Photocatalytic dye degradation and biological activities of the Fe2O3/Cu2O nanocomposite

Article abstract of DOI:10.1039/c8ra09929d

The present study reports the synthesis of the Fe₂O₃/Cu₂O nanocomposite via a facile hydrothermal route. The products were characterized using X-ray diffractometry (XRD), Fourier-transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), high-resolution transmission electron microscopy (HR-TEM), energy dispersive spectroscopy (EDS) and Brunauer-Emmett-Teller (BET) techniques. The composition, morphology and structural features of the nanoparticles were found to be size-dependent due to the temperature response in the particular time log during hydrothermal synthesis. HR-TEM confirmed the formation of hexagonal rod-shaped bare Cu₂O, rhombohedral-shaped Fe₂O₃ and composite assembly. Rhodamine-B (RB) and Janus green (JG) were chosen as model dyes for the degradation studies. Photocatalytic degradation of the dyes was deliberated by altering the catalyst and dye concentrations. The results showed that the Rhodamine-B (RB) and Janus green (JG) dyes were degraded within a short time span. The synthesized materials were found to be highly stable in the visible light-driven degradation of the dyes; showed antibacterial activity against E. coli, P. aeruginosa, Staph. aureus and B. subtilis; and exhibited less toxicity against the Musmusculus skin melanoma cells (B16-F10). The fusion of these advantages paves the way for further applications in energy conversion, biological applications as well as in environmental remediation.

Full text of DOI:10.1039/c8ra09929d

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Photocatalytic dye degradation and biological
activities of the Fe2O3/Cu2O nanocomposite†
Cite this: RSC Adv., 2019, 9, 8557
a
Gangadhar Akshathaab
*
Mavinakere Ramesh Abhilash,
and Shivanna Srikantaswamyab
The present study reports the synthesis of the Fe2O3/Cu2O nanocomposite via a facile hydrothermal route.
The products were characterized using X-ray diffractometry (XRD), Fourier-transform infrared
spectroscopy (FTIR), dynamic light scattering (DLS), high-resolution transmission electron microscopy
(HR-TEM), energy dispersive spectroscopy (EDS) and Brunauer–Emmett–Teller (BET) techniques. The
composition, morphology and structural features of the nanoparticles were found to be size-dependent
due to the temperature response in the particular time log during hydrothermal synthesis. HR-TEM
confirmed the formation of hexagonal rod-shaped bare Cu2O, rhombohedral-shaped Fe2O3 and
composite assembly. Rhodamine-B (RB) and Janus green (JG) were chosen as model dyes for the
degradation studies. Photocatalytic degradation of the dyes was deliberated by altering the catalyst and
dye concentrations. The results showed that the Rhodamine-B (RB) and Janus green (JG) dyes were
degraded within a short time span. The synthesized materials were found to be highly stable in the
visible light-driven degradation of the dyes; showed antibacterial activity against E. coli, P. aeruginosa,
Staph. aureus and B. subtilis; and exhibited less toxicity against the Musmusculus skin melanoma cells
(B16-F10). The fusion of these advantages paves the way for further applications in energy conversion,
biological applications as well as in environmental remediation.
Received 3rd December 2018
Accepted 28th February 2019
DOI: 10.1039/c8ra09929d
rsc.li/rsc-advances
constitutes nearly 46% of the astral band.14 In recent years, the
investigation of semiconductors for harvesting energy in the
1 Introduction
visible spectrum has led to the progression of the invention of
non-titania-based semiconducting photocatalysts for dye
degradation.15,16 In addition, the plasmon-resonance of ne
metal oxide nanoparticles (MNP's) also enhances the photo-
catalytic process by absorbing incident photons much more
effectively. Cu2O has been used as a prolic p-type photo-
catalyst with a through band gap in the visible range of 2.0–
2.2 eV.17–19 However, Cu2O suffers from photo-wavering and
photo-corrosion, which can over time cause deactivation in
solar irradiation.20 Iron oxides (Fe2O3), on the other hand, are
an extremely constant n-type semiconductor under standard
ambient conditions.21 The valence band-edge of Fe2O3 is an
outstanding catalyst for the photo-squalor of unrened pollut-
ants and is not only abundant in nature but also mass-
producible.22 However, photocatalytic activity in Fe2O3 is oen
overwhelmed by its hole diffusion length (approximately, 10
nm) and an extremely short electron–hole recombination
time.23–25 Most studies focus on the antibacterial activity of
metal nanoparticles and its composites, including those that
are against resistant strains of microorganisms. However, there
is no exact mechanism for the antimicrobial action of metal
oxide nanoparticles.26 The anti-microbial activity of metal oxide
nanoparticles was enhanced with increasing surface area and
volume of the nanoparticles and decreasing particle size.27
Photocatalysis is a fascinating tool for energy conversion and
environmental decontamination due to its conspicuous
nature.1 Using metal-semiconductors for photocatalysis is an
environmentally safe process that simulates the photosynthesis
process to speed up chemical reactions that require or involve
light.2,3 The visible light photocatalytic process has been
implemented in semiconductor integrated circuits for harvest-
ing solar energy.4 To overcome the band gap problem, metal
oxides can be used as a photocatalyst, which, upon exposure to
UV lights, visible lights or both, allows photo-excited electrons
to be promoted to the conduction band to speed up the
recombination of huge charge carriers.5–7 Broad band gap issues
can also be overcome by introducing a hetero or homo barrier
made with a combination of different metal oxides,8 nanowire,9
core–shells9 and nanowires,10 which offers superior charge
transfer competence and commotion compared to a sole metal
oxide semiconductor.11–13 This allows the competent and viable
catalysts to perform under visible light irradiation, which
aDepartment of Studies in Environmental Science, University of Mysore,
Manasagangotri, Mysore 570006, India
bCentre for Materials Science and Technology, Vijnana Bhavan, University of Mysore,
Manasagangotri, Mysore 570006, India. E-mail: abilashmrenvi@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra09929d
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Inimitable properties and the assessment of interactions product were investigated. Temperature plays a crucial role in
between nanomaterials and the biological system are essen- the formation of a well-dened spherical product. The autoclave
tial.28 In recent years, the cell line toxicity of human cell lines containing these chemicals was naturally cooled to room
using several synthesized metal oxide is a more fascinating temperature, and the precipitates were then washed with
topic in understanding the relationship between nanoparticle– distilled water and isolated under a magnet. The nal products
cell interactions.29,30 The size, shape, and morphology of were dried at 60 ꢀC and characterized. In the hydrothermal
nanoparticles, play a vital role in mammalian cell culture process, the following reaction takes place.
