Applications of Phosphorene and Black Phosphorus in Energy Conversion and Storage Devices

Review

Black Phosphorus www.advenergymat.de Applications of Phosphorene and Black Phosphorus in Energy Conversion and Storage Devices

Jinbo Pang, Alicja Bachmatiuk, Yin Yin, Barbara Trzebicka, Liang Zhao, Lei Fu, Rafael G. Mendes, Thomas Gemming, Zhongfan Liu, and Mark H. Rummeli*

to the extreme high-pressure heating con- The successful isolation of phosphorene (atomic layer thick black phos- ditions required for its synthetic prepara- phorus) in 2014 has currently aroused the interest of 2D material researchers. tion, as well as a lack of knowledge of its applications. Then, a decade ago, Park In this review, first, the fundamentals of phosphorus allotropes, phos- and Sohn[2] exploited high-energy ball phorene, and black phosphorus, are briefly introduced, along with their struc- milling to fabricate black phosphorus as tures, properties, and synthesis methods. Second, the readers are presented anode materials for rechargeable lithium with an overview of their energy applications. Particularly in electrochemical ion batteries. Eventually, bulk crystals of energy storage, the large interlayer spacing (0.53 nm) in phosphorene allows black phosphorus were successfully fab- the intercalation/deintercalation of larger ions as compared to its graphene ricated in large batches by Nilges and co-workers[3–5] using a chemical vapor counterpart. Therefore, phosphorene may possess greater potential for high transport approach. This prompted its electrochemical performance. In addition, the status of lithium ion batteries exploration in energy-related devices such as well as secondary sodium ion batteries is reviewed. Next, each applica- as secondary ion batteries, supercapaci- tion for energy generation, conversion, and storage is described in detail tors, and solar cells. with milestones as well as the challenges. These emerging applications Few layer or monolayer black phosphorus possess a larger specific area than include supercapacitors, photovoltaic devices, water splitting, photocatalytic its 3D parents. Early works termed the hydrogenation, oxygen evolution, and thermoelectric generators. Finally material as nanosheets, ultrathin,[6,7] atomic the fast-growing dynamic field of phosphorene research is summarized and layer thin,[8] few layer,[9] and monolayer perspectives on future possibilities are presented calling on the efforts of black phosphorus.[10] Thus, we will need to chemists, physicists, and material scientists know the correct terminology from a strict chemical view. The word suffix ene is usually adopted for naming a molecular struc- 1. Introduction ture which consists of π electrons such as graphene. But there is no π electrons in black phosphorus, so that Black phosphorus was first synthesized by Bridgman in 1914[1] International Union of Pure And Applied Chemistry (IUPAC) but has been less intensively studied in the past century due name will be 2D phosphorane as suggested by Castro Neto

Dr. J. Pang, Prof. A. Bachmatiuk, Y. Yin, Dr. R. G. Mendes, Prof. A. Bachmatiuk, Prof. B. Trzebicka, Prof. M. H. Rummeli Dr. T. Gemming, Prof. M. H. Rummeli Centre of Polymer and Carbon Materials The Leibniz Institute for Solid State and Materials Research Dresden Polish Academy of Sciences (CMPW PAN) (IFW Dresden) ul. M. Curie-Sklodowskiej 34, Zabrze PL-41-819, Poland Helmholtzstr. 20, Dresden D-01069, Germany Prof. L. Fu E-mail: [email protected] College of Chemistry and Molecular Science Prof. A. Bachmatiuk, L. Zhao, Dr. R. G. Mendes, Prof. Z. Liu, Wuhan University Prof. M. H. Rummeli Wuhan 430072, China Key Laboratory of Advanced Carbon Materials and Wearable Prof. Z. Liu Energy Technologies of Jiangsu Province Center for Nanochemistry (CNC) Soochow University Beijing Science and Engineering Center for Nanocarbons Suzhou 215006, China College of Chemistry and Molecular Engineering Prof. A. Bachmatiuk, L. Zhao, Dr. R. G. Mendes, Prof. Z. Liu, Peking University Prof. M. H. Rummeli Beijing 100871, China Soochow Institute for Energy and Materials InnovationS (SIEMIS) School of Energy Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology Soochow University Suzhou 215006, China

DOI: 10.1002/aenm.201702093

Adv. Energy Mater. 2017, 1702093 1702093 (1 of 43) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advenergymat.de and co-workers[11] and Martel and co-workers.[12] But gradually over time, phosphorene[13] became accepted by the community Jinbo Pang is presently a post- and we will use this terminology, that of phosphorene for mon- doctoral fellow in the Leibniz olayer or few layer black phosphorus in this review. Institute for Solid State and Early in 2014, the theoretical work of few layer black phos- Materials Research Dresden, phorus pioneered by Castro Neto and co-workers[14] showed Germany. He received its band gap with tunable capability under strain. Later, phos- M. Eng. from the Institute phorene, a monolayer sheet of black phosphorus, was afterward of Photoelectronics, Nankai successfully experimentally exfoliated in 2014 by Zhang and co- University, China in 2011 and workers[8] as well as Ye and co-workers[13] in which they imple- Ph.D. in Materials Science mented the scotch-tape microcleavage method, which is well from Dresden University of known for graphene isolation. After these pioneering works, Technology (TU Dresden) in numerous 2D research teams began rapidly exploiting this new 2017. His research focuses material over the previously intensively studied 2D materials on the synthesis, device fabrication, and energy device such as graphene,[15–22] transition metal dichalcogenides,[23–30] applications of graphene and other novel 2D materials as hexagonal boron nitride,[31–38] MXenes,[39–47] and silicene.[48–52] well as characterizations with electron microscopy. Since energy storage and conversion have become so crucial for the future of consumer electronics, electrical vehicles, and grid storage, electrochemical energy devices based on phos- Alicja Bachmatiuk currently phorene are intensively studied. We therefore review the work heads the graphene group at thus far conducted on phosphorene in energy applications such Polish Academy of Sciences as secondary ion batteries, supercapacitors, solar cells, photo- (CMPW PAN) in Zabrze catalysts for solar fuel production such as hydrogen from water and looks after a double Cs splitting and hydrocarbon from carbon dioxide reduction, the corrected TEM in Wroclaw oxygen evolution reaction and hydrogenation, and thermoelec- Research Centre EIT+, Poland. tric power generators. In the review, the concepts and chal- She is a guest professor lenges in energy devices are presented and we hope to inspire at the Soochow Institute readers toward the future possibilities and opportunities with for Energy and Materials this material. We begin with a look at the fundamentals of InnovationS (SIEMIS), phosphorene. Soochow University. She obtained her Ph.D. in chemistry at Szczecin University and joined the IFW Dresden with a Humboldt postdoctoral 1.1. Phosphorus Allotropes fellowship.

Phosphorus is an earth-abundant element from group 15 in the period table of elements (also termed pnictogen group). Mark H. Rummeli heads the Because phosphorus easily loses electrons resulting in high electron microscopy labs reactivity, straightforward natural access of the free element on at the Soochow Institute Earth is yet to be reported. Typically it exists in an oxidized state for Energy and Materials InnovationS (SIEMIS), within phosphate rocks, e.g., Ca3(PO4)2. Phosphorus has four allotropes (Figure 1), namely, red, Soochow University. He white, violet, and black phosphorus, named according to their also oversees the gas appearance. White phosphorus and red phosphorus are the two sensor laboratory at Polish major allotropes. White phosphorus, consisting of tetrahedral Academy of Sciences, Zabzre. He earned his Ph.D. P4 molecules, can be easily obtained by the sintering of mineral phosphate rocks in the presence of coke and silica. It is reac- from London Metropolitan tive and volatile, and ignites in air at 34 °C; hence, it requires University and then worked as a Research Fellow at the German Aerospace Center. His water sealing for storage. Red phosphorus is a derivative of P4 wherein a PP bond dissociates and forms a new bond with an research focuses on the growth mechanisms of nanostructures and their functionalization and their application in adjacent tetrahedron P4; this eventually yields a chained structure similar to that of a polymer. Amorphous red phosphorus electronic, biomedical, and energy storage. can be prepared by heating white phosphorus in N2 at ≈300 °C or exposing it to sunlight. Further heating yields crystalline red phosphorus. Violet phosphorus can be synthesized by the long- time annealing of red phosphorus at 550 °C with the assistance from white phosphorus under extreme high-pressure heating of molten lead. conditions (1.2 GPa at 200 °C).[1] Black phosphorus bulk crys- Black phosphorus is the thermodynamically stable allotrope. tals consist of stacked layer structures, termed phosphorene.[53] Analogous to graphite in appearance, black phosphorus is shiny The interlayer interactions between these stacking layers are black and has good electrical conductance. It can be produced comparable to van der Waals interactions,[54–56] which can be

Figure 1. Phosphorus allotropes, their important compounds, and transformation reactions. easily broken to obtain monolayer phosphorene.[12,57–59] Phos- of phosphorene monolayers. The lattice constants for bulk phorene has now become a part of the 2D membrane family. phosphorene are a1 = 0.34 nm, a2 = 0.45 nm, and a3 = [130–132] In terms of phosphorous sources, diphosphorus (P2), the 1.12 nm. The sheet-sheet spacing between two phos- dimer of phosphorus,[60–66] exists in a gaseous state, and can be phorene layers is 0.53 nm (Figure 2a,d),[133–136] which is greater obtained by the thermal decomposition of white phosphorus than the interlayer spacing of graphene (0.33 nm).[137–141] The at 800 °C. Further cracking of P2 at 2000 °C yields the atomic relatively large spacing is due to its puckered structure, as well as monomer in the vapor phase.[67–69] The other important sources the AB Bernal stacking of two phosphorene layers in the unit cell. for elemental phosphorus are adenosine triphosphate,[70–74] The phosphorene layers are held together via a weak interlayer −1 [142–144] phosphoric acid (H3PO4), the fertilizer salt monopotassium interaction (20 meV atom ), which can be easily broken. [75] [76–81] phosphate, KH2PO4, and phosphane (PH3). The top view shows that the phosphorene monolayer (Figure 2f,g) has a honeycomb lattice structure analogous to that of graphene, but with an anisotropy along one basic vector due 1.2. Type and Properties of Phosphorene to the nonplanar structural ridges. The 2D orthogonal lattice constants for the phosphorene monolayer are a1 = 0.34 nm and [149–151] Phosphorene has a monolayer sheet structure and is the a2 = 0.46 nm for both the basic vectors. In the anisotropic building block for black phosphorus. According to its layer structure, different from symmetrical graphene, phonons, pho- numbers, phosphorene can be classified as monolayer,[82–84] tons, and electrons show highly anisotropic behavior;[82,152–158] bilayer,[85–87] and few-layer phosphorene.[88–90] Based on its hence, phosphorene has great potential for thin film elec- morphology, phosphorene can be categorized as quantum tronics[88,159–162] and infrared photoelectronics.[132,163–167] dots,[91–95] nanoribbons,[96,97]nanowires,[98–100] nanorods,[101–103] microrods/platelets,[100] nanotubes,[104,105] nanoparticles,[106] nanoflakes,[56,57,107–109] and microscale flakes.[110,111] Further, in 1.2.2. Atomic Orbital Hybridization terms of atomic configuration, phosphorene can be classified as black phosphorene (commonly termed phosphorene),[112–116] The sp3 orbital hybridization[168] formed by the three σ bonds blue phosphorene,[117–123] and red phosphorene.[124,125] To and one lone pair bond gives rise to the nonplanar puckered understand its potential applications, one first needs to under- structure of phosphorene. The nonbonding lone pair bond sta- stand the properties of phosphorene. bilizes the geometrical distortion[169] in pristine phosphorene. Because of the out-of-plane orbitals, the lone pair of electrons lead to interlayer interactions,[55] often approximated as van der 1.2.1. Crystal Structure Waals interactions.[55,170] Chemisorbed[171–174] oxidizing gases such O2 and NO2 often reside on the lone pair. In addition, dec- Phosphorene possesses a puckered structure,[11,126–129] as oration with metal particles[114,118,175,176] leads to orbital hybridi- observed from a side view normal to the armchair direction zation with the lone pair. Under high pressures (≈5 GPa), the (Figure 2a). Few-layer phosphorene forms from the stacking lone pairs cause structural rearrangement resulting in phase

