Ultrasonic-assisted preparation of two-dimensional materials for electrocatalysts

Highlights • The development status of ultrasonic wave and cavitation effect are introduced.• Various inorganic materials prepared under the assistance of ultrasound are summarized.• The ultrasonic-assisted preparation of two-dimensional materials are discussed.• The effects of the ultrasonic wave on the electrocatalytic performances of 2D electrocatalysts are illustrated.


Introduction
In order to tackle the current energy dilemma and environmental contamination, developing clean and efficient sustainable energy technologies has become a major research focus [1][2][3][4]. The development of the advanced energy conversion technologies, such as water decomposition systems, fuel cell systems, and metal-air battery systems, is an important part of the sustainable development. Since hydrogen, oxygen, and water are readily available in nature and are environmentally friendly, electrochemical reaction systems are built around these substances in energy conversion systems. Electrochemical oxygen reduction (ORR) [5][6][7], oxygen evolution (OER) [8][9][10], hydrogen evolution (HER) [11][12][13], nitrogen reduction (NRR) [14][15], and carbon dioxide reduction (CO 2 RR) [16][17] are the core components of these new energy technologies. For example, HER and OER are the cathode and anode reactions of the water splitting. ORR is the cathode reaction of the fuel cell. However, these electrochemical reactions often require amount of precious metals or their oxides as electrocatalysts to facilitate energy conversion or fuel production, limiting their widespread application in new energy technologies. Therefore, developing cheap metal electrocatalysts with abundant reserves and equivalent or higher activity to replace them has become especially important. In the past, researchers have committed significant effort to studying the cheap metal electrocatalysts. Nevertheless, the activity of most cheap metal electrocatalysts remains low compared to that of the precious metal Pt [18][19][20][21][22][23].
The 2D materials have attracted great attentions in all kinds of areas from the first finding of graphene [24][25][26]. Continuous exploration of these materials has promoted their application as electrocatalysts. In addition to graphene, a number of 2D materials have been employed to electrocatalytic reaction [27][28][29][30][31]. 2D nanomaterials have unique advantages as electrocatalysts contrasted with other structural materials: fully exposed edge sites and tunable electrochemical properties. Therefore, nitrogen-doped graphene electrocatalysts applied in zinc-air batteries, and their catalytical properties are already identical to those of traditional electrocatalysts [32][33]. Moreover, nitrogen-doped graphene is cheap and easy to prepare on a large scale. However, the highquality graphene is hard to be obtained under the existing technology. Another 2D material, MoS 2 , is considered the most prospective HER catalyst because S sites on the material edge have a very low adsorption energy for hydrogen protons, even near to the best Pt electrocatalyst [34][35][36]. Nevertheless, it is still a challenge to precisely control the active sites of the transition metal sulfides. The low cost and good performances of NiFe LDH electrocatalyst have attracted much attention, but their poor conductivity limits their wide applications. Similarly, MXene has the advantages of large specific surface area, adjustable band gap, good conductivity and abundant chelating sites. However, MXene is easily rusty in wet environment, which affects their electrical conductivity. In addition to good catalytic performance, 2D nanomaterials also have good mechanical performances and a high surface area-to-volume ratio, making them desired electrocatalysts with large-scale use.
Nevertheless, the electrocatalytic activities of most pure 2D nanomaterials are still low. For example, pure graphene has almost no activity for HER. Because the electronic structure of the electrocatalytic site at the heterophase interface is unsuitable, which weakens the adsorption strength of the reaction intermediate. Therefore, scientists have exploited various methods to ameliorate the electro-catalytical activity of graphene and other 2D nanomaterials, such as doping, defects, interfaces, alloys, additives, heterojunctions, stress, and nanostructures, which are used to regulate the electron structure of 2D nanomaterials and thus improve their electrocatalytic activities [37][38][39][40][41][42]. These strategies also provide significant room for potential improvement in the electrocatalytic activities of 2D nanomaterials.
