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Bioinspired heterostructured bead-on-string fibers via controlling the wet-assembly of nanoparticles† Lin Zhao, Cheng Song, Miaoxin Zhang and Yongmei Zheng*
Received 4th July 2014, Accepted 20th July 2014 DOI: 10.1039/c4cc05156d www.rsc.org/chemcomm
A kind of bioinspired heterostructured bead-on-string fiber (BHBF), composed of poly-(methyl methacrylate) (PMMA) and titanium tetrachloride (TiCl4) hydrolyzed nanoparticles, was prepared via integrating a wet-assembly system, including PMMA electrospinning, fog of nanoparticles and water coalescence at multi-stages. The wetassembly of BHBF was regulated by the difference in surface energy and Laplace pressure. Especially, BHBF is characteristic of a hydrophilic rough bead for excellent water collection ability.
Heterostructures of bead-on-string fibers have drawn considerable attention for manufacturing cutting-edge micro-/nanodevices as ‘‘function centers,’’ which achieve many unique abilities such as directional water collection1,2 and special adhesion,3 responsive microreactor,4,5 smart sensor6 and selective catalyst.7–9 The heterostructures of bead-on-string at a micro- and nano-level have been developed by various methods, e.g., microfluidic10–12 and pneumatic6 or mechano-electro13 spin facilities, and the Plateau– Rayleigh instability in solid–liquid systems6,14–18 and solid–solid systems.19 Especially, natural bead-on-string in wet-rebuilt process endowed in cribellate spider capture silk for water collection is exhibited with unique spindle-knot and joint composed of porous nanofibrils and aligned nanofibrils,16,20 which inspire researchers to design the novel structured materials.21 Up to now, it is still challenging to fabricate wet-assembly bead-on-string heterostructures conveniently and controllably. Herein, we present a kind of bioinspired heterostructured bead-on-string fiber (BHBF) composed of poly-(methyl methacrylate) (PMMA) and titanium tetrachloride (TiCl4) hydrolyzed nanoparticles, which can be prepared via integrating a wet-assembly system, including PMMA electrospinning, fog of nanoparticles and water coalescence at multi-stages (this refers to several adsorption-wetting-drying processes). Thus, BHBFs can be effectively prepared by controlling
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, 100191, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental section, Fig. S1 to S6, and Movie S1. See DOI: 10.1039/c4cc05156d
This journal is © The Royal Society of Chemistry 2014
the wet-assembly parameters, e.g., assembly time in humidity via water coalescence into droplets. In particular, water coalescence invigorates the nanoparticles to gather and form periodically separated nanoparticle-in droplets, which can be wet-assembled at multiple stages. The study offers an insight to the design of novel heterostructured materials via bioinspired wet-assembly. The findings are significant enough to be extended into realms such as water collection, fog-harvesting engineering, humidity sensor, micro-reactor, and filtration. The BHBFs were fabricated by combining the control of TiCl4 hydrolysis and water condensation (as illustrated in Fig. S1, ESI†). Initially, bare PMMA fibers (with diameters of 0.549 0.030 mm) were fabricated in an electrospinning system via an injected fluid consisting of a solution of PMMA in the solvent of N,N-dimethyl formamide (DMF). The foggy nanoparticles containing Ti(NPCTi) that result from the hydrolysis products of TiCl4 are used in BHBFs fabrication (see Experimental section in the ESI†). The sizes of nanoparticles are 0.282 0.072 mm via hydrolysis of 0.8 ml of TiCl4 (the optimization process is shown in Fig. S2, ESI†). As PMMA fiber captures the nanoparticles (Fig. 1a) and is then placed in a humid atmosphere; thus, water condensation appears around the NPCTi. Subsequently, small water droplets coalesce to form nanoparticle-in droplets. After the nanoparticle-in-droplets were dried, the nanoparticles assembled into beads along the fiber. Moreover, the method can be applied to a cross structure of two contact electrospinning fibers, as shown in Fig. S3 (ESI†). Fig. 1b and c shows the SEM images of the BHBFs in sequence. To distinguish the chemical components, energy dispersive spectrometer (EDS) mapping analysis was used to indicate NPCTi (Fig. 1d–f), where titanium (see scatters) is the main element of the nanoparticles in the beads. The outline of the scatters corresponded to SEM images of rough beads (Fig. 1a–c) composed of nanoparticles. It shows that the ‘‘bead’’ is mainly composed of titanium, while the ‘‘string’’ is PMMA, indicating that as-fabricated BHBFs are excellent heterostructures. To control the geometric structure of BHBFs, the wetting parameters and multi-stage wet-assembly were regulated. We firstly modulate the wetting parameters, e.g., at a relative humidity (RH) from 50% to 90% with a wetting time from 2 s to 16 s. Fig. S4 (ESI†)
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Fig. 1 SEM images and EDS mapping analysis of the PMMA-Ti fiber before and after wetting. (a) SEM image of PMMA-Ti fiber before wetting, which is densely coated by nanoparticles containing Ti with a diameter of 0.282 0.072 mm. (b) SEM image of the ‘‘bead-on-string’’ fiber after wetting. (c) Magnified SEM image of the ‘‘bead-on-string’’ fiber after wetting. (d–f) Elemental mapping of the same nanofiber region corresponding to that found in (a–c), indicating the spatial distribution of Ti (red). (d) Nanoparticles are effectively captured on the PMMA fiber. (e) The ‘‘bead-on-string’’ structure forms after wetting. (f) The magnified bead is composed of nanoparticles with Ti components.
