1. Introduction
In the global industrialization process, the excessive consumption of fossil fuels has led to increasingly severe issues of resource depletion and environmental pollution. Particularly prominent is the climate crisis triggered by greenhouse gas emissions, which has evolved into a common challenge faced by all humanity [
1,
2]. To achieve sustainable development goals, the research and development of efficient energy conversion and storage technologies, such as water electrolysis (HER/OER), metal-air batteries (ORR), carbon dioxide reduction (CO
2RR), and methane oxidation (CH
4OR), is of great strategic significance [
3-
6]. As a core component of these energy conversion technologies, electrocatalysis can efficiently convert renewable electricity into high-value chemical energy by lowering the activation energy barrier of reactions and enhancing reaction selectivity [
7].
In the field of nanomaterials science, defect engineering has evolved into a cutting-edge research direction for the precise regulation of the electronic structure of electrocatalysts. Research ideas have shifted from passively accepting inherent defects to actively designing defects [
8,
9]. By intentionally introducing structural defects such as vacancies, doped heteroatoms, grain boundaries, and lattice distortions, it is possible to effectively break the rigid symmetry of ideal crystals, reconstruct the local charge density distribution, and optimize the adsorption energy of reaction intermediates [
10-
13]. This atomic-scale structural strategy provides a multidimensional design space to overcome the activity bottleneck of traditional catalytic materials.
However, traditional defect construction methods still face many constraints in practical applications. The high-temperature calcination method based on thermodynamic equilibrium usually relies on extremely high reaction temperatures, which not only easily triggers the sintering and agglomeration of nanoparticles but also leads to the loss of active sites and a decrease in structural stability [
14]. At the same time, high-temperature conditions promote the transformation of metastable defects into stable configurations, making it difficult to effectively retain thermodynamically unstable high-activity defects in the products [
15].
On the other hand, typical wet chemical synthesis routes, such as sol-gel methods and hydrothermal reactions, often require a large number of reagents, produce a significant amount of waste liquid, and have long reaction cycles. The use of acid-base reagents can easily cause irreversible damage to the material's structure [
16]. More critically, these methods struggle to achieve precise quantitative control of specific types of surface defects while maintaining the overall performance of the materials [
17].
Plasma-assisted synthesis technology, as an emerging material preparation strategy, demonstrates significant advantages compared to traditional preparation methods [
18]. As the fourth state of matter, plasma is rich in highly active components such as high-energy electrons, reactive free radicals, and excited species [
19]. These active components can reduce the activation energy barrier of reactions and achieve precise construction of active sites at the atomic level through dual regulation of "etching and peeling" and "component reconstruction" on the material surface [
20,
21]. This technology features rapid synthesis rates, environmental friendliness, and strong interfacial stability, making it an important approach for constructing high-performance electrocatalytic materials with rich defect structures.
Plasma, as the fourth state of matter, is an ionized gas composed of ions, electrons, neutral particles, and free radicals [
22]. According to the energy distribution and thermal equilibrium state of the system, plasma can be classified into high-temperature plasma (Thermal plasma) and low-temperature plasma (Low-Temperature Plasma, LTP) [
23]. In the field of catalysis, low-temperature plasma (cold plasma) has received widespread attention due to its significant non-thermodynamic equilibrium characteristics. The average kinetic energy of electrons is between 1-10 eV, and the corresponding electron temperature (Te ≈ 10
4-10
5 K) is much higher than the gas temperature of ions and neutral molecules [
24]. Due to this significant temperature difference, plasma can generate highly chemically active reactive species at near-room temperature conditions, leveraging inelastic collisions between high-energy electrons and gas molecules, thereby facilitating chemical restructuring processes that are difficult to occur under thermodynamic equilibrium conditions [
25,
26].
The plasma treatment process primarily entails generating a stable electric field between the anode and cathode using a power source (
Fig.1a). This ionizes the gas and creates a plasma region enriched with high-energy electrons, ions, and free radicals on the material surface [
27]. Positively charged ions are directed and accelerated by the electric field of the cathode, bombarding the material surface placed at the cathode at high speeds, inducing defect structures at the atomic scale [
28]. The formation, density, and stability of these plasma-induced defect structures are further governed by the tunable physical parameters of the plasma process [
18]. Discharge power affects the electron temperature, ionization degree, sheath potential, and energy flux delivered to the material surface, thereby determining the probability of bond breaking, atom removal, and surface reconstruction [
8,
19,
22]. Gas composition controls the chemical nature of reactive species; for example, inert gases such as Ar mainly promote physical sputtering, H
2 plasma favors the reductive removal of lattice oxygen and the formation of low-valence metal sites, whereas O
2 or air plasma promotes oxidative reconstruction and the generation of oxygen-containing surface groups [
2,
5,
13]. Gas pressure, collision frequency, and ion bombardment energy, which influence whether defect generation is dominated by physical impact or chemical reactions [
22,
23]. Treatment time determines the balance between defect generation, defect saturation, and over etching. Therefore, precise control of defect density and thermodynamic stability requires coordinated regulation of plasma power, atmosphere, pressure, time, and temperature [
2,
4,
8]. The advantages and disadvantages of the six plasma technologies and their applicable scenarios are presented in
Table 1.
In the field of material synthesis, plasma technology has the following unique advantages (
Fig.1b): 1) High reactivity: it can generate a large number of high-energy particles, active free radicals, and ultraviolet photons to break strong chemical bonds [
22,
26,
32]. 2) Rapid synthesis and processing: reactions are typically completed in seconds to minutes, achieving high energy utilization efficiency [
35,
36]. 3) Synthesis at room temperature or low temperatures: it avoids high-temperature thermal stress, making it suitable for flexible substrates and thermosensitive materials [
20,
25,
37]. 4) Surface-selective modification: the depth of action is controllable, primarily affecting the surface and subsurface of the material, while the bulk structure remains unaffected [
38,
39,
48]. 5) Environmentally friendly: it usually requires little or no chemical solvents, thereby reducing pollution [
20,
24,
40]. This article summarizes the recent research progress on plasma-assisted synthesis of defect-rich electrocatalysts, including the physical principles of plasma technology, the microscopic mechanisms of defect formation, strategies, and challenges.
