Hierarchical Metal Oxide Topographies Replicated from Highly Textured Graphene Oxide by Intercalation Templating

Two-Dimensional Oxide Crystals for Device Applications: Challenges and Opportunities

Atomically thin two-dimensional (2D) oxide crystals have garnered considerable attention because of their remarkable physical properties and potential for versatile applications. In recent years, significant advancements have been made in the design, preparation, and application of ultrathin 2D oxides, providing many opportunities for new-generation advanced technologies. This review focuses on the controllable preparation of 2D oxide crystals and their applications in electronic and optoelectronic devices. Based on their bonding nature, the various types of 2D oxide crystals are first summarized, including both layered and nonlayered crystals, as well as their current top-down and bottom-up synthetic approaches. Subsequently, in terms of the unique physical and electrical properties of 2D oxides, recent advances in device applications are emphasized, including photodetectors, field-effect transistors, dielectric layers, magnetic and ferroelectric devices, memories, and gas sensors. Finally, conclusions and future prospects of 2D oxide crystals are presented. It is hoped that this review will provide comprehensive and insightful guidance for the development of 2D oxide crystals and their device applications.

Reversible flowering of CuO nanoclusters via conversion reaction for dual-ion Li metal batteries

Dual-ion Li metal batteries based on non-flammable SO₂-in-salt inorganic electrolytes ( Li-SO₂ batteries) offer high safety and energy density. The use of cupric oxide (CuO) as a self-activating cathode material achieves a high specific capacity with cost-effective manufacturing in Li-SO₂ batteries, but its cycle retention performance deteriorates owing to the significant morphological changes of the cathode active materials. Herein, we report the catalytic effect of carbonaceous materials used in the cathode material of Li-SO₂ batteries, which act as templates to help recrystallize the active materials in the activation and conversion reactions. We found that the combination of oxidative-cyclized polyacrylonitrile (PAN) with N-doped carbonaceous materials and multi-yolk-shell CuO (MYS-CuO) nanoclusters as cathode active materials can significantly increase the specific capacity to 315.9 mAh g^- 1 (93.8% of the theoretical value) at 0.2 C, which corresponds to an energy density of 1295 Wh kg_CuO^-1, with a capacity retention of 84.46% at the 200th cycle, and the cathode exhibited an atypical blossom-like morphological change.

Snap-through of graphene nanowrinkles under out-of-plane compression

Nanowrinkles (i.e. the buckled nanoribbons) are widely observed in nano-devices assembled by two-dimensional (2D) materials. The existence of nanowrinkles significantly affects the physical (such as mechanical, electrical and thermal) properties of 2D materials, and thus further, impedes the applications of those devices. In this paper, we take the nanowrinkle formed in a monolayer graphene as a model system to study its deformation behaviours, especially the configuration evolution and the snap-through buckling instabilities, when subjected to the out-of-plane compression. By performing molecular dynamics simulation, the graphene nanowrinkles with or without self-adhesion (which are notated as 'clipped' state or 'bump' state, respectively) are obtained depending on the geometric size and the applied axial compressive pre-strain. The elastica theory is employed to quantify the shape of 'bump' nanowrinkles, as well as the critical condition of the transition between 'clipped' and 'bump' states. By applying out-of-plane compression to the generated graphene nanowrinkle, it flips to an opposite configuration via snap-through buckling. We identify four different buckling modes according to the configuration evolution. An unified phase diagram is constructed to describe those buckling modes. For the cases with negligible van der Waals interaction getting involved in the snap-buckling process, i.e. without self-adhesion, the force-displacement curves for nanowrinkles with same axial pre-strain but different sizes can be scaled to collapse. Moreover, the critical buckling loads can also be scaled and predicted by the extended elastica theory. Otherwise, for the cases with self-adhesion, which corresponds to the greater axial pre-strain, the van der Waals interaction makes the scaling collapse break down. It is expected that the analysis about the snap-through buckling of graphene nanowrinkles reported in this work will advance the understanding of the mechanical behaviours of wrinkled 2D materials and promote the design of functional nanodevices, such as nanomechanical resonators and capacitors.

