Infrared Light-Driven Single Atom

Subscriber access provided by Nottingham Trent University

Communication

Metal-Free B@g-CN: Visible/Infrared Light-Driven Single Atom Photocatalyst Enables Spontaneous Dinitrogen Reduction to Ammonia Xingshuai Lv, Wei Wei, Fengping Li, Baibiao Huang, and Ying Dai Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02572 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Metal-Free B@g-CN: Visible/Infrared Light-Driven Single Atom Photocatalyst Enables Spontaneous Dinitrogen Reduction to Ammonia Xingshuai Lv, Wei Wei,* Fengping Li, Baibiao Huang and Ying Dai* School of Physics, State Key Laboratory of Crystal Materials, Shandong University, 250100 Jinan, China * Corresponding

authors:

[email protected] (W. Wei) [email protected] (Y. Dai)

ABSTRACT: Conversion of naturally abundant dinitrogen (N2) to ammonia (NH3) is one of the most attractive and challenging topics in chemistry. Current studies mainly focus on electrocatalytic nitrogen reduction reaction (NRR) using metal-based electrocatalysts, while metal-free and solar-driven photocatalysts have been rarely explored. Here, on basis of the “σ donation − π* backdonation” concept, single B atom supported on holey g-CN (B@g-CN) can serve as metal-free photocatalyst for highly efficient N2 fixation and reduction under visible and even infrared spectra. Our results reveal that N2 can be efficiently activated and reduced to NH3 with extremely low overpotential of 0.15 V and activation barrier of 0.61 eV, lower than most of metalbased NRR catalysts, thereby guaranteeing low energy cost and fast kinetics of NRR. The inherent properties of B@g-CN, such as centralized spin-polarization on the B atom, efficient prohibition of competitive hydrogen evolution reaction (HER), and reduced exciton binding energy, are responsible for the high selectivity and Faradic 1

ACS Paragon Plus Environment

Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

efficiency for NRR under ambient conditions. Moreover, for the first time, we theoretically disclose that the external potential provided by photogenerated electrons for NRR/HER endowing B@g-CN spontaneous NRR and inaccessible HER. This work may provide a promising lead for designing efficient and robust metal-free single atom catalysts toward photocatalytic NRR under visible/infrared spectrum.

KEYWORDS: Nitrogen reduction reaction, metal-free photocatalysts, single B atom supported on g-CN, selectivity, infrared light absorption

1. INTRODUCTION Nitrogen fixation in which the abundant N2 (makes up 78% of the atmosphere) is converted to NH3 is a vital chemical synthesis technique for sustaining all life forms. However, N2 is extremely stable and thus difficult to utilize because of the inert N≡N triple bond.1−3 Currently, NH3 synthesis is dominated by the industrial Haber−Bosch reaction usually necessitating rigorous conditions and heavy energy consumption, which is harmful and unsustainable.4−7 It is therefore of paramount importance to develop green techniques to replace Haber-Bosch production of NH3. Comparing to the conventional Haber−Bosch process, encouragingly, electrocatalytic and/or photocatalytic reduction of N2 can be carried out under ambient conditions, enabling energy-saving and environmentally-benign processes for sustainable NH3 production.8−12 In regard to electrocatalytic and photocatalytic NRR, efficient catalysts are definitely the core components to enhance the reaction rate, increase the 2


Page 2 of 33

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

selectivity and decrease energy consumption.13,14 In this field, solar driven N2 fixation is currently at the forefront since it inherits the natural photosynthesis virtues that are green and sustainable. In a photocatalytic process, photons are absorbed as the driving force to propel the N2 activation and reduction. However, the efficiency is still far from satisfactory because of the weak binding strength of N2 and inefficient electron transfer from photocatalyst to N2 (N2 + e− → N2•−), therefore the N≡N bond is difficult to be activated.15 To improve the NRR activity, photocatalysts should facilitate the chemisorption of N2 to guarantee the sufficient activation of the inert N≡N triple bond. On account of the coexisted empty and occupied d orbitals, transition metal (TM) atoms can not only accept the lone-pair electrons of N2 to strengthen the TMnitrogen bond, but also donate electrons into the antibonding orbitals of N2 to weaken the N≡N bond,16−19 see Figure 1a. In accordance to such a concept, non-metal elements which can also function as TM atoms to activate N2 are theoretically filtered. It is of great interest that sp2-hybridized B atom containing three occupied sp2 and one empty sp2 orbitals comes in sight, and could be an ideal element to realize single atom photocatalysts (SAPCs). In contrast to the B@g-C2N used as electrocatalyst where electric conductivity plays a curial role,14 semiconducting carbon nitrides should be more competitive as photocatalysts. To our knowledge, the electrical conductivity is crucial for electrocatalysts. In this respect, semiconductors may hold greater promise in photocatalysts. Recently, against conventional wisdom, non-metal boron-based catalysts such as dicoordinate borylene and B doped graphene have arisen great 3


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

attention from N2 fixation and reduction.1,20 Results show that the boron sites can provide enhanced binding capability to N2 molecules, high NH3 production rate and Faradaic efficiencies. These studies present convincing evidence for B-based catalysts for challenging NRR. In this regard, it is highly imperative to incorporate B atoms into appropriate photocatalysts for efficient N2 fixation. In comparison to electrocatalytic NRR, photocatalytic NRR, especially based on metal-free photocatalysts, possesses many superiorities including high activity, long durability, lost cost and environmental friendliness. Thus, the key to propelling the photochemical reactions lies in searching photocatalysts that enable N2 conversion to NH3. In this work, a novel kind of twodimensional (2D) graphitic carbon nitrides, i.e., g-CN, was convinced to be an efficient photocatalyst for N2 fixation. g-CN has high surface-to-volume ratio and porous structure, which are ideal for anchoring B atoms. As shown in Figures 1b, sp2-bonded N atoms at the vacancy hole edge can provide coordination sites for B atoms, leading to strong covalent B–N bonds as far as possible to prohibit B aggregation. It can be anticipated that one half-occupied sp2 orbital and one empty sp2 orbital of B atom can facilitate the binding and activation of N2, as illustrated in Figure 1c. Besides, 2D g-CN shows predominating ultraviolet light activity together with thermodynamic stability and engineerability.21,22 g-CN has already been fabricated through the reaction of C3N3Cl3 and Na by a simple solvothermal method.23 It is worth noting that various of g-C2N and g-C3N4 supported SACs have been already realized experimentally for photocatalytic and electrocatalytic reactions and 4


