Recent progress on degradation mechanism and antioxidation of low-dimensional black phosphorus

Among the allotropes of phosphorus, black phosphorus (BP) is one of the most thermodynamically stable structures. Due to its unique physical and chemical properties, BP has shown considerable potential in many applications, such as field-effect transistors, energy storage and conversion, and photocatalysis. However, low-dimensional BP is easily corroded by oxygen and water owing to the large specific surface area and unbonded lone pair electrons on the surface, which reduces its chemical stability in the environment. As a result, different passivation approaches, relying on noncovalent bonding, covalent functionalization, and surface coordination, are employed to enhance the stability and performance of BP. In this review, the degradation mechanisms of BP are first analyzed for the material in both its ground state and excited state. Subsequently, the promising strategies for improving stability are overviewed. A comprehensive and in-depth understanding of the oxidation mechanisms and protection strategies of BP will provide guidance for the large-scale applications of BP and its derivatives.


Introduction
With Moore's Law approaching the limit, traditional silicon-based semiconductor devices have encountered the development bottleneck, and it is urgent to find new materials to continue Moore's law [1].Two-dimensional semiconductor materials have the advantages of achieving ultra-thin limit channel thickness, high carrier mobility, and tunable energy band, and are expected to be developed in the field of microelectronic devices in the post-Moore era [2].Two-dimensional BP is considered one of the most promising next-generation semiconductor materials due to its suitable carrier mobility and switching ratio, which can effectively improve device operation speed and reduce transistor power consumption [3,4].Furthermore, BP possesses variable bandgap, significant in-plane anisotropy, good transmittance, and mechanical flexibility, which shows good development prospects in the fields of energy conversion and storage, photoelectric sensing, photo/electrocatalysis, and flexible devices [5][6][7][8].
Compared with bulk BP, low-dimensional BP structures, such as BP nanosheets, BP nanowires, and BP quantum dots, exhibit quantum effects such as quantum confinement, showing novel physical and chemical properties.Although the novel properties of low-dimensional BP have great potential for a wide range of applications, its poor chemical stability limits its practical applications.It is generally accepted that thinning of BP will exacerbate its oxidative degradation [9].In the literature, the stability of black phosphorene (one monolayer of BP) was found to be significantly weaker than that of violet phosphorus (VP) (or violet phosphorene) [10,11].Although BP has shown promising potential for various applications, such as in-suit tissue regeneration [12], lubrication [13], different protection methods are needed to delay its degradation in order to maintain its excellent semiconductor properties.To date, a variety of physical and chemical passivation strategies, including passivation coating and covalent functionalization, have been proposed to battle against oxidants, to mitigate the degradation of BP in ambient conditions [14][15][16][17][18].
There have been multiple review articles focusing more on the material synthesis and properties of BP [19].In this review, we provide a comprehensive overview of the degradation mechanisms and different protection strategies for the material.After a brief discussion on the oxidation origin of BP, including structural factors and environmental factors, we offer a summary of recent advances in BP passivation methods.In addition, the challenges and future development of antioxidation strategies for BP are presented.

The structures of BP
There are multifarious allotropes of phosphorus, such as white phosphorus (WP), red phosphorus (RP), VP, blue phosphorus, and BP.BP further has various variants, including orthorhombic structure, hexagonal structure, simple cubic structure and amorphous [20,21].BP belongs to the orthorhombic structure at room temperature and under ambient pressure.The orthorhombic BP can be transformed into other phases by high pressure, from a semiconductor orthorhombic phase to semi-metallic hexagonal phase, and to metallic simple cubic phase.The two high-pressure phases of BP exhibit superconductivity at low temperatures [22].The ambient phase of BP is a layered semiconductor material, and its lattice structure is shown in figures 1(a)-(d) [23].In the monolayer BP crystal, P atoms are located in two different planes.Each P atom hybridized by the sp 3 orbitals in the layer is covalently bonded to three adjacent P atoms, which forms a folded honeycomb pattern that breaks the individual bonds of the P 4 unit (figure 1(e)) [24], resulting in two different bond lengths and bond angles.The bond length R 1 of the in-plane P-P bond connecting in-plane P atoms is 2.224 Å and the bond angle θ 1 is 96.3 • .Another bond length R 2 of the out-of-plane P-P bond connecting P atoms in the upper and lower sublayer is 2.244 Å and the bond angle θ 2 is 102.095• .In the layered bulk BP, the adjacent monolayers are coupled by the weak van der Waals forces to form layered structures, with a P-P distance around 5.5 Å.There are four possible stacking configurations between the two adjacent monolayers.Among them, the AB stacking is the energetically most favorable [25].

