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Nanostructures of layered transition metal dichalcogenide (TMD) alloys with tunable compositions are promising candidates for a broad scope of applications in electronics, optoelectronics, topological devices, and catalysis. Most TMD alloy nanostructures are synthesized as films on substrates using gas-phase methods at high temperatures. However, lower temperature solution routes present an attractive alternative with the potential for larger-scale, higher-yield syntheses of freestanding, higher surface area materials. Here, we report the direct solution synthesis of colloidal few-layer TMD alloys, MoxW1-xSe2 and WS2ySe2(1-y), exhibiting fully tunable metal and chalcogen compositions that span the MoSe2- WSe2 and WS2-WSe2 solid solutions, respectively. Chemical guidelines for achieving the targeted compounds are presented, along with comprehensive structural characterizations (X-ray diffraction, electron microscopy, Raman, and UV-visible spectroscopies). High-resolution microscopic imaging confirms the formation of TMD alloys and identifies a random distribution of the alloyed elements. Analysis of the tilt-angle dependency of the intensities associated with atomic-resolution annular dark field imaging line scans reveals the types of point vacancies present in the samples, thus providing atomic-level insights into the structures of colloidal TMD alloy nanostructures that were previously only accessible for substrate-confined films. The A excitonic transition of the TMD alloy nanostructures can be readily adjusted between 1.51 and 1.93 eV through metal and chalcogen alloying, correlating the compositional modulation to the realization of tunable optical properties. [ABSTRACT FROM AUTHOR]

The optical properties of titanate nanotubes (TiNts) intercalated with rare earths (RE) ions were intensively investigated in this study. To prepare the nanomaterials, sodium titanate nanotubes (Na-TiNts) were submitted to ion exchange reactions with different rare earth elements (RE: Pr3+, Er3+, Nd3+, and Yb3+). To the best of our knowledge, it is the first time that these RE-TiNts were synthesized. All samples were characterized by Raman spectroscopy, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). Furthermore, the optical properties were examined using photoluminescence spectroscopy (PL) and UV-Vis-NIR absorption spectroscopy. The PL intensity (visible range) of the RE-TiNt samples showed a strong dependence when the temperature was decreased down to 20 K. This PL enhancement probably was promoted by electronic modifications in titanate layer band gap and/or interface charge transfers due to RE ions intercalation. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides such as WS2 show exciting promise in electronic and optoelectronic applications. Significant variations in the transport, Raman, and photoluminescence (PL) can be found in the literature, yet it is rarely addressed why this is. In this report, Raman and PL of monolayered WS2 produced via different methods are studied and distinct features that indicate the degree of crystallinity of the material are observed. While the intensity of the LA(M) Raman mode is found to be a useful indicator to assess the crystallinity, PL is drastically more sensitive to the quality of the material than Raman spectroscopy. We also show that even exfoliated crystals, which are usually regarded as the most pristine material, can contain large amounts of defects that would not be apparent without Raman and PL measurements. These findings can be applied to the understanding of other two-dimensional heterostructured systems. [ABSTRACT FROM AUTHOR]

Activated Carbon (AC) with high specific surface area and elevated porosity was synthesized from babassu mesocarp by hydrothermal carbonization, followed by KOH activation-coupled pyrolysis and subjected to heat-treatment at three different temperatures to generate AC-1100, AC-1400 and AC-2200. Raman spectroscopy, X-Ray diffraction (XRD), transmission electron microscopy (TEM) and N 2 adsorption-desorption were used to characterize structurally the synthesized materials and electrochemical studies to evaluate the capacitance properties. The Raman spectra showed sharpening of graphitic bands and decrease in intensities of amorphous bands with the increase of treatment temperature. AC-2200 showed a significant decrease of I D /I G due to the crystal growth in ab plane and drastic increase of I 2D /I G due to graphitic ordering in c direction, further proven by sharpening of characteristic XRD peaks and visualization of few-layer well-formed graphite micro-crystals by TEM. Although gradual heat treatment resulted in an increase in sample crystallinity, the surface area and porosity decreased, leaving AC-2200 practically non-porous. These changes were reflected in their electrochemical properties as AC-1400 showed highest double layer capacitance owing to an optimization between structural ordering and retention of porosity, while the capacitance, as well as the resistance, of AC-2200, decreases dramatically due to loss of pores and increased crystallinity brought in by graphitization. Image 1 [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Herein an activated carbon (AC) with high surface area and microporosity was synthesized from the starch-rich mesocarp of the babassu coconut, an abundant biomass from north and northeastern Brazil. The synthesis of AC was realized by chemical activation using potassium hydroxide (KOH) coupled with pyrolysis at 750 °C and the produced material was further characterized by scanning (SEM) and transmission electron (TEM) microscopy, Fourier transform infrared (FTIR) and Raman spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis. SEM and TEM showed the formation of a thin-layer porous morphology of AC; whereas the nitrogen (N 2) and carbon dioxide (CO 2) adsorption experiments identified a high surface area and microporosity. Raman spectra obtained by various laser lines revealed that AC has graphite-like microstructures with characteristic bands with features dependent on laser excitation energy. The amorphous nature from AC was further proven by XRD whereas FTIR showed the presence of surface-active oxygen functional groups. This AC material, produced from Brazillian Biomass showed a great potential as a CO 2 sensor using a optical technique, In-situ Raman spectroscopy and their D and G vibrational modes shifting in the presence of CO 2. [ABSTRACT FROM AUTHOR]

