Gunda et al 2018-simple green and high yield production of boron based nanostructures

Simple, Green, and High-Yield Production of Boron-Based
Nanostructures with Diverse Morphologies by Dissolution
and Recrystallization of Layered Magnesium Diboride
Crystals in Water
Harini Gunda, Saroj Kumar Das, and Kabeer Jasuja*[a]
1. Introduction
The element boron has always intrigued the scientific com-
munity on account of its rich inherent properties—low density,
high mechanical strength but lighter weight, high thermal re-
sistance, high specific resistance at ordinary temperature, high
melting point, ability to absorb neutrons, and high resistance
to chemical attack.[1–6]
Recent advances in the science of two-
dimensional (2D) materials have motivated researchers to in-
vestigate the prospects of realizing planar nanostructures con-
stituted from boron.[5,7–13]
Such nanoconstructs are expected to
serve as promising platforms for utilizing the inherent proper-
ties of boron to their full potential.[14–19]
Several computational
modeling studies have predicted the feasibility of realizing
quasiplanar boron-based clusters and 2D boron
sheets.[5,7,8,12,20,21]
Quasiplanar clusters of boron were first pre-
dicted by Boustani by using first-principle calculations.[7]
This
study was followed by several theoretical reports predicting
various forms of 2D boron sheets.[8–11]
Later, by using first-prin-
ciple calculations, Liu et al. predicted that boron nanosheets
could be grown on metals (such as Cu, Ag, and Au) and metal
boride (MgB2 and TiB2) substrates.[12]
Although there are a sub-
stantial number of theoretical studies on planar forms of
boron, experimental investigations on realizing 2D forms of
boron are in their incipient stages.[20]
In 2014, Piazza et al. dem-
onstrated (experimentally and theoretically) the synthesis of a
36-atom planar cluster of boron atoms and proposed that it
could be used as a basis for realizing 2D sheets of boron (boro-
phenes).[22]
The last two years have witnessed several newer
methods for the synthesis of 2D boron; these include an effec-
tive vapor–solid process through the thermal decomposition
of diborane,[19]
high-temperature growth with the use of metal
boride precursors,[23]
deposition of elemental boron on metal
substrates under ultrahigh-vacuum conditions,[24]
and chemical
vapor deposition (CVD) techniques involving varied sources of
boron.[25,26]
It is pertinent to note that the aforementioned re-
search efforts for realizing planar forms of boron are bottom-
up vapor-phase approaches. Although the 2D forms obtained
from these approaches are stable, the boron atoms present in
these planar structures are not arranged in a honeycomb pat-
tern; instead, the boron atoms form B7, B12, or B36 clusters ar-
ranged in a 2D form. Recently, we demonstrated two top-
down approaches that enabled the synthesis of few-layer-thick
boron-based nanosheets by exfoliation of magnesium diboride
(MgB2) by using the aides of either ultrasonication or chela-
tion.[27,28]
The material MgB2 represents an entire family of lay-
ered metal borides (of the form MB2) that have metal atoms
sandwiched between boron honeycomb planes.[29–32]
MgB2 is
Layered metal diborides that contain metal atoms sandwiched
between boron honeycomb planes offer a rich opportunity to
access graphenic forms of boron. We recently demonstrated
that magnesium diboride (MgB2) could be exfoliated by ultra-
sonication in water to yield boron-based nanosheets. However,
knowledge of the fate of metal boride crystals in aqueous
phases is still in its incipient stages. This work presents our pre-
liminary findings on the discovery that MgB2 crystals can un-
dergo dissolution in water under ambient conditions to result
in precursors (prenucleation clusters) that, upon aging, under-
go nonclassical crystallization preferentially growing in lateral
directions by two-dimensional (2D) oriented attachment. We
show that this recrystallization can be utilized as an avenue to
obtain a high yield (%92%) of boron-based nanostructures, in-
cluding nanodots, nanograins, nanoflakes, and nanosheets.
These nanostructures comprise boron honeycomb planes
chemically modified with hydride and oxy functional groups,
which results in an overall negative charge on their surfaces.
This ability of MgB2 crystals to yield prenucleation clusters that
can self-seed to form nanostructures comprising chemically
modified boron honeycomb planes presents a new facet to
the physicochemical interaction of MgB2 with water. These
findings also open newer avenues to obtain boron-based
nanostructures with tunable morphologies by varying the
chemical milieu during recrystallization.
[a] H. Gunda, S. K. Das, Dr. K. Jasuja
Department of Chemical Engineering
Indian Institute of Technology Gandhinagar
Palaj, Gandhinagar, 382355 (India)
E-mail: kabeer@iitgn.ac.in
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cphc.201701033.
ChemPhysChem 2018, 19, 880 – 891 2018 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim880
ArticlesDOI: 10.1002/cphc.201701033

primarily known for its superconducting properties, and until
our earlier studies, it had remained unexplored for exfolia-
tion.[33,34]
One critical challenge in extracting boron planes
from MgB2 crystals is the removal of interplanar Mg atoms,
which are strongly bonded relative to the interplanar bonds in
van der Waals layered materials. We demonstrated earlier that
ultrasonication of standard MgB2 crystals in aqueous medium
generated normal and shear forces that could displace a frac-
tion of the Mg atoms in a few crystals to yield Mg-deficient
boron-based nanosheets.[27]
However, as we will explain ahead,
we realized that there was an entirely different facet to the in-
teraction of MgB2 crystals with water that has remained unex-
plored.[35]
In the present study, we demonstrate that a large
fraction of MgB2 crystals can undergo dissolution in water
under ambient conditions to result in nanocrystal precursors
(prenucleation clusters) that recrystallize in a nonclassical fash-
ion upon aging. We show that this recrystallization can be uti-
lized as an avenue to obtain a high yield (%92%) of boron-
based nanostructures, including nanodots (with dimensions
10 nm), nanograins (few-layer-thick with lateral dimensions
%50–400 nm), and nanoflakes (few-layer-thick with lateral di-
mensions of several micrometers). These nanostructures are
found to exhibit a deficiency of Mg, relative to standard MgB2,
and are found to have boron honeycomb planes modified
with hydride, oxide, and hydroxy groups. This discovery consti-
tutes a fundamental set of findings in the science of MgB2 and
forms the central merit of this study. These results also set a
platform for exploring the possibility of obtaining boron-based
nanostructures with tunable morphologies by varying the
chemical milieu during recrystallization.
2. Results and Discussion
Ultrasonication of MgB2 flakes in water was observed to result
in a dark black suspension (Figure 1a), which if allowed to
stand for 24 h started to separate as black sediments and a
pale-yellow colored phase (Figure 1b). To remove the sedi-
ments, this suspension was passed through a 0.22 mm filter
paper, after which a golden yellow-colored filtrate was ob-
tained (Figure 1c). At that stage, the homogeneous golden
yellow color filtrate did not exhibit a Tyndall effect. Upon al-
lowing the filtrate to age, we made an unexpected observa-
tion: the homogenous filtrate gradually lost its golden yellow
color and turned transparent after about 72 h (Figure 1c–f); in-
terestingly, it also started to exhibit a Tyndall effect. As shown
in Figure 1g, irradiating a laser beam through the fresh filtrate
(golden yellow color) did not leave any discernible track. How-
ever, upon aging for 72 h, the filtrate turned transparent and
exhibited a strong Tyndall effect (Figure 1h). This suggested
the gradual formation of a dispersed phase upon aging, likely
through nucleation and growth, which are the underlying phe-
nomena in the solution-phase synthesis of nanostructures.[36–38]
To obtain preliminary insight into the formation of a dis-
persed phase, we analyzed the filtrate at different aging times
(i.e. 0, 24, 48, and 72 h) under high-resolution transmission
electron microscopy (HRTEM). The samples for HRTEM were
prepared by immersing the TEM grids into the dispersions (at
different stages of aging). The TEM images obtained at t=0 h
(golden-yellow-colored solution) indicate the presence of
nanodots (10 nm) (Figures 1i and 2a); the HRTEM images
(Figure 2d) and the corresponding selected area electron dif-
fraction (SAED) pattern (inset of Figure 2a) shows that these
nanodots are crystalline in nature and undergo oriented at-
tachment (as shown in Figure 2d). We see that these nano-
dots/nanocrystals resemble the prenucleation clusters that are
commonly observed in the nonclassical crystallization of vari-
ous biominerals (e.g. calcium carbonate, calcium phosphate,
oxides/hydroxides of iron, and silica).[36,39,40]
Thus, we can term
these nanodots/nanocrystals as prenucleation clusters.
Figure 1. Schematic describing how dissolution of MgB2 followed by recrystallization can be used to synthesize various morphologies of boron-based nano-
structures: a) a dark black suspension immediately after ultrasonication; b) separation into sediments and a homogeneous phase at the end of 24 h;
c) vacuum filtration of the suspension results in a filtrate, which is golden yellow in color at zero hour; d–f) filtrate gradually loses its color upon aging and fi-
nally becomes colorless (transparent) at the end of about 72 hours; g,h) Tyndall effect for the filtrate at zero hour (golden yellow color) does not show any
discernible track (upon irradiating with a laser beam), whereas the filtrate aged for 72 h (colorless) shows a strong scattered beam of light indicating the for-
mation of dispersed phase; i–l) TEM images of the filtrate dispersion aged from 0 to 72 h shows nanodots (at 0 h), nanograins (at 24 h), coalescence of nano-
grains (at 48 h), and nanoflakes/nanosheets (at 72 h).
ChemPhysChem 2018, 19, 880 – 891 www.chemphyschem.org 2018 Wiley-VCH Verlag GmbH Co. KGaA, Weinheim881
Articles

