nanomaterials
Article
Hydrothermal Synthesis of Nanooctahedra MnFe2O4
onto the Wood Surface with Soft Magnetism, Fire
Resistance and Electromagnetic Wave Absorption
Hanwei Wang 1 , Qiufang Yao 1 , Chao Wang 1 , Zhongqing Ma 1 , Qingfeng Sun 1,2, *, Bitao Fan 1 ,
Chunde Jin 1,2 and Yipeng Chen 1
1
2
*
School of Engineering, Zhejiang Agriculture & Forestry University, Lin’an 311300, China;
[email protected] (H.W.); [email protected] (Q.Y.); [email protected] (C.W.);
[email protected] (Z.M.); [email protected] (B.F.); [email protected] (C.J.);
[email protected] (Y.C.)
Key Laboratory of Wood Science and Technology, Hangzhou 311300, China
Correspondence: [email protected]; Tel./Fax: +86-571-6373-2718
Academic Editor: Jordi Sort
Received: 9 February 2017; Accepted: 8 May 2017; Published: 23 May 2017
Abstract: In this study, nanooctahedra MnFe2 O4 were successfully deposited on a wood surface via
a low hydrothermal treatment by hydrogen bonding interactions. As-prepared MnFe2 O4 /wood
composite (MW) had superior performance of soft magnetism, fire resistance and electromagnetic
wave absorption. Among them, small hysteresis loops and low coercivity (<±5 Oe) were observed
in the magnetization-field curve of MW with saturation magnetization of 28.24 emu/g, indicating
its excellent soft magnetism. The MW also exhibited a good fire-resistant property due to its initial
burning time at 20 s; while only 6 s for the untreated wood (UW) in combustion experiments.
Additionally, this composite revealed good electromagnetic wave absorption with a minimum
reflection loss of −9.3 dB at 16.48 GHz. Therefore, the MW has great potential in the fields of special
decoration and indoor electromagnetic wave absorbers.
Keywords: wood; MnFe2 O4 /wood composite; hydrothermal; soft magnetism; fire resistance;
electromagnetic wave absorption
1. Introduction
Wood/inorganic hybrids fall under the category of new functional nanocomposite materials
because of being a combination between organic and inorganic materials with their respective
advantages and excellent performance. Synergistic effects resulting from the physical or chemical
interactions between the inorganic and wood components have produced such properties as improved
thermal stability, UV-resistance, hydrophobic, mechanical and dimensional stability [1]. Currently,
deposition of a thin solid inorganic nanomaterial coating imbedded onto the wood surface has great
potential to improve the inherent wood defects (like moisture deformation, easily burnt, etc.) and
simultaneously to grant novel performances (like super-hydrophobic, self-cleaning, UV–resistance, or
others) [2]. The present focus among the thin solid inorganic nanomaterial coatings is mainly metal
semiconductor materials, like TiO2 [3,4], SiO2 [5], ZnO [6], CeO2 [7], and so on [8–13]. Several research
works paid less attention to magnetic nanostructure materials deposited onto the wood surface.
With the rapid development of wireless communications indoors and outdoors, the
electromagnetic interference (EMI) pollution has become much more serious. The electromagnetic
waves may cause interception and malfunction of the performance of electrical equipment in medical,
military and aircraft systems or even lead to radiative damage of the human body [7,14]. Therefore, it
Nanomaterials 2017, 7, 118; doi:10.3390/nano7060118
www.mdpi.com/journal/nanomaterials
Nanomaterials 2017, 7, 118
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is necessary to exploit new types of microwave-absorption materials with excellent properties, such as
a wide frequency range, strong absorption, low density, high resistivity, etc.
Magnetic nanomaterials/wood hybrids would be potential candidates for microwave absorption,
especially when wood serves as interior decorations due to its renewability, attractive surface, sound
insulation, temperature- and humidity-controlling performances. It may be a reasonable choice for
wave absorption if the wood surface is embedded with a thin solid magnetic film with a trivial
change of appearance. Previous studies have been conducted showing that the wood surface can be
considered as an effective substrate containing plentiful hydroxyl groups for the nucleation and growth
of inorganic nanomaterials. Publicly reported pathways for the deposition of magnetic materials are
the sol-gel method [15–17], electroless deposition [18], the hydrothermal process [19,20] and physical
padding. Among these methods, the hydrothermal method was a feasible and efficient pathway for
growing magnetic nanomaterials with high product purity and homogeneity, crystal symmetry, narrow
particle size distributions, a lower sintering temperature, a wide range of chemical compositions and
single-step processes, as well as for the growth of crystals with polymorphic modifications [21–27].
Herein, we employed a facile low temperature hydrothermal process for the growth of nanooctahedra
MnFe2 O4 on the wood surface. The as-prepared MnFe2 O4 /wood (MW) composite showed a superior
soft magnetism, fire resistance and electromagnetic absorption. The saturation magnetization of the
MW was 28.24 emu/g with extremely small hysteresis loops, and low coercivity indicated that this
composite was an excellent soft-magnetic material. The MW also exhibited a good fire resistance
property due to it not being burnt in the first 20 s, while only 6 s were needed for the untreated
wood. Additionally, this composite is a good electromagnetic absorption material due to its minimum
reflection loss of −9.3 dB at 16.48 GHz. Thus, the MW has great potential in the fields of special
decoration and indoor electromagnetic wave absorbers.
