Superhydrophobic coating of silica with photoluminescence properties synthesized from rice husk ash

Superhydrophobic coating of silica with photoluminescence properties synthesized from rice husk ash

Progress in Organic Coatings 111 (2017) 29–37 Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: www.elsevier...

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Progress in Organic Coatings 111 (2017) 29–37

Contents lists available at ScienceDirect

Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat

Superhydrophobic coating of silica with photoluminescence properties synthesized from rice husk ash

MARK



M.U.M. Junaidia,b, S.A. Haji Azamana, N.N.R. Ahmada, C.P. Leoa, , G.W. Lima, D.J.C. Chana, H.M. Yeec a b c

School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia Faculty of Civil Engineering, Universiti Teknologi MARA, 13500 Permatang Pauh, Penang, Malaysia

A R T I C L E I N F O

A B S T R A C T

Keyword: Superhydrophobic Photoluminescent Ash Concrete

Water penetration into concrete can cause the degradation of concrete strength, leading into structure failure. Superhydrophobic coatings on concrete received tremendous attention in the recent years as these coatings repel water like lotus leaves. In this work, rice husk ash (RHA) was used to prepare the superhydrophobic coating. Rice husk was calcined at 550 °C and 650 °C to form silica particles with a small amount of carbon residue. RHA 550 sample showed slightly higher photoluminescence (PL) intensity than RHA 660 sample as shown in fluorescent images and PL spectra. Such difference could be related to the variation of carbon content measured using scanning electron microscope-energy-dispersive X-ray spectroscopy. The carbon residue in nano size was detected in transmission electron microscope images. RHA with PL properties was further mechanochemically modified using 1H,1H,2H,2H-perfluorodecyltriethoxysilane (HFDS) or stearic acid in ethanol. However, PL properties of RHA 550 was slightly reduced due the successful grafting of hydrophobic groups on silica particles. The modified RHA in ethanol was later spray coated on a layer of commercial adhesive to form superhyrophobic coatings on glass slides and concrete. The superhydrophobic coating on concrete with the water contact angle as high as 157.7° was recorded.

1. Introduction The ingression of water into concrete not only causes the destructive expansion, but also results in the penetration of corrosive chloride ions. Hence, concrete degradation and structure failure due to the ingression of water into the hydrophilic concretes with pores and micro-cracks should be prevented. Different types of water resistant materials can be used to alter the surface energy on the concrete surface, pores and cracks for reducing concrete wetting. In the recent years, superhydrophobic coatings with water contact angles larger than 150° have been promoted since the superhydrophobic coatings can repel water droplets, dew drops and even dust particles [1]. The superhydrophobic coatings enhanced the anti-wetting, anti-corrosion, anti-icing, anticontamination and self-cleaning properties of concrete to combat the adverse weather conditions [2–5]. A wide range of hydrophobic chemicals can be used to reduce the surface energy of concrete, including silane, siloxanes and silicones [6]. These silicon based chemicals contains the hydrophobic alkyl groups to prevent wetting. However, the water repellence of superhydrophobic



Corresponding author. E-mail address: [email protected] (C.P. Leo).

http://dx.doi.org/10.1016/j.porgcoat.2017.05.009 Received 27 January 2017; Received in revised form 22 March 2017; Accepted 13 May 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.

coatings at Cassie-Baxter state requires the creation of surface roughness to capture a thin layer of air which can minimize water contact [1]. Many roughness creation methods such as sol-gel [7], self-assembly [8], templating [9], lithographic patterning [10], chemical etching [11], electrospinning [12] and chemical vapour deposition [13] have been reported. The addition of nanoparticles remains to be popular since it is time and cost saving. Expensive equipment or rigorous condition is not required as well. Hence, the popular formulation of superhyrophobic coatings usually consists of the hydrophobic polymer and the inorganic nanoparticles dispersed in the solvent. Various types of hydrophobic polymers were recently studied in the development of superhydrophobic coatings [14]. The superhydrophobic thin film made of polyvinylidene fluoride (PVDF) and SiO2 nanoparticles with fluoroalkyl groups was successfully coated on the glass surface using the surface functionalizing agent, aminopropyltriethoxysilane [14]. The superhydrophobic PVDF coating not only exhibited the satisfactory durability, the surface hydrophobicity was maintained even after UV irradiation. Incorporating the TiO2 nanoparticles with fluoroalkyl groups into PVDF coating, the superhydrophobic surface could be easily turned into