Page 4 of 33

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

the synthesis method turns out to be mature.24−26 It will be similar to g-C2N and gC3N4, therefore, g-CN supported SACs including B@g-CN hold great promise to be synthesized experimentally. In this respect, our work can give a hint for the coming experimental explorations. By virtue of these superiorities, we reported the strategy to achieve greatly boosted photocatalytic NRR activity based on B atom decorated metal-free g-CN (B@g-CN), as a new kind of SAPC. The first-principles density functional theory (DFT) calculations and a computational hydrogen electrode (CHE) model were used to explore the stability of B@g-CN and the NRR thermodynamics. With the rationale of “σ donation − π* backdonation” as a guideline, B@g-CN can efficiently promote N2 activation and reduction through an enzymatic pathway with a record low overpotential of 0.15 V and an activation barrier of 0.61 eV. The competitive hydrogen evolution reaction (HER) could be effectively suppressed on B@g-CN, indicative of the high selectivity for NRR. Remarkably, B@g-CN is demonstrated to be an efficient and robust visible/infrared light-driven (230 nm) SAPC for N2 fixation.

2. COMPUTATIONAL METHODS All spin-polarized first-principles DFT calculations were performed using Vienna ab initio simulation package.27,28 In order to describe the ion−electron interactions, projector-augmented wave (PAW)29 potentials were adopted. The Perdew, Burke, and Ernzerhof (PBE)30 exchange−correlation functional within the generalized gradient approximation (GGA) was employed. To take into account the van der Waals (vdW) 5


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

interactions, dispersion corrected DFT–D2 scheme31 was used. The cutoff energy for the plane-wave basis was set to 500 eV, and the convergence criteria for residual force and energy was set to 0.01 eV/Å and 10−5 eV, respectively. The Brillouin zone of 2×2×1 g-CN supercell was sampled by 5×5×1 and 11×11×1 k-point meshes for structure optimization and electronic property calculations, respectively. Ab initio molecular dynamics (AIMD) simulations were conducted to evaluate the thermodynamic stability of the catalysts. Heyd–Scuseria–Ernzerhof approach was considered to describe the band structures and the optical properties of g-CN and [email protected] Calculation details for optical absorption based on many-body perturbation theory (GW+BSE), Gibbs free energy and Faradaic efficiency can be found in the Supporting Information (SI).

3. RESULTS AND DISCUSSION Struture and stability of B@g-CN. Figures S1a and S1b show the optimizied structures of pristine and B embedded g-CN nanosheets, respetively. In respect to a single B atom decoration on g-CN, five possible anchoring sites are taken into account (Figure S1a). As summarized in Table S1, binding energies show that single B atom prefers to be stably adsorbed at the hole edge of g-CN via two B−N bonds with bond lengths around 1.45 Å. After B decoration, g-CN maintains its planar structure. For SAPC, diffusion and aggregation of single atoms are problematic issues, which can reduce the catalytic activity and the stability of catalytic cycles. To address this, binding energy (Eb) of B atom on g-CN is examined according to Eb = 6


Page 6 of 33

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

EB@g-CN – Eg-CN – EB, with EB@g-CN, Eg-CN, and EB being the energies of the adsrobed structure, pristine g-CN, and single B atom, respectively. The calculated binding energy is −5.67 eV, comparable to previous calculation results for various SACs, which is large enough to prohibit B aggregation and suggests high durability. Kinetically, diffusion of B atom from one anchoring site to a neighboring one is also examined. As shown in Figure S2, interestingly, the diffusion is unlikely to occur since the barrier can be as high as 2.28 eV. In addition, the thermal stability of B@gCN is evaluated by AIMD simulations. As can be found in Figure 1d, energy and temperature ossillate within small ranges and no significant structure distortion occurs even at elevated temperature of 800 K, which is a clear indication of high thermodynamic stability of B@g-CN. In this repect, the hole edge of g-CN can serve as stable anchoring sites for single B atoms, thereby meeting the prerequisites for gCN as a promising candidate for SAPC and holding great promise for synthesis. The electronic properties of catalysts are key factors to promote N2 adsorption and activation.8,33 In the light of Bader charge analysis, about 1.42|𝑒| is transferred from B atom to g-CN, and each of two nearest neihboring N atoms gains 0.40|𝑒|. In this case, B atom is positively charged and thus acts as ideal activation center, laying the foundation for N2 selectivity and activition. B@g-CN has a magnetic ground state with a spin moment of 1.00 μB, with B atom mainly contributing to the spin charge density (90%, Figure S1c). Therefore, B@g-CN is of great interest because of the rather centralized spin moment, which plays a vital role for N2 activation.33 Band structures of pristine g-CN and B@g-CN are shown in Figure 1e. The large band gap 7


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of 3.21 eV restricts pristine g-CN to mainly harvest ultraviolet light, accounting for less than 7% of the solar energy.23 For B@g-CN, however, the band gap is greatly reduced to 0.63 eV due to the in-gap states steming from hybridized B and N 2p orbitals. The significantly small band gap means that decoration of B atom on g-CN can effectively enhance the visible and infrared light absorption. It is worth noting that the majority of photocatalysts are susceptible to ultraviolet light, while missing the contribution from inrfared spectrum accounting for 43% of the solar energy.34 In this respect, B@g-CN can to a great extent utilize the solar energy and notably improve the apparent quantum yield (AQY). As discussed later, of course the infrared band gap of B@g-CN is still larger than the potential of overall redox reaction. After photoexcitation, charge transfer excitons can be formed with electrons trapped on BN2 units while holes left within valence bands. As exicton dissociation occurs, energetic electrons could be facilely injected into the antibonding orbitals of adsorbed N2 to trigger the photocatalytic NRR over B@g-CN. Thus, B atom-induced in-gap states playing a role as electron trapping center can with large possibility inhibite photoexcited carrier recombination. Besides, highly dispersive empty and occupied states where optical transition happens give rise to efficient electron transportation, corresponding to high solar energy conversion efficiency. In brief, electronic properties of B@g-CN can desirably promote N2 activation, and rationally tend to lower the overpotential and activation barrier for NRR (as discussed above).

