Parsing the basic principles to build efficient heterostructures toward electrocatalysis

Abstract

Electrocatalysis can facilitate the reaction kinetics in many electrochemical energy conversion devices. Heterostructural catalysts are important candidates due to their sufficient tunability of components and electronic states. So far however, no basic principles have been provided to guide the rational selection of components and catalytic mechanisms of such high-efficiency heterostructures. This article is aimed at parsing the inherent principles of universal component selection strategies from the perspective of compatibility in electron structures or physical properties. The catalytic mechanism of these as-obtained heterostructural catalysts is also systematically analyzed, and the foreseeable research on the structure–activity relationship of heterostructural catalysts is then discussed. This would enable researchers in this field to conduct related works in a guiding way.

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Electrocatalysis refers to a class of chemical reactions where the modifier on the electrode surface, namely, an electrocatalyst, can significantly affect the electron–proton (or hydroxyl) coupled transfer reaction, while the modifier itself does not undergo any changes.¹ The purpose of studying electrocatalysis is to find an activation path with a low energy barrier, so that the desired electrode reaction can occur at a high current density near the equilibrium potential as much as possible.² The core of electrocatalysis is the design of catalysts, which are expected to possess low reaction energy barriers. In terms of the multifarious electrocatalytic reactions (e.g., hydrogen oxidation reaction, HOR; hydrogen evolution reaction, HER; oxygen reduction reaction, ORR; oxygen evolution, OER; nitrogen reduction reaction, NRR; and carbon dioxide reduction reaction, CO₂RR) in different reaction media (acidic, neutral, and alkaline electrolytes), the applicative electrocatalysts and their reaction pathway must be effectively analyzed.

With the development of customized catalysts, many catalyst families, such as noble metals, alloys, semiconducting metal oxides, metal complexes, and carbon nanomaterials, are constantly emerging.³ Among them, heterostructures composed of inorganic components show excellent application prospects in various electrocatalytic reactions.⁴ The “heterostructure” is originally from the concept of “heterojunction” in semiconductor physics. Both have the same structural characteristics, in which the chemical compositions vary with locations near the interfaces. The difference is that the components of heterojunctions are always semiconductors, while the components of heterostructures can be metals, semiconductors and even insulators.⁵ In terms of component properties, the “heterostructure” is a relatively broader concept. Heterostructural catalysts generally refer to the mixed-phase nanocatalysts obtained by forming well-defined interfaces between two kinds of components with different chemical compositions.⁶ Normally, their intrinsic catalytic activity can be significantly enhanced owing to the modulated electronic structure around the newly bonded heterointerfaces. Differently, the mixed-phase materials, formed by mechanical mixing of different components, could only achieve the macroscopic homodisperse nature. The immanent microstructures are still mutually isolated with little interaction between different components. Thus, their improvement of intrinsic catalytic activity is limited. According to the different stacking directions, the heterostructures can be roughly divided into two types: vertical and lateral ones (Fig. 1). Vertical heterostructures signify the multilayer structures formed by the perpendicular stacking of two material phases, while the lateral ones indicate the single-layer structures formed by the chemical bonding of two material phases at similar horizontal height.⁷ For the vertical heterostructures, supported, coated, and core–shell structures are all suitable microstructures. Thus, some reasonable construction strategies, such as stepwise assembly and layer-by-layer CVD deposition, are feasible. For lateral heterostructures, localized transformation and marginal epitaxial growth of layered two-dimensional materials are more appropriate strategies. After obtaining the heterostructure, it is necessary to determine its stacking direction by combining the construction strategy and structural information (especially electron microscopy results), to match appropriate models in DFT simulations. In theoretical simulation, the adsorption of vertical ones always occurs on the surface atoms of the top layers, while the interfacial atoms in lateral ones can be extra exposed as the possible adsorption sites. The specific effect of each structure should be discussed prudently. The discrimination criterion should be the specific structural configuration, and the exposed adsorption surface should be directly accessible to the electrolyte. In addition, according to the crystal structure and composition of components, the heterostructures can be classified into (1) metal–metal type, (2) metal–compound type, (3) compound–compound type, and (4) nonmetal–metal/compound type. The inner components can be either crystalline or amorphous. Owing to the tunability of each component in the heterogeneous catalysts, there will be a large number of combinations available for researchers. With the “heterostructure” and “electrocatalyst” as the search terms, thousands of research papers can be retrieved. Quite a lot of research studies have been conducted on material compositing and concomitant heterointerfaces between metallic simple substances, compounds, complexes, and carbon materials.⁸ Such heterostructural catalysts have become the research hotspot in electrocatalysis. However, no basic principle could be offered to guide the rational component selection and interface bonding of those heterostructures with high electrocatalytic activity. Research studies in this field are more like trials and errors or exhaustive methods. Besides, the inherent structural merits and catalytic mechanisms are still vague and controversial. Therefore, the academic community must have an in-depth understanding of the fundamental principles of heterostructural catalysts, so as to make related basic research studies and engineering development more directional.