The dispersion aged for t=24 h shows the presence of
nanostructures, which are 100 nm in lateral dimension (Fig-
ures 1j and 2b). The HRTEM image (Figure 2e) reveals that
these nanostructures are formed by the oriented attachment
of the prenucleation clusters. To obtain further insight into
these nanostructures, we conducted thickness measurements
by atomic force microscopy (AFM) analysis. The sample for
AFM was prepared by spin coating on a freshly cleaved mica
substrate. The tapping-mode AFM scans of the nanostructures
prepared by aging for 24 h are shown in Figure 3a,b. The
height profiles corresponding to marked lines i and ii in Fig-
ure 3a are shown in Figure 3d,e, which measures the thickness
of the selected nanostructures as roughly 6–7 nm with average
lateral dimensions of around 60–70 nm, which indicates that
these are grain-like nanostructures. A representative 3D view
of these nanograins is shown in Figure 3c, and Figure 3 f pres-
ents a histogram of the height profiles obtained for all of the
nanograins (that appear in Figure 3a), which indicates that the
nanograins exhibit a thickness in the range of about 4 to
10 nm. It is likely that these nanograins represent the nuclea-
tion clusters, which are usually formed by aggregation of pre-
nucleation clusters.[40–42]
A quasiplanar nature of these nano-
structures suggests that the as-formed prenucleation clusters
exhibit a tendency to grow preferentially in lateral dimensions.
From the TEM and HRTEM images obtained at t=24 h (Fig-
ure 2b, e; also see Figure S1ic,d in the Supporting Informa-
tion), we observe that the nanograins appear to coalesce with
each other to resemble bridged nanostructures that are com-
monly observed as intermediates during postnucleation in
nonclassical crystallization and exhibit 2D oriented attachment
growth.[38,39,43–45]
On further aging the dispersion till t=48 h, we observe the
presence of nanograins with a higher degree of coalescence
resembling bridged nanostructures[38]
as well as mesocrys-
tals[36,38,40,45–47]
(Figures 1k and 2c). The HRTEM image (Fig-
ure 2 f) of the prepared nanostructures at t=48 h also shows
the presence of internal prenucleation clusters arranged in a
2D oriented fashion, which lets the nanostructures grow later-
ally.
Further, the dispersion aged for t=72 h (colorless disper-
sion) indicates the presence of flake-like nanostructures exhib-
iting localized darker spots (Figures 1l and 4a,b). Similar
darker spots were also reported for TiS2 and SnO2 nanostruc-
tures, and these thicker areas were attributed to crystalline re-
gions, which was indicative of the transformation from a
random arrangement into a 2D oriented arrangement.[48,49]
The
HRTEM image of a typical nanosheet/nanoflake is shown in
Figures 4c, and 4d clearly shows the presence of the prenu-
Figure 2. TEM/HRTEM/SAED analysis of nanostructures obtained at different stages of aging the filtrate sample: a–c) typical TEM images and the correspond-
ing SAED patterns of the nanostructures prepared by aging the filtrate sample for t=0 h (nanodots), 24 h (nanograins), and 48 h (nanoflakes) respectively; d–
f) HRTEM images of the aforementioned three samples drawn from the area denoted by red boxes in panels a–c; the dashed red lines correspond to the in-
terfaces of the nanocrystals (prenucleation clusters) in the magnesium boride nanostructures; g,h) represent the HRTEM images of the regions I and II shown
in panel d; i) HRTEM image of the area denoted by dashed blue box in panel f. From these images, it can be seen that the prenucleation clusters undergo ori-
ented attachment to form nanograins, which further grow into nanosheet or flake-like structures upon aging through a 2D oriented attachment mode.
Articles

cleation clusters arranged in a 2D oriented fashion to form a
hierarchical nanostructure. The aforementioned results suggest
that the nanostructures observed after dissolution of MgB2 in
water are formed by nonclassical crystallization, in which
Figure 3. Tapping mode AFM images of the golden-yellow-colored filtrate (t=0) deposited on mica substrate by the spin-coating method: a) filtrate deposit-
ed on freshly cleaved mica substrate by spin coating at a speed of 2000 rpm for 30 s and drying overnight by keeping it in a desiccator shows the uniform
distribution of nanograins; b) high-resolution image of the two closely spaced nanograins; c) three-dimensional view of the closely packed and uniformly dis-
tributed nanograins on a freshly cleaved mica substrate; d) height profile of a single nanograin showing a diameter of 60–70 nm with a height of 6–7 nm;
e) height profile for the two closely spaced nanograins also shows they are equal in diameter with an average height of 8–9 nm; f) average height of the
nanograins was found to be roughly 7 nm.
Figure 4. TEM/HRTEM/SAED/EDX/EELS/STEM analysis of a nanoflake/nanosheet obtained at 72 h of aging the filtrate sample: a) typical TEM image and corre-
sponding SAED pattern of a nanosheet prepared by aging the filtrate sample for 72 h and b) TEM image of the selected red box in panel a; c,d) HRTEM
images of the selected red box in panels b and c, respectively; e) TEM/EDX, f) boron EELS (192–214 eV), and g–j) element mapping of the selected dashed
white box region of the nanosheet in panel b.
Articles

