Fabrication of superhydrophobic surface on filter paper by a facile coating method and application in environmental treatment

VIETNAM NATIONAL UNIVERSITY, HA NOI VIETNAM JAPAN UNIVERSITY DINH DAM KHANH FABRICATION OF SUPERHYDROPHOBIC SURFACE ON FILTER PAPER BY A FACILE COATING METHOD AND APPLICATION IN ENVIRONMENTAL TREATMENT MASTER’S THESIS VIETNAM NATIONAL UNIVERSITY, HA NOI VIETNAM JAPAN UNIVERSITY DINH DAM KHANH FABRICATION OF SUPERHYDROPHOBIC SURFACE ON FILTER PAPER BY A FACILE COATING METHOD AND APPLICATION IN ENVIRONMENTAL TREATMENT MAJOR: ENVIRONMENTAL ENGINEERING CODE: 8520320.01 RESEARCH SUPERVISORS: Dr TRAN THI VIET HA Dr NGUYEN MINH VIET Hanoi, 2021 ACKNOWLEDGEMENTS For the accomplishment of this master thesis, I have received plenty of invaluable support First of all, I would like to express my deep gratitude and sincere thanks to Dr Tran Thi Viet Ha, my principal supervisor, for giving me an opportunity to this research and extend my knowledge I am extremely grateful to her for providing such an indispensable support and guidance throughout my thesis procedure Secondly, I would like to thank Dr Nguyen Minh Viet, my co-supervisor, for his instruction and enthusiasm to help me complete my master thesis successfully He helped me gain lots of experiences in working with analysis equipment Last but not least, I am deeply grateful to my parents, my lecturers in Vietnam Japan University and my classmates for their support and encouragement during all the time I have been studying here and carrying out my thesis, which motivates me a lot to successfully complete my master course and master thesis TABLE OF CONTENTS CHAPTER 1: INTRODUCTION CHAPTER 2: LITERATURE REVIEW 2.1 Wettability and water contact angle 2.2 Fabrication methods of superhydrophobic surface 2.2.1 Chemical etching method 2.2.2 Sol-gel method 2.2.3 Dip-coating method 2.2.4 Electrochemical deposition method 10 2.2.5 Plasma-etching method 11 2.2.6 Hydrothermal method 12 2.2.7 Self-assembly method 13 2.3 Techniques for superhydrophobic surface analysis 13 2.3.1 Scanning electron microscope (SEM) 13 2.3.2 Energy-dispersive X-ray spectroscopy (EDX or EDS) 15 2.3.3 Fourier-transform infrared spectroscopy (FTIR) 17 2.3.4 X-Ray Diffraction (XRD) 18 2.3.5 Water contact angle measurement 20 2.4 Previous studies about application of superhydrophobic material in oil-water separation 21 CHAPTER 3: MATERIALS AND METHODOLOGIES 24 3.1 Objectives and contents of the study 24 3.1.1 Objectives of the study 24 3.1.2 Contents of the study 24 3.2 Materials 24 3.3 Investigation of fabrication parameters 24 3.3.1 Investigation of ZnO coating procedure 25 3.3.2 Investigation of ZnO coating solvent 26 3.3.3 Comparison between one-step coating and two-step coating 26 3.3.4 Investigation of ZnO coating pH 27 3.3.5 Investigation of ZnO coating cycle number 27 3.3.6 Investigation of stearic acid coating cycle number 27 3.4 Fabrication of superhydrophobic surface on filter paper 28 3.5 Characterization of fabricated filter paper surface 28 3.6 Applicability of fabricated filter paper in environmental treatment 29 3.7 Durability and reusability of fabricated filter paper 30 3.7.1 Durability of fabricated filter paper 30 3.7.2 Reusability of fabricated filter paper 31 CHAPTER 4: RESULTS AND DISCUSSIONS 32 4.1 Investigation of fabrication parameters 32 4.1.1 Investigation of fabrication procedure 32 4.1.2 Investigation of fabrication solvent 32 4.1.3 Comparison between one-step coating and two-step coating 33 4.1.4 Investigation of coating pH 33 4.1.5 Investigation of ZnO coating cycle number 34 4.1.6 Investigation of stearic acid coating cycle number 35 4.2 Fabrication of superhydrophobic surface on filter paper 36 4.3 Characterization of fabricated filter paper surface 36 4.3.1 Scanning electron microscope (SEM) analysis 36 4.3.2 Energy-dispersive X-ray spectroscopy (EDX) analysis 37 4.3.3 X-Ray Diffraction (XRD) analysis 38 4.3.4 Fourier-transform infrared spectroscopy (FTIR) analysis 39 4.3.5 Proposal of superhydrophobic coating formation mechanism 40 4.3.6 Water contact angle (WCA) and water shedding angle (WSA) 41 4.4 Applicability of fabricated filter paper in environmental treatment 42 4.5 Durability and reusability of fabricated filter paper 43 4.5.1 Durability of fabricated filter paper 43 4.5.2 Reusability of fabricated filter paper 46 CHAPTER 5: CONCLUSION 48 LIST OF TABLES Table 4.1 Recovery rates in oil-water separation experiments 43 Table 4.2 Recovery rates in oil-water separation after durability tests 46 Table 4.3 Recovery rates in reusability tests 47 i LIST OF FIGURES Figure 1.1 Superhydrophobic surfaces in nature Figure 2.1 Hydrophilic and hydrophobic surface Figure 2.2 Schematic of (a) Young’s equation, (b) Wenzel’s model and (c) CassieBaxter’s model (Edalatpour et al., 2018) Figure 2.3 Back-scattered electron image showing the difference