Highly Aligned Molybdenum Trioxide Nanobelts for Flexible Thin-Film

Feb 19, 2019 - Carrier mobility of aligned film along nanobelt axis and perpendicular directions are about 400 and 30 cm2/(V·s), respectively. The pr...
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Highly Aligned Molybdenum Trioxide Nanobelts for Flexible Thin-Film Transistors and Supercapacitors: Macroscopic Assembly and Anisotropic Electrical Properties Linpeng Li, Hongwei Fan, Chengyi Hou, Qinghong Zhang, Yaogang Li, Hao Yu, and Hongzhi Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b02342 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Highly Aligned Molybdenum Trioxide Nanobelts for Flexible Thin-Film Transistors and Supercapacitors: Macroscopic Assembly and Anisotropic Electrical Properties Linpeng Li†, Hongwei Fan†, Chengyi Hou* ,†, Qinghong Zhang‡, Yaogang Li‡, Hao Yu*,†, and Hongzhi Wang†

†State

Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Materials Science and Engineering, Donghua University, Shanghai 201620, China

‡Engineering

Research Center of Advanced Glasses Manufacturing Technology, MOE, Donghua

University, Shanghai 201620, China

KEYWORDS: molybdenum trioxide nanobelts, anisotropy, macroscopic assembly, nano yarns, carrier mobility

ABSTRACT: One dimensional (1D) nanomaterials have attracted a lot of attention owing to its intriguing performance. However, cost-efficient macroscopic assembly of 1D nanomaterials into free-standing and well-aligned macroscopic structures that are anisotropic and highly ordered in microscopic appearance is still a challenge. Here, we report a general and versatile technology to directly assemble MoO3 nanobelts into macroscopic structures, including film and yarn, without using any extra functionalization agent nor crosslinking agent. MoO3 nanobelts are close-packed as parallel arrays with longitudinal axes aligned perpendicular to the capillary force.

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Carrier mobility of aligned film along nanobelt axis and perpendicular directions are about 400 and 30 cm2/(V·s), respectively. The promising carrier mobility raised from its high-order alignment indicates potential application in thin film transistors. High degree order of assembled MoO3 yarn also enables large tensile strength of up to 120 MPa, which is superior to that of many similarly structured nature and artificial fibers/yarns.

Introduction One dimensional (1D) nanostructures are attractive building blocks for hierarchical assembly of funcional nanodevices that could overcome fundamental limitation of conventional fabrication owing to their anisotropic nature1-5. Uniformly aligned nanostructures possess enhanced electrical and physical properties, which offer great potential for applications in nanoelectronics6-8, nanomedicine9,10, nanoenergy11-15, et al. For instance, in many semiconductor devices such as high-performance field effect transistors, substantial potential of 1D nanostructures has been achieved by controlled assembly of well-ordered structures, in which charges travel from source to drain within single crystals, thus ensuring high carrier mobility. Further assembling 1D nanostructures into two dimensional (2D) or macrostructure, such as film or yarn, bridges the gap from the nano- into macro-world hence is essential for a variety of engineering applications. Up to now, numerous assembly methods including

Langmuir-Blodgett16-18,

self-assembly19-21,

electrospinning22,23,

electric/magnetic field driving24-27, bubble-blowing28, etc. have been explored for the

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assembly of nanoscale building blocks. As a mature approach, Langmuir-Blodgett technique was utilized to align silver nanowire16, tungsten oxide nanowire17, single walled carbon nanotube18, et al., but the requirement of the hydrophobic group on nanostructures surface and high-price of the equipment restrict its extensive application. Electrospinning with using high speed roller facilitates the assembly of polymer nanostructures, such as PVDF22 and PVP23, but it requires large external electronic fields (thousands of volts), which is uneconomical and not human-friendly. Bubbleblowing is an ingenious method for fabrication of uniformly aligned and large-scale nanowires. As reported by Lieber group28, bubble-blowing offers shear force to align carbon nanotubes, silicon nanowires. However, the organic component in bubble film may cause contamination of the nanowire and possibly degrade the performance of the inorganic nanomaterial. In comparison, self-assembly is a more attractive approach that has been used for fabricating aligned silver nanowires19, carbon nanotubes20, et al., owing to the low-cost, simple and high efficient fabrication process. However, it is also notable that each approach is particularly applicable to certain materials. The ordered assembly of some special nanomaterials ramains a challenge. Molybdenum trioxide (MoO3) are versatile oxide compounds with well recongized applications in catalysis29, sensors30, field emmison devices31,32, electronics33-35, and electrochromic systems36. The controlled alignment of MoO3 nanobelts is one key prerequisite in materials science when fabricating materials and semiconductor devices with anisotropic physical properties, whereas the alignment of 1D nanostructure of MoO3 has yet been reported.

