Distinctive Sandwich-Type Composite Film and Deuterogenic Three-Dimensional Triwall Tubes Affording Concurrent Aeolotropic Conduction, Magnetism, and Up-/Down-Conversion Luminescence.

Compared to single functional materials, multifunctional materials with electrical conduction, magnetism, and luminescence are more attractive and promising, so it has become an important subject. A distinctive sandwich-type composite film (STCF) with dual-color up- and down-conversion luminescence, magnetism, and aeolotropic conduction is prepared by layer-by-layer electrospinning technology. Macroscopically, STCF is assembled by three tightly bonded layers, including a [polypyrrole (PPy)/poly(methyl methacrylate) (PMMA)]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt array layer as the first layer, a CoFe2O4/polyacrylonitrile (PAN) nanofiber nonarray layer as the second layer, and a Na2GeF6:Mn4+/polyvinylpyrrolidone (PVP) nanofiber nonarray layer as the third layer. This unique macropartition effectually confines conductive aeolotropy, magnetism, and luminescence in different layers. Microscopically, a Janus nanobelt is used as a construction unit to restrict the luminescent and conductive materials to their microregions, thus achieving highly conductive aeolotropy and green luminescence. The high integration of the micro-subarea and macro-subarea in the STCF can efficaciously avoid the mutual disadvantageous effects among different materials to obtain splendid polyfunctional performance. The conductive anisotropy and magnetism of the STCF can be adjusted by changing the contents of PPy and CoFe2O4. When the PPy content reaches 70%, the conductance ratio in the conductive direction to insulative direction is 108. The 2D STCF can be crimped by four different methods, and the 3D TWTs have the same excellent polyfunctional performances as 2D STCF. This unique design idea and construction technology can be applied to the preparation of other multifunctional materials to avoid harmful interference among various functions.

Introduction

Aeolotropic conductive films (ACFs) display electrical conductivity differences in different directions,[1] which are widely used in high-tech fields such as novel electrodes, electronic packing, and biomedicine.[2−4] With the fast development of society, the increasing demand for high-tech electronic products is more and more urgent, so the design and construction of novel ACFs have become research hotspots in the current materials science field. At present, the following types of ACFs have been reported. Type I ACFs are conductive and insulating along the direction of the film thickness and surface, respectively.[5] Type II ACFs have two different conductivity values along two mutually perpendicular directions on the surface of the film.[6] The former type I ACFs have been widely used in electronics, but type II ACFs are still in the exploration and research stage and are not widely used in industry. Chen et al.[7] prepared a large-scale aeolotropic conductive film by a simple method of gravity sedimentation. The novel aeolotropic conductive film made of PS-Ag core–shell particles has good conductivity and insulation on the bottom and top surfaces, respectively. Li et al.[8] synthesized aeolotropic conductive compounds by electric-field-induced assembly. The prepared compounds exhibit remarkable conductive aeolotropy. Qian et al.[9] fabricated aeolotropic conductive bacterial cellulose-PSS/PEDOT composite hydrogels. The conductivity in the parallel orientation is about 4.1 times that in the vertical orientation. Qi et al.[10] designed and constructed a 2D Janus pellicle with aeolotropic conductive, magnetic, and fluorescent trifunctionality. The electrical conductance of the pellicle was determined to be 10–2 S, and the conductance ratio between the conductive and insulative directions reaches 108.

Multifunctional materials have two or more functions, so they have broader application prospects than their counterpart single-functional materials. Therefore, multifunctional materials have attracted more and more attention from researchers.[11−17] Nanomaterials with two or three properties of luminescence, conductance, and magnetism have become the focus of researches in the current materials science field. For example, Li et al.[18] prepared a PEDOT/Fe3O4/PLGA magnetic–conductive fibrous scaffold with dual functions by the in situ polymerization of EDOT on Fe3O4/PLGA fibers, which possess excellent magnetic and conductive properties. Furthermore, the cell viability under electromagnetic double stimulation is significantly higher than that under single electric or magnetic stimulation. The PEDOT/Fe3O4/PLGA fiber scaffold has great potential in the field of bone tissue engineering. Yang et al.[19] successfully synthesized magnetic–fluorescent bifunctional microparticles which have important applications in biological detection, such as protein detection and analysis. Tian et al.[20] prepared microfibers with joint properties of fluorescence, anisotropic conduction, and magnetism by a conjugate electrospinning technique.

Polypyrrole (PPy)[21−23] with good conductivity and thermal stability has become one of the most important conductive polymers. A large number of researchers have devoted themselves to the investigation of PPy, which can be widely used in antistatic coatings, batteries, supercapacitors, and other fields. Photoluminescence (PL) materials have excellent luminescence properties, showing great potential in the fields of biomarkers, medical imaging, and fluorescence coding. Rare-earth-doped upconversion nanomaterials can convert long-wavelength near-infrared light to short-wavelength visible light and have become very important fluorescent materials. Among various upconversion materials, NaYF4:Yb3+, Er3+ nanomaterials have received much attention and have been proven to be one of the best near-infrared–visible upconversion materials.[24] In recent years, transition-metal Mn4+-activated narrow-emission red phosphors have caused widespread concern owing to their important applications in optoelectronic devices and lighting. Under ultraviolet light excitation, the Mn4+-activated phosphors exhibit red fluorescence with a wavelength of 620–750 nm.[25] In recent years, magnetic materials with nanostructures have been widely studied and used in many fields. In particular, CoFe2O4 nanoparticles (NPs) have attracted special attention in the fields of supercapacitors, electrocatalysts, magnetic recording materials, and electromagnetic wave absorption owing to their outstanding chemical and physical stability, biocompatibility, and high saturation magnetization.[26]

