Nickel oxide nanoparticles grafted on reduced graphene oxide (rGO/NiO) as efficient photocatalyst for reduction of nitroaromatics under visible light irradiation

Article abstract of DOI:10.1016/j.jphotochem.2016.12.023

Nickel oxide nanoparticles were grafted on reduced graphene oxide via simultaneous reduction of graphene oxide and nickel salt in a single step reaction. The synthesized material (rGO/NiO) was found to be efficient visible light active photocatalyst for the reduction of nitroaromatic derivatives to their corresponding amino compounds. Hydrazine monohydrate provided necessary protons and electrons for the targeted reaction. After completion of the reaction, the photocatalyst could readily be recovered by simple external magnet and could be reused for six runs without any significant loss of its activity. More importantly, the photocatalyst did not show any leaching during the reaction as confirmed by ICP-AES analysis of the recovered catalyst.

Full text of DOI:10.1016/j.jphotochem.2016.12.023

Accepted Manuscript
Title: Nickel oxide nanoparticles grafted on reduced graphene
oxide (rGO/NiO) as efficient photocatalyst for reduction of
nitroaromatics under visible light irradiation
Author: <ce:author id="aut0005"
author-id="S1010603016308899-
1
714eff1fa2e2191e8b070497683038c"> Amer
Al-Nafiey<ce:author id="aut0010"
author-id="S1010603016308899-
3
3fcbcd974b858e82502423a5b0d6bf3"> Anurag
Kumar<ce:author id="aut0015"
author-id="S1010603016308899-
8
cb243b95bd10c7b6cd0324e151afa36"> Malika
Kumar<ce:author id="aut0020"
author-id="S1010603016308899-
2
e77300be51d3e9a388fd020e7852aa7"> Ahmed
Addad<ce:author id="aut0025"
author-id="S1010603016308899-
b3c64ddecd655dcd4c246dadaa7914cf"> Brigitte
Sieber<ce:author id="aut0030"
author-id="S1010603016308899-
8
2066d2161dca6ed5d14a7700fc9d0ef"> Sabine
Szunerits<ce:author id="aut0035"
author-id="S1010603016308899-
5
c4b80588cd9e5f17441881dadc46b20"> Rabah
Boukherroub<ce:author id="aut0040"
author-id="S1010603016308899-
6
42fd502b810b65a0264c66861965b67"> Suman L.
Jain
PII:
S1010-6030(16)30889-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.12.023
JPC 10478
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
10-10-2016
17-12-2016
25-12-2016

Please cite this article as: Amer Al-Nafiey, Anurag Kumar, Malika Kumar,
Ahmed Addad, Brigitte Sieber, Sabine Szunerits, Rabah Boukherroub,
Suman L.Jain, Nickel oxide nanoparticles grafted on reduced graphene oxide
(rGO/NiO) as efficient photocatalyst for reduction of nitroaromatics under visible
light irradiation, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.12.023
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Nickel oxide nanoparticles grafted on reduced graphene oxide
rGO/NiO) as efficient photocatalyst for reduction of
nitroaromatics under visible light irradiation
(
1
,†
2
2
3
3
Amer Al-Nafiey, Anurag Kumar, Malika Kumar, Ahmed Addad, Brigitte Sieber, Sabine
1
1*
2*
Szunerits, Rabah Boukherroub and Suman L. Jain
1Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 - IEMN, F-59000
Lille, France
2
CSIR Indian Institute of Petroleum, Haridwar Road Mohkampur Dehradun 248005 India
UMET, UMR CNRS 8207, Université Lille 1, 59655 Villeneuve d'Ascq Cédex, France
3
rabah.boukherroub@iemn.univ-lille1.fr; suman@iip.res.in
†
Current address: University of Babylon, Collage of science for women, Department of laser,
Babylon, Iraq
1

Graphical Abstract
Nickel oxide nanoparticles grafted on reduced graphene oxide (rGO/Ni) as
efficient photocatalyst for reduction of nitroaromatics under visible light
irradiation
Highlights of the research

Facile synthetic methodology for large scale production of nickel nanoparticles
decorated reduced graphene oxide via simultaneous reduction of simultaneous
chemical reduction of GO and NiCl
2
with sodium borohydride (NaBH )
4

Photocatalytic reduction of nitroaromatics to corresponding amino compounds under
visible light irradiation



Facile recovery of the photocatalyst via external magnetic effect
Efficient recycling of the photocatalyst without significant loss in activity.
Cost effective and mild synthetic methodology for the decomposition of organic
pollutants to safer compounds by visible light.
2

