Functional surfactant-directing ultrathin metallic nanoarchitectures as high-performance electrocatalysts

Jinyu Zheng , Xin Xiang , Dongdong Xu * and Yawen Tang *
Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, P. R. China. E-mail: ddxu@njnu.edu.cn; tangyawen@njnu.edu.cn

First published on 12th August 2024

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Jinyu Zheng

Jinyu Zheng is currently a MS in the School of Chemistry and Materials Science at Nanjing Normal University. Her current research interests relate to the design and synthesis of two-dimensional noble metal nanomaterials and their applications in energy storage and conversion.

Xin Xiang

Xin Xiang received her bachelor's degree from Nanjing Normal University in 2023. She is currently pursuing a master's degree in the School of Chemistry and Materials Science at Nanjing Normal University.

Dongdong Xu

Dongdong Xu is currently an associate professor in the School of Chemistry and Materials Science at Nanjing Normal University, China. He received his BS degree in chemistry from Nankai University in 2009, and PhD from Shanghai Jiao Tong University in 2014, under the supervision of Prof. Shunai Che. His research interests mainly focus on the construction of inorganic metallic architecture based on the self-assembly of designed surfactants, and electrocatalytic applications.

Yawen Tang

Yawen Tang received his BS degree and MS degree from Nanjing Normal University in 1992 and 2002, respectively, and his PhD from Nanjing University of Science & Technology in 2011. He is currently a full professor at the College of Chemistry and Materials Science at Nanjing Normal University. His main research interests are the synthesis and assembly of nanomaterials, and their applications in batteries, fuel cells, and photocatalysis.

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1. Introduction

As we strive to achieve a sustainable and recyclable energy future, fuel cells are the cornerstone of sustainable energy solutions with their versatility, efficiency and environmental benefits, and have long been at the forefront of modern energy solutions as a clean energy source.^1,2 Direct alcohol fuel cells utilize methanol, ethanol, and glycerol as anode fuels. Additionally, the hydrogen evolution reaction plays a crucial role in fuel cell technology by facilitating the conversion of hydrogen energy into electricity. Precious metal nanomaterials possess unique electronic structures and surface characteristics, high conductivity, and controllable synthesis strategies that can significantly enhance the kinetic parameters of electrode reactions. These factors have sparked considerable interest in the field of electrocatalysis.^3–6 Nanocatalysts can enhance reaction rates and increase conversion efficiency by reducing activation energy; however, many catalysts encounter challenges such as limited active sites and susceptibility to corrosion. Since the discovery of graphene in 2004, layered ultrathin two-dimensional nanomaterials have been extensively investigated.⁷ Two-dimensional ultrathin nanosheets possess several advantages, including a high specific surface area, complete exposure of active sites, and straightforward synthesis methods. These characteristics render nanosheets suitable catalysts for various electrochemical reactions. Meticulous design of the morphology and size of two-dimensional nanosheet electrocatalysts can significantly enhance the activity and durability during the reaction.^8–10

In recent years, extensive research has been conducted on the synthesis and self-assembly of noble metal nanosheets, exploring diverse approaches that each present unique advantages and challenges. These approaches aim to achieve controlled growth of nanosheets with precise morphology, thickness, and active performance. Among these strategies, the “top-down” and “bottom-up” routes have garnered significant attention.^11–13 The top-down methods focus on overcoming the interlayer van der Waals interactions within nanomaterials by harnessing external energy sources.¹⁴ This involves applying external forces to dismantle bulk nanomaterials into smaller nanosheets, often achieved through techniques such as thermal treatment, organic reactions, and oxidation etching (e.g., ball milling, lithography, chemical etching, or exfoliation processes). This approach offers precise control over size and shape, making it advantageous for high-yield mass production.^15–21 Conversely, the bottom-up approach involves controlling the growth process of metals to construct and assemble into target nanosheets.^22–25 This method represents a frequently used, simple, and scalable strategy for synthesizing novel nanomaterials. Key methods within this framework include gas-phase deposition, wet-chemical synthesis, solution phase synthesis (such as hydrothermal and solvent heat treatment), and template-guided self-assembly synthesis. Ultrathin two-dimensional nanomaterials possess exceptional properties attributed to their high surface area and quantum confinement of two-dimensional electrons. Among the bottom-up methods, template assistance stands out as a simple and widely adopted technique for controlling the morphology and thickness of two-dimensional noble metal nanosheets. Common templates include silica, layered clay, graphene oxide, or LDH. Physical or chemical deposition (PVD/CVD) has evolved into a versatile technology capable of achieving precise control over nanosheet growth kinetics by adjusting deposition parameters.^25–37 Tan et al. provided a detailed and comprehensive review of wet-chemical synthesis and applications of non-layered two-dimensional nanosheets, emphasizing the effectiveness of wet-chemical methods in producing high-yield, large-scale, and cost-effective nanosheets. They introduced strategies such as two-dimensional template synthesis and water/solvent thermal synthesis, while also offering a glimpse into the self-assembly synthesis of two-dimensional nanomaterials and the soft template method.^7,38

