Plasma surface modifications of microchannels

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Sealing of a microreactor

One of the important properties of PDMS is that it can be sealed to itself or other substrates both reversibly and irreversibly without an adhesive. Therefore, PDMS is easy to be used to enclose channels [45]. Furthermore, for permanent sealing a broad range of surface modification techniques such as plasma, wet chemistry, and photochemistry is developed. For instance, the use of oxygen plasma and heating process could briefly provide the ideal permanent bonding for PDMS chips [40]. However, the drawbacks of PDMS, including adsorption of hydrophobic molecules, short-term stability after surface treatment, swelling in organic solvents, water permeability, and incompatibility with very high pressure operations, strongly limits its wider application in industry.
As a consequence, thermoplastics have been preferred as low-cost and mass-producible alternatives. Among this class of polymers, cyclic olefin copolymer (COC) is taken into account [46]. The hot pressuring process is a conventional method for introducing thermal fusion bonding on COC surface with a relative high strength [47]. However, the deformation and/or collapse of the channel might occur as the substrates are heated to temperatures near or above the glass transition temperature at a high pressure. Alternatively, the drawback of this method could be resolved by solvent-based bonding techniques which don‘t require a high temperature. Thus, the hot pressuring technique usually consists in the use of a compatible solvent that is applied on the COC substrates for the sealing of such microsystems.

Plasma surface modifications of microchannels

Definition of a plasma

Plasma is a partially ionized gas consisting of an equal numbers of positive and negative charges, with a different number of un-ionized neutral molecules [48]. It has to be noted that a plasma-like behavior occurs once a remarkably small fraction of the gas has undergone ionization. Thus, fractionally ionized gases exhibit most of the exotic phenomena characteristic of fully ionized gases. In fact, at temperatures near or exceeding atomic ionization energies, atoms similarly decompose into electrons and positively charged ions. When the charges move, they generate electric currents along with magnetic fields. As a result, they are affected by each other‘s fields. Nevertheless, because the charges are no longer bound, their assemblage becomes capable of collective motions of great vigor and complexity and such an assemblage is termed plasma. Scientists have successfully developed plasma technologies in materials science since the 1960s. As plasma demonstrates quite different properties from those of common substances in the gaseous, liquid or solid state due to its high energy level, plasma is commonly admitted as the fourth state of matter. In fact, 99% of the substances in the universe exist in a state of plasma, mostly in countless giant stellar stars [49].

Applications of plasma

Plasma can be divided into two categories as it can be generated at different temperatures [50]: cold plasma and thermal plasma (Table I-1). Depending on different working power, the plasma enables various applications for plasma technologies: surface coatings, waste destruction, gas treatments, and chemical synthesis and so on. Thermal plasmas can be artificially generated using a high voltage, high temperature arc, which is the basis for the corona discharge process and for the plasma torch used to vaporize and redeposit metals. Cold plasmas are always used in surface modifications and organic cleaning. This type of plasma is generated within a vacuum chamber where atmospheric gases have been evacuated (typically below 0.1 torr); the low pressure allowing a relatively long free path of accelerated electrons and ions. Since the ions and neutral particles are at/near ambient temperatures, the long free path of electrons, which are at high temperature or electron volt levels, have relatively few collisions with molecules at this pressure and the reaction remains at a low temperature.
In the plasma surface modification process, glow discharge plasma is created by applying a voltage between electrodes in a reaction chamber containing with low-pressure gas. The gas is then energized by one of the following types of energy: radio frequency (13.56 MHZ), microwaves (2.45 GHz), and alternative or direct current. The energetic species in plasma include ions, electrons, radicals, metastables, neutrals species in different excited levels, and photons in the short-wave ultraviolet (UV) range. Surfaces in contact with the gas plasma are bombarded by these energetic species and their energy is transferred from the plasma to the solid. These energy transfers are dissipated within the solid by a variety of chemical and physical processes. In terms of the highly unusual and reactive chemical environment of plasma, many plasma-surface reactions occur and the high-density of ionized and excited species in the plasma can change the surface properties of materials [51]. The application for plasma interaction on the surface can be divided into four categories [50]: (i) to tailor interfacial properties between reinforcement and matrix materials; (ii) to increase wear or erosion resistance of surface; (iii) to enhance adhesion strength or minimize porosity of the coatings; (iv) to functionalize the surface of materials, e.g. plasma treatment including cleaning, etching, crosslinking, grafting and plasma deposition, respectively.