The properties of BP
For the energy band structure, the bandgap varying from 0.3 eV to 2.0 eV can be adjusted by thinning the thickness or doping, resulting in the variation in electronic and optical properties, but the bandgap remains direct [26,27].The narrow bandgap of BP builds a bridge between the zerobandgap graphene (GR) and the relatively widebandgap transition metal dichalcogenides (TMDCs), enabling BP to exhibit good photoresponse properties in the infrared region.As shown in figure 2(a), the electric transport performance of BP is between GR and TMDCs.Despite of its well-known very high mobility, due to the lack of a bandgap, the on-off ratio of GR-field effect transistors (GR-FET) is usually below 10.However, a majority of TMDC based FETs have high on-off ratios that can even exceed 10 8 , but their mobility is generally less than 100 cm 2 V −1 s −1 [28].In comparison, BP has a suitable level of carrier mobility and on-off ratio.Theoretically, the carrier mobility of a thin-layer BP-FET can be as high as 10 000-26 000 cm 2 V −1 s −1 , and the on-off ratio is between 10 3 and 10 5 [28][29][30].In literature, the carrier mobility and on-off ratios have been reported in the range of 10 2 -10 3 cm 2 V −1 s −1 and >10 5 , respectively [31].These results indicate that BP has great potential for broad applications in photoelectric conversion and low power devices.Unlike most two-dimensional materials, BP exhibits the in-plane anisotropy associated with the folded honeycomb structure.The different crystal structures of the armchair and zigzag direction also lead to anisotropy in the effective mass of electron and hole as exhibited in figure 2(b) [32].Therefore, BP devices with special applications can be designed by considering their anisotropic characteristics.
Although the material properties of BP have been extensively studied in recent years, the long-term stability of the material, particularly of the few-layer BP, remains inadequate for most real-world applications.Compared with WP and RP, bulk BP is generally considered chemically stable and hardly reacts with other substances.However, it does not mean that the exposed surface of a bulk BP can remain pristine or is not reactive with O 2 .As we know, Si will

The degradation mechanisms
A material may degrade spontaneously without subjecting to any forms of energetical excitation, such as light, temperature, current injection.This type of degradation can be classified as ground state degradation, which may be caused by either intrinsic mechanisms, such as the material does not have an adequately large formation energy, or extrinsic mechanisms, such as surface oxidation and those driven by structural defects.Often external energetic excitations can accelerate the degradation, because an excited state of a system is understandably less stable.This type of degradation can be classified as excited state degradation.Photo-degradation is often observed in semiconductor materials, including the extensively studied halide perovskites [34] and the few-layer BP.Below we offer an overview for different degradation mechanisms in BP.

Characterization of BP oxidation
There is a variety of techniques available to monitor the oxidation of BP.Raman spectroscopy, XPS, AFM, optical/electron microscopy, electron energy loss spectroscopy (EELS), nuclear magnetic resonance (NMR) spectroscopy and contact angle measurement can be used not only to study the properties of the material itself, but also to characterize the oxidation/degradation of BP.Moreover, other methods to characterize the optical and electrical properties of BP-based devices, such as PL spectroscopy, UV-vis spectroscopy, I-V characteristics of BP-FET can also identify the degradation of BP.BP oxidation can be distinguished at two different levels by Raman measurement.On the one hand, the integrated peak ratio A 1 g /A 2 g > 0.2 of the Raman spectrum is considered as low oxidation levels [35,36].On the other hand, different oxidation degrees and oxide geometries lead to inconsistent peak positions of high/lowintensity peaks in BP Raman spectra [37].Of these, the latter remains to be further investigated.The generation of P-O bonds can be used to determine the oxidation by P 2p signal of the XPS spectrum of BP [38,39].The bumps on the BP surface can be visually observed by AFM image, which can be analyzed for the surface roughness and thickness changes before and after oxidation [38,40].Another intuitive characterization method is light/electron microscopy, including optical microscopy, TEM and STEM, which can detect the oxidation of BP by the shape and color changes of the flakes [41][42][43].EELS equipped by TEM can also identify the presence of BP in oxygen [35,44].NMR spectroscopy is also used to identify the presence of phosphoric acid on the surface of BP to determine oxidation [45].Since the surface of unoxidized BP is hydrophobic, contact angle tests can also monitor BP oxidation [44].This measurement time is short-lived because water also degrades BP and makes it hydrophilic [46].
Furthermore, the electrical and optical performance of different types of BP-based devices is also found to be sensitive to the appearance of oxygen defects [41,44,45,47,48].For example, the conductivity and response time of a BP gassensor will change when the charge transfer occurs between the gas-sensitive material and the gas molecule in the resistive gas sensor.Adsorption of electron-withdrawing NO 2 molecules leads to gas sensitive response due to p-type doping of bare-BP.The electron-rich region on the surface of BP increases with the aging of BP.The electron-absorbing NO 2 prefers to attach to the electron-rich region, and the sensitivity of the BP gas-sensitive device increases.As being oxidized further, BP is consumed, and the gas sensitivity becomes worse [48].

