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General applicability of nanocrystalline Ni2P as a noble-metal-free
cocatalyst to boost photocatalytic hydrogen generation
Ni2P cocatalyst can boost hydrogen generation over TiO2, CdS, and
C3N4 photocatalysts, demonstrating the good catalytic property
and general applicability. The optimized Ni2P/TiO2 sample
exhibited an 85 times higher activity compared to single TiO2,
leading to the apparent quantum e ciency of 11.6 % at 360 nm.
As featured in:
See Yubin Chen and Zhixiao Qin,
Catal. Sci. Technol., 2016, 6, 8212.
www.rsc.org/catalysis
Catalysis
Science &
Technology
PAPER
Cite this: Catal. Sci. Technol.,2016,
6,8212
Received 1st August 2016,
Accepted 15th September 2016
DOI: 10.1039/c6cy01653g
www.rsc.org/catalysis
General applicability of nanocrystalline Ni
2
Pasa
noble-metal-free cocatalyst to boost
photocatalytic hydrogen generation†
Yubin Chen*and Zhixiao Qin
To replace the role of noble-metal cocatalysts (e.g. Pt) in photocatalytic hydrogen generation, low-cost al-
ternatives with earth-abundant elements should not only possess high catalytic activities, but also have
general applicability. Herein, nanocrystalline Ni
2
P cocatalysts are used to modify CdS, TiO
2
, and C
3
N
4
host
photocatalysts. It is observed that the Ni
2
P cocatalyst boosts hydrogen generation over all the host photo-
catalysts, which demonstrates its good catalytic property and general applicability. To investigate its action
mechanism, nanocrystalline Ni
2
P was successfully integrated with TiO
2
nanorods (TiNR) for the first time.
The optimized Ni
2
P/TiNR sample exhibits an 85 times higher activity compared to single TiNR, and its ap-
parent quantum efficiency was calculated to be 11.6% at 360 nm. Among the varied nickel-based semicon-
ductor cocatalysts, Ni
2
P is also proven to be the best cocatalyst. Photoluminescence and electrochemical
results reveal that the Ni
2
P cocatalyst promotes the charge transfer both in the photocatalyst and at the
photocatalyst/solution interface, as well as accelerates the surface reaction. The enhanced charge transfer
efficiency and improved surface reaction rate finally result in a dramatically improved performance. It is be-
lieved that the present work can provide basic principles for the development of noble-metal-free cocata-
lysts with high catalytic activity and general applicability.
1. Introduction
Since Honda and Fujishima discovered photocatalytic water
splitting on TiO
2
electrodes in 1972,
1
photocatalytic hydrogen
generation over semiconductor photocatalysts using solar en-
ergy has been considered as a promising and challenging
clean-energy technology.
2–8
In general, multi-component
photocatalysts composed of efficient host photocatalysts and
optimal cocatalysts are necessary to achieve a high photocata-
lytic performance. Cocatalysts loaded onto the host photo-
catalysts can effectively capture photogenerated charges to
suppress charge recombination, provide delicately designated
sites for the surface redox reaction, and subsequently en-
hance the photocatalytic activities.
9–11
However, to date, the
most efficient cocatalysts remain dominated by noble metals
(e.g. Pt and Au), which suffer from high costs and low re-
serves, in terms of large-scale application. As a consequence,
the development of efficient cocatalysts based on earth-
abundant elements is quite appealing with the aim of provid-
ing cost-competitive hydrogen.
In recent years, a number of materials based on low-cost
and earth-abundant elements (e.g. Ni, Mo, W and Fe) have
been widely investigated as cocatalysts to promote photocata-
lytic hydrogen production,
12–15
and high efficiency has been
obtained, which is even comparable to noble-metal cocata-
lysts. Inspired by the active [NiFe] hydrogenase,
16–18
Ni-based
cocatalysts have received particular interest for the hydrogen
evolution reaction (HER). For instance, Ni,
19–21
NiO,
22,23
NiIJOH)
2
,
24
and NiS
25–28
were reported as efficient cocatalysts
to significantly enhance photocatalytic activity. However,
most of these Ni-based catalysts are not stable enough in
harsh aqueous solution.
29
Nickel phosphide, which is composed of low-cost and
earth-abundant elements, is a highly active hydro-
desulfurization (HDS) catalyst, and is predicted to be a suit-
able HER catalyst. Density functional theory (DFT) calcula-
tions reveal that the Ni hollow site and Ni–P bridge site
exhibit an ensemble effect to facilitate hydrogen generation.