Although numerous reports have extensively discussed the synthesis of ultrathin two-dimensional nanosheets, there remains a significant gap in detailed reviews specifically addressing the nanosheets facilitated by the self-assembly of surfactants.^39–42 Functional surfactants play a pivotal role in nanosheet synthesis, not only controlling their growth and morphology but also serving as templates to shape nanostructures with precise dimensions and geometries. By carefully selecting surfactants with specific molecular structures and fine-tuning their concentration, it is possible to manipulate the molecular arrangement to preferentially bind to designated crystal planes of metals, thereby exerting a profound influence on the nanosheets' growth kinetics.^43–46 The ultimate nanostructure that emerges depends on intricate interactions between ligands, metals, and the hydrophobic tail groups of the surfactants. In the realm of catalysis, ultrathin noble metal nanosheets that are synthesized through surfactant-mediated self-assembly have demonstrated exceptional catalytic activity and selectivity across a range of chemical reactions. These nanosheets boast ultrathin architectures, manifesting exceptional quantum phenomena that greatly facilitate advancements in both electrical and photonic device technologies. Their large specific surface area markedly enhances the reactivity of electrode materials, making them ideal candidates for energy storage and conversion processes. They efficiently catalyse a diverse array of battery reactions, enhancing energy density and electrochemical reaction efficacy. Furthermore, the fully exposed active sites of these two-dimensional nanosheets enable optimized catalytic performance through precise modulation of their thickness, defects, and compositional makeup. Consequently, they serve as exceptional catalysts for electrocatalytic oxygen reduction, hydrogen evolution, alcohol oxidation, among others, and demonstrate remarkable efficacy in the photolysis of water and degradation of organic pollutants. Moreover, the heightened sensitivity of two-dimensional nanosheets to gas and biomolecular adsorption surpasses that of other materials, thereby laying the groundwork for the development of highly sensitive nanosheet-based sensors. This versatility also translates into immense potential for applications within the biomedical realm, promising a wide spectrum of innovative solutions and advancements. The surfactant is adeptly adsorbed onto the surface of the nanomaterial, effectively diminishing surface energy and tension within the solution. This not only fosters a uniform dispersion of the sample, but also prevents stacking and agglomeration due to excessive surface energy, which could otherwise obscure active sites. Furthermore, it plays a pivotal role in stabilizing the nanosheet's shape, ensuring that the desired morphology and functionality are maintained.^47–51 The surfactant-driven self-assembly process is highly sensitive to surfactant concentration. As described earlier, the surfactants are driven by hydrophobic interactions to form various micelles and further condense into stable micelle phases. However, if the concentration of surfactant is increased further, the concentration of single molecules or ions in the solution cannot be significantly increased; instead, more micelles can be formed, and eventually, these micelles aggregate into large massive micelles. These unique characteristics underscore the indispensability of surfactants in fabricating uniform and controllable nanomaterials.^52–56

Herein, the objective of this article is to provide a comprehensive overview of the current research landscape regarding the self-assembly synthesis of noble metal ultrathin two-dimensional nanosheets through the utilization of functional surfactants, as schematically illustrated in Fig. 1. Initially, we delve into the growth mechanism underlying the surfactant-mediated synthesis of nanosheets, highlighting significant research outcomes in this domain. Furthermore, we present a compilation of the electrocatalytic performance demonstrated by nanosheets synthesized via this approach, encompassing, but not limited to, the hydrogen evolution reaction (HER) and alcohol oxidation reaction (AOR) (Table 1). Finally, we offer a recapitulation of the present research status and advancements, alongside an exploration of the challenges and potential avenues for future exploration in this promising field.

