Solution Plasma for Surface Design of Advanced Photocatalysts

Abstract

Rational design of the surface of photocatalysts can conveniently modulate the photo-stimulated charge separation, influence the surface reaction kinetics, and other pivotal factors in the photocatalytic processes for efficient photocatalysis. Solution plasma, holding promise for mild modification of the surface structure of materials, has recently been recognized as an emerging technology for surface engineering of high-performance photocatalysts. In this review, we will briefly introduce the fundamentals of solution plasma and its applications in materials preparation and summarize the recent research progress in the surface design of advanced photocatalysts by solution plasma. Lastly, we will indicate some possible new directions. This review is expected to provide an instructive guideline for the surface design of heterogeneous photocatalysts by solution plasma.

1. Introduction

The surface of photocatalysts, as the reaction site of the photocatalytic process, determines the efficiency and selectivity of photocatalysis [1,2,3,4]. The surface structure of photocatalysts, such as vacancies, heteroatoms, amorphousness, crystal facets, etc., essentially dominates surface electronic states [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22] and then controls almost all factors in photocatalysis, including light absorption and charge separation of photocatalysts, adsorption of reactants, as well as the path and kinetics of surface reactions. Therefore, the surface design of photocatalysts is the core issue in photocatalysis, which has received continuous attention from researchers, thereby developing a variety of surface modification methods [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46].

Plasma is an ionized system consisting of electrons, positive and negative ions, neutral or excited atoms and molecules, radicals, and photons [47]. Plasma generated in solution at normal temperature and pressure, that is, solution plasma (SP) [48], is regarded as a novel and unique technology in the surface modification of photocatalysts, taking advantage of multiple physical fields (electric fields, thermal fields, light fields, liquid annealing, etc.) involved and abundant active species with redox capacity (e_aq⁻, ·H, ·OH, ·O, O₂⁻, etc.). These can synergistically impact on the surface of photocatalysts for modification. More importantly, the high density and strong internal pressure of the solution environment lead to weaker kinetic energy of the plasma, which facilitates the gentle surface modification of photocatalysts without changing the bulk properties. Therefore, SP has attracted extensive attention in recent years as a powerful tool for the surface structure design of photocatalysts [49,50,51,52,53,54,55,56,57].

In this article, we will briefly introduce the fundamentals of SP, including the principles, the configurations of the experimental setup, the physiochemical properties of SP, and its applications in materials preparation. Then we will review the recent research progress in the surface design of advanced photocatalysts by SP. Lastly, we will suggest some new directions. This review is expected to provide an instructive guideline for the materials science applications of the emerging SP technology, especially on the surface design of heterogeneous photocatalysts.

2. SP Fundamental

The SP installations can be divided into the following categories according to the type and position of the electrode, as shown in Figure 1. The reactors for plasma discharging in liquid mainly include rod-rod, needle-plate, wire-plate, and plate-hole (in an insulation plate)-plate types (Figure 1a–d). In addition, the gas-liquid plasma installations can be roughly divided into needle-plate, multi-needle-plate, and plate-plate types (Figure 1e–g). The configurations of the electrode generally affect plasma temperature, active species content, the characteristics of physical fields, and other parameters in SP, furthering the surface modification of photocatalysts, which has been systematically reviewed in the previous literature [58,59,60].

In this article, we focus on the rod-rod reactor, a typical configuration of in-liquid plasma, which has the advantages of simple construction, convenient operation, and a low ambient temperature. For generating plasma in liquid, specifically by applying a high voltage pulse to metal electrodes, bubbles around the electrode tip are created due to the local Joule heating effect [61,62]. When the applied energy exceeds a certain threshold in bubbles, an electric breakdown occurs, thereby forming a plasma. Subsequently, accompanied by the extension and expansion of plasma, a roughly circular discharge of plasma appears along the gas-liquid interface (Figure 2a). Bubbles are critical in plasma formation because they lower the threshold for electric breakdown [63]. In this regard, a series of gases, such as N₂, O₂, Ar, He, etc., are usually introduced into the solution to help plasma generation.

2.1. Physical and Chemical Properties of SP

In the SP process, as shown in Figure 2b, reaction places involve the plasma phase, gas phase, liquid phase, plasma-gas interface, and gas-liquid interface. Among them, the plasma phase, liquid phase, and gas-liquid interface, with various physical and chemical effects, play a more significant role in the surface modification of photocatalysts [58].

In SP, the photoelectric effect and inelastic collision between high-energy electrons and surrounding molecules result in its ionization, excitation, and decomposition, which then generate various active species, such as e_aq⁻, ·H, ·OH, ·O, O₂⁻, H₂O⁺, H₃O⁺, H₂O₂, etc. The possible fundamental reactions in SP are summarized in

Table 1. A brief summary of the possible fundamental reactions in SP.

No.	Reaction	No.	Reaction
1	H2O → H2O *	12	eaq− + •H + H2O→ H2 + OH−
2	H2O * → •H + •OH	13	e− + O2 → O(3P) + O(1D) + e−
3	H2O * → H2 + •O	14	•OH + H2O2 → H2O + •HO2
4	H2O → H2O+ + e−	15	O(1D) + H2O → 2•OH
5	e− + H2O → eaq−	16	•H + O2 → •O + •OH
6	e− + O2 → O2−	17	•H + •OH → H2O
7	H2O + e− * → •OH + •H + e−	18	•OH + •OH → H2O2
8	H2O + e *− → H2O+ + 2e−	19	•H + •HO2 → H2O2
9	H2O+ + H2O → H3O+ + •OH	20	•H + •H → H2
10	eaq− + •OH → OH−	21	H2O+ + eaq− → H2O
11	eaq− + H2O → •H + OH−

* excited state; • radical; O₂⁻: superoxide radical; O(³P): triplet oxygen; O(¹D): singlet oxygen.

. These active species with redox capacity are very important for the surface modification of photocatalysts. The density of plasma reaches 10¹⁷–10²⁰ cm⁻³ [64]. The active species differ in their spatial distribution. The ·OH, ·O, O₂⁻ and other oxygen-containing ions have a high mass, which causes slow migration speeds, finally remaining in plasma. Differently, ·H with a small mass can move quickly to the gas-liquid interface (Figure 2b) [65]. The optical emission spectrum (OES) is a useful method to detect free radicals and can offer effective information such as their composition and quantity [66]. Yu et al. utilized OES to investigate active radicals in water plasma (Figure 3a) [67]. They observed emission lines of H_α (3d → 2p: 656.6 nm), H_β (4d → 2p: 486.1 nm), and O_Ⅰ (5p → 5s⁰: 777.5 nm; 3p → 3s⁰: 844.6 nm). Moreover, the electron density (N_e) was deduced from the full width at half-maximum (fwhm) of the H_β line, and the calculated formula was determined as $\mathrm{fwhm}=4{.8\times (\mathrm{N}}_{\mathrm{e}}{/10}^{23}{)}^{0.68116}$ by Bratescu et al. [68].

