We enter 2018 with a new set of article categories for Surface Innovations, which includes Invited Feature Articles, Expert Opinions, Innovation Letters, Original Research reports and Comments. Invited Feature Articles are mainly state-of-the-art reviews with an up-to-date summary of knowledge on a particular subject or issue in the field of surfaces/interfaces and representing an overview of recent developments. These articles are written by experts in a field who have made important contributions to a specific topic in recent years, who are invited by the Editor-in-Chief and/or Associate Editors. Researchers who are not invited but are willing to prepare such a contribution are encouraged to email a one-page proposal for an Invited Feature Article to the Editor-in-Chief for consideration.
We also encourage experts in the field to prepare Expert Opinions for our journal. Three types of contributions are typically considered under this category, including i) practical guidelines on experimental methods and protocols related to surface characterizations; ii) critical analysis of terminology used in surface/interfacial science; and iii) critical analysis of research directions and interpretations in surface/interfacial science. These articles are written by experts in a field who have made important contributions to a specific topic in recent years, and who are invited by the Editor-in-Chief and/or Associate Editors. Researchers who are not invited but are willing to prepare such a contribution are encouraged to email a brief proposal for an Expert Opinion to the Editor-in-Chief for consideration.
Innovation Letters have replaced Communications and are brief reports of significant, original and timely research results on the science and engineering of surfaces or interfaces, detailing preliminary results and warranting rapid publication. They can also include technical notes, relating to the latest technological, experimental or methodological developments within the author’s field and should refer to the intended application. We would like to encourage the submission of letters that present intriguing observations and innovations, even if they are not complete pieces of scientific/engineering art and difficult to explain or implement. Such contributions should be mailed directly to the Editor-in-Chief who, together with the Editorial Board, will make a determination on fast publication of the letter.
Original Research includes technical reports that detail techniques and approaches and the work’s contribution to the field. Finally, any brief responses, including presentation of alternative points of view, discussion of intriguing results and observations, and analysis of errors in experimentation or interpretation in the published article(s) will be published under Comments as well.
In these combined two first issues of 2018, we offer several interesting and often inspirational articles. The issue opens with an Invited Feature Article on Raman studies of two-dimensional materials, silicene and germanene, prepared by a researcher from the Institute for Superconducting and Electronic Materials at the University of Wollongong in Australia, led by Dr. Yi Du, in collaboration with Professor Weichang Hao from the Beihang University in Beijing, China.1 The synthesis of atomically thin two-dimensional materials such as silicene and germanene is driven by scaling down nanoelectronic devices and circuits with improved electrostatic control. These two materials do not exist in nature and are an example of human innovation. This timely review of new electronic materials concentrates on analysis of materials using Raman spectroscopy. A limited experimental study on Raman vibrational modes, detailed in this invited contribution, is well complemented by the authors with theoretical analysis factors affecting the vibrational modes such as the strain, doping, coverage and defect effects. The review concludes with a personalized overview of research done with Raman spectroscopy on silicene and germanene and challenges for this field. It should attract attention not only from researchers working on electronic materials, but also the broader advanced material characterization community and surface chemistry in general.
In the first Expert Opinion, Wen et al. discuss emerging surface coating strategies, which could enhance electrochemical performance of lithium (Li) ion battery electrode materials.2 As well articulated by the authors, the coatings are intended to add a specifically designed structure with goals of improving the stability and conductivity of the electrodes together with suppressing the structural transformations in electrodes. If successful, it could materialize with Li ion batteries of enhanced cycle stability and rate capability. The authors offer a very fresh look into the challenges of research on surface coating technologies for batteries. They also offer new concepts of ‘ultrathin conformal coating, continuous phase coating, and multi-functional coating’, together with perspectives on future developments.
In the second Expert Opinion of this issue, Drelich and Marmur offer a comprehensive personal prospective on misconceptions of contact angles and the importance of additional fundamental studies in the area of mineral particles flotation.3 As pointed out by the authors, many practices adopted by the mineral processing community rely on either incorrect methodologies of contact angle measurements or questionable selection of contact angles important to the particle flotation process or its individual steps. Although this opinion does not cover all controversies surrounding contact angle measurements and their interpretations in mineral flotation systems, it summarizes major challenges for the flotation research community. Recommendations offered by the authors could help researchers working on separation of particles using flotation and other methods to select or develop more appropriate measurement techniques and improve interpretation of contact angles – important for drawing correlations between contact angles and flotation mechanisms.