Plasma enhanced chemical vapor deposition (PECVD)

As known to all, plasma enhance chemical vapor deposition (PECVD) is a kind of chemical vapor deposition (CVD) technique based on the reaction between the gaseous precursor that is introduced into the chamber (or vapor of a liquid carried by gas) and another molecule in the gas phase. The most important advantage compared with CVD is the reaction can occur at much lower temperature due to the assistance of the plasma reactive media for the dissociation of the precursor [52].
Furthermore, controlling the plasma chemical reactions and plasma-surface interactions, could optimize the film composition and microstructure. The processes leading to the deposition of thin films in the plasma environment include reactions in the gas phase, transport toward the surface involving specific energetic considerations, and reactions at the surface, giving rise to film formation and microstructural evolution, and providing specific film functional properties.
Numerous reports show that this method can be used in various areas for different application, i.e. deposition inorganic films like oxides, nitrides and carbides of metals; deposition of organic thin films like polymers, hard carbon films, crystalline diamond; even deposition of inorganic-organic mixture thin films like metal-organic catalysts. It could be also employed for the treatment of different substrates such as stainless steel, NiTi SMA, glass, PDMS, silicon, COC and so on. Concerning the deposition on organic materials, the glow discharge is efficient at creating a high density of free radicals, both in the gas phase and in the surface of organic materials, including the most stable polymers. These surface free radicals are created by vacuum ultraviolet light generated in the primary plasma. The surface free radicals then are able to react either with each other, or with species in the plasma environment. Thus, optimization of the deposition involves identification of discharge characteristics giving rise to the formation of large densities of free radicals that diffuse toward the surface.
Of course, in the deposition processes discharge characteristics are not the only controlling parameters. If we consider the whole PECVD system and the properties of coating, plenty of parameters have to be discussed. They can be generally divided into two catalogues: external parameters including pressure, gas flow, discharge excitation frequency, power supply and the resulting ‗internal‘ plasma characteristics like the electron (plasma) density, the electron energy distribution function (EEDF), electrical potentials, and fluxes of different species toward the surfaces exposed to plasma. As we mentioned before, the nature of plasma is the balance between collisional ionization and recombination. This makes EEDF as an essential parameter for plasma processing because it represents how many electrons are available for the ionization and other plasma reactions. However, EEDF is affected by all external parameters in a complex way, especially, the discharge field frequency is the important factor influencing on how the plasma interacts with the exposed surface. Therefore, plasma systems are varied due to different frequency: Low-, medium- and radio-frequency (LF, MF, and RF (standard RF frequency is 13.56 MHz)) deposition systems. Note that higher frequency leads to higher power efficiency, which could avoid surface charging and plasma instabilities. It means the ionization and dissociation rates are higher in the microwave plasma than in RF plasma. As a consequence, with the development of PECVD microwave discharge is introduced into plasma system to provide high electron (plasma) density and high ion flux. Based on this system, MW/RF dual mode plasma, remote MW/RF plasma, electron cyclotron resonance plasma is developed as well.
Actually, for a certain system, the frequency is almost fixed; the other parameters play a very important role to affect the characteristics of coating including mechanical, optical, electrical and tribological properties. The details will be discussed in the following section.