Abstract
Nickel oxide nanoparticles were grafted on reduced graphene oxide via simultaneous
reduction of graphene oxide and nickel salt in a single step reaction. The synthesized
material (rGO/NiO) was found to be efficient visible light active photocatalyst for the
reduction of nitroaromatic derivatives to their corresponding amino compounds. Hydrazine
monohydrate provided necessary protons and electrons for the targeted reaction. After
completion of the reaction, the photocatalyst could readily be recovered by simple external
magnet and could be reused for six runs without any significant loss of its activity. More
importantly, the photocatalyst did not show any leaching during the reaction as confirmed
by ICP-AES analysis of the recovered catalyst.
Keywords: Photocatalyst, reduced graphene oxide, nickel oxide nanoparticles, reduction,
nitrobenzene
1
. Introduction
Development of green and sustainable methodologies for organic transformations has
become a prime need in current day chemistry. Among the known approaches, light assisted
transformations particularly using visible light are of prime importance due to its plentiful
availability and inexpensive nature [1, 2]. Apart from the photodegradation of organic
pollutants and waste water treatments [3-5], the development of photocatalytic chemical
conversions is gaining considerable interest and growing rapidly worldwide in current years
[
6, 7].
The reduction of nitrocompounds to the corresponding amines is an important
transformation owing to the pharmaceutical importance of these compounds as reactive
3

intermediates in the synthesis of various bioactive compounds [8-10]. Apart from the
conventional approaches which generate copious amounts of undesirable waste [11, 12],
photocatalytic reduction using UV light in the presence of TiO based semiconductor
2
photocatalysts has also been well documented in the literature [13]. However, the main
drawback is the use of UV radiation which consists of only 5% part of solar spectrum and
requires special reaction vessels. Thus, the visible light assisted methodology for the
reduction of nitrocompounds to amines is highly desired. To achieve this goal, a number of
surface modifications of the TiO
2
photocatalyst by doping metal or metal oxides, oxide
halides (i.e., PbPnO X (Pn = Bi, Sb; X = Br, Cl) and sensitization with dyes have been
2
reported to perform the reaction under visible light [14-17]. In addition to these reports,
Fe(bpy) +3/rGO [19], Ni@npg-C
Ag/Bi
2
MoO
6
[18]
,
2
3
4
N [20] and Ni(II)-porphyrin metal–
organic framework [21] have also been recently reported for this transformation. However,
the tedious and multi-step synthesis of these photocatalysts leaves a scope for the further
development in this area.
Graphene, owing to its outstanding thermal, mechanical and electrical properties has
come out to be an exciting and promising material in recent decades for diversified
applications in various fields, including adsorption, photocatalysis and heterogeneous
catalysis, etc [22, 23]. Properties such as high surface area and absence of mass transfer
barriers make this material extremely suitable in catalysis as a new form of carbon material
[
23]. The solution-based chemical reduction of exfoliated graphene oxide is the most suitable
approach for the efficient production of reduced graphene oxide (rGO) sheets owing to its
low-cost and massive scalability and an affordable technique for the large-scale production of
2
3
rGO sheets. In addition, owing to the presence of sp carbon sheets and sp carbons bonded
with additional oxygen groups graphene oxide/reduced graphene oxide behave like a
2
semiconductor. The sp carbon network has mobile electrons due to π conjugated system that
4

3
behaves like conduction band while sp carbons have tightly held electrons which behave like
valence band [24-25]. Thus, because of the presence of conduction and valence band,
GO/rGO have a band gap and act like a semiconductor in photocatalytic transformations. The
presence of oxygen functional groups in GO/rGO can also act as nucleation centers or
anchoring sites for the landing of nanoparticles [26], limiting the nanoparticles growth and
improving their stability and dispersion on the rGO surface. At the same time, these
nanoparticles can help to enlarge the interplanar spacing of the rGO sheets, limiting their
stacking, and thus maintain the excellent properties of individual rGO nanosheets [26]. Some
of the current studies on graphene-based nanocomposites have shown that a synergistic
combination of metal/metal oxide nanoparticles with graphene sheets enhances their
properties and performances, which make them utilizable in various fields.
Nickel-based nanoparticles (Ni NPs) have received considerable attention recently,
because of their low cost and remarkable catalytic performance in hydrogenation of
nitrobenzene and nitrophenol, reduction of oxygen and oxidation of olefins [27-28], etc.
Recently, several reports described the synthesis of Ni NPs/graphene nanocomposites. The
most common synthetic methodology to produce these nanocomposites is based on the
simultaneous reduction of both nickel salt and GO [29], or the reduction of nickel salt in the
presence of rGO. For example, Ji and co-workers [30] synthesized reduced graphene
oxide/Ni (rGO/Ni) nanocomposites via one step reduction of a mixture of nickel chloride
(
NiCl
atmosphere. Yang et al. [31] used hydrothermal method for the preparation of rGO-Ni(OH)
composites from nickel nitrate [Ni(NO ·6H
The dried rGO-Ni(OH) composite was calcined at 250°C for 4 h in air to obtain the final
black colored product, the rGO–NiO composite. Zhu et al. [32] prepared a rGO/NiO
2
) and graphene oxide (GO) with hydrazine hydrate at 100 °C for 3 h under N
2
2
3
)
2
2
O] mixed with GO in DMF at 180 °C for 12 h.
2
composite by homogeneous co-precipitation and subsequent annealing. Urea and NiCl
5
2

