Photocatalysis in Two-Dimensional Black Phosphorus: The Roles of

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Photocatalysis in Two-Dimensional Black Phosphorus: The Roles of Many-Body Effects Hui Wang, Xiaodong Zhang,* and Yi Xie*

ACS Nano 2018.12:9648-9653. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/10/18. For personal use only.

Hefei National Laboratory for Physical Science at Microscale, iChEM, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ABSTRACT: Two-dimensional (2D) black phosphorus (BP) has drawn tremendous attention in solar-light-driven catalytic processes for its intriguing chemical and physical properties. Benefiting from the highly anisotropic electronic structure induced by its puckered crystal geometry, 2D BP tends to have greater confinement with respect to traditional inorganic nanomaterials, thereby leading to robust many-body effects. Such Coulomb-interaction-mediated effects dominate the electronic and optical properties of 2D BP-based nanosystems, where exotic correlations between photoinduced species give rise to unique photoexcitation processes that are closely associated with the involved photocatalytic behavior. In this Perspective, we highlight the critical role of many-body effects in 2D BP-based photocatalysis and exemplify the relationships between the correlated photoinduced species-dominated photoexcitation processes and photocatalytic behavior involved therein. The relevant challenges and opportunities in pursuing efficient 2D BP-based solar energy utilization are also discussed.

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lead to band structures that are distinctly different from those of other 2D systems with strong many-body effects.11,12,16 However, in terms of 2D BP-based photocatalytic research, the impacts of many-body effects have not yet been adequately considered, which greatly hinders the comprehensive understanding of the relevant photoexcitation processes.

olar-light-driven catalytic processes have attracted increasing attention due to their great potential for solving problems such as the energy crisis and environmental pollution.1−4 With regard to the design of advanced photocatalysts, low-dimensional materials show promise by virtue of their advantages in maximizing surface areas and tailoring electronic structures.3,4 Recently, the emerging two-dimensional (2D) material black phosphorus (BP) has been studied extensively due to its excellent transport properties, broadband light absorption, and tunable electronic band structure; it shows great promise in the field of solarlight-driven catalysis.5−7 Although some progress has been made in 2D BP-based photocatalysis, traditional viewpoints focusing on the behavior of photogenerated free charge carriers are not straightforwardly applicable, due to many-body effects. Many-body effects, which originate from the interactions between particles or quasiparticles (electrons, holes, excitons, etc.), dictate the electronic and optical properties of materials and, hence, impact the photoexcitation-related applications.8−10 Compared to bulk counterparts, low-dimensional materials possess more significant many-body effects due to promoted quantum confinement and reduced screening.9,10 In a series of experimental and theoretical investigations, researchers have demonstrated that robust many-body effects can be expected in 2D BP, where the resulting excitonic effects and self-energy corrections dominate the photoexcitation processes.11−16 Benefiting from these effects, 2D BP exhibits promising light-driven applications in photodetectors, photon polarizers, and field-effect transistors.13−16 In addition, 2D BP has novel electronic structures resulting from its highly anisotropic crystal structure (i.e., in-plane anisotropy), which © 2018 American Chemical Society

Understanding the robust many-body effects inherent in two-dimensional black phosphorus will not only provide insight into the involved photocatalytic behavior but will also offer guidance for improving solar energy utilization. In this Perspective, we present the critical role of many-body effects in 2D BP-based photocatalysis. By understanding the interactions between photoinduced species (including electrons, holes, and excitons), we highlight the unique photoexcitation processes and the resulting photocatalytic behavior in this system. We discuss the significance of regulating different many-body correlations for photocatalytic performance, noting potential regulation strategies for gaining efficient solar energy utilization. Many-Body Effects Lead to Strong Excitonic Effects in Two-Dimensional Black Phosphorus. Excitonic effects mediated by Coulomb interactions between photogenerated electrons and holes lead to the formation of neutral Published: September 19, 2018 9648

