Phase transformation-controlled synthesis of CuO nanostructures and their application as an improved material in a carbon-based modified electrode

Article abstract of DOI:10.1039/c5ra22297d

Column-shaped CuO nanorods have been synthesized by a two-step "precursor formation-crystallization" process using a hydrothermal method with advantages of being template- and surfactant-free. The regular particle morphology of the as-prepared material was explored to be produced through a good transformation process coupled with a series of phase changes from CuCl, to Cu₂(OH)₃Cl, to Cu(OH)₂, which rely on heat by using NaOH and n-butylamine solution in a sealed vessel, and finally to CuO. Scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD), and Raman spectroscopy were employed to characterize the morphology and structures of our samples. The as-prepared CuO nanostructures have been employed to modify a glassy carbon electrode for nonenzymatic glucose oxidation. Compared with the bare glassy carbon electrode, the CuO modified electrode exhibits satisfactory performance with an apparent rate constant of κ as high as 231.0 M^-1 s^-1 due to its high specific surface area and especially good electron delivery capability of the CuO nanorods.

Full text of DOI:10.1039/c5ra22297d

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PAPER
Phase transformation-controlled synthesis of CuO
nanostructures and their application as an
improved material in a carbon-based modified
electrode†
Cite this: RSC Adv., 2016, 6, 12829
a
a
b
a
a
a
Zhonghua Xue,* Mengqian Li, Honghong Rao, Bo Yin, Xibin Zhou, Xiuhui Liu
and Xiaoquan Lu*a
Column-shaped CuO nanorods have been synthesized by a two-step “precursor formation-crystallization”
process using a hydrothermal method with advantages of being template- and surfactant-free. The regular
particle morphology of the as-prepared material was explored to be produced through a good
2 3 2
transformation process coupled with a series of phase changes from CuCl, to Cu (OH) Cl, to Cu(OH) ,
which rely on heat by using NaOH and n-butylamine solution in a sealed vessel, and finally to CuO.
Scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDX), X-ray diffraction (XRD),
and Raman spectroscopy were employed to characterize the morphology and structures of our samples.
The as-prepared CuO nanostructures have been employed to modify a glassy carbon electrode for
Received 24th October 2015
Accepted 20th January 2016
nonenzymatic glucose oxidation. Compared with the bare glassy carbon electrode, the CuO modified
DOI: 10.1039/c5ra22297d
ꢀ1 ꢀ1
electrode exhibits satisfactory performance with an apparent rate constant of k as high as 231.0 M
s
www.rsc.org/advances
due to its high specific surface area and especially good electron delivery capability of the CuO nanorods.
10–13
morphological structures as well as unique characteristics.
Nowadays, there has been considerable effort in the fabrication
Introduction
Copper oxide (CuO), an important semiconductor material with of nanostructured CuO with various morphologies to enhance
a narrow band gap, is of particular interest as well as a unique its performance in currently existing applications such as
monoxide compound, due to its inexpensiveness and non- nanorods, nanowires, nanosheets, nanoparticles, nanotubes,
toxicity, in both fundamental investigations and practical nanowhiskers, nanoneedles, nanoshuttles, nanobelts and
1
–4
applications. Currently, it is considered to be the best support nanoleaves prepared by various simple and inexpensive
1
4–17
for heterogeneous catalysts owing to its high catalytic activity as routes.
Due to their unique properties, 1D CuO nanoscaled
5–8
well as nontoxic nature and affordable price. For example, in materials showed dramatically enhancing performance in its
many important chemical processes such as degradation of existing applications and have attracted extensive attention
nitrous oxide, selective catalytic reduction of nitric oxide with during the past few years. Until now, many strategies have
ammonia, and oxidation of carbon monoxide, hydrocarbon and already been employed for the preparation of 1D CuO nano-
phenol in supercritical water, CuO has been employed as scaled materials by using different fabrication technique, which
9
18
19
a powerful heterogeneous catalyst. However, such practical includes hydrothermal method, sol–gel technique, gas-
applications of CuO are challenged by its shape, size and phase oxidation, micro-emulsion, and so forth, in which
20
21
preparation method, which signicantly inuence its the hydrothermal process as an easy approach to obtain the
effectiveness.
products with good yield has been considered as the most
Therefore, many efforts have been made on the preparation promising route due to its advantages of low-temperature, low
of CuO based nanomaterials and composites with different energy, simplicity and cost-effective as well as large-scale
22–24
25
production.