Fig. 1 Schematic of (a) vertical and (b) lateral heterostructures.

As for the construction of heterostructural catalysts, the most important consideration should be the selection of separate components. Due to the characteristics of electrocatalytic reactions, involving the multi-electron–proton (hydroxyl) transfer process and the adsorption/desorption behavior of key reaction intermediates, the heterostructure must be customized from a certain theoretical perspective.

➢ From the perspective of band structures: According to the band gap between the valence band maximum and conduction band minimum, nanomaterials can be divided into conductors (∼0 eV), insulators (>4.5 eV) and intervening semiconductors (normally 0.2–3.5 eV). Among them, the insulators cannot be utilized in electrocatalytic processes owing to their restricted electron transmission over materials surfaces with extremely high electrical resistances. Importantly, the most representative one is the conductor–semiconductor heterostructure (Fig. 2a). The distinctive interfaces between these two components would result in a Mott–Schottky effect, thereby inducing interfacial electron redistribution and accelerating the electron transfer rate.^9,10 Note that the conductors do not merely denote the elementary metals, because many metallic compounds also display metallic properties with no band gap. In future research studies, the band properties of each component should be clearly identified by theoretical simulations of the band gap or spectroscopy techniques such as ultraviolet photo-electron spectroscopy (UPS). The conventional heterostructures composed of two semiconductors can be distinguished into three types based on the locations of valence bands and conduction bands, which have the straddling gap (type-I), staggered gap (type-II) and broken gap (type-III), respectively (Fig. 2b–d).¹¹ In the field of photocatalysis, a general consensus is that only staggered gap can achieve effective interfacial transfer and spatial separation of electrons and holes under photoexcited conditions.¹² But for electrocatalysis, there is still no related study dedicated to the in-depth connection between the band structures of components and the electrocatalytic activity of integrated heterostructures. However, for the bimetallic ones, alloys can always be obtained. Even for the separated conducting components, the zero band gaps urge us to explore new perspectives.

Fig. 2 (a) Conductor–semiconductor-type heterostructure; semiconductor–semiconductor-type heterostructures with (b) straddling gap-type I, (c) staggered gap-type II, and (d) broken gap-type III.

➢ From the perspective of work function: The physical meaning of work function (WF) is the minimum energy required for an electron at the Fermi level to escape to the vacuum.¹³ The metals with smaller WF can endow more electrons with the ability to flee. In short, it can simply be understood as the ability of an object to possess or capture electrons. When two components with different WFs (unequal Fermi levels) combine together to form heterointerfaces, the carrier concentration and potential would be rearranged (Fig. 3). Generally, the electron transfer occurs from one component with a lower WF (higher Fermi level) to the other component with a higher WF (lower Fermi level), until the Fermi level is the same at all points in these two components.¹⁴ A concomitant contact potential difference compensates for the initial Fermi level difference. For example, according to our investigation, an in-built electric field can be formed in Os-OsSe₂ heterostructures in terms of the WF difference between Os and OsSe₂, leading to an interfacial redistributed electron structure and a resultant balanced adsorption behavior toward hydrogen intermediates.¹⁵ Importantly, the WF of materials is mainly related to the location of the Fermi level, rather than the valence-band top and conduction-band bottom; thus, the WF strategy applies to almost all kinds of heterostructures. The identification of the WF for all materials should be based on theoretical calculations, as well as experimental UPS and/or thermionic emission methods (J = ATe).