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2. The effect of surfactants on the growth of nanosheets

Surfactant materials stand out as environmentally sustainable and eco-friendly options, making them a potent tool for synthesizing two-dimensional ultrathin metal nanosheets with exceptional properties and diverse applications. These surfactants offer numerous advantages, particularly their high degree of adjustability in the synthesis of two-dimensional nanosheets, as exemplified in Fig. 2a and b.^77–79 By manipulating surfactant concentrations, researchers can fine-tune the size, shape, and structure of the products, enabling precise control over the synthesis of single-layer, multi-layer, or hexagonal nanosheets. Moreover, surfactants possess scalability and versatility, allowing for widespread adaptability in various synthesis techniques. Surfactants are inherently hydrophilic and tend to assemble in solution, with their hydrophobic tails oriented away from the aqueous phase. This unique property enables them to combine with a range of precursors, forming micelles of diverse shapes that serve as effective templates for nanosheet synthesis, thereby producing various nanomaterials for electrocatalysis, biosensing, and other applications.^80–82 The use of surfactants in chemical synthesis offers a highly efficient method for producing a significant quantity of nanomaterials, a prerequisite for their commercialization and industrialization. Furthermore, surfactants distinguish themselves as exceptional templates for facilitating a controlled self-assembly process, directing the orderly arrangement of precursors into different structural architectures. Upon achieving saturation in the solution, these surfactants coalesce into stable micelles, consisting of numerous ions or molecules transitioning from a molecular or ionic dispersion state. As depicted in Fig. 2a, the surfactant's head group binds with the precursor, leveraging their interactive forces to align along designated crystal surfaces, orchestrating nucleation, and ultimately yielding diverse nanosheets under the subsequent reduction by reducing agents. The nanosheets synthesized through this methodology, exemplified by the TEM image in Fig. 2a, exhibit exceptional characteristics, including ultrathin thickness, uniform dispersion and size. This success stems from the surfactant's distinctive amphiphilicity and selectivity, enabling preferential adsorption onto specific crystal facets to hinder their growth—a pivotal attribute in the precise fabrication of controllable nanosheets. Furthermore, surfactants possess versatility among their numerous benefits, underscoring their widespread applicability in nanomaterial synthesis.^83–85

Drawing from the aforementioned reasons, researchers are able to devise an array of innovative synthetic routes for controlling nanostructures. To date, surfactants have been categorized in numerous ways, with the most frequently utilized method in research and application being the classification into nonionic and ionic types, depending on the ion types present in an aqueous solution. Both types can effectively self-assemble with metal elements, promoting their growth in a specified direction into extremely thin nanosheets.^59,86,87 Within this synthetic framework, our research group has examined several surfactants that enable metal precursors to grow along special crystal planes, ultimately yielding two-dimensional ultrathin nanosheets. These surfactants possess a long-chain alkyl hydrophobic tail (illustrated in Fig. 2c and d), facilitating their self-assembly into the desired layered micelles. Functional groups, such as pyridine, carboxyl, and quaternary ammonium, serve as hydrophilic heads, guiding the epitaxial growth of the precursors into ultrathin nanosheets in a controlled direction. Notably, these surfactants also contain halogen ions (primarily Cl⁻ and Br⁻), which contribute to the growth process and facilitate the formation of a stable two-dimensional structure. Furthermore, we have expanded our research to produce non-metallic doped two-dimensional nanosheets. Phosphorus can seamlessly integrate into the nanosheets at the atomic level, modulating their internal structure without compromising the original morphology. This approach offers exciting possibilities for the development of novel nanomaterials with enhanced properties and diverse applications.