H radicals, with strong reducing abilities, are critical for the surface modification of photocatalysts and the generation of metal nanoparticles. Its density is easily affected by the SP configuration. In SP, bubbling inert gases (Ar, N₂, He, etc.) into solution can produce a higher concentration of the ·H radical. According to the reports from Sun et al. [69], the elastic collision of inert gas increased the average energy of electrons and then promoted the decomposition rate of water molecules for producing the ·H radical. The addition of alcohol in SP also plays a positive role in the ·H radical. Wang et al. found that the hydrogenation degree of P25-TiO₂ treated with alcohol plasma was stronger than that treated with water plasma [70]. Sudare et al. reported that adding alcohol to the HAuCl₄ solution increased the reduction rate of Au nanoparticles by SP [71]. Coherently, alcohol plasma exhibited a higher reduction rate of graphene oxide than reported by Takeuchi et al. [62]. This can be traced to the difference in the physical properties of alcohol and water. Compared with water, alcohol has a lower dielectric constant, which is conducive to trigger discharging [72]. The low boiling point and surface tension of alcohol induce denser and smaller bubbles [62], which fills the electrode gap, thereby leading to more sustained discharging. Besides, the C-H and O-H bond energies in alcohol are lower than the O-H bond energies in water [73] and are more likely to break in SP. Therefore, plasma discharging in alcohol can generate more ·H radicals for material preparation and modification. The electrode composition influences the discharge process. Miron et al. found that the Ta electrode system created fewer ·H radicals compared to the W electrode system [66]. Given that they reported the higher energy required for discharging breakdown in the Ta electrode system [66], the decrease in ·H radical was due to the poor discharging effect.

O species, including ·OH, ·O, O₂⁻, and H₂O₂, are responsible for the synthesis of metal oxides and the surface structure design of photocatalysts. Yu et al. examined OES spectra in N₂- and O₂-bubbled SP and found that the concentration of O species in O₂-bubbled SP was enhanced by 4.9 times compared with N₂-bubbled SP (Figure 3a) [67]. In addition, the production of H₂O₂ in O₂-bubbled SP outweighed that in N₂-bubbled SP (Figure 3b) [67]. This manifested O₂ favored the formation of oxygen species. The main reason was that O₂ itself reacted with H atoms to form more ·O and ·OH radicals. Xing et al. observed that the enhanced oxygen species in O₂-bubbled SP tended to heal the surface oxygen vacancies (Ovs) of photocatalysts [74]. The conductivity and pH of the solution also affect the formation of the ·OH radicals. Sun et al. investigated the emission intensity of ·OH radical at different conductivities in KCl and KOH solutions [75]. Results indicated that when the conductivity ranged from 10 to 80 μS/cm, the concentration of the ·OH radical reached its maximum. Both too-low and too-high conductivity impeded discharging. Moreover, the concentration of ·OH radical in the KOH solution was higher than that in the KCl solution, suggesting that ·OH radical preferred neutral and slightly alkaline conditions. Making use of a Cu electrode to discharge in water, Hayashi et al. found that sputtered Cu atoms might help accelerate the formation of the ·OH radical [76].

The temperature of the plasma region reaches 10⁴–10⁵ K [64], which is essential for material modification. The high-temperature region provides a pathway for the nucleation growth of metals, the phase transition of oxides, and the thermal decomposition, polymerization, and growth of organic precursors into carbon nanomaterials. On the other hand, the huge difference in temperature between the plasma phase and the liquid phase constitutes a unique physical process, i.e., solution annealing [70]. This process can freeze and retain the modified metastable structure. In addition, the instantaneous high temperature in the plasma region with the applied high voltage pulse makes the pressure in the discharging channel rise sharply. Then this causes the plasma channel to expand outward rapidly at a high speed (10²~10³ m/s) [77], forming a shock wave. The shock wave can act on the surface of materials to cause the crystal to break and rearrange.

In general, SP, as a novel material processing technology, integrates a variety of physical fields and chemical reactions. Its physicochemical effects can be conveniently adjusted by altering the plasma configurations, which provides great potential for surface structure modification of advanced materials.

2.2. SP for Materials Preparation

2.2.1. Oxide Materials

SP has been reported for the synthesis of semiconductor oxides with various physicochemical structures, such as vacancies, disordering, and foreign element doping. Pitchaimuthu et al. treated anatase-TiO₂ with water plasma in N₂ [78]. The surface amorphous layer and bulk brookite phase were created by being bombarded with high-energy ·H radicals. The crystal phase transformation and lattice rearrangement enhanced the yield of CO₂ from 50.9% to 96.6% in photocatalytic acetaldehyde degradation under ultraviolet light. Mizukoshi et al. prepared blue Ovs-TiO₂ by ammonia solution plasma [79]. Under visible light, the ·OH yield of SP-treated anatase-TiO₂ and P25-TiO₂ was higher than that of original TiO₂. This indicated that ammonia solution plasma enhanced the photooxidation capacity of anatase-TiO₂ and P25-TiO₂ under visible light. In addition to vacancies and amorphization, foreign element doping of materials has also been realized by SP. In a solution containing Ti sources and different F sources (Ti source: TiOSO₄ solution; F source: NaF, NaF with [C₄MIM]HSO₄, NaF with [C₄MIM]BF₄), plasma discharging synthesized F-doped anatase-TiO₂ [80]. The results showed that the addition of [C₄MIM]HSO₄ and [C₄MIM]BF₄ was conducive to the doping of F ions of NaF into the TiO₂ lattice. Moreover, the MB degradation rate of F-doped anatase-TiO₂ ([C₄MIM]BF₄ as an F doping agent) was improved by 2.9 times.

Besides, making use of the corresponding metal electrodes, plasma discharging in different electrolytes can be employed to prepare metal oxides with various morphologies. The black TiO₂ nanosphere was prepared by SP with a Ti electrode [81]. Additionally, it had a high specific surface area (~120 m² g⁻¹) and mesopore structure, which facilitated MB photodegradation. By replacing water with HNO₃ or KCl solution, the generated rate of black TiO₂ was enhanced, and in the HNO₃ solution, it reached the highest [82,83]. This was because of the high acidic property and conductivity of HNO₃. Kim et al. used coil-needle Zn electrodes in water plasma and synthesized bullet-like ZnO nanoparticles with the main exposed (100) crystal facet [84]. Saito et al. reported that plasma discharging with Zn electrodes prepared flower-like ZnO nanoparticles in a high-temperature and unstirred K₂CO₃ solution [85].

Table 2. A brief summary of various SP-prepared oxides.