30
Recently, several groups have demonstrated that the nickel
8212 |Catal. Sci. Technol.,2016,6,8212–8221 This journal is © The Royal Society of Chemistry 2016
International Research Center for Renewable Energy, State Key Laboratory of
Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Shaanxi
710049, PR China. E-mail: ybchen@mail.xjtu.edu.cn
†Electronic supplementary information (ESI) available: Synthesis of different
samples; electrode fabrication; electrochemical and photoelectrochemical mea-
surements; summary of noble-metal-free cocatalysts; BET specific surface areas;
fitted values of all parameters in the equivalent circuit for the Nyquist imped-
ance plots; TEM image of Pt nanoparticles; photocatalytic hydrogen production
over Pt and Ni
2
P modified samples; TEM image and XRD pattern of as-prepared
H
2
Ti
3
O
7
; TEM images and EDX spectrum of Ni
2
P/TiNR; TEM images of Ni
2
P/
CdS and Ni
2
P/C
3
N
4
samples; Raman spectra; FT-IR spectra and photocatalytic
activities of 0.4Ni
2
P/TiNR samples before and after annealing; TEM images of
NiO, Ni(OH)
2
, and NiS samples; and Photocatalytic activities of 0.4Ni
2
P/TiNR un-
der different sacrificial solutions. See DOI: 10.1039/c6cy01653g
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phosphide electrocatalyst shows excellent activity and stabil-
ity for the HER, even in aqueous solution with a wide pH
value of 0–14.
31–33
As is known, efficient electrocatalysts can
be utilized as cocatalysts to dramatically promote photocata-
lytic hydrogen evolution. Cao et al. first employed mono-
dispersed Ni
2
P nanoparticles as cocatalysts to modify CdS
photocatalysts, and the hybrid system exhibited apparently
improved activity and excellent stability in lactic acid aque-
ous solution.
34
Similarly, Du's group reported an integrated
heterostructure by tightly anchoring crystalline Ni
2
P as the
cocatalyst onto CdS nanorods, which displayed extraordi-
narily high photocatalytic activity and durability.
29
It was
found that Ni
2
P as a cocatalyst is in favour of charge separa-
tion at the cocatalyst/photocatalyst interface to enhance
photocatalytic activity. Nevertheless, to date, the relative stud-
ies on photocatalytic hydrogen generation using nickel phos-
phide cocatalysts are all based on CdS host photocatalysts. As
is known, CdS is a good visible-light response photocatalyst
with the band gap of ca. 2.4 eV. However, cadmium is highly
toxic, and the stability of CdS is poor. These disadvantages
restrict its application. On the other hand, to replace the role
of noble-metal cocatalysts, low-cost alternatives should not
only possess high catalytic activity, but also have general ap-
plicability.
35
Therefore, coupling nickel phosphide cocatalysts
with other host photocatalysts (especially stable oxides and
nitrides) to examine their general applicability and detailed
mechanism is of great importance. TiO
2
has been widely
studied for photocatalytic hydrogen production due to its
long-term stability, non-toxicity, low-cost, and easy
availability.
36–38
Recent studies have demonstrated signifi-
cantly improved photocatalytic properties by extending its
light absorption to the visible-light region.
39,40
Meanwhile,
C
3
N
4
with a band gap of ∼2.7 eV can harvest visible light. It
is an earth-abundant and low-cost photocatalyst with suitable
band positions for both hydrogen and oxygen production. It
is also commercially available and can be easily fabricated.
41
As a consequence, if we can significantly improve the photo-
catalytic properties of TiO
2
and C
3
N
4
photoharvesters by load-
ing nickel phosphide cocatalysts, it will pave new ways for
promoting the application of photocatalytic hydrogen
production.
Herein, CdS, TiO
2
, and C
3
N
4
are selected as the represen-
tatives of sulfide, oxide, and nitride host photocatalysts. Ni
2
P
nanoparticles with uniform dispersity were prepared via a
solution-based method, and coupled with these typical host
photocatalysts for hydrogen production. It is demonstrated
that nanocrystalline Ni
2
P could serve as an efficient
cocatalyst to boost photocatalytic hydrogen generation over
all the host photocatalysts, which indicates its good catalytic
property and general applicability. To investigate the underly-
ing mechanism for the improved properties, the Ni
2
P/TiO
2
hybrid photocatalyst was chosen as a model. In terms of the
morphology of photocatalysts, one-dimensional nanorods
provide a large surface area with a high length-to-diameter ra-
tio, as well as fast charge separation efficiency with short ra-
dial distance, which are beneficial for facilitating the photo-
catalytic reaction.
29
For the first time, nanocrystalline Ni
2
Pas
a cocatalyst is integrated with TiO
2
nanorods (TiNR) for
photocatalytic hydrogen production. Detailed characteriza-
tion on the morphology, structure, chemical state, and opti-
cal property of the hybrid photocatalysts was carried out. By
optimizing the Ni
2
P loading concentration and sacrificial re-
agents, the Ni
2
P/TiNR sample showed an 85 times higher ac-
tivity than that of the pure TiNR photocatalyst. The action
mechanism of the N
2
P cocatalyst is thoroughly studied
via photoluminescence spectroscopy and electrochemical
measurements.
