Article pubs.acs.org/cm
Exfoliation of Stable 2D Black Phosphorus for Device Fabrication Yuqin Zhang,†,# Ningning Dong,‡,# Hengcong Tao,† Chao Yan,§ Jiawei Huang,‡ Tengfei Liu,§ Alex W. Robertson,∥ John Texter,⊥ Jun Wang,*,‡ and Zhenyu Sun*,† †
State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China § School of Material Science & Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China ∥ Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom ⊥ School of Engineering Technology, Eastern Michigan University, Ypsilanti, Michigan 48197, United States ‡
S Supporting Information *
ABSTRACT: Discovering stabilizers that protect phosphorene from oxidative degradation is critically required for dispersion processing of black phosphorus (BP). It is equally important to also find environmentally friendly, low-cost, and practical exfoliating media. Herein, we demonstrate the yield of remarkably stable phosphorene by coating with a polymer to shield the nanosheets from reaction with water and air. The polymer shell suppresses the rate of BP degradation more efficiently than previously reported systems. We present for the first time a simple kinetic model for exfoliation of BP in polyvinylpyrrolidone (PVP) ethanol solution that appears to quantitatively fit BP exfoliation data, and it illuminates mechanistic aspects of exfoliation. Exfoliated flakes consist of a high level of 51% crystalline single layers that are free from structural disorder or oxidation. A successive centrifugation and redispersion strategy is developed affording dispersions with high phophorene-to-stabilizer ratio, which is very useful for further applications. We also demonstrate that PVP-stabilized phosphorene dispersions possess saturable absorption at both 515 and 1030 nm, which have potential use as ultrafast broadband absorbers. Furthermore, such phosphorene dispersions were processed to prepare new metal/phosphorene nanocomposites that have potential for use as electrocatalysts in electrolytic cells. for bilayers7 and further to ∼2.1−2.2 eV for the monolayer (single sheet).8 Such unique properties make this new twodimensional (2D) material especially attractive for both electronic and optoelectronic devices.9 In the last two years, there have been demonstrations of the material’s application in transistors,10 photodetectors,11 solar cells,12 and rechargeable batteries.13 To realize its potential applications, however, production of high-quality phosphorene in a cost-effective, industrial-scale process needs to be developed. BP is characterized by strong covalent in-plane bonds but weak van der Waals interactions
1. INTRODUCTION Black phosphorus (BP), a layered allotrope of phosphorus, has recently regained interest because single- and few-layer black phosphorus were isolated by micromechanical exfoliation (known as the “Scotch-tape” method) in 2014.1,2 Bulk BP exhibits p-type carrier mobilities of 350 cm2 V−1 s−1.3 Singlelayered black phosphorus, termed phosphorene, was found to have significantly enhanced and highly anisotropic hole mobilities of 1000 cm2 V−1 s−1 and higher on/off current ratios of ∼104−105.2,4 In contrast to graphene with zero bandgap5 and materials such as MoS2 that display indirect bandgap in thick layers,6 BP possesses an intrinsic direct bandgap in single-layer, few-layer, and bulk forms. A blueshift of the bandgap energy with reduction of layer thickness has been observed from 0.33 ± 0.02 eV for the bulk to 1.88 ± 0.24 eV © 2017 American Chemical Society
Received: May 15, 2017 Revised: July 7, 2017 Published: July 7, 2017 6445
DOI: 10.1021/acs.chemmater.7b01991 Chem. Mater. 2017, 29, 6445−6456
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Chemistry of Materials
Figure 1. (a) Structure of black phosphorus (BP). (b) Photographs of phosphorene dispersions obtained after centrifugation (CF) for 30 min at varying speeds (from left to right: 500, 1000, 3000, 5000, 7000, and 9000 rpm). (c) A465/l and CBP for phosphorene dispersions versus PVP concentration (CPVP) in ethanol (initial BP concentration CI = 1 mg mL−1; tsonic = 3 h; CF = 3000 rpm, 30 min). For comparison purposes, A465/l data for dispersions that were prepared in a similar manner in water and PVP aqueous solution (CPVP = 0.2 mg mL−1) are also shown. (d) Absorbance spectra of phosphorene dispersions (CPVP = 0.2 mg mL−1; CI = 1 mg mL−1; tsonic = 12 h) that were prepared by centrifugation at 500 rpm (17 g), 1000 rpm (69 g), 3000 rpm (625 g), 5000 rpm (1738 g), 7000 rpm (3406 g), and 9000 rpm (5631 g) for 30 min.
