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Low-dimensional copper oxide nanostructures are very promising building blocks for various functional materials targeting high-demanded applications, including energy harvesting and transformation systems, sensing and catalysis. Featuring a very high surface-to-volume ratio and high chemical reactivity, these materials have attracted wide interest from researchers. Currently, extensive research on the fabrication and applications of copper oxide nanostructures ensures the fast progression of this technology. In this article we briefly outline some of the most recent, mostly within the past two years, innovations in well-established fabrication technologies, including oxygen plasma-based methods, self-assembly and electric-field assisted growth, electrospinning and thermal oxidation approaches. Recent progress in several key types of leading-edge applications of CuO nanostructures, mostly for energy, sensing and catalysis, is also reviewed. Besides, we briefly outline and stress novel insights into the effect of various process parameters on the growth of low-dimensional copper oxide nanostructures, such as the heating rate, oxygen flow, and roughness of the substrates. These insights play a key role in establishing links between the structure, properties and performance of the nanomaterials, as well as finding the cost-and-benefit balance for techniques that are capable of fabricating low-dimensional CuO with the desired properties and facilitating their integration into more intricate material architectures and devices without the loss of original properties and function.

As famed quantum physicist W. Pauli once said, “The surface was invented by the devil”. The nonequilibrium state of particles forming the surface, and the presence of dangling bonds transform the surfaces into a 2D reactor with high physical and chemical reactivity. When two such active surfaces are matched, their interface becomes even more reactive, giving rise to novel properties or enhanced performance. For this reason, much effort is applied to design nanoengineered interfacial systems for applications spanning all facets of human life. This review article discusses recent, mostly within two years, progress in the design of complex, sophisticated carbon-based interfacial material systems for energy and photonics applications, with the aim to emphasize some of the most interesting and important examples of such systems. Differences in the processes that take place on flat and 3D (curved) surfaces are discussed, with the view of guiding the design and construction of complex functional interfaces, focusing on several points that are of particular importance to the ongoing development of advanced interfacial material systems.

Sub-stoichiometric tungsten oxide (i.e., WO3-x) materials are advanced multipurpose semiconducting materials. However, it is still a challenge to tune their structure and morphology. Here we report the structural and morphological conversion and the photoluminescence (PL) enhancement of the WO3-x materials synthesized in the mixed acetic and oleic acids using tungsten hexachloride precursor. We demonstrate the structural conversion from nanorods to nanodots, accompanied by the morphological change from the mushroom-like particles to the spherical particles. Importantly, the PL emission from the particles aggregated by the WO3-x nanodots is much stronger than the particles aggregated by the WO3-x nanorods. The structural and morphological conversion mechanism is related to the growth rate reduction of WO3-x nuclei and the steric repulsion effect, which are caused by oleic acid. The PL enhancement arises from the reduction of non-radiative recombination caused by the structural conversion and the surface passivation of WO3-x nanodots caused by oleic acid. These results contribute to the commonly needed ability to control the structure of metal oxide nanomaterials and to the development of next-generation functionalities and devices for energy, electronics, optics, and other fields.

Different structured WO3-x nanomaterials were synthesized in ethanoic acid using the WCl6 and Ti(OC4H9)4 precursors. Advanced characterizations reveal that the incorporation of TiO2 leads to the granulation and structural conversion of WO3-x due to the reduction of surface tension on the WO3-x nanoparticles (NPs) and the fast nucleation of the WO3-x NPs caused by the adsorption of Ti(OC4H9)4 molecules on the WO3-x NPs. The photoluminescence (PL) studies indicate that the WO3-x − TiO2 hybrid nanomaterials generate ultraviolet, blue, green, red and near-infrared emissions, and that the PL emission is remarkably enhanced. The emissions can be attributed to the band − band, defect state − state and defect level − band transitions. The substantial PL enhancement originates from trapping the strong PL emission from TiO2 by the WO3-x nanomaterials.

