Advanced strategies for controlling three-phase boundaries in photocatalysis

Abstract

This review delves into the latest advancements in controlling three-phase boundaries (TPBs) in photocatalytic systems, with a focus on photo(electro)catalytic processes for nitrogen reduction, oxygen reduction, and water reduction. We critically analyze various strategies and advanced materials designed to enhance TPB performance, evaluating their impact on catalytic efficiency and identifying gaps in the existing literature. By examining sophisticated triphasic systems that integrate superwetting materials, we emphasize their essential role in improving light absorption, charge separation, and mass transfer. Key challenges in TPB optimization are discussed, and future research directions are proposed to advance photocatalytic technologies for sustainable energy applications. This review highlights the crucial importance of TPBs in photo(electro)catalysis, aiming to inspire further innovation for more efficient and scalable solutions.

---

Lagnamayee Mohapatra

Dr Lagnamayee Mohapatra earned her Ph.D. degree from the CSIR-Institute of Minerals and Materials Technology, in the Academy of Scientific and Innovative Research (AcSIR), India. She was awarded the DST-DFG award to participate in the 63rd Meeting of Nobel Laureates and Students in Lindau, Germany, in 2013. Currently, she is a research professor at Jeonbuk National University (JBNU), Republic of Korea. Dr Mohapatra's research is at the forefront of nanoscience, specializing in nanostructured materials, layered materials, and quantum dots. Her work includes artificial photosynthesis, photocatalytic water purification, and advanced oxidation processes. Currently, she is advancing superwetting materials for sustainable energy and environmental applications.

Lekha Paramanik

Dr Lekha Paramanik is a post-doctoral research fellow at the Department of Quantum System Engineering, Jeonbuk National University, Republic of Korea. She earned her master's degree from Ravenshaw University in India and completed her PhD in Chemistry at the Centre for Nano Science and Nano Technology, S‘O'A Deemed to be University in Odisha, India, in 2021. Dr Paramanik's research focuses on the synthesis of photocatalytic and photoelectrocatalytic semiconductor materials, with applications in environmental and energy fields.

Subhashree Sabnam

Subhashree Sabnam graduated with an integrated master's degree in chemistry from the University of Hyderabad, India, in 2020. She is currently pursuing her Ph.D. at Jeonbuk National University, Republic of Korea, starting in 2022. Her research area focuses on two key aspects: the photocatalytic degradation of organic pollutants, with the objective of crafting effective and environmentally sustainable techniques for purifying water via photo-induced chemical reactions, and the generation of hydrogen energy, emphasizing novel methods to produce clean, renewable energy through the employment of hydrogen as an alternative fuel source. Additionally, she is proficient in fabricating various nano-materials employing different techniques to augment their efficacy in these applications.

Seung Hwa Yoo

Prof. Seung Hwa Yoo has been a professor at Jeonbuk National University (JBNU), Republic of Korea since 2020. He earned his Ph.D. from the Korea Advanced Institute of Science and Technology (KAIST) in 2013. He is a prominent researcher with diverse expertise in various fields. His areas of specialization include photocatalysis, radiation processing technology, and the fabrication and application of advanced materials including carbon, composites, and polymers. In his current research, Prof. Yoo focuses on the fabrication and application of light-responsive nano-semiconductors for sustainable energy and environmental solutions. In particular, he focuses on hydrogen production, aiming to develop innovative approaches for generating clean and renewable energy through the utilization of hydrogen as a fuel source. With his extensive knowledge and expertise, Prof. Yoo is making valuable contributions to advancing cutting-edge technologies in environmental remediation and sustainable energy production. His work holds great promise for addressing critical challenges and creating a more sustainable future.

---

1. Introduction

In the quest for sustainable energy solutions, photocatalysis has emerged as a promising technology for harnessing solar energy to drive chemical reactions.^1,2 However, the efficiency of photocatalytic systems critically depends on the effective interaction between the three involved phases: the photocatalyst (solid phase), the reactant solution (liquid phase), and the generated gases or products (gas phase).^3,4 The interfaces where these phases meet, known as three-phase boundaries (TPBs), are crucial in determining the overall performance of photocatalytic processes. One significant challenge in these multiphase systems is the transport of reactants and products across the interfaces. Interfacial diffusion and mass transfer often become rate-limiting steps that hinder the overall reaction rates and efficiency.⁵ The TPB is the region where the gas, liquid, and solid phases converge, and it plays a critical role in photocatalytic reactions by representing a geometrical class of phase boundaries where these phases intersect.⁶ A simple analogy is a coastline, where land, air, and sea meet, forming a dynamic zone influenced by solar, wind, and wave energies, supporting diverse ecosystems.

Recently, there has been a groundbreaking shift in materials design with the exploration of three-phase interfacial catalysts. This growing interest is driven by the crucial role of interfacial chemistry in enhancing energy and mass transfer in light-driven multiphase catalytic systems.⁷ In photocatalysis, most photo(electro)catalysts are in the solid state, leading to typical configurations involving solid–liquid or solid–gas diphasic systems. These conventional configurations are fundamental to traditional photocatalytic (PC) and photoelectrochemical (PEC) mechanisms.⁸ The transition from conventional biphasic systems to advanced triphasic systems, which incorporate superwetting materials, represents a significant advancement in photocatalysis.⁹ By improving interactions among the solid, liquid, and gas phases, these advanced systems achieve superior light absorption, charge separation, and reactant/product transfer. Mastering the control of TPBs and utilizing superwetting materials are essential for optimizing photocatalytic efficiency. Continued research and development, inspired by biological phenomena, will lead to innovative solutions for energy and environmental applications, paving the way for more effective and scalable photocatalytic technologies.^10–12

Controlling the TPBs to maximize photocatalytic efficiency involves optimizing the interactions between the solid, liquid, and gas phases to ensure effective light absorption, charge separation, and the transfer of reactants and products.¹³ This requires a thorough understanding of catalyst material properties, the dynamics of reactants in different phases, and the environmental conditions under which the reactions occur. Recent advances in materials science and nanotechnology have introduced new methods for engineering these boundaries, including novel nanostructured photocatalysts with tailored surface properties, innovative reactor designs that enhance phase contact, and the strategic use of co-catalysts and additives.¹⁴ Developing an efficient TPB is crucial for achieving high catalytic performance, but our current understanding of TPB structures is still limited, hindering the rational construction of these systems. Increased focus on TPB studies is essential to gain insights into mass transport, electron conduction, and reaction kinetics, which could guide the design and enhancement of TPB performance, ultimately reducing resource utilization and improving catalytic efficiency.¹¹

Our group has recently published a comprehensive review on the emerging field of superwetting materials and their applications in photocatalytic organic pollutant degradation, highlighting their significant potential for environmental remediation and sustainable energy production.¹⁵ In this review, we will shift our focus to the state of research on controlling TPBs in photocatalytic systems, specifically targeting photo(electro)catalytic processes for nitrogen reduction, oxygen reduction, and water reduction. We will critically examine various strategies and materials designed to enhance TPB performance, assess their impact on overall catalytic efficiency, and address gaps identified in our previous review. Additionally, we will identify key challenges and propose future research directions to advance this crucial aspect of photocatalytic technology, particularly in the energy sector. This review aims to underscore the importance of TPBs in photocatalysis and inspire further innovations for more efficient and scalable applications.

2. Fundamental concepts of three-phase boundaries

The TPB concept (Fig. 1a) is particularly important in heterogeneous catalysis and electrochemical systems because it creates a unique environment that enhances catalytic reactions through the simultaneous interaction of all three phases.^15–19 These effects can lead to enhanced reaction rates, better selectivity, and improved overall performance of the catalytic processes. Hence, the significance of the TPB is as follows:

(i) Enhanced reaction rates: At the TPB, reactants from the gas phase can adsorb onto the solid catalyst surface, while reactants or ions from the liquid phase can also interact at the same site. This simultaneous presence facilitates rapid reaction kinetics, as all necessary reactants are in close proximity to the active sites of the catalyst.

(ii) Increased catalytic efficiency: The efficiency of catalysts can be significantly improved at TPBs due to the higher availability of reactants at the active sites. This leads to more effective utilization of the catalyst material and can reduce the amount of catalyst required for a given reaction.

(iii) Improved selectivity: The controlled environment at the TPB can help in steering the reaction pathways towards the desired products by minimizing side reactions. Specific interactions at the TPB can lead to a higher selectivity for certain products, which is crucial in many industrial processes.

Fig. 1 Schematic of (a) three-phase contact line where the electrolyte–water droplet meets the solid surface and the surrounding gas. Schematic of (b) hydrophilic, (c) superhydrophilic, (d) hydrophobic, and (e) superhydrophobic surfaces with different wetting/non-wetting states. Reprinted (adapted) with permission.¹⁵ Copyright 2024, Elsevier. Schematic of a liquid drop under (f) Cassie state, (g) Wenzel state, and (h) Cassie–Wenzel state.

2.1. Surface wettability in triphasic catalytic systems

Wettability plays a crucial role in triphasic catalytic systems in which solid surfaces simultaneously interact with two immiscible phases, typically gases and liquids. Manipulating the surface wettability significantly influences the distribution and interaction of the reactants at the interfaces, thereby affecting the reaction rates and selectivity.²⁰ Optimizing the wettability properties is essential for enhancing interfacial mass transfer and improving the overall catalytic efficiency.²¹ For catalytic reactions involving structured and chemically stable solid substrates, conventional methods for modifying the surface wettability are often limited by the inherent inability to tune the solid surface tension. Therefore, structured surfaces require innovative strategies to achieve tunable surface wettability independent of the solid phase. This approach is essential for optimizing the catalytic efficiency and product selectivity of triphasic catalytic systems.^22,23 Wettability refers to the affinity of a solid substrate for wetting by a fluid and is influenced by the surface polarity or geometric structure.²⁴ Understanding and categorizing wettability states based on contact angles (θ) help in designing and optimizing surfaces for various applications, including catalysis, where controlling how liquids interact with solid catalysts can profoundly impact reaction efficiency and selectivity.^25,26 The degree of wettability can be categorized into four main states based on the contact angle (θ) formed by a liquid droplet on the solid surface (Fig. 1b–e): (i) superhydrophilic (0° < θ < 10°): on superhydrophilic surfaces, water or other polar liquids spread extensively due to strong adhesive forces, resulting in complete wetting; (ii) hydrophilic (10° < θ < 90°): these surfaces facilitate relatively high spreading and wetting, although not as extensively as superhydrophilic surfaces; (iii) hydrophobic (90° < θ < 150): hydrophobic surfaces repel water and other polar liquids to a significant extent, causing droplets to bead up and minimally wet the surface; and (iv) superhydrophobic (θ > 150°): superhydrophobic surfaces are extremely water-repellent, causing water droplets to bead tightly and roll off the surface easily. These surfaces often exhibit self-cleaning properties because of the minimal contact area between the water droplet and the surface.^15,27–29

2.2. Wetting states and mass transfer processes

The wettability of a catalyst surface plays a crucial role in catalytic processes by influencing the interaction and transfer of reactants across interfaces. The wetting state of the catalyst can be tailored by altering its chemical or physical properties, leading to three distinct states that significantly affect the interfacial mass transfer:^30–32 (i) Wenzel state: in the Wenzel state, the catalyst surface is fully wetted by the liquid phase, with minimal trapped air at the liquid/solid interface. This configuration results in limited three-phase contact and sluggish gas diffusion through the liquid phase, which is slower than that through the gas phase (Fig. 1(f)). The Wenzel state is characterized by reduced efficiency in the mass transfer of gases, potentially limiting catalytic reaction rates;³³ (ii) Cassie state: conversely, the Cassie state describes a surface that is completely non-wetting to the liquid phase, creating a barrier that hinders gas diffusion into the liquid (Fig. 1g). This poor wetting condition limits the interaction between the gas reactants and the liquid phase, thereby restricting the photocatalytic activity and overall catalytic performance;³⁴ and (iii) Cassie–Wenzel coexistent state: the Cassie–Wenzel coexistent state represents an intermediate condition in which the catalyst surface exhibits mixed wetting characteristics. The liquid partially wets the solid surface, leaving pockets of trapped air at the interface. This configuration maximizes the creation of multiple three-phase contact points, facilitating the efficient mass transfer of reactants in both the liquid and gas phases. The coexistence of the Cassie and Wenzel states enhances interfacial interactions, promoting enhanced catalytic performance by improving the accessibility of reactants to active sites on the catalyst surface (Fig. 1h).^35–37 Understanding and controlling these wetting states are crucial for optimizing catalytic processes, particularly in photocatalysis, where efficient mass transfer between the gas and liquid phases is essential. By manipulating surface wettability, researchers can tailor catalyst designs to maximize interfacial contact and enhance the overall catalytic efficiency.³⁸

2.3. Strategies for controlling three-phase boundaries

TPBs are crucial for optimizing catalytic and photocatalytic processes.^25,26,39,40 The effective manipulation of TPBs can enhance the efficiency of these systems by improving the reactant interactions, mass transfer, and overall reaction rates. Strategies for controlling the TPBs in the context of catalysis and photocatalysis are as follows.

