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Please cite this article as: Shivani Dangwal et al., International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2024.08.101
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Direct Z-scheme based WS
2
/TiO
2
heterostructures for hydrogen
evolution reactions
Shivani Dangwal
a
, Saurabh Rawat
a
, Deb Kumar Rath
b
, Charu Dwivedi
c
, Mohit Sharma
d
,
Rajesh Kumar
b
, Himani Sharma
a
,
*
a
Functional Nanomaterial Laboratory, Department of Physics, Doon University Dehradun, Uttarakhand, 248001, India
b
Materials and Device Laboratory, Department of Physics, Indian Institute of Technology, Indore, 453552, India
c
Department of Chemistry, Doon University Dehradun, Uttarakhand, 248001, India
d
Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis #08-03, Singapore
138634, Republic of Singapore
ARTICLE INFO
Handling Editor: Dr A Bhatnagar
Keywords:
Transition metal di chalcogenides (TMDC)
Z-Scheme
Linear sweep voltammetry (LSV)
Electrochemical impedance spectroscopy (EIS)
ABSTRACT
Hydrogen as a clean fuel is increasingly sought after for its potential to replace non-renewable energy sources,
with the hydrogen evolution reaction (HER) presenting a sustainable method for its production. This study fo-
cuses on enhancing HER efciency through the fabrication of a Z-scheme based tungsten di sulde/titanium di
oxide (WS
2
/TiO
2
) heterostructure via hydrothermal synthesis. Transition metal dichalcogenides like WS
2
, known
for their unique properties, are integrated with TiO
2
nanorods to create a robust photocatalytic system. Char-
acterization techniques such as X-ray diffraction (XRD), Raman spectroscopy, Field-emission scanning electron
microscopy (FE-SEM) and Brunauer-Emmett-Teller (BET) were performed. X-ray photoelectron spectroscopy
(XPS) and Ultraviolet photoelectron spectroscopy (UPS) provided comprehensive insights into the electronic
interactions and charge transfer kinetics. A shift in peak positions in XPS spectra indicate the enhancement in
catalytic active sites which is in corroboration with the UPS studies. An altered energy environment causing the
Z-scheme charge transfer in heterostructure was proved, enhancing the hydrogen production. From the UPS
studies, a lower work function for heterostructure i.e. 5.47 eV as compared to 5.5 eV for pristine WS
2
indicates
improvement in charge transfer. Electrochemical measurements including linear sweep voltammetry (LSV) and
electrochemical impedance spectroscopy (EIS) conrmed the improved HER performance of heterostructure with
lower value of onset potential (0.031 V) and charge transfer resistance (3.5 k Ω) as compared to pristine samples.
Thus, proving WS
2
/TiO
2
heterostructure to be a potential candidate for sustainable hydrogen production.
1. Introduction
The escalating energy demand has become a signicant societal
concern due to the nite nature of fossil fuels like coal and oil, which on
combustion leads to severe hazards. Consequently, there’s an urgent
need for sustainable solutions that can generate renewable and eco-
friendly energy resources. Among the array of emerging energy sour-
ces, hydrogen derived from water electrolysis has emerged as a highly
promising option due to its cost-effectiveness, renewability, and scal-
ability. Traditionally, noble metals like platinum and palladium serve as
catalysts in hydrogen production, exhibiting zero overpotential in the
hydrogen evolution reaction (HER) [1–3]. However, their scarcity and
high cost hinder large-scale commercial utilization. Recently,
two-dimensional transition metal dichalcogenides (2D-TMDs) have
gained attention for their exceptional properties such as high conduc-
tivity and stability [4,5]. Thus, can be substituted over expensive ma-
terials for the HER. This shift not only overcomes the cost and scarcity
concerns but also proves as a promising substitute for industrial appli-
cations where large-scale production is required.
Among various TMDCs, sulde-based materials such as molybdenum
disulde (MoS₂) and tungsten disulde (WS₂) have garnered signicant
attention in HER due to their exceptional electronic and optical prop-
erties. Extensive studies on MoS₂-based structures for the hydrogen
evolution reaction (HER) has been observed [6,7]. But not much work
has been reported regarding the performance of WS
2
based material for
HER. While WS
2
is less recognized compared to MoS
2
, it also possesses
* Corresponding author.
E-mail address: himanitiet427@gmail.com (H. Sharma).
Contents lists available at ScienceDirect
International Journal of Hydrogen Energy
journal homepage: www.elsevier.com/locate/he
https://doi.org/10.1016/j.ijhydene.2024.08.101
Received 7 January 2024; Received in revised form 9 July 2024; Accepted 6 August 2024
International Journal of Hydrogen Energy xxx (xxxx) xxx
2
properties suitable for undergoing HER. It possesses good conductivity
and hence facilitates charge transfer [8]. However, the fewer exposed
edge sites and the weak ionic conductivity of WS
2
can limit its electrical
transfer and thus reduce the overall catalytic activity. Various modi-
cations have been explored to increase the catalytic sites in WS
2
nano-
structures through structural alterations such as combining with metals
and conductive materials to enhance its mobility. However, less work
has been reported regarding the interfacial interactions and optimiza-
tion of active sites [5,9,10].
