U.S. patent application number 17/045570 was filed with the patent office on 2021-11-25 for metal-free few-layer phosphorous nanomaterial: method for its preparation and use thereof.
The applicant listed for this patent is INSTITUT NATIONAL DE LA RECHERCHE SCIENTIFIQUE. Invention is credited to Mohamed CHAKER, Dongling MA, Qingzhe ZHANG.
Application Number | 20210362135 17/045570 |
Document ID | / |
Family ID | 1000005810720 |
Filed Date | 2021-11-25 |
United States Patent
Application |
20210362135 |
Kind Code |
A1 |
ZHANG; Qingzhe ; et
al. |
November 25, 2021 |
METAL-FREE FEW-LAYER PHOSPHOROUS NANOMATERIAL: METHOD FOR ITS
PREPARATION AND USE THEREOF
Abstract
A method for preparing a metal-free few-layer phosphorous
nanomaterial. The method comprises an ice-assisted exfoliation
process (or solvent ice-assisted exfoliation process). The method
allows for the preparation of a few-layer phosphorous nanomaterial
with improved yield and reduced duration and exfoliation power. The
few-layer phosphorous nanomaterial is used in the preparation of a
photocatalyst. The photocatalyst exhibits a long-term stability,
high photocatalytic H.sub.2 evolution efficiency from water, and
good stability under visible light irradiation.
Inventors: |
ZHANG; Qingzhe; (Longueuil,
CA) ; MA; Dongling; (Pointe-Claire, CA) ;
CHAKER; Mohamed; (Montreal, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INSTITUT NATIONAL DE LA RECHERCHE SCIENTIFIQUE |
Quebec |
|
CA |
|
|
Family ID: |
1000005810720 |
Appl. No.: |
17/045570 |
Filed: |
June 10, 2019 |
PCT Filed: |
June 10, 2019 |
PCT NO: |
PCT/CA2019/050813 |
371 Date: |
October 6, 2020 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62685371 |
Jun 15, 2018 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 37/0072 20130101;
B01J 37/04 20130101; C01P 2002/72 20130101; B01J 27/24 20130101;
B01J 27/14 20130101; C01B 25/02 20130101; B01J 37/009 20130101;
B01J 35/004 20130101; C01B 3/042 20130101; B01J 37/06 20130101;
C01P 2004/24 20130101 |
International
Class: |
B01J 27/14 20060101
B01J027/14; C01B 25/02 20060101 C01B025/02; B01J 27/24 20060101
B01J027/24; B01J 37/04 20060101 B01J037/04; B01J 37/06 20060101
B01J037/06; B01J 37/00 20060101 B01J037/00; B01J 35/00 20060101
B01J035/00; C01B 3/04 20060101 C01B003/04 |
Claims
1. Method for preparing a few-layer phosphorous nanomaterial from a
bulk layer-structured phosphorous material, comprising an
ice-assisted exfoliation process or solvent ice-assisted
exfoliation process.
2. Method for preparing a few-layer phosphorous nanomaterial from a
bulk layer-structured phosphorous material, comprising a
combination of the following steps: grinding, dispersion in a
solvent, freezing, melting, separation, purification.
3. Method for preparing a few-layer phosphorous nanomaterial,
comprising: (a) providing a bulk layer-structured phosphorous
material; (b) grinding the bulk phosphorous material; (c)
dispersing the grinded material into a first solvent to obtain a
first dispersion; (d) freezing the first dispersion for a period of
time, preferably using liquid nitrogen; (e) melting the frozen
dispersion, preferably by sonication for a period of time to obtain
a second dispersion; and (f) submitting the second dispersion to a
separation step, preferably involving centrifugation for a period
of time, to obtain the nanomaterial.
4. Method according to claim 3, further comprising a purification
step; preferably the purification step comprises: (g) washing the
nanomaterial using a second solvent, optionally repeating step (g)
a number of time, preferably 2-6 times, or 3 times, or 4 times; and
(h) dispersing the nanomaterial into a third solvent, wherein the
second and third solvents are the same or different.
5. Method according to claim 3 or 4, wherein steps (d) and (e) are
repeated a number of time, preferably 2 to 6 times, or 3 times or 4
times.
6. Method according to claim 3, wherein the freezing time period at
step (d) is about 3-15 minutes, or about 4-14 minutes, or about
5-13 minutes, or about 5-12 minutes, or about 5-11 minutes, or
about 5-10 minutes, or about 6-8 minutes.
7. Method according to claim 3, wherein the sonication time period
at step (e) is about 5-15 minutes, or about 6-14 minutes, or about
7-13 minutes, or about minutes 8-12 minutes, or about 9-11 minutes,
or about 10 minutes.
8. Method according to claim 3, wherein the centrifugation at step
(f) is performed at 7000 rpm and the time period is about 10-20
minutes, or about 12-18 minutes, or about 14-16 minutes, or about
15 minutes.
9. Method according to any one of claims 1 to 8, wherein the bulk
layered structure phosphorous material is black phosphorous (BP),
red phosphorous (RP), violet phosphorous (VP).
10. Method according to any one of claims 1 to 9, wherein the bulk
layer-structured phosphorous material is a black phosphorous (BP)
material, and the few-layer phosphorous nanomaterial is a few-layer
black phosphorous (BP) nanomaterial.
11. Method according to claim 1, wherein the solvent is an organic
solvent; preferably the organic solvent is selected from the group
consisting of N-methyl-2-pyrrolidone (NMP), alcohols such as
methanol, ethanol and isopropanol (IPA), diethyl ether, chloroform,
tetrahydrofuran, cyclohexane, toluene, dimethylformamide, and
combinations thereof; more preferably the solvent is
N-methyl-2-pyrrolidone (NMP).
12. Method according to claim 3 or 4, wherein: the first solvent is
selected from the group consisting of N-methyl-2-pyrrolidone (NMP),
alcohols such as methanol, ethanol and isopropanol (IPA), diethyl
ether, chloroform, tetrahydrofuran, cyclohexane, toluene,
dimethylformamide, and combinations thereof; preferably the first
solvent is N-methyl-2-pyrrolidone (NMP); the second solvent is
selected from the group consisting of isopropanol (IPA), other
alcohols such as methanol and ethanol; diethyl ether, chloroform,
tetrahydrofuran, cyclohexane, toluene, dimethylformamide, and
combinations thereof; preferably the second solvent is isopropanol
(IPA); and the third solvent is selected from the group consisting
of isopropanol (IPA), other alcohols such as methanol and ethanol;
diethyl ether, chloroform, tetrahydrofuran, cyclohexane, toluene,
dimethylformamide, N-methyl-2-pyrrolidone (NMP), and combinations
thereof; preferably the second solvent is isopropanol (IPA).
13. Method according to any one claims 1 to 12, wherein
substantially no oxidation occurs.
14. Method according to any one claims 1 to 12, wherein the
few-layer phosphorous nanomaterial is metal-free.
15. A few-layer phosphorous nanomaterial obtained by the method as
defined in any one of claims 1 to 14.
16. A few-layer black phosphorous (BP) nanomaterial obtained by the
method as defined in any one of claims 1 to 14.
17. A few-layer phosphorous nanomaterial as defined in claim 15 or
16, having 4 to 10 layers, or 5 to 9 layers, or 6 to 8 layers, or 7
layers, or 6 layers.
18. A few-layer phosphorous nanomaterial as defined in any one of
claims 15 to 17, having a thickness which is less than about 12 nm,
or less than about 10 nm; or which is about 9 nm, or about 8 nm, or
about 7 nm, or about 6 nm, or about 5 nm.
19. Use of a few-layer phosphorous nanomaterial as defined in any
one of claims 15 to 18, in the development of photocatalysts,
transistor devices, photodetector devices, solar cells, or in
bio-imaging, or in phototherapy.
20. A method for preparing a photocatalyst, comprising coupling the
few-layer phosphorous nanomaterial as defined in any one of claims
15 to 18, with a 2D material; preferably the 2D material is
selected from the group consisting of poly (methyl methacrylate),
graphene or hexagonal boron nitride which may be nitrogen-doped,
molybdenum disulfide, a carbon nitride nanomaterial; more
preferably the 2D material is graphitic carbon nitride
(g-C.sub.3N.sub.4).
21. A method for preparing a photocatalyst, comprising coupling the
few-layer black phosphorous (BP) nanomaterial as defined in claim
20, with graphitic carbon nitride (g-C.sub.3N.sub.4).
22. Use of the few-layer phosphorous nanomaterial as defined in any
one of claims 15 to 18, in the preparation of a photocatalyst.
23. Use of the few-layer black phosphorous (BP) nanomaterial as
defined in claim 16, in the preparation of a photocatalyst.
24. A photocatalyst obtained by the method as defined in claim 20
or 21.
25. A photocatalyst obtained by the method as defined in claim 21,
which is few-layer black phosphorous
nanomaterial/g-C.sub.3N.sub.4.
26. Use of the photocatalyst as defined in claim 24 or 25, for
water splitting (H.sub.2 evolution).
