# Photocatalytic water splitting ability of Fe/MgO-rGO nanocomposites towards
hydrogen evolution

Fahmida Sharmin, Dayal Chandra Roy and M. A. Basith<EMAIL_ADDRESS>Nanotechnology Research Laboratory, Department of Physics, Bangladesh
University of Engineering and Technology, Dhaka-1000, Bangladesh.

DOI: 10.1016/j.ijhydene.2021.09.072

###### Abstract

Photocatalytic water splitting has greatly stimulated as an ideal technique
for producing hydrogen (H2) fuel by employing two renewable sources, i.e.,
water and solar energy. Here, we have adopted a facile hydrothermal approach
for the successful synthesis of reduced graphene oxide (rGO) incorporated
Fe/MgO nanocomposites followed by thermal treatment at inert atmosphere to
investigate their ability for photodegradation and photocatalytic hydrogen
evolution via water splitting. Transmission Electron Microscopy images of
Fe/MgO-rGO nanocomposite ensured the distribution of Fe/MgO nanoparticles
throughout rGO sheets. Notably, all rGO supported nanocomposites, especially
the one, thermally treated at 500 ∘C at Argon (Ar) atmosphere has demonstrated
significantly higher photocatalytic efficiency towards the photodegradation of
a toxic textile dye, rhodamine B, than pristine MgO and commercially available
Degussa P25 titania nanoparticles as well as other composites. Under solar
irradiation, Fe/MgO-rGO(500) nanocomposite exhibited 86% degradation of
rhodamine B dye and generated almost four times higher H2 via photocatalytic
water splitting compared to commercially available P25 titania nanoparticles.
This promising photocatalytic ability of the Fe/MgO-rGO(500) nanocomposite can
be attributed to the improved morphological and surface features due to heat
treatment at inert atmosphere as well as escalated charge carrier separation
with increased light absorption capacity imputed to rGO incorporation.

##  I Introduction

With the industrial revolution and an increase in the world population, the
consistent growth in energy consumption, coupled with high pollution levels,
has sparked tremendous research interest in alternative and renewable energy
fuels Jain ; Tama ; Tong ; Abe . Solar energy is considered as a potential
alternative energy source due to its abundance and sustainability. The
efficient conversion of solar energy into chemical fuel is the ultimate
challenge in confronting the energy crisis of today’s world. Nowadays, the
production of H2 through photocatalytic water splitting Fu ; Sub  using solar
energy is gaining sincere attention in the scientific community because they
are simple, efficient and have significant potential for future development Ma
; Lv ; Nayak ; Winter . Owing to its low cost and environmentally friendly
nature, hydrogen has emerged as the most feasible substitute for traditional
fossil fuels as a green and efficient energy source with high storage capacity
Basith2 ; Shaheer ; Jain1 ; Bicakova ; Mori . Though at present, renewable
energy contributes only $\sim$5% of commercial hydrogen production mainly
through water electrolysis Cao2 , the remaining $\sim$95% hydrogen is
primarily generated from fossil fuels Dawood  which is neither cost-effective
nor eco-friendly. Therefore, the development of efficient and cost-effective
photocatalysts using renewable sources remains challenging. On the other hand,
photodegradation using photocatalysts for environmental remediation has been
recognized as an effective approach for converting hazardous industrial
contaminants into non-toxic byproducts Reddy ; Patil ; Yang . In this context,
several quantified photocatalysts, including TiO2, ZnO, CdS, SrTiO3,
CdS/TiO2/Pt, and TiO2/Ag2O, have been widely used PA ; Ga ; Lang ; Saad ; Sola
; Mazierski . Unfortunately, these existing photocatalysts suffer from
limitations such as large band gap and quick recombination of $e^{-}$ \-
$h^{+}$ pairs, limiting their photocatalytic performance and solar energy
utilization efficiency.
Magnesium oxide (MgO) with high surface reactivity, extensive absorption
capacity, and ease of production has been used recently for various
applications, including catalysis, ceramics, waste-water remediation, anti-
bacterial agents, and so on Mohamed ; Mou ; Sutradhar . However, due to its
large band gap energy and fast charge carrier recombination, bare MgO has
negligibly lower photocatalytic performance Mag . These issues were resolved
by adding low concentrations of transition metal ions at the conducting MgO
interface, which prevents the formation of recombination centers for photo-
generated $e^{-}$ \- $h^{+}$ pairs Pin . Among transition metal ions, iron
(Fe) has a high transition ability and can trap multiple types of electron-
hole pairs Ba ; Ot ; Man . Thus incorporating Fe on MgO may improve the
separation of the maximum number of electrons and holes, thereby avoiding
recombination Borhade . In recent years, graphene, a two-dimensional (2D)
network of hexagonally structured sp2 -hybridized carbon atoms, has sparked
significant research interest in various energy adaptation applications as
well as in the field of photocatalysis Ramirez ; Xu ; Marlinda . The
importance of graphene is attributed to its excellent charge carrier mobility,
electrical conductivity, high specific surface area and superior chemical
stability Purwajanti ; Go . Graphene has two oxidative derivatives, between
which reduced graphene oxide (rGO) demonstrates enhanced photocatalytic
activity compared to graphene oxide (GO) when they are being tailored as
supporting material to make a composite with photocatalysts Wang2 ; Hafeez .
The oxygen components over a pure GO surface may give a reverse action against
its high electron conductivity, turning GO into an insulator Sadhukhan ; 1Yu .
Several attempts have been adopted to synthesize pure graphene by reducing
oxygen epoxy; among them, thermal treatment at inert atmosphere has been
considered to be quite promising for producing high strength reduced graphene
oxide by eliminating oxygen-containing functional groups from the surface of
GO Shi . Besides, thermal treatments may significantly modify the properties
of photocatalysts since they have a notable impact on the physicochemical
characteristics of the respective materials Feng ; Yeoh .
Motivated by manifestations mentioned above, in this investigation, a simple
hydrothermal route has been adopted for the synthesis of rGO supported Fe/MgO
nanocomposites followed by heat treatment at different temperatures at Argon
(Ar) atmosphere. Several extensive studies have been carried out to synthesize
nanoscale MgO powders using different synthesis techniques, such as solid
state, sol-gel, chemical precipitation method, etc Hong ; Karnaukhov ; Kumar1
. Among various methods, the hydrothermal technique is regarded as one of the
most successful approaches to obtain the desired structures with high phase
purity and uniform size distribution with an additional advantage of being low
cost Basith1 . According to our acknowledgement, here, for the first time, we
report the application of hydrothermally synthesized Fe/MgO-rGO nanocomposite
photocatalysts on wastewater remediation and generation of H2 through water
splitting. Furthermore, the influence of heat treatment on the structure,
morphology as well as the catalytic properties of Fe/MgO-rGO nanocomposites
has also been explored. The photocatalytic performance of widely used Degussa
titania (P25) nanoparticles has also been studied under similar experimental
conditions for comparative analysis. It has been observed that Fe/MgO-rGO
after heat treatment at 500 ∘C at Ar atmosphere demonstrated higher potential
in H2 production via solar driven water splitting compared to other
synthesized nanocomposites and P25 nanoparticles. In most of the previous
investigations Bri ; Sol , the photocatalytic performance of rGO based
composites have been observed to be improved either by addition of H2O2 doses
in dye and photocatalyst mixture, or by enhancing the power of light
irradiation and controlling the pH of the solution, etc. However, in this
investigation, Fe/MgO-rGO(500) nanocomposite has demonstrated satisfactory
performance in RhB degradation and H2 evolution without using any additional
reagents or external energy input and in neutral pH condition. We anticipate
that this unique property might distinguish our synthesized composite from
those reported by others. Finally, a plausible reaction mechanism for the
profound photocatalytic activities of Fe/MgO-rGO(500) has been suggested.

##  II Materials and Methods

###  II.1 Materials

The chemical reagents used were analytical grade Manganese nitrate hexahydrate
[Mg(NO3)3.6H2O], Ferric nitrate nonahydrate [Fe(NO3)3.9H2O], Graphite flakes,
Sodium nitrate [NaNO3], Sodium hydroxide [NaOH], Potassium permanganate
[KMnO4], Sulfuric acid [H2SO4], Hydrochloric acid [HCl] and Hydrogen peroxide
[H2O2]. All the reagents were manufactured by Sigma-Aldrich, Germany.

