Eco-Friendly Synthesis of Water-Glass-Based Silica Aerogels Via Catechol-Based Modiﬁer

nanomaterials

Article Eco-Friendly Synthesis of Water-Glass-Based Silica Aerogels via Catechol-Based Modiﬁer

Hyeonjung Kim 1 , Kangyong Kim 1 , Hyunhong Kim 1, Doo Jin Lee 2 and Jongnam Park 1,3,*

1 School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea; [email protected] (H.K.); [email protected] (K.K.); [email protected] (H.K.) 2 DPG Project and Chemical Business Planning Team, SKC, Seoul 03142, Korea; [email protected] 3 Department of Biomedical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea * Correspondence: [email protected]; Tel.: +82-52-217-2927

Received: 15 October 2020; Accepted: 27 November 2020; Published: 1 December 2020

Abstract: Silica aerogels have attracted much attention owing to their excellent thermal insulation properties. However, the conventional synthesis of silica aerogels involves the use of expensive and toxic alkoxide precursors and surface modifiers such as trimethylchlorosilane. In this study, cost-effective water-glass silica aerogels were synthesized using an eco-friendly catechol derivative surface modifier instead of trimethylchlorosilane. Polydopamine was introduced to increase adhesion to the SiO2 surface. The addition of 4-tert-butyl catechol and hexylamine imparted hydrophobicity to the surface and suppressed the polymerization of the polydopamine. After an ambient pressure drying process, catechol-modified aerogel exhibited a specific surface area of 377 m2/g and an average pore diameter of approximately 21 nm. To investigate their thermal conductivities, glass wool sheets 1 1 were impregnated with catechol-modified aerogel. The thermal conductivity was 40.4 mWm− K− , 1 1 which is lower than that of xerogel at 48.7 mWm− K− . Thus, by precisely controlling the catechol coating in the mesoporous framework, an eco-friendly synthetic method for aerogel preparation is proposed.

Keywords: water-glass aerogel; catechol coating; eco-friendly; trimethylchlorosilane substitute; ambient pressure drying; thermal insulation

1. Introduction Silica aerogels are highly mesoporous materials composed of interconnected silica backbones. Due to their porous nanostructures with air pockets, they have high specific surface areas 2 1 3 1 1 (400–1000 m g− ), low densities (<0.05 gcm− ), and low thermal conductivities (<20 mWm− K− )[1–3]. Thus, owing to their unique properties, aerogels have been widely applied in various industries as catalysts, absorbents, and heat-insulating materials [4–10]. However, conventional aerogel synthesis is difficult to commercialize because it requires expensive and toxic alkoxide precursors such as tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS) for the sol–gel reaction and supercritical drying (SCD) processes, which require special instruments for high-pressure conditions to maintain their porous framework without shrinkage. To solve these problems, researchers have developed wet gels using inexpensive sodium silicates, called water-glass, and applied the ambient pressure drying (APD) method instead of the supercritical process [11–16]. Minimization of the shrinkage of gels during APD requires a “spring-back” effect that implies repulsion throughout the framework through hydrophobization of the silica surface. Such hydrophobization can be achieved either by using co-precursors such as methyltrimethoxysilane (MTMS), phenyltriethoxysilane (PTES), or dimethyldiethoxysilane (DMDES), or by treating silylation

