Communicationwww.small-journal.com

Self-Anti-Stacking 2D Metal Phosphide Loop-Sheet
Heterostructures by Edge-Topological Regulation for Highly
Efficient Water Oxidation
Quan Quan, Zhengxun Lai, Yan Bao, Xiuming Bu, You Meng, Wei Wang,
Tsunaki Takahashi, Takuro Hosomi, Kazuki Nagashima, Takeshi Yanagida, Chuntai Liu,
Jian Lu, and Johnny C. Ho*system still lies in the sluggish oxygen evolution reaction (OER) because of its highenergy barrier for the OH bond breaking
and the attendant OO bond formation.[2]In this regard, efficient OER electrocatalysts are urgently required to address thischallenge by effectively utilizing multiple
proton and electron transfers for oxygen
evolution at low overpotentials.[3,4] At present, many noble metal catalysts (e.g.,
RuO2 and IrO2) are well-acknowledged tobe efficient for OER, but they can hardly
satisfy the scale-up applications due totheir scarcity and high cost.[5] Recently,
cost-effective and highly efficient nonnoble-metal alternatives, transition metal
phosphides (TMPs), have been emerged asa new family of OER catalysts.[6] Although
there are significant progresses achieved in
exploiting these advanced phosphide nanostructures for efficient OER, their OER
performance is quite far from satisfactory.
In general, ingeniously engineering the TMPs with welldefined geometries and functional nanoarchitectures has been
demonstrated as an effective method to promote their electrocatalytic activities.[7,8] For the most part, 2D TMP nanosheets
offer unique properties originating from their ultrathin

2D metal phosphide loop-sheet heterostructures are controllably synthesizedby edge-topological regulation, where Ni2P nanosheets are edge-confined by
the N-doped carbon loop, containing ultrafine NiFeP nanocrystals (denotedas NiFeP@NC/Ni2P). This loop-sheet feature with lifted-edges prevents thestacking of nanosheets and induces accessible open channels for catalytic
site exposure and gas bubble release. Importantly, these NiFeP@NC/Ni2P
hybrids exhibit a remarkable oxygen evolution activity with an overpotentialof 223 mV at 20 mA cm−2 and a Tafel slope of 46.1 mV dec−1, constituting therecord-high performance among reported metal phosphide electrocatalysts.
The NiFeP@NC/Ni2P hybrids are also employed as both anode and cathodeto achieve an alkaline electrolyzer for overall water splitting, delivering a current density of 10 mA cm−2 with a voltage of 1.57 V, comparable to that of thecommercial Pt/C||RuO2 couple (1.56 V). Moreover, a photovoltaic–electrolysis
coupling system can as well be effectively established for robust overall water
splitting. Evidently, this ingenious protocol would expand the toolbox fordesigning efficient 2D nanomaterials for practical applications.

Electrocatalytic water splitting powered by renewable energy
sources is considered as an attractive technology to produce
clean fuels, which can relieve the ever-growing energy demands
from fossil fuels and the associated environmental crisis.[1]However, the bottleneck for a fully integrated water-splitting
Q. Quan, Z. Lai, Dr. X. Bu, Y. Meng, W. Wang, Prof. J. C. Ho
Department of Materials Science and Engineering
City University of Hong Kong
Hong Kong SAR 999077, China
E-mail: johnnyho@cityu.edu.hk
Dr. Y. Bao, Prof. J. Lu
Department of Mechanical Engineering
City University of Hong Kong
Hong Kong SAR 999077, China
Dr. T. Takahashi, Dr. T. Hosomi, Dr. K. Nagashima,
Prof. T. Yanagida
Department of Applied Chemistry
School of Engineering
University of Tokyo
Tokyo 113-8654, JapanThe ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202006860.

DOI: 10.1002/smll.202006860
Small 2021, 17, 2006860

Prof. T. Yanagida, Prof. J. C. Ho
Institute for Materials Chemistry and Engineering
Kyushu University
Fukuoka 816-8580, Japan
Prof. C. Liu
Key Laboratory of Advanced Materials Processing & Mold (Zhengzhou
University)
Ministry of Education
Zhengzhou 450002, China
Prof. J. Lu
Centre for Advanced Structural Materials
City University of Hong Kong Shenzhen Research InstituteGreater Bay Joint Division
Shenyang National Laboratory for Materials Science
Shenzhen 518057, China
Prof. J. C. Ho
State Key Laboratory of Terahertz and Millimeter Waves
City University of Hong Kong
Hong Kong SAR 999077, China

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thickness, large surface area, and high surface-to-volume atom
ratios, which are beneficial for enhancing the active site exposure and enlarging the contact between reactant molecules
and active sites to boost their OER performance.[9–11] Despite
the great promise, the 2D structural precursors tend to easily
aggregate or sinter due to lattice strains arising from heteroatom substitution (i.e., phosphorus) and stochastic strain
release during the phase transformation process.[12] In thisway, these 2D phosphide nanosheets or nanoplates are typicallyin situ grown on conductive substrates for immobilization,
where the material growth is highly dependent on the external
media, limiting their versatile utilization. On the other hand,the freestanding 2D nanosheets are usually inclined to stack
together through the week van der Waals interaction to form
bundles and agglomerations, severely compromising the electrochemical properties.[13,14] Considering the instability during
synthetic processes and the intrinsic stacking problem, rationally appending building blocks anchored onto the peripheryof 2D nanosheets by a targeted edge-topologic regulation is
anticipated to be a multieffective strategy to resolve theseissues; meanwhile, the synchronously aroused electronic
structure modulation can further optimize the electrocatalytic
performance of the nanosheets.[15]Here, for the first time, as a proof of concept, we haveedge-selectively grown the Ni–Fe PBA nanocubes along
the periphery of ultrathin Ni(OH)2 nanosheets by controlling thecoordination reaction kinetics in order to construct a loop-sheet
heterostructure. During the phosphorization step, the edgeloop topology is found to compensate the stress induced in thephase transformation process. In this case, the obtained metal
phosphides are well-maintained with their 2D heterostructure,in which the interconnected polycrystalline Ni2P nanoparticles
as sheet structures are edge-confined by the N-doped carbon
loop containing ultrafine uniform NiFeP nanocrystals (denotedas NiFeP@NC/Ni2P). Benefiting from the conspicuously accessible active sites and the largely improved mass/charge transfer
based on the self-anti-stacking loop-sheet heterostructures, theachieved NiFeP@NC/Ni2P exhibits an overpotential of 223 mV
at a current density of 20 mA cm−2 and a small Tafel slope of
46.1 mV dec−1, ranking the top tier among the state-of-the-art
metal phosphide electrocatalysts reported to date. Importantly, the NiFeP@NC/Ni2P couple can also be employed asbifunctional catalysts to drive the overall water splitting in alkaline solution, requiring a cell voltage of 1.57 V and 1.83 V at
10 mA cm−2 and 100 mA cm−2, respectively. A solar-cell-driven
overall water-splitting device is as well constructed to illustrate
its effective and robust operation. As a result, the controllable
synthetic methodology developed in this work would pave a
new way to synthesize versatile and efficient 2D-morphological
TMP heterostructure, which does not only enrich the 2D material family but also significantly promote their practical use fora wide range of energy and environment-related applications.
Figure 1a schematically illustrates the two-step preparation
procedure of the NiFeP@NC/Ni2P loop-sheet heterostructure. The Ni–Fe PBA/Ni(OH)2 loop-sheet precursors are first
constructed through the [Fe(CN)6]3+ coordinating with theedge-released Ni2+ ions from ultrathin Ni(OH)2 nanosheets
under the assistance of H+, followed by the phosphorization
treatment via a gas-solid reaction. At first, as shown in the

