Asymmetrically strained hcp rhodium sublattice stabilized by 1D covalent boron chains as an efficient electrocatalyst

Article abstract of DOI:10.1039/d1cc00774b

Intermetallic rhodium boride (RhB) comprising an asymmetrically strained hcp Rh sublattice is synthesized. The covalent interaction of interstitial boron atoms is found to be the main contributor to the generation of asymmetric strains and the stabilization of the hcp Rh sublattice. In addition, RhB is identified as a hydrogen-evolving eletrocatalyst with Pt-like activity, because the Rh(d)-B(s,p) orbital hybridization induces an optimized electronic structure.

Full text of DOI:10.1039/d1cc00774b

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Asymmetrically strained hcp rhodium sublattice
stabilized by 1D covalent boron chains as an
efficient electrocatalyst†
Cite this: DOI: 10.1039/d1cc00774b
Received 10th February 2021,
Accepted 14th April 2021
b
Zhenyu Li,‡ab Xuan Ai, ‡a Hui Chen,‡a Xiao Liang,a Xiaotian Li,
Dong Wangc
a
and Xiaoxin Zou
*
DOI: 10.1039/d1cc00774b
rsc.li/chemcomm
Intermetallic rhodium boride (RhB) comprising an asymmetrically metals, and to explore their atomic configuration-dependent proper-
strained hcp Rh sublattice is synthesized. The covalent interaction ties and functionalities.
of interstitial boron atoms is found to be the main contributor to
Herein, we report the synthesis of phase-pure intermetallic
the generation of asymmetric strains and the stabilization of the rhodium boride (RhB) comprising an asymmetrically strained
hcp Rh sublattice. In addition, RhB is identified as a hydrogen-evolving hcp-Rh sublattice, in which the interstitial boron atoms are
eletrocatalyst with Pt-like activity, because the Rh(d)–B(s,p) orbital covalently bonded in a single chain and different chains with-
hybridization induces an optimized electronic structure.
out direct bonding interaction are aligned parallel to the c axis.
We find that the covalent interaction of interstitial boron atoms
Noble metals commonly form close-packed crystal structures. plays the most vital role in the stabilization of the hcp Rh
Some metal atoms (e.g., Ru and Os) prefer the hexagonal close- sublattice, compared to other factors such as unit cell expan-
packed (hcp) structure, and the others (e.g., Rh, Ir, Pd, Pt, Ag sion, asymmetric strains and Rh–B interatomic interaction. We
and Au) form the cubic close-packed (ccp, also known as face- also show that RhB is a highly active eletrocatalyst for the
centered cubic or fcc) structure. Recently, many research hydrogen evolution reaction (HER) due to its optimized electro-
attempts have been made to stabilize new metallic lattices nic structure mainly governed by Rh–B interatomic orbital
other than the aforementioned conventional ones, and the vast hybridization. Despite recent reports on the catalytic properties
majority of successful cases include the realization of fcc- of some transition metal borides, RhB has never been investi-
metals in hcp atomic packing or vice versa.1,2 The formation gated experimentally to catalyze any reactions previously.7–11
of unconventional metallic lattices are mostly accessed by
We synthesized intermetallic RhB by a solid state reaction
making changes in the environment of noble metal crystallites’ between K3RhCl6 and MgB2 (details are provided in the Experi-
growth. Suitable environmental modulators, such as growth mental section, ESI†). For the comparative purpose, we also
templates, surface ligands, structural defects and interstitial synthesized metallic Rh and B-doped Rh with disordered
atoms, have been confirmed in some cases.3,4 In other cases, interstitial boron distribution (dubbed B–Rh) as reference
e-beam irradiation and external pressure also can cause metallic samples (Fig. S1 and S2, ESI†). The powder X-ray diffraction
lattice transition from the conventional to unconventional (XRD) patterns in Fig. 1a show that both metallic Rh and B–Rh
structure.5,6 Despite recent significant successes, the stabilization possess the fcc structure. Despite the similarity, the diffraction
of unconventional metallic lattices is still a trial-and-error process. peaks of B–Rh slightly shift toward the small angle side
More efforts are required to provide a fundamental understanding compared to those of metallic Rh, suggesting that B atoms
of unconventional metallic lattices and crystallochemistry of noble are interstitially incorporated in the fcc-Rh lattice for B–Rh and
expand the lattice volume. Unlike B–Rh, the diffraction peaks of
RhB are completely different from those of metallic Rh, due to
the interstitial boron ordering and hcp-Rh sublattice. The XRD
pattern of RhB measured experimentally is highly consistent
with the fitting plot (Fig. S3a and Table S1, ESI†), indicating the
high purity of intermetallic RhB. Energy-dispersive X-ray
spectroscopy (EDS) exhibits that the Rh : B atomic ratio in both
RhB and B–Rh is about 1 : 1, revealing that the interstitial boron
ordering in Rh–B alloy is necessary for the generation of the
a State Key Laboratory of Inorganic Synthesis and Preparative Chemistry,
College of Chemistry, Jilin University, Changchun 130012, China.
