Understanding How the Structure of Boron Oxynitride Affects its Photocatalytic Properties

2023/05/302023/06/06

Synchrotron studies show that tuning the synthesis of boron oxynitride can improve its performance as a photocatalyst and semiconductor

Carbon dioxide (CO2) is often in the news these days. As a greenhouse gas, released during the combustion of fossil fuels, it is fuelling climate change, and reducing our CO2 emissions is critical to a sustainable future. CO2 is also a by-product of many industrial processes, including the production of ammonia used for fertilisers. On the other hand, many industries need a regular supply of CO2, and shortages have caused problems in recent years. It makes sense, therefore, to find ways to recycle some of the waste CO2 we produce into useful products. However, CO2 conversion reactions are energy-intensive, and new catalysts are needed to make the reactions more efficient. Photocatalysts absorb light energy, creating a charge separation that can then drive a chemical reaction. A team of researchers from Imperial College London are researching CO2 conversion using photocatalysis. In work recently published in Chemistry of Materials, they investigated how oxygen doping affects the photocatalytic and optoelectronic properties of boron nitride. Their results provide valuable insights into the photochemistry of boron oxynitride (BNO) at the fundamental level.

By clarifying the importance of paramagnetism in BNO semiconductors and providing fundamental insight into their photophysics, this study paves the way to tailoring its properties for CO2 conversion photocatalysis. The group has also recently used a similar methodology to investigate phosphorus doping of boron nitride, which they will explore in a future publication.

Read more on the Diamond Light Source website

Image: Combined experimental (EPR, NEXAFS) + computational study (DFT)

Credit: Image via Chem. Mater. 2023, 35, 5, 1858-1867

Ultrafast all-optical spin injection in silicon revealed at FERMI

2022/11/282022/11/29

A revolutionary and energy-efficient information technology encoding digital data in electron spin (spintronics) by combining semiconductors and ferromagnets is being developed worldwide. Merging of memory and logic computing of magnetic based storage devices and silicon-based logic transistors is expected to ultimately lead to new computing paradigms and novel spin-based multifunctional devices. The advantages of this new technology would be non-volatility, increased data processing speed, reduced electric power consumption. All of them are essential steps towards next generation quantum computers.

To create spin-based electronics with potential to revolutionize information technology, silicon, the predominant semiconductor, needs to be integrated with spin functionality. Although silicon is non-magnetic at equilibrium, spin polarized currents can be established in Si by a variety of approaches such as the use of polarized light, hot electrons spin injection, tunnel spin injection, Seebeck spin tunneling and dynamical spin pumping methods, as had been demonstrated recently. In general, spin polarized currents refer to the preferential alignment of the spin angular momentum of the electrons in a particular direction.

Advancing materials science with the help of biology and a dash of dish soap

2022/01/192022/01/27

High-speed X-ray free-electron lasers have unlocked the crystal structures of small molecules relevant to chemistry and materials science, proving a new method that could advance semiconductor and solar cell development.

Compounds that form tiny crystals hold secrets that could advance renewable energy generation and semiconductor development. Revealing the arrangement of their atoms has already allowed for breakthroughs in materials science and solar cells. However, existing techniques for determining these structures can damage sensitive microcrystals.

Now scientists have a new tool in their tool belts: a system for investigating microcrystals by the thousands with ultrafast pulses from an X-ray free-electron laser (XFEL), which can collect structural information before damage sets in. This approach, developed over the past decade to study proteins and other large biological molecules at the Department of Energy’s SLAC National Accelerator Laboratory, has now been applied for the first time to small molecules that are of interest to chemistry and materials science.

Researchers from the University of Connecticut, SLAC, DOE’s Lawrence Berkeley National Laboratory and other institutions developed the new process, called small molecule serial femtosecond X-ray crystallography or smSFX, to determine the structures of three compounds that form microcrystal powders, including two that were previously unknown. The experiments took place at SLAC’s Linac Coherent Light Source (LCLS) XFEL and the SACLA XFEL in Japan.

Read more on the SLAC website

Image: Artist’s rendition of the X-ray beam illuminating a solution of powdered metal-organic materials called chalcogenolates.

Credit: Ella Maru Studios

Astonishing diversity: Semiconductor nanoparticles form numerous structures

2021/08/052021/08/19

X-ray study reveals how lead sulphide particles self-organise in real time

The structure adopted by lead sulphide nanoparticles changes surprisingly often as they assemble to form ordered superlattices. This is revealed by an experimental study at PETRA III. A team led by the DESY scientists Irina Lokteva and Felix Lehmkühler, from the Coherent X-ray Scattering group headed by Gerhard Grübel, has observed the self-organisation of these semiconductor nanoparticles in real time. The results have been published in the journal Chemistry of Materials. The study helps to better understand the self-assembly of nanoparticles, which can lead to significantly different structures.