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field-emission scanning electron microscopy (FESEM) image in
Figure 1b, the starting Ni(OH)2 precursor is confirmed to havea high 2D anisotropy with a lateral dimension of ≈400 nm. The
atomic force microscopy (AFM) image reveals the thickness of
Ni(OH)2 being ≈2.5 nm, demonstrating its ultrathin nanosheet
structure (Figure S1, Supporting Information).[12] Next, under
the assistance of exotic H+ and [Fe(CN)6]3+ species at 40 °C,
the Ni–Fe PBA nanocubes are in situ and directionally grownon the peripheral edge of Ni(OH)2 nanosheets (Figure 1c andFigure S2, Supporting Information). The X-ray powder diffraction (XRD) results of the obtained samples further illustrate
additional patterns of the face-centered cubic structure of Ni–Fe
PBAs assigned to Ni3[Fe(CN)6]2 phase (JCPDS No. 46-0906)
besides the peaks of Ni(OH)2 (JCPDS No. 14-0117), confirming the formation of pure Ni–Fe PBA phase onto Ni(OH)2
nanosheets (Figure 1d).[12,16] Moreover, as compared with theRaman peaks of Ni(OH)2, the obtained spectrum of Ni–Fe
PBA/Ni(OH)2 shows an additional band at ≈2250 cm−1, whichis corresponded to the vibrational frequency of CN bond(Figure S3, Supporting Information),[17] being in good agreement with the XRD results.
The control experiments and temperature-dependent reactions are then carried out to unravel the underlying growthmechanism of Ni–Fe PBA. To be specific, without the additionof [Fe(CN)6]3+ species, the size of Ni(OH)2 nanosheets exhibitsa shrinkage and their edges become rough and irregular as
only reacting with H+ (Figure S4a,b, Supporting Information),
manifesting the Ni2+ being etched from the edge of Ni(OH)2
nanosheets. This observation is in conformity with the previous report that the edge is more vulnerable than smoothplane surface under H+ attack owing to more exposed defects
in the edge.[18] In addition, when [Fe(CN)6]3+ is only participated in the reaction without adding H+, the sparse Ni–Fe PBA
nanocubes are randomly dispersed on the Ni(OH)2 nanosheets(Figure S4c,d, Supporting Information) and the corresponding
XRD pattern of Ni–Fe PBA phase is not discerned because of
the sluggish production rate (Figure S5, Supporting Information). It is disclosed that the H+ is essential for the edge-selectivity etch. For comparison, the exotic Ni2+ using NiCl2 as asource is directly utilized to react with [Fe(CN)6]3− instead of
applying Ni(OH)2 nanosheets as the template. In this way, thepure Ni–Fe PBA is formed, which is confirmed by the XRD pattern (Figure S6a, Supporting Information). However, the pure
Ni–Fe PBA is present in the form of severely aggregative particles (Figure S6b, Supporting Information), demonstratingthat it is beneficial for the synthesis of well-ordered Ni–Fe
PBA nanocubes through dissolving the Ni2+ from Ni(OH)2
nanosheets by H+ etching.[19] At the same time, the temperaturedependent reactions are further performed to explore the different reaction stages for the formation of Ni–Fe PBA/Ni(OH)2.
Explicitly, as compared with the as-prepared product at 40 °C,
the smaller and discrete Ni–Fe PBA nanocubes are anchoredon the edge of Ni(OH)2 obtained at the lower temperature (i.e.,
20 °C) (Figure S7a,b, Supporting Information), while the bigger
and much more PBA nanocubes are formed and thus cover
the Ni(OH)2 surface at the higher temperature (i.e., 60 °C)(Figure S7c,d, Supporting Information). All these findings arealso supported by the gradually increased XRD diffraction intensity of Ni3[Fe(CN)6]2 in Figure S8 (Supporting Information) as

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Figure 1. a) Schematic illustration of the preparation process of NiFeP@NC/Ni2P loop-sheet heterostructures. FESEM images of b) Ni(OH)2 nanosheets
and c) Ni–Fe PBA/Ni(OH)2. d) XRD patterns of Ni(OH)2 and Ni–Fe PBA/Ni(OH)2. e) TEM image of Ni–Fe PBA/Ni(OH)2 (Inset: TEM image of a single
Ni–Fe PBA nanocube). HRTEM images of f) the single Ni–Fe PBA nanocube and g) the Ni(OH)2 in Ni–Fe PBA/Ni(OH)2 heterostructure (Inset: FFT
patterns of the Ni–Fe PBA nanocube and Ni(OH)2, respectively). h) STEM–EDX element mapping of Ni–Fe PBA/Ni(OH)2.

well as their color change from the light green of Ni(OH)2 into
yellow brown in Figure S9 (Supporting Information).
Above all, the above results evidently indicate that this coordination reaction initially occurred at the edge of nanosheets withthe assistance of foreign H+.[20] In detail, the Ni2+ ions released
from edge sites would in situ coordinate with [Fe(CN)6]3+
to form Ni–Fe PBA nanocubes at the solid-liquid interface(Figure S10, Supporting Information). Also, the reaction
rate is highly dependent on the reaction temperature.[21] The
Ni–Fe PBA/Ni(OH)2 synthesized at 40 °C displays that the wellordered Ni–Fe PBA nanocubes are edge-selectively aligned on
the Ni(OH)2 nanosheets and the phosphatized product exhibits
the optimal catalytic activity; therefore, the obtained product ischosen as the typical sample for the subsequent characterization. Notably, the AFM image of such Ni–Fe PBA/Ni(OH)2 heterostructure illustrates the height of two overlapped loop-sheets
gets increased to two orders of magnitudes more than that of
the two overlapped ultrathin Ni(OH)2 nanosheets (Figure S11,
Supporting Information). This increase of the height is due

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to the lifted-edges formed by the Ni–Fe PBA loop, where theloop prevents the 2D plane-to-plane stacking and induce theconstruction of open channels.
To get more insights into the detailed structure achievedby the edge-selective synthesis strategy, transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX)
elemental mapping are conducted. As depicted in Figure 1e,each individual PBA nanocube, also attaching with neighboring
nanocubes, is implanted along the edge of sheet-template toconstruct a cube-to-loop architecture. Besides, high-resolution TEM (HRTEM) images in Figure 1f,g are recorded fromthe edge and surface region marked by the yellow frames in
Figure 1e, respectively. In region f, the lattice spacing is determined to be 0.51 nm, which is attributed to the (200) face of
Ni-Fe PBA (Figure S12, Supporting Information).[22] The corresponding fast Fourier transform (FFT) pattern presents distinct bright spots with a cubically arranged shape (the inset
of Figure 1f), suggesting the formation of pure Ni–Fe phaseand being consistent with the XRD results.[22] Simultaneously,

