Stable functionalized phosphorenes with photocatalytic HER activity

Article abstract of DOI:10.1039/c8ta08497a

Phosphorene, a mono-elemental 2D material of phosphorus, can catalyze the hydrogen evolution reaction (HER) owing to the suitable position of its conduction band minimum but the ambient instability of phosphorene is the major challenge for its application. We have functionalized phosphorene with indium(iii) chloride, tris(pentafluorophenyl) borane and a benzyl group. The functionalized phosphorenes show stability under ambient conditions as well as good dispersibility in water and exhibit hydrogen yields as high as 6597 μmol h^-1 g^-1. The HER activity of the functionalized phosphorenes is much superior compared to pristine phosphorene and black phosphorus-based catalysts reported earlier.

Full text of DOI:10.1039/c8ta08497a

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Stable functionalized phosphorenes with
photocatalytic HER activity†
Cite this: DOI: 10.1039/c8ta08497a
Pratap Vishnoi,
Uttam Gupta,
Richa Pandey
and C. N. R. Rao
*
Received 1st September 2018
Accepted 14th December 2018
DOI: 10.1039/c8ta08497a
rsc.li/materials-a
Phosphorene, a mono-elemental 2D material of phosphorus, can content is attributed to the effective separation of electron–hole
catalyze the hydrogen evolution reaction (HER) owing to the suitable pairs in the photocatalysts as well as the availability of more
position of its conduction band minimum but the ambient instability of active sites on the catalyst surface. Bare BP nanostructures
8
,11,16
phosphorene is the major challenge for its application. We have produce trace amounts of H
2
.
Pt-deposited BP showed
17
functionalized phosphorene with indium(III) chloride, tris(penta- improved H evolution from water. But the ambient instability
2
fluorophenyl) borane and a benzyl group. The functionalized phos- of phosphorene remains a challenge for its applications. In the
phorenes show stability under ambient conditions as well as recent past, a number of efforts have been made to understand
good dispersibility in water and exhibit hydrogen yields as high as the mechanism of phosphorene degradation and its prevention.
ꢀ1
ꢀ1
6
597 mmol h
g . The HER activity of the functionalized phosphor- One strategy to suppress phosphorene degradation is to func-
enes is much superior compared to pristine phosphorene and black tionalize its surface with different chemical entities through
1
8,19
20,21
phosphorus-based catalysts reported earlier.
covalent
and coordination bonds
as well as van der Waals
2
2
interaction. It was our special interest to stabilize phosphor-
ene under ambient conditions by using chemical routes and to
study the efficiency of stabilized phosphorene for photochem-
Aer the discovery of phosphorene, a 2D monolayer sheet of
black phosphorus (BP), there have been studies on BP and its
2
ical H evolution.
1
–6
3
composites.
The phosphorene sheet is composed of sp
We have employed different means for the functionalization
hybridized phosphorus atoms, wherein each phosphorus atom
is covalently bonded to three neighbouring phosphorus atoms
to form a puckered honeycomb structure. The phosphorus
atoms contain lone-pair electrons. Owing to the unique struc-
ture, phosphorene exhibits excellent charge carrier mobility as
well as a thickness tuneable band-gap in the 0.3–2.0 eV
of solvent exfoliated few-layer phosphorene (Scheme 1). The

rst one uses the ability of group-III trivalent molecules with
vacant orbitals (n) to form h–n adducts with the lone-pair elec-
23
trons (h) of phosphorene as in triphenylphosphine (PPh
this purpose, we have employed indium trichloride (InCl
tris(pentauorophenyl) borane (B(C ). We have also carried
3
). For
3
) and
6 5 3
F )
7–10
range.
The conduction band minimum (CBM) of BP lies
out the reaction of few-layer phosphorene nanosheets with
benzyl bromide to form phosphonium ylide. Under ambient
conditions, the functionalized few-layer phosphorenes show
signicantly improved stability compared to the pristine phos-
phorene. They show enhanced water dispersibility as well. We
have examined the dye-sensitized photocatalytic HER activity of
the stable phosphorenes in visible light. We have used Eosin-Y
(EY) as the photosensitizer. The HER activity of these
compounds is signicantly higher than those reported for bare
BP nanostructures as well as the semiconductor-based photo-
catalysts hybridized with BP.
above the water reduction potential (0.0 V vs. RHE). These
properties suggest that few-layer BP can effectively catalyze the
7,10
hydrogen (H
2
) evolution reaction.
