In situ fabrication of a 2D Ni2P/red phosphorus heterojunction for efficient photocatalytic H2 evolution

Article abstract of DOI:10.1016/j.materresbull.2019.03.011

Highly available and active photocatalysts and cocatalysts play key roles in the of efficient and stable visible-light-driven photocatalytic systems. In this work, a nickel phosphide (Ni₂P)/red phosphorus (RP) heterojunction composite was fabri

Full text of DOI:10.1016/j.materresbull.2019.03.011

Accepted Manuscript
Title: In situ fabrication of a 2D Ni2P/red phosphorus
heterojunction for efficient photocatalytic H2 evolution
Authors: Enzhou Liu, Lulu Qi, Jibing Chen, Jun Fan, Xiaoyun
Hu
PII:
S0025-5408(18)33024-1
DOI:
Reference:
https://doi.org/10.1016/j.materresbull.2019.03.011
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Accepted date:
21 September 2018
10 March 2019
10 March 2019
Please cite this article as: Liu E, Qi L, Chen J, Fan J, Hu X, In situ fabrication of a 2D
Ni2P/red phosphorus heterojunction for efficient photocatalytic H2 evolution, Materials
Research Bulletin (2019), https://doi.org/10.1016/j.materresbull.2019.03.011
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In situ fabrication of a 2D Ni2P/red phosphorus
heterojunction for efficient photocatalytic H2 evolution
Enzhou Liuab*, Lulu Qia, Jibing Chena, Jun Fana, Xiaoyun Huc*
a School of Chemical Engineering, Northwest University, Xi’an, Shaanxi, China, 710069
b Institute of Modern Physics, Northwest University, Xi’an, Shaanxi, China, 710069
c School of Physics, Northwest University, Xi’an, Shaanxi, China, 710069
Corresponding author e-mail: liuenzhou@nwu.edu.cn; hxy3275@nwu.edu.cn

Graphical abstract
Highlights:

Ni2P/red phosphorus heterojunction was fabricated by a simple in situ hydrothermal
strategy.




Ni2P/red phosphorus with 2D nanostructure has good contact interface and stability.
The surface edge of red phosphorus is the active sites during the photocatalytic reaction.
Ni2P as co-catalyst can induce more active sites and improve the charge separation.
Ni2P can greatly enhance the photocatalytic H2 production efficiency of red phosphorus.

Abstract: Highly available and active photocatalysts and cocatalysts play key roles in
the of efficient and stable visible-light-driven photocatalytic systems. In this work, a
nickel phosphide (Ni2P)/red phosphorus (RP) heterojunction composite was fabricated
for the first time by an in situ hydrothermal method using nanosized RP and nickel
chloride as the starting materials. The results indicated that the heterojunction exhibits
a two-dimensional (2D) nanostructure with a good contact interface and stability. The
surface edges of pure RP are the active sites during the photocatalytic reaction. The
introduction of Ni2P could improve the light-harvetsing ability of RP and facilitate
charge separation, leading to an enhanced photocurrent intensity and excellent
photocatalytic performance for H2 evolution. The H2 production rate of the 3% Ni2P/RP
-1 -1
.
sample was 2183 μmol.g h under visible light irradiation, which was higher than that
of Pt/RP, indicating that Ni2P could be used as an efficient noble-metal-free cocatalyst.
In addition, a possible mechanism for H2 production was proposed for Ni2P/RP
composite. Further work should be conducted in the exploration of P-based transition
metal cocatalysts and RP-based photocatalysts.
Keywords: Red phosphorus; Nickel phosphide; Heterojunction; Cocatalyst
1. Introduction
Energy shortages and environmental pollution have become major global
challenges, with a profound impact on sustainable development worldwide [1-11]. The
replacement of fossil resources with green and renewable sources is the most effective