E-mail: xxzou@jlu.edu.cn
b College of Materials Science and Engineering, Jilin University, Changchun 130022,
China
c College of Chemical Engineering, Sichuan University, Chengdu 610065, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc00774b
‡ These authors contributed equally to this work.
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Fig. 1 (a) XRD patterns of Rh, B–Rh and RhB, using the standard cards of
RhB (JCPDS 13-0289) as reference. (b) Rh 3d XPS spectra of the Rh, Rh2O3
and RhB. (c) B 1s XPS spectra of B, NaBO2 and RhB.
hcp-Rh sublattice. In addition, another intermetallic rhodium
boride with a lower boron content (Rh7B3) was also synthesized
(Fig. S4–S6 and Table S2, ESI†). The Rh atoms in Rh7B3 form a
close-packed structure, neither a fcc nor a hcp structure. This
result suggests that besides interstitial ordering, a suitable
amount of interstitial boron atoms is also critical for the
realization of the hcp-Rh sublattice.
The scanning electron microscopy (SEM) image shows that
the RhB sample is composed of particles that are 100–700 nm
in size (Fig. S3b, ESI†). Further analysis using elemental map-
ping images (Fig. S3c, ESI†) confirms the uniform distribution
of Rh and B elements over the entire particles. In the transmis-
sion electron microscopy (TEM) image (Fig. S3d, ESI†), the
lattice spacing of 0.28 nm can be ascribed to the (100) crystal-
Fig. 2 (a) Crystal structure of RhB. In order to clarify the stacking pattern
of the Rh sublattice, the Rh atoms are represented using blue and green
spheres on the basis of the difference in their position. (b) ELF of the (110)
crystallographic plane for RhB. (c) COHP curves for Ru–B and B–B pairs in
RhB. (d) Equations of state for fcc-Rh and hcp-Rh lattices. Inset in (d)
shows the fcc and hcp atomic arrangements of the Rh lattice. The fcc
structure is an a–b–c. . . stacking pattern, and the hcp structure is an
a–b. . . stacking pattern. (e) The total energy as a function of strain exerted
on the hcp-Rh lattice along the c axis at a fixed cell volume of RhB
(20.44 Å3). The inset shows a strained hcp-Rh lattice model. (f) The
formation energies of RhB, RhC and RhN; and the negative integrated
COHP (–ICOHP) values of Rh–X, X–X (X = B, C and N) in the three models.
lographic planes of RhB. The surface elemental composition (COHP) curves (Fig. 2c). The results show that there are
and chemistry of RhB were analyzed using X-ray photoelectron significant contributions of bonding states below the Fermi
spectroscopy (XPS). Fig. 1b displays Rh 3d XPS spectra of RhB energy for Rh–B and B–B bonds, and the bonding strength of
and the reference samples (Rh and Rh2O3), and Fig. 1c shows Rh–B is weaker than that of B–B.