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Herein, we report a general and versatile technology to directly assemble MoO3 nanobelts into macroscopic structures, including film and yarn, without using any extra functionalization agent or crosslinking agent. MoO3 nanobelts were close-packed as parallel arrays with longitudinal axes aligned perpendicular to the capillary force, which can be processed over various bases, hence enables applications in flexible and wearable electronics. Carrier mobility of aligned film along nanoblets axis and perpendicular directions are 412 and 29 cm2/(V·s), respectively, with more than 14-fold difference between them. MoO3 yarns can be fabricated through a dimension-reduced process. Owing to high order degree in its microscopic structure, the MoO3 yarn has a large tensile strength of about 120 MPa. In addition, the method reported in this work can be readily applied to other 1D nanostructures, e.g. silver-nanowire yarns and bismuth sulfide-nanobelt yarns. Results and Discussion

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Fig. 1. Digital photograph (a) and FESEM image (b) of MoO3 nanobelts dispersion. Inset: XPS Mo3d spectra of the MoO3 nanobelts. (c) Distance from the center dependence of compactness and alignment ratio of the nanobelts. (d) Schematic diagram of the assemble process on side view. (e) Ordered distribution of nanobelts on water surface. FESEM images was taken at the different areas on polyethylene base.

MoO3 nanobelts were synthesized through hydrothermal method, and disorderly dispersed in water (Fig. 1a). The process is described in brief: Molybdenum power and hydrogen peroxide were dispersed in deionized water, and transferred into Teflon-lined autoclave, which was maintained at 220 oC for 72 hours. The micro-morphologies of the products were observed by field emission scanning electron microscope (FESEM)

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and high resolution transmission electron microscope (HRTEM). Fig. 1b and S1 are the images of wire-like particles of MoO3 after drying, showing that nanobelts are uniform in width and length. The dimensions of nanobelts were estimated according to the FESEM or HRTEM images in Fig. 1b and S1d (~50 um in length), S1a (~50 nm in thickness) and S1b (~300 nm in width). As shown in Fig. 1b, the doublets in X-ray photoelectron spectroscopy (XPS) are attributed to the binding energies of the 3d3/2 and the 3d5/2 orbital electrons of Mo6+. Selected area electron diffraction (SAED) patterns indicates that the nanobelts are single crystal (Fig. S1c). The powder samples were characterized with X-ray diffraction (XRD, Fig. S2), the results show that the nanobelts are orthorhombic α-MoO3. The alignment of MoO3 nanostructures was carried out through a self-assembly method. Certain amount of dichloromethane (CH2Cl2) was added into the container to act as the bottom phase. MoO3 nanobelts were then added in CH2Cl2 to prepare a dispersion with a concentration of 2 mg/mL. A few minutes later, a sparkling film appeared on water surface, indicating that the nanobelts were successfully assembled in this process (Fig. S3). The assembly process was analyzed by investigating sample morphology in different cell areas. Polyethylene base was put on the water surface to adhere MoO3 firmly onto the film surface and brought out carefully to investigate the assembly condition of nanobelts through FESEM. As shown in Fig. 1c, compactness and alignment ratio of assembled materials in different areas were calculated (Fig. S4, Note S1).

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Corresponding morphologies are shown in Fig. S5. The compactness means the number of nanobelts per micron, and alignment ratio means the number of nanobelts along the alignment direction divided by the gross in 500 um. As the distance increases, both values decrease linearly. Hence, the order of nanobelt assembly increase as it is closer to the central ring as illustrated in Fig. 1e. Interestingly, the capillary force shows a same trend. The self-assembled ordered structure was realized in air-water-CH2Cl2 phase, in which interfacial force is the main driving factor (Fcapillary = 1.28 × 10-3 mN, Felectrostatic < 3.45 × 10-11 mN) as discussed in Note S2. This assembly approach can be described as a compression process, in which the interaction between nanobelts plays an important role. As shown in Fig. 1d, nanobelts was brought to the water surface owing to the capillary flow of water dispersion. The strong interparticle capillary interactions drove MoO3 to self-assemble on water surface (see Movie S1).37 Dispersion concentration is an important factor in assembly process. Well-dispersed MoO3 of 0.8‒15 mg/mL lead to close-packed parallel arrays, but higher concentration over 15 mg/mL will prevent the nanobelt flowing while lower concentration below 0.8 mg/mL cannot offer sufficient building blocks. We also compared the effect of underlying solvent including CCl4, CH2Cl2, and CHCl3. The MoO3 nanobelts show best alignment when CH2Cl2 is used as the underlying solvent, while very loose assembly is observed in CHCl3 case (Fig. S6), and no obvious assembly happens when CCl4 is used as the underlying solvent.