2. Experimental
2.1. Synthesis
Ni
2
P nanoparticles were synthesized via a solution-based
method. Typically, 0.98 mmol of nickel acetylacetonate, 2 mL
of tri-n-octylphosphine, 6.4 mL of oleylamine, and 4.5 mL of
1-octadecence were added to a three-necked flask with stir-
ring. The reaction mixture was heated at 120 °C for 1 h under
vacuum to remove water and other low-boiling impurities.
The solution was then placed under Ar, heated to 320 °C,
and kept at this temperature for 2 h. After the mixture cooled
to the room temperature, isopropanol was added and the sus-
pension was centrifuged at 8000 rpm for 10 min. The isolated
powder was re-dispersed using n-hexane/isopropanol (volume
ratio of 1 : 3) and subsequently centrifuged. This re-
dispersion and centrifugation process was repeated three
times to remove the excess ligands. The obtained Ni
2
P nano-
particles were finally dispersed in toluene.
TiO
2
nanorods (denoted as TiNR) were prepared by
annealing hydrogen trititanate (H
2
Ti
3
O
7
) nanotubes at 450 °C
for 5 h, and H
2
Ti
3
O
7
nanotubes could be synthesized via the
hydrothermal method. In a typical procedure, 1 g of Degussa
P25 TiO
2
was dispersed in 80 mL of 10 M NaOH aqueous so-
lution under stirring. The suspension was then transferred to
a 110 mL Teflon-lined stainless steel autoclave and heated at
130 °C for 20 h. The obtained product was suspended in de-
ionized water, washed with ethanol and 0.1 M HCl aqueous
solution three times, and dried at 80 °C for 12 h to obtain
H
2
Ti
3
O
7
nanotubes.
Ni
2
P nanoparticles were anchored onto the surface of
TiNR according to a reported method.
42
Appropriate amounts
of Ni
2
P and TiNR were dispersed in toluene under stirring.
The suspension was stirred for 1 h, separated by centrifuga-
tion, and washed with acetone. The obtained Ni
2
P/TiNR sam-
ple was finally dried at 80 °C for 12 h under vacuum. The
Ni
2
P/P25 sample was also prepared using a similar proce-
dure, and replacing TiNR with P25 TiO
2
. The detailed syn-
thetic processes for the Ni
2
P/CdS, Ni
2
P/C
3
N
4
, Ni(OH)
2
/TiNR,
NiO/TiNR, NiS/TiNR, Pt/TiNR, Pt/CdS, and Pt/C
3
N
4
samples
can be found in the ESI.†
2.2. Characterization
X-ray powder diffraction (XRD) patterns were obtained on a
PANalytical X'pert MPD Pro X-ray diffractometer. The Raman
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scattering study was carried out on a Jobin YvonLab RAM HR
spectrometer. UV-visible (UV-vis) absorption spectra were
measured on a HITACHI U-4100 spectrophotometer. Photo-
luminescence (PL) spectra were carried obtained using a PTI
QM-4 fluorescence spectrophotometer with an excitation
wavelength of 320 nm. Transmission electron microscope
(TEM) images were obtained using a FEI Tecnai G2 F30
S-Twin microscope attached with an OXFORD MAX-80 energy
dispersive X-ray spectrometer (EDX) system. Scanning
electron microscope (SEM) images were recorded on a JEOL
JSM-7800F microscope. The Brunauer–Emmett–Teller (BET)
approach using adsorption data obtained on a Micromeritics
ASAP 2020 nitrogen adsorption apparatus was employed to
determine specific surface area. X-ray photoelectron spectro-
scopy (XPS) measurements were conducted on an Axis Ultra,
Kratos (UK) multifunctional X-ray spectrometer. Binding en-
ergies were calibrated relative to the C 1s peak (284.8 eV)
from the adsorbed adventitious carbon. Fourier transform in-
frared spectroscopy (FT-IR) patterns were measured by time-
resolved spectroscopy using an FT-IR spectrometer, model
VERTEX series S510.
2.3. Photocatalytic measurement
Photocatalytic hydrogen generation was performed in a side
irradiation Pyrex cell with magnetic stirring. Typically, 20 mg
of as-prepared photocatalyst was added to 80 mL of aqueous
solution containing the sacrificial reagents. Before irradia-
tion, in the dark, nitrogen was purged through the reaction
cell for 30 min to remove air. The reaction temperature was
kept at 35 °C. A 300 W Xe-lamp was employed to provide the
light irradiation. The amount of generated hydrogen was de-
termined with a TCD gas chromatograph. The average hydro-
gen production rate was calculated based on hydrogen gener-
ation amount in the first 5 h. The apparent quantum
efficiency (AQE) was calculated according to eqn (1).
