Cascade electron transfer in ternary CuO/α-Fe2O3/γ-Al2O3 nanocomposite as an effective visible photocatalyst

Article abstract of DOI:10.1016/j.jpcs.2020.109899

Highly efficient ternary heterojunction of CuO/α-Fe₂O₃/γ-Al₂O₃ was effectively fabricated by a facile and cost effective chemical route. The structural, chemical composition, morphology, optical and photocatalytic properties of as-prepared CuO/α-Fe₂O₃/γ-Al₂O₃ photo catalyst were compared to pristine and binary samples by various characterization. Existence of all the dominant peaks of CuO, α-Fe₂O₃ and γ-Al₂O₃ are noticeable in XRD spectrum of CuO/α-Fe₂O₃/γ-Al₂O₃ ternary photo catalyst which confirms the successful formation of the photocatalyst. SEM and HRTEM results revealed the spherical shape CuO nanoparticles with distorted α-Fe₂O₃ agglomerated plates which led to complete diffusion with γ-Al₂O₃. The band gap of ternary nanocomposite was found to be 1.9 eV elucidated by UV-DRS. Brunauer-Emmett-Teller (BET) analysis showed that as-fabricated ternary CuO/α-Fe₂O₃/γ-Al₂O₃ nanocomposite exhibited the porous structure with large surface area and small pore volume as compared to pristine γ-Al₂O₃.due to the unique ternary nanocomposite structure and synergistic effect among various components. The photocatalytic activity was examined by monitoring the deterioration of methyl orange under simulated solar light irradiation. CuO/α-Fe₂O₃/γ-Al₂O₃ exhibited superior photocatalytic efficacy as compared to CuO/γ-Al₂O₃ and α-Fe₂O₃/γ-Al₂O₃ binary and pure oxides of γ-Al₂O₃, CuO and α-Fe₂O_3. The marvelous photocatalytic activity of CuO/α-Fe₂O₃/γ-Al₂O₃ ternary nanocomposite samples can be ascribed to their close contact, strong interfacial hybridization and proficient charge transfer capacity. The electrochemical studies such as linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were carried out to explore the charge transfer behavior and support the high photo activity of ternary nanocomposite CuO/α-Fe₂O₃/γ-Al₂O₃. LSV measurements manifested that CuO/α-Fe₂O₃/γ-Al₂O₃ exhibited 4.3 folds higher current density than bare γ-Al₂O₃ which confirmed the faster electron transfer from CuO to γ-Al₂O₃ via mediated α-Fe₂O₃ through the interfacial potential gradient in conduction band. Cyclic voltammetry (CV) results showed that pair of anodic and cathodic peaks in CuO/α-Fe₂O₃/γ-Al₂O₃ appeared which affirm the efficient increase in photo-induced e^?/h⁺ separation and suppress recombination rate of electron-hole pair. This work demonstrated that CuO/α-Fe₂O₃/γ-Al₂O₃ ternary nanocomposite is found to be a promising candidate as an efficient adsorbent for organic dye removal from waste water.

Full text of DOI:10.1016/j.jpcs.2020.109899

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Cascade electron transfer in ternary CuO/α-Fe O /γ-Al O nanocomposite as an
2 3 2 3
effective visible photocatalyst
Aisha Kanwal, Shamaila Sajjad, Sajjad Ahmed Khan Leghari, Zameela Yousaf
PII:
S0022-3697(20)32799-2
https://doi.org/10.1016/j.jpcs.2020.109899
PCS 109899
DOI:
Reference:
To appear in: Journal of Physics and Chemistry of Solids
Received Date: 5 July 2020
Revised Date: 30 October 2020
Accepted Date: 10 December 2020
Please cite this article as: A. Kanwal, S. Sajjad, S.A. Khan Leghari, Z. Yousaf, Cascade electron transfer
in ternary CuO/α-Fe O /γ-Al O nanocomposite as an effective visible photocatalyst, Journal of
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Physics and Chemistry of Solids, https://doi.org/10.1016/j.jpcs.2020.109899.
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Credit author statement
Aisha Kanwal: Conceptualization, Methodology, Data curation, Writing- Original draft
preparation
Shamaila Sajjad: Supervision, Validation, Resourses, Project administration
Sajjad Ahmed Khan Leghari: Visualization, Investigation, Writing- Reviewing and Editing
Zameela Yousaf: Writing- Reviewing and Editing

Graphical Abstract

Cascade electron transfer in ternary CuO/α-Fe2O3/γ-Al2O3
nanocomposite as an effective visible photocatalyst
Aisha Kanwala, Shamaila Sajjada*, Sajjad Ahmed Khan Legharib, Zameela Yousafa
a International Islamic University, H-10 Islamabad, Pakistan
b Pakistan Institute of Engineering and Applied Sciences, Islamabad, Pakistan.
Corresponding author e-mail. shalisajjad@yahoo.com
Corresponding Author Address: Associate Professor Faculty of Basic and Applied Sciences,
International Islamic University Islamabad. 44000, Pakistan.
Phone No. 0092-51-9019813
1