2.3.1. Surface modification and functionalization.
2.3.1.1. Chemical treatment. Surface modification by chemical treatment can alter the wettability and adsorption properties of the catalyst surface.⁴¹ For instance, introducing hydrophilic or hydrophobic functional groups can help selectively enhance the interactions with liquid or gas phases, respectively.^42–44 In particular, the introduction of hydroxyl groups, oxides, or other polar functional groups onto the catalyst surface can increase its hydrophilicity. Hydrophilic surfaces enhance liquid-phase reactions by promoting the spread and adsorption of the liquid reactants. Moreover, the application of non-polar substances such as silanes, fluorinated compounds, or alkyl chains can make the surface hydrophobic or even superhydrophobic. This is useful for trapping air, thereby promoting gas-phase reactions and inhibiting liquid contact.
2.3.1.2. Application of coating materials. Applying coatings or creating composite layers on the catalyst surface can improve its interactions with different phases. For example, coating photocatalysts with materials that enhance light absorption or charge separation can enhance their efficiency.^28,45,46

2.3.2. Nanostructuring and texturing. Nanostructuring and texturing significantly influence the superwetting behavior of catalysts and the formation of three-phase boundaries (TPBs). The creation of nanostructures, such as nanorods, nanotubes, or nanopillars, increases the surface area, providing more active sites at the TPBs, which are essential for catalytic reactions. These nanostructures enhance the interaction between the gas, liquid, and solid phases by offering a larger interface for mass transfer and improving the reactant distribution and conversion efficiency.^45,47,48 Moreover, in the context of photocatalysis, these structures can trap light more effectively, increasing the absorption of photons and enhancing the generation of electron–hole pairs. The unique geometries of the nanostructures also help reduce charge recombination by facilitating more efficient charge separation and transport to the active sites. Consequently, nanostructuring and texturing not only optimize the physical and chemical interactions at the TPBs, but also enhance the overall photocatalytic efficiency through improved light harvesting and charge dynamics. Thus, nanostructured catalysts are highly effective for various applications, including environmental remediation and energy conversion.^49–51

Surface roughness plays a crucial role in defining the three-phase boundaries (TPBs) and significantly affects catalytic performance. Adjusting the surface roughness of materials through mechanical or chemical etching can significantly affect their wettability and phase contact properties. By modifying the surface topology, it is possible to enhance interactions between the gas, liquid, and solid phases, which are crucial for catalysis.^52–54 For example, hydrophobic materials can become superhydrophobic with appropriate roughening, thereby enhancing their ability to trap air and facilitate gas-phase interactions. The combination of micro- and nanoscale features can further enhance the desired wetting properties by promoting liquid spread or air trapping. This multiscale texturing approach allows the fine-tuning of surface characteristics to create optimal three-phase boundaries, thereby improving the mass transfer, reactant distribution, and overall catalytic efficiency.

2.3.3. Wettability control. Wettability and surface energy are crucial factors that influence the formation of TPBs and the superwetting behavior of catalysts, significantly impacting photocatalysis. By adjusting the wettability and surface energy of a catalyst, interactions between the gas, liquid, and solid phases can be optimized, thereby enhancing reactant adsorption and mass transfer. Superhydrophilic surfaces enable extensive wetting by aqueous solutions, which maximizes contact and improves the transfer of liquid-phase reactants. On the other hand, superhydrophobic surfaces trap air, promote gas-phase interactions, and reduce liquid barriers. Achieving the right balance between these properties can create an ideal TPB configuration for the most efficient catalytic reactions. Tailoring surface properties in this way boosts light absorption, charge separation, and overall photocatalytic activity, leading to improved efficiency and selectivity in photocatalytic processes. This customization ensures that the catalyst performs effectively under various environmental conditions, making it more versatile and robust for practical applications.⁸ Adjusting a catalyst's wettability by altering its chemical composition or surface texture can control the distribution and interaction of gas and liquid phases. Superhydrophobic surfaces can trap air and promote gas-phase reactions, while hydrophilic surfaces enhance liquid-phase reactions.^27,55

Additionally, patterning the surface with hydrophobic and hydrophilic regions is a strategic method for creating controlled TPBs that optimize mass transfer and reactant distribution in catalytic systems.^56,57 Selective modification of surface properties, such as wettability and surface energy, allows for tailored interactions with the gas, liquid, and solid phases. Hydrophobic regions repel water, promoting efficient gas-phase interactions and minimizing liquid coverage.⁵⁸ Conversely, hydrophilic regions ensure thorough wetting by aqueous solutions, enhancing liquid–solid interactions and facilitating rapid mass transfer of reactants. This spatial control over surface characteristics allows for the creation of well-defined TPBs, directing reactants to active sites and improving catalytic efficiency and selectivity. Such patterning techniques are essential for optimizing catalyst performance in photocatalysis and other catalytic processes.⁵⁹

2.4.4. Catalyst morphology and structure. The morphology and structure of the catalyst play a pivotal role in determining the triphasic superwetting behavior, which significantly affects the catalytic performance. The physical and chemical characteristics of the catalyst surface influence how the gas, liquid, and solid phases interact, thus affecting the overall efficiency of the catalytic and photocatalytic processes.^60–63

Metal–organic frameworks (MOFs), zeolites, and mesoporous silica are highly porous materials that provide extensive three-phase boundaries (TPBs) and exhibit superwetting behavior, which is crucial for catalytic applications. Their high surface areas and tunable pore structures enhance the interactions between the gas, liquid, and solid phases, facilitating efficient mass transfer and reactant diffusion. MOF catalysts with customizable pore sizes and functional groups can be engineered for specific interactions and wettability properties to optimize the contact with all phases. Zeolites, known for their uniform pore structures and high thermal stability, offer selective adsorption sites that enhance catalytic reactions at TPBs. Owing to its large pore volume and adjustable surface chemistry, mesoporous silica can be modified to achieve superhydrophobic or superhydrophilic surfaces, thereby promoting optimal wetting states for different catalytic processes. These properties can create extensive TPBs, and their adaptable wetting properties make them ideal candidates for enhancing catalytic efficiency and selectivity.^{25,61,64–67} Moreover, designing core–shell catalysts, where a reactive core is coated with a shell material, can enhance the stability and activity of the catalyst at TPBs. The shell can be engineered to improve specific interactions with the gas or liquid phase.^68–71

Dual-phase catalysts, such as Janus catalysts, have distinct surface properties, favoring superwetting behavior that enhances photocatalysis. These catalysts feature one side engineered for hydrophilicity and the other for hydrophobicity, creating a unique interface that promotes efficient interactions among the gas, liquid, and solid phases.⁷² This duality ensures optimal light absorption and charge separation, which are crucial for photocatalytic reactions. The hydrophilic side maximizes contact with the aqueous phase, facilitating the transfer of reactants, whereas the hydrophobic side improves gas-phase interactions and reduces the liquid barrier. This strategic design enhances the generation and utilization of reactive species, thereby boosting the overall efficiency and effectiveness of photocatalytic processes. The ability of Janus catalysts to create tailored wetting states and extensive three-phase boundaries makes them highly effective in optimizing photocatalytic reactions.^68,73,74

The use of bimetallic or trimetallic nanoparticles can create synergistic effects that significantly enhance both catalytic activity and stability. These nanoparticles, composed of two or three different metals, offer unique properties that single-metal catalysts cannot achieve.⁷⁵ By carefully selecting and combining metals, these nanoparticles can be designed to optimize the interactions at three-phase boundaries (TPBs).^76,77 One metal may provide high catalytic activity, but there is another issue of stability or resistance to deactivation of the catalyst. Additionally, the combination of different metals can create new active sites and modify electronic properties, leading to improved reaction kinetics.⁷⁸ The tailored surface characteristics of bimetallic and trimetallic nanoparticles also facilitate better contact between the gas, liquid, and solid phases, promoting efficient mass transfer and increasing the overall catalytic performance.^79,80 This synergistic approach not only increases the catalytic efficiency, but also extends the durability and operational lifespan of the catalyst, making it highly effective for various catalytic applications, including photocatalysis.

2.4.5. Innovative fabrication techniques. Innovative fabrication techniques such as electrodeposition and electrospinning play a pivotal role in tailoring catalysts with controlled surface properties and morphologies, thereby enhancing their superwetting behavior and creating three-phase boundaries (TPBs).⁸¹ Electrodeposition allows for the precise deposition of metallic nanoparticles onto catalyst surfaces, influencing their wettability and interaction with both the liquid and gas phases.⁸² This technique can be used to create catalysts with tailored surface roughness and composition, and to optimize TPBs for efficient mass transfer and catalytic activity. The growing issue of oily wastewater and organic pollutants necessitates efficient oil–water separation. A superwetting, visible-light catalytic Cu@C₂O film was developed via one-step electrodeposition, and it displayed underwater superoleophobicity and underoil superhydrophobicity after pre-wetting. Its unique wettability and small pore size enable the effective separation of water-in-oil and oil-in-water emulsions under gravity. Performance was optimized at the TPB, which enhanced the mass transfer and separation efficiency. In addition, the visible light catalytic properties of the film enabled pollutant degradation during the separation process, thus enabling both oil and contaminant removal.⁸³

Chemical vapor deposition (CVD) is typically performed in a sealed chamber under controlled conditions to deposit specific chemicals onto substrates. CVD has been widely used to increase surface roughness and thereby aid in the development of hydrophobic or superhydrophobic materials for oily wastewater separation. Graphene-based porous materials fabricated via CVD have garnered significant attention because of their high chemical stability, large surface area, and three-dimensional (3D) networks that exhibit extreme water repellence. These properties make them highly effective for oil–water separation.⁸⁴ Moreover, the formation of TPBs in these materials further enhances the mass transfer and separation efficiency by optimizing the interactions between the solid surface, oil, and water.^85–87

On the other hand, electrospinning enables the fabrication of nanofibrous structures with large surface areas and high porosity, ideal for promoting superhydrophilic or superhydrophobic properties depending on the application. Hydrophobic materials, such as polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF), and fluorinated silane compounds, and hydrophilic materials, such as poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), and poly(vinylpyrrolidone) (PVP), have been widely used in the development of electrospun membranes for oil–water separation. These membranes effectively leverage surface wettability to achieve efficient separation by repelling or attracting water and oil, depending on their composition.^88–90 By leveraging these advanced fabrication methods, catalysts can be engineered to exhibit enhanced superwetting behavior, facilitating superior performance in photocatalytic applications through improved reactant accessibility and reaction kinetics at the TPBs.^91–93

Self-assembly methods provide a powerful approach for creating structured surfaces or composite materials that optimize three-phase boundaries (TPBs) to enhance catalytic performance.^8,94 These methods utilize natural molecular interactions or external stimuli to spontaneously organize molecules into ordered structures at various scales. Many superwetting particle coatings have been developed for efficient oil–water separation using this technique. For example, 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) has been employed to increase both the water contact angle (WCA) and oil contact angle (OCA) by forming larger particles, resulting in a superhydrophobic surface. This modification enabled a treated stainless steel mesh to achieve effective oil/water separation. Additionally, superhydrophobic and superoleophilic polytetrafluoroethylene (PTFE)-coated porous metal fiber sintered felt (PMFSF) was fabricated via a layer-by-layer (LBL) assembly method that utilized electrostatic interactions to enhance separation performance.^95,96 Moreover, this technique enables precise control of the surface morphology, porosity, and chemical composition, all of which are critical for catalytic activity. By directing the assembly process, catalysts can be engineered with tailored TPBs, in which the gas, liquid, and solid phases interact efficiently. For example, self-assembled monolayers (SAMs) on surfaces can modify the surface chemistry to promote specific interactions with reactants, whereas self-assembled nanomaterials can create hierarchical structures that maximize the surface area and expose catalytically active sites. Such approaches not only improve mass transfer and reactant distribution, but also enhance stability and durability, making self-assembly a promising strategy for advancing catalytic technologies across various industrial and environmental applications.⁹⁷

In addition to the common fabrication methods discussed above, several other techniques, such as spray coating, phase separation, electroplating, and grafting, have been employed to construct novel superwetting materials with exceptional properties to optimize wettability and improve performance in various applications.^98,99

3. Importance of TPBs in photocatalysis

Three-phase boundary interfaces (TPBs) play a crucial role in photocatalysis by involving the convergence of the solid, liquid, and gas phases.⁴ Typically, these boundaries include a photocatalyst in the solid phase, an aqueous solution in the liquid phase, and gases such as oxygen or hydrogen in the gas phase.^8,100 TPBs are essential for facilitating key processes such as light absorption, charge separation, and redox reactions, which are fundamental to the efficiency of photocatalytic systems. The characteristics of TPBs, including their surface areas, structural properties, and chemical compositions, critically influence the effectiveness of these processes.¹⁰¹ Optimal TPBs promote enhanced contact between phases, facilitating efficient transfer of reactants and products and thereby improving overall reaction rates.

In engineering designs focused on three-phase systems, efforts are concentrated on optimizing the interfaces between the gas, solid, and liquid phases to enhance the interfacial mass-transfer processes essential for catalytic efficiency.^8,102 Manipulating the wettability of the surfaces within these configurations can significantly impact reaction kinetics, further enhancing the performance of photocatalytic systems.

3.1. Role of three-phase boundaries (TPBs) in photocatalysis mechanism

Three-phase boundaries (TPBs) play a crucial role in the photocatalytic mechanism, where the solid, liquid, and gas phases converge to facilitate key processes. The TPBs can enhance the efficiency of photocatalytic systems by offering several advantages.

