An aminolytic approach toward hierarchical β-Ni(OH)2 nanoporous architectures: A Bimodal forum for photocatalytic and surface-enhanced Raman scattering activity

Article abstract of DOI:10.1021/ic1015065

A surfactantless, trouble-free, and gentle wet chemistry approach has been used to interpret the precisely controlled growth of β-Ni(OH)₂ with the assistance of ammonia and nickel acetate from seedless mild hydrothermal conditions. A thorough investigation of the reaction kinetics and product morphology with varied concentration of NH₃ and different reaction times suggests that a putative mechanism of dissolution, recrystallization, and oriented attachment supports the intelligent self-assembly of nanobuilding blocks. Associated characterizations (FTIR, PXRD, FESEM, EDAX, HRTEM, and Raman) have identified it to be pure β-Ni(OH) ₂ without any signature of contamination. The assembled units result in porous frameworks (nanoflowers and nanocolumns) and are indeed full of communally intersecting nanopetals/nanoplates with both lengths and widths on the order of micrometer to nanometer length scale. The as-synthesized material could also be used as a precursor for nanometric black NiO under calcination. The hydroxide has been found to be a potent and environmentally benign material because it warrants its photocatalytic activity through dye mineralization. Finally, Ni(OH)₂ has been photochemically derivatized with dosages of silver nanoparticles bringing a competent composite authority Ag@Ni(OH) ₂, to give a full-proof enhanced field effect of prolific SERS activity. In a nutshell, these results are encouraging and fetch new promise for the fabrication of a low-cost and high-yielding greener synthetic protocol for a functional material with promising practicability.

Full text of DOI:10.1021/ic1015065

Inorg. Chem. 2010, 49, 8813–8827 8813
DOI: 10.1021/ic1015065
An Aminolytic Approach toward Hierarchical β-Ni(OH)2 Nanoporous
Architectures: A Bimodal Forum for Photocatalytic and Surface-Enhanced
Raman Scattering Activity
Sougata Sarkar,† Mukul Pradhan,† Arun Kumar Sinha,† Mrinmoyee Basu,† Yuichi Negishi,‡ and Tarasankar Pal*,†
†Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India, and
‡Tokyo University of Science, 1-3 Kagurazaka, Shinjyuku-ku, Tokyo 162-8601, Japan
Received May 13, 2010
A surfactantless, trouble-free, and gentle wet chemistry approach has been used to interpret the precisely controlled
growth of β-Ni(OH)2 with the assistance of ammonia and nickel acetate from seedless mild hydrothermal conditions. A
thorough investigation of the reaction kinetics and product morphology with varied concentration of NH3 and different
reaction times suggests that a putative mechanism of dissolution, recrystallization, and oriented attachment supports
the intelligent self-assembly of nanobuilding blocks. Associated characterizations (FTIR, PXRD, FESEM, EDAX,
HRTEM, and Raman) have identified it to be pure β-Ni(OH)2 without any signature of contamination. The assembled
units result in porous frameworks (nanoflowers and nanocolumns) and are indeed full of communally intersecting
nanopetals/nanoplates with both lengths and widths on the order of micrometer to nanometer length scale. The
as-synthesized material could also be used as a precursor for nanometric black NiO under calcination. The hydroxide
has been found to be a potent and environmentally benign material because it warrants its photocatalytic activity
through dye mineralization. Finally, Ni(OH)2 has been photochemically derivatized with dosages of silver nanoparticles
bringing a competent composite authority Ag@Ni(OH)2, to give a full-proof enhanced field effect of prolific SERS
activity. In a nutshell, these results are encouraging and fetch new promise for the fabrication of a low-cost and
high-yielding greener synthetic protocol for a functional material with promising practicability.
Introduction
meet different needs. During the last 2 decades, preferentially
TiO2, ZnO, Fe2O3, Fe3O4, and CeO2 in their nanoregime
3
An exponential growth of research efforts have been made
to realize systematic control over the size, shape, composition,
pattern, and functionality of inorganic nanocrystals owing to
their potential and promising applications in diverse areas.1
Thus, a spectacular variety of inorganic nanocrystals with
tailor-made geometries2 are of profound interest for their
subsequent utilization as building blocks for the fabrication
of hierarchical nanomaterials with complex architectures to
have been paid more care in the midst of oxide semiconduc-
tors because of their structural diversity with innumerable
purposeful involvement with other extensively investigated
chalcogenide (S2-, Se2-, and Te2-) nanostructures. Now-a-
days, owing to their fundamental and technological signifi-
cance, ever more efforts continue toward “soft” nanocrystal
assembly motifs of other transition-metal oxides and hydrox-
ides. Nickel hydroxide [Ni(OH)2], a typical layered double-
hydroxide-type material with two well-identified polymorphs
(R and β), is fetching interest for its intended advanced
exploitation in electrode materials for high-energy-density
batteries, electrochemical supercapacitors, magnetic materials,
etc.4 Ni(OH)2 has also been found to be a suitable solid-
state precursor material for metallic nickel as well as NiO.5
Boosted by these beneficial implementations, an assortment
of nanofabrication techniques have been viewed as successful
*To whom correspondence should be addressed. E-mail: tpal@chem.
iitkgp.ernet.in.
€
(1) (a) Gratzel, M. Nature 2001, 414, 338. (b) Rodríguez-Lorenzo, L.;
ꢀ
ꢀ
Alvarez-Puebla, R. A.; Pastoriza-Santos, I.; Mazzucco, S.; Stephan, O.; Kociak,
ꢀ
M.; Liz-Marzan, L. M.; Abajo, F. J. G. de J. Am. Chem. Soc. 2009, 131, 4616. (c)
Albrecht, T. A.; Sauvage, F.; Bodenez, V.; Tarascon, J.-M.; Poeppelmeier, K. R.
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r
2010 American Chemical Society
Published on Web 09/08/2010
pubs.acs.org/IC

8814 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
strategies for the artwork of an unprecedented diverse spec-
trum of anisotropic Ni(OH)2 superstructures including nano-
belts, nano/microtubes, nanorods, microspheres, nanoflowers,
nanocarnations, oriented nanocolumns, ribbon- and boardlike
nanostructures, and even more.6 Colloidal synthetic and process-
ing methods have been believed to be flexible enough to
allow promising options for large-scale productions of these
superlattice assemblies and hierarchical structures and, of
course, provide a demanding arsenal over sophisticated equip-
ment-based techniques or rigid experimental conditions. A
rational synthetic protocol involving the traditional use of soft
and hard templates has been manifested for a rich variety of
nanocrystalline materials, although they do present challenges
like removal of the templates, etc.7 In fact, hydrothermal-
assisted “one-pot” synthesis has been attempted as a real
solution to these challenges and has expanded the arena of
achieving the hierarchical frameworks of nanostructures. Also,
Ostwald ripening, the Kirkendall effect, dissolution and recrystalli-
zation, oriented attachment, etc., are different mechanisms
familiarized with this tool for the self-assembly of alleviated
three-dimensionally ordered nanostructures. Although the
above literature surveys the emergence of new particle aniso-
tropies, there is still the need for a simple, environmentally
benign, and less-expensive methodology for the clever organiza-
tion of anisotropic building blocks to scale-up the rapid devel-
opment of desired nanostructured materials.
When the NH3 concentration in the reaction media is tuned,
the shape transformation of Ni(OH)2 from porous nano-
flowers to hexagonal nanoplates, with the intermediacy of
truncated triangular nanoplates, has been achieved, and a
probabilistic explanation has been arranged on the basis of the
surface energy to explain this fascinating assembly. Stacking
interactions between the nanopetals and nanoplates are ob-
served to play a key role in the production of the super-
structures, and such assembly-directed crystal engineering to
fabricate novel organic/inorganic superstructures has been
8
€
well reported by Colfen et al. A quantitative idea of the
specific surface area of the porous frameworks has been made
with nitrogen adsorption-desorption isotherm studies. A
notable finding is the formation of nanoholes caused by
dehydration of the hydroxide material during TEM analyses.
Photochemical mineralization of methyl red, an organic
dye, has been made under UV irradiation with nanoflower
morphology, which substantiates the photoactivated catalyt-
ic nature of the semiconductor material, and a comparative
account of its catalytic activity has also been manifested,
in contrast with nanoporous NiO (derived by annealing
Ni(OH)2) and commercial NiO.
