Tuning the Catalytic Property of Phosphorene for Oxygen Evolution

Jun 4, 2019 - Black phosphorus (BP), as a new kind of two-dimensional (2D) layer ... and defects; and schematic diagram for neighboring O atoms (PDF) ...
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Cite This: J. Phys. Chem. Lett. 2019, 10, 3440−3446

Tuning the Catalytic Property of Phosphorene for Oxygen Evolution and Reduction Reactions by Changing Oxidation Degree Xiong-Xiong Xue,† Shiyu Shen,† Xingxing Jiang,† Phoxay Sengdala,† Keqiu Chen,† and Yexin Feng*,†,‡ †

School of Physics and Electronics, Hunan University, Changsha 410082, People’s Republic of China Hunan Provincial Key Laboratory of Low-Dimensional Structural Physics & Devices, Hunan University, Changsha 410082, People’s Republic of China

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‡

S Supporting Information *

ABSTRACT: The development of inexpensive metal-free catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desirable for fuel cells and rechargeable metal−air batteries. Black phosphorus (BP), as a new kind of twodimensional (2D) layer material, has recently shown excellent OER electrocatalytic activity. However, atomistic understanding of the catalytic mechanism is lacking. Here, on the basis of ab initio calculations, we find that pristine phosphorene shows poor ORR/ OER performances. However, oxidation can effectively tune the adsorption strength of reactive intermediates and thus change its OER/ORR electrocatalytic performance. For OER, the higher the local oxidation degree (DOlocal) of phosphorene, the better the OER activity. Therefore, the oxidized phosphorene site with highest DOlocal shows the best OER catalytic property. In contrast, there exists an optimum DOlocal for ORR. These findings provide new insights for better understanding and improving the catalytic performances of BP-based electrocatalysts and could stimulate more theoretical and experimental studies in this area.

T

exhibits great potential for many emerging applications, including optoelectronic devices,31 thermoelectric generators,32,33 photocatalytic hydrogenation,34 etc. Jiang et al. have suggested that BP supported on Ti foil and a carbon nanotube network matrix exhibited efficient OER activity.35 Moreover, it was recently reported that a BP nanosheet can act as a promising electrocatalyst for OER.36 Its excellent electrochemical activity is equivalent to those of precious metal-based electrocatalysts, such as RuO2 and Pt/C.36 However, the underlying catalytic mechanism is still vague, especially concerning the specific active sites for OER, which is crucial for further improving the catalytic efficiency of BP nanosheets. On the other hand, the pristine BP sheets could be easily chemically modified. In ambient conditions, the BP surface is prone to oxidation.37,38 Theoretically, people have suggested that controlled oxidation can engineer the band gap of BP and tune the anisotropic carrier effective mass for electrons and holes.39−41 In experiments, it has been demonstrated that phosphorene oxide can act as a potential functional material in optoelectronics and photoluminescence.42,43 Additionally, various kinds of vacant defects are unavoidable in BP layers during fabrication. The influence of native point defects on structural and electronic properties has been widely investigated by scanning tunneling microscopy/spectroscopy

he oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are two key electrode reaction processes for various energy conversion and storage technologies.1−3 However, both OER and ORR reactions suffer from sluggish electronic transfer kinetics and require efficient electrocatalysts to accelerate them.4,5 Nowadays, noble metals and metallic oxides, such as Pt, RuO2, IrO2, etc., have been widely employed as electrocatalysts.6−9 However, the expensive price, limited natural resources, and poor stability of them seriously hinder the applications of these new technologies. Therefore, exploring the inexpensive and sustainable metal-free electrocatalysts with high performance is urgent. Recently, various metal-free electrocatalysts for ORR/OER have been extensively examined, including many kinds of 2D layered materials.10−14 Among them, graphenebased materials, such as N-doped graphene15−18 and Ncontaining co-doped graphene with a second heteroatom (B, P, S, Fe, Co, Ni, etc.),19−24 have shown potential in catalyzing the ORR and OER with high activity. Transition metal dichalcogenides (TMDs), such as GeS, GeSe, etc., have also been identified as potential efficient electrocatalysts.25 Black phosphorus (BP), as a new kind of 2D layered material, has recently attracted extensive attention, owing to its unique structural, electronic, and optical properties.26,27 BP is an anisotropic layered semiconductor with a puckered honeycomb structure, relatively high carrier mobility, thickness-dependent tunable band gap energy, and anisotropic dispersion.28−30 On the basis of these distinct properties, BP © 2019 American Chemical Society