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Figure 2. Structural characterization of NiFeP@NC/Ni2P heterostructures. a) SEM image. b) AFM image of overlapped NiFeP@NC/Ni2P loop-sheet
heterostructures c) with corresponding height profile and d) schematic illustrations. e) TEM image. f) HRTEM image, g) SAED pattern, and h) EDX
spectrum of the selected plane structure in NiFeP@NC/Ni2P heterostructures. i) HRTEM image, j) SAED pattern, and k) EDX spectrum of the selectedloop structure in NiFeP@NC/Ni2P heterostructures. l) STEM–EDX elemental mapping of NiFeP@NC/Ni2P loop-sheet heterostructures.

the well-resolved lattice fringes with a spacing of 0.23 nm
can be indexed to the (101) planes of Ni(OH)2 in the region g(Figure S13, Supporting Information).[12] The single crystalline
nature of Ni(OH)2 nanosheets is evidently validated by its FFTpattern (the inset of Figure 1g). Moreover, the scanning TEM(STEM) and the corresponding EDX mapping are employed toevaluate the spatial element distribution in this heterostructure(Figure 1h).[7] It is witnessed that the Fe, C, and N elements
clearly occupy the edges. Also, the O element is evenly distributed across the subsheet while the Ni element is stretchedthrough the entire loop-sheet structure, corroborating that theedge-etching Ni2+ ions from Ni(OH)2 nanosheets are indeed
coordinated with [Fe(CN)6]3+. Therefore, the above results canjointly affirm the successful edge-selective formation of Ni–Fe
PBA loop onto Ni(OH)2 nanosheets.
After that, the obtained Ni–Fe PBA/Ni(OH)2 loop-sheet
hybrids are phosphatized via a facile gas-solid phase reaction
to form NiFeP@NC/Ni2P. The typical SEM images of final
products suggest that the loop-sheet heterostructure is wellinherited with the rough surface (Figure 2a and Figure S14,
Supporting Information). The corresponding AFM image further reveals the lifted-edges at the periphery of nanosheets(Figure 2b,c), illustrating the geometrically restricted stacking
property and the concurrently formed open channels for

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effective gas release (Figure 2d). The XRD pattern of hexagonal
Ni2P (JCPDS No. 89-2742) is well discerned without diffraction peaks of precursors remained, manifesting the complete
transformation of Ni–Fe PBA/Ni(OH)2 after the phosphorization process (Figure S15, Supporting Information). Notably, thepure Ni2P diffraction peaks do not only represent the phosphorization of Ni(OH)2 but also indicate a single-phase matter of
NiFeP originated from Ni–Fe PBA rather than from a mixtureof two solid phases,[23] which is resulted from the coexistence ofNi and Fe atoms in Ni–Fe PBA and their synchronous phasetransformation.[24] As a comparison, the XRD pattern of pure
Ni–Fe PBA after phosphorization reveals the coexistence of
two-phase separated Ni2P (JCPDS No. 89-2742) and FeP (JCPDS
No. 65-2595), which can be attributed to the different stoichiometric ratio of elements in the precursors (Figure S16, Supporting Information).[25] The TEM analysis is as well conducted
to trace the changes of phase and interior structure for the asobtained NiFeP@NC/Ni2P hybrid. As displayed in Figure 2e,an individual loop-sheet heterostructure discloses that thesubsheet is consisted of an assembly of arbitrarily oriented
and interconnected Ni2P nanoparticles with a high density ofinterparticle grain boundaries, thereby fully activating the basal
planes. Besides, the ultrafine NiFeP nanocrystals are confinedin the amorphous N-doped carbon loop derived from the Ni–Fe

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PBA cube-to-loop architecture, endowing abundant active sites
distributed at edge-loop as well. The marked plane region f andloop region i in Figure 2e are further characterized, respectively.
In Figure 2f, the HRTEM image of the plane region illustrates
that the interplanar spacing of the nanoparticles are 0.21 and
0.51 nm, corresponding to the (201) and (100) planes of Ni2P
crystals, accordingly.[26] It is interesting that Moiré patterns
can be clearly distinguished because of the overlap of Ni2P
nanoparticles at small mis-orientation angles, which designate
the lattice distortion of Ni2P obtained from the phosphorization process of single crystal Ni(OH)2.[27,28] The corresponding
selected-area electron diffraction (SAED) image suggests its
polycrystalline structure (Figure 2g), while the EDX spectrum
implies the existence of Ni, P, and O elements (Figure 2h). The
O element is come from the surface oxidation of Ni2P due to air
exposure.[29,30] Meanwhile, the HRTEM image of the loop regionin Figure 2i delineates that the ultrafine nanocrystals are dottedamong the amorphous N-doped carbon matrix with an average
diameter of ≈3.2 nm (Figure S17, Supporting Information).
Besides, the distinguished lattice fringes of 0.22 nm canbe ascribed to the (111) plane of NiFeP crystals.[31] The SAED
patterns and EDX spectrum of this marked loop region signify the polycrystalline structure of NiFeP nanocrystals andthe coexistence of Ni, Fe, P, N, and O elements, respectively(Figure 2j,k), demonstrating that the Fe element is still locatedat loop region after phosphorization. The STEM-EDX elemental
mapping also demonstrates the homogeneous distribution of
Ni and P elements and the distinctly edge-occupied Fe, N, C
elements (Figure 2l). All these confirm the fully inherited loopsheet heterostructure and spatial element distribution after
phosphorization reaction. Additionally, the incorporation ofcarbon matrix is further verified by Raman spectroscopy. The
two broad peaks centered at around 1350 cm−1 (D band) and1560 cm−1 (G band) are observed to confirm the presence ofcarbonaceous species in this NiFeP@NC/Ni2P heterostructure,
which can be credited to the carbonization of cyano groups of
Ni–Fe PBA (Figure S18, Supporting Information).[4,32,33]
For a comparison, the pure Ni(OH)2 nanosheets are as welldirectly undergone phosphorization reaction under identicalconditions. As expected, the obtained Ni2P product is failed tomaintain the nanosheet shape and fall apart as severely aggregated nanoparticles owing to their random stress transfer in
the interior to reach a mechanical equilibrium (Figure S19,
Supporting Information).[12] In this way, it is revealed that theedged-loop is crucial to facilitate the stress transfer and compensate the stress derived from the lattice mismatch in thephase transformation process. Therefore, the 2D nanosheet
structure of as-obtained interconnected Ni2P nanoparticles
with abundant grain boundaries is well preserved, contributing
to the exposure of solution-accessible active sites. Moreover,
the two counterparts (i.e., Ni–Fe PBA/Ni(OH)2-20 and Ni–Fe
PBA/Ni(OH)2-60, where the numbers of 20 and 60 designate
the synthesis temperature) are gone through the identical
phosphorization process. Their corresponding products (i.e.,
NiFeP@NC/Ni2P-20 and NiFeP@NC/Ni2P-60) present thewell-maintained 2D heterostructure as disclosed by the overview SEM images (Figure S20, Supporting Information), further indicating the confined effect of the Ni–Fe PBA subarchitecture in order to maintain the 2D structure. In particular,

