Catalyst Chemical Vapor Deposition Process Research Articles

Deposition of ZIF-67 and polypyrrole on current collector knitted from carbon nanotube-wrapped polymer yarns as a high-performance electrode for flexible supercapacitors

Polymeric fiber fabrics with inherent mechanical flexibility, lightweight and porous textile structure are highly attractive as an ideal substrate for building flexible electrodes. However, their insulating nature limits their direct application as current collector. Here, we designed and fabricated a flexible conductive fabric from polymer yarns. This was achieved through wrapping polymer yarns with a carbon nanotube (CNT) cylinder, which was continuously prepared from a floating catalyst chemical vapor deposition process, and a subsequent knitting process. The derived CNT-wrapped polymer yarn fabric (CPYF) could directly serve as current collector/substrate to load zeolitic imidazolate framework-67 (ZIF-67) and polypyrrole (PPy) through in-situ growth and chemical polymerization. The resultant CPYF-ZIF-67-PPy displayed a maximum areal capacitance of 2308.8 mF cm−2 at 0.5 mA cm−2. Moreover, the assembled supercapacitor achieved a maximum areal energy density of 112 μW h cm−2 at the power density of 201.5 μW cm−2. Meanwhile, the device demonstrated an extraordinary flexibility with stable electrochemical properties after 5000 cycles of bending and 1000 cycles of stretching. This work therefore offers a new strategy that can be used to develop flexible conductive fabric for flexible supercapacitors.

Highly Oriented Direct-Spun Carbon Nanotube Textiles Aligned by In Situ Radio-Frequency Fields.

Carbon nanotubes (CNTs) individually exhibit exceptional physical properties, surpassing state-of-the-art bulk materials, but are used commercially primarily as additives rather than as a standalone macroscopic product. This limited use of bulk CNT materials results from the inability to harness the superb nanoscale properties of individual CNTs into macroscopic materials. CNT alignment within a textile has been proven as a critical contributor to narrow this gap. Here, we report the development of an altered direct CNT spinning method based on the floating catalyst chemical vapor deposition process, which directly interacts with the self-assembly of the CNT bundles in the gas phase. The setup is designed to apply an AC electric field to continuously align the CNTs in situ during the formation of CNT bundles and subsequent aerogel. A mesoscale CNT model developed to simulate the alignment process has shed light on the need to employ AC rather than DC fields based on a CNT stiffening effect (z-pinch) induced by a Lorentz force. The AC-aligned synthesis enables a means to control CNT bundle diameters, which broadened from 16 to 25 nm. The resulting bulk CNT textiles demonstrated an increase in the specific electrical and tensile properties (up to 90 and 460%, respectively) without modifying the quantity or quality of the CNTs, as verified by thermogravimetric analysis and Raman spectroscopy, respectively. The enhanced properties were correlated to the degree of CNT alignment within the textile as quantified by small-angle X-ray scattering and scanning electron microscopy image analysis. Clear alignment (orientational order parameter = 0.5) was achieved relative to the pristine material (orientational order parameter = 0.19) at applied field intensities in the range of 0.5–1 kV cm–1 at a frequency of 13.56 MHz.

Ultrasensitive Airflow Sensors Based on Suspended Carbon Nanotube Networks.

High-performance airflow sensors are in great demand in numerous fields but still face many challenges, such as slow response speed, low sensitivity, large detection threshold, and narrow sensing range. Carbon nanotubes (CNTs) exhibit many advantages in fabricating airflow sensors due to their nanoscale diameters, excellent mechanical and electrical properties, and so on. However, the intrinsic extraordinary properties of CNTs are not fully exhibited in previously reported CNT-based airflow sensors due to the mixed structures of macroscale CNT assemblies. Herein, this article presents suspended CNT networks (SCNTNs) as high-performance airflow sensors, which are self-assembled by ultralong CNTs and short CNTs in a one-step floating catalyst chemical vapor deposition process. The SCNTN-based airflow sensors achieved a record-breaking short response time of 0.021 s, a high sensitivity of 0.0124 s m-1 , a small detection threshold of 0.11 m s-1 , and a wide detection range of ≈0.11-5.51 m s-1 , superior to most of the state-of-the-art airflow sensors. To reveal the sensing mechanism, an acoustic response testing system and a mathematical model are developed. It is found that the airflow-caused intertube stress change resulted in the resistance variation of SCNTNs.

