Science in China Series B: Chemistry
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Synthesis and characterization of boron nitride sponges
as a novel support for metal nanoparticles
ZHENG MingTao, LIU YingLiang†, GU YunLe & XU ZiLin
Department of Chemistry and Institute of Nanochemistry, Jinan University, Guangzhou 510632, China
This paper describes a simple synthetic route for the synthesis of hexagonal boron nitride (h-BN)
powders with high specific surface area, in which BBr3, NH4Cl and Al powders are used as starting
materials. The structure and composition of the powders were characterized by electron diffraction,
Fourier transformation infrared spectroscopy and X-ray photoelectron spectroscopy in the selected
area. X-ray diffraction shows wide peaks of crystalline h-BN with the particle size on the nanometer
scale, and transmission electron microscopy reveals that the products have a novel spongy morphology. Silver nanoparticles loaded h-BN sponges were prepared via a one-step synthesis method. Different reaction conditions for the formation of h-BN sponges were also investigated.
boron nitride, synthesis, characterization, catalyst supports
1 Introduction
Boron nitride (BN) is well known as one of the most
important technical ceramic materials with interesting
properties, such as high strength, low density, high
melting point, high mechanical strength, good resistance
to corrosion, excellent chemical stability, and outstanding thermal and electrical properties. These make
BN an attractive candidate for a wide range of technical
applications, especially for applications under extreme
－
conditions[1 3]. Recently, a number of studies have been
reported on the preparation of boron nitride materials
with special morphologies, such as one-dimensional
nanostructures[4,5], hollow spheres[6], nanocapsules[7],
nanocages[8], porous structures[9] to obtain new interesting properties and potential applications.
For instance, porous BN materials have attracted particular interests due to their high surface area, low density and narrow pore size distribution. Porous BN powders with a high specific surface area (between 300 and
600 m2·g−1) have been synthesized for catalytic applica－
tions[10 13]. BN of other structures with high surface area
and narrow pore size distribution have also been reported. For example, highly regular BN nanomesh with
a 2 nm hole size was obtained by self-assembly on a
Rh(111) single crystalline surface[14]. BN nanotubes
were synthesized by chemical vapor deposition (CVD)
using BCl3 and NH3 at 923 K within the channels of
mesoporous silica SBA-15[15].
In our previous work, we synthesized hexagonal bo－
ron nitride[16 18]. Herein, spongy boron nitride powders
with high surface area (378 m2·g-1) were prepared via a
simple chemical route without using templates. The reaction conditions were optimized for the synthesis of
spongy boron nitride suitable to be used as catalyst supports, especially for noble metal catalysts. A simple
one-step method for loading noble metal nanoparticles
was also developed.
2 Experimental
All reagents used in our experiments were of analytical
pure grade. Synthesis experiments were carried out in a
Received July 29, 2007; accepted September 22, 2007
doi: 10.1007/s11426-008-0026-3
†
Corresponding author (email: [email protected])
Supported by the Key S&T Special Projects of Guangdong Province (Grant No.
2005A11001001) and the Natural Science Union Foundations of China and Guangdong Province (Grant No. U0734005)
Sci China Ser B-Chem | Mar. 2008 | vol. 51 | no. 3 | 205-210
N2 flowing glove box. For the preparation of bare BN
sponges, an appropriate amount of NH4Cl (0.150 mol),
Al powders (0.100 mol, 500 mesh) and BBr3 (0.050 mol)
were put into a stainless autoclave of 50 mL volume
(route 1). For the preparation of noble metal particles
(such as Ag) loaded sponges, NH4Cl (0.150 mol), Al
powders (0.105 mol), BBr3 (0.050 mol) and AgCl 2.5
mmol) were added into the autoclave (route 2). In both
cases, the autoclaves were sealed and heated from room
temperature to 500℃ at a ramping rate of 10℃/min in a
furnace and maintained at 500℃ for 10 h. After the
autoclave was cooled to room temperature naturally, the
crude product was collected and washed in turn with
dilute hydrochloric acid，distilled water and absolute
ethanol for several times. The products were dried in
vacuum at 80℃ for 10 h. White powders were obtained.
X-ray powder diffraction (XRD) pattern was collected in a 2θ range from 10° to 80° at a scan speed of
2° (2θ )/min on an MSAL-XD2 X-ray diffractometer
using Cu-Kα radiation (40 kV, 20 mA, λ = 1.5406 Å).
Transmission electron microscopy (TEM) images, high
resolution transmission electron microscopy (HRTEM)
images and selected area electron diffraction (SAED)
pattern were obtained on a JEOL JEM-2010HR and a
Philips Tecnai-10 microscope. TEM samples were prepared by casting a droplet of the product suspension in
ethanol onto amorphous carbon-coated copper grids.