Ground state degradation
The low corrosion resistance of bulk BP is closely related to its properties.Each P atom has a valence electron configuration of 3s 2 3p 3 in its outermost shell so that it has five valance electrons available for bonding.In the BP structure, the special bonding configuration of P atoms results in the existence of one unsaturated lone pair of electrons on the surface of each P atom, which leads to a high affinity of BP flakes for oxidation.As a promising two-dimension layered material, BP retains the natural property of the two-dimension material with a large specific surface area, which provides more active sites for oxidation.Notably, the oxidative degradation problem becomes more severe while the thickness is reduced to a few layers [9].The bandgap of thin layer BP has a good energy level matching relationship with O 2 /O − 2 redox potential due to the change of band gap [40,49].In addition, degradation caused by crystal structure cannot be underestimated.On the one hand, the etching rates of crystallographic directions for BP are not isotropic.A preferential degradation along the [001] and [001] crystal orientations was proved by experiment and theory [50].On the other hand, the high reactivity of BP to water and oxygen was observed owing to the presence of lattice defects [51,52].
Oxygen is the most important factor leading to the degradation of BP.No matter whether the BP flakes have defects or not, they can produce P x O y species on the BP surface.Ziletti et al [46] used firstprinciples calculations to show that the degradation of phosphorene is essentially attributable to oxygen.They speculated that one pathway for BP oxidation is related to the transition of oxygen from triplet to singlet state acted to oxidize phosphorene in ambient conditions.And BP defects to which oxygen atoms are chemisorbed enhance the hydrophilicity of the thin BP.In 2020, Oh et al [39] confirmed that long-term exposure to high O 2 content could aggravate the oxidation degree of few-layered BP, which was more obvious at low temperature as probed by low-temperature core-level photoelectron spectroscopy under ultrahigh vacuum conditions.Further insights were proposed by Naclerio et al [50] by taking into account the crystal orientation and the influence of beam energy during testing.They believe that there is an energy barrier for the oxidation of BP, and the oxidation could only occur with the help of other factors.BP can be etched by highly oxidizing ozone.Kwon et al [38] applied ozone produced by ultraviolet radiation in a high-purity oxygen atmosphere to thin BP.In addition, singlet oxygen ( 1 O 2 ) and O radicals can also contribute to the degradation of BP [38,53].Therefore, the stability of BP is reduced in practical applications.
Another non-negligible factor that weakens the stability of BP is H 2 O.However, whether BP's surface is hydrophilic or not remains uncertain.Some studies have shown that the degradation of BP in a wet environment is due to the hydrophilicity of the surface, which accelerates BP's instability [54,55].As shown in figure 3(a), thin layers of BP are rapidly showing signs of oxidation in the air within 24 h.In figures 3(b) and (c), the freshly exfoliated BP flakes were visibly rived after being placed in the air for two weeks, due to the long-time adsorption of H 2 O. Owing to the presence of a strong dipole moment on the surface of BP, the destruction of BP  [54].(d), (e) The contact angle of water droplets on the surface of BP changes with time [44].Reproduced from [54].© IOP Publishing Ltd.All rights reserved.Reprinted with permission from [44].Copyright (2016) American Chemical Society.
in figure 3(c) is responsible for the prolonged water absorption, which implies that BP is hydrophilic [54].Contrary to the above view, other studies revealed that BP's surface was hydrophobic [44,51,52], and the change of BP's surface from hydrophobic to hydrophilic in figures 3(d) and (e) was caused by oxygen [44], because H 2 O was apt to degrade defective phosphorene [52].Moreover, H 2 O prefers to drag BP lattices with surface defects such as steps, crystal boundaries, edges, oxidized BP, etc.In general, the degradation of BP by water is not controversial.

