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ReseaRch aRticle

Hydroplastic Micromolding of 2D Sheets

Fan Guo, Yue Wang, Yanqiu Jiang, Zeshen Li, Zhen Xu,* Xiaoli Zhao, Tingbiao Guo, Wei Jiang, and Chao Gao*

DOI: 10.1002/adma.202008116

1. Introduction

2D sheets are a broad class of atomically thin materials, as exemplified by graphene, graphene oxide, and MoS2. The extraordi-nary physical and chemical properties of 2D sheets have been expected to translate into outstanding performances of their macroscopic assemblies. However, the intrinsic interlayer interactions of intact 2D sheets result in the poor solid process-ability. An alternative method is solution processing from homogeneous disper-sions of 2D sheets aided by surfactants or surface modification.[1,2] Solution pro-cessing methods, including casting, spin-ning and additive manufacturing,[2–4] have been used to process dilute dispersions into macroscopic forms, such as films, fibers, and aerogels.[5] The solid content of homogeneous dispersions of 2D sheets is usually restricted to a range from 0.1 to 5.0 wt% due to their high aspect ratio and low affinity with common solvents. During the drying process, these sparse solid net-

works are too weak to resist compressive capillary stress and exhibit considerable volume shrinkage ratios, up to 1000 (for 0.1  wt% dispersion). More importantly, the extreme shape asymmetry of 2D sheets brings about considerable anisotropic shrinkage, resulting in randomly shaped dried solids. In the case of wet-spun graphene oxide (GO) fibers, the average dia-meter of solid fibers is only one thirtieth the spinneret size and fiber sections are ellipses or irregular shapes with rough periph-eries.[6] To date, the common solution processing methods face great challenges in meeting the manufacturing requirements of precise structures with high fidelity at high speed. Hence, a processing strategy that enables fine structures to be easily con-structed from 2D materials is urgently demanded.

Plastic forming is a powerful processing method that has been widely used in industry, covering both metal forging and polymer molding.[7] In addition to a high processing velocity and large build volume, spatial accuracy has been greatly improved to enable microscale and nanoscale fabrication, as typified by nanoimprinting.[7] However, plastic forming has not been extensively applied to newly emerging 2D sheets. Typical plastic forming techniques (e.g., forging, rolling, and imprinting) operate through the plastic deformation of bulk materials to shape them into designed parts. Plasticity arises from grain-boundary sliding or dislocation glide in metals[8] as well as thermally activated motion of chains in polymers.

Processing 2D sheets into desired structures with high precision is of great importance for fabrication and application of their assemblies. Solution processing of 2D sheets from dilute dispersions is a commonly used method but offers limited control over feature size precision owing to the extreme volume shrinkage. Plastic processing from the solid state is therefore a pref-erable approach to achieve high precision. However, plastic processing is intrinsically hampered by strong interlayer interactions of the 2D sheet solids. Here, a hydroplastic molding method to shape layered solids of 2D sheets with micrometer-scale precision under ambient conditions is reported. The dried 2D layered solids are plasticized by intercalated solvents, affording plastic near-solid compounds that enable local plastic deformation. Such an intercalated solvent-induced hydroplasticity is found in a broad family of 2D materials, for example graphene, MoS2, and MXene. The hydroplastic molding enables fabrication of complex spatial structures (knurling, origami) and microimprinted tubular structures down to diameters of 390 nm with good fidelity. The method enhances the structural accuracy and enriches the structural diversity of 2D macroassemblies, thus providing a feasible strategy to tune their electrical, optical, and other functional properties.

