Triphenylphosphine-Assisted Transformation of NiS to Ni2P through a Solvent-Less Pyrolysis Route: Synthesis and Electrocatalytic Performance

Article abstract of DOI:10.1021/acs.inorgchem.1c01325

Straightforward synthetic routes to the preparation of transition metal phosphides or their chalcogenide analogues are highly desired due to their widespread applications, including catalysis. We report a facile and simple route for the preparation of a pure phase nickel phosphide (Ni2P) and phase transformations in the nickel sulfide (NiS) system through a solvent-less synthetic protocol. Decomposition of different sulfur-based complexes (dithiocarbamate, xanthate, and dithiophosphonate) of nickel(II) was investigated in the presence and absence of triphenylphosphine (TPP). The optimization of reaction parameters (nature of precursor, ratio of TPP, temperature, and time) indicated that phosphorus- and sulfur-containing inorganic dithiophosphonate complexes and TPP (1:1 mole ratio) produced pure nickel phosphide, whereas different phases of nickel sulfide were obtained from dithiocarbamate and xanthate precursors in the presence or absence of TPP. A plausible explanation of the sulfide or phosphide phase formation is suggested, and the performance of Ni2P was investigated as an electrocatalyst for supercapacitance and overall water-splitting reactions. The performance of Ni2P with the surface free of any capping agents is not well explored, as common synthetic methods are solution-based routes; therefore, the electrocatalytic performance was also compared with metal phosphides, prepared by other routes. The highest specific capacitance of 367 F/g was observed at 1 A/g, and the maximum energy and power density of Ni2P were calculated to be 17.9 Wh/kg and 6951 W/kg, respectively. The prepared nickel phosphide required overpotentials of 174 and 316 mV along with Tafel slopes of 115 and 95 mV/dec to achieve a current density of 10 mA/cm2 for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively.

Full text of DOI:10.1021/acs.inorgchem.1c01325

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Article
Triphenylphosphine-Assisted Transformation of NiS to Ni2P through
a Solvent-Less Pyrolysis Route: Synthesis and Electrocatalytic
Performance
Gwaza E. Ayom, Malik D. Khan, Ginena B. Shombe, Jonghyun Choi, Ram K. Gupta, Werner E. van Zyl,
and Neerish Revaprasadu*
Cite This: Inorg. Chem. 2021, 60, 11374−11384
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ABSTRACT: Straightforward synthetic routes to the preparation
of transition metal phosphides or their chalcogenide analogues are
highly desired due to their widespread applications, including
catalysis. We report a facile and simple route for the preparation of
a pure phase nickel phosphide (Ni2P) and phase transformations in
the nickel sulfide (NiS) system through a solvent-less synthetic
protocol. Decomposition of different sulfur-based complexes
(dithiocarbamate, xanthate, and dithiophosphonate) of nickel(II)
was investigated in the presence and absence of triphenylphosphine
(TPP). The optimization of reaction parameters (nature of
precursor, ratio of TPP, temperature, and time) indicated that phosphorus- and sulfur-containing inorganic dithiophosphonate
complexes and TPP (1:1 mole ratio) produced pure nickel phosphide, whereas different phases of nickel sulfide were obtained from
dithiocarbamate and xanthate precursors in the presence or absence of TPP. A plausible explanation of the sulfide or phosphide
phase formation is suggested, and the performance of Ni2P was investigated as an electrocatalyst for supercapacitance and overall
water-splitting reactions. The performance of Ni2P with the surface free of any capping agents is not well explored, as common
synthetic methods are solution-based routes; therefore, the electrocatalytic performance was also compared with metal phosphides,
prepared by other routes. The highest specific capacitance of 367 F/g was observed at 1 A/g, and the maximum energy and power
density of Ni2P were calculated to be 17.9 Wh/kg and 6951 W/kg, respectively. The prepared nickel phosphide required
overpotentials of 174 and 316 mV along with Tafel slopes of 115 and 95 mV/dec to achieve a current density of 10 mA/cm2 for the
hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively.
challenging reactions.19 The focus has been on developing
cheaper, recyclable alternatives to the scarce, expensive, and
efficient noble metal catalysts in these electrochemical
INTRODUCTION
■
Transition-metal phosphides continue to attract research
attention due to their applications in photovoltaic devices,
reactions. Supercapacitors are sought after as electrochemical
energy storage devices due to their superior performance in
power density and energy and cycle stability compared to
batteries,20 but they also employ noble metal electrodes for
efficient performance.21 In this regard, nickel is considered an
earth-abundant metal, and nickel phosphides are emerging as
cost-effective catalysts in replacing noble metals in energy
generation and storage devices.18
biomedical imaging, catalysis, energy generation, and stor-
age.1−10 Nickel phosphides, as an important transition metal
phosphide, have been explored as catalysts in different
chemical reactions.11−13 It is a binary system that has been
synthesized in different compositions and phases, including
NiP, Ni2P, Ni3P, NiP2, NiP3, Ni5P4, and Ni12P12.14 The
multiplicity of phases of nickel phosphide is a constraint to the
preparation of the desired/pure phase nickel phosphide
system.
Nickel phosphides have been prepared via different routes.