Excited state degradation
In general, the oxidative degradation of BP in the atmospheric environment mainly originates from the co-existence of the external stimuli, such as light, electron-beam (e-beam) energy, thermal energy (including laser, annealing, and heating), chemicals.Visible light plays a vital role in the corrosion of BP.It accelerates the oxidation of the BP flake, while the degradation of BP is significantly reduced under dark conditions [56].To explore the mechanism of light on BP degradation, Ahmed et al [53] determined the different spectral wavelengths ranging from ultraviolet to infrared on the degradation of BP in 20-30 nm thick.Figure 4 shows the surface morphology changes of BP after 120 min of exposure at different wavelengths.As can be seen from the figure, the holes on the BP surface are obvious under ultraviolet light (280 nm), blue light (455 nm) can also generate holes on the BP surface but smaller, and other longer wavelengths of light have negligible effect on BP degradation.The result indicates that the high photon energy in ultraviolet and blue light is the archrival of BP stability.Besides absorbing energy from light, BP can also absorb e-beam energy and thermal energy to obtain the high reactivity of water and oxygen [21,39,50].For the oxidation of BP, light-induced degradation mechanism are widely reported [49].The photodegradation process can be divided into three steps (figure 5 .Moreover, the faster oxidation rate of thinner BP was also confirmed in figure 5(c).Subsequently, similar photo-degradation mechanisms were advocated by Kuntz et al [52] and Ahmed et al [53].
Generally, thermal degradation of BP has been considered negligible.In some previous reports, thermal annealing was shown to enhance the performance of BP [57,58].After thermal annealing (250 • C) for 5 min in ambient conditions, the hole mobility of BP-FET increases from 15 cm 2 V −1 s −1 to 160 cm 2 V −1 s −1 [58].The high temperature treatment can remove the oxide layer on the surface of BP, leading to a decrease in the thickness of the BP channel, and indicating that the structure of BP can be affected by temperature.Even though BP has relatively high thermodynamic stability, other studies have revealed that BP may be degraded into amorphous phosphorus once the temperature exceeds 400 • C [10,11,21].Thermal induced degradation of BP may occur at even lower temperatures in atmosphere [36].After a high-powered laser is applied, the BP flakes will gradually sublimate, owing to the conversion of absorbed light energy to heat energy [47].Another worth mentioned energy source that may cause degradation of BP is e-beam energy.Studies have shown that high-energy e-beam will contribute to BP degradation [50,59,60], which suggests that the sample should be kept away from radiation damage during the testing.In the literature, the FET prepared when fully covered with perfluorosulfonic acid (BP-PFSA) degraded rapidly once bias voltage was applied [61].Therefore, we speculate that carrier injection may also cause the degradation of few-layer BP, but this needs to be confirmed by further studies.
It was reported that pH played an important role in the BP's degradation [62].The research revealed that the degradation rate was the fastest in an alkaline environment, but slower in a neutral and acidic environment.Interestingly, OH − not only accelerated the dissolution of PO x generated on the BP surface but also directly interacted with BP in the aging process of BP.In addition, BP can be skived in organic solvent by some electron-deficient chemicals such as 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) and triphenylcarbenium tetrafluorobor ([Ph 3 C]BF 4 ) [63].

