One-step hydrothermal synthesis of Fe single atom doped 1T-MoS2 nanosheets for high-performance seawater hydrogen production

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## Geng et al. Energy Mater. 2025, 5, 500009 Energy Materials

DOI: 10.20517/energymater.2024.46

Article Open Access

# One-step hydrothermal synthesis of Fe single atom doped 1T-MoS2 nanosheets for high-performance seawater hydrogen production

Xun Geng1, Yitong Cao1, Zhixuan Li1, Mengyao Li2, Chaojie Cao1, Chenxi Yu1, Xueze Chu1, Long Hu2,

Yunlong Sun2, Liang Qiao3, Xiaojiang Yu4, Mark B. H. Breese4, Jang Mee Lee1, Danyang Wang2, Dewei

Xun Geng

2, Liang Qiao3, Xiaojiang Yu4, Mark B. H. Breese4, Jang Mee Lee1, Danyang Wang2, Dewei

Chu

2, Jiabao Yi1,*

1Global Innovative Centre for Advanced Nanomaterials, School of Engineering, University Drive, The University of Newcastle,
Callaghan, NSW 2308, Australia.
2School of Material Science and Engineering, The University of New South Wales (UNSW), Sydney 2033, Australia.
3School of Physics, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China.
4Singapore Synchrotron Light Source, National University of Singapore, Singapore 117603, Singapore.

- Correspondence to: Prof. Jiabao Yi, Global Innovative Centre for Advanced Nanomaterials, School of Engineering, University

Drive, The University of Newcastle, University Drive, Callaghan, NSW 2308, Australia. E-mail: Jiabao.yi@newcastle.edu.au

How to cite this article: Geng, X.; Cao, Y.; Li, Z.; Li, M.; Cao, C.; Yu, C.; Chu, X.; Hu, L.; Sun, Y.; Qiao, L.; Yu, X.; Breese, M. B. H.;
Lee, J. M.; Wang, D.; Chu, D.; Yi, J. One-step hydrothermal synthesis of Fe single atom doped 1T-MoS2 nanosheets for highperformance seawater hydrogen production. _Energy Mater._ 2025, _5_ [, 500009. https://dx.doi.org/10.20517/energymater.2024.](https://dx.doi.org/10.20517/energymater.2024.46)

[46](https://dx.doi.org/10.20517/energymater.2024.46)

Received: 13 May 2024 First Decision: 11 Jun 2024 Revised: 13 Aug 2024 Accepted: 21 Oct 2024 Published: 7 Jan 2025

Academic Editor: Sining Yun Copy Editor: Ping Zhang Production Editor: Ping Zhang

Abstract

In the pursuit of sustainable and clean energy sources, the development of efficient electrocatalysts for hydrogen
evolution reaction has gained significant attention. In this work, we synthesized single-atom Fe-doped 1T MoS2
(sFe-1T/MoS2) nanosheets using a one-step hydrothermal method, harnessing the synergistic effects of ironintercalation to enhance hydrogen production through an abundance of active sites. Notably, 10 _at._ % sFe-1T/MoS2
exhibited excellent hydrogen evolution reaction performance with a low onset potential of 190 mV and a Tafel

slope of 55 mV/dec in acidic solution. High performance was also achieved in alkaline solutions. Additionally, these

catalysts demonstrated excellent efficiency in seawater splitting. This work not only offers a cost-effective and

scalable method for producing high-quality electrocatalysts but also sets a precedent for the application of this

technology across various catalytic systems, marking a significant advancement in clean energy research.

Keywords: MoS2, hydrogen production, single-atoms, seawater splitting

© The Author(s) 2025. Open Access This article is licensed under a Creative Commons Attribution 4.0
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Page 2 of 14 Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46

## INTRODUCTION

The search for clean energy sources remains the most critical challenge of our time, especially in light of
serious consequences induced by greenhouse gas emissions [1] . Hydrogen gas is widely recognized as the
cleanest alternative to fossil fuels, as its combustion only produces water. Traditional methods to produce
hydrogen are not environmentally friendly due to significant CO2 emissions. Catalysis is promising to
achieve the production of hydrogen with minimal or no greenhouse emissions utilizing processes such as
photocatalysis, electrocatalysis, photothermal catalysis, _etc_ . [2-5] . Among these, electrocatalysis, a process that
produces hydrogen from water using catalysts with a small amount of energy input, has shown great
potential for hydrogen clean energy. However, achieving efficient and cost-effective hydrogen production
remains challenging, as the expensive Pt catalyst is still the best catalyst due to its outstanding watersplitting efficiency during electrocatalysis (Tafel slope: 30 mV/dec). However, its high cost and scarcity limit
its wide applications. Efforts have been made to search for other catalysts as the working electrodes for
efficient water splitting, such as graphene, MXene, porous carbons, molybdenum disulfide (MoS2), _etc_ . [6,7] .
Among them, MoS2 has gained considerable attention due to its high conductivity, low synthesis cost and
ultra stability during electrocatalysis. MoS2 features a stable semiconductive phase (2H) and a metastable
metallic phase (1T) [8,9] . The 2H phase has limited active sites, making the water splitting less favorable [10] .
Specially, the Tafel reaction in water splitting must overcome a high activation energy of 0.98 eV to initiate
the Heyrovsky reaction on 2H-MoS2 basal plane, while 1T phase has a much lower energy barrier of 0.16
eV, followed by a smaller Heyrovsky reaction activation energy (0.62 eV) due to the vibration (S-H edge) at
sulfur site in the 1T phase, resulting in a higher hydrogen generation rate on the basal edge of 1T MoS2[11-14].

