Manipulation and Formation Mechanism of Silica One-Dimensional

Feb 14, 2014 - ABSTRACT: A roller electrospinning technique is combined with sol−gel chemistry to fabricate silica and polymeric materials on conduc...
0 downloads 0 Views 4MB Size
Download PDF
Article pubs.acs.org/Langmuir

Manipulation and Formation Mechanism of Silica One-Dimensional Periodic Structures by Roller Electrospinning Yongtao Yao,† Weilong Yin,† Jungang Cao,‡ Min Yang,§ Jianjun Li,*,† Shuyuan Zhao,† Yibin Li,† Xiaodong He,† and Jinsong Leng† †

National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin 150080, P. R. China ‡ College of Chemistry, Jilin University, Changchun 130012, P. R. China § School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150080, P. R. China S Supporting Information *

ABSTRACT: A roller electrospinning technique is combined with sol−gel chemistry to fabricate silica and polymeric materials on conductive and nonconductive substrates to verify its ability for controlling the long-range periodic structure of the final product. According to the experimental results, formation of the one-dimensional periodic silica structure was dependent on the electrical conductivity of the collector substrate. The periodic density seems to be related to the width of silica product. No effect from the electrical conductivity of collector substrate on the structure of polymeric system was observed. An energy transformation model was proposed to investigate the formation mechanism of this periodic structure. The theoretical simulation indicates that large width-to-thickness ratio of the product and high-energy transformation efficiency favor the formation of the long-range periodic structure.

■

INTRODUCTION Creation of one-dimensional structures, such as fibers, belts, tubes, etc., has gained considerable attention over the past years. Numerous experimental and theoretical studies have been performed and developed for their fabrications and applications.1 Cost-effective electrospinning is a general method to produce continuous one-dimensional structures with controllable diameters.2 Current research has been focused on the understanding of the electrospinning process, the manipulation of the structures, and the properties of final product. Electrospun products have attracted a lot of interest for a variety of applications including chemical catalysis,3,4 adsorption,5 filtration,6,7 sensors,8,9 and biomedical research.10,11 Although electrospinning technique is generally used in the system of organic polymer and inorganic/polymer hybrid composite with viscoelastic behavior,12−16 several one-dimensional ceramic structures have been prepared from the conventional sol−gel precursor solutions.17−22 In the early stage of electrospinning technique, a solution with viscoelastic behavior was forced through a hollow needle and one-dimensional structures formed after applying a high voltage. In order to satisfy the high volume industrial application, the needleless (roller) electrospinning was developed by Jirsak et al.23 and Elmarco Company started to commercialize machinery for the production of nanofibers. This is now known as a Nanospider and can produce one-dimensional products at industrial scale. © 2014 American Chemical Society

Fabrication and manipulation of matter at the nano- and micrometer level can create new classes of materials with enhanced mechanical, optical, transport, and magnetic properties. The self-assembling technique is often used to realize the objective of the nano- and microstructure control. Self-assembly of long-range periodic arrays and morphology control of one-dimensional structures24−30 have been a subject of intense interest and continue to present challenges for the nano- and microengineering applications.31−35 For example, helical gel formation from evaporation of colloidal solutions from a small vertical or inclined capillary was confirmed, and the physical mechanism was also proposed.36 Self-assembly of small molecules into one-dimensional nanostructures has been reviewed and researchers can use the fundamental principles of supramolecular chemistry to craft the size, shape, and internal structure of nanoscale objects.37 More recently, a quite general and direct synthesis method for one-dimensional periodic structure preparation and controllable wetting behaviors of a silica belt (ribbon shaped product) was explored successfully by a combination of sol−gel chemistry and electrospinning techniques. According to our previous report, the controllable wetting behaviors were realized by modifying the Received: June 28, 2013 Revised: February 12, 2014 Published: February 14, 2014 2335

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 1. Illustration of the roller electrospinning in our experiments with conductive and nonconductive collector substrates.