Besides, phosphorus is
an earth-abundant and environmentally benign non-metal
element. Therefore, it is of great interest to exploit the poten-
tial of phosphorene as a metal-free photocatalyst for H
2
evolu-
tion. It has been reported that the H production activity of
2
11,12
11,12
semiconducting CdS,
ZnS,
and g-C N photocatalysts can
3 4
be signicantly improved by hybridizing with a small amount of
13–15
BP.
The enhanced photocatalytic activity at an optimum BP
We will rst discuss the features of the h–n adducts of
phosphorene with InCl and B(C F ) . Few-layer phosphorene
New Chemistry Unit, International Centre for Materials Science, Sheikh Saqr
3
6 5 3
Laboratory, Jawaharlal Nehru Centre for Advanced Scientic Research, Jakkur P. O., nanosheets were prepared by ultrasonic exfoliation of
Bangalore-560 064, India. E-mail: cnrrao@jncasr.ac.in
0
black phosphorus crystals in deaerated anhydrous N,N -dime-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta08497a
thylformamide (DMF) under an inert atmosphere. The exfoliated
1
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Scheme 1 (a) Schematic of the preparation of phosphorene nanosheets by exfoliation of black phosphorus. (b) Schematic of functionalization of
phosphorene nanosheets with indium(III) chloride, tris(pentafluorophenyl) borane and a benzyl group.
nanosheets separated from the non-exfoliated bulk by centrifu-
gation at 400g. The top 75% of the supernatant containing few-
layer phosphorene was collected and used for characterization
using scanning electron microscopy (SEM), transmission
electron microscopy (TEM), Raman spectroscopy, X-ray
3
1
photoelectron spectroscopy (XPS) and P NMR spectroscopy
see Fig. S1–S3, ESI† for details of characterization). To carry out
(
functionalization reactions, we removed the DMF solvent from
the dispersion by centrifugation and re-dispersed the exfoliated
material in tetrahydrofuran (THF). The room temperature reac-
tion of the THF dispersion with InCl yielded the P$InCl adduct
3 3
which was characterized by SEM, TEM, EDS, XPS, Raman spec-
troscopy and NMR spectroscopy. Fig. 1a shows the Raman
spectra of pristine phosphorene along with that of the P$InCl
3
1
2
adduct. Pristine phosphorene shows A , B and Ag Raman
g
ꢀ
2g
1
bands at 361.7, 438.0 and 465.9 cm , respectively. While
ꢀ1
P$InCl
3
exhibits these bands at 360.0, 435.8 and 462.0 cm ,
3,4
respectively. All three bands of P$InCl
3
soen compared to
those of pristine phosphorene. The Raman characteristics of
functionalized phosphorene suggest that the adduct formation
takes place without signicantly changing the structure of
phosphorene sheets. The chemical composition of P$InCl was
examined by EDS (Fig. 1b) which shows the presence of P, In and
Cl elements. SEM and TEM images show the layered nature of
3
Fig. 1 (a) Raman spectra of phosphorene and P$InCl
dispersive X-ray spectrum (EDS) of P$InCl . (c) TEM image of a P$InCl
nanosheet, the inset shows the HR-TEM image with lattice fringes
3
. (b) Energy-
3
3
the material (Fig. 1c and S4, ESI†). Fig. 1c shows the TEM image corresponding to the (004) crystallographic plane of black phos-
31
of a P$InCl
3
nanosheet along with the HR-TEM image. The HR- phorus. (d) CP-MAS P NMR spectrum of P$InCl
3
.
TEM image shows a d-spacing of 2.65 nm corresponding to the
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004) crystal plane of black phosphorus. We recorded the cross-
Journal of Materials Chemistry A
(
3
1
polarization magic angle spinning (CP-MAS) P NMR spectrum
3
1
of P$InCl3 (Fig. 1d) which shows a P chemical shi of
95 ppm. The sharp signal with a chemical shi of ꢁ18 ppm is
ꢁ
24
due to the non-coordinated phosphorus. The broad hump
around ꢁ5 ppm is due to H PO and H PO formed by auto-
oxidation of phosphorene. XPS of P$InCl shows signals due
to P, In and Cl elements (Fig. 2a). The P 2p spectrum shows the
3/2 and 2p1/2 states at 129.8 and 130.7 eV, respectively, due to
3
3
3
4
25
3
2p
the bare phosphorene (Fig. 2b). The peak at 133 eV suggests
interaction between phosphorus and InCl , consistent with
3
6 5 3
Fig. 3 Spectral characterization of P$B(C F ) showing (a) Raman
20,21
31
metal-coordinated black phosphorus reported earlier.
