Flower-like NiO hierarchical microspheres self-assembled with nanosheets: Surfactant-free solvothermal synthesis and their gas sensing properties

Article abstract of DOI:10.1016/j.jallcom.2015.02.191

Flower-like NiO hierarchical microspheres self-assembled with thin nanosheets were successfully synthesized by surfactant-free solvothermal method. The obtained product was characterized by means of X-ray diffraction, scanning electron microscopy, and ultraviolet-visible spectroscopy. The results showed that flower-like NiO hierarchical microspheres with a cubic structure were assembled by a number of thin nanosheets with a thickness of about 10 nm. Reaction time was an important parameter for the synthesis of perfect flower-like microspheres. The band gap energy of NiO hierarchical microspheres was estimated to be 3.68 eV. The gas sensing properties showed that the sensor exhibited the highest sensitivity of 3.2 for 100 ppm ethanol at an operating temperature of 250 °C. The reversible response to ethanol at various operating temperatures and gas concentrations and good selectivity were obtained, implying a potential application for the fabrication of high performance ethanol sensors.

Full text of DOI:10.1016/j.jallcom.2015.02.191

Journal of Alloys and Compounds 636 (2015) 357–362
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Letter
Flower-like NiO hierarchical microspheres self-assembled with
nanosheets: Surfactant-free solvothermal synthesis and their gas
sensing properties
Xiaoguang San a, Guosheng Wang a, Bing Liang b, Jiao Ma a, Dan Meng a, , Yanbai Shen c,
⇑
⇑
a College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
b College of Materials Science and Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China
c College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Flower-like NiO hierarchical microspheres self-assembled with thin nanosheets were successfully syn-
thesized by surfactant-free solvothermal method. The obtained product was characterized by means of
X-ray diffraction, scanning electron microscopy, and ultraviolet–visible spectroscopy. The results showed
that flower-like NiO hierarchical microspheres with a cubic structure were assembled by a number of
thin nanosheets with a thickness of about 10 nm. Reaction time was an important parameter for the syn-
thesis of perfect flower-like microspheres. The band gap energy of NiO hierarchical microspheres was
estimated to be 3.68 eV. The gas sensing properties showed that the sensor exhibited the highest sensi-
tivity of 3.2 for 100 ppm ethanol at an operating temperature of 250 °C. The reversible response to etha-
nol at various operating temperatures and gas concentrations and good selectivity were obtained,
implying a potential application for the fabrication of high performance ethanol sensors.
Ó 2015 Elsevier B.V. All rights reserved.
Received 12 January 2015
Received in revised form 24 February 2015
Accepted 25 February 2015
Available online 3 March 2015
Keywords:
Nickel oxide
Hierarchical architecture
Solvothermal
Sensing property
1. Introduction
which causes contamination in the products and subsequently
affects various semiconductor device applications. Thus, a facile
In recent years, the novel three-dimensional (3D) hierarchical
architectures assembled by low dimensional nano-building blocks
such as nanoparticles, nanowires, and nanosheets, have attracted
considerable attention due to its unique properties and wide prac-
tical application [1,2]. Because of their high specific surface areas
and excellent structural stability, as well as their outstanding
transportation properties, these kinds of nanostructures have
exhibited superior performance for catalysis, solar cell, sensor,
etc [3–7]. For example, Ko et al. [3] reported that tree-like hierar-
chical ZnO nanostructures assembled by ZnO nanowires demon-
strated potential application in dye-sensitized solar cell. Li et al.
[5] reported that hierarchical meso-macroporous SnO2 exhibited
higher gas response and shorter response/recovery times in detect-
ing indoor air pollutants. Up to now, various routes have been
employed successfully to assemble building blocks into 3D hierar-
chical architectures [3–7]. The method of hydrothermal/solvether-
mal is widely used to synthesize hierarchical architectures.
However, some surfactants or templates are usually introduced,
surfactant- or template-free method for producing hierarchical
architecture materials is great significance from the view of both
scientific research and practical application.
Nickel oxide (NiO), a p-type metal oxide semiconductor, has
been widely used for different fields, such as catalysts [4,8], super-
capacitors [9], lithium ion batteries [10], electrochromic devices
[11] and so on. Moreover, NiO is one of the promising materials
used as gas sensors due to its good sensing properties [12,13].
From the viewpoint of gas sensor, hierarchical architectures with
high specific surface areas and porous structures are advantageous
for improving the gas sensing performance [14]. Recently, a variety
of metal oxide semiconductors such as SnO2 [5,15], WO3 [16], and
ZnO [17] with hierarchical structures have been synthesized and
showed good sensing properties to the detected gases. However,
there is little report about gas sensing properties for NiO hierarchi-
cal architectures, though which has been widely investigated in
many other application fields [4,18,19]. In this study, we report
on the synthesis of flower-like NiO hierarchical microspheres com-
posed of thin nanosheets through solvethermal process without
using any surfactant. Ethanol gas sensing properties of these flow-
er-like microspheres were investigated.
⇑
Corresponding authors.
E-mail addresses: mengdan0610@hotmail.com (D. Meng), shenyanbai@mail.neu.
edu.cn (Y. Shen).
http://dx.doi.org/10.1016/j.jallcom.2015.02.191
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