For example, Xiao et al. reported the forma-
tion of CuO nanorods via the hydrothermal process using
CuSO $5H O, sodium citrate and NaOH in a 50 mL Teon-lined
autoclave for 20 min to 12 h at 160 C. Sambandam Anandan
a
Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province,
College of Chemistry Chemical Engineering, Northwest Normal University,
Lanzhou, 730070, China. E-mail: xuezh@nwnu.edu.cn; luxq@nwnu.edu.cn; Fax:
4
2
&
ꢁ
26
et al. demonstrated the synthesis of CuO nanorods on the basis
of Cu foils immediately immersed into water containing
+
869317971323; Tel: +869317975276
b
Lanzhou City University, Lanzhou, 730070, China
†
Electronic supplementary information (ESI) available: Related SEM and XRD ammonia and NaOH solutions for 96 h reaction time at room
2 2 3 2
analyses on CuCl , CuCl, Cu (OH) Cl, and Cu(OH) were further performed and
temperature (pH is 11 at this concentration). Obviously, in the
presented in Fig. S1 and S2. See DOI: 10.1039/c5ra22297d
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previous reports, high temperature, and high pH of the solution powder was dispersed into 40 mL twice distilled water to form
and long reaction time are generally needed for the preparation green suspension solution with stirring. And then 10 mL 0.1 M
of various CuO nanostructures. And therefore, it is considerably NaOH solution was hereby added drop by drop into the above
necessary to develop a simple and effective method for the solution under magnetic stirring condition at room tempera-
synthesis of CuO nanomaterials, especially in large-quantity at ture. Meantime, a blue colored solution with the presence of
2
low-temperature and short time as well as the various expected a precipitate of Cu(OH) is rstly formed and observed, then it
advantages. In addition, direct growth of CuO nanorods and transformed into a blue colored suspension. Aer 3 mL of n-
nanowires in the wet-chemistry method such as hydrothermal butylamine was added drop by drop in the above suspensions
27
method usually is difficult; the typical synthesis route is the solution, then the whole mixed solution was transferred into 50

rst growth of cupric compound nanostructures and then mL Teon-lined stainless steel autoclave and sealed. The reac-
28,29
ꢁ
transformation to desirable CuO nanomaterials.
tion mixture was maintained at 100 C for 2 h. Aer this, it was
Recently, chemical reactions have been made to control the cooled to room temperature. The precipitates were ltered and
3
0,31
32
crystallization process of oxide nanomaterials.
Xue et al.
washed with distilled water three times followed by ethanol,
ꢁ
also pointed out that the control of chemical transformation nally dried at 60 C for 4 h in a vacuum and the goal product of
reactions could be a novel way to synthesize CuO nanowires in CuO nanorods was gained.
a mild way and successfully gained the products of CuO
Typically, 5 mg of as-prepared CuO nanostructures were
nanowires at room temperature using the proposed method, well-dispersed in 1 mL double distilled water by ultrasonic
in which the low-temperature chemical transformation of treatment to form a homogenous suspension. Prior to casting,
Cu(OH)2 nanowires can reserve the nanowires morphology the glassy carbon electrode was polished with 0.3 and 0.05 mm
and make the synthesized CuO nanowires more active in alumina powders, respectively, and then successively soni-
electrochemical reactions. Therefore, a spontaneous chemical cated in deionized water, ethanol and deionized water, and
transformation may be an effective strategy to prepare CuO nally dried under nitrogen. Then 5 mL as-dispersed CuO
nanorods materials via adjusting crystallization of copper was dropped onto the surface of glassy carbon (GC) disc
oxide achieved by a two-step “precursor formation-crystalli- electrode (diameter: 3 mm). Aer that, 5 mL of as-diluted
zation” process.