Hydrogen/oxygen production via water splitting is a cheap
and clean alternative to traditional fossil fuels. Researchers
have explored this energy generation route to produce
hydrogen at the cathode, generally referred to as hydrogen
evolution reactions (HERs) or oxygen at the anode, termed
oxygen evolution reactions (OERs).15−18 The production of
cheap and clean energy through water splitting is, however,
restricted by the scarcity and cost of noble metal catalysts,
usually employed to overcome the thermodynamically
Xie et al. used white phosphorus and nickel salt to prepare
Received: May 1, 2021
Published: July 14, 2021
https://doi.org/10.1021/acs.inorgchem.1c01325
© 2021 American Chemical Society
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Ni2P via a solvothermal route.22 Qian and co-workers used the
less toxic red phosphorus and nickel salt as starting materials to
prepare dinickel phosphide nanomaterials.23 These earlier
solvothermal routes were, however, restricted by the difficulty
of phase control, purity, toxicity, reaction times, and the need
for highly reactive phosphide sources.24 Decomposition of
metal−organic precursors as single-source precursors has been
explored to overcome the challenges of the earlier cumbersome
protocols in the preparation of nickel phosphides. A few
metal−organic precursors have been used as single-source
precursors, including phosphines,25−27 dithiophosphinates,28
and dithiophosphonates18 to prepare nickel phosphides. The
strengths of the single-source precursor route over the dual or
multiple sources are found in their ease and simplicity. Despite
the advantages of the single-source metal−organic precursors
in the formation of nickel phosphides, this synthetic protocol
employs a large amount of nongreen solvents in both the
ligand and metal complex preparation and also in the
formation of the metal phosphides.
Reports on solvent-less synthetic protocols to obtain nickel
phosphides are scarce. Shanmugam et al., for example, used an
inorganic complex [(triphenylphosphine)nickel(II) chloride]
to prepare nickel phosphides by solvent-less pyrolysis, but their
attempt yielded a mixture of Ni2P and Ni5P4 phases.25 Sodium
hypophosphite and nickel chloride hexahydrate have been used
in solid to solid solvent-less thermal decomposition to form
nickel phosphides.29 This synthetic protocol, however, requires
the initial decomposition of the employed phosphite to
generate the toxic phosphine gas and a filtration step with
deionized water to remove impurities. Zhou et al. also used
sodium hypophosphite as a phosphorus source to prepare
nickel phosphide in a lyophilization reaction with a long
reaction time, ultimately yielding a mixture of nickel phosphide
phases.30 Coleman et al. used toxic and poisonous precursors
such as red phosphorus and pyrophoric molecular white
phosphorus in a direct solid to solid reaction with nickel
chloride to form nickel phosphides.31 This synthetic route to
nickel phosphides was also restricted by high temperatures
employed, long reaction times, and the formation of impure
phases. Furthermore, it has been reported that the presence of
surfactants on the surface of nanomaterials act as insulating
layers and deteriorate their catalytic activity. Therefore,
solvent-less synthesis is important not only for the scalable
synthesis of catalytic materials but also to avoid the negative
effect of surfactants on the electrocatalytic performance of
electrocatalysts.
catalytic activity of Ni2P prepared by other routes to provide a
better understanding of the effect of the synthetic route on
electrocatalytic performance. We also report the effect of
solvents on the electrochemical behavior of catalysts by
comparing the performance of our solvent-less prepared
catalyst with other commonly prepared metal phosphides
formed through the wet route.
EXPERIMENTAL SECTION
■
Materials. Potassium ethyl xanthogenate (96%), sodium dieth-
yldithiocarbamate trihydrate, triphenylphosphine (PPh3), phosphorus
pentasulfide (P2S5, 99%), anisole (99%), nickel chloride hexahydrate,
nickel acetate tetrahydrate, Celite, deionized water, chloroform, and
acetone were purchased from Sigma Aldrich and used without further
purification. Diethyl ether, THF, toluene, and hexane were distilled
under nitrogen over a Na wire to form a benzophenone ketyl
indicator. Dichloromethane was distilled over P4O10. Methanol was
distilled from I2/Mg turnings.
Preparation of Metal Complexes. The syntheses of [Ni{S2CN-
(Et)2}2] (1), [Ni{(S2COEt)}2] (2), [Ni{S2P(OH)(4-C6H4OCH3)}2]
(3), and [Ni{S2P(OCH3)(4-C6H4OCH3)}2] (4) have been carried
out by following the reported procedures with slight modification, and
the details of synthetic protocols are provided in the Supporting
Information.
Preparation of Nickel Phosphide or Nickel Sulfide. Complex
3 or 4 with TPP in the mole ratio of 1:1 was ground together using a
ceramic boat and pestle. The homogenized solid mixture was then
placed in a ceramic combustion boat. The combustion boat with the
homogenized solid mixture was then heated in a quartz glass tube at
10 °C/min to 400 °C under a N2 flow for 1 h. The thermally
decomposed mixtures were then allowed to cool to room temperature
under N2 and isolated.
Replacing complexes 3 and 4 (which formed Ni2P) with 1 and 2 in
the same procedure as above yielded nickel sulfides.
The decomposition of complexes 1, 2, 3, and 4 without TPP
following the same procedure above and at temperatures between 250
and 400 °C also afforded nickel sulfides.
INSTRUMENTATION
■
Proton nuclear magnetic resonance (1H NMR) spectra of
complexes 1, 2, 3, and 4 were obtained from a Bruker Advance
instrument (400 MHz) with tetramethylsilane as an external
reference at 25 °C. Powder X-ray diffraction patterns of the
prepared materials were obtained from a Bruker D8 Discover
Diffractometer (Cu Kα radiation source) and matched to
standard reference patterns obtained from the International
Center for Diffraction Data (ICDD). Thermogravimetric
analysis (TGA) was performed using a TGA/DSC instrument
(Mettler-Toledo), with nitrogen gas employed for an inert
atmosphere at a flow rate of 10 mL/min. Transmission
electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM) images of the prepared
materials were obtained via a JEOL TEM (1400) and JEOL
HRTEM (2100). The preparation of samples for (HR)TEM
was carried out by dropping the diluted solution of the sample
onto coated Formvar grids for TEM and holey carbon grids for
HRTEM. These samples so prepared were allowed to dry at
room temperature and then examined for TEM (120 kV
voltage) and HRTEM (200 kV). Images were processed using
iTEM software for TEM and Gatan software for HRTEM.
