Preparation of nickel oxide and carbon nanosheet array and its application in glucose sensing

Article abstract of DOI:10.1016/j.jssc.2011.08.008

Nickel oxide and carbon (NiO/C) nanosheet array was fabricated on Ti foil for the first time by calcining the precursor, which was synthesized through the hydrothermal reaction of nickel acetate, urea and glucose. The slow release of OH^- by the hydrolysis of urea aided in the direct nucleation and adhesion of precursor seeds on Ti substrate. The presence of carbon ensured a large specific surface area and good conductivity of the final NiO/C composite. The prepared NiO/C nanosheet array exhibited higher catalytic oxidation activity of glucose compared with the pure NiO nanosheet at a detection limit of 2 μM, linear range up to 2.6 mM (R²=0.99961), and sensitivity of 582.6 μAm M^-1 cm^-2. With good analytical performance, simple preparation and low cost, this composite is promising for nonenzymatic glucose sensing.

Full text of DOI:10.1016/j.jssc.2011.08.008

Journal of Solid State Chemistry 184 (2011) 2738–2743
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Preparation of nickel oxide and carbon nanosheet array and its application in
glucose sensing
n
Xin Li a,b, Anzheng Hu a, Jian Jiang a, Ruimin Ding a, Jinping Liu a, Xintang Huang a,
a Center for Nanoscience and Nanotechnology, Central China Normal University, Wuhan 430079, China
b Department of Applied Physics, Wuhan University of Science and Technology, Wuhan 430081, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel oxide and carbon (NiO/C) nanosheet array was fabricated on Ti foil for the first time by calcining
the precursor, which was synthesized through the hydrothermal reaction of nickel acetate, urea and
glucose. The slow release of OHꢀ by the hydrolysis of urea aided in the direct nucleation and adhesion
of precursor seeds on Ti substrate. The presence of carbon ensured a large specific surface area and good
conductivity of the final NiO/C composite. The prepared NiO/C nanosheet array exhibited higher
catalytic oxidation activity of glucose compared with the pure NiO nanosheet at a detection limit of
Received 9 March 2011
Received in revised form
25 July 2011
Accepted 10 August 2011
Available online 22 August 2011
2
m
M, linear range up to 2.6 mM (R2¼0.99961), and sensitivity of 582.6 Am Mꢀ1 cmꢀ2. With good
m
Keywords:
Nickel oxide
analytical performance, simple preparation and low cost, this composite is promising for nonenzymatic
glucose sensing.
Nanosheet array
Carbon
Crown Copyright & 2011 Published by Elsevier Inc. All rights reserved.
Hydrothermal method
Glucose sensing
1. Introduction
problem that occurred when using NiO to modify electrodes is
the poor charge transport.
Glucose determination has attracted considerable interest in
biotechnology, clinical diagnostics and the food industry [1]. A
number of studies have focused on developing electrochemical
glucose sensors over the past few decades [2] and enzymatic
glucose biosensors with good selectivity and sensitivity have been
successfully developed [3,4]. However, these biosensors lack long-
term stability [5]. Thus, different nonenzymatic glucose sensors
should be explored [6–10].
Nonenzymatic electro-oxidation of glucose using Ni-based
material has been extensively studied, since Fleishman and his
co-workers [11] demonstrated that organic compounds could be
partially oxidized at a nickel anode in alkaline solution. Glucose
sensors based on Ni metallic nanomaterials exhibited high cata-
lytic oxidation activity over glucose [12–14]. However, Ni metallic
nanomaterials are unstable and easily oxidized in air and solu-
tion. Therefore, considerable efforts have been focused on the use
of nickel oxide to modify the electrodes. Cheng et al. [15] devised
NiO nanoparticles modified carbon paste electrode. Liu et al. [16]
prepared Ni/NiO nanoparticle-loaded carbon nanofiber paste
electrode. These proposed electrodes exhibited long-term stabi-
lity towards the glucose determination. However, a common
Carbon materials have received considerable attention because
of their unique properties, such as excellent conductivity, high
surface area-to-volume ratio, and good physicochemical stability
[17]. Carbon nanotubes, diamond like carbon and graphene have
been used to improve the conductivity of composite in the
glucose determination [18–20]. However, the preparation of these
carbon materials is not simple and economy. Considering this
point, partially graphitized carbon would be a good substitute. An
important example was demonstrated by Huang et al. [21]. The
electronic conductivity of NiO/C composite was improved after
the incorporation of partially graphitized carbon. The prepared
composite exhibited much better performance as lithium-ion
battery electrode material than pure NiO. However, to our knowl-
edge, using NiO and partially graphitized carbon composite for
nonenzymatic glucose sensing has never been discussed before.
Stimulated by this result, NiO and partially graphitized carbon
composite nanosheet array was synthesized in our laboratory.
In this article, NiO/C nanosheet array was successfully fabri-
cated on Ti substrate using a two-step method. The slow release
of OHꢀ by the hydrolysis of urea was assumed to explain
the nucleation and adhesion of precursor seeds on Ti substrate.
The fabrication process was simple and cheap. Furthermore, the
presence of partially graphitized carbon ensured a large specific
surface area and good conductivity of the final NiO/C composite.
As a result, the NiO/C nanosheet array exhibited shorter response
time, lower detection limit and higher sensitivity towards the
n Corresponding author. Fax: þ86 027 67861185.
E-mail addresses: lixin2010@hotmail.com (X. Li),
xthuang@phy.ccnu.edu.cn (X. Huang).
0022-4596/$ - see front matter Crown Copyright & 2011 Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.08.008

