Biomimetic Superhydrophobic Biobased Polyurethane-Coated

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Biomimetic Superhydrophobic Biobased Polyurethane-Coated Fertilizer with Atmosphere “Outerwear” Jiazhuo Xie,† Yuechao Yang,*,†,§ Bin Gao,‡ Yongshan Wan,† Yuncong C. Li,§ Jing Xu,∥ and Qinghua Zhao# †

National Engineering Laboratory for Efficient Utilization of Soil and Fertilizer Resources, National Engineering & Technology Research Center for Slow and Controlled Release Fertilizers, College of Resources and Environment, Shandong Agricultural University, Tai’an, Shandong 271018, China ‡ Agricultural and Biological Engineering, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida 32611-0570, United States § Department of Soil and Water Science, Tropical Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, Homestead, Florida 33031, United States ∥ College of Chemistry and Material Science, Shandong Agricultural University, Tai’an, Shandong 271018, China # Department of Basic Courses, Shandong Medicine Technician College, Tai’an, Shandong 271000, China S Supporting Information *

ABSTRACT: The development of efficient biobased controlled-release fertilizers has captured much research attention because of the environmental concerns and food scarcity problems. In this work, a biomimetic superhydrophobic biobased polyurethanecoated fertilizer (SBPF) was successfully fabricated by increasing surface roughness and reducing surface energy of polyurethane (PU) coating. The green PU coating was synthesized from low-cost, biodegradable, and renewable cottonseed oil. The nutrient release longevity of SBPF revealed 2-fold enhancement compared with the normal biobased PU-coated fertilizer (BPF). The significant improvement of nutrient release characteristics can be attributed to the atmosphere “outerwear” which ensured the nonwetting contact of water with superhydrophobic surfaces in gas state instead of in liquid state. The new concept introduced in this study can inform the development of the next generation of biobased controlled release fertilizers. KEYWORDS: superhydrophobic surface, biobased polyurethane-coated fertilizer, controlled-release fertilizer, atmosphere “outerwear”, nonwetting contact, cottonseed oil

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fluoroalkylsilanes,11 alkylsilanes, thiols, fatty acids with long alkyl chains, perfluorinated polymers, polydimethylsiloxanebased polymers, silicones,12 and carbon tetrafluoride.13 For instance, Zhang et al.14,15 successfully prepared durable superhydrophobic and superamphiphobic surfaces by spray methods using various clays (e.g., attapulgite and rodlike palygorskite) and organosilane. Currently, biomimetic superhydrophobic materials have been designed for various applications including antifouling,16 anticorrosion,17 antisnow,18 antifreezing,19,20 antimicrobial,21 and self-cleaning.22,23

INTRODUCTION

In nature, many species release various materials with superhydrophobic surface in order to avoid the invasion of water droplets. Observing the microstructure of superhydrophobic surface of many species, such as lotus leaf, water strider’s legs, and butterfly’s wings,1−4 researchers have found that the unique micronanoscale structures with surface energy provide the superhydrophobic characteristics. Biomimicry technologies, inspired by these natural creatures to fabricate biomimetic superhydrophobic surfaces, often rely on both increasing surface roughness such as spray coating,5 templation,6 lithography,7 plasma treatment,8 laser ablation,9 micromachining, and deposition10 and decreasing the surface energy by grafting the reagents with low surface energy, for instance, © XXXX American Chemical Society