Photocatalytic properties of titanium dioxide (TiO2) have been known for a few decades, and major activities with this material concentrate on formulation of new structures and particles with enhanced catalytic capabilities and high specific surface area. Core-shell particles, nanowire, nanotubes and other structures were invented in recent years. One-dimensional nanowire TiO2 structures are under intensive study because they promote migration and separation of photogenerated charges. Such a one-dimensional nanowire can be synthesized on (100) crystal faces of anatase. To explain why on this (100) face, Wang et al. analyze the adsorption of hydroxyl groups on different planes of anatase and energy barriers associated with esterification and hydrazinolysis on these planes using molecular simulation and first-principles approach.4 It is shown that the esterification reaction on anatase crystal face occurs between oleic acid through OH functionality to form the ester layer when oleic acid and hydrazine hydrate coexist as a surface coordinating regulating agent. Calculations indicate that the (100) face has the maximum peak of status density and the strongest affinity towards adsorption of OH functionality. Also in the hydrazinolysis of hydrazine hydrate and ester layer occurs preferentially on the (100) crystal face, leading to preferential formation of anatase nanowire structures on this crystal face.
The band gap of 3·0 eV for rutile, 3·2 eV for anatase and 3·4 eV for brookite is too big to absorb visible light efficiently, and researchers explored ways to reduce the band gap value by coupling TiO2 with other oxides such as tungsten trioxide (WO3). The research team from China shows how to formulate a hollow microsphere with a mesoporous wall made of WO3/TiO2 composite by a cost-effective spray-drying approach and using 10–20 nm TiO2 and WO3 nanoparticles.5 These new structures demonstrate tuned optical, photoelectric and photocatalytic properties, which solely depend on the tungsten/titanium ratio, crystallinity, diameter of crystallites and their contact relationship at a mesoscale.
Porous materials are commonly used as drug delivery vehicles and serve as a support for catalysts. The enlarged specific surface area of such porous materials is also utilized for adsorption of pollutants and in devices for sensing. Copper oxide (CuO) is a low-cost, high-stability and nontoxic semiconductor with a narrow band-gap. Li et al. demonstrate that nanoparticles of CuO doped with manganese (Mn) can be produced by a hydrothermal technology.6 The CuO–Mn nanoparticles can be then converted into hierarchical porous microspheres and microplates with a specific surface area of about 17 m2/g, and a pore size distribution from microporous to macroporous. Li et al. also demonstrate that the porous microplates adsorb potassium ethyl xanthate (KEX) at quantities of >5·2 g/g. The KEX reagent is a widely used collector in flotation of minerals and the main pollutant in water discharged from mineral processing operations.
Hou et al. explore sodium (Na) ion batteries, which could replace Li ion-based batteries, mainly due to abundance and availability of Na resources.7 Unfortunately, sluggish kinetics of Na ions insertion and release during the charge/discharge processes make the development of Na batteries challenging. A breakthrough could come after the invention of a new electrode material with enlarged Na ion capacity and ion exchange kinetics. Hou et al. explored the synthesis of sodium p-toluenesulfonate (TsONa)-doped polypyrrole (PPy) on iron (Fe) foil, which could serve as the cathode active material for a Na ion battery. The cauliflower-like TsONa-doped PPy structures produced on Fe foil in this study demonstrate promising electrochemical performance, which the authors attribute to Na-storage activity of PPy and high conductivity induced by doping of TsONa.
Zinc phthalocyanine (ZnPc) is an organic semiconducting material with a high absorption coefficient and good photochemical and thermal stabilities. This material is explored for photovoltaic applications, gas sensing and optoelectronic devices. In a new contribution by Kaur et al., ZnPc was grown on glass substrates using a thermal evaporation technique.8 Thin films of deposited ZnPc were characterized regarding their structural, optical and electrical properties, and as a function of glass substrate temperature during deposition. It was found that temperature of substrate during thin film deposition has significant effects on structure, and consequently on optical and electrical properties. These effects are caused by structural phase transformations and molecular orientations observed in ZnPc films.