solutions were slowly and sequentially added to GO and heated at 90 °C for 1.5 h. However,
long experimental time and high-temperature calcination are required during all the
procedures.
Herein, we report a facile low-temperature solution method for the large-scale synthesis
of rGO/NiO nanocomposites and their application as photocatalysts for reduction of nitro-
compounds to corresponding amines under visible light irradiation (Scheme 1).
2
. Experimental
Materials
The chemical reagents are of analytical grade and were used as received without further
purification. Graphite powder (< 20 micron), potassium permanganate (KMnO
acid (H SO ), phosphoric acid (H PO ), hydrogen peroxide (H ), nickel chloride (NiCl
sodium borohydride (NaBH ) were purchased from Sigma-Aldrich. All substrates and
4
), sulphuric
2
4
3
4
2
O
2
2
),
4
solvents were purchased from Merck India Ltd (Mumbai, Maharashtra, India) and used
without further purification.
Preparation of graphene oxide (GO)
GO was prepared from graphite according to the modified Hummers’ method. Briefly,
graphite flakes (2 g) were placed in a round bottom flask and the flask was immersed in an
ice bath. Then H
stirred for 5 min. After that KMnO
days. 100 mL diluted H SO (5 wt %) were added to this and heated at 90 °C for 2 h with
continuous stirring. After that 30 wt % H solution (approximately 5.4 mL) was added and
stirred for 2 h at room temperature. GO was collected by centrifugation (6000 rpm) of the
mixture and washed with H SO (3 wt%), H (0.5 wt%) and HCl (3 wt%). Finally washing
was done with distilled water until pH of washing became neutral. A homogeneous yellow
2
SO
4
(68 mL) and sodium nitrate (1.50 g) were added simultaneously and
4
(9.0 g) was added slowly to this mixture and stirred for 5
2
4
2
O
2
2
4
2
O
2
-
1
brown suspension (1 mg mL ) of GO sheets in deionized water (DI) was achieved by
6

ultrasonication for 3 h. Preparation of rGO/NiO nanocomposite was achieved by
simultaneous chemical reduction of GO and NiCl with sodium borohydride (NaBH ).
Typically, to 5 mL of GO (1 mg mL ) in DI water, sonicated for about 20 min to form a
homogeneous dispersion, was added NiCl (9, 19 or 38 mg) and the mixture was sonicated
for 30 min. Then 5 mL of NaBH (0.1 M) aqueous solution was added to the mixture at room
2
4
-1
2
4
temperature. A spontaneous formation of a black precipitate was observed. The precipitate
was washed repeatedly with water and separated by centrifugation. The products were signed
as rGO/NiO-1, rGO/NiO-2 and rGO/NiO-3 according to the feeding amount of 9, 19, and 38
mg of NiCl
2
.6H O, respectively. The nickel content in the synthesized composites was found
2
to be 0.2, 0.35 and 0.5 mmol/g as determined by ICP-AES analysis.
Characterization
X-Ray Diffraction (XRD)
X-ray diffraction patterns for determining the phase structure and crystalline properties of the
materials were obtained on a Bruker D8 Advance diffractometer (Billerica, MA, USA)
working at 40 kV and 40 mA with Cu K
α
radiation (λ = 0.15418 nm).
Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) imaging was performed with a FEI Tecnai G2-20
twin operating at 200 kV. For that purpose, the rGO/NiO composites were dispersed on a thin
carbon film of a 3 mm diameter copper grid. It was equipped with a CCD camera and a
Digital Zicrograph software for the acquisition of electron diffraction patterns, and the dark-
field and high-resolution imaging.
FTIR spectroscopy
Fourier transform infrared (FTIR) spectra were recorded using a Thermo Scientific FTIR
instrument (Nicolet 8700). Dried rGO/NiO sample (1 mg) was mixed with KBr powder (100
mg) in an agate mortar. The mixture was pressed into a pellet under 10 tons load for 2–4 min,
7

and the spectrum was recorded immediately. Sixteen accumulative scans were collected. The
signal from a pure KBr pellet was subtracted as the background.
Thermogravimetric analysis (TGA)
Thermogravimetric analysis (TGA) measurements were carried out in Al
2
3
O crucibles in a
nitrogen atmosphere at a heating rate of 10°C/min using a TA Instruments Q50
thermogravimetric analyzer.
X-ray photoelectron spectroscopy (XPS)
For X-ray photoelectron spectroscopy (XPS) analysis, the samples were prepared by casting
5
0 μL of an aqueous solution of rGO/NiO composite onto a clean silicon wafer followed by
drying in an oven at 80°C to remove the solvent. XPS measurements were performed with an
ESCALAB 220 XL spectrometer from Vacuum Generators featuring a monochromatic Al Kα
X-ray source (1486.6 eV) and a spherical energy analyzer operated in the CAE (constant
analyzer energy) mode (CAE=100 eV for survey spectra and CAE=40 eV for high-resolution
spectra), using the electromagnetic lens mode. The detection angle of the photoelectrons is
3
0°, as referenced to the sample surface. After subtraction of the Shirley-type background,
the core-level spectra were decomposed into their components with mixed Gaussian-
Lorentzian (30:70) shape lines using the CasaXPS software. Quantification calculations were
performed using sensitivity factors supplied by PHI.
Raman measurements
Micro-Raman measurements were carried out using a visible Labram HR spectrometer
(
Horiba Gr, France). The Raman backscattering was excited with 532 nm excitation
wavelength. The beam was focused on the sample surface through an optical objective to a
spot less than 1 μm. The samples were prepared by casting 50 μL of an aqueous solution of
rGO/NiO composite onto a clean silicon wafer followed by drying in an oven at 80 °C to
remove the solvent.
8

