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

One-dimensional (1D) ZnO nanostructures have been studied intensively and extensively over the last decade not only for their remarkable chemical and physical properties, but also for their current and future diverse technological applications. This article gives a comprehensive overview of the progress that has been made within the context of 1D ZnO nanostructures synthesized via wet chemical methods. We will cover the synthetic methodologies and corresponding growth mechanisms, different structures, doping and alloying, position-controlled growth on substrates, and finally, their functional properties as catalysts, hydrophobic surfaces, sensors, and in nanoelectronic, optical, optoelectronic, and energy harvesting devices.

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One-dimensional ZnO nanostructures: Solution growth and functional properties

ZnO nanostructures for optoelectronics: Material properties and device applications

Zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices

High intensity ultrasound can be used for the production of novel materials and provides an unusual route to known materials without bulk high temperatures, high pressures, or long reaction times. Several phenomena are responsible for sonochemistry and specifically the production or modification of nanomaterials during ultrasonic irradiation. The most notable effects are consequences of acoustic cavitation (the formation, growth, and implosive collapse of bubbles), and can be categorized as primary sonochemistry (gas-phase chemistry occurring inside collapsing bubbles), secondary sonochemistry (solution-phase chemistry occurring outside the bubbles), and physical modifications (caused by high-speed jets or shock waves derived from bubble collapse). This tutorial review provides examples of how the chemical and physical effects of high intensity ultrasound can be exploited for the preparation or modification of a wide range of nanostructured materials.

Sonochemical synthesis of nanomaterials

Morphology-controlled growth of ZnO nano- and microstructures was achieved by microwave irradiation. Various basic ZnO structures, including nanorods, nanocandles, nanoneedles, nanodisks, nanonuts, microstars, microUFOs, and microballs were simply synthesized at a low temperature (90 °C) with low power microwave-assisted heating (about 50 W) and a subsequent aging process. These results could be obtained by changing the precursor chemicals, the capping agents, and the aging times. Even more complex ZnO structures, including ZnO bulky stars, cakes, and jellyfishes, were constructed by microwave irradiation to a mixture of the as-prepared basic ZnO structures and the solution I, IV, or V. This is a fast, simple, and reproducible method which does not require any template, catalyst, or surfactant but can control the morphology of ZnO crystals from simple to complex. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (X...

Morphology-Controlled Growth of ZnO Nanostructures Using Microwave Irradiation: from Basic to Complex Structures

A simple and facile sonochemical route has been demonstrated for the shape-selective preparation of highly crystalline ZnO nanostructures, such as nanorods, nanocups, nanodisks, nanoflowers, and nanospheres. The concentration of precursor chemicals, the kind of hydroxide anion-generating agents, the ultrasonication time, and the use of a capping agent are key factors in the morphological control of ZnO nanostructures. This method is fast, simple, convenient, economical, and environmentally benign. On the basis of our shape-control of the ZnO nanostructures by the sonochemical technique, growth mechanisms of ZnO nanostructures were also proposed. We believe this technique will be readily adopted in realizing other forms of various nanostructured materials.

Sonochemical Preparation of Shape-Selective ZnO Nanostructures

A facile and fast sonochemical route for the fabrication of a resistive-type ZnO gas sensor has been demonstrated. Vertically aligned ZnO nanorod arrays were grown on a Pt-electrode patterned alumina substrate under ambient conditions. The average diameter and length of the ZnO nanorods were 50 and 500 nm, respectively. Sonochemically grown ZnO nanorod gas sensor was highly sensitive to NO2 gas with a very low detection limit of 10 ppb at 250 °C; further, its response and recovery time were short. Considering the advantageous properties of this sonochemical technique, we believe that it can be used to fabricate high-performance gas sensors.

High-performance NO2 gas sensor based on ZnO nanorod grown by ultrasonic irradiation

We have studied the precursor effects of citric acid and various citrates-including triethyl citrate, tripotassium citrate, trisodium citrate and triammonium citrate-on the formation of ZnO crystals in alkaline solution These citrate-related chemicals could be divided into three groups (group A, triethyl citrate; group B, tripotassium citrate and trisodium citrate; and group C, citric acid and triammonium citrate) based on their activity for modifying the ZnO growth direction and solution pH dependency on their concentration We could obtain ZnO structures with various distinct morphologies by simply changing the concentration of citric acid or citrate additive dissolved in the alkaline reaction solution On the basis of the results, we propose the growth mechanisms underlying the formation of the various ZnO structures in the absence and presence of citric acid or citrate additives

Seung-Ho Jung

Papers

Morphology-Controlled Growth of ZnO Nanostructures Using Microwave Irradiation: from Basic to Complex Structures

Sonochemical Preparation of Shape-Selective ZnO Nanostructures

High-performance NO2 gas sensor based on ZnO nanorod grown by ultrasonic irradiation

Precursor effects of citric acid and citrates on ZnO crystal formation.

A Sonochemical Method for Fabricating Aligned ZnO Nanorods