###  II.2 Sample preparation

####  II.2.1 Synthesis of Fe/MgO nanocomposites

Fe/MgO nanocomposites were synthesized by adopting a facile and low-cost
hydrothermal technique Basith1 . The first step was the synthesis of MgO
nanoparticles, for which 1 mmol Mg(NO3)3.6H2O were dissolved in 50 ml of 5M
NaOH solution and magnetically stirred for about 4 hours to form a homogeneous
mixture. The solution was then transferred into a Teflon-lined autoclave and
heated at 180 ∘C for 12 hours. The suspension was allowed to cool down to room
temperature naturally. Afterwards, the final products were washed several
times with deionized water and ethanol followed by centrifugation, and dried
in the oven for 10 hours at 100 ∘C. In the next step for incorporating Fe on
MgO nanoparticles, 1 mmol Mg(NO3)3.6H2O and 0.03 mmol Fe(NO3)3.9H2O were
dissolved in 50 ml of 5M NaOH solution and the rest of the procedure was
similar to that described earlier. After the hydrothermal process, both MgO
and Fe/MgO were treated at various temperatures at Ar atmosphere for further
characterization.

Figure 1: A schematic diagram of the synthesis scheme for rGO supported Fe/MgO
nanocomposites

####  II.2.2 Synthesis of Fe/MgO-rGO nanocomposites

Initially, 1 mmol Mg(NO3)3.6H2O and 0.03 mmol Fe(NO3)3.9H2O were dissolved in
50 ml of 5M NaOH solution and stirred for 4 hours to get suspension I. In the
next step, graphite oxide, prepared by modified Hummer’s method Paulchamy ,
was sonicated for 2 hours in DI water to obtain graphene oxide (GO). Then 10
mg of GO was dispersed in 10 ml DI water to make suspension II . The
suspension I and II were combined and stirred for an additional 1 hour. The
mixture was then transferred into a Teflon-lined autoclave and heated at 180
∘C for 12 hours. The hydrothermally synthesized products were washed and
dried, and we have obtained the final composites after heat treatment of the
dried products at 400 and 500 ∘C at Ar atmosphere. A schematic representation
of the synthesis process has been depicted in Fig. 1.

###  II.3 Characterization techniques

The Crystalline structure and phase purity of the samples were characterized
by the powder X-ray diffraction (XRD) spectra using a diffractometer
(PANalytical Empyrean, UK; Cu-$K_{\alpha}$,$\lambda$ = 1.5418 Å).
Additionally, the Rietveld refinement approach was conducted for detailed
analysis of the experimentally acquired XRD data using FullProf Suite Software
Basith1 . The surface morphology were investigated from field emission
scanning electron microscopy (FESEM) imaging along with the energy dispersive
X-ray (EDX) analysis using a scanning electron microscope (XL30SFEG; Philips,
Netherlands) and transmission electron microscopy (TEM) (Talos F200X; Thermo
fisher scientific, USA) at an operating voltage of 200 kV. With an aim to
observe the optical properties, the absorption spectra of the as-synthesized
samples were investigated in the range of 200–600 nm using a UV-visible
spectrophotometer (UV-2600; Shimadzu, Japan). Steady-state photoluminescence
(PL) analysis was carried out using a Spectro Fluorophotometer (RF-6000;
Shimadzu, Japan) to study the rate of photoinduced electron-hole recombination
during the photocatalytic process Tama .

###  II.4 Photocatalytic degradation of dye

The photocatalytic performance of the synthesized nanocomposites was assessed
by monitoring the degradation of rhodamine B (RhB) dye in DI (deionized) water
solution without the presence of any sacrificial agents. A 500W Xe lamp
(Hamamatsu Photonics, Japan) having irradiance value of 100 mW cm-2 was used
as a solar simulator Basith2 . For each trial, 80 mg of photocatalyst powder
was dissolved in 15 mg/L of RhB solution. Prior to irradiation, the solution
was magnetically stirred in the dark for 60 minutes to attain an adsorption
equilibrium of RhB on the surface of the photocatalysts. After the lamp was
turned on, 4 ml of the solutions were extracted at 1 hour intervals and
centrifuged to filter out the catalyst particles. The absorbance of the
solution at a specific wavelength was measured using a UV-vis
spectrophotometer to investigate the changes in RhB concentration caused by
solar irradiation. The photodegradation process was repeated four times under
identical experimental procedures to check the stability and reusability of
the photocatalysts.

###  II.5 Photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution experiment was conducted in a slurry type
reactor, where 20 mg of photocatalyst were mixed with 30 ml DI water and kept
under magnetic stirring before being illuminated by a 500W Hg-Xe lamp.
Meanwhile, the system was purged with argon gas for 30 minutes to ensure an
inert atmosphere for the splitting process. The evolved hydrogen (H2) was
extracted at one-hour intervals under illumination, then analyzed and
quantified using the GC (Shimadzu, Japan) device equipped with the thermal
conductance detector (TCD) and gas analyzer. The GC programming was reverse
polarized in order to attain hydrogen peaks in the upward direction to allow a
comparative analysis with the peak intensities of the various gases produced
Basith2 .

##  III Results and discussions

###  III.1 Structural characterization

####  III.1.1 Crystal structure

The XRD patterns of GO, Mg(OH)2 and Fe/Mg(OH)2-GO are shown in ESI (electronic
supporting information) Fig. S1. The preparation of GO was confirmed from the
characteristic peak (002) at Bragg position around 12 $\theta$ Jalil , as
shown in ESI Fig. S1(a). During the preparation of MgO following the
hydrothermal method, hexagonal Mg(OH)2 were formed at 180 ∘C temperature by
redox reactions Devaraja . Fig. S1(b) and (c) inserted in ESI demonstrate that
the incorporation of Fe and GO into MgO during hydrothermal treatment resulted
in the formation of Fe/Mg(OH)2-GO at the same temperature. In the XRD pattern
of Fe/Mg(OH)2-GO, we observe a peak for GO which indicates that the GO was not
reduced. This can be attributed to the absence of any chemically reducing
agent during hydrothermal preparation.
The Rietveld refinement method was used for detailed structural analysis of
the samples after heat treatment at Ar atmosphere. Fig. 2(a), (b), (c), and
(d) display the Rietveld refined powder XRD patterns of synthesized materials
after heat treatment at Ar atmosphere. MgO and Fe/MgO nanoparticles heated at
500 ∘C are represented by MgO(500) and Fe/MgO(500), and Fe/MgO-rGO
nanocomposites heated at 400, and 500 ∘C are represented by Fe/MgO-rGO(400)
and Fe/MgO-rGO(500), respectively. It is confirmed that following the heat
treatment at 500 ∘C at Ar atmosphere, the synthesized hexagonal structured
Mg(OH)2 was converted into cubic MgO which is consistent with a previous
investigation Devaraja .

Figure 2: XRD patterns of (a) MgO(500), (b) Fe/MgO(500) nanoparticles, and (c)
Fe/MgO-rGO(400), (d) Fe/MgO-rGO(500) nanocomposites after heat treatment at Ar
atmosphere

Moreover, it can be observed that all samples display characteristics
diffraction peaks indexed at (111), (200), (220), (311) and (222) planes which
are in good accordance with cubic MgO with Fm-3m space group (JCPDS 1-75-447
Petnikota ). Fig. 2(a) and (b) show that due to the incorporation of Fe in
MgO(500) nanoparticles, the peak positions remained the same, which suggested
that Fe did not alter MgO crystal structure and was suitably deposited on the
surfaces of MgO Borhade . Though the presence of an additional iron oxide
phase $\alpha$-Fe2O3 was observed in Fe/MgO(500) samples (Fig. 2(b)) due to
iron substitution, which was eliminated when GO was added into the sample and
heated at inert atmosphere as shown in Fig. 2(c) Pin .
Comparing the XRD patterns of ESI Fig. S1(c) and Fig. 2(c), the formation of
Fe/MgO-rGO is assured at 400 ∘C temperature with an absence of GO peak
indicating the total reduction of GO. This is consistent with the reduction
mechanism of GO by thermal deoxygenation, in which an increase in temperature
facilitates the removal of oxygen-containing functional groups from GO Ganguly
; Jin . This also explains the disappearance of $\alpha$-Fe2O3 phase due to GO
substitution. With the increased temperature at Ar atmosphere, slightly
decreased peak broadening as well as an increase in peak intensities was
observed (Fig. 2(c) and (d)). This decreasing trend of peak broadening
indicates the increase in particle size, suggesting agglomeration at a higher
temperature Kayani ; Sinornate .
Table 1 presents different structural parameters along with constituent phases
for as-synthesized samples. Notably, the calculated lattice parameters are
well consistent with the values reported in the literature Devaraja1 . The
lattice parameters and volume corresponding to the MgO phase remain almost
unaltered throughout the synthesis process, which confirmed that the crystal
structure was not distorted. The refined and observed XRD patterns for the
cubic MgO phase are in reasonable agreement according to the fitting
parameters (Rwp, Rp, RExp and $\chi^{2}$) and atomic coordinates (ESI Table
S1).