Experimental procedure for the photocatalytic reduction of nitroaromatics
Photocatalytic activity experiments were performed under visible light using a 20 watt LED
(
Model No. HP-FL-20W-F-Hope LED Opto-Electric Co., Ltd (Shenzhen, China), λ > 400
nm). Intensity of the light at vessel was measured by light meter (Lutron Electronic Light
2
Meter) and was found to be 70 W/m . In a typical reaction, a round bottom flask was charged
with 50 mg of rGO/NiO photocatalyst, 1 mmol of nitrobenzene and 2 mmol of hydrazine
monohydrate in 20 mL solvent mixture of acetonitrile/DCM/methanol. The resulting mixture
was subjected to sonication and then irradiated under visible light. All the experiments were
performed under the inert atmosphere in the presence of nitrogen. Progress of the reaction
was monitored by withdrawing the samples at certain time intervals and analyzed by gas
chromatography-flame ionization detector (GC-FID). After the completion of the reaction,
the photocatalyst was recovered by external magnet and the resulting filtrate was
concentrated under reduced pressure. Thus obtained crude product was isolated using column
chromatography. The identification of product was done by gas chromatography-mass
spectrometry (GC-MS).
3. Results and discussion
Synthesis and characterization of rGO/NiO nanocomposites
In the present study, the required rGO/NiO nanocomposites having different nickel
contents were prepared by simultaneous chemical reduction of GO and NiCl
borohydride (NaBH ), as depicted in Scheme 2. The obtained products were signed as
rGO/NiO-1, rGO/NiO-2 and rGO/NiO-3 according to the feeding amount of 9, 19 and 38 mg
of NiCl .6H O, respectively. The nickel content in the synthesized composites was found to
2
with sodium
4
2
2
be 0.2, 0.3 and 0.5 mmol/g, respectively as determined by ICP-AES analysis. Among all the
three composites, rGO/NiO-3 showed the highest catalytic activity, therefore its detailed
characterization and catalytic results are discussed in the following section.
9

The HR-TEM images of the synthesized rGO/NiO-3 nanocomposite are displayed in Fig.
1
. They reveal that the shape of the NiO NPs varies from nearly spherical to ellipsoidal and
irregular morphologies; they are closely attached to rGO sheets with an average diameter of
-10 nm.
2
The HRTEM image and SAED pattern of rGO/NiO-3 composite are provided to further
reveal the structure of Ni. As shown in Fig. 2 (a,d), the HRTEM images have atomic
columns at positions (see the fast Fourier transform (FFT) in the insets of Fig. 2b and c)
compatible with the structure of rGO with the lattice fringe spacing of 0.37 nm,
corresponding to the interplanar spacing of (002) planes. The selected area electronic
diffraction pattern (SAED) displays rings that reveal the ultra-small NiO NPs on rGO with a
polycrystalline nature (inset of Fig. 2d), in accordance with reported data [33, 34].
X-ray photoelectron spectroscopy (XPS) analysis also confirms the formation of rGO/NiO-3
nanocomposite. The XPS survey spectrum of the nanocomposite comprises carbon (C1s),
oxygen (O1s) and nickel (Ni2p) in full accordance with the chemical composition of the
material with a high Ni atomic concentration (14.58%) as shown in Fig. 3A.
The Ni2p high resolution XPS spectrum comprises four distinctive peaks at 854.4, 860.4,
8
72.5 and 878.6 eV. The peaks at 854.4 and 860.4 eV are assigned to Ni2p3/2 and its satellite,
while the features at 872.5 and 878.6 eV are due to Ni2p1/2 and its satellite. The binding
energy values are consistent with those reported for NiO [35-37]. The absence of a peak at
8
52.6±0.4 eV indicates that the amount of Ni metal in the composite is negligible [37, 38].
Similarly, the absence of an XPS peak at 856.6 eV and its satellite at 862.4 eV due to Ni2p3/2
37] suggests that Ni(OH) was not formed during the chemical process.
[
2
1
0

