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Article https://doi.org/10.1038/s41467-023-43331-x
Self-activated superhydrophilic green
ZnIn
2
S
4
realizing solar-driven overall water
splitting: close-to-unity stability for a full
daytime
Wei-Kean Chong
1
, Boon-Junn Ng
1
,YongJiehLee
1
, Lling-Lling Tan
1
,
LutfiKurnianditia Putri
1
, Jingxiang Low
1,2
, Abdul Rahman Mohamed
3
&
Siang-Piao Chai
1
Engineering an efficient semiconductor to sustainably produce green hydro-
gen via solar-driven water splitting is one of the cutting-edge strategies for
carbon-neutral energy ecosystem. Herein, a superhydrophilic green hollow
ZnIn
2
S
4
(gZIS) was fabricated to realize unassisted photocatalytic overall water
splitting. The hollow hierarchical framework benefits exposure of intrinsically
active facets and activates inert basal planes. The superhydrophilic nature of
gZIS promotes intense surface water molecule interactions. The presence of
vacancies within gZIS facilitates photon energy utilization and charge transfer.
Systematic theoretical computations signify the defect-induced charge redis-
tribution of gZIS enhancing water activation and reducing surface kinetic
barriers. Ultimately, the gZIS could drive photocatalytic pure water splitting by
retaining close-to-unity stability for a full daytime reaction with performance
comparable to other complex sulfide-based materials. This work reports a self-
activated, single-component cocatalyst-free gZIS with great exploration value,
potentially providing a state-of-the-art design and innovative aperture for
efficient solar-driven hydrogen production to achieve carbon-neutrality.
Hydrogen (H
2
), the smallest molecule with the largest specific energy
content, is an emerging alternative to the traditional fossil energy for
achieving carbon neutrality. Inspired by the natural photosynthesis,
utilization of semiconductors to drive photocatalytic water splitting
for clean H
2
and oxygen (O
2
) production has been widely explored1,2.
Photocatalytic water splitting could serve as an important step
towards a more sustainable energy future, as it allows the conversion
of abundant photon energy into chemical energy as well as the storage
of solar energy in the form of high energy-content H
2
.Withinthis
framework, the selection of an appropriate photocatalyst is extremely
critical as it directly governs the efficiency of the solar-driven H
2
generation. Lately, metal chalcogenide semiconductors have gained
enormousattentioninthisfield owing to the favorable visible-light
response ability, large abundancy, and diversified chemical
structures3,4.
In particular, two-dimensional (2D) hexagonal ZnIn
2
S
4
(ZIS), a
well-known ternary metal chalcogenide, has garnered significant
attention in photocatalytic water splitting. The appealing features of
ZIS such as tunable band gap, high photosensitivity towards visible
light and favorable conduction band potential with a strong reduction
Received: 9 June 2023
Accepted: 6 November 2023
Check for updates
1
Multidisciplinary Platform of Advanced Engineering, Departmentof Chemical Engineering,School of Engineering, Monash University Malaysia, Jalan Lagoon
Selatan, 47500 Bandar Sunway, Selangor, Malaysia.
2
Department of Applied Chemistry, University of Science and Technology of China (USTC), 96 Jinzhai
Road, Hefei, Anhui 230026, PR China.
3
School of Chemical Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal Pulau Pinang, Malaysia.
e-mail: chai.siang.piao@monash.edu
Nature Communications | (2023) 14:7676 1
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capability making it a prominent candidate for H
2
evolution reaction
(HER)5,6. Different morphologies of ZIS have been developed till date,
with dominance of microspheres and nanosheets attributed to the
ease of fabrication, structural stability and diverse application7.
Nonetheless, microspherical ZIS suffers from low exposure of active
surface area due to the bilayer self-assembly into a three-dimensional
(3D) microsphere, covering some active sites in the core. Despite 2D
nanosheet ZIS possessing more surface area per unit volume, it suffers
from severe aggregation and interlayer stacking with surrounding
nanosheets to modulate its high surface energy8,9.Thisleadsto
reduction of available active surface sites and worsened surfacecharge
separation, deteriorating the photocatalytic performance. Thus, it is
critical to alter the morphology of ZIS to minimize the rate of charge
carrier recombination, while simultaneously possessing high avail-
ability of active surface area for the photocatalytic reaction. Further-
more, the self-oxidation of ZIS and deficiency of catalytic O
2
evolution
reaction (OER) sites serve as long hidden bottleneck issues that limit
the application of ZIS in overall water splitting10. Several strategies
could be employed in addressing the aforesaid criteria. Primarily, the
structure of ZIS could be designed to enhance light scattering
for improved light utilization and to prevent interlayer aggregation.
Secondly, the introduction of S vacancy (S
v
) into the framework to
suppress electron-hole pair recombination, promote charge redis-
tribution and facilitate photoreaction11,12. Following that, the density of
the active edge S atoms in (110) facet should also be increased via
selective unleashing of the edge sites to favor the photogeneration of
H
2
13,14. Furthermore, the surface hydrophilicity could be improved to
facilitate water molecule interactions to drive a more vigorous water-
splitting reaction. Concomitantly, high surface wettability could pro-
mote efficient mass transfer of water molecules to active surface and
expedite instantaneous release of generated gas bubble to maintain
ubiquitous availability of active sites15–18.
Herein, a distinctive superhydrophilic green ZnIn
2
S
4
(gZIS) was
constructed in this work via a one-step in-situ solvothermal synthetic
route. The gZIS with hollow hierarchical framework is found to possess
higher specific surface area with more exposed active facets. The
superhydrophilic surface enhances interaction with surrounding water
molecules to drive water decomposition. Besides, gZIS experiences an
optical absorption property analogous to natural leaves, utilizing both
the high and low wavelength of solar light to generate electron-hole
pairs for photoreaction. The defects within the structure further reg-
ulate the charge redistribution and activate the inert basal plane with
facile charge transfer and enhanced surface reaction. The first-
principle calculations provide theoretical insights and verify the sig-
nificant roles of vacancies in electronic properties modulation, water
molecules interaction, HER and OER surface kinetics improvement. As
a result, this self-activated gZIS demonstrated its capability in cata-
lyzing solar-driven overall water splitting with close-to-unity stability
for a full daytime reaction. In addition, the single-component cocata-
lyst-free gZIS exhibited an apparent quantum yield (AQY) and solar-to-
hydrogen conversion efficiency (STH) that is comparable to other
noble-metal loaded and complex sulfide-based photocatalysts. These
groundbreaking deliveries represent a significant breakthrough in
addressing the longstanding concealed obstacles of sulfide-based
materials, particularly the unassisted overall water splitting capability
and photostability. This discovery will pave a way towards the devel-
opment of high-performing photocatalysts to achieve efficient and
sustainable overall water splitting without the incorporation of
expensive noble metal cocatalysts.