Electrospinning technology is considered to be a simple, efficient, and convenient technique for the preparation of continuous and uniform nanofibers/nanoribbons,[27−32] and core/shell or Janus structures can also be obtained by using improved electrospinning devices. In recent years, a lot of work on electrospinning technology has been carried out by researchers, and the electrospun products more and more tend to possess multilayer intricate structures. For example, Miao et al.[33] fabricated a three-layer PU/(PU–HPAN)/HPAN fiber membrane with a directional water transport property by the facile electrospinning technique. The performance of the three-layer membrane is much better than that of the double-layer PU/HPAN fiber membrane.

In this work, based on the ACFs, a unique {[polypyrrole (PPy)/poly(methyl methacrylate) (PMMA)]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt array}/{[CoFe2O4/polyacrylonitrile (PAN)] nanofiber nonarray}/{[Na2GeF6:Mn4+/ polyvinylpyrrolidone (PVP)] nanofiber nonarray} sandwich-typed composite film (abbreviated as CLJA/MN/LN STCF) with dual-color luminescence, magnetism, and aeolotropic conductive was creatively constructed by layer-by-layer electrospinning technology. The STCF consists of three layers, of which the first layer is made up of [PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelts. The two sides of the Janus nanobelt are composed of PPy/PMMA (conductive side) and NaYF4:Yb3+, Er3+/PMMA (upconversion luminescent insulative side). In addition, the second layer (magnetic layer) and the third layer (down-conversion luminescent layer) are made up of CoFe2O4/PAN magnetic disordered nanofibers and Na2GeF6:Mn4+/PVP down-conversion luminescent disordered nanofibers, respectively. Microscopically, a Janus nanobelt for segregating upconversion luminescent substances from conductive PPy with two independent microregions is used as the conductive and building unit of the first layer, which ensures the highly conductive aeolotropy and strong upconversion luminescence of the STCF. Macroscopically, the STCF possesses three independent macroregions, which can limit different functional substances in their respective macroregions and shun adverse interactions among them. Therefore, the neoteric film can successfully separate the luminescent material from the conductive and magnetic materials via the combination of micropartition with macropartition so that it can possess polyfunctions of excellent luminescence, aeolotropic conduction, and magnetism. More importantly, the 2D film can be crimped by different means to obtain the 3D triwall tubes (abbreviated as TWTs). The performances of the 3D TWTs are similar to those of the 2D films. This distinctive design idea successfully realizes the transformation of materials from 1D to 2D and then to 3D. Moreover, 2D STCF and 3D TWT can be used in nanodevices, drug targeting and electromagnetic interference shielding.

Experimental Section

Chemicals

The chemical reagents are summarized in the Supporting Information (SI).

Preparation of Na2GeF6:Mn4+ Nanoparticles (NPs), NaYF4:Yb3+, Er3+ NPs, CoFe2O4 NPs, and PMMA

The detailed preparation processes of Na2GeF6:Mn4+ NPs, NaYF4:Yb3+, Er3+ NPs, and CoFe2O4 NPs are shown in the SI. PMMA was synthesized on the basis of ref (34).

Preparation of Spinning Fluids

The preparation processes of spinning fluids for fabricating CLJA/MN/LN STCF and contrast samples are listed in the SI.

Fabrication and Forming Mechanism of CLJA/MN/LN STCF and Contrast Samples

The abbreviations for the target sample and the four contrast samples are listed in Table . The detailed electrospinning process of preparing CLJA/MN/LN STCF is shown in Figure a. First, the first layer with the [PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt as a building unit was prepared by biaxial electrospinning. Spinning fluid A (2.5 mL) and spinning fluid B (2.5 mL) were respectively injected into two plastic injectors connected to a homemade parallel spinneret to form a Janus structure at the tip of the spinneret, which was connected to the positive pole of a high direct current (DC) voltage power supply, and the rotary drum as a collector was connected to the ground pole. The distance between the collecting device and the spinneret was 12 cm, and the DC voltage was set at 8 kV. Figure b exhibits the formation mechanism of the Janus nanobelt. In the electrospinning process, two kinds of spinning fluids formed the parallel structure at the bottom of the spinneret, and then they were stretched to form a parallel-structured Taylor cone and Janus-structured jet under the tension of an electric field force. The Janus-structured jet was cylindrically shape at the time when it was stretched out of the Taylor cone under the electric field force, and the positive charges were evenly distributed on the jet. Subsequently, a tiny transformation happens owing to the Coulomb repulsion force. At this time, the positive charge was concentrated in the place with a larger curvature radius, which produced a larger Coulomb repulsion to transversely stretch the jet into a striplike shape. Furthermore, the Janus jet was solidified with the volatilization of solvent, and thus a Janus nanobelt was prepared. In addition, the Janus nanobelt swung and whipped in the air owing to the instability of the electrospinning process, and a Janus nanobelt array film with a certain width was collected on the surface of a rotary drum. When the spinning fluid was completely consumed, the first layer of CLJA/MN/LN STCF was obtained, taken off, and cut into 4 × 4 cm2 pieces. The film was placed on the wire mesh as the receiving device, which was connected to the grounded pole. Conventional single-axis electrospinning was used to form the second layer and the third layer by respectively using spinning fluid C (5 mL) and spinning fluid D (5 mL). The distance between the collecting device and the spinneret was 16 cm, and the DC voltage was set at 14 kV. After the two-step electrospinning process, the second layer and the third layer were successfully prepared and cut into the same size as the first layer so that the CLJA/MN/LN STCF was successfully prepared.