way to alleviate the above problems. Hydrogen energy is one of the best choices
because of its high-energy intensity and pollution-free features [12-17]. However,
hydrogen is usually produced by steam reforming of natural gas, involving high
temperatures and energy-consuming separation processes. There is an urgent need to
develop energy-saving and eco-friendly H2 production processes.
Since the discovery of H2 evolution from water splitting on a TiO2 electrode under
UV light irradiation by Fujishima and Honda, it has provided a way to convert solar
energy directly into sustainable H2 energy [18]. During the past few decades, a
substantial amount of effort has been devoted to the development of efficient
photocatalytic systems with the aim of improving the H2 evolution efficiency. The
major obstacles to achieving high solar-to-hydrogen energy conversion efficiency are
the lack of visible light harvesting materials and the recombination of charge carriers
due to the inherent defects in photocatalysts. For instance, the traditional TiO2
photocatalyst has a large band gap ( 3.2 eV), which results in TiO2 only being active
under ultraviolet light, leading to a low usage of solar energy [19-20]. Therefore,
visible-light-driven (VLD) photocatalysts have received a substantial amount of
attention, such as g-C3N4 [21], CdS [22], BiVO4 [23], CdXZn(1-X)S [24], and ZnInS4
[25]. However, the absorption edge of most photocatalysts is below 500 nm, and
photocatalysts cannot fully utilize visible light. In addition, the high recombination rate
of charge carriers during the reaction limits the quantum efficiency of the excited
electrons and holes. Many strategies have been developed to solve the above problems,
such as constructing heterojunctions [26-27], elemental doping [28-29], structural
regulation [30], photosensitization [31], and cocatalyst loading [32-33]. The
exploration of photocatalysts with excellent visible light harvesting and H2 production
abilities is still a very active and interesting field.
In 2012, Wang et.al reported that red phosphorus (RP), which has a narrow band
gap smaller than 2.0 eV, could achieve photocatalytic water splitting under visible light,
and H2 and O2 were obtained over RP due to the suitable positions of its conduction and
valence band edges. The nontoxic, stable and easily available properties of RP make it

a promising candidate for photocatalytic H2 production. Most investigations have
focused on improving charge carrier separation and increasing the specific surface area
of RP. Additionally, YPO4 [34], g-C3N4 [35], Ni(OH)2 [36], black phosphorus (BP) [37],
TiO2 [38], CdS [39], MoS2 [40], Cr2O3 [41] and graphene [42] have been employed to
improve the activity of RP by constructing heterojunctions. However, most
heterojunction composites with microsized structures suffer from small contact areas
and a low number of active sites, leading to poor photocatalytic performance[43].
Zhang et al. increased the specific surface area of RP by heating amorphous RP in a
vacuum tube furnace, but the maximum specific surface area was only 39ꢀm2/g [44]. In
our previous work, we successfully prepared nanosized RP by a hydrothermal method,
its specific surface area was 54.3 m2/g, and the obtained nanosized RP exhibited
excellent photocatalytic activity and adsorption performance. However, RP-based
photocatalysts are still far from satisfactory for large-scale application. Developing
highly active RP-based photocatalysts is an urgent need.
Extensive experiments have shown that introducing appropriate cocatalysts can not
only enhance light harvesting but also favor the desired reactions and hinder the back
reaction[45-46]. Introducing cocatalysts may create enough reactive sites for efficient
utilization of the charge carriers and ensure the long-term stability of the composite [47-
50]. Pt is the most commonly used cocatalyst for H2 evolution, whereas its scarcity and
high cost greatly limit the wide application of Pt-containing photocatalysts.
Consequently, a substantial amount of attention has been focused on developing
cocatalysts with earth-abundant nonprecious elements.
Recently, the use of transition metal phosphatides (such as Ni2P and Co2P) as
nonprecious metal cocatalysts has sparked wide attention. For example, Cao et al.
discovered that nickel phosphide (Ni2P) is an efficient cocatalyst for photocatalytic H2
evolution, and its hybrid system exhibited enhanced photocatalytic hydrogen-
generating activity and excellent stability in an aqueous lactic acid solution [50]. Sun
et al. prepared Ni2P/CdS NRs in ethylenediamine by a solvothermal method, the highest
H2 production rate was 1200 mmol g-1·h-1, and the apparent quantum yield was 41% at

450 nm [51], In addition, Zhao et al. reported g-C3N4 modified by a Ni2P cocatalyst,
which exhibited enhanced H2 production activity under visible light irradiation, the
photoinduced electron in the CB of g-C3N4 could rapidly transfer to the metallic Ni2P
through the heterostructured interfaces between g-C3N4 and Ni2P. [52]. Chen et al.
employed dispersed Ni2P cocatalysts to boost the photocatalytic hydrogen generation
of TiO2, the photogenerated electrons can also transfer to the Ni2P cocatalyst, since the
conduction band edge of Ni2P (0 V vs. RHE) is much lower than that of anatase TiO2
( − 0.26 V vs. RHE)[53]. In addition, Ni2P has been employed to construct
heterojunctions with CdXZn(1-X)S [54], ZnO [55], CaIn2S4 [56] and graphene [57]. It is
commonly believed that Ni2P has the potential to replace Pt as a H2-releasing cocatalyst.
(the use of Ni2P as a cocatalyst in the literature is summarized in Table 1). However,
Ni2P is mainly fabricated via high temperature calcining method with inert gas
protection, and then it was combined with the semiconductor to form heterogeneous
junction, leading to a poor contact interface, which can’t take full advantage of Ni2P.
In this work, Ni2P/RP heterojunctions with 2D nanostructures were synthesized
for the first time by an in situ hydrothermal method using nanosized red phosphorus as
the phosphor source. The light-harvesting ability of RP and the separation of the charge
carriers can be improved after the introduction of Ni2P. The composites exhibit an
obviously enhanced H2 evolution rate under visible light irradiation (λ>420 nm)
compared to those of pure RP and Pt/RP. The interfacial charge transfer between Ni2P
and RP was investigated, and a reasonable photocatalytic mechanism was proposed for
the Ni2P/RP system during the H2 evolution reaction.