However, 1T phase MoS2 is a metastable phase and achieving long term stable hydrogen production is still
challenging. Moreover, its water-splitting performance still has a distance to Pt metals. Therefore, further
research is necessary to enhance the performance of 1T-phase MoS2 by advanced material strategies, such as
doping, intercalation, surface modification, _etc_ . [3] .

Strain engineering has emerged as an effective strategy for stabilizing metastable structures. Our previous
work successfully stabilized 1T MoS2 with hydrazine intercalation. The as-synthesized 1T dominant MoS2
nanosheets exhibit significantly improved hydrogen evolution reaction (HER) performance (64 mV/dec)
with increased active sites, which makes 1T-MoS2 a bright future for highly efficient water production [15] .
Doping, another widely used efficient strategy, introduces multiple lattice strains and defect centers through
surface modification. Single-atom doping in electrocatalysis has shown a promising future in terms of
catalytic activity, selectivity and stability due to the increased active sites on the catalyst surface [16] . By
selecting the appropriate dopants, the electronic properties of the host material can be tailored to optimize
catalytic performance. Transition metal featuring a high _d_ -orbital status shows a similar effect to Pt on
electrocatalysis [17] [, especially Ni, Cu, V and W, as reported in previous millstones [Supplementary Table 1],](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
showing advanced water splitting efficiency. For example, by doping 5% V, the doped 1T-MoS2 exhibited a
low onset potential of 102 mV and a Tafel slope of 54 mV/dec [7] . The results indicated that doping with
transition metals in MoS2 could effectively modulate the energy level, which decreases the energy barrier for
electron transfer and consequently reduces the overpotential. The performance and stability of 1T-MoS2 can
also be improved by intercalating single atoms into its layers. For example, a high concentration of single
atoms of copper (98 _at_ .%) was successfully intercalated into the layers of 1T MoS2 by simple stirring. The
synthesized Cu-1T MoS2 optimal sample showed a low onset potential of 104 mV and a small Tafel slope of
52 mV/dec with a high stability and the 1T phase can be maintained up to 800 o C. The enhanced catalytic
performance was attributed to improved electrical conductivity, enhanced active sites, and modified
electronic structure with weakened H-S bond [18] . Another example includes the work done by Zhao _et al_ .,
who incorporated FeS2 clusters with MoS2, demonstrating a high HER performance with a Tafel slope of 82
mVdec -1 and onset potential of 136 mV at a current density of 10 mAcm -2[19] .

Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46 Page 3 of 14

The advancement of electrocatalysts for HER in seawater splitting represents a significant leap toward
sustainable hydrogen production. The high efficiency achieved through active ion concentration
manipulation underscores the potential for scaling this methodology for industrial applications. Fe is one of
the promising transition metals for efficient water splitting due to the S2p-Fe3d interaction [20] . In this work, we
develop a single-atom system of Fe-doped 1T MoS2 (sFe-1T/MoS2) nanosheets, a material that has recently
gained attention for its promising hydrogen electrocatalysis due to the S2p-Fe3d interaction. Notably, the
synthesized sFe-1T/MoS2 nanosheets exhibit a unique structural evolution from single-atom intercalation to
site-replacement doping, a phenomenon not extensively documented in previous studies. This structural
transformation by doping contributes to a remarkable electrochemical performance, particularly for the 10
_at._ % sFe-1T/MoS2, which achieves an onset potential of 190 mV and a Tafel slope of 55 mV/dec in acidic
solutions. In alkaline solutions, the material displays an extraordinarily low onset potential of 80 mV
alongside a slightly higher Tafel slope compared to that in acidic solutions. This dual-performance
characteristic in varying pH environments highlights the material's versatility and robustness. The
significant enhancement in catalytic activity is attributed to the morphological effects that induced more
active sites due to the change in the coordination environment, as confirmed through advanced Near Edge
X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Moreover, the synthesized sFe-1T/MoS2
demonstrates exceptional performance in ocean water splitting, an area of growing interest due to the
increasing focus on sustainable and scalable hydrogen production methods. This research provides a novel,
facile, and cost-effective strategy for developing highly efficient electrocatalysts, which could revolutionize
the field of sustainable energy. Our work not only underscores the potential of sFe-1T/MoS2 in various
electrochemical applications but also paves the way for further exploration of single-atom doped catalysts.
The insights gained from this study could inspire the development of next-generation catalytic systems with
enhanced performance and durability.

