Y.L. Kim et al. / Journal of Alloys and Compounds 663 (2016) 574e580
575
example, polycrystalline rhodium nanowire formation using elec-
2.3. Electrochemical measurements
trodeposition through template synthesis [17]. Up to now, the
template-free, synthesis of rhodium oxide nanofibers(Rh2O3 NF)
with a large scale has not been reported. Ruthenium oxide (RuO2)
has excellent chemical and thermal stability so that its nano-
structures have been used for many applications such as high en-
ergy density capacitors, hydrogenation and the oxidation of CO. In
addition, high electrical conductivity of ruthenium oxide often
helps to facilitate electrochemical processes when conjunction
with platinum and rhodium [18]. Despite the fact that the ruthe-
nium oxide nanowire has excellent properties, very few hierar-
chical hetero-structure formations with other materials have been
demonstrated so far. Recently, we reported ruthenium oxide
nanostructure growth on carbon fibers/carbon nanotubes and
ruthenium oxide nanowire growth on graphene substrates
[19e22].
Hydrogen peroxide (H2O2, 35 wt%), potassium nitrate (KNO3),
sodium phosphate monobasic (NaH2PO4), sodium phosphate
dibasic (Na2HPO4), and Nafion (5%) were purchased from Sigma-
eAldrich (St. Louis, MO). Oxygen (O2) and argon (Ar) gases were
obtained from Dong-A Gas Co. (Seoul, Korea). As synthesized Rh2O3
nanofibers (Rh2O3 NF) or RuO2 nanowires grown on Rh2O3 NF
(RuO2 NWeRh2O3 NF) were dispersed in deionized water
(2 mg mLꢁ1). 6
mL of the dispersed sample was loaded on a cleaned
glassy carbon (GC) disk electrode (A ¼ 0.071 cm2 Bioanalytical
Systems Inc.) followed by drying. This sample loading/drying pro-
cedure was repeated 5 times, and therefore 30
mL of the sample
suspension in total was loaded on a GC electrode. Then, 10
mL of
0.05% Nafion (SigmaeAldrich, St. Louis, MO) solution was applied
on the electrode and dried.
Herein, we have first prepared rhodium oxide nanofibers by
electrospinning and then grown highly crystalline RuO2 nanowires
directly on rhodium oxide nanofibers successfully. Additionally, we
examined the electrochemical performances of pure Rh2O3 nano-
fibers (Rh2O3 NF) and RuO2 nanowireeRh2O3 nanofiber hybrid
nanostructures exhibited different electroactivities toward H2O2
electrochemical reactions.
For the following electrochemical experiments, the GC disk
electrode loaded with either Rh2O3 NF or RuO2 NWeRh2O3 NF,
saturated calomel electrode (SCE, Bioanalytical Systems Inc.), and
coiled Pt wire (0.5 mm in diameter) were used as the working,
reference, and counter electrodes, respectively. The electro-
activities of Rh2O3 NF and RuO2 NWeRh2O3 NF were studied using
rotating disk electrode (RDE) voltammetry. RDE voltammetry was
carried out with a GC electrode loaded with either Rh2O3 NF and
RuO2 NWeRh2O3 NF in 0.05 M phosphate buffer solution (pH 7.4)
containing 1 mM H2O2. RDE voltammetry of RuO2 NWeRh2O3 NF
was additionally performed in O2 saturated 0.05 M phosphate
buffer solution to study its activity for oxygen reduction reaction.