Abstract
Highly efficient ternary heterojunction of CuO/α-Fe2O3/γ-Al2O3 was effectively fabricated by a
facile and cost effective chemical route. The structural, chemical composition, morphology,
optical and photocatalytic properties of as-prepared CuO/α-Fe2O3/γ-Al2O3 photo catalyst were
compared to pristine and binary samples by various characterization. Existence of all the
dominant peaks of CuO, α-Fe2O3 and γ-Al2O3 are noticeable in XRD spectrum of CuO/α-
Fe2O3/γ-Al2O3 ternary photo catalyst which confirms the successful formation of the
photocatalyst. SEM and HRTEM results revealed the spherical shape CuO nanoparticles with
distorted α-Fe2O3 agglomerated plates which led to complete diffusion with γ-Al2O3. The band
gap of ternary nanocomposite was found to be 1.9 eV elucidated by UV-DRS. Brunauer-
Emmett-Teller (BET) analysis shown that due to the unique ternary nanocomposite structure and
synergistic effect among various components, as-fabricated ternary CuO/α-Fe2O3/γ-Al2O3
nanocomposite exhibited the porous structure with large surface area and small pore volume as
compared to pristine γ-Al2O3. The photocatalytic activity was examined by monitoring the
deterioration of methyl orange under simulated solar light irradiation. CuO/α-Fe2O3/γ-Al2O3
exhibited superior photocatalytic efficacy as compared to CuO/γ-Al2O3 and α-Fe2O3/γ-Al2O3
binary and pure oxides of γ-Al2O3, CuO and α-Fe2O3. The marvelous photocatalytic activity of
CuO/α-Fe2O3/γ-Al2O3 ternary nanocomposite samples can be ascribed to their close contact,
strong interfacial hybridization and proficient charge transfer capacity. The electrochemical
studies such as linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were carried out
to explore the charge transfer behavior and to support the high photo activity of ternary
nanocomposite CuO/α-Fe2O3/γ-Al2O3. LSV measurements manifested that CuO/α-Fe2O3/γ-
Al2O3 exhibited 4.3 folds higher current density than bare γ-Al2O3 which confirmed the faster
2

electron transfer from CuO to γ-Al2O3 via mediated α-Fe2O3 through the interfacial potential
gradient in conduction band. Cyclic voltammetry (CV) results shown that pair of anodic and
cathodic peaks in CuO/α-Fe2O3/γ-Al2O3 which affirm the efficient increase in photo-induced e-
/h+ separation and suppress recombination rate of electron-hole pair. This work demonstrated
that CuO/α-Fe2O3/γ-Al2O3 ternary nanocomposite found to be a promising candidate as an
efficient adsorbent for organic dye removal from waste water.
Keywords:
CuO/α-Fe2O3/γ-Al2O3; Ternary composite; Interfacial charge transfer;
Environmental remediation; Cascade electron transfer.
3

1. Introduction
In past few years, heterostructure semiconductor nanomaterials have captivated much
consideration of reader due to its potential employment in environmental applications.
Particularly, water becomes the main source of environmental pollution as it is the primary
carrier of all contaminations such as heavy metals, carcinogenic dyes, pharmaceutical
desolations and industrial wastes [1, 2]. To combat such issues, numerous approaches have been
adopted to treat effluents from water and to develop various processes such as photo-Fenton,
adsorption wet-air oxidation, Fenton, and electrochemical oxidation, ozonation, evaporation,
photocatalytic purification, microwave catalysis and chemical process. On the other hand, each
approach has some limitations which generally include infuriation of large amount of solid
wastes and production of toxic byproducts [3-6]. Recently, heterogeneous catalysis is an
environmentally amiable and cost efficacious technique to expedite harmful compounds
(pollutants) into the effective and carbonaceous products [7, 8]. The basic principle of photo
catalysis involves the creation of electron hole (e-h) pair in semiconductor under visible light
irradiation [9]. The heterostructures with different band gap materials have been employed to
improve photo generated charge carrier separation efficiency in photo catalysis [4, 10, 11].
Gamma aluminum oxide (γ-Al2O3) is a supporting catalyst in the field of catalysis due to
photochemical inertness, marvelous thermal stability, exquisite surface characteristics and
unique acidic features [12-15]. Even, there are few drawbacks in γ-Al2O3 such as its low photo
response and recombination rate due to a wide band gap (7.0-9.5 eV). Numerous approaches
have been established to enhance photocatalytic efficiency and tuned optical properties to
increase the light absorption capacity of γ-Al2O3 toward visible light range and suppress e-h pair
recombination during photoreaction. This is achieved by composite fabrication, doping by metals
4

and non-metals and addition of transition metals or coupling of γ-Al2O3 with narrow band gap
materials. It is important to modify the band gap of γ-Al2O3 which results in improvement of
sensing, electrical and photo catalytic properties [7, 16]. Major instance has been paid to the
coupling of γ-Al2O3 with some materials that shows superior charge transport properties to enrich
the photocatalytic performance by eliminating the agglomeration of semiconductor. Recently, it
was reported that γ-Al2O3 have defect sites which could accept photo-induced electrons to
suppress the rate of recombination of (e-h) thus enhanced the photocatalytic activity [17, 18]. In
this regard, hematite (α-Fe2O3) and cupric oxide (CuO) have been chosen to couple with γ-Al2O3
for increasing the dispersion of active sites to promote surface area, improved charge carrier
efficiency that enhanced the photocatalytic activity. Cupric oxide (CuO) is a p-type
semiconductor which exhibits a small optical band gap of 1.3-2.0 eV [19]. On the other hand, α-
Fe2O3 is an n-type semiconductor that has low band gap in the range of 1.9eV-2.2 eV has more
absorbing ability of visible light. Under the ambient atmosphere, α-Fe2O3 is the most stable form
of iron oxide which is extensively used in water treatments, catalysis, sensors and lithium ion
batteries [20-22]. CuO and α-Fe2O3 are the promising photo catalyst that have been widely used
for the photo deterioration of organic contaminations and harmful products. A profusion of
reports revealed that there are many binary and ternary hybrid structures based on α-Fe2O3 and
CuO such as CuO/Cu2O [23], Ag/Fe2O3 [21], SiO2 and TiO2 modified Fe2O3 [24], ZnO/CuO
[16], TiO2/Al2O3[18], Fe3O4@Fe2O3/Al2O3 [22], g-C3N4/Al2O3[17], WO3/NiO/Fe2O3 [25],
CuO/CeO2 [26] and CuO/Al2O3 [27] etc. Literature study revealed that binary and ternary
heterojunctions have been prepared by adopting various synthetic routes. The main focus of this
study is to adopt an effective wet chemical synthetic route with same concentration of each oxide
to prepare a novel ternary CuO/α-Fe2O3/γ-Al2O3 photo catalyst with marvelous optical, structural
5

