Abstract
Defects can induce drastic changes of the electronic properties of two-dimensional transition metal dichalcogenides and influence their applications. It is still a great challenge to ...characterize small defects and correlate their structures with properties. Here, we show that tip-enhanced Raman spectroscopy (TERS) can obtain distinctly different Raman features of edge defects in atomically thin MoS
2
, which allows us to probe their unique electronic properties and identify defect types (e.g., armchair and zigzag edges) in ambient. We observed an edge-induced Raman peak (396 cm
−1
) activated by the double resonance Raman scattering (DRRS) process and revealed electron–phonon interaction in edges. We further visualize the edge-induced band bending region by using this DRRS peak and electronic transition region using the electron density-sensitive Raman peak at 406 cm
−1
. The power of TERS demonstrated in MoS
2
can also be extended to other 2D materials, which may guide the defect engineering for desired properties.
Surface and interfaces play key roles in heterogeneous catalysis, electrochemistry and photo(electro)chemistry. Tip-enhanced Raman spectroscopy (TERS) combines plasmon-enhanced Raman spectroscopy ...with scanning probe microscopy to simultaneously provide a chemical fingerprint and morphological information for the sample at the nanometer spatial resolution. It is an ideal tool for achieving an in-depth understanding of the surface and interfacial processes, so that the relationship between structure and chemical performance can be established. We begin with the background of surfaces and interfaces and TERS, followed by a detailed discussion on some issues in experimental TERS, including tip preparation and TERS instrument configuration. We then focus on the progress of TERS for studying the surfaces and interfaces under different conditions, from ambient, to UHV, solid-liquid and electrochemical environments, followed by a brief introduction to the current understanding of the unprecedented high spatial resolution and surface selection rules. We conclude by discussing the future challenges for TERS practical applications in surfaces and interfaces.
TERS offers the high spatial resolution to establish structure-function correlation for surfaces and interfaces.
Tip‐enhanced Raman spectroscopy can provide molecular fingerprint information with ultrahigh spatial resolution, but the tip will be easily contaminated, thus leading to artifacts. It also remains a ...great challenge to establish tip‐enhanced fluorescence because of the quenching resulting from the proximity of the metal tip. Herein, we report shell‐isolated tip‐enhanced Raman and fluorescence spectroscopies by employing ultrathin shell‐isolated tips fabricated by atomic layer deposition. Such shell‐isolated tips not only show outstanding electromagnetic field enhancement in TERS but also exclude interference by contaminants, thus greatly promoting applications in solution. Tip‐enhanced fluorescence has also been achieved using these shell‐isolated tips, with enhancement factors of up to 1.7×103, consistent with theoretical simulations. Furthermore, tip‐enhanced Raman and fluorescence signals are acquired simultaneously, and their relative intensities can be manipulated by changing the shell thickness. This work opens a new avenue for ultrahigh resolution surface analysis using plasmon‐enhanced spectroscopies.
Combined tip‐enhanced Raman and fluorescence spectroscopy using shell‐isolate tips is presented. With this method, clear and strong Raman and fluorescence signals can be obtained, and their relative intensities can be tuned by changing the shell thickness.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Abstract
Surface plasmons (SPs) of metals enable the tight focusing and strong absorption of light to realize an efficient utilization of photons at nanoscale. In particular, the SP-generated hot ...carriers have emerged as a promising way to efficiently drive photochemical and photoelectric processes under moderate conditions. In situ measuring of the transport process and spatial distribution of hot carriers in real space is crucial to efficiently capture the hot carriers. Here, we use electrochemical tip-enhanced Raman spectroscopy (EC-TERS) to in situ monitor an SP-driven decarboxylation and resolve the spatial distribution of hot carriers with a nanometer spatial resolution. The transport distance of about 20 nm for the reactive hot carriers is obtained from the TERS imaging result. The hot carriers with a higher energy have a shorter transport distance. These conclusions can be guides for the design and arrangement of reactants and devices to efficiently make use of plasmonic hot carriers.