Received: March 28, 2019 Accepted: June 4, 2019 Published: June 4, 2019 3440

DOI: 10.1021/acs.jpclett.9b00891 J. Phys. Chem. Lett. 2019, 10, 3440−3446

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The Journal of Physical Chemistry Letters

Figure 1. Top and side views of (a) pristine phosphorene and structures with adsorbed intermediate (b) OH*, (c) O*, and (d) OOH*. Purple (red, pink) balls are P (O, H) atoms. (e,f) Free-energy diagrams for OER (U = 1.23 and 4.26 V) and ORR (U = 1.23 and −0.79 V) elementary steps, respectively.

(STM/STS) and theoretical calculations.44−46 Single and double vacant defects can introduce unoccupied localized states into the band gap and result in magnetic moments in phosphorene.45 K adsorption can tune the BP from a moderate-gap semiconductor to a band-inverted semimetal.47,48 Therefore, chemical modification of BP, such as oxidation and including various kinds of defects and heteroatoms, may greatly affect the electrocatalytic performances.49 In this study, the OER and ORR catalytic properties of a single-layer BP sheet (phosphorene) and the influences from oxidation and various defects are systematically studied using density functional theory (DFT) calculations. The moderate adsorption strength of intermediate O* (not too strong nor too weak) is a key factor for good ORR/OER performance. The too strong adsorption of O* on pristine phosphorene and defects leads to poor ORR and OER performances. Oxidation of phosphorene, however, can effectively weaken the adsorption strength of O* and thus tune the OER and ORR activities. The catalytic performance is closely related to an order parameter, the local oxidation degree of phosphorene, DOlocal. The definition of this order parameter will be given below. For OER, higher DOlocal leads to better OER activity. The highest oxidation state, a local P2O5-like sheet, possesses the best OER catalytic performance. However, for ORR, there is an optimum DOlocal (3.4) corresponding to the best catalytic activity. These findings are expected to be instrumental in better understanding the catalytic mechanism and improving the catalytic performances of BP-based electrocatalysts. Spin-polarized calculations based on DFT methods were performed with the Vienna ab initio simulation package (VASP).50,51 The Perdew−Burke−Emzerhof (PBE) functional with the generalized gradient approximation (GGA) was employed to describe the exchange−correlation energy.52 The projector augmented wave (PAW) potentials were used with a

cutoff energy of 450 eV. We constructed the phosphorene supercell with 4 × 4 surface periodicity including 64 atoms to evaluate the ORR and OER performances. A vast variety of defect structures were considered by us. The Brillouin zone was sampled within the Monkhorst−Pack scheme by 4 × 4 × 1. All atoms were allowed to relax until the forces were less than 0.02 eV/Å. We created a vacuum region of 15 Å in the slab model to ensure negligible interaction between phosphorene and its mirror images. The formation energies (Ef’s) of various defects in phosphorene are calculated by Ef = Etot(m , n) − Etot(0, 0) − mμ N − nμC + m′μP

(1)

where Etot(m,n) and Etot(0,0) are total energies of defect structures and pristine phosphorene, respectively. m and n are the number of introduced N and C atoms in one supercell, and m′ is the number of substituted P atoms. The symbols μN, μC, and μP are the chemical potentials of N, C, and P. μN is calculated as in gas N2, μC in graphene, and μP in the bulk phase of BP. For oxidized phosphorene, we calculate Ef per oxygen atom, defined as Ef = (EO−P − E P − NOμO)/NO