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located at Kuala Selangor, Malaysia. Citric acid and ethanol were purchased from Merck, Malaysia. On the other hand, 1H,1H,2H,2Hperfluorodecyltriethoxysilane (HFDS) from Gelest Inc. and SA (C18H36O2) at the analytical grade with 95% purity from Sigma Aldrich were used as the hydrophobic modifier. The commercial adhesive, 3 M Spray Mount™ Artist’s adhesive was used in spray coating.

hydrophilic under UV irradiation [15]. Thermal treatment could be used to restore the superhydrophobic surface of PVDF/TiO2 coating. In order to improve the durability of superhydrophobic coating, Wang et al. [16] utilized polydimethylsiloxane (PDMS) which is a common sealant to bond a mixture of SiO2 nanoparticles and fluoroalkylsilane coating on glass slide via spray coating. The commercial adhesive containing hydrocarbon resin had also been used to attach the hydrophobic silica nanoparticles on substrate strongly without the requirement of heat treatment at high temperature [5,17]. Besides looking into the chemical selection and roughness creation, the sustainability of superhydrophobic coatings was further studied. The SiO2 nanoparticles in waste ash [18–21] could be used to improve the sustainability of superhydrophobic coating as reported in our previous work [5]. Meanwhile, the cost effective and safe chemicals such as fatty acids were used to modify the silica nanoparticles which were later applied as the concrete admixture [22,23]. The green solvent, water was even promoted by selecting the water soluble fuoroacrylic copolymer as the hydrophobic agent [24]. Pantoja et al. [25] emphasized that the solvent selection is important to disperse the nanoparticles for roughness creation. The higher water contact angle was achieved by using white spirit instead of ethanol or ethanol-water to form the superhydrophobic bentonite coating. Moreover, the superhydrophobic coating with satisfactory transparency was successfully synthesized by dip-coating a mixture of SiO2 and methyltrimethoxysilane on polyurethane (PU) surface [26]. The PU/SiO2 coating with three-dimensional networking structure allowed the light transmittance as high as 94.38%. The multiple layer of silica coatings followed by the chemical vapour deposition of 1H,1H,2H,2H-perfluorooctyltriethoxysilane also resulted in the superhydrophobic coating with excellent transparency [27]. The mesopores in silica nanosheets and the hollows in silica nanospheres reduced the refractive index and increased the light transmission. On the other hand, the photocatalytic properties of the near superhydrophobic sol-gel coating of TiO2-SiO2 on bricks was investigated [28]. This coating degraded at least 70% of methylene blue strains within 4 h of light irradiation. Besides transparency and photocatalytsis, the luminescence of coating is interesting to be studied since it is useful in safety signs and markings. Phosphors are the most commonly used luminescent materials as they can convert energy into electromagnetic radiation, generally in the visible energy range [29]. In other words, phosphors emit photons when they are energized by an external energy source such as sun light. The incoming radiation can be converted to visible light by the presence of phosphor during dark. Photoluminescent SiO2 materials were recently reported due to their wide applications in the bioanalytical assays, labelling, chemical sensing, lighting, drug delivery and etc. [30]. The luminescent silica can be chemically produced from alkoxysilanes as well as physically doped with the expensive dyes or metal activators [29]. Due to the trapped carbon in the silica framework, rice husk ash (RHA) exhibited photoluminescence (PL) as well [31]. Fluorescence sensing and imaging could be helpful in the detection or analysis of corrosion in reinforced concrete using these photoluminescent materials available in abundance and low cost [32]. The major aim of this work is to synthesize superhydrophobic coating with PL. The PL can be a useful indicator of coating presence and even durability. The photoluminescent SiO2 nanoparticles from RHA were used to engineer the superhydrophobic coating for concrete. The effects of calcination temperature on the properties of RHA was first studied. The effects of fluoroalkyl silane and stearic acid (SA) on RHA and coating properties were further investigated. Furthermore, the superhydrophobic coating was applied on the concrete cube.

