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Initial Stages in the Formation of Nickel Phosphides
Rodrigo García-Muelas, Qiang Li, and Núria López*

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Institute of Chemical Research of Catalonia, ICIQ, The Barcelona Institute of Science and Technology, Av. Països Catalans, 16, 43007
Tarragona, Spain
ABSTRACT: Metal phosphides have emerged as a new
powerful class of materials that can be employed as
heterogeneous catalysts in transformations mainly to generate
new energy vectors and the valorization of renewables. Synthetic
protocols based on wet techniques are available and are based
on the decomposition of the organic layer decorating the
nanoparticles. For nickel, the phosphine of choice is
trioctylphosphine, and this leads to the formation of NiPx
materials. However, the temperature at which the decomposition starts has been found to depend on the quality of the
nickel surface. Density functional theory, DFT, holds the key to
analyze the initial steps of the formation of these phosphide
materials. We have found how clean nickel surfaces, either (111)
or (100), readily breaks the ligand P−C bonds. This triggers the
process that leads to the replacement of a surface nickel atom by P and concomintantly forms a Ni adatom on the surface
surrounded by two methyl groups, thus starting the formation of the NiPx phase. The whole process requires low energies, in
agreement with the low temperature found in the experiments, 150 °C. In contrast, if the surface is oxidized, the reaction does
not proceed at low temperatures and oxygen vacancies need to be created first to start the P−C bond breaking on the Ni-clean
patches. Our results show that the cleaner the surface is, the milder the reactions are required for the NiPx formation, and thus
they pave the way for gentler synthetic protocols that can improve the control of these materials.

■

INTRODUCTION
Metal phosphides are attracting a large amount of attention, as
they have appealing properties in different areas of physics and
chemistry.1 The applications extend from optics2,3 to
magnets4,5 and for energy storage.6,7 Among these applications,
one of the fastest growing areas is the use of metal phosphides
as catalysts. Initially, they were used in hydrotreatment and
selective hydrogenations,8−10 but nowadays, energy vector
generation is the blooming area.11−13 Particularly, growing
attention has been put in the hydrogen evolution reaction
(HER) of the Ni2P and CoP compositions,14−23 but also on the
H2 production from BH3NH3.24,25 These properties appear as a
consequence of the modifications in the ensemble, and the
electronic structure of these materials.26 Thus, the P atoms in
the lattice can be seen as embedded lattice poisons. In some
cases, these materials present an almost continuum phase
diagram with different P contents27,28 and thus some stability
issues can be foreseen. The nature of the particular Ni−P phase
is highly relevant in the catalytic performance; for instance,
different performances are found for the HER reaction on Ni2P,
Ni12P5, and Ni5P4,21 and thus, a strict control on the synthetic
procedures that lead to the formation of the active phase is
required.
Very recently, colloidal protocols to produce these materials
have been generalized. The synthetic routes are based on the
formation of nanoparticles decorated by phosphine ligands that
evolve toward the formation of the phosphides by thermal
© 2017 American Chemical Society

treatment. For instance, the tri-n-octylphosphine, TOP, is
employed both to decorate the nanoparticles and to control
their size and as the source for P. The reaction then is
performed at medium temperatures of about 250 °C and
pressures close to the normal, ambient pressure29,30 and can
generate phosphides for a wide variety of reactive metals. For
Ni, two phases, Ni2P and Ni12P5, can be prepared by changing
the conditions.31,32 The decomposition of the phosphide
precursor depends on the metal, and for Ni, the starting
temperature for decomposition was reported to be 210 °C,33
while the complete decomposition and crystallization of the
phosphide occurs at 230 °C.34 In a deep XPS study, Salmeron
et al.35 were able to describe more precisely how the wet
synthesis of the Ni-based phosphide proceeds. In summary,
when a Ni metal foil is clean, the surface can be decorated with
TOP ligands that limit the growth of the nanoparticles. Upon
heating to 150 °C, the phosphines start degrading, and as a
consequence, the P appears. However, this process is slightly
more complex. Particularly, when the synthesis starts from
nanoparticles, the decomposition of the organic fragment leads
to the formation of a C-containing Ni core and the outermost
surface is mostly NiP with Ni2P and Ni5P4 stoichiometries.
Special Issue: Miquel B. Salmeron Festschrift
Received: June 19, 2017
Revised: September 5, 2017
Published: September 7, 2017
672

