Different synthesis procedures including hydrothermal, solvothermal, and chemical reduction routes have been previously developed by our group for the production of bismuth nanotubes (BiNTs). It has been shown that a post synthesis treatment with sodium borohydride greatly improves nanotube structure and yield. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy were employed to characterize the bismuth nanotubes. Bismuth has a low melting point and this contributes to energy induced structural changes during in situ TEM and Raman observations. Here, HRTEM and Raman techniques were employed to characterize the products and to gain more insight by using in situ observation of an energy-induced morphology change (EiMC) of the bismuth nanotubes. It is shown that the change of the morphology of the bismuth nanotubes is accompanied by (a) an oxidation process (Bi° → Bi3+) and (b) a progressive rhombohedral-Bi → β-Bi2O3 → α-Bi2O3 phase transformation.

Metal incorporated MCM-41 has proven to be a valuable template for the growth of narrow distributions of single-walled carbon nanotubes (SWNT), producing samples with a wide range of different mean diameters. The ability to obtain narrow diameter distributions at different mean diameters is important for applications that require particular (n,m) nanotubes. Another advantage of this system is the ease of cleaning and low metal content as compared to bimetallic systems. In this Article, we show that Co-MCM-41 allows diameter tuning of SWNT produced over a broad diameter range (from 0.6−0.8 to 1.8−2.0 nm) by changing reaction temperature. The lower temperature reaction provides a robust means to obtain very small diameter SWNT. X-ray absorption experiments show that the change in SWNT diameter correlates with the change in metal particle size.

Understanding how nano-dimensionality impacts iron oxide based catalysis is central to a wide range of applications. Here, we focus on hematite nanosheets, nanowires and nanoparticles as applied to catalyze the reverse water gas shift (RWGS) probe reaction. We introduce a novel approach to synthesize ultrathin (4–7 nm) hematite nanosheets using copper oxide nanosheets as a hard template and propose a reaction mechanism based on density functional theory (DFT) calculations. Hematite nanowires and nanoparticles were also synthesized and characterized. H2 temperature programmed reduction (H2-TPR) and RWGS reactions were performed to glean insights into the mechanism of CO2 conversion to CO over the iron oxide nanomaterials and were compared to H2 binding energy calculations based on density functional theory. While the nanosheets did exhibit high CO2 conversion, 28% at 510 °C, we found that the iron oxide nanowires had the highest CO2 conversion, reaching 50% at 750 °C under atmospheric pressure. No products besides CO and H2O were detected.

CuO is a nonhazardous, earth-abundant material that has exciting potential for use in solar cells, photocatalysis, and other optoelectronic applications. While progress has been made on the characterization of properties and reactivity of CuO, there remains significant controversy on how to control the precise band gap by tuning conditions of synthetic methods. Here, we combine experimental and theoretical methods to address the origin of the wide distribution of reported band gaps for CuO nanosheets. We establish reaction conditions to control the band gap and reactivity via a high-temperature treatment in an oxygen-rich environment. SEM, TEM, XRD, and BET physisorption reveals little to no change in nanostructure, crystal structure, or surface area. In contrast, UV–vis spectroscopy shows a modulation in the material band gap over a range of 330 meV. A similar trend is found in H2 temperature-programmed reduction where peak H2 consumption temperature decreases with treatment. Calculations of the density of states show that increasing the oxygen to copper coverage ratio of the surface accounts for most of the observed changes in the band gap. An oxygen exchange mechanism, supported by 18O2 temperature-programmed oxidation, is proposed to be responsible for changes in the CuO nanosheet oxygen to copper stoichiometry. The changes induced by oxygen depletion/deposition serve to explain discrepancies in the band gap of CuO, as reported in the literature, as well as dramatic differences in catalytic performance.

A safe, scalable method for producing highly conductive aligned films of single-walled carbon nanotubes (SWNTs) from water suspensions is presented. While microfluidic assembly of SWNTs has received significant attention, achieving desirable SWNT dispersion and morphology in fluids without an insulating surfactant or toxic superacid is challenging. We present a method that uniquely produces a noncorrosive ink that can be directly applied to a device in situ, which is different from previous fabrication techniques. Functionalized SWNTs (f-SWNTs) are dispersed in an aqueous urea solution to leverage binding between the amine group of urea and the carboxylic acid group of f-SWNTs and obtain urea-SWNT. Compared with SWNTs dispersed using conventional methods (e.g., superacid and surfactants), the dispersed urea-SWNT aggregates have a higher aspect ratio with a rodlike morphology as measured by light scattering. The Mayer rod technique is used to prepare urea-SWNT, highly aligned films (two-dimensional nematic order parameter of 0.6, 5 μm spot size, via polarized Raman) with resistance values as low as 15–1700 Ω/sq in a transmittance range of 2–80% at 550 nm. These values compete with the best literature values for conductivity of SWNT-enabled thin films. The findings offer promising opportunities for industrial applications relying on highly conductive thin SWNT films.

