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Functional Protection of Exfoliated Black Phosphorus by Noncovalent Modification with Anthraquinone Rui Gusmaõ , Zdeněk Sofer, and Martin Pumera* Downloaded via UNIV OF TOLEDO on June 15, 2018 at 18:02:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Center for the Advanced Functional Nanorobots, Department of Inorganic Chemistry, University of Chemistry and Technology, Technická 5, Praha 6, 16000 Czechia S Supporting Information *
ABSTRACT: Few and monolayer black phosphorus (phosphorene) is currently an intensively researched material. Shear exfoliated black phosphorus (BPSE) nanosheets were functionalized with the redox active antraquinone (AQ) that can provide additional charge storage capacity. The noncovalent interaction of BP with AQ occurs due to van der Waals interactions. X-ray photoelectron spectroscopy results show that AQ coverage of BPSE nanosheets led to a stabilization against BPSE degradation. Electrochemistry of the BPSE-AQ shows that AQ is stably anchored onto BPSE and exhibits redox peaks stable for more than 100 cycles. The surface coverage by AQ on BPSE is estimated to be 1.25 nmol AQ/mg BP and electrontransfer rate constant (kET) of 33 s−1. Furthermore, the proposed modification greatly increases the gravimetric capacitance of BPSE-AQ with respect to the starting BPbulk. Such coating of BP not only protects BP from degradation but also brings electroactive functionality to this two-dimensionally layered material. KEYWORDS: black phosphorus, layered materials, anthraquinone, exfoliation, noncovalent functionalization, pseudocapacitance
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efforts to attain efficient BP passivation have been proposed, such as its incorporation in van der Waals heterostructures.15 Another useable strategy can be the covalent and noncovalent modification of BP,16−19 with the latter having the advantage of avoiding degradation of the electronic structure of the original material.19 It was recently demonstrated that the puckered regular atomic-scale grooves of BP enable the controlled anchoring of a wide sort of soft materials, including rod-like molecules.18 Quinone derivatives of polyaromatic hydrocarbons, such as anthraquinone (AQ), are hydrophobic and have an extended πelectron system that can increase the strength of noncovalent attractions. Specifically, AQ is a redox active small organic molecule (Figure 1B), frequently utilized for electrochemical labeling of biomolecules or providence of additional charge storage capacity.20,21 It has been demonstrated that the role of edge planes on the adsorption of substituted AQ on graphitic electrode materials,22 with the carbonyl group and defect/edge plane electron density on activated carbon electrode, is also
he layered allotrope of phosphorus, orthorhombic black phosphorus (BP, Figure 1A), was originally synthesized by Bridgman more than a century ago.1 This involved a high-pressure conversion of white phosphorus. Based on a vapor-phase growth method, Krebs reported an alternative synthetic route for BP in 1955.2 A recent proposed adaptation, avoiding convoluted experimental setups or toxic catalysts, yielded high-quality BP crystals in larger scale.3 This advent made BP synthesis much less intricate and later commercially available to researchers, thus soon being rediscovered in the new wave of two-dimensionally (2D) layered nanomaterials.4 More elements belonging to the pnictides or pnictogens group have since also been gaining momentum.5,6 BP synthesis leads to divergences in its crystallinity, stability, or the presence of impurities,7 which consequentially also strongly influences its delamination.8 BP can be exfoliated by different strategies to a few or single sheet structure (also designated as phosphorene).9−11 Bulk and exfoliated BP surfaces are highly susceptible to oxidation when exposed to oxygen or humidity,12 which can be observed just within a few hours.13 It is known that BP degrades upon exposure to ambient conditions, being oxidized on its surface to PxOy−.14 This can be a major holdback for BP applications, thus several © XXXX American Chemical Society
Received: February 25, 2018 Accepted: June 4, 2018
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DOI: 10.1021/acsnano.8b01474 ACS Nano XXXX, XXX, XXX−XXX
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Figure 1. (A) Lateral view of the armchair structure of BP sheets. (B) Commonly described mechanism of the electrochemical redox process of AQ. (C) SEM micrograph of the starting BP bulk crystal. (D) STEM micrograph of shear exfoliated BP nanosheets. Scale bars represent 20 μm and 500 nm.
