Two-Dimensional Supramolecular Nanoarchitectures of Polypseudorotaxanes Based on Cucurbit[8]uril for Highly Efficient Electrochemical Nitrogen Reduction

Electrochemical Nitrogen Reduction

Article

Two-Dimensional Supramolecular Nanoarchitectures of

Polypseudorotaxanes Based on Cucurbit[8]uril for Highly Eﬃcient

Cai-Cai Zhang, Xiaolu Liu, Yu-Ping Liu,* and Yu Liu *

Cite This: Chem. Mater. 2020, 32, 8724 −8732

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sı Supporting Information

ABSTRACT: Supramolecular assemblies with two-dimensional

2D) topology have drawn great attraction in the development of

(

functional materials through a modular approach. Herein, a novel

organic 2D polypseudorotaxane has been constructed on the basis

of cucurbit[8]uril (CB[8]) through host-stabilized charge-transfer

(CT) interactions of naphthol-modiﬁed porphyrin (TPP-Np) and

viologen derivatives (DMV). Interestingly, the 2D polypseudor-

otaxanes could serve as a platform for preparation of ultraﬁne Pt

nanoparticles with favorable and homogeneous dispersion through

a simple self-metallized process, leading to the formation of

supramolecular hybrid materials (PtNPs@(CB[8]/DMV/TPP-

Np)). Surprisingly, the PtNPs@(CB[8]/DMV/TPP-Np) could be applied to eﬃcient electrochemical nitrogen reduction reaction,

−

−1

with a NH yield rate up to 23.2 μg h mg_P_tat −0.2 V versus a reversible hydrogen electrode under ambient conditions. Notably,

our results not only demonstrate the feasible construction of 2D polypseudorotaxanes but also deepen the understanding of

structure−activity relationships in supramolecular nanoarchitectures, as well as pave an eﬀective and low-energy input strategy for

artiﬁcial nitrogen ﬁxation.

INTRODUCTION

process. Although the design and construct of 2D supra-

molecular nanoarchitectures of (pseudo)rotaxanes or poly-

(

■

Two-dimensional (2D) nanoarchitectures have drawn a great

deal of attention in nanotechnology and materials science

because of their fascinating physical and chemical properties as

well as potential application in catalysis, electronics, sensing,

separation, and other related ﬁelds.

eﬀorts have been devoted to the formation and development of

D nanoarchitectures. Among these various structures, organic

D nanomaterials constructed through noncovalent interac-

tions or supramolecular assembly were considered remarkable

because of their ﬂexible design routes, potential additional

functionality, and highly modular nature.

taking advantage of the host−guest interactions for the

formation of stable (pseudo)rotaxanes or poly(pseudo)-

rotaxanes, a series of 2D nanoarchitectures with a single-layer

arrangement and high structural regularity were constructed.

For instance, Li and Zhao reported remarkable results using

mild conditions for the construction of 2D supramolecular

pseudo)rotaxanes have been explored using diverse methods,

their potential as reactors has rarely been studied, despite the

fact that the 2D nanostructures have highly accessible active

sites, homogeneous features, and large surface areas, which can

provide a well-deﬁned platform for substance formation and

exchange.

−3

To date, numerous

On the other hand, integrating building blocks with special

features into 2D nanoarchitectures to realize the generation of

functional materials is becoming of widespread interest and

general concern. In artiﬁcial systems, supramolecular assembly

strategies using various noncovalent interactions provide the

possibility to achieve optimal performance from simple

elements under mild conditions, which is particularly

important for the discussion of structure−activity relation-

−7

Signiﬁcantly,

ships. Porphyrin and its derivatives with unique physical,

chemical, biological, and catalytic properties have attracted

growing interest in the ﬁelds of nanotechnology and materials

organic frameworks (SOFs) in solution. Moreover, Feng et al.

utilized interface synthesis to obtain the large-area free-

Received: August 22, 2020

Revised: September 21, 2020

Published: October 2, 2020

standing monolayers by host−guest enhanced donor−acceptor

interactions. In addition, our group designed photocontrolled

and interconvertible 2D/1D supramolecular nanostructures

with diﬀerent ﬂuorescence enhancement eﬀects for Nile Red.