The C1s high resolution XPS spectrum of the initial GO is displayed in Fig. 4A. It consists of
2
peaks due to Csp (283.9 eV), C-O (286.2 eV) and C=O (287.4 eV). After reaction of GO
with NiCl
2
in the presence of NaBH , the peak at 283.9 eV becomes predominant, while the
4
peaks at 286.2 and 287.4 eV are greatly decreased, suggesting that the chemical process
removes most of the oxygen containing groups and therefore restores partially the graphenic
network (Fig. 4B). A new peak at 290.8 eV appears in the C1s XPS spectrum assigned to O-
C=O.
The Raman spectra of GO and rGO/NiO-3 are depicted in Fig. 5. The Raman spectrum
-1
-1
of GO displays two prominent peaks at 1345 cm (D band) and 1607 cm (G band),
2
corresponding to the E2g vibrational mode of sp -bonded carbon atoms in a two-dimensional
hexagonal lattice and the breathing mode of κ-point photons of A1g symmetry, respectively.
The peak intensity ratio of the D and G band (I
D
/I
G
) is commonly used as a measure of the
/I of 0.72.
disorder degree in the materials. The starting GO shows an I
D
G
-1
-
The Raman spectrum of rGO/NiO exhibits the D band (1358.6 cm ) and G band (1584.6 cm
1)
/I ) of 1.09,
of the chemically reduced graphene oxide (rGO) with an intensity ratio (I
D
G
-1
along with peaks at 360, 536, 681 and 1073 cm due to the shaking peaks of NiO [39-40].
-
1
The peaks at 360 and 536 cm could be assigned to the first order transverse optical (1TO)
and longitudinal optical (1LO) phonon modes of NiO, respectively. The peaks at 680 and
-1
1
073 cm could be attributed to second order transverse optical (2TO) and longitudinal
optical (2LO) phonon modes of NiO [39].
−
1
Comparing with GO, the G band of rGO/NiO is found to shift from 1607 to 1584.6 cm ,
which is attributed to the recovery of the hexagonal network of carbon atoms in rGO/NiO as
reported in the literature [41].
1
1

The thermal properties of the rGO/NiO-3 nanocomposite were investigated by TG–DSC
analysis (Fig. 6). The rGO/NiO-3 nanocomposite exhibits a three-step thermal
decomposition. The first mass loss (4.5 wt%) below 200 °C is due to the removal of adsorbed
water and evaporation of the intercalated water molecules. The second weight loss (25.3
wt%) in the temperature range of 200–600°C is attributed to the decomposition of the labile
oxygen-containing functional groups present in rGO. The weight loss (2.4 wt%) observed
above 600 °C results from the pyrolysis of the carbon skeleton of rGO nanosheets.
The FTIR spectra of GO and rGO/NiO-3 nanocomposite are depicted in Fig. 7. The main
-1
peaks observed at 3412, 1742, 1630, 1422, 1232 and 1068 cm in the FTIR spectrum of GO
are attributed to the stretching vibrations of O−H, C=O, C=C, C-C, C−O and C−O−C,
respectively. The intensities of these peaks decreased significantly after formation of NiO
nanoparticles on rGO sheets due to the reduction of the oxygen containing functional groups
during the chemical process. Indeed, after reduction of GO with NaBH
4
, the peaks due to O–
H, C–O (epoxy) and C–O (alkoxy) stretchings decreased dramatically, some of them
−
1
disappeared entirely. The disappearance of C=O peak at 1742 cm in the FTIR spectrum of
rGO/NiO-3 further supports GO partial reduction to rGO. In addition, the appearance of the
-1
peak at 615 cm related to NiO confirmed the presence of NiO NPs on rGO sheets (Fig. 7)
The XRD patterns of the synthesized fresh and recovered rGO/NiO are shown in Fig. 8. It is
clear from (Fig. 8) that the photocatalyst consists of crystalline NiO phases. All the
diffraction peaks in this pattern can be indexed to a single phase of face-centered cubic NiO,
1
2

which matches well with JCPDS card no. 47-1049. The absence of peaks related to Ni(OH)
in the XRD pattern, confirmed that Ni(OH) was not formed during the reduction process.
Photocatalytic reduction of nitrobenzenes
2
2
The photocatalytic activity of the synthesized NiNPs, rGO and rGO/NiO 1-3 was tested for
the reduction of nitrobenzene as a model substrate using hydrazine monohydrate as a proton
source under visible light irradiation. The results of these optimization experiments are
summarized in Table 1. There was no conversion obtained with pristine nickel nanoparticles
(
Ni NPs) (Table 1, entry 1). However, the yield of product was found to be 15.6 % when rGO
was used as a photocatalyst (Table 1, entry 2). In order to check the effect of nickel content
on the photocatalytic reactivity, three samples having different Ni-content i.e. 9, 19, and 38
mg of NiCl
2
.6H O, respectively were synthesized. The yield of product increased with the
2
nickel content and was found to be 54.6 % and 72.6 %, respectively, when rGO/NiO-1 (0.2
Ni mmol/g) and rGO/NiO-2 (0.35 Ni mmol) were used as photocatalysts under identical
conditions (Table 1, entry 3-4). In case of using rGO/NiO-3 (0.5 Ni mmol/g) as photocatalyst
almost quantitative conversion with 97.2% yield of aniline was obtained within 8 h (Table 1,
entry 5). Further increase in nickel content (0.75 mmol/g) did not show any significant effect
on the activity and afforded almost similar yield (97.5 %). However, a large excess of Ni (1.0
mmol/g) on the GO surface makes the photocatalyst less reactive (78.5% conversion), which
is believed to be due to increased electron-hole recombination and reaching of saturation
point where further transfer of electrons from Ni to substrate might not be possible (Table 1,
entry 7). Therefore, 0.5 mmol/g was the optimum loading and used for the detailed study.
Furthermore, the reaction did not take place in dark conditions at ambient temperature in all
o
cases as shown in Table 1 (entries 1-5). At higher temperature (65 C) under identical
conditions in dark, the reaction was found to occur and afforded moderate product yield
(
62%) after 24 h. However, the use of visible light under mild conditions represents the
1
3