stable prenucleation clusters undergo nucleation and 2D ori-
ented growth postnucleation to result in hierarchical super-
structures.[36,38,47]
Similar oriented attachment growth leading
to the formation of nanosheets was also observed in Co9Se8,
SnO2, SnS, and CeO2.[43,49,50]
Several more representative TEM
and HRTEM images corresponding to different aging times are
presented in Figures S1–S4. We observed that these nanostruc-
tures were extremely sensitive to the electron beam, and
hence, it was challenging to record their energy-dispersive X-
ray (EDX) spectra for boron. A similar challenge was reported
earlier by Pickering et al. for boron nanoparticles with dimen-
sions less than 45 nm.[51]
To obtain insight into the chemical
constitution of these nanostructures, we performed boron
electron energy-loss spectroscopy (EELS) and TEM/EDX for
other elements. The representative TEM/EDX and boron EELS
results of a nanosheet are shown in Figure 4e,f, respectively.
TEM/EDX analysis shows the presence of C, O, Mg, Cu, and Si.
The elements Mg and O are attributed to the oxy-functional-
ized nanosheets. The C and Cu peaks arise from the lacey
carbon-coated copper TEM grid. The minor Si peak is likely
from an impurity. The boron EEL spectrum shows the presence
of two boron peaks (K-edges) at about 194 and 202 eV. The
obtained EEL spectrum looks identical to the boron K-edge in
MgB4O7 (from WEELS, web source for electron energy-loss
spectra) and matches well with the boride and borate com-
pounds,[52–55]
which suggests that the first K-edge is attributed
to the absorption by p* states and the other K-edge is attrib-
uted to the s* states. The presence of p* and s* states im-
plies the existence of sp2
hybridization, which indicates that
these nanostructures contain a boron honeycomb network.
The chemical characterization results for all of the other stages
are presented in Figures S1–S4. The nanostructures obtained
at all stages showed chemical characterization (TEM/EDX/EELS)
results similar to those explained above for a typical nanosheet
prepared at 72 h of aging. Further, to obtain more insight into
the chemical make-up of the nanostructures, we conducted in-
ductively coupled plasma atomic emission spectroscopy (ICP-
AES) analysis of the filtrate that was allowed to age for differ-
ent times. The results, as summarized in Table 1, indicate that
the nanostructures are significantly deficient in Mg content
compared with the standard MgB2 crystals, which suggests a
distinct chemical constitution. Element mapping for these
nanostructures was conducted by scanning transmission elec-
tron microscopy (STEM) analysis (a typical elemental mapping
for a nanosheet is shown in Figure 4g–j), which shows the uni-
form distribution of the elements B, O, and Mg; this suggests
that these are functionalized magnesium boride nanostruc-
tures. STEM analyses for other stages are reported in Fig-
ures S1–S4. To obtain insight into the crystallinity of these
nanostructures, we performed SAED analysis for all the stages
(shown as insets in Figures 2a–c and 4a) and found that they
are crystalline in nature. For some instances, we found that
they also exhibited an amorphous nature (see Figure S5 for
more information). Such a growth of the nanostructures, from
oriented attachment of nanodots and nanograins to form
nanoflakes/nanosheets, is characteristic to the phenomenon of
nonclassical crystallization.[40]
Thus, we posit that these nano-
structures are formed by nonclassical crystallization in a
bottom-up way.
The inferences above are also supported by analysis of the
HRTEM images and chemical characterization, as explained in
subsequent sections. Although it was challenging to obtain
the HRTEM images of the nanostructures due to their sensitivi-
ty to the electron beam, we obtained HRTEM images from rela-
tively thicker areas and processed the micrographs by using
GATAN software to obtain fast-Fourier transforms (FFTs) and in-
verse FFTs. The FFT (Figure 5b) obtained from the selected
region of the HRTEM image of the nanostructure (Figure 5d)
resembles a truncated hexagon with a [111] hexagonal close-
packed (hcp) structure; this is indicative of its crystalline nature
and signifies the presence of lattice defects. The mask applied
to obtain the inverse FFT is presented as an inset in Figure 5e.
The inverse FFT (Figure 5e) of the selected region reveals the
occurrence of growth in different directions with certain lattice
defects. In Figure 5e, the defects are shown by arrows; the
red-colored arrows indicate the line defects, the blue-colored
arrows indicate vacancies, and the green circles show the re-
gions with a hexagonal arrangement of atoms with an atom
enclosed at the center. We were also able to identify the hex-
agonal honeycomb lattice arrangement (selected yellow
region) within this mesocrystal, which is magnified and shown
in the inset of Figure 5e. Using the line profile on these pat-
terns, we calculated the d spacing as 2.8 Š (Figure 5 f–g) and
found that this value does not match with any d spacing
values reported for standard MgB2, which is indicative of a con-
stitutional modification. Thus, we refer to these nanostructures
as functionalized nanostructures derived from MgB2. The de-
tailed HRTEM analysis of the thicker flake-like structures (poly-
crystalline in nature, as shown in Figures S5 f and S6), formed
after aging for 72 h, shows the presence of few-layer-thick
boron-based shells. They exhibit a d spacing of roughly 3.3 Š,
which closely matches the d spacing value of multiwalled
boron nanotubes (MWBNs) (%3.2 Š) shown by Liu et al.[56]
(see
Figure S6b). We also found the presence of nanodots embed-
ded in these thicker flake-like structures with a d spacing value
of about 2.9 Š (see Figure S6c,e, and inset ii).[47]
All these ob-
servations reaffirm that these nanostructures are hierarchical
superstructures formed by a nonclassical crystallization growth
mechanism. This study constitutes the first report showing
that a range of morphologies of functionalized nanostructures
can be obtained from MgB2 on account of its dissolution and
recrystallization.
Table 1. ICP-AES analysis of the filtrate at different aging times.[a]
Sample[b]
Concentration [mgLÀ1
] Stoichiometric ratio
Mg B Mg/B
0 h 377.31 817.72 0.41:2
24 h 385.14 824.87 0.42:2
48 h 373.30 839.20 0.40:2
72 h 369.37 813.79 0.40:2
[a] Stoichiometric ratio of Mg and B in standard MgB2 is Mg/B=0.99:2.
[b] Samples are labeled by the time for which the filtrate was aged.
Articles

We note that throughout the ultrasonication of MgB2 in
water at 258C, there was continuous evolution of gas, which
suggested that a chemical reaction between the MgB2 crystals
and water constituted one aspect of their interaction. To
obtain insight into the chemical reaction between MgB2 and
water during ultrasonication, we collected the gases evolving
during the process (Figure 6, insets i,ii) and stored them in a
gas sampling bag (see Figure S9 for detailed information). The
stored gases were analyzed by GC–MS. The spectrum (shown
in Figure 6) exhibits two strong signals (corresponding to m/
z=32 and 40) along with several weak signals in the m/z=30–
44 range. The weak signal observed at m/z=44 is attributed
to tetraborane(4) in its ionic form (i.e. B4H4
+
).[57]
The strong sig-
nals observed at m/z=30 and 42 along with the other weak
signals are likely attributed to the fragments derived from
B4H4
+
, which has a native tendency to undergo fragmentation
due to metastable transitions during GC–MS analysis, as shown
by Norman et al.[57]
We posit that some such possible frag-
ments could be 10
B4H2
+
, 11
B3H9
+
, 10
B3H12
+
, 11
B2
10
BH10
+
,
11
B10
B2H11
+
, 10
B4H+
, 10
B3H11
+
, 10
B3H10
+
, 11
B3H7
+
, 10
B3H4
+
,
11
B10
B2H3
+
, 11
B2
10
BH2
+
, 10
B3H2
+
, 11
B10
B2H+
, 10
B2H10
+
, and
11
B10
BH9
+
. We confirmed that the evolved gas was not CO2, as
the fragmentation pattern of CO2 results in signals with m/z
values of 12, 16, 22, 28, and 44.[58]
Also, the evolved gases
were found to exhibit a characteristic foul odor. To understand
the reason behind the formation of boron hydrides, we revisit-
ed the reaction kinetics of MgB2 with water. Zhao demonstrat-
ed that MgB2 did not exhibit any visible reaction with water at
temperatures 398C.[35]
It, however, underwent an exothermic
reaction with water at temperatures !398C,[35]
as described by
[Eq. (1)]:[35]
3 MgB2 þ 6 H2O ! 4 B þ 3 Mg OHð Þ2 þ B2H6 ð1Þ
Although the ultrasonication of MgB2 was performed in an
aqueous medium maintained at 258C, local hot spots (with
high temperatures and pressures) are expected to form within
the aqueous medium due to the phenomena of cavitation that
occurs during ultrasonication.[59]
Thus, we anticipate that these
local hot spots facilitate a type of reaction between the MgB2
crystals and water similar to that mentioned by Zhao et al. to
result in the formation of diborane.[35]
However, the GC–MS
analysis suggests that the evolved gases are heavier boron hy-
drides. Thus, we posit that the diborane formed in Equation (1)
may dissociate to form heavier boron hydrides [as shown in
Equation (2)],[60]
and depending on the nuclearity of the boron
hydrides, they form as volatile compounds,[60]
which are re-
leased as gases [Eq. (2)].
B2H6 ! heavier boron hydrides þ H2 ð2Þ
Furthermore, the as-formed diborane shown in Equation (1)
may react with oxygen and water present in the reaction
medium to form oxides of boron (the presence of the BÀO
bond is corroborated by FTIR spectroscopy, as detailed below)
as shown in Equations (3) and (4).[60]
Figure 5. HRTEM, FFT, inverse FFT, and line-profile imaging of nanostructures: a,c) TEM images and d) HRTEM image of the edge of the nanostructure; b) FFT
of the selected red-colored region of the HRTEM image showing the hexagonal pattern with the (101) plane direction and indicating the crystalline nature of
the sample; e) inverse FFT of the selected region of the HRTEM image indicating the presence of honeycomb lattice structure with line defects (red arrow)
and vacancies (blue arrow), and the inset shows the mask applied on FFT; the selected yellow region is magnified and shown in the inset, for which a clear
hexagonal arrangement is seen; f) magnified inverse FFT showing the selected region for line profile; g) line profile for the selected line in its inverse FFT
image indicating its d spacing as 2.8 Š.
Articles