Recently, Scherman and Barrio reported platelet-like aggre-

gates and 2D nanoﬁbers undergo a stepwise self-assembly

2020 American Chemical Society

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3−15

−2 22,23

science.

Because of signiﬁcant and excellent features, a

constants (K_a≥ 10 M );

the electron-rich unit

great deal of porphyrin-based nanostructures have been

presented in the past decade.

naphthalene (Np) in TPP-Np and electron-deﬁcient unit

methylviologen (MV) in DMV could be simultaneously

encapsulated in the cavity of CB[8] and provide the major

driving force for the formation of 2D polypseudorotaxanes.

Construction and Characterization of 2D Polypseu-

dorotaxanes. To ensure the formation of polypseudorotax-

anes, we ﬁrst explored the host−guest interactions between

16,17

In our previous work, we

constructed a series of porphyrin-based supramolecular

assemblies with well-deﬁned morphologies, taking advantage

of hydrophilic−hydrophobic as well as host−guest inter-

8−20

actions.

Considering the topological characteristics of 2D

nanomaterials, the design and synthesis of porphyrin-based 2D

supramolecular nanoarchitectures are in high demand.

DMV and CB[8] by H NMR experiments and UV/vis

Herein, we report a convenient, facile, and modular

approach for the construction of functional 2D polypseudor-

otaxanes in which a stable ternary complex could be formed on

the basis of cucurbit[8]uril via host-stabilized charge-transfer

spectroscopy. As shown in Figure S12b , the signals for the

protons of viologen units in DMV were signiﬁcantly shifted

upﬁeld by addition of 2 equiv of CB[8]. This phenomenon

provided evidence for the formation of a 2:1 inclusion complex

between CB[8] and DMV, in which the viologen units were

located in the cavity of the host and inﬂuenced by the shielding

eﬀect of CB[8]. The Job’s plot was employed to ﬁnd the

binding stoichiometry of DMV and CB[8]. The top point of

the curve appeared at the molar fraction X_g_u_e_s_t= 0.33,

indicating the 2:1 binding stoichiometry between CB[8] and

(

CT) interactions of naphthol-modiﬁed porphyrin and

viologen derivatives. Because of the steric eﬀect of macrocycles

of cucurbit[8]uril, the 2D polypseudorotaxanes exit as single-

layer ﬁlms with a lateral size, in which porphyrin units prevent

intermolecular aggregation. Interestingly, taking advantage of

the photocatalytic properties of porphyrin, the 2D supra-

molecular nanoarchitectures can be self-metallized with Pt

nanoparticles, enabling their further use as an excellent catalyst

for electrochemical nitrogen reduction reaction with superior

Faradaic eﬃciency and a satisfactory yield rate at −0.2 V vs

RHE under ambient conditions, exhibiting high catalyst

stability and excellent selectivity. The in situ Fourier transform

infrared spectroscopic data indicated that the supramolecular

system follows the associative mechanism during the nitrogen

reduction reaction process. This work not only deepens the

understanding of structure−activity relationships in 2D

supramolecular nanoarchitectures but also opens up oppor-

tunities for the development of supramolecular polypseudor-

otaxanes to realize eﬃciency in electrochemical nitrogen

reduction.

DMV (Figure S13 ), which corresponds to the result of H

NMR experiments. Furthermore, the association constant (K )

−1

of DMV and CB[8] was calculated as K = 2.59 × 10 M , K

−1

= 2.12 × 10

according to the result of UV/vis

spectroscopic titrations (Figure S14). H NMR experiments

were utilized to collect information about the polypseudor-

otaxanes. With the addition of TPP-Np to a solution of

CB[8]/DMV (Figure S12c ), the signals of viologen units of

DMV were further shifted upﬁeld and broadened, meaning

that a higher-level supramolecular assembly was formed.