DOI: 10.1021/acsnano.8b06723 ACS Nano 2018, 12, 9648−9653

Perspective

Cite This: ACS Nano 2018, 12, 9648−9653

ACS Nano

Perspective

Excitonic effects determine the different photocatalytic mechanisms. Compared with standard charge-transfer processes, exciton-based energy-transfer processes achieve effective energy utilization by means of exchange or dipole−dipole interactions.17−19 Such processes do not involve net chargecarrier-transfer between donor and acceptor, and will promise interesting photocatalytic behavior. For instance, with regard to photocatalytic singlet oxygen (1O2) generation, the traditional view focuses on charge-transfer processes and attributes this reaction to the oxidation of superoxide radicals by photogenerated holes.20 However, the high quantum yields that are observed in some cases are difficult to explain in this way. Indeed, 1O2 generation tends to undergo nonradiative transfer of energy from long-lived triplet excitons to groundstate oxygen molecules.19 Bearing this process in mind, we proposed that 2D BP with strong excitonic effects could be a promising photosensitizer for achieving efficient 1O2 generation under visible- and near-infrared-light illumination.21 We fabricated ultrathin BP nanosheets by means of liquid exfoliation, using water as the dispersion solvent. As compared with their bulk counterparts, the ultrathin BP nanosheets exhibited higher 1O2 yields under visible- and near-infraredlight illumination by virtue of promoted excitonic effects and suitable band gaps. The prominent excitonic effects also influence the quantum efficiencies of carrier-based photocatalysis in 2D BP-based materials. Strong excitonic effects lead to low yields of free charge carriers and limit the mobility of charge carriers,22 both of which are associated with charge-carrier-based photocatalytic quantum yields. In this case, low-efficiency carrierbased photocatalytic reactions (e.g., water splitting, carbon dioxide reduction, and nitrogen fixation) can be expected in pristine 2D BP. In Addition to Excitonic Effects, Many Other Higher Order Correlations in Two-Dimensional Black Phosphorus Are Closely Related to Its Photocatalytic Behavior. Many-body correlations between multiple particles (including electrons, holes, and excitons) would result in higher order correlated states that dominate the nonlinear optical responses of 2D BP.13,15,23−26 For instance, the correlations between three charge carriers (i.e., two electrons and one hole, or two holes and one electron) would lead to the formation of trions (also known as charged excitons); the correlations between four charge carriers (i.e., two electrons and two holes) would lead to the formation of biexcitons. Recently, Plochocka and co-workers demonstrated that the radiative decay of trions plays a critical role in the photoluminescence of monolayer BP, even under low excitation power density and high temperature (Figure 2a−c), confirming the involvement of strong higher order correlations in the 2D system.13 These higher order multiexcitons traditionally exhibit much faster decay rates than do single excitons undergoing nonradiative, carrier−carrier interaction-mediated Auger processes. Thus, strong depopulation of photoinduced species can be expected in 2D BP, which is undoubtedly detrimental to the photocatalytic efficiency. The multiexcitons also possess faster resonance energy-transfer rates than do excitons, due to their more suitable dipole moments. Experimental results have demonstrated that the energy-transfer efficiencies between multiexcitons and single excitons are comparable to those between single excitons.27,28 Thus, it is reasonable to seek potential pathways to promote solar energy collection via multiexcitonbased energy-transfer processes.

quasiparticles, excitons (or bound electron−hole pairs), which endow BP with quite different excitation processes that are closely related to photocatalytic behavior.11−13,16 Note that excitons differ from what are generally termed photogenerated electron−hole pairs in most photocatalytic research. Excitons are bound states of electrons and holes mediated by Coulomb interactions, whose energy levels are lower than those of photogenerated electron−hole pairs. This energy difference is typically defined as exciton binding energy (Eb) and can be employed to estimate the strength of excitonic effects in the system. A series of recent theoretical and experimental achievements in the investigation of excitonic aspects of 2D BP demonstrated that excitonic effects can greatly impact 2D BP’s optical properties. 11−13 Tran et al. theoretically investigated the quasiparticle band gaps and excitons in 2D BP with different stacking layers and demonstrated thicknessdependent excitonic effects in the system (Figure 1a).11 They

Figure 1. (a) The calculated band gap evolution of black phosphorus (BP) with different numbers layers based on different calculation methods. (b) The calculated optical absorption spectra with/without (solid/dash lines) considering electron−hole interactions of monolayer, bilayer, trilayer, and bulk BP for the incident light polarized along the armchair direction. Simulated distributions of the square of the electron wave functions of the (c) first and (d) second bound excitonic states over the (x,y) plane in monolayer BP, suggesting the quasi-one-dimensional confinement in the two-dimensional material. Adapted with permission from ref 11. Copyright 2014 American Physical Society.