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the HRTEM images and STEM–EDX elemental mapping of
NiFeP@NC/Ni2P-20 demonstrate the well-defined loop-sheet
heterostructure and the distinctly edge-occupied Fe element,
further validating the efficient stress-relaxation effect of theedged Ni–Fe PBA architecture during the phase transformation process from Ni(OH)2 to Ni2P (Figure S21, Supporting
Information).
Collectively, the above characterization results illustrate
three aspects of the structural information: i) The Ni–Fe PBA
nanocubes are edge-selectively implanted along the peripheryof ultrathin Ni(OH)2 nanosheets, topologically lifting the edges
to prevent sheet-to-sheet stacking; ii) after phosphorization, theinterconnected Ni2P nanoparticles with abundant grain boundaries maintain the 2D sheet structure and simultaneously activate
the basal planes, while the Ni–Fe PBA cube-to-loop architecture isconverted into N-doped carbon loop decorated with ultrafine
NiFeP nanocrystals rendering abundant active sites distributed
at edge-loop as well; iii) the obtained NiFeP@NC/Ni2P is wellmaintained with its 2D loop-sheet heterostructure owing to
the edged-loop topology compensating the stress induced in thephase transformation process.
To shed light on the valence states and electronic structureof NiFeP@NC/Ni2P, the detailed X-ray photoelectron spectroscopy (XPS) analysis is performed. The survey XPS spectra
are collected to convey the coexistence of Ni, Fe, P, C, and N
in NiFeP@NC/Ni2P as well as O elements due to the inevitablesurface oxidation of phosphides coming from air exposure(Figure S22a, Supporting Information).[34] The depth-profiling
XPS spectra of Ni 2p can be deconvoluted into four peaks
besides the satellites (Figure 3a). During Ar ion sputtering, it isobserved that the intensity of the two peaks at 852.9 (Ni 2p3/2)
and 870.2 eV (Ni 2p1/2) for Ni2+ in Ni–P gets gradually increased,
which confirms the interior existing in the form of phosphides.[12] Simultaneously, the intensity of other two peaks at
856.8 (Ni 2p3/2) and 874.6 eV (Ni 2p1/2) decreases because the oxidized Ni species (Ni3+) located at surface would contact with theambient air rather than the interior.[35] For the case of Fe, similar changes in the depth-profiling XPS spectra of Fe 2p suggest
the intrinsic phosphide to form NiFeP with surface oxidation as
well (Figure S22b, Supporting Information).[36,37] The intensityof P−M (M = Ni, Fe) bond is increased for the increasing Ar
ion sputtering duration, agreeing well with the above discussion(Figure S22c, Supporting Information).[38] It is also noted thatthe depth-profiling N 1s and C1s spectra signify the uniform distribution of N-doped carbon matrix (Figure S22d,e, Supporting
Information), being consistent with the Raman and EDX element mapping results discussed above. As compared with
the interior valence states and chemical environment of Ni2P,
the binding energies of Ni 2p and P 2p for NiFeP@NC/Ni2P areshifted negatively, indicating the charge redistribution occurs
after introducing the edge NiFeP@NC loop due to the strong
interfacial interaction (Figure 3b,c).[39] Such effects suggest thestrong electronic coupling of Ni2P nanosheet and NiFeP@NC
loop, which is beneficial for electron transport during OER.[40]With the aim to further assess the subtle change in theelectronic state for the loop-sheet heterostructure, elementselective X-ray absorption fine structure (XAFS) spectroscopy
experiments are conducted, including X-ray absorption nearedge structure (XANES) and extended X-ray absorption fine2006860 (5 of 11)© 2021 Wiley-VCH GmbH

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Figure 3. a) Depth-profiling XPS spectra of Ni 2p for NiFeP@NC/Ni2P after Ar ion etching with different duration. High-resolution XPS for b) Ni 2p
and c) P 2p spectra of NiFeP@NC/Ni2P and Ni2P under Ar ion etching for 360 s. d) Ni K-edge XANES spectra. e) Fourier transform of Ni K-edge
EXAFS spectra at R space and the corresponding EXAFS fitting at R space. f) Wavelet transform of the k2-weighted EXAFS data of NiFeP@NC/Ni2P.
g) Fe K-edge XANES spectra. h) Fourier transform of Fe K-edge EXAFS spectra at R space and the corresponding EXAFS fitting at R space. i) Wavelet
transform of the k2-weighted EXAFS data of NiFeP@NC/Ni2P.

structure (EXAFS).[41,42] Figure 3d shows the Ni K-edge XANES
spectra of NiFeP@NC/Ni2P, Ni2P, NiO, and Ni foil. The absorption edges of NiFeP@NC/Ni2P and Ni2P are found lower thanthat of NiO but higher than the one of Ni foil, which indicatesthe average charge state values of Ni element in NiFeP@NC/
Ni2P and Ni2P being less than +2.[43] Moreover, as comparedwith Ni2P, a negative edge shift of XANES peak of NiFeP@
NC/Ni2P is observed along with a decrease of the Ni oxidation
state, indicating the occurrence of electron transfer to Ni sites
and engendering the electron enrichment on the Ni sites.[44,45]
The reverse trend is observed for the positive edge shift of the
Fe K-edge XAENS spectra of NiFeP@NC/Ni2P as comparedwith that of Ni2P–FeP@NC (Figure 3g). It confirms the oxidation state of Fe increased in the edge NiFeP@NC loop, which
is beneficial for the oxidation reaction.[46] Also, a comparisonof the Fourier transforms of EXAFS and fitting results at theNi K-edge of the different samples is applied to extract the

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structural parameters (Figure 3e).[47–48] The Ni−Ni(Fe) bond distance in NiFeP@NC/Ni2P is slightly shorter than that of Ni2P
(Table S1, Supporting Information), meaning the Ni−Ni(Fe)bond strengthening and demonstrating an intimate interfacial
contact of edge-occupied Fe species with interconnected Ni2P
nanoparticles.[43,44] The Fourier transforms of EXAFS and fitting
results at the Fe K-edge reveal a distinguished position difference of Fe−P and Fe−Fe(Ni) bond in NiFeP@NC/Ni2P as compared with those in Ni2P–FeP@NC (Figure 3h and Table S2,
Supporting Information). This distinct difference indicates the dissimilar coordination spheres of Fe atoms,
which in turn confirms the single-phase NiFeP existing in
NiFeP@NC/Ni2P as distinguished from the two-phase Ni2P
and FeP in Ni2P–FeP@NC. The wavelet transform (WT) of Ni
K-edge EXAFS oscillations of NiFeP@NC/Ni2P demonstrates
only one intensity maximum at 5.7 Å−1 related to the Ni−P
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Figure 4. a) Polarization curves of NiFeP@NC/Ni2P, Ni–Fe PBA/Ni(OH)2, Ni(OH)2, and RuO2. b) The corresponding Tafel plots derived from thepolarization curves in (a). c) Comparison of overpotentials at 20 mA cm−2 and Tafel plots for NiFeP@NC/Ni2P and other reported TMPs for OER.
d) Nyquist plots of NiFeP@NC/Ni2P, Ni–Fe PBA/Ni(OH)2, Ni(OH)2, and RuO2 (Inset: equivalent circuit). e) The double-layer capacitance (Cdl) of
NiFeP@NC/Ni2P, Ni–Fe PBA/Ni(OH)2, and Ni(OH)2. f) Multicurrent steps chronopotentiometry curve of NiFeP@NC/Ni2P without iR correction.
g) Stability test of NiFeP@NC/Ni2P on NF via chronoamperometry curve for 24 h without iR correction (Inset: polarization curves of NiFeP@NC/Ni2P on
NF before and after chronoamperometry measurement without iR correction). h) Schematic illustration of the comparison of 2D sheets and loop-sheets.

