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Black phosphorus: Degradation favors lubrication Shuai Wu, Feng He, Guoxin Xie, Zhengliang Bian, Jianbin Luo, and Shizhu Wen Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02092 • Publication Date (Web): 01 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018
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Black phosphorus: Degradation favors lubrication Shuai Wu1, Feng He1, Guoxin Xie1*, Zhengliang Bian2, Jianbin Luo1, Shizhu Wen1 1
State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University,
Beijing 100084, China 2
Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China
*Corresponding author, E-mail:
[email protected] TOC figure
ABSTRACT Due to its innate instability, the degradation of black phosphorus (BP) with oxygen and moisture was considered as the obstacle for its application in ambient conditions. Here, reduced friction force by about 50% at the degraded area of BP nanosheets was expressly observed using atomic force microscopy due to the produced phosphorus oxides during degradation. EDS mapping analyses corroborated the localized concentration of oxygen on the degraded BP flake surface where friction reduction was observed. Water absorption was discovered to be essential for the degraded characteristic as well as the friction reduction behavior of BP sheets. The combination of water molecules as well as the resulting chemical groups (P-OH bonds) formed on the oxidized surface may account for the friction reduction of degraded BP flakes. It is indicated that besides its layered structure, the ambient degradation of BP significantly favors its lubrication behavior.
KEYWORDS: Nano-friction, Black phosphorus, Chemical instability, Degradation, Lubrication
1
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Black phosphorus (BP) is the most air stable allotrope of phosphorus and bulk BP has an orthorhombic layered structure via van der Waals interlayer interactions
1-2
, similar to bulk graphite, molybdenum
disulfide (MoS2), and cubic boron nitride (h-BN). Exfoliated few-layer BP nanosheet has emerged to be an exciting member of the two-dimensional (2D) material family and drawn tremendous research interest 3-5
. In contrast to other types of 2D materials 6, the unique anisotropic nature
7
and semiconducting
8
properties of BP make it especially attractive for the applications in transistors of electronic devices. Beyond its impressive properties for the applications in nanoelectronics and optoelectronics, one of the motivating interests in BP is its potential applications as lubricants. Due to their strong in-plane bonds in conjunction with weak van der Waals interlayer interactions, 2D materials, such as MoS2 and graphene, display excellent lubrication properties with low friction even to superlubric regimes ubiquitously used as solid lubricants
10-11
or lubricant additives in composites
12
9
and have been
for practical applications.
Recently, researches into the lubrication properties of layered BP nanosheets functioning as lubrication additives
13
or fillers in self-lubricating materials
14-15
declared the promising applications of BP in
lubrication practices. From the microscopic view, previous studies
16
on the atomic-scale friction
behavior of BP using atomic force microscopy (AFM) revealed that BP flakes exhibited similar thickness-dependent behaviors as other 2D materials. Obvious anisotropy in the friction forces along the armchair and zigzag directions of BP sheets were observed both experimentally and theoretically 17. However, one of the biggest challenges in the researches and applications of BP remain its innate instability under ambient conditions
18-21
. Few-layer BP surface was prone to oxidize upon exposure to
22
water, oxygen and visible light . Ambient degradation of exfoliated sheets was reported to rapidly occur within a few hours and resulted in both compositional and physical changes of the BP flake
23
, which
24
would hamper their optical and electronic applications . The reported atomic-scale friction experiments of BP flakes were specially conducted within 2 h to reduce the influence of sample degradation 16. Great efforts have been made on understanding the oxidation details as well as the protection methods to preserve its intrinsic properties for device applications 25-26. The high chemical reactivity and modifiable surface of BP sheets, from the dialectical point of view, can also be beneficial under certain circumstances. Researchers have taken advantage of the environmental instability of BP and proposed using BP as gas sensors 27, humidity sensors
2
as well as
28
