The document summarizes research on synthesizing Fe2TiO5–TiO2 heterostructures for efficient electrocatalytic hydrogen evolution. Key points: - Hierarchically arranged nanostructures with interconnected nano-petals of thickness around 50 nm were obtained using a simple hydrothermal technique with natural ilmenite as the source material. - The electrocatalytic properties of the synthesized heterostructures were enhanced following a cathodization technique. This led to an overpotential of 301 mV for 10 mA/cm2 hydrogen evolution compared to 928 mV without cathodization. - The enhancement is attributed to defect-rich Fe2-xTiO5-x-TiO2-x heterostructures

1. Original Research Report
Pseudobrookite based heterostructures for efficient electrocatalytic
hydrogen evolution
Niranjala Fernando a
, Jayashree Swaminathan b
, Francisco Carlos Robles Hernandez c,d,b
,
Gayan Priyadarshana e
, Chanaka Sandaruwan f
, Wenli Yang d
, Veranja Karunaratne h
,
Zixing Wang b
, Gehan A.J. Amaratunga g
, Nilwala Kottegoda b,h,*
, Ashokkumar Meiyazhagan b,**
,
Pulickel M. Ajayan b,***
a
Department of Design and Engineering, Faculty of Science and Technology, Bournemouth University, Dorset, BH12 5BB, UK
b
Department of Materials Science & NanoEngineering, Rice University, Texas, 77005, USA
c
Department of Mechanical Engineering Technology, University of Houston, Houston, TX, 77204, USA
d
Materials Science & Engineering, Cullen College of Engineering, University of Houston, Houston, TX, 77204, USA
e
Department of Materials and Mechanical Technology, Faculty of Technology, University of Sri Jayewardenepura, Pitipana, Homagama, Sri Lanka
f
Sri Lanka Institute of Nanotechnology, Pitipana, Homagama, Sri Lanka
g
Department of Engineering, University of Cambridge, Cambridge, CB2 1TN, UK
h
Department of Chemistry, Center for Advanced Materials Research, University of Sri Jayewardenepura, Gangodawila, Nugegoda, Sri Lanka
A R T I C L E I N F O
Keywords:
Heterostructures
Cathodization
Green catalyst
2D materials
Hydrogen evolution
Electrocatalysis
Pseudobrookite
A B S T R A C T
Synthesis of ultrathin heterostructures has received much attention in the recent past due to their unique physical
and chemical properties. In this work, we report the synthesis of Fe2TiO5–TiO2 heterostructures using a simple
hydrothermal technique employing natural ilmenite as the source. Hierarchically arranged nanostructures with
interconnected nano-petals of thickness around 50 nm are obtained. The electrocatalytic properties of the synthe-
sized Fe2TiO5–TiO2 heterostructures are enhanced following the cathodization technique. The observed enhance-
ment in the synthesized materials’ electrocatalytic property can be attributed to the defect-rich Fe2-xTiO5-x-TiO2-x
heterostructures. The current approach and technique discussed in this work offer a simple method to synthesize a
nanostructured heterostructure material and create defects for enhancing electrocatalytic activity.
1. Introduction
The discovery of two-dimensional (2D) graphene has unlocked
investigation in multifold areas of materials science and nanotech-
nology.1,2
Among them, exfoliation of a single-layered 2D sheet from
bulk has received significant interest due to their outstanding optical,
magnetic, and electronic transport characteristics.3,4
Until now, various
strategies have been proposed to create atomically thin layers of 2D
materials using techniques such as mechanical exfoliation,2,5,6
intercalation by ionic species,7,8
ultrasonication,9,10
etc. These exfoliated
atomically thin sheets have the potential in transforming technologies
and engineering next-generation ultrathin nanoelectronics and opto-
electronics devices.11,12
Amongst broader classification of the reported
2D materials, ilmenite (FeTiO3) is considered as a naturally occurring 2D
non-van der Waals material, which is formed autonomously during the
process of slow cooling of magma chambers.13
It exists in nature as iron
titanates, and in three primary forms of minerals (i.e., ilmenite (FeTiO3),
pseudobrookite (Fe2TiO5), and ulvite (Fe2TiO4)), and it most commonly
* Corresponding author. Department of Materials Science & NanoEngineering, Rice University, Texas, 77005, USA.
** Corresponding author. Department of Materials Science & NanoEngineering, Rice University, Texas, 77005, USA.
*** Corresponding author. Department of Materials Science & NanoEngineering, Rice University, Texas, 77005, USA.
E-mail addresses: nilwala@sjp.ac.lk (N. Kottegoda), ma37@rice.edu (A. Meiyazhagan), ajayan@rice.edu (P.M. Ajayan).
