Inorganic Nanowires-Assembled Layered Paper as the Valve for

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Inorganic Nanowires-Assembled Layered Paper as the Valve for Controlling Water Transportation Fei-Fei Chen,†,‡ Ying-Jie Zhu,*,†,‡ Zhi-Chao Xiong,*,†,‡ Tuan-Wei Sun,†,‡ Yue-Qin Shen,†,‡ and Ri-Long Yang†,‡ †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Layered materials with open interlayer channels enable various applications such as tissue engineering, ionic and molecular sieving, and electrochemical devices. However, most reports focus on the two-dimensional nanosheets-assembled layered materials, whose interlayer spacing is limited at the nanometer scale. Herein, we demonstrate that one-dimensional inorganic nanowires are the ideal building blocks for the construction of layered materials with open interlayer channels as well, which has not aroused much attention before. It is found that the relatively long inorganic nanowires are capable of assembling into free-standing layered paper with open interlayer channels during the filtration process. The spacings of interlayer channels between adjacent layers are up to tens of micrometers, which are much larger than those of the two-dimensional nanosheets-assembled layered materials. But the closed interlayer channels are observed when the relatively short inorganic nanowires are used as building blocks. The mechanism based on the relationship between the structural variation and the nanowires used is proposed, including the surface charge amplified effect, surface charge superimposed effect, and pillarlike supporting effect. According to the proposed mechanism, we have successfully fabricated a series of layered paper sheets whose architectures (including interlayer channels of cross section and pores on the surface) show gradient changes. The as-prepared layered paper sheets are employed as the valves for controlling water transportation. Tunable water transportation is achieved by the synergistic effect between in-plane interlayer channels (horizontal transportation) from the open to the closed states, and through-layer pores (vertical transportation) without surface modification or intercalation of any guest species. KEYWORDS: inorganic nanowires, nanostructured materials, layered paper, interlayer channels, water transportation

1. INTRODUCTION It is well-known that the materials architectures, to a great extent, affect the performance and thus behaviors of materials in the practical applications.1 Hence, the rational design of materials with the desirable architectures is of both theoretical and technological interest. Recently, the layered materials with open interlayer channels have attracted much attention as they exhibit various excellent characteristics including open porous network, size exclusion, capillary force, ion selection, space confinement, interface reflection, alleviative volume change, and energy dissipation, etc. Therefore, the layered materials with desirable properties are promising for applications in various fields. For example, by using the layered graphene hydrogel membrane with nanochannels as the barrier membrane, the open porous network resulted from larger interlayer channels allows the inflow of liquid and nutrients that are necessary for apatite formation and simultaneously restricts the outflow of cells, and thus benefits the bone generation, whereas the dried membrane with collapsed channels shows inferior performance.1 The layered materials with the open porous network are also employed in the electrochemical energy conversion and storage.2−5 The © XXXX American Chemical Society

open porous network provides more accessible pathways toward electrolyte to shorten greatly the ion diffusion length and maximize the utilization of active materials, which is responsible for the enhanced electrochemical performance. Additionally, the layered structure provides opportunities to achieve molecular and ion sieving by means of controllable interlayer spacing and capillary force, which give rise to size exclusion and frictionless flow of fluids.6−8 By adjusting the size of interlayer channels processed by the surface modification of the starting building blocks or intercalation of guest species, precise separation of target molecules and ions is realized.8 Moreover, in the innovative design of lithium metal anode reported by Cui et al.,9 the capillary force created by nanoscale interlayer channels of layered reduced graphene oxide enables the fast metallic lithium intake and uniform lithium deposition, which greatly alleviates the volume change during cycles. Recently, there is much attention focused on the ion-selective nanofluidic layered materials because of their unique surfaceReceived: January 25, 2017 Accepted: March 7, 2017