neuroprotective nanodrug in biomedical applications . It is important to note that surface chemistry of the contact surfaces could significantly affect the friction behavior 9 through the study of environmental contribution and surface chemical modification to the lubrication behavior of 2D materials. The formed oxides at the edge sites and defects of MoS2 film due to environmental degradation reduced the interlayer Coulombic repulsive interaction and impede lamellar shear, resulting in a slight increase in the initial friction (∼20%).29-31 Owing to the testing environment with hydrogen gas, graphene exhibited substantially lower wear rate for sliding surfaces as a result of the passivated dangling carbon bonds by hydrogen
32
. In comparison to pristine graphene, though the adhesion between the AFM tip and the 2
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fluorinated graphene decreased, the friction force of fluorinated graphene increased adsorbed water molecules on the 2D material surface tribochemical reactions
10, 38-39
36-37
33-35
. Especially, the
would also alter the involved dynamics in
. For instance, water was proved to have a high affinity for bridging
between two MoS2 planes at higher humidity, which would lead to the increase of both friction and wear.30, 40 This study explored the nano-friction behavior of degraded BP sheets against a silicon nitride tip by using AFM and considered the contribution of its chemical reactivity on lubrication properties from the aspect of friction-induced chemical reactions. We demonstrated that friction was evidently diminished in the degraded area of the BP nanosheets and ambient degradation significantly favored the lubrication behavior of BP flakes. Especially, the combination of water as well as the chemical groups formed on the oxidized surface was deemed to contribute to the friction reduction of degraded BP flakes, which threw light on the mechanism of BP functioning as the lubricant at a fundamental level.
Orthorhombic BP is formed by puckered layers held together through weak van der Waals force (a=3.313 Å, b=10.473 Å, c=4.374 Å)
41-42
. Figure 1a presents the typical schematic structure of the
single-layer BP nanosheet, bilayer top view from [010] and side view from [100]. The layered structure of BP sample in this study is confirmed by the scanning electron microscope (SEM) image shown in Figure 1b where ordered stacking of layers are observed. Low-magnification transmission electron microscope(TEM) image (Figure 1c) of the peeled BP flake shows sheet-like morphology and the selected area electron diffraction (SAED) pattern shown in the inset confirms the highly crystalline nature of the BP nanosheet. High resolution (HR)TEM image (Figure 1d) shows the ordered crystal structure of pristine BP flake, which correlates well with the reported results in literature 43-45. The exfoliated BP flakes were stored in ambient conditions (25°C, 15%RH) with continuous light illumination. Degradation of the peeled BP flakes occurred due to the direct exposure, which can be seen from the optical images (Figure 1e) of a representative flake (thickness ~95 nm). After exposed for 2 h, the BP flake surface was smooth while only few blobs emerged at the edges, indicating that defects or edges of the flakes might be preferentially oxidized
46
. From the flake surface after exposure of 40 h
shown in Figure 1e, it could be seen that small blobs would grow and coalesce as the exposure time increased, resulting in less but larger blobs on the surface of the BP flake. Similar degradation behavior of another flake (thickness ~42nm) shown in Figure S1 of Supporting Information also confirmed the spontaneous oxidation and water absorption of BP flakes during degradation. However, from the optical image of BP flake after 7 days exposure (Figure 1e) under the ambient conditions in this study, it was noticeable that the bump-like structures on the BP surface tended to disappear with longer exposure due to the possible evaporation of water. As shown in Figure 1e, something like residues might be left on the BP surface where water blobs ever existed. 3
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Figure 1. (a)Typical schematic structure of BP: single-layer, top view and side view of bilayer ; (b) SEM image of the layered bulk BP (scale bar: 1 µm); (c) low magnification TEM image of the peeled BP nanosheets (scale bar: 500 nm), inset: SAED pattern of the BP nanosheets (scale bar: 5 1/nm); (d) HRTEM image taken from freshly exfoliated BP flake (scale bar: 1 nm); (e) optical images of pristine BP flakes (~95 nm) on the SiO2/Si substrate and exposed in ambient conditions for 2 h, 40 h and 7 days, respectively (scale bar: 20 µm); (f) P2p and (g) O1s XPS spectra of pristine BP flakes and exposed in ambient conditions for 2 h, 40 h and 7 days, respectively. XPS measurements were performed to determine the chemical composition on the BP surface at various oxidation stages. The obtained P2p and O1s core-level spectra of pristine (newly exfoliated within 30 min) BP sample, BP flakes exposed for 2 h, 40 h and 7 days are illustrated in Figure 1f and g. As shown in Figure 1f, strong P2p3/2 and P2p1/2 peaks at 129.2 and 130.0 eV represent the characteristic peaks of crystalline BP, being in good agreement with the literature 47. The P2p3/2 and P2p1/2 peaks are dominant in P2p spectra of all the degraded BP surfaces, indicating that a majority of P-P bonds are 4