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Materials Reports: Energy
journal homepage: www.keaipublishing.com/en/journals/materials-reports-energy
https://doi.org/10.1016/j.matre.2021.100020
Received 11 September 2020; Accepted 18 January 2021
Available online 29 March 2021
2666-9358/© 2021 Chongqing Xixin Tianyuan Data & Information Co., Ltd. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an
open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Materials Reports: Energy 1 (2021) 100020

2. presents orthorhombic and monoclinic crystal structures.14,15
In general,
pseudobrookite is identified as an n-type semiconductor with a narrow
bandgap of ~2 eV, which possesses numerous potential applications,
including their use as an anode material for the photoelectrolysis of
water.16
The other exciting features of Fe2TiO5 are its unique magnetic
properties such as paramagnetic to ferromagnetic transition,17
aniso-
tropic spin-glass behavior with the shift at 53 K18
, and electrical
properties.19,20
Several groups have reported the synthesis of Fe-doped TiO2 based
materials and described their variations in structural features and their
relation to different properties.21,22
For example, Tao et al. synthesized
FeTiO3 nanoflowers through a hydrothermal technique, which displayed
enhanced pseudocapacitance behavior due to their highly organized 3D
architecture.21
Another study reports excellent dye removal properties
due to their layered flower-like 3D structures.22
The wet chemical syn-
thesis of different types of layered nanostructures has proven attractive
Fig. 1. (a) Powder X-ray diffraction patterns, (b) Raman spectra, (c) XPS survey scan, (d) high resolution Ti 2p spectrum, (e) high resolution Fe 2p spectrum and (f)
high resolution O1s spectrum of Fe2TiO5–TiO2 heterostructure samples calcined at 800
C for 2 h.
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
2

3. due to easy scalability, repeatability, presence of more active sites, and
enhanced porosity. The strong in-plane bonds and weak interactions
between the neighboring layers are the striking features of these layered
nanostructures, which can be delaminated or exfoliated into thin atomic
layers. Some of the synthesized 3D nanostructures have shown signifi-
cant enhancements in electronic and photocatalytic properties, including
Bi2WO6,
23
anatase TiO2,
24
and TiS2
25
nanoflowers.
Here we report a green, hydrothermal synthesis technique to prepare
Fe2TiO5–TiO2 heterostructures using natural ilmenite sand as a precur-
sor. The chemical synthesis methods of Fe2TiO5 have been mostly carried
out using ceramic formation techniques, or solid-state processes, where
the samples are treated at high temperatures (1000
C) for extended time
durations (~20 h). The other method using sol-gel techniques has also
been carried out for synthesizing Fe2TiO5.26,27
The drawback of these
methods is the use of hazardous organic salts (such as titanium tetra-
chloride, titanium isopropoxide, titanium butoxide, and tetra-n-butyl
titanate) during synthesis with inorganic salts such as iron (III) nitrate
and iron acetylacetonate and inorganic oxides used as an iron
source.17,26,27
More importantly, chemically synthesized Fe2TiO5 via all
the above mentioned methods had been directed towards 1D nano and
microstructures.
In this work, we focus on the synthesis of architectural Fe2TiO5–TiO2
nanostructures. The facile, in-situ formation of a mixture of Fe2TiO5 and
TiO2 with an easily accessible surface can lead to the creation of highly
active catalysts, which are attractive due to their nontoxicity and stability
toward corrosion, together with the abundance and hence cost-
effectiveness of the precursor material (naturally occurring ilmenite).
Inspired by the performance of some non-noble metal based catalysts for
water splitting,28–33
we attempted to investigate and improve the cata-
lytic activity of Fe2TiO5–TiO2 for electrochemical water splitting appli-
cations. In general, ilmenite concentrate is reported to reduce to metallic
iron and reduced titania at higher temperatures (1073 K) in the presence
of a hydrogen atmosphere.34–36
However, in our work, we observed that
Fe2TiO5–TiO2 heterostructure is possibly reduced to metal-rich oxides at
the surface level (Fe2-xTiO5-x-TiO2-x) due to the electrochemical cathod-
ization process. The derived heterostructure displayed enhanced
hydrogen evolution activity on 3h cathodization and afforded an over-
potential of 301 mV to achieve a current density of 10 mA cm2
.