Results and discussions
Morphological design fundamentals and structural
characterization
A conventional yellow 3D microspherical ZIS was synthesized via
hydrothermal route as shown in Fig. 1a. In detail, H
2
O molecules
donate lone pair electrons to Zn2+ via dative bonding. Zn-aquo com-
plex is then formed, followed by the transformation into tetrahedral
[Zn(TAA)
4
]2+ complex in the solution upon ligand exchange. Mean-
while, the formation of two distinctive aquo complexes ofIn3+ give rise
to In-TAA complexes with varying coordination number, i.e., tetra-
hedral [In(TAA)
4
]3+ and octahedral [In(TAA)
6
]3+19,20.Subsequently,
three metal sulfide species (Zn-S
4
,In-S
4
, and In-S
6
) are formed
under the hydrothermal condition, which spontaneously combine into
2D hexagonal bilayer ZIS nanosheets in S-Zn-S-In-S-In-S stacking
sequence. Due to the high surface energy of 2D layered structure, the
ZIS nanosheetswould self-assembleinto 3D microspheres tomoderate
the surface energy for a more thermodynamically stable form. On
the other hand, a unique green gZIS was successfully fabricated by
employing ethylene glycol (EG) as the solvent. In the presence of EG,
Zn2+ and In3+ are subjected to the formation of metal glycol complexes
with larger radii than the respective metal aquo complexes21.During
the solvothermal reaction, the complexes interact with S2- to form
different metal sulfide species that lead to nanolayer construction and
subsequently self-assemble into unique gZIS framework. In this con-
text, EG serves pivotal roles in the formation of the final structure: (i)
Firstly, it acts as a surfactant that moderates the surface tension at
the boundaries between particles, lowering particle aggregation and
promoting anisotropic assembly into hollow hierarchical cavity-
network configurations19,22, (ii) it also reduces the valence state of
Zn2+ and In3+ leading to a decrease in coordination abilities with
S2-21,23,24, and (iii) lastly, it reacts with the exposed S on the terminated
surface inducing surface S removal, which collectively incur S
v
into the
gZIS structure25,26.
The morphologies and microstructures of the as-synthesized ZIS
andgZISwereanalyzedbyfield emission scanning electron micro-
scopy (FESEM). As displayed in Fig. 1b, d and Supplementary Fig. 1, ZIS
exhibits a basic configuration composing of closely assembled
nanosheets flowerlike structure with densely packed core, which
would deteriorate the exposure and utilization of active surface sites.
In the absence of any surfactant, the morphology of ZIS is highly
irregular and consists of micro-sized spheres varying from 3 to 10μm.
Conversely, EG-assisted solvothermal reaction is more conducive
towards generating hollow gZIS lamellar framework from the loose
interwoven of nanoflakes (see Fig. 1c, e and Supplementary Fig. 1). The
numerous sparsely intersecting nanosheets not only exposes more
expansive active surface sites for the photoredox reactions, but also
ameliorates the multilevel reflection and scattering of incident pho-
tons for the enhancement of light absorption27. Ascribed to the pre-
sence of EG, the growth of gZIS is more uniform with equivalent
diameters ranging from 1 to 2 μm. This would lead to a more homo-
genous distribution of active sites for photocatalytic reactions. The
elemental compositions of ZIS and gZIS were investigated using
energy-dispersive X-ray (EDX) spectroscopy. The EDX spectrum of
pristine ZIS in Fig. 1f clearly reveals the even and spherical distribution
of Zn, In, and S elements with Zn:In:S atomic ratio of 1.07:2.00:3.99,
which is close to the ideal stoichiometric ratio of 1:2:4 (see Supple-
mentary Table 1). The EDX mapping also confirmed the uniform
coexistence of Zn, In, and S in the gZIS structure, with an S defective
atomic ratio (see Fig. 1g and Supplementary Table 1). Citing to the
normal levels of In atoms, the S
v
concentration of gZIS could be
determined by comparing the S:In ratio of gZIS to that in pristine ZIS28.
The EDX analysis reflects the presence of ca.3%S
v
across the gZIS
hierarchical framework as depicted in the EG-assisted synthesis reac-
tion. The S
v
in the framework could act as electron traps to facilitate
vectorial transport of photogenerated electrons as well as induce
favorable lattice defects for charge redistribution and electronic
properties modification29–31.
Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) were employed to investigate the microcosmic char-
acteristics of ZIS and gZIS. In concordance with the observation in
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 2
Content courtesy of Springer Nature, terms of use apply. Rights reserved
FESEM,TEM image of ZIS manifests the packed layer-by-layerassembly
of nanosheets in constructing a large microstructure with a dense core
(see Fig. 2a). Antithetical to the packed solid core ZIS structure, there
are loose interwoven thin nanosheets in the smaller hollow hierarchical
gZIS configurations (Fig. 2b). The 3D flimsy layer-to-layer inter-
connection of thin nanosheets of gZIS is advantageous in exposing
more active sites for reactant species interaction, and at the same time
preventing the undesired stacking and aggregation as-of observed in
the standalone single phase nanosheet system32.Comparatively,the
migration distance of photogenerated charge carriers could also be
shortened in thesmaller gZIS particle, favoring the participation of the
electron-hole pairs in the photocatalytic reactions33.TheHRTEM
image in Fig. 2c presents the characteristic d-spacings of 0.32 and
0.19 nm, corresponding to the (102) and (110) of ZIS, respectively.
There is nearly imperceptible distortion observed along the lattices of
ZIS, signifying the successful construction of a pure pristine ZIS crystal.
Directing attention to Fig. 2d, the lattice spacing of gZIS is ca.0.19nm
which is assigned to the (110) plane. Unlike the pristine ZIS, lattice
fringe distortions and defects are noticeable in the structure of gZIS
owing to the presence of S
v
. The asymmetrical distortions and defects
could potentially induce instantaneous dipole moment which enhance
the charge separation efficiency34. To shed light onto the structural
atomic insights, first-principle density functional theory (DFT) was
utilized to replicate the theoretical models for ZIS (ZIS
T
)andgZIS
Fig. 1 | Catalyst synthesis and morphological characterizations. a Schematic of
the formation of ZIS and gZIS. The charges of the complexes are omitted in the
figure for clarity. M denotes the metal ions, either Zn2+ or In3+, present in the
solution. False-colored FESEM images for (b)ZISand(c) gZIS. Magnified false-
colored FESEM view for (d)ZISand(e) gZIS, with the insets showing the original
FESEM images. EDX elemental mappings for (f)ZISand(g) gZIS.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 3
Content courtesy of Springer Nature, terms of use apply. Rights reserved
(gZIS
T
). As demonstrated in Supplementary Fig. 2, different S
v
posi-
tions were introduced into the ZIS
T
, with the formation favorability
evaluated by the magnitude of formation energy (E
form
). Removal of S
from the (001) basal plane position (SV1) leads to a lower E
form
,
denoting a more energetically favorable structure of sulfur-vacant
gZIS
T
.PristineZIS
T
resembles a perfect hexagonal bilayer structure
without any bond dislocation (Fig. 2e and Supplementary Fig. 3a–c). In
contrast, the presence of Sv in the framework of gZIST alters the
arrangement of neighboring atoms as illustrated in Fig. 2fandSup-
plementary Fig. 3d–f. The occurrence of theoretical lattice distortion is
coherent with the experimental HRTEM observations that eventually
distorts the endogenous hexagonality. In pursuit of disclosing further
insights onto the structural information of gZIS, spherical aberration-
corrected bright field scanning TEM (BF-STEM) was employed to
provide atomic arrangement information of the framework as eluci-
dated in Fig. 2g. Congruently, an obvious distortion was observed
along the atomic alignment of gZIS attributed to the presence of S
v
which altered the intrinsic arrangement. Fast Fourier transformation
(FFT) was then conducted to provide an amplified resolution of the
atomic imaging. In point of fact, the relatively smaller S atoms, over-
laying on top or underneath the Zn and In atoms (see Supplementary
Fig. 4a), could be hardly visible under microscopic vision. Thus, the
visibility of S atoms in the modeled structure is toggled-off in Fig. 2g
(right-most) and Supplementary Fig. 4b for equitably juxtaposition.