X. Li et al. / Journal of Solid State Chemistry 184 (2011) 2738–2743
2739
detection of glucose compared with pure NiO nanosheet. With
3. Results and discussion
good analytical performance, simple preparation and low cost,
this NiO/C composite is promising for nonenzymatic glucose
sensing.
3.1. Preparation and morphology of NiO/C nanosheet array grown
directly on Ti substrate
NiO/C nanosheet array film was fabricated on Ti substrate in
two steps, as shown in Fig. 1. According to recent report, the
polymerization and dehydration of the hydroxyl group in glucose
molecules under hydrothermal condition could result in a com-
plicated mixture of organic compounds [22]. The organic com-
pounds were transformed into partially graphitized carbon under
anneal treatment in protective gas [23]. In the hydrothermal
process, both urea and glucose played important roles. The
hydrolysis of urea provided a source of anions during the forma-
tion of the precursor Ni2(OH)2(CO3) [24]. The polymerization and
dehydration of glucose under hydrothermal treatment generated
a mixture of organic compounds. The organic compounds uni-
formly coated on the precursor Ni2(OH)2(CO3). During the next
anneal in N2 gas, the precursor decomposed into NiO [25]. The
organic compounds were transformed into a carbon shell coating
on the NiO nanosheets. Different kinds of metal oxide and carbon
composites were prepared using similar method [26–28]. The
possible precipitation reactions were as follows:
2. Experimental
2.1. Reagents
All chemicals, including nickel acetate, urea, glucose, and
sodium hydroxide (NaOH), were obtained from Sinopharm Che-
mical Reagent Company and used as received without further
purification. All aqueous solutions were prepared in distilled
water. The titanium foil was 99% pure.
2.2. Apparatus
Products were characterized by scanning electron microscopy
(SEM; JSM-6700 F, 5.0 kV), powder X-ray diffraction (XRD; Bruker
˚
D-8 Avance, Cu K
a radiation, 1.5418 A), Raman spectroscopy
(Confocal Raman Microspectroscopy, RM-1000, 514.5 nm), trans-
mission electron microscopy (TEM; JEM-2010FEF, 200 kV), and
Nitrogen adsorption–desorption measurements (Micromeritics
Tristar 3000 analyzer).
All electrochemical experiments including electrochemical
impedance spectroscopy (EIS) were carried out using a CHI
660C Electrochemical Work Station (CH Instrument Company of
Shanghai, China). A conventional three-electrode system was
adopted with NiO/C nanosheet array modified Ti foil as the
working electrode, Pt wire as counter electrode, and saturated
calomel electrode (SCE) as the reference electrode. All potential
values were referred to the SCE.
CO(NH2)2þH2O-NH4þ þOHꢀ þCO32ꢀ
Ni2þ þOHꢀ þCO32ꢀ-Ni2(OH)2(CO3)k
Ni2(OH)2(CO3)-NiOþH2OþCO2
(1)
(2)
(3)
The typical morphology of the prepared NiO/C nanosheet array
is illustrated in Fig. 2(A–C), as observed by SEM. The prepared
NiO/C composite is uniformly distributed on Ti foil on a large-
scale. The film consists of interconnected nanosheets with 90–
120 nm thickness and lateral sizes of 600–900 nm. These
nanosheets have rough surfaces. They are decorated with small
sphere particles with a diameter of 60–90 nm.
2.3. Fabrication of NiO/C or pure NiO nanosheet array on Ti foil
The direct growth of NiO/C nanosheet array on Ti foil was
ascribed to urea. The slow release of OHꢀ by the hydrolysis of
urea aided in the nucleation and adhesion of precursor seeds on Ti
substrate. To confirm this point, pure NiO nanosheet array was
prepared without the additive of glucose in the first reaction
solution.
The typical morphology of pure NiO nanosheet array is illu-
strated in Fig. 2(D and E). The structure is similar to that of NiO/C
nanosheet array. However, the pure NiO nanosheets have smooth
surfaces. No other materials, such as small sphere particles, are
observed. These nanosheets are aligned vertically to the Ti foil,
enabling robust mechanical adhesion and electrical contact of
every nanosheet to the substrate. This result implies that glucose
has no relationship with the direct growth of nanosheet array on
Ti substrate.
NiO/C nanosheet array was prepared on Ti substrate by first
dissolving nickel acetate, urea and glucose in 60 ml distilled water
to achieve 0.05, 0.25 and 0.01 M, respectively. Ti foils were placed
vertically in the solution. The mixing solution was transferred
into a 100 ml Teflon-lined stainless steel autoclave and kept at
120 1C for 10 h. The substrate was then washed, dried at 60 1C and
further annealed in N2 gas at 500 1C for 2 h. A uniform NiO/C
nanosheet array film was then formed on Ti foil. Carbon in the
NiO/C composite was incorporated by the hydrothermal reaction
of glucose in the first mixing solution. The glucose used here
could be replaced by sucrose or cellulose.
To understand the role of carbon in final composite, pure NiO
nanosheet array was prepared on Ti substrate for contrast. NiO
nanosheet array was prepared by first dissolving nickel acetate
and urea in 60 ml distilled water to achieve 0.05 and 0.25 M,
respectively. Ti foils were placed in the solution as substrate. The
solution was kept at 120 1C for 10 h. Then Ti foils were washed,
dried at 60 1C and further annealed in N2 gas at 500 1C for 2 h.
Pure NiO nanosheet array film was then uniformly formed on
Ti foil.
In other two contrast experiments, urea was replaced by
ammonia or sodium hydroxide. Nothing was obtained on Ti
substrate, as observed from Fig. 2(F) and its inset. This result
confirmed that urea played an important role in the direct growth
2.4. Electrochemical experiments
The electrochemical behavior of the prepared electrode was
investigated by cyclic voltammetry (CV). NiO/C nanosheet array
modified Ti foil (NiO/C–Ti) and pure NiO nanosheet array mod-
ified Ti foil (NiO–Ti) were evaluated as the working electrode.
Amperometric curves were recorded after adding the glucose
solution to the electrolyte.
Fig. 1. Fabrication processes of the NiO/C nanosheet array film grown directly on
Ti foil.