Table 1

Abbreviations for Target Sample and the Four Contrast Samples

samples	abbreviations
{[polypyrrole (PPy)/poly(methyl methacrylate) (PMMA)]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt array}/{[CoFe2O4/polyacrylonitrile (PAN)] nanofiber nonarray}/{[Na2GeF6:Mn4+/ polyvinylpyrrolidone (PVP)] nanofiber nonarray} sandwich-typed composite film	CLJA/MN/LN STCF
{[PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt nonarray}/{[CoFe2O4/PAN] nanofiber nonarray}/{[Na2GeF6:Mn4+/PVP] nanofiber nonarray} sandwich-typed composite film	CLJN/MN/LN STCF
{[PPy/NaYF4:Yb3+, Er3+/PMMA] composite nanobelt array}/{[CoFe2O4/PAN] nanofiber nonarray}/{[Na2GeF6:Mn4+/PVP] nanofiber nonarray} sandwich-typed composite film	CLCA/MN/LN STCF
{[PPy/NaYF4:Yb3+, Er3+/PMMA] composite nanobelt nonarray}/{[CoFe2O4/PAN] nanofiber nonarray}/{[Na2GeF6:Mn4+/PVP] nanofiber nonarray} sandwich-typed composite film	CLCN/MN/LN STCF
{[PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt array}/{[CoFe2O4/Na2GeF6:Mn4+/PVP/PAN] nanofiber nonarray} dual-layered composite film	CLJA/[M-L]N dual-layered composite film

Figure 1

Sketched maps of (a) the electrospinning process for preparing CLJA/MN/LN STCF and (b) the formation mechanisms of Janus nanobelts and array.

At the same time, four contrast samples consisting of the {[PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt nonarray}/{[CoFe2O4/PAN] nanofiber nonarray}/{[Na2GeF6:Mn4+/PVP] nanofiber nonarray} (abbreviated as CLJN/MN/LN) STCF, the {[PPy/NaYF4:Yb3+, Er3+/PMMA] composite nanobelt array}/{[CoFe2O4/PAN] nanofiber nonarray}/{[Na2GeF6:Mn4+/PVP] nanofiber nonarray} (abbreviated as CLCA/MN/LN) STCF, the {[PPy/NaYF4:Yb3+, Er3+/PMMA] composite nanobelt nonarray}/{[CoFe2O4/PAN] nanofiber nonarray}/{[Na2GeF6:Mn4+/PVP] nanofiber nonarray} (abbreviated as CLCN/MN/LN) STCF, and the {[PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt array}/{[CoFe2O4/Na2GeF6:Mn4+/PVP/PAN] nanofiber nonarray} dual-layered composite film (defined as the CLJA/[M-L]N dual-layered composite film) were also designed and prepared to prove the superiority of CLJA/MN/LN STCF. The preparation processes of contrast samples by electrospinning are described in the SI.

Characterization

The characterization methods are described in the SI.

Results and Discussion

X-ray Diffraction Analysis

Figure demonstrates the XRD patterns of NaYF4:Yb3+, Er3+ NPs, CoFe2O4 NPs, Na2GeF6:Mn4+ NPs, and CLJA/MN/LN STCF. The results show that the diffraction peaks of NaYF4:Yb3+, Er3+ NPs, CoFe2O4 NPs, and Na2GeF6:Mn4+ NPs correspond to the standard cards of NaYF4 (PDF no. 77-2042), CoFe2O4 (PDF no. 22-1086), and Na2GeF6 (PDF no. 35-0814), respectively. In addition, the diffraction peaks of other impurities are not detected. The characteristic diffraction peaks of NaYF4:Yb3+, Er3+ NPs, CoFe2O4 NPs, and Na2GeF6:Mn4+ NPs are observed when the first and third layers of CLJA/MN/LN STCF face the X-ray source to obtain diffraction results. However, CoFe2O4 NPs are dispersed only in the middle second layer of CLJA/MN/LN STCF, so the characteristic peak intensity of CoFe2O4 NPs is very weak.

Figure 2

XRD patterns of (a) NaYF4:Yb3+, Er3+ NPs, (b) CoFe2O4 NPs, (c) Na2GeF6:Mn4+ NPs, (d) the third layer, and (e) the first layer of CLJA/MN/LN STCF facing the X-ray source.