2.1. The effect of various surfactants on the growth of nanosheets

2.2. The influence of surfactant structure on the growth of nanosheets

2.3. Other ways to manipulate nanosheets

Besides exploring the impacts of diverse surfactant types and structures in synthesizing nanosheets, we have also focused on the introduction of non-metallic doping, with phosphorus (P) being the most commonly used and highly effective element to date. In 2022, our team successfully crafted atomic-level phosphorus-doped PtP nanocrystals by employing surfactants and utilizing the disproportionation of sodium phosphite. Fig. 7a illustrates the synthesis process of this P-doped system. Once the highly branched and ultrathin nanodendrites (NDs) are synthesized, the dispersed P atoms infiltrate the dendrites without compromising their morphological integrity. Analysis of HRTEM, SAED, and HAADF-STEM images of PtP NDs (Fig. 7b) reveals that P is uniformly distributed within the nanosheets (NSs). Furthermore, the lattice spacing and the exposed (110) facet indicate that the crystal structure of P-doped NSs remains similar to its original form, thereby preserving the crystallinity of the nanomaterials. Subsequently, we fine-tuned the P doping rate through a straightforward and operable post-phosphating method, enabling the production of nanosheets with varying P concentrations while maintaining consistent morphologies. Additionally, we successfully doped phosphorus (P) into Pd NSs grown epitaxially on distinct crystal planes, yielding PdP NSs exhibiting remarkable tensile strain, indicative of altered lattice spacing in the metal Pd. Fig. 7f displays the XRD analysis of the sample, complemented by mapping observations that confirm the successful and uniform doping of P. Moreover, through meticulous experimentation with varying amounts of sodium hypophosphite, along with precise control of phosphating temperature and duration, we have mastered the art of precisely regulating the P doping quantity. In 2024, the incorporation of phosphorus into PdPt NDs resulted in products exhibiting superior morphology, crystallinity, and a uniform distribution of constituent elements. Apart from phosphorus, we have also ventured into other doping experiments, such as anchoring amorphous MoS₃ onto Pt NSs in 2023. Fig. 7c and d delve into the formation mechanisms. Given the extraordinary electronegativity of element sulphur, it effectively attracts electrons. Its interaction with Pt accelerates electron transfer, transforming the Pt–S bond from a covalent to an ionic character. HAADF-STEM element mapping reveals a homogenous distribution of Pt, Mo, and S elements, and the analysis of their FFT images further underscores the presence of small molecular units of α-MoS₃ as depicted.^60,70,71

3. Electrocatalytic application

Energy issues have garnered significant attention across diverse fields in recent years, with fuel cells emerging as potent materials due to their unparalleled advantages in environmental protection and utilization efficiency. Electrocatalytic reactions, a crucial aspect of fuel cell operations, have been extensively deliberated. Enhancing the catalytic activity and durability of nanosheets as nanocatalysts stands as a pivotal area of research. A fundamental approach involves refining their stability by meticulously adjusting nanosheet dimensions and morphologies. Additionally, optimizing synthesis conditions to bolster crystallinity can significantly augment stability. Within the realm of material synthesis, prevalent strategies encompass the alloying of diverse metals, the incorporation of heterogeneous and core–shell architectures to modify the electronic structure of nanosheets, and the surface modification of ultrathin nanosheets. These well-established methods are highly effective in amplifying both catalytic activity and stability, thus empowering nanosheets to make outstanding contributions in the vast landscape of catalysis. In this regard, our work offers an insightful overview of anodic reactions in fuel cells (Table 2), emphasizing the structural and compositional benefits of two-dimensional ultrathin nanosheets synthesized via surfactant-guided self-assembly techniques in the realm of electrocatalysis.^113–115

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3.1. Single metal nanosheets

