Advanced Coating Materials

By Liang Li and Qing Yang

This book covers the recent advances in coating materials and their novel applications at the cross-section of advanced materials both current and next-generation. Advanced Coatings Materials contains chapters covering the latest research on polymers, carbon resins, and high-temperature materials used for coatings, adhesives, and varnishes today. Concise chapters describe the development, chemical and physical properties, synthesis and polymerization, commercial uses, and other characteristics for each raw material and coating detailed. A comprehensive, yet practical source of reference, this book provides an excellent foundation for comparing the properties and performance of coatings and selecting the most suitable materials based on specific service needs and environmental factors.

• Language: English
• Publisher: Wiley
• Release date: Nov 15, 2018
• ISBN: 9781119407645

Book preview

Preface

Coatings is an area with great variety which has developed to be a quite significant technique for protecting existing infrastructure from corrosion and erosion, maintaining and enhancing the performance of equipment, and providing novel functions such as smart coating. In recent years, coating techniques entered an age of rapid development, greatly benefiting the medical device, energy industry, automotive and construction industries.

The mechanisms, usage, and manipulation of cutting edge coating methods are the focus of this book. Not only are the working mechanisms of coating materials explored in great detail, but also craft designs for further optimization of more uniform, safe, stable, and scalable coatings.

A group of leading experts in different coating technologies were invited to summarize the major developments in their discipline, demonstrate their main applications, identify the key bottlenecks, and prospects for the future. Their efforts are reflected in this book, Advanced Coating Materials, which broadly covers the coating techniques, including cold spray, plasma vapor deposition, chemical vapor deposition, sol–gel method, etc., and their significant applications in microreactor technology, super(de)wetting, joint implants, electrocatalyst, etc. Numerous kinds of coating structures are addressed, including nanosize particles, biomimicry structures, metals and complexed materials, along with the environmental and human compatible biopolymers resulting from microbial activities. This book divides the collection of diversified topics related to coating materials into three parts: (1) Materials and Methods: Design and Fabrication, (2) Coating Materials: Nanotechnology, and (3) Advanced Coating Technology and Applications.

The first part of the book, ‘Materials and Methods: Design and Fabrication’, describes the most promising approaches illustrated in coating techniques, with Chapter 1 broadly covering the adaptation of new coating techniques by explaining the science behind the molecular precursor method. Information regarding 3D cold spray modeling in an advanced coating process is covered in Chapter 2. The effects of laser process parameters (HLPP) on alloy characteristics are described in Chapter 3 and Chapter 4 focuses on physicochemical properties and electrocatalytical reactivity in oxygen transfer reactions, suggesting that oxidative degradation of organic substances is likely due to an increase in the amount of strongly bound oxygen-containing species on the electrode surface. Chapter 5 discusses nuclear fuel durability enhancement by using polycrystalline diamond (PCD) coating protection, which has been found by nuclear reactor research to be appropriate for prolonging the lifetime of nuclear cladding, and consequently enhancing nuclear fuel burnup as a passive element for nuclear safety. High-performance WC-based coatings for narrow and complex geometries are well defined in Chapter 6.

The topics in the second part of the book, ‘Coating Materials: Nanotechnology’, are related to dimensional properties of coating materials. The role of nanotechnology in paints and coatings is discussed in Chapter 7, which is representative of the recent technology enhancements which show an astonishing influence of dimensions on antimicrobial properties. Chapter 8 explains anodic oxide nanostructures and theories of anodic nanostructure self-organization (growth mechanism of oxide film). Next, in Chapter 9, the potential prospects of nanodiamond, epoxy, and important epoxy/ND hybrids for coatings and their significant applications are discussed. Nano-dimension coatings are an important coating technology that offer significant benefits for electrocatalytic applications in nanostructured metal–metal oxides, as described in detail in Chapter 10.

The last part of the book, ‘Advanced Coating Technology and Applications’, mainly focuses on the use of advanced coating technologies in applications of utmost significance to future advancements in the field. Chapter 11 describes solid-phase microextraction coatings based on tailored materials (e.g., molecularly imprinted polymers), which are found to be a significant contributor to the field. The focus of Chapter 12 is the effect of laser processing on hardening of titanium alloy. Engineering involved in scalable fabrication of super(de)wetting coatings is described in Chapter 13, along with prospects and guidelines for the upgraded development. In Chapter 14, some of the widely used polymers are discussed in detail and further research is suggested that can lead to their modification as coating materials for biomedical applications.

This book is written for readers from diverse backgrounds across nanotechnology, biomedical engineering, chemistry, physics, engineering, medical, environmental, and materials science fields. Since it offers a comprehensive view of innovative research in advanced coating materials and their technological importance, the book will be of benefit to scientists, researchers, and technologists in advanced coating materials; those in industrial sectors intending to fabricate materials employing state-of-the-art techniques; and students of PhD, master’s and undergraduate-level courses on surface materials processing, properties, and applications of multidisciplinary subjects.

The editors would like to thank the International Association of Advanced Materials, the eminent authors for their contributions to this book as well as the efforts of the publishing team.

Editors

Liang Li, PhD

Qing Yang, PhD

July 2018

Part I

MATERIALS AND METHODS: DESIGN AND FABRICATION

Chapter 1

The Science of Molecular Precursor Method

Hiroki Nagai and Mitsunobu Sato*

Department of Applied Physics, School of Advanced Engineering, Kogakuin University, Tokyo, Japan

*Corresponding author: [e-mail]

Abstracts

The metal complexes are used in various applications such as catalysts, luminescent materials, and medicines. In 1996, one of the authors, M.S., focused on the thin-film fabrication of various metal oxides and phosphate compounds, using coating solutions involving stable metal complexes of industrially available multidentate ligands. This is the molecular precursor method (MPM). The method is based on the facile preparation of coating solutions involving the metal complex anions and alkylammonium cations. The stability, homogeneity, miscibility, coatability, and other characteristics of the coating solutions are practical advantages, as compared to the conventional sol–gel method. This is because metal complex anions with high stability can be dissolved in volatile solvents by combining with appropriate alkylamines. Furthermore, the resultant solutions can form excellent precursor films through various coating procedures including spin-coating. The precursor films obtained by the coating process on various substrates should be amorphous, just as with the metal/organic polymers in the sol–gel processes; otherwise, it would not be possible to obtain the resulting metal-oxide or metal-phosphate thin films spread homogeneously on substrates by heat treatment. The advantages of the molecular precursor solutions will be also explained through detailed results of thin film fabrication in this chapter.