2 B2H6 þ 3 O2 $ 2 B2O3 þ 6 H2 ð3Þ
B2H6 þ 3 H2O $ B2O3 þ 6 H2 ð4Þ
The GC–MS analysis along with our observations led us to
conclude that a chemical reaction happens between MgB2 and
water during ultrasonication even at a temperature of 258C.
This is expected to disintegrate the crystals of MgB2 and result
in their dissolution. The recrystallization observed after the dis-
solution is also supported by the z potential of colloidal dis-
persions obtained at various aging times: À12 to À16 mV
(0 h), À14 to À22 mV (24 h), À18 to À26 mV (48 h), and À7 to
À12 mV (72 h), which correspond to incipient instability (see
Figure S10). The negative z potential values indicate the pres-
ence of ionizable functional groups on the surface of the nano-
structures. Chemical characterization of the filtrate was per-
formed by FTIR spectroscopy, X-ray diffraction (XRD), and
Raman spectroscopy to obtain further insight. These character-
izations were performed on the powder forms of the nano-
structures (see a representative image in Figure S11a), which
were obtained by lyophilizing the filtrate at various hours (in
each case, we used to get %2.75 mgmLÀ1
, that is, yield
%92%). Figure 7a shows the Raman spectra of the standard
MgB2 crystals and the powder forms of the nanostructures pre-
pared from different aging times of the filtrate sample. The
spectra were acquired in the n˜ =100–700 cmÀ1
range by using
an excitation wavelength of 785 nm. The obtained Raman
bands are in good agreement with the previous reports on the
Raman characterization of MgB2.[61–68]
Standard MgB2 shows
bands at n˜ %552, 590, and 650 cmÀ1
(as shown in Figure 7a)
corresponding to the doubly degenerate E2g Raman mode; this
confirms the presence of in-plane, antiphase stretching and
hexagon-distorting displacements of the boron atoms. The
Raman E2g mode for standard MgB2 is typically reported in the
n˜ %570–630 cmÀ1
range.[63,68]
The filtrate samples at different
aging times shown in Figure 7a also exhibit bands in the n˜ =
550–650 cmÀ1
range corresponding to the E2g band of MgB2,
and this is indicative of the presence of boron honeycombs.
The band observed at n˜ %247 cmÀ1
in standard MgB2 matches
with the Raman-active bands of MgB2 excited at l=785 nm as
reported by Alarco et al.[63]
The presence of boron-based hon-
eycombs is also supported by the inverse FFT shown in
Figure 5. Figure 7b shows the FTIR spectra of the standard
MgB2 powder and the powder form of the filtrate samples at
different aging times. The FTIR spectrum of standard MgB2
powder exhibits a weak band at n˜ =405.59 cmÀ1
that corre-
sponds to the IR-active A2u mode (that is indicative of B and
Mg planes moving against each other),[61,62]
as shown in Fig-
ure S11g. The other IR-active mode, the E1u mode (B and Mg
planes sliding along x, y),[61,62]
usually occurs at n˜ %327 cmÀ1
;
this band could not be detected due to the limitations of the
instrument. The FTIR spectrum of the standard MgB2 powder
also shows a weak band at n˜ =668.14 cmÀ1
, which is indicative
of the presence of the BÀB bond.[69]
The weak band at n˜ =
1635.45 cmÀ1
denotes the presence of the MgÀB bond.[70]
The
bands at n˜ =3628.04 and 3710.38 cmÀ1
correspond to the
stretching vibrations of OÀH derived from water.[70,71]
The
bands at n˜ =982.77, 1456.68, and 1635.45 cmÀ1
correspond to
multiples of the absorption band at n˜ =485 cmÀ1
, which sug-
gests native MgB2, as shown by Sundar et al.[72]
The band at
n˜ =982.77 cmÀ1
can also be attributed to the in-plane bending
and out-of-plane bending of the BÀOÀH bond.[73]
The bands at
Figure 6. Relative abundance versus m/z (GC–MS) spectrum of gases evolved in the head space during ultrasonication of MgB2 with water at 258C: Inset i
shows the standard MgB2 powder taken for ultrasonication in water at 258C; inset ii is the customized setup used to collect the evolved gases during ultra-
sonication. In the graph of relative abundance versus m/z, the signals indicate the release of B4H4 [tetraborane(4)] gas (metastable) at m/z=44 that fragments
into its lower masses and becomes relatively stable at m/z=40 and 32. The signals at m/z=32 and 40 could be attributed to the presence of B3H2 (highly
stable) and B3H10 gases.
Articles