In this paper, we report the one-step synthesis of the TiO2/WO3−x hybrid nanomaterials in ethanoic acid using the tungsten hexachloride (WCl6) and titanium butoxide precursors. The characterization results indicate that the WO3−x incorporation in TiO2 nanomaterials leads to the morphology conversion of TiO2 nanomaterials from the nanoparticles to the nanowire beams. The conversion mechanism is based on enhancing the nucleation rate of TiO2 and the viscosity of the solution containing the mixed WCl6 powder and titanium butoxide precursors. The photoluminescence (PL) studies reveal that the as-synthesized TiO2/WO3−x hybrid nanomaterials generate the UV, blue, green and red light, and these emissions are related to the band-band transition, surface states, oxygen vacancies and the transition between the gap state and valence band, respectively. Furthermore, WO3−x also leads to the PL quenching due to the oxygen vacancy migration and charge transfer across the TiO2/WO3−x interfaces and the enhancement of nonradiative recombination centers. Finally, the application of the as-synthesized hybrid nanomaterials in the area of photocatalysis was explored.

The substoichiometric tungsten oxide (WO3-x) nanostructures are important for the catalytic, energy and optoelectronic applications, yet it remains challenging to control their morphologies and oxygen vacancies. Here we address the challenge of synthesizing the WO3-x nanoflakes and nanorods in ethanol and ethanoic acid by solvothermal method, where tungsten hexachloride (WCl6) is used as the precursor of tungsten. The results characterized by different analytical methods indicate that the WO3-x nanoflakes and nanorods are crystalline structures composed of different phases, respectively. The formation of different WO3-x nanostructures is related to the difference in the formation mechanisms of WO3-x monomers in ethanol and ethanoic acid. Due to the difference in the mobility of WO3-x monomers caused by the properties of ethanol and ethanoic acid, the WO3-x nanoflakes are formed in ethanol depending on the interface between gas and liquid ethanol. In contrast, the WO3-x nanorods are formed in ethanoic acid due to the anisotropic intermediate product formed by the reaction of WCl6 and ethanoic acid. Furthermore, the photoluminescence (PL) of WO3-x nanoflakes and nanorods is studied at room temperature and low temperature. Both the WO3-x nanostructures generate the ultraviolet, green, red and near-infrared light at room temperature, which are related to the transition from the defect resonant state level in the conduction band to the valence band, the transition between defect state levels and relaxation of polarons, respectively. The PL emission at low temperature was enhanced due to the weak lattice vibration and the increase of oxygen vacancy concentration. Our results not only enrich the synthesis and optical properties of WO3-based nanomaterials but also contribute to the development of WO3-based optoelectroni

Properties of substoichiometric tungsten oxide (WO3-x) semiconducting nanomaterials in diverse applications are morphology-dependent. However, it remains challenging to control morphology and structure of WO3-x nanomaterials. Here, we report the solvothermal synthesis of WO3-x nanostructures with different morphologies in tungsten hexachloride-ethanol solution through adjusting the filling fraction of ethanol in autoclave. The structural and morphological conversion mechanisms are related to the changes in the viscosity and Brownian motion caused by the concentration of tungsten hexachloride in ethanol. The studies on the photoluminescence (PL) properties reveal that the WO3-x nanostructures emit ultraviolet, blue, green and red emissions. The PL emissions are attributed to the recombination between the electron occupying the resonant defect state in the conduction band and the hole in the valence band and the transitions between the conduction and valence bands as well as the transition between the oxygen vacancy states. The enhancement of PL intensity is related to the increase of oxygen vacancies and the phonon suppression during the PL emission as well as the nanocavity structure. Furthermore, the potential applications in the catalysis fields are discussed. These results provide a new technique to control the structure of WO3-x nanomaterials and contribute to the development of next-generation PL emission nanodevices based on vacancy- and structure-engineered WO3 materials and the rational design of catalytic materials.

Substoichiometric tungsten oxide (WOx, 2 < x < 3) nanomaterials are important nanoscale semiconductor materials with broad applications in optoelectronic devices; however a controllable modification of their physical properties remains challenging task. Here, we report an alternative green synthesis of hybrid nanostructures composed by WOx and ZnWO4 phases in ethanol via a solvothermal process using ZnO and WCl6 as the precursors. The results indicate that the formation of ZnWO4 leads to the structural conversion of WOx from nanorods to nanoparticles due to the effect of ZnWO4 on the side growth of WOx nanostructures. Moreover, the surface morphology tailored by ZnWO4 and crystalline phase transformation induce different photoluminescence (PL) emission from pure WOx nanostructures and WOx/ZnWO4 hybrid nanostructures due to the increased number of oxygen vacancies.