Figure 2. Fundamentals of phosphorene: a) crystal structure, b) electronic band structure, c) AFM image, HR-TEM of d) side view and g) top view, e) EELS, f) side view and top view of atomic structure, h) SAED, i) optical micrograph, and j) Raman spectra. (a) Reproduced with permission.[145] Copyright 2017, American Chemical Society. (b) Reproduced with permission.[146] Copyright 2015, American Chemical Society. (c) Reproduced with permission.[13] Copyright 2014, American Chemical Society. (d) Reproduced with permission.[147] Copyright 2015, AIP Publishing LLC. (e,f,g,h) Reproduced with permission.[148] Copyright 2016, IOP Science. (i) Reproduced with permission.[90] Copyright 2017, Macmillan Publishers limited. (j) Reproduced with permission.[12] Copyright 2015, Macmillan Publishers limited. transformation into rhombohedral crystals.[168,177,178] Therefore, selected area electron diffraction (SAED) using a transmission the lone pair electrons can be considered the basis for chemical electron microscope (TEM).[21] The first three nearest reflexes reactivity and phase transformation. in the [001] zone axis[147] represent the (101), (002), and (200) planes, respectively.[59,179–182] This renders a standard determination approach for black phosphorus single crystals as confirmed 1.2.3. Electron Diffraction with X-ray diffraction.[183] Indeed, in a black phosphorus film consisting of nanocrystals complex facet information is observed, The electron diffraction pattern of phosphorene is shown in e.g., the (021), (040), and (117) directions.[184–186] This nonsingle Figure 2h. Such diffraction patterns can be easily obtained by crystal feature is attributed to the rotational stacking and vertical

Adv. Energy Mater. 2017, 1702093 1702093 (4 of 43) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advenergymat.de stacking of 2D materials or zone axis adjustments.[94,147,187,188] rotation angles[206] between the phosphorene sheets can also Moreover, the relative intensity ratio, I(101)/I(002), between the (101) tune the band gap of bilayer phosphorene. Its band gap can and (002) dots can provide a layer number estimation; that is, 2.6 also be manipulated with strain[207,208] as well as external elec- for monolayer, 0 for bilayer, and 0.3 for trilayer phosphorene.[59] trical field.[209,210] Moreover, in a phosphorene ribbon, the Overall, TEM and SAED enable straightforward determination of edges terminated with zigzag or armchair configuration[211–213] atomic structures and layer numbers of phosphorene. as well as functional groups[214] have a different bandgap as compared to its microscale counterpart. The ease of bandgap tunability in phosphorene renders it a rich platform for 1.2.4. Electronic Band Structure novel physical phenomena such as the integral quantum hall effect,[215–217] superconductivity,[218–222] fermions,[223–226] spin- The phosphorene monolayer sheet has a direct band gap of tronics,[227–231] and topological insulators.[232,233] 2.05 eV,[189,190] which matches that for visible light.[191–193] Hence, phosphorene can be applied to photovoltaic devices and solar cells.[194–196] The band gap of phosphorene can be tuned 1.2.5. Electrical Conductivity in various ways. First, the band gap correlates linearly to the inverse of the layer number of phosphorene (Figure 3b).[197–200] Charge carrier mobility is a core parameter for electron-hole For instance, in a density functional theory (DFT) transport and has a reciprocal relationship with effective calculation[201] with CASTEP code, the bandgap decreased mass. Theoretical studies[189] predict an asymmetry and anisot- with increasing layer numbers; e.g., monolayer phosphorene ropy for effective electron mass and mobility, i.e., 0.17 m0 for [86,189] 2 −1 −1 2 −1 −1 (1.67 eV), bilayer phosphorene (1.08 eV), and trilayer 1140 cm V s in the x direction and 1.12 m0 for 80 cm V s phosphorene[189,201] (0.74 eV) and bulk black phosphorus in the y direction. In the case of holes, the effective mass of the 2 −1 −1 (0.40 eV). 3D bulk black phosphorus has a direct band gap hole and its mobility are 0.15 m0 and 700 cm V s in the x-axis, [86,189,190] 2 −1 −1 of 0.31–0.36 eV. Multilayers leads to band splitting at respectively, and 6.35 m0 and 26000 cm V s in the y-axis, the Brillouin zones.[202,203] At Γ points, the conduction band respectively.[189] This indicates hole-dominant transport in mon- and valence band consist of a mixture of s and p orbitals olayer phosphorene, which is attributed to the extremely small [189,234] (pz component). Orbital hybridization in phosphorene is dif- deformation potential (0.15 eV) despite a high effective ferent from that of graphene whose bands are composed mass. The extraordinary charge carrier mobility is significant [204,205] solely of pz orbitals. In addition to layer number, the for the separation and transport of electrons and holes.

Figure 3. Synthesis of black phosphorus crystals with mineralizer-assisted chemical vapor transport methods. a) Resultant black phosphorus in a sealed ampoule. Inset b) is the EDS spectrum showing predominant phosphorus element. c) STM image showing the atomic surface of black phosphorus. d) SEM graph, e) optical micrograph, and f) XRD profile of black phosphorus. (a,d) Reproduced with permission.[3] Copyright 2014, Elsevier. (b,f) Reproduced with permission.[333] Copyright 2017, Wiley-VCH. (c) Reproduced with permission.[334] Copyright 2014, American Chemical Society. (e) Reproduced with permission.[145] Copyright 2017, American Chemical Society.

[12] 2 1.2.6. Optical Absorption red-shifted for all the three modes; particularly for the Ag mode at 470, 469, 468, and 468 cm−1 for bilayer, trilayer, Phosphorene exhibits a strong absorption spectrum in the quadruple-layer, and bulk phosphorene, respectively.[59,240] In [235] [236–240] 2 ultraviolet region. With layer number modulation, addition, bilayer phosphorene exhibits a distinctive Ag mode, the wavelength can extend to the entire absorption spectrum which can split into two peaks.[12] Because two out-of-phase with significant enhancement in absorption rate.[199,241,242] monolayer infrared modes are allowed to combine, the bilayer From DFT calculations,[203,236] the optical band gaps of mono phosphorene can generate a Raman-allowed mode, which is layer, bilayer, and trilayer phosphorene are estimated as 1.04, forbidden in monolayer and bulk 2D materials.[267–270] Recent 0.6, and 0.31 eV, respectively. The optical band gaps obtained advances low frequency Raman techniques[259,268,271–274] from direct optical absorption measurements are 1.38, 1.23, allow for the acquisition of Raman modes below 100 cm−1, 1.05, 0.85, and 0.72 eV for 1–5 layer phosphorene.[56] A 4% which are straightforward and more sensitive for the layer compressive strain[243] shifts the optical band gap to the infrared determination of 2D materials as compared to the high fre- [244] 3 region between 1.2 and 1.7 eV. Moreover, a 4% tensile strain quency Raman modes. The interlayer breathing modes (Ag at −1 4 −1 enables absorption in the entire visible range. For a photo- 15–35 cm ; Ag at 38 cm ) of black phosphorus have been catalyst, efficient diffusion of the photon is key. The prototype observed experimentally.[275] Two modes residing around orientation of phosphorene[203] determines the optical absorp- 70–90 cm−1 have been preliminarily assigned as the B mode tion; for example, armchair phosphorene nanoribbons have a (76 and 86 cm−1)[276] or the first order interlayer compression H −1 [277] direct optical band gap, while zigzag nanoribbons are optically mode (A1 interlayer mode at 71 cm ). Thus, Raman spec- inactive. In addition, the absorption coefficients depend on the troscopy is widely exploited as a facile nondestructive examina- directions, i.e., the absorption coefficient is 10 times smaller tion tool for phosphorene. in the zigzag direction than in the armchair direction.[245–247] Infrared absorption spectroscopy can reveal informa- Compared with the zigzag direction, the armchair direction tion about the bonds in molecules.[168] For example, the PO facilitates a 16 times greater photon diffusion, which is benefi- bond stretching can be confirmed from the appearance of an cial in enhancing photocatalytic reactions.[245] These tailorable absorption band at 1200 cm−1 in the Fourier transform infrared optical properties render phosphorene a rising star in elec- spectra.[260,278] In addition, the formation of phosphorene oxide trochemistry[248–251] and photochemistry[203,252–254] for energy broadens the vibrational mode at 880 cm−1, which is attributed applications in hydrogen evolution[203,255,256] and solar fuel to the stretching of the PO ester bond.[190,261,279] Furthermore, production.[257,258] doping with N or F[261,279] can be confirmed from the presence of absorption bands corresponding to the stretching of PN and PF bonds, respectively. 1.2.7. Stability

Few-layer phosphorene prepared by mechanical exfoliation 1.2.9. Other Properties often degrades under ambient conditions due to humidity and air, as observed by labs of Castro Neto and co-workers[259] and Aside from electronics applications, phosphorene is of Hersam and co-workers.[260] Then, Martel and co-workers[12] interest in the fields of chemistry and biomedical engi- demonstrated that three environmental conditions, i.e., neering for application as gas sensors,[81,280,281] medical oxygen, water, and visible light, are required concurrently to imaging agents,[282] photothermal therapy,[283–285] and drug initiate the degradation of monolayer phosphorene. Moreover, delivery.[286,287] In the field of physics, phosphorene has been its oxidation rate is strongly correlated to the oxygen concen- applied in mechanics,[288–292] thermal physics,[110] acous- tration, light intensity, and energy gap. We will expand on this tics,[293–298] nonlinear optics,[167,299,300] photonics,[115,193,301,302] in the methods section for an understanding of the degrada- electronics,[166,303–306] photoelectronics,[165,307–310] thermoelec- tion mechanism and air stable electronic and electrochemistry tronics,[311–316] spintronics,[216,230,317] magnetics,[318–320] and elec- devices. tromagnetics.[146,321–324] In material sciences, phosphorene can be used as functional flexible substrates[292,325,326] and electrical enhanced composites.[327] These represent bare testament to 1.2.8. Vibrational Spectroscopic Modes the high level of interest in phosphorene. Prior to exploitation in applications, the synthesis of the Vibrational Raman and infrared spectroscopies pro- material with high quality, uniformity, and large scale is of vide detailed information of phosphorene such as layer vital importance, especially during mass production. There- numbers,[132,197,261–263] crystal orientations,[59,89,264] and fore, we first discuss the synthetic approaches for black strain.[14,244,265] Three vibrational modes of phosphorene are phosphorus. [132,135] 1 −1 observed in the Raman spectra, namely, the Ag (362 cm ), −1 2 −1 B2g (439 cm ), and Ag (471 cm ) modes, which correspond to the in-plane or perpendicular phosphorene vibrations shown 2. Synthesis Methods in Figure 2j. The wavenumber position[59,240] and full width at [12,266] 2 half maximum of the Ag mode are highly dependent on Here, we discuss the synthesis methods for both bulk crystal the layer numbers. With increasing layer numbers (the optical 3D black phosphorus and 2D phosphorene preparation. In micrograph in Figure 2i), the Raman wavenumbers become addition, based on the successful synthesis concepts of other