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. Provisional Patent
Application No. 62/685,371, filed on Jun. 15, 2018, the content of
which is incorporated herein in its entirety by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to few-layer
phosphorous nanomaterials. More specifically, the present invention
relates to a metal-free few-layer black phosphorous (BP)
nanomaterial. The method for its preparation comprises an
ice-assisted exfoliation process. The BP nanomaterial according to
the invention may be used, among others, in the development of
photocatalysts.
BACKGROUND OF THE INVENTION
[0003] Solar water splitting for H.sub.2 evolution has shown great
potential as a green technology in solving energy crisis [1].
Taking economic and environmental factors into consideration, the
development of efficient, low-cost, stable and nontoxic
photocatalyst is highly desired for a widespread implementation of
solar fuel technology. In this regard, visible-light-responsive
graphitic carbon nitride (g-C.sub.3N.sub.4), a two-dimensional (2D)
metal-free photocatalyst, has been extensively explored in
photocatalysis. Though g-C.sub.3N.sub.4 was discovered to be
feasible for photocatalytic water splitting, achieving an
acceptable efficiency in H.sub.2 evolution still relies largely on
the loading of noble metal co-catalysts. This is necessary because
of the high recombination rate of the charge carriers in
g-C.sub.3N.sub.4 [2]. Furthermore, the relatively wide bandgap (2.7
eV) confines its light response mainly into the ultraviolent (UV)
range and only slightly into a small portion of the visible light
range (.lamda.<460 nm) [3]. To solve these problems, numerous
strategies have been developed, mainly including morphology tuning,
doping with metal/non-metal ions, and heterojunction creation [4].
However, quite limited progresses have been achieved thus far.
Aiming to enhance the harvesting of solar light efficiently and
economically, the development of g-C.sub.3N.sub.4-based metal-free
photocatalysts with a broader photo-response range is of great
significance.
[0004] Black phosphorus (BP), a layered material that consists of
corrugated atomic planes with strong intra-layer chemical bonding
and weak interlayer Van der Waals interactions, has attracted the
interest of material scientists. Since the successful preparation
of 2D BP with atom-thick layer in early 2014, it has provoked a
surge of research with its enticing electrical and optical
properties [5]. Differentiating from previously reported 2D
nanomaterial such as graphene, BP possesses a tunable
thickness-dependent bandgap that spans from about 0.3 eV (bulk) to
about 2.0 eV (monolayer) in addition to sufficiently high carrier
mobility and photo-electronic response [5b-d, 5f, 5g, 5i]. These
favorable properties render BP, particularly few-layer BP
nanosheets (10 nm in thickness), a good candidate for diverse
applications in transistor and photodetector devices, solar cells,
bio-imaging and phototherapy [5i, 6]. Notably, BP has demonstrated
its great potential as a broadband photocatalyst for the harvesting
of solar energy due to its narrow and direct bandgap [7].
[0005] However, certain inherent problems existing in the typical,
exfoliated BP nanosheets bring practical challenges for its actual
application. For example, BP is very reactive to moisture and
ambient oxygen, and can be easily oxidized due to the exposed lone
pairs at its surface [5f, 6e, 7e, 8]. The roughening caused by the
exfoliation can further accelerate the surface oxidation, which may
proceed exponentially during the first hour after exfoliation [8b].
As a consequence, the semiconducting properties of BP deteriorate
rapidly, reflected from significantly increased contact resistance
and reduced carrier mobility [8a, 8b, 8e]. It is thus importance to
develop effective strategies to retard or eliminate the degradation
of BP.
[0006] Recently, several approaches were developed to protect BP
from oxidation with various levels of success [5i, 9]. Among these
approaches, the non-covalent surface coverage of BP with other
inert 2D materials, such as poly (methyl methacrylate), graphene or
hexagonal boron nitride, was proposed [8e, 9b].
[0007] For the preparation of few-layer BP nanosheets, the
mechanical and liquid exfoliation from bulk BP is known in the art
[8a, 8c, 10]. As BP possesses stronger interlayer interactions
compared to graphene or other 2D materials, the exfoliation by
ultrasonication would be difficult and would require a long
processing time (>15 hours), or would require a sonicator with
high power [8a, 8c, 10]. The yield obtained for the preparation of
few-layer BP nanosheets is still low [8a, 10c]. As the P-P bond is
weaker than the C--C bond, such long duration or high power of
sonication are known to generate nanosheets with reduced lateral
size and structural defects [8a, 11]. In addition to the
instability, such structural defects also restrict the practical
applications of BP obtained by these methods.
[0008] There is a need for few-layer phosphorous nanomaterials that
are stable, that have structures free of defects, and that are
environment-friendly. There is a need for efficient methods for the
preparation of such few-layer phosphorous nanomaterials.
SUMMARY OF THE INVENTION
[0009] The inventors have designed and performed a method for
preparing a metal-free few-layer phosphorous nanomaterial. The
method comprises an ice-assisted exfoliation process (or solvent
ice-assisted exfoliation process). The method according to the
invention is novel, and allows for the preparation of a few-layer
phosphorous nanomaterial with improved yield and reduced duration
and exfoliation power.
[0010] In embodiments of the invention, the inventors have designed
and performed a method for preparing a metal-free few-layer black
phosphorous (BP) nanomaterial. In these embodiments, the
ice-assisted exfoliation process involves use of a solvent.
Preferably, the solvent is an organic solvent, for example
N-methyl-2-pyrrolidone (NMP).
[0011] In other embodiments of the invention, a photocatalyst is
prepared. In these embodiments, the few-layer BP nanomaterial and
graphitic carbon nitride (g-C.sub.3N.sub.4) are integrated into a
single, 2D-on-2D architecture (BP/g-C.sub.3N.sub.4). The
thus-obtained metal-free BP/g-C.sub.3N.sub.4 photocatalyst exhibits
a long-term stability, high photocatalytic H.sub.2 evolution
efficiency from water, and good stability under visible light
irradiation.
[0012] The invention thus provides the following according to
aspects thereof: [0013] (1) Method for preparing a few-layer
phosphorous nanomaterial from a bulk layer-structured phosphorous
material, comprising an ice-assisted exfoliation process or solvent
ice-assisted exfoliation process. [0014] (2) Method for preparing a
few-layer phosphorous nanomaterial from a bulk layer-structured
phosphorous material, comprising a combination of the following
steps: grinding, dispersion in a solvent, freezing, melting,
separation, purification. [0015] (3) Method for preparing a
few-layer phosphorous nanomaterial, comprising: (a) providing a
bulk layer-structured phosphorous material; (b) grinding the bulk
phosphorous material; (c) dispersing the grinded material into a
first solvent to obtain a first dispersion; (d) freezing the first
dispersion for a period of time, preferably using liquid nitrogen;
(e) melting the frozen dispersion, preferably by sonication for a
period of time to obtain a second dispersion; and (f) submitting
the second dispersion to a separation step, preferably involving
centrifugation for a period of time, to obtain the nanomaterial.
[0016] (4) Method according to (3) above, further comprising a
purification step; preferably the purification step comprises: (g)
washing the nanomaterial using a second solvent, optionally
repeating step (g) a number of time, preferably 2-6 times, or 3
times, or 4 times; and (h) dispersing the nanomaterial into a third
solvent, wherein the second and third solvents are the same or
different. [0017] (5) Method according to (3) or (4) above, wherein
steps (d) and (e) are repeated a number of time, preferably 2 to 6
times, or 3 times or 4 times. [0018] (6) Method according to (3)
above, wherein the freezing time period at step (d) is about 3-15
minutes, or about 4-14 minutes, or about 5-13 minutes, or about
5-12 minutes, or about 5-11 minutes, or about 5-10 minutes, or
about 6-8 minutes. [0019] (7) Method according to (3) above,
wherein the sonication time period at step (e) is about 5-15
minutes, or about 6-14 minutes, or about 7-13 minutes, or about
minutes 8-12 minutes, or about 9-11 minutes, or about 10 minutes.