However, when the same operation was handled in D O

solution of DMV alone, neither chemical shifts of the protons

on DMV nor TPP-Np displayed, indicating that there was no

obvious interaction between TPP-Np and DMV in the absence

of CB[8] (Figure S15). According to the above experiment

results, we believe that CB[8] can be employed as the module

of the “molecular handcuﬀ” to combine the two guest

molecules together to form network-like polypseudorotax-

RESULTS AND DISCUSSION

■

Design and Synthesis of Naphthol-modiﬁed Porphyr-

in (TPP-Np) and Viologen Derivatives (DMV). Synthesis of

the guest molecule TPP-Np was performed with commercially

available 5,10,15,20-tetra(4-pyridyl)porphyrin. Saliﬁcation was

conducted in dimethylformamide at 90 °C for 12 h in a

nitrogen environment with 2-(2-bromoethoxy)-naphthalene.

Subsequently, the counterions were exchanged with chloride

ions and ultimately give TPP-Np in yields of 60%. Another

guest molecule, DMV, was synthesized by the Zinke reaction,

starting with 1,5-naphthalenediamine, which was reacted with

anes.

More detailed information on the polypseudorotaxanes was

investigated by spectroscopy experiments. As shown in Figure

1, a remarkable intensity decrease was observed at peaks at 219

and 267 nm in the UV/vis spectra of CB[8]/DMV/TPP-Np

compared to that of the individual components, which are

attributed to the naphthalene units and viologen units,

respectively. Moreover, there was an increase in the range of

450−700 nm, exhibiting the formation of the proposed

polypseudorotaxanes via host-stabilized CT interactions.

Fluorescence spectra provided further insight into the

interactions between each part of the building blocks in the

supramolecular nanoarchitectures. The peaks centered at 680

and 720 nm displayed a slight decline with the formation of

polypseudorotaxanes, also suggesting cucurbit[8]uril is capable

of drawing close the two guest molecules so that photoinduced

electron transfer arises from the porphyrin moieties of TPP-Np

-(2,4-dinitrophenyl)-4,4′-bipyridinium in ethanol and re-

ﬂuxed for 72 h. After methylation with methyl iodide and

ion metathesis procedures, DMV was obtained as a yellowish

solid. The detailed synthesis route of TPP-Np and DMV is

shown in the Supporting Information . Characteristics for the

design of TPP-Np and DMV as building modules, respectively,

include the following: (i) Porphyrin as a functionalized group

in the center of TPP-Np facilitates polypseudorotaxanes with

photoactivity performance. (ii) The ﬂexible hydrocarbon

between the porphyrin core and naphthalene periphery in

TPP-Np enables 2D supramolecular nanoarchitectures with

pliable features. (iii) DMV as the linear connection element

with a rigid structure provides the basic prerequisite for the

formation of 2D supramolecular nanoarchitectures. Typically,

cucurbit[8]uril, possessing a large cavity, is capable of

simultaneously accommodating an electron-deﬁcient guest

and an electron-rich guest by host-stabilized CT interactions

to form a 1:1:1 ternary complex with high association

to the viologen units of DMV (Figure S16).

Dynamic light scattering (DLS), transmission electron

microscopy (TEM), and atomic force microscopy (AFM)

were subsequently utilized to identify the size and morphology

of the 2D polypseudorotaxanes. The data from the DLS

experiments indicate that the large size of CB[8]/DMV/TPP-

Np formed indeed via CB[8]-mediated host−guest complex-

ations, whereas there were no detectable signals in the solution

of TPP-Np or CB[8]/DMV (Figures S17 and S18). TEM is an

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height of the 2D supramolecular nanoarchitectures was

symmetrical and uniform. The height of CB[8]/DMV/TPP-

Np was measured to be ∼2.2 nm at diﬀerent positions,

approaching the outer diameter of CB[8], strongly suggesting

that the formation of single-layer 2D nanostructures resulted

from a cross-link by CB[8]. Meanwhile, the CB[8]