Keywords: Molecular precursor method, stability, homogeneity, miscibility, coatability, functional thin films

1.1 Metal Complex

Metal complexes (coordination compounds) are one of the most important chemical compounds and form the basis of coordination chemistry. Coordination chemistry is being considered a science only after the formulation of the coordination theory proposed by A. Werner [1, 2]. After Werner, enormous metal complexes were obtained, characterized, and widely applied. Especially, their syntheses, structures, and properties have been investigated.

Metal complexes consisted of a central metal atom (ion) and ligands connected to the metal atom. The combination of metal atom and ligand produces the coordination sphere, which is formed by coordination bonds having donor–acceptor interactions. A coordination bond is mostly formed as a result of the overlapping of atomic orbitals (AO) of ligands, filled with electrons and/or vacant AO of the central metal atom. Lewis acid can form a new covalent bond by accepting a pair of electrons, and Lewis base can form a new covalent bond by donating a pair of electrons. The fundamental Lewis acid–base theory is described by a direct equilibrium, leading to the complex formation as follows.

Thus, the coordination (donor–acceptor) bond between the central metal (M) and each joining group (ligand, L) is formed by the electron pair. The conventional theory by Lewis made a considerable contribution in understanding the reaction with participation of Lewis acids and bases.

The HSAB (Hard and Soft Acids and Bases) principle is one of the important theories for coordination chemistry, formulated by Pearson in 1963 [3]. The following three statements are the basis of HSAB.

Chemical reactions, in particular complex formation, can be classified as acid–base ones; the resulting products can be examined as complexes of the type Lewis acids and bases.

All acids and bases can be divided into hard, soft, and/or intermediate.

The HSAB principle itself is the following: the acid–base reactions take place in such a way that hard acids prefer to be connected with hard bases, meanwhile soft acids react with soft bases.

The classification of HSAB is summarized in Table 1.1.

Table 1.1 HSAB classification of metal and ligand.

The HSAB principle emphasizes the preference for hard–hard and soft–soft interactions, and the highest thermodynamic stability of complexes formed as a result is achieved.

The rows shown below indicate that the hardness of the elements (donor atoms in ligands) decreases from left to right:

Ligands with N, O, F, Cl donor atoms containing a combination of these elements are hard bases according to Pearson. On the contrary, containing elements further to the right are soft bases. The hardness and softness of acids depend considerably on the oxidation number of the metal center.

The HSAB conception has been widely used to explain various coordination modes in the complexes of di-and polydentate ligands. The solvent nature can be also an important factor. The most favorable conditions to control the localization mode of a coordination bond with participation of ligands containing hard and soft donor atoms are created when complex-formation reactions are carried out in aprotic nonaqueous solvents.

Ligands, as the main part of metal complexes, are the object of a great deal of attention in coordination and organometallic chemistry. The reaction control should be emphasized among the reaction conditions of competitive complex formation. It is necessary to take into account that it is possible to determine, and frequently predict, the direction of the electrophilic attack to the donor atom of di- and polyfunctional donors (ligands) only in the case when the thermodynamically stable products are formed under conditions of kinetic control.

Thus, the thermodynamic stability of complexes is discussed, when the bond between the metal and di- and polydentate ligands is localized in the place of primary attack on the donor atoms by the electrophilic reagent, without further change of coordination mode in the reaction of complex formation.

1.2 Molecular Precursor Method

In 1996, one of the authors, M.S., focused on the thin-film fabrication of various metal oxides and phosphate compounds using the stable metal complexes [4–54]. This is the Molecular Precursor Method (MPM), which is one of the chemical processes used for thin-film fabrication. In those days, most of the researchers in the field of thin-film formation by chemical processes preferred to use rather unstable metal complexes. It is easy to imagine the capability of polymers to form films because we use polymer films every day. In fact, well-adhered precursor films involving metal ions can be formed on various substrates by coating the solution dispersing the produced oligomers and polymers including metallic species provided by hydrolyzing the unstable metal complexes. These results led us to believe for a long time that only the oligomers and polymers can form precursor films, but the stable metal complexes having a discrete molecular weight would not be useful in the fabrication of such thin films. The MPM was a challenge to this central belief.

The MPM, pertinent to coordination chemistry and materials science including nanoscience and nanotechnology, has been used to fabricate various high-quality thin films with appropriate film thicknesses. As a result, the MPM represents a facile procedure for thin-film fabrication of various metal oxides or phosphates, which are useful as electron and/or ion conductors, semiconductors, dielectric materials such as In2O3, ZnO, LiCoO2, Li3Fe2(PO4)3, TiO2, Cu2O, Co3O4, SrTiO3, ZrO2, SiO2, BaTiO3, and Ca10(PO4)6(OH)2. The MPM aims to develop many functional materials by surface modification of various substrates including glasses, metals, and ceramics, through chemical fabrication of thin films. One of the features related to this method is the low-cost manufacture involving the chemical process, which can save both resource and production energy.

1.3 Counter Ion (Stability)

The appropriate alkyl groups in the used amines play an essential role. This principle of the MPM is absolutely different from that of the conventional sol–gel method, which needs and uses the mixture of oligomers and polymers for the identical purpose. Amino group itself is usually very reactive, forming simple salts with metal complex anions. The stability of these salts is dependent on the basicity of amine and pH in the used solvent. Most of these salts are rather soluble in both water and aprotic organic solvents. Additionally, the presence of the ligands in metal complex anions and alkylammonium cations in the precursor films generally affects the properties of resultant thin films, as expected. It is very interesting that the thermal reactions between them and metallic species are quite sensitive to the reaction conditions during heat treatment for fabricating the final thin films.