Received: February 15, 2017 Accepted: April 25, 2017 Published: April 25, 2017 A

DOI: 10.1021/acsami.7b02244 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Chemical Group Ltd., China), dibutyltin dilaurate (95%, Aladdin), diatomite powder (2000 mesh, 6.5 μm in diameter, Aladdin), sodium silicate (19.3% Na2O and 22.8% SiO2, Aladdin), hydrochloric acid (HCl) (37%, Aladdin), n-hexane (99%, Aladdin), 1H,1H,2H,2HPerfluorodecyltrichlorosilane (PFDS) (96%, Aladdin) were used as received. Synthesis of BPF from Cottonseed Oil. Cottonseed oil based BPF was prepared by a two-step reaction. The first step was to synthesize the cottonseed oil polyol (COP) by the reaction of cottonseed oil (276.80 g, 0.32 mol) with glycerol (58.88 g, 0.64 mol) using sodium hydroxide (0.64 g, 0.016 mol) as the catalyst. The mixture was stirred in an instrumental setup comprising a 500 mL three-neck round-bottom flask and a condenser pipe under nitrogen atmosphere for 1.5 h on a 220 °C silicone bath. The resulting product was washed 3 times with deionized water and purified by centrifugation to obtain the COP. In the second step, the BPF was fabricated by the polymerization reaction of COP and PAPI in a 0.8:1 mass ratio with 0.8 kg of urea prills. The mixture of coating materials was sprayed uniformly to the constantly rolling urea prills, which was preheated at 60 ± 2 °C for 10 min in a rotating coating machine (WKY-300, Jingcheng Medical Equipment Manufacture Ltd., China). The process conditions for urea coating are as follows: coating time, 10 min; flow rate of hot air, 380 m3/h; flow temperature of hot air, 65 °C; spray rate, 4 g/min; spray temperature, 65 °C; rotational speed of the pan, 40 rpm. After the heat-curing reaction at 60 ± 2 °C for 10 min, the formed PU coating (0.5% with respect to the urea fertilizer) would be attached onto the surface of urea prills. Three types of BPF with different coating rates (3%, 5%, and 7%) were prepared by repeating the coating process 6, 10, and 14 times. In the last coating process, the mass ratio of COP/PAPI was adjusted to 1:1 to obtain the PU coating with redundant OH groups. The synthesis mechanisms of COP and PU coating are illustrated in Scheme 1. Preparation of SiO2 and Diatomite Hydrosols. The SiO2 and diatomite hydrosols were prepared by the following procedures. First, the SiO2 nanosol was synthesized by adding concentrated HCl solution to the sodium silicate solution (10 g/L) with magnetic stirring until the pH of the mixed solution reached 8−9. Then, 100 mL of prepared SiO2 nanosol was added to 100 mL of diatomite hydrosol

All these functions can be attributed to the three-phase (solid− vapor−liquid) interface in wetted superhydrophobic surfaces. This has been observed visually using synchrotron-radiationbased X-ray phase-contrast imaging in recent research.24 Incorporating the nonwetting properties of superhydrophobic surfaces through the newly developed superhydrophobic surface technology to the research area of controlling nutrient release for better agricultural production plays an increasingly important role in the near future.25−27 In recent decades, increasing attention has been poured into developing controlled-release fertilizers throughout the world. This is mainly because the conventional chemical fertilizers have low nutrient use efficiency and thus present serious nonpoint source pollution problems.28,29 Coated with waterproof materials, the controlled-release fertilizers can enhance the nutrient use efficiency to the maximum by balancing nutrient release and crop requirement. Despite their nonrenewable and nonbiodegradable properties, petroleum-based polymers have been applied as the current coating materials of controlled-release fertilizers for their abilities in delaying nutrient release.30 Great efforts also have been devoted to develop low-cost and ecofriendly biopolymers that are biodegradable and renewable through synthesis of biobased coating materials (e.g., biobased PU).31−33 Several well recognized ecofriendly biopolymers, including starch,34 chitosan,35 alginate,36 lignin and cellulose,37,38 vegetable oil,39 L-aspartic acid,40 and their counterparts have been widely chosen and studied to fabricate the biobased controlled-release fertilizer. However, the imbalance between crop long-term requirement and the short nutrient release longevity (often 4) belonging to fatty acid and the absorption bands (1143−800 cm−1) derived from glycerol appeared simultaneously, confirming the successful synthesis of COP. To better adjust the mass ratio of raw coating materials to fabricate the PU coating with optimal performance, the physical properties of COP and PAPI were obtained (Table 1). On the basis of the OH value and NCO content, it can be concluded that the optimal mass ratio of COP and PAPI was 0.8:1. The appropriate melting point, viscosity, and quite lower moisture content of raw coating materials can be used to determine the best reactive condition (e.g., reaction temperature). The molecular weight distribution of COP and PAPI measured by