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Fig. 2. Low- (a) and high- (b) magnification FESEM images of aligned MoO3 nanobelts film. Inset: Digital photograph of freestanding MoO3 nanobelts film. (c) Carrier mobility of MoO3 nanobelts film in different direction (00 and 900 represent the directions parallel and perpendicular to nanobelt axis). The error bars represent the standard deviation of the measurements. FESEM images in lower panel show double layer MoO3 nanobelts assemblies with twist angle of 90o (d), 45o (e) and 0o (f).

A flat substrate was vertically dipped into the water to transfer assembled MoO3 nanobelts film. With sufficient strength, freestanding film was then peeled off, giving a two dimensional single-component MoO3 nanobelts assembly. The freestanding film was observed by FESEM. As shown in Fig. 2a and b, the film consists of a high density of nanobelts with remarkable alignment parallel to each other. The alignment direction

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of nanobelts is parallel to the applied capillary force. Freestanding MoO3 films larger than 2 cm2 were successfully fabricated (Fig. 2b). The as-assembled aligned film can be transferred onto conductive glass, silicon wafer, polyethylene, et al., with Van der Waals force acting on the interface. Successful transfer of aligned MoO3 nanobelts film provides a test for the suitability of the method for advanced applications in energy-storage and electronic devices. Carrier mobility is one of the most significant properties of semiconductors, which determines the conductivity of semiconductor and affects the working speed of many semiconductor devices such as thin film transistors. Carrier mobility of aligned MoO3 nanobelts film was measured by van der Pauw method38.

μ = 𝑅 𝐻𝜎 RH is hall coefficient measured by Lakeshore 8400, σ is electrical conductivity. Through measurement of aligned MoO3 nanobelts film, the average value of RH in parallel and perpendicular direction are 179.6 and 21.66 m3/C, respectively. The average value of σ in parallel and perpendicular direction are 2.29×10-4 and 1.34×10-4 S/m, respectively. μ is thus calculated to be ~400 and 30 cm2/(V·s) for parallel and perpendicular direction respectively, and there is more than 14-fold difference between them (Fig. 2c), showing the superior anisotropic electrical property. The anisotropy ratio of our MoO3 assembly (14) is slightly lower than a calculated result for a previously reported few-layer MoO3 (anisotropy ratio of 20-30)[39], but is remarkably larger than other experimental measured results for MoO3 materials (anisotropy ratio

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of 2)[40]. In addition, transferring two layers of aligned MoO3 nanobelts into double layer structures with designable twist angles are illustrated in Fig. 2d-2f. The nanobelts assembly was transferred directly to a planar substrate, by turning the deposition angle of different layer, various twist angles from 0 to 90° can be obtained.

Fig. 3. (a) Schematic diagram of the pull process of MoO3 nanobelts yarn. Crosssection FESEM image (b) and digital photograph of straight (c) and curved (d) MoO3 nanobelts yarn. (e) FESEM image of knotted MoO3 nanobelts yarn. (f) Mechanical test of MoO3 nanobelts yarn.

More importantly, we first utilize this method to fabricate free-standing aligned

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MoO3 yarns. As shown in Fig. 3a, aligned free-standing yarn was fabricated through a vertical pulling process. When the beaker was filled with water, close-packed film floated on the water surface, which can be continuously pulled up by a dropper. Owing to the surface tension, during pulling process, close-packed film will fold and become flat after drying. Cross-sectional FESEM image of the yarn clearly shows a bilayer structure (Fig. 3b), the constitute units of which are aligned nanobelts. Fig. 3c and d show the digital photograph of MoO3 yarn after drying. Curve-state yarn demonstrates its good flexibility. FESEM images in Fig. S7 show high order degree of close-packed nanobelt yarns. It is worth mentioning that the yarns have considerable mechanical strength owing to the close-packed structure, which promise its application in flexible electronic devices. For instance, flexible fiber-shaped supercapacitor was fabricated using MoO3 nanobelt yarns and other functional materials (Fig. S8). The electrochemical redox process was evaluated via cyclic voltammetry (CV) measurements over a range of scan rates of 5-100 mV/s between 0 and 1.0 V, with using PVA/H2SO4 as a gel electrolyte. Owing to the highly ordered microstructure, the yarn is strong and flexible enough to form a tightened knot (Fig. 3e) and would not break with hand rubbing. Fig. 3f shows that the tensile strength of the yarn is 120 MPa, which is higher than that of pure natural cotton yarn (30 MPa), and is close to the value for pure graphene oxide fiber (184~501 MPa)41. Although natural cotton yarn (directly cut from cotton sliver without twisting) has the analogous 1D constitutional unit as same as MoO3 nanobelts yarn, tensile strength of artificial nanobelts structure is about 4 times higher than that of cotton yarn