The magnified post-FFT atomic imaging convincingly manifests the
presence of distortion within the gZIS structure, which is in agreement
with the simulated result. On one hand, the In-In hexagonal ring
remains high regularity, implying the undisturbed arrangement of In
atoms across the structure owing to the location of S
v
far from the In
hosts. On another hand, two consecutive alternating Zn-In hexagonal
rings experience slight deformation, signifying the presence of S
v
close
to Zn atom which eventually induces disorderness around Zn hosts.
The observations in the experimental findings align with the theore-
tical simulations, which verified the existence of S
v
within gZIS struc-
ture andaffirm the location of S defect close to Zn atomalong the basal
plane. The existence of S
v
inevitably induces structural defects in gZIS
framework which eventually lead to a charge redistribution.
Surface electronic and physiochemical characteristics
The surface chemical states and elemental compositions of ZIS and
gZIS were analyzed by X-ray photoelectron spectroscopy (XPS). As
evident from thefull survey scan spectrum in Supplementary Fig. 5, the
presence of Zn, In and S peaks clearly delineate the coexistence of
these elements in both the ZIS and gZIS sample, which is consistent
with that in EDX analysis. Accentuated from Fig. 3a,theZn2pmetallic
peak in ZIS splits into two individual peaks of 2p
3/2
(1021.38 eV) and
2p
1/2
(1044.38 eV). The Zn 2ppeaks of gZIS moderately upshift to
higher binding energy values due to the loss of electrons and the
Fig. 2 | Structural characterizations and analysis. TEM images for (a)ZISand(b)
gZIS. HRTEM images for (c) ZIS and (d) gZIS, with an inset showing the enlarged
region withlattice distortion and defects in gZIS. Theoretical structural models for
(e) pristine ZIS
T
and (f) S-vacant gZIS
T
.gAtomic-resolution spherical aberration-
corrected BF-STEM imaging of gZIS with pre- and post-FFT. The magnified view
shows the atomic arrangement with distorted hexagonal in concordance to the
simulated result.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 4
Content courtesy of Springer Nature, terms of use apply. Rights reserved
reduction in coordination number of Zn to maintain the charge equi-
librium with surrounding S elements35,36, suggesting the presence of S
v
around Zn atom. Nonetheless, the spin-orbit splitting of S 2pconfers
bi-deconvoluted peaks in ZIS sample, namely 2p
3/2
(161.65 eV) and 2p
1/2
(162.87eV) as shown in Fig. 3b. The intensity of the S 2pcharacteristic
peaks in gZIS is patently weaker than that of ZIS, clearlydemonstrating
a lesser S content in gZIS. The XPS elemental composition suggests a
consistent S
v
percentage in gZIS as previously observed in the EDX
analysis (Supplementary Fig. 6). The presence of S
v
is further testified
by the electron paramagnetic resonance (EPR) in Fig. 3c. The gZIS
sample displays a sharp signal at a g-factor of ca. 2.004, which is not
evident in pristine ZIS due to existence of S
v
only in the gZIS
framework37. In addition, the two S 2ppeaks of gZIS in Fig. 3b
experience negative shift to lower binding energy, signifying the
enrichment of electron cloud density around the S atoms38,39.The
higherelectronegativityof S contributes to a better tendency to attract
electrons during the charge redistribution brought by S
v
.DFTwasthen
utilized to investigate the effect of S
v
on the charge distribution. As
elucidated in the charge density difference from Fig. 3d, there is a
noticeable charge redistribution in the gZIS
T
framework, with gray
area showing the electron depletion zone and green area marking the
electron accumulation region. It is not astonishing to observe the
electron depletion zone in the S
v
location due to the loss of S atom.
Coherent with the XPS finding, there is also a visible electron depletion
around the Zn atom near to the S
v
along the basal plane. Additionally,
the electron density not only increases in the intrinsically active S
atoms at the (110) surface, but also gathers along the inherently
unreactive S atoms at the basal plane. This defect-induced favorable
charge redistribution could activate the inert basal plane for photo-
reactions as well as fur ther boost the efficiency of H
2
production at the
intrinsic active sites.
An essential aspect of assessing the catalytic performance is a
thorough analysis of the crystalcharacteristics and surface properties
of the samples. Therefore, X-ray diffraction (XRD) is utilized to acquire
the crystal features. Harmonious with ZIS as presented in Fig. 4a, gZIS
still preserves the two main hexagonal peaks in thestructure, which are
assigned to the (102) and (110) planes following PDF #065-202340.
