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Successive effect of rolling up, oriented attachment and Ostwald ripeningon the hydrothermal formation of szaibelyite MgBO2(OH) nanowhiskers

Wancheng Zhu,*ab Shenlin Zhua and Lan Xiang*a

Received 20th March 2009, Accepted 27th May 2009

First published as an Advance Article on the web 12th June 2009

DOI: 10.1039/b905698j

The successive effects of rolling up, oriented attachment and Ostwald ripening on the hydrothermal

formation of szaibelyite MgBO2(OH) nanowhiskers (diameter: 20–60 nm, aspect ratio: 10–70) from

amorphous precursor obtained at room temperature are investigated in this paper, and the

MgBO2(OH) nanowhiskers successively experience three various stages in the course of the

hydrothermal treatment, dominated via rolling up at an early heating stage, head-to-head overlapped

and side-by-side oriented attachment at medium-term crystal growth, and Ostwald ripening at late

hydrothermal coarsening, which lead to rudimental 1D MgBO2(OH), lotus root-like MgBO2(OH) with

a concavo-convex surface, and uniform MgBO2(OH) nanowhiskers with a smooth surface and a high

aspect ratio, respectively.

1. Introduction

As one of the most thriving research fields in nanoscience and

nanotechnology, one-dimensional (1D) nanostructures such as

nanotubes, nanowires, nanorods, and nanobelts have attracted

extraordinary attention and intensive interest due to their unique

structures, fantastic properties and great potential applications.1–5

Varieties of synthetic techniques have been developed for the

synthesis of 1D nanostructures,6–12 among which hydrothermal

technology has been emerged as a competitive method for the

synthesis of 1D nanostructures6–9 owing to its distinct advantages

such as versatility, energy saving, better control of nucleation

and shape, lower temperature of operation, etc.13 For instance,

the hydrothermal method has been extensively utilized in the

synthesis of 1D hydroxyl or hydrated compounds, and the cor-

responding 1D anhydrous compounds, e.g. oxides, such as

Co3O4 nanorods,12 MnO2/Mn2O3/Mn3O4 nanorods,14 Eu2O3

nanorods,15 Tb4O7 and Y2O3 nanotubes,16,17 Y2O3 : Eu nano-

belts,18 Dy2O3 nanotubes,19 MgO nanorods20 and Al2O3 nano-

rods,21,22 etc., supplemented by subsequent calcination or

annealing. It is essential to attain 1D hydroxyl or hydrated

compound via hydrothermal synthesis with a controllable shape,

size and crystallinity so as to obtain the corresponding functional

1D oxide via morphology and crystallinity preservation in the

course of the subsequent thermal conversion. To date, it is still of

great significance, albeit intractable, to ascertain the oriented

growth or 1D assembly of the amorphous precursors (generally

acquired at room temperature) under hydrothermal conditions

into 1D nanostructured crystals.

Generally, the crystal growth of the bulk materials complies

with the traditional Ostwald ripening or coarsening mecha-

nism,23–25 i.e. dissolution–reprecipitation mechanism,26 which

often leads to the isotropic growth of particles with regular

aDepartment of Chemical Engineering, Tsinghua University, Beijing,100084, China. E-mail: zhuwc04@mails.tsinghua.edu.cn; xianglan@mail.tsinghua.edu.cn; Fax: +86-10-62772051; Tel: +86-10-62788984bDepartment of Chemical Engineering, Qufu Normal University,Shandong, 273165, China

1910 | CrystEngComm, 2009, 11, 1910–1919

elliptical or quasi-spherical shape and relatively smooth surface

and fringes.27,28 However, Ostwald ripening mechanism has been

proven not unique in controlling the growth of nanocrystals. In

the last decade, the oriented attachment was indicated as another

kind of typical and significant growth mechanism for the growth

of nanocrystals,29–33 which has now been found also effective and

responsible for the formation of shuttle-like,34 multipod-like,35

dendritic36 and many other heterogeneous nanostructures, and

has especially been widely used in the formation of nanowires37,38

and hydrothermal growth of nanorods.39–44 Compared to the

Ostwald ripening mechamism, oriented attachment growth

attributed to the attachment and further junction of the adjacent

two or more nanoparticles on the specific crystallographic

direction often results in the anisotropic growth of complicated

microstructures with irregular shape, zigzag concavo-convex

fringes, and defections such as twin boundaries and stacking

faults.30,33 In contrast with Ostwald ripening and oriented

attachment, Li et al. proposed recently another kind of rolling-up

mechanism and explained well the hydrothermal formation of Bi

nanotubes,6 WS2 nanotubes8 and W nanowires45 from natural or

artificial lamellar structures, which has subsequently been proven

and even designed by Wang et al. in the hydrothermal synthesis

of MnO2 nanowires/nanorods,46 Ln(OH)3 nanowires47 and also

silicate nanotubes.9 It is worth noting that oriented attachment

and Ostwald ripening mechanisms have been found both to exist

in the hydrothermal coarsening of ZnS nanoparticles,27,48

however little work has been done concerning the joint effect of

the above three mechanisms on the hydrothermal growth of

other nanocrystals, e.g. magnesium borates.

