Fabrication of Subnanometer-Precision Nanopores in Hexagonal Boron Nitride

Production of biocompatible and stable porous materials, e.g., boron nitride, exhibiting tunable and enhanced porosity is a prerequisite if they are to be employed to address challenges such as drug delivery, molecular separations, or catalysis. However, there is currently very limited understanding of the formation mechanisms of porous boron nitride and the parameters controlling its porosity, which ultimately prevents exploiting the material's full potential. Herein, we produce boron nitride with high and tunable surface area and micro/mesoporosity via a facile template-free method using multiple readily available N-containing precursors with different thermal decomposition patterns. The gases are gradually released, creating hierarchical pores, high surface areas (>1900 m2/g), and micropore volumes. We use 3D tomography techniques to reconstruct the pore structure, allowing direct visualization of the mesopore network. Additional imaging and analytical tools are employed to characterize the materials from the micro- down to the nanoscale. The CO2 uptake of the materials rivals or surpasses those of commercial benchmarks or other boron nitride materials reported to date (up to 4 times higher), even after pelletizing. Overall, the approach provides a scalable route to porous boron nitride production as well as fundamental insights into the material's formation, which can be used to design a variety of boron nitride structures.

Solid-state nanopores have drawn considerable attention for their potential applications in DNA sequencing and nanoparticle analysis. However, fabrication of nanopores, especially those of diameter below 30 nm, requires sophisticated techniques. Here, a versatile method to controllably reduce the diameter of prefabricated large-size pores down to sub-30 nm without greatly increasing the effective pore depth from the original membrane thickness is shown. This method exploits carbon deposition achieved via hydrocarbon evaporation, induced by an incident beam of electrons, and subsequent dissociation of hydrocarbon to solid carbon deposits. The carbon deposition employs a conventional scanning electron microscope equipped with direct visual feedback, along with a stable hydrocarbon source nearby the sample. This work systematically studies how electron beam accelerating voltage, imaging magnification, initial pore size and membrane composition affect the process of pore size reduction. Secondary electrons generated in the membrane material are confirmed to be the main cause of the dissociation of hydrocarbon. Thicker carbon deposited on one side than on the other of the membrane results in an asymmetric nanopore shape and a rectifying ionic transport. A physico-phenomenological model combined with Monte Carlo simulations is proposed to account for the observed carbon deposition behaviors.

In this article, we explore the role of nanotechnology in addressing water scarcity through water desalination. The scope of nanotechnology in water treatment is discussed, emphasizing the potential of 2D nanomaterials such as hexagonal boron nitride (h-BN), graphene, and quantum dots in revolutionizing desalination technologies. Various water desalination techniques, including membrane distillation (MD), solar-powered multi-stage flash distillation (MSF), and multi-effect distillation (MED), are analyzed in the context of nanomaterial applications. The review highlights the energy-intensive nature of conventional water treatment methods and underscores nanomaterials’ potential to enhance efficiency and sustainability in water desalination processes. Challenges facing desalination, such as scalability and environmental impact, are acknowledged, setting the stage for future research directions.

Electrolyte adsorption in graphene and hexagonal boron nitride nanochannels

Two-dimensional (2D) nanomaterials hold significant promise for reducing energy consumption in water desalination. This study investigates the influence of pore size and shape on the slip behavior of saline water at the interface of two promising 2D nanomaterials: hexagonal boron nitride (hBN) and molybdenum disulfide (MoS2). Slip length, a key parameter governing fluid flow at the nanoscale, is highly dependent on interfacial properties. Here, we explore how the pore characteristics in these 2D nanomaterials can impact slip length, aiming to gain a fundamental understanding of the role of pore size and shape in optimizing desalination efficiency. We performed quantum mechanical calculations to compute the partial atomic charges on atoms in hBN and MoS2 containing pores. Our DFT calculations reveal a spatially varying charge distribution on these 2D nanomaterials with pores, which we then incorporate into molecular dynamic simulations to elucidate their influence on the 2D nanomaterial-water interface. Our results reveal a significant impact of pore size on friction for nanomaterials containing hexagonal pores, while pore size had no effect on nanomaterials containing triangular pores. Moreover, friction increases with pores in both materials. This research contributes to the development of efficient and energy-saving desalination technologies through the manipulation of interfacial properties in 2D nanomaterials.

Two dimensional (2D) materials and biomaterials for water desalination; structure, properties, and recent advances