In recent years, various chemical synthesis methods have made great strides in development. Among them, ultrasound-assisted preparation, as a promising material synthesis tactics with the features of green, innovation, and low cost, is becoming popular. Ultrasound waves are widely used in creasming [43], foodstuff ptocessing [44], extraction [45], and other areas. The import of ultrasound can offer additional energies in the mixing, drying, and extraction processes, which has better mass transfer than those solutions that are not ultrasonically treated. In addition, various materials have been successfully synthesized via ultrasound-assisted methods, such as nanostructured materials [46], photocatalysts [47], and conductive polymers [48]. The selfassembled Ni nanocrystals have been synthesized through a simple ultrasound-assisted chemical reduction method, exhibiting good HER activity [49]. Jun et al. dispersed MXene by using 28 kHz and 580 kHz ultrasonic frequencies to obtain a new type of nanostructured material Ti 3 C 2 T x MXene materials [50]. The Co 4 S 3 /Co 9 S 8 nanosheets with mixed crystal phases have been successfully prepared for the first time by using a solvent-thermal method with ultrasonic stripping technology. Fe-Co 4 S 3 /Co 9 S 8 nanosheets were formed after doping Fe 3+ ions by a onestep impregnation method, which showed excellent OER activity [51].
As we all know, the constitute and the micro-structure of the 2D nanomaterials have a direct influence on the performances of the electrocatalytic reactions. Hence, the design and preparation of the 2D nanomaterials are essential. Though many kinds of 2D nanomaterials have been synthesized and applied in the electrocatalytic reactions, there are still various challenges to be addressed, such as the regulation of the surface electronic structure, the precise electrocatalytic active sites, the control of the lattice distortion and so on. The main content of this review is following. We first introduce the importance of developing 2D nanomaterials for electrocatalytic reactions and the research results of ultrasound-assisted synthesis of 2D materials. Ultrasound cavitation effect and its adhibition in the preparation of inorganic materials have been overviewed. The micro-structure, performances, advantages, and existing problems of the samples obtained by the above routes have been depicted. In the third part, the representative 2D nanomaterials for example TMDs, LDHs, graphene, MXenes, and their catalytic performance as electrocatalysts have been detailly introduced. Finally, we point out some problems of 2D nanomaterials as electrocatalysts and make prospects for their development.

Ultrasonic wave and cavitation
The frequency of ultrasonic waves is above 20 kHz, and their applications are widespread. Ultrasonic waves can detect relevant information in a propagating medium when their intensity is low. This is called monitoring ultrasonics, for example, in ultrasonic inspection of casting shrinkage and porosity. When the intensity of ultrasonic waves is high, they can act as energy to impact a substance, known as power ultrasonics. This review only refers to power ultrasonic waves.
As the ultrasonic waves are introduced into liquid, it causes periodic pressure fluctuations in the liquid. If the amplitude of the alternating sound pressure exceeds a certain value (cavitation threshold), the nuclei of cavitation forms, grows, and ultimately cracks within the liquid or at the liquid-solid interface. Studies have shown that the crack of cavitation bubbles generates elevated temperatures, pressures, microjets [52][53][54][55][56][57]. In a review article on acoustic cavitation published in 1980, Neppiras used the term "ultrasonic cavitation" as an academic term.
The study of cavitation can be traced back to the phenomenon of blade damage caused by cavitation on the propeller of a British destroyer in 1902. After years of research, Rayleigh published an article in 1917 titled "The Pressure Developed in a Liquid during the Collapse of a Spherical Cavity". In the above article, he established equations that described the motion of free bubbles in incompressible fluids and indicated that the collapse of bubbles in high-speed water flow would result in extremely high pressures. Despite a century of research, the mechanism behind cavitation remains to be explored. The study of cavitation mainly focuses on two aspects: the formation of bubbles and the problem of bubble collapse based on this formation.
The formation of voids indicates the stretching of the liquid, making it a function of the liquid's tensile strength. The theoretical calculation for water's tensile strength is around 170-320 MPa. However, even in pristine water without impurities, its tensile strength only reaches 30 MPa during experiments. Scientists have proposed several hypotheses to explain this physical phenomenon, with the most widely accepted explanation being the crevice model introduced by Harvey in 1947 [58]. This theory claims that voids formed in the concave crevices of hydrophobic solid surfaces are stable in water due to their negative surface tension, making them a stable cavitation nucleus (similar to the heterogeneous nucleation theory of metal crystallization). When pressure exceeds the cavitation threshold, the voids in the crevices begin to grow.
Greenspan et al. filtered out particles (>0.2 μm) in the water, and the tensile strength of the water increased to nearly 20 MPa, which indicated that the particles in the water played a role in stabilizing nucleation [59]. The different types of particles have been added into water to study the effect of particles on cavitation [60]. The research results show that polyamide and polystyrene have significantly higher cavitation strength than other particles, because they aren't wet with water and the surface is rough. The non-reproducibility of heterogeneous nucleation on solid particle surfaces renders the survey and study of cavitation quite tough. To investigate the effect of pits on cavitation formation, Bremond et al. used etched pits on silicon wafers [61][62]. The experimental study proves that pit-induced effects can make cavitation occur repeatedly and controllably.