Nanomaterials 2020, 10, 2406; doi:10.3390/nano10122406 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 2406 2 of 10 agents such as trimethylchlorosilane (TMCS), but both methods are expensive and not environmentally friendly [17,18]. Therefore, the development of an inexpensive and eco-friendly surface treatment method is required, but research studies on alternatives of silylation agents on the surface of water-glass aerogels are limited. As a different approach for eco-friendly aerogel, many studies have been conducted to develop the aerogels based on organic networks with low toxicity such as cellulose, lignin, and chitosan-based aerogels and increase their mechanical properties [19–23]. However, these materials still have a disadvantage of relatively low flame retardancy than silica aerogel; it is necessary to find a proper surface modifier imparting the hydrophobicity to the inorganic silica aerogel without the silylation agents. Catechol derivatives, which have a high affinity for metal oxide surfaces, have been studied as environmentally friendly surface modifiers. Das et al. found that a hydrophobic self-assembled monolayer (SAM) could be formed on silica using mussel-derived catechol molecules [24]. In addition, Wang et al. improved the mechanical strengths and textural properties of silica aerogels by using catechol molecules as co-precursors for gelation [25]. However, there is still a lack of research on the use of catechol molecules as hydrophobic surface coating agents for APD-processed aerogels. In this work, we focused on applying the good adhesion properties of catechol derivatives to silica frameworks in order to form hydrophobic surfaces without toxic TMCS. Examples of catechol derivatives include 4-tert-butylcatechol (TBC) and polydopamine (PDA). The degree of polymerization and hydrophobicity can be controlled by alkylamine treatment, thereby optimizing the environment-friendly surface treatment method. Catechol surface-treated silica wet gel was dried at ambient pressure, and the presence and sizes of pores were compared with those of xerogel using a scanning electron microscope (SEM) and Barrett–Joyner–Halenda (BJH) analysis. In addition, the synthesized aerogel was impregnated into a glass wool sheet for the fabrication of a thermal insulation blanket. The thermal conductivity analysis results showed a high reduction in thermal conductivity for this blanket compared with that of the xerogel–glass wool composite. Thus, instead of toxic hydrophobic modifiers, catechol derivatives were used for the synthesis of APD-processed aerogels, which was found to be an eco-friendly and economical synthetic method.

2. Materials and Methods

2.1. Materials

Sodium silicate solution (water-glass, SiO2 content of 28–30 wt%, SiO2:Na2O = 3.52:1) was purchased from Young Il Chemical Co., Ltd., Incheon, Korea. Glass wool sheet was purchased from GF Chem., Pyeongtaek, Korea. TBC ( 99.0%), dopamine hydrochloride, hexylamine (HA, 99%), ≥ TMCS ( 98.0%), and hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich, Seoul, Korea. ≥ Methanol, ethanol, n-hexane, and isopropanol were obtained from Samchun ChemicalsPohang, Korea. All chemicals were used without further puriﬁcation.

2.2. Preparation of Water-Glass Based Silica Wet Gel and Aerogel-Impregnated Glass Wool Sheet All steps of wet-gel preparation were conducted under ambient temperature and atmosphere. First, 60 mL of the water-glass solution was mixed with 350 mL of deionized water. Next, 10 mL of diluted water-glass solution was mixed with 3.5 mL of 10 vol% HCl solution and poured into a mold. The gelation process was completed within 1 h. Once the wet gel had formed, we added 5 mL of deionized water and then aged the gel at room temperature for 2 days to strengthen the gel network. After aging, the wet gel was pulverized into small pieces and washed 3 times with 20 mL of with deionized water before further surface modification. Aerogel-impregnated glass wool sheets were prepared by the following method. Prior to gelation and surface modification, glass wool sheets were cut to 4 cm 6 cm 1 cm × × cuboids. Subsequently, the glass wool sheets were soaked in 30 mL of diluted water-glass solution. Next, the gelation process was carried out by adding 10.5 mL of acidic HCl solution. Next the wet gel–glass wool composite was aged for 2 days. After aging, the composite was washed 3 times with deionized Nanomaterials 2020, 10, 2406 3 of 10 water before further surface modification. The hydrophobization and drying steps were performed in the same manner as the aerogel surface modification steps described below.

2.3. Preparation of TMCS-Modified Aerogel and Xerogel For hydrophobization with TMCS, the wet gels formed in Section 2.2 were used. Firstly, the water in the wet gel was replaced with the isopropanol/hexane solution through several solvent exchange processes. After the solvent exchange process, the wet gels were chemically modified in 6.25 vol% TMCS/hexane solution for 24 h. The TMCS-modified wet gel was washed 3 times with hexane to remove residual modifiers and by-products. After the solvent was decanted, the hydrophobized wet gel was dried for 12 h at room temperature and ambient pressure; then, it was further dried in an oven at 60 ◦C for 12 h to obtain a TMCS-modified aerogel. The xerogel could be produced by the same drying process without surface modification.

2.4. Preparation of Catechol-Modified Aerogel The wet gel prepared in Section 2.2 was used for the surface modification applying catechol. Then, the water in the wet gel was replaced with alcohol solvents such as methanol and ethanol through several solvent exchange processes. After the solvent exchange process, the wet gels were chemically modified in 10.5 mM catechol solution (TBC or a mixture of dopamine hydrochloride and TBC, 1:5 molar ratio) for 24 h. The 210 mM HA reagent was added to the catechol solution for hydrophobicity. The catechol-modified wet gel was washed 5 times with alcohol to remove residual modifiers and by-products. Finally, the surface-modified aerogel was dried for 12 h at room temperature and ambient pressure; then, it was further dried in an oven at 60 ◦C for 12 h.