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X. Li et al. / Journal of Solid State Chemistry 184 (2011) 2738–2743
Fig. 2. (A, B and C) SEM images of NiO/C nanosheet array on Ti foil; (D and E) SEM images of pure NiO nanosheet array on Ti foil; and (F) SEM image of Ti foil after the
reaction using ammonia as precipitation reagent, inset: SEM image of Ti foil after the reaction using sodium hydroxide as precipitation reagent.
of nanosheet array on Ti substrate. Using ammonia or sodium
hydroxide as precipitation reagent, the OHꢀ was abundant in the
solution. The nucleation and growth of precursor rapidly occurred
in the solution. The coprecipitation reaction was so fast that it
was hard for precursor seeds to adhere on Ti substrate. Using urea
as precipitation reagent, the OHꢀ was gradually released by the
hydrolysis of urea. The coprecipitation reaction was relatively
mild that it was possible for precursor seeds to adhere on Ti
substrate. As a result, a uniform nanosheet array film was formed
on the Ti foil.
graphitization degree, respectively. This confirms the presence of
partial graphitized carbon in the final composite. Unlike other
complex preparation methods, carbon in the composite is incor-
porated through the hydrothermal reaction of glucose in the first
mixing solution. This method has been proven to be simple,
cheap, and effective.
The structure of NiO/C composite is further investigated by
TEM. One piece of platelet examined is shown in Fig. 3(C). The
nanosheet is actually composed of small NiO nanoparticles with
numerous pores in the structure. As indicated by arrows in
Fig. 3(D), NiO nanoparticles are homogeneously surrounded by a
carbon shell. The carbon shell offers a conductive network for NiO
nanoparticles that facilitates the charge transport. The inset of
Fig. 3(D) shows the corresponding selected-area electron diffrac-
tion pattern of NiO in the composite.
Porous nanomaterials are more attractive in the detection of
glucose. Their large specific surface area can effectively increase
the adsorption of glucose. In order to investigate the porosity of
NiO/C nanosheet array, nitrogen adsorption–desorption isotherm
and BET analysis were performed and compared to that of the
pure NiO nanosheet. As shown in Fig. 4(A), both NiO/C and pure
NiO nanosheet array exhibit distinct hysteresis loop, indicating
their porous structure. The N2 uptakes of the initial part for the
3.2. Structure characterizations of NiO/C nanosheet array grown
directly on Ti substrate
The XRD pattern of NiO/C nanosheet array is presented in
Fig. 3(A) showing characteristic diffraction peaks at 37.2751 and
43.3161, which correspond to the (101) and (012) crystalline
planes of NiO material, respectively (JCPDS file no. 44-1159).
Other strong diffraction peaks can be assigned to the crystalline
planes of Ti substrate (JCPDS file no. 01-1197).
Fig. 3(B) shows the Raman spectra with two strong peaks at
1369 (D band) and 1598 cmꢀ1 (G band), which are due to the
disordered defects of the carbon material and relatively high