Whereas, pristine Fe2TiO5–TiO2 heterostructure required 928 mV to
afford a current density of 10 mA cm2
. On studying the effect of cath-
odization via a series of experiments, we understood that the enhanced
catalytic activity is attributed to the co-existence of iron (Fe), titanium
(Ti), and oxygen (O) in reduced oxidation states.
2. Results and discussion
Fig. 1a shows the PXRD patterns of the heterostructure product
derived after calcination at 800
C for 2 h. The precursor (i.e.) ilmenite
sand reveals the crystalline structure of the FeTiO3 (ICDD PDF no: 01-
075-1204) (Fig. S1 a b). The absence of intense diffraction peaks in
the diffractogram of the precursor indicates the amorphous nature of the
initial product synthesized through hydrothermal reaction (Fig. S1 a).
However, after calcination, the product becomes more crystalline, as
seen in XRD shown in Fig. 1a. The diffraction peaks are indexed ac-
cording to an orthorhombic Fe2TiO5 with the lattice parameters (a ¼
3.73 Å, b ¼ 9.77 Å, and c ¼ 9.97 Å (ICDD PDF no: 01-076-1158 (c)) and
TiO2 (ICDD PDF no: 01-021-1276) in rutile form. The absence of other
crystalline phases in the PXRD patterns demonstrates the purity of the
composite mixture of TiO2 and Fe2TiO5.
We further studied the calcined material using Raman spectroscopy to
elucidate the structural features, which exhibited the presence of both
rutile TiO2 and pseudobrookite Fe2TiO5 phases (Fig. 1b). The peaks
observed at 440 and 608 cm1
correspond to Eg and A1g modes of crys-
talline rutile phases of TiO2.37,38
More importantly, the peak at 250 cm1
did not match anatase or rutile phases, and it was reported as a result of
iron doping.39
On the other hand, the pseudobrookite (Fe2TiO5) shows
intense peaks around 197, 333, and 654 cm1
.40
The extended signals
observed around 830 cm1
are related to symmetric stretching move-
ments of the short Ti–O bonds in distorted TiO6 octahedra, (O2–Ti2–O3
atoms).41
Detailed Raman studies showing traces of sodium-based min-
erals are shown in Fig. S2.
The calcined samples’ chemical state was further evaluated using the
X-ray photoelectron spectroscopy (XPS) technique. Fig. 1c shows the
survey scan results demonstrating the presence of iron (Fe), titanium (Ti),
oxygen (O), and traces of sodium (Na). Fig. 1d shows Ti 2p peaks at
~457 eV, which corresponds to Ti 2p3/2, and the peak observed around
464.3eV resembles Ti 2p1/2 consisting of Ti in a fully oxidized Ti4þ
state.42
Fig. 1e shows Fe 2p region with binding energies of ~725 eV
corresponding to Fe 2p1/2, and the other peak observed around 712 eV
corresponds to Fe 2p3/2 with a shake-up satellite peak observed at ~718
eV which feature the presence of Fe3þ
in Fe2O5.43
The O1s spectrum
displays two peaks around 529 and 532 eV (Fig. 1f). The peak at 529 eV
can be attributed to O2
binding oxygen coordinated with Ti and Fe
particles.44
Another peak observed around 532 eV might be caused by
oxygen presence at the interface or other organics such as Na2Ti3O7.44
The traces of Na (Fig. 1c) might be attributed to the formation of sodium
titanate nanoflowers, which happens thanks to the addition of ilmenite
powder into NaOH during the hydrothermal reaction.45,46
The surface morphology of the samples was studied using scanning
electron microscopy (SEM). Fig. 2a is the SEM image of pristine ilmenite
sand, which exhibits granular morphology with particle sizes of few
microns. The morphology of the material resulting after the hydrother-
mal treatment of ilmenite followed by calcination to obtain the hetero-
structure can be seen from Fig. 2b–c. The calcined product reveals a
uniform 3D hierarchical micron-sized flower-like morphologies extend-
ing outwards from the center of the microstructure. The self-directed
growth of Fe2TiO5–TiO2 nanoflowers is attributed to the dissolution
process of ilmenite in high alkali conditions and the release of Feþ3
and
[TiO6
]-
octahedrons. The Feþ3
ions stabilize the negatively charged
layered titanate octahedrons by occupying the interlayer regions, leading
to layered Fe2TiO5–TiO2 heterostructures. These petal-like structures are
connected through the center to form a 3D hierarchical flower-like
structure. Each flower structure consists of several interconnected
petals with a few microns in thickness. However, the exact mechanism of
formation of the uniform 3D flower-like architecture remains unclear.