Benefitting from the intrinsic porous network, the flower
allows the platform to accommodate a secondary material
like metal nanoparticles, and this typical characteristic lends a
hand to house silver nanoparticles over the nanoframeworks
in our effort. The hybrid material has been characterized with
proper care and, finally, the state-of-the-art efficiency of the
composite material [Ag@Ni(OH)2] lies with their manifesta-
tion as a superior surface-enhanced Raman scattering (SERS)
substrate for suitable nearby molecules. Analyzing the ob-
served spectroscopic results, we suggest the augmented charge
transfer from metal to ligand for the composite when com-
piled with Ni(OH)2.
Inorganic nanomaterials with structural porosity therein
could lead to a wide variety of promising applications such as
gas sensing, and therefore emerging synthetic attempts have
been made to induce porosity within the superstructures
without any assisting templates.
Herein, entirely simple, cost-effective, completely aqueous
solution-based “bottom-up” chemistry has been judged for its
implications in the anisotropic assembly through an ammonia-
prompted hydrolytic route for controllable syntheses of hier-
archical Ni(OH)2 with only the assistance of nickel acetate as
the metal ion precursor. A series of reaction parameters, the
molar ratio of Ni2þ to NH3 and reaction time, upon morpho-
logical and crystallographic phase evolution have been synthet-
ically and systematically studied to rationalize the structural
configuration. Of course, because NH3 has been employed as a
common synthetic reagent for different nanomaterials, the use
of soft templates like surfactants as structure-directing agents
becomes a necessary tradition. Nonetheless, here only NH3
fuels the fabrication of diverse structural analogues. The
surface attachment of NH3 molecules has been authenticated
and their spatial participation in hydrogen-bonding interac-
tion has been supposed to regulate the self-templated assembly
for microscopic construction with nanometric precision.
Thus, to this end, an attempt has been made successfully to
lay out the fabrication of a material with tunable hierarchy
with the assistance of molecular linkage, and the material
with its parallel bimodal activity, photocatalysis, and SERS
characteristics deserves special attention.
Experimental Section
Materials. Nickel acetate [Ni(OCOCH3)2 H2O] and ammonia
3
were purchased from Sisco Research Laboratory, Mumbai, India.
Hexamethylenetetramine (HMT/hexamine; C6H12N4) and pyri-
dine were purchased from Merck, Gurgaon, India. Silver nitrate
was purchased from Aldrich. All glassware was cleaned using
aqua regia, subsequently rinsed with a copious amount of double-
distilled water, and dried well prior to use. Double-distilled water
was used throughout the course of the investigation.
Synthesis. Nanosized two-dimensional (2D) and three-
dimensional (3D) hierarchical Ni(OH)2 nanostructures have been
synthesized by adapting amine-assisted hydrolysis of aqueous
nickel acetate with the aid of a modified hydrothermal (MHT)
protocol.9a In a typical preparation of a β-Ni(OH)2 nanoflower,
(6) (a) Dong, L.; Chu, Y.; Sun, W. Chem.;Eur. J. 2008, 14, 5064. (b) Tao,
F.; Guan, M.; Zhou, Y.; Zhang, L.; Xu, Z.; Chen, J. Cryst. Growth Des. 2008, 8,
2157. (c) Yuan, C.; Zhang, X.; Su, L.; Gao, B.; Shen, L. J. Mater. Chem. 2009,
19, 5772. (d) Ni, X.; Zhang, Y.; Tian, D.; Zheng, H.; Wang, X. J. Cryst. Growth
2007, 306, 418. (e) Xu, L.; Ding, Y.-S.; Chen, C.-H.; Zhao, L.; Rimkus, C.;
Joesten, R.; Suib, S. L. Chem. Mater. 2008, 20, 308. (f) Yang, L.-X.; Zhu, Y. J.;
Tong, H.; Liang, Z. H.; Wang, W. W. Cryst. Growth Des. 2007, 7, 2716. (g) Zhu,
J.; Gui, Z.; Ding, Y.; Wang, Z.; Hu, Y.; Zou, M. J. Phys. Chem. C 2007, 111,
5622. (h) Yang, D.; Wang, R.; He, M.; Zhang, J.; Liu, Z. J. Phys. Chem. B 2005,
109, 7654. (i) Tan, Y.; Srinivasan, S.; Choi, K.-S. J. Am. Chem. Soc. 2005, 127,
3596. (j) Ni, X.; Zhao, Q.; Zhang, Y.; Song, J.; Zheng, H.; Yang, K. Solid State
Sci. 2006, 8, 1312.
€
(8) (a) Wohlrab, S.; Colfen, H.; Antonietti, M. Angew. Chem., Int. Ed.
€
2005, 44, 4087. (b) Jones, F.; Colfen, H.; Antonietti, M. Biomacromolecules
€
2000, 1, 556. (c) Wohlrab, S.; Pinna, N.; Antonietti, M.; Colfen, H. Chem.;Eur.
€
J. 2005, 11, 2903. (d) Yu, S.-H.; Colfen, H.; Antonietti, M. J. Phys. Chem. B
€
2003, 107, 7396. (e) Wang, T.; Colfen, H.; Antonietti, M. J. Am. Chem. Soc.
€
2005, 127, 3246. (f) Colfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44,
5576.
(9) (a) Sinha, A. K.; Jana, S.; Pande, S.; Sarkar, S.; Pradhan, M.; Basu,
M.; Saha, S.; Pal, A.; Pal, T. CrystEngComm 2009, 11, 1210. (b) Creighton,
J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 1979, 75,
790.
(7) (a) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (b)
Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (e)
Orikasa, H.; Karoji, J.; Matsui, K.; Kyotani, T. Dalton Trans. 2007, 3757.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8815
Table 1. Reaction Conditions for the Growth of a Couple of β-Ni(OH)2 Diverse Nanoarchitectures
vol. of nickel acetate final concn of nickel
acetate (mM)
vol. of amine
final concn of
entry (0.1 M) taken (mL)
(NH3) taken (mL) amine (mM) [Ni2þ]/[NH3]
products’ morphology
an unripened basement
porous nanoflower on a discrete and
porous basement
1a
1b
2.9
2.7
97
90
0.1
0.3
500
1500
0.194
0.06
1c
1d
1e
2.5
2.3
2.0
83
76
67
0.5
0.7
1.0
2500
3500
5000
0.03
stacked triangular nanoplates on
a porous basement
regularly stacked quasi-hexagonal
nanoplates on a porous basement
stacked perfectly hexagonal nanoplates
on a porous basement
0.022
0.013
2þ
a precursor solution of the nickel ammine complex [Ni(NH3)6
was prepared by dripping ammonia (0.3 mL) into 2.7 mL of a 0.1
]
bulk NiO] in each reactor. The system was allowed to stand for
15 min in the dark to equilibrate and then was irradiated under
UV light from the top (15 cm from the solution surface). The
radiating source was a 15 W Philips medium-pressure mercury
lamp (λ = 365 nm). The degradation kinetics was monitored
from time to time with UV-visible spectroscopy.
M Ni(OCOCH3)2 H2O aqueous solution under continuous-stir-
3
ring conditions (entry 1b). The resulting blue solution was sealed
into a 15-mL-capacity screw-capped closed glass vial with a
submerged glass slide (1 cm ꢀ 0.5 cm), where the upper edge of
the slide leveled off exactly with the air-water interface and was
subjected to overnight (∼12 h) heat treatment using a tungsten
bulb (100 W) illumination in a closed wooden box. This results in
the subsequent formation of a uniform greenish-white thin film
over the slide. The clear supernatant was decanted off, and the
slide was courteously rinsed with water and dried in an air oven at
50 °C for 6 h. A greenish pastelike precipitate was also separated
out at the bottom from the reaction medium, which was washed
thoroughly with water and dried under a vacuum at 50 °C. Table 1
summarizes the variable reaction conditions of the amminolysis
experiments conducted for the achievement of a couple of diverse
Ni(OH)2 nanoarchitectures.