2D materials, we suggest possibilities for future approaches for 2.1.2. Organic Liquid Exfoliation large-area phosphorene membrane fabrication. Black phosphorus has an orthorhombic crystalline struc- Liquid phase exfoliation has been long employed for the fab- ture from the Cmca space group,[146,328] and is the 3D parent rication of large weight 2D materials. Few-layer phosphorene of 2D phosphorene membranes for which there is a high flakes have been isolated first by Coleman and co-workers demand for its fabrication with low cost, facile apparatus, and through exfoliation in organic solvent medium (Figure 4) such simple protocols. In a mineralizer-assisted gas vapor trans- as N-methyl-2-pyrrolidone,[57,58,342] dimethylformamide,[56,368] formation, the heating of red phosphorus in the presence dimethyl sulfoxide,[56]N-cyclohexyl-2-pyrrolidone,[342] and iso- [341] of SnI4/Sn in a sealed ampoule leads to the production of propanol. The size of phosphorene nanoflakes can be con- black phosphorus via a safe and simple method. Nilges and trolled with processing time and sonication power by Hersam co-workers[3–5] first report the formation of black phosphorus and co-workers[57] and Salehi-Khojin and co-workers.[57] Several from red phosphorus via a single reaction (Figure 3). This optimizations have been made in this approach. For instance, a reported protocol is often termed the chemical vapor trans- prestep grinding of the bulk crystal with ball milling by Chung port method.[329–331] In this method, the sizes of the bulk black and co-workers[369] and Jeong and co-workers[370] leads to the crystals are reported to increase. Recently, centimeter-scale formation of nanoparticles of black phosphorus, which allows synthesis has been reported by Ren and co-workers.[329,332] easy sonication in the next step. Moreover, probe sonication is efficient and completes the exfoliation in 5 [57,342]h as compared to bath sonication, which requires 24 h. In addition, the encap- 2.1. Phosphorene Monolayer Nanosheets sulation of phosphorene surfaces by organic molecules[57,342] provides protection from air oxidation, thus preserving its Phosphorene thin films can be fabricated via two strategies: performance without degradation. Further, synthetic phos- one is the so-called top down approach, which involves the phorene mixed with organic solvents can be used by Dai and exfoliation of its 3D parent, black phosphorus, and another co-workers[371] and Cui and co-workers[372] in electrochemical is the bottom-up approach, which involves thermal deposi- devices, which require large quantities of phosphorene. tions or assemblies from small phosphorus precursors. The exfoliation approaches are based on the weak interlayer interactions[142] between two sheets of phosphorene, which can 2.1.3. Surfactant-Assisted Dispersion be easily overcome with external forces. Depending on the external forces, the exfoliations can be classified into mechan- The use of chemical surfactants such as soaps can break the ical exfoliation,[335,336] sonication exfoliation,[58] and microwave interlayer interaction of phosphorene and separate the indi- exfoliation.[337–340] The sonication exfoliation can be further vidual phosphorene sheets during sonication (Figure 4a,b). One categorized into organic liquid,[57,58,109,341,342] aqueous sur- of the typical surfactants in use by Hersam and co-workers is factant,[343] and water exfoliation according to the assisting sodium dodecyl sulfate.[343] Phosphorene dispersions or suspen- agents.[332,344] sions are widely used in nonlinear optics and physical chemistry research such as optical adsorption,[115,154,373] photolumin escence,[89,129,374,375] pump probe spectroscopy,[376–378] transient 2.1.1. Mechanical Exfoliation bleaching,[378] and exciton dynamics[375,379,380] such as trion lifetime,[375,381] charge transfer,[153,171] and exciton quenching.[374] The microcleavage approach has been successfully exploited in the first isolations of many 2D membranes such as gra- [345–349] [350,351] [352,353] [354,355] phene, MoS2, WS2, h-BN, and 2.1.4. Water Exfoliation MXene.[44,356] Due to the ease of isolation by a Scotch tape,[8,13] phosphorene has been vastly employed in the research of funda- To obtain pristine phosphorene, water has been employed as mental physics and electronic devices. Because the as-exfoliated a medium for sonication exfoliation. This simple, clean route phosphorene usually has a few layers, argon[240] or ozone[357,358] developed by the Ren and co-workers[332] may pave the way for plasma can be used for surface cleaning and layer thinning to the application of phosphorene as anodes in secondary electro- eventually obtain monolayer phosphorene. To avoid organic chemical batteries. residues from the adhesive tapes, Castellanos-Gomez[359] developed a modified exfoliation method with an all-dry transfer technique, which employed a thermal-release tape[360–362] or a 2.1.5. Alkali Metal Intercalation Exfoliation polydimethylsiloxane (PDMS) stamp.[59,363–366] However, the mechanical fabrication is slow and the yield is low,[8,13] not only The lithiation reaction results in the intercalation of layered [382] [383] because of the repeating multistep exfoliations (adhesion and materials such as graphene and MoS2, and the subse- splitting), but also due to the necessary layer thickness determi- quent immersion in aqueous solution generates hydrogen nation step with optical microscopy[90,367] and Raman spectros- bubbles, which isolate the monolayers. Given that the lithia- copy[12,240] after each exfoliation. Thus, this method has limited tion of phosphorene leads to a 300% volume expansion,[384,385] potential in mass production. However, mechanical exfoliation the interlayer interaction is weakened and phosphorene fulfills its mission in academic laboratories for fundamental layer splitting becomes feasible with lithium reaction in water. studies. Alternatively, large sodium ions were used to intercalate and

Figure 4. Phosphorene preparation with aqueous solution exfoliation and organic liquid exfoliation. Upper panel: a) Protocol of aqueous exfoliation with chemical sodium dodecyl sulfate. b) Photographs of the black phosphorus solution and few-layer phosphorene dispersions. TEM observation of phosphorene: c) low-magnification TEM micrograph, d) high-resolution TEM micrograph, and e) SAED pattern. (a–e) Reproduced with permission.[343] Copyright 2016, National Academy of Sciences, U.S.A. Bottom panel showing organic liquid exfoliation: f) photograph of phosphorene solution, g) low-magnification TEM, h,i) high-resolution TEM. j) XPS spectra. (f–j) Reproduced with permission.[342] Copyright 2015, Macmillan Publishers Limited.

[386,387] [388,389] [390] exfoliate graphene, MoS2, and antimony (Sb). 2.2.1. Thermal Decomposition of Phosphine The volume change in sodiated phosphorene[385] indicates that the interlayer interactions are weakened, and therefore, sub- In the course of silicon microelectronics, phosphorus has been sequent ultrasonication enables monolayer exfoliation. The intensively studied for the n-type doping of Si:P.[77,394–396] Phos- intercalation exfoliation is advantageous to the investigations phorus adatoms have been fabricated along the Si (100) surface [80,397–399] of the reaction mechanisms of new morphologies such as the terraces via the thermal decomposition of PH3. The oriented self-assembly of scrolls.[391–393] deposition of phosphorus film on single crystal metals such as Au (111)[400,401] and Cu (111) has also been reported.[400]

2.2. Chemical Vapor Deposition 2.2.2. Preliminary Chemical Vapor Deposition Large-area layered materials have been successfully fabricated by chemical vapor deposition (CVD). Here, we summarize the Phosphorus thin films deposited on SiOx substrates have been CVD strategies for phosphorene in terms of morphologies. grown in Ji and co-workers’ lab.[186] The early chemical vapor

Adv. Energy Mater. 2017, 1702093 1702093 (8 of 43) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advenergymat.de deposition protocol includes two steps to mimic the vapor 2.2.5. Reaction Kinetics transport mineralization method. The first is the deposition of amorphous red phosphorus at 600 °C on SiOx in low vacuum, Epitaxy or surface-mediated deposition techniques can be and the second is the transformation of black phosphorus film considered for phosphorene fabrication analogous to the syn- [416,417] from red phosphorus in the presence of SnI4/Sn at 27.2 bar thesis routes of other 2D materials. These deposition (2.76 MPa) in Ar ambience. This protocol allows for the direct techniques provide essential information about the growth growth of phosphorus thin film (four-layer phosphorene) on a rate such as monolayer coverage rate.[406,418,419] Furthermore, dielectric substrate. Xia and co-workers[330] were able to transfer dynamic phosphorene evolution can be recorded with advanced this technology to flexible substrates. The analogous reaction in situ microscopy[420,421] and spectroscopy.[407] Another avenue path over a polyethylene terephthalate (PET) substrate was: is the chemical equilibrium calculation[422,423] for reactive [424,425] red phosphorus powder → red phosphorus film (by thermal radicals (e.g., P2 concentration) that accounts for the evaporation) → black phosphorus film (pressurization reac- 2D membrane formation. tion in the anvil cell under 8 GPa). It is noted that these early phosphorus films consist of nanocrystalline structures and a number of defects.[290] A large portion of grain boundaries[290] 2.2.6. Growth Mechanisms and defects[402,403] suppress the electronic and optical performances of 2D materials, which is unfavorable for utilizing their As per the crystal growth theory, the deposition of 2D films pristine properties. includes early nucleation, island expansion, and complete film formation. In an atomic view, the formation reactions require the bond dissociation of precursor molecules, adsorption, [426] 2.2.3. Choice of Substrates migration, edge-attachment, and desorption. Recently, P2 dimers[65] were found to be the smallest units for phosphorene The phosphorene–substrate interaction plays an essential role sublimation (a reverse reaction of deposition) over the black in the early nucleation step. Theoretical calculations with ab phosphorus surface. This may shed light on the phosphorene initio molecular dynamics[404,405] provide insights into the sta- deposition kinetic constants with elevated concentrations of the bilization of phosphorene clusters (P27) over a substrate sur- reactive radicals of diphosphorus. Indeed, the dimer building face. For example, the binding interaction (0.75 eV atom−1) theory is valid for interpreting graphene formation with an with Cu (111) arises from a strong chemical adsorption, edge-attachment mechanism in which the C2 dimer radicals which results in the collapse of the phosphorene cluster. In act as the building block.[427–429] This comprehensive study of contrast, the interaction (0.063 eV atom−1) with h-BN surface material formation sheds light on phosphorene, and thus, one is through van der Waals forces; however, it fails to stabi- may hypothesize that the phosphorene reaction originates from lize the nanocluster. It has been found that an artificial BN the competitive adsorption and desorption of P2. More theo- interaction with a bonding energy of 0.35 eV can preserve retical and experimental observations should be conducted to the puckered phosphorene structure well. Nanoclusters have better understand large-area phosphorene growth with chem- been experimentally synthesized with molecular beam epi- ical vapor deposition approaches. taxy. In addition, phosphorene nanoribbons were successfully The dissolution of carbon atoms and segregation of graphene synthesized over Cu (110) surfaces as well as Te-modified over Ni substrates is a commonly argued growth mechanism Au (111).[400,406,407] Moreover, the formation of phosphorene for graphene.[430–432] In brief, the carbon radicals dissolve into triangular islands was revealed via in situ observation using a bulk Ni at elevated temperatures and, upon cooling, the gra- scanning tunnelling microscope (STM) and low-energy elec- phene diffuse toward the surface and segregate. Here, the grain tron microscope. boundaries[433–435] of Ni typically yield thicker layers of graphene than the grain surfaces do. Analogous to graphene segregation, the surface coverage of elemental phosphorus occurs 2.2.4. Thermodynamics at the grain boundaries of steels (Fe alloys, model 17-4PH) was reported in 2003.[436] To determine phosphorus formation, The reaction paths require a comprehensive understanding a metallographic approach using a picric acid-based reagent of thermodynamics with theoretical calculation methods. was employed. The authors found a linear relationship between Indeed, the precise energy levels need to be calculated for phosphorus segregation and the depth of intergranular etching all the phosphorus states in precursors such as white phos- processed from grooves. From a thermodynamic aspect, the [395,408–410] phorus (P4) or PH3, or other molecules, interme- formation of phosphorus at 600 °C at the iron grain bounda- diate radicals, and final product phosphorene. For example, ries corresponds to a Gibbs free energy of −43.1 kJ mol−1 Zhang and co-workers[411] and Cui and co-workers[400] have (0.45 eV atom−1). proposed a reaction path for phosphorene formation over Further, thermodynamic parameters such as enthalpy and BN and GaN surfaces. Further, the presence of catalysts entropy were determined for phosphorus segregation.[437] and plasma, which decrease the reaction energy barrier, Auger electron spectroscopy determined the grain boundary should be taken into consideration.[22,412] Attention should concentrations to correspond with the high density of disloca- also be paid to the edge termination either with foreign tions, which can be stabilized with carbide precipitates and pro- atoms[334,413,414] in zigzag or armchair directions, or with vide reservoirs for phosphorus.[438] A first principles theoretical functional groups.[415] study determined the binding energy of phosphorus with the