GPC (Figure 1C,D) can be used to speculate the microstructure (e.g., degree of cross-linking) of PU coating. To confirm the formation of PU coating during the polymerization reaction of COP and PAPI, FTIR was performed to detect the structure change of raw coating materials (COP, PAPI) and PU coating. A comparison of the FITR spectra of COP, PAPI, and PU coating (Figure 2A−C) revealed evident differences among them. The emerging absorption peaks N−H (ν, 3320 cm−1), C−N (ν, 1315 cm−1), C−O (νs and νas, 1217 and 1076 cm−1) derived from newly formed PU, the already-existing absorption peaks −CH2− (ν, 2921 cm−1), −CH3 (ν, 2854 cm−1), CO (ν, 1731 cm−1) caused by the COP, and CC (νs and νas, 1604 and 1523 cm−1), C−H (β and ρ, 816 and 756 cm−1) attributed to the PAPI indicated the successful fabrication of PU coating. Furthermore, the disappeared characteristic peaks of NC O (ν, 2281 cm−1) belonging to PAPI revealed the polymerization reaction proceeded thoroughly. To further confirm the formation of PU coating, 13C NMR was employed to detect the change of chemical shift of C atoms in raw coating materials (COP, PAPI) and formed PU coating. For COP (Figure 2D), the peaks at δ = 174.1 ppm, δ = 126.8 ppm, δ = 31.4−22.6 ppm, and δ = 14.2 ppm correspond to the carbonyl carbons of ester linkages, olefinic carbons, methylene carbons, and methyl carbons of fatty acid chains, respectively. For PAPI (Figure 2E), the peaks at δ = 130.1 ppm and δ = 122.6 ppm are assigned to the aromatic carbons of benzene rings, while the peak at δ = 37.9 ppm is attributed to the methylene carbons connected with benzene rings.42 For PU coating (Figure 2F), the peaks attributed to COP and PAPI still can be observed. Furthermore, the peaks at δ = 67.9 ppm and δ = 72.4 ppm can be ascribed to the newly emerging carbon atoms directly attached to oxygen atoms. These facts further indicated the successful fabrication of PU coating. Compared E


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Figure 4. (A−C) SEM images and corresponding WCAs of (A) BPF, (B) HBPF, and (C) SBPF at 5000× magnification. (D, E) SEM images of SBPF at different magnifications (D, 25 000×; E, 100 000×). (F) Schematic diagram of SBPF with rough surface.

Figure 5. (A−C) ATR-FTIR spectra, (D−F) EDX spectra, and (G−I) EDX maps corresponding to the SEM images for surface elemental compositions and distributions of BPF, HBPF, and SBPF, respectively.

with the 13C NMR spectra of PAPI (Figure 2E) and PU coating (Figure 2F), the disappeared peak at δ = 124.7 ppm which can be attributed to the carbonyl carbons of isocyanate groups revealed that the polymerization reaction had proceeded thoroughly. On the basis of the FTIR and 13C NMR results, it can be concluded that PU coating has been successfully fabricated by a thorough polymerization reaction. The XRD and TEM analyses were employed to investigate the crystallinity and the microstructure of the SiO2 and

diatomite. The SiO2 and diatomite powders were dried from the prepared SiO2 nanosol and diatomite hydrosol, respectively, to detect the corresponding crystallinity by XRD. For SiO2, the intense and characteristic peak at 22.2° was observed (Figure 3A), and this has been reported in pervious works.43 The Bragg reflections of diatomite at 20.8° and 35.0° (Figure 3B) revealed crystallization of the diatomite.44 The microstructures of SiO2 and diatomite were also characterized by TEM, which were deposited from the SiO2 nanosol and diatomite hydrosol, F


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Figure 6. Elemental composition and corresponding valence state on the surface of PU coating: XPS spectra of (A) BPF, (B) HBPF, (C) SBPF, and (D) PFDS.