shows the SEM images of the controlled fabrication of the ‘‘beadon-string’’ under different conditions. It can be concluded that only with an optimized RH and wetting time, there appears typical BFBFs. Fig. 2 shows statistic graphs of the geometric structures with RH from 60% to 90% and wetting times from 6 s to 14 s via wet-assembly at a single stage. Fig. 2a and b show the ‘‘bead’’ volume and ‘‘inter-bead’’ distance versus RH and wetting time. It shows that the volume of the ‘‘bead’’ and the distance between ‘‘beads’’ increases with an appropriate wetting time and humidity (from 6 s to 14 s in 60% and 65%; from 6 s to 12 s in 70% and 75%; and from 6 s to 10 s in 80%). There was no
Fig. 2 (a and b) Statistic graph of the BHBFs on volume (a) and distance (b) of beads regulated by RH from 60% to 80% and wetting times from 6 s to 14 s in a single stage. (c) SEM images of BHBFs via wet-assembly from 1 to 5 stages (frames numbered with 1–5) in an RH of 70%. (d) Statistical graph of BHBFs with increasing volumes of beads from 4.856 0.384 mm3 to 58.891 2.595 mm3 via wet-assembly from 1 to 5 stages.
10652 | Chem. Commun., 2014, 50, 10651--10654
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obvious ‘‘bead-on-string’’ structure under a wetter environment (14 s in 70% and 75%; 12 s and 14 s in 80%). There was a tendency that beads become bigger and more isolated due to the aggregation of nanoparticles in the wet-assembly process. The beads size is relevant not only with wetting parameters, such as RH and wetting time, but also with the multi-repetition of adsorption of nanoparticles in the wet-assembly process. Fig. 2c shows the microstructure on the surface of the ‘‘beadon-string’’ fibers after the multi-stage wet-assembly from stage 1 to 5. Here, the bead volume (Vb) is expressed with Vb = plw2/6, where l and w represent the length and width of the bead, respectively. As shown in Fig. 2d, it was estimated that Vb jumps from 4.856 0.348 mm3 to 58.892 2.595 mm3, as a result of the multi-stage wet-assembly at the fifth stage, which indicates the controllability of BHBFs fabrication. It is interesting how microscale beads are formed from nanoparticles via an multi-stage wet-assembly. Fig. 3 shows the in situ optical microscopic observation of the wet-assembly process with three stages of BHBFs in the fog. In the first stage (Fig. 3a–d), the PMMA fiber is densely coated with NPCTi by the hydrolysis of 0.8 mL of TiCl4 (at B54.94 s). The small water droplets condensed on the NPCTi, grow and coalesce with each other into bigger droplets (see arrows) during water condensation in a RH of 70% from B80.49 s to B84.80 s. The ‘‘bead’’ appears and was linked by ‘‘string’’ after drying at B97.80 s. To control the size of the bead, the wetting process above is repeated via wet-assembly (Fig. 3e–h). Nanoparticles are absorbed mostly on the bead due to the interception effect of the fiber22 (at B152.00 s). By water condensation and coalescence (from B163.11 s to 170.68 s), bigger beads composed of nanoparticles take on fiber via wet-assembly in the second-stage (at B186.85 s). Repeated continuously
Fig. 3 Optical images of the increasing bead-on-string via a multi-stage wet-assembly. (a–d) The first-stage. (a) NPCTi are uniformly captured on the fiber at B54.94 s. (b) Water condenses around the NPCTi and forms small droplets at B80.49 s. (c) Water-condensed droplets sweep the nanoparticles to assemble into ‘‘bead’’ on fiber (see the arrows) via coalescence of small water droplets at B84.80 s. (d) By controlling the wetting time, there appears small ‘‘bead’’ alternates ‘‘string’’ after drying at B97.80 s. (e–h) The secondstage. (e) Nanoparticles are mainly captured on the small bead at B152.00 s. (f) Water condenses around the NPCTi at B163.11 s. (g) After the same wetting conditions, droplets condensed on the fiber and coalesced around the ‘‘bead’’ at B170.68 s. (h) The ‘‘bead’’ becomes bigger after the secondstage at B186.85 s. (i–l) The third-stage. (i) Nanoparticles are mostly adsorbed around bead at B226.40 s. (j–k) Water-condensed droplets are formed on the fiber at B242.78 s and coalesced around the ‘‘bead’’ at B245.75 s. (l) Beads become bigger after the third-stage at B260.99 s. Nanoparticles, including NPCTi components, are obtained by hydrolyzing 0.8 mL of TiCl4. A RH of 70% is maintained during the wet-assembly process.
