Composition-controlled ultrathin holey TiO1−xNx nanosheets as powerful hybridization matrices for highly mass-efficient electrocatalysts

https://doi.org/10.1016/j.cej.2022.135415Get rights and content

Highlights

  • A hybridization strategy is developed to optimize mass activity of metal nanocluster.

  • Holey 2D TiO1−xNx nanosheet shows superior efficiency as a hybridization matrix.

  • Defects and interfacial anion linkers are crucial in maximizing hybridization effect.

  • 2D Pt−holey-TiO1−xNx nanohybrid shows an unusually high gravimetric HER efficiency.

Abstract

Highly porous holey inorganic nanosheets have received growing attention as emerging 2D nanostructures because of their excellent functionalities as active materials and hybridization matrices. Here we report the composition-controlled synthesis of ultrathin holey metal oxynitride nanosheets with subnanometer-level thickness of ∼0.8 nm and tunable defect/surface structures. The application of the obtained holey TiO1−xNx nanosheets as immobilization substrates allowed to maximize mass electrocatalytic activity of hybridized Pt nanoclusters via enhanced interfacial interaction at defective sites. The strong electronic coupling between positively-charged Pt nanoclusters and holey TiO1−xNx nanosheets with interfacial oxygen linkers enabled to achieve a superior electrocatalytic performance for hydrogen evolution reaction with an unusually high gravimetric efficiency (20.8 AmgPt−1), i.e., one of the most efficient values for Pt nanostructures. Density functional theory calculations and in-situ Raman analysis emphasize the significant contributions of interfacial oxygen linker in holey TiO1−xNx substrates and positive charge of Pt nanocluster to optimizing the electrocatalytic activity. The present study underscores that employing composition-controlled holey TiO1−xNx nanosheets as immobilization substrates provides a novel efficient methodology to explore high-performance electrocatalysts via the improvement of charge/mass transport and electrocatalytic kinetics, and the optimization of d-band center upon hybridization.

Introduction

Highly anisotropic 2D nanosheets (NSs) of layered inorganic solids have attracted special attention because of their unique physicochemical properties and valuable functionalities [1], [2], [3]. Many redoxable inorganic NSs boast promising functionalities as electrodes, electrocatalysts, and photocatalysts, which play crucial roles in many emerging energy technologies like fuel cells, metal−O2 batteries, and electrolyzers [4], [5], [6]. In addition to the role of active material, the conductive inorganic NSs can function as hybridization matrices for improving the catalyst performance of anchored nanospecies [7]. In one instance, the immobilization of metal nanoclusters on conductive NS offers effective way to improve their gravimetric electrocatalytic activities for the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) [8], [9]. To further optimize the beneficial effect of hybridization with conductive substrate, it is necessary to enhance an interfacial electronic coupling with hybridized species via the fine-control of the surface bonding character of inorganic NS. Considering the fact that surface defects with coordinatively-unsaturated geometry can act as anchoring sites for immobilizing inorganic species, subnanometer-thick holey 2D inorganic NS with plenty of surface vacancies and unusually high surface-to-volume ratio can be used as efficient hybridization matrix for optimizing an interfacial chemical interaction in the resulting nanohybrids [10], [11]. The fine-tuning of the surface anion structure of ultrathin holey NS substrate could be useful in terms of reinforcing the interfacial chemical interactions with the immobilized metal nanoclusters via strong interfacial coordinative bonding assisted by intervened linker anion species [12], [13]. Also, the partial introduction of interfacial cationic charge into metal nanoclusters is supposed to enhance the interfacial interaction with inorganic substrates [14]. In addition, the holey 2D NS offers additional advantage of effective mass diffusion pathways for the reactants and products of electrocatalysis. This synergistic combination of interfacial charge and anion structure control in holey NS substrate is expected to provide a valuable opportunity to maximize the electronic coupling with metal catalysts, and to ultimately establish design principles and synthetic methods for high-mass-activity electrocatalysts.