gation.31 Concentrations of almost 0.9 mg mL−1 (0.09% by weight) were obtained after 100 h of ultrasonication after centrifugation. It was proposed that the solvation shell of solvent molecules could act as a barrier to oxygen/water, offering partial protection of exfoliated nanosheets from deterioration.29 CHP and NMP have been shown to be useful, but isopropanol (IPA) was less effective. NMP was demonstrated to suppress BP oxidation in ambient conditions.26 Very dilute dispersions of BP have also been produced in γ-butyrolactone (GBL).32 A concentration of 0.1 mg mL−1 was obtained using GBL after 10 h of probe ultrasonication (300 W), followed by centrifugation at 2000 rpm for 60 min. Most exfoliated BP nanosheets have lateral sizes less than 1 μm. Unlike CHP and NMP, GBL did not provide an effective passivation shell for BP nanosheets. Although BP presents a dipolar moment out of plane due to uncharacterized surface modification during exfoliation, making it somewhat hydrophilic, a measured contact angle of water on BP of ∼57° lies between those for graphene oxide (∼27°) and transition metal dichalcogenides (∼90°).33 Another issue is that it is difficult to fully exclude solvated O2, leading to potential reactions between BP and water. Warren and co-workers examined 18 solvents for BP exfoliation and concluded that benzonitrile was the best, yielding the highest concentration (0.11 mg mL−1) after centrifugation.7 However, the degree of oxidation or of other sonolysis surface-modification effects of exfoliated nanosheets in benzonitrile and other solvents after exposure to air remains to be determined. Despite recent attempts in liquid exfoliation of BP, there are still a number of challenging issues. The best dispersion solvents (NMP, benzonitrile) for BP reported earlier are toxic and relatively costly. Finding environmentally friendly, low-cost, and practical exfoliating media is, therefore, critically required for dispersion processing of BP. It will be helpful to find stabilizers that protect phosphorene from oxidative degradation. The yield of BP monolayers afforded by liquid exfoliation and centrifugation is currently rather low, being typically 1, suggesting the structure to be monolayer.27 After background subtraction, we measured this ratio to be ca. 2.3, in agreement with the computational value of 2.577 for monolayer BP.39 Atomic force microscopy (AFM) measurements show that most flakes have smooth surfaces (Figure 3). Approximately 51% of measured
r
dGm = −(m − 1)kGm + 2k ∑ Gl dt l=m+1
where Gm is the number density of m-sheet flakes and r represents the thickest flakes in the dispersion. This model assumes irreversible exfoliation, whereby increases in Gm can only occur by splitting of a flake Gl where l > m. This model is analytically solvable without matrix inversion, and one can solve this model with rate constants derived experimentally.31 Experimental effective absorbance spectra and effective optical absorbances at 465 nm, A465 (t), are illustrated in Figure S9, and there we see that the absorbance growth kinetics appear to behave as one would expect for a nominally firstorder growth process. When we apply a semilogarithmic kinetic model, which reveals first-order time constants when such a process dominates dynamics over a given time interval, to the data illustrated in Figure S9b, we obtain the result illustrated in Figure 5a. This