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preserved. However, because of the interactions between P atoms and O2/H2O in the environment, the binding energy of P2p3/2 and P2p1/2 peaks increase to 129.5 eV and 130.4eV after degradation. Additionally, broad sub-bands at around 134 eV are observed in the degraded samples (Figure 1f), which could be attributed to the produced oxidized phosphorus (PxOy) on the surface of the degraded BP flakes 48-50. However, no spectral feature is observed at about 134 eV in the P2p spectrum of the pristine BP surface. Owing to its wide range of oxidation states for phosphorus, the oxides on the BP flake surface contained phosphorus of low oxidation states ( III and lower) as well as high oxidation state (V), corresponding to different core-level binding energies 51. Normally, the peak at a lower binding energy corresponds to a lower oxidation state. After 2 h exposure, the peak at 134.1 eV of the maximum intensity indicates that the main production of the degraded BP was oxidized phosphorus with valence III 18, 51
. In addition, phosphorus of lower oxidation states (133.2eV) and high oxidation states (135.0eV)
were produced, taking up only 20% among all the oxidized phosphorus. The band of phosphorus oxides (PxOy) dramatically increases with the peak at 135.0eV becoming the predominant component, indicating that the largest amount of oxidized phosphorus V was produced with a longer exposure time. As for the pristine BP flake , only dominant O 1s peak located at 532.7eV is observed (Figure 1g), indicating the presence of the intermediate structure of P-O-P owing to the unavoidable oxidation upon exposure to O2 at room temperature 19. After exposure for 2 h, 40 h and 7 days, the asymmetric line shape of O 1s peaks indicates the presence of different oxygen species. In addition to the dominant peak located at about 532.9eV, another peak arises at about 531.6eV, which is reported to be consistent with terminal oxygen of P=O bonding 52 . From the relative intensity of the two peaks, it can be inferred that oxygen is mostly bonded within the structure of P-P bond on the degraded BP surfaces and the amount of bridging (P-O-P) oxygen is larger than that of dangling P=O bonds. With limited supply of oxygen, phosphorus trioxide (P4O6) is mainly formed with only bridging oxygen (P-O-P bonding) in the molecules. While the complete oxidation product phosphorus pentoxide (P4O10), the most stable oxide for phosphorus, contains both bridging oxygen and terminal oxygen atoms 53. After exposure for 2 h, 40 h and 7 days, terminal oxygen (P=O bond) was induced on the surface and phosphorus oxide of the higher valence state (V state) was increasingly produced, corresponding to the increasing peak at 135.0 eV in the P2p spectra. Moreover, the induced dangling oxygen (P=O bond) would increase the hydrophilicity of the BP surface 54 and lead to the formation of obvious surface blobs as well as degradation by water molecules 48, 51
. As XPS spectra are obtained in a high vacuum condition (10–9 Torr) where combined water molecules
are probably removed, it is likely that the dissolution of the PxOy species into varied phosphorus oxyacids occurs in ambient conditions in consideration of water absorption 48, 55. Lateral force mode (LFM) is frequently used to study the friction force between the AFM tip and 2D materials. When an AFM tip is sliding on the sample surface forward and backward at a constant normal loading force, lateral signal trace and retrace curves are recorded to form a friction loop. The average value of the friction force can be computed by the ratio between the friction loop area and the total tip 5
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displacement
56
. In this study, the BP surface was scanned under the lateral force mode to obtain the
height image as well as the friction trace image simultaneously. Figure 2a displays the AFM height image of a BP flake (about 52±3 nm thick) on a SiO2/Si substrate exposed to air for 2 h. The AFM height image of the micro-domains on the BP surface is given in Figure 2b1, from which randomly distributed micro-bumps are observed. Figure 2c1 shows the morphology of one micro-bump, of which the height profile indicated by the marked dashed line is shown in Figure 2c3. The height profile of the micro-bump reveals that the bump is 4.5 nm higher than the flat sheet and about 245 nm in diameter, which is consistent with the size of typical blobs formed on the degraded BP surface in previous observations
55, 57
. However, the BP flake displays the flat surface without any sign of
degradation when scanning under dry nitrogen (N2) atmosphere for 2 h (Figure S2 in Supporting Information).