exhibits well-defined and few-layer platelet-like nanosheets of a few micrometers. Other image acquisitions for BP materials are shown in Figure S2. The relatively large and thinner BPSE nanosheets represent an opportunity for the anchoring of different organic molecules and thus giving interesting functionalities to BPSE. The size distribution of BPSE nanosheets was performed by dynamic light scattering (DLS) showing the downsizing of BP crystals, exhibiting a broad size distribution profile in the size ranging from 100−700 nm and with maxima around 390 nm (Figure 2A). The thickness of BPSE nanosheets studied by optical profilometer indicates an average height profile of the nanosheets of 19.1 ± 9.7 nm (Figures 2B and S4), which corresponds to few-layer BPSE nanosheets. The structure, morphology, and composition of exfoliated BP were further in detail investigated by TEM in combination with EDS, as shown in Figures 2 and S4. BPSE has high degree of anisotropy and thus forms large sheets with visible wrinkled structure consisting of few-layer sheets (Figure 2C,D). Elemental distribution mapping shows the presence of BPSE in the obtained nanosheets and also oxygen due to partial the surface oxidation (Figure S4D). BPSE was noncovalently modified by dispersing the exfoliated material BPSE and AQ together in the same organic media. The suspension was then centrifuged and washed several times to remove the excess of AQ. At the end of the process, BPSE noncovalently modified with adsorbed AQ (BPSE-AQ) is obtained. Full details with respect to procedures of the BP synthesis, exfoliation, and modification is given in experimental section (see Methods). X-ray photoelectron spectroscopy (XPS) is useful to analyze the composition and oxidation state of materials. Figure S5
reported to assist the efficient adsorption of aromatic compounds through π−π static and electrostatic interactions.23 Quinone-based redox systems involve interfacial protoncoupled electron transfers, which are fast and stable, therefore facilitating greater power densities than other redox systems involving larger and slower Li+ or OH− ions.24 A number of methods have been developed to attach AQ to carbon-based materials, which can increase energy density by up to 86%.25,26 Here, we report a method of facile, noncovalent functionalization of shear exfoliated BP (BPSE) nanosheets with the redox probe AQ (referred in the text as BPSE-AQ). Although there are numerous examples of functionalization of carbon nanomaterials with AQ for different purposes, examples with other 2D layered materials are still unexplored. AQ is expected to adsorb at BPSE nanosheets through noncovalent attractions. The addition of such redox active components can hold more efficient charge storage by adding a pseudocapacitive and/or fast faradaic charge transfer to BP.
RESULTS AND DISCUSSION Orthorhombic black phosphorus (BPbulk) crystals that were synthesized by vapor-phase growth (Figure S1A)27 have characteristic stacked millimeter-size platelet sheets, as observed by SEM and shown in Figures 1C and S2. In the EDS spectra of the BPbulk (Figure S3), besides the presence of P elements in great majority, also O and Sn are present due to prone oxidation of the material and by product of the BP synthesis, respectively. BPbulk was submitted to shear exfoliation in aqueous surfactant sodium cholate for 2 h (SC 5 g/L) using an immersion kitchen hand blender and ice bath, as shown in Figure S1B. The morphology of the shear exfoliated BP (BPSE) was observed by STEM images as shown in Figure 1D, which B
DOI: 10.1021/acsnano.8b01474 ACS Nano XXXX, XXX, XXX−XXX
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High-resolution XPS spectra of the P 2p signal enable a more in-depth assessment of the oxidation state of P. It can be seen in Figure 3A that BPbulk has well-defined P 2p doublet, which can be deconvoluted into the two binding energy signals P 2p3/2 and P 2p1/2 at 129.7 and 130.2 eV, respectively. The wide signal centered ca. 134.1 eV usually is attributed to phosphorus pentoxide (P2O5).28 This is very common for BP, which is rather unstable in atmospheric conditions, to readily produce oxide layers on its surface. BPSE and BPSE-AQ have less intense P 2p signals, which has a 1:1 contribution to the P oxides (Figure 3B,C and Table S2). The signal centered at about 134.1 eV, corresponding to P oxides, is evidently more intense for BPSE (Table S2). This suggests that the exfoliated BPSE nanosheets alone are more prone to oxidation. The lower oxidation percentage verified here for BPSE-AQ is feasible, since it has been previously