Generally speaking, hierarchical flower-like 3D crystal structures are
formed by homocentric self-assembly, which first nucleates through the
initially formed 2D arrangements due to various factors, such as elec-
trostatic and dipolar fields, van der Waals forces, hydrophobic in-
teractions, and hydrogen bonds. The presence of easily accessible open
interfaces is expected to contribute to enhanced catalytic activity. The
elemental composition of derived powder was also analyzed using elec-
tron energy loss spectroscopy (EELS) and found to be Ti, O, and Fe pri-
marily (See Fig. S3).
High-resolution transmission electron microscopy (HRTEM) and EDX
analyses were carried out to investigate the morphology, crystallinity,
and elemental composition of the synthesized Fe2TiO5–TiO2 hetero-
structure. Fig. 3a shows the elemental mapping, which indicates a high
degree of homogeneity. Fig. 3b and c shows a lower magnification TEM
image of the flower petals within the structure. The average inter-atomic
layer distance is found to be approximately 0.21, 0.23, and 0.32 nm,
corresponding to the planes (210), (200) and (110) of rutile TiO2,
respectively (Fig. 3d), as per the ICDD PDF no: 01-021-1276.
The structure was further confirmed with simulated projection using
the Vesta software for an orientation (zone axis) along [001].47
The area
denoted (ii) is identified as Fe2TiO5, and it is further confirmed through
the characteristic d-spacing 0.21, 0.44, and 0.31 nm (Fig. 3e) for the
respective (042), (021), and (012) planes which are revalidated with the
projected plane in the [010] zone axis. The areas marked (iii) and (iv) are
the evidence for the other additional phases that were observed. The
region marked iii (Fig. 3f) matches the d-spacing of 0.78 and 0.27 nm,
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
3

4. which confirms the presence of Na2Ti3O7, and it is confirmed through the
projection generated using the respective (010) and (003) planes with
d-spacing of 0.80 and 0.29 nm, correspondingly. This data was used to
identify the right projection [010] to match the simulated atomic dis-
tribution and compare it with the actual image. In the area with caption iv
(Fig. 3g), the identification corresponds to Fe2TiO5, but in this case, it is
oriented along the [111] zone axes. The observed interlayer distance is
attributed to the (001) plane of Fe2TiO5, as shown in the PXRD pattern
(Fig. 1a). However, the back folded edges of Fe2TiO5 (Fig. 3g), clearly
provides evidence for the presence of fringes of stacked polyanion sheets
that appear due to interconnected [TiO6] octahedra with the interlayer
distance of 0.8 nm. The chemical composition of the synthesized nano-
flower petals was analyzed using EDX mapping, and the results are shown
in Fig. 3.
Interestingly, the Fe2TiO5–TiO2 sample is comprised mainly of TiO2
(Fig. 3d) according to observation from the top view of the sample. The
EDS results clearly show highly homogeneous particles rich with Ti, O,
and Fe, with traces of Si, Na, and Ca present in the derived Fe2TiO5–TiO2.
Fig. 2. SEM images of (a) pristine ilmenite and (b–c) shows flower-like morphology of the formed calcined pseudobrookite structure.
Fig. 3. (a) HADDF and EDS mapping of the Fe2TiO5–TiO2 heterostructures, showing the independent elemental distribution (scale 100 nm). (b, c) Low magnification
HRTEM of Fe2TiO5–TiO2 heterostructures displaying different phases corresponding to (i) TiO2-rutile, (ii) Fe2TiO5, (iii) Na2Ti3O7 and (iv) Fe2TiO5. High magnification
HRTEM of (d) TiO2-rutile orientated along [001] zone axis, (e, and g) Fe2TiO5, (f) Na2Ti3O7.
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
4

5. From our observations, we understand that our synthesized samples are
predominantly composed of a mixture of TiO2 and Fe2TiO5. The absence
of peaks corresponding to Na2Ti3O7 in PXRD analysis, suggests that this
phase presents in traces or is of insignificance. Another interesting
observation is the high homogeneity with a clear mix of the different
phases. This could be a crucial factor for improved performance in the
observed catalytic activity.
