Kei Hasegawa

Millimeter-tall single-walled carbon nanotubes rapidly grown with and without water.

Millimeter-tall vertically aligned single-walled carbon nanotubes (SWCNTs) were grown in 10-15 min by chemical vapor deposition from C(2)H(2)/Ar with or without water addition using Fe catalyst supported on an Al-Si-O underlayer. Using combinatorial catalyst libraries coupled with the real-time monitoring of SWCNT growth, the catalyst and chemical vapor deposition conditions were systematically examined, and millimeter-tall SWCNTs were obtained even without water addition. The key for millimeter-scale growth of SWCNTs is to limit the C(2)H(2) supply to below a certain partial pressure to retain an active catalyst. Water prolongs the catalyst lifetime under excess C(2)H(2) supply, whereas it deactivates small catalyst particles and degrades the quality of SWCNTs at the same time. We also observed a gradual increase in the diameter of SWCNTs with growth because of the coarsening of catalyst particles and found that water had no effect on this phenomenon. We demonstrate millimeter-tall SWCNTs grown by simply using C(2)H(2)/Ar gas without water addition, which revealed the mysterious role of water, and we show a practical route for the large-scale production of SWCNTs.

Diameter Increase in Millimeter-Tall Vertically Aligned Single-Walled Carbon Nanotubes during Growth

Diameter increase was found during the rapid, vertically aligned growth of millimeter-long single-walled carbon nanotubes (SWCNTs). The diameters continuously increased on average from 1.7 nm at the top to 3.7 nm at the bottom of vertically aligned SWCNTs. The SWCNT structures ranged from straight and bundled at the top to bent and isolated and even collapsed at the bottom. Our findings show the importance of suppressing catalyst coarsening to obtain thin SWCNTs, as well as a potential production route of thick SWCNTs and edgeless bilayer graphene ribbons.

Real-Time Monitoring of Millimeter-Tall Vertically Aligned Single-Walled Carbon Nanotube Growth on Combinatorial Catalyst Library

The rapid growth dynamics of millimeter-tall, vertically aligned single-walled carbon nanotubes (VA-SWCNTs) was studied using a simple real-time monitoring method. By using combinatorial catalyst libraries, VA-SWCNT growth curves under various catalyst conditions were obtained in a single chemical vapor deposition (CVD) run. VA-SWCNTs grew at constant or gradually decreasing rates for several minutes and then abruptly ceased growth. This unusual behavior of the growth occurred under wide ranges of catalyst and CVD conditions.

Carbon nanotube 3D current collectors for lightweight, high performance and low cost supercapacitor electrodes

Self-supporting hybrid electrodes were fabricated through the systematic combination of activated carbon (AC), a low cost capacitive material, with sub-millimetre long few-wall carbon nanotubes (FWCNTs). After an easy three-step (mixing, dispersion and filtration) process, robust self-supporting films were obtained, comprising 90% AC particles wrapped in a 3-dimensional FWCNT collector. The 10% FWCNTs provide electrical conductivity and mechanical strength, and replace heavier metal collectors. The FWCNT matrix effectively improved the capacitance of the inexpensive, high surface area AC to 169 F g−1 at a slow scan rate of 5 mV s−1, and to 131 F g−1 at a fast scan rate of 100 mV s−1, in fairly thick (∼200 μm) electrodes. Connection to a metallic collector at the film edge only, which significantly reduced the use of metal, retained much larger capacitance for the AC-FWCNT hybrid film (107 F g−1) than for the conventional AC electrode with binder and conductive filler (3.9 F g−1) at a practical voltage scan rate, 100 mV s−1. Transport measurements in three- and two-electrode cells show that the FWCNT matrix can greatly enhance the conductivity of the AC-based films.

Methane-Assisted Chemical Vapor Deposition Yielding Millimeter-Tall Single-Wall Carbon Nanotubes of Smaller Diameter

We examined the use of low purity H2 (96 vol % H2 with 4 vol % CH4) in chemical vapor deposition (CVD) using a C2H2 feedstock, and obtained vertically aligned single-wall carbon nanotubes (VA-SWCNTs) with unexpectedly smaller diameters, larger height, and higher quality compared with those grown using pure H2. During the catalyst annealing, carbon deposited at a small amount from CH4 on the Fe particles, which kept them small and dense. During CVD, CH4 prevented the Fe particles from coarsening, resulting in an enhanced growth lifetime and suppressed diameter increase of growing SWCNTs. These effects were observed only for CH4, and not for C2H4 or C2H2. CH4-assisted CVD is an efficient and practical method that uses H2 containing CH4 that is available as a byproduct in chemical factories.

