Metal-Organic Framework-Based Nanomaterials for Electrocatalytic Oxygen Evolution

Oxygen evolution reaction (OER) is an energy-determined half-reaction for water splitting and many other energy conversion processes, such as rechargeable metal-air batteries and CO₂ reduction, due to its four-electron sluggish process. To reduce the energy consumption and cost of these advanced technologies, various transition metal-based nanomaterials, like metal oxides/hydroxides, nitride, and phosphide are synthesized. Among these, metal-organic framework (MOF)-based materials are considered as the ideal candidate for the fabrication of efficient OER electrocatalysts owing to their unique physicochemical properties. In this review, the fundamental catalytic mechanisms and key evaluation parameters of OER in acidic and alkaline media are presented first. Then, design strategies for MOF-based OER catalysts and research progress in the study of the structure-performance relationship are summarized. Subsequently, the recent research advances of MOF-based OER electrocatalysts in alkaline, acidic, and neutral electrolytes are overviewed. Finally, current challenges and future opportunities are provided under the frame of materials design, theoretical understanding, advanced characterization techniques, and industrial applications.

Exceptional Deformability of Wurtzite Zinc Oxide Nanowires with Growth Axial Stacking Faults

To ensure reliability and facilitate the strain engineering of zinc oxide (ZnO) nanowires (NWs), it is significant to understand their flexibility thoroughly. In this study, single-crystalline ZnO NWs with rich axial pyramidal I (π₁) and prismatic stacking faults (SFs) are synthesized by a metal oxidation method. Bending properties of the as-synthesized ZnO NWs are investigated at the atomic scale using an in situ high-resolution transmission electron microscopy (HRTEM) technique. It is revealed that the SF-rich structures can foster multiple inelastic deformation mechanisms near room temperature, including active axial SFs' migration, deformation twinning and detwinning process in the NWs with growth π₁ SFs, and prevalent nucleation and slip of perfect dislocations with a continuous increased bending strain, leading to tremendous bending strains up to 20% of the NWs. Our results record ultralarge bending deformations and provide insights into the deformation mechanisms of single-crystalline ZnO NWs with rich axial SFs.

Atomic-Scale Fabrication of In-Plane Heterojunctions of Few-Layer MoS2 via In Situ Scanning Transmission Electron Microscopy

Layered MoS₂ is a prospective candidate for use in energy harvesting, valleytronics, and nanoelectronics. Its properties strongly related to its stacking configuration and the number of layers. Due to its atomically thin nature, understanding the atomic-level and structural modifications of 2D transition metal dichalcogenides is still underdeveloped, particularly the spatial control and selective precision. Therefore, the development of nanofabrication techniques is essential. Here, an atomic-scale approach used to sculpt 2D few-layer MoS₂ into lateral heterojunctions via in situ scanning/transmission electron microscopy (STEM/TEM) is developed. The dynamic evolution is tracked using ultrafast and high-resolution filming equipment. The assembly behaviors inherent to few-layer 2D-materials are observed during the process and included the following: scrolling, folding, etching, and restructuring. Atomic resolution STEM is employed to identify the layer variation and stacking sequence for this new 2D-architecture. Subsequent energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy analyses are performed to corroborate the elemental distribution. This sculpting technique that is established allows for the formation of sub-10 nm features, produces diverse nanostructures, and preserves the crystallinity of the material. The lateral heterointerfaces created in this study also pave the way for the design of quantum-relevant geometries, flexible optoelectronics, and energy storage devices.

Nanowire Length, Density, and Crystalline Quality Retrieved from a Single Optical Spectrum

We propose spectral domain attenuated reflectometry (SDAR) for fast characterization of nanomaterial growth. The method is demonstrated here for zinc oxide (ZnO) nanowires (NWs) which are grown vertically in random forest fashion showing that it is not limited to well-ordered NWs. We show how SDAR can provide, on the basis of a single measured spectrum, simultaneous information on nanowire length, nanowire density (through nanowire/air filling ratio), and crystalline quality (through band gap). The robustness of the proposed method is assessed first through comparison with information obtained from SEM and XRD taken as reference. In SDAR, the process for fast extraction of NW thickness and filling ratio values  makes use of the interference pattern contrast and the spectral periodicity in the reflection response which involve a best fit of the measured spectra with simple theoretical modeling based on the effective medium approach, achieved with a mean square error down to 0.1%. The results also suggest the existence of either 2 or 3 layers of different effective refractive index, hence providing insight on possible growth mechanisms.

Synthesis and modelling of the mechanical properties of Ag, Au and Cu nanowires

The recent interest to nanotechnology aims not only at device miniaturisation, but also at understanding the effects of quantised structure in materials of reduced dimensions, which exhibit different properties from their bulk counterparts. In particular, quantised metal nanowires made of silver, gold or copper have attracted much attention owing to their unique intrinsic and extrinsic length-dependent mechanical properties. Here we review the current state of art and developments in these nanowires from synthesis to mechanical properties, which make them leading contenders for next-generation nanoelectromechanical systems. We also present theories of interatomic interaction in metallic nanowires, as well as challenges in their synthesis and simulation.