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DOI: 10.1021/acsami.7b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces charge-governed ion transportation.10−13 Unipolar species transport of the layered materials by allowing the permeation of counterions and repelling ions with like charge can enhance ionic or proton conductivity. Such a way of transportation will be potentially applied in the generators, biosensors, and electrochemical devices.10 In addition, another state-of-the-art research toward layered materials is biomimetic artificial nacres that simultaneously have the outstanding strength and toughness.14 The layered brick-and-mortar architecture of natural nacres contributes to energy dissipation and crack bridging, which plays a key role in their unique mechanical performance.15 Inspired by the architecture of natural nacres, efforts have been made to improve further the mechanical performance of artificial nacres through the synergistic interfacial interactions coupled with synergistic building blocks.16 With increasing interest and rapid development of layered materials, more potential fields are excavated such as the space confinement strategy for the preparation of nanosheets by using the layered materials as templates,17,18 and enhanced electromagnetic interference shielding by using layered transition metal carbides with more available interlayer reflecting surface as the multilevel shield.19 However, the development and applications of layered materials are hindered currently by at least three issues as follows: (1) most of the building blocks for the construction of layered materials reported are two-dimensional (2-D) nanosheets. Only rare reports involve the one-dimensional (1-D) nanowires-assembled layered materials.20,21 Additionally, the comprehensive investigation on the phenomena of 1-D nanowires-assembled layered materials and in-depth insights into the mechanisms of the structural formation have not been documented; (2) the sizes of interlayer channels of 2-D nanosheets-assembled layered materials are limited at the nanometer scale even through the intercalation of large-sized polymer molecules, nanoparticles or nanofibers.8 It is expected that the larger interlayer spacings are able to further facilitate the diffusion of fluids (e.g., electrolyte in the electrochemical devices) into the channels and thus bring them into contact with active species faster, and allow the fluid flux to be tunable in a broad spectrum; (3) very few accessible vertical pathways exist in the 2-D nanosheets-assembled layered materials except for some defects like voids and dislocations.10 Hence, the flow of fluids is primarily in the way of horizontal transportation and how to increase the vertical transportation remains a challenge.10 Herein, we prepare and investigate the 1-D inorganic nanowires-assembled layered paper. Two types of layered paper with open or closed interlayer channels are obtained by using four kinds of inorganic nanowires with different lengths as building blocks. During the vacuum-assisted filtration process, the relatively long nanowires assemble into the layered paper sheets with open interlayer channels. The interlayer spacing is as large as tens of micrometers. However, the closely packed cross section of paper sheets is observed in the cases of using relatively short nanowires as the building blocks. On the basis of the experimental results, we give the deep insights into the mechanism of the structural formation and propose three related effects that are surface charge amplified effect, surface charge superimposed effect, and pillarlike supporting effect. According to the proposed mechanism, a series of layered paper sheets with interlayer channels in different states, from the open to the closed, have been successfully fabricated by simply mixing two kinds of inorganic nanowires with different lengths.

By using the nanowires as building blocks, the porosity of the vertical direction of layered paper is dramatically increased and the pore size can be adjusted. The as-prepared layered paper sheets are employed as the valves for controlling the water transportation. The synergistic effect between in-plane interlayer channels (horizontal transportation) and throughlayer pores (vertical transportation) allows tunable water flux in a wide range through the layered paper. In this work, the tunable water transportation is achieved simply by adjusting the materials architecture, avoiding the tedious surface modification or intercalation of any guest species.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Manganese(II) sulfate monohydrate (MnSO4· H2O, analytical reagent (AR)), potassium chlorate (KClO3, AR), potassium acetate (CH3COOK, AR), acetic acid (CH3COOH, AR), sodium hydroxide (NaOH, AR), hydrochloric acid (HCl, AR), oleic acid (C18H34O2, AR), calcium chloride (CaCl2, AR), and sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received without purification. 2.2. Synthesis of MnO2 Nanowires (Mn-NWs). Mn-NWs were synthesized according to a previously reported method.22 In brief, MnSO4·H2O (2.028 g), KClO3 (2.574 g), and CH3COOK (2.061 g) were dissolved in deionized water (180 mL) under ultrasonic treatment to form a clear solution. After the addition of CH3COOH (9.6 mL), the solution was transferred into three Telfon-lined stainless steel autoclaves (100 mL). The autoclaves were sealed and treated under hydrothermal conditions at 160 °C for 8 h. After being naturally cooled to room temperature, the product was washed with ethanol two times and then deionized water three times at room temperature. Finally, the Mn-NWs were stored in deionized water for further use. 2.3. Synthesis of H2Ti5O11·H2O Nanowires (Ti-NWs). Ti-NWs were synthesized via a modified method reported previously.23 In brief, TiO2 (P25, 3.000 g) was dispersed in 400 mL aqueous solution containing 160.000 g of NaOH. The dispersion was treated by magnetic stirring for 30 min, followed by hydrothermal treatment in six Telfon-lined stainless steel autoclaves (100 mL) at 180 °C for 72 h. After being naturally cooled to room temperature, the product was washed with HCl aqueous solution (0.2 mol L−1) two times and then deionized water three times at room temperature. Finally, the asprepared Ti-NWs were stored in deionized water. 2.4. Synthesis of Relatively Long Hydroxyapatite (Ca10(PO4)6(OH)2) Nanowires (L-Ca-NWs). L-Ca-NWs were synthesized via a modified method previously reported by this research group.24,25 In brief, a mixture of oleic acid (100.000 g) and ethanol (140.000 g) was treated by magnetic stirring, followed by the addition of CaCl2 (2.200 g) aqueous solution (200 mL), NaOH (10.000 g) aqueous solution (200 mL), and NaH2PO4·2H2O (2.800 g) aqueous solution (100 mL) at intervals of 20 min, respectively. The resulting mixture was transferred into a Telfon-lined stainless steel autoclave (1 L volume), which was sealed and treated under solvothermal conditions at 180 °C for 24 h. After being naturally cooled to room temperature, the product was washed with ethanol and deionized water. To eliminate the effect of adsorbed oleic acid molecules on L-Ca-NWs, the as-prepared L-Ca-NWs were treated by the reaction-dissolution method to remove oleic acid molecules adsorbed on the surface of L-Ca-NWs according to a modified method reported previously.26 The detailed formation is shown in the Supporting Information. Finally, the as-prepared L-Ca-NWs were stored in deionized water for further use. 2.5. Synthesis of Relatively Short Ca10(PO4)6(OH)2 Nanowires (S-Ca-NWs). The synthesis procedure of S-Ca-NWs was the same as that of L-Ca-NWs except for using different amounts of oleic acid (80.000 g) and ethanol (160.000 g). The S-Ca-NWs were treated by the reaction-dissolution method as well. Finally, the as-prepared S-CaNWs were stored in deionized water for further use. B