Structural Integrity Preserving and Residue-Free Transfer of Large-Area Wrinkled Graphene onto Polymeric Substrates

Wrinkled graphene offers many advantageous features resulting from modifying the structural and physical properties as well as the chemical reactivity of graphene. However, its inadequate transferability to other substrates has limited its usability. This paper reports a roll-based clean transfer approach that enables the damage-free and contamination-free transfer of large-area wrinkled graphene onto polymeric substrates without compromising the integrity of wrinkle structures. The method implements the simultaneous imidazole-assisted etching and doping of chemical vapor-deposited graphene to fabricate multilayer graphene on a thermoplastic polystyrene (PS) substrate coated with a water-soluble poly(4-styrenesulfonic acid) (PSS) sacrificial layer via a roll-based transfer process. The compliant PSS layer affords the conformal contact between the PS substrate and graphene during the wrinkle formation process, enabling the controllable fabrication of graphene wrinkle structures on a large area. The water-soluble properties of PSS simplify the typically difficult separation of wrinkled graphene from the PS substrate after its transfer onto a target substrate. This improves the transferability of wrinkled graphene, rendering the transfer process solvent-free and residue-free. This work demonstrates the feasibility of the formulated method by transferring centimeter-scale wrinkled graphene onto currently used transparent flexible substrates (i.e., polyethylene terephthalate and polydimethylsiloxane). The results indicate that the transferred wrinkled graphene possesses the desirable combination of superior stretchability, optical transmittance, sheet resistance, and electromechanical stability, rendering its suitable application to transparent flexible and stretchable electronics.

Controlled preparation and gas sensitive properties of two-dimensional and cubic structure ZnSnO3

Two-dimensional (2D) ZnSnO₃ is a promising candidate for future gas sensors due to its high chemical response and excellent electronic properties. However, the preparation of 2D ZnSnO₃ nanosheets by utilizing soluble inorganic salts and nonorganic solvents remains a challenge. In this work, 2D ZnSnO₃ was synthesized via a facile graphene oxide (GO)-assisted co-precipitation method, in which inorganic salts in the aqueous phase replaced metal organic salts in a non-aqueous system. Meanwhile, a "dissolution and recrystallization" mechanism was proposed to explain the transformation from 3D nanocubes to 2D nanosheets. In comparison, the 2D ZnSnO₃ nanosheets showed a higher response to formaldehyde (HCHO) at low operating temperature (100 °C). The response (R_a/R_g) of the 2D ZnSnO₃ sensor to 10 ppm HCHO was as high as 57, which was approximately 5 times the response of the ZnSnO₃ nanocubes sensor. However, the ZnSnO₃ nanocubes sensor showed better gas sensing performance to ethanol at high temperature (200 °C). Different gas-sensitive properties were attributed to the different gas diffusion and adsorption processes caused by the morphology and nanostructure. Moreover, both sensors could detect either 0.1 ppm HCHO or ethanol at their optimum operating temperature. This work presents a relatively economical method to prepare 2D compound metal oxides, provides a novel "dissolution and recrystallization" mechanism for 2D multi-metal oxide preparation, and sheds light on the great potential of high-efficiency HCHO and/or ethanol gas sensors.

Electronic Modulation of Non-van der Waals 2D Electrocatalysts for Efficient Energy Conversion

The exploration of efficient electrocatalysts for energy conversion is important for green energy development. Owing to their high surface areas and unusual electronic structure, 2D electrocatalysts have attracted increasing interest. Among them, non-van der Waals (non-vdW) 2D materials with numerous chemical bonds in all three dimensions and novel chemical and electronic properties beyond those of vdW 2D materials have been studied increasingly over the past decades. Herein, the progress of non-vdW 2D electrocatalysts is critically reviewed, with a special emphasis on electronic structure modulation. Strategies for heteroatom doping, vacancy engineering, pore creation, alloying, and heterostructure engineering are analyzed for tuning electronic structures and achieving intrinsically enhanced electrocatalytic performances. Lastly, a roadmap for the future development of non-vdW 2D electrocatalysts is provided from material, mechanism, and performance viewpoints.

