Robust hydrophobic coating and deposition of carbon thin layer for the electrode of energy storage devices by using atmospheric pressure plasma

Abstract

This thesis focused on improving the robustness of the thin film for hydrophobic surface and deposition of carbon thin layer for the electrode of energy storage devices by using non-thermal plasma (NTP). NTP can effectively produce a protective or hydrophobic coating layer on the surfaces of various materials, namely glasses, porous surface, fabrics, powders, and polymers with low cost. Thermal coating presents low efficiency with a non-uniform coating layer, and the sol-gel method shows poor visibility along with a non-uniform coating layer. However, the gas phase coating by NTP is an alternative method, that presents a uniform coating layer with excellent visibility. In this study, it is shown that NTP is useful to generate uniform robust thin film on the various surfaces using gas phase precursors at ambient condition. Hydrophobic surfaces have various applications such as self-cleaning windshields, for anti-contamination, anti-sticking of snow for windows and antennas, anti-biofouling paints for boats, anti-icing, anti-corrosion, etc. The surface wettability is determined by the measurement of the water contact angle (WCA). A surface is said to be wetted if a liquid spread over the surface evenly without forming the droplets. If the water spreads over the surface and it does not form droplets, then the surface is called hydrophilic. Regarding surface static water contact angle, a surface having water contact angle (WCA) < 90° is known as hydrophilic surface but that having WCA ≥ 90° is known as hydrophobic. In addition, surface having WCA >150° and sliding angle (SA) ≤ 10° is called super-hydrophobic owing to its outstanding self-cleaning capability. Both, the WCA and the SA show the performance of the thin film surface that how effective it is in the self-cleaning function. The wettability of a solid surface is a property that depends on both surface roughness and surface chemistry and directly related to the surface free energy. Basically, materials with low surface energies are used to prepare hydrophobic surfaces and materials rich in hydrocarbon are suggested to make anode materials. Many research groups have tried to make durable superhydrophobic surfaces over metals, organic or inorganic substrates. Long-lasting hydrophobic coatings are not readily achievable with only low surface energy materials, mainly because of their poor adhesion nature to solid surfaces. Low surface energy materials such as hexamethyldisiloxane (HMDSO, O[Si(CH3)3]2), pentamethyldisiloxane (PMDSO, C5H16OSi2), tetramethyldisiloxane (TMDSO, [(CH3)2SiH]2O), tetramethylsilane (TMS, Si(CH33)4), trimethylsilane (TriMS, HSi(CH3)3) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) are well-known organosilicon precursors capable of forming hydrophobic layers. On the other hand, methane (CH4), ethylene (C2H4), propane (C3H6), butane (C4H8), pentane (C5H10), hexane (C6H12), and heptane (C7H14) are rich in hydrocarbon. So, these hydrocarbons are good candidates to form a carbon polymer layer on the surfaces. Conventional chemical polymerizations have a drawback of poor adhesion on solid surfaces, and as well, the coating layers created do not function well after several scratches due to their chemical nature. For industrial applications, wear resistance, and adhesive properties are critical factors. The poor adhesive property may be improved by incorporating aminopropylethoxysilanes that are silanating mediators for modifying the silica-based materials' surface. The aminosilanes anchor to the surface and establishing a Si-O-Si covalent bond and hydrogen bonding with the amino group. Among aminosilanes, (3-aminopropyl)triethoxysilane (APTES, H2N(CH2)3Si(OC2H5)3) and 3-aminopropyl(diethoxy)methylsilane (APDMES, CH3Si(OC2H5)2(CH2)3NH2) have widely been used along with low surface energy materials. After studying all those chemicals, TMS, HMDSO, APTES, and APDMES were chosen to use as precursors for the robust hydrophobic surfaces and n-heptane and ethylene was used for the anode materials. The plasma reactor was operated with a high-voltage alternating current (AC) power source whose frequency output was set to 11.5 kHz. In all the cases, noble gas argon (Ar) and the same power source were used to generate plasma. Dielectric barrier discharge (DBD) configuration was used in both hydrophobic coatings and preparing of graphitic carbon materials for the anode cases. The cylindrical plasma jet was used for hydrophobic coatings, but in the case of preparing anode materials, the plasma configuration was changed to planar DBD plasma. This thesis divided into seven chapters dealing with plasma polymerization of different precursors. Glass substrates have been used to deposit polymer layer for hydrophobic coatings and nickel foams to deposit hydrocarbon to prepare anode materials for the energy storage devices. In chapter one and two, literature review to support this thesis has been discussed widely. In chapter three, robust hydrophobic coating with excellent visibility has been investigated using HMDSO and APTES precursors. In this study, the H/A (HMDSO/APTES) ratio between two precursors has been varied from 3/1 to 1/3 to check the WCA and robustness of the coatings. The highest WCA was measured to be 143° at the ratio of 3/1. The detailed discussion has been done in chapter two. In chapter four, TMS was used along with APTES and the plasma configuration was changed from dielectric barrier discharge (DBD) to corona discharge so that the surface having any kind of thickness can be treated. But corona discharge has shown noticeable drawback over DBD plasma in terms of treated surface area and treatment time. The treated surface area by the corona discharge is smaller and the treatment time increased from 5 to 6 times with compare to DBD. The T/A (TMS/APTES) ratio has been varied from 3.0 to 7.0 and the obtained WCA angle and coating robustness were almost similar to the DBD case. Superhydrophobic surface has been investigated in chapter five. DBD configuration has been used in order to treat large surface area within short treatment time. Two precursors namely TMS and APDMES were used in the plasma polymerization process and deposited onto the glass substrate. Despite the hydrophilic nature of APDMES, however, the coatings prepared with the mixture of these two precursors exhibited increases in the WCA from 154 to 163° as the A/T (APDMES/TMS) ratio was increased from 1 to 1.7. But, further increase in the A/T ratio to 2.4 led to a decrease in the WCA, suggesting that the coating layer begins to change toward hydrophilic at around this A/T ratio. Preparing of carbon material has been studied in chapter six using planar DBD configuration reactor. For this study, n-heptane was used as precursor to deposit hydrocarbon on the nickel foam. The plasma treatment time was varied from 5 to 120 min to find the optimal coating thickness and then pyrolysis was performed under nitrogen environment at 800 ℃ for 6 hours to convert hydrocarbon to carbon on the nickel foam. The formation of the carbon is confirmed by the Raman spectra. The detailed discussion has been done in chapter six. The coating films formed under different conditions were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), static water contact angle (WCA), and scratch test.