Excellent electroconductivity, fire resistance, and mechanical properties are several important indexes for electromagnetic shielding sealing materials. In this work, a novel functionalized graphene ...decorated with a nickel-Schiff base (rGO-Salen-Ni) was first synthesized
via
a chemical modification process, and applied in enhancing the mechanical properties, electroconductivity, and fire resistance of a thermoplastic polyurethane elastomer (TPU). The experimental result illustrated that the rGO-Salen-Ni significantly improved the above vital properties of TPU. First, the tensile strength, elongation at break, and storage modulus of the TPU containing only 1.5 wt% rGO-Salen-Ni (TPU/rGO-Salen-Ni1.5) were respectively increased by 1.50 times, 1.58 times, and 2.17 times; second, the electrical conductivity of the TPU/rGO-Salen-Ni1.5 was increased by 5 orders of magnitude in comparison with that of a pure TPU, up to 7.3 × 10
−5
S m
−1
; finally, the TPU/rGO-Salen-Ni1.5 displayed excellent flame retardancy; for instance, the peak of heat release rate, total heat release, and total smoke production were respectively decreased by 43%, 22%, and 28% compared with the corresponding values of pure TPU. The analysis of the enhancement in mechanical properties and electroconductivity of TPU/rGO-Salen-Ni revealed that the uniform dispersion of rGO-Salen-Ni, resulting from the incorporation of the nickel-Schiff base into the GO, was the leading reason for their improvements. The study on the flame-retardant mechanism demonstrated that a more continuous and compact protective layer related to the nickel-Schiff base played the key role in the excellent fire resistance of TPU/rGO-Salen-Ni. The rGO-Salen-Ni shows great potential for application in electromagnetic shielding sealing materials.
Excellent electroconductivity, fire resistance, and mechanical properties are several important indexes for electromagnetic shielding sealing materials.
Active oxygen species (AOS) play key roles in many important catalytic reactions relevant to clean energy and environment. However, it remains challenging to characterize the active sites for ...producing AOS and to image the surface properties of AOS, especially on multicomponent metallic catalyst surfaces. Herein, we utilize tip-enhanced Raman spectroscopy (TERS) to probe the local generation and diffusion of OH radicals on a Pd/Au(111) bimetallic catalyst surface. The reactive OH radicals can be catalytically generated from hydrogen peroxide (H2O2) at the metal surface, which then oxidizes the surface adsorbed thiolate, a reactant that is used as the TERS probe. By TERS imaging of the spatial distribution of unreacted thiolate molecules, we demonstrate that the Pd surface is active for generation of OH radicals and the Pd step edge shows much higher activity than the Pd terrace, whereas the Au surface is inactive. Furthermore, we find that the locally generated OH radicals at the Pd step edge could diffuse to both the Au and the Pd surface sites to induce oxidative reactions, with a diffusion length estimated to be about 5.4 nm. Our TERS imaging with few-nanometer spatial resolution not only unravels the active sites but also characterizes in real space the diffusion behavior of OH radicals. The results are highly valuable to understand AOS-triggered catalytic reactions. The strategy of using reactants with large Raman cross sections as TERS probes may broaden the application of TERS for studying catalysis with reactive small molecules.
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IJS, KILJ, NUK, PNG, UL, UM
Phytic acid mainly exists in seeds, roots and stems of plants, which has a potential value in flame-retarding polymers due to the high content of phosphorus. In this work, a novel bio-based phytic ...acid salt PHYPI was prepared through a salt formation reaction between phytic acid and piperazine. The structure of the bio-based PHYPI was verified using 1H NMR spectrascoppy. When PHYPI was used to fabricate flame-retardant polypropylene (PP), it showed high efficiency in combustion tests. The limiting oxygen index (LOI) value for PP containing 18.0 wt% PHYPI is 25.0%, showing a 38.9% increase compared to 18.0% for PP containing no additive. Moreover, it passed the UL-94 V-0 rating in the vertical burning test, superior to the no rating for pure PP. Obviously, the flame-retarding efficiency of PHYPI is higher than that of typical traditional intumescent flame retardant containing ammonium polyphosphate or pentaerythritol. Cone calorimeter test revealed that the heat release and smoke production of PP were efficiently restrained by the presence of PHYPI. The peak of heat release rate (PHRR), total heat release (THR), and the peak of smoke release rate (PSPR) for PP containing 20 wt% PHYPI were decreased by 65.6%, 13.5%, and 32.8%, respectively, compared to the same values for PP alone. Fourier transform infrared spectroscopy (FTIR) was used to investigate the changes which accompanied the thermal degradation of the polymer containing PHYPI. Changes in the infrared spectra for the polymer undergoing degradation indicate that structures containing C=C and P-N-C were formed as a consequence of the presence of PHYPI. These transformations enhanced char formation to provide condensed phase protective action. At the same time, non-combustion volatile gases such as water and carbon dioxide may be released to dilute the fuel load in the gas phase. All evidences illustrate that PHYPI is an effective flame retardant for PP.