The XRD spectra exhibit a conspicuous absence of any impurities
peak, signifying the attainment of pure phases of ZIS and gZIS. It
is widely recognized that the (110) facet of ZIS represents the
mostconducivesiteforHER,thusgreaterexposureofthe(110)planeis
desirable. Fascinatingly, the (110)-to-(102) peak intensity ratio increa-
ses from 0.92 in ZIS to 1.11 in gZIS. The greater-than-unity peak ratio
clearly exhibits that (110) plane attains dominance in gZIS, providing
higher exposure of HER active site. Following that, the Brunauer-
Emmett-Teller (BET) specific surface area and pore size distribution
of the samples were evaluated by the nitrogen adsorption-desorption
isotherms. As manifested from Fig. 4b, pristine ZIS displays a con-
ventional type IV isotherm with a calculated BET surface area
of 71.83 m2∙g−1. Besides, the isotherm elucidates a large lag of H
3
hysteresis loop in the relative pressure ranged from 0.5 to 1.0,
indicating the presence of wide distribution of non-uniformly shaped
mesopores in the structure41. Such observation is consistent with the
morphology previously observed in Fig. 1b, as well as the broad
Barrett–Joyner–Halenda (BJH) pore size distribution in the inset of
Fig. 4b. Conversely, the presence of EG facilitates even construction of
gZIS with a relatively larger exposure of surface area (117.04 m2∙g−1)
from its hollow hierarchical framework and a comparatively more
uniform pore distribution concentrating at the smaller pore size. The
well-dispersed gZIS exhibits higher specific surface area, which in turn
offers more active sites for photocatalytic reaction. The hydrophilicity
of the photocatalyst surface was also examined via the static water
contact angle measurements (Fig. 4c). Pristine ZIS is found to be
inherently hydrophilic with a contact angle of 68.6°, while gZIS
transforms into superhydrophilic nature with a contact angle as low as
8.1°. The observed enhancement in the surface wettability ofgZIS may
be attributed to the higher degree of surface area exposure with
increased surface roughness resulting from the dense small meso-
pores distributed throughout the large surface area of hollow frame-
work. Furthermore, water dispersion test was conducted on ZIS and
gZIS as shown in Supplementary Fig. 7. Both the samples were
homogenously dissolved in DI water at similar concentrations. After
Fig. 3 | Surface chemical and charge properties. High-resolution XPS spectra of
(a)Zn2pand (b)S2pfor th e as-synthesized sa mples. cEPR spectra for ZIS and gZIS
indicating the presence of S
v
.dComputed 3D charge density difference for gZIS
T
¸
with the top showing the whole bilayer structure and the bottoms focus on the
monolayer where S
v
is present. Gray and green areas dictate the charge depletion
and accumulation isosurfaces, respectively.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 5
Content courtesy of Springer Nature, terms of use apply. Rights reserved
stewing for 5 min, there is observable precipitation of ZIS sedimented
at the bottom, while gZIS remains well-dispersed in the solution. Some
pale green precipitation of gZIS becomes discernible only after an
elapsed time of 30 min. The increase in hydrophilicity and dispersion
of gZIS would benefit the water adsorption capability and enhance the
interface contact42. On top of the physical structural modification of
gZIS promoting water engagement, the effect of S
v
onto the water
interaction was also explored from DFT computation. A free water
molecule was primarily modeled as illustrated in Fig. 4d, with a bond
angle of 104.5° and O-H bond length (L
OH
) of 0.96 Å. The water inter-
action study wasthen conducted on the theoretical structures of ZIS
T
and gZIS
T
,specifically on the basal plane where the S
v
is located. It is
found that the water molecule experiences a weak physisorption and
mild activation (Fig. 4e and Supplementary Fig. 8a–c) upon interacting
with the Zn atom of pristine ZIS
T
structure. On the other hand, the
unsaturated Zn atom of gZIS
T
possesses a lower electron density as
discussed in Fig. 3d, and thus a relatively higher partial positive charge
(δ+) from the asymmetric distribution of electrons. From the nature of
higher electronegativity of O as compared to Hatom, the O atom holds
a partial negative charge (δ-) in the water molecule and tends to attract
to any oppositely charged atom. Consequently, the δ+-Zn in gZIS
T
could drive a more intensewater interaction with the δ--O from water.
Comparing the water interactivity on the basal plane of the structures,
gZIS
T
is more competent in activating the water molecule than ZIS
T
(see Fig. 4e, f and Supplementary Figs. 8a–f) as demonstrated by the
greater bond angle expansion and L
OH
elongation. The water adsorp-
tion free energy ΔEH2O*
at the basal plane of gZIS
T
(−1.40 eV) is also
foundtobemorenegativethanthatofZIS
T
(−0.97 eV), demonstrating
a more favorable adsorption of water towards activated basal plane of
gZIS
T
from thermochemical perspective. Interestingly, the defect
location of S
v
is able to accommodate water molecule adsorption (see
Fig. 4g and Supplementary Figs. 8g–i). The unsaturated Zn atom near
to S
v
strongly interacts with the adsorbed water molecule, leading to
more sizable L
OH
lengthening and bond angle stretching to ease the
O-H bond breaking. In conjunction with the exposure of more surface
active sites, gZIS could further elevate the photocatalyticefficiency by
promoting water molecule interaction and propagating the cleavage
of O-H bond for H+deprotonation to drive both the HER and OER
forward.
Electrochemical attributes and charge transfer properties
Transient photocurrent responses and electrochemical impedance
spectroscopy (EIS) Nyquist analysis were conducted to assess the
charge transfer dynamics of the samples. As depicted in Fig. 5a, the
measured photocurrent intensity of gZIS is approximately twice as
much as the pristine ZIS, contributing to a striking enhancement of
photogenerated electron-hole pairs separation. EIS Nyquist plot is also
provided in Fig. 5b to consolidate the charge transfer kinetics. The plot
is fitted according to an equivalent Randle circuit consisting of inter-
facial charge transfer resistance (R
CT
), series resistance (R
S
)andcon-
stant phase element (CPE) for the electrolyte-electrode interface (see
inset of Fig. 5b and Supplementary Table 2). In detail, the arc diameter
of the plot correspondsto the charge transfer impedance whereby the
smaller the arc size of the semicircle, the lower the R
CT
value, the faster
the charge transfer and separation of the photogenerated charge
carriers43–45. As observed, gZIS displays a smaller semicircle arc
accompanied by a reduction of R
CT
, proving that the S
v
-induced
charge redistribution and asymmetric dipole moment facilitating the
charge transfer within the framework of gZIS. Moreover, steady-state
photoluminescence (PL) spectrums for both ZIS and gZIS were
recorded in Fig. 5c. The pristine ZIS manifests a single prominent peak,
emitting from the band-to-band transition across the band gap fol-
lowed by the recombination of photogenerated charge carriers. Con-
trarily, gZIS displays two distinctive peaks which are attributed to the
intrinsic band-to-band radiative transition of excited electron from
conduction band (CB) to empty state in valence band (VB), and the
extrinsic sub-band from defect state introduced by S
v
to the ground
state46. Not only that, the intensity of gZIS PL spectra is quenched due
to the presence of S
v
acting as electron trap to restrain the electron-
hole pairs recombination, and simultaneously expedite the charge
Fig. 4 | Physical properties and water interaction study. a XRD spectra for ZIS
and gZIS. bNitrogen adsorption-desorption isotherms of ZIS and gZIS with inset
showing the respective pore size distribution. cSurface wettability static contact
angle measurements for ZIS and gZIS; error bars represent the standard deviation
from three independent runs. dFree water molecule with its respective O-H bond
length and H-O-H bond angle. Theoretical modeling of water adsorption along the
basal plane: (e)onZnatomofZIS
T
,(f) on Zn atom of gZIS
T
,and(g)inS
v
position
of gZIS
T
.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 6
Content courtesy of Springer Nature, terms of use apply. Rights reserved
transfer process for active participation in the photocatalytic reaction.