DOI: 10.1021/acs.jpcb.7b06020
J. Phys. Chem. B 2018, 122, 672−678

The Journal of Physical Chemistry B

Article

Care shall be taken also, as Ni is relatively easy to oxidize.
When starting by oxidized nanostructures, the phosphine
decomposition starts only at 250 °C. The process is
schematically represented in Figure 1. Density functional

Figure 2. Top views of the models employed for the Ni(111) and
(100) surfaces and for the oxide phase NiO(100) and the labels for the
corresponding surface atoms. Gray Ni, red O, and fading O the
position where the vacancy is formed.

nudged elastic band method.55 All of the structures can be
retrieved from our ioChem-BD repository.56

■

RESULTS
Four model systems have been used to characterize the fosfine
decomposition and initial steps of the formation of the
phosphide phase. As the trioctylphosphine (TOP) employed
in the experiments is very large, instead a surrogate compound
trimethylphosphine P(CH3)3 (TMP) has been used as reacting
phosphine. The replacement would have an effect provided that
the packing or stabilization contributions with respect to the
solvent were investigated, but for the purpose of analyzing the
decomposition, the effect is minor, as the interactions are local
in nature. Phosphine decomposition in the model systems has
been summarized in Figure 3. The first model corresponds to a

Figure 1. Required conditions for the formation of the NiP phase from
Ni. The different color codes show the dominating phases all
throughout the manuscript (metallic Ni gray, oxidized green,
phosphide orange). Adapted from Carenco et al.35

theory simulations have been extensively used to analyze the
chemical properties of materials including phosphides.
However, DFT is less often applied in the study of wet
chemistry and synthetic protocols, although we have shown the
potentiality of the approach in a wide series of systems.36−39
For instance, we have analyzed the role of surfactants when
driving the structure of nanoparticles, controlling the catalytic
properties therein.40−46 However, in all of the cases considered
so far, the surfactants kept their entity under reaction
conditions and the surface was poorly reactive. In the present
work, we aim at investigating the evolution of the ligand on a
relatively active surface as well as its decomposition and
integration in the lattice as the first step in the formation of the
phosphide phase.

■

COMPUTATIONAL DETAILS
The initial steps for the formation of phosphide under wet
synthesis conditions were investigated by density functional
theory, DFT, applied to slabs representing different nickel
phases. The calculations were performed with the VASP47 code,
and the functional of choice was PBE.48 Inner electrons were
replaced by PAW,49 and the valence electrons were expanded in
plane waves with a cutoff energy of 450 eV. The pure Ni(111)
and (100) surfaces were described by a four-layer slab in a
p(3 × 3) supercell with spin polarization; see Figure 2. The
NiO structure was constructed similarly: the slab contained
four layers, and the U parameter was set to 5.4 eV following
previous computational studies.50,51 The k-point were sampled
by a Γ-centered 4 × 4 × 1 Monkhorst−Pack52 grid. The
adsorbates were only placed in one side of the slabs and
interleaved by at least 14 Å. Dipole correction along the z
direction has been considered,53 and vdW interaction have
been employed via the DFT-D3 method developed by Grimme
et al.54 The large phosphines used in the experimental synthetic
protocol were replaced by small surrogates P(CH3)3. Transition
states were located with the climbing image version of the

Figure 3. Reaction energy profile for the decomposition of P(CH3)3
on Ni(111) and Ni(100) (gray-orange) and NiO(100), both on the
clean and oxygen-depleted surface (green continuous and dashed,
respectively). The insets represent the most relevant structures along
the reaction coordinates. Gray spheres stand for Ni, red for O, orange
for P, black for C, and H are white.

