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Cite this: J. Mater. Chem. A, 2020, 8,21833

Eﬃcient photocatalytic CO2 reduction mediated bytransitional metal borides: metal site-dependent
activity and selectivity†
Li Shi,‡a Pei Wang,‡b Qi Wang,ac Xiaohui Ren,ac Fumihiko Ichihara,ac Wei Zhou, eHongwei Zhang,f Yasuo Izumi, f Ben Cao,b Shengyao Wang, ab Hao Chen *b
and Jinhua Ye *acd
The search for novel and low-cost cocatalysts that can achieve high eﬃciency, while maintaining highselectivity in photocatalytic CO2 reduction is highly desirable, yet challenging. Herein, we demonstratethat transitional metal borides (TMBs) Ni3B, Co3B and Fe2B can serve as eﬀective and low-cost
cocatalysts to enhance the performance of photocatalytic CO2 reduction under visible light, whileother TMBs (TaB2, NbB2 and MoB) are almost ineﬀective. The performance of photocatalytic CO2
reduction depends on the metal site in TMBs, among which Ni3B exhibits the highest selectivity andactivity compared to those of other TMBs. Speciﬁcally, Ni3B exhibits a CO evolution rate of 157.7mmol h1 with selectivity of 93.0% in the presence of [Ru(bpy)3]Cl2 as a light absorber under visible
light, representing state-of-the-art cocatalyst. The excellent activity of Ni3B can be ascribed to its

Received 20th July 2020
Accepted 30th September 2020

metallic feature that can eﬃciently transport photogenerated electrons from the light absorber, as
well as the unique Ni–B bond that acts as an electron accumulator to provide abundant long-lived
electrons to be injected into Ni0 active sites for the activation of CO2. These ﬁndings may provide the

DOI: 10.1039/d0ta07072f

principle guidance for the search and design of eﬃcient cocatalytic materials and systems for solar

rsc.li/materials-aenergy conversion.

1. Introduction
The photocatalytic conversion of CO2 to valuable chemical
fuels (such as CO, CH4 and HCOOH) with solar energy as thesole energy input has attracted intensive attention, as it not
only address energy shortage but also alleviates the greenhouseeﬀect.1,2 Achieving highly eﬃcient and selective photocatalytic
CO2 conversion remains a critical issue because of the highthermodynamic stability of CO2 molecules, the limitedaInternational Center for Materials Nanoarchitectonics (WPI-MANA), National
Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki, 305-0044,
Japan. E-mail: Jinhua.YE@nims.go.jpbCollege of Science, Huazhong Agricultural University, Wuhan, 430070, P. R. China.
E-mail: hchenhao@mail.hzau.edu.cn

cGraduate School of Chemical Sciences and Engineering, Hokkaido University,
Sapporo, 060-0814, Japand

TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin
University, 92 Weijin Road, Nankai District, Tianjin 300072, P. R. Chinae

Department of Applied Physics, Tianjin Key Laboratory of Low Dimensional Materials
Physics and Preparing Technology, Faculty of Science, Tianjin University, Tianjin
300072, P. R. Chinaf

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33,
Inage-ku, Chiba 263-8522, Japan† Electronic supplementary
10.1039/d0ta07072f

information(ESI)

available.

‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2020

See

DOI:

separation eﬃciency of electron–hole pairs of photocatalysts,
the sluggish kinetics of the multiple proton-coupled electrontransfer processes, and the erce competition of the favorable
H2 evolution reaction.3,4 Recently, both the selectivity andactivity of the photocatalytic CO2 conversion were found to begreatly improved by cocatalysts via lowering the overpotential
for facilitating the CO2 reduction and assisting the separationof photogenerated charges.5 The research of eﬀective and lowcost cocatalysts for photocatalytic CO2 reduction is of intense
academic and industrial interest.6 Research eﬀorts havefocused on exploring nonprecious and earth-abundant transition metals, e.g., Ni, Co and Fe, and their derivative oxides,
sulde and molecular complexes as alternatives to expensive
noble metal cocatalysts for improving the photocatalytic CO2
reduction eﬃciency and selectivity.7–9 However, the development of advanced cocatalysts with satisfactory activity andselectivity still face huge challenges to realize their practicalapplications.
For the transition metal-based cocatalysts, the metal sites areusually regarded as active sites, which tend to bond with the C
and O atoms of the adsorbed CO2 via strong hybridization
between the 2p and 3d orbitals, followed with a successive
protonation process to form hydrocarbon products.9 During thephotocatalytic reactions, the transition metal cations in cocatalytic materials would progressively reduce from high valence

