Fabrication of Zn(OH)2/ZnO nanosheet-ZnO nanoarray hybrid structured films by a dissolution-recrystallization Route

Article abstract of DOI:10.1111/j.1551-2916.2009.03502.x

Zn(OH)₂/ZnO nanosheet-ZnO nanoarray hybrid structured films were fabricated on F-doped SnO₂-coated glass by a dissolution- recrystallization route without any additives. The as-deposited c-axis-oriented ZnO nanowhisker films were soaked in hot distilled water for various periods of time. X-ray diffraction, field-emission scanning electron microscopy, and transmission electron microscopy clarified that the dissolution of ZnO induced Zn(OH)₂ to grow on the surface of the ZnO nanowhiskers film by heterogeneous nucleation and growth. The morphology of the Zn(OH)₂ nanosheets was found to be controllable by the dissolution-recrystallization of as-deposited ZnO nanowhiskers in hot distilled water. ZnO with an almost identical morphology could also be produced by a dehydration reaction. Given the highly porous nature and the strong adsorption properties of the as-fabricated hybrid structured films, it is expected that they will have a variety of potential applications including dye-sensitized gas sensors and solar cells, and photocatalysis systems.

Full text of DOI:10.1111/j.1551-2916.2009.03502.x

J. Am. Ceram. Soc., 93 [3] 881–886 (2010)
DOI: 10.1111/j.1551-2916.2009.03502.x
r 2009 The American Ceramic Society
ournal
J
Fabrication of Zn(OH)2/ZnO Nanosheet-ZnO Nanoarray Hybrid
Structured Films by a Dissolution–Recrystallization Route
Xiulan Hu,*,w Yoshitake Masuda,* Tatsuki Ohji,** and Kazumi Kato*
Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST),
2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 436-8560, Japan
Zn(OH)2/ZnO nanosheet-ZnO nanoarray hybrid structured
films were fabricated on F-doped SnO2-coated glass by a disso-
lution–recrystallization route without any additives. The as-de-
posited c-axis-oriented ZnO nanowhisker films were soaked in
hot distilled water for various periods of time. X-ray diffraction,
field-emission scanning electron microscopy, and transmission
electron microscopy clarified that the dissolution of ZnO induced
Zn(OH)2 to grow on the surface of the ZnO nanowhiskers film
by heterogeneous nucleation and growth. The morphology of the
Zn(OH)2 nanosheets was found to be controllable by the disso-
lution–recrystallization of as-deposited ZnO nanowhiskers in hot
distilled water. ZnO with an almost identical morphology could
also be produced by a dehydration reaction. Given the highly
porous nature and the strong adsorption properties of the as-
fabricated hybrid structured films, it is expected that they will
have a variety of potential applications including dye-sensitized
gas sensors and solar cells, and photocatalysis systems.
hierarchical and complex ZnO micro/nanoarchitectures have
stimulated much attention because such architectures combine
the features of micrometer- and nanometer-scale building blocks
and exhibit unique properties not found in mono-morphological
structures.13,14 ZnO nanowire-layered basic zinc acetate (LBZA)/
ZnO nanoparticle composite films with different nanoparticle
content, which were synthesized using a simple wet-chemical
route, have been shown to be promising photoanodes for use in
DSSCs.15 In addition, it has been reported that large arrays of
oriented ZnO nanorods with controlled aspect ratios, complex
film morphologies made up of oriented nanocolumns, and nano-
plates and complex bilayers could be prepared using controlled
seeded growth with citrate anions that selectively adsorb on
ZnO basal planes as the structure-directing agent.12 Gao et al.16
I. Introduction
HE physical and technical limitations of the current lithogra-
phy-based electronic and photonic devices have motivated
T
intensive research on nanostructure systems for use as building
blocks or basic units of device structures.1 Zinc oxide (ZnO) is
used in a wide range of functional devices, nanostructure va-
ristors, UV absorbers, gas sensors, dye-sensitized solar cells
(DSSCs), and industrial additives due to its wide band gap
(3.37 eV) and large exciton binding energy (60 meV).2,3 Proto-
type devices based on one-dimensional (1D) ZnO nanoarrays
oriented along the c-axis, such as gas sensors and solar cells, have
attracted considerable interest.4–11 The properties of ZnO depend
closely on its nanostructure including the crystal size, orientation
and morphology, aspect ratio, and even the crystalline density.
Surface area and morphology also play a crucial role in many
applications such as photo-emitters, varistors, sensor, catalysts,
and solar cells.12 Presently, low-temperature solution deposition,
which offers the advantages of simplicity, reproducibility, cost
effectiveness, and suitability for producing large-area thin films, is
attracting considerable attention. Well-aligned 1D ZnO nano-
structures have been successfully synthesized on various sub-
strates using
a
low-temperature (o1001C) and low-cost
chemical bath deposition technique to produce vertical 1D nano-
structures for use in gas sensors and DSSCs, which provide direct
electron-conducting channels to the electrodes. However, enrich-
ment of the surface region of the 1D nanostructures is necessary
to further enhance the performance of such devices. Recently,
I. Lloyd—contributing editor
Manuscript No. 26442. Received July 8, 2009; approved October 19, 2009
Supported by METI, Japan, as part of the R&D for high-sensitivity environmental
sensor components.
*Member, The American Ceramic Society.
Fig. 1. X-ray diffraction patterns (a) and field-emission scanning elec-
tron microscopic image (b) of a ZnO film following deposition in an
aqueous solution.
wAuthor to whom correspondence should be addressed. e-mail: xiulan-hu@aist.go.jp,
whoxiulan@gmail.com
**Fellow, The American Ceramic Society.
881

