Morphology control of nickel oxalate by soft chemistry and conversion to nickel oxide for application in photocatalysis

Article abstract of DOI:10.1039/c3ra21978j

The present work provides an effective methodology for controlled room-temperature aqueous synthesis of nickel oxalate (NiOX) nanosheets and nanoflakes in the presence of anion rich self-assembled bilayers of catanionic surfactant comprising of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB). Encouragingly alteration of the CTAB/SDS ratio played an extraordinary role to form nanoflakes and nanosheets of NiOX. Our synthetic approach is combined with calcination to produce antiferromagnetic spherical and hexagonal nickel oxide (NiO) nanoparticles (NPs) as the end product. Synthesized nanostructured NiOX and NiO were characterized by X-ray diffraction study (XRD), energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). TEM studies illustrated that spherical NiO NPs have an average size around 5-10 nm and that of hexagonal NiO NPs have average width of about 22-27 nm. Temperature and field dependent magnetic properties of spherical and hexagonal NiO nanomaterials (NMs) were measured by using a SQUID magnetometer which revealed canted antiferromagnetic and spin glass nature, respectively. In addition, we report photocatalytic activity of NiO NMs, investigated on the photodegradation of phenol under ambient conditions, and as expected, the NiO having largest surface area showed best catalytic efficiency. This biomimetic catanionic surfactant inspired approach which require only metal ions as reactants have a definite potential towards an alternative, simple way of synthesizing metal oxide NMs.

Full text of DOI:10.1039/c3ra21978j

RSC Advances
View Article Online
View Journal
PAPER
Morphology control of nickel oxalate by soft chemistry
and conversion to nickel oxide for application in
photocatalysis3
Cite this: DOI: 10.1039/c3ra21978j
Soumyadipta Rakshit,a Sayantani Chall,a Soumya Sundar Mati,a
Anirban Roychowdhury,b S.P. Moulika and Subhash Chandra Bhattacharya*a
The present work provides an effective methodology for controlled room-temperature aqueous synthesis
of nickel oxalate (NiOX) nanosheets and nanoflakes in the presence of anion rich self-assembled bilayers of
catanionic surfactant comprising of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylam-
monium bromide (CTAB). Encouragingly alteration of the CTAB/SDS ratio played an extraordinary role to
form nanoflakes and nanosheets of NiOX. Our synthetic approach is combined with calcination to produce
antiferromagnetic spherical and hexagonal nickel oxide (NiO) nanoparticles (NPs) as the end product.
Synthesized nanostructured NiOX and NiO were characterized by X-ray diffraction study (XRD), energy
dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FESEM) and transmission
electron microscopy (TEM). TEM studies illustrated that spherical NiO NPs have an average size around 5–
10 nm and that of hexagonal NiO NPs have average width of about 22–27 nm. Temperature and field
dependent magnetic properties of spherical and hexagonal NiO nanomaterials (NMs) were measured by
using a SQUID magnetometer which revealed canted antiferromagnetic and spin glass nature, respectively.
In addition, we report photocatalytic activity of NiO NMs, investigated on the photodegradation of phenol
under ambient conditions, and as expected, the NiO having largest surface area showed best catalytic
efficiency. This biomimetic catanionic surfactant inspired approach which require only metal ions as
reactants have a definite potential towards an alternative, simple way of synthesizing metal oxide NMs.
Received 29th August 2012,
Accepted 5th February 2013
DOI: 10.1039/c3ra21978j
www.rsc.org/advances
novel intrinsic properties and applications of nanomaterials
(NMs).
1. Introduction
Morphology-controlled synthesis of inorganic one-dimen-
sional (1D) and two-dimensional (2D) nanoscale building
blocks with controlled size and shape into 2D complex
structures is of extensive research interest. It is expected that
if the morphology and assembly mode of the original building
units can be controlled, then those of their secondary
architecture would be adjustable. Electronic, optical, magnetic
and catalytic properties of a functional material are highly
responsive on tuning the morphology (including dimension-
ality and shape) of nanostructured materials.1–5 Therefore,
research on cost effective morphology-controlled and assem-
bly-designed synthesis needs to be emphasized for enabling
Transition metal oxides are key materials in catalysis,
microelectronic, optical and magnetic applications.6–9 In our
work we have focused on nickel oxide since it is one of the
relatively few metal oxides with p-type semiconductivity and
having a stable wide band gap. It has extensive applications as
a catalyst,10 as electrode material for lithium ion batteries and
fuel cells,11–13 in electrochromic films,14 electrochemical
supercapacitors,15,16 magnetic materials,17 gas sensors18 and
biosensors.19
Morphology controlled transformation of precursors has
proven to be a promising approach for the synthesis of NiO
nanostructures. Recent reports on solid-state calcination of
well designed precursors into nanoporous NiO clearly exem-
plify a very close relationship between precursors and products
and further inspire the concept of precursor controlled
synthesis of well defined NiO nanostructures. Nickel hydro-
xides (Ni(OH)2) and oxalates (NiOX) are the mostly used
precursors for synthesising NiO nanoparticles (NPs) after
thermal conversion. Gao et al. reported synthesis of
a-Ni(OH)2 and NiO with different morphologies through a
ethylene glycol mediated self-assembly approach,20 synthesis
aDepartment of Chemistry, Jadavpur University, Kolkata 700032, India.