1D nanostructured magnesium borates, including MgB4O7

nanowires,49 Mg3B2O6 nanotubes,50 nanobelts51 and nanorods,52

and Mg2B2O5 nanowires,53–55 nanorods,56 whiskers57,58 and

nanowhiskers,59,60 etc., have attracted much attention in recent

years due to their potential applications as reinforcements in

electronic ceramics,49 wide band gap semiconductors,53 antiwear

additive,54 and plastics or aluminum/magnesium matrix alloys.58

Traditionally, 1D mirco-/nanostructured magnesium borates

were prepared via chemical vapor deposition (CVD)49–51,53,54,56 or

This journal is ª The Royal Society of Chemistry 2009

Fig. 1 Hydrothermal temperature–time phase diagram for the forma-

tion of MgBO2(OH). Section I: Mg7B4O13$7H2O; section II:

MgBO2(OH) + Mg7B4O13$7H2O; section III: MgBO2(OH).

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molten salt synthesis (MSS)55,57,58 at temperature as high as 850–

1250 �C or solution-based method under rigorous supercritical

conditions.52,61 Recently, szaibelyite MgBO2(OH) has attracted

much interest as a promising precursor for Mg2B2O559–61 and

novel luminescent material.62,63 In a previous work, we have

obtained high-crystallinity Mg2B2O5 nanowhiskers59,60 on the

basis of the hydrothermal synthesis of MgBO2(OH) nano-

whiskers.64,65 Here we report for the first time, to the best of our

knowledge, the hydrothermal formation mechanism of

MgBO2(OH) nanowhiskers, which was successively controlled

via rolling up, oriented attachment and Ostwald ripening at

various stages of the hydrothermal treatment. The various

mechanisms controlled 1D growth phenomena might also be

existent in the course of the oriented growth or 1D assembly of

other amorphous precursors under hydrothermal conditions into

1D nanostructured crystals (especially those with a highly

anisotropic structure) in absence of any surfactants/capping

reagents or templates, and thus be especially helpful for under-

standing the fantastic conversion from the irregular amorphous

nanoparticles to 1D nanostructures.

2. Experimental

Formation of MgBO2(OH) nanowhiskers

MgBO2(OH) nanowhiskers were synthesized by room-tempera-

ture coprecipitation of Mg7B4O13$7H2O in MgCl2 (2 mol L�1),

H3BO3 (3 mol L�1) and NaOH (4 mol L�1) solutions with a molar

Mg : B : Na ratio of 2 : 3 : 4, followed by hydrothermal treat-

ment of the slurry (Mg7B4O13$7H2O, 40 mL) within a Teflon-

lined stainless steel autoclave (capacity: 70 mL) at 240 �C for

18.0–30.0 h.64,65 All of the reagents were analytical grade without

further purification. After the hydrothermal treatment, the

resultant white precipitate was washed with distilled water,

filtered and dried at 105 �C for 12.0 h, and finally collected for

further characterization. To investigate the hydrothermal

formation of the MgBO2(OH) nanowhiskers, hydrothermal

temperature and time were adjusted within the range of 60–240�C and 0–168 h, respectively, to acquire various hydrothermal

products, with the other conditions kept the same. The pH value

of the hydrothermal solution corresponding to various temper-

ature moments was monitored via a precise indicator paper,

meanwhile the yield of the solid phase was calculated through

dividing the practical mass of the hydrothermal product by the

theoretical mass of the product, taking into consideration that all

MgCl2 had participated in the formation of MgBO2(OH).

Characterization

The structure of the sample was identified using an X-ray powder

diffractometer (XRD, D8-Advance, Bruker, Germany) using

a Cu Ka radiation (l ¼ 1.54178 A), a fixed power source (40.0

kV, 40.0 mA) and an aligned silicon detector. The morphology

and microstructure of the samples were examined by field emis-

sion scanning electron microscopy (FESEM, JSM 7401F, JEOL,

Japan) operated at an accelerating voltage of 1.0 kV, and high-

resolution transmission electron microscopy (HRTEM, TEM-

2010, JEOL, Japan) performed at an accelerating voltage of

120.0 kV. The average diameter and length of the hydrothermal

products were estimated by direct measuring of about 200

This journal is ª The Royal Society of Chemistry 2009

particles from typical FESEM images with magnifications of

15 000–40 000.