Passivation coating treatments
To solve the problem that the thin-layer BP is readily degraded because of a large specific surface, a proper surface passivation coating can effectively isolate the direct contact between BP and O 2 or H 2 O.Typically, BP surface does not interact chemically due to the density of the film, van der Waals force, electrostatic interaction, and other physical factors.The passivation scheme is to cover the BP surface with a stable oxide layer, which mainly consists of P 2 O 5 ,  [66].Reprinted from [68], Copyright (2019), with permission from Elsevier.Reprinted with permission from [66].Copyright (2020) American Chemical Society.A1 2 O 3 , HfO 2 , MgO, SnO 2 , and ZnO in the ambient environment [64][65][66][67][68][69][70][71].P 2 O 5 can also exist stably in an arid environment, which protects BP against the degradation.For example, Nan et al [64] found that P 2 O 5 formed a passivation layer to protect BP and the native oxidation layer was more uniform than plasma-induced oxidation.The FET based on BP-P 2 O 5 retained the carrier mobility of 220 cm 2 V −1 s −1 after 2 weeks of storage in air.However, H 2 O is still an irreducible factor, leading to BP degradation in practical applications.The water-adsorbing P 2 O 5 separates from the surface of BP, which will bring about the corrosion of the underlying BP.To avoid this situation, Wu et al [68] deposited a thin hydrophobic SnO 2 film on BP's surface by electron-beam evaporation (EBE).As shown in figures 6(a)-(c), the performance of BP-FET passivated by SnO 2 did not degrade significantly after exposure to air for 15 d, but the pure BP-FET device current became nonconductive.The results demonstrated that SnO 2 passivation is promising.Limited by the characteristics of the EBE deposited film, the loose oxide film allows O 2 to invade the surface, so atomic layer deposition (ALD) and magnetron sputtering are employed for obtaining a dense oxide layer.At present, Al 2 O 3 and HfO 2 films deposited on BP's surface via ALD and magnetron sputtering have obtained considerable application prospects [66,70,71].The Al 2 O 3 (or HfO 2 ) film was encapsulated on the BP surface by ALD to prevent the erosion by O 2 and H 2 O [66].A serious degradation appeared in BP flakes without Al 2 O 3 passivation after one day of ambient exposure (figure 6(d)).In contrast, passivated BP had no obvious surface changes after 15 d under environmental conditions (figure 6(e)).The results displayed that Al 2 O 3 passivated BP layer could reduce the affinity of BP to O 2 and H 2 O. Similar results were observed in figures 6(f) and (g) which represents the hole mobility and I on /I off ratio for encapsulated and pure BP-FET with different air exposure time.Additionally, the FET based on BP-Al 2 O 3 has good electric transport performance with a hole mobility about 420 cm 2 V −1 s −1 , an electron mobility about 80 cm 2 V −1 s −1 , and a switching ratio more than 10 3 .At present, the BP-FET with Al 2 O 3 encapsulation can exhibit a stable performance for at least 17 months, which is the longest survival time reported so far [72].
Besides the single-sided passivation, the doublesided passivation can be achieved by wrapping the BP film in a sandwich structure, providing increased stability of BP.In 2019, Liu et al [73] prepared the ZnO-BP-ZnO structure by ALD.About 85% of the photoluminescence (PL) intensity was retained when BP was in the air for a month, which indicated that only slight degradation was observed by the stability test.The results were consistent with those of Li et al [74] reported in 2021, where the structure of hBN-BP-hBN was fabricated by using a combination of O 2 plasma etching, hBN sandwiching, and thermal annealing process, and the remaining 80% of the PL intensity after a period of air exposure.Most importantly, a record-long lifetime remaining more than seven months was achieved by the structural design.In general, both single-sided protection and sandwich structures can protect BP against O 2 and H 2 O. Therefore, the stability of BP can be greatly improved by covering BP with passivation material containing a stable structure.
Taking advantage of the large specific surface area of two-dimensional materials, the BP surface is encapsulated with other stable two-dimensional materials such as hBN, GR quantum dots (GQDs), and MoS 2 , SnSe 2 , and GaN [75][76][77][78][79][80] to form a heterojunction structure for passivation.Moreover, such vertically stacked heterojunction structures are not sensitive to the lattice mismatch between twodimensional materials and other passivation materials, which is beneficial to applications.

Noncovalent protection
Unlike covalent functionalization, non-covalent functionalization is another effective passivation method.Covalent functional materials may distort the BP lattice due to the strong chemical interactions between the functional material and BP, even leading to crystal structure damage.In contrast, non-covalent modification can well preserve the crystal structure integrity of phosphorene or thin BP.
In 2019, Wild et al [88] tried the noncovalent protection of thin-layer BP using perylene diimides (PDI).BP/PDI flakes could survive for 7 d, after which oxidization appeared.The stability improvement of BP-PDI is because of PDI keeping H 2 O, O 2 out and scavenging photo-induced electrons from the conduction band of BP.The PDI encapsulation can indeed improve the environmental stability of BP, but its protection efficiency is dissatisfactory and cannot meet the requirements of practical applications.Lloret et al [89] proposed a noncovalent functionalization strategy using the PDI with different periamide aromatic side chains (R1-R3).The pure BP (10 nm thick) was degraded after 72 h.BP/PDI-R1 could be stabilized up to 7 d.When the BP was passivated by PDI-R2 and PDI-R3, the hybrids could survive more than 22 d at ambient moisture.Compared with PDI-R1 and PDI-R2, BP/PDI-R3 has stronger van der Waals adsorption capacity.The 30 nm thick flakes showed no signal of oxidation after 58 d, which indicated that the passivation of PDI-R3 was much more effective.
In general, lattice defects (vacancies) tend to occur during the production of low dimensional BP.Due to the high activity and mobility of these vacancies, the thinner BP is more easily degraded under environmental conditions.Therefore, the passivation of P vacancy on the BP surface can reduce its high affinity for environmental molecules [90].The expandable π electronic structure of the anthraquinone (AQ) molecule can enhance the non-covalent interaction with phosphorene, which enables the AQ molecule to adsorb preferentially on the active vacancy of the BP surface.The P vacancies can be passivated using AQ nanowire encapsulation to enhance oxygen resistance [48].The light absorption of BP-AQ was only a 25% reduction after 6 d of storage at RT and RH = 25%-30%, while the absorbance of pure BP decreased by nearly 50%.Even after 16 d, BP-AQ had a higher absorbance (close to 50%) than pure BP (less than 10%).The gas sensor fabricated by BP-AQ achieved good selective recognition for NO 2 and NH 3 .To sum up, noncovalent methods include sequestration of H 2 O and O 2 , removal of photoexcited electrons, and passivating defects.These pathways slow the aging of BP, so noncovalent protection is a feasible method to delay the high degradation activity of BP.

