Induction of Superhydrophobicity in a Cellulose Substrate by LbL Assembly of Covalently Linked Dual-Sized Silica Nanoparticles Layers

Micro/nano texturized oxidized cellulose membranes (MNOCM) were constructed by layer-by-layer (LbL) assembly in which a base cellulose film was modified by covalent linkages to amino-functionalized silica nanoparticles (amino-SiO2 NPs, 260 nm diameter) and epoxy-functionalized silica nanoparticles (epoxy-SiO2 NPs, 30 nm diameter). The amino-SiO2 NPs grafted onto the MNOCM surface through a standard amidation reaction between the amino groups of the SiO2 NPs and the carboxyl groups of the MNOCM surface in the presence of EDC and NHS consequently forming a first layer of large (260 nm) nanoparticles; subsequently, it was reacted with smaller (30 nm) epoxy-SiO2 NPs. Continuous repetitions of these alternating sized silica NPs through a standard LbL approach lead to a highly micro/nano-texturized MNOCM film as shown by SEM, which was ultimately sealed with a layer of hydrophobic PFOTES (1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane). Although the wettability of MNOCM was no longer hydrophilic, it was found that at five layers deep of NPs, it became superhydrophobic as evidenced by a water contact angle of 151 ̊ ± 2 ̊ and slide angle of 4 ̊. The change in wettability was attributed to increases in final LbL layer surface roughness induced by the sufficient LbL layering of alternating sizes of NPs akin to what is observed in a lotus leaf surface. It was also noted that these superhydrophobic-MNOCM materials displayed good self-cleaning.


Materials that display superhydrophobicity may demonstrate the unique property
dual-size roughness and low surface energy [1]; thus, control of surface roughness and low surface energy via chemical composition are the two principal motifs to induce superhydrophobicity [2] [3] [4] [5].A number of methods have emerged to endow hierarchical dual-size roughness structured surfaces, including solution-immersion process [6] [7], electrospinning [8], chemical vapor deposition [9] [10], and layer-by-layer assembly (LbL) [2] [5] [11] [12] [13].LbL is popular among these methods to imbue stable rough surfaces on various sizes of silica particles especially via continuous covalent bonding of functionalized silica nanoparticles instead of using charged polymers to conglutinate the particles.
Guittard et al. [2] found that the hydrophobicity increased with the number of layers and the static contact angle with water could reach 150˚ ± 3˚ and the contact angle hysteresis could reach 12˚ with the alternation of nine layers.
In general, such silica particle-based superhydrophobic materials are based on hard base substrates such as silicon, glass, or metals.Recently, however, the development of soft superhydrophobic materials based on cellulose including textiles [5] [14] [15] and paper [16] is attractive due to its low-cost, biodegradability, renewability and environmental friendliness.However, NPs attached to paper substrates possess no intrinsic stability and detach from the substrate when applied.The objective of this work was to prepare a stable substrate membrane hosting a suite of carboxyl groups on the surface to provide active sites for NPs and using inspiration from nature (the lotus leaf) thereby construct a dual NP-sized architecturally rough structure.
The primary hydroxyl group on the C-6 of glucose in cellulose can be selectively oxidized to carboxylate groups with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)-mediated oxidation [17] [18] [19] [20] [21] that can couple amino group from amino-functionalized polymers through carbodiimide-mediated reactions [22].An added bonus is that the intrinsic wet strength of TEMPO-oxidized cellulose membranes is significantly enhanced [23] [24].However, compared to plastic, TEMPO-oxidized cellulose is highly hydrophilic and cannot meet the requirement of high hydrophobicity for many applications.
Herein, we present a facile and direct method to produce TEMPO-oxidized cellulose-based superhydrophobic membranes characterized by covalent layer-bylayer assembly of dual-sized nanoparticles.The cellulose fibers were first oxidized by TEMPO-mediated oxidation to form carboxylate groups followed by coupling to the amino group of large amino-SiO

Preparation of TEMPO-Oxidized Cellulose Membranes (MNOCM)
BSKP (10 g, dry weight) was dispersed into 1 L of deionized water with continuous mechanical stirring at 300 rpms, followed by addition of NaBr (0.1 g/g d.w.p.) and TEMPO (0.015 g/g d.w.p.) [25].The reaction was started by addition of NaClO solution (12%, 6, 8 and 10 mmol/g d.w.p.) at 25˚C.pH of the reaction was maintained at 10 ± 0.2 by addition of 0.5 M NaOH until no further decrease.
Finally, the pulp was washed with deionized water and filtered by a Büchner funnel with a filter cloth (400-mesh) to obtain TEMPO-oxidized cellulose (OC) suspension at a concentration of 0.5 wt%.
TEMPO-oxidized cellulose membranes (MNOCM) were prepared by vacuum filtering 50 ml of TEMP-oxidized cellulose suspension on a 0.45 μm filter membrane and dried at room temperature.