## EXPERIMENTAL SECTION

Materials
Fe-doped MoS2 was synthesized by a hydrothermal reaction. All those chemicals were purchased from
Sigma-Aldrich without any further purification. Ammonium heptamolybdate [(NH4)6Mo7O24 · 4H2O] was
used as the Mo source ( _s_ Mo) since it matches the microstructure of 1T-MoS2 because of its fine Mo-centre
octahedral structure. Fe(NO3)3 ∙ 9H2O was chosen as the Fe-dopant and the doping concentration varied in
a ratio of 2 _at._ %, 5 _at._ %, 10 _at._ % and 20 _at._ %, respectively. First, 10 mmol sulfur powders and 5 mmol _s_ Mo
were mixed and fully dissolved in 10 ml hydrazine monohydrate (N2H4 · H2O) via 20 min ultrasonication.
The Fe-dopant was dissolved in 50 ml of deionized (DI) water in another beaker. Then, those two
precursors were mixed in 80 mL stainless steel autoclaves for 48 h at 180 o C. The impurities in the product
were fully washed using 0.5 M H2SO4 and DI water, accompanied by 5000 rpm centrifugation for 15 min.
Then, the wet sample was transferred to a vacuum oven for drying for 48 h at room temperature. The
pristine 1T-MoS2 is marked as S0 as the reference compared to the Fe-doped MoS2 labeled as S2 (2 _at._ %), S5
(5 _at._ %), S10 (10 _at._ %) and S20 (20 _at._ %), respectively. The acidic corrosion to iron-doped MoS2 was also
checked by acidic treatment (2M HCL wash). The prepared samples (800 mg) were put in 50 ml 2M HCL
solution with 5 mins stirring. The washed samples were dried in the same way as the sample preparation.

Characterization
The surface morphology was investigated by a JEOL scanning electron microscope (JEOL-SEM) at room
temperature. The crystal structure of sFe-1T/MoS2 was investigated by a JEOL F200 Transmission Electron
Microscope (F200-TEM) at 200 kV voltage. X-ray diffraction analysis (XRD) was carried out by a
Panalytical Empyrean instrument with λ = 1.54 Å (Cu _Kα_ ) that generates at 40 kV voltage and 40 mA
current. The sample was loaded on a circular holder and the angle of measurements is within a 2θ scan
range of 5° to 60°. The scan step size is 0.006° with a time-per-step of 97.92s. Raman measurement was

Page 4 of 14 Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46

carried out on a Witec alpha-300 spectrometer. A 532 nm laser source was used as the excitation source for
1 h measurement under 0.1% laser power. The textural Brunauer-Emmett-Teller (BET) surface area was
determined by the N2 adsorption-desorption analysis conducted on the Micromeritics ASAP 2420
instrument. The samples were degassed under a constant vacuum at 200 °C before analysis. The degassed
samples were analyzed at the liquid nitrogen temperature (-196 °C). The BET method was used to calculate
the total specific surface area. Total pore volume was obtained at a relative pressure (P/P0) of 0.99. The
micropore characteristics, including the micropore surface area and the micropore volume, were obtained
using the t-plot method. The coordination information was also obtained by the low-energy (soft) and highenergy (hard) near-edge X-ray absorption fine structure (NEXAFS) analysis in a total fluorescence yield
(TFY) mode at the Synchrotron of Singapore University and the Australia Synchrotron, respectively. X-ray
Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) were measured by the
SPECS Phobis 100 in Chengdu, China. The UPS is performed in He (I) spectrum. All measurements above
were taken at room temperature.

Measurements
The electrochemical measurement was carried out using a three-electrode system on CHI 760E
electrochemical workstation. The working electrode is the as-deposit samples on glassy-carbon rod
electrode (sample loading amount ~ 40 µg·cm -2 ), Ag/AgCl (3.5 M KCl) and Pt metal was applied as the
reference electrode and counter electrode, respectively. 30 mL of 0.5M H2SO4 and 1M KOH were used
separately as acidic and alkaline electrolytes. Linear sweep voltammetry (LSV) and chronoamperometry
were applied with a scan rate of 5.0 mV s -1 . The measurements were performed from -0.1 V to -0.8 V ( _vs_ .
Ag/AgCl) in 0.5 M H2SO4 and from -0.9 V to -1.6 V ( _vs_ . Ag/AgCl) in 1.0 M KOH. The LSV performance
was also checked in the seawater environment which was picked from the eastern coastline of Newcastle
Beach, Australia. All the recorded potentials were then calibrated _vs_ . reversible hydrogen electrode (RHE)
based on the Nernst equation:

The electrochemical impedance spectroscopy (EIS) measurement was carried out at an overpotential of
-0.2 V _vs_ . RHE over a frequency range from 100 MHz to 10 Hz with an amplitude of 0.005 V. The durability
test was applied at the overpotential point of 10 mA cm -2 for 43200 seconds (12 h). The electrochemical
double-layer capacitance ( _C_ dl) was obtained by scanning cyclic voltammetry (CV) within the non-faradaic
range, which is equal to the slope of the functional summary between scan rates and average potential (0.2 V
_vs_ . RHE). Six increasing scan rates (5, 10, 25, 50, 75 and 100 mV/s) were chosen for this measurement.
Electrochemically active surface area (EASA) was calculated with the electrochemical double-layer
capacitance ( _C_ dl) by EASA = _C_ dl/ _C_ s, where _C_ s is standard specific capacitance (0.02 mF/cm 2 ) on carbon
electrode with a unit surface area.