Table 1: Structural parameters of synthesized nanomaterials and nanocomposites. Sample | Constituent | Crystallographic phase | Space group | a = b (Å) | c (Å) |  V (Å3)
---|---|---|---|---|---|---
MgO (500) | MgO | Cubic | F m -3 m | 4.220 | 4.220 | 75.176(0.005)
Fe/MgO (500) | MgO | Cubic | F m -3 m | 4.227 | 4.227 | 75.576(0.005)
|  $\alpha$-Fe2O3 | Trigonal | R -3 c | 5.430 | 5.429 | 138.649(0.001)
Fe/MgO-rGO (400) | MgO | Cubic | F m -3 m | 4.250 | 4.250 | 76.794(0.008)
Fe/MgO-rGO (500) | MgO | Cubic | F m -3 m | 4.221 | 4.221 | 75.243(0.005)

####  III.1.2 Surface morphology

The FESEM image in Fig. 3(a) demonstrates randomly aggregated, thin, crumpled
sheets like structure of GO Jalil . In Fe/Mg(OH)2-GO samples, without heat
treatment (ESI Fig. S2(a)), numerous Fe/Mg(OH)2 hexagons with different
particle sizes were predominately observed to be grown and randomly
distributed on the GO surface.

Figure 3: FESEM images of (a) GO, (b) Fe/MgO(500), (c) Fe/MgO-rGO(400),and (d)
Fe/MgO-rGO(500)

Fig. 3(b), (c), and (d) represent the FESEM images of Fe/MgO sample heated at
500 ∘C and Fe/MgO-rGO samples heated at 400 and 500 ∘C at Ar atmosphere,
respectively. It can be seen that for Fe/MgO-rGO(400) sample, the surface are
not homogeneous, and the particles growth is not up to the mark, which can be
ascribed to the fact that hexagonal Fe/Mg(OH)2 did not completely transformed
into cubic Fe/MgO at lower heating temperature (400 ∘C). However, due to
graphene incorporation, a large number of Fe/MgO nanocrystallites nucleate and
grow into fine seed particles at the early stage of hydrothermal treatment;
with time, this is followed by the growth of Fe/MgO nuclei Turabik ; Li .
Surprisingly, when composites are formed with rGO, such consumption of finer
seed particles that lead to the growth of nuclei is interrupted due to steric
blockage of seed particle transportation caused by the interpenetrating
graphene networks WLi . However, for Fe/MgO-rGO nanocomposite at 500 ∘C
temperature, the particle size distribution seems to be more homogeneous than
the samples treated at 400 ∘C. For a further increase in temperature to 600
∘C, resulted in a significant amount of agglomeration KumarA , as can be
observed in ESI Fig. S2(b). This result implies that 500 ∘C is the optimized
Ar atmosphere required to form Fe/MgO-rGO nanocomposites with a higher degree
of uniformity in shape and size. To get further insight into the morphology of
the rGO supported Fe/MgO nanocomposite at 500 ∘C, TEM imaging was performed at
various magnifications (Fig. 4). The TEM image, Fig. 4(a) clearly demonstrated
the distribution of Fe/MgO nanoparticles on rGO sheets. The higher
magnification images, Fig. 4(b) and (c) confirmed the presence of graphene
sheets. In Fig. 4(d) we have observed bright concentric rings in the selected
area electron diffraction (SAED) pattern, which implied the polycrystalline
nature of the synthesized nanocomposite.

Figure 4: TEM images (a-c), and SAED pattern (d) of Fe/MgO-rGO(500)
nanocomposite

####  III.1.3 Elemental composition

The compositions of the synthesized samples were identified utilizing EDX
analysis. The experimental details in ESI Fig. S3 show that the desired
elements C, Mg, O and Fe are present in the synthesized samples, and no
additional element was found. The EDX spectra were taken at three distinct
positions, and the average atomic ratios of Mg, Fe, O and C were observed to
be very close to nominal composition within the instrumental accuracy.

###  III.2 Optical characterization

####  III.2.1 Optical band gap

Figure 5:  (a) UV-visible absorbance and (b) Tauc plots for band gap
estimation for MgO, Fe/MgO nanoparticles heated at 500 ∘C and Fe/MgO-rGO
nanocomposites heated at 400 and 500 ∘C at Ar atmosphere

The optical properties of all the synthesized samples were studied using UV-
visible diffuse reflectance spectroscopy (DRS) in the wavelength range of
200–600 nm. Fig. 5(a) demonstrates the room temperature UV-visible absorption
spectra of pure MgO, Fe/MgO nanoparticles and Fe/MgO-rGO nanocomposites after
heat treatment at different reaction temperatures. All the samples exhibited
continuous absorption across the UV region, at the same time, the addition of
rGO showed an increase in the optical absorption compared to pure MgO and
Fe/MgO samples, indicating higher light absorption ability of the synthesized
nanocomposites. Moreover, MgO nanoparticles exhibit a prominent absorption
peak at 286 nm, as shown in Fig. 5(a). In contrast all Fe-incorporated samples
show a blue shift in the absorption peak, which can be attributed to the
formation of donor energy levels in the actual band gap of MgO.
The optical band gap $E_{g}$ of the samples were calculated using Tauc’s
relation Basith2

$F(R)\times h\nu=A(h\nu-E_{g})^{n}$ (1)

where $h\nu$, A and $E_{g}$ denote the energy of the photon, proportionality
constant and optical band gap, respectively and F(R) is a parameter calculated
from DRS by Kubelka–Munk function Basith1 . From Fig. 5(b), the band gap
energy was determined by extrapolating the straight-line portion to the
abscissa at zero absorption co-efficient. The band gap of MgO is found to be
3.9 eV which has been reduced to 3.3 eV when Fe was incorporated into MgO.
Incorporation of rGO suppressed the crystal growth, resulting in smaller
particle size and the band gap energy of Fe/MgO-rGO(400) nanocomposite was
reduced to 2.78 eV Jalil . As the temperature rises from 400 to 500 ∘C, there
is an increase in band gap from 2.78 to 3.0 eV due to quantum confinement
effect Nanakkal . Another reason is that extremely high temperature promotes
the generation of strong bonds between the crystallites, leading to the
agglomeration of particles Zhang . Interatomic bonds are tightened and high
energy is required to break a bond, and move an electron to the conduction
band.

####  III.2.2 Photoluminescence (PL) spectra at steady-state

A photocatalyst must possess the ability to inhibit the electron-hole pair
recombination efficiently. To study this property, we have conducted the PL
analysis of our samples for an excitation wavelength of 200 nm as represented
in ESI Fig. S4. The optical band gaps from the position of the PL peaks are
observed to be 3.40, 2.69 and 3.03 eV for Fe/MgO(500), Fe/MgO-rGO(400) and
Fe/MgO-rGO(500) samples, respectively. These values are in accordance with the
band gaps obtained from UV-visible spectroscopy (Fig. 5(b)). Notably, the peak
intensity of both rGO supported samples are lower than Fe/MgO samples, which
suggests that the separation of photogenerated electron-hole pairs can be
facilitated through rGO incorporation. This result is also consistent with our
previous band gap estimation by DRS, where the band gaps of all rGO assisted
nanocomposites are smaller than those of Fe/MgO.
Moreover, for Fe/MgO-rGO(500), the peak intensity is lowest as compared to
other samples, which imply the successful separation and transfer of
photogenerated charge carriers. As a result, the photocatalytic activity of
this nanocomposite under solar irradiation increases by inhibiting electron-
hole pair recombination Ghosh . The outcome of our experiment is also in good
agreement with previous reports, where PL emission is observed to be quenched
due to rGO substitution He ; Hareesh

###  III.3 Photocatalytic degradation of rhodamine B (RhB) dye

Figure 6: Spectral changes during the deterioration of RhB over (a) Fe/MgO-
rGO(400), and (b) Fe/MgO-rGO(500) nanocomposites under solar irradiation;(c)
RhB concentration change as a function of irradiation time, (d) First-order
kinetic plots for the photodegradation of RhB, (e) Change in RhB concentration
under different wavelength conditions, and (f) Recyclability test of Fe/MgO-
rGO(500) for four successive runs

To evaluate the photocatalytic performance of the as-synthesized materials, at
first, a blank test was conducted without any catalyst. RhB did not show
significant degradation for four hours indicating the self-degradation ability
of RhB is extremely low. After that, the catalyst samples were blended with
RhB solution and kept in the dark under constant magnetic stirring in order to
achieve adsorption equilibrium. In Fig. 6(a) and (b), we have presented the
time-dependent UV-visible absorbance spectra of RhB at one-hour intervals in
the presence of Fe/MgO-rGO(400) and Fe/MgO-rGO(500) photocatalysts. The
decrease in absorbance peak as irradiation time extends, indicates the
decomposition of RhB under solar irradiation. It was observed that
incorporation of rGO, and heat treatment has resulted in a significant
improvement in degradation efficiency of the nanocomposites and the maximum
degradation of RhB was observed at 500 ∘C at Ar atmosphere. This result may be
attributed to the relationship between the stability of the crystal structure
and the effectiveness of reduced graphene oxide for solar light absorption
Zhang .
The degradation efficiency as well as degradation rate were calculated by
using the following equation

