Facile synthesis of single-crystalline hollow α-Fe2O 3 nanospheres with gas sensing properties

Article abstract of DOI:10.1039/c4ra05651e

High-quality single-crystalline hollow α-Fe₂O₃ nanospheres were prepared, using the ZnS-CHA (CHA = cyclohexylamine) nanohybrid as an additive through a solvothermal reaction, which avoids tedious steps and a high temperature calcination process. The formation process of these hollow nanospheres can be divided into two stages: (i) formation of solid Fe ₂O₃ nanospheres and (ii) preferential inside-out dissolution of the solid nanoparticles to form hollow nanospheres. Due to the unique single-crystalline hollow structure, the as-obtained α-Fe ₂O₃ nanomaterial exhibits enhanced gas sensing properties. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05651e

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Facile synthesis of single-crystalline hollow a-
Fe O nanospheres with gas sensing properties†
2
3
Cite this: RSC Adv., 2014, 4, 38707
a
a
a
a
a
Pei-Pei Wang, Xiaoxin Zou, Liang-Liang Feng, Jun Zhao, Pan-Pan Jin,
b
c
a
a
Rui-Fei Xuan, Ye Tian, Guo-Dong Li and Yong-Cun Zou*
Received 12th June 2014
Accepted 7th August 2014
DOI: 10.1039/c4ra05651e
www.rsc.org/advances
21
High-quality single-crystalline hollow a-Fe
O
2 3
nanospheres were consuming. Combining the functionality of Fe O3 with its
2
prepared, using the ZnS–CHA (CHA ¼ cyclohexylamine) nanohybrid as single-crystalline hollow structure is of essential for function
an additive through a solvothermal reaction, which avoids tedious optimum. Quest for simple, controllable and scalable synthesis
steps and a high temperature calcination process. The formation of single-crystalline hollow Fe
process of these hollow nanospheres can be divided into two stages: tremendous attention.
2 3
O materials have been obtained
(
i) formation of solid Fe
dissolution of the solid nanoparticles to form hollow nanospheres. prepare single-crystalline hollow Fe
Due to the unique single-crystalline hollow structure, the as-obtained hybrid as an additive. The procedure was conducted without
2
O
3
nanospheres and (ii) preferential inside-out
In this paper, we report a facile template-free approach to
2 3
O
with ZnS–CHA nano-
a-Fe
2
O
3
nanomaterial exhibits enhanced gas sensing properties.
post-high temperature calcination process within 24 h. The
hollow nanospheres are in diameters of 200–300 nm with well-
dened shape and size, and the size of the shells are around 20–
2 3
As an important n-type semiconductor, a-Fe O has been widely
studied for its comprehensive applications such as for gas
sensors, Li-ion batteries, pigments, magnetic recorders and
catalysts.
develop methods for delicate control over the morphology, size
and functions of a-Fe . Most of the reported work evolves
around methodologies of constructing hollow or single-
50 nm. And the as-obtained Fe
2 3
O exhibits promising chemical
sensing properties. The hollow single-crystalline a-Fe
2 3
O
nanospheres show good gas sensing property towards ethanol,
and exhibits good response characteristics towards multiple of
target gases. As the raw material, ZnS–CHA nanocomposite, was
easy to sale up, the current contribution may offer a versatile
approach for the development of a series of metal oxide for
advanced applications.
1–11
Considerable research efforts have been devoted to
2 3
O
2,5,12–19
crystalline Fe
Fe was the best representative.
hollow Fe was reported by Eswaramoorthy's group, using
2
O
3
,
among which single-crystalline hollow
17,20
2
O
3
The rst single-crystalline
2 3
Fig. 1A shows the SEM image of the single-crystalline Fe O
2 3
O
with hollow spheres in uniformed size, and the hollow structure
was further determined by TEM analysis, which exhibits the
diameter in the range of 200–300 nm with wall thickness
around 20–50 nm (Fig. 1B). High resolution TEM (HRTEM)
show the particles with continuous lattice fringes measured
from (101) plane to be 0.42 nm and (006) plane to be 0.23 nm in
carbonaceous spheres as sacricial templates with a sol–gel
process. The single-crystalline hollow spheres were homoge-
neous, but they were collected by high temperature post-
calcination which was a bit energy consumption. Fan and his
coworkers reported a shape-controlled method making hollow
5
ꢀ
single-crystalline Fe
2
O
3
by hydrothermal treatment at 220 C for
(Fig. 1C), and the selected area electron diffraction (SAED)
60 h, which process was easy to realize but a bit time-
pattern (Fig. 1D) evidently gives the single-crystalline nature of
the nanospheres.