Normal Raman Scattering (NRS) and Surface-Enhanced Raman
Scattering (SERS) Measurement. Raman spectra of the
samples were obtained with a Renishaw Raman microscope,
equipped with a He-Ne laser excitation source emitting at a
wavelength of 632.8 nm and a Peltier-cooled (-70 °C) charge-
coupled-device camera. A Leica DMLM microscope was at-
tached and fitted with three objectives (5ꢀ, 20ꢀ, and 50ꢀ). For
our experiments, the 20ꢀ objective was used. The laser power at
the sample was 20 mW, and the data acquisition time was 30 s.
The holographic grating (1800 grooves mm-1) and the slit
provided a spectral resolution of 1 cm-1
.
Synthesis of a Silver Colloid. Colloidal silver nanoparticles
were synthesized following the ever-addressed Creighton’s
protocol9b by the reduction of silver nitrate (1 mL, 1 mM) with
sodium borohydride (3 mL, 2 mM) in ice-cold conditions with
vigorous shaking to aid monodispersity.
Analytical Measurements. UV-Visible Spectroscopy. Absorp-
tion spectra were recorded in a Spectrascan UV 2600 spectro-
photometer (Chemito, Mumbai, India), and the solutions were
taken in a 1 cm well-stoppered quartz cuvette.
Fourier Transform Infrared Spectroscopy (FTIR). FTIR spec-
tral characteristics of samples were collected in KBr pellets in
reflectance mode with a Nexus 870 Thermo-Nicolet instrument
coupled with a Thermo-Nicolet Continuum FTIR microscope.
X-ray Diffraction (XRD) Study. XRD patterns of the differ-
ent samples were recorded on a Philips PW-1710 X-ray diffractro-
Synthesis of a Colloidal Ag@Ni(OH)2 Nanocomposite. A
weighed (0.015 g) amount of the as-prepared powder Ni(OH)2
sample (nanoflower; sample no. 1b; Table 1) was well dispersed
in 5 mL of water through sonication followed by the addition of
5 mL of a 50 mM aqueous solution of AgNO3. Reduction of
AgNO3 within the Ni(OH)2 matrix was carried out in a manner of
photocatalytic reduction under UV-light irradiation. After com-
pletion of the reduction, the yellowish-green sol was repetitively
washed with water through centrifugation, and finally the com-
posite material was redispersed in 5 mL of water for further use.
Preparation of Samples for SERS. A total of 0.5 mL of the as-
prepared colloidal silver sol or the composite materials dis-
persed in water was placed in a small glass vial, into which 0.5
mL of a stock solution of 4-MP (with the desired concentration)
was added, and this substrate-probe assembly was incubated
overnight to ensure binding of 4-MP on the nanoparticle’s
surface. A total of 15 μL of the above suspension was dropped
on a microscope slide, and SERS measurements were started as
soon as the solvent evaporated.
˚
meter (40 kV, 20 mA) using Cu KR radiation (λ = 1.5418 A) in
the 2θ range of 10-90° at a scanning rate of 0.5° min-1. The
XRD data were analyzed using JCPDS software.
Field-Emission Scanning Electron Microscopy (FESEM). The
morphology of the samples was analyzed by FESEM using a
Supra 40 (Carl Zeiss Pvt. Ltd.) microscope at an accelerating
voltage of 20 kV. Compositional analysis of the sample was
done by an energy-dispersive X-ray microanalyzer (Oxford ISI
300) attached to the scanning electron microscope.
Transmission Electron Microscopy (TEM). TEM analysis of
the samples was carried out on a Hitachi H-9000 NAR trans-
mission electron microscope, operating at 100 kV. Samples were
prepared by sonication of the powders with alcohol and then
placement of a drop of solution on a carbon-coated copper grid
followed by solvent evaporation in a vacuum.
Results and Discussion
Gas Sorption Measurement. Nitrogen adsorption and desorp-
tion measurements were performed at 77 K using a Quanta-
chrome Autosorb Automated Gas Sorption System utilizing
Brunauer-Emmett-Teller (BET) calculations for the surface
area and Barrett-Joyner-Halenda (BJH) calculations for the
pore-size distribution after the samples were degassed in a
vacuum overnight.
To realize promising high-performance applications, hier-
archical organizations of Ni(OH)2 nanoarchitectures and
their composite materials must meet a wide variety of
requirements in terms of the particle size, size distribution,
shape and morphology, crystallinity, and phase purity. The
different solution-phase synthetic routes assembled in the
past decade noticeably involve the high-temperature-driven
hydro/solvothermal-assisted hydrolysis and condensation of
nickel salts for the fabrication of those Ni(OH)2 nanostruc-
tures with dimensions ranging from a few tens to several
hundreds of nanometers.6 Owing to the moisture or water
Photocatalytic Degradation of Methyl Red. The photocatalyt-
ic degradation of methyl red was performed under an ambient
atmosphere in a reactor equipped with water refrigeration and
magnetic stirring. An aliquot of 40 mL of an aqueous solution of
methyl red (10-4 M) was taken separately in three reactors with
10 mg of catalyst [Ni(OH)2 nanoflower or nanoporous NiO or

8816 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
Scheme 1. Recommended Aminolysis-Condensation Combined Scheme toward the Stepwise Evolution of Ni(OH)2 Nanoarchitectures from the
Crystallites through a Polycondensation Mechanism
sensitivity, commonly observed for metal salts in most cases,
very fast hydrolytic progression at low temperature yields
amorphous products with polydispersity and thus high
temperature (hydrothermal processing or calcinations) is
necessary to bring crystallization.
Therefore, the layout of new preparative protocols could
be a suitable alternative to circumventing the existing prob-
lems encountered in conventional hydrolytic routes.
The present “aminolytic” challenge ensures the control
and tailor making over all of these parameters and properties
simultaneously under a one-pot synthetic approach and thus
extends the pathway of obtaining hierarchical Ni(OH)2
nanostructures with tunable shape and size.
For the present system discussed here, we have put for-
warded a combined aminolysis-condensation mechanism,
as illustrated in Scheme 1. In brief, the nucleophilic attack of
ammonia onto the electrophilic carboxylate carbon (acetate
group) center results in the release of Ni(OH)2 and the
formation of an amide. The metal-center-bound hydroxyl
groups (an essential requirement for the formation of the
Ni-O-Ni bond) then promote the polycondensation reac-
tion with concomitant elimination of water, and the asso-
ciated outcome is the formation of extensive Ni-O-Ni
networks, the key step of crystallization for the nanostruc-
tured building block.
Figure 1a illustrates the crystallographic structural char-
acteristics of the as-synthesized samples investigated by XRD
analysis. All of the reflection peaks could be indexed entirely
to a brucite or C6-type structure (space group: P3m1), with
the hexagonal crystal phase of β-Ni(OH)2 having unit cell of
˚
˚
dimensions a0 = 3.126 A and c0 = 4.605 A containing one
Figure 1. XRD pattern of as-synthesized (a) β-Ni(OH)2 and (b) NiO
using Cu KR radiation (λ = 1.5418 A).
formula unit of Ni(OH)2.10 These peaks at d = 4.59, 2.70,
˚
˚
2.34, 1.75, 1.54, 1.48, 1.35, 1.34, 1.29, and 1.16 A correspond
broadening of the peaks also advocates that the synthe-
sized material is replete with stacking faults.11 No obvious
peaks from other impurities like R-Ni(OH)2 authenticate the
high purity of the synthesized product.12 The representative
to the (001), (100), (101), (102), (110), (111), (200), (103),
(201), and (202) planes and are in good agreement with the
standard values (JCPDS file no. 14-0117), and the broad-
ening of the peaks in the pattern indicates the nanoscale
dimensionality of the crystallites. Again this nonuniform
(11) Ramesh, T. N.; Kamath, P. V.; Shivakumara, C. Acta Crystallogr.
2006, B62, 530.
(12) (a) Jeevanandam, P.; Koltypin, Y.; Gedanken, A. Nano Lett. 2001, 1,
263. (b) Wang, D.; Song, C.; Hu, Z.; Fu, X. J. Phys. Chem. B 2005, 109, 1125.
(10) Cai, F.-S.; Zhang, G.-Y.; Chen, J.; Gou, X.-L.; Liu, H.-K.; Dou, S.-X.