Figure 2. (a) AFM height image of a BP flake on a SiO2/Si substrate exposed for 2 h (scale bar: 10 µm); (b1) AFM height and (b2) the corresponding friction image of the BP flake surface with micro-bumps (scale bar: 1 µm); (c1) AFM height and (c2) the corresponding friction image of one micro-bump on the BP surface (scale bar: 200 nm); (c3) height profile, in situ lateral trace/retrace signal and the calculated friction force profile of the marked line in (c1). The LFM results were obtained with an applied normal force of 1.9 nN. (d1) STEM bright-field image of BP flake degraded for 2 h; (d2) STEM-EDS spectrum of local region in (d1) (scale bar: 500 nm); (d3) P map and (d4) O map of the area marked by the rectangle in (d1) (scale bar: 200 nm). The friction image of the bump area is shown in Figure 2c2, where the flat area and the micro-bump show obvious differences in friction force. From the in situ lateral trace/retrace signal and the calculated friction force profile of the micro-bump area (Figure 2c3), it is noticeable that the bump area exhibits the 6
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friction force of about 3.8 nN, which is around 50% of that of the flat area (7.8 nN). The lower friction of the micro-bump formed on the BP flake surface could also be seen from other flakes within 2 h after exfoliation, as shown in Supporting Information, Figure S3. The atomic-scale roughness of the micro-bumps did not result in the higher friction force as expected, indicating that the contribution of topography-induced variation to the friction force of the bump was negligibly small. Therefore, the chemical reactions in the bump zone should be especially concerned. Even though XPS results confirmed the formation of oxidized phosphorus on the surface of the BP flake exposed in air for 2 h, transmission electron microscope (TEM) analyses, including energy dispersive spectrometer (EDS) via scanning transmission electron microscope (STEM), of the BP flakes were also needed to clarify the microstructure as well as the local chemical composition at a finer length scale. Because of the slight oxidation within 2 h, BP flakes still displayed good crystallinity under TEM (Figure S4b in Supporting Information). However, as shown in Figure 2d1, dark spots with diameters of around 100 nm or 253nm are observed on the flake surface, which is quite similar to the size of surface micro-bumps characterized by AFM (Figure 2b1 and 2c1). In addition to P, STEM-EDS spectrum (Figure 2d2) reveals the presence of O in this region. To further examine the element distribution, STEM area mapping was carried out. The P map (Figure 2d3) and O map (Figure 2d4) of the area marked by the rectangle in Figure 2d1 provide direct evidences of localized oxygen-enrichment of the dark spot. It can be deduced that the observed local bump formed on the BP flake surface within 2 hours is composed of oxidized P atoms, which is consistent with the reported results through high-angle annular dark field (HAADF) and transmission electron microscope-electron energy loss spectroscopy (TEM-EELS) data 55. The oxidation process of BP seemed to exhibit a nucleation step, with the micro-bumps acting as the nucleation spots for the occurrences of oxidation. Based on the chemical analysis on the BP flake surface, it can be concluded that the friction reduction of the micro-bump might be closely related with the oxidized phosphorus. The introduction of oxygen into the P-P structure of BP, on the one hand, would change its original bond length or angle and cause local physical deformation to form micro-bumps on the surface. On the other hand, the interaction between oxygen and phosphorus would break down the ordered crystalline structure of BP. The relative smooth area shows the original crystalline structure of BP while the locally oxidized area is mainly amorphous (Figure S4d2 in Supporting Information). In addition, the interaction between water molecules and pristine phosphorene is not strong unless the phosphorene is oxidized Water molecules would be selectively absorbed in strongly oxidized areas interaction with the induced O
19, 22
23, 58
19
.