## RESULTS AND DISCUSSION

The XRD patterns of sFe-1T/MoS2 are presented in Figure 1A. The broad peak at 9.5 o indicates the (002)
orientation of 1T/MoS2. Compared to the 2H domain peak at 17.5 o, the left-shift peak position indicates the
structure change from 2H hexagonal to 1H monolayers, as described in previous works [4,21,22] . The peak
position moves slightly to a lower angle when the dopant concentration increases, which indicates the
enlarged interlayer space caused by Fe doping. This trend is consistent with our previous work on the Cu
single-atom system and suggests the possible formation of single-atom Fe coordination. The impurity peaks
in sample S20 indicate the formation of FeSx clusters (surface aggregation), which is beyond the ideal
concentration of iron doping. Raman measurement was also employed to investigate the phase variation of

Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46 Page 5 of 14

Figure 1. The characterization of sFe-1T/MoS2. (A) XRD patterns; (B) Raman spectra; (C) High-resolution TEM image of sample S10;
(D) XANES curves of Fe _L_ 3-edge; (E) NEXAFS curves of Fe _K_ -edge with Fe foil reference; (F) Fourier-transform (FT) _k_ 2-weighted

(D) XANES curves of Fe _L_ 3-edge; (E) NEXAFS curves of Fe _K_ -edge with Fe foil reference; (F) Fourier-transform (FT) _k_ 2-weighted

NEXAFS spectra of sFe-1T/MoS2 with Fe foil reference; Wavelet transform (WT) of _k_ 2-weighted NEXAFS spectra for (G) S2, (H) S5, (I)

S10 and (J) S20 based on the mode of Morlet wavelet function (κ = 3; σ = 0.5) for the first coordination shells, where 1 ≤ (R+α) ≤ 3 Å
and 2 ≤ _k_ ≤ 10 Å-1. XRD: X-ray diffraction analysis; TEM: Transmission electron microscope; NEXAFS: Near edge X-ray absorption fine

structure; XANES: X-ray absorption near edge structure.

sFe-1T/MoS2. As displayed in Figure 1B, 1T- dominant bond vibration is confirmed by intense peaks at the
Raman shift of 147 and 324 cm -1 for 1T-phase J1 (out of plane) and J3 (in-plane) Mo-S phonon modes,
respectively [23] . As the dopant concentration increases, a leftward shift of the J1 peak (J1 → J1*) with a

respectively [23] . As the dopant concentration increases, a leftward shift of the J1 peak (J1 → J1*) with a

broadened peak area (S0 → S10) indicates changes in anisotropic symmetry due to Fe doping, while a J2
mode is also presented in sample S20. Generally, the J2 vibrational mode is related to in-plane Mo-S
displacements (defects). This indicates weak zigzag (S-Mo-S) displacements at low doping concentrations
supported by single-atom intercalation [24] . While the strong site replacement effect is observed at high
doping concentrations (S20) instead of layer intercalation. Additionally, peak splitting is also observed for
the in-plane J3 vibrational mode (J3 → J3* and J3 → J3’, respectively) and the J3 peak suddenly disappears in

- and J3 → J3’, respectively) and the J3 peak suddenly disappears in

Page 6 of 14 Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46

sample S20. The pronounced peak splitting highlights the twofold symmetric variation between single-atom
doping and site replacement doping and also indicates doping-induced phase variation from 1T to 2H [25] .
Furthermore, the peak at 407 cm -1 corresponds to the out-of-plane S-Mo-S opposite moving (A1g) of MoS2.
The peak intensity of A1g mode decreases without a noticeable peak position shift, suggesting a weakening of
out-of-plane S-Mo-S vibrations due to a formation of S-Fe-S bulks (phase aggregation), agreeing with the
SEM results described above. The absence of the E1g mode is primarily attributed to low laser density as a
laser-induced 1T to 2H phase transition can be triggered under high laser density conditions [26] . The deep
lattice pattern is further demonstrated by transmission electron microscopy (TEM) from sample S0 to
sample S10. The high-resolution TEM image of sample S10 is displayed in Figure 1C and that of others is
[demonstrated in Supplementary Figure 1. The apparent wrinkles indicate the fine sheet structure of sFe-](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
1T/MoS2 at low doping concentrations. Some Fe atoms are found between interlayers of MoS2 without
prominent Fe clusters, suggesting the formation of single atoms. In addition, there is a plane expansion
(002) in doped MoS2 1T-dominance, from 0.83 nm to 0.98 nm, when the doping concentration increases
from 0 to 10 _at_ .%. Those observations prove the Fe single-atoms intercalation between MoS2 layers,
consistent with our previous works on Cu intercalated 1T/MoS2[18].