2.2. RHA preparation and characterization The rice husk was chemically washed according to literature [33]. The raw rice husk was first washed with tap water, then it was further rinsed with distilled water up to 3 times. After drying at 110 °C for 24 h, 20 g of rice husk was immersed in 500 ml of citric acid solution (5 wt. %) and stirred for 2 h at 50 °C. The rice husk was then filtered, rinsed and dried before calcination. In order to generate the photoluminescent silica, the washed rice husks were calcined in a muffle furnace (Carbolite) at two different calcination temperatures, 550 °C and 650 °C for 6 h under the heating rate of 10 °C/min and atmospheric condition as reported by Liu et al. [31]. RHA calcined at 550 °C was denoted as RHA 550 while RHA calcined at 650 °C was denoted as RHA 650 (Table 1). The RHA samples were examined using scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX, Quanta FEG 450) which was operated at an accelerating voltage 5 kV and transmission electron microscope (TEM, CM200 FEG Philips). RHA samples were sonicated in ethanol, dispersed onto a carbon grid and air dried prior to TEM imaging. 2.3. Silica modification, coating and characterization The RHA samples calcined at 550 °C were further modified using HFDS and SA and they are designated as RHA 550 HFDS and RHA 550 SA, respectively. The modification involved mechanochemical grafting which was conducted in the ball milling machine (PM 100, Retsch, Germany). The chemical grafting and size reduction occurred simultaneously by grinding 2 g of RHA in the HFDS/ethanol mixture (volume ratio 1:50 ml) for 1 h or 50 ml of SA/ethanol solution (8 mM) for 5 h at 600 rpm. The unmodified and modified RHA samples were also characterized using Fourier Transform Infrared (FTIR) spectroscopy (Nicolet Nexus 670, Thermo Scientific, USA) in order to investigate the chemical properties. The spectra were generated from 32 scans within the wavenumber range of 425–4000 cm−1 at a resolution of 4.00 cm−1. The fluorescent and microscopic images of these RHA samples were recorded using a fluorescence microscope (365 nm, Olympus BX53, Japan). RHA samples dispersed in ethanol at a concentration of 0.1 g/ ml were used to prepare the slides for imaging. Meanwhile, PL intensity of the unmodified and modified RHA samples was quantified using a spectoflourometer (Perkin Elmer Lambda S55 spectrofluorometer using a Xe lamp). The PL measurement was conducted using RHA samples dispersed in ethanol at a concentration 4.5 × 10−9 mg/ml. The spectra were scanned under 365 nm light excitation. The superhydrophobic coating was formed by spray coating the ethanol solution containing the modified RHA on a layer of adhesive which was pre-coated on the glass slide. A layer of adhesive coating was sprayed on the glass slide before the ash coating was applied. The same coating method was applied on the concrete cubes Table 1 Description of all RHA samples applied in this work. Sample

Description

RHA RHA RHA RHA

Calcined Calcined Calcined Calcined

2. Experimental 2.1. Materials The raw rice husk was collected from Padiberas Nasional Berhad, 30

550 650 550 HFDS 550 SA

RHA RHA RHA RHA

at at at at

550 °C 650 °C 550 °C and grafted with HFDS 550 °C and grafted with SA

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3. Results and discussion

(100 × 100 × 100 mm). The concrete cubes were prepared from a homogenized mixture of coarse aggregate (1264.8 kg/m3), fine aggregate (595.2 kg/m3), cement (380 kg/m3) and free water (190 kg/m3). The ratio of free water to cement (w/c) was fixed at 0.5. The slump test was conducted according to BS 1881: Part 102: 1983 to determine the workability of fresh concrete, while the concrete was further cured for 28 days based on BS 1881: Part 111: 1983. The water contact angle of ash coating on glass slide and concrete was determined using a ganiometer (250-F1, Ramé-Hart Instruments Co.). A drop of deionized water were placed on the coatings to measure water contact angle after 30 s. The average value of water contact angle was determined using three replicated coatings on glass slides or concrete cubes. The coating morphology were studied using SEM (HITACHI Tabletop Microscope instrument, TM-3000-Japan) operated at 15 kV while the fluorescent images of coatings were captured using a fluorescence microscope (Olympus BX61- UMWU2, Japan)

3.1. The effects of calcination temperature RHA samples calcined at 550 °C and 650 °C consist of two major elements, namely silicone and oxygen as shown in SEM-EDX results (Fig. 1). Carbon element was evidently shown in the SEM-EDX results of both RHA 550 and RHA 650 samples as well. RHA 550 sample contains 7.39% of carbon, while RHA 650 contains 5.18% of carbon. Rice husk usually contains 15–25 wt.% silica and a large amount of organic compounds such as lignin, cellulose, and hemicellulose [34]. The carbon residue in RHA samples originated from these organic compounds which were mainly released in the form of CO2 during calcination. The higher calcination temperature resulted in the lower carbon content in ash. The carbon element distributed in the ash samples appeared as the red dots in C distribution images. TEM images (Fig. 2) of RHA 550 and RHA 650 samples showed the existence of large particles with irregular size and small particles with circular shape. RHA 550 contains more small particles than RHA 650. These small particles could be related to the formation of carbon-based nanomaterials. Carbon-based nanomaterials especially carbon nanodot (CND)

Fig. 1. SEM images and elements scanned graph for (a) RHA 550 and (b) RHA 650.