N2 chemisorption and NRR pathways. N2 adsorption on catalysts is the first step of 8


Page 8 of 33

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

NRR and a fundamental requisite to start the reaction process.2 Figure S3 illustrates the configurations of N2 adsorbed on B@g-CN via end-on and side-on patterns. It suggests a spontaneous process when N2 going from gas phase to chemisorbed state with nagative adsorption energies of −1.36 and −0.86 eV for end-on and side-on adsorption, respectively. Thus, it is indicative of the strong N2 philicity of B@g-CN. As CO2 may usually exist in the environment and CO2 reduction on B@g-CN will compete against NRR, comparison between CO2 and N2 adsorption on B@g-CN is informative. Results show that CO2 can be captured with adsorption energy of −0.02 eV, indicative of rather weak interaction with B@g-CN. In this respect, B@g-CN prefers to adsorb and activate N2 compared with CO2, verifing substantially high N2 selectivity and offering encouraging perspectives for the use of B@g-CN in aqueous environment. In addition, the effects of H2O and O2 on the stability of B decorated on g-CN were examined. As shown in Figure S4, H2O and O2 can be adsorbed on B atoms with adsorption energies of −2.85 and −2.63 eV, respectively, which are lower than that of N2 (−1.37 eV). As H2O and O2 are captured, B−N bonds are slightly elongated and there is however no significant structure distortion. It appears to be that, therefore, active B sites could be of passivation in the presence of traces amounts of water and oxygen. In general, H2O/O2 scubber can be usually employed to efficiently reduce the water and oxygen before the NRR.35 In environment containing traces amounts of water and oxygen, although a fraction of active sites may be poisoned by water and oxygen, most of the B atoms are still active sites for NRR. Indeed, many photo/electrocatalysts encounter the aforementioned issues, but 9


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

experimental explorations verified the remarkable performance of TM- and/or Bbased SACs towards NRR. For example, Ru single atoms distributed on carbon nitride SACs achieve a Faradaic efficiency of 29.6% for NH3 production and a yield rate of 120.9 μg/mg/h.36 Légaré et al. recently presented the observation of N2 binding and reduction by a dicoordinate borylene.1 Especially, Zheng and co-workers also demonstrated the promoted electrochemical N2 fixation by metal-free B doped graphene (BG) under ambient conditions.20 At a doping level of 6.2%, BG achieved an encouraging performance, that is, a NH3 production rate of 9.8 μg/hr/cm2 and one of the highest reported Faradaic efficiencies up to 10.8%. It therefore powerfully suggests the great potential of metal-free B species for efficient N2 reduction, even though the inevitable existence of traces amounts of water and oxygen. In order to provide perspective on photon-driven NRR, it is useful to examine the thermodynamic driving forces which govern the process.37 For an efficient NRR photocatalysts, the essential prerequisites are low overpotential and energy barrier for N2 conversion to NH3 to guarantee low energy cost and excellent kinetics. In general, NRR occures through three possible pathways, including distal, alternating and enzymatic mechanisms, in which six consecutive protonation and reduction prcessess are involved (Figure 2a). For end-on adsoprption, N2 conversion to NH3 follows either distal or alternating mechanisms, while NRR will proceed through the enzymatic mechanism for side-on adsorption. Figures S5–S7 present the intermediate configurations of each elementary step on B@g-CN via three pataways, and the corresponding free energy diagrams are summarized in Figures 2b–2d. If NRR 10


Page 10 of 33

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

follows the distal or alternating trajectories (Figure 2b or 2c), adsorbed N2 could be easily hydrogenated by a proton coupled with an electron (H+/e−) to form *N–NH (Figure S5a or S6a) due to the slightly large free energies (from −0.80 to −0.39 eV). The N−H bond length is 1.06 Å and the N−N bond is further elongated to 1.22 Å. For the subsequent elementary steps through distal pathway, H+/e− pair continually attacks the distal N atom of *N–NH. Then, *N–NH2 is formed and the free energy decreases from −0.39 to −0.82 eV. The free energy for *N–NH2 reduction to *N–NH3 is, interestingly, slightly uphill by 0.04 eV. The first NH3 will be released after the fourth H+/e− participation, and one *NH remains on the surface of B@g-CN. During this process, the free energy of *N–NH3 reduction to *NH + NH3 is downhill by 0.73 eV. In the subsequent steps, the remaining *NH will be consecutively hydrogenated and form *NH2 and *NH3 with downhill free energies of −3.72 and −3.79 eV, respectively. Finally, the second NH3 will be desorbed from the surface of B@g-CN after overcoming a positive ∆G of 2.23 eV. It is known that NRR is a six-electron reduction process,8,9 in which the final step is not related to the overpotentials as there is no hydrogenation involved. To recover the catalysts, the produced NH3 should be desorbed from the catalysts immediately through, in experiments, applied potential. In consideration that the rapid removal of the produced NH3 is crucial for NRR, therefore, ∆G of the final step should be as low as possible.10 In addition, in the next step the adsorbed *NH3 would be further protonated to form NH4+, which is not considered in this work since it requires more detailed modelings of the solvated NH4+.38 The solvation of NH3 will release energy to stabilize the desorbed NH3, thus 11