In contrast to conventional synthetic routes to holey 2D NSs such as plasma etching, electron irradiation, thermal reduction, and sacrificial template method [15], [16], [17], [18], the aliovalent substitution of divalent O2− ion with trivalent N3− ion allows not only to create surface holes and anion vacancies in 2D metal oxide NSs but also to enhance its electrical conductivity via the narrowing of bandgap energy [19]. The accompanying increase of surface polarity and surface electron density upon partial nitrogen substitution is supposed to be beneficial in improving the electrocatalyst functionality of immobilized metal nanoclusters due to the enhanced adsorption of reactants [20], [21]. Such a controllability of the surface chemical structure of the oxynitride NS substrate can offer an important opportunity to study the profound function of interfacial charge and anion linkers in optimizing the electrocatalyst functionality and the interfacial electronic coupling of immobilized metal nanoclusters [22], [23]. It is also informative to systematically investigate the evolution of the chemical bonding nature of substrate NS during the electrocatalytic process using in-situ spectroscopic analysis for attaining valuable insights for the design and synthesis of high-efficiency metal-based electrocatalysts. Despite intense research efforts devoted to 2D nanostructured materials, at the time of this submission, we are unaware of any other study on the composition-controlled synthesis of ultrathin holey metal oxynitride NSs and their application as hybridization matrices for exploring novel mass-efficient electrocatalysts.

In the present study, a novel synthetic method to subnanometer-thick holey 2D metal oxynitride NSs with tunable chemical compositions was developed by finely-controlled ammonolysis for exfoliated titanium oxide NSs with high morphological stability. The obtained ultrathin holey titanium oxynitride NSs were employed as immobilization substrates for Pt nanoclusters to verify their usefulness in optimizing the electrocatalyst functionality of anchored metal nanoclusters. Density functional theory (DFT) calculations were also carried out for the Pt−holey TiO1−xNx nanohybrids to probe the profound influences of the interfacial oxygen linkers and crystal defects of TiO1−xNx NS on the electronic structure and electrocatalytic activity of the Pt nanoclusters. Finally, the operation mechanism responsible for the excellent HER electrocatalyst performances of the Pt−holey TiO1−xNx nanohybrids was systematically investigated using a series of in-situ surface enhanced Raman scattering and DFT analyses to elucidate dominant factors for improving the electrocatalytic performance of the metal catalyst.

Section snippets

Synthesis

The precursor material of exfoliated titanate NS was prepared in the form of aqueous colloidal suspension by the reaction between protonated derivative of lepidocrocite-type Cs0.67Ti1.830.67O2 and tetrabutylammonium hydroxide solution for 10 days, as reported previously [24]. The obtained colloidal suspension of exfoliated layered titanate NS was restacked by adding 1 M HCl solution and then the obtained precipitates were washed with distilled water and freeze-dried. The restacked titanate NS

Composition-controlled synthesis of subnanometer-thick holey TiO1−xNx NSs

The holey TiO1−xNx NSs were prepared by heat treatment of the precursor titanate (TiO2) NS at 700, 800, and 900 °C under a flow of NH3 (the obtained materials were denoted as TNS700, TNS800, and TNS900, respectively). The precursor titanate NSs restored by proton restacking showed typical Bragg reflections of lepidocrocite-type layered titanate (Fig. S1). As shown in Fig. 1a, all the TNS NSs commonly exhibited intense Bragg reflections of TiN phase, clearly demonstrating the phase transition

Conclusions

In this work, a novel and powerful synthetic route to composition-tailored holey 2D metal oxynitride NSs with subnanometer-level thickness of ∼0.8 nm was developed by finely-controlled ammonolysis for exfoliated metal oxide NSs. The partial nitridation of exfoliated TiO2 NSs provided a useful method not only to induce a phase transition to the cubic TiO1−xNx structure but also to form subnanometer-thick holey 2D NS morphology with introduction of interfacial oxygen linkers and anion vacancies.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF‐2020R1A2C3008671, No. NRF‐2017R1A5A1015365). This work was also supported by National R&D Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (No. 2021M3H4A1A03049662). T. Lee and A. Soon gratefully acknowledge support from the Basic Science Research Program (2020R1F1A1063070) through the NRF. Computational resources

References (58)

  • N. Kim et al.