Scanning electron microscopy (SEM) analyses of particles
were performed using a FEG-SEM (Philips XL30), while
energy-dispersive X-ray spectroscopy (EDX) was carried out
using a FEG-SEM instrument equipped with a DX4 detector.
All samples analyzed by the SEM technique were carbon-
Herein, we report a facile and straightforward solvent-less
thermal decomposition route for the formation of dinickel
phosphide and phase transformations in the nickel sulfide
system. We have investigated different nickel organic
precursors to prepare nickel phosphides. We used a previously
reported15 dithiocarbamate complex, [Ni{S2CN(Et)2}2] (1),
and a xanthate complex, [Ni{(S2COEt)}2] (2), as the nickel
precursors with triphenylphosphine (TPP) as the phosphorus
source via the melt method. This resulted in the formation of
nickel sulfide and its transformations. The solvent-less
decomposition of the previously reported dithiophosphonate
complexes,18 [Ni{S2P(OH)(4-C6H4OCH3)}2] (3) and [Ni-
{S2P(OCH3)(4-C6H4OCH3)}2] (4), which are sulfur and
phosphorus-containing complexes, together with TPP, how-
ever, formed pure phase Ni2P. The as-prepared uncapped Ni2P
was investigated as a catalyst in overall water splitting and
supercapacitance, and the performance is compared with the
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Scheme 1. Schematic Flow to the Solvent-Less Formation of Ni2P
coated by an Edwards coating system (E306A) before the
analysis. The X-ray photoelectron spectroscopy (XPS) spectra
were recorded on a Kratos Axis Ultra DLD spectroscopy. The
curve fitting and quantitative analysis was obtained by
measuring the area under the elemental and synthesized
peaks. Data analysis was conducted using the NIST XPS
database for peak assignments.
(TPP/1) shows the initiation of decomposition at 173 °C,
which is lower than the onset decomposition temperature of
TPP (200 °C) and that of complex 1 (292 °C). Complex 2
showed a two-step decomposition profile with a decom-
position onset temperature of 149 °C and an offset
temperature of 228 °C with a % residue weight of 29%,
matching the calculated weight residue of 32% for NiS. The
TGA of TPP/2 showed a two-step decomposition profile
similar to that of complex 2 with the onset and offset
decomposition temperatures of 105 and 322 °C. The TGA
profile of complex 3 shows multiple decomposition steps,
which set in at about 100 °C. The TGA profile of TPP/3
showed a single-step decomposition profile, unlike that of
complex 3, which has multiple steps. The thermogram of TPP/
3 indicates the onset and offset decomposition temperatures of
183 and 366 °C, respectively. The thermal decomposition
analysis of complex 4 shows a three-step decomposition profile
with the onset and offset decomposition temperatures of 200
and 800 °C, while that of TPP/4 is a one-step decomposition
profile with decomposition starting at 180 °C and ending at
320 °C, as summarized in Table S1. These results suggest that
TPP distills off completely without any residue, and when
mixed with complexes 1, 2, 3, and 4, it lowers the onset and
offset decomposition temperatures of the complexes. The TGA
also shows that the thermal stability of these complexes is in
the order 4 > 3 > 1 > 2.
Synthesis, p-XRD, and UV−Vis Analyses. Synthetic
procedures for the preparation of dithiocarbamates,34,35
xanthates,15,36,37 and dithiophosphonates18,38,39 are well
reported, and the synthesis of complexes 1 to 4 followed
these procedures (Supporting Information). We have also
previously reported the molecular structures of complexes 3
and 4.18 Nickel phosphide or sulfide particles were prepared by
the solvent-less decomposition of complexes 1 to 4 in the
presence or absence of TPP. The procedure for the solvent-less
decomposition of the mixture of complexes 1 to 4 with TPP
required the homogenization of the required complexes with
TPP by grinding the complexes and TPP (1:1) using a ceramic
mortar and pestle. The homogenized mixtures were then
decomposed in an inert atmosphere between 250 and 400 °C
without any solvents and isolated as black powders, as
illustrated in Schemes 1 and S1.
Electrochemical Studies. Electrochemical characteriza-
tion of the prepared Ni2P electrodes was performed using a
Gamry Potentiostat, which employed a three-electrode system.
Sample preparation for these electrochemical examinations was
achieved by forming pastes of the Ni2P materials using (80 wt
%) poly(vinylidene difluoride) (PVDF, 10 wt %) and acetylene
black (10 wt %), which were prepared using N-methyl
pyrrolidinone (NMP) as a solvent. The prepared paste was
then applied to cleaned and weighed nickel foam and allowed
to dry. The dried pastes were then deployed as working
electrodes. Nickel foams (MTI Corporation, 99.99% purity)
were used for this electrochemical analysis. We also used
commercial carbons as conducting acetylene black (MTI
Corporation) with a particle size of 35−40 nm. Platinum wires
and saturated calomel electrodes (SCE) were used as counter
and reference electrodes, respectively. Supercapacitance
examinations of the solvent-less prepared Ni2P electrodes
were carried out using a 3 M KOH electrolyte. Charge storage
capacity was recorded using cyclic voltammetry (CV) and
galvanostatic charge−discharge (GCD) techniques at different
scan rates and current densities. The electrocatalytic properties
of the prepared electrodes were studied by employing cyclic
voltammetry and linear sweep voltammetry (LSV), with LSV
performed at a scan rate of 2 mV/s for OER measurements.
Electrochemical impedance spectroscopy (EIS) was performed
in the frequency range of 0.05 Hz to 10 kHz at an applied AC
amplitude of 10 mV.