Dr. F. Guo, Y. Wang, Prof. W. JiangNational Special Superfine Powder Engineering Research CenterNanjing University of Science and Technology1 Guanghua Road, Nanjing 210094, P. R. ChinaDr. F. Guo, Dr. Y. Jiang, Z. Li, Prof. Z. Xu, Prof. C. GaoMOE Key Laboratory of Macromolecular Synthesis and FunctionalizationDepartment of Polymer Science and EngineeringKey Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang ProvinceZhejiang University38 Zheda Road, Hangzhou 310027, P. R. ChinaE-mail: zhenxu@zju.edu.cn; chaogao@zju.edu.cnDr. X. ZhaoSchool of Materials Science and EngineeringTongji UniversityShanghai 200123, ChinaT. GuoCentre for Optical and Electromagnetic ResearchCollege of Optical Science and EngineeringZhejiang UniversityHangzhou 310058, P. R. ChinaProf. C. GaoGraphene Industry and Engineering Research InstituteXiamen UniversityNo. 422 Siming Road, Xiamen 361005, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.202008116.

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Most 2D sheet solids have a temperature-independent elasticity, expressed as intrinsic brittleness,[9,10] which does not meet the fundamental requirement for plastic deformation. This brit-tleness is attributed to the restrained motion of sheets owing to the strong interlayer attractions, which are a dimensional effect of 2D sheets. Recently, Yeh  et  al. applied aerosolized water mists into freeze-drying GO foams to get a moldable dough.[11] Liu  et  al. embossed hemispherical caps from GO-cellulose membranes.[12] In addition to these attempts, our group reported on an intercalation-modulated plasticity of GO lamellate solids. Random wrinkles were eliminated in graphene fibers and papers and their overall performances improved by stretching plastic lamellate solids.[13] Although these efforts indicate the potential for a plastic state in solvated 2D sheets, high-precision plastic manufacturing similar to that applied to metals and polymers has not been achieved yet.

Here, we report a general plastic near-solid state of 2D sheets induced by solvent intercalation and realize plastic manufacturing with sub-micrometer size resolution. Using graphene oxide papers (GOPs) as a research model, we reveal that the hydroplastic GO papers (Hp-GOPs) undergo a tran-sition from a brittle to ductile state, accompanied by a 500% increase of their ultimate tensile strain, as solvent intercala-tion expands the interlayer spacing from 0.85 to 1.3 nm. The activated interlayer sliding endows the Hp-GOPs with both global and local plastic deformation abilities. We hereby con-duct molding and microimprinting processes on hydroplastic 2D sheets to obtain fine structures of assemblies including origami, embossing, and periodic arrays with size resolution ranging from 200 µm to 390 nm. This proposed micromolding strategy enables precise manufacturing of microstructures of 2D sheets assemblies and modulation of properties, including surface hydrophilicity, ion transport, and light absorption. Fur-thermore, we convert graphene laminates with metallic luster into black body materials with 1% hemispherical reflectance in the visible range. Hydroplastic molding extends the process-ability of 2D sheets and further overcomes size resolution constraints of their assembled structures, opening up new pos-sibilities toward rational structural design with high precision for broader applications.

2. Results and Discussion

2.1. Hydroplastic Molding of 2D Sheets

Although the interlayer van der Waals interactions of 2D sheets are much weaker than the in-plane covalent bonding, the accu-mulated forces, over the large contact area, are still too strong for free interlayer sliding motion,[14–16] leading to a global brittle-ness in layered solids. For example, in GO laminate papers, the adhesion energy of GO sheets is ≈0.2 J m−2,[17] which is slightly greater than that between poly(dimethylsiloxane) (PDMS) and a glass substrate (0.1–0.2 J m−2).[18] Adhesion between adjacent sheets constrains their relative motion, providing laminate films with a high modulus but limited elongation and tough-ness.[11] The poor deformability of layered solids composed of 2D sheets prevents the use of plastic manufacturing applicable to polymers and metals.

