Recent progresses on Janus electrocatalysts for water electrolysis: A critical review

Janus particles (JPs) are named after the Roman two-faced God of the same name. The Nobel laureate Pierre-Gilles de Gennes first coined the term in his Nobel Laureate speech called Soft Matter,¹ describing Janus particles as having two sides, one polar and the other apolar. Since then, the term Janus has transcended its original amphiphilic definition to be regarded as particles composed of two or more components that differ in chemical or physical properties and are often antipodal.^2–4 The individual components of each JP are present in a single particle, and those individual components retain their properties that are seldom altered, thus allowing these particles to exhibit the unique multifunctional properties of each constituent.⁵ Such differences in properties are not only limited to electrical,⁶ chemical,⁷ magnetic,⁸ or optical⁹ properties; however, they broadly extend toward other functional properties, such as catalysis, electrocatalysis, and so on. The asymmetric nature of the particle surface, known as Janus balance, is the ratio of the surface area devoted to the types of different surfaces, which can vary from half-and-half hemispheres to interspaced small patches.¹⁰ It has been established that materials with asymmetric structures and anisotropic properties can bring considerable advantages toward bi-functional and multi-functional catalysis.¹¹ Specifically, the amphiphilic (having both hydrophilic and hydrophobic properties) properties of JPs have accumulated much attention for being used as functional materials. Over the last few years, numerous Janus particles have been developed to use them as electro-catalysts in the catalytic splitting of water. Such catalysts should have a low over potential, high stability, low Tafel slope, and high Faradic efficiency, where Janus particles perfectly fit the above criteria.

Hydrogen has been addressed as one of the promising green alternatives to our energy needs, as hydrogen shows credible solutions to climate change and other related issues. Being a clean fuel, hydrogen and its production and use do not produce any greenhouse gases, and the by-product of the hydrogen fuel is carbon-free. Moreover, among all the available fuels, hydrogen has the highest gravimetric energy density¹² and is easy to use. Coal gasification, steam reforming, and water electrolysis are readily available technologies to produce hydrogen. Other hydrogen production methods, such as reforming sugar and ethanol, water bio-photolysis, and photochemical splitting of water, are still in the developmental stage. With renewable energy taking the forefront, the electrocatalytic production of hydrogen via water splitting has already been in the limelight.¹³ However, during the splitting of water, it is required to provide additional energy to overcome the energy barrier known as dynamic over potential; thus, we need catalysts to minimize the electrocatalysis’s overpotential, making the process economically feasible.¹⁴ Specifically, overall water splitting is in demand where bi-functional catalysts could play a significant role in improving efficiency by simultaneously proceeding with electrocatalytic hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs).

Over the past few decades, several materials have been developed for water electrolysis, including zero-, one-, two-, and three-dimensional materials and combinations of two or more of these materials.^15–20 It is observed that some nanomaterials with an asymmetric heterostructured junction can produce remarkable properties, which cannot be realized in homogeneous or symmetric nano-scale materials.^21,22 To this end, with their multi-functionality, JPs come up as suitable contenders. It has been established that catalysts with an asymmetric structure and properties can bring considerable advantages in overall water electrolysis.¹¹ In this regard, the amphiphilic nature of the JPs has garnered much attention, particularly for water electrolysis. Over the last few years, numerous Janus particles have been synthesized to use them as electro-catalysts for water splitting. These Janus catalysts exhibit close-to-zero overpotential, remarkable stability, lower Tafel slope, and extraordinary Faradic efficiency and are economically feasible for their scalability.

In this paper, we focus on systematically reviewing the status of the developments of Janus materials for electrocatalysis and photo-electrocatalysis. Starting with summarizing the different designs and preparation of Janus particles, we have discussed the compositions and types of Janus materials (such as polymeric Janus materials, inorganic Janus materials, and polymeric/inorganic hybrid Janus materials). After that, the synthesis processes of Janus materials were cited, followed by classifications of different approaches to synthesize them with a detailed review of the respective process parameters, multifunctional properties, and present status of their development. In addition, such comprehensive properties of the Janus particles are summarized in this Review, subjected to their applications towards catalytic hydrogen evolution reactions (HERs), oxygen evolution reactions (OERs), and photo-electrocatalysis. Finally, a thorough summary is presented on the synthesis and applications of Janus particle, while the respective challenges and outlooks are also discussed.

Generally, the structure and the composition of a Janus particle (JP) synergistically determine the particle’s properties. Hence, to build Janus materials, structural complexities are a prerequisite, along with diverse compositional configurations.^23,24 For instance, it was observed that dumbbell-shaped JPs have a greater surfactancy than their spherical counterparts. It was hypothesized that the jamming of the dumbbell-shaped particles at the interface in time scales smaller than the droplet’s relaxation time, which leads to the tendency of these particles to the covering of the interface with closed-packed arrangements, thereby leading to the stabilization of these emulsion droplets that are non-spherical in morphology.²⁵ 

To date, numerous morphologies of different JPs have been reported, including spheres,^26–29 disks,³⁰ rods,³¹ dumb-bells,^25,32 snowman,³³ sheets,³⁴ raspberry,³⁵ and many more. Typically, some commonly found morphologies of JPs are shown in Fig. 1.

It is already revealed that a JP’s morphology and chemical composition portray its applications and properties. Not only Janus particles are limited to the complex morphologies, but also the individual components in the particle retain their intrinsic properties (i.e., optical, magnetic, and electronic). As a consequence of the amalgamation of different components, JPs are bestowed with the unique capability to integrate antipodal components and utilize the properties of all the individual constituent components. Moreover, depending on the applications, the properties of the JPs can be altered. For instance, we can combine two opposite characteristics of hydrophilicity and hydrophobicity in the same JPs, as reported in many recent works.³⁶ Janus particles are also used for delivering catalysts to specific phases as phase transfer vehicles.^37,38 We can also achieve extraordinary catalytic activity by combining two or more components with extraordinary catalytic properties. For instance, metal–semiconductor hybrid catalysts can be fabricated for applications in photocatalysis, energy devices, and solar energy conversions.³⁹ Janus particles are used for stabilizing Pickering emulsions and are also employed for interfacial catalysis owing to their enhanced interfacial activities.^38,40 In another example, JPs could also have been imparted with specialized properties, such as piezoelectric properties. Janus MXY mono- and multi-layered particles are predicted to have both out-of-plane and in-plane piezo-electric polarization. Based on the first-principles calculations, different piezoelectric Janus materials have been designed with various multifunctional properties.^41,42 It was discovered that these JPs have a comparatively high piezoelectric coefficient, and their broken-mirror symmetry was attributed to their out-of-plane piezoelectric polarization. For example, Janus MoSTe has an out-of-plane piezoelectric polarization of 5.7–13.5 pm/V, which is even greater than many common 3D piezoelectric materials.^41,43

Most of the studies conducted on Janus materials suggest that the particle sizes of these materials vary from a few hundred nanometers to a few micrometers. No doubt, the synthesis and processing of Janus particles smaller than 100 nm are a challenging and complex issue,^44,45 and very few such Janus particles below 100 nm have been reported to date. Zhu et al.⁴⁶ synthesized three-component CdS–Au₂S–Au Janus-satellite heterostructure nanocrystals (HNCs) of an average particle size of 19.8 nm. Schick et al.⁴⁷ synthesized a highly biocompatible and multifunctional Au@MnO Janus particle of 25 nm. Generally, the morphology and size of a JP can be controlled by the process of its synthesizing parameters, as discussed in detail in Sec. III.