medium, and the nanoparticle are much more toxic than the
hydrothermally synthesized nanocomposite semiconductor.31,32
2FeCl3$6H2O + 2FeCl2$4H2O + 10NH4OH
/ 2Fe2O3 + 10NH4Cl + 14H2O + H2[
(1)
In this experiment, we focus on exploiting and utilizing some
multi-tasking facile materials with high photocatalytic activity
and biological applications. To evade these drawbacks, in an
attempt to tackle the above issues, we have developed a cost-
effective method for the synthesis of non-toxic Fe2O3 impuri-
ties on the Cu2O nanocomposite for various applications (Fig. 1
and 14). Fe2O3 burdened onto Cu2O and the nanocomposite
was prepared by a facile method. The (p–n) hetero-junction
nanocomposite was used for the effective degradation of
organic pollutants; including Rhodamine-B (RB) and Janus
green (JG), bacterial degradation and some additional biolog-
ical screening applications.
2.2. Hydrothermal preparation of Cu2O
Cu2O nanocomposites were synthesized by using the reaction
between copper sulfate and sodium hydroxide. 1.5 g of CuSO4-
$5H2O was rst added into 80 ml of distilled water under
mechanical stirring at 60 ꢀC for 30 min, followed by the addition
of 10 ml of NaOH aqueous solution (1 M) into the solution. Aer
1 minute, a pinch of anhydrous glucose (0.3 M) was quickly
added and was mixed by mechanical stirring for about 30
minutes. The nal solution was transferred into a Teon-lined
autoclave and hydrothermally treated at 80 ꢀC for 5 hours.
The resulting precipitate was collected, washed with distilled
water followed by ethanol and vacuum dried at 60 ꢀC for 6
hours.
2 Experimental
2.1. Hydrothermal preparation of Fe2O3
The hydrothermal technique has been found to be one of the
best techniques to prepare Fe2O3 nanoparticles of the desired
size with homogeneity in composition and an extreme degree of
crystalline particles. The primary nanoparticles were synthe-
sized by taking advantage of a hydrothermally enabled reaction
between FeCl3 and NaOH. Ammonia (Aldrich, India), 10.14 g
(37.5 mol) of FeCl3$6H2O and 7.45 g (37.5 mmol) of FeCl2$4H2O
were dissolved into 25 ml of distilled water. 25 ml of 25%
ammonia was added to the salt solution under stirring at
700 rpm for 2 min. Next, 15 ml of the mixture was put into
a Teon-lined stainless steel Morey autoclave. The autoclave
was heated to 160 ꢀC in an oven and maintained with a 12 hour
reaction time. The effects of the reaction temperature on the
2CuSO4$5H2O + 2NaOH / Cu2O + 2NaSO4 + 6H2O (2)
2.3. Preparation of Fe2O3/Cu2O
Nanocatalysts such as Fe2O3 and Cu2O synthesized from the
above method were used to synthesize Fe2O3/Cu2O nano-
composites by a simple ultra-sonication method, in which the
calculated 1 : 1 molar ratio of Fe2O3 : Cu2O nanoparticles was
sonicated in 50 ml of CHCl3 solution for 20 minutes. During
sonication, the nanoparticles were dispersed uniformly into
a composite structure. The sonicaꢀted solution was ltered as
wet solids, which were dried at 80 C for 5 hours.
2.4. Photocatalytic experiments of the removal of dyes
The photocatalytic behaviour of the synthesized nanomaterials
was evaluated by the removal of Rhodamine-B (RB) and Janus
green (JG) dyes (200 ml of aqueous solution of dyes (1 ꢁ
10ꢂ5 mol lꢂ1)) under UV light radiation. The light source used
was a 150 W Xe (Xenon) lamp, and the distance between the UV
source and the photo-reaction vessel was 10 cm. Prior to irra-
diation, the suspensions were magnetically stirred in the dark
for 30 min. Then, the photoreaction vessel was exposed to UV
irradiation under standard ambient conditions. The selected
dyes were used in conjunction with 10 mg of catalyst nano-
particles in the photo-removal experiment. At regular time
intervals, 3 ml of the suspension was taken for centrifugation to
separate the photocatalyst and for further evaluation using
a UV-Vis absorption spectrometer. The photo-removal efficiency
Fig. 1 Symmetric representation of the hydrothermal growth of
Fe2O3/Cu2O nanocomposites; the photocatalytic activity of dye
degradation and recyclability.