Figure 2. Splitting process in different types of materials and the samples were collected on nonconductive substrate. Silica (molar ratio, TEOS:H2O = 1:1; PEO, 0.5 wt % and Mn = 900 000): (a, b) with scale bar 10 μm; PMMA: (c, d) with scale bar 50 μm.

onto collector substrates with different electrical conductivity, and a detailed investigation into the dependence of the morphology of the product on various parameters was conducted. Based on the experimental results, an energy transformation model was proposed.

functional groups on the backbone and one-dimensional periodic structures were directly fabricated by using collector substrates with different electrical conductivity.38 These results demonstrated the utility of roller electrospinning in the fabrication of controllable one-dimensional periodic structures. However, further investigation was required to elucidate mechanistic details and to develop an appropriate model in order to reveal the intrinsic physics behind the long-range one-dimensional periodic structure. Herein, silica and polymeric system were electrospun

■

EXPERIMENTAL SECTION

As described previously,38 the polymerized silica sol was prepared by hydrolyzing tetraethyl orthosilicate (TEOS-98%, Sigma-Aldrich Chemical Co.) single precursor. Ethanol (99.7%) and HCl (36.38%) 2336

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 3. SEM images of the electrospun products from the silica sols (molar ratio, TEOS: H2O = 1:1; PEO, 0.5 wt % and Mn = 900 000) and PMMA solutions on nonconductive substrate. Silica: (a, b, c, d) were taken from the same sample with scale bar 50, 5, 50, and 50 μm, respectively; PMMA: (e, f) with scale bar 50 and 10 μm.

■

were supplied by Beijing Chemical Company. N,N-Dimethylformamide (DMF, 99.5%) was from Tianjin Chemical Company. Poly(ethylene oxide) (PEO, Mn = 100 000, 900 000 and 2 000 000) was purchased from Changchun Jinghua Company. Poly(methyl methacrylate) (PMMA, Mn = 40 000) was obtained from Aladdin Industrial Corporation. In a typical experiment, only TEOS was mixed with ethanol, water, and HCl solution (molar ratio in the solution; TEOS:ethanol:H2O:HCl = 1:3.1:x:7 × 10−4; where x was between 1 and 3), and the mixtures were refluxed at 70−80 °C for 3−4 h. Finally, the solvent was removed by distillation to get the silica sols with viscoelastic behavior. A small amount of poly(ethylene oxide) should be added into the silica sols in order to favor the formation of the electropsun silica product. Pure PEO was diluted into 1 wt % solution in deionized water, and the solution was continuously stirred at room temperature for 4 h. Then the PEO solution was added into the silica sols with continuous stirring for 24 h to completely dissolve the polymer. Final PEO concentration is 0.1, 0.5, or 1 wt % in the sols. Poly(methyl methacrylate) (PMMA) was also diluted into 30 wt % solution in N,N-dimethylformamide and employed as polymeric sample to have a comparison with ceramic silica. Roller experimental apparatus of NS Lab 500s (Nanospider, Elmarco Company) was used to fabricate nonwoven membranes. The applied voltage between collector and electrode was 30 kV. The electrode-to-collector distance was kept at 15 cm, and the electrode rotation rate was controlled at 3 rpm. The morphologies of the final products were investigated on a Hitachi SU8000 scanning electron microscope (SEM). A Ni-run RDV-3 (Shanghai, China) digital viscometer was used for the measurements of solution viscosity parameters at room temperature (25 °C).

RESULTS AND DISCUSSION Figure 1 illustrates the roller electrospinning process in the experiments. Two different collector substrates were employed to collect the electrospun products. One is commercially available “conductive paper”, whose electrical resistivity falls in the range of 102−106 ohm/cm. The actual value of the electrical resistivity is about 105 ohm/cm as shown by a digital multimeter. Aluminum or copper foil is chosen as the opposite collector substrate with very low electrical resistivity. Thus, in this study, the commercially available “conductive paper” was regarded as a nonconductive substrate. According to the comparison between the products on the nonconductive substrate and the products on the conductive substrate, the influence resulting from the different electrical conductivity of the collector substrate on final morphology of the different types of materials can be verified. When high voltage is applied, the silica sols or polymeric solutions are electrospun from many Taylor cones on the surface of a rotating spinning electrode, and one-dimensional product will deposit onto the collector substrate. The jet is initiated from the Taylor cone and starts to travel in air. Its diameter decreases due to the simultaneous effect of stretching and evaporation of the solvent. Also, the high repulsive forces from the increased charge density split the jet into fibers with smaller diameter.39 The SEM images in Figure 2 reveal the 2337

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 4. SEM images of the electrospun products from the silica sols (molar ratio, TEOS:H2O = 1:1; PEO, 0.5 wt % and Mn = 900 000) and the PMMA solutions on aluminum conductive substrate: Silica: (a, b) with scale bar 100 μm; PMMA: (c, d) with scale bar 50 and 10 μm.