advantage of the present methodology over thermally catalyzed reaction at higher
temperature. These results indicated that although reaction may occur at thermally higher
temperature, light illumination is essential to progress the reaction efficiently. The presence
of hydrazine hydrate as a source of protons and electrons was found to be vital and no
reaction occurred in its absence (Table 1, entry 5). Furthermore, 50 mg of photocatalyst
amount was found to be optimum and further increase in catalyst amount did not influence
the reaction to any significant extent. We have used solvent mixture
(
acetonitrile/dichloromethane/methanol) for the photocatalytic reactions owing to the better
solubility of the substrates and good dispersion of the photocatalyst in the reaction medium.
In case of using acetonitrile and methanol mixture, slightly poor product yield (88.5%) was
obtained which we assume due to the poor mixing/solubility of the substrates. Whereas, the
reaction was found to be slow in mixture of dichloromethane and methanol, which is
attributed to the poor mixing/solubility in both solvents. The reaction in acetonitrile alone
also provided poor conversion (65%) which is most likely due to the poor dispersion of the
catalyst. The presence of methanol was found to be beneficial for better dispersion of the
photocatalyst during the reaction. However, the mixing of ternary solvent system
(
acetonitrile, dichloromethane and methanol) provided better homogenous medium along
with better solubility of the substrates and good dispersion of the photocatalyst for the
reaction, thus we have chosen this combination as reaction medium for the experiments.
Further, the reaction was explored for various substituted nitrobenzene derivatives under
optimized reaction conditions and the results are summarized in Table 2. All the substrates
showed high to excellent conversion to corresponding amino derivatives within 8-10 h
without revealing any significant effect of the substituents. More particularly, the substrates
having electron donating groups required comparatively lesser reaction times than the
1
4

substrates having electron withdrawing groups. In the present study, hydrazine hydrate has
been used as sacrificial agent that provided necessary electron and protons for the reduction
of nitrocompounds.
Recycling experiments were also conducted in order to study the photocatalytic stability
and activity. Under the above optimized reaction conditions, reduction of nitrobenzene was
performed to check the recycling of the photocatalyst. After completion of the reaction, the
photocatalyst was recovered by external magnetic effect and the resulting filtrate was
subjected to usual workup and GC analysis. The recovered photocatalyst was washed with
methanol, dried and used for subsequent runs. The recovered photocatalyst was reused for
subsequent six runs. The results of these experiments are summarized in Fig. 9. No
significant loss was observed in the activity of the recycled catalyst, and the product yield
remained almost unchanged even after six recycling experiments, which confirmed the true
heterogeneous nature of the developed photocatalyst. Furthermore, an inductively coupled
plasma-atomic emission spectrometry (ICP-AES) analysis of the photocatalyst after six
recycling experiments showed a nickel content of 0.48 mmol/g which is nearly similar to that
of a fresh catalyst (0.5 mmol/g). These results confirmed that the developed photocatalyst had
not shown any leaching and was truly heterogeneous in nature (Fig. 9).
Mechanism of the reaction
The exact mechanism of the reaction is not known at this stage; however, based on the
analytical results and existing literature, a plausible mechanism is proposed as shown in
Scheme 3. According to the existing reports, reduction of nitrobenzene to aniline can proceed
by two possible reaction pathways [24,42]. The first direct route includes nitrosobenzene and
N-phenylhydroxylamine intermediates, while second condensation route include
1
5

condensation of nitrosobenzene and N-phenylhydroxylamine to give azoxybenzene,
azobenzene and hydrazobenzene intermediates. In order to confirm the reaction pathway, we
had withdrawn aliquots from the reaction mixture after 8 and 16 h for HPLC analysis
(
Agilent 1120 compact LC equipped with a Qualisil BDS C-18 column with a flow rate of 1
mL/min and the eluent consisted of 70% methanol and 30% water solution) (Figure 10). The
appearance of nitrosobenzene, N-phenylhydroxylamine and aniline peaks in the HPLC
chromatogram confirmed the formation of these intermediates (direct route). Next, we
performed the photoreduction of azobenzene under similar reaction conditions. After the
reaction only hydrazobenzene can be obtained and no aniline was detected, which further
confirmed that photoreduction of nitrobenzene proceeded through direct route with the
formation of nitrosobenzene and N-phenylhydroxylamine as reactive intermediates.
Furthermore, rGO/NiO-3 exhibits a large absorbance in the 1.5-6.5 eV range (Figure 11). The
peak close to 6 eV could be due to rGO and/or to NiO [43]. The very large absorption in the
1
.5 – 4.7 eV range could result only from the presence to defects present in NiO [44] as the
absorption of rGO at these low energies has much decreased. Under visible light, electronic
transitions between the valence band (VB) and defects energy levels located in the band gap
of NiO leave free holes in the VB. Similarly, transitions between defects energy levels and
the conduction band (CB) create free electrons in the CB. Therefore free carriers are able to
reach the NiO-rGO interface, thus becoming available for redox reaction.
Reduced graphene oxide (rGO) due to the presence of extensive network of conjugated sp2
carbons provides excellent mobility of electrons which prevents the electron-hole
recombination. Thus due to the better charge separation on the rGO sheets the electrons are
easily available for the reduction of nitrobenzene however the holes generated in the valence
band of NiO provide oxidation of hydrazine hydrate. Hydrazine works as sacrificial electron
and proton donor for the reduction (Scheme 3).
1
6