pure Ni(111) surface that represents the clean surface model
(dark gray); this is complemented by using the second lowest
energy surface Ni(100) as the second model for which the
reaction has also been inspected (gray). A third model
represents a completely oxidized nickel system shown by the
NiO(100) surface (green); this is the lowest energy surface for
the oxide. The fourth model corresponds to a NiO(100)
surface where an oxygen vacancy site has been created (green
dashed), as oxides can lose oxygen atoms and they create
673

DOI: 10.1021/acs.jpcb.7b06020
J. Phys. Chem. B 2018, 122, 672−678

The Journal of Physical Chemistry B

Article

Table 1. Magnetic Moments, μB, for the Most Relevant Ni Atoms during the Formation of the Phosphide Phase

a

Ni(111)

1

2

3

4

5

6

7

8

9

total

IS-1
TS1
IS-2
TS-2
IS-3
TS-3
FS
Ni(100)

0.60
0.40
0.52
0.53
0.48
0.30
0.29
1

0.65
0.63
0.49
0.57
0.59
0.52
0.44
2

0.66
0.65
0.67
0.67
0.67
0.66
0.66
3

0.46
0.41
0.53
0.30
0.23
0.30
0.15
4

0.32
0.31
0.29
0.34
0.46
0.08
0.04
5

0.61
0.62
0.67
0.65
0.50
0.60
0.58
6

0.64
0.62
0.54
0.52
0.21
0.62
0.63
7

0.61
0.28
0.32
0.38
0.32
0.34
0.36
8

0.63
0.63
0.66
0.69
0.55
0.54
0.30
9

5.18
4.55
4.69
4.65
4.01
3.96
3.45
total

IS-1
TS1
IS-2
TS-2
IS-3
TS-3
FS
NiO(100)

0.72
0.70
0.72
0.71
0.67
0.66
0.67

0.34
0.68
0.27
0.34
0.34
0.33
0.30
7

0.66
0.72
−0.68
0.67
0.70
0.68
0.64
8

5.54
5.79
5.02
4.69
4.14
3.71
3.88
total

1.80
1.80
1.79
7

1.80
1.80
1.79
8

0.05
0.13
0.27
total

0.72
0.71
0.73
0.71
0.72
0.64
0.71

0.69
0.70
0.68
0.43
0.34
0.30
0.33

0.30
0.26
0.14
0.21
0.03
0.02
0.16

0.70
0.70
0.68
0.66
0.55
0.43
0.33

0.72
0.70
0.0.68
0.49
0.35
0.26
0.31
6

1

2

3

4

5

−1.75
−1.68
−1.55
1

IS-1
TS-1
FS-1
NiO(100)-v

−1.79
−1.79
−1.79
2

1.79
1.79
1.80
3

1.80
1.79
1.80
4

−1.80
−1.78
−1.77
5

−1.80
−1.80
−1.80
6

1.79
1.79
1.79

1.80
1.80
1.78

−1.71
−1.74
−1.76

−1.80
−1.80
−1.80

−1.67
−1.69
−1.73

IS-1
TS-1
FS-1
a

0.69
0.62
0.43
0.47
0.44
0.39
0.43

−1.80
−1.80
−1.80

1.66
1.26
1.72

1.66
1.66
0.95

−0.07
−0.52
−0.85

The atoms are numbered as in Figure 2.

Table 2. Bader Charges, in |e−|, for the Surface Ni Atoms during the Formation of the Phosphide Phasea
Ni(111)

1

2

3

4

5

6

7

8

9

total

P

IS-1
TS1
IS-2
TS-2
IS-3
TS-3
FS
Ni(100)