$Photocatalytic\;\;degradation(\%)=\frac{C_{0}-C}{C_{0}}\times 100$ (2)

Fig. 6(c) demonstrates the degradation characteristics of RhB as a function of
solar irradiation time in the presence of as-synthesized MgO, Fe/MgO(500),
Fe/MgO-rGO(400) and Fe/MgO-rGO(500) photocatalysts as well as commercially
available Degussa TiO2 (P25) nanoparticles for comparison. C0 and C represent
the primary and retained concentrations of RhB in the solution, respectively.
The degradation efficiency of pure MgO, Fe/MgO(500), Fe/MgO-rGO(400) and
Fe/MgO-rGO (500) was $\sim$25% , $\sim$42%, $\sim$80% and $\sim$86%,
respectively. The photodegradation curve of commercial titania (P25) was also
plotted in Fig. 6(c) for comparison. Interestingly, the degradation percentage
of all the samples except MgO is higher compared to P25. The large band gap
and quick charge carrier recombination might be responsible for this poor
degradation efficiency of MgO. Due to the presence of Fe on MgO, the band gap
was reduced, the active surface of MgO was increased, and the RhB dye
degradation efficiency was enhanced accordingly Ot ; Man . Moreover, for
comparison we have also evaluated the photodegradation ability of Fe2O3,
Fe2O3-MgO, and Fe2O3-rGO samples for comparison. It was observed that under
four hours of solar irradiation, the degradation efficiency was $\sim$38%,
$\sim$43%, and $\sim$52% for the respective samples, which was significantly
lower than that of Fe/MgO-rGO nanocomposites. The synthesis procedure of the
above-mentioned samples and the experimental findings of the photodegradation
experiments have been attached in the supplementary (ESI Fig. S5). From these
observations, it can be elucidated that the inclusion of rGO in Fe/MgO-rGO
nanocomposite significantly improved their photocatalytic performance. The
highest photocatalytic degradation efficiency of $\sim$86% was observed for
Fe/MgO-rGO nanocomposite, which was heated at 500 ∘C at Ar atmosphere. We
think that at optimum temperature of 500 ∘C maximum number of oxides were
removed from GO and resulted in the successful migration of electrons from the
conduction band (CB) of Fe/MgO to rGO Arshad . This migration inhibited
electron-hole pair recombination and resulted in an outstanding photocatalytic
performance of Fe/MgO-rGO(500) nanocomposite Gayathir . In addition, from the
FTIR analysis of Fe/MgO-rGO(500) nanocomposite we can observe peaks at 1450
and 3390 cm-1, indexed to –OH and H–O–H vibration bonds, respectively (ESI
Fig. S6). These oxygen-containing groups have an effective role in the
photocatalytic activity of catalysts by generating more hydroxyl radicals to
facilitate the photocatalytic process Arshad ; Isari .
We have incorporated the Pseudo first-order kinetics model to quantitatively
study the kinetics of the photocatalytic degradation using the following
equationSub :

$ln(C\textsubscript{0}/C)=kt$ (3)

Where C0 and C represent the same as previously stated, and k (min -1) is the
pseudo-first order rate constant of photodegradation. The rate constant was
calculated by using a linear fit to the plot of ln(C0/C) versus irradiation
time. Based on the results from Fig. 6(d), the highest degradation rate was
observed for Fe/MgO-rGO(500) with a rate constant of k=7.072×10-3 min-1, which
is 2.86 times higher than that of Fe/MgO (k=2.479×10-3 min-1). Considering the
degradation efficiency and rate constant, Fe/MgO-rGO(500) can be regarded as a
superior solar light driven photocatalyst to be used for environmental
remediation purpose. The mechanism behind this profound performance of Fe/MgO-
rGO(500) has been discussed subsequently.
Furthermore, to study the role of light with different wavelengths, the
photodegradation ability of Fe/MgO-rGO(500) nanocomposite was also evaluated
using a UV cut-off filter ($\lambda\geq$ 420 nm) where the efficiency was
obtained to be only $\sim$64% (Fig. 6(e)) which is lower than that obtained
under UV-visible irradiation. This result justifies our findings from optical
characterizations, where continuous absorption across the UV region was
exhibited by Fe/MgO-rGO composites.

####  III.3.1 Photostability of Fe/MgO-rGO(500) nanocomposite

The stability and re-usability of the Fe/MgO-rGO(500) nanocomposite has been
checked, employing a recyclability test to validate their applicability in
practical fields Basith1 . It has been observed from Fig. 6(f) that after four
consecutive photo-degradation cycles, the change in dye degradation efficiency
of the catalyst was trivial, which could be related to the loss in material
during the recovery process. The outcome of this study anticipates the
prospect of this material for repeated application in waste-water management
and other photocatalytic purposes.

###  III.4 Photocatalytic hydrogen generation

We have evaluated the efficiency of the synthesized photocatalysts as a worthy
candidate for hydrogen generation via water splitting under solar irradiation
Basith2 . Fig. 7 demonstrates the amount of H2 gas produced by our synthesized
catalysts as a function of solar irradiation time. For comparison, the
hydrogen generation under identical conditions by titania P25 nanoparticles
was also included. The hydrogen production of commercially available P25
nanoparticles was observed to be $\sim$445 $\mu$mol g-1 h-1. As shown in Fig.
7, Fe/MgO photocatalyst’s hydrogen evolution is slightly higher than that of
P25 nanoparticles. Notably, the rate of hydrogen production increased
significantly when rGO was introduced into Fe/MgO, and it was observed that
the total amount of H2 evolved under 4 hours of solar irradiation were
$\sim$1517 and $\sim$1741 $\mu$mol g-1 h-1 for the Fe/MgO-rGO(400) and Fe/MgO-
rGO(500) nanocomposites, respectively.

Figure 7: Amount of H2 evolved via water splitting with respect to irradiation
time

It is well established that Photocatalytic H2 generation mainly includes three
basic steps: (1) absorption of photons by the photocatalyst to form electron-
hole pairs; (2) separation and migration of the photo-induced charges to
different photocatalyst sites; and (3) reduction of water to evolve H2 on the
surface of the photocatalyst by the photo-generated electrons. In our case,
Fe/MgO-rGO(500) nanocomposite absorbed a significant amount of photons to
produce electron-hole pairs due to its narrow band gap and graphene, with its
high work function (4.42 eV) and superior conductivity, played the crucial
role by accepting photo-generated electrons and rapidly migrating these
electrons through its 2D plane to reactive sites, which resulted in a
significant amount of H2 evolution Liu ; Wang . For further insight, a
comparative analysis between our experimental findings and some previous
related investigations has been presented in Table 2. Noticeably, our Fe/MgO-
rGO(500) nanocomposite exhibited a comparatively increased H2 evolution rate
(HER) than other rGO supported and MgO based nanocomposites. Additionally, in
most cases, various sacrificial reagents were used as electron donors to
enhance H2 generation rate. However, in this present investigation, we have
obtained a notable result on solar hydrogen generation by water splitting
without any additional reagent.

Table 2:  An overview of the photocatalytic H2 production ability of
photocatalysts reported in recent literature.