Adv. Energy Mater. 2017, 1702093 1702093 (9 of 43) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advenergymat.de host Fe grain boundaries.[439] The P–Fe interaction is shown Such three-fold configured phosphorus atoms processing a as an electrostatic embedding-like bonding, resulting from the lone electron pair, tend to provide active sites that allow for the large spatial extension of the P 3p wave function; thus, it is bond formation with an oxygen atom.[482] As the metals stored saturated without a dangling bond over P for P–Fe (111). Fur- in ambient conditions, a surface thin oxide prevents further oxi- ther, the orientation of the substrate also affects the segrega- dation which stabilized the surface. However, the Castro Neto tion reaction. The fcc crystalline structured substrates promote and co-workers[193,483–485] showed that phosphorene oxidation phosphorus segregation, while the reaction is not preferential occur continuously and the pristine material vanish eventu- for the bcc plane.[440,441] Also, elemental incorporation can ally. The oxidation rate is determined by several external condi- influence the segregation level. For instance, boron suppresses tions[12,486] such as ambient humidity, light exposure as well as phosphorus segregation, while C, Mn, and Cr do not noticeably the phosphorene layer numbers. In addition, the phosphorus influence the reaction.[442] A high-temperature treatment over sublimation dynamics such as P atom removal and migration the alloy substrates can increases the segregation level with have been investigated under high temperatures.[180,426,487,488] increasing processing temperatures.[443] These early studies are What the reaction chemistry for the oxidation of phos- surprisingly analogous to aspects of Cu substrates for graphene phorene became clear in a theoretical work headed by Barrazu- formation.[412] Lopez and co-workers.[489] They emphasized that such oxidation Indeed, greater structural details and even an atomic level processes are driven by the intrinsic defects and curvature in mechanism can be provided by employing state-of-the-art in the phosphorene crystals. In the ab intio atomic optimizations, situ techniques such as low-energy electron microscopy,[444,445] the chemisorption and dissociation of oxygen molecules have low-energy electron diffraction,[446] STM,[447,448] scanning trans- been thoroughly unveiled for an equilibrium oxide phase for- mission electron microscopy,[449] conventional TEM,[393] and mation. Started with the planar or conical phosphorene phase, scanning electron microscopy (SEM)[450,451] and X-ray photo- the intrinsic defects decrease the chemisorption energy bar- electron spectroscopy (XPS).[452] rier compared to a perfect phosphorene crystal. Therefore, it With regard large-scale growth with CVD, it is argued that allows the occurrence of oxygen adatoms chemisorption upon the large-scale homogeneous phosphorene is at a critical time a visible or ultraviolet photon absorption. Interestingly, the and requires more efforts from the experimental and theoret- exothermic reaction accounts for the continuous oxidation and ical scientists of the 2D research communities.[453] amorphization processes on the newly generated defects sites, Referring to the synthesis roadmap of the graphene coun- which eventually break down the phosphorene and transform terpart, the fabrication of a large-area planar graphene film into aqueous phosphoric acid phases in the presence of water. by chemical vapor deposition was reported in 2009,[454] This work determined the curvature of phosphorene as well five years after exfoliated microflakes were first reported in lattice defects accounts for the photo induced degradation of 2004.[346] We list the timeframe for sophisticated 2D materials: phosphorene. [22,346,454,455] 2D materials, e.g., graphene (5–12 years), MoS2 After understanding the chemistry nature of such degrada- [350,351,456–461] [352,353,462–467] (7–10 years), WS2 (5 years), MoSe2 tion, we now look at the strategies that can suppress these effects. [468–472] [350,353,462,473–477] (5–6 years), WSe2 (5–7 years), and h-BN Castro Neto and co-workers showed the graphene contacts can (3–7 years).[478–481] Thus, one may deduce that the CVD syn- preserve the phosphorene heterostructures transistors perfor- thesis for phosphorene is incubating and breakthroughs are mances for more than two months.[6] Also, the van der Waals likely to arrive soon as it is now three years since its first isola- interactive encapsulation of phosphorene between two h-BN tion. In other words, there is a huge opportunity for promoting layers[194,238,490] can preserve successfully the electrical proper- the CVD fabrication of large-scale phosphorene. ties from degradation. Another avenue is the passivation of phosphorene with an AlOx protection layer (by atomic layer deposition). Hersam and 2.3. Environmental Stability Strategies co-workers reported a phosphorene transistor that can retain charge carrier mobility of 100 cm2 V−1 s−1 and high on/off Photocatalyst materials are required to be highly stable ratio of 103 for more than two weeks in ambient conditions.[260] against humidity and oxidation for water splitting and other Akiwande and co-workers showed a flexible transistor which photochemical reactions. In order to pave the way for the retains electrical performance for weeks long durations and electrochemistry utilizations, the environmental stability mechanical robustness under 2% uniaxial tensile strain over of phosphorene becomes very important research themes. 5000 bending cycles.[336] Indeed, such a material has taken two years of the commu- Third, few-layer phosphorene nanosheets produced by nity to obtain a comprehensive understanding of the under- organic exfoliation[56,115,343,344] show improved water and light lying oxidation mechanisms and protection strategies that stability. This progress in material synthesis renders phos- succeed in long term stability. Here we give credits to early phorene a promising long lifetime photocatalyst.[203,307] Indeed, works that address the stability concerns and the passivation these passivation strategies and synthesis progresses are approaches. reflected in the following discussion of electrochemistry devices We start with the oxidation phenomena during the phos- such as phosphorene–graphene composite, and encapsulating phorene materials preparation and device fabrication. Upon as interlayers between other inert films. exposure in air for 30 min, the mechanically exfoliated phos- After understanding the long term stability of phosphorene phorene experienced severe oxidation and degraded in the elec- in devices, we now look at the multiple roles of phosphorene in trical properties as observed by Castro Neto and co-workers.[259] secondary ion batteries as well as other energy systems.

3. Anodes in Lithium-Ion Batteries binary/ternary alloys[512–515] were explored to replace the rare metal cobalt. To further reduce cost with abundant iron, oli- [516–523] [524,525] Rechargeable lithium ion batteries with their stable cycling per- vine-structured LiFePO4 and LiFeMnPO4 having a formance,[491–494] high storage capacity,[495–497] and high energy higher cell potential and a flat voltage curve were used, wherein density[498–500] are the dominant energy storage units for con- the oxygen was substituted with phosphate; these are suitable sumer electronics[501,502] such as smartphones and tablets. A for electrical vehicles and grid storage. secondary lithium-ion battery is fabricated with an anode, a Portable electronics require better space utilization to make cathode, a separator, and electrolytes. Both the electrodes act as smaller devices so that a higher energy density is achieved. The lithium ion hosts with a separator membrane to avoid a short cathode materials do not change the specific Li capacity much circuit while the electrolyte supplies lithium ions. The specific due to the stoichiometric ratio of Li in these crystals. Therefore, energy of a battery is determined by the specific capacities of the improvement in the capacity of anodes is one of the most the cathode and anode materials. Both consumer electronics intensively studied issues in battery research. For three dec- and electrical vehicles demand batteries with large energy den- ades, graphite has been the commercial anode material due to sities. Therefore, the strategies for exploring cathode and anode low cost and high abundance. However, graphite, with a theo- [526–528] −1 materials are of vital importance in battery research. retical specific capacity of 372 mA h g (with LiC6 final Before discussing the anode materials, we first recall how a product), has a limited specific energy[529] of 200 mW h g−1. For lithium-ion battery works (Figure 5). During charging, the elec- this reason, intensive efforts have been made in developing new trical potential from an external power source draws the Li ions anode materials such as Ge,[530–533] Sn,[534,535] Si,[527,528,536,537] from the cathode into the ionic electrolyte, and the Li ions dif- Pb,[538] and graphene foam.[539–542] fuse toward the anodes. Eventually, these ions intercalate into Most recently, pure phosphorus has become a rising star [503] the anode, e.g., graphite, and result in LixC formation, which in lithium batteries with a high power density and long cycle is termed as lithiation. After the charging process, electrons are life. Elemental P possesses a high theoretical capacity[543] of −1 transferred into the anodes, and thus, electricity is stored in the 2596 mA h g (with Li3P as the end compound) and a low form of electrochemical energy. During the discharging pro- diffusion energy barrier of 0.08 eV for lithium ion.[250] Thus, cess, Li ions are extracted from the anode by the electrochemical black phosphorus[2] was employed in anodes and exhibited an potential difference between both the electrodes and transferred improved charge capacity of 1279 mA h g−1 with a first cycle back to the cathode. After discharging, the anode loses electrons efficiency of 57%. To improve the electrical conductivity of phos- and the cathode acquires them by charge conservation. When phorus, which is a semiconductor, carbon was incorporated to connected with a resistance load, electrons are generated and form a black phosphorus-carbon composite (Super P). Indeed, flow through the load toward the cathode. The potential differ- the black P-carbon composite[2] improved the first discharge and ence gradually decays with the delithiation of anode and eventu- charge capacities of 2010 and 1814 mA h g−1, respectively. The ally needs to be rebuilt, which requires a recharging reaction. high coulombic efficiency of 90% of the black phosphorus com- This is the working principal of lithium-ion batteries. posite anode is attributed to the nearly full reversibility of black P [2] −1 Conventional lithium-ion batteries consist of graphite and Li3P transformation. The capacity remains at 600 mA h g anodes and lithium metal oxide cathodes. We first take a look at after 100 cycles when cycling at a voltage between 0.8 and 2 V. the cathode materials. Since the 1990s, lithium cobalt oxide has However, the capacity drops severely when cycling is performed been the dominant cathode[504–507] material in the batteries of between 0 and 2 V. Upon periodic lithiation and delithiation, [132,385,544,545] consumer electronics. The layered structured LiCoO2 possesses capacity decay occurs due to the severe breakage of structural stability, ultrahigh energy density, and high com- anode materials owing to huge volume changes, as well as the pact density. For low cost and long cycle life, spinel-structured loss of electrical contact. The volume change of the composite oxides of nickel,[508,509] manganese,[510] vanadium,[511] or their anode can be reduced by coating the phosphorus on porous carbon.[331] Moreover, thermal annealing[331] of the composite at 450 °C increases the capacity to 2413 mA h g−1. The problem of structural stability can be tackled by forming phosphorus-carbon bonds in a chemical-mechanical reaction process. Indeed, the PC bond strategy (Figure 6) facilitates a high initial discharge capacity of 2786 mA h g−1 at 0.2 C and excellent cycling performances with 80% capacity retention after 100 cycles.[545] In addition, the capacity remains nearly unaffected after the first cycle between 0 and 2 V with a current density of 0.2 C. The mechanical flexibilities of phosphorene– graphene batteries[332] can provide electrical power for wearable flexible electronics.