2 3
High-quality single-crystalline hollow a-Fe O nanospheres
a
State Key Lab of Inorganic Synthesis & Preparative Chemistry, College of Chemistry,
Jilin University, Changchun 130012, P. R. China. E-mail: zouyc@jlu.edu.cn; Fax: +86-
were prepared using ZnS–CHA nanohybrid as an additive. In the
typical system, zinc acetate dihydrate and thiourea functioned
as the zinc source and the sulfur source, respectively, whereas
cyclohexylamine (CHA) acted as both the solvent and the reac-
tive agent. In comparison with the previous synthetic condition
of ZnS–CHA, we just decreased the concentration of the zinc
4
31-85168624; Tel: +86-431-85168318
b
College of Materials Science and Engineering, China University of Mining and
Technology, Xuzhou 221116, China
c
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of
Chemical Engineering, Tianjin University, Tianjin 300072, China
ꢁ
1
ꢁ1
†
Electronic supplementary information (ESI) available: Characterization details source (75 mmol L in referenced paper and 37.5 mmol L in
2 3
of the ZnS–CHA nanocomposite, SEM and TEM of the relevant Fe O products.
this paper) in the reaction mixture to get a high-quality ZnS–
See DOI: 10.1039/c4ra05651e
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nanosphere was shown in Scheme S1.† Based on the above
results, the formation process of these hollow nanospheres can
be mainly divided into two stages: (i) formation of solid Fe O3
2
nanospheres and (ii) preferential inside-out dissolution of the
solid nanoparticles to form hollow nanospheres.
3
Fe + H
+
+
2
O / Fe(OH)
3
/FeOOH + H
(1)
(2)
Fe(OH) /FeOOH / Fe O + H O
3
2
3
2
Under a hydrothermal reaction condition, the formation of
3
+
Fe
2
O
3
nanoparticles from Fe ions is based on the eqn (1) and
(
2). Then the 3-D oriented attachment of nanocrystals was
4,5,24
formed,
the nanocrystals began to self-assembly well-
dened single-crystalline a-Fe O solid nanospheres. Accom-
2
3
panying this process, the pH value of the reaction system will
decrease due to the generation of protons (eqn (1); the pH value
of the nal reaction system is as low as 2.4). This is will provide
an acidic condition to dissolve the ZnS–CHA hybrid and to
Fig. 1 (A) SEM; (B) TEM; (C) HRTEM images and (D) SAED pattern of the
obtained single-crystalline a-Fe O nanospheres.
2 3
selectively inside-out dissolve the solid Fe
2 3
O nanospheres
because the inner part was less stable compared with the outer
CHA sample, while maintaining the molar ratio of zinc to sulfur
1 : 2). The structure of as-prepared ZnS–CHA was similar to
5,25
surface layer of Fe O nanocrystal. At last, the hollow single-
2
3
(
crystalline nanospheres were formed.
that in the previous report, as demonstrated by powder X-ray
diffraction (XRD), infrared spectroscopy (IR), and X-ray photo-
electron spectroscopy (XPS) (see Fig. S1 in the ESI†). The ZnS–
CHA nanocomposite was formed from the very small ZnS
nanoparticles through assembly with the CHA molecules. The
XRD patterns were shown in Fig. 2A. It was clear that the
2 3
product was consists of a-Fe O and ZnS at 1 h, and the pure a-
Fe (JCPDS card no.: 33-0664) with no impure peaks was
2 3
O
formed aer 6 h. The product we obtained aer 24 h was in
good crystalline with narrow peaks and smooth base line.
Raman spectra was also carried out to determine the crystal
phase and crystalline condition (Fig. 2B). The characteristics
roman bands were typical a-Fe O . The base lines of the prod-
2
+
2ꢁ
zinc and sulfur species were Zn and S , respectively, and the
CHA molecules were not protonated in the ZnS–CHA nano-
composite. Based on the thermogravimetric (TG) result
2
3
(Fig. S1C†), the empirical composition of the as-prepared ZnS–
ucts in 1 h and 6 h were a bit rough revealing the low crystalline.