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(from B226.40 s to 245.75 s), ‘‘beads’’ grow much bigger in the third-stage at B260.99 s, which shows that the nanoparticles aggregate effectively and spontaneously. The fabrication indicates that the bead-on-string structures are controlled effectively by regulating wet-assembly at stages. The dynamic process of the multi-stage wet-assembly can also be seen in Movie S1 (ESI†). In fact, the wet-assembly of BHBFs was related to a driving coalescence of particles-in-droplets in humidity as illustrated in Fig. 4. When the PMMA fiber with NPCTi was placed in humidity (Fig. 4a, Frame 1–2), water condenses around the hydrophilic NPCTi to form small nanoparticle-in-droplets. Subsequently, the tiny droplets grow and coalesce with each other to form larger ones (Fig. 4a, Frame 3–5) for the sake of shrinking the superficial area and decreasing surface energy.23 The separated nanoparticle-in-droplets are periodically distributed along the PMMA fiber. Beads composed of nanoparticles were formed after water evaporation (Fig. 4a, Frame 6). For controlling the size of the bead, a second-assembly was carried out by NPCTi second-capture via the hydrolysis of TiCl4. The ‘‘dendritic’’ structure composed of NPCTi appears on the bead because of the roughnessinduced nanoparticles re-capture interception effect22 (Fig. 4b, Frame 1). At a given RH, small water droplets coalesce on the fiber (Fig. 4b, Frame 2–3). NPCTi gather around the ‘‘bead’’ due to the surface energy and curvature difference between the bead and string (Fig. 4b, Frame 4). Thus, a larger bead was formed after a multi-stage wet-assembly process (Fig. 4b, Frame 5).
Fig. 4 Illustration of BHBF fabrication. (a) ‘‘Bead-on-string’’ structure formation in a single-stage process. Water condenses around the NPCTi due to its hydrophilicity and forms small droplets (Frame 1). The droplets grow and coalesce with each other (Frame 2–4). The continuous coalescence of droplets induces the formation of a large droplet (Frame 5). Water evaporation induces the wet-rebuilt ‘‘bead-on-string’’ structure (Frame 6). (b) Controlling on size of ‘‘bead’’ in a multi-stage process. The ‘‘bead-onstring’’ absorbs NPCTi again via the hydrolysis of TiCl4. There is ‘‘dendritic’’ structure on the bead (Frame 1). Water condenses around the NPCTi to form droplets and move directionally to the ‘‘bead’’ (Frame 2–3). Finally, NPCTi gather around the ‘‘bead’’ with condensation of droplets (Frame 4). Thus, a larger bead is formed after drying (Frame 5). (c) Details of the mechanism on formation of larger beads. A water drop on the ‘‘string’’ location (less hydrophilic due to a small content of NPCTi) moves towards ‘‘bead’’ location (more hydrophilic due to a large content of NPCTi) (Frame 1). yA and yR indicate the advancing and receding angles of the water drops on the fiber, respectively. Another driving force arises from the conicalshaped geometry of the ‘‘bead’’ location, which causes a difference in the Laplace pressure. R0, r1 and r2 indicate the radius of the water droplet, ‘‘string’’ location and ‘‘bead’’ location, respectively. The arrows indicate the direction of moving water droplets (Frame 2).