n˜ =1456.68, 1472.36, and 1488.91 cmÀ1
correspond to BÀO
stretching.[69,74]
The strong band at n˜ =2359.66 cmÀ1
corre-
sponds to the typical BÀH stretching vibration of boron hy-
drides [BH4]À1
, and the band at n˜ %1640 cmÀ1
can be ascribed
to motion of the hydrogen atoms in the BÀHÀB bridge.[74–76]
The IR bands obtained for standard MgB2 and the filtrate sam-
ples at different aging times are summarized in Table 2. The
bands located at n˜ 1000 cmÀ1
agree well with the modes of
b-rhombohedral boron, as shown by Sundar et al.,[72]
and the
A2u mode of MgB2 appears at n˜ %405 cmÀ1
(see Figure S11g).
The bands at n˜ 1000 cmÀ1
for the filtrate samples at different
aging times indicate the presence of the MgÀB, BÀB, BÀOH,
BÀO, BÀH, BÀHÀB, and OÀH bonds.[69–71,74–77]
The presence of
hydrides, oxides, and hydroxide functional groups on the stan-
dard MgB2 powder sample is attributed to its hygroscopic
nature. However, these functional groups are more pro-
nounced in the spectra of the filtrate than in the spectrum of
the standard MgB2 powder (see Figure 7b). This observation
suggests that these functional groups are likely attained due
to the chemical reaction between MgB2 and water, which is
supported by the GC–MS results (as mentioned above). XRD
analysis of the samples is shown in Figures S14 and S15. XRD
analysis of the nanostructures suggests that these are weakly
crystalline magnesium boride nanostructures. The Raman spec-
tra of these boron-based nanostructures indicate the presence
of the E2g mode (corresponding to BÀB in-plane stretching),
and it reconfirms that the honeycomb lattice arrangements
seen in the HRTEM are composed of boron. We conclude from
the TEM and AFM images that the nanostructures tend to
grow in lateral directions, which is also evident from XRD anal-
ysis (see Figure S12). These boron-based nanostructures are
decorated with hydrides and oxy functional groups (as shown
from the FTIR spectroscopy analysis). The aqueous dispersion
of these nanostructures exhibits high transparency with exclu-
sive absorption in the UV regime (l305 nm) (Figure S14). The
spectrum consists of four daughter bands at l=191, 211,
254.7, and 302 nm, which confirms that these boron-based
nanostructures can be utilized for making transparent UV-pro-
tective coatings.
With evidence from GC–MS analysis and the TEM micro-
graphs, we initially postulated that ultrasonication aided inter-
action between MgB2 and water and that this was the reason
Figure 7. Raman and FTIR spectra of various samples: a) Raman spectra of standard MgB2 and lyophilized filtrate samples obtained at different aging times
(with excitation at l=785 nm) show bands in the n˜ =550–650 cmÀ1
range. These correspond to the doubly degenerate Raman mode, E2g, that arises from in-
plane, antiphase stretching and hexagon-distorting displacements of the boron atoms; b) FTIR spectra of standard MgB2 and filtrate sample at different aging
times are plotted and compared with the standard compounds B2O3, MgB2O4, B(OH)3, and Mg(OH)2. The spectra depict the presence of OÀH, BÀH, BÀHÀB, BÀ
O, MgÀB, BÀB, and BÀOÀH functional groups in the nanostructures.
Table 2. IR bands of standard MgB2 and filtrate at different aging times.
Sample Range [cmÀ1
]
400–1000 1001–2000 2001–3000 3001–4000
A2u mode (MgÀB) BÀB/BÀOH BÀO MgÀB/BÀHÀB BÀH OÀH
MgB2 405.59 668.14, 982.77 1456.68, 1472.36, 1488.91 1635.45 2359.66 3628.04, 3710.38
filtrate at 0 h – 579.78 1486.22 1651.76 2489.54 3585.69
filtrate at 24 h 404.08 494.92, 584.38, 1053.52 1472.90 1651.69 2485.54 3566.35
filtrate at 48 h 402.71 490.64, 586.25, 643.21 1486.51 1661.86 2482.16 3566.70
filtrate at 72 h 408.95 505.53, 581.43 1495.19 1652.17 2487.17 3563.97
Articles

for the dissolution of the crystals to yield crystalline precursors,
which subsequently underwent nonclassical crystallization to
yield boron-based nanostructures with diverse morphologies.
However, upon simple addition of MgB2 crystals to water
under ambient conditions (at 258C) in the absence of ultra-
sonication, we observed a similar phenomenon of the continu-
ous evolution of gas. We also performed TEM analysis of the
resultant dispersions (at all stages of aging) obtained without
ultrasonication and found identical growth phenomena (the
detailed TEM and HRTEM micrographs and SAED patterns are
presented in Figures S16 and S17). These images also indicated
that the nanostructures tended to grow in lateral directions by
2D oriented attachment. This similarity suggests that the MgB2
crystals investigated by us in this study strongly interact with
water even at 258C and undergo dissolution, followed by re-
crystallization. We acknowledge that this differs with the con-
clusion made by Zhao,[35]
who noted that MgB2 crystals reacted
with water only at temperatures greater than 398C and that
no visible reaction occurred at room temperature. This differ-
ence could be because of the variation in the sizes of the crys-
tals (the MgB2 crystals used in this study: 150 mm). We antici-
pate that this chemical reaction between MgB2 and water at
258C is instrumental to disintegration of the crystals, which in
turn results in the formation of boron-based hierarchical super-
structures following a nonclassical crystallization path.[50,78–80]
One major difference between the nanostructures formed with
and without the ultrasonication step is their degree of defects.
We observed that the nanostructures obtained by the proce-
dure in which ultrasonication was applied exhibited a signifi-
cantly higher degree of defects (as can be seen by comparing
the inverse FFT patterns; Figure 5 and Figure S17). The pres-
ence of defects is expected, as prenucleation clusters generat-
ed during ultrasonication are more likely to be functionalized
due to the presence of local hotspots generated during the
ultrasonication. The feasibility of acquiring functional groups
during ultrasonication was also shown by Lin et al. in their
study, for which the ultrasonication of h-BN in water resulted
in the formation of the BÀOH functionality near the defect
sites in BN.[81]
This observation indicates that ultrasonication
can increase the degree of functional groups and defect sites.
Further experimental investigations are required to understand
the effect of the dissolution aide being used on the morpholo-
gy of boron-based nanostructures. Due to the presence of
functionalized boron, these nanostructures exhibit properties
that are distinct from those of parent MgB2.[82]
To obtain nano-
structures that are chemically identical to parent MgB2 it
would be promising to explore its exfoliation in the presence
of organic solvents.[83]
Green et al. recently demonstrated the
possibility of obtaining pristine nanosheets by exfoliating lay-
ered metal diborides in organic solvents.[84]
3. Conclusions
We presented a simple method to obtain functionalized
boron-based nanostructures of diverse morphologies by utiliz-
ing the dissolution–recrystallization of layered MgB2 crystals in
the aqueous phase under ambient conditions. The ability to
synthesize a high yield (92%) of nanostructures comprising
chemically modified boron honeycomb planes forms the cen-
tral merit of this work. This study also opens up avenues for
tuning the shape of nanostructures by facilitating directed
growth of the formed crystalline precursors in the presence of
surfactants; we have initiated research in this pursuit. Further-
more, the ability of these boron-based nanostructures to
absorb UV radiation selectively makes these promising candi-
dates for developing transparent UV-absorbing films. These
functionalized magnesium boride nanostructures (containing
hydrides, oxides, and hydroxide functional groups) are also
promising candidates for engineering hydrogen-storage mate-
rials.[74,85,86]
In the near future, we hope to utilize these func-
tionalized quasiplanar nanostructures as templates for metal
nanoparticles to develop hybrid interfaces that can be applied
in catalyzing reactions; this potentiality is aided by the extraor-
dinarily high yield of nanostructures obtained by this method.
We anticipate that this simple method of synthesizing a high
yield of boron-based nanostructures with diverse morpholo-
gies from layered MgB2 crystals will add impetus to the grow-
ing science on 2D boron nanomaterials. We envisage that the
synthetic procedure reported by us can be extended to other
metal diborides with geometry of the type AlB2 (e.g. ScB2, TiB2,
ZrB2),[87]
and it would be promising to investigate this possibili-
ty. It would also be interesting to extend this procedure to
pseudo-diborides of the type Y2ReB6, which share some as-
pects with AlB2-type borides and contain flat sheets of boron
arranged in five-, six-, and seven-membered rings.[88]
Experimental Section
Chemicals and Materials
Standard MgB2 powder (Sigma–Aldrich, !99% purity, À100 mesh
size) was used as the parent material. The ultrasonication of MgB2
was performed in deionized water (Ultrapure Millipore water-Type
I). In a typical synthesis, standard MgB2 powder (1.3 g) was added
to deionized water (440 mL). The mixture was placed in a custom-
ized double-walled vessel (for details, see Figure S8) and was ex-
posed to a probe ultrasonicator (Sonic Vibracell-VC505, 500 W,
20 kHz, operating at an amplitude value of 30% with 10 s on/10 s
off pulse) for 30 min. At the end of the ultrasonication, a dark
black suspension was obtained (represented in Figure 1a), which
was left undisturbed for 24 h at room temperature of 258C (repre-
sented as Figure 1b). Subsequently, the suspension was vacuum fil-
tered through a 0.22 mm filter paper (Durapore GVWP 47 mm) to
remove the sediments. The resultant filtrate exhibited a golden
yellow color (represented as Figure 1c). It was stored at 258C for
further investigation.
TEM, HRTEM, EELS, EDX, and SAED
Transmission electron microscopy (TEM) images were obtained by
using a FEI Tecnai F20 operated at 200 kV and FEI Tecnai G2 F30
operated at 300 kV. HRTEM images were obtained by using a JEM-
2100F or JEM-2200 FS operated at 200 kV. High-contrast images
with a point-to-point resolution of 1.2 Š were obtained. Selected
area electron diffraction (SAED) patterns, EELS, and EDX were ob-
tained for all of these. Samples for TEM were prepared by immers-
ing the TEM grids into a drop of dispersion for a few seconds. The
Articles