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ACS Applied Materials & Interfaces 2.6. Fabrication of Layered Inorganic Paper Sheets. In a typical preparation process, a mixture of Mn-NWs (or L-Ca-NWs) aqueous suspension and Ti-NWs (or S-Ca-NWs) aqueous suspension at different weight ratios of 1:0, 5:1, 2:1, 1:1, 1:2, 1:5, 0:1 was magnetically stirred for 5 min at room temperature to form a homogeneous suspension. The resulting mixture was filtrated on the cellulose filter paper (Whatman) under vacuum (Figure S1, Supporting Information). After being dried at 60 °C, the free-standing layered inorganic paper sheets with a diameter of 39 mm were obtained. 2.7. Measurement of Water Flux through the Layered Inorganic Paper. The free-standing layered inorganic paper sheets with a diameter of 39 mm were prepared as the above-mentioned procedure. Then, a vacuum was applied to drive a certain volume (30 or 50 mL) of water through the as-prepared layered inorganic paper sheet. The time (t) for water (volume (V)) going through the layered inorganic paper sheet (diameter (d)) was recorded and the water flux was calculated by a formula 4 V/(πd2·t). 2.8. Characterization. Powder X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Rigaku D/max 2550 V). Fourier transform infrared (FTIR) spectra were recorded on a FTIR spectrometer (FTIR-7600, Lambda Scientific, Australia). Thermogravimetric (TG) and differential scanning calorimetry (DSC) curves were taken on a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) with a heating rate of 10 °C min−1 in flowing air. Transmission electron microscopy (TEM) images were collected with a transmission electron microscope (Hitachi H-800, Japan). Scanning electron microscopy (SEM) images were collected on field-emission scanning electron microscopes (Hitachi S-4800, Japan; Hitachi SU8220, Japan; FEI Magellan 400, USA). The detailed information for SEM images is listed in Table S1 (Supporting Information). Energy dispersive spectroscopy (EDS) elemental mapping was obtained using a field-emission scanning electron microscope (FEI Magellan 400 equipped with X-MAXN, Oxford Instrument, UK).