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Journal of the American Ceramic Society—Hu et al.
Vol. 93, No. 3
Fig. 2. Field-emission scanning electron microscopy images (top view) of films formed in water at 651C for various periods. (a, b) 5 days; (c, d) 25 days;
(e, f) 2 months.
synthesized successfully several unique morphologies of ZnO
from an aqueous solution containing ZnCl2 and a mixture com-
plex agent (amine and ammonia) at about 901C by regulating the
dissolution–recrystallization process during soaking, and pointed
out one can purposely optimize morphology, crystallography,
and organization by kinetically controlling the nucleation and
crystallization processes.
Fibrous zinc hydroxide (Zn(OH)2) strongly adsorbs nega-
tively charged DNA and proteins, and thus has a large potential
for practical applications such as surgical dressings, separation
of proteins, nanofibrous membranes, and a precursor for ZnO in
industry. Nanostrand Zn(OH)2 has already been synthesized
using a dilute aqueous solution of zinc nitrate and aminoetha-
nol.17 Fibrous Zn(OH)2 has also attracted increasing attention
due to its nonhazardous nature and high stability in air and
vacuum compared with toxic cadmium hydroxide nanostrands.
In the present study, Zn(OH)2 nanosheet layers were fabri-
cated on the surface of c-axis-oriented ZnO nanowhisker arrays
by a simple dissolution–recrystallization route in hot distilled
water without any additional raw materials or special catalysts.
The morphology of the Zn(OH)2 nanosheet films can be con-
trolled by varying the dissolution–recrystallization periods of as-
deposited ZnO nanowhiskers in the hot water. Hybrid struc-
tured Zn(OH)2/ZnO–ZnO films consisting of nanoarrays and
nanosheets have a variety of potential applications including
dye-sensitized gas sensors and solar cells, and photocatalysis
systems, due to their unique pore structure and high adsorption
abilities. In this study, the formation mechanism and resulting
microstructure of the nanosheet Zn(OH)2 films were investi-
gated in detail.
II. Experimental Procedure
The starting materials for preparing ZnO nanowhisker films were
zinc acetate dihydrate (Zn(CH3COO)2 ꢀ 2H2O, 99%), zinc nitrate
hexahydrate (Zn(NO3)2 ꢀ 6H2O, 99%), hexamethylenetetramine
(HMT, C6H12N4, 99%), and polyethylenimine (PEI, (C2H5N)n,

