Gas Chromatography — an overview, ScienceDirect Topics

Gas Chromatography

Gas chromatography (GC) is the technique of preference for separation of volatile polyphenols like hydroxycinnamic-acid-derived flavors (Thurston and Tubb, 1981; Callemien et al., 2006).

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Controlled Atmosphere Storage Gas Chromatography

Gas chromatography is used to separate and measure various types of gases ( Fig. 13.19 ). It is a sensitive technique, can analyze small samples, and can be automated, but is also relatively expensive and requires technical knowledge. The analysis of gases such as ethylene at low concentrations ( Fig. 13.20 ).

Fig. 13.19 . Gas chromatograph used for controlled atmosphere rooms.

Fig. 13.20 . Controlled atmosphere (CA) laboratory cabinets preassembled inside a 40′ container.

Electrochemical sensing and optical sensing instruments for ethylene measurement also exist, but they are rarely used in commercial CA storage applications due to being less robust or more expensive than gas chromatography.

Carbohydrate Reactions

Use of alditols in carbohydrate analysis

Gas-liquid chromatography (GLC) is often used to determine monosaccharides—both qualitatively and quantitatively. To determine the monosaccharides present in a food product, they are first extracted (with water) from the product. To determine the monosaccharide composition of an oligo- or polysaccharide, the saccharide is hydrolyzed with aqueous acid to release the monosaccharides. The monosaccharides in the mixture of sugars are then reduced to alditols using sodium or potassium borohydride. The next step is complete acetylation of the resulting mixture of alditols with acetic anhydride to make what are called peracetate esters of the alditols. The alditol peracetates are volatile enough, that mixtures of them can be analyzed by gas-liquid chromatography (GLC). (Unsubstituted carbohydrates are nonvolatile.) This process is discussed more thoroughly in the section on Esters in this chapter.

Drinking Water Detection

10.2.5 Gas Chromatography

Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, or separating the different components of a mixture. In some situations, GC may help in identifying a compound. In preparative chromatography, GC can be used to prepare pure compounds from a mixture ( Pavia, 2005 ).

In gas chromatography, the mobile phase is a carrier gas, usually an inert gas such as helium or an unreactive gas such as nitrogen. Helium remains the most commonly used carrier gas in about 90% of instruments although hydrogen is preferred for improved separations. The stationary phase is a microscopic layer of liquid or polymer on an inert solid support, inside a piece of glass or metal tubing called a column. The instrument used to perform gas chromatography is called a gas chromatograph.

The gaseous compounds being analyzed interact with the walls of the column, which is coated with a stationary phase. This causes each compound to elute at a different time, known as the retention time of the compound. The comparison of retention times is what gives GC its analytical usefulness.

Gas chromatography is in principle similar to column chromatography, but has several notable differences. First, the process of separating the compounds in a mixture is carried out between a liquid stationary phase and a gas mobile phase, whereas in column chromatography the stationary phase is a solid and the mobile phase is a liquid. Second, the column through which the gas phase passes is located in an oven where the temperature of the gas can be controlled, whereas column chromatography has no such temperature control. Finally, the concentration of a compound in the gas phase is solely a function of the vapor pressure of the gas. Gas chromatography is also similar to fractional distillation, since both processes separate the components of a mixture primarily based on boiling point differences. However, fractional distillation is typically used to separate components of a mixture on a large scale, whereas GC can be used on a much smaller scale. Gas chromatography is also sometimes known as vapor-phase chromatography (VPC), or gas-liquid partition chromatography (GLPC). These alternative names, as well as their respective abbreviations, are frequently used in the scientific literature. Strictly speaking, GLPC is the most correct terminology and is thus preferred by many authors ( Pavia, 2005 ).

Alkaloid Chemistry Gas chromatography

Gas chromatography is a similar method to that of HPLC. It provides a quick and easy way of determining alkaloids in a mixture. The only requirement is some degree of stability at the temperature necessary to maintain the substance in the gas state 290 .