found that excitonic effects dominate the optical properties of 2D BP and that effective optical response spans the visible and near-infrared regions and can be tuned by controlling the number of stacking layers (Figure 1b). Compared to other 2D systems, such as graphene and transition-metal dichalcogenides nanosheets, BP exhibits a large E b , several hundred millielectron volts, which can be ascribed to the strong selfenergy corrections arising from the highly anisotropic band dispersion in 2D BP structure (Figure 1c,d). The large Eb leads to excitons, rather than charge carriers, tending to be the primary photoinduced species dominating the relevant photocatalytic behavior in BP. Understanding the influence of excitonic effects on the photoexcitation processes involved in 2D BP is necessary to achieve efficient solar-light-driven catalytic performance. 9649

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These higher order correlations can also improve photocatalysis. For instance, the presence of exciton−exciton annihilation provides a potential scattering process for multiplication of the photoinduced species.29 One absorbed photon with high energy could give rise to multiple electron− hole pairs or excitons, which can be useful for high-efficiency photocatalysis. In addition, Auger recombination, a nonradiative many-body scattering mechanism involving three charge carriers, has drawn attention in photocatalysis for its role in the hot-carrier transfer process.30 These higher order correlations hold promise for avoiding energy loss during cooldown processes and extending the effective excitation spectra of photocatalytic systems. These higher order correlations are dependent on the concentrations of photoinduced species and become at high concentrations. The higher order correlated states determine the photoexcitation processes at high excitation power densities.

Higher order correlated states in twodimensional black phosphorus enable exotic photoexcitation processes, but also complicate photocatalytic investigations.

Figure 2. (a) Microphotoluminescence spectra and (b) integrated emission intensities of the trion (T) and exciton (X) on the basis of Gaussian fitting of monolayer black phosphorus (BP) under different excitation power densities. (c) Temperature-dependent microphotoluminescence spectra of monolayer BP. Adapted with permission from ref 13. Copyright 2016 American Physical Society. (d) Time-resolved photoluminescence spectra of exciton and corresponding fits to single exponential, double exponential, and bimolecular models under different excitation power densities of two-dimensional BP. (e) Decay ratios of the two exponential components under different pump fluences/initial exciton densities. Adapted with permission from ref 25. Copyright 2016 American Physical Society.

Band-Structure Modifications Present Another Prototypical Impact of Many-Body Effects on Two-Dimensional Black Phosphorus-Based Photocatalysis. According to theoretical calculations, 2D BP has a highly anisotropic band structure, where flat band dispersion can be expected along the zigzag direction of the anisotropic material with extremely large effective mass.11,12 This band dispersion leads to strong, quasi-one-dimensional confinement of particles along the armchair direction, which supplies 2D BP with a large self-energy correction. Moreover, the quasi-one-dimensional band dispersion and strong many-electron effects result in singularities in the electronic density of states (i.e., Van Hove singularities), which is a notable difference between 2D BP and other traditional 2D materials.31,32 In this regard, the emergence of sub-band structures can be expected in 2D BP, hinting at distinct photoexcitation processes to be obtained by controlling the adopted excitation. Recently, Zhang et al. carried out an infrared study on 2D BP, where the nontrivial sub-band structures in BP with different thicknesses were highlighted (Figure 3a).33 The sub-band structures render a series of absorption resonances in the infrared spectra that serve as infrared fingerprints, important for identifying optical transitions for photocatalysis. Moreover, the asymmetrical subband structures in BP nanosheets enable photogenerated charge carriers with different redox potentials to be obtained for certain photocatalytic reactions. Recently, we demonstrated the presence of optically switchable photocatalytic oxygen activation in 2D BP, where •OH and 1O2 were identified as the dominant reactive oxygen species generated under ultraviolet- and visible-light excitation, respectively.7 With a combination of ultrafast spectroscopy and photoluminescence spectroscopy, we confirmed different relaxation processes under the two excitation energies. On the basis of experimental observations, we presented a method for controlling the carrier-based charge-transfer process and exciton-based energy-transfer process in 2D BP by adopting