Figure 5. Morphology difference between the final silica product (molar ratio, TEOS:H2O = 1:1.5; PEO, 0.5 wt % and Mn = 900 000) adhering onto copper substrate surface and the silica product in the top layer: (a) copper substrate coated with silica product; (b) silica product in the top layer; (c) silica product adhering onto copper substrate surface; (b1 and b2): SEM images of silica product in the top layer with scale bar 500 and 100 μm; (c1 and c2): SEM images of the silica product adhering onto copper substrate surface with scale bar 500 and 100 μm.

dependent on the electrical conductivity of the collector substrate. When the nonconductive substrate was used, silica ribbon shaped products with a wide width distribution were prepared as observed in Figure 3a,b. Electrospinning can produce ribbon structures from silica in which large ribbon widths and high periodicity (Figure 3c), or small ribbon widths and lower periodicity (Figure 3d) are observed. Here the periodic density refers to the number of repeated periodic structures per unit length in the silica product. In the electrospinning process, the electrospun silica products can hit the conductive fillers in a nonconductive substrate. The charges transfer and balance are lost. Finally, the ribbon-shaped product

splitting process in different types of materials. Silica and poly(methyl methacrylate) show no obvious differences in these images. In the electrospinning process, a one-dimensional structure with large diameter is produced first, which is further split, leading to the formation of a one-dimension ribbon shaped product or fiber with smaller diameter. This process may repeat several times resulting in a variety of fiber widths, as shown in Figure 2c. We studied the influence of the electrical conductivity of the collector substrate on the final morphology of different types of materials. For the silica sols (molar ratio, TEOS:H2O = 1:1; PEO, 0.5 wt % and Mn = 900 000), the final structure strongly 2338

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 6. SEM images of the electrospun products from the silica sols (molar ratio, TEOS:H2O = 1:1.5; PEO, 0.5 wt %) on copper substrate with different PEO molecular weight: (a, b) Mn = 100 000 with scale bar 500 and 100 μm; (c, d) Mn = 900 000 with scale bar 500 and 100 μm; (e, f) Mn = 2 000 000 with scale bar 1 mm and 200 μm.

distribution by roller electrospinning (Figure 3e,f). Another polymeric system, polyacrylonitrile, shows similar results (Figure S1). If a conductive aluminum foil is employed for the electrospinning, the ratio of the one-dimensional periodic structures to the normal ribbon-shaped product increased (Figure 4a,b, compare with Figure 3a,b). As displayed in Figure 4c,d, no obvious effect from the collector substrate on the structure of the polymeric system was observed. When needle electrospinning instead of roller electrospinning was applied to the silica sol, silica fibers with periodic structure were never produced (Figure S2), no matter what kind of substrate was used.38 The fibers produced with a needle

Table 1. Viscosity of Silica Sols with Different Molar Ratio between TEOS and H2O and Altering Concentration of PEO (mPa·s) TEOS/H2O (molar ratio)

0% PEO

0.1% PEO

0.5% PEO

1% PEO

1:1.5 1:2 1:3

29 35 47

37 46 59

64 86 106

121 162 205

spontaneously contracts into the one-dimensional periodic structure by undergoing a structural rearrangement. Compared with the silica sols, the polymeric PMMA system tends to form a fiber structure with a much narrower diameter 2339

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 7. SEM images of the electrospun products from the silica sols (molar ratio, TEOS:H2O = 1:1.5; PEO, Mn = 900 000) on copper substrate with different PEO concentration: (a, b, c) 0.1 wt % PEO with scale bar 500, 100, and 50 μm; (d, e, f) 0.5 wt % PEO with scale bar 500, 100, and 50 μm; (g, h, and i) 1 wt % PEO with scale bar 500, 100, and 100 μm.

show a narrow diameter distribution and the average diameter is about 2 μm. If the solvent is highly volatile, during jetting a dry skin is formed at the surface in roller electrospinning, which leads to a ribbon-shaped product. According to our experimental results, formation of one-dimensional periodic silica structures seems to be dependent on the width of the product, possibly explaining why one-dimensional periodic structures were only observed with roller electrospinning. In order to further confirm the long-range periodic structure dependence of the silica product on the electrical conductivity of the collector substrate, copper foil was used as another conductive collector substrate. According to the results (Figure S3), the electrical conductivity of the copper foil is similar to that of the aluminum foil. We investigated the morphology difference between the final silica product adhering onto the copper substrate surface and silica product in the top layer. In this experiment, the molar ratio between TEOS and H2O is 1 to 1.5 and the PEO (Mn = 900 000) concentration is 0.5 wt %. First, copper substrate coated with silica product was prepared as shown in Figure 5a. Then the coated silica product was separated from the copper substrate carefully, giving the pure silica product (Figure 5b) and the copper substrate with silica product adhering onto its surface (Figure 5c). The SEM images of the silica product in the top layer are shown in Figures 5b1 and 5b2. Very few one-dimensional periodic structures were observed. This phenomenon likely arises from the fact that the