Morphology and Structure

The SEM images and histograms of the particle size distributions of NaYF4:Yb3+, Er3+ NPs and Na2GeF6:Mn4+ are exhibited in Figure S1, and a relevant description is also given in the SI. Figure a indicates that CLJA/MN/LN STCF possesses an obvious tightly combined three-layer structure. An SEM image of Janus nanobelts in the first layer is exhibited in Figure b. Conductive material (PPy) and fluorescent material (NaYF4:Yb3+, Er3+ NPs) are distributed on both sides of the Janus nanobelt, which is ca. 8.51 ± 0.06 μm in width (Figure g). Figure e,f reveals the OM image and EDS line-scan analysis of a single [PPy/PMMA]//[NaYF4:Yb3+, Er3+/PMMA] Janus nanobelt, respectively. PPy is distributed on the dark side of the left half, and NaYF4:Yb3+, Er3+ is distributed on the transparent side of the right half. EDS line-scan analysis is carried out to analyze the distribution of elements to further demonstrate the structure of the Janus nanobelt. The S element stands for the distribution of dark-colored conductive material (PPy), and the Y element represents the distribution of fluorescent material (NaYF4:Yb3+, Er3+). For the S element, it exists only on the left side of the Janus nanobelt, and the Y element lies on only the right side of the nanobelt. The above analysis results fully indicate that Janus nanobelts have been successfully prepared, and the conductive and fluorescent materials are respectively distributed on the two sides of the Janus nanobelts. Therefore, the Janus structure is helpful for the effective separation of PPy from NaYF4:Yb3+, Er3+ NPs. Figure c,d exhibits SEM images of the second and third layers, respectively. As can be seen from the figure, these layers are composed of disordered nanofibers. The diameters of nanofibers in the second and third layers are 560 ± 3.2 and 540 ± 7.8 nm (Figure h,i), respectively.

Figure 3

SEM images of (a) the cross section, (b) the first layer, (c) the second layer, and (d) the third layer of CLJA/MN/LN STCF. Histograms of (g) the width distribution of Janus nanobelts in first layer and the diameter distribution of nanofibers in (h) the second layer and (i) the third layer. (e) OM image and (f) EDS line-scan analysis of the single Janus nanobelt in the first layer.

The morphologies of the CLCA/MN/LN STCF, CLCN/MN/LN STCF, CLJN/MN/LN STCF, and CLJA/[M-L]N dual-layered composite film are provided in Figure S2. It can be observed that the nanobelts in CLCN/MN/LN STCF and CLJN/MN/LN STCF are disordered (Figure S2b,c) but the nanobelts in CLCA/MN/LN STCF are ordered (Figure S2a). Figure S2d indicates the SEM image of the magnetic–luminescent layer of the CLJA/[M-L]N dual-layered composite film, and the magnetic–luminescent layer is composed of disordered nanofibers. The widths of nanobelts in CLCA/MN/LN STCF, CLCN/MN/LN STCF, and CLJN/MN/LN STCF are 7.60 ± 0.09, 5.07 ± 0.05, and 4.91 ± 0.05 μm (Figure S2e–g, respectively). The diameter of the nanofiber in the magnetic–luminescent layer of the CLJA/[M-L]N dual-layered composite film is 680 ± 16.3 nm (Figure S2h).

As demonstrated in Figure a, CLJA/MN/LN STCF can be bent at any angle, meaning that CLJA/MN/LN STCF possesses good flexibility. Figure b,c shows physical pictures of the first layer and the third layer, respectively. It can be seen from the figures that the first layer and the third layer are black and white under natural light, respectively. The first layer emits green fluorescence under 980 nm laser irradiation, and the third layer exhibits significant red fluorescence under 461 nm visible light excitation.

Figure 4

Physical digital photographs of (a) the folded film, (b) the first layer, and (c) the third layer under natural light and emission light of (d) the first layer and (e) the third layer in darkness, respectively, under a 980 nm laser and 461 nm light excitation.

Magnetism

Figure shows the hysteresis loops of CoFe2O4 NPs and CLJA/MN/LN with different CoFe2O4 NPs contents. The corresponding saturation magnetizations are listed in Table . CoFe2O4 NPs possess strong magnetism, and their saturation magnetization is 36.1 emu·g–1. When the mass ratio of CoFe2O4 to PAN changes from 0.3:1 to 1:1, the saturation magnetization of CLJA/MN/LN increases from 2.1 to 6.9 emu·g–1, indicating that the magnetic properties of CLJA/MN/LN STCF can be regulated by adjusting the content of CoFe2O4 NPs.

Figure 5

Hysteresis loops of CoFe2O4 NPs and CLJA/MN/LN with different mass ratios of CoFe2O4 NPs to PAN.

Table 2

Saturation Magnetization of CLJA/MN/LN with Different Mass Ratios of CoFe2O4 NPs to PAN

samples	saturation magnetization (Ms) (emu·g–1)
CoFe2O4 NPs	36.1
CLJA/MN/LN ([SA-2//SB-2]/SC-3/SD; CoFe2O4:PAN = 1:1)	6.9
CLJA/MN/LN ([SA-2//SB-2]/SC-2/SD; CoFe2O4:PAN = 0.5:1)	4.7
CLJA/MN/LN ([SA-2//SB-2]/SC-1/SD; CoFe2O4:PAN = 0.3:1)	2.1

Fluorescent Performance

The fluorescent performances of NaYF4:Yb3+, Er3+ NPs and CLJA/MN/LN STCF were studied in detail. Figure a gives the upconversion fluorescence spectra of the NaYF4:Yb3+, Er3+ NPs excited by a 980 nm laser with different pump powers. The upconversion fluorescence intensity increases with the increase in pump power. The green emission and red emission peaks at 526, 545, and 660 nm are respectively from the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 energy-level transitions of Er3+, respectively. It can be seen from the figure that the intensity of green light is obviously stronger than that of red light, meaning that the NaYF4:Yb3+, Er3+ NPs mainly display green light emission. The double natural logarithm curve between the upconversion luminescence intensity and pump power is presented in Figure b. The slopes of green emission and red emission are 1.786 and 1.927, respectively, close to 2, indicating that the upconversion luminescence process of the NaYF4:Yb3+, Er3+ NPs is a two-photon process. It can be found from Figure c that the CIE chromaticity coordinates of NaYF4:Yb3+, Er3+ NPs are ascertained to be (0.22, 0.74), belonging to green fluorescence.