In addition to WS
2
, TiO
2
stands out as the most renowned member,
specically in photocatalysis due to its potent redox capabilities, wide-
spread availability, and exceptional stability when subjected to chal-
lenging photocatalytic processes. It has thus been extensively studied in
various photocatalytic reactions for its stability and cost effectiveness
[11,12]. On forming its heterostructure with narrow band gap semi-
conductor such as WS
2
results in improvement of the overall photo-
catalytic performance. This can be attributed to the modications in
electronic band structure after formation of composite which facilitates
light absorption into the visible spectrum [13,14]. Therefore, combining
sulde-based materials with TiO
2
proves to be signicant in HER as their
synergistic effect provides them with enhanced charge separation ef-
ciency and unique catalytic properties for hydrogen production. They
possess high stability and durability under harsh environmental condi-
tions. The synergistic effect provides them with higher charge separa-
tion efciency and thus improving the overall catalytic performance for
hydrogen evolution [15–18].
Furthermore, the direct Z-scheme conguration between hetero-
structures have demonstrated robust redox effectiveness and superior
photocatalytic performance due to generation of efcient charge car-
riers [19,20]. Thus, the novel WS
2
/TiO
2
heterojunctions, characterized
by their favourable interfacial connection between the two components,
possess inherent benets within the Z-scheme heterojunction. This
conguration further enhances the rate of hydrogen production [10].
This study thus aims to devise a two-step method for synthesis of WS
2
nanosheets with number of catalytic sites and large specic surface area,
and combining them with TiO
2
nanorods. This approach amalgamates
the remarkable catalytic attributes of WS
2
with the high charge carrier
properties of TiO
2
nanorod arrays. The synthesis involved creating the
WS
2
/TiO
2
structure on a conductive glass substrate using a hydrother-
mal synthesis, resulting in a highly stable electrocatalyst for hydrogen
production [21]. The hydrothermal synthesis is specically followed as
it produces material with high purity and controlled morphology. Other
benets include cost effectiveness, energy efciency, large-scale equip-
ment usability, nucleation control, pollution avoidance, high dispersion,
rapid reaction rates and low operation temperatures in various solvents.
Besides this, solvents used can have varied behavior under hydrothermal
conditions, such as pH variation, viscosity, density, etc. Depending on
growth condition of materials. Thus, hydrothermal reactions can be
conducted in different pH environment and under varied temperature
and pressure, providing diverse conditions for synthesis. Thus, following
this route ensures fabrication of high purity nanomaterial and their
various heterostructures for large scale production and applications of
HER [22–25].
As of now, not much work regarding the HER performance of hex-
agonal 2H WS
2
grown on rutile TiO
2
and their interfacial interactions
has been observed [5,26,27]. The outcomes obtained for HER are sub-
stantial within the realm of non-precious electrocatalysts, offering a
promising avenue for producing efcient, cost-effective, and environ-
mentally friendly electrocatalysts on a large scale.
2. Experimental section
2.1. Synthesis of WS
2
nanosheets
WS
2
nanosheets were synthesized using the hydrothermal method as
detailed in the cited reference [28,29]. To create the solution, sodium
tungstate, hydroxylamine hydrochloride and sulfourea 0.005, 0.001 and
0.02 mol respectively were dissolved in deionized water, the quantity of
which was taken to be 30 ml. Surfactants (CTAB, 0.24 g) which is an
important precursor in maintaining the morphology was introduced
while continuous stirring of the solution. The pH of the resultant ho-
mogeneous solution was maintained by adding either hydrochloric acid
(2 mol/L) or ammonia water. The WS
2
nanosheets synthesized were
designated as WNS and the synthesis method is illustrated in Fig. 1a.
2.2. Synthesis of 1D TiO
2
nanorods
TiO
2
nanorods (TNR) were prepared using hydrothermal method
(Fig. 1b) [30]. Firstly, uorine doped tin oxide (FTO) was cleaned for 10
min in IPA, acetone and DI. Stirring of 5 ml HCL was done with DI.
Furthermore, titanium isopropoxide was added and nally poured on
autoclave having cleaned FTO. The synthesis was carried at 180 ◦C for 4
h in hot air oven and nally the substrate was cleaned with DI and dried.
2.3. Synthesis of TiO
2
-WS
2
heterostructure
WS
2
nanosheets were grown on TiO
2
nanorods (WNS/TNR) through
hydrothermal method (Fig. 1c) [21]. As mentioned previously the so-
lution of sodium tungstate, hydroxylamine hydrochloride and sulfourea
0.005 mol, 0.001 mol and 0.02 mol respectively were dissolved in
deionized water (30 ml). Surfactants (CTAB, 0.24 g) was introduced
while continuous stirring of the solution. The pH of the resultant ho-
mogeneous solution was maintained by adding either hydrochloric acid
(2 mol/L) or ammonia water. The obtained homogeneous solution was
then poured in autoclave containing already prepared TiO
2
nanorods.