conductive copper substrate will be covered with silica product if electrospinning continues for some time, which makes the substrate nonconductive. Figures 5c1 and 5c2 display SEM images of the final silica products adhering onto the copper substrate surface, and obvious long-range periodic morphology of silica was confirmed. The morphology comparison results of the silica product in Figure 5 clearly demonstrate the long-range periodic structure dependence on the electrical conductivity of the collector substrate. The viscosity of silica sols is directly related to the hydrolysis and polymerization degree of TEOS, the concentration, and the molecular weight of polymer in the sols.40 The effects of molecular weight on the fiber structure of electrospun poly(vinyl alcohol) (PVA) have been studied, and the molecular weight and the solution concentration have significant effects on the structure of the electrospun polymer.41,42 The molecular weight of PEO in this research is also a key parameter, and different PEO with the number-average molecular weight ranging from 100 000 to 2 000 000 were used. Figure 6 shows the effect of molecular weight of PEO (0.5 wt %) on the final silica product on copper substrate surface. At a molecular weight of Mn = 100 000, fibers with average diameter 1 μm are observed (Figure 6a,b). When the PEO molecular weight increases to 900 000, ribbon-shaped products with long-range periodic structure were observed (Figure 6c,d). If the molecular weight reaches 2 000 000, the electrospinning process becomes 2340

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 8. SEM images of the electrospun products from the silica sols (molar ratio, TEOS:H2O = 1:2; PEO, Mn = 900 000) on copper substrate with different PEO concentration: (a, b, c) 0.1 wt % PEO with scale bar 500, 100, and 100 μm; (d, e, f) 0.5 wt % PEO with scale bar 500, 100, and 100 μm; (g, h, i) 1 wt % PEO with scale bar 500, 100, and 100 μm.

difficult, and a broad distribution of final products was found (Figure 6e,f). At a constant concentration, PEO with higher molecular weight leads to a higher sol viscosity, and the electrospinning process is strongly dependent on the viscosity. According to the results, PEO with molecular weight of Mn = 900 000 was chosen for further experiments. As alkoxide precursor, the hydrolysis and the polymerization degree of TEOS will depend on the molar ratio between TEOS and H2O in the solution. Also, the concentration of PEO in the solution may affect the formation of long-range periodic structure. To investigate the effect of the ratio of water to alkoxide precursor and the concentration of PEO on structure, a series of electrospun silica samples were prepared. The viscosity of silica sols with different molar ratio between TEOS and H2O and altering concentration of PEO are shown in Table 1, and the corresponding SEM images are displayed in Figures 7, 8, and S4. Figure 7 shows the SEM images of the products generated from the sols containing different PEO concentrations on the copper substrate, and the molar ratio between TEOS and H2O is 1 to 1.5. When the concentration of PEO is 0.1 wt % in the sols, silica fibers cannot form due to the low viscosity. When the concentration of PEO increases to 0.5 wt %, interesting one-dimensional periodic structures in ribbon-shaped silica were obtained. If the concentration of PEO reaches 1 wt %, long-range periodic structure disappears and beads are instead observed. The presence of beads in

electrospun fibers is a common problem and can be attributed to the action of the viscosity, surface tension, and density of charges.43 Figure 8 shows SEM images obtained for sample prepared at higher water content and the molar ratio TEOS:H2O = 2. The results in Figures 8a−c and 8d−f clearly indicate the formation of long-range periodic structure in the silica product with 0.1 and 0.5 wt % PEO, respectively, while higher concentration of PEO did not favor the formation of long-range periodic structure (Figure 8g−i). When the molar ratio between TEOS and H2O was further increased to 3, long-range periodic structures were not observed for any PEO concentration (Figure S4). From these results, we can see that the formation of longrange periodic structures in silica depends not only on the concentration of PEO but also on the hydrolysis and polymerization degree of TEOS, possible through effects on the viscosity. Furthermore, these factors could determine the mechanical behavior of the products, which may be another key parameter for the formation of long-range periodic structure in the silica product. Based on the results for formation of long-range periodic silica structures on the conductive substrate, it seems that the periodic density depends on the width of the ribbon shaped silica. Figure 9 demonstrates the formation of one-dimensional periodic structures with different periodic densities. The results from Figure 9a−d appear that bigger width may result in higher 2341