In general, the metal oxides are known for their intrinsically low
electrical conductivity, and hence most popularly studied only for solar
water splitting, and no previous evaluations have been carried out on
understanding the electrocatalytic behavior of Fe2TiO5–TiO2 nano-
structures.48,49
We used a simple cathodization technique (See experi-
mental section) to improve the electrical conductivity of the synthesized
poorly conducting mixed oxides. In brief, we induced oxygen vacancy in
the synthesized Fe2TiO5–TiO2 using inherently generated hydrogen
during the electrochemical water splitting process. Impedance spectra
were recorded at different intervals of cathodization to confirm the
improvement in conductivity due to the cathodization of Fe2TiO5–TiO2,
and their corresponding Nyquist plot is shown in Fig. 4a. The acquired
impedance spectra were fitted with an equivalent circuit composed of
solution resistance (RS), charge transfer resistance (Rct), constant phase
element (Q), and its values are summarized in Table S1.
The 2 h cathodized Fe2TiO5-x-TiO2-x displays the lowest Rct (58 Ω),
compared to 1 h cathodized (95 Ω), 3 h cathodized (69 Ω), and pristine
Fe2TiO5–TiO2 (165 Ω). We observed a dramatic decrease in Rct due to the
cathodization process (Table S1), demonstrating the rapid electron
transfer ability of Fe2TiO5-x-TiO2-x. This indicates the augmentation in
conductivity of Fe2TiO5-x-TiO2-x on cathodization. Besides, the colossal
improvement in electron density from 5.03 1018
cm3
to 12.8 1022
cm3
based on Mott-Schottky analysis (Fig. S4) of Fe2TiO5-x-TiO2-x
attributed to cathodization is consistent with the electrochemical
impedance analysis. Furthermore, the shift in flat band potential from
0.43 to 0.80 V vs. RHE for 2h of the cathodization process further con-
firms an increase in donor density and facilitates charge transfer. How-
ever, the slight enhancement in Rct on 3h cathodized material could
result in scattering because of excessive donor densities. The enhanced
donor density and conductivity stimulate us to further explore its elec-
trocatalytic activity, especially the hydrogen evolution reaction (HER)
behavior. Hence, Linear sweep voltammograms (LSV) were recorded
using a 0.1 M KOH solution at a scan rate of 5 mV s1
to evaluate the HER
performance of the cathodized Fe2TiO5-x-TiO2-x hybrid interface. Fig. 4b
shows the LSV of as-synthesized Fe2TiO5–TiO2 and the cathodized
Fe2TiO5-x-TiO2-x sample compared to a benchmark Pt catalyst. The
pristine Fe2TiO5–TiO2 are intrinsically HER inert and display HER ac-
tivity only at high onset potential of ~545 mV, while 1, 2 and 3 h
cathodized samples exhibit lower onset potential around 362, 236, and
332 mV, respectively. The required overpotential to afford a current
density of 10 mA cm2
is reduced from 928 mV to 398, 301, and 373 mV
through 1, 2, and 3h cathodization, respectively.
We further carried out Tafel analysis to understand Vo vacancies’
influence on HER kinetics (Fig. 4c). The Tafel slope is determined from
the linear region of the polarization curve, and it is found to be 258, 167,
69, and 159 mV for 0, 1, 2, 3 h cathodized samples of Fe2TiO5-x-TiO2-x,
respectively. It is observed that 2 h cathodized samples displayed a low
Tafel slope of 69 mV decade1
, and hence the rate-determining step for
HER mechanism is found to be electrochemical desorption oriented
Volmer-Heyrovsky mechanism.50
This signifies the importance of cath-
odization and the tremendous improvement in HER catalytic activity of
Fe2TiO5–TiO2 on cathodization.
We carried out a post-cathodization analysis of Fe2TiO5–TiO2 using
different techniques to understand the structural and chemical compo-
sition of cathodized Fe2TiO5–TiO2 samples and to evaluate possible
reasons for the improvement in catalytic activity. As seen through XRD
(Fig. S5), the patterns of pristine and 2 h treated samples look similar,
which indicates unnoticeable change in crystallinity of the cathodized
samples and no major changes occurred in the bulk phase of the sample.
Hence, XPS analysis was carried out to explore surface composition of
Ti2p, Fe2p, O1s regions, and the results are shown in Fig. S6a-d,
respectively. The survey scan results of the 2 h cathodized sample are
shown in Fig. S6a. As can be seen, the binding energies of Fe, Ti, and O
Fig. 4. (a) Impedance spectra, (b) LSV, (c) Tafel analysis of Fe2TiO5–TiO2 and its cathodized sample (Fe2TiO5-x-TiO2-x) in 0.1 M KOH solution. (d) Effect of cath-
odization on the HER activity of Fe2TiO5-x-TiO2-x.
