Black Phosphorus Synthesis Path Strongly Influences Its

Article Cite This: ACS Appl. Energy Mater. 2018, 1, 503−509

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Black Phosphorus Synthesis Path Strongly Influences Its Delamination, Chemical Properties and Electrochemical Performance Rui Gusmaõ ,† Zdeněk Sofer,‡ Daniel Bouša,‡ and Martin Pumera*,† †

Division of Chemistry & Biological Chemistry, School of Physical Mathematical Science, Nanyang Technological University, 637371, Singapore ‡ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: We show here that synthetic approaches to layered black phosphorus (BP), namely, high-pressure conversion of red phosphorus (RP) and vapor growth of BP from RP in the presence of Sn/SnI4, lead to dramatically different crystallinity of BP which in turn strongly influences its exfoliation, chemical and electrochemical properties. In particular, we demonstrate that black phosphorus (BP) shear exfoliation in aqueous surfactant media can be used to obtain well-defined and few-layer BP nanosheets and that this depends on the crystallinity of BP. The process can proceed without the need to use glovebox or purging solutions. BP consisting of large well-developed crystals obtained by vapor growth methods and containing trace amounts of Sn is easily exfoliated. On other hand, highly pure nanocrystalline BP obtained by the high-pressure method could not be extensively exfoliated by shear mixing. Starting material BP crystallinity furthermore influences its counterpart shear exfoliated BP surface chemistry and electrochemical performance. KEYWORDS: black phosphorus, pnictogen, layered material, shear exfoliation, hydrogen evolution reaction

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Au.15 Later, the method was further improved by using only Sn and SnI4 for conversion of red P to BP at a high temperature.16 Since the electronic properties can be strongly influenced by unintentional doping during crystal growth, the use of highpurity BP prepared by high pressure can bring new and interesting properties to exfoliated BP. Shear exfoliated nanosheets can be produced using both dedicated rotor/stator high-shear mixers (laminar regime) and kitchen blenders (turbulent regime).17−19 Shear force mixing has been reported to exfoliate BP and other pnictogens20 in different purged organic solvents with diverse solvent surface tension (Table S1).17,21,22 Methods can have a straightforward 30 min shear exfoliation time22 or an intricate and alternate combination of shear mixing and sonication up to 8 h (Table S1).23 Results have varied from few-layer nanosheets24 to quantum dots (BPQDs).17 Kitchen blenders can be used when exfoliation is performed in aqueous surfactants, which makes this a low-cost, accessible, and scalable method. Nevertheless, many studies perform shear exfoliation in organic media, known to severely attack polymeric components of household kitchen blenders (Table S1). Aqueous and surfactant assisted exfoliation of BP crystals has been obtained using sonication25

INTRODUCTION The bulk form of black phosphorus (BP) was first synthesized more than a century ago,1 not receiving much attention until it was very recently rediscovered in 2014.2 BP then became a newcomer to the wave of 2D layered materials,3,4 and others from the pnictogens group are now also emerging in the field.5 Thermodynamically stable at room temperature, orthorhombic BP is composed of puckered layers built of P6 rings in a chair conformation (Scheme S1).6 Similar to graphite, multilayer orthorhombic BP is made of vertically stacked sheets of graphene/phosphorene held together by weak van der Waals forces in-between layers.2 BP has been shown to be a 2D layered material with applications in the fabrication of humidity, vapor, and gas sensor devices, and photodetection.7 BP has also been reported as a potential alternative nanomaterial in materials for water splitting, batteries, transistors, and photovoltaic solar cells, among others.8−11 BP can be exfoliated to a single sheet structure with tunable direct band, semiconducting property, and high carrier mobility at room temperature, and a highly anisotropic layered structure.12 Exfoliated BP also belongs to a group of materials known as topological insulators.13,14 Most BP crystals used in liquid phase exfoliation across the literature originate primarily from vapor growth methods (Table S1). These methods are based on vapor growth from red P using Sn and its alloys with © 2018 American Chemical Society

Received: November 7, 2017 Accepted: January 17, 2018 Published: January 27, 2018 503

DOI: 10.1021/acsaem.7b00106 ACS Appl. Energy Mater. 2018, 1, 503−509

Article

ACS Applied Energy Materials

Figure 1. Morphological characterization of BPbulk and BPSE. Top row corresponds to results for BPhP, and bottom row corresponds to results for BPVG. SEM images of the starting materials BPhPbulk (a, b) and BPVGbulk (e, f). STEM images of the shear exfoliated BPhPSE (c, d) and BPVGSE (g, h). Scale bars in the images represent 10 and 1 μm.