and morphological properties. The remarkable enhancement in photocatalytic performance
towards methyl orange (MO) under sunlight irradiation is observed. This study elucidates that
ternary nanocomposite of CuO/α-Fe2O3/γ-Al2O3 exhibits increased photocatalytic degradation as
compared to pure oxide samples and respective binary composite. Current research explains that
high photocatalytic efficiency of ternary nanocomposite can be accredited to the synergetic effect
between defect sites of γ-Al2O3, CuO and α-Fe2O3 nanoparticles which further promotes the
minor recombination of photo created holes and electrons, and increases the mobility of charge
carriers. As per our knowledge, no study has described the utilization of ternary composite of
CuO/α-Fe2O3/γ-Al2O3 as a photocatalyst with cascade electron transfer. This study meticulously
explain the comprehensive methodologies which can improve the selectivity of photo catalysis
for crucial industrial processes.
2. Experimental
2.1. Materials and method
Aluminium foil, Copper nitrate Cu (NO3)2, Ferrous chloride (FeCl3), Sodium hydroxide (NaOH)
and Methyl orange were obtained from Sigma Aldrich. All the chemicals were analytically pure
(99.99%) and were used as received. The deionized water was employed for synthesis of all
aqueous solutions.
2.2. Synthesis of Gamma Alumina (γ-Al2O3), Hematite (α-Fe2O3) and Cupric Oxide (CuO)
In first step, pure γ-Al2O3, Hematite (α-Fe2O3) and Cupric Oxide (CuO) were fabricated by facile
wet chemical route using Aluminum foil, Iron chloride and Copper nitrate as precursors,
respectively. Typically, 6.0 g of Aluminum foil, 5.0 g Iron chloride and 5.0 g Copper nitrate
were separately deliquesce in 100 mL water and each solution was kept under robust stirring
6

until homogeneous mixtures were obtained. Now sodium hydroxide (NaOH) solution is added
drop by drop to each solution. All suspensions were kept under fast stirring at 60 °C for 3 h until
γ-Al2O3, Hematite (α-Fe2O3), Cupric Oxide (CuO) nanoparticles were formed. The γ-Al2O3, α-
Fe2O3 and CuO precipitates were then obtained by centrifugation. Finally several washings were
done with deionized water to adjust the optimum pH value and to eliminate the residual
reactants. Consequently, the nanoparticles were desiccated at 40 °C for 7h and then followed by
calcination of dried powder samples at 800 °C for 4 h (γ-Al2O3), 500 °C for 2h (CuO) and 300
oC for 5h (α-Fe2O3).
2.3. Synthesis of α-Fe2O3/γ-Al2O3 and CuO/γ-Al2O3 binary composites
The binary nanocomposite of α-Fe2O3/γ-Al2O3 and CuO/γ-Al2O3 were fabricated by chemical
method. As prepared 0.5 g of CuO and 0.5 g of α-Fe2O3 nanopowders were deliquesce separately
in 50 mL deionized water with the γ-Al2O3 (0.5 g) in both solutions. The suspensions were
o
stirred vigorously for 1 h at 40 C. The precipitates were then washed many time with double
distilled water by centrifugation to eliminate the excess salts from both solutions. Finally,
collected binary composites of α-Fe2O3/γ-Al2O3 and CuO/γ-Al2O3 precipitates were dried in
vacuum oven at 50 oC for 4 h prior for characterization.
2.4. Synthesis of CuO/α-Fe2O3/γ-Al2O3 ternary composite
Wet chemical method was adopted for the formation of ternary nanocomposite CuO/α-Fe2O3/γ-
Al2O3. In a typical synthesis route, 0.5 g γ-Al2O3, 0.5 g α-Fe2O3 and 0.5 g CuO were added to
100 mL of distilled water. The prepared solution was magnetically stirred for approximately 120
o
min at 40 C. Finally, the product were collected by centrifugation and then dried in vacuum
7

o
oven at 50 C for 4 h to get CuO/α-Fe2O3/γ-Al2O3 nano-powder. The synthesis of ternary
nanocomposite is given in Scheme 1.
Scheme 1. Diagrammatic demonstration for the fabrication of CuO/α-Fe2O3/γ-Al2O3 ternary
composite.
2.5. Material characterization
The crystallinity and crystal phase of prepared samples were analyzed by powder Rigaku
D/MAX 2550 X-ray diffractometer using Cu Kα radiations (λ = 0.154056 nm) at ambient
temperature in a broad range of Braggs angle from 10o to 80o operating at 30 mA with 40 kV.
Surface features and textural properties of synthesized specimens were studied by Scanning
Electron Microscopy (JEOL-JAD-2300). Energy Dispersive X-Ray (EDX) coupled with SEM
was used to study the elemental configuration of as fabricated specimens. The structural
morphologies of fabricated ternary nanocomposite was accomplished by a JEM-2100 (HRTEM)
operated at an acceleration voltage of 200 kV. Fourier transform infrared spectroscopy was
8