Gas chromatography is divided into two subclasses, according to the nature of the stationary phase. One of this in GSC (gas–solid chromatography). The fixed phase consists of a solid material such as granual silica, alumina or carbon. Gas–solid chromatography is an important method in the separation of permanent gases and low-boiling hydrocarbons 290 . The second subclass, more important for lupine alkaloid analysis, is gas–liquid chromatography (GLC). A gas chromatograph is needed for the analysis. Basically, a gas chromatograph consists of six parts as follows: (1) a supply of carrier gas in a high-pressure cylinder with attendant pressure regulators and flow meters, (2) a similar injection system, (3) the separation column, (4) detectors, (5) an electrometer and strip-chart recorder (integrator), and (6) separate thermostated compartments for housing the columns and the detector so as to regulate their temperature. Helium is the preferred carrier gas 287 . For alkaloid analysis, a nitrogen detector is needed.

Gas–liquid chromatography is a qualitative, but also quantitative, method of alkaloid analysis. It is very sensitive. The only problem concerns the distribution of the alkaloid mixture in the chromatographic process and the identification of alkaloids, which must be achieved by a different technique 120, 121 . A very positive characteristic is the possibility of totally computerizing this method of alkaloid detection.

Groundwater Quality Detection

11.3.2 Gas Chromatography

Gas chromatography is a chromatographic detection method for gas as a mobile phase. As early as the last century, about 40 years ago, the British biologist Martin and others proposed the use of gas chromatography through a variety of scientific research experiments ( Bengtsson and Annadotter, 1989 ; Potter and Pawliszyn, 1992 ; Thurman et al., 1990 ). Gas chromatography has now become an indispensable analytical technique in the organic analytical industry. The general workflow is that the treated groundwater samples are transported from the injection unit to the gasification chamber and then the gasified samples with mobile phase go into the chromatogram column system. Because of the gas chromatographic column temperature that will change and the carrier gas flow that is not fixed, the adsorption and distribution between the stationary phase and samples are in a state of dynamic equilibrium. And the specificity of the material that is not significantly different from the macroscopic difference is more pronounced, which promotes the separation of materials. Gas chromatography is mainly applied to the determination of organic or inorganic substances with strong thermal stability and relatively easy gasification ( Mestres et al., 1987 ).

Organochlorine pesticides (OCPs) are an organic compound with highly toxic, refractory and easily residual characteristics, They were used in large quantities as pesticides in China’s agricultural production process, resulting in large amounts of soil residues and groundwater permeation from irrigation. Most OCPs have been listed as the first polluters to be controlled and reduced in the Stockholm Convention on Persistent Organic Pollutants, such as DDT, chlordane, aldrin, dieldrin, endrin, heptachlor, and hexachlorobenzene. They can be ingested through the food chain, which causes great harm to the environment and human health. Due to the serious shortage of per capita water resources in China, plus the surface water pollution problem, many areas can only use groundwater as the major water resource. Gas chromatography has thus been applied to groundwater OCP monitoring, a necessary step. Depending on the physicochemical characteristics of the pollutants, Estévez et al. (2012) employed gas chromatography methods for sample analysis. The results show that sorption and degradation processes in soil account for more compounds being detected in reclaimed water than in groundwater. With chemical analyses of gas chromatography-mass spectrometry (GC-MS) and headspace-gas chromatography (HS-GC), Hildenbrand et al. (2016) found that some constituents remained stable over time but others experienced significant variation over the period of study. Notable findings include significant changes in total organic carbon and pH along with ephemeral detections of ethanol, bromide, and dichloromethane after the initial sampling phase. Cabeza et al. (2012) used analytical methods of gas chromatography-mass spectrometry/mass spectrometry and gas chromatography-high-resolution mass spectrometry; the result showed that a number of chemicals, namely 10 pesticides and 10 pharmaceuticals, were only present in groundwater samples, confirming a different origin than the injected treated wastewater (TWW). In addition, in the literature ( Bono-Blay et al., 2012 ), with SPE and GC-MS, the 21 target compounds were satisfactorily recovered (77%–124%) and limits of quantification were between 0.0004 and 0.029 μg/L for pesticides. Shahsavari et al. (2012) described how gas chromatography-flame detection technology was used to analyze pesticide concentration in groundwater, and the results displayed that hydrogeology and anthropogenic activities have an impact on the groundwater quality. Silva et al. (2012) used gas chromatography-mass spectrometry (GC-MS) to determine 14 herbicides, 4 insecticides, and 2 metabolites. Mochalski et al. (2006) described the application of gas chromatography to the simultaneous determination of Ne, Ar, and N in groundwater. In addition, Liang et al. (2012) developed fingerprint identification of volatile organic compounds in gasoline-contaminated groundwater using gas chromatography differential ion mobility spectrometry.