Oxides			SP Conditions		Ref.
Samples	Characteristics	Performance	Solution	Electrode
Anatase /Brookite TiO2	surface amorphous layer, bulk brookite phase and oxygen defects	96.6% of CO2 conversion rate in degradation of the gaseous acetaldehyde under a fluorescent lamp with 8000 lux intensity	water	W	[78]
Blue TiO2	oxygen defects	105 μmol of H2 generated in 2 h irradiated by a xenon lamp with 5 mW/cm2	aqueous ammonia	W	[79]
Fluorine-doped TiO2	anatase phase TiO2 with interstitial fluorine atoms	~98% of MB degradation under stimulated sunlight for 30 min	TiOSO4 solution with NaF, a mixed solution of NaF and [C4MIM]BF4, or a mixed solution of NaF and [C4MIM]HSO4	stainless steel	[80]
Black TiO2−x spheres	a mixture of rutile, anatase, and oxygen-deficient TiO2; disordered structure; oxygen defects and Ti3+	90% of MB degradation under visible-light irradiation for 3 h	water	Ti	[81]
Black TiO2	disordered structure and Ti3+	58.49% of glycerol conversion under a 120W mercury lamp for 24 h	KCl or HNO3	Ti	[82,83]
ZnO nanobullets	the main exposed (100) facet, and the (101) facet at the inclined surfaces in the edges	—	water	Zn	[84]
ZnO flowers	ZnO nanoflowers with a small number of Zn particles	100% of MB degradation under UV light irradiation for 120 min	K2CO3 solution	Zn	[85]
CuO nanorods	highly anisotropic CuO nanorods with 2–4 nm of diameter and 14–16 nm of length	—	NaNO3 solution	Cu	[86]
Cu2O	large particles with sizes of 300–500 nm	—	NaNO3 solution and ascorbic acid	Cu	[86]
Nanoporous MnO2	a spherelike shape; nanoparticles and thin layered structure	>99% of MB degradation for 2 min	KMnO4 solution with glucose, fructose, or sucrose	W	[87]
WO3 nanoparticles	monoclinic phase and an average particle diameter of ~60 nm	100% of RB degradation under UV irradiation for 50 min	HNO3, NaOH, NaCl and glucose solution	W	[88]
ZnO nanospheres	nanospheres with about 17 nm in diameter	100% of MO degradation for 8 min under UV irradiation	ZnCl2, KCl and CO(NH2)2 aqueous solution	W	[89]

summarizes various oxides prepared by SP in brief [78,79,80,81,82,83,84,85,86,87,88,89].

2.2.2. Carbon Materials

Adopting organic solvents for discharging, SP can be employed to synthesize diversified advanced carbon materials. In SP, the bombardment of high-energy species and the thermolysis effect in the high-temperature plasma region can dissociate organic molecules into various unsaturated organic fragments. Subsequently, through heating integration in the plasma region and polymerization at the gas-liquid interface, various carbon materials can be synthesized. In 2013, Kang et al. prepared carbon nanospheres in a benzene solution for discharging [90]. Aromatic benzene with delocalized π electrons, which are considered to be easily excited by plasma, is rapidly decomposed to synthesize carbon materials in SP. As the applied frequency increased from 25 to 65 kHz, the carbon nanospheres grew from amorphous carbon to graphene. The reason was that the increase in frequency raised the temperature in the plasma region, exceeding the temperature critical point for nanocrystalline carbon formation. Recently, Romero Valenzuela et al. connected the SP reactor to an annealing chamber. Thus, SP-generated gaseous soot containing carbon seeds was blown by Ar to a nickel foil substrate in the annealing chamber, on which carbon fibers were grown at 400 °C in H₂/Ar [91]. Using benzene as the precursor, the SP-synthesized crude carbon nanofibers with a diameter of 150–220 nm were characteristic of a bumpy surface and random distribution. For dichlorobenzene, the SP-synthesized single fiber was grown in a columnar fashion and formed bundle structures ranging from 90 to 140 nm. This morphological difference was caused by the chlorine species of dichlorobenzene softening the surface of the Ni substrate and subsequently altering the nucleation mode of carbon nanofibers. In terms of ethanol and methanol, the SP-synthesized carbon materials displayed thin fiber clusters (diameters: <76 nm). The volatility of alcohols may be the reason for the small size of carbon fibers. Besides, as shown in

Table 3. A brief summary of various SP-synthesized carbon materials.

Samples	Characteristics	SP Conditions	Ref.
Carbon nanospheres	amorphous carbon nanoparticles (25 kHz), turbostratic carbon nanoparticles (65 kHz)	benzene; W electrodes; 1.3 kV, 2.0 µs and 25–65 kHz; 20 min	[90]
Carbon fibers	thick-bump fibers with a diameter of 150–220 nm; fiber bundles with a diameter of 90–140 nm; worm-like fibers with a diameter of 80–114 nm; thin fiber cluster with a diameter of <38 nm; flower-like fiber clusters with a diameter of 72–76 nm	benzene, o-dichlorobenzene, N-methylpyrrolidone, ethanol, or methanol; W electrodes; 5 kV, 20 kHz, and 1.0 µs; 15 min	[91]
Graphite-like and amorphous carbon	graphite-like carbon: a lamellar stack structure; amorphous carbon: turbostratic carbon surrounding by multi-layer fullerene-like carbon	tributyl borate; W electrodes; 1.5 kV, 100 kHz, and 0.5 µs; 60 min	[92]
Short carbon nanotubes	short carbon nanotubes of 100–400 nm in length	multi-wall carbon nanotubes in KCl solution; W electrodes; 450 V and 12 kHz; 30 min	[93]
Oxygen-containing nanocarbon	carbon particles with a diameter of 20 to 50 nm, the graphite-like layered structure	benzene and 1,4-dioxane; W electrodes; 20 kHz, 1 µs and 1.2–1.6 kV; 10 min	[94]
Fluorescent carbon dots	monocrystalline and semicrystalline	fructose, glucose, and sucrose; W electrodes; 20 kHz, and 0.9 µs; 20 min	[95]

, SP also prepared other carbon nanomaterials, such as graphite-like carbon polymer, carbon black, and carbon quantum dots [90,91,92,93,94,95]. Overall, the design of organic precursor structure (functional groups, physical properties, etc.) is the key to the preparation of target carbon materials by SP.

Furthermore, by selecting organic precursors with heteroatoms (such as N, B, S, F, et al.), SP can help heteroatoms incorporate or co-incorporate into carbon materials. Plasma discharging in a solution of pyridine and boric acid prepared N/B co-doped nanocarbon, as reported by Lee et al. [96]. The formation of an uncoupled B-C-N bond altered the electronic structure of nanocarbon and formed new active sites for enhanced oxygen reduction reaction (ORR) performance. Li et al. chose pyridine and acrylonitrile as precursors, respectively, to synthesize graphitic-N and amino-N-doped nanocarbons with SP [97]. They found that graphitic-N bonds promoted the four-electron transfer of ORR, and amino-N bonds shifted the ORR onset potential to more positive values. Panomsuwan et al. conducted plasma discharging in a mixture of toluene and trifluorotoluene and obtained F-doped amorphous nanocarbons with enhanced electrocatalytic activity in ORR [98]. They suggested the promotion of catalytic performance was related to the ionic C-F and semi-ionic C-F bonds on the surface of nanocarbon. Moreover, P-doped carbon balls, graphitic N-doped graphene, B-doped carbon nanoparticles, B/F co-doped carbon nanoparticles, and others were also synthesized by SP [96,97,98,99,100,101,102,103,104,105,106,107,108,109,110]. As summarized in

Table 4. A brief summary of various SP-synthesized foreign element-doped carbon materials.