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It is understood that both the cooperation of surface energy gradient and Laplace pressure act as a driving force for the directional movement of water drops.20 During the re-capture of nanoparticles (Frame 1 of Fig. 4c), the higher NPCTi concentration on the ‘‘bead’’ induces the hydrophilic property more than that of the ‘‘string,’’ which gives rise to a surface energy gradient along the fiber.24 A driving force (F) propels the water drops to move towards the location of the ‘‘bead’’ from that of the ‘‘string’’, F is expressed as follows:25,26 ð xbead dF ¼ gðcos yA cos yR Þdx (1) xstring
where g is the surface tension of water, x is the integrating variable along the axis of the fiber from string to bead, yA and yR represent the advancing and receding water contact angles of the surfaces with higher and lower NPCTi concentration (yA o yR), respectively. The surface energy gradient resulting from the NPCTi concentration will drive water to move from the less Ti component region to a more Ti component one. The second driving force to be considered is the gradient of Laplace pressure. The beads can be considered as two oppositely curved and joined conical objects (Frame 2 of Fig. 4c). Each conical object induces a difference in the Laplace pressure (DP) functioning on the water drop as follows:27 ð r2 2g DPcurvature ¼ sin a dz (2) 2 r1 ðr þ R0 Þ where r is the local radius of the fiber, R0 is the drop radius (R0 = (3V/4p)1/3, V is the drop volume), a is the half apex-angle of the bead, and z is the integrating variable along the diameter of the bead. The Laplace pressure on the high curvature position (the ‘‘string’’ with a local radius r1) is larger than that on the low curvature position (the ‘‘bead’’ with a local radius r2; r1 o r2), which results in a difference of Laplace pressure acting on the water drops to move from the ‘‘string’’ to ‘‘bead’’ region. Thus, the cooperation of Laplace pressure and surface gradient resulting from the chemical composition act together as driving forces to form bigger droplets, which corresponds to bigger beads on the fiber after drying. The drying time of the as-prepared first-stage BHBF leads to similar results during the second-stage process (as shown in Fig. S5, ESI†), which was attributed to the adsorption of nanoparticles at the water–air interface providing a synergic effect during wet-assembly. To examine this method, we compared the wetting behavior of bare PMMA fibers and the PMMA/Ti fibers (Fig. S6, ESI†). In addition, other particles such as TiO2 (P25) and ZnO (diameter of B100 nm) were used to fabricate ‘‘bead-on-string’’ fibers by flowing with N2 current at ambient temperature (Fig. S7a–d, ESI,† respectively). This indicates an effective method to design heterostructures for several applications. The water collection ability was examined on BHBF (also see in Experimental section). As shown in Fig. 5, Z has a linear increase tendency with an increase of S (for an approximately ellipsoidal bead,28 S = p(l2 + aw2/tan a)/2, where a = arc cos(l/w), l and w are the length and width of the bead, respectively). The Z value increases up to B8 times when beads increase S
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Notes and references
Fig. 5 Water collecting ability of BHBFs. With an increase of bead surface area (S) from 91.6 mm2 to 514.7 mm2, the water collection efficiency of beads has a linear growth, from 1.82 103 mm3 s1 to 1.35 104 mm3 s1. The insets are optical images of water drops collected on beads at B120 s.
from B91.6 mm2 to B514.7 mm2. This demonstrates that BHBFs with larger beads are more efficient in water collection than those with smaller beads due to increase in the rough surface. The insets show the optical images of typical water collection of BHBFs. In conclusion, we have controllably fabricated BHBFs at the micro- and nano-level and analyzed the mechanism of waterevaporation-induced self-assembly. This study is significant and helpful towards the design of smart heterostructured materials, and towards the development of materials with novel functionality that can be extended in realms such as water collecting device, fog-harvesting engineering, humidity-sensor, micro-reactor, filtration and others. This work is supported by the National Research Fund for Fundamental Key Project (2013CB933000), the National Natural Science Foundation of China (21234001), the Doctoral Fund of Ministry of Education of China (20121102110035), and the Graduate Student Innovation Practice Foundation of Beihang University.