with that of Ni2P (Figure 3f and Figure S23, Supporting
Information).[49] The WT of Fe K-edge EXAFS oscillations of
NiFeP@NC/Ni2P as well displays one intensity maximum
at 4.2 Å−1 assigned to the Fe−P coordination, which is shiftedto a smaller value as compared with that of Ni2P–FeP@NC(Figure 3i and Figure S24, Supporting Information).[50] Allthese results clearly illustrate that the controllable construction of the edge-loop concurrently regulates the local electronic
configuration in NiFeP@NC/Ni2P heterostructure, which is
conducive to oxidation reaction and electron transfer.
In this way, the electrocatalytic performance of NiFeP@
NC/Ni2P for OER is thoroughly evaluated by a typical threeelectrode setup under alkaline condition (1 m KOH, pH 13.6).
For a comparison, the obtained Ni–Fe PBA/Ni(OH)2, Ni(OH)2,and the commercial RuO2 are measured at identical condition. As shown in Figure 4a, the polarization curve reveals
that the as-obtained NiFeP@NC/Ni2P exhibits an overpotential of 223 mV for OER at 20 mA cm−2, which is much smaller

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than those of Ni–Fe PBA/Ni(OH)2 (η20 = 310 mV), Ni(OH)2(η20 = 343 mV) as well as the commercial RuO2 (η20 = 284 mV).
Moreover, the NiFeP@NC/Ni2P product requires a much
lower overpotential of 257 mV to afford a high current density of 100 mA cm−2, whereas the commercial RuO2 needs
an overpotential of 394 mV to yield the same current densityof 100 mA cm−2. Besides, NiFeP@NC/Ni2P shows a muchenhanced mass activity at an overpotential of 300 mV, which
is about 9, 21, and 45 times higher than those of the benchmark RuO2, binary Ni–Fe PBA/Ni(OH)2, and pristine Ni(OH)2,
respectively (Figure S25, Supporting Information). All theseresults suggest the ultrahigh efficiency of NiFeP@NC/Ni2P forelectrocatalytic OER. To further investigate the OER kinetics,
the corresponding Tafel plots are measured. As displayed in
Figure 4b, the Tafel slope of NiFeP@NC/Ni2P is determined
to be 46.1 mV dec−1 that is much lower than those of RuO2
(78.2 mV dec−1), Ni–Fe PBA/Ni(OH)2 (71.5 mV dec−1), andNi(OH)2 (98.5 mV dec−1), implying the favorable OER kinetics of2006860 (7 of 11)© 2021 Wiley-VCH GmbH

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the NiFeP@NC/Ni2P hybrids. The electrocatalytic OER activityof different counterparts are also measured under the identical alkaline condition. As a result, the NiFeP@NC/Ni2P has
superior OER performance than Ni2P–FeP@NC (η20 = 244 mV;
Tafel slope: 54.9 mV dec−1), NiFeP@NC/Ni2P-60 (249 mV;
64.5 mV dec−1), NiFeP@NC/Ni2P-20 (277 mV; 65.0 mV dec−1),
and Ni2P (308 mV; 78.5 mV dec−1) (Figures S26 and S27, Supporting Information). Those results corroborate that the synergistic composition effects of hybrid provide a crucial factor to
the superior electrocatalytic OER performance,[51] in which theedge-topologic regulation on NiFeP@NC/Ni2P catalyst induces
Ni2P nanosheets confined by NiFeP@NC loop exhibiting the
optimal synergistic effects of the components. Impressively,
the low overpotential and Tafel slope point toward the OER
performance of NiFeP@NC/Ni2P being superior to almost all
TMP catalysts reported previously, placing the NiFeP@NC/
Ni2P catalyst among the top tier of water oxidation catalysts(Figure 4c and Tables S3 and S4, Supporting Information).
On the other hand, to elucidate the origins of the high
OER performance of NiFeP@NC/Ni2P, we carry out the electrochemical impedance spectroscopy (EIS) and double-layer
capacitance (Cdl) measurement. In explicit, EIS analysis is
first employed to uncover the interfacial reaction kinetics.
The Nyquist plot reveals that the NiFeP@NC/Ni2P catalyst
exhibits the lowest charge transfer resistance (4.5 Ω) as compared with those of RuO2 (8.8 Ω), Ni–Fe PBA/Ni(OH)2 (15.1 Ω),and Ni(OH)2 (16.2 Ω), implying the superior charge-transfer
capacity of NiFeP@NC/Ni2P because the self-anti-stacking
loop-sheet feature with lifted-edges can induce open channels to facilitate mass transfer (Figure 4d and Table S5, Supporting Information).[52] Subsequently, the values of Cdl aremeasured to estimate the electrochemically active surface area
(ECSA) in order to assess the corresponding intrinsic activityof OER electrocatalysts. The cyclic voltammetry (CV) curves of
NiFeP@NC/Ni2P, Ni–Fe PBA/Ni(OH)2, and Ni(OH)2 with different scan rates are recorded (Figure S28, Supporting Information). As presented in Figure 4e, the Cdl value of NiFeP@NC/
Ni2P is found to be 4.56 mF cm−2, being larger than that of
Ni–Fe PBA/Ni(OH)2 (3.08 mF cm−2) and Ni(OH)2(2.10 mF cm−2). It is suggested that a higher density of catalytically active sites is obtained in NiFeP@NC/Ni2P, which canbe ascribed to the formation of abundant grain boundaries in
the 2D Ni2P sheet and the gain of ultrafine NiFeP nanocrystals in the edge-loop. More importantly, the loop-sheet structure can prevent the stacking of 2D nanosheets from blocking
the active sites. Instead, it can create accessible open channels
for the better exposure of these catalytic sites. In addition,
Figure 4f illustrates the multistep chronopotentiometriccurve of NiFeP@NC/Ni2P with the current density increased
stepwise from 20 to 80 mA cm−2. Each increasing step, witha duration of 500 s, shows the instantly stable state as the current changes, verifying the excellent ion/mass transport (i.e.,
inward diffusion of OH− and outward diffusion of oxygen bubbles) of NiFeP@NC/Ni2P loop-sheet heterostructures with open
channel construction.[53,54]
The stability of NiFeP@NC/Ni2P is further evaluated via
a chronoamperometry curve for 24 h. It is noted that the Ni
foam (NF) is used as the substrate for this stability test to prevent the catalyst from detachment during a long-time opera-