2. Plasma-assisted synthesis of vacancy defect strategies
2.1 Carbon Vacancies
Carbon-based materials are favored in the field of electrochemistry due to their excellent conductivity, tunable structure, and high specific surface area. Utilizing plasma for physical etching and chemical activation of the carbon framework can quickly break the originally inert sp
2 hybridized conjugated network [
40,
41], generating a large number of carbon vacancies, topological distortion defects, and highly active unsaturated edge sites in situ [
42-
44]. These structural defects not only effectively increase the material's electrochemical active surface area but also disrupt the electrical neutrality of the carbon surface by inducing local charge redistribution, creating a unique surface microenvironment [
45,
46].
Current research on graphene, a typical two-dimensional carbon material, focuses on defect regulation to alter its intrinsic electronic structure. Carbon atoms can be directly impacted by Ar plasma to remove them from sp
2 or sp
3 hybridization, leaving vacancies [
47]. NH
3 andN
2 are also commonly used as plasma sources to create carbon vacancy defects, removing carbon atoms from the lattice through collisions to produce active sites such as single vacancies (SV) and double vacancies (DV). Low voltage radio frequency inductively coupled plasma technology is used to generate highly chemically active nitrogen ions, atoms, and free radical species in NH
3 or N
2 gas environment, inducing the formation of defect structures on the surface of graphene nanosheets, such as single vacancies and double vacancies (
Fig. 2a) [
48]. In addition to conventional physical sputtering, the chemical etching with reactive plasma can dynamically control the size of vacancy defects. Single layer graphene was chemically etched using oxygen plasma, and atomic level carbon vacancies were gradually expanded into tunable nanoscale pores by controlling the etching time [
49]. Furthermore, when targeting carbon supports with complex networks and porous structures, if the plasma energy is sufficient to induce rearrangement of lattice atoms rather than merely removing them, it often accompanies the generation of topological defects [
50]. Directional bombardment of highly reactive carbon atoms at the edges of graphene pores with nitrogen plasma effectively induces the transformation of hexagonal rings into pentagonal and heptagonal topological defects, while maintaining the original three-dimensional honeycomb conductive structure (
Fig. 2b) [
51].
Unlike the planar structure of graphene, carbon nanotubes (CNTs) possess a unique one-dimensional high symmetry and curvature of the tube walls [
52]. Plasma etching can not only introduce point defects but also induce local geometric distortion by disrupting the electronic delocalization balance of the tube walls [
53,
54]. Relevant studies have utilized plasma technologies such as O
2 and N
2 for deep directional etching of multi-walled carbon nanotubes, achieving efficient construction of vacancy defects in the tube walls (
Fig. 2c-d). In an O
2 plasma field, high-energy reactive particles engage in vigorous chemical reactions with the carbon network to directly create vacancy defects on the tube walls [
55]; in contrast, in N
2 plasma, non-equilibrium high-energy particles strip lattice carbon atoms through intense physical collisions, forming vacancy defects [
56]. Both types of plasma etching treatments significantly disrupt the one-dimensional high symmetry of carbon nanotubes, greatly enhancing the defect density of the samples, and enabling efficient physical etching and vacancy activation of the carbon substrate while retaining the overall tubular framework [
57]. The intrinsic carbon vacancies induced by plasma in carbon fiber composites possess unpaired electrons and extremely high surface energy, making them ideal metal anchoring sites [
58]. A significant number of carbon defects can be in situ etched on the surface of flexible carbon cloth utilizing plasma technology (
Fig. 2e) [
59]. These plasma-induced carbon vacancy defect sites serve as anchoring centers, not only introducing oxygen-containing groups to improve hydrophilicity but also successfully suppressing the aggregation of metal precursors, achieving efficient anchoring and high dispersion of ultra-low loading PtRu alloy nanoclusters.
2.2 Oxygen Vacancies
For transition metal oxides and their derivatives, plasma technology provides an efficient pathway for constructing defects at room temperature. This technique not only avoids the sintering issues associated with traditional high-temperature treatments but also introduces a high concentration of oxygen vacancies on the lattice surface through high-energy ion bombardment [
60]. These oxygen vacancies are capable of effectively modulating the local electronic structure of materials, accelerating charge transfer, and playing a crucial role in providing active anchoring points for heteroatoms [
61]. Jang et al. directly treated MnCuOx precursors with Ar or O
2 plasmas, successfully preparing defect-type materials with precisely controllable oxygen vacancies at room temperature [
62]. In perovskite-type oxides, high concentrations of oxygen vacancy defect structures were generated on the surface of LaCoO
3 using H
2 plasma, while the original crystal structure was fully maintained (
Fig. 3a) [
63]. Moreover, these vacancies created by plasma can be directly used to accommodate other foreign atoms. For instance, exposure to Ar plasma generates numerous oxygen vacancies on the surface of Co
3O
4 nanosheet arrays. Subsequently, nitrogen and sulfur atoms from the thermal decomposition of thiourea occupy these vacancies, leading to the formation of a confined structure (
Fig. 3b) [
64]. Similarly, platinum atoms can be introduced into the oxygen vacancies on the surface of D-Co
3O
4 nanosheets to synthesize platinum single-atom catalysts (
Fig. 3c) [
65].
