Hot Electrons from Catalysis

SURFACE SCIENCE

Imagine being able to tap the energy released during exothermic reactions on surfaces. Usually, this energy is channeled into the surface, resulting in a slight temperature rise. According to a new study, however, the reactions can be used to generate a steady flow of electricity.

For years, scientists have known that energy liberated in reactions occurring on solid surfaces can be transferred into the solid, thereby generating short-lived energetic electrons. Carrying more than 0.5 eV of kinetic energy, these so-called hot electrons can zip around the interior of a material--often traveling several nanometers before losing the excess energy to their surroundings.

Generally, hot electrons cool off within mere picoseconds. Despite the short timescale, a number of researchers in the past decade have succeeded in detecting hot electrons. In most cases, though, the energetic particles were detected as tiny transient signals.

Now, a research team has constructed a nanoscale device known as a Schottky diode and used it to measure a continuous flow of hot electrons generated by catalytic surface reactions. Specifically, the group--which includes chemistry professor Gabor A. Somorjai and postdoc Xiaozhong (Eric) Ji of the University of California, Berkeley, and Anthony Zuppero and Jawahar M. Gidwani of San Francisco-based NeoKismet--measured a continuous current of 40 amp produced via oxidation of carbon monoxide on a platinum electrode for more than half an hour (Nano Lett. 2005, 5, 753).

The essential features of the diode include a 150-nm-thick layer of TiO₂ that supports a 5-nm-thick platinum film that serves as a catalyst and an electron source. As CO is converted to CO₂, the exothermic reaction generates hot electrons at the Pt surface. If the Pt film is no more than a few nanometers thick, the electrons can migrate through the metal to the semiconductor (TiO₂) interface and retain their high energy. Electrons with sufficient energy can cross the potential barrier into the semiconductor and be measured as a diode current. The team reports that, under the conditions used in the demonstration, for every four molecules of CO₂ produced, they collected three electrons.

Using Pt/GaN nanodiodes in a related study, Somorjai and coworkers showed that the Pt films must be of high quality and less than 10 nm thick to collect hot electrons efficiently (J. Am. Chem. Soc., published online March 31 at dx.doi.org/10.1021/ja050945m).

Francisco Zaera, a chemistry professor at UC Riverside, remarks that "the study could open new avenues for generating electrical signals directly from catalytic processes and can be used, for example, to design new types of chemical sensors." The observations also raise interesting questions regarding the role of hot electrons in catalytic reactions, he adds.

Somorjai agrees emphatically. "In light of the large currents of hot electrons measured in the study, we need to revisit the influence of electron transport on surface bonding and catalytic activity at metal and metal oxide surfaces," he says.

And given the similarities between the catalytic nanodiodes and nanosized metal-on-metal-oxide catalysts, the tiny devices are poised to serve as new model systems.