Single crystals of the metal complex can be obtained from the precursor solution in several cases when the alkyl groups in the alkylamines are sufficiently small, for example, an ethyl group. The model structure of the amorphous precursor films formed on substrates can be examined by means of crystal engineering and based on the crystal structures. For example, an ORTEP view of the precursor complex having the EDTA (ethylenediaminetetraacetic acid) and peroxo ligands linked to the central Ti⁴+ ion is shown in Figure 1.1. The molecular structure was determined by an X-ray single crystal structure analysis of the diethylammonium salt of the complex. The single crystals of the identical orange-yellow color could be obtained from a reacted solution of the complex with the diethylamine. The single crystal was {(C2H5)2NH2}[Ti(O2)(Hedta)]·1.5H2O; in a monoclinic crystal system, P21/c with a = 8.583(1) Å, b = 6.886(1) Å, c = 36.117(2) Å, and β = 92.780(3)°. The full-matrix least-squares refinement on F² was based on 3206 observed reflections that were measured at 250 K by using an imaging plate as a detector and converged with unweighted and weighted agreement factors of R = 0.054 and Rw = 0.061, respectively, and GOF = 1.63. Two Ti–N(edta) bond lengths of 2.307 and 2.285 Å are slightly longer than the bond length of 2.12 Å in the TiN single crystal.

Figure 1.1 An ORTEP view of the precursor complex having the EDTA and peroxo ligands linked to the central Ti⁴+ ion.

Results indicated that EDTA acts as a pentadentate ligand in the complex, and the peroxo ligand linked to the Ti⁴+ ion has a side-on coordination structure.

1.4 Conversion Process from Precursor Film to Oxide Thin Film

A stable metal complex anion in the precursor solution is dissolved at a molecular level. The metal complex salt in the precursor film must be amorphous before heat treatment in order to fabricate thin films without cracks and pinholes. The alkylammonium cations play an important role in obtaining an amorphous salt in the precursor film. The plausible packing of the metal complex in the precursor film formed on the substrate can be theoretically explained using molecular dynamics and crystal engineering. The shrinkage rate of the film in the vertical direction can be easily estimated from the model structure before heat treatment based on the crystal structure of the metal complex salt, which can be obtained as a single crystal when the alkyl groups in the amines are short enough. The shrinkage rate in the sol–gel method is usually considered to be around 10 times. However, it is roughly estimated to be 10–15 times in the case of MPM, on the basis of the crystal structures (Figure 1.2). Thus, the densification degrees of the precursor films during heat treatment in the process of MPM are similar to those in sol–gel procedures, even though the precursor films involve alkylamines and ligands.

Figure 1.2 Schematic for shrinkage models of metal complex film (left: Ti–EDTA complex; right: Ti–NTA complex).

1.5 Anatase–Rutile Transformation Controlled by Ligand

Titanium dioxide, the only naturally occurring oxide of titanium at atmospheric pressure, exhibits three polymorphs, rutile, anatase, and brookite. While rutile is the stable phase, both anatase and brookite are metastable. Rutile and anatase are industrially available. The poor photoreactivity and chemical stability of rutile, which is a useful pigment for white paint, are generally known. Anatase is contrarily an attractive material as the photoreactive material. Anatase transforms irreversibly to rutile at elevated temperatures. This transformation does not have a unique temperature, and the processes that are involved in the transformation as well as the methods to inhibit or promote this transformation have not been reviewed comprehensively to date. The phase transition temperature from anatase to rutile is different between the MPM and sol–gel method. Using both MPM and conventional sol–gel method, the anatase phase appears during the heat treatment of both precursor films at a temperature between 400 and 500 °C. By using MPM, anatase can be transformed to the rutile one between 600 and 800 °C, while a conventional sol–gel process showed that anatase could not be transformed to the rutile one, even when heat-treated at 900 °C. Shibuya et al., founded that the nuclear number of the metal complex in the precursor solution will be related to the transformation from anatase to rutile [54]. By a single-crystal X-ray structure determination, the complex in the precursor solution using NTA (nitrilotriacetic acid) ligands was tetranuclear complex with structure of [Ti4O4(nta)4]⁴–. Moreover, if this solution reacted for longer time, the dinuclear complex was obtained. Table 1.2 shows the complex structure in these precursor solutions, and relation between crystal phase transition temperature from anatase to rutile and oxo-unit of the complex.

Table 1.2 Relationship between the structure of complexes in the precursor solutions and the anatase–rutile transition temperatures.

The structure of the [Ti4O4(nta)4]⁴– had four Ti-O-Ti bonds (oxo-unit), and the phase transition temperature from the anatase to rutile was 800 °C. In addition, as for the ingredient of solution provided by a long-time reaction, the phase transition temperature was 650–750 °C from dinuclear complexes. The complex of EDTA ligand in the precursor solution was a mononuclear complex, and the phase transition temperature was 600–700 °C. In addition, phase transition temperature is known to be 900–1000 °C by using the typical sol–gel method.

The transformation of anatase to rutile might be influenced by the rearrangement of the atoms in the lattices. The most important factor affecting the phase transformation is the presence and amount of defects on the oxygen sublattice. Ease of rearrangement and transformation are enhanced by relaxation of the large oxygen sublattice through the increased amount of oxygen vacancies. The experimental results also suggested that the difference of the phase transition temperature might be related to the number of oxo-units in the complex.

1.6 Homogeneity

The film thickness is an important factor for electronic devices. Figure 1.3 shows the relation of the concentration of the complex and the thickness of the resulting anatase thin film. The anatase thin film was fabricated by heat-treating the precursor film, which included the dinuclear complex of Ti–NTA at 500 °C for 30 min in air. The film thickness of anatase is easily controlled by adjusting the concentration.

Figure 1.3 The relation of the concentration of the complex and the thickness of the resulting anatase thin film.

An excellent perovskite-type SrTiO3 thin film was fabricated using a mixed precursor solution from a titania precursor solution containing a Ti complex of EDTA and an SrO precursor solution containing an Sr complex of EDTA [51]. The metal complex ions dissolve independently in each precursor solution, and the homogeneity of the mixed solution can be kept at the molecular level. In fact, a mixed precursor solution containing exact amounts of Ti and Sr can be easily prepared due to the excellent miscibility of the solutions. This is the essential difference between the MPM and conventional sol–gel methods in which the hydrolyzed polymers are heterogeneous because of the different rates of hydrolysis of each metal ion.

The mixed solution of the titania precursor solution and the ethanol solution obtained from reaction of Sr(H2edta) with dialkylamine in ethanol lead to a suitable readily handled precursor solution for the preparation of perovskite strontium titanate thin films. It was elucidated by XRD that perovskite-type SrTiO3 film formation was attainable by employing the mixed precursor derived from the stable Ti(IV) metal complex and Sr(II) salt, with no evidence for polymerization.