Influence of the parameters during PECVD process

The reality in plasma process is that a steady state population of atoms is continuously fed by gas flow and continuously pumped in the vacuum system. Thus, it is important to describe the amount of gas flowing in pumping system: on one hand it is the volume of gas passing per second, which is measured by pumping speed S; on the other hand it‘s the number of molecules contained in that volume gas, which is defined by its pressure [53]. Kim et al. report that the characteristics of the coatings changed by increasing the total pressure in the deposition processing [54], as shown in Fig. I-5. Increase in pressure improves the grain structure and size due to the change of favorable growth plane. Furthermore, the cause of enhancement of the diamond film quality with increasing total pressure was investigated. The results show enhancement is related to the increase in promoter intensity (in this case is CH) in the plasma.
Fig. I-5 Raman spectra of thin films at total pressures of (a) 27.5, (b) 40, (c) 60, (d) 150 and (e) 250Torr [54].
Moreno et al.[55] found that the deposition pressure and the SiH4/H2 flow rate ratio had an important influence on the structural, optical and electric characteristics of the films. AFM results showed that higher gas flow rate ratios resulted in an increment on silicon clusters density on the film surface. The effects of the CH4 flow rate on properties of a-SiCx:H films are also investigated by Lien et al.[56]. As increasing the carbon content in the films, the band gap and deposition rate also increased, while the absorption coefficient and conductivity of the films decreased. It was found that the grain size becomes larger with an addition of the incorporation of carbon atoms into the films. By investigating the influence of Ar flow rate on deposition rate and structural properties of hydrogenated silicon germanium (SiGe:H) films, Tang et al. show that the addition of Ar in the diluted gas efficiently improve the deposition rate and crystallinity due to an enhanced dissociation of source gases and bombardment on growth surface [57].
Thus, increase of RF power is predictable to generate higher kinetic energy ions when the pressure is fixed in plasma process; thereafter the ionic species in plasma sheath bombard the substrate [61] and force the catalyst droplets on the growth surface into large number of small droplets. Guzenda et al. had calculated the values of RMS (calculated for images of comparable quality (Fig. I-6)) were 4.6 nm, 6.4 nm and 11.6 nm for titanium oxide films deposited at power of 100 W, 200 W and of 300 W respectively [64]. In this work, the film roughness measured with AFM is clearly dependent on the deposition power, which was also observed in Ref. [65]. Furthermore, Chong et al. describe the transformation of SiNWs microstructures using different RF power (Fig. I-7), two types of SiNWs were observed on film [63]. These results imply it is controllable to fabricate coating with different microstructure and size by adjusting plasma generator power.
Fig. I-8 EDX spectra scan on catalyst droplet and nanowire stem at different rf power. The inset in each spectrum is the high magnification FESEM view of SiNW [63].
Except for the ion bombardment, another significant effect of power difference is dissociation of molecules that influences on chemical composition of the ionic species. Actually, different step of reactions occurs after dissociation in the plasma deposition. Increase in RF power resulted in an increase in primary dissociation and limited the following reaction. It is reported that after SiHn (n=1, 2, 3, which depending on the energy of electron. The threshold energy for SiH3 generation (8.75 eV) is the lowest compared to SiH2 and SiH [66]) is generated from dissociation of SiH4 molecules, second gas-phase reaction occurred between SiH4 species and SiH4 parent molecules to form higher silane species (SinHm).
This reaction usually involved short lifetime species of SiH2 and SiH rather than SiH3, increase in RF power reduces the formation of SiH3 species in the plasma and thus reduced the formation of nucleation sites for Si film growth as a result of abstraction of Si-H bonds and the growth of ordered Si:H film as a result of diffusion of SiH3 radicals into these growth sites. It indicates RF power is also a key parameter to influence on the chemical composition of coating, as shown in Fig. I-8.
The growth rate of coating are investigated by Sun et al. [65] and Seo et al. [67], respectively. Both of them figure out the deposition rate increase with the rise of RF power in a certain range. Moreover, in the work of Seo et al., a stable deposition rate state was sustained when the power was continuously increased (Fig. I-9 (a)). However, contrary results are also described in ref. [60], this decrease may be due to the back etching that occurs during the deposition process (Fig. I-9 (b)).
Fig. I-9 Effect of RF power variation on the growth rate of the deposited film (a) reported by Seo et al. [67]; (b) reported by Mahajan et al. [60].
Other parameters like working time (Fig. I-10 [68]), inter-electrode distance (Fig. I-11 [69]) and so on also take efforts on the characteristics of coating. Some results are exhibited as follow:

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Amino functionalization by PECVD method