RESULTS AND DISCUSSION
■
TGA Analysis. The thermal stability and decomposition
profile of the synthesized complexes, TPP and the ground
mixture of TPP with the complexes (TPP/complexes) in an
inert atmosphere, were studied and are presented in the
electronic Supporting Information (Figures S1, S2 and Table
S1). TPP decomposes cleanly without any residue, with
decomposition setting in at about 200 °C with completion at
320 °C, consistent with previous reports.32,33 The TGA of
complex 1 shows a one-step decomposition profile with a
decomposition onset temperature of 292 °C and an offset
temperature of 389 °C with a % residue weight of 25
(calculated 18%) corresponding closely with NiS. The
decomposition profile of the mixture of complex 1 and TPP
We have explored different classes of metal−organic
precursors, namely, dithiocarbamates, xanthates, and dithio-
phosphonates, in our attempt to prepare nickel phosphides via
the solvent-less route. This is interesting, particularly for the
dithiophosphonate ligand, which contains a phosphorus atom,
unlike the xanthate or dithiocarbamate ligands (Figure 1). The
solvent-less decomposition of the mixture of complex 1 with
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Table 1. Nickel Sulfide or Phosphide Nanoparticles Formed
by the Solvent-less Decomposition of Complexes 1, 2, 3,
and 4 with or without TPP for 1 h
complex TPP temperature (°C)
formed phase
NiS (hex)
Ni3S2 (haezlewoodite)
NiS (hex)
Ni3S2 (haezlewoodite)
Ni3S2 (haezlewoodite)
Ni3S2 (haezlewoodite)
NiS (hex)
reference
1
1
2
2
2
2
3
3
4
4
no
yes
no
yes
yes
yes
no
yes
no
300
300
250
250
350
400
400
400
400
400
this study
this study
15
this study
this study
this study
18
this study
18
this study
Figure 1. Inorganic ligands in deprotonated form explored in this
study.
TPP (TPP/1; 1:1) at 300 °C for 1 h yielded nickel sulfide. In
the solvent-less decomposition of TPP/1, we attempted to
prepare nickel phosphide with complex 1 (dithiocarbamate
complex) as the metal precursor and TPP as the phosphorus
source.
Ni2P (hex)
NiS (hex)
Ni2P (hex)
The p-XRD analysis of the formed nickel sulfide confirms
the formation of the Ni3S2 (heazlewoodite) (ICDD no. 01-
073-0698) phase, as shown in Figure S3. Further heating of the
TPP/1 mixture for an extended duration of 2 h at 300 or 400
°C still yielded Ni3S2. The ratio of TPP/1 (2:1, 3:1, 4:1) was
also varied to facilitate phosphide product formation, but in all
cases, the Ni3S2 (haezlewoodite) phase was formed. It is
interesting to note that the solvent-less decomposition of 1 in
the absence of TPP at 300 °C for 1 h resulted in the formation
of a pure hexagonal NiS (ICDD no. 01-076-2306) phase, as
shown in Figure S4. The solvent-less decomposition of the
TPP/1 mixture, therefore, does not form nickel phosphide but
nickel sulfide, and the presence of TPP mediates the formation
of a different nickel sulfide phase compared to its solvent-less
decomposition without TPP. These results indicate that the
solvent-less pyrolysis of dithiocarbamates in the presence of
TPP as a phosphorus source may not form nickel phosphides.
We have also explored the suitability of complex 2 (xanthate
complex) as the nickel precursor and TPP as the phosphorus
source to form nickel phosphide. The solvent-less pyrolysis of
the TPP/2 (1:1) mixture at 400 °C for 1 h formed nickel
sulfide just like the solvent-less decomposition of TPP/1. The
as-prepared nickel sulfide matched well with the haezlewoodite
(Ni3S2) (ICDD no. 01-073-0698) phase (Figure S5). It is
important to note that the ratio of TPP to complex 2 or the
reaction time has no effect on the phase of NiS formed, which
is similar to results obtained by the solvent-free solid
decomposition of TPP/1. Furthermore, the solvent-less
pyrolysis of the TPP/2 (1:1) mixture at 250 and 350 °C
also resulted in the formation of the Ni3S2 phase (Figures S6
and S7). We have reported temperature-mediated phase
transformations of the nickel sulfide system upon the
solvent-less decomposition of complex 2.15 Interestingly, the
solvent-less decomposition of complex 2 in the absence of
TPP, at 250 °C for 1 h, forms pure hexagonal NiS (Figure S8),
and an increase in temperature to 400 °C resulted in the
formation of the millerite NiS phase. However, in the presence
of TPP, Ni3S2 was the only phase formed at different
temperatures. These results of the solvent-less decomposition
of complexes 1 and 2 with or without TPP are summarized in
Table 1.
yes
for 1 h yielded nickel phosphide, which was indexed to pure
hexagonal Ni2P (ICDD no. 03-065-1989) as shown in Figure 2
and Table 1.
Figure 2. Powder XRD of Ni2P formed from the solvent-less
decomposition of TPP/3 at 400 °C for 1 h.
The solvent-less thermolysis of complex 3 (dithiophospho-
nate complex) without TPP formed hexagonal NiS (ICDD no.
01-075-0613), which is an expected product of decomposition
of sulfur-based complexes of nickel (Figure S9).18 It is
pertinent to note that the reaction time, temperature, and
the ratio of TPP to 3 had no effect on the solvent-less pyrolysis
of TPP/3. Complex 3 contains an −OH group; therefore, we
used another dithiophosphonate complex, 4, in which −OH
was replaced with an −OCH3 group and decomposed it in the
presence of TPP to gain further insight into the solvent-less
preparation of Ni2P employing a dithiophosphonate complex.
The decomposition of TPP/4 (1:1) at 400 °C for 1 h also
formed Ni2P (ICDD no. 03-065-1989), just like the solvent-
less decomposition of TPP/3 (Figure S10). The solvent-less
decomposition of complex 4 without TPP, however, formed
hexagonal NiS (ICDD no. 01-075-0613) similar to complex 3
as expected (Figure S11 and Table 1). We have also analyzed
the diffraction patterns of complex 4, TPP, and a mixture of
TPP/complex 4 before the melt reactions and the p-XRD of
TPP/complex 4 after solvent-less decomposition at 400 °C for
1 h to check the purity of our solvent-less formed Ni2P
(Figures S10 and S12). A comparative p-XRD analysis of these
materials, as shown in Figure S12, showed no residual peaks
from the dithiophosphonate complexes and TPP employed.