Conclusion
The present paper demonstrated a facile methodology for large scale synthesis of nickel oxide
nanoparticles decorated reduced graphene oxide (rGO/NiO) via simultaneous chemical
reduction of GO and NiCl
2
with sodium borohydride (NaBH ). The developed material was
4
found to be as efficient visible light active photocatalyst for the reduction of nitroaromatic
derivatives to their corresponding amino compounds. Hydrazine monohydrate was used as
the source of protons and electrons for the targeted reaction. After completion of the reaction,
the photocatalyst could readily be recovered by simple external magnet and could be reused
for six runs without any significant loss of its activity. The developed synthetic procedure and
application of rGO/NiO will provide a viable solution for the decomposition of organic
pollutant such as nitrobenzenes under mild visible light irradiation.
References
[
1]
K. Teegardin, J.I. Day, J. Chan, J. Weaver, Org. Process Res. Dev. 20 (2016) 1156-
163.
1
[
[
2]
3]
D. Sun, L. Ye, Z. Li, App. Catal. B: Environ. 164 (2015) 428-432.
S. Murgolo, F. Petronella, R. Ciannarella, R. Comparelli, A. Agostiano, M.L. Curri,
G. Mascoloa, Catal. Today 240 (2015) 114-124.
[
[
[
[
4]
5]
6]
7]
J. Xiao, Y. Xie, H. Cao, Chemosphere 121 (2015) 1-17.
M. Vallejo, M.F.S. Román, I. Ortiz, A. Irabien, Chemosphere 118 (2015) 44-56.
X. Lang, X. Chen, J. Zhao, Chem. Soc. Rev. 43 (2014) 473-486.
C.K. Prier, D.A. Rankic, D.W.C. MacMillan, Chem. Rev. 113 (2013) 5322-5363.
1
7

[
8]
9]
A.J. Wang, H.Y. Cheng, B. Liang, N.Q. Ren, D. Cui, N. Lin, B.H. Kim, K. Rabaey,
Environ. Sci. Technol. 45 (2011) 10186-10193.
[
[
A. Corma, P. Concepcion, P. Serna, Angew. Chem. Int. Ed. 46 (2007) 7266-7269.
10] J. Wang, Z. Yuan, R. Nie, Z. Hou, X. Zheng, Ind. Eng. Chem. Res. 49 (2010) 4664-
669.
4
[
[
[
[
11] A.S. Kulkarni, R.V. Jayaram, Appl. Catal. A 252 (2013) 225-230.
12] W.Y. Xu, T.Y. Gao, J.H. Fan, J. Hazard. Mater. 123 (2005) 232-241.
13] A. Agrawal, P.G. Tratnyek, Environ. Sci. Technol. 30 (1995) 153-160.
14] A. Tanaka, Y. Nishino, S. Sakaguchi, T. Yoshikawa, K. Imamura, K. Hashimoto, H.
Kominami, Chem. Commun. 49 (2013) 2551-2553.
[
[
15] S. Füldner, P. Pohla, H. Bartling, S. Dankesreiter, R. Stadler, M. Gruber, A. Pfitzner,
B. König, Green Chem. 13 (2011) 640-643.
16] G. Richner, J.A. Bokhoven, Y.M. van Neuhold, M. Makosch, K. Hungerbühler, Phys.
Chem. Chem. Phys. 13 (2011) 12463-12471.
[
[
[
17] H. Huang, J. Zhou, H. Liu, Y. Zhou, Y. Feng, J. Hazard. Mater. 178 (2010) 994-998.
18] R.M. Mohameda, F.M. Ibrahimc, J. Ind. Eng. Chem. 22 (2015) 28-33.
19] A. Kumar, P. Kumar, C. Joshi, M. Manchanda, R. Boukherroub, S.L. Jain,
Nanomaterials 6 (2016) 59.
[
[
20] A. Kumar, P. Kumar, S. Paul, S.L. Jain, Appl. Surf. Sci. 386 (2016) 103-114.
21] M.S. Deenadayalan, N. Sharma, P.K. Verma, C.M. Nagaraja, Inorg. Chem. 55 (2016)
5
320-5327.
22] R. Mas-Balleste, C. Gomez-Navarro, J. Gomez-Herrero, F. Zamora, Nanoscale 3
2011) 20-30.
[
(
[
[
23] D.R. Haag, H.H. Kung, Topics Catal. 57 (2014) 762-773.
24]. K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, Nature Chem. 2 (2010) 1015-1024.
1
8