the B 1s XPS spectra of RhB and the reference samples (B and
It is worth emphasizing that the cell volume of the hcp-Rh
NaBO2). The Rh 3d and B 1s peaks of RhB do not shift in sublattice (20.44 Å3) of RhB is obviously larger than the opti-
binding energy compared to those of elemental Rh and B, mized unit cell volume for hcp-Rh (14.11 Å3), and the a and b
suggesting a negligible charge transfer between Rh and B lattice constants of the former are greater (2.73 Å vs. 3.37 Å),
atoms in RhB. The XPS results also suggest that an obvious with the c lattice constant of the former being smaller (4.37 Å
surface oxide layer of RhB does not exist, again confirming the vs. 4.14 Å). In other words, huge asymmetric lattice strains are
high-purity of the obtained RhB.
exerted on the hcp-Rh sublattice of RhB: 24% tensile strain
RhB possesses P63/mmc symmetry and a hcp-Rh sublattice, along the a and b axes, and 5% compressive strain along the
in which boron atoms occupy all of the octahedral interstices c axis (see method for strain calculation and Table S3 and S4,
and form 1D covalent boron chains along the c axis (Fig. 2a). ESI†). The direction of B–B bonding interaction is consistent
The electron location function (ELF) value between two adja- with that of compressive strain, implying that the B–B covalent
cent boron atoms along the c axis is ca. 0.6, suggesting the bond is possibly the origin of asymmetrically strains.
covalent characteristic of the B–B bond (Fig. 2b). And the ELF
In order to better understand why the metallic sublattice of
values in a range of 0.4 to 0.5 for Rh–Rh and Rh–B bonds RhB is the unconventional hcp-Rh, we considered three possi-
demonstrate that both Rh–Rh and Rh–B bonds are metallic. ble factors: lattice expansion, asymmetric strains and boron-
The strong orbital hybridization in the Rh–B and B–B pairs can relevant electronic effect. We first compared the equations of
also be reflected by crystal orbital Hamiltonian population state for fcc-Rh and hcp-Rh (Fig. 2d and Fig. S7 in ESI†), which
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show their total energies as a function of unit cell volume. The
result demonstrates that in a wide volume range, fcc-Rh always
has a lower total energy than hcp-Rh, suggesting the higher
thermodynamic stability of fcc-Rh, regardless of the expansion
or compression in the cell volume. Hence, the lattice expansion
is not the main reason for the stabilization of the hcp-Rh
sublattice in RhB.
We also explored the effect of strain on the thermodynamic
stability of the hcp-Rh lattice with a fixed unit cell volume of
RhB. The total energy of the hcp-Rh lattice as a function of
strain along the c axis is presented in Fig. 2e. The result shows
that hcp-Rh can become more stable than fcc-Rh only when the
compressive strain along the c axis is larger than 28%. But the
compressive strain (17%) produced by interstitial boron atoms
does not reach this critical point. Thus, the strain is also not the Fig. 3 (a) Linear sweep voltammetry (LSV) curves toward HER of RhB, Rh,
B–Rh, and Pt in acidic solution with 85% iR-compensation. (b) Comparison
critical reason behind the formation of the hcp-Rh sublattice
in RhB.
In order to investigate the boron-relevant electronic effect,
we theoretically constructed two control structural models (i.e.,
of ECSAs and overpotentials at 0.1 mA cmÀ2 current density (normalized
by ECSAs). (c) Tafel slopes toward HER of RhB, Rh, B–Rh, and Pt.
(d) Chronopotentiometric curves of BhB, Rh, and B–Rh as catalysts at
10 mA cmÀ2 current density. (e) Faraday efficiency of hydrogen production
RhC and RhN) by replacing interstitial boron atoms of RhB with over RhB at a current density of 20 mA cmÀ2 in 0.5 M H2SO4 solution.