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1 Y. Zheng, H. Bai, Z. Huang, X. Tian, F.-Q. Nie, Y. Zhao, J. Zhai and L. Jiang, Nature, 2010, 463, 640–643. 2 X. Tian, Y. Chen, Y. Zheng, H. Bai and L. Jiang, Adv. Mater., 2011, 23, 5486–5491. 3 H. Bai, J. Ju, Y. Zheng and L. Jiang, Adv. Mater., 2012, 24, 2786–2791. 4 H. B. Eral, J. de Ruiter, R. de Ruiter, J. M. Oh, C. Semprebon, M. Brinkmann and F. Mugele, Soft Matter, 2011, 7, 5138–5143. 5 A. L. Yarin, W. Liu and D. H. Reneker, J. Appl. Phys., 2002, 91, 4751–4760. 6 X. Tian, H. Bai, Y. Zheng and L. Jiang, Adv. Funct. Mater., 2011, 21, 1398–1402. 7 T. Gilet, D. Terwagne and N. Vandewalle, Eur. Phys. J. E: Soft Matter Biol. Phys., 2010, 31, 253–262. 8 M. Reches, K. A. Mirica, R. Dasgupta, M. D. Dickey, M. J. Butte and G. M. Whitesides, ACS Appl. Mater. Interfaces, 2010, 2, 1722–1728. 9 T. L. Owens, J. Leisen, H. W. Beckham and V. Breedveld, ACS Appl. Mater. Interfaces, 2011, 3, 3796–3803. 10 E. Kang, G. S. Jeong, Y. Y. Choi, K. H. Lee, A. Khademhosseini and S.-H. Lee, Nat. Mater., 2011, 10, 877–883. 11 B. G. Chung, K.-H. Lee, A. Khademhosseini and S.-H. Lee, Lab Chip, 2012, 12, 45–59. 12 S. Cho, T. S. Shim and S.-M. Yang, Lab Chip, 2012, 12, 3676–3679. 13 C.-F. Sun, K. Karki, Z. Jia, H. Liao, Y. Zhang, T. Li, Y. Qi, J. Cumings, G. W. Rubloff and Y. Wang, ACS Nano, 2013, 7, 2717–2724. 14 L. Rayleigh, Proc. Lond. Math. Soc., 1878, s1–10, 4–13. 15 M. Xu, F. Wang, Y. Zhang, S. Yang, M. Zhao and X. Song, Nanoscale, 2013, 5, 8067–8072. 16 H. Bai, X. Tian, Y. Zheng, J. Ju, Y. Zhao and L. Jiang, Adv. Mater., 2010, 22, 5521–5525. 17 Y. Zhao and L. Jiang, Adv. Mater., 2009, 21, 3621–3638. 18 J.-H. Park and P. V. Braun, Adv. Mater., 2010, 22, 496–499. 19 R. Wu, B. Zha, L. Wang, K. Zhou and Y. Pan, Phys. Status Solidi A, 2012, 209, 553–558. 20 Y. Zheng, X. Gao and L. Jiang, Soft Matter, 2007, 3, 178–182. 21 C. Song, L. Zhao, W. Zhou, M. Zhang and Y. Zheng, J. Mater. Chem. A, 2014, 2, 9465–9468. 22 K. Kosmider and J. Scott, Filtr. Sep., 2002, 39, 20–22. 23 B. Ling, X. Sun, Y. Shen and Z. Dong, Appl. Phys. A: Mater. Sci. Process., 2010, 98, 91–96. 24 A. Fujishima, T. N. Rao and D. A. Tryk, J. Photochem. Photobiol., C, 2000, 1, 1–21. 25 M. K. Chaudhury and G. M. Whitesides, Science, 1992, 256, 1539–1541. 26 S. Daniel, M. K. Chaudhury and J. C. Chen, Science, 2001, 291, 633–636. ´re ´, J. Colloid Interface 27 K. Piroird, C. Clanet, E. Lorenceau and D. Que Sci., 2009, 334, 70–74. 28 J. L. Viovy, D. Beysens and C. M. Knobler, Phys. Rev. A: At., Mol., Opt. Phys., 1988, 37, 4965–4970.
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Bioinspired heterostructured bead-on-string fibers via controlling the wet-assembly of nanoparticles† Lin Zhao, Cheng Song, Miaoxin Zhang and Yongmei Zheng*
Received 4th July 2014, Accepted 20th July 2014 DOI: 10.1039/c4cc05156d www.rsc.org/chemcomm
A kind of bioinspired heterostructured bead-on-string fiber (BHBF), composed of poly-(methyl methacrylate) (PMMA) and titanium tetrachloride (TiCl4) hydrolyzed nanoparticles, was prepared via integrating a wet-assembly system, including PMMA electrospinning, fog of nanoparticles and water coalescence at multi-stages. The wetassembly of BHBF was regulated by the difference in surface energy and Laplace pressure. Especially, BHBF is characteristic of a hydrophilic rough bead for excellent water collection ability.
Heterostructures of bead-on-string fibers have drawn considerable attention for manufacturing cutting-edge micro-/nanodevices as ‘‘function centers,’’ which achieve many unique abilities such as directional water collection1,2 and special adhesion,3 responsive microreactor,4,5 smart sensor6 and selective catalyst.7–9 The heterostructures of bead-on-string at a micro- and nano-level have been developed by various methods, e.g., microfluidic10–12 and pneumatic6 or mechano-electro13 spin facilities, and the Plateau– Rayleigh instability in solid–liquid systems6,14–18 and solid–solid systems.19 Especially, natural bead-on-string in wet-rebuilt process endowed in cribellate spider capture silk for water collection is exhibited with unique spindle-knot and joint composed of porous nanofibrils and aligned nanofibrils,16,20 which inspire researchers to design the novel structured materials.21 Up to now, it is still challenging to fabricate wet-assembly bead-on-string heterostructures conveniently and controllably. Herein, we present a kind of bioinspired heterostructured bead-on-string fiber (BHBF) composed of poly-(methyl methacrylate) (PMMA) and titanium tetrachloride (TiCl4) hydrolyzed nanoparticles, which can be prepared via integrating a wet-assembly system, including PMMA electrospinning, fog of nanoparticles and water coalescence at multi-stages (this refers to several adsorption-wetting-drying processes). Thus, BHBFs can be effectively prepared by controlling