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percentage was calculated from the equation given below.
Aerward, recycling experiments were carried out for ve
repeated cycles to inspect the permanence of the photocatalytic
Fe2O3/Cu2O nanocomposite. The composite catalyst was
centrifuged, washed with ethanol and deionised water, and was
dried before reuse for the next trial. The photo-removal effi-
ciency percentage was calculated from the equation given
below;
% Photo-removal efficiency ¼ C0 ꢂ C/C0 ꢁ 100
where C0 is the initial concentration of dye and C is the
concentration of dye aer photo-irradiation (nal).
2.5. Determination of antibacterial activity and live and
dead cell analysis
The Fe2O3, Cu2O and the Fe2O3/Cu2O nanocomposite under-
went bacterial screening along with the control by a disc
diffusion method,33–35 against Gram-positive Bacillus subtilis,
(MTCC 121) and Staphylococcus aureus (MTCC 7443) and Gram-
negative Escherichia coli, (MTCC 7410) and Pseudomonas aeru-
ginosa sp., (MTCC 733) bacteria. Hydrothermally synthesized
Fe2O3, Cu2O and Fe2O3/Cu2O and the control were prepared in
sterile distilled water (stock solution) over a range of different
concentrations in 100 mg mlꢂ1. The analysis was performed to
distinguish dead and viable bacterial cells upon treatment with
Fe2O3, Cu2O and Fe2O3/Cu2O NPs according to earlier reports,36
with minor modications.
Fig. 2 X-ray diffraction profile of Fe2O3, Cu2O and Fe2O3/Cu2O.
33.15 and 35.61 correspond to the (104) and (110) planes of
rhombohedral Fe2O3, respectively (33-0664-JCPDS).38 The crys-
talline phase of the nanocomposite Fe2O3/Cu2O photocatalyst
suggests that the hexagonal rod-shaped Cu2O with
rhombohedral-shaped Fe2O3 would be evenly distributed and
agglomerated over the entire surface. However, the photo-
catalyst Fe2O3/Cu2O exhibited
a different structure with
a different crystalline phase. The HR-TEM results matched with
the XRD patterns, conrming that the experimental processes
were ample.
FT-IR spectrum of the Cu2O, Fe2O3 and Fe2O3/Cu2O photo-
catalysts are shown in (Fig. 3). The nanocatalyst with Cu2O and
Fe2O3 are respectively associated with the O–H bond stretching
and bending vibration modes.39 The characteristic broad
absorption peak adjusted from 700 cmꢂ1 to 400 cmꢂ1 as
assigned to the ngerprint of the Fe2O3/Cu2O is 701 cmꢂ1 in the
Cu2O nanocomposite, allocated to the infrared active mode of
Cu2O40 and the vigorous mode of Cu2O.41 For bare Fe2O3, the
absorption peaks at 572 cmꢂ1 and 454 cmꢂ1 correspond to the
photocatalysis reaction of Fe3+. FT-IR spectra of Cu2O, Fe2O3,
2.6. Cell culture and treatment
Musmelanoma cells (B16-F10) were seeded in tissue culture
asks and full-grown in Dulbecco's modied Eagle's medium
(DMEM; Thermo Fisher, USA Gibco), balanced with 5% fetal
bovine serum (FBS, Thermo Fisher; Gibco) and 1% penicillin/
streptomycin blend (Santa Cruz Biotechnology, USA).
detailed experiment procedure was clearly provided in S2 in
A
ESI.†
2.7. Statistical analysis
Systematic data replicates (three) were analyzed for every
attempt and for every analysis of discrepancy (ANOVA) using
SPSS-Inc. 16.0. Trivial effects of the treatments were resolved by
F values (p # 0.05).
3 Results and discussions
3.1. Characterization results of the synthesized Fe2O3/Cu2O
nanocomposite
The XRD spectrum of the Fe2O3/Cu2O photocatalyst with JCPDS
data for Cu2O and Fe2O3 are shown in (Fig. 2). The prominent
diffraction peaks at the angle 2q ¼ 29.55, 36.41, 42.29, 52.4,
61.34 and 73.53ꢀ respectively correspond to the (110), (111),
(200), (211), (220) and (311) crystal planes of the hexagonal rod-
shaped Cu2O (05-0667-JCPDS). The (111) crystal plane of Cu2O
in the composite is predicted to yield better performance in
Fig. 3 FT-IR spectrum of different Cu2O, Fe2O3 and Fe2O3/Cu2O
terms of photocatalytic activity.37 The two peaks found at 2q ¼ photocatalysts.
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and the Fe2O3/Cu2O composite; O2ꢂ bond stretching in the
FeO6 octahedron; and Fe3+ and O2ꢂ bond stretching in the FeO4
tetrahedron, respectively, are shown.42 Fe2O3/Cu2O shows the
presence of IR peaks caused by both photocatalysts, Cu2O and
Fe2O3.