Pt nanomaterials have long been regarded as exceptional catalysts for electrocatalysis. Given their similarity in electronic structure to Pt, Pd nanomaterials have also emerged as contenders, exhibiting remarkable electrocatalytic performance as precious metal catalysts. Therefore, our team embarked on exploring the potential of Pd NSs in the catalytic alcohol oxidation. In alkaline media, we conducted a comparative analysis of MOR performance among Pd NSs-1 (∼150 nm), Pd NSs-2 (∼20 nm), and commercial palladium carbon. The CV curves obtained in a 1 M glycerol solution revealed significantly higher current densities than those observed in commercial cases (Fig. 8a and b), attributed to the ultrathin nature of our nanomaterials. Furthermore, utilizing CV measurements in a 1 M KOH solution, we discovered that Pd NSs-1 exhibited the highest ECSAs due to its enlarged edge size and abundant active sites. After i–t tests, the nanosheets demonstrated a slower decline rate compared to commercial Pd/C, with Pd NSs-1 retaining the highest residual current density (Fig. 8c). Remarkably, our single metal Pd NSs also exhibited promising catalytic potential for the hydrogen evolution reaction (HER). We tested nanosheets with diverse crystal faces under acidic conditions. As evident from Fig. 8d, the catalytic activity of Pd NSs across all crystal planes surpassed that of commercial catalysts. Notably, Pd nanosheets exposing (100) crystal faces demonstrated superior catalytic activity and stability for the HER, achieving a minimal overpotential of 67 mV. After a stability test, the Pd NSs maintained superior electrocatalytic stability compared to commercial Pd/C (Fig. 8e and f). Notably, this study marks the first successful demonstration of controlling the synthesis of ultrathin Pd NSs with specific crystal planes. In 2019, we further extended our investigations to assess the electrocatalytic performance of single metal Pt NSs in the HER. Leveraging the numerous structural advantages of ultrathin single-crystal Pt NSs, we observed a significant enhancement in their electrocatalytic performance. The HER curves collected in an N₂-saturated sulfuric acid solution revealed that Pt NSs significantly outperformed commercial Pt/C (Fig. 8g and h). Furthermore, Pt NSs demonstrated exceptional performance in subsequent stability tests (Fig. 8i), underscoring their potential as robust electrocatalysts.^57,58,67

3.2. Alloy nanosheets

Drawing upon the two-dimensional ultrathin morphology, the introduction of other metals to form alloys with precious metal bases achieves remarkable outcomes. This approach accelerates electron transfer, boosts active site availability, and vastly improves the atomic utilization efficiency of precious metals. Ultrathin polymetallic nanosheets possess a natural affinity for stabilizing toxic CO intermediates that arise during reactions, while the incorporation of non-precious metals binds effectively with OH_ads, purging toxic intermediates from active sites, significantly enhancing catalyst performance, and bolstering the anti-poisoning ability. In 2019, we successfully synthesized alloyed PdPtCu NSs through a one-pot method and subjected them to a rigorous series of electrocatalytic tests. This ternary alloy composition endows the nanocatalyst with a dual functional effect, promoting OH_ads adsorption, accelerating intermediate oxidation, and elevating electrocatalytic performance. In comparison with other nanosheets (Fig. 9a and b), PdPtCu NSs exhibit the lowest reduction potential and best activity. Additionally, Fig. 9b presents the results of our anti-CO poisoning test, demonstrating that our catalyst possesses superior anti-poisoning capabilities and the largest active surface area. These results indicate that the synergistic effect and compositional advantage of alloy nanosheets can enhance the electrocatalytic performance.⁶³ We also synthesized bimetallic PtRu NSs with an optimal composition and structure for methanol oxidation catalysis. The alloys' synergistic effect and enhanced electron transfer significantly boosted the activity and stability of the PtRu NSs catalyst. Fig. 9c illustrates the electrochemical tests conducted on PtRu NSs with varying Pt and Ru content ratios. As the amount of Ru increased, so did its methanol oxidation activity. However, once a critical balance of components was achieved, further increases in Ru content began to detract from the overall catalytic performance. This investigation revealed that PtRu NSs exhibited the most outstanding electrocatalytic performance compared to other nanosheets, highlighting the versatility and scalability of this strategy for Pt-based nanosheet synthesis.⁶⁸