(1)
3. Results and discussion
3.1. General applicability of Ni
2
P cocatalyst
The morphology of the as-prepared Ni
2
P nanoparticles was
examined by TEM analysis. As shown in Fig. 1a, the Ni
2
P
nanoparticles show a sphere-like shape with a diameter of ca.
17 nm. The synthesized nanoparticles are uniformly dis-
persed and have a relatively narrow size distribution. Close
examination of the TEM image further reveals that the syn-
thesized nanoparticles are hollow and multifaceted. The hol-
low morphology should be attributed to a nanoscale
Kirkendall pathway during the synthetic process.
43
The high-
resolution TEM (HR-TEM) image in Fig. 1b demonstrates that
the Ni
2
P nanoparticles are single crystalline. The observed
lattice fringes with the interplanar spacing of 0.19 nm
corresponded to the {210} planes of Ni
2
P. The XRD pattern of
nanocrystalline Ni
2
P (Fig. 1c) shows that all the diffraction
peaks matched well with hexagonal Ni
2
P (JCPDS 03-0953). In
addition, the corresponding EDX spectrum (Fig. 1d) verifies
that the Ni/P molar ratio is close to 2 :1, which indicates the
successful formation of an Ni
2
P single phase.
To replace the role of noble-metal cocatalysts in photo-
catalytic hydrogen production, noble-metal-free alternatives
should not only possess high catalytic activity, but also have
general applicability. We synthesized CdS, TiO
2
, and C
3
N
4
as
the representatives of sulfide, oxide, and nitride host photo-
catalysts. Ni
2
P nanoparticles were coupled with each of these
typical host photocatalysts for hydrogen production. The
comparative test demonstrated that Ni
2
P could not generate
hydrogen alone, which indicates its character as a cocatalyst.
As shown in Fig. 1e, nanocrystalline Ni
2
P could serve as an
efficient cocatalyst to boost photocatalytic hydrogen genera-
tion over all the host photocatalysts. The enhancement fac-
tors after loading Ni
2
P cocatalysts onto TiO
2
, CdS, and C
3
N
4
were 85, 34, and 30, respectively. These results indicate the
good catalytic property and general applicability of the Ni
2
P
cocatalyst for hydrogen production.
In order to make a comparison of the catalytic activities
of Ni
2
P and Pt, we prepared Pt nanoparticles, and mixed
them with all three photoharvesters. The two-step synthetic
method is similar to that for the Ni
2
P loaded photo-
catalysts. The TEM image in Fig. S1†reveals that the as-
prepared Pt nanoparticles show a nano-flower morphology
with the size of ca. 25 nm. As shown in Fig. S2,†all the
Ni
2
P modified samples show higher activities compared to
the photocatalysts modified with Pt, which indicate the su-
perior activity of Ni
2
P nanoparticles for hydrogen produc-
tion in the present study. A comparative summary of noble-
metal-free cocatalysts and their catalytic activities for hydro-
gen production is provided in Table S1.†For the various
host photocatalysts (TiO
2
, CdS, or C
3
N
4
), Ni
2
P was proven
to be among the best cocatalysts for hydrogen production.
These results demonstrate its good catalytic activity and
general applicability. Another advantage of Ni
2
P as a good
catalyst is its stability. Nickel phosphide shows excellent
stability for hydrogen production in aqueous media over a
wide pH value of 0–14.
32
3.2. Characterization of Ni
2
P/TiO
2
nanorods
To investigate the action mechanism of the Ni
2
P cocata-
lysts, TiO
2
nanorods (TiNR) were chosen as a representative
host photocatalyst due to the advantages of TiO
2
and its
one-dimension morphology.
29,36
TiNR were prepared by
annealing the H
2
Ti
3
O
7
nanotubes from the hydrothermal
treatment, and the Ni
2
P/TiNR photocatalyst was obtained
by anchoring the as-prepared Ni
2
P nanoparticles onto the
TiO
2
nanorods. The morphologies of H
2
Ti
3
O
7
, TiNR, and
Ni
2
P/TiNR were investigated by TEM. H
2
Ti
3
O
7
exhibits a
nanotubular structure with an almost uniform diameter of
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Fig. 1 (a) TEM image, (b) HR-TEM image, (c) XRD pattern, (d) EDX spectrum of Ni
2
P nanoparticles, and (e) photocatalytic hydrogen production
over different samples. Reaction conditions: 20 mg of photocatalyst, the weight percentage of Ni
2
P was kept at 0.4 wt%, 80 mL of aqueous solu-
tion containing different sacrificial reagents (50 vol% methanol for TiO
2
and Ni
2
P/TiO
2
, 20 vol% lactic acid for CdS and CdS/TiO
2
, 10 vol%
triethanolamine for C
3
N
4
and Ni
2
P/C
3
N
4
), and a 300 W Xe lamp as the light source (with a 420 nm cut-off filter for CdS, Ni
2
P/CdS, C
3
N
4
and
Ni
2
P/C
3
N
4
).