macrocycles eﬀectively prevent the interlayer packing, leading

to the 2D polypseudorotaxanes remaining as single-layer ﬁlms

with a lateral size. Moreover, for characterization of the ﬁne

microstructure arrangement inside the CB[8]/DMV/TPP-Np,

grazing incidence small-angle X-ray diﬀraction (GI-SAXS)

experiments were carried out. A sharp scattering peak

corresponding to a d spacing of 4.1 nm is observed in Figure

S19 , which is assigned to the spacing of the quadrangular pores

in the molecular model of the 2D polypseudorotaxanes, which

is consistent with the proposed supramolecular assembly mode

(

Scheme 1).

−

Figure 1. UV/vis spectra of TPP-Np ([TPP-Np] = 5 × 10 M, blank

Preparation and Characterization of PtNPs@(CB[8]/

line), DMV ([DMV] = 1 × 10⁻M, red line), CB[8]/DMV/TPP-Np

DMV/TPP-Np). Size-controlled and conﬁned growth of such

ultraﬁne metal nanoparticles have attracted extensive research

interests because of excellent properties for applications in

−5

(

CB[8]:DMV:TPP-Np = 2:1:0.5, [TPP-Np] = 5 × 10 M, blue line)

−5

and TPP-Np + DMV (TPP-Np:DMV = 0.5:1, [TPP-Np] = 5 × 10

M, purple line) in aqueous solution (light path = 1 mm).

27,28

heterogeneous catalysis and electrocatalysis.

The porphyr-

in-based and well-deﬁned 2D supramolecular nanoarchitec-

tures have potential in the preparation of ultraﬁne Pt

eﬃcacious method to intuitively characterize the morphology

of the supramolecular assembly. As expected, the TEM images

of CB[8]/DMV/TPP-Np indicate the formation of ﬁlmlike

structures with some wrinkles, on account of the ﬂexible

structure between porphyrin and naphthalene units in TPP-Np

nanoparticles through a succession of light harvesting and

29,30

photochemical cycles (Scheme S3 ).

First of all, platinum-

(

II) solution was prepared by dissolving K PtCl in ultrapure

2 4

water under ambient conditions and was aged at least 24 h in

the dark. The photoinduced reduction of platinum salts by

CB[8]/DMV/TPP-Np was accomplished in the case of

ascorbic acid as an electron donor under visible-light

irradiation. Scanning electron microscope (SEM) images and

TEM images show that the Pt nanoparticles were generated

with uniform size (approximately 3 nm) and ﬁne dispersion

(Figure 2 A). The thickness of the nanomaterial is a very

(

Figure 3 and Figure S20). The high-resolution TEM image

and selected area electron diﬀraction pattern exhibit the

polycrystalline nature and characteristic diﬀraction rings of Pt

nanoparticles. TEM-EDS also illustrated that Pt nanoparticles

were prepared successfully (Figure S21). In addition, the Pt

content in PtNPs@(CB[8]/DMV/TPP-Np) was estimated to

be about 4.17 wt % based on ICP-AES measurement, which is

consistent with the theoretical amount. As shown in FTIR

spectra (Figure S22), the absorption at 3368 cm⁻assigned to

the N−H stretching vibration in porphyrin units was obviously

−

attenuated; meanwhile, the peak at 1722 cm corresponding

to CO of cucurbit[8]uril stretching vibration shifted to 1715

−

cm , suggesting that the Pt nanoparticles were immobilized by

D polypseudorotaxanes. Furthermore, the Pt nanoparticles

were characterized via X-ray photoelectron spectroscopy

XPS). The spectrum in the Pt 4f region showed peaks

centered at 70.9 eV for Pt 4f₇ _/₂ and 74.2 eV for Pt 4f₅_/₂, which

(

could be assigned to Pt(0) (Figure S23 ). However, no Pt

nanoparticles were observed in the absence of the CB[8]/

DMV/TPP-Np or without light irradiation. On the another

hand, the obtained Pt nanoparticles were large in size and easy

to reunite in the case of TPP-Np operating alone. The above

control experiments demonstrated that the photoactivity of the

porphyrin and the two-dimensional conﬁguration of the

CB[8]/DMV/TPP-Np are both critical for the development

of functional supramolecular nanostructures.

