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Products of Degradation of Black Phosphorus in Protic Solvents Jan Plutnar, Zdeněk Sofer, and Martin Pumera* Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 1668 28 Prague, Czech Republic
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S Supporting Information *
ABSTRACT: Deterioration of the surface of black phosphorus (BP) caused by ambient atmosphere is an undesired process, limiting broader use of BP in many areas. The mechanism of BP degradation was explained theoretically, and the oxidized materials were thoroughly characterized experimentally. However, the surface analysis techniques introduce only a limited insight into the real state of the material. Here, we report a thorough analysis of the composition of mixtures obtained after a prolonged exposure of suspensions of BP to atmospheric oxygen with the aim to further disclosure the processes involved in the decomposition process. The results are compared with the predicted structures of the oxidized material and confirm the results of the theoretical calculations. The comparison of reactivity of BP with reactivity of white phosphorus under similar conditions concludes a similar distribution of the products in both cases. KEYWORDS: black phosphorus, phosphorene, degradation, oxidation products, NMR surface of BP,16 contact of this layer with moisture (either atmospheric vapors or liquid) must be avoided. Several other approaches were involved in the elimination of the undesired degradation process, including stabilization in the form of an aerogel with graphene oxide,17 electrochemical fluorination,18 or adsorption of metal ions on the surface of BP.19 Moisture (in addition to light irradiation) plays a significant role in the degradation processupon contact with water, the oxide dissolves and the BP surface is exposed to further oxidation.14 A thorough solid-state NMR study revealed the role of water in more detail.20 A synergy of oxygen and moisture is necessary to deteriorate the properties of BP, resulting in the ultimate destruction of the structure of BP. As shown, the sole water does not react with BP and could be used for its manipulation under strict exclusion of the presence of oxygen. Although there have been several studies aiming to determine the BP degradation mechanism published so far, the vast majority of these focused on characterization of the changes of the BP itself. Various techniques were involved in these studies, including X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy, energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy, electrochemical methods, and others; however, most of them provide information about the actual structure of the deteriorated
B
lack phosphorus (BP) and its single-layer form, phosphorene, have attracted the attention of researchers in the area of material chemistry for many decades, particularly since the development of low-temperature methods for its preparation in 2007.1 Black phosphorus is a direct band gap semiconductor with the band gap value tunable in the range between 0.3 and 2 eV by simply lowering the number of phosphorus layers in the material2,3 or by applying mechanical strain.4 Single-layer and multilayer BP could be utilized in the construction of many types of electronic devices, as already demonstrated for high on/off current ratio field-effect transistors,5 supercapacitors,6 or various types of sensors, including photodetectors,7 selective vapor sensors,8 and electrochemical sensors.9 However, the high sensitivity of the surface of BP to air hinders further expansion of the use of this material. The oxidative degradation of BP under ambient conditions is an undesirable process, limiting the direct use of pristine BP in the manufacture of electronic devices. Much effort was already put into detailed understanding of the oxidation process. Several theoretical works10,11 explain possible mechanisms of the process as well as the structures of the resulting products in the solid state. The oxidation was experimentally observed by optical methods,12 infrared spectroscopy,13 or Raman spectroscopy,14 and its influence on the electronic properties of BP was also studied.12,15 Although the controlled oxidation of BP leads to formation of a stable oxide layer suitable for passivation of the material or for a further deposition of other materials upon the © XXXX American Chemical Society
Received: May 18, 2018 Accepted: August 7, 2018
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DOI: 10.1021/acsnano.8b03740 ACS Nano XXXX, XXX, XXX−XXX
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Hz). In addition to these signals, a minor intensity signal at δ = 8.20 ppm was also present and assigned to hypophosphoric acid (H4P2O6),33 which is known to be formed from white phosphorus under similar conditions.34 The intensity ratio of the two most intense signals is approximately 48:39, corresponding to a slightly higher content of phosphonic acid over phosphoric acid. The remaining fraction of 13% phosphorus compounds present in the mixture is assigned to phosphinic and hypophosphoric acid (Table 1). The intensity ratio of the two major signals is not
surface only. The other products of the degradation process remain ignored, or just very indirect methods for their characterization were used so far.21−23 The majority of these studies conclude with the statement that phosphoric or phosphonic acids are the products of the oxidative degradation process, in some cases accompanied by such improbable products such as phosphane (PH3).21 Contrary to the results rendered by these techniques, NMR study of the decomposition products obtained under different reaction conditions in solution could provide insight into the structure of the oxidized BP, as the solution NMR technique allows a relatively straightforward and unequivocal identification of the phosphorus-containing compounds. In our study, we focused on a detailed characterization of the products of oxygen-induced degradation of BP in water and methanolsolvents suggested and tested as environments for ultrasonic exfoliation of BP24using the NMR technique. The oxidative decomposition of white phosphorus in alcohols is a well-documented process potentially involved in many industrial applications, including production of dialkyl phosphites25 or phosphonic acid (H3PO3).26 The NMR technique was involved in several former studies of oxidative decomposition of white phosphorus with peroxides in aqueous27 as well as alcoholic environments,28 and its application facilitated the proposal of reaction mechanisms of these processes. As the possible effects of the oxidative process on the structure of BP are well-documented, our aim was to study the composition of the oxidative degradation products only and to offer some possible explanations of the origin of the individual components of the mixture.
