Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


The hunt for electron holes

A molecular glance on solar water splitting
Hydrogen production by solar water splitting in photoelectrochemical cells (PEC) has long been considered the holy grail of sustainable energy research. Iron oxide is a promising electrode material. An international team of researchers led by Empa, the Swiss Federal Laboratories for Materials Science and Technology, have now gained in-depth insights into the electronic structure of an iron oxide electrode – while it was in operation. This opens up new possibilities for an affordable hydrogen production from solar energy.

Hematite, the mineral form of iron oxide (or trivially, rust), is a promising anode material for photoelectro-chemical cells (PEC) because of its affordability, availability, high stability and good spectral match to the solar spectrum. Although it has the potential of a 15% solar-to-hydrogen energy conversion efficiency, its actual efficiency is lower than that of other metal oxides. This is due to hematite’s electronic structure, which only allows for ultrashort electron-hole excited-state lifetimes.

(Image source: iStock)

Helpful holes in hematite

Electrons are well-known (negative) charge carriers, indispensable in our daily lives, but they do not play this role alone. When an electron leaves its assigned place, it leaves behind a hole that can effectively behave like a positive charge carrier, provided that the electron and hole remain separated and do not recombine. In modern semiconductor electronics, holes are important charge carriers, without which devices like batteries, capacitors, fuel cells, solar cells, and PEC could not operate. PEC electrodes typically form electron-hole pairs when struck by sunlight. In PEC photoanodes made of hematite, the generated holes must diffuse to the semiconductor surface, where they can oxidize water and form oxygen.

However, the electronic structure of hematite is such that the photo-generated holes tend to recombine with the electrons before reaching the surface. As a result, the resulting photocurrent is limited by the relatively few holes that actually do reach the surface. Recent efforts to optimize the nanostructured morphologies of hematite photoanodes have led to significant improvements in performance, but in spite of these efforts, the overall energy conversion efficiency in hematite remains at only about a third of its potential. An intelligent management of electron and hole transport is, therefore, critical for a better materials performance.

In this context, a better understanding of hole states at the hematite surface has been the subject of much interest as well as debate. It has long been suspected that in hematite, two types of holes with different water-splitting power are formed. The existence of different types of holes with disparate reactivity toward water oxidation has broad implications for the ultimate performance of hematite. But it is quite difficult to detect such holes, and studies of this phenomenon are complicated by numerous technical constraints. Moreover, the holes are transitional and quite elusive.

Not all holes are created equal

In a recent study published in the «Journal of Physical Chemistry C», Empa researchers Artur Braun and Debajeet Bora and their colleagues from EPF Lausanne, the University of Basel, China and the US studied the nature of photoelectrically generated holes in a PEC that had been specially designed for gathering data while the cell is in operation («operando»). They recorded soft X-ray absorption spectra under simulated sunlight and in the dark and identified two new spectral signatures corresponding to two different hole transitions, an O 2p hole transition into the charge-transfer band and an Fe 3d-type hole transition into the upper Hubbard band. According to Braun, this is the first time that the electronic structure of a PEC photoanode has been analyzed while it was in real water splitting action, i.e. in contact with electrolyte, under anodic bias and illuminated by visible light. «The preparations for this extremely complex experiment took us three years», says Braun. «After all, soft X-ray spectroscopy works only in ultra-high vacuum, and photoelectrochemistry works only in liquids. Combining both was technically a great advancement. Yet, I would say we were very fortunate to discover the two electron holes in an operating PEC.»

Schematic drawing of a photoelectrochemical cell with a 30 nm thin hematite photoanode (orange), properly wired, in contact with electrolyte (blue), illuminated with visible light, separated by a 100 nm thin membrane from the vacuum environment, and analyzed in situ with soft X-rays in the ALS synchrotron.

Their groundbreaking experiment demonstrated the formation of two different types of electron holes at the semiconductor-liquid interface under the exact conditions, at which the photocurrent arises. Quantitative analysis of their spectral signatures revealed that both types of holes, contrary to earlier speculation and historical perception, contribute to the resulting photocurrent. «This is a milestone in the understanding of solar water splitting and encouraging news for researchers worldwide who are working to optimize hematite for PEC photoanodes», says Braun.

Artur Braun | EurekAlert!
Further information:

More articles from Materials Sciences:

nachricht 3-D-printed structures shrink when heated
26.10.2016 | Massachusetts Institute of Technology

nachricht From ancient fossils to future cars
21.10.2016 | University of California - Riverside

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Etching Microstructures with Lasers

Ultrafast lasers have introduced new possibilities in engraving ultrafine structures, and scientists are now also investigating how to use them to etch microstructures into thin glass. There are possible applications in analytics (lab on a chip) and especially in electronics and the consumer sector, where great interest has been shown.

This new method was born of a surprising phenomenon: irradiating glass in a particular way with an ultrafast laser has the effect of making the glass up to a...

Im Focus: Light-driven atomic rotations excite magnetic waves

Terahertz excitation of selected crystal vibrations leads to an effective magnetic field that drives coherent spin motion

Controlling functional properties by light is one of the grand goals in modern condensed matter physics and materials science. A new study now demonstrates how...

Im Focus: New 3-D wiring technique brings scalable quantum computers closer to reality

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...

Im Focus: Scientists develop a semiconductor nanocomposite material that moves in response to light

In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...

Im Focus: Diamonds aren't forever: Sandia, Harvard team create first quantum computer bridge

By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.

"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...

All Focus news of the innovation-report >>>



Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

Agricultural Trade Developments and Potentials in Central Asia and the South Caucasus

14.10.2016 | Event News

World Health Summit – Day Three: A Call to Action

12.10.2016 | Event News

Latest News

Greater Range and Longer Lifetime

26.10.2016 | Power and Electrical Engineering

VDI presents International Bionic Award of the Schauenburg Foundation

26.10.2016 | Awards Funding

3-D-printed magnets

26.10.2016 | Power and Electrical Engineering

More VideoLinks >>>