On the whole, gas chromatography provides the advantages of high sensitivity, high accuracy, and precision, in addition to simple operation, and it is suitable for batch analysis. However, the key to whether the components can be separated is the chromatographic column. Whether the components can be identified after separation is based on the detector, so the separation system and the detection system are the core of the instrument.

Gas Chromatography and Gas Chromatography—Mass Spectrometry

Eric Stauffer, . Reta Newman, in Fire Debris Analysis , 2008

8.3.1 Principle

Gas chromatography (GC) is a separation technique capable of separating highly complex mixtures based primarily upon differences of boiling point/ vapor pressure and of polarity. Even though chromatography was invented at the beginning of the twentieth century and Martin and Synge did not see any reason why the mobile phase should not be a gas in their 1941 publication, GC was not developed until 1952 [ 16 ]. That year, James and Martin published the first article demonstrating the use of GC to separate volatile fatty acids [ 17 ]. Gas chromatography—also referred to as gas-liquid chromatography (GLC)—is a specific type of chromatography that utilizes an inert gaseous mobile phase and a liquid stationary phase. 3 Instrumentation continues to improve, but the basics of a gas chromatograph—the instrument used to perform GC that bears the same abbreviation—have not changed and remain fairly simple. The GC is composed of several basic components as shown in Figure 8-7 . A photograph of a modern GC is shown in Figure 8-8 .

FIGURE 8-7 . A gas chromatograph and its main components.

FIGURE 8-8 . An Agilent 6890N-5975 gas chromatograph-mass spectrometer. An Agilent 7683B autosampler is present above the GC on the left.

A GC operates by introducing a sample via an injection port into the inlet (also called injector). Carrier gas, which is the mobile phase, passes through the inlet, and sweeps the sample onto the column, where the stationary phase is. The column is enclosed in a temperature-controlled oven. The chromatographic separation takes place as the mixture travels through the column. When the separated components of the sample exit the column, they enter a detector, which provides an electronic signal proportional to the amount of eluting analytes. Each constituent is described in more detail in the following subsections.

Six-membered Rings with Three or more Heteroatoms, and their Fused Carbocyclic Derivatives Chromatographic Behavior

Gas chromatography –orthogonal acceleration time-of-flight mass spectrometry (GC–oaTOFMS) facilitated the identification of 1,3,5-dithiazine 33 from extracts of marine natural products . Similarly, GC coupled to MS allowed identification of dihydro-2,4,6-triethyl-(4H)-1,3,5-dithiazine 34 as one main component of seed oil in Azaridachta indica . By the same token, GC–MS techniques made possible the identification of each isomer of 1,3,5-dithiazine 35 (thialdine) .

Sol Gel Process

The sol–gel process is a wet chemical technique also known as chemical solution deposition, and involves several steps, in the following chronological order: hydrolysis and polycondensation, gelation, aging, drying, densification, and crystallization.