in contrast due to the atomic number: (A) observed material and (B) image of back-scattered electrons (Kim et al., 2010) 14 Figure 2.4 Schematic representation of the types of X-ray spectrum emitted upon bombardment of fast electron (Reichelt, 2007) 16 Figure 2.5 Basic component in Fourier transform infrared spectrometer (Mohamed et al., 2017) 17 Figure 2.6 Basic features of a typical XRD experiment (Toney, 1992) 19 Figure 2.7 Contact angle goniometer system 21 Figure 3.1 Coating steps in fabrication of superhydrophobic surface 25 Figure 3.2 Water shedding angle determination experiment 29 Figure 3.3 Vacuum filter holder 30 Figure 4.1 The filter papers after coated using different solvents 33 Figure 4.2 The WCAs of paper with (a) ZnO coating, (b) stearic acid coating and (c) ZnO-stearic acid coating 33 Figure 4.3 The WCAs of filter paper with (a) coating cycles and (b) coating cycles of ZnO 34 Figure 4.4 WCA measurement of filter paper with (a) coating cycle and (b) coating cycles of stearic acid 35 Figure 4.5 SEM pictures of (a) bare filter paper, (b) filter paper with ZnO coating and (c) filter paper with ZnO and stearic acid coating 37 Figure 4.6 EDX analysis of (a) bare filter paper, (b) filter paper with ZnO coating and (c) filter paper with ZnO and stearic acid coating 38 Figure 4.7 XRD analysis of bare and coated papers 39 Figure 4.8 FTIR analysis of bare and coated papers 40 Figure 4.9 Formula structure of (a) cellulose and (b) stearic acid 40 Figure 4.10 Schematic illustration of the superhydrophobic coating 41 Figure 4.11 Water shape in WCA measurement 41 Figure 4.12 The mixture of oil and water (a) before filtration and (b) after filtration 42 Figure 4.13 Water drop shape on fabricated paper (a) in the beginning and (b) after months 43 Figure 4.14 The tape after adhesive tape test 44 Figure 4.15 Water drop shape on fabricated paper (a) in the beginning and (b) after adhesive tape test 44 ii Figure 4.16 Water drop shape on fabricated paper (a) in the beginning and after immersion in solution with (b) pH = 1, (c) pH = and (d) pH = 13 45 Figure 4.17 Recovery rates in oil-water separation after durability tests 46 Figure 4.18 Recovery rates in reusability tests 47 iii LIST OF ABBREVIATIONS EDX: Energy-dispersive X-ray spectroscopy FTIR: Fourier-transform infrared spectroscopy SEM: Scanning electron microscope WCA: Water contact angle WSA: Water shedding angle XRD: X-Ray Diffraction iv CHAPTER 1: INTRODUCTION Nature is the inspiration to many scientists and engineers to create remarkable inventions for human life There are numerous materials, structures and systems in the natural world which human observe and imitate to design and invent new products This creative process is known as biomimicry One notable example for biomimicry is the superhydrophobic surface In nature, we can find many superhydrophobic surfaces of plants and animals, such as lotus leaves and butterfly wings (Figure 1.1), with waterproof, self-cleaning and anti-adhesion properties These properties are studied and applied in many fields of life such as making anti-fogging materials (Das et al., 2018), anti-freezing materials (Chevallier et al., 2011), self-cleaning materials (Satapathy et al., 2018), or environmental treatment (Sriram et al., 2020) Figure 1.1 Superhydrophobic surfaces in nature Recently, superhydrophobic surfaces with high applicability have received more attention from researchers Various methods have been developed to prepare superhydrophobic surfaces, for example, sol-gel method (Fan et al., 2012), selfassembly (Song et al., 2010), chemical etching (Qian & Shen, 2005), plasma etching (Ji et al., 2009), vapor deposition (Rezaei et al., 2014) and so on Moreover, there are also many studies on fabricating superhydrophobic surfaces on different substrates such as fabric (Ge et al., 2020), glass (Shi et al., 2005), silicon (Song et al., 2010) or metal surface (Fan et al., 2012; Movahedi & Norouzbeigi, 2019), etc with diverse applications Based on the basis of the substrate, the appropriate fabricating method will be chosen and developed differently to successfully prepare the superhydrophobic surface Figure 4.7 XRD analysis of bare and coated papers 4.3.4 Fourier-transform infrared spectroscopy (FTIR) analysis The functional groups on the surface of filter paper samples were determined using FTIR technique FTIR results of these samples are demonstrated in Figure 4.8 For bare filter paper sample, peaks appear at positions 3335 cm-1, 2913 cm-1 and 1028 cm-1, indicating –OH, –CH2–, –COC– functional groups (Kim & Peppas, 2003; Skenderidis et al., 2019) of cellulose molecule, the main composition of filter paper After fabrication, the peak intensity at position 2913 cm-1 increased remarkably, and new peaks appears at positions 2847 cm-1, corresponding to –CH2– functional groups (Skenderidis et al., 2019), 1552 