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about 10 nm, and the length of the nanotubes is several
hundred nanometers (Fig. S3†). As displayed in Fig. 2a,
the synthesized TiNR has a nanorod morphology with a
diameter of ca. 8 nm and length of ca. 100 nm, which
indicate that the annealing process led to the partial de-
struction of the nanotubular H
2
Ti
3
O
7
.
44
For the hybrid
Ni
2
P/TiNR photocatalysts, Ni
2
P nanoparticles with a
sphere-like shape (marked by yellow circles) are dispersed
on the surface of the TiO
2
nanorods (Fig. 2b). To evaluate
the elemental distribution, STEM (Fig. 2c) and elemental
mapping of the Ti, O, P, and Ni species (Fig. 2d–g) were
performed for Ni
2
P/TiNR, which reveal that the Ni
2
P nano-
particles were successfully distributed on the TiO
2
nano-
rods. To study the dispersity of the Ni
2
P nanoparticles, we
examined the TEM images of Ni
2
P/TiNR with different
magnifications. As shown in Fig. S4,†the TiO
2
nanorods
tended to aggregate together. However, there was no ap-
parent agglomeration of Ni
2
P nanoparticles. EDX measure-
ment revealed an Ni
2
P loading concentration of 0.45 wt%,
which is a little higher than the initial value of 0.4 wt%
in the synthetic process. Additionally, the morphologies of
the Ni
2
P/CdS and Ni
2
P/C
3
N
4
samples were also examined.
As shown in Fig. S5a and b,†a large agglomeration of
Ni
2
P nanoparticles was found on the surface of CdS poly-
hedrons in Ni
2
P/CdS. For Ni
2
P/C
3
N
4
, some Ni
2
P nano-
particles were well dispersed on the surface of C
3
N
4
sheets
(Fig. S5c†), while some nanoparticles aggregated together
(Fig. S5d†).
The obtained Ni
2
P/TiNR photocatalysts are denoted as
xNi
2
P/TiNR, where xrepresents the weight percentage
(wt%) of Ni
2
PinNi
2
P/TiNR. The crystal structures of the
as-prepared samples were investigated by an XRD study.
The diffraction peaks of the H
2
Ti
3
O
7
nanotubes could be
well assigned to a monoclinic structure (JCPDS 47-0561)
(Fig. S6†). As shown in Fig. 3, an anatase phase (JCPDS
21-1272) was formed for the TiO
2
nanorods with the typi-
cal diffraction peak at 2θ= 25.4°corresponding to the
(101) plane. For all xNi
2
P/TiNR photocatalysts, there were
only diffraction peaks ascribed to anatase TiO
2
, which is
possibly due to the low amount and fine dispersion of
Ni
2
P nanoparticles. In addition, Raman spectra of the
H
2
Ti
3
O
7
, TiNR, and Ni
2
P/TiNR samples were measured to
further examine their microstructures. As seen in Fig. S7,†
the Raman scatting peaks of the TiNR and Ni
2
P/TiNR
samples at 144 cm
−1
corresponds to the E
g
band in ana-
tase TiO
2
.
45
The Raman peaks at 156, 265, 446, and 671
cm
−1
of the H
2
Ti
3
O
7
sample are associated with the mono-
clinic phase.
46
These results are in good agreement with
the XRD data.
XPS measurements were conducted to investigate the sur-
face chemical states of TiNR and Ni
2
P/TiNR. As shown in
Fig. 4a, the binding energies of Ti 2p
3/2
and Ti 2p
1/2
for TiNR
and Ni
2
P/TiNR were located at 458.4 and 464.3 eV, respec-
tively, which correspond to the typical values for Ti
4+
in TiO
2
.
Meanwhile, the peak at 529.6 eV of O 1s for TiNR and Ni
2
P/
TiNR could be assigned to the binding energies of Ti–Oin
TiO
2
(Fig. 4b). As indicated in Fig. 4c, the Ni 2p
3/2
XPS spec-
trum of Ni
2
P/TiNR involves two contributions. The peak at
852.3 eV is assigned to Ni
δ+
in the Ni
2
P phase and the other
peak at 855.6 eV possibly corresponds to Ni
2+
ions interacting
with phosphate ions as a result of surface oxidation.
32,47
The
broad peak at 861.0 eV should be the Ni 2p satellite peak. In
the P 2p XPS spectrum (Fig. 4d), the peak at 129.3 eV is
assigned to P
δ−
on the metal phosphides and the peak at
133.5 eV corresponds to nickel phosphate species from the
surface oxidation.
