Boron-Doped Nanocrystalline Diamond-Carbon Nanospike Hybrid Electron Emission Source

Cathodic Activation of Titania-Fly Ash Cenospheres for Efficient Electrochemical Hydrogen Production: A Proposed Solution to Treat Fly Ash Waste

Fly ash (FA) is a waste product generated in huge amounts by coal-fired electric and steam-generating plants. As a result, the use of FA alone or in conjunction with other materials is an intriguing study topic worth exploring. Herein, we used FA waste in conjunction with titanium oxide (TiO2) to create (FA-TiO2) nanocomposites. For the first time, a cathodic polarization pre-treatment regime was applied to such nanocomposites to efficiently produce hydrogen from an alkaline solution. The FA-TiO2 hybrid nanocomposites were prepared by a straightforward solvothermal approach in which the FA raw material was mixed with titanium precursor in dimethyl sulfoxide (DMSO) and refluxed during a given time. The obtained FA-TiO2 hybrid nanocomposites were fully characterized using various tools and displayed a cenosphere-like shape. The synthesized materials were tested as electrocatalysts for the hydrogen evolution reaction (HER) in 0.1 M KOH solution in the dark, employing various electrochemical techniques. The as-prepared (unactivated) FA-TiO2 exhibited a considerable HER electrocatalytic activity, with an onset potential (EHER) value of −144 mV vs. RHE, a Tafel slope (−bc) value of 124 mV dec−1 and an exchange current density (jo) of ~0.07 mA cm−2. The FA-TiO2′s HER catalytic performance was significantly enhanced upon cathodic activation (24 h of chronoamperometry measurements performed at a high cathodic potential of −1.0 V vs. RHE). The cathodically activated FA-TiO2 recorded HER electrochemical kinetic parameters of EHER = −28 mV, −bc = 115 mV dec−1, jo = 0.65 mA cm−2, and an overpotential η10 = 125 mV to yield a current density of 10 mA cm−2. Such parameters were comparable to those measured here for the commercial Pt/C under the same experimental conditions (EHER = −10 mV, −bc = 113 mV dec−1, jo = 0.88 mA cm−2, η10 = 110 mV), as well as to the most active electrocatalysts for H2 generation from aqueous alkaline electrolytes.

Nitrogen-Incorporated Boron-Doped Nanocrystalline Diamond Nanowires for Microplasma Illumination

The origin of nitrogen-incorporated boron-doped nanocrystalline diamond (NB-NCD) nanowires as a function of substrate temperature (T_s) in H₂/CH₄/B₂H₆/N₂ reactant gases is systematically addressed. Because of T_s, there is a drastic modification in the dimensional structure and microstructure and hence in the several properties of the NB-NCD films. The NB-NCD films grown at low T_s (400 °C) contain faceted diamond grains. The morphology changes to nanosized diamond grains for NB-NCD films grown at 550 °C (or 700 °C). Interestingly, the NB-NCD films grown at 850 °C possess one-dimensional nanowire-like morphological grains. These nanowire-like NB-NCD films possess the co-existence of the sp³-diamond phase and the sp²-graphitic phase, where diamond nanowires are surrounded by sp²-graphitic phases at grain boundaries. The optical emission spectroscopy studies stated that the CN, BH, and C₂ species in the plasma are the main factors for the origin of nanowire-like conducting diamond grains and the materialization of graphitic phases at the grain boundaries. Moreover, conductive atomic force microscopy studies reveal that the NB-NCD films grown at 850 °C show a large number of emission sites from the grains and the grain boundaries. While boron doping improved the electrical conductivity of the NCD grains, the nitrogen incorporation eased the generation of graphitic phases at the grain boundaries that afford conducting channels for the electrons, thus achieving a high electrical conductivity for the NB-NCD films grown at 850 °C. The microplasma devices using these nanowire-like NB-NCD films as cathodes display superior plasma illumination properties with a threshold field of 3300 V/μm and plasma current density of 1.04 mA/cm² with a supplied voltage of 520 V and a lifetime stability of 520 min. The outstanding plasma illumination characteristics of these conducting nanowire-like NB-NCD films make them appropriate as cathodes and pave the way for the utilization of these materials in various microplasma device applications.

Hydrogenation of diamond nanowire surfaces for effective electrostatic charge storage

We report a novel versatile method for writing charged areas on diamond nanowire (DNW) surfaces using an atomic force microscopy (AFM) tip. Transmission electron microscopy (TEM) investigations revealed the existence of abundant plate-like diamond aggregates, which were encased in layers of graphite, forming nano-sized diamond-graphite composites (DGCs) on DNW surfaces. These DGCs are the main feature, acting as charge-trapping centers and storing electrostatic charge. A hydrogenation process has been observed effectively enhancing the charge-trapping properties of these DNW materials. The effective charge trapping properties with hydrogenation are ascribed to the disintegration of the DGCs into smaller pieces, with an overall increase in the metallic nanographitic phase fractions in a dielectric diamond matrix. Moreover, the written charge on the surface can be easily modified, re-written, or completely erased, enabling application in diamond-based re-writable electronic devices. However, excessive hydrogenation degrades the charge-trapping properties, which is attributed to the etching of the DGCs from the surface. This study demonstrates the potential importance of a simple hydrogenation process in effective electrostatic charge trapping and storage for diamond related nanocarbon materials and the role of DGCs to further enhance it.