These results indicate that the solvent-less decomposition of
dithiophosphonate complexes in the presence of TPP yields
Ni2P, which are rarely formed in a pure phase via the solvent-
Therefore, it can be inferred that the solvent-less thermolysis
of nickel dithiocarbamate and xanthate complexes yields nickel
sulfides, and the presence of TPP results in the phase
transformation. It is interesting to note that generally in
solvent-less synthesis, the phase is mostly transformed with
respect to temperature. There are not many reports on the
addition of solid additives to transform the synthesized phase
by this route. The solvent-less decomposition of the mixture of
TPP and complex 3 (dithiophosphonate complex) at 400 °C
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Figure 3. SEM image (a), elemental distribution of Ni (b) and P (c), TEM image (d), HRTEM image (e), and the selected area electron
diffraction (SAED) pattern (f) of solvent-less formed Ni2P.
less protocol. The thought-provoking question then is why is
the solvent-less decomposition of TPP/1 and TPP/2 not
successful in the formation of phosphides, unlike TPP/3 and
TPP/4. One of the plausible arguments for the solvent-less
formation of nickel phosphide with TPP/3 and TPP/4 is the
thermal stability of TPP and complexes 3 and 4. TPP has been
explored as a phosphorus source in the preparation of
phosphides in wet40−43 and dry thermolysis25 though its
mechanism of phosphorization is not well understood. The
phosphorization mechanism of TPP requires the breaking of its
P−C bonds to release phosphorus at elevated temperatures.44
It is important to note that the exploration of TPP as a
phosphorus agent requires a minimum temperature of 300
°C.25,27,40−44 The decomposition of TPP to release phospho-
rus at elevated temperatures of 300 °C or more is also
corroborated by its TGA with the onset and offset temper-
atures of 200 and 320 °C, respectively. Complexes 1 and 2 are
thermally less stable compared to complexes 3 and 4. For
example, TPP/1 and TPP/2 decompose completely at
temperatures less than 400 °C, unlike TPP/3 and TPP/4
that are highly stable and decompose completely beyond 400
°C. We therefore hypothesize that complexes 1 and 2 begin to
decompose (breaking of the N−C bonds in complex 1 and O−
C bonds in complex 2) to form NiS at temperatures lower than
300 °C before the release of the phosphorus atoms from the
breaking of the P−C bond of TPP at temperatures ≥300 °C.
The presence of phosphorus atoms after the formation of the
Ni−S bond most likely mediates a phase change from NiS
(hex) formed in the absence of TPP to Ni3S2 (heazlewoodite)
formed in the presence of TPP for the solvent-less
decomposition of complexes 1 and 2. The presence of these
phosphorus atoms after the formation of NiS may also be the
source of impurities in the formed Ni3S2 (Figures S3 and S5−
S7). Trialkylphosphines have been reported to act as a
desulfurizing agent in colloidal synthesis;45 therefore, it is
assumed that the presence of TPP leads to the partial removal
of sulfur from NiS and resulted in the sulfur deficient phase
(i.e., Ni3S2). It was noted that the Ni3S2 phase formed was not
obtained with high purity, and the presence of other nickel
sulfide phases as minor impurities was also observed. The
decomposition of TPP/2 at 250 °C yielded the Ni3S2 phase
with relatively better purity as only three low-intensity peaks
were observed as impurities (Figure S6). The higher thermal
stability of complexes 3 and 4 plausibly delays its
decomposition to form nickel sulfide compared to complexes
1 and 2. The delayed decomposition to form NiS hence affords
the opportunity for less competition for the formation of Ni-P
compared to the Ni−S bonds in TPP/3 and TPP/4 solvent-
less pyrolysis. The high-temperature stability may also allow
the interaction of TPP with sulfur before the formation of NiS
as well, which may result in an easy/better desulfurization
process. Likewise, the concentration of phosphorus in the
TPP/3 and TPP/4 system compared to the TPP/1 and TPP/
2 system is another plausible argument for the formation of
phosphides in the former system compared to the latter. For
example, the availability of the intramolecular phosphorus
atoms from the decomposition of complexes 3 and 4 in
addition to the one from the breakdown of TPP, at
temperatures of 300 °C or more, seems suitable for the
formation of the Ni-P bond compared to the Ni−S bond. This
plausible argument gains more weight by the formation of NiS
(hex) from the solvent-less pyrolysis of the phosphorus and
sulfur-containing complexes 3 and 4 without TPP. In the
absence of TPP as a desulfurizing agent and reagent for
providing phosphorus, the preformed bonds between Ni and
sulfur in the molecular precursor favor the formation of the
Ni−S bond over the Ni-P bond.
The electronic absorption properties of complex 3, TPP, and
TPP/complex 3 were analyzed by ultraviolet−visible (UV−
vis) spectroscopy (Figure S13) to examine any interactions
between the dithiophosphonate complexes and TPP prior to
the solvent-less decomposition reactions. Complex 3, TPP, and
TPP/complex 3 all absorb light in the UV region between 300
and 350 nm with an absorption maxima of 316, 306, and 328
nm, respectively. The change in the shape and a red-shift in the
TPP/complex 3 mixture’s absorption spectrum may indicate
some interactions between the dithiophosphonate complex
and TPP before the decomposition reactions. The lone pair on
phosphorus may show interaction with the nickel in the
metal−organic complex.
Particle Analysis. The morphology of the synthesized
Ni2P was examined by scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) and is given in
Figure 3. SEM images of the Ni2P particles show highly
agglomerated and aggregated particles that form large blocks.