Surface coordination
To suppress the lone-pair electron corrosion of fewlayer BP, surface ligands are expected to deplete the lone pair electrons on the BP's surface.Lone pair electrons on the BP surface can be removed by orbitals provided by surface coordination ions and groups to reduce electron density and reactivity on BP surface, thus improving environmental stability.In 2019, Jia et al [91] employed titanium sulfonate ligand (TiL 4 ) to coordinate with the BP surface lone pair electrons to reduce BP instability in air and water.In figure 7(a), the BP flakes without coordination treatment were degraded within 24 h at RH = 30%, while, the BP-TiL 4 flakes showed no signs of degradation after 96 h at RH = 99% (figure 7(b)).The lone pair electrons on the BP's surface were transferred to TiL 4 , which is the reason for the excellent stability of BP to H 2 O and O 2 .The results imply that passivation of the lone pair electrons on BP's surface is effective.In the same year, the coordinate BP sheets using two electron-deficient reagents was stabilized for 4 months [63].
Traditional functionalized treatment can only achieve unidirectional adjustment of BP's reactivity.It is difficult to realize reversible regulation of nanomaterials reactivity.Liu et al [41] imitated an organic protective-deprotective process and developed a method for rationally regulating the reactivity of BP, as shown in figure 8(a).They first prepared the BP/Al 3+ coordination complexes, during which the common noble metal ions, heavy metal ions, and light metal ions were systematically screened to coordinate BP protection, but their passivation ability to BP was weaker than that of Al 3+ .Subsequently, the BP/Al 3+ coordination complexes were encapsulated through hydrophobic 1,2benzenedithiol (BDT) forming the Al-S bonds.In the stability test of BP and BP/Al 3+ /BDT complex shown in figure 8(b), the bubbles appeared on the BP's surface after 1 d of ambient exposure and the surface became rougher after 7 d.Contrary to the pure BP, the BP/Al 3+ /BDT did not appear aging after the flakes were exposed to the same conditions for 60 d.These results indicated that the surface coordination combined with passivation coating was triumphant in reducing the reactivity of the BP surface.The ultrastability compound could be preserved in ambient conditions for up to 1 yr, which was attributed to two factors.On the one hand, the binding of Al 3+ to BP reduced the surface electron density of BP, resulting in a decrease in chemical reactivity.This dealt with an internal cause of oxidation of BP.On the other hand, the hydrophobic BDT layer could effectively prevent BP from etching by H 2 O and O 2 , which solved the external factors of BP's oxidation.More interestingly, the high reactivity of BP can be recovered by a Chelator treatment (EDTA-4Na), which could remove the Al 3+ and hydrophobic layer (BDT) on the surface of BP.Hence, the high surface electron density and high reactivity of BP was restored via the process shown in figure 8(a).

Covalent functionalization
Both surface coordination and covalent functionalization can host the lone pair electrons on the BP surface, but the biggest difference lies in the way of occupying the lone pair electrons.Surface coordination employs vacant orbitals of ligands to accommodate lone pair electrons, while covalent functionalization is the interaction between functionalized materials and P atoms forming chemical bonds to host the lone pair electrons on the BP surface.Liu et al [92] used 1 H,2 H,2 Hperfluorooctyltrichlorosilane (PFDTS) to covalently functionalize the BP, and obtained oil-and waterphobic few-layer BP.PFDTS was regarded as an ideal fluorinated coating, because its main chain contains plentiful -CF 2 and -CF 3 groups.The BP-PFDTS was both hydrophobic and oleophobic by high concentrations of PFDTS.As shown in figures 9(a) and (b), the BP-PFDTS sheets had both water and oil contact angle greater than 90 • (>1 mmol L −1 ), and even had ultra-amphiphobicity (>5 mmol L −1 ).Based on this characteristic, the BP-PFDTS could be stable in RH = 95% for more than 3 months, and the BP-PFDTS with added oleic acid could survive in aqueous environment for more than 2 months.Even though fluorination treatment is a promising passivation method, the conductivity of BP-based devices decreases with the increase of the thickness of the fluorinated layer.To solve the conflict between the thickness of the fluorinated layer and the conductivity, Hsieh et al [61] optimized the fluorination pathway to obtain BP sheets fluorinated with PFSA.In figure 9(c), the fully covered BP was obtained by spin-coating and annealing treatment (route 1), and ultrathin BP-PFSA was fabricated by ultrasound treatment (route 2).Compared to fully passivated BP-PFSA, the surface of ultra-thin BP-PFSA was slightly hydrophilic.However, the FET based on ultra-thin BP-PFSA could maintain stable electrical performance even when bias voltage was applied.In contrast, the FET prepared by the fully covered BP-PFSA degraded rapidly once bias voltage was applied.The formation of strong P-F covalent bond on the surface of BP effectively prevented the erosion of water and oxygen.Thus, the BP-PFSA device was stored for 5 months under ambient conditions.