Samples	Doping Amount	SP Conditions	Ref.
Boron-carbon-nitrogen nanocarbon	N: 3.8 at% B: 1.2 at%	pyridine, boric acid; W electrodes; 2 kV, 0.5 µs and 100 kHz; 30 min	[96]
Graphitic-N and amino-N in nitrogen-doped carbon	N: 2.01 at%; 2.50 at%; 2.54 at%; 2.97 at%	anthracene, pyridine, acrylonitrile; W electrodes; 2.0 kV, 1.0 µs and 20 kHz; 20 min	[97]
Fluorine-doped carbon	F: 4.52 at%	toluene, trifluorotoluene; W electrodes; ~1.0 kV, 0.60 µs and 20 kHz; 20 min	[98]
Nitrogen-doped carbon nanoparticles	N: 4.78 at%; 1.33 at%	acrylonitrile; W electrodes; ~1.5 kV, 1.0 µs and 20 kHz; 30 min	[99]
Nitrogen-doped carbons	N: 0.58 at% 1.29 at% 1.36 at%	cyanobenzene, 2-cyanopyridine, cyanopyrazine; W electrodes; 1.2–1.5 kV, 0.80 µs and 20 kHz; 30 min	[100]
Graphitic nitrogen-doped graphene	N: 18.79 at%	benzene and N, N-dimethylformamide; W electrodes; 1.6 kV, 1.0 µs and 180 kHz; 15 min	[101]
Cationic nitrogen-doped graphene	N: 13.4 at%	1-ethyl-3-methylimidazolium dicyanamide, dimethylformamide; W electrodes; 2.0 kV, 200 kHz and 1.0 µs; 5 min	[102]
Phosphorus-doped carbon balls	P: 4 at%	triphenylphosphine; WC electrodes; 2.0 kV, 100 kHz and 1.0 µs; 10 min	[103]
Nitrogen-Doped Carbon Nanoparticle/Carbon Nanofiber Composite	N: 1.35 at%	a suspension of carbon nanofiber and 2-cyanopyridine; W electrodes; 20 kHz and 0.8 µs; 30 min	[104]
Boron-doped, and boron/fluorine co-doped carbon nanoparticles	B: 0.91 at%, F: 1.95 at%; B: 1.08 at%, F: 3.38 at%	toluene, 2,4,6-Triphenylboroxin, 2,4,6-Tris(4-fluorophenyl) boroxin, 2,4,6-Tris(3,4-difluorophenyl) boroxin, 2,4,6-Tris(3,4,5-trifluorophenyl) boroxin; W electrodes; 1.5 kV, 0.50 µs and 100 kHz; 20 min	[105]
Nitrogen-carbon nanosheets	N: 1.3 at%	N-methyl-2-pyrrolidone; W electrodes; 2.0 kV, 1.0 µs and 200 kHz; 5 min	[106]
Nitrogen-doped carbon nanoparticles	N: 0.58 at% 0.51 at% 0.39 at%	benzene, pyrazine; W electrodes; 1.5 kV, 1.0 µs and 20 kHz; 20 min	[107]
Boron-doped carbon nanoparticles	B: 0.67 at%	benzene, triphenyl borate; W electrodes; 1.0 µs and 20 kHz; 20 min	[108]
Nitrogen-doped carbon nanoparticles	N: 1.94 at% 1.85 at%, 0.63 at%	benzonitrile, 2-cyanopyridine, cyanopyrazine; W electrodes; 0.80 µs and 20 kHz; 30 min	[109]
Nitrogen, Boron, or Phosphorus doped carbon; Nitrogen-Boron co-doped carbon; Nitrogen-Phosphorus co-doped carbon	—	benzene, pyridine, phenylboronic acid, phenylphosphonic acid; W electrodes	[110]

, by selecting suitable organic precursors and SP configurations, SP can flexibly and conveniently synthesize multiple types of advanced carbon materials.

2.2.3. Metal Materials

The reducing radicals in SP, for example, the ·H radical, can reduce metal ions into metal particles of various types, sizes, and shapes. In 2008, Hieda et al. reported that HAuCl₄ solution plasma synthesized Au nanoparticles with triangular, pentagonal, and hexagonal shapes [111]. The higher applied voltage and lower HAuCl₄ concentration resulted in a size reduction and anisotropic shapes of Au nanoparticles. Then, Bratescu et al. further adjusted the pH value in HAuCl₄ solution plasma and controlled the size of Au nanoparticles [68]. As the pH value increased from 3 to 12, the size of Au nanoparticles decreased from ~10 nm to ~1–2 nm. Bratescu found that at a high pH value, the reduction potential of Au⁰ nanoparticles decreased to ~0.60 eV. Meanwhile, the enriched negative ions (AuO⁻, AuOH⁻, and Au₂O⁻) were absorbed on the Au surface to create a repulsive force among the Au nanoparticles. These prevented excessive agglomeration of Au nanoparticles. Besides, the addition of reducing solvent promoted the generation rate of metal particles in SP. Sudare et al. reported that when the molar fraction of ethanol was 0.089, the reduction rate of Au nanoparticles was improved by 35.2 times [71,112]. Under the action of plasma, ethanol molecules split into stable reducing ethanol free radicals (·CH(CH₃)OH). As described in Section 2.1, ethanol facilitated plasma discharging and easily produced more ·H radicals. Other short-chain alcohols had the same effect [71]. Thus, plasma discharging in a metal salt solution can be employed to quickly prepare metal nanoparticles of various sizes and shapes.

Except for plasma discharging in a metal salt solution, the sputtering of the metal electrode surface in SP is also an effective means to prepare metal nanoparticles. During SP, the high-energy species in the plasma region continuously attack the metal electrode surface, resulting in the sputtering of metal ions. Then, metal ions nucleate and grow into metal particles. In 2012, Hu et al. utilized Au electrodes sputtering in liquid nitrogen and successfully prepared well-crystalline spherical Au clusters with a mean diameter of ~1.25 nm [113], which represented the characteristics of multiple twins and were near monodispersed. When two electrodes adopt different metal compositions, SP can be used to prepare bimetallic alloys. Zhang et al. adopted Pt and Pd electrodes for plasma sputtering in a solution of water and methanol to synthesize PtPd alloy nanoparticles with sizes ranging from 2 nm to 5 nm [114]. In their study, methanol played a role in inhibiting the aggregation of PtPd nanoparticles.

Table 5. A brief summary of various metal materials prepared by SP.