The homogeneous precursors (complexes) at the molecular level in the solution affect for the fabrication process of metal oxides. Hao and coworkers synthesized a spinel-type Li4Ti5O12 by a modified sol–gel method using oxalic acid as a chelating agent. According to the TG–DTA results of the report, the final weight loss from 700–800 °C is mainly attributed to the thermal decomposition of residual carbonate phases and the completion of the crystallization to Li4Ti5O12. These results correspond to the following reaction:

However, in the case of this present MPM, it is clear from the TG–DTA results that no reaction occurs between 700 and 800 °C (Figure 1.4). Thus, it is suggested that the chelating ligand in the molecular precursors successfully prevents any unexpected segregation of the metal compounds during oxide formation because the discrete molecular precursor complexes provide homogeneous and ideal mixtures of the different metal species at the molecular level in the precursor films.

Figure 1.4 The TG–DTA curves of the LTO precursor gel obtained by evaporating the precursor solution. The measurement temperature was increased from 25 °C to 1000 °C at a rate of 10 °C/min using an air flow rate of 0.1 dm³/min.

1.7 Miscibility

In a typical sol–gel protocol, the process starts with a solution consisting of metal compounds, such as a metal alkoxide, acetylacetonate, carboxylate, and soluble inorganic species as the source of cations in the target oxide. Additional reactants include water as the hydrolysis agent, alcohols as the solvent, and an acid or base as a catalyst. Metal compounds undergo hydrolysis and polycondensation near room temperature, giving rise to a sol in which polymers or colloidal particles are dispersed without precipitation. Further reaction connects the fine particles, solidifying the sol into a wet gel, which still contains water and solvents. Vaporization of solvents and water produces a dry gel. Thus, the rigorous exclusion of water from the system is essential for the synthesis and conservation of the precursor alkoxides and their solutions, since the process is based on partial or complete hydrolysis of such metal alkoxides. From this point of view, a novel wet process, in which films can be formed by facile coating procedures of precursor solutions of a water-resistant precursor, may afford practical advantages.

1.8 Coatability (Thin Hydroxyapatite Coating of Ti Fiber Web Scaffolds)

Molecular precursor solutions can be used in spin-, dip-, or spray-coating on various material surfaces to form precursor films. It is performed by deposition of a precursor solution with enough amounts to be evenly spread on the substrate surface. The precursor solutions employed in can easily be washed away with water from substrate surface and the substrate can be recycled. Recycling of such a substrate in an industrial application would constitute a large improvement in the loading process.

Spin-coating is one of the most popular coating methods as it is simple and easy to realize the intended film thickness. The solution dispensed on the substrate can be spread by a centrifugal force. In general, the film thickness depends on the viscosity and concentration of the solution. The wet film thickness can be expressed in the following equation.

hf: the wet film thickness, x: rate of the solid contacting on the substrate, µ: viscosity of the solution, ρ: solution concentration, ω: angular speed.

However, very small amounts of the solution remain on the substrate to form the film during the spinning process. The solution is totally wasted. As a result, material utilization in the spin-coating process is commonly too low. Therefore, that is problem as a practical manufacturing process.

In the case of sol–gel method, that is difficult to collect the wasted solution because the gel film was rapidly produced. Moreover, the viscosity of the solution will be changed by the polymerization. On the other hand, the MPM could not produce the polymerization in the solution, which means that the viscosity is not changed for long time, and the wasted solution might be easily collected.

The thin-film fabrication for biomaterial is often required on the metal of complicate shapes. M. Hirota et al., attempted the hydroxyapatite (HA) coating of Ti fiber mesh scaffolds. The fiber diameter was 20 µm and the porosity was 87%. The mean pore size was 80–110 µm (average 95 µm) and was performed by molecular precursor methods [25]. The precursor solution was prepared by mixing calcium–ethylenediaminetetraacetic acid/amine complex and dibutylammonium diphosphate salt in ethanol at a calcium–phosphorous ratio of 1.67. The Ti fiber mesh scaffolds were dipped in this precursor solution for 20 min with ultrasonic treatment. Subsequently, the scaffolds were preheated at 60 °C for 20 min followed by heating at 600 °C for 2 h under atmospheric conditions. As a result, field-emission scanning electron microscopy (FE-SEM) imaging suggested that changes in fiber thickness and porosity were rarely occurred after HA coating, indicating that the coated layer was thin. Element mapping showed that Ca and P were distributed with Ti, indicating that the Ti fibers were coated with calcium phosphate. The results indicate that a thin HA coating on Ti fiber mesh scaffolds enhances osteoblast maturation to a final stage of cell differentiation for early bone formation. Also, the HA-coating enabled Ti fiber mesh scaffolds to firmly connect with the bone. This enhanced osteoconductivity rapidly creates a bone–Ti fiber mesh scaffolds complex with rigidity and resistance to mechanical stress.

C. Mochizuki et al., reported fabrication of HA films on a Ti plate using an aqueous spray method. The airbrush was used to spray the aqueous solution. A spray solution with sufficient concentrations of Ca²+ and PO4³– ions by mixing phosphoric acid with a calcium hydrogen carbonate solution was obtained by passing CO2 through a calcium hydroxide solution. The results of elemental analyses and Fourier transform infrared spectroscopy of the powders that were mechanically collected from the surface of the sprayed film suggest that the film was Ca10(PO4)6(CO3)·2CO2·3H2O. The presence of the carbonate ion and the lattice CO2 molecule was confirmed via the aforementioned analyses; the finding was also consistent with the X-ray diffraction patterns of the films and the chemical identity of the sprayed and heat-treated films that were measured using X-ray photoelectron spectroscopy. The sprayed film comprises a characteristic network structure, which contains round particles within the networks, as it was observed by FE-SEM. A scratch test indicated that the shear stress of the sprayed film (21 MPa) significantly improved to 40 and >133 MPa after heat treatment at 600 °C and 700 °C, respectively, under Ar gas flow for 10 min.