respectively. In the TEM images, the uniformly dispersed SiO2 displayed nanoscale size (Figure 3C) while the micrometersized diatomite can be observed (Figure 3D). Therefore, rough micronanoscale structure of the superhydrophobic surface could be fabricated by spraying the micrometer-sized diatomite and nanosized SiO2 onto the PU coating. The SEM with three different magnifications (5000×, 25 000×, and 100 000×) was performed to observe the surface morphology of BPF, HBPF, and SBPF (Figure 4). For the image of BPF in Figure 4A, the flat and smooth surface with hydrophilic property (WCA ≈ 86.1°) can be observed. After being sprayed with microsized diatomite and nanosized SiO2 hydrosols and heat curing process, the formed micronanoscale structure on the surface of PU coating increased the roughness of the surface dramatically, as illustrated in Figure 4C−F. The WCA on the rough surface of SBPF was about 155.8° after a subsequent silanization process for decreasing surface energy, indicating the formation of the superhydrophobic surface. As a comparison, the hydrophobic surface of HBPF with a WCA of about 142.2° is shown in Figure 4B. To understand the changes that happened in PU coating after the surface modification, ATR-FTIR was conducted to compare the characteristic functional groups on the surfaces of BPF, HBPF, and SBPF (Figure 5A−C). For the FTIR spectrum of BPF (Figure 5A), the characteristic absorption peaks belonged to the stretching vibration of O−H, declaring the existence of redundant OH groups at the outermost layer fabricated by the enhanced mass ratio of COP/PAPI (1:1) in

the last coating process. After being sprayed with SiO2 and diatomite hydrosols and immersed with PFDS, the differences in the FTIR spectra of BPF and SBPF were obvious. The emerging absorption peaks O−Si−O (ν, 814 cm−1), C−F (ν, 1142 cm−1), and C−Si (ν, 710 and 651 cm−1) and the disappeared absorption band of OH group at 3400 cm−1 all indicated the existence of PFDS molecule on the surface of SBPF (Figure 5C). The FTIR spectrum of HBPF (Figure 5B) was similar to that of SBPF (Figure 5C). The EDX spectra (Figure 5D−F) and maps (Figure 5G−I) were used to study the surface elemental compositions and distributions of BPF, HBPF, and SBPF. The EDX results can not only prove the elemental composition revealed in ATRFTIR curves but also further display the elemental distribution. For the surfaces of BPF (Figure 5D,G), only equally distributed C and O elements are observed. After the silanization, the uniformly distributed F and Si elements appeared on the surfaces of HBPF (Figure 5E,H) and SBPF (Figure 5F,I). The newly emerging F and Si elements further confirmed the presence of PFDS molecules after the silanization process. XPS was employed to study the elemental compositions and corresponding elemental valence states of BPF, HBPF, SBPF, and PFDS. As shown in Figure 6A, O 1s (531.8 eV), N 1s (398.4 eV), and C 1s (285.0 eV) on the XPS spectrum of BPF can be observed. Compared with the XPS spectrum of BPF, the appearance of Si 2s, Si 2p, and F 2s further suggested the presence of PFDS molecules on the surface of HBPF (Figure 6B) and SBPF (Figure 6C). The XPS spectra can only provide G


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Figure 7. Cumulative nitrogen release rate of different coated fertilizers at 25 °C in water. Release curves of BPF, HBPF, and SBPF for (A) 3%, (B) 5%, and (C) 7% coating percentage, respectively. (D) WVTs of (a) biobased PU films, (b) hydrophobic biobased PU films, and (c) superhydrophobic biobased PU films with similar treatment with BPF, HBPF, and SBPF, respectively.