358
X. San et al. / Journal of Alloys and Compounds 636 (2015) 357–362
2. Experimental
crystallinity of NiO is obtained. Especially, no diffraction peaks of
-Ni(OH)2 are observed, indicating that the -Ni(OH)2 was com-
a
a
Flower-like NiO hierarchical microspheres were synthesized by the surfactant-
free solvothermal method. In a typical procedure, 0.466 g of nickel nitrate hexahy-
drate (Ni(NO3)2Á6H2O) was dissolved in 40 mL N, N dimethyl formamide (DMF)
under magnetic stirring at room temperature to get a clear solution. Then the solu-
tion was transferred into a 50 ml Teflon-lined stainless autoclave, which was sealed
and maintained at 180 °C for 12 h and subsequently cooled down to room tem-
perature naturally. A green-blue precipitate was obtained, which was washed with
distilled water and ethanol for several times, and dried in vacuum at 60 °C for 4 h.
Finally, the black product was obtained through annealing at 400 °C for 4 h in air.
The obtained product was characterized using an X-ray diffractometer (XRD,
PANalytical X’Pert Pro), a field emission scanning electron microscope (FESEM,
ZEISS Ultra Plus) equipped with energy dispersive X-ray spectroscopy (EDS), and
ultraviolet–visible spectrometer (UV–Vis, UV-2550, Shimadzu).
pletely transformed to NiO after annealing. To demonstrate the
chemical composition of the flower-like microspheres, EDS analy-
sis of the annealed product was performed and the EDS spectrum
is illustrated in Fig. 2(b). It can be seen that only oxygen and nickel
elements exist in the microspheres with O/Ni molar ratio of nearly
1.
It is well known that reaction time has a great effect on the
structure and morphology of the product in solvothermal method
[20–22]. SEM images of the as-synthesized products at different
reaction times are shown in Fig. 3. No precipitation is obtained
when the reaction time is less than 3 h. At the reaction time of
4 h, many irregular shapes consist of sheets, rods, and cubics are
obtained as shown in Fig. 3(a). Some of them aggregate with each
other and form the flower-like microspheres. After increasing reac-
tion time to 8 h, as shown in Fig. 3(b), many incompact aggrega-
tions with flower-like structures are obtained, which are
assembled by a few nanosheets. At the reaction time of 12 h, the
perfect flower-like microspheres consisted of abundant thin
nanosheets are found (Fig. 1(a) and (b)). The products remain the
flower-like structures at 18 h and show stone-like structures at
24 h, while the diameter of these flower-like microspheres increas-
3. Results and discussion
3.1. Structure characterizations
SEM images of the as-synthesized and annealed products are
shown in Fig. 1. Fig. 1(a) and (b) indicate that the products before
annealing are composed of the flower-like microspheres with a dia-
meter ranging from 1 to 3 lm. These flower-like microspheres are
constructed by many thin nanosheets with smooth surface and about
10 nm in thickness, which are densely packed and formed a multilay-
ered structure. It is interesting that the follower-like microspheres
are successfully sustained after annealing at 400 °C for 4 h (Fig. 1(c)
and (d)). In addition, it should be noted that the diameter of these
flower-like microspheres does not change after annealing process.
The XRD pattern observed for the as-synthesized and annealed
products are shown in Fig. 2(a). All the diffraction peaks of the as-
es from 2 to 8 lm (Fig. 3(c) and (d)). Such results reveal that the
reaction time is the main factor to form nanosheet based NiO
microspheres. In the present work, hydrate Ni(NO3)2Á6H2O was
used as Ni2+ ion source. Thus, the water molecules existed in
Ni(NO3)2Á6H2O are free after it dissolves into the DMF solvent. In
the solvothermal process, the water molecules provide OHÀ ions
for the formation of Ni(OH)2 nuclei, which subsequently grows to
nanosheets. As the reaction time increases, in order to reduce the
surface energy, the thin nanosheets gradually self-oriented and
assembled to flower-like hierarchical Ni(OH)2 structures [21].
Ostwald ripening process maybe plays an important role in this
procedure [20–22]. The size increase can significantly decrease
the surface free energy and enhance the stability energy of
nanosheets, thus leading to the size increase of flower-like micro-
spheres and formation of stone-like structures as extending reac-
tion time. After annealing at 400 °C, the Ni(OH)2 precursors are
decomposed and NiO flower-like microspheres are formed.
synthesized products could be indexed to single phase
a-Ni(OH)2
with a hexagonal crystalline structure (JCPDS card No. 22-0444).
No impurity peaks are found in the XRD pattern, suggesting a high
purity of the as-synthesized a-Ni(OH)2. The wide peaks and weak
peak intensities imply that the crystalline structure of the as-syn-
thesized products is not good. After annealing at 400 °C for 4 h, five
main peaks corresponding to the crystallographic planes of (111),
(200), (220), (311), and (222) are observed (Fig. 2(a)), which
characterize a cubic structure of the NiO (JCPDS card No. 47-
1049). The sharp diffraction peaks suggest that the high
Fig. 1. SEM images of the as-synthesized and annealed products. (a) and (b) as-synthesized products. (c) and (d) products annealed at 400 °C.