3.1. Lithium Sulfur Batteries

[546–550] Figure 5. Schematic depicting a lithium-ion battery with a phosphorene Lithium–sulfur cathodes with specific capacities of 3861 −1 anode and a LiCoO2 cathode. and 1672 mA h g for Li and S, respectively, have attracted

Figure 6. Schematic, TEM, and performance of black phosphorus–graphene composites in lithium ion batteries. a) Black phosphorus composite pulverizes after cycling without phosphorus–carbon bonds. b) Black phosphorus composite retains its morphology after cycling due to strong phosphorus–carbon bonds. d,e) High-resolution TEM graph of black phosphorus–graphene interface. f) Atomic configuration of their interface, g) charge– discharge profiles, h) cycling performance and Coulombic efficiency, i) Nyquist plots and equivalent circuit, and j) rate performance. Reproduced with permission.[545] Copyright 2014, American Chemical Society. increased attention. However, Li–S batteries have several limita- compared to phosphorene-free cathodes, the phosphorene in tions[551–555] such as the low electrical and ionic conductivities of the cathode matrix significantly reduces polarization, acceler- sulfur, high volume change upon lithiation, sulfur loss due to dis- ates the redox reactions for fast charging/discharging, and solution in the electrolyte, and sulfur shuttling between the anode increases sulfur utilization. and cathode. All these result in low reversibility and poor sulfur Furthermore, phosphorene can modify the separator to utilization. To solve this problem, sulfur composites were fabri- trap and activate the polysulfides, leading to a much enhanced cated by adding conductive matrices such as graphene,[556–558] capacity and improved cyclability.[569] carbon nanotubes,[559–561] and porous carbon.[562–567] Compared with nanocarbon materials, phosphorene has advantages such as anchoring/immobilizing sulfur with strong 4. Sodium-Ion Batteries and Beyond PS bonds[568] to enable its efficient utilization. Ren and co-workers[333] found that phosphorene acts as a sulfur immo- Sodium-ion batteries are attracting a fair bit of attention bilizer and electrocatalyst, prolonging the cycle life of lithium owing to their low cost[543,570–577] and the abundance of Na. sulfur batteries with enhanced capacities (Figure 7). In addition, Sodium-ion batteries are promising as an alternative for LIBs.

Figure 7. Phosphorene in lithium-sulfur batteries. a) Schematic depicting few-layer phosphorene–carbon nanofiber matrix as a host for lithium polysulfide catholyte. b) Electrochemical rate properties at different current densities (1 C= 1675 mA g−1). c) Galvanostatic charge–discharge profiles of the first cycle at 0.2 C. d) Cycling stability performance and coulombic efficiency. Inset is the utilization ratio of sulfur at a current density of 1 C. e) Theoretical calculations of lithium polysulfide adsorption at phosphorene. f) Binding energy between lithium polysulfide and phosphorene by DFT [333] calculation. g) Adsorption measurement of pure Li2S6, Li2S6 + CNF, and Li2S6 + phosphorene (from left to right). Reproduced with permission. Copyright 2017, Wiley-VCH.

Kulish et al.[578] performed an ab initio calculation and found directions. Phosphorene can retain its structural stability[580,581] [582] that sodium ions diffuse fast along the zigzag direction and by forming Mg0.5P. Moreover, a phosphorus alloy can serve overcome a small energy barrier of 0.04 eV. The charging and as an anode for a potassium-ion battery[583] with abundant and discharging of sodium-ion batteries is analogous to those for low cost potassium. In addition, selenium[584–586] may serve lithium-ion batteries. as cathodes for sodium-ion batteries with high capacity and Cui and co-workers[372] fabricated the first sodium-ion bat- good cycling ability. It is worth noting that all-solid batteries teries with phosphorene/graphene composite anodes (Figure 8). are gradually becoming important due to safety concerns.[587] The sodium-ion battery with a phosphorene/graphene hybrid In future, further improvement in the performances of sec- anode presented a reversible capacity of 2440 mA h g−1 at ondary-ion batteries can be expected. Therefore, phosphorene 0.02 C. These composites consist of phosphorene/graphene holds promise as anode materials for metal-ion batteries and sandwiched structures, which shorten the diffusion path of selenium batteries. ions and electrons resulting in enhanced rate performance. Moreover, the stacking of phosphorene with graphene enables the accommodation of volume expansion with elastic 5. Electrodes in Supercapacitors buffer spacing. Further, graphene allows electron transport from the phosphorene redox reaction to the current collec- Supercapacitors are of great interest owing to their remark- tors. The sodium batteries[372] retain 85% of the capacity of able performances such as high power density, long cycle life, 2080 mA h g−1 at 0.02 C rate after 100 cycles. and fast charge/discharge rate.[588–590] Often, high-capacitance Another confinement approach is the encapsulation of phos- double-electrode capacitors require materials with a large phorene with h-BN nanosheets;[579] this strategy is promising specific surface area such as 2D layered graphene.[591,592] For due to its high theoretical capacity and minimal diffusion example, stacked graphene nanosheets showed a high volu- barrier. metric capacitance of 1 F cm−3.[580,593,594] Beyond sodium-ion batteries, magnesium ions can be With the development of phosphorene dispersions[82,341] adsorbed and diffuse along the phosphorene surface. The mag- and drop coating techniques,[596–599] Wen and co-workers[595] nesium ions have a diffusion barrier of 0.08 eV along the zigzag fabricated phosphorene films over PET substrates (Figure 9)

Figure 8. Sodiation/desodiation mechanism in black phosphorus and the electrochemical performance of the composite anodes for sodium-ion batteries. a) Schematic of pristine black phosphorus, b) initial sodium ion diffusion, and c) alloy reaction in Na3P. d) Structural evolution during sodiation and reversible desodiation. e) High-resolution TEM of pristine black phosphorus. f–h) Time-lapse TEM micrographs of the sodiation process in black phosphorus. i) Reversible desodiation capacities of the composite anodes with different carbon/phosphorus molar ratios. j) Galvanostatic discharge- charge curves of the anode after the 1st, 2nd, and 50th cycles. k) Volumetric and specific capacities at various current densities. Reproduced with permission.[372] Copyright 2015, Macmillan Publishers Limited. and obtained a supercapacitor with a capacitance of 13.75 F cm−1 6. Photovoltaic Device and Solar Cells at a scanning rate of 0.01 V s−1. These double electrodes of phosphorene were highly flexible and exhibited little loss of In a conventional solar cell, the semiconducting p–n junction current density. The maximum current density and power separates the photogenerated electron–hole pairs, while the density that could be achieved were 2.47 mW h cm−3 and dual terminal electrodes collect the charges and supply them to 8.83 W cm−3, respectively. external circuits such as storage batteries and integrated grids. Future optimization of fabrication protocols will render The photovoltaic effect occurs at the interface of the p–n junc- phosphorene highly attractive for energy devices in wearable tion, and the charges drift toward the electrodes driven by the electronics. built-in electrical potentials due to their band gap differences.

Figure 9. Black phosphorus as electrodes in flexible supercapacitors. a) Schematic of the fabrication of all-solid-state supercapacitors. b) Galvanostatic charging/discharging profiles for single and 4-series supercapacitors. Insets are the photographs of four light-emitting diodes of yellow, white, red, and green light, driven by four linked device series. Electrochemical properties of black phosphorus supercapacitor: c,d) cyclic voltammograms at different scan rates. e) Galvanostatic charge/discharge curves at various current densities. f) Stack capacitances extracted from scan-rate-dependent CV performances. g) Bending experiments for different angles. h) Cyclic voltammograms at different bending angles shown in (g), demonstrating a stable changing–discharging behavior independent of the bending angles. Reproduced with permission.[595] Copyright 2016, Wiley-VCH.

To date, the commercialized solar cells are based on bulk for the deposition of conjugated polymer poly(3,4-ethyl- materials of monocrystalline Si[600–609] and polycrystalline enedioxythiophene) (PEDOT):polystyrene sulfonate (PSS) Si,[610–613] as well as thin films of CdTe,[614–623] Cu(In,Ga) (electron donor)[712–720] for an organic solar cell, as well as [624–631] [721–723] Se2, and tandem amorphous-Si/microcrystalline- for a nanocrystalline Cu(Zn,Sn)S2 film for a CZTS Si.[632–639] These first and second generation photovoltaic solar device. Further, the dye, which is the so-called photo- devices possess excellent photoelectron efficiency and long- sensitizer, can be bound to the photoanode TiO2 by soaking term stability; however, their disadvantages arise from either it in dye solutions.[724–731] Moreover, the nanocrystals,[732–739] the toxicity of cadmium metal[640–643] or the low abundance of obtained from wet chemistry assemblies, can significantly indium metal,[644–648] as well as complex manufacturing pro- enhance the solar device performance and reduce the fabrica- tocols.[649–658] Such obstacles can be overcome with third gen- tion cost. eration photovoltaics, that is, the organic solar cells,[659–668] Phosphorene is an ideal photovoltaic material with the advan- dye-sensitized solar cells,[669–678] perovskite solar cells,[679–692] tages of high electrical conductivity[123,129,166,189,217,326,740,741] and and van der Waals heterostructure solar cells.[310,693–706] Quite readily tunable band gaps.[84,90,199,742–744] Pioneering theoretical often, nonvacuum fabrication techniques[707–711] are employed and experimental studies illustrate its potential in organic solar for the deposition of high abundance materials as optical cells,[195] dye-sensitized solar cells,[196] and heterostructure solar absorbers. For instance, spin coating was adopted dominantly devices.[194,745]

Figure 10. Black phosphorus in perovskite solar cells. a) Device configuration of planar perovskite. b) Energy level diagram of the device. c) SEM of the cross section of solar cell. d) Current density–voltage characteristics of black phosphorus quantum dot based perovskite solar cell. e) External quantum efficiency. f) Photoelectron conversion efficiency of solar cells with and without black phosphorus quantum dots. Reproduced with permission.[770] Copyright 2017, American Chemical Society.