been reported that the noncovalent interaction of BP with a perylene diimide (polycyclic aromatic scaffold), mainly due to van der Waals interactions, also led to considerable stabilization of the BP against oxygen degradation.29 High-resolution XPS spectra of the C 1s core levels for AQ (Figure 3D) and BPSE-AQ (Figure 3E) brings further insight into the chemical composition of the oxygen-containing groups. For AQ, the spectrum showed a double peak profile at 284.5 and 286.5 eV, while for BPSE-AQ there was an asymmetrical tail at higher energies. The C 1s spectra were deconvoluted into three peaks: the sp2-hybridized carbon (CC) at 284.4 eV, the sp3-hybridized carbon (C−C) at 285.5 eV, and the carbonyl or quinone group (CO) group at 287.1 eV. From the deconvolution (Table S2) there is a high relative abundance of CO groups of ca. 19% for BPSE-AQ. Disparities in the C 1s core profile for BPSE-AQ are related to eventual contribution of the underlying carbon tape. In order to evaluate the long-term stability of shear exfoliated BP and BP-AQ nanosheets, these were observed 30 days after their preparation. It is known that BP oxidation causes not only chemical but also physical changes in its properties.30 Figure S6 shows the SEM and STEM of BP materials after being exposed
Figure 2. (A) DLS BPSE size distribution by the number of particles. (B) Profile of the BPSE nanosheests obtained from the optical profilometer, from drop cast on to ITO glass (zmax = 60 nm, scale bar represents 25 μm). (C and D) The TEM and HR-TEM images of BPSE. The scale bar corresponds from left to right to 500 and 5 nm.
shows the XPS survey spectra of the bulk crystal BP, BPSE-AQ BPSE nanosheets processed without modification and AQ alone. Clearly, phosphorus (P) is detected in all BP samples. The presence of Sn in BPbulk is due to the use of an Au/Sn alloy as mineralizer in the vapor growth synthesis (Figure S5A). The BPSE-AQ holds much lower amounts of residual elements than the starting material and BPSE, thus they are not detected by XPS (Table S1). The higher presence of carbon for BPSE-AQ is due to the successful modification of BPSE nanosheets, but it is worth remaking that BPSE-AQ also withholds the lowest percentage of oxygen of BP materials. It should be mentioned that there is an additional contribution to C signal originated from the underlying carbon tape and Al from the sample holder (see Methods section).
Figure 3. High-resolution XPS of the P 2p core level for (A) BPbulk, (B) BPSE, and (C) BPSE-AQ. High-resolution XPS of the C 1s core level for (D) AQ and (E) BPSE-AQ. C
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Figure 4. (A) Anodic scan CV for the inherent electrochemistry of the BP materials. (B) Cathodic scan 100 CVs of BPSE-AQ within the potential window in which AQ is active. (C) Variation of ΔEp and E°′ along the 100 CVs. (D) Ipeak variation along the 100 CVs. All CVs done in aqueous electrolyte 0.1 M KCl at 100 mV/s, reference Ag/AgCl.
corresponds to the oxidation of P0 to P5+ (Figure 4A), likely forming H3PO4. This signal is indistinguishable for BPSE due to the more extensive material oxidation, as shown previously in the XPS data. Notice that if the bare BPSE nanosheets are for the most part in the POx oxidized form, it will not possible to observe the P oxidation peak at +600 mV. On the other hand, BPSE-AQ electrochemical oxidation signal is observed in the first cycle, due to lower oxidation of BP in this sample. In the following cycles, only the AQ redox process remains. Thus, AQ adsorption onto the BPSE acts as a protective layer in coherence with XPS results. Furthermore, BPSE-AQ has an additional and stable redox faradaic signal ca. −500 mV. The redox peaks observed on the cyclic voltammogram (CV) of the BPSE-AQ are attributed to the well-known 2 protons, 2 electrons quinone redox interconversion in acid media (mechanism shown in Figure 1B). The characterization of the electrochemical processes in terms of the stability was carried out by performing 100 cycles (Figure 4B). The peak to peak separation (ΔEp) along the 100 CVs averages at 35 ± 4 mV, while the formal redox potential (E°′) was −509 ± 17 mV (Figure 4C). The stability of the signal can also be seen by the low variation of peaks current after initial cycles (Figure 4D). An ideal reversible electrochemical behavior results when both the reduced and oxidized forms of the redox species are strongly adsorbed to the electrode surface. The CV should show symmetric peaks, a linear relationship between the peak current and scan rate, and ΔEp equal to 0 at low scan rates for a reversible system.