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing, 100191, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental section, Fig. S1 to S6, and Movie S1. See DOI: 10.1039/c4cc05156d
This journal is © The Royal Society of Chemistry 2014
the wet-assembly parameters, e.g., assembly time in humidity via water coalescence into droplets. In particular, water coalescence invigorates the nanoparticles to gather and form periodically separated nanoparticle-in droplets, which can be wet-assembled at multiple stages. The study offers an insight to the design of novel heterostructured materials via bioinspired wet-assembly. The findings are significant enough to be extended into realms such as water collection, fog-harvesting engineering, humidity sensor, micro-reactor, and filtration. The BHBFs were fabricated by combining the control of TiCl4 hydrolysis and water condensation (as illustrated in Fig. S1, ESI†). Initially, bare PMMA fibers (with diameters of 0.549 0.030 mm) were fabricated in an electrospinning system via an injected fluid consisting of a solution of PMMA in the solvent of N,N-dimethyl formamide (DMF). The foggy nanoparticles containing Ti(NPCTi) that result from the hydrolysis products of TiCl4 are used in BHBFs fabrication (see Experimental section in the ESI†). The sizes of nanoparticles are 0.282 0.072 mm via hydrolysis of 0.8 ml of TiCl4 (the optimization process is shown in Fig. S2, ESI†). As PMMA fiber captures the nanoparticles (Fig. 1a) and is then placed in a humid atmosphere; thus, water condensation appears around the NPCTi. Subsequently, small water droplets coalesce to form nanoparticle-in droplets. After the nanoparticle-in-droplets were dried, the nanoparticles assembled into beads along the fiber. Moreover, the method can be applied to a cross structure of two contact electrospinning fibers, as shown in Fig. S3 (ESI†). Fig. 1b and c shows the SEM images of the BHBFs in sequence. To distinguish the chemical components, energy dispersive spectrometer (EDS) mapping analysis was used to indicate NPCTi (Fig. 1d–f), where titanium (see scatters) is the main element of the nanoparticles in the beads. The outline of the scatters corresponded to SEM images of rough beads (Fig. 1a–c) composed of nanoparticles. It shows that the ‘‘bead’’ is mainly composed of titanium, while the ‘‘string’’ is PMMA, indicating that as-fabricated BHBFs are excellent heterostructures. To control the geometric structure of BHBFs, the wetting parameters and multi-stage wet-assembly were regulated. We firstly modulate the wetting parameters, e.g., at a relative humidity (RH) from 50% to 90% with a wetting time from 2 s to 16 s. Fig. S4 (ESI†)
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Fig. 1 SEM images and EDS mapping analysis of the PMMA-Ti fiber before and after wetting. (a) SEM image of PMMA-Ti fiber before wetting, which is densely coated by nanoparticles containing Ti with a diameter of 0.282 0.072 mm. (b) SEM image of the ‘‘bead-on-string’’ fiber after wetting. (c) Magnified SEM image of the ‘‘bead-on-string’’ fiber after wetting. (d–f) Elemental mapping of the same nanofiber region corresponding to that found in (a–c), indicating the spatial distribution of Ti (red). (d) Nanoparticles are effectively captured on the PMMA fiber. (e) The ‘‘bead-on-string’’ structure forms after wetting. (f) The magnified bead is composed of nanoparticles with Ti components.
shows the SEM images of the controlled fabrication of the ‘‘beadon-string’’ under different conditions. It can be concluded that only with an optimized RH and wetting time, there appears typical BFBFs. Fig. 2 shows statistic graphs of the geometric structures with RH from 60% to 90% and wetting times from 6 s to 14 s via wet-assembly at a single stage. Fig. 2a and b show the ‘‘bead’’ volume and ‘‘inter-bead’’ distance versus RH and wetting time. It shows that the volume of the ‘‘bead’’ and the distance between ‘‘beads’’ increases with an appropriate wetting time and humidity (from 6 s to 14 s in 60% and 65%; from 6 s to 12 s in 70% and 75%; and from 6 s to 10 s in 80%). There was no
Fig. 2 (a and b) Statistic graph of the BHBFs on volume (a) and distance (b) of beads regulated by RH from 60% to 80% and wetting times from 6 s to 14 s in a single stage. (c) SEM images of BHBFs via wet-assembly from 1 to 5 stages (frames numbered with 1–5) in an RH of 70%. (d) Statistical graph of BHBFs with increasing volumes of beads from 4.856 0.384 mm3 to 58.891 2.595 mm3 via wet-assembly from 1 to 5 stages.