3. Results

3.1. Room temperature coprecipitation and hydrothermal

conversion

According to the analysis on the precipitate,64 the coprecipitation

at room temperature resulting in the slurry containing

Mg7B4O13$7H2O can be written in ionic form as follows:

H3BO3(s) + H2O / B(OH)4�(aq.) + H+(aq.) (1)

MgCl2(aq.) / Mg2+(aq.) + 2Cl�(aq.) (2)

NaOH(aq.) / Na+(aq.) + OH�(aq.) (3)

4B(OH)4�(aq.) + 7Mg2+(aq.) + 10OH�(aq.) /

Mg7B4O13$7H2O(s) + 6H2O (4)

The hydrothermal conversion can be expressed as follows, defi-

nitely showing the necessary basic medium for the hydrothermal

formation of the szaibelyite MgBO2(OH) phase:66

Mg7B4O13$7H2O(s) + 3B(OH)4�(aq.) /

7MgBO2(OH)(s) + 3OH�(aq.) + 8H2O (5)

3.2. Temperature–time phase diagram

In order to get deeper insight into the formation process of the

target MgBO2(OH), phase diagram concerning the stabilities of

Mg7B4O13$7H2O and MgBO2(OH) were systematically investi-

gated. As illustrated in Fig. 1, the temperature–time phase

diagram (Mg : B : Na ¼ 2 : 3 : 4; cMg2+ ¼ 0.33 mol L�1) can be

divided into three sections according to the crystalline phase of

the hydrothermal product. Section I shows the stable region for

the existence of the hydrothermal precursor Mg7B4O13$7H2O in

case of relatively low temperature and short time; section II

indicates the mixed region for the coexistence of MgBO2(OH)

and Mg7B4O13$7H2O corresponding to either relatively low

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temperature and long time or relatively high temperature and

short time, due to the partial conversion from Mg7B4O13$7H2O

to MgBO2(OH); section III demonstrates the stable region for

the existence of MgBO2(OH) owing to the complete conversion

from Mg7B4O13$7H2O to MgBO2(OH).

Fig. 2 Morphology evolution of the hydrothermal products obtained at

various temperature moments in the course of the heating procedure.

Temperature (�C): (a) 60, (b) 100, (c) 140, (d) 160, (e) 180, (f) 200, (g) 220,

(h) 240.

3.3. Morphology and crystalline phase evolution

A wide range of temperature and time were employed in the

hydrothermal treatment of the precursor slurry so as to attain the

morphology/crystalline phase evolution of the product and

further understand the hydrothermal formation of MgBO2(OH)

nanowhiskers. Fig. 2 and Fig. 3 indicate the evolution of the

morphology and crystalline phase of the hydrothermal products

obtained at various temperature moments in the course of the

heating procedure, respectively. The precipitate acquired at room

temperature was composed of particulate Mg7B4O13$7H2O

(Fig. 3o) with irregular shape and poor crystallinity.64 With

temperature going up to 60 �C, the precipitate turned into

irregular plate-like Mg7B4O13$7H2O (Fig. 2a1 and Fig. 3a),

coexisting with sporadic thin 1D nanoflakes with either smooth

surface and regular/irregular ends or rough surface and head-to-

head coalesced stem (Fig. 2a1–a2). Meanwhile, the gradual shift

and broadening of the X-ray diffraction peaks of the product

with the temperature raised from room temperature to 60 �C

(Fig. 3o–a) revealed the crystallinity degradation and gradual

dissolution of Mg7B4O13$7H2O. With temperature going up to

100 �C, irregular plate-like Mg7B4O13$7H2O further dissolved

and recrystallized,63 leading to a new phase of MgBO2(OH) with

a locally rolled-up surface and rudimental 1D morphology

(Fig. 2b1 and 3b). Besides, adjacent newly formed 1D nano-

particles could further coalesce both on sides and ends, resulting

in aggregation of MgBO2(OH) with relatively sharp ends, wide

middle, certain aspect ratio and also leaf-like profile (Fig. 2b2 and

3b). With temperature going up within 100–120 �C, the yield of

MgBO2(OH) also increased gradually at the cost of

Mg7B4O13$7H2O (Fig. 3c–d), and the phase conversion was

approximately accomplished at 130 �C (Fig. 3e). With tempera-

ture going up to 140 �C, irregular plate-like Mg7B4O13$7H2O

disappeared thoroughly and the hydrothermal product consisted

of either dispersive 1D MgBO2(OH) nanoflakes with a rough

surface, relatively wide diameter and low aspect ratio or

congregated 1D MgBO2(OH) nanoparticles with a curved

surface, relatively thin diameter and high aspect ratio (Fig. 2c

and 3f). With temperature going further up to 140–200 �C, more

and more 1D nanoflakes or nanoleaves of MgBO2(OH) appeared

and the surfaces became smoother, whereas congregated 1D

MgBO2(OH) nanoparticles decreased (Fig. 2d–f, Fig. 3g–i). With

temperature going up to 220 �C, 1D MgBO2(OH) nanoflakes

displayed a general head-to-head overlapped growth phenom-

enon, leading to multitude of lotus root-like 1D MgBO2(OH)

nanostructures with a relatively high aspect ratio and local

accidented surfaces. Additionally, congregated 1D MgBO2(OH)

nanoparticles significantly decreased (Fig. 2g and 3j). When

temperature was as high as 240 �C, 1D MgBO2(OH) nano-

particles tended to be smooth on the surfaces, with fewer head to

head overlaps and local lotus root-lke phenomena observed

(Fig.2h, Fig.3k).