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Journal of Materials Chemistry A
states to low valence states upon accepting electrons fromphotoexcited photocatalysts prior to driving the CO2 reaction(for example, NiII to NiI/0, CoII to CoI/0, and FeIII to FeII/I/0).10–12
This reaction mechanism reveals that the lower valent transition metal cations with partially occupied 3d orbitals should beconsidered as more catalytically active to drive CO2 reduction.
Namely, the performance of photocatalytic CO2 reduction canbe improved by increasing the content of low oxidation transition metal species in cocatalytic materials. Nevertheless, the low
oxidation transition metal species, especially involved in reactions are thermally unstable and easily oxidize to higher
valency, thereby greatly limiting the development of such kindof cocatalytic materials for eﬃcient photocatalytic CO2
reduction.
As is known to all, the electronic states of transition metals
largely depend on their coordination environment. In
general, the transition metals coordinating with atoms
composed by highly electronegative O, S, and N could easily
lead to the highly oxidized state, which probably decreases
the amount of active sites and suppresses CO2 conversion.5
Transitional metal borides (TMBs) raise our attention due totheir fascinating properties, including good chemical
stability, high thermal and electrical conductivities, as well as
a variety of technological applications, such as metal–air
battery, hydrogen and oxygen evolution catalyst.13–15 TMBs
comprise a diverse group of compounds, among which the
Ni, Co, Fe borides have received intensive research interest
because of the easy preparation, low-cost and environmentally friendly properties.16 Previous reports have revealed thatthe coordination between B and transition metals would
result in a partial electron transfer from B to the vacant dorbital of the transition metals, making the transition
metals electron rich and the alloying B electron-decient.13,14,17–19 In this case, the electron-decient alloying B
can eﬀectively protect transition metals from oxidation, andthe low oxidation transition metal active species arepreserved.13,14 Due to the existence of the low oxidationtransition metal species, the utilization of TMBs as eﬀective
cocatalysts for photocatalytic CO2 reduction is promising, but
not explored yet.
Herein, we attempted to explore the cocatalytic eﬀect of the
TMBs for photocatalytic CO2 reduction in this work. It was
found that Ni3B, Co3B, Fe2B were eﬀective cocatalysts in thepresence of [Ru(bpy)3]Cl2 (abbreviated as [Ru]) as lightabsorbers under visible light, while other TMBs (TaB2, NbB2 andMoB) were almost ineﬀective. The performance evaluation
indicates the metal site-dependent activity and selectivity of thephotocatalytic CO2 reduction over TMBs, among which Ni3B
exhibits the highest CO evolution rate of 157.7 mmol h1 witha selectivity of 93.0%, outperforming the state-of-the-art cocatalysts in the [Ru]-photosensitized systems. The excellent activityof Ni3B can be ascribed to its metallic feature that can bringeﬃcient transport of photogenerated electrons from the lightabsorber, as well as the unique Ni–B bonds that act as anelectron accumulator to provide abundant long-lived electrons
to be injected into the Ni0 active sites for the activation of CO2during photocatalytic reactions.

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Paper

2.