1.9 Oxygen-Deficient Rutile Thin Films

The ligands in the precursor metal complexes provide important functions to the metal oxide thin films. We achieved direct fabrication of O-deficient rutile thin films with high photoreactivity using a MPM. Rutile is the most stable crystal form of titania. Since Nishimoto et al., showed that anatase is more sensitive to UV light than rutile in photoreactions, rutile was believed to be inferior to anatase in terms of photoreactivity [55]. Anatase is important for photocatalysis in pollutant degradation and in the development of photofunctional materials such as films with hydrophilic surfaces under UV light irradiation. The poor photoreactivity and photosensitivity of rutile is generally believed to be due to its crystal structure. Rutile is primarily known as a useful pigment for white paint, due to its chemical stability [56, 57]. Because the band edge of a rutile single crystal is 3.0 eV, rutile has the potential to respond to visible light. The thin films were formed by heat-treating the precursor films after spin-coating onto a quartz glass substrate. Molecular precursor solution involving Ti–EDTA complex and conventional sol–gel solution were applied in an Ar gas flow. The transparent precursor films formed by spin-coating the solutions and preheating in a drying oven at 70 °C for 10 min were heat-treated at 700 °C for 30 min in a furnace made from a quartz tube with an Ar gas flow rate of 0.1 dm³/min. When the molecular precursor solution was used, a transparent rutile thin film was formed. When sol–gel solution was used, a transparent anatase thin film was formed. The film thickness was 100 nm in both cases.

A coordination skeleton of (TiO4N2) or (TiO5N2) can be assumed in the EDTA complex as a precursor molecule from the structural study of a Ti complex [Ti(H2O)(edta)]·1.5H2O also reported by Fackler et al., [58]. In the precursor films, two N and at least four O atoms link to one Ti ion. As a result of heat-treating the precursor complex in an Ar gas flow, neighboring complexes reacted with each other. In this process, several O atoms linked to one Ti ion could be covalently bonded by other Ti ions, and the anatase lattice was gradually created. By eliminating large amounts of C, H, and N atoms with O atoms, oxide ion sites of the anatase lattice were partially occupied by a rather stable nitride ion derived from the coordinated N atom originally belonging to the ligand.

The selectivity was due to the O-vacant sites in the oxide thin films formed at different levels due to the differences between the amounts of oxygen in the two precursors. In this case, the oxygen source required to structure titania was available only in the precursor films when these thin films were fabricated. Therefore, crystallization into rutile, which has many O-vacant sites, and the accompanying rapid elimination of organic residues from the molecular precursor film, occurred because of the heat treatment.

In contrast, the amount of oxygen available to Ti⁴+ in titanoxane polymers, though significant, was sufficient to develop stoichiometric TiO2 from sol–gel solution. The oxygen defects in an anatase lattice generally lower the temperature of the phase transformation from anatase to rutile [59, 60]. Thus, selective formation occurred according to the differing degrees of O deficiency.

The photoreactivities of the thin films were evaluated by the decoloration rates of MB (Methylene Blue) solutions, which served as a model for organic pollutants in water. The results measured under Vis and UV light irradiation are summarized in Table 1.3, along with those measured under dark conditions (reference values). The data show the effects of adsorption on the samples, vessels, and self-decoloration of MB under each condition. Moreover, the photoreactivity of rutile fabricated by MPM was extremely high under both Vis and UV irradiation and higher than the photoreactivity of anatase fabricated by sol–gel method.

Table 1.3 The reaction rate v of the decoloration reaction in an aqueous solution containing 0.01 mol/dm³ of methylene blue under visible- and UV-light irradiation and under dark conditions. Calculated standard deviations are presented in parentheses.

1.10 Cu Thin Film

The copper precursor solution containing a Cu²+ complex of EDTA and a Cu²+ complex of propylamine derived from formic acid and the amine was prepared by mixing the two precursor solutions. The concentration of total copper in the ethanolic precursor solution was adjusted to 0.35 mmol/g. The spin-coating method was used for precursor film formation on a Na-free glass substrate. The spin-coated precursor films were preheated in a drying oven at 70 °C for 10 min and then heat-treated at 350 °C for 15 min under an Ar gas flow of 1.5 dm³/min to fabricate thin films in a tubular furnace with a quartz glass tube. The resultant thin film is hereby denoted as A. The rate of temperature increase was controlled by a proportional–integral–derivative program preinstalled in the furnace. Before increasing the temperature, the tubular furnace was filled with Ar gas. The thickness of the resultant films was measured using a stylus profilometer. A flat and same-sized quartz glass plate was placed on the resulting thin film A in the tubular furnace and then postannealed at 350 °C for 20 and 40 min in an Ar gas flow of 1.5 dm³/min. The resulting thin film is hereby denoted as AP. The XRD patterns of A and AP with a thickness of 40 nm, the peaks at 2θ = 36.6° and 42.5° for A can be assigned to the (111) and (200) phases of Cu2O, respectively, and an additional peak at 43.5° for A is assigned to the (111) phase of copper (JCPDS card No. 04-0836). The single peak at 2θ = 43.7° for AP is attributed to the (111) phase of copper. Thus, the Cu2O phase decreased gradually with increasing postannealing time, and no peak owing to any impurity phase such as Cu2O and CuO could be detected in the annealed AP film. The cell parameter of the Cu cubic lattice in A, which was determined by the Wilson and Pike method, is a = 3.71(3) Å, and the crystallite size of Cu crystals formed in the film can be determined as 11(1) nm; the estimated standard deviations are presented in parentheses. The cell parameter of the Cu cubic lattice in AP is a = 3.72(3) Å, and the crystallite size of the Cu crystals can be determined as 13(1) nm. The surface morphology of the A and AP thin films was observed using FE-SEM. The grain size of the Cu particles also increased from 50 nm (A) to 70 nm (AP) upon annealing. The Auger spectra of the resulting thin films suggested broad peaks were observed at 264 eV for carbon, 509 eV for oxygen, and 764, 835, and 914 eV for copper atoms. The kinetic energies of the copper atoms are identical to those in other films fabricated by the MPM. The result indicates that the amount of carbon atoms in the AP thin film was reduced to half by the postannealing treatment of A. The electrical resistivity of the A and AP thin films is 7.5 × 10–4 and 4.7 × 10–5 Ω cm, respectively. Thus, highly conductive translucent copper thin films could be obtained in commercially available Ar gas. A plausible scheme for copper lattice formation, which can be deduced from the XRD and Auger spectra, is presented in Scheme 1.1.

Scheme 1.1 Plausible scheme for the formation of a Cu thin film from the precursor film.