The challenge in molybdenum oxides intended for integration into electronic, optical, energy conversion, and other devices is tuning of their photoluminescence (PL) properties. Despite some initial studies on possibilities of tuning PL properties, this issue remains unresolved. For this reason, we explored the tailoring of PL properties of molybdenum oxide nanomaterials with a dopant through a robust synthesis by hot filament chemical vapour deposition. Here the MoO3-x nanomaterials are doped with tungsten in a low concentration, and the room temperature PL properties are studied. The obtained results highlight the interaction of Mo5+ defects and oxygen vacancies, which are playing an essential role in the enhancement of green PL emission. Moreover, the used doping method is fast and straightforward and can be used to control the structure as well as the PL properties of molybdenum oxide nanomaterials.

Exploring the fast synthesis of Zn-doped MoO3-x nanostructures in a N2 environment by the hot filament chemical vapour deposition using a mixture of Zn and MoO3 powders is reported, whereas targeted transformation of nanomaterials is done via Zn addition in MoO3 powder. During this process, MoO3-x nanostructures convert from nanoparticles to nanofilms. The structural conversion of MoO3-x nanostructures is the results of the predominant diffusion of MoO3-x and MoO3 molecules along the substrate surface caused by reduced evaporation of MoO3 due to the melting of Zn particles. At room temperature, the pure and Zn-doped MoO3-x nanostructures generate the photoluminescence (PL) emission in the ultraviolet to infrared range, while the PL emission in the visible range has a great change at low temperature. The PL emission is related to the bandgap transition, Mo5+ d-d transition, intervalence charge transfer transition, and transition between the intermediate and valence bands. Furthermore, the PL emission of MoO3-x nanostructures is enhanced by Zn doping. The strong PL emission from Zn-doped MoO3-x nanostructures compared to pure MoO3-x nanostructures results from the increase of oxygen vacancies caused by the Zn incorporation. The enhancement of PL emission at low temperature compared to room temperature is due to the increase of vacancy concentration, weak lattice vibration and relaxation of polarons. Our present results could be used to control the structures of MoO3-x nanomaterials and impact the development of optoelectronic devices based on MoO3 nanomaterials, such as organic solar cells, photovolatic devices and light-emitting diodes.

Vertically oriented graphenes have been grown for more than a decade, but until now the chemical and physical mechanisms underlying their growth have not been fully defined and understood. For this reason, we build a multi-scale, multi-factor model which is thoroughly verified using a large body of experimental data to provide a significant insight into the chemical and physical processes that determine nucleation, growth and structure formation of vertically aligned graphenes in plasma environments. Roles of chemical and physical processes that cannot be directly characterized using presently available experimental techniques, e.g. surface diffusion of adatoms and radicals, are also studied using this model. The leading role of surface diffusion fluxes, rather than direct influx from the gas phase, is confirmed, with ion bombardment being a key factor in 'switching' the growth modes by generating surface defects and hence, increasing the surface adsorption energy. Thus, the hydrocarbon radicals generated on a substrate as a result of bombardment are shown to diffuse to the nanoflakes and catalyze the reactions, and serve as the primary source of material to build the nanoflakes.

In this paper, we examine several different types of miniaturized plasma sources that have been developed for nonpropulsion applications, but could be useful for the advancement of electric propulsion. With the same or similar physical principles and often similar design solutions, such sources suggest useful pathways for modernization and integration of presently available well established as well as emerging miniaturized plasma sources into space thruster systems. Features related to miniaturization and optimization of the technological plasma sources will provide useful insights for consideration by the electric propulsion specialists. It is not the aim of this paper to show an entire spectrum of technological microplasma systems, but rather to outline possible future trends and perspectives for the miniaturized technological plasmas in relation to space micropropulsion systems.