[
[
25]. G. Eda, C. Mattevi, H. Yamaguchi, H. Kim, and M. Chhowalla, J. Phys. Chem. C
13 (2009) 15768–15771.
1
26] Y. Yamada, Y. Suzuki, H. Yasuda, S. Uchizawa, K. Hirose-Takai, Y. Sato, K.
Suenaga, S. Sato, Carbon 75 (2014) 81-94.
[
[
[
[
[
27] Y. Du, H. Chen, R. Chen, N. Xu, Appl. Catal. A 277 (2004) 259-264.
28] H. Liu, J. Deng, W. Li, Catal. Lett. 137 (2010) 261-266.
29] G. Chen, F. Wang, F. Liu, X. Zhang, Appl. Surf. Sci. 316 (2014) 568-574.
30] Z. Ji, X. Shen, G. Zhu, H. Zhou, A. Yuan, J. Mater. Chem. 22 (2012) 3471–3477.
31] H. Wang, H. Sanchez Casalongue, Y. Liang, H. Dai, J. Am. Chem. Soc. 132 (2010)
7
472–7477.
[
[
[
[
32] X. Zhu, H. Dai, J. Hu, L. Ding, L. Jiang, J. Power Sources 203 (2012) 243–249.
33] K. Bhowmik, A. Mukherjee, M.K. Mishra, G. De, Langmuir 30 (2014) 3209−3216.
34] N. Hussain, P. Gogoi, P. Khare, M.R. Das, RSC Adv. 5 (2015) 103105–103115.
35] P. Subramanian, J. Niedziolka-Jonsson, A. Lesniewski, Q. Wang, M. Li, R.
Boukherroub, S. Szunerits, J. Mater. Chem. A 2 (2014) 5525-5533.
[
[
[
36] M.A. Peck, M.A. Langell, Chem. Mater. 24 (2012) 4483−4490.
37] S. Oswald, W. Brückner, Surf. Interface Anal. 36 (2004) 17-22.
38] A.P. Grosvenor, M.C. Biesinger, R.S.C. Smart, N.S. McIntyre, Surf. Sci. 600 (2006)
1
771-1779.
[
39] N. Mironova-Ulmane, A. Kuzmin, I. Steins, J. Grabis, I. Sildos, M. Pärs, J. Phys.:
Conference Series (2007) 012039.
[
[
40] M. Zhou, H. Chai, D. Jia, W. Zhou, New J. Chem. 38 (2014) 2320-2326.
41] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'homme, I.A. Aksay, R. Car, Nano
Lett. 8 (2008) 36-41.
1
9

[
42]. X. Jie Y. B. Chen, L. Q. Zheng, L. Z. Wu and C. H. Tunga, Green Chem. 16 (2014)
082–1086
1
[
[
43]. J. S. Fain, J.W. Mares and S.M. Weiss, J. Nanopart. Res 17 (2015) 331-341 .
44]. N. M. Hosny, Polyhedron 30 (2011) 470–476
2
0

Fig. 1: High resolution transmission electron microscopy (HRTEM) images of rGO/Ni
nanocomposite.
2
1

Fig. 2: HRTEM images (a), fast Fourier transform (FFT) (b), high profile of NiO NPs on
rGO, and (d) SAED of rGO/NiO nanocomposite.
2
2

(B)
1
7 ev
890
880
870
860
850
Binding Energy (eV)
Fig. 3: X-ray photoelectron spectroscopy (XPS) analysis: (A) survey and (B) Ni2p high
resolution spectra of rGO/NiO-3.
2
3

2
(
A)
(B)
Csp
C-OH
286
C=O
288
O=C-OH
2
94
292
290
284
282
280
2
94 292 290 288 286 284 282 280
Binding energy (ev)
Binding Energy (eV)
Fig. 4: C1s high resolution XPS spectra of (A) GO and (B) rGO/NiO-3 nanocomposite.
2
4

rGO-Ni NPs
GO
-1
-
1
-
1
D 1358.6 cm
G 1584.6 cm
-
1
G 1607.2 cm
D 1339 cm
-
1
2
D 2920 cm
-1
2
D 2712.8 cm
5
00
1000
1500
2000
2500
3000
3500
4000
5
00
1000
1500
2000
2500
3000
3500
4000
-
1
-
1
Raman shift (cm )
Raman shift (cm
)
Fig. 5. Raman spectra of GO and rGO/NiO nanocomposite.
2
5

1
1
08
00
Mass/%
DTG/(%/min) 0.3
0
mass change : - 4.49 %
-
0.3
9
8
7
6
2
4
6
8
-0.6
mass change : - 25,32 %
-
-
-
-
0.9
1.2
1.5
1.8
mass change : -2,36 %
Reisdual mass : 67.83 % 978.6°C
-2.1
5
0
150 250 350 450 550 650 750 850 950
Temp./°C
Fig. 6: TGA and DTG curves of rGO/NiO nanocomposite.
2
6

Fig. 7: FTIR spectra of GO and rGO/NiO-3 nanocomposite.
2
7

Fig.8: Wide angle XRD of: a) Fresh rGO/NiO; b) Recovered rGO/NiO.
2
8