The scheme indicates that four Cu complexes are required to construct one FCC (Face centered cubic) copper unit cell. During the heat treatment of the precursor complexes in Ar gas flow containing less than 2 ppm of oxygen as impurity, neighboring complexes react with each other. The valency of copper was reduced from +2 to 0 by the thermal decomposition of the complexes of EDTA and butylamine ligands in Ar gas. In the process, Cu2O involving Cu and the neutral carbon atom is produced in the A thin film. During the reaction from the A thin film to the resultant AP thin film by postannealing, when the oxygen content is below 2 ppm in the Ar gas, it cannot react with the film, because the quartz glass plate placed on the A thin film can prevent the film from encountering the oxygen molecule. In fact, the copper thin film, which was separately prepared by a vacuum plating method, was not oxidized by postannealing under an identical condition. Thus, it is accepted that the reduction reaction occurred because of the materials inside the A thin film. Under these conditions, only one candidate that can act as a reductant for Cu²+ ion remains on the carbon atoms in film A.

The polycrystalline Cu lattices were gradually structured by reducing the valency of the Cu²+ ion with carbon atoms, and the Cu grains were simultaneously grown by annealing. This reaction mechanism involving the reduction reaction caused by carbon atoms may be comparable to the modern and indirect steel-making system using corks. The tensile strength of the AP adhered onto the Na-free glass substrate was 36(12) MPa as determined from the stud-pull-adherence tests, indicating strong adhesion to the glass substrate. The tensile strength of the Cu film deposited onto an identical Na-free glass substrate by a vacuum plating method was 1.7(5) MPa after an identical heat treatment of the AP thin film. Thus, the tensile strength of the AP thin film on the Na-free substrate was more than 20 times higher than that of the Cu thin film deposited by the vacuum plating method. The covalent bonds between the trace amounts of Cu²+ ion present locally at the interface between the thin film AP, and the O²– ions belonging to the Na-free glass molecules may assist in the formation of a robust interface between the Na-free glass substrate and the AP thin film. In fact, the tensile strength of the adhesion of the Cu2O thin film to the substrate fabricated using the MPM was 83(2) MPa.

The transmittance spectra of AP having 40 nm film thickness are more than ~30% in the visible region. The infrared reflectance of AP is higher than 40% and reached 100% in the far-infrared region, whereas the reflectance of A was low, 20–30%, over this region. The MPM can facilely control the film thickness by adjusting the concentration of Cu ion in the precursor solution under identical spin-coating conditions. When the Cu thin film is 100 nm thick, the conductivity is 1.8 × 10–5 Ω cm and the transparency in the visible region is below 5%. Thus, a thicker film indicates higher conductivity, but reduced transparency.

Recently, we attempted to embed copper in narrow trenches (0.2–1.0 µm wide and 5.0 µm deep) by using the MPM. A new precursor solution was prepared by dispersing the Cu nanopowder (20–40 nm) into the above-mentioned Cu precursor solution. Si substrates with the trenches were immersed in this precursor solution under ultrasonic vibration for 1 min and then slowly withdrawn from the solution. The dip coating and heat treatment steps were repeated twice. The cross-sectional FE-SEM images of the single- and double-layer films on the Si substrates with trenches indicated that the embedded particles filled the trenches of (i) 1.0, (ii) 0.5, and (iii) 0.2 µm widths and 5 µm depth (Figure 1.5). The trenches were found to be filled from the bottom upward. The bottom-up fill of copper by electroless deposition has been demonstrated using a plating bath containing a deposition inhibitor [61, 62]. The embedded copper having 0.2 µm width and sufficiently low resistivity could be facilely fabricated with no electrical contact or pretreatment of the Si substrate. Thus, this method may be useful for fabricating copper interconnects of ultra large-scale integrated circuits.

Figure 1.5 Cross-sectional FE-SEM images of the single- and double-layer films on the Si substrate with trenches of (a) 1.0, (b) 0.5, and (c) 0.2 µm widths and 5 µm depth.

1.11 Applications Using the Molecular Precursor Method

We attempted to fabricate a transparent thin-film LIB (Lithium ion battery) using the MPM. A transparent LFP thin-film cathode of 80 nm thickness was fabricated on a conductive FTO precoated glass substrate by heat-treating a precursor ethanolic solution containing a Li(I) complex of nitrilotriacetic acid (NTA), an Fe(III) complex of ethylenediaminetetraacetic acid (EDTA), and (dibutylammonium)2H2P2O7·0.5H2O at 550 °C for 10 min in air. A transparent LTO thin-film anode of 90 nm thickness was also fabricated on the substrate by heat-treating a precursor ethanolic solution containing a Li(I) complex of NTA, a Ti(IV) complex of NTA, and hydrogen peroxide, at 550 °C for 30 min in air. The rechargeability of the assembled sandwich-type battery using an electrolytic solution dissolving LiPF6 was measured by a repeated charge and discharge test.

The repeated charge and self-discharge tests of the assembled LIB were successfully performed at a constant current of 10 µA, and the curve of the voltage change is shown in Figure 1.6. A maximum voltage of 3.6 V was recorded when the current was applied at intervals of 20 s.

Figure 1.6 The colorless battery before charge and after discharge (left) and blue-gray battery after charge (right).

When the battery was charged from an external source, the colorless battery drastically changed color to blue-gray. The color changes were repeatable and occurred simultaneously with the charge and discharge operations. This unprecedented phenomenon suggests a two-step reaction based on the Ti⁴+/Ti³+ redox coupling with the intercalation of Li+ ions into the spinel-type LTO electrode. The electrochemical reaction can be described as follows [20].

The intercalation of Li+ ions occurred in the vacant sites of the LTO spinel skeleton through the electrolytic solution, and an equivalent amount of Li+ ions was supplied from the LFP electrode by the charge operation. The coloration of the LTO to blue-gray in the process suggests that some of the Ti⁴+ sites were simultaneously reduced to Ti³+ ions by the electrons supplied from the power source. The Ti³+ ions could again be oxidized to Ti⁴+ ions along with the discharge of the battery, when the intercalated Li+ ions returned to the LFP electrode through the electrolytic solution. Thus, the reversible LIB reaction could be visualized by using the novel thin-film electrodes. This monitoring system might be useful for clarifying the reaction mechanism of the novel LIB and contribute to solving multiple problems such as thermal runaway, explosion, or fire.