2. Experimental Details
2.1. Materials
All chemicals were supplied by Boyle chemical Co. Ltd., Shanghai, China. and used without
further purification. The wood slices were cut with sizes of 20 mm (length) × 10 mm (width) × 5 mm
(height), and then, the slices were ultrasonically rinsed in deionized water for 30 min and dried at
80 ◦ C in a vacuum.
2.2. One-Pot Hydrothermal Synthesis of MW
In a typical synthesis, FeCl3 ·6H2 O and MnSO4 ·H2 O in a stoichiometric ratio of 2:1 were dissolved
in 80 mL of deionized water under magnetic stirring at room temperature. The obtained homogeneous
mixture was transferred into a 100 mL Teflon-lined stainless autoclave. Wood specimens were
subsequently placed into the above reaction solution, and the pH value was adjusted via adding
a certain amount of ammonia solution. The Teflon-lined stainless-steel autoclave was sealed and
heated to 120 ◦ C for 8 h. Subsequently, the autoclave was left to cool down to room temperature.
Finally, the prepared magnetic wood samples were removed from the solution, ultrasonically rinsed
with deionized water for 30 min and dried at 45 ◦ C for over 24 h in a vacuum.
2.3. Characterizations
The surface morphologies of the samples were characterized by scanning electron microscopy
(SEM, Quanta 200, FEI, Eindhoven, The Netherlands). Crystalline structures of the samples were
identified by the X-ray diffraction technique (XRD, D/MAX 2200, Rigaku, Tokyo, Japan) operating with
Cu Kα radiation (λ = 1.5418 Å) at a scan rate (2θ) of 4◦ min−1 , an accelerating voltage of 40 kV and the
applied current of 30 mA ranging from 10–80◦ . Changes of chemical groups were recorded via a Fourier
transform infrared spectroscopy (FT-IR, Magna-IR 560, Nicolet, Madison, WI, USA). XPS analysis
was characterized by an X-ray photoelectron spectrometer (XPS, ESCALAB 250 XI, Thermofisher Co.,
containing composite materials and paraffin wax with the mass ratio of 2:3 was pressed into toroidalshaped samples (Φout = 7.00 mm, Φin = 3.04 mm, thickness = 2 mm) for microwave measurement.
The simulated reflection loss (RL) was calculated from the measured parameters according to the
transmission line theory.
Nanomaterials 2017, 7, 118
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3. Results and Discussion
Bridgewater,
USA).
magnetic
of the composites
measured
by acomposite.
vibrating
Figure 1aNJ,
shows
theThe
XRD
patternsproperties
of the untreated
wood and were
the MnFe
2O4/wood
sample
magnetometer
(VSM,atModel
7404,22.6°
LakeShore
Cryotronics
Inc.,crystalline
Westerville,
OH,of
USA)
at 300 K.
The strong
diffraction peaks
16.1° and
were equivalent
to the
region
the cellulose
The
thermal
the MW the
were
examinedpeaks
usingat
a thermal
gravimetric
analysis
of the
wood performances
(Figure 1a). Inofaddition,
diffraction
17.9°, 30.3°,
35.6°, 43.3°,
53.5°,(TGA,
57.1° SDT
and
Q600,
TA Instruments,
New
Castle,
DE, USA)
heating
of (311),
4 ◦ C/min
under
N2(440),
flow rate
of
62.8° could
be attributed
to the
diffractions
of the
(111),rate
(220),
(400),and
(422),
(511)anand
which
50
mL/min.the
The
relativeof
permeability
permittivity
were
obtained
on an Agilent
N5244A PNA-X
confirmed
presence
the MnFe2Oand
4 (JCPDS
73-1964)
with
a phase-pure
spinel structure
on the
network
analyzer
Inc.,
TX, USA)
in on
thethe
frequency
range of
wood surface.
This(VNA,
result Agilent
revealedTechnologies
that the MnFe
2O4Richardson,
was successfully
grown
wood surface.
1bthe
showed
the FTIR
spectra (400–4000
of coaxial
the untreated
wood (UW) and the
MW.
2–18 Figure
GHz for
calculation
of reflection
loss (RL)cm
by−1)the
reflection/transmission
method
−1
−1
−1,
The main
absorption
bands of the MWmethod.
were located
at 3416
cm , 1168
cm and
1050 cm
based
on the
NRW (Nicolson-Ross-Weir)
The sample
containing
composite
materials
and
corresponding
to the
vibrations
ofpressed
O–H, C=O
and
C–O, respectively,
which
were=attributed
paraffin
wax with
thestretching
mass ratio
of 2:3 was
into
toroidal-shaped
samples
(Φout
7.00 mm,
to the
wood
substrate
(hemicellulose,
cellulose
and lignin).
The peak
of (RL)
UW
Φin
= chemical
3.04 mm,contents
thicknessof=the
2 mm)
for
microwave
measurement.
The simulated
reflection
loss
−1
at 3431
cm wasfrom
ascribed
to stretching
vibrations
of the hydroxyl
groups of the
wood.
Then, this peak
was
calculated
the measured
parameters
according
to the transmission
line
theory.
shifted to the lower wavenumbers of 3416 cm−1 after coating MnFe2O4 on the wood surface, indicating
3.
and of
Discussion
theResults
formation
the hydrogen bonding interaction between the nanooctahedra MnFe2O4 and the
−1 and 2950 cm−1 were caused by the stretching vibrations of –CH3
woodFigure
[28]. The
peaks the
at 2922
1a shows
XRDcm
patterns
of the untreated wood and the MnFe2 O4 /wood composite.