In addition to traditional metal oxides, this plasma preparation strategy is also applicable to structurally complex transition metal derivatives, thus avoiding the generation of metal nanoparticles due to high-temperature pyrolysis. For instance, oxygen-rich vacancy defects can be engineered on the surface of Cu-MOF using low-temperature plasma [
66]. This process exposes numerous low-coordination metal centers and facilitates the preparation of highly active single-atom copper catalysts while maintaining the integrity of the original porous structure (
Fig. 3d).
2.3 Nitrogen Vacancies
Nitride materials (such as carbon nitrides, transition metal nitrides, etc.) have received widespread attention in the field of defect engineering. Plasma technology offers an efficient room temperature modification method that not only circumvents the limitations of traditional high-temperature pyrolysis [
67], but can also rapidly introduce a high concentration of intrinsic nitrogen vacancies on the material's surface through physical sputtering and chemical activation [
68]. These vacancies effectively break the symmetry of the lattice and adjust the local electronic states, playing a crucial role in constructing highly active geometric sites and complex defect interfaces [
69].
The static vacancy etching strategy for single non-metal systems was developed, introducing adjacent double N
2C site nitrogen vacancies (ACN) into amorphous carbon nitride by treating melamine-formaldehyde precursors with H
2 plasma [
70]. Concurrently, the high-energy bombardment from the plasma further enabled the material to form a hierarchical continuous nanosheet network structure with an ultra-high specific surface area (
Fig. 4a). For the rapid remodeling strategy of transition metal multilayer heterojunctions, a double-layer Ni
3N nanoporous sandwich heterojunction (Ni
3N/Ni/Ni
3N) with metal nickel layers inserted can be quickly constructed by using radio frequency plasma bombardment to electrodeposit nickel particles [
71]. The physical etching from the high-energy plasma facilitated the production of abundant intrinsic nitrogen vacancies on the surface of the heteromaterial (
Fig. 4b). In the reactive plasma strategy of dynamically generating vacancies, in-situ nitration of the WO
3 array surface under H
2/N
2 plasma can accurately construct a multiphase interface of WO
xN
y/WO
3, which is conducive to electron transfer (
Fig. 4c) [
72]. In terms of the collaborative construction strategy of multi anion defects, the TiN/TiO
2 interface structure was treated with H
2 plasma to form nitrogen vacancies on the TiN surface and oxygen vacancies on the TiO
2 surface, thereby constructing a composite material rich in nitrogen and oxygen anion double vacancies (
Fig. 4d) [
73].
2.4 Sulfur and Selenium Vacancies
For transition metal sulfides, selenides, and phosphides, plasma selectively removes surface heteroatoms through physical or chemical etching, resulting in anion vacancies while exposing unsaturated coordination metal sites [
74,
75]. This method facilitates the thinning of materials and the construction of defects. In constructing sulfur vacancies, plasma physical etching primarily relies on the continuous bombardment of high-energy particles in inert gas plasmas to remove sulfur atoms. For instance, the reconstruction of sulfur vacancies on the surface of cobalt doped Ni
3S
2 nanoarrays was achieved through Ar plasma (
Fig. 5a） [
76]. Based on this physical etching mechanism, precise removal of sulfur atoms on the surface of CoS
2 nanoparticles has been successfully achieved, resulting in sulfur vacancies while maintaining the integrity of the CoS
2 crystal structure [
77]. Furthermore, plasma stripping and modulation strategies can achieve interlayer stripping of precursor materials and modulation of microscopic defects under mild environmental conditions. DBD cold plasma technology effectively produced a significant number of sulfur vacancies on the surfaces of two-dimensional nanosheets derived from bulk MoS
2 [
78]. They dynamically and controllably constructed macroscopic stripping of two-dimensional materials and the concentration of microscopic sulfur vacancies by adjusting the plasma treatment time (
Fig. 5b).
In addition, high concentration Se vacancy defects can be constructed on the surface of MoSe
2 through physical etching using plasma technology [
79]. This physical etching strategy also demonstrates a high degree of controllability in other two-dimensional transition metal chalcogenides (
Fig. 5c). On the premise of maintaining the stability of the WSe
2 crystal skeleton through radio frequency plasma treatment, precise exfoliation of surface selenium atoms and introduction of selenium vacancies were achieved (
Fig. 5d) [
80].
2.5 Cation Vacancies
Compared to carbon materials, metal lattices have higher atomic migration energy barriers, making it difficult for traditional chemical methods to precisely construct metal cation vacancies under mild conditions. Plasma technology, with its non-equilibrium characteristics of high-energy particles, provides a powerful means for the regulation of metal vacancies [
81,
82]. Because low-temperature plasma operates under non-thermodynamic equilibrium conditions, high-energy particle bombardment provides violent, localized kinetic impacts [
22,
26]. These impacts efficiently knock metal cations out of their equilibrium lattice nodes [
4,
41], successfully overcoming high migration energy barriers without relying on high-temperature calcination [
28]. In alloy systems, constructing metal vacancies using plasma still faces certain challenges. However, through selective etching strategies, precise construction of metal vacancies can be achieved in complex systems. The treatment of SnCoFe perovskite hydroxides with Ar plasma preferentially and selectively removes Sn atoms, resulting in a high density of metal cation vacancies at the atomic scale and the induction of oxygen vacancies [
83].
In layered double hydroxide (LDH) systems, plasma technology can induce the generation of high-density cation vacancies while facilitating material exfoliation. Liu et al. used DBD water plasma to treat CoFe LDH precursors, rapidly achieving the exfoliation of layered materials [
84]. During the exfoliation process, the etching effect of the plasma in situ generated various vacancy defects, including Co and Fe metal cations as well as O anions.