The TEM image of Ni2P reveals its flaky nature. The
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Figure 4. High-resolution XPS spectra of (a) Ni 2p and (b) P 2p regions.
Figure 5. (a) CV curves at various scan rates, (b) charge−discharge behavior at various current densities, (c) variation of specific capacitance versus
current density, and (d) the Ragone plot for the Ni2P material.
agglomeration and aggregation of these flaky particles are
attributed to the absence of any surface capping agent in the
dry solvent-less pyrolysis protocol. The elemental composition
of Ni2P was examined using SEM-EDX, which showed only
the presence of nickel and phosphorus. The Ni2P sample also
showed a good elemental distribution of nickel and
phosphorus (Figure 3b,c). Weight and atomic percentages of
nickel and phosphorus were found to be 38.87, 61.13, and
54.65, 45.35, respectively. Selected area electron diffraction
analysis of the prepared Ni2P material (Figure 3e) indicated its
crystallinity with spots well matched with the Ni2P standard
pattern (ICDD no. 03-065-1989). The microstructure of the
prepared Ni2P was analyzed by HRTEM, which indicated the
single crystallinity of the prepared nickel phosphide with well-
defined lattice fringes (Figure 3e).
Surface State Analysis. X-ray photoelectron spectroscopy
(XPS) was performed to check the purity of the solvent-less
formed Ni2P and is presented in Figure 4. The XPS results
show a peak at 852.7 eV from the Ni 2p3/2 spectrum window
typical of the Ni in the nickel phosphide system.46 The satellite
peaks at 855.0 and 855.1 eV in the Ni 2p3/2 spectrum region
are due to surface oxidation.46 The peak at 852.7 eV assigned
to Ni in Ni2P is close to the binding energy of zero valence
state Ni and positively shifted with reference to it, which
suggests that the Ni species have a small positive charge. The
peak at 870 eV in the Ni 2p1/2 window is assigned to Ni in
Ni2P, and the satellite peaks are due to the surface oxidation.44
There is one peak in the P 2p window, which is assigned to
phosphorus in the solvent-less formed Ni2P, indicative of a
small positive charge on the phosphorus species.46 The XPS
results therefore indicate a partial electron transfer from Ni to
P and purity of solvent-less formed Ni2P since no sulfur peaks
were detected, in agreement with p-XRD and SEM-EDX
results. Furthermore, XPS analysis of the Ni3S2 material
formed from the decomposition of TPP/complex 2 at 250 °C
was also carried out to check for any phosphorus
contamination, and the spectrum is shown in Figure S14.
The XPS analysis indicates that phase-transformed Ni3S2
material assisted by TPP has no phosphorus contamination
as no phosphorus peaks were detected. This is also in
agreement with the SEM-EDX results. The presence of oxygen
is due to the surface oxidation of the uncapped materials.
Electrochemical Properties of Ni2P for Supercapaci-
tance. The electrochemical behavior of Ni2P was analyzed
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Figure 6. (a) Polarization curve, (b) Tafel slope, and (c) stability test for the Ni2P sample for the HER process.
Figure 7. (a) Polarization curve, (b) Tafel slope, (c) stability test, (d) EIS curves at various potentials, and (e) chronoamperometry measurement
for the Ni2P sample for the OER process.
using cyclic voltammetry (CV) and galvanostatic charge−
discharge (GCD) measurements. It is important to note that
the electrochemical behavior of the solvent-less prepared nickel
phosphide is not well explored. The CV of Ni2P, i.e., a plot of
the current density and electric potential at different scan rates
of 2−30 mV/s, is given in Figure 5a. The Ni2P material
showed a typical redox-type behavior with the presence of
oxidation and reduction peaks, as seen in Figure 5a. The
charge−discharge behavior of Ni2P is given as a plot at
different current densities ranging from 1 to 30 A/g (Figure
4b). The plot shows a typical redox nature, which is in
agreement with the CV measurements. The discharge time
increases with decreasing current density, suggesting improved
charge storage capacity at lower current densities. The
variation of specific capacitance as a function of the discharge
current is shown in Figure 5c. The specific capacitance of our
Ni2P material was estimated via galvanostatic charge−
discharge measurements using the following equation.
I × Δt
Csp =
(1)
ΔV × m
where I is the discharge current (A), Δt is the discharge time
(s), ΔV is the potential window (V), and m is the mass (g) of
Ni2P. The highest specific capacitance of 367 F/g was observed
at 1 A/g, which decreased to 179 F/g on increasing the current
to 30 A/g. The variation of energy and power density of the
Ni2P electrode, which is given as a Ragone plot, is shown in
Figure 5d. The energy density (E) and power density (P) of
the Ni2P material were calculated using the following
expressions
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C × ΔV2
The electrocatalytic performance of the Ni2P electrode
toward the OER was also investigated in 1 M KOH. Figure
7a,b shows the polarization curve and the Tafel slope of the
Ni2P electrode for the OER. An overpotential of 316 mV was
required to reach the current density of 10 mA/cm2. In
addition, the Ni2P electrode showed a Tafel slope of 95 mV/
dec. The OER activity of Ni2P synthesized by the solvent-less
route compares favorably or outperforms the activity of
reported nickel phosphides or other metal phosphides. For
example, Shanmugam et al. prepared a mixture of Ni2P and
Ni5P2 nickel phosphides from the solvent-less decomposition
of a bis(diphenylphosphine)nickel(II)chloride complex that
required an overpotential of 380 mV, higher than our
electrode, to reach a current density of 10 mA/cm2 in 0.1 M
KOH.25 Similarly, Shanmugam et al. prepared CoP/Co2P and
FeP/FeP2 through a solvent-less route that required higher
overpotentials than our electrode of 370 and 550 mV,
respectively, to reach a current density of 10 mA/cm2 (Table
S2). Justus et al. also prepared Co/Co2P materials from the
reductive thermal decomposition of dichloro-
(triphenylphosphine) cobalt ((PPh3)2CoCl2) in N2 and H2
and tested their electrochemical properties.51 The solvent-less
formed Co/Co2P electrode needed an overpotential of 390
mV, higher than our electrode, to achieve a current density of
10 mA/cm2. The electrochemical performance of our electrode
for the OER also compared well with other electrodes prepared
via the traditional wet route. For example, Ho-Wing et al.