10652 | Chem. Commun., 2014, 50, 10651--10654
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obvious ‘‘bead-on-string’’ structure under a wetter environment (14 s in 70% and 75%; 12 s and 14 s in 80%). There was a tendency that beads become bigger and more isolated due to the aggregation of nanoparticles in the wet-assembly process. The beads size is relevant not only with wetting parameters, such as RH and wetting time, but also with the multi-repetition of adsorption of nanoparticles in the wet-assembly process. Fig. 2c shows the microstructure on the surface of the ‘‘beadon-string’’ fibers after the multi-stage wet-assembly from stage 1 to 5. Here, the bead volume (Vb) is expressed with Vb = plw2/6, where l and w represent the length and width of the bead, respectively. As shown in Fig. 2d, it was estimated that Vb jumps from 4.856 0.348 mm3 to 58.892 2.595 mm3, as a result of the multi-stage wet-assembly at the fifth stage, which indicates the controllability of BHBFs fabrication. It is interesting how microscale beads are formed from nanoparticles via an multi-stage wet-assembly. Fig. 3 shows the in situ optical microscopic observation of the wet-assembly process with three stages of BHBFs in the fog. In the first stage (Fig. 3a–d), the PMMA fiber is densely coated with NPCTi by the hydrolysis of 0.8 mL of TiCl4 (at B54.94 s). The small water droplets condensed on the NPCTi, grow and coalesce with each other into bigger droplets (see arrows) during water condensation in a RH of 70% from B80.49 s to B84.80 s. The ‘‘bead’’ appears and was linked by ‘‘string’’ after drying at B97.80 s. To control the size of the bead, the wetting process above is repeated via wet-assembly (Fig. 3e–h). Nanoparticles are absorbed mostly on the bead due to the interception effect of the fiber22 (at B152.00 s). By water condensation and coalescence (from B163.11 s to 170.68 s), bigger beads composed of nanoparticles take on fiber via wet-assembly in the second-stage (at B186.85 s). Repeated continuously
Fig. 3 Optical images of the increasing bead-on-string via a multi-stage wet-assembly. (a–d) The first-stage. (a) NPCTi are uniformly captured on the fiber at B54.94 s. (b) Water condenses around the NPCTi and forms small droplets at B80.49 s. (c) Water-condensed droplets sweep the nanoparticles to assemble into ‘‘bead’’ on fiber (see the arrows) via coalescence of small water droplets at B84.80 s. (d) By controlling the wetting time, there appears small ‘‘bead’’ alternates ‘‘string’’ after drying at B97.80 s. (e–h) The secondstage. (e) Nanoparticles are mainly captured on the small bead at B152.00 s. (f) Water condenses around the NPCTi at B163.11 s. (g) After the same wetting conditions, droplets condensed on the fiber and coalesced around the ‘‘bead’’ at B170.68 s. (h) The ‘‘bead’’ becomes bigger after the secondstage at B186.85 s. (i–l) The third-stage. (i) Nanoparticles are mostly adsorbed around bead at B226.40 s. (j–k) Water-condensed droplets are formed on the fiber at B242.78 s and coalesced around the ‘‘bead’’ at B245.75 s. (l) Beads become bigger after the third-stage at B260.99 s. Nanoparticles, including NPCTi components, are obtained by hydrolyzing 0.8 mL of TiCl4. A RH of 70% is maintained during the wet-assembly process.
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(from B226.40 s to 245.75 s), ‘‘beads’’ grow much bigger in the third-stage at B260.99 s, which shows that the nanoparticles aggregate effectively and spontaneously. The fabrication indicates that the bead-on-string structures are controlled effectively by regulating wet-assembly at stages. The dynamic process of the multi-stage wet-assembly can also be seen in Movie S1 (ESI†). In fact, the wet-assembly of BHBFs was related to a driving coalescence of particles-in-droplets in humidity as illustrated in Fig. 4. When the PMMA fiber with NPCTi was placed in humidity (Fig. 4a, Frame 1–2), water condenses around the hydrophilic NPCTi to form small nanoparticle-in-droplets. Subsequently, the tiny droplets grow and coalesce with each other to form larger ones (Fig. 4a, Frame 3–5) for the sake of shrinking the superficial area and decreasing surface energy.23 The separated nanoparticle-in-droplets are periodically distributed along the PMMA fiber. Beads composed of nanoparticles were formed after water evaporation (Fig. 4a, Frame 6). For controlling the size of the bead, a second-assembly was carried out by NPCTi second-capture via the hydrolysis of TiCl4. The ‘‘dendritic’’ structure composed of NPCTi appears on the bead because of the roughnessinduced nanoparticles re-capture interception effect22 (Fig. 4b, Frame 1). At a given RH, small water droplets coalesce on the fiber (Fig. 4b, Frame 2–3). NPCTi gather around the ‘‘bead’’ due to the surface energy and curvature difference between the bead and string (Fig. 4b, Frame 4). Thus, a larger bead was formed after a multi-stage wet-assembly process (Fig. 4b, Frame 5).
Fig. 4 Illustration of BHBF fabrication. (a) ‘‘Bead-on-string’’ structure formation in a single-stage process. Water condenses around the NPCTi due to its hydrophilicity and forms small droplets (Frame 1). The droplets grow and coalesce with each other (Frame 2–4). The continuous coalescence of droplets induces the formation of a large droplet (Frame 5). Water evaporation induces the wet-rebuilt ‘‘bead-on-string’’ structure (Frame 6). (b) Controlling on size of ‘‘bead’’ in a multi-stage process. The ‘‘bead-onstring’’ absorbs NPCTi again via the hydrolysis of TiCl4. There is ‘‘dendritic’’ structure on the bead (Frame 1). Water condenses around the NPCTi to form droplets and move directionally to the ‘‘bead’’ (Frame 2–3). Finally, NPCTi gather around the ‘‘bead’’ with condensation of droplets (Frame 4). Thus, a larger bead is formed after drying (Frame 5). (c) Details of the mechanism on formation of larger beads. A water drop on the ‘‘string’’ location (less hydrophilic due to a small content of NPCTi) moves towards ‘‘bead’’ location (more hydrophilic due to a large content of NPCTi) (Frame 1). yA and yR indicate the advancing and receding angles of the water drops on the fiber, respectively. Another driving force arises from the conicalshaped geometry of the ‘‘bead’’ location, which causes a difference in the Laplace pressure. R0, r1 and r2 indicate the radius of the water droplet, ‘‘string’’ location and ‘‘bead’’ location, respectively. The arrows indicate the direction of moving water droplets (Frame 2).