Figure 6

(a) Upconversion emission spectra of NaYF4:Yb3+, Er3+ NPs with different pump powers, (b) natural logarithm diagram of upconversion emission intensity vs pump power, and (c) CIE chromaticity coordinates diagram of NaYF4:Yb3+, Er3+ NPs.

To study the influence of different contents of NaYF4:Yb3+, Er3+ NPs, PPy, and CoFe2O4 NPs on the upconversion luminescence properties, a series of samples were prepared by electrospinning technology, and the fluorescent performances of the samples were studied in detail. The emission spectra of the first layer of CLJA/MN/LN STCF under 980 nm laser excitation are indicated in Figure . When the mass ratios of CoFe2O4 NPs/PAN, Na2GeF6:Mn4+ NPs/PVP, and PPy/PMMA in CLJA/MN/LN STCF are respectively fixed at 0.5:1, 1:1, and 50%, the ratio of NaYF4:Yb3+, Er3+ NPs to PMMA is changed from 0.5:1 to 1:1 to 2:1. As demonstrated in Figure a, the upconversion luminescence intensity of the first layer of CLJA/MN/LN STCF is enhanced significantly with the increase in NaYF4:Yb3+, Er3+ NPs content in the Janus nanobelts, implying that the upconversion luminescence intensity of the first layer of CLJA/MN/LN STCF can be changed by adjusting the contents of NaYF4:Yb3+, Er3+ NPs.

Figure 7

Upconversion emission spectra of the first layer of CLJA/MN/LN STCF with different contents of (a) NaYF4:Yb3+, Er3+, (b) PPy, and (c) CoFe2O4 NPs.

Furthermore, we also explored the influence of PPy and CoFe2O4 NPs content on the upconversion luminescence of the first layer of CLJA/MN/LN STCF. When the effect of PPy content on the upconversion luminescence is discussed, the mass ratios of Na2GeF6:Mn4+ NPs to PVP, NaYF4:Yb3+, Er3+ NPs to PMMA, and CoFe2O4 NPs to PAN are fixed at 1:1, 1:1, and 0.5:1, respectively. The corresponding upconversion emission spectra are demonstrated in Figure b. The luminescence intensity of the first layer of CLJA/MN/LN STCF decreases with the increase in PPy content. To better explain this phenomenon, Figure displays the mechanism diagram of excitation and emission light of CLJA/MN/LN STCF with different PPy content. The physical color of the first layer of CLJA/MN/LN STCF becomes darker and darker with the increase in PPy content in the Janus nanobelts owing to the dark-green color of PPy itself. Because the dark-colored PPy strongly absorbs light, including excitation and emission light,[35] the greater the PPy content, the stronger the light absorption; therefore, the luminescence intensity of the first layer of CLJA/MN/LN STCF significantly decreases with increasing PPy content. However, the change in CoFe2O4 NPs content has basically no effect on the upconversion luminescence performance of the first layer of CLJA/MN/LN STCF, as shown in Figure c. This is because CoFe2O4 NPs and NaYF4:Yb3+, Er3+ NPs are arranged in different layers of CLJA/MN/LN STCF, as indicated in the diagrammatic sketch of Figure , proving that macroscopic partitioning has advantages in the construction of multifunctional nanomaterials.

Figure 8

Diagrammatic sketches of excitation and emission light of CLJA/MN/LN STCF at various percentages of PPy.

Figure 9

Diagrammatic sketches of excitation and emission light of CLJA/MN/LN STCF with various ratios of CoFe2O4 NPs.

In the same way, we continued to explore the influence of the PPy and CoFe2O4 NPs with dark color on the down-conversion luminescence properties of the third layer of the CLJA/MN/LN STCF. It can be seen in Figure a,c that the excitation spectra of CLJA/MN/LN STCF are obtained by monitoring at a wavelength of 629 nm, which is the wavelength of the maximum intensity emission peak in the emission spectrum. Two broad excitation peaks at 360 and 461 nm are attributed to spin-allowed 4A2g → 4T1g and 4A2g → 4T2g transitions of Mn4+, respectively. Under the excitation of 461 nm light, significant red emission in the range of 575–675 nm can be observed, which is attributed to the 2Eg → 4A2g spin-forbidden transition of Mn4+,[36] as shown in Figure b,d. At the same time, the peak at 619 nm is the zero phonon line (ZPL) caused by the Mn4+ electric dipole forbidden transition in the emission spectrum. Two short-wavelength peaks at 596 and 611 nm are assigned to anti-Stokes vibronic modes v3 (t1u) and v6 (t2u), respectively. Two emission peaks at 629 and 643 nm in the long-wavelength range are respectively ascribed to the Stokes v6 (t2u) and v3 (t1u) vibronic modes, and the emission peak at 629 nm is the strongest. When the contents of other components are kept unchanged, the mass percent of PPy to PMMA is adjusted from 30 to 50 to 70. It can be observed that the change in PPy content in the first layer has little influence on the down-conversion luminescence properties of the third layer (Figure a,b). For the change in CoFe2O4 NPs contents in the second layer, the same results can be observed in Figure c,d, viz., the change in the contents of CoFe2O4 NPs in the second layer has no influence on the luminescence performance of the third layer. The mechanism diagrams in Figures and 9 can well explain this phenomenon. It can be seen from the diagrams that the CLJA/MN/LN STCF has an obvious three-layer structure and that each layer is independent of each other, viz., the change in the deep-colored substances of the first layer and second layer will basically not affect the luminescence of the third layer, adequately demonstrating that macroscopic partition has superiority in the construction of multifunctional nanomaterials via shunning baleful reciprocal influences among various functions.