Numerous studies have delved into the strain effects, leading us to successfully fabricate bent PdPt NDs endowed with long-range compressive strain. A density functional theory (DFT) analysis, depicted in Fig. 9g, reveals that this strain effectively diminishes the adsorption strength of catalytic intermediates, thereby accelerating the kinetics of catalytic reactions. Additionally, it fine-tunes the local coordination environment of active sites, theoretically leading to a substantial enhancement in catalytic performance. Remarkably, under standard testing conditions, the linear sweep voltammetry (LSV) curve of the PdPt NDs, measured in a 0.5 M sulfuric acid solution, exhibited a commendable overpotential of just 10.8 mV. This represents a significant improvement in both electrocatalytic activity and stability compared to unstrained and flat PdPt NDs. The surfactant-induced strategy for synthesizing ultrathin nanosheets with extensive strain holds immense potential for diverse applications, paving the way for the production of a wider array of high-performance electrocatalysts. In 2022, PdIr NSs with different structures and compositions were prepared using self-synthesized surfactants to investigate their synergistic advantages for electrocatalytic alcohol oxidation. We conducted extensive measurements across various alcohol types, including methanol, ethanol, and glycerol, to assess the universality of this approach. The results, presented in Fig. 9d, e, and f, are promising. Ultrathin PdIr NSs with a thickness of 2.1 nm exhibited the highest mass activity, and interestingly, an increase in branching degree led to a corresponding enhancement in alcohol oxidation reaction (AOR) performance. This underscores the intricate relationship between elemental composition, material morphology, and electrocatalytic performance. Therefore, the meticulous design of two-dimensional nanomaterials, harnessing the synergistic control of self-assembled surfactants and halides, offers a powerful tool for elevating electrocatalytic activity. This approach not only expands our understanding of nanoscale catalysis but also points to new inspiration in materials science and catalysis research.^61,69

3.3. Nonmetallic-doped nanosheets

It has been conclusively demonstrated that the incorporation of non-metallic elements, particularly N, P, and S, significantly enhances both the stability and catalytic activity of catalysts. Among these elements, phosphorus holds a unique position due to its five electrons in the outer shell and a distinct atomic radius from most metals. When phosphorus is integrated into the material, it effectively regulates the electronic structure of metal orbitals by either reducing or increasing the interatomic distance. Locally, these charge alterations serve to fine-tune the electrocatalytic behavior. The high electronegativity of phosphorus atoms in the alloy attracts electrons, redirecting their flow towards the phosphorus-occupied sites. Consequently, in acidic environments, the negatively charged phosphorus acts as a base, attracting protons, while the positively charged metal sites attract hydroxides on the catalyst's surface. This innovative doping technique efficiently activates water molecules and tailors the internal electronic structure, ultimately optimizing electrocatalytic kinetics.^116–120 In 2020, our team successfully synthesized PdP NSs, which exhibited remarkable electrocatalytic activity, low reaction activation energy, excellent resistance to CO poisoning, and robust stability. As depicted in Fig. 10b, the alcohol oxidation CV curves clearly reveal that the P-doped nanosheets exhibit a higher mass activity compared to single-metal Pd NSs. This superior performance can be attributed to the enhanced oxygen content on the electrocatalyst's surface, which boosts the catalytic activity for alcohol oxidation. Furthermore, P doping might also contribute to oxidizing intermediate products formed during alcohol electrocatalysis. We also delved into the electrocatalytic behavior of PdP NSs grown through various crystal plane epitaxy. Intriguingly, the nanosheets grown along the (100) crystal plane emerged as the most active. This phenomenon can be explained by the presence of abundant defects on the ultrathin (100) facet, which confers suitable adsorption energy and significantly enhances the intrinsic activity. Additionally, the incorporation of non-metallic P modifies the electronic state of the precious metal Pd, further fortifying its resistance to poisoning. This illustrates that P doping increases the concentration of OH_ads, promoting its subsequent oxidation reaction with CH₃CO_ads, a crucial step in alcohol oxidation catalysis. The synergistic structure and compositional effects of the P-doped PdP NSs enable the catalyst to achieve superior performance, bringing it closer to the ideal electrocatalyst.