The hydrothermal process will be carried out at 24h and temperature
was kept 180 ◦C.The obtained precipitate was washed with DI water and
dried to obtain the heterostructure.
2.4. Characterization methods
The morphologies of the samples were assessed using eld emission
scanning electron microscopy (FESEM), utilizing Apreo Field Emission
Scanning Electron Microscope with Low Vacuum. X-ray diffraction using
Rigaku, Smart lab X-ray Diffractometer was employed for phase analysis
of the samples. Raman Spectroscopy was utilized for investigating phase
transformation using Horiba-Jobin Yvon LABRAM spectrometer,
UV–Visible spectra were collected using a UV–Vis spectrometer (Lab-
man double beam spectrometer, LMSP-UV1900S). X-ray Photoelectron
Spectroscopy (XPS) analysis was conducted on the samples to determine
the material surface composition and elemental identication, utilizing
a monochromatic Al K
α
source on a VG ESCALAB 220I-XL instrument.
Brunaner-Emmett-Teller (BET) was utilized using Belsorp (X mini) to
determine the surface area of the catalyst. Ultraviolet Photoemission
Spectroscopy (UPS) using Prevac was utilized to investigate the inter-
facial interactions and determination of the work function.
2.5. Electrochemical measurements
This was carried out in 1 M KOH solution using 3 electrodes. Plat-
inum wire served as counter electrode, Ag/AgCl (saturated KCl) as
reference electrode and the prepared WNS/TNR heterostructure on FTO
was used directly as working electrode. The electrolyte employed during
the measurements was 1 M KOH aqueous solution. The experiment was
conducted using Admiral Squidstat electrochemical workstation. A solar
ux simulator (Holmarc) was used for conducting electrochemical
measurements under light illumination. LSV was conducted at scan rate
of 10 mV/s. EIS was performed for samples at frequency range of 100
kHz to 0.01Hz using an input sine wave of 10 mV amplitude [26,31].
The potentials that were obtained against the Ag/AgCl reference elec-
trode were converted to reversible hydrogen electrode (RHE). Nernst
equation (eq) was employed to obtain potential against RHE [2,32].
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
3
E
RHE
=E
Ag/AgCl
+0.059*pH +0.1976 (1a)
Here E
Ag/AgCl
is the applied potential vs Ag/AgCl and the standard po-
tential value of Ag/AgCl is 0.1976V at 25 ◦C. Cyclic Voltammetry was
obtained at different scan rates of 10 mV/s, 25 mV/s, 50 mV/s, 100 mV/
s and 200 mV/s.
2.6. Results and discussion
The samples’morphologies and structures underwent evaluation
through FESEM. Analysis of these images (Fig. 2a) unveiled that WNS
displayed a sheet-like morphology [29,33]. Moreover, observations
from SEM images of TNR grown on the FTO revealed nanorods with
average diameter of 356 nm as shown in Fig. 2b [30].
Hydrothermal synthesis was utilized for the uniform growth of WNS
on these nanorods, which is depicted in Fig. 2c. Notably, TNR were
covered completely by WNS, a small region of TNR could be observed as
highlighted in Fig. 2d. The formation of the heterostructure was
conrmed using energy dispersive X-ray (EDX) analysis, which veried
the presence of W, S, Ti, and O at weight percentages of 33.7%, 5%,
30.0%, and 29.3%, respectively (Fig. 2e).
The X-ray diffraction (XRD) pattern was obtained to conrm the
crystal structure of the samples i.e. pristine WNS, TNR and the resulting
WNS/TNR heterostructure. For TNR the peaks observed at 27.2◦, 36◦,
39.2◦, 41.3◦, 44◦, 54.4◦and 56.5◦correspond to (110), (101), (200),
(111), (210), (211) and (220) planes, conrming the rutile phase as
indicated in the JCPDS le 86–0147 [30,34]. In case of WNS, the specic
lattice planes indexed as (002), (004), (100), (102), (006), (106) and
(105) correspond to 14.36◦, 28.56◦, 32.86◦, 36.22◦, 44.08◦, 50.2◦and
56.3◦respectively. These indexing values align with the hexagonal phase
of WNS, as referenced in the JCPDS le 08–0237 [28,29,35]. The
presence of high and distinct peaks in Fig. 3 indicates the sample’s high
crystallinity. This XRD analysis strongly suggests that the synthetic
conditions employed have successfully yielded high-quality crystalline
WNS. In the XRD patterns of the WNS/TNR heterostructure, diffraction
peaks corresponding to rutile phase of TiO
2
[30,36]can be observed
corresponding to (110), (101) and (200) at 27◦, 34.6◦and 38.03◦, ac-
cording to JCPDS le 86–0147. The remaining peaks observed at 14.4◦,
28.5◦, 43.7◦and 50.23◦are assigned to (002), (004), (006) and (105),
plane of WNS. The XRD analysis of the WNS/TNR composites does not
show few peaks as present in pristine [37,38] WNS, indicating that the
WS
2
nanosheets attached to the TiO
2
nanorods might be extremely thin,
possibly below the detection limit of XRD. This can also be veried
through the values of crystallite size and strain obtained from Wil-
liamson Hall method (discussed further), where the value of crystallite
size for WNS/TNR was found to be lower (18.14 nm) than the crystallite
size of WNS (20.22 nm), indicating that the growth of WNS nanosheets
in heterostructure might be inhibited. This is also supported by higher
value of strain obtained in heterostructure (1.37 x 10
−3
) as compared to
pristine WNS (1.07 x 10
−3
) nanosheets. This higher strain might be
because of WNS sheets experiencing lattice distortion when hetero-
structure formation takes place [39–42].