C and N atoms, respectively, and then compared their crystal
and electronic structures with those of RhB. The ELF and COHP
results of RhC and RhN are shown in Fig. S8 and S9 (ESI†), activity than metallic Rh. Particularly, the overpotential
respectively. A comparison of the negative integrated Crystal required by RhB (15 mV) is even slightly lower than that
Orbital Hamilton Population values (–ICOHP, a quantitative required by the Pt catalyst to achieve 10 mA cmÀ2 current
description of bonding strength) and formation energies for density. The intrinsic activities of RhB, Rh, B–Rh, and Pt are
RhB, RhC and RhN (Fig. 2f and Table S5, ESI†) produces three further compared by normalizing their HER currents with the
important results. (i) Only RhB is thermodynamically stable corresponding electrochemical active surface area ( jECSA). As
(reflected by its negative formation energy), while RhC and RhN shown in Fig. 3b, their intrinsic activities for HER follow the
are thermodynamically unfavorable. Moreover, the phononic same trend as their apparent activities.
dispersion curves also show that only RhB is dynamically
The Tafel slopes for HER of RhB, Rh, B–Rh, and Pt are also
stable, because there is no imaginary frequency in phonon compared (Fig. 3c). Tafel slope of RhB is 27.8 mV decÀ1, which
dispersion for RhB (Fig. S10, ESI†). (ii) The bonding strengths is comparable to that of Pt (26.2 mV decÀ1) and much smaller
between Rh and B/C/N in three models are all great, with Rh–C than those of Rh and B–Rh. This result demonstrates the rapid
having the strongest bonding strength. This result, together HER catalytic kinetics for RhB during the reaction process.12–15
with the result (i), shows that the electronic interaction between The Tafel slope of E30 mV decÀ1 also suggests that the hydrogen
Rh lattice and interstitial atoms is not the key cause for the evolution on the RhB surface proceeds via a Volmer–Tafel mecha-
stabilization of the hcp-Rh lattice. (iii) It is unique that there is nism, in which the recombination of the two adsorbed hydrogen
a strong B–B bonding in RhB, compared with the extremely atoms (i.e., Tafel step, H* + H* - H2) is the rate-determining
weak C–C and N–N interactions in RhC and RhN, respectively step.16–18
(Fig. 2f). Additionally, there is an asymmetrical strain for only
In addition to high catalytic activity, RhB exhibits good cata-
RhB (Table S4, ESI†). These results overall demonstrate that the lytic and structural stabilities during electrocatalysis. As shown in
strong B–B covalent interaction is the main contributor to the Fig. 3d, while the catalytic activities of Rh and B–Rh decrease
generation of asymmetrically strains and the stabilization of slowly, RhB can stably electrocatalyze HER for at least 10 h,
the hcp-Rh sublattice in RhB.
demonstrating its high catalytic stability. The structure of RhB
The electrocatalytic properties of RhB toward HER in acidic after a 10 h-long HER electrocatalysis was characterized. Its XRD
electrolyte were investigated (see the Experimental section and pattern and HRTEM image show that RhB maintains its original
Fig. S11 in the ESI†). For comparison, metallic Pt particles were structure after HER (Fig. S13 and S14, ESI†). A comparison of RhB
also studied (Fig. S12, ESI†). The electrocatalytic activities of with some representative Rh-based electrocatalysts reveals that
RhB, metallic Pt, metallic Rh and B–Rh were evaluated under RhB is among the most efficient Rh-based catalysts for HER both
the same conditions. Fig. 3a shows the polarization curves of in terms of activity and stability (Table S6, ESI†). During the
RhB, Rh, B–Rh and Pt toward HER. As expected, Rh has a electrocatalysis, the hydrogen evolution rate of RhB is in good
moderate catalytic activity, and Pt has a remarkable catalytic agreement with the theoretical value (Fig. 3e), indicating a com-
activity. Pt requires an overpotential of 20 mV at 10 mA cmÀ2 plete electricity-to-hydrogen conversion.
current density (normalized by the geometric area of the
In order to get better insights into the good HER catalytic
electrode). In addition, RhB and B–Rh exhibit higher catalytic activity of RhB, we calculated the hydrogen adsorption free
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antibonding states become more occupied and the adsorption
strength of hydrogen is weakened. This explains why RhB is
able to possess a Pt-like catalytic activity for HER.