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

promoting the desorbtion of produced NH3.39 After NRR occurs for some time, the obtained NH4+ can be detected and quantified by Nessler’s reagent. Nessler’s analysis shows that the NH3 yield rate based on the active mass changes under different applied voltage.40–42 With respect to the alternating pathway, protonation occurs alternatively to the two N atoms of N2, resulting in the release of the first NH3 at the fifth step (Figure 2c). It can be found that, therefore, protonation of *N2 to form *N– NH is the rate-limiting step (∆G = 0.41 eV) among all elementary steps of distal/alternating pathway. As NRR goes along the enzymatic pathway, protonation also occurs alternatively to the two N atoms of N2. As shown in Figure 2d, hydrogenation of N2 is extremely easy with a uphill free energy of 0.31 eV. Then, strikingly, all the subsequent elementary steps are exothermic. In cases of *NH–*NH2 and *NH2–*NH2 along the enzymatic pathway, after structural optimization, one N atom of the adsorbed species bonds the B atom while the other one tends to break the N–B bond and goes away from the B atom, which turns out to be similar with the alternating pattern. Such a switch in the pathway has also been found in B@C2N for NRR.14 Release of the first NH3 takes place at the fifth step. Similarly, hydrogenation of *N2 to form *N–NH is also the rate-limiting step, demanding an energy injection of 0.31 eV. The NRR overpotential (η) on B@g-CN was examined on the basis of free energy diagrams of three mechanisms. The lower the overpotential is, the better the catalytic activity is. According to the computational hydrogen electronde (CHE) model, the overpotential can be determined by8,19,43–47 12


Page 12 of 33

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

η = Ueuqilibrium – Ulimiting where Ueuqilibrium is the equilibrium potential of NRR (about −0.16 V vs NHE for the reaction N2 + 6H+ + 6e− → 2NH3), and Ulimiting is the required potential to overcome the maximum positive ∆G (∆Gmax) during NRR process defined as Ulimiting = – ∆Gmax/e. As discussed above, the limiting potentials for distal, alternating and enzymatic mechanisms are –0.41, –0.41 and –0.31 V, respectively. Thus, the corresponding NRR overpotentials are 0.25, 0.25 and 0.15 V, respectively. Therefore, NRR on B@g-CN prefers to proceed via the enzymatic mechanism due to the lowest overpotential. It is even lower than those of B@C2N (0.29 eV), metal-based NRR catalysts (larger than 0.20 eV) and the well-established Re(111) surface (0.50 V).2,8,14,43−47 Therefore, B@g-CN could be expected as a promising NRR catalyst with low energy cost. To our knowledge, previous discussions relied merely on the established thermodynamic facts as the relevant thermodynamic free energies rely solely on gasand liquid-phase products and reactants.37,43–47 However, a full understanding on photocatalytic NRR requires a detailed examination of the kinetic characteristics of adsorbed intermediates. In the current work, therefore, full reaction energies and transition states in the enzymatic pataway are addressed and presented in Figure 3. The largest reaction energy in the entire NRR process is 0.61 eV (TS1) for the first hydrogenation of N2, being the kinetic rate-determining step (KRDS). It is obviously lower than that on flat Ru (1.08 eV). In addition to the quite low overpotential, interestingly, excellent kinetics can be expected from B@g-CN. 13


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In a process of nitrogen fixation, the nitrogen oxitation reaction (NOR) can probably compete againest the NRR, which has been neglected in previous discussions toward NRR.13,14,43–47 As shown in Figure S8, the reductive pathway (N2→N2H) is more thermodynamically favorable than the oxidative pathway (N2→N2O), indicative of the high selectivity of NRR.

Mechanism of high NRR activity on B@g-CN. N2 activation has been theoretically48 and experimentally49 proven to be the rate-limiting step in the Haber−Bosch process. As N2 is adsorbed on B@g-CN, N−N bond length is stretched from 1.11 Å in gas phase to 1.13 (end-on) and 1.21 Å (side-on), proving the weakening of the N≡N triple bond and N2 activation. Charge density difference of B@g-CN reveals obvious charge transfer between B atom and N2 for both adsorption patterns (Figure S3). Interestingly, charge accumulation and depletion can be concurrently observed for both B atom and N2, well consistent with aforementioned “σ donation − π* backdonation” mechanism, as shown in Figure 1c. It is thus conclusive that B atom can function as a TM atom. In detail, the empty sp2 orbital of B atom will accept the lone-pair electron of N2 and, at the same time, the occupied sp2 ∗ orbital donates electron into the 𝜋2p antibonding orbital of N2.

In order to further reveal the superior catalytic activity of B@g-CN, variation of atomic charge in each elementary step is evaluated. According to previous studies,8,33 each intermediate can be divided into three moieties: g-CN (moiety 1), BN2 unit composed of B atom and its surrounding two N atoms (moiety 2), and adsorbed NxHy 14


Page 14 of 33

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(moiety 3), as illustrated in Figure 4a. The charge variation based on Bader charge difference is plotted in Figures 4b–4d, where the charge variation at step 0 represents the transferred charge after N2 adsorption. For following hydrogenation steps, BN2 unit may be involved in the reaction or only serve as electron transmitter. For example, at the third step of the enzymatic pathway, adsorbed *NxHy gains ~0.7|𝑒| from g-CN via the electron-transfer function of BN2 unit. Generally, g-CN serves as an electron reservoir during NRR, while BN2 unit is the active site for N2 reduction and the transmitter for electron transfer between *NxHy and g-CN. In addition, variation of N−N bond lengths in each step along the favorable enzymatic pathway is also examined. As reduction reaction begins, N−N bond is continuously stretched to 1.48 Å before emitting the first NH3 (Figures S5–S7), indicating the feasibility of N2 activation on B@g-CN.