Addi- and (b) CP-MAS P NMR spectra.
tionally, a peak at 134.3 eV is observed due to oxidized phos-
21
x y
phorene (P O ). It is possible that during the exfoliation and
functionalization reaction, phosphorene is oxidized to some
extent. Fig. 2c shows the In 3d spectrum with In 3d5/2 and 3d3/2
peaks at 446.0 and 453.8 eV, respectively, and the Cl 2p signal at
broad signal at 133 eV corresponds to phosphorus coordinated
to B(C F ) as well as oxidized phosphorus. Fig. 4b-d show the C
6
5 3
1s, F 1s and B 1s peaks at 287.3, 688.0 and 187.8 eV, respectively,
26
200 eV (Fig. 2d).
due to the coordinated B(C F ) .
6
5 3
We synthesized the P$B(C F ) adduct from few-layer phos-
6
5 3
Covalent functionalization of BP with an aryl diazonium salt
phorene and B(C F ) under reaction conditions similar to that
18
6
5 3
was reported by Hersam et al. who found that the function-
for P$InCl
3
. As shown in Fig. 3a, the Raman spectrum of
alization occurs through the formation of a P–C bond between
the BP nanosheet and the aryl group. Trivalent phosphorus
1
2
P$B(C
6
F
5
)
3
ꢀ
shows A
g
, B2g and A
g
bands at 360.8, 436.7 and
1
464.6 cm , respectively. These Raman bands are soened by
compounds such as PPh
3
are known to form alkyl-
ꢀ
1
ꢁ
2 cm compared to the pristine sample. The layered and
was
triphenylphosphonium ylides with alkyl halides. Phosphonium
ylides are important reagents in the Wittig reaction, and are
crystalline nature of the phosphorene sheets in P$B(C
6
F
5
)
3
conrmed by SEM and TEM images as well as the SAED pattern
28
used in the synthesis of olens from ketones or aldehydes. We
3
1
(
Fig. S5a–c, ESI†). The CP-MAS P NMR spectrum of P$B(C F )
6
5 3 have carried out the reaction of few-layer phosphorene
shows a broad signal around ꢁ85 ppm due to the phosphorus
with benzyl bromide to synthesize benzylphosphonium ylide
27
6 5 3
atoms coordinated to B(C F ) (Fig. 3b). EDS (Fig. S5d, ESI†)
P$CH–C
6 5
H on the phosphorene nanosheets (Fig. 5a). The
1
and XPS (Fig. 4 and S6, ESI†) measurements show the presence
of B, C, F and P elements in the adduct. The P 2p spectrum
Raman spectrum of P$CH–C
6
H
5
(Fig. S7a, ESI†) shows A
g
, B2g
2
g
ꢀ1
and A
bands at 360.4, 437.0 and 464.7 cm , respectively. The
(Fig. 4a) shows 2p3/2 and 2p1/2 states at 129.9 and 130.8 eV,
SEM and TEM images as well as the SAED pattern show the
layered morphology as well as the crystallinity of the product
respectively, due to non-coordinated phosphorus, while the
3
Fig. 2 XPS of the P$InCl adduct: (a) survey spectrum, (b) high reso-
lution P 2p spectrum, (c) high resolution In 3d spectrum and (d) high Fig. 4 XPS of P$B(C
6
F
5
)
3
showing (a) P 2p (b) C 1s, (c) F 1s and (d) B 1s
resolution Cl 2p spectrum.
bands.
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Fig. 5 (a) Schematic of the synthesis of benzylphosphonium ylide of phosphorene and its Wittig reaction with benzaldehyde to form stilbene. (b)
GC-MS of stilbene formed in the Wittig reaction (molecular mass of stilbene is 180.2).
(
2
Fig. S7b–d, ESI†). In the XPS of P$CH–C
p signal is signicantly broad and can be deconvoluted into where we observed that the oxide (P
three bands due to P–P, P–C and P–O bonds at 130, 133 and the case of P$B(C and P$CH–C
34 eV, respectively. We treated the isolated P$CH–C solid water droplets absorbed on pristine phosphorene, while the
6
H
5
(Fig. S8, ESI†), the P exposure. These results are consistent with the XPS results,
) peak is much smaller in
. In addition, we found
x y
O
H
6 5
6
F
5
)
3
1
6 5
H
with n-BuLi followed by the reaction with benzaldehyde to functionalized samples did not show such droplets (Fig. S9,
prepare stilbene. We conrmed the formation of stilbene by GC- ESI†). These studies suggest that the functionalized phosphor-
MS (Fig. 5b). This reaction further provides proof that the benzyl ene samples are stable in humid air. They show good dis-
group is present in P$CH–C
6
H
5
ylide.