E-mail: sbjuchem@yahoo.com; scbhattacharyya@chemistry.jdvu.ac.in; Fax: 91(033)
24146584; Tel: 033 2414 6223
bUGC - DAE Consortium for Scientific Research, Kolkata Centre, 111/LB-8,
Bidhanangar, Kolkata 700098, India
Electronic supplementary information (ESI) available: Figures showing energy
3
dispersive X-ray spectra of NiOX I and NiO I, FESEM images of nickel oxides, plot
of xT vs. T for NiO I, photoluminescence of residual phenol in presence of NiO I
over three cycles, FESEM image and XRD pattern of commercial NiO have been
included. See DOI: 10.1039/c3ra21978j
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.

View Article Online
Paper
RSC Advances
of NiO nanobelts, nanowires, nanoplates and hollow spheres
were also reported by various other groups.21,22 Vaidya et al.
reported solvent effects in reverse micelle templated shape
controllable synthesis of nano-spheres, -cubes and -rods of
nickel oxalate to prepare NiO NPs.23 However, insurmountable
limitations of most of the synthesis methods (excluding
thermal decomposition) in terms of large scale production,
complicated synthesis steps and comparatively high cost,
triggered us to opt for an alternative facile and promising
route. Moreover this approach is suitable for mass production
for future large-scale application.
aqueous medium could be the basis of waste treatment.32,33
Semiconductor photocatalysts offer a huge potential for
elimination of organic pollutants in water.34 The photocataly-
tic activity of NiO with high surface area so far has been
insignificant.35 BET surface areas of NiOs were also measured
to correlate their photocatalytic activity with surface area.
2. Experimental
2.1. Chemicals
Since surface active compounds enable soft chemical and
biomimetic approaches,24 self-assembled well defined struc-
tures of surfactants has been taken advantage of for designing
and synthesis of inorganic nanodimensional materials. They
are used as a kind of template for the synthesis and have the
ability for compartmentalization of the synthesised nanos-
tructures, which alters the product states and reduces the
reaction dimensionalities.25 Aqueous mixtures of oppositely
charged surfactants exhibit interesting phase behaviour and
properties because of the tunability of the oppositely charged
head groups. At high surfactant concentrations and above the
Krafft boundary of the mixtures, these systems can form
several types of microstructures, namely lamellar and cubic
phases, vesicles, spheres, rod like structures which are not
present in the individual surfactant–water binary system.26,27
At low concentrations (generally below 5.0 wt%) the behaviour
of these catanionic systems is dominated by phase separation,
with the formation of a highly insoluble precipitate (solid
catanionic) at and around equimolarity.28 However, with an
excess of one of the surfactants the precipitate dissolves to
yield heterogeneous self-assembled aggregates of different
structures. In recent years efforts have been made on the
synthesis of nanoparticles within unique catanionic reverse
micelles.29–31 However despite the development of strategies
for synthesis of nanosheets, we are reporting for the first time
a large-scale room-temperature soft chemical nonhazardous
synthesis of nickel oxalate polycrystalline nanosheets.
In the present work, we have synthesized NiOX in dispersed
catanionic systems which show a promising potential in
morphological and hierarchical synthesis of nanostructured
materials. Here, we discuss the role of catanionic surfactant in
morphology controlled synthesis. Pure phase synthesis of the
desired material was characterised by employing X-ray
diffraction (XRD) study, thermogravimetry–differential ther-
mal analysis (TG–DTA), field emission scanning electron
microscopy (FESEM) and transmission electron microscopy
(TEM) analysis. Furthermore, we have assessed the photo-
catalytic performance of NiO NPs during the photoinduced
oxidation of phenol.
The precursor metal salts, nickel(II) nitrate hexahydrate
(Ni(NO3)2?6H2O, 99.9%) and sodium oxalate (Na2C2O4, 98%)
were purchased from E. Merck. The two surfactants, cetyl-
trimethylammonium bromide (CTAB, 99.9%) and sodium
dodecyl sulfate (SDS, 99.9%) were purchased from Aldrich
and BDH, respectively, and were used as received. Chloroform
and acetone of analytical grade used in this work were
purchased from E. Merck and dried by standard procedures.36
Commercial NiO nanopowder (98%, spherical) was purchased
from SRL, India. The water used was treated with a Millipore-Q
water purification system.
2.2. Synthesis of nickel oxalate and nickel oxide nanomaterials
In the present work, nanostructured NiO was synthesized in
two steps. In the first step, nanocrystalline precursor NiOX was
prepared using two different synthetic strategies namely
dispersion method (method I) and in situ method (method II)
and in the next step, oxalates (NiOX I obtained from method I
and NiOX II from method II) were subjected to thermal
decomposition. In the first synthetic approach, growth of
nickel oxalate was realised through templated dispersed
catanionic surfactant; however, in the second approach, the
mode of synthesis of the catanionic was different. In both the
methods we used three different SDS/CTAB volume ratios (60/
40, 65/35, 70/30) to investigate the role of catanionic surfactant
during the synthetic procedure.
In method I, SDS (0.05 mol dm23) and CTAB (0.05 mol
dm23) were added in a volume ratio 65/35 in water to prepare a
dispersed catanionic solution (dispersion method). The final
concentration of SDS and CTAB in the solution were 0.8125
and 0.4375 mM which were lower than their respective critical
micellar concentrations (CMC).37,38 To this solution Ni(NO3)2
(0.05 mol dm23) and Na2C2O4 (0.05 mol dm23) were added
and stirred for 6 h. The product i.e., nickel oxalate was
separated by centrifugation, washed first with chloroform and
then with acetone to make it surfactant free. The washed
material was dried at 40 uC in an oven.