Angew. Chem., Int. Ed. 2004, 43, 4212.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8817
crystal structure presented in Figure 1a (inset in Figure 1a)
consists of stacked layers, with each layer having a hexagonal
arrangement of nickel atoms with octahedral coordination of
the oxygen atoms, and the atoms have the fractional coordi-
nates: nickel 0, 0, 0; oxygen 1/3, 2/3, z and 2/3, 1/3, z, where z =
0.25.13 The intrinsic crystal structure possesses an interlayer
˚
spacing of 4.605 A and a Ni
Ni distance within the layer
3 3 3 3 3
˚
of 3.12 A. After being calcined at 350 °C for 4 h in a tube
furnace under ambient pressure, the synthesized hydroxide
(green) was completely transformed into nickel oxide (NiO,
gray-black), explicitly bearing out the well-crystalline nature
of the thermally treated product (Figure 1b). The crystalline
nature of the calcined sample is well understood owing to the
topotactic conversion of the (001) crystallographic plane for
Ni(OH)2 to the (111) plane for NiO because cubic NiO
affords stacking like that of Ni2þ and O2- layers along the
(111) direction as does Ni(OH)2 along the (001) direction and
also because the d value of the Ni(OH)2 (002) plane agrees
well with that of the NiO (111) plane.
FTIR (Figure 2) was employed as an additional probe to
evince the presence of major functional groups in β-Ni(OH)2
(trace a) and annealed Ni(OH)2 for 4 h at 350 °C (trace b).
The narrow and sharp band for trace a at ∼3645 cm-1 is the
birthmark for the stretching vibrational mode of non-hydro-
gen-bonded, geminal, free hydroxyl groups (νO-H) of the
brucite-like sheet structure, while the broad band at ∼3437
cm-1 for both samples is assigned to the νO-H vibration of
hydrogen-bonded hydroxyl groups located in the interlamel-
lar space of the samples. This feature presents a clear demon-
stration of the complete dehydration of surface-adsorbed
water molecules in the case of the annealed sample and, in
turn, corroborated by the sole presence of the 517 cm-1 (in-
Figure 2. FTIR spectrum of (a) the β-Ni(OH)2 nanoflower and (b) NiO
collected in a KBr pellet.
plane deformation mode of free hydroxyl groups; δO-H
)
band only for trace a. Here it is worth mentioning that
this 517 cm-1 band is exclusive for the β phase because this
band appears at ∼656 cm-1 for the R phase.14 The band at
∼1630 cm-1 is also ascribed to the bending vibrational mode
of surface-adsorbed/trapped (hydrogen-bonded) water mol-
ecules. The weak appearance of 1563 and 1405 cm-1 bands,
indexed to the CdO vibration of the acetate ions (CH3-
COO-), accompanied with very weak signatures from 2937
and 2864 cm-1 bands (-CH2 vibration), gives foolproof
evidence of the presence of trace acetate ions, which are
wrapped in the as-synthesized and annealed nanocrystallites
and could not be completely washed out. Also, the band
discernible at ∼666 cm-1 is strictly assigned to the bending
vibration of the C-O species and authenticates the surface
attachment of acetate ions once more. The peak at about
1384 cm-1 can be ascribed to a τ(NH2) twisting vibration
individually. Additionally, the basic character of Ni(OH)2
results in the adsorption of atmospheric CO2, and therefore
the presence of carbonate anions on their surface could not
be avoided, as suggested from the small peak (broad) at
fractured and porous pedestal. This SEM micrograph shows
that architectures are favorably comprised of densely packed
all-but-uniform nanosheets with a thickness of about <100 nm.
The microspherical (diameter ∼2 μm) flowery architec-
ture in Figure 3b is observed to be built up from plentiful
patterned and aligned radiating nanopetals with a border
length of about 1 μm. More intriguingly, these petals are
interlinked with each other, and a closer inspection of the
porous structures reveals the self-assembly-directed oriented
attachment of the nanopetals having wavy and rough edges
(Figure 3c). A high-magnification SEM micrograph in
Figure 3d represents a well-defined and discrete nanoflower
having a multilayered and highly ordered texture. The
surfaces of the petals are smooth16 (a probable outcome of
Ostwald ripening) and are standing along the radial direc-
tions from the center of the microsphere, and they seem to be
quite flexible, reflecting their ultrathin feature. The reticu-
lated basement from where the flowers are rooted upwardly is
also (Figure 3d) formed by way of interconnecting nanopet-
als. The entire typical structural aspects of the thin film on
the glass slide are again verified with the greenish powder,
separated out during synthesis (Figure 3e). The formation of
such anisotropic superstructures could be rationalized by a
diffusion-limited-aggregation model, where the irreversible
aggregation of randomly moving particles results in self-
clustering growth and the formation of fractal dimensions.
∼1384 cm-1 15
.
Archetypal SEM images of the synthesized β-Ni(OH)2 (for
entry 1b, Table 1) are shown in Figure 3. The panoramic
image in Figure 3a is an overview of the samples with porous
flower-mimetic morphology, likely drizzled over a generally
(13) McEwenl, R. S. J. Phys. Chem. 1971, 76, 1782.
(14) (a) Li, Y.; Xie, X.; Liu, J.; Cai, M.; Rogers, J.; Shen, W. Chem. Eng. J.
2008, 136, 398. (b) Yang, L.-X.; Zhu, Y.-J.; Tong, H.; Liang, Z.-H.; Li, L.; Zhang,
L. J. Solid State Chem. 2007, 180, 2095.
(16) Cheng, Y.; Wang, Y. S.; Zheng, Y. H.; Qin, Y. J. Phys. Chem. B 2005,
109, 11548.
(15) Freitas, M. B. J. G. J. Power Sources 2001, 93, 163.

8818 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
Figure 3. Panoramic FESEM images of β-Ni(OH)2 over glass slides at (a) lower and (b) medium magnifications, (c) an enlarged view of a bunch of
nanoflowers madeby an orientedattachment of the nanopetals havingwavy and rough edges, (d) a well-defined and discretenanoflower. FESEM images of
(e) greenish powder separated out during synthesis[conditions:[Ni2þ] = 90mMand[Ni2þ]/[NH3] = 0.06;MHTovernight(∼12h) usingtungsten bulb (100
W) illumination] and (f) gray-black porous NiO obtained after calcination of Ni(OH)2 at 350 °C for 4 h in a tube furnace under ambient pressure. (g) EDX
spectrum of the flowery sample.
The porous framework even persists after calcination of the
sample (Figure 3f).
Compositional analysis of the sample Ni(OH)2 was achieved
with energy-dispersive X-ray (EDX) analysis, revealing that the

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8819
Figure 4. Bright-field TEM images of nanoflowers at (a) low and (b) medium magnifications.
coexistence of nickel and oxygen through hydrogen detection
was unattainable because the tool could only resolve elements
after boron (Figure 3g).17
nanostructures are very sensitive to high-energy electron
beam irradiation under high-vacuum conditions during
TEM sampling and are transformed to porous structures
grown with very tiny nanoparticles. As shown in Figure 5e,
after being irradiated by an electron beam, copious amounts
of holes with distinct atomic planes are generated. In agree-
ment with this, lattice fringes of (111) planes [of Ni(OH)2] can
be viewed in HRTEM with a lattice gap of 0.148 nm that
corresponds to the separation between the (220) planes of
face-centered-cubic NiO. This fascinating feature is a result
of dehydration/thermal decomposition of Ni(OH)2 to NiO
upon electron beam irradiation, and a literature survey6a,19
equally highlights this thermodynamically favorable event.
Such irradiation can also render electroreduction, as reported
by Liu et al., who observed the reduction of R-Ni(OH)2
nanosheets to nickel nanoparticles.20
To understand the texture and crystalline orientation of
the as-prepared nanostructured sample, further characteriza-
tion was executed with TEM. Parts a and b of Figure 4 show
the low-magnification TEM images of the products after
hydrothermal treatment for 12 h, manifesting the formation
of flowerlike microstructures abundantly and also revealing
that these arrangements are indeed made up of clearly
distinguishable nanopetals protruding out from the center
having edge-width ranges from 5 to 20 nm. Again, the sharp
contrasts among the dark edges of these petals and their
faded area underneath unambiguously confirm the well-
defined porous structures of the flower and also substantiate
the compactness of the petals preferentially in the central
region of the flower rather than in the exterior, indicating the
density variation of the nanosheets inside the porous archi-
tectures. These features associated with the interweaving
petal subunits are in good agreement with our FESEM
observation. The flowery structures are stable, and no evi-
dence of obvious cracking or destruction into individual
nanoplates even under high-intensity ultrasonication could
be addressed, signifying that the integrating force betweenthe
adjoining nanopetals is the chemical binding at their con-
tacting surfaces as well as feeble van der Waals interactions.18
Parts a-d of Figure 5 are typical higher magnification
TEM images taken on a piece of extended nanopetal of the
flowerlike structures (Figure 5a) at varied magnifications.