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
5

6. shift towards lower energies due to the cathodization process. Fig. S6c
exhibits deconvoluted Fe peaks which shows reduction of Fe2TiO5–TiO2
due to cathodization effect. According to previous reports, Fe3þ
(Fe2O3),
Fe2þ
(Fe3O4), Feþ
(FeO) show characteristic peaks of Fe 2p3/2 at 710.2,
709.8 and 708.4 eV respectively.48
On the other hand, the Ti4þ
corre-
sponding to Ti 2p3/2 seen at 464.3 eV is reduced to Ti3þ
, after the
cathodization process, thereby leading to an increase in the intensity of
Ti3þ
peak seen at 463.7 eV.49
Moreover, lattice oxygen ion (OL), chem-
isorbed oxygen (OOH), oxygen vacancies (OV) show characteristic O1s
peak at 529, 531.1, 532 eV, respectively.50
On quantitative analysis, of
O1s spectra, the relative surface atomic concentration of OL: OOH: Ov is
found to be 68:13:19, 26:47:27, 17:45:38, for 1, 2 and, 3h of cathod-
ization. It indicates increased oxygen vacancies and enhanced average
oxidative ability of iron and titanium species in Fe2TiO5–TiO2 on
cathodization.
Further, SEM analyses were carried out to understand the surface
morphological changes of the post-cathodized samples, and their results
are shown in Fig. 5. The surface morphology of Fe2TiO5–TiO2 hetero-
structures at different cathodization durations reveals the occurrence of a
high degree of segregation due to cathodization. As seen from Fig. 5a, the
pristine Fe2TiO5–TiO2 displayed interconnected nanoflowers with high-
orientation morphology. The inset confirms the uniform arrangement.
Whereas the 1 h cathodized samples (Fig. 5b and inset) revealed slight
disintegration of high-order orientation, but it still retained the structure
with minimal collapse. Interestingly, after 2h cathodization (Fig. 5c and
inset), the edges of Fe2TiO5–TiO2 flower-like heterostructures were
collapsed, showing a significant difference in their surface morphology. It
could be possibly caused by the reduction of iron species, on the edges of
original ilmenite grains.51
This indicates the increasing rate of reduction
of Fe2TiO5–TiO2 heterostructures on cathodization. We further analyzed
the post-cathodized samples with EDX to confirm that the phase was
depleted in oxygen and understood the degree of reduction by cathod-
ization. For better understanding, the elemental weight (%) of Fe, Ti, and
O of pristine and 2h cathodized samples were compared, and the results
are shown in Fig. S6d. Besides correlating the SEM and XPS analyses, we
can infer that the reduction of Fe2TiO5–TiO2 heterostructure proceeds
topochemically. Initially, the large particle size of Fe2TiO5–TiO2 heter-
ostructure slows down the diffusion of Hþ
ion and hence hinders its
reduction at the initial cathodization time. As reduction proceeds, par-
ticle size reduction occurs, leading to reduced oxide sites favoring faster
Hþ
ion diffusion, thereby increasing the reduction rate.52
Even though enhanced metallic iron concentration is observed in 3h
of cathodized samples, the slight decrease in the catalytic activity and
HER kinetics could be due to the scattering of excessive Vo vacancies, as
evidenced through impedance spectra (Fig. 4a). The relative perfor-
mance and catalytic efficiency of the prepared samples are summarized
in Fig. 4d. Overall, the cathodized samples displayed noticeable onset
shift and reduction in overpotential and optimal Vo vacancies, which
resulted in a substantial HER activity. Thus, on combining LSV, and post-
cathodization analysis, the high electrocatalytic performance can be
attributed to (Vo) vacancy centers and reduced oxides with highly
accessible nanopetal architecture.53
The beneficial action at the surface
vacancy centers brings about the improvement in electrocatalytic HER
activity. As catalytic reaction is a surface reaction, the induced surface
oxygen vacancies by cathodization are sufficient to promote the catalytic
performance. This finding also suggests the appropriate ratio of reduced
oxides is an essential factor in determining the catalytic activity, which
needs to be considered in the future catalyst design process. Indeed, the
interconnected petals of Fe2TiO5–TiO2 heterostructure provide addi-
tional support for electron confinement, efficient diffusion pathways, and
superior transport properties.
Our findings reveal the importance of the cathodization technique,
and the atomic and electronic structure rearrangements by cathodization
create potentially viable Fe2TiO5-x-TiO2-x heterostructure based catalysts.
Accordingly, our engineered Fe2TiO5-x-TiO2-x nanostructures effectively
overcome the limitation of poor conductivity of oxides and possess great
potential for practical applications since these oxides are intrinsically
anti-corrosive and abundant in nature.