To understand the electron environment of iron intercalation, the _L_ 3-edge orbital status of iron is explained
by low-energy (soft) NEXAFS measurement to determine the dynamic process of p-d dipole excitation, as
demonstrated in Figure 1D. The 2p → 3d electron transfer occurs between 708 eV and 710 eV without
obvious t2g-eg orbital splitting in all samples. Additionally, only an _L_ 3 p-d electron transfer peak is observed
at 709 eV, indicating the absence of strong crystal field splitting effect. This behavior is mainly attributed to
the low spin state (unoccupied eg dominant) of Fe 2+[27] . A slightly right shift of peak position (S2 → S10)
indicates a lower coordination environment and higher oxidization state since a lower valence band
position can be predicted [28] . However, there is no shift for sample S20, which is associated with the
structural aggregation mentioned above. The peak intensity increases and the peak width also broadens
symmetrically aligned with the rise in Fe concentrations (S2 → S10). Generally, the higher peak intensity
indicates a higher unoccupied density state (more empty orbitals) and lower covalency caused by higher
dopant concentration [29] . In contrast, the enlarged peak width indicates the expanded unoccupied orbitals
and enhanced orbital contributions caused by multi-coordination [30] . It is observed that there is no obvious
post-edge shift from samples S10 to S20, while a pre-edge shift is noticeable. This indicates the new subband
formed aligning with the transition from a low spin multiplet (unoccupied eg dominant) to high spin
multiplet (t2g/eg equal contribution) transition when the Fe concentration keeps increasing (new phase
generated) [31] . Fe _K_ -edge (1s → 4p dipole electron transition) is further checked by high energy (hard)
NEXAFS and the normalized adsorption edge is displayed in Figure 1E. Generally, the shift of edge position
is associated with the change of oxidization state, such as the transition from ferrous to ferric. All samples
show a higher edge position than the reference curve of Fe foil, which confirms the similar oxidization state
(Fe 2+ ) of iron in sFe-1T/MoS2. A pre-edge peak at 7,113 eV for sample S20 is observed compared to a flat
appearance in samples with lower doping concentrations. The pre-edge feature originates from a
quadrupole (1s to 3d) transition, indicating a 3d→4p geometric coordination hybridization from octahedral
to tetrahedral ( _T_ d) symmetry, accordingly [32] . It also has a rising edge level (7,115 ~ 7,120 eV) from S2 to S20,
while this edge lifting between 7,115 eV and 7,120 eV, in our case, is mainly affected by the pre-edge feature
aligning with the increased dopant concentration since there is no noticeable edge position shift to Fedoped samples. The feature peak at 7,120 eV in sample S20 indicates the bulk formation of FeS2. This is
[confirmed by SEM images [Supplementary Figure 2]. The nanoflower appearance (S0 → S10) indicates fine](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
nanosheet structure of MoS2, shrinking in dry conditions, while a bulk-like morphology is observed in
sample S20. This proves a sheet to bulk transition with increased doping concentration, which is widely
observed in other doping systems. The intensity of the adsorption peak at 7,127 eV decreases, accompanied
by an elevated peak intensity at 7,135 eV when dopant concentration grows to 5 _at._ %, which corresponds to

Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46 Page 7 of 14

the standard FeS to FeS2 transition [33] . A slightly decreased peak intensity from samples S5 to S20 is mainly
caused by orbital distortion, corresponding to the pre-edge changes. The distance of the first atom shell is
obtained by Fourier _k_ 2 -weighted transform ( _k_ 2 -FT), as shown in Figure 1F. Compared to the Fe-Fe
scattering path in Fe foil (2.2 Å), the absence of Fe–Fe interaction indicates the single-atom configuration
for sFe-1T/MoS2. The peak at approximately 1.7 Å is mainly attributed to the first shell Fe-S scattering path
coordinated at the S site between the basal planes [34] . In addition, there is a slight peak shifting to a shorter
scattering path to 1.5 Å with the increased Fe concentration where a decreased peak intensity of the Fe-S
distribution is aligned, which is associated with a reduced Fe-S coordination number and the oscillation
from octahedral to tetrahedral coordination environment [35] . The reliability of Fourier transform (FT) is
further confirmed by the forward-backward FT ( _k_ → _R_ → _q_ ). The good matching between k-space and
[q-space curves confirms the reliable results of FT, as displayed in Supplementary Figure 3. Wavelet-](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
transformed analysis of _R_ -space curves by the Morlet function is also conducted to differentiate the spatial
interactions of the Fe atoms. As displayed in Figure 1G-J, the energy maximum ( _k_ -space) for all samples is
located in the range of 5-7 Å -1, falling in the standard Fe-S bonding range [36,37] . This again supports that the
Fe species are individually dispersed as Fe-S single-atom coordination. This status of Fe is also proved by
XPS surface characterization. The positions of Fe2p, Mo3d and S2p peaks are labeled in the XPS survey curves
[shown in Supplementary Figure 4. The C1s and O1s peaks arise from the carbon tape background due to](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
sample loading. No pronounced Fe peaks are examined except in sample S20, which indicates the
embedded Fe-single atoms between interlayers of MoS2 at low doping concentrations.

The catalysis performance for HER was investigated by three-electrode LSV measurement. As displayed in
Figure 2A, the HER performance of the sFe-1T/MoS2 (S2, S5 and S10) is gradually improved by the
increased dopant concentration compared to pristine 1T/MoS2 nanosheets in an acidic solution. Iron-doped
samples exhibit better HER performances with decreased overpotentials. In particular, sample S10 shows
the best performance with an onset potential of 190 mV _vs_ . RHE and an overpotential of 215 mV _vs_ . RHE

[[Supplementary Table 2]. Tafel slope is generally used to identify the HER mechanism consisting of the](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
Volmer-Tafel and Volmer-Heyrovsky process and determine the rate-controlling step for a certain catalyst.
It has been reported that 1T/MoS2 follows standard Volmer-Heyrovsky process [38] . As displayed in
Figure 2B, the low Tafel slope of sample S0 (70 mV/dec) indicates high catalytic performance of 1T/MoS2
corresponding to our previous report on ultra-stable 1T-phase MoS2 nanosheets [15] . Volmer process (H3O + +
e - = Hads) is a fast process with a low energy barrier, but if it is under rate-limiting control, the Tafel slope
would be much higher [12] . It has been reported that if a fast discharge reaction (Volmer) is followed by a
rate-limiting Tafel combination reaction, Heyrovsky (Hads + H3O + + e - = H2) dominates, leading to a low
Tafel slope; While if a fast Volmer discharge reaction is followed by a slow Heyrovsky electrochemical
desorption reaction, Tafel process (Hads + Hads = H2) dominates, leading to a high Tafel slope [39] . After doping
with Fe, the Tafel slope of MoS2 decreases from 70 to 55 mV/dec in sample S10, suggesting the increased
coverage of H and adsorption-desorption kinetics. In electrocatalysis, inherently excellent conductivity
tends to afford rapid electron transport and extend electrons to more active sites, thus resulting in increased
utilization efficiency of active sites [40] . The decreased Tafel slope also shows a far lower electrochemical
desorption process on sFe-1T/MoS2 surface indicating higher EASA. As EASA can be an indicator of the
number of active sites, it confirms that Fe dopant further induces rich active sites on 1T-MoS2. Therefore,
the EASA is calculated from the non-Faradaic region of the CV curves with the increased scan rates