X. San et al. / Journal of Alloys and Compounds 636 (2015) 357–362
359
Fig. 2. (a) XRD patterns of the as-synthesized and annealed products. (b) EDS pattern of the product annealed at 400 °C.
Fig. 3. SEM images of the as-synthesized products at different reaction times. (a) 4 h. (b) 8 h. (c) 18 h. (d) 24 h.
UV–Vis absorption spectrum of the NiO hierarchical micro-
spheres annealed at 400 °C is shown in Fig. 4. The absorption in
the UV region can be observed, which may be attributed to the
band gap absorption in NiO [23]. The band gap energy (Eg) is usu-
ally calculated by the optical absorption spectrum using the fol-
lowing equation
(
a
hv)n = B(hv À Eg), where
a is absorption
coefficient, hv is the photo energy, B is a constant relative to the
material and n is either 2 for direct transition or 1/2 for an indirect
transition [24]. NiO is generally a direct transition semiconductor
[25]. Hence, the direct band gap can be obtained by extrapolating
the linear portion of the (a
hv)2 versus hv curve to zero, as shown
in inset Fig. 4. The derived band gap energy of the NiO hierarchical
microspheres was estimated to be 3.68 eV, which is lower than
that of the bulk NiO (ꢀ3.9 eV) [26]. This is due to the chemical
defects or vacancies present in the intergranular regions, forming
a new energy level to reduce the band gap energy [25,27].
3.2. Gas sensing properties
Fig. 4. UV–Vis absorption spectrum of the NiO hierarchical microspheres. The inset
The unique hierarchical microspheres with many petals, pores,
and intervals are very good candidates for gas sensors [14]. In this
study, gas sensor was fabricated by coating the annealed product
onto an alumina ceramic tube attached with a pair of Au electrodes
and four Pt wires. A Ni–Cr alloy coil was inserted through the tube
to control the operating temperature of the sensor. The gas sensing
properties were performed on a static system with the constant
is the (a
hv)2 – hv curve.
loop voltage of 5 V. A diagram of NiO hierarchical microspheres
based gas sensor device and measurement electric circuit are
shown in Fig. 5(a). The sensitivity is defined as Rg/Ra, where Ra
and Rg are the resistances in air and in the tested gas, respectively.