Plasma reshapes the surface structure of materials at the atomic scale through "top-down" etching and "bottom-up" deposition [
40]. The formation of defects significantly alters the local electronic density distribution, optimizing the adsorption and desorption energies of reaction intermediates (such as adjusting the adsorption strengths of *O, *OH, and *H), thereby enhancing the intrinsic catalytic activity [
41]. This approach avoids the severe limitations present in traditional defect construction methods during thermodynamic equilibrium processes. The main types of defects include carbon vacancies, oxygen vacancies, nitrogen vacancies, sulfur, selenium vacancies, and cation vacancies (
Table 2).
3 Plasma-assisted doping strategy
Plasma doping is a key method for achieving the functionalization and performance enhancement of materials [
85]. In experiments, this method typically involves discharging in an atmosphere containing target elements (such as N
2, NH
3, H
2S, PH
3, etc.), or directly exposing precursors to these plasma environments [
86-
88]. Electrons dissociate gas molecules through inelastic collisions, generating doping species with extremely high chemical reactivity (such as atoms, ions, or free radicals) [
89]. These species can not only directly embed into the material's lattice or replace existing atoms, but the vacancies that are simultaneously generated during the plasma etching process often become preferred anchoring points for heteroatom doping, leading to multi-element co-doping and defect complexes such as diverse structures [
90].
In plasma-induced defect engineering, the electronic structure can be effectively tuned by embedding heteroatoms into specific point defect sites [
48]. The selection of different plasma precursor gas sources enables precise introduction of specific alloyed atoms and directional control over their configuration. Nitrogen is currently the most widely used doping element in research. For example, controllable doping of nitrogen atoms in 1T/2H MoS
2 heterostructures can be achieved using room temperature N
2 plasma technology [
91]. By substituting sulfur atoms in the lattice with N ions generated from the plasma, Mo-N bonds are formed without disrupting the overall morphology of the material, while also utilizing defects such as vacancies and dislocations induced by the plasma (
Fig. 6a). In contrast, NH
3 plasma can produce a large number of reactive hydrogen species (such as H*, NH
2*) during the discharge process, which can efficiently achieve nitrogen doping when combined with nitrogen-containing solid precursors. Ti
3C
2T
x MXene was successfully exfoliated and doped with nitrogen atoms through NH
3/Ar plasma technology [
92]. The electron temperature generated by the plasma induces instantaneous decomposition of interlayer NH
4HCO
3, producing high-pressure gas that rapidly disrupts van der Waals forces for MXene exfoliation; simultaneously, the plasma converts NH
3 into metastable ions and radicals such as NH
2, NH, N, and H, which combine with active sites generated by Ar ion bombardment to achieve nitrogen doping (
Fig. 6b).
Additionally, doping of elements with large atomic radii can also be achieved through plasma sources such as H
2S and PH
3. Ma et al. utilized DBD plasma technology to achieve efficient in-situ synthesis of sulfur-doped graphitic carbon nitride in an H
2S atmosphere [
93]. This process not only resulted in a high amount of sulfur doping but also induced the interlayer separation of g-C
3N
4, significantly enhancing the specific surface area (
Fig. 6c). Similarly, plasma doping using PH
3 employs phosphorus species produced by high-energy dissociation as a doping source, facilitating in-situ phosphorus doping of nanocrystalline silicon thin films [
94]. Plasma treatment can not only introduce single foreign atoms but also optimize the charge distribution in materials by forming defect complexes that combine foreign atoms with vacancies [
95,
96]. In this coupling mechanism, the anionic vacancies generated by plasma etching provide preferential anchor points for the embedding of foreign atoms [
97]. Carbon based conductive agents with nitrogen atom doping and topological vacancy composite structures were constructed in situ in a mixed atmosphere of Air and NH
3 through cold plasma [
98]. For transition metal compound systems, N
2/Ar plasma surface treatment of NiCo bimetallic hydroxides successfully introduced nitrogen atoms into the oxide lattice and generated vacancy structures in situ [
99]. Similarly, nitrogen doping and sulfur vacancies were successfully constructed simultaneously in Bi
2S
3 nanorods using NH
3/Ar plasma treatment strategy (
Fig. 6d) [
100]. The active nitrogen species embedded at lattice sites altered the local electronic density and coordination environment together with the sulfur vacancies generated simultaneously. For more complex heterogeneous interface systems, NH
3 plasma treatment is used to treat Co
9S
8 graphene composite materials [
101]. Through physical etching and chemical doping of nitrogen heteroatoms, composite defect structures with vacancies and doping defects can be constructed (
Fig. 6e). Further research indicated that directional construction of defect types could be realized by adjusting plasma parameters. Low power N
2 plasma was used to etch the carbon lattice while introducing nitrogen atoms, resulting in the in-situ construction of nitrogen doped composite structures rich in vacancies (
Fig. 6f) [
102].
4. Other plasma-assisted strategies for defect engineering4.1 Plasma-induced reduction and oxidation
In addition to vacancy creation and heteroatom doping discussed in Sections 2 and 3, plasma reduction and oxidation represent another important class of plasma-assisted strategies for defect engineering. This technology mainly involves controlling the plasma atmosphere (such as H
2, O
2, etc.) to utilize the strong reducing or oxidizing radicals generated by dissociation, directly breaking chemical bonds on the material's surface under mild conditions, thereby enabling the controlled construction of oxygen vacancies, cation vacancies, low-valence metal species, and oxides at the atomic scale [
104-
106].
During the plasma reduction process, H
2 plasma can provide a very strong non-equilibrium reducing environment [
107], effectively seizing oxygen from metal oxide lattices, thus forming oxygen vacancies and partially reduced metallic states [
108]. This reduction mechanism effectively breaks local charge symmetry and reshapes electron transport pathways [
109]. The highly reducing hydrogen radicals (H*) produced by H₂ plasma rapidly react with oxygen atoms on the surface of Cu-Co bimetallic oxides, resulting in the formation of Cu-Co surface alloys [
110]. This process not only achieves in-situ reduction of the metal centers but also induces the formation of abundant unsaturated coordination defects at the atomic scale (
Fig. 7a).