The complexities in Janus materials are not only limited to their morphology and size but also to their compositions, which can be classified as polymeric Janus, inorganic Janus, and polymeric/inorganic Janus, as shown in Fig. 2.^51,58 The classifications of various Janus particles based on their compositions are discussed in the following sub-sections.

Polymeric Janus materials have attracted much attention in recent years. These particles are entirely composed of long-chain polymers,¹⁰ which show excellent performance as catalysts,⁵² biological and chemical sensors,⁵³ colloidal surfactants,^54,55 and so on. Moreover, polymers are soft, and the ease of tailoring their asymmetric structure allows one to control their physicochemical properties.¹¹ Polymeric Janus materials are generally synthesized via the phase separation approach. However, self-assembly and microfluidic strategies are also used for synthesizing polymeric JPs. De Leon et al.⁵⁶ demonstrated a preparation method based on an asymmetrically functionalized dumbbell-shaped Janus material by a combination of polymers and Pickering emulsion, as shown in Fig. 3(A). Similarly, graphene oxide (GO) nanosheets were assembled on wax beads and (polymethyl methacrylate) PMMA, which was selectively grafted on the exposed face to create such a dumbbell-shaped Janus particle. This provided an amphiphilic characteristic to the Janus particle. The as-fabricated JPs showed a marked improvement in stabilizing the oil–water interface by lowering interfacial tension, which was determined using pendant drop tensiometry of a chloroform drop in water. In addition, such JPs found a relatively more hydrophilic nature of PMMA when compared to chloroform. The incredibly superior interfacial activity was due to its Janus nature, where the PMMA-functionalized phase preferred the chloroform phase and the non-modified face of the Janus nanosheet preferred water. Kim et al.⁵⁷ employed a seeded polymerization approach using the precursors poly (butyl methacrylate) and polystyrene to fabricate anisotropic dumbbell-shaped Janus particles and established that the chemical anisotropy of non-spherical particles could be achieved by employing surface treatments and using pairs of immiscible polymers, which opened up new avenues for assembling of complex structures and in the long run may even help in tuning the curvature of the various interfaces developed in the anisotropic Janus particles; the synthesis technique is illustrated in Fig. 3(B). Deng et al.⁴⁹ synthesized an amphiphilic Janus particle with P4VP protuberances on its surface employing the co-assembly approach using the diblock copolymer polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP). Their study established that the Janus balance of the nanoparticles (JPs) is tunable by changing the block-chain length ratio of diblock copolymers. The fabrication process is depicted in Fig. 3(C). Tang et al.⁵⁵ synthesized a Janus polyacrylonitrile (PAN)/PS colloid by seeded emulsion polymerization using precursors polyacrylonitrile and styrene/divinylbenzene. They demonstrated that the composition and the microstructures of the Janus particles can be controlled by selective modification of Janus polymer colloids, as depicted in Fig. 3(D).

Inorganic Janus particles comprise one or more inorganic components, with the advantage of retaining their shapes in various solvents. Thus, JPs can be employed in multiple practical applications, viz., as catalysts in electro and photocatalytic water splitting, in the biomedical field, as sensors, and as drug delivery vehicles (agents).⁵⁸ There are two subcategories of inorganic Janus particles: (i) Janus particles composed of two or more inorganic constituents and (ii) Janus particles with a single inorganic constituent but different surfaces or other chemical/physical properties.³ Inorganic Janus particles have been reported for their applications in catalysis,⁵⁹ magnetic,⁵¹ and optical applications.⁶⁰ 

Primarily, inorganic Janus nanoparticles are synthesized through different approaches, including the method called surface-controlled nucleation and masking/immobilization, particle synthesis via metal evaporation,⁹ electrostatic deposition,⁶¹ and chemical modifications.³ In addition to the inherent properties of the constituent elements, several new properties are realized from the surface chemistry and the morphology of such newly assembled multi-functional inorganic JPs.⁶² The difference in surface chemistry due to the combination of two or more elements leads to multi-functionality in JPs, and orthogonal functionalization of the surfaces due to the directed assembly is the most common reason for the change in properties that are observed in Janus particles when compared to their constituent elements.^23,47,63 The epitaxial interaction between the two components enhances and sometimes even generates new properties or alters the materials properties altogether. Xuan et al.⁶⁴ used chemical vapor deposition to synthesize mesoporous Janus nanoparticles, wherein the surface of mesoporous silica particles was deposited with Au. Rowe et al.⁶⁵ synthesized SiO₂-overcoated gold nanorods (GNRs) as inorganic JPs [Fig. 4(A)]. They demonstrated that the size of the SiO₂ lobes could be controlled up to a minimum lobe size, below which total encapsulation takes place, as shown in TEM images in Fig. 4A (a-f). It was observed that the alcohol concentration in the reaction mixture leads to the CTAB(cetyltrimethylammonium bromide)-stabilized GNR (CTAB-GNR) core, which is fully encapsulated by the SiO₂ shell or Au core is exposed on the sides by the deposition of patchy lobes of SiO₂ (Fig. 4A (g-h)). They demonstrated that the morphology of JPs can be tweaked by variations in synthesis parameters [Fig. 4(A)]. McConnell et al.⁶⁰ synthesized a patchy and optically active multi-region gold on silica as JPs with silica on the bottom, gold patches on the top surface, and gold patches around the center of the particle, as depicted in Fig. 4(B). Such multi-region gold silica JPs were synthesized by employing a unique hierarchical self-assembly process. Inorganic Janus particles can be synthesized into various shapes and sizes. Lyubarskaya and Shestopalov⁶⁶ reported a method for preparing multi-component, shape-controlled particles. First, they synthesized an array of gold–nickel–titanium structures on an oxidized silicon oxide layer by physical vapor deposition (PVD) and photolithography. These structures were enveloped by a SiO₂ layer, which was later patterned by a reactive ion etching. A simple sonication with ethanol removed the exposed gold particles, producing immobilized ordered particle arrays on a flat surface. This pre-arrangement allows for selective patterning, orthogonal functionalization of the surface, and compartmentalization of the Janus particle, as depicted in Fig. 4(C). In the similar fashion, Ma et al.⁶¹ synthesized a self-propelled Janus nanomotor by electron beam deposition of a Pt layer (2 nm) on the surface of mesoporous silica nanoparticles, which led to the formation of two faces on the particles. The meso-pores at the non-coated side were still accessible to small molecules, as Pt was deposited on only one side. Transmission electron microscopy (TEM) images depicted that Pt forms a patchy island-like deposition instead of a smooth layer, which exhibited better catalytic activities for the decomposition of H₂O₂ into H₂O and O₂ with the diffusion coefficient value being enhanced by nearly 100% [Fig. 4(D)].