Figure S3a,b in the Supporting Information) can be indexed to the crystal phases of MnO2 (JCPDF 72-1982), H2Ti5O11·H2O (JCPDF 44-0131), and Ca10(PO4)6(OH)2 (JCPDF 74-0566), respectively. In this paper, the as-prepared MnO2 nanowires, H2Ti5O11·H2O nanowires, relatively long Ca10(PO4)6(OH)2 nanowires, and relatively short Ca10(PO4)6(OH)2 nanowires are designated as Mn-NWs, Ti-NWs, L-Ca-NWs, and S-CaNWs, respectively. MnO2, H2Ti5O11·H2O (further dehydration and recrystallization into TiO2 after calcination at high temperatures),23 Ca10(PO4)6(OH)2 (hydroxyapatite, HAP) are chosen as building blocks because they are typical electrochemical,27,28 photocatalytic,29,30 and biomedical materials.31,32 Therefore, the experimental results demonstrated herein may be applied in the related fields. As a common technology used in papermaking, vacuumassisted filtration is a low-cost and easy method for preparing paperlike bulk materials. Herein, two types of layered inorganic paper are fabricated using the aqueous suspensions of four kinds of inorganic nanowires via vacuum-assisted filtration. SEM images from the top view and cross-sectional view and digital images of four layered inorganic paper sheets are shown in Figure 2. The well-defined layered structure with large interlayer spacings in the range of 10−20 μm (Figure 2a1,2) is clearly observed in the Mn-NWs paper, whereas the layered inorganic paper with the closely packed cross section is obtained when the Ti-NWs are used as building blocks (Figure 2b1,2). To verify further that the structural variation of these two types of nanowires-assembled paper is not associated with the different nanowires used, the nanowires with different lengths but with the same chemical composition of Ca10(PO4)6(OH)2 are adopted to prepare the layered inorganic paper as well. As a result, relatively long Ca10(PO4)6(OH)2 nanowires-assembled paper has obvious large interlayer channels with interlayer spacings of 5−10 μm (Figure 2c1,2). In contrast, relatively short Ca10(PO4)6(OH)2 nanowires assemble into closely packed paper (Figure 2d1,2). It is worth mentioning that the distortion of layers to some extent results from the cutting by the sharp blade in the sample preparation for SEM characterization. Except for the totally different cross-sectional structure, the variation of the surface morphology of two types of the asfabricated layered inorganic paper is distinct from each other. In the Mn-NWs paper and L-Ca-NWs paper, some Mn-NWs and L-Ca-NWs assemble into bundles with lengths of more than 100 μm during the vacuum-assisted filtration (Figure 2a4,c4). And then the bundles intertwine with each other to form the porous network with pore sizes of greater than 1 μm (Figure 2a5,c5). In the Ti-NWs paper and S-Ca-NWs paper, there is lack of enough crossing and interweaving among the Ti-NWs or S-Ca-NWs assembled spindles because of their shorter lengths of less than 10 μm (Figure 2b4,d4; Figure S2 in the Supporting Information). Hence, the pore sizes of the Ti-NWs paper and S-Ca-NWs paper are smaller (below 500 nm, Figure 2b5,d5). Compared with a small number of defects on the surface of 2-D nanosheets-assembled layered materials, through-layer (vertical direction) pores formed by interweaving of nanowires are observed on the whole surface in the asprepared layered inorganic paper. Therefore, 1-D nanowiresassembled layered paper reported herein has more accessible vertical porous pathways. With the combination of micrometersized interlayer channels and vertical porous pathways, 1-D nanowires-assembled layered paper possesses more open porous network with both horizontal and vertical porous

3. RESULTS AND DISCUSSION 3.1. Fabrication of Two Types of Layered Inorganic Paper. We first synthesized four kinds of inorganic nanowires with different lengths as building blocks by the solvothermal treatment. The synthesis method and characterization details for the samples are described in the Experimental Section. TEM images of the as-prepared inorganic nanowires are shown in Figure S2 (Supporting Information). The X-ray diffraction (XRD) patterns of the inorganic nanowires (Figure 1 and

Figure 1. XRD patterns: (a) Mn-NWs; (b) Ti-NWs; (c) L-Ca-NWs; (d) S-Ca-NWs. C

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Figure 2. (a1, b1, c1, d1) High-magnification SEM images and (a2, b2, c2, d2) low-magnification SEM images of the cross section of the corresponding paper sheets. (a3, b3, c3, d3) Digital images: (a3) Mn-NWs paper; (b3) Ti-NWs paper; (c3) L-Ca-NWs paper; (d3) S-Ca-NWs paper. The diameter of the layered inorganic paper in all cases is 39 mm. (a4, b4, c4, d4) Low-magnification SEM images and (a5, b5, c5, d5) highmagnification SEM images of the surface morphology of the corresponding paper sheets. Scale bars: (a1, b1, c1, d1) 20 μm; (a2, b2, c2, d2) 200 μm; (a4, c4) 50 μm; (a5, c5) 5 μm; (b4, d4) 25 μm; (b5, d5) 2.5 μm.