[[Supplementary Figure 5]. The summarized fitting plots have been displayed in](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf) Figure 2C for _C_ dl
calculation. The EASA value is dramatically enhanced, from 4,165 cm 2 to 5,853 cm 2, by the increased dopant
concentration from samples S0 to S10 caused by the increased active sites of basal planes. The EASA value
drops significantly for sample S20, which indicates a morphology change under a high doping level. To
[further evaluate this property, BET surface area was measured as shown in Supplementary Figure 6 in the](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
supporting information. The surface area of sFe-1T/MoS2 rises from 8.6 m²/g to 17.38 m²/g from sample S2

Page 8 of 14 Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46

Figure 2. Electrochemical measurements for all Fe-1T/MoS2 samples. (A) LSV measurements in 0.5 M H2SO4 solution; (B) Tafel slopes
derived from LSV curves; (C) Fitting plots of C _dl_ from CV measurements for the EASA calculation; (D) The EIS in 0.5 M H2SO4 solution;
(E) The stability measurements for the acidic and alkaline solution for sample S10; (F) 24 h stability measurement at a constant RHE
potential of - 0.2 V (equal to 10 mA/cm2 current density in acidic (0.5 M H2SO4) and alkaline (1M KOH) solutions, respectively. LSV:

Linear sweep voltammetry; RHE: Reversible hydrogen electrod; CV: Cyclic voltammetry; EASA: Electrochemically active surface area;
EIS: Electrochemical impedance spectra.

to sample S10, consistent with the trend in CV measurement, thus supporting the presence of the more
active sites in the S10 sample. In addition, BET surface area decreases to 6.35 m 2 /g with rising doping
concentration (S20), corroborating surface aggregation under high doping concentrations, as demonstrated
in XRD results [41] . Electrochemical impedance spectra (EIS) are demonstrated in Figure 2D. The decreased
semi-circle diameter, from samples S0 to S10, indicates the reduced resistance with enhanced iron
concentration [42] . This is primarily caused by the high concentration of iron single atoms in MoS2 layers,
leading to excellent LSV results and catalytical performance. The enlarged contact resistance of sample S20
is mainly caused by surface-aggregated FeSx bulks. For the non-doped sample, there is minor Tafel slop
variation and a change of onset potential. Figure 2E shows the stability measurement in acidic and alkaline
environments. Since the iron site is sensitive to catalytical environment, the long-term stability of sample
S10 is further checked in both acidic (0.5 M H2SO4) and alkaline solution (1M KOH) referring to the non[doped sample (S0) in Supplementary Figure 7 of supporting information. In alkaline solution, after 1,000](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
cycles, there is not much change in terms of onset potential and Tafel slope, suggesting that the active sites
on MoS2 basal are mainly attributed to the iron doping because of lower chemical corrosion in alkaline
environment. However, for stability in an acidic solution, the LSV curves change significantly after 1,000
cycles. This may be due to the acidic corrosion of single Fe atoms. To understand the mechanism, the Fe1T/MoS2 is treated with 1M HCL solution and we observed the structural changes. After acidic treatment, a
[broader peak is observed for all samples [Supplementary Figure 8A-E]. When the reaction between iron](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
active sites and acidic solution happens, the iron between MoS2 layers will be extracted, causing layer
[collapse and d-spacing narrowing [Supplementary Figure 8F]. This high acidic sensitivity should be](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
associated with single-atom nature of Fe in MoS2 layers. Figure 2F shows the time-dependent measurement
of current density at an overpotential of -0.2 V. In acidic solution, the current density decays continuously

Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46 Page 9 of 14

until stable after 6 h. In alkaline solution, there is an apparent decline in current density at the beginning
and then the current density gradually becomes stable. The overall stability of the sample in alkaline
solution is better than that in acidic solution, which may be due to the corrosion in the acidic environment.
Stability tests were carried out at the RHE potential of -0.3 V and -0.5 V, respectively, as shown in
[Supplementary Figure 9 in the supporting information. When the potential is at -0.3 V, the sample showed](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
an excellent stability, while when the potential reached -0.5 V, the current density continuously decreased in
the first five hours. The sample current density became stable.