Polymeric–inorganic hybrid Janus particles combine the properties of both polymers and inorganic constituents. These particles present numerous advantages compared to other types of Janus particles, mainly in photoelectric, responsive, and magnetic properties, along with flexibility and ease of processing.²⁴ This type of Janus particle is generally fabricated by immobilization/masking,⁶⁷ phase separation,⁵¹ emulsion polymerization,⁶⁸ and seeded growth.⁶⁹ The morphology of these particles is typically snowman, dumbbell, or eccentric core-shell structures. Faria et al.⁷⁰ synthesized two types of Janus particles by anchoring Pd on the surface of nano-hybrids made up of silica oxide and carbon nanotubes. Pd was anchored throughout the surface on one surface, whereas on the hydrophobic side on the other, as depicted in Fig. 5(A). The particles thus obtained showed high phase selectivity and a remarkable ability to stabilize oil in water emulsions. Chen et al.⁷¹ synthesized PS-Fe₃O₄@SiO₂-polyethylene oxide (PEO) Janus particles by transferring self-assembled PS-b-PAA copolymer membranes at an emulsion interface, where preferential adsorption of magnetic Fe₃O₄@SiO₂ core shells got amine capped by specific molecular interactions, as shown in Fig. 5(B). The exposed side of the core-shell particles was modified to conjugate aldehyde-capped polyethylene oxide (PEO). Covalent bonding made the connections robust, and the hydrophobic and hydrophilic chains were distinctly compartmentalized on either side of the particle. Liu et al.⁷² demonstrated dual functionalized, anisotropic Janus particles’ hyaluronic acid (HA)-JMSN/DOX-2, 3-dimethylmaleic anhydride (DMMA) by modifying one side of MSN (mesoporous silica nanoparticle)-NH₂ with hyaluronic acid (HA) and the other side with charge reversal group 2, 3-dimethylmaleic anhydride (DMMA), as illustrated in Figs. 5(C) and 5(D). The as-prepared JPs were superior to isotropous nanoparticles. When intravenously injected in mice, the particle inhibited tumor growth and improved the survival rate of tumor-infected rats. As a mechanism, they explained that the negatively charged ligand of this particle detaches on entering the weakly acidic microenvironment of a tumor cell. The particle thus exhibits a strong positive charge with HA on the surface that binds to CD44 receptors on the tumor cells, thereby inhibiting tumor growth. Chen et al.⁷³ synthesized Janus cages with bilayer inorganic–polymeric composite shells by synthesizing silicon precursors through a sol-gel process followed by the grafting of polymers on the interior surface of the cage, Fig. 5(E). It was reported that the internal hydrophobic layer was tuneable for oil collection, whereas the exterior hydrophilic layer ensured good dispersibility of the microspheres in water. Liu et al.⁷⁴ modified mono-dispersed colloids of silica with aminopropyl-trimethoxysilane (APTMS) that was allowed to cool to form wax/silica–NH₂ composite spheres. These were then immersed in NH₄F, and the exposed side is etched Fig. 5(F). The process yielded a Janus particle with a coarse and smooth side having clear circular etching groove separating them. A selective area energy-dispersive X-ray spectroscopy (EDX) revealed that the smooth side had the presence of amino group, whereas no amino group was present on the coarser side. Moreover, both the sides of the JPs were connected by covalent bonds, which preserved the Janus characteristics of the particles.

In the last few years, tremendous progress has been made in formulating various new strategies for synthesizing JPs.^3,45 More specifically, wide ranges of JPs can be prepared by a plethora of robust methods, including techniques exploiting topo-selective surface modification (masking),⁷⁵ phase separation,^57,76 surface nucleation,⁷⁷ modification at Pickering emulsions,^28,78,79 self-assembly,⁸⁰ sputtering,⁸¹ microfluidics,⁸² and seeded growth.^83,84

“Immobilization/masking/selective modification at the interface” is one of the most popular techniques for JP synthesis. The technique employs a temporary immobilization template for breaking the symmetry of the particle by partial masking of one face.⁸⁵ This concept has its roots in (i) 2D planar substrates for inducing asymmetry in the particles, (ii) immobilization of the protein capsids and the smallest NPS, and (iii) topo-selective modification of (sub)-micrometer particles at the Pickering emulsions.² In this synthesis technique, homogeneous particles are kept fixed on an interface or substrate; subsequently, one or both surfaces of the particle are functionalized to fabricate the JPs by employing a template, mask, or the shadow of the adjacent particle, as shown in Fig. 6(A).⁸⁶ After that, physical or chemical processes are employed to modify the unshielded side of the particles to form desired JPs.² In due course, removing the coating yields an anisotropic Janus particle.⁸⁷ A wide variety of JPs of different natures and sizes can be synthesized by using this method, such as metallic particles,⁸⁸ silica particles,⁸⁹ and polymeric beads.⁹⁰ Although straightforward and simple, this technique gives us a lower yield in multifunctional compounds. A typical batch can sometimes generate only milligram quantities of Janus particles;⁹¹ the process is sometimes labor-intensive. Masking and desymmetrization can also be achieved by employing sequential deposition of materials within channels of two-dimensional arrays of cylindrical pores and by subsequently dissolving the templates to recover Janus materials.⁹² Several research groups have prepared hybrid organic/inorganic rods by sequential electrodeposition of materials inside the pores of nanoporous alumina templates, followed by their dissolution to generate Janus particles.⁹³ Kloberg et al.⁹⁴ demonstrated the use of a 2D nanomaterial, silicane, as a solid state masking substrate for the anisotropic modification of silicon quantum dots using trimethyl(vinyl)silane; the pictorial description of the process is depicted in Fig. 6(B). The resultant Janus particle did not render a clear dispersion like silicon quantum dots (QDs) and exhibited a very different PL characteristic from its constituent elements. Andala et al.⁹⁵ used gold nanospheres stabilized with dodecyl amine (DDA) dispersed in a toluene–water mixture; the same amount of hydrophobic dodecanethiol (DDT) and hydrophilic, fully deprotonated mercaptoundecanoic acid (MUA) were simultaneously added to the mixture, which lead to the formation of JPs at the interface. The thermodynamic aspects of the process were studied by adding the ligands in different order; in both the cases, the same JP was obtained, Fig. 6(C).

Microfluidic is a relatively new method developed recently to fabricate JPs.⁹⁶ Microfluidic technique provides many vital characteristics unavailable in traditional batch processes, such as shorter mixing and residence time, high mass transfer and mixing rate, and clear-cut reaction temperatures owing to the eccentric behavior of fluids in a micrometer scale.⁸² The JPs synthesized via microfluidics are typically formed by solidifying Janus droplets using polymerization and separating the two phases inside the particles.^82,97,98 For example, mono-dispersed JPs consisting of two immiscible organic phases are generated in a co-flowing aqueous phase. In particular, the mono-dispersed, anisotropic polymer particles are synthesized by subsequent off-chip polymerization, as reported elsewhere.⁹⁹ Similarly, for synthesizing a JP in a sheath system, a “Y” shaped channel is employed to generate a two-phase organic stream, introduced in a co-flowing aqueous stream to produce a Janus droplet.⁹⁹ Nisisako and Torii have demonstrated a unique setup by dispersing titanium oxide and pigments of carbon black dispersed in an acrylic monomer to prepare separate white and black monomers. After the formation of droplets and off-chip polymerization, a Janus particle is obtained with electrical and color anisotropies.⁶ A recent report demonstrated that this synthesis process could fabricate anisotropic magnetic JPs.¹⁰⁰ Wang et al.¹⁰¹ adopted a microfluidic strategy to synthesize (Au nanorod@Ag)-polyaniline [(AuNR@Ag)-PANI] Janus nanoparticles, where a seed-mediated growth method was utilized to create the gold nanorod (Au NR) in an aqueous solution. Subsequently, the AuNR, polyaniline, and AgNO₃ were injected into the microfluidic machine. Silver (Ag) growth was observed on one side of the nanorods and polyaniline on the other, generating a Janus nanoparticle. On another note, JPs with a sharp interface between the constituting phases can be synthesized using a continuous microfluidic synthesis technique. Shah et al. demonstrated a synthesized process to develop a new type of JPs with a PNIPAm poly(N-isopropyl acrylamide) microgel-rich side and a PAAm-rich side (polyacrylamide) by injecting two immiscible monomers, N-isopropyl acrylamide, and NIPAm, through the main channel of the microfluidic machine with side channels containing sodium dodecyl sulfate (SDS) aqueous solution acting as sheath flows. In this process, a sharp interface is developed within the JPs composed of the two constituent monomers. In contrast, the JP retains this sharp interface in the subsequent polymerization steps.¹⁰² 