The second aspect of cavitation effect concerns the exploration of the collapse of cavitation bubbles. Rayleigh first reported that the collapse of bubbles is spherically symmetrical and generates a tremendous shock wave that causes plastic deformation of the material, leading to damage in blades. In 1944, the theory of micro-jets had been proposed, which stated that the collapse of bubbles could be non-symmetrical (flattened or diamond-shaped) under pressure gradients or near boundaries. The moment before collapse produces high-speed micro-jets that strike the wall and damage the material.
It has been observed that the pressure produced by collapsing cavitation bubbles is directly proportional to the ratio of the minimum and maximum radius of the bubbles at the moment of collapse. That is to say the smaller the minimum radius and the larger the maximum radius at collapse, the greater the generated pressure. Scholars worldwide have measured the pressure generated by the crack of cavitation bubbles using different methods. For instance, the peak pressure generated by the crack of of bubbles in degassed water have been measured under ultrasound frequency of 23.5 kHz using a piezoelectric quartz detector, which is approximately 400 MPa [63]. The peak pressure of the pulse initially increased rapidly with the amplitude of the tool rod's end face before slowing down. Using high-speed photography, Brujan et al. studied the crack of bubbles and suggested that the highest velocity of micro-jets produced at collapse can reach 500 m s − 1 , causing a water hammer pressure of 1000 MPa [64]. The mechanical mechanisms of cavitation involve the shock wave and micro-jet effects. Whether cavitation-induced damage occurs depends on whether the maximum pressure produced exceeds the material's strength and whether the collapse happens near its boundary. This is because both shock waves and micro-jets have limited effective ranges. A vast amount of theoretical and experimental studies indicate that the cavitation bubbles can damage to the material surface it only when they are at a certain distance [65][66][67].
Further research is needed to fully understand the ultrasonic cavitation phenomena in aluminum melts due to their high temperatures and invisibility. Recently, the synchrotron X-ray technology is attempted to study the cavitation phenomena of Al-Cu melts [68]. The generation of bubbles at the end face of the tool bar has been observed. The use of a higher acoustic power of 1500 W cm − 2 has resulted in larger bubble diameters compared to those measured in water (120 W cm − 2 ). Experimental results have shown that ultrasonic cavitation can ameliorate the grain structure of Al alloys. The potential and mechanism of ultrasonic cavitation for grain refinement in Al-Cu alloys have been studied from a theoretical perspective. Fig. 1 illustrates the concept of shock waves formed via the crack of bubbles close to dendrite crystals during solidification in Al-Cu alloys [69]. The theoretical simulation has indicated that the pressure produced by the cavitation bubble collapse decreases with the increased distance from the bubble. The maximum pressure was calculated to be 235 MPa near the bubble, decreasing to 2.5 MPa at a distance of 200 µm. However, even at this distance, the pressure still exceeded the strength of the dendrite crystal of 1.037 MPa, leading to crystal fragmentation and grain refinement during solidification.
The unique cavitation effect generated when materials workers use ultrasound has resulted in crucial achievements in the application of ultrasound in new materials preparation.

The adhibition of ultrasound in preparing inorganic materials
The introduction of ultrasonic treatment into chemical reactions has accelerated or controlled chemical reactions to prepare micro-or nano-powders, which has become an independent subject-sonochemistry. Suslick's research group has carried out many creative works in sonochemistry [70][71][72]. This section mainly introduces the preparation of powder materials by ultrasonic pulverization. The use of ultrasonic cavitation has help to reduce the size of solid particles in liquids. Ultrasound is used to crush cells and is widely used in laboratories. In addition, ultrasound is also used to crush minerals [73][74][75], such as, kaolin, graphite, mica, coal powder, etc., metal Zn particles, carbon nanotubes, alumina and other ceramics, explosive materials, etc.
The influences of 20 kHz ultrasound power, processing time, and particle content on the average diameter and size distribution of Al particles have been studied [76]. Fig. 2a presents the change of the size distribution curve of alumina particles with increasing ultrasonic action time. The particle size distribution curve is observed to shift to the left, and the size of particles obviously reduced as the action time of the ultrasonic wave increased. In Fig. 2b, it is illustrated that the average particle size continuously decreased with the extension of processing time and the increasement of ultrasonic power. The rise in particle content resulted in an increase in particle fragmentation, indicating that the increased content of particles would not affect the occurrence of cavitation. In short, the mean diameters of the Al particles reduced with the increasement of the ultrasonic action time under the same ultrasonic power. And the average particle sizes decreased with increasing the ultrasonic power at the same ultrasonic action time.