Controlling nanochannel orientation and dimensions in graphene-based nanofluidic membranes

There is great interest in exploiting van der Waals gaps in layered materials as nanofluidic channels. Graphene oxide (GO) nanosheets are known to spontaneously assemble into stacked planar membranes with transport properties that are highly selective to molecular structure. Use of conventional GO membranes in liquid-phase applications is often limited by low flux values, due to intersheet nanochannel alignment perpendicular to the desired Z-directional transport, which leads to circuitous fluid pathways that are orders of magnitude longer than the membrane thickness. Here we demonstrate an approach that uses compressive instability in Zr-doped GO thin films to create wrinkle patterns that rotate nanosheets to high angles. Capturing this structure in polymer matrices and thin sectioning produce fully dense membranes with arrays of near-vertically aligned nanochannels. These robust nanofluidic devices offer pronounced reduction in fluid path-length, while retaining the high selectivity for water over non-polar molecules characteristic of GO interlayer nanochannels.

Vertically stacked graphene oxide sheets are promising structures for molecular sieving technologies. By folding large planar sheets in an accordion-like manner, Liu et al. fabricate a thin robust filter with near-vertically aligned nanochannels geared towards commercial separation membranes.

Synergistic Antimicrobial Titanium Carbide (MXene) Conjugated with Gold Nanoclusters

Bacterial resistance toward antibiotics is a world-wide problem, and one potential solution to fight against the resistance is to develop multi-mechanism antimicrobial agents to achieve synergistic performance. Titanium carbide (MXene) is an emerging 2D nanomaterial with antimicrobial ability to physically damage bacterial membrane and chemically induce oxidative stress, and it can be further conjugated with nanomaterials to improve its antibacterial performance. Herein, a synergistic antimicrobial agent is developed through conjugation of the ultra-small gold nanoclusters (AuNCs) on MXene nanosheets. The conjugated AuNCs are effectively delivered into bacteria after bacterial membrane damage caused by MXene, generating localized reactive oxygen species (ROS) of high concentration to effectively oxidize bacterial membrane lipid for enhanced membrane broken, as well as bacterial DNA for violent fragmentation. Thus, the synergistic physical (via MXene) and chemical (via MXene and AuNCs) antimicrobial mechanisms lead to eventual bacterial death of both Gram-positive and Gram-negative bacteria, with low IC₅₀ values of 11.7 µg mL^-1 of MXene and 0.04 µm of AuNCs. Moreover, the crumpled MXene-AuNCs structure is constructed to inhibit biofilm formation, which hold synergistic antibacterial ability of MXene-AuNCs conjugation, hydrophobic surface to prevent bacterial attachment, and large surface area containing higher density of bactericides.

Recent advances in the template-confined synthesis of two-dimensional materials for aqueous energy storage devices

The template-confined synthesis strategy is a simple and effective methodology to prepare two-dimensional nanomaterials. It has multiple advantages including green process, controllable morphology and adjustable crystal structure, and therefore, it is promising in the energy storage realm to synthesize high-performance electrode materials. In this review, we summarize the recent advances in the template-confined synthesis of two-dimensional nanostructures for aqueous energy storage applications. The material design is discussed in detail to accommodate target usage in aqueous supercapacitors and zinc metal batteries. The remaining challenges and future prospective are also covered.

Bioinspired Multiscale Wrinkling Patterns on Curved Substrates: An Overview

The surface wrinkling of biological tissues is ubiquitous in nature. Accumulating evidence suggests that the mechanical force plays a significant role in shaping the biological morphologies. Controlled wrinkling has been demonstrated to be able to spontaneously form rich multiscale patterns, on either planar or curved surfaces. The surface wrinkling on planar substrates has been investigated thoroughly during the past decades. However, most wrinkling morphologies in nature are based on the curved biological surfaces and the research of controllable patterning on curved substrates still remains weak. The study of wrinkling on curved substrates is critical for understanding the biological growth, developing three-dimensional (3D) or four-dimensional (4D) fabrication techniques, and creating novel topographic patterns. In this review, fundamental wrinkling mechanics and recent advances in both fabrications and applications of the wrinkling patterns on curved substrates are summarized. The mechanics behind the wrinkles is compared between the planar and the curved cases. Beyond the film thickness, modulus ratio, and mismatch strain, the substrate curvature is one more significant parameter controlling the surface wrinkling. Curved substrates can be both solid and hollow with various 3D geometries across multiple length scales. Up to date, the wrinkling morphologies on solid/hollow core-shell spheres and cylinders have been simulated and selectively produced. Emerging applications of the curved topographic patterns have been found in smart wetting surfaces, cell culture interfaces, healthcare materials, and actuators, which may accelerate the development of artificial organs, stimuli-responsive devices, and micro/nano fabrications with higher dimensions.