The identifiable disparity in luminosity in Figure 5b again
beefs up the poriferous skeletal subordinate by the intercon-
nected nanopetals of the flowery topology. An enlarged view
in Figure 5c clearly discloses that the petals are thin enough.
Close inspections of Figure 5d fetch the lattice-resolved
HRTEM images of different regional parts surrounded
by yellow frames, and the insets therein unequivocally
illustrate the (01-10) atomic planes,5a,6a with a fringe spacing
of 0.27 nm. Keen observation excavates that the synthesized
Parts a and b of Figure S1 in the Supporting Information
are more TEM and HRTEM images taken on different sides
of the nanopetals that advocate (i) the promising crystallinity
with clearly resolvable interplanar spacing, (ii) the interlock-
ing nanopetals, and (iii) the “electron-beam-issued” porosity
once more within the synthesized hierarchies.
The selected-area electron diffraction (SAED) pattern in
Figure5fdemonstratestheappearance ofperiodicdiffraction
spots, which, in turn, specify the polycrystalline nature of the
sample. Interestingly, a few diffraction spots, e.g., at d =
˚
0.874 and 0.78 A, though these are very weakly intense, also
appear with diffraction spots from concentric rings therein,
i.e., from (102), (110), and (202) planes. These phenomena
have been underpinned in a recent report,21 where some
1
normally forbidden diffraction spots, such as /2{110} and
1/2{102}, also appeared besides {110} and {102} diffraction
spots. Figure 5g is an emblematical image of the crystal
structure, projected along the crystallographic c axis, of
β-Ni(OH)2 crystallizing inthe hexagonal system (space group
P3m1 and symmetry D33d) common to several halogenides
(CdI2 type), with ABAB oxygen packing.22 The structure can
(19) Matsui, K.; Kyotani, T.; Tomita, A. Adv. Mater. 2002, 14, 1216.
(20) Liu, B.-H.; Yu, S.-H.; Chen, S.-F.; Wu, C.-Y. J. Phys. Chem. B 2006,
110, 4039.
(21) Li, Y.; Tan, B.; Wu, Y. Chem. Mater. 2008, 20, 567.
(22) Oliva, P.; Leonardi, J.; Laurent, J. F.; Delmas, C.; Braconnier, J. J.;
Figlarz, M.; Fievet, F.; De Guibert, A. J. Power Sources 1982, 8, 229.
(17) Zhuo, L.; Ge, J.; Cao, L.; Tang, B. Cryst. Growth Des. 2009, 9, 1.
(18) (a) Liang, J.; Bai, S.; Zhang, Y.; Li, M.; Yu, W.; Qian, Y. J. Phys.
Chem. C 2007, 111, 1113. (b) Mo, M.; Yu, J. C.; Zhang, L. Z.; Li, S. K. A. Adv.
Mater. 2005, 17, 756.

8820 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
Figure 5. HRTEM images of (a) a segment of a nanoflower and (b) intersecting nanopetals, (c) an enlarged view of the enclosed area (marked with red) of
part b with a width typically in the range of 5-20 nm and lengths up to several tenths of nanometers. (d) Lattice-resolved HRTEM images of different
regional parts (surrounded by the yellow frames) of the nanopetals with a fringe spacing of 0.27 nm and (e) an enlarged view of the enclosed area (marked
with a red frame) of partd with copious amounts of holes with a lattice gap of0.148 nmafter beingirradiated byanelectron beam during high-vacuum TEM
sampling conditions. (f) SAED pattern and (g) representative crystal structure of β-Ni(OH)2.
be visualized as a layered structure, with each layer having an
hexagonal planar arrangement of nickel(II) ions with octa-
hedral coordination of oxygen atoms (three lying above the
nickel plane and three lying below), and the layers are stacked
up along the c axis. The hydrogen atoms dwell in the
tetrahedral sites, although not perfectly at the central posi-
tion, of the remaining layers between oxygen atoms and are
engaged in tight binding with one of the four oxygen atoms
surrounding the tetrahedral sites.
view of the nitrogen adsorption/desorption isotherm (Figure 6)
and the BJH pore-size distribution (inset in Figure 6). The
isotherm can be categorized as type IV with a distinct but narrow
hysteresis loop in the relative pressure (P/P0) range 0.2-1.0.23
Hysteresis at the desorption moment often results from
hindered desorption and is observed in particulate samples.24
(23) Xiong, S.; Yuan, C.; Zhang, X.; Xi, B.; Qian, Y. Chem.;Eur. J. 2009,
15, 5320.
(24) (a) Wang, Z.; Heising, J. M.; Clearfield, A. J. Am. Chem. Soc. 2003,
125, 10375. (b) Subbia, A.; Pyle, D.; Rowland, A.; Huang, J.; Narayanan, R. A.;
Thiyagarajan, P.; Zon, J.; Clearfield, A. J. Am. Chem. Soc. 2005, 127, 10826.
Quantification of the porosity of the flowerlike architectures
could be achieved with BET gas sorptometry measurement in

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8821
causes favorable self-assembly between the initial tiny nano-
crystals in order to reduce the surface energy.27 Also, it is
expected that a series of mononuclear and polynuclear com-
2-
plexes like NiOHþ, Ni(OH)2, Ni(OH)3-, Ni(OH)4
,
Ni2OH3þ, Ni4(OH)44þ, etc., may form in solution in the
course of the reaction, and with a change in the solution pH,
these species encourage the formation of oligomeric hydroxo
moieties through an olation pathway, which elucidates the
rapid development of 2D petal-like/platelike building units.28
Growth of the nanocrystals in the dynamic reaction
environment is typically monitored by the presence of NH3,
which controls the hydroxide precipitation along with surface
passivation of the nanocrystals, and its responsibility is veri-
fied with the products in different time domains (Figure 7) at
the standard synthesis conditions. As shown in Figure 7a, at
the early stage (t = 4 h), the sample is made up with plentiful
but interlocking nanopetals and gives rise to the construction
of a porous basement. Eventually, no trace of a nanoflower is
observed at this stage. As the reaction proceeds (Figure 7b;
t = 6 h), nanoflowers are observed to grow over the
microporous platform and retain a hollow interior. The
petals are sticking out from the central part of the flower
and fewer numbers of petals are engaged in the building of a
nanoflower. After 8 h of reaction (Figure 7c; t = 8 h), the
growth of the 3D structures happens to be complete, and
these are now filled with densely packed nanopetals. We have
already described the pattern of the products obtained after
12 h of reaction time, and when the aging time is increased to
24 h (Figure 7d-f; t = 24 h), colonial growth of nanopetals
brings forth nanoflowers with more compact architectures.
The thicker petals (average thickness ∼ 70-100 nm; marked
by red arrows) in the structures are grown with self-stacking
thin individuals.
Following the whole evaluation process, we are inclined to
illustrate the patterning of the Ni(OH)2 textures involving a
sequence of dissolution, recrystallization, and oriented
attachment.29 Under hydrothermal conditions, the tiny par-
ticles experience a mass relocation by “dissolution and
crystallization”;30 the temperature and autogenous pressure
increase the solubility of the initially grown fine but amor-
phous Ni(OH)2 nanoparticles to make a supersaturated
growth solution; the process is thermodynamically further
driven toward nucleation and crystallization of the tiny
nanocrystals with an increase in the reaction time. Moreover,
it is a fair assumption that with time the escape of ammonia
from the reaction environment trims down its surface passiv-
ation effect and the pH of the solution, and when the pH
reaches the limit of pzc, spontaneous and quick aggregation
between the petals becomes favorable and gives rise to the
construction of dense nanostructures.