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tion because the deposited catalyst would easily fall off fromthe small-area glass carbon electrode (GCE) under the influence of generated O2 gas bubbles.[55] As a result, the current
remains constant without any obvious degradation in thislong-duration electrochemical process, indicating an excellent
stability of this NiFeP@NC/Ni2P loop-sheet heterostructure(Figure 4g). The morphology of NiFeP@NC/Ni2P subjectedto the stability test is also examined. The corresponding TEM
image illustrates the catalysts sustained the original framework, demonstrating the structural stability of such 2D loopsheet heterostructures (Figure S29, Supporting Information).
This enhanced stability is anticipated due to the fact that the
N-doped carbon looped-skeleton would hold the sheet morerobustly with strong mechanical reinforcement.[56] Later, theprocessed NiFeP@NC/Ni2P catalyst after OER stability testfor 24 h is further characterized by the depth-profiling XPS toevaluate the OER mechanism (Figure S30a, Supporting Information).[57] Specifically, without any Ar ion etching, the characteristic peaks related to Ni−P and Fe−P bonding are completely disappeared, whereas there are only the oxidized peaks
observed in the Ni 2p and Fe 2p spectra (Figure S30b,c, Supporting Information), indicating the surface is completely oxidized on the phosphides after the reaction. It can be ascribedto the formation of oxy/hydroxides (M−OOH, M = Ni, Fe) as
active sites for OER,[58] which is consistent with the previous
reports of TMPs for OER.[59–61] Likewise, for P 2p spectrum
without any Ar ion etching, the disappearance of the phosphidecomponents (≈129 eV) further confirms the oxidation of phosphides on the surface (Figure S30d, Supporting Information).[4]Furthermore, the peaks ascribed to Ni−P and Fe−P bonding in
the Ni 2p and Fe 2p spectra, respectively, are present and intensified under the prolonged etching time, indicating the corematerials remained unchanged.[62] In this way, the OER catalytic capability of NiFeP@NC/Ni2P for OER can be originatedfrom the formation of catalytically active oxo/hydroxide on
the surface. Previous computational study of (Ni,Fe)OOH has
indicated that the converted Fe(IV) stabilizes the O radical byexchange interaction to generate it efficiently, while the Ni(IV)catalyzes the O–O coupling.[63] The two components assume
the two different critical functionalities necessitated for OER
and this synergy can deliver the optimal OER performance.[63,64]
Alternatively, the core phosphides cooperated with the conductive carbonaceous looped-skeleton are particularly beneficial forthe faster electron transport in the heterostructure.[26,65]
Taken all together, the 2D loop-sheet heterostructure caneffectively prevent sheet-to-sheet stacking with the lifted-edges,
while the conventional 2D nanosheets are prone to stack
together under the influence of van der Waals interaction,[66,67]
as schematically illustrated in Figure 4h. Therefore, this loopsheet heterostructure is advantageous to the exposure of active
sites and the enhanced contact area with electrolytes. Suchstructure with open channels can also promote the diffusionof water molecules and release of gaseous products to further
avail the OER activity of catalysts. Apart from the exterior geometric superiority, the interior compositional synergism accompanied with electronic regulation is also critical to the excellent
OER performance.[68] Specifically, the N-doped carbon loopedskeleton can provide improved electrical conductivity, whereas
the ultrafine NiFeP nanocrystals and Ni2P nanoparticles

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Figure 5. a) Schematic illustration of the two-electrode setup for overall water splitting. b) Polarization curves of NiFeP@NC/Ni2P||NiFeP@NC/Ni2P
and Pt/C||RuO2 in a two-electrode configuration for water splitting. c) Digital photograph of the NiFeP@NC/Ni2P||NiFeP@NC/Ni2P couple driven bya solar cell. d) Magnified photograph of the two-electrode setup and the electrodes.

provide abundant active Fe and Ni sites. Therefore, the appropriate control of NiFeP@NC edged-loop on the Ni2P sheet can
substantially lower the activation barrier with the optimal synergetic effect of Fe and Ni sites to achieve the superior OER
performance of NiFeP@NC/Ni2P.[7,69]
The electrocatalytic hydrogen evaluation reaction (HER)
performance of NiFeP@NC/Ni2P is also investigated in thealkaline solution (1 m KOH). It is found that the NiFeP@NC/
Ni2P catalyst exhibits an overpotential of 257 mV to afford acurrent density of 10 mA cm−2 with outstanding stability for
HER (Figure S31, Supporting Information). In this case, a twoelectrode configuration employing NiFeP@NC/Ni2P as bothanode and cathode is constructed for overall water splitting in
1.0 m KOH electrolyte (Figure 5a). As shown in Figure 5b, the
NiFeP@NC/Ni2P couple shows a cell voltage of 1.57 V to reacha benchmark current density of 10 mA cm−2, which is comparable to that of the commercial Pt/C||RuO2 couple (1.56 V). It isnoted that this voltage is lower than the simple addition of overpotentials from the HER and OER half-reaction, which could
be ascribed to the fact that 3D NF as supporting electrode has alarger surface area and better electrical conductivity as comparedwith the ones of GCE.[70] Notably, the NiFeP@NC/Ni2P coupledelivers a high current density of 100 mA cm−2 achieved by a cellvoltage of 1.83 V that is lower than that of Pt/C||RuO2 (1.86 V).
Furthermore, the durability of this electrolyzer is examined ata current density of 10 mA cm−2. The chronopotentiometriccurve illustrates that the NiFeP@NC/Ni2P||NiFeP@NC/Ni2P
couple maintains a steady cell voltage at 10 mA cm−2 over

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24 h (Figure S32, Supporting Information), indicating thegood mechanical robustness of NiFeP@NC/Ni2P as bifunctional electrocatalysts. More importantly, this two-electrodedevice is also integrated with a commercial silicon solar cell todemonstrate the practical utilization of NiFeP@NC/Ni2P for
solar-driven water electrolysis. Interestingly, the NiFeP@NC/
Ni2P||NiFeP@NC/Ni2P couple is efficiently powered by the solar
panel, which provides a real-time potential output of ≈1.68 V
under sunlight irradiation (Figure 5c). A large amount of bubbles is witnessed to be emerged from both electrodes (Figure 5d
and Movie S1, Supporting Information). As a result, a solarcell-driven overall water-splitting device can be effectively androbustly achieved by the NiFeP@NC/Ni2P couple, which renders it with a good potential for industrial applications.
In summary, Ni–Fe PBA nanocubes are edge-selectivelyaligned along the periphery of ultrathin Ni(OH)2 nanosheets
to constitute a self-anti-stacking 2D loop-sheet assembly. The
subsequent phosphorization treatment generates uniquephosphides based heterostructures, with the architecture composing interconnected Ni2P nanosheets and N-doped carbon
loop decorated with ultrafine NiFeP nanocrystals. The key tothis synthesis is owing to the edged-loop topology compensating the induced stress in the phase transformation process;therefore, the sheet-to-sheet stacking is prevented with lotsof the open channels resulted. This edge-topologic and electronic regulation on the 2D metal phosphide heterostructure
would contribute to the conspicuously accessible active sites
and the largely improved ion/mass transfer. In this way, the2006860 (9 of 11)© 2021 Wiley-VCH GmbH

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heterostructure exhibits a remarkable oxygen evolution activity
with an overpotential of 223 mV at 20 mA cm−2 and a small
Tafel slope of 46.1 mV dec−1, ranking the top tier among themetal phosphide electrocatalysts reported to date. Moreover,
the NiFeP@NC/Ni2P heterostructures are employed as bothanode and cathode for overall water splitting in alkaline electrolytes, exhibiting a current density of 10 mA cm−2 with a cellvoltage of 1.57 V as well as excellent durability. More intriguingly, a solar-cell-driven overall water-splitting device can berobustly established based on the heterostructures. This workpaves a promising pathway for preparing durable and vastly
active 2D transition-metal heterostructures towards versatile
applications in electrochemical energy storage and conversion.

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (Grants 51672229), the General Research Fund(CityU 11204618) and the Theme-based Research (T42-103/16-N) of
the Research Grants Council of Hong Kong SAR, China, the Science
Technology and Innovation Committee of Shenzhen Municipality (Grant
JCYJ20170818095520778), the Foshan Innovative and Entrepreneurial
Research Team Program (No. 2018IT100031) as well as the City
University of Hong Kong (Project No. 9610427).