X-ray diffraction (XRD) prole analysis was also utilized to deter-
mine parameters such as interplanar distance, crystallite size, disloca-
tion density, micro-strain and lattice parameters [41,43,44]. The
Fig. 1. Schematic of (a) Synthesis of WS
2
nanosheets (WNS) by hydrothermal method. (b) Synthesis of TiO
2
nanorods (TNR) by hydrothermal method. (c) Synthesis
of WS
2
/TiO
2
heterostructure (WNS/TNR) by hydrothermal method.
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
4
interplanar distance is dened as perpendicular distance between suc-
cessive planes.
Bragg’s law was used to determine the crystal parameters:
2d sinΘ=nλ(1b)
Where n is the order of reection (1 for rst peak), λis wavelength of
X rays (0.154 nm for Cu K
α
), d is interplanar spacing, Θis the diffraction
angle [45].
For WNS having hexagonal structure, the relationship between
interatomic distance (d) and lattice parameters is given as [46]:
1
d2=4
3h2+k2+l2
a2+l2
c2(2)
Where hkl represent the lattice planes or Miller indices.
The lattice parameter for WNS were determined to be a =0.31 nm, c
=1.23 nm. The volume of unit cell =
3
√
2a2cwas found to be 0.102
nm
3
.
For TNR having tetragonal system the interplanar spacing is given as
[45,46],
1
d2=h2+k2
a2+l2
c2(3)
The lattice parameters are a =0.45 nm, c =0.29 nm. The volume of
unit cell =a
2
c was found to be 0.058 nm
3
.
To determine the average crystallite size, Debye-Scherrer equation
was used as follows:
D=Kλ
βDD Cos Θ(4)
Where D is the average crystallite size, K is the shape factor (0.94), λis
the wavelength of X-rays (1.5406 Å), Θis the peak position or diffraction
Fig. 2. (a) SEM images of WS
2
nanosheets (WNS) (b) SEM images of TiO
2
nanorods (TNR) (c) SEM images of WS
2
/TiO
2
heterostructure (WNS/TNR) (d) SEM images
of TNR magnied view in WNS/TNR heterostructure (e) EDX spectra of WNS/TNR showing corresponding weight ratio of W, S, Ti and O.
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
5
angle, β
D
is full width at half maxima. D values are tabulated in Table 1.
The average crystallite size for WNS, TNR and WNS/TNR obtained was
26.67 nm, 16.87 nm and 25.85 nm respectively [47,48].
Dislocation density can be calculated using Williamson and Small-
man’s formula as follows:
δ=1
D2(5)
Where D is the crystallite size [46]. The value of dislocation density is
listed in Table 1 for different crystallographic planes of WNS, TNR and
WNS/TNR.
The Williamson-Hall method (WH) was utilized to determine the
crystalline size and lattice strain present in the sample [46,49,50]. The
crystalline size can be determined by eliminating the instrumental peak
broadening. The total peak broadening is given as follows:
βhkl =βs+βD(6)
Where β
s
=contribution of lattice strain
β
D
=contribution due to crystallite size
The strain in the synthesized powders, caused by crystalline defects
and distortion [44], was calculated using the equation:
ϵ=βS
tan Θ(7)
Therefore, strain leading to broadening is given as
βS=ϵtan Θ(8)
From equations (3) and (7) it can be observed that the changes in
crystallite size and strain as determined from peak width can be
expressed as 1/cos Θand tan Θrespectively [42,43].
Therefore, the peak broadening β
hkl
=β
s
+β
D
can now be written
as:
βhkl =Kλ
D cos Θ+4ϵtan Θ(9)
βhkl cos Θ=Kλ
D+4ϵsin Θ(10)
This is the W–H equation for uniform deformation model (UDM) [43,
49,50]. It is assumed that the strain is uniform in all geometrical di-
rections. Thus, the strain and crystallite size are obtained by plotting
4 sin Θalong x-axis and βhkl cos Θalong y-axis. The slope of linear t and
y-intercept values can be used to determine the strain and crystallite size
[51]. The value of crystallite size and strain obtained are listed in
Table 2.