prepared Ni2P, NiCoP, NiMnP, and NiMoP using olelyamine,
trioctylphosphine, and trioctylphosphine oxide, which required
overpotentials of 390, 410, 550, and 430 mV to attain a 20
mA/cm2 current density, which our solvent-less prepared Ni2P
outperforms with an overpotential of 338 mV required for a
similar current density (Table S2). Similarly, Fe2P and
Fe2−xMnxP (x = 0.9) nanorods52 and CoP polyhydron53
formed through the wet decomposition method recorded
overpotentials of 590, 440, and 400 mV, respectively, to
achieve a current density of 10 mA/cm2 compared to our
electrode with an overpotential of 316 to attain the same
current density. Table S2 gives a summary of the electro-
chemical performance of our solvent-less formed electrode in
comparison to other metal phosphides prepared by the dry and
wet decomposition routes. Figure 7d displays the Nyquist plot
for the Ni2P electrode. It was observed that the diameter of the
semicircle decreases with increasing potential, which indicates
the gradual decrease of charge transfer resistance at the
interface between the Ni2P electrode and 1 M KOH
electrolyte. When a high potential of 0.6 V (V, SCE) was
applied, the electrode showed a low resistance of 1.4 Ω/cm2,
explaining the electrode’s high activity. The long-term
durability test for the Ni2P electrode was carried out by
comparing the polarization curve of the 1st and 2000th cycles
(Figure 7c), along with chronoamperometry (CA) measure-
ments over 18 h (Figure 7e). As can be seen, the graphs of the
1st and 2000th cycles are well matched, and the current
density measurements showed a minimal decrease from 23 to
11 mA/cm2 over a long period. These results indicate the
suitability of our electrode for practical applications in OER
technology.
E (Wh/kg) =
(2)
(3)
7.2
E × 3600
P (W/kg) =
t
where C is the specific capacitance of the Ni2P electrode
determined using charge−discharge measurements, ΔV is the
potential window (V), and t is the discharge time (s). The
maximum energy and power density of Ni2P were calculated to
be 17.9 Wh/kg and 6951 W/kg, respectively.
Electrochemical Properties of Ni2P for Overall Water
Splitting. The electrochemical behavior of Ni2P was also
investigated for its application in the water-splitting process.
Generating hydrogen and oxygen via water splitting is one of
the cleanest ways to generate green energy. The thermody-
namic potential for water splitting is 1.23 V; however, in
reality, a much higher potential is required to split water. The
extra potential is called overpotential, which increases the
production cost of water splitting for green fuel. Ni2P was
therefore investigated as an electrocatalyst for its ability to
reduce this constraining overpotential. Ni2P was first tested as
an electrocatalyst in HERs. It is important to note that dinickel
phosphide is a good catalyst for the HER due to its highly
exposed (001) facets, the thermostability of these exposed
sites, and its hexagonal structure.47 Linear sweep voltammetry
(LSV) measurements were used to analyze the Ni2P material
activity and stability properties in 1 M KOH electrolyte. The
Ni2P electrode required 174 mV to deliver a current density of
10 mA/cm2, along with a Tafel slope of 115 mV/dec (Figure
6a,b), indicative of fast reaction kinetics. Kucernak and
Sundaram48 prepared Ni2P and Ni5P4 materials by ball milling
and a hydrothermal method, respectively, and examined their
electrochemical properties in the HER. The prepared Ni2P and
Ni5P4 materials recorded approximate overpotentials of 350
and 450 mV to achieve a current density of 10 mA/cm2,
respectively, in an acidic medium. These results indicate that
our Ni2P electrodes outperform their electrodes by 50 and
62%, respectively. Paquin et al.49 fabricated Ni2P materials,
which required an overpotential of about 225 mV, higher than
ours, to reach a current density of 10 mA/cm2. Iron phosphide
and iron phosphide on carbon support used by Suliman et al.
were similarly outperformed by our electrode with a recorded
overpotentials of 385 and 235 mV to achieve a current density
of 10 mA/cm2. Mabayoje et al. used a phosphine-containing
metal−organic framework (MOF) to form phosphides on
carbon support by a solvent-less route (annealing under
hydrogen), which required overpotentials of 120, 202, and 212
mV to achieve a current density of 10 mA/cm2 comparable to
our results.50 Table S1 also summarizes the comparisons
between the performance of our Ni2P electrodes synthesized
by the solvent-less method and other reported transition metal
phosphides. These electrochemical property comparisons show
that our Ni2P electrode has lower overpotential as well as a low
Tafel slope indicative of high activity and fast reaction kinetics.
The long-term stability and hence practical application of an
electrode is a function of its durability. We therefore
investigated the durability of Ni2P electrodes by measuring
the polarization curves in the first and 2000 cycles of recycling
CV measurements (Figure 6c). The two curves display
negligible deviation, proving the Ni2P electrode as a highly
stable electrocatalyst for HER.