employed to identify the functional groups and bond structures in the range of 400-4000 cm-1 by
using FT-IR spectrometer of manufacturing model Nicolet 740. To examine the optical
properties of fabricated specimens, Cary 100 UV-Vis spectrophotometer helps to collect the UV-
Vis DRS spectra of prepared specimens with BaSO4 as reflectance sample and a spectral
reflectance standard is over a wavelength range from 200 to 700 nm. Kubelka-Munk function
was employed to determine the optical band gap energies of prepared specimens. The
electrochemical measurements were performed by using core test potentiostat.
2.6. Photocatalytic activity test
The photo activity of fabricated specimens was investigated by detoxification of methyl orange
(MO) (toxic dye) upon exposure to solar light irradiation i.e visible light. All fabricated samples
(0.05 g) were added into 50 mL of MO solution. The obtained solution was placed in dark and
stirred for 30 min to achieve desorption/adsorption equilibrium of toxic dye methyl orange on the
surface of ternary heterojunction of CuO/α-Fe2O3/γ-Al2O3. Then solution was irradiated for 4h
under solar light to observe photocatalytic degradation efficiency. After the specific time period,
a 5.0 mL of aliquot of the colloidal suspension was collected and centrifuged to separate the
catalyst for further analysis. The clear transparent solution was then monitored by Schimadzu
2700 spectrophotometer in the range of 450-750 nm. The maximum absorption wavelength of
MO at 464.0 nm.
3. Result and discussion
3.1. Characterization of as-prepared samples
The phase purity, crystalline structure and crystallite size of prepared samples were analyzed by
X-ray powder diffraction technique. Figure 1(A) reveals the XRD pattern of as fabricated bare γ-
9

Al2O3, Hematite (α-Fe2O3) and Cupric Oxide (CuO). Fig. 1(B) depicts XRD profile of binary
nanocomposites (α-Fe2O3/γ-Al2O3, CuO/γ-Al2O3) and ternary heterojunction (CuO/α-Fe2O3/γ-
Al2O3). Fig. 1A(a) shows the diffraction peaks of pure γ-Al2O3 which can be ascribed to the
cubic structure of γ-Al2O3 (JCPDS No. 10-0425) and the 2θ value along with the hkl values are
32.0o (220), 37.5o (311), 39.0o (222), 46.0o (400) and 67.0o (440). The broad peaks shows the
amorphous nature and low crystalline degree of γ-Al2O3 [7, 28]. The mean crystallite size of γ-
Al2O3 has been computed by Debye Scherer formula and is found to be approximately 9.0 nm
.
Fig. 1A (b) shows XRD photograph of Fe2O3 nanoparticles confirming the hematite phase (α-Fe-
2O3). The clearly prominent characteristic peak of α-Fe2O3 indexed at 2θ value of 23.7o, 32.0 o,
36.0 o, 40.0 o, 49.0 o, 53.6 o, 62.0 o and 63.0 o corresponds to the crystal planes (012), (104), (110),
(113), (024), (116), (214) and (300) which coincides well with JCPDS No. 39-1346. The average
particle size of the fabricated NPs is ∼14.0 ± 3.0 nm [29]. In case of pure CuO, it has two main
peaks at the 35.2ᵒ and 38.3ᵒ associated to (-111) and (111) diffraction crystal planes as displayed
in Fig. 1A (c). The other characteristics peaks appeared at 32.3ᵒ (110), 48.8ᵒ (-202), 53.2ᵒ (020),
57.9ᵒ (202), 61.3ᵒ (-113), 66.0ᵒ (022), 67.9ᵒ (220) and 75.2ᵒ (203) affirming fabrication of CuO
nanoparticles with single monoclinic phase (JCPDS No. 045-0937) as no peaks of
contaminations or other forms of CuO are observed. The intensity or sharpness of the peaks
indicate the high degree of crystallinity of synthesized CuO nanoparticles [30, 31]. No impurity
and remnant peaks have been observed in all three pure samples which manifests the high grade
purity of synthesized samples. The average particle size of CuO is approximately 8.0±2.0 nm.
Figure 1B(a) illustrates that the XRD pattern of binary CuO/γ-Al2O3 nanocomposite implicate
that all the main peaks of bare CuO and γ-Al2O3 exists in the pattern confirming the successful
formation of CuO/γ-Al2O3. No peak shift has been observed which shows the stability of binary
10

structure. XRD pattern of α-Fe2O3/γ-Al2O3 nanocomposite is depicted in Fig. 1B (b). The peak
appeared at 2θ=46.0o and 67.0o is ascribed to γ-Al2O3 (JCPDS card 29-0063) whereas much
diminished peak of α-Fe2O3 has been observed in binary nanocomposite. The main reason for
disappearance of α-Fe2O3 in α-Fe2O3/γ-Al2O3 is the encapsulation of α-Fe2O3 crystallites by γ-
Al2O3. Thus, X-ray diffraction peak of α-Fe2O3 is insignificant in hybrid structure of α-Fe2O3/γ-
Al2O3 [32, 33]. Figure 1B (c) displays the diffraction pattern of CuO/α-Fe2O3/γ-Al2O3 ternary
nanocomposite. The heterostructure CuO/α-Fe2O3/γ-Al2O3 is composed of three kinds of phases,
cubic γ-Al2O3, α-Fe2O3 and monoclinic CuO confirming the successful formation of the ternary
heterostructure. On the other hand, there was no prominent peak shift as well as no other
impurities were found and no traces of other phases were spotted, suggesting the high purity of
the samples and confirming that CuO, α-Fe2O3 and γ-Al2O3 coupled together successfully and
without other phases.
The existence of defect sites in γ-Al2O3 can be justified by the results obtained from the XRD.
XRD results as depicted in Fig. 1 clearly displays the broad peaks at 47.0o and 67.0o in all
synthesized samples (pristine, binary and ternary nanocomposites). The broadening of peaks
describes the low degree of crystallinity and the indication of amorphous structure. Amorphous
structure of γ-Al2O3 possess a lot of surface active sites and defects which are responsible for
capturing the electrons more effectively in hybrid system [28].
11