Conflict of Interest
The authors declare no conflict of interest.

Keywords
bifunctional electrocatalysts, edge-selective synthesis, loop-sheets, overallwater splitting, oxygen evolution reaction, transition metal phosphides
Received: November 3, 2020
Revised: December 8, 2020
Published online: January 22, 2021

[1] B. You, Y. Sun, Acc. Chem. Res. 2018, 51, 1571.
[2] K. Jiang, M. Luo, M. Peng, Y. Yu, Y.-R. Lu, T.-S. Chan, P. Liu,
F. M. F. de Groot, Y. Tan, Nat. Commun. 2020, 11, 2701.
[3] L.-A. Stern, L. Feng, F. Song, X. Hu, Energy Environ. Sci. 2015, 8,
2347.
[4] X.-Y. Yu, Y. Feng, B. Guan, X. W. Lou, U. Paik, Energy Environ. Sci.
2016, 9, 1246.
[5] D. Li, H. Baydoun, C. N. Verani, S. L. Brock, J. Am. Chem. Soc. 2016,
138, 4006.
[6] M. Sun, H. Liu, J. Qu, J. Li, Adv. Energy Mater. 2016, 6, 1600087.
[7] B. Qiu, L. Cai, Y. Wang, Z. Lin, Y. Zuo, M. Wang, Y. Chai, Adv. Funct.
Mater. 2018, 28, 1706008.
[8] H. Guo, Q. Feng, K. Xu, J. Xu, J. Zhu, C. Zhang, T. Liu, Adv. Funct.
Mater. 2019, 29, 1903660.

Small 2021, 17, 2006860[9] D. Deng, K. S. Novoselov, Q. Fu, N. Zheng, Z. Tian, X. Bao, Nat.
Nanotechnol. 2016, 11, 218.
[10] W. Xu, F. Lyu, Y. Bai, A. Gao, J. Feng, Z. Cai, Y. Yin, Nano Energy
2018, 43, 110.
[11] M. Zhao, Y. Huang, Y. Peng, Z. Huang, Q. Ma, H. Zhang, Chem.
Soc. Rev. 2018, 47, 6267.
[12] S.-H. Li, N. Zhang, X. Xie, R. Luque, Y.-J. Xu, Angew. Chem., Int. Ed.
2018, 57, 13082.
[13] C. Tan, X. Cao, X.-J. Wu, Q. He, J. Yang, X. Zhang, J. Chen, W. Zhao,
S. Han, G.-H. Nam, M. Sindoro, H. Zhang, Chem. Rev. 2017, 117, 6225.
[14] J. Yu, Q. Wang, D. O’Hare, L. Sun, Chem. Soc. Rev. 2017, 46, 5950.
[15] C. Hu, L. Zhang, Z. J. Zhao, A. Li, X. Chang, J. Gong, Adv. Mater.
2018, 30, 1705538.
[16] X. Su, Y. Wang, J. Zhou, S. Gu, J. Li, S. Zhang, J. Am. Chem. Soc.
2018, 140, 11286.
[17] K. W. Chapman, P. J. Chupas, C. J. Kepert, J. Am. Chem. Soc. 2006,
128, 7009.
[18] X. Y. Yu, L. Yu, H. B. Wu, X. W. Lou, Angew. Chem., Int. Ed. 2015,
54, 5331.
[19] Y. Zeng, G.-F. Chen, Z. Jiang, L.-X. Ding, S. Wang, H. Wang, J. Mater.
Chem. A 2018, 6, 15942.
[20] Y. Wang, J. Ma, J. Wang, S. Chen, H. Wang, J. Zhang, Adv. Energy
Mater. 2019, 9, 1802939.
[21] J. Nai, B. Y. Guan, L. Yu, X. W. D. Lou, Sci. Adv. 2017, 3, e1700732.
[22] Z. Chen, B. Fei, M. Hou, X. Yan, M. Chen, H. Qing, R. Wu, Nano
Energy 2020, 68, 104371.
[23] R.-Q. Li, B.-L. Wang, T. Gao, R. Zhang, C. Xu, X. Jiang, J. Zeng,
Y. Bando, P. Hu, Y. Li, X.-B. Wang, Nano Energy 2019, 58, 870.
[24] G. Yilmaz, C. F. Tan, Y.-F. Lim, G. W. Ho, Adv. Energy Mater. 2019,9, 1802983.
[25] B. Cao, Y. Cheng, M. Hu, P. Jing, Z. Ma, B. Liu, R. Gao, J. Zhang,Adv. Funct. Mater. 2019, 29, 1906316.
[26] F. Yu, H. Zhou, Y. Huang, J. Sun, F. Qin, J. Bao, W. A. Goddard,
S. Chen, Z. Ren, Nat. Commun. 2018, 9, 2551.
[27] X. Wang, Z. Wang, J. Zhang, X. Wang, Z. Zhang, J. Wang, Z. Zhu,
Z. Li, Y. Liu, X. Hu, J. Qiu, G. Hu, B. Chen, N. Wang, Q. He,
J. Chen, J. Yan, W. Zhang, T. Hasan, S. Li, H. Li, H. Zhang, Q. Wang,
X. Huang, W. Huang, Nat. Commun. 2018, 9, 3611.
[28] S. H. Lee, D. Y. Kim, J. Lee, S. B. Lee, H. Han, Y. Y. Kim, S. C. Mun,
S. H. Im, T.-H. Kim, O. O. Park, Nano Lett. 2019, 19, 5437.
[29] L. Yu, J. Zhang, Y. Dang, J. He, Z. Tobin, P. Kerns, Y. Dou, Y. Jiang,
Y. He, S. L. Suib, ACS Catal. 2019, 9, 6919.
[30] Q. Liang, L. Zhong, C. Du, Y. Luo, J. Zhao, Y. Zheng, J. Xu, J. Ma,
C. Liu, S. Li, Q. Yan, ACS Nano 2019, 13, 7975.
[31] W. Zhang, Y. Zou, H. Liu, S. Chen, X. Wang, H. Zhang, X. She,D. Yang, Nano Energy 2019, 56, 813.
[32] Q. Quan, T. Zhang, C. Lei, B. Yang, Z. Li, J. Chen, C. Yuan, L. Lei,
Y. Hou, Nanoscale 2019, 11, 22261.
[33] P. Ge, S. Li, H. Shuai, W. Xu, Y. Tian, L. Yang, G. Zou, H. Hou, X. Ji,
Adv. Mater. 2019, 31, 1806092.
[34] F.-X. Ma, C.-Y. Xu, F. Lyu, B. Song, S.-C. Sun, Y. Y. Li, J. Lu, L. Zhen,
Adv. Sci. 2019, 6, 1801490.
[35] X. Wang, L. Chai, J. Ding, L. Zhong, Y. Du, T.-T. Li, Y. Hu, J. Qian,
S. Huang, Nano Energy 2019, 62, 745.
[36] Y. Wu, X. Tao, Y. Qing, H. Xu, F. Yang, S. Luo, C. Tian, M. Liu, X. Lu,
Adv. Mater. 2019, 31, 1900178.
[37] X. F. Lu, L. Yu, X. W. Lou, Sci. Adv. 2019, 5, eaav6009.
[38] F. Yang, H. Gao, J. Hao, S. Zhang, P. Li, Y. Liu, J. Chen, Z. Guo, Adv.
Funct. Mater. 2019, 29, 1808291.
[39] S.-H. Yu, R. Wu, B. Xiao, Q. Gao, Y. Zheng, X. Zheng, J. Zhu,
M.-R. Gao, Angew. Chem., Int. Ed. 2018, 57, 15445.
[40] J.-X. Feng, S.-Y. Tong, Y.-X. Tong, G.-R. Li, J. Am. Chem. Soc. 2018,
140, 5118.
[41] Y. Tong, Y. Guo, P. Chen, H. Liu, M. Zhang, L. Zhang, W. Yan,
W. Chu, C. Wu, Y. Xie, Chem 2017, 3, 812.