(i) Enhanced surface interaction: Enhanced reactant contact facilitated by TPBs is crucial for photocatalysis, in which these boundaries create a specialized interface. TPBs enable direct interactions between reactants from the solid, liquid, and gas phases and the photocatalyst surface, thereby enhancing the availability of reactants and promoting efficient catalytic reactions. Moreover, TPBs increase the number of active adsorption sites, which are essential for processes such as hydrogen evolution, oxygen reduction, and the degradation of organic pollutants. This dual role of TPBs in promoting surface interactions and providing abundant adsorption sites underscores their significance in optimizing the photocatalytic efficiency and performance.^15,103

(ii) Optimizing light absorption and charge separation: To facilitate key processes in photocatalysis, three-phase boundaries (TPBs) play a critical role in enhancing light absorption and charge separation mechanisms. In TPBs, photocatalysts can absorb light more effectively, initiating the generation of electron–hole pairs, which are crucial for subsequent redox reactions. This efficient light absorption is pivotal for the activation of photocatalytic processes. Additionally, TPBs enhance the separation and migration of photogenerated electrons and holes, thereby minimizing recombination losses and significantly increasing the overall photocatalytic activity. By optimizing both the light absorption and charge separation, TPBs contribute substantially to improving the efficiency and effectiveness of photocatalytic systems.^15,104

(iii) Optimized mass transfer: TPBs optimize the diffusion pathways for reactants and products across solid–liquid–gas interfaces, significantly enhancing the dissolution of gas-phase reactants into the liquid phase and the desorption of liquid-phase products into the gas phase. Additionally, TPBs minimize diffusion resistance by promoting improved contact between phases, thereby enabling faster and more efficient mass-transfer processes. The dual capabilities of TPBs to facilitate reactant diffusion and minimize resistance underscore their pivotal role in enhancing the overall efficiency of catalytic systems.¹⁰⁵

(iv) Redox reaction sites facilitate redox reactions; TPBs serve as active sites where oxidation and reduction reactions can occur simultaneously.^106,107 For example, in processes such as water splitting, TPBs facilitate the reduction of protons to hydrogen gas and the oxidation of water to oxygen gas. Moreover, TPBs enhance the transfer of electrons to the adsorbed reactants, thereby accelerating redox reactions and significantly improving the overall photocatalytic performance. TPBs play a crucial role in enhancing the effectiveness and efficiency of photocatalytic systems.¹⁰⁸

(v) Chemical composition: The chemical composition of the TPBs plays a critical role in determining the binding strength of the reactants and intermediates, which in turn affects the reaction pathways and product selectivity. By influencing the interaction of reactants with the catalytic surface, the chemical nature of TPBs directly affects the efficiency and specificity of the catalytic processes. Understanding and manipulating the chemical composition of TPBs are essential for optimizing the reaction pathways and enhancing the selectivity of the desired products in various catalytic applications.¹⁰⁹

Therefore, understanding and optimizing TPBs are crucial for designing highly efficient photocatalytic systems that enhance reactant contact, facilitate key processes, optimize mass transfer, and provide efficient redox reaction sites.

4. Recent advances and case studies

4.1. Photo/photoelectrocatalytic N₂ reduction

Green ammonia is increasingly emerging as a key future energy carrier in the drive toward a net-zero carbon landscape. The nitrogen reduction reaction (NRR) is a multifaceted catalytic process involving several proton-coupled electron transfer steps.¹¹⁰ As a result, the efficiency of NRR pathways is often hindered by slow N₂ adsorption and activation kinetics, competing reductions with hydrogen and ammonia, and the limited capabilities of current photo(electro)catalysts. These challenges collectively limit the overall effectiveness of NRR strategies, highlighting the complexity and intricacy involved in enhancing nitrogen fixation technologies.

Recent advancements in catalyst engineering have focused on the rational control of interfacial chemistry in light-driven multiphase systems, emphasizing the development of three-phase interface catalysts as a promising approach. These catalysts can mitigate mass transfer limitations observed in conventional two-phase systems by enhancing the efficiency of the interface between the gas, liquid, and solid phases. This enhancement leads to improved N₂ adsorption and activation, facilitates better separation and migration of photogenerated charge carriers, and ultimately boosts the overall efficiency of the nitrogen reduction reaction (NRR). The unique structure of three-phase interface catalysts promotes the diffusion of reactants and products across the gas–liquid–solid interfaces, reducing diffusion resistance and accelerating reaction kinetics. Furthermore, the interface design optimizes the separation of photogenerated electrons and holes, thereby minimizing recombination losses and enhancing photocatalytic activity.

Through meticulous optimization of the catalyst's surface area and structure, more active sites for N₂ adsorption and activation are made available, resulting in heightened catalytic performance. In conclusion, the strategic engineering of three-phase interface catalysts holds great promise for advancing solar-driven NRR technology. This approach addresses inherent process challenges and advances more efficient and sustainable ammonia synthesis, thereby significantly contributing to the future landscape of green energy.^111,112

The low solubility and slow diffusion of nitrogen (N₂) in water significantly hinder the efficiency of nitrogen fixation into ammonia (NH₃). Sun et al. developed a bio-inspired hierarchical assembly, carbonized loofah sponge@BiOBr–OV/Au (CL@BiOBr–OV/Au), to enhance nitrogen diffusion by immobilizing hydrophilic BiOBr–OV/Au onto a hydrophobic CL sponge. In this case, the AuNPs generated high-energy electrons that were injected into the conduction band of BiOBr–OV, leaving holes in the valence band. The electrons reduce adsorbed N₂, whereas the holes oxidize H₂O. CL@BiOBr–OV/Au formed a triphase interface, enhancing the N₂ supply, light absorption, and nitrogen fixation efficiency through synergistic effects. This hydrophobic CL sponge established an optimized directional pathway for N₂ diffusion to the triphase interface (N₂ gas, H₂O liquid, and BiOBr–OV/Au solid), thereby providing ample N₂ for the photocatalytic reaction and mitigating the slow diffusion of N₂ through the liquid phase. The continuous supply of N₂ effectively captured the generated electrons, preventing electron–hole recombination and facilitating N₂ activation. Incorporating Au nanoparticles broadened the photoresponse range of BiOBr–OV, increased the oxygen vacancy concentration, and improved N₂ adsorption and activation. As a result, CL@BiOBr–OV/Au exhibits outstanding nitrogen fixation performance with high selectivity and recyclability, achieving 2.64 mmol gcat⁻¹ h⁻¹, which is 10.2 and 8.0 times greater than that of CL@BiOBr–OV and BiOBr–OV/Au, respectively. This study introduces a practical method for developing bio-inspired triphase catalysts with a directed N₂ transport channel to enhance the nitrogen fixation efficiency and demonstrates significant potential for other reactions involving gas consumption (Fig. 2a–e).¹¹³

Fig. 2 Schematic illustration of photocatalytic nitrogen fixation in (a) a triphase system of CL@BiOBr–OV/Au and (b) a diphase system of Loofah@BiOBr–OV/Au. (c) Photocatalytic nitrogen fixation activities of CL@BiOBr–OV/Au, loofah@BiOBr–OV/Au, BiOBr–OV/Au, CL@BiOBr–OV, CL and control experiments with loofah@BiOBr–OV/Au. (d) The recycling runs of the photocatalytic nitrogen fixation over CL@BiOBr–OV/Au. (e) Schematic illustration of the N₂ transport and triphase interface, as well as plasmonic hot electron generation, injection, and N₂ reduction processes in the photocatalytic nitrogen fixation with CL@BiOBr–OV/Au under visible light irradiation. Reprinted (adapted) with permission.¹¹³ Copyright 2022, Elsevier.

Photocatalytic NH₃ production from N₂ and H₂O at room temperature and atmospheric pressure offers a safe, clean, and sustainable method for ammonia synthesis. However, the poor solubility and slow diffusion rate of N₂ in water significantly hinder nitrogen activation efficiency. To address this issue, Fan et al. (2019) developed a photocatalyst with a hydrophilic–hydrophobic structure, creating a gas–liquid–solid triphase reaction interface. They deposited hydrophilic Bi₄O₅Br₂ onto the surface of hydrophobic ZIF-8, forming a novel Bi₄O₅Br₂/ZIF-8 composite photocatalyst. In conventional diphase systems, the photocatalyst's surface is fully wetted by the reaction solution, preventing direct contact between the catalyst and air. As a result, only dissolved N₂ is available, which is limited by its poor solubility and slow diffusion rate in water. In contrast, the triphase system supplies N₂ directly from the air phase. The hydrophilic–hydrophobic structure of Bi₄O₅Br₂/ZIF-8 creates triphase interfaces, enabling direct N₂ delivery from air to the photocatalytic reaction site, thereby improving electron–hole pair separation and nitrogen fixation performance (Fig. 3a–c). This architecture bypasses the need for N₂ to diffuse through the liquid phase, significantly enhancing the nitrogen fixation rate and the overall efficiency of the photocatalytic process.¹¹⁴ Chen et al. conducted experiments demonstrating that doping MIL-88A with transition metals enhances its responsiveness to visible light by reducing the band gap from 1.93 eV to 1.85 eV for Mn-MIL-88A and 1.78 eV for Zn-MIL-88A. Additionally, the conduction band positions of the synthesized catalysts were favorable for CO₂ reduction and N₂ fixation. The experimental results revealed efficient electron transfer in the doped catalysts, improving charge-separation efficacy and consequently enhancing photoactivity. The poor solubility and slow diffusivity of gases in the aqueous surface layer of the hydrophilic catalyst in the triphase photocatalyst system controlled the product formation rate. Detailed in situ studies identified the active reaction intermediates, helping to elucidate a plausible mechanism for the photocatalytic reactions. A systematic comparative study using manganese (Mn) and zinc (Zn) as dopants in an iron-based MOF, MIL-88A, was conducted to address the role of d-orbitals and the poor electron conductivity of MOFs. MIL-88A, a three-dimensional framework composed of iron(III) octahedral trimers linked to fumarate dianions, is known for being non-toxic, inexpensive, chemically stable, and responsive to visible light. Efficient charge transfer at the interface between the transition metals and semiconductors significantly enhances photocatalyst efficiency. The study investigated the photoactivity of Mn- and Zn-doped MIL-88A for CO₂ reduction and N₂ fixation using water under UV–vis irradiation and ambient conditions. The apparent quantum yield of Zn-MIL-88A was found to be 2.16 times higher than that of pristine MIL-88A. Furthermore, Zn-MIL-88A exhibited the highest NH₄⁺ generation rate. Analysis revealed that electrons were efficiently transferred from the doped transition metals to the Fe–O cluster of MIL-88A (Fig. 3d–f). The hydrophilic surfaces of the tested catalysts in the tri-phase photocatalyst system indicated that the process was controlled by mass transport, influencing product distribution and reaction kinetics. Experimental results also demonstrated that incorporating transition metals enhances the stability, charge separation, and overall efficiency of the photocatalysts.¹¹⁵

Fig. 3 (a) Schematic illustration of the tri-phase. (b) and (c) Photocatalytic mechanism of Bi₄O₅Br₂/ZIF-8 (30%). Reprinted (adapted) with permission.¹¹⁴ Copyright 2019, Elsevier. (d) Contact angle measurement of water on the tested catalysts. All tested catalysts are hydrophilic and can readily form an aqueous surface layer. (e) Tri-phase photocatalyst system: the schematic displays slow diffusion and poor solubility of CO₂/N₂ in water, which makes the process essentially controlled by mass-transfer, and (f) schematic illustration of charge migration mechanism in Zn-MIL-88A. Here, TM stands for transition metals (i.e., Mn and Zn) doped in MIL-88A. Under illumination, electrons transfer from O²⁻ to Fe³⁺ of pristine MIL-88A. As shown in the schematic, TM acts as the electron donor for Fe–O clusters that substantially facilitates the charge separation efficacy. Reprinted (adapted) with permission.¹¹⁵ Copyright 2021, Elsevier.

The direct plasmonic photocatalytic nitrogen fixation (P2NRR) method reported by Chen et al. converts inert N₂ into NH₃ using solar energy, in which Au nanoparticles (AuNPs) are encapsulated within porous (MOFs) membranes. Upon visible light irradiation, hot electrons generated on AuNPs are injected into N₂ molecules adsorbed on Au surfaces and activated by the localized surface plasmon resonance (LSPR) field, resulting in a supralinear intensity dependence of ammonia evolution with a higher apparent quantum efficiency and lower activation energy under stronger irradiation. The gas-permeable Au@MOF membranes with numerous interconnected nanoreactors ensured AuNP dispersion and stability, facilitated the mass transfer of N₂ molecules and (hydrated) protons, and enhanced plasmonic photocatalytic reactions at the gas–membrane–solution interface. Consequently, an ammonia evolution rate of 18.9 mmol g_Au⁻¹ h⁻¹ was achieved under visible light (>400 nm, 100 mW cm⁻²) with an apparent quantum efficiency of 1.54% at 520 nm. Unlike previous systems using plasmonic nanoparticles solely as antennas, each AuNP in the MOF matrix acts as both a photosensitizer and a cocatalyst, harvesting light and generating electrons to catalyze dinitrogen reduction. Synergistic LSPR effects, including electron transfer, energy transfer, and localized electric field polarization, activate the surface-adsorbed N₂ molecules and reaction intermediates. Porous MOF substrates such as UiO-66 stabilize and disperse AuNPs while ensuring their accessibility to N₂ molecules and (hydrated) protons. The specially designed gas–membrane–solution interface increases N₂ availability near the AuNPs, enabling efficient ammonia production under ambient conditions with high efficiency.¹¹⁶

Photocatalytic N₂ fixation provides a low-cost route to generate ammonia, although poor photon-to-ammonia conversion efficiency due to the low N₂ concentration hinders its progress. Herein, we demonstrate a photocatalyst with surface oxygen vacancies (Vo) and F-modified surfaces that facilitate N₂ adsorption and activation on the catalyst surface. The Vo sites promoted the chemical adsorption of N₂ and electron transfer from the catalyst to N₂, whereas F modification altered the TiO₂ surface properties from hydrophilic to aerophilic, enhancing N₂ adsorption (Fig. 4a). This modification suppresses the hydrogen evolution reaction (HER), as protons struggle to adsorb onto the F-capped surfaces. Consequently, the optimal NH₃ production rate reaches 206 μmol h⁻¹ g⁻¹, approximately 9 times higher than that of pure TiO₂ nanoparticles (∼23 μmol h⁻¹ g⁻¹) (Fig. 4b and c). Introducing oxygen vacancies onto the TiO₂ surface via NaBH₄ treatment creates active sites for high N₂ adsorption and activation. The modification of F changed the surface wettability to form an aerophilic surface, enriching N₂ molecules on the catalyst surface and limiting proton adsorption, thereby slowing the HER. Both experimental and theoretical calculations confirmed that oxygen vacancies enhance N₂ chemical adsorption and electron transfer, whereas F modification facilitates N₂ physical adsorption, leading to an NH₃ production rate approximately 10 times higher than that reported for TiO₂ samples. This strategy effectively overcomes mass-transfer limitations and achieves high conversion efficiency, as shown in Fig. 4(c). The detailed mechanism for photocatalytic N₂ fixation on the hydrophobic surface of treating defective TiO₂ NPs with F and Vo is shown in Fig. 4(d).¹¹⁷

Fig. 4 (a) Contact angle measurement of water on Vo–TiO₂ and F–Vo–TiO₂. (b) H₂ production rate of pristine TiO₂, Vo–TiO₂ and F–Vo–TiO₂; (c) NH₃ production rate of pristine TiO₂, Vo–TiO₂-n (n = NaBH₄ treatment time) and F–Vo–TiO₂; (d) photocatalytic N₂ fixation mechanism on the surface of hydrophobic treated defective TiO₂ NPs with F and Vo. Reprinted (adapted) with permission.¹¹⁷ Copyright 2021, Elsevier.