Photocatalyst | Light source | Reactant solution | H2 yield | Reference(year)
---|---|---|---|---
|  |  | ($\mu$ mol g-1 h-1) |
SrTiO3-rGO | 300 W Xe lamp | H2PtCl6 | 363 |  He  (2016)
ZnO/rGO | 300 W Xe lamp | Triethylamine, alcohol | 616 |  Kang  (2016)
TiO2-rGO | 300 W Xe lamp; $\lambda$>300 nm | Methanol | 720 |  El (2017)
TiO2/Pt/rGO | 4 Philips PL-S (9 W) lamp; 315<$\lambda$<400 nm | Methanol | 259 |  Rivero (2018)
MgO/MgCr2O4 | Hg lamp; $\lambda\geq$ 400 nm | Methanol | 420 |  Nayak  (2018)
Bi25FeO40-rGO | 500 W Xe lamp; $\lambda$ <400 nm | Deionized water | 1562 |  Basith2  (2018)
CdS/rGO | 300 W Xe lamp; $\lambda\textgreater$420 nm | Na2S, Na2SO3 | 500 |  Hareesh  (2019)
TiO2 /rGO/LaFeO3 | 300 W Xe lamp | Methanol | 893 |  Lv  (2019)
PANI-TiO2 /rGO | 300 W Xe lamp | Triethanolamine | 806 |  Ma  (2020)
TiO2 /In0.5WO3-rGO | Xe arc lamp | Glycerol | 304 |  Shaheer  (2021)
Fe/MgO-rGO | 500 W Xe lamp | Deionized water | 1741 | Present work

###  III.5 Photocatalytic mechanism

In order to gain a better perspective of the profound photocatalytic
performance of Fe/MgO-rGO nanocomposites, a plausible mechanism for the
photodegradation of RhB under solar illumination is proposed with the help of
a schematic illustration (Fig. 8). The band edge positions were calculated
using the Mulliken electronegativity method using the following equations
Nethercot

$E_{CB}=\chi-E_{c}-\frac{1}{2}E_{g}$ (4) $E_{VB}=E_{CB}+E_{g}$ (5)

where $E_{CB}$ and $E_{VB}$ are conduction and valence band edge potentials
with respect to Normal Hydrogen Electrode (NHE) ; $\chi$ is the absolute
electronegativity, expressed as the geometric mean of the absolute
electronegativity of the constituent atoms; $E_{g}$ is the band gap of the
sample and $E_{c}$ is the energy of free electrons on the hydrogen scale
($\sim$4.5 eV). It is well established that, along with optical band gap, CBM
(conduction band minimum) and VBM (valance band maximum) play crucial roles in
catalyzing the redox reactions in the electrolyte system. Efficient
photocatalytic degradation ability of the nanocomposites can be better
understood by analyzing the redox reactions in the electrolyte system that
govern the degradation of RhB dye. For successful degradation of RhB under
solar irradiation two redox reactions are particularly important. First, the
photogenerated electron can react with the surface adsorbed oxygen (redox
potential: -0.16 eV vs. NHE) to form $\cdot O_{2}^{-}$ radicals, which will
further react with RhB to cause degradation. Second, the photogenerated holes
can react with the $OH^{-}$ radicals, ionized from the water molecules to
produce $\cdot OH$ (redox potential: 2.38 eV vs. NHE). This $\cdot OH$ can
further oxidize the RhB molecules. Therefore, a photocatalyst needs to possess
a CBM<-0.16 eV to drive the frst reaction and VBM>2.38 eV to drive the second
one efficiently Jalil .

CBM and VBM for MgO has been determined to be -1.24 and 2.66 eV, respectively,
which imply that MgO can perform the redox reactions of RhB degradation since
it has fulfilled the above mentioned criteria. However, lower absorption of
photons due to the large value of optical band gap in MgO, resulted in poor
photocatalytic activity. Introduction of iron Fe3+ ions into the MgO surface,
effectively leads to band gap narrowing, creating a sub level above the
valance band (VB) of MgO. Under solar illumination, electrons in the VB of MgO
can firstly transfer to Fe intermediate band through absorbing photons with
longer wavelength, and these electrons then transfer from intermediate band to
conduction band (CB) leaving holes in the VB. Moreover, the Fe3+ ions can be
reduced to Fe2+ ions by the photogenerated electrons during the
photocatalysis,

Figure 8: Schematic representation of electron transfer mechanism in the
photodegradation of RhB over Fe/MgO-rGO nanocomposites under solar irradiation

which greatly improves the charge separation with increased lifetime of
excited electrons and holes LiJ ; Liang . Fe2+ ions are relatively unstable
when compared to Fe3+ ions, which have half-filled 3d5 orbital. Therefore, the
trapped charges can easily release from Fe2+ ions and then migrate to the
surface to initiate the photocatalytic reaction. In addition, Fe2+ ions can be
oxidized to Fe3+ ions by transferring electrons to absorbed molecular oxygen
in the reaction solution, resulting in the formation of reactive superoxide
radicals ($\cdot O_{2}^{-}$) which can further degrade RhB Liang .
The Fe/MgO nanoparticles are supposed to be well attached to the rGO sheets as
can be seen in the TEM images. Due to its excellent electron conductivity,
graphene can greatly promote interfacial charge transfer and significantly
improve the photoactivity of Fe/MgO-graphene nanocomposites. Furthermore, due
to the $\pi$–$\pi$ conjugated interactions between the RhB molecules and the
graphene rings, graphene sheets have an exceptional ability to adsorb dye
molecules in the aqueous medium Ma . Besides, density-functional theory (DFT)
calculations revealed that the work function (WF) of graphene is 4.42 eV,
indicating the redox potential of rGO/rGO$\cdot^{-}$ is (-0.08 eV versus NHE)
more positive than the CB potential of Fe/MgO (-1.24 eV versus NHE) LiQ ; Chen
. This will result in a thermodynamically convenient transfer of
photogenerated electrons from the CB of Fe/MgO to the rGO surface, giving the
electrons more opportunities to interact with reactants and participate in
photocatalytic reactions. The photo generated electron in rGO could react with
adsorbed O2 to form $\cdot O_{2}^{-}$ radicals. The majority of $\cdot
O_{2}^{-}$ radicals will be involved in the degradation of RhB molecules,
while others may react with $H_{2}O$ to produce $\cdot OH$ radicals.
Consequently, the produced $\cdot O_{2}^{-}$ and $\cdot OH$ radicals, as well
as photogenerated electrons and holes will react with dye molecules and
degrade it into non-toxic byproducts.
The tentative mechanism can be summarized as follows

$Fe/MgO+h\nu\rightarrow Fe(e^{-})/MgO(h^{+})$ (6) $Fe^{3+}+e^{-}\rightarrow
Fe^{2+}$ (7) $Fe^{2+}+O_{2}\rightarrow Fe^{3+}+\cdot O_{2}^{-}$ (8)
$MgO(h^{+})+H_{2}O\rightarrow H^{+}+\cdot OH$ (9) $e^{-}+rGO\rightarrow
rGO(e^{-})$ (10) $rGO(e^{-})+O_{2}\rightarrow\cdot O_{2}^{-}$ (11) $RhB+(\cdot
O_{2}^{-}+\cdot OH+e^{-}+h^{+})\rightarrow Degraded\;\;products$ (12)

A similar model can be suggested for understanding the mechanism behind the
superior ability of Fe/MgO-rGO(500) nanocomposite for solar $H_{2}$ production
through water splitting (ESI Fig. S7). For successful water splitting reaction
to take place, CBM potential of the photocatalyst needs to be lower than the
proton reduction potential (0 eV vs NHE) whereas the VBM potential needs to be
greater than the oxidation potential of hydroxyl ion (1.23 eV vs NHE) Jalil .
Notably, our rGO supported nanocomposites fulfil both requirements
efficiently. Therefore, the photogenerated holes from the nanocomposite with
sufficient potential can react with the water molecules and produce $H^{+}$
along with O2 and the photogenerated electrons evolve $H_{2}$ gas by reducing
$H^{+}$.

##  IV Conclusions

We have demonstrated that incorporation of a potential electron acceptor and
reservoir rGO is an excellent choice of material to maintain $e^{-}$ and
$h^{+}$ pairs separated for a long period by suppression of charge carrier
recombination. This also resulted in an improved optical absorption and
reduction in band gap, all of which contributed to enhance the photocatalytic
performance of the Fe/MgO-rGO nanocomposites as compared to MgO and widely
used P25 nanoparticles. Notably, the hydrogen production capability via water
splitting of the synthesized nanocomposites, particularly thermally treated
Fe/MgO-rGO at 500 ∘C is significantly higher compared to that of MgO, Fe/MgO,
commercial widely used P25 nanoparticles as well as other related rGO based
composites. Based on the current findings, the synthesized nanocomposites can
be considered as a promising candidate for numerous photocatalytic
applications such as purification of water by removing harmful pollutants,
solar water disinfection, and carbon-free hydrogen production via renewable
sources like water and solar energy. Our findings with profound insights into
the kinetics of charge transfer may pave the way for researchers to construct
and optimize new graphene-based nanocomposite photocatalysts to fulfill the
modern technological requirements in environmental and energy-related
applications.

## Acknowledgements

We sincerely acknowledge Committee for Advanced Studies and Research (CASR),
Bangladesh University of Engineering and Technology for financial assistance.
The Atomic Energy Center, Dhaka is also acknowledged for providing TEM
facilities.

## Appendix A. Supplementary data

Supplementary data to this article can be found online under the DOI:
10.1016/j.ijhydene.2021.09.072.

## Data availability

The raw and processed data required to reproduce these findings cannot be
shared at this time due to technical or time limitations.