As hybrid nanomaterials have myriad of applications in modern technology, different functionalization strategies are being intensely sought for preparing nanocomposites with tunable properties and structures. Multi-Walled Carbon Nanotube (MWNT)/CdSe Quantum Dot (QD) heterostructures serve as an important example for an active component of solar cells. The attachment mechanism of CdSe QDs and MWNTs is known to affect the charge transfer between them and consequently to alter the efficiency of solar cell devices. In this study, we present a novel method that enables the exchange of some of the organic capping agents on the QDs with carboxyl functionalized MWNTs upon ultrasonication. This produces a ligand-free covalent attachment of the QDs to the MWNTs. EXAFS characterization reveals direct bond formation between the CdSe QDs and the MWNTs. The amount of oleic acid exchanged is quantified by temperature-programmed decomposition; the results indicate that roughly half of the oleic acid is removed from the QDs upon functionalized MWNT addition. Additionally, we characterize the optical and structural properties of the QD-MWNT heterostructures and investigate how these properties are affected by the attachment. The steady state photoluminescence response of QDs is completely quenched. The lifetime of the PL of the QDs measured with time resolved photoluminescence shows a significant decrease after they are covalently bonded to functionalized MWNTs, suggesting a fast charge transfer between QDs and MWNTs. Our theoretical calculations are consistent with and support these experimental findings and provide microscopic models for the QD binding mechanisms.

Direct fabrication of MgxBy nanostructures is achieved by employing Ni–Mg incorporated MCM-41 in the Hybrid Physical–Chemical Vapor Deposition (HPCVD) reaction. Different reaction conditions are tested to optimize the fabrication process. TEM analysis shows the fabrication of MgxBy nanostructures starting at a reaction temperature of 600 °C, and the yield of the nanostructures increases with the reaction temperature. The as-synthesized MgxBy nanostructures have the diameters in the range of 3–5 nm, which do not increase with the reaction temperature. EELS analysis of the template removed nanostructures confirms the existence of B and Mg with minimal contamination of Si and O. NEXAFS and Raman spectroscopy analyses suggested a concentric layered structure for our as-synthesized MgxBy nanotube/nanowire, which is in good agreement with the theoretical calculations. Ni K-edge XAS indicates that the formation of MgNi alloy particles is important for the Vapor–Liquid–Solid (VLS) growth of MgxBy nanostructures with fine diameters, and the presence of Mg vapor not just Mg in the catalyst is crucial for the formation of Ni–Mg clusters. Physical templating by MCM-41 might also help to confine the diameter of the nanostructures. DC magnetization measurements indicate possible superconductive behaviors in the as-synthesized sample.

Many applications in nanotechnology require short and unentangled single-walled carbon nanotubes (SWCNT). Liquid-phase oxidative cutting gives nonuniform short tubes and causes significant material loss. Mechanical cutting is good for shortening SWCNT, but it leaves collapsed tube ends and might not be favorable for further manipulation. Solid-state reaction cutting is better for multi-walled carbon nanotubes than for SWCNT. Herein, we present a method combining mechanical and oxidative cutting. The SWCNT sample was first ground with a Wig-L-Bug grinding mill for 30 min, introducing structural defects into the side walls of SWCNT. The treated SWCNT were then soaked in a Piranha solution, in which the oxidants attack the existing side wall defects and give a complete cut. According to statistical analysis from transmission electron microscopy, most of the shortened SWCNT fall in the range of 50–200 nm. The material loss is 12.2 wt%. The functional groups on the tube surface introduced by shortening were removed by refluxing in a soda lime/water suspension. Then, the carbon nanotubes were further annealed by sonicating in ethanol. After annealing, the defect level of shortened carbon nanotubes was reduced significantly, as determined by Raman spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis.