Raman spectroscopy was employed for brief investigation of phase
compositions of the prepared samples. Fig. 4a and b illustrates the
Raman spectra of WNS and TNR respectively. For WNS peaks observed
at 349.38 cm
−1
and 416 cm
−1
correspond to the in-plane vibrational E
1
2g
mode and out-of-plane vibrational A
1g
modes of 2H-WS
2
, thus con-
rming its formation [52,53]. From the Raman spectra of TNR, peaks
corresponding to second order scattering (SOS) at 233.4 cm
−1
, E
g
vibrational mode at 443 cm
−1
and A
1g
mode at 609.85 cm
−1
can be
observed. Thus, the formation of tetragonal rutile mode of TNR has been
conrmed as reported previously [8,30,54–58].
The specic surface area of WNS and WNS/TNR were also analyzed
using N
2
adsorption and desorption isotherms (as illustrated in Fig. 5a).
The surface area for TNR, WNS and WNS/TNR was found to be 26.4 m
2
/
g, 51.6 m
2
/g and 84.5 m
2
/g respectively [59]. The corresponding pore
volume (Fig. 5b) was 0.08 cm
3
/g, 0.22 cm
3
/g and 0.33 cm
3
/g. Table 3
illustrates the surface area and pore volume of TNR, WNS and
WNS/TNR. The increase in surface area of heterostructure implies it will
provide higher catalytic active sites and thus will result in improved
HER performance [35,57,58,60,61].
In order to estimate the thermal stability and decomposition
behavior of the WNS/TNR heterostructure, the weight loss of the com-
posite was examined using thermogravimetric analysis (TGA). The re-
sults are presented in the attached Fig. S2. The TGA curve of WNS/TNR
shows three distinct weight loss steps. The rst weight loss of 1.37%
occurs around 180 ◦C–430 ◦C, which can be attributed to the removal of
adsorbed water and surface-bound contaminants [62]. The second
weight loss of 0.95% occurs between 430 ◦C and 600 ◦C, which is likely
due to the decomposition of organic residues or possible oxidation of
surface impurities [63]. The third and most signicant weight loss of
Fig. 3. Shows X-Ray Diffraction pattern of TiO
2
, WS
2
and WS
2
/TiO
2
hetero-
structure in which curve a represents XRD peaks of TNR conrming the rutile
phase of TiO
2
, curve b represents XRD peaks of WNS validating the 2H phase of
WS
2
and curve c represents XRD peaks of heterostructure WNS/TNR.
Table 1
Variation of 2Θ, FWHM, crystallite size and dislocation density corresponding to
different planes of WNS, TNR and WNS/TNR.
Sample 2ΘFWHM D (nm) δ(10
−3
) (lines/nm
2
)
WNS 14.52 0.380 22.01 2.06
28.57 0.381 22.50 1.97
32.88 0.258 33.53 0.88
36.17 0.298 29.24 1.16
44.02 0.354 25.28 1.56
50.09 0.376 24.32 1.69
56.29 0.316 29.79 1.56
TNR 27.2 0.554 15.40 4.21
36.0 0.439 19.85 2.53
39.2 0.462 19.08 2.74
41.3 0.469 18.89 2.80
44.0 0.547 16.36 3.73
54.4 0.617 15.11 4.38
56.5 0.702 13.43 5.54
WNS/TNR 14.4 0.299 27.92 1.28
26.89 0.332 25.70 1.51
28.41 0.365 23.44 1.81
34.48 0.328 26.44 1.43
38.03 0.319 27.50 1.32
43.7 0.356 25.25 1.56
50.18 0.371 24.67 1.64
Table 2
Calculated values of crystallite size and lattice strain using Williamson-Hall
(W–H) Uniform Deformation model (UDM).
Sample Crystallite size (nm) Strain (ϵ) (10
−3
)
TNR 12.41 1.68
WNS 20.22 1.07
WNS/TNR 18.14 1.37
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
6
1.64% occurs between 600 ◦C and 800 ◦C, which can be attributed to the
decomposition of any remaining organic material and the thermal
degradation of the composite material [64]. Notably, WNS and TNR
have remarkable heat stability; the weight loss in the composite is pri-
marily caused by impurities and organic residues added during the
production process [62,64]. The nding demonstrates that the
WNS/TNR heterostructure is appropriate for applications requiring
great thermal stability since it retains its structural integrity at high
temperatures. This stability is important for HER, as it ensures that the
heterostructure can withstand thermal and chemical stresses during its
prolonged operations. Thus, proving it to be a promising candidate for
HER.
