Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilinesat room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex asphotocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity thanhomogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. Theheterogeneous photocatalyst was found to be robust and could easily be recovered and reused for severalruns without any significant loss in photocatalytic activity

1. Applied Surface Science 386 (2016) 103–114
Contents lists available at ScienceDirect
Applied Surface Science
journal homepage: www.elsevier.com/locate/apsusc
Visible light assisted reduction of nitrobenzenes using Fe(bpy)3
+2
/rGO
nanocomposite as photocatalyst
Arvind Kumara
, Pawan Kumara
, Subham Paulb
, Suman L. Jaina,∗
a
Chemical Science Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India
b
Refinery Technology Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India
a r t i c l e i n f o
Article history:
Received 1 February 2016
Received in revised form 24 May 2016
Accepted 25 May 2016
Available online 3 June 2016
Keywords:
Reduced graphene oxide
Photocatalysis
Nitrobenzenes
Anilines
Iron complex
a b s t r a c t
Visible-light-induced photocatalytic reduction of aromatic nitrobenzenes to the corresponding anilines
at room temperature using reduced graphene oxide (rGO) immobilized iron(II) bipyridine complex as
photocatalyst is described. The rGO-immobilized iron catalyst exhibited superior catalytic activity than
homogeneous iron(II) bipyridine complex and much higher than metal free rGO photocatalysts. The
heterogeneous photocatalyst was found to be robust and could easily be recovered and reused for several
runs without any significant loss in photocatalytic activity.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
The reduction of nitrobenzenes to the corresponding anilines is
an industrially important reaction because anilines are important
intermediates for the synthesis of dyes, biologically active com-
pounds, pharmaceuticals, rubber, photographic and agricultural
chemicals [1,2]. The traditional methods for reduction of nitroben-
zenes include catalytic hydrogenation, electrolytic reduction, and
metal catalyzed reductions [3–5]. However these processes suffer
from certain drawbacks such as the use of potentially explosive H2
gas, high pressure reactors, hazardous and harmful reagents like
mineral acids etc. Furthermore, the catalytic reduction of a nitro
compound using hydrogen gas is generally carried out at a high
temperature (100–150 ◦C) and high pressure (10–50 bar) which
provide low selectivity of the product mainly due to the nonse-
lective hydrogenation of other functional groups [6,7].
In recent years, photocatalytic reduction of nitrobenzene into
aniline using semiconductor photocatalysts has gained consider-
able interest as these reactions occur under mild and ambient
temperature conditions [8,9]. Among the known semiconductors,
TiO2 based heterogeneous photocatalysts have been widely used
for reduction of nirtrobenzenes [10–12]. However, these photo-
catalysts work only under UV irradiation, which is a small part of
∗ Corresponding author.
E-mail addresses: suman@iip.res.in, sumanjain@hotmail.com (S.L. Jain).
the solar spectrum and also need special reaction vessels. In order
to improve their efficiency in the visible region, surface modifica-
tion of the TiO2 photocatalyst by doping of metal or metal oxides,
oxide halides i.e. PbPnO2X (Pn = Bi, Sb; X = Br, Cl) and sensitization
with dyes has also been demonstrated [13–15]. However, transition
metal doping commonly generates a discrete level in the forbidden
band of the photocatalyst, which causes low-mobility of electrons
and holes in the dopant level and thus provide limited activity
enhancement. Recently metal complexes owing to their fascinating
properties such as higher stability in the reaction medium, higher
visible light absorbance and better charge separation have been
distinguished to be superior and efficient photocatalysts in organic
transformations over conventional organic synthesis [16]. Among
the known metal complexes, [Ru(bpy)3]2+ complex has widely been
used, however its limited accessibility, high cost and toxic nature
makes its utility limited from practical viewpoints [17,18]. Further-
more, homogeneous nature of the catalyst and its higher solubility
in common organic solvents along with water makes its recovery
and recycling difficult. To address the issue associated with the
catalyst recovery, immobilization of these metal complexes to pho-
toactive semiconductor supports constitute a logical and promising
approach [19]. The immobilization of metal complexes to photoac-
tive support not only enhances their efficiency but also make their
recovery and recycling feasible [20,21].
Since its discovery in 2004, graphene has attracted consider-
able interest owing to its unique mechanical, thermal, optical, and
electrical properties [22]. The unique two dimensional structures
http://dx.doi.org/10.1016/j.apsusc.2016.05.139
0169-4332/© 2016 Elsevier B.V. All rights reserved.