Figure 10

(a, c) Excitation and (b, d) emission spectra of the third layer of CLJA/MN/LN STCF doped with different contents of PPy (a, b) in the first layer and with different contents of CoFe2O4 NPs (c, d) in the second layer.

To further prove the superiority of the performance and structure of CLJA/MN/LN STCF, CLJA/MN/LN STCF and the contrasting samples are studied. The emission spectra of the first layer of CLJA/MN/LN STCF and the contrast samples are shown in Figure a. The first layer of CLJA/MN/LN STCF possesses the strongest luminescence intensity. Compared with CLJA/MN/LN STCF, CLJN/MN/LN STCF has weaker luminescence intensity, and CLCA/MN/LN STCF and CLCN/MN/LN STCF have much weaker luminescence intensities. To better explain the difference in the luminescence properties, we propose a mechanism schematic diagram, as shown in Figure . In CLJA/MN/LN STCF, Janus nanobelts are closely arranged with almost no gaps among them. When the excitation light reaches the first-layer surface of CLJA/MN/LN STCF, it is hard for the excitation light to reach the lower layer through the upper layer, so the emission light mainly comes from the surface and the luminescence intensity is not weakened. However, the Janus nanobelts in CLJN/MN/LN STCF are disordered. There are many voids among Janus nanobelts, so the surface of the film is loose. When the excitation light reaches the film surface, part of the exciting light can reach the lower layer through the voids among the Janus nanobelts of the upper layer, and thus some of the light is absorbed. At the same time, the emission light from the lower layer is also absorbed when the light is emitted out of the lower layer through the voids among the Janus nanobelts in the upper layer, resulting in a further reduction of the emission light. Because the luminescent region and conductive region of Janus nanobelts are still separated from each other, CLJN/MN/LN STCF still has excellent luminescence performance. However, the composite nanobelts in CLCA/MN/LN STCF and CLCN/MN/LN STCF are prepared by uniaxial electrospinning, so the luminescent region and conductive region are blended together without separation. The fluorescent substance (NaYF4:Yb3+, Er3+) is completely mixed with a dark-colored conductive substance (PPy) which has strong light absorption, and the excitation light and emission light are strongly absorbed and weakened, resulting in a significant reduction in the luminescence intensity of CLCA/MN/LN STCF and CLCN/MN/LN STCF. The luminescence intensity of the CLCA/MN/LN STCF is slightly higher than that of CLCN/MN/LN STCF for the same reason as for CLJA/MN/LN STCF and CLJN/MN/LN STCF. Consequently, compared to three contrast samples, CLJA/MN/LN STCF has the strongest luminescence intensity, fully implying that Janus nanobelts as building units have advantages in constructing polyfunctional materials.

Figure 11

(a) Upconversion emission spectra of the first layer of CLJA/MN/LN STCF together with three contrasting samples. (b) Excitation and (c) emission spectra of the third layer of CLJA/MN/LN STCF (A) and the second layer of the CLJA/[M-L] dual-layered composite film (B). (d) CIE chromaticity coordinates diagram of the first layer and third layers of CLJA/MN/LN STCF.

Figure 12

Sketched maps of excitation and emission light of CLJA/MN/LN STCF together with four contrast samples.

The CIE chromaticity coordinates of the first layer and third layer of CLJA/MN/LN STCF are ascertained to be (0.23, 0.68) and (0.65, 0.35), which correspond to green and red fluorescence, respectively. In addition, to further exhibit the superiority of the CLJA/MN/LN STCF target sample, the luminescence intensities of the third layer of the CLJA/MN/LN STCF and the magnetic–luminescent layer of the CLJA/[M-L]N dual-layered composite film are also compared. The magnetic–luminescent layer of the CLJA/[M-L]N dual-layered composite film displays very weak excitation and and a very weak emission peak compared to the down-conversion luminescent layer of CLJA/MN/LN STCF (Figure b,c). The schematic diagram in Figure can well explain this result. As shown in the diagram, the luminescent substance in the magnetic–luminescent layer of the CLJA/[M-L]N dual-layered composite film is directly mixed with dark-colored CoFe2O4 NPs. The literature reports that the dark-colored materials will strongly absorb excitation and emission light, hence the luminescence intensity will be greatly reduced.[37] The luminescent material is totally separated from the magnetic material in CLJA/MN/LN STCF, so the luminescence intensity is not affected. The above results indicate that CLJA/MN/LN STCF possesses better luminescence performance than the CLJA/[M-L]N dual-layered composite film. On the basis of the above analysis, it can be seen that CLJA/MN/LN STCF has better luminescence performance than the above four contrast samples.