The surface of heterogeneous catalyst takes a particular place among the surfaces with specific requirements. Because of economic and environmental concerns: catalytic processes require continuous improvements. One way to achieve these improvements is to find out new modes for the preparation of catalytic materials that promote the desired reactions [70]. In decades, scholars pay attention to develop a modified surface for catalysts attachment. In order to gain an immobilizable coating with good adhesion strength, organic compounds are introduced into the surface. Conventionally, silanization agents with functional end-groups, e.g. –NH2, -SH, -COOH … are often used for functionalizing surface [23, 71-74]. For example, organic amines allylamine is usually chosen most often as a plasma medium to obtain amine functionalities, which are important for the immobilization of catalytic metal particles [75]. With the development of research, plasma polymerization of other chemicals containing amine groups such as VTES, HMDSO, APTES, TES, HBDS and so on also show the potential for plasma functionalization.
Among those candidates, 3-aminopropyltriethoxysilane (APTES) (CAS Number: 919-30-2) with a chemical structure as presented in Fig. I-12 is considered as the most popular functionalization reagent and has been widely used in current research [73, 76, 77]. As one of the silane-coupling agents, it even exhibits the enhanced adhesion to inorganics like silica, whereas there was no activity at all without the plasma treatment. On one hand APTES exhibits good adhesion to substrate such as glass, PDMS, COC, etc. through the formation of Si-O bonds, on the other hand it provides the amino groups that could bound to catalytic nanoparticles through electrostatic interaction [78]. However, few publications focus on introducing this chemical into plasma system, the most commonly used method is through a wet chemical treatment [22]. Since Barbarossa et al.[79] reported the deposition of thin film by plasma polymerization of AA/3-APTES mixture in 1992, few developments continued in recent years. Four years later, Akovali et al.[80] reported the different exhibitions of silicon and tin containing organic compounds including VTES, HMDS, APTES, TES and HBDS, which were used as the precursor in plasma surface modification. In 2010, two important publications both reported the successful deposition by PECVD with good adhesion, respectively [81, 82]. Various surface characterization techniques have been employed and aimed to understand and further control the fabrication of APTES nanostructures by PECVD on thermoplastic substrates in ref.[81]. Gandhiraman et al.[82] confirms the presence of siloxane functionality is essential for film adhesion to substrate and indicates the presence of amine and amide functionalities, which are both important to further immobilization. In recent years, the use of PECVD for functionalizing APTES has undergone enormous expansion, however it remains only a few reports and still needs to be intensively studied [83-87]. Especially, the parameters such as gas selection, deposition time, working pressure, working power, substrate materials and so on, which strongly affect the resulting structure, coverage of the layers, etc., haven‘t been systemic investigated yet.

Catalysts and their immobilization

Gold nanoparticles (AuNPs) as catalysts

In the perspective of chemical engineering, heterogeneous catalysis has acquired a vital role since it is a very efficient green approach. The main concepts of green catalysis are as follows: (i) usage of eco-friendly solvents; (ii) avoidance of hazardous wastes; (iii) usage of recyclable catalysts; (iv) mild reaction conditions; and (v) high efficiency and selectivity. Over the past few years, heterogeneous catalysis by nano-gold catalysts has attracted the attention of researchers because it is a highly proficient substitute for non-separable and pollutant homogeneous catalysis. Actually gold is usually viewed as the most stable metal, but surprisingly it has been found that Au nanoparticles less than 3~5 nm in diameter are catalytically active for CO oxidation below room temperature [88]. Henceforward, the usage of gold nanoparticles (AuNPs) as an important catalyst for different kinds of chemical reactions (e.g. oxidation [89, 90], hydrogenation [91], hydroamination [92], ring expansion [93, 94], coupling reactions [95, 96], etc.) has been studied after 90s. Besides, it has long been known that gold nanoparticles can strongly adsorb visible light due to the surface plasmon resonance (SPR) effect, which features a collective oscillation of conducting electrons of AuNPs with the electromagnetic field of the incident light. The feature indicates the AuNPs could become a quite efficient visible-light photocatalyst at ambient conditions in the photooxidation.