Fig. 5. Microscopic images of (a) pristine Fe2TiO5–TiO2 before cathodization, (b) Fe2TiO5–TiO2 after 1h cathodization and (c) Fe2TiO5–TiO2 after 2h cathodization
(insets show higher magnification). (d) Proportion change in the element of Fe2TiO5–TiO2 before and after cathodization process.
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
6

7. 3. Conclusions
In brief, we have demonstrated a green method for synthesizing
layered arrangements of Fe2TiO5–TiO2 heterostructures employing
naturally available ilmenite sand without using additional templates. The
uniformly distributed heterostructures of Fe2TiO5–TiO2 were character-
ized by various analytical techniques, and their catalytic and magnetic
performances were evaluated. The XRD and TEM results reveal the
crystalline nature of the synthesized materials, and the SEM observation
displays a flower-like structure with more accessible open sites. More
importantly, we demonstrated a simple cathodization procedure to
enhance the catalytic performance of the derived Fe2TiO5–TiO2 nano-
heterostructures inherently. The cathodization process creates more de-
fects contributing to additional vacant sites, thereby leading to enhanced
electrocatalytic activity. The current research finding provides a new
approach to synthesize a new generation of green, hetero-structured
functional materials that could be used as a potentially active material
for a wide range of applications related to energy storage, magnetism,
and the development of functional materials.
4. Experimental section
4.1. Materials
Ilmenite sand (100 mesh) was obtained from Sri Lanka mineral sand
Ltd, Sri Lanka, and sodium hydroxide, 98% (Sigma-Aldrich analytical
grade) were used as starting materials without any further purification
unless otherwise specified.
4.2. Preparation of Fe2TiO5–TiO2 nanoflowers
The preparation of Fe2TiO5 nanoflowers was achieved by the hy-
drothermal technique using natural ilmenite granules (FeTiO3). In the
extraction technique, 2 g of ilmenite sand and 30 mL of 10 M NaOH
solution were placed in a Teflon tube, and the content was hydrother-
mally treated at 300
C for 2 h under autogenous pressure. After the
reaction, the product was allowed to cool to room temperature, and a
reddish-brown precipitate was obtained. The amorphous precipitate was
separated from centrifugation and washed with deionized water several
times to remove basic impurities and dried at 50
C in an oven. Then, the
product was calcined at 800
C for 2 h in air.
4.3. Preparation of defect-rich Fe2-xTiO5-x-TiO2-x nanoflowers
To induce vacancies in Fe2TiO5–TiO2, the electrochemical cathod-
ization (reduction) process is conducted at 0.8 V vs. RHE. It was per-
formed in a three-electrode system with Pt wire, Hg/HgO as the counter,
and a reference electrode. The derived samples were washed in excess
water before cathodization process to remove impurities and residual
contaminants and dried in a vacuum oven for 48 h. A few milligrams of
the dried Fe2TiO5–TiO2 were sonicated in 5 mL ethanol and coated on
removable glassy carbon, which acted as a working electrode. During the
cathodization process, the hydrogen ions adsorbed by Fe2TiO5–TiO2 lead
to a reduction of oxygen ions in the lattices. Due to the creation of oxygen
vacancies, reduction in metal ion (oxidation state) occurs, which helps to
maintain local electrostatic neutrality.51,54
Fe2TiO5–TiO2 þ H2→(FeTiO3–FeO–TiO2) þ H2O→Fe2-xTiO5-x-TiO2-x (FexOy
-TiO2-x) þ H2O
4.4. Characterization of the synthesized Fe2TiO5–TiO2 nanoflowers
The phase and crystallinity of the resultant products were analyzed
using powder X-ray diffraction techniques (Bruker D8 Focus) with Cu
Kα (λ ¼ 0.154 nm) irradiation in the 2θ range of 5–90
at a scan rate of
0.02
. Raman analysis was carried out using a JYHoriba LabRAM HR. A
laser power of 28 μW was used at an excitation wavelength of 514 nm.