Table 1 The photocatalytic systems containing Ni2P as a cocatalyst that have been reported in the literature
Phosphorus
source
Sacrificial AQY
Materials
Preparation method
Application
Results
Ref.
agent
[%]
Ni2P/CdS
exhibited
a
better
photocatalytic H2 evolution rate than that
of both of the components. The rate of 0.5
wt% Ni2P loaded sample was [48]
Photocatalytic
water
splitting
Ni2P/CdS
nanorods
(NRs)
It
was
prepared
by
in
a
Yellow
Phosphorous
Na2S,
Na2SO3
41
under visible light
irradiation ( 420
nm).
ethylenediamine
solvothermal method.
(450 nm) approximately 22 times higher than that
of the CdS NRs alone, providing an
extraordinary H2 evolution rate of ~553
μmol·h-1·mg-1.
Photocatalytic
reduction of 4-
nitrotoluene into its
corresponding
The composite was fabricated by
grinding CdS and Ni2P
nanoparticles using a mortar in a NaH2PO2
certain ratio to form a uniform
powder.
For the system containing 5 wt% Ni2P,
Ni2P/CdS
hybrid
Na2S,
the reaction rate for nitro reduction was
No
[58]
amino
organics Na2SO3
determined to be 7146 μmol·h-1·mg-1 in
under 30 × 3 W
LED light (λ> 420
nm).
the first 10 h.
The
10%Ni2P@CdS
photocatalyst
displayed the best photocatalytic activity
for hydrogen production of the samples,
251.4 μmol of H2 was evolved over the
Photocatalytic
Ni2P was prepared by a simple
solvothermal method. CdS were White
suspended in the process of phosphorous
preparing Ni2P.
water
splitting
3.89
under visible light
irradiation ( 420
nm).
Ni2P@CdS
No
[59]
[60]
(430 nm) 10%Ni2P@CdS catalyst within 180 min,
which was 28.7 times higher than that of
1%Pt@CdS (8.7 μmol) under the same
reaction conditions.
Photocatalytic
The 3.5 wt% sample exhibited the highest
photocatalytic activity of the samples,
water
splitting
Ni2P with an average size of 10.3 NaH2PO2
nm was blended with g-C3N4 by and
a simple ultrasonication method. NH4H2PO4
15 nm Ni2P/g-
C3N4
3.2
under visible light
irradiation ( 420
nm).
Ethanol
(435 nm) which achieved a H2 evolution rate of
474.7 μmol·h-1·g-1.

H2 evolution rate over Ni2P/g-C3N4-
0.48% was approximately 1418 and 29
times higher than that over the pure g-
C3N4 and Ni(OH)2/g-C3N4 precursors, [49]
which represented a better H2 evolution
activity compared with that of g-C3N4/Pt-
0.48%.
Photocatalytic
water splitting
under visible
light
irradiation (
400 nm).
The Ni2P decorated g-C3N4 composite Monohydrat
was calcined at 300°C for 2 h at a sodium
heating rate of 2°C/min in an Ar hypophosph
Ni2P/g-C3N4
e
TEOA
No
atmosphere.
ite
It was obtained by an in situ growth
process, which contained solvent heat
evaporation and calcination at 543 K NaH2P-O2
in an Ar atmosphere.
Photocatalytic
water splitting
under visible TEOA
light
The 5% Ni2P/Fe-CNN composite had a
No
high H2 evolution rate of 397 μmol g-1 h−1,
[61]
Ni2P/Fe3+-
doped g-C3N4
which is 71 times higher than that of
BCN.
irradiation (
420 nm).
The fixation of
N2 to prepare
NH3 ( 400
nm).
After Ni2P was loaded onto the surface of
The composite was synthesized by a
solvothermal method at 140°C for 12
h.
Ni2P
/Cd0.5Zn0.5S
Yellow
phosphorous
4.32
Cd0.5Zn0.5S, the NH3 generation rate was
No
[62]
(420 nm) 101.5 μmol g-1 h−1 (35.7 times higher than
that of Cd0.5Zn0.5S) under visible light.
Ni2P-Cd0.9Zn0.1S
was
prepared
Photocatalytic
water splitting
under visible
light
irradiation (
420 nm).
The composite showed
performance for photocatalytically
a
great
through two-step hydrothermal
a
Ni2P-
method. Ni(NO3)2·6H2O and red
Red
phosphorus
Na2S,
Na2SO3
73.2
splitting water, and a high hydrogen
Cd0.9Zn0.1S/ g- phosphorus (the molar ratio of Ni to P
[63]
(420 nm) production rate of 2100.3 μmol mg-1 h−1,
which was ~ 17 times higher than that of
Cd0.9Zn0.1S/g-C3N4.
C3N4
was 1:5) were mixed in an
ethylenediamine solution and
maintained for 24 h at 200°C.
Photocatalytic
water splitting
under
ultraviolet
light.
0.4 wt% Ni2P/TiO2 had a high hydrogen
Ni2P nanoparticles were synthesized
production rate of 9.38 mmol h−1 g−1,
via a solution-based method. Ni2P/Ti Trinoctylph
NRs were synthesized by a thermally osphine
decomposing metal method.
11.6
(360 nm)
Ni2P/TiO2
Methanol
which is ∼85 times higher than that of the [50]
pure TiNR photocatalyst (0.11 mmol g-1
h−1).