The nanoparticles synthesized by hydrothermal method
resulting in the formation of nano/mesopore structure show
excellent photocatalytic performance.43,44 HR-TEM of the
synthesized nanoparticles was shown in (Fig. 4); the high-
quality images of the hexagonal rod-shaped Cu2O,
rhombohedral-shaped Fe2O3 and Fe2O3/Cu2O composite
nanoparticles with partial agglomeration are shown. Addition-
ally, the nanocomposite results in the formation of mesopores
between the particles, which were clearly conrmed by BET
surface area and pore volume measurements. The sizes of both
individual nanoparticles were between 30–60 nm, which are in
accordance with the results of XRD. It was also observed that the
d-spacing measurements of the twin domains were measured to
Fig. 5 The elemental composition in the EDS of (a) Fe2O3, (b) Cu2O,
and (c) Fe2O3/Cu2O.
˚
˚
be approximately 0.2 A to 0.4 A nm and corresponded to the
(111) and (220) planes of Fe2O3 and Cu2O, respectively, and the
SAED patterns conrmed the compact arrangement of the
nanoparticles (Fig. S3 in ESI†). Energy dispersive spectroscopy
(EDS) analysis conrmed the phase transparency of the Fe2O3,
Cu2O and Fe2O3/Cu2O photocatalysts, as shown in (Fig. 5). The
characteristic peaks of Fe, Cu, and O appear in the spectrum of
Fe2O3/Cu2O and conrm its successive formation and purity
(Fig. S2 in ESI†). The carbon peak appears due to the carbon
tape used sample holder.
during interaction with other biological systems as well as
environmental degradation. The particles examined presently
possessed zeta potentials of ꢂ0.1 mV for Cu2O, 13.9 mV for
Fe2O3, and ꢂ46.2 mV for the Fe2O3/Cu2O composite. Nano-
particles with a zeta potentials between ꢂ10 and +10 mV have
a neutral charge, while if it is greater than +30 mV or less than
ꢂ30 mV, it is considered to be strongly cationic or anionic.
Surface area is an important parameter to determine the
photocatalytic activity of the nanoparticles. A photocatalyst with
a high surface area is likely to absorb more dye molecules and
react faster (Table 1); the BET surface area as a function of the
pore volume of the prepared samples undoubtedly demonstrates
a similar type II curve. The BET surface area of Fe2O3, Cu2O and
The particle size distribution was analyzed using a dynamic
light scattering instrument. The Fe2O3/Cu2O particles were
dispersed in a solvent. The distribution of particle sizes when
immersed in the solvent ranged from 42–593 nm, with
approximately 266 nm being the mean particle size. Histogram
of the DLS analysis for the particle size distribution of Fe2O3/
Cu2O composites is depicted in (Fig. S1 in ESI†). Zeta potential
measurements by DLS were considered as an authentic tech-
nique for evaluating the particle size and zeta potentials of
nanoparticles in a suspension and also plays a crucial role
Fe2O3/Cu2O were found to be 5.676, 9.90, and 10.401 m2 gꢂ1
,
respectively (Fig. 6). According to the hysteresis loop in the rela-
tive pressure region around 0.4–0.9, the nitrogen adsorption/
desorption isotherms showed that the Fe2O3/Cu2O exhibited
a similar type IV curve. In other words, the Fe2O3/Cu2O nano-
composite existed with a mesoporous structure. The surface area
of the Fe2O3/Cu2O repressed a huge difference in the bare
nanoporous Fe2O3 and Cu2O. With an increase in the surface area
and decrease in pore size, as well as the large total pore volume,
the Fe2O3/Cu2O is expected to have a high photocatalytic activity.
The common Tauc approach was used to approximate the
band gap energy (Eg) values. It was found that the band gap
energy values of Fe2O3, Cu2O and the Fe2O3/Cu2O nano-
composite were respectively 1.96 eV, 1.89 eV and 1.85 eV. The
surface charge on the metal oxides in aqueous solution
becomes obvious: recognition to the progression of hydration,
Table 1 Surface parameters of Fe2O3, Cu2O and Fe2O3/Cu2O
Pore volume
Photocatalyst
SBET (m2 gꢂ1
)
(cm3 gꢂ1
)
Fig. 4 HR-TEM of (a) Cu2O nanoparticles, (b) Fe2O3 nanoparticles, (c)
hexagonal rod-shaped Cu2O and rhombohedral-shaped Fe2O3; (d)
Cu2O/Fe2O3 nanocomposite assembly, scale bar is in nm.
Fe2O3
Cu2O
Fe2O3/Cu2O
5.676
9.90
10.401
1.304
2.275
2.389
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Fig. 7 Photo-removal of (a) Rhodamine-B (RB) and (b) Janus green
(JG) using Cu2O, Fe2O3 and the Fe2O3/Cu2O photocatalysts.
Fig. 6 BET, N2 adsorption isotherms of (a) Fe2O3, (b) Cu2O, and (c)
Fe2O3/Cu2O.