It is generally accepted that the empirical lifetime of electron (τ0
e)is
approximated by the equation of τ0
e=1
2πfmax,wherebyf
max
corresponds
to the frequency of the maximum Bode-phase peak47.Basedonthe
reciprocal correlation and Bode-phase plot in Supplementary Fig. 9,
the negative shift in f
max
of gZIS implies a prolonged electron lifetime
as compared to that of ZIS, which is associated to the augmented
charge carrier separation. Time-resolved PL (TRPL) was then carried
out to verify the lifetime behavior of the samples. As depicted in Fig. 5d
andSupplementaryTable3,boththefluorescence decay curves follow
a bi-exponential decay model, whereby gZIS possesses longer average
lifetime (τ
ave
) than pristine ZIS. Thus, it can be deduced that the charge
separation efficiency of gZIS is significantly enhanced, allowing more
photogenerated charge carriers to actively participate in reactions.
Luminescent-electrochemical properties evaluation
It is crucial to investigate the photo-absorption characteristics and the
energy band positions of the samples in order to correlate to the
performance of photocatalytic H
2
production. The optical absorption
properties were examined by ultraviolet-visible (UV-Vis) diffuse
reflectance spectraas shown in Fig. 6a. Pristine ZIS is found to have the
ability to absorb the blue-to-green region of the visible light spectrum,
reflecting a large portion of unutilized yellow-to-red visible light and
appearing to be bright orangish-yellow as shown inthe inset. The gZIS
sample, however, experiences a blue shift of intrinsic absorption edge
due to the size reduction as observed14,48. Astoundingly, gZIS exhibits
an extended absorption tail up to the red visible and near infrared
(NIR) region, where pristine ZIS displays nil responses beyond its
absorption edge. This intriguing phenomenon may be attributed to
the presence of S
v
within gZIS framework, introducing an additional
defect state that can utilize both the high and low photon energy to
drive two-step photoexcitation of electrons. In other words, gZIS has
the tendency to capture the short and long wavelength of light, fea-
turing its intrinsic band gap and the extrinsic defect sub-band. This
unique light-absorbing property is analogous to that of natural chlor-
ophyll, which absorbs the blue and red region of the light spectrum,
giving leaves their characteristic green color. Therefore, gZIS also
exists in a green hue as opposed to the customary yellow color. The
optical band gaps (E
g
) of the samples are obtained from the Kubelka-
Munk function vs. the incident photon energy plots as elucidated in
Fig. 6b49. It is found that both ZIS and gZIS possess visible-light-active
E
g
of 2.31 and 2.63eV, respectively. In addition, the energy levelof the
band tail in gZIS, or commonly known as Urbach’s tail, is calculated
using the Urbach equation to locate the position of defect state (see
details in Supplementary Fig. 10a)50.Theinverseslopofthelinearized
Urbach equation dictates an Urbach energy (E
u
)of0.34eVfromthe
CB, representing the location of defect sub-band as a shallowtrap state
near to the CB of gZIS. Besides, transition energy (E
t
)evaluationcould
also serve as a technique to represent the intraband state, whereby
the numerical value of E
t
could be obtained via extrapolation of the
Tauc plot to the x-axis51. Concordant with the E
u
evaluation, E
t
suggests
the exact same level of defect state which is situated well below the CB
of gZIS (see Supplementary Fig. 10b). In short, the presence of S
v
-
induced defect sub-band is verified by the PL spectra, E
u
calculation
and E
t
computation (see Supplementary Fig. 11), signifying the cap-
ability of gZIS to harvest the high energy photon to promote photo-
excitation of electron from the ground state, and concomitantly
provide an alternate lower energy excitation route to facilitate sec-
ondary excitation of electron by utilizing the long wavelength elec-
tromagnetic radiation.
With the aim of acquiring fundamental understanding of the
electrochemical properties of the samples, Mott-Schottky (MS) mea-
surements were performed as presented in Fig. 6c. The positive slopes
observed in the samples validate the n-type behaviors, as is usually
reported52. Furthermore, it is unequivocally presented that the gra-
dient of gZIS is lower than that of ZIS, indicating an elevated con-
centration of donor charge carrier (N
D
) of gZIS based on the inverse-
proportionality relationship between gradient and charge density.
Specifically, the MS gradient and charge density are related following
the equation of ND=2
qεε01
gradient, whereby the governing constants q,
ε,andε
0
represent the elementary charge constant, material dielectric
constant and vacuum permittivity, respectively53. Upon utilizing the
aforementioned equation, it is intriguing to observe that the N
D
of
gZIS (8.23 × 1021 cm−3) exceeds that of ZIS (6.94 × 1021 cm−3), attributed
Fig. 5 | Photoelectrochemical and charge transfer characteristics. a Transient photocurrent responses, (b) EIS Nyquist plot with the equivalent Randle circuit, (c)
steady-state PL emission spectra and (d) transient TRPL decay spectra of ZIS and gZIS.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 7
Content courtesy of Springer Nature, terms of use apply. Rights reserved
to the favorable S
v
-induced charge redistribution and electron delo-
calization. The mechanism is capable of generating a denser amount of
charge carriers that can contribute in elevating photocatalytic per-
formances. On top of that, the valence band edge (E
VB
)ofthesamples
was evaluated via ultraviolet photoelectron spectroscopy (UPS) ana-
lysis. As shown in Supplementary Fig. 12, ZIS and gZIS possess E
VB
of
1.34 and 1.70 V vs. NHE at pH 7, respectively. The conduction band
edge (E
CB
) of the samples can be further evaluated through the
expression of E
CB
=E
VB
–E
g
, in conjunction with the values obtained.
Leveraging all the antecedently acquired outcomes, the proposed
band structures of the samples are illustrated in Fig. 6d. As depicted,
the pristine ZIScould utilizeonly the photon energy equal to or greater
than its E
g
for photogeneration of electrons and holes to drive the
respective photocatalytic reactions at the CB and VB. On the contrary,
gZIS could exploit both the high and low energy light spectrum to
drive the photocatalytic reaction, by primarily harvesting high-energy
photons to initiate the photoexcitation of ground-state electrons
directly to the CB for the photoreductive reaction. Concurrently, gZIS
provides an alternative lower energy channel through absorption of
medium-long wavelength photons to excite electrons from VB to CB
via the defect state. Furthermore, the presence of the defect state will
trap energy-lost electrons from the CB, thereby preventing them from
returning to the ground state and recombining with photogenerated
holes. The temporarily captured electrons in the defect state could
undergo secondary photonic excitation back to the CB to drive the
photoreduction reaction by absorbing low-energy light illumination,
signifying the benefits of the defect state in utilizing solar energy to its
full advantage to generate charge carriers. DFT calculations were
performed to elucidate the electronic potential of S
v
-induced defect
state onto the structure. In agreement with the experimental findings,
the density of state (DOS) computations in Fig. 6eunveilsthepresence
of shallow trap state near to the CB of sulfur-vacant gZIS
T
.TheDOS
profiles alsodemonstratethe enhancement of electron density around
VB of gZIS
T
, which implies a greater proportion of readily available
ground state electrons to be photoexcited for photocatalytic HER.