For more complex layered bimetallic hydroxides, NiFe LDH is treated synergistically through cation doping and plasma reduction [
111]. It can promote the formation of Ni
0 nanoparticles and rich vacancies defects on the surface to form a heterojunction composite structure of nickel nanoparticles/V doped NiFe LDH nanosheets. Similarly, plasma reduction treatment of monolayer MoS
2 formed isolated metal single atom sites with unsaturated coordination environment on its surface (
Fig. 7b) [
112].
In contrast, when performing plasma oxidation, O
2 or air is used as the gas source to controllably introduce oxygen-containing groups and oxidative defects into the substrate [
113]. These strongly oxidizing species not only rapidly fill inherent anionic vacancies in the material but also form oxygen coordination bonds through strong chemical adsorption at lattice edges or defects [
114], further inducing metal surface species to evolve to higher valence states [
115,
116], forming a heterogeneous interface between metals and oxides [
117]. Utilizing the strong oxidation capability of high-energy oxygen species in the plasma field, we can construct a Cu-CuO heterojunction interface in situ on the surface of copper metal (
Fig. 8c) [
118]. For carbon-based materials, the intense oxidizing etching effect of plasma can effectively break the inert carbon framework, achieving functionalization of the material surface [
119,
120]. Plasma treatment of carbon nanotubes (CNTs) can induce the formation of carbon dot topological defects and simultaneously graft oxygen-rich functional groups, including hydroxyl, carboxyl, and carbonyl groups, onto broken carbon bonds through robust chemical interactions.
4.2 Plasma-assisted in situ synthesis
Plasma-assisted in situ synthesis represents another important route for defect engineering, distinct from post-treatment strategies such as vacancy creation and heteroatom doping. In this route, plasma is directly involved in precursor decomposition, nucleation, crystallization, and material growth. The resulting non-equilibrium growth environment can promote the formation of amorphous phases, ultrafine nanostructures, grain boundaries, unsaturated coordination sites, and heterogeneous interfaces [
121,
122]. Therefore, plasma-assisted in situ synthesis is particularly suitable for constructing intrinsic defects during bottom-up material growth [
123,
124].
In this plasma-driven synthesis process, the formation of non-equilibrium thermodynamic conditions often prevents atoms from perfectly arranging within the lattice [
125]. As a result, this method easily introduces a large number of intrinsic defects during the material growth process [
126]. Unlike the long-range ordered lattice materials obtained through traditional high-temperature thermal synthesis, plasma-assisted synthesis tends to induce the formation of amorphous phases, ultrafine nanostructures, and small grains [
127]. At the interfaces of these nanostructures, a significant number of dislocations, unsaturated coordination, and topological defects are inevitably generated [
124,
128].
When preparing carbon-based films or nanostructures, plasma serves not only as an energy source for the decomposition of precursors but also produces reactive species that facilitate rapid nucleation and uneven growth on substrates, thereby endowing the deposited carbon framework with abundant edge sites and a porous defect network [
129]. Furthermore, when constructing composite films using plasma sputter deposition, the plasma-driven non-equilibrium growth mode allows for the in-situ construction of specialized amorphous/crystalline grain boundary networks, enabling deep control of grain boundary engineering [
130]. For example, plasma magnetron co-sputtering, nitriding, selenization, and other methods can be used to prepare a series of catalysts such as B-Cu amorphous composite films [
131], Ni@Ni
3N/NCNT@CC, and p-NiSe
2 (
Fig. 7d) [
132]. Additionally, the defects and surface modifications produced by plasma on carbon substrates create favorable conditions for the synthesis and growth of transition metal hydroxide derivatives. Two steps atmospheric pressure DBD air plasma treatment was used to pretreat carbon cloth, and a three-dimensional lotus shaped heterostructure (IrO
2-P-Coch/PCC) with oxygen rich vacancy defects was directly constructed (
Fig. 7e) [
133].
5. Electrochemical applications
5.1 Water Electrolysis
Electrocatalytic water splitting is an effective way to produce clean hydrogen energy; however, its energy conversion efficiency is limited by the kinetics of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER), requiring additional overpotentials to overcome the energy barriers of these two half-reactions [
137,
138]. For HER, the ideal catalyst's hydrogen adsorption free energy (ΔG
H) should be close to zero to achieve kinetic balance between adsorption and desorption [
139]; however, conventional non-precious metal materials have a limited density of active sites, and ΔG
H is often either too high or too low (strong adsorption leads to difficulties in desorption, while weak adsorption results in inadequate surface hydrogen coverage) [
140]. For OER, this process involves complex four-electron transfers, with high adsorption energy barriers for various intermediates (*O, *OH, *OOH), leading to increased polarization overpotential [
141,
142]. Additionally, catalysts are prone to structural collapse, loss of electrochemical active surface area, and deactivation in strongly oxidizing and acidic environments [
143]. Plasma surface engineering introduces defects and regulates local coordination environments and electronic structures, effectively overcoming the kinetic barriers of HER and OER through increased active site density, optimized adsorption energies, and controlled reaction microenvironments [
144].