The Langmuir–Hinshelwood model was engaged to investi-
gate the kinetics of RB and JG photo-removal. The photo-
catalytic experiments were carried out under optimal reaction
conditions [Fe2O3 ¼ 0.75 g lꢂ1, both RB and JG ¼ 9 mM and pH
5]. Fig. 8 shows the logarithmic plot of RB and JG concentration
as a function of irradiation time. The photocatalytic removal of
protonation, and deprotonation of the surface groups.45 RB is
a cationic dye.46 JG is basic dyes which, together with (RB), are
scarcely absorbed on the Fe2O3/Cu2O surface due to its negative
zeta potential. However, the deliberate zeta potential was found
to be ꢂ46.2 mV, suggesting the presence of a negative charge on
the surface of the Fe2O3/Cu2O photocatalyst.47 The negative
surface charge favours the adsorption of cationic as well as
basic dyes due to the increased electrostatic force of attraction.
The surface conductivity of the Fe2O3/Cu2O photocatalyst was
found to be 0.229 mS cmꢂ1
.
The surface properties of Fe2O3, Cu2O were affected by
changes in pH. The pzc (point of zero charge) values for Fe2O3
and Fe2O3, Cu2O were found to be 6.9 and 7.2 respectively. It
was observed that RB and JG in an acidic environment can
advantageously increase the electrostatic attraction between the
protons from the catalysts, including the dyes, RB and JG. Thus,
photo-removal activity is high. At low pH (below 5), the chances
for agglomeration are high, thus reducing the active surface
area available for dye adsorption and photon absorption. At an
optimum pH, the predominant iron site, namely Fe(OH)2+, not
only forms Fe(II), the major catalytic species in the photo-
removal reactions, but also produces additional ꢂOH respon-
sible for dye removal. When the pH value was greater than pzc,
the surface of Fe2O3/Cu2O became negatively charged. There-
fore, the negatively charged dye molecules were repelled by the
catalyst surface, leading to a decrease in the effective photo-
catalytic activity. Highly alkaline conditions are favourable for
the generation of a large number of less reactive high-valence
composite iron species (Fig. 7).
Fig. 8 Kinetics of (a) Rhodamine-B (RB) and (b) Janus green (JG)
photo-removal using Cu2O, Fe2O3 and the Fe2O3/Cu2O photocatalyst.
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RB and JG follows pseudo-rst-order kinetics; the pragmatic was a much more effective photocatalyst that enhances the
rate constant for Fe2O3/Cu2O of 1.21 ꢁ 10ꢂ2 sꢂ1 is signicantly removal rate of Rhodamine-B and Janus green.
higher than those of Fe2O3 (4.36 ꢁ 10ꢂ3 sꢂ1) and CuO2 (6.45 ꢁ
10ꢂ3 sꢂ1). Hence, the activity of the Fe2O3 nano-porous material
is about approximately 2.6 times higher than that of other
demonstrated materials in a systematic manner. DRS was used
to investigate the light-harvesting nature of the Fe2O3/Cu2O
3.2. Total organic carbon (TOC) and chemical oxygen
demand (COD) studies of Rhodamine-B (RB-B) and Janus
green (JG)
photocatalyst. Estimating the conduction-band minimum
(CBM) and the valence-band maximum (VBM) is vital to
understanding the mechanism of the photocatalytic degrada-
tion of the photocatalyst. To investigate the CBM and VBM of
Fe2O3, Cu2O, or Fe2O3/Cu2O, the UV-DRS spectra were used to
record the spectrum. As shown, the associated band gap values
were calculated using the following eqn:48
This analysis is necessary because the disappearance of dye
colour alone cannot be used as a measure to determine the
complete mineralization of the dyes.51–54 Noxious and long-
lasting reaction intermediates were formed during the photo-
removal of the dyes. Simultaneously, it was essential to calcu-
late the degree of degradation of Rhodamine-B (RB) and Janus
green (JG) during photo-removal using the Fe2O3/Cu2O photo-
catalyst. Aer the degradation process, it was a necessity to
measure the chemical oxygen demand (COD) and total organic
carbon (TOC) to assess the purity of degraded dyes before the
discharging process. Total organic carbon (TOC) analysis was
performed in order to determine the extent of mineralization of
Rhodamine-B (RB-B) and Janus green (JG) during the photo-
catalytic degradation process. The present work shows the
signicantly declining performance in the particular functioned
period (Fig. 10). Furthermore, the photocatalytic degradation
process can result in the formation of colourless dye interme-
diates resulting in the disappearance of colour, which may
actually be more toxic than the dye itself. The present study
revealed that the colour disappearance of the dye was faster
than the degree of mineralization with maximum TOC removal
(Table S3 in ESI†). The rapid loss of colour might arise from the
cleavage of the azo bond, while the high TOC value may be due
to difficulty in converting the N-atom of the dye into oxidized
nitrogen compounds. This mechanism clearly explained that
the dye molecules were converted to other intermediates and
that the dye was systematically decolourized in 120 min, which
may lead to complete mineralization. Similarly, the reduction of
ahn ¼ A(hn ꢂ Eg)n
The calculated band gaps of bare Fe2O3 and Cu2O were
found to be 1.96 and 1.89 eV, (Fig. 9) respectively, which are
consistent with similar results obtained from related work.49
The observed small red-shi in the band gap value (1.85 eV) of
the Fe2O3/Cu2O photocatalyst is possibly due to the formation
of a p–n heterojunction between the p-type Cu2O and the n-type
Fe2O3.50
In the exploration of the photocatalytic dye removal activity, it
was found that the 1.5% loading of Fe2O3 on Cu2O exhibits
sophisticated performance compared to those of 0.8 and 2.4%
loading. Fig. 7 displays the rate of the adsorption and photo-
removal of Rhodamine-B and Janus green dyes for Cu2O, Fe2O3
and Fe2O3/Cu2O. Due to its negative surface charge, the optimized
photocatalyst Fe2O3/Cu2O was found to have adsorbed the dyes
under visible-light irradiation. The Fe2O3/Cu2O photocatalyst had
effectively degraded the RB and JG dyes. It can be clearly seen that
1.5% Fe2O3 loaded onto the Cu2O photocatalyst is much more
efficient than bare Cu2O towards cationic as well as basic dyes like
RB and JG removal. Compared to bare Fe2O3 or Cu2O, Fe2O3/Cu2O
Fig. 9 The band gap calculation using the Tauc plot of pristine Fe2O3, Fig. 10 TOC and COD measurements of (a) Rhodamine-B (RB-B) and
Cu2O and the Fe2O3/Cu2O photocatalyst.