We attempted to fabricate a novel thin-film LIB that could be charged by light irradiation based on these results [11]. This novel, translucent, solar-chargeable LIB was fabricated using titania (anode) and LiCoO2 (cathode) thin films prepared by MPM as the active materials on the abovementioned conductive glass substrate. The precursor solutions containing the corresponding complexes capable of producing the anode and cathode active materials were easily prepared. Precursor films of TiO2 and LiCoO2 on the FTO precoated glass substrate were separately formed via a spin-coating method at ambient temperature using a two-step process, and they were preheated in a drying oven at 70 °C for 10 min. Then, the precursor films were heat-treated in air for 30 min at 500 °C and 550 °C, respectively.

The X-ray diffraction peaks of the resulting thin films can be attributed to anatase and LiCoO2. The optical transmittance of the assembled device was 50% at 700 nm, which is the longest wavelength in the visible region; hence, the device is translucent. A typical charge/discharge cyclic test was performed with a DC voltage source/monitor and was repeated 10 times at 20-s intervals. The averaged potential at 2.34 V was observed by applying a constant current of 1.0 mA. Then, that at 2.01 V was detected after 20 s during the sequential self-discharge process (Figure 1.7).

Figure 1.7 The charge/discharge cyclic test of the assembled battery. The lines indicate the following: —; a constant current of 0.2 mA, —; 1-sun irradiation.

Based on these plateau values, the potential difference between TiO2 and LiCoO2 can be theoretically estimated in the range of 2.3–2.0 V. Therefore, a device constructed of these active materials on a FTO precoated glass substrate could be operated as a typical LIB because the detected potentials in the charge/discharge cycles are in good agreement with the theoretical values. The charge and self-discharge test of the LIBs performed here study was also conducted under light irradiation and in the dark with no electrical supply 30 times at 60-s intervals. The 1-sun irradiation was achieved using a solar simulator and monitoring with a DC voltage monitor. The irradiated area of the LIB was 4.0 cm². The averaged voltages were 1.32 V during 1-sun irradiation and 1.29 V in the dark during the self-discharge process. Based on the calibration curve of the charging voltages over constant currents ranging from 0–1.0 mA, the detected value (1.38 V) can be theoretically reduced to the charging operation by applying a constant current of approximately 60 µA.

1.12 Conclusion

The author (M.S.) developed the molecular precursor method approximately 20 years ago. At the beginning, we reported the fabrication of Co3O4 and the TiO2 films. Afterward, we reported it mainly on the fabrication of various metal oxides such as SrTiO3, Cu2O, SiO2, ZnO, and the apatite films. In addition, the embedding of the Cu metal in the trench also is successfully fabricated by this method. It is an interdisciplinary domain including coordination chemistry, materials science, nanoscience, and the nanotechnology. It is important that the process of this method is resource saving, energy saving. This method will be proposed for supersmart society and will develop the film manufacture and the application for more effective device manufacture in future. Moreover, this method might provide the various films with high quality and be continued for sustainable society.

References

1. Gregory Jackson, W., McKeon, J.A., Cortez, S., Alfred Werner’s inorganic counterparts of racemic and mesomeric tartaric acid: A milestone revisited. Inorg. Chem., 43, 6249, 2004.

2. Bowman-James, K., Alfred Werner revisited: The coordination chemistry of anions. Acc. Chem. Res., 38, 671, 2005.

3. Pearson, R., Hard and soft acids and bases. J. Am. Chem. Soc., 85, 3533, 1963.

4. Nagai, H., Sato, M., Molecular Precursor Method for Fabricating p-Type Cu2O and Metallic Cu Thin Films, Modern Technologies for Creating the Thin-film Systems and Coatings, N.N. Nikitenkov (Ed.), INTECH, 1, 3, 2017.

5. Nagai, H., Sato, M., Highly Functionalized Lithium-Ion Battery, Alkali-ion Batteries, D. Yang (Ed.), INTECH, 6, 111, 2016.

6. Nagai, H., Sato, M., Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films, Heat Treatment - Conventional and Novel Applications, F. Czerwinski (Ed.), INTECH, 13, 297, 2012.

7. Hirota, M., Mochizuki, C., Sato, M., Hayakawa, T., Influence of heating conditions for formation of a thin apatite film on zirconia using molecular precursor method. Coatings, 7, 69, 2017.

8. Taka, D., Onuma, T., Shibukawa, T., Nagai, H., Yamaguchi, T., Jang, J-.S., Sato, M., Honda, T., Fabrication of Ag dispersed ZnO films by molecular precursor method and application in GaInN blue LED. Phys. Stat. Sol., 214, 1600598, 2016.

9. Nagai, H., Suzuki, T., Nakano, T., Sato, M., Embedding of copper into submicrometer trenches in a silicon substrate using the molecular precursor solutions with copper nano-powder. Mater. Lett., 182, 206, 2016.

10. Seino, H., Kondo, T., Mochizuki, C., Tokunaga, C., Yamaguchi, M., Sato, M., Structural determination, DFT calculation, and formation mechanism of ethyl 2-cyano-3-alkoxypent-2-enoates synthesized via Ru-mediated coupling reaction between α,β-unsaturated acetals and cyanoacetate. Bull. Chem. Soc. Jap., 90, 79, 2017.

11. Nagai, H., Suzuki, T., Takahashi, Y., Sato, M., Photovoltaic lithium-ion battery fabricated by molecular precursor method. Funct. Mater. Lett., 9, 1650046, 2016

12. Hirota, M., Shima, T., Sato, I., Ozawa, T., Iwai, T., Ametani, A., Sato, M., Noishiki, Y., Ogawa, T., Hayakawa, T., Tohnai, I., Development of a biointegrated mandibular reconstruction device consisting of bone compatible titanium fiber mesh scaffold. Biomaterials, 75, 223, 2016.

13. Nagai, H., Mita, S., Takano, I., Honda, T., Sato, M., Conductive and semi-transparent Cu thin film fabricated using molecular precursor solutions. Mater. Lett., 141, 235, 2015.

14. Mochizuki, C., Hara, H., Oya, K., Aoki, S., Hayakawa, T., Fujie, H., Sato, M., Behaviors of MC3T3-E1 cells on carbonated apatite films, with a characteristic network structure, fabricated on a titanium plate by aqueous spray coating. Mater. Sci. Eng. C, 39, 245, 2014.

