Facile synthesis of nickel oxide nanotubes and their antibacterial, electrochemical and magnetic properties

Article abstract of DOI:10.1039/b914898a

Nickel oxide nanotubes with great antibacterial activities, electrochemical capacitance, and magnetic properties have been synthesized through a precursor method with dimethylglyoxime as precipitant for the precursor, and the method has been developed for

Full text of DOI:10.1039/b914898a

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Facile synthesis of nickel oxide nanotubes and their antibacterial,
electrochemical and magnetic propertiesw
Huan Pang,a Qingyi Lu,*a Yecheng Lia and Feng Gao*b
Received (in Cambridge, UK) 22nd July 2009, Accepted 14th October 2009
First published as an Advance Article on the web 6th November 2009
DOI: 10.1039/b914898a
Nickel oxide nanotubes with great antibacterial activities,
electrochemical capacitance, and magnetic properties have been
synthesized through a precursor method with dimethylglyoxime
as precipitant for the precursor, and the method has been
developed for the synthesis of Ni/C nanorods.
magnetic and may be used in fields of MR imaging and
magnetic drug delivery.
Dimethylglyoxime CH3(CQNOH)2CH3 is
a common
organic precipitant for Ni2+ in industry.17 Herein, the
precursor was easily synthesized by mixing dimethylglyoxime
and Ni(OAc)2 in aqueous solution with vigorous stirring
(see ESIw for experimental details). X-ray diffraction (XRD)
analysis and scanning electron microscopy (SEM) observation
confirm that the obtained product is Ni(dmg)2 and has a
one-dimensional morphology, with an average diameter of
600 nm (XRD pattern and SEM images are shown in Fig. 1a
and ESIw Fig. SI1, respectively). Dimethylglyoxime would
chelate with Ni2+ ion to form a square planar configuration
with Ni2+ in the center. The central metal Ni2+ ions interact
strongly with the adjacent molecules and the square planes
stack face-to-face, resulting in a one-dimensional columnar
structure as shown in Fig. SI2.w18 This inherent columnar
structure would lead to the formation of Ni(dmg)2 nanorods.
After the precursor was calcined at 450 1C in air, NiO
nanotubes can be obtained. The XRD analysis (Fig. 1b)
confirms that the Ni(dmg)2 nanorod precursor has been
completely transformed to NiO (JCPDS-780643). The XRD
peaks are very broad, indicating that the NiO nanocrystallites
are in the range of the nanometre scale. The rod-like morphology
is maintained on a large scale, as evidenced by SEM image in
Fig. 2a. But unlike the precursor, these 1D crystals are tubes,
which can be clearly seen from Fig. 2b as there are holes of the
broken ends. Fig. 2c displays an SEM image with a higher
magnification, confirming that the product is one-dimensional
nanotubes, and the surface is very rough, suggesting that the
nanotubes might be composed of NiO nanoparticles. The
Due to low mass density, high porosity, and increased surface
area, hollow inorganic nanomaterials constitute an important
family of nanostructures with interesting properties and
potential applications, such as catalysts, host materials,
sensors, drug-delivery carriers, acoustic insulation, biomedical
diagnosis agents, and chemical reactors.1–4 Up to now, some
synthetic strategies have been developed to fabricate hollow
inorganic nanostructures, such as the Kirkendall effect,5
self-curling mechanism,6,7 template-directed synthesis,8,9 etc.
However, these methods need complicated multi-steps such as
removal of the template or selective etching in an appropriate
solvent, which would make the product impure and limit the
extensive applications of the nanotubes.
Nickel oxide (NiO) has been under extensive investigations
as an important functional inorganic material and has diverse
technological applications in fuel cell electrodes, hetero-
geneous catalysis, antiferromagnetic layers, etc.10–13 The
valuable functions greatly depend on the composition and
structure. Of the wide range of nanostructures, nanotubes
represent a perfect one-dimensional topology that hold the
foreground as functionalized materials as a result of both
shape-specific and quantum size effects. However, to the best
of our knowledge there are few reports about the production
of NiO nanotubes.14,15 In the study presented here, we
developed a simple and convenient approach for the synthesis
of NiO nanotubes through one-dimensional metal complex
bis(dimethylglyoximato) nickel(II), Ni(dmg)2. The obtained
NiO nanotubes show good antibacterial properties compared
to commercial NiO and synthetic NiO nanoflowers. Also, the
NiO nanotubes exhibit effective electrochemical capacitance,
which can be used as a supply of energy in the exothermic
system. The magnetic measurement demonstrates that
unlike antimagnetic bulk NiO,16 the NiO nanotubes are
a State Key Laboratory of Coordination Chemistry,
Coordination Chemistry Institute, Nanjing National Laboratory of
Microstructures, Nanjing University, Nanjing, 210093, P. R. China.