To investigate the mechanism of the enhanced HER, the valence band structure was examined by UPS
[(HeI) measurement and the overall spectra are displayed in Supplementary Figure 10. There is no obvious](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
drift in ionization energy for all 1T-MoS2 samples, indicating that the enhancement of HER is related to
more active sites induced by coordination environment change instead of doping-related work function
variation [40] . The zoom-in spectra of UPS are demonstrated in Figure 3A to check the variation in 2p
orbitals. In the Fe single atom system, the peak of 2p orbitals shifts slightly to lower binding energy from
6.5 eV to 6.3 eV from samples S0 to S2. This is mainly caused by the newly formed van der Waals force
between Fe and S. There is no peak shift from S2 to S10 with slightly decreased peak intensity. This indicates
a reproductive single-atom state for our samples. The decreased peak intensity is mainly caused by layer
expansion-induced orbital variation that resulted from higher intercalation concentration. The peak of S20
shows dramatic shifting on both peak position and intensity, which results from the transition from Fe-S
Van der Waals force to covalent Fe-S bond which is promoted by the p-d overlapping in atom replacement
doping system and surface aggregation as mentioned before [43] . This process is also demonstrated by dorbital variation in Figure 3B. The peak at 1.18 eV is contributed by d orbital of MoS2. The peak intensity
decreases from S0 to S10 and increases significantly for sample S20, confirming the observation in Figure 3A
that the enhanced orbital hybridization is mainly caused by new-formed FeSx bulks [44] . The peak variation in
UPS also indicates the change of coordination environment. The coordination environment of Fe is fitted
by NEXAFS curves and summarized in Figure 3C. The fitting curve well matches both real and imaging
[parts of the fitting curve [Supplementary Figure 11] and low Debye-Waller factors [Supplementary Table 3],](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
indicating reliable fitting results. The coordination number decreases from 6.5 to 5 from S2 to S20. In a
typical FeS2 crystal, Fe atoms are coordinated by six ligands, forming a stable and sulfur-rich coordination
environment. A higher coordination number in low iron concentration sample (S2) suggests the coexistence of N-Fe-S interlayer chains. These chains represent a significant electron supply from the standard
FeS2 configuration. The formation of these interlayer chains not only increases the coordination number but
also introduces new types of Fe sites that are likely more catalytically active due to their altered electronic
structure [45,46] . The observed reduction in coordination numbers in sample S10 signifies a diminish of the
electronegative environment around the Fe atoms, which effectively increases the local electron density at
Fe sites, which is crucial to engage in electron transfer reactions and push the Fe sites to be more reactive in
an acidic environment. As mentioned above, the HER process of MoS2 relies on one primary discharge step
and at least one kind of desorption step. The activation effect on hydroxonium ion (H3O + ) goes through
synergistic interaction along Fe-S-H chain, as roughly represented in Figure 3D. The catalysis kinetics
depend on a delicate balance between the dissociation rate of H3O + on Fe δ+ site in acidic solution, the
recombination rate of Hads on S δ- site and the desorption rate of H2O, which helps to accommodate the
adsorption of the next H3O + molecules. During the discharge step, H3O + diffuses to the catalyst surface and
then H3O + picks up electrons and generates hydrogen adsorbed on the catalyst surface. In the current
system, due to the synergistic interaction between Fe and H, the concentration of available H3O + is
increased, hence facilitating the discharge process [47] . While in an alkaline solution, OH - directly pushes free
Hads to the S site, jumping over the process of synergistic interaction along the Fe-S-H chain. In this case, the
electron-rich center moves from Fe to S, which can explain the difference between acidic and alkaline
solutions.

Page 10 of 14 Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46

Figure 3. (A)-(B) UPS (HeI) spectra of s-Fe-1T/MoS2; (C) Coordination number changes with increased Fe concentration; (D)
Schematic diagram of catalysis process of sFe-1T/MoS2. UPS: Ultraviolet photoelectron spectroscopy.

To further validate our findings, we applied two additional RHE potentials of -0.3 V and -0.5 V during the
stability test. The X-ray absorption spectroscopy (XAS) results for sample S10 after catalysis are presented in
[Supplementary Figure 12 in the supporting information. As shown in Supplementary Figure 12A, there are](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
no significant changes in the pre-edge peak before and after catalysis at an RHE potential of -0.3 V,
indicating minimal variation in the coordination environment after catalysis. However, an increase in the
quadrupole pre-edge peak intensity was observed with prolonged catalysis at -0.5V, suggesting a shift
towards a higher electronegativity and a more asymmetric coordination environment. The edge position
also shifts to a higher energy range, from 7,120 eV to 7,125 eV, reflecting an increase in the Fe oxidation
state and the formation of a higher electronegativity center, consistent with the pre-edge changes.
Additionally, a new merged white peak at 7,130 eV emerges post-catalysis compared to the split peak
observed before catalysis, indicating a newly formed dominant p orbital contribution. This p orbital
contribution diminishes when the catalysis was performed at high potential, which suggests the S-site
[peeling off at high-voltage catalysis. The R-space curves, displayed in Supplementary Figure 12B, further](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
support these observations. A reduction in atomic distance from approximately 1.8 Å to 1.4 Å is observed,
along with a decrease in peak intensity. It suggests a reduced coordination number of the Fe site with an
indicative transition from FeSx to FeOx.

More active sites induced by iron single atoms also promote significant enhancement in real industrial
applications, such as ocean water splitting. In normal ocean water, lower OH - /H + concentration (neutral
pH) leads to a low catalytical efficiency. Thus, to evaluate the efficiency of ocean water splitting, 10% v/v and
30% v/v 0.5M H2SO4 and 1M KOH solutions were added to the ocean water, respectively. The performance
in pure acidic and alkaline solutions is remarked as S-H100 and S-OH100, respectively, as the standard
reference. The LSV curves of ocean water splitting are demonstrated in Figure 4A and the overpotential at

Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46 Page 11 of 14

Figure 4. (A) Seawater HER performance with different volume ratios (v/v) of 0.5M H2SO4 and 1M KOH. The concentration is 10% v/v
(S-H10/S-OH10), 30% v/v (S-H30/S-OH30) and 100% v/v (pure solution, S-H100/S-OH100), respectively; (B) The long-term
stability in such a solution is tested at a constant RHE potential of -0.2 V within 24 h. HER: Hydrogen evolution reaction; RHE:
Reversible hydrogen electrode.