The key to improve the deformation ability of layered solids is to weaken interlayer interactions and activate continuous interlayer slide. We used the common solvent ethanol as a plasticizer (Figure  1a). Water was not used because GOPs swell infinitely, redisperse, and form a gel or solution, which results in insufficient strength to resist disintegration under applied stresses (Figures S1–S3, Supporting Information).[19] Ethanol was chosen as an intercalator due to its spontaneous intercalation into GO interlayers, as well as the convenience of removal by low temperature drying (Figure S3, detailed sol-vent selection criteria, see the Supporting Information). GOPs were immersed into an ethanol solution for 5 min (Figure S2, Supporting Information), allowing ethanol molecules to dif-fuse into the interlayer spacing and reach a saturated state. The typical GO weight concentration after intercalation was ≈54% (Figure S4 and Table S1, Supporting Information) and the inter-layer spacing (d) expanded, which translated into a cubic reduc-tion of the interlayer van der Waals forces (Figure 1b). Slippage of 2D sheets was activated, manifesting as softening, ductility, and good processability of GOPs (Figure 1a). This brittle-to-duc-tile transition in Hp-GOPs enabled further plastic processing of the 2D assemblies. Starting from Hp-GOPs, a wide range of designed shapes and structures were achieved, including ori-gami with stereostructures (Figure 1c), embossing (Figure 1d), and nanometer-sized periodic tube arrays (Figure  1e). Notably, the maximum stamping depth was greater than 180  µm, i.e., ten times the GOP thickness (Figure S5a, Supporting Infor-mation). The lateral resolution was as low as 390  nm, which is two orders of magnitude smaller than the GO sheet size (≈10 µm). Compared with previous dilute solution processing, this new plastic processing method offers higher working accu-racy, freedom, and fidelity.[2–5] Furthermore, the method is also generally applicable to many 2D layered solids, such as conduc-tive graphene derivatives, MXene, and semiconductor MoS2 (Figure 1f–h, Figures S5–S7, Supporting Information).

2.2. Overall Plasticity of Hp-GOPs

We performed X-ray diffraction (XRD) and mechanical tests to investigate the relationship between d and the deformation capability. GOPs showed typical elastic deformation in ten-sile testing with a 1.3% breaking elongation (Figure  2a). This intrinsic brittleness completely prevents the application of plastic forming to GOPs owing to their insufficient elongation. When the GOPs were molded, catastrophic cracking occurred at stress concentration areas. When the GOPs were immersed in ethanol solution, d expanded from 0.8 to 1.3  nm as calcu-lated from the (001) peak in the XRD patterns of GOPs and Hp-GOPs (Figure 2b). According to the van der Waals force (F) for two planar surfaces

π=

area 6132

03

F A

z (1)

where A132 is the Hamaker constant for surfaces “l” and “2” in the presence of medium “3”, and z0 is the separation distance. In our system, “l” and “2” represent GO sheets, the medium is ethanol, and z0 indicates d. When d increased from 0.8 to

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1.3  nm, the corresponding interlayer van der Waals force per unit area of Hp-GOPs (fHp) decreased to 23.3% of the original force (fo).

The small interlayer expansion greatly weakened interlayer interactions, liberating GO sheets from the constraints of neighboring sheets.[20] Hp-GOPs have distinctive features of plastic deformation on both macro- and microscopic levels. The macroscopic elongation, a characteristic parameter defining plastic deformation, reached 10% with a broad plateau after 4% elongation. The molded GOPs, i.e., the dried GOPs after molding and solvent removal, became stronger but more brittle with no notable changes in the overall elongation and perfor-mance compared with the original casting GOPs (Figure S8, Supporting Information).[13] Microscopic sheet slippage was observed by fracture morphology analysis using a scanning electron microscope (SEM) (Figure  2c–f). Fractures occurred around the outer perimeter of the neck at an angle of ≈45° with a tensile axis by shear deformation (Figure 2e), accompanied by delamination and buckling of GO sheets. The central interior region of the fracture had an irregular and fibrous appearance,