2.5. Surface Coating of SiO2 Wafer To demonstrate the properties of the catechol-coated surface, the coating on the silicon wafer was performed by a simple dip-coating method. We selected the silicon wafer with a 200 nm oxide layer as a substrate because its components are similar to that of silica aerogel surfaces, and the smoothness of the surface is helpful for elaborating ﬁlm analysis. The coating solution contained 10.5 mM of dopamine hydrochloride (or a mixture of dopamine hydrochloride and TBC, 1:5 molar ratio) and 210 mM of HA in methanol (or ethanol). SiO2 substrates were immersed vertically in the coating solution for 12 h. The substrate was carefully removed with argon blowing. Non-coated materials were washed out by a stream of distilled water, and the substrate was gently dried by argon blowing. Finally, we repeated the washing step three times and stored the samples in a desiccator for analysis.

2.6. Characterization The mesoporous structure of the aerogel was investigated by field-emission scanning electron microscopy (FE-SEM, Hitachi, Tokyo, Japan, SU8220) and transmission electron microscopy (TEM, JEOL, Tokyo, Japan, JEM-2100). The surface of the aerogel was analyzed by Fourier transform-infrared spectroscopy (FT-IR, Shimadzu, Kyoto, Japan, IRTracer-100) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA, ESCALAB 250XI). XPS spectra were recorded by Al Kα radiation (1486.6 eV) under ultrahigh vacuum (1.0 10 10 torr). The contact angle of a × − water droplet was measured using a drop shape analyzer (Krüss, Hamburg, Germany, DSA-100) to verify changes in hydrophobicity of the catechol-coated surfaces. Nitrogen adsorption–desorption measurements were carried out at 196 C using a BELSORP-Mini II instrument. The samples were − ◦ degassed at 120 ◦C for 24 h before the measurements. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The thermal conductivity of the aerogel-impregnated glass wool sheet was measured using a modified transient plane source (MTPS) sensor in a thermal conductivity analyzer (C-Therm, Fredericton, NB, Canada, TCi). Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by the Dunnett’s post hoc test. The probability value (p) < 0.05 was considered to indicate significant difference. Nanomaterials 2020, 10, 2406 4 of 10