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Fig. 2. TEM images and carbon distribution images generated from EDX for (a) RHA 550 and (b) RHA 650 particles.

ash calcined at low temperatue emmitted the higher PL intensity (225–415 nm) compared to the rice husk ash calcined at high temperature. They related the PL of RHA to the trapped carbon. From their elemental analysis, RHA calcined at 700 °C yielded a carbon content of 0.084 wt.% whereas RHA calcined at 550 °C produced a carbon content of 0.477 wt.% which resulted in the higher PL intensity. Graphene quantum dots could be also hydrothermally synthesized from rice husk carbon which was produced under the nonoxidation atmosphere [37]. However, PL in the range of 330–340 nm of porous silica prepared by sol-gel route was related to the nonbridging oxygen hole centers of surface hydroxyls [38]. The PL of this porous silica was associated with the increase of −OH groups absorbed on the surface.

below the size of 10 nm have been long studied due to their PL [35]. Most of the researchers believe that the PL emission of CND is caused by the radiative recombination of excitons located at surface energy traps although the PL of CND was not fully understood [36]. The PL intensity of RHA samples produced at different calcination temperatures were observed using a fluorescence microscope. The fluorescent images of RHA 550 sample and RHA 650 samples in Fig. 3 showed the slight difference in term of fluorescence intensity. The fluorescence intensity of RHA 550 particles was slightly stronger than RHA 650 particles. The difference could be related to the calcination temperature which caused the varied carbon content in the ash samples [31]. In order to quantify the difference in these fluorescent images, a spectrofluorometer was used to measure the PL intensity of RHA 550 and RHA 660 samples. Fig. 4 presents the PL spectra of the RHA samples calcined at different temperatures. The emission peak at 365 nm appeared in these spectra under the excitation of 435 nm light. Moreover, the PL intensity of RHA 550 was slightly higher than RHA 650 samples. Liu et al. [31] reported that the rice husk

3.2. The effects of mechanochemical grafting In this work, RHA 550 sample was further chemically modified into hydrophobic SiO2 using HFDS or SA before spray coating on any substrate. RHA 550 sample was chosen because it exhibited similar PL 32

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Fig. 3. Microscopic images of (a) RHA 550 without UV light, (b) RHA 550 with UV light, (c) RHA 650 without UV light and (d) RHA 650 with UV light.

fluorescence intensity of RHA 550 HFDS sample reduced signification after grafting. The PL intensity of the modified RHA particles was further measured. From the PL graph (Fig. 7), the highest PL intensity at 365 nm was recorded for RHA 550 sample without modification followed by RHA 550 SA and RHA 550 HFDS samples. However, the broad peak in the range of 400–500 nm grew when RHA 550 sample was modified. The PL emission of CND with shorter wavelength was related to the intrinsic state emission or electron-hole recombination, while the PL emission of CND with longer wavelength was linked to the defect state emission or surface energy traps [40]. The PL properties of CND could be easily varied by changing the surface functional groups, but other researchers mainly focused on amino-containing groups [41]. This is because the water soluble properties of CND are required in the common applications of CND.

intensity at a lower calcination temperature in comparison to RHA 650. The FTIR spectrum of RHA samples calcined at 550 °C exhibited OeH stretching at 3383 cm−1, SieOeSi vibrational stretching at 1076 cm−1 and SieO vibrations at 800 cm−1 (Fig. 5) [39]. These absorption peaks are consistent with the typical FTIR spectra of the SiO2 samples. In this work, the RHA samples were further modified in mechanochemical reaction. Two new peaks appeared at 2960 cm−1 and 2854 cm−1 in the FTIR spectrum of RHA 550 SA sample due to the symmetric and antisymmetric stretching vibrations of eCH. These peaks indicated the successful grafting of hydrocarbon groups on silica [23]. After ball milling RHA with HFDS, a new peak occurred at 1201 cm−1 due to the introduction of the eCF groups [5]. Fig. 6 shows the fluorescence images RHA 550 samples modified with the hydrophobic agent, either HFDS or SA. Apparently, the

Fig. 4. PL intensity of RHA samples calcined at different temperatures.

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Fig. 5. FTIR spectra of different RHA samples.