analysis shows that the long-time kinetics fit an apparent pseudo-first-order process with rate constant k = 7.3 × 10−6 s−1 and half-life, τ1/2 = 1580 min. This value is slightly larger than 6.0 × 10−6 s−1 reported for graphene exfoliation in water31 with a nanolatex stabilizer and 2.0 × 10−6 s−1 for MoS2 observed in water with the same nanolatex stabilizer. The effective optical absorption spectra used in constructing Figure 5a, given in Figure S9a of the Supporting Information, approach an apparent maximum absorbance level at 465 nm of 21 absorbance units per cm (path length), although further slow increases with much longer sonication would be expected, as has been seen in graphene.41 If all of the BP in this dispersion were exfoliated into few-sheet flakes, we would expect to see an absorbance approaching 33.4 per cm, consistent with the absorption coefficient derived in Figure S9b for a suspension of 1 mg of BP/mL at 465 nm. This discrepancy is accounted for mainly by the presence of thicker and readily sedimenting BP particles in our “beforecentrifugation” dispersion of Figure S9. When we centrifuge to get the spectra and absorption data of Figure S9a, we sediment ∼90% of the BP flakes, including large particulates and an unknown fraction of the few-sheet flakes. The absorption spectra we see in the supernatant is characteristic of small flakes as studied and reported by Hanlon, Backes, and
Figure 3. (a, b) Tapping mode AFM images of phosphorene dispersions after being subjected to a cascade cleaning procedure to remove free PVP molecules (from 7000 to 500 rpm for 30 min, illustrated in “Removal of PVP” part), deposited on a SiO2/Si wafer. The flake with apparent AFM height of 1.6 nm shown in image (b) appears to be a two-layer nanosheet.
flakes have apparent AFM height of 0.6−0.8 nm that are likely monolayers (Figure 3a and Figure S7a, b, and c). Although the step height (0.8 nm) is slightly larger than the theoretical value of 0.6 nm for phosphorene, we expect that the AFM-measured thickness value of a single-layer 2D crystal on a SiO2/Si wafer is higher than the theoretical value, which has been reported in graphene and MoS2.40 A number of few-layer nanosheets were also observed, as shown in Figure 3b and Figure S7d. Raman spectra of bulk BP and exfoliated flakes both showed three typical peaks between 300 and 500 cm−1 including an out-of-plane vibrational mode Ag1 (363.0 cm−1) and two inplane modes of B2g (440.5 cm−1) and Ag2 (467.7 cm−1) (Figure 4a).2,17,29 Exfoliated nanosheets have higher peak values of fullwidth at half-maximum (fwhm) (5.62, 5.43, and 5.0 cm−1 of Ag1, B2g, and Ag2, respectively) than bulk BP (4.25, 5.39, and 6448
DOI: 10.1021/acs.chemmater.7b01991 Chem. Mater. 2017, 29, 6445−6456
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Figure 4. (a) Raman spectra of bulk BP and exfoliated nanosheets. (b) Raman intensity map of Ag1, B2g, and Ag2 modes. (c) Raman Ag1/Ag2 integrated peak ratio map of phosphorene dispersions deposited on a SiO2/Si wafer. A 50 μm × 50 μm area where a number of large nanosheets of several micrometers appear was selected to collect sufficient information for oxidation analysis. (d) Histogram of Ag1/Ag2 integrated peak ratio.
Figure 5. (a) Semilogarithmic kinetic analysis of ln[1 − A465(t)/A465(max)]; see Figure S9b for experimental data. (b) Diffusion scaling analysis of experimental absorbance data.