Furthermore, Yin et al.⁹⁷ synthesized single-phase, mono-dispersed Janus superballs using microfluidic synthesis. Here, a microfluidic setup was built up with a single needle micro-channel and an internal microfluidic solution of a dichloromethane solution of poly(methyl methacrylate-co-2-hydroxyethyl methacrylate)/cadmium sulfide quantum dot, poly(MMA-co-HEMA)/CdS QD–polymer hybrid as the discontinuous phase (oil phase) and polyvinyl alcohol in the aqueous solution, as the continuous phase. The constant flow of the water phase cuts off the oil phase, and the surface tension between water and oil forms a perfectly spherical droplet. Seo et al.¹⁰³ used one microfluidic step process to develop a novel Janus micro-hydrogel, as shown in Fig. 7(A). Based on N-isopropyl acrylamide (NIPAAm), N-rich and N-poor NIPAAm were pumped simultaneously using a two-channel microfluidic setup. In this approach, cross-linking of two separate streams was prevented by using liquid–liquid phase separation, which allowed for the fabrication of compartmentalized structure in a single step. This results in a JP consisting of N-rich and N-poor phases, which exhibited anisotropic thermos-responsive behavior. Khan et al.¹⁰⁴ fabricated drug-loaded poly(methyl acrylate)/poly(acrylamide) Janus particles by employing a side-by-side capillary microfluidic setup. As depicted in Fig. 7(B), silicon oil 500 cSt was used as the continuous phase to generate biphasic Janus droplets, as shown in Figs. 7(C1) and 7(C2). Similarly, a two-channel microfluidic system was utilized for the fabrication of magnetic fluorescent bifunctional JPs, as demonstrated in Fig. 7(D). Similarly, this microfluidic synthesis process can also be adopted for synthesizing microhydrogels as reported in various recent reports.^105,106 Nisisako and Torii⁹⁹ showed that the volume ratio of two organic phases in the Janus droplets could be controlled by varying the flow rates of two organic phases via the microfluidic synthesis technique. They demonstrated a planar microfluidic system to generate biphasic Janus particles [Figs. 7(E1) and 7(E2)], employing polymerizable 1, 6-hexanediol diacrylate as a precursor.⁹⁹ 

The microfluidic synthesis method has also been applied to synthesize bi-colored Janus particles with electrical anisotropy. Nisisako et al.⁶ also demonstrated bi-colored droplets with two hemispheres using isopropyl as the precursors. Two parallel streams of the microfluidic system were injected simultaneously: one doped with carbon (black colored) and the other with TiO₂ (white colored). Finally, the resultant JPs obtained via this process exhibit exceptional features, while the JPs are made from two distinct sections. Such JPs showed superior weather-resistant properties compared to conventional organic dyes. In addition, carbon black and TiO₂ exhibit opposite electrical charging capabilities, thereby providing the opposing surfaces of the JP with asymmetric charge distribution.

In the natural world, numerous molecular structures arise through a self-assembly process, viz, self-assembled monomer layers and protein folding. In colloidal chemistry, the patchy surfaces are made out of “molecular colloids,” which leads to their self-assembly in a complex structure.¹⁰⁷ The self-assembly process of nanomaterials synthesis is an approach in which constituent elements form an ordered structure at the interface or in the solution.^23,108 This simple method can fabricate particles with delicate structures in which JPs retain the original properties of the constituents.² Chen et al.¹⁰⁹ developed JPs with one negatively charged hemisphere and another hydrophobic as a prototypical system to study the kinetics of self-assembly at a single particle level based on theory and molecular dynamics coupled with experimental visualization. It was demonstrated that the kinetic data obtained were consistent with a reversible first-order reaction. Moreover, they observed that in their system, the octahedral shape was stable, but growth proceeded by the rotation of particles in the feeder clusters as the particles in the end had the largest rotational freedom, which led to the formation of elongated structures. In self-assembly, the small nanoparticle core is typically the first to be fabricated, followed by the deposition of the second material via the polymerization process.¹¹⁰ The morphology and nano-structures of the assembled JPs depend on the varied solubility of the polymers in the solvent and their intrinsic physical properties.¹¹¹ Investigations on self-assembled block copolymers have garnered interest in recent times.¹¹² Self-assembly of block copolymers is a very agile process to fabricate a nanoscale supramolecule in bulk or solution. Depending on the interaction strength, the obtained microstructure is tunable, where one can synthesize various shapes, such as spheres to lamellar and cylinders.¹¹³ The self-assembly process could yield JPs from block polymers containing two or more mutually incompatible segments with a high molecular weight. This copolymer can be pre-dissolved in a co-solvent of all the segments, while an unfavorable solvent (e.g., water) is added in the mixture. After that, the solution mixture became non-conducive to the dissolution of hydrophobic segments, which leads to the aggregation and formation of micelles. With the reduction in interfacial energy between the solvent and the micelles, the hydrophilic segments become fixed with multiple morphologies and a solution of self-assembly particles is formed.¹¹¹ The state of aggregation formed during this approach is dependent on three primary parameters: (i) the volume fraction of various segments, (ii) Flory–Huggins interaction parameter, and (iii) the degree of polymerization of the copolymer. Song et al.¹¹⁴ developed bifunctional JPs containing Au–Fe₃O₄ NP grafted with hydrophobic polystyrene on Fe₃O₄ and hydrophilic polyethylene glycol on Au. This self-assembly approach resulted in the formation of doubled layer plasmonic magnetic vesicles.

Self-assembly makes monitoring the morphology, pore properties, and composition of Janus Particles relatively easy.¹¹⁵ Nie et al.¹¹⁶ synthesized amphiphilic polymeric JPs by a single-pot synthesis method. The prepared nanoparticles were found to be self-assembled into large micelles. A hydrophobic divinyl cross-linker, a hydrophobic free-radical initiator, and a hydrophobic divinyl cross-linker were solubilized in the hydrophobic polymer layer (HPL), and a hydrophilic monomer remained in the aqueous phase. Hydrophobic spheres are composed of the resulting polymer grown in HPL during polymerization of the hydrophobic cross-linker. Polymerization of a water-soluble monomer was initiated by exposing one side of the sphere to water. In contrast, the other side of the hydrophobic sphere was embedded in and protected by HPL. Thus, they successfully fabricated an amphiphilic Janus particle with one side embedded with polymeric chains on a hydrophobic sphere. Moreover, this process is easily scalable, and JPs can be prepared in large quantities owing to the large interfacial area of the water/HPL interface, Fig. 8(A). Liang et al.¹¹⁷ synthesized Janus camptothecin–floxuridine conjugate (JCFC) NCs by the self-assembly in dimethyl sulfoxide (DMSO). Pentaerythritol linked to two hydrophilic floxuridine molecules and two hydrophobic camptothecin molecules by hydrolyzable ester linkage was used to fabricate bi-layered JCFC, Fig. 8(B). This Janus particle represents a strategy to preserve and deliver a 1:1M ratio of two drugs in a liposomal manner. Gröschel et al.¹¹⁸ developed a solution-based method for fabricating Janus particles via the self-assembly of ABC triblock tetra-polymers into multi-compartmental micelles, Fig. 8(C). First, a cross-linkage is formed between these compartments, which then gets disassembled to form Janus Particles. It was also demonstrated that JPs produced by this method exhibit tunable physical properties owing to their varying structures. Song et al.¹¹⁴ synthesized Janus Au–Fe₃O₄ NPs by self-assembly technique. Herein, the hydrophobic polystyrene (PS) was grafted on Fe₃O₄, and hydrophilic polyethylene glycol (PEG) was grafted on Au by self-assembling. The amphiphilic graft distribution of polymeric grafts led to the formation of double-layered plasmonic magnetic vesicles. These vesicles demonstrated enhanced magnetic and optical properties, which were attributed to the coupling and interactions of the JPs. Moreover, these served as dual functional probes for in vivo dual magnetic resonance (MR) and photoacoustic imaging with high accuracy and resolution. Cheng et al.¹¹⁹ fabricated polymeric JPs by utilizing efficient intramolecular cross-linking of the middle P₂VP block using 1, 4-dibromobutane in N, N-dimethylformamide (DMF) solvent from a polystyrene-b-poly(2-vinylpyridine)-b-poly(ethylene oxide) triblock copolymer [Fig. 8(D)].