Doping
By applying doping to adjust the band alignments between BP and O 2 /O − 2 redox potential, BP's oxidation resistance can be significantly improved.Chemical vapor transport (CVT) method is suitable for BP modification because of its strong operability and mild reaction conditions.Sn and Pb are acted as mineralizing agents, participating in the mineralization reaction during the preparation of BP via the CVT method, so they are naturally incorporated in the synthesis of doped BP by CVT.Sn-doped BP (Sn-BP) and Pb doped BP (Pb-BP) were successfully synthesized by Yu et al [93] using this method.The FETs based on Pb-BP were superior to those prepared by Sn-BP in purity, carrier mobility, switching ratio, electrical transport polarity.
These results indicate that the preparation of high-purity BP can be realized by optimizing the mineralizers.In the same year, Izquierdo et al [94] prepared a BP film with dimensions of 10 µm × 85 µm × 115 nm on silicon substrate by the CVT method.XPS measurement showed that a slight Sn doping existed in the BP film.It is noteworthy that there was no degradation of the BP film after 4 months in the ambient condition, indicating that Sn doping could effectively passivate the defects of the film and improve the stability of thin-layer BP.Although new progress has been achieved in the doped BP, it is still a challenge to grow doped BP crystals with high doping concentration and high quality.Therefore, Liu et al [40] fabricated BP doped with different dopants, including Te, Se, Bi, Sb.In figures 10(a) and (b), because the pure thin BP band edge states are partially overlapping with the O 2 /O − 2 redox potential, the surface shows apparent degradation after 16 d of exposure to air.However, the value of the conduction band edge of Te-BP and Sb-BP is lower than O 2 /O − 2 redox potential, which implies that the electrons in the doped BP conduction band are hard to transfer to the O 2 on the surface [49].Hence the Te-BP and Sb-BP showed a good stability and no degradation even after being left in the air for 16 d (figures 10(c) and (d)).On the contrary, the conduction band edge of As-BP is higher than the O 2 /O − 2 redox potential and, thus, degraded seriously after 8 d under the same conditions (figure 10(e)).Although the Se-BP bandgap is not conducive to the long-term stability of BP, adding Se can still effectively modify the electronic structure of BP.Viti et al [95] used Se-BP for terahertz photoelectric detection, showing a high potential for the development of BP-based electro-optical devices in the far-infrared region.In addition to the CVT method, other pathways such as surface transfer doping, thermal deposition, and ball milling can also modify BP.Although Xiang et al [96] did not perform stability tests, the bipolar properties of few-layer of BP were effectively modulated by doping caesium carbonate and molybdenum trioxide.Wang et al [97] obtained n-type modification for BP by thermal deposition of Al atoms (thickness was about 2 nm) under high vacuum conditions.Additionally, the modified BP was very stable within 10 d without significant degradation, while the pure BP flakes were slightly oxidized on the surface after 1 d and degraded severely after 4 d in the ambient conditions.First-principles calculations showed that the high stability of Al-BP benefited from the lowering of BP's conduction band edge and the formation of self-limiting Al 2 O 3 oxidation layer.

Rebuilding edge structure
Since BP tends to be oxidized preferentially at the edge, Zhu et al [98] employed ball milling technology to stabilize BP flakes via edge-selective bonding of sacrificial C 60 molecules by rebuilding BP's edge. Figure 11 exhibited the formation mechanism of the BP-C 60 hybrid.The P-P bonds are broken, producing reactive species (radicals and ions) at the BP's edges.In the meantime, C 60 is activated due to high-energy impact.Ultimately, the edge-passivated BP-C 60 hybrid is prepared with covalent P-C bonds.The degradation rate of BP-C 60 hybrid was slowed down by a factor of 4.6 compared to the mixture of BP and C 60 (M-BP) in water, which significantly improved the stability of BP via edge passivation.Furthermore, BP-C 60 hybrid had superior photoelectrochemical and photocatalytic activities than pure BP, pure C 60 , and M-BP.