Figure 2. Characterization of 2D polypseudorotaxanes. (A) TEM

image of CB[8]/DMV/TPP-Np. (B) Molecular size of CB[8]. (C)

Tapping-mode AFM image of CB[8]/DMV/TPP-Np.

important parameter to evaluate the 2D nanostructures.

Therefore, the as-prepared solution of CB[8]/DMV/TPP-Np

was dropped onto a mica wafer and then investigated by

tapping-mode AFM. As can be seen in Figure 2C, the

graphene-like nanostructure of CB[8]/DMV/TPP-Np was

observed by AFM. Cross-section analysis indicated that the

Electrochemical Nitrogen Reduction Performance of

PtNPs@(CB[8]/DMV/TPP-Np). Recently, the electrochemical

nitrogen reduction reaction (NRR) to produce ammonia

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Scheme 1. Schematic Illustration of the Construction of the 2D Polypseudorotaxanes Based on CB[8] via Host-Stabilized

Charge-Transfer (CT)

Figure 3. Characterization of 2D PtNPs@(CB[8]/DMV/TPP-Np). (a) Low-magniﬁcation TEM image of PtNPs@(CB[8]/DMV/TPP-Np). (b)

Size distribution proﬁle of Pt nanoparticles. (c) High-resolution TEM image of PtNPs@(CB[8]/DMV/TPP-Np). (d) TEM-EDS mapping of

composition elements C, N, Pt, and O.

32,33

(

NH ) at atmospheric pressure and moderate temperature

of intense research.

Haber-Bosch process, which produces NH from N and H

Diﬀerent from the current industrial

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Figure 4. Electrocatalytic NRR activity of PtNPs@(CB[8]/DMV/TPP-Np). (a) Chronoamperometric curves at various potentials in N -saturated

.1 M K SO . (b) NH yields (left axis) and Faradaic eﬃciencies (FEs) (right axis) of PtNPs@(CB[8]/DMV/TPP-Np) at each given potential in

2 4 3

.1 M K SO . The error bars correspond to the standard deviations of measurements over three separately prepared samples under the same

conditions. (c) Corresponding NH yield rates and Faradaic eﬃciencies in the three electrolytes at −0.2 V. (d) N isotope labeling experiment. H

NMR spectra for the postelectrolysis 0.1 M K SO electrolytes with N and N as the feeding gases. Also shown are the spectra for NH and

NH standard samples.

catalyzed by iron-based catalysts under high temperature

350−550 °C) and high pressure (150−350 atm), the

electrochemical NRR is considered a sustainable and

The NH yield rate was normalized on the basis of the

(

weight of Pt loading. The produced NH and N H were

measured according to the reported indophenol blue method

and Watt and Chrisp method, respectively (the corresponding

34,35

promising approach for NH production.

Recent reports

39,40

illustrated that various materials including noble metals

calibration curves are shown in Figures S25 and S26 ).

The

36,37

production of hydrazine was below the detection limit,

showed favorable activity for NH production.

Signiﬁ-

indicating high selectivity for NH over PtNPs@(CB[8]/

cantly, we found that the Pt-based catalysts, which were

DMV/TPP-Np) (Figure S27 ). Time-dependent current

thought to not be appropriate for NRR, could be eﬃcient NRR

density curves for 2.5 h from 0 to −0.4 V are presented in

catalysts. Inspired by previous results and PtNPs@(CB[8]/

DMV/TPP-Np) with high-density and ﬁnely dispersed Pt

nanoparticles, extensive explorations and investigations toward

the catalytic reactivity of hybrid materials for electrochemical

NRR were conducted. To the best of our knowledge, there was

no prior work based on supramolecular nanoarchitectures for

nitrogen ﬁxation, which is challenging but potentially valuable

research.