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owing to the elaborate nano-building structures. In addition, MoO3 yarn shows a hysteresis fracture process, which is the result of 1D primary units and its interactions. Normally, the interaction between anisotropic particles is more than two orders of magnitude stronger than the attraction between spheres30, therefore we expect that the proposed method is suitable in the assembly of other 1D inorganic materials that are anisotropic with high aspect ratio. As shown in Fig. S9 and S10, silver nanowires and bismuth sulfide (Bi2S3) nanobelts yarns were successfully fabricated. However, the length of the resulting silver nanowires yarn was only millimeter-scale due to the weak interaction force, while Bi2S3 nanobelts yarn can be several centimeters in length. Simultaneously, these nanostructures keep high order degrees. CONCLUSIONS In summary, we report a general and cost-effective approach for assembling 1D nanostructures with high efficiency and good controllability. Macroscopic MoO3 nanobelt assemblies with aligned architectures, including film and yarn, are fabricated from the nanobelts suspension. Owing to the high order degree in microscopic structure, MoO3 nanobelts yarn has an adequate tensile strength of 120 MPa. Assembled MoO3 nanobelts film has a promising anisotropic electrical performance, hence holds the potential for application in thin-film transistors. Experimental Section Synthesis of molybdenum trioxide nanobelt dispersion: Molybdenum trioxide

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and vanadium pentoxide were synthesized via a modified hydrothermal method. MoO3 nanobelts: Typically, 2 g molybdenum powder was added into 10 mL deionized water to form a uniform mixture. Then 20 mL 30% (wt%) H2O2 was added slowly. The solution was stirred for 30 minutes to react thoroughly. After that, the solution was transferred to a Teflon-lined stainless steel autoclave and heated to 220 oC for 72 h. The precipitate was filtered and rinsed by deionized water and ethanol for several times. MoO3 was dispersed in water. Aligned and MoO3 nanobelts film: 20 mL CH2Cl2 was added in 60 mL beaker, and a certain amount of MoO3 nanobelts dispersion was added on the CH2Cl2 to form a ring-like structure. After 1 hour, an amount of water was added, making the water phase totally cover the surface. The close packed assembled nanobelts was transferred to the substrate through the pulling process and drying in the oven. Free-standing aligned nanobelts film was obtained by using adhesive tape on one side of the film and taking it off. Polyethylene film was used to cover the whole water surface to study nanobelt distribution. Free-standing aligned MoO3 yarn: Close-packed nanobelts yarn was pulled through a dropper or carbon fiber. When it attached one side on water surface, subsequent film would roll into yarn shape, thus free-standing aligned yarn was made. Synthesis of bismuth sulfide nanobelts dispersion: The bismuth sulfide nanobelts was synthesized according to the method previously reported.[42] Fabrication of coaxial fiber supercapacitor: Carbon fiber was used to pull out of

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the MoO3 yarn to make a coaxial fiber. PVA/H2SO4 gel electrolyte was used to cover onto MoO3. (2 g PVA and 1 M H2SO4 were dispersed in 20 mL water, and magnetic stirred for 1 h in 90 ℃.) Cellulose separator was used to block electron transfer. Characterization and measurements: The morphology of the samples was characterized by field emission scanning electron microscope (FESEM, S-4800, Hitachi, Japan). XRD pattern was carried on a Rigaku D/max 2550 V X-ray diffractometer using Cu Kα irradiation (λ = 1.5406 Å). The operating voltage and current were kept at 40 kV and 300 mA, respectively. The tensile stress of the aligned yarn was measured on a universal testing machine with 5 N sensing element and 10 N clamp (Instron Model 5969, Instron), which sample is oblate with area of 200 um2 and length of 2 cm. The microscope was carried on an optical microscope. Electrochemical testing of fiber supercapacitor was carried on electrochemical workstation (VSP-300). Hall measurment of carrier mobility was carried on the Lake Shore 8400 series using movable-type contacts (probe pins) with 1 cm2 (1 cm × 1 cm) sample area and the tightest sample compactness (4). ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Fig. S1-S10, Note S1, Note S2 (PDF)

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Movie S1 (Self-assembly process of MoO3 nanobelts on water surface, AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support by Science and Technology Commission of Shanghai Municipality (16JC1400700), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00055). C.H. thanks the Shanghai Natural Science Foundation (16ZR1401500), the Shanghai Sailing Program (16YF1400400), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program (LZB2019002).

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Synthesis of Ultralong Bi2S3 Nanoribbons via a Solvothermal Process. Adv. Mater. 2003, 15, 936-940.

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