Visible-light-driven H2 production and decomposition of 4-nitrophenol over nickel phosphides

Article abstract of DOI:10.1039/c8ra06770h

Photocatalytic H₂ production and photocatalytic decomposition are efficient and economical methods to obtain hydrogen fuel and dispose of organic pollutants. In this paper, amorphous nickel phosphide (Ni₂P) is synthesized by an extremely simple precipitation method under low temperature. The prepared nickel phosphide was not only used to produce H₂ in the presence of Eosin Y as sensitizer and triethanolamine (TEOA) as a sacrificial electron donor but also to degrade 4-nitrophenol under visible light illumination. The rate of hydrogen evolution is 34.0 μmol h^?1 g^?1 and the degradation efficiency is 25.5% within 4 h at initial 5.0 mg L^?1 4-nitrophenol. Probable photocatalytic mechanisms were discussed. The present work is expected to contribute toward the hydrogen evolution and disposal of highly toxic pollutants by cost-effective photocatalytic means.

Full text of DOI:10.1039/c8ra06770h

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PAPER
Visible-light-driven H2 production and
decomposition of 4-nitrophenol over nickel
phosphides
Cite this: RSC Adv., 2018, 8, 34259
ab
c
a
a
a
Lanhua Zhao, Haixia Wang, Hua Lai, Gang Peng, Junhua Li,a
* *
Xing Liu,
Zhengji Yia and Kang Chena
Photocatalytic H2 production and photocatalytic decomposition are efficient and economical methods to
obtain hydrogen fuel and dispose of organic pollutants. In this paper, amorphous nickel phosphide (Ni2P) is
synthesized by an extremely simple precipitation method under low temperature. The prepared nickel
phosphide was not only used to produce H2 in the presence of Eosin Y as sensitizer and triethanolamine
(TEOA) as a sacrificial electron donor but also to degrade 4-nitrophenol under visible light illumination.
The rate of hydrogen evolution is 34.0 mmol hꢀ1 gꢀ1 and the degradation efficiency is 25.5% within 4 h at
initial 5.0 mg Lꢀ1 4-nitrophenol. Probable photocatalytic mechanisms were discussed. The present work
is expected to contribute toward the hydrogen evolution and disposal of highly toxic pollutants by cost-
effective photocatalytic means.
Received 12th August 2018
Accepted 27th September 2018
DOI: 10.1039/c8ra06770h
rsc.li/rsc-advances
Transition metal phosphides have attracted considerable
attention in the elds of electrochemical catalysis due to many
1. Introduction
advantages such as earth-abundance, high catalytic activity,
good stability and facile synthesis.17–20 Nickel phosphide is one
of the most promising candidates among transition metal
phosphides because it is the most active mono-metallic phos-
phide.21–23 Inspired by the high activity of phosphides in elec-
trochemical H2 evolution, they were subsequently used as
cocatalysts in photocatalytic H2 evolution.24–31 For example, Cao
et al. employed Ni2P as cocatalysts to modify CdS, and the
composite photocatalysts exhibited remarkably improved
activity and excellent stability in lactic acid aqueous solution.32
However, it is a new attempt for using nickel phosphide to
photocatalytic degradation organic contaminants 4-nitrophenol
and photosensitized hydrogen evolution from water splitting.
In this work, nickel phosphide (Ni2P) material was prepared
by a simply precipitation method. The prepared nickel phos-
phide was used to degrade 4-nitrophenol under visible light
illumination and produce H2 in the presence of Eosin Y (EY) as
sensitizer and triethanolamine (TEOA) as a sacricial electron
donor.
Energy and the environment are two major topics in today's
society. Molecular hydrogen (H2) is an ideal candidate as
a carbon free fuel for the replacement of fossil fuels in the
future. The photocatalytic approach is considered as one of the
most promising ways to produce H2 from water splitting.1–3
4-Nitrophenol, used as a raw material or intermediate for the
manufacture of agrochemicals, dyes, drugs etc., is one of the
most difficult organic contaminants to dispose of, with high
toxicity, stability and bioaccumulation.4–6 Photocatalytic degra-
dation of organic compounds, including 4-nitrophenol, is one
of the most popular approaches of pollution control and
disposal.7–12
For photocatalytic hydrogen evolution and degradation of
organic compounds, many semiconductor photocatalysts, like
TiO2, can be only activated by UV light due to their high-energy
band gaps. The UV light accounts for only ca. 4% to 6% of the
solar spectrum, while visible light is about 43%. Thus, the
development of visible-light-driven photocatalysts is
needed.13–16
2. Experimental
Synthesis of nickel phosphide
aKey Laboratory of Functional Metal-Organic Compounds of Hunan Province & Key
Laboratory of Functional Organometallic Materials of College of Hunan Province,
Hengyang Normal University, Hengyang, 421008, China. E-mail: liuxing1127@sina.