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 10. Plot of the periodic density versus the width of the silica products by summarizing the results from Figure 9e−h. The periodic density refers to the number of repeated periodic structures per 50 μm silica product. Periodic density 5 for (e), 2 for (f), 1.5 for (g), and 1 for (h), while the width 25 μm for (e), 8 μm for (f), 4 μm for (g), and 13 μm for (h).

of one-dimensional periodic structure is complicated, and many parameters can affect the final morphology of the product.44 Here we try to explain its growth mechanism based on the energy transformation model. In this model, the energy per unit volume is assumed and a general estimation of the total energy is given. After the thin films containing many Taylor cones were stretched and split into final one-dimensional structures, they carried away excess charges (Figure 11). After they touch conductive substrate, the kinetic energy and electrostatic energy will transform into elastic energy. In the electrospinning process, it is obvious that the electrostatic energy will be dominant. If the charge density on the surface and the velocity are regarded as constant, the kinetic energy is proportional to the cross-sectional area, while the electrostatic energy depends on the square of the charge on the surface area; the elastic energy stored in the final product is proportional to the crosssectional area and the square of the change in length. For cylindrical fibers, this gives

Figure 9. Periodic density dependence on the width of the silica products: (a, b, c, d) SEM images of the electrospun products from the silica sols (molar ratio, TEOS:H2O = 1:1.5; PEO, 0.5 wt % and Mn = 900 000) with scale bar 100, 100, 50, and 50 μm; (e, f, g, h) SEM images of the electrospun products from the silica sols (molar ratio, TEOS:H2O = 1:2; PEO, 0.1 wt % and Mn = 900 000) with scale bar 50, 50, 50, and 50 μm.

E kinetic ∝ πd 2/4

(1)

2

periodic density. Figure 10 presented the periodic density dependence on the width, summarizing the results from Figure 9e−h. Here the periodic density refers to the number of repeated periodic structures per 50 μm silica product. However, no obvious correlation between the structure width and periodic density is observed. For example, the periodic density of the product shown in Figure 9h is smaller than those shown in Figure 9f,g, while the width of the product shown in Figure 9h is bigger than those shown in Figure 9f,g. The correlation between the structure width and periodic density and abnormal case in Figure 9h will be explained in the next theoretical discussion. According to the experimental results of the one-dimensional structure in silica, we propose a model to get an insight into this electrospinning process. It should be noted that the growth

Eelectro ∝ (πd)

(2)

Eelastic ∝ (Δl)2 πd 2/4

(3)

Here d is the diameter of the fiber and Δl is the change in length of the fiber (which also reflects the periodic density). The kinetic energy, the electrostatic energy, and the elastic energy are all proportional to the square of the diameter for the cylindrical fiber. This shows that the change in length in the cylindrical fiber is not dependent on its diameter. For the ribbon-shaped silica with different width, the energy was considered as

E kinetic ∝ wt

(4) 2

2342

Eelectro ∝ 4(w + t )

(5)

Eelastic ∝ (Δl)2 wt

(6) dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

Figure 11. An energy transformation model to illustrate the formation mechanism of one-dimensional periodic structures: (a) polymeric sample; (b) silica product.

w and t are the width and the thickness of the ribbon-shaped product, respectively. If we define n as the width-to-thickness ratio w = nt

(n > 1)

(7)

then the energy transformation in the ribbon-shaped product is (Δl)2 wt ∝ 4(w + t )2 + wt

(8)

Formula 8 is based on an ideal conductive collector substrate in the electrospinning process. In fact, the energy transformation efficiency should be taken into account in this model. For the ideal conductive substrate, both the kinetic energy and electrostatic energy transform into elastic energy (Figure 12a). If a nonconductive substrate is used, only the kinetic energy can transform into elastic energy or thermal energy (Figure 12b). Thus, a coefficient k is considered to reflect the effect resulting from the electrical conductivity of collector substrate. Then formula 8 is changed to be (Δl)2 wt ∝ k 4(w + t )2 + wt

(0 ≤ k ≤ 1)

(9)

Finally, the change in length should be Δl ∝

[k 4(n + 1)2 + n]/n

(10)

This formula reveals that the change in length in the ribbonshaped product is proportional to the ratio between width and thickness (n). In the case of w ≫ t, the change in length will increase correspondingly, favoring the spontaneous formation of a periodic structure. When n is smaller, it becomes difficult to spontaneously form a periodic structure. According to formula 10, it is found to be difficult for the spontaneous formation of

Figure 12. (a, b) Illustration for energy transformation on conductive and nonconductive substrate, respectively. (c) A general simulation of periodic density dependence on width−thickness ratio with different coefficient k.

the one-dimensional periodic structure for the cylindrical fibers (n = 1). It is shown that coefficient k depends on the electrical 2343