X. Li et al. / Journal of Solid State Chemistry 184 (2011) 2738–2743
2741
Fig. 3. (A) XRD pattern; (B) Raman spectra; (C and D) TEM images of NiO/C nanosheet, inset: selected-area electron diffraction pattern of the NiO/C composite.
Fig. 4. (A) Nitrogen adsorption–desorption isotherm and (B) pore-size distribution of pure NiO and NiO/C nanosheet.
two isotherms are very low, indicating the micro-pores may be
ignored in the two samples. The isotherm of NiO nanosheet
belongs to a mixed type in the IUPAC classification [29]. The part
of isotherm at high relative pressure P/P040.8 corresponds to
type II, typical of non-porous or macro-porous materials. A
hysteresis loop can be seen in the range P/P040.6, associated
with capillary condensation in meso-pores, which is characteristic
of type IV isotherms. The isotherm of NiO/C composite
nanosheet also belongs to a mixed type of II and IV with a
hysteresis loop of type H3. The hysteresis loop is bigger than that
of NiO nanosheet, indicating the higher meso-porosity of NiO/C
composite nanosheet sample.
specific surface area of NiO/C composite. This improvement is
valuable for the enhancement of sensitivity towards the detection
of glucose.
3.3. Amperometric determination of glucose at NiO–Ti and NiO/C–Ti
electrode
Fig. 5(A) shows the CV curves of the prepared NiO–Ti and NiO/
C–Ti electrode in 0.1 M NaOH solution at scan rate of 0.10 V/s.
Both of them exhibited a pair of redox peaks. The redox peak
potentials at 0.41 V and 0.31 V were assigned to the Ni2þ/Ni3þ
redox couple formation in the alkaline medium [30].
Fig. 4(B) gives the plot of the pore size distribution of NiO/C
and pure NiO nanosheet array. Both of them contain small micro-
pores (o2 nm), meso-pores (from 2 to 50 nm) and macro-pores
(450 nm). The average pore diameter of NiO/C nanosheet is
15.8 nm, smaller than that of pure NiO nanosheet (29.7 nm). The
calculated BET surface areas of NiO and NiO/C nanosheet are 79.6
and 194 m2/g while their pore volumes are 0.59 and 0.76 cm3/g,
respectively. The presence of carbon obviously increases the
The EIS results presented in Fig. 5(B) show that the electron
transfer resistance at the NiO/C–Ti electrode is about 134
diameter of semicircle), much smaller than that of the NiO–Ti
), indicating its improved conductivity. The
O (the
electrode (ꢁ332
O
conductivity of the NiO/C composite is obviously improved due to
the incorporation of carbon.
Fig. 6(A and B) give the detection limit and response time of
NiO–Ti and NiO/C–Ti electrode under desired potential of 0.41 V.