In method II, a substantially modified approach of the
previous strategy was employed where the template and
product were generated in situ (in situ method). During the
synthesis, two individual solutions were prepared. Solution A
containing CTAB (0.05 mol dm23), Ni2+ (0.05 mol dm23) and
Photocatalysis is an effective method for the treatment of
contaminants and pollutants, especially for the removal of
organic compounds, since it works in ambient conditions of
temperature and pressure without forming any kind of sludge
which otherwise create different pollutant problems.
Degradation of these toxic substances by photocatalysis in
22
solution B comprising of SDS (0.05 mol dm23), C2O4 (0.05
mol dm23) in water were mixed together at the previous
surfactant concentrations and continuously stirred for 6 h.
Similar SDS/CTAB ratios were also investigated. The product
was centrifuged followed by washing and drying as before. In
RSC Adv.
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
both routes the concentration of NiC2O4 was 4.0 mmol dm23
.
The prepared green powders were calcined at 450 uC for 3 h to
obtain black NiO powders. The optimal annealing temperature
was determined by thermogravimetric measurements to
ensure complete conversion of precursor oxalates to oxide.
2.3. Instrumentation
The surface morphology of NiO nanoparticles was examined
by high-resolution transmission electron microscopy (HRTEM,
Model: JEOL-JEM-2100) at an accelerating voltage of 200 kV
and by field emission scanning electron microscopy (Hitachi
S-4800, Japan). The structural studies of the precursor oxalates
as well as the calcined oxide products were performed with an
X-ray diffractometer using Cu-Ka radiation (l = 0.15406 nm).
Thermogravimetry–differential thermal analysis was carried
out in a Perkin Elmer Pyris Dyamond TGA/DTA system in air
with a heating rate of 10 uC min21. BET (Brunauer–Emmett–
Teller) surface area measurements were performed with an
Autosorb iQ (Quantamchrome Inc., USA) gas sorption system
at 77 K. Magnetic properties of nickel oxide were measured by
a SQUID magnetometer [MPMS XL7, Quantum Design, USA].
Absorption spectra of as synthesised NiO were recorded with a
Shimadzu UV-1601 PC UV-Vis spectrophotometer using
matched quartz cells. The photoluminescence spectral analy-
sis of the residual phenol was performed using a Fluorolog
FIIA spectrofluorimeter (Spex Inc, NJ, USA) with a slit width of
2.5 mm.
2.4. Photocatalytic measurement
Fig. 1 X-Ray diffraction pattern of (a) as-synthesized NiOX I, NiOX II and (b) NiO
The photocatalytic activity of as-synthesized NiO nanoparticles
(NPs) was measured by decomposing phenol at room
temperature. A tubular beaker (capacity 50 mL) was used as
the photoreactor. The reaction system contained 22.0 mg of
NiO nanoparticles and 30 mL of a dilute aqueous phenol
solution (0.02 mg L21), which was initially stirred in the dark
for 1 h to reach the adsorption equilibrium of phenol. The
reaction system was then exposed to a source of UV radiation
and the degradation of the photoluminescent peak of phenol
at y305 nm was monitored with time.
I, NiO II.
the same phenol concentration (0.02 mg L21) and degradation
of phenol was monitored by photoluminescence.
3. Results and discussion
After the first use of the NiO, it was filtered and then washed
with water along with ultrasonication several times and it was
then centrifuged. The centrifuged NiO was then dried at 120
uC for 5 h in an electric oven. Recovered NiO having same size
and shape was then reused for new degradation batch having
3.1. Characterization of NiOX and NiO
The overall crystallinity and phase composition of the as
synthesized nanostructured NiOX and NiO were initially
analysed by X-ray powder diffraction (XRD) (Table 1).
Table 1 Values of d-spacings (Å) obtained from XRD patterns and HRTEM analysis of nickel oxalate and nickel oxide prepared by methods I and II
Sample
NiOX I
Space group
hkl
Obs. dhkl (XRD)/Å
Obs. dhkl (TEM)/Å
Standard dhkla/Å
Cccm
202
004
400
202
004
400
111
200
111
200
4.711
3.906
2.953
4.727
3.918
2.953
2.413
2.087
2.413
2.086
—
—
—
—
—
—
2.41
2.08
2.42
2.08
4.770
3.940
2.959
4.770
3.940
2.959
2.410
2.088
2.410
2.088
NiOX II
Cccm
¯
NiO I
Fm3m
¯
Fm3m
NiO II
a
JCPDS 14-0742 and 47-1049.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.

View Article Online
Paper
RSC Advances
According to the JCPDS card (14-0742), all the diffraction
peaks as displayed in Fig. 1a, could readily be indexed to a
pure phase NiOX having orthorhombic lattice structure (lattice
parameters: a = 11.84 Å, b = 5.345 Å, c = 15.716 Å, Z = 8, space
group Cccm). For NiOX I and II the diffraction peak of (202)
was the strongest of the diffraction peaks shown in the XRD
pattern (Fig. 1a), which indicated a preferential growth. In the
earlier works of Wang et al.21 and Vaidya et al.,23 they reported
a monoclinic phase of nickel oxalate dihydrate prepared by
solvothermal and reverse micellar routes, respectively, having
lattice parameters a = 11.77 Å, b = 5.333 Å, c = 9.833 Å, b =
127.2u, Z = 4, space group C2/c which were contrary with our
present findings. In Fig. 1b, the three major diffraction peaks
could perfectly be indexed as (111), (200) and (220) crystal
planes of the cubic structure of highly crystalline NiO which
was in good agreement with the literature value (JCPDS 47-
1049) and no obvious diffraction peaks assignable to other
impurities could be detected. The average grain sizes of NiO I
and NiO II, calculated from X-ray line broadening (using the
Debye–Scherrer equation23) on the basis of the strongest (200)
reflection are 11 and 20 nm, respectively.
indicated an atomic ratio of 86% Ni and 14% O in NiOX and
76.7% Ni and 23.3% O in NiO.