persibility in the aqueous medium compared to the pristine
The ambient stability of the functionalized samples was one.
assessed by Raman spectroscopy. The samples were deposited
We examined the HER activity of pristine and functionalized
on glass slides and kept under ambient conditions. As shown in phosphorenes in visible light. We carried out an Eosin Y (EY)
Fig. 6, we recorded Raman spectra at 0, 5, 10 and 20 days. We sensitized photocatalytic reaction in an aqueous solution of

nd that the intensity of the spectra of the pristine lm triethanolamine (TEOA, 20%; v/v). TEOA has been used as an
decreased drastically in 5 days and was completely suppressed efficient sacricial electron donor in dye sensitized photo-
3
1,32
in 10 days. Interestingly, the functionalized samples retain their catalysts.
On visible light irradiation, the photocatalyst (EY +
evolution which increased
intensity ratio in the Raman spectrum is steadily with time. A plausible mechanism of the H evolution
a good parameter to evaluate the degree of phosphorene reaction is shown in Fig. 7a. EY, in the presence of visible light
Raman intensity during the experiments. According to a few phosphorene + TEOA) showed H
2
1
g
2
g
reports, the A
/A
2
29,30
degradation.
We analysed the integrated intensities of the and TEOA, goes through a series of changes to generate a highly
Raman spectra aer their baseline subtraction. The pristine reducing species EY which then transfers an electron to cata-
phosphorene showed a decrease in the A /A intensity ratio lyze the HER over phosphorene sheets. Fig. 7b shows the
ꢀ
1
g
2
g
from 0.58 (0 days; fresh sample) to 0.42 in 5 days. The intensity amount of H2 evolved in the four different photocatalyst
ratio in the case of P$InCl showed a small change from 0.48 systems for a period of 6 h. In the presence of EY and TEOA,
fresh sample) to 0.38 in 20 days. The intensity ratio of pristine phosphorene shows an activity of 621 mmol g
P$B(C and P$CH–C did not change much on ambient which is much less compared to that of the functionalized
3
ꢀ1
ꢀ1
(
h
6
F
5
)
3
6 5
H
3 6 5
Fig. 6 Raman spectra of the samples deposited on glass slides; (a) pristine phosphorene, (b) P$InCl and (c) P$CH–C H . The spectra were
collected for the fresh sample from 0 day to up to 20 days of exposure to air.
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Fig. 7 Photocatalytic H
reaction, (b) comparison of H
ESI† for the cycling study of other photocatalysts) and (d) SEM image of P$B(C F ) after the H
6 5 3 2
2
evolution from water by phosphorene based photocatalysts: (a) schematic of a plausible mechanism of the H
evolution activities of different photocatalysts for 6 h, (c) cycling study on P$B(C for 36 h (see Fig. S10–S12,
evolution reaction; the inset shows the cor-
2
evolution
2
6 5 3
F )
responding Raman spectrum.
3 6 5 3
samples. The hydrogen yields of P$InCl , P$B(C F ) and P$CH– nanosheets by inhibiting re-stacking which is a common
ꢀ
1
ꢀ1
33
C
6
H
5
are 2058, 6597 and 5691 mmol g
P$B(C exhibits the highest activity which is nearly 10.6 ciency (AQE) of pristine phosphorene, P$InCl
fold higher than that of the pristine sample. The H yields P$CH–C is found to be 0.8, 2.1, 8.4 and 7.3%, respectively.
increase in the pristine phosphorene ꢂ P$InCl < P$B(C F ) ꢁ Our control experiments on individual components EY, phos-
h
, respectively. The phenomenon in 2D materials. The apparent quantum effi-
6
F
5
)
3
3
6 5 3
, P$B(C F ) and
2
6 5
H
3
6 5 3
P$CH–C H order which is consistent with the stability order phorene and the starting reagents of the functional groups in
6
5
suggested by Raman spectroscopy and XPS. The superior HER the presence of TEOA showed no H evolution (Tables S1 and
2
3 6 5 3 6 5 2
activity of P$InCl , P$B(C F ) and P$CH–C H could be due to S2, ESI†). We did not observe H evolution from the catalysts
their enhanced ambient stability, and dispersibility in water. in the dark, indicating that it is a photocatalytic reaction and
The surface functional groups also stabilize the few-layer that the phosphorene sheet is the HER site. To examine the
a
Table 1 A comparison of the photocatalytic H
2
yields of various phosphorene based photocatalysts
Photocatalytic H
2
1
ꢀ
ꢀ1
Sr. no.