Lee et al. designed an advanced three-phase boundary (TPB) engineered PNRR system using oxygen-vacant TiO₂ photocatalysts embedded in poly(N-isopropyl acrylamide) (PNIPAm). The oxygen vacancies in the TiO₂ lattice generate Ti³⁺ species that function as active sites. The physicochemical properties of PNIPAm, including its porosity and hydrophilic–hydrophobic nature, change with the environmental temperature, thus adjusting the relative concentrations of N₂ and H₂O around the oxygen-vacant TiO₂ photocatalysts for an efficient TPB. TiO₂ with PNIPAm exhibited a significantly high NH₃ production rate (11.12 μmol g⁻¹ h⁻¹) at the lower critical solution temperature (LCST; 32 °C). This study is the first to combine the defect engineering of TiO₂ photocatalysts and TPB engineering using a thermoresponsive polymeric hydrogel for effective PNRRs. Experimental results indicated that the Sollen-collapsed transition of PNIPAm controls the mass transfer of reactants (N₂ and H₂O) around the photocatalyst, with TiO₂ Vo@PNIPAm showing reversible switching of its surface characteristic from hydrophilic to hydrophobic depending on the temperature. This transition changed the pore size of the PNIPAm network, thereby enhancing NH₃ production. At the LCST, TiO₂ Vo@PNIPAm exhibited a hydrophobic contact angle (CA) of 110°, compared to a hydrophilic CA of 66° below the LCST and a predominantly hydrophobic CA of 130° above the LCST. These temperature-dependent changes in PNIPAm's physicochemical properties increase the relative concentration of N₂ around the photocatalyst, forming abundant TPBs for enhanced NH₃ production.⁵⁵

Sun et al. suggested modifying defective TiO₂ surfaces with alkyl acids with different carbon chain lengths (C2, C5, C8, C11, and C14) to adjust the surface properties of the catalyst. The presence of defect sites notably boosts N₂ adsorption and activation, whereas the wettability of the catalyst can shift from hydrophilic to hydrophobic depending on the length of the alkyl chain. A hydrophobic surface enhances N₂ adsorption and local concentration owing to its aerophilic characteristics, and simultaneously decreases proton adsorption, thereby reducing the HER. This combined effect significantly advanced the nitrogen reduction reaction (NRR). Among the modified samples, n-octanoic acid-defective TiO₂ (C8–Vo–TiO₂; Vo = oxygen vacancy) demonstrated the best NRR performance, with an ammonia production rate reaching up to 392 μmol g⁻¹ h⁻¹. This paper presents a novel approach for efficient ammonia synthesis at a three-phase interface using photocatalyst technology.¹¹⁸

Photoelectrochemical nitrogen reduction reaction (PEC NRR) is a gas-consuming reaction in which the diffusion rate of N₂ molecules significantly affects the reaction efficiency. Traditional two-phase NRR catalytic systems reduce N₂ molecules dissolved in the electrolyte to NH₃. However, owing to the low solubility of N₂ and slow diffusion in the electrolyte, there is an insufficient supply of N₂ on the catalyst surface. Additionally, a high concentration of H⁺ in the liquid phase enhances the hydrogen evolution reaction (HER), thereby limiting the NRR efficiency. A feasible strategy for enhancing the NRR is to transfer gaseous N₂ directly to the photoelectrochemical reaction interface via a hydrophobic surface, thereby increasing the N₂ concentration and reducing the H⁺ concentration at the reaction center. The hydrophobic interface provides an efficient three-phase reaction interface for N₂ (gas), the electrolyte (liquid), and the catalyst (solid). Nanocomposites (SNP/MSO) were prepared by loading hydrophilic SnO₂ nanoparticles (SNP) onto hydrophobic MoS₂ (MSO) surface-modified with silicomolybdic acid. This hydrophobic interface eliminates the N₂ transport barrier in the liquid phase to the reaction center, inhibits the HER by lowering the contact area with liquid protons, and accelerates effective electron transfer through the heterojunction structure formed between MSO and SNP. This synergistic effect results in a high NH₃ yield (19.6 μg h⁻¹ mg⁻¹) and a faradaic efficiency (FE) of 40.34% for SNP/10MSO in the PEC system. Compared with the nanocomposites (SNP/MSI) prepared using hydrophilic MoS₂, the FE increased by 10 percentage points, demonstrating the effectiveness of the hydrophobic interface and heterojunction structure in enhancing the PEC NRR efficiency, as shown in Fig. 5(a–c).¹¹⁹

Fig. 5 Schematic diagram of the three-phase reaction interface of N₂, electrolyte and catalyst (a) hydrophilic interface and (b) hydrophobic interface. (c) Schematic diagram of the SNP/MSO heterostructure PEC NRR mechanism. Reprinted (adapted) with permission.¹¹⁹ Copyright 2022, Elsevier.

4.2. Photo/photoelectrocatalytic O₂ reduction

H₂O₂ is widely used as a clean oxidant in various industries such as organic synthesis, water treatment, and pulp bleaching. Photocatalysis offers an eco-friendly method for H₂O₂ production, in which photogenerated electrons reduce O₂ to form H₂O₂ and the holes oxidize water or other hole scavengers, such as ethanol or formic acid. Traditionally, this reaction occurs at the solid–liquid interface, where O₂ must diffuse from the liquid phase, which has a low concentration and diffusion coefficient. This limited interfacial O₂ concentration often leads to significant recombination of photogenerated electron–hole pairs, resulting in low electron utilization efficiency and hindering of the efficiency of the reaction under high light intensity. To address this issue, conducting photocatalytic reactions at a gas–liquid–solid interface is a rational solution. The O₂ concentration in the gas phase at this tri-phase interface is much higher than that in water, and O₂ can be delivered to the catalyst surface at a diffusion rate significantly higher than that in water. This setup facilitates rapid O₂ reduction, overcomes the sluggish O₂ supply in the aqueous phase, and promotes the separation of electron–hole pairs by forming reactive oxygen species, thus enhancing the photocatalytic kinetics. Additionally, O₂ from air can reach the reaction interface through an atmosphere-connected porous substrate without the need for aeration support. Therefore, photocatalysts with porous structures, chemical stabilities, and hydrophobic properties are ideal candidates for the formation of stable triphase boundaries and efficient air-diffusion channels.¹²⁰

Au nanoparticle-decorated TiO₂ (Au–TiO₂) has garnered attention as a visible-light-active photocatalyst, although its efficiency has been constrained, particularly under visible-light irradiation. Feng et al. introduced an effective triphasic photocatalytic system for visible-light-driven H₂O₂ synthesis using Au–TiO₂.¹²¹ This system operates at an air–liquid–solid triphasic interface, allowing for a direct and ample supply of O₂ from the ambient atmosphere, which significantly increases its concentration at the reaction site. This triphasic photocatalytic system enhances reaction kinetics and results in a higher steady-state H₂O₂ concentration. Remarkably, only one-tenth of the light intensity is required to achieve a comparable H₂O₂ formation rate to that of conventional diphasic systems (Fig. 6a and b). This triphasic approach provides a novel platform for exploring intrinsic photocatalyst kinetics and plasmonic mechanisms. The triphase system allows O₂ to diffuse directly from the air phase to the reaction interface, rather than through slow diffusion in the liquid phase. This interface architecture significantly increases the photocatalyst's accessibility to O₂, enhancing the reaction rate between O₂ and photogenerated electrons, suppressing electron–hole recombination, increasing charge utilization efficiency, and reducing degradation reactions between H₂O₂ and photogenerated electrons, thereby greatly enhancing H₂O₂ production rates. Inspired by nature, the same group fabricated superhydrophobic substrates by utilizing the cooperative effect of low surface energy and rough surface structure. They constructed a triphase photocatalytic interface architecture by immobilizing Au-decorated TiO₂ nanoparticles onto a polytetrafluoroethylene-treated superhydrophobic porous membrane composed of carbon fiber. The interface environment generally governs the performance of interfacial catalytic reactions. The interfacial environment plays a critical role in the performance of interfacial catalytic reactions. In this study, the authors overcome these limitations by introducing a reaction system that utilizes an air–liquid–solid triphase interface to optimize the conditions and thereby significantly enhance the catalytic activity (Fig. 6c–f).¹⁰⁰

Fig. 6 (a) Schematic representation of the triphase system for plasmonic photocatalytic synthesis of H₂O₂. (b) Enlarged view of the reaction zone with an air–liquid–solid triphase interfacial microenvironment where reactant O₂ is available directly from the air phase. Based on such a triphasic system, the enhanced accessibility of electron acceptor O₂ to the catalysts can effectively suppress the recombination of photo-generated charge carriers and boost the photocatalytic reaction. Reprinted (adapted) with permission.¹²¹ Copyright 2021, Elsevier. (c) Rate of H₂O₂ formation under different light intensities using triphase (red line) and diphase (dark line) systems. (d) H₂O₂ formation rates based on different operational conditions using these two systems under illumination of 60 mW cm⁻². (e) IAQYs for the two systems under 1 mW cm⁻², 9 mW cm⁻², and 60 mW cm⁻² illumination. Data of IAQYs for 1, 9, and 60 mW cm⁻² illumination are 17.29 ± 0.44, 15.23 ± 0.57, and 5.49 ± 0.21, respectively, in the triphase system and 2.45 ± 0.64, 0.55 ± 0.08, and 0.08 ± 0.009, respectively, in the diphase system. (f) Schematic illustration of the H₂O₂ formation rates based on different operational conditions (i, ii, iii, and iv). Reprinted (adapted) with permission.¹⁰⁰ Copyright 2019, Elsevier.

The triphenylbenzene–dimethoxyterephthaldehyde covalent organic framework (TPB–DMTP-COF) demonstrated high activity for H₂O₂ generation by reducing O₂ in pure water under visible light irradiation. Importantly, a triphase reaction interface was created by exploiting the hydrophobic nature of this polymeric semiconductor, which was achieved by immobilizing TPB–DMTP-COF onto porous carbon fiber paper. Owing to the hydrophobic nature of the TPB–DMTP-COF immobilized on the porous carbon fiber paper, a triphase reaction interface was created. This configuration achieved a record H₂O₂ generation rate of 2882 μmol gcat⁻¹ h⁻¹, surpassing diphasic systems by 15 times. Enhanced mass transfer of O₂ through the porous carbon support elevated O₂ levels at the triphase interface, facilitating improved separation of charge carriers (holes and electrons) and enhancing photocatalytic kinetics. TPB–DMTP-COF operates effectively across a wide pH range without sacrificial agents and maintains long-term photochemical stability. Density functional theory calculations identified the triphenylbenzene moiety as the primary site for O₂ activation and electron transfer. This COF-based photocatalyst achieved a solar-to-chemical conversion efficiency of 0.76% for H₂O₂ generation. Its hydrophobic and mesoporous properties enable the formation of a gas–liquid–solid triphase interface, boosting H₂O₂ production to approximately 2.9 mmol gcat⁻¹ h⁻¹ by mitigating O₂ mass-transfer limitations in water. Theoretical and photoelectrochemical analyses confirmed that the increased yield stems from enhanced mass transfer and higher interfacial O₂ concentration, validating the triphenylbenzene component as the reactive site (Fig. 7a–g).¹²²

Fig. 7 Photocatalytic O₂ reduction in a triphase system. (a) Schematic illustration of the diphase and triphase photocatalytic conditions. Photocatalytic H₂O₂ generation in a (i) diphase system (pure water presaturated with oxygen) and in a (ii) triphase system. The local oxygen concentration at the catalyst surface for the (iii) diphase and (iv) triphase reaction systems. The scale bar indicates the oxygen concentration in the reaction system. (b) The SEM image of polytetrafluoroethylene (PTFE)-treated superhydrophobic carbon paper substrate loaded with TPB–DMTP-COF catalyst. (c) The inset of (b) is the photograph of water droplets placed on CFP loaded with COF catalyst. (d) Time course of H₂O₂ production under different reaction conditions under visible light irradiation in pure water. Theoretical calculations. (e) Decomposed charge density isosurface for the valence band (VB) edge and conduction band (CB) edges. (f) Structure of O₂-COF before activation. (g) Structure of O₂-COF after activation. Carbon atoms are yellow, oxygen atoms are red, hydrogen atoms are white, and nitrogen atoms are green. Reprinted (adapted) with permission.¹²² Copyright 2021, John Wiley and Sons.