through hydrogen-bond
, which also enhances the surface deformation of BP flakes. Due to
the increased hydrophilicity of the degraded BP surface, both the quantity and size of the blobs on the BP surface increase after exposure for 16 h on account of the water absorption of phosphorus oxides, and the BP flake is covered with blobs of about 4 nm in height, as shown in Figure 3a1. The surface roughness is highly increased and the friction force profile in the blob area mainly corresponds to its height profile 7
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(Figure 3a2 and a3). However, the degraded edges of the BP flake, without surface blobs, are observed to exhibit lower friction force than the flat area (Figure S5 in Supporting Information). It can be inferred that the measured friction force under LFM was dominated by the tip-water interface when scanning the BP flake surface with water blobs. Therefore, the enhanced friction force in the blob area might be caused by the enhanced surface roughness as well as the strengthened adhesion of the water droplet.
Figure 3. (a1) AFM height image of a BP flake exposed to air for 16 h (scale bar: 1 µm); (a2) friction image (scale bar: 200 nm) of the blobs on the BP surface marked by the square in (a1); (a3) height and in-situ friction force profiles of the blob area; (b1) AFM height image and (b2) the corresponding friction image of BP flake (exposed for 16 h) without surface blobs (scale bar: 500 nm); (b3) height and in-situ friction force profiles of the porous phosphorus oxide layer on the degraded BP surface in (b1); (c) TEM image of the degraded (12 h) BP flake with surface blobs (scale bar: 200 nm); (d) porous structure of the BP flake degraded for 16 h under TEM (scale bar: 200 nm); (e1) STEM bright-field image of the BP flake degraded for 16 h (scale bar: 200 nm) ; Insets show the EDS line scan profiles (green for P and yellow for O); (e2) P map and (e3) O map of the area marked by the rectangle in (e1) (scale bar: 100 nm); (f1) AFM height image of the BP flake degraded for 7 days (scale bar: 1 µm); (f2) deflection image, (f3) the corresponding friction image and (f4) adhesive force mapping of the marked area in (f1) (scale bar: 200 nm); (f5) height profile and in-situ friction profile of the BP flake degraded for 7 days. All the AFM results here were obtained with an applied normal force of 1.3 nN. 8
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In particular, the blobs formed on the degraded BP surface could disappear during the exposure in our study, which was different from the observations reported by other researchers
20, 52, 59
. We noticed the
relative humidity (15%) of the ambient environment in this study was lower than the reported conditions 20, 45
, which might influence the water absorption taking into consideration of the evaporation of water on
the BP surface 60. In this study, water absorption seemed to be critical for the surface morphology of a degraded BP flakes. Figure 3b1 shows the AFM height image of degraded (16 h) BP flake surface where the blob structure disappeared but a porous layer of about 1 nm in height formed instead. The formation of the surface layer as a result of water evaporation could be confirmed by the observation of the surface layer (shown in Figure 3c) of BP flakes under TEM, where the surface blobs were removed in the high vacuum condition. In addition, plenty of pit structures on the surface of the degraded BP flakes are observed under TEM and the average pit diameter seems to increase with the degradation time (Figure S6 in Supporting Information). As seen from Figure 3d, the BP flake degraded for 16h displays surface pits of around 500 nm in diameter, being similar to the observed porous layer shown in Figure 3b1. STEM bright-field image of the pit area on the degraded (16 h) BP flake surface and in-situ EDS line scan profiles (green for P and yellow for O) are shown in Figure 3e1. O enrichment can be observed at the pit edge while there is no change of P concentration in this region. Furthermore, STEM-EDS area mapping (Figure 3e2 and e3) confirms that O is not distributed homogeneously but heavily concentrated along the pit outlines. Therefore, it could be said that the observed porous layer on the degraded BP surface (Figure 3b1) was attributed to the surface pitting oxidation mechanism described in previous study
42
and the
composition of the porous layer was mainly the produced phosphorus oxides owing to degradation. The XPS results given in Figure 1f also confirm the existence of phosphorus oxides on the degraded BP surface due to the absence of water. In consideration of the presence of water molecules in ambient conditions, it can be inferred that the pit area was the place where water blobs ever existed and the pit structure was induced by the removal of water molecules. Owing to the partial evaporation of water