Experimental section

2.1. Preparation of the metal borides
The transitional metal borides Ni3B, Co3B and Fe2B nanoparticles were prepared by a simple chemical reduction andthermal annealing process. For the synthesis of Ni3B, twoseparated solutions were prepared. Solution A was prepared bydissolving 0.8 g NiCl2$6H2O in 40 ml deionized water. Solution
B was prepared by dissolving 0.4 g of NaBH4 in 30 ml deionizedwater. Aer that, solution B was slowly dropped into solution A
under vigorous stirring under the protection of Ar gas. The
mixed solution was further stirred for 30 min. Aer that, theblack powder was obtained by centrifugation and washing with
deionized water for ve times, following with drying undervacuum at 50  C. The obtained powder was annealed under Ar
at 300  C for 2 hours. For the synthesis of Co3B and Fe2B,
NiCl2$6H2O was replaced by CoCl2$6H2O and FeCl3, and theannealing temperatures were set at 400  C and 200  C, respectively. TaB2, NbB2 and MoB powders were commercially obtained from High Purity Chemicals Corporation (Japan). Air
treated Ni3B was prepared via thermal annealing of Ni3B in air
at 300  C for 2 hours. Ni nanoparticles were prepared by thermal
treatment of Ni3B in H2 at 300  C for 2 hours. NiO wascommercially obtained from Wako Corporation (Japan).
2.2. Characterization
The XRD patterns were recorded by powder XRD on the X-ray
diﬀractometer (X'Pert Powder, PANalytical B. V., Netherlands).
The morphology of the metal borides was obtained by eld
emission scanning electron microscope (JSM 6701F, JEOL).
TEM and HRTEM images of the samples were received from the
TEM machine (FEI, Talos F200x). UV-Vis absorption spectra
were obtained by Ultraviolet-visible spectroscopy (UV-2600 Shimadzu Corp., Japan). The decay time spectra were achieved ona Hamamatsu instrument (Hamamatsu C5680, Japan), and thewavelength of the excitation light was 400 nm. The chemical
states of the prepared samples were recorded via X-ray photoelectron spectroscopy (XPS, VG-ESCA Mark II). The in situ
Fourier-transform infrared (FTIR) spectroscopy was conductedby a JASCO FT-IR-6300h instrument in CO2 atmosphere. The Ni
K-edge X-ray absorption ne structure (XAFS) measurements
were conducted at the Photon Factory Advanced Ring in Tsukuba, Japan. Ni metal foil, NiO and Ni3B samples of analyticalgrade were measured. The ICP-OES data was obtained on an
Agilent 7900 ICP-MS apparatus.
2.3. Photocatalytic experiments
The photocatalytic CO2 reduction reactions were conducted ina liquid–solid reaction mode in a 100 ml quartz reactor underambient temperature and atmospheric pressure, and theexperimental setup is displayed in Fig. S2.† Typically, 5 mg
metal boride cocatalysts, 30 mg [Ru(bpy)3]Cl2$6H2O (commercially obtained from Sigma-Aldrich), 6 ml deionized water,
18 ml acetonitrile (MeCN) and 6 ml triethanolamine (TEOA)
were mixed and sonicated in a gas-closed quartz reactor. The
reaction system was evacuated completely (no O2 or N2 can be

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detected), and then was lled with 100 kPa pure CO2 gas. The
light source was employed by a 300 W xenon arc lamp combinedwith a UV-cut lter (L42 lter, l > 420 nm) and cooling waterlter. The evolved H2 and CO gas products were detected andquantied by using gas chromatographs (GC-8A, Shimadzu Co.,
Japan) and (GC-14B, Shimadzu Co., Japan), respectively. The
carrier gas of GC was pure Ar gas. The isotope-labeled experiment was conducted using 13CO2 instead of 12CO2, and thegenerated gas was detected by the gas chromatograph-mass
spectrum (GC-MS, JEOL-GCQMS, JMS-K9 and 6890N Network
GC system). The cycling test of the experiment was conducted asfollows. The Ni3B powder was received by centrifugation andwashing with MeCN for three times, then re-dispersed in a fresh
reaction solution (30 mg [Ru(bpy)3]Cl2$6H2O, 6 ml water, 18 ml
MeCN and 6 ml TEOA) for the next cycling experiment for a totalof 5 cycles. The measurement of the apparent quantum yield(AQY) was conducted by applying a 300 W xenon arc lamp withmonochromatic light band-pass lters (MIF-W, Optical Coatings Japan Co.). The radiant power energy meter (Ushio Spectroradiometer, USR-40) was applied to measure the number ofincident photons. The AQY of CO evolution was calculated by
the equation listed below:
AQY ð%Þ ¼¼

3.

