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Rice team demonstrates plasmonic hot-electron solar water-splitting technology; simpler, cheaper and efficient

5 September 2015

Researchers at Rice have demonstrated an efficient new way to use solar energy for water splitting. The technology, described in a paper in the ACS journal Nano Letters, relies on a novel plasmonic photoelectrode architecture of light-activated gold nanoparticles that harvest sunlight to drive photocatalytic reactions by efficient, non-radiative plasmon decay into “hot carriers”—highly excited electrons.

In contrast to past work, the new architecture does not utilize a Schottky junction—the commonly used building block to collect hot carriers. Instead, the team observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay.

Master.img-001-2
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

September 5, 2015 in Hydrogen Production, Nanotech, Solar, Solar fuels | Permalink | Comments (12)

Comments

I reckon I understood about one word in three of that.

From the little I understood my questions would be:

How efficient is it?

How much gold does it use?

New ways are being discovered, every other day, to better use solar energy to fill our future needs for H2, liquid fuels, chemicals, etc.

Interesting if it can be done in huge quantities at an affordable cost?

This is a really creative and cute lab experiment and in about 10 to 20 years someone might fund it and bring it to market, maybe! In the meantime, it keeps scientists employed. Hey, I would be the first to sign off for producing hydrogen for a hydrogen car if you don't burn hydrocarbons in the atmosphere and you use photons to create the gasses.

I often think how inefficient it is to generate electricity using solar or wind then create hydrogen using the electricity in an inefficient electrolysis process, compress and store the hydrogen which uses more energy, then use the hydrogen in an inefficient fuel cell to create electricity to drive an EV and call the device a hydrogen car. Am I missing something here or is that just dumb?

I like the simple life: create electricity using solar or wind, store it in a buffer battery and quick charge your battery EV as you need too. Notice the key to all this is a good inexpensive long-life, high-density battery. Isn't that what we are all waiting for?

@Lad,
Making Hydrogen is easier than you think. Running an electric current between two electrodes in pure water and you get Hydrogen. Any high school student can do that.

Newer electrolyzers can output Hydrogen to 80 bar of pressure, which can directly go into high-pressure pipeline for underground storage without further compression. Using two simple stages of compression and you can further get this to 980-bar high-pressure H2 dispenser.

H2 storage tank inside a vehicle and a few valves are far simpler than a complex battery pack with 7,000 cells with zillion of battery management circuitry.

pure water..not always

It's been a long time that I say to produce synthetic gasoline here in montreal for my gasoline car at a better price than actual petroleum gasoline. I need it and it is actually too costly so im tire of paying high doller for it. Also the provincial government here is criminal and charge a lot of tax on every liters of gasoline. don't put any tax on non-polluting synthetic gasoline so it gonna cost 4 time as low as actual petroleum gasoline

Getting hydrogen chiper then oil could be one alternative to battery and safe storage could go long way in making world greener.

Roger:
I'll buy hydrogen if you don't burn hydrocarbons in any of the processes to create.

After you have the H2 and O2; why not recombine them in a controlled environment, i.e., in an aircraft engine or jet. And, you quite polluting the upper atmosphere. Yes, I know if you get it wrong...BAM! But, the byproduct is H2O and that's good.

@Lad:

Presumably you are off grid entirely and just rely on your solar panels then, as if you plug in you sure are buying electricity which has used hydrocarbons to make it.

There is another write up here:

http://newenergyandfuel.com/http:/newenergyandfuel/com/2015/09/08/researchers-demo-a-sunlight-powered-water-splitter/

It sounds as though using the energy of hot electrons means that this may(?) enable higher efficiency than photovoltaic cells, and of course hydrogen is much more storable than electricity.

"Thomann said the results warrant further study..."

This is a lab experiment that shows the physics, not a ready for production device.

All told this could be taken as a form of thermionic device, which is an abortive dream from the 70's: Conversion of waste heat to electricity has been on the order of 5%.

To see what I mean, imagine the following:

Your "light source" can be a glow plug or mantle of cadmium or whatever,

the ultimate heat/energy source is nuclear, steam, or flue gas therms,

The hydrogen is immediately consumed by the water-originated or air derived oxygen, creating an electric junction. All incidental heat can be utilized within the plasmons to assist the energy barrier leaps,

Mass flows will be along those of decades old designs of thermionic gas flues, but

This could accommodate or anticipate carbon based reactions such as the water gas shift, or to use direct chemical coupling rather than direct combustion of hydrogen.

I'm really not thinking of storing hydrogen at all, but rather of storing accumulated heat for on-demand plasmonic startups.

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