nucleophilic substitution reacting with the OH groups to decrease the surface energy. In theory, the ability of insulating water from hydrophobic surface of HBPF and superhydrophobic surface of SBPF would give HBPF and SBPF, especially SBPF, tremendous potential to increase the controlled-release ability. To study the influences of superhydrophobic surfaces on the nutrient release, BPF, HBPF, and SBPF with 3%, 5%, and 7% coating percentage were measured in water at 25 °C. The results can be used to predict the N release characteristics of coated fertilizers in soils as indicated in previous work.45,46 As shown in Figure 7A−C, the N release characteristics of BPF and SBPF were significantly different. For BPF and SBPF with 3% coating material, the 24 h N release rates were 16.7% and 2.5%, respectively. The N release longevity of SBPF was 25 days, much longer than that of BPF (13.5 days). The same trends of 24 h N release rates and N release longevity were observed for the other samples with higher coating percentage (Figure 7A). With 5% coating, the 24 h N release rates of BPF and SBPF were 3.8% and 0.3%, respectively, and their N release longevity was 22.7 days and 41.8 days, respectively (Figure 7B). Similarly, with 7% coating, the 24 h N release rates of BPF and SBPF were even lower (1.6% and 0.1%, respectively) and their N release longevity increased (34.2 days and 66.3 days, respectively, Figure 7C). Because the nutrient release longevity of SBPF with different coating percentage almost doubled that of the corresponding BPF, the superhydrophobic surfaces clearly enhanced the controlled-nutrient release ability of SBPF. As shown in Figure 7A−C, the nutrient release characteristics

the information on PFDS molecular chains attached onto the surface of PU coating but not the accurate information on the PU coating due to the thin detection depth (usually within a few nanometers) of XPS probe. Therefore, the elimination of N 1s can indirectly verify the existence of PFDS molecules on the surface. Furthermore, the significant improvement of the binding energy of Si 2s (154.13 eV) and Si 2p (103.28 eV) on the surface of SBPF (Figure 6C), in comparison to the binding energy of Si 2s (152.88 eV) and Si 2p (102.04 eV) of PFDS (Figure 6D), revealed that the PFDS molecules had reacted with the OH groups on the micrometer−nanoscale diatomite-SiO2 structure and the outermost PU coating by the nucleophilic substitution (as illustrated in Scheme 2). The rising atomic inner electron binding energy might be obtained when the atoms bonded to other atoms with greater electronegativity, induced by the reduced peripheral electron cloud density of the atoms. The improvement of the binding energy of Si atoms thus can be attributed to the fact that the Si atoms bonded to O atoms with greater electronegativity on the surface of SBPF. On the other hand, the Si atoms bonded to Cl atoms with lower electronegativity on the surface of PFDS. These results indicated that the PFDS molecules were grafted to the surface of the PU coating through chemical reaction with the surface OH groups rather than through simple physical absorption onto the surface. According to the above-mentioned analyses, it can be concluded that the superhydrophobic PU coating surface had been fabricated by constructing the micronanoscale structure to increase surface roughness and grafting fluoroalkylsilanes by the H


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Figure 8. (A) Optical photograph and (B) model diagram of (a) BPF and (b) SBPF when being immersed in water. (C) Schematic diagram of the atmosphere “outerwear” between water and the superhydrophobic surface of SBPF.

Figure 9. (A) Photo of fertilizers used in the experiment: (a) urea, (b) BPF, (c) SBPF, and (d) swollen SBPF. (B) Controlled-release process of SBPF: (a) moisture permeating into fertilizer core in gas state; (b) release of dissolved nutrient driven by developing osmotic pressure. (C) SEM images with corresponding WCAs of the coating surface of BPF after being immersed in water for 70 days. (D) SEM images with corresponding WCAs and (E) elemental composition and distribution of the coating surface of SBPF after being immersed in water for 70 days.

characteristics, the WVTs of biobased PU films, hydrophobic biobased PU films, and superhydrophobic biobased PU films were almost the same. The similarity of WVTs was derived from the similar contact modes of water with the PU films in gas state. Therefore, it can be concluded that the differences of nutrient release characteristics of BPF, HBPF, and SBPF can be attributed to the different contact modes of water with the PU coating surfaces. Figure 8 shows the mechanism of how superhydrophobic surface enhances the nutrient release longevity of SBPF. When BPF with hydrophilic coating is immersed into water, water contacted directly the surfaces in liquid state to promote the nutrient release. When SBPF is immersed into water, however, water cannot entirely wet the superhydrophobic surface of