2. 104 A. Kumar et al. / Applied Surface Science 386 (2016) 103–114
Scheme 1. Visible light assisted photoreduction of nitrobenzenes.
with its high specific surface area have made graphene an attractive
photocatalyst as well as ideal support for constructing new type of
graphene-based photocatalysts for photocatalytic reactions [23].
In this regard, extensive research work has done on the develop-
ment of novel graphene-based semiconductor photocatalysts for
photocatalytic hydrogen generation and CO2 reduction [24,25]. For
example Xiang et al., recently published a review on semiconductor
graphene based photocatalysts for solar fuel production including
hydrogen generation and CO2 reduction [26]. Putri et al., reviewed
the applications of heteroatom doped graphene in photocatalysis
[27]. Min et al., reported dye-cosensitized graphene/Pt photocata-
lyst for high efficient visible light hydrogen evolution [28]. Kong
et al., reported novel Pt–Sn alloy decorated graphene nanohy-
brid [29] and amorphous CoSnxOy decorated graphene nanohybrid
photocatalyst for highly efficient photocatalytic hydrogen evolu-
tion [30]. In another report they described dye-Sensitized NiSx
catalyst decorated on graphene for highly efficient reduction of
water to hydrogen under visible light irradiation [31]. Similarly
a number of reports are known on photocatalytic CO2 reduction
using graphene based photocatalysts [32]. In this regard, we have
recently reported graphene oxide immobilized cobalt phthalocya-
nine [33] and ruthenium trinuclear polyazine complex [34] for the
photo-reduction carbon dioxide to methanol under visible light
irradiation.
In the present paper we have synthesized a low cost, easily avail-
able and environmentally benign iron bipyridne [Fe(II)(bpy)3]2+
complex which subsequently grafted to rGO support to make
it recoverable and recyclable. The synthesized heterogeneous
(Fe(bpy)3@rGO 3) catalyst was used for the photoreduction of
nitrobenzenes to corresponding anilines using hydrazine hydrate
at room temperature under visible light irradiation (Scheme 1).
2. Experimental section
2.1. Materials
Iron(II) chloride (98%), 2,2 -bipyridine (99%), graphite flakes,
ammonium hexafluoro phosphate (99.9%) was purchased from
Aldrich were of analytical grade and used without further purifi-
cation. All other chemicals were of A.R. grade and used without
further purification.
2.2. Synthesis of [Fe(II)(bpy)3](PF6)2 complex [35]
In a typical synthesis, 0.80 mmol iron(II) chloride (0.1 g) was
dissolved in a minimum amount of water and in another solution
2.56 mmol bipyridine (0.4 g) was dissolved in a minimum amount
of ethanol. Both solutions were mixed together under stirring and
then an aqueous solution of ammonium hexafluorophosphate was
added to obtain a deep red color precipitate. The obtained precip-
itate was collected by filtration and washed with cold water and
ethanol. Yield: 75%, UV–vis (␭max) = 285 nm, and 523 nm.
2.3. Synthesis of graphene oxide [36]
Exfoliated graphene oxide was synthesized by oxidation of
graphite with KMnO4 and H2SO4 according to the literature pro-
cedure. Briefly, in a round bottom flask immersed in a ice bath, 2 g
graphite flakes were taken and then 68 mL H2SO4, 1.50 g sodium
nitrate was added; the resulting suspension was stirred for 5 min.
Then 9.0 g KMnO4 was added slowly to this mixture and the result-
ing mixture was stirred for additional 5 days. Next to this 5% diluted
H2SO4 (100 mL) was added and heated at 90 ◦C for 2 h with continu-
ous stirring. To this mixture 30 wt% H2O2 solution (approximately
5.4 mL) was added and stirred for 2 h at room temperature. The
raw GO was isolated by centrifugation (6000 rpm) and washed with
H2SO4 (3 wt%), H2O2 (0.5 wt%) and HCl (3 wt%). Final washing was
done with distilled water until pH of filtrate became neutral to get
exfoliated graphene oxide.
2.4. Synthesis of reduced graphene oxide [37]
Reduced graphene oxide was synthesized by hydrothermal
method by using water and ethanol as a solvent. GO (400 mg)
was dispersed in water/ethanol (60 mL/30 mL) mixture and soni-
cated for 2 h. The obtained suspension was transferred to a 100 mL
teflon-sealed autoclave and maintained at 120 ◦C for 24 h. This
hydrothermal treatment reduces oxygen carrying functionalities of
graphene oxide. The resulting reduced graphene oxide (rGO) was
recovered by filtration, washed by water, and dried at 60 ◦C for 24 h.
