Sustainable hydrogen production is an essential prerequisite of a future hydrogen economy. Water electrolysis driven by renewable resource-derived electricity and direct solar-to-hydrogen conversion based on photochemical and photoelectrochemical water splitting are promising pathways for sustainable hydrogen production. All these techniques require, among many things, highly active noble metal-free hydrogen evolution catalysts to make the water splitting process more energy-efficient and economical. In this review, we highlight the recent research efforts toward the synthesis of noble metal-free electrocatalysts, especially at the nanoscale, and their catalytic properties for the hydrogen evolution reaction (HER). We review several important kinds of heterogeneous non-precious metal electrocatalysts, including metal sulfides, metal selenides, metal carbides, metal nitrides, metal phosphides, and heteroatom-doped nanocarbons. In the discussion, emphasis is given to the synthetic methods of these HER electrocatalysts, the strategies of performance improvement, and the structure/composition-catalytic activity relationship. We also summarize some important examples showing that non-Pt HER electrocatalysts could serve as efficient cocatalysts for promoting direct solar-to-hydrogen conversion in both photochemical and photoelectrochemical water splitting systems, when combined with suitable semiconductor photocatalysts.

Noble metal-free hydrogen evolution catalysts for water splitting

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

/pdf/alternative-plasmonic-materials-beyond-gold-and-silver-1p8fd7ol50.pdf

Alternative Plasmonic Materials: Beyond Gold and Silver

The number of research reports published in recent years on electrochemical water splitting for hydrogen generation is higher than for many other fields of energy research. In fact, electrochemical water splitting, which is conventionally known as water electrolysis, has the potential to meet primary energy requirements in the near future when coal and hydrocarbons are completely consumed. Due to the sudden and exponentially increasing attention on this field, many researchers across the world, including our group, have been exerting immense efforts to improve the electrocatalytic properties of the materials that catalyze the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode, aided by the recent revolutionary discovery of nanomaterials. However, the pressure on the researchers to publish their findings rapidly has caused them to make many unnoticed and unintentional errors, which is mainly due to lack of clear insight on the activity parameters. In this perspective, we have discussed the use and validity of ten important parameters, namely overpotential at a defined current density, iR-corrected overpotential at a defined current density, Tafel slope, exchange current density (j0), mass activity, specific activity, faradaic efficiency (FE), turnover frequency (TOF), electrochemically active surface area (ECSA) and measurement of double layer capacitance (Cdl) for different electrocatalytic materials that are frequently employed in both OER and HER. Experimental results have also been provided in support of our discussions wherever required. Using our critical assessments of the activity parameters of water splitting electrocatalysis, researchers can ensure precision and correctness when presenting their data regarding the activity of an electrocatalyst.

Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment

Hybrid nanostructures composed of semiconductor and plasmonic metal components are receiving extensive attention. They display extraordinary optical characteristics that are derived from the simultaneous existence and close conjunction of localized surface plasmon resonance and semiconduction, as well as the synergistic interactions between the two components. They have been widely studied for photocatalysis, plasmon-enhanced spectroscopy, biotechnology, and solar cells. In this review, the developments in the field of (plasmonic metal)/semiconductor hybrid nanostructures are comprehensively described. The preparation of the hybrid nanostructures is first presented according to the semiconductor type, as well as the nanostructure morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then elucidated. Lastly, possible future research in this burgeoning field is discussed.

Metal/Semiconductor Hybrid Nanostructures for Plasmon-Enhanced Applications

We examine the optical properties of nanostructures comprised of titanium nitride, TiN, an electrically conducting intermetallic-like compound. This material can be deposited in the form of durable films by physical vapor deposition. Use of nanosphere templating techniques extends the range of nanostructures that can be produced to include the versatile semi-shell motif. The dielectric properties of TiN1 − x depend upon stoichiometry and are favorable for plasmon resonance phenomena in the mid-visible to near-infrared range of the spectrum and for x≈0. We analyze the optical phenomena operating in such structures using a combination of experiment and simulation and show that semi-shells of TiN exhibit a tunable localized plasmon resonance with light. The material is, however, unsuitable for applications in which a long-distance surface plasmon polariton is desired.

/pdf/optical-properties-and-plasmon-resonances-of-titanium-1zo6j6d6yk.pdf

Optical properties and plasmon resonances of titanium nitride nanostructures.

The synthesis of nanocrystalline ZnO by thermal decomposition of zinc hydroxyacetate, Zn5(OH)8(CH3CO2)2·nH2O, was investigated The decomposition process was examined using X-ray diffraction, thermogravimetric analysis, mass spectrometry, electron microscopy, Brunauer–Emmett–Teller surface area analysis, and solid-state NMR spectroscopy Intermediate Zn5(OH)8(CH3CO2)2·nH2O phases form at temperatures up to 110 °C from the starting compound Zn5(OH)8(CH3CO2)2·2H2O by partial dehydration At ∼110 °C, 4 equiv of ZnO and 1 equiv of Zn(CH3CO2)2 are formed Further heating causes Zn(CH3CO2)2 to decompose to acetone, acetic acid, acetic anhydride, and ZnO Notably, a portion of Zn(CH3CO2)2 sublimes during the process Overall, the product of the calcination is equiaxed ZnO nanocrystals of 20–100 nm diameter

Zinc hydroxyacetate and its transformation to nanocrystalline zinc oxide.

An acoustic impedance spectrometer was used to measure the frequencies R1 and R2 of the first two resonances of the vocal tract. The measurement was made just outside the mouth, in parallel with the free field, using a new technique that provides precise information about the acoustic response of the vocal tract in real time. Values measured for native speakers for a particular vowel were used as target parameters for subjects who used a visual display of an impedance spectrum of their own vocal tracts as real-time feedback to realise the vocal tract configuration required to pronounce the target vowel. We report the values (R1,R2) for eleven non-nasalised vowels of French. These values are similar to the formant frequencies measured previously for these vowels, and their relative positions in the (R2,R1) plane are similar to those of the same vowels in the (F2,F1) formant plane. The confusion and correct identification of these vowels are shown to be strongly related to their separation in the (R2,R1) plane. We report the results of attempts to imitate six of these vowels by monolingual anglophone subjects. One group used a traditional method of learning pronunciation: they heard the vowel sounds and then attempted to imitate them. Another group also heard the sounds, but were assisted by the vocal tract feedback described above when imitating the target sounds. The acoustic properties and recognizability of the vowels were significantly superior when the subjects used vocal-tract feedback.

/pdf/learning-to-pronounce-vowel-sounds-in-a-foreign-language-4genl017aj.pdf

Learning to pronounce vowel sounds in a foreign language using acoustic measurements of the vocal tract as feedback in real time.

A compact, high-power emitter of half-cycle terahertz (THz) radiation is demonstrated. The device consists of an epitaxial InAs emitter upon a GaAs prism and produces THz pulses that are 20 times more powerful than those from conventional planar InAs emitters. This improvement is a direct result of reorienting the transient THz dipole such that its axis is not perpendicular to the emitting surface.

/pdf/generation-of-high-power-terahertz-pulses-in-a-prism-4155pcswwj.pdf

Annette Dowd

Papers

Optical properties and plasmon resonances of titanium nitride nanostructures.

Zinc hydroxyacetate and its transformation to nanocrystalline zinc oxide.

Learning to pronounce vowel sounds in a foreign language using acoustic measurements of the vocal tract as feedback in real time.

Generation of high-power terahertz pulses in a prism.

Magnetic metal phosphide nanorods as effective hydrogen-evolution electrocatalysts