Building on Kass's research, the grinding effect of different ultrasound delivery approaches (20 kHz tool rod delivery and 192 kHz ultrasonic cleaning equipment) on aluminum oxide particles (74-80 μm average particle size) has been explored [77]. The results show that lowfrequency tool rod delivery, high power, and longer ultrasonic processing can achieve better grinding results. Due to the agglomeration of the generated nano-aluminum oxide particles, the particle size could not be detected. However, TEM results have revealed the presence of aluminum oxide particles are smaller than 100 nm.
The effects of ultrasound on the comminution of explosives and crystal particles such as NTO, AN, NaCl, HNS, HMX and RDX in heptane liquid and water have been investigated [78]. The consequence has demonstrated that the size of AN and NaCl particles didn't change significantly, while other particles reduced from 140 μm to 20 μm after 40 min of ultrasonic treatment. At a particle size of about 40 μm, the ultrasonic input energy increased exponentially. Teipel's study on the crystal structure, surface condition and morphology of these particles have indicated that NaCl has a smooth cubic structure, making it less susceptible to ultrasonic comminution [78]. HNS is needle-shaped and requires only a small energy input to be comminuted in its longitudinal direction. HMX and RDX have irregular surface morphology, making them more susceptible to comminution.
In 2004, Geim et al. first obtained single-layer graphene via a manual peeling method. The specifical procedure was following: the transparent tape was firstly used to stick to the surface of the graphite. Then the transparent tape was fold in half, sticked on and pulled off. In this way, both sides of the transparent tape were stuck with a graphite layer. And the graphite layer became thin again. Repeated this many times, when the graphite layer on the transparent tape was as thin as one carbon atom, it became graphene. Subsequently, researchers have conducted extensive creative work to prepare graphene for large-scale production. The ultrasonic exfoliation has been utilized to peel graphite layers in a liquid medium [74]. The ultrasonic energy can exfoliate the graphite layer by layer to obtain up to 12 wt% high-quality non-oxidized singlelayer graphene without introducing any defects, which benefit the excellent properties of graphene. The crushing effect and mechanism of ultrasound on aspirin crystals have been studied [79]. The polarized light images of aspirin particles at different ultrasonic treatment times have been show in Fig. 3. With the increasement of the ultrasonic treatment time, the particle size reduces while the number of particles per unit area rises significantly. Since the size of the particle is less than 200 μm, the bubbles don't rupture asymmetrically, and the shock waves formed via cavitation crack are the dominating mechanism for crushing aspirin particles. The above results have indicated that the pulverization effects of ultrasound on particles are influenced by various parameters, including particle size and shape, particle strength, properties of liquid media, ultrasonic action mode and intensity, processing time, liquid volume, particle content and so on.
Ultrasonic cavitation is mainly used for degassing of metals and refinement of solidified structures in the early stage. With the progress of high-power ultrasonic facility and the embedded research of the acting regime of the ultrasonic wave, the cavitation effect produced by strong ultrasonic waves in liquids is gradually applied to improve wettability and disperse micro-particles and even nanoparticles. We will concentrate on the ultrasonic-assisted preparation of 2D materials and their application in electrocatalytic reactions.

Ultrasound-assisted synthesis of 2D nanomaterial electrocatalysts
2D nanomaterials, especially TMDs and graphene, have garnered broad attention due to the following numerous advantages. Notably, they are easily accessible and cost-effective, and possess tunable physicochemical properties that enable their development as replacements for precious metal electrocatalysts [80][81][82][83]. For instance, researchers have found that nitrogen-doped graphene exhibits greatly enhanced ORR performance in alkaline condition compared to pure graphene, which is close to the commercial 20 wt%Pt/C [84]. Additionally, MoS 2 demonstrates high HER activity in acidic condition [85]. Despite some progress, the electrocatalytic activity and stability of most 2D materials still require improvement. Currently, apart from Pt, the majority of materials exhibit inferior acidic ORR activity. The graphene shows much lower activity in acidic ORR than alkaline ORR [86][87]. Moreover, transition metal dichalcogenide materials continue to possess higher HER reaction energy barriers than metal Pt, leading to greater energy consumption in hydrogen production [88][89]. The electrocatalytic reactions of 2D material occur at the active sites of heterogeneous interfaces, and the electron structure of these activated sites directly  influences the size of the reaction energy barrier, thereby determining the starting potential for electrochemical reactions and the reaction rate, which ultimately affects energy consumption. The following sections will focus on four types of 2D nanomaterial catalysts: TMDs, LDHs, graphene, and MXenes.