3.2. Thermogravimetric (TG–DTA) investigation
To elucidate the exact chemical structure of nickel oxide
precursors, we studied the thermal behaviour of NiOX I and
NiOX II. The thermogravimetry–differential thermal analysis
(TG–DTA) curves of the as-prepared precursors are shown in
Fig. 2a and b. Both the curves of the sheet-like oxalate
precursors showed two step weight losses. For NiOX I, the
initial weight loss of about 19.8% at 165–220 uC associated
with an endothermic peak at 214.2 uC represented the removal
of the two crystal water molecules. Likewise, for NiOX II, in the
first step, 18.3% weight loss occurred at 165–225 uC with an
endothermic maximum at 216.6 uC. For both the precursors, a
second step weight loss at around 325 uC (40.76 and 36.7%,
respectively) corresponded to the elimination of CO2.39 Release
of CO2 molecules from the oxalates at high temperature region
was exothermic in nature and gave rise to peaks at 346.1 and
356.1 uC, respectively. This slight difference in peaks could be
assigned to their difference in sizes.
Corresponding EDX spectra further confirmed the elemental
3.3. Electron microscopy analysis: FESEM and TEM
composition of the precursor NiOX (Fig. S1a, ESI ) and the
3
After ascertaining the synthesis of chemically pure phase NiOX
and NiO NMs we further attempted to visualize the variation of
morphologies and to conclusively determine the structural
patterns of the precursors and final products, FESEM and TEM
measurements were carried out. Morphology evolution of
nickel oxalate samples prepared at an SDS/CTAB ratio of 60/40
showed nanoflakes with an average size of 230 ¡ 20 nm for
method I and of 190 ¡ 20 nm for method II (Fig. 3a and 4a).
Fig. 3b and 4b show representative FESEM images of the single
nanosheet structures (SDS/CTAB = 65/35) of NiOX I and II
having average size of 210 ¡ 20 and 490 ¡ 40 nm. At an SDS/
CTAB ratio of 70/30, nickel oxalate formed multilayered
nanosheets with average size of 260 ¡ 20 nm for NiOX I
(Fig. 3e) and 260 ¡ 25 nm for NiOX II (Fig. 4e). We report the
sizes of NiOXs with standard deviations (Table 2) after
repeating the preparations three times, showing excellent
reproducibility of this catanionic system. Clearly the ratios of
SDS/CTAB catanionic template undoubtedly control the
morphology and size of the precursor oxalate. Furthermore,
the geometrical structures of NiO were elucidated by transmis-
sion electron microscopy (TEM) which clearly reinforced that
no structure variation of NiO was obtained after calcination
(Fig. 3d, f and 4d, f) for oxalate nanoflakes and multilayered
nanosheets of precursor oxalates. However, for single
nanosheets we obtained granular (NiO I) and hexagonal nickel
oxide (NiO II) (Fig. 3d and 4d, SDS/CTAB = 65/35).
final product, NiO (Fig. S1b, ESI ). Fig. S1b highlighted the
3
3
presence of only nickel and oxygen peaks, with no other
relevant elements were present which clearly demonstrated
that the proposed method was a clean, simple and effective
chemical process. Analysis of the EDX of NiOX I (Fig. S1a, ESI )
3
and NiO I (Fig. S1b, ESI ) prepared in 65/35 SDS/CTAB ratio
3
During subsequent synthesis we preferably chose 65/35
(SDS/CTAB) volume ratio for our synthesis since in this
particular ratio the dispersed anionic rich catanionic surfac-
tant gave rise to highest stability along with a two dimensional
structure.40
On the basis of reaction conditions and parameters we
herein propose a plausible formation mechanism of nickel
oxalate (Scheme 1). Surfactant concentrations are one of the
controlling parameters of the phase behavior of cationic/
anionic surfactant mixtures40 which in turn organize pattern-
Fig. 2 TG–DTA curves of (a) NiOX-I and (b) NiOX-II nanosheets prepared at a
SDS/CTAB ratio of 65/35.
RSC Adv.
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Fig. 3 FESEM images of as-synthesized NiOX I (dispersion method) with SDS/CTAB ratios (a) 60/40, (c) 65/35, (e) 70/30; TEM images of corresponding NiOs prepared
after calcination of NiOX I, SDS/CTAB ratios (b) 60/40, (d) 65/35, (f) 70/30.
ing of different morphologies of the nanomaterials. As a
consequence of dynamic electrostatic interaction, between two
oppositely charged head groups (here, CTAB and SDS)
catanionic complexes formed in solutions. In combination
CTAB and SDS are known to form uni- and multi-lamellar
vesicles which with higher proportion of SDS progressively
become unstable as the molar composition tends towards 1 : 1
with precipitation.40–43 Inorganic ions are known to form plate
and sheet like morphologies44 in such media. In both the
methods I and II (in catanionic template forming media)
formation of NiOX is governed by the above route. The mixed
lamellar vesicular template supported formation of large
nanosheets of NiOX as schematically presented. Penetration
of both Ni2+ and C2O422 in the bilayered head group region of
Fig. 4 FESEM images of as-synthesized NiOX II (in situ method) with SDS/CTAB ratios (a) 60/40, (c) 65/35, (e) 70/30; TEM images of corresponding NiOs prepared
after calcination of NiOX II, SDS/CTAB ratios (b) 60/40, (d) 65/35, (f) 70/30.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.