com
The nickel phosphide was synthesized by a simple precipitation
method using nickel acetate (Ni(Ac)2$4H2O) and sodium hypo-
phosphite (NaH2PO2) as the starting materials, and all of the
used reagents were of analytical grade and were used without
further purication. Typically, 5.0 mmol of Ni(Ac)2$4H2O,
100.0 mmol of NaAc$3H2O and a given NaH2PO2 (the mole ratio
bCollege of Chemistry and Chemical Engineering, Hunan University, Changsha,
410082, People's Republic of China
cInstitue of Pathogenic Biolog, Hunan Provincical Key Laboratory for Special Pathogens
Prevention and Control, University of South China, Hengyang, 421001, People's
Republic of China. E-mail: zhaolanhua@126.com
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of P/Ni ¼ 20/1, 15/1, 10/1 and 5/1) were dissolved into 50.0 mL of
distilled water by stirring for 30 min at room temperature, and
then the pH of aqueous solution was adjusted to 8.0 with 1.0 M
KOH, the reaction system was transferred into a 150 mL three-
necked ask and heated at 90 ꢁ 95 ꢂC for 12 h. The black
products were ltered under vacuum, washed carefully with
water and anhydrous ethanol for three times in sequence, and
nally dried in a vacuum oven at 60 ꢂC for 5 h. The nickel
phosphide catalyst for H2 production and 4-nitrophenol
degradation was obtained aer grinding in an agate mortar.
Characterization and instruments
X-ray diffraction (XRD) analysis was carried out using
a BRUCKER D8 polycrystalline X-ray diffractometer with Cu Ka
radiation as the X-ray source (l ¼ 0.15406 nm) at a scan rate of
6ꢂ minꢀ1. Scanning electron microscopy (SEM) image was taken
on a EVO-10 (Carl Zeiss AG, Germany) working at 30 kV, the
sample was mounted onto carbon adhesive pad attached to
aluminum stub. X-ray photoelectron spectroscopy (XPS) anal-
yses were performed using an ESCALAB250xi photoelectron
spectrometer (Thermon Scientic) with a monochromatized Mg
Ka X-Ray resource (150 W). The C 1s peak (284.8 eV), was used
as an internal reference for absolute binding energy.
Photocatalytic test
Photocatalytic reaction was conducted at room temperature,
and a high pressure Hg lamp (150 W) was used as the light
source, which was equipped with a cutoff lter (l > 420 nm) to
3. Results and discussion
remove the radiation below 420 nm. The IR fraction of the beam XPS spectra of Ni2P were shown in Fig. 1, from the element
was removed by a cool water lter to ensure illumination of measurement from XPS, the atom ratio of Ni/P is 2, this
visible light only.
conrms that the prepared nickel phosphide is Ni2P. For Ni 2p
In a typical H2 production experiment,33 0.10 g of nickel region, three peaks at 852.6, 855.5 and 861.2 eV, which is
phosphide catalyst was added into 80.0 mL of aqueous solution ascribed to Nid+ (0 < d < 2) in Ni2P, oxidized Ni species (Ni2+) and
containing TEOA (9.5 ꢃ 10ꢀ2 mol Lꢀ1) and EY (pH was adjusted the satellite of the Ni 2p1/2 peak, respectively.29 And the peaks at
as 7.0 with hydrochloric acid). Before irradiation, the catalyst 873.1 and 879.7 eV is corresponding to Nid+ in Ni2P, oxidized Ni
was dispersed in an ultrasonic bath for 5 min and N2 was species and the satellite of the Ni 2p3/2 peak, respectively. For
bubbled through the reaction mixture for 30 min to remove the P 2p, the peak at 129.9 eV is a mark of metal–P bonds in
oxygen. The top of the cell was sealed with a silicone rubber metal phosphides (i.e. Ni2P),29 while the peak at 133.1 eV can be
septum. Sampling was operated intermittently through the attributed to the oxidized P species due to air contact.
septum during experiments. The amount of hydrogen evolved
was determined on a gas chromatograph (TCD, 13X molecular showed no obvious diffraction peaks, which indicates that the
sieve column, N2 gas carrier).