(b) Janus green (JG) degradation by the Fe2O3/Cu2O photocatalyst.
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COD reects the extent of removal or mineralization of an
organic species (Table S4 in ESI†), the percentage change in
COD and TOC during photo-removal was measured under
optimum reaction conditions [Rhodamine-B (RB) and Janus
green (JG) concentration 9 mM, catalyst concentration 0.75 g
lꢂ1, pH ¼ 5 and irradiation time of up to 120 min]. The solutions
obtained aer 120 min of photo-removal showed a signicant
decrease in COD and TOC concentration. It has been observed
that Rhodamine-B (RB) and Janus green (JG) molecules were
partially degraded to intermediates, and only a small fraction
was subjected to complete mineralization; the COD showed
a related emergent action similar to TOC.
Fig. 12 Photo-degradation intermediates of Janus green (JG).
acid, 2-(6-(ethylamino)-3-imino-3H-xanthen-9-yl)benzoic acid
and 2-(6-(diethylamino)-3-imino-3H-xanthen-9-yl) benzoic acid,
as clearly demonstrated in (Table S1 in ESI†). In Fig. 12, S16 and
S17 in ESI,† photocatalytic degradation of Janus green (JG)
resulted in the evolution of nitrogen, which took prominence
over deethylation, leading to the formation of dinitrogen,
benzene-1-ylium, 4-(dimethylamino)benzene-1-ylium, N4,N4-
3.3. Photodegradation intermediates of Rhodamine-B (RB-
B) and Janus green (JG)
The intermediates and nal products would help to gure out
the details of the reaction process. Because of the spectral
overlap between the original dye and its degradation interme-
diates. Temporal variations during the photooxidation of
Rhodamine-B (RB) and Janus green (JG) were systematically
executed by LC-MS. According to earlier reports,55–57 small
molecules were mineralized to form CO2 and H2O, which can be
proven by the results of the TOC measurement. The two
competitive processes occurred simultaneously during the
photoreaction: N-deethylation and destruction of dye chromo-
phore structure, under visible light irradiation. Most of the N-
deethylation processes were preceded by the formation of
a nitrogen-centered radical, while the destruction of dye chro-
mophore structures is preceded by the generation of a carbon-
centered radical. The photogenerated active species such as
$OH could directly attack the central carbon of Rhodamine-B
(RB) (Fig. S4 in ESI†) and Janus green (JG) (Fig. S18 in ESI†),
for the degradation of the dye. These dynamic species work on
any N-deethylation intermediates to continue the deethylation
process and turn the adsorption of Rhodamine-B (RB) and
Janus green (JG) on the Fe2O3/Cu2O catalyst surface. Interme-
diates of the photocatalytic degradation of Rhodamine-B (RB)
(Fig. 11) and (Fig. S5–S15 in ESI†), using Fe2O3/Cu2O are ben-
zoic acid, benzylidyneoxonium, (E)-4-amino-2-(prop-1-en-1-yl)
phenol, 3-imino-3H-xanthen-6-amine, (Z)-3-(ethylimino)-3H-
diethylbenzene-1,2,4-triamine
cation,
8(diethylamino)
phenazine-2-diazonium intermediates (Table S2 in ESI†). In the
present context, the cleavage of the dye chromophores by Fe2O3/
Cu2O has been hypothesized.
To evaluate the stability and re-usability of the Fe2O3/Cu2O
photo-catalyst, ve additional cycles of dye RB and JG removal
were performed with it. Fig. 13 shows the good recyclability of
the Fe2O3/Cu2O photo-catalyst for ve consecutive cycles. A
slight decrease in activity for the h cycle of RB and JG may be
due to the loss of catalyst due to its recyclability. This result
xanthen-6-amine,
(E)-N,N-diethyl-3-(diethylammonium)-3H-
xanthen-6-amine, 2-(6-amino-3-imino-3H-xanthen-9-yl)benzoic
Fig. 13 Role of different radical scavengers on the photo-removal of
Fig. 11 Photodegradation intermediates of Rhodamine-B (RB-B).
(a) Rhodamine-B (RB) and (b) Janus green (JG).