immersion was repeated thrice to ensure the deposition of nano-
structures. Two types of TEM grids were used: 1) lacey carbon
coated on a 300 mesh copper grid (Ted Pella); 2) ultrathin carbon
film supported by a lacey carbon coated on a 400 mesh copper
grid (Ted Pella). Before TEM and HRTEM examination, the grids
were dried under an infrared lamp for about 15–20 min. The
HRTEM micrographs were processed by using Gatan Digital Micro-
graph for obtaining the fast Fourier transform (FFT) and inverse
FFT. Detailed explanation on how to process the micrographs to
find the direction of the plane from the FFT and d spacing from
the inverse FFT are included in the Supporting Information (Sec-
tion S11).
FESEM
Field-emission scanning electron microscopy (FESEM) was per-
formed with a JEOL (JSM-7600F) field-emission scanning electron
microscope operated at 5–15 kV. Energy-dispersive X-ray (EDX)
spectroscopy was used to measure the elemental constituents by
collecting the spectrum in a silicon drift detection system. The
powdered form of the nanostructure was sprinkled on carbon ad-
hesive tape, on which the colloidal solutions were drop casted on
plasma treated silicon (with 300 nm coated SiO2) substrate. The
sample was dried in a desiccator. Prior to examination, the sample
was sputter coated with platinum in a high-vacuum chamber for
about 60 s to form an ultrathin layer to make the sample conduc-
tive.
UV/Vis Absorption Spectroscopy
The UV/Vis absorption spectra were recorded with a spectrometer
(Shimadzu, UV-1700 Pharma Spec UV/Vis) over the wavelength
range of 190 to 1100 nm in a quartz cuvette (path length of 1 cm).
The filtrate was observed periodically up to 72 h. The spectra were
plotted by using Origin Pro 8.0 and were analyzed by using
Fityk 0.9.8 software.
Lyophilization
Lyophilization was performed by taking an aqueous dispersion
(10 mL) in a 15 mL glass vial and placing it in a deep freezer at
À188C overnight. The frozen sample was subsequently placed in a
lyophilizer (CHRIST, Alpha 2–4 LD plus) for 3 d to obtain the
powder form of the sample.
GC–MS
A Supel-inert foil gas sampling bag with Thermogreen LB-2 Septa
was purchased from Sigma–Aldrich. Before use, it was filled with
air and evacuated as recommended. The gas evolving during the
ultrasonication of MgB2 crystals in water were collected in the sam-
pling bag. These were analyzed by GC–MS (Agilent 5975 GC/MSD
with 7890A GC system) at a programmed oven temperature rang-
ing from 50 to 2508C. Helium was used as the carrier gas at a flow
rate of 1 mLminÀ1
in a Hewlett Packard (HP) À5 capillary column
with a length and diameter of 60 m and 250 mm, respectively. The
temperature of the ion source was maintained at 2308C while op-
erating in the electron ionization mode.
FTIR Spectroscopy
The infrared (IR) spectra were recorded in the range of n˜ =400–
4000 cmÀ1
with a Thermo Scientific Nicolet iS50 FTIR spectrometer.
The standard MgB2 powder (2 mg) was mixed with KBr (300 mg),
whereas the lyophilized filtrate powder sample was taken directly
in a sample holder without mixing with KBr (pellets were not pre-
pared in either case). Multiple scans were obtained by using
OMNIC software.
ICP-AES
Qualitative full-scan analyses of colloidal dispersions and powder
samples were obtained by using ICP-AES (ARCOS from M/s Spec-
tro, Germany) with the spectrometer wavelength ranging from 130
to 770 nm. Quantitative measurements of the Mg and B concentra-
tions in the colloidal dispersions were obtained by using Optima
3300 DV, PerkinElmer, US. For colloidal samples, a volume of 10 mL
was used for analysis. For powder samples, the sample (0.2–0.5 g)
was dissolved in acidic solution according to the standard proce-
dures. Ultrapure Millipore water (Type I) was used as a control
during all the measurements.
Raman Spectroscopy
Raman spectra were collected with a Renishaw Raman microscope
by using a near-IR diode laser operating at a wavelength of
785 nm. For the standard MgB2 powder, the spectra were recorded
by using varying laser powers (5, 10, 50, and 100%). The presence
of bands was apparent if a 50% laser power source was used. At
100% laser power, we could not acquire any data due to overheat-
ing of the sample. Therefore, the Raman spectra for all samples
were further acquired only by using 50% laser power source
through a 20” objective lens. Each Raman spectrum consists of
two accumulations with a 10 s exposure per scan. Spectrum noise
and background were corrected by using the smoothing and base-
line functions in the WIRE software.
AFM
Atomic force microscopy (AFM) images were obtained by using
the NT-MDT (Moscow, Russia) model. The images were acquired
with the aid of a silicon cantilever (NSG 10-Tip, spring constant
3.08 NmÀ1
, resonating frequency 140 kHz). Samples for AFM analy-
sis were prepared by static dispensing of the filtrate (30 mL) on a
freshly cleaved mica substrate (9.9 mm diameter PELCO mica discs)
and spin coated at a speed of 2000 rpm for 30 s and allowed to
dry in a desiccator for about 12 h. All the images were obtained by
using Nova 1.1.0.1780 software and were further processed by
using WSxM 5.0 Develop 7.0 software.
XRD
X-ray diffraction analysis was performed by using a Bruker-D8-Dis-
cover (Germany) diffractometer with the 2q angle varying from 5
to 908 with a step size of 0.02 at a scan rate of 0.2 s with 40 V,
30 mA power having Cu X-ray source of wavelength 1.5406 Š. The
data of the standard MgB2 powder and lyophilized filtrate at vari-
ous hours were analyzed with the ICDD (International Centre for
Diffraction Data) database.
Articles