Preparation of Amino-Functionalized Silica Nanoparticles (Amino-SiO2 NPs)
A variety of sizes of silica nanoparticles were obtained by polymerization of TEOS using the Stöber method [26].Typically, a fixed volume of TEOS was added drop wise to a flask under magnetic stirring containing ammonia solution, water, and ethanol (see Table 1) followed by addition of 5 mL of APS and heating at 70˚C for 6 h.The resulting silica nanoparticles were separated by centrifugation and washed by ethanol in triplicate.Finally, the obtained particle was dried in vacuo at 50˚C overnight.added drop wise with mechanical stirring, and was kept at 126˚C under N 2 atmosphere for 3 h.The resulting silica nanoparticles were separated by centrifugation and washed with toluene in triplicate.Finally, the epoxy-functionalized silica nanoparticles were dried in vacuo at 50˚C overnight.

Covalent LbL Assembly to Derive the Micro/Nano Surface Texture
Micro/nano surface architecture was obtained by inter-grafting the two sizes of nanoparticles as obtained from the LbL assembly process.The first step was grafting amino-SiO 2 NPs onto the MNOCM through carbodiimide-mediated coupling of the amino group from amino-SiO 2 NPs to the carboxyl group of the TEMPO-oxidized cellulose membranes.First, 0.02 g of AS was dispersed into 8 mL of ethanol and ultrasonicated for 10 mins.Subsequently, 0.1 g EDC, 0.1 g NHS, and MNOCM were added and kept at room temperature for 4 h.After reaction, the MNOCM@SiO 2 -NH 2 was removed and washed with ethanol for three times.
The second step was reacting MNOCM@SiO 2 -NH 2 and epoxy-SiO 2 NPs.0.02 g of epoxy-SiO 2 NPs by dispersing into 8 mL of ethanol and ultrasonicating for 10 mins.Then the MNOCM@SiO 2 -NH 2 was immersed in the solution at 40˚C for 12 h to afford surface 1 (denoted as 1 layer).Several cycles were repeated to afford 2 to 5 additional layers.

Silanization of MNOCM Using Chemical Vapor Deposition (CVD)
The as-prepared MNOCMs were treated with PFOTES by chemical vapor phase deposition [9].The samples were placed in a 250 mL metal bottle containing two small open bottles holding 200 μL of PFOTES and 1 mL of deionized water.The metal bottle was then sealed and heated inside an 80˚C oven for 6 h.Subsequently, the surface-treated samples were taken out and placed in a vacuum oven for more than 1 h at 60˚C to remove unreacted silane.

Transmission Electron Microscopy (TEM)
The size distribution of SiO 2 NPs was obtained from TEM images.Samples were dispersed in ethanol (0.02% w/v) and deposited on a carbon-coated copper grid.

Morphology and Nanostructure
The MNOCM were first sectioned into pieces.These pieces were fixed on sample holders and coated with Au for SEM (Merlin, Zeiss, Germany) at an operating voltage of 10 kV in secondary electron mode.

Contact Angle
Water contact angle (WCA) measurements were carried out on an OCA20 Micro (Data physics Instruments) to evaluate hydrophobicities by using a droplet (5 μL) of deionized water as an indicator at room temperature.The values reported were taken after the contact angle equilibrated.

Preparation of Silica Nanoparticles
In general, the pre-eminent physical factors that contribute to the phenomenon referred to as superhydrophobicity are (1) roughness or surface texture and ( 2) attenuated (low) surface energy [29].The silica nanoparticles to induce the micro/nano texturized structure on the MNOCM surface were prepared by the Stober procedure which is characterized by catalyzed hydrolysis and condensation of TEOS under alkaline conditions [12] [30] (Figure 1(a)).It is noteworthy that in the process of TEOS hydrolysis, the ammonia content, alcohol dosage, temperature and the time of the reaction influence the sizes of silica nanoparticles.
Figure 1.Scheme for the preparation of silica nanoparticles.respectively [32], whereas the peak at ~955 cm −1 corresponded to the bending vibration of Si-OH [11].
When the surface silica nanoparticles were grafted subsequently AMS or GPS, new peak appeared at ~2945 cm −1 , corresponds to the C-H stretching of -CH 2 groups [11], which confirmed that AMS or GPS had been successfully grafted because both groups contain -CH 2 -.Moreover, the peaks (blue line) at ~1538 cm −1 (N-H bending) and ~1400 cm −1 (C-H bending) further proved amino groups existed in the modified silica particles.The color of these modified particles changed blue-purple within a few min when added into 5% ninhydrin aqueous solution.This reaction is the typical method to identify the amino group existence.