−1 (the
◦
and
–CH
2
,
but
these
peaks
in
the
MW
spectra
significantly
decreased;
and the peak
atof
1740
The strong diffraction peaks at 16.1 and 22.6◦ were
equivalent
to the crystalline
region
the cm
cellulose
◦
◦
◦
◦
◦
carbon
and (Figure
oxygen 1a).
double
bonds) only
existed inpeaks
the UW
spectra
of
the wood
In addition,
the diffraction
at 17.9
, 30.3 [29].
, 35.6These
, 43.3phenomena
, 53.5 , 57.1◦were
and
◦
attributed
to
the
hydrolysis
of
fatty
acids
by
hydrothermal
reaction
under
the
alkaline
conditions.
62.8 could be attributed to the diffractions of the (111), (220), (311), (400), (422), (511) and (440), which
−1
Simultaneously,
the absorption
at 73-1964)
587 cmwith
were
intrinsicspinel
vibrations
the
confirmed
the presence
of the MnFe2peaks
O4 (JCPDS
a phase-pure
structureofon the
manganese
ferrite
wood
surface.
This[29].
result revealed that the MnFe2 O4 was successfully grown on the wood surface.
Figure 1. (a) XRD patterns and
and (b)
(b) FTIR
FTIR spectra
spectra of
ofthe
theuntreated
untreatedwood
wood(UW)
(UW)and
andthe
theMnFe
MnFe
2O
/wood
2O
44/wood
composite (MW).
−1 )scanning
The surface
morphologies
the UW
and MW
electronwood
microscopy
arethe
exhibited
in
Figure
1b showed
the FTIR of
spectra
(400–4000
cmby
of the untreated
(UW) and
MW. The
−
1
−
1
−
1
Figure
2.
Some
fibrils
and
the
inner
surface
of
the
lumen
could
be
clearly
observed
on
the
main absorption bands of the MW were located at 3416 cm , 1168 cm and 1050 cm , corresponding
microstructure
a longitudinal
section
theC–O,
UW respectively,
surface. Afterwhich
the hydrothermal
process,
wood
to
the stretchingofvibrations
of O–H,
C=Oof
and
were attributed
to thethe
chemical
contents of the wood substrate (hemicellulose, cellulose and lignin). The peak of UW at 3431 cm−1 was
ascribed to stretching vibrations of the hydroxyl groups of the wood. Then, this peak shifted to the
lower wavenumbers of 3416 cm−1 after coating MnFe2 O4 on the wood surface, indicating the formation
of the hydrogen bonding interaction between the nanooctahedra MnFe2 O4 and the wood [28]. The
peaks at 2922 cm−1 and 2950 cm−1 were caused by the stretching vibrations of –CH3 and –CH2 , but
these peaks in the MW spectra significantly decreased; and the peak at 1740 cm−1 (the carbon and
oxygen double bonds) only existed in the UW spectra [29]. These phenomena were attributed to the
hydrolysis of fatty acids by hydrothermal reaction under the alkaline conditions. Simultaneously, the
absorption peaks at 587 cm−1 were intrinsic vibrations of the manganese ferrite [29].
The surface morphologies of the UW and MW by scanning electron microscopy are exhibited in
Figure 2. Some fibrils and the inner surface of the lumen could be clearly observed on the microstructure
Nanomaterials 2017, 7, 118
Nanomaterials 2017, 7, 118
4 of 11
4 of 11
surface
was compactly
covered
MnFe
2O4 in Figure 2b. The inset of (b) shows the size distribution
of a longitudinal
section
of thebyUW
surface.
After the hydrothermal process, the wood surface
of
nanooctahedra
MnFe
2O4 with an average particle size of 0.68 nm and a size distribution width of
was compactly covered by MnFe2 O4 in Figure 2b. The inset of (b) shows the size distribution of
0.5–1
μm. FigureMnFe
2c shows
the MnFe2O4 with an octahedral shape aggregated over the wood surface
nanooctahedra
2 O4 with an average particle size of 0.68 nm and a size distribution width of
by
a
compact
manner,
and
nanoparticles
also
were
equippedover
on this
nanooctahedra
0.5–1 µm. Figure 2c shows themany
MnFe2small
O4 with
an octahedral
shape
aggregated
the wood
surface by
MnFe
2O4 surface. In order to further investigate the fine features of the nanooctahedra MnFe2O4, the
a compact manner, and many small nanoparticles also were equipped on this nanooctahedra MnFe2 O4
MW
wasInstudied
byfurther
a zoomed-in
SEM the
afterfine
ultrasonic
with 1800 WMnFe
for 30 O
min (Figure 2d).
surface.
order to
investigate
featurestreatment
of the nanooctahedra
2 4 , the MW was
The
zoomed-in
SEM
showed
the
nanooctahedra
MnFe
2O4 still tightly covered the wood surface, but
studied by a zoomed-in SEM after ultrasonic treatment with 1800 W for 30 min (Figure 2d). The
azoomed-in
small fraction
of the MnFe
2O4 and the bare wood surface appeared to rupture. These results
SEM showed
the nanooctahedra
MnFe2 O4 still tightly covered the wood surface, but a
indicated
that of
thethe
nanooctahedra
2O4 were closely integrated with the wood surface by a strong
small fraction
MnFe2 O4 andMnFe
the bare
wood surface appeared to rupture. These results indicated
interaction.