Electrical Conduction Analysis

The second and third layers of CLJA/MN/LN STCF do not contain conductive materials, so the second and third layers are insulating. Therefore, only the conductance of the first layer (containing conductive material) is discussed. For the convenience of discussion and analysis, the conductive direction parallel to the nanobelt length is defined as the C direction, and the insulative direction (viz. nonconductive) perpendicular to the nanobelt length (the width direction) is marked as the N direction.

The film is cut into an area of 1 × 1 cm2. Two tin sheets (used as electrodes) coated with conductive adhesive are placed on the surface of the film. Then the two styles of the Hall effect measurement system are attached to the two tin sheets. The conductances of CLJA/MN/LN STCF and the contrast samples are reported in Table . As is known to all, the conductivity of PPy is determined by the charge transfer capability provided by the connected conductive network. With the increase in PPy content, the conductance in the conductive direction will be increased because more continuous conductive networks will be formed. For a series of tested samples, the content of PPy is increased from 30 to 50 to 70%, but the content of other substances remains unchanged. The conductance in the conductive direction increases from 10–6 to 10–2 S, whereas that in the insulative direction remains almost unchanged at 10–10 S. It is shown that the conductance in the conductive direction can be significantly enhanced and the conductance in the insulative direction is almost invariable. When the PPy content reaches 70%, the conductance in the conductive direction is 8 orders of magnitude higher than that in the insulative direction. From the schematic diagram of Figure , it can be seen that the Janus nanobelts in the first layer of CLJA/MN/LN STCF are composed of a conductive region and an upconversion luminescent–insulative region. The direction of electron movement is fixed, viz., along the length direction of the nanobelt, thus the movement of the electron is not hindered, resulting in high conductance in the C direction. Nevertheless, because of the existence of the upconversion luminescent–insulative region, the electron movement along the width direction of the nanobelt is hindered, so it is insulated in the N direction. Therefore, the film with Janus nanobelts as the construction unit possesses high conductive aeolotropy. Furthermore, the conductance ratio of the conductive direction to the insulative direction (C/N) can be noticeably enhanced by increasing the PPy content, meaning that the conductive aeolotropy can be regulated. At the same time, we also research the influence of CoFe2O4 NPs content on the conductance of the first layer, and the results show that the change in CoFe2O4 NPs content does not affect the conductance. When the content of CoFe2O4 NPs changes, the conductance ratio of the two perpendicular directions remains almost unchanged, so the change in CoFe2O4 NPs content in the second layer has no significant influence on the conductance value.

Table 3

Conductances of the First Layer of CLJA/MN/LN STCF with Different Contents of PPy and CoFe2O4 NPs and the Contrasting Samples

	conductance (S)
samples	C	N	conductance ratio (C/N)	aeolotropy level
CLJA/MN/LN STCF (30% PPy) (CoFe2O4:PAN = 0.5:1)	7.83 × 10–6	2.17 × 10–10	3.61 × 104	moderate
CLJA/MN/LN STCF (50% PPy) (CoFe2O4:PAN = 0.5:1)	6.87 × 10–4	3.64 × 10–10	1.89 × 106	high
CLJA/MN/LN STCF (70% PPy) (CoFe2O4:PAN = 0.5:1)	6.43 × 10–2	3.32 × 10–10	1.94 × 108	highest
CLJA/MN/LN STCF (CoFe2O4:PAN = 0.3:1) (50% PPy)	5.73 × 10–4	4.67 × 10–10	1.27 × 106	high
CLJA/MN/LN STCF (CoFe2O4: PAN = 1:1) (50% PPy)	8.42 × 10–4	5.68 × 10–10	1.48 × 106	high
CLJN/MN/LN STCF (CoFe2O4:PAN = 0.5:1) (50% PPy)	3.69 × 10–4	2.12 × 10–4	1.74	none
CLCA/MN/LN STCF (CoFe2O4:PAN = 0.5:1)	5.65 × 10–7	8.29 × 10–10	681	low
CLCN/MN/LN STCF (CoFe2O4:PAN = 0.5:1)	8.74 × 10–7	7.21 × 10–7	1.21	none
CLJA/[M-L]N dual-layered composite film (CoFe2O4:PAN = 0.5:1) (50% PPy)	5.17 × 10–4	4.66 × 10–10	1.11 × 106	high

Figure 13

Sketched maps of the conductive mechanism of CLJA/MN/LN STCF with various contents of (a) PPy and (b) CoFe2O4 NPs.