Figure 4a. The NH yield and corresponding Faradaic

eﬃciency revealed that the PtNPs@(CB[8]/DMV/TPP-Np)

was an eﬃcient catalyst for the ﬁxation of inert N molecules

into valuable NH . As shown in Figure 4b, the NH yield

increased with more negative potential until −0.2 V, where the

−

maximum value of the NH yield was calculated as 23.2 μg h

−

mg_P_t, and the Faradaic eﬃciency displayed the highest value

of 10.39% at −0.1 V and then decreased gradually, which was

ascribed to the competitive selectivity toward the hydrogen

The N reduction activity of PtNPs@(CB[8]/DMV/TPP-

Np) was carried out at room temperature and atmospheric

pressure in an H-shaped electrochemical cell, which was

separated by a Naﬁon membrane at room temperature and

atmospheric pressure. The NRR activity of the electrode was

evaluated using controlled potential electrolysis with N₂-

saturated electrolytes. All potentials mentioned in this work

were converted as the value versus RHE unless speciﬁed

otherwise. The linear sweep voltammetry (LSV) curves of

PtNPs@(CB[8]/DMV/TPP-Np) exhibited diﬀerent current

evolution reaction (HER). For determination of the active

sites of PtNPs@(CB[8]/DMV/TPP-Np) for electrocatalytic

NRR, bare Pt nanoparticles and 2D polypseudorotaxanes as

the controlled samples were also studied at the same catalytic

potential (Figure S28). The results conﬁrmed that the Pt

nanoparticles with high density and ﬁne dispersion in

PtNPs@(CB[8]/DMV/TPP-Np) was the actual active sites

for electroreduction of N₂under ambient conditions.

According to a previous report, Pt nanoparticles were stabilized

by oxygen atoms at the edge of macrocycle structures of CB[8]

and displayed positive charge, which can favorably chemisorb

densities in the range from 0 to −0.70 V in 0.1 M K SO

solution with Ar-saturated and N -saturated electrolytes,

respectively (Figure S24 ). The current density in N -saturated

and activate the N₂. In addition, macrocycle structures of

electrolyte is higher than that in Ar-saturated electrolyte,

indicating that PtNPs@(CB[8]/DMV/TPP-Np) possesses

CB[8] provided locally hydrophobic sites,

which is

unfavorable to the HER process. As shown in Figure 4c, the

catalytic activity toward N electrochemical reduction.

ﬁndings of the electrocatalytic behaviors of PtNPs@(CB[8]/

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Figure 5. Stability tests. (a) Time-dependent current density curve. (b) Recycling test of PtNPs@(CB[8]/DMV/TPP-Np) at −0.2 V under

ambient conditions.

DMV/TPP-Np) in diﬀerent N -saturated electrolytes reveal

For investigation of the activation of N on the surface of

that, compared with that in neutral electrolyte (0.1 M K SO ,

PtNPs@(CB[8]/DMV/TPP-Np) in our work, in situ time-

resolved Fourier transform infrared (FTIR) spectroscopy was

employed to demonstrate time-dependent changes of the N

pH 7) at −0.2 V, the NH yield and the Faradaic eﬃciency of

PtNPs@(CB[8]/DMV/TPP-Np) were much lower than those

in acid (0.05 M H SO , pH 1) and basic electrolytes (0.1 M

contained after injection of N . As shown in Figure 6a, the

KOH, pH 14), indicating that K SO should be a promising

absorption band that can be assigned to −N H intermediates

electrolyte for electrochemical NRR because of its eﬀective

suppression of HER (Figure S29). Therefore, all of the

gradually enhanced, exhibiting that N can be eﬃciently

adsorbed and activated on the surface of the PtNPs@(CB[8]/

DMV/TPP-Np). More concretely, absorption bands appearing

following NRR experiments were conducted in 0.1 M K SO .

