A comprehensive review on anticorrosive/antifouling superhydrophobic coatings: Fabrication, assessment, applications, challenges and future perspectives.

Superhydrophobicity (SHP) is an incredible phenomenon of extreme water repellency of surfaces ubiquitous in nature (E.g. lotus leaves, butterfly wings, taro leaves, mosquito eyes, water-strider legs, etc). Historically, surface exhibiting water contact angle (WCA) > 150° and contact angle hysteresis <10° is considered as SHP. The SHP surfaces garnered considerable attention in recent years due to their applications in anti-corrosion, anti-fouling, self-cleaning, oil-water separation, viscous drag reduction, anti-icing, etc. As corrosion and marine biofouling are global problems, there has been focused efforts in combating these issues using innovative environmentally friendly coatings designs taking cues from natural SHP surfaces. Over the last two decades, though significant progress has been made on the fabrication of various SHP surfaces, the practical adaptation of these surfaces for various applications is hampered, mainly because of the high cost, non-scalability, lack of simplicity, non-adaptability for a wide range of substrates, poor mechanical robustness and chemical inertness. Despite the extensive research, the exact mechanism of corrosion/anti-fouling of such coatings also remains elusive. The current focus of research in recent years has been on the development of facile, eco-friendly, cost-effective, mechanically robust chemically inert, and scalable methods to prepare durable SHP coating on a variety of surfaces. Although there are some general reviews on SHP surfaces, there is no comprehensive review focusing on SHP on metallic and alloy surfaces with corrosion-resistant and antifouling properties. This review is aimed at filling this gap. This review provides a pedagogical description with the necessary background, key concepts, genesis, classical models of superhydrophobicity, rational design of SHP, coatings characterization, testing approaches, mechanisms, and novel fabrication approaches currently being explored for anticorrosion and antifouling, both from a fundamental and practical perspective. The review also provides a summary of important experimental studies with key findings, and detailed descriptions of the evaluation of surface morphologies, chemical properties, mechanical, chemical, corrosion, and antifouling properties. The recent developments in the fabrication of SHP -Cr-Mo steel, Ti, and Al are presented, along with the latest understanding of the mechanism of anticorrosion and antifouling properties of the coating also discussed. In addition, different promising applications of SHP surfaces in diverse disciplines are discussed. The last part of the review highlights the challenges and future directions. The review is an ideal material for researchers practicing in the field of coatings and also serves as an excellent reference for freshers who intend to begin research on this topic.

Understanding the structural and chemical changes in vertical graphene nanowalls upon plasma nitrogen ion implantation.

Shallow plasma ion implantation is a versatile method for nitrogen incorporation in vertical graphene nanowalls (VGNs). However, the defects introduced by the process and the preference of nitrogen to occupy various locations in the 2D layered structure make the characterization complex. We have simplified the analysis of 2 kV nitrogen plasma ion implanted VGNs by correlating the binding energy of N1s electrons with the chemical state of nitrogen as lone-pair localized (N1), lone-pair de-localized (N2) and quaternary nitrogen (N3). This new approach helps to understand the electronic nature of implanted VGNs, based on the occupancy of structural locations by nitrogen. The C1s photoelectron spectra and G-peak intensity normalized comparison of the entire Raman spectra revealed large scale sp2C to sp3C conversion and generation of defects upon implantation. The increase in relative stiffness of implanted VGNs, as observed in atomic force acoustic microscopic studies, was correlated with the formation of graphitic CNx (N2), crosslinking of layers by nitrogen (N3) and interlayer sp3 carbon.

Local crystal structure and mechanical properties of sputtered Ti-doped AlN thin films.

In this article, we predominantly report the investigation of the local crystal structure around a Ti dopant by X-ray absorption spectroscopy (XAS) and the nano-mechanical properties of co-sputtered Al1-xTixN (x = 0 to 4%) thin films. Grazing incidence X-ray diffraction (GIXRD) results show that these films are crystallized with the hexagonal wurtzite structure of AlN. Surface chemical analysis and morphology analysis of Al1-xTixN films are executed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques, respectively. X-ray absorption near-edge structure (XANES) shows that a Ti atom replaces Al in the AlN crystal and forms localized distorted tetrahedral TiN species, leading to a tensile strain. The bond lengths (Ti-N)ax and (Ti-N)bs are found to be moderately decreased with increasing Ti concentration, which is extracted from the extended X-ray absorption fine structure (EXAFS) analysis. However, the Ti-Al bond length in the second coordination sphere having Al vacancies is unaffected by Ti concentration. The hardness (H) and modulus (E) of Al1-xTixN films are measured by the nano-indentation technique, and increase from 17.5 to 27.6 GPa and 231 to 293 GPa, respectively with x = 0 to 4 at% because of the improvement of p-d hybridization between the Ti and N atoms.

Local crystal structure in the vicinity of Cr in doped AlN thin films studied by X-ray absorption spectroscopy.

This article reports the detailed X-ray absorption spectroscopy (XAS) study of Al1-xCrxN (x = 4, 6, 11%) thin films synthesized by the reactive magnetron co-sputtering technique. All these films were crystallized with a hexagonal wurtzite structure with preferential orientation along the a-axis without the formation of any secondary phases. Surface chemical analysis to evaluate the Cr concentration was carried out using X-ray photoelectron spectroscopy. The study confirmed the presence of AlN and Cr in bonding with N. The local crystal structure around the Cr dopant in the as-synthesized and annealed thin films has been analyzed by both the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. From XAS, it was found that Cr replaced the Al atom in the AlN lattice and led to a localized CrN species with distorted tetrahedral AlN in the absence of Cr clustering. The bond lengths of (Cr-N)ax, (Cr-N)bs and Cr-Al, extracted from the EXAFS fitting, were found to decrease with the Cr concentration for both the as-synthesized and annealed thin films due to the enhancement of p-d hybridization between the dopant and the host atoms. However, in the annealed 11% Cr film, the bond lengths are larger than the other and tend to match the Cr-N geometry in CrN.