Synthesis of Black Phosphorene/P-Rich Transition Metal Phosphide NiP₃ Heterostructure and Its Effect on the Stabilization of Black Phosphorene

Abstract

Black phosphorus (BP), as a direct band gap semiconductor material with a two-dimensional layered structure, has a good application potential in many aspects, but the surface state of it is extremely unstable, especially that of single-layer black phosphorus. In this study, BP crystals and two-dimensional black phosphorus (2D BP) are prepared by a mechanical ball-milling–liquid-phase exfoliation method. The X-ray diffraction (XRD) spectrum and high-resolution transmission electron microscopy (HRTEM) results showed that red phosphorus (RP) successfully turned to BP by the mechanical ball-milling method. The spectrophotometric analysis has detected absorption peaks at 780 nm, 915 nm, and 1016 nm, corresponding to single, double, and three-layer BP bandgap emission. A simple solvothermal strategy is designed to synthesize in-plane BP/P-rich transition metal phosphide (TMP) heterostructures (BP/NiP₃) by defect/edge-selective growth of NiP₃ on the BP nanosheets. HRTEM analysis indicates that the metal ions are preferentially deposited on the defects of 2D BP such as edges and unsaturated sites, forming a 2D BP/NiP₃ in-plane heterojunction.

1. Introduction

Two-dimensional (2D) nanomaterials have attracted great attention in the fields of material science, condensed matter physics, and nanotechnology since the first mechanical stripping process of graphene.

Black phosphorus (BP) has shown excellent electrical and optical properties as a direct band gap semiconductor material with a 2D layered structure in recent years [1,2]. It is not only used in field effect transistors [3,4] and other fields but also due to the large surface area, long charge-carrier diffusion path, high carrier mobility, etc. BP nanosheets have also been used for water electrolysis to generate hydrogen and potential electrocatalysts for oxygen [5,6]. Additionally, because of its large surface area and theoretical capacity, phosphorene is a good candidate for electrochemical energy storage devices, such as batteries and supercapacitors [7,8]. With the development of liquid-phase exfoliation technology, monodisperse BP nanosheets have been synthesized for lithium-ion battery, photovoltaic, sensing, and catalysis research fields [9]. Moreover, the application of black phosphorene in solar cells [10,11,12,13,14] and the study of the surface stability of black phosphorene have become a hot topic.

Black phosphorene has good application potential in many aspects, but the surface state of black phosphorene is extremely unstable, especially monolayer black phosphorene. The thinner the monolayer black phosphorene is, the lower the stability is, and it is easily degraded in the natural environment, which severely limits its future application in the industrial field [15,16,17]. There are several methods for preparing 2D black phosphorus, including mechanical stripping, the liquid-phase exfoliation method, and the pulsed laser deposition method. Among them, liquid-phase stripping is a commonly used method with high yield and low equipment requirements. How to improve the stability of BP is one of the challenges in BP research. Most believe that the reason for the instability of black phosphorene is that the water and oxygen in the atmosphere react with the surface atoms of the black phosphorene, converting the phosphorus into phosphoric acid and causing the material to be corroded [18,19,20]. However, Sang, et al. [21] mentioned that visible light and oxygen are the main contributing factors to the phosphorene degradation and, therefore, the reaction can be hampered by limiting either light or oxygen. The photoenergy hastens the degradation process of phosphorene to form PxOy on the surface, and the oxygen concentration, energy gap, and light intensity influence the limiting step in the oxidation rate. Nevertheless, the water and phosphorene surface interact weakly. Recently, Baboukani, et al. [22] conducted a TEM investigation on bipolar electrochemical exfoliated phosphorene nanosheets and obtained direct evidence of point defects, grain boundaries, and amorphous regions on the phosphorene. Moreover, density function theory (DFT) calculations reveal that the amorphization occurs because of surface oxidation of phosphorene. They found that the oxygen affinity of the vacancy in phosphorene is much stronger than the perfect phosphorene lattice site because, upon exposure of an oxygen molecule to a vacancy in phosphorene, the O-O bond dissociates, and the resulting oxygen atoms react with P atoms. So, they conclude that phosphorene’s fast degradation owing to oxidization is more likely to happen at edges and grain boundaries where vacancies tend to form.