cm-1 and 1420 cm-1, corresponding to –COO– functional group (Kim & Peppas, 2003) These two functional groups are the characteristic functional groups of the stearic acid molecule Therefore, the change in peak intensity at position 2913 cm-1 and the appearance of new peaks after surface modification indicate the existence of stearic acid on the fabricate filter paper 39 Figure 4.8 FTIR analysis of bare and coated papers a b Figure 4.9 Formula structure of (a) cellulose and (b) stearic acid 4.3.5 Proposal of superhydrophobic coating formation mechanism Based on characterization results, the formation of superhydrophobic coating on filter paper surface is proposed as in Figure 4.10 After first coating step with ZnO, there were –OH functional groups attached to ZnO micro and nano structures (the existence of peak at position 335, corresponding to –OH functional group) When the filter paper 40 was coated with stearic acid in the next step, CH3(CH2)16COO– groups would replace these –OH groups, from which the superhydrophobic coating is formed on the filter paper surface Figure 4.10 Schematic illustration of the superhydrophobic coating 4.3.6 Water contact angle (WCA) and water shedding angle (WSA) The wettability of fabricated filter paper was analyzed based on static water contact angle and water shedding angle After fabrication, the filter paper had an average water contact angle of 154° (left WCA = 154.7°, right WCA = 153.5°) and a water shedding angle of 8°± 1° Because the WCA exceeded 150° and the WSA was below 10°, the superhydrophobic surface was successfully fabricated on the filter paper The picture of water shape in WCA measurement is shown in Figure 4.11 Figure 4.11 Water shape in WCA measurement 41 4.4 Applicability of fabricated filter paper in environmental treatment After filtering the solution of 50 mL water and 50 mL kerosene with the fabricated paper, the oil part was successfully separated from the mixture (Figure 4.12), with the oil part permeating through the filter paper to go down, and the water part still remaining on the surface of the paper The volume of the oil part was measured to calculate the recovery rate This process was then repeated with other fabricated papers The recovery rates are shown in Table 4.1, which are all greater than 92% The achieved recovery rates are quite similar with results reported in several published studies In a study conducted by Lathe et al (2020), the separation efficiency of the superhydrophobic pellet was 95± 2% for kerosene-water mixture Yin et al (2020) obtained the recovery rate of about 96% when using fabricated steel mesh for kerosene-water separation Overall, the fabricated filter paper in this study showed satisfactory separation efficiency, confirming the excellent applicability in environmental treatment of the fabricated filter papers as oil-water separating filter a b Figure 4.12 The mixture of oil and water (a) before filtration and (b) after filtration 42 Table 4.1 Recovery rates in oil-water separation experiments No Oil in mixture Oil separated Recovery 50 mL 47 mL 94% 50 mL 48 mL 96% 50 mL 46 mL 92% 50 mL 47 mL 94% 4.5 Durability and reusability of fabricated filter paper 4.5.1 Durability of fabricated filter paper In the first durability test, the fabricated papers were stored in normal condition and its wettability of was checked monthly, up to months The pictures of water drop shape on fabricated filter paper in the beginning and after months are demonstrated in Figure 4.13 It can be clearly seen that there were no significant change in the droplet shape, in other words, no significant change in the wettability of the filter paper after months stored in normal conditions, meaning the paper was stable after long time storage a b Figure 4.13 Water drop shape on fabricated paper (a) in the beginning and (b) after months In the second durability test – adhesive tape test, the adhesive tape was attached to the fabricated paper surfaces and then stripped 10 times continuously After that, the 43 wettability of the papers was checked The pictures of the tape after stripping, the water drop shape on fabricated filter paper in the beginning and after adhesive tape test are shown in Figure 4.14 and Figure 4.15 According to the pictures, only small amount of coating agents (ZnO and stearic acid) were attached to the adhesive tape after stripping and the WCA on the paper surface only decreased slightly after the test, indicating that the coating layers were firmly coated on the filter paper surface Figure 4.14 The tape after adhesive tape test a b Figure 4.15 Water drop shape on fabricated paper (a) in the beginning and (b) after adhesive tape test In the third durability test, the fabricated papers were immersed in solutions with different pH (pH = 1, 7, 13) in hour and dried After that, their wettability was checked The water drop