The Ni2P electrode exhibited high HER and OER
performances in the three-electrode system. We therefore
fabricated the electrolyzer cell using the Ni2P electrodes as
both anode and cathode to further study its activity in water-
splitting reactions using the two-electrode system. In this
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Figure 8. (a) Polarization curve and (b) chronoamperometry measurement for electrolyzer fabricated using Ni2P electrodes (inset figure shows the
evaluation of oxygen and hydrogen during electrolysis).
system, hydrogen and oxygen gases can be generated via water-
splitting reactions at the cathode and anode, respectively. The
overpotential of 500 mV was required to produce a current
density of 10 mA/cm2 for the Ni2P electrodes (Figure 8a),
which correlates to its overpotential for both HER and OER, as
shown in Figures 6 and 7. To investigate the long-term stability
of our fabricated two-electrode cell (using the Ni2P cathode
and anode), chronoamperometry measurements were recorded
over 43 h and are shown in Figure 8b. The small current
density loss from 21 to 18 mA/cm2 over a long period of 43 h
is indicative of electrode durability and suitability for usage
even when deployed as both the anode and cathode in an
electrolyzer cell. Small fluctuations in chronoamperometry
measurements observed in the graph (Figure 8b) are due to
the generation of hydrogen and oxygen gas. The good activity
and performance of the Ni2P electrode prepared via our
solvent-less protocol could be due to the absence of any
solvents, surfactants, or capping agents in its formation.
Though the use of surfactants in tuning the size and shape
of nanomaterials is crucial and widely employed, their surface
passivation has been reported to have a detrimental effect on
the electrocatalytic properties of prepared electrodes.54
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01325.
■
sı
Thermogravimetric analyses of complexes 1, 2, 3, and 4
alone, triphenylphosphine (TPP) and the composites of
TPP with complexes 1, 2, 3, and 4 respectively;
summary of TGA of TPP and TPP/complexes 1−4
composites; schematic flow to the formation of Ni2P; p-
XRD patterns of the solvent-less decomposition of TPP/
complex 1 at 300 °C, complex 1 at 300 °C, TPP/
complex 2 at 400 °C, 250, and 350 °C, complex 2 at 250
°C, complex 3 at 400 °C, TPP/complex 4 at 400 °C,
complex 4 at 400 °C all for 1 h; p-XRD of complex 4,
TPP and TPP/complex 4 at room temperature and
TPP/4 at 400 for 1 h; UV−vis of TPP, complex 3, and
TPP/complex 3; XPS spectrum of the decomposition of
TPP/complex 2 at 250 °C for 1 h; comparison tables of
the HER and OER of our solvent-less formed Ni2P with
other reported transition metal phosphides; NMR of
complexes 1, 2, 3, and 4 (PDF)
AUTHOR INFORMATION
Corresponding Author
■
CONCLUSIONS
■
Neerish Revaprasadu − Department of Chemistry, University
of Zululand, KwaDlangezwa 3880, South Africa;
orcid.org/0000-0001-5730-1232;
A facile and simple solvent-less protocol for the preparation of
a nickel phosphide is described. This protocol requires the
decomposition of the ground solid mixture of dithiophosph-
onate complexes (complexes 3 and 4) and triphenylphosphine
at 400 °C for an hour. We suggest that TPP has a dual role by
acting as a reagent to provide phosphorus and a desulfurizing
agent. The study also indicates that the presence of solid
additives (TPP) can result in a phase transformation in the
nickel sulfide system. The possibility of the formation of Ni2P
from other commonly used metal complexes of nickel was
ruled out as the decomposition of the solid mixtures of
complexes 1 (dithiocarbamate) and 2 (xanthate) with TPP
formed nickel sulfide. The absence of sulfur-based impurities
and phase purity of Ni2P was also confirmed by XPS analysis.
The uncapped nickel phosphide showed good electrochemical
activity as a catalyst in supercapacitance and overall water-
splitting investigations. The precise control over size in the
absence of surfactants is challenging; however, the absence of
surfactants also improved the HER and OER performances of
the synthesized Ni2P as compared to Ni2P prepared by other
colloidal methods. The study indicates that, besides scalability,
solvent-less synthesis has the potential to enhance the
electrocatalytic performance of electroactive materials.
Email: RevaprasaduN@unizulu.ac.za
Authors
Gwaza E. Ayom − Department of Chemistry, University of
Zululand, KwaDlangezwa 3880, South Africa
Malik D. Khan − Department of Chemistry, University of
Zululand, KwaDlangezwa 3880, South Africa; Institute of
Physical Chemistry, Polish Academy of Sciences, Warsaw 01-
224, Poland
Ginena B. Shombe − Department of Chemistry, University of
Zululand, KwaDlangezwa 3880, South Africa; Chemistry
Department, University of Dar-es-salaam, Dar-es-salaam,
Tanzania
Jonghyun Choi − Department of Chemistry, Pittsburg State
University, Pittsburg, Kansas 66762, United States
Ram K. Gupta − Department of Chemistry, Pittsburg State
University, Pittsburg, Kansas 66762, United States;
orcid.org/0000-0001-5355-3897
Werner E. van Zyl − School of Chemistry and Physics,
University of KwaZuluNatal, Westville Campus, Chiltern
Hills, Private Bag, Durban 4000, South Africa
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Kongmanklang, C.; Butburee, T.; Kiatphuengporn, S.;
Faungnawakij, K.; Chanlek, N.; Wittayakun, J.; Khemthong, P.
Influential properties of activated carbon on dispersion of nickel
phosphides and catalytic performance in hydrodeoxygenation of palm
oil. Catal. Today 2020, 153−164.
(14) Callejas, J. F.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak,
R. E. Synthesis, Characterization, and Properties of Metal Phosphide
Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater. 2016,
28, 6017−6044.
(15) Shombe, G. B.; Khan, M. D.; Zequine, C.; Zhao, C.; Gupta, R.
K.; Revaprasadu, N. Direct solvent free synthesis of bare α-NiS, β-NiS
and α-β-NiS composite as excellent electrocatalysts: Effect of self-
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01325
Author Contributions
This manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Research Foundation (NRF)
South African Research Chair Initiative (SARChI) for financial
support (Grant No. 64820). AGE is also thankful to the
University of Zululand and NRF for Research fellowship
funding.
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