Figure 6. Nitrogen adsorption (9) and desorption (b) isotherms of the
porous Ni(OH)2 nanoflowers. The insets display the corresponding pore-
size distribution plot with a formulaic expression of determination of the
average thickness of the nanopetals from a specific surface area (S0)
[a distinct but narrow hysteresis loop in the relative pressure (P/P0) range
˚
0.2-1.0; pore-size distribution centered at ∼15 A].
The BET specific surface area is found to be 163.5 m2 g-1, and
the average pore diameter, as calculated from the BJH pore-size
distribution plot, addresses the presence of mostly micropores
and partly mesopores within the framework with the pore-size
˚
distribution centered at ∼15 A. This isothermal result also
provides the advantage of obtaining an idea about the average
thickness of the nanopetals. A recent report25 has interpreted
the methodology where the specific surface area (S0) could be
described as S0 = 2S/m, where S and m are the single surface
area and mass of the nanopetals, respectively. Assuming that F
and d are the mass density and thickness of the petals, we have
S0 = 2/Fd, where m = FSd, and this equation provides the
average thickness of the nanopetals as 3.08 nm, taking S0 =
163.5 m2 g-1 and F = 3.97 g cm-3 into account (schematically
presented in the inset of Figure 6). The result agrees well with our
examined HRTEM images.
Now, a plausible mechanistic interpretation is necessary for
an analytical investigation of the reaction mechanism. Initial-
2þ
ly, the precursor solution (entry 1b) contains Ni(NH3)6
with excess dissolved NH3 (pH ∼ 14), and in the ammonia-
rich environment, the poorly crystallized nanoparticles are
first formed owing to the kinetic advantage through hydrolysis
of the hexamine species and result in the formation of solid
Ni(OH)2 product (separated from the liquid phase as a
powder/thin film) according to the following reaction:
With a view to obtaining a more comprehensive under-
standing concerning the role of ammonia, test experiments
with varying concentrations of ammonia (t = 12 h) havebeen
performed, and the results are shown in Figure 8. When we
have introduced 0.1 mL of NH3 in the reaction system
([Ni2þ]/[NH3] = 0.194; entry 1a of Table 1), neither any
2þ
NiðNH3Þ ðaqÞ þ 2OH- ðaqÞ f NiðOHÞ ðsÞ þ 6NH3ðaqÞ
6
2
After completion of the reaction, pH measurement (observed
to be ∼10) indicates a decrease in the ammonia concentration
through evaporation/solvation with the progression of the
reaction, and this value of the pH is close to the “point of zero
charge (pzc)” of Ni(OH)2, as was estimated earlier (pzc = 10.7
at 60 °C).26 Thus, this spontaneous decrease in the pzc value
€
(27) (a) Colfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (b)
Yin, Y.; Alivisatos, A. P. Nature 2005, 437, 664.
(28) Coudun, C.; Grillon, F.; Hochepied, J.-F. Colloids Surf., A 2006,
280, 23.
(29) Zhong, L.-S.; Hu, J.-S.; Liang, H.-P.; Cao, A.-M.; Song, W.-G.;
Wan, L.-J. Adv. Mater. 2006, 18, 2426.
(30) Chen, L.-Y.; Zhang, Z.-D. J. Phys. Chem. C 2008, 112, 18798.
(25) Luo, Y.; Duan, G.; Li, G. J. Solid State Chem. 2007, 180, 2149.
(26) Tewari, P. H.; Campbell, A. B. J. Colloid Interface Sci. 1976, 55, 531.

8822 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
Figure 7. FESEM images of synthesized β-Ni(OH)2 at different reaction time domains: after (a) 4 h (inset gives the impression of “just initialization of
budding” of the flowers), (b) 6 h at higher magnification bearing a hollow interior within the nanoflowers, (c) 8 h having full grown nanoflowers fabricated
with densely packed nanopetals over a porous platform, and (d-f) 24 h where nanoflowers with more compact architectures are made with thicker but
stacked nanopetals [conditions: [Ni2þ] = 90 mM and [Ni2þ]/[NH3] = 0.06].
trace of flower nor even well-nurtured nanopetals are ob-
served. The view presents (Figure 8a) only the appearance of
an unripened basement made with undergrown nanopetals.
Well-defined flowerlike morphology over a discrete and por-
ous basement can only be attained with 0.3 mL of NH3
([Ni2þ]/[NH3] = 0.06; entry 1b of Table 1), as was already
elaborated on in our previous discussion. A definitive alter-
ation in the product morphology is noted with higher con-
centrations of NH3. The nanoflowers exit completely from
the architectures, and nanoplates enter the framework. Tri-
angular nanoplates (0.5 mL of NH3; [Ni2þ]/[NH3] = 0.03;
entry 1c of Table 1) are seen (Figure 8b) to stick to one
another to yield triangular but short-length nanocolumns of
length g200 to e800 nm. An intimate view speaks for the
large-scale and uniform productivity of the nanocolumn.
Practically all of the triangular faces of the columns (as
randomly indicated by red arrows) have an equilateral
geometry with side lengths in the range of ∼500 nm and all
angles measuring 60°. The porous framework of the structure
is unveiled from Figure 8c, where we could also observe the
covering of the basement with a columnar construction. A
similar interlamellar stacking showing growth with “pan-
cake” morphology for β-Ni(OH)2 was noted in an earlier
report.31 The next approach with 0.7 mL of NH3 ([Ni2þ]/
[NH3] = 0.022; entry 1d of Table 1) could evolute nanoplates
again in a cumulated fashion (Figure 8d). Here the nanocol-
umns seem to result from the regular stacking of quasi-
hexagonal nanoplates. The large scalability of the product
(31) Coudun, C.; Hochepied, J.-F. J. Phys. Chem. B 2005, 109, 6069.

Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8823
Figure 8. FESEM images of as-synthesized β-Ni(OH)2 products with varying [Ni2þ]/[NH3] ratios: (a) [Ni2þ]/[NH3] = 0.194; no nanoflower formation
is observed; (b and c) [Ni2þ]/[NH3] = 0.03; triangular but short-length nanocolumn built up from stacking triangular nanoplates, their uniform
productivity, and porous frameworks; (d and e) [Ni2þ]/[NH3] = 0.022; nanocolumns of stacking quasi-hexagonal nanoplates at different magnifications;
(f and g) [Ni2þ]/[NH3] = 0.013; dozens of nanocolumns with a “honeycomb” outlook cultivated with communally stacking hexagonal nanoplates, where
the structures are partly engrossed in the porous basement [conditions: ammonothermal treatment overnight (∼12 h) using tungsten bulb (100 W)
illumination].
is again demonstrated by this figure, and the enlarged detail
(Figure 8e) enables the mean thickness to be estimated as
∼200 nm. The faces with truncated triangular geometry (as
randomly indicated by red arrows; Figure 8d) stand with

8824 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
discrete edges having a short-arm length of around 500 nm
and a long-arm length in the region of ∼800 to ∼900 nm.
When we switch over with the introduction of 1.0 mL of
NH3 ([Ni2þ]/[NH3] = 0.013; entry 1e of Table 1) to the
nutrient solution to track its effect on the product mor-
phology, we come up with dozens of nanocolumns
(Figure 8f) typically hundreds of nanometers in length
scale (thickness), thoroughly spread over the porous base-
ment. The “honeycomb” outlook of each of the columns is
a consequence of back-to-back stacking interactions
among the hexagonal nanoplates (as randomly indicated
by red arrows). The face diagonal of a perfect hexagon is
estimated to be ∼1 μm, with an internal angle of 120°
(Figure 8g). Again, parts of the glued superstructures are
observed to be linked with one another in an irregular
fashion, accompanied by their partially embedded struc-
tural segments within the basement. It is worth mentioning
that all of the stacking merely follow an “eclipsed” manner
of attachment. The self-assembly route again encourages
the columnar micropillars to sustain the porosity within
them. TEM images of the hexagonally grown nanocol-
umns have been displayed in Figure S5 in the Supporting
Information, and their subsequent porosity measurement
study (Figure S6 in the Supporting Information) has also
been elucidated. These statistics thus corroborate the
inherent mesoporosity of this assemblage.