Moreover, higher order correlations would establish robust, nonradiative decay processes such as exciton−exciton annihilation and Auger recombination in 2D BP,25,26 leading to depopulation of photoinduced species that would be unfavorable to the promotion of quantum yields of both exciton- and carrier-based photocatalytic reactions. Taking exciton−exciton annihilation as an example, the collision between two excitons would lead to the nonradiative transfer of energy and momentum from one exciton to another, leaving a lower excited-state (generally, ground-state) exciton and a higher excited-state exciton. Subsequently, the generated higher excited-state exciton (sometimes known as hot exciton) would relax to lower energy states, undergoing a rapid, electron−phonon interaction-mediated cooling process. Thus, exciton−exciton annihilation would lead to exciton depopulation and energy loss in the system, which would reduce photocatalytic performance. Recently, Surrente et al. identified the onset of exciton−exciton annihilation in 2D BP.25 By monitoring the excitation-power-dependent time-resolved microphotoluminescence at low temperature, they observed a fast, nonexponential decay component associated with exciton−exciton annihilation in exciton dynamics under high injection density (Figure 2d). An upper limit of ∼6.1 × 1012 cm−2 was estimated, after which the exciton−exciton annihilation tended to impact the quantum yield of the photoexcitation applications of 2D BP (Figure 2e). This value might be variable in different cases, due to the dielectric environment-dependent feature of Eb in 2D BP.13 9650

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interaction-mediated effects would provide new perspectives on the design and optimization of 2D BP-based photocatalysts. Given the magnitude of many-body effects in 2D BP, exciton-based energy-transfer processes provide an alternative pathway for solar energy harvesting in this system. Considering that the efficiency of exciton-based energy-transfer processes is closely associated with the excitonic factors of donors and acceptors (such as spin states and energy levels), unfavorable energy utilization can be anticipated once these excitonic factors are ill-matched. In this regard, the addition of other materials with suitable excitonic factors that act as the media between donors and acceptors could help achieve highefficiency energy utilization, with two successive energytransfer processes (from the excited donors to ground-state media and from the excited media to ground-state acceptors) replacing the original unfavorable direct energy-transfer processes between excited donors and ground-state acceptors. Therefore, as photocatalysts with strong excitonic effects, the above medium-assisted energy-transfer processes can provide an alternative pathway to optimize photocatalytic performance. The addition of co-catalysts with suitable excitonic factors to promote the efficiency of energy transfer between the main photocatalysts and reactants could be important for gaining high-efficiency solar energy utilization. Moreover, the mediumassisted energy-transfer processes could be effective in promoting light harvesting. This strategy has been widely employed in the design and optimization of solar cells, where semiconductors with wide and high absorption spectra act as the light collection medium.38,39 In this case, co-catalysts can be employed to improve the light absorption undergoing exciton-based energy-transfer processes. That is, co-catalysts harvest the solar energy that cannot be effectively absorbed by the main photocatalysts. When excitonic factors are matched, the energy in photoexcited co-catalysts can resonantly transfer to the main photocatalysts, thus improving solar energy utilization. With regard to its wide and tunable light absorption, 2D BP serves as a promising co-catalyst for extending photocatalytic energy conversion to the visible and near-infrared regions, and the identification and optimization of appropriate main catalysts will be an important step.

Figure 3. (a) The optical conductivities for two-dimensional black phosphorus (BP) with six layers under incident light with different polarization angles, where E11 and E22 denote the first and second sub-band transitions. Adapted with permission from ref 33. Copyright 2017 Nature Publishing Group. (b) Schematic illustration of the sub-band structure and the resulting photocatalytic behavior in BP nanosheets. Adapted from ref 7. Copyright 2018 American Chemical Society.

appropriate excitation scenarios (Figure 3b). Note that the excitonic effects still dominate the photoexcitation processes of ultraviolet-light-excited 2D BP and the energetic matches are critical to the relevant photocatalytic behavior. The matched energy between valence band edge and redox potential enables the oxidation of water molecules into •OH, whereas the matched energy between the excitonic state and 1 O 2 conversion energy enables the activation of ground oxygen molecule into 1O2. Beyond the sub-band structures, band gap renormalization is another universal electronic band modification induced by many-body effects that has been widely observed in lowdimensional semiconductors.34−36 Due to Coulombic interactions between photogenerated charge carriers (including electrons in the conduction band and/or holes in the valence band), exchange−correlation corrections lead to a reduced band gap. Such a reduction effectively renormalizes the band gap of photoexcited systems, which is critical to the relevant nonlinear optical response. Recently, Gao and Yang theoretically investigated the band gap renormalization of free carrierdoped 2D BP; they suggested that different many-electron interactions are responsible for the band gap renormalization at different doping densities.37 The Coulomb-hole and screenedexchange self-energies determine the band gap renormalization of monolayer BP under low and high doping densities, respectively. Benefiting from its highly anisotropic electronic band dispersion, monolayer BP possesses a small band-edge density of states, which endows this 2D material with large screened-exchange interactions, leading to much larger band gap renormalization at high doping density than that of other 2D systems. Given that band gaps determine the absorption properties, band gap renormalization should be taken into account when dealing with 2D BP-based photocatalysis, which also opens potential methods for optimizing photocatalytic behavior via controllable photoexcitation.