Ramsden and Gotch¹²⁰ and Pickering¹²¹ discovered that particles with correct hydrophobicity could be used to stabilize the interface of two immiscible liquids and form a Pickering emulsion (PE). The particles at the interface are divided into two parts by this interface,¹²² which presents an opportunity to modify the parts to achieve the desired properties selectively.^91,123 Hong et al.¹²⁴ introduced the PE-interface method for the synthesis of asymmetric Janus materials. This method is used to fabricate asymmetric Janus particles of varied sizes and shapes, such as polymer-based micro-gel,¹²⁵ molecularly imprinted polymer,¹²⁶ polymersomes,¹²⁷ graphene nanosheets,¹²⁸ and metallic nanoparticles.¹²⁹ 

Templating with emulsions has several advantages for the synthesis of Janus particles; this method is easily scalable, and there is no requirement for expensive instruments or complex synthesis procedures. However, nano-sized particles with sizes less than 100 nm are hard to fabricate through this method as it is difficult to form a Pickering emulsion with nano-sized particles.⁹² To date, PE has been utilized to synthesize Janus particles at the interfaces by many research groups. For example, Hong et al.¹²⁴ fabricated a bipolar Janus particle by dispersing fused silica in oil (wax)–water emulsion, as illustrated in Fig. 9(A). The silica particles aggregate to form clusters due to capillary forces as depicted in Fig. 9(B) whose external surface was then modified to functionalized one hemisphere with (aminopropyl) trimethoxysilane (APS) and the other hemisphere with n-octadecanetrichlorosilane (OTS). The as-produced particles were hydrophobic on the OTS side and cationic on the APS side. Saad et al.¹³⁰ synthesized a Janus calcium carbonate nano-particle with the ability to self-propel themselves in a Newtonian fluid by the Pickering emulsion approach, which was utilized to impart anisotropy to the particle (introduce silica on calcium carbonate using sodium silicate solution), as depicted in Fig. 9(C). Chen et al. employed the Pickering emulsion approach to fabricate Janus PS-Fe₃O₄@SiO₂-PEO, where a PS-b-PAA copolymer was used to emulsify the paraffin(oil)–water mixture. On cooling, the amine capped Fe₃O₄@SiO₂ particles were preferentially absorbed on the copolymer due to electrostatic attraction. The exposed side was selectively chemically modified with hydrophilic polymers (PEO-CHO). The as-prepared Janus particles were magnetically responsive and amphiphilic in nature, as shown in Fig. 9(D).

Phase separation is considered one of the most efficient techniques for the fabrication of Janus particles.¹³² Preparation of Janus particles via this process is more straightforward than the masking or immobilization strategy.¹³³ In this process, two or more immiscible polymers are dissolved in a solvent, employing this organic solution as the disperse phase of a Mini emulsion, later forming Janus particles with different segments having different functionalities. This is made possible due to the controlled phase separation of the polymer blend on the length scales of tens of nanometers.¹³⁴ Steinhaus et al.¹³⁵ employed ABC tri-block-tetra polymers to fabricate Janus nanorings. The step-by-step formations of such unique nanostructures are schematically illustrated in Fig. 10(A). Triblock tetra polymers were used with equal size A and C end blocks and a varying B fraction to form a lamella-like structure. PS-b-PB-b-PMMA (SBM) was self-assembled (evaporation induced) into a lamella ring morphology, which underwent cross-linkages to create Janus ring-like structures, Figs. 10(B)–10(D).

Liquid–liquid phase separation has also been utilized for the fabrication of Janus materials. Zhang et al.¹³⁶ synthesized Janus droplets by employing an evaporation-driven liquid–liquid phase separation approach. They used two immiscible liquids (octanol and water), which formed a homogeneous mixture, but with the addition of co-solvent (ethanol), liquid–liquid phase separation takes place; subsequently, the volatile ethanol evaporates, and the droplet slowly changes from homogeneous composition to a heterogeneous one. Water nucleation occurs along with the evaporation of ethanol and spontaneous de-wetting transition. Such phenomena occurred at the outer layer interfaces, while these two processes simultaneously occur until no more ethanol remains and Janus half water, a half organic droplet is formed. Yuan et al.¹³⁷ fabricated Janus droplets by promoting phase separation in single-phase droplets composed of a two-phase aqueous system, which took place at room temperature. Due to the evaporation of water [Fig. 10(E)], these Janus droplets exhibited clear interfaces between the constituent phases, and the bioactivity of the encapsulated bio ingredient is also preserved.

In this method, the synthesis of Janus particles relies on the seed-growth emulsion polymerization (SEP) with a semi-batch process. First, a seed (initial latex particle) is fabricated by conventional emulsion polymerization. The process is then continued by the addition of a second monomer. This then leads to the onset of phase separation under ambient conditions, hence facilitating the formation of a completely phase-separated latex particle.² Pfau et al.¹³⁸ were the first to apply this process for the fabrication of acrylic composite particles composed of a soft PnBuA side and a hard PMMA part. This method can also be utilized to fabricate anisotropic Janus colloids, as demonstrated by Tang et al., who showed the development of Janus PAN (polyacrylonitrile)/PS (polystyrene) colloids via seeded emulsion polymerization of styrene onto cross-linked PAN (polyacrylonitrile) hollow seed spheres.⁵⁵ Li et al.¹³⁹ synthesized dumbbell-shaped Janus nanoparticles by using seeded emulsion polymerization using polystyrene as the precursor in a methanol–water mixture, as shown in Fig. 11(A). They observed that by varying the methanol concentration, the morphology of the particles could be changed from spherical to dumbbell. In lower methanol concentrations, spherical seeds were obtained, whereas in higher methanol concentrations, spherical to dumbbell transformations are occurred. It was proposed that the final shape is determined by mobility of protrusions on the seed particle surface and the thermodynamic equilibrium.