[0020] (8) Method according to (3) above, wherein the
centrifugation at step (f) is performed at 7000 rpm and the time
period is about 10-20 minutes, or about 12-18 minutes, or about
14-16 minutes, or about 15 minutes. [0021] (9) Method according to
any one of (1) to (8) above, wherein the bulk layered structure
phosphorous material is black phosphorous (BP), red phosphorous
(RP), violet phosphorous (VP). [0022] (10) Method according to any
one of (1) to (9) above, wherein the bulk layer-structured
phosphorous material is a black phosphorous (BP) material, and the
few-layer phosphorous nanomaterial is a few-layer black phosphorous
(BP) nanomaterial. [0023] (11) Method according to (1) above,
wherein the solvent is an organic solvent; preferably the organic
solvent is selected from the group consisting of
N-methyl-2-pyrrolidone (NMP), alcohols such as methanol, ethanol
and isopropanol (IPA), diethyl ether, chloroform, tetrahydrofuran,
cyclohexane, toluene, dimethylformamide, and combinations thereof;
more preferably the solvent is N-methyl-2-pyrrolidone (NMP). [0024]
(12) Method according to (3) or (4) above, wherein: the first
solvent is selected from the group consisting of
N-methyl-2-pyrrolidone (NMP), alcohols such as methanol, ethanol
and isopropanol (IPA), diethyl ether, chloroform, tetrahydrofuran,
cyclohexane, toluene, dimethylformamide, and combinations thereof;
preferably the first solvent is N-methyl-2-pyrrolidone (NMP); the
second solvent is selected from the group consisting of isopropanol
(IPA), other alcohols such as methanol and ethanol; diethyl ether,
chloroform, tetrahydrofuran, cyclohexane, toluene,
dimethylformamide, and combinations thereof; preferably the second
solvent is isopropanol (IPA); and the third solvent is selected
from the group consisting of isopropanol (IPA), other alcohols such
as methanol and ethanol; diethyl ether, chloroform,
tetrahydrofuran, cyclohexane, toluene, dimethylformamide,
N-methyl-2-pyrrolidone (NMP), and combinations thereof; preferably
the second solvent is isopropanol (IPA). [0025] (13) Method
according to any one (1) to (12) above, wherein substantially no
oxidation occurs. [0026] (14) Method according to any one (1) to
(12) above, wherein the few-layer phosphorous nanomaterial is
metal-free. [0027] (15) A few-layer phosphorous nanomaterial
obtained by the method as defined in any one of (1) to (14) above.
[0028] (16) A few-layer black phosphorous (BP) nanomaterial
obtained by the method as defined in any one of (1) to (14) above.
[0029] (17) A few-layer phosphorous nanomaterial as defined in (15)
or (16) above, having 4 to 10 layers, or 5 to 9 layers, or 6 to 8
layers, or 7 layers, or 6 layers. [0030] (18) A few-layer
phosphorous nanomaterial as defined in any one of (15) to (17)
above, having a thickness which is less than about 12 nm, or less
than about 10 nm; or which is about 9 nm, or about 8 nm, or about 7
nm, or about 6 nm, or about 5 nm. [0031] (19) Use of a few-layer
phosphorous nanomaterial as defined in any one of (15) to (18)
above, in the development of photocatalysts, transistor devices,
photodetector devices, solar cells, or in bio-imaging, or in
phototherapy. [0032] (20) A method for preparing a photocatalyst,
comprising coupling the few-layer phosphorous nanomaterial as
defined in any one of (15) to (18) above, with a 2D material;
preferably the 2D material is selected from the group consisting of
poly (methyl methacrylate), graphene or hexagonal boron nitride
which may be nitrogen-doped, molybdenum disulfide, a carbon nitride
nanomaterial; more preferably the 2D material is graphitic carbon
nitride (g-C.sub.3N.sub.4). [0033] (21) A method for preparing a
photocatalyst, comprising coupling the few-layer black phosphorous
(BP) nanomaterial as defined in (20) above, with graphitic carbon
nitride (g-C.sub.3N.sub.4). [0034] (22) Use of the few-layer
phosphorous nanomaterial as defined in any one of (15) to (18)
above, in the preparation of a photocatalyst. [0035] (23) Use of
the few-layer black phosphorous (BP) nanomaterial as defined in
(16) above, in the preparation of a photocatalyst. [0036] (24) A
photocatalyst obtained by the method as defined in (20) or (21)
above. [0037] (25) A photocatalyst obtained by the method as
defined in (21) above, which is few-layer black phosphorous
nanomaterial/g-C.sub.3N.sub.4. [0038] (26) Use of the photocatalyst
as defined in (24) or (25) above, for water splitting (H.sub.2
evolution).
[0039] Other objects, advantages and features of the present
invention will become more apparent upon reading of the following
non-restrictive description of specific embodiments thereof, given
by way of example only with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawing(s) will be provided by the Office
upon request and payment of the necessary fee.
[0041] In the appended drawings:
[0042] FIG. 1: (a) Schematic illustration of the preparation of BP
nanosheets with ice-assisted exfoliation method. (b) TEM image of
BP nanosheets and (c) EDX spectrum of (b). (d) Tapping mode AFM
topographical image of few-layer BP nanosheets. Scale bars in b)
and (d) are 500 nm. (e) The height profiles of BP nanosheets along
the blue Line 1 and green Line 2 in (d). (f) Statistical thickness
distribution calculated from the height profiles of 150 BP
nanosheets in AFM images.
[0043] FIG. 2: Photographs of BP nanosheets in isopropanol (IPA)
(a) at the first day, (b) after four weeks, (c) after adding
g-C.sub.3N.sub.4, and (d) after the incubation at room temperature
for 30 minutes. (e) The zeta potentials of BP and g-C.sub.3N.sub.4
nanosheets in IPA.
[0044] FIG. 3: P2p XPS spectra of BP and BP/g-C.sub.3N.sub.4
samples after water splitting under visible light irradiation for
24 hours.
[0045] FIG. 4: Representative TEM images of (a) g-C.sub.3N.sub.4
and (b-d) BP/g-C.sub.3N.sub.4 with different magnifications. (e)
High-angle annular dark field (HAADF) scanning TEM (STEM) image of
(d), (f-i) STEM-EDX mapping of C, N, P, and the overlay of all the
elements of the selected area in (e). (j) HRTEM image of
BP/g-C.sub.3N.sub.4, and (k) EDX spectrum of (j). Scale bars: (a)
and (c-i), 250 nm; (b), 1 .mu.m; (j), 5 nm. The grid used in (a),
(j) and (k) are carbon film on copper, and that used in the other
figures is lacey carbon film on nickel.
[0046] FIG. 5: (a) XPS survey spectra of g-C.sub.3N.sub.4 and
BP/g-C.sub.3N.sub.4 nanosheets. High-resolution (b) C1 s, (c) Nis,
and (d) P2p XPS spectra of BP/g-C.sub.3N.sub.4 sample.
[0047] FIG. 6: (a) XRD patterns of bulk BP, BP nanosheets,
g-C.sub.3N.sub.4 and BP/g-C.sub.3N.sub.4 samples. (b) Amplification
of XRD patterns of bulk BP and BP nanosheets in the low-angle range
which is marked by the dashed rectangle in (a). (c) UV-vis-NIR
absorption spectra of BP nanosheets in IPA, and g-C.sub.3N.sub.4
and BP/g-C.sub.3N.sub.4 powder samples. Insets in (c) are the
photos of g-C.sub.3N.sub.4 (bottom) and BP/g-C.sub.3N.sub.4 (top)
powders.
[0048] FIG. 7: (a) Photocatalytic water splitting for H.sub.2
evolution and (b) H.sub.2 evolution rate by BP (orange),
g-C.sub.3N.sub.4 (blue) and BP/g-C.sub.3N.sub.4 (red)
photocatalysts under visible light irradiation (.lamda.>420 nm).
(c) EIS Nyquist plots of g-C.sub.3N.sub.4 and BP/g-C.sub.3N.sub.4
with and without illumination. (d) Transient photocurrent density
response of g-C.sub.3N.sub.4 and BP/g-C.sub.3N.sub.4 during light
on/off cycles under a 0.2 V bias versus Ag/AgCl electrode.
[0049] FIG. 8: P2p XPS spectra of BP and BP/g-C.sub.3N.sub.4
samples after water splitting under visible light irradiation for
24 hours.
[0050] FIG. 9: Valence band UPS cut-off spectra of (a) BP and (b)
g-C.sub.3N.sub.4 samples. (c) Schematic energy diagram of
BP/g-C.sub.3N.sub.4 photocatalyst and proposed possible mechanism
for the photocatalytic H.sub.2 evolution from water splitting under
visible light irradiation.
DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0051] Before the present invention is further described, it is to
be understood that the invention is not limited to the particular
embodiments described below, as variations of these embodiments may
be made and still fall within the scope of the appended claims. It
is also to be understood that the terminology employed is for the
purpose of describing particular embodiments, and is not intended
to be limiting. Instead, the scope of the present invention will be
established by the appended claims.
[0052] In order to provide a clear and consistent understanding of
the terms used in the present specification, a number of
definitions are provided below. Moreover, unless defined otherwise,
all technical and scientific terms as used herein have the same
meaning as commonly understood to one of ordinary skill in the art
to which this disclosure pertains.
[0053] As used herein, the term "exfoliation" refers to a process
which allows for the separation of layers of a layer-structured
material. The process may involve dispersing the material into a
solvent. The process is herein referred to as "ice-assisted
exfoliation" or "solvent ice-assisted exfoliation". The expressions
"ice-assisted exfoliation" and "solvent ice-assisted exfoliation"
are used herein interchangeably.
[0054] As used herein the expression "few-layer black phosphorous
(BP) nanomaterial" is used interchangeably with the expression
"few-layer black phosphorous (BP) nanosheets" to refer to the
material prepared by the method according to the invention. As will
be understood by a skilled person, the "few-layer black phosphorous
(BP) nanomaterial" according to the invention comprises
nanosheets.