−0.03
0.09
0.11
0.11
0.08
0.17
0.15
1

−0.04
−0.02
0.09
0.08
0.09
0.03
0.06
2

−0.05
−0.06
−0.05
−0.04
−0.02
−0.03
−0.04
3

−0.01
0.00
0.00
−0.07
−0.10
−0.08
0.12
4

−0.04
0.01
−0.03
0.11
0.17
0.23
0.29
5

−0.05
−0.06
−0.03
−0.03
0.11
−0.05
−0.04
6

−0.03
−0.02
0.10
0.12
0.11
0.04
0.02
7

−0.03
−0.04
−0.03
−0.01
−0.02
−0.05
0.00
8

−0.03
−0.03
0.00
0.02
0.11
0.15
0.15
9

−0.31
−0.13
0.16
0.29
0.53
0.41
0.71
total

1.19
0.86
0.75
0.48
0.40
0.41
−0.09
P

−0.09
0.00
−0.03
−0.04
0.01
−0.03
0.04

−0.05
−0.02
0.13
0.12
0.09
0.09
0.16

−0.01
0.02
−0.04
0.01
0.03
0.04
0.02

−0.03
−0.02
0.02
0.04
−0.01
−0.04
0.05

−0.06
0.09
0.14
0.25
0.34
0.31
0.29

−0.04
−0.07
0.03
−0.06
0.11
0.09
0.15

0.00
−0.04
−0.04
0.02
−0.02
0.16
0.12

0.01
0.00
−0.05
0.02
0.03
−0.02
0.06
7

0.06
−0.03
−0.04
−0.02
−0.05
0.01
0.03
8

−0.21
−0.07
0.12
0.34
0.53
0.61
0.92
total

1.32
0.99
0.84
0.48
0.31
0.02
−0.22
P

IS-1
TS1
IS-2
TS-2
IS-3
TS-3
FS
NiO(100)
IS-1
TS-1
FS-1
NiO(100)-v
IS-1
TS-1
FS-1
a

1

2

3

4

5

6

1.12
1.09
0.83
1

1.21
1.19
1.20
2

1.24
1.23
1.18
3

1.24
1.24
1.18
4

1.21
1.21
1.18
5

1.22
1.22
1.16
6

1.24
1.24
1.18
7

1.23
1.24
1.23
8

9.71
9.66
9.14
total

1.34
0.85
0.60
P

1.23
1.21
1.16

0.93
0.91
1.09

0.93
1.04
0.63

8.58
8.87
8.67

1.27
0.81
0.86

0.87
0.98
1.07

1.21
1.23
1.22

1.23
1.25
1.20

1.23
1.19
1.22

0.95
1.06
1.08

The atoms are numbered as in Figure 2.

in contact with one Ni and effectively quenching the magnetic
moment of the Ni atom to which it is directly adsorbed due to
the donor nature of the lone pair from the P atom. From this
starting point, the phosphine can break; see Figure 3. The first
P−C breaking shows a rather small barrier of about 0.99 eV and
results in a configuration where the methyl group migrates to

vacancies that present completely different properties when
increasing the temperature.57 The magnetic moments and
Bader charges58 of the surface Ni atoms and P are presented in
Tables 1 and 2.
The phosphines adsorb on the most stable surface
termination of Ni(111) by −2.14 eV, with the P atom directly
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DOI: 10.1021/acs.jpcb.7b06020
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Figure 4. Transition state structures (TS-3) leading to the formation of the phosphide with the most relevant distances for the first Ni replacement
by P (left (111) surface, right (100) surface). The origin of the stabilization of adatoms by the methyl groups is highlighted. Gray spheres stand for
Ni, orange for P, black for C, and white for H. The atoms are numbered as in Figure 2.