Amperometric responses of Rh2O3 NF and RuO2 NWeRh2O3 NF for
H2O2 reduction were measured at 0.12 V vs. SCE in 0.05 M phos-
phate buffer solution (pH 7.4) under stirring into that a standard
H2O2 solution was added successively to alter the H2O2 concen-
tration. To investigate capacitive current behaviors depending on
potential scan rate, cyclic voltammetry (CV) was carried out in
deaerated 1 M KNO3 solution (SigmaeAldrich, St. Louis, MO) so-
2. Experimental
2.1. Growth of Rh2O3 nanofibers
2.1.1. Synthesis of Rh2O3 nanofibers
To prepare the rhodium oxide electrospun fibers, 0.1 g of
rhodium chloride hydrate (RhCl3$xH2O, SigmaeAldrich) and 0.2 g
of polyvinylpyrrolidone (PVP, Mw z 1,300,000, SigmaeAldrich)
were dissolved in 2.2 ml of the mixed solvent including water and
ethanol with a volume ratio of 1:1 respectively. After stirring the
mixed solution for 24 h, the solution was loaded in a syringe which
was attached to the needle tip at the end of the syringe. The flow
lution at different scan rates ranged from 10 to 200 mV sꢁ1
All aqueous solutions were prepared by using deionized water
(18 M cm). RDE-2 rotor/Epsilon electrochemical analyzer (Bio-
.
rate of the solution on the needle tip was operated at 5 ml/min and
U
the distance between the end of needle tip and the surface of a
grounded aluminum plate was 7.5 cm via an electrospinning sys-
tem (NanoNC ESR200R2). The syringe needle was connected to a
high voltage power source and applied voltage was 7.0 kV. The
stainless steel needle of gauge 25 was used as the syringe needle.
The electrospun fibers were dried for 10 min at 60 ꢀC and then the
calcination of the RhCl3/polymer nanofibers was performed at
700 ꢀC for 2 h after reaching 700 ꢀC with the ramping speed of
1.6 ꢀC/min.
analytical Systems Inc.) was used for RDE experiments and
amperometry and CHI 920C potentiostat was used for amperom-
etry and CV.
3. Results and discussion
3.1. The characterization of electrospun Rh2O3 nanofibers and RuO2
nanowires on Rh2O3 electrospun nanofibers
Fig. 1 indicates SEM images of electrospun Rh2O3 nanofibers,
which was prepared from an ethanol and water solution containing
RhCl3$H2O and PVP, before and after thermal annealing process at
700 ꢀC in the air. Fig. 1(a) and (b) indicate that electrospun fibers
have the smooth surface and their diameters are mostly in range
from 100 to 400 nm. In contrasts, Fig. 1(c) and (d) show that the
morphology of these nanofibers were dramatically changed after
annealing at 700 ꢀC for 2 h. The surface of electrospun fibers was
relatively rough and the diameters of Rh2O3 nanofibers were
greatly reduced to about 50 nm. It is likely because the removal of
the matrix precursor of PVP and the oxidation process of the
RhCl3$xH2O/PVP composite nanofibers into polycrystalline Rh2O3
phase took place during the thermal annealing process. Fig. 2
represents SEM images of hierarchically grown RuO2 nanowires
on electrospun Rh2O3 nanofibers by a thermal annealing process
from Ru(OH)3∙xH2O precursor at low temperature. SEM images of
high magnification show that a great density of RuO2 nanowires
were randomly grown by covering the surface normal plane of
2.2. Growth of RuO2 nanowires on Rh2O3 nanofibers
The Ru(OH)3$xH2O precursor were prepared though a chemical
reaction by carefully dropping diluted NaOH solution to 5 mM
ruthenium chloride hydrate (RuCl3$xH2O, SigmaeAldrich, 99.98%)
aqueous acidic solution until pH 11.0 at room temperature. During
the procedure, the amorphous Ru(OH)3$xH2O precursor was
precipitated. To rinse the Ru(OH)3$xH2O precursor, the solution
was centrifuged for 20 min 5 times and then 0.1 mg of the precursor
was dispersed in 10 mL of water. A few drops of the Ru(OH)3∙xH2O
precursor solution was directly spread on about 1 mg of the
rhodium oxide nanofibers. After drying at 60 ꢀC for 1 h, the sample
was placed on the middle of the furnace at 300ꢀC for 4 h. The
product was characterized by scanning electron microscopy (SEM),
X-ray Diffraction (XRD), micro-Raman spectroscopy and high res-
olution transmission electron microscopy (HRTEM; FEI Titan TEM/
STEM at 300 kV) at room temperature.