ConspectusThe researchof new porous materials for applications in interfacialprocesses is key to addressing global energy and sustainability challenges.For example, porous materials can be used to store fuels such as hydrogenor methane or to separate chemical mixtures reducing the energy currentlyrequired by thermal separation processes. Their catalytic propertiescan be exploited to convert adsorbed molecules into valuable or lesshazardous chemicals, thereby reducing energy consumption or pollutantsemissions. Porous boron nitride (BN) has appeared as a promising materialfor applications in molecular separations, gas storage, and catalysisowing to its high surface area and thermal stability, as well as itstunable physical properties and chemistry.However, the productionof porous BN is still limited to the laboratoryscale, and its formation mechanism, as well as ways to control porosityand chemistry, are yet to be fully understood. In addition, studieshave pointed toward the instability of porous BN materials when exposedto humidity, which could significantly impact performance in industrialapplications. Studies on porous BN performance and recyclability whenemployed in adsorption, gas storage, and catalysis remain limited,despite encouraging preliminary studies. Moreover, porous BN powdermust be shaped into macrostructures (e.g., pellets) to be used commercially.However, common methods to shape porous materials into macrostructuresoften cause a reduction in the surface area and/or mechanical strength.In recent years, research groups, including ours, have startedaddressing the challenges discussed above. Herein, we summarize ourcollective findings through a selection of key studies. First, wediscuss the chemistry and structure of BN, clarifying confusion aroundterminology and discussing the hydrolytic instability of the materialin relation to its structure and chemistry. We demonstrate a way toreduce the instability in water while still maintaining high specificsurface area. We propose a mechanism for the formation of porous BNand discuss the effects of different synthesis parameters on the structureand chemistry of porous BN, therefore providing a way to tune itsproperties for selected applications. While the syntheses coveredoften lead to a powder product, we also present ways to shape porousBN powders into macrostructures while still maintaining high accessiblesurface area for interfacial processes. Finally, we evaluate porousBN performance for chemical separations, gas storage, and catalysis.While the above highlights key advances in the field, further workis needed to allow deployment of porous BN. Specifically, we suggestevaluating its hydrolytic stability, refining the ways to shape thematerial into stable and reproducible macrostructures, establishingclear design rules to produce BN with specific chemistry and porosity,and, finally, providing standardized test procedures to evaluate porousBN catalytic and sorptive properties to facilitate comparison.

Abstract

The single molecule detection associated with DNA sequencing has motivated intensive efforts to identify single DNA bases. However, little research has been reported utilizing single-layer hexagonal boron nitride (hBN) for DNA sequencing. Here we employ molecular dynamics simulations to explore pathways for single-strand DNA (ssDNA) sequencing by nanopore on the hBN sheet. We first investigate the adhesive strength between nucleobases and the hBN sheet, which provides the foundation for the hBN-base interaction and nanopore sequencing mechanism. Simulation results show that the purine base has a more remarkable energy profile and affinity than the pyrimidine base on the hBN sheet. The threading of ssDNA through the hBN nanopore can be clearly identified due to their different energy profiles and conformations with circular nanopores on the hBN sheet. The sequencing process is orientation dependent when the shape of the hBN nanopore deviates from the circle. Our results open up a promising avenue to explore the capability of DNA sequencing by hBN nanopore.

Mesoporous boron nitride in contact with water - Chemical stability and adsorption properties

In this paper we report an improved three-step wet etching method to achieve individual nanopore and nanopore arrays at wafer scale. Nanopore arrays and individual nanopores with feature size down to 14nm were obtained. To improve the precise control of the pore-opening point, we used a home-made device to monitor the traversing current, which presents pore-opening event. The uniformity of the nanopore arrays have also been analyzed.

This paper presents an improved three-step wet etching method for the fabrication of single-crystal silicon nanopores and nanoslists. A diffusion model was built to analyze the influence of the color-based feedback mechanism on the final pore size. Reference structures were added aside normal pore patterns, to obtain a more precise control of the pore size during the pore opening process. By using this method, square nanopores with the minimum size of 8 nm × 8 nm, rectangle nanopores and nanoslits with feature sizes down to 5 nm were successfully obtained. Focused ion beam cutting revealed that the nanopore profile keeps well the inverted-pyramid shape, with an included angle of 54.7°.

Sliding guideways have received renewed interest in recent years as machine tool linear motion guides, due to a demand for machine tools to have good dynamic performance, which is of vital importance when machining difficult-to-cut materials. While the traditional fabrication approach of the sliding surface is grinding, this paper investigated the possibility of an alternative cubic boron nitride (CBN) milling-based manufacturing approach while utilizing Al and Mg additives in the cast iron material for better machinability and productivity. Machining results have shown a dramatic improvement in machinability especially in terms of tool wear at certain cutting conditions with the refined hardened cast iron and a CBN tool. It was found by the post experimental analysis that oxide films of the Al and Mg additives were generated at the cutting edge of the CBN tool to protect the tool from wear. Because of suppression of tool wear, a very constant surface roughness can also be achieved. A case study has also demonstrated the effectiveness of the CBN milling-based manufacturing approach with the refined cast iron and the found high-speed cutting conditions.

A new procedure to produce poly(vinylidene fluoride)/boron nitride hybrid membrane is presented for application in membrane distillation (MD) process. The influence of hexagonal boron nitride (h-BN) incorporation on the performance of the polymeric membranes is studied through the present investigation. For this aim, h-BN nanopowders were successfully synthesized using the simple chemical vapor deposition (CVD) route and subsequent solvent treatments. The resulting h-BN nanosheets were blended with poly(vinylidene fluoride) (PVDF) solution. Then, the prepared composite solution was subjected to phase inversion process to obtain PVDF/h-BN hybrid membranes. Various examinations such as scanning electron microscopy (SEM), wettability, permeation flux, mechanical strength and liquid entry pressure (LEP) measurements are performed to evaluate the prepared membrane. Moreover, Air gap membrane distillation (AGMD) experiments were carried out to investigate the salt rejection performance and the durability of membranes. The results show that our hybrid PVDF/h-BN membrane presents higher water permeation flux (~18 kg/m2 h) compared to pristine PVDF membrane. In addition, the experimental data confirms that the prepared nanocomposite membrane is hydrophobic (water contact angle: ~103 degree),has a porous skin layer (>85%), as well competitive fouling resistance and operational durability. Furthermore, the total salt rejection efficiency was obtained for PVDF/h-BN membrane. The results prove that the novel PVDF/h-BN membrane can be easily synthesized and applied in MD process for salt rejection purposes.

Effect of granulated sugar as pore former on the microstructure and mechanical properties of the vitrified bond cubic boron nitride grinding wheels