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Fabrication of Zn(OH)2/ZnO by Dissolution–Recrystallization Route
883
Fig. 3. Field-emission scanning electron microscopy images (cross-sectional view) of hybrid films. (a, b) 25 days; (c, d) 2 months. (b, d) magnified views
at the top region.
branched mean molecular weight of 600, 99%). All chemicals
(Wako Pure Chemical Industries Ltd., Osaka, Japan) were used
as received without further purification.
A c-oriented ZnO nanowhisker film, as shown in Fig. 1, was
deposited on the F-doped SnO2-coated glass (FTO, 50 mm ꢁ 26
mm ꢁ 1.1 mm, sheet resistivity: 9.5 O/&, Asahi Glass, Tokyo,
Japan) by forced-hydrolysis-initiated nucleation of anhydrous
zinc acetate using a 200 mL aqueous solution of zinc nitrate
hexahydrate, HMT, and PEI at 881C for 1 h.4,18 The formation
processes of ZnO nanowhiskers film were described as follows in
the present study: (1) an anhyrous zinc acetate template
layer was prepared previously on the FTO substrate by spin-
coating using 0.01M zinc acetate dihydrate–anhydrous
ethanol solution followed by a dehydration reaction by ultravi-
olet (UV) or low-temparature drying at 651C; (2) the in situ
formation of ZnO nuclei from the forced hydrolysis of anhy-
drous zinc acetate in the aqueous solution at an initial stage,
and further fabrication of ZnO nanowhiskers film of about
1.2 mm thickness by heterogeneous nucleation and growth on
the substrate.
Subsequently, the as-deposited ZnO nanowhisker films were
soaked in hot distilled water for a desired time without stirring.
Per sheet of FTO substrate coated with ZnO nanowhiskers film
was fixed in 60 g water. Finally, the substrates were drawn from
the water, rinsed with distilled water, and air-dried at 651C in an
oven or subjected to a dehydration reaction at 3501C for 2 h
before characterization.
The products were characterized using X-ray diffraction
(XRD; RINT-2100V, Rigaku, Tokyo, Japan) with CuKa radi-
ation (40 kV, 30 mA) at a scan rate of 21/min, field-emission
scanning electron microscopy (FE-SEM; JSM-6335FM, JEOL
Ltd., Tokyo, Japan) with an accelerating voltage of 5 kV and
emission current of 12 mA, and transmission electron micros-
copy (TEM; Hitachi HF-2000, Hitachi Kyowa Engineering Co.
Ltd., Tokyo, Japan). The chemical compositions were analyzed
by FE-SEM (FE-SEM; S-4300, Hitachi Kyowa Engineering Co.
Ltd.) in conjunction with energy dispersive X-ray (EDX) mea-
surements. The specimen surfaces were analyzed at room tem-
Fig. 4. Energy dispersive X-ray (EDX) spectrum of as-fabricated hy-
brid film (as shown in Fig. 2(e)). The inset is an EDX spectrum of the
bare FTO substrate.