Encouraged by the above findings, we conducted ethanol catalytic oxidation tests on (100)-PdP NSs, which further validated the exceptional catalytic capabilities of this material.^71,93 The mass activity and stability is optimized when the P content reaches 3.75%. Volcano plots were constructed based on the catalytic performance exhibited by different concentrations of phosphorus doping (Fig. 10i and j). This study was further extrapolated to synthesize other multicomponent alloy nanosheets doped with phosphorus, aiming to enhance their electrocatalytic performance.⁷¹ When compared with other catalysts (Fig. 10c), PdPtP NDs doped with phosphorus exhibit the lowest initial potential, as well as the highest mass and specific activity, during ethanol oxidation reactions. This underscores the pivotal role of phosphorus doping in alloy regulation.⁶² Due to the significant difference in electronegativity between Pt and P, a substantial amount of electron transfer occurs during the formation of nanosheets. This transfer not only facilitates the hydrogen evolution reaction (HER) but also significantly enhances the methanol oxidation reaction (MOR) performance. We delve into the underlying mechanism (Fig. 10h). Compared to Pt NDs without phosphorus doping, our PtP nanosheets exhibit superior reaction kinetics in both the HER and MOR. The introduction of phosphorus leads to electron transfer activity from Pt to P, resulting in optimized electronic states. During the HER catalytic process, the phosphorus sites are more adept at adsorbing H⁺ in acidic environments. Conversely, in the MOR process, positively charged Pt atoms readily combine with methanol molecules to form adsorbed carbon monoxide species (CO_ads). Concurrently, both Pt and oxygenophilic P atoms facilitate the formation of adsorbed hydroxyl species (OH_ads), which subsequently react rapidly with adjacent CO_ads. Therefore, the key to enhancing catalyst performance through phosphorus doping lies in the augmentation of reaction kinetics.

Based on the aforementioned insights, we conducted a dual functional analysis of PtP nanosheets. Fig. 10d and e demonstrate their catalytic oxidation activity towards methanol, revealing a regular trend in mass activity depending on the amount of phosphorus doping. Accordingly, a volcano plot was generated to visualize the impact of phosphorus atom content on catalytic performance. Similarly, the catalytic analysis of the HER (Fig. 10f and g) also yields volcano plots exhibiting distinct patterns. These tests collectively indicate that appropriate phosphorus doping can significantly enhance the catalytic performance of nanosheets; however, excessive doping leads to a decline in catalytic activity due to reduced catalyst conductivity and an increase in positively charged Pt sites, which hinder the removal of adsorbed hydrogen (H_ads) and carbon monoxide (CO_ads) species during the HER and MOR. Doping phosphorus into nanosheets represents an effective approach to enhance the electrocatalytic performance of catalysts. Analogously, anchoring MoS₃ is another research objective of ours. The high dispersion of MoS₃ significantly boosts the activity of Pt catalysts under both acidic and alkaline conditions. Notably, Pt-α-MoS₃ nanosheets maintain excellent HER catalytic activity even after 24 hours of operation, with impressive η₁₀ potentials (Fig. 10h–l). The stability of Pt-α-MoS₃ nanosheets surpasses that of commercial Pt/C and nanosheets without α-MoS₃ doping. Our work underscores the importance of atomic engineering of active sites. Although the electrocatalytic performance of precious metals and precious metal-based alloys doped with phosphorus has been significantly improved, numerous challenges remain. As a nonmetal, phosphorus exhibits limited conductivity, hence excessive doping can detrimentally affect the electrocatalytic performance. Moreover, P atoms are prone to oxidation during the electrocatalytic process, highlighting the need for discovering catalysts with more stable structures.^60,70

4. Conclusion and outlook

In this comprehensive review, we strive to underscore the pivotal role played by functional surfactants as both domain-limiting and structural-directing agents in the self-assembly synthesis of two-dimensional ultrathin nanosheets, especially their contribution to enhancing electrocatalytic performance. By delving into illustrative cases of surfactant-assisted nanosheet preparation, we provide an encompassing perspective on the underlying synthesis mechanisms and the electrochemical applications of the resulting products. The utilization of surfactants in the nanosheet synthesis process is paramount in shaping their intended morphology, structure, and specific crystal planes. Among the surfactants commonly employed for synthesizing ultra-thin nanosheets, ionic and non-ionic surfactants stand out, offering unique physical and chemical properties thanks to their hydrophobic tails and hydrophilic heads, making them ideal candidates as soft templates. Firstly, surfactants facilitate the controllable self-assembly of micelles towards special structures and morphologies that exhibit optimal catalytic performance. Secondly, the nanosheets obtained through this surfactant-guided approach possess large sizes and extremely thin thicknesses, thereby maximizing the exposure of active sites, crucial for achieving high-performance catalysis across various fields. Furthermore, surfactants exert a protective effect, enabling the preparation of two-dimensional nanosheets with stable and uniform morphologies, along with superior electrocatalytic performance. Overall, this review underscores the significant impact of surfactants in nanosheet synthesis, not only in shaping their morphological and structural features but also in enhancing their electrochemical properties, thus broadening their potential applications in catalysis and beyond.^121–124