which indicates a typical plastic fracture morphology (Figure S9a,b, Supporting Information). By contrast, cracking of the GOPs occurred perpendicular to the tensile axis and yielded a rela-tively flat and neat fracture surface, denoting a brittle fracture (Figure  2c). For Hp-GOPs, the average deformation band was ≈16.2 µm (Figure 2f), which is comparable to the average size of the constituent GO sheets, indicating high mobility of the GO sheets.[13] However, for dried GOPs, the average deforma-tion band was measured to be 1.4  µm (Figure  2d), only one tenth that of the Hp-GOPs. This notable difference indicates that sheet slippage is greatly activated by the intercalation of solvents, reflecting a typical transition from elastic-to-plastic deformation and a brittle-to-ductile fracture (with solid contents over 50%).

Plastic deformation can also be observed in flexure and bending cases (Figure  2g–k), which are the main deforma-tion models in molding processes. The plasticization effect is embodied by a weakening of folding strength (Figure S9c,d, Supporting Information).[21] We used SEM imaging to further investigate the crease structure of the folded papers as a function

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Figure 1. Schematics illustrating the hydroplastic molding of 2D sheets and demonstrations of their processability. a) Plasticizing process of 2D macroassemblies and b) the corresponding schematic diagram of interlayer expansion during the plasticizing process. c–e) Demonstrations of the processability of hydroplastic 2D assemblies using different templates. c) Graphene paper with millimetric Miura-origami tessellation. d) Elaborate patterns of the ZJU logo in micrometers. e) Nanosized graphene tube array with ≈390 nm diameter. f–h) The universal hydroplastic forming of various 2D sheets assemblies, including: f) MoS2, g) MXene, and h) GO papers. The ZJU logo in (d) is reproduced with permission of Zhejiang University.

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of folding angle (θf) and identify the underlying microdeforma-tion mechanism. The elastic GOPs buckled with typical chevron or box folds and delamination under flexural stress, arising from interlayer 2D confined deformation (Figure  2h, Figure S10a, Supporting Information). As θf increased to 90°, outer arc tension stress exceeded the strength and led to catastrophic sur-face cracking, whereas the inner arc compression stress caused tight folds and severe delamination (Figure 2i, Figure S10b,c,e, Supporting Information). Conversely, the increased bending deformation was easily accommodated by plastic slippage in Hp-GOPs. The outer and inner surface remained almost completely parallel with ≈150° interlimb angles when θf = 45° (Figure  2j, Figure S10d, Supporting Information), indicating full dissipation of the bending work by sheets sliding. When folded in half, the plastic flexural slip deformation partially dis-sipated the outer arc tension stress and avoided cracking. Buck-ling was still generated at the inner arc, but with more open fold and thinner delaminated lamella (Figure  2k) than pure flexural deformation in dried GOPs (Figure  2i, Figure S10b,c,

Supporting Information). This interlayer “decoupling” behavior allows for great deformability under complex flexural deforma-tion. The transition from pure flexural deformation of GOP to flexural slip deformation of Hp-GOP indicates the same plastic deformation in the bending cases, which is a precondition for direct plastic manufacturing of graphene origami structures (Figure 1b).

2.3. Local Plasticity of Hp-GOPs

Overall plasticity can be easily achieved by building laminates composed of individual brittle layers alternating with ductile inter-layers, as exemplified by natural seashell or artificial bulletproof glass.[22] However, these materials are not suitable for nano -meter-sized plastic processing due to the absence of local plas-ticity. We used piezoelectric nanoindentation micromechanical analysis to evaluate whether local plasticity exists in Hp-GOPs. This method has been previously reported to investigate

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Figure 2. Overall plasticity of Hp-GOPs. a) Stress–strain contrast curves in tensile test of GOP and Hp-GOP. b) XRD patterns of unstretched GOP (blue) and Hp-GOP (red). c–f) Surface and transverse cross-section morphology of GOPs (c,d) and Hp-GOPs (e,f) after tensile breaking. The insets to the right show brittle fracture of the GOP and ductile fracture of the Hp-GOP. g) Schematic illustration of paper folding test where θf represents the folding angle. h–k) Longitudinal cross-section morphology of GOPs (h,i) and Hp-GOPs (j,k) after folding. The insets to the right are schematic illustrations showing the morphology of inner and outer sheets in GOP (i) and Hp-GOP (k).