number of reacted electrons
 100%
number of incident photons
number of evolved CO molecules  2
 100%
number of incident photons

Results and discussion

Ni, Co and Fe borides were prepared by a facile chemical
reduction of metal chloride solution using NaBH4 at room

temperature, followed with thermal annealing procedures
under Ar. X-ray diﬀraction (XRD) patterns (Fig. 1a–c) reveal thatthe as-prepared Ni, Co and Fe borides have crystalline metal
borides phase structures Ni3B (ICSD: 614985), Co3B (ICSD:
44339) and Fe2B (ICSD: 391328), respectively. Inductively
coupled plasma optical emission spectroscopy (ICP-OES) wasused to investigate the atomic ratio of Ni : B in the as-prepared
Ni3B materials, and the measured atomic ratio of Ni : B is
2.89 : 1, which is close to the stoichiometric ratio of Ni : B in
Ni3B. Fig. S1† shows the energy dispersive X-ray spectroscopy(EDX) of the as-prepared Ni3B, and the atomic ratio of Ni : B was
measured to be 4.52 : 1, revealing that Ni was the exposed site in
the as-prepared Ni3B. The morphology of the metal borides wasinvestigated by scanning electron microscopy (SEM) images.
Fig. 1d–f show that the as-prepared Ni3B, Co3B and Fe2B
samples present as aggregated particles, with the size in therange of tens of nanometers to hundreds of nanometers. The
transition electron microscopy (TEM) image in Fig. S2† shows
the typical aggregated nanoparticle shape of Ni3B, Co3B andFe2B, consistent with the SEM observation. In the highresolution transition electron microscopy (HRTEM) images,
well-resolved lattice fringe spaces of 0.235 nm, 0.205 nm and
0.201 nm are observed for Ni3B, Co3B and Fe2B, corresponding
to the (121), (220) and (121) crystallographic planes of the Ni3B,
Co3B and Fe2B phases, respectively.
The prepared Ni3B, Co3B and Fe2B were then investigated as
cocatalysts for photocatalytic CO2 reduction. These studies wereconducted in the reaction system with [Ru] as the light absorberand charge generator, and the system is shown in Fig. S3.† The
main products of this photocatalytic system were CO and H2,
and no formation of CH4 or formic acid was observed.20,21 The
time-dependent CO and H2 evolution of metal borides photosensitized by [Ru] are shown in Fig. S4.† The evolution rates forFig. 1 XRD patterns of (a) Ni3B, (b) Co3B and (c) Fe2B. The left insets in panels (a), (b), and (c) show the crystal structures of Ni3B, Co3B and Fe2B,
respectively. SEM images of (d) Ni3B, (e) Co3B, and (f) Fe2B. Scale bar: (d) 200 nm, (e) 400 nm, (f) 400 nm.