## References

  * (1) Jain IP. Hydrogen the fuel for 21st century. Int J Hydrogen Energy 2009;34(17):7368-78. https://doi.org/10.1016/j.ijhydene.2009.05.093.
  * (2) Tama AM, Das S, Dutta S, Bhuyan MD, Islam MN, Basith MA. MoS2 nanosheet incorporated $\alpha$-Fe2O3/ZnO nanocomposite with enhanced photocatalytic dye degradation and hydrogen production ability. RSC Adv 2019;9(69):40357-67. https://doi.org/10.1039/C9RA07526G.
  * (3) Tong H, Ouyang S, Bi Y, Umezawa N, Oshikiri M, Ye J. Nano-photocatalytic materials: possibilities and challenges. Adv Mater 2012;24(2):229-51. https://doi.org/10.1002/adma.201102752.
  * (4) Abe JO, Popoola AP, Ajenifuja E, Popoola OM. Hydrogen energy, economy and storage: review and recommendation. Int J Hydrogen Energy 2019;44(29):15072-86.https://doi.org/10.1016/j.ijhydene.2019.04.068.
  * (5) Fu X, Huang C, Wen J, Du Y, Zheng X. One-pot sulfurized synthesis of ZnIn2S4/S, N-codoped carbon composites for solar light driven water splitting. Int J Hydrogen Energy 2021;46(34):17697-17707. https://doi.org/10.1016/j.ijhydene.2021.02.177.
  * (6) Das S, Dutta S, Tama AM, Basith MA. Nanostructured LaFeO3-MoS2 for efficient photodegradation and photocatalytic hydrogen evolution. Mater Sci Eng B 2021;271:115295. https://doi.org/10.1016/j.mseb.2021.115295.
  * (7) Ma J, Dai J, Duan Y, Zhang J, Qiang L, Xue J. Fabrication of PANI-TiO2/rGO hybrid composites for enhanced photocatalysis of pollutant removal and hydrogen production. Renew Energy 2020;156:1008-18. https://doi.org/10.1016/j.renene.2020.04.104.
  * (8) Lv T, Wang H, Hong W, Wang P, Jia L. In situ self-assembly synthesis of sandwich-like TiO2/reduced graphene oxide/LaFeO3 Z-scheme ternary heterostructure towards enhanced photocatalytic hydrogen production. Mol Catal 2019 ;475:110497. https://doi.org/10.1016/j.mcat.2019.110497.
  * (9) Nayak S, Pradhan AC, Parida KM. Topotactic transformation of solvated MgCr-LDH nanosheets to highly efficient porous MgO/MgCr2O4 nanocomposite for photocatalytic H2 evolution. Inorg Chem 2018;57(14):8646-61. http://dx.doi.org/10.1021/acs.inorgchem.8b01517.
  * (10) Winter CJ. Hydrogen energy—Abundant, efficient, clean: A debate over the energy-system-of-change. Int J Hydrogen Energy 2009;34(14):S1-52. https://doi.org/10.1016/j.ijhydene.2009.05.063.
  * (11) Basith MA, Ahsan R, Zarin I, Jalil MA. Enhanced photocatalytic dye degradation and hydrogen production ability of Bi25FeO40-rGO nanocomposite and mechanism insight. Sci Rep 2018;8(1):1-11. https://doi.org/10.1038/s41598-018-29402-w.
  * (12) Shaheer AM, Vinesh V, Lakhera SK, Neppolian B. Reduced graphene oxide as a solid-state mediator in TiO2/In0.5WO3 S-scheme photocatalyst for hydrogen production. Sol Energy 2021;213:260-70. https://doi.org/10.1016/j.solener.2020.11.030.
  * (13) Jain IP, Lal C, Jain A. Hydrogen storage in Mg: a most promising material. Int J Hydrogen Energy 2010;35(10):5133-44. https://doi.org/10.1016/j.ijhydene.2009.08.088.
  * (14) Bičáková O, Straka P. Production of hydrogen from renewable resources and its effectiveness. Int J Hydrogen Energy 2012;37(16):11563-78. https://doi.org/10.1016/j.ijhydene.2012.05.047.
  * (15) Moriarty P, Honnery D. Prospects for hydrogen as a transport fuel. Int J Hydrogen Energy 2019;44(31):16029-37. https://doi.org/10.1016/j.ijhydene.2019.04.278.
  * (16) Cao L, Iris KM, Xiong X, Tsang DC, Zhang S, Clark JH, Hu C, Ng YH, Shang J, Ok YS. Biorenewable hydrogen production through biomass gasification: A review and future prospects. Environ Res 2020;186:109547. https://doi.org/10.1016/j.envres.2020.109547.
  * (17) Dawood F, Anda M, Shafiullah GM. Hydrogen production for energy: An overview. Int J Hydrogen Energy 2020;45(7):3847-69. https://doi.org/10.1016/j.ijhydene.2019.12.059.
  * (18) Reddy P, Cui H, Vattikuti SP, Suh Y, Park SH. Influence of Gd doping on the visible-light photocatalytic activity and magnetic properties of BiFeO3 particles. Mater Res Express 2019;6(11):115044. https://doi.org/10.1088/2053-1591/ab463d.
  * (19) Patil SB, Naik HB, Nagaraju G, Viswanath R, Rashmi SK. Sugarcane juice mediated eco-friendly synthesis of visible light active zinc ferrite nanoparticles: Application to degradation of mixed dyes and antibacterial activities. Mater Chem Phys 2018;212:351-62. https://doi.org/10.1016/j.matchemphys.2018.03.038.
  * (20) Yang H, Yang J. Photocatalytic degradation of rhodamine B catalyzed by TiO2 films on a capillary column. RSC adv 2018;8(22):11921-9. https://doi.org/10.1039/C8RA00471D.
  * (21) Peña PA, Ortega DR, Araque DG, Gordillo AH, Zanella R, Reguera E. Boosting the photocatalytic hydrogen production of TiO2 by using copper hexacyanocobaltate as co-catalyst. Int J Hydrogen Energy 2021;46(17):10312-23. https://doi.org/10.1016/j.ijhydene.2020.12.135.
  * (22) Galdámez-Martínez A, Bai Y, Santana G, Sprick RS, Dutt A. Photocatalytic hydrogen production performance of 1-D ZnO nanostructures: Role of structural properties. Int J Hydrogen Energy 2020;45(56):31942-51. https://doi.org/10.1016/j.ijhydene.2020.08.247.
  * (23) Lang D, Cheng F, Xiang Q. Enhancement of photocatalytic H2 production activity of CdS nanorods by cobalt-based cocatalyst modification. Catal Sci Technol 2016;6(16):6207-16. https://doi.org/10.1039/C6CY00753H.
  * (24) Saadetnejad D, Yıldırım R. Photocatalytic hydrogen production by water splitting over Au/Al-SrTiO3. Int J Hydrogen Energy 2018;43(2):1116-22. https://doi.org/10.1016/j.ijhydene.2017.10.154.
  * (25) Solakidou M, Giannakas A, Georgiou Y, Boukos N, Louloudi M, Deligiannakis Y. Efficient photocatalytic water-splitting performance by ternary CdS/Pt-N-TiO2 and CdS/Pt-N, F-TiO2: interplay between CdS photo corrosion and TiO2-dopping. Appl Catal B 2019 ;254:194-205. https://doi.org/10.1016/j.apcatb.2019.04.091.
  * (26) Mazierski P, Malankowska A, Kobylanski M, Diak M, Kozak M, Winiarski MJ, Klimczuk T, Lisowski W, Nowaczyk G, Zaleska-Medynska A. Photocatalytically active TiO2/Ag2O nanotube arrays interlaced with silver nanoparticles obtained from the one-step anodic oxidation of Ti–Ag alloys. ACS Catal 2017;7(4):2753-64. https://doi.org/10.1021/acscatal.7b00056.
  * (27) Mohamed RM, Shawky A, Mkhalid IA. Facile synthesis of MgO and Ni-MgO nanostructures with enhanced adsorption of methyl blue dye. J Phys Chem Solids 2017;101:50-7. https://doi.org/10.1016/j.jpcs.2016.10.009.
  * (28) Moussavi G, Mahmoudi M. Removal of azo and anthraquinone reactive dyes from industrial wastewaters using MgO nanoparticles. J Hazard Mater 2009 ;168(2-3):806-12. https://doi.org/10.1016/j.jhazmat.2009.02.097.
  * (29) Sutradhar N, Sinhamahapatra A, Pahari SK, Pal P, Bajaj HC, Mukhopadhyay I, Panda AB. Controlled synthesis of different morphologies of MgO and their use as solid base catalysts. J Phys Chem C 2011;115(25):12308-16. https://doi.org/10.1021/jp2022314.
  * (30) Mageshwari K, Mali SS, Sathyamoorthy R, Patil PS. Template-free synthesis of MgO nanoparticles for effective photocatalytic applications. Powder Technol 2013;249:456-62. https://doi.org/10.1016/j.powtec.2013.09.016.