(2)

where EO−P and EP are total energies of oxidized and pristine phosphorene, respectively. NO is the number of O atoms, and μO is calculated as in gas O2.39 Both OER and ORR reactions were investigated in this study. The OER and ORR progresses can occur along the four electron reaction paths,53,54 as described in the Supporting Information (SI). For each reaction step, the reaction Gibbs free energy ΔG is calculated according to the following equation ΔG = ΔE + ΔZPE − T ΔS + ΔG U + ΔGpH 3441

(3)

DOI: 10.1021/acs.jpclett.9b00891 J. Phys. Chem. Lett. 2019, 10, 3440−3446

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Figure 2. Atomic structures of (a) VP, (b) V2P-1, (c) V2P-2, (d) C1P, (e) N1P, and (f) C1PN1P defects in phosphorene. The optimal active sites with minimum ηOER (in red) and ηORR (in black) are also marked by arrows, and the values of ηOER and ηORR are shown in Table 1. Brown and green balls denote C and N atoms, respectively.

whereΔE is the energy difference between the initial and final states, obtained from the DFT total energy calculations. ΔZPE is the zero-point energy correction. T is the temperature (298.15 K), and ΔS is the vibrational entropy change. ΔGU is equal to −eU, where e is transferred charge and U is the electrode potential. ΔGpH = kBT ln 10 × pH is the correction of the H+ free energy, where kB is the Boltzmann constant and pH = 0 is assumed in the acidic environment in this study. The free energy of (H+ + e−) in solution is estimated as the energy of (1/2)H2 according to the computational hydrogen electrode model.54 The free energy of H2O is obtained from the gasphase H2O at 0.035 bar, which is in equilibrium with liquid H2O at 298.15 K.53 The free energy of O2 is derived from the reaction O2 + 2H2 → H2O to be 4.92 eV at T = 298.15 K and a pressure of 0.035 bar.55 The entropies and zero-point energy of the molecules in the gas phase are taken from the NIST database.56 However, for the adsorbed species, the entropies and zero-point energy are obtained from the vibrational frequencies from the DFT calculations. Because at any pH and U, the computational hydrogen electrode can describe the chemical potentials of protons and electrons, the calculated overpotentials are independent of the pH and U and are also applicable to the alkaline conditions.54 To better understand the ORR/OER catalytic mechanism of the BP nanosheet, we take phosphorene as a prototype to study its catalytic property with DFT calculations. The OER/ ORR catalytic activity can be characterized by overpotentials ηOER and ηORR, which can be calculated from the reaction freeenergy diagrams.53,54 Figure 1e,f shows the free-energy diagrams for OER and ORR on pristine phosphorene under a certain electrode potential U. Pristine phosphorene with and without adsorption intermediates (OH*, O*, and OOH*) is shown in Figure 1a−d. In Figure 1e, at U = 1.23 V (ideal water splitting potential), the steps of OH* → O* and OOH* → O2 are downhill and exothermal. However, for the ratedetermining step (RDS) of O* → OOH*, a free-energy gain of 3.03 eV has to be overcome. Only when the U increases to 4.26 V, as shown by the blue lines, are all elementary steps