In addition,
the surface
of nanooctahedra MnFe2O4 became very clean, which might
that the nanooctahedra
MnFe
2 O4 were closely integrated with the wood surface by a strong interaction.
reveal
that
small
nanoparticles
were
adsorbed
onOthebecame
surfacevery
of nanooctahedra
MnFe2reveal
O4 by athat
physical
In addition, the surface of nanooctahedra MnFe
clean, which might
small
2 4
process.
nanoparticles were adsorbed on the surface of nanooctahedra MnFe O by a physical process.
2
4
Figure 2.2.SEM
of (a)
of the of
UW;
(b)UW;
the surfaces
the MW of
with
Figure
SEMimages
images
of the
(a) surfaces
the surfaces
the
(b) theofsurfaces
thelow-magnification;
MW with low(c)
the
surfaces
of
the
MW
without
ultrasonic
treatment
and
(d)
the
surfaces
of
the
afterof
ultrasonic
magnification; (c) the surfaces of the MW without ultrasonic treatment and (d) theMW
surfaces
the MW
treatment.
The
inset
of
(b)
is
the
size
distribution
of
nanooctahedra
MnFe
O
.
2 4
after ultrasonic treatment. The inset of (b) is the size distribution of nanooctahedra
MnFe2O4.
X-ray photoelectron spectroscopy (XPS) is a powerful technique for studying the elemental
composition and chemical oxidation state of the
the surfaces.
surfaces. Figure
Figure 3a
3a shows the survey-scan XPS
spectra of UW and MW; elemental O and C coexisted in both samples, where the C element was
attributed to
to the
the wood
wood substrate
substratefrom
fromcellulose,
cellulose,lignin
ligninand
andhemicellulose.
hemicellulose.InInaddition,
addition,
besides
and
besides
OO
and
C
C
elements,
the
survey-scanspectrum
spectrumofofthe
theMW
MWrevealed
revealediron
ironand
andmanganese
manganese peaks,
peaks, which
which should
elements,
the
survey-scan
come from MnFe22O
O44 nanoparticles
nanoparticleson
on the
the wood
wood surface.
surface. Figure
Figure 3b,c
3b,c shows
shows the
the Fe2p
Fe2p and the Mn2p
spectra, respectively. For Fe, the two
two main
main peaks
peaks Fe2p
Fe2p1/2
1/2 and
at
724.73
eV
and Fe2p
Fe2p3/2
at
724.73
eVand
and711.13
711.13 eV,
eV,
3/2
respectively, indicate the
with
thethe
Fe2p
3/2 3/2
peaks
of Fe
3 and
the presence
presence of
ofFe
Fe3+3+cations
cationsbybycomparison
comparison
with
Fe2p
peaks
of2OFe
O
2 3
Fe
[30–33].
TwoTwo
sharp
peaks
forfor
thetheMn2p
1/21/2
and
Mn2p
3/2 3/2
at at
653.23
andmetal
Fe metal
[30–33].
sharp
peaks
Mn2p
and
Mn2p
653.23eVeVand
and641.43
641.43 eV,
eV,
2+ cations by comparison with the MnO [34].
respectively,
indicate the
the presence
presence of
of Mn
Mn2+
respectively, indicate
Figure 3d and Table 1 illustrated the O1s spectra and binding energy of the UW and MW
samples, respectively. The broad peak of O1s at the UW spectrum could be fitted by two peaks at
binding energies of 533.18 eV and 531.83 eV, respectively. The largest peak at 533.18 eV was attributed
to the carbon-oxygen bond on the wood, such as cellulose, lignin and hemicellulose. The other peak
at 531.83 eV was assigned to the other oxygen from OH, H2O and carbon-oxygen double bonds from
energies of 530.23 eV, which was compatible with oxygen in metal oxides, such as Fe–O and Mn–O
[35,36]. Furthermore, it is easily observed that the peak at 531.82 eV had been shifted to 531.55 eV and
relatively higher than before the reaction. That strongly indicated that the MnFe2O4 nanoparticles had
been successfully located on the wood surface by the hydrogen bond after hydrothermal treatment
Nanomaterials 2017, 7, 118
5 of 11
under
this work.
Figure 3. (a) Survey-scan XPS spectra of the UW and the MW; (b) Fe2p XPS spectra of the MW;
Figure
3. (a)
Survey-scan
XPS
spectra
of the
and the
MW;
Fe2p
XPS spectra of the MW; (c)
(c) Mn2P
XPS
spectra of the
MW;
(d) O1s
XPSUW
spectra
of the
UW(b)
and
MW.
Mn2P XPS spectra of the MW; (d) O1s XPS spectra of the UW and MW.