Recently, Lou et al. designed a two-layered (2L) Janus fiber membrane photocatalyst with asymmetric hydrophobicity for efficient H₂O₂ production. The top layer consists of superhydrophobic PTFE fibers with dispersed modified carbon nitride (mCN) photocatalyst, onto which an amphiphilic Nafion (Naf) ionomer was sprayed to achieve moderate hydrophobicity, featuring both hydrophobic –CF2 and hydrophilic –SO₃⁻ groups (Fig. 8a). The bottom layer consists of highly hydrophobic PTFE fibers, while the top layer modulates its hydrophobicity. This configuration ensured that most of the mCN was exposed at the gas–liquid–solid interfaces, enhancing the mass transfer of gaseous O₂ within the membrane and increasing the local O₂ concentration near the mCN photocatalyst (Fig. 8b and c). Consequently, the optimized 2 L-mCN/F-Naf membrane achieved a remarkable H₂O₂ production rate of 5.38 mmol g⁻¹ h⁻¹ under visible light irradiation.¹²³

Fig. 8 (a) Schematic illustration of the synthesis route for the Janus 2L-mCN/F-Naf fiber membrane. Note that the aluminum (Al) foil substrate supporting the membrane is omitted for clarity. (b) Schematic illustration of the two-layered structure of the 2L-mCN/F-Naf fiber membrane and its advantageous features. (c) Schematic illustration of TPI in the 2L-mCN/F-Naf fiber membrane for photocatalytic O₂ reduction to H₂O₂. Reprinted (adapted) with permission.¹²³ Copyright 2024, American Chemical Society.

Developing high-performance bio-photoelectrochemical (bio-PEC) assay systems requires bio-photoelectrodes with rapid charge- and gas-phase mass-transport capabilities. Chen et al. demonstrated a solid–liquid–air triphase bio-photoelectrode (TBP-electrode) by immobilizing glucose oxidase (GOx), a model enzyme, onto one-dimensional single-crystalline TiO₂ nanowire arrays grown on a superhydrophobic carbon textile substrate. This triphase configuration allows continuous oxygen supply from the air phase to enhance enzymatic reactions, thereby improving oxidase kinetics. In addition, the photogenerated electrons are rapidly transported and efficiently collected along the TiO₂ nanowire arrays. Our TBP electrode system showed a detection range 100 times greater than that of conventional diphase bio-photoelectrodes, with sensitivity over 10 times higher and detection limits 30 times lower than those of nanoparticle-based TBP-electrodes. The bio-PEC assay operates at a low potential (+0.1 V vs. Ag/AgCl), approximately 0.5 V lower than traditional electrochemical systems, making it resistant to many easily oxidizable interferences. This triphase bio-PEC assay system presents a powerful platform for environmental analysis and clinical applications, with its design principles broadly applicable for fabricating other high-performance triphase bio-photoelectrodes.¹²⁴

4.3. Photo/photoelectrocatalytic water reduction

Solar water splitting provides a sustainable and environmentally benign route for the production of a storable chemical fuel in the form of clean hydrogen from sunlight, and has attracted tremendous research interest since the report by Fujishima and Honda in 1972.¹²⁵ Superhydrophilic materials significantly enhance photocatalytic water reduction by promoting efficient water interactions and improving the establishment of the TPB between the solid catalyst, liquid water, and gaseous products. This optimized TPB facilitated enhanced charge separation and transfer, leading to improved reaction kinetics and overall photocatalytic performance. Padmanabhan et al. reported the morphology-dependent spatial separation of charge carriers in TiO₂ and graphene hybrids were synthesized using a tailor-made hydrothermally modified sol–gel method. TiO₂/graphene hybrids were created at various weight percentages of graphene oxide (GO), with TiO₂ growing in situ while GO was simultaneously reduced to reduced graphene oxide (rGO). The hybrids were analyzed for photocatalytic H₂ generation, photodegradation under UV and visible-light irradiation, and photoinduced superhydrophilicity. The spatial charge separation of photogenerated electron–hole pairs in TiO₂ was achieved through exposed crystal facets with different surface potentials, which play a critical role in the TPB between the solid, liquid, and gas phases. The high-energy {010}/{100} and {001} facets of TiO₂ endowed the hybrids with self-cleaning properties, whereas an optimal mix of these high-energy and low-energy {101} facets enhanced H₂ production.¹²⁶

Remarkably, a mere 0.1% w/w graphene loading significantly boosted the photoactivity of the hybrids by establishing a chemical linkage between the {010} facet of TiO₂ and the graphene layers, thus further enhancing the TPB and promoting effective charge separation and hydrogen evolution.¹²⁵

A 2D/2D S-scheme NiTe₂/g-C₃N₄ heterojunction was successfully synthesized using an in situ deposition strategy with g-C₃N₄ nanosheets as the substrate. In this case, the TPB plays a crucial role in enhancing catalytic performance, particularly in photocatalytic hydrogen evolution reactions. The 2D contact interface between NiTe₂ and g-C₃N₄ provides an ideal platform for TPB formation. TPBs facilitate the migration of photogenerated carriers and subsequent redox reactions. Pristine g-C₃N₄ and NiTe₂ exhibit WCAs of 8.5° and 121.1°, respectively, highlighting the excellent hydrophilicity of g-C₃N₄. However, the WCA of the NiTe₂/g-C₃N₄ composite increases to 149.6°, approaching superhydrophobicity, likely owing to the influence of organic solvents during the in situ NiTe₂ growth. After mercaptopropionic acid treatment, the WCA of the composite decreases to approximately 0°, indicating the superior superhydrophilicity of NiTe₂/g-C₃N₄. Improved hydrophilicity increases the contact area between the liquid phase (water) and solid catalyst surface, thereby optimizing the surface proton reduction reactions occurring at the TPB. This enhanced hydrophilicity promotes selective contact between reactant molecules and catalytic active sites, thereby boosting hydrogen evolution reaction (HER) activity. Moreover, S-scheme charge transfer minimized carrier recombination while maximizing redox potential. Enhanced hydrophilicity further improves H₂O adsorption on the catalyst surface, optimizes surface redox kinetics, and boosts the photocatalytic water-splitting performance. Therefore, this study presents a novel strategy for advancing surface redox kinetics and developing efficient and stable 2D/2D semiconductor heterojunction photocatalysts.¹²⁷

Despite its potential as a photocatalyst, pristine graphitic carbon nitride (g-C₃N₄) suffers from limited light absorption, insolubility, small specific surface area, and rapid electron–hole pair recombination. This study addresses these issues by developing a hydroxyl-grafted oxygen-linked tri-s-triazine-based polymer (HGONTP) through the polycondensation of hydrothermally pretreated dicyandiamide (DCDA) by the Luo group. By adjusting the degree of cyclization and hydrolysis of DCDA, the content of C–O–C linkers and terminal OH groups in the HGONTP could be controlled by replacing the pendant NH₂ groups with OH groups. The resulting oxygen species of the HGONTP photocatalyst affected the hydrophilicity and facilitated the photoexcited separation of charge carriers. Moreover, HGONTP exhibits exceptional light absorption from UV to near-IR, a narrow band gap of 2.18 eV, a large specific surface area of 96.1 m² g⁻¹, and reduced charge recombination. Consequently, HGONTP demonstrates a hydrogen evolution rate 27.7 times higher than that of pristine g-C₃N₄ (6.54 versus 0.236 mmol g⁻¹ h⁻¹), with apparent quantum yields of 12.6% at 420 nm and 4.1% at 500 nm. This innovative approach opens new avenues for the development of a HGONTP, enhances hydrophilicity, and facilitates the formation of a three-phase boundary (TPB) that significantly improves the separation of photoexcited charge carriers and promotes hydrogen evolution reaction (HER) efficiency.¹²⁸ An optimal reaction environment at the water–catalyst interface is essential for high-efficiency semiconductor-based water splitting. Traditionally, the hydrophilic surface of a semiconductor catalyst has been considered crucial for effective mass transfer and proper water contact. Recently, Jiang et al. developed a superhydrophobic PDMS–Ti³⁺/TiO₂ (P–TTO) interface with nanochannels formed by nonpolar silane chains that significantly enhanced the TPB interactions. Remarkably, the P–TTO interface achieved a tenfold increase in water-splitting efficiency under both white light and simulated AM1.5G solar irradiation compared with the hydrophilic Ti³⁺/TiO₂ (TTO) interface. Additionally, the electrochemical water splitting potential on the P–TTO electrode decreased from 1.62 V to 1.27 V, approaching the thermodynamic limit of 1.23 V. In situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) detected a nanochannel-induced water configuration transition, and density functional theory (DFT) calculations confirmed the lower reaction energy at the water/PDMS–TiO₂ interface. This work demonstrates that efficient overall water splitting can be achieved through nanochannel-induced water configurations without altering the bulk properties of the semiconductor catalyst, highlighting the significant impact of the interfacial water status on the water-splitting reaction efficiency. The P–TTO catalyst, with its superhydrophobic surface, showed over 13.5 times improvement under white light and 8.7 times under AM1.5G simulated solar irradiation, with reduced reaction potentials for both the oxygen evolution reaction (OER) and overall water splitting, compared to untreated TTO.¹²⁹

A novel high-performance photoelectrochemical (PEC) water oxidation photoanode material, a Co-nanoparticle-modified Ti₃C₂ MQD (Co-MQD) Schottky catalyst, was rationally designed, demonstrating significantly enhanced PEC water oxidation performance.¹³⁰ The Co content in the Co-MQD product could be efficiently controlled by optimizing the ion thermal anchoring temperature of the initial cobalt precursor. The surface plasmon effect introduced by the cobalt terminals broadened the optical response properties of the Co-MQDs. Acting as water oxidation reaction centers, the cobalt terminals effectively accelerated the surface water oxidation kinetics of the pristine MQD. Moreover, the Schottky junction introduced by the Janus Co-MQDs enhances the photogenerated carrier separation and injection efficiency. Consequently, the Co-MQD-48 Schottky-catalyst exhibits a boosted photocurrent density of 2.99 mA cm⁻² and a carrier migration efficiency of 87.56%, representing improvements of 194% and 236%, respectively, compared to the pristine MQD. After 10 hours of cycling, a photocurrent density of 2.79 mA cm⁻² is maintained, confirming the good photostability of Co-MQD. This outstanding PEC performance is among those of the state-of-the-art photoanode materials. The Janus structure photocatalysts, characterized by their dual-sided morphology with distinct chemical or physical properties, play pivotal roles in optimizing TPBs during photocatalytic reactions as well as improving mass transfer and charge separation for photoenergy conversion applications.

5. Challenges and future perspectives

Controlling TPBs in photocatalytic systems presents several challenges that directly affect catalytic efficiency. One of the primary difficulties lies in the precise engineering of the TPB interfaces to ensure optimal contact among the solid catalyst, liquid reactant, and gaseous reactant. Achieving a uniform distribution and stable maintenance of these interfaces is crucial for maximizing the active surface area and facilitating efficient charge transfer. The inherent complexity of multi-phase interactions often leads to issues such as poor wettability, agglomeration of catalysts, and formation of non-uniform active sites, all of which can significantly reduce catalytic performance.

Another challenge is the stability of the TPBs under operational conditions. Photocatalytic processes, particularly those involving nitrogen reduction, oxygen reduction, and water splitting, often require harsh environments such as high temperatures, extreme pH values, and intense illumination. These conditions can degrade the catalyst and disrupt the integrity of the TPB, leading to decreased efficiency and shorter operational lifetimes. Moreover, the dynamic nature of photocatalytic reactions can cause continuous changes in the TPB configuration, which complicates the control and optimization of these systems.

Several future research directions can be explored to address the challenges in enhancing TPB performance. One promising avenue is the development of advanced materials with tailored surface properties to promote stable and uniform TPBs. For instance, the incorporation of superwetting materials, which we previously reviewed in the context of organic pollutant degradation, could be extended to TPB applications. Potential innovations include superwettability-patterned, superwettability-responsive, Janus (super)wettable, slippery wettable, and superamphiphobic photocatalysts. These advanced materials offer promising capabilities for TPB-related applications and can significantly enhance catalytic performance through improved surface interactions.^131–136 These materials significantly improve wettability and ensure better contact between different phases, which enhances overall photocatalytic efficiency. Another important research avenue is the use of nanostructured catalysts. Nanostructures offer high surface-area-to-volume ratios and can be engineered to possess specific surface properties conducive to stable TPB formation. For example, the design of catalysts with hierarchical structures can facilitate the simultaneous presence and interaction of all three phases at the nanoscale, leading to more efficient charge separation and transfer.

Integrating advanced characterization techniques and computational modeling can provide deeper insights into the TPB dynamics and guide the rational design of more effective photocatalytic systems. In situ characterization methods can be used to monitor the real-time behavior of TPBs under operational conditions, revealing critical information about phase interactions and catalyst stability. Coupled with computational models, these insights can help predict optimal TPB configurations and identify key parameters for enhancing the catalytic performance.