Samples	Characteristics	Electrode	Precursor Solution	Ref.
Au	~50 nm, and hexagonal shape; ~10 nm, and triangular, pentagonal and hexagonal shapes	W	0.3 mM and 0.65 mM HAuCl4	[111]
Au	PH = 12, 1~2 nm PH = 3, ~10 nm PH = 6, ~4 nm	Pt	HAuCl4·3H2O, hexadecyltrimethylammoni-um chloride, and NaOH	[68]
Au	χ(ethanol) = 0, ~32 nm χ(ethanol) = 0.089, <10 nm χ(ethanol) = 1.0, ~25 nm	W	HAuCl4·3H2O, ethanol, methanol, 2-propanol	[71]
Au	TA = 0 mM, 56.6 ± 19 nm TA = 0.42 mM, 47.8 ± 17.8 nm TA = 1.25 mM, 21 ± 6.1 nm TA = 5 mM, 11.2 ± 4.1 nm	Cu	HAuCl4 solution with L-cysteine, glucosamine, salicylic acid, or terephthalic acid, terephthalic acid (TA)	[115]
Ag/chitosan	spherical shape and a mean diameter of <20 nm	Graphite	AgNO3 and chitosan solution	[116]
Ag/alginate	spherical shape and size range of 5–40 ± 2.0 nm	W	5 mM AgNO3 and 0.3%(w/w) alginate solution	[117]
Au	spherical shape and the mean diameter of 1.25 ± 0.5 nm	Au	liquid nitrogen and pure water	[113]
Pd	the average size of ~2 nm	Pd	Ketjen Black aqueous solution	[118]
Au	the average particle diameters of 4.32 ± 0.85 nm, 3.54 ± 1.04 nm, and 2.87 ± 0.77 nm	Au	0.2%, 0.5%, and 0.9% sodium alginate aqueous solution	[119]
Sn	in pure water, ~5 nm Sn particles; in gelatin aqueous solution, Sn particles with a diameter of 40 to 400 nm	Sn	pure water or gelatin aqueous solution	[120]
PtAu	1.5 ± 1.0 nm clusters	Pt and Au	water or ethanol solution	[121]
PdAu	the average size of 2–5 nm	Pd and Au	Ketjen Black aqueous solution	[122]
CoNi	the size range of ~3–200 nm	Co and Ni	liquid nitrogen	[123]
PtPd	the size range of 2–5 nm	Pt and Pd	water and methanol solution	[114]

briefly summarizes SP-prepared metal materials [68,71,111,113,114,115,116,117,118,119,120,121,122,123].

2.2.4. Composite Materials

SP with easily turned configurations can be employed to prepare composite materials with complex composition and/or morphology. The reported SP-prepared metal/oxide structures include Au/hollow fibrous TiO₂, Pt/ZnO, Ag/Co₃O₄, Au/coating TiO₂, etc. [124,125,126,127,128,129]. In these works, the SP-prepared metals are characterized by clusters. Additionally, there have been reports of a series of SP-prepared oxide/carbon structures, such as cubic Co₃O₄/reduced graphene, ZnO/carbon fiber, and TiO_x/carbon nanosheets [130,131,132]. In SP-prepared metal/carbon structures, except for metal particles dispersed on the surface of carbon materials [118,122], SP was also employed to synthesize a core-shell structure with a few layers of graphene as the shell and metal nanoparticles (Cu, Pt, PtAu, PtAg, and PtPd) as the core [133,134]. This core-shell structure was prepared by metal electrode sputtering in a dimethylformamide (DMF) solution. Aliphatic DMF without delocalized π electrons exhibited a low synthesis rate of carbon, and then it preferred to obtain layered graphene rather than carbon nanoparticles. Furthermore, the N and O atoms in the DMF structure formed terminal bonds, thereby terminating the growth of carbon. In SP, the monolayer carbon was first adsorbed on the metal surface, followed by the growth and crystallization of the carbon shell in the high-temperature plasma region.

Table 6. A brief summary of various composite materials prepared by SP.

Samples	SP Conditions	Ref.
Au/hollow fibrous TiO2	A solution of TiO2 hollow fibrous power and adipic acid; Au electrodes; 2.0 μs and 15 kHz; 4 min	[124]
Pt/ZnO	Zn and Pt electrodes; 15–20 kHz and 1–2 μs	[125]
Ag/Co3O4	distilled water; Ag and Co electrodes; 0.9 kV, 2.0 μs and 20 kHz; 300 s	[126]
Au/coating TiO2	A solution of TiO2 particles and 0.01 M NaOH; Au electrodes; 400 V; 1 h	[127]
Co3O4/reduced graphene	A solution of Co(NO3)2·6H2O and reduced graphene oxide; graphite as the anode, and Cu as the cathode; 190 V; 180 s	[130]
ZnO/carbon fiber	ZnSO4 solution; a carbon fiber fabric as the negative electrode and a stainless-steel needle as the positive electrode; 1.5 kV	[131]
TiOx/carbon nanosheets	Titanium tetraisopropoxide, distilled water, nitric acid, and ethanol; W electrodes; five capacitors in parallel with 0.1 μF capacitance, 100 times of discharging, the stored energy of 10–90 J per pulse in capacitor	[132]
Pd/Ketjen Black	Ketjen Black aqueous solution; Pd electrodes; 15–20 kHz and 1–2 μs	[118]
PdAu/Ketjen Black	Ketjen Black aqueous solution; Pd and Au electrodes; 15–20 kHz and 1–2 μs	[122]
a core-shell structure with graphene as shell and Cu nanoparticles as core	Dimethylformamide; Cu electrodes; 1.0 kV, 1.0 μs and 100, 200 kHz; 60 min	[133]
a core-shell structure with graphene as shell and metal nanoparticles (Pt, PtAu, PtAg or PtPd) as core	Dimethylformamide; Pt, Au, Ag, Pd electrodes; 1.0 μs and 30 kHz; 60 min	[134]

provides a summary of a series of SP-prepared composite materials [118,122,124,125,126,127,130,131,132,133,134].

3. Surface Design of Advanced Photocatalysts by SP

SP is recognized as an effective means of engineering the surface structure of photocatalysts. A solution with a high internal pressure would induce a short mean free path of electrons and ions in SP [135] and then result in a low collision energy between SP-generated species and the surface of photocatalysts. This is conducive to mild and wet etching on the surface of photocatalysts. Furthermore, the dense solvent molecules in the solution would contribute to the weak kinetic energy of the plasma phase, which favors gently modifying surface structure. Besides, the high density and variety of active species provide a high rate and multiple angles of surface modification for photocatalysts [64]. Due to these merits, SP has great potential in the surface structure design of advanced photocatalysts.

3.1. Surface Modification of Semiconductor Oxides

3.1.1. Surface Electronic State Regulation

The surface electronic state is closely related to the light absorption and charge separation of photocatalysts, which can be facilely tuned by electron traps [1,2]. However, light absorption capacity and efficient charge separation generally have a trade-off relationship. Photocatalysts are mostly nanosized, and various traps are possibly present in the photocatalyst, resulting in the coexistence of shallow-energy-level and deep-energy-level electron traps [136,137,138,139]. The electron traps with shallow- and deep-energy levels would be beneficial to broaden light absorption from ultraviolet to visible, even near-infrared. Unfortunately, deep-energy-level electron traps serve as centers for charge recombination. Therefore, to promote efficient charge separation and ensure high light absorption ability, it is indispensable to engineer the electron traps by maintaining or introducing shallow-energy-level electron traps and eliminating deep-energy-level electron traps in the nanosized photocatalyst [2].