[0055] The use of the word "a" or "an" when used in conjunction
with the term "comprising" in the claims and/or the specification
may mean "one", but it is also consistent with the meaning of "one
or more", "at least one", and "one or more than one". Similarly,
the word "another" may mean at least a second or more.
[0056] As used in this specification and claim(s), the words
"comprising" (and any form of comprising, such as "comprise" and
"comprises"), "having" (and any form of having, such as "have" and
"has"), "including" (and any form of including, such as "include"
and "includes") or "containing" (and any form of containing, such
as "contain" and "contains"), are inclusive or open-ended and do
not exclude additional, unrecited elements or process steps.
[0057] As used herein when referring to numerical values or
percentages, the term "about" includes variations due to the
methods used to determine the values or percentages, statistical
variance and human error. Moreover, each numerical parameter in
this application should at least be construed in light of the
number of reported significant digits and by applying ordinary
rounding techniques.
[0058] The inventors have designed and performed a method for
preparing a metal-free few-layer phosphorous nanomaterial. The
method comprises an ice-assisted exfoliation process (or solvent
ice-assisted exfoliation process). The method according to the
invention is novel, and allows for the preparation of a few-layer
phosphorous nanomaterial with improved yield and reduced duration
and exfoliation power.
[0059] In embodiments of the invention, the inventors have designed
and performed a method for preparing a metal-free few-layer black
phosphorous (BP) nanomaterial. In these embodiments, the
ice-assisted exfoliation process involves use of a solvent.
Preferably, the solvent is an organic solvent, for example
N-methyl-2-pyrrolidone (NMP).
[0060] In other embodiments of the invention, a photocatalyst is
prepared. In these embodiments, the few-layer BP nanomaterial and
graphitic carbon nitride (g-C.sub.3N.sub.4) are integrated into a
single, 2D-on-2D architecture (BP/g-C.sub.3N.sub.4). The
thus-obtained metal-free BP/g-C.sub.3N.sub.4 photocatalyst exhibits
a long-term stability, high photocatalytic H.sub.2 evolution
efficiency from water, and good stability under visible light
irradiation.
[0061] The present invention is illustrated in further details by
the following non-limiting examples.
EXPERIMENTAL SECTION
[0062] Materials. BP crystals of high-purity (99.998%) were
purchased from Smart Elements, N-methyl-2-pyrrolidone (NMP, 99.5%,
anhydrous), isopropanol (IPA, 99.5%, anhydrous), urea
(NH.sub.2CONH.sub.2), nitric acid (HNO.sub.3),
N,N-dimethylformamide (DMF) and triethanolamine (99.0%) were
purchased from Sigma-Aldrich and used as received without further
purification. The ultrapure water (18.2 MO cm, 25.degree. C.),
obtained from a Millipore Ultrapure water system, was used
throughout the current study.
[0063] Example 1--Ice-Assisted Preparation of BP Nanosheets. BP
nanosheets were synthesized by developing a "NMP ice"-assisted
exfoliation method. Specifically, 25 mg of bulk BP was ground into
fine powder and dispersed into 25 mL of NMP solvent. The dispersion
was completely frozen with a liquid nitrogen bath for 5-10 minutes,
and then sonicated in a bath sonicator (BRANSONIC, 70 W, 40 kHz)
for about 10 minutes to make the "ice" melt. The procedure of
freezing and melting was repeated 3 times. To protect the BP from
oxygen and water, the dispersion was sealed in a vial, and all the
experimental manipulations were performed in a glovebox or with
nitrogen bubbling. Afterwards, the dispersion was centrifuged at
7000 rpm for 15 minutes to remove the residual un-exfoliated BP.
The light yellow supernatant was decanted gently, which was the
dispersion of BP nanosheets in NMP. The obtained BP nanosheets were
washed with IPA by centrifugation at 12000 rpm, 2 times. The
collected precipitate was re-dispersed into 25 mL of IPA. The
concentration of BP in this dispersion was determined to be 0.75 mg
mL.sup.-1 by Inductively Coupled Plasma Atomic Emission
Spectroscopy (ICP-AES).
[0064] Example 2--Preparation of g-C.sub.3N.sub.4 Nanosheets. The
g-C.sub.3N.sub.4 nanosheets were synthesized by our reported
thermal polymerization method [12]. Generally, urea (30 g) was
placed into a covered alumina crucible and then heated in a quartz
tube furnace with a heating rate of 2.degree. C. min.sup.-1 to 250,
350, and 550.degree. C., and maintained at these three target
temperatures for 1, 2, and 2 hours, respectively. After being
naturally cooled down to room temperature, the yellow powder was
collected and washed, three times with HNO.sub.3 (0.1 mol L.sup.-1)
and water to remove potential alkaline residue (e.g., ammonia).
After centrifugation, the precipitate was dried in the vacuum at
80.degree. C. overnight.
[0065] Example 3--Preparation of BP/g-C.sub.3N.sub.4
Photocatalysts. BP/g-C.sub.3N.sub.4 nanosheets were prepared by
dispersing 10 mg of g-C.sub.3N.sub.4 powder into 0.5 mL of BP
nanosheets dispersion in IPA. The mixture was stirred for 2 hours
to couple BP nanosheets with g-C.sub.3N.sub.4 nanosheets under the
protection of N.sub.2. Subsequently, the sample was collected by
centrifugation at 6000 rpm for 5 minutes, and then washed
completely with isopropanol. The final product was obtained by
drying the washed sample in an oven under vacuum at 60.degree. C.
overnight.
[0066] Example 4--Characterization. A transmission electron
microscope (TEM, JEOL 2100F), equipped with an energy-dispersive
X-ray (EDX) spectrometer, was employed and operated at an
accelerating voltage of 200 kV to study the microstructure and
composition of the prepared samples. The topography image of the BP
nanosheets on the pre-cleaned glass was observed by an atomic force
microscopy (AFM, Bruker, MultiMode 8) in a tapping mode. Zeta
potential of the as-prepared BP and g-C.sub.3N.sub.4 nanosheets in
IPA was recorded with a Brookhaven ZetaPlus system in a standard 10
mm all-side-transparent polymethyl methacrylate cuvette. The
crystalline structure was analyzed by an X-ray diffraction system
(XRD, PANalytical X'Pert MRD, operated at 45 kV and 40 mA) with a
Cu K.alpha. radiation source (A=0.15406 nm). X-ray photoelectron
spectroscopy (XPS) was taken on a VG Escalab 220i-XL spectrometer
equipped with a twin anode X-ray source. All the XPS spectra were
calibrated with the C1s peak at 284.8 eV as reference. Ultraviolet
photoelectron spectroscopy (UPS) measurements were carried out with
an unfiltered Helium (21.22 eV) gas discharge lamp to determine the
valence band (VB) position of the as-prepared BP and
g-C.sub.3N.sub.4 samples. The UV-visible-near infrared (UV-vis-NIR)
absorption spectra of the BP nanosheets dispersion and
BP/g-C.sub.3N.sub.4 powder were obtained using a scan spectrometer
(Varian Cary 5000). The concentration of BP nanosheets in IPA
dispersion and the content of P in the composite samples were
determined by an IRIS Intrepid II XSP ICP-AES (Thermal Scientific,
USA).
[0067] Example 5--Photoelectrochemical Measurements.
Photoelectrochemical (PEC) properties were measured with a standard
three electrode system in an electrochemical workstation (CHI 660E,
CH Instruments). The working electrode was prepared by coating the
as-synthesized sample on fluorine-doped tin oxide (FTO) glass with
its boundaries being protected by Scotch tape. Specifically, 2 mg
of powder sample was dispersed into 2 mL of DMF under sonication
for 30 minutes to obtain evenly dispersed slurry, which was
drop-casted onto the FTO glass. After drying under ambient
condition, the epoxy resin glue was used to isolate the uncoated
part of the FTO glass. A Pt wire and a Ag/AgCl electrode were used
as the counter and reference electrode, respectively. The 0.2 M of
Na.sub.2SO.sub.4 (pH=6.8) aqueous solution pre-purged with nitrogen
for 30 minutes was used as an electrolyte. A solar simulator
equipped with an AM1.5G filter (LCS-100, Newport) was utilized as
the light source. Nyquist plots were recorded over the frequency
range of 100 mHz to 100 kHz at a bias of 0.2 V.
[0068] Example 6--Photocatalytic H.sub.2 Evolution. Photocatalytic
H.sub.2 evolution experiment was performed in a 500 mL Pyrex
top-irradiation reactor with a quartz cover. A 300 W Xenon lamp
equipped with a cut-off filter (420 nm) was used to provide the
irradiation source in the visible wavelength range. Typically, 10
mg of photocatalysts were dispersed in 100 mL of aqueous solution
containing 10% of triethanolamine (TEOA) as sacrificial reagents.