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

An energy transformation model was proposed to explain the growth mechanism and accounted for the observation of anomalous structures in which the width did not correlate with the periodic density. Future work should be directed toward controlling the long-range periodic structure precisely.

conductivity of the collector substrate. Higher electrical conductivity of the collector substrate leads to a larger coefficient k and faster electrostatic charge dissipation, which favors the formation of one-periodic structure with high periodic density over the same time interval. Furthermore, lower electrical conductivity of the collector substrate may result in the formation of one-dimensional periodic structure with low periodic density. A general simulation of periodic density dependence on width-thickness ratio with different coefficient k is indicated in Figure 12c. The charge on the surface area is proportional to the width of ribbon shaped silica. With increase in width of the product, the electrostatic charge dissipation may also become faster. Unfortunately, now we cannot measure the time interval between the ribbon-shaped silica touching the substrate and the periodic structure forming on the substrate. The incident angle should be also introduced and considered as another key parameter. Formula 10 can be modified as Δl ∝

2

[k 4(n + 1) + n]/n sin θ

■

ASSOCIATED CONTENT

S Supporting Information *

SEM images of the electrospun polyacrylonitrile-PAN, SEM images of the electrospun silica products by needle and roller electrospinning, and electrical resistivity of copper foil. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel 86-451-86403786; Fax 86-451-86403786; e-mail ljj8081@ gmail.com (J.L.).

(11)

Notes

The authors declare no competing financial interest.

The formation of periodic structure dependence on the incident angle is illustrated in Figure 13a−c. From formula 11, a general

■

ACKNOWLEDGMENTS This research is supported by “National Natural Science Foundation of China” (No. 21301040 and No. 21203043) and “the Fundamental Research Funds for the Central Universities” (Grants HIT. NSRIF. 2012027 and 2012029).

■

REFERENCES

(1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15, 353−389. (2) Greiner, A. J.; Wendorff, H. Electrospinning: A Fascinating Method for the Preparation of Ultrathin Fibers. Angew. Chem., Int. Ed. 2007, 46, 5670−5703. (3) Patel, A. C.; Li, S. X.; Wang, C.; Zhang, W. J.; Wei, Y. Electrospinning of Porous Silica Nanofibers Containing Silver Nanoparticles for Catalytic Applications. Chem. Mater. 2007, 19, 1231−1238. (4) Formo, E.; Lee, E.; Campbell, D.; Xia, Y. N. Functionalization of Electrospun TiO2 Nanofibers with Pt Nanoparticles and Nanowires for Catalytic Applications. Nano Lett. 2008, 8, 668−672. (5) Im, J. S.; Park, S. J.; Kim, T. J.; Kim, Y. H.; Lee, Y. S. The Study of Controlling Pore Size on Electrospun Carbon Nanofibers for Hydrogen Adsorption. J. Colloid Interface Sci. 2008, 318, 42−49. (6) Qin, X. H.; Wang, S. Y. Filtration Properties of Electrospinning Nanofibers. J. Appl. Polym. Sci. 2006, 102, 1285−1290. (7) Barhate, R. S.; Ramakrishna, S. Nanofibrous Filtering Media: Filtration Problems and Solutions from Tiny Materials. J. Member. Sci. 2007, 296, 1−8. (8) Li, Z. Y.; Zhang, H. N.; Zheng, W.; Wang, W.; Huang, H. M.; Wang, C.; MacDiarmid, A. G.; Wei, Y. Highly Sensitive and Stable Humidity Nanosensors Based on LiCl Doped TiO2 Electrospun Nanofibers. J. Am. Chem. Soc. 2008, 130, 5036−5037. (9) Wang, W.; Huang, H. M.; Li, Z. Y.; Zhang, H. N.; Wang, Y.; Zheng, W.; Wang, C. Zinc Oxide Nanofiber Gas Sensors Via Electrospinning. J. Am. Ceram. Soc. 2008, 91, 3817−3819. (10) Kim, H. W.; Lee, H. H.; Knowles, J. C. Electrospinning Biomedical Nanocomposite Fibers of Hydroxyapatite/poly(lactic acid) for Bone Regeneration. J. Biomed. Mater. Res., Part A 2006, 79A, 643− 649. (11) Agarwa, S.; Wendorff, J. H.; Greiner, A. Use of Electrospinning Technique for Biomedical Applications. Polymer 2008, 49, 5603− 5621. (12) Agarwal, S.; Wendorff, J. H.; Greiner, A. Chemistry on Electrospun Polymeric Nanofibers: Merely Routine Chemistry or a Real Challenge? Macromol. Rapid Commun. 2010, 31, 1317−1331.