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The pH value of the hydrothermal solution and the yield of the

solid phase obtained at various hydrothermal temperature

moments were also monitored, as shown in Fig. 4. With the

temperature going up gradually, pH value dropped rapidly from

11.8 (room temperature) to 9.3 (100 �C), then to 8.8 (130 �C), and

This journal is ª The Royal Society of Chemistry 2009

Fig. 3 XRD patterns of the precipitate at room temperature (o) and

hydrothermal products obtained at various temperature moments in the

course of the heating procedure (a–k). Temperature (�C): (a) 60, (b) 100,

(c) 110, (d) 120, (e) 130, (f) 140, (g) 160, (h) 180, (i) 200, (j) 220, (k) 240;

crystalline phase: (o)–(a) Mg7B4O13$7H2O (*), (b)–(d) MgBO2(OH) +

Mg7B4O13$7H2O, (e)–(k) MgBO2(OH) (A).

Fig. 4 The pH value of the hydrothermal solution and yield of the solid

phase obtained at various hydrothermal temperature moments.

Fig. 5 Morphology evolution of the hydrothermal products obtained

after the hydrothermal treatment at various temperatures for 6 h.

Temperature (�C): (a) 60, (b) 80, (c) 100, (d) 120, (e) 140, (f) 160, (g) 180,

(h) 220.

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subsequently dropped slowly and finally kept approximately

stable around 8.2 when the temperature was higher than 180 �C.

Increasing the temperature favored the ionization of the weak

acid H3BO3 (eqn (1)), resulting in more B(OH)4�(aq.) and further

accelerating the reaction trend of the coprecipitation (eqn (4))

and hydrothermal conversion (eqn (5)) which finally led to the

decrease in the pH of the hydrothermal solution with the gradual

consumption of NaOH. At the same time, the yield of the solid

phase rapidly increased from 26.3% (room temperature) to

95.1% (130 �C), then to 97.6% (140 �C), and then fluctuated

around 98%, probably due to the dissolution–recrystallization of

MgBO2(OH) within 140–240 �C. The remarkable changes of pH

of the hydrothermal solution and yield of the solid phase within

120–130 �C were attributed to the accomplishment of conversion

from Mg7B4O13$7H2O to MgBO2(OH) within the same

temperature range (Fig. 3d–e).

This journal is ª The Royal Society of Chemistry 2009

In addition, the morphology/crystalline phase evolution of the

hydrothermal products obtained at various temperatures for 6 h

were also tracked, as shown in Fig. 5 and 6, respectively. In

contrast with that obtained at 60 �C moment (Fig. 2a), the

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product acquired at 60 �C for 6 h mainly consisted of irregular

plate-like Mg7B4O13$7H2O, coexisting with a few 1D nanoflakes

joined by irregular plates (Fig. 5a and 6a). When treated at 80 �C

for 6 h, the irregular plate-like Mg7B4O13$7H2O almost dis-

appeared and the product consisted of MgBO2(OH) nano-

particles with a tiny diameter, rolled up trend and rudimental 1D

morphology, though poor crystallinity (Fig. 5b and 6b). When

treated at 100 �C for 6 h, more 1D MgBO2(OH) nanoflakes were

formed with improved crystallinity, and a comparatively

remarkable particle–particle attachment phenomenon had

emerged (Fig. 5c and 6c). When treated at 120 �C for 6 h, the

product comprised MgBO2(OH) nanoflakes with more defined

1D morphology, broad middle, partial rolled up fringes, rela-

tively rough surface and improved crystallinity (Fig. 5d and 6d).

The rolled-up MgBO2(OH) nanostructures were also observed

by Liu et al. at 120 �C.63 When treated at 140 �C for 6 h,

a multitude of 1D MgBO2(OH) nanoflakes with head to head

overlapped and lotus root-like morphology have emerged

(Fig. 5e and 6e), and the head to head overlapped phenomenon

became more distinct for the product treated at 160 �C for 6 h,

leading to the general existence of the lotus root-like

MgBO2(OH) nanostructures (Fig. 5f and 6f). It is worth noting

that the specific growth phenomenon existed for the product

treated at 140–160 �C for 6 h was quite similar to that observed at

the moment of 220 �C in the course of the heating procedure

(Fig. 2g). Also similar to the morphology of the product obtained

at the moment of 240 �C (Fig. 2h), the 1D MgBO2(OH) nano-

particles tended to be smooth on the surfaces and uniform in the

diameter along the axial direction when treated at 180 �C for 6 h,

with the lotus root-like morphology disappearing (Fig. 5g and

6g). Strikingly, the MgBO2(OH) nanowhiskers with uniform

morphology, smooth surfaces, uniform diameter along the axial

direction, relatively high aspect ratio, and significantly improved

crystallinity were obtained after a hydrothermal treatment of the

precursor slurry at 220 �C for 6 h (Fig. 5h and 6h).

Fig. 6 XRD patterns of the precipitate (o) and hydrothermal products

obtained after the hydrothermal treatment at various temperatures for 6

h (a–h). Temperatures (�C): (a) 60, (b) 80, (c) 100, (d) 120, (e) 140, (f) 160,

(g) 180, (h) 220. Crystalline phase: (o)–(a) Mg7B4O13$7H2O (*), (b)–(d)

MgBO2(OH) + Mg7B4O13$7H2O, (e)–(k) MgBO2(OH) (A).