View Article Online
Paper
RSC Advances
Table 2 Structural variation of nickel oxalate (NiOX) and nickel oxide (NiO) with the variation of volume ratio of SDS/CTAB
Ratio
SDS
Dispersion
In situ
Sample
NiOX
CTAB
Morphology
Size/nm
Morphology
Size/nm
60
65
70
40
35
30
Nanoflakes
Nanosheet
Nanosheet with layers
200–250
180–230
250–280
Nanoflakes
Nanosheet
Nanosheet with layers
170–210
450–530
240–280
NiO
60
65
70
40
35
30
Granular
Granular
Granular
14–16
5–10
12–14
Granular
Hexagonal
Granular
15–17
22–27
12–14
the systems produce NiOX, and depths of the lamellar vesicles
determine the sheets’ dimensions until destabilization of the
overall structure arose for solubility reasons. The volume ratio
of SDS and CTAB has a primary influence on the formation
and size of the products. In case of a 60/40 ratio the numbers
of catanionics formed were a maximum, which indicated
denser catanionic situation and low concentration of stabilizer
SDS, resulting in quicker formation of thinner flaky products.
Initially formed larger plate-like structures were amenable to
form larger products upon removal of surfactant and calcina-
tion under controlled thermal condition.
Scheme 1 Schematic representation of formation of nickel oxalate by methods I and II in dispersed catanionic (SDS/CTAB ratio 65/35).
RSC Adv.
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
prepared in method II (hexagonal shaped) for SDS/CTAB ratio
65/35. The measured surface areas are given in Table 3. The
higher surface area for NiO I may be attributed to its smaller
grain size (Fig. S2a, ESI ).
3
3.4. Optical properties
UV-Vis absorption spectroscopy is an important method to
reveal the energy structures and optical properties of semi-
conductor nanocrystals. The absorption spectra of as-synthe-
sized granular and hexagonal NiO prepared at SDS/CTAB ratio
65/35 is illustrated in Fig. 7. The above spectra showed a 9 nm
blue shift for NiO I relative to NiO II, which was due to
significant difference in crystal size associated with NiO
nanoparticles. The strong absorption peaks at 322 nm for
NiO I and 331 nm for NiO II gave the absorption energy
corresponding to a band gap (Eg) of 3.85 and 3.74 eV,
respectively. The increase in the band gap of the NiO
nanoparticles compared to bulk material was indicative of a
quantum confinement effect of the nanocrystallites.
Fig. 5 HRTEM images of granular and hexagonal nickel oxide nanoparticles
prepared by method I (a) and method II (b) having interplanar spacing (dhkl
2.08 Å [200]; inset shows the SAED patterns.
)
A large difference in size of the precursors can result a
difference in grain size and morphology in the synthesis of
secondary architectures.45 Nickel oxalates with different
morphology but comparable size led to granular NiO with a
very little difference in grain size but nickel oxalate having size
in the range 450–530 nm resulted in hexagonal NiO. As the size
difference of NiO nanosheets can produce two distinct
morphologies, we can conclude in a generalised way that the
size of precursor played the key role towards the morphology
of calcined product rather than the morphology of the
precursor.
Fig. 5a, b and 6a, b display the most prominent lattice
planes present and have been indexed and correspond to 200
and 111 planes obtained from careful HRTEM observation,
consistent with our XRD results. This observation coupled
with selected area electron diffraction patterns, shown as an
inset in Fig. 5a, b and 6c, d give the idea of the overall
polycrystallinity of the nickel oxides.
3.5. Magnetic properties
Magnetic properties of both the nickel oxides were measured
by a SQUID magnetometer over a wide range of temperature
and magnetic field. We observed a huge change in magnetic
properties with the change in morphology and grain size.
Temperature-dependent magnetization measurements were
carried out from 10 to 300 K at an applied field of 100 Oe for
both the samples under zero-field-cooled (ZFC) and field-
cooled (FC) conditions.
For NiO I ZFC and FC magnetization (MZFC and MFC) curves
followed the same nature and both showed a sharp increase
after 30 K and increased monotonically up to the lowest
experimentally possible temperature. A bifurcation between
the ZFC and FC curves was observed in NiO II which was not
pronounced in the ZFC/FC curves of NiO I. Similar (Fig. 8a)
behavior of the ZFC and FC curves were reported earlier for
other oxide materials.46,47 A plot of x21 vs. T of NiO I (Fig. 8a
inset) presents antiferromagnetic behaviour. The increase in
magnetization value at lower temperature suggested the
presence of uncompensated antiferromagnetic spin at the
The texture properties were further elucidated by N2
sorption analysis. The specific surface area of NiO prepared
in method I (granular shaped) was much higher than that
nanoscale.48 Furthermore a plot of xT vs. T (Fig. S3, ESI )
3
clearly demonstrated canted antiferromagnetic behaviour.49 In
order to determine the magnetic property more fully hysteresis
loops of the NiO I were measured at 10, 100 and 300 K (Fig. 8c).
A clear M–H loop was observed at 10 K which clearly showed a
nanoscale ferromagnetic (FM) interaction in consequence of
canting in the surface region.