Figure 13. (a−c) Periodic growth dependence on incident angle. (d) A general simulation of periodic density dependence on width− thickness ratio with different incident angles and k = 1.

simulation of periodic density dependence on width−thickness ratio with different incident angles and k = 1 is displayed in Figure 13d. According to the theoretical results, it is possible to find silica products with the same width but having different periodic densities. The simulation results give a good explanation for the abnormal case observed in Figure 9h.

■

CONCLUSION Silica and polymeric materials were studied to determine the influence of collector substrate electrical conductivity on the final morphology of the electrospun product obtained by roller electrospinning. Interesting long-range periodic structures were observed for silica but not for organic polymers such as PMMA and PAN. It has been clearly demonstrated that spontaneous formation of the one-dimensional silica periodic structure can occur on the conductive collector substrate. 2344

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345

Langmuir

Article

(34) Lv, H. B.; Gou, J. H. Fabrication and Electroactive Responsive Behavior of Shape-memory Nanocomposite Incorporated with Selfassembled Multiwalled Carbon Nanotube Nanopaper. Polym. Adv. Technol. 2012, 23, 1529−1535. (35) Lv, H. B.; Liang, F.; Gou, J. Nanopaper Enabled Shape-memory Nanocomposite with Vertically Aligned Nickel Nanostrand: Controlled Synthesis and Electrical Actuation. Soft Matter 2011, 7, 7416− 7423. (36) Veretennikov, I.; Indeikina, A.; Chang, H. C.; Marquez, M.; Suib, S. L.; Giraldo, O. Mechanism for Helical Gel Formation from Evaporation of Colloidal Solutions. Langmuir 2002, 18, 8792−8798. (37) Palmer, L. C.; Stupp, S. I. Molecular Self-Assembly into OneDimensional Nanostructures. Acc. Chem. Res. 2008, 41, 1596−1608. (38) Yao, Y. T.; Cao, J. G.; Yang, M.; Li, J. J.; Zhao, S. Y.; Yin, W. L.; Li, Y. B.; He, X. D.; Leng, J. S. Exploration of the Novel Stacked Structure and One-step Fabrication of Electrospun Silica Microbelts with Controllable Wettability. RSC Adv. 2013, 3, 12026−12030. (39) Doshi, J.; Reneker, D. H. Electrospinning Process and Applications of Electrospun Fibers. J. Electrost. 1995, 35, 151−160. (40) Toskas, G.; Cherif, C.; Hund, R. D.; Laourine, E.; Fahmi, A.; Mahltig, B. Inorganic/Organic (SiO2)/PEO Hybrid Electrospun Nanofibers Produced from a Modified Sol and Their Surface Modification Possibilities. ACS Appl. Mater. Interfaces 2011, 3, 3673−3681. (41) Koski, A.; Yim, K.; Shivkumar, S. Effect of Molecular Weight on Fibrous PVA Produced by Electrospinning. Mater. Lett. 2004, 58, 493−497. (42) Tao, J.; Shivkumar, S. Molecular Weight Dependent Structural Regimes during the Electrospinning of PVA. Mater. Lett. 2007, 61, 2325−2328. (43) Fong, H.; Chun, I.; Reneker, D. H. Beaded nanofibers formed during electrospinning. Polymer 1999, 40, 4585−4592. (44) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. Bending Instability of Electrically Charged Liquid Jets of Polymer Solutions in Electrospinning. J. Appl. Phys. 2000, 87, 4531−4547.