Naon (0.05 wt% in ethanol) was dropped and covered onto
Here, CuO nanorods were successfully synthesized by a two- the surface of CuO, and nally dried under nitrogen. The as-
step reaction process, during which Cu(OH)2 nanorods are prepared modied electrode is labeled as CuO/GCE in this
employed as the precursor for transformation to CuO nano- manuscript. Prior to the electrochemical measurements, the
rods. Typically, the as-prepared CuO nanostructures were CuO nanorods modied electrode was immersed in twice
gained through a well transformation process from the irreg- distilled water for 2 hours to make the Naon sufficiently wet
ular particle morphology coupled with a series of phase and ideally conductive.
changes from CuCl, through Cu
2
(OH)
3
Cl, to Cu(OH)
2
, and
The morphology of as-obtained product of CuO was char-

nally to CuO which rely on heat by using NaOH, n-butylamine acterized by an ultra plus scanning electron microscopy (SEM,
ꢁ
solution in sealed vessel at 100 C for 2 h without any surfac- Zeiss, Oberkochen, Germany), operating at 5 kV voltage
tant or template. These CuO nanorods can be perfectly served accompanied with an energy-dispersive X-ray spectrometry
as the electrode materials for glucose electrocatalytic oxida- (EDX) (Aztec-X-80, Oxford). A X-ray photoelectron spectroscopy
tion with high performance, demonstrating that the proposed (XPS, PHI5702, USA) equipped with an aluminum/magnesium
two-step “precursor formation-crystallization” process for CuO dual anode and a monochromated aluminum X-ray sources
nanorods is promising for producing advanced electrode was employed to investigate the detail informations on the
materials.
component, crystal formation of the as-prepared CuO nano-
rods. X-ray diffraction (XRD) patterns of CuO nanorods were
recorded on a Bruker D8 Focus X-ray diffractometer (Germany)
Experimental
using Cu Ka radiation (l ¼ 0.1542 nm), operating at 40 kV and
Copper chloride (CuCl
2
), n-butylamine and glucose were 150 mA. The Raman spectra of the sample was measured and
purchased from Shanghai Chemical Co. Ltd. (Shanghai, China). recorded on a Invia Raman Microscope (Renishaw) in back-
All other chemicals were of analytical grade, and were received scattering geometry using a 633 nm laser excited. All electro-
without any further purication. Twice distilled water (with chemical measurements such as cyclic voltammograms (CVs)
a resistivity of 18 MU cm) from an all-quartz still was used and chronoamperometry were performed on a CHI660E elec-
throughout and all the experiments were performed at room trochemical station (CHI Instruments Inc. USA) with a three-
temperature.
.6 g CuCl
electrode system, where the bare glass carbon electrode
was dissolved in 40 mL of water, and then 10 mL (3 mm diameter) and the CuO nanorods modied electrode
0
2
0
.28 M ascorbic acid (AA) solution was added into the above were used as working electrode, platinum wire and Ag/AgCl
solution with magnetic stirring for 1 h until the white CuCl (saturated KCl) was employed as counter and reference elec-
precipitates were obtained. Then the as-obtained precipitates trodes, respectively. Electrochemical impedance spectroscopy
were ltered and dried at room temperature, during which the (EIS) measurements were performed on a VMP2 Multi-
green Cu
2 3
(OH) Cl powders transformed from the white CuCl potentiostat (Princeton Applied Research, USA) by using 5
27
precipitates were gained. 0.1 g of as-prepared Cu
2
3 3 6 4 6
(OH) Cl mM K [Fe(CN) ]/K [Fe(CN) ] (1 : 1) in 0.1 M KCl solution.