Development of synthetic routes to control the morphology and composition of nanostructured thermoelectric materials and to leverage their unique performance enhancements presents challenges in the realization of practical thermoelectric systems. We report here the fabrication of intricate networks of nanostructured tellurium, bismuth telluride, and bismuth-rich compounds with diverse morphologies. The nanostructured networks synthesized via solution-phase techniques consist of nanocrystalline Bi2Te3 with a grain size of about 15–20 nm, 3–5 nm thick rolled-up nanosheets of Te forming tubular structures, nanotubes of Bi2Te3 about 300–400 nm in diameter, Te and Bi4Te3 nanowires ranging from 50 to 200 nm diameter, and microspheres of 3–7 μm diameter composed of self-assembled BiOCl nanorods. The formation and crystallinity of Bi-rich and Te-rich compounds were investigated using powder X-ray and electron back-scattered diffraction. We present the first detailed analysis of micro-Raman scattering of BixTey nanostructures of above morphologies using six different laser wavelengths. The BixTey nanostructures exhibit the most intense infrared (IR) active A1u mode at 120 cm–1 in the Raman spectra, which disperses with a change in the chemical composition and laser power. In addition, we observe new internal strain-induced peaks in the Raman spectra of BixTey nanostructures. The rich morphologies and compositions present within the nanostructured Bi–Te compounds are expected to result in novel thermoelectric materials.

We present here the first experimental evidence of a significant enhancement in the thermoelectric properties of networks containing bismuth nanotubes and nanowires fabricated via aqueous chemical routes. The as-prepared samples revealed two dominant morphologies: double wall nanotubes with 4–6 nm diameter and nanowires of 15–30 nm diameters. Transport measurements in the range 15–300 K were performed on cold pressed pellets which contained these bismuth nanostructures. We find that the cold pressed networks of nanostructured bismuth exhibit a non-metallic behavior, the corresponding room temperature resistivities vary from ∼0.54 to 5.8 mΩ-cm, depending on the detailed nanomorphology. The measured thermal conductivity for the networks of bismuth nanotubes and nanowires is almost 5 times lower compared to bismuth powder at room temperature. The observed improvement in the thermoelectric performance has been attributed to the reduced lattice thermal conductivity of nanostructured bismuth networks via interface scattering.Graphical AbstractHighlights► Synthesis of double wall Bi nanotubes and Bi nanowires using facile aqueous chemical routes. ► Lowest room temperature electrical resistivity is obtained for Bi nanotubes. ► We present the first experimental study of thermoelectric properties of arrays of Bi nanotubes. ► A five-fold decrease in the thermal conductivity of Bi nanostructured networks is reported.

Subnanometer single-walled carbon nanotubes (sub-nm SWNTs) were synthesized at different temperatures (600, 700, and 800 °C) using CoMn bimetallic catalysts supported on MCM-41 silica templates. The state of the catalyst was investigated using X-ray absorption, and the (n,m) indices of the sub-nm SWNTs were determined from Raman spectroscopy and photoluminescence measurements. We find that the size of the metallic particles that seed the growth of sub-nm SWNTs (diameter ∼0.5−1.0 nm) is highly sensitive to the reaction temperature. Low reaction temperature (600 °C) favors the growth of semiconducting tubes whose diameters range from 0.5 to 0.7 nm. These results were also confirmed by electrical transport measurements. Interestingly, dominant intermediate frequency modes on the same intensity scale as the Raman breathing modes were observed. An unusual “S-like” dispersion of the G-band was present in the Raman spectra of sub-nm SWNTs with diameters <0.7 nm.

In this paper, we demonstrate a new route for synthesis of bismuth nanotubes (BiNTs) based on chemical reduction using sodium borohydride. A comparison and an optimization of different synthesis procedures including hydrothermal and solvothermal synthesis for the production of BiNTs are presented. The effects of parameters such as the bismuth precursor, temperature, and reagent mixing process on nanomaterial structure were evaluated. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy were employed to characterize the products from the different reaction schemes. A postsynthesis sodium borohydride treatment improved both yield and nanotube structure for all syntheses studied. Different morphologies of the bismuth nanostructures were observed depending on the synthesis method selected.

Single-walled carbon nanotubes (SWNT) with encapsulated nanosized cobalt particles have been synthesized by a facile and scalable method. In this approach, SWNT were filled with a cobalt acetylacetonate solution in dichloromethane by ultrasonication. In a second step, exposure to hydrogen at different temperatures released discrete cobalt particles of controllable size inside the SWNT cavity. The SWNT-Co particles systems were characterized by transmission electron microscopy, X-ray absorption spectroscopy, Raman spectroscopy, and thermal gravimetric analysis.