Figure 3d and Table 1Table
illustrated
the composition
O1s spectraof
and
of the UW and MW samples,
1. Surface
thebinding
UW andenergy
the MW.
respectively. The broad peak of O1s at the UW spectrum could be fitted by two peaks at binding
UW531.83 eV, respectively. The largest
MW
energies of 533.18 eV and
peak at 533.18 eVAssignment
was attributed to
The excited electron
Binding
The excited
the carbon-oxygen
bond on the
wood,energy
such as cellulose,
lignin and Binding
hemicellulose. The other peak at
electron
energy (eV) double bonds from the
531.83 eV wasO1s
assigned to the other(eV)
oxygen from OH,
H2 OO1s
and carbon-oxygen
O1
533.18
O3
533.15
C–O energies
wood substrate. By contrasts, the main peak on the O1s spectrum of MW
appeared at binding
O4 oxides, such
531.55
O–H,
C=O [35,36].
of 530.23 eV, which was compatible with oxygen in metal
as Fe–O and
Mn–O
O2
531.83
O5
530.23
Mn–O,
Fe–O
Furthermore, it is easily observed that the peak at 531.82 eV had been shifted to 531.55 eV and
relatively
higher than before the reaction. That strongly indicated that the MnFe2 O4 nanoparticles had been
A mechanism
ofon
thethe
formation
of theby
nanooctahedra
O4 might
be expressed
by reaction
successfully
located
wood surface
the hydrogen MnFe
bond 2after
hydrothermal
treatment
under
Equations
(1)–(3):
this work.
–
–
Fe3+ + 3OH
(s); Mn2+ + 2OH
(s)
Table⇋Fe(OH)
1. Surface 3composition
of the ⇋
UWMn(OH)
and the 2MW.
MW
Fe(OH)3(s) +UW
(n-3)OH–⇋ Fe(OH)n3-n; Mn(OH)2(s)
+ (n-2)OH–⇋ Mn(OH)Assignment
n2-n
The excited
electron O1s
Binding energy
The excited
(1)
(2)
Binding energy
(eV)n2-n + Fe(OH)
electron
Mn(OH)
n3-n →O1s
MnFe2O4 + H(eV)
2O
(3)
On the O1
basis of the results
a schematic
the creation of
533.18 mentioned above,
O3
533.15illustration of C–O
O4
531.55
O–H, C=Oprocess is
nanooctahedra MnFe2O4 on the wood surface through
a low temperature
hydrothermal
O2
531.83
530.23
Mn–O,
Fe–O
proposed in Figure 4. The precursors (FeCl3·6H2OO5
and MnSO4·H2O)
were dissolved
into the
deionized
A mechanism of the formation of the nanooctahedra MnFe2 O4 might be expressed by reaction
Equations (1)–(3):
Fe3+ + 3OH− Fe(OH)3 (s); Mn2+ + 2OH− Mn(OH)2 (s)
(1)
Nanomaterials 2017, 7, 118
6 of 11
Fe(OH)3 (s) + (n-3)OH− Fe(OH)n 3-n ; Mn(OH)2 (s) + (n-2)OH− Mn(OH)n 2-n
(2)
Mn(OH)n 2-n + Fe(OH)n 3-n → MnFe2 O4 + H2 O
(3)
On the basis of the results mentioned above, a schematic illustration of the creation of
nanooctahedra
MnFe
Nanomaterials 2017, 7,
118 2 O4 on the wood surface through a low temperature hydrothermal process
6 of 11
is proposed in Figure 4. The precursors (FeCl3 ·6H2 O and MnSO4 ·H2 O) were dissolved into the
2+ and Fe3+ ions at first. After adding ammonia2+ solution,
2+ and Fe3+
deionized
water andMn
provided
Mn
the
water and provided
ions
at first. After adding ammonia solution, the Mn and Fe3+ ions
2+
3+
Mn
and hydroxyl
Fe ions with
ionstransformed
in the solution
transformed
into Mnand
hydroxides
and Fe
with the
ions the
in hydroxyl
the solution
into
Mn hydroxides
Fe hydroxides,
hydroxides,
Thehydroxides
Fe and Mn dissolved
hydroxides
the mixed
solution
because
of the
respectively.respectively.
The Fe and Mn
indissolved
the mixedinsolution
because
of the
presence
of
2-n
3-n
2-n
3-n
presence
of high concentrations
of the
ammonia
andtoconverted
to the
Mn(OH)
Fe(OH)
high concentrations
of the ammonia
and
converted
the Mn(OH)
n
and
Fe(OH)
with
massive
n n and
n
2-n and
3-n , as the growth unit, formed the
with
massive
hydroxyl
groups.
Then, the
Fe(OH)
hydroxyl
groups.
Then,
the Mn(OH)
n2-n Mn(OH)
and Fe(OH)
n3-n, as
the growth
unit, formed the MnFe2O4
n
n
MnFe
O
nanocrystal
nucleus
by
the
dehydration
reaction.
Additionally,
the
plentiful
hydroxyl
nanocrystal
nucleus by the dehydration reaction. Additionally, the plentiful
hydroxyl
groupsgroups
of the
2 4
of
the wood
absorbed
the 2MnFe
for its
the OH-rich
that be
could
wood
surfacesurface
absorbed
the MnFe
O4 nanocrystal
for its the
OH-rich
surfacesurface
that could
duebe
todue
the
2 O4 nanocrystal
to
the
high
surface
activity
and
large
surface-to-volume
atomic
ratio
on
the
MnFe
O
nanoparticle’s
high surface activity and large surface-to-volume atomic ratio on the MnFe2O4 nanoparticle’s
surface.
2 4
surface.
Finally,
the hydrothermal
advanced,
developed
a more
stable
Finally, with
thewith
hydrothermal
advanced,
the MnFethe
2O4MnFe
nanocrystal
developed
a more stable
MnFe
2O4
2 O4 nanocrystal
MnFe
layer
on the
wood
surface
and
formed
MnFe2composite.
O4 /wood composite.
layer on
wood
surface
and
formed
the
MnFe2the
O4/wood
2 O4the
Figure
of the
the MW.