Expanding the calculation from the DOS, S 3p band center (ε
p
)could
be evaluated as shown in Fig. 6f, whereby D
S3p
(E) denotes the energy-
dependent DOS projected onto the p orbitals of S element and E
f
dictates the Fermi level of the system (conventionally set to 0eV). The
theoretical ε
p
of gZIS
T
(−3.483 eV) is discovered to positively shift
and approach the E
f
as compared to that of ZIS
T
(−4.998 eV). In
accordance with the p-band center correlation, ε
p
governs the surface
activity, i.e., a smaller deviation of ε
p
from E
f
indicates a greater elec-
tron accumulation around the species with a stronger adsorption
capability54–56. Moreover, the gZIS possesses a reduction in work
function with the introduction of S
v
,whichisbeneficial for surficial
electron transfer (Supplementary Figs. 13 and 14). These empirical
findings collectively suggest a higher charge accumulation around
active S sites to facilitate H* adsorption and promote photoelectron
transfer for augmented HER.
Water splitting mechanisms and performance
To unravel a more comprehensive insight towards HER, H* adsorption
Gibbs free energy ΔGH*
was evaluated (see details under Supple-
mentary Fig. 15). According to the Sabatier principle, an ideal HER
photocatalyst should compromise both the adsorption and deso-
rption kinetic barriers in expediting electron transfer to the bonded H*
as well as spontaneous release of generated H
2
from the surface,
indicated by a thermoneutral value of ΔGH*57,58.Analogizingthemost
active (110) facet of the structure, gZIS
T
endows a more favorable
theoretical HER kinetics than pristine ZIS
T
as shown by the closer-to-
zero ΔGH*in Fig. 7a, which favors the adsorptive reduction of H+to
form H
2
through intimate electron transfer. Credited to the S
v
-induced
charge redistribution around S atoms, the inert basal plane is activated
to be more propitious towards H* adsorption on the surface with a
lower HER barrier. Interestingly, the unoccupied defect location (S
v
)
also allows an additional binding of H* for catalyzing the reductive
generation of H
2
as shown in Supplementary Fig. 15d. In brief, the
introduction of S
v
into the ZIS structure not only diminishes the
adsorption-desorption barriers at the intrinsically active (110) site to
better drive the HER, but also simultaneously triggers the catalytic
activity of inert basal planes as to provide additional reactive HER
centers. DFT calculations were extended to elucidate the effect of S
v
toward interfacial O
2
evolution reaction (OER) mechanisms (refer
Supplementary Figs. 16 and 17 fordetails).As summarized in Fig. 7b, it
could be observed that the basal (110) plane of pristine ZIS
T
does not
Fig. 6 | Optoelectronic properties and band structure. a UV-Vis diffuse reflec-
tance spectra with inset showing the actual color of the samples, (b) KM function
for band gap determination, and (c) MS plot for ZIS and gZIS. dSchematic of the
electronic band structures of ZIS and gZIS with light absorption properties and
photogeneration electron-holes pair formation mechanisms. eTheoretical calcu-
lated DOS and (f) respective ε
p
for ZIS
T
and gZIS
T
.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 8
Content courtesy of Springer Nature, terms of use apply. Rights reserved
favor OER with a large overpotential (ηOER )of5.27eVintheprocessof
O
2
production from HOO*. Conversely, the unsaturated Zn atom of
gZIS
T
is found to be more conducive towards OER with a generally
diminished energy barrier. Despite the O
2
disengagement process
persisting as the rate-determining step (RDS), the ηOER is incredibly
reduced to 2.03 eV which signifies the advantage of S
v
towards
decreasing OER energy barriers, facilitating the H
2
Ooxidationand
simultaneously escalating HER by providing more H+from the
deprotonation of H
2
O. Magnificently, the deprotonation process of
H
2
O on the unsaturated Zn to form HO* intermediate is barrierless for
gZIS
T
(low negative value of −0.65 eV), representing a thermo-
dynamically favorable process. Thus, the H
2
Ooxidationprocessis
capable of competing with self-oxidation of sulfide from high oxida-
tion potential of photogenerated holes, which in turn impeding pho-
tocorrosion of sulfide and catalyzing stable OER process59,60.Besides,
the defect location induced by S
v
formerly found to be capable in
activating H
2
Omolecule(seeFig.4g) could also provide an alternative
pathway for OER with an ηOER (3.68 eV) lower than that of pristine ZIS
T
.
In order to scrutinize the catalytic performance of ZIS and gZIS, pho-
tocatalytic half-reactions (HER and OER) were primarily performed.
Under Na
2
S/Na
2
SO
3
sacrificial conditions as shown in Fig. 7c, pristine
ZIS presented a H
2
evolution of 338.33 μmol∙g−1undersix-hourcon-
tinuous light radiation. Remarkably, gZIS exhibited distinctive per-
formance enhancement with six-hour continuous H
2
evolution of
2036.41 μmol∙g−1, that is more than 6-fold performance than the pris-
tine counterpart. A controlled experiment was performed to validate
that H
2
generation was indeed driven by the reduction of H+by the
photogenerated electrons in the CB (see details in Supplementary
Fig. 18). Briefly, additional NaIO
3
was introduced into the system to
scavenge electrons as to inhibit the photoreductive production of H
2
.
It was evidently reflected that nil H
2
couldbeobservedunderNa
2
S/
Na
2
SO
3
+ NaIO
3
sacrificial conditions, confirming the generation of H
2
was driven by photoelectrons in the CB of gZIS whereby the S2−/SO
3
2−is
irreversibly oxidized by holes to form S
2
O
3
2−/SO
4
2−(ref. 61). Following
that, pristine ZIS did not exhibit any O
2
evolution under NaIO
3
sacri-
ficial conditions, whereas gZIS demonstrated a significant O
2
produc-
tion of 1239.92 μmol∙g−1under 6-h of irradiation. Extending from
the capability of gZIS catalyzing both the HER and OER process,
solar-driven overall pure water splitting experiment was conducted
without any sacrificial reagent. The gZIS employed the competence to
drive photocatalytic pure water splitting with H
2
and O
2
yield of 36.04
and 18.96 μmol∙g−1∙h−1, respectively (close to a stoichiometric ratio of
2:1), while pristine ZIS did not deliver any appreciable yield. Additional
controlled experiments were also performed in the dark and without
photocatalyst to eliminate the potential false-positive observation
from the background and photolysis of water. The controlled experi-
ments did not present any H
2
evolution (Supplementary Fig. 19).
Moreover, there is a consistency trend between the optical absorption
property of gZIS with its respective AQY at different monochromatic
wavelengths (Supplementary Fig. 20), indicating the H
2
is in fact gen-
erated via photon utilization of gZIS in solar-driven water splitting.