interlayer channels. It is well-known that the lateral sizes of 2-D nanosheets are usually at the micrometer scale.19 If 2-D nanosheets are broken into 1-D nanowires or zero-dimensional (0-D) nanoparticles, the surface area to volume increases significantly and thus a great deal of surface charge confined in 2-D nanosheets is released,35 as shown in Figure 3a. We call this dramatic increase in surface charge as the surface charge amplified effect. However, the experimental results show that the Ti-NWs and S-Ca-NWs are not able to assemble into the layered paper with open interlayer channels, and thus the surface charge amplified effect alone cannot well explain the structural formation of the layered paper. As mentioned above, more effective interweaving of the Mn-NWs and L-Ca-NWs are observed than that of the Ti-NWs and S-Ca-NWs. As a result, the more effective interweaving, the more surface charges are superimposed, the larger electrostatic repulsive force is formed, which is defined as the surface charge superimposed effect (Figure 3b). Generally, the layered structure of 2-D nanosheetsassembled bulk materials collapses easily after drying, and the interlayer spacing declines sharply due to the loss of interaction forces.1,34 However, the Mn-NWs and L-Ca-NWs paper sheets can preserve the well-defined interlayer channels of the layered structure even after drying (Figure 2a1,c1). On the one hand, inhomogeneous distribution of electrostatic repulsive force occurs inevitably, leading to the localized collapse of layers in the regions with smaller electrostatic repulsive force. On the other hand, the layers assembled by the Mn-NWs and L-CaNWs exhibit a little curved morphology due to nanowires

channels, making the both horizontal and vertical transportation possible. The available vertical porous passages, micrometer-sized interlayer channels, together with unique characteristics of 1-D nanowires and simple template-free fabrication process, enable the 1-D nanowires-assembled layered inorganic paper to be an ideal functional material for applications in various fields. 3.2. Structural Formation Mechanism of Layered Inorganic Paper. The mechanism of structural formation in the 2-D nanosheets-assembled layered bulk materials has been established. The functionalized regions of 2-D nanosheets can both adsorb water molecules to induce repulsive hydration forces and create intersheet electrostatic repulsion caused by the same charged groups.33 Therefore, the functionalized regions serve as spacers to prevent the aggregation or restacking of 2-D nanosheets due to the van der Waals attraction during the fabrication process.34 In this study, the Mn-NWs or L-CaNWs assembled layered paper sheets have the larger interlayer channels with interlayer spacings of tens of micrometers. Therefore, the mechanism investigation, based on the aforementioned experimental results and established formation mechanism of 2-D nanosheets-assembled layered materials, is necessary. Given that the interlayer spacings of the as-prepared layered inorganic paper with open interlayer channels are tens of micrometers, it is hard to attribute the structural formation to short-range repulsive hydration force, whereas the long-range electrostatic repulsive force is more likely to induce such large D