For NiO II ZFC and FC magnetization curves were found to
bifurcate at around 206 K (Tbf) (Fig. 8b). It is well known that
the bifurcation temperature Tbf corresponds to the largest
particles in the size distribution. At 12 K (T2) and 56 K (T1) in
the MZFC curve two distinct maxima were observed (Fig. 8b) for
which the first (T2) was very sharp compared to the other (T1).
The MFC curve showed a steep low temperature rise starting at
about 25 K and then increased monotonically till the lowest
temperature of measurement. The spin glass like behavior of
the MZFC curve at around 12 K indicated collective freezing of
disordered spins in the surface region and that around 58 K
was attributed to the presence of a ferromagnetic core
Fig. 6 HRTEM images of NiO prepared by method I (a) and method II (b) having
interplanar distances of 2.41 and 2.42 Å [111], respectively, and (c), (d) their
corresponding SAED patterns.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.

View Article Online
Paper
RSC Advances
Table 3 Structural and catalytic information of nanostructured NiO.
Sample
dXRD/nm
dTEM/nm
SBET/m2 g21
k/min21
% PhOH degraded
NiO I
NiO II
Commercial NiO
11
20
45
5–10
66
28
11
1.72 6 1022
1.17 6 1022
5.82 6 1023
82.10
69.22
44.10
22–27
32–37a
a
Obtained from FESEM (Fig. S5a, ESI ).
3
consisting of a non-saturating component.50 The origin of
such a maximum at low temperature (12 K) might be due to
the ordering of small magnetic clusters at the particle surface,
inducing strong intraparticle interactions, which led to a
frustrated magnetic state.51 Beyond T1 the thermal energy
surpassed the applied field effect leading to superparamag-
netic relaxation.52 To obtain a more detailed view of this
interesting magnetic property the hysteresis loops were
measured at different temperatures (10, 100 and 300 K). At
10 K a hysteresis loop with a low value of coercivity was evident
but at 300 K it almost became a straight line along with a high
value of coercivity (Fig. 8d) which supported the previous
discussion along with an increase in surface anisotropy at
higher temperature. Uncompensated spins at the surface
In the present work we investigated the dependence of
morphology of NiO NPs with photocatalytic efficiency by
monitoring the characteristic fluorescence emission of phenol
at 305 nm (Fig. 9). In every case the emission peak at 305 nm
diminished gradually as the exposure time increased. For NiO
I the emission peak intensity decreased up to 82% which
indicated the percentage of phenol degraded. Conversely for
commercial NiO nanopowder the level of decrease was y44%.
The decrease of the emission peak intensity at 305 nm is
attributed to the degradation of phenol under the irradiation
of ultraviolet light and hence our synthesized NiO NPs (Fig. 9b,
c) showed good photocatalytic reactivity in the decomposition
of phenol. Nickel oxides were used as catalyst for another three
cycles (Fig. S4, ESI ). For each cycle the percentage of phenol
3
produced
a ferromagnetic like behavior, giving rise to
degraded is presented in Fig. 9f.
remanence and coercivity in the hysteresis loops.2
Degradation of phenol corresponds to hydroxylation of the
phenyl ring55 as promoted by hydroxyl radicals ( OH). The OH
radicals are formed by combination of water with electron–
hole pairs in nickel oxide formed due to adsorption of phenol
on the surface of NiO and irradiation by UV light, while
?
?
4. Implication for waste water disposal:
photodecomposition of phenol
electrons convert oxygen to superoxide radicals ( O22), which
?
We studied the semiconductor photocatalyst activity of NiO for
the photodegradation of phenol. Phenol is a major pollutant of
surface and/or groundwater and according to environment
protection rules of World Health Organization (WHO) the
exposure limit of phenol in inland water should be lower than
0.02 mg L21 (1994) and its degradation to a safe level in this
range is very difficult due to its stability and solubility in water.
Though NiO is reported as a catalyst for oxidation of numerous
alcohols,53 its photocatalytic activity was rarely explored.20,54
are also highly active oxidizing species. In aqueous solution
the holes are scavenged by surface hydroxyl groups to generate
?
strongly oxidizing hydroxyl radicals ( OH). Some other reactive
?
species such as HO2 and H2O2 were also formed during the
reaction. The mechanistic pathway is shown in Scheme 2.
When phenol molecules are adsorbed on the surface of
excited NiO nanoparticles, they obtain the necessary activation
56
?
energy to react with OH radicals and produce CO2 and H2O
where intermediates viz., catechol, hydroxyhydroquinone,
hydroquinone, benzoquinone etc. were also expected to
form.57,58
The Brunauer–Emmett–Teller (BET) surface area of NiO
prepared by method I (granular shaped) was much higher than
that prepared by method II (hexagonal shaped) for SDS/CTAB
ratio 65/35 elucidated by N2 sorption analysis (Table 3). The
increase in surface area for NiO I may be attributed to its
smaller grain size. The number of active surface sites is high in
NiO I and consequently the surface charge carrier transfer rate
in photocatalysis was higher than for NiO II. Since there are a
very few reports on the photocatalytic activity of NiO on water
pollutants, our findings suggested a useful environmental
application of nanostructured NiO for removal of organic
pollutant, air purification, and waste-water remediation.
Fig. 7 Absorption spectra of NiO synthesised by method I (a) and method II (b).
RSC Adv.