Photocatalysts
yields (mmol h
g
)
AQE (%)
References
1
2
3
4
5
6
7
8
9
Few-layer phosphorene
621
2058
6597
5691
64
45
571 and 427
138
138
0.8
2.1
8.4
7.3
NA
NA
This work
This work
This work
This work
8
8
13 and 14
16
17
Phosphorene$InCl
Phosphorene$B(C
3
6
F
5
)
3
Phosphorene$CH–C
BP nanosheets
6
H
5
BP nanoparticles
BP nanoakes/g-C
BP nanoakes
3
N
4
1.2 and 1.0–3.5
NA
NA
BP nanosheet/Pt
10
11
12
BP quantum dots/g-C
BP nanosheets/CoP
3
N
4
1900 and 271
735
160
NA, NA
5.5
0.89
15 and 35
36
37
BP nanosheets/BiVO
4
a
NA: data not available.
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ꢀ7
Fig. 8 (a) Photoluminescence quenching profile of an aqueous solution of EY (concentration ¼ 1.4 ꢃ 10 M; excitation wavelength ¼ 490 nm)
ꢀ1
in the presence of pristine and functionalized phosphorenes (10 mg mL ) and (b) photoluminescence decay curves of aqueous solution of EY in
the presence of pristine and functionalized phosphorenes. For lifetime measurements, the samples were excited at 480 nm and the emission was
monitored at 535 nm.
stability of the photocatalysts during the HER, we conducted purpose we have the advantage of the lone-pair electrons of
cycling studies. The functionalized phosphorene catalysts phosphorus to react phosphorene with indium(III) chloride,
retained their HER activities throughout the study (36 h, in tris(pentauorophenyl) borane and a benzyl group. The func-
that 24 h is the light period and 12 h is the dark period) (Fig. 7c tional groups protect the phosphorene sheets from oxidation as
and S10–S12, ESI†). In the case of pristine phosphorene, well as re-stacking. They show signicant photocatalytic
a decrease in the activity was observed in the second cycle. We HER activity with high hydrogen yields of 2058, 6597 and
ꢀ1
ꢀ1
recovered the catalysts aer the HER and assessed their 5691 mmol h
g
and apparent quantum efficiency (AQE) in
structural stability by SEM and Raman spectroscopy (Fig. 7d, the 0.8–8.4% range. The high activity of these functionalized
S13 and S14, ESI†). Both SEM images and Raman spectra show phosphorenes arises from their enhanced ambient stability and
no observable changes aer the reaction, conrming the water dispersibility as well as the efficient sensitization effect of
stability of the catalysts. We have compared our results of dye- EY. Electron transfer from EY to phosphorene is crucial for the
sensitized photocatalytic activity with those reported for bare photocatalytic proton reduction on phosphorene nanosheets.
few-layer BP and its composites at room temperature (Table 1). Our method of functionalization provides not only metal-free
Our dye-based photocatalysts show superior HER performance HER photocatalysts with high activity, but also new avenues
compared to other few-layer BP based photocatalysts. It is to work with phosphorene nanomaterials under ambient
2
noteworthy that the H evolution activity of these photo- conditions.
catalysts is higher than that of BP-loaded semiconductor-
based photocatalysts.
Conflicts of interest
Electronic absorption spectra show that the phosphorenes
absorb very little in the visible light, while EY shows intense
absorption in this region (Fig. S15 and S16, ESI†). To under-
stand the photo-excited electron transfer between EY and
phosphorene, we measured steady-state and time-resolved
photoluminescence (PL) spectra. The aqueous solution of EY
shows an emission band at 535 nm which was quenched on the
addition of phosphorene (Fig. 8a and S17, ESI†). It is worth
mentioning that the functionalized phosphorenes quench the
PL more efficiently compared to the pristine phosphorene.
Time-resolved photoluminescence spectra (Fig. 8b) suggested
that the excited state lifetime of EY decreased signicantly in
the presence of phosphorenes (Table S3†). The decays of EY and
EY–phosphorenes show mono- and bi-exponential behaviour,
respectively. This suggests a dynamic quenching mechanism
with an efficient photoinduced electron transfer process from
The authors declare no conicts of interest.
Acknowledgements
PV thanks the Nano Mission, Department of Science and
Technology, Govt. of India for a postdoctoral fellowship.
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These electrons eventually facilitate
proton reduction on the phosphorene sheet. These results
support our HER mechanism shown in Fig. 7a.
In conclusion, we have successfully stabilized phosphorene
under ambient conditions by functionalization. For this
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