Tmds
Various TMDs have also been prepared by the ultrasound-assisted method [90][91][92][93][94][95] and studied as electrocatalysts, especially MoS 2 -based materials. DFT calculations have shown that its electronic structure is very similar to that of Pt. Additionally, similar to other transition metal phosphides, S interacts with protons, promoting the HER reaction [96]. Since the active sites at MoS 2 edges have extremely supernal catalytic activities, the focus of research has been on preparing nanomaterials with high edge site densities and defect formations in their structures [97].
The three-dimensional (3D) nano-flower-like porous CoMoS 4 electrocatalysts have been synthesized through a facile ultrasound-assisted hydrothermal method (as shown in Fig. 4a) [98]. Because of the luxuriant active places, small electrolyte contact resistance, and rapid electron transition rate on CoMoS 4 electrocatalysts (Fig. 4b), they exhibit excellent electrocatalytic activity. When CoMoS 4 is used as the cathode and anode in electrolysis, a 10 mA cm − 2 current density is achieved at 1.51 V. Additionally, the OER overpotential of CoMoS 4 electrode is 250 mV (Fig. 4c) and the HER overpotential is 141 mV. Furthermore, the vesicular structure of CoMoS 4 material offers a perfect pathway for the spread of the electrolyte, improves its reaction dynamics, and promotes gas emission.
A composite material of ZnS nanoparticles (ZnS NPs) coated with RGO has been synthesized using ultrasonic bath [99]. The preparation process is shown in Fig. 4d, where the reaction was carried out in an ultrasonic bath for 1 h (50 kHz and 60 W), followed by calcination to obtain ZnS NPs@RGO nanocomposites. Because of the strong synergistic action between RGO and ZnS NPs, the electrode material modified with ZnS NPs@RGO exhibits high stability and excellent sensitivity, making it suitable for determining 3,4-dihydroxycinnamic acid in different foods ( Fig. 4e-f).
Yang et al. exploited a new route for constructing a threedimensional rGO/Co 9 S 8 composites with extraodinary synergetic actions between the two components [100]. This was achieved by employing a simple process that combined ultrasound atomization drying and thermal treatment (Fig. 5a). The resulting Co 9 S 8 /rGO composite material exhibits an excellent in-plane shrinking structure, leading to a large specific surface area and abundant potential electrocatalytic places (Fig. 5b). The optimized electrocatalyst shows 0.308 V overpotential and of 130.0 mV dec -1 Tafel slope at 10 mA cm − 2 , and also demonstrates excellent cycling consistency in 1 M KOH (Fig. 5c). The above work offers a potential large-scale tactic for the preparation of TMDs oxygen evolution reaction electrocatalysts, which will contribute to the progress of hydrogen energy study.
Ultrathin structures of transition metal dichalcogenides possess various superior versatile performances and has huge potential in energy conversion fields. However, their simple preparation and volume production are still tough. MoS 2 nanoflakes with 100 nm lateral size and 10-20 nm thickness have been prepared using inorganic matrixfacilitated deposition and ultrasonic exfoliation techniques (Fig. 5d) [101]. They exhibit representative various absorptions, suggesting the particular band structure features of MoS 2 2D nanosheets (Fig. 5e). In addition, the electron structure of slim MoS 2 nanosheets can be regulated by tunning the surrounding environment in which they are assembled. In the interlayer interfacial assembly of (PEDOT:PSS/MoS 2 ) n multilayer films, the charge transition procedure enhances the catalytic efficiency of MoS 2 nanosheets. The catalytic performance of the assembly is optimized through a well-regulated layer-by-layer procedure, and the assembly exhibits efficient and stabilized HER performances (Fig. 5f). We anticipate the above preparation tactic of MoS 2 2D nanosheets can facilitate the study of versatile materials, which can be generalized to synthesizing other nanosheets.