3. Results and Discussion In general, APD-processed hydrophobic silica aerogel is produced by substituting the solvent in the wet gel with a hydrophobic solvent and then treating with a silylation agent such as TMCS. However, for surfaceNanomaterials modification 2020, 10 with, x catechol derivatives, it is important to select a suitable exchanging4 of 11 solvent considering3. Results the strength and Discussion of interaction with the modifier. Figure1a is a schematic illustration of aerogel production through surface modification of catechol derivatives. First, we chose TBC and methanol as In general, APD-processed hydrophobic silica aerogel is produced by substituting the solvent in the surfacethe modifier wet gel with and a solvent, hydrophobic respectively. solvent and The thentert treating-butyl with group a silylation of TBC agent induces such hydrophobicityas TMCS. on the silica surfaces,However, for similar surface to modification the methyl with group catechol of TMCS. derivatives, Methanol it is important dissolves to theselect TBC a suitable molecules well and assistsexchanging in their permeation solvent considering into the the gelstrength network. of intera Inction Figure with 2the, the modifier. morphologies Figure 1a is of a schematic the silica aerogels illustration of aerogel production through surface modification of catechol derivatives. First, we are obtainedchose via TBC SEM and accordingmethanol as tothe the surface surface modifier modification and solvent, methodsrespectively. and Thethen tert-butyl are compared.group of In the case of xerogelTBC induces without hydrophobicity surface modification, on the silica surfaces, pores similar disappear to the methyl due to group shrinkage of TMCS. of Methanol the gel networks during thedissolves APD process the TBC molecules (Figure2 wella). However,and assists in the thei TBC-r permeation and TBC into andthe gel HA-modified network. In Figure aerogels 2, show the morphologies of the silica aerogels are obtained via SEM according to the surface modification the characteristics of type IV mesoporous materials in N2 physisorption isotherms (Figure S1) [26]. methods and then are compared. In the case of xerogel without surface modification, pores disappear After TBCdue surface to shrinkage treatment, of the gel mesoporous networks during silica the networksAPD process with (Figure an 2a). average However, pore the TBC- size and of 23 nm are 2 preservedTBC with and a specificHA-modified surface aerogels area show of 308the characteristics m /g. In comparison, of type IV mesoporous the average materials pore sizein N2 of xerogel was 3.23 nm,physisorption and the isotherms surface area(Figure was S1) [26]. 98 m After2/g. TBC (Figure surface2b, treatment, Table1). mesoporous These results silica indicatenetworks that the tert-butylwith group an ofaverage TBC pore contributes size of 23 tonm the are surface preserved hydrophobicity with a specific surface for a “spring-back”area of 308 m2/g. eInff ect. Next, comparison, the average pore size of xerogel was 3.23 nm, and the surface area was 98 m2/g. (Figure we add HA2b, toTable the 1). TBC These solution results indicate with an that expectation the tert-butyl of group two of roles. TBC contributes One is the tofabrication the surface of a thin catechol coatinghydrophobicity layer. for The a “spring-back” catechol derivatives effect. Next, we are add easily HA to oxidizedthe TBC solution and with polymerized an expectation under basic conditionsof [ 27two]. roles. In these One is polymerization the fabrication of a processes, thin catechol the coating primary layer. The amine catechol suppresses derivatives the are polymerizationeasily of catecholoxidized molecules and polymerized by Michael under addition basic conditions and Schi ff[27].base In reactionsthese polymerization [28–30]. processes, By adding the HA to the primary amine suppresses the polymerization of catechol molecules by Michael addition and Schiff TBC coatingbase solution, reactions [28–30]. we can By adjust adding the HA thickness to the TBC co ofating the coatingsolution, we layer can andadjust prevent the thickness the mesoporeof the from clogging withcoating coating layer and materials. prevent the mesopore The second from clogging role of with the additioncoating materials. of HA The to second the TBC role of solution the is the enhancementaddition of hydrophobicityof HA to the TBC solution owing is tothe the enhanc alkylement chain of hydrophobicity exposed on owing the surface. to the alkyl As chain a result of the treatmentexposed of TBC on and the HA,surface. the As specific a result of surface the treatm areaent and of TBC average and HA, pore the specific diameter surface increase area and to 340 m2/g average pore diameter increase to 340 m2/g and 25 nm, respectively, compared to those of the TBC- and 25 nm,modified respectively, aerogel (Figure compared 2c, Table to those1). of the TBC-modified aerogel (Figure2c, Table1).

Figure 1. Schematic of (a) the synthesis process of catechol-modiﬁed water-glass-based silica aerogel, and (b) the proposed surface structure of 4-tert-butylcatechol (TBC) + hexylamine (HA) case (left) and polydopamine (PDA) + TBC + HA case (right). Nanomaterials 2020, 10, x 5 of 11

Figure 1. Schematic of (a) the synthesis process of catechol-modified water-glass-based silica aerogel, Nanomaterialsand (b)2020 the, proposed10, 2406 surface structure of 4-tert-butylcatechol (TBC) + hexylamine (HA) case (left) and5 of 10 polydopamine (PDA) + TBC + HA case (right).

FigureFigure 2.2. ScanningScanning electronelectron microscopymicroscopy (SEM)(SEM) imagesimages ofof ((aa)) xerogel,xerogel, ((bb)) TBC,TBC, ((cc)) TBCTBC+ + HA,HA, andand ((dd)) trimethylchlorosilanetrimethylchlorosilane (TMCS)-modified (TMCS)-modified aerogel. aerogel. (e (,ef),f Fourier) Fourier transform-infrared transform-infrared spectroscopy spectroscopy (FT-IR) (FT- spectraIR) spectra of xerogel, of xerogel, TBC-modified TBC-modified aerogel, aerogel, TBC + HA-modifiedTBC + HA-modified aerogel, aerogel, and TMCS-modified and TMCS-modified aerogel. aerogel. Table 1. Results of Brunauer–Emmett–Teller (BET) analysis.