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It is understood that both the cooperation of surface energy gradient and Laplace pressure act as a driving force for the directional movement of water drops.20 During the re-capture of nanoparticles (Frame 1 of Fig. 4c), the higher NPCTi concentration on the ‘‘bead’’ induces the hydrophilic property more than that of the ‘‘string,’’ which gives rise to a surface energy gradient along the fiber.24 A driving force (F) propels the water drops to move towards the location of the ‘‘bead’’ from that of the ‘‘string’’, F is expressed as follows:25,26 ð xbead dF ¼ gðcos yA cos yR Þdx (1) xstring
where g is the surface tension of water, x is the integrating variable along the axis of the fiber from string to bead, yA and yR represent the advancing and receding water contact angles of the surfaces with higher and lower NPCTi concentration (yA o yR), respectively. The surface energy gradient resulting from the NPCTi concentration will drive water to move from the less Ti component region to a more Ti component one. The second driving force to be considered is the gradient of Laplace pressure. The beads can be considered as two oppositely curved and joined conical objects (Frame 2 of Fig. 4c). Each conical object induces a difference in the Laplace pressure (DP) functioning on the water drop as follows:27 ð r2 2g DPcurvature ¼ sin a dz (2) 2 r1 ðr þ R0 Þ where r is the local radius of the fiber, R0 is the drop radius (R0 = (3V/4p)1/3, V is the drop volume), a is the half apex-angle of the bead, and z is the integrating variable along the diameter of the bead. The Laplace pressure on the high curvature position (the ‘‘string’’ with a local radius r1) is larger than that on the low curvature position (the ‘‘bead’’ with a local radius r2; r1 o r2), which results in a difference of Laplace pressure acting on the water drops to move from the ‘‘string’’ to ‘‘bead’’ region. Thus, the cooperation of Laplace pressure and surface gradient resulting from the chemical composition act together as driving forces to form bigger droplets, which corresponds to bigger beads on the fiber after drying. The drying time of the as-prepared first-stage BHBF leads to similar results during the second-stage process (as shown in Fig. S5, ESI†), which was attributed to the adsorption of nanoparticles at the water–air interface providing a synergic effect during wet-assembly. To examine this method, we compared the wetting behavior of bare PMMA fibers and the PMMA/Ti fibers (Fig. S6, ESI†). In addition, other particles such as TiO2 (P25) and ZnO (diameter of B100 nm) were used to fabricate ‘‘bead-on-string’’ fibers by flowing with N2 current at ambient temperature (Fig. S7a–d, ESI,† respectively). This indicates an effective method to design heterostructures for several applications. The water collection ability was examined on BHBF (also see in Experimental section). As shown in Fig. 5, Z has a linear increase tendency with an increase of S (for an approximately ellipsoidal bead,28 S = p(l2 + aw2/tan a)/2, where a = arc cos(l/w), l and w are the length and width of the bead, respectively). The Z value increases up to B8 times when beads increase S
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Notes and references
Fig. 5 Water collecting ability of BHBFs. With an increase of bead surface area (S) from 91.6 mm2 to 514.7 mm2, the water collection efficiency of beads has a linear growth, from 1.82 103 mm3 s1 to 1.35 104 mm3 s1. The insets are optical images of water drops collected on beads at B120 s.
from B91.6 mm2 to B514.7 mm2. This demonstrates that BHBFs with larger beads are more efficient in water collection than those with smaller beads due to increase in the rough surface. The insets show the optical images of typical water collection of BHBFs. In conclusion, we have controllably fabricated BHBFs at the micro- and nano-level and analyzed the mechanism of waterevaporation-induced self-assembly. This study is significant and helpful towards the design of smart heterostructured materials, and towards the development of materials with novel functionality that can be extended in realms such as water collecting device, fog-harvesting engineering, humidity-sensor, micro-reactor, filtration and others. This work is supported by the National Research Fund for Fundamental Key Project (2013CB933000), the National Natural Science Foundation of China (21234001), the Doctoral Fund of Ministry of Education of China (20121102110035), and the Graduate Student Innovation Practice Foundation of Beihang University.
10654 | Chem. Commun., 2014, 50, 10651--10654
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