Phosphorene was first incorporated in organic solar cells in Lau dots were used to bridge the PEDOT:PSS and perovskite and co-workers’ lab.[195] The inverted device architecture included layers. As a hole transport layer, black phosphorus quantum glass/indium tin oxide (ITO)/ZnO/phosphorene/PTB7:PC71BM/ dots can enhance the extraction efficiency of holes due to high MoO3/Ag, exhibiting an improved energy efficiency (8.25%) charge mobility and a suitable energy band level. Moreover, compared to the phosphorene-free control device (7.37%). Such the external quantum efficiencies and current density-voltage bridging phosphorene has a well-matched band structure with curves can be significantly improved (Figure 10). Considering both the electron transport layer (ZnO) and the photoactive its thermal sublimation,[426] phosphorene incorporation and [771–782] layer (PTB7:PC71BM), which enhances the charge transport and subsequent device fabrication should be carried out with decreases charge recombination. An optimal thickness for the low temperature processes (<400 °C). phosphorene film is given as three times spin coating. However, The photovoltaic performances of dye-sensitized solar a further increase in film thickness degraded the device efficiency cells can be enhanced with phosphorene addition. Yu and (7.94%), probably due to the increased series resistances[746] and co-workers[308] adopted phosphorene as a photocathode in dye decreased optical transparency.[747,748] sensitized solar cells, in which few-layer phosphorene was Perovskite solar cells have attracted great interest since incorporated in a polymer to enhance charge transport and 2009[749] due to their high conversion efficiency (22.1%)[750–752] light absorption. The phosphorene-modified electrode facilitated and low-cost absorber materials (organometal halide).[753–763] a short circuit current of 24.31 mA cm−2 and a conversion effi- Phosphorene can play an enhancement role when incorpo- ciency of 6.85%, which was 20% higher than that of the refer- [764] rated in perovskite solar cells, where mesoporous TiO2 is ence device. Notably, the few-layer phosphorene quantum dots employed for electron collection and transfer. Again, phos- were prepared by the exfoliation of black phosphorus in eth- phorene incorporation not only accords well-matched energy anol, which minimized the fabrication cost by wet chemistry. [196,783] bands with perovskite absorbers, but also enhances charge Moreover, the phosphorene-modified TiO2 can resolve [765] transfer by suppressing charge recombination. Further- the problems of conventional TiO2 electrodes in dye sensitized more, doping phosphorene with surface adsorbates such as Cu solar cells. With high charge carrier mobility, the surface-coated adatoms[766,767] can improve the carrier concentration, which is phosphorene can suppress charge recombination, enhance an effective strategy to further enhance the device efficiency. In electron transfer, and promote light absorption. In such device addition, planar TiO2 perovskite solar cells have been reported architectures, phosphorene can effectively bridge the TiO2 and with an efficiency of 19.3% by Yang and co-workers.[768,769] It dye with its readily tunable band gap. is reasonable to expect an improvement when phosphorene is Van der Waals heterostructure solar cells[272,695,702,784,785] inserted between the blocking (yttrium-doped) TiO2 layer and are devices containing planar 2D materials that can form [743,746,786,787] the perovskite layer (CH3NH3PbIxCl3−x). Schottky junctions. Band gap alignment can Planar ITO perovskite solar cells demonstrated an 18% facilitate the design of heterostructures by matching the improvement[770] in efficiency when phosphorene quantum easily tunable band gaps of phosphorene with a variety of 2D

Adv. Energy Mater. 2017, 1702093 1702093 (16 of 43) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advenergymat.de materials. Recently, a phosphorene/h-BN heterostructure[788] adatom decorations (Pt and Au)[118] on the phosphorene sur- was successfully fabricated for photovoltaic devices such as face can enhance the conversion efficiency due to the improved solar cells and photodetectors. The primary external quantum crystal structures and electrical properties.[114] In short, the efficiency of the phosphorene/h-BN device is 0.1% at 640 nm. phosphorene/n-Si configuration is expected to show high effi- [310] In another work, the phosphorene/MoS2 junction exhib- ciency in device performance. ited an improved quantum efficiency of 0.3% at 633 nm. Phosphorene can combine with graphene for device These relatively low quantum efficiencies are attributed to the enhancement by enabling a more efficient charge separation low light absorbance[789] owing to the limited absorber thick- and transfer in graphene/n-Si solar cells. The phosphorene/gra- [790] [791,792] [832] ness (<1 nm). Similarly, 2D films of MoS2 and WS2 phene combination can utilize the dual merits of the excel- in solar cells demonstrated low photoelectron conversion effi- lent electrical conductivity of graphene and the readily tunable ciencies (<3.3%). Such a van der Waals heterostructure can band gap of phosphorene. Such heterostructure devices can be enhance the light absorption with a large surface area[793] and designed based on the semiconductor physics of doping levels promote ultrafast charge transfer,[794] which are favorable fac- and Schottky barriers, which have been calculated by Padilha tors for solar efficiency improvement. Despite the low effi- et al.[833] and Hu et al.[834] Thus, device fabrication involving ciency, ultrathin heterostructure solar cells exhibit the highest graphene transfer and subsequent phosphorene deposition is power densities (2.5 MW kg−1),[789] exceeding three orders of an experimental study remaining to be done. Moreover, the sta- magnitude more than those of the GaAs and Si solar cells bility of phosphorene can be enhanced by encapsulating it in with absorber thicknesses greater than 1 µm. Therefore, the graphene.[194] optical absorption should be further enhanced with light Overall, the incorporation of phosphorene can enhance the management strategies such as light trapping textures[795–806] device performances in all types of solar cells, as suggested on rough substrates rather than on ultraflat polished (SiOx) by comprehensive theoretical studies. Since only a few experi- surfaces.[22] In addition, the commercial, low cost soda mental works have been reported until now, experimental lime glass[807–809] can help with texturing surface with large studies on phosphorene solar cells are lacking. roughness. A large number of theoretical studies have been reported for heterostructure solar cells. Band-gap matching is an effi- 7. Photocatalysts for Solar Fuel Production cient strategy for exploring the possibilities of phosphorene in photovoltaics, which saves time and materials as compared Photocatalysts convert light energy into chemical energy, either to trial-and-error experimental processes.[810,811] For instance, reducing or oxidizing, through a redox reaction.[835–839] The [86] bilayer phosphorene/MoS2 heterostructures have a theo- photo-induced chemical reactions proceed by the transfer of retical conversion efficiency of up to 18%, which exceeds that charge carriers, where the solar light generates electron-hole of graphene/ transition metal dichalcogenide (TMDC) coun- pairs in a semiconductor surface. Initially, the electrons in terparts.[812–814] In addition, sandwiched phosphorene/TMDC/ semiconductors are excited by photons to hop from the valence phosphorene structures,[310,490,815] which are analogous to band to the conduction band, resulting in the generation of graphene/WS2/graphene structures, are expected to provide holes in the valence band. The electron transfer leads to the [272] − remarkable performances. formation of superoxide radicals (•O2 ) or the initialization of Besides 2D heterostructures, phosphorene with p-type and reduction with a matching reduction potential. Simultaneously, n-type doping can form p-n junction for efficient solar energy con- hole transfer produces hydroxyl radicals (•OH) or induces an version. A DFT calculation by Hu et al.[745] indicated that hydrogen- oxidation reaction with a suitable oxidation potential. The edged phosphorene (n-type) and fluorine-edged phosphorene scheme of redox potentials for H2 production and CO2 reduc- (p-type) have matching energy bands (i.e., the highest occupied tion is given in Figure 11. The essential steps of a photocatalytic molecular orbital and the lowest unoccupied molecular orbital). electrochemical reaction are light absorption and charge carrier Such edge termination with H or F can align band structures, excitation. The light utilization efficiency, which evaluate the which satisfies the electron-hole separation for fast charge transfer solar energy conversion, is determined by the band structure of and extraction. This device exhibited a high theoretical conversion the photocatalytic semiconductors. efficiency (20%), which can greatly motivate experimental scien- There are four typical routes for photocatalytic solar energy [840–845] tists to carry out further fabrication and performance studies. conversion. First, the overall water splitting into H2 Phosphorene with Si can form heterojunction solar cells, and O2 represents a typical route for photocatalytic solar which is compatible with commercialized n-doped bulk Si photo energy conversion. Second, the photocatalyts for CO2 reduc- voltaics.[816,817] On the one hand, 2D semiconductors/Si heter- tion[846–853] convert light energy into solar fuels with the for- [854] [855] ojunctions boast the advantages of pure Si devices with high mation of combustible gases such as CH3OH, CH4, [818–820] [821] [856] [857–859] conversion efficiency. A MoS2/Si device shows a short- and CO. Third, photocatalytic hydrogenation can circuit current of 22.36 mA cm−2 and an efficiency of 5.23%, raise the heating value of fuels for combustion, which maxi- which exceeds those of TMDC Schottky solar cells.[822–828] mizes the fuel utilization efficiency. Last but not the least, On the other hand, the alternative atomically thin 2D layers can photocatalytic detoxification[860–867] of wastewater can reduce minimize material and energy consumption during (doped) Si the energy consumption of water recycling treatments such as film deposition.[829] Here, the Si crystal accounts for the effi- photocatalytic water disinfection.[868–875] Therefore, we review cient optical absorption[830,831] as well as device enhancements the photocatalytic roles of phosphorene in these aforemen- such as short-circuit current and quantum efficiency. Further, tioned redox reactions.

Compared to bulk phosphorus, phosphorene has a wider band gap and its valence band has a positive shift, which enhances the electron reduction ability and suppresses electron–hole recombination. The improvement in catalytic activities was attributed to edge termination with hydroxyl groups which provide spacing for exposing the catalytic surfaces. In future, decoration with other catalysts could increase the hydrogen yields.[256,898–901] Further developments on the synthetic approaches for phosphorene such as, for example, the use low cost exfoliation agents such as sodium hydroxide solution. Another strategy to increase H2 yields is to optimize the architecture of water splitting devices such as the phosphorene dispersion on mesoporous matrices. Apart from water splitting for hydrogen generation, phosphorene as a photocatalyst holds promise for several other applications such as carbon dioxide reduction, hydrogenation, organic pollutant degradation, and bacteria disinfection.

Figure 11. Schematic energy diagram of photocatalytic semiconductor for water splitting and carbon dioxide reduction reactions. The 7.2. Catalytic Conversion of Carbon Dioxide into Fuels redox potentials are referenced with normal hydrogen electrode (NHE) at pH = 7. Reproduced with permission.[839] Copyright 2016, Royal Society of Chemistry. The direct transformation of carbon dioxide into a useful fuel is the ultimate goal in the management of exhaust gases. How- 7.1. Photocatalysts for Hydrogen Production ever, the chemical inertness of CO2 poses difficulties in its reduction, which requires complicated catalysts and reactions [876] −1 [902] Hydrogen, with a high energy capacity of 143 MJ kg , is paths. Generally, the photocatalytic reduction of CO2 is anal- a clean combustion fuel, which generates only water without ogous to water reduction for H2 production. The single electron − carbon emission. Early studies employed ultraviolet light reduction of CO2 to CO 2 (anion radical) requires a strongly (5% intensity in the total solar spectrum) for H2 fuel production negative reduction potential of −1.9 V versus the normal [877–880] [903] for TiO2 photocatalytic water splitting. A full reaction of hydrogen electrode (NHE). Virtually no semiconductor pro- water splitting has a large redox potential, which requires the vides such a potential to allow a single electron transfer to a [904] incorporation of low wavelength photons (ultraviolet, <180 nm) free CO2 molecule. Theoretical calculations have shown and corresponds to a large band gap (6.9 eV) of the semicon- that proton-assisted multiple electron transfer could favor CO2 ductor catalyst. However, this is not practical for the efficient reduction.[846,849,904–906] In the following equations, the electro- utilization of visible and near infrared light (>380 nm). There- chemical potentials are listed for the production of methanol, + fore, a half reaction (from H to H2) has been adopted for methane, and carbon monoxide. Indeed, many semiconductors H2 production (as shown below), while the oxidation reaction have more negative conduction band minima to allow multiple (other than O2 production) is compensated with the oxidation electron transfer for the feasibility of these reactions. How- of hole sacrificial agents. This suggests that a narrow band gap ever, limited evidence has been reported for these reactions.[907] semiconductor can promote the reduction reaction when the Therefore, these reduction reactions are likely to consist of a conduction band minima is more negative than −0.41 V at a series of single electron transfer steps, wherein the initial elec- pH = 7 (Figure 11) tron transfer poses a severe hurdle for the total reaction and contributes to the rate limiting step.[908,909] +− 0 He+→1/2H2rE edox =−0.41 VvsNHE (1) −− 0 CO22+→eCO Eredox =−1.90 VvsNHE (2) Hydrogen evolution can occur on transition metals such as [881,882] [883–885] Pt and Ni, which are costly and limited resources. +− 0 CO23++6H 6e →+CH OH HO2rE edox =−0.38 VvsNHE Graphitic carbon nitride opened a new avenue for H2 evolution on a nonmetal catalyst.[886–888] However, limited success has (3) been achieved with nonmetal elemental photocatalysts such as [889] [890] [891] [892,893] +− 0 boron, silicon, selenium, sulfur, and red phos- CO24++8H 8e →+CH 2H2rO E edox =−0.24 VvsNHE (4) phorus.[894,895] Recently, the layer-dependent bandgaps of phosphorene from 0.3 to 2 eV facilitated a broad light absorption as CO ++2H+−2e →+CH HO E 0 =−0.53 VvsNHE (5) possible visible-light photocatalysts in water splitting.[191,203,896] 242redox However, bulk phosphorus exhibited little photocatalytic activity.[257] Recently, Salehi-Khojin and co-workers[910] reported that an [897] Ji and co-workers revisited few-layer phosphorene as well artificial leaf of WSe2 electrocatalyst can reduce CO2 to CO fuel. as functionalized phosphorene as a photocatalyst (Figure 12), The low overpotentials and fast electron transfer of the TMDC which showed 18 times higher efficiency than bulk phosphorus. flakes account for the excellent performance in catalytic CO2