The stability of gZIS in photocatalytic pure water splitting was also
examined as shown inFig. 7e. The gZIS retained 99.6% performance for
a full daytime irradiation (12-hour equivalence) and still possessed
more than 90% performance after one full solar day (24-h) reaction,
with negligible changes in post-reaction characterizations (Supple-
mentary Figs. 21–23). This fascinating observation suggested gZIS
refraining from the serious photocorrosion issues which is commonly
experienced by other sulfide-based catalysts. On one hand, this single-
component cocatalyst-free gZIS displays a remarkable H
2
half-reaction
AQY of 5.34% (420 nm) that is comparable to other noble-metal
loaded and complex ZIS-based heterostructure systems as reported in
Supplementary Table 4. On the other hand, the ability of this self-
activated and stable gZIS without introduction of heteroatom nor
noble metal to drive photocatalytic overall water splitting reaction
with AQY (0.17%, 420 nm) and STH (0.002%), can be on par with
the other modified and assisted sulfide-based photocatalysts
(Supplementary Table 5).
In summary, a unique superhydrophilic green gZIS was success-
fully constructed via an in-situ solvothermal strategy. In-depth
experimental investigations and theoretical computations conducted
in this study systematically unraveled the fundamental insights on the
critical roles of morphology transformation, surface modification, and
vacancy engineering. The efficient photocatalytic water splitting
activity of self-activated gZIS is attributed to the exclusive hollow
hierarchical framework, exposing more intrinsically active facet
Fig. 7 | Water splitting mechanism and performance. Gibbs free energy maps for
(a)HERand(b)OERforZIS
T
and gZIS
T
.cPhotocatalyticHER and OER half-reaction
under different sacrificial conditions. dTime-dependent solar-driven overall water
splitting performance and (e) long-term photocatalytic stability performance of
gZIS. Error bars represent the standard deviation from two independent runs.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 9
Content courtesy of Springer Nature, terms of use apply. Rights reserved
and activating the inert basal plane, as well as the presence of super-
hydrophilic surface enhancing water interaction. These intriguing
occurrences allow gZIS to maximize the utilization of surface areas.
Additionally, the presence of S
v
within the structure propagates sig-
nificant charge redistributionand induces asymmetricdipole moment,
which consequently boosts the charge transfer, reduces the surface
HER and OER kinetic barriers. The existence of defect state in the
electronic band structure of gZIS further expands the optical absorp-
tion properties and mediates photoexcitation of electrons via alter-
native two-step process. Besides exhibiting more than 6-fold
enhancement in photocatalytic half-reaction of H
2
production than
conventional yellow pristine ZIS, this gZIS could also catalyze solar-
driven overall water splitting reaction with high stability, and perfor-
mance comparable to other complex sulfide-based photocatalysts.
This self-activated high activity single-component noble-metal-free
gZIS contains high value of exploration and could open up a brand-
new design opportunity. It is believed that this could encourage the
generation of novel ideas toconceive and devise a highly efficientgZIS-
based photocatalyst to sustainably drive large-scale green H
2
pro-
duction for achieving a carbon-neutral future.
Methods
Materials
Analytical grade reagents were used directly without any purification.
Zinc chloride (ZnCl
2
, Merck, ≥98%), indium (III) chloride tetrahydrate
(InCl
3
∙4H
2
O, Sigma Aldrich, ≥97%), thioacetamide (C
2
H
5
NS, Nacalai
Tesque, ≥99%), ethylene glycol (C
2
H
6
O
2
,SigmaAldrich,≥99%), ethanol
(C
2
H
5
OH, Fisher Scientific, ≥96%). Deionized water (DIwater, resistivity
≥18 MΩ∙cm) used in this experiment was obtained from Millipore Milli-
Qwaterpurification system.
Synthesis of pristine and hollow ZnIn
2
S
4
microsphere
Pristine ZnIn
2
S
4
(ZIS) was synthesized via one-step hydrothermal
method, where stoichiometric ratio of 0.5 mmol ZnCl
2
, 1.0 mmol
InCl
3
∙4H
2
Oand2.0mmolC
2
H
5
NS were dissolved homogeneously in
30 mL DI water. The solution was transferred into a Teflon vessel held
in a stainless-steel autoclave maintained at 160 °C for 12 h. After cool-
ing to room temperature, the solution was subjected to thorough
washing with ethanol and DI water to completely remove any
unreacted precursor.Yellow ZIS powder was obtained upon overnight
freeze drying. A similar process was used to obtain green hollow
ZnIn
2
S
4
(gZIS) powder by replacing DI water with ethylene glycol (EG)
in a solvothermal synthesis process.
Materials characterizations
The surface morphology and elemental composition of the sam-
ples were analyzed by field emission scanning electron micro-
scopy (FESEM) using the Hitachi SU8010 microscope equipped
with an energy-dispersive X-ray (EDX). Transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM) imaging
were taken using the JEOL, JEM-2100 F microscope. Atomic-
resolution spherical aberration-corrected bright field scanning
TEM (BF-STEM) imaging with fast Fourier transformation (FFT)
was obtained from the Hitachi HD-2700. The crystallographic
properties and information of the samples were obtained via
X-ray diffraction (XRD) analysis by utilizing the Bruker D8 Dis-
covery X-ray diffractometer with an Ni-filtered Cu Kαradiation.
X-ray photoelectron spectroscopy (XPS) analysis of surface che-
mical states were obtained using the Thermo Fisher Scientific
Nexsa G2 XPS with monochromatic Al-Kα(hν=1486.6eV) X-ray
source. The binding energies were referenced to adventitious
carbon signal (C 1 s peak) at 284.6 eV prior to peak deconvolution.
Ultraviolet photoelectron spectroscopy (UPS) analysis was per-
formed using the Thermo Fisher Scientific Nexsa G2 surface
analysis system by using vacuum UV radiation for induction of
photoelectric effects. The photon emission possessed an energy
of 21.22 eV through He I excitation. The contact potential differ-
ences of the materials were obtained through Kelvin probe force
microscopy (KPFM) using Bruker Multimode 8 atomic force
microscope (AFM) electric mode. The sample powders were
evenly spray-coated on fluorine-doped tin oxide (FTO) glass and
mounted onto AFM sample stage with silver paste to ensure
uninterrupted electrical connection. The surface area information
of the samples was obtained from the multipoint Brunauer-
Emmett-Teller (BET) N
2
adsorption-desorption isotherm at 77 K
using the Micrometrics ASAP 2020. The samples were subjected
to degassing at 150 °C for 8 h to remove any adsorbed species
prior to the analysis. Surface wettability test and water contact
angle measurement were conducted using the Ramè-hart Co
Model 250 goniometer. In this context, 3 μL droplets of DI water
was adapted as working medium to drop onto sample-coated FTO
glass slide to perform contact angle analysis with triplicate mea-
surement data collected. The electron paramagnetic resonance
(EPR) measurements were performed at room temperature using
a spectrometer (JEOL, JES-FA200). Ultraviolet-visible (UV-Vis)
diffused reflectance spectra of the samples were obtained from
the Agilent Cary 100 UV-Vis spectrophotometer equipped with an
integrated sphere and BaSO
4
as reflectance standard. The optical
band gap was obtained from the Kulbeka-Munk relationship.
Steady-state photoluminescence (PL) spectra was acquired from
the Perkin Elmer LS55 fluorescent spectrometer. Time-resolved PL
(TRPL) spectra was recorded using the DeltaPro Fluorescence
lifetime system (Horiba Scientific)withanexcitationwavelength
of 317 nm.