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Novel approaches for preparation of nanoparticles

Bolla G. Rao, . Benjaram M. Reddy, in Nanostructures for Novel Therapy , 2017

2.2 Sol–Gel Method

Sol–gel method is one of the well-established synthetic approaches to prepare novel metal oxide NPs as well as mixed oxide composites. This method has potential control over the textural and surface properties of the materials. Sol–gel method mainly undergoes in few steps to deliver the final metal oxide protocols and those are hydrolysis, condensation, and drying process. The formation of metal oxide involves different consecutive steps, initially the corresponding metal precursor undergoes rapid hydrolysis to produce the metal hydroxide solution, followed by immediate condensation which leads to the formation of three-dimensional gels. Afterward, obtained gel is subjected to drying process, and the resulting product is readily converted to Xerogel or Aerogel based on the mode of drying. Sol–gel method can be classified into two routes, such as aqueous sol–gel and nonaqueous sol–gel method depending on the nature of the solvent utilized.

If water is used as reaction medium it is known as aqueous sol–gel method; and use of organic solvent as reaction medium for sol–gel process is termed as nonaqueous sol–gel route. The reaction pathway for the production of metal oxide nanostructures in the sol–gel method is shown in Fig. 1.5 . In the sol–gel approach, nature of metal precursor and solvent plays a remarkable role in the synthesis of metal oxides NPs.

Figure 1.5 . The Reaction Pathway for the Production of Metal Oxide Nanostructures in the Sol–Gel Method

2.2.1 Aqueous sol–gel method

In aqueous sol–gel method, oxygen is necessary for the formation of metal oxide, which is supplied by the water solvent. Generally, metal acetates, nitrates, sulfates, chlorides, and metal alkoxides are employed as the metal precursors for this method. However, metal alkoxides are widely used as the precursors for the production of metal oxide NPs, due to high reaction affinity of alkoxides toward water ( Bradley et al., 2001; Turova and Turevskaya, 2002 ). However, some difficulties are associated with the aqueous sol–gel method. The key steps, such as hydrolysis, condensation, and drying take place simultaneously in a number of cases resulting in difficulty in controlling particle morphology, and reproducibility of the final protocol during the sol–gel process ( Corriu and Leclercq, 1996 ). The aforementioned difficulties, however, do not affect much of the synthesis of metal oxides in bulk, but strongly affect the preparation of nanooxides. Therefore, it is believed that the aqueous sol–gel route is highly recommended for the synthesis of bulk metal oxides rather than their nanoscale counterparts ( Niederberger, 2007 ).

2.2.2 Nonaqueous sol–gel method

Nonaqueous or nonhydrolytic sol–gel method is devoid of some of the major drawbacks found in aqueous sol–gel method. In nonaqueous sol–gel process, oxygen required for the formation of metal oxide is supplied from the solvents, such as alcohols, ketones, aldehydes, or by the metal precursors. Furthermore, these organic solvents not only serve as oxygen providers but also offer a versatile tool for tuning several key components like morphology, surface properties, particle size, and composition of the final oxide material. Although, nonaqueous sol–gel approach is not as popular as aqueous sol–gel method; nonaqueous sol–gel routes have shown excellent impact on the production of nanooxides compared to that of aqueous sol–gel route. The nonaqueous sol–gel route can be divided into two important methodologies, namely, surfactant-controlled and solvent-controlled approaches for the production of metal oxide NPs. Surfactant-controlled strategy involves direct transformation of metal precursor into the respective metal oxide at higher temperature range in hot injection method. This method permits outstanding control over the shape, growth of the NP, and avoids the agglomeration of particles. Few examples of surfactant-controlled synthesized NPs are mentioned here for understanding. Song and Zhang (2004) have demonstrated the simple nonhydrolytic route to synthesize high-quality spherical-shaped CoFe2O4 NPs with 8-nm size. However, the spherical morphology can be changed to cubic shape with 10-nm edge length during the seed-mediated growth. Heating rate and growth temperature played a pivotal role in controlling the shape of CoFe2O4 nanomaterial ( Fig. 1.6 A–B).

Figure 1.6 . TEM images of (A) 8-nm sized spherical CoFe2O4 NPs and (B) cube-like CoFe2O4 NPs. TEM images of (C) cube-like and (D) polyhedron-shaped MnFe2O4NPs. (E) TEM image of MnO multipods (inset, hexapod). (F) TEM image of Tungsten oxide nanorods.