Selectivity of the NRR. To date, catalysts that can produce NH3 in significant yields with high Faradaic efficiencies (FEs) are indeed still scarce, with one of the grand plague being that catalysts active for N2 conversion to NH3 in aqueous environment are also highly active for hydrogen evolution at the same potential.35 The majority of protons and electrons go towards HER rather than NRR, resulting in a severe selectivity issue.50 Free energy change (ΔGH) and applied potential needed to initiate the HER on B@g-CN is examined and compared, reaction with lower free energy is assumed to be more selective. The computational details are given in SI. For B@gCN, there are four possible sites for H adsorption, which are labeled as C, B, N1 and 15


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

N2, as shown in Figure S1b. ΔGH at these sites are 1.53, 0.47, −0.45 and −0.72 eV (Figure 5a), respectively, which are significantly larger than those of NRR (0.25, 0.25 and 0.15 eV). In this respect, the Faradaic efficiency of B@g-CN is estimated as close to 100% at room temperature. Therefore, the competitive HER could be efficiently suppressed, being a compelling evidence for the high selectivity of B@gCN for NRR.

Photcatalytic NRR. In order for a semiconductor to be active for photofixation of N2, the band edges must be aligned with the potentials of the redox half-reactions.37 On the other hand, the magnitude of the external potentials provided by photogenerated carriers directly determines whether the photocatalytic NRR/HER can proceed spontaneously.51 The potential of photogenerated electrons for NRR/HER (Ue) is defined as the energy difference between the hydrogen reduction potential (H+/H2) and the electron acceptor states, which is found to be −0.45 V for B@g-CN at pH = 0 and is much more negative than the theoretical potential of NRR (−0.16 V vs NHE) (Figure 5b). High Ue unravels that photogenerated electrons of B@g-CN would prefer to be transferred to react with H+ rather than with itself, contributing to a good resistance to the photoinduced corrosion.51,52 In consideration of the external potential that can be provided by photogenerated electrons, amazingly, all NRR intermediate steps turn out to be exothermic, indicating the spontaneous NRR upon light irradiation (Figure 5c). However, the potential of photogenerated electrons is insufficient to drive the HER over B@g-CN due to its much higher overpotential (average 0.79 V). 16


Page 16 of 33

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

In this respect, NRR can proceed spontaneously with efficient suppression of HER, guaranteeing the high selectivity and Faradaic efficiency of NRR. In order to initiate the photocatalytic N2 conversion to NH3, an efficient photocatalyst should possess high photoconversion efficiency. As shown in Figure 6, optical absorption of g-CN maximizes at 230 nm, indicative of limited visible light harvesting.53 On account of the decreased band gap, notably, B decoration on g-CN expands the optical absorption into visible and even infrared regions, therefore photons within a wider energy range can be utilized. It should be pointed out that, however, the band gap reduction does not mean necessarily the photoactivity in the visible and infrared regions, since light absorption needs to be followed by efficient charge transfer and extraction/utilization for photocatalytic processes to take place. In most cases, these processes are highly hindered by defects introduced by dopants, and hence most approaches of improving visible light activity result in reduced photocatalytic efficiency. In the case of B@g-CN, several factors such as the exciton binding energy and carrier effective mass guarantee the enhanced visible/infrared light response and carrier transfer. In addition, experimental explorations towards incorporating various non-metal atoms, including B, into g-C3N4 have validated that photogenerated electrons could easily jump from the impurity states to the conduction band or from the valence band to the impurity states.54,55 Results indicated that the photoactivity towards H2 generation and CO2 reduction could be greatly improved after introducing impurity states. In this respect, B@g-CN hints a higher photoconversion efficiency than g-CN, ensuring abundent photogenerated carriers and 17


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

high solar energy conversion efficiency. To unravel the charge transfer mechanism from B@g-CN to N2, projected band structure on N2 adsorbed on B@g-CN is shown in Figure S9. Once the N2 is captured by B@g-CN, significant hybridization appears within the band gap between N2 and B@g-CN (blue lines in Figure S9), indicating that photoexcited electrons from B@g-CN will be transferred substantially to these ingap N2 states to photoactivate N2. In respect to the photocatalytic NRR, efficient charge transfer and extraction are necessary. As a common phenomenon in low-dimensional materials, excitonic absorption dominates the optical response of g-CN and B@g-CN due to the strong electron–hole Coulomb interactions, with bound excitons featuring the spectra, see Figure S10. For clean g-CN, the exciton binding energy is 1.08 eV, smaller than the well-known g-C3N4 of 1.32 eV.56 In case of B@g-CN, interestingly, the exciton binding energy is significantly reduced to 0.62 eV. It is an indication of reduced electron–hole Coulumb interactions, and thus decreased photoexcited carrier recombination. In this situation, steering of electron tranportation could be more efficient. In addition, electrons and holes with high mobility are crucial for efficient carrier seperation. Herein, we explored the effective masses (m*) for g-CN and B@gCN. As shown in Table S3, carriers in B@g-CN present significantly smaller effective mass than those in clean g-CN, indicative of higher carrier mobility. As a result, the smaller carrier effective mass would lead to faster migration of photogenerated electrons and holes, which will quickly reach the surface of B@g-CN to participate in the NRR. In addition, for B@g-CN the hole effective mass is two 18


Page 18 of 33

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

times larger than that of electrons in both directions, then the anisotropy could further reduce the proability of the recombination of photogenerated carriers.