•A novel bio-based flame retardant named PHYPI from phytic acid.•High flame-retarding efficiency of PHYPI for polypropylene.•Flame-retardant mechanism of polypropylene/PHYPI.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In the past work, few study considers an effect of nanoparticle as a synergistic flame retardant on the interfacial interaction between flame retardants and polymer matrix, as well as further ...influence on the flame retardancy and mechanical properties of flame-retarding polymers. In this work, the electrostatic action was used to prepare a novel ammonium polyphosphate@layered double hydroxide (APP@LDH) in which the LDH nanoparticles accumulated at the surface of APP to achieve the highly-efficient synergistic action of LDH in thermoplastic polyurethane/APP (TPU/APP) and reduce the deterioration of APP to mechanical properties of TPU. Scanning electron microscope, etc., demonstrated that the APP@LDH was prepared successfully. Combustion tests results showed that the APP@LDH had much higher flame-retarding efficiency than the APP/LDH which was prepared through a simple physical blending process. Only 1.0 wt% LDH made the TPU pass the V-0 rating with no dripping in the UL-94 test and a limiting oxygen index (LOI) of 29.2% in the case of 7.0 wt% APP@LDH. However, the TPU/APP/LDH with 1.0 wt% LDH did not pass the V-0 rating, and a dripping behavior also existed at 7.0 wt% APP/LDH. In cone calorimeter test, the heat release and smoke production of TPU/APP@LDH were also lower than the corresponding values of TPU/APP/LDH under equal amount of LDH. The analysis of flame-retardant mechanism proved that an enhanced condensed action induced by the interfacial LDH dominated the better flame retardance of APP@LDH system than that of APP/LDH system. Another important aspect is that the mechanical properties of APP@LDH system showed remarkable improvements in comparison with those of APP/LDH system. The tensile strength of TPU with 7.0 wt% APP@LDH was 32.5% higher than that of TPU with 7.0 wt% APP/LDH under equal 1.0 wt% LDH. Meanwhile, the elongation at break for the former was maintained at 863.0%, almost equal to that of TPU. This work illustrates that the interfacial accumulation of LDH may enhance its synergistic flame-retarding efficiency and meanwhile achieve the mechanical enhancement for TPU/APP.
•Electrostatic action induced ammonium polyphosphate@layered double hydroxide.•Highly-efficient synergistic flame retardance and mechanical enhancement for TPU.•Mechanisms for the improved flame retardance and mechanical properties.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•A review on flame-retarded thermoplastic polyurethane elastomer.•From organic materials to nanocomposites and new prospects.•Inherent and additive-type flame-retarded thermoplastic polyurethane ...elastomer.•Organic, inorganic, compound, and hybridized flame retardants.
Thermoplastics polyurethane elastomers (TPUs) have been widely used in auto parts, electronic appliances, wires and cables, et al. However, they are flammable, thus seriously threating the safety of human beings and property. To achieve the flame retardancy of TPUs and reduce the hazards from smoke and toxic gases as well as severe droplet, much work was carried out in the past two decades, especially in the recent decade. In this review, we focus on the fire safety of TPU and deeply review the past and current status concerning the flame retardation of TPU, covering organic materials to nanocomposites and new prospects. Starting from the structural characteristic, several basic aspects of TPU are overviewed, including thermal properties, combustion performance, and flame-retardant method. Then, on the basis of published literatures, the development course, current research hot spot, and difficulty in achieving the flame retardancy of TPU are analyzed by detailed statistic data. Further, existing inherently flame-retarded TPU and already used flame retardants in TPU are introduced thoroughly. For the most important additive-type flame-retardant mode currently, the structural design and flame-retarding effect of vital flame retardants on the fire safety of TPU are discussed in detail, which includes mono-component organic and inorganic flame retardants, modified and hybridized flame retardants, compound flame retardants, and nano flame retardants. For inherently flame-retarded TPU, this review focuses on the structural design and flame-retardant mechanism. In addition, special processes to the flame retardation of TPU are also discussed. On the basis of the past research, the flame retardation of TPU without the incorporation of traditional flame retardant elements and the bio-based flame retardation of TPU are prospected in the final part.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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