Although black phosphorene has unique and excellent properties, its surface instability does bring obstacles to its development. Whether this difficulty can be overcome will become a key issue for its wide application. Therefore, monolayer black phosphorene needs to be prepared in the absence of water and oxygen, or the reaction will disappear immediately. In terms of coating protection, researchers [23,24,25] used ALD deposition technology to obtain Al₂O₃ film, which was coated on the surface of black phosphorene. The surface did not react after being placed at room temperature for more than 1 week, and its electron mobility did not significantly attenuate. In this way, although the stability is improved, after BP coating the dielectric layer, the impurity charge state will inevitably appear, decreasing electron mobility. Luo Zhong Zhen et al. [26] synthesized a BP-based 0D–2D heterostructure containing Ni₂P nanoparticles homogeneously embedded in the BP nanosheets, which was prepared for Li storage and acidic hydrogen evolution, and found that it has excellent performance compared with the bare BP. Yu Xue Feng et al. [27], by controlling the cobalt atoms on the BP unsaturated sites with selective deposition, found BP/cobalt in-plane heterojunction phosphide, optimization of BP defects at the same time, improvement in the stability, provision of more active electric catalytic sites, better BP/Co₂P nanopiece in hydrogen evolution and oxygen evolution reaction, and more stable electric catalytic activity. However, the synthesis of in-plane BP/NiP₃ heterostructures has yet to be reported.

Transition metal phosphide (TMP) has attracted much attention due to its abundant active sites, unique physical and chemical properties, and adjustable composition structure [28,29]. Phosphorus-rich compounds are semiconductors and are less stable than metal-rich compounds. NiP₃, rich in P element, is an Im$\overline{3}$ cubic crystal system, with many P-P bonds and forms multiple phosphates. A slight change in the stoichiometric ratio of metal to phosphorus can significantly change its structure and thus show different physicochemical properties.

Recently, researchers also found that phosphate-rich metallic phosphates such as transition metal diphosphates are considered one of the ideal materials to replace platinum (Pt)-based noble metal catalysts in the future due to their high electrical conductivity, electrocatalytic activity of hydrogen evolution, and electrochemical stability [30]. J. Fullenwarth et al. [31] prepared NiP₃ by ball-milling and ceramic routes. They found the NiP₃ electrode possesses a very promising capacity with a reversible storage capacity higher than 1000 mA h/g after 50 cycles for LiB and 900 mA h/g after 15 cycles for NaB, which represents one of the highest capacities ever sustained in Na-ion batteries.

This work, starting from improving the phase stability of 2D BP, presents a simple construction strategy of BP nanosheets and P-rich TMP binding heterojunctions (BP/NiP₃). Creating heterogeneous structures of BP nanosheets and TMP nanoparticles promises to bring some beneficial characteristics, which will not only provide an idea to improve the performance of BP by fabricating heterojunctions at defects but also makes new progress in the field of phase stabilization of 2D BP. It will likely benefit future experimental and theoretical efforts on applications of phosphorene nanosheets.

2. Materials and Methods

2.1. Preparation of Samples

2.1.1. Preparation of BP

First, the BP was synthesized from red phosphorus (RP) by a high-energy mechanical milling method in a ball-mill apparatus. Twenty grams of RP was used as the starting material. Red phosphorus (RP) is an allotrope of black phosphorus, purplish red or brownish amorphous powder, stable at room temperature. The preparation process was carried out in a stainless-steel container with 20 stainless steel balls (14.4 mm in diameter and 13 g in weight) in an Ar atmosphere and rotated for 3.5 h at a speed of 400 rpm. The container was kept in the ball-milling machine for 2 days to cool, and then the lid was opened in a glove box. The BP was soaked in petroleum ether to preserve.

2.1.2. Preparation of Black Phosphorene

The black phosphorene was prepared by liquid-phase exfoliation of BP single crystals. The bulk BP crystals were exfoliated into nanosheets in N, N-dimethylformamide (DMF, ≥99.5%) under sonication. Then, 10 mg BP was placed in 100 mL DMF medium and placed in an ultrasonic cell fragmentation instrument. The ultrasonic power was adjusted to 400 W, and the ultrasonic time was set to 4 h (2 s of ultrasound, pause for 4 s). The ultrasonic liquid was put into a centrifuge and centrifuged at a centrifuge rate of 6000 r/min for 30 min. When the supernatant was centrifuged for a second time, the centrifugation rate was 11,000 r/min, and the centrifugation time was 20 min. The supernatant was taken again, and black phosphoene was distributed in this supernatant.

2.1.3. Deposition of TMPs on the 2D BP

The heterostructure is synthesized by using BP nanosheets as a precursor (exfoliated from the BP crystals) to react with NiCl₂ × 6H₂O, resulting in NiP₃ nanocrystals (NCs) homogeneously embedded in the BP nanosheets.