The mixture was deaerated by N.sub.2 gas for 20 minutes and
sonicated for 5 minutes. The system was sealed and vacuumed prior
to photocatalysis. During the irradiation, the suspension was
stirred continuously and kept at a constant temperature by
circulating cooling water. The evolved H.sub.2 was analyzed by a
gas chromatography (GC, 7890B, Agilent Technologies) equipped with
a thermal conductivity detector. For stability measurements, the
photocatalysts were collected from the final reaction slurry by
centrifugation, and then washed with ethanol and water thoroughly.
Subsequently, the recycled sample underwent the photocatalytic
H.sub.2 evolution experiment under the identical conditions and
repeated for 5 cycles with a total irradiation time of 120
hours.
Results and Discussions
[0069] Preparation of BP Nanosheets and BP/g-C.sub.3N.sub.4
Photocatalysts
[0070] To prepare BP nanosheets, bulk BP crystals are exfoliated in
NMP using ice-assisted ultrasonication as outlined above in Example
3 above, and schematically illustrated in FIG. 1a. When the bulk BP
powder is dispersed into NMP, the spaces between BP layers are
filled with this solvent. As the melting point of NMP is
-24.degree. C., after being placed into direct contact with liquid
nitrogen bath, the dispersion starts to freeze. The gradual growth
of NMP ice crystals intercalates into BP layers to enlarge the
interlayer spacing of BP, which reduces the interlayer Van der
Waals interactions and will be favourable for the exfoliation
process to generate BP nanosheets.
[0071] Subsequently, the frozen dispersion undergoes
ultrasonication, and the BP nanosheets are exfoliated from the bulk
BP. The ultrasonic vibration of NMP ice between the layers also
facilitates the exfoliation process. The required total time is
less than 2 hours and the output power of the sonicator is less
than 70 W. Compared with the conventional liquid phase exfoliation
[8a, 8c, 10], both the processing time and the sonication power are
reduced in the method according to the invention. As a result, the
BP nanosheets obtained is a good quality, with larger lateral size
and less anomalous structural defects are obtained [8a, 11].
Furthermore, the few-layer BP nanosheets are obtained in good
yield. According to the ICP-AES analysis, 18.75 mg of few-layer BP
nanosheets were obtained from 25 mg of bulk BP with the yield of
75%, which is higher than the values reported in the art; see Table
1 below. The obtained BP nanosheets dispersion in IPA is brown and
is stable. Indeed, no aggregation or color change is observed
during storage for over four weeks (FIGS. 2a-2b).
TABLE-US-00001 TABLE 1 Few-layer BP nanosheets yield with different
exfoliation methods. Few- Sonication Bath Tip Sonicator layer Power
Time Power Time BP Reference (W) (h) (W) (h) yield ACS Nano, 2015,
9, 8869 70 13 26% Adv. Mater. 2016, 28, 380 20 30% 510 ACS Catal.
2016, 6, 8009 -- 8 15% J. Am. Chem. Soc. 2017, 10 4 20% 139, 13234
Angew. Chem. Int. Ed. 10 4 20% 2018, 57, 1 The invention 70 2
75%
[0072] To form the 2D-on-2D assembly, the g-C.sub.3N.sub.4 powder
was introduced into the BP dispersion (FIG. 2c). The large amount
of precipitate was soon observed at the bottom of the solution with
the supernatant turning to colorless and transparent after the
incubation at room temperature for 30 minutes (FIG. 2d), suggesting
the successful integration and coupling of BP nanosheets with
g-C.sub.3N.sub.4 nanosheets. FIG. 2e presents the zeta potentials
of BP and g-C.sub.3N.sub.4 in IPA, which are positive and negative,
respectively. A strong electrostatic interaction between them is
noted. This contributes to their integration.
[0073] Morphological and Structural Characterization
[0074] The morphologies of the as-prepared BP nanosheets were
characterized by TEM (FIG. 1b-1h). The typical TEM image of BP
nanosheets shows a lamellar morphology with the lateral size of 50
nm-3 .mu.m (FIG. 1b and FIGS. 3a-3d). Only the peaks of C, Cu and P
elements were observed in the EDX spectrum (FIG. 1c), indicating
that the pure BP without oxidation was obtained via the
ice-assisted exfoliation method. The BP nanosheets thickness
distribution was investigated using AFM height measurements (FIGS.
1d-f). Lines 1 and 2 in FIG. 1d are randomly selected and their
corresponding height profiles are displayed in FIG. 1e. Assuming
the thickness of monolayer BP is 0.53 nm [6a, 6b, 8d], the number
of layers of the generated BP nanosheets could be estimated from
the AFM height measurements. FIG. 1f shows the statistical
histogram of the number of BP layer distribution, which was
obtained from the height profiles of 150 randomly selected
individual BP nanosheets in AFM images. The mean number of layers
was determined to be <N>=5.9.+-.1.5, and about 93% of the
observed BP nanosheets have the thickness of less than 10 nm.
[0075] The g-C.sub.3N.sub.4 shows a free-standing graphene-like
wrinkled nanosheet structure (FIG. 4a). As displayed in FIGS.
4b-4d, the initial morphologies of BP and g-C.sub.3N.sub.4
nanosheets were not altered by their integration. The nanosheets
marked with arrows in FIG. 4d are supposed to be BP considering
their relatively regular edges, which are further corroborated by
the high-angle annular dark field (HAADF) scanning TEM (STEM) image
(FIG. 4e) and its corresponding STEM-EDX elemental mappings (FIGS.
4f-4i). The STEM-EDX mapping of C, N and P clearly confirms the
co-existence of g-C.sub.3N.sub.4 and BP, and evidently shows the
stacking of these two components. The high-resolution TEM (HRTEM)
image reveals lattice fringes of 0.34 nm and 0.26 nm, attributed to
the (021) and (040) planes of the BP crystals (FIG. 4j) [6e]. The
presence of C, N and P peaks indicates the successful preparation
of BP/g-C.sub.3N.sub.4 hybrid nanosheets with high purity and
without detectable oxidative degradation (FIG. 4g), which is
consistent with the STEM-EDX mapping results and is further
verified by the following XPS analysis.
[0076] The composition and the chemical states of the as-prepared
samples are assessed using XPS (FIG. 5). In the XPS survey spectra
of BP/g-C.sub.3N.sub.4 (FIG. 5a), only the peaks assigned to C, N,
O and P elements were observed, signifying the high purity of the
prepared samples and the successful integration of BP and
g-C.sub.3N.sub.4 nanosheets. As outlined above, 01s peak was
observed in the XPS spectrum of g-C.sub.3N.sub.4, which is
attributed to the 0 element in the adsorbed O.sub.2 or H.sub.2O on
the sample surface [13]. The similar atomic 0 percentages of
g-C.sub.3N.sub.4 (3.61%) and BP/g-C.sub.3N.sub.4 (3.59%)
illustrates that no further oxidation occurred in the preparation
of BP/g-C.sub.3N.sub.4 hybrid sample; see Table 2 below. In
addition, the concentration of BP in BP/g-C.sub.3N.sub.4 nanosheets
was detected to be 3.3% by XPS, which is quite close to that of
3.61% measured by ICP-AES and the nominal value of 3.75%.
TABLE-US-00002 TABLE 2 Atomic composition of g-C.sub.3N.sub.4 and
BP/g-C.sub.3N.sub.4 photocatalysts. C atom N atom O atom P atom
Sample (%) (%) (%) (%) g-C.sub.3N.sub.4 46.71 49.68 3.61 0
BP/g-C.sub.3N.sub.4 46.70 47.41 3.59 3.30
[0077] These results suggest the effective coupling between BP and
g-C.sub.3N.sub.4 nanosheets. To specify the bond formation in the
prepared BP/g-C.sub.3N.sub.4 sample, peak deconvolution was
performed for the C1 s, N1s and P2p XPS spectra (FIGS. 5b-5d). The
high-resolution C1s XPS spectrum presents two distinct peaks at
284.8 and 288.3 eV (FIG. 5b), which can be assigned to the
graphitic sp.sup.2 C.dbd.C bonds in the surface adventitious
carbonaceous environment and in the C--N aromatic heterocycles,
respectively [4c, 14]. The main N1s peak was deconvoluted into
three peaks (FIG. 5c), located at 398.6, 399.4 and 401.1 eV, which
are assigned to the sp.sup.2 hybridized N in triazine rings
(C.dbd.N--C), tertiary N (N--(C).sub.3) and amino group (C--N--H),
respectively [15]. As shown in FIG. 5d, the fitting result of P2p
spectrum shows two peaks at binding energies of 129.8 and 130.9 eV,
corresponding to P2p.sub.3/2 and P2p.sub.1/2, respectively. It is
worth noting that the peak in the range of 133.5-134.0 eV,
originating from oxidized P (P.sub.xO.sub.y) [7c, 7d, 16], was not
observed in the P2p XPS spectrum, indicating that P was not
oxidized during both the exfoliation of bulk BP to BP nanosheets
and the preparation of BP/g-C.sub.3N.sub.4 hybrid sample. The
time-efficient ice-assisted exfoliation method according to the
invention plays an important role in protecting BP from oxidation
by largely shortening the ultrasonication time and further reducing
the possibility of exposure to O.sub.2.
