmonic particles, (II) plasmonic near-field energy transfer, and (III) nonradiative plasmon decay into hot carriers. In particular this last mechanism is emerging as a hot topic since the utilization of hot carriers to drive chemical reactions may open up pathways that are inaccessible by conventional methods such as heating or applying an electrochemical bias.

Past demonstrations of hot-carrier utilization focused on employing a Schottky barrier for hot-electron extraction. In this scenario, only hot-electrons with sufficient energy to overcome the Schottky barrier can be collected from the conduction band of the semiconductor and spatially separated from the holes left behind in the metal nanostructure. … In this Letter, we describe an alternative architecture to achieve efficient and direct injection of hot electrons from plasmonic gold nanoparticles to adsorbed water molecules for solar water splitting in a Schottky junction-free device.

—Robatjazi et al.

Hot electrons have the potential to drive very useful chemical reactions, but they decay very rapidly, and people have struggled to harness their energy, explained lead researcher Isabell Thomann, assistant professor of electrical and computer engineering and of chemistry and materials science and nanoengineering at Rice. Most of the energy losses in the best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat, she added. Capturing these high-energy electrons before they cool could allow solar-energy providers to increase significantly the solar-to-electric power-conversion efficiencies.

In the light-activated nanoparticles studied by Thomann and colleagues at Rice’s Laboratory for Nanophotonics (LANP), light is captured and converted into plasmons, waves of electrons that flow like a fluid across the metal surface of the nanoparticles. Plasmons are high-energy states that are short-lived, but researchers at Rice and elsewhere have found ways to capture plasmonic energy and convert it into useful heat or light. Plasmonic nanoparticles also offer a promising means of harnessing the power of hot electrons, and LANP researchers have made progress toward that goal in several recent studies.

To use the hot electrons, Thomann’s team first had to find a way to separate them from their corresponding “electron holes,” the low-energy states that the hot electrons vacated when they received their plasmonic jolt of energy.

One reason hot electrons are so short-lived is that they have a strong tendency to release their newfound energy and revert to their low-energy state. The only way to avoid this is to engineer a system where the hot electrons and electron holes are rapidly separated from one another.

The standard way for electrical engineers to do this is to drive the hot electrons over an energy barrier that acts like a one-way valve. Thomann said this approach has inherent inefficiencies, but it is attractive to engineers because it uses Schottky barriers, a tried-and-true component of electrical engineering.

Rather than driving off the hot electrons, Thomann and her colleagues designed a system to carry away the electron holes. In effect, the setup acts like a sieve or a membrane. The holes can pass through, but the hot electrons cannot, so they are left available on the surface of the plasmonic nanoparticles.

The setup features three layers of materials. The bottom layer is a thin sheet of shiny aluminum. This layer is covered with a thin coating of transparent nickel-oxide, and scattered atop this is a collection of plasmonic gold nanoparticles about 10 to 30 nanometers in diameter.

When sunlight hits the discs, either directly or as a reflection from the aluminum, the discs convert the light energy into hot electrons. The aluminum attracts the resulting electron holes and the nickel oxide allows these to pass while also acting as an impervious barrier to the hot electrons, which stay on gold.

By laying the sheet of material flat and covering it with water, the researchers allowed the gold nanoparticles to act as catalysts for water splitting. In the current round of experiments, the researchers measured the photocurrent available for water splitting rather than directly measuring the evolved hydrogen and oxygen gases produced by splitting, but Thomann said the results warrant further study.

… our device is capable of efficient direct hot electron injection for photocatalysis, and the produced photocurrent is similar to that of recent reports for driving the hydrogen evolution reaction, but in contrast to previous designs, our device architecture possesses a relatively small catalytic surface area (basically a planar geometry); it does not utilize expensive platinum as a cocatalyst, it uses a minimum amount of gold, and importantly, it does not rely on a Schottky junction to collect the hot-electrons, thereby opening possibilities for cheaper, simpler, and more efficient devices.

—Robatjazi et al.

Thomann said the team is confident that it can optimize the system to improve significantly upon the results.

Resources

• Hossein Robatjazi, Shah Mohammad Bahauddin, Chloe Doiron, and Isabell Thomann (2015) “Direct Plasmon-Driven Photoelectrocatalysis” Nano Letters doi: 10.1021/acs.nanolett.5b02453