Mechanochemical engineering of 2D materials for multiscale biointerfaces

Atomically thin nanomaterials represent a unique paradigm for interfacing with biological systems due to their mechanical flexibility, exceptional interfacial area, and ease of chemical functionalization. In particular, these two-dimensional (2D) materials are able to bend, curve, and fold in response to biologically-generated forces or other external stimuli. Such origami-like folding of 2D materials into wrinkled or crumpled topographies allows them to withstand large deformations by accordion-like unfolding, with implications for stretchable and shape-changing devices. Here, we review how mechanically manipulated 2D materials can interact with biological systems across a multitude of length scales. We focus on recent work where wrinkling, crumpling, or bending of 2D materials permits new chemical and material properties, with four case studies: (i) programming biomolecular reactivity and enhanced sensing, (ii) directed adhesion and encapsulation of bacteria or mammalian cells, (iii) stimuli-responsive actuators and soft robotics, and (iv) stretchable barrier technologies and wearable human-scale sensors. Finally, we consider future directions for manufacturing, materials and systems integration, as well as biocompatibility. Taken together, these 2D materials may enable new avenues for ultrasensitive molecular detection, biomaterial scaffolds, soft machines, and wearable technologies.

Multifunctional metallic backbones for origami robotics with strain sensing and wireless communication capabilities

The tight integration of actuation, sensing, and communication capabilities into origami robots enables the development of new-generation functional robots. However, this task is challenging because the conventional materials (e.g., papers and plastics) for building origami robots lack design opportunities for incorporating add-on functionalities. Installing external electronics requires high system integration and inevitably increases the robotic weight. Here, a graphene oxide (GO)–enabled templating synthesis was developed to produce reconfigurable, compliant, multifunctional metallic backbones for the fabrication of origami robots with built-in strain sensing and wireless communication capabilities. The GO-enabled templating synthesis realized the production of complex noble metal origamis (such as Pt) with high structural replication of their paper templates. The reproduced Pt origami structures were further stabilized with thin elastomer, and the Pt-elastomer origamis were reconfigurable and served as the multifunctional backbones for building origami robots. Compared with traditional paper and plastic materials, the reconfigurable Pt backbones were more deformable, fire retardant, and power efficient. In addition, the robots with conductive Pt-elastomer backbones (Pt robots) demonstrated distinct capabilities—such as on-demand resistive heating, strain sensing, and built-in antennas—without the need for external electronics. The multifunctionality of Pt robots was further demonstrated to extend beyond the capabilities of traditional paper-based robots, such as melting an ice cube to escape, monitoring/recording robotic motions in real time, and wireless communications between robots. The development of multifunctional metallic backbones that couple actuation, sensing, and communication enriches the material library for the fabrication of soft robotics toward high functional integration.

Ti-Doped Tunnel-Type Na4Mn9O18 Nanoparticles as Novel Anode Materials for High-Performance Supercapacitors

Nanomaterials with tunnel structures are extremely attractive to be used for electrode materials in electrochemical energy storage devices. Tunnel-structured Ti-doped Na₄Mn₉O₁₈ nanoparticles (TNMO-NPs) were synthesized by a facile and high-production method of the solid-state reaction with a high-energy ball-milling process. As electrode materials in the supercapacitor cell, the as-synthesized TNMO-NPs exhibit a high specific capacity of 284.93 mA h g^-1 (0.57 mA h cm^-2/1025.75 F g^-1). A superior rate capability with a decay of 36% is achieved by increasing the scan rates from 2 to 25 mV s^-1. To further explore the storage mechanism of Ti-doped Na₄Mn₉O₁₈ materials, density functional theory (DFT) calculations were used to calculate the activation energy for the ion immigration in the electrode, and the results show that the minimum ion diffusion barrier energy is 0.272 eV, indicating that the sodium ions could insert into the system easily. Through the scan-rate-dependent cyclic voltammetry analysis, the capacity value indicates a mixed charge storage of capacitive behavior and Na⁺ intercalation progress. A maximum energy density of 77.81 W h kg^-1 at a power density of 125 W kg^-1 is achieved, and a high energy density of 54.79 W h kg^-1 is maintained even at an ultrahigh power density of 3750 W kg^-1. The TNMO-NP supercapacitors show excellent flexibility at various bent (0-180°) states. The capacitive performance of the TNMO-NPs makes them promising cathode materials for flexible supercapacitors with high specific capacities and high energy densities.