[0078] FIG. 6 shows the XRD patterns of bulk BP, exfoliated BP
nanosheets, g-C.sub.3N.sub.4 and BP/g-C.sub.3N.sub.4 samples. As
illustrated in FIG. 6a, the diffraction peaks shown in the patterns
of bulk BP and BP nanosheets can be indexed to the orthorhombic BP
with space group Cmca (64) according to the standard pattern of BP
(JCPDS No. 73-1358) [6d, 6f]. Furthermore, the low-angle peak
originated from the periodic stacking of layers exhibits a
downshift from 16.95.degree. of the BP bulk counterpart to
15.89.degree. of the exfoliated BP nanosheets, corresponding to the
inter-plane distance increasing from the 5.2 .ANG. to 5.6 .ANG.,
respectively (FIG. 6b). This result shows that intercalation of ice
crystals can enlarge the inter-planar spacing of BP, and further
benefit its exfoliation by reducing the interlayer Van der Waals
interactions. In the XRD pattern of g-C.sub.3N.sub.4, the two peaks
at 13.0.degree. and 27.4.degree. are ascribed to the in-planar
arrangement of the tri-s-triazine unit and the inter-planar
stacking of the conjugated aromatic system, respectively [2a, 4c,
12a, 15c, 17]. For the diffractogram of BP/g-C.sub.3N.sub.4 sample,
both the characteristic diffraction peaks of BP and
g-C.sub.3N.sub.4 were observed, explicitly confirming their
successful integration once again.
[0079] The optical properties of BP nanosheets in IPA,
g-C.sub.3N.sub.4 and BP/g-C.sub.3N.sub.4 nanosheets were
investigated as displayed in the UV-vis-NIR absorption spectra
(FIG. 6c). The BP nanosheets show a quite broad absorption band
from UV to NIR regions with the absorption edge of 910 nm,
corresponding to its bandgap of about 1.36 eV. The g-C.sub.3N.sub.4
exhibits a typical semiconductor-like absorption spectrum in the UV
and blue regions with the absorption edge of around 459 nm,
representing the bandgap of about 2.70 eV [2a, 12a]. For the
BP/g-C.sub.3N.sub.4 2 D-on-2D assembled nanosheet photocatalyst, in
addition to the absorption of g-C.sub.3N.sub.4, an enhanced tail
absorption in the visible and NIR regions was observed due to the
introduction of BP nanosheets. This can be propitious to the
visible light-driven photocatalytic water splitting for H.sub.2
production.
[0080] Photocatalytic H.sub.2 Evolution
[0081] The photocatalytic H.sub.2 production from water splitting
by BP, g-C.sub.3N.sub.4 and BP/g-C.sub.3N.sub.4 photocatalysts
under visible light irradiation and the stability measurement of
BP/g-C.sub.3N.sub.4 are shown in FIGS. 7a-7b. All the samples show
H.sub.2 evolution from water containing triethanolamine, which acts
as the sacrificial electron donor to quench the photoinduced holes
under visible light irradiation (.lamda.>420 nm). The
as-prepared BP/g-C.sub.3N.sub.4 photocatalyst exhibits much larger
H.sub.2 evolution amount (93.14 .mu.mol), compared to that of BP
(13.18 .mu.mol) and g-C.sub.3N.sub.4 samples (20.43 .mu.mol) after
24 hours of light irradiation.
[0082] As displayed in FIG. 7b, the highest H.sub.2 evolution rate
was achieved by BP/g-C.sub.3N.sub.4 (384.17 .mu.mol g.sup.-1
h.sup.-1), which is about 7 times and 4.5 times higher than that of
pure BP (54.88 .mu.mol g.sup.-1 h.sup.-1) and g-C.sub.3N.sub.4
(86.23 .mu.mol g.sup.-1 h.sup.-1). The fast recombination of
photo-generated charge carriers in BP and g-C.sub.3N.sub.4 is
probably responsible for their poorer activity. The integration of
g-C.sub.3N.sub.4 and BP nanosheets improved the visible light
photocatalytic activity in water splitting. The excited electrons
in conduction band (CB) of g-C.sub.3N.sub.4 can be transferred to
BP nanosheets and suppress the recombination of charge carriers in
g-C.sub.3N.sub.4, and further enhance the photocatalytic activity.
The H.sub.2 production rate obtained by BP/g-C.sub.3N.sub.4 is
comparable to or higher than that of the photocatalyst with the
loading of precious metal as co-catalyst reported in the art; see
Table 3 below.
TABLE-US-00003 TABLE 3 Photocatalytic H.sub.2 production rate under
visible light (.lamda. > 420 nm) irradiation. H.sub.2 evolution
rate (.mu.mol g.sup.-1 References Metal Catalysts h.sup.-1) Nat.
Mater. 2009, 8, 3 wt % Pt C.sub.3N.sub.4 106.94 76 Chem. Mater.
2015, 1 wt % Pt H.sub.2 treated g-C.sub.3N.sub.4 29.63 27, 4930 J.
Catal. 2016, 342, 55 1 wt % Pt g-C.sub.3N.sub.4 anatase/ 29.97
brookite TiO.sub.2 Appl. Catal., B 2016, 3 wt % Pt Br-modified
g-C.sub.3N.sub.4 960 196, 112 Adv. Mater. 2017, 3 wt % Pt
crystalline CN 1060 1700008 nanosheets Appl. Catal., B 2018, 3 wt %
Pt O-doped C.sub.3N.sub.4 732 224, 1 nanorods Science 2015, 347,
free CDots-C.sub.3N.sub.4 105 970 Angew. Chem. Int. Ed. free
BP/BiVO.sub.4 160 2018, 57, 6 The invention free
BP/g-C.sub.3N.sub.4 384.17
[0083] Furthermore, only about 2% decrease was observed in the
H.sub.2 evolution by the as-synthesized BP/g-C.sub.3N.sub.4
photocatalyst after 120 hours of visible light irradiation,
suggesting that it possesses good stability in water under light
illumination. The XPS spectra of BP and BP/g-C.sub.3N.sub.4 after
photocatalytic experiment were measured (FIG. 8). One additional
peak at about 134 eV, assigned to the oxidized P, was observed in
their P2p XPS spectra compared to the spectra before water
splitting, which accounts for 21.64% and 7.56% in the three peaks
of BP and BP/g-C.sub.3N.sub.4, respectively; see Table 4 below,
indicating that the introduction of g-C.sub.3N.sub.4 inhibits the
oxidation of BP. Though the P in BP/g-C.sub.3N.sub.4 was slightly
oxidized, the photocatalytic activity was not distinctively
affected. These results suggest that the as-prepared
BP/g-C.sub.3N.sub.4 is an economic, efficient and stable,
metal-free photocatalyst, without introducing any metal as
co-catalyst, for H.sub.2 evolution from water splitting under
visible light.
TABLE-US-00004 TABLE 4 The atomic composition of P1, P2, and P3 of
BP and BP/g-C.sub.3N.sub.4 photocatalysts in FIG. 8. P1 P2 P3
Sample (%) (%) (%) BP 21.64 21.18 57.19 BP/g-C.sub.3N.sub.4 7.56
39.86 52.57
[0084] PEC Measurements
[0085] The PEC properties of the as-prepared g-C.sub.3N.sub.4 and
BP/g-C.sub.3N.sub.4 samples were evaluated by electrochemical
impedance spectroscopy (EIS) and transient photocurrent responses
(FIGS. 7c-7d). Some useful information for the charge transfer
resistance can be shown in the high frequency region of Nyquist
plots. The decreased arc radii were exhibited in the EIS Nyquist
plots of BP/g-C.sub.3N.sub.4 compared to that of g-C.sub.3N.sub.4
both in the dark and under simulated solar light irradiation (FIG.
7c), suggesting that the introduction of BP leads to enhanced
electronic conductivity and thus increase the interfacial charge
transfer rate in BP/g-C.sub.3N.sub.4 sample [2d, 12a, 15c, 18].
[0086] To further verify the charge separation transfer
performance, the transient photocurrent responses for more than ten
light on-off cycles were measured under simulated solar light
irradiation (FIG. 7d). The photocurrent density rapidly increases
to a saturation value and remains constant once the light is
switched on, and immediately returns to nearly zero when the light
is turned off. The saturated photocurrent density of
BP/g-C.sub.3N.sub.4 (about 5.28 .mu.A cm.sup.-2) is about 4.8 times
higher than that of plain g-C.sub.3N.sub.4 photocatalysts (about
1.11 .mu.A cm.sup.-2). The increased photocurrent density shows
that the introduction of BP nanosheets can increase the mobility,
facilitate the separation or elongate the life time of the
photo-generated charge carriers [2d, 19], and/or enhance the
visible light absorption due to the narrower bandgap. Altogether
they contribute to the improved photocatalytic H.sub.2 evolution
rate of water splitting under visible light irradiation. It is
worth noting that almost no decrease in the photocurrent density
was observed after about 2000 s of the light on-off tests, which
shows that the as-synthesized g-C.sub.3N.sub.4 and
BP/g-C.sub.3N.sub.4 samples possesses good stability under light
irradiation.