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Journal of the American Ceramic Society—Hu et al.
Vol. 93, No. 3
Fig. 5. (a) Typical transmission electron microscopic image of as-fabricated film (651C, 25 days); (b) and (c) are selected area electron diffraction
patterns of the circled regions in (a).
perature by X-ray photoelectron spectroscopy (ESCA-3400,
Shimadzu Co., Tokyo, Japan) with a monochromatic MgKa
radiation source.
in good agreement with the JCPDS card (36-1451) for a typical
wurtzite-type ZnO crystal (hexagonal, P63mc). The significantly
higher intensity of the 0002 diffraction peak indicated that the
films were preferentially oriented along the c-axis direction. Fur-
thermore, 1011, 1012, 1013 diffraction peaks were detected.
Figure 1(b) shows well-orientated ZnO nanowhiskers (observed
from the top view), where it can be seen that each nanowhisker
has a smooth surface and pointed tip.
Figure 2 shows top view FE-SEM images of films obtained by
hot water soaking at 651C for different periods. Following 5
days of soaking, the ZnO nanowhiskers were still isolated as in
the as-deposited film, although small localized nanosheets had
formed on their surface, especially near the tips, resulting in a
rough surface (Figs. 2(a) and (b)). After soaking for 25 days, all
of the ZnO nanowhiskers were covered by wavy nanosheets as
shown in Figs. 2(c) and (d). The nanowhiskers themselves could
not be observed from the top view. When the soaking time
was increased to 2 months, a dense nanosheet layer was formed
on the surface of the ZnO nanowhiskers as shown in Figs. 2(e)
and (f).
ꢀ
ꢀ
ꢀ
III. Results and Discussion
Figure 1 shows an XRD pattern and FE-SEM image of a ZnO
nanowhisker film following deposition on a FTO substrate in an
aqueous solution at 881C for 1 h. All ZnO diffraction peaks are
Figure 3 shows cross-sectional FE-SEM images of the hybrid
structured film. Figs. 3(a) and (b) show the nanosheet formed on
the top part of the ZnO nanowhiskers. Formation of the nano-
sheet caused the tip of the nanowhiskers to disappear and the
surfaces to become rough. These results indicate the dissolution
of the ZnO nanowhiskers and the formation of a nanosheet
phase at the water–ZnO interface. As the soaking time increased
to 2 months, the nanosheet layer became thicker, and the its
morphology became more regular, as shown in Figs. 3(c) and
(d), indicating further dissolution of the nanowhiskers and for-
mation of nanosheet material at the water–ZnO interface. The
nanosheet layer was determined to be Zn(OH)2 by later inves-
tigations. It should be noted that ZnO nanosheets with an al-
most identical morphology to that of Zn(OH)2 could be easily
produced by a dehydration reaction over a wide range of tem-
peratures from 1201 to 4501C.
To further confirm the chemical composition of the nano-
sheets, EDX analysis was carried out. The EDX spectrum
shown in Fig. 4 identifies the elements Zn and O in the as-fab-
ricated hybrid films, in addition to the elements Si, Al, and Sn in
the FTO substrate (Fig. 4, inset).
Figure 5(a) shows a typical cross-sectional TEM image of a
hybrid film fabricated in distilled water at 651C for 25 days. The
film consisted of nanowhiskers (lower part) and nanosheets (up-
per part) growing on the surface of the nanowhiskers. Figure
5(b) shows a selected area electon diffraction (SAED) pattern of
the nanosheet region labeled (b) in Fig. 5(a), which confirms that
the nanosheet is single crystal Zn(OH)2. The SAED pattern in
Fig. 6. X-ray diffraction pattern of as-fabricated hybrid film (651C,
2 months).