Moreover, the integration of advanced reaction devices that optimize the interactions between the solid, liquid, and gas phases is essential to maximizing the potential of TPBs in photocatalysis. Innovative designs, such as structured photoreactors and flow reactors, facilitate enhanced mass transfer and improved light penetration, which increases the effectiveness of TPBs. For example, microfluidic devices can create controlled environments that enable the precise manipulation of dynamics at the TPB, resulting in significantly improved reaction rates and selectivities.^137–139 Additionally, the incorporation of novel materials, such as nanostructured photocatalysts with maximized surface areas, can further increase the availability of TPBs.¹⁴⁰ This strategy not only addresses current challenges in photocatalytic efficiency but also lays the groundwork for future advancements in sustainable energy and environmental remediation technologies. The development of such reaction devices is critical for overcoming the existing limitations and optimizing the roles of TPBs in photocatalytic processes.

Finally, exploring hybrid systems that combine photocatalysis with other catalytic processes, such as electrocatalysis or biocatalysis, could offer synergistic effects and new pathways for improving the TPB efficiency. Such hybrid approaches can leverage the strengths of different catalytic mechanisms, leading to more robust and versatile systems capable of operating under a wider range of conditions.

By addressing the current challenges and pursuing future research directions, significant advancements can be made in controlling TPBs, ultimately leading to more efficient and scalable photocatalytic applications. This will not only enhance our understanding of photocatalytic processes but also contribute to the development of sustainable energy solutions and environmental remediation technologies.

6. Conclusions

This review emphasizes the critical roles of TPBs in enhancing the photo/photoelectron catalytic efficiencies of processes such as nitrogen reduction, oxygen reduction, and water splitting. The key strategies for optimizing TPBs include the use of advanced materials with tailored surface properties, nanostructured catalysts, and hybrid catalytic systems. In situ characterization techniques and computational modeling provide valuable insights into the dynamic behavior of TPBs, and guide the design of more effective photocatalytic systems.

Despite the challenges of maintaining stable and uniform TPBs under operational conditions, these advancements present promising pathways for improving the catalytic performance. Understanding and controlling TPBs are critical for advancing photo/photoelectrocatalytic processes, particularly nitrogen reduction, oxygen reduction, and water splitting. Control of TPB dynamics can significantly enhance the catalytic efficiency by optimizing the interactions between the solid catalyst, liquid reactant, and gaseous reactant. Despite the challenges posed by the complexity and stability of TPBs under operational conditions, innovative material designs, nanostructuring, and advanced characterization techniques offer promising pathways to overcome these obstacles. Future research focused on these strategies could lead to substantial improvements in photocatalytic performance, paving the way for more efficient and scalable applications in sustainable energy production and environmental remediation. Understanding and controlling TPBs not only advance the fundamental science of photocatalysis but also drive technological innovations essential for addressing global energy and environmental challenges.

Author contributions

Lagnamayee Mohapatra: conceptualization and writing – original draft preparation; Lekha Paramanik: review and editing. Subhashree Sabnam: review and editing; Seung Hwa Yoo: conceptualization, review & editing, project administration, and funding acquisition.

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

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) (NRF-2022R1I1A3064533, RS-2023-00221529) and Commercialization Promotion Agency for R&D Outcomes (COMPA) grant funded by the Korean Government (Ministry of Science and ICT) (RS-2023-00304743).