Continuous synthesis of high-crystalline carbon nanotubes by controlling the configuration of the injection part in the floating catalyst chemical vapor deposition process

Continuous synthesis of high-crystalline carbon nanotubes (CNTs) is achieved by reconfiguring the injection part in the reactor that is used in the floating catalyst chemical vapor deposition (FC-CVD) process. The degree of gas mixing is divided into three cases by adjusting the configuration of the injection part: Case 1: most-delayed gas mixing (reference experiment), Case 2: earlier gas mixing than Case 1, Case 3: earliest gas mixing. The optimal synthesis condition is obtained using design of experiment (DOE) in the design of Case 1, and then is applied to the other cases to compare the synthesis results. In all cases, the experiments are performed by varying the timing of gas mixing while keeping the synthesis conditions constant. Production rate (Case 1: 0.63 mg/min, Case 2: 0.68 mg/min, Case 3: 1.29 mg/min) and carbon content (Case 1: 39.6 wt%, Case 2: 57.1 wt%, Case 3: 71.6 wt%) increase as the gas-mixing level increases. The amount of by-products decreases stepwise as the gas-mixing level increases. The IG/ID ratio increases by a factor of 7 from 10.3 (Case 1) to 71.7 (Case 3) as the gas-mixing level increases; a high ratio indicates high-crystalline CNTs. The radial breathing mode (RBM) peak of Raman spectrograph is the narrowest and sharpest in Case 3; this result suggests that the diameter of the synthesized CNTs is the most uniform in Case 3. This study demonstrates the importance of configuration of the injection part of the reactor for CNT synthesis using FC-CVD.

Impact of aligned carbon nanotubes array on the magnetostatic isolation of closely packed ferromagnetic nanoparticles

We investigate the influence of carbon nanotubes (CNT) aligned array on the magnetic properties of ensemble of densely packed Co nanoparticles (NPs) embedded inside CNT. Each CNT contains only one nanosized Co. Such a special structure was formed by catalyst chemical vapor deposition (CCVD) activated by current discharge plasma and hot filament. The Co NPs, previously deposited onto SiO2/Si substrate, acted as a catalyst. By varying the parameters of the CCVD process, we were able to also sputter the substrate instead of CNT growth. Co NPs were used as a mask and the structure of Si-based nanocones with Co NPs on the top of each cone was formed. Exhaustive investigation of the structural, morphology and crystalline properties of Co nanoparticles were performed. The magnetic properties of two kinds of samples, Co on the Si-based nanocone and Co inside CNT, were differ drastically. In the former case, the magnetic anisotropy of thin-film-type has been observed with large magnetic domains. Whereas for the Co-CNT samples ferromagnetic NPs were magnetically isolated. It was established that the magnetic anisotropy of nanosized Co plays more dominant role than the dipole interaction between Co NPs. The role of the CNT container in this is discussed.

Layered manganese oxides-decorated and nickel foam-supported carbon nanotubes as advanced binder-free supercapacitor electrodes

Three-dimensional carbon nanotubes@MnO2 core–shell nanostructures grown on Ni foam for binder-free capacitor electrodes have been fabricated by a floating catalyst chemical vapor deposition process and a facile hydrothermal approach. Ultrathin layered MnO2 nanosheets are uniformly coated on the surface of the carbon nanotubes (CNTs), directly grown on Ni foam. This unique well-designed binder-free electrode exhibits a high specific capacitance (325.5 F g−1 at a current density of 0.3 A g−1), good rate capability (70.7% retention), and excellent cycling stability (90.5% capacitance retention after 5000 cycles), due to the high conductivity of the close contact between CNTs and Ni foam, as well as the moderate specific surface area of the CNTs@MnO2 core–shell nanostructures. The developed synthetic strategy may provide design guidelines for constructing advanced binder-free supercapacitors electrode.

Synthesis of Brushlike Carbon Nanotubes Using Wet-Processed Catalyst

For a large-scale process of synthesizing vertically aligned (brushlike) multiwalled carbon nanotubes using a thermal catalyst chemical vapor deposition process, a wet process for coating Fe catalyst onto a substrate has been investigated. Dimethylformamide was used for dissolving Fe(NO3)3 to form uniform films of Fe catalyst, and hence to synthesize uniform brushlike nanotubes on 6-in. Si substrates. The advantages of amine series solvent are discussed in terms of solubility in water and formation of Fe complexes.