Figure 1. (A) XRD diffraction patterns of pristine oxides, binary and ternary nanocomposites;
(a) γ-Al2O3 (b) α-Fe2O3 (c) CuO; (B) (a) CuO/γ-Al2O3 (b) α-Fe2O3/γ-Al2O3 (c) CuO/α-Fe2O3/γ-
Al2O3.
The surface morphology of nanomaterials and their ternary nanocomposite CuO/α-Fe2O3/γ-
Al2O3 were examined by scanning electron microscope. Figure 2 depicts the SEM photographs
of bare γ-Al2O3, α-Fe2O3, CuO and ternary heterostructure synthesized with same concentration
of each oxide. Figure 2A illustrates the dispersed leafy morphology of pure γ-Al2O3. Figure 2B
demonstrates that pure CuO has spherical morphology with uniform distribution [34]. Irregular
plate like architect with smooth surface are observed for pure α-Fe2O3 with small agglomeration
as shown in Fig. 2C [35]. Textural features of ternary heterostructure CuO/α-Fe2O3/γ-Al2O3 are
manifested in Fig. 2D. It can clearly be seen that ternary nanocomposite consists of spherical
particles and plate like morphology. CuO and α-Fe2O3 are well incorporated with Al2O3 matrix
and decorated on surface of γ-Al2O3. This promotes the intimate interfacial connection between
them to provide the more active sites for maximum degradation of dye molecule to enhance
photocatalytic performance.
12

Figure 2. SEM images of as-prepare samples; (A) γ-Al2O3, (B) CuO, (c) α-Fe2O3, and (D)
CuO/α-Fe2O3/γ-Al2O3.
Fourier transform infrared spectroscopy evidenced the various surfaces functional groups and
confirmed the chemical structure of as-prepared samples. Figure 3 (A-D) exhibits the FT-IR
spectra for pure γ-Al2O3, binary heterojunctions (α-Fe2O3/γ-Al2O3, CuO/γ-Al2O3) and ternary
nanocomposite of CuO/α-Fe2O3/γ-Al2O3. Figure 3A depicts the FTIR spectra of γ-Al2O3. The
peaks between 500-800 cm-1 is attributed to the γ-Al2O3. The most evident peak at 645 cm-1 is
assigned to the stretching vibrational mode of Al-O. The peak appeared at 1000-1032 cm-1 is
ascribed to Al-O bending vibrational mode. A peak at 1062 cm-1 is corresponding with
symmetrical bending mode of Al-O-H group [36- 38]. FT-IR results of binary nanocomposite α-
Fe2O3/γ-Al2O3 is given in Fig. 3B. It displays the characteristics peaks of α-Fe2O3 phase in the
range of 400-700 cm-1. The strong peak below the 700 cm-1 is accredited to Fe-O stretching
mode. The strong and sharp bands appeared at 466 and 576 cm-1 are ascribed to Fe-O stretching
13

mode [39-42]. The intense peak occurred at 644 cm-1 can be associated to Al-O stretching
vibration mode and a peak at 1041 cm-1 can be related with Al-O-H group [36-38, 43]. The
bands ranging from 3300-3500 cm-1 is accredited to OH group and water molecules that are
adsorbed on surface respectively [44]. The above mentioned peaks of both hematite (α-Fe2O3)
and γ-Al2O3 confirmed the successful formation of binary nanocomposite α-Fe2O3/γ-Al2O3.
Figure 3C demonstrates the FT-IR spectrum of CuO/γ-Al2O3 binary nanocomposite. The three
main absorption peaks shows the vibrational mode of CuO in the range of 500-700 cm-1. The
dominant and sharp vibration peak for CuO was noticed at 434 cm-1 and the peak at 525 cm-1
linked to the stretching mode of CuO [45]. The band at 802 cm-1 can be assigned to Cu-OH [46].
The hump shown at 647 cm-1 is attributed to Al-O stretching mode in γ-Al2O3. The peak located
at 1313 cm-1 may arise due to existence of CO2 which usually resides on the surface of sample by
adsorption from air or may be due to organic moieties [4]. These characteristic vibrations and
bending modes reappear in the FTIR spectrum of the CuO/α-Fe2O3/γ-Al2O3 (Fig. 3D) which
demonstrates successful formation of ternary nanocomposite. The peak occurred at 434 cm-1 to
525 cm-1 may be related to Cu-O stretching vibration mode [45, 47]. The band appeared at 879
cm-1 is associated to Cu-OH vibrational mode of CuO [46]. Fe-O stretching mode of α-Fe2O3 is
observed at 446 cm-1 and 563 cm-1 [48, 49]. The γ-Al2O3 is confirmed in the ternary
nanocomposite by the valley between 1000-400 cm-1 in the spectral range and the peak at 778
cm-1 corresponds to the vibrational bending of Al-O. The peak at 645 cm-1 can be accredited to
Al-O-Al in γ-Al2O3 [50]. The band appeared in all FT-IR spectra in the range between 1200-
2000 cm-1 arises due to organic moieties and bands in the range of 3300-3500 cm-1 is attributed
to hydroxyl group as vibrational stretching from absorbed water molecules [4, 51].
14

Figure 3. FT-IR of pristine oxides, binary and ternary composites (A) pure γ-Al2O3; (B) α-
Fe2O3/γ-Al2O3; (C) CuO/γ-Al2O3; (D) CuO/α-Fe2O3/γ-Al2O3.
The elemental configuration and distribution of as synthesized materials were evaluated and
affirmed by energy dispersive spectroscopy and is displayed in Fig. 4(A-D). Figure 4A shows
the existence of Al and O in bare sample of γ-Al2O3. EDX spectra of binary heterojunction α-
Fe2O3/γ-Al2O3 are illustrated in Fig. 4B. According to EDX pronounced peaks of Al, Fe and O
elements are distinctly seen. In case of CuO/γ-Al2O3, the peaks are ascribed to Al, Cu and O
without any impurity peaks as shown in Fig. 4C. X-ray elemental mapping profile of ternary
nanocomposite (CuO/α-Fe2O3/γ-Al2O3) is shown in Fig. 4D and manifested the successful
fabrication of pure-phase of ternary nano-hybrid structure by the existence of Fe, Cu, Al and O.
Presence of all elements (Al, Cu, Fe and O) implies the uniform distribution over the surface of
ternary hybrid structure. Moreover, no impurity peaks have been noticed in EDX spectra of all
15