−2.36 eV. The reaction proceeds in the same manner as
described for the (111) surface, with the barriers for the
different P−CH3 cleavages being 0.74, 0.75, and 0.74 eV. The
overall process is more exothermic than that on (111), and the
barriers are smaller, since the coordination number of the (100)
surface is smaller than that of the (111), which allows larger
reorganizations at a lower energy cost. The magnetic moments
behave similarly to the (111) case, as from an initial
magnetization of 5.54 μB, the value decreases to 5.02, 4.14,
and 3.88 μB during the different cleavages. The corresponding
charges for P are positive by 1.3 |e−| in the starting
configuration and −0.2 |e−| in the final state, while the surface
goes from slightly negative, −0.2, to 0.9 |e−| in the surface layer.
Again, as for the (111) surfaces, the last step ends with the
replacement of the surface Ni by a P atom and the formation of
the adatom stabilized by the two methyl groups. The last
transition states where the replacement occurs are represented
in Figure 4, and show that the Ni atom is less displaced upward
in the (100) surface because the surface is less compact than
the (111).
On the nickel oxide, TMP adsorption is very exothermic,
−3.16 eV, and takes place on top of the Ni cations. The
dissociation could only occur heterolytically, as the activation in
many oxides,62 and ends up forming a cationic CH3+ fragment
that is attached to a lattice oxygen and the negatively charged
P(CH3)2 still bonded to the cation. Although the phosphine
adsorption is very exothermic, the dissociation is very
endothermic, about 2.24 eV. This is due to two fundamental
terms: (i) the C−O bond formation is not very effective and
(ii) the phosphine cannot coordinate properly to two cations
on the surface, as they are far apart in the structure. In the initial
structure, the P is positively charged, 1.3 |e−|, and this charge is
reduced along the reaction path to 0.6 |e−|, which is
compensated by the charge variation in one of the Ni atoms
that also reduces its magnetization. The consequence is that the
fully oxidized NiO surface is inert with respect to phosphines
and then an alternative mechanism needs to be identified. As
oxides are known to lose oxygen when the temperature is
raised, we included a model showing a surface oxygen vacancy.
In the case of NiO, the vacancy formation requires 2.32 eV. On
the model system close to a surface oxygen vacancy, the
adsorption of the basic P(CH3)3 is less favorable than Ni or
NiO. The first step in the demethylation is now endothermic by
2.10 eV, and the barrier is 2.35 eV. Therefore, the process is
also unlikely on this surface under the reaction conditions
described in the experiments. The interpretation of the results
is thus clear: the NiO surface can trap the surfactant but would
require extensive reduction with the formation of Ni patches

the surface and the remaining P(CH3)2 fragment changes from
a top to a bridge-like configuration to satisfy the valence of P.
The overall reaction is slightly exothermic. This finding agrees
with the experimental observations on Ni nanoparticles or
organometallic compounds with phosphine ligands for which
bond cleavage was observed at temperatures about 100 °C.59,60
The elimination of the second methyl group requires a slightly
larger energy barrier of 1.07 eV. In that case, the overall
thermodynamic cost for this step is endothermic by 0.51 eV. In
this configuration, one of the Ni atoms from the surface is
already displaced out of the equilibrium position, by 0.16 Å, and
thus points toward the formation of an adatom.61 The dimethyl
coordinated phosphine is then reactive enough to start
penetration into the lattice. The final step leads to the bond
breaking of the third P−C bond and the full replacement of one
of the Ni atoms from the surface by the P, thus becoming the
seed of the nickel phosphide formation. The reaction is uphill
by 1 eV, and the activation barrier is 1.09 eV. In the final
structure, one of the methyl groups sits on the Ni surface and
the total magnetic moment is reduced due to the Ni−C bond
formation, from the starting value of 5.18 μB for the surface
layer down to 4.69, then 4.01 after one and two methyl cleaves,
and 3.45 μB for the final structure. In parallel, the initial positive
charge of P, about 1.2 |e−|, gets reduced down to the final state
where it is slightly negatively charged, −0.1, while the metal
surface changes from slightly negative to positive 0.7 |e−|. In the
final configuration, detailed in Figure 4, the adatom is
coordinated to the two methyl groups (distances 2.13 and
1.96 Å and the Ni atom is extracted from the plane by 2.40 Å).
The energy cost of forming the adatom (roughly 2/3 of the
cohesive energy) is counterbalanced by the increasing
interaction with the methyl fragments and PCH3, which also
moves toward a fcc-like configuration on the surface to saturate
as many bonds as possible. The overall energy profile starting
from the adsorbed phosphine is 1.37 eV uphill with the largest
barrier being 1.47 eV, which agrees very well with the
temperature of 150 °C reported in the experiments needed
to see the initial spectroscopic features corresponding to the
formation of the nickel phosphide phase. Further adsorption of
the phosphine on the adatom is much less exothermic, around
0.7 eV. Therefore, scavenging atoms from another area of the
clean surface would be more favorable than reacting on the
formed adatom.
Alternatively, the second most stable surface (100) has been
set as the initial state. The coverage considered for the
phosphine is the same as that in the previous case. Initially, the
phosphine adsorbs to a single Ni atom on the surface, and the
binding energy is slightly larger than that for the (111) surface,
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DOI: 10.1021/acs.jpcb.7b06020
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The Journal of Physical Chemistry B