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X. San et al. / Journal of Alloys and Compounds 636 (2015) 357–362
The sensitivities of the sensors made of flower-like NiO hierar-
It is well known that NiO belongs to p-type semiconductor,
which is a surface controlled process, that is, the sensitivity greatly
depends on the adsorption and desorption of gas molecules [12]. A
possible sensing mechanism is proposed in Fig. 6. The oxygen
molecules from the ambient atmosphere are initially adsorbed on
the surface of NiO in the form of OÀ2 , OÀ, and O2À, depending on
the ambient temperature [28,29]. The high coverage with adsorbed
oxygen causes the accumulation of holes near the surface of NiO,
and thus showing a relatively low resistance state in air ambient.
When NiO hierarchical microspheres are exposed to a reducing
gas such as ethanol, these gas molecules could react with surface
oxygen species by C2H5OH(gas) + 6OÀ(ads) ? 2CO2 + 3H2O + 6eÀ
[30]. This process releases the electrons back to NiO, which induces
a decrease of the hole concentration by electron-hole recombina-
tion, leading to an increase in the resistance. That is the character-
istic of p-type semiconductor [31]. The flower-like NiO structure
may be beneficial to enhance the probability to absorb target gases
and provide an effective gas diffusion path via well-aligned porous
architectures, therefore, resulting in good sensing properties [14].
Here, it should be noted that a high response to reducing gas is
relatively difficult to accomplish in p-type semiconductors, even
with the considerably large surface changes due to the conduction
chical microspheres upon exposure to 100 ppm ethanol gas are
shown in Fig. 5(b) as a function of the operating temperature.
The sensor has an optimal operating temperature, at which the
sensor exhibits the highest sensitivity to ethanol gas. The highest
sensitivity is estimated to be 3.2 at 250 °C. The gas response and
recovery behavior of the sensor measured at different operating
temperatures to 100 ppm ethanol is shown in Fig. 5(c). The resis-
tance quickly increases when the sensor is exposed to ethanol
gas. The response times (the time for the resistance increases to
90% of the maximum) are less than 1 min at all operating tem-
peratures. It is also noted that the resistance can recover to the ini-
tial values after removing ethanol gas, indicating
a good
reversibility. Fig. 5(d) shows the temporal responses of the sensor
upon exposure to different concentration ethanol gases (50–
1000 ppm) at 250 °C. As the gas concentration increases, the
response is enhanced and the resistance can recover to the initial
value after removal of ethanol, exhibiting a good reversibility.
Gas responses to 100 ppm different volatile organic compound
gases were also measured at 250 °C as shown in Fig. 5(e). The result
shows that the sensitivity to ethanol is higher than that to other
gases, implying that selective detecting of ethanol is possible.
Fig. 5. (a) Schematic illustration of the gas sensor and measurement electric circuit. (b) Sensitivity and (c) temporal responses upon exposure to 100 ppm ethanol gas at
different operating temperatures. (d) Temporal responses to ethanol gases with various concentrations at 250 °C. (e) Selectivity to different gases with concentration of
100 ppm.

X. San et al. / Journal of Alloys and Compounds 636 (2015) 357–362
361
Fig. 6. Schematic illustration of gas sensing mechanism.
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4. Conclusions
High-performance flower-like NiO hierarchical microspheres
assembled with thin nanosheets were successfully synthesized
by surfactant-free solvethermal method. The structure characteris-
tics and ethanol sensing properties of NiO hierarchical micro-
spheres were investigated. The results showed that NiO
hierarchical microspheres with a cubic structure were assembled
by a number of thin nanosheets with a thickness of about 10 nm.
The band gap energy of NiO hierarchical microspheres was esti-
mated to be 3.68 eV. Ethanol sensing measurements indicated that
NiO hierarchical microspheres exhibited the highest sensitivity of
3.2–100 ppm ethanol at an operating temperature of 250 °C and
reversible response to ethanol at various operating temperatures
and concentrations. The results indicate a potential application
for the fabrication of high-performance ethanol sensors.
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Acknowledgements
The authors are grateful for the support of the National Natural
Science Foundation of China (61403263, 51422402) and Liaoning
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