15. Nagai, H., Suzuki, T., Mochizuki, C., Takano, I., Honda, T., Sato, M., Formation mechanism of p-type Cu2O thin films via intermediate Cu0 species derived from Cu(II) complex of ethylenediamine-N,N,N’N’-tetraacetic acid. Sci. Adv. Mater., 6, 603, 2014.

16. Onuma, T., Yasuno, T., Takano, S., Goto, R., Fujioka, S., Hatakeyama, T., Oda, T., Hara, H., Mochizuki, C., Nagai, H., Yamaguchi, T., Sato, M., Honda, T., Cathodoluminescence spectra of Ga-In-O polycrystalline films fabricated by molecular precursor method. J. J. Appl. Phys., 53, 05FF02, 2014.

17. Kaneko, H., Sasaki, H., Honma, S., Hayakawa, T., Sato, M., Yajima, Y., Yoshinari, M., Influence of thin carbonate-containing apatite coating with molecular precursor method to zirconia on osteoblast-like cell response. Dent. Mater. J., 33, 39, 2014.

18. Hirota, M., Hayakawa, T., Ohkubo, C., Sato, M., Hara, H., Toyama, T., Tanaka, Y., Bone responses to zirconia implants with a thin carbonate-containing hydroxyapatite coating using a molecular precursor method. J. Biomedi. Mater. Res. B: Appl. Biomater., 102, 1277, 2014.

19. Daniel, L.S., Nagai, H., Sato, M., Absorption spectra and photocurrent densities of Ag nanoparticle/TiO2 composite thin films with various amounts of Ag. J. Mater. Sci., 48, 7162, 2013.

20. Daniel, L.S., Nagai, H., Yoshida, N., Sato, M., Photocatalytic activity of vis-responsive Ag-nanoparticles/TiO2 composite thin films fabricated by molecular precursor method (MPM). Catalysts, 3, 625, 2013.

21. Ueno, D., Sato, M., Hayakawa, T., Guided bone regeneration using hydroxyapatite-coated titanium fiber web in rabbit mandible: Use of molecular precursor method. J. Hard Tissue Biol., 22, 329, 2013.

22. Nagai, H., Hara, H., Enomoto, M., Mochizuki, C., Honda, T., Takano, I., Sato, M., Synchronous electrochromism of lithium ion battery with chemically fabricated transparent thin films. Funct. Mater. Lett., 6, 1341001, 2013.

23. Mochizuki, C., Hara, H., Takano, I., Hayakawa, T., Sato, M., Application of carbonated apatite coating on a Ti substrate by aqueous spray method. Mater. Sci. Eng. C, 33, 951, 2013.

24. Hayakawa, T., Mochizuki, C., Hara, H., Amemiya, T., Hirayama, S., Yang, F., Shen, H., Wang, S., Hamada, Y., Sato, M, Cortical bone response towards porous composites of PLGA and apatite prepared from calcium complexes. J. Hard Tissue Biol., 21, 345, 2012.

25. Hirota, M., Hayakawa, T., Yoshinari, M., Ametani, A., Shima, T., Monden, Y., Ozawa, T., Sato, M., Koyama, C., Tamai, N., Iwai, T., Tohnai, I., Hydroxyapatite coating for titanium fibre mesh scaffold enhances osteoblast activity and bone tissue formation. Int. J. Oral Maxillofac. Surg., 41, 1304, 2012.

26. Nagai, H., Suzuki, T., Hara, H., Mochizuki, C., Takano, I., Honda, T., Sato, M., Chemical fabrication of p-type Cu2O transparent thin film using molecular precursor method. Mater. Chem. Phys., 137, 252, 2012.

27. Likius, D.S., Nagai, H., Aoyama, S., Mochizuki, C., Hara, H., Baba, N., Sato, M., Percolation threshold for electrical resistivity of Ag nanoparticle/titania composite thin films fabricated using molecular precursor method. J. Mater. Sci., 47, 3890, 2012.

28. Monden, Y., Hirota, M., Hayakawa, T., Sato, M., Murata, S., Sato, Y., Maegawa, J., Tohnai, I., Thin hydroxyapatite coating on porous beta-tricalcium phosphate (β-TCP) enhances osteoblast function activity. J. Hard Tissue Biol., 21, 9, 2012.

29. Shukur, H.A., Sato, M., Nakamura, I., Takano, I, Characteristics and photocatalytic properties of TiO2 thin film prepared by sputter deposition and post-N+ ion implantation. Adv. Mater. Sci. Eng., 2012, 1, 2012

30. Zheng, L., Yang, F., Shen, H., Hu, X., Mochizuki, C., Sato, M., Wang, S., Zhang, Y., The effect of composition of calcium phosphate composite scaffolds on the formation of tooth tissue from human dental pulp stem cells, Biomaterials, 32, 7053, 2011.

31. Hirota, M., Hayakawa, T., Ametani, A., Kuboki, Y., Sato, M., Tohnai, I., The effect of hydroxyapatite-coated titanium fiber web on human osteoblast functional activity. Int. J. Oral Maxillofac Implants, 26, 245, 2011.

32. Honda, T., Oda, T., Mashiyama, Y., Hara, H., Sato, M., Fabrication of c-axis oriented Ga-doped MgZnO-based UV transparent electrodes by molecular precursor method. Phys. Status Solidi. C, 7, 2471, 2010.

33. Nagai, H., Aoyama, S., Hara, H., Mochizuki, C., Takano, I., Honda, T., Sato, M., Photoluminescence and photoreactivity affected by oxygen defects in crystal-oriented rutile thin film fabricated by molecular precursor method. J. Mater. Sci., 45, 5704, 2010.

34. Hayakawa, T., Mochizuki, C., Hara, H., Yang, F., Shen, H., Wang, S., Sato, M., In vivo evaluation of composites of PLGA and apatite with two different levels of crystallinity. J. Mater. Sci: Mater. Med., 21, 251, 2010.

35. Furuya, N., Hayakawa, T., Yamanishi, Y., Fukushima, T., Yoshinari, M., Sato, M., Effect of bisphosphonate immobilization of apatite coated titanium web on trabecular bone response. J. Oral. Tissue. Engin., 7, 73, 2009.

36. Mochizuki, C., Sasaki,