Self-Limiting Growth of Two-Dimensional Palladium between Graphene Oxide Layers

The ability of different materials to display self-limiting growth has recently attracted an enormous amount of attention because of the importance of nanoscale materials in applications for catalysis, energy conversion, (opto)electronics, and so forth. Here, we show that the electrochemical deposition of palladium (Pd) between graphene oxide (GO) sheets result in the self-limiting growth of 5-nm-thick Pd nanosheets. The self-limiting growth is found to be a consequence of the strong interaction of Pd with the confining GO sheets, which results in the bulk growth of Pd being energetically unfavorable for larger thicknesses. Furthermore, we have successfully carried out liquid exfoliation of the resulting Pd-GO laminates to isolate Pd nanosheets and have demonstrated their high efficiency in continuous flow catalysis and electrocatalysis.

Self-Assembled Functionally Graded Graphene Films with Tunable Compositions and Their Applications in Transient Electronics and Actuation

The facile fabrication of functionally graded all-graphene films using a single-step casting process is reported. The films consist of a self-assembled graphene oxide (GO) precursor that can be reduced to different levels on an active metal substrate. Control of processing conditions such as the underlying substrate metal and the film-drying environment results in an ability to tailor the internal architecture of the films as well as to functionally grade the reduction of GO. A gradient arrangement within each film, where one side is electrically conductive reduced GO (rGO) and the other side is insulating GO, was confirmed by scanning electron microscopy, Raman, X-ray diffraction, Fourier transform infrared, and X-ray photoelectron spectroscopy characterization studies. All-graphene-based freestanding films with selectively reduced GO were used in transient electronic applications such as flexible circuitry and RFID tag antennas, where their decommissioning is easily achieved by capitalizing on GO's ability to readily dissociate and create a stable suspension in water. Furthermore, the functionally graded structure was found to exhibit differential swelling behavior, and its potential applications in graphene-based actuators are outlined.

Graphene Oxide-Enabled Synthesis of Metal Oxide Origamis for Soft Robotics

Origami structures have been widely applied in various technologies especially in the fields of soft robotics. Metal oxides (MOs) have recently emerged as unconventional backbone materials for constructing complex origamis with distinct functionalities. However, the MO origami structures reported in the literature were rigid and not deformable, thus limiting their applications to soft robotics. Herein, we reported a graphene oxide (GO)-enabled templating synthesis to produce complex MO origami structures from their paper origami templates with high structural replication. The MO origami structures were next stabilized with elastomer, and the MO-elastomer origamis were able to be adapted into multiple actuation systems (including magnetic fields, shape-memory alloys, and pneumatics) for the fabrication of MO origami robots. Compared with conventional paper origami robots, the MO robots were lightweight, mechanically compliant, fire-retardant, magnetic responsive, and power efficient. We further demonstrated the legendary phoenix-fire-reborn concept in the soft robotics fields: a paper origami robot sacrificed itself in a fire scene and transformed itself into a downsized Al₂O₃ robot; the Al₂O₃ robot was able to crawl through a narrow tunnel where the original paper robot was unfit. These MO reconfigurable origamis provide an expanded material library for building soft robotics, and the functionalities of MO robots can be systematically engineered via the intercalation of various metal ions during the GO-enabled synthesis.

A direct route to activated two-dimensional cobalt oxide nanosheets for electrochemical energy storage, catalytic and environmental applications

Two-dimensional Co₃O₄ nanosheets have emerged as attractive materials for use in a number of relevant technological applications. To exhibit a competitive performance in such uses, however, their structure needs to be activated, which is frequently accomplished via post-synthesis reduction strategies that introduce oxygen vacancies and increase the number of active Co(II) sites. Here, we investigate a direct route for the synthesis of activated Co₃O₄ nanosheets that avoids reduction post-treatments, yielding materials with a high potential towards energy- and environment-related applications. The synthesis relied on an interim amorphous cobalt oxide material with nanosheet morphology, which upon calcination afforded Co₃O₄ nanosheets having Co(II) sites in quantities similar to those usually found for Co₃O₄ nanostructures activated by reduction post-treatments. When tested as electrodes for charge storage, the nanosheets demonstrated a competitive behavior in terms of both capacity and rate capability, e.g., a gravimetric capacity of ∼293 mAh g^-1 at 1 A g^-1 with 57% retention at 60 A g^-1 was measured for nanosheets calcined at 350 °C. The materials were shown to be efficient catalysts for the reduction of nitroarenes (4-nitrophenol and 4-nitroaniline), outperforming other Co₃O₄ nanostructures, as well as effective adsorbents for the removal of organic dyes (methyl orange, methylene blue) from water.

