The chemical effects of ultrasound derive primarily from acoustic cavitation. Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into heating of the contents of the bubble. The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. A diverse set of applications of ultrasound to enhance chemical reactivity has been explored with important uses in synthetic materials chemistry. For example, the sonochemical decomposition of volatile organometallic precursors in low-volatility solvents produces nanostructured materials in various forms with high catalytic activities. Nanostructured metals, alloys, oxides, carbides and sulfides, nanometer colloids, and nanostructured supported catalysts can all be prepared by this general route. Another important application of sonochemistry in materials chemistry has been the preparation of biomaterials, most notably protein microspheres. Such microspheres have a wide range of biomedical applications, including their use in echo contrast agents for sonography, magnetic resonance imaging, contrast enhancement, and oxygen or drug delivery. Other applications include the modification of polymers and polymer surfaces.

Applications of ultrasound to materials chemistry

Recent advances in nanostructured materials have been led by the development of new synthetic methods that provide control over size, morphology, and nano/microstructure. The utilization of high intensity ultrasound offers a facile, versatile synthetic tool for nanostructured materials that are often unavailable by conventional methods. The primary physical phenomena associated with ultrasound that are relevant to materials synthesis are cavitation and nebulization. Acoustic cavitation (the formation, growth, and implosive collapse of bubbles in a liquid) creates extreme conditions inside the collapsing bubble and serves as the origin of most sonochemical phenomena in liquids or liquid-solid slurries. Nebulization (the creation of mist from ultrasound passing through a liquid and impinging on a liquid-gas interface) is the basis for ultrasonic spray pyrolysis (USP) with subsequent reactions occurring in the heated droplets of the mist. In both cases, we have examples of phase-separated attoliter microreactors: for sonochemistry, it is a hot gas inside bubbles isolated from one another in a liquid, while for USP it is hot droplets isolated from one another in a gas. Cavitation-induced sonochemistry provides a unique interaction between energy and matter, with hot spots inside the bubbles of approximately 5000 K, pressures of approximately 1000 bar, heating and cooling rates of >10(10) K s(-1); these extraordinary conditions permit access to a range of chemical reaction space normally not accessible, which allows for the synthesis of a wide variety of unusual nanostructured materials. Complementary to cavitational chemistry, the microdroplet reactors created by USP facilitate the formation of a wide range of nanocomposites. In this review, we summarize the fundamental principles of both synthetic methods and recent development in the applications of ultrasound in nanostructured materials synthesis.

Applications of Ultrasound to the Synthesis of Nanostructured Materials

We discuss recent developments in nanostructured molybdenum sulfide catalysts for the electrochemical hydrogen evolution reaction. To develop a framework for performing consistent and meaningful comparisons between catalysts, we review standard experimental methodologies for measuring catalyst performance and define two metrics used in this perspective for comparing catalyst activity: the turnover frequency, an intrinsic activity metric, and the total electrode activity, a device-oriented activity metric. We discuss general strategies for synthesizing catalysts with improved activity, namely, increasing the number of electrically accessible active sites or increasing the turnover frequency of each site. Then we consider a number of state-of-the-art molybdenum sulfide catalysts, including crystalline MoS2, amorphous MoSx, and molecular cluster materials, to highlight these strategies in practice. Comparing these catalysts reveals that most of the molybdenum sulfide catalysts have similar active site turnov...

Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials

Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production

The hydrogen evolution reaction plays a decisive role in a range of electrochemical and photoelectrochemical devices. It requires efficient and robust electrocatalysts to lower the reaction overpotential and minimize energy consumption. Over the last decade, we have witnessed a rapid rise in new electrocatalysts, particularly those based on non-precious metals. Some of them approach the activity of precious metal benchmarks. Here, we present a comprehensive overview of the recent developments of heterogeneous electrocatalysts for the hydrogen evolution reaction. Detailed discussion is organized from precious metals to non-precious metal compounds including alloys, chalcogenides, carbides, nitrides, borides and phosphides, and finally to metal-free materials. Emphasis is placed on the challenges facing these electrocatalysts and solutions for further improving their performance. We conclude with a perspective on the development of future HER electrocatalysts.

T.A. Pecoraro

Papers

Hydrodesulfurization catalysis by transition metal sulfides

Structure and properties of molybdenum sulfide: Correlation of O2 chemisorption with hydrodesulfurization activity☆

Transition metal sulfide catalysis: Relation of the synergic systems to the periodic trends in hydrodesulfurization

Electron spin resonance of supported and unsupported molybdenum hydrotreating catalysts: I. Model system studies

Electron spin resonance of supported and unsupported molybdenum hydrotreating catalysts