Photoelectrochemical analysis
Photoelectrochemical (PEC) measurements including transient pho-
tocurrent response, electrochemical impedance spectroscopy (EIS)
and Mott-Schottky plots were conducted using Metrohm Autolab
electrochemical workstation. A conventional three-electrode PEC
setup was adapted with 0.5 M Na
2
SO
4
(pH = 7) as the electrolyte solu-
tion. Platinum (Pt) served as the counter electrode whereby Ag/AgCl
saturated with 3.0 M KCl was utilized as the reference electrode. The
working electrode was prepared by uniformly coating the sample onto
FTO glass substrate with an active square area of 1 cm by 1 cm. The
working electrode was illuminated by a 500 W Xe arc lamp with a fixed
sample-to-lamp distance of 10 cm during the PEC analysis. A potential
of +0.2 V was applied for the transient photocurrent and the working
electrode was exposed to the light source at an intermittent light on-off
rate of 20 s interval. Subsequently, EIS measurements were performed
across a frequency range from 10 mHz to 100 kHz, with an equivalent
Randle circuit was fitted according to the obtained Nyquist plot. Lastly,
the Mott-Schottky plots were measured in the range from –1.0 to 0.8 V
vs. Ag/AgCl with a potential step of 50 mV at a frequency of 1kHz. For
standardization, normal hydrogen electrode (NHE) scale at pH 7 was
adapted as in ENHE =E
Ag=AgCl +0:059pH + 0:1967V61.
Photocatalytic hydrogen and oxygen evolution half-reaction
In photocatalytic H
2
evolution half-reaction, 30 mg of photocatalyst
was dispersed homogenously in a 60 mL aqueous solution containing
0.35 M Na
2
S/Na
2
SO
3
. The solution was then transferred into a Pyrex
top-irradiated vessel with quartz window. The outlet of the vessel was
connected to the Agilent 7820 A gas chromatography (Ar carrier gas)
for gas measurement at hourly sampling interval. Prior to photo-
catalytic performance analysis, the system was purged with a high
flowrate of N
2
gas for at least half an hour. The reactor was illuminated
using 500 W Xe arc lamp with AM1.5 filter (c.a. 1 Sun illumination)
during the reaction. Photocatalytic O
2
evolution half-reaction was
carried out under the same conditions except that 0.1M NaIO
3
was
adopted as the sacrificial reagent.
Article https://doi.org/10.1038/s41467-023-43331-x
Nature Communications | (2023) 14:7676 10
Content courtesy of Springer Nature, terms of use apply. Rights reserved
Photocatalytic overall water-splitting reaction
Photocatalytic overall water splitting reaction was carried out under
the exact same condition as photocatalytic hydrogen and oxygen
evolution half-reaction as described, except that the solution consists
of pure DI water without the presence of any sacrificial reagent.
Besides, the apparent quantum yield (AQY) was evaluated under dif-
ferent monochromatic light under various band pass filters (355, 420
and 500 nm) following the equation:
AQY %ðÞ=2NH2
Np
×100%= 2rH2NAhc
IStλ×100% ð1Þ
in which NH2= total number of H
2
molecules evolved, Np=total
number of incident photons, rH2=amountofH
2
molecule generated at
time t (in mol), N
A
= Avogadro constant, h = Planck constant, c = speed
of light, I = light intensity, S = irradiation area and λ= wavelength of
monochromatic light. Following that, solar-to-hydrogen (STH) con-
version efficiency was determined at 1 Sun illumination with AM 1.5
filter in concordance to the equation:
STH %ðÞ=RH2ΔGr
PSunS×100% ð2Þ
whereby RH2=rateofH
2
evolution (in mol∙s−1), ΔGr= Gibbs free energy
change of water splitting reaction and P
sun
= energy flux of the
incident ray.
Computational details
Density functional theory (DFT) computations were conducted using
Vienna Ab initio simulation package (VASP)62. Exchange-correlation
potential was described using generalized gradient approximation
(GGA) with Perdew-Burke-Ernzerhof (PBE) parameterization63.500eV
energy cut-off, 1 × 10−5energy convergence and 0.01 eV∙Å−1force
converged wereadapted as plane wave basis settings in this study. The
Monkhorst-Pack k-point mesh was set at 3 × 3 × 1. A theoretical 2 × 2
bilayer unit cell of pristine ZIS (ZIS
T
) and sulfur-vacant gZIS (gZIS
T
)
structures were modeled by removing different intrinsic S atoms from
the framework. All the atoms were allowed to relax with an additional
15 Å vacuum layer added perpendicular to the surface to eliminate any
potential periodic image interaction. Hybrid functional Heyd-Scuseria-
Ernzerhof (HSE06) was employed in the density of state and band edge
evaluations64. For H
2
adsorption and water interaction study, 4-by-4
supercell was adopted as the substrate to neglect any interactions of
adsorbates with adjacent unit cells. Grimmer’s DFT-D3 method was
accounted as additional van der Waals (vdW) correction for higher
accuracy in computing the interatomic forces, stress tensor and
potential energy65. Reaction Gibbs free energy (ΔGr) calculations for
the HER and OER processes were evaluated utilizing:
ΔGr=ΔEr+ΔZPE TΔSðÞ
reU ð3Þ
wherein ΔEris the reaction adsorption energy, ΔZPE is the zero-point
energy correction factor, TΔS is the temperature dependent entropy
contribution and eU is the external bias accounting the elementary
proton-coupled transfer step.
Data availability
The data supporting the findings of this study are available within the
article and its Supplementary Information. The sourcedata is available
from the corresponding author upon reasonable request.
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Acknowledgements
This research project was funded by the Malaysia Research University
Network (MRUN) from the Ministry of Higher Education Malaysia (Grant
No. 304/PJKIMIA/656501/K145) and MUM-ASEAN Research Grant
Scheme (Ref. No. ASE-000010) from Monash University Malaysia. This
work was also supported by the High Impact Research Support Fund
(HIRSF) (Ref. No. REU00354) and Advanced Computing Platform (APC)
from Monash University Malaysia. We thank Hong Yuan Tok from Hi-Tech
Instruments Sdn. Bhd. for the spherical aberration-corrected BF-STEM
measurements.
Author contributions
W.-K.C. carried out the sample synthesis, characterization, and theore-
tical calculations as well as wrote the paper. B.-J.N., Y.J.L., L.-L.T., L.K.P.,
and S.-P.C. discussed and validated the experimental and theoretical
results. J.L. performed EPR analysis and validated the results. B.-J.N.,
L.-L.T., A.R.M., and S.-P.C. supervised the project. All authors con-
tributed to the overall scientific interpretation and revised this paper.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains
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Correspondence and requests for materials should be addressed to
Siang-Piao Chai.
Peer review information Nature Communications thanks Juncheng Hu
and the other, anonymous, reviewer(s) for their contribution to the peer
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