synthesized samples which assure the purity of samples. Table 1 describes the relative
configuration of prepared samples.
Table 1. Elemental composition of the fabricated specimens of pure oxides, binary and ternary
composites.
γ-Al2O3
CuO/γ-Al2O3
α-Fe2O3/γ-Al2O3
CuO/α-Fe2O3γ-Al2O3
Elements
Al
At. % Wt. % At. % Wt. % At. %
Wt. %
43.83
At. %
31.76
Wt. %
38.69
33.04 45.42 34.49 42.35
32.74
------
------
------
------
4.59
-----
13.29
-------
-----
1.41
65.85
------
3.90
52.27
3.07
3.02
62.15
8.81
7.62
44.89
Cu
Fe
O
66.96 54.58 60.92 44.36
100 100 100 100
100
100
100
100
Total
Figure 4. EDX analysis of (A) pristine γ-Al2O3; (B) α-Fe2O3/γ-Al2O3; (C) CuO/γ-Al2O3 and (D)
CuO/α-Fe2O3/γ-Al2O3.
16

To study the optical properties and to probe the relationship between the electronic structures of
synthesized samples, UV-Vis DRS of pure samples, binary and ternary nano-hybrid structures
are depicted in Figure 5(A-F). Kubelka-Munk function helps to calculate the optical band gap
energies of prepared samples. The Kubelka-Munk function is given as;
F(R) =K/S= (1-R) 2/ 2R
Where, R = reflectance of material, S = scattering coefficient, K= molar absorption coefficient.
It is approximated by drawing a tangent to the curve on to X-axis. Figure 5(A-C) illustrates the
calculated band gaps to be 3.4 eV, 2.04 eV and 1.8 eV for pure samples of γ-Al2O3, α-Fe2O3 and
CuO, respectively which coincided well with reported literature [7, 52, 53]. It can be seen that
the addition of α-Fe2O3 and CuO influence the optical property of light absorption of binary and
ternary heterostructure significantly. The band gap values of binary heterostructures are found to
be 2.1 eV and 3.0 eV for α-Fe2O3/γ-Al2O3 and CuO/γ-Al2O3, respectively as demonstrated in
Fig. 5(D, E). The calculated band gap energy value of as prepared γ-Al2O3 is 3.4 eV which is far
smaller than previously reported for insulators >5.0 eV. The significantly low band gap value of
γ-Al2O3 may be ascribed to surface defects in the γ-Al2O3 particles. This existence of defects can
be associated with the hybridization of the sp3 orbitals of γ-Al2O3 [7, 54]. The band gap
narrowing of host material i.e γ-Al2O3 may be due to the intimate interfacial interaction which
suggests that the combination of α-Fe2O3 and CuO nanoparticles with γ-Al2O3 which could
probably make modifications to γ-Al2O3 in the basic process of e-h pair formation under sunlight
illumination. The band gap of ternary nanocomposite CuO/α-Fe2O3/γ-Al2O3 is 1.9 eV which is
calculated by transformed Kubelka-Munk function. From the results (Fig. 5F), it is clear that
there is a remarkable narrowing in the band gap of the ternary heterostructure (CuO/α-Fe2O3/γ-
Al2O3) in comparison with pure and binary samples. This may be ascribed to the synergic effect
17

of the composite constituent’s i.e γ-Al2O3, CuO and α-Fe2O3 leading to strong interfacial
interaction between constituents causing formation of new molecular orbitals of lower energy.
Therefore, photocatalytic phenomena by ternary nanocomposite could be better than binary
composite.
Figure 5. Kubelka-Munk function (KMF) of pure oxides, binary and ternary nanocomposites;
(A) γ-Al2O3, (B) α-Fe2O3, (C) CuO, (D) α-Fe2O3/γ-Al2O3, (E) CuO/γ-Al2O3, (F) CuO/α-Fe2O3/γ-
Al2O3.
To investigate the structure of CuO/α-Fe2O3/γ-Al2O3 ternary nanocomposite, TEM and HRTEM
images of CuO/α-Fe2O3/γ-Al2O3 ternary nanocomposite at different resolutions are shown in
Figure 6(A-C). Figure 6(A, B) displays TEM image of CuO/α-Fe2O3/γ-Al2O3 catalyst has a little
distorted spherical shape and plate like morphology which is in accordance with SEM results.
Cu, and Fe could be distinctly seen on the porous surface of γ-Al2O3 as well which evidenced the
successful fabrication of ternary catalyst. From TEM photographs, it could be expected that an
18

intimate interfacial channel exist in CuO/α-Fe2O3/γ-Al2O3 ternary heterojunction, which would
support the efficient charge carriers transfer thus causing enhancement in the photocatalytic
activity. High resolution TEM image (HRTEM) in Fig. 6(C) exhibits lattice fringes of the
CuO/α-Fe2O3/γ-Al2O3 ternary hybrid structure which have three major noticeable fringes with an
inter planer spacing of 0.25, 0.23 and 0.33 nm, associated to the crystalline nature of α-Fe2O3,
CuO coordinated with γ-Al2O3, respectively [53,24]. XRD results is in corroborate with the TEM
results.
Figure 6. TEM (A, B) and HRTEM (C) images of ternary nanocomposite of CuO/α-Fe2O3/γ-
Al2O3.
Cyclic voltammetry (CV) is a powerful technique to elucidate the redox behavior,
electrochemical properties and electron transfer kinetics of prepared materials. The cyclic
voltammetry analysis was performed at a scan rate of 10 mV/s in the potential range from 0.7 V
to 1.6 V using 1.0 M of NaOH as electrolyte. Figure. 7 delineates the CV scan of pristine γ-
19