2.5. Synthesis of iron(II) bipyridine and reduced graphene oxide
nanocomposite
For the synthesis of Fe(bpy)3@rGO nanocomposite 0.25 g of
[Fe(bpy)3]PF6 and 1.0 g reduced graphene oxide was added in
250 mL acetonitrile/water (1/1) mixture. This mixture was soni-
cated for 30 min to dispersing the reduced graphene oxide. Then
obtained mixture was stirred for 24 h at room temperature. The
obtained catalyst was filtered with PTFE filter and washed with ace-
tonitrile and water. Fe content of synthesized catalyst was found
to be 1.1 wt% (0.197 mmol/g) as determined by ICP-AES analysis.
Thus the calculated value of loading of Fe(bpy)3(PF6)2 complex in
composite should be 16 wt%.
2.6. Photocatalytic reduction of nitrobenzene
The photocatalytic activity of synthesized catalyst was checked
under visible light by using 20 W LED (Model No. HP-FL-20W-F-
Hope LED Opto-Electric Co., Ltd., ␭ >400 nm). In a borosil round
bottom flask 25 mg of Fe(bpy)3@rGO catalyst was taken and 25 mL
of acetonitrile/DCM(dichloromethane)/methanol was added. The
resulting mixture was sonicated for 10 min to disperse the cata-
lyst. Next to this 0.1 mmol aromatic nitro compound and 1 mmol
hydrazine monohydrate was added to round bottomed flask and
sealed with a rubber septum. The reaction mixture was irradiated
in visible light with the collection of samples at regular intervals.
The collected samples were analyzed by TLC and GC to monitor
the progress of reaction. After completion of reaction solvent was
removed under reduced pressure and the crude product was puri-
fied by column chromatography. The identification of product was
done by GC–MS and 1H NMR.
2.7. Chemical and structural characterizations
The rough structure of materials was determined with the help
of scanning electron microscopy (SEM) image collected on FE-SEM
(Jeol Model JSM-6340F). To get fine morphologies of the synthe-
sized materials, high resolution transmission electron microscopy
was performed by using FEI-TecnaiG2 TwinTEM operating at an
acceleration voltage of 200 kV. The samples for HR-TEM were
obtained by dispersing them into a minimum amount of water
and deposited carbon coated copper grid. Vibrational spectra of
samples for executing various functional groups was collected by
using Fourier transform infrared spectroscopy and recorded on

3. A. Kumar et al. / Applied Surface Science 386 (2016) 103–114 105
Scheme 2. Synthetic procedure of Fe(bpy)3
+2
/rGO catalyst 3.
PerkinElmer spectrum RX-1 IR spectrophotometer having potas-
sium bromide window. The phase and crystalline structure phase
was determined with powder X-ray diffraction pattern recorded on
Bruker D8 Advance diffractometer at 40 kV and 40 mA with Cu K␣
radiation (␭ = 0.15418 nm). Raman spectra of materials were col-
lected at room temperature using a Raman microprobe (HR-800
Jobin-Yvon) with 532 nm Nd-YAG excitation source. XPS measure-
ments were obtained on a KRATOS-AXIS 165 instrument equipped
with dual aluminum–magnesium anodes by using MgK␣ radiation
(h␯ = 1253.6 eV) operated at 5 kV and 15 mA with pass energy 80 eV
and an increment of 0.1 eV. To overcome the charging problem, a
charge neutralizer of 2 eV was applied and the binding energy of C1s
core level (BE ≈ 284.6 eV) of adventitious hydrocarbon was used as
a standard. The XPS spectra were recorded by using a nonlinear
square method with the convolution of Lorentzian and Gaussian
functions, after subtracting a polynomial background from the raw
spectra. Absorption profile of iron(II) bipyridine complex in ace-
tonitrile and solid UV of other samples was collected on Perkin
Elmer lambda-19 UV–vis-NIR spectrophotometer using a 10 mm
quartz cell, using BaSO4 as reference. The BET surface area(SBET),
BJH porosity, mean pore diameter and other surface properties of
materials were examined by N2 adsorption-desorption isotherm
at 77 K by using VP; Micromeritics ASAP2010. Thermo gravimetric
analysis for calculating the thermal degradation pattern material
and various functionalities present was carried out using a thermal
analyzer TA-SDT Q-600. Analysis was carried out in the temper-
ature range of 40–800 ◦C under nitrogen flow with heating rate
10 ◦C/min. Proton (1H) and carbon (13C) NMR of the iron complex
was taken at 500 MHz by using Bruker Advance-II 500 MHz instru-
ment. The iron content of catalyst was determined by inductively
coupled plasma atomic emission spectrometer (ICP-AES, DRE, PS-
3000 UV, Leeman Labs Inc., USA). The photoreduction products
were analyzed and quantified with GC–MS.