2 Experimental
2.1. Synthesis of RP
The nanosized RP was synthesized by a facile hydrothermal method using commercial
microsized RP as the raw material. The detailed preparation process is shown in a
previous report [64].
2.2. Preparation of 2D Ni2P-decorated RP (Ni2P/RP)
Ni2P/RP with a 2D nanostructure was synthesized by an in situ hydrothermal method
using nanosized (RP) and (NiCl2·6H2O) as the raw materials. All reagents were of
analytical grade and were used without further purification. First, NiCl2·6H2O (x g) and
1.30 g of RP were dissolved in a mixture containing 50 mL of ethanol under magnetic
stirring for 3 h; then, the mixture was transferred into a Teflon-lined stainless steel
autoclave with a volume of 100 mL, heated and maintained at 180 °C for 1 h. After
cooling to room temperature, the solid samples were washed several times by vacuum
filtration and then dried at 60°C for 6 h. Finally, different composites with weight ratios
of NiCl2·6H2O to RP of 0.0001:1.3, 0.0003:1.3, 0.0005:1.3, 0.001:1.3, 0.003:1.3,
0.005:1.3, 0.01:1.3, 0.6:1.3 and 1:1.3 were fabricated and labeled as 0.1% Ni2P/RP, 0.3%
Ni2P/RP, 0.5% Ni2P/RP, 1% Ni2P/RP, 3% Ni2P/RP, 5% Ni2P/RP, 10% Ni2P/RP, 60%
Ni2P/RP and 100% Ni2P/RP.
2.4. Characterization techniques
A Shimadzu XRD-6000 powder diffractometer (Cu Kα radiation) was used to
characterize the crystalline structures of the samples. The microstructures of the
samples were observed using scanning electron microscopy (SEM, Carl Zeiss SIGMA)
and a Tecnai G2 F20S-TWIN for transmission electron microscopy (TEM). X-ray
photoelectron spectroscopy (XPS, Kratos AXIS NOVA spectrometer) was performed
on a Kratos AXIS NOVA spectrometer. The UV-Vis diffuse reflectance spectra were
obtained on a Shimadzu UV-3600 UV/Vis/NIR spectrophotometer. The Brunauer–
Emmett–Teller (BET) surface area was measured by a Quantachrome NOVA 2000e
sorption analyzer based on N2 adsorption-desorption isotherms at 77 K. The
photoelectrochemical performance was measured with an electrochemical system (CHI
660E, China) and a mixed electrolyte containing 0.5 M Na2SO4. The sample electrodes