Al2O3 and CuO/α-Fe2O3/γ-Al2O3 ternary nanocomposites. Negligible current response and no
redox peak have been found for bare γ-Al2O3 as shown in Fig. 7A. CuO/α-Fe2O3/γ-Al2O3 hybrid
structure (Fig. 7B) possess maximum current for both forward and reverse scans. The existence
of pair of particular redox (anodic and cathodic) peaks and significant increase in current density
of the ternary nanocomposite may be accredited to the synergistic interaction of individual
components in ternary nanocomposite which creates more number of catalytic sites and
enhances electron transfer. Moreover, the enhancement in current density of CuO/α-Fe2O3/γ-
Al2O3 suggested the existence of appropriate electronic channels between CuO, α-Fe2O3 and γ-
Al2O3 to effectively increase the photo-induced e--h+ separation and simultaneously decreased
the recombination rate of e-/h+ pair [55-57].
Figure 7. CV of pristine and ternary nanocomposite, (a) bare γ-Al2O3 (b) ternary nanocomposite
of CuO/α-Fe2O3/γ-Al2O3 at scan rate of 10 mV s−1.
Linear sweep voltammetry (LSV) helps to explain e- and h+ recombination rate and to study the
transfer of photo induced charge carriers [58]. Figure 8 depicts the linear sweep voltammetry
20

(LSV) plots of pure γ-Al2O3 and ternary heterojunction CuO/α-Fe2O3/γ-Al2O3 was carried out at
scan rate of 10 mV/s sweeping potential range from 0.7V to 1.8V in the 1M NaOH electrolyte
solution. As presented in Fig. 8a, the current density of pristine γ-Al2O3 is very small. However,
Fig. 8b illustrates that the inclusion of CuO and α-Fe2O3 to γ-Al2O3 significantly increases the
current density which in turn effectively promotes the electron mobility of ternary photo catalyst
CuO/α-Fe2O3/γ-Al2O3. This enhancement in photocurrent density affirmed the increase in
transfer of photo generated charge carrier through interfacial hybridization and reduce the e-/h+
pair recombination. By interfacial transfer of charge, photocurrent density exhibited by ternary
heterojunction is 69 μA/cm2 at a potential of 1.76 V which is about 4.3 folds greater than those
of the pure γ-Al2O3 (16 μA/cm2). This supports that the efficient charge transfer occurs from
CuO to Al2O3 via α-Fe2O3. The synergetic interaction of CuO and α-Fe2O3 with Al2O3 causes the
increase in electron mobility (photocurrent) which provides more active site and hence resulting
in excellent photocatalytic activity [56, 58].
21

Figure 8. Linear sweep voltammetry curves of pristine and ternary composite, (a) pure γ-Al2O3
(b) CuO/α-Fe2O3/γ-Al2O3 at scan rate of 10 m/v.
In order to determine the surface area, pore volume and pore diameter of catalysts are performed
by the N2 adsorption-desorption isotherms [59]. The adsorption in porous materials may be
attributed to the specific properties such as specific surface area, pore diameter and total pore
volume of porous stricture [60]. The hysteresis loop of γ-Al2O3 and CuO/α-Fe2O3/γ-Al2O3
(Figure 9) at relative pressure (0.4-1.0) P/Po represents the type II isotherm which further
confirms the porous structure of nanomaterial. CuO/α-Fe2O3/γ-Al2O3 ternary nanocomposite
shows the higher surface area 156.2 cm2/g than pristine γ-Al2O3 (143.4 cm2/g). Pore volume of
pure γ-Al2O3 and α-Fe2O3/CuO/γ-Al2O3 is noted to be 0.0023 and 0.0020 cm3/g respectively.
High surface area, small pore volume and pore diameter of CuO/α-Fe2O3/γ-Al2O3 ternary hybrid
structure exhibits that light may scatter efficiently and increase photo catalytic activity under
visible light irradiation and rapid charge transfer ability in catalytic activity.
Figure 9. N2 Adsorption desorption of (a) γ-Al2O3 and (b) CuO/α-Fe2O3/γ-Al2O3 at 77 K.
22

3.2. Photocatalysis
Methyl orange (organic pollutant) is used to assess the photocatalytic activity of synthesized
specimens upon exposure to visible light. For comparison, the photo catalytic activity of the pure
oxides, γ-Al2O3, α-Fe2O3 and CuO was also measured. The photocatalytic activity of pure oxide,
binary and ternary nanostructures is manifested in Fig 10A. Prior to irradiation, the samples were
placed in dark for half an hour to acquire adsorption-desorption equilibrium. As shown in Fig.
10Aa γ-Al2O3 alone show a very low photocatalytic activity 8.0%. This low photocatalytic
activity is due to large band gap which makes it less effective harvester of solar light and that is
the drawback of γ-Al2O3 for the use of sunlight [7]. Pure CuO shows 57.0% degradation of MO
as clear from Fig. 10Ab. Earlier studies revealed that CuO nanoparticles show very low photo
activity towards decomposition of MO and MB dyes. The reason for low decomposition
efficiency of CuO under sunlight is the quick recombination of e-h pair due to very small band
gap energy [53, 61]. Photocatalytic activity of bare α-Fe2O3 is shown in Fig. 10Ac and only
68.0% degradation of MO is observed. Pure α-Fe2O3 has photocatalytic activity which is limited
by some features like fast recombination rate of e-h, poor conductivity and low diffusion lengths
of holes which leads to low performance of iron oxide. These drawbacks of α-Fe2O3 can
overcome by lowering the recombination rate by designing heterostructures that can cause
increase in conductivity by coupling with suitable materials and increasing the charge transfer
capability [62]. The coupling of γ-Al2O3 with α-Fe2O3 and CuO to form binary nanocomposites
of α-Fe2O3/γ-Al2O3 and CuO/γ-Al2O3 shows improved photocatalytic performance of γ-Al2O3 as
illustrated in Fig. 10A(d, e). α- Fe2O3/γ-Al2O3 exhibits better photocatalytic efficiency than
CuO/γ-Al2O3 under sunlight irradiation. The appreciable photocatalytic performance of α-
Fe2O3/γ-Al2O3 is due to synergetic effect between α-Fe2O3 and γ-Al2O3. The interfaces in the
23