Fourier transformation infrared (FTIR) spectra were recorded on a BRUKER EQUINOX 55 Fourier transmission IR spectrometer, and a KBr wafer was used as the
substrate. X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250spectrometer (Thermao
Electron Corp.) using Al Kα as the excitation source.
For the calibration of binding energies, carbon C 1s at
285.5 eV was used as the reference. Nitrogen adsorption/desorption isotherms were measured on a Tristar
3000 adsorption analyzer. Prior to the adsorption measurements, the samples were degassed for 6 h at 300℃
in the degas port of the adsorption analyzer.
reported in literature (JCPDS card file, No. 85-1068).
The peak broadening can be attributed to its small particle size which is shown to be on the nanometer scale.
Calculation reveals the lattice constants are a = 2.511
and c = 6.764 Å, well consistent with the reported values
(a = 2.510 and c = 6.690 Å).
Figure 1 XRD pattern of the BN sample prepared by the reaction of
BBr3, NH4Cl and Al powders at 500℃ for 10 h.
Figure 2 shows the FTIR spectrum of the sample.
Two strong characteristic absorption bands centered at
1390 and 810 cm−1 were observed, similar to the reported absorption bands for h-BN [19,20]. The band at
1390 cm−1 results from the in-plane B—N TO model of
the sp2-bonded BN, while the band at 810 cm−1 can be
attributed to the out-of-plane B—N—B bond vibrations.
The absorptions at 3430 cm−1 and the weak peak at 1635
cm−1 can be ascribed to O—H and N—H bonds respec-
3 Results and discussion
Figure 1 shows a typical XRD pattern of the sample
prepared by route 1. All three diffraction peaks can be
indexed as h-BN (002), (100) and (110) planes, corresponding to their inter-plane spacing of 3.38, 2.18 and
1.26 Å respectively. These values agree well with those
206
Figure 2 FTIR spectrum of the BN sample prepared by the reaction of
BBr3, NH4Cl and Al powders at 500℃ for 10 h.
ZHENG MingTao et al. Sci China Ser B-Chem | Mar. 2008 | vol. 51 | no. 3 | 205-210
tively resulting from the adsorbed H2O and the surface
—NH2 groups.
The composition of the products was also investigated. Shown in Figure 3 is the wide-scan XPS spectrum
of the as-prepared h-BN sample. In the survey scan
(Figure 3(a)), peaks at 190.21, 397.92, 284.58 and
532.49 eV, characteristic of B1s, N1s, C1s and O1s respectively, are displayed. The B1s peak at about 190.21
eV (Figure 3(b)) and the N1s peak at about 397.92 eV
(Figure 3(c)) indicate the presence of BN, and the binding energies are in good agreement with the reported
values for bull BN[21]. Quantification of B1s and N1s
peaks gives the average B/N atomic ratio of approximately 1.00:1.05 corresponding to the stoichiometric
composition of BN. Oxygen and carbon may come from
water absorption and contamination by species in air.
Figure 3 XPS spectra of the as-prepared BN sponges: (a) survey spectrum; (b) B1s region; (c) N1s region.
Based on the above XRD patterns, XPS and FTIR spectra, it can be concluded that the as-synthesized products
are pure- phase h-BN.
The morphology of the products was investigated by
TEM. Figure 4(a) and (b) show the representative TEM
images of the as-prepared BN products. It can be seen
that the products have a sponge-like morphology and a
porous structure. A magnified TEM image of BN
sponges is shown in Figure 4(c). It is evident that the
product consists of a large quantity of the nanocrystalline flakes of 400－800 nm long and 20－40 nm thick
(Figure 4(b) and (c)), and the spongy structures are
formed by the agglomeration of such flakes. Figure 4(d)
shows the SAED pattern of the spongy sample. The diffraction rings corresponding to d-spacings of 3.38, 2.18,
1.26 Å, match the crystalline (002), (100) and (110)
planes of h-BN. This is in good agreement with the
XRD result. Figure 4(e) shows the HRTEM image of the
sample. It reveals that the average distance between the
neighboring fringes is about 0.34 nm. It also matches the
(002) d-spacing of h-BN.
Figure 5 shows nitrogen adsorption-desorption isotherms and corresponding pore size distribution of the
boron nitride sponges. The adsorption isotherm is of
type IV with H2 hysteresis loop at a relative pressure
between 0.4 and 1.0. From the isotherms, the specific
surface area of 378 m2·g−1 and pore volume of 0.27
cm3·g−1 are derived. The pore size distribution shows
two well separated peaks, indicating the existence of two
types of pores with the average pore size of about 2.7
nm and 50 nm in diameter respectively.