shape on fabricated filter papers in the beginning and after pH durability test is presented in Figure 4.16 From the pictures, the wettability of fabricated filter papers did not change much after immersion in acidic and neutral solutions (Figure 4.16b, c) However, the WCA decreased considerably after immersing the paper in basic solution (Figure 4.16d) Thus, it can be concluded that 44 the fabricated filter papers were durable to acidic and neutral solutions but affected by basic solutions a b c d Figure 4.16 Water drop shape on fabricated paper (a) in the beginning and after immersion in solution with (b) pH = 1, (c) pH = and (d) pH = 13 After all durability tests, all of the examined filter papers were used to filter a solution of 50mL water and 50mL kerosene and the recovery rates were recorded in order to estimate their applicability after being affected by different factors fabricated papers were employed for each durability tests in order to evaluate the precision of the experimental results The recovery rate results were shown in Table 4.2 and Figure 4.17 According to the table, all recovery rates in oil-water separation experiments after durability tests were greater than 90%, even for papers immersed in solution with pH = 13 Generally, the fabricated papers could be considered as durable material under physical impact, different pH environments and long-time storage, with their oil-water separation ability remaining under these conditions 45 Table 4.2 Recovery rates in oil-water separation after durability tests Fabricated paper Recovery rate (%) After month storage 94 94 98 96 After adhesive tap test 94 94 96 94 After immersion in solution with pH = 94 92 96 94 After immersion in solution with pH = 94 96 94 96 After immersion in solution with pH = 13 92 92 94 94 Figure 4.17 Recovery rates in oil-water separation after durability tests 4.5.2 Reusability of fabricated filter paper In this test, fabricated filter papers were used to filter solutions containing 50mL water and 50mL kerosene The papers were then dried and reused for times to evaluate the reusability of the fabricated paper as oil-water separation filter The recovery rates in each time of filtration are shown in Table 4.3 and Figure 4.18 All of the examined papers could perform well in oil-water separation even after being 46 reused times, with the recovery rates higher than 90% Therefore, it can be concluded that the fabricated filter papers have high reusability Table 4.3 Recovery rates in reusability tests Recovery rate (%) Time of filtration 1st time 92 94 94 96 2nd time 94 96 96 96 3rd time 92 94 94 96 4th time 92 94 96 96 5th time 96 96 92 98 6th time 92 94 94 94 7th time 92 96 96 98 8th time 94 94 96 96 9th time 92 96 96 94 10th time 92 96 96 96 Figure 4.18 Recovery rates in reusability tests 47 CHAPTER 5: CONCLUSION In recent year, oil pollution is a big problem to the environment, which can seriously affect marine life, human health and the ecology if it does not receive proper treatment Among plenty of treatment methods (combustion, chemical treatment, bioremediation, etc.), utilization of superhydrophobic material for oil-water separation is considered as one of the most effective method because it can solve the problem without causing any secondary pollution to the environment In this study, a cost-effective and environment friendly method for fabrication of superhydrophobic surface on filter paper was proposed, in which the surface roughness was enhanced with ZnO micro/nano structures and the surface chemistry was modified with stearic acid Different material analysis techniques including SEM, EDX, XRD and FTIR were then employed to confirm that ZnO and stearic acid were successfully coated onto the filter paper surface After fabrication, the achieved static water contact angle and water shedding angle were 154.1° and 8°±1°, indicating that the fabricated paper has achieved superhydrophobic state Moreover, the applicability, the durability and the reusability of the fabricated paper were also evaluated After the filtration test, it was shown that the fabricated paper can perform well in oil-water separation, with all recovery rates were higher than 90% The filter paper also showed good durability to physical impact, different pH environments, long-time storage, and the oil-water separation ability still remained even after affected by these conditions In addition, 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.. Characterization of fabricated filter paper surface  Evaluation of applicability of fabricated filter paper in environmental treatment  Evaluation of durability and reusability of fabricated filter paper. .. (WCA) and water shedding angle (WSA) The wettability of fabricated filter paper was analyzed based on static water contact angle and water shedding angle After fabrication, the filter paper had