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
Fig. 8 Magnetization curve of NiO I (a) and NiO II (b) for ZFC and FC in a 50 Oe applied field as a function of temperature (inset shows a plot of x21 vs. T for NiO I) and
(c), (d) their corresponding magnetic hysteresis loops (inset shows magnified hysteresis loops from 21000 to 1000 Oe).
Fig. 9 Photoluminescence spectra of residual phenol under UV light excitation in presence of (a) no catalyst, (b) NiO I, (c) NiO II and (d) commercial NiO nanopowder,
(e) intensity of residual phenol as a function of UV irradiation time, (f) value of percentage of phenol degraded for each cycle for NiO I as catalyst.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.

View Article Online
Paper
RSC Advances
References
1 V. Protasenko, D. Bacinello and M. Kuno, J. Phys. Chem. B,
2006, 110, 25322.
2 R. H. Kodama, S. A. Makhlouf and A. E. Berkowitz, Phys.
Rev. Lett., 1997, 79, 1393.
3 S. Chall, A. Saha, S. K. Biswas, A. Dutta and S.
C. Bhattacharya, J. Mater. Chem., 2012, 22, 12538.
4 N. Tian, Z. Y. Zhou, S. G. Sun, Y. Ding and Z. L. Wang,
Science, 2007, 316, 732.
5 C. Burda, X. B. Chen, R. Narayanan and M. A. El-Sayed,
Chem. Rev., 2005, 105, 1025–1102.
6 J. T. Sampanthar and H. C. Zeng, J. Am. Chem. Soc., 2002,
124, 6668.
7 Y. Hou, H. Kondoh, M. Shimojo, T. Kogure and T. Ohta, J.
Phys. Chem. B, 2005, 109, 19094.
Scheme 2 Possible photodegradation pathway of phenol over NiO.
8 X. Wang, X. Chen, L. Gao, H. Zheng, M. Ji, C. Tang, T. Shen
and Z. Zhang, J. Mater. Chem., 2004, 14, 905.
9 F. Cai, G. Zhang, J. Chen, X. Gou, H. Liu and S. Dou, Angew.
Chem., Int. Ed., 2004, 43, 4212.
10 J. Park, E. Kang, S. U. Son, H. M. Park, M. K. Lee, J. Kim, K.
W. Kim, H. J. Noh, J. H. Park, C. J. Bae, J. G. Park and
T. Hyeon, Adv. Mater., 2005, 17, 429.
11 X. Wang, J. Song, L. Gao, J. Jin, H. Zheng and Z. Zhang,
Nanotechnology, 2005, 16, 37.
12 P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J.
M. Tarascon, Nature, 2000, 407, 496.
13 X. Wang, L. Li, Y. G. Zhang, S. T. Wang, Z. D. Zhang, L.
F. Fei and Y. T. Qian, Cryst. Growth Des., 2006, 6, 2163.
14 M. C. A. Fantini, F. F. Ferreira and A. Gorenstein, Solid
State Ionics, 2002, 152–153, 867.
15 T. Ahmad, K. V. Ramanujachary, S. E. Lofland and A.
K. Ganguli, Solid State Sci., 2006, 8, 425.
16 H. Pang, Q. Lu, Y. Lia and F. Gao, Chem. Commun., 2009,
7542.
17 S. D. Tiwari and K. P. Rajeev, Thin Solid Films, 2006, 505,
113.
Conclusion
In summary, we designed a soft and facile catanionic
surfactant controlled approach for preparation of various
nickel oxalate nanostructures, including single layered
nanosheets, multilayered nanosheets and nanoflakes.
Compared with other conventional solution methods, this
technique avoids large concentration gradients and gives
better control of the particle growth in chemical processing
systems. The role of the catanionic surfactant in controlling
the morphology of nickel oxalate has been studied in detail in
the anionic rich region. After calcination in air, the as-
prepared nickel oxalate single layered nanosheets are con-
verted to NiO nanostructures with different grain size and
morphology. Magnetic study of NiO with size 5–10 nm shows
canted antiferromagnetism but for 22–27 nm grain size, we
find surface spin disorder resulting in spin glass behaviour.
The as-obtained NiO nanostructures show high photocatalytic
activity to decompose phenol dissolved in water displaying
great potential in the application of environmental pollutant
cleanup. This easy but efficient approach presented here, not
only can be extended for the synthesis of other metal oxides
with controlled shape and structure but also fundamentally
18 X. Song, L. Gao and S. Mathur, J. Phys. Chem. C, 2011, 115,
21730.
19 S. Mohan, P. Srivastav, S. N. Maheshwari, S. Sundarb and
R. Prakash, Analyst, 2011, 136, 2845.
20 X. Wang and L. Gao, J. Phys. Chem. C, 2008, 112, 15299.
21 D. Wang, C. Song, Z. Hu and X. Fu, J. Phys. Chem. B, 2005,
109, 1125.
22 L. Dong, Y. Chu and W. Sun, Chem.–Eur. J., 2008, 14, 5064.
23 S. Vaidya, P. Rastogi, S. Agarwal, S. K. Gupta, T. Ahmad, A.
M. Antonelli Jr., K. V. Ramanujachary, S. E. Lofland and A.
K. Ganguli, J. Phys. Chem. C, 2008, 112, 12610.
24 M. S. Bakshi, G. Kaur, P. Thakur, T. S. Banipal,
F. Possmayar and N. O. Petersen, J. Phys. Chem. C, 2007,
111, 5932.