1914 | CrystEngComm, 2009, 11, 1910–1919

3.4. Microstructure analysis

Since the hydrothermal formation of the MgBO2(OH) nano-

whiskers unavoidably experienced the transitional 1D

MgBO2(OH) nanoflakes with relatively broad middle and sharp

ends, an embedded characterization of the MgBO2(OH) nano-

flake was crucial to full understanding of the hydrothermal

formation of the MgBO2(OH) nanowhiskers. Fig. 7 shows the

TEM and corresponding HRTEM images of the typical

MgBO2(OH) nanoflakes. The nanoflake seemed to be assembled

by thinner 1D flake-like building blocks with an obvious

different contrast due to their difference in thickness (Fig. 7a).

The HRTEM image (Fig. 7b) corresponding to the dashed

rectangular region I in the nanoflake indicated that there existed

Fig. 7 TEM images (a), (d) and the corresponding HRTEM images (b),

(c), (e) showing the head-to-head and side-by-side oriented growth of the

MgBO2(OH) nanoflakes obtained at 200 �C for 12 h.

This journal is ª The Royal Society of Chemistry 2009

Fig. 8 TEM images (a), (c) and the corresponding HRTEM images (b),

(d) revealing the head-to-head oriented growth of the MgBO2(OH)

nanowhiskers obtained at 240 �C for 30 h.

Fig. 9 TEM image (a) and the corresponding HRTEM images (b)–(d)

demonstrating the side-by-side oriented growth of the MgBO2(OH)

nanowhiskers obtained at 240 �C for 30 h.

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discontinuity and dislocation in the legible lattice fringes across

the dashed rectangular region III approximately along the axis of

the nanoflake, and series of such breaking off of the lattice fringes

led to the thickness fringes in a direction approximately

perpendicular to the axis of the nanoflake, showing the variation

of the thickness of the nanoflake in a direction parallel to its axis

(defined as 1st-type thickness fringes hereafter) due to the head to

head overlapped growth in the hydrothermal process. Mean-

while, there were also remarkable thickness fringes across the

dotted rectangular region IV approximately along the axis of the

nanoflake, showing the variation of the thickness of the nano-

flake in a direction normal to its axis (defined as 2nd-type

thickness fringes hereafter) owing to the side by side oriented

growth in the hydrothermal treatment. The HRTEM image

(Fig. 7c) corresponding to the dashed rectangular region II

clearly showed the explicit lattice fringes on the left side and also

the distinct 1st-type thickness fringes (indicated by dashdotted

lines) across the whole nanoflake in the dashed rectangular

region V. On the other hand, several 2nd-type thickness fringes

were also detected in the dotted rectangular region VI. The head

to head overlapped and side by side oriented growth were

particularly distinct in the diameter changing region VII of the

nanoflake (Fig. 7d), and the desultory and ladder-like lattice

fringes detected from the gradually thinned section (Fig. 7e)

seemed quite like the terraces tiled on the hill, indicated by the

dashed rectangular regions VIII–XI containing 1st-type thick-

ness fringes. Besides, the interplanar spacings of 0.598 nm

detected from the legible lattice fringes in various sections of the

nanoflakes (Fig. 7b, c, e) were quite similar to those of the (200)

planes of the standard monoclinic MgBO2(OH) (PDF No.39–

1370), revealing the preferential growth direction of the nano-

flakes parallel to the (200) planes, in accordance with that of the

MgBO2(OH) nanowhiskers along the c axis65 and also the growth

habit of the natural szaibelyite (MgBO2(OH)).67 Moreover, the

existing (200) planes detected from the dashed rectangular

regions as IV and VI containing 2nd-type thickness fringes also

demonstrated that the nanoflake was formerly grown by the side-

by-side oriented attachment of the corresponding flake-like

building blocks along the (200) planes.

Similar head-to-head overlapped and side-by-side oriented

attachment growth phenomena were also captured in the

hydrothermally synthesized MgBO2(OH) nanowhiskers

obtained at 240 �C for 30 h (diameter: 20–60 nm; aspect ratio:

10–70),65 as shown in Fig. 8 and 9, respectively. As a result of the

hydrothermal treatment at a relatively high temperature for

a long time, the 1st-type thickness fringes were more likely

observed in an individual MgBO2(OH) nanowhisker. The

explicit thickness fringes (indicated by the dash-dotted lines) on

the both sides of the dashed rectangular region denoted on the

straight nanowhisker (Fig. 8a–b) or curved one (Fig. 8c–d),

especially the concavo-convex surface of the curving section on

the curved nanowhisker (Fig. 8d) with a formed obtuse angle

(approximately 169 �) definitely revealed the previously occurred

head-to-head overlapped growth of the individuals I and II.