    Lattice Engineering to Simultaneously Control the Defect/Stacking Structures of Layered Double Hydroxide Nanosheets to Optimize Their Energy Functionalities

    ACS Nano

    (2021)
  • X. Liu et al.

    Plasma-Induced Defect Engineering and Cation Refilling of NiMoO4 Parallel Arrays for Overall Water Splitting

    ACS Appl. Mater. Interfaces

    (2021)
  • G. Huang et al.

    Defect Engineering of Cobalt-Based Materials for Electrocatalytic Water Splitting

    ACS Sustainable Chem. Eng.

    (2018)
  • J. Pan et al.

    Vanadium Oxynitrides as Stable Catalysts for Electrochemical Reduction of Nitrogen to Ammonia: the Role of Oxygen

    J. Mater. Chem. A

    (2020)
  • K.L. Zhou et al.

    Platinum Single-Atom Catalyst Coupled with Transition Metal/Metal Oxide Heterostructure for Accelerating Alkaline Hydrogen Evolution Reaction

    Nat. Commun.

    (2021)
  • Q. Yao et al.

    Molecular Reactivity of Thiolate-Protected Noble Metal Nanoclusters: Synthesis, Self-Assembly, and Applications

    Chem. Sci.

    (2021)
  • J. Bai et al.

    Graphene Nanomesh

    Nat. Nanotech.

    (2010)
  • C. Nottbohm et al.

    Holey Nanosheets by Patterning with UV/Ozone

    Phys. Chem. Chem. Phys.

    (2010)
  • L. Peng et al.

    Holey Two-Dimensional Transition Metal Oxide Nanosheets for Efficient Energy Storage

    Nat. Commun.

    (2017)
  • Y.-Q. Cao et al.

    TiOxNy Modified TiO2 Powders Prepared by Plasma Enhanced Atomic Layer Deposition for Highly Visible Light Photocatalysis

    Sci. Rep.

    (2018)
  • M. Bele et al.

    Increasing the Oxygen-Evolution Reaction Performance of Nanotubular Titanium Oxynitride-Supported Ir Nanoparticles by a Strong Metal-Support Interaction

    ACS Catal.

    (2020)
  • Y. Wu et al.

    3D Ordered Macroporous Copper Nitride-Titanium Oxynitride as Highly Efficient Electrocatalysts for Universal-pH Hydrogen Evolution Reaction

    J. Mater. Chem. A

    (2021)
  • M.E. Kreider et al.

    Nitride or Oxynitride? Elucidating the Composition−Activity Relationships in Molybdenum Nitride Electrocatalysts for the Oxygen Reduction Reaction

    Chem. Mater.

    (2020)
  • T. Sasaki et al.

    Osmotic Swelling to Exfoliation. Exceptionally High Degrees of Hydration of a Layered Titanate

    J. Am. Chem. Soc.

    (1998)
  • G. Kresse et al.

    Ab Initio Molecular Dynamics for Liquid Metals

    Phys. Rev. B

    (1993)
  • G. Kresse et al.

    Efficient Iterative Schemes for Ab Initio Total-Energy Calculations using a Plane-Wave Basis Set

    J. Phys. Rev. B

    (1996)
  • G. Kresse et al.

    From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method

    Phys. Rev. B

    (1999)
  • J.P. Perdew et al.

    Generalized Gradient Approximation Made Simple

    Phys. Rev. Lett.

    (1997)
  • A. Tkatchenko et al.

    Accurate Molecular Van der Waals Interactions from Ground-State Electron Density and Free-Atom Reference Data

    Phys. Rev. Lett.

    (2009)
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    1

    These authors contributed equally to this work.

    2

    Present address: Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA

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