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CO and H2 decrease aer 1 hour continuous testing (Fig. S4†).
This stagnation eﬀect is mainly due to the photobleaching of
the [Ru] light absorber that has been commonly reported inprevious works.20,21 Fig. 2a summarizes the photocatalytic
performances of metal boride cocatalysts in the initial one hour.
Ni3B/[Ru] (Ni3B as the cocatalyst and [Ru] as the light absorber)
exhibits a considerable CO evolution rate of 157.7 mmol h1with a H2 production rate of 11.8 mmol h1 during the 1 hreaction period, obtaining a high CO selectivity of 93.0%. The
apparent quantum yield (AQY) of the CO evolution along withthe wavelength of the incident light was investigated. As shownin Fig. 2b, the variation tendency of AQY matches well with thelight absorption spectrum of [Ru], indicating the CO2 reduction
reaction occurs photocatalytically by the [Ru] light absorber.
Notably, an AQY at the monochromatic light of 420 nm wasmeasured as high as 4.5%. The achieved photocatalytic CO2
reduction performance outperforms the state-of-the-art cocatalysts under comparable conditions in the literature, as shown in
Table S1.†20–27 Without metal borides in the photocatalytic
system, [Ru] only exhibits the CO evolution rate of 2.9 mmol h1
together with CO selectivity of 37.7%. The CO and H2 evolutions
are completely terminated in the absence of [Ru]. These results
indicate the cocatalytic eﬀect of metal borides in the photocatalytic CO2 reduction reaction. Co3B and Fe2B can also act as
the cocatalysts for photocatalytic CO2 reduction. However, theirPaper
CO evolution rates and selectivities are much lower than thoseof Ni3B (Fig. 2a).
Given that Ni3B achieves high activity, its stability was thenevaluated. The used Ni3B catalyst was collected and re-dispersed
in a fresh reaction solution with newly added [Ru] for stability
tests. As presented in Fig. 2c, the used Ni3B cocatalyst retained
94% of its original performance aer ve cycle tests, demonstrating its high stability for photocatalytic CO2 reduction. The
SEM image and XRD pattern of Ni3B aer photocatalytic reaction further conrm the high stability (Fig. S5 and S6†). The
evolution of CO was not observed under either Ar or without
light, indicating that the CO product indeed originated from thephotocatalytic conversion of CO2 molecules (Fig. S7†). To
further certify the carbon source of the produced CO, 13Clabelled isotopic experiment using 13CO2 was conducted in
the Ni3B/[Ru] system, and the product was then analyzed by GCMS machine. The CO product appeared at the GC peak of
6.06 min, and the other two peaks in the GC spectrum are O2
and N2, respectively, derived from the air of the sampling needle(Fig. 2d). The analysis of CO revealed a value of m/z ¼ 29, corresponding to 13CO. This result clearly demonstrates that thegenerated CO indeed comes from the reduction of CO2.
The above results demonstrate that the Ni, Co and Fe borides
act as eﬃcient cocatalysts for the photoreduction of CO2. It isvery interesting to investigate the cocatalytic eﬀect of otherFig. 2 (a) Photocatalytic CO2 reduction activities over diﬀerent catalysts in one hour. The percentage of each catalyst represents the selectivityof CO. (b) Wavelength dependence of the AQY over Ni3B/[Ru], and the UV-Vis absorption spectrum of the [Ru] light absorber. (c) Cycling stability
test over Ni3B/[Ru]. Each test was conducted for one hour. (d) Gas chromatography-mass spectra (GC-MS) of 13CO generated in the photocatalytic reduction of 13CO2 over Ni3B/[Ru].

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metal borides. TaB2, NbB2 and MoB were selected as they also
have stable structures. The photocatalytic performances of the
TaB2, NbB2 and MoB cocatalysts in the presence of [Ru] in theinitial hour are shown in Fig. S8.† It clearly indicates that TaB2,
NbB2 and MoB were not eﬀective cocatalysts for CO2 photoreduction, since the CO evolution rates were not improved. These
results indicate that the cocatalytic performance is dependent
on the metal sites of the metal borides. The high cocatalytic
activities of the Ni, Co and Fe borides can be attributed to theintrinsic catalytic properties of the Ni, Co and Fe active sites forCO2 activation.28,29
Generally speaking, the photocatalytic conversion of CO2 to
CO undergoes the formation of a formate intermediate(COOH*) via CO2* + H+ + e / COOH*.22 To investigate the CO2
photoreduction process on the Ni3B/[Ru] system, an in situ
Fourier-transform infrared spectroscopy (FTIR) investigation
was conducted. As shown in Fig. 3a, a strong broad absorption
peak centered at 2329 cm1 appears during CO2 photoreduction, which is ascribed to the absorbed CO2 molecule.30 In
addition, the broad absorption peaks centered at 1522 cm1
and 1354 cm1 are assigned to the carbonate-type species andthe adsorbed formate (COOH*) species, respectively.30
Furthermore, the broad absorption peaks in the range of 1670–
1820 cm1 with a center located at 1731 cm1 correspond to theasymmetric stretching of O–C]O, which also belong to theintermediate formate (COOH*) species.31 It can be seen thatwith the increase of the irradiation time, the intensities of thesepeaks gradually increase, illustrating the formation of theformate intermediate during the CO2 photoreduction process.
CO* is then produced from the COOH* conversion by a proton–
electron transfer reduction process (COOH* + H+ + e / CO* +H2O), and CO is nally released from the dissociation of theabsorbed CO* adduct (CO* / CO).22 The cocatalytic performances of the metal borides follow the order: Ni3B > Co3B >Fe2B. In order to investigate the origin of the metal-sitedependent performance of the metal borides for CO2 photoreduction, density functional theory (DFT) calculations were thenconducted (see Computational details in ESI†). The optimized
bulk Ni3B, Co3B, and Fe2B lattice parameters were in good
agreement with the experimental value, as shown in Table S2.†