co-workers.29 They also found that larger flakes (greater lateral dimensions) had differently shaped absorption spectra, so there is probably no unique absorption coefficient for BP, unless one specifies lateral and thickness dimensions. It does appear clear from empirical studies and from explicit measurements of extinction, absorption, and scattering coefficients29 that absorption approaches extinction as flake size decreases and scattering decreases concomitantly. Taking our limiting and before-centrifugation absorption as an extinction coefficient of 21 cm2/mg, it is surprisingly close to the 26.5 cm2/mg reported by Hanlon, Backes, and co-workers29 after centrifugation. Our estimate of 33.4 cm2/mg obtained after centrifugation, about twice the 15 cm2/mg reported earlier, perhaps is due to our ethanol/PVP milieu being less damaging than the CHP sonication environment used earlier or simply due to effects of solvent on visible excitation. Clearly more experimental work, such as done for graphene by Nair and co-workers,42 and theoretical modeling are needed. Some
interesting initial progress in theoretical absorption has been reported by Tran and co-workers, who showed that many-body effects, for example, electron−hole interactions with excitons, can significantly modify absorption selection rules, and we cannot yet find comments on oscillator strengths for visible light polarized perpendicular to BP lateral planes, although no obvious symmetry property appears to exclude such excitation and phosphorene is much more polarizable than graphene. Figure 5b illustrates increases in absorbance as a function of t1/2. These data show that there are two discernible domains of behavior: a long-time behavior (t > 3 h) and an initial time behavior. These two domains exhibit slopes of 0.00282 s−1/2 (t < 5 h) and 0.00114 s−1/2 (t > 5 h). These linear domains suggest these kinetic processes are diffusion-controlled, and the exfoliation appears rate-limiting as diffusion-controlled processes. Two similar domains observed in exfoliation of graphene in water, 0.0026 s−1/2 and 0.00086 s−1/2, respectively, 6449
DOI: 10.1021/acs.chemmater.7b01991 Chem. Mater. 2017, 29, 6445−6456
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Figure 6. AFM images along with AFM height profiles of exfoliated BP flakes (CPVP = 1 mg mL−1; CI = 1 mg mL−1; tsonic = 3 h; CF = 3000 rpm, 30 min) on a SiO2/Si substrate after exposure to air with an average temperature of 26 °C and a relative humidity of 100% for (a, b) 0 h, (c, d) 12 h, and (e, f) 24 h. The flakes likely comprise two individual nanosheets that stack on top of each other. Those brighter dots on the surface of the flake shown in a, c, and e may originate from the polymer PVP. No bubble formation in the flakes indicative of irreversible BP oxidation was observed after 24 h in ambient conditions.
dispersion remained unchanged after this freezing treatment (Figure S10). There is only a very slight decrease in absorbance after heating at 80 °C for 12 h. It is known that BP oxidizes and converts to PxOy species, which can further react with moisture to form phosphoric acid.17,43 From this scenario, an ambient degradation study was performed using AFM. Figure 6 shows AFM images along with AFM height profiles of exfoliated 2-nm-thick BP flakes after exposure to air for 0, 12, and 24 h. No signs of apparent degradation are observable even after 24 h of ambient exposure with an average temperature of 26 °C and a relative humidity of 100%. We further used X-ray photoelectron spectroscopy (XPS) to assess whether chemical modifications take place in exfoliated BP samples upon ambient exposure. Figure 7b shows that BP nanosheets have a single spin-orbit P 2p3/2 and P 2p1/2 doublet at ∼129.1 and 130.0 eV, respectively, characteristic of crystalline BP. No oxidized phosphorus sub-bands at ∼134 eV are observed in pristine sample, indicating its high quality. Notably, we found that, despite 3 and 7 days of ambient exposure, the BP nanosheets have a low oxide content of ca. 3.9 atom %, as estimated by peak integration software. This contrasts to a PxOy content of 15% reported for BP nanosheets in N-cyclohexyl-2-pyrrolidinone.29 In addition, no increase in the full-width at half-maximum (fwhm = ca. 1.6 eV) was observed, suggesting that the long-range order was preserved. Both aspects suggest significantly higher stability of our BP nanosheets against ambient oxidation than those exfoliated BP reported in the literature.7,26,29,33 This indicates that the PVP shell acts as a barrier to prevent oxidative species reaching the nanosheet surface. Protection of the nanosheets using this polymer therefore may allow large-scale production and storage in ambient conditions.