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plasticity and ductility in GO sheets.[22] In a typical load–depth curve, the indenter penetrated the sample, was held for 1 s and then unloaded. The microindenter generated a permanent

inverted cone hole with a depth of 1  µm and a diameter of 3  µm on the Hp-GOP surface (Figure  3a–c). By contrast, the strong interlayer interactions in the GOP restricted slippage

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Figure 3. Local plasticity of Hp-GOPs. a–c) Morphology inspection of indentation on Hp-GOP surface after nanoindentation test. a) SEM image, b) white light interferometer topography image, and c) the corresponding height profile. The upper inset is a false color 3D topographic view of indent. d,e) Force versus depth curves of GOPs (d) and Hp-GOPs (e) by piezoelectric nanoindentation. f) Energy loss coefficients and effective Young’s modulus of GOPs and Hp-GOPs. g,h) Cross-section images of casting GOPs (g) and molded GOPs (h) after nanoindentation. i) Azimuth angle dis-tribution histogram of GO sheets in (g) and (h). Left inset is a schematic diagram of azimuth angle of the GO sheet and right one is the distribution of the absolute azimuth angle.

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of the sheets and there was no obvious irreversible deforma-tion. Load–depth curves were further analyzed to reveal the microscale mechanism of hydroplasticity (Figure  3d,e). Upon unloading, Hp-GOP barely rebounded with 60% reduction of the loading force (Figure  3e, Figure S11, Supporting Informa-tion). The plastic index (ψ) is a parameter to quantitatively char-acterize the plastic deformation of materials under an external load.[23] ψ can be calculated from

A A

Aψ = −1 2

1

(2)

where A1 and A2 are the areas under the loading and unloading curves, respectively (Figure 3d,e). A value of ψ closer to 1 indi-cates a greater proportion of plastic deformation. The ψ value of Hp-GOP was calculated to be 0.91, whereas ψ of GOP was only 0.29 (Figure  3f). Likewise, the indentation creep (Cit), defined as the relative change in the indentation depth under constant loading, increased from 11% to 17% (Figure S11, Supporting Information).[24] The increases of ψ and Cit for Hp-GOP con-firmed that the plastic deformation of 2D sheets was enhanced after hydro-plasticization, which is an essential deformation requirement for high accuracy plastic fabrication. Hp-GOP sys-tems dissipated external mechanical work by sheet sliding and re-orientation, leading to irreversible plastic deformation.

Furthermore, we used azimuth angles (θa) between the normal vector of sheets (ns) and the direction axis (nd) to quantify the microarea deformation resistance of GO sheets (Figure  3g,h). θa is positive in the counterclockwise rotation (Figure  3i, inset); θa  = 0 represents parallel alignment with the director axis and a hard deformation system, and |θa| = π/2 represents a low deformation resistance with good plasticity. Figure S12 (Supporting Information) shows processed false color images of the orientations of the GOPs and Hp-GOPs, where the hue represents the azimuth angle. A distinctly mixed hue was observed in Hp-GOPs, suggesting easier sliding of the GO sheets. Similarly, Figure  3i shows the distribution of the azimuth angle. More than 90% of GO sheets were deflected by more than 20°, and <θa> was 28° in Hp-GOP. By con-trast, GOP maintained a good alignment with <θa> = 14°, and 85% of sheets deflected within 20°. Apart from the improved deformability of GO assemblies, the modulus of Hp-GOPs also decreased by three orders of magnitude. The diminished effective modulus implies easier deformation under a lower load. The hydroplasticity was also independent of the loading frequency, reflecting in stable modulus at frequencies ranging from 1 to 100  Hz (Figure S13, Supporting Information). The reduction of the modulus and increase of the deformability from local plasticity suggest potential for microimprinting.