The SEP has also been utilized to fabricate Janus magnetic nanoparticles (MNPs) with high magnetic properties. Besides the advantages of being controlled by a magnetic field, this particle can be used to encapsulate drugs in the polymeric matrix without any outer magnetic interference. Moreover, the morphologies of Janus MNPs provide advanced multi-functionalities in composition, hydrophilicity and hydrophobicity, and other modular functionalities, enabling them to be used in multiple applications, a feature absent in their symmetrical counterparts.¹⁴⁰ Janus MNPs were also developed using the same SEP technique.¹⁴¹ First, solvent evaporation was used to synthesize Janus nano-gels, followed by the modification/functionalization of one face of this gel using a thermos-response polymer. Such functionalization resulted in the formation of nano-gels with two different sides, as shown in Fig. 11(B).

Similarly, a tri-block JP has been synthesized with SEP technique and customized SEM methods, as reported several times recently.¹⁰⁶ Yu et al.¹⁴² employed two-step SEP to synthesize a tri-block JP [as schematically depicted in Fig. 11(C)] using polystyrene (PS) and PS/PDVB (polystyrene/polymeric divinylbenzene) hollow particle as the precursor [Fig. 11(D)]. The as-synthesized snowman-like diblock Janus particle using this method [Fig. 11(C)] is depicted in Fig. 11(E).

Sputtering is a process in which a solid surface is bombarded with high-energy ions, leading to the dislodgment of atoms from the host surface of the solid (known as sputtering target) and deposited toward the desired substrates/surfaces.¹⁴³ In this process, a Janus particle can be synthesized by modifying the surface properties of the exposed surface of a uniform nanomaterial, whereas the other surface is left unaltered.¹⁴⁴ This approach can be used to control the morphology of the metal film by controlling its deposition time. Although the process yields a low quantity of JPs.¹⁴⁵ however, the speed of the movement of Janus particle-based micro-motors can be varied by tuning the thickness of the sputtered layer and the rectifying voltage, as reported by Meng et al.¹⁴⁶ A monolayer of JPs was synthesized by sputtering gold onto a thin polystyrene substrate, and the as-prepared particles were then re-dispersed in an aqueous medium.¹⁴⁷ As depicted in Fig. 12(A), it was observed that by changing the sputtering condition, red, purple, black, and gold colored dispersions of JPs were obtained. The difference in color was attributed to the surface plasmon resonance absorption of gold nanoparticles. With the change in sputtering conditions, the morphology was altered, which resulted in the above changes. Maric et al.¹⁴⁸ obtained various Janus particles by sputtering novel metals, such as Pt, Au, Ag, and Cu on TiO₂, and obtained UV-activated micro-motors; they displayed this UV induced motion by a self-electrophoresis process, as depicted in Fig. 12(B). It was observed that the velocity of these particles strongly depended on the metal that was sputtered on TiO₂, with Pt/TiO₂ displaying the highest speed. Liu et al.¹⁴⁹ fabricated an alkaline-driven GaInSn liquid metal Janus micro-motor by sputtering both metallic and non-metallic materials on the liquid metal (LM), as demonstrated in Fig. 12(C). The metal-coated JPs moved quicker than their non-metallic counterparts, which was due to the change in driving mechanism when sputtering materials are metallic or non-metallic. For example, the particle is self-electrophoresis when sputtering materials are metallic (here galvanic corrosion reaction takes place), and the other one is self-diffusiophoresis (here chemical corrosion reaction takes place) when sputtering materials are non-metallic. The liquid metal Janus micro-motor developed by them can flip between these two modes by varying the sputtered material. This switching of driving mechanism finds application in various biochemical scenarios.

Overall, we can see that Janus particles can be synthesized with many synthesis and processing techniques, which are unique and versatile for fabricating such multifunctional particles with different sizes, shapes, architectures, morphologies, and so on. All these synthesis and processing techniques have their own merits and demerits based on the type of particle produced with sets of functionalities. Table I summarizes all these synthesis and processing techniques for developing several Janus particles with different compositions, sizes, and morphologies, while Table II lists the various synthesis techniques and different precursors for developing morphology-controlled Janus particles.

The catalytic splitting of water to generate hydrogen and oxygen is one of the green energy sources that attracted more attention in very recent times. The key parameters are evaluated to determine the catalyst’s performance as follows:¹⁹³ (i) over potential, (ii) Tafel slope, (iii) stability, (iv) Faradic efficiency, (v) turnover frequency, and (vi) mass and specific activities. A wide range of catalysts have been developed for the splitting of water over the past decade, including zero-, one-, two-, and three-dimensional materials (including metals, semiconductors, 2D transition metal di-chalcogenides, hybrid structures, and various heterostructure) and combinations of two or more of these nanoparticles.^15,19–21 It is observed that some nanomaterials with an asymmetric heterostructured junction at the nanoscale can produce remarkable properties, which cannot be realized in homogeneous or symmetric nanomaterials.¹⁹⁴ With their complex morphologies and ability to amalgamate the properties of various constituent components, Janus particles show promising results in the electrocatalysis of water for hydrogen and oxygen generation. Using the JPs, it is possible to have simultaneous HERs and OERs in a similar system with long-term stability. Specifically, overall efficient water splitting can be done employing bi-functional Janus catalysts that plays a pivotal role in enhancing water splitting efficiency by simultaneously proceeding with both hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs). In the upcoming sections i.e., in section A-C, there is a comprehensive summary of different applications of JP-based electrocatalysts and photocatalysts that have been fabricated to date.

Janus materials come up as suitable contenders of catalysts for the following reasons: (i) their Janus geometry increases the number of active sites, and so they are highly accessible to the reactant molecules;²⁰³ (ii) their high interfacial energy gives them a unique advantage while being used as an interfacial catalyst;²⁰⁴ (iii) their multi-functionality makes them suitable to be used as catalysts for the overall splitting of water;²⁰⁵ (iv) ease of separation of products; and (v) an enhanced rate of mass transfer between two phases, which accelerates the rate of reaction.^70,206

Transition metal particles were generally used to fabricate Janus particles for hydrogen evolution. Zhang et al.¹⁶⁸ synthesized Janus NiS₂/MoS₂, which showed an overpotential of 135 mV to achieve a current density of 10 mA cm⁻² in alkaline conditions. In contrast, precursors MoS₂ nano-sheets and NiS₂ nanoparticles have an overpotential of 434 and 315 mV, respectively, displaying a cooperative interaction between MoS₂ and NiS₂. To better understand this synergistic effect in hereto-interfaces of yolk-shell NSs and NiS₂/MoS₂, a linear sweep voltammetry (LSV) curve of physically mixed MoS₂ nanosheets and NiS₂ nanoparticles was carried out. The Janus particle was observed to have better HER activity than the physically mixed sample. Thus, this validates that Janus materials act as better catalysts than their individual components owing to their multi-functionality. The schematic synthesis diagram and HER performance plots are depicted in Fig. 13.

Liu et al.¹⁵³ synthesized unique JP having multifunctional compositions of CoP/Co₂P@NC/Ti. They used a masking approach to grow lamellar cobalt nitrate hydroxide on Ti foil. One side of the foil saw microspheres having an average diameter of ∼2.7–6.3 µm. In contrast, perpendicular pores of the honeycomb structure were observed on the other side, as depicted in Fig. 14. The subsequent phosphidation of CoP/Co₂P@NC/Ti was formed. High-angle annular dark field-scanning transmission electron microsopic (HAADF-STEM) image of the JPs showed numerous bright rods of Co₂P and CoP, which confirmed the formation of Co₂P and CoP nanocrystals in the porous honeycomb wall.