MW.
Figure 4.
4. Possible
Possible schematic
schematic illustration
illustration of
of the
the preparation
preparation of
Figure 5 shows the magnetization-field curves of MW at 298 K. The saturation magnetization of
Figure 5 shows the magnetization-field curves of MW at 298 K. The saturation magnetization
MW was 28.24 emu/g, and it exhibited small hysteresis loops and low coercivity (<±5 Oe), as shown
of MW was 28.24 emu/g, and it exhibited small hysteresis loops and low coercivity (<±5 Oe), as
in the inset (a). Therefore, the MW showed a soft magnetism behavior [37–39]. The lower right corner
shown in the inset (a). Therefore, the MW showed a soft magnetism behavior [37–39]. The lower right
illustration shows the camera images of the magnet attracting samples. In the inset (b), the UW lacks
corner illustration shows the camera images of the magnet attracting samples. In the inset (b), the UW
the response by a magnet, and the MW could be easily removed from the desktop by a magnet. These
lacks the response by a magnet, and the MW could be easily removed from the desktop by a magnet.
results revealed that the MW is an excellent soft magnetic material with sensitive magnetic response
These results revealed that the MW is an excellent soft magnetic material with sensitive magnetic
[40].
response [40].
The thermal stability of MW and UW was determined by TG and DTG under nitrogen atmosphere
as shown in Figure 6. In the first stage (20 ◦ C–110 ◦ C), a little weight loss (UW of 7.43%; MW of 4.52%)
was observed, which was related to the release of moisture from the samples. Thus, the water
content of the UW and MW was 7.43% and 4.52%, respectively. In the second stage (110–410 ◦ C),
the weight/mass-loss rates of UW and MW were approximately 68.37% and 16.80%, respectively.
The main weight loss was mainly attributed to the pyrogenic decomposition of hemicellulose and
cellulose. The pyrolysis maximum rates of UW and MW occurred at 364 and 380 ◦ C, respectively.
Furthermore, the maximum rate of UW of −0.2189%/◦ C was higher than MW −1.049%/◦ C on the
DTG curves, which could be caused by the higher content of nanooctahedra MnFe2 O4 on the samples.
In the third stage (410–800 ◦ C), the weight loss of the MW reaches, 10.15% and the maximum pyrolysis
rate occurred at 570 ◦ C in which the nanooctahedra MnFe2 O4 protects and improves the pyrolysis
temperature of the wood. Finally, the MW still had larger residues (68.53%) with carbon residues
and MnFe2 O4 , while the remainder of the UW with carbon residues was only 14.31% at 800 ◦ C. This
Figure 4. Possible schematic illustration of the preparation of the MW.
Figure 5 shows the magnetization-field curves of MW at 298 K. The saturation magnetization of
MW was 28.24 emu/g, and it exhibited small hysteresis loops and low coercivity (<±5 Oe), as shown
Nanomaterials 2017, 7, 118
7 of 11
in the inset (a). Therefore, the MW showed a soft magnetism behavior [37–39]. The lower right corner
illustration shows the camera images of the magnet attracting samples. In the inset (b), the UW lacks
the responseindicated
by a magnet,
MW could be MnFe
easily 2removed
from the
by surface
a magnet.
These
phenomenon
thatand
thethe
nanooctahedra
O4 deposited
on desktop
the wood
and
had a
results
revealedinthat
MW
an excellent
magnetic
material
with
sensitive
magnetic
response
large
proportion
thethe
MW.
Inissummary,
thesoft
thermal
stability
of the
wood
had been
improved
after
[40].
the hydrothermal reaction and the existence of the potential in the fire-resistant performance.
Nanomaterials 2017, 7, 118
7 of 11
The thermal stability of MW and UW was determined by TG and DTG under nitrogen
atmosphere as shown in Figure 6. In the first stage (20 °C–110 °C), a little weight loss (UW of 7.43%;
MW of 4.52%) was observed, which was related to the release of moisture from the samples. Thus,
the water content of the UW and MW was 7.43% and 4.52%, respectively. In the second stage
(110–410 °C), the weight/mass-loss rates of UW and MW were approximately 68.37% and 16.80%,
respectively. The main weight loss was mainly attributed to the pyrogenic decomposition of
hemicellulose and cellulose. The pyrolysis maximum rates of UW and MW occurred at 364 and 380
°C, respectively. Furthermore, the maximum rate of UW of −0.2189%/°C was higher than MW
−1.049%/°C on the DTG curves, which could be caused by the higher content of nanooctahedra
MnFe2O4 on the samples. In the third stage (410–800 °C), the weight loss of the MW reaches, 10.15%
and the maximum pyrolysis rate occurred at 570 °C in which the nanooctahedra MnFe2O4 protects
and improves the pyrolysis temperature of the wood. Finally, the MW still had larger residues
(68.53%) with carbon residues and MnFe2O4, while the remainder of the UW with carbon residues
was only 14.31% at 800 °C. This phenomenon indicated that the nanooctahedra MnFe2O4 deposited
on the wood surface and had a large proportion in the MW. In summary, the thermal stability of the
Figure
Magnetization
ofofafter
the
aafunction
appliedand
magnetic
field.The
Theinset
inset
shows
Figure
5. Magnetization
theMW
MW
function of
of the
applied
magnetic
field.