32,47
Fig. 5 shows the UV-vis absorption spectra of TiNR and
Ni
2
P/TiNR. Pure TiNR displays a sharp absorption edge at
around 400 nm, which corresponds to a band gap of 3.1
eV. It was noticed that the absorption edges of all the
Ni
2
P/TiNR samples were close to that of single TiNR. Com-
pared to TiO
2
nanorods, the Ni
2
P/TiNR samples exhibit
strong absorption in the region of 400–800 nm, and the ab-
sorption intensity gradually increased with an increase in
the amount of Ni
2
P. This enhanced absorption could be
Fig. 2 TEM images of (a) TiNR and (b) Ni
2
P/TiNR. (c) STEM image of
Ni
2
P/TiNR. (d–g) Elemental mapping of Ti, O, P, and Ni species,
respectively, in Ni
2
P/TiNR.
Fig. 3 XRD patterns of TiNR and xNi
2
P/TiNR, where xrepresents the
weight percentage (wt%) of Ni
2
PinNi
2
P/TiNR.
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ascribed to the contribution of Ni
2
P, since its band gap is
reported to be 1.0 eV.
48
3.3. Photocatalytic hydrogen generation over Ni
2
P/TiO
2
nanorods
The photocatalytic activities of the as-prepared TiNR and
xNi
2
P/TiNR samples with different weight percentages of
Ni
2
P were investigated in methanol aqueous solution. As
shown in Fig. 6a, the hydrogen production rate of pure
TiNR was rather low. Ni
2
P coupling boosted the photocata-
lytic hydrogen evolution over TiO
2
nanorods. With a gradu-
ally increased amount of Ni
2
P cocatalyst, the photocatalytic
activity of Ni
2
P/TiNR initially increased and then underwent
a decrease due to the negative shielding effect of the excess
Ni
2
P. In particular, the 0.4Ni
2
P/TiNR sample with a Ni
2
P
concentration of 0.4 wt% showed the highest hydrogen pro-
duction rate of 9.38 mmol h
−1
g
−1
, which is ∼85 times
higher than that of the pure TiNR photocatalyst (0.11 mmol
h
−1
g
−1
). The apparent quantum efficiency of the 0.4Ni
2
P/
TiNR sample was calculated to be 11.6% at 360 nm. It
should be noted that the as-prepared Ni
2
P nanoparticles
were capped by hydrophobic ligands, which might have an
influence on the photocatalytic hydrogen production. There-
fore, the obtained Ni
2
P/TiNR photocatalysts were annealed
at 450 °C for 2.5 h under an Ar atmosphere to remove the
ligands. As shown in the Fourier transform infrared
spectroscopy (FT-IR) patterns (Fig. S8a†), the peaks at 2924
and 2853 cm
−1
corresponding to the C–H stretching modes
disappeared after annealing, and no peaks attributed to the
organic compounds could be observed.
31
Subsequently, we
compared the photocatalytic hydrogen production over the
TiNR and Ni
2
P/TiNR photocatalysts before and after
annealing (Fig. S8b†). It was found that the photocatalytic
activities before and after the annealing treatment were
quite close, which indicates that the presence of organic li-
gands in the Ni
2
P cocatalysts did not have an apparent
Fig. 4 (a) Ti 2p and (b) O 1s XPS spectra of TiNR and Ni
2
P/TiNR. (c) Ni 2p
3/2
and (d) P 2p XPS spectra of Ni
2
P/TiNR.
Fig. 5 UV-vis absorption spectra of TiNR and xNi
2
P/TiNR.
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influence on the hydrogen production. Since the Ni
2
P/TiNR
photocatalysts were washed three times before the photo-
catalytic test, the amount of residual ligands on the surface
of Ni
2
P nanoparticles could be quite low (as determined
from the weak peaks in the FT-IR spectra), which probably
accounts for the almost unchanged activities. As shown in
Fig. 6b, loading the Ni
2
P cocatalyst could also significantly
enhance the photocatalytic performance of P25 TiO
2
. Never-
theless, 0.4Ni
2
P/P25 showed a much lower photocatalytic
activity compared to 0.4Ni
2
P/TiNR, with the hydrogen pro-
duction rate of only 2.97 mmol h
−1
g
−1
. It should be noted
that the specific surface areas play a key role in the photo-
catalytic activities. As shown in Table S2,†0.4Ni
2
P/TiNR
exhibited a much larger specific surface area (121.1 m
2
g
−1
)
than the 0.4Ni
2
P/P25 sample (49.5 m
2
g
−1
) due to the high
length-to-diameter ratio of the TiO
2
nanorods. This high
surface area provides sufficient active sites for the interface
reaction, which lead to improved photocatalytic activity.
49
As is reported, nickel-based semiconductors, such as
NiO, NiIJOH)
2
, and NiS, are also efficient cocatalysts to en-
hance photocatalytic activity.