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Fabrication of Zn(OH)2/ZnO by Dissolution–Recrystallization Route
885
Fig. 7. X-ray photoelectron spectroscopy data of O 1s peak and its gaussian-resolved component. (a) as-deposited ZnO nanowhiskers film;
(b) as-fabricated Zn(OH)2 nanosheet film.
Fig. 5(c) is from the nanowhisker region labeled (c) in Fig. 5(a),
and indicates that the nanowhisker is a single crystal ZnO with a
hexagonal structure. Thus, these results confirm that the as-
grown nanosheets are composed of crystalline Zn(OH)2.
Figure 6 shows the XRD pattern of a hybrid film formed
in hot distilled water over a period of 2 months. The pattern is
very similar to that of the as-deposited ZnO nanowhisker
film shown in Fig. 1(a). However, the intensity of the 0002 dif-
fraction peak has obviously decreased. No other diffraction
peaks were detected excluding the peaks originating from the
FTO substrate and ZnO. These results suggest that the Zn(OH)2
nanosheets have low crystallinity and are difficult to detect by
XRD.
Figure 7 shows the O 1s peak and its gaussian-resolved com-
ponent for the as-deposited ZnO nanowhisker film and the as-
fabricated Zn(OH)2–ZnO hybrid structured film. The O 1s
core-level spectra of the nanowhisker film can be resolved into
two components by Gaussian fitting, as shown in Fig. 7(a). The
component at the low binding energy (530.5 eV) can be assigned
to O2– ions in the wurtzite structure of a hexagonal Zn21 ion
array. The high binding energy (532.1 eV) peak corresponds
to chemisorbed or dissociated ꢂOH and OHyO types of
surface species, which cannot be easily removed even by high-
temperature annealing.19,20 The O 1s core-level spectra of the
hybrid structured film were resolved into only one component
by Gaussian fitting, as shown in the Fig. 7(b). The much
stronger high-binding energy (532.5 eV) shoulder on the O 1s
peak is probably attributable to OH.21 Thus, these results con-
firm that a nanosheets Zn(OH)2 overlyer was formed on the
surface of ZnO nanowhiskers.
metal ion aqua complex: (i) positively charged metal ion
hydroxocomplexes are the stable forms in acidic solutions; (ii)
negatively charged metal ion hydroxocomplexes are the stable
forms in alkaline solutions; and (iii) uncharged hydroxocom-
plexes tend to be the thermodynamically stable forms in neutral
pH solutions.28 In the present study, because the as-deposited
ZnO nanowhiskers were kept in neutral pH water, the thermo-
dynamic equilibium in Eq. (1) moved to the left so that ther-
modynamically stable Zn(OH)2 was formed by the principle of
least energy. Thus, layered Zn(OH)2(s) grew by heterogeneous
nucleation on the surface of the ZnO nanowhiskers accompa-
nied by the further dissolution of ZnO(s). The Zn(OH)2 was se-
lected nucleation on the ZnO tip and formed as a nanosheet. It
was considered that the polar (0002) crystal plane having high
surface energy easily dissolved and became the heterogeneous
nucleation site for Zn(OH)2 with low thermodynamic free en-
ergies to decrease its surface energy. Thus, Zn(OH)2 sheets were
readily selective for nucleation and growth on the tip at the
solid–liquid interface. It should be noted that, at room temper-
ature, dissolution–recrystallization did not occur for periods of
up to 6 months. However, the dissolution–recrystallization pro-
cess occurred quickly at 901C, as shown in Fig. 8. Following 25
days soaking at this temperature, a large amount of Zn(OH)2
nanosheets grew on the ZnO nanowhiskers, eventually connect-
ing them together. The difference morphologies were deduced to
the increasing solubility of ZnO at high temperature. Because
high solubility resulted in the increasing of Zn cations concen-
tration in the water, thus the growth rate of Zn(OH)2 was pro-
moted. In general, slow growth rate gave regular morphology.
Therefore, it is difficult to obtain fine morphology at a high
temperature of 901C. While regular morphology Zn(OH)2 sheets
were formed easily as shown that at low temperature of 651C
(Fig. 2). These results are in good agreement with the previously
reported solubility behavior of ZnO in aqueous solutions.28–30
As is well known, the most stable crystal structure
of Zn(OH)2 is transparent double pyramids containing
tetrahedrally coordinated zinc ions with an unlayered
lattice. Double-pyramid Zn(OH)2 is easily produced from highly
alkaline zinc salt solutions, and as a ZnO precursor.22
Other layer lattice Zn(OH)2 are not easy to obtain, and have
not yet been well explored structurally.23 The thermodynamic
free energies (DGf1) are ꢂ318.3 and ꢂ553.6 kJ molꢂ1 for ZnO(s)
and Zn(OH)2(s) at 251C, respectively. Thus Zn(OH)2 is
readily formed as a thermodynamically stable material during
the initial stages. At elevated temperatures, the precursor
Zn(OH)2 transforms to ZnO, depending on the degree of
supersaturation and the chemical potential of OH–, by the
IV. Conclusions
Zn(OH)2/ZnO nanosheet-ZnO nanoarray hybrid structured
films were easily fabricated by a simple dissolution–recrystalli-
zation technique in neutral hot distilled water without any ad-
ditives or catalysts. The morphology and thickness of the
Zn(OH)2 could be controlled by the soaking time and water
temperature. As the water temperature increased, both the ZnO
dissolution rate and the Zn(OH)2 growth rate also increased.
The dissolution–recrystallization process did not occur at room
temperature. ZnO nanosheets with almost identical morphology
to that of Zn(OH)2 could be easily produced by a dehydration
reaction above 1201C. Given the highly porous nature and the
strong adsorption properties of the as-fabricated hybrid struc-
tured films, it is expected that they will have a variety of poten-
tial applications including dye-sensitized gas sensors and solar
cells, and photocatalysis systems.
following reaction10,24–26
:
Zn2þ þ 2OHꢂ2ZnðOHÞ2ðsÞ2ZnOðsÞ þ H2O
(1)
The solubility of ZnO is almost the same as that of Zn(OH)2
at 251C from the phase stability diagrams of ZnO(s)–H2O and
Zn(OH)2(s)–H2O as a function of pH.27 As has been reported,
the solution pH (at a particular temperature) is the primary
factor determining the stable hydrolytic state of a solubilized

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