[0087] UPS Measurement and Mechanism of Photocatalytic H.sub.2
Evolution
[0088] To better understand the nature of BP/g-C.sub.3N.sub.4 as an
efficient photocatalyst for H.sub.2 evolution, UPS measurements
were performed to determine the energy levels of BP and
g-C.sub.3N.sub.4 nanosheets (FIGS. 9a-9b). The intersections of the
extrapolated linear portion at high and low binding energies with
the baseline give the edges of the UPS spectra, from which the UPS
widths of BP and g-C.sub.3N.sub.4 are determined to be 15.99 eV and
14.95 eV, respectively [14a]. Then the VB energy (E.sub.VB) values
of BP and g-C.sub.3N.sub.4 are calculated to be 5.23 eV and 5.96
eV, respectively, by subtracting the width of the UPS spectra from
the excitation energy (21.22 eV). Combining with the measured
bandgap energy (E.sub.g) from the absorption spectra, the CB energy
values (E.sub.CB) of BP (3.87 eV) and g-C.sub.3N.sub.4 (3.26 eV)
are estimated from E.sub.CB=E.sub.VB-E.sub.g [7c, 14a]. These
values in eV are all converted to electrochemical energy potentials
in V according to the reference standard for which -4.44 eV vs.
vacuum level equals 0 V vs. reversible hydrogen electrode (RHE)
[14a], which are all displayed in FIG. 9c.
[0089] Being based on the UPS measurement results, the possible
mechanism for the largely enhanced photocatalytic activity in
H.sub.2 evolution of BP/g-C.sub.3N.sub.4 photocatalysts is
proposed. As schematically illustrated in FIG. 9c, the CB energy
level of BP is more negative than that of g-C.sub.3N.sub.4, and
both are more positive than the reduction potential of
H.sup.+/H.sub.2. In addition, the VB energy level of BP is higher
than that of g-C.sub.3N.sub.4. These properly positioned bands are
suitable for the transfer of charge carriers for water splitting,
corroborating the capability of BP/g-C.sub.3N.sub.4 as a metal-free
photocatalyst for H.sub.2 evolution. Under visible light
irradiation, mainly the electrons in the VB of g-C.sub.3N.sub.4 are
excited to its CB, leaving behind the positive-charged holes in the
VB. Afterwards, the excited electrons can be further transferred
into the CB of adjacent BP, suppressing the recombination of charge
carriers and promote the reduction of H.sub.2O to produce H.sub.2.
At the same time, the holes in the VB of g-C.sub.3N.sub.4 can be
immediately captured by the hole-sacrificial agent TEOA to generate
its oxide. In this process, BP plays a role as the electron sink to
inhibit the charge carriers recombination and leads to efficient
H.sub.2 evolution under visible light irradiation, which is in
agreement with the PEC measurement.
[0090] As will be understood by a skilled person, other allotropes
of BP may be used as starting materials. Such materials which
generally present a layered structure include but are not limited
to red phosphorous (RP) and violet phosphorous (VP).
[0091] As will be understood by a skilled person, other organic
solvents may be used in the ice-assisted process. Such solvents are
suitably selected such as not to allow for any oxidation to occur.
In particular, such solvents include but are not limited to
alcohols such as methanol, ethanol and isopropanol (IPA), diethyl
ether, chloroform, tetrahydrofuran, cyclohexane, toluene,
dimethylformamide and the like, and combinations thereof, in
addition to N-methyl-2-pyrrolidone (NMP).
[0092] As will be understood by a skilled person, other organic
solvents may be used for the purification of the nanosheets formed,
i.e., in the washing and re-dispersion steps. Such solvents are
suitable selected to allow dispersion of the formed nanosheets. The
solvent for these separations steps may be the same or different.
Such solvents are suitably selected such as not to allow for any
oxidation to occur. In particular, for example the solvents for the
washing step include but are not limited to other alcohols such as
methanol, ethanol, in addition to isopropanol (IPA); diethyl ether,
chloroform, tetrahydrofuran, cyclohexane, toluene,
dimethylformamide, and the like, and combinations thereof. And the
solvents for the re-dispersion step include but are not limited to
other alcohols such as methanol, ethanol, in addition to
isopropanol (IPA); diethyl ether, chloroform, tetrahydrofuran,
cyclohexane, toluene, dimethylformamide, N-methyl-2-pyrrolidone
(NMP), and the like, and combinations thereof. Accordingly, as will
be understood by a skilled person, the solvent used in the
purification step (washing and/or re-dispersion steps) may be the
same as the solvent used in the ice-assisted process.
[0093] As will be understood by a skilled person, any suitable 2D
material may be coupled with the few-layer phosphorous nanomaterial
according to the invention such as to obtain a photocatalyst. Such
material may be poly (methyl methacrylate), graphene or hexagonal
boron nitride which may be nitrogen-doped, molybdenum disulfide, a
carbon nitride nanomaterial, and the like, in addition to graphitic
carbon nitride (g-C.sub.3N.sub.4).
[0094] The scope of the claims should not be limited by the
preferred embodiments set forth in the examples, but should be
given the broadest interpretation consistent with the description
as a whole.
[0095] The present description refers to a number of documents, the
content of which is herein incorporated by reference in their
entirety.
REFERENCES
[0096] [1] a) A. Fujishima, K. Honda, Nature 1972, 238, 37-38; b)
K. Maeda, K. Teramura, D. L. Lu, T. Takata, N. Saito, Y. Inoue, K.
Domen, Nature 2006, 440, 295-295; c) A. Kudo, Y. Miseki, Chem. Soc.
Rev. 2009, 38, 253-278; d) Q. Zhang, D. Thrithamarassery
Gangadharan, Y. Liu, Z. Xu, M. Chaker, D. Ma, J. Materiomics 2017,
3, 33-50. [0097] [2] a) X. Wang, K. Maeda, A. Thomas, K. Takanabe,
G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2009,
8, 76-80; b) G. G. Zhang, Z. A. Lan, X. C. Wang, Angew. Chem. Int.
Ed. 2016, 55, 15712-15727; c) H. H. Ou, P. J. Yang, L. H. Lin, M.
Anpo, X. C. Wang, Angew. Chem. Int. Ed. 2017, 56, 10905-10910; d)
D. Zheng, X. N. Cao, X. Wang, Angew. Chem. Int. Ed. 2016, 55,
11512-11516. [0098] [3] J. S. Zhang, X. F. Chen, K. Takanabe, K.
Maeda, K. Domen, J. D. Epping, X. Z. Fu, M. Antonietti, X. C. Wang,
Angew. Chem. Int. Ed. 2010, 49, 441-444. [0099] [4] a) X. F. Chen,
J. S. Zhang, X. Z. Fu, M. Antonietti, X. C. Wang, J. Am. Chem. Soc.
2009, 131, 11658-11659; b) L. Sun, M. J. Yang, J. F. Huang, D. S.
Yu, W. Hong, X. D. Chen, Adv. Funct. Mater. 2016, 26, 4943-4950; c)
V. W.-h. Lau, I. Moudrakovski, T. Botari, S. Weinberger, M. B.
Mesch, V. Duppel, J. Senker, V. Blum, B. V. Lotsch, Nat. Commun.
2016, 7; d) W. J. Ong, L. L. Tan, Y. H. Ng, S. T. Yong, S. P. Chai,
Chem. Rev. 2016, 116, 7159-7329; e) J. Fu, J. Yu, C. Jiang, B.
Cheng, Adv. Energy Mater. 2017, 1701503. [0100] [5] a) Y. X. Deng,
Z. Luo, N. J. Conrad, H. Liu, Y. J. Gong, S. Najmaei, P. M. Ajayan,
J. Lou, X. F. Xu, P. D. Ye, ACS Nano 2014, 8, 8292-8299; b) L. Li,
Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, Y.
Zhang, Nat. Nanotechnol. 2014, 9, 372-377; c) H. Liu, A. T. Neal,
Z. Zhu, Z. Luo, X. F. Xu, D. Tomanek, P. D. Ye, ACS Nano 2014, 8,
4033-4041; d) E. S. Reich, Nature 2014, 506, 19; e) F. N. Xia, H.
Wang, D. Xiao, M. Dubey, A. Ramasubramaniam, Nat. Photonics 2014,
8, 899-907; f) X. Ling, H. Wang, S. X. Huang, F. N. Xia, M. S.
Dresselhaus, PNAS 2015, 112, 4523-4530; g) L. Z. Kou, C. F. Chen,
S. C. Smith, J. Phys. Chem. Lett. 2015, 6, 2794-2805; h) H. Liu, Y.
C. Du, Y. X. Deng, P. D. Ye, Chem. Soc. Rev. 2015, 44, 2732-2743;
i) C. R. Ryder, J. D. Wood, S. A. Wells, Y. Yang, D. Jariwala, T.