Taking the two brims, i.e. top and bottom, existed on each end of

the nanowhisker into consideration, the variation of the number

of the observed 1st-type thickness fringes on the joint section

may be ascribed to the difference of the angles between the

electron beams and the surface of the nanowhisker employed for

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Fig. 10 Variation of the average diameter, average length (a) and cube

of average length (b) of the MgBO2(OH) nanowhiskers obtained at 240�C with the hydrothermal time.

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the HRTEM observation. In contrast to the head-to-head

overlapped growth phenomenon captured in the individual

nanowhisker, the side-by-side oriented attachment growth

phenomenon was often observed in the profile attached or

bundled nanowhiskers (Fig. 9). The selected so-called nano-

whisker (Fig. 9a) was grown from three adjacent thinner nano-

whiskers, I, II and III. The HRTEM image corresponding to the

upside of the nanowhisker (Fig. 9b) showed that the thinner

nanowhiskers I and II had almost coalesced with each other, and

there existed a distinct gap between the thinner nanowhisker III

and the other two. The HRTEM image corresponding to the

middle of the nanowhisker (Fig. 9c) indicated that the thinner

nanowhiskers I and II had completely coalesced with one

another, meanwhile the gap between the thinner nanowhisker III

and the other two became narrower, showing the approaching

trend of the thinner nanowhisker III to the thinner nanowhisker I

and II. The HRTEM image corresponding to the downside of the

nanowhisker (Fig. 9d) demonstrated that the gap had dis-

appeared and the adjacent three thinner nanowhiskers had ulti-

mately coalesced together, leading to another thicker

nanowhisker. The legible lattice fringes parallel to the axis of the

nanowhiskers with the detected interplanar spacings of 0.598 nm

(Fig. 9b–d) reconfirmed the side-by-side oriented attachment

growth of the nanowhiskers along the (200) planes. It was worth

noting that, there still existed 2nd-type thickness fringes along

the axis of the newly formed nanowhisker (Fig. 9b,d) although

the adjacent thinner nanowhiskers finally coalesced into a single

one.

3.5. 1D Growth of the MgBO2(OH) nanowhiskers

Although the side-by-side oriented attachment growth was found

for the MgBO2(OH) nanowhiskers obtained at 240 �C for 30 h, in

more cases the 1D growth of the MgBO2(OH) nanowhiskers at

high temperature as 240 �C was dominantly controlled by some

other mechanism rather than oriented attachment. The influence

of the reaction time on the morphology of the nanowhiskers at

240 �C65 showed that tiny floccules occurred at the initial stage of

the hydrothermal treatment, which became thicker and longer

after 2.0 h of reaction and finally led to the uniform nano-

whiskers with a smooth surface and a larger size after the

prolongation of the reaction time up to 6.0–30.0 h. Corre-

spondingly, the average diameter and also average length of the

MgBO2(OH) nanowhiskers increased in an approximately

parabola mode with the hydrothermal time at 240 �C (Fig. 10a).

At the same time, the cube of the average length increased almost

linearly (Fig. 10b) with the hydrothermal time bearing a corre-

lation coefficient of 0.99213, higher than 0.98265 and 0.98349

correlated from the relation of the square and biquadratic of the

average length with the hydrothermal time, respectively.66 Thus

the variation of the length, or in other words, the 1D growth of

the MgBO2(OH) nanowhiskers at 240 �C complied with the

traditional Ostwald ripening mechanism.23–25,69 Taking �L (mm)

and �L0 (mm) as the average length of the MgBO2(OH) nano-

whiskers ripened at 240 �C for t h and 0 h, respectively, the 1D

growth of the MgBO2(OH) nanowhiskers at 240 �C could be

expressed as:

�L3 � �L03 ¼ kt (6)

1916 | CrystEngComm, 2009, 11, 1910–1919

where k was the rate constant for ripening, mm3 h�1. Accord-

ingly, the 1D growth of the MgBO2(OH) nanowhiskers at 240 �C

could be correlated as:

�L3 � (0.389)3 ¼ 0.117t (7)

Correspondingly, the correlated average length of the nano-

whiskers at the initial stage of the ripening was 0.389 mm, in good

agreement with that obtained by the experiment (Fig. 10a). Since

the morphology evolution and 1D growth of the MgBO2(OH)

nanowhiskers at 240 �C revealed the dominant Ostwald ripening

mechanism existed at the high temperature hydrothermal

formation of the MgBO2(OH) nanowhiskers, the captured head-

to-head overlapped, as well as side-by-side oriented attachment

growth (Fig. 8–9) should be the remnant evidence of the early

hydrothermal growth in the heating procedure rather than the

sole straightforward result of the late high temperature hydro-

thermal treatment, considering the similar growth phenomena

existed in the heating process (Fig. 2 and 4). The head-to-head

overlapped and side-by-side oriented attachment growth could

bring on the crystallinity degradation of the MgBO2(OH)

nanowhiskers on the overlapped and attached section, similar to

the crystal defects such as stacking faults and dislocations

generally resulted in the oriented attachment growth,29,30 which

however could be improved by Ostwald ripening at high

temperature hydrothermal treatment.