For the analysis of electronic interactions at the interface of WNS and
WNS/TNR heterostructure, XPS characterization was utilized. As illus-
trated in Fig. 6, peaks corresponding to W 4f, S 2p, Ti 2p and O 1s are
observed in the obtained heterostructure, conrming the presence of
elements in the samples. Fig. 6a illustrates the XPS scan of W in pristine
WNS and WNS/TNR heterostructure. The binding energy observed at
37.3 eV and 35.2 eV corresponds to W 4p
3/2
and W 4f
5/2
respectively [8,
57,58,65]. Furthermore, peaks at 163.17 eV and 162.07 eV corresponds
to S 2p
1/2
and S 2p
3/2
of S orbital. In the corresponding heterostructure a
slight shift in binding energy is observed for W and S towards lower
region indicating the coupling between WS
2
and TiO
2
components, thus
conrming the formation of heterostructure as has been studied before
[34,66]. Additionally, spectra of Ti with peaks at 464.4 eV and 458.7 eV
corresponding to Ti 2p
1/2
and Ti 2p
3/2
conrms its presence. The oxygen
spectrum for O1s with peaks at 530 eV and 531.6 eV belong to lattice
and non-lattice oxygen was also obtained [30,54,66]. The binding en-
ergy shifts in XPS indicate a change in the electronic environment due to
the interaction between WNS and TNR, which aligns with the Z-scheme
charge transfer in the heterostructure as explained further using UV–vis
and UPS studies. Under illumination, electron-hole pairs are generated
in both WNS and TNR. In the Z-scheme mechanism (illustrated in Fig. 9),
electrons in the CB of TNR have tendency to recombine with holes in the
VB of WNS, leading to effective charge separation with electrons
remaining in the CB of WNS and holes in the VB of TNR [19,20,67]. The
observed binding energy shifts towards lower values for W and S in WNS
suggest an increase in electron density around WNS, consistent with the
proposed Z-scheme electron transfer [68]. This electron transfer en-
hances the photocatalytic activity of the heterostructure, supporting the
Fig. 4. Raman spectra of (a) WS₂nanosheets (WNS) displaying characteristic peaks with E
1
2g
and A
1g
modes(b) TiO₂nanorods (TNR) with notable peaks representing
SOS, E
g
, and A
1g
vibrational modes indicating successful synthesis and distinct vibrational modes of the respective materials.
Fig. 5. BET surface area analysis of samples: (a) N
2
adsorption desorption isotherm plots for TNR, WNS and WNS/TNR. (b) Pore size distribution results calculated
from BJH desorption pore volume data for respective samples.
Table 3
BET surface area and Pore volume of TNR, WNS and WNS/TNR.
Sample S
BET
(m
2
/g) V
p
(cm
3
/g)
TNR 26.49 0.08
WNS 51.61 0.22
WNS/TNR 84.52 0.33
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
7
efcient hydrogen production process [69]. The correlation between
XPS results and the Z-scheme mechanism validates the effective
coupling in the WNS/TNR heterostructure, conrming its potential for
enhanced photocatalytic applications.
To comprehensively investigate the behavior of photogenerated
charge separation and transfer in the WNS heterostructure, UV–vis ab-
sorption spectroscopy and UPS were conducted. Fig. 7 represents the UV
absorption spectra for TNR, WNS and WNS/TNR heterostructure. The
absorption for pristine samples and heterostructure was found to be in
the range 300–800 nm [8,34,58]. The band gaps of TNR, WNS and
WNS/TNR were determined using Tauc plots. These plots were deter-
mined by plotting (
α
h
ν
)
2
and energy (h
ν
). The band gap was determined
by extrapolating the graph at the intercepts. In the case of WNS the band
gap energies were determined to be approximately 2.68 eV after
extrapolating on the energy intercept [10]. These values suggest that the
photocatalysts possess the ability to be activated by visible light. The
band gap for TNR and WNS/TNR was 2.9 eV and 1.9 eV respectively.
From literature review we know that pristine TiO
2
is a wide bandgap,
restricting its light absorption to the UV region only [21,70–72].
Conversely, pristine WNS and the WNS/TNR heterostructure has the
ability to absorb light in the visible as well as the ultra violet region of
the spectrum. Thus, enhancement in light absorption of the WNS/TNR
structure will facilitate the electrons to get excited easily, resulting in an
enhanced photoelectrochemical performance for H
2
production [31,73].
To understand the charge ow direction, the band structures of WNS
and WNS/TNR were investigated through ultraviolet photoelectron
spectroscopy (UPS) (Fig. 8). The work function was determined using
the equation reported in literature, considering the laser light (21.22 eV)
as incident ultraviolet photons and using the fermi energy (E
F
) as well as
the cutoff energy of secondary electrons (E
cutoff
) [34,74–76]. The
magnied view of secondary electron cut off region and valence band
edges for WNS and WNS/TNR heterostructure is shown. The valence
band maxima for WNS [77] and WNS/TNR were observed at 2.25 eV
and 3.08 eV, respectively, as depicted in Fig. 8. The work function values
for bare WNS and WNS/TNR were calculated as 5.55 eV and 5.47 eV,
respectively. Hence, a decrease in work function was observed on
fabrication of the heterostructure. This decrease implies that the heter-
ostructure facilitates rapid charge transfer at the interface as compared
to pristine WNS. Thus, an increase in charge transfer will in turn result in
improvement in electrocatalytic activity, leading to an improved HER
performance. A lowered work function implies lowered energy barrier,
rapid charge transfer and enhanced HER [74,78].