First, 10 mg of nickel chloride hexahydrate (NiCl₂ × 6H₂O, ≥98%) powder is poured into a beaker, then 10 mL of DMF is used to dissolve it, then it is poured into 9 ml of centrifuged black phosphorene DMF solution, the sample is shaken to mix well, and then poured into a two-necked bottle. Because the reaction needs to be carried out at high temperature, a condenser should be installed on the two-necked bottle, and the reaction process should be placed in an argon gas environment to prevent the reaction with oxygen molecules and water molecules in the air. The whole reaction device is placed on a magnetic stirrer and stirred evenly during the reaction with constant heating. The temperature is set to 180 °C, the speed of the stirrer is 280 r/min. When the temperature reaches 150 °C, the reaction is timed for 4 h. During the reaction, the color of the sample changes from light blue to dark blue. After natural cooling, the sample is placed in a centrifuge tube, and the resulting solution is subjected to a third centrifugal treatment at a centrifugal rate of 6000 r/min and a centrifugation time of 20 min.

2.2. Measurement

The phase composition was analyzed by an X-ray diffractometer (Empyrean, Panalytical, Alemlo, Holland) using copper Ka radiation, 30 kV, 30 mA.

The solution obtained by the liquid-phase exfoliation method was subjected to ultraviolet–visible light (UV, Shanghai Youke Instrument Co., Ltd., Shanghai, China) light absorption analysis, and the wavelength detection range was 400–1100 nm.

The morphology of the BP was characterized using scanning electron microscopy (SEM, SU-8010, Hitachi, Ibaraki Prefecture, Japan). The interplanar spacing and orientation of the BP were characterized by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Tokyo, Japan) with a point resolution of 0.19 nm, lattice resolution of 0.14 nm, and an operating voltage of 120 kV. The BP was diluted by absolute alcohol and dropped on a carbon film supported by a copper grid.

3. Results

3.1. XRD and Optical Absorption Analysis

Figure 1 shows the X-ray diffraction (XRD) spectrum of amorphous red phosphorus and the BP prepared by the ball-milling method. It indicates that the BP has been successfully crystallized from amorphous RP when the ratio of ball to material is 13:1, rotated for 3.5 h at a speed of 400 rpm. From the XRD peak spectrum, it was also found that the diffraction peak is wide; there is a small part of amorphous RP mixed with BP.

Then, the BP/NiP₃ nanosheets prepared by liquid-phase exfoliation–solvothermal method were also analyzed by XRD. The diffraction peaks of BP and NiP₃ were not detected. It may be because the BP/NiP₃ nanosheets are too thin and penetrated by the X-ray during XRD analysis, so the diffraction peaks of the BP or NiP₃ cannot be obtained. Therefore, we need to use other detection methods to determine the phase. Therefore, we subsequently adopted other detection methods, such as HRTEM, to determine the composition of phase.

It is known that the BP atomic layers have a thickness-dependent direct bandgap. The direct bandgap of bulk BP is 0.3 eV, and the bandgaps of 1–5-layer BP are 1.51 ev (1 layer), 1.38 eV (2 layers), 1.23 eV (3 layers), 1.05 eV (4 layers), and 0.85 eV (5 layers) [32,33,34,35]. Therefore, it can be concluded that the absorption peak corresponding to one layer of 2D BP is λ = 821 nm, the absorption peak corresponding to two-layer BP is λ = 898 nm, that of three-layer phosphorus is λ = 1008 nm, that of four-layer phosphorus is λ = 1180 nm, and that of five-layer phosphorus is λ = 1458 nm.

Figure 2 shows the UV–Vis absorption spectrum of BP nanosheet solution prepared by the liquid-phase exfoliation method. We found the spectra absorption peaks at 780 nm, 915 nm, and 1016 nm, respectively. According to Eg = 1240/λ, the value of the bandgap can be calculated. It was estimated to be 1.58 eV, 1.36 eV, and 1.22 eV, respectively; these peaks roughly correspond to the bandgap edge emission from single, double, and three-layer BP. This shows that a single-layer material has basically been formed. Because the wavelength of the ultraviolet–visible light tester is only in the range from 400–1100 nm, we did not observe the absorption peak of BP in more than three layers, including the bulk BP.

A 2D BP nanosheet cannot be stably maintained in the air for a long time and can only be preserved for a few hours or days before being oxidized. Therefore, some passivation encapsulation techniques are adopted to solve the stability problem of 2D BP nanosheets. Therefore, we use the solvothermal method to deposit the TMP heterojunction on the 2D BP nanosheet.