E-mail: qylu@nju.edu.cn
b Department of Materials Science and Engineering, Nanjing
University, Nanjing, 210093, P. R. China. E-mail: fgao@nju.edu.cn
w Electronic supplementary information (ESI) available: Experimental
details; SEM images, molecular structure, and TG curve of Ni(dmg)2
nanorods; SEM images of NiO nanoflowers and commercial NiO;
variation of specific capacitance with cycle number; XRD pattern, SEM
and TEM images of Ni/C nanocomposite. See DOI: 10.1039/b914898a
Fig. 1 XRD patterns of (a) Ni(dmg)2 nanorods; (b) NiO nanotubes;
(c) commercial NiO; (d) NiO nanoflowers.
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diameter of an individual nanotubes is about 300 nm, much
smaller than the diameter of the original Ni(dmg)2 nanorods,
This is reasonable because during the thermal decomposition
the remarkable shrinkage would occur as gases (such as CO,
CO2, and H2O) are generated, and the weight loss is as high as
about 75%, as the TG curve of Ni(dmg)2 shows (Fig. SI3).w
The transmission electron microscopy (TEM) image shown in
Fig. 2d also demonstrates that the product has tube-like
nanostructures with an average inner diameter of 200 nm
and the NiO nanotubes are composed of nanoparticles, which
leads to the polycrystalline SAED pattern (Fig. 2d right-inset).
A high-resolution HRTEM image (Fig. 2d left-inset) displays
clear lattices of NiO crystal, confirming the crystalline nature
of NiO nanotubes. During the process with Ni(dmg)2 nano-
rods acting as a self-sacrificial template for NiO nanotubes, the
transformation from Ni(dmg)2 to NiO would start at the
surface of the solid nanorods. Subsequently, NiO nanocrystals
crystallize and form a polycrystalline layer on the surface of
the nanorods. In the following growth process, Ni(dmg)2
precursors continue to be consumed and the transformation
continues toward the interior of the nanorods. Thus, the
smooth solid nanorods gradually convert into rough porous
nanorods and finally to completely porous hollow nanotubes
because of the large mass lost during the transformation.
NiO nanomaterials have wide applications in many fields,
however little literature has reported the antimicrobial
activities of NiO. In this study, we studied the antimicrobial
activities of NiO nanotubes against B. subtilis, S. aureus,
S. faecalis, P. aeruginosa, and E. cloacae by determining the
minimum inhibitory concentrations (MIC, mg mLÀ1) through
B. subtilis, S. aureus and P. aeruginosa, while the commercial
NiO and the NiO nanoflowers have no or very weak
antibacterial abilities to the tested five bacteria. Although
the real mechanism of the antibacterial effect is still uncertain,
there is no doubt that this effective antibacterial ability of NiO
nanotubes would arise from its special nanoparticle-composed
tube-structure, in which its hollow interior might inhibit the
growth of bacterial and the high specific surface area would
increase the efficiency of antimicrobial performance (the BET
surface areas are 2.9 m2 À1, 10.3 m2 À1, 27.0 m2 gÀ1 for
g g
commercial NiO, NiO nanoflowers, and NiO nanotubes,
respectively).21
The electrochemical properties of NiO nanotubes were
investigated by cyclic voltammetry (CV), and chrono-
potentiometry (CP). Fig. 3a shows CV curves, with different
scanning rates, of the NiO nanotubes. The shapes are different
from that of electric double layer capacitance, which normally
shows an approximately rectangular shape. Typically, a
couple of redox peaks can be observed within the potential
range 0.3–0.5 V, and the peak currents are linearly propor-
tional to the sweep rate, suggesting that the capacity mainly
results from pseudocapacitive capacitance, which is based on a
redox mechanism. Fig. 3b shows the typical chronopotentiograms
of the NiO nanotubes electrode measured in the potential
range of 0–0.5 V. It obviously displays two variation ranges
during the discharge steps: a linear variation of the time
dependence of the potential (below about 0.30 V) indicating
the double-layer capacitance behavior, which is caused by the
charge separation taking place between the electrode and
the electrolyte interface, and a slope variation of the time
dependence of the potential (from 0.50 V to 0.30 V), indicating
a typical pseudocapacitance behavior, which results from
the electrochemical redox reaction at an interface between
electrode and electrolyte.22 The specific capacitance of the
NiO nanotube electrode can be calculated to be 80.49, 73.39
and 46.74 F gÀ1 when the current densities are 50, 80, and
280 mA gÀ1, respectively. On the other hand, as a long cycle
life is very important for electrochemical capacitors, the cycle
charge/discharge test has also been employed to examine
the service life of the NiO nanotubes. Fig. SI5w shows the
variation of specific capacitance with cycle number at 4.5 mA
and reveals that the NiO nanotube electrode has good cycle
properties as an excellent electrode material for electrochemical
capacitors.