Journal of Materials Chemistry A
The (010) surface was chosen for simulation, as the Ni3B (010)
surface had the lowest barrier of 0.49 eV. The corresponding
surface energy and structure of the intermediate along thereaction path are shown in Fig. S9 and S10,† respectively. The
reaction free energy diagrams of the CO2 reduction over Ni3B,
Co3B and Fe2B are plotted in Fig. 3b. As for Ni3B, Co3B and Fe2B,
their rate-determining steps of the reaction are the dissociationof CO* adduct, which are 0.49 eV, 0.81 eV and 1.33 eV, respectively. The free energy barrier of the CO2 reduction shows the
expected trend of Ni3B < Co3B < Fe2B, consistent with theexperimental results that Ni3B shows higher CO2 reduction
activity than Co3B and Fe2B.
The above results have revealed the highly active Ni3B for
promoting the photocatalytic CO2 reduction reaction, and now
we are in a position to disclose the advantages of this structure.
Noting that the electronic structure and chemical state of
cocatalysts play signicant roles in inferring their photocatalytic CO2 reduction performances, X-ray photoelectron
spectroscopy (XPS) was utilized to reveal the chemical states of
Ni3B. Fig. 4a shows the high-resolution XPS spectrum of Ni 2p of
Ni3B. The peak located at the binding energy of 852.2 eV can be
attributed to Ni0, originating from the Ni–B bond.32 Notably, thecoordination between B and the transition metals would resultin a partial electron transfer from B to the vacant d-orbital of thetransition metals. This would make the transition metals
electron-rich and the alloying B electron-decient. Thus, the low
oxidation transition metals are formed.13 A weak peak at
a higher binding energy of 855.8 eV corresponds to the Ni2+
signal, resulting from the inevitable surface oxidation in air
storage.33 The XPS spectrum of B 1s of Ni3B is displayed inFig. S11,† and the peaks located at 191.9 eV and 187.7 eVcorrespond to B–O bonding and B–Ni bonding, respectively.32
The formation of B–O bonding is derived from the surface
oxidation in air. To probe the chemical environment of the Ni
element in Ni3B, the Ni K-edge extended X-ray absorption nestructure (EXAFS) and X-ray absorption near-edge structure(XANES) spectroscopies were measured. Fig. 4b shows thenormalized XANES spectra collected at the Ni K-edge for the Ni
metal, NiO and Ni3B. The pre-edge region of Ni3B consists ofa small peak at 8334 eV, whose shape and position are similar

Fig. 3 (a) In situ FTIR spectra of photocatalytic CO2 reduction under visible light over [Ru]-photosensitized Ni3B. (b) Reaction free energy
diagrams of the CO2 reduction over Ni3B, Co3B and Fe2B.

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Fig. 4 (a) Ni 2p XPS spectrum of Ni3B. (b) Ni K-edge XANES spectra and (c) the Fourier transform of angular wave number (k)3-weighted EXAFS
spectra for the Ni metal, NiO and Ni3B. (d–f) Calculated DOS of Ni3B, Co3B and Fe2B, respectively.