J. Marks, G. C. Schatz, M. C. Hersam, Nat. Chem. 2016, 8, 598-603.
[0101] [6] a) F. N. Xia, H. Wang, Y. C. Jia, Nat. Commun. 2014, 5,
4458; b) M. Buscema, D. J. Groenendijk, S. I. Blanter, G. A.
Steele, H. S. J. van der Zant, A. Castellanos-Gomez, Nano Lett.
2014, 14, 3347-3352; c) J. Sun, G. Y. Zheng, H. W. Lee, N. Liu, H.
T. Wang, H. B. Yao, W. S. Yang, Y. Cui, Nano Lett. 2014, 14,
4573-4580; d) H. Wang, X. Z. Yang, W. Shao, S. C. Chen, J. F. Xie,
X. D. Zhang, J. Wang, Y. Xie, J. Am. Chem. Soc. 2015, 137,
11376-11382; e) Z. Sun, H. Xie, S. Tang, X. F. Yu, Z. Guo, J. Shao,
H. Zhang, H. Huang, H. Wang, P. K. Chu, Angew. Chem. Int. Ed. 2015,
54, 11526-11530; f) X. Zhang, H. Xie, Z. Liu, C. Tan, Z. Luo, H.
Li, J. Lin, L. Sun, W. Chen, Z. Xu, L. Xie, W. Huang, H. Zhang,
Angew. Chem. Int. Ed. 2015, 54, 3653-3657; g) Y. Yang, J. Gao, Z.
Zhang, S. Xiao, H. H. Xie, Z. B. Sun, J. H. Wang, C. H. Zhou, Y. W.
Wang, X. Y. Guo, P. K. Chu, X. F. Yu, Adv. Mater. 2016, 28,
8937-8944. [0102] [7] a) M. Z. Rahman, C. W. Kwong, K. Davey, S. Z.
Qiao, Energy Environ. Sci. 2016, 9, 709-728; b) W. Y. Lei, T. T.
Zhang, P. Liu, J. A. Rodriguez, G. Liu, M. H. Liu, ACS Catal. 2016,
6, 8009-8020; c) M. Zhu, S. Kim, L. Mao, M. Fujitsuka, J. Zhang, X.
Wang, T. Majima, J. Am. Chem. Soc. 2017, 139, 13234-13242; d) M. S.
Zhu, X. Y. Cai, M. Fujitsuka, J. Y. Zhang, T. Majima, Angew. Chem.
Int. Ed. 2017, 56, 2064-2068; e) X. J. Zhu, T. M. Zhang, Z. J. Sun,
H. L. Chen, J. Guan, X. Chen, H. X. Ji, P. W. Du, S. F. Yang, Adv.
Mater. 2017, 29; f) W. Hu, L. Lin, R. Zhang, C. Yang, J. Yang, J.
Am. Chem. Soc. 2017, 139, 15429-15436. [0103] [8] a) A. H. Woomer,
T. W. Farnsworth, J. Hu, R. A. Wells, C. L. Donley, S. C. Warren,
ACS Nano 2015, 9, 8869-8884; b) A. Ziletti, A. Carvalho, D. K.
Campbell, D. F. Coker, A. H. C. Neto, Phys. Rev. Lett. 2015, 114,
046801; c) J. Kang, J. D. Wood, S. A. Wells, J. H. Lee, X. L. Liu,
K. S. Chen, M. C. Hersam, ACS Nano 2015, 9, 3596-3604; d) A.
Favron, E. Gaufres, F. Fossard, A. L. Phaneuf-L'Heureux, N. Y. W.
Tang, P. L. Levesque, A. Loiseau, R. Leonelli, S. Francoeur, R.
Martel, Nat. Mater. 2015, 14, 826-832; e) A. Hirsch, F. Hauke,
Angew. Chem. Int. Ed. 2017, 57, 4338-4354. [0104] [9] a) J. D.
Wood, S. A. Wells, D. Jariwala, K. S. Chen, E. Cho, V. K. Sangwan,
X. L. Liu, L. J. Lauhon, T. J. Marks, M. C. Hersam, Nano Lett.
2014, 14, 6964-6970; b) R. A. Doganov, E. C. T. O'Farrell, S. P.
Koenig, Y. T. Yeo, A. Ziletti, A. Carvalho, D. K. Campbell, D. F.
Coker, K. Watanabe, T. Taniguchi, A. H. C. Neto, B. Ozyilmaz, Nat.
Commun. 2015, 6; c) W. N. Zhu, M. N. Yogeesh, S. X. Yang, S. H.
Aldave, J. S. Kim, S. Sonde, L. Tao, N. S. Lu, D. Akinwande, Nano
Lett. 2015, 15, 1883-1890; d) Y. T. Zhao, H. Y. Wang, H. Huang, Q.
L. Xiao, Y. H. Xu, Z. N. Guo, H. H. Xie, J. D. Shao, Z. B. Sun, W.
J. Han, X. F. Yu, P. H. Li, P. K. Chu, Angew. Chem. Int. Ed. 2016,
55, 5003-5007. [0105] [10] a) J. R. Brent, N. Savjani, E. A. Lewis,
S. J. Haigh, D. J. Lewis, P. O'Brien, Chem. Commun. 2014, 50,
13338-13341; b) P. Yasaei, B. Kumar, T. Foroozan, C. H. Wang, M.
Asadi, D. Tuschel, J. E. Indacochea, R. F. Klie, A. Salehi-Khojin,
Adv. Mater. 2015, 27, 1887-1892; c) L. Chen, G. M. Zhou, Z. B. Liu,
X. M. Ma, J. Chen, Z. Y. Zhang, X. L. Ma, F. Li, H. M. Cheng, W. C.
Ren, Adv. Mater. 2016, 28, 510-517. [0106] [11] M. Batmunkh, C. J.
Shearer, M. J. Biggs, J. G. Shapter, J. Mater. Chem. A 2016, 4,
2605-2616. [0107] [12] a) Q. Zhang, J. Deng, Z. Xu, M. Chaker, D.
Ma, ACS Catal. 2017, 7, 6225-6234; b) Z. Xu, M. G. Kibria, B.
AlOtaibi, P. N. Duchesne, L. V. Besteiro, Y. Gao, Q. Zhang, Z. Mi,
P. Zhang, A. O. Govorov, L. Mai, M. Chaker, D. Ma, Appl. Catal., B
2018, 221, 77-85. [0108] [13] a) F. Dong, Z. W. Zhao, T. Xiong, Z.
L. Ni, W. D. Zhang, Y. J. Sun, W. K. Ho, ACS Appl. Mater.
Interfaces 2013, 5, 11392-11401; b) Y. Q. Cao, Z. Z. Zhang, J. L.
Long, J. Liang, H. Lin, H. X. Lin, X. X. Wang, J. Mater. Chem. A
2014, 2, 17797-17807. [0109] [14] a) J. Liu, Y. Liu, N. Y. Liu, Y.
Z. Han, X. Zhang, H. Huang, Y. Lifshitz, S. T. Lee, J. Zhong, Z. H.
Kang, Science 2015, 347, 970-974; b) H. J. Kong, D. H. Won, J. Kim,
S. I. Woo, Chem. Mater. 2016, 28, 1318-1324. [0110] [15] a) C. Ye,
J.-X. Li, Z.-J. Li, X.-B. Li, X.-B. Fan, L.-P. Zhang, B. Chen,
C.-H. Tung, L.-Z. Wu, ACS Catal. 2015, 5, 6973-6979; b) J. Q.
Zhang, X. H. An, N. Lin, W. T. Wu, L. Z. Wang, Z. T. Li, R. Q.
Wang, Y. Wang, J. X. Liu, M. B. Wu, Carbon 2016, 100, 450-455; c)
G. Peng, L. Xing, J. Barrio, M. Volokh, M. Shalom, Angew. Chem.
Int. Ed. 2017, 56, 1-7; d) H. J. Yu, R. Shi, Y. X. Zhao, T. Bian,
Y. F. Zhao, C. Zhou, G. I. N. Waterhouse, L. Z. Wu, C. H. Tung, T.
R. Zhang, Adv. Mater. 2017, 29, 1605148. [0111] [16] M. Zhu, Z.
Sun, M. Fujitsuka, T. Majima, Angew. Chem. Int. Ed. 2018, 57, 1-6.
[0112] [17] a) D. J. Martin, P. J. T. Reardon, S. J. A. Moniz, J.
W. Tang, J. Am. Chem. Soc. 2014, 136, 12568-12571; b) Q. Han, B.
Wang, J. Gao, Z. Cheng, Y. Zhao, Z. Zhang, L. Qu, ACS Nano 2016,
10, 2745-2751. [0113] [18[ M. X. Li, W. J. Luo, D. P. Cao, X. Zhao,
Z. S. Li, T. Yu, Z. G. Zou, Angew. Chem. Int. Ed. 2013, 52,
11016-11020. [0114] [19] D. Shi, R. Zheng, M. J. Sun, X. Cao, C. X.
Sun, C. J. Cui, C. S. Liu, J. Zhao, M. Du, Angew. Chem. Int. Ed.
2017, 56, 14637-14641.
* * * * *