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nanowires. Therefore, controlling the layered structure by integrating Mn-NWs (or L-Ca-NWs) with Ti-NWs (or S-CaNWs) can be accomplished. In this work, we use the simple mixing method to fabricate a series of layered paper sheets with interlayer channels in different states. In brief, the homogeneous aqueous mixture of two kinds of inorganic nanowires with different lengths in a series of weight ratios is adopted as the raw material to prepare the corresponding free-standing layered paper sheets. As a representative example, the Mn-NWs and Ti-NWs are selected as building blocks to prepare the freestanding layered paper for detailed investigation. SEM images of the as-prepared Mn-NWs/Ti-NWs layered paper sheets with a series of weight ratios from the top view and cross-sectional view are shown in Figure 4. With increasing the weight ratio of Mn-NWs/Ti-NWs, both surface morphology and cross section of the layered paper change obviously, as illustrated in Figure 4. When the weight ratios are 5:1 and 2:1, the well-defined layered structure of the Mn-NWs/Ti-NWs paper is obtained (Figure 4a1,2,b1,2). Although the interlayer spacings are still at the micrometer scale (Figure 4a1,b1), compared with the singlephase Mn-NWs layered paper (Figure 2a1), the interlayer channels are not uniform, indicating the trend for the change of the interlayer channels from the open state to the closed state. At the Mn-NWs/Ti-NWs weight ratio of 1:1, the collapse of the interlayer channels occurs (Figure 4c1,2). The thicker layers that are assembled by many single layers are observed (Figure 4c1). Energy dispersive spectroscopy (EDS) elemental mapping (Figure 4f1−4) shows the uniform distribution of the oxygen, manganese, and titanium over the cross section of the layered paper at a weight ratio of 1:1, suggesting the homogeneous mixing of Mn-NWs and Ti-NWs. By increasing the weight ratio of Mn-NWs/Ti-NWs, the layered structure gradually switches from the open state to the closed state (Figure 4d1,2,e1,2). Schematic illustration of the left column in Figure 4 shows the evolution of interlayer channels from the open state to the closed state with increasing weight ratio of Mn-NWs/Ti-NWs. A similar evolution of interlayer channels from the open state to the closed state can also be observed in the layered paper sheets assembled by the L-Ca-NWs and S-CaNWs (Figure S5, Supporting Information). Increasing the weight ratio of Mn-NWs/Ti-NWs not only weakens the surface charge superimposed effect and pillarlike supporting effect, leading to the transition of the interlayer channels from the “on” state to the “off” state, but also changes the surface morphology of the corresponding paper at the same time, as shown in Figure 4a3,b3,c3,d3,e3. The schematic illustration of the right column in Figure 4 shows the evolution of the surface morphology. The Ti-NWs (represented by blue lines) are randomly distributed in the paper surface, and filling the pores that are formed by the interweaving of the Mn-NWs (represented by yellow lines). With increasing content of TiNWs, the pore sizes of the layered paper decrease, as indicated b y t h e h ig h -m ag n ifi ca t ion S EM i m a ges (F i g u r e 4a4,b4,c4,d4,e4). However, the existence of a small proportion of Mn-NWs in the paper at a weight ratio of 1:5 can still maintain a slight surface charge superimposed effect and pillarlike supporting effect to prevent the total collapse of interlayer channels (Figure 4e2). 3.4. Tunable Water Transportation through the Layered Inorganic Paper. According to the proposed mechanism, a series of layered inorganic paper sheets with interlayer channels in different states have been successfully prepared. In addition, the through-layer pore sizes can be

Figure 3. Schematic illustration of the structural formation mechanism of the inorganic nanowire paper. (a) The surface charge amplified effect. The combination of circle and straight line represents negative charge; (b) the surface charge superimposed effect. The length of blue arrows represents the magnitude of electrostatic repulsive force; (c) the pillarlike supporting effect. The horizontal curve represents a single layer of the layered paper.

interweaving and high flexibility, which can be observed in Figure 2a2,c2. Both of these factors create the supporting points, through which the adjacent layers are connected with each other, as shown in Figure 3c. The supporting points play the pillarlike role to prevent the collapse of interlayer channels of the layered paper after drying. During the vacuum-assisted filtration process, negatively charged Mn-NWs and L-Ca-NWs (Figure S4, Supporting Information) intertwine with each other and deposit as the first layer of paper. The rest of the nanowires in aqueous suspension are difficult to deposit further onto the first layer due to strong electrostatic repulsive force caused by the surface charge amplified effect and surface charge superimposed effect. Such strong electrostatic repulsive force results in the large interlayer spacings in the Mn-NWs layered paper and L-Ca-NWs layered paper. Finally, in the presence of the localized collapse and the curved layers, the pillarlike supporting effect occurs to stabilize the layered architecture after drying. In the case of the Ti-NWs paper and S-Ca-NWs paper, the surface charge superimposed effect is weak, resulting in smaller electrostatic repulsive force (Figure 3b). Furthermore, less interweaving and the relatively rigid nature of Ti-NWs and S-Ca-NWs produce feeble pillarlike supporting effect. As a result, their interlayer channels collapse and switch to the closed state (Figure 2b2,d2). 3.3. Fabrication of Layered Inorganic Paper with Interlayer Channels in Different States. To meet the requirements of various applications, the layered inorganic paper with tunable layered structure is desirable. According to the proposed formation mechanism, it is possible to design and fabricate a series of layered inorganic paper sheets with interlayer channels in different states. As mentioned above, the surface charge superimposed effect and pillarlike supporting effect are weakened by reducing the interweaving effect of E