LDHs
LDHs is a kind of ionic layered compounds composed of solvent molecules, interlayer anions, and positively charged main layer [102]. The bivalent positive charges are partially replaced by trivalent cations coordinated by hydroxyl octahedrons, and the supererogatory cations are balanced by interlayer negative ions. The common symbol of LDHs is [M 2+ 1-x M 3+ x (OH) 2 ] z+ A nz/n ⋅mH 2 O, in which M 2+ and M 3+ are + 2 and + 3 metal cations, and A nis interlayer negative ion [103]. Every hydroxyl radical in the LDH layer towards the interlayer area, which may generate hydrogen bonds with water molecules or interlayer negative ions. Various LDHs have been synthesized via the ultrasound-assisted method [104][105][106][107][108][109][110][111][112]. Various metals can be imported into LDHs, and the distances and negative ions between layers can be regulated by the exchange of the negative ions. Hence, the constitute of the LDHs can be adjusted at will, which further widen the applications of the LDHs [113][114]. In addition, the stripping of the liquid can expand the distance between layers and weaken the interaction between motherboards, thus forming ultra-thin nanosheets with coordinated unsaturated active sites and improving the OER performances. Ni/Fe-LDH is considered to be an effective OER catalyst in alkaline solution.
Chen et al exploited a facile ultrasonic synthesis route, which can produce flower-like cobalt base-like dihydroxide nanowires on a large scale within 30 min (Fig. 6a-b) [115]. The nanoflower has excellent electrocatalytic performance and excellent stability under alkaline conditions. The overpotential, the Tafel slope and the charge transfer resistance are 300 mV, 110 mV⋅dec -1 and 21.4 Ω at 10 mA⋅cm − 2 , which is superior to the commercialized RuO 2 nanoparticles (Fig. 6c). This is because of the affluent active places and redox coupling brought by the 2D ultra-thin structure of cobalt-like base-like dihydrooxides nano-sheet aggregates, which further improves the OER activities.
NiFeLDH/ carbon black (CB) bulk materials have been stripped into monolayer or small-layer NiFeLDH/CB nanoflakes via a simple ultrasonic peeling method (Fig. 6d-e) [116]. The ultrasonic stripped NiFeLDH/CB nanowires in pure water have excellent OER activity, which is better than other cheap metal catalysts. In alkaline solution, the overpotential of the exfoliated NiFeLDH/CB nanowires is 220 mV (Fig. 6f) and 35 mV dec -1 Tafel slope, which is superior to the bulk NiFeLDH/CB (280 mV overpotential and 48 mV dec -1 Tafel slope at 10 mA cm − 2 ). Meanwhile, after 12 h, the structure of the stripped NiFeLDH/CB remains stable and the potential is nearly constant, suggesting its high stability. Similarly, Li et al. used ultrasound-assisted method to insert formamide into the interlayer of NiFeLDH materials to expand the interlayer spacing, which increased the active places and provided greater space for the spread of reactants, bubbles [117]. The results show that after formamide is inserted at 80 ℃, the distance of NiFeLDH interlayer enlarged from 7.8 to 9.5, which improve the OER activities (210 mV overpotential), interfacial connectivity and stability. Highly active Ru nanoparticles (RuNPs) have been loaded on NiFe LDH (Ru/NiFe-LDH) by ultrasound-assisted route with no reducing and stabilizing agents (Fig. 7a) [118]. Ultrasound excites the rich hydroxyl radicals on the NiFe-LDH surface to form hydrogen free groups (•H), reducing Ru from 3 valence to zero valence (Fig. 7b). In Fig. 7c, the Nethylcarbazole (NEC) hydrogenation rate firstly increased and then decreased with the rise of ultrasonic power. When the ultrasonic power was 300 W, the hydrogenation rate was the fastest. And after the NEC hydrogenation reaction of 40 min, the mass hydrogen storage capacity remained unchanged. Oxygen-containing groups enhance the interaction between layered NiFe-LDH carriers and Ru nanoparticles, thus making the structure of Ru/NiFe-LDH more stable and ameliorating the electrocatalytic activities of hydrogenation (Fig. 7c).
The petal-shaped NiCoFe layered double hydroxides (LDHs) have been prepared after 60 min of high-power ultrasound [119]. The morphology of nano-petal-like NiCoFe 50 LDH will be affected by ultrasonic time, and 60 min is the best time to synthesize NiCoFe-LDH. The above consequences manifest that compared with NiCoFe-LDH nanoparticles (synthesized in 30, 120 mins by coprecipitation and ultrasonic method), the nano-flake NiCoFe-LDH has better OER properties (0.46 V starting voltage, 227 mV overpotential and 234 mV⋅dec -1 Tafel slope at10 mA⋅cm − 2 ) and stable cycling performances in 0.1 M KOH. The above means of synthesizing nanometer petal NiCoFe LDH is simple and easy to operate, which can be extended to large-scale industrial production.