of HBPF were between SBPF and BPF, further confirming the importance of the superhydrophobic surface to the controlledrelease effect. The cumulative N release curves of BPF were “inverted-L” shape. In contrast, the curves of SBPF changed from “C” to “inverted-C” shape with the increasing coating percentage (from 3% to 7%). These results suggested that the coating percentage also had strong effects on the nutrient release. Better nutrient release effects of SBPF could be obtained if more coating materials were added. To explore the nutrient release mechanisms, the WVTs of biobased PU films, hydrophobic biobased PU films, and superhydrophobic biobased PU films from BPF, HBPF, and SBPF, respectively, were measured (Figure 7D). Although the fertilizer samples showed very different nutrient release I


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mode of hydrophilic coating of BPF after being immersed into water. As a result of this mechanism, it can effectively enhance the nutrient release longevity.50 In the second stage (b), with the accumulation of moisture in the fertilizer core of SBPF after the first release stage, the nutrient of SBPF gradually dissolved to form saturated solution. The resulting remarkable difference of osmotic pressure inside and outside of the coating caused the accelerating diffusion of dissolved nutrient from the fertilizer− PU interface to the external environment. The accelerating release stage of nutrient (b) can be attributed to two factors. On the one hand, the high pressure in the interior of SBPF caused by nutrient dissolution made the membrane of SBPF swelling, resulting in the increasing cracks and holes in coating surface (Figure 9A,D). Similarly, the hydrophilic BPF film (WCA ≈ 86.3°) also had large number of cracks and holes on coating surface (Figure 9C). As a result, water and nutrient molecules can easily pass through the SBPF coating, causing the accelerating nutrient release (as illustrated in Figure 9B). On the other hand, after being immersed into water for a long time, the micronanoscale structure of SBPF coating was partially damaged because of the separation of the diatomite-SiO2 system (Figure 9D) with the expanding SBPF. Furthermore, compared with F (34.77%) and Si (4.49%) element content of SBPF before being immersed into water (Figure 5F), the reduced F (21.75%) and Si (2.83%) element content (Figure 9E) can be observed. The decreasing of micronanoscale bumps would also reduce PFDS that had been grafted to the micronanoscale structure, making the SBPF no longer superhydrophobic (WCA ≈ 116.3°). After the SBPF lost its superhydrophobicity, water drops wetted the coating surface again and diffused into the interior core directly in liquid state, promoting the accelerating nutrient release of SBPF. Therefore, it can be concluded that SBPF enhanced the nutrient release longevity by slowing the permeation speed of water through atmosphere “outerwear” in gas molecules instead of liquid phase on the first stage while maintaining the accelerated nutrient release on the second stage. In this study, the accelerating release of nutrient was used as the indicator to determine the durability of the SBPF. For SBPF with 7% coating rate, the durability thus was 28 days (Figure 7C). Further investigations should be conducted to test other methods that can directly measure the durability of the coatings that thus can inform the development of superhydrophobic biobased controlled-release fertilizers with controllable durability. In addition, previous studies have pointed out the potential environmental risks of PFDS,12 and environmentally friendly reagents such as silanes and silicones with hydrocarbon groups thus should also be tested as a replacement of PFDS in the synthesis to construct superhydrophobic coatings of controlled-release fertilizer.

SBPF due to the presence of atmosphere membrane between water and superhydrophobic surface. The atmosphere membrane functions just like an atmosphere “outerwear”, covering the superhydrophobic surface of SBPF. This atmosphere “outerwear” only allows water vapor (instead of liquid) to enter the internal urea core of SBPF (as illustrated in Figure 8C). The speed of water accumulation in vapor phase is slower than that in liquid phase. Therefore, the atmosphere “outerwear” can significantly slow down nutrient release by reducing water accumulation in the internal urea core of SBPF (as illustrated in Figure 9B). Cassie and Baxter have developed the following eq 1 describing this state of solid surface:47,48 cos θCB = f1 cos θY − f2