A silica-supported cobalt catalyst has been developed via incipient wetness impregnation for high-yield synthesis of single-walled carbon nanotubes (SWNTs). Co/SiO2-impregnated catalysts have not been observed to be efficient for SWNT synthesis. Using an appropriately chosen precursor, we show that effective catalysts can be obtained for SWNT synthesis with yields up to 75 wt %. Detailed characterization indicates that the active sites for SWNT synthesis are small cobalt particles resulting from the reduction of a highly dispersed surface cobalt silicate species. The SWNTs produced by this catalyst are of high quality and easy to purify, and the process is simple and scalable.Keywords: chemical vapor deposition (CVD); Co/SiO2-impregnated catalyst; high-yield synthesis; highly dispersed surface cobalt silicate; single-walled carbon nanotubes (SWNTs)

Mg incorporated mesoporous alumina with high surface area and narrow pore size distribution was prepared by the sol−gel method to use as a template for the synthesis of boron-based nanostructures by the CVD method. The mesoporous nature of the Mg−Al2O3 was confirmed by low-angle XRD and N2 adsorption analyses. The randomly ordered pores were observed by TEM. The growth of boron-based nanostructures, such as nanowires and nanotubes, over Mg−Al2O3 is evidenced by TEM analysis. Use of BCl3 as the boron source resulted in nanowires of around 20 nm in diameter and several micrometers in length. When using B2H6 as the boron source, the diameter of the obtained nanostructures was in the range 3−4 nm, reflective of the pore diameter of Mg−Al2O3. Further loading of Mg in the form of Rieke Mg by sonication over 1% Mg−Al2O3 resulted in the growth of straight nanotubes with a uniform diameter of 3−4 nm. The NEXAFS of the boron K-edge of purified boron-based nanostructures shows characteristic peaks for the existence of Mg−B and Al−B bonding. The STEM and EELS of the obtained individual nanostructures confirm the presence of B, Mg, and Al in the nanostructures. The appearance of a boron prepeak at 186 eV in EELS indicates that the boron hole states are not filled by the aluminum present in the nanostructures. Both field dependent DC magnetization (SQUID) and AC magnetic susceptibility measurements of MgxBy nanostructures grown over 2 wt % Rieke Mg sonicated 1% Mg−Al2O3 using B2H6 as the boron source show evidence of a diamagnetic transition at about 80 K. The diamagnetic nature of MgxBy nanostructures is suppressed significantly when the grown MgxBy nanostructures are washed with NaOH solution to remove the template.

The effect of manganese addition to the Co-MCM-41 catalyst on the synthesis of single wall carbon nanotubes (SWNT) by CO disproportionation was characterized. The ratio between the two metals in the MCM-41 framework was varied, and its effect on the resultant SWNT distribution was studied and compared with the results obtained for the monometallic Co-MCM-41 catalyst. Methods including temperature-programmed reduction, X-ray absorption fine structure, thermogravimetric analysis, TEM imaging, and Raman and fluorescence spectroscopy were employed to characterize the behavior of the catalysts under the SWNT synthesis conditions and the diameter and structure distribution of the resultant nanotubes. We found that addition of Mn to the Co-MCM-41 catalyst promotes the growth of SWNT, leading to synthesis of high yield, small diameter SWNT. Manganese does not act in the nucleation of SWNT but acts as an anchoring site for cobalt particles formed during the synthesis process as shown by X-ray absorption.

Highly ordered MCM-41 mesoporous molecular sieves in which silicon was isomorphously substituted with 0.5–4 wt.% cobalt were synthesized using an alkyl template with a 16 carbon atoms alkyl chain length. These materials were used as catalysts for the synthesis of uniform diameter single wall carbon nanotubes (SWNT) by CO disproportionation (Boudouard reaction). The SWNT synthesis conditions were identical for all Co-MCM-41 samples, and consisted of pre-reduction of the Co-MCM-41 catalyst in hydrogen at 500 °C for 30 min followed by reaction with pure CO at 800 °C and 6 atm for 1 h (conditions previously optimized for 1 wt.% Co-MCM-41). The SWNT grown in the Co-MCM-41 catalysts were characterized by TGA, multi-excitation energy Raman spectroscopy and TEM. The state of the catalyst and the size of the metallic cobalt clusters formed in Co-MCM-41 during the SWNT synthesis were characterized by X-ray absorption spectroscopy. The mechanism controlling the diameter distribution of the SWNT produced is related to the size uniformity of the cobalt clusters nucleated in the Co-MCM-41 catalytic template: the SWNT growth selectivity and size uniformity is influenced by the cobalt concentration in the framework. If the cobalt is not initially strongly stabilized in the MCM-41 framework during template synthesis, the catalyst produces SWNT with a wider diameter distribution. Co-MCM-41 catalysts with up to 3 wt.% cobalt can be used to grow SWNT with a diameter distribution similar to that obtained with 1 wt.% Co-MCM-41, but at yields greater by a factor of approximately 2.4.