Ni2P has an amorphous structure.30 The morphology of Ni2P is
As shown in Fig. 2, the XRD diffraction pattern of Ni2P
In a 4-nitrophenol degradation experiment, 0.10 g of nickel revealed using SEM. As shown in Fig. 3A and B, the Ni2P sample
phosphide photocatalysts was suspended into 100 mL of 4- is composed of nanospheres with various sizes, and nano-
nitrophenol aqueous solution and stirred under dark for 30 min spheres are stacked together. The statistical particle size
to reach adsorption equilibrium. The mixed solution was irra- distributions of Ni2P were shown in Fig. 3C, the average diam-
diated under continuous stirring. Samples were taken out every eter of Ni2P nanospheres is 800 ꢄ 30 nm.
60 min, and centrifuged at speed of 10 000 rpm for 10 min in
A noble-metal-free hydrogen production system was con-
order to remove the photocatalysts, the obtained clear solution structed with the as-prepared Ni2P as a catalyst, EY as a photo-
was analyzed using spectrophotometric method on a 7SS2 sensitizer, and TEOA as a sacricial electron donor. It is
spectrophotometer (made in Shanghai, China), and the absor- observed from Fig. 4 that Ni2P (without photosensitizer)
bance was measured at the wavelength of 315 nm.34
displays very low photocatalytic H2 evolution activity, while no
Fig. 1 X-ray photoelectron spectroscopy (XPS) spectra of Ni2P.
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photons, and electrons in the HOMO (highest occupied
molecular orbital) can be excited to the LUMO (lowest unoc-
cupied molecular orbital), and resulting in excited EY*. EY* is
prevailingly quenched reductively by electron donor TEOA to
form radical EYcꢀ anion, the surplus electrons of EYcꢀ can be
trapped by Ni2P, and then the electrons react with H+ adsorbed
on Ni2P to generate H2 gas. A possible reaction mechanism of
the hydrogen evolution is shown in Fig. 5. However, the
produced EYcꢀ anion could also undergo self-degradation by
debromination, the self-degradation decreases the sensitization
activity.33,35 In Fig. 4, it can be found that the H2 evolution
activity decreased in the last two hours, this phenomenon
suggests the above discussion.
Based on the above discussions, the EY plays a critical role in
number of photo-induced electrons, which results in the pho-
tocatalytic hydrogen evolution. Fig. 6A shows the impact of EY
concentration on hydrogen evolution. The photocatalytic
activities increases rst and then declines with increase of EY
Fig. 2 XRD pattern of Ni2P.
concentration, and reached a maximum at 3.1 ꢃ 10ꢀ4 mol Lꢀ1
.
H2 can be observed in the system containing only EY (without
nickel phosphides). And the EY–Ni2P (EY sensitized Ni2P)
system has a remarkable activity under the same experimental
If no dye is added, only a little evolved hydrogen is observed,
this demonstrates that hydrogen generation is indeed driven by
dye sensitization. When concentration of EY increases from 0 to
3.1 ꢃ 10ꢀ4 mol Lꢀ1, the antenna effect of dye to absorb light is
boosted, more and more radical EYcꢀ anion generates, thus the
hydrogen evolution is improved. Nevertheless, when the
concentration of EY is extremely high, numerous free dye
molecules in solution absorb the input light but would not
contribute to hydrogen evolution reaction. Moreover, greater
collisional deactivation of the 3EY* excited state happens due to
condition, the rate of hydrogen generation is 34.0 mmol hꢀ1 gꢀ1
.
The result indicates that EY can markedly enhance the
hydrogen generation activity.
From the UV-Vis absorption spectra of EY (the inset of Fig. 4),
EY can strongly absorb visible light during the range of 450 ꢁ
560 nm with the absorption centre at 515 nm. When the EY
photosensitizer is irradiated by visible light, EY absorbs
Fig. 3 SEM image (A and B) and particle size distribution (C) of Ni2P.