are about 17% and 75% slower, highlighting slower exfoliation for graphene. Exfoliating BP in ethanol using PVP as stabilizer appears to be more facile than exfoliating graphene in water using an effective nanolatex stabilizer. The half-life for producing singlesheet BP here is ∼1580 min, less than 2/3 that for graphene in water, 2660 min.31 The slopes obtained in the diffusion scaling illustrated in Figure 5b are proportional to the square root of the effective diffusion coefficient for exfoliation, and the ratio 0.00114/0.00086 ≈ 1.3 suggests a ratio of exfoliation diffusion lengths of 1.32 ≈ 1.7 for BP exfoliation in ethanol compared to that for graphene in water. This ratio is quantitatively also reflected in the long-time half-lives (2660/1580 ≈ 1.7). This more facile exfoliation of BP is surprising in part because the cohesive energy for BP is ∼151 meV per P atom,14 while it is 61 meV per C in graphene.15 It is difficult to explain this paradox at this early stage of quantifying exfoliation kinetics, but the phosphorene molecular structure is inherently more flexible. Temperature-dependent rate studies to examine activation energies will be helpful in furthering our understanding of exfoliation. Dispersion Stability. For practical applications, dispersions with high concentration and good stability are preferred. We evaluated the stability of our phosphorene dispersions against aggregation under inert conditions by monitoring A465/l as a function of sedimentation time. Figure 7a shows the sedimentation profile for a phosphorene dispersion with CBP ≈ 0.15 mg mL−1. Despite some decay with extension of incubation time, ∼90% of BP nanosheets in the dispersion remained stably dispersed against sedimentation over long periods up to 480 h. Moreover, no sedimentation of the dispersion was observed, even after exposure to low (−5 °C) or high (80 °C) temperatures for 12 h. The absorbance of the 6450
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Figure 7. (a) CBP versus sedimentation time for a phosphorene dispersion in a glovebox (CPVP = 0.2 mg mL−1; CI = 1 mg mL−1; tsonic = 12 h; CF = 3000 rpm, 30 min). (b) P 2p core-level XPS spectra of BP nanosheets at 0, 3, and 7 days ambient exposure. After 3 or 7 days of ambient exposure, a weak doublet with a large fwhm appears at ca. 134 eV, which can be assigned to oxidized phosphorus species. (c) Time evolution of Raman peak intensities of Ag1, B2g, and Ag2 modes for phosphorene dispersions deposited on a SiO2/Si substrate. Raman spectra of bulk BP treated under similar conditions were also measured for comparison. (d) Tracking BP degradation by UV−vis spectroscopy. Relative absorbance, A465(t)/A465(0), monitoring as a function of time (t) of phosphorene dispersions for different volume fractions of water added. The relative absorbances for BP dispersions in CHP with addition of 3, 5, 10, and 25% H2O at t = 50 h reported by Coleman and co-workers were added for comparison.29
mechanically cleaved BP in water.29 We note that the PVPstabilized phosphorene dispersions showed significantly slower degradation kinetics with equivalent addition of water compared to the dispersions in CHP demonstrated by Coleman and co-workers (Figure 7d).29 This suggests more effective protection of such polymer coating than the best stable solvent reported so far. The need to have both water and oxygen present in order to induce oxidative degradation of BP has been substantiated in multiple studies,17,29,43,44 and Favron and co-workers17 further found that visible light must be present for oxidative degradation. Exhaustive experimentation showing that light is absolutely required has yet to be reported, at least in liquid dispersion systems. Mechanisms for passivating BP to oxidation in the presence of both water and oxygen appear to be physical. These mechanisms include condensing an overlayer of alumina, AlOx,44 forming a surface layer of solvent (N-cyclohexyl-2pyrrolidone, CHP),29 forming a surface layer of adsorbing planar molecules, such as 7,7,8,8-tetracyano-p-quinodimethane (TCNQ),45 and forming covalently attached monolayers of nitrophenyl and methoxyphenyl moieties.46 It is clear that physical obstruction of water and oxygen can be implemented by diverse approaches, as well as that such obstructions stabilize BP from oxidation. The effects of solvents29 or of additives45 that tend to physically bind by van der Waals forces parallel to the BP sheet