Fig. 3 Electron transfer trend derived from the work function difference.

➢ From the perspective of adsorption ability: Adsorption/desorption behaviors are the most important features of electrocatalysis, concerning the reactant supply, active site utilization, and continuous catalysis. The adsorption behavior on different crystal faces is also different; therefore, the crystal plane consideration is very significant. The thermodynamically stable crystal faces are usually widespread in crystalline materials, accounting for the vast majority of exposed surfaces. A crystal face with lower surface energy normally possesses a closely packed surface atomic configuration and a slower normal growth rate; thus it could be better retained in the final crystal.¹⁶ All these stable crystal faces have deterministic adsorptive strengths toward reactants, products and key intermediates in electrocatalysis. According to the Sabatier principle, moderate adsorption strength is beneficial for the adsorption/desorption behaviors of intermediates and stepwise progress of electrocatalytic reactions.¹⁷ However, most of the single-phase materials are usually limited by their too strong/weak adsorption capacity, while the construction of a heterostructure is an effective method to adjust the adsorption strength. Predictably, it is feasible to couple a strongly adsorptive component with another weakly adsorptive component. According to our research, the strongly adsorptive Ru is combined with the weakly adsorptive RuS₂ component to obtain the expected Ru/RuS₂ heterostructure, which displays more moderate hydrogen adsorption free energy and excellent intrinsic HER activity.¹⁸ Owing to the interfacial electronic modulation, the d-band center of potential active atoms can be adjusted, leading to strengthened/weakened adsorption behavior over interfacial active sites and resultant better actual catalytic performance.¹⁹

There are still many indistinct principles remaining to be investigated, which can guide the accurate component selection. These principles can be from the perspective of theoretical electronic structures and/or macroscopic physical properties for both sides. Plenty of future studies should concentrate on this research direction.

After obtaining the desired heterostructural catalysts, it is convenient to explore their electrocatalytic activity, reaction kinetics, and stability performance, which can be compared with those of the two single-phase components to determine the electrocatalytic advantages of heterostructural catalysts. The exploration of their catalytic mechanism is crucial for the parsing of the in-depth structure–activity relationship (Fig. 4).

Fig. 4 Exploration of the possible catalytic mechanism over the as-prepared heterostructural catalysts.

(1) Synergistic effect: The bonding between different components can effectively modify the electron conductivity, hydrophilic/hydrophobic properties, chemical/electrochemical stability, and active site density of the whole composite materials.^8,20 The catalytic species with high intrinsic activity are essential components for electrocatalytic reactions, but the mechanical properties, electrical conductivity, wettability, corrosion resistance and other properties of such species may be very poor. Therefore, by interfacially coupling the active species with substrates, the basic physicochemical properties of the whole composite material can be optimized. Such an improvement can ensure the electrocatalytic potential of these heterostructural materials.