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Results and discussion
Here, by using a two-step “precursor formation-crystallization”
process, CuO nanorods for producing advanced electrode
materials with high electrocatalytic activity and stability were
fabricated, in which chemical reactions have been employed
and provided to control the crystallization process of copper
oxide nanomaterials. Scheme 1 shows the synthesis process of
CuO nanorods and the corresponding SEM and XRD analyses
on CuCl , CuCl, Cu (OH) Cl, and Cu(OH) were further per-
2
2
3
2
formed and presented in Fig. S1 and S2 in ESI.† Initially, white
CuCl precipitates were obtained aer ascorbic acid was added
into CuCl
CuCl.
2
solution due to the relatively low solubility of
In the second case, green Cu (OH) Cl was formed by
the oxidization reaction during the drying process under the
30,32
2
3
27
air. Thirdly, copper hydroxide was easily formed by the reac- Fig. 1 SEM images of as-prepared CuO nanorods at low (ꢂ30 000, (a))
tion between the Cu (OH) Cl powders and sodium hydroxide. and high (ꢂ50 000, (b)) and (ꢂ85 000, (c)) magnification. EDX spec-
2
3
trum of the same sample (d), insert of (d) TEM image of CuO nanorods.
Finally, CuO nanorods were obtained upon decomposition of
the copper hydroxide, during which the precipitates of Cu(OH)
2
ꢁ
was dried at 100 C in a vacuum to gain the ne product of CuO
nanorods. Because it is recognized that the form of the copper
oxide nanorods could be inuenced greatly by hydrothermal
treatments during the synthesis, where the morphological
features of the CuO nanorods are highly dependent upon the
temperature of the treatment during the phase changes from
typical TEM image of the sample synthesized by the as-
proposed method, which illustrates that the obtained product
is of column-shaped CuO nanorods. As well known, EDX is an
effective method for analysis the chemical composition for
most of the nanostructures materials. The mapping of the
elements in the as-prepared samples (Fig. 2) also demonstrates
that the composition primarily contained copper and oxygen
elements by EDX, and element distribution is uniform.
Additionally, XRD analysis is also used to determine the
structure and phase of the as-prepared samples. As indicated in
Fig. 3a, the XRD pattern of CuO nanorods demonstrates all
characteristic peaks of a pure phase CuO nanocrystals accord-
ing to the proposed preparation conditions compared with the
standard diffraction peaks from (JCPDS card no. 05-0661).
33
Cu(OH) to CuO. Though the uniform ne nanorods of CuO
2
can be produced at room temperature through the proposed
phase changes process, but these ne nanorods upon this
condition usually tend to aggregate in spindle-like bundles
34
because of their high surface energy or van der Waals forces.
In addition, at higher temperature, another situation would
occur that bulk copper oxide nanorods can be obtained. Both of
two are not beneting for constructing advanced electrode
modier materials with good electrocatalytic activity.
The morphology and structure of the resulting CuO nano-
rods with column-shaped crystals were elucidated by scanning
electron microscopy (SEM). As seen in Fig. 1, the SEM images of
CuO nanorods revealed a rod-like texture with uniform sizes
with an average size about 700 nm in length and about 100 nm
in width. Interestingly, we can observe many CuO nanorods
with branched side edges as clearly seen in the magnied image
ꢁ
ꢁ
Especially, the major peaks located at 2q ¼ 35.5 and 38.7 are
characteristics for the phase pure monoclinic CuO crystallites
22,35–37
and are good agreement with previous reports.
same time, no other noticeable peaks are detected and
depicted belonging to the impurities such as Cu(OH) , Cu O or
At the
2
2
Cu (OH) Cl, suggesting the high purity of as-prepared samples.