Table 1. Final Composition of the Reaction Mixtures after 2 Weeks of BP Oxidation and Solvolysis As Detected by 31P NMR Spectroscopy
a
compound
oxidation in water
oxidation in methanol
oxidation in air
H3PO2 H4P2O6 H3PO3 H3PO4 Me(H)P(O)OMe (MeO)P(O)(H)(OH) (MeO)2P(O)(H) OP(OH)2(OMe) OP(OH)(OMe)2 OP(OMe)3
10% 3% 48% 39% npb npb npb npb npb npb
10% nda 30% 1% 2% 29% 10% 4.5% 12% 1%
nda nda 77% 23% npb npb npb npb npb npb
Not detected. bNot present.
affected by exclusion of light from the reaction mixture and only negligibly changes over the time of the reaction (see Figure 1). Determined reaction kinetics results were compared to the results recently published as determined using anionexchange chromatography.23 Because the reactions follow pseudo-first-order kinetics, the values of the respective reaction rate constants were determined to be 0.020, 0.075, and 0.046 for H3PO2, H3PO3, and H3PO4 formation, respectively. These values are slightly higher than the reported values (0.019, 0.034, and 0.023). The value of the rate constant for the H4P2O6 formation reaction could not be determined with a reasonable error due to a relatively low abundance of this species in solution and high uncertainty in determination of its contents. The ratio of the intensities of the signals is even more pronounced in favor of phosphonic acid (H3PO3 to H3PO4 signal ratio 7:3) in the solution obtained by sonication of the solid sample of BP decayed under ambient atmosphere. The different ratio of these two acids indicates the prevailing formation of the oxide phase with phosphorus in a formal +3 oxidation state. Its further oxidation to the +5 state most probably proceeds in the droplets of moisture formed upon exposition of the hydrophilic35 oxidized BP surface toward the moist air12 as oxidation of phosphonic acid dissolved in the liquid of the droplet to phosphoric acid. However, the progress of the process is limited by the relatively slow exchange of oxygen between the static liquid phase and the atmosphere, driven by diffusion only. On the other hand, in the stirred suspension, the distribution of dissolved oxygen is wellprovided, thus a larger part of phosphorus compounds oxidizes to the higher oxidation state. Surprisingly, no additional product was detected in this experiment. Oxidation of BP in Methanolic Suspension. The NMR spectrum of the solution revealed the presence of a broad
RESULTS AND DISCUSSION Oxidation of BP in Aqueous Suspension. The solution 31 P NMR data recorded for experiments run under inert nitrogen revealed the absence of any phosphorus-containing compounds, after aging of BP either in methanol or in water. This observation confirmed the absence of reactivity of BP toward these solvents asfor wateralready reported.20 On the other hand, the 31P{1H} NMR spectra of residual solutions after the experiments were run in water in the presence of oxygen contained signals of several compounds. The most intense signals at δ = 0.86 and 3.52 ppm were assigned to phosphoric and phosphonic acid, respectively. The identity of the latter compound is confirmed by the presence of the H−P doublet (1JPH = 653.0 Hz) in the 31P NMR spectrum of the sample.29 A low-intensity nonbinomial 1:1:1 triplet of signals is observed at δ = 3.25 (1JPD = 102.0 Hz) assigned to the deuterated form of phosphonic acid, ascribed to the H−D exchange between the deuterated solvent and protic phosphonic acid. In addition to the major components of the mixture, phosphinic acid was also identified in the product mixture. Due to a relatively rapid H/D exchange under highly acidic conditions,30−32 this acid is present exclusively in its deuterated forms (D3PO2, δ = 4.68 ppm, nonbinomial quintet with 1JPD = 80.6 Hz; HDP(O)OD, δ = 5.03 ppm, doublet of nonbinomial triplets with 1JPH = 521.4 Hz, 1JPD = 82.1 Hz). Concentration of the other protic form, H2PO(OD), was very low, and we did not detect its low-intensity signals. The presence of the phosphinic acid is also supported by the presence of its respective methyl-d3 esters (both P-HD as well as P-D2) formed upon reaction of the acid with methanol-d3 (δ = 19.75, nonbinomial quintet, 1JPD = 88.5 and δ = 20.15 ppm, doublet of nonbinomial triplets, 1JPH = 580.8 Hz, 1JPD = 87.8 B
DOI: 10.1021/acsnano.8b03740 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. Kinetics of the BP oxidation in water in the presence of oxygen. The relative concentrations of the respective acids (left) and the molar fractions of the acids (right) in the reaction mixture as determined by the 31P NMR spectroscopy.