To conﬁrm the origin of the produced NH , we carefully

−1

in the range from 1050 to 1500 cm can be assigned to H−

examined the nitrogen source through control experiments.

First, the PtNPs@(CB[8]/DMV/TPP-Np) was immersed in a

N -saturated electrolyte without applied potentials, and no

NH₃was detected using the indophenol blue method.

Furthermore, the control experiments were carried out in Ar-

saturated electrolyte at −0.2 V in 0.1 M K SO solution, and

only negligible NH could be detected (Figure S30). And the

pristine carbon paper was used as a working electrode in N₂-

saturated electrolyte at −0.2 V in 0.1 M K SO solution; no

NH was detected. The above experiment results suggest that

the NH₃was produced from electroreduction of N by

PtNPs@(CB[8]/DMV/TPP-Np). In addition, N (99 atom

N) isotopic labeling experiment was executed as an

alternative method to verify the nitrogen source of the

produced NH generated in our experimental conditions.

As shown in Figure 4d, the standard samples illustrated a

triplet coupling for NH₄and a doublet coupling for NH₄⁺

in H NMR spectra. When N or N was supplied as the

feeding gas, NH or NH₄could be detected, respectively,

suggesting that the produced NH₃originated from the

electroreduction of N instead of decomposition of catalysts

or other contaminations.

The stability of the PtNPs@(CB[8]/DMV/TPP-Np) for

electrochemical NRR was assessed by consecutive recycling

electrolysis at −0.2 V. Under sustained N gas ﬂow, no obvious

change in current density could be observed during electrolysis

within 8 h. Furthermore, the catalytic activity displayed only a

slight decrease in NH yields and Faradaic eﬃciencies during

eight consecutive cycles (Figure 5), suggesting that

PtNPs@(CB[8]/DMV/TPP-Np) maintained satisfactory elec-

trocatalytic activity. According to the high-resolution Pt XPS

spectrum, the Pt chemical state of PtNPs@(CB[8]/DMV/

TPP-Np) almost remained unchanged after electrocatalysis for

NRR (Figure S31). The good stability of PtNPs@(CB[8]/

DMV/TPP-Np) may be attributed to the strong host−guest

interactions.

Figure 6. (a) Electrochemical in situ time-resolved Fourier transform

infrared (FTIR) spectra for nitrogen reduction reaction (NRR) on the

PtNPs@(CB[8]/DMV/TPP-Np) electrode. (b) Schematic illustra-

tion of the possible NRR process on the PtNPs@(CB[8]/DMV/

TPP-Np) electrode.

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The Supporting Information is available free of charge at

Article

−

−1

N−H bending (1450 cm ), −NH wagging (1301 cm ),

conducted at −0.2 V vs RHE for 6 h in 10 mL of 0.1 M K SO

2 4

−

−1

aqueous solution in a circulation setup. Then 0.5 mL of the above

adsorbed NH (1200 cm ), and N−N stretching (1070 cm )

solution mixed with 0.05 mL of d -DMSO was used for H nuclear

models of −N H intermediates, respectively. The in situ time-

magnetic resonance (NMR) spectroscopy measurement (Bruker

AVANCE III 600 MHz).

resolved FTIR data provide strong evidence that the nitrogen

reduction reaction on PtNPs@(CB[8]/DMV/TPP-Np) surfa-

ces follows an associative mechanism. Speciﬁcally, the

possible NRR process over PtNPs@(CB[8]/DMV/TPP-Np)

cathode can be reasonably deduced, as shown in Figure 6b. N₂

molecules were ﬁrst adsorbed on the surface, and then the

adsorbed N reacted with the dissociated proton (H ) from the

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.chemmater.0c03425.

■

electrolyte to form intermediates, resulting in the facilitation of

dissociation of N≡N triple bonds.