2
3
Furtherly, additional information on the structure of the as-
prepared CuO nanostructures was characterized by the Raman
(Fig. 1C), which may provide more active sites for some elec-
ꢀ
1
spectroscopy in the range of 100–1000 cm measured at room
trode reactions and enhance the electron delivery capability of
CuO nanorods materials. In addition, the energy dispersive
X-ray (EDX) spectra of the CuO nanostructure in Fig. 1d shows
that only Cu and O are the elemental components, and also Au,
which comes from the SEM testing process, indicating no other
impurity in fabricated samples. Insert of Fig. 1d shows the
temperature and depicted in Fig. 3b. In general, the modes at
ꢀ1
ca. 274, 320, and 617 cm for the sample can be assigned to the
1
g
2
g
A , B , and B modes, and modes, respectively. Obviously, three
g
Scheme
1 Synthesis process of CuO nanorods (RT: room Fig. 2 SEM images of a portion of as-prepared CuO nanorods and the
temperature).
corresponding EDX mapping data of Cu, O, and Au, respectively.
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accordance with the previous reports, also demonstrating that
the as-products are composed of only CuO.
As well known, electrochemical technology was extensively
applied both in preparation and characterization of various
nanomaterials. Here, the as-prepared CuO/GCE were furtherly
characterized and conrmed by using CVs. As seen in Fig. 5a,
a pair of redox current peaks corresponding to the probe of
potassium ferricyanide is clearly observed for CVs response on
the bare GCE (black curve) in 0.1 M KCl solution. Interestingly,
it was almost disappeared on the CuO/GCE (red curve) and only
two small redox peaks derived from CuO nanostructures
appeared at 0.6 mV and 0.8 mV, respectively, indicating that the
introduction of the CuO played an important role in the
decrease of the electroactive surface area and providing the
Fig. 3 Typical XRD pattern (a) and Raman spectrum (b) of the as-
prepared CuO nanorods.
Raman-active normal modes (A + 2B ) indicate that the CuO
g
g
nanostructures that we prepared have a single phase and are
highly crystalline in nature, which is consistent with the
3
ꢀ/4ꢀ
35,38,39
conducting bridges for the electron-transfer of Fe(CN)
6
. On
previous ndings.
the other hand, highlighting CuO nanorods can be successfully
modied and exists on the electrode surface. The resistance of
a modied electrode may be changed due to the modier on the
Subsequently, in order to further investigate the purity and
composition of the samples, the XPS measurement was carried
out and the results are shown in Fig. 4. The indexed peaks
corresponded to C, O, and Cu can be clearly observed in Fig. 4a
for the as-obtained sample, where C 1s peaks were corrected to
be 292.7 eV and, all the other peaks were corrected accordingly,
demonstrating the exclusive presence of Cu, O, and C elements,
43–45
electrode surface.
Electrochemical impedance spectroscopy (EIS) is an efficient
and facile tool for studying the interface properties of the
46
capability of electron transfer. As reported and well known, the
charge-transfer resistance (Rct) of a certain electrode is equal to
the semicircle diameter of EIS and can be used to describe the
electrical conductivity properties of the electrode interface.
Thus, the surface and interface properties of the different
modied electrodes were further investigated by EIS experi-
ments. As depicted in Fig. 5b, a small semicircle and almost
a straight line featuring a diffusion-limiting step of the
38,40
corroborating the high purity of the samples.
In addition,
the Cu 2p core-level XPS spectrums derived by using Gaussian
curve-tting analysis are presented and depicted in Fig. 4b,
where the two main peaks of Cu 2p3/2 and 2p1/2 corresponding
to the spin–orbit split at 934.4 eV and 954.3 eV are well dis-
played, respectively. Furthermore, the difference between spin–
orbit coupling energy for Cu 2p3/2 and 2p1/2 is 20 eV, which is
4
ꢀ/3ꢀ
40–42
Fe(CN)
6
processes for the bare GCE (black curve) is clearly
consistent with the standard value for CuO.