Among other things, lead sulphide nanoparticles are used in photovoltaic cells, light-emitting diodes and other electronic devices. In the study, the team investigated the way in which the particles self-organise to form a highly ordered film. They did so by placing a drop of liquid (25 millionths of a litre) containing the nanoparticles inside a small cell and allowing the solvent to evaporate slowly over the course of two hours. The scientists then used an X-ray beam at the P10 beamline to observe in real time what structure the particles formed during the assembly.

To their surprise, the structure adopted by the particles changed several times during the process. “First we see the nanoparticles forming a hexagonal symmetry, which leads to a nanoparticle solid having a hexagonal lattice structure,” Lokteva reports. “But then the superlattice suddenly changes, and displays a cubic symmetry. As it continues to dry, the structure makes two more transitions, becoming a superlattice with tetragonal symmetry and finally one with a different cubic symmetry.” This sequence has been never revealed before in such detail.

Read more on the DESY website

Image: The lead sulphide nanoparticles, which are about eight nanometres (millionths of a millimetre) in size, initially arrange themselves into a layer with hexagonal symmetry

Credit: (Credit: University of Hamburg, Stefan Werner)

First detailed look at how charge transfer distorts a molecule’s structure

2021/05/042021/05/27

Charge transfer is highly important in most areas of chemistry, including photosynthesis and other processes in living things. A SLAC X-ray laser study reveals how it works in a molecule whose lopsided response to light has puzzled scientists for nearly a decade.

When light hits certain molecules, it dislodges electrons that then move from one location to another, creating areas of positive and negative charge. This “charge transfer” is highly important in many areas of chemistry, in biological processes like photosynthesis and in technologies like semiconductor devices and solar cells.

Even though theories have been developed to explain and predict how charge transfer works, they have been validated only indirectly because of the difficulty of observing how a molecule’s structure responds to charge movements with the required atomic resolution and on the required ultrafast time scales.

In a new study, a research team led by scientists from Brown University, the Department of Energy’s SLAC National Accelerator Laboratory and the University of Edinburgh used SLAC’s X-ray free-electron laser to make the first direct observations of molecular structures associated with charge transfer in gas molecules hit with light.

Molecules of this gas, called N,N′-dimethylpiperazine or DMP, are normally symmetric, with a nitrogen atom at each end. Light can knock an electron out of a nitrogen atom, leaving a positively charged ion known as a “charge center.”

Read more on the SLAC website

Image: In experiments with SLAC’s X-ray free-electron laser, scientists knocked electrons out of a molecule known as DMP to make the first detailed observations of how a process called charge transfer affects its molecular structure. Left: DMP is normally symmetric. Center: When a pulse of light knocks an electron out of one of its nitrogen atoms (blue spheres), it leaves a positively charged ion known as a charge center, shown in pink. This creates a charge imbalance that shifts the positions of atoms. Right: But within three trillionths of a second, the charge redistributes itself between the two nitrogen atoms until it evens out and the molecule becomes symmetric again.

Credit: Greg Stewart/ SLAC National Accelerator Laboratory

Fe Cations Control the Plasmon Evolution in CuFeS2 Nanocrystals

2021/02/092021/02/25

Research on the synthesis of CuFeS₂, an exciting semiconductor, outlines a method to verify its phase purity and investigate its properties.

Plasmonic semiconductor nanocrystals have become an appealing avenue for researching nanoscale plasmonic effects due to their wide spectral range (visible to infrared) and great tunability compared to traditional precious metal nanocrystals. CuFeS₂ is an exciting semiconductor that has a prominent plasmon absorption band in the visible range (∼498 nm). In this work, the researchers determined the origin of the plasmonic behaviour in CuFeS₂ by characterizing the nucleation and growth stages of the reaction through a series of ex situ and in situ probes (e.g., X-ray absorption spectroscopy and X-ray emission spectroscopy). They showed that the plasmon formation is driven by band structure modification from Fe(II) incorporation into the nanocrystals. Mixed oxidation state of Cu(I)/Cu(II) and Fe(II)/Fe(III) was observed. Using these results, the researchers proposed a reaction mechanism for synthesis of CuFeS₂ and outlined a method to verify the phase purity of the material.

Read more on the CHESS website