Part (A–B): Reproduced from Song, Q., Zhang, Z.J., 2004. Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals. J. Am. Chem. Soc. 126, 6164–6168. Copyright 2004, American Chemical Society. Part (C–D): Reprinted from Zeng, H., Rice, P.M., Wang, S.X., Sun, S.H., 2004. Shape-controlled synthesis and shape-induced texture of MnFe2O4 nanoparticles. J. Am. Chem. Soc. 126, 11458–11459. Copyright 2004, American Chemical Society. Part (E): Reproduced from Zitoun, D., Pinna, N., Frolet, N., Belin, C., 2005. Single crystal manganese oxide multipods by oriented attachment. J. Am. Chem. Soc. 127, 15034–15035. Copyright 2005, American Chemical Society. Part (F): Reproduced from Seo, J.-W., Jun, Y.-W., Ko, S.J., Cheon, J., 2005. In situ one-pot synthesis of 1-dimensional transition metal oxide nanocrystals. J. Phys. Chem. B. 109, 5389–5391. Copyright 2005, American Chemical Society.

The resulting materials were subjected to shape-dependent magnetic properties. Zeng et al. (2004) have extensively studied the shape-controlled synthesis of MnFe2O4 nanomaterial. The relative ratio between surfactant and Fe(acac)3 showed a remarkable role in controlling the final morphology of MnFe2O4. TEM analysis revealed the formation of cube-like or polyhedron-type morphology for MnFe2O4 ( Fig. 1.6 C–D). In addition, size of MnFe2O4 particle is dependent on the concentration of metal precursors. Novel cone-shaped ZnO was obtained by decomposition of TOPO–Zn(OAc)2 complex resulting in the formation of hierarchically ordered spheres of cone-shaped ZnO nanocrystals ( Joo et al., 2005 ). Li et al. (2006) fabricated titanium oxide nanorods with 3.3-nm diameter and a length of 25 nm using appropriate amounts of reaction ingredients, such as titanium butoxide, triethylamine, linoleic acid, and cyclohexane. Reaction temperature, time, and concentration of the reactant were found to show huge effect on the shape and size of the TiONPs. Preparation of high-quality single crystalline MnO multipods with homogeneous size and shape, involved decomposition of Mn(oleate)2 in the presence of oleic acid and n-trioctylamine ( Fig. 1.6 E) ( Zitoun et al., 2005 ). Tungsten oxide nanorods were generated by treatment of WCl4 with oleylamine and oleic acid ( Fig. 1.6 F) ( Seo et al., 2005 ). Solvent-controlled sol–gel route, involves the reaction between metal halide and alcohols to produce metal oxide nanostructures. For example, porous SnO2 NPs were prepared by the addition of tin chloride to benzylalcohol under stirring condition, which was immediately dispersed in THF solution, producing sol. The subsequent addition of block polymer to sol allowed mesoporous nanostructure for SnO2 by the elimination of solvent molecule ( Ba et al., 2005 ).

Preparation of Catalysts VII


Sol-gel process provides a new approach to the preparation of new materials. This process allows a better control of the whole reactions involved during the synthesis of solids. Homogenous multi-component systems can be easily obtained, particulary homogenous mixed oxides can be prepared by mixing the molecular precursors solutions ( 1 ).

The chemistry of the sol-gel process is based on hydrolysis and polycondensation reactions. Metal alcoxides [M(OR)3] are versatile molecular used to obtain oxides, on account of their ability to form homogeneous solution in large variety of solvents and in the presence of other alcoxides or metallic derivatives and also for their reactivity toward nucleophilic reagents such as water ( 2 ).

The essential aim for the most investigations reported was the final product and its applications without interested to the synthesis conditions and reactionnel mechanism involved to obtain gels, although the properties of a gel and its behaviour on heat treatment could be very sensitive to the structure already created during the sol stage. Therefore, the formation of colloidal aggregates determines the main properties of resulting powder. By varing the chemical conditions under which materials is polymerized, the structure and the morphology of samples formed are drastically affected. However, many earlier investigations try to developp a good understanding of gelification phenomenon.The purpose of this work is to better understand the chemistry involved during the preparation of mixed oxides by Sol-gel process, in order to have a good control of the properties of the final material.