Figure 12. H2 generation performance based on phosphorene photocatalyst for water splitting. a) Schematic of phosphorene functionalization. b,c,d) XPS spectra of P 2p of bulk phosphorus and OH functionalized phosphorene. e) Schematic depicting photocatalytic H2 evolution. f,g,h) XPS spectra of O 1s for different types of phosphorene. i) Mott–Schottky plots of black phosphorus and functionalized phosphorene at a frequency of [897] 1 kHz. i,j,k) H2 production yields of different types of phosphorene. Reproduced with permission. Copyright 2017, Wiley-VCH.

reduction. Analogous to 2D WSe2, phosphorene shows promise phosphorene/red phosphorus hybrids can efficiently photogen- for CO2 reduction with its high electron transport properties erated electrons and transfer charges to the red phosphorus and high electrocatalytic activities. through their interfaces; this holds great potential for solar fuel Another straightforward strategy is the reduction of carbon generation. A recent work by the Yu and co-workers[257] presents dioxide on photocatalysts under solar irradiation,[911–917] even- a black phosphorus-red phosphorus heterostructure for effi- tually yielding solar fuels such as methanol or methane. 2D cient photocatalytic activity comparable to that of CdS. Further, layered structures such as graphene,[918,919] TMDCs,[910,920,921] a hybrid of phosphorene nanosheets with red phosphorus may and MXenes[922] can serve as photocatalysts for carbon dioxide provide better yields of photocatalytic products due to the pres- [923] reduction. Moreover, graphitic carbon nitride (g-C3N4) ence of more active sites, as well as readily tunable bands. There- shows great potential in combination with transition metal fore, phosphorene, as a new member of the 2D family, can act as [924–926] complexes. Indeed, red phosphorus and g-C3N4 hybrids an electrocatalyst and photocatalyst for carbon dioxide reduction prepared by Xue and co-workers[258] demonstrated a suitable with the advantage of high efficiency and utilization of full spec- [90,743,927,928] energy band diagram for the conversion of CO2 into methane tral range of solar energy. under solar irradiation (Figure 13). A small portion of g-C3N4 on a red phosphorus surface improves the photocatalytic activity for CH4 production. The enhancement is attributed to 7.3. Hydrogenation the effective separation of photogenerated electrons and holes at the red phosphorus/g-C3N4 interfaces. Hydrogenation involves the addition of hydrogen molecules The conduction band minimums of phosphorene and to unsaturated carbon bonds such as alkene and alkyne. g-C3N4 (Figure 13b) are comparable. This indicates that black The increased hydrogen content raises the heat value of

Figure 13. Black phosphorus and red phosphorus for CO2 conversion into CH4. a) Energy band diagram of phosphorus-graphitic C3N4 and scheme of electron-hole separation under solar irradiation. Reproduced with permission.[258] Copyright 2013, Elsevier. b) Energy level diagrams of layer-dependent few-layer phosphorene. Reproduced with permission.[743] Copyright 2014, Macmillan Publishers Limited. combustion in the final products. Earlier, homogeneous 7.4. Photocatalysts for Organic Pollutant Degradation hydrogenation employed transition metals such as Pd, Pt, Rh, and Ru.[929–931] However, incorporation of phosphorus The steady growth of global industries has led to a gradual enhanced the hydrogenation efficiency as compared to heavy increase in the release of wastewater with organic pollut- metals.[932–939] Non-noble-metal catalysts have been proposed ants such as pharmaceutical[954,955] and endocrine disrupting for environmental and economic concerns.[940] Often these chemicals.[956,957] These long-term endurable organic pollut- catalysts emerge as an acid/base pair, the so-called frustrated ants keep cycling in the ecosystem, and their biomagnifica- Lewis pair, such as C/B, P/B, P/Al, and P/Zn.[941–944] However, tion in animals and human beings cause serious food safety these catalysts are disadvantaged by steric hindrance due to the and health problems.[958] Various conventional strategies[959–962] bulky substituents, which suppress hydrogenation because of have been developed to treat organic pollutants such as physical increased activation energy barriers.[945] Therefore, the stabili- absorption, ion exchange, and biochemical degradation. How- zation of frustrated Lewis pair catalysts is necessary to enhance ever, these approaches have either low efficiency or high cost. the hydrogenation efficiency. Au (111) as well as metal–organic Photocatalytic degradation[963,964] is a novel solution that uses frameworks can graft the catalyst pairs and promote the hydro- semiconductor nanoparticles whose wide band gaps allow the [946–948] [965–970] genation of CO2. In addition, photocatalytic hydro- utilization of ultraviolet light. genation can significantly improve the reaction efficiency and Recently, Xie and co-workers[254] proposed that a singlet oxygen product yields.[949–952] state is promoted from the oxygen ground state upon illumina- Recently, Chen and co-workers,[931] using DFT computa- tion with a medium based on phosphorene nanosheets. Phos- tions, proposed that 2D phosphorene together with boron phorene as a photosensitizer has a high quantum yield of 0.91 for dopants as a frustrated P/B Lewis pair acts as an efficient cat- singlet oxygen generation over the entire visible-light range.[252] alyst for the hydrogenation of ethylene, ketones, and nitriles. This photocatalytic reaction enables efficient decomposition of B-doped phosphorene facilitates a two-step hydrogenation organic molecules such as 1,3-diphenylisobenzofuran and methyl reaction (Figure 14): H2 dissociation into hydridic and protic orange in wastewater, thereby facilitating disinfection of water as H atoms, followed by atomic hydrogen transfer to C2H4 well as photodynamic therapy. Therefore, phosphorene is highly molecules for the formation of C2H6. Here, hydrogen split- promising for high-efficiency low-cost photocatalytic degradation ting was the rate-limiting step, which has a lower activation because its tunable band gap guarantees high catalytic activity energy in the presence of electron-deficient boron. Substitu- with the utilization of the full solar spectrum. tion with Al-doped phosphorene exhibited even higher catalytic activities. A theoretical study has shown that B/Al doping in phos- 8. Electrocatalyst for Oxygen Evolution Reaction phorene results in relatively low formation energies and reasonable thermal stabilities. While experimental studies are The oxygen evolution reaction (OER)[971–980] and oxygen reduc- lacking, a recent experimental work showed efficient hydrogen- tion reaction (ORR)[558,981–988] play key roles in electrochemical ation with Ni-doped phosphorene.[953] energy conversion and storage, mainly in fuel cells[989–995] and

Figure 14. Hydrogenation at phosphorene surface. a) H2 dissociation reaction paths for phosphorene. b) Scheme of phosphorene catalyzed hydrogenation for alcohol and hydrocarbon production from unsaturated small molecules. c) Reaction pathways for ethanol (C2H5OH) and ethane (C2H6) production. Reproduced with permission.[931] Copyright 2017, American Chemistry Society. metal-air batteries.[982,996–1003] The OER is an important pro- large electrochemical surface area of 6.62 mF cm2 is deter- cess related to electrocatalysts for water splitting devices. In a mined from the overall adsorption electron charge in cyclic − [1014,1015] basic solution, the OER proceeds as follows: 4OH → 2H2O voltammograms. Electrochemical impedance spec- − [1016] + 4e + O2. The reverse process is the ORR. In an alkaline troscopy was employed to determine the ohmic resistance − − medium, the ORR is O2 + 2H2O + 4e → 4OH . For both and charge transfer resistance, where black phosphorus over reactions, the electrocatalyst should have good electrical charge Ti (263.4 Ω) exhibited a much lower resistance than pure Ti conductivity, a large specific area, and high activity. and phosphorus. Furthermore, a carbon nanotube matrix was State-of-the-art catalysts include platinum[981,1004,1005] and applied as a support for black phosphorus, which improved transitional metal oxides[1006–1010] and nitrides[1011,1012] but their the OER performance with a Tafel slope of 72.88 mV dec−1 and high cost and low abundance restrict mass production. Hence, charge transfer resistance of 191.4 Ω. The more efficient cata- there is a high demand for novel electrocatalysts based on low- lytic kinetics resulted in a larger specific area in 3D networks of cost abundant elements. First, black phosphorus was reported the black phosphorus/CNT matrices. as a promising OER catalyst, with an onset potential of 1.48 eV A four-electron transfer pathway accounts for the rapid versus the reversible hydrogen electrode and a current den- increase in current density, as determined using a rotating sity of 10 mA cm−2 at 1.6 V, which is comparable to that of the ring-disk electrode technique.[1017–1019] In addition, the time- [1013] sophisticated commercial RuO2. dependent chronoamperometric response shows a highly stable Initially, Wang and co-workers[972] investigated the OER current density with a small drop of 3.4% after continuous performance of black phosphorus deposited on titanium foil operation for 10,000 s. Therefore black phosphorus with high support. Tafel plots were employed to examine the catalytic structural stability,[1020] low charge transfer resistance, and effi- kinetics, where a lower Tafel slope represents faster reac- cient catalytic kinetics is an important electrocatalyst for oxygen tion kinetics. A Tafel slope of 91.52 mV dec−1 was achieved oxidation reactions. for black phosphorus on titanium, which indicated improved Recently, Zhang and co-workers[973] presented few-layer catalytic kinetics as compared with reported values.[978,981] A phosphorene nanosheets for preferable electrocatalytic OER

Figure 15. Oxygen evolution reaction of phosphorene nanosheets. a) Polarization curves of phosphorene nanosheets in KOH electrolyte with various concentrations (ranging from 0.05 to 1 m); inset shows the Tafel plots. b) Calculated Tafel slope as a function of the OH− concentration, at a current density 10 mV cm−2 and potential of 1.6 V. c) Electrochemical cell based on a three-electrode system with a glassy carbon electrode for contacting the working materials. d) Photograph of the working electrode with O2 bubble generation in electrochemical cell. e) Rotating ring-disk electrode voltammograms for determining the reaction pathway of electron transfer. (a,b,d) Reproduced with permission.[973] Copyright 2017, Wiley-VCH. (c,e) Reproduced with permission.[1021] Copyright 2009, American Association for the Advancement of Science.