Table of contents :

Chapter I: Bibliography
1.1 Introduction
1.2 General introduction of microfluidic systems
1.2.1 The development of microsystems
1.2.2 Microfluidic systems for chemical engineering
1.2.3 Materials for microreactors fabrication
1.2.4 Fabrication methods
1.2.5 Other components of a microfluidic system
1.2.6 Sealing of a microreactor
1.3 Plasma surface modifications of microchannels
1.3.1 Definition of a plasma
1.3.2 Applications of plasma
1.3.3 Plasma enhanced chemical vapor deposition (PECVD)
1.3.4 Influence of the parameters during PECVD process
1.3.5 Amino functionalization by PECVD method
1.4 Catalysts and their immobilization
1.4.1 Gold nanoparticles (AuNPs) as catalysts
1.4.2 Zeolite as a support material
1.4.3 Synthesis of AuNPs
1.4.4 Immobilization of AuNPs
1.4.5 Deposition zeolite and gold@zeolite on substrate surface
1.5 Application: oxidation in microreactors
1.5.1 Liquid phase oxidation of benzyl alcohol and its products
1.5.2 Influence of reaction conditions
1.5.3 Recent studies of using microsystem for benzyl alcohol oxidation
1.6 Objectives of this work
1.7 Outline
Chapter II Characterization methods
2.1 Contact angle measurement
2.2 Fourier Transform infrared spectroscopy (FTIR)
2.3 X-ray Photoelectron Spectroscopy (XPS)
2.4 Spectroscopic ellipsometer
2.5 Field-emission scanning electron microscopy (FESEM)
2.6 Transmission electron microscopy (TEM)
2.7 Zeta potential
2.8 X-ray diffraction (XRD)
2.9 Ultraviolet-visible spectroscopy (UV-Vis)
2.10 High performance liquid chromatography (HPLC)
Chapter III Deposition of amine groups by means of APTES PECVD process 
3.1 Abstract
3.2 Introduction
3.3 Experimental
3.3.1 Materials and chemicals
3.3.2 APTES plasma polymerization
3.3.3 Characterization
3.4 Results and discussion
3.4.1 Active gas selection
3.4.2 Influence of various substrates
3.4.3 Influence of deposition time
3.4.4 Influence of working pressure
3.4.5 Influence of working power
3.5 Conclusion
Chapter IV A comparison study of two methods for glass surface functionalization and their application in gold nanoparticles (AuNPs) immobilization
4.1 Abstract
4.2 Introduction
4.3 Experimental
4.3.1 Materials and chemicals
4.3.2 Deposition of APTES using the wet chemistry method
4.3.3 Deposition of APTES using PECVD method
4.3.4 Synthesis and immobilization of AuNPs
4.3.5 Characterization of modified surfaces
4.4 Results and discussion
4.4.1 APTES deposition and surface Characterization
4.4.2 Study of the immobilization of AuNPs
4.5 Conclusion
Chapter V Deposition of Y-zeolite and Au@Y-zeolite on amine functionalized surface 
5.1 Abstract
5.2 Introduction
5.3 Experimental
5.3.1 Materials and chemicals
5.3.2 Synthesis and immobilization of AuNPs
5.3.3 Immobilization of gold on zeolite surface using APTES and MPTES
5.3.4 Deposition of APTES using PECVD method
5.3.5 Deposition of zeolite and Au@zeolite
5.3.6 Coating stability test in flowing water
5.3.7 Characterizations
5.4 Results and discussion
5.4.1 Characterization of Y zeolite
5.4.2 Immobilization of AuNPs on Y type zeolite using APTES and MPTES
5.4.3 Deposition of zeolite and Au@zeolite on COC surface
5.5 Conclusion
Chapter VI Oxidation of benzyl alcohol in catalytic microreactors
6.1 Abstract
6.2 Introduction
6.3 Experimental
6.3.1 Fabrication of the microreactor
6.3.2 Amine functionalization of microchannel by plasma enhanced chemical vapor deposition
6.3.3 Synthesis and immobilization of AuNPs
6.3.4 Synthesis of Au@zeolite
6.3.5 Deposition of zeolite and Au@zeolite
6.3.6 Sealing of the catalytic microreactor
6.3.7 Experimental set up of microsystem for oxidizing benzyl alcohol
6.3.8 Working conditions in microsystem
6.4 Results and discussion
6.4.1 Identification of standard retention time and peak area
6.4.2 Catalytic activity of gold immobilized microreactor
6.4.3 The influence of Y-zeolite on gold in microsystem
6.5 Conclusion
Chapter VII General conclusions and perspectives
7.1 General conclusions
7.2 Perspectives

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