Keeping in mind the above information, a qualitative
and probabilistic explanation could be recommended to
explain the alteration in the hierarchy with a change in the
ammonia concentration. The FTIR spectra of the flower
(Figure 2a) and hexagonal nanocolumn (Figure S7 in the
Supporting Information) show the presence of vibrational
bands at ∼990 and ∼1055 cm-1, which arise from the
deformation modes of hydrogen-bonded ammonia
molecules,32 and these are accompanied again by the
N-H stretching modes in the region of ∼3300-3400
cm-1 (broad feature). These spectroscopic features indi-
cate the retention of ammonia molecules on the surface of
the hydroxides, where the surface passivation of the nano-
crystallites may occur in different ways: (i) through
hydrogen bonding involving its hydrogen atom to the
oxygen atom of surface-anchored hydroxyl groups;
(ii) through hydrogen bonding by way of its nitrogen atom
with the hydrogen atom of surface hydroxyl groups;
(iii) through coordinative interactions with surface metal
ions (coordinatively unsaturated). Now, a higher NH3
concentration results in adequate surface coverage, and
then they happen to be more prone to engaging in a
sufficient amount of hydrogen bonding, which perhaps
facilitates the stitching up of the surfaces with each other
to cause a more compact architecture. A recent study33
on the self-assembly tuned growth of β-Ni(OH)2 single
crystals with concave polyhedron structures has de-
scribed the cross-linking role of hydrazine cleverly intro-
duced into a reaction system in which also the existence
Scheme 2. Schematic Presentation of the Shape Transformation of a
Triangular to a Hexagonal Ni(OH)2 Nanocolumn through Truncated
Triangular/Quasi-Hexagonal Nanocolumnar Intermediate
the hexagonal nanocolumnar structure confirms the superi-
or coverage of their surface with NH3 beyond a doubt.
Of definite significance is the distinctively observed ammo-
nia-assisted shape transformation of triangular nanocolumns
to hexagonal ones through truncated triangular intermediates
(pictorially represented in Scheme 2). Earlier reports have
highlighted the phenomenon for metal nanocrystals,34 and a
recent study35 has reported the transformation from triangular
to hexagonal r-BN nanoplates through a simple thermal
treatment. Following these literature results, we can come up
with a qualitative explanation that suggests the following: for a
hexagonal crystal lattice, stable (lowest surface energy) cleav-
age planes are known to be (10-10) and (01-10) atomic
planes with angles of 120°, and therefore the triangular
nanoplates, bearing a high surface energy at their tips, thermo-
dynamically transform into the hexagonal ones through the
intermediacy of truncated triangular/quasi-hexagonal nano-
structures in order to lower the energy. The hexagonal crystal
system with four crystallographic axes, three equatorial axes at
120°, and one vertical (c) axis that is perpendicular to the other
three withstands the growth direction of the nanocrystals
during shape transformation, although in a real state of affairs,
this might be more complicated.
Of particular interest, the introduction of N2H4 as a
bidentate linker molecule to pile up Ni(OH)2 nanoplates
to achieve columnar packing and the manipulation of the
column length are described with a concentration varia-
tion of N2H4.33 To realize the unique role of N2H4, they
introduced NH3 in lieu of N2H4, but only flowerlike
structures composed of randomly stuck nanoplates
resulted. They have addressed their finding with a justi-
fication of the molecular structure of N2H4 and NH3:
because a N2H4 molecule has two nitrogen atoms, layers
of nickel hydroxide can be assembled, engrossing its
bridging action. However, the NH3 molecule with only
one nitrogen atom cannot serve the bridging function
and, consequently, the nickel hydroxide layers mount up
randomly to give only a flowerlike structure.
Again, another recent report36 has described the crucial
and essential role of an anionic surfactant in getting the
microspherical hierarchy of β-Ni(OH)2 congregated with
nanoplates from the Ni(NH3)62þ precursor under hydrother-
mal conditions (120 °C, 12 h), and the key point to be
of (N-H
O) hydrogen bonding was supported from
experimental results. Moreover, the broad band at ∼550
3 3 3 3
(34) (a) Wang, Z. L.; Petroski, J. M.; Green, T. C.; El-Sayed, M. A. J.
Phys. Chem. B 1998, 102, 6145. (b) Chou, N. H.; Schaak, R. E. J. Am. Chem.
cm-1 (νNi-N; Figure S7 in the Supporting Information) for
ꢀ
Soc. 2007, 129, 7339. (c) Metraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano
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Lett. 2003, 3, 519. (d) Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.;
Mirkin, C. A. Nature 2003, 425, 487.
(35) Li, M.; Xu, L.; Sun, C.; Ju, Z.; Qian, Y. J. Mater. Chem. 2009, 19,
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Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8825
addressed here is the formation of no obvious hierarchy
under the same experimental conditions but without any
surfactant.
the photocatalytic activity by allowing multiple reflections
of UV light within the internal cavity and also provides
immediate contact of the dye molecules through their
efficient transport to the interior void.40 It is expected
that, being a p-type wide-band-gap semiconductor with a
distorted rock salt structure and the most stable (100)
plane, nanometric NiO could be promising in a wide
variety of optical properties, and this may well explain
the small variance in the efficiency in relation to the
hydroxide homologue, as observed in our case. The photo-
degradation kinetics is followed with UV-visible spectro-
scopy and is represented in Figure 9a-c, where we can
observe the time-dependent change in its absorption profile
that corresponds with a steady decrease in λmax (∼525 nm)
of MR with the concomitant enhancement of a new and
broad peak centered at ∼425 nm. The fate of β-Ni(OH)2
upon adsorption of the dye is noticed and is given in
Figure 9a. Nonetheless, the dye alone (without catalyst)
shows no trace of degradation under identical conditions,
and degradation is only achieved with the hydroxide and
oxide nanocatalysts within the experimental time scale. A
detail representation regarding possible degradation prod-
ucts was mentioned earlier.38b Under the experimental
conditions, the degradation pathways follow first-order
reaction kinetics (Figure 9d) where the rate dependency
could be well expressed by the Langmuir-Hinshelwood
kinetic model.41 In the model, the reaction rate (R) is
proportional to the surface coverage (θ) of the hetero-
geneous nanomaterials as expressed below:
However, to the best of authors’ knowledge, this is the first
report on the columnar growth (triangular, quasi-hexagonal,
and hexagonal) of β-Ni(OH)2 with a regular stacking of
nanoplates with the assistance of only NH3. It has been
established that explicit regulation in the structural arrange-
ments of the hydroxide material (from flower to different
nanocolumnar structures) meets the need for a simple ma-
nipulation in the ammonia concentration. No special and
selective assistance (for instance, the presence of a bridging
ligand with two successive donor sites like N2H4 and any
structure-directing agent like surfactant) is needed to dish up
the purpose.
We have judiciously introduced HMT and pyridine (no
N-H bonds) separately in lieu of ammonia to judge their
effect on the growth pattern. No obvious formations of
well-patterned flower as well as platelike/columnarlike
morphologies are observed (Figure S8 in the Supporting
Information). This result supports the essential require-
ment of N-H bond(s) within the structural backbone of
added hydrolyzing amine/base for making such 3D super-
structures.
Semiconductor-driven photocatalysis, a promising tool
for implementing the large-scale purification of waste-
waters through extensive mineralization of a variety of
environmental pollutants, has received a considerable
challenge among the advanced oxidation process. How-
ever, the efficiency depends upon their higher crystallinity
to minimize electron-hole pair loss owing to the trapping
of charge carriers at the defect sites.37 Although TiO2,
ZnO, and Fe3O4 have been appreciated as esteemed photo-
active catalysts,38 small endeavors have been made to date
to explore that possibility with the oxide materials of
nickel.39 Benefitting from the chemical stability of the
synthesized hydroxide materials in an aqueous solution
even in a strong alkaline medium20 and the nonphotocor-
rosivity, we have employed the material (nanoflower) to
photocatalyze the degradation of methyl red (MR; color
changes from red to yellow) under UV irradiation as a
model compound, and a comparative account of its cata-
lytic activity has also been manifested, in contrast with
nanoporous NiO [derived from annealing Ni(OH)2] and
bulk NiO. Nearly complete degradation (92.5% with
nanoflower and 94.03% with nanoporous NiO) becomes
possible with the nanophase materials, but the contribu-
tion from bulk NiO is only 48.4% even after a long UV
exposure time. This enhanced activity of the synthesized
materials over the commercial NiO believably results from
their high surface-to-volume ratio in the nanostage and
porous frameworks. The porosity significantly promotes
R ¼ - dC=dt ¼ krθ ¼ krKC=ð1 þ KCÞ
where C, kr, and K denote the reaction rate constant,
adsorption coefficient of the reactant, and reactant con-
centration, respectively. With a very low concentration (C)
of the reactant, KC becomes small enough so that the
denominator reduces to 1 and the equation thus describes
just first-order kinetics. Assuming C = C0 at t = 0 (the
start of irradiation), integration of the above equation
results:
-lnðC=C0Þ ¼ k0t
where k0 is the apparent first-order rate constant evaluated
as 1.35 ꢀ 10-2, 2.03 ꢀ 10-2, and 8.15 ꢀ 10-4 min-1 for
Ni(OH)2 nanoflowers, nanoporous NiO, and bulk NiO,
respectively. The well-accepted mechanism for the photo-
catalytic reaction engrosses photoexcited electron
transfer3a from the valence band to the conduction band
of the semiconductor under consideration, able to initiate
the oxidation/reduction processes of a surface-adsorbed
substrate. In aqueous solutions, the holes in the valence
band are scavenged by surface hydroxyl groups, generat-
ing a strong oxidizing hydroxyl radical, which can promote
the oxidation and subsequent mineralization of the dye
molecule.