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Figure 4. Characterization of layered inorganic paper sheets assembled by a mixture of Mn-NWs and Ti-NWs with a series of weight ratios. (a1, b1, c1, d1, e1) High-magnification SEM images and (a2, b2, c2, d2, e2) low-magnification SEM images of the cross section of the corresponding layered inorganic paper sheets. (a3, b3, c3, d3, e3) Low-magnification SEM images and (a4, b4, c4, d4, e4) high-magnification SEM images of the surface morphology of the corresponding layered inorganic paper sheets. (f1, f2, f3, f4) EDS elemental mapping of the cross section of the layered inorganic paper at a Mn-NWs/Ti-NWs weight ratio of 1:1. Schematic illustration on the left column shows the cross section of the corresponding paper sheets. The yellow lines represent the layers in the layered structure. Schematic illustration on the right column shows the surface morphology of the corresponding layered inorganic paper sheets. The yellow lines represent Mn-NWs nanowires and the blue lines represent Ti-NWs nanowires. Scale bars: (a1, b1, c1, d1, e1) 20 μm; (a2, b2, c2, d2, e2) 200 μm; (a3, b3, c3, d3, e3) 50 μm; (a4, b4, c4, d4, e4) 5 μm; (f1, f2, f3, f4) 50 μm.

adjusted as discussed above. In this work, we investigated the synergistic effect of interlayer channels and through-layer pores on the water flux through the as-fabricated layered inorganic paper. Water is chosen as a model liquid for investigation because it is one of the most common fluids and an essential constituent in organisms. Importantly, controlling the water transportation through smart gating membranes plays important roles in various fields such as substance separation, desalination, valves, and drug delivery.35,36 However, it is difficult for the existing gating membranes to achieve tunable water transportation because these gating membranes require sophisticated and tedious fabrication steps, complicated surface modification or intercalation of guest species. These preparation processes are time-consuming and high cost, thus

hindering their applications. In this work, we demonstrate that the tunable water transportation through the as-prepared layered inorganic paper can be realized only by the adjustment of the layered paper architecture without surface modification or intercalation of any guest species. The experimental details for the water transportation are described in the Experimental Section. In brief, a vacuum is applied to drive a certain volume of water through the asprepared layered paper. The time at which water completely goes through the layered paper is recorded and the water flux is obtained, as shown in Figure 5. With increasing content of TiNWs or S-Ca-NWs and decreasing content of Mn-NWs or LCa-NWs, four kinds of the layered paper show the similar trend that the water flux is gradually reduced. For the Mn-NWs/TiF

DOI: 10.1021/acsami.7b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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transportation only), as an uncommon route for the flow of water molecules, are dependent on the size of through-layer pores to a great extent, whereas tortuous pathways (both vertical and horizontal transportation), as a primary route, are related with both interlayer channels and pore size. In the layered paper with the open interlayer channels and large through-layer pores, for example, the layered paper containing higher weight ratio of Mn-NWs or L-Ca-NWs, more water molecules can be accommodated in pores to speed up the vertical transportation (Figure 6a). Additionally, the open interlayer channels provide more accessible pathways along the horizontal direction, as indicated by blue dashed lines in Figure 6a, and thus improve the horizontal water transportation. However, both vertical and horizontal transportation of water molecules through the layered paper constructed by more TiNWs or S-Ca-NWs are limited due to their closed interlayer channels and small through-layer pores. Therefore, compared with the Ti-NWs or S-Ca-NWs dominated layered paper, the layered paper containing more Mn-NWs or L-Ca-NWs allows the passage of a larger quantity of water molecules per unit time by enhanced vertical and horizontal transportation owing to the open interlayer channels and larger through-layer pores. 1-D inorganic nanowires have been considered as ideal building blocks for the next generation of advanced devices due to their unique physical and chemical properties, and excellent thermal and mechanical stability.37 The advantages such as unique characteristics of 1-D inorganic nanowires, available vertical porous passages, micrometer-scale interlayer channels, tunable water transportation, as well as simple template-free fabrication process, enable the 1-D inorganic nanowiresassembled layered paper to be very promising for various applications such as gating for separation or sieving, high performance electrodes for energy storage and conversion, barrier membranes for guided bone generation, and effective electromagnetic interference shielding. Moreover, some previous reports have demonstrated that the organic nanowires (for example, cellulose nanofibrils) are capable of assembling into layered paper with open interlayer channels as well.38−40 The mechanism of the structural formation established in this work also provides the rational explanation for the cellulose nanofibrils-assembled layered paper. More importantly, the layered organic−inorganic composite materials can thus be fabricated and extend their applications in various fields by combining their respectively unique characteristics.