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Fig. 4 Time courses of photocatalytic H2 production of EY–Ni2P and
Ni2P. Conditions: 0.10 g Ni2P, 9.5 ꢃ 10ꢀ2 mol Lꢀ1 TEOA, light source:
150 W high pressure Hg lamp, l > 420 nm. The inset is UV-Vis
absorption spectra of EY.
serried dye molecules,35,36 so the activity of hydrogen evolution
would not increase. Fig. 6B reveals the effect of mole ratio of
initial P/Ni on hydrogen evolution over EY sensitized Ni2P, the
highest photo-activity occurs at P/Ni ¼ 10. All Ni2P samples
prepared with different ratio of initial P/Ni has exactly similar
XRD diffraction patterns, indicating that they are amorphous.
When the ratios of initial P/Ni are 5/1, 10/1, 15/1, 20/1, the BET
specic surface areas of the prepared nickel phosphides
samples are 18.9, 27.7, 23.1, 20.5 m2 gꢀ1, respectively. This
suggests that the specic surface areas of nickel phosphide with
ratio of initial P/Ni ¼ 10 is largest. Since the hydrogen evolution
reaction is surface-dependent, a large surface area should
provide more surface active sites for the adsorption of reactants,
making the photocatalytic H2 production more efficient.
Fig. 6 Effect of (A) initial EY concentration and (B) mole ratio of initial
P/Ni on hydrogen evolution of EY sensitized Ni2P.
The photocatalytic performance of Ni2P was also evaluated
by degradation of 4-nitrophenol under visible light illumina-
tion. As shown in Fig. 7A, self-degradation of 4-nitrophenol is
almost negligible when the initial concentrations of 4-nitro-
phenol is 5 mg Lꢀ1. In the presence of the Ni2P, the remarkable
degradation was found, and the degradation efficiency is 25.5%
within 4 h. In 4-nitrophenol degradation, the activity decrease
in second cycle degradation is not obvious (see the inset of
Fig. 7A), this indicates that nickel phosphides is stable. A
control experiment reveals that the degradation of 4-nitro-
phenol over Ni2P under dark condition is also negligible, this
result conrms that the degradation is indeed a photochemical
reaction. The degradation efficiency of 4-nitrophenol over Ni2P
is not high, which can be explained as following: rstly, charge
recombination happens easily in Ni2P when it is excited by
visible light, thus the generated oxidative species (see the
following discussion) is limited. Secondly, the light source in
our experiments was a 150 W high pressure Hg lamp (equipped
with a cutoff lter (l > 420 nm)), compared with xenon lamp, its
light intensity was very low, so the photocatalytic activity is low.
In addition, the BET specic surface areas of prepared nickel
phosphides is relatively small (<28 m2 gꢀ1), so photocatalytic
active sites are limited.
According to previous study, the kinetic behavior of the
photocatalytic reaction obeys pseudo-rst-order kinetics. To
conrm this speculation, the natural logarithm of 4-nitro-
phenol concentration ln(C/C0) is plotted as a function of irra-
diation time (C and C0 are the concentrations of 4-nitrophenol
at time t and zero, respectively). The kinetics for the photo-
degradation of 4-nitrophenol was shown in Fig. 7B, the
Fig.
5 Schematic illustration of probable mechanism of photo-
sensitized hydrogen production. Noted that EY*, EYcꢀ, TEOA represent
the triplet excited state of Eosin Y, the radical anion of Eosin Y and
triethanolamine, respectively.
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Fig. 8 Photocatalytic degradation of 4-nitrophenol over Ni2P at
different initial concentration. Experimental conditions as in Fig. 7.
ꢀ
degradation of 4-nitrophenol.34 It is found that $OH and $O2
radicals are the main reactive species in the photocatalytic
degradation. The intermediate products of 4-nitrophenol
photodegradation include 4-nitrocatechol, 1,2,4-benzenetriol,
hydroquinone, which have been demonstrated by previous
works containing ours.34,37,38 The ultimate products are carbon
dioxide, water, nitrate aer complete mineralization.
Fig. 8 illustrates the performance of Ni2P when the initial
concentrations of 4-nitrophenol vary from 2.5 to 15.0 mg Lꢀ1
.
The dependence of the photodegradation yield of 4-nitrophenol
on the initial concentration may be based on the fact that the
degradation reaction mainly occurs on Ni2P surface. On the
surface of catalyst, the reaction happens between $OH radicals
and 4-nitrophenol molecules. When the initial 4-nitrophenol
concentration is low, the transfer rate plays an important role,
the adsorption of 4-nitrophenol on the catalyst surface
increases as the initial concentration increases, which in favour
Fig. 7 (A) Degradation curves of p-nitrophenol over (a) Ni2P, (b) self-
degradation without photocatalysts. The inset is second cycle for Ni2P.