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clearly indicates that both dyes degraded into carbon dioxide
and water as the nal products. The experiments were carried
out under optimal reaction conditions (RB and JG ¼ 9 mM,
Fe2O3, Cu2O and Fe2O3/Cu2O ¼ 0.75 g lꢂ1 and irradiation time ¼
120 min) in the presence of scavengers (2 mM for 200 ml of
respective dye solution) such as t-BuOH for $OH,58 benzoqui-
none (BQ) for O2c,59 and potassium iodide (KI) for holes and
$OH.60 The effect of t-BuOH, BQ and KI on the photo-removal
percentage of RB and JG is shown in (Fig. 6). It was clearly
observed that the photo removal percentage of RB was reduced
to 35.79%, 39.78%, 43.11% and 91.25% aer the addition of KI,
t-BuOH, BQ and blank, respectively. Then again, the Janus
green (JG) is as follows: t-BuOH-43.65%, BQ-44.78%, KI-38.65%,
and blank-89.14%. The photocatalytic activity of the nano-
material surprisingly concealed the scavenge ring effect, indi-
cating that both O2c and the $OH are actively implicated in the
photo-removal process.
Fig. 15 Antibacterial activity of different Fe2O3, Cu2O and Fe2O3/Cu2O
nano-composites (a) E. coli; (b) P. aeruginosa; (c) Staph. aureus; (d) B.
subtilis; [C] sterile distilled water.
3.5. Probable photocatalytic mechanism with Fe2O3/Cu2O as
the photocatalyst
3.4. Determination of the antibacterial activity of Cu2O,
Fe2O3 and the Fe2O3/Cu2O photocatalyst nanocomposite
This section clearly explains how the movement of the photo-
induced charge carriers occurring between Fe2O3 and Cu2O
have been systematically estimated using Mulliken-
electronegativity theory.61 This theory helps explain how a p–n
heterojunction functions, according to Anderson's model.62,63
The Mulliken-electronegativity of the semiconductor, ECB and
EVB, are respectively the conduction and valence band edge
potential, where the band gap of the semiconductor Eg and Ee is
the free energy of electrons on the H2 (4.5 eV), for Cu2O and
Cu2O, Fe2O3 and the Fe2O3/Cu2O were further tested for their
potential to inhibit test bacterial pathogens by the disc diffusion
method (Fig. 15). The result of antibacterial activity showed that
there exists a signicant zone of inhibition against test pathogens
(Table 3). It should be noted that, among the test samples, due to
its large surface area, Fe2O3/Cu2O composites respectively yields
the maximum inhibition zones of 20.13, 21.09, 08.23 and 20.60
for Staph. aureus, P. aeruginosa, B. subtilis and E. coli. In the case
of B. subtilis, whilst only a slight response to Fe2O3/Cu2O nano-
composite is observed, we have not observed any zone of inhi-
bition in SDW (Sterile Distilled Water) diffused discs against test
pathogens. This study clearly suggests that the Fe2O3/Cu2O
nanocomposite inhibits bacterial pathogens by rupturing the
outer and inner walls of the cell, which leads to disorganization
and leakage of the cell membrane (Fig. 16).
Fig. 16 Selected live and dead cell representative fluorescent micro-
scopic images (40 X) and scanning electron microscopic images of
dead cells, (a, e and i) E. coli, Fe2O3; (b, f and j) Pseudomonas sp., Cu2O;
(c, g and k) Staph. aureus, Fe2O3/Cu2O; (d, h and i) B. subtilis, Fe2O3/
Cu2O in-general both of the series; green dots represent live bacterial
cells, and yellow/orange dots represent dead cells.
Fig. 14 Hydrothermal growth of the Fe2O3/Cu2O nanocomposites
and the bacterial degradation and cell viability testing studies.
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Fe2O3. The electronegativity values were reported to be 5.32 and
5.88 eV, respectively. The probable band gap values of Cu2O and
Fe2O3 from the UV-Vis spectrum determine the diffuse reec-
tance. The conduction and the valence band edges of the Fe2O3/
Cu2O photocatalyst are given in Table 2. Both the conduction
band edge of Cu2O and Fe2O3 are respectively negative and
positive with respect to the hydrogen reduction potential on the
normalized hydrogen scale (Fig. 17). Likewise, the valence band
edge of Fe2O3 is more positive than that of Cu2O. Both semi-
conductors with different electronegativities and bands posi-
tions in an internal electric eld at either side of the junction
will build up, being directed from the Fe2O3 surface to the Cu2O
surface and become exposed to visible spectrum (400 nm). The
photo-generated electrons will, under the inuence of an inner
electric eld, shi from p-type Cu2O to n-type Fe2O3, and the
photo-created holes force to transfer from the valence bands of
Fe2O3 to the valence band of Cu2O. These processes in effect
separate and mobilize the photo-generated electron and holes,
thereby enhancing the photocatalytic activity of the Fe2O3/Cu2O
photocatalysts.
Fig. 17 Projected photocatalytic removal mechanism under visible
light irradiation.
photo-reaction very much depends on the efficiency of the
adsorption of untreated organic contaminants on the photo-
catalysts and the process of splitting photo-created electron–hole
pairs. The holes can either react or be adsorbed through surface
hydroxyl to form hydroxyl radicals. As a result, the adsorption
equilibrium was destroyed, permitting dye molecules to move from
single elucidation to the interface and to consequently decompose
into CO2, H2O and other raw materials through redox reactions.