Ceramics and Glasses, Sol–Gel Synthesis of

The sol–gel process represents an elegant chemical route that can be used for the low-temperature synthesis of single- or multiple-component ceramic materials (including glasses) in the form of thin solid films, ultrafine powders, high surface area porous materials, dense abrasive minerals, and continuous ceramic and glass fibers. The sol–gel process involves the preparation of a precursor mix (a sol or a solution), which is ultimately converted into a final product via a stage that may involve drying, chemical reactions, gelation of the precursors, and curing (often using heat). The major advantages associated with the sol–gel processes include lower processing temperatures, high levels of purity, control of dopant concentrations, and the ability to synthesize multicomponent compositions in different product forms.

Preparation of Catalysts VII


The sol–gel method has been explored to prepare a catalytic membrane. Photocorrelation spectroscopy, nitrogen adsorption, scanning electron microscopy (SEM), scanning electron microscopy wave dispersive X-ray analysis (SEM-WDX), and gas permeation measurement were used to characterize the preparation of the catalytic membrane. In the sol–gel process, the noble metal ion-modified boehmite sols were produced by adsorption of the noble metal complexes at the liquid/solid interface of the boehmite sol particles. The thickness of the catalytic membrane was increased by a multiple sol–gel process. It was confirmed that the distribution of the catalytst precursors exhibited a uniformity in the direction of the thickness of as well as along the surface of the catalytic membrane. The gas permeation measurement showed that the catalytic membrane prepared had a good thermostability.

Sol–Gel Process and Applications

2.3 Reaction for Nonsilica Oxides

Sol–gel method is useful for fabrication of functional materials, such as photocatalyst, nonlinear optical materials, ferroelectrics, and superconductors. For this purpose, sol–gel preparation of simple and complex nonsilica oxides, including TiO2, ZrO2, Al2O3, ZnO, WO3, Nb2O5, rare earth oxides, and so on, have to be studied. Metal alkoxides are often employed for those oxides. Most of them, however, are unstable, that is, they are rapidly hydrolyzed and are easily precipitated, which makes it difficult to form homogeneous multicomponent oxide products. Methods of controlling reactivity of the transition metal alkoxides are introduced below.

Preparation of Catalysts VII


Sol–gel methods have been used by different authors working on catalysts preparation ( 1 ). The advantages of the applications of these methods to the design of catalytic materials have been already described in the literature ( 1 , 2 ).

Catalysts based on supported vanadium oxide are frequently used in selective oxidation reactions due to their considerable activity for the oxidation of aromatics, alkanes, and alcohols ( 3–5 ). Differences in their catalytic behaviour are generally explaines on the basis of the nature and distribution of vanadium species, which are influenced by the vanadium loading, the preparation procedure and the acid–base character of the support ( 6–9 ).

The interaction of vanadia with silica and the structure of the VOx units on the support have been studied by a number of investigators ( 5 , 7 , 9–17 ). It has been observed that in general highly dispersed monomeric surface vanadia species were found on silica-supported catalyst with very low loadings of vanadia ( 7 , 12–13 ). Crystalline V2O5 has been detected on catalysts well bellow the so-called “monolayer” coverage as the vanadium loading was increased . The formation of crystalline V2O5 in the V2O5/SiO2 system reflects the weaker interaction of vanadium oxide with the SiO2 supports relative to others like Al2O3 and TiO2 ( 5 , 14 , 18 ).

To obtain a catalyst with V-SiO2 interactions stronger than those developed in impregnations solids we have studied the preparation of these materials by means of the sol–gel process.

Neuman et al. have previously prepared amorphous metallosilicalite xerogels by the sol–gel method using metal alkoxides precursors ( 4 ). This work have shown that vanadium silicate xerogel are active catalysts for the activation of aqueous hydrogen peroxide for a variety of reactions including epoxidation of alquenes, oxidation of secondary alcohols to ketones and the hydroxylation of phenol.