3. Results and discussion
3.1. Synthesis and characterization of Fe(bpy)3@rGO
nanocomposite 3
During the present study, exfoliated graphene oxide was syn-
thesized from graphite by following the modified Hummer method
[36]. The harsh oxidation of graphite with KMnO4 and H2SO4 gen-
erates lot of oxygen containing functionalities at the surface of
graphene oxide that facilitate the separation of sheets due to the
repulsion between oxygen atoms. The synthesized graphene oxide
was subjected to hydrothermal treatment with ethanol and water
to convert it into reduced graphene oxide (rGO) 1 [37]. The larger
surface area and high electron mobility of electrons due to the
conjugated structure make reduced graphene oxide a better pho-
toactive support for anchoring of homogeneous metal complex
[38]. The iron(II) bipyridine complex 2 as synthesized by follow-
ing the literature procedure [34] was grafted to rGO surface by the
␲-␲ interaction between bipyridine and graphite sheets as shown
in Scheme 2.
The surface morphology of synthesized GO, rGO and 3 was deter-
mined with the scanning electron microscopy as shown in Fig. 1.
The SEM image of GO (Fig. 1a) shows crumpled, twisted structure
due to the folding of exfoliated sheets. After reduction of GO to
rGO the wrinkles and folds were found to be increased without
having any significant change in surface morphology (Fig. 1b). The
non-covalent attachment of 2 to rGO nanosheets generate some
erupted structures as shown in Fig. 1c which are probably due to the
␲–␲ interaction between iron(II) bipyridine complex and graphene
sheets. The EDX pattern of Fe(bpy)3@rGO 3 clearly indicates the
presence of iron in the synthesized material (Fig. 1d).
The overlapped, transparent sheets in the HR-TEM image of GO
suggested few layer thickness of sheets due to the better oxidation
of graphite nanosheets (Fig. 2a). After the reduction of graphene
oxide to rGO no major change in the structure was observed
(Fig. 2b). Similarly, immobilization of Fe(bpy)3
+2 complex units
on the rGO support did not make any significant change in the
morphology of catalyst (Fig. 2c). Selected area electron diffraction
pattern of the synthesized composite 3 indicated that the material
was amorphous in nature (Fig. 2d). Furthermore, the number of
only few rings in the SAED pattern confirms that most of the sheets
were single layered.
FT-IR spectra of the samples are shown in Fig. 3. Graphite exhibit
a very less intense peak at 3432 cm−1 due to the presence of inter-
calated water in the form of moisture Fig. 3a [39]. Graphene oxide
shows a characteristic peak at 3430 cm−1 corresponding to the
combined vibration of OH groups located on the edge of sheets
and OH of COOH groups. The peaks at 1727 cm−1 is due to the
stretching vibration of C O of carboxylic groups and the peaks
at 1267 and 1052 cm−1 are attributed to the epoxide groups and
C O C bonds respectively. The peak at 1614 cm−1 is attributed to
C C of aromatic skeleton vibrations as shown in Fig. 3b [40]. For
reduced graphene oxide the peak at 1577 cm−1 is corresponding to
the C C of aromatic skeleton vibrations of graphene sheets and a

4. 106 A. Kumar et al. / Applied Surface Science 386 (2016) 103–114
Fig. 1. FE-SEM images of: (a) GO; (b) rGO; (c) Fe(bpy)3@rGO 3; and (d) EDX pattern of 3.
small peak at 1724 cm−1 is attributed to the C O vibration of resid-
ual COOH as shown in Fig. 3c. The significant reduction of OH
peak at 3430 cm−1 clearly indicates that most of the oxygen carry-
ing groups were reduced during the hydrothermal treatment [41].
The FTIR spectrum of 3 as shown in Fig. 3d is found to be almost
similar to the rGO, which is probably due to the low loading of the
complex on the rGO surface.
The Raman spectra of pristine graphite, GO, rGO, and
Fe(bpy)3@rGO are shown in Fig. 4. Graphite shows its character-
istic G band at 1590 cm−1 as shown in Fig. 4a. In case of graphene
oxide, the characteristic D (1353 cm−1) and G bands (1591 cm−1) of
nearly equal intensity are obtained (Fig. 4b). Due to the presence of
several defects in GO in the form of sp3 carbons D band is observed.