4. Discussion

4.1. Formation mechanisms of the MgBO2(OH) nanowhiskers

Thus, the overall hydrothermal formation of the MgBO2(OH)

nanowhiskers could be depicted on the basis of the above anal-

ysis, as shown in Fig. 11, with three temperature–time regions

successively dominated by various mechanisms. Section I: from

amorphous irregular Mg7B4O13$7H2O nanoparticles to rudi-

mental 1D MgBO2(OH) with poor crystallinity and curved

surface at relatively low temperature (#120 �C), mainly

controlled by rolling up mechanism. The amorphous

This journal is ª The Royal Society of Chemistry 2009

Fig. 11 Hydrothermal formation of the MgBO2(OH) nanowhiskers

successively controlled via rolling up, oriented attachment, and Ostwald

ripening.

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Mg7B4O13$7H2O nanoparticles gradually and locally dissolved

into irregular plate-like Mg7B4O13$7H2O of poor crystallinity

with the increase in temperature; and with the gradual successive

increase in temperature, the plate-like Mg7B4O13$7H2O recrys-

tallized and began the phase conversion and partial rolling up,

leading to MgBO2(OH) with rudimental 1D morphology and

curved surface. During the phase conversion and original 1D

morphology formation of MgBO2(OH), the special chain-like

structure units existed in the bulk crystal structure of szaibelyite

(Fig. 12) should be considered. The structure consists of distorted

Mg–O octahedra, which share edges to form a chain with two

octahedra in width parallel to the c axis (Fig. 12a). Two such

nonequivalent chains, i.e. A chain and B chain, share corners to

form a sheet parallel to (200) planes (Fig. 12b), and the sheets are

further held together by the pyroborate ions [B2O4(OH)]3�.67 The

specific anisotropic crystal structure was believed to be respon-

sible for the early rolling up and further formation of the original

1D MgBO2(OH) nanostructures. Most recently, the rolling up

phenomenon has also been observed in the hydrothermal

formation of MgBO2(OH) nanobelts at 120–180 �C,63 quite

Fig. 12 Crystal structure of szaibelyite (a) containing sheets formed by

nonequivalent A chain and B chain (b), projection along the c axis.

This journal is ª The Royal Society of Chemistry 2009

similar to those occurring in the present work (Fig. 5b–d). As

a matter of fact, the anisotropic crystal structure induced 1D

growth has also been found for many other 1D nanostructures,

such as Se nanowires68 and Te nanowires and nanotubes.69

However, the attempt to acquire szaibelyite nanotubes was failed

in this system, which ultimately led to the stable MgBO2(OH)

nanowhiskers on the present conditions, probably due to the

failure of the thermal energy to overcome the potential energy

barrier associated with the induced strain by rolling up

MgBO2(OH) in present cases.63

Section II: from MgBO2(OH) nanostructures with rudimental

1D morphology to MgBO2(OH) 1D nanostructures with a con-

cavo-convex surface and lotus root-like morphology within 120–

220 �C, dominantly controlled by oriented attachment. The

rudimental 1D MgBO2(OH) nanostructures began the prevalent

head-to-head overlapped and side-by-side oriented growth with

an increase in temperature and length of time, resulting in lotus

root-like MgBO2(OH) nanostructures with a concavo-convex

surface and also flake-like MgBO2(OH) nanostructures with

relatively broad middle and sharp ends. Although stress/strain

might cause contrast fringes due to the thin thickness of 1D

nanostructures such as nanobelts in some cases,70 the present

fringes in HRTEM observation (Fig. 7b–c,e and Fig. 8b,d) were

believed to be thickness fringes rather than stress/strain caused

contrast fringes, which indicated the thickness change caused by

the head-to-head coalesced growth, considering the formerly

existing particle–particle attachment phenomenon (Fig. 5c) and

further grown head-to-head overlapped and lotus root-like

nanostructures (Fig. 2g and 5e–f).

Section III: from lotus root-like MgBO2(OH) nanostructures

to MgBO2(OH) nanowhiskers with uniform 1D morphology,

smooth surface, and high aspect ratio at relatively high temper-

ature as 240 �C, dominantly controlled by Ostwald ripening. The

1st-type thickness fringes (approximately perpendicular to the

longitudinal axis) or 2nd-type thickness fringes (approximately

along the longitudinal axis) of the MgBO2(OH) nanowhiskers

owing to the previously formed head-to-head or side-by-side

oriented attachment growth at section II, respectively, were

observed, bringing on the well crystallized MgBO2(OH) nano-

whiskers with well defined facets at the ends and also uniform

diameter along the axis of the nanowhiskers.