22–28
In order to make a com-
parison, we synthesized NiO/TiNR, Ni(OH)
2
/TiNR, and NiS/
TiNR photocatalysts (ESI†) and examined their activities un-
der the same reaction conditions as that for Ni
2
P/TiNR.
TEM images of the NiO, Ni(OH)
2
, and NiS nanoparticles are
shown in Fig. S9.†It is observed that the sizes of the NiO,
Ni(OH)
2
, and NiS particles are around 100, 100, and 200
nm, respectively. The particle size and other characters of
cocatalysts affect their catalytic property. However, it is
quite difficult to synthesize different cocatalysts with the
same size, shape, and dispersity. In our present study, the
Ni
2
P/TiNR photocatalysts showed the highest hydrogen pro-
duction rate (Fig. 6c), which indicates the superior catalytic
activity of Ni
2
P for hydrogen evolution compared to other
nickel-based semiconductors. Sacrificial electron donors are
always added to consume photogenerated holes to improve
charge separation. As shown in Fig. S10,†different sacrifi-
cial electron donors were investigated, and methanol was
demonstrated to be the optimal donor for the 0.4Ni
2
P/TiNR
photocatalyst. Meanwhile, the photocatalytic activities of
0.4Ni
2
P/TiNR in the aqueous solution with different concen-
trations of methanol were also studied. The best activity
was achieved in an aqueous solution with 50 vol% metha-
nol (Fig. S11†). In addition to photocatalytic activity, the
durability of photocatalysts is also essential to their practi-
cal application. A long-time photocatalytic test was carried
out to investigate the stability of 0.4Ni
2
P/TiNR. As shown in
Fig. 6d, over the 20 h reaction, 0.4Ni
2
P/TiNR did not show
an apparent decrease in photocatalytic activity, which indi-
cates its good stability.
Fig. 6 (a) Photocatalytic hydrogen production over the as-prepared TiNR and xNi
2
P/TiNR samples. (b) Photocatalytic hydrogen production rates
of P25, 0.4Ni
2
P/P25, TiNR, and 0.4Ni
2
P/TiNR samples. (c) Photocatalytic hydrogen production rates of TiNR photocatalysts modified with NiO,
NiIJOH)
2
, NiS, and Ni
2
P cocatalyst. (d) Long-time photocatalytic test of 0.4Ni
2
P/TiNR. Reaction conditions: 20 mg of photocatalyst, and 80 mL of
aqueous solution containing 50 vol% methanol, 300 W Xe lamp.
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3.4. Mechanism study
To achieve high activity, a photocatalytic system should have
good light-absorption ability, efficient charge transfer, and
rapid surface reaction. Since the absorbed photons by Ni
2
P
nanoparticles do not contribute to the photocatalytic reac-
tion, it is inferred that Ni
2
P modification should have a great
influence on the charge transfer and surface reaction of TiO
2
nanorods. PL spectra of the different samples were measured
to examine the charge separation and migration behaviors in
the semiconductor photocatalyst. As shown in Fig. 7a, TiNR
and xNi
2
P/TiNR all exhibited broad emission peaks centered
at around 430 nm, which correspond to the absorption onset
of TiO
2
(ca. 400 nm).
50
It was noticed that the Ni
2
P/TiNR
samples showed apparently decreased PL intensity compared
to pure TiNR, and the PL intensity gradually decreased with
an increase in the Ni
2
P content in Ni
2
P/TiNR. This result in-
dicates that Ni
2
P loading could facilitate the charge transfer
so as to inhibit the charge recombination in TiNR.
In addition to the charge-transfer behavior in the photo-
catalyst, the charge-transfer property at the photocatalyst/
solution interface is also crucial to the photocatalytic perfor-
mance. Therefore, TiNR and 0.4Ni
2
P/TiNR electrodes were
fabricated via the electrophoretic deposition (EPD) route
(ESI†), and electrochemical impedance spectroscopy (EIS)
was performed to elucidate their charge-transfer resistances.
As displayed in Fig. 7b, the Nyquist impedance plots for the
electrodes could be fitted to an equivalent circuit (inset in
Fig. 7b) consisting of the series resistance (R
s
), recombina-
tion resistance at the electrode interface (R
rec
), charge-
transfer resistance from the electrode to the electrolyte (R
ct
),
and constant phase elements (CPE1 and CPE2). As summa-
rized in Table S3,†the values of R
ct
and R
rec
for 0.4Ni
2
P/TiNR
were dramatically lower than that for TiNR, which uncovered
Fig. 7 (a) PL spectra of TiNR and xNi
2
P/TiNR samples. (b) Nyquist impedance plots of TiNR and 0.4Ni
2
P/TiNR measured at −1.0 V vs. RHE (inset
shows the equivalent circuit). (c) Linear sweep voltammogram curves for TiNR and 0.4Ni
2
P/TiNR. (d) Transient photocurrent responses of TiNR and
0.4Ni
2
P/TiNR measured at 1.0 V vs. RHE. The electrochemical tests were carried out in N
2
-saturated 0.5 M Na
2
SO
4
solution. A 500 W xenon lamp
coupled with an AM 1.5 filter was used as the light source for photocurrent measurement.