Scheme 1. Possible Pathways of Solvolytic Decomposition of Various Oxidized Motifs in the Structure of BP Leading to Formation of the Acids (A−C and a) and the Respective Methyl Esters (b−f)
variety of compounds. As expected, the portfolio of the products present in the reaction mixture was much broader than that in the previous experiments due to the presence of products of methanolysis of oxidized BP (Scheme 1). The most intense signals at δ = 1.58, 3.66, and 7.75 ppm in the 31 1 P{ H} NMR spectrum (Figure 2) were assigned to dimethyl phosphate, phosphoric acid, and monomethyl phosphite, respectively. In addition to these major signals, a variety of low-intensity signals belonging to compounds containing phosphorus in +1, +3, and +5 oxidation states was detected (Table 1). The concentration of phosphonic acid is surprisingly high (30%), considering the absence of water in the reaction mixture. Scheme 1 depicts hydrolytic/methanolytic routes of decomposition of all possible oxygen-containing moieties theoretically predicted as products of oxidation of black phosphorus by oxygen.10,11 The initial oxygen-binding mode was predicted to be the “dangling” mode (structure IV in Scheme 1), where a single oxygen atom is attached to phosphorus, forming a PO bond. A subsequent oxidation of the material results in insertion of additional oxygen atoms into the P−P bonds, forming “bridging”-mode oxygen defects in the structure (structures V−VII in Scheme 1). The opposite sides of the “bridges” might be represented by the same oxygen-rich
Figure 2. Part of the 31P{1H} NMR spectrum of the mixture obtained after reaction of BP with oxygen in methanol containing signals of the most abundant products.
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Figure 3. Kinetics of the BP oxidation in methanol in the presence of oxygen. The relative concentrations of the respective compounds (left) and the molar fractions of the compounds (right) in the reaction mixture as determined by the 31P NMR spectroscopy. MMP, monomethyl phosphite; DMP, dimethyl phosphite; MMPO, monomethyl phosphate; DMPO, dimethyl phosphate; TMPO, trimethyl phosphate.
Scheme 2. Proposed Pathway of Transformation of Methyl Ester of Phosphinic Acid to Methyl Ester of Methylphosphinic Acida
a
Transformation of the tautomeric form of methyl phosphinate is relatively slow as methyl-d3 ester of phosphinic acid, but neither methyl-d3 phosphinic acid nor its methyl ester was observed in methanol-d3 solutions of the BP oxidation products.