Consequently, the hydrothermal formation of the

MgBO2(OH) nanowhiskers experienced a procedure successively

controlled via rolling up, oriented attachment and Ostwald

ripening, respectively. However, it did not mean that the three

temperature–time regions were entirely different, and the char-

acteristics of each section depended on the corresponding

dominant growth mechanism. For instance, rolling up domi-

nated the crystal growth at the early heating stage (i.e. section I)

whereas sporadic leaf-like aggregation (oriented attachment) and

thin 1D nanoflakes (Ostwald ripening) also existed (Fig. 2a–b);

oriented attachment dominated the medium-term crystal growth

(section II) nevertheless Ostwald ripening might affect the

formation of each segment of the lotus root-like nanostructures;

Ostwald ripening dominated the late hydrothermal coarsening

(section III) however the growth phenomenon at high tempera-

ture for a long time (Fig.9) and especially the formation of the

broad leaf-like MgBO2(OH) nanostructures under a high filling

ratio (e.g. 80%) and high temperature (e.g. 260 �C)66 revealed the

CrystEngComm, 2009, 11, 1910–1919 | 1917

Fig. 13 Extrapolation and validation of the hydrothermal formation of the MgBO2(OH) nanowhiskers. (a): 100 �C/30 h; (b): 100 �C/168 h; (c): 140 �C/

30 h; (d): 140 �C/168 h; (e): 100 �C–30 �C–240 �C/18 h; (f): 220 �C–30 �C–240 �C/18 h.

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existence of the oriented attachment growth within the Ostwald

ripening dominated section. Undoubtedly, the head-to-head and

side-by-side oriented attachment growth at a medium-term stage

bridged the rolling up dominated early growth and Ostwald

ripening controlled late coarsening. The higher the hydrothermal

temperature reached, the earlier the oriented attachment growth

terminated and also the more significant the dominant effect of

the Ostwald ripening was. This phenomenon was analogous to

that existing in the hydrothermal growth of ZnS nano-

crystals,27,48 which were successively controlled via oriented

attachment and Ostwald ripening mechanism, respectively.

4.2. Further identification of the formation mechanisms

The hydrothermal formation of MgBO2(OH) nanowhiskers was

further identified by extensive experiments. Compared to that

obtained at 100 �C for 6 h (Fig. 5c), the hydrothermal product

acquired at 100 �C for 30 h with the other conditions unchanged

(Fig. 13a) exhibited higher crystallinity and more remarkable 1D

morphology accompanied by a more particle–particle attached

phenomenon, and the 1D morphology of the hydrothermal

product became more significant with the time prolonged to 168

h (Fig. 13b). Analogous head-to-head overlapped growth

phenomenon and lotus root-like 1D nanostructures with a bigger

size of most attached individuals and improved crystallinity were

also observed for the hydrothermal products formed at 140 �C

for 30 h (Fig. 13c) and 168 h (Fig. 13d), in contrast to that

acquired at 140 �C for 6 h (Fig. 5e). Both of the hydrothermal

growth phenomena at 100–140 �C for 30–168 h confirmed the

head-to-head overlapped and side-by-side oriented attachment

growth of MgBO2(OH) existing even under an extrapolated

1918 | CrystEngComm, 2009, 11, 1910–1919

conditions, i.e. relatively low temperature and long time.

Particularly, either the 1D MgBO2(OH) obtained after the

hydrothermal treatment at 240 �C for 18 h with the system

previously ceased at 100 �C and cooled down to room temper-

ature in the course of the heating procedure and thereafter

restarted the heating (Fig. 13e), or those acquired after the

hydrothermal treatment at the same temperature for the same

time whereas with the system previously ceased at 220 �C and

kept other conditions unchanged (Fig. 13f) indicated that the

MgBO2(OH) nanoparticles with poor 1D morphology

(Fig. 2b,g) could continue their hydrothermal growth via

restarting the heating procedure to form ultimate MgBO2(OH)

nanowhiskers with uniform 1D morphology (Fig. 13e–f) and

a higher aspect ratio at a relatively high temperature (e.g. 240 �C)

for a relatively long time (18 h) and therefore undoubtedly

reconfirmed the reliability of the growth mechanism presented in

Fig. 11.

5. Conclusions

Based on the hydrothermal temperature–time phase diagram, the

evolution of the morphology/crystalline phase, and also

the microstructure analysis of the hydrothermal products, the

hydrothermal formation mechanism of szaibelyite MgBO2(OH)

nanowhiskers (diameter: 20–60 nm; aspect ratio: 10–70) from

amorphous precursor coprecipitating at room temperature was

investigated in this paper. It successively experienced three

various stages dominated by rolling up at early heating stage,

head-to-head overlapped and side-by-side oriented attachment

at medium-term crystal growth and Ostwald ripening at late

hydrothermal coarsening, respectively. The various mechanisms

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successively controlling 1D growth phenomena may also be

existent in the hydrothermal formation of other 1D nano-

structures (especially those with a highly anisotropic structure)

from amorphous precursors obtained at room temperature in

absence of any surfactants/capping reagents or templates, and is

thus helpful for understanding and further intensifying the

controllable hydrothermal synthesis of 1D nanostructured

materials.

Acknowledgements

This work is supported by the National Natural Science Foun-

dation of China (NSFC; No. 50574051, 50874066). The authors

appreciate the constructive help and discussions from Prof. Xun

Wang, Department of Chemistry, Dr. Qiang Zhang and Xiaobo

Wei, Department of Chemical Engineering, Tsinghua Univer-

sity, and also the constructive suggestions from the reviewers.

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