were illuminated with a 300 W Xe lamp.
2.5. Photocatalytic hydrogen production
The photocatalytic H2 production experiments were carried out by a closed on-line
detection system (Lab Solar-IIIAG, Beijing Perfect Light Technology Co. Ltd, China)
using a 300 W Xe lamp equipped with a UV cut-off filter (λ > 420 nm) as the light
source. Typically, 30 mg of each sample was dispersed in a 100 mL aqueous solution
containing a mixed sacrificial agent (0.35 M Na2S and 0.25 M Na2SO3). The evolved
hydrogen was measured by gas chromatography (Tianmei, GC7900). To test the
stability of the samples, a cycling experiment was performed in which the residual H2
was removed by purging with nitrogen gas for 30 min before starting every new cycle.
3 Results and discussion
3.1 Formation mechanism of the Ni2P/RP sample
A possible formation mechanism was proposed to understand the evolution of the RP
and nickel ions during the hydrothermal process, as shown in Fig. 1, during the
hydrothermal process, nickel ions are absorbed onto the surface of RP, then it can be
reduced to metal nickel particles as the increase of reaction temperature (Equation1-
1)[65], XPS indicates the existence of very small metal Ni (Fig. 5). which can be
attributed to the excellent adsorption and reduction performance of RP. Besides, it well
known that RP can react with water under high temperature to produce PH3 and H3PO4
(Equation 1-2)[26], the obtained PH3 may react with Ni particle to form intermediate
(Ni-PH3), and then Ni2P (Equation 1-3)[65].
Ni2 2e  NiEquation1-1
O2
8P12H2O 5PH3 3H3PO4 Equation1-2
NiPH3  Ni2P(NiPH3)Equation1-3
3.2 Structural and compositional analysis
The X-ray diffraction (XRD) patterns of the Ni2P/RP samples with different ratios of
Ni2P were carried out to reveal their crystal structures. As displayed in Fig. 2., it can be
observed that the five distinct diffraction peaks of Ni2P in the 100% Ni2P/RP sample
were located at 40.71°, 44.61°, 47.36°, 54.20° and 55.00°, which can be accurately

indexed to the (111), (201), (210), (300) and (211) planes, respectively, of the hexagonal
structure of Ni2P (JCPDS#74-1385) [66]. In addition, a characteristic diffraction peak
at 2θ=14.95°, belonging to amorphous RP, was also detected, suggesting that Ni2P/RP
was successfully constructed. However, the characteristic diffraction peaks of Ni2P in
the Ni2P/RP composites gradually weakening with decreasing Ni2P content, and finally,
only the diffraction peak of RP could be seen. The above results reveal that the Ni2P/RP
composite was successfully obtained (see SEM and TEM results below for more
detailed information).
3.3 SEM and TEM analysis
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
were utilized to visualize the surface morphology and microstructure of the 3% Ni2P/RP
sample. Fig. 3Adisplays the typical microstructure of bare nanosized RP. Fig. 3B shows
the SEM image of the as-obtained 3% Ni2P/RP composite at the same magnification. It
can be seen that irregularly shaped Ni2P was uniformly grown across the surface of RP.
Additionally, the original microscopic morphology of RP was well preserved. As shown
in Fig. 3C-D, the elemental mapping results of the 3% Ni2P/RP sample further
demonstrate that Ni and P coexisted and were uniformly distributed.
The typical TEM images of RP and 3% Ni2P/RP are given in Fig. 4. As shown in Fig.
4A, pure RP possesses a two-dimensional porous structure. Fig. 4B and Fig. 4C show
that Ni2P nanoparticles were uniformly grown and distributed across the surface of the
RP, forming a tight interface with RP. To further visualize the crystal lattice on the
surface of the sample, high-resolution TEM (HRTEM) was employed, as shown in Fig.
4D. Highly crystalline individual Ni2P nanosheets with an average diameter of 5 nm
can be clearly observed. The spacing of the lattice fringes is 0.22 nm, which
corresponds to the (111) plane of Ni2P [62]. Importantly, Ni2P with a 2D structure was
embedded onto the surface of RP due to the above in situ preparation process. It can be
clearly seen that Ni2P also has a 2D structure, suggesting that a tightly connected 2D
heterojunction formed between Ni2P and RP.

For comparison, a Pt/RP composite was prepared using a photochemical reduction
method (see the supporting materials). As shown in Fig. 4E and 4F, Pt nanoparticles
with diameters of approximately 3 nm were mainly distributed along the surface edges
of pure RP. This result means that the surface edges are the active sites of pure RP
during photocatalytic processes. In contrast to the Pt deposition, Ni2P was uniformly
distributed across the RP nanosheets by in situ growth, which may greatly increase the
number of active sites on the surface of the RP.
3.4 XPS analysis
To further obtain the surface chemical composition and the chemical state, the X-
ray photoelectron spectroscopy (XPS) survey spectra of the 3% Ni2P/RP were analyzed.
As displayed in Fig. 5A, P, Ni, O and C were detected, and the existence of C and O
originate from surface adsorbed impurities and oxygen (Fig. 5A), which is attributed to
the adsorption performance of RP. The C 1s peak at 284.6 eV was used to correct all
the binding energies. In the high-resolution XPS spectrum of the P 2p peak, shown in
Fig. 5B,the peak located at 129.6 eV (P 2p3/2) was assigned to Pδ- in the form of metal
phosphides (i.e., Ni2P) [67]. Notably, as shown in Fig. 5C, the RP has a peak at 130.4
eV (P 2p3/2), which is consistent with the characteristic peak position of elemental
phosphorus (see Fig. 5E), In addition, the peak at a binding energy of 133.4 eV
originated from P 2p1/2, which was ascribed to oxidized P species, likely due to surface
oxidation or the interaction between RP and H2O at room temperature [67]. The high-
resolution XPS spectrum of the Ni 2p region (Fig. 5C) shows peaks centered at 853.1
eV (Ni 2p1/2) and 870.0 eV (Ni 2p3/2), which are consistent with Niδ+ in the Ni2P phase.
The peaks at 856.2 eV and 874.1 eV were possible to attribute to the oxidized Ni species
(Ni2+) of the Ni 2p1/2 and Ni 2p3/2 peaks. Moreover, two satellite peaks appeared at 860.5
eV and 880.2 eV [68].
3.5 UV-Vis analysis
UV-Vis diffuse reflectance spectroscopy was used to evaluate the optical properties