550 SA. The significant difference in surface morphology between RHA 550 HFDS and RHA 550 SA coatings was not detected. The fluorescent image of RHA550 HFDS coating on glass slide exhibited slightly lower PL intensity compared to the fluorescent image of RHA 550 SA coating on glass slides as expected (Fig. 10). In comparison to literature [42], the PL intensity of ash coating requires further improvement. RHA 550 HFDS coating with PL and superhydrophobic properties was further coated on a concrete cube to study the feasibility of superhydrophobic RHA coating on concrete. The concrete cubes with a dimension of 100 mm × 100 mm × 100 mm was used to perform this feasibility test. The concrete cubes were coated with a layer of

3.3. Hydrophobicity coatings using the modified RHA The images of water droplet on RHA 550 HFDS and RHA SA coatings were captured in Fig. 8. As expected, the surface tension of eCH2 and eCH3 groups in the alkyl chain are higher than fluorocarbon groups (–CF2 and eCF3), resulting in less hydrophobic effect compared to HFDS. The surface energy of hydrophobic groups on silica particles decrease in the following order: eCH2 (36 dyn/cm) > –CH3 (30 dyn/ cm) > CF2 (23 dyn/cm) > CF3 (15 dyn/cm) [1]. Furthermore, SEM images of these coating surface in Fig. 9 shows the rough surface formed by modified SiO2 particles, namely RHA 550 HFDS and RHA

Fig. 6. Microscopic images of (a) RHA 550 HFDS without UV light, (b) RHA 550 HFDS with UV light, (c) RHA 550 SA without UV light and (d) RHA 550 SA with UV light.

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Fig. 7. PL intensity of RHA 550 HFDS and RHA 550 SA.

poly(alkyl siloxane) dissolved in white spirit to create the superhydrophobic surface on concrete with the high water contact angles ranged from 150° to 162° [2]. PMMA coating without silica nanoparticles only achieved the water angle less than 140°. Besides inorganic nanoparticles, polyvinyl alcohol (PVA) fibers could be blended into concrete to create the hydrophobic and icephobic properties [4]. By adjusting the sand and PVA fiber amount, The low ice adhesion strength was accomplished on the superhydrophobic concrete with a water contact angle as high as 151°. The surface roughness of ultra-high performance concrete could be also enhanced using the templating effects of micro-pillared moulds made of polydimethylsiloxanes [3]. The superhydrophobic concrete with a water contact angle of 164° was achieved after spray-coating with oligomeric siloxane in ethanol solution. The mentioned works showed the importance of both surface roughness and surface energy in the creation of superhydrophobic surface on concrete. 4. Conclusions RHA containing SiO2 particles and a small amount of carbon residue was successfully converted into superhydrophobic coating via mechanochemical modification and spray coating. The variation of PL intensity emitted by RHA samples calcined at different temperature could be related to the carbon content. The carbon residue in nano scale was detected in EDX and TEM images. The emission peak at 365 nm appeared in the PL spectra of RHA samples under the excitation of 435 nm light. The PL intensity of RHA calcined at 550 °C was slightly higher than the PL intensity of RHA calcined at 650 °C. After mechanochemical modification of RHA 550 sample using HFDS and SA, fluorescence intensity of RHA reduced as shown in the fluorescent images and PL spectra. This is because that different hydrophobic groups such as −CH2, −CH3, −CF2 and −CF3 had been successfully grafted on the silica particles as proven in the FTIR spectra. However, the superhydrophobic coating could be only formed on glass slide and concrete by spray coating RHA 550 HFDS on a layer of commercial adhesive. The coated concrete showed a water contact angle up to 157.7°. However, PL intensity of superhydrophobic ash coating should be further improved for more practical use in construction and building.

Fig. 8. Goniometer images of the static water droplets (5 μl) on the surface of (a) RHA 550 HFDS and (b) RHA 550 SA coatings on glass slide.

commercial adhesive followed by RHA 550 HFDS coating. The water droplets wetted the surface of unmodified concrete as usual since concrete is porous and hydrophilic (Fig. 11). Meanwhile, the coated concrete surface remained dry as the water droplets were unable to wet on the superhydrophobic surface. A water contact angle on the coated concrete up to 157.7° was recorded. In literature, other researchers also reported the blending of silica nanoparticles into the hydrophobic polymers, poly(methyl methacrylate) (PMMA) dissolved in toluene or

Acknowledgements The authors would like to convey great gratitude to the Ministry of Education Malaysia for their financial support via FRGS 2015-1 grant (Flood Disaster Management, 203/PJKIMIA/6071315) and My Brain 15 (My Ph. D) scholarship awarded to Ms. Nor Naimah Rosyadah Ahmad. Finally, the authors would like to express acknowledgment to Padiberas 35

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Fig. 9. SEM images of (a) RHA 550 HFDS and (b) RHA 550 SA coatings on glass slide.

Fig. 10. Fluorescent images of (a) RHA 550 HFDS and (b) RHA 550 SA coatings on glass slide.

Fig. 11. The water droplets placed on the (a) uncoated concrete surface and (b) RHA 550 HFDS coated concrete surface.

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