4. CONCLUSION In summary, by means of spin-polarized DFT calculations combined with a CHE model, we rationally designed and highlighted that B@g-CN as metal-free single atom photocatalyst presents high activity and selectivity for N2 fixation, which is hardly obtainable from TM-based catalysts. Our results revealed that the excellent NRR catalytic activity originates from the “σ donation − π* backdonation” process, which plays a vital role in capture and activation of N2. Importantly, NRR prefers to occur through the enzymatic pathway with a quite low overpotential of 0.15 V and activation barrier of 0.61 eV. In addition, the potential of photogenerated electrons provided by B@g-CN is sufficient to drive NRR against HER, with significantly reduced exciton binding energy. Our findings not only demonstrate the possibility of high-efficiency N2 reduction accompanied with vigorous hydrogen evolution, but also provide new insights for the rational design of NRR catalysts, which is expected to motivate more research efforts to explore metal-free photocatalysts.



ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (No. 51872170 and 21333006), the Taishan Scholar Program of Shandong Province, the Natural Science Foundation of Shandong Province (No. ZR2019MEM013), and the Young Scholars Program of Shandong University (YSPSDU).

19


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



REFERENCES

(1) Légaré, M.-A.; Bélanger-Chabot, G.; Dewhurst, R. D.; Welz, E.; Krummenacher, I.; Engels, B.; Braunschweig, H. Nitrogen Fixation and Reduction at Boron. Science 2018, 359 (6378), 896−900. (2) Azofra, L. M.; Li, N.; MacFarlane, D. R.; Sun, C. Promising Prospects for 2D d2– d4 M3C2 Transition Metal Carbides (MXenes) in N2 Capture and Conversion into Ammonia. Energy Environ. Sci. 2016, 9 (8), 2545−2549. (3) Xue, X.; Chen, R.; Chen, H.; Hu, Y.; Ding, Q.; Liu, Z.; Ma, L.; Zhu, G.; Zhang, W.; Yu, Q.; Liu, J.; Ma, J.; Jin, Z. Oxygen Vacancy Engineering Promoted Photocatalytic Ammonia Synthesis on Ultrathin Two-Dimensional Bismuth Oxybromide Nanosheets. Nano Lett. 2018, 18 (11), 7372−7377. (4) Kitano, M.; Inoue, Y.; Yamazaki, Y.; Hayashi, F.; Kanbara, S.; Matsuishi, S.; Yokoyama, T.; Kim, S.-M.; Hara, M.; Hosono, H. Ammonia Synthesis Using a Stable Electride as an Electron Donor and Reversible Hydrogen Store. Nat. Chem. 2012, 4 (11), 934−940. (5) Honkala, K.; Hellman, A.; Remediakis, I.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Ammonia Synthesis from First-Principles Calculations. Science 2005, 307 (5709), 555−558. (6) van Kessel, M. A.; Speth, D. R.; Albertsen, M.; Nielsen, P. H.; Op den Camp, H. J.; Kartal, B.; Jetten, M. S.; Lucker, S. Complete Nitrification by a Single Microorganism. Nature 2015, 528 (7583), 555−559. 20


Page 20 of 33

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(7) Shi, M. M.; Bao, D.; Wulan, B. R.; Li, Y. H.; Zhang, Y. F.; Yan, J. M.; Jiang, Q. Au Sub−Nanoclusters on TiO2 toward Highly Efficient and Selective Electrocatalyst for N2 Conversion to NH3 at Ambient Conditions. Adv. Mater. 2017, 29 (17), 1606550. (8) Zhao, J.; Chen, Z. Single Mo Atom Supported on Defective Boron Nitride Monolayer as an Efficient Electrocatalyst for Nitrogen Fixation: A Computational Study. J. Am. Chem. Soc. 2017, 139 (36), 12480−12487. (9) Liu, C.; Li, Q.; Wu, C.; Zhang, J.; Jin, Y.; MacFarlane, D. R.; Sun, C. SingleBoron Catalysts for Nitrogen Reduction Reaction. J. Am. Chem. Soc. 2019, 141 (7), 2884−2888. (10)Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355 (6321), eaad4998. (11)Zhang, N.; Jalil, A.; Wu, D.; Chen, S.; Liu, Y.; Gao, C.; Ye, W.; Qi, Z.; Ju, H.; Wang, C.; Wu, X.; Song, L.; Zhu, J.; Xiong, Y. Refining Defect States in W18O49 by Mo Doping: A Strategy for Tuning N2 Activation towards Solar-Driven Nitrogen Fixation. J. Am. Chem. Soc. 2018, 140 (30), 9434−9443. (12)Banerjee, A.; Yuhas, B. D.; Margulies, E. A.; Zhang, Y.; Shim, Y.; Wasielewski, M. R.; Kanatzidis, M. G. Photochemical Nitrogen Conversion to Ammonia in Ambient Conditions with FeMoS-chalcogels. J. Am. Chem. Soc. 2015, 137 (5), 2030−2034. (13)Ling, C.; Niu, X.; Li, Q.; Du, A.; Wang, J. Metal-Free Single Atom Catalyst for 21


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

N2 Fixation Driven by Visible Light. J. Am. Chem. Soc. 2018, 140 (43), 14161−14168. (14)Cao, Y.; Deng, S.; Fang, Q.; Sun, X.; Zhao, C.; Zheng, J.; Gao, Y.; Zhuo, H.; Li, Y.; Yao, Z.; Wei, Z.; Zhong, X.; Zhuang, G.; Wang, J. Single and Double Boron Atoms Doped Nanoporous C2N–h2D Electrocatalysts for Highly Efficient N2 Reduction Reaction: A Density Functional Theory Study. Nanotechnology 2019, 30 (33), 335403. (15)Zhu, D.; Zhang, L.; Ruther, R. E.; Hamers, R. J. Photo-Illuminated Diamond as a Solid-State Source of Solvated Electrons in Water for Nitrogen Reduction. Nat. Mater. 2013, 12 (9), 836−841. (16)Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501 (7465), 84−87. (17)Yandulov, D. V.; Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center. Science 2003, 301 (5629), 76−78. (18)Chen, Z. W.; Lang, X. Y.; Jiang, Q. Discovery of Cobweb-Like MoC6 and its Application for Nitrogen Fixation. J. Mater. Chem. A 2018, 6 (20), 9623−9628. (19)Liu, C.; Li, Q.; Zhang, J.; Jin, Y.; MacFarlane, D. R.; Sun, C. Conversion of Dinitrogen to Ammonia on Ru Atoms Supported on Boron Sheets: A DFT Study. J. Mater. Chem. A 2019, 7 (9), 4771−4776. (20)Yu, X.; Han, P.; Wei, Z.; Huang, L.; Gu, Z.; Peng, S.; Ma, J.; Zheng, G. BoronDoped Graphene for Electrocatalytic N2 Reduction. Joule 2018, 2 (8), 1610−1622. (21)Srinivasu, K.; Ghosh, S. K. Photocatalytic Splitting of Water on s-Triazine Based 22