of RP with different amounts of Ni2P. As displayed in Fig. 6A, pure RP had an
absorption edge at approximately 690 nm, corresponding to a band gap of 1.8 eV for
RP. After introducing the Ni2P cocatalyst, the composites exhibited significantly
enhanced absorption in the 650-1200 nm region compared with that of RP, which can
be attributed to the introduction of black Ni2P, which has an excellent optical absorption
[48]. In addition, the results are in good accordance with the color change from red (RP)
to black (Ni2P) with increasing Ni2P content. The favorable optical absorption
properties of Ni2P greatly increased the visible light absorption of RP. The N2
adsorption-desorption isotherms of RP and 3% Ni2P/RP are presented in Fig. 6B. The
calculated specific surface areas of RP and 3% Ni2P/RP were 54.3 and 40.8 m2/g,
respectively. It should be noted that the loading of Ni2P led to a slight decrease in the
surface area compared to that of pure RP. This might be due to the Ni2P filling and
blocking the pores of RP [69].
3.6 Transient photocurrent response
To validate the photoelectronic performance of the samples, the photocurrents and
electrochemical impedance spectroscopy (EIS) behaviors of 3% Ni2P/RP and RP were
studied, and the results are displayed in Fig. 7A and B. The photocurrent density of the
3% Ni2P/RPelectrode was 11.96 μA/cm2, which is approximately 2.26-fold greater than
that of the bare RP electrode (5.30 μA/cm2). As shown in Fig. 7B, the EIS Nyquist plots
of 3% Ni2P/RP have a much smaller arc radius than that of RP, indicating that loading
of the Ni2P cocatalyst onto RP greatly reduced the charge transfer resistance, thus
significantly improving the charge transfer and separation efficiencies of RP. This result
is in good agreement with the results of the transient photocurrent responses.
3.7 3.7 Photocatalytic hydrogen production
The visible-light-induced (λ ≥ 420 nm) photocatalytic activity for H2 generation of
the different photocatalysts was measured under a 300 W Xe lamp irradiation using
Na2S and Na2SO3 as sacrificial agents. Fig. 8A shows the H2 evolution rates of the x%
Ni2P/RP (x=0.1, 0.3, 0.5, 1, 3, 5, 10, 60 and 100) catalysts after 3 h of irradiation.
Notably, the rate of H2 evolution increased with increasing Ni2P loading (from 0.1 to