References

1. D. Gunawan, J. Zhang, Q. Li, C. Y. Toe, J. Scott, M. Antonietti, J. Guo and R. Amal, Materials advances in photocatalytic solar hydrogen production: Integrating systems and economics for a sustainable future, Adv. Mater., 2024, 2404618.
2. L. Mohapatra and K. Parida, A review on the recent progress, challenges and perspective of layered double hydroxides as promising photocatalysts, J. Mater. Chem. A, 2016, 4, 10744–10766.
3. M. Li, F. Wang and Z. Guo, The fabrication and application of a triphasic reaction interface based on superwettability for improved reaction efficiency, J. Mater. Chem. A, 2024, 12, 2561–2582.
4. Y. Tang, Z. Qin, Y. Zhong, S. Yin, S. Liang and H. Sun, Three-phase interface photocatalysis for the enhanced degradation and antibacterial property, J. Colloid Interface Sci., 2022, 612, 194–202.
5. S.-S. Liu, L. C. Saha, A. Iskandarov, T. Ishimoto, T. Yamamoto, Y. Umeno, S. Matsumura and M. Koyama, Atomic structure observations and reaction dynamics simulations on triple phase boundaries in solid-oxide fuel cells, Commun. Chem., 2019, 2, 48.
6. M. Zhu, H. Ge, X. Xu and Q. Wang, Investigation on the variation law of gas liquid solid three phase boundary in porous gas diffusion electrode, Heliyon, 2018, 4, e00729.
7. W. Xu, Z. Lu, X. Sun, L. Jiang and X. Duan, Superwetting electrodes for gas-involving electrocatalysis, Acc. Chem. Res., 2018, 51, 1590–1598.
8. Y. Wu, J. Feng, H. Gao, X. Feng and L. Jiang, Superwettability-based interfacial chemical reactions, Adv. Mater., 2019, 31, 1800718.
9. K. Liu, M. Cao, A. Fujishima and L. Jiang, Bio-inspired titanium dioxide materials with special wettability and their applications, Chem. Rev., 2014, 114, 10044–10094.
10. H. Zhu, S. Cai, G. Liao, Z. F. Gao, X. Min, Y. Huang, S. Jin and F. Xia, Recent advances in photocatalysis based on bioinspired superwettabilities, ACS Catal., 2021, 11, 14751–14771.
11. M. Liu, S. Wang and L. Jiang, Nature-inspired superwettability systems, Nat. Rev. Mater., 2017, 2, 17036.
12. Y. Wei, F. Wang and Z. Guo, Bio-inspired and metal-derived superwetting surfaces: Function, stability and applications, Adv. Colloid Interface Sci., 2023, 314, 102879.
13. Z. Zhu, S. Zheng, S. Peng, Y. Zhao and Y. Tian, Superlyophilic interfaces and their applications, Adv. Mater., 2017, 29, 1703120.
14. R. S. Sutar, X. Wu, S. S. Latthe, B. Shi, R. Xing and S. Liu, Efficient separation of oil-water emulsions: Competent design of superwetting materials for practical applications, J. Environ. Chem. Eng., 2023, 11, 111299.
15. L. Mohapatra and S. H. Yoo, Superwetting materials as catalysts in photocatalysis: State-of-the-Art review, J. Chem. Eng., 2024, 481, 148537.
16. X. Yao, Q. Cheng, X. Bai, B. Davaasuren, G. Melinte, N. Morlanes, J. L. Cerrillo, V. K. Velisoju, H. O. Mohamed, P. D. Kolubah and L. Zheng, Enlarging the three-phase boundary to raise CO₂/CH₄ conversions on exsolved Ni–Fe alloy perovskite catalysts by minimal Rh doping, ACS Catal., 2024, 14, 5639–5653.
17. Y.-C. Wang, W. Huang, L.-Y. Wan, J. Yang, R.-J. Xie, Y.-P. Zheng, Y.-Z. Tan, Y.-S. Wang, K. Zaghib, L.-R. Zheng and S. H. Sun, Identification of the active triple-phase boundary of a non-Pt catalyst layer in fuel cells, Sci. Adv., 2022, 8, eadd8873.
18. M. Shishkin and T. Ziegler, Direct modeling of the electrochemistry in the three-phase boundary of solid oxide fuel cell anodes by density functional theory: a critical overview, Phys. Chem. Chem. Phys., 2014, 16, 1798–1808.
19. J. Xu, X. Xiao, Z. Zhang, Y. Wu, D. T. Boyle, H. K. Lee, W. Huang, Y. Li, H. Wang, J. Li and Y. Zhu, Designing a nanoscale three-phase electrochemical pathway to promote Pt-catalyzed formaldehyde oxidation, Nano Lett., 2020, 20, 8719–8724.
20. Z. Cai, Y. Zhang, Y. Zhao, Y. Wu, W. Xu, X. Wen, Y. Zhong, Y. Zhang, W. Liu, H. Wang, Y. Kuang and X. Sun, Selectivity regulation of CO₂ electroreduction through contact interface engineering on superwetting Cu nanoarray electrodes, Nano Res., 2019, 12, 345–349.
21. J. Zhang, X. Sheng, J. Jin, X. Feng and L. Jiang, High performance metal oxide based sensing device using an electrode with a solid/liquid/air triphase interface, Nano Res., 2017, 10, 2998–3004.
22. Y. Wu, J. Feng, H. Gao, X. Feng and L. Jiang, Superwettability-based interfacial chemical reactions, Adv. Mater., 2019, 31, 1800718.
23. Z. Lu, W. Zhu, X. Yu, H. Zhang, Y. Li, X. Sun, X. Wang, H. Wang, J. Wang, J. Luo, X. Lei and L. Jiang, Ultrahigh hydrogen evolution performance of under-water “superaerophobic” MoS₂ nanostructured electrodes, Adv. Mater., 2014, 26, 2683–2687.
24. Y. Li, H. Zhang, T. Xu, Z. Lu, X. Wu, P. Wan, X. Sun and L. Jiang, Under-water superaerophobic pine-shaped Pt nanoarray electrode for ultrahigh-performance hydrogen evolution, Adv. Funct. Mater., 2015, 25, 1737–1744.
25. C. Chen, L. Fei, L. Lu, B. Li, S. Raza, L. Shen and H. Lin, Superwetting graphene-based materials: From wettability regulation to practical applications, Mater. Today Chem., 2023, 29, 101452.
26. D. Aebisher, D. Bartusik, Y. Liu, Y. Zhao, M. Barahman, Q. Xu, A. M. Lyons and A. Greer, Superhydrophobic photosensitizers, Mechanistic studies of ¹O₂ generation in the plastron and solid/liquid droplet interface, J. Am. Chem. Soc., 2013, 135, 18990–18998.
27. Z. Jin, H. Mei, L. Pan, H. Liu and L. Cheng, Superhydrophobic self-cleaning hierarchical micro-/nanocomposite coating with high corrosion resistance and durability, ACS Sustainable Chem. Eng., 2021, 9, 4111–4121.
28. S. Parvate, P. Dixit and S. Chattopadhyay, Superhydrophobic surfaces: Insights from theory and experiment, J. Phys. Chem. B, 2020, 124, 1323–1360.
29. R. Shi, L. Shang, C. Zhou, Y. Zhao and T. Zhang, Interfacial wettability and mass transfer characterizations for gas–liquid–solid triple-phase catalysis, Exploration, 2022, 2, 20210046.
30. K. Wang and M. Pera-Titus, Microstructured gas-liquid-(solid) interfaces: A platform for sustainable synthesis of commodity chemicals, Sci. Adv., 2024, 10, eado5448.
31. E. Bormashenko, T. Stein, G. Whyman, Y. Bormashenko and R. Pogreb, Wetting properties of the multiscaled nanostructured polymer and metallic superhydrophobic surfaces, Langmuir, 2006, 22, 9982–9985.
32. X. Deng, L. Mammen, H.-J. Butt and D. Vollmer, Candle soot as a template for a transparent robust superamphiphobic coating, Science, 2012, 335, 67–70.
33. J. Lee, S.-H. Hwang, S.-S. Yoon and D.-Y. Khang, Evaporation characteristics of water droplets in Cassie, Wenzel, and mixed states on superhydrophobic pillared Si surface, Colloids Surf., A, 2019, 562, 304–309.
34. M. S. Bobji, S. V. Kumar, A. Asthana and R. N. Govardhan, Underwater sustainability of the “Cassie” state of wetting, Langmuir, 2009, 25, 12120–12126.
35. F. Huang, B. Motealleh, D. Wang and C. J. Cornelius, Tailoring intrinsic hydrophobicity and surface energy on rough surface via low-T Cassie–Wenzel wetting transition method, AIChE J., 2023, 69, e17908.
36. M. T. Rauter, S. K. Schnell and S. Kjelstrup, Cassie–Baxter and Wenzel States and the effect of interfaces on transport properties across membranes, J. Phys. Chem. B, 2021, 125, 12730–12740.
37. X. Zhong, S. Xie and Z. Guo, The challenge of superhydrophobicity: Environmentally facilitated Cassie–Wenzel transitions and structural design, Adv. Sci., 2024, 11, 2305961.
38. R. Shi, L. Shang and T. Zhang, Three phase interface engineering for advanced catalytic applications, ACS Appl. Energy Mater., 2021, 4, 1045–1052.
39. Y. Zhao, Y. Liu, Q. Xu, M. Barahman, D. Bartusik, A. Greer and A. M. Lyons, Singlet oxygen generation on porous superhydrophobic surfaces: effect of gas flow and sensitizer wetting on trapping efficiency, J. Phys. Chem. A, 2014, 118, 10364–10371.
40. Y. Zhao, Y. Liu, Q. Xu, M. Barahman and A. M. Lyons, Catalytic, self-cleaning surface with stable superhydrophobic properties: Printed Polydimethylsiloxane (PDMS) arrays embedded with TiO₂ nanoparticles, ACS Appl. Mater. Interfaces, 2015, 7, 2632–2640.
41. L. Nurmi, K. Kontturi, N. Houbenov, J. Laine, J. Ruokolainen and J. Seppälä, Modification of surface wettability through adsorption of partly fluorinated statistical and block polyelectrolytes from aqueous medium, Langmuir, 2010, 26, 15325–15332.
42. B. Petrovic, M. Gorbounov and S. Masoudi Soltani, Impact of surface functional groups and their introduction methods on the mechanisms of CO₂ adsorption on porous carbonaceous adsorbents, Carbon Capture Sci. Technol., 2022, 3, 100045.
43. J. A. Syed, S. Tang and X. Meng, Super-hydrophobic multilayer coatings with layer number tuned swapping in surface wettability and redox catalytic anti-corrosion application, Sci. Rep., 2017, 7, 4403.
44. J. Ran, M. Li, C. Zhang, F. Xue, M. Tao and W. Zhang, Synergistic adsorption for parabens by an amphiphilic functionalized polypropylene fiber with tunable surface microenvironment, ACS Omega, 2020, 5, 2920–2930.
45. J. Ryu and D. W. Lee, Tailoring hydrophilic and hydrophobic microenvironments for gas–liquid–solid triphase electrochemical reactions, J. Mater. Chem. A, 2024, 12, 10012–10043.
46. W. Qing, F. Liu, H. Yao, S. Sun, C. Chen and W. Zhang, Functional catalytic membrane development: A review of catalyst coating techniques, Adv. Colloid Interface Sci., 2020, 282, 102207.
47. R. Deng and Q. Zhang, Cu embedded in Co–P nanosheets with super wetting structure for accelerated overall water splitting under simulated industrial conditions, Adv. Energy Mater., 2024, 2401444.
48. K. Wang, S. Yu, W. Li, Y. Song, P. Gong, M. Zhang, H. Li, D. Sun, X. Yang and X. Wang, Superhydrophobic and photocatalytic synergistic self-cleaning ZnS coating, Appl. Surf. Sci., 2022, 595, 153565.
49. X. Wang, M. Deng, Z. Zhao, Q. Zhang and Y. Wang, Synthesis of super-hydrophobic CuO/ZnO layered composite nano-photocatalyst, Mater. Chem. Phys., 2022, 276, 125305.
50. T. Kamegawa, Y. Shimizu and H. Yamashita, Superhydrophobic surfaces with photocatalytic self-cleaning properties by nanocomposite coating of TiO₂ and polytetrafluoroethylene, Adv. Mater., 2012, 24, 3697–3700.
51. K. Li, X. Zeng, H. Li, X. Lai, C. Ye and H. Xie, Study on the wetting behavior and theoretical models of polydimethylsiloxane/silica coating, Appl. Surf. Sci., 2013, 279, 458–463.
52. X. Wang and Q. Zhang, Role of surface roughness in the wettability, surface energy and flotation kinetics of calcite, Powder Technol., 2020, 371, 55–63.
53. Y. Li, G. Zhao, B. Hong, S. Zhao, X. Han and M. Pera-Titus, Unraveling particle size and roughness effects on the interfacial catalytic properties of pickering emulsions, Colloids Surf., A, 2020, 599, 124800.
54. D. Schweigert, B. Damson, H. Lüders, P. Stephan and O. Deutschmann, The effect of wetting characteristics, thermophysical properties, and roughness on spray-wall heat transfer in selective catalytic reduction systems, Int. J. Heat Mass Transfer, 2020, 152, 119554.
55. C. Lee, H. Kim and Y. J. Jang, Three phase boundary engineering using hydrophilic–hydrophobic poly(N-isopropylacrylamide) with oxygen-vacant TiO₂ photocatalysts for photocatalytic N₂ reduction, ACS Appl. Energy Mater., 2022, 5, 11018–11024.
56. D. Xing, R. Wang, F. Wu and X. Gao, Confined growth and controlled coalescence/self-removal of condensate microdrops on a spatially heterogeneously patterned superhydrophilic–superhydrophobic surface, ACS Appl. Mater. Interfaces, 2020, 12, 29946–29952.
57. H. Notsu, W. Kubo, I. Shitanda and T. Tatsuma, Super-hydrophobic/super-hydrophilic patterning of gold surfaces by photocatalytic lithography, J. Mater. Chem., 2005, 15, 1523–1527.
58. C. Gao, L. Zhang, Y. Hou and Y. Zheng, A UV-resistant heterogeneous wettability-patterned surface, Adv. Mater., 2023, 35, 2304080.
59. S. Jang, Y. S. Kang, D. Kim, S. Park, C. Seol, S. Lee, S. M. Kim and S. J. Yoo, Multiscale architectured membranes, electrodes, and transport layers for next-generation polymer electrolyte membrane fuel cells, Adv. Mater., 2023, 35, 2204902.
60. P. Liu, J. Liu, J. Liu, X. Dong and R. Wang, Controlling the morphological evolution of iron oxide/alkoxide particles and their application in a superwetting surface, J. Alloys Compd., 2018, 769, 873–880.
61. D. Zhang, Z. Cheng, H. Kang, J. Yu, Y. Liu and L. Jiang, A smart superwetting surface with responsivity in both surface chemistry and microstructure, Angew. Chem., Int. Ed., 2018, 57, 3701–3705.
62. G. Liu, W. S. Y. Wong, M. Kraft, J. W. Ager, D. Vollmer and R. Xu, Wetting-regulated gas-involving (photo)electrocatalysis: biomimetics in energy conversion, Chem. Soc. Rev., 2021, 50, 10674–10699.
63. X. Bai, Z. Yuan, C. Lu, H. Zhan, W. Ge, W. Li and Y. Liu, Recent advances in superwetting materials for separation of oil/water mixtures, Nanoscale, 2023, 15, 5139–5157.
64. X. Gong, L. Zhang, Y. Liu and M. Zhu, A review on zeolitic imidazolate framework-8 based materials with special wettability for oil/water separation, J. Environ. Chem. Eng., 2023, 11, 111360.
65. J. Di, L. Li, Q. Wang and J. Yu, Porous membranes with special wettabilities: designed fabrication and emerging application, CCS Chem., 2021, 3, 2280–2297.
66. J. Xue, M. Xu, J. Gao, Y. Zong, M. Wang and S. Ma, Multifunctional porphyrinic Zr-MOF composite membrane for high-performance oil-in-water separation and organic dye adsorption/photocatalysis, Colloids Surf., A, 2021, 628, 127288.
67. Y. Liu, Y. Liu, M. Chen, S. Liu, B. Lai and W. Tu, Strategies for the construction of special wettability metal organic framework membranes: A review, J. Water Proc. Engineering, 2023, 51, 103374.
68. H. He, Y. Liu, Y. Zhu, T. C. Zhang and S. Yuan, Underoil superhydrophilic CuC₂O₄@Cu-MOFs core-shell nanosheets-coated copper mesh membrane for on-demand emulsion separation and simultaneous removal of soluble dye, Sep. Purif. Technol., 2022, 293, 121089.
69. Y. Hou, Y. Liu, M. Liu and S. Fu, Asymmetrically superwetting Janus Double-layer fabric for synchronous oil removal and catalytic reduction of aromatic dyes, Sep. Purif. Technol., 2021, 255, 117663.
70. Y. Yu, W. Cui, L. Song, Q. Liao, K. Ma, S. Zhong, H. Yue and B. Liang, Design of organic-free superhydrophobic TiO₂ with ultraviolet stability or ultraviolet-induced switchable wettability, ACS Appl. Mater. Interfaces, 2022, 14, 9864–9872.
71. X. Chen, Y. Zhan, A. Sun, Q. Feng, W. Yang, H. Dong, Y. Chen and Y. Zhang, Anchoring the TiO₂@crumpled graphene oxide core–shell sphere onto electrospun polymer fibrous membrane for the fast separation of multi-component pollutant-oil–water emulsion, Sep. Purif. Technol., 2022, 298, 121605.
72. X. Zhang, C. Wei, S. Ma, C. Zhang, Y. Li, D. Chen, Z. Xu and X. Huang, Janus poly(vinylidene fluoride)-graft-(TiO₂ nanoparticles and PFDS) membranes with loose architecture and asymmetric wettability for efficient switchable separation of surfactant-stabilized oil/water emulsions, J. Membr. Sci., 2021, 640, 119837.
73. X. Zhang, Y. Liu, F. Zhang, W. Fang, J. Jin and Y. Zhu, Nanofibrous Janus membrane with improved self-cleaning property for efficient oil-in-water and water-in-oil emulsions separation, Sep. Purif. Technol., 2023, 308, 122914.