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors

Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300-530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion.

Graphene template-induced growth of single-crystalline gold nanobelts with high structural tunability

Assembling Au nanocrystals with tunable dimensions and shapes on graphene templates has attracted increasing attention recently. However, directly growing anisotropic Au nanobelts on a graphene support has been rarely reported. Here, a facile, one-pot, and surfactant-free route is demonstrated to synthesize well-defined Au nanobelts with the induction of a multilayer graphene (mlG) template. The obtained Au nanobelts are single-crystalline with a preferable (111) orientation. More importantly, their structural evolution starting from Au clusters is systematically investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results confirm that mlG consistently induces the growth of Au nanobelts from nucleation to the growth completion. The interfacial interaction between Au atoms and the graphene lattice is a predominant factor to direct the shapes and structures of Au nanocrystals, which makes the structures of Au nanobelts highly tunable with the surface modification of the mlG template. The assembly of mlG-Au nanobelts also presents extraordinary detection sensitivity when employed as a flexible surface-enhanced Raman scattering (SERS) substrate, suggesting their great potential application in high-performance sensors. This report strengthens the fundamental understanding of the interactions between noble metals and carbon interfaces, which paves the way to construct and manipulate the complex structures of metals on graphitic substrates.

Ultrastretchable Graphene-Based Molecular Barriers for Chemical Protection, Detection, and Actuation

A wide range of technologies requires barrier films to impede molecular transport between the external environment and a desired internal microclimate. Adding stretchability to barrier films would enable the applications in packaging, textiles, and flexible devices, but classical barrier materials utilize dense, ordered molecular architectures that easily fracture under small tensile strain. Here, we show that textured graphene-based coatings can serve as ultrastretchable molecular barriers expandable to 1500% areal strain through programmed unfolding that mimics the elasticity of polymers. These coatings retain barrier function under large deformation and can be conformally applied to planar or curved surfaces, where they are washfast and mechanically robust to cycling. These graphene-polymer bilayer structures also function as sensors or actuators by transducing chemical stimuli into mechanical deformation and electrical resistance change through asymmetric polymer swelling. These results may enable multifunctional fabrics that integrate chemical protection, sensing, and actuation, with further applications as selective barriers, membranes, stretchable electronics, or soft robotics.

Universal Method for Creating Hierarchical Wrinkles on Thin-Film Surfaces

One of the most interesting topics in physical science and materials science is the creation of complex wrinkled structures on thin-film surfaces because of their several advantages of high surface area, localized strain, and stress tolerance. In this study, a significant step was taken toward solving limitations imposed by the fabrication of previous artificial wrinkles. A universal method for preparing hierarchical three-dimensional wrinkle structures of thin films on a multiple scale (e.g., nanometers to micrometers) by sequential wrinkling with different skin layers was developed. Notably, this method was not limited to specific materials, and it was applicable to fabricating hierarchical wrinkles on all of the thin-film surfaces tested thus far, including those of metals, two-dimensional and one-dimensional materials, and polymers. The hierarchical wrinkles with multiscale structures were prepared by sequential wrinkling, in which a sacrificial layer was used as the additional skin layer between sequences. For example, a hierarchical MoS₂ wrinkle exhibited highly enhanced catalytic behavior because of the superaerophobicity and effective surface area, which are related to topological effects. As the developed method can be adopted to a majority of thin films, it is thought to be a universal method for enhancing the physical properties of various materials.