2.4. Microimprinting of Hp-GOPs

Microimprinting by thermoplastic forming is a prevailing pro-cessing method in metals and polymers owing to its advantage of low-cost fabrication of functionalized surfaces and nano-structured devices. Although 2D materials generally lack ther-moplasticity due to their temperature independent mechanical properties,[10] we successfully performed microimprinting on

2D assemblies relying on intercalation-induced local plasticity. We selected anodic aluminum oxide (AAO) membranes with periodic hexagonal cavities and narrow diameter distribution (≈390 nm) as templates. Notably, the cavity diameter was only one-tenth or even one-hundredth the size of the GO sheets. Usu-ally, AAO membranes are used as filter membranes to intercept GO sheets. But we found that the plasticized GO sheets entered the nanoscale channel in the form of rolls or folds through the microimprinting process (Figure 4). Hp-GOPs were placed on a smooth substrate and covered with AAO templates, where con-stant a compression force (35 kN, ≈263.5 MPa) was applied and held for a certain time. The whole process was performed in a closed container to prevent plasticizer volatilization (Figure S14, Supporting Information). After drying, chemical reduction, and template removal, reduced graphene oxide (rGO) tube arrays with hexagonal order were fabricated (Figure  4a, Figure S15, Supporting Information). Changing the pressing time (t) effec-tively controlled the aspect ratio of the tubular structure (L/d), ranging from 1.5 to 6. At the initial stage, L/d and t followed a power law, which was reminiscent of the case in metal nanoim-printing.[7] Notably, at a later stage (t > 8 h), L/d approached an equilibrium value of ≈6.2 (Figure 4b, Figures S12, S16, and S17, Supporting Information).

Our imprinted structures were not solid columnar arrays but rather hollow tube arrays. The tubular structures were mainly determined by interfacial interactions, namely the good wetta-bility between hydroplastic Hp-GOPs and AAO pore surfaces. First, GO sheets and solvent plasticizer quickly climbed up the pore wall and formed a wetting film with a thickness of a few tens of nanometers on a time scale of seconds to minutes. Sheets at the central region did not flow into the cavity but were squeezed by moving neighbors, which caused a compression of the central region and corresponding convex profile.[25] During the drying process, the interlayer forces between the GO sheets increased with volatilization of intercalated solvent and the tubular structure was eventually fixed (Figure 4c).

The morphology of a single tube is schematically illus-trated in Figure  4d–g. SEM images from the top view reveal that the rGO tube array exhibited a well-replicated hexagonal structure from AAO templates, with a uniform wall thickness around 10  nm (Figure  4d). At the bottom of these nanosized rGO tubes, a typical convex profile was observed (Figure  4f), which was generated under the central compressive stress in the imprinting procedure. Notably, GO sheets tended to form random wrinkles during evaporation of the plasticizer, which explains the formation of helical crumples in the TEM images of rGO tubes (Figure 4h,i). In the electron diffraction pattern of the tube wall area, a broad (002) diffraction ring was observed, and no higher-order diffraction (e.g., 004, 006) rings. This result indicates a loss of periodicity along the c-axis direction of the hexagonal lattice structure and a turbostratic lamellar structure owing to less ordered stacking of GO sheets in the confined nanoscale curved space (Figure 4i,j).