Conclusion
Table 1 summarizes the passivation methods and their effects as well as related applications.The protection strategies can reduce the degradation rate of the fewlayer BP, although they cannot completely suppress the collapse of BP crystal, which is a reference for choosing reasonable passivation approaches.Low-dimensional BP has excellent properties and broad application prospects, but its poor environmental stability limits its development.Many factors that can lead to BP degradation, which are mostly related to the oxidation of the exposed surface or edge, may or may not involve external perturbations.In terms of the external perturbations, light (especially ultraviolet or blue light), water, oxygen, heat, e-beam energy, even electric injection, pH, and other factors may result in the oxidative degradation of the low-dimensional BP.Even in the ground state, various factors, including the large specific surface area, surface-active lone pair electrons, a good match between the BP band edge states and O 2 /O − 2 redox potential, thickness, crystal orientation, and crystal defects, also contribute to the degradation or ambient stability of BP.Due to the convolution of all these factors, the oxidation mechanisms of BP in air become complex and need further research.
Numerous protection methods addressing the degradation mechanisms of BP have been proposed and verified.The protection methods can reduce the degradation of BP to some degree.However, they still cannot adequately eliminate the BP degradation problem in the air.The densified passivation layer can somewhat effectively isolate BP from oxygen and water vapor contact.When BP is exposed to air for a long time, oxygen and water vapor can still penetrate the passivation layer and corrode BP.Noncovalent protection can guarantee lattice integrity, but the interaction force relying on electrostatic adsorption, van der Waals interaction, and other weak interactions is weak between the passivation agents and BP.Additionally, the interaction between the passivation agents and BP is destructed by the P x O y layer formed through rapid oxidation of BP surface in the air, which leads to the oxidative degradation of BP.Contrasting with the noncovalent protection, the introduction of strong covalent bonds by covalent functionalization results in significant lattice distortion, which damages the crystal structure and properties of the original BP.In addition, most of the covalent functional materials are organic reagents, which are toxic and pollutants to the environment and are not conducive to the large-scale use of the low-dimensional BP.Nevertheless, the covalent functionalization is considered to be one of the most effective methods to inactivate BP from oxidation.Similar to the covalent functionalization, the surface coordination removes the lone pair electrons on the surface of BP, reducing the electron density on the surface of BP and effectively improving the stability of BP.Furthermore, doping not only protects the integrity of the BP crystal structure but also effectively delays the degradation of BP.In general, although various protection strategies can passivate BP to a certain extent, the research on the degradation and protection of BP is still in its infancy, and its implementation in various fields still needs further development.In the future, large-scale preparation, large-scale transfer, and effective protection of few-layer BP will contribute to commercial processes.In that sense, the protection strategy of doping via the CVT pathway combined with passivation coating treatments is promising.Based on the mechanism of BP degradation, we look forward to developing more high-performance passivation strategies to cooperatively passivate BP to shorten the commercialization process.

Figure 3 .
Figure 3. Oxidation characterization of BP flakes and contact angle of water drops on the surface of BP. (a)-(c) Optical images of BP flakes oxidized in ambient conditions over time[54].(d), (e) The contact angle of water droplets on the surface of BP changes with time[44].Reproduced from[54].© IOP Publishing Ltd.All rights reserved.Reprinted with permission from[44].Copyright (2016) American Chemical Society.
(a)): (I) O 2 induces a charge transfer reaction on BP surface to form superoxide radical O − 2 under ambient light.(II) Phosphorene is oxidized by O − 2 to form PO x .(III) The acidic PO x falls off eventually, due to the adsorption of H 2 O reacted with the surface PO x by the hydrogen bond.The oxidation process is accelerated due to a good energy level match between BP's bandgap and the redox potential of O 2 /O − 2 (figure 5(b))

Figure 6 .
Figure 6.Structure, stability, and electrical characterization of passivated/pure BP-FETs.(a) Three-dimensional schematic view of FET based on BP-SnO2.(b) Transfer curves of the FETs based on BP or BP-SnO2 measured in air, V ds = −0.5 V. (c) After exposure in air for 15 d, transfer curves of the FETs based on BP or BP-SnO2, V ds = −0.1 V [68].The AFM images of (d) pure BP flakes and (e) passivated BP flakes by Al2O3.(f) The hole mobility, and (g) Ion/I off ratio for passivated and pure BP-FET with different air exposure times[66].Reprinted from[68], Copyright (2019), with permission from Elsevier.Reprinted with permission from[66].Copyright (2020) American Chemical Society.

Figure 10 .
Figure 10.The energy band diagrams and antioxidation of pure BP and doped BP.(a) The band alignments of pure BP and doped BP.(b) The surface morphology of BP (thickness about 25 nm), Sb-BP (thickness about 28 nm), Te-BP (thickness about 31 nm), and As-BP (thickness about 26 nm) changes with time [40].Reprinted from [40], Copyright (2020), with permission from Elsevier.