Characterizations of 2D polypseudorotaxanes and NRR

catalytic performance of PtNPs@(CB[8]/DMV/TPP-

Np), including NMR spectra, MS spectra, UV/vis

spectra, and ﬂuorescence spectra, SEM and AFM

images, FT-IR and GI-SAXS patterns, LSV curves, and

XPS spectra (PDF )

CONCLUSIONS

■

In conclusion, novel 2D polypseudorotaxanes have been

constructed by host-stabilized charge-transfer interactions by

CB[8]. The functional and structural features allow the 2D

supramolecular nanomaterial to be used as a platform for

preparation of ultraﬁne Pt nanoparticles, which has been

proven to pave a new way for electrochemical ammonia

synthesis from nitrogen under ambient conditions with high

yield and favorable stability. To the best of our knowledge, this

is the ﬁrst construction to realize artiﬁcial nitrogen ﬁxation

through supramolecular assembly strategy. As it is convenient,

facile, and eﬀective, the modular supramolecular assembly

approach exhibits great potential for designing and construct-

ing various functional 2D nanomaterials, which could give rise

to new opportunities for electrochemical nitrogen reduction

reaction and lead to various other applications.

AUTHOR INFORMATION

Corresponding Authors

■

Yu Liu − College of Chemistry, State Key Laboratory of

Elemento-Organic Chemistry, Nankai University, Tianjin

00071, P.R. China; orcid.org/0000-0001-8723-1896;

Email: yuliu@nankai.edu.cn

Yu-Ping Liu − College of Chemistry, Research Center for

Analytical Sciences, Key Laboratory of Advanced Energy

Materials Chemistry (Ministry of Education), Nankai

University, Tianjin 300071, P.R. China; Email: liuypnk@

nankai.edu.cn

Authors

EXPERIMENTAL SECTION

Cai-Cai Zhang − College of Chemistry, State Key Laboratory of

Elemento-Organic Chemistry, Nankai University, Tianjin

300071, P.R. China; College of Chemistry and Materials

Science, Hebei Normal University, Shijiazhuang 050024, P.R.

China.

Xiaolu Liu − College of Chemistry, Research Center for

Analytical Sciences, Key Laboratory of Advanced Energy

Materials Chemistry (Ministry of Education), Nankai

University, Tianjin 300071, P.R. China

■

Preparation of PtNPs@CB[8]/DMV/TPP-Np. Platinum(II)

solution was prepared by dissolving K PtCl in water under ambient

conditions and was aged 24 h before use; 125 μL of aqueous K PtCl

(

20 mM), 10 mL solution of CB[8]/DMV/TPP-Np (0.05 mM of

TPP-Np), and 125 μL of aqueous ascorbic acid (200 mM) were

added into a glass vial together. The samples were irradiated by visible

light through a 400 nm cutoﬀ ﬁlter from a 300 W Xe lamp for 10 min.

Calculation of NH Faradaic Eﬃciency (FE), NH Yield Rate.

The ammonia synthesis rate was calculated using the following

equation:

Complete contact information is available at:

VC_N_H₃

https://pubs.acs.org/10.1021/acs.chemmater.0c03425

ν_N_H₃

m t

Author Contributions

FE was calculated according to the following equation:

C.-C.Z. and X.L. contributed equally to this work.

Fn_N_H₃

Notes

FE =

The authors declare no competing ﬁnancial interest.

where V is the volume of the electrolyte, C_N_H₃is the NH₃

ACKNOWLEDGMENTS

We thank the National Natural Science Foundation of China

Nos. 21432004, 21772099, and 21861132001) for ﬁnancial

support.

■

concentration, t is the reduction reaction time, A is the geometric

area of the cathode, F is the Faraday constant, m_c_a_t_.is the loading mass

of the catalyst, and Q is the quantity of applied electricity.

(

Isotope-Labeled Experiments. The N and N isotopic

labeling experiments were conducted using N and N as the

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