To investigate the effect of different reaction conditions on the formation of the sponges, we changed the
reaction temperature and time length and found that
temperature and time affected the yields of h-BN
sponges significantly. At the temperature below 400℃,
nanoparticles instead of sponges were obtained. At the
temperature above 600℃, mainly flake-like boron nitride
and few sponges were obtained, and its surface area was
reduced remarkably. In contrast, at 500℃, h-BN sponges
were the predominant product. It was also found that
reaction time of 10 h produced more h-BN sponges than
either of 6 or 24 h. Thus, the most favorable temperature
for synthesizing h-BN sponges is around 500℃, and the
optimal reaction time is about 10 h. The main chemical
reactions involved here can be written as follows:
3 NH4Cl → 3 NH3 + 3 HCl
(1)
ZHENG MingTao et al. Sci China Ser B-Chem | Mar. 2008 | vol. 51 | no. 3 | 205-210
207
Figure 4 Representative TEM characterizations of the as-prepared BN sponges: (a) and (b) TEM images; (c) and (d) magnified TEM image of the
sponges and its SAED pattern; (e) corresponding HRTEM image taken from the BN sponges. Inserted magnified HRTEM image of the square area shows
the average inter-planar spacing is 0.34 nm.
Figure 5 Nitrogen adsorption/desorption isotherms and pore size distribution of boron nitride sponges.
208
ZHENG MingTao et al. Sci China Ser B-Chem | Mar. 2008 | vol. 51 | no. 3 | 205-210
Al + 3 HCl → AlCl3 + 1.5 H2
(2)
(3)
BBr3 + NH3 → h-BN + 3 HBr
(4)
Al + 3 HBr → AlBr3 + 1.5 H2
BBr3 + 3 NH4Cl + 2 Al
(5)
→ h-BN + AlCl3 + AlBr3 + 2 NH3 + 3 H2
It is well-known that NH4Cl can be decomposed to
NH3 upon heating as described in Eq. (1). The freshly
produced NH3 instantly reacts with BBr3 to form h-BN
nanocrystals, as described in Eq. (3). From the thermodynamic point of view, the reaction between BBr3 and
NH4Cl, described as Eqs. (1) and (3), is thermodynamically favored and also endothermic (ΔHfº= 98.62 kJ/mol;
ΔGfº= −42.5 kJ/mol). Reactions described in Eqs. (2)
and (4) are exothermic (Eq. (2): ΔHfº= −427.3 kJ/mol,
ΔGfº= −342.9 kJ/mol; Eq. (4): ΔHfº= −418.33 kJ/mol,
ΔGfº= −328.3 kJ/mol. The heat produced could be transferred to the reaction in Eq. (3), and this ensures that the
reaction in Eq. (3) proceeds smoothly. In addition, BN
nanocrystallites anneal to form sponges in the presence
of superfluous heat and prevent nanocrystallites from
growing into larger crystallites. Shapes of the spongy
h-BN may be determined by the template effect of Al or
AlX3 (X = Cl, Br) particles. The total reaction can be
described by Eq. (5). Previously, Xu et al. reported the
synthesis of fullerene-like h-BN with vessel, hollow
sphere, peanut, and onion structures by reacting BBr3
with the synergic nitrogen sources NaNH2 and NH4Cl[22].
However, in our experiments, it was found that the
Figure 6
products were amorphous BN without using Al powders
under the same conditions. It indicated that Al powders
played an important role in producing the novel
sponge-like BN.
This novel material is expected to be used as a spongy
catalyst support. As described in the experiment part,
when AgCl (2.5 mmol) was added into the reaction system (route 2), silver nanoparticle loaded BN sponges
could be produced. The XRD pattern of the resulted
sample is shown in Figure 6(a). In addition to the diffraction peaks of h-BN, strong diffraction peaks of Ag
(111), (200), (220) and (311) were observed. Further, the
composite was characterized by TEM. Figure 6(b) is a
typical TEM image of BN sponges supported Ag
nanoparticles synthesized by route 2. Ag nanoparticles
with the diameter of about 10－50 nm are well dispersed
and embedded in BN sponges. Obviously, this one-step
synthesis route is an efficient measure to load noble
metal nanoparticles on BN.
4 Conclusions
In summary, we have successfully synthesized spongy
h-BN powders with high specific surface area by the
reaction of BBr3, NH4Cl and Al powders at 500℃. Silver nanoparticles loaded BN sponges have been prepared via a one-step synthesis route. Due to its interesting composite nanostructure, this novel material may
have potential use as a catalyst for new applications.
(a) XRD pattern and (b) typical TEM image of the Ag/BN sample synthesized by route 2.
ZHENG MingTao et al. Sci China Ser B-Chem | Mar. 2008 | vol. 51 | no. 3 | 205-210
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