A decrease in work function upon fabrication of the heterostructure
typically results in an increase in hydrogen evolution reaction (HER)
performance. This can be explained as a lower work function would
facilitate more efcient transfer of electrons at the heterostructure
interface, promoting enhanced electrocatalytic activity for the HER [78,
79]. This lowered energy barrier allows easy electron transfer across the
interface, and since this movement is directly favourable for HER, hence
its kinetics is improved. This electron transfer in the heterostructure can
be easily explained using the band energy diagram. Thus, a decrease in
work function is correlated with improved HER performance due to
enhanced electron transfer and catalytic activity at the heterostructure
interface [2,80].
Fig. 6. XPS Plots of (a) W in WS
2
assigned as WNS and WS
2
/TiO
2
assigned as WNS/TNR (b) S in WNS and WNS/TNR (c) T in WNS/TNR and O in WNS/TNR showing
corresponding peaks for each elements.
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
8
The bandgap of WNS has been found to be equal to 2.68 eV, on
combining it with the valence band position obtained from UPS spectra,
the conductive band of WNS was found to be equal to −0.35 eV [10,31].
The band gap of TNR was found to be 2.9 eV. According to literature
review the conduction band for TNR is at −0.21 eV. Thus, the band
diagram for the resulting heterostructure as also reported in literature is
shown in Fig. 9 [10]. The band diagram proves to be that of a direct Z
scheme heterojunction. In direct Z scheme photocatalyst, on formation
of heterostructure, the negative charge present on the reductive semi-
conductor tend to combine with those of oxidative semiconductor to
achieve fermi level equilibrium. Thus, inactive electron and hole pairs
are consumed whereas highly active electron and holes are left out for
photocatalytic reactions [19,20,67].
Hence, when exposed to light (as depicted in Fig. 9), the excited
electrons present in the CB of TNR will tend to combine with the holes
present in VB of WNS. This process will lead to the consumption of
inactive photogenerated electrons and holes, allowing the preservation
and effective utilization of highly reactive photogenerated electrons and
holes for various photocatalytic reactions. These photogenerated charge
carriers will thus participate in hydrogen evolution [5,19,81,82].
2.7. Hydrogen evolution reaction studies
The catalytic activity of TNR, WNS and WNS/TNR heterostructure
was examined in 1 M KOH alkaline solution utilizing a three-electrode
system. Linear sweep voltammetry was performed, as shown in
Fig. 10a. A rapid increase in current density can be observed with rise in
potential [3,7,83]. A lower value of onset potential for heterostructure
(0.031 V) as compared to 0.05 V and 0.06 V for WNS and TNR proves its
superior catalytic activity. The Tafel slope was also obtained to evaluate
the performance of prepared samples by replotting overpotential and
logarithm of current density. The Tafel slope for TNR, WNS and
WNS/TNR were found to be 126 mV/dec, 110 mV/dec and 99 mV/dec
respectively (Fig. 10b). Generally, a lower Tafel slope signies a lower
required overpotential to produce an adequate current, which governs
the kinetics of the Hydrogen Evolution Reaction (HER). The hetero-
structure had a lower Tafel slope which indicates it provides a better
charge transfer [84–86]. According to literature, Tafel slopes around
120 mV/decade, 40 mV/decade, and 30 mV/decade correspond to the
Volmer, Heyrovsky, and Tafel reaction mechanisms, respectively [4,27,
87,88]. Therefore, in our case the tafel slope of TNR, WNS and
WNS/TNR heterostructure falls between the Volmer Hervosky range,
proving the reaction to be of Volmer- Hervosky type.
Cyclic Voltammetry was also performed at different scan rates at 10
mV/s, 25 mV/s, 50 mV/s, 125 mV/s and 200 mV/s respectively as
shown in Fig. 11. The WNS/TNR displayed higher values for current and
also area as compared to pristine WNS [89]. The CV results indicate that
the heterostructure has higher value of oxidation and lower reduction as
compared to pristine WNS. An excellent reversibility of electrochemical
reactions for electrode materials can be conrmed from the CV graphs
[90].
The investigation of photocurrent response is pivotal in evaluating
Fig. 7. UV–Vis absorption spectra (a) WS₂nanosheets (WNS), TiO₂nanorods (TNR) and WS₂/TiO₂heterostructure (WNS/TNR). (b–d) The Tauc plots are used to
govern the band gap energies of WNS, TNR, and WNS/TNR heterostructure respectively.
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
9
electron-hole separation and the photocatalytic efciency of materials
[58,91]. In this study, experiments were conducted under
solar-simulator irradiation to validate the response of WNS, TNR, and
the WNS/TNR heterostructure to light (Fig. 12). Transient current
measurements were performed at a constant bias voltage with alter-
nating light signals (off/on) at 300 W cm⁻
2
. The current-time (I-t) plot, as
illustrated in Fig. 12, shows a fast response to the switching of the light
on-off signal for all the samples.