3%). In addition, it is worth noting that the 3% Ni2P/RP photocatalyst had the highest
photocatalytic activity of all the samples. However, a decay in the rate is observed when
the amount of Ni2P was further increased to 5% and 10%. For both the 60% Ni2P/RP
and 100% Ni2P/RP composites, the H2 production rates are below that of RP, which was
ascribed to a “shielding effect” from the excess Ni2P loaded on RP [70-71]. In addition,
the average rate of H2 evolution was calculated and is shown in Fig. 8B. The average
-1 -1
.
H2 production rate of 3% Ni2P/RP was 2183.32 μmol.g h , which is 7.41 times higher
-1 -1
.
than that of RP (294.76 μmol.g h ). To compare the H2 evolution ability of Ni2P with
that of Pt, 3wt% Pt/RP and 3% Ni2P/RP, using TEOA (triethanolamine) as a scavenger,
were tested by a 300 W Xe lamp equipped with a UV cut-off filter (λ > 420 nm). As
displayed in Fig. 8C, the average photocatalytic H2 evolution rate of 3% Ni2P/RP
-1 -1
.
(240.93 μmol.g h ) was approximately 2.95 times higher than that of 3wt% Pt/RP
-1 -1
.
(81.63 μmol.g h ). In addition, the rate of 3% Ni2P/RP was 17.33 times higher than
-1 -1
.
that of RP (13.90 μmol.g h ). The above results clearly indicated that Ni2P was an
effective noble-metal-free and HER cocatalyst compared to RP. The enhanced
photocatalytic activity was attributed to the abundance of active reaction sites caused
by the introduction of Ni2P and its charger transfer properties. The stability of a
photocatalyst during photocatalytic reactions is an essential factor for its practical
application. Therefore, a cycling experiment for 3% Ni2P/RP was carried out, as shown
in Fig. 8D. The 3% Ni2P/RP sample still retains high H2 evolution activity after five
cyclic photoreactions. The Fig. 9 shows a comparison of the XRD patterns of 3%
Ni2P/RP before and after the photocatalytic reaction, which indicated that the crystal
structure of 3% Ni2P/RP composite did not change.
3.8 Photocatalytic mechanism
The band gap energy of RP was estimated to be 1.8 eV based on the results of a
Kubelka-Munk plot (Fig. 10A). Furthermore, the valence band was calculated to be
1.59 eV by XPS analysis (Fig. 10B); therefore, the corresponding conduction band was
-0.21 eV. These results are in agreement with those reported in the literature [42].
Consequently, a possible photocatalytic mechanism for the Ni2P-decorated RP is

depicted in Fig. 11. Under visible light illumination, the excited electrons of RP were
transferred from the valence band (VB) to the conduction band (CB), leaving holes in
the valence band (VB). On the one hand, the photogenerated holes can be effectively
2-
consumed by S2-/SO3 . On the other hand, the electrons can be quickly transferred to
Ni2P due to its electron capture property and good interfacial contact. Therefore,
photogenerated charge separation can be achieved, and the recombination process of
the electron-hole pairs is restrained. In addition, Ni2P, which has an abundance of active
sites, can reduce the activation potentials for H2 evolution, thus easily reducing H+ into
H2.
4. Conclusions
In conclusion, the 2D Ni2P cocatalyst-decorated RP was successfully synthesized by
an in situ hydrothermal method and exhibited an excellent ability to photocatalytically
evolve H2 from water under visible light irradiation. The introduction of Ni2P could
produce a greater number of active sites on the surface of RP for H2 evolution, and the
tightly combined 2D heterojunction can shorten the transmission distance of the
photoelectrons. Therefore, the enhanced photocatalytic activity could be attributed to
the effective separation of the charge carriers and greater number of exposed active sites
for promoting the reaction of solar-driven water splitting. The 3% Ni2P/RP sample
-1 -1
.
exhibited an average H2 production rate of 2183 μmol.g h , which was 7.41 times
higher than that of RP (S2-/SO32- was used as the sacrificial agent). In addition, the H2
production rate over 3% Ni2P/RP was 2.95 times greater than that of Pt-modified RP
under visible light irradiation (TEOA was used as the sacrificial agent). This work
demonstrates that Ni2P is a potential substitute for noble metal-based cocatalysts for
photocatalytic H2 production.
Acknowledgement:
This work was supported by the National Natural Science Foundation of China

(21676213, 21476183, and 51372201), the China Postdoctoral Science Foundation
(2016M600809), the Natural Science Basic Research Plan in Shaanxi Province of
China (2017JM2026) and the Graduate student innovation Funds of Northwest
University (No.YZZ17134).

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100% Ni2P-RP
60% Ni P-RP
3Ni+2PH3
Ni P+(Ni-PH )
2
2 3
Ni2++2e-
hydrothermal process
Ni
hydrothermal result
hydrothermal process
Ni2P
10% Ni2P-RP
Fig. 1 The possible formation mechanism process of Ni2P/RP the composite.
3% Ni2P-RP
RP
Ni2P-PDF#74-1385
10
20
30
40
50
60
70
80
2 Theta (dgree)
Fig. 2 XRD patterns of the pure RP, 3% Ni2P/RP, 10% Ni2P/RP, 60% Ni2P/RP, and
100% Ni2P/RP samples.
Fig. 3 SEM images of the (A)pure RP, and (B)3% Ni2P/RP, and (C) and (D) the

corresponding EDS mapping of the 3% Ni2P/RP sample for the region in image B.
(A)
(C)
(B)
(D)
Ni2P
RP
Ni2P
Ni2P
d (111)= 0.22
nm
Ni2P
RP
Fig. 4 TEM images of (A) pure RP, and (B) and (C) 3% Ni2P/RP (D) HRTEM images
of 3% Ni2P/RP, (E) TEM image of Pt/RP and (F) HRTEM images of Pt/RP .