Installation of NPs on the Surface of Oxidized Cellulose Membranes (OCM)
In general, the rough micro/nano surface texture of the final MNOCM was created by covalent LbL assemblies of alternating large silica amino-NPs and smaller silica epoxy-functionalized-NPs. Figure 4 showed the scheme of reaction.TEMPO-mediated oxidation was carried out to produce varying levels of surface carboxyl on cellulose.The acid enriched sites were exploited to chemically anchor amino groups on the amino-SiO 2 NPs by activation with EDC and NHS (Figure 3).Additionally, free amino groups can react with the epoxy groups of epoxy-SiO 2 NPs by ring opening to afford layer 1 [2].Subsequently, free epoxy groups can react with amino-SiO 2 NPs again.These reactions can be repeated as many times as desired to form n layers.Finally, samples were treated with PFOTES through vapor phase deposition to provide a surface capping layer as stated previously (vide supra).The EDX of MNOCM after treatment with PFOTES is presented in Figure 9.
Only carbon, oxygen, fluorine, silicon and sodium elements were detected.The carbon and oxygen were of course from MNOCM, silica, and PFOTES.Sodium was the counterion from the carboxyl groups to demonstrate that a fraction of the surface neat carboxylic groups remained carboxylates, silicon originated from silica and PFOTES, whereas fluorine was exclusively from PFOTES.These results provided further confirmation that silica NPs and PFOTES were on the MNOCM surface.
The sizes of amino-SiO 2 NPs were varied for the covalent layer-by-layer assembly on the MNOCM surface to interrogate how particles sizes affect hydrophobicity (Table 2).Although, the 70 nm and 260 nm amino-SiO 2 NPs showed superhydrophobicity, applying 500 nm (~twice the second sample size) amino-SiO 2 , the water contact angle was attenuated to 140˚ ± 2˚.Therefore, the hydrophobicity of the raspberry-like structure for the silica NP coated surfaces is not only a function of the numbers of layers, but also of the size of the silica na-    Self-cleaning is a property that is typically associated with superhydrophobicity as evidenced in the self-cleaning of mud from lotus leaves.Therefore, the self-cleaning of superhydrophobic MNOCM was evaluated according to a simple construct shown in Figure 10.The superhydrophobic-MNOCM films were attached to a glass slide onto which a dry slurry of activated carbon was placed to simulate contaminants (Figure 10

Conclusion
How to cite this paper: Yu, C.H., Wang, F., Lucia, L.A. and Fu, S.Y. (2017) Induction of Superhydrophobicity in a Cellulose Substrate by LbL Assembly of Covalently Linked Dual-Sized Silica Nanoparticles C. H. Yu et al.DOI: 10.4236/ampc.2017.712031396 Advances in Materials Physics and Chemistry of self-cleaning as typified by a pristine lotus leaf surrounded by mud.It has been determined that the surface of such materials possess a hierarchical

Figure 3 .
Figure 3. FT-IR spectra of silica nanoparticles before (after Stöber modification) and after AMS and GPS surface grafting.

Figure 4 .
Figure 4. Schema of constructing rough micro/nano texture by covalent LbL assemblies.

Figure 6 .
Figure 6.SEM images of cellulose films surfaced onto which SiO 2 -NPs are installed.

Figure 7 (
c) and Figure 7(d) showed the particle distribution in C. H. Yu et al.DOI: 10.4236/ampc.2017.712031404 Advances in Materials Physics and Chemistry

Figure 8 .
Figure 8.The sliding process for a water droplet (8 μL) on the MNOCM.
(a)).Upon the disposition of water to the surface, the intrinsic surface tension of water on such an ultralow energy surface caused it to roll along down the incline due to gravity.During its roll, the contaminants on the surface were adsorbed (Figures 10(b)-10(e)), leaving a clean surface without any ostensible trace of contaminants (Figure 10(f)).
Superhydrophobic surfaces on MNOCM were successfully prepared via covalent LbL assembly of amino-SiO 2 NPs and epoxy-SiO 2 NPs.The amino-SiO 2 NPs were grafted onto the MNOCM surface through a condensation reaction between the amino functionalities of the SiO 2 NPs and the carboxyl functionalities of the MNOCM surface followed by covalent LbL assembly of epoxy-SiO 2 NPs.The change in wettability was attributed to increases in final LbL layer surface .Yu et al.DOI: 10.4236/ampc.2017.712031407 Advances in Materials Physics and Chemistry

Figure 10 .
Figure 10.Simplified self-cleaning test for the superhydrophobic MNOCM films using a gently inclined glass slide upon which water droplets were added. 2

Table 1 .
Preparation of silica nanoparticles.

Table 2 .
The water contact angles (WCA) of the surface with different sizes of the SiO 2 nanoparticles after assembly of 5 layers.