downhill and can spontaneously occur, corresponding to an OER overpotential ηOER of 3.03 V. For ORR in Figure 1f, at U = 1.23 V, the RDS O* → OH* is an endothermic process with the highest free-energy gain of 2.02 eV. As shown by blue lines, at U = −0.79 V, this free-energy gain will disappear, determining ηORR = 2.02 V. The high ηOER (3.03 V) and ηORR (2.02 V) indicate that pristine phosphorene does not shows effective OER and ORR catalytic properties. Besides, the dispersion interactions within DFT-D2 and DFT-D3 schemes are also tested to confirm our results, as shown in Figure S1, which indicates that the dispersion interactions have little influence on calculated OER/ORR overpotentials. In fact, the catalytic activities strongly depend on the adsorption strength of reaction intermediates (OH*, O*, and OOH*), which should be not too strong nor too weak. The weak adsorption of intermediates will result in low efficiency of reaction, while too strong adsorption will starve reactions by blocking the catalytic surface. According to OER (O* → OOH*) and ORR (O* → OH*) RDSs on pristine phosphorene, we can find that adsorbed intermediate O* bears the critical responsibility for poor OER and ORR activity. The presence of a lone pair of electrons for P atoms results in too strong O* adsorption,40 which suppresses further reactions. Therefore, the key to obtain better OER and ORR catalytic properties for phosphorene is to weaken the adsorption of reaction intermediate O*. Many kinds of defects, including native defects and heteroatoms, are inevitable in obtained BP nanosheets in experiment, and the influence of these defects on structural and electronic properties has also been widely investigated by both experiment and theory.44−46,57 Intuitively, these defects are likely to play an important role in OER and ORR performances. We first construct intrinsic defects including the single-vacant defect (VP) and two kinds of double-vacant defects (V2P-1 and V2P-2), as shown in Figure S2. The atomic structures of them before structural relaxation are also shown in Figure S2. Considering that phosphorene has a puckered honeycomb structure, we consider two kinds of double-vacant 3442

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Table 1. Formation Energy Ef, Optimal ηOER and ηORR of Pristine Phosphorene, VP, V2P-1, V2P-2, C1P, N1P, C1PN1P, and Oxidized Phosphorene DOlocal Ef (eV) ηORR min (V) ηOER min (V)

Pri-P

VP

V2P-1

V2P-2

C1P

2.02 3.03

1.76 1.22 1.70

3.24 1.06 2.05

1.28 1.54 2.49

1.84 0.84 1.62

N1P

C1PN1P

1

1.8

2.6

3.4

P2O5

0.74 2.92 3.22

2.59 0.81 1.60

−2.11 3.41 2.24

−2.15 3.37 2.25

−2.18 2.76 1.48

−2.09 1.48 0.69

−2.72 0.70 1.82

Figure 3. (a) Adsorption energy of the O atom in pristine and different defect structures. Corresponding structures are shown in Figures S2 and S3. (b) Adsorption energy of the O atom on oxidized phosphorene as a function of local oxidation degree (DOlocal).

doped phosphorene, C sites possess better OER/ORR catalytic performances than P and N sites. Previous studies have reported that N atoms are not good active sites due to high electron affinity, compared with C atoms.15,16 The calculated ΔEO* at C and nearest-neighbored P sites are shown in Figure 3. We can see that the smaller ηOER and ηORR on C-containing structures are accompanied by the weaker O* adsorption, which could be further confirmed by free-energy diagrams in Figure S4. However, steps of O* → OOH* and O* → OH* are still the RDSs for OER and ORR, which implies that further lowering ΔEO* probably can improve the OER/ORR catalytic performances of phosphorene. As mentioned in the introduction part, the BP surface is prone to oxidation, which usually leads to degradation of the structure and electronic properties.37−40 However, could oxidation of phosphorene weaken the adsorption of O* and then improve the catalytic activity? On the basis of this hypothesis, many kinds of oxidized configurations were considered, as shown in insets of Figure 3b. In Table 1, negative Ef’s of these structures indicate that these oxidized defects are energetically favored. To evaluate the oxidation level more precisely, we introduce an order parameter DOlocal to describe the local oxidation degree of phosphorene. DOlocal is defined as DOlocal = NON + ωS × NOS + ωT × NOT , where NON(NOS , NOT ) is the number of nearest (second-nearest, third-nearest) O atoms of the P active site (indicated by N, S, and T in Figure S5) and ωS (ωT) is the weight factor capturing the importance of oxidation at the second-nearest (third-nearest) sites. The adsorption energy ΔEO* of O* at the P active site as a function of DOlocal with ωS (ωT) set to 0.8 (0.2) is plotted in Figure 3b. Changing ω only influences the value of DOlocal but does not change the trend of ΔEO*, as shown in Figure S6. As shown in Figure 3b, a different oxidation degree of phosphorene can change ΔEO* within a very wide rage. When 0 ≤ DOlocal ≤ 1.8, the O* adsorption strength slightly increases. When DOlocal ≥