74. Y. Duan, X. Zhao, M. Sun and H. Hao, Research advances in the synthesis, application, assembly, and calculation of Janus materials, Ind. Eng. Chem. Res., 2021, 60, 1071–1095.
75. R. Kalusulingam, P. Koilraj, C. Angel Antonyraj and K. Srinivasan, Recent advances on the fabrication of superwettable layered double hydroxides materials for oil–water separation, Mater. Today: Proc., 2023 DOI:10.1016/j.matpr.2023.08.292.
76. N. Yadav, R. R. Yadav and K. K. Dey, Microwave assisted formation of trimetallic AuPtCu nanoparticles from bimetallic nano-islands: Why it is a superior new age biocidal agent compared to monometallic & bimetallic nanoparticles, J. Alloys Compd., 2022, 896, 163073.
77. K. K. Karuppanan, M. K. Panthalingal and P. Biji, Chapter 26 - Nanoscale, Catalyst Support Materials for Proton-Exchange Membrane Fuel Cells, in Handbook of Nanomaterials for Industrial Applications, ed. C. Mustansar Hussain, Elsevier, 2018, pp. 468–495.
78. L. Ma, Z. Wei, C. Zhao, X. Meng, H. Zhang, M. Song, Y. Wang, B. Li, X. Huang, C. Xu, M. Feng, P. He, D. Jia, Y. Zhou and X. Duan, Hierarchical superhydrophilic/superaerophobic 3D porous trimetallic (Fe, Co, Ni) spinel/carbon/nickel foam for boosting oxygen evolution reaction, Appl. Catal., B, 2023, 332, 122717.
79. Y. Du, H. Tang, D. Zhang, H. Liu, Y. Chen, Z. Zhu, W. Yang, Z. Li, Y. Tang and C. Liu, Boosting electrocatalytic oxygen evolution: Superhydrophilic/superaerophobic hierarchical nanoneedle/microflower arrays of Ce_xCo_3−xO₄ with oxygen vacancies, ACS Appl. Mater. Interfaces, 2021, 13, 42843–42851.
80. Y. Du, Z. Chen, Y. Gong, Q. Li, W. Xiao, G. Xu, B. Li, X. Wang, Z. Wu and L. Wang, 1D Copper-based nanowire decorated with trimetallic nanoparticles affording abundant edges toward hydrogen generation in wide pH range, Fuel, 2023, 341, 127719.
81. A. Kumar and D. Nanda, Chapter 3 - Methods and fabrication techniques of superhydrophobic surfaces, in Superhydrophobic Polym. Coatings, ed. S. K. Samal, S. Mohanty and S. K. Nayak, Elsevier Inc, 2019, pp. 43–75.
82. H. Liu, L. Zhang, J. Huang, J. Mao, Z. Chen, Q. Mao, M. Ge and Y. Lai, Smart surfaces with reversibly switchable wettability: Concepts, synthesis and applications, Adv. Colloid Interface Sci., 2022, 300, 102584.
83. B. Zhan, Y. Liu, S. Li, C. Kaya, T. Stegmaier, M. Aliabadi, Z. Han and L. Ren, Fabrication of superwetting Cu@Cu2O cubic film for oil/water emulsion separation and photocatalytic degradation, Appl. Surf. Sci., 2019, 496, 143580.
84. V. Chabot, D. Higgins, A. P. Yu, X. C. Xiao, Z. W. Chen and J. J. Zhang, A review of graphene and graphene-oxide sponges: material synthesis and applications to energy and the environment, Energy Environ. Sci., 2014, 7, 1564–1596.
85. A. Khan, M. R. Habib, R. R. Kumar, S. M. Islam, V. Arivazhagan, M. Salman, D. R. Yang and X. G. Yu, Wetting behavior and applications of Metal-catalyzed CVD Grown graphene, J. Mater. Chem. A, 2018, 6, 22437–22464.
86. X. C. Dong, J. Chen, Y. W. Ma, J. Wang, M. B. Chan-Park, X. M. Liu, L. H. Wang, W. Huang and P. Chen, Superhydrophobic and superoleophilic hybrid foam of graphene and carbon nanotubes for selective removal of oils or organic solvents from the surface of water, Chem. Commun., 2012, 48, 10660–10662.
87. J. Bong, T. Lim, K. Seo, C. A. Kwon, J. H. Park, S. K. Kwak and S. Ju, Dynamic graphene filters for selective gas–water–oil separation, Sci. Rep., 2015, 5, 14321,  DOI:10.1038/srep14321.
88. M. Ge, C. Cao, J. Huang, X. Zhang, Y. Tang, X. Zhou, K. Zhang, Z. Chen and Y. Lai, Rational design of materials interface at nanoscale towards intelligent oil-water separation, Nanoscale Horiz., 2018, 3, 235–260,  10.1039/C7NH00185A.
89. J. C. Zhang, F. Zhang, J. Song, L. F. Liu, Y. Si, J. Y. Yu and B. Ding, Electrospun flexible nanofibrous membranes for oil–water separation, J. Mater. Chem. A, 2019, 7, 20075–20102,  10.1039/C9TA07296A.
90. J. Li, C. C. Xu, H. F. Tian, F. Zha, W. Qi and Q. Wang, Blend electrospun poly (vinylidene fluoride)/stearic acid membranes for efficient separation of water-in-oil emulsions, Colloids Surf., A, 2018, 538, 494–499,  DOI:10.1016/j.colsurfa.2017.11.043.
91. T. Pisuchpen, N. Chaim-ngoen, N. Intasanta, P. Supaphol and V. P. Hoven, Tuning Hydrophobicity and water adhesion by electrospinning and silanization, Langmuir, 2011, 27, 3654–3661.
92. B. Zhan, Y. Liu, S.-Y. Li, C. Kaya, T. Stegmaier, M. Aliabadi, Z.-W. Han and L.-Q. Ren, Fabrication of superwetting Cu@Cu₂O cubic film for oil/water emulsion separation and photocatalytic degradation, Appl. Surf. Sci., 2019, 496, 143580.
93. W. Zheng, J. Huang, S. Li, M. Ge, L. Teng, Z. Chen and Y. Lai, Advanced materials with special wettability toward intelligent oily wastewater remediation, ACS Appl. Mater. Interfaces, 2021, 13, 67–87.
94. X. Cao, Y. Li, X. Zhang, A. Gao, R. Xu, Y. Yu and X. Hei, Surface wettability and emission behavior tuned via solvent in a supramolecular self-assembly system based on a naphthalene diimides derivative, Appl. Surf. Sci., 2020, 501, 144256.
95. X. Y. Li, D. Hu, K. Huang and C. F. Yang, Hierarchical rough surfaces formed by LBL self-assembly for oil-water separation, J. Mater. Chem. A, 2014, 2, 11830–11838.
96. C. L. Chen, C. A. Du, D. Weng, A. Mahmood, D. Feng and J. Wang, Robust superhydrophobic polytetrafluoroethylene nanofibrous coating fabricated by self-assembly and its application for oil/water separation, ACS Appl. Nano Mater., 2018, 1, 2632–2639.
97. J. Liu, R. Zhang, L. Wang, Y. Liu, X. Tian, X. Dai, J. Pan and J. Dai, Tannic acid-mediated layered double hydroxide hybridizing PVDF superwetting catalytic membranes constructed via ion diffusion-induced interfacial self-assembly for boosting water purification, Sep. Purif. Technol., 2023, 319, 124051.
98. T. T. Fan, J. L. Miao, Z. H. Li and B. W. Cheng, Bio-inspired robust superhydrophobic-superoleophilic polyphenylene sulfide membrane for efficient oil/water separation under highly acidic or alkaline conditions, J. Hazard. Mater., 2019, 373, 11–22.
99. C. J. Wei, L. G. Lin, Y. P. Zhao, X. Y. Zhang, N. Yang, L. Chen and X. J. Huang, Fabrication of pH-sensitive superhydrophilic/underwater superoleophobic poly(vinylidene fluoride)-graft-(SiO₂ nanoparticles and PAMAM dendrimers) membranes for oil–water separation, ACS Appl. Mater. Interfaces, 2020, 12, 19130–19139.
100. Z. Liu, X. Sheng, D. Wang and X. Feng, Efficient hydrogen peroxide generation utilizing photocatalytic oxygen reduction at a triphase interface, iScience, 2019, 17, 67–73.
101. M. Li, J. Wei, L. Ren, Y. Zhao, Z. Shang, D. Zhou, W. Liu, L. Luo and X. Sun, Superwetting behaviors at the interface between electrode and electrolyte, Cell Rep. Phys. Sci., 2021, 2, 100374.
102. H. Zhou, X. Sheng, Z. Ding, X. Chen, X. Zhang, X. Feng and L. Jiang, Liquid–liquid–solid triphase interface microenvironment mediates efficient photocatalysis, ACS Catal., 2022, 12, 13690–13696.
103. S. N. Wan Ikhsan, N. Yusof, F. Aziz, A. F. Ismail, J. Jaafar, W. N. Wan Salleh and N. Misdan, Superwetting materials for hydrophilic-oleophobic membrane in oily wastewater treatment, J. Environ. Manage., 2021, 290, 112565.
104. R. Qu, X. Li, H. Zhai, S. Zhao, Y. Wei and L. Feng, Integration of catalytic capability and pH-responsive wettability in a V_xO_y-based dual-mesh system: towards solving the trade-off between the separation flow rate and degradation efficiency, J. Mater. Chem. A, 2021, 9, 5454–5467.
105. R. Xu, X. Wang, C. Zhang, Y. Zhang, H. Jiang, H. Wang, G. Su, M. Huang and A. Toghan, Engineering solid–liquid-gas interfaces of single-atom cobalt catalyst for enhancing the robust stability of neutral Zn-air batteries under high current density, Chem. Eng. J., 2022, 433, 133685.
106. M. Ketkaew, S. Assavapanumat, S. Klinyod, A. Kuhn and C. Wattanakit, Bifunctional Pt/Au Janus electrocatalysts for simultaneous oxidation/reduction of furfural with bipolar electrochemistry, Chem. Commun., 2022, 58, 4312–4315.
107. S. T. Aziz, M. Ummekar, I. Karajagi, S. K. Riyajuddin, K. V. R. Siddhartha, A. Saini, A. Potbhare, R. G. Chaudhary, V. Vishal, P. C. Ghosh and A. Dutta, A Janus cerium-doped bismuth oxide electrocatalyst for complete water splitting, Cell Rep. Phys. Sci., 2022, 3, 101106.
108. A. Chauhan, M. Rastogi, P. Scheier, C. Bowen, R. V. Kumar and R. Vaish, Janus nanostructures for heterogeneous photocatalysis, Appl. Phys. Rev., 2018, 5, 041111.
109. R. Shi, L. Shang and T. Zhang, Three phase interface engineering for advanced catalytic applications, ACS Appl. Energy Mater., 2021, 4, 1045–1052.
110. L. Paramanik, S. Sultana and K. M. Parida, Photocatalytic and photo-electrochemical ammonia synthesis over dimensional oriented cobalt titanate/nitrogen-doped reduced graphene oxide junction interface catalyst, J. Colloid Interface Sci., 2022, 625, 83–99.
111. H. Zhu and F. Xia, A Janus wetting catalyst, Chem, 2022, 8, 2589–2590.
112. W. Zong, H. Gao, Y. Ouyang, K. Chu, H. Guo, L. Zhang, W. Zhang, R. Chen, Y. Dai, F. Guo and J. Zhu, Bio-inspired aerobic-hydrophobic janus interface on partially carbonized iron heterostructure promotes bifunctional nitrogen fixation, Angew. Chem., Int. Ed., 2023, 62, e202218122.
113. X. Wang, B. Wang, S. Yin, M. Xu, L. Yang and H. Sun, Highly efficient photocatalytic nitrogen fixation on bio-inspired triphase interface with improved diffusion of nitrogen, J. Cleaner Prod., 2022, 360, 132162.
114. J. Liu, R. Li, X. Zu, X. Zhang, Y. Wang, Y. Wang and C. Fan, Photocatalytic conversion of nitrogen to ammonia with water on triphase interfaces of hydrophilic-hydrophobic composite Bi₄O₅Br₂/ZIF-8, Chem. Eng. J., 2019, 371, 796–803.
115. N. Ojha and S. Kumar, Tri-phase photocatalysis for CO₂ reduction and N₂ fixation with efficient electron transfer on a hydrophilic surface of transition-metal-doped MIL-88A (Fe), Appl. Catal., B, 2021, 292, 120166.
116. L.-W. Chen, Y.-C. Hao, Y. Guo, Q. Zhang, J. Li, W.-Y. Gao, L. Ren, X. Su, L. Hu, N. Zhang, S. Li, X. Feng, L. Gu, Y.-W. Zhang, A.-X. Yin and B. Wang, Metal–organic framework membranes encapsulating gold nanoparticles for direct plasmonic photocatalytic nitrogen fixation, J. Am. Chem. Soc., 2021, 143, 5727–5736.
117. R. Guan, D. Wang, Y. Zhang, C. Liu, W. Xu, J. Wang, Z. Zhao, M. Feng, Q. Shang and Z. Sun, Enhanced photocatalytic N₂ fixation via defective and fluoride modified TiO₂ surface, Appl. Catal., B, 2021, 282, 119580.
118. R. Guan, X. Cheng, Y. Chen, Z. Wu, Z. Zhao, Q. Shang, Y. Sun and Z. Sun, Wettability control of defective TiO₂ with alkyl acid for highly efficient photocatalytic ammonia synthesis, Nano Res., 2023, 16, 10770–10778.
119. H. Yang, C. Nan, N. Gao, W. Zhou, F. Gao, D. Dong, D. Dou, Y. Liu, Z. Liang and D. Yang, Three-phase interface of SnO₂ nanoparticles loaded on hydrophobic MoS₂ enhance photoelectrochemical N₂ reduction, Electrochim. Acta, 2022, 430, 141086.
120. E. L. Cussler, Diffusion: Mass transfer in fluid systems, Cambridge University Press, 2009.
121. W. Xu, X. Sheng, H. Zhou, D. Wang, Z. Ding and X. Feng, Enhanced plasmonic photocatalytic synthesis of hydrogen peroxide at an air-liquid-solid triphasic interface, Chem. Eng. J., 2021, 410, 128342.
122. L. Li, L. Xu, Z. Hu and J. C. Yu, Enhanced mass transfer of oxygen through a gas–liquid–solid interface for photocatalytic hydrogen peroxide production, Adv. Funct. Mater., 2021, 31, 2106120.
123. Y. Li, Z. Pei, D. Luan and X. W. D. Lou, Triple-phase photocatalytic H₂O₂ production on a Janus fiber membrane with asymmetric hydrophobicity, J. Am. Chem. Soc., 2024, 146, 3343–3351.
124. L. Chen, X. Sheng, D. Wang, J. Liu, R. Sun, L. Jiang and X. Feng, High-performance triphase bio-photoelectrochemical assay system based on superhydrophobic substrate-supported TiO₂ nanowire arrays, Adv. Funct. Mater., 2018, 28, 1801483.
125. A. Fujishima and K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature, 1972, 238, 37–38.
126. N. T. Padmanabhan, P. Ganguly, S. C. Pillai and H. John, Morphology engineered spatial charge separation in superhydrophilic TiO₂/graphene hybrids for hydrogen production, Mater. Today Energy, 2020, 17, 100447.
127. Q. Zhang, X. Bai, X. Hu, J. Fan and E. Liu, Efficient photocatalytic H₂ evolution over 2D/2D S-scheme NiTe₂/g-C₃N₄ heterojunction with superhydrophilic surface, Appl. Surf. Sci., 2022, 579, 152224.
128. J. Yuan, X. Yi, Y. Tang, C. Liu and S. Luo, Efficient photocatalytic hydrogen evolution and CO₂ reduction: Enhanced light absorption, charge separation, and hydrophilicity by tailoring terminal and linker units in g-C₃N₄, ACS Appl. Mater. Interfaces, 2020, 12, 19607–19615.
129. E. Jiang, C. Guo, X. Zhao, Y. Chao, Y. Chao, D. Ma, P. Huo, Y. Yan, P. Zhou and Y. Yan, Nanochannel-induced efficient water splitting at the superhydrophobic interface, ACS Nano, 2023, 17, 10774–10782.
130. R. Tang, S. Zhou, C. Li, R. Chen, L. Zhang, Z. Zhang and L. Yin, Janus-structured Co-Ti₃C₂ MXene quantum dots as a Schottky catalyst for high-performance photoelectrochemical water oxidation, Adv. Funct. Mater., 2020, 30, 2000637.
131. Y. Wang, X. Wang, C. Lai, H. Hu, Y. Kong, B. Fei and J. H. Xin, Biomimetic water-collecting fabric with light-induced superhydrophilic bump, ACS Appl. Mater. Interfaces, 2016, 8, 2950–2960.
132. F. Li, W. Kong, B. Bhushan, X. Zhao and Y. Pan, Ultraviolet-driven switchable superliquiphobic/superliquiphilic coating for separation of oil-water mixtures and emulsions and water purification, J. Colloid Interface Sci., 2019, 557, 395–407.
133. Z. Zhu, L. Zhong, X. Chen, W. Zheng, J. Zuo, G. Zeng and W. Wang, Monolithic and self-roughened Janus fibrous membrane with superhydrophilic/omniphobic surface for robust antifouling and antiwetting membrane distillation, J. Membr. Sci., 2020, 615, 118499.
134. Q.-K. Zhang, W.-H. Zhao, Z.-P. Zhou, L.-M. Cao, W.-J. Yin, X.-L. Wei, Z.-K. Tang and H. Zhang, 2D Janus MoSSe/MoGeSiN4 vdW heterostructures for photovoltaic and photocatalysis applications, J. Alloys Compd., 2023, 938, 168708.
135. S. Wooh and H.-J. Butt, Photocatalytically active lubricant-impregnated surface, Angew. Chem., Int. Ed., 2017, 56, 4965–4969.
136. Y. Pan, S. Huang, F. Li, X. Zhao and W. Wang, Coexistence of superhydrophilicity and superoleophobicity: Theory, experiments, and applications in oil/water separation, Mater. Chem. A, 2018, 6, 15057–15063.
137. S. Bai, H. Qiu, G. He, F. Wang, Y. Liu and L. Guo, Porous fixed-bed photoreactor for boosting C–C coupling in photocatalytic CO2 reduction, eScience, 2022, 2, 428–437.
138. Y. Su, N. J. W. Straathof, V. Hessel and T. Noël, Photochemical transformations accelerated in continuous-flow reactors: Basic concepts and applications, Chem. – Eur. J., 2014, 10562–10589.
139. P. Zhu and L. Wang, Microfluidics-enabled soft manufacture of materials with tailorable wettability, Chem. Rev., 2021, 122, 7010–7060.
140. X. Cui, Q. Ruan, X. Zhuo, X. Xia, J. Hu, R. Fu, Y. Li, J. Wang and H. Xu, Photothermal nanomaterials: A powerful light-to-heat converter, Chem. Rev., 2023, 123, 6891–6952.

---