  * (31) Pinilla JL, Utrilla R, Karn RK, Suelves I, Lázaro MJ, Moliner R, García AB, Rouzaud JN. High temperature iron-based catalysts for hydrogen and nanostructured carbon production by methane decomposition. Int J Hydrogen Energy 2011;36(13):7832-43. https://doi.org/10.1016/j.ijhydene.2011.01.184.
  * (32) Ba-Abbad MM, Kadhum AA, Mohamad AB, Takriff MS, Sopian K. Visible light photocatalytic activity of Fe3+-doped ZnO nanoparticle prepared via sol–gel technique. Chemosphere 2013;91(11):1604-11. https://doi.org/10.1016/j.chemosphere.2012.12.055.
  * (33) Othmen WB, Hamdi A, Addad A, Sieber B, Elhouichet H, Szunerits S, Boukherroub R. Fe-doped SnO2 decorated reduced graphene oxide nanocomposite with enhanced visible light photocatalytic activity. J Photochem Photobiol A 2018;367:145-55. https://doi.org/10.1016/j.jphotochem.2018.08.016.
  * (34) Mansingh S, Padhi DK, Parida KM. Enhanced photocatalytic activity of nanostructured Fe doped CeO2 for hydrogen production under visible light irradiation. Int J Hydrogen Energy 2016;41(32):14133-46. https://doi.org/10.1016/j.ijhydene.2016.05.191.
  * (35) Borhade AV, Kanade KG, Tope DR, Patil MD. A Comparative study on synthesis, characterization and photocatalytic activities of MgO and Fe/MgO nanoparticles. Res Chem Intermed 2012;38(8):1931-46. https://doi.org/10.1007/s11164-012-0515-z.
  * (36) Ramirez S, Chan K, Hernandez R, Recinos E, Hernandez E, Salgado R, Khitun AG, Garay JE, Balandin AA. Thermal and magnetic properties of nanostructured densified ferrimagnetic composites with graphene-graphite fillers. Mater Des 2017;118:75-80. https://doi.org/10.1016/j.matdes.2017.01.018.
  * (37) Xu YF, Yang MZ, Chen BX, Wang XD, Chen HY, Kuang DB, Su CY. A CsPbBr3 perovskite quantum dot/graphene oxide composite for photocatalytic CO2 reduction. J Am Chem Soc 2017;139(16):5660-3. https://doi.org/10.1021/jacs.7b00489.
  * (38) Marlinda AR, Yusoff N, Sagadevan S, Johan MR. Recent developments in reduced graphene oxide nanocomposites for photoelectrochemical water-splitting applications. Int J Hydrogen Energy 2020;45(21):11976-94. https://doi.org/10.1016/j.ijhydene.2020.02.096.
  * (39) Purwajanti S, Huang X, Liu Y, Yang Y, Noonan O, Song H, Zhang J, Zhang J, Fu J, Liang C, Yu C. Mg(OH)2–MgO@ reduced graphene oxide nanocomposites: the roles of composition and nanostructure in arsenite sorption. J Mater Chem A 2017;5(46):24484-92. https://doi.org/10.1039/C7TA07629K.
  * (40) Gollu SR, Sharma R, Srinivas G, Kundu S, Gupta D. Incorporation of silver and gold nanostructures for performance improvement in P3HT: PCBM inverted solar cell with rGO/ZnO nanocomposite as an electron transport layer. Org Electron 2016;29:79-87. https://doi.org/10.1016/j.orgel.2015.11.015.
  * (41) Wang K, Tan J, Lu Z, Chen S, She X, Zhang H, Yang D. Nanoscale engineering MoP/Fe2P/RGO toward efficient electrocatalyst for hydrogen evolution reaction. Int J Hydrogen Energy 2018;43(30):13939-45. https://doi.org/10.1016/j.ijhydene.2018.02.012.
  * (42) Hafeez HY, Lakhera SK, Shankar MV, Neppolian B. Synergetic improvement in charge carrier transport and light harvesting over ternary InVO4-g-C3N4/rGO hybrid nanocomposite for hydrogen evolution reaction. Int J Hydrogen Energy 2020;45(13):7530-40. https://doi.org/10.1016/j.ijhydene.2019.05.235.
  * (43) Sadhukhan S, Ghosh TK, Rana D, Roy I, Bhattacharyya A, Sarkar G, Chakraborty M, Chattopadhyay D. Studies on synthesis of reduced graphene oxide (RGO) via green route and its electrical property. Mater Res Bull 2016;79:41-51. https://doi.org/10.1016/j.materresbull.2016.02.039.
  * (44) Yu Z, Shi Z, Xu H, Ma X, Tian M, Yin J. Green chemistry: Co-assembly of tannin-assisted exfoliated low-defect graphene and epoxy natural rubber latex to form soft and elastic nacre-like film with good electrical conductivity. Carbon 2017;114:649-60. https://doi.org/10.1016/j.carbon.2016.12.049.
  * (45) Shi P, Wang X, Zhou X, Min Y, Fan J, Yao W. Influence of calcination temperature on the catalytic performance of Co3O4/GO nanocomposites for Orange II degradation. RSC Adv 2015;5(43):34125-33. https://doi.org/10.1039/C5RA02680F.
  * (46) Feng J, Ding Y, Guo Y, Li X, Li W. Calcination temperature effect on the adsorption and hydrogenated dissociation of CO2 over the NiO/MgO catalyst. Fuel 2013;109:110-5. https://doi.org/10.1016/j.fuel.2012.08.028.
  * (47) Yeoh WM, Lee KY, Chai SP, Lee KT, Mohamed AR. Effective synthesis of carbon nanotubes via catalytic decomposition of methane: Influence of calcination temperature on metal–support interaction of Co–Mo/MgO catalyst. J Phys Chem Solids 2013;74(11):1553-9. https://doi.org/10.1016/j.jpcs.2013.05.023.
  * (48) Zhang H, Hu J, Xie J, Wang S, Cao Y. A solid-state chemical method for synthesizing MgO nanoparticles with superior adsorption properties. RSC adv 2019;9(4):2011-7. https://doi.org/10.1039/C8RA09199D.
  * (49) Karnaukhov TM, Vedyagin AA, Cherepanova SV, Rogov VA, Stoyanovskii VO, Mishakov IV. Study on reduction behavior of two-component FeMgO oxide system prepared via a sol-gel technique. Int J Hydrogen Energy 2017;42(52):30543-9. https://doi.org/10.1016/j.ijhydene.2017.11.036.
  * (50) Kumar A, Nazzario R, Torres-Castro L, Pena-Duarte A, Tomar MS. Electrochemical properties of MgO-coated 0.5 Li2MnO3-0.5 LiNi0.5Mn0.5O2 composite cathode material for lithium ion battery. Int J Hydrogen Energy 2015;40(14):4931-5.https://doi.org/10.1016/j.ijhydene.2015.01.104.
  * (51) Basith MA, Yesmin N, Hossain R. Low temperature synthesis of BiFeO3 nanoparticles with enhanced magnetization and promising photocatalytic performance in dye degradation and hydrogen evolution. RSC adv 2018;8(52):29613-27. https://doi.org/10.1039/c8ra04599b.
  * (52) Brillas E, Sirés I, Oturan MA. Electro-Fenton process and related electrochemical technologies based on Fenton’s reaction chemistry. Chem Rev 2009;109(12):6570-631. https://doi.org/10.1021/cr900136g.
  * (53) Soltani T, Tayyebi A, Lee BK. Photolysis and photocatalysis of tetracycline by sonochemically heterojunctioned BiVO4/reduced graphene oxide under visible-light irradiation. J Environ Manage 2019;232:713-21. https://doi.org/10.1016/j.jenvman.2018.11.133.
  * (54) Paulchamy B, Arthi G, Lignesh BD. A simple approach to stepwise synthesis of graphene oxide nanomaterial. J Nanomed Nanotechnol 2015;6(1):1. https://doi.org/ 10.4172/2157-7439.1000253.
  * (55) Jalil MA, Chowdhury SS, Alam SM, Yousuf SMEH, Ashik EK, Firoz SH, Basith MA. Temperature-dependent phase transition and comparative investigation on enhanced magnetic and optical properties between sillenite and perovskite bismuth ferrite-rGO nanocomposites. J Appl Phys 2017;122(8):084902. https://doi.org/10.1063/1.4985840.
  * (56) Devaraja PB, Avadhani DN, Prashantha SC, Nagabhushana H, Sharma SC, Nagabhushana BM, Nagaswarupa HP. Synthesis, structural and luminescence studies of magnesium oxide nanopowder. Spectrochim Acta A 2014;118:847-51. http://dx.doi.org/10.1016/j.saa.2013.08.050.