Page 22 of 33

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Graphitic Carbon Nitride: An Ab Initio Investigation. J. Mater. Chem. A 2015, 3 (45), 23011−23016. (22)Liang, D.; Jing, T.; Ma, Y.; Hao, J.; Sun, G.; Deng, M. Photocatalytic Properties of gC6N6/gC3N4 Heterostructure: A Theoretical Study. J. Phys. Chem. C 2016, 120 (42), 24023−24029. (23)Li, J.; Cao, C.; Hao, J.; Qiu, H.; Xu, Y.; Zhu, H. Self-Assembled OneDimensional Carbon Nitride Architectures. Diamond Relat. Mater. 2006, 15 (10), 1593–1600. (24)Mahmood, J.; Li, F.; Jung, S.-M.; Okyay, M. S.; Ahmad, I.; Kim, S.-J.; Park, N.; Jeong, H. Y.; Baek, J.-B. An Efficient and pH-Universal Ruthenium-Based Catalyst for the Hydrogen Evolution Reaction. Nat. Nanotech. 2017, 12 (5), 441−446. (25)Zheng, Y.; Jiao, Y.; Zhu, Y.; Cai, Q.; Vasileff, A.; Li, L. H.; Han, Y.; Chen, Y.; Qiao, S.-Z. Molecule-Level gC3N4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions. J. Am. Chem. Soc. 2017, 139 (9), 3336−3339. (26)Jiao, Y.; Zheng, Y.; Chen, P.; Jaroniec, M.; Qiao, S.-Z. Molecular Scaffolding Strategy with Synergistic Active Centers to Facilitate Electrocatalytic CO2 Reduction to Hydrocarbon/Alcohol. J. Am. Chem. Soc. 2017, 139 (49), 18093−18100. (27)Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54 (16), 11169−11186. (28)Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to The Projector 23


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Augmented-Wave Method. Phys. Rev. B 1999, 59 (3), 1758−1775. (29)Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50 (24), 17953−17979. (30)Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Perderson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of The Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46 (11), 6671−668 (31)Liechtenstein, A. I.; Anisimov, V. I.; Zaanen, J. Density-Functional Theory and Strong Interactions: Orbital Ordering in Mott-Hubbard Insulators. Phys. Rev. B 1995, 52 (8), R5467−R5470. (32)Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118 (18), 8207−8215. (33)Li, X.-F.; Li, Q.-K.; Cheng, J.; Liu, L.; Yan, Q. Wu, Y.; Zhang, X.-H.; Wang, Z.Y.; Qiu, Q.; Luo, Y. Conversion of Dinitrogen to Ammonia by FeN3-Embedded Graphene. J. Am. Chem. Soc. 2016, 138 (28), 8706−8709. (34)Zhao, P.; Ma, Y.; Lv, X.; Li, M.; Huang, B.; Dai, Y. Two-Dimensional III2-VI3 Materials: Promising Photocatalysts for Overall Water Splitting under Infrared Light Spectrum. Nano Energy 2018, 51, 533−538. (35)Cui, X.; Tang, C.; Zhang, Q. A Review of Electrocatalytic Reduction of Dinitrogen to Ammonia under Ambient Conditions. Adv. Energy Mater. 2018, 8 (22), 1800369. (36)Geng, Z.; Liu, Y.; Kong, X.; Li, P.; Li, K.; Liu, Z.; Du, J.; Shu, M.; Si, R.; Zeng, 24


Page 24 of 33

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

J. Achieving a Record-High Yield Rate of 120.9 μgNH3 mgcat−1 h−1 for N2 Electrochemical Reduction over Ru Single-Atom Catalysts. Adv. Mater. 2018, 30 (40), 1803498. (37)Medford, A. J.; Hatzell, M. C. Photon-Driven Nitrogen Fixation: Current Progress, Thermodynamic Considerations, and Future Outlook. ACS Catal. 2017, 7 (4), 2624−2643. (38)Skúlason, E.; Bligaard, T.; Gudmundsdóttir, S.; Studt, F.; Rossmeisl, J.; AbildPedersen, F.; Vegge, T.; Jónsson, H.; Nørskov, J. K. A Theoretical Evaluation of Possible Transition Metal Electro-Catalysts for N2 Reduction. Phys. Chem. Chem. Phys. 2012, 14 (3), 1235−1245. (39)Zhao, W.; Zhang, L.; Luo, Q.; Hu, Z.; Zhang, W.; Smith, S.; Yang, J. Single Mo1(Cr1) Atom on Nitrogen-Doped Graphene Enables Highly Selective Electroreduction of Nitrogen into Ammonia. ACS Catal. 2019, 9 (4), 3419−3425. (40)Cheng, H.; Ding, L.-X.; Chen, G.-F.; Zhang, L.; Xue, J.; Wang, H. Molybdenum Carbide Nanodots Enable Efficient Electrocatalytic Nitrogen Fixation under Ambient Conditions. Adv. Mater. 2018, 30 (46), 1803694. (41)Chen, G.-F.; Cao, X.; Wu, S.; Zeng, X.; Ding, L.-X.; Zhu, M.; Wang, H. Ammonia Electrosynthesis with High Selectivity under Ambient Conditions via a Li+ Incorporation Strategy. J. Am. Chem. Soc. 2017, 139 (29), 9771−9774. (42)Luo, Y.; Chen, G.-F.; Ding, L.; Chen, X.; Ding, L.-X.; Wang, H. Efficient Electrocatalytic N2 Fixation with MXene under Ambient Conditions. 2019, 3 (1), 279−289. 25