Physical PropertyTable 1. Xerogel Results of TBC Brunauer–Emmett–Teller TBC + HA PDA + (BET)TBC analysis. PDA + TBC + HA TMCS BETPhysical surface areaProperty (m2/g) Xerogel98 308 TBC TBC 340 + HA PDA 375 + TBC PDA + 377 TBC + HA TMCS 473 BETPore surface volume area (cm3 (m/g)2/g) 0.0898 1.77308 2.13340 2.14 375 1.93 377 4.15 473 AveragePore pore volume diameter (cm3 (nm)/g) 3.230.08 22.901.77 25.022.13 22.82 2.14 20.52 1.93 35.11 4.15 Average pore diameter (nm) 3.23 22.90 25.02 22.82 20.52 35.11 The surface structure of the modified aerogels is analyzed by FT-IR spectroscopy (Figure2e,f). 1 The absorptionThe surface peaksstructure near of the 1635, modified 1070, aerogels 948, and is analyzed 799 cm− byappear FT-IR spectroscopy commonly in(Figure allsilica 2e,f). aerogelThe absorption structures peaks and representnear 1635, the 1070, physically 948, and adsorbed 799 cm− water1 appear molecules commonly deformation in all silica vibrations, aerogel Si-O-Sistructures asymmetric and represent stretching the physically vibrations, adsorbed Si-O in-plane water stretchingmolecules vibrations,deformation and vibrations, Si-O symmetric Si-O-Si 1 vibrations,asymmetric respectively stretching [vibrations,31]. The TBC-containing Si-O in-plane aerogelsstretching show vibrations, peaks at and 2980, Si-O 2960, symmetric 1466, and vibrations, 1380 cm− , whichrespectively are not [31]. seen The in the TBC-containing xerogel (yellow aerogels and blue show boxed peaks region). at 2980, These 2960, peaks 1466, indicate and 1380 C-H asymmetriccm−1, which stretching,are not seen symmetric in the xerogel stretching, (yellow and and deformation blue boxed vibrations region). These originating peaks fromindicate the C-Htert-butyl asymmetric group in TBC molecules. In addition, we performed XPS analysis because it was difficult to find distinct changes between TBC and TBC + HA modified aerogels in FT-IR due to the small amount of the Nanomaterials 2020, 10, x 6 of 11 stretching, symmetric stretching, and deformation vibrations originating from the tert-butyl group in TBC molecules. In addition, we performed XPS analysis because it was difficult to find distinct Nanomaterials 2020, 10, 2406 6 of 10 changes between TBC and TBC + HA modified aerogels in FT-IR due to the small amount of the coating layer. In the N 1s XPS spectra, the TBC + HA aerogel showed a peak at 400.4 eV, which is coatingindicating layer. a C-N In bond the N of 1s secondary XPS spectra, amine the (Figure TBC + HAS2) [32]. aerogel This showed peak indicates a peak the at 400.4 reaction eV, whichbetween is indicatingTBC and HA a C-N and bondis evidence of secondary of the two amine roles (Figure of HA: S2) suppression [32]. This of peak polymerization indicates the and reaction enhancement between TBCof hydrophobicity. and HA and is The evidence control of theTMCS-modified two roles of HA: aerogel suppression exhibited of the polymerization absorption peaks and enhancement at 1258, 845, ofand hydrophobicity. 758 cm−1 are attributed The control to the TMCS-modified symmetric deformation aerogel exhibited vibrations the and absorption stretching peaks vibrations at 1258, of 845, Si- 1 andC bonds. 758 cm In− addition,are attributed it showed to the a specific symmetric surface deformation area of 473 vibrations m2/g and and pore stretching diameters vibrations of ≈35 nm of with Si-C 2 bonds.the characteristic In addition, of ittype showed Ⅳ mesoporous a specific surface materials area in of the 473 N m2 physisorption/g and pore diameters isotherm. of(Figure35 nm 2d with and ≈ theFigure characteristic S1, Table 1). of type IV mesoporous materials in the N2 physisorption isotherm. (Figure2d and FigureTBC S1, coating Table1). has shown potential for the hydrophobization of silica aerogel; however, it is known that catecholTBC coating molecules has shown have potential weak binding for the hydrophobizationaffinity to SiO2 surfaces of silica and aerogel; strong however, binding it affinity is known to thatmetal catechol oxides molecules(e.g., Al2O have3, Fe2 weakO3, and binding TiO2) a surfacesffinity to (Figure SiO2 surfaces 1b) [33–35]. and strong To improve binding the affi nityadhesion to metal to oxidesSiO2, we (e.g., introduce Al2O3, Fecatecholamine2O3, and