3.6. Photo-removal mechanism of dyes
Hydrogen generation from photo-induced water break-down is
increasingly seen as a feasible choice to concurrently solve
energy and environmental problems. Various photo-induced H2
generation techniques and photocatalytic water splitting was
demonstrated in the visible light spectrum.64–68 The low cost
and high sustainability features of the reaction system in the
photo-removal mechanism of dyes are explained in S1 in ESI.†
Electron–hole pairs in the excited Fe2O3 could be efficiently
separated to facilitate an efficient shi into photo-induced elec-
trons between Fe2O3 and Cu2O. This radical splitting process
makes a crucial task in the removal of dyes. The effectiveness of the
3.7. Re-usability of photocatalysts
The sustainability of a photocatalyst is the most important
concern for the booming industry. In order to investigate the
stability and durability of Fe2O3/Cu2O, nanomaterial recycling
experiments were conducted for the photo-removal of RB and
JG. Aer the completion of each cycle, the photocatalyst was
collected using an external magnet, washed with double
Table 2 The electronegativity, band gap, conduction band (CB) edge
and valence band (VB) edge potential of the catalysts on a normalized
hydrogen scale
Semiconductor
catalyst
x (eV)
Eg (eV)
ECB (eV)
EVB (eV)
Fe2O3
Cu2O
5.88
5.32
1.96
1.88
ꢂ0.12
2.355
1.76
0.395
Table 3 Antibacterial activity of the Fe2O3/Cu2O nano-composite
against test pathogensa
Zone of inhibition (in mm)
Test sample B. subtilis
Staph. aureus P. aeruginosa E. coli
Fe2O3
Cu2O
04.53 ꢃ 0.11 15.21 ꢃ 0.33 17.24 ꢃ 0.12 17.41 ꢃ 0.06
02.22 ꢃ 0.24 16.24 ꢃ 0.33 17.10 ꢃ 0.23 18.17 ꢃ 0.07
Fe2O3/Cu2O 08.23 ꢃ 0.14 20.13 ꢃ 0.11 21.09 ꢃ 0.32 20.60 ꢃ 0.11
Fig. 18 Recycling performance of the photo-removal of (a) Rhoda-
mine-B (RB) (b) Janus green (JG), and XRD patterns of (c) Rhodamine-
B (RB): Fe2O3/Cu2O (d) Janus green (JG): Fe2O3/Cu2O before and after
successful cycles of recycling.
Std. DW
00.00 ꢃ 0.00 00.00 ꢃ 0.00 00.00 ꢃ 0.00 00.00 ꢃ 0.00
a
Values are means of three independent replicates; ꢃ indicate standard
error.
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distilled water, dried overnight, and reused.66,67 The photo- concentration, the lowest concentration of the three respec-
removal percentages of RB and JG for ve successive cycles tive samples used in the assay did not reduce the cell viability
were found to be 79.15%, 74.65%, 75.45%, 74.98% and 73.24%, noticeably as compared to other increased concentrations,
respectively, and are shown in Fig. 18.
among which Fe2O3/Cu2O was found to be low in a systematic
The Fe2O3/Cu2O nanomaterial exhibits an average of manner. Therefore, Fe2O3/Cu2O is the safest and the super-
76.24% for photocatalytic activity aer ve successive cycles. lative alternative in terms of toxicity in biological
The reduction in activity aer two cycles is due to the loss of application.
catalyst during the washing process.69 In addition, there is no
obvious change observed in the XRD pattern of Fe2O3 in
(Fig. 18) aer ve cycles. These results indicated that the
4 Conclusions
Fe2O3/Cu2O materials could be used as a stable photo-catalyst In this experimental work, a unique and visible light-driven
for the removal of organic pollutants in the form of dyes in Fe2O3/Cu2O composite photo-catalyst was prepared by
industrial wastewater.
a simple, eco-friendly and cost-effective hydrothermal method
for multi-purpose application. Rhodamine-B (RB) and Janus
green (JG) could be easily decolorized by Fe2O3/Cu2O under
visible light irradiation. Furthermore, the photocatalyst was
recycled several times without any observable loss of photo-
catalytic activity, making it suitable for use in dye removal in
wastewater. Moreover, the synthesized materials were found
to be highly stable in the photocatalytic process; displayed
antibacterial properties against E. coli, P. aeruginosa, Staph.
aureus and B. subtilis; and anti-cancer properties against
Musmusculus skin melanoma cells (B16-F10). These results
suggest that these composite materials can be utilized in the
biomedical eld and for catalysis and energy conversion
systems.
3.8. Toxicity tests of Fe2O3, Cu2O and Fe2O3/Cu2O
To further illustrate the bio-compatible advantage of the
proposed photocatalyst, up-conversion materials for emit-
ting visible light were used as candidates for both in vivo and
in vitro bio-imaging as well as in photodynamic therapy. We
measured the toxicity of Fe2O3, Cu2O and Fe2O3/Cu2O using
Musmusculus skin melanoma cells of various concentrations
(5–500 g mlꢂ1). The results suggested that the viability of
(B16-F10) cells decreased in a dose-dependent manner in
each sample (Fig. 19), clearly showing that, at a 5 g mlꢂ1
Conflicts of interest
The authors declare no competing nancial interests.
Acknowledgements
One of the author M. R. Abhilash (IF160104) thanks the
Department of Science and Technology, Govt. of India, for
awarding nancial assistance through the DST – INSPIRE
Fellowship to carry out the research work.
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