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.

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

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 μm; (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 (λ>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° 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° C. min.sup.−1 to 250, 350, and 550° 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° 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° 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 Kα 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° 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 μm (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═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═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° of the BP bulk counterpart to 15.89° of the exfoliated BP nanosheets, corresponding to the inter-plane distance increasing from the 5.2 Å to 5.6 Å, 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° and 27.4° 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 (λ>420 nm). The as-prepared BP/g-C.sub.3N.sub.4 photocatalyst exhibits much larger H.sub.2 evolution amount (93.14 μmol), compared to that of BP (13.18 μmol) and g-C.sub.3N.sub.4 samples (20.43 μmol) 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 μmol g.sup.−1 h.sup.−1), which is about 7 times and 4.5 times higher than that of pure BP (54.88 μmol g.sup.−1 h.sup.−1) and g-C.sub.3N.sub.4 (86.23 μmol 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 (λ > 420 nm) irradiation. H.sub.2 evolution rate (μmol 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 μA cm.sup.−2) is about 4.8 times higher than that of plain g-C.sub.3N.sub.4 photocatalysts (about 1.11 μA 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.

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