(13) Ma, M. L.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.; Rutledge, G. C. Decorated Electrospun Fibers Exhibiting Superhydrophobicity. Adv. Mater. 2007, 19, 255−259. (14) Wang, M.; Jin, H. J.; Kaplan, D. L.; Rutledge, G. C. Mechanical Properties of Electrospun Silk Fibers. Macromolecules 2004, 37, 6856− 6864. (15) Asmatulu, R.; Ceylan, M.; Nuraje, N. Study of Superhydrophobic Electrospun Nanocomposite Fibers for Energy Systems. Langmuir 2011, 27, 504−507. (16) Pirzada, T.; Arvidson, S. A.; Saquing, C. D.; Sakhawat Shah, S.; Khan, S. A. Hybrid Silica-PVA Nanofibers via Sol-Gel Electrospinning. Langmuir 2012, 28, 5834−5844. (17) Sigmund, W.; Yuh, J.; Park, H.; Maneeratana, V.; Pyrgiotakis, G.; Daga, A.; Taylor, J.; Nino, J. C. Processing and Structure Relationships in Electrospinning of Ceramic Fiber Systems. J. Am. Ceram. Soc. 2006, 89, 395−407. (18) Li, D.; Xia, Y. N. Direct Fabrication of Composite and Ceramic Hollow Nanofibers by Electrospinning. Nano Lett. 2004, 4, 933−938. (19) Larsen, G.; Velarde-Ortiz, R.; Minchow, K.; Barrero, A.; Loscertales, I. G. A Method for Making Inorganic and Hybrid (Organic/Inorganic) Fibers and Vesicles with Diameters in the Submicrometer and Micrometer Range via Sol-Gel Chemistry and Electrically Forced Liquid Jets. J. Am. Chem. Soc. 2003, 125, 1154− 1155. (20) Li, X.; Yu, M.; Hou, Z. Y.; Wang, W. X.; Li, G. G.; Cheng, Z. Y.; Chai, R. T.; Lin, J. Preparation and Luminescence Properties of Lu2O3:Eu3+ Nanofibers by Sol-gel/electrospinning Process. J. Colloid Interface Sci. 2010, 349, 166−172. (21) Maensiri, S.; Nuansing, W.; Klinkaewnarong, J.; Laokul, P.; Khemprasit, J. Nanofibers of Barium Strontium Titanate (BST) by Sol-Gel Processing and Electrospinning. J. Colloid Interface Sci. 2006, 297, 578−583. (22) Sui, R. H.; Rizkalla, A. S.; Charpentier, P. A. Formation of Titania Nanofibers: A Direct Sol-Gel Route in Supercritical CO2. Langmuir 2005, 21, 6150−6153. (23) Jirsak, O.; Sanetrnik, F.; Lukas, D.; Kotek, V. ; Martinova, L. ; Chaloupek, J. A Method of Nanofibres Production from a Polymer Solution Using Electrostatic Spinning and a Device for Carrying Out the Method. European Patent 2004; EP 1 (673 493). (24) Chang, G. Q.; Song, G. X.; Yang, J.; Huang, R. S.; Kozinda, A.; Shen, J. Y. Morphology Control of Nanohelix by Electrospinning. Appl. Phys. Lett. 2012, 101 (263505), 1−3. (25) Grasl, C. Arras, Matthias M. L.; Stoiber, M.; Bergmeister, H.; Schima, H. Elctrodynamic Control of the Nanofiber Alignment during Electrospinning. Appl. Phys. Lett. 2013, 102 (053111), 1−4. (26) Kessick, R.; Tepper, G. Microscale Polymeric Helical Structures Produced by Electrospinning. Appl. Phys. Lett. 2004, 84, 4807−4809. (27) Yu, J.; Qiu, Y. J.; Zha, X. X.; Yu, M.; Yu, J. L.; Rafique, J.; Yin, J. Production of Aligned Helical Polymer Nanofibers by Electrospinning. Eur. Polym. J. 2008, 44, 2838−2844. (28) Koombhongse, S.; Liu, W. X.; Reneker, D. H. Flat Polymer Ribbons and Other Shapes by Electrospinning. J. Polym. Sci., Polym. Phys. 2001, 39, 2598−2606. (29) Ma, M. L.; Titievsky, K.; Thomas, E. L.; Rutledge, G. C. Continuous Concentric Lamellar Block Copolymer Nanofibres with Long Range Order. Nano Lett. 2009, 9, 1678−1683. (30) Ma, M. L.; Krikorian, V.; Yu, J. H.; Thomas, E. L.; Rutledge, G. C. Electrospun Polymer Nanofibers with Internal Periodic Structure Separation of Cylindrically Confined Block Copolymers. Nano Lett. 2006, 6, 2969−2972. (31) Lv, H. B. A Simulation Method to Analyze Chemo-mechanical Behavior of Swelling-induced Shape-memory Polymer in Response to Solvent. J. Appl. Polym. Sci. 2012, 123, 1137−1146. (32) LeMieux, M. C.; McConney, M. E.; Lin, Y. H.; Singamaneni, S.; Jiang, H.; Bunning, T. J.; Tsukruk, V. V. Polymeric Nanolayers as Actuators for Ultrasensitive Thermal Bimorphs. Nano Lett. 2006, 6, 730−734. (33) Sun, J. Y.; Bhushan, B. Hierarchical Structure and Mechanical Properties of Nacre: A Review. RSC Adv. 2012, 2, 7617−7632. 2345

dx.doi.org/10.1021/la4037277 | Langmuir 2014, 30, 2335−2345