heterojunction has a vital impact on supporting the overall photocatalytic performance of binary
nanocomposite [52]. Furthermore, ternary CuO/α-Fe2O3/γ-Al2O3 nanocomposite (Fig. 10Af)
exhibits higher degradation efficiency as compared to both binary nanocomposite (α-Fe2O3/γ-
Al2O3 and CuO/γ-Al2O3) and pure samples under visible light irradiation. Under the sunlight,
ternary nanocomposite degraded approximately 98.0% of MO. While α-Fe2O3/γ-Al2O3 and
CuO/γ-Al2O3 nanocomposites degraded only 92.0% and 85.0% of MO, respectively. This
increase in MO degradation efficiency using ternary heterostructure is attributed to the existence
of the interfaces between CuO, α-Fe2O3 and γ-Al2O3. Photocatalytic oxidation reduction reaction
occurs on the surface of photo catalyst and significantly makes surface properties better which in
turn influence the performance of catalyst [5]. The synergetic effect of CuO, α-Fe2O3 in γ-Al2O3
structure in the degradation of MO is associated to these species in suppressing electron hole
recombination rate and improving the reactive species [63]. Consequently, the superb CuO/α-
Fe2O3/γ-Al2O3 photo catalyst is much more effective under solar light due to two main reasons;
(1) strong optical absorbance property, (2) large surface area of ternary nanocomposite [7, 64].
24

Figure 10. (A) Time course decaying profile, (B) Kinetic studies of the degradation of MO
under sunlight by pristine oxides, binary and ternary nanocomposites; (a) pristine γ-Al2O3, (b)
pure CuO, (c) pure α-Fe2O3, (d) CuO/γ-Al2O3, (e) α-Fe2O3/γ-Al2O3, (f) CuO/α-Fe2O3/γ-Al2O3
composites. (C) Comparison of photocatalytic degradation of MO by CuO/α-Fe2O3/γ-Al2O3
ternary nanocomposite at different pH values. (D) The reusability of CuO/α-Fe2O3/γ-Al2O3
ternary nanocomposite after degradation process.
3.2.1. Kinetic studies
The modified Langmuir-Hinshelwood model provides the pseudo-first-order kinetics for
photocatalysis. Photocatalytic activity can be estimated by apparent rate constant (k app) at
25

different light intensities. The first order kinetics determining that photoactivity is obtained by
the equation given below [65].
ln (C/Co) = k app
t
Where C= concentration of solution at irradiation time t and Co= initial concentration at t=0 and
kapp is apparent rate constant. The change in ln (Co/C) as a function of irradiation time are given
in Figure 10B.
3.2.2. Influence of solution pH on dye degradation
In the degradation of organic dye, pH of the solution is considered to be key factor in
photocatalysis experiment. Figure 10C indicated the impact of pH of the solution on the
decolourization of MO. The ternary composite CuO/α-Fe2O3/γ-Al2O3 showed the maximal
photocatalytic activity of 98.0% at low pH=4 of the solution whereas activity was found to be
40.0%, 60.0%, 65.0%, 89.0%, 85.0% and 36.0% at pH values of 1.0, 2.0, 3.0, 5.0, 7.0 and 9.0.
The results indicated that the solution in lower pH (acidic conditions) favors increased photo
reduction ability for MO than in alkaline media as in accordance with the literature. The dye
degradation phenomena is closely related to pH of the solution which in turn effects the surface
charge of adsorbents. In alkaline condition, the photocatalytic activity reduces because the
catalyst surface becomes negatively charged and repels the dye molecules to absorb on the
CuO/α-Fe2O3/γ-Al2O3 surface, thus suppressing the reduction rate of MO. Conversely, in acidic
condition (low pH) surface of CuO/α-Fe2O3/γ-Al2O3 becomes highly protonated, providing the
strong electrostatic attraction to the active and main molecules of MO and positively charged
surface of CuO/α-Fe2O3/γ-Al2O3 which would result in increased photocatalytic activity of
CuO/α-Fe2O3/γ-Al2O3 [66, 67].
3.2.3. Reusability and structure of the photocatalyst
26

The reusability of ternary CuO/α-Fe2O3/γ-Al2O3 nanocomposite is very essential for practical
application. There is no such report on the long term durability of CuO/α-Fe2O3/γ-Al2O3 for MO
photo reduction. The reusability of ternary CuO/α-Fe2O3/γ-Al2O3 hybrid structure was studied by
repetitive photocatalytic detoxification of MO by ternary heterojunction under solar light for five
cycles. Figure 10D illustrates that after five cycle’s degradation rate of MO was not less than
92.0%. The slight decrease of the 6.0% is mainly due to filtration loss and fouling of catalyst
[68]. The synthesized catalyst shows good stability, durability and reusability. Hence ternary
composite CuO/α-Fe2O3/γ-Al2O3 has proved to be an efficient photo catalyst that prevent the
environment from being polluted. Moreover, the ternary heterojunction (CuO/α-Fe2O3/γ-Al2O3)
after the photocatalysis was subjected to XRD to evaluate the structural stability. The
corresponding XRD spectra of recovered sample as shown in Figure 11 delineates that there is
absolutely no change in peak position but there is slight decreases in peak intensity of CuO/α-
Fe2O3/γ-Al2O3 compared to as prepared materials. These results suggests that crystal structure of
the photo catalyst remains unchanged before and after photocatalysis which means that the
internal structure of the catalyst remain same and will not break after catalytic process. Hence,
prepared ternary heterojunction exhibits the excellent stability after degradation process.
27