Supercritical Fluid-Facilitated Exfoliation and Processing of 2D Materials

Since the first intercalation of layered silicates by using supercritical CO2 as a processing medium, considerable efforts have been dedicated to intercalating and exfoliating layered two-dimensional (2D) materials in various supercritical fluids (SCFs) to yield single- and few-layer nanosheets. Here, recent work in this area is highlighted. Motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed. Further, advances in formation and application of 2D material-based composites with assistance from SCFs are presented. These discussions address chemical transformations accompanying SCF processing such as doping, covalent surface modification, and heterostructure formation. Promising features, challenges, and routes to expanding SCF processing techniques are described.

Stable and tunable plasmon resonance of molybdenum oxide nanosheets from the ultraviolet to the near-infrared region for ultrasensitive surface-enhanced Raman analysis

Preparation of color-tunable and stable plasmonic MoO3 nanomaterials remains challenging, due to the lack of an effective preparation strategy and surface protection in heavily doped MoO3. Herein, we report a facile and reliable method for synthesis of oxygen-deficient MoO3 (MoO3-x ) nanosheets using dopamine as the reducing agent and precursor for the formation of a polydopamine (PDA) surface coating. The PDA-coated MoO3-x nanosheets show stable and tunable localized surface plasmon resonance (LSPR) from the ultraviolet to the near-infrared region (361-809 nm) via altering the pH value of the medium, accompanying the generation of multicolor nanosheet dispersions, such as deep blue, faint bluish, orange, yellow and black. Importantly, the resulting PDA-coated MoO3-x nanosheets are quite stable even in the presence of oxidants, and they can be used as an ultrasensitive surface-enhanced Raman scattering (SERS) substrate. The limit of detection for rhodamine 6G (R6G) dye is down to 0.3 fM concentration, and the corresponding Raman enhancement factor reaches 1 × 1010. The coupling of charge transfer between R6G and PDA-coated MoO3-x nanosheets and molecular resonances may be responsible for the strong SERS effect.

A colorimetric assay for ultrasensitive detection of copper (II) ions based on pH-dependent formation of heavily doped molybdenum oxide nanosheets

Here we report a simple and low-cost colorimetric assay for ultrasensitive detection of copper (II) ions (Cu²⁺) based on pH-dependent formation of plasmonic MoO_3-x nanosheets. The reaction between ascorbic acid (AA) and Cu²⁺ gives birth to hydrogen ions, which remarkably promotes the reduction of MoO₃ nanosheets by AA to form oxygen vacancies-rich MoO_3-x nanosheets that increase the free carrier concentration, generating a strong local surface plasmon resonance (LSPR) absorption. The Cu²⁺-triggered LSPR is utilized for development of the colorimetric assay. Under the optimal experimental conditions, this colorimetric assay can be used for selective detection of Cu²⁺, and the limit of detection is 0.8 nM, which is lower than that of most of the reported methods. Additionally, the present colorimetric assay has also been successfully employed for Cu²⁺ determination in real serum and human hair samples with satisfactory recoveries ranging from 96% to 108%.

Electrophoresis Assembly of Novel Superhydrophobic Molybdenum Trioxide (MoO₃) Films with Great Stability

This work presents a hydrothermal synthesis approach to produce novel schistose molybdenum trioxide (MoO₃) powders with wide application, and introduces a facile electrophoresis assembly technique to construct the superhydrophobic MoO₃ films (SMFs) with contact angle up to 169 ± 1° at normal pressure and temperature. The microstructures and chemical compositions of product were analyzed by field emission scanning electron microcopy (FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD). The wettability and stability studies indicate that the SMFs all show great resistance in various environments with adjusting factors, including droplets with different surface tension, pH, relative humidity, etc., and the stability can be maintained at least for five months. Notably, this paper will provides a valuable reference for designing novel oxide powders and their high-efficient hydrophobic film formation with self-cleaning or water proof properties.

NIR light-activated upconversion semiconductor photocatalysts

Harvesting of near infrared (NIR) light in the abundant and environmentally friendly solar spectrum is particularly significant to enhance the utilization rate of the cleanest energy on earth. Appreciating the unique nonlinear optical properties of upconversion materials for converting low-energy incident light into high-energy radiation, they become the most promising candidates for fabricating NIR light-active photocatalytic systems by integrating with semiconductors. The present review summarizes recent NIR light-active photocatalytic systems based on a sequence of NaYF₄-based, fluoride-based, oxide-based and Ln³⁺ ion-doped semiconductor-based photocatalysts for degradation of organic molecules. In addition, we provide an in-depth analysis of various photocatalytic mechanisms and enhancement effects for efficient photo-redox performance of different upconversion semiconductor photocatalysts. We envision that this review can inspire multidisciplinary research interest in rational design and fabrication of efficient full-spectrum active (UV-visible-NIR) photocatalytic systems and their wider applications in solar energy conversion.