(B) Plots of pseudo-first-order kinetics for photocatalytic degradation of the subsequent photocatalytic degradation. However, with
over Ni2P. Experimental conditions: Ni2P was prepared at mole ratio of
further increase of 4-nitrophenol concentration, the rate of
P/Ni ¼ 10/1, initial concentration of 4-nitrophenol is 5.0 mg Lꢀ1
.
photocatalytic degradation decreases due to the reduced ratio of
$OH (or $O2)/4-nitrophenol. Therefore, the highest photo-
catalytic degradation efficiency is obtained when the initial 4-
nitrophenol concentration is 5.0 mg Lꢀ1. This nding demon-
strates that the degradation kinetics of 4-nitrophenol is indeed
not of simple rst order but pseudo rst order within the
experimental range, which is consistent with many previous
reports.37–40
regression curve is roughly linear, indicating that the kinetics of
4-nitrophenol degradation over the photocatalyst is indeed
pseudo-rst-order, and the reaction rate constant k was calcu-
lated to be 0.16 hꢀ1
.
As is known, the photocatalyst Ni2P can be directly excited
to generate electron (eꢀ)–hole (h+) pairs aer visible light
illumination. The photoinduced holes react with surface-
bound H2O and produce the hydroxyl radical $OH species
that are extremely strong oxidant for the mineralization of
organic pollutions, including 4-nitrophenol. Meanwhile, the
photoinduced electrons can react with the adsorbed molecular
oxygen to yield superoxide radical $O2ꢀ, which combine with
H+ to produce HO2c. The HO2c can further react with the
trapped electrons to generate $OH radicals. To further study
the photocatalytic mechanism and identify the main oxidative
species (h+, $OH and $O2ꢀ) in the photocatalytic process, we
studied the effect of active-species scavengers on the
4. Conclusions
In conclusion, nickel phosphide was synthesized by a simple
precipitation method, XRD and SEM characterization results
demostrate that the Ni2P is an amorphous nanospheres. Eosin
Y sensitized nickel phosphide displays relatively high photo-
catalytic activity for hydrogen evolution with the rate of 34.0
mmol hꢀ1 gꢀ1. The Ni2P also exhibits 4-nitrophenol degradation
under visible light illumination with degradation efficiency of
25.5% within
4
at initial 5.0 mg Lꢀ1 4-nitrophenol
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concentration. Probable mechanisms of photocatalytic 10 A. Bhattacharjee and M. Ahmaruzzaman, CuO
hydrogen evolution and photo-degradation of 4-nitrophenol
were discussed.
nanostructures: facile synthesis and applications for
enhanced photodegradation of organic compounds and
reduction of p-nitrophenol from aqueous phase, RSC Adv.,
2016, 6, 41348–41363.
Conflicts of interest
11 P. V. R. K. Ramacharyulu, S. J. Abbas, S. R. Sahoo and
S. C. Ke, Mechanistic insights into 4-nitrophenol
degradation and benzyl alcohol oxidation pathways over
MgO/g-C3N4 model catalyst systems, Catal. Sci. Technol.,
2018, 8, 2825–2834.
12 H. G. Lee, G. Sai-Anand, S. Komathi, A. I. Gopalan,
S. W. Kang and K. P. Lee, Efficient visible-light-driven
photocatalytic degradation of nitrophenol by using
graphene-encapsulated TiO2 nanowires, J. Hazard. Mater.,
2015, 283, 400–409.
13 J. Wang, P. Wang, C. Wang and Y. Ao, In-situ synthesis of
well dispersed CoP nanoparticles modied CdS nanorods
composite with boosted performance for photocatalytic
hydrogen evolution, Int. J. Hydrogen Energy, 2018, 43,
14934–14943.
14 J. Willkomm, K. L. Orchard, A. Reynal, E. Pastor,
J. R. Durrant and E. Reisner, Dye-sensitised
semiconductors modied with molecular catalysts for
light-driven H2 production, Chem. Soc. Rev., 2016, 45, 9–23.
There are no conicts to declare.
Acknowledgements
The authors appreciate nancial support from the National
Natural Science Foundation of China (21603065, 41773133), the
Support Plan for Talents in Hengyang Normal University (2017),
the China Postdoctoral Science Foundation (2017M612547), the
Aid Programs for Science and Technology Innovative Research
Team on Functional Organometallic Compounds in Higher
Educational Instituions of Hunan Province.
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