Synergistic additive-mediated CVD growth and chemical modification of 2D materials

This review summarizes significant advances in the use of typical synergistic additives in growth of 2D materials with chemical vapor deposition, and the corresponding performance improvement of field effect transistors and photodetectors.

Interlayer electrical resistivity of rotated graphene layers studied by in-situ scanning electron microscopy

Interlayer electrical transport between two-dimensional atomic crystals can be strongly modulated by the rotational misalignment between them. However, the experimental study on the interlayer electrical transport between rotated two-dimensional atomic crystals with variable rotation angles is challenging. Here, an in-situ scanning electron microscopy method is developed to study the interlayer electrical transport between rotated graphene layers. We employ nanoprobes installed in a scanning electron microscope to function as both "fingers" to induce interlayer rotation of a microfabricated metal-graphite-metal sandwiched island and also electrical probes to measure interlayer electrical resistivity of the rotated graphene layers. Interlayer electrical resistivity of the rotated graphene layers is found to increase monotonically by three orders of magnitude from ∼0.1 to ∼100 Ω cm when the rotational misalignment angle increases from 0° to 30°. This phenomenon can be well described by phonon-mediated electrical transport model. The large-magnitude tunability of interlayer electrical resistivity by mechanical rotation implies the potential applications of rotated graphene layers in nanoelectromechanical systems. Our results also provide a method for studying and tuning interlayer electrical transport between rotated two-dimensional atomic crystals.

Simultaneous atomic-level visualization and high precision photocurrent measurements on photoelectric devices by in situ TEM

Herein, a novel in situ transmission electron microscopy (TEM) method that allows high-resolution imaging and spectroscopy of nanomaterials under simultaneous application of different stimuli, such as light excitation, has been reported to directly explore structure–activity relationships targeted towards device optimization. However, the experimental development of a photoelectric system capable of combining atomic-level visualization with simultaneous electrical current measurement with picoampere-precision still remains a great challenge due to light-induced drift while imaging and noise in the electrical components due to background current. Herein, we report a novel photoelectric TEM holder integrating an LED light source covering the whole visible range, a shielding system to avoid current noise, and a picoammeter, which enables stable TEM imaging at the atomic scale while measuring very small photocurrents (pico ampere range). Using this high-precision photoelectric holder, we measured photocurrents of the order of pico amperes for the first time from a prototype quantum dot solar cell assembled inside a TEM and obtained atomic-level imaging of the photo anode under light exposure. This study paves the way towards obtaining mechanistic insights into the operation of photovoltaic devices by providing direct information on the structure–activity relationships that can be used in device optimization.

A photoelectric system is capable of simultaneous atomic-level visualization and pico-ampere-precision.

Optical and Optoelectronic Property Analysis of Nanomaterials inside Transmission Electron Microscope

In situ transmission electron microscopy (TEM) allows one to investigate nanostructures at high spatial resolution in response to external stimuli, such as heat, electrical current, mechanical force and light. This review exclusively focuses on the optical, optoelectronic and photocatalytic studies inside TEM. With the development of TEMs and specialized TEM holders that include in situ illumination and light collection optics, it is possible to perform optical spectroscopies and diverse optoelectronic experiments inside TEM with simultaneous high resolution imaging of nanostructures. Optical TEM holders combining the capability of a scanning tunneling microscopy probe have enabled nanomaterial bending/stretching and electrical measurements in tandem with illumination. Hence, deep insights into the optoelectronic property versus true structure and its dynamics could be established at the nanometer-range precision thus evaluating the suitability of a nanostructure for advanced light driven technologies. This report highlights systems for in situ illumination of TEM samples and recent research work based on the relevant methods, including nanomaterial cathodoluminescence, photoluminescence, photocatalysis, photodeposition, photoconductivity and piezophototronics.

Nanofiber electrospinning in samarium complex-doped PMMA

Herein, an electrospinning process of samarium complex-doped PMMAs was carried out to fabricate ultrafine fibers with a uniform diameter of about 230 nm.

Evidence for Core Oxygen Dynamics and Exchange in Metal Oxide Nanocrystals from In Situ 17O MAS NMR

Long-term stability of the properties of nanocrystals (NCs) is of paramount importance for any applicative development. However, these are jeopardized by chemical and structural alterations of the NCs induced by the environment and the working conditions. Among the species that alter the NCs properties, water molecules are of tremendous importance. We used ¹⁷O solid-state NMR spectroscopy to follow this process and the dynamics of O atoms in metal oxide NCs. Using ZnO as reference material, different chemical environments for the O atoms are characterized and a dynamic exchange process between the NCs and the O atoms from water is evidenced. The exchange does not involve only surface atoms but also ones located deeper inside the ZnO core of the NCs. Finally, a postsynthesis process based on watering/drying cycles is proposed that may greatly improve the long-term stability of metal oxide NCs.