In the present work we report the obtention of VOx-SiO2 catalyst from vanadium acetyl acetonate and tetraethoxysilane. These catalysts were characterized and evaluated in the oxidative dehydrogenation of n-butane.

Polymer–Ceramic Nanocomposites: Ceramic Phases

The sol–gel process has been extensively studied and employed as the most important route in forming new hybrid inorganic–organic materials ( Mark 1996 , Mark et al. 1995 , Sharp 1998 , Sanchez and Ribert 1994 , Novak 1993 , Chujo and Saegusa 1992 , Schmidt 1994 , Schaefer and Mark 1990 ). This is because this process can introduce ceramic phases into composites through chemistry. Very mild reaction conditions and low reaction temperatures are particularly useful for incorporating inorganic moieties into organic materials or organic materials into inorganic matrices. Thus, the organic polymers involved can survive during the sol–gel process that eliminates sintering temperatures used in the preparation of typical inorganic ceramics (>1200°C).

Sol-Gel Synthesis of Metal Nanoparticle Incorporated Oxide Films on Glass

6.3.5 Inorganic-Organic Hybrid Hosts

Sol-gel process is also used for synthesis of metal NPs doped inorganic-organic hybrid films such as organically modified silica (ORMOSIL). In this case, organically modified metal alkoxides (e.g., 3-(glycidoxypropyl) trimethoxysilane, GLYMO) are co-hydrolyzed with different alkoxides. Later on, UV or thermal curing induces organic polymerization leading to the formation of ORMOSILs. These hybrid materials exhibit composite hybrid microstructures and can also be easily molded as unsupported thin transparent films [ 80–82 ]. The curing temperature applied during post film fabrication should not be high to maintain the hybrid inorganic-organic structure. The curing technique can have significant role in governing the size of metal NPs inside host matrices. Figure 6.9 shows one such case in which Au NPs were synthesized inside SiO2-TiO2-polyethyleneoxide hybrid films coated on extra dense flint glass substrates by using UV light of different energies [ 28 ]. The refractive index of the matrix can be increased by increasing the UV curing energy. A gradual shifting of Au-SPR ( Figure 6.9a ) was explained due to the increase of refractive index of the matrix and plasmon coupling of the densely populated Au NPs as observed in the two representative TEM images ( Figure 6.9b and c ) [ 28 ].

Figure 6.9 . (a) Absorption spectra of Au NPs doped hybrid SiO2-TiO2-polyethylene oxide films coated on extra dense flint glass substrates after UV curing with different energies. (b) and (c) Representative TEM images of the films cured with UV energy 5.3 ± 0.1 J cm − 2 (film color: red) and 53.0 ± 0.1 J cm − 2 (film color: purple), respectively. Histograms of Au NPs embedded in the films are presented in the respective insets of (b) and (c). In (c) a marked area (A) is highlighted to show the growth of few larger particles through coalescence of smaller ones.

Reproduced with permission from Ref. [ 28 ], Copyright © American Chemical Society.

Corrosion Protective Coatings for Ti and Ti Alloys Used for Biomedical Implants

17.3 Sol-Gel Method

Sol-gel process is a method for producing solid materials from small molecules that is suitable for preparing different coatings (e.g., silicium and titanium oxides) on the surface of Ti-based materials. It involves conversion of small molecules (precursors) into a colloidal solution (sol) and then into an integrated network (gel) consisting of either discrete particles or network polymers.

The main advantages of the sol-gel method are 18 : (1) low processing temperature (avoiding volatilization of entrapped species), (2) the possibility to cast coatings in complex shapes, and (3) the use of compounds that do not introduce impurities into the end product. Among the disadvantages of this method one can count the relatively long period of time for processing flow and the difficulties related to phase separation occurring especially in hybrid coating synthesis.

The sol-gel process involves four stages: (1) hydrolysis, (2) condensation/polymerization of monomers, (3) growth of particles, and (4) gel formation. 18 These processes are influenced by several experimental parameters such as pH, temperature, concentration of the reactants, and presence of additives.