The ratio of D and G band intensity (ID/IG) is found to be nearly 1.00,
suggesting that large number of defects is presented in the form of
epoxide and other oxygen containing functionalities on GO sheet
[42]. In case of reduced graphene oxide the intensity of D band is
decreased mainly due to the partial removal of oxygen contain-
ing groups during the hydrothermal treatment (Fig. 4c) [43]. This
was a clear indication of restoration of ␲ conjugated system. The
non covalent attachment of [Fe(bpy)3]+2 complex on rGO did not
change the position or intensity of D and G bands in Fe(bpy)3
+2/rGO
catalyst 3 (Fig. 4d).
XRD diffractogram of graphite gave a sharp peak at 2␪ value
26.6◦ due to 002 plane reflections with 0.34 nm interlayer spacing
(Fig. 5a). GO gave a characteristics peak at 2␪ value 10.2◦ corre-
sponding to the 001 plane with the 0.87 nm interlayer distance
(Fig. 5b) [44]. After the reduction of graphene oxide the peak at
10.2◦ was disappeared due to the significant reduction of oxygen
carrying functionalities and a very small peak at 2␪ value 24.8◦ was
observed, probably due to some degree of sheets stacking at some
points [45]. After the functionalization of rGO with iron complex
moieties, the XRD pattern remained almost unchanged which is
mainly due to the lower loading of the complex.
In order to confirm the presence of iron complex 2 in the catalyst
3 and to investigate the surface chemical properties, XPS analysis
was performed (Fig. 6). The wide survey scan of catalyst 3 confirmed
the presence of C1s, N1s, O1 s and Fe2p in the catalyst (Fig. 6a). High
resolution XPS of Fe(bpy)3@rGO 3 in Fe 2p region gave two charac-
teristic peaks due to Fe2p3/2 and Fe2p1/2 at binding energy 710.7 eV
and 724.1 eV, respectively [46]. The high-resolution C1 s spectrum
for GO exhibited four peak components at 284.7, 286.4, 287.9 and
288.9 eV, which were specific to C C, C O, C O, and COOH groups
respectively (Fig. 6c) [47]. While in the C1 s spectrum of catalyst 3
the significant reduction in the peak intensity of oxygen contain-
ing function groups confirmed the successful removal of oxygen
functionalities during the reduction of GO to rGO (Fig. 6d).
Nitrogen adsorption desorption isotherm was used for deter-
mining the surface properties of the synthesized materials. The
Type (IV) loop of isotherm for GO, rGO, and 3 confirmed
the mesoporous nature of the material [48]. BET surface area
(SBET), total pore volume (VP) and mean pore diameter (rp)

5. A. Kumar et al. / Applied Surface Science 386 (2016) 103–114 107
Fig. 2. TEM images of: (a) GO; (b) rGO; (c) 3, and (d) SAED pattern of 3.
Fig. 3. FTIR Spectra of: (a) Graphite; (b) GO; (c) rGO and (d) catalyst 3.

6. 108 A. Kumar et al. / Applied Surface Science 386 (2016) 103–114
Fig. 4. Raman spectra of: (a) graphite; (b) GO; (c) rGO and (d) 3.
was found to be 87.247 m2 g−1, 0.1212 cm3 g−1 and 5.5566 nm
for GO, 32.72 m2 g−1, 0.074 cm3 g−1 and 9.046 nm for rGO and
13.16 m2 g−1, 0.070 cm3 g−1 and 21.308 nm for 3 respectively. The
Fig. 5. XRD Pattern of: (a) graphite; (b) GO; (c) rGO and (d) 3.
changes observed in the surface properties clearly depicted the suc-
cessful intercalation of iron complex units 2 between the layers of
rGO sheets (Fig. S1).
Fig. 6. XPS spectra: (a) survey scan of 3; (b) high resolution Fe 2p spectrum of 3; (c) C1 s spectrum of GO; (d) C1 s spectrum of 3.

7. A. Kumar et al. / Applied Surface Science 386 (2016) 103–114 109
Fig. 7. UV–vis absorption spectra of: (a) 2; (b) GO; (c) rGO; (d) 3.
The UV–vis spectra of synthesized materials are shown in Fig. 7.
The electronic UV–vis spectra of Fe(bpy)3](PF6)2 in acetonitrile
gave a intense peak at 284 nm due to inter ligand ␲ → ␲* (LLCT)
transition. The weak peak at 523 nm was attributed to the metal
d(␲) → ligand(␲*) (MLCT) transition (Fig. 7a). UV–vis spectra of GO
(Fig. 7b) shows its characteristics intense absorption band near to
230 nm due to ␲ → ␲* transition of aromatic ring electrons and a
small hump near to 300 nm was due to n → ␲* of carbonyl group
[49]. Reduced graphene oxide shows a sharp intense peak at 315 nm
due to the restoration of conjugated ␲ aromatic system (Fig. 7c).