Figure 5. Water flux through the layered inorganic paper made from (a) Mn-NWs and Ti- NWs; (b) Mn-NWs and S-Ca-NWs; (c) L-CaNWs and Ti-NWs; (d) L-Ca-NWs and S-Ca-NWs at various weight ratios.

NWs layered paper (Figure 5a), the reason for a sharp decrease of water flux through the layered paper at a weight ratio of 5:1 is still not well understood. The largest water flux is 8947.40 ± 726.28 L m−2 h−1 for the single-phase Mn-NWs layered paper, whereas the smallest one is 543.71 ± 23.13 L m−2 h−1 corresponding to the single-phase Ti-NWs paper. It is expected that more precise control of water flux can be accomplished by only adjusting the weight ratio of two kinds of nanowires in a wide range. Figure 6 shows the schematic illustration of the proposed mechanism of tunable water transportation through the layered inorganic paper. Water molecules permeate through the layered inorganic paper by the following two pathways: (I) straight pathways or (II) tortuous pathways. Straight pathways (vertical

4. CONCLUSION In summary, 1-D inorganic nanowires-assembled layered paper with open or closed interlayer channels and through-layer pores is fabricated. The relatively long MnO 2 or long Ca10(PO4)6(OH)2 nanowires-assembled paper has a layered structure with open interlayer channels, and the interlayer spacings between adjacent layers is tens of micrometers, much larger than those in the 2-D nanosheets-assembled layered materials. However, the closed interlayer channels are observed when relatively short H2Ti5O11·H2O or short Ca10(PO4)6(OH)2 nanowires are used as building blocks to fabricate the free-standing layered inorganic paper. The structural formation mechanism of the layered inorganic paper including the surface charge amplified effect, surface charge superimposed effect, and pillarlike supporting effect is proposed and discussed. According to the proposed mechanism, a series of layered inorganic paper sheets are prepared to adjust the sizes of interlayer channels and through-layer pores

Figure 6. Schematic illustration for the proposed mechanism of tunable water transportation through the layered inorganic paper. (a) The layered paper with open interlayer channels and larger throughlayer pores are able to accommodate more water molecules; (b) the layered paper with closed interlayer channels and smaller through-layer pores accommodate less water molecules. Yellow lines on the left column represent the nanowires or nanowires-assembled bundles, and blue dashed lines represent the flow pathways of water molecules. Yellow lines on the right column represent Mn-NWs or L-Ca-NWs, and blue lines represent Ti-NWs or S-Ca-NWs. G

DOI: 10.1021/acsami.7b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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simultaneously and employed as the valves for controlling the water transportation. Tunable water flux through the valve-like layered inorganic paper has been achieved by the synergistic effect between interlayer channels and through-layer pores. We have achieved the tunable water transportation by only adjusting the weight ratios of the constituent nanowires with different lengths, without surface modification or intercalation of any guest species. Furthermore, 1-D inorganic nanowiresassembled layered paper reported in this work may also be used as the valves for controlling the flux of many other kinds of liquids.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01326. Digital image of a vacuum filter apparatus for making layered inorganic paper sheets; detailed information for SEM images; TEM images and ζ-potentials of Mn-NWs, Ti-NWs, L-Ca-NWs, and S-Ca-NWs; XRD patterns, FTIR spectra and TG-DSC curves of L-Ca-NWs and SCa-NWs; SEM images of layered paper sheets assembled by the mixture of L-Ca-NWs and S-Ca-NWs with different weight ratios (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Y.-J. Zhu. E-mail: [email protected]. *Z.-C. Xiong. E-mail: [email protected]. ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Ying-Jie Zhu received funding from Science and Technology Commission of Shanghai (Grant number 15JC1491001); ZhiChao Xiong received funding from the National Natural Science Foundation of China (Grant number 21601199) and Shanghai Sailing Program (Grant number 16YF1413000). We acknowledge these financial supports.

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REFERENCES

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DOI: 10.1021/acsami.7b01326 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX