Revealing the aging process of solid electrolyte interphase on SiOx anode.

As one of the most promising alternatives to graphite negative electrodes, silicon oxide (SiOx) has been hindered by its fast capacity fading. Solid electrolyte interphase (SEI) aging on silicon SiOx has been recognized as the most critical yet least understood facet. Herein, leveraging 3D focused ion beam-scanning electron microscopy (FIB-SEM) tomographic imaging, we reveal an exceptionally characteristic SEI microstructure with an incompact inner region and a dense outer region, which overturns the prevailing belief that SEIs are homogeneous structure and reveals the SEI evolution process. Through combining nanoprobe and electron energy loss spectroscopy (EELS), it is also discovered that the electronic conductivity of thick SEI relies on the percolation network within composed of conductive agents (e.g., carbon black particles), which are embedded into the SEI upon its growth. Therefore, the free growth of SEI will gradually attenuate this electron percolation network, thereby causing capacity decay of SiOx. Based on these findings, a proof-of-concept strategy is adopted to mechanically restrict the SEI growth via applying a confining layer on top of the electrode. Through shedding light on the fundamental understanding of SEI aging for SiOx anodes, this work could potentially inspire viable improving strategies in the future.

A flavin-Cu2+ supramolecular complex for highly selective sorting of semiconducting single-walled carbon nanotubes with specific chiralities.

We introduce thio-substituents at the 7- and 8-positions of a flavin analogue, which allows for the coordination of metal ions, and demonstrate, based on both experimental and theoretical methods, that the formed Cu2+-based supramolecular assembled complex shows very high selective sorting of semiconducting single-walled carbon nanotubes with (8,6)- and (9,4)-chiralities.

Carbon Nanotube Photoluminescence Modulation by Local Chemical and Supramolecular Chemical Functionalization.

ConspectusCarbon nanotubes (CNTs) have been central materials in nanoscience and nanotechnologies. Single-walled CNTs (SWCNTs) consisting of a cylindrical graphene show a metallic (met) or semiconducting (sc) property depending on their rolling up manner (chirality). The sc-SWCNTs show characteristic chirality-dependent optical properties of their absorption and photoluminescence (PL) in the near-infrared (NIR) region. These are derived from their highly π-conjugated structures having semiconducting crystalline sp2 carbon networks with defined nanoarchitectures that afford a strong quantum confinement and weak dielectric screening. Consequently, photoirradiation of the SWCNTs produces a stable and mobile exciton (excited electron-hole pair) even at room temperature, and the exciton properties dominate such optical phenomena in the SWCNTs. However, the mobile excitons decrease the PL efficiency due to nonradiative relaxation including collision with tube edges and relaxation to lower-lying dark states. A breakthrough regarding the efficient use of the mobile exciton for PL has recently been achieved by local chemical functionalization of the SWCNTs, in which the chemical reactions introduce local defects of oxygen and sp3 carbon atoms in the tube structures. The defect doping creates new emissive doped sites that have narrower band gaps and trap the mobile excitons, which provides locally functionalized SWCNTs (lf-SWCNTs). As a result, the localized exciton produces E11* PL with red-shifted wavelengths and enhanced PL quantum yields compared to the original E11 PL of the nonmodified SWCNTs.In this Account, we describe recently revealed fundamental properties of the lf-SWCNTs based on the analyses by photophysics, theoretical calculations, and electrochemistry combined with in situ PL spectroscopy. The new insight allows us to expand the wavelength regions of the NIR E11* PL derived from the localized exciton, in which upconversion generates a higher energy PL through thermal activation and proximal doped site formation using bis-aryldiazonium modifiers provides a much lower energy PL than typical E11* PL. Moreover, owing to the chemical reaction-dominant doping process, the molecular structure design of modifiers succeeds in producing functionalized lf-SWCNTs; namely, molecular functions are incorporated into the doped sites for their PL modulation. The wavelength changes/switching in the E11* PL selectively occurs by a supramolecular approach using molecular recognition and imine chemistry. Therefore, the local chemical functionalization of the SWCNTs is a key to designing the properties and creating their new functions of the lf-SWCNTs. Fundamental understanding of the doped site properties of the lf-SWCNTs and molecularly driven approaches for exciton and defect engineering would unveil the intrinsic natures of these materials, which is crucial for elevating the SWCNT-based nanotechnologies to the next stage. The resulting materials are of interest in the fields of high performance NIR-II imaging and sensing for bio/medical analyses and single-photon emitters in quantum information technology.

Supramolecular Chemistry-Based One-Pot High-Efficiency Separation of Solubilizer-Free Pure Semiconducting Single-Walled Carbon Nanotubes: Molecular Strategy and Mechanism.

Single-walled carbon nanotubes (SWCNTs) have the potential to revolutionize nanoscale electronics and power sources; however, their low purity and high separation cost limit their use in practical applications. Here we present a supramolecular chemistry-based one-pot, less expensive, scalable, and highly efficient separation of a solubilizer/adsorbent-free pure semiconducting SWCNT (sc-SWCNT) using flavin/isoalloxazine analogues with different substituents. On the basis of both experimental and computational simulations (DFT study), we have revealed the molecular requirements of the solubilizers as well as provided a possible mechanism for such a highly efficient selective sc-SWCNT separation. The present sorting method is very simple (one-pot) and gives a promising sc-SWCNT separation methodology. Thus, the study provides insight for the molecular design of an sc-SWCNT solubilizer with a high (n,m)-chiral selectivity, which benefits many areas including semiconducting nanoelectronics, thermoelectric, bio and energy materials, and devices using solubilizer-free very pure sc-SWCNTs.

Tailoring Different Molecular Weight Phenylene-Polybenzimidazole Membranes with Remarkable Oxidative Stability and Conductive Properties for High-Temperature Polymer Electrolyte Fuel Cells.

Polybenzimidazole (ph-PBI) polymer was synthesized with different molecular weights (MWs) and casted into conductive films for use in high-temperature fuel cells (FCs). A comprehensive study on the influence of polymer MW on membrane cast efficiency, chemical stability, thermal behavior, tensile strength, conductivity, FC performance, and durability was reported. The synthesized materials were characterized by different techniques, including, nuclear magnetic resonance spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, impedance microscopy, and scanning electron microscopy. The results showed the importance of manufacturing ph-PBI membranes with controlled properties to achieve high efficiency FCs. High MW ph-PBI membrane (119 kDa) showed a slower rate of chemical degradation, remarkable mechanical properties, and an improved FC performance compared to low MW ph-PBI membrane (39 kDa), thanks to the architecture of high MW ph-PBI. A gain of 91% in proton conductivity with a 47% in FC power density was obtained for the ph-PBI membrane with MW 119 kDa.

Electrochemical Degradation of Pt-Ni Nanocatalysts: An Identical Location Aberration-Corrected Scanning Transmission Electron Microscopy Study.

The evolution of Pt-Ni nanoparticles supported on amorphous carbon is investigated before and after electrochemical potential cycling (0.6-1.1 V), using aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). During voltage cycling and due to the dissolution of  nanoparticles, single ions/atoms and ionic/atomic clusters emerge and diffuse across the carbon support toward larger nanoparticles, where they redeposit. We observe that the preferred locations for the dissolution are the steps and corners of the nanoparticles. On the other hand, the redeposition process happens often on {111} type planes. In addition, contrary to the conventional view, where larger particles grow isotropically from smaller ones, this research work shows that anisotropic growth of smaller particles occurs during potential cycling. The reason for this behavior seems to be related to the fact that smaller particles with thicker Pt-rich shells trigger the nucleation and deposition of Pt.

Multistep Wavelength Switching of Near-Infrared Photoluminescence Driven by Chemical Reactions at Local Doped Sites of Single-Walled Carbon Nanotubes.

Local chemical functionalization is used for defect doping of single-walled carbon nanotubes (SWNTs), to develop near-infrared photoluminescence (NIR PL) properties. We report the multistep wavelength shifting of the NIR PL of SWNTs through chemical reactions at local doped sites tethered to an arylaldehyde group. The PL wavelength of the doped SWNTs is modulated based on imine chemistry. This involves the imine formation of aldehyde groups with added arylamines, imine dissociation reaction, exchange reaction of bound arylamines in the imine, and the Kabachnik-Fields reaction of imine groups using diisopropyl phosphite. Using doped sites as a localized chemical reaction platform can exploit the versatile molecularly driven functionality of carbon nanotubes and related nanomaterials.

Synthesis of Single-Walled Carbon Nanotubes Coated with Thiol-Reactive Gel via Emulsion Polymerization.

Single-walled carbon nanotubes (SWNTs) have unique near-infrared absorption and photoemission properties that are attractive for in vivo biological applications such as photothermal cancer treatment and bioimaging. Therefore, a smart functionalization strategy for SWNTs to create biocompatible surfaces and introduce various ligands to target active cancer cells without losing the unique optical properties of the SWNTs is strongly desired. This paper reports the design and synthesis of a SWNT/gel hybrid containing maleimide groups, which react with various thiol compounds through Michael addition reactions. In this hybrid, the method called carbon nanotube micelle polymerization was used to noncovalently modify the surface of SWNTs with a cross-linked polymer gel layer. This method can form an extremely stable gel layer on SWNTs; such stability is essential for in vivo biological applications. The monomer used to form the gel layer contained a maleimide group, which was protected with furan in endo-form. The resulting hybrid was treated in water to induce deprotection via a retro-Diels-Alder reaction and then functionalized with thiol compounds through Michael addition. The functionalization of the hybrid was explored using a thiol-containing fluorescent dye as a model thiol, and the formation of the SWNT-dye conjugate was confirmed by energy transfer from the dye to SWNTs. Our strategy offers a promising SWNT-based platform for biological functionalization for cancer targeting, imaging, and treatment.

Control of the Near Infrared Photoluminescence of Locally Functionalized Single-Walled Carbon Nanotubes via Doping by Azacrown-Ether Modification.

Doped semiconducting single-walled carbon nanotubes (SWNTs) through local chemical functionalization (lf-SWNTs) show fascinating photoluminescence (PL) that appears with a longer wavelength and enhanced quantum yield compared to the original PL of non-modified SWNTs. In this study, we introduce an azacrown ether moiety at the doped sites of lf-SWNTs (CR-lf-SWNTs), and observe selective PL wavelength shifts depending on different interaction modes of silver ion inclusion and protonation of the amino group in the ring. Interestingly, their different values of the wavelength shifts show a clear correlation with calculated electron density of the nitrogen atom in the azacrown moiety in case of the inclusion form and the protonated form. This newly-observed responsiveness based on molecular interactions is expected to create doped sites that can versatilely control the PL functions based on molecular systems.

Surfactant-Dependent Charge Transfer between Polyoxometalates and Single-Walled Carbon Nanotubes: A Fluorescence Spectroscopic Study.

Hybridizations of redox-active polyoxometalates (POMs) with single-walled carbon nanotubes (SWNTs) have been widely investigated for their diverse applications. For the purpose of constructing high-quality electronic devices, controlling charge transfer within POM/SWNT hybrids is an inevitable issue. As determined by means of fluorescence spectroscopy, electron transfer between SWNTs and a common POM dopant, phosphomolybdic acid (PMo12 ), can be tuned simply by an alteration of nanotube surfactant type from anionic to nonionic. The mechanism is attributed to the influence of surfactant type on the stabilization of the electron donor-acceptor hybrid and effect of surfactant-nanotube interactions. These results will be important to control charge-transport behavior in nanohybrids consisting of carbon nanotubes.

Near infrared photoluminescence modulation by defect site design using aryl isomers in locally functionalized single-walled carbon nanotubes.

We newly introduce positional isomeric structures at the defect sites of locally-functionalized single-walled carbon nanotubes (lf-SWNTs) showing unique near infrared photoluminescence (PL). The observed PL is significantly different from that of typical para-aryl modified lf-SWNTs; i.e., (i) an extraordinary PL wavelength shift of the meta-aryl modified lf-SWNTs, and (ii) remarkably red-shifted PL from the ortho-aryl modified lf-SWNTs are revealed.

Gold Nanoparticles on Polymer-Wrapped Carbon Nanotubes: An Efficient and Selective Catalyst for the Electroreduction of CO2.

Multiple approaches will be needed to reduce the atmospheric CO2 levels, which have been linked to the undesirable effects of global climate change. The electroreduction of CO2 driven by renewable energy is one approach to reduce CO2 emissions while producing chemical building blocks, but current electrocatalysts exhibit low activity and selectivity. Here, we report the structural and electrochemical characterization of a promising catalyst for the electroreduction of CO2 to CO: Au nanoparticles supported on polymer-wrapped multiwall carbon nanotubes. This catalyst exhibits high selectivity for CO over H2 : 80-92 % CO, as well as high activity: partial current density for CO as high as 160 mA cm-2 . The observed high activity, originating from a high electrochemically active surface area (23 m2 g-1 Au), in combination with the low loading (0.17 mg cm-2 ) of the highly dispersed Au nanoparticles underscores the promise of this catalyst for efficient electroreduction of CO2 .

Substituent effects on the redox states of locally functionalized single-walled carbon nanotubes revealed by in situ photoluminescence spectroelectrochemistry.

Single-walled carbon nanotubes (SWNTs) with local chemical modification have been recognized as a novel near infrared (NIR) photoluminescent nanomaterial due to the emergence of a new red-shifted photoluminescence (PL) with enhanced quantum yields. As a characteristic feature of the locally functionalized SWNTs (lf-SWNTs), PL wavelength changes occur with the structural dependence of the substituent structures in the modified aryl groups, showing up to a 60 nm peak shift according to an electronic property difference of the aryl groups. Up to now, however, the structural effect on the electronic states of the lf-SWNTs has been discussed only on the basis of theoretical calculations due to the very limited amount of modifications. Herein, we describe the successfully-determined electronic states of the aryl-modified lf-SWNTs with different substituents (Ar-X SWNTs) using an in situ PL spectroelectrochemical method based on electrochemical quenching of the PL intensities analyzed by the Nernst equation. In particular, we reveal that the local functionalization of (6,5)SWNTs induced potential changes in the energy levels of the HOMO and the LUMO by -23 to -38 meV and +20 to +22 meV, respectively, compared to those of the pristine SWNTs, which generates exciton trapping sites with narrower band gaps. Moreover, the HOMO levels of the Ar-X SWNTs specifically shift in a negative potential direction by 15 meV according to an enhancement of the electron-accepting property of the substituents in the aryl groups that corresponds to an increase in the Hammet substituent constants, suggesting the importance of the dipole effect from the aryl groups on the lf-SWNTs to the level shift of the frontier orbitals. Our method is a promising way to characterize the electronic features of the lf-SWNTs.

Highly Microporous Nitrogen-doped Carbon Synthesized from Azine-linked Covalent Organic Framework and its Supercapacitor Function.

Porous carbons with nitrogen-doped (N-doped) structures are promising materials for advanced energy conversion and storage applications, including supercapacitors and fuel cell catalysts. In this study, microporous N-doped carbon was successfully fabricated through carbonization of covalent organic frameworks (COFs) with an azine-linked two-dimensional molecular network (ACOF1). In the carbonized ACOF1, micropores with diameters smaller than 1 nm are selectively formed, and a high specific surface area (1596 cm2 g-1 ) is achieved. In addition, the highly porous structure with N-doped sites results in enhancement of the electrochemical capacitance. Detailed investigation for the micropore-forming process reveals that the formation of nitrogen gas during the thermal degradation of the azine bond contributes to the microporous structure formation. Therefore, the present direct carbonization approach using COFs allows the fabrication of microporous heteroatom-doped carbons, based on molecularly designed COFs, toward future electrochemical and energy applications.

Self-Organized Micro-Spiral of Single-Walled Carbon Nanotubes.

Single-walled carbon nanotubes (SWCNTs) are reported to spontaneously align in a rotational pattern by drying a liquid droplet of toluene containing polyfluorene as a dispersant. By situating a droplet of an SWCNT solution around a glass bead, spiral patterns are generated. The parallel alignment of SWCNTs along one stripe of such a pattern is confirmed using scanning electron microscopy and polarized optical microscopy. The orientation order increases toward the outer edge of a stripe. The stripe width in the pattern is proportional to the solute concentration, and the width and position of the stripes follow geometric sequences. The growth of the rotational pattern is also observed in real time. The process of spiral pattern formation is visualized, indicating the role of the annihilation of counter-traveling accompanied by continuous depinning. The geometric sequences for the stripe width and position are explained by the near-constant traveling speed and solute enrichment at the droplet periphery.

Decorating unoxidized-carbon nanotubes with homogeneous Ni-Co spinel nanocrystals show superior performance for oxygen evolution/reduction reactions.

We present a new concept for homogeneous spinel nanocrystal-coating on high crystalline pristine-carbon nanotubes (CNTs) for efficient and durable oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Oxidized CNTs have widely been used to functionalize with metal or metal oxides since the defect sites act as anchoring for metal oxide binding. However, such defects on the tubes cause the decrease in electrical conductivity and stability, leading to lower catalyst performance. In the present study, at first, pristine multi-walled carbon nanotubes (MWNTs) were wrapped by pyridine-based polybenzimidazole (PyPBI) to which uniform NixCo3-xO4 nanocrystals were homogeneously deposited by the solvothermal method without damaging the MWNTs, in which PyPBI acted as efficient anchoring sites for the deposition of spinel oxide nanocrystals with ~5 nm size. The obtained catalyst (MWNT-PyPBI-NixCo3-xO4) outperformed most state-of-the-art non-precious metal-based bifunctional catalysts; namely, for OER, the potential at 10 mA cm-2 and Tafel slope in 1 M KOH solution were 1.54 V vs. RHE and 42 mV dec-1, respectively. For ORR, the onset and half-wave potentials are 0.918 V and 0.811 V vs. RHE, respectively. Moreover, the MWNT-PyPBI-NixCo3-xO4 demonstrates an excellent durability for both ORR and OER.

Near infrared photoluminescence modulation of single-walled carbon nanotubes based on a molecular recognition approach.

We present a concept to modulate near infrared photoluminescence (NIR-PL) from locally-functionalized single-walled carbon nanotubes (local-f-SWNTs) based on a molecular recognition approach using newly synthesized phenylboronic acid (PB)-functionalized local-f-SWNTs (PB-SWNTs) and saccharides, in which a selective PL spectral shift is observed by addition of the saccharides.

Specific Molecular Interaction and Recognition at Single-Walled Carbon Nanotube Surfaces.

Carbon nanotubes (CNTs) are carbon clusters (polymers) with huge molecular weight and have been the central material in the field of nanomaterials science and nanotechnology because of their remarkable electronic, thermal, mechanical, optical, and electrical properties. In this review article, we first focus on the formation of self-assembled CNT superstructures and spontaneous conductive CNT-honeycomb structure formation from CNT/long-chain ammonium lipids by simple solvent casting. We also summarized our recent studies on specific molecular interactions and recognition at single-walled carbon nanotube surfaces and CNT chirality recognition using specific polymers. For such studies, the key issue is to develop a methodology to solubilize/disperse them in solvent because as-synthesized CNTs form tightly bundled structures as a result of their strong van der Waals interactions and are insoluble in many solvents. For the analysis of molecules and CNT surfaces, the introduction of thermodynamic treatment and an HPLC method using CNT-coated silica as a stationary phase was powerful.

Emergence of new red-shifted carbon nanotube photoluminescence based on proximal doped-site design.

Single-walled carbon nanotubes (SWNTs) show unique photoluminescence (PL) in the near-infrared (NIR) region. Here we propose a concept based on the proximal modification in local covalent functionalization of SWNTs. Quantum mechanical simulations reveal that the SWNT band gap changes specifically based on the proximal doped-site design. Thus, we synthesize newly-designed bisdiazonium molecules and conduct local fucntionalisation of SWNTs. Consequently, new red-shifted PL (E11(2*)) from the bisdiazonium-modified SWNTs with (6, 5) chirality is recognized around 1250 nm with over ~270 nm Stokes shift from the PL of the pristine SWNTs and the PL wavelengths are shifted depending on the methylene spacer lengths of the modifiers. The present study revealed that SWNT PL modulation is enable by close-proximity-local covalent modification, which is highly important for fundamental understanding of intrinsic SWNT PL properties as well as exciton engineering-based applications including photonic devices and (bio)imaging/sensing.

Improved Durability of Electrocatalyst Based on Coating of Carbon Black with Polybenzimidazole and their Application in Polymer Electrolyte Fuel Cells.

Improvement of durability of the electrocatalyst has been the key issue to be solved for the practical application of polymer electrolyte membrane fuel cells. One of the promising strategies to improve the durability is to enhance the oxidation stability of the carbon-supporting materials. In this report, we describe in detail the mechanism of the stability improvement of carbon blacks (CBs; Vulcan and Ketjen) by coating with polybenzimidazole (PBI). Nitrogen adsorption experiments reveal that the PBI coating of CBs results in the capping of the gates of the CB-micropores by the PBI. Since the surface of the micropores inside the CBs are inherently highly oxidized, the capping of such pores effectively prevents the penetration of the electrolyte into the pore and works to avoid the further oxidation of interior of the micropore, which is proved by cyclic voltammogram measurements. Above mechanism agrees very well with the dramatic enhancement of the durability of the membrane electrode assembly fabricated using Pt on the PBI-coated CBs as an electrocatalyst compared to the conventional Pt/CB (PBI-non coated) catalyst.