moieties (i.e., structures IV−VII) considering formation of the oxygen “bridge” between P atoms on the surface of the multilayer material or by moieties where the PO bond is absent (structures I−III in Scheme 1). The presence of these motifs in the multilayered structure is justified by hindrance of the lone pair at the phosphorus atoms by the inner layers of phosphorus in the structure of BP. Presence of some of the mentioned structural motifs could be excluded or their abundance concluded as negligible, based on the analyses of the NMR spectra obtained from both the aqueous as well as methanolic experiments. Thus, the low amount of phosphoric acid signifies very low abundance of the structural motif VII in the oxidized material in the presence of MeOH. As there is no other direct route to this acid (with the exception of phosphonic acid oxidation, however, kinetically unfavorable under the reaction conditions), its amount in the reaction mixture is proportional to the abundance of the mentioned moiety. Analogously, the very low concentration of methylphosphinic acid and of its oxidation product, methylphosphonic acid, together with relatively low amount of dimethylphosphite in the product mixture is indicative of low susceptibility of the structural motif I toward methanolysis and/or its low resilience toward further insertion of oxygen to P−P bonds. Similar conclusions could be applied to structures IV and V based on comparison of the content of their possible methanolytic productsmethylphosphinic acid, dimethyl phosphite, and trimethyl phosphate. The remaining three structural motifs (i.e., II, III, and VI) represent the main fraction of oxidized fragments involved in the decomposition of the oxidized BP by water and/or methanol. The presence of phosphinic acid is indicative of solvolytic decomposition of a P−P bond (because there is no other possible reaction pathway toward this compound) and justifies involvement of structure II in the decomposition process. Similarly, the detection of hypophosphoric acid in the hydrolytic mixtures justifies involvement of structure VI. The solvolysis of
structural motifs III and VI results in formation of phosphonic and phosphoric acid (in the case of hydrolysis) and in an additional formation of monomethyl- and dimethyl phosphite together with monomethyl and dimethyl phosphate (in addition to their possible formation via methanolysis of VII). Although the compounds may undergo further oxidation in the solution (namely, H3PO2 to H3PO3 and further to H3PO4), the presence of the relatively large amount of the low oxidation state species suggests the rate of these processes is rather slow under our experimental conditions. Also the fraction of the tautomeric forms of H3PO2 and H3PO3 without the P−H bond is very low (the equilibrium constants for the HP(O) to P(OH) transformation are known to be on the order of 10−12)36 and can be neglected. Similarly to the experiment run in an aqueous environment, a kinetic study of the methanolic decomposition was attempted. However, due to a very low concentration of some of the products, namely, the methyl phosphinates, only the concentrations of the major products were followed. Thus, dimethyl phosphite, monomethyl phosphite, and phosphonic acid were evaluated, as well as monomethyl-, dimethyl-, and trimethyl phosphate and phosphinic and phosphoric acid. Obviously, the degradation process in methanol follows different kinetics than the degradation in water (Figure 3). In the case of the methanolic mixture, the components may undergo mutual equilibrium reactions, namely, hydrolysis of the esters with H3PO2, H3PO3, and H3PO4 and esterification of the acids by methanol. The phosphorus +1 compounds represent the least abundant group of the products, approximately 10% of all the compounds in the mixture. Phosphinic acid is detected in its dideuterated (P−D) as well as monodeuterated forms because of the relatively rapid H/D exchange (nonbinomial quintet, δ = 8.7, 1JPD = 88 Hz and doublet of nonbinomial triplets at δ = 9.06 ppm, 1JPH = 558.9 Hz, 1JPD = 85.9 Hz in the 31 P NMR spectrum), analogously to the oxidation reaction in water. Also the respective methylesters-d3 are observed D
DOI: 10.1021/acsnano.8b03740 ACS Nano XXXX, XXX, XXX−XXX
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Table 2. 31P NMR Chemical Shifts and the Respective P−H and P−D Coupling Constants of the Compounds Identified in the Reaction Mixtures formula
δ31Pa
D3PO4 (MeO)P(O)(OD)2 (MeO)2P(O)OD (MeO)3P(O) HD2PO3 D3PO3(DP(O)(OD)2) (MeO)P(O)(H)OD (MeO)P(O)(D)OD (MeO)2P(O)H (MeO)2P(O)D (DO)P(O)(H)D (DO)P(O)D2 (DO)P(O)HD (CD3O)P(O)(H)(D) (CD3O)P(O)D2 (MeO)P(O)(CH3)(H) (CD3O)P(O)(CH3)(D)e (CD3O)P(O)(CH3)(D) D4P2O6 MeP(O)(OH)2 MeP(O)(OH)(OMe) MeP(O)(OMe)2
0.86 1.18 1.58 2.15 3.53/3.66b 3.25 7.75 7.46 12.02 11.67 9.06 4.68/8.69b 5.00 20.14 19.74 31.07 30.92 30.86 8.20 27.20 24.67 31.52
1
JPH
np np np np 653.0/667.0b np 680.0 np npc np 558.9 np 527.1 npd np 562.8 np 541.8 np np np np
1
JPD
np np np np nd 102.0 np 104.0 np 108.6 85.9 80.6/85.9b 83.3 90.9 88.0 np 85.4 np np np np np
3
JPH
np 11.3 11.2 10.9 np np 12.0 11.4 11.9 12.0 np np np np np 14.5 np np np np ndf ndf
a
All chemical shifts are in ppm, and the coupling constants are in Hz. bReaction in water/methanol, respectively. cOne of the signals of this compound overlaps with the signal of monomethyl phosphinate in the 31P spectrum. dThe intensities of the signals are very low in the 31P{1H} spectrum, and the signals cannot be detected in the 31P spectrum. eThe signals of P−CH3 and P−CD3 phosphinate are overlapping. fThe intensity of the signals is too low to allow determination of the coupling constant; nd, not detected; np, not present.