The disappearance of peaks at 230 and 300 nm of GO and appear-
ance of a new peak at 315 nm was in well concordance with the
literature and suggested the significant deoxygenating of GO dur-
ing the reduction process. As shown in Fig. 7c, reduced graphene
oxide gave strong absorption in the visible region which is mainly
attributed to the black body effect of the graphene sheets having
conjugated sp2 carbon–carbon structure with lower oxygen to car-
bon atomic ratio [50]. However, graphene oxide (GO) has higher
oxygen to carbon atomic ratio due to the presence of a number of
sp3 carbons bonded with various oxygen functional groups. Owing
to the presence of sp3 carbons, graphene oxide possesses less con-
jugated structure than rGO and therefore shows poor absorption in
visible light (Fig. 7b). The UV–vis spectra of iron complex function-
alized rGO 3 was found to be similar to rGO, which indicates the
lower loading of Fe(bpy)3](PF6)2 2 in composite and therefore the
peak due to MLCT transition of iron complex is suppressed by the
absorption of rGO (Fig. 7d).
Tauc’s plots of synthesized materials were obtained to confirm
the visible light activity of the photocatalysts (Fig. 8). It can be seen
from Tauc’s plot (Fig. 8a) that iron complex 2 exhibited three band
gap values i.e at 2.02 eV, 3.60 eV due to MLCT and 3.99 eV due to
LLCT which suggested that the complex 2 can absorb well in the
visible region. For graphene oxide the band gap was found to be
at 2.2 eV due to ␲–␲* transition and 3.6 eV due to n–␲* transition,
which clearly indicated that the visible light mediated transitions
were not possible (Fig. 8b). While for rGO due to residual oxy-
genated zone on sheets the value of band gap was found to be
1.3 eV and 2.8 eV, suggesting the visible light activity (Fig. 8c). After
the attachment of iron complex 2 to rGO, the band gap value of 3
remained almost nearly same probably due to low loading of the
iron complex to the support (Fig. 8d).
Thermal stability of the synthesized materials was determined
by thermogravimetric (TG) analysis as shown in Fig. 9. Thermogram
of graphite (Fig. 9a) indicated the stability of the material up to
700 ◦C and then a sharp decomposition due to the formation of CO
and CO2. In case of GO three major weight losses were observed, the
first weight loss at 100 ◦C was due to the loss of water, the second
major weight loss from 150 to 250 ◦C was due to the loss of oxy-
gen containing functionalities. Subsequently a linear weight loss
pattern was obtained due to the degradation of carbon of aromatic
sheets (Fig. 9b) [51]. The thermogram of rGO shows a small weight
loss in the region between 200 and 475 ◦C which is attributed to
the removal of most of the oxygen containing functionalities dur-
ing reduction. After 475 ◦C, there is a sharp and steady weight loss
because of the degradation of carbon containing sheets (Fig. 9c)
[52]. The almost similar degradation behaviour of rGO and graphite
clearly indicates the successful synthesis of reduced graphene oxide
after reduction process. The catalyst 3 also exhibited similar degra-
dation pattern as rGO because of low loading of iron complex.
3.2. Photo-catalytic activity
After the successful synthesis and characterization, the rGO
immobilized iron catalyst 3 was tested for the photocatalytic reduc-
tion of nitrobenzene as the representative substrate under the
visible light irradiation using hydrazine hydrate as a source of pro-
ton. For the comparison purpose, the same reaction was performed
using graphene oxide, reduced graphene oxide and homogeneous
iron(II) bipyridine complex 2 as photocatalysts under identical
experimental conditions. A time vs. product yield graph for all cat-
alyst components is shown in Fig. 10. Furthermore, the results of
these experiments are summarized in Table 1. As shown in Fig. 10
and Table 1, the catalyst 3 was found to be more active in compari-
son to the homogeneous iron complex 2 as catalyst. The less activity
of homogeneous complex 2 in comparison to rGO immobilized cat-
alyst 3 may be due to the short lived excited state of iron complex
which makes it difficult to transfer electrons efficiently to nitroben-
zene due to faster recombination of charge. However, after the

8. 110 A. Kumar et al. / Applied Surface Science 386 (2016) 103–114
Fig. 8. Tauc plots of: (a) 2; (b) GO; (c) rGO; (d) 3.
Table 1
Effect of conditions on reduction of nitrobenzene to anilinea
.