At last, the hollow Fe in better quality crystalline was
CHA nanocomposite was close to ZnS$CHA, in which the
amount of CHA was obviously higher than that reported previ-
ously, probably due to the higher CHA : Zn ratio in this reaction
condition. In addition, the morphology of the as-prepared ZnS–
CHA with a short rod-shape (see SEM image in Fig. S2†) was also
different from that reported previously (irregular powder
2 3
O
obtained aer 24 h. The Raman spectra matched very well with
the XRD patterns.
Surface elements of the hollow nanospheres were examined
by XPS spectra shown in Fig. S4.† High-resolution of Fe 2p1/2
and Fe 2p3/2 peaks of the hollow a-Fe O (Fig. S4A†) were located
22
2 3
sample). It should be noted that, the exact crystal structure
was still not available since the large single crystal of ZnS–CHA
can't be obtained. Regardless of its exact crystal structure, ZnS–
CHA has been applied as an efficient precursor for the prepa-
at 725.3 and 711.2 eV, respectively. The shakeup satellite at
3
+
4,8
7
19.9 eV was the typical Fe in Fe O . High-resolution of O 1s
2 3
was shown in Fig. S4B.† The vibrations of crystal lattice O
occupied nearly 82.8%, and the adsorption of molecule oxygen
takes only 27.2%.
23
ration of inorganic nanosheet material. It was also used as a
novel additive for formation of the well-dened a-Fe O nano-
2
3
spheres, as demonstrated as below.
In order to understand the formation process of the uniform
hollow a-Fe , time-dependent experiments were carried out
2 3
O
carefully on basis of the above experimental procedures. At
given time interval, the powder products were harvested by
centrifuging and washing with deionized water for further
characterization. Corresponding SEM photographs were shown
in Fig. S3.† As shown in the pictures, the nanoparticles were so
small that we couldn't see them clearly during 1 h (Fig. S3A†).
Aer that, the solid products we collected were with diameters
in the range of 200–300 nm (Fig. S3B and C†). It doesn't form
the completely hollow nanospheres until 24 h later (Fig. S3E and
F†), and the diameters of the nanospheres were not changed
2 3
Fig. 2 (A) XRD patterns and (B) Raman spectra of the a-Fe O nano-
obviously. The schematic diagram for formation of the hollow spheres obtained at 1 h, 6 h and 24 h.
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In order to determine the role of ZnS–CHA nanohybrid in
formation of the well-dened single-crystalline hollow a-Fe O ,
2
3
four different control experiments were performed for
comparative studies. (1) The rst control experiment involved
the synthesis of materials with no ZnS–CHA being used as the
above synthetic procedure, this resulted in only uneven a-Fe
nanoparticles formed with average size of ꢂ300 nm (Fig. S5A†).
The product we collected in this process was called Fe -1; (2)
2 3
O
2 3
O
instead of ZnS–CHA, the same amount of pure CHA (0.015 g)
was used under identical reaction conditions. However, the
synthesis also resulted in irregular a-Fe O nanoparticles with
2
3
average size of ꢂ100 nm (Fig. S5B†) which was called Fe O -2;
2
3
ꢀ
(
3) treating the ZnS–CHA nanocomposite at 160 C in water
solution to synthesis the pure ZnS, and then taking the same
amount of ZnS instead of the ZnS–CHA nanohybrid, the nal
product was also uneven a-Fe
2
O
3
nanoparticles with average
-3; (4) the as
size of ꢂ300 nm (Fig. S5C†) which was called Fe
2 3
O
prepared pure ZnS (0.015 g) with pure CHA (0.015 g) were used
to react at the same condition as mentioned above, without
exception, the nal product was also uneven nanoparticles with
average size of ꢂ100 nm (Fig. S5D†) named as Fe
2 3
O -4. There-
Fig. 3 Gas sensing property toward 5–2000 ppm ethanol of (A) the
fore, we can conclude that the ZnS–CHA nanohybrid plays an single-crystalline Fe
2
O
3
and (B) the Fe
2
O
3
-1; (C) concentration–
(B) and the Fe -1
O) towards ethanol; and (D) response of the single-crystalline Fe
criss-crossed rectangle) and the Fe -1 (open rectangle) towards
response curves of the single-crystalline Fe
2
O
3
2 3
O
O
2 3
essential role in preparation of the well-dened single-
crystalline hollow a-Fe nanospheres.
(
(
2 3
O
2 3
O
To further conrm the relationship between the iron content
multiple gases.
with the single-crystalline hollow Fe O , controlled experiments
2
3
were conducted by using 0.32 g, 0.48 g and 0.64 g FeCl $6H O.