Two-dimensional black phosphorus serves as a promising co-catalyst for extending photocatalytic energy conversion to the visible and near-infrared regions. Given the strong correlations between photoinduced species, the behavior of free charge carriers is inevitably influenced by many-body effects, opening important optimization strategies for gaining high-efficiency carrier-based photocatalysis. In view of the competitive generation of excitons and free charge carriers, the yields of photogenerated charge carriers can be optimized by regulating the strength of excitonic effects in 2D BP. In this case, promoted charge carrier yields can be expected in multilayer 2D BP, due to the thickness-dependent Eb in the framework. However, the concomitant reduction in electronic bandgap with increasing thickness inevitably limits this layer control strategy. In addition to layer control, there are other potential methods for regulating the strength of excitonic effects of BP. It has

PERSPECTIVES AND OUTLOOK The unique electronic structure of BP endows this emerging 2D material with intriguing optical properties that underscore its potential in photocatalysis; however, the ignored manybody effects preclude comprehensive understanding of relevant photocatalytic processes. Insight into the impact of Coulomb9651

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Sponsorship Program by CAST, Anhui Provincial Natural Science Foundation (1708085QB24, 1808085QB34), the National Postdoctoral Program for Innovative Talents (BX201700219), and China Postdoctoral Science Foundation Funded project (2017M620262).

been demonstrated that the strength of the excitonic effects in BP is dependent on the surrounding dielectric environment.13 This feature enables excitonic regulation to be achieved via substrate control. Thus, in terms of heterojunction design, the selection of alternative components beyond BP in the heterostacks is also important. By selecting materials with suitable dielectric properties, the energy levels of excitonic states and the yield of excitons in 2D BP are regulated, thereby modifying the photoexcitation processes and photocatalysis. Moreover, the electronic structure of BP can be tailored by other structural modifications including stacking order,40 strain,41 and elemental doping,42 which enables optimizing catalysis. Given that excitons are bound electron−hole pairs, it is sensible to promote carrier-based photocatalytic efficiency by dissociating excitons. Traditionally, exciton dissociation tends to occur in areas with disordered energy landscapes; structural factors, such as order−disorder interfaces, defects, and heterojunction interfaces have been demonstrated to be beneficial for dissociating excitons.43−45 Note that the conventional strategies that promote the separation of charge carriers also favor the efficiency of carrier-based photocatalysis. The selective extraction of electrons and holes toward different regions or components of photocatalysts shifts the equilibrium between excitons and charge carriers, leading to increased charge carrier yields for photocatalysis. Thus, the construction of heterojunctions is a meaningful method for gaining efficient charge carrier yields in BP-based photocatalysts. Additional problems related to many-body effects in 2D BP deserve attention. For instance, for the excitonic processes, the complex profiles of dark and bright exciton states in 2D BP are not fully understood, which makes it difficult to track the conversion pathway of the photoinduced species. The optically dark exciton states play dominant roles in the entire excitonic process, and the nonradiative transition between different states limits the photocatalytic quantum yield. The many-body correlations between photoinduced species and lattice vibrations would also complicate the photocatalytic investigation, due to their influence on the electronic band structure.46 Moreover, it has been demonstrated that BP possesses strong light−matter coupling,47 where the correlations between many-body-effect-mediated photoinduced species and photons might establish some effective pathways for solar energy utilization. Overall, the robust many-body effects involved in 2D BP-based materials leave both challenges and opportunities in improving the efficiency of photocatalysis.

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaodong Zhang: 0000-0002-8288-035X Yi Xie: 0000-0002-1416-5557 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (21331005, U1532265, U1632149, 11621063), Key Research Program of Frontier Sciences (QYZDY-SSW-SLH011), the Youth Innovation Promotion Association of CAS (2017493), Young Elite Scientists 9652

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DOI: 10.1021/acsnano.8b06723 ACS Nano 2018, 12, 9648−9653