Important factors for effective use of carbon nanotube matrices in electrochemical capacitor hybrid electrodes without binding additives

Various capacitive particles are currently available for use in electrochemical capacitors, but their electrical conductivities are typically low and require enhancement. Carbon nanotube (CNT) matrices can be used to fabricate self-supporting electrodes without binding or conducting additives. Herein, liquid dispersion and subsequent vacuum filtration were used to prepare thick (∼100 μm) hybrid electrodes of activated carbon (AC) and CNTs. Factors including CNT type, AC particle size, solvent, and surfactant strongly affected the capacitance and rate performance of the hybrid electrodes. Different solvents and types of CNTs were best suited for pure CNT electrodes and AC–CNT hybrid electrodes; single-wall CNTs (SWCNTs) with a strong dispersant produced CNT electrodes with the best performance among pure CNT electrodes. Meanwhile, few-wall CNTs (FWCNTs) with a weak dispersant produced AC–CNT hybrid electrodes with the best performance among hybrid electrodes. Addition of 10 wt% FWCNTs to AC yielded a self-supporting hybrid electrode with improved performance (132 F g−1, 58 F cm−3, 0.70 F cm−2 at 100 mV s−1) compared with that of a conventional AC electrode with conducting and binding additives (74 F g−1, 26 F cm−3, 0.60 F cm−2 at 100 mV s−1), and a pure electrode of expensive SWCNTs (52 F g−1, 26 F cm−3, 0.26 F cm−2 at 100 mV s−1).

Denser and taller carbon nanotube arrays on Cu foils useable as thermal interface materials

To achieve denser and taller carbon nanotube (CNT) arrays on Cu foils, catalyst and chemical vapor deposition (CVD) conditions were carefully engineered. CNTs were grown to ~50 µm using Fe/TiN/Ta catalysts in which Ta and TiN acted as diffusion barriers for Cu and Ta, respectively. A tradeoff was found between the mass density and height of the CNT arrays, and CNT arrays with a mass density of 0.30 g cm−3 and height of 45 µm were achieved under optimized conditions. Thermal interface materials (TIMs) with CNT array/Cu foil/CNT array structures showed decreasing thermal resistance from 86 to 24 mm2 K W−1 with increasing CNT array mass densities from 0.07–0.08 to 0.19–0.26 g cm−3 for Cu and Al blocks with surfaces as rough as 20–30 µm. The best CNT/Cu/CNT TIMs showed thermal resistance values comparable to that of a typical indium sheet TIM.

50–100 μm-thick pseudocapacitive electrodes of MnO2 nanoparticles uniformly electrodeposited in carbon nanotube papers

To overcome the tradeoff between the gravimetric capacitance and loading density of pseudocapacitive MnO2, we electrodeposited MnO2 nanoparticles on the carbon nanotube (CNT) surfaces in 18–37 μm-thick self-supporting CNT papers. We examined the electrodeposition conditions including constant potential, constant current, and potential pulses, and obtained MnO2–CNT hybrid electrodes containing MnO2 nanoparticles uniformly deposited at 60–90 wt% with an expanded CNT matrix. The MnO2–CNT hybrid electrode with a thickness of 62 μm, density of 1.09 g cm−3, areal mass of 6.75 mg cm−2, and 82 wt% MnO2 load showed a total gravimetric capacitance of 120 and 51 Ftotal gelectrode−1, volumetric capacitance of 131 and 56 Ftotal cm−3 and areal capacitance of 0.81 and 0.34 Ftotal cm−2 at scan rates of 2 and 200 mV s−1, respectively. The large thickness, moderately high mass density, and fairly conductive CNT matrix realized such high values of gravimetric, areal and volumetric capacitances that are important for practical devices.

Carbon nanotube–silicon heterojunction solar cells with surface-textured Si and solution-processed carbon nanotube films

Carbon nanotube (CNT)–silicon (Si) heterojunction solar cells are fabricated with surface-textured Si substrates. Using a dilute alkaline solution, common etchant in the Si solar cell industry, we formed a pyramidal texture on the Si substrate surface. The texture effectively enhances the absorption of the incident light, improving the short-circuit current density by ∼1.3-fold, up to 33.1 mA cm−2. We fabricated CNT–Si solar cells with a power conversion efficiency (PCE) of 10.4% without any anti-reflective coatings or doping of the CNTs. Moreover, the CNT films were prepared from commercialized CNT agglomerates by a mild solution-based process, which is well suited for the fabrication of CNT–Si solar cells with large area. We also achieved a PCE of 9.57% for a flat cell with careful removal of surfactant from and doping by nitric acid of the CNT films. These findings suggest that with the combination of surface-textured Si and solution-processed CNT films, efficient and low-cost CNT–Si solar cells may be realized.

Rapid vapour deposition and in situ melt crystallization for 1 min fabrication of 10 μm-thick crystalline silicon films with a lateral grain size of over 100 μm

We developed a film deposition method which yielded continuous polycrystalline Si films with large lateral grain sizes of over 100 μm and thicknesses of ∼10 μm in 1 min on growth substrates other than silicon wafers in a single-step process. The silicon source is heated to ∼2000 °C, much higher than the melting point of Si, which enables a high deposition rate. Controlling the temperature of the growth substrate, initially above and later below the melting point of Si, allows the seamless lateral to vertical growth of crystalline silicon grains. Thermally and chemically stable substrates of quartz glass and alumina with a 0.1 μm-thick amorphous carbon layer were effective; liquid silicon wetted well by forming a thin SiC interlayer while substrates stayed stable. Such large-grain polycrystalline silicon films synthesized rapidly in 1 min may be used for low-cost, stable and flexible thin film photovoltaic cells.