3.2. SEM and HRTEM Analysis

The BP/Ni²⁺ nanosheet solution was deposited on the defect of the 2D BP nanosheet by the solvothermal method. Figure 3 shows the SEM morphology of the BP nanosheets suspended in liquid at different magnifications. The samples analyzed by SEM were obtained by drop-casting the nanosheets suspended in liquid on a Si substrate. In Figure 3, the randomly distributed dark regular geometric shapes are BP/Ni²⁺ nanosheets which indicate the 2D nanosheets are broken into multiple pieces after ultrasonication. For the 2D BP nanosheet, in addition to its layered structure, its size is also a factor reflecting its quality, which determines its practical application possibility in 2D materials. As shown in Figure 3a, the size of 2D BP is about 15 μm × 5 μm and it is distributed uniformly. And when zooming in further, it can be seen from Figure 3b that the dark BP/Ni²⁺ nanosheets also have subtle bright cracks at a higher magnification. They were split into about 2 μm pieces. The results show that suiTable 2D BP has been prepared by liquid-phase exfoliation of DMF.

The morphological and structural details of the 2D BP/NiP₃ heterostructure nanosheets were analyzed by HRTEM with selective area electron diffraction (SAED). The nanosheet is ultrathin as revealed by TEM in Figure 4a. It can be seen from the figure that there are some black spheres scattered on the amorphous matrix with light gray contrast, among which the light gray matrix is the amorphous black phosphoene under TEM irradiation. At the same time, we also took many HRTEM images of BP crystals that were not destroyed by electron irradiation on the matrix at the initial stage of TEM observation. Figure 4b illustrates the d-spacing and most of the crystals measured 2.41 Å or 1.60 Å, the lattice plane indices corresponded to BP {111} and {151}, respectively, and the insets of electron diffraction patterns in the lower right correspond to the characteristic diffraction peak of BP, such as {111}, {021}, and {151}, which were consistent with the values reported for JCPDS card No. 73-1358. HRTEM results show that the crystal size of most of the BP samples is about 5~6 nm in diameter.

Figure 4c shows an HRTEM image of the particle in Figure 4a, with SAED. We can see the lattice fringes and the diffraction patterns of particles in the samples. First, the d-spacing and the angle between every two clusters of the crystal plane were detected. The d-spacings of the crystal were measured as 0.314 nm, 0.318 nm, and 0.367 nm. The angle between every two clusters of crossed fringes was ~50° and ~65° (SAED embedded in the lower right). According to the d-spacing and the angle between crystal planes, the corresponding lattice plane indices of the crystal plane were deduced, and the phase was determined. The lattice plane indices of the crystal correspond to cubic NiP₃ (21$\overline{1}$), (2$\overline{1}\overline{1}$), and (020) (the theoretical angle between them is 48.2° and 65.9°), when the viewing direction is [102]. It is consistent with the values results reported in the JCPDS card (No 73-1242).

Figure 4d shows the HRTEM image of a BP/NiP₃ nanosheet at another position. For the particle in the top right corner (with SAED pattern), the d-spacings were measured as 0.383 nm and 0.237 nm, the angle between the two clusters of crossed fringes was ~91° (SAED embedded in the lower right). It is in good agreement with the (020) and (30$\overline{1}$) lattice of NiP₃ (theoretically 90°), when the viewing direction is [103]. As can be seen from the figure, some another crystal fringes appear at the edge of the large slice; the average d-spacing is 0.303 nm. According to the above analysis results and the crystal plane spacing, it corresponds to the {211} interplanar of the NiP₃.

Figure 4e is also an HRTEM image of the edge of the nanosheet. For the particles in the edges (with SAED pattern), The d-spacing of most of the grain was about 0.278 nm. It corresponded to the {220} interplanar of the NiP₃. Additionally, it can be seen from the figure that there are many edge grains and lattice distortions. This may be because the single-layer grain of the two-dimensional material easily deformed under specific forces. Figure 4d,e both show that NiP₃ nanocrystals selectively grow on the edge of BP nanosheets to form BP/NiP₃ in-plane heterostructures due to the reactive edge defect of BP nanosheets.