The ultrafine ZnCo-LDH nanowires (ZnCo-UF) with 3.5 nm average particle size and 0.5 nm thickness have been successfully exfoliated from the massive ZnCo materials by simple ultrasonic peeling method [120]. The monolayer ZnCo-UF nanowires are rich in oxygen vacancies and unsaturated coordination, which endow them with semi-metallic properties, enhance the charge transfer, improve the positivity of the nanowires, and enhance the adsorption capacity and OER activities of the intermediates. The OER activities of these ultra-fine ZnCo-UF nanowires (overpotential 340 mV@5 mA⋅cm − 2 ) are dramatically superior to bulk materials (overpotential 530 mV@5 mA⋅cm − 2 ). This synthesis method is universal and has been successfully applied to NiFe-LDH, CoMn-LDH and CoFeAl-LDH materials.

Graphene
Non-metallic materials are composed of metal-free atoms, such as various carbon materials, which are widely used in ORR, OER and HER [121][122][123]. At present, scientists have put a great deal of energy to develop non-metallic materials into catalysts to replace Pt and Ru/Irbased materials. Generally speaking, 2D graphene nanosheets have large specific surface area and thinner thickness, layered structure, porous structure, excellent physical, chemical properties, which render them the most competitive research objects in the next generation of electrochemical energy devices. A mass of literatures have accounted for the successful synthesis of 2D graphene nanosheets with specific physical chemistry properties via ultrasound-assisted method [124][125][126].
The Pt and Sn precursors, graphene oxide (GO) have been treated by the ultrasonic wave with 20 kHz frequency. The Pt/Sn/rGO catalysts have been prepared by loading the monometallic Pt or the bimetallic Pt-Sn nanoparticles on the surface of rGO [127]. This sonochemical method is a simple method to obtain monoatomic and diatomic nanoparticles with different compositions, which can distribute these metal nanoparticles more evenly on rGO. Graphene as the matrix can better fix metal particles, increase the active places on the surface of the electrode, and significantly improve the catalytic efficiency of metal nanoparticles. The nitrogen-sulfur co-doped graphene/Fe 3 C (NS-GR/Fe 3 C) nanocomposites have been prepared by the assist of the ultrasound with homogenization dispersion advantage (Fig. 8a) [128]. The results show that the NS-GR/Fe 3 C catalyst prepared under the ultrasonic bath of 150 W and 40 kHz has outstanding performance for both ORR and OER processes (Fig. 8b-c). Due to the influence of heteroatom doping, Fe 3 C has a strong synergistic effect with graphene sheets. The doped matrix has high electrical conductivity and electrocatalytic activity, which makes the NS-GR/Fe 3 C catalyst show excellent catalytic performance in aqueous and non-aqueous electrolytes.
Mai et al reported that layered N-doped carbon spheres derived from bimetallic zeolite imidazole skeleton (BMZIF) were grappled by graphene-coated cobalt nanocrystals (Co@C) and cobalt monoatom (CoSAs) (Co-NCS) for efficient catalysis of ORR [129]. The authors use ultrasound-assisted technology to disperse BMZIF on phenolic resin microspheres and obtain Co-NCS-x after pyrolysis (Fig. 9a). The high pressure and temperature caused by the ultrasonic wave promotes  nucleation and greatly shortens the synthesis time. The ultrasonic method can obtain smaller particles and more uniform distribution than traditional methods (Fig. 9b). BMZIF with interconnected porous carbon skeleton can promote electronic transmission and contains a mass of active places, so that Co-NCS-2 shows excellent ORR activities (Fig. 9c). The nickel manganese oxides modified partially rGO (NiMnO@pr-GO) nanocomposites have been synthesized by sonochemical means for the modification of glassy carbon electrode (GCE) [130]. CV outcomes show that the NiMnO@pr-GO nanocomposites present higher electrocatalytical activities and lower voltage than that of single NiMnO and GO.
Co-Sn-Pd supported on reduced graphene has been prepared through ultrasonic radiation [131]. Even at very low content of Pd, tribasic Co-Sn-Pd/rGO electro-catalysts exhibit higher ORR activity and stability than dibasic Pd-Co/rGO catalysts and 20 wt% Pt/C catalysts. 2D Pd nanowires have been loaded on GO (Pd/rGO-u) by sonochemical method [132]. By contrast with Pd/rGO-c synthesized by traditional method, Pd/rGO-u presents superior ORR electrocatalytic properties and follows 4ereaction dynamics.