2742
X. Li et al. / Journal of Solid State Chemistry 184 (2011) 2738–2743
Fig. 5. (A) CV curves of NiO–Ti (lower curve) and NiO/C–Ti (upper curve) electrode recorded 0.1 M NaOH solution at a scan rate of 0.10 V/s; (B) EIS measurements of 5 mM
[Fe(CN)6]3ꢀ/4ꢀ in 0.1 M KCl using NiO–Ti and NiO/C–Ti electrodes.
Fig. 6. (A) Detection limit of NiO–Ti electrode, inset: response time at 6 s; and (B) current–time response of NiO/C–Ti electrode with an addition of low concentration
glucose solution, inset: response time within 4 s.
Fig. 7. (A) Current–time curves of Ti foil, NiO–Ti and NiO/C–Ti electrodes obtained at 0.41 V with the successive addition of same concentration glucose solution; and (B)
corresponding calibration curves of glucose concentration on NiO–Ti and NiO/C–Ti electrodes.
The detection limit of the NiO–Ti electrode is at 5
response time is 6 s. The NiO/C–Ti electrode exhibits better
performance. Fast response time within 4 s is observed and the
detection limit is at 2 mM at a signal-to-noise ratio of 3. In
addition, the NiO/C–Ti electrode exhibits a linear response with
m
M and the
Ni3þ þGlucose-Ni2þ þGluconic acid
(5)
First, Ni2þ could be electro-oxidized to Ni3þ in alkaline
solution. In this process, the release of electron resulted in the
formation of oxidation peak current. Second, glucose (C6H12O6)
could be oxidized to gluconic acid (C6H12O7) by Ni3þ. In this
process, Ni3þ was deoxidized to Ni2þ. Therefore, the presence of
glucose could lead to an increase in current.
Pure Ti foil exhibited no amperometric response with the
addition of glucose. It was stable enough to be used as a novel
substrate in electrochemistry. NiO/C–Ti electrode exhibited
higher response currents than NiO–Ti electrode, indicating its
better electrocatalytic oxidation activity of glucose. The corre-
sponding calibration curves of glucose concentration are shown in
Fig. 7(B). The NiO–Ti electrode exhibited a linear range up to
the addition of low concentration glucose solution.
Fig. 7(A) presents the current–time curves of NiO–Ti electrode
and NiO/C–Ti electrode with successive additions of glucose. Each
injection increased the glucose concentration at a 0.2 mM step.
The glucose molecule contained aldehyde group (–CHO). The
aldehyde group could be generally oxidized to a carboxyl group
(–COOH). The oxidation mechanism of glucose by Ni-based
materials could be represented by the following reactions:
Ni2þ þOHꢀ-Ni3þ þeꢀ
(4)

X. Li et al. / Journal of Solid State Chemistry 184 (2011) 2738–2743
2743
3.0 mM with a sensitivity of 229.3
The NiO/C–Ti electrode exhibited
m
Am Mꢀ1 cmꢀ2 (R2¼0.99906).
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582.6
mated as 2.6 mM.
m
Am Mꢀ1 cmꢀ2 (R2¼0.99961). Its linear range was esti-
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This work was financially supported by the National Natural
Science Foundation of PR China (No. 50872039).