Plasma can directly dislodge specific lattice atoms through high-energy bombardment to create vacancy defects. In the cathodic hydrogen evolution reaction, Wu et al. adopted a plasma etching strategy to precisely control the concentration of oxygen vacancies on electrochemically inert two-dimensional Bi
2O
3 nanosheets (
Fig. 8a) [
145]. Studies showed that a moderate concentration of oxygen vacancies can significantly enhance the material's free charge carrier density and optimize H adsorption energy. The optimized Bi
2O
3 catalyst exhibited excellent HER performance in 1 M KOH alkaline solution, requiring only 174.2 mV overpotential at a current density of 10 mA▪cm
-2, with a Tafel slope as low as 80 mV▪dec
-1 (
Fig. 8b), successfully activating the originally inert Bi
2O
3 into an efficient alkaline HER catalyst. In the anodic oxygen evolution reaction, plasma-induced anionic vacancies also demonstrate significant abilities to reconstruct electronic structures. Through collaboration between sulfur vacancies and cobalt atoms constructed by plasma, Peng et al. effectively adjusted the electronic structure of the active metal center's Ni d-band, significantly reducing the energy barrier for the rate-determining step of O to OOH* conversion and greatly accelerating OER kinetics [
76]. For high entropy oxides (HEO), oxygen vacancies were precisely constructed on the surface of HEO using direct current argon plasma. It could optimize the electronic structure and improved the electrode surface charge transfer rate and reaction kinetics during the electrolysis of water reaction process (
Fig. 8c) [
103]. Furthermore, this vacancy engineering is applicable not only to traditional metal oxides or compounds; plasma etching techniques can also induce abundant vacancies in porous materials like MOFs (
Fig. 8d), exhibiting excellent kinetic performance in water electrolysis [
147].
Plasma technology not only manufactures vacancy defects through high-energy particle bombardment, but another significant advantage is its ability to achieve physical etching and chemical doping in a synergistic manner in one step, allowing precise control over the surface chemical properties and electronic structure of catalysts [
96,
147]. By using plasma-induced vacancies as templates and employing dopant atoms for in-situ filling, the coordination environment can be adjusted, which significantly enhances catalytic efficiency through the synergistic effect between the dopant atoms and metals [
64,
148]. For instance, to address the challenge of high energy barriers for the adsorption of oxygen-containing intermediates in the oxygen evolution reaction (OER), N-doped NiCoPBA nanocages with rich oxygen vacancies were prepared using Ar/N
2 plasma etching with H₂ thermal reduction method [
149]. The dual modification of oxygen vacancies and nitrogen doping, along with the porous structure, effectively modulated the electronic states of the metal centers, significantly increasing the active surface area and charge transfer efficiency. In OER tests conducted in a 1 M KOH solution, this catalyst demonstrated a low overpotential of 289 mV, a Tafel slope of 70 mV▪dec
-1, and long-term stability, surpassing commercial RuO₂ in overall performance. To meet the more complex and demanding dual-function catalytic requirements for overall water splitting, Zha et al. employed an RF plasma-assisted phosphorization strategy, effectively inducing the generation of a large number of anionic P vacancies through high-energy species produced by the plasma [
150]. The P vacancies exhibited a synergistic effect with metal Ni doping and the carbon support. This not only optimized the adsorption free energy of hydrogen atoms on the surface of the catalyst, promoting the hydrogen evolution reaction, but also significantly enhanced the electronic coupling between the underlying phosphides and the surface oxide layer formed in the OER (
Fig. 8e). This local charge transfer mechanism at the interface endows the catalyst with exceptional intrinsic activity and long-term stability in overall water splitting (
Fig. 8f).
In addition to the direct modulation of the intrinsic active sites of the catalysts, plasma defect engineering also shows great potential in tuning the strong interactions at the interface between the catalyst and the support, such as N-doped antimony tin oxide (ATO) with rich oxygen vacancies [
151]. These interfacial defects effectively modulated the strong interactions between the catalyst and the support, significantly shortening the interatomic distance between Ir and the Mn bimetallic active centers and enhancing the local electronic coupling (
Fig. 8g). This synergistic effect mediated by interfacial defects successfully stimulated a lower energy barrier asymmetric oxygen coupling reaction pathway, greatly enhancing the OER activity and long-term stability of the catalyst in acidic environments (
Fig. 8h).
5.2 Electrocatalytic reduction of CO2
The electrocatalytic reduction reaction of carbon dioxide (CO
2RR) driven by renewable electricity is a highly promising approach to converting the greenhouse gas CO
2 into high-value chemicals or hydrocarbon fuels, which is essential for achieving carbon cycling and dual carbon goals [
152]. However, the highly stable C=O double bond within the CO
2 molecule makes its thermodynamic and kinetic activation extremely challenging [
153]. Furthermore, CO
2RR involves complex multi-proton-electron coupled transfer processes and faces significant competition from hydrogen evolution reactions (HER) in aqueous electrolytes [
154], leading to common challenges such as high overpotential, poor product selectivity, and insufficient stability for traditional catalysts [
155]. In light of these issues, plasma defect engineering has demonstrated remarkable capabilities in structuring surface interfaces [
156]. By bombarding the catalyst surface with high-energy particles, plasma technology can precisely create vacancy defects, construct grain boundaries, or modulate the coordination environment of metal atoms [
157,
158]. This process effectively reshapes the electronic structure of active centers, thereby optimizing the adsorption free energy of key reaction intermediates. It not only significantly lowers the activation energy barrier for CO
2 but also effectively suppresses competitive HER reactions through spatial hindrance or electronic effects induced by defects [
159], greatly enhancing the Faradaic efficiency and catalytic stability of target products [
160].