Yu et al. utilized the N₂ bubbled-SP to adjust the surface electronic state of as-synthesized rich-Ovs TiO₂-AB photocatalysts (a mixture of TiO₂ of anatase and TiO₂(B) phases as shown in Figure 4a) [140]. Results indicated that SP helped to incorporate H heteroatoms into the TiO₂ lattice and created shallow-level traps above the deep level of Ovs, eventually achieving both visible photoabsorption capacity and high catalytic efficiency. The Ovs, located at 0.75–1.18 eV in the forbidden band [141], as visible absorption sites, were introduced to TiO₂-AB by hydrothermal synthesis. After SP, hydrogen species derived from plasma-induced water splitting were adsorbed or inserted into the surface lattice of rich-Ovs TiO₂-AB (Figure 4b) and generated shallow-level traps near the CBB with enhanced light absorption (Figure 4c), definitively supported by reversed double-beam photoacoustic spectroscopy (RDB-PAS) (Figure 4d). Surprisingly, the introduced shallow-level traps of H heteroatoms were below CBB and above the deep-level traps of Ovs. Therefore, with the help of external energy input, such as heat, trapped photogenerated electrons can be pulled out of the deep-level traps and transferred to the CBB through shallow-level traps (Figure 4e). This improved the utilized yield of photogenerated charges for enhanced photocatalysis. The results of photothermal catalysis of CO₂ conversion were exactly confirmed above the viewpoints. Under solar light, the photothermal catalysis of CO and CH₄ yield over SP-treated TiO₂-AB was respectively increased by 299.9 and 108.5 times compared to that over TiO₂-AB in photocatalysis (Figure 4f). Moreover, SP-treated TiO₂-AB displayed 3.51 µmol g⁻¹ h⁻¹ of CO yield and 1.16 µmol g⁻¹ h⁻¹ of CH₄ yield under visible light. Besides, they expanded the investigative oxides from TiO₂ to ZnO, WO₃, and Ta₂O₅. The SP-treated ZnO, WO₃, and Ta₂O₅ photocatalysts all exhibited an enhanced CO₂ production rate in photothermal acetaldehyde degradation, increasing by 3.4, 2.8, and 2.9 times, respectively. This indicated that SP was universal for semiconductor oxides. Therefore, SP can introduce H heteroatoms into the surface of photocatalysts and regulate the surface electronic states, thereby promoting catalytic activity.

Yu et al. further changed N₂ bubbles into O₂ bubbles in SP and treated commercial N-TiO₂ photocatalysts [67]. N-TiO₂ is the most classical visible light photocatalyst [142], and its energy level of N heteroatoms is located above the valence band top. Compared with N₂ bubbles, the O₂-bubbled SP introduced more H heteroatoms into the N-TiO₂ lattice (Figure 5a). Correspondingly, RDB-PAS spectra displayed the more shallow-level traps of H heteroatoms below the CBB (Figure 5b). In brief, O₂-bubbled SP is more favorable to introducing hydrogen dopants and forming shallow-level traps. Moreover, the more shallow-level traps would form more compact channels for smoothly and efficiently extracting trapped photogenerated electrons and then realizing efficient catalytic activity (Figure 5c). Coherently, the photothermal acetaldehyde mineralization rate of O₂ bubbles-treated N-TiO₂ was 82.69%, which was higher than that of N₂ bubbles-treated N-TiO₂ (61.47%) under visible light. In conclusion, SP is an effective means to regulate the surface electronic states of photocatalysts and can be flexibly controlled by altering the conditions of SP.

3.1.2. Crystal Facet Modulation

Different exposed crystal facets of polyhedral photocatalysts would separate redox sites for effective charge separation. For example, Pan et al. reported that anatase TiO₂ represented the discrete oxidation and reduction sites on its {0 0 1} and {1 0 1} facets [14]. Tan et al. found the oxidation site and reduction site of Cu₂O, respectively, were located on its {1 0 0} facet and {1 1 1} facet [15]. Domen et al. utilized the oxidation site—{1 1 0} facet and the reduction site—{1 0 0} facet of SrTiO₃ to realize efficient charge separation for photocatalysis [16]. Li et al. reported that, for monoclinic BiVO₄, the {1 1 0} facet was the oxidation site and the {0 1 0} facet was the reduction site [17]. Thus, modulating the exposed facet structure of photocatalysts is one of the useful means to achieve photogenerated charge separation and improve photoactivity.

In a recent work, Che et al. reported that SP can conduct defect engineering over the crystal facet of decahedral BiVO₄ and then improve its crystal facet-dependent photoactivity [143]. In this work, because of the abundant bulk defects brought by hydrothermal synthesis, the pristine as-synthesized decahedral BiVO₄ showed severe photogenerated charge recombination, which caused poor facet-dependent photoactivity. As shown in Figure 6a, the deposited Pt nanoparticles and MnO_x nanosheets were randomly distributed on the {0 1 0} and {1 1 0} facets of pristine decahedral BiVO₄. In contrast, after SP treatment, the Pt nanoparticles and MnO_x nanosheets can be selectively and separately deposited on the {0 1 0} and {1 1 0} facets (Figure 6b). Moreover, the density and dispersion of {1 1 0} facet-deposited MnO_x nanosheets were significantly increased. Then, through a series of precise characterizations, they elucidated the mechanism by which SP enhanced facet-dependent photoactivity. From the positron annihilation spectroscopy (Figure 6c), SP-treated BiVO₄ showed a decreased lifetime component of τ1 and an increased lifetime component of τ2, which indicated a decrease in bulk defects and an increase in surface defects. Combined with the electron spin resonance (ESR) signal of metal vacancy at g = 1.994 (Figure 6d) and the decreased molar ratio of V/Bi in SP-treated BiVO₄ (from 1.119% to 1.044%), the created surface defects were confirmed as vanadium vacancies. The DFT calculation manifested the formation energy of vanadium vacancy on the {1 1 0} facet (6.56 eV) was smaller than that on the {0 1 0} facets (7.79 eV). This demonstrated vanadium vacancies were mainly distributed on the {1 1 0} facet, which coincided with the improved density and dispersion of deposited MnO_x nanosheets. Therefore, SP can minimize bulk defects and introduce {1 1 0} facet-located vanadium vacancies. Such defect engineering enhanced the separation efficiency of photogenerated charges at the {1 1 0}/{0 1 0} interface, sequentially increasing the crystal facet-dependent photoactivity. In the meantime, the SP-treated BiVO₄ exhibited a 1.5-times improvement in oxygen evolution rate from photocatalytic water oxidation referred to pristine BiVO₄ (Figure 6e). From the above, it can be seen that SP works well in crystal facet modulation and has potential for improving the crystal facet-dependent photoactivity of other photocatalysts.