defects, V2P-1 with vacancies in different layers and V2P-2 with vacancies in the same layer, as shown in Figure 2b,c, respectively. Like previous heteroatom-doped (B, N, P, etc.) graphene, we also introduce different external defects, including defects CxP and NxP, in which a P atom is replaced by x (x = 1−3) C or N atoms and co-doped defects containing both C and N atoms, CxPNyP (C1PN1P, C1PN2P, and C2PN1P), as shown in Figure S3. Because the activity of P atoms around vacancies is easily affected, the ORR and OER performances at different active sites around vacancies are investigated by us. Figure 2 shows some representative defect structures and the corresponding optimal OER and ORR reactive sites. More complete data for reactive sites and overpotentials are illustrated in Figures S2 and S3. In Table 1, results of optimal overpotentials indicate that these vacant structures (VP, V2P-1 and V2P-2) indeed possess smaller ηOER and ηORR than pristine phosphorene, but nonideal overpotentials still do not endow phosphorene with promising OER/ORR electrocatalyst properties. As shown in Figure S4, the OER/ORR RDSs on vacant structures are the same as those on pristine phosphorene. The reduction in ηOER and ηORR is mainly attributed to the strengthening of OOH* and OH* adsorption. Next, we focus on the OER/ORR catalytic properties of Cand N-doped phosphorene. The example structures of C1P, N1P, and C1PN1P are shown in Figure 2d−f, respectively. The atomic structures of the rest of the clusters are shown in Figure S3. The adsorption of O* is a critical step for OER and ORR; therefore, we summarize the adsorption energy of O* (ΔEO*) on different kinds of structures, as shown in Figure 3a. ΔEO* is defined as ΔEO* = EO* − E* − EO2, where EO*, E*, and EO2 are the total energies of surface adsorbed with O*, the clear surface, and O2. As shown in Table 1, C-containing defects show smaller ηOER and ηORR than pristine phophorene and vacancies. Our calculations also suggest that on C- and N3443

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and OER performance can been finely tuned by DOlocal. With the increase of DOlocal, ηORR’s and ηOER’s first increase and then decrease. For ORR, the best catalytic activity with the lowest ηORR occurs at DOlocal = 3.4, while for OER, the higher local oxidation degree of phosphorene, the better OER activity. P2O5, the highest local oxidation state, possesses the lowest ηORR. The calculated overpotentials ηORR’s and ηOER’s for pristine phosphorene, various defects, and oxidized phosphorene are all summarized in Figure 5a,b. ηORR’s and ηORR’s of these structures show typical volcano shapes. This indicates that by oxidation or introducing defects into phosphorene, the OER/ ORR catalytic activity can be tuned within a wide range. It can be seen that data of ηORR and ηOER for pristine phosphorene all locate at the bottom of the volcano shapes, indicating the poor catalytic performances. Although various kinds of defects (VP, V2P, CxP, NxP, and CxPNyP) could improve the ηORR and ηOER, the value is still far away from the peak of the OER and ORR volcano shapes. However, for oxidized phosphorene (O−P), as shown in Figure 5a, close to the peak of the OER volcano, the ηOER for P2O5 is as low as 0.70 V. Similarly, in Figure 5b, close to the peak of the ORR volcano, the ηORR for O−P can be reduced to as low as 0.69 V when DOlocal = 3.4. The reactive overpotentials can been better understood by plotting the reaction free-energy diagrams. Figure 5c,d shows the free-energy diagrams for OER on P2O5 and ORR on O−P with DOlocal = 3.4, respectively. In Figure 5c, for OER on P2O5, at U = 1.23 V, thanks to the weak O* adsorption, the RDS has translated to H2O → OH* with a free-energy gain of 0.70 eV, corresponding to an ηOER of 0.70 V. In Figure 5d, for ORR