Entry Reaction condition Time/h Catalyst Yield (%)b
1 Dark 12 2 0.6c
GO 0
rGO 0
3 1.6
2 Visible light, DCM as
solvent
12 2 6.4c
GO 4.2
rGO 0
3 18.5
3 Visible light, methanol
as solvent
12 2 12.8c
GO 5.8
rGO 3.2
3 47.0
4 Visible light,
Acetonitrile as solvent
12 2 23.0c
GO 7
rGO 9
3 88.0
rGO + 2 29.0d
a
Reaction conditions; nitrobenzene (0.1 mmol); catalyst, 25 mg; hydrazine
monohydrate (1 mmol); Irradiation, White cold LED ␭ > 400 nm, Power at reaction
vessel 70 W/m2
.
b
determind by GCMS; DCM = dichloromethane.
c
equivalent amount as presented in catalyst 3.
d
Physical mixtuture of rGO + 2, Fe(bpy)2(PF6)2 = 0.197 mmol/g cat.
immobilization of complex to rGO, the higher conductive surface
of rGO provide better mobility of electrons and therefore led to the
better charge separation on the surface. These photogenerated elec-
trons can be used for the reduction of nitrobenzene to aniline. The
reduction of nitrobenzene did not occur while using GO and rGO as
catalysts under described reaction conditions. The absence of any
reaction product by using rGO as catalyst suggested that the iron is
the real catalyst and rGO is mainly improving the catalytic activity
of complex 2 via providing better charge separation. Similarly, in
dark the reduction of nitrobenzene using catalyst 3 afforded negli-
gible yield of aniline, which confirms the visible light driven nature
of reaction (Table 1, entry 1). Furthermore, in an additional reaction
by using a physical mixture of rGO and 2 in equimolar ratio as in cat-
alyst 3, the comparable product yield as homogeneous catalyst was
achieved. This finding clearly depicted that the immobilization of
complex 2 on rGO support provides better transportation of charge
on rGO’s surface and therefore gives significant enhancement in
the activity (Table 1, entry 4). To evaluate the effect of solvent, the
reduction of nitrobenzene was performed in different solvents such
as dichloromethane, methanol and acetonitrile (Table 1, entry 2–4).
Among the various solvents studied, acetonitrile was found to be
most suitable solvent for this reaction.
However the maximum loading of Fe(bpy)3(PF6)2 complex on
reduced graphene oxide that can be achieved without leaching was
16 wt% having 1.1 wt% iron content as determined by ICP-AES anal-
ysis. However to explore the effect of amount of Fe(bpy)3(PF6)2
on the product yield, a number of Fe(bpy)3/rGO composites with
variable iron content as 0.1, 1.0, 1.5, 2.0 and 3.0 wt% were synthe-
sized and tested under identical reaction conditions. It has been
observed that the yield of aniline is increased with increasing the
iron content of Fe(bpy)3/rGO composite from 0.5 to 2.0 wt%. How-
ever further increase in the amount to 3.0 wt% does not increase

9. A. Kumar et al. / Applied Surface Science 386 (2016) 103–114 111
Fig. 9. TGA thermogram of: (a) graphite; (b) GO; (c) rGO and (d) 3.
Fig. 10. Time vs product yield for reduction of nitrobenezene using different pho-
tocatalysts.
the yield, indicating that the saturation point is reached where fur-
ther transfer of electrons from complex to reduced graphene oxide
sheets is not possible (Fig. 11).
After completion of these optimization studies, the reaction was
generalized to various substituted nitrobenzenes and the results of
these experiments are summarized in Table 2. The developed cata-
lyst 3 afforded moderate to excellent yield of the corresponding
anilines under visible light irradiation. Among the various sub-
stituents, the substrates containing electron donating groups were
Fig. 11. Product yield vs Fe(bpy)3(PF6)2 amount in catalyst after 6 h of irradiation.
found to be more reactive and provided higher yield of the corre-
sponding anilines (Table 2, entry 2–6).
Further we have carried out the recycling experiments to check
the stability and reusability of the photocatalyst 3. The catalyst
was separated from reaction mixture by centrifugation and washed
with ethanol and dried at 50 ◦C. The loss of the catalyst during cen-
trifugation was considered during the addition of fresh reactant in
recycling run. The recovered catalyst was used for subsequent six
runs under described experimental conditions (Fig. 12). As shown
in Fig. 12, the catalytic activity of the photocatalyst remained nearly
same even after the six runs without any significant change in the

10. 112 A. Kumar et al. / Applied Surface Science 386 (2016) 103–114
Table 2
Photocatalytic reduction of nitro compoundsa
.