(a)(a)
shows
a a in the
wood
had5. been
improved
theasas
hydrothermal
reaction
the existence
of
the
potential
magnification
one
segmentofofthe
theMW
MWmagnetization-field
magnetization-field curves,
that
thethe
magnification
ofof
one
segment
curves,and
andthe
theinset
inset(b)
(b)shows
shows
that
fire-resistant
performance.
UW
and
MW
samples
wereattracted
attractedby
byaamagnet.
magnet.
UW
and
MW
samples
were
Figure 6. (a) Thermogravimetric (TG) and (b) differential thermogravimetric (DTG) curves of the UW
and
the MW
under
a nitrogen atmosphere.
Figure
6. (a)
Thermogravimetric
(TG) and (b) differential thermogravimetric (DTG) curves of the UW
and the MW under a nitrogen atmosphere.
In order to test the fire-resistant property, it was of great significance to test the ignitability of
Inand
order
to on
testan
thealcohol
fire-resistant
property,
it was
of great
to test
theUW
ignitability
the UW
MW
lamp flame.
Figure
7 shows
thesignificance
digital photos
of the
and theof
theon
UW
and
MW on
an alcohol
lamp
7 shows
photoson
of the
the
MW
MW
the
alcohol
lamp
flame for
theflame.
first 20Figure
s. In Figure
7a,the
thedigital
UW caught
fireUW
at 6 and
s and
was
on
the
alcohol
lamp
flame
for
the
first
20
s.
In
Figure
7a,
the
UW
caught
on
fire
at
6
s
and
was
incinerated to ash after 70 s. After removing the alcohol lamp at 20 s, the flames still burned strongly
incinerated
to
ash
after
70
s.
After
removing
the
alcohol
lamp
at
20
s,
the
flames
still
burned
strongly
until 35 s. After 35 s, the flame had been gradually extinguished, and it became ash. By contrast, the
until
35 s.not
After
s, the
hadlamp
beenburning
gradually
and
it became
ash. removing
By contrast,
MW
could
be 35
lit by
theflame
alcohol
forextinguished,
the first 20 s in
Figure
7b. After
thethe
MW
could
not
be
lit
by
the
alcohol
lamp
burning
for
the
first
20
s
in
Figure
7b.
After
removing
the
alcohol lamp, the shape and volume of the MW had no apparent change. These results indicated that
alcohol
the shape
and volume
of thesuitable
MW had
no apparent
change.
These
that
the
MnFe2lamp,
O4 /wood
composite
was more
than
the untreated
wood
andresults
mightindicated
be applied
the
MnFe
2O4/wood composite was more suitable than the untreated wood and might be applied in
in building and special decoration materials. This enhanced fire resistance could be ascribed to the
building
and special
decoration
materials.
This
enhanced
resistance
could
the
special
construction
of the
sample by
which the
dense
MnFe2fire
O4 layer
formed
overbe
theascribed
cell walltoon
of the oxidation
sample byand
which
dense from
MnFeproceeding
2O4 layer formed over the cell wall on the
thespecial
woodconstruction
surface prevented
heatthe
transfer
into the inner portion of the
wood
surface
prevented
oxidation
and
heat
transfer
from
proceeding
into the inner portion of the
wood cell walls, which could be observed from the scanning electron microscope.
wood cell walls, which could be observed from the scanning electron microscope.
Nanomaterials 2017, 7, 118
Nanomaterials 2017, 7, 118
Nanomaterials 2017, 7, 118
8 of 11
8 of 11
8 of 11
Figure 7. Fire-resistant properties of (a) the UW and (b) the MW.
Figure 7.
7. Fire-resistant
Fire-resistant properties
properties of
of (a)
(a) the
the UW
UW and
and (b)
Figure
(b) the
the MW.
MW.
To investigate the electromagnetic absorption property of the UW and the MW, the reflection
To
the
electromagnetic
property
of of
thethe
UW
andand
thethe
MW,
the reflection
loss
To investigate
investigate
thecalculated
electromagnetic
absorption
property
UW
MW,
the reflection
loss (RL)
values were
by theabsorption
transmission
line theory
[41].
(RL)
values
werewere
calculated
by the
line theory
[41]. [41].
loss (RL)
values
calculated
bytransmission
the transmission
line theory

  2πfd 
Zin = Z 0 μr / ε p
 μr εr 
r tanh j 2πfd
2πμf dε √
c
Zin = Z 0Z μ=r /Zε0r tanh
j





r r  µr ε r
µr /εr tanh j
in

  c  c
Zin − Z0
RL(dB) = 20 log Zin −ZZ0 − Z 0
in
Zlog
) =(dB
RL(dBRL
20)log
= 20
in + Z 0
Zin +ZZ0in + Z0 (4)
(4)
(4)
(5)
(5)
(5)
where Zin is the input impedance of the samples, Z0 is the impedance of air, μr is the complex
where
is the
the input
input impedance
impedance of
of the
the samples,
samples, ZZ0 0isisthe
theimpedance
impedanceofofair,
air, μ
µr is
the complex
where Z
Zin
in is
r is the complex
is the
complexpermittivity,
permittivity,d disisthe
the thickness
thickness of
of the
frequency
of
permeability, r εisr the
permeability,ε
complex
the samples,
samples,f fisisthe
the
frequency
permeability, ε r is the complex permittivity, d is the thickness of the samples, f is the frequency of
microwaves
and
c is
by
of
microwaves
and
c isthe
thevelocity
velocityofoflight.
light.Figure
Figure8 8shows
showsRL
RLcurves
curves of
of the
the UW
UW and MW by
microwaves
and cand
is the
velocity of
light. in
Figure
8 shows range
RL
curves
of GHz.
the UW
and MW
by
three-dimensional
and
color-filling
patterns
in
the frequency
frequency
range
of 2–18
2–18
GHz.