(37) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
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While SERS has enjoyed varied applications to in situ
interfacial chemistry mostly with “free-electron-like” metals,
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8826 Inorganic Chemistry, Vol. 49, No. 19, 2010
Sarkar et al.
Figure 9. UV-visible spectra of the time-dependent degradation of methyl red (MR) under UV irradiation with (a) synthesized Ni(OH)2 nanoflower,
(b) nanoporous NiO, and (c) bulk NiO. Digital photographs present the color change upon degradation. (d) Comparative bar-diagrammatic representation
of the first-order reaction kinetics with the above materials [conditions: [MR] = 10-4 M; radiating source, a 15 W Philips medium-pressure mercury lamp
(λ = 365 nm)].
mainly silver, gold, and copper,42 and a few transition metals
(Ni, Pd, Pt, Co, Fe, Ga, etc.),43 as many people had
anticipated, in some quarters, semiconductor nanoparticles
have also been observed to contribute in SERS with a typical
enhancement factor of 101-103. From this perspective,
Lombardi’s group has made an assortment of significant
contributions to embellish the pathway of obtaining SERS
from the so-called “ill-defined” semiconductor substrates.44
Encouraged by the puffy structures of the flowery β-
Ni(OH)2, a secondary material could easily be deposited
within the framework to result in the formation of a func-
tional composite material,45 and taking this structural ad-
vantage, we have introduced silver nanoparticles within the
inorganic matrix and the material Ag@Ni(OH)2 has been
addressed as a pertinent candidate for greeting SERS from
suitable nearby molecules located on (or within) the latter
material. The hybrid material bears a similar morphology,
only differentiating in the thickness aspect, where we could
notice a relative fatty nature of the petals, and this is the
possible outcome of the deposition of in situ produced silver
nanoparticles (Figure S9 in the Supporting Information).
The derivative material has been further characterized by
XRD and EDX analysis to show the presence of the second-
ary materials (Ag) beyond a doubt (Figure S10 in the
Supporting Information).
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Here the study presents the SERS spectrum of 4-mercap-
topyridine (4-MP) from the nanocomposite, and their char-
acteristic manifestation prescribes more diagnostic features
related to the composite-analyte interaction. Table S1 in the
Supporting Information summarizes the frequencies and
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Article
Inorganic Chemistry, Vol. 49, No. 19, 2010 8827
deposited and the semiconductor, may have a bearing on
the said SERS enhancement.47
Conclusions
In summary, a trouble-free and simple ammonothermal
methodology has been accepted as a straightforward proto-
col for the exploitation of highly anisotropic Ni(OH)2 nano-
structures. Of special interest is the explicit regulation in
structural arrangements of the hydroxide material with a
simple manipulation in the ammonia concentration and
reaction time. Both parameters play a major role in the
reaction dynamics. Ni(OH)2 is observed to crystallize in the
β phase with a brucite structure and form a flowerlike
morphology built up with plentiful patterned and aligned
nanopetals. A conformational change from flower to trun-
cated triangular stacked nanoplates to finally hexagonally
stacked nanoplates with columnar morphology is exclusively
achievable with an increase in the NH3 concentration with an
explanation from the surface coverage, hydrogen bonding,
and surface energy. Microporous and mesoporous behaviors
of the different nanostructures have been ascertained by BET
gas sorption measurements. Following the whole evaluation
process, we are inclined to illustrate the patterning of the
Ni(OH)2 textures involving a sequence of dissolution, recrys-
tallization, and oriented attachment. Ultimately, the UV-
activated photocatalytic property of the material has been
bestowed with complete mineralization of an organic dye
molecule and also provides an opportunity to make a
secondary functional material with embedded silver nano-
particles, and the derived composite gives rise a more refined
description of the SERS activity with 4-MP in its low
concentration level. Thus, in the approach with the advan-
tages of inexpensive experimental setup and simple and
greener synthetic routes, the impressive increase in the variety
of particle shapes promises to be transformed into a pro-
liferation of revolutionary structures and materials that may
readily be scaled up for industrial production.
Figure 10. SERS spectrum of 4-MP from a Ag@Ni(OH)2 nanocompos-
ite [conditions: [4-MP] = 5 ꢀ 10-7 M; excitation source, He-Ne laser
emitting at a wavelength of 632.8 nm].
vibrational assignments of Raman bands of 4-MP. As we can
see from Figure 10, the NRS spectrum of solid 4-MP signifi-
cantly differs from the SERS spectrum in vibrational figures of
merit. Noticeable changes are observed in the frequencies. The
spectral analysis is presented in the Supporting Information.
To evaluate the limit of detection of 4-MP by employing
this analytical tool by the composite, we have extended our
study with its further lower concentration where we could
observe the competence of the material to detect the concen-
tration down to a nanomolar level (Figure S11 in the
Supporting Information). The enhanced vibrational modes
at ∼1387 and ∼1142 cm-1 (b2 vibrational modes) compared
to that of only silver nanoparticles imply an improved metal-to-
ligand charge transfer for the material.
The general consensus attributes the observed enhance-
ment to contributions from two mechanisms: a long-range
electromagnetic (EM) effect and a short-range chemical
(CHEM) effect, although for the semiconductor particles
because they often have plasmon resonance far from the
excitation wavelength used, here these SERS spectra are
dominated by a charge-transfer (CT) mechanism. The reality
of this proposal is readily verified with the blue shifting of the
bands at ∼1595 and ∼1089 cm-1 of pure 4-MP to ∼1583 and
∼1065 cm-1, respectively, in the SERS spectrum (Figure 10).
This information authenticates that CT operates from the
conduction band of the deposited silver nanoparticles to the
π* level (LUMO) of the adsorbed molecule. In a true sense,
the CHEM and EM mechanisms collectively contribute to
the SERS enhancement and are not readily separable.46 Any
theoretical approach that challenges their separation will
probably fail in one or more limits. Also, the “Schottky
barrier”, a dipolar layer juxtaposed between the metal
Acknowledgment. The authors are thankful to the CSIR,
UGC, and DST (New Delhi, India) and Indian Institute of
Technology, Kharagpur, for financial assistance.
Supporting Information Available: TEM and HRTEM images
on a different sides of the nanopetals, Raman spectrum of the
sample with analysis and a schematic presentation of the Raman-
active modes, magnetic property of the sample with corresponding
magnetization plots, TEM and HRTEM analyses of hexagonal
columnar structures, BET isotherms with pore-size distribution
and an analysis for hexagonally stacked nanocolumns, FTIR
spectrum of the β-Ni(OH)2 hexagonal nanocolumn, FESEM
images of Ni(OH)2 synthesized with hexamethylenetetramine
and pyridine, FESEM images of a Ag@Ni(OH)2 nanocomposite
with its characterized XRD and EDX patterns, analytical explana-
tion of the SERS spectrum, the SERS spectrum of 4-MP at the
nanomolar level of concentration over a Ag nanoparticle and the
nanocomposite material, and Table S1 describing frequencies and
vibrational assignments of Raman bands of 4-MP. This material is
available free of charge via the Internet at http://pubs.acs.org.
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