to ambient conditions for one month. As expected, the degradation of BPSE is much more extensive than in BPbulk, in which the oxidation is considered to be confined to outward layers. It is possible to observe that for the aged BPbulk and BPSE nanosheets, there is an abundance of degradation bubbles and even there is loss in the definition of the sheets shapes. These samples degrade to oxygenated phosphorus (POx) in the presence of H2O and O2, due to BP hydrophilicity. On the other hand, the bubble-like features were sparse or even not observable in the BPSE-AQ, indicating that BP noncovalent modification with the hydrophobic AQ can be advantageous to ameliorate BP degradation. TEM images (Figures S7 and S8) are consistent with the above observations. BPSE nanosheets exhibit extensive degradation and engorgement deforming the nanosheet shape. The EDX mapping of elements shows a more abundant presence of oxygen for BPSE than BPSE-AQ. AFM images of representative aged BP flakes (Figures S9A and S10A) indicated moisture accumulation on the surface of the bare BP, which resulted in substantial local volume expansion and irregular surface. On the other hand, BP-AQ has much more smooth terraces (Figures S9B and S10B). The height profiles (Figure S10C) indicate that the bubbles at the bare BP flakes caused variations within the surface that can reach up to 40 nm. In contrast, BP-AQ flake sustains smooth surface, which is indicative of lower BP degradation. We focused on the electrochemistry of the BP materials in aqueous electrolyte. BPbulk shows a significant irreversible oxidation peak at ca. + 600 mV (vs Ag/AgCl) which D
DOI: 10.1021/acsnano.8b01474 ACS Nano XXXX, XXX, XXX−XXX
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Figure 5. (A) Cathodic scans CVs of BPSE-AQ within the potential window in which AQ is active at scan rates (ν) from 10 to 500 mV/s. (B) Linear variation of faradaic process of AQ, Ipeak with the scan rates (R2 = 0.997). (C) Blank corrected anodic current of the recorded CVs at −0.30 V (vs Ag/AgCl) vs scan rate for BPbulk (R2 = 0.992), BPSE (R2 = 0.988), and BPSE-AQ (R2 = 0.989). All CVs done in aqueous electrolyte 0.1 M KCl, reference Ag/AgCl.
(Figure S11C). As scan rate increases, ΔEp increases, which the data should yield a linear curve. The value obtained for α was 0.7. Following the seminal work of Chidsey,33 Marcus−Hush theory has been widely applied in describing the redox activity of species adsorbed on an electrode surface.34 Kinetic inhomogeneity may be attributed to many causes such as roughness of the electrode surface or surely the disordered drop casted layer of BPSE-AQ. Likewise, AQ adsorbed species at BPSE nanosheets of different sizes or the presence of different functional groups (P2O5) could alter the thermodynamic properties of the redox system. The value for kET determined is equal to 33 s−1, which is greater, but of the same order, than values reported for CNT-AQ of 10 s−1 and much greater than the value of 1 s−1 obtained for AQ attached to GC electrode through the different linker immobilized by electrochemical activation.35,36 In part, due to its recent rediscovery, but mostly due to material degradation, examples of BP as supercapacitor and pseudocapacitor are still scarce compared with other layered 2D materials. Most efforts involve a combination of BP with carbon nanomaterials37,38 or organic polymer coating.39−41 It is therefore of interest to study the variations in these BP materials pseudocapacitance. Figure 5C shows the comparison of the blank corrected capacitive current for the GC electrodes modified with BPbulk, BPSE, and BPSE-AQ. The values were obtained from the cathodic capacitive current measured at −0.3 V in the CVs shown in Figures 5A and S12. A visual comparison between the voltammograms shows that CVs of BPSE and BPSE-AQ exhibit higher capacitance than the BPbulk. From the linear slope of each curve, it is possible to obtain the capacitance of each material (see equation in SI). The value obtained for BPSE-AQ was the highest of the series (3.8 F/g), followed by BPSE (2.6 F/g), while BPbulk yielded negligible capacitive value (0.9 F/g). Although the capacitance value registered for BPSE-AQ may be far from the highest registered values for BP materials,39 it is nevertheless an improvement that quadruples the value of the original BPbulk.