3
2
As a result, the obtained Fe O nanospheres were poly-
crystalline with increased diameters and the corresponding
SEM images were given in Fig. S6.†
2
3
formaldehyde than the contrast Fe
the stability of the single-crystalline hollow Fe
2
O
3
-1. In order to estimate
, time
2 3
O
depended gas sensing experiments were taken every one day for
ten days. Slight variations were found during the ten days
measurement towards 5, 500 and 2000 ppm ethanol (Fig. S10†).
This results give the nal conclusion that the sensors exhibit
good repeatability to ethanol.
Gas sensing performance was employed by a CGS-8 gas
sensing measurement system (Beijing Elite Tech Company
Limited). The whole process was the same as mentioned
26–28
before.
Sensitivity was designed as R /R , where R was the
a g a
resistance of sensors in air, R was the resistance of the sensors
g
2 3
The sensing mechanism of Fe O sensors was the same as
9,29,30
in the target gases.
crystal hollow a-Fe
Gas sensing properties of the single
2 3
O nanospheres were researched towards
the traditional semiconductor sensors. The main mechanism
was based on the reaction between the detected gas molecules
and the chemisorbed oxygen species on Fe O surface. The
ethanol. The concentration–sensitivity relationship experi-
2
3
ꢀ
ments were executed from 5 ppm to 2000 ppm at 300
C
Fe O sensors will exhibits a relatively high resistance state by
2
3
(Fig. 3A). The sensitivity was increased with increasing the
2ꢁ
ꢁ
32
generate O or O in the air condition, since the adsorbed
oxygen molecules will capture electrons from the Fe layer
conduction band of the Fe ). When the target reductive gases
were introduced at an moderate operating temperature (e.g.,
ethanol), the Fe surface layer oxygen species will have a
concentration of ethanol. Excepting the sensitivity, there were
also some vital parameters for the sensing materials, such as
response time, recovery time and the stability. The magnica-
tion curve of the single-crystalline hollow Fe O towards 500
2 3
O
(
2 3
O
2
3
2 3
O
ppm ethanol was shown in Fig. S9.† The response time was
dened as the time from R to R ꢁ R ), and the
ꢁ 90% ꢃ (R
recovery time was dened as the time from R to R
+ 90% ꢃ (R
). The single-crystalline hollow Fe exhibits short
response time for 5 s and fast recovery time for 2 s, both of
reaction with them, and then the electrons will be given back
into the Fe O layer, giving rise to the reduced surface oxygen
a
a
a
g
2
3
g
g
a
species along with the decreased surface resistance. The reac-
tion equation was presented as follows:
ꢁ
R
g
2 3
O
16,31
which were much better than the reported results.
The
ꢁ
ꢁ
O + 6e
C
H
2 5
OH + 6O 4 2CO
2
+ 3H
2
(3)
dynamic response curves of the contrast Fe O -1 mentioned
2
3
above (control experiment (1)) to ethanol was shown in Fig. 3B.
The enhanced sensing performance may attribute to the
Concentration–response curves of the two sensors were shown hollow structure of the single-crystalline Fe O . TEM and
2
3
in Fig. 3C. The single-crystalline hollow Fe
rior response towards ethanol compared to contrast Fe
shown in Fig. 3D. The single-crystalline hollow Fe also gives Fe O owns larger BET surface area, the BET surface area of the
2 3
2 3
O shows the supe- HRTEM images of Fe O -1 were shown in Fig. S7,† which
2 3
O -1 further conrmed its solid structure. Single-crystalline hollow
2 3
O
2
3
higher response to other gases such as acetone, methanol, and
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2
ꢁ1
single-crystalline hollow Fe
2
O
3
was 31 m
g
(Fig. S8A†),
9 P. Sun, Z. Zhu, P. Zhao, X. Liang, Y. Sun, F. Liu and G. Lu,
CrystEngComm, 2012, 14, 8335.
2
remarkably higher than that of the Fe O -1 (Fig. S8B,† 12 m
2
3
ꢁ1
g
). The hollow structure with higher surface to volume ratio 10 N. V. Long, Y. Yang, M. Yuasa, C. M. Thi, Y. Cao, T. Nann and
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O
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1
Conclusion
2 3
Single-crystalline hollow hematite (Fe O ) nanospheres were
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967.
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