The HER performance of the catalyst was evaluated in 0.5M H₂SO₄, and it showed an overpotential of 31 mV to achieve a current density of 10 mA cm⁻² and high stability (3000 cycles) under harsh acidic conditions. At the same time, the standard Pt/C electrode requires an overpotential of 26 mV. The HER characteristic plots and the SEM images are shown in Fig. 15.

Fu et al.¹⁷⁰ fabricated Janus MoReS₃ nanosheets using sodium molybdate, sodium perrhenate, and thiourea as precursors by employing a modified hydrothermal method followed by annealing of the product. Janus MoReS₃ showed a highly enhanced HER activity compared to their components of ReS₂ and MoS₂. Specifically, the enhanced performance was because Janus MoReS₃ provides a larger surface area and small interfacial electron transfer resistance than their individual counterparts. At higher current densities of 150 mA cm⁻², it showed an overpotential of 189 mV, smaller than the commercial Pt/C electrode (∼261 mV at ∼150 mA cm⁻²).¹⁷⁰ 

Janus particles composed of noble metals and transition metals as constituents have also been reported. Lai et al. developed multifunctional JPs PtFe–Fe₂C Janus particles using platinum (II) acetylacetonate and Fe (CO)₅ as the substrate through a one-pot approach, followed by carbonizing the as-obtained PtFe–Fe₃O₄ particles (during one pot synthesis) in octadecyl amine under Ar atmosphere at 320 °C. They observed that the catalyst promotes a barrier-free interfacial electron transfer, boosting electrocatalysis. The catalyst displays an impressive overpotential of ∼4 mA/cm² and a Tafel slope of only 23 mV dec⁻¹ in 0.5M H₂SO₄.¹⁵⁹ Several other Janus particle electrocatalysts have been reported to date for HER, and Table III summarizes the list of various other Janus particles used for HER (in different media) with their potential performances, such as over potential, Tafel slope, and stability (Fig. 16).

A few metal oxides can withstand the oxidative potentials in acidic conditions; numerous research groups have been looking for non-noble metal oxides for OER electrocatalysts under alkaline conditions where metal hydroxide and oxides are chemically stable.²²⁰ Although many such non-noble metal oxides were synthesized, those materials could not be used for the overall splitting of water. In this regard, Janus particles, with their asymmetric multifunctional characteristics, proved to be a considerable impact as OER electrocatalysts. Xue et al.¹⁵¹ demonstrated a synthesis and applications of Janus Co/CoP nanoparticles [as depicted in Fig. 17(A-i–v)], which exhibited an overpotential of 340 mV at 10 mA/cm² and a Tafel slope of 79.5 mV per decade. The as-prepared Janus particle was active for both HER and OER, as designated in Fig. 17(B-i–iv), where the Volmer–Heyrovsky process was found as the main reaction path for HER. It was observed that the synergistic effect between CoP and Co components leads to an enhancement of both OER and HER, while the as-fabricated electrodes were active at all the pH values, as shown in Fig. 17(C-i–vi). Similarly, Liu et al. demonstrated Ni–P/CF-based JPs by electrodepositing a Ni–P nanoparticle film on copper foam (CF); the as-prepared JP-based electrode exhibited simultaneous catalytic activity of both HER and OER, showing an overpotential of 325 and 98 mV to achieve a current density of 10 mA/cm² for OER and HER, respectively. Moreover, when used as both cathode and anode, the electrode obtained a current density of ∼10 mA/cm² at 1.68 V.¹⁸⁹ 

Li et al.²²¹ designed Janus structured NiS/NiO nanoparticles, which were in situ encapsulated inside an N-doped carbon nanotube nanofiber with a trunk-shaped super-structure by electrospinning followed by calcination and vulcanization; due to the simultaneous effect of nano-architectonics and interfacial engineering, the as-synthesized Janus particles have increased oxygen vacancies, optimized electronic configuration, and good structural robustness and promoted mass diffusion channels. Moreover, they exhibited outstanding OER activity with a small overpotential of 269 mV at ∼10 mA/cm² and a Tafel slope of 48.4 mV/decade, as depicted in Fig. 18. Several other JPs have been reported so far for OER, and Table IV lists a series of Janus particles used for OER along with various electrochemical parameters reported so far.

Photocatalysts for water splitting are preferred over electrocatalysts as they (electro-catalysts) require external energy to split water. Most of the time, this additional energy is supplied from non-renewable sources. A photocatalyst should have a suitable bandgap, effective carrier mobility, and band edge positions. By virtue of their band edge positions, large surface area, and tunable bandgap, a plethora of Janus materials have been suggested for photocatalytic applications for the generation of green hydrogen.

For a photocatalyst to utilize solar energy, the bandgap must be higher than 1.23 eV but lower than 3 eV.²²⁵ Incident photons having an energy equal to or greater than the material’s bandgap generates holes (in the valance band) and electrons (in the conduction band). Redox reactions then occur by migrating these photo-generated charged particles to the surface of the particles.²²⁶ The disadvantages of a single-phase semiconductor are its numerous shortcomings, such as chemical back radiation, low quantum efficiency, and charge recombination.²²⁷ Therefore, combining elements is often employed to provide favorable photocatalytic sites for redox reactions and increase efficiency.²²⁸ Moreover, as heterogeneous catalysis is a surface phenomenon, the change in morphology, size, shape, and functionality of the particle drastically alters the properties and efficiency of the catalysts,²²⁹ which has brought attention to JPs for photocatalytic evolution of hydrogen.

Photocatalysts are catalytically active species that can simultaneously perform reduction and oxidation reactions. When light is irradiated on a photocatalyst, the electrons’ energy in the material’s valance shell absorbs the energy of the photons. These excited electrons then jump to the conduction band, thus forming a hole in the valance band. The excited electron acts as a reducing agent, whereas the hole acts as an oxidizing agent.²³⁰ However, not all the photons that are incident on the catalysts lead to the generation of an exciton; some of them may recombine to generate heat or might be trapped as ions.²³¹ 

The absorption coefficient of an ideal semiconductor is given as

αE = α₀ √ (E − E_g)/E_g (for direct semiconductor)or

αE = α₀ (E − E_g)/E_g (for indirect semiconductor),where “α” is the absorption coefficient, “α₀” is the intrinsic absorption coefficient of the semiconductor, and E represents the photon’s energy. From the above equations, it can be established that visible light can penetrate deeper into the material to develop electron–hole pairs than photons with higher wavelengths. One must also be careful while selecting the morphology of the catalyst. While a large cross-sectional area of the particles improves the photon-to-electron–hole conversion, smaller particle sizes lead to an increase in the surface area, which increases the number of incident photons and reduces migration distances of the electrons and holes.