to those of the Ni metal, indicating predominance of the Ni0
state in the sample. Fig. 4c shows the Fourier-transformed
EXAFS spectra of the Ni metal, NiO and Ni3B. For the Ni
metal, the peak at 2.20 Å corresponds to the Ni–Ni bond. Ni3B
exhibits a much weaker peak centered at 2.16 Å, which is
attributed to the Ni–Ni scattering pair in Ni3B.34,35 This peak
position is smaller than that of Ni–Ni (2.20 Å) in the Ni foil
standard sample, which results from the chemical interaction
involving the hybridization and electron transfer between the B
2p states with the Ni metal d orbitals.34,35 The Ni–O bond at 1.65
Å and Ni–O–Ni bond at 2.58 Å are absent in Ni3B, indicating thatthe portion of surface Ni2+ oxidation species in Ni3B is very low.
Ni3B displays a metallic feature, which can be conrmed by DFT
calculations. As shown in Fig. 4d, the total density of states(DOS) of Ni3B are mainly ascribed to the contribution from thed orbitals of the Ni atom, and the values of DOS of Ni3B aroundthe Fermi level (Ef) are all continuous without an obvious
bandgap, demonstrating the metallic feature of Ni3B. In addition, the p orbitals of B also displays the continuous nature,indicating the covalent interaction between B and Ni in themetallic state.36 Similarly, Co0 and Fe0 also exist in Co3B andFe2B, respectively, which can be conrmed by their XPS spectra(Fig. S12†), consistent with previous reports.37,38 The metallic
features of Co3B and Fe2B are also conrmed by their calculated
DOS distributions (Fig. 4e and f). The metallic character of theNi3B, Co3B and Fe2B materials could bring an eﬃcient transportof charge carriers. Thus, they can act as highly active cocatalysts
to promote photocatalytic performances.
Fig. 5a shows the valence band XPS (VB-XPS) spectrum of
Ni3B. It can be seen that the Ef of Ni3B is occupied by continuous

21838 | J. Mater. Chem. A, 2020, 8, 21833–21841

electronic occupied states, further disclosing its metallic
feature. Fig. 5b shows the calculated charge distribution of
Ni3B, which indicates that charges are accumulated on the Ni–B
bond. This unique feature enables the Ni–B bond acting as anelectron accumulator to store electrons, which can provide
abundant long-lived electrons to be injected into the Ni0 active
sites for the activation of CO2 during photocatalytic reactions.
Time-resolved PL decay spectroscopy was then utilized toexperimentally reveal the dynamic charge behavior of the photocatalytic system.39 Fig. 5c shows the decay curves of [Ru] withand without Ni3B. The tted average lifetimes are shown in
Table S3,† which indicates that the PL average lifetime
increases in the following order: pure [Ru] < Ni3B/[Ru]. The
lengthened lifetime of [Ru] in the presence of Ni3B is ascribed tothe electron transfer from [Ru] to Ni3B, and the electrons areaccumulated on Ni3B. The mechanism for the photocatalytic
CO2 reduction is displayed in Fig. 5d and S13† based on above
results, and the previous reports in the Ru-photosensitizedsystems.22 Under visible light irradiation, the electrons generated from the photosensitizer [Ru] can be transferred to themetal boride cocatalysts. Then, the CO2 molecules absorbed on
the surface of the metal borides are reduced to CO. The H2evolution is regarded as a competing reaction to the CO evolution, and Ni boride is demonstrated as the most eﬀective
cocatalyst to suppress the H2 evolution and simultaneously
maintain a record high CO evolution rate.
To demonstrate the dominant role of the Ni–B bond in Ni3B
for the photocatalytic CO2 reduction, the cocatalytic performances of Ni3B, air-treated Ni3B, Ni nanoparticles and NiO were
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Fig. 5 (a) VB-XPS spectrum of Ni3B. (b) Charge density distribution of Ni3B. The green ball is B and the grey ball is Ni, and the value of theisosurface is 0.01 e Å3. (c) Time-resolved PL decay spectra of [Ru] with and without Ni3B. (d) Proposed mechanism for the photocatalytic
reduction of CO2 to CO over [Ru]-photosensitized Ni3B under visible light irradiation.