Traditional precursors for sol-gel coatings are alkoxysilanes such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS), but efforts are made in order to find less toxic and more environmental friendly precursors.

TiO2 can be prepared, for example, starting from precursor solutions containing an alkoxide (e.g., tetrabutylorthotitanate) and diethanolamine dissolved in ethanol 16 and mixing them with water (with or without additives) in a certain ratio.

SiO2 coatings can be prepared starting from a precursor solution consisting of tetraethylorthosilicate Si(OC2H5)4, H2O, C2H5OH, and HCl, mixed at a certain molar ratio. 19 Different compounds (inhibitors, pigments, etc.) can be added in order to improve the physicochemical properties of the coating.

Irrespective of the nature of the film, the two main techniques used to apply a sol-gel coating on the surface of a metallic substrate are dip-coating and spin-coating.

17.3.1 Dip-coating

By this technique, the material from which the film is produced is put into solution, and then the substrate is progressively dipped into and is extracted from the solution at a controlled rate ( Figure 17.1 ). After the solvent evaporates, a thin and homogeneous film is produced. The thickness of deposited liquid film coatings depends on the coating solution properties such as density, viscosity, and surface tension, as well as surface withdrawal speed from the coating solution. The thickness of the film is generally bigger than that prepared by spin-coating with the same solutions.

Figure 17.1 . Schematics of a dip-coating process. 20

Dip-coating was successfully used, for example, to prepare sol-gel-derived Al2O3 films on γ-TiAl-based alloys, 21 porous TiO2 films, 16 hydroxyapatite (HA) coatings, 22 and SrO-SiO2-TiO2 on NiTi, 23 and so on.

17.3.2 Spin-coating

In the case of spin-coating, an amount of solution is placed on the substrate that is rotated at high speed in order to spread the fluid by centrifugal force ( Figure 17.2 ). After the evaporation of the solvent, a thin, homogeneous film is formed. As in the case of dip-coating, final film thickness and other properties will depend on the nature of the sol-gel coating (viscosity, drying rate, surface tension, etc.) and on the parameters chosen for the spin process. Higher spin speeds and longer spin times give birth to thinner films. 25 Generally, a moderate spinning speed is recommended. The drying rate should also be slow in order to ensure film uniformity. A supplementary drying step is sometimes necessary after the high-speed spin step to further dry the film without substantially thinning it.

Figure 17.2 . Schematics of a spin-coating process. 24

By using spin-coating, SiO2 films were deposited on Ti-48Al alloy, 19 sphene (CaTiSiO3) ceramics on Ti-6Al-4V 26 alkoxide-based HA nanocoatings on Ti and Ti-6Al-4V substrates, 27 HA 28 and HA/polymer coatings on Ti, 29 TiO2 on Ti-6Al-4V, 30 and many others.

Surface Coating Processes Sol-gel process

The sol-gel process is used for manufacturing superhydrophobic surfaces from many types of materials. 65–70 In many investigations, due to the presence of materials with low surface energy in the sol-gel process, there is no other hydrophobizing process. For example, Shirtcliff et al. 67 produced a porous sol-gel foam of Organo-trithoxysile that shows both superhydrophobic behavior and superhydrophilic behavior when exposed to high temperatures. Shang et al. 69 presented a method whereby transparent superhydrophobic surfaces were produced by modifying silicate-based films using fluorinated silanes instead of mixing low-energy materials in sol. In the same case, Wu et al. 70 produced a ZnO-based surface in a microstructure using chemical processing; after coating, superhydrophobia was achieved using long-chain alkanoic acids (see Figure 32 ).

Figure 32 . Superhydrophobic surfaces produced by sol-gel process. (a) Scanning electron microscope (SEM) image of MTEOS sol-gel foam. Inset pictures show fenol fetalein in water on the sol-gel foam, tempered at 390 °C (left) and 400 °C (right). 67 (b) Atomic-force microscopy (AFM) picture of sol-gel film containing 30 wt% silicate colloid. Picture area: 55 µm 2 . Inset picture shows the chilled water vapor on this film. 68

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