3.1.3. Surface Amorphization

A surface-amorphous TiO₂ catalyst was first reported by Chen et al. [144]. They reported that surface amorphization induced the CB and VB band-tail electronic states to narrow the bandgap of TiO₂ to ~1.54 eV, which then enhanced the photocatalytic activity of hydrogen evolution. Subsequently, Yang et al. constructed a heterojunction of TiO₂ and amorphous WO₃ nanoparticles [145]. They found the decoration of amorphous WO₃ allowed a rapid electron transfer to adsorbed O₂. Interestingly, the TiO₂-crystalline WO₃ heterojunction showed no electron transfer tendency, due to the fact that the CBB of crystalline WO₃ was lower than the reduction potential of O₂. They suggested the CBB of amorphous WO₃ may move up, exceeding the reduction potential of O₂. Moreover, Ohtani et al. recently observed that surface amorphization of TiO₂ caused a slight uplift of the CBB [146,147,148]. Therefore, amorphous oxides may favor the photocatalytic reduction reaction, and regretfully, this remains ambiguous.

Wang et al. utilized alcohol plasma to successfully introduce a thin amorphous layer (~0.7 nm) on the surface of commercial P25 (Figure 7a) [70]. The subsequent characterizations confirmed that the surface amorphous layer would passivate surface Ovs and raise the energy level of electron traps for enhanced separation and reduction capacity of photogenerated charges. As shown in Figure 7b, alcohol-plasma-treated P25 displayed a marked decrease in the ESR signal of g = 2.004 assigned to Ovs. Then, they investigated the distribution of Ovs by electron energy loss spectroscopy (EELS). For pristine P25, the Ti-L_2,3 edges collected in the surface region obviously shifted to the lower energy (Figure 7c), which illustrated that Ovs were mainly located on the surface. Disparately, the Ti-L_2,3 edges of alcohol-plasma-treated P25 barely shifted with a scan from the bulk to the surface (Figure 7d). These manifested Ovs are evenly distributed in alcohol-plasma-treated P25. Therefore, combined with HRTEM and ESR, the surface amorphous layer passivated the surface Ovs for alcohol-plasma-treated P25. Correspondingly, the RDB-PAS spectra of alcohol-plasma-treated P25 visually showed that the density of electron traps decreased. Furthermore, the energy level of electron traps was simultaneously uplifted (Figure 7e). This verified that the electron-donating ability of alcohol-plasma-treated P25 was improved, which was attributed to the surface amorphization of P25. This contributed to facilitating photocatalytic reduction. From the photocatalytic evolution of hydrogen under simulated solar irradiation, the hydrogen evolution rate of alcohol-plasma-treated P25 (1.982 mmol g⁻¹ h⁻¹) was improved by 124 times compared with P25 (0.016 mmol g⁻¹ h⁻¹) (Figure 7f). The apparent quantum efficiency (AQE) of it loaded with 0.3 wt% Pt reached 37.37% at 365 nm (Figure 7g). In addition, alcohol-plasma-treated anatase- and rutile-TiO₂ also exhibited structural and catalytic performance trends consistent with P25. This demonstration of alcohol plasma was generally applicable to other oxides. Moreover, compared with water-plasma-treated TiO₂ samples, alcohol plasma was superior in activating TiO₂.

The formation of the surface amorphous layer was closely related to the solution annealing. In their work, the alcohol solution split into H•, OH•, and CH(CH₃) OH• radicals under the action of plasma. When P25 was added, the ESR signal intensity of H• and OH• radicals was obviously weakened. This result indicated that H• and OH• species interacted with the surface of P25 to result in surface amorphization. The above processes tended to be carried out in a high-temperature plasma region. Subsequently, under stirring, a dramatic quenching of modified P25 in a low-temperature liquid phase led to the freezing of the surface amorphous structure. To sum up, SP can efficiently realize the surface amorphization of semiconductor oxide with improved photocatalytic reduction performance, which is expected to be popularized by a variety of catalysts.

3.2. Single-Atom Metal Synthesis and Support Modulation

As summarized in Table 5, the metal particles prepared by SP often display the characteristics of clusters. Compared with metal clusters, single-atom metals have a higher utilization rate of atoms, stronger metal-support interaction, and a unique electronic structure for excellent catalytic performance [149,150]. However, the high surface free energy of single-atom metals results in their easy aggregation to form clusters [151,152].

Recently, Xing et al. first utilized the N₂-bubbled SP to load single-atom Au on oxygen-deficient CeO₂ nanosheets within a short period of 2 min [74]. In their work, SP constructed Ovs on ultrathin and multiporous 2D CeO₂ nanosheets (Figure 8a), which acted as anchoring sites for single-atom Au. Thus, the synchronously SP-reduced Au in solution was rapidly and dispersedly captured on the surface of CeO₂ nanosheets in the form of a single atom (Figure 8b). For the synthesis of Au₁/CeO₂, the construction of Ovs on CeO₂ was a decisive factor. The reason was that by changing N₂ bubbles into O₂ bubbles in SP, Xing et al. found that Ovs were decreased, and then Au atoms were agglomerated into clusters (Figure 8c). In terms of SP-prepared Au₁/CeO₂, the Au L₃-edge X-ray absorption near edge structure (XANES) spectra displayed a higher intensity of the white line peak than Au foil (Figure 8d). This indicated the SP-prepared Au₁/CeO₂ was mainly composed of ionic Au^δ+ species. Besides, the radial distance space spectrum χ(R) of SP-prepared Au₁/CeO₂ exhibited two distinct peaks located at ~1.97 and 2.87 Å, assigned to the scattering path of Au-O bonds, and assuredly no Au-Au scattering (Figure 8e). These results further confirmed that the loaded Au existed as a single atom and was coordinated with the O atom in CeO₂ nanosheets.

The SP-prepared Au₁/CeO₂ displayed prominent photopromoted low-temperature CO oxidation performance with a remarkable catalytic temperature decrease of ~100 °C. In the photocatalytic temperature-dependent CO oxidation experiment under solar light irradiation, the CO conversion over Au₁/CeO₂ was as high as 91.7% at 25 °C (Figure 8f). While in the dark, the lowest temperature to achieve 90% CO conversion required 125 °C (Figure 8f). These indicated that light helped enhance the catalytic activity. Xing et al. next selected a 375 nm laser as the light source because it avoided the introduction of photoinduced heat. Results indicated that the value of n, rooted in the sublinear relationship as CO₂-yiled rate∝light intensityⁿ, was 0.72 (Figure 8g), which meant the reaction was a photodriven process. Therefore, the high CO oxidation activity over Au₁/CeO₂ under solar irradiation was dominated by the photoexcitation process. Furthermore, the reaction mechanism of the photodriven CO oxidation over Au₁/CeO₂ was adequately surveyed to be the MvK mechanism. By labeling Au₁/CeO₂ with ¹⁸O, the mass spectra detected a signal of ¹⁸O¹³C¹⁶O (Figure 8h), which indicated lattice oxygen in Ce¹⁸O₂ support reacted with reactant CO. This was an important feature of the MvK mechanism. Therefore, the process of photodriven CO oxidation over Au₁/CeO₂ can be described as follows (Figure 8i). Reactant CO adsorbed at the single-atom Au site reacted with activated surface lattice oxygen to produce CO₂, simultaneously forming Ovs. Afterward, O₂ was adsorbed on the Ovs site to form the O₂⁻ anion and then dissociated into the O atom, refilling the Ovs.