Figure 2. Structural and chemical characterization of BPSE. The top row corresponds to results of BPhP and the bottom row to those of BPVG. Raman spectra of bulk and exfoliated BPVG (a) and BPVG (c) with an excitation wavelength of 514 nm. High-resolution X-ray photoelectron spectrum and peak deconvolution of BPhPSE (b) and BPVGSE (d) for P 2p region.

and electrochemical methods.26,27 Although there is the issue of humidity degradation, exfoliated BP nanosheets can be stable for weeks,28 making them suitable for a number of envisaged applications, such as photothermal treatment.29 Herein we demonstrate that BP shear exfoliation with kitchen blenders operating in turbulent and laminar regimes is possible in aqueous surfactant media, without the need to use a glovebox or purging solutions. The results vary according to the synthesis of BP bulk. Well-defined and few-layer BP exfoliated nanosheets were obtained from vapor phase growth BP. Morphological and chemical characterization of the exfoliated

BP shows a decrease in thickness, sheet size, and partial oxidation. The electrochemical performance in terms of inherent electrochemistry and heterogeneous electron transfer is tested. Energy related applications are evaluated by the hydrogen evolution reaction (HER).

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RESULTS AND DISCUSSION Two types of synthesized orthorhombic black phosphorus (BP) were selected for shear exfoliation as shown in Figure 1 and Scheme S1. High-pressure-converted BP (BPhPbulk) is a dense nanocrystallite material (Figure 1a,b), with stacked millimeter504


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Figure 3. (a) Inherent electrochemistry of GC modified with each of the BPbulk and BPSE layered materials. Cyclic voltammograms were recorded in purged 0.1 M PBS at pH 7.2 and scan rate of 100 mV/s, starting at −1.0 V (vs Ag/AgCl reference) in the anodic direction. (b) Cyclic voltammograms obtained for 1.0 mM ferro/ferricyanide [Fe(CN)6]3−/4− redox probe in KCl 0.1 M for GC electrode (black continuous line), BPbulk starting materials (discontinuous lines), and BPSE (continuous line). (c) Average peak-to-peak separation and standard deviation calculated from triplicate CVs (asterisks indicate no redox peaks). (d) LSV corresponding to HER in 0.5 M H2SO4 for the different BPbulk and BPSE and (e) the corresponding average of the overpotential at a current density of −10 mA/cm2. Error bars correspond to standard deviations based on triplicate measurements. Scan rate: 5 mV/s.

turbulent regimes, as shown in Scheme S1. More details are given in the Methods section. The morphologies of the shear exfoliated (BPSE) samples were observed by STEM images as shown in Figure 1, which shows evidence of totally different results. While BPhPSE (Figure 1c,d) has arrays of thick and heterogeneous fragments with undefined structure, BPVGSE (Figure 1g,h) exhibits well-defined and few-layer platelet-like nanosheets. This brings to our attention the fact that the quality of obtained shear exfoliated

size platelet sheets which are characteristic for vapor phase grown BP (BPVGbulk). This morphological difference holds the key for the shear exfoliation results described. EDS elemental distribution mapping shows the presence of phosphorus in both BPbulk species, and oxygen due to partial surface oxidation as shown in Figure S1. BPhPbulk and BPVGbulk were submitted to shear dispersion and exfoliation in aqueous surfactant sodium cholate for 2 h (SC 5 g/L), alternating between two types of kitchen blenders, thus undergoing through laminar and 505