2006860 (10 of 11)© 2021 Wiley-VCH GmbH

www.small-journal.com

www.advancedsciencenews.com[42] L. Zhao, Y. Zhang, L.-B. Huang, X.-Z. Liu, Q.-H. Zhang, C. He,
Z.-Y. Wu, L.-J. Zhang, J. Wu, W. Yang, L. Gu, J.-S. Hu, L.-J. Wan,Nat. Commun. 2019, 10, 1278.
[43] E. Cao, Z. Chen, H. Wu, P. Yu, Y. Wang, F. Xiao, S. Chen, S. Du,
Y. Xie, Y. Wu, Z. Ren, Angew. Chem., Int. Ed. 2020, 59, 4154.
[44] Q. He, D. Tian, H. Jiang, D. Cao, S. Wei, D. Liu, P. Song, Y. Lin,
L. Song, Adv. Mater. 2020, 32, 1906972.
[45] D. Zhao, K. Sun, W. C. Cheong, L. Zheng, C. Zhang, S. Liu, X. Cao,
K. Wu, Y. Pan, Z. Zhuang, B. Hu, D. Wang, Q. Peng, C. Chen, Y. Li,
Angew. Chem., Int. Ed. 2020, 59, 8982.
[46] X. Wang, Y. Jia, X. Mao, D. Liu, W. He, J. Li, J. Liu, X. Yan, J. Chen,
L. Song, A. Du, X. Yao, Adv. Mater. 2020, 32, 2000966.
[47] Y. Pan, K. Sun, Y. Lin, X. Cao, Y. Cheng, S. Liu, L. Zeng,
W.-C. Cheong, D. Zhao, K. Wu, Z. Liu, Y. Liu, D. Wang, Q. Peng,
C. Chen, Y. Li, Nano Energy 2019, 56, 411.
[48] X. Liu, F. Liu, J. Yu, G. Xiong, L. Zhao, Y. Sang, S. Zuo, J. Zhang,
H. Liu, W. Zhou, Adv. Sci. 2020, 7, 2001526.
[49] J. Cai, Y. Song, Y. Zang, S. Niu, Y. Wu, Y. Xie, X. Zheng, Y. Liu, Y. Lin,
X. Liu, G. Wang, Y. Qian, Sci. Adv. 2020, 6, eaaw8113.
[50] H. Zhang, W. Zhou, J. Dong, X. F. Lu, X. W. Lou, Energy Environ. Sci.
2019, 12, 3348.
[51] T. Liu, P. Li, N. Yao, G. Cheng, W. Luo, S. Chen, Y. Yin, Angew.
Chem., Int. Ed. 2019, 58, 4679.
[52] Y. Guo, J. Tang, Z. Wang, Y. Sugahara, Y. Yamauchi, Small 2018, 14,
1802442.
[53] S. Gao, Y. Zhang, Y. Zhang, B. Wang, S. Yang, Small 2018, 14,
1804388.
[54] X. Lu, C. Zhao, Nat. Commun. 2015, 6, 6616.
[55] W. Ni, A. Krammer, C.-S. Hsu, H. M. Chen, A. Schüler, X. Hu,
Angew. Chem., Int. Ed. 2019, 58, 7445.
[56] J. Wang, D. Liu, H. Huang, N. Yang, B. Yu, M. Wen,
X. Wang, P. K. Chu, X. F. Yu, Angew. Chem., Int. Ed. 2018,57, 2600.

Small 2021, 17, 2006860[57] H. Q. Fu, L. Zhang, C. W. Wang, L. R. Zheng, P. F. Liu, H. G. Yang,
ACS Energy Lett. 2018, 3, 2021.
[58] H. J. Song, H. Yoon, B. Ju, G.-H. Lee, D.-W. Kim, Adv. Energy Mater.
2018, 8, 1802319.
[59] L. Jiayuan, Y. Ming, Z. Xuemei, H. Zheng-Qing, X. Zhaoming,
C. Chun-Ran, M. Yuanyuan, Q. Yongquan, Adv. Funct. Mater. 2016,
26, 6785.
[60] H. B. Zhang, W. Zhou, J. Dong, X. F. Lu, X. W. D. Lou, Energy
Environ. Sci. 2019, 12, 3348.
[61] P. Ji, H. Jin, H. Xia, X. Luo, J. Zhu, Z. Pu, S. Mu, ACS Appl. Mater.
Interfaces 2020, 12, 727.
[62] J. Xu, X.-K. Wei, J. D. Costa, J. L. Lado, B. Owens-Baird, L. P. L. Gonçalves,
S. P. S. Fernandes, M. Heggen, D. Y. Petrovykh, R. E. Dunin-Borkowski,
K. Kovnir, Y. V. Kolen’ko, ACS Catal. 2017, 7, 5450.
[63] H. Xiao, H. Shin, W. A. Goddard, Proc. Natl. Acad. Sci USA 2018,
115, 5872.
[64] Y. Zhang, N. Li, Z. Zhang, S. Li, M. Cui, L. Ma, H. Zhou, D. Su,
S. Zhang, J. Am. Chem. Soc. 2020, 142, 8490.
[65] H. Liang, A. N. Gandi, D. H. Anjum, X. Wang, U. Schwingenschlögl,
H. N. Alshareef, Nano Lett. 2016, 16, 7718.
[66] M. Qin, S. Li, Y. Zhao, C.-Y. Lao, Z. Zhang, L. Liu, F. Fang, H. Wu,
B. Jia, Z. Liu, W. Wang, Y. Liu, X. Qu, Adv. Energy Mater. 2019, 9,1803060.
[67] J. Yang, Z. Zeng, J. Kang, S. Betzler, C. Czarnik, X. Zhang, C. Ophus,
C. Yu, K. Bustillo, M. Pan, J. Qiu, L.-W. Wang, H. Zheng, Nat. Mater.
2019, 18, 970.
[68] G. Guan, M.-Y. Han, Adv. Sci. 2019, 6, 1901837.
[69] D. Friebel, M. W. Louie, M. Bajdich, K. E. Sanwald, Y. Cai,
A. M. Wise, M. J. Cheng, D. Sokaras, T. C. Weng, R. Alonso-Mori,R. C. Davis, J. R. Bargar, J. K. Norskov, A. Nilsson, A. T. Bell, J. Am.
Chem. Soc. 2015, 137, 1305.
[70] Y. Lian, H. Sun, X. Wang, P. Qi, Q. Mu, Y. Chen, J. Ye, X. Zhao,
Z. Deng, Y. Peng, Chem. Sci. 2019, 10, 464.

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