SP is a general approach to preparing other single-atom or double-atom metals. Xing et al. then successfully obtained Pd₁/CeO₂, Rh₁/CeO₂, AuPd/CeO₂, and AuRh/CeO₂ photocatalysts, and they all displayed the highlighted photocatalytic performance in the water gas shift reaction [74]. Moreover, they combined peristaltic pumps with the SP reactor to achieve rapid continuous-flow preparation of single-atom metal, which contributed to the large-scale production [153]. In conclusion, SP has great research potential in the fabrication of single-atom metals, double-atom metals, multiple-atom metals, and even high-entropy alloys for various valuable catalytic reactions.

3.3. Construction of Multidimensional Heterojunctions

In addition to the single-atom metal, the photocatalyst surface can be embellished with SP-prepared carbon materials by changing the precursor solution into an organic solution in SP. Huang et al. recently reported that alcohol plasma modulated the defects of 2D ultrathin HNb₃O₈ nanosheets and synchronously anchored carbon dots (CDs) on the surface of HNb₃O₈ nanosheets in situ [154]. In this work, they found that plasma discharging in an ethanol solution can produce typical CDs with a lattice spacing of 0.24 nm. With the presence of 2D HNb₃O₈ nanosheets, the SP-produced carbon dots were dispersedly and tightly anchored to the surface of the HNb₃O₈ nanosheets to form a 0D/2D HNb₃O₈/C heterojunction, as shown in the HRTEM images in Figure 9a,b. The Raman spectra of the HNb₃O₈/C heterojunction detected two new peaks assigned to the D-band and G-band of CDs (Figure 9c), which again confirmed the decoration of HNb₃O₈ by CDs. Besides, both the O1s XPS peaks and the Nb 3d XPS peaks displayed an obvious shift to higher binding energy in the HNb₃O₈/C heterojunction (Figure 9d,e). These results manifested electron transfers from HNb₃O₈ to CDs at the interface of the HNb₃O₈/C heterojunction. Thus, the decoration of CDs on HNb₃O₈ enhanced the separation of charges. Besides, the HNb₃O₈/C heterojunction revealed the upshifted fermi energy level (E_f) and the downshifted valence band. The upward shift of E_f should result from the generation of shallow energy-level traps of hydrogen dopants. Additionally, the downward shift of the valence band may be caused by the passivation of defect states in pristine HNb₃O₈ nanosheets. Such defect modulation enhanced the redox capacity of charges. Therefore, the accelerated charge separation by CDs and defect modulation synergistically boosted the photocatalytic performance over the 2D HNb₃O₈ nanosheets. As shown in Figure 9f, the H₂ evolution rate over the HNb₃O₈/C heterojunction was 317.7 times higher than that over pristine HNb₃O₈ nanosheets. In addition, CDs, the mixture of CDs and untreated HNb₃O₈ nanosheets, and water-plasma-treated HNb₃O₈ nanosheets all showed much lower photocatalytic H₂ evolution performance compared with HNb₃O₈/C heterojunction. It proved that CD decoration and defect modulation were the pivotal reasons for the enhanced catalytic activity of the HNb₃O₈ nanosheets.

In addition to the HNb₃O₈/C heterojunction, SP also performed well in constructing other CDs/2D-layered transition metal oxoacid heterojunctions. For example, Huang et al. successfully fabricated the H₄TiO₄/C heterojunction by treating titanate nanosheets with alcohol plasma [154]. Besides ethanol, solution discharging with propanol and butanol also produced CDs [154]. This manifested SP can regulate the surface structure and simultaneously construct the heterojunction structure for advanced photocatalysts.

In conclusion, SP can modify the surface structure of photocatalysts and then regulate the crystal facet-dependent photoactivity and surface electronic states for efficient photocatalysts. Meanwhile, with the replacement of solutions, SP can synthesize single-atom metals, double-atom metals, and CDs as surface additives for photocatalysts. More interestingly, vacancies, amorphous phases, and others created by SP would have a positive impact on the morphology, dispersity, and electronic structure of additives to optimize photocatalysts for high-performance photocatalysis.

4. Conclusions and Future Perspectives

In summary, the green, safe, and efficient SP has exhibited great advantages for the surface structure design of advanced photocatalysts. Its rich chemical active species and a variety of physical fields can synergistically act on the surface of photocatalysts for surface designs. At present, SP has been applied for surface electron state regulation, surface amorphization, crystal facet modulation, and additive decoration of photocatalysts. The physicochemical structure of SP-modified photocatalysts can be easily regulated by altering installation parameters (voltage, frequency, pulse width, reactor type, etc.) and the discharging environment (bubble species, electrode compositions, precursor solution, etc.). On the whole, SP has achieved a series of advanced progress in the surface modulation of photocatalysts, as shown in Figure 10.

Based on the numerous reported achievements of SP so far, we briefly discuss the challenges and opportunities for the SP field in the coming years, as follows:

• The material systems are expanded from photocatalysis to electrocatalysis, thermal catalysis, dye-sensitized solar cells, etc. Recently, Li et al. utilized SP to modify transparent MoO_x film, loading it with Pt to act as counter electrodes for bifacial dye-sensitized solar cells (DSSCs). The fabricated DSSCs exhibited excellent double-sided electrocatalytic efficiency of 7.56% and 6.41% with front- and rear-side illumination, respectively [155]. Therefore, SP has the potential to prepare diversified materials for various fields. In addition, it is reasonably expected that the materials suitable for SP processing can be extended from oxides to sulfides, carbides, nitrides, etc., and even polymer systems, such as covalent-organic frameworks, metal-organic frameworks, porous organic polymers, etc.
• SP has been successfully employed to prepare single-atom metals and double-atom metals for catalyst applications. It is reasonable to expect that multi-atom metal catalysts of metal kinds over three as well as high-entropy alloy catalysts could be prepared by SP in the future. In addition, non-metallic materials with the advantages of low cost and abundant reserves also have broad application prospects. SP can be attempted to synthesize non-metallic single atoms, non-metallic multiple atoms, and even non-metallic high-entropy alloys.
• Plasma catalysis has achieved a series of achievements in nitrogen fixation, waste degradation, and fuel synthesis [156,157,158]. The combination of SP with various types of catalysis, such as photo-, thermal-, and electro-catalysis, may possibly achieve rapid mass production and directional selection of products with low energy consumption. Extra energy input can accelerate the reaction rate for mass production. Moreover, different energy fields may affect the proportion of active species and alter the catalytic reaction paths, ultimately achieving precise product orientation.