a
colorimetric method using the dye MTT (3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide).19,20
For comparison, the antimicrobial activities of commercial
NiO and synthetic NiO nanoflowers (the synthesis details are
described in the ESIw and the XRD patterns and morphologies
of commercial NiO and NiO nanoflowers are displayed in
Fig. 1 and Fig. SI4w) have also been investigated. The results
are presented in Table 1 and demonstrate that the NiO
nanotubes can kill all of the tested five bacteria well, especially
The magnetic properties of 1D nanomaterials are believed
to be highly dependent on the structure, morphology, and
geometry of the materials, such as the diameter and aspect
ratio. Fig. 4 shows the field dependence of magnetization of
NiO nanotubes measured by using quantum design MPMS
SQUID XL-7. From Fig. 4, there exist small hysteresis loops
at lower field at both temperatures, indicating the existence of
a weak ferromagnetic component. This result suggests that the
obtained NiO nanotubes are ferromagnetic and different from
the bulk NiO, which is antiferromagnetic. The magnetic
hysteresis loops show the coercive force (Hc) values and the
saturated magnetic (Ms) moment at 1.8 K are 130.50 Oe and
0.133 emu gÀ1, and 61.58 Oe and 0.097 emu gÀ1 at 300 K,
respectively. This ferromagnetic behavior might be attributed
to their 1D tube structures and very big aspect ratio.
Fig. 2 (a)–(c) SEM images of NiO nanotubes obtained by calcining
the Ni(dmg)2 nanorods; (d) TEM image of NiO nanotubes (inset:
HRTEM image and SAED pattern).
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Table 1 MICs (minimum inhibitory concentrations) (mg mLÀ1) of NiO nanotubes, NiO nanoflowers, and commercial NiO
Minimum inhibitory concentration/mg mLÀ1
Gram positive
Gram negative
B. subtilis
S. aureus
S. faecalis
P. aeruginosa
E. cloacae
Samples
NiO nanotubes
NiO nanoflowers
Commercial NiO
6.25
450
450
6.25
50
450
25
450
450
6.25
50
450
25
50
450
materials, stainless steel materials. We believe that research
of the electrochemical capacitance and magnetic properties of
NiO nanotubes are useful, which may be expected to lead to
their use in a number of applications that involve magnetic
drug delivery, MR imaging, nanoscale encapsulation, and
so forth.
This work is supported by the National Natural Science
Foundation of China (Grant No. 20671049, 20721002 and
50772047), the National Basic Research Program of China
(Grant No. 2007CB925102), the Natural Science Foundation
of Jiangsu Province (Grant No. BK2007129), and Program for
New Century Excellent Talents in University.
Fig. 3 (a) Cyclic voltammetric (CV) curves at different scan rates
within a potential window of 0 to 0.6 V vs. Ag/AgCl; (b) discharge
curves in the potential range from 0 to 0.5 V at different galvanostatic
currents.
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This method not only provides us with NiO nanotubes with
multi-functionalities, but is also an effective and facile route
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Ni/C nanocomposites can be obtained when using N2 as
an inert atmosphere to calcine Ni(dmg)2 nanorods with
preservation of their morphology. The experimental details
are described in the ESIw. Fig. SI6 displays its XRD pattern,
SEM and TEM images, suggesting that one-dimensional Ni/C
nanostructure has also been successfully synthesized.
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