z Potential Analysis
The z potential of the filtrate at various hours was analyzed by
using a Zetasizer Nano-ZS apparatus (Malvern Instruments Ltd.,
Malvern UK). Samples were taken in a clear disposable z cell, and
two measurements were made for each sample. The number of
runs per measurement, attenuation, and optical measurement po-
sition were automatically determined. Data were analyzed by using
Zetasizer Software Version 6.34 (Malvern Instruments Ltd).
Acknowledgements
The authors thank Dr. Manish Dixit and Dr. Bhavesh Bharatiya
(DDU, Nadiad, India) for GC–MS and AFM analysis, Bharati Patro
and Aaradhana (IIT Bombay, SAIF) for HRTEM analysis, Vikas
Patel (SICART, Anand, India) for help with TEM imaging, and
Kumud Arora (IIT Delhi, India) for help with the Raman Spectros-
copy measurements. We are grateful to the research institute C-
MET, Pune, for providing the HRTEM with EELS facility. The staff
of the IITGN Mechanical Department is acknowledged for helping
in designing the customized cap for the ultrasonicator assembly.
We appreciate the help extended by Narendra Bandaru and Sas-
mita Majhi (Gatan DigitalMicrograph); Narendra Bandaru (spin
coating); Veera Bhadraiah P and Abhijeet Ojha (UV/Vis spectros-
copy); Anuj Bisht and Bhanu Pratap (FTIR spectroscopy and
XRD); Sophia Varghese and Komal Pandey (XRD); Pallawi Gupta
(FTIR spectroscopy); and Varsha Thambi, Vikram Karde, and Awa-
neesh Upadhyay (FESEM and EDX). We also thank Prof. Sameer V.
Dalvi for helpful discussions on the phenomenon of nonclassical
crystallization. We are also thankful to Mr. Supresh Shashikant
Thaleshri and Mr. Suvakanta Barik for their kind help in process-
ing the purchase of the chemicals. This work was supported by
seed funding from IIT Gandhinagar; Fast Track Research Grant
for Young Scientists (SB/FTP/ETA-114/2013) by Science and Engi-
neering Research Board, Department of Science and Technology,
India; and INSPIRE Faculty Award Research Grant (DST/INSPIRE/
04/2014/001601) by Department of Science and Technology,
India. The authors deeply acknowledge the central facilities pro-
vided by IIT Gandhinagar for conducting various experiments.
Conflict of interest
The authors declare no conflict of interest.
Keywords: boron · crystallization · layered compounds ·
nanostructures · ultrasonication
[1] E. Weintraub, J. Ind. Eng. Chem. 1911, 3, 299–301.
[2] E. Weintraub, J. Ind. Eng. Chem. 1913, 5, 106–115.
[3] A. W. Laubengayer, D. T. Hurd, A. E. Newkirk, J. L. Hoard, J. Am. Chem.
Soc. 1943, 65, 1924–1931.
[4] “Boron: Inorganic Chemistry”D. M. Schubert, R. J. Brotherton in Encyclo-
pedia of Inorganic Chemistry (Ed.: R. B. King), 2006, Wiley, New York, .
[5] I. Boustani in Chemical Modelling: Applications and Theory, (Ed.: M.
Springborg), RSC, Cambridge, 2011, pp. 1–44.
[6] J. P. Scheifers, Y. Zhang, B. P. T. Fokwa, Acc. Chem. Res. 2017, 50, 2317–
2325.
[7] I. Boustani, Surf. Sci. 1997, 370, 355–363.
[8] I. Boustani, A. Quandt, E. Hernµndez, A. Rubio, J. Chem. Phys. 1999, 110,
3176–3185.
[9] K. C. Lau, R. Pandey, J. Phys. Chem. C 2007, 111, 2906–2912.
[10] H. Tang, S. Ismail-Beigi, Phys. Rev. Lett. 2007, 99, 115501.
[11] E. S. Penev, S. Bhowmick, A. Sadrzadeh, B. I. Yakobson, Nano Lett. 2012,
12, 2441–2445.
[12] Y. Liu, E. S. Penev, B. I. Yakobson, Angew. Chem. Int. Ed. 2013, 52, 3156–
3159; Angew. Chem. 2013, 125, 3238–3241.
[13] S. Carenco, D. Portehault, C. Boissi›re, N. MØzailles, C. Sanchez, Chem.
Rev. 2013, 113, 7981–8065.
[14] J. Wang, H. Y. Zhao, Y. Liu, ChemPhysChem 2014, 15, 3453–3459.
[15] B. Zheng, H. T. Yu, Y. Xie, Y. F. Lian, ACS Appl. Mater. Interfaces 2014, 6,
19690–19701.
[16] S. Banerjee, G. Periyasamy, S. K. Pati, J. Mater. Chem. A 2014, 2, 3856.
[17] Q. Peng, L. Han, X. Wen, S. Liu, Z. Chen, J. Lian, S. De, Phys. Chem.
Chem. Phys. 2015, 17, 2160–2168.
[18] I. Boustani, Phys. Rev. B 1997, 55, 426–438.
[19] J. Xu, Y. Chang, L. Gan, Y. Ma, T. Zhai, Adv. Sci. 2015, 2, 1500023.
[20] X.-B. Li, S.-Y. Xie, H. Zheng, W. Q. Tian, H.-B. Sun, Nanoscale 2015, 7,
18863–18871.
[21] X. Yang, Y. Ding, J. Ni, Phys. Rev. B 2008, 77, 41402.
[22] Z. A. Piazza, H.-S. Hu, W.-L. Li, Y.-F. Zhao, J. Li, L.-S. Wang, Nat. Commun.
2014, 5, 1–6.
[23] R. B. Patel, T. Chou, Z. Iqbal, J. Nanomater. 2015, 2015, 7.
[24] A. J. Mannix, X.-F. Zhou, B. Kiraly, J. D. Wood, D. Alducin, B. D. Myers, X.
Liu, B. L. Fisher, U. Santiago, J. R. Guest, M. J. Yacaman, A. Ponce, A. R.
Oganov, M.C. Hersam, N. P. Guisinger, Science 2015, 350, 1513–1516.
[25] G. Tai, T. Hu, Y. Zhou, X. Wang, J. Kong, T. Zeng, Y. You, Q. Wang, Angew.
Chem. Int. Ed. 2015, 54, 15473–15477; Angew. Chem. 2015, 127, 15693–
15697.
[26] H. Feng, Z. Hu, X. Liu, Chem. Commun. 2015, 51, 10961–10964.
[27] S. K. Das, A. Bedar, A. Kannan, K. Jasuja, Sci. Rep. 2015, 5, 10522.
[28] A. L. James, K. Jasuja, RSC Adv. 2017, 7, 1905–1914.
[29] J. O. G. Reim, E. S. K. C. Gmbh, C. Kg, A. C. Company, Ullmann’s Encycl.
Ind. Chem. 2011, 100C, 121–145.
[30] M. Pissas in Low-Dimensional Solids (Eds.: D. W. Bruce, D. O’Hare, R. I.
Walton), 2010, Wiley, Chichester, pp. 229–286.
[31] G. Akopov, M. T. Yeung, R. B. Kaner, Adv. Mater. 2017, 29, 1604506.
[32] B. Albert, H. Hillebrecht, Angew. Chem. Int. Ed. 2009, 48, 8640–8668;
Angew. Chem. 2009, 121, 8794–8824.
[33] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature
2001, 410, 63–64.
[34] S. L. Bud’ko, G. Lapertot, C. Petrovic, C. E. Cunningham, N. Anderson,
P. C. Canfield, Phys. Rev. Lett. 2001, 86, 1877–1880.
[35] F. Zhao, Am. J. Anal. Chem. 2011, 2, 270–275.
[36] F. C. Meldrum, H. Colfen, Nanoscale 2010, 2, 2326–2327.
[37] S. E. Bates, W. E. Buhro, C. A. Frey, S. M. L. Sastry, K. F. Kelton, J. Mater.
Res. 1995, 10, 2599–2612.
[38] H. Imai in Biomineralization I: Crystallization and Self-Organization Pro-
cess (Ed.: K. Naka), Springer, Berlin, 2007, pp. 43–72.
[39] E. M. Pouget, P. H. H. Bomans, J. A. C. M. Goos, P. M. Frederik, G. de With,
N. A. J. M. Sommerdijk, Science 2009, 323, 1455–1458.
[40] D. Gebauer, M. Kellermeier, J. D. Gale, L. Bergstrçm, H. Cçlfen, Chem.
Soc. Rev. 2014, 43, 2348–2371.
[41] R. Viswanatha, D. Sarma in Nanomaterials Chemistry: Recent Develop-
ments and New Directions (Eds.: C. N. R. Rao, A. Müller, A. K. Cheetham),
Wiley-VCH, Weinheim, 2007, pp. 139–170.
[42] X. Zhang, J. Zhang, J. Zhao, B. Pan, M. Kong, J. Chen, Y. Xie, J. Am.
Chem. Soc. 2012, 134, 11908–11911.
[43] J. Wang, G. Lian, Z. Xu, C. Fu, Z. Lin, L. Li, Q. Wang, D. Cui, C. P. Wong,
ACS Appl. Mater. Interfaces 2016, 8, 9545–9551.
[44] D. Gebauer, A. Vçlkel, H. Cçlfen, Science 2008, 322, 1819–22.
[45] D. Gebauer, H. Cçlfen, Nano Today, 2011, 6, 564–584.
[46] E. R. Leite, R. Caue, Crystallization and Growth of Colloidal Nanocrystals,
Springer, New York, 2012.
[47] H. Cçlfen, S. Mann, Angew. Chem. Int. Ed. 2003, 42, 2350–2365; Angew.
Chem. 2003, 115, 2452–2468.
[48] S. Prabakar, C. W. Bumby, R. D. Tilley, Chem. Mater. 2009, 21, 1725–1730.
[49] C. Wang, G. Du, K. Ståhl, H. Huang, Y. Zhong, J. Z. Jiang, J. Phys. Chem. C
2012, 116, 4000–4011.
[50] W. Yang, X. Zhang, Y. Xie, Nano Today 2016, 11, 793–816.
Articles