TiO and2) surfaces dopamine, (Figure which1b) [can33– 35be]. universally To improve coated the adhesion on any tosurface SiO2, we[27,36]. introduce However, catecholamine another problem and dopamine, arises when which only can dopamine be universally is used coated as a onsurface anysurface modifier. [27 ,The36]. However,mesoporous another structure problem collapses arises similar when only to that dopamine of xerogel: is used by asthe a surfaceuncontrollable modifier. polymerization The mesoporous of structuredopamine collapses and its hydrophilic similar to thatproperties of xerogel: (Figure by 3) the. This uncontrollable problem cannot polymerization be solved even of dopamine when the andcoating its hydrophilictime is reduced properties from 24 (Figure h to 103 min.). This To problemaddress this cannot problem, be solved we apply even TBC when and the HA coating to the timePDA iscoating. reduced TBC from and 24 h toHA 10 are min. expected To address to thissuppress problem, the we formation apply TBC of and PDA HA and to the impart PDA coating.hydrophobicity TBC and (Figure HA are 1b). expected To verify to this suppress hypothesis, the formation we obtain of contact PDA andangle impart data for hydrophobicity PDA, PDA + (FigureHA, PDA1b). + ToTBC, verify and this PDA hypothesis, + TBC + HA we coatings obtain contact on SiO angle2 wafers data (Figure for PDA, 4a). PDA The contact+ HA, PDAangle+ ofTBC, the andPDA PDA film +isTBC 48.2°,+ HAwhich coatings indicates on SiOinsufficient2 wafers (Figurehydrophobicity4a). The contactfor the angle“spring-back” of the PDA effect. film We is 48.2confirm◦, which an increase indicates in insu hydrophobicityfficient hydrophobicity as the contact for the angle “spring-back” of the PDA e ff+ect. HA We and confirm PDA + an TBC increase films inincrease hydrophobicity from 48.2° as to the 67.6° contact and angle 70.4°, of respectively. the PDA + HA However, and PDA in+ TBCthe case films of increase PDA + from HA, 48.2 the◦ tomesoporous 67.6◦ and 70.4silica◦, network respectively. is not However, maintained in thebecaus casee of PDAweak +hydrophobizatiHA, the mesoporouson (Figure silica S3). network On the isother not hand, maintained we observe because a mesoporous of weak hydrophobization morphology in (Figure PDA + S3).TBC On modified the other aerogels, hand, we which observe has a 2 mesoporousspecific surface morphology area of 375 in m PDA2/g and+ TBC pore modified diameters aerogels, of ≈23 which nm (Figure has a specific4b, Table surface 1). The area associated of 375 m N/g2 andadsorption–desorption pore diameters of 23isotherm nm (Figure and 4BJHb, Table pore1). size The distribution associated Ngraphsadsorption–desorption are shown in Figure isotherm S4. We ≈ 2 andinferBJH that pore the sizetert-butyl distribution group graphs provides are shownbetter inhydropho Figure S4.bicity We than infer HA that and the tertsuppresses-butyl group the providespolymerization better hydrophobicityof dopamine effectively. than HA andWhen suppresses TBC and the HA polymerization are used simultaneously, of dopamine the eff ectively.contact Whenangle is TBC the andhighest HA areon the used SiO simultaneously,2 wafer due to the contactsynergetic angle effect, is the and highest the specific on the surface SiO2 wafer area dueof the to theaerogel synergetic increases effect, slightly and the to specific377 m2/g surface compared area to of that the aerogel of the PDA increases + TBC slightly aerogel to (Figure 377 m2/ 4c,g compared Table 1). toPolymerization that of the PDA suppression+ TBC aerogel of PDA (Figure is also4 confirmedc, Table1). by Polymerization observing the color suppression of the catechol-modified of PDA is also confirmedaerogels (Figure by observing S5). The the color color fade of thewith catechol-modified an increasing ratio aerogels of TBC (Figure to PDA S5). and The with color the fade addition with anof increasingHA. This pale ratio color of TBC indicates to PDA a and thin with catechol the addition coating of and HA. the This possibility pale color of indicates increased a thinpore catechol size by coatingcontrolling and catechol the possibility polymerization. of increased pore size by controlling catechol polymerization.