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BP were subsequently performed. Cyclic voltammograms of modified GC electrodes with bulk and exfoliated BP to evaluate their inherent electrochemical properties are shown in Figure 3a. BPbulk shows a significant oxidation peak at +0.6 V, which is much more prominent for BPVGbulk, corresponding to the conversion of P0 to P5+.35 This oxidation process is irreversible because it is absent in subsequent cycles. This also explains why for BPSE the characteristic oxidation peak is absent. This has to do with the fact that the exposed layer of BPSE is readily oxidized upon GC electrode modification. To probe electron transfer properties of BPSE, samples were tested with the inner-sphere redox probe ferro/ferricyanide, [Fe(CN)6]3−/4− (Figure 3b). Analyses of the voltammetric profiles were taken in terms of peak-to-peak potential (ΔEp) of the oxidation and reduction processes, where 59 mV/e− is the reversible limit. The bare GC electrode has an average ΔEp of 182 mV. Considering the starting bulk materials, BPhPbulk and BPVGbulk have irreversible electron transfer processes (Figure 3c). BPVGbulk has a second oxidation corresponding to its inherent oxidation which is absent in the subsequent cycles. Whereas in the case of BPhPSE no redox peaks were observed, for BPVGSE only an oxidation peak was detected at higher potentials. Upon BP shear exfoliation, new edges and defects quickly became oxidized. In aqueous media, phosphate surface groups coating BPSE nanosheets are most likely deprotonated, are negatively charged, and repel the anionic [Fe(CN)6]3−/4−, leading to a decrease of the species concentration near the electrode surface. This correlates accordingly with reports that the inner-sphere redox electron transfer kinetics is slowed at oxygenated carbon edge plane sites.36,37 Hydrogen evolution reaction (HER) is the cathodic half reaction of water splitting and the applications of energy conversion devices including water electrolysis and artificial photosynthetic cells. Noble metals, such as Pt, show the highest efficiency for HER; thus, there is currently the need for alternative low-cost and efficient materials. The HER polarization curves of BPbulk and BPSE in acidic media are shown in Figure 3d. HER measurements of a bare GC electrode and Pt curves are also shown as performance references, with onset potential at −10 mA/cm2 of −1.18 and −0.06 V versus RHE, respectively (Figure 3e). The HER onset potential for BPbullk is higher than that of GC. BPSE species have improved catalytic performance with respect to the corresponding BPbulk. Substantial differences were observed in BP catalytic activities, while HER at BPVGSE has an improved onset potential in 250 mV with respect to BPVGbulk. Reduction at BPhPSE started at −0.5 V, a much lower potential than that verified for BPVGbulk, although there was no significant difference verified for the onset potential.

material is highly dependent on the starting BP. Other image acquisitions are shown in Figure S2. EDS elemental distribution mapping confirms the presence of phosphorus in both BPSE species; also, oxygen is much more present in the case of BPhPSE (Figure S2). To further study the crystal structure and thickness of BPbulk and BPSE, Raman spectra of substrate dispersed on silicon wafer were measured (Figure 2a,c). Pictures taken of the acquired spectral area, where the decrease in flake size for BPSE is also observable, are shown in Figure S3. Raman scattering conveys valuable information about BP as a result of the three distinct peaks corresponding to the A1g, B2g, and A2g modes at 365, 442, and 470 cm−1 , respectively, vibrational modes of the phosphorus atoms in BP structure as shown in Scheme S2.30 Raman peaks of BPbulk match well with their predicted spectra. Peak parameters obtained by Lorentzian fitting are summarized in Table S2 (position, intensity, area, and full width at halfmaximum, fwhm). Peaks have red-shifts in their position, and there are much lower intensities and peak area for BPSE because of the exfoliation process. Changes in fwhm have been reported to be dependent on the number of layers of BP.31 In this case, the fwhm for BPSE peaks shows a clear broadening of all modes, particularly for A1g modes. Although BPhPSE STEM images indicate that the material is poorly exfoliated, there is also a great decrease in signal intensities with respect to BPhPbulk. This dependence of the Raman intensities has been previously reported to be related to the flake thickness for exfoliated BP.30,32 It has been shown that the intensity ratio of the A1g/A2g phonon sensitively depends on sample degradation.33 The spectra of BPSE materials have A1g/A2g < 0.6, thus indicating that the basal planes are partially oxidized. Photoelectron spectroscopy (XPS) is a much more surface sensitive technique than EDS and was thus employed to characterize BP surfaces. Wide-scan X-ray photoelectron spectra for the different materials are shown in Figure S4. The presence of P 2p in bulk and exfoliated materials was detected. Other elements were also detected in the wide-range XPS spectra as described in Table S3. Specifically, for BPVGbulk, Sn is present at ∼9%, which is absent after exfoliation. Both BPSE species have similar P/O ratios reflecting an increased oxidation relative to that of the respective BPbulk. Sodium was detected at ≤0.5%, which means that the aqueous washing step of the BPSE materials was effective in the removal of the surfactant. The chemical bonding characteristics of the BPSE were also investigated by the high-resolution XPS spectrum (Figure 2b,d). Deconvolution analysis of the bonding modes, with the component positions of the P 2p region, is summarized in Table S4. A broad peak at 134 eV can be attributed to P−O bonds in orthophosphate (PO4−), and the smaller peak at 129 eV corresponds to elemental P0. BPSE undergoes partial oxidation, due to the higher surface area exposed and sensitivity to ambient humidity and increased band gap resulting from the decreased thickness of BP.33 Furthermore, BP exfoliated in aqueous solutions came in contact constantly with water; yet, degradation appears to be limited to around 85% of the total mass of BP, comparable with other exfoliation approaches in organic solvents.25 It has been proposed that P−O−P layers can act as a native capping layer, leaving underneath intact BP layers.34 Only since 2014 has BP been used more regularly in electrochemical sensing and energy related applications.2,7 Electrochemical fundamental studies of bulk and exfoliated