1.8, the O* adsorption strength sharply decreases with the increase of DOlocal. ORR/OER activities of these oxidized phosphorene layers have been systematically studied, with ηOER and ηORR shown in Table 1. We can find that P2O5, the highest oxidation state, exhibits the best OER catalytic properties with ηOER = 0.70 V. For ORR, the oxidized structure with DOlocal = 3.4 possesses the best activity with ηORR = 0.69 V. To show the relation between ORR/OER catalytic properties and DOlocal more clearly, we illustrate the ηORR’s and ηOER’s of oxidized phosphorene as a function of DOlocal in Figure 4. We can clearly see that the ORR

Figure 4. ORR and OER overpotentials for oxidized phosphorene as a function of local oxidation degree (DOlocal). The corresponding atomic structures are shown in insets of Figure 3b.

Figure 5. Volcano plots of (a) OER overpotentials ηOER versus the difference between the adsorption energies of O* and OH* and (b) ORR overpotentials ηORR versus the adsorption energies of O* for different decorated structures. The data of ηOER for clusters of Ni12CN4C from ref 22 are also shown to illustrate the volcano-shaped curve more clearly. Free-energy diagrams for (c) the most optimal OER (U = 1.23 and 1.93 V) on P2O5 and (d) the most optimal ORR (U = 1.23 and 0.54 V) on local oxidized BP (O-BP) with DOlocal = 3.4. The corresponding structures with adsorbed intermediate OH* are also shown as insets. Blue balls represent the O atoms belonging to adsorbed intermediate. 3444

DOI: 10.1021/acs.jpclett.9b00891 J. Phys. Chem. Lett. 2019, 10, 3440−3446

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The Journal of Physical Chemistry Letters progress on O−P with DOlocal = 3.4, the RDS has changed to O2 → OOH* with a free-energy gain of 0.69 eV, corresponding to an ηORR of 0.69 V. These results clearly indicate that the local oxidation can effectively tune the adsorption strength of reactive intermediates and thus improve the OER/ORR catalytic activity. In addition, density of states results in Figure S9 clearly show that partially oxidized phosphorenes exhibit semiconductor properties, and a fully oxidized one is an insulator, which indicates that partially oxidized structures possess better electron transport property. In conclusion, by using DFT calculations, we systematically studied the OER and ORR catalytic properties of pristine and different chemically modified phosphorene sheets. Strong adsorption of O* on pristine phosphorene results in poor ORR and OER performances. However, catalytic performance strongly depends on the local oxidation of phosphorene. Local oxidation can effectively reduce the adsorption strength of O* and thus finely tune the ORR/OER catalytic activity within a wide range. For OER, catalytic activity improves with the increase of DOlocal. Therefore, the OER catalytic activity could be maximized on a local P2O5-like surface. However, for ORR, the catalytic performance could be optimized on partially oxidized phosphorene with DOlocal = 3.4. This study offers a rationalization of the observed excellent OER catalytic performances of BP-based electrocatalysts and paves the way for further improving catalytic performances of them.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b00891.

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Further computational setups, detailed atomic structures, reactive sites, and calculated ORR/OER overpotentials of a series of defects; free-energy diagrams of pristine, oxidized phosphorene, and defects; and schematic diagram for neighboring O atoms (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiong-Xiong Xue: 0000-0003-2342-1003 Keqiu Chen: 0000-0001-8627-0498 Yexin Feng: 0000-0001-5925-8645 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.X.F. and K.Q.C. are supported by the National Basic Research Programs of Ch ina under Grant No. 2016YFA0300900 and the National Science Foundation of China with Grant Nos. 11604092, 11634001, and 11674092. The computational resources were provided by the supercomputer TianHe-1 in Changsha, China.

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REFERENCES

(1) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the Rational Design of NonPrecious Transition Metal Oxides for Oxygen Electrocatalysis. Energy Environ. Sci. 2015, 8, 1404−1427. 3445

DOI: 10.1021/acs.jpclett.9b00891 J. Phys. Chem. Lett. 2019, 10, 3440−3446

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DOI: 10.1021/acs.jpclett.9b00891 J. Phys. Chem. Lett. 2019, 10, 3440−3446