•EXAFS and XRD analysis indicates that after adding water, cobalt was oxidized and amorphous carbon formation was suppressed.•TGA data demonstrate that the SWCNT yield increases by about 100% with a water concentration of 7%.•Raman and PLE spectroscopic data demonstrate that the SWCNT diameter increases with the water/ethanol ratio.•We discussed influences of water on the yield and diameter of SWCNT.Single-walled carbon nanotubes (SWCNT) were synthesized using Co-MCM-41 catalysts through water-assisted ethanol pyrolysis. The effect of varying the water to ethanol ratio in the reactant mixture was studied. The data from X-ray absorption spectroscopy indicate that with the addition of water, the cobalt metal particles were oxidized. X-ray diffraction analysis of CoCx suggests that the formation of amorphous carbon on the surface of the Co particles was suppressed. Cobalt magnetization measurements were performed to study the size and anisotropy of cobalt particles. Thermogravimetric analysis data demonstrate that with a water concentration of 7%, the yield increases by about 100% relative to pure ethanol synthesis. Raman and photoluminescence excitation spectroscopic data demonstrate that the SWCNT diameter increases (within the diameter range detected) with the water/ethanol ratio. From the statistical SWCNT diameter distribution obtained from transmission electron microscopy, 65% of the SWCNT synthesized with 20% water in ethanol have diameters larger than 1.5 nm, but there is a severe decrease in yield and a modest decrease in selectivity of SWCNT. The mechanism of how the water influences the yield and diameter of SWCNT is discussed.Graphical abstractDownload high-res image (118KB)Download full-size image

We present a study on CoCr–MCM-41 catalysts with stable nm-sized particles. The focus is their promoting effect on the synthesis of Single-Wall Carbon Nanotubes (SWNT).Bimetallic CoCr–MCM-41 catalysts were synthesized by combined grafting and incorporation of metals in the framework. This synthesis allowed an increase in the maximum metal loading in the MCM-41 framework while maintaining nm-sized Co particles stable in high-temperature reactive environments. The SWNT yield was increased by more than 100% from the Co–MCM-41 catalyst.Cobalt is responsible for SWNT nucleation. The role of chromium is to anchor small cobalt particles during reduction and prevent their sintering into large unreactive particles. A larger fraction of the cobalt in the bimetallic catalysts becomes available for the SWNT synthesis when compared to the monometallic one, leading to a significant yield increase. Another effect of the addition of Cr was a shift of the SWNT diameter distribution to smaller nanotubes.Characterization of SWNT synthesized on various catalytic systems: Co grafted on MCM-41 (CogMCM-41), Co incorporated on MCM-41 (Co–MCM-41), Cr grafted on Co–MCM-41 (CrgCo) and Co grafted on Cr–MCM-41 (CogCr). Left – resultant SWNT yield. Right – Resonant Raman Spectroscopy of SWNT.Download high-res image (97KB)Download full-size image

Point of zero charge is a useful measurement to assess the surface acidity of multi-walled carbon nanotubes and to characterize functional groups on the multi-walled carbon nanotube surface. Knowledge of point of zero charge also assists the choice of an appropriate metal precursor to be used for electrostatic adsorption to prepare metal particles supported on multi-walled carbon nanotubes. Multi-walled carbon nanotubes with points of zero charge that range from 2.2 to 11.8 have been prepared and characterized. An efficient reduction method, utilizing ethanol at 20 atm and 180 °C, is described and can be used to reduce nitric acid functionalized multi-walled carbon nanotubes to convert the functional groups to mostly hydroxyls.

Direct fabrication of MgxBy nanostructures is achieved by employing Ni–Mg incorporated MCM-41 in the Hybrid Physical–Chemical Vapor Deposition (HPCVD) reaction. Different reaction conditions are tested to optimize the fabrication process. TEM analysis shows the fabrication of MgxBy nanostructures starting at a reaction temperature of 600 °C, and the yield of the nanostructures increases with the reaction temperature. The as-synthesized MgxBy nanostructures have the diameters in the range of 3–5 nm, which do not increase with the reaction temperature. EELS analysis of the template removed nanostructures confirms the existence of B and Mg with minimal contamination of Si and O. NEXAFS and Raman spectroscopy analyses suggested a concentric layered structure for our as-synthesized MgxBy nanotube/nanowire, which is in good agreement with the theoretical calculations. Ni K-edge XAS indicates that the formation of MgNi alloy particles is important for the Vapor–Liquid–Solid (VLS) growth of MgxBy nanostructures with fine diameters, and the presence of Mg vapor not just Mg in the catalyst is crucial for the formation of Ni–Mg clusters. Physical templating by MCM-41 might also help to confine the diameter of the nanostructures. DC magnetization measurements indicate possible superconductive behaviors in the as-synthesized sample.