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CONCLUSION To summarize, shear force exfoliation of layered materials has advantages over electrochemical and ultrasonication in terms of scalability, although the latter approach has been more extensively explored. We have demonstrated the possibilities of applying shear force exfoliation for two different types of synthesized BP in aqueous surfactant media. BPSE does undergo partial oxidation, but nonetheless, the quality of the obtained material is not worse when compared to other methods that involve manipulation of BP in a glovebox or shear exfoliation in purged media (Table S1). Furthermore, we have also demonstrated that the crystal structure of the starting BPbulk highly influences results and electrochemical properties of BPSE, 506


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materials, SEM was operated in the gentle beam mode at 2 kV. The materials investigated were affixed on an aluminum sample stub using conductive carbon tape for imaging. To reduce the charging effect, the prepared sample stubs of shear exfoliated materials were sputtercoated with a thin layer of platinum (10 nm). EDS measurements were conducted at a higher acceleration voltage of 20 kV. STEM images were obtained using 30 kV acceleration voltage. The XPS spectra were obtained using an X-ray photoelectron Phoibos 100 MCD-5 spectrometer (SPECS) with monochromatic Mg Kα radiation (SPECS XR50, hυ = 1253 eV, 200 W) as the X-ray source; the spectra were calibrated to the C 1s peak at 284.5 eV. An InVia Raman microscope (Renishaw) was used for Raman spectroscopy measurements in backscattering geometry with a CCD detector. Nd:YAG laser (532 nm, 50 mW) and 50× objectives were used for the measurement. The instrument calibration was achieved using a silicon reference which gave the peak position at 520 cm−1 and a resolution of less than 1 cm−1. To ensure a sufficiently strong signal and to avoid radiation damage to the samples, the laser power used for these measurements was 5 mW. Electrochemical Measurements. Electrochemical measurements were carried out at room temperature using an Autolab PGSTAT204 (Eco Chemie, Utrecht, The Netherlands) controlled by NOVA Version 2.1 software (Eco Chemie) and three-electrodes arrangement. Glassy carbon (GC) electrode (3 mm diameter from CH Instruments, Texas) was used as a working electrode, Pt as counter electrode, and Ag/AgCl as reference electrode (CH Instruments, Texas). Each material was dispersed in DMF at a concentration of 5 mg/mL and sonicated initially for 15 min in an ultrasonic ice bath (T < 20 °C). Prior to GC modification, each suspension was mechanically stirred for 1 min using a vortex. Working electrode modification was done by drop casting 2 μL of each suspension. Studies of the inherent electrochemistry of modified electrodes were made in 0.1 M phosphate-buffered solution (PBS, pH 7.2) using cyclic voltammetry at 0.1 V/s scan rate, in a potential window of −1 to +1 V. Solutions were purged with nitrogen gas before measurements. The electron transfer measurements were performed at a scan rate of 0.1 V/s, for 1.0 mM of the ferro-/ferricyanide redox probe in a 0.1 M KCl solution. The hydrogen evolution reaction (HER) was performed by linear sweep voltammetry (LSV) at a scan rate of 0.005 V/s, performed in 0.5 M H2SO4 (acidic media).

although their oxidations are of the same order (P/O ratio). BPVG yields exfoliated few-layer nanosheets that have improved HER performance, while for BPhP there is no improvement.