Figure 3. SEM images of PDA-modified PDA-modiﬁed aerogel incubated for (a) 10 minmin andand ((b)) 2424 h.h.

Nanomaterials 2020, 10, 2406 7 of 10 Nanomaterials 2020, 10, x 7 of 11

Figure 4. ((aa)) Water Water contact anglesangles onon catechol-coatedcatechol-coated SiO SiO22wafers wafersincubated incubated for for 12 12 h h with with PDA, PDA, PDA PDA+ +HA, HA, PDA PDA+ +TBC, TBC, and and PDA PDA+ +TBC TBC+ + HA.HA. ((pp << 0.05, one-way ANOVA) SEMSEM imagesimages ofof ((bb)) PDAPDA ++ TBC and ( c) PDA + TBCTBC ++ HA-modifiedHA-modiﬁed aerogel. aerogel.

Aerogels are effective effective thermal insulators because aerogels are mainly composed of air in small nanopores. BeforeBefore analyzing analyzing the thermalthe thermal insulating insu property,lating property, we confirmed we confirmed whether the whether hydrophobic the hydrophobicsurface treatment surface contributed treatment to contributed the preservation to the of pr theeservation mesoporous of the structure mesoporous through structure the “spring-back” through theeffect “spring-back” through TEM effect analysis. through As TEM a result analysis. of the As TEM a result analysis, of the in TEM the analysis, case of xerogel, in the case the of mesopore xerogel, thewas mesopore not preserved was not at all,preserved but the at samples all, but treatedthe samples with catecholtreated with modifiers catechol showed modifiers mesoporous showed mesoporousstructures similar structures to TMCS-modified similar to TMCS-modified aerogels (Figure aerogels5a–d). (Figure These 5a–d). results These are direct results evidence are direct of evidencethe formation of the of formation a hydrophobic of a hydrophobic thin layer onthin the la silicayer on skeleton the silica without skeleton clogging without the clogging mesopore. the mesopore.Therefore, theTherefore, synthesized the synthesizedaerogel is impregnated aerogel is impr intoegnated glass wool into glass sheets wool for thesheets fabrication for the fabrication of thermal ofinsulation thermal blankets;insulation then, blankets; the thermal then, the conductivity thermal co isnductivity measured is and measured analyzed and to analyzed see the effi tociency see the of efficiencycatechol modification. of catechol modificati We thenon. modify We then the aerogel-impregnatedmodify the aerogel-impregnated glass wool sheetglass withwool TBCsheet+ withHA, TBCPDA ++ HA,TBC PDA+ HA + ,TBC and + TMCS HA, and (Figure TMCS S6). (Figure Thermal S6). Thermal conductivities conductivities are measured are measured using anusing MTPS an MTPSsensor sensor after placing after placing a 1 kg a weight 1 kg weight on the on fabricated the fabricated glass glass wool wool sheets sheets (Figure (Figure5e). The5e). xerogelThe xerogel has 1 1 hasthe highestthe highest thermal thermal conductivity conductivity of 48.7of 48.7 mWm mWm− K−1−K−1because because it itonly only containscontains aa smallsmall volume with air. The The TBC TBC ++ HA-HA- and and PDA PDA + TBC+ TBC + HA-modified+ HA-modified samples samples show show thermal thermal conductivities conductivities of 37.9 of 37.9and 1 1 40.4and 40.4mWm mWm−1K−1−, respectively,K− , respectively, lower lower than than that that of ofthe the xe xerogelrogel (Figure (Figure 5f).5f). These These values showedshowed comparable performance to aerogel-impregnated blankets measured under the similar conditions in previously reported papers [[37,38].37,38]. The thermal conductivityconductivity of the eco-friendly catechol-modifiedcatechol-modified aerogel is higher than that of the TMCS-modified TMCS-modified one but shows considerably higher heat insulation compared toto thatthat ofof xerogel. xerogel. This This economical economical water-glass water-glass aerogel aerogel suggests suggests possible possible applications applications in the in thethermal thermal insulation insulation blanket blanket industry. industry.

