This Feature Article gives an overview of microwave-assisted liquid phase routes to inorganic nanomaterials. Whereas microwave chemistry is a well-established technique in organic synthesis, its use in inorganic nanomaterials' synthesis is still at the beginning and far away from having reached its full potential. However, the rapidly growing number of publications in this field suggests that microwave chemistry will play an outstanding role in the broad field of Nanoscience and Nanotechnology. This article is not meant to give an exhaustive overview of all nanomaterials synthesized by the microwave technique, but to discuss the new opportunities that arise as a result of the unique features of microwave chemistry. Principles, advantages and limitations of microwave chemistry are introduced, its application in the synthesis of different classes of functional nanomaterials is discussed, and finally expected benefits for nanomaterials' synthesis are elaborated.

Microwave chemistry for inorganic nanomaterials synthesis

Colloidal inorganic nanocrystals stand out as an important class of advanced nanomaterials owing to the flexibility with which their physical-chemical properties can be controlled through size, shape, and compositional engineering in the synthesis stage and the versatility with which they can be implemented into technological applications in fields as diverse as optoelectronics, energy conversion/production, catalysis, and biomedicine. The use of microwave irradiation as a non-classical energy source has become increasingly popular in the preparation of nanocrystals (which generally involves complex and time-consuming processing of molecular precursors in the presence of solvents, ligands and/or surfactants at elevated temperatures). Similar to its now widespread use in organic chemistry, the efficiency of "microwave flash heating" in dramatically reducing overall processing times is one of the main advantages associated with this technique. This Review illustrates microwave-assisted methods that have been developed to synthesize colloidal inorganic nanocrystals and critically evaluates the specific roles that microwave irradiation may play in the formation of these nanomaterials.

Microwave-assisted synthesis of colloidal inorganic nanocrystals.

Microwave-assisted preparation of inorganic nanostructures in liquid phase.

A review is given on arc cathode spots, mainly based on the investigation of arcs in a vacuum with cold cathodes. For the latter and after a short description of general features and theoretical concepts, experiments are presented that study the temporal and spatial behaviour of the spots with high time and space resolution of less than 10 ns and less than 5 µm, respectively. With the help of these observations the various spot types described in the literature are ordered into three levels: level A corresponding to the proper spot with typical diameters of 50-100 µm, level B associated with spot fragments having a size of 10-20 µm and level C comprising a substructure of the fragments. The structures undergo periodic fluctuations of brightness and position with characteristic times that can be arranged in a hierarchy from a few nanoseconds through about 100 µs. The analysis of these fluctuations shows that the spot operates in cycles that include both extremely non-stationary periods with time constants of less than 10 ns and more stationary periods in the microsecond range. In the presence of an external magnetic field, the latter periods lead to unstable plasma configurations that give rise to retrograde motion. Finally, for vacuum arc spots the basic parameters are summarized. After that, the peculiarities of spots in gases with cold electrodes are discussed, followed by a presentation of spots with hot cathodes at high pressures.

https://iopscience.iop.org/article/10.1088/0022-3727/34/17/202/pdf

Topical Review: Cathode Spots of Electric Arcs

After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to volume ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS2, in the first years of 2010s. Along with MoS2, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS2, WSe2, MoSe2, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the number of layers. On the other hand, 2D materials constituted by one atomic species have been synthetized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS2 and WS2 and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.

2D Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS2, WS2 and Phosphorene

This paper reports the local detection of buried calibrated metal defects in metal samples by a new experimental technique, scanning microwave microscopy. This technique combines the electromagnetic measurement capabilities of a microwave vector network analyzer with the subnanometer-resolution capabilities of an atomic force microscope. The network analyzer authorizes the use of several frequencies in the range 1--6 GHz, allowing three-dimensional tomographical investigation, which is useful for the detection of bulk defects in metal materials.

Detection of defects buried in metallic samples by scanning microwave microscopy

Rapid synthesis of tin (IV) oxide nanoparticles by microwave induced thermohydrolysis

This work presents new developments in the microwave transduction and its application to gas sensors. Microwave measurements were extended to reflection/transmission coefficients through the use of a microstrip interdigital capacitor design. A sensitive layer composed of commercial TiO2 nanoparticles was deposited on the sensor surface, in order to detect the ammonia target gas. A complete analysis methodology is proposed. It allows to identify without ambiguity all the sensor frequencies of interest, as well as a full characterization of usual gas sensor parameters such as reproducibility, stability, response time, etc. Furthermore, it involves a new method of representation which significantly limits the amount of unexploited data collected during microwave measurements. The proposed sensor exhibits a strong correlation between response values and injected ammonia concentration between 100 and 500 ppm, with a good reversibility and stability of the measure.

Microwave gas sensing with a microstrip interDigital capacitor: Detection of NH3 with TiO2 nanoparticles

The conductivity of CoPc (cobalt phthalocyanine) and Co[(SO3Na)2,3Pc] was measured under a flow of two different gases (NH3 and O3), during exposure/recovery cycles. It appears that the relative responses are linearly related to the concentration, in the 20–200 ppb range for O3 and in the 20–200 ppm range for NH3. Observed during time, the sensing parameters allow a qualitative understanding of the kinetics. The comparative study of those products under both different gases gave interesting results for sensor applications. Whereas CoPc is sensitive to both gases, its sulfonated counterpart is only sensitive to NH3.

Differential study of substituted and unsubstituted cobalt phthalocyanines for gas sensor applications

This work presents a novel approach in gas detection by an original method of microwave transduction. The design of the sensor includes a coplanar grounded wave guide with a gas sensing material to study its sensitivity to ammonia in argon flux. The sensing material can play the role of the substrate or can be deposited as a thin layer on a microstrip structure used in electronics. Submitted to an electromagnetic excitation in microwave energies, the sensor response in the presence of a gas results in a specific modification of the reflected wave (real and imaginary parts). The goals of this study include an examination of the form of the sensitive material and its influence on the response of the microwave gas sensor. Two cases are considered: bulk or thin layer. In bulk case, the material plays the role of the substrate of the microstrip structure. In the second case, a thin layer is deposited on the sensor. We showed how, in the presence of ammonia, the reflected wave is related to its concentration. The response to the material–gas interaction depends on the excitation frequency. The parameter used as the sensor response is the ratio of the reflected wave on the incident wave at each frequency. The study deals with the influence of molecular sensing materials (CoPc) on the response of the sensor in the presence of ammonia. All the measurement were carried out at room temperature.

Jérôme Rossignol

Papers

Detection of defects buried in metallic samples by scanning microwave microscopy

Rapid synthesis of tin (IV) oxide nanoparticles by microwave induced thermohydrolysis

Microwave gas sensing with a microstrip interDigital capacitor: Detection of NH3 with TiO2 nanoparticles

Differential study of substituted and unsubstituted cobalt phthalocyanines for gas sensor applications

Development of microwave gas sensors