Copper-based materials are among the few heterogeneous catalysts capable of efficiently catalyzing C-C coupling, and plasma technology can greatly stimulate and stabilize their catalytic potential, leading to breakthroughs in synthesizing multicarbon products from CO
2RR [
161]. Nitrogen cold plasma technology was employed to treat an Ag@Cu core-shell structure, successfully stabilizing the Cu
0/Cu
+-on-Ag interface in an electrochemical environment by forming non-stoichiometric CuN
x species in situ (
Fig. 9a) [
162]. The Cu
+ sites provided by CuN
x significantly enhanced the adsorption of the key intermediate *CO and cooperated with the interface to substantially reduce the rate-determining step and C-C coupling barrier. This enabled the catalyst to achieve a long-term stability of 96 hours while improving the Faradaic efficiency for C2 products (
Fig. 9b). Similarly, based on the strategy of constructing cooperative active interfaces, Ar/H
2 mixed plasma was used to controllably reduce Cu
2O, successfully creating an oxygen vacancy-rich Cu
0/Cu
+ interface (
Fig. 9c) [
163]. The dual approach involving interface engineering and oxygen vacancy formation also greatly optimized the electronic environment of active centers, promoting C-C coupling kinetics by optimizing the adsorption of key intermediates and providing multiple highly active sites through oxygen vacancies. Recently, in the development of bimetallic synergistic systems, H
2 cold plasma was utilized to preactivate Cu-CoPBA precursors, creating unique CN vacancies [
164]. The spatial hindrance effect induced by vacancy defects and the rearrangement of local electronic density at the interface enabled homogeneous dispersion of the key intermediate CO across copper atoms (
Fig. 9e), further enhancing C-C coupling and improving the selective conversion of CO
2 to low-carbon alcohols (
Fig. 9f). In two-dimensional metal, two-dimensional CuMnO
2 nanosheets were activated by O
2 plasma technology to generate a high concentration of oxygen vacancies [
5], effectively reshaping the local electronic structure and significantly lowering the C-C coupling barrier, thus achieving efficient electrocatalytic conversion of CO
2 to multicarbon products.
In addition to synthesizing carbon products on copper-based catalysts, plasma defect engineering can also deeply regulate non-copper-based catalysts for effectively promoting the conversion of CO
2 into carbon products, such as amorphous manganese oxide (
Fig. 9g) [
60]. These oxygen vacancies not only optimized the local electronic structure of the catalyst, significantly enhancing its adsorption and activation ability for CO
2 molecules, but also maintained a lower binding strength to *CO intermediates, facilitating the rapid desorption of the target products. The prepared defect-rich MnO
x catalyst can efficiently convert CO
2 into CO at lower overpotentials, achieving a Faradaic efficiency as high as 94.8% (
Fig. 9h).
5.3 Methane Electrooxidation
At room temperature and pressure, the electrochemical conversion of methane into high-value-added products through electrocatalysis holds broad applications [
165]. However, traditional catalysts face the bottleneck of insufficient activity and poor selectivity in practical applications. Firstly, the methane molecule exhibits a highly symmetrical, nonpolar tetrahedral structure, with a C-H bond energy as high as 439 kJ/mol, which renders it very inert [
166]. Additionally, intermediates such as *CH
3 are prone to deep oxidation, ultimately converting to CO
2 [
167]. Plasma introduces defects like vacancies or doping through defect engineering, effectively tuning the local electronic structure of active sites [
147], significantly lowering the energy barrier for the first C-H bond cleavage during methane activation [
168]. On the other hand, the unique steric hindrance and electronic confinement effects at defect sites can significantly reduce the binding strength of target products with the catalyst surface, allowing C1, C2, and other products to rapidly desorb after formation, fundamentally reducing the excessive oxidation of intermediates [
169].
To overcome the selectivity bottleneck of methane electrooxidation at room temperature, a highly stable Ni
+/Ni
0 interface was successfully constructed using nitrogen cold plasma technology [
170]. The interface electronic state reconstruction induced by this plasma not only accelerated the initial activation of methane C-H bonds at Ni sites but also enhanced the adsorption of key intermediates such as *CH
3 and *CH
2OH via Ni
+ sites, thereby effectively promoting critical C-C coupling reactions (
Fig. 10a). This achieved a highly selective conversion of methane to lower alcohols (
Fig. 10b).
In a multi-site tandem catalysis, Ni-Co bimetallic catalysts with CN vacancy were prepared by H
2 cold plasma [
171]. The plasma-induced CN vacancy defects optimize the local electronic structure, shifting the active Ni d-band center upward closer to the Fermi level (
Fig. 10c). This shift lowers the energy barrier for CH
4 activation by strengthening the chemisorption of the reactant onto the low-coordinated Ni sites. The enhanced metal-adsorbate interaction effectively weakens the C-H bond, drastically reducing the initial cleavage barrier from 0.78 eV to 0.47 eV. This accelerates the generation of key intermediates (CH
3 and CH
2) and drives CH
2 overflow to adjacent Co sites for efficient tandem esterification into methyl formate.
Benefiting from this bimetallic tandem synergistic effect, intermediates on Ni and Co sites underwent efficient coupling esterification reactions, achieving the first efficient conversion of methane to methyl formate under room-temperature electrocatalytic conditions (
Fig.10d). Furthermore, by constructing a high concentration of oxygen vacancies in two-dimensional porous NiCo
2O
4 using H
2 plasma technology [
172], the d-band centers of Ni and Co were induced to shift upward, significantly enhancing the adsorption of key carbon-containing intermediates, further promoting C-C coupling reactions, and realizing the highly selective, efficient, and long-term stable conversion of methane into high-value-added chemicals.
5.4 Other electrocatalytic reactions
In addition to the aforementioned applications, plasma-modified defect-rich catalysts have also demonstrated excellent performance in reactions such as electrochemical nitrogen reduction and phenol hydrogenation. For example, in the field of electrochemical nitrogen reduction (NRR), the low solubility of N
2 at the water interface and the extremely slow diffusion rate have long limited the efficiency of ammonia synthesis. To overcome this kinetic bottleneck, Xu et al. innovatively employed plasma etching technology to construct a large number of pore defects in situ on the catalyst substrate [
173]. The surface pores induced by plasma not only enhanced the surface heterogeneity of the catalyst but also significantly improved the van der Waals interactions between the catalyst and nitrogen molecules. This unique microspatial confinement effect can directionally attract and effectively anchor nitrogen nanobubbles. Moreover, this clever interfacial gas confinement mechanism significantly increased the supply rate of local reactants at the catalytic sites, enabling efficient ammonia synthesis under ambient temperature and pressure while achieving outstanding Faradaic efficiency (
Fig. 10e).