The steady-state photocurrent density was found to be highest in the
case of WNS/TNR, measuring 0.015
μ
A/cm
2
. TNR followed with a
photocurrent density of 0.011
μ
A/cm
2
, and the lowest value was ob-
tained for WNS at 0.005
μ
A/cm
2
. The highest photocurrent density
observed in the WNS/TNR can be attributed to the efcient charge
separation and reduction in recombination after the formation of het-
erostructure. This efcient charge separation is well-validated by the Z-
scheme formation, as proven by the band diagram [19,20,81].
Electrochemical impedance spectroscopy (EIS) was also employed to
examine the electrode kinetics involved in the Hydrogen Evolution Re-
action (HER) process. The value of charge transfer resistance (Rct)
determined by tting the circuit was found as 3.52 kΩ, 8.61 kΩand
555.54 kΩfor WNS/TNR, WNS and TNR respectively [92,93]. Illus-
trated in Fig. 13 WNS/TNR demonstrates the lowest charge transfer
resistance (Rct) as compared to pristine samples, indicating that high
efciency electron transfer takes place on the surface of WNS/TNR [2,
26,31,84]. Thus, proving heterostructure to be a superior catalyst for
HER as compared to pristine TNR and WNS nanosheets. These ndings
show the heterostructure has played an important role in inuencing
HER performance. A close interaction between TNR and WNS has
resulted in overall increase in conductivity [94–96].
Fig. 8. UPS spectra of (a) WNS showing valence band edge and (b) secondary electron cut off for WNS nanosheets and (c) WNS/TNR showing valence band edge and
(d) secondary electron cutoff for WNS/TNR.
Fig. 9. Shows Possible band diagram for Z scheme based WNS/TNR hetero-
structure and electron ow direction.
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
10
3. Conclusion
In conclusion, WS
2
nanosheets were successfully synthesized using
hydrothermal method and were grown on TiO
2
nanorods to create WS
2
/
TiO
2
heterostructure. Interfacial investigations were conducted to study
the charge transfer mechanism and their impact on the electrocatalytic
and photo-electrochemical performance of the obtained heterostructure.
The combination of WS
2
and TiO
2
caused structural modications and
electronic interactions as proven by X-ray photoelectron spectroscopy
(XPS) and ultraviolet photoelectron spectroscopy (UPS) analysis.
Electrochemical measurements illustrated that the WS
2
/TiO
2
heter-
ostructure exhibited higher electrical conductivity and henceforth su-
perior HER performance as compared to pristine samples. The lower
Fig. 10. Shows (a) Linear sweep voltammetry curves for TiO
2
(TNR), WS
2
(WNS) and WS
2
/TiO
2
(WNS/TNR) heterostructure and (b) shows corresponding Tafel plots
for TNR, WNS and WNS/TNR.
Fig. 11. Shows Cyclic voltammetry (CV) curves of (a) WS₂(WNS) and (b) WS₂/TiO₂(WNS/TNR) at scan rates 10, 25, 50, 125 and 200 mV/sec.
Fig. 12. Shows photocurrent density response of WS
2
(WNS), TiO
2
(TNR) and
WS
2
/TiO
2
(WNS/TNR) under the presence of external light.
Fig. 13. Shows EIS analysis with equivalent circuit model and Nyquist plots of
experimental and tted results for TiO
2
(TNR), WS
2
(WNS) and WS
2
/TiO
2
(WNS/TNR).
S. Dangwal et al.
International Journal of Hydrogen Energy xxx (xxxx) xxx
11
impedance value of the heterostructure further conrmed its superior
catalytic performance. The rapid response of heterostructure to light
proved the effective formation of a Z-scheme based heterojunction.
CRediT authorship contribution statement
Shivani Dangwal: Conceptualization, Data curation, Formal anal-
ysis, Investigation, Methodology, Resources, Software, Visualization,
Writing –original draft, Writing –review &editing. Saurabh Rawat:
Conceptualization, Data curation, Formal analysis, Methodology, Vali-
dation, Writing –original draft. Deb Kumar Rath: Conceptualization,
Data curation, Software, Visualization. Charu Dwivedi: Supervision,
Validation, Visualization. Mohit Sharma: Supervision, Validation,
Visualization. Rajesh Kumar: Supervision, Validation, Visualization.
Himani Sharma: Project administration, Supervision, Validation,
Visualization, Writing –review &editing.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgement
The authors are extremely thankful to Doon University Dehradun for
providing facilities and support. The authors acknowledge the Depart-
ment of Science and Technology (DST), Government of India for
providing funds to carry out research under PURSE scheme (SR/PURSE/
2023/199(G)) and Inter-University Accelerator Centre (IUAC) (UFR-
72307) for providing necessary facilities and funds to carry out research.
Author Shivani Dangwal acknowledges the Department of Science and
Technology (DST), Government of India for providing fellowship under
le no. DST/INSPIRE Fellowship/2021/IF210425. One of the author
Deb Kumar Rath acknowledges PMRF (ref. 2103355) for fellowship,
Government of India. The authors acknowledge the Raman Spectros-
copy facility received from the Department of Science and Technology
(DST), Government of India under the FST scheme (SR/FST/PSI225/
2016).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ijhydene.2024.08.101.
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