Nevertheless, this value ΔEp is predicted for a covalently bound surface specie. In real situations, the adsorbed species may be weakly adsorbed. In this case, the redox specie (AQ) is noncovalently bond to BP on the surface of GC electrode. Moreover, a suspension of AQ was also drop casted onto a bare GC electrode, and CVs were performed in the same potential window as BPSE-AQ as shown in Figure S11A, which confirms that AQ is leaked from GC surface alone within just 20 cycles. Values of full width at half of the peak maximum height (fwhm) average at 90 ± 0.13 mV are larger than theoretical 90.6 mV/n (all equations described in SI). Deviations have been attributed to electrostatic effects incurred by neighboring charged species.31 Since AQ is an electroactive redox species, the surface concentration of AQ present on BPSE at the electrode surface Γ (mol/cm2) can be evaluated based on the charge of the voltammetric peak at 100 mV/s using Faraday’s law. Based on surface concentration measurements, the extent of AQ adsorption on BP can be estimated. From the average value of anodic peak area of the last 50 scans in Figure 4B and considering the amount of BPSE-AQ drop cast onto the GC surface, the determined value is 1.25 ± 0.24 nmol AQ/mg BP. The characterization of the BPSE-AQ electrochemical processes in terms of the rate-limiting step (diffusion or adsorption) and heterogeneous electron-transfer kinetics was also carried out. Figure 5A shows a series of CVs at different scan rates in 0.1 M KCl where both peaks shape remain almost unchanged over the chosen scan rate range. The anodic and cathodic currents of the faradaic process increase linearly with scan rate (Figure 5B) which is consistent with surface immobilization of the AQ redox probe at BPSE surface. The plot of log Ip vs log ν was found to linear with a slope of 1.03 and 0.91 (correlation coefficient of 0.998), which is very close to the expected theoretical slope of 1 for immobilized thin-layer voltammetry (Figure S11B). The Laviron method is widely used for determining the electron-transfer rate constant, however, it is subject to a number of constraints (equations shown in SI).32 First, at slow scan rates the ΔEp should again be 0 because the redox center is adsorbed onto the electrode and diffusion does not play a role. In this case, the ΔEp was 12 mV at a scan rate of 10 mV/s. Second, this method relies on α, the transfer coefficient, which is a measure of the symmetry of the energy barrier of the redox reaction. Ideally, α = 0.5 for all overpotentials, however in many cases α shifts from 0.5. Therefore, determination of α is crucial to estimate the electron-transfer rate constant (kET). To determine α, the peak potential Ep is plotted vs log ν. Epa and Epc are plotted separately in this way to give two branches
CONCLUSIONS Regardless of the remarkable interest and research load on BP, its degradation upon long-term exposure in ambient conditions is a major hurdle when further considering tangible applications. To tackle this obstacle and considering desirable improvements in the performance of BP, various strategies of covalent and noncovalent modification of BP have been formulated as shown in Table S3. Ideally, the proposed functionalization can enhance the stability of BP while E
DOI: 10.1021/acsnano.8b01474 ACS Nano XXXX, XXX, XXX−XXX
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using a JEOL JEM 2200FS microscope (200 kV) in combination with EDX SDD detector (X-Max, Oxford Instruments) for mapping of elements. With a monochromator Mg Kα radiation source (hv = 1253 eV, 200 W), the XPS spectra were measured using Phoibos 100 MCD5 spectrometer (SPECS, Germany). The analysis of the BPSE thickness profiles were measured with an optical profilometer (Sensofar, Spain) by drop casting BPSE (2 μL, 0.5 mg/mL) onto an ITO glass surface. Data treatment was done using MountainsMap 6.2.6266 version (Digital Surf, France). The thickness of BPSE nanosheets was estimated from 5 different height profile curves of the surface. Freshly cleaved BP and BP-AQ crystals were placed on a carbon tape strip. A drop of water was drop casted on top of each crystal, and left to dry in ambient conditions. After 1 month, the crystals were dried under vacuum prior to SEM and AFM measurements. The AFM measurements (Ntegra Spectra, NT-MDT) were operated in tapping mode (semicontact), under ambient conditions at a scan rate of 1 Hz and scan lines of 512. Silicon cantilevers had a strain constant of 1.5 kN/m equipped and curvature radius