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(43)Zhang, X.; Chen, A.; Zhang, Z.; Zhou, Z. Double-Atom Catalysts: Transition Metal Dimer-Anchored C2N Monolayers as N2 Fixation Electrocatalysts. J. Mater. Chem. A 2018, 6 (38), 18599−18604. (44)Chen, Z.; Zhao, J.; Cabrera, C. R.; Chen, Z. Computational Screening of Efficient Single-Atom Catalysts Based on Graphitic Carbon Nitride (g-C3N4) for Nitrogen Electroreduction. Small Methods 2019, 3 (6), 1800368. (45)Choi, C.; Back, S.; Kim, N.-Y.; Lim, J.; Kim, Y.-H.; Jung, Y. Suppression of Hydrogen Evolution Reaction in Electrochemical N2 Reduction Using Single-Atom Catalysts: A Computational Guideline. ACS Catal. 2018, 8 (8), 7517−7525. (46)Li, Q.; Qiu, S.; Liu, C.; Liu, M.; He, Z.; Zhang, X.; Sun, C. Computational Design of Single-Molybdenum Catalysts for the Nitrogen Reduction Reaction. J. Phys. Chem. C 2019, 123 (4), 2347−2352. (47)Li, Q.; He, L.; Sun, C.; Zhang, X. Computational Study of MoN2 Monolayer as Electrochemical Catalysts for Nitrogen Reduction. J. Phys. Chem. C 2017, 121 (49), 27563−27568. (48)Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Ammonia Synthesis from First-Principles Calculations. Science 2005, 307 (5709), 555−558. (49)Dahl, S.; Taylor, P. A.; Törnqvist, E.; Chorkendorff, I. The Synthesis of Ammonia over a Ruthenium Single Crystal. J. Catal. 1998, 178 (2), 679−686. (50)Singh, A. R.; Rohr, B. A.; Schwalbe, J. A.; Cargnello, M.; Chan, K.; Jaramillo, T. F.; Chorkendorff, I.; Nørskov, J. K. Electrochemical Ammonia Synthesis−The 26


Page 26 of 33

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Selectivity Challenge. ACS Catal. 2017, 7 (1), 706−709. (51)Qiao, M.; Liu, J.; Wang, Y.; Li, Y.; Chen, Z. PdSeO3 Monolayer: Promising Inorganic 2D Photocatalyst for Direct Overall Water Splitting Without Using Sacrificial Reagents and Cocatalysts. J. Am. Chem. Soc. 2018, 140 (38), 12256−12262. (52)Chen, S.; Wang, L.-W. Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution. Chem. Mater. 2012, 24 (18), 3659−3666. (53)Srinivasu, K.; Modak, B.; Ghosh, S. K. Porous Graphitic Carbon Nitride: A Possible Metal-Free Photocatalyst for Water Splitting. J. Phys. Chem C 2014, 118 (46), 26479−26484. (54)Wang, K.; Li, Q.; Liu, B.; Cheng, B.; Ho, W.; Yu, J. Sulfur-Doped g-C3N4 with Enhanced Photocatalytic CO2-Reduction Performance. Appl. Catal. B: Environ. 2015, 176, 44−52. (55)Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H. M. Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4. J. Am. Chem. Soc. 2010, 132 (33), 11642−11648. (56)Wei, W.; Jacob, T. Strong Excitonic Effects in the Optical Properties of Graphitic Carbon Nitride g-CN from First Principles. Phys. Rev. B 2013, 87 (8), 085202.

27


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1 (a) Simplified schematic of N2 bonding to transition metals. (b) Design concept of B atom decoration on g-CN. B atom bonds with two N atoms of g-CN to form two B-N bonds, leaving one half-occupied sp2 orbital and one empty sp2 orbital. Therefore, the as-designed B@g-CN can facilitate the binding and activation of N2. (c) Schematic of N2 bonding to B atom on B@g-CN. (d) Variations of energy and temperature versus the AIMD simulation time for B@g-CN. The insets denote the top and side view of B@g-CN after AIMD simulation lasting for 10 ps at T = 800 K. Band structures of (e) pristine g-CN and (f) B@g-CN. The Fermi level is set to zero.

28


Page 28 of 33

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2 (a) Schematic depiction of three mechanisms for N2 reduction to NH3 on B@g-CN, including distal, alternating and enzymatic pathways. Corresponding free energy diagrams through (b) distal, (c) alternating, and (d) enzymatic pathways at different applied potentials.

29


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3 Minimum energy path for N2 conversion to NH3. The intermediates and transition states (TSs) are indicated.

30


Page 30 of 33

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4 (a) Three moieties of, for example, the B-N=NH2 intermediate, and the charge variation of the three moieties along the (b) distal, (c) alternating and (d) enzymatic pathways, respectively.

31


Nano Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5 (a) Gibbs free energy diagram of HER on B@g-CN. (b) Schematic illustration of band edge positions of B@g-CN relative to the normal hydrogen electrode (NHE) at pH = 0. EAVS and ENHE represent energy level relative to the absolute vacuum scale (AVS) and NHE. (c) Gibbs free energy diagrams of NRR on B@g-CN at U = 0 and U = –0.45 V. U = –0.45 V correspond to the applied potential provided by B@g-CN under light iradiation.

32


Page 32 of 33

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 6 Optical absorbance of pure g-CN and B/g-CN. The shadow represents the reference solar spectrum irradiance at Air Mass 1.5.

Table of Contents

33


Infrared Light-Driven Single Atom

Recommend Documents