Otherly, as
displayed. With the modication of CuO onto GCE, the diam-
eter of the semicircle increased and which indicates a higher
charge-transfer resistance value than that of bare GCE. Thus, by
shown in Fig. 4c, three Gaussians (marked as a, b, and g) of O 1s
core level spectrum were resolved by using a curve-tting
2
ꢀ
procedure. Among this, curve a can be identied as O

tting to R(Q(RW)) equivalent circuit model using the ZsimpWin
binding with Cu at 529.9 eV, and the other peaks observed in
47
program, the interfacial electron transfer resistance (R ) of the
ct
curves b and g with higher binding energy values at 531.4 and
ꢀ
bare GCE and CuO/GCE are calculated to be ꢃ77.5 and 7756 U,
respectively, revealing the high electron transfer (ET) resistance
of the CuO/GCE surface, which is agreement with previous
5
32.2 eV can be attributed to the O–O and OH , respectively,
which may be due to the H O molecules adsorbed on the
2
surface of the sample. All the results gained by XPS are in
48–51
literatures.
Since the ET resistance (Rct) of the bare and CuO/
GCE was gained, the apparent ET rate constant (kapp) of the
probe molecule on different electrodes was also calculated
Fig. 5 Cyclic voltammograms (a) and electrochemical impedance
spectroscopy (b) of 5 mM K
at bare GCE and CuO/GCE (black curves for the bare electrode, red
Fig. 4 XPS spectra of the as-prepared CuO nanorods (a), curving- curves for the ERGO modified electrode). Insert of (b) is employed
fitting spectra of Cu 2p3/2 and Cu 2p1/2 (b) and O 1s (c). equivalent circuit model.
3 6 4 6
[Fe(CN )]/K [Fe(CN )] in 0.1 M KCl solution
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5
5–58
according to the literature using the equation by Lakshminar- specied potential region.
ayanan (formula (1)).
When 5 mM glucose was present,
52
an obvious oxidation peak corresponding to the irreversible
glucose oxidation was observed (red curve) on the nanorods-
structured CuO modied glassy carbon electrode, where the
oxidation of glucose started at approximately +0.30 V, with
a peak at about +0.54 V. The results demonstrate that the as-
prepared nanorods-structured CuO on the modied electrode
surface have greatly improved and enhanced the electro-
chemical performance of the electrode towards glucose oxida-
tion in alkaline medium, which may be attributed to their large
surface area, high surface energy, and enhanced electron
RT
k
app
¼
(1)
2
F Rct
C
where kapp is the apparent electron transfer rate constant, R is
the gas constant, T is the absolute temperature, F is Faraday's
constant, Rct is the interfacial ET resistance, and C is the
concentration of the redox probe of K [Fe(CN) ]/K [Fe(CN) ].
3
6
4
6
The values of kapp on the bare GCE and CuO/GCE are calculated
ꢀ7
ꢀ9
to be 6.868 ꢂ 10 and 6.863 ꢂ 10 , respectively. Obviously,
compared to the bare electrodes, the CuO/GCE can reduce the
apparent ET rate of the probe due to its semiconductor char-
acteristics, which are good accord with the above results of the
Fig. 5a.
54,55,59–62
transferability.
Furtherly, the effect of the scan rate on the performance of
CuO/GCE toward glucose oxidation was further studied to
provide more evidences. Fig. 6b depicts the electrochemical
behaviour of 5 mM glucose at the CuO/GCE in 0.1 M NaOH at
different scan rates in the range of 30–140 mV s . It is obvious
that the oxidation currents of glucose increase with increasing
As well known that CuO nanostructures can be applied to
modify the carbon-based electrodes and widely employed in
ꢀ1
51–54
49
glucose detection as non-enzymatic sensors.