The interaction of the noble atoms (Ar and Xe) with single-walled carbon nanotube (SWCNT) bundles are investigated using Raman spectroscopy in conjunction with computational modeling known as ReaxFF force field. SWCNT bundles were deposited on transmission electron microscopy (TEM) grids, and different noble gases were adsorbed onto the nanotubes at 20 K. Raman spectra acquired show significant frequency blueshifts of the radial breathing mode (RBM), G- and G′(or 2D)-bands due to gas solidification within the external groove sites (free spaces between the tubes in the bundle) and external surfaces of the bundles. This solid shell formed by the adsorbed gases contributes with a hydrostatic pressure to the system. We show from Raman measurements that the frequencies found after gas adsorption exhibit almost the same shifts indicating that the interactions between SWCNTs bundles and the gases (Ar or Xe) are nearly identical. [ABSTRACT FROM AUTHOR]

Hexaniobate nanosheets derived from the parent compound K 4 Nb 6 O 17 have been decorated with CeO 2 nanoparticles by ion exchange with aqueous cerium (IV) solution. Very homogeneous CeO 2 nanoparticle decoration of the hexaniobate sheets can be achieved by this method and the resulting composites may absorb visible light. HRTEM images show that ∼3.0 nm diameter CeO 2 nanoparticles adhere to hexaniobate nanosheets that are exfoliated and then restacked prior to Ce deposition. The interfacial interaction between CeO 2 nanoparticles and nanosheets would be due to an electrostatic attraction mechanism. Raman and XRD measurements have given strong evidence that CeO 2 nanoparticles have fluorite structure. EDS, FTIR and XPS results suggest almost complete exchange of TBA + and K + by Ce 4+ . Cerium ion exchange on the acid exchanged parent compound, H 2.9 K 1.1 Nb 6 O 17 , revealed that the extent of Ce ion exchange is much greater in case of nanosheets, which may be rationalized by the larger surface area available after exfoliation. XPS measurements show that the ratio of Ce 4+ /Ce 3+ is around 4.4, in agreement with the formation of fluorite structure (CeO 2 ). Thus, these CeO 2 nanoparticle/nanosheet composites may be useful for catalytic processes. [ABSTRACT FROM AUTHOR]

We report the study of the growth of CeO 2 nanoparticles on the external walls and Ce 4+ intercalation within the titanate nanotubes. The materials were fully characterized by multiple techniques, such as: Raman spectroscopy, infrared spectroscopy (FTIR), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The ion exchange processes in the titanate nanotubes were carried out using different concentrations of Ce 4+ in aqueous solution. Our results indicate that the growth of CeO 2 nanoparticles grown mediated by the hydrolysis in the colloidal species of Ce and the attachment onto the titanate nanotubes happened and get it strongly anchored to the titanate nanotube surface by a simple electrostatic interaction between the nanoparticles and titanate nanotubes, which can explain the small size and even distribution of nanoparticles on titanate supports. It was demonstrated that it is possible to control the amount and size of CeO 2 nanoparticles onto the nanotube surface, the species of the Ce ions intercalated between the layers of titanate nanotubes, and the materials could be tuned for using in specific catalysis in according with the amount of CeO 2 nanoparticles, their oxygen vacancies/defects and the types of Ce species (Ce 4+ or Ce 3+ ) present into the nanotubes. [ABSTRACT FROM AUTHOR]