Synthetic diamonds with high-quality surfaces, down to an atomically smooth surface, are in demand. Synthesis of diamonds with a perfect surface is, however, a very challenging task. Instead, reactive ion etching with hydrogen- and oxygen-rich plasmas is commonly used in smoothing the imperfect diamond surfaces along different crystal faces. The research team from Russia used the quantum chemical calculations of the adsorption and desorption activation energy, interaction and bonding energy for H, O, CH2 and CO with point defects on the reconstructed hydrogenated diamond surface C(100)-(2×1).9 The authors describe the most probable mechanisms of etching and restoration of an ordered surface and demonstrate capability of the quantum chemical method in analyzing the physical and chemical processes on the diamond surface during reactive ion etching.
Zinc alloys have been under extensive development for biodegradable medical devices since 2013. Zinc exhibits near-ideal biocorrosion behavior and good antiatherogenic properties. However, pure zinc has a much lower strength than metals currently used for structural medical applications and its strength must be improved through an alloying process. Only several different Zn alloys were formulated in the last few years, and current understanding of alloying elements on the corrosion of zinc materials in biologically relevant media is very limited. The research team from KTH Royal Institute of Technology in Sweden studied the corrosion of zinc–silver (Zn–Ag) and zinc–magnesium (Zn–Mg) alloys using electrochemical impedance spectroscopy.10 They demonstrate that the presence of secondary phases such as intermetallics in the alloy microstructure is the source of selective microgalvanic corrosion of Zn that results in accelerated degradation rate of Zn matrix and enrichment in secondary phases. They suggest that resistance of secondary phase in the Zn–Ag alloy to corrosion might result in post-surgery complications when the degraded volume of an implant must be replaced with regenerated biological tissue.
A curiosity on meaning of apparent contact angles such as advancing and receding contact angles, and contact angle hysteresis, reported frequently in the scientific and technical literature over the last two centuries, drives scientific debate and research in this area in the last several years. In spite of significant progress made in the past, it is only recently that the adhesion of a liquid to a solid surface of varying surface characteristics is better understood and more broadly investigated. This progress is mainly attributed to advances made in fabrication of substrates with pre-defined surface topography and chemistry. Progress is also driven by the fact that it became possible in recent years to measure the adhesion between a liquid droplet and a solid surface directly using a microbalance or other force-measuring system and correlate this adhesion with measured contact angles. In a new collaborative study, Sun et al. measured adhesion forces between water droplets and polymer substrates of different surface wetting characteristics and topography, and correlated these forces with capillary and surface tension forces as well as apparent contact angles.11 They found nearly excellent agreement between directly measured and calculated capillary/surface tension forces for polymers with smooth surfaces. Interpretation of forces measured on patterned polymers having pores and pillars required an understanding of triple contact line geometry. As a result, a normalized contact line length was introduced to the equation describing surface tension forces, allowing quantification of forces measured directly with a microbalance.
The term superhydrophobicity was introduced in 1996 and describes exceptionally weak interactions of solid surfaces with water, controlled entirely by surface topography and material chemistry. An explosion of research on fabrication of superhydrophobic surfaces and coatings was noticed almost immediately after the concepts appeared in the technical literature, with hundreds of reports now published annually. The interest in this new class of surfaces/coatings is not slowing down and is driven by an emerging market for water-repellant, snow- and ice-phobic products and formulations, anti-fogging screens, windows and lenses, anti-fouling coatings, microfluidic devices, coatings for enhanced boiling heat transfer, foils for food packaging, and many other products with improved durability. In the last contribution to this issue, Chen et al. reveal one-step electrochemical deposition technology for fabrication of a superhydrophobic zirconium palmitate coating with a hierarchical structure on steel.12 The superhydrophobic zirconium palmitate coating exhibited good chemical and mechanical stabilities, with encouraging mechanical abrasion resistance, and protected steel from corrosion.
We hope that the content of this issue will inspire further advances and stimulate new ideas in many research laboratories. We would like to take this opportunity and invite you to publish cutting-edge research results from your laboratories as well as your thorough reviews in Surface Innovations.