The photocatalytic splitting of water for the generation of hydrogen consists of four primary steps: (a) generation of electron–hole pairs by irradiation of light on the photo-anode; (b) oxidation of water by the hole on the surface of the anode to produce H⁺ and O²⁻; (c) transfer of the as generated electron through an external circuit to the cathode; and finally, (d) reduction of H⁺ ions by the photoelectron at the cathode surface to generate H₂.²³² 

Photocatalysts offer many advantages over electrocatalysts, viz., (a) it has a very simple setup where no special electrolytes or external circuits are required;²³³ (b) the reaction proceeds at normal temperature and pressure (NTP), which is less energy intensive;²³³ and (c) this reaction does not need to require any fuel-based energy sources and is eco-friendly.²³⁴ However, the disadvantages include (a) low quantum efficiencies;²³⁵ (b) low reproducibility of the reported rates;²³⁶ (c) scalability and fabrication of the materials;²³⁷ and (d) separation of products is difficult as generation of oxygen and hydrogen in the same reactor leads to undesired reverse reaction and generates water.²³³ In an typical example, Sun et al.²³⁸ reported the synthesis of [TiO₂/C]//[Bi₂WO₆/C] carbon-based Janus nanofiber heterojunction, and the process is schematically illustrated in Fig. 19(A). The as-synthesized fiber’s carbon content was analyzed via thermogravimetric analysis (TGA) [Fig. 19(B)], and the pore size distributions (whether the structure containing any micropores, mesopores, and macropores) were evaluated using Brunauer–Emmett–Teller (BET) surface area. The specific surface area and average pore diameter of the Janus hybrid were found to be 29.64 m² g⁻¹ and 24 nm, respectively [Fig. 19(C)].²³⁸ The as-synthesized hybrid Janus fibers grew toward a certain orientation to form an array of nanofiber [Figs. 19(D) and 19(E)] and those Janus fibers showed excellent photoelectrocatalytic hydrogen production, as depicted in Figs. 19(F)–19(I). Numerous first principle studies have been conducted to determine the feasibility of using Janus photocatalysts for hydrogen evolution; however, due to the complex nature of materials fabrication techniques, only a few have been synthesized until date, as listed in Table V.²³⁹ 

In summary, Janus, a class of materials, has been widely researched in recent years. These materials, along with posing a mirror asymmetry, also display a wide range of significantly unique electronic properties, such as piezoelectric properties, tunable bandgap, and the Rashba effect, thus putting these materials to great use in electronic devices, such as sensors, actuators, spintronics, energy harvesting, and so on. Posing two or more different faces, JPs can be used for a vast pool of applications, including biomedical applications (especially for drug delivery and tissue regeneration), optics, and catalysis (especially electrocatalysis). Moreover, one significant advantage that Janus poses over other nanomaterials is that a single Janus particle can be utilized to attach different chemical functionalities on its different faces. Interestingly, clean and green hydrogen shows us a beacon of hope in the world amid an energy crisis.

In contrast, commercial adaptation of hydrogen as an alternative to fossil fuels requires using less expensive, earth-abundant elements as catalysts. Over the past few years, numerous breakthroughs have been achieved in fabricating materials with near noble metal-like catalytic efficiencies. In this regard, it has been observed that Janus particles, with their anisotropy and multi-functionality, offer catalytic performance over their counterparts. This Review summarizes the different synthesis processes for fabricating Janus particles, including microfluidics, self-assembly, seeded nucleation, and surface nucleation. Next, we summarize the applications of Janus particles in the electrocatalytic splitting of water. Specifically, we focus on evaluating the performances of various JPs in various segments of other water electrolysis, which results in superior catalytic performances. Indeed, JPs combine the properties of the individual components, coupled with their asymmetry, anisotropy, and multi-functionality. This could be extremely useful for devices involving multi-directional reactions (e.g., electrolyzer, HER, and OER). Janus particles outperform most non-noble metal-based catalysts, particularly for multi-functional applications. However, there is still much room for improvement in various aspects of JP synthesis and applications. For instance, not all Janus catalysts are pH universal; most of them operate in chloro-alkali, acid-water, water-alkali, or neutral electrolysis cells. The pH universality of the Janus HER catalyst will broaden the scope of application of these catalysts even further.

Similarly, the majority of the Janus catalysts are not used for overall water splitting, while some Janus electrocatalysts can be used for both HER and OER, which serves as a cost-effective way for water electrolysis. Successfully fabricating more JPs would pave the way for the accelerated adaptation of these catalysts for real-life applications. Since it is challenging and expensive to fabricate JPs, it is required to develop a cost-effective and simple fabrication process for JP synthesis while scaling them up toward commercial application.

Since these materials are a very recently researched domain, there are many challenges associated with the fabrication and use of these materials, apart from their application in the biomedical. Theoretically, many advances have been made to this field of special-class of emerging materials; however, practically, limited such materials have been synthesized to date. For example, one of those is two-dimensional (2D) MoSSe, which manifests that the fabrication of such a particular class of materials needs more profound study. Regardless of the many versatilities in a single particle, considerable advancements are still necessary to make Janus particles in currently manifested immensely complicated synthesis techniques and small-scale production. From the materials science perspective, evaluating several theories and methodologies to aim and broaden the synthesis techniques of Janus particles methodically is of the most significant importance. Thus, more consistent research and repetition of existing methods will amplify the know-how while continuously innovating several physical and chemical properties of Janus materials, which are not yet explored. In brief, composition, particle size, morphology, and surface modification will typically affect the JP’s applications toward the design, synthesis, and catalytic applications; thereby, strategies need to be developed to systematically arrange those functionalities toward a certain goal of specialized applications. Several major issues and challenges related to water electrolysis applications still prevail, and those have to be addressed. First, it is rather challenging to form multifaceted or multifunction JPs with diverse properties, which could have immense potential to open up many other novel applications. Second, very few JPs are available, which could act as catalysts on overall water splitting, i.e., a JP could act as both HER and OER catalysts. Focusing on such aspects will significantly reduce the catalysts’ cost and accelerate commercial adaptation. In addition, there is still room to improve the catalytic activity of JPs and bring down the overpotential and Tafel slope while improving stability. Finally, it is essential to develop JPs with high catalytic activities in all pH ranges, which could have added advantages of improving water electrolysis efficiencies without any pH effect.

Despite many intriguing advances in JPs for catalytic applications that have already been demonstrated, many other challenges persist in developing simple and scalable synthesis strategies and constructing non-corrosive and non-toxic JPs with excellent HER/OER efficiency. It is also crucial to design JPs with both self-propelled and self-controlled properties and understand JPs’ working mechanism with all catalytic applications. In particular, many published papers reported that Janus particles present superior performance as electrocatalysts (both HER/OER) compared with conventional homogeneous particles; however, in major cases, the mechanism of how Janus particles work is entirely unknown. With the rapidly growing interest in JPs, it is highly required to understand the working mechanism of how the surface and structural anisotropy of JPs work for the water electrolysis reactions. The basic understanding will propel further toward designing JPs, benefiting JPs’ catalytic applications. Last but not least, the above discussions undoubtedly need a long way to go, while meticulous efforts are necessary to address these challenges one by one. To this end, we hope this Review will pave the way for many novel Janus particle syntheses while providing an integral understanding of the design, processing, and functionalities of Janus catalysts, simultaneously stimulating further interest in expanding the horizon in water electrolysis.

Dr. Santanu Das (S.D.) gratefully acknowledges the STARS Project from the Ministry of Education, Government of India (Grant No. STARS/APR2019/NS/428/FS), and SERB Core Research grant (Grant No. CRG/2022/003204) for providing the financial support for this work. S.D. also acknowledges the UGC-DAE consortium (Grant No. CRS-IC-ISUM-41/CRS-324/2019-20/1376) for providing financial support for this work.

The authors have no conflicts to disclose.

Sayak Roy: Conceptualization (supporting); Data curation (lead); Formal analysis (lead); Investigation (equal). Ummiya Qamar: Data curation (supporting); Formal analysis (equal); Investigation (supporting); Methodology (supporting). Assa Aravindh Sasikala Devi: Conceptualization (supporting); Methodology (lead); Resources (supporting); Software (equal); Validation (supporting). Santanu Das: Conceptualization (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Project administration (equal); Resources (equal); Supervision (equal); Validation (equal).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.