10 mAcm -2 [has been summarized in Supplementary Table 4 supported by previous milestones in MoS2-](https://oaepublishstorage.blob.core.windows.net/articlepdfpreview202501/4046-SupplementaryMaterials.pdf)
based seawater splitting. Tafel slope keeps increasing when pH is close to a neutral level, which is mainly
caused by reduced active catalytic ions in the environment. This also indicates the reliability of electrocatalytical performance for sFe-1T/MoS2. The long-term stability was also checked, as displayed in
Figure 4B. Catalysts in acidic solution show a similar trend with different H + concentrations, indicating
stable performance of acidic ocean splitting. A serious decay is observed in the initial stage (within 3 h) for
alkaline-modified ocean water compared to pure 1M KOH solution. This is mainly caused by the newly
formed Ca/Mg-based carbonates in the ocean water, which block the active surface of MoS2 and
significantly reduce the performance of water splitting. This also explains why this phenomenon is not
obvious in acidic solution since it is corroded immediately in H + environment. The overpotential at 10
mA/cm 2 reaches 242 mV for the S-OH30 sample, which is relatively lower compared to previous MoS2
catalysis. In addition, chloride ions in seawater will also significantly affect the HER by competing with
hydrogen ions for adsorption sites on the electrode surface, thereby reducing the efficiency of hydrogen
production. This explains the significant decrease in current density observed in seawater compared to pure
acidic and alkaline solutions. In addition, the chloride ion in seawater also significantly influences HER
performance by competing with hydrogen ions on adsorption sites of electrodes, reducing the efficiency of
hydrogen production consequently. This competition results in a notable decrease in current density
observed in seawater environment compared to pure acidic and alkaline solutions. Furthermore, Cl - can
induce electrode corrosion. Although the corrosive effect in alkaline solutions is less effective than in neutral
or acidic environments, chloride-induced sample peeling off is the primary concern since Cl ions can
penetrate protective oxide layers on the electrode surface, leading to localized corrosion and subsequent the
degradation of the electrode's structural integrity.

## CONCLUSION

In this work, single-atom iron-modified 1T/MoS2 was synthesized using a scalable hydrothermal method.
The synthesized samples showed structure transformation from single-atom intercalation to site
replacement doping with increasing doping concentrations. Our results demonstrate that doping
concentration can significantly influence the HER performance of the doped 1T-phase MoS2. The 10% _at._
sFe/1T/MoS2 exhibited superior electrochemical activity toward HER with an onset potential of 190 mV and

Page 12 of 14 Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46

a Tafel slope of 55 mV/dec in acidic solution. Additionally, it shows an exceptionally low onset potential of
80 mV, albeit with a slightly higher Tafel slope in alkaline solution, indicating distinct HER mechanisms.
The substantial enhancement is attributed to the morphological effect-induced rich active sites due to the
change of coordination environment. Additionally, the synthesized product also exhibited high efficiency in
ocean water splitting with an overpotential of 242 mV at 10mA/cm 2 in a 30%v/v mixed KOH/seawater
environment. This high efficiency of seawater splitting is primarily attributed to the active ions. Our present
work provides a facile and cost-effective strategy for designing efficient electrocatalysts for HER, promising
broad applicability of this methodology to other catalytic systems. However, overcoming electrode
corrosion and achieving high-power output ocean catalysis remain significant challenges requiring further
investigation.

## DECLARATIONS

Authors’ contributions
Mainly supervised this project: Yi, J.
Carried out this project: Geng, X.
Contributed to sample characterization: Cao, Y.
Performed TEM measurements: Li, Z.
Assisted with catalysis measurement: Li, M.; Chu, D.
Contributed to the materials characterization and revised the manuscript: Cao, C.; Yu, C.; Chu, X.; Hu, L.;
Sun, Y.; Wang, D.; Qiao, L.
Conducted the NEXAS measurements: Yu, X.; Breese, M. B. H.
Conducted the EXAFS measurements: Lee, J. M.

Availability of data and materials
The data that support the findings of this study are available from the corresponding author upon
reasonable request.

Financial support and sponsorship
This work was supported by the Australian Research Council (DP220103045). The authors would like to
acknowledge the Singapore Synchrotron Light Source for providing the facility necessary for conducting the
research. The Laboratory is a National Research Infrastructure under the National Research Foundation,
Singapore. Any opinions, findings, conclusions or recommendations expressed in this material are those of
the author(s) and do not reflect the views of the National Research Foundation, Singapore.

Conflicts of interest
All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate
Not applicable.

Consent for publication
Not applicable .

Geng _et al. Energy Mater._ 2025, _5_, 500009 https://dx.doi.org/10.20517/energymater.2024.46 Page 13 of 14

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One-step hydrothermal synthesis of Fe single atom doped 1T-MoS<sub>2</sub> nanosheets for high-performance seawater hydrogen production

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