To show that CLJA/MN/LN STCF has the highest conductive aeolotropy, the conductances of the contrast samples are also analyzed. Figure exhibits a schematic diagram of the conductive mechanism of CLJA/MN/LN STCF and the contrast samples. Compared to CLJA/MN/LN STCF, CLJN/MN/LN STCF possesses no conductive aeolotropy because the Janus nanobelts of CLJN/MN/LN STCF are randomly arranged, so they are conductive in every direction and the conductance values are similar. Therefore, CLJN/MN/LN STCF has no conductive aeolotropy. The conductances of CLCA/MN/LN STCF and CLCN/MN/LN STCF are significantly lower than those of CLJA/MN/LN STCF because of the existence of nonconductive insulative material NaYF4:Yb3+, Er3+ in the composite nanobelts of CLCA/MN/LN STCF and CLCN/MN/LN STCF, and the existence of nonconductive material will seriously block the formation of the PPy conductive network. In CLCA/MN/LN STCF, the interfaces among the composite nanobelts are used as the insulative medium, but the aeolotropy is weak owing to the poor insulation. For CLCN/MN/LN STCF, the nanobelts are arranged in a disorderly manner in the composite film, so it has no conductive aeolotropy. On the basis of the above analysis, target sample CLJA/MN/LN STCF possesses a more excellent building unit than the above three contrast samples, so CLJA/MN/LN STCF has the highest conductive aeolotropy. The conductance of the CLJA/[M-L]N dual-layered composite film is similar to that of CLJA/MN/LN STCF because its first layer has the same composition and structure as the first layer of CLJA/MN/LN STCF.

Figure 14

Sketched maps of the conductive mechanism of CLJA/MN/LN STCF together with the four contrasting samples.

Construction and Characteristics of 3D TWT

Figure exhibits four kinds of 3D TWTs obtained by four different crimping methods. The four sides of CLJA/MN/LN are labeled I–IV. The third layer of CLJA/MN/LN is the core layer, and the first and second methods are adopted to curl STCF from I to II and from III to IV until it is completely closed (marked as tubes A and B), respectively. For the third and fourth methods, the first layer of CLJA/MN/LN is used as the core layer and STCF is curled from III to IV and from I to II, respectively, until it is completely closed to obtain tubes C and D. Therefore, four kinds of 3D TWTs are successfully fabricated by the above methods. Furthermore, 3D TWTs with different sizes and thicknesses can be constructed by changing the preparative process parameters.

Figure 15

Sketched map of the construction process of 3D TWTs by different curling methods

Table lists sketched maps, characteristics, and physical photographs of the four kinds of 3D TWTs. It can be clearly observed from the sketched maps that the exterior walls of tubes A and B have upconversion fluorescence and conductive aeolotropism. The middle wall and the interior wall possess magnetic isotropy and down-conversion fluorescence, respectively. When the infrared laser irradiates the exterior walls of tubes A and B, it can emit upconversion green fluorescence. The interior walls of tubes A and B can emit down-conversion red fluorescence irradiated by ultraviolet light. The exterior wall of tube A conducts electricity and insulates along the axis and circumference directions. The exterior wall of tube B conducts electricity and insulates along the circumference and axis directions, respectively. As for tubes C and D, the interior walls possess upconversion fluorescence and conductive aeolotropism. The middle wall and the exterior wall possess magnetic isotropy and down-conversion fluorescence, respectively. When the infrared laser irradiates the interior walls of tubes C and D, it can emit upconversion green fluorescence. The exterior walls of tubes C and D can emit down-conversion red fluorescence illuminated by ultraviolet light. The interior wall of tube C conducts electricity and insulates along the circumference and axis direction, respectively. The interior wall of tube D conducts electricity and insulates along the axis and the circumference direction, respectively. Furthermore, the conductivity, fluorescence, and magnetism of the 3D TWTs are the same as those of the 2D CLJA/MN/LN. In the 3D TWT, the combination of the micro-subarea and macro-subarea is also realized. The design idea and preparation method realize the transformation of multifunctional materials from 2D to 3D and provide theoretical support and technical methods for the construction of other 3D multifunctional materials.

Table 4

Sketched Map, Characteristics, and Physical Photographs of 3D TWTs

Conclusions

To summarize, the novel 2D STCF and 3D TWTs with dual-color luminescence, magnetism, and aeolotropic conduction are devised and constructed by a layer-by-layer electrospinning technique. The basic building unit of the first layer of STCF is a Janus nanobelt, which endows the layer with conductive aeolotropy and green upconversion luminescence. This distinctive Janus structure is an important factor in ensuring the high conductance and conductive aeolotropy of STCF. The conductance in the conductive direction is 8 orders of magnitude higher than that in the insulative direction. The second and third layers are made of disordered nanofibers with magnetic and red down-conversion luminescence properties, respectively. The conductive aeolotropy and magnetism of STCF can be regulated by respectively adjusting the contents of PPy and CoFe2O4 NPs. In addition, the upconversion luminescence of rare earth ions and down-conversion luminescence of transition-metal ions are concurrently realized in the STCF. Moreover, micro- and macropartitions exist simultaneously in the 2D STCF and 3D TWTs, thus the combination of a micropartition and a macropartition is realized. The micropartitioning of the Janus nanobelt guarantees that the first layer has a high conductive aeolotropy and intense upconversion fluorescence, whereas the macropartitioning of the whole film ensures that the three layers are independent and do not interfere with each other, with both high integration and mutual separation of polyfunctions being actualized. Four kinds of 3D TWTs are obtained by crimping the film via four different strategies. The conversion of materials from 1D nanobelts/nanofibers to a 2D film and then to a 3D triwall tube is successfully realized with the help of this unique design idea. More importantly, this design concept and construction method offer technical and theoretical support for the design and preparation of other multifunctional materials.