4. Discussion

Compared with bulk black phosphorus in the previous research literature [36] of our research group, phosphorene is challenging to detect by XRD and TEM due to the few layers of the structure. The UV–Vis absorption spectrum of the BP nanosheet solution displayed spectra absorption peaks at 780 nm, 915 nm, and 1016 nm, respectively. According to Eg = 1240/λ, the bandgap value was estimated to be 1.58 eV, 1.36 eV, and 1.22 eV, respectively, corresponding to the bandgap edge emission from single, double, and three-layer BP. This shows that a few layers of material have been prepared.

The degradation mechanism of black phosphene has been studied in the following aspects. Chief among them is degradation caused by defects and edge effects. The chemical instability of phosphorene is considered to be influenced by the lone pair of electrons in phosphorus atoms [21]. Phosphorene fabricated by mechanical exfoliation is prone to degradation in ambient environments due to the presence of humid air, and this demonstrated that oxygen, water, and photon energy (light) are needed concurrently for the degradation process to take place on the surface of phosphorene [37]. Wang et al. [38] found that the BP degradation process involves three steps: first, the formation of unstable oxide with visible illumination; second, detachment of unstable oxide; third, separation of unstable oxide under water activity. Researchers found the edges of phosphorene flakes are also prone to degradation because of the edge-induced mechanism, which is instigated by moisture and the presence of impurity traces at the edges of the flake, and the high affinity of moist air by dangling bonds on the edges leads to a high degradation rate. Two-dimensional materials inevitably incorporate different structural defects such as point defects, grain boundaries (GBs), impurities, and dislocations [21]. For the effect of the defects, Baboukani et al. [22] deduced that the oxygen affinity of the vacancy in phosphorene is much stronger than the perfect phosphorene lattice site. Upon exposure of an oxygen molecule to a vacancy in phosphorene, the O-O bond dissociates, and the resulting oxygen atoms react with P atoms. DFT calculations reveal that the amorphization occurs because of surface oxidation of phosphorene.

Combined with the above research, phosphorene’s fast degradation due to oxidization is more likely to happen at edges and grain defects where vacancies easily form. The reason why nano-BP has many defects is that, when preparing the BP nanosheets by the liquid-phase exfoliation method, bulk BP is gradually peeled off after a long ultrasonic treatment, resulting in edge and surface defects. These defects lead to phase instability as degradation of BP nanosheets and destroy the conductivity, electrochemical activity, and other properties. We can see from Figure 4 that the NiP₃ crystals were distributed on the edge of the BP nanosheet. It is an original defect of the BP nanosheet. It corresponded well with a TEM image [22] which shows the oxidation and amorphization on the edges of the phosphorene nanosheet, which is likely related to the presence of phosphorus oxide along the edge of the nanosheets. It occupies the defects of BP nanosheets, enhancing their stability. Additionally, it provides more active electrocatalytic sites and improves electrical conductivity, therefore BP/NiP₃ nanosheets show more stable electrocatalytic activity in hydrogen evolution and oxygen evolution reactions.

Also, we found that part of the BP in the central area is amorphous. This is because 2D BP crystals become amorphous under TEM electron irradiation even at a low voltage of 120 V. However, the edge remained crystalline, which may be due to the deposition of NiP₃ in the defects of BP, which played a stabilizing role. Therefore, the formation of BP/NiP₃ heterojunctions also plays a good role in the phase stabilization of BP. NiP₃ NCs maintain a good crystalline state in this heterostructure, with an average diameter of about 4–16 nm. Additionally, transition metal phosphide has attracted much attention due to its excellent properties and adjustable composition structure, including NiP₃ [31].

The black phosphorene/P-rich transition metal phosphide NiP₃ heterostructure nanosheets from our study will likely benefit future experimental and theoretical efforts to study phosphorene and transition metal phosphide nanosheets.

5. Conclusions

The XRD and HRTEM results showed that RP successfully turned into BP by the mechanical ball-milling method. The UV–Vis absorption spectra displayed absorption peaks at 780 nm, 915 nm, and 1016 nm, respectively, corresponding to single, double, and three-layer BP bandgap emission. This indicates that a few layers of BP nanosheets have been prepared by liquid-phase exfoliation of the bulk BP.

A simple solvothermal strategy is designed to synthesize in-plane black phosphorus/P-rich TMP (BP/NiP₃) heterostructures by defect/edge-selective growth of NiP₃ on the BP nanosheets. The HRTEM and SAED results showed that P-rich TMP NiP₃ formed and was deposited on the BP matrix. Nickel ions are preferentially deposited on the edges and unsaturated sites of 2D BP, forming a 2D BP/NiP₃ in-plane heterojunction, which plays a role in stabilizing the BP phase.