In the case of selective hydrogenation of phenol, plasma surface reconstruction technology can also be utilized to build carbon defects catalysts. Plasma-induced defects, as efficient anchoring sites, can enhance the interaction between the metal and the support, facilitating the uniform dispersion of nanoparticles and effective modulation of their electronic structure [
174]. Under mild conditions, the catalyst exhibited excellent hydrogenation activity and selectivity (
Fig. 10f).
Plasma defect engineering is a non-thermal equilibrium and environmentally friendly surface modification technique. Due to its ability to precisely adjust the local electronic structure of materials and construct high-energy surface-active sites, it shows great application potential in various electrochemical energy conversion technologies. This section will systematically overview the application progress and intrinsic mechanisms of defect-rich catalysts modified by plasma in water electrolysis, carbon dioxide reduction, methane oxidation, and other electrochemical reactions (
Table 3).
6. Summary, Challenges, and Future Perspectives
Plasma-assisted synthesis has emerged as a promising route for the efficient and controllable preparation of defect-rich electrocatalysts. As a typical non-thermodynamic equilibrium strategy, plasma technology leverages high-energy electrons, reactive ions, and free radicals to overcome the kinetic limitations imposed by conventional thermodynamic reaction barriers under mild, near-room-temperature conditions. Through distinct physical sputtering and chemical etching processes, it enables precise sculpting and modification of material surfaces at the atomic scale, generating high concentrations of highly active, metastable defect structures that are difficult to achieve using traditional methods. This strategy has demonstrated substantially enhanced performance over conventional catalysts in a wide range of electrochemical energy conversion reactions, including water electrolysis, metal-air batteries, carbon dioxide electroreduction, and methane oxidation. Despite these advantages, the development of plasma-based defect engineering still faces several critical challenges:
(1) Design and precise characterization of defects
Currently, accurately identifying and quantitatively analyzing specific complex defect types (such as vacancies of different sizes, multi-element coordinated defect complexes, etc.) remains very challenging. Conventional characterization methods often only reflect the steady-state average structure before and after plasma treatment. In the future, advanced technologies such as spherical aberration-corrected transmission electron microscopy, synchrotron radiation (XAFS), angle-resolved XPS, and in-situ spectroscopy need to be combined to trace the atomic migration trajectories during plasma treatment in real-time, as well as the dynamic interactions between defect sites and reaction intermediates during electrocatalysis, thereby establishing a clearer and more realistic dynamic structure-property-performance relationship of "defect structure-electronic properties-catalytic performance." Meanwhile, to overcome the limitations of current empirical explorations, there is an urgent need to integrate machine learning and first-principles calculations to predict optimal defect configurations with superior catalytic performance and stability at the atomic level. This theoretical framework can provide reverse guidance on experimental synthesis parameters, transitioning from blind trial-and-error approaches to rational design of highly active catalysts.
(2) Precise control of plasma treatment processes
The influence mechanism of plasma parameters (such as discharge power, reaction gas pressure, gas composition, and treatment time) on the types and densities of defects generated is extremely complex, posing challenges to precise process control. There is an urgent need to develop real-time plasma diagnostic technologies (such as optical emission spectroscopy, in-situ mass spectrometry, etc.) to deeply reveal the reaction dynamics at the gas-solid interface and achieve precise process regulation. Based on achieving precise control, future synthesis strategies should transcend the limitations of constructing single defects, utilizing plasma composite processes to precisely build hierarchical structure catalysts that encompass multi-dimensional defect synergy, including vacancies, heteroatom doping, grain boundaries, and heterogeneous interfaces. This approach not only maximizes the synergistic effects of charge transfer and cascading reactions but also significantly enhances the overall performance of catalysts through the spatial confinement effects of hierarchical structures.
(3) Multiscale defect synergistic structure construction and intrinsic stability enhancement
Highly active defect sites are often accompanied by extremely high surface energy, existing in a thermodynamically metastable state. During prolonged electrochemical processes with high current density, particularly under conditions of high polarization potential, strong redox environments, and extreme acid or alkaline conditions, the materials are prone to dissolution, reconfiguration, or agglomeration, leading to deactivation. Therefore, it is crucial to reveal the dynamic evolution patterns of defects under operational conditions and to deliberately design highly stable defect protection and anchoring structures. Specific strategies may include constructing strong metal-support interactions (SMSI) or introducing heteroatom co-doping to thermodynamically anchor vacancies securely; alternatively, utilizing mesoporous structures or ultra-thin carbon layers to create spatial confinement effects, physically blocking the migration of active sites. This structural design can fundamentally enhance the corrosion resistance and sintering resistance of defect sites in harsh environments. Based on this foundation, it is important to fully leverage the universal advantages of plasma defect engineering, which is efficient and environmentally friendly, actively expanding application boundaries and exploring the potential of defect-rich catalysts in broader frontier fields such as photonic catalysis, multiphase catalysis, advanced secondary batteries, and energy storage.
(4) Engineering barriers and industrial applications of scalable manufacturing
Current research primarily relies on small vacuum equipment at the laboratory level. Scaling up existing plasma processes to continuous, uniform, large-area industrial production faces significant costs and engineering barriers. In the future, it will be necessary to design new roll-to-roll and fluidized bed plasma reactors, achieving an optimal balance between energy efficiency and manufacturing costs in large-scale production. More critically, to truly realize industrial applications, research perspectives should extend beyond merely the synthesis of powder materials, aiming to directly apply plasma technology to the in-situ growth or activation of catalyst layers in core devices such as membrane electrode assemblies (MEAs) and gas diffusion electrodes (GDEs), thereby constructing efficient binder-free electrodes and transitioning from basic materials to practical electrochemical devices.
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