In conclusion, the synthesis and structural characterization
of phase-pure intermetallic rhodium boride comprising an
asymmetrically strained hcp-Rh sublattice have been pre-
sented. The strong B–B covalent intereaction is responsible
for the stabilization of the unconventional hcp Rh sublattice of
RhB. RhB is found to be a highly efficient eletrocatalyst for HER
owing to its optimized surface electronic structure mainly
governed by the Rh–B interatomic orbital hybridization. The
results reported herein further our understanding of metallic
lattices and boride-based catalysts.
X. Z. and H. C. thank the financial supports from
the National Natural Science Foundation of China (NSFC)
Grant No. 21922507, 21771079 and 21901083 and Fok Ying
Tung Education Foundation, Grant No. 161011. The authors
also thank the Fundamental Research Funds for the Central
Universities, NSFC (21621001) and the 111 Project (B17020) for
the additional financial support.
Fig. 4 Structural models for the stable hydrogen adsorption sites on
(a) hcp-Rh(0001) and (b) RhB(001) surfaces. (c) The calculated free-
energy diagrams at equilibrium potential for RhB(001), hcp-Rh(0001),
fcc-Rh(111) and Pt(111) surfaces at a 100% H* coverage. (d) The reaction
pathway for Tafel step on the RhB(001) surface. (e) Density of states (DOS)
of hcp-Rh(0001) and RhB(001) surfaces. The vertical dotted line denotes
the position of the Fermi energy, and the solid lines indicate the position of
d-band centers.
Conflicts of interest
energy (DGH*, a theoretical descriptor of HER activity) of close-
packed surfaces for RhB and hcp-Rh. For comparison, fcc-Rh
and Pt were also studied (the details of the modeling method
are shown in the ESI†). The structural models for the H
adsorption sites on RhB(001) and hcp-Rh(0001) surfaces are
shown in Fig. 4a and b and the structural models for the H
adsorption sites on fcc-Rh(111) and Pt(111) surfaces are pre-
sented in Fig. S15, ESI.† The results show that the most stable
H adsorption sites of all model surfaces are the Rh3-hollow sites
(Table S7, ESI†). The calculations (Fig. 4c) reveal that RhB
exhibits a smaller absolute DGH* value (0.18 eV) than fcc-Rh
(0.30 eV) and hcp-Rh (0.36 eV), and RhB exhibits a slightly
stronger hydrogen adsorption ability than Pt (DGH* = À0.11 eV).
The calculated activation energy for HER on the RhB(001)
surface on the basis of the Volmer–Tafel mechanism is
0.47 eV (Fig. 4d), which is lower than that on the Pt(111) surface
(0.85 eV).16 These theoretical results confirm the experimental
observation on the good activity of RhB. Additionally, these
theoretical results also suggest that the hcp-Rh sublattice itself
does not rationalize RhB’s good catalytic activity.
We next investigated the Rh–B interatomic orbital inter-
action on surface electronic structure and surface hydrogen
adsorption properties. We calculated the density of states (DOS)
of RhB(001) and hcp-Rh(0001). As revealed in Fig. 4e, there is a
DOS overlap between Rh-4d band and B-2sp band, indicating
the orbital hybridization between the Rh and B atoms. This is
in agreement with the COHP result (Fig. S16, ESI†). Moreover,
the d-band width (2.70 eV) of RhB is larger than that (2.37 eV)
of hcp-Rh (Table S8, ESI†). And the d-band center of RhB
There are no conflicts to declare.
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