60
, phosphorus oxide species that ever
dissolved in water blobs might concentrate at the blob boundary where the evaporation rate is larger 61, resulting in the formation of porous PxOy layer. Since the oxidation of BP flake was considered to initially occur at the topmost layer
18, 55, 62
, the PxOy layer formed on the surface was observed to barely
influence the crystalline structure of the layers beneath (Figure S6d in Supporting Information). With the longer degradation time, the thickness of the oxide layer was also increased. Such a kind of oxide layer was also observed from the BP flake surface exposed for 7 days (Figure 3f1) and the average height of the observed oxide layer increases to 3.1nm (Figure 3f5). From the corresponding friction image in Figure 3b2, it seems that the lower friction area is detected along the outline of the porous oxide layer. The friction force of the flake surface covered with an oxide layer is about 16.2 nN while the average friction force of the flat surface without an oxide layer is 33.8 9
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nN. As for the BP flake degraded for 7 days, the friction force of the oxide layer is about 15.5 nN while that in the area without an oxide layer is 32.9 nN (Figure 3f3 and f5), confirming that the observed friction reduction was mainly caused by the degradation product (PxOy) layer in air. Moreover, the adhesive force mapping image (Figure 3f4) of the degraded BP surface, in-situ obtained by using the same AFM tip, shows that the oxide layer area exhibits the lower adhesive force (~9 nN, dark blue area in Figure 3f4), which basically corresponds to the area with the lower friction force (dark area in Figure 3f3). The force-distance curves measured on the degraded BP surface upon approach and retraction are given in Figure S7 of Supporting Information. As described by the Bowden-Tabor adhesion model for interfacial friction 63, the friction force can be written as Ff = τA + Fp, where the friction force Ff under a constant normal load is determined by the true contact area A and the shear strength τ, regardless of the load-dependent ploughing force Fp. The shear strength τ scales with the measured adhesion force. Therefore, the reduced friction force of the oxide layer was directly related with its reduced atomic interaction and decreased shear strength with the AFM tip. Nevertheless, continuous scans of the same area on the degraded BP surface by the AFM tip would lead to the removal of the oxide layer and re-exposure of the bare surface (Supporting Information, Figure S8), suggesting the weak bonding of the oxide layer with the flake surface. From the LFM tests of the degraded BP flake surface, it can be concluded that the degraded area with phosphorous oxides covered exhibits a reduced friction force of nearly 50% of the flat areas. Therefore, we hypothesize that the reduction of friction force of degraded areas correlates with the oxidized phosphorus. In addition, a similar conjecture has been mentioned during the friction process of composites with BP nanosheets as additives, where the friction coefficient was dramatically diminished due to the added BP nanosheets, with a transfer film composed of phosphorus oxide or phosphoric acid formed on the counterpart surface 14-15. However, due to the hygroscopicity of the phosphorus oxide layer, the influence of the moisture combination in the oxidized area or the water condensation between AFM tip and the sample surface on the nano-friction behavior of the degraded BP flake should also be considered
63
. The adhesion force at the AFM tip-sample interface in ambient conditions mainly arises
due to van der Waals (FVdW), electrostatic (FES) and attractive capillary meniscus (Fmen) forces.64 Since the commonly formed water meniscus between the tip and the sample, Fmen is known as the main contribution to the strong adhesion during the measurements in air. For instance, the measured value of Fmen for few layer graphene accounts for about 60% of the total adhesion.65 However, when both the tip and sample are immersed in water, Fmen can be effectively eliminated at the interface.37, 64 To verify the role of water molecules, the degraded BP flake already exposed for 7 days was treated in a dry condition to merely remove the combined water molecules, and then the LFM test was conducted in a humidity cell with a low relative humidity of