Hu et al.
pointed out that both CuO nanoowers and 1D nanorods
modied graphite electrodes display greatly improved perfor-
mances and increase the electrocatalytic ability towards glucose
oxidation compared with single graphite electrode, which may
be attributed to their large surface area, high surface energy,
and enhanced electron transferability. In addition, they also
found that the CuO nanostructure-modied electrodes also
have excellent selectivity towards glucose in the presence of
dopamine and ascorbic acid. Hence, the nanorods-structured
scan rate. And oxidation peak currents (I ) have a linear rela-
pa
tionship with n (r ¼ 0.9930, where n is the scan rate) (insert of
Fig. 6b) with a regression equation as follows, indicating the
electrochemical oxidation of glucose at the CuO/GCE was
controlled by adsorption.
Ipa ¼ 5.3005 ꢂ 10ꢀ n + 0.456
4
(2)
Additionally, the electrocatalytic oxidation of glucose in
CuO was electrochemically tested by cyclic voltammetry to alkaline electrolyte at the nanorods-structured CuO modied
establish its sensitivity to glucose under non-enzymatic milieu. electrode was further conrmed by cyclic voltammetry and
The cyclic voltammetry prole of nanorods-structured CuO shown in Fig. 6c. It is clear that the oxidation current of glucose
modied glassy carbon electrode in 0.1 M NaOH solution at increase with increasing glucose concentration from 1 mM to 5
ꢀ
1
a scan rate of 100 mV s is presented in Fig. 6a, which shows mM, demonstrating a good response and relatively stable
well broad redox behavior (black curve) in about 0.20–0.8 V (vs. electrochemical oxidation capability toward the target of
Ag/AgCl). Broad redox peaks can be assigned to the well known glucose.
Cu(II)/Cu(III) redox couple in the alkaline solution under the
For the enhanced nonenzymatic glucose performance of
CuO/GCE in our work, the mechanism can be elucidated
63,64
according to previsouly.
Initially, copper oxide was oxidized
into powerful trivalent copper ions by electrochemical oxidation
in the alkaline solution, which can be observed with a broad
reduction peak in Fig. 6a and, where the corresponding reaction
process can probably be interpreted as below:
CuO + OH ꢀ eꢀ / CuO(OH)
ꢀ
(3)
Then, with the existence of glucose, trivalent copper ions can
be reduced into copper oxide and, meanwhile glucose also be
oxidized into gluconolactone, which also can be clearly seen
with a well oxidation peak observed at 0.54 V in Fig. 6a and,
where the following reactions can be employed to depict the
corresponding reaction process:
2
CuO(OH) + glucose(R –CHOH–R ) /
1 2
2
CuO + gluconolactone(R –CHO–R ) + H O
1
(4)
2
2
Fig. 6 CV curves of as-prepared CuO nanorods modified GCE in 0.1 M
NaOH before (black trace) and after (red trace) the injection of 5 mM
glucose (a), in 0.1 M NaOH solution with 5 mM glucose at various scan
rate with 30, 50, 100, 120, and 140 mV s (b), and in 0.1 M NaOH
solution with different concentration of 1, 2, 3, 4, 5, 6 mM glucose.
Insert of (b) plot of the peak currents vs. the scan rate.
Obviously, we can concluded that the Cu(II)/Cu(III) redox
couple is the essential factor in the whole reaction process,
where glucose lose an electron in order to transform into glu-
conolactone and, a Cu(III) ion derived from copper oxide at
ꢀ1
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Scheme 2 The schematic representation of the mechanism of the
glucose oxidation based on CuO/GCE.
alkaline condition obtains an electron and acts as an electron
delivery system, and it is an irreversible process. The corre- Fig. 8 Amperometric response of with (black curves) and without (red
curves) 5 mM glucose containing in NaOH at the bare GCE (a), CuO/
GCE (b), respectively. (c) and (d) shows I /I vs. t plot obtained for the
c l
sponding sensing mechanism schematic is shown in Scheme 2.