Despite notable advancements in the synthesis of two-dimensional nanosheets, numerous challenges persist in developing straightforward methods for fabricating nanosheets with tunable morphologies and significantly enhanced catalytic performance. A notable hurdle lies in the purification step, where the complete removal of surfactants from the samples remains elusive. Even trace amounts of surfactant residue can obstruct active sites on the nanosheets, thereby impeding accurate characterization and testing of their various properties. Additionally, while laboratory-scale synthesis and evaluation of nanosheets have reached satisfactory levels, scalable production of high-quality 2D nanosheets and the precise engineering of their morphology, size, and composition remain crucial bottlenecks hindering large-scale commercialization and industrial utilization.^125–127

As we gaze ahead into the future, the potential of nanosheet electrocatalysts and their practical applications appear vast and limitless, offering boundless opportunities:

(1) Firstly, researchers must consistently delve into surfactant molecules (including novel structure designs and functionalities) to enhance the precision of self-assembly. They must strive to develop surfactants that promote greater stability, repeatability, reliability, and ease of removal from metallic nanosheet catalysts. Additionally, the quest for efficient electrocatalysts capable of driving fundamental energy conversion processes remains paramount. The main aim is to synthesize metallic nanosheets with a cleaner surface by reducing surfactant adsorption on electrocatalysts.

(2) Secondly, among the numerous promising avenues, nanosheet catalysts offer the tantalizing possibility of being tailored and modulated to meet specific functional and application requirements. This customization extends to various aspects, including but not limited to element composition and microelectronic motion. Their adaptability in terms of chemical properties enables them to accommodate diverse energy conversion processes. Their remarkable structural attributes and performance have the potential to revolutionize numerous energy conversion and storage technologies. Consequently, this may pave the way for unprecedented and highly efficient clean energy conversion methods. Central to this endeavor are efforts to refine the electronic motion within nanosheets and fine-tune their active sites, ultimately enhancing the activity and stability of these catalysts.

(3) Thirdly, the commercialization, scalability, and seamless integration of two-dimensional nanosheets into functional hardware represent crucial areas of research. Given their exceptional performance and tunable properties, the future of these nanosheets is poised to intersect with numerous diverse fields. In particular, their application in electrocatalysis exhibits remarkable sensitivity, selectivity, and catalytic activity, making them suitable for the detection and degradation of pollutants, toxic gases, and harmful chemicals. Notably, fuel cells stand as the cornerstone of sustainable energy technology, and as such, the precise control of self-assembly processes and the accelerated translation of research outcomes into practical applications play a pivotal role in addressing urgent environmental challenges, mitigating climate change, and promoting energy recycling efforts.

(4) Fourthly, there is an ongoing necessity for focused research efforts to delve deeper into exploring synthesis routes that offer increased convenience, reproducibility, and high yields for high-performance catalysts. This pursuit must be complemented by a constant quest to minimize costs and expand production capabilities. Leveraging advancements in nanotechnology, researchers are now able to fine-tune the molecular-level characteristics of fuel cell components, thereby optimizing their performance and addressing longstanding challenges. The development of scalable production technologies and manufacturing processes will serve as the linchpin for the widespread commercial and industrial adoption of two-dimensional nanosheets.

In essence, ongoing research in these areas is anticipated to further deepen our comprehension of the synthesis and self-assembly mechanisms for two-dimensional nanosheets. This, in turn, will fully unlock their catalytic potential, laying the foundation for the creation of novel nanomaterials with superior performance and functionality that are scalable for mass production.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22232004 and 22279062). We also thank the financial supports from Priority Academic Program Development of Jiangsu Higher Education Institutions, National and Local Joint Engineering Research Center of Biomedical Functional Materials.

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