[51] A. L. Pickering, C. Mitterbauer, N. D. Browning, S. M. Kauzlarich, P. P.
Power, Chem. Commun. 2007, 2, 580–2.
[52] H. Sauer, R. Brydson, P. N. Rowley, W. Engel, J. M. Thomas, Ultramicrosco-
py 1993, 49, 198–209.
[53] R. F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Micro-
scope, Springer, Boston, MA, 2011.
[54] R. C. Yu, S. C. Li, Y. Q. Wang, X. Kong, J. L. Zhu, F. Y. Li, Z. X. Liu, X. F.
Duan, Z. Zhang, C. Q. Jin, Phys. C 2001, 363, 184–188.
[55] O. Stephan, P. M. Ajayan, C. Colliex, P. Redlich, J. M. Lambert, P. Bernier,
P. Lefin, Science 1994, 266, 1683–1685.
[56] F. Liu, C. Shen, Z. Su, X. Ding, S. Deng, J. Chen, N. Xu, H. Gao, J. Mater.
Chem. 2010, 20, 2197.
[57] A. D. Norman, R. Schaeffer, A. B. Baylis, G. A. Pressley, Jr., F. E. Stafford, J.
Am. Chem. Soc. 1966, 88, 2151–2156.
[58] P. J. Linstrom, W. G. Mallard, NIST Chemistry WebBook, NIST Standard
Reference Database Number 69, 2000.
[59] L. Thompson, L. Doraiswamy, Ind. Eng. Chem. Res. 1999, 38, 1215–1249.
[60] E. T. Flaherty, J. Marshall, P. Albert, A. M. Brzychcy, D. Forbes, R. Halver-
son, J. Electrochem. Soc. 1993, 140, 1709 .
[61] J. Kortus, I. I. Mazin, K. D. Belashchenko, V. P. Antropov, L. L. Boyer, Phys.
Rev. Lett. 2001, 86, 4656–4659.
[62] K. P. Bohnen, R. Heid, B. Renker, Phys. Rev. Lett. 2001, 86, 5771–5774.
[63] J. A. Alarco, A. Chou, P. C. Talbot, I. D. R. Mackinnon, Phys. Chem. Chem.
Phys. 2014, 16, 24443–24456.
[64] H. Martinho, C. Rettori, P. G. Pagliuso, A. A. Martin, N. O. Moreno, J. L.
Sarrao, Solid State Commun. 2003, 125, 499–502.
[65] A. F. Goncharov, V. V. Struzhkin, E. Gregoryanz, J. Hu, R. J. Hemley, H.
Mao, G. Lapertot, S. L. Bud’ko, P. C. Canfield, Phys. Rev. B 2001, 64,
100509.
[66] X. K. Chen, M. J. Konstantinovic, J. C. Irwin, D. D. Lawrie, J. P. Franck,
Phys. Rev. Lett. 2001, 87, 157002.
[67] T. Yildirim, O. Gülseren, J. W. Lynn, C. M. Brown, T. J. Udovic, Q. Huang,
N. Rogado, K. A. Regan, M. A. Hayward, J. S. Slusky, T. He, M. K. Haas, P.
Khalifah, K. Inumaru, R. J. Cava, Phys. Rev. Lett. 2001, 87, 37001.
[68] J. Hlinka, I. Gregora, J. Pokorny, A. Plecenik, P. Kus, L. Satrapinsky, S. Be-
nacka, Phys. Rev. B 2001, 64, 140503.
[69] H. T. Tsou, W. Kowbel, Surf. Coatings Technol. 1996, 79, 139–150.
[70] S. Angappan, N. Kalaiselvi, R. Sudha, A. Visuvasam, Int. Scholarly Res.
Not. 2014, 123194.
[71] N. Kumar, S. Das, C. Bernhard, G. D. Varma, Supercond. Sci. Technol.
2013, 26, 95008.
[72] C. S. Sundar, A. Bharathi, M. Premila, T. N. Sairam, S. Kalavathi, G. L. N.
Reddy, V. S. Sastry, Y. Hariharan, T. S. Radhakrishnan, 2001, arXiv:cond-
mat/0104354, 1–15.
[73] T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y. K. Gun’ko, J. Cole-
man, J. Am. Chem. Soc. 2012, 134, 18758–18771.
[74] H. W. Li, Y. Yan, S. I. Orimo, A. Züttel, C. M. Jensen, Energies 2011, 4,
185–214.
[75] J. F. Mao, Z. P. Guo, H. K. Liu, S. X. Dou, J. Mater. Chem. A 2013, 1, 2806–
2811.
[76] D. Y. Kim, Y. Yang, J. R. Abelson, G. S. Girolami, Inorg. Chem. 2007, 46,
35–38.
[77] M. B. Kadam, B. B. Sinha, R. S. Kalubarme, S. H. Pawar, J. Alloys Compd.
2009, 478, 467–473.
[78] J. J. Geuchies, C. van Overbeek, W. H. Evers, B. Goris, A. de Backer, A. P.
Gantapara, F. T. Rabouw, J. Hilhorst, J. L. Peters, O. Konovalov, A. V. Petu-
khov, M. Dijkstra, L. D. A. Siebbeles, S. van Aert, S. Bals, D. Vanmaekel-
bergh, Nat. Mater. 2016, 15, 1248–1254.
[79] K. Bian, J. J. Choi, A. Kaushik, P. Clancy, D. M. Smilgies, T. Hanrath, ACS
Nano 2011, 5, 2815–2823.
[80] F. Wang, X. Wang, Nanoscale 2014, 6, 6398–414.
[81] Y. Lin, T. V. Williams, T. B. Xu, W. Cao, H. E. Elsayed-Ali, J. W. Connell, J.
Phys. Chem. C 2011, 115, 2679–2685.
[82] B. Z. Xu, S. P. Beckman, 2D Mater. 2016, 3, 031003.
[83] Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. T. Mc-
Govern, B. Holland, M. Byrne, Y. K. Gun’Ko, J. J. Boland, P. Niraj, G. Dues-
berg, S. Krishnamurthy, R. Goodhue, J. Hutchison, V. Scardaci, A. C. Fer-
rari, J. N. Coleman, Nat. Nanotechnol. 2008, 3, 563–568.
[84] A. Green, A. Yousaf, A. Debnath, 2017.
[85] A. Remhof, Y. Yan, D. Rentsch, A. Borgschulte, C. M. Jensen, A. Züttel, J.
Mater. Chem. A 2014, 2, 7244.
[86] Boron Science: New Technologies and Applications (Ed.: N. S. Hosmane),
CRC Press, Boca Raton, FL, 2012.
[87] J. K. Burdett, G. J. Miller, E. Canadell, J. Am. Chem. Soc. 1986, 108, 6561–
6568.
[88] Y. Kuz’ma, S. Svarichevskaya, Sov. Phys. Crystallogr. 1972, 17, 569–571.
Manuscript received: September 21, 2017
Accepted manuscript online: January 3, 2018
Version of record online: February 16, 2018
Articles

Gunda et al 2018-simple green and high yield production of boron based nanostructures

Recommended

Recommended

More Related Content

What's hot

What's hot (19)

Similar to Gunda et al 2018-simple green and high yield production of boron based nanostructures

Similar to Gunda et al 2018-simple green and high yield production of boron based nanostructures (20)

Recently uploaded

Recently uploaded (20)

Gunda et al 2018-simple green and high yield production of boron based nanostructures