The excitation radiation of 514.5 nm was employed with the Lexel-SHG
95 argon-ion laser. XPS analysis was carried out using a PHI Quantera X-
ray photoelectron spectrometer with a chamber pressure of 5 109
Torr, and an Al cathode was used as the X-ray source. The pass energies
were set to 26.00 eV for the core-level scan, and the source power was
set at 100 W. The surface morphology and elemental analysis of the
products were studied using a scanning electron microscope (SEM,
Hitachi SU 6600) equipped with energy dispersive X-ray spectrometer
(Oxford X-act, EDX). The atomic arrangement of the sample was
observed through high-resolution transmission electron microscope
(HRTEM, JEOL JEM-2100F, operating at 200 kV) facilitated with EDX
(AMETEK-Octane T optima - 60 EDX detector) and electron energy loss
spectrometer (Gatan GIF 963 EELS spectrometer) at 0.05 eV/channel
dispersion). The synthesized samples were sonicated in methanol,
placed on a holey carbon-coated Cu grid, and dried completely under
vacuum conditions before TEM analysis. The magnetic measurement
was carried using a magnetic property measuring system (MPMS) fitted
with a superconducting quantum interference device (SQUID) attach-
ment. The room temperature measurements and the ZFC-FC were car-
ried out from 2 to 250 K.
4.5. Electrochemical characterization
Reference electrode potential (EHg/HgO) was converted to a reversible
hydrogen electrode (ERHE) using the following equation:
ERHE ¼ EHg=HgO þ 0:098V þ 0:059pH (1)
Electrochemical impedance was carried out at an AC amplitude of 10
mV from 1 MHz to 100 mHz. The Mott-Schottky analysis was performed
in the potential 2 to 1 V vs. Hg/HgO at a frequency of 1 Hz. Later, the
carrier density (ND) and flat-band potential (EFB) was determined using
the following equation:61
ND ¼
2C2
eεε0A2
ðE EFBÞ
KBT
e
(2)
Where e, ϵ0, KB, T are the elementary electron charge (1.6 1019
C),
the permittivity of vacuum (8.86 1012
F m1
), Boltzmann constant
(1.38 1023
J K1
), and temperature (298 K), respectively. Moreover, E
is the applied bias potential and ϵ dielectric constant of the sample, EFB is
the built-in voltage (flat band potential); and A is the surface area of the
electrode.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
The authors wish to acknowledge Eshan Malintha, Amila Prabath,
and Nadeesha Gamage for their continuous support and Sri Lanka Insti-
tute of Nanotechnology (Pvt) Ltd for research facilities. NK is grateful to
the Fulbright Commission for the Fulbright fellowship to Rice University,
USA.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.matre.2021.100020.
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
7

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N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
8

9. Niranjala Fernando is currently pursuing her Ph.D. in
advanced energy materials at Bournemouth University. She
received her B.Sc. degree in chemistry from the University of
Ruhuna (Sri Lanka, 2015) and was awarded Prof. R. H. Wije-
nayake memorial gold medal in chemistry for the best perfor-
mance. In 2015, Niranjala joined the Sri Lanka Institute of
Nanotechnology (SLINTEC) as a research scientist. Her current
research interests are in advanced materials and electro-
chemical energy applications.
Nilwala Kottegoda obtained a first-class BSc Special Degree in
Chemistry from the University of Peradeniya and a Ph.D. in
Materials Chemistry from Cambridge, UK. She is a Professor in
Chemistry, and the Head/Department of Chemistry at the Uni-
versity of Sri Jayewardenepura, Sri Lanka. She was a founder
and principal scientist at Sri Lanka Institute of Nanotechnology
(SLINTEC), which emerged as the first-ever private-public
partnership research institute in Sri Lanka. She has significantly
contributed to smart agriculture, advanced materials, water
purification, and nanocomposites. Her pioneering research
work on smart agriculture and value addition to natural mate-
rials have been awarded several US patents.
Ashok Kumar Meiyazhagan holds a Ph.D. degree in Chemistry
from Madras University. Dr. Ashok is currently affiliated with
Rice University, and he has several years of research experience
in the interdisciplinary fields of chemistry, material science, and
environmental science. His research interest includes function-
alized 2D materials and 3D composites for nanoelectronics,
catalysis, and water desalination applications. He is a recipient
of prestigious fellowships and holds invited editorial positions in
international journals.
Pulickel M. Ajayan received Ph.D. in Materials Science and
Engineering from Northwestern University. He is currently the
chair and Benjamin M. and Mary Greenwood Anderson, Pro-
fessor of Department of Materials Science and NanoEngineering
at Rice University. He is one of the pioneers in the field of car-
bon nanotubes and was involved in the early work on the topic.
His research interests include synthesis and engineering of
carbon-based nanomaterials, 2D materials and phase stability,
bio-mimetic materials synthesis, flexible thin-film devices,
electron microscopy, and materials characterization. He has
invited positions in numerous institutes and acted on the boards
of several journals, startups, and international conferences.
N. Fernando et al. Materials Reports: Energy 1 (2021) 100020
9