[1061] activity with structural robustness. Compared to black phos- Other catalysts for the OER are MgAl2O4 spinels, [1062] [1063,1064] phorus, the electrochemical activity of phosphorene nanosheets CaMnO3 perovskite, oxygen vacancies MnO2, (Figure 15) shows great improvement with an onset potential mesoporous nanocarbon foams,[981] and graphene/CNT of 1.45 V and Tafel slope of 88 mV dec−1. Indeed, the nano- hybrids.[1065] Phosphorus doping[1066] imparts the matrix with structures are advantageous for the extraordinary electrochem- good ORR performance and better tolerance to methanol oxi- ical OER and show long-term stability. Furthermore, the thick- dation, high durability and comparable Tafel slopes. Therefore, ness of phosphorene has been tuned with density centrifuga- the potential applicability of pure phosphorene as an electrocat- tion selection. As a result, the reduction of phosphorene layers alyst in the ORR should be explored.[980,1067–1069] Moreover, the improved the OER performance. The authors propose that the catalytic capability of phosphorene for carbon dioxide reduction increased number of active edge sites of the nanosheets com- needs to be examined because large specific area structures [919,1070,1071] [910] pared with its bulk counterpart accounts for the enhancement such as graphene, WSe2, and silver/copper deco- in the electrocatalytic activities. rated nanosurfaces[1072–1075] successfully transformed carbon Further studies may focus on surface coatings with phos- dioxide into burnable fuels. After realizing the ORR, low-cost phorene. One avenue is low-cost supporting matrices with elemental abundant phosphorene can broaden the avenue of [979,1022–1031] [1076–1079] [1075,1080] a large specific area, such as porous TiO2, zeo- lithium air batteries and solar fuel cells. lites,[1032–1036] and other porous matrices.[1037–1041] Second, the development of phosphorene dispersion[57,58] have merits for blending with other electrical conductive nanosheets such as 9. Thermoelectric Conversion reduced graphene oxides,[1042–1049] and liquid exfoliated graphene dispersions.[472,1050–1055] In addition, the layer depend- A thermoelectric converter is a solid-state device that converts ence[59,168] as well as facet influence[1056,1057] in phosphorene heat flux to electricity via the Seebeck effect. In the thermoelec- should be taken into account for evaluating its catalytic activity tric effect, also termed the Seebeck effect, a temperature grain the OER. The electrocatalytic activities of phosphorene can dient in a thermal conducting material leads to heat flow, which be improved by doping[1058–1061] with foreign atoms such as drives the diffusion of charge carriers. Eventually, the flow of boron, carbon, nitrogen, and sulfur. charge carriers between the cold and hot electrodes creates a

Adv. Energy Mater. 2017, 1702093 1702093 (22 of 43) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advenergymat.de potential difference, and this principle is employed in waste require complicated multistep lithography[1089,1090] and material heat utilization to increase fuel efficiency. To date, thermoelec- deposition.[1091] tric energy generators have been confined to niche applications However, the intrinsic anisotropy of the structure and due to the low efficiency of the current materials. electrical properties of phosphorene provide an excellent The overall conversion efficiency is determined by the figure strategy to satisfy the requirements of small thermal con- of merit, abbreviated as ZT. ductivity[110,116,132] and large electrical conductance[11,1092,1093] along the armchair direction. Black phosphorus shows a ZT ST2 /k measured Seebeck coefficient[1094]S 335 V K−1, with a = σ (6) = µ ZT[1095] of 0.14 at room temperature, which is not competitive where S is the Seebeck coefficient;σ is the electrical conduc- due to the high lattice thermal conductivity.[1096] The strain tivity; T is the work temperature; k is the thermal conductivity. induced band convergence[154,314] can increase the Seebeck Over half a century of commercial applications,[1081] the coefficient and electrical conductivity and improve ZT to figure of merit of thermoelectric materials has remained steady 0.87.[1097] The calculation by Hicks and Dresselhaus[1098,1099] near ZT = 1 with a device efficiency of 10% at a temperature of predicted an increase in the thermoelectric performance ΔT = 300 K. To maximize the figure of merit, the material must ZT, due to the improved power factors caused by quantum have both high electrical conductance and low thermal conduc- confinement and decreased thermal conductivity resulting tivity,[1082–1084] which validate the most conventional bulk semi- from phonon boundary scattering. Indeed, black phosphorus conductor materials. Thus, nanostructured materials such as nanoribbons in an experimental measurement[1100] showed super lattices[1085–1087] and multilayer thin films[1088] can be an improvement in the Seebeck coefficient (Figure 16). The used for better electrical conductance and low thermal con- anisotropic structure of black phosphorus shows a higher ducting. These artificial anisotropic multilayers or nanostructures Seebeck coefficient, higher electrical conductivity, higher

Figure 16. Experimental measurement of the thermoelectric performance of black phosphorus ribbons. a) Diagram indicating the thermal transport measurement. b) Schematic and optical graph of the black phosphorus ribbon over a suspended-pad microdevice. c) SEM image of thermoelectric device. d,e,f) Seebeck coefficients, electrical conductivity, and power factor, for armchair and zigzag ribbons with temperature dependence. g) Electrical conductivity and thermal conductivity of an armchair ribbon. h,i) Temperature-dependent thermal conductivities for both armchair and zigzag black phosphorus ribbons. Reproduced with permission.[1100] Copyright 2015, Macmillan Publishers Limited.

Scheme 1. Applications of phosphorene to energy systems. power factor, and lower thermal conductance than those in co-workers[1103] showed exciting performance in vaporizing the the zigzag directions. aqueous surface into steam under sunlight irradiation, which With a further smaller dimension calculation by Lu and is a facile route for the efficient collection of drinkable water. co-workers,[313] monolayer phosphorene under strain presents Because phosphorene has better phototherapy potential[284,1104] a figure of merit of 2.1 in the armchair direction at room than graphene, the phosphorene/polymer matrix may find temperature. At 500 K, monolayer phosphorene has a theo- promising application in solar steam production with high effi- retical ZT of 2.5 along the armchair direction.[315] Ultimately, ciency and low cost. the nanoribbons of phosphorene[316] have a figure of merit ZT = 6.4 and S = – 492 µV K−1 at room temperature, at a doping level 2.3 × 1019 cm−3, as revealed in the calculation by Liu and 9.2. Exhaust Gas Treatment co-workers. The corresponding energy conversion efficiency is above 38%, which satisfies industrial requirements. This sug- Acidic exhaust gases from industrial establishments and vehi- gests that phosphorene nanoribbons are very promising high- cles cause air pollution and global warming, so their capture performance candidates for thermoelectric applications. and treatment are of vital importance. A recent density func- We also list a few future possibilities for energy applications tional theory calculation[1105] demonstrated that phosphorene (Scheme 1) based on an analysis of the perspectives of phos- decorated with lithium possesses a low adsorption energy phorene as well borrowing concepts from the well-established (0.376 eV) for CO2, which favors high-efficiency carbon capture. energy devices of other 2D materials: The strong adsorption of CO2 on light metal-decorated phosphorene suggests its potential utility in the filtration and collection of exhaust gases.[1106] Subsequently, the CO -rich gases 9.1. Solar Water Steam Generation 2 can be transformed into gaseous fuel with a novel complex cat- The graphene aerogel set afloat on seawater surface by Zhu alyst.[902] Therefore, this integrated protocol has a bright future [1101] [1102] and co-workers, Luo and co-workers, and Wang and for suppressing overall CO2 emission. In addition, other acidic

[1121] exhaust gases such as NO2 and SO2 can be adsorbed on light with TEM statistics. In addition, the low frequency Raman metal-decorated phosphorene[1105] and decomposed into less spectroscopy can assist the layer number determination of hazardous fragments with Pt-decorated phosphorene. Future phosphorene nanosheets.[1122] experiments can be designed and optimized to achieve the These advances in production methods will pave the way for phosphorene potentials. applications such as printed electronics[1123] which require uni- formly sized, and electrically identical phosphorene dispersions. This will also produce high quality transparent heaters,[1124] 9.3. Nanogenerator from Cyclic Molecular Charge Transfer high capacity secondary batteries,[1125,1126] hydrogen production[1127] and reinforcement in mechanics when blending into Triboelectric and piezoelectric energy nanogenerators[1107–1110] nanocomposites.[1128,1129] can harvest motion energy into electricity. These devices trans- A noticeable optical saturation absorption of phosphorene form motion-induced mechanical stress into accumulation of may lead to application in laser devices.[1130] The phosphorene charges,[1111–1113] are compatible for integration with sensor showed a very low ratio of excited state absorption over ground devices, and hold promise for the direct charging of wearable state absorption compared with the other 2D materials. Such electronic devices.[1114–1117] As a good electrical conductor with a high ground state transition probability together with a low a high specific area ratio, phosphorene is proposed as a host loss of excited absorption renders a good absorption saturation charge collector between the donor benzyl viologen (BV) and feature. This fundamental understanding shows accessibility of acceptor tetracyanoquinodimethane in a concept device.[1118] photonics such as fiber laser mode locking and Q switching. The phosphorene nanogenerator can generate power with con- As phosphorene can be explored for use in all types of tinuous charge transfer from the external force-driven motion energy devices, its growth theories aimed at higher quality, of the donor and acceptor polymers. The power nanogenerators better homogeneity, and large scale require more attention. present a concept of cyclic molecular (charge carrier) doping.

Acknowledgements 10. Outlook and Conclusions The following are gratefully acknowledged. The National Science We briefly review the phosphorus allotropes, types, and proper- Foundation China (NSFC. Project 51672181), the National Science Center for the financial support within the frame of the Sonata Program ties of phosphorene, and their emerging energy applications. (Grant agreement 2014/13/D/ST5/02853), and the Opus program Herein we see fast-growing knowledge of phosphorene energy (Grant agreement 2015/19/B/ST5/03399). J.P. is thankful for an IKM applications, together with breakthroughs in facile synthesis fellowship at the IFW Dresden. approaches. With its structural anisotropy, easy surface reaction, and large layer spacing, phosphorene plays important roles in electrochemical storage (lithium/sodium ion batteries Conflict of Interest and supercapacitors), energy conversion (photovoltaic cells and thermoelectronic devices), as well as photocatalysts for The authors declare no conflict of interest. hydrogen generation, hydrogenation, and the oxygen evolution reaction. Most recently Coleman and co-workers developed a cascade Keywords centrifugation method for liquid exfoliated nanosheets of 2D materials which allow the formation of monolayer enriching black phosphorus, lithium ion batteries, phosphorene, supercapacitors, water splitting dispersion.[1119] After an initial ultrasonication, a slow rate centrifugation (1.5 krpm) is conducted and then a top suspension Received: August 1, 2017 is collected. Then, the second step centrifugation with higher Revised: September 12, 2017 rate (2 krpm) is performed and again the top suspension is col- Published online: lected. Gradually with accelerated centrifugation rate for several steps, a final step (10 krpm) is carried on and eventually the top nice dispersion features the final product. Indeed, AFM height statistics showed 74% monolayer ratio and the optical extinc- [1] P. W. Bridgman, J. Am. Chem. Soc. 1914, 36, 1344. tion spectra determined a 70% monolayer volume fraction. [2] C. M. Park, H. J. Sohn, Adv. Mater. 2007, 19, 2465. Emerging in situ optical spectroscopy tools shed light on the [3] M. Kopf, N. Eckstein, D. Pfister, C. Grotz, I. Kruger, M. Greiwe, exfoliated 2D nanosheets. Thanks to an edge and confinement T. Hansen, H. Kohlmann, T. Nilges, J. Cryst. Growth 2014, 405, 6. effect, the lateral size and thickness of 2D materials nanosheets [4] S. Lange, P. Schmidt, T. Nilges, Inorg. Chem. 2007, 46, 4028. can be monitored in real time with optical extinction, absorb- [5] T. Nilges, M. Kersting, T. Pfeifer, J. Solid State Chem. 2008, 181, ance and scattering spectra.[1120] This general characterization 1707. [6] A. Avsar, I. J. Vera-Marun, J. Y. Tan, K. Watanabe, T. Taniguchi, technique will largely simplify the basic examination of phos- A. H. Castro Neto, B. Ozyilmaz, ACS Nano 2015, 9, 4138. phorene dispersions during the organic solvent exfoliation, [7] S. P. Koenig, R. A. Doganov, H. Schmidt, A. H. C. Neto, especially for the applications based on the size dependent B. Ozyilmaz, Appl. Phys. Lett. 2014, 104, 103106. properties such as fluorescence spectroscopy. Indeed, the lateral [8] L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, size obtained from dynamic optical spectra matches precisely Y. Zhang, Nat. Nanotechnol. 2014, 9, 372.

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