Fig. 8 Schematic illustration of photocatalytic hydrogen production
over Ni
2
P/TiNR.
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that the Ni
2
P cocatalyst could efficiently promote the charge
transfer at the photocatalyst/solution interface.
Linear sweep voltammetry (LSV) was subsequently
performed to examine the catalytic reductive abilities of TiNR
and 0.4Ni
2
P/TiNR for the HER (Fig. 7c). Compared to TiNR,
which has an onset potential of ca. −0.8 V vs. RHE, 0.4Ni
2
P/
TiNR exhibited a more positive onset potential of around
−0.5 V vs. RHE. This positive shift indicates that the catalytic
reductive ability of TiNR is greatly enhanced by coupling
Ni
2
P.
51–53
In addition, 0.4Ni
2
P/TiNR exhibits a significant im-
provement in current density beyond −0.6 V vs. RHE and a
lower overpotential (−0.98 V vs. RHE) to drive 10 mA cm
−2
than pure TiNR (−1.31 V vs. RHE). These results further prove
that Ni
2
P could accelerate the surface reaction for hydrogen
production.
The transient photocurrent responses of TiNR and
0.4Ni
2
P/TiNR were also investigated to examine their photo-
electrochemical properties. As shown in Fig. 7d, 0.4Ni
2
P/TiNR
exhibits a much higher photocurrent density compared to
TiNR, which is in good agreement with the photocatalytic
performance. On the basis of the above discussion, it can be
summarized that the Ni
2
P cocatalyst could promote the
charge transfer both in the photocatalyst and at the photo-
catalyst/solution interface, as well as accelerate the surface re-
action for hydrogen evolution. The enhanced charge transfer
efficiency and improved surface reaction rate finally lead to a
significantly improved photocatalytic activity.
Consequently, the proposed photocatalytic process over
Ni
2
P/TiNR photocatalysts is illustrated in Fig. 8. Under light
illumination, electrons are excited from the valence band to
the conduction band of the TiO
2
nanorods. The photo-
generated electrons can be efficiently transferred to the Ni
2
P
cocatalyst, since the conduction band edge of Ni
2
P(0Vvs.
RHE) is much lower than that of anatase TiO
2
(−0.26 V vs.
RHE).
48
It was reported that the injection of electrons into
phosphides could upshift the Fermi level, thereby providing
a large driving force for hydrogen evolution,
54
which could
ensure efficient charge transfer at the photocatalyst/solution
interface. Finally, the accumated electrons in the Ni
2
P
cocatalyst reduce H
+
ions to generate molecular hydrogen,
and the good catalytic ability demonstrated by the electro-
chemical measurement could promote the surface reaction.
4. Conclusions
In summary, nanocrystalline Ni
2
P particles with uniform
dispersity have been synthesized, which serve as a cocatalyst
to modify CdS, TiO
2
, and C
3
N
4
host photocatalysts for hydro-
gen production. It was observed that the Ni
2
P cocatalyst
could boost hydrogen generation over all the host photo-
catalysts, which indicates its good catalytic property and gen-
eral applicability. To investigate the underlying mechanism,
nanocrystalline Ni
2
P as a cocatalyst was successfully inte-
grated with TiO
2
nanorods (TiNR) for the first time. By opti-
mizing the Ni
2
P loading concentration and sacrificial re-
agents, the Ni
2
P/TiNR sample showed the highest H
2
production rate of 9.38 mmol h
−1
g
−1
, which is ∼85 times
higher than that of the pure TiNR photocatalyst. The appar-
ent quantum efficiency was calculated to be 11.6% at 360
nm. Among the different nickel-based semiconductor cocata-
lysts, Ni
2
P was demonstrated to be the best cocatalysts.
Photoluminescence spectroscopy and electrochemical mea-
surements reveal that the Ni
2
P cocatalyst could promote the
charge transfer both in the photocatalyst and at the photo-
catalyst/solution interface, as well as accelerate the surface re-
action for hydrogen evolution. The enhanced charge transfer
efficiency and improved surface reaction rate lead to a signifi-
cantly improved performance. It is expected that the present
work can provide basic principles for the development of
noble-metal-free cocatalysts with high catalytic activity and
general applicability.
Acknowledgements
The authors thank the financial support from the National
Natural Science Foundation of China (No. 21606175), the
grant support from the China Postdoctoral Science Founda-
tion (No. 2014M560768), and the China Fundamental Re-
search Funds for the Central Universities (xjj2015041).
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