Raman spectra of BP nanosheets are shown in Figure S11 at 0, 15, 30, 45, and 60 min after exposure in air at λ = 532 nm. In stark contrast to quenching of Raman density reported for mechanically exfoliated BP,17 no change in Raman intensity, position, and fwhm was observed for BP nanosheets deposited on a SiO2/Si substrate (Figure 7c), suggesting no occurrence of degradation, in accordance with XPS results. It has been demonstrated that exfoliated BP degrades upon exposure to O2 and H2O, limiting its processing and application.17,29,43 To explore whether the polymer coating can suppress the degradation process, we have monitored the stability of phosphorene dispersions by tracking the variation of absorbance (A) versus time (Figure 7d and Figure S12). Only a slight fall (120 mV dec−1, indicating that the Volmer reaction is the rate-determining step. The 30 wt % Cu/few-layer BP catalyst retained stable albeit a slight increase in overpotential at 10 mA cm−2 after 1000 cycles, as shown in Figure 11c. Such superior HER performance may originate from the strong electron transfer from phosphorene to the metal, in combination with merits of high carrier mobility, large surface area, and hydrophilicity of BP nanosheets. We believe that these preliminary results offer the possibility of using such interesting material in catalysis. Further work in this direction is underway in our lab. Our Cu/BP results, while superior to similar Cu/FLG composite catalysts, exhibit an overpotential of about −550 mV at 10 mA cm−2 and a Tafel slope of 139 mV dec−1, while 20% Pt on carbon exhibits only a −90 mV overpotential and a slope of 34 mV dec−1. Nitrogen and sulfur codoped graphene yields an overpotential of −280 mV at a current density of 10 mA cm−2 and a Tafel slope of 80.5 mV dec−1.52 Even better is the performance of g-C3N4 on N-doped graphene with an HER overpotential of −240 mV and a Tafel slope of 51.5 mV dec−1.53 Very competitive with these carbon nitride results are MoS2 nanosheets treated with butyl lithium, to yield a similar overpotential of about −200 mV and a lower slope of about 43 mV dec−1.54 A competitive MoS2 alternative catalyst utilizing MoS2 nanoplatelets grown on reduced graphene oxide also yields an overpotential of about −200 mV and a Tafel slope of 41 mV dec−1 at 10 mA cm−2.55 These results are very close to matching Pt/C performance,24 and some other hybridization of BP will be needed to match these MoS2-based performances.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01991. Dispersion preparation, characterization, nonlinear optical measurements, hydrogen evolution reaction measurements, dispersion data, optical images, SEM images and lateral size distributions extracted by SEM counting, TEM images, AFM images and thickness distribution, Raman mapping data, UV/vis absorbance versus ultrasonic time, optical absorbance before and after heating and freezing treatments, Raman spectra after air exposure, UV/vis absorbance versus time at varying contents of added water, open-aperture Z-scan results, and TEM images of Cu/few-layer BP (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jiawei Huang: 0000-0001-5108-9904 Zhenyu Sun: 0000-0001-5788-9339 Author Contributions #
Y.Z. and N.D. contributed equally to this study and share first authorship.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the State Key Laboratory of Organic−Inorganic Composites (no. oic-201503005) and the Fundamental Research Funds for the Central Universities (no. buctrc201525). Z.S. thanks the support from Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, CAS. J.W. thanks the support from NSFC (61675217 and 61522510), the Strategic Priority Research Program of CAS (XDB16030700), the Key Research Program of Frontier Science of CAS (QYZDB-SSW-JSC041), the Program of Shanghai Academic Research Leader (no. 17XD1403900), and the Youth Innovation Promotion Association, CAS.
3. CONCLUSION We have shown that BP can be effectively exfoliated by ultrasonication in ethanol with the polymer PVP to yield stable dispersions comprising a large fraction of single sheets. We found that exfoliation kinetics fit an apparent pseudo-first-order process that generates single sheets more quickly than aggregated BP. Using HRTEM, XPS, and Raman spectroscopy, we observed that exfoliated nanosheets are crystalline free of oxidation. The BP nanosheets are remarkably stable with an oxide content as low as 3.9 atom % even after exposure to air for 7 days, probably due to protection by the polymer coating. Such PVP-stabilized BP nanosheets showed higher resistance against degradation with addition of water compared to solventexfoliated BP. We also developed a successive centrifugation
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