In Figure
Figure
8a, the
the
three-dimensional
color-filling
patterns
the
of
In
8a,
three-dimensional
and
patterns
theatfrequency
range
GHz. In
8a,were
the
minimum RL
RL values
values
ofcolor-filling
the UW
UW reached
reached
−3.0
16.64 GHz,
and of
the2–18
RL values
values
ofFigure
the UW
UW
were
minimum
of
the
−
3.0indB
of
the
minimum
RL
values
of
the
UW
reached
−3.0
dB
at
16.64
GHz,
and
the
RL
values
of
the
UW
were
close to
to zero
zero at
at another
another frequency,
frequency,which
whichindicated
indicatedthat
thatthe
themain
mainloss
lossmechanism
mechanismof
ofthe
the UW
UW was
was only
only
close
close
to zero atof
another
frequency,
which
indicated
the
main
loss
mechanism
of the
UW was
only
the resonance
resonance
of
thewood
wood
certain
frequency
16.64
GHz.
By
contrast,
theMW
MW
reached
dB
the
the
atataacertain
frequency
atatthat
16.64
GHz.
By
contrast,
the
reached
−−9.3
9.3
dB
the
resonance
of
the
wood
at
a
certain
frequency
at
16.64
GHz.
By
contrast,
the
MW
reached
−9.3
dB
at 16.48
16.48 GHz
GHz with
with aa thickness
thickness of
of 33 mm
mm in
in Figure
Figure 8b.
8b. Moreover,
Moreover, itit could
could be
be observed
observed that
that both
both the
the
at
at
16.48 GHz
with aloss
thickness
of
3 the
mm
in Figure bandwidth
8b.
Moreover,
itthe
could
be
observed
that bothhad
the
minimum
reflection
loss
valueand
and
theabsorption
absorption
bandwidth
MnFe
2
O
4
/wood
composite
had
minimum
reflection
value
ofofthe
MnFe
O
/wood
composite
2 4
minimum
reflection
loss value
and
the
absorption
bandwidth
of that
the MnFe
2O4/wood composite had
improvedgreatly
greatlycompared
compared
with
the
untreated
wood,
indicating
that
the electromagnetic
electromagnetic
absorption
improved
with
the
untreated
wood,
indicating
the
absorption
improved
greatly
compared
with
the
untreated
wood,
indicating
that
the
electromagnetic
absorption
property of
of the
the MnFe
MnFe22O44/wood
growth
ofof
the
MnFe
2O24O
on
property
/woodcomposite
compositehad
hadbeen
beenenhanced
enhancedthrough
throughthe
the
growth
the
MnFe
4
property
ofThus,
theThus,
MnFe
4/wood composite had been enhanced through the growth of the MnFe2O4 on
thethe
wood.
it might
provide
a potential
application
in theinfield
of indoor
electromagnetic
wave
on
wood.
it2Omight
provide
a potential
application
the field
of indoor
electromagnetic
the
wood.
Thus, it might provide a potential application in the field of indoor electromagnetic wave
absorbers.
wave
absorbers.
absorbers.
Figure
Frequencydependence
dependenceofof
reflection
(RL)
(a) UW
the UW
andthe
(b)MW
the by
MW
by
Figure 8.
8. Frequency
thethe
reflection
lossloss
(RL)
for for
(a) the
and (b)
threethree-dimensional
and
color-filling
patterns
in
the
frequency
range
of
2–18
GHz.
Figure
8.
Frequency
dependence
of
the
reflection
loss
(RL)
for
(a)
the
UW
and
(b)
the
MW
by
threedimensional and color-filling patterns in the frequency range of 2–18 GHz.
dimensional and color-filling patterns in the frequency range of 2–18 GHz.
Nanomaterials 2017, 7, 118
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4. Conclusions
In summary, we had successfully synthesized the MnFe2 O4 /wood composite via a
low-temperature hydrothermal method. The nanooctahedra MnFe2 O4 had been tightly located on the
wood surface through the hydrogen bonding interactions. The MnFe2 O4 /wood composite expressed
superior performance of soft magnetism, fire resistance and electromagnetic wave absorption, which
could be applied to some fields for special decoration and indoor electromagnetic wave absorption.
Acknowledgments: This research was supported by the Zhejiang Provincial Natural Science Foundation of China
under Grant No. LZ15C160002, the Scientific Research Foundation of Zhejiang Agriculture & Forestry University
(Grant No. 2014FR077), the Special Fund for Forest Scientific Research in the Public Welfare (Grant No. 201504501)
and the Fund for Innovative Research Team of Forestry Engineering Discipline (101-206001000713).
Author Contributions: Qingfeng Sun conceived of the project and revised the whole manuscript. Hanwei Wang,
Qiufang Yao and Zhongqing Ma designed the experiments and wrote the paper. Chao Wang, Bitao Fan, and
Yipeng Chen prepared Figures 1–8. Chunde Jin provided the project support and revised the partial manuscript.
All authors reviewed the manuscript.
Conflicts of Interest: The authors declare that they have no conflict of interest.
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