Entry Reactant Product Time/h Conv (%)b
Yield (%)c
TOF (h−1
)
1 8 90 88 11
2 8 93 90 11.25
3 8 92 89 11.12
4 8 94 92 11.50
5 8 95 92 11.5
6 8 97 96 12
7 10 86 83 8.3
8 12 80 75 6.25
9 12 82 78 6.50
10 12 76 72 6.00
a
Reaction conditions; nitro compound, 0.1 mmol; catalyst, 25 mg; hydrazine monohydrate, 1 mmol; Irradiation, White cold 20 W LED ␭ > 400 nm, Power at reaction vessel
70 W/m2
.
b
Conv. (%) =
(n◦
x
−nx,
)
n◦
x
x 100.
c
Yield (%) =
mole of product
mole of substrate
x 100 .
where nx
◦
is the number of moles of substrate before the reaction and nx’ is the number of moles of substrate after the reaction.
Fig. 12. Recycling experiments.
yield of the desired product. Furthermore, the recovered catalyst as
obtained after six runs was analyzed by ICP AES and the iron con-
tent of catalyst was found to be 1.0 wt% in comparison to 1.1 wt%
for freshly synthesized catalyst.
It is well documented that reduction of nitrobenzene to aniline
can proceed by two possible reaction pathways [53]. The first direct
route include nitrosobenzene and N-phenylhydroxyl amine inter-
mediates while second condensation route include condensation
of nitrosobenzene and N-phenylhydroxylamine to give azoxyben-
zene, azobenzene and hydazobenzene intermediates (Scheme 3). In
order to confirm that reduction of nitrobenzene to aniline proceed
via direct route we have carried out photoreduction of azobenzene
under similar reaction conditions. After the reaction only hydra-
zobenzene can be isolated and no aniline was detected, confirms
that reduction of nitrobezene proceeds through direct route.
Although the exact mechanism of the reaction is not clear at this
stage, however a single electron transfer initiated plausible mech-
anism was proposed on the basis of existing reports (Scheme 4)
[54,55]. Reduction of graphene oxide transform sp3 carbons into
sp2 carbons and changes semiconductor sheet of graphene oxide
to conductive reduced graphene oxide [56–58]. Reduced graphene
oxide (rGO) due to presence of extensive network of conjugated sp2
carbons provides excellent mobility of electrons. After the absorp-
tion of visible light iron(II) complex on the surface of rGO gets
excited from singlet to triplate state via MLCT transition. The exited
triplet state transfer electrons to nitrobenzene and oxidized to Fe3+
state. Reduced graphene oxide due to conductive surface provides
high mobility to photogenerated electrons which prevent charge
recombination [59,60]. The oxidized Fe3+ complex receive elec-
trons from hydrazine through reductive quenching to convert back
into Fe2+ state. Hydrazine works as sacrificial electron and proton

11. A. Kumar et al. / Applied Surface Science 386 (2016) 103–114 113
Scheme 3. Reaction pathways for reduction of nitrobenzene to anilines.
Scheme 4. Plausible reaction mechanism of reduction of nitro compounds using photocatalyst 3.
donor which finally after donating six protons and six electrons get
converted to its oxidation product “nitrogen”.
3/2NH2NH2 → 3N2 + 6e−
+ 6H+
As the electron transfer from Fe(II) complex is a single elec-
tron process, the reduction of nitrobenzene to aniline occurs in a
step-wise manner through the formation of nitrosobenzene and
N-phenylhydroxylamine as intermediates by using six electrons
and six protons. Thus, reduced graphene oxide increases catalytic
activity of the iron complex through facilitating electron transfer to
reactant molecule along with the extensive sites for the attachment
[61].
4. Conclusions
The present paper describes a first successful example of hetero-
genized homogeneous photocatalyst for the reduction of aromatic
nitro compounds to corresponding amines under visible light irra-
diation at room temperature. Iron complex because of its good
visible light absorbance and rGO due to better charge transporta-
tion on its surface exhibited synergistic effect to boost the reduction
of nitrobenzenes to corresponding anilines. The rGO-immobilized
iron catalyst exhibited higher catalytic activity in comparison to
homogeneous iron(II) bipyridine and much higher than metal free
rGO photocatalysts. The heterogenized photocatalyst was found to
be highly stable and could easily be recovered and reused for several
runs without any significant loss in photocatalytic activity.
Acknowledgements
We kindly acknowledge Director, CSIR-IIP for his permission to
publish these results. AK and PK are thankful to UGC and CSIR, New
Delhi, respectively for research fellowship. Authors also thanks to
analytical sciences division of CSIR-IIP, Dehradun for providing sup-
port in analysis.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.05.
139.
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