thermal treatment of Ni3B in air at 300  C for 2 hours, and Ni
nanoparticles were prepared by thermal treatment of Ni3B in H2
at 300  C for 2 hours. The XRD pattern indicates that the airtreated Ni3B also displays a crystalline Ni3B phase structure(Fig. S14†), and the H2-treated Ni3B displays a crystalline Ni
phase structure (Fig. S15†). The SEM images show that the airtreated Ni3B and H2-treated Ni3B exhibit aggregated particles,
which have similar morphology compared with Ni3B (Fig. S16†).
However, the content of the Ni–B bond on the surface of the airtreated Ni3B is signicantly decreased, as revealed by XPS
spectra (Fig. S17†). The decreased content of the Ni–B bond iscaused by the thermal oxidation of Ni0 to Ni2+ in air. The photocatalytic performances of Ni3B, air-treated Ni3B, Ni nanoparticles and NiO in the presence of [Ru] are shown in Fig. S18.†
It is obvious that Ni3B exhibits much higher activity and selectivity than air-treated Ni3B, Ni nanoparticles and NiO, demonstrating the dominant role of the Ni–B bond for promoting thephotocatalytic CO2 activation and reduction. The lower photocatalytic performance of NiO is due to its semiconducting
features, whose conductivity and electron transfer eﬃciency aremuch lower than that of Ni3B (Fig. S19†). The existence of the
Ni–B bond in Ni3B is benecial for the desorption of the COmolecule, which can be revealed by the DFT calculations. The
calculated CO adsorption energy was 0.27 eV over the Ni3B
(010) surface, which is much weaker than that over Ni (111)(0.41 eV and 0.72 eV on the top site and hollow site,

This journal is © The Royal Society of Chemistry 2020

respectively) (see computational details). The charge density
analysis in Fig. S20† further veries the lower adsorption energyof CO over the Ni3B (010) surface compared to that over the Ni(111) surface, indicating that the CO product is much easier torelease from the Ni3B surface, which is one of the reasons why
Ni3B exhibits much higher activity than Ni nanoparticles forphotocatalytic CO2 reduction (Fig. S18†).

4. Conclusion
In summary, we have demonstrated that the TMBs Ni3B, Co3B,
Fe2B are eﬀective cocatalysts for photocatalytic CO2 reduction
under visible light, while other TMBs (TaB2, NbB2 and MoB) arealmost ineﬀective. Experimental results reveal that the cocatalytic CO2 reduction performances of TMBs exhibit metal sitedependent activity and selectivity, among which Ni3B isdemonstrated as the most eﬀective cocatalyst, obtaining both
highest CO evolution rate and highest selectivity compared toother TMBs. Specically, a record high CO evolution rate of
157.7 mmol h1 with selectivity of 93.0% was obtained over the
Ni3B cocatalyst. The mechanism investigation indicates that theexcellent activity of the Ni3B cocatalyst can be ascribed to its
metallic feature, which can bring an eﬃcient transport ofphotogenerated electrons from the light absorber, as well as theunique Ni–B bond that acts as an electron accumulator toprovide abundant long-lived electrons to be injected into the Ni0

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Journal of Materials Chemistry A
active sites for the activation of CO2 during photocatalytic
reactions. This work may provide fundamental guidance for thedesign of eﬃcient cocatalytic materials and systems for solar
water splitting, CO2 reduction and many other energy conversion reactions.

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Author contributions
L. Shi and P. Wang contributed equally to this work. L. Shi and J.
Ye conceived and designed the experiments. L. Shi fabricatedthe materials. P. Wang, Q. Wang and W. Zhou completed thetheoretical computations and discussed the theoreticalresults. L. Shi and X. Ren designed the experimental devices andperformed the photocatalytic experiments. F. Ichihara conducted the time-resolved PL decay measurement. H. Zhang and
Y. Izumi performed the EXAFS and XANES measurements andanalyzed the data. B. Cao and S. Wang conducted the TEM
measurements and analyzed the data. L. Shi, P. Wang and S.
Wang wrote the manuscript. J. Ye and H. Chen supervised theresearch work and revised the manuscript. All authors
contributed to the general discussion.

Conﬂicts of interest
The authors declare no competing nancial interests.

Acknowledgements
This work received nancial support from the World PremierInternational Research Center Initiative (WPI Initiative) on
Materials Nanoarchitectonics (MANA), MEXT (Japan), JSPS
KAKENHI (JP18H02065), the Photo-excitonix Project at Hokkaido University, National Natural Science Foundation of China(No. 21633004, No. 52073110, and No. 2190020614), and theFundamental Research Funds for the Central Universities (No.
NE2019103 and No. 2662018QD041).

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