(2) Confinement effect: The confinement effect over these heterostructures can resolve the aggregation phenomenon of active species during synthesis and catalysis.²¹ The pore or defect site can well anchor the original crystal nuclei and further limit their overgrowth to obtain more appropriate size and higher atomic utilization. Besides, the confinement effect can also stabilize the highly active metastable phase, making them stably exist during electrocatalysis.²²

(3) Strain effect: Due to the different chemical compositions and lattice mismatch between the bonded components, tensile or compressive strain will be exerted on each other. The crystal phases near the heterointerface show larger or smaller interplanar spacing, thereby affecting the d-orbital electronic structure of these atoms. Generally, the compressive strain would lead to the width of the d-band to increase in order to maintain a d-orbital filled with electrons. The d-band center is further away from the Fermi level, and the interaction between the active site and the adsorbate is weakened. Conversely, the tensile strain would result in the upshift of the d-band center and the enhancement of adsorptive action.²³ Such a strain effect can directly influence the surface adsorption of heterostructural catalysts, thus causing the change in catalytic activity.

(4) Electronic state modulation: Owing to the new bonding over the heterointerfaces, the bonding effect between different components can destroy the originally stable bonding structure and charge states, therefore resulting in an obvious charge density rearrangement near the interface. Such a rearrangement phenomenon would affect at least several atomic layers, making their electronic structures display completely different properties from the body phase and also the pristine surface phase.^18,24 Due to the electronegativity difference, electrons would be transferred through these newly bonded structures near the interface, resulting in the modification of various electronic properties. Therefore, spectroscopy means and theoretical calculations should be used to analyze the electronic states of heterostructural catalysts, so as to understand the inherent merits. Normally, the study of electronic states includes the band structure, density of states, work function, differential charge density and so on.

In essence, both the bonding effect and strain effect are microstructural phenomena that occur at these heterointerfaces. They can realize the electronic state modulation by virtue of the electron transfer around the heterointerface. Owing to such electronic state modulation, the catalytic pathway can be altered to result in tandem catalysis and active site swapping, etc.

(5) Tandem catalysis: Due to the multi-proton/electron coupled progress of electrocatalytic reactions, and the multi-selectivity of active sites in heterostructural catalysts, different active sites possess different adsorption strengths toward multiple key reaction intermediates; thus, tandem catalysis is very likely to occur. For example, for the Ni(OH)₂–Pt(111) interfaces, Ni(OH)₂ can adsorb and dissociate the water molecule, and the resultant H_ad can transfer to the adjacent Pt sites near the interface through the spillover effect. Two neighbouring H_ad over the Pt surface can be combined into a hydrogen molecule and then desorbed.²⁵ A similar mechanism may also exist for the four-step OER, where the first two steps occur in one component while the last two steps in another component.²⁶ It is not hard to see that the spillover phenomenon, intermediate migration path, and the migration energy barrier are the research cores for such mechanisms.

(6) Active site swapping: Because of the electronic state modulation, the adsorption behaviors of key intermediates at different active sites would be regulated. This means that the originally inactive sites could be also endowed with catalytic activity and serve as new catalytic centers. For example, according to our studies, for the hybrid Ni-based heterostructure, the inactive S sites become the new active center for the HER due to their electronic state optimization.²⁷ Systematically researching the transformation of adsorption behaviors at different active centers can provide a better understanding of the possible distribution of active sites.

The structure–activity relationship analysis can verify the effectiveness of construction strategies and further guide the refining of new component selection tactics.

Conclusions

In summary, heterostructural catalysts have great application prospects in electrocatalysis, because their fully adjustable components could provide a variety of research systems for different electrocatalytic reactions. Herein, we sort out and summarize the possible general principles for component selection to some extent. Whether from the perspective of physical properties or theoretical electronic structures of components, compatibility is always the most important principle. Not all free collocation will bring ideal results; therefore, the unification of construction principles provides necessary guidance for the reasonable construction of future heterostructural catalysts. Moreover, in-depth research on the reaction mechanism of the as-constructed heterostructural catalysts needs the right direction. Considering different catalytic mechanisms comprehensively and finding out the most suitable one are favorable for the parsing of structure–activity relationship. This Chemistry Frontiers article hopes to guide the research on the related topics by virtue of such individual combing and opinions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 22075223 and 22179104), the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (2022-ZD-4) and the Fundamental Research Funds for the Central Universities (No. 2020-YB-012).

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