  * (57) Petnikota S, Srikanth VV, Toh JJ, Srinivasan M, Bobba CV, Adams S, Reddy MV. Electrochemistry-related aspects of safety of graphene-based non-aqueous electrochemical supercapacitors: a case study with MgO-decorated few-layer graphene as an electrode material. New J Chem 2019;43(25):9793-801. https://doi.org/10.1039/C9NJ00991D.
  * (58) Ganguly A, Sharma S, Papakonstantinou P, Hamilton J. Probing the thermal deoxygenation of graphene oxide using high-resolution in situ X-ray-based spectroscopies. J Phys Chem C 2011;115(34):17009-19. https://doi.org/10.1021/jp203741y.
  * (59) Jin M, Jeong HK, Yu WJ, Bae DJ, Kang BR, Lee YH. Graphene oxide thin film field effect transistors without reduction. J Phys D: Appl Phys 2009;42(13):135109. https://doi.org/10.1088/0022-3727/42/13/135109.
  * (60) Kayani ZN, Saleemi F, Batool I. Effect of calcination temperature on the properties of ZnO nanoparticles. Appl Phys A 2015 ;119(2):713-20. https://doi.org/10.1007/s00339-015-9019-1.
  * (61) Sinornate W, Mimura H, Pecharapa W. Structural, Morphological, Optical, and Electrical Properties of Sol–Gel Derived Sb-Doped ZnO Thin Films Annealed Under Different Atmospheres. Phys Status Solidi 2021;218(1):2000233. https://doi.org/ 10.1002/pssa.202000233.
  * (62) Devaraja PB, Avadhani DN, Nagabhushana H, Prashantha SC, Sharma SC, Nagabhushana BM, Nagaswarupa HP, Prasad BD. Luminescence properties of MgO: Fe3+ nanopowders for WLEDs under NUV excitation prepared via propellant combustion route. J Radiant Res Appl Sci 2015;8(3):362-73. http://dx.doi.org/10.1016/j.jrras.2015.02.001.
  * (63) Turabik M, Simsek UB. Effect of synthesis parameters on the particle size of the zero valent iron particles. Inorg Nano-Met Chem 2017;47(7):1033-43. https://doi.org/10.1080/15533174.2016.1219869.
  * (64) Li X, Staykov A, Yoshizawa K. Orbital views of the electron transport through polycyclic aromatic hydrocarbons with different molecular sizes and edge type structures. J Phys Chem C 2010;114(21):9997-10003. https://doi.org/10.1021/jp102280r.
  * (65) Li W, Bu Y, Jin H, Wang J, Zhang W, Wang S, Wang J. The preparation of hierarchical flowerlike NiO/reduced graphene oxide composites for high performance supercapacitor applications. Energ Fuel 2013;27(10):6304-10. https://doi.org/10.1021/ef401190b.
  * (66) Kumar A, Kumar J. On the synthesis and optical absorption studies of nano-size magnesium oxide powder. J Phys Chem Solids 2008;69(11):2764-72. https://doi.org/10.1016/j.jpcs.2008.06.143.
  * (67) Nanakkal AR, Alexander LK. Graphene/BiVO4/TiO2 nanocomposite: tuning band gap energies for superior photocatalytic activity under visible light. J Mater Sci 2017;52(13):7997-8006. https://doi.org/10.1007/s10853-017-1002-0.
  * (68) Zhang J, Huang X, Zhang H, Xue Q, Xu H, Wang L, Feng Z. The effect of powder grain size on the microstructure and electrical properties of 8 mol${\varkappa}$ Y2O3-stabilized ZrO2. RSC Adv 2017;7(62):39153-9. https://doi.org/10.1039/C7RA06710K.
  * (69) Ghosh S, Kar P, Bhandary N, Basu S, Maiyalagan T, Sardar S, Pal SK. Reduced graphene oxide supported hierarchical flower like manganese oxide as efficient electrocatalysts toward reduction and evolution of oxygen. Int J Hydrogen Energy 2017;42(7):4111-22. http://dx.doi.org/10.1016/j.ijhydene.2016.12.008.
  * (70) He GL, Zhong YH, Chen MJ, Li X, Fang YP, Xu YH. One-pot hydrothermal synthesis of SrTiO3-reduced graphene oxide composites with enhanced photocatalytic activity for hydrogen production. J Mol Catal A 2016;423:70-6. https://doi.org/10.1016/j.molcata.2016.05.025.
  * (71) Hareesh K, Dhole SD, Phase DM, Williams JF. One-step bacterial assisted synthesis of CdS/rGO nanocomposite as Hydrogen production catalyst. Mater Res Bull 2019;110:82-9. https://doi.org/10.1016/j.materresbull.2018.10.012.
  * (72) Arshad A, Iqbal J, Siddiq M, Mansoor Q, Ismail M, Mehmood F, Ajmal M, Abid Z. Graphene nanoplatelets induced tailoring in photocatalytic activity and antibacterial characteristics of MgO/graphene nanoplatelets nanocomposites. J Appl Phys 2017;121(2):024901. https://doi.org/10.1063/1.4972970.
  * (73) Gayathri S, Jayabal P, Kottaisamy M, Ramakrishnan V. Synthesis of ZnO decorated graphene nanocomposite for enhanced photocatalytic properties. J Appl Phys 2014;115(17):173504. https://doi.org/10.1063/1.4874877.
  * (74) Isari AA, Payan A, Fattahi M, Jorfi S, Kakavandi B. Photocatalytic degradation of rhodamine B and real textile wastewater using Fe-doped TiO2 anchored on reduced graphene oxide (Fe-TiO2/rGO): Characterization and feasibility, mechanism and pathway studies. Appl Surf Sci 2018;462:549-64. https://doi.org/10.1016/j.apsusc.2018.08.133.
  * (75) Liu Z, Yin Z, Cox C, Bosman M, Qian X, Li N, Zhao H, Du Y, Li J, Nocera DG. Room temperature stable COx-free H2 production from methanol with magnesium oxide nanophotocatalysts. Sci Adv 2016 ;2(9):e1501425. https://doi.org/10.1126/sciadv.1501425.
  * (76) Wang X, Zhi L, Müllen K. Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano lett 2008;8(1):323-7. https://doi.org/10.1021/nl072838r.
  * (77) Kang W, Jimeng X, Xitao W. The effects of ZnO morphology on photocatalytic efficiency of ZnO/RGO nanocomposites. Appl Surf Sci 2016;360:270-5. https://doi.org/10.1016/j.apsusc.2015.10.190.
  * (78) El-Bery HM, Matsushita Y, Abdel-moneim A. Fabrication of efficient TiO2-RGO heterojunction composites for hydrogen generation via water-splitting: Comparison between RGO, Au and Pt reduction sites. Appl Surf Sci 2017;423:185-96. https://doi.org/10.1016/j.apsusc.2017.06.130.
  * (79) Rivero MJ, Iglesias O, Ribao P, Ortiz I. Kinetic performance of TiO2/Pt/reduced graphene oxide composites in the photocatalytic hydrogen production. Int J Hydrogen Energy 2019;44(1):101-9. https://doi.org/10.1016/j.ijhydene.2018.02.115.
  * (80) Nethercot Jr AH. Prediction of Fermi energies and photoelectric thresholds based on electronegativity concepts. Phys Rev Lett 1974;33(18):1088. https://doi.org/10.1103/PhysRevLett.33.1088.
  * (81) Li J, Liu Y, Zhou Y, Liu S, Liang Y, Luo T, Dai G. Enhanced visible-light photocatalytic activity of Bi2O2CO2 nanoplates by Fe-doping in the degradation of rhodamine B. Mater Res Bull 2018;107:438-45. https://doi.org/10.1016/j.materresbull.2018.08.018.
  * (82) Liang J, Zhu G, Liu P, Luo X, Tan C, Jin L, Zhou J. Synthesis and characterization of Fe-doped $\beta$-Bi2O3 porous microspheres with enhanced visible light photocatalytic activity. Superlattices Microstruct 2014;72:272-82. https://doi.org/10.1016/j.spmi.2014.05.005.
  * (83) Li Q, Li X, Wageh S, Al-Ghamdi AA, Yu J. CdS/graphene nanocomposite photocatalysts. Adv Energy Mater 2015;5(14):1500010. https://doi.org/10.1002/aenm.201500010.
  * (84) Chen Y, Wang Y, Zhou X, Zhao Y, Peng W. Defected graphene as effective co-catalyst of CdS for enhanced photocatalytic activities. Environ Sci Pollut Res 2020;27:26810-6. https://doi.org/10.1007/s11356-020-09066-8.