1/2
In addition, since the stability of the electrode is an impor-
tant parameter to decide the fate of a modied electrode, it was
further investigated using amperometric technique. To access
the reliability of goodly electrochemical oxidation capability of
nanorods-structured CuO modied electrode toward the target
of glucose, the amperometric response of 0.1 M (black curve)
and 0.1 mM (red curve) glucose at CuO/GCE was studied by
keeping the oxidation potential a constant at 0.67 V as showed
in Fig. 7, respectively. As clearly seen, for either high or low
concentration of the sample, both constant amperometric
responses were achieved for 15 minutes indicating the anti-
fouling effects and stability of the CuO/GCE toward glucose
oxidation, on the other hand, demonstrating a relative stable
electrocatalytic oxidation capability of the as-proposed CuO/
GCE.
corresponding electrodes.
different electrodes can be determined by plotting the values of
1
/2
47
I /I vs. t according to the following formula:
c
l
1
2
1
2
1
2
1
2
I
c
¼
g
p
¼ p
ðkC
0
tÞ
ðg ¼ kC
0
tÞ
(5)
I
l
where I /I is the ratio between the faradaic current measured
c
l
aer and before glucose addition. C is the concentration of
0
glucose in the bulk in M and k is the catalytic rate constant in
ꢀ1 ꢀ1
M
s
and t is the time elapsed. The apparent rate constants
(
k) calculated for the glucose oxidation reaction was calculated
ꢀ1 ꢀ1
ꢀ1 ꢀ1
to be 27.197 M
s
and 231.0 M
s
for bare and nanorods-
structured CuO modied GCE, respectively. It is clearly that the
apparent rate constants (k) of glucose oxidation on the CuO/
GCE is enlarged 4 times than that on bare GCE, demon-
strating that the as-prepared nanorods-structured CuO modi-
Furtherly, chronoamperometry technique is an effective tool
for investigating the kinetics of target molecules via calculating
47,65
the apparent rate constant (k) and so on.
Therefore, chro-
noamperometry performances of the bare and CuO/GCE
modied electrodes were performed and employed to calcu-
late the corresponding kinetics of the apparent rate constant. In
this work, by using double potential step method and holding
the electrode potential at 0 V for 10 s and forcing it to the

ed electrode possesses a stable electrocatalytic oxidation
capability toward the target of glucose, which is a good agree-
ment with the results of CVs shown in Fig. 6a.
glucose oxidation potential i.e. 0.6 V (for bare GCE and CuO Conclusions
nanorods modied GCE), a steady state response was gained
and corresponding current vs. time (i–t) curves are shown in
Fig. 8. The apparent rate constant (k) of glucose oxidation on the
In summary, CuO nanorods with column-shaped crystals were
synthesized successfully by a simple and efficient method
containing a phase transformation process of “precursor
formation-crystallization”. A series of phase changes from CuCl,
2 3 2
through Cu (OH) Cl, to Cu(OH) , and nally to CuO have been
demonstrated. The characterization of as-prepared nano-
structures by SEM, EDX, XRD and Raman shows the materials
possess abundant channel-type uniform porosity, a single
phase and highly pure monoclinic crystalline. The electro-
chemical results demonstrate that the ne and pure crystalline
CuO nanorods exhibit a high electrocatalytic apparent rate
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
constants of 231.0 M
s
, compared to 27.197 M
s
for the
bare carbon substrate towards glucose oxidation due to its high
Fig. 7 Amperometric response of 0.1 M (a) and 0.1 mM (b) glucose at
CuO/GCE for 15 min in 0.1 M NaOH. Applied potential: 0.67 V.
specic surface and especially a good electron delivery
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capability. The present method is hopeful and applicable to the 19 L. Armelao, D. Barreca, M. Bertapelle, G. Bottaro, C. Sada
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Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (no. 21265009, 21265018, 21165016), Program 23 S. Li, H. Zhang, Y. Ji and D. Yang, Nanotechnology, 2004, 15,
for Chang Jiang Scholars and Innovative Research Team,
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Ministry of Education, China (Grant no. IRT1283), Research 24 Y. Liu, Y. Chu, M. Li, L. Li and L. Dong, J. Mater. Chem., 2006,
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