(nonbinomial quintet, δ = 19.74 ppm, 1JPD = 88.0 Hz, nonbinomial triplet, δ = 20.14 ppm, 1JPD = 90.9 Hz, the P−H product could not be detected due to a low intensity of the signals in the 31P{1H}, and their complete absence in the 31P NMR spectra) in addition to the acid. We did not observe the presence of (CH3O)P(O)H2 or of any of its P−D analogues in the spectra; however, the signals of the respective methyl phosphinates were detected, though in trace amounts, indicating a transformation of the ester to the methylphosphinate (Scheme 2). Four compounds with chemical shifts of ∼31 ppm were identified and tentatively assigned as (CH3O)P(O)(H)(CH3), (CD3O)P(O)(H)(CH3), (CH3O)P(O)(D)(CH3), and (CD3O)P(O)(D)(CH3) (see Table 2).28 The compounds with phosphorus in the formal +3 oxidation state represent the majority of compounds detected in the reaction mixture with total intensity of the respective signals representing approximately 70% of the overall intensity of all signals in the spectra. In addition to the presence of the abovementioned phosphonic acid (H3PO3), all its respective methyl esters were observed, as well. Thus, the presence of monomethyl phosphite (CH3OP(O)(H)OH), dimethyl phosphite ((CH3O)2P(O)(H)), as well as their respective P−D analogues was confirmed in the spectra. The abundance ratio of the acid, monomethyl ester, and dimethyl ester is approximately 30:55:15, respectively. This ratio well corresponds with the ratio of these compounds obtained in peroxide-assisted oxidation of white phosphorus in MeOH (23:55:21).28 The last group of compounds present in the reaction mixture consists of compounds containing phosphorus in the formal +5 oxidation state. This group represents approximately 20% of all the products. The most abundant of these
compounds is dimethyl phosphate (representing the vast majority of all the P(V) compoundsapproximately 64%), followed by monomethyl phosphate (24%), trimethyl phosphate, and phosphoric acid (6% each). Several additional minor intensity signals were observed at δ = 31.52, 27.20, and 24.67, assigned to methylphosphonic acid dimethyl ester, methylphosphonic acid, and methylphosphonic acid monomethyl ester. However, the intensity of these signals was very low (less than 0.1% each).
CONCLUSION Our analysis of the solutions obtained from solvolytic reactions of oxidized BP in our opinion supports the mechanism of BP oxidation theoretically predicted by Ziletti10,11 and Sutter.15 The initial attack of the structure by oxygen results in formation of the “dangling” oxygen moieties on the surface of the material. The subsequent insertion of the oxygen atoms into the P−P bonds disintegrates the structure of BP. The presence of a reactive solvent (water or methanol) assists the decomposition of the material via solvolytic cleavage of the “bridging” P−O−P bonds. The presence of low-valency phosphorus compounds (phosphinic and hypophosphoric acid) indicates that the remaining P−P bonds become susceptible to direct cleavage by the solvent. Although the solubility of oxygen in methanol is 1 order of magnitude higher than that in water,37 the oxidation of phosphorus compounds is more pronounced in water; the ratio of the low oxidation state compounds (+1, +3) to the compounds with phosphorus atoms in the +5 oxidation state is much higher in methanol than in water. This effect could be ascribed to a more pronounced tautomerization of phosphinic and phosphonic acid in acidic aqueous environments and the related facile E
DOI: 10.1021/acsnano.8b03740 ACS Nano XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This work was supported by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR), by the Czech Science Foundation (GACR No. 16-05167S) and by the Neuron Foundation for Science.
METHODS Black phosphorus samples were prepared according to the previously described procedure39 and characterized by HRTEM/EDX, XPS, Raman spectroscopy, and X-ray powder diffraction (see the Supporting Information for more details). The crystals of BP were subsequently smeared in an agate mortar and further exfoliated in anhydrous acetonitrile using high-energy shear force milling under inert atmosphere of argon for 10 h. The milled material was stored in argon-filled glovebox prior to the experiments to avoid its uncontrolled oxygen-assisted degradation. For the degradation experiments, the powdered BP (50 mg) was suspended in demineralized water or HPLC purity methanol (containing