(A)
0
200
400
600
800
Banding energy (eV)
XPS:Ni 2p
XPS:P 2p
(C)
(B)
+
Niδ
P 2p
129.6 eV
Ni 2p
Ni2+
853.1 eV
3/2
1/2
Ni 2p
870.0 eV
P 2p
130.4eV
3/2
3/2
+
Niδ
Ni2+
satellite
P 2p
133.4eV
1/2
satellite
125
130
135
140
145
845
850
855
860
865
870
875
880
885
Banding energy (eV)
Banding energy (eV)
XPS:P 2p
(E)
(D)
P 2p
130.4 eV
C1s
3/2
P
2pP2s
O1s
126
128
130
132
134
136
138
140
0
200
400
600
800
Banding energy (eV)
Banding energy (eV)
Fig. 5 XPS spectra of the as-prepared 3% Ni2P/RP including the (A) survey spectrum,
(B) high-resolution spectrum of the Ni 2p region, (C) high-resolution spectrum of the
P 2p region, (D) Survey spectrum of pure RP and (E) high-resolution spectrum of the P
2p region.

(A)1.2
160
140
120
100
80
(B)
RP
3% Ni2P/RP
1.0
0.1% Ni P/RP
2
0.8
0.6
0.4
0.2
0.0
0.3% Ni P/RP
2
0.5% Ni P/RP
2
1.0% Ni P/RP
2
3.0% Ni P/RP
2
60
5.0% Ni P/RP
2
40
10 % Ni P/RP
2
60 % Ni P/RP
2
20
100% Ni P/RP
2
RP
0
0.0
0.2
0.4
0.6
0.8
1.0
200
400
600
800
1000
1200
Relative pressure (P/P0)
Wavelength (nm)
Fig. 6 (A) UV-vis diffuse reflectance absorption spectra of the samples, (B) Nitrogen
adsorption-desorption isotherms of the pure RP and 3% Ni2P/RP sample.
(A)16
14
3% Ni P/RP
2
12
10
8
6
4
2
0
RP
5
15
25
35
45
55
65
75
85
Time (s)
(B)2500
RP
3% Ni P/RP
2000
2
1500
1000
500
0
0
500
1000
1500
2000
2500
3000
Z'/(ohm)
Fig. 7 (A) Transient photocurrent responses (I−t curves) of the photocatalysts in an
aqueous 0.5 M Na2SO4 aqueous solution under visible-light irradiation, and (B) the
Nyquist plots of different electrodes in an aqueous 0.5 M Na2SO4 aqueous solutions.

(B) 2500
(A) 3000
RP
0.1% Ni2P/RP
1.0% Ni2P/RP
0.3% Ni2P/RP
3.0% Ni2P/RP
0.5% Ni2P/RP
5.0% Ni2P/RP
10 % Ni2P/RP
60 % Ni2P/RP
100% Ni2P/RP
2500
2000
1500
1000
500
2000
1500
1000
500
0
0
0
0.0
0
0
0
1
3
.
1
5
1
0
0.5
1.0
1.5
2.0
2.5
3.0
.
3
6
0
.
5
100%
Ni
.
0
RP
.
0
.
%
0
%
%
%
%
%
%
%
P
2
Time (h)
(C) 300
(D) 2500
270
240
210
180
150
120
90
3% Ni2P/RP
2000
1500
1000
500
0
Pt/RP
60
30
RP
1.0
I
II
III IV
V
0
0
2
4
6
8
10
12
14
0.5
1.5
2.0
2.5
Time (h)
Time (h)
Fig. 8 (A) Photocatalytic H2 evolution rate of the samples under visible light irradiation
using Na2S and Na2SO3 as sacrificial agents, (B) the average H2 production rates after
3 h, (C) photocatalytic generation of H2 over Pt/RP and 3% Ni2P/RP using TEOA as a
sacrificial agent, and (D) cyclic H2 evolution curves over the 3% Ni2P/RP sample under
visible light irradiation.

After the photocatalytic hydrogen production
3% Ni2P/RP
Before the photocatalytic hydrogen production
3% Ni2P/RP
10
20
30
40
50
60
70
80
90
Banding energy (eV)
Fig. 9 The XRD patterns of 3% Ni2P/RP before and after the photocatalytic reaction.
(A)
(B)
RP
Eg=1.80 eV
1.59 eV (RP)
1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7
14
12
10
8
6
4
2
0
h (eV)
Banding energy (eV)
Fig. 10 (A) Kubelka-Munk plots of bare RP and (B) XPS valence band spectrum of RP

Fig. 11 A possible schematic diagram for the charge carrier transfer in the Ni2P/RP
catalyst.