Nanomaterials 2020, 10, 2406 8 of 10 Nanomaterials 2020, 10, x 8 of 11

FigureFigure 5. Transmission 5. Transmission electron electron microscopy microscopy images images of of ( a(a)) xerogel,xerogel, ( b)) TBC TBC ++ HA,HA, (c ()c PDA) PDA + TBC+ TBC + HA,+ HA, and (andd) TMCS-modified(d) TMCS-modified silica silica aerogel aerogel (magnification (magnification ofof 13.5k). ( (ee) )Schematic Schematic of ofthermal thermal conductivity conductivity analyzeranalyzer using using an an modified modified transient transient plane planesource source (MTPS)(MTPS) sensor (TCi, (TCi, C-Therm). C-Therm). (f) (Thermalf) Thermal conductivitiesconductivities on glasson glass wool wool sheets sheets impregnated impregnated by by surface-modifiedsurface-modified silica silica aerogel. aerogel.

4. Conclusions4. Conclusions In summary,In summary, we we have have developed developed a a novel novel catechol-modified catechol-modified water-glass water-glass aerogel aerogel without without the theuse use of expensiveof expensive and and hazardous hazardous materials. materials. CatecholCatechol derivatives are are used used instea insteadd of ofTMCS TMCS as surface as surface modifiers. The tert-butyl group of TBC and the alkyl chain of HA impart hydrophobicity and modifiers. The tert-butyl group of TBC and the alkyl chain of HA impart hydrophobicity and suppress suppress the polymerization of the catechol group. Dopamine is introduced to increase the binding the polymerization of the catechol group. Dopamine is introduced to increase the binding strength strength between SiO2 and catechol. A combination of TBC, PDA, and HA is able to produce a water- betweenglass SiOaerogel2 and similar catechol. to the A TMCS-modified combination ofversion. TBC, PDA,Finally, and we HAimpregnate is able tothe produce water-glass a water-glass aerogel aerogelinto similarglass wool to the sheets TMCS-modified and modify them version. with ca Finally,techol derivatives. we impregnate This thetechnique water-glass has commercial aerogel into glasspotential wool sheets as a cost-effective and modify themthermal with insulation catechol blanket. derivatives. This technique has commercial potential as a cost-effective thermal insulation blanket. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: N2 adsorption–desorption isotherms and the corresponding BJH pore size distributions of (a,b) xerogel, (c,d) TBC, Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/12/2406/s1, (e,f) TBC + HA, and (g,h) TMCS-modified aerogel. Figure S2: N 1s XPS spectra of TBC and TBC + HA-modified Figure S1: N2 adsorption–desorption isotherms and the corresponding BJH pore size distributions of (a,b) xerogel, (c,d)aerogel. TBC, (e,f) Figure TBC S3:+ SEMHA, images and (g,h) of PDA TMCS-modified + HA-modified aerogel. aerogel at Figure different S2: magnifications: N 1s XPS spectra (a) 50k of and TBC (b) and 100k. TBC + HA-modifiedFigure S4: aerogel.N2 adsorption–desorption Figure S3: SEM images isotherms of PDAand the+ HA-modified corresponding aerogelBarrett–Joyner–Hal at differentenda magnifications: (BJH) pore size (a) 50k and (b)distributions 100k. Figure of (a,b) S4: PDA N2 adsorption–desorption + TBC and (c,d) PDA + isothermsTBC + HA-modified and the corresponding aerogel. FigureBarrett–Joyner–Halenda S5: Digital images of (BJH)various pore sizecatechol-modified distributions powder of (a,b) aerogels PDA + afterTBC residual and (c,d) modifier PDA washing+ TBC + steps.HA-modified Figure S6: Digital aerogel. images Figure of S5: Digital images of various catechol-modified powder aerogels after residual modifier washing steps. Figure S6: Digital images of glass wool sheets impregnated by (a) xerogel, (b) TBC + HA, (c) PDA + TBC + HA, and (d) TMCS-modified silica aerogel. Nanomaterials 2020, 10, 2406 9 of 10

Author Contributions: Conceptualization, J.P.; Data curation, J.P. and D.J.L.; Funding acquisition, J.P.; Investigation, H.K. (Hyeonjung Kim), K.K., D.J.L. and H.K. (Hyunhong Kim); Project administration, J.P.; Writing—original draft, H.K. (Hyeonjung Kim), K.K. and D.J.L.; Writing—review and editing, J.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the Research Project Funded by U-K Brand (1.200030.01) of UNIST and by a grant of the Korea Industrial Complex Corp. (KICOX, No. RUS20002). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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