Graphene Inks as Versatile Templates for Printing Tiled Metal Oxide Crystalline Films

There is great interest in exploiting van der Waals gaps in layered materials as confinement reaction vessels to template the synthesis of new nanosheet structures. The gallery spaces in multilayer graphene oxide, for example, can intercalate hydrated metal ions that assemble into metal oxide films during thermal oxidation of the sacrificial graphene template. This approach offers limited control of structure, however, and does not typically lead to 2D atomic-scale growth of anisotropic platelet crystals, but rather arrays of simple particles directionally sintered into porous sheets. Here, a new graphene-directed assembly route is demonstrated that yields fully dense, space-filling films of tiled metal oxide platelet crystals with tessellated structures. The method relies on colloidal engineering to produce a printable "metallized graphene ink" with accurate control of metal loading, grain size/porosity, composition, and micro/nanomorphologies, and is capable of achieving higher metal-carbon ratio than is possible by intercalation methods. These tiled structures are sufficiently robust to create free standing papers, complex microtextured films, 3D shapes, and metal oxide replicas of natural biotextures.

Synthesis and Characterization of Graphene/ITO Nanoparticle Hybrid Transparent Conducting Electrode

The combination of graphene with conductive nanoparticles, forming graphene-nanoparticle hybrid materials, offers a number of excellent properties for advanced engineering applications. A novel and simple method was developed to deposit 10 wt% tin-doped indium tin oxide (ITO) nanoparticles on graphene. The method involved a combination of a solution-based environmentally friendly electroless deposition approach and subsequent vacuum annealing. A stable organic-free solution of ITO was prepared from economical salts of In(NO₃)₃ ^·H₂O and SnCl₄. The obtained ITO nanostructure exhibited a unique architecture, with uniformly dispersed 25-35 nm size ITO nanoparticles, containing only the crystallized In₂O₃ phase. The synthesized ITO nanoparticles-graphene hybrid exhibited very good and reproducible optical transparency in the visible range (more than 85%) and a 28.2% improvement in electrical conductivity relative to graphene synthesized by chemical vapor deposition. It was observed that the ITO nanoparticles affect the position of the Raman signal of graphene, in which the D, G, and 2D peaks were redshifted by 5.65, 5.69, and 9.74 cm^-1, respectively, and the annealing conditions had no significant effect on the Raman signatures of graphene.

From Flatland to Spaceland: Higher Dimensional Patterning with Two-Dimensional Materials

The creation of three-dimensional (3D) structures from two-dimensional (2D) nanomaterial building blocks enables novel chemical, mechanical or physical functionalities that cannot be realized with planar thin films or in bulk materials. Here, we review the use of emerging 2D materials to create complex out-of-plane surface topographies and 3D material architectures. We focus on recent approaches that yield periodic textures or patterns, and present four techniques as case studies: (i) wrinkling and crumpling of planar sheets, (ii) encapsulation by crumpled nanosheet shells, (iii) origami folding and kirigami cutting to create programmed curvature, and (iv) 3D printing of 2D material suspensions. Work to date in this field has primarily used graphene and graphene oxide as the 2D building blocks, and we consider how these unconventional approaches may be extended to alternative 2D materials and their heterostructures. Taken together, these emerging patterning and texturing techniques represent an intriguing alternative to conventional materials synthesis and processing methods, and are expected to contribute to the development of new composites, stretchable electronics, energy storage devices, chemical barriers, and biomaterials.

Graphene and its derivatives as versatile templates for materials synthesis and functional applications

The obvious incongruity between the increasing depletion of fossil fuel and the finite amount of resources has motivated us to seek means to maintain sustainability in our society. Developing renewable and highly efficient energy conversion and storage systems represents one of the most promising and viable methods. Although the efficiency of energy conversion and storage devices depends on various factors, their overall performances strongly rely on the structure and functional properties of materials. Graphene and its derivatives as versatile templates for materials synthesis have garnered widespread interest because of their flexible capability to tune the morphology and structure of functional materials. Herein, we have demonstrated recent progress on graphene and its derivatives as versatile templates for materials synthesis, particularly highlighting the basic fundamental roles of graphene in the materials preparation process. Then, a concise overview of the functional applications of materials obtained from graphene-templated approaches has been presented with a few selected examples to show the wide scope of potential in energy storage and conversion. Finally, a brief perspective and potential future challenges in this burgeoning research area have been discussed.

Tunable in-plane torsional strength of surface functionalized two dimensional nanomaterials

In this paper, the in-plane torsional properties of two dimensional nanomaterials are revealed to be tunable by surface functionalization using molecular dynamics simulations.