(1)

where θCB is the Cassie−Baxter contact angle, f i is the percentage of total area of solid under the drop to unit projected area under the droplet (f1 + f 2 = 1), and θY is the Young’s contact angle or flat surface contact angle. According the Cassie−Baxter theory, liquids contact the solid surface only through the top of the asperities and air pockets are trapped underneath the liquid. In this “composite” state, the surface underneath the air can be considered perfectly nonwetting. From a thermodynamic point of view, substance will exist with the lowest energy state. For instance, the formation of hydrogen bond is an effective way to reduce chemical energy. When placed on the rough surface with low surface energy, water molecules can only form hydrogen bonds with each other due to the nonpolar property of superhydrophobic surface. The surface tension on the interface of water and superhydrophobic surface makes water suspending just like being supported by an invisible hand. These equilibrium states are the minima of Gibbs free energy of the water droplet above the rough superhydrophobic surface. Recently, a few researchers observed visually the atmosphere “outerwear” on natural lotus leaves and artificial carbon nanotube films in three dimensions using fluorescence microscopic imaging49 and synchrotron radiation-based X-ray phase-contrast imaging,24 verifying the existence of the threephase solid−vapor−liquid interface on the superhydrophobic surface with protruding micrometer−nanoscale structure. In this work, due to the unique superhydrophobic characters and micrometer−nanoscale diatomite-SiO2 structures of SBPF, it thus can form the air membrane at the interface of water and superhydrophobic surface when being immersed into water (Figure 8A,B). Water slowly permeated into the fertilizer core through the atmosphere “outerwear” in vapor (gas phase) instead of liquid phase (Figure 8C) and thus effectively enhanced the nutrient release longevity of SBPF. On the basis of the results, the controlled-release process of SBPF was divided into two stages (Figure 9A): (a) slow permeation of water molecules into fertilizer core in gas state and (b) accelerating release of dissolved nutrient driven by developing osmotic pressure. In the first stage (a), water slowly permeated into the fertilizer core through the atmosphere “outerwear” between water and superhydrophobic surface as gas (vapor) instead of liquid phase (Figure 8C). The atmosphere “outerwear” acted as a barrier between water and SBPF, which reduced the liquid−solid contact area and inhibited the penetration of water into the internal urea core of SBPF. Consequently, the dissolution, migration, and release of nutrient from SBPF were restricted. This stage lasted for a much longer time in comparison to that of the direct contact

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CONCLUSION A biomimetic biobased controlled-release fertilizer with superhydrophobic surface (SBPF) was successfully fabricated via a two-step process by tailoring surface topography and surface chemical compositions. Micrometer−nanoscale diatomite-SiO2 structure was developed on the coating surface of SBPF to increase surface roughness, and PFDS subsequently substituted the OH groups to decrease the surface energy. When being immersed in water, water on the SBPF surface was in vapor (gas phase) instead of liquid phase and formed the atmosphere “outerwear” at the interface of water and superhydrophobic surface. This “outerwear” reduced the speed of nutrient release J


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and thus enhanced the nutrient release longevity of SBPF. As a result, the nutrient release longevity of SBPF revealed 2-fold enhancement compared with that of untreated BPF. The idea of using superhydrophobic surface treatment to produce SBPF can provide new strategies for the development of novel biobased controlled-release fertilizers in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02244. Chemical compositions of standard solution used in GC (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-538-824 2900. E-mail: yangyuechao2010@163. com. ORCID

Yuechao Yang: 0000-0003-4045-0252 Bin Gao: 0000-0003-3769-0191 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the National Natural Science Foundation of China (Grant 31572201), the Natural Science Foundation of Shandong Province (Grant ZR2015CM035), Shandong Agricultural Innovation Team (Grant SDAIT-1704), the Projects of Commercialization of Research Findings of Shandong Province (Grant [2014] 183), the Great Innovation Projects in Agriculture of Shandong Province (Grant [2013] 136), the National Key Research and Development Program (Grant 2016YFB0302403), the Project of Shandong Province Education Department (Grant ZR2014JL023), and Shandong Youth Education Science Program for College Students (Grant 17BSH113).

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REFERENCES

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Biomimetic Superhydrophobic Biobased Polyurethane-Coated

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