An efficient method to investigate the microstructure and spatial distribution of nitrogen and nitrogen-vacancy (N-V) defects in detonation nanodiamond (DND) with primary particle sizes ranging from approximately 3 to 50 nm is presented. Detailed analysis reveals atomic nitrogen concentrations as high as 3 at% in 50% of diamond primary particles with sizes smaller than 6 nm. A non-uniform distribution of nitrogen within larger primary DND particles is also presented, indicating a preference for location within the defective central part or at twin boundaries. A photoluminescence (PL) spectrum with well-pronounced zero-phonon lines related to the N-V centers is demonstrated for the first time for electron-irradiated and annealed DND particles at continuous laser excitation. Combined Raman and PL analysis of DND crystallites dispersed on a Si substrate leads to the conclusion that the observed N-V luminescence originates from primary particles with sizes exceeding 30 nm. These findings demonstrate that by manipulation of the size/nitrogen content in DND there are prospects for mass production of nanodiamond photoemitters based on bright and stable luminescence from nitrogen-related defects.
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Non-precious-metal catalysts are
promising alternatives for Pt-based
cathode materials in low-temperature fuel cells, which is of great
environmental importance. Here, we have investigated the bifunctional
electrocatalytic activity toward the oxygen reduction reaction (ORR)
and the oxygen evolution reaction (OER) of mixed metal (FeNi; FeMn;
FeCo) phthalocyanine-modified multiwalled carbon nanotubes (MWCNTs)
prepared by a simple pyrolysis method. Among the bimetallic catalysts
containing nitrogen derived from corresponding metal phthalocyanines,
we report the excellent ORR activity of FeCoN-MWCNT and FeMnN-MWCNT
catalysts with the ORR onset potential of 0.93 V and FeNiN-MWCNT catalyst
for the OER having
E
OER
= 1.58 V at 10
mA cm
–2
. The surface morphology, structure, and
elemental composition of the prepared catalysts were examined with
scanning electron microscopy, X-ray diffraction, and X-ray photoelectron
spectroscopy. The FeCoN-MWCNT and FeMnN-MWCNT catalysts were prepared
as cathodes and tested in anion-exchange membrane fuel cells (AEMFCs).
Both catalysts displayed remarkable AEMFC performance with a peak
power density as high as 692 mW cm
–2
for FeCoN-MWCNT.
Cobalt-
and nitrogen-doped carbide-derived carbon/carbon nanotube
(CDC/CNT) composites are prepared and used as oxygen reduction reaction
(ORR) electrocatalysts for an anion exchange membrane fuel cell (AEMFC)
cathode. For the doping, high-temperature pyrolysis is applied using
a cobalt salt and a nitrogen precursor (either dicyandiamide, urea,
or melamine). During the doping, (i) new mesopores are formed as confirmed
by the N2 physisorption results, (ii) atomically dispersed
cobalt is present on the catalysts as detected by scanning transmission
electron microscopy, and (iii) N-pyridinic and Co–N4 are the dominant N-containing species as shown by X-ray photoelectron
spectroscopy. This indicates that using the composite of CDC and CNTs
as well as the cobalt salt and nitrogen precursor is advantageous
for the preparation of electrocatalysts. All three catalyst materials
demonstrate similarly good electrocatalytic activity toward O2 electroreduction in alkaline medium and excellent stability
after 10000 repetitive potential cycles. The Co-N-CDC/CNT catalyst
as the cathode material together with a hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI) membrane exhibits excellent
AEMFC performance by reaching maximum power density of 577 mW cm–2.
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