Two-dimensional amorphous NiO as a plasmonic photocatalyst for solar H2 evolution

Amorphous materials are usually evaluated as photocatalytically inactive due to the amorphous nature-induced self-trapping of tail states, in spite of their achievements in electrochemistry. NiO crystals fail to act as an individual reactor for photocatalytic H₂ evolution because of the intrinsic hole doping, regardless of their impressive cocatalytic ability for proton/electron transfer. Here we demonstrate that two-dimensional amorphous NiO nanostructure can act as an efficient and robust photocatalyst for solar H₂ evolution without any cocatalysts. Further, the antenna effect of surface plasmon resonance can be introduced to construct an incorporate antenna-reactor structure by increasing the electron doping. The solar H₂ evolution rate is improved by a factor of 19.4 through the surface plasmon resonance-mediated charge releasing. These findings thus open a door to applications of two-dimensional amorphous NiO as an advanced photocatalyst.

While photocatalysis offers a means to store solar energy as chemical fuels, photocatalysts typically require crystalline structures and expensive noble-metal cocatalysts. Here, authors prepare 2D amorphous nano-nickel oxide capable of plasmonic, photodriven H₂ evolution without cocatalysts.

CO2 -Assisted Conversion of Crystal Two-Dimensional Molybdenum Oxide to Amorphism with Plasmon Resonances

Localized surface plasmon resonances (LSPRs) of ultra-thin two-dimensional (2D) nanomaterials have opened up a new regime in plasmonics in the last several years. 2D plasmonic materials are currently concentrated on the crystal structure, with amorphous materials hardly being reported because of their limited preparation methods rather than undesired plasmonic properties. Taking molybdenum oxides as an example, herein, we elaborate the 2D amorphous plasmons prepared with the assistance of supercritical CO₂ . In brief, we examine the reported characteristic plasmonic properties of molybdenum oxides, and applications of supercritical CO₂ in formations of 2D layer materials as well as introduced phase and disorder engineering based on our research. Furthermore, we propose our perspective on the development of 2D plasmons, especially for amorphous layer materials in the future.

Ultrasensitive Surface-Enhanced Raman Spectroscopy Detection Based on Amorphous Molybdenum Oxide Quantum Dots

Surface-enhanced Raman spectroscopy (SERS) based on plasmonic semiconductive material has been proved to be an efficient tool to detect trace of substances, while the relatively weak plasmon resonance compared with noble metal materials restricts its practical application. Herein, for the first time a facile method to fabricate amorphous H_x MoO₃ quantum dots with tunable plasmon resonance is developed by a controlled oxidization route. The as-prepared amorphous H_x MoO₃ quantum dots show tunable plasmon resonance in the region of visible and near-infrared light. Moreover, the tunability induced by SC CO₂ is analyzed by a molecule kinetic theory combined with a molecular thermodynamic model. More importantly, the ultrahigh enhancement factor of amorphous H_x MoO₃ quantum dots detecting on methyl blue can be up to 9.5 × 10⁵ with expending the limit of detection to 10^-9 m. Such a remarkable porperty can also be found in this H_x MoO₃ -based sensor with Rh6G and RhB as probe molecules, suggesting that the amorphous H_x MoO₃ quantum dot is an efficient candidate for SERS on molecule detection in high precision.

Plasmonic behaviour and plasmon-induced charge separation of nanostructured MoO3-x under near infrared irradiation

Plasmon-induced charge separation (PICS) allows direct conversion of localized surface plasmon resonance (LSPR) to electron flows and photoelectrochemical reactions. However, PICS has only been achieved using plasmonic noble metal nanoparticles, not with compound nanoparticles. In order to achieve compound PICS, MoO_3-x nanostructures were prepared that exhibit LSPR in the near infrared region by using metal oxides or metal nanoparticles as templates. Solid-state cells based on the MoO_3-x nanostructure were developed. Their photoresponse to 700-1400 nm infrared light was investigated and analyzed on the basis of their PICS mechanisms.

Rapid room-temperature preparation of MoO3−x quantum dots by ultraviolet irradiation for photothermal treatment and glucose detection

Oxygen deficient molybdenum oxide (MoO_3−x) spurred intense scientific interest in biomedical research owing to the strong localized surface plasmon resonance (LSPR) effect in NIR region.

A negative feedback loop based on proton-driven in situ formation of plasmonic molybdenum oxide nanosheets

A negative feedback loop is developed based on proton-driven in situ formation of plasmonic MoO_3−x nanosheets.