Article

temperatures at which the experiments are carried out. In
addition, the reaction paths encompass the extraction of Ni
atoms from the surface and their replacement by the reactive P
from the ligand. However, nickel reacts easily with oxygen;
therefore, many surfaces are better described as oxides. On Ni
oxides the phosphidation reaction is very endothermic and thus
cannot proceed. Even when they are partially oxygen depleted,
oxide surfaces are not reactive enough to allow the
decomposition of the phosphine. Therefore, the reaction can
only proceed by increasing the overall temperature and will take
place on the Ni patches of the surface. The number of potential
simultaneous paths leading to full decomposition and
phosphide formation calls for an integrated effort where
detailed characterization with in situ techniques can be coupled
to density functional theory models to fully describe the
reaction mechanisms involved. The present results open a new
path toward the rational understanding of synthetic protocols
and pave the way for establishing new, more suitable ligands
that can allow milder reaction conditions and thus more control
on the synthesis.

before the reaction occurs. In summary, our results are
compatible with the 250 °C temperature needed to start the
phosphine decomposition effectively on the Ni-only patches of
the surface of the oxides that are already oxygen depleted. As
we have indicated in the Introduction, our calculations only
represent the very first stages of the formation of the NiP phase.
The methyl groups on the surface could decompose further
following the paths that have been already described in the
literature.63

■

DISCUSSION
For the metal Ni(111) and (100) surfaces, the barriers for all of
the steps follow two Brønsted−Evans−Polanyi64 relationships
(see Figure 5); this indicates the local nature of the bond that

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: nlopez@iciq.es.
ORCID

Figure 5. Brønsted−Evans−Polanyi (BEP) relationship between the
activation energy for each of the P−C steps as a function of the
reaction energy for each particular step on the Ni(111) and Ni(100)
surfaces. The numbers correspond to the transition state structures
identified in Figure 3. Notice that NiO(100) surface would not follow
the same BEP, as the bond-breaking transition state corresponds to a
heterolytic path.

Rodrigo García-Muelas: 0000-0002-2219-5027
Núria López: 0000-0001-9150-5941
Notes

The authors declare no competing financial interest.
Data-set collection of computational results, including all the
structures, are available in: http://dx.doi.org/10.19061/
iochem-bd-1-42.

drives the formation of the adatom and the phosphide
structure. The reason for the lower values found for Ni(100)
can be traced back to the more open nature of the surface that
allows lower energy processes and is slightly more flexible.
Therefore, this would allow one to couple the present results to
the coordination dependence rules to expand the thermodynamic and kinetic requirements to finite size nanoparticles and
low coordinated sites.65 However, it shall be noticed that the
low barriers found for all of the steps imply that the activation
can easily occur even in the denser packed surfaces; thus, no
important size contributions are expected.
The present replacement mechanism is related to several
other observations in the literature. For instance, in
homogeneous catalysis, the reductive cleavage of P−C bonds
and the concomitant formation of phosphido species is
extensively reported in the literature for low-valent metal
complexes.66 The formation of adatoms in reactive environments has been identified in other contexts, like the adsorption
of S-containing thiols on gold,67 or under reaction conditions
by reaction−deposition−elimination cycles68 or the identification of organometallic-like compounds as intermediates on
surface C−C couplings.61

■

ACKNOWLEDGMENTS
The authors thank MINECO CTQ2015-68770-R project for
financial support. The authors gratefully acknowledge the
generous computing time and assistance provided by the
Barcelona Supercomputing Center and the Spanish Supercomputing Network.

■

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CONCLUSIONS
Density functional theory simulations have been employed to
unravel the initial states of the decomposition of ligands based
on phosphines to form metal phosphides. We have found that
the reaction is slightly endothermic on the bare Ni(111) and
(100) surfaces, but it proceeds smoothly with relatively small
energy barriers. The values obtained are compatible with the
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