Solar-driven Hydrogen Economy

Hydrogen as a fuel source, rather than hydrocarbons like oil and coal, offers many benefits. Burning hydrogen produces harmless water with the potential to eliminate carbon dioxide emissions and their environmental burden. In pursuit of technologies that could lead to a breakthrough in achieving a hydrogen economy, a key issue is making hydrogen cheaply. Using catalysts to split water is the ideal way to generate hydrogen, but doing so usually requires an energy input from other chemicals, electricity, or a portion of sunlight which has high enough energy.

Now researchers at Osaka University have developed a new catalytic system for efficiently splitting water and making hydrogen with energy from normal sunlight. Their study was recently reported in Angewandte Chemie International Edition.

It has not been possible to use visible light for photocatalysis, but our approach of combining nanostructured black phosphorus for water reduction to hydrogen and bismuth vanadate for water oxidation to oxygen lets us make use of a wide range of the solar spectrum to make hydrogen and oxygen with unprecedented efficiency,” lead author Mingshan Zhu says.

Black phosphorus has a flat, two-dimensional structure similar to that of graphene and strongly absorbs light across the whole of the visible spectrum. The researchers combined the black phosphorus with bismuth vanadate, which is a well-known water oxidation catalyst.

In the same way that plants shuttle electrons between different structures in natural photosynthesis to split water and make oxygen, the two components of this new catalyst could rapidly transfer electrons excited by sunlight. The amounts of the two components was also optimized in the catalyst, leading to production of hydrogen and oxygen gases in an ideal 2:1 ratio.


Nano-based Material Is 60 Times More Efficient To Produce Hydrogen

Global climate change and the energy crisis mean that alternatives to fossil fuels are urgently needed. Among the cleanest low-carbon fuels is hydrogen, which can react with oxygen to release energy, emitting nothing more harmful than water (H2O) as the product. However, most hydrogen on earth is already locked into H2O (or other molecules), and cannot be used for power.

Hydrogen can be generated by splitting H2O, but this uses more energy than the produced hydrogen can give back. Water splitting is often driven by solar power, so-called “solar-to-hydrogenconversion. Materials like titanium oxide, known as semiconductors with the wide band-gap, are traditionally used to convert sunlight to chemical energy for the photocatalytic reaction. However, these materials are inefficient because only the ultraviolet (UV) part of light is absorbed—the rest spectrum of sunlight is wasted.

Now, a team in Osaka University has developed a material to harvest a broader spectrum of sunlight. The three-part composites of this material maximize both absorbing light and its efficiency for water splitting. The core is a traditional semiconductor, lanthanum titanium oxide (LTO). The LTO surface is partly coated with tiny specks of gold, known as nanoparticles. Finally, the gold-covered LTO is mixed with ultrathin sheets of the element black phosphorus (BP), which acts as a light absorber.

BP is a wonderful material for solar applications, because we can tune the frequency of light just by varying its thickness, from ultrathin to bulk,” the team leader Tetsuro Majima says. “This allows our new material to absorb visible and even near infrared light, which we could never achieve with LTO alone.”

By absorbing this broad sweep of energy, BP is stimulated to release electrons, which are then conducted to the gold nanoparticles coating the LTO. Gold nanoparticles also absorb visible light, causing some of its own electrons to be jolted out. The free electrons in both BP and gold nanoparticles are then transferred into the LTO semiconductor, where they act as an electric current for water splitting.

Hydrogen production using this material is enhanced not only by the broader spectrum of light absorption, but by the more efficient electron conduction, caused by the unique interface between two dimensional materials of BP and LTO. As a result, the material is 60 times more active than pure LTO.


After Graphene, New 2D Materials To Play With

Dozens of new two-dimensional materials similar to graphene are now available, thanks to research from the University of Manchester (U.K.) scientists. These 2D crystals are capable of delivering designer materials with revolutionary new properties. The problem has been that the vast majority of these atomically thin 2D crystals are unstable in air, so react and decompose before their properties can be determined and their potential applications investigated.  By protecting the new reactive crystals with more stable 2D materials, such as , via computer control in a specially designed inert gas chamber environments, these materials can be successfully isolated to a single atomic layer for the first time.
2D materials

The team created devices to stablise 2D materials

Combining a range of 2D materials in thin stacks give scientists the opportunity to control the properties of the materials, which can allow ‘materials-to-order’ to meet the demands of industry.  High-frequency electronics for satellite communications, and light weight batteries for mobile energy storage are just two of the application areas that could benefit from this research. The breakthrough could allow for many more atomically thin materials to be studied separately as well as serve as building blocks for multilayer devices with such tailored properties.

The team, led by Dr Roman Gorbachev, used their unique fabrication method on two particular two-dimensional crystals that have generated intense scientific interest in the past 12 months but are unstable in air: black phosphorus and niobium diselenide. The technique the team have pioneered allows the unique characteristics and excellent electronic properties of these air-sensitive 2D crystals to be revealed for the first time.

The isolation of graphene in 2004 by a University of Manchester team lead by Sir Andre Geim and Sir Kostya Novoselov led to the discovery of a range of 2D materials, each with specific properties and qualities. Dr Gorbachev said: “This is an important breakthrough in the area of 2D materials research, as it allows us to dramatically increase the variety of materials that we can experiment with using our expanding 2D crystal toolbox”. The more materials we have to play with, the greater potential there is for creating applications that could revolutionise the way we live.” Sir Andre Geim added.

Writing in NanoLetters, the University of Manchester team demonstrate how tailored fabrication methods can make these previously inaccessible materials useful.


Black Phosphorus Instead Of Silicon

Silicon Valley in Northern California got its nickname from the multitude of computer chip manufacturers that sprung up in the surrounding area in the 1980’s.  Despite its ubiquity as a chip building material, silicon may be facing some competition from a new version of an old substance.

Researchers working at the Institute for Basic Science (IBS) Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea, led in part by Director Young Hee Lee, have created a high performance transistor using black phosphorus (BP) which has revealed some fascinating results.

Transistors are made up of materials with semiconducting properties, which come in two varieties: n-type (excess electrons) and p-type (excess holes). With the BP crystal, researchers have discovered that they can change its thickness and/or the contact metals and that will determine if it is high performance n-type, p-type, or ambipolar (function as both n- or p-type) material.

Atomic structure of black phosphorus monolayer

Sibplicon has to be extrinsically doped (inserting another element into its crystal structure) to make it n-type or p-type in order for it to work in a semiconductor chip.   The BP crystals can operate as both n-type and p-type or something in between, but don’t require extrinsic doping.  This means that instead of having to fabricate a silicon-arsenic crystal sandwiched between silicon-boron crystals, a transistor can have a single, lightweight, pure black phosphorus logic chip — no doping required.

Additionally, changing the metals used to connect the chip to the circuit has an influence on whether BP will be n- or p-type.  Instead of doping to make an n- and p-type material, both n- and p-type BP can be put all together on one chip just by changing its thickness and the contact metal used.