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METHODS

Reagents. Red phosphorus (99.999%) and tin (99.999%) were obtained from Sigma-Aldrich. Iodine (99.9%), carbon disulfide (99.99%), and chloroform (99.9%) were obtained from Penta. Gold (99.99%) was from Safina. N,N-Dimethylformamide (DMF), potassium ferrocyanide, potassium chloride, potassium hydroxide, potassium phosphate dibasic, potassium phosphate monobasic, potassium chloride, sulfuric acid, and sodium cholate (SC) were purchased from Sigma-Aldrich. Synthesis of the Black Phosphorus. Black phosphorus materials were synthesized using two methods, high-pressure conversion (BPhP) and vapor phase growth (BPVG). For the first method, 10 g of red phosphorus was wrapped in graphite foil and was exposed to highpressure (6 GPa) and high-temperature (600 °C) conditions using a uniaxial pressing apparatus of 1-in. size. The heating and cooling rate were 100 °C/min. Finally, the resulting ceramic piece (20 mm × 5 mm) was mechanically removed from the graphite foil. BPVG was prepared by placing 40 mg of Sn, 20 mg SnI4, and 1000 mg of red phosphorus inside of a quartz ampule, followed by sealing of the ampule using an oxygen/hydrogen torch. Subsequently, the ampule was heated at 400 °C for 1 h using muffle furnace and left in these conditions for 2 more hours. The temperature was then increased to 650 °C for 24 h. Finally, the muffle furnace was cooled down to 500 °C over 10 h. The resulted plates (up to 5 mm × 2 mm) were washed with CS2 to remove the remains of the white phosphorus formed during the synthesis and SnI4. This washing step was originally done with hot toluene retaining the reaction promoter SnI4 in the works of Nigels et al.15,16 Nevertheless, CS2 is an efficient solvent for dissolution of both white phosphorus38 and SnI4 at room temperature.39 Shear Exfoliation. Starting material BP bulk crystals have mixed composition and may contain impurities; therefore, there was a pretreatment step introduced. Typically, 1 g of starting materials was initially sonicated in 5 mg/mL aqueous surfactant sodium cholate (SC) in an ice bath for 20 min. Suspensions were then centrifuged with a Beckman Coulter Allegra 64R centrifuge for 30 min at 3 krpm with the supernatant being decanted and discarded and the sediment being retained. The powders were then dried at 60 °C in a vacuum oven. Shear exfoliation was done using 6 stainless steel blades, 650 W Tefal Blendforce Maxi (BL 233865), equipped with glass jug for bottom mixing. For top mixing an immersion 750 W hand blender, all stainless-steel foot with 4 blades, from Bosch was used (MSM 67190GB) (Scheme S1). The pretreated powders were submitted to shear mixing and exfoliation in aqueous surfactant 5 g/L SC for 2 h alternating between two types of kitchen blenders. Each pretreated material was added to the cold blender jug with 200 mL of SC. The blenders were operated at full speed for the required amount of time. Such kitchen household blenders are not designed for long periods of continuous operation at high speeds due to excessive heating. To avoid degradation of plastic components and overheating of the rotor appliances, 2 min on/1 min off cycles and stoppage for every 10 min of effective blending were followed. The jug of the kitchen blender was kept in the freezer during these 10 min off cycles. Overall, materials were mixed for 1 h, for each blender (2 h in total). After shear exfoliation, the suspensions were centrifuged at 1 krpm for 1 h. The top 75% of the dispersions were separated for aqueous surfactant, by aqueous washing and centrifugation at different rotation speeds. Sediments of the shear exfoliated materials were then vacuum-dried at 60 °C for characterization and electrochemical performance studies. For the remainder of the text, starting materials will be referred to as BPbulk and exfoliated materials as BPSE. The production yield of material recovered from the top 75% suspensions was 7.3% for BPhPSE and 7.9% BPVGSE. Structural and Morphological Characterization. To obtain SEM micrographs, scanning electron microscopy (JEOL 7600F, Japan) was used, at an acceleration voltage of 5 kV. For bulk

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00106. Scheme of exfoliation procedure, table of BP exfoliation and experimental conditions, EDS mapping of elements for bulk and exfoliated BP, micrographs used in the Raman acquisition, table with Raman shifts and peak analysis, and XPS spectra with respective table individual elements assignation and deconvolution of P 2p region (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 507


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ACKNOWLEDGMENTS The project was funded by Tier 1 (99/13) from the Ministry of Education, Singapore. Z.S. and D.B. were supported by the Czech Science Foundation (GACR No. 17-11456S) and by specific university research (MSMT No. 20-SVV/2017). This work was created with the financial support of the Neuron Foundation for science support. This work was supported by the project Advanced Functional Nanorobots (ref CZ.02.1.01/ 0.0/0.0/15_003/0000444 financed by EFRR).

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ABBREVIATIONS BP, black phosphorus BPbulk, starting material bulk BP BPSE, shear exfoliated BP BPhPbulk, high-pressure conversion BP BPVGbulk, vapor phase growth BP BPhPSE, shear exfoliated BPhPbulk BPVGSE, shear exfoliated BPVGbulk

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Black Phosphorus Synthesis Path Strongly Influences Its

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