2.5. Applications of Carbon Arrays

Carbon materials with a vertical periodic structure are useful for applications as electrodes, selective absorbents and in thermal

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management. These materials are usually prepared by two methods, namely templating[26] and chemical vapor deposition growth.[27] In the templating method, carbon precursors (e.g., polyacrylonitrile, PDMS) are extruded into the designed tem-plate. After demolding and carbonization, the desired structures are obtained. However, the main constituent of carbonized materials is glassy carbon, which has poor electrical conduc-tivity and thermal conductivity. Carbon nanotube forests are usually prepared by chemical vapor deposition methods. Con-ductivity has been improved in recent years; but wider applica-tions are still limited by low packing densities. In this context, it is hard to shape carbon materials into elaborate structures while maintaining their good conductivity and thermal stability.

In our work, carbon arrays were directly prepared from gra-phene derivatives. Highly integrated graphene materials could

be obtained by a high-temperature thermal treatment. The constructed nanoscale tubular structures resulted in distinct surface, optical, and electrical properties (Figure 5a). We evalu-ated the surface wettability by the static sessile drop method. The contact angles decreased from 81° for planar rGO papers (rGOPs) to ≈31° for imprinted rGOPs with nanometer-sized tube arrays. After a further high temperature thermal treatment, the surface of the imprinted graphene papers (GPs) became hydrophobic with a ≈132° static contact angle (Figure  5b, Figure S18, Supporting Information). As electrodes, the verti-cally aligned tubes showed a large exposed active area, smooth electrolyte channels as well as low inner resistance and charge transfer resistance. Nyquist plots were obtained from two-elec-trode double layer capacitor cells, which were constructed with symmetrically imprinted rGOP electrodes in 1 m H2SO4 over

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Figure 4. Microimprinting of Hp-GOPs. a) Schematic of microimprinting process with through-hole AAO template with 390 nm diameter. b) Length–diameter ratio versus holding time under constant applied force (35 kN). c) Schematic showing plastic flow traces and sheet alignment in nanoscale AAO cavity. d–g) SEM images of imprinted nanosized rGO tube morphologies from top (d), bottom (f), and side (g) views. e) Schematic of single nanosized rGO tube. h,i) TEM images of nanosized rGO tubes with helical crumples. j) Selected-area diffraction pattern of tube walls in (i).

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the frequency range from 100  kHz to 0.01  Hz (Figure  5c). A Nyquist plot of the imprinted rGOP showed a nearly vertical curve, indicating an ideal capacitive performance. In the equiv-alent circuit, the internal resistance (Rs) and charge transfer resistance (Rct) of imprinted rGOP were 1 and 7 Ω, respectively, whereas Rs and Rct of the planar rGOPs were 5 and 23 Ω. These low resistance values indicate the high electron conductivity along the imprinted tubes and high-speed ion diffusion within the channels. Carbon materials are ideal black materials for thermal management due to their high light absorption and excellent thermal stability. The imprinted GP with vertically aligned tube arrays acted as ultra-black absorbers with reflec-tance well below 1% in the visible light range (Figure  5d). By comparison, planar GP exhibited a reflectance of over 20%. The superior antireflection properties of the nanosized tube arrays derived from multiple reflections within the upright channels. The low reflectance of the imprinted GPs imparted a higher efficiency for solar energy photothermal conversion. Under one sun illumination, the temperature of the imprinted GP was

62 °C at equilibrium state, which was 17 °C higher than that of planar GP (Figure 5e).

3. Conclusion

We have reported a general strategy of an intercalation-induced hydroplasticity in 2D assemblies, which enables direct plastic processing of 2D layered solids with micrometer-scale preci-sion. Solvent intercalation weakens the interlayer attractions and activates block slippage, resulting in a near-solid plastic state. This hydroplasticity endows 2D sheets with good deform-ability under complex deformations at the micrometer scale, enabling direct micromolding of 2D layered solids. 2D assem-blies with fine structures including origami, embossing, and periodic arrays, over size resolutions ranging from 200  µm to 390  nm, are achieved by applying different templates. The hydroplastic reprocessing of 2D assemblies shows high size accuracy, controllability, efficiency, and general feasibility. This

Figure 5. The altered surface functionalities of the imprinted papers with nanosized tube arrays. a) Schematics of distinct surface, optical, and electrical properties. b) Droplet profiles and static contact angle on imprinted rGOP and imprinted GP, exhibiting Wenzel and Cassie contact states respectively. c) Nyquist plots of planar and imprinted rGOP electrodes. Insets are the Randles’ equivalent circuit (left) and SEM images of imprinted rGOP (right). d) Measured total hemispherical absorptivity of planar GP and imprinted GP. The insets are the corresponding optical images. e) Infrared images of planar and imprinted GP under one sun illumination.