25 K. Holmberg, J. Colloid Interface Sci., 2004, 274, 355.
26 A. Caria and A. Khan, Langmuir, 1996, 12, 6282.
27 H. Edlund, A. Sadaghiani and A. Khan, Langmuir, 1997, 13,
4953.
28 E. F. Marques, O. Regev, A. Khan, M. G. Miguel and
B. Lindman, J. Phys. Chem. B, 1998, 102, 6746.
29 H. Shi, L. Qi, J. Ma, H. Cheng and B. Zhu, Adv. Mater., 2003,
15, 1647.
30 Y. Xing, M. Li, S. A. Davis, A. J. Patil and S. Mann, Soft
Matter, 2006, 2, 603.
provides
a new route towards fabrication of inorganic
materials into well-defined morphologies for potential appli-
cations.
Acknowledgements
S. R and S. S. M acknowledges UGC for providing JRF. S. C, S. B
and S. P. M greatly appreciate the funding from DST (SR/S1/
PC-63/2009). Authors are grateful to Dr K. K. Chattopadhyay,
Mr N. S. Das and Mr D. Sarkar of Jadavpur University, India for
their kind help in performing TEM and FESEM measure-
ments. The authors also thank Dr S. Koner, Jadavpur
University, India for his help in BET surface area measure-
ment. The authors are also thankful to the reviewers for their
valuable suggestions.
RSC Adv.
This journal is ß The Royal Society of Chemistry 2013

View Article Online
RSC Advances
Paper
31 H. Shi, X. Wang, N. Zhao, L. Qi and J. Ma, J. Phys. Chem. B,
2006, 110, 748.
44 S. S. Kim, Y. Liu and T. J. Pinnavaia, Microporous
Mesoporous Mater., 2001, 44–45, 489.
45 D. Wang, Q. Wang and T. Wang, Inorg. Chem., 2011, 50,
6482.
32 S. Ahmed, M.G. Rasul, W. N. Martens, R. Brown and
M.A. Hashib, Desalination, 2010, 261, 3.
46 K. Karthik, G. K. Selvan, M. Kanagaraj, S. Arumugam and
N. V. Jaya, J. Alloys Compd., 2011, 509, 181.
47 V. Bisht, K. P. Rajeev and S. Banerjee, Solid State Commun.,
2010, 150, 884.
48 S. A. Makhlouf, F. T. Parker, F. E. Spada and A.
E. Berkowitz, J. Appl. Phys., 1997, 81, 5561.
49 R. K. Mishra, D. K. Pradhan, R. N. P. Choudhary and
A. Banerjee, J. Magn. Magn. Mater., 2008, 320, 2602.
50 S. Xiang, X. Wu, J. Zhang, J. Zhang, R. Fu, S. Hu and X. Z.
X. Zhang, J. Am. Chem. Soc., 2005, 127, 16352.
51 M. Y. Ge, L. Y. Han, U. Wiedwald, X. B. Xu, C. Wang,
K. Kuepper, P. Ziemann and J. Z. Jiang, Nanotechnology,
2010, 21, 425702.
52 H. Bi, S. Li, Y. Zhang and Y. Du, J. Magn. Magn. Mater.,
2004, 277, 363.
53 S. Thota and J. Kumar, J. Phys. Chem. Solids, 2007, 68, 1951.
54 S. T. Christoskova and M. Stoyanova, Water Res., 2002, 36,
2297.
55 T. Zhu, J. S. Chen and X. W. (David) Lou, J. Phys. Chem. C,
2012, 116, 6873.
56 D. Li and H. Haneda, Chemosphere, 2003, 51, 129.
57 A. Sobczynski, L. Duczmal and W. Zmudzinski, J. Mol.
Catal. A: Chem., 2004, 213, 225.
33 S. L. Koro and L. Dekany, Colloids Surf., A, 2006, 280, 146.
34 J. Sato, H. Kobayashi and Y. Inoue, J. Phys. Chem. B, 2004,
108, 4369.
35 L. V. Dearden and E. M. Woolley, J. Phys. Chem., 1987, 91,
2404.
36 B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A.
R. Tatchell, Vogel’s Textbook of Practical Organic Chemistry,
Longman Singapore Publishers Pte. Ltd, Singapore, 5th
edn., p. 408.
37 P. Mukerjee and K. J. Mysels, NSRDS-NBS 36, US
Government Printing Office, Washington, D. C., 1971.
38 K. Kalyanasundaram and J. K. Thomas, J. Am. Chem. Soc.,
1977, 99, 2039.
39 S. Thota and J. Kumar, J. Phys. Chem. Solids, 2007, 68, 1951.
40 B. Tah, P. Pal, M. Mahato and G. B. Talapatra, J. Phys.
Chem. B, 2011, 115, 8493.
41 P. Andreozzi, S. S. Funari, C. La Mesa, P. Mariani, M.
G. Ortore, R. Sinibaldi and F. Spinozzi, J. Phys. Chem. B,
2010, 114, 8056.
ˇ
ˇ ´
´
´
´
42 V. Tomasic, I. Stefanic and N. Filipovic-Vincekovic, Colloid
Polym. Sci., 1999, 277, 153.
43 C. Leitiza, P. Andreozzi, A. Scipioni, C. La Mesa,
A. Bonincontro and E. Spigone, J. Phys. Chem. B, 2007,
111, 898.
58 K. I. Okamoto, Y. Yamamoto, H. Tanaka and M. Tanaka,
Bull. Chem. Soc. Jpn., 1985, 58, 2015.
This journal is ß The Royal Society of Chemistry 2013
RSC Adv.