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plastic forming technology may open up new possibilities for tunable structural design of 2D sheets and facilitate their wider applications in catalysis, nanoelectronics, and optical devices.

4. Experimental SectionPreparation of Hp-GOP: Aqueous GO dispersion was purchased

from Hangzhou Gaoxi Technology Co. Ltd (www.gaoxitech.com). The average size of the GO was measured as 9.4 µm (Figure S1, Supporting Information). Typically, GO suspension (6 mg g−1, 10 mL) was cast-dried at ambient temperature with 50% relative humidity for slow dehydration. The freestanding dried GOP was then immersed into ethanol for about 5 min to get Hp-GOP in a saturated plastic state.

Microimprinting Process: The as-prepared Hp-GOP was placed on a flat and smooth plate and covered with AAO template. Typically, the diameter of AAO template is 13  mm and periodic through-hole size is ≈390  nm. A constant loading rate of 0.1  mm min−1 was applied on Hp-GOP/AAO pile until the maximum force reached 35  kN (≈263.8  MPa). Hold the maximum stress for a certain time. The composite disk was then reduced by hydriodic acid at 90 °C for 10 h. After reduction, the disk was immersed into phosphoric acid solution (H3PO4, 3 wt %) at 60 °C for 300 min. The demolded sample was soaked in ethanol (4 h, twice) and n-hexane (4 h, twice) successively for thoroughly solvent replacement. Finally, the air-dried sample was placed into a vacuum oven to remove residual solvent.

Characterization Methods: Mechanical tests were taken on REGER-6000. SEM inspections were taken on Hitachi S4800 field emission system. TEM images of nanosized tubes were taken on a JEM-2100 HR-TEM. Nanoindentation curves were taken on Piuma Chiaro Optics11 BV. XPS spectra were collected on Axis supra. X-ray diffraction data were collected on a X’Pert Pro (PANalytical) diffractometer using monochromatic Cu 17 Kα1 radiation (λ = 1.5406 Å) at 40 kV. The Ocean Optics STS-VIS spectrometer mounted with an integrating sphere ISPREF was used to measure the hemispherical (specular and diffuse) reflectance from 450  to 850  nm. The reflectance was normalized to a standard whiteboard. The normal incident reflectance was measured with a 15× objective with NA = 0.4.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was financially supported by the National Key R&D Program of China (No.2016YFA0200200), the Fundamental Research Funds for the Central Universities (Nos. 30920041106 and 30919011271), the National Natural Science Foundation of China (Nos. 51973191, 52090030, 51703194, and 51803177), the National Key R&D Program of China (No. 2016YFA0200200), the Hundred Talents Program of Zhejiang University (No. 188020*194231701/113), the Fujian Provincial Science and Technology Major Projects (No. 2018HZ0001-2), the Key Research and Development Plan of Zhejiang Province (No. 2018C01049), the Key Laboratory of Novel Adsorption and Separation Materials and Application Technology of Zhejiang Province (No. 512301-I21502), the Fundamental Research Funds for the Central Universities (No. K20200060). Thanks for the technical support by the Core Facilities, State Key Laboratory of modern optical instruments, Zhejiang University.

Conflict of InterestThe authors declare no conflict of interest.

Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.

Keywords2D materials, high design flexibility, high precision, hydroplasticity, microimprinting

Received: December 1, 2020Revised: March 22, 2021

Published online:

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