MXene
Thanks to the high conductivity, high hydrophilicity, adjustable band structure and 2D nanostructures conducive to charge transport, MXene has attracted extensive attention in the area of electrocatalysis since its advent. Researchers have made great progress in HER, OER, ORR and so on. Compared with hydrothermal and solvothermal methods, mechanically assisted ultrasound doesn't need the harsh conditions of high pressure and temperature. The reaction mechanism depends on the exfoliation and cracking of the layer. Among them, the mode of ultrasound (probe ultrasound, water bath ultrasound), ultrasonic power and time affect the final formation and yield of MXene. At present, a series of MXene-based composites have been synthesized by ultrasound-assisted method [133][134][135].
Ti 3 C 2 T x MXene is usually obtained by etching the main group elements in its corresponding transition metal carbide MAX phase. Because of its particular physical and chemical performances, it shows excellent electrochemical performance in the electrocatalytic reaction. Nevertheless, the lowered efficiencies of getting rid of dominating radical elements seriously limits the adhibition prospect of MXene and MXenebased composites. In view of this, Ti 3 C 2 T x MXene materials have been synthesized using Ti 3 AlC 2 as the precursor via an ultrasound-assisted method [136]. Under the condition of ultrasound, the chemical etching efficiency of Ti 3 AlC 2 is greatly improved, and the etching time in dilute HCl solution is shortened from 24 h to 8 h. In particular, with the assistance of ultrasound, a higher etching efficiency can be achieved in 2 % HF solution, while the 2 % concentration is lower than the currently reported etching environment. The Ti 3 C 2 T x MXene material obtained by 8-hour ultrasound shows the best electrochemical performances. This ultrasound-assisted process can remove the AlF 3 impurities formed on the etchant surface more efficiently, and the acoustic cavitation effect produced in the ultrasonic treatment process can destroy the hydrogen bond between the two adjacent layer functional groups.
Han et al exploited a facile and rapid means to continuously regulate the nitrogen doping of MXene through cryogenic ultrasound [137]. Firstly, a simple ultrasonic method with adjustable temperature is used to realize the continuous control of nitrogen concentration in MXene (Fig. 10a). Secondly, nitrogen doped into MXene not only provides an  [129]. Copyright © 2021, Elsevier. efficient electronic transition path, but also increases the spacing of MXene interlayer, thus exposing additional active places and producing excellent electrocatalytic performance (Fig. 10b). Through the generation of Ti-N bond at the optimal ultrasonic temperature, the surface of MXene can be embellished and the HER activities can be further enhanced. Third, the optimized nitrogen-doped MXene catalyst has low overpotential (162 mV), low Tafel slope and good durability at 10 mA⋅cm − 2 , thus providing excellent HER electrocatalytic properties (Fig. 10c).
heteroatom doping and vacancy engineering of MXene-based composites have attrested wide attention. Nandi et al used S vacancy regulated nickel-doping Co 4 S 3 nanocubes/N-doping V 2 CT x MXene nanowires (Ni-Co 4 S 3 (S v )/N-V 2 CT x ) electrocatalysts. And the vulcanized process is based on the basis of ultrasonic treatment (as shown in the Fig. 11a) [138]. The effects of sulfur vacancy, hollow structure and Ni-N doping on the electrocatalytic activity were studied systematically (Fig. 11b). The vacancies in Ni-Co 4 S 3 (S v ) hollow nanocubes reduces the contact resistance and promotes the electrons immitting from the surface to the active center. This particular porous construction can provide ample land areas to promote electrolyte infiltrating and effective transition of the electrons and ions. Heterodoping can availably solve the stacked issuse of MXene, expose abundant active places and accelerate electron transition. Therefore, the prepared Ni-Co 4 S 3 (S v )/N-V 2 CT x has excellent HER activities with 127 mV overpotential (Fig. 11c). Overall, MXene-based electrocatalysts have the remarkable HER electrocatalytic properties due to their sulfur vacancies, hollow porous structure and Ni-N doping. MXene with CoBDC nanowires have been coupled to obtain tightly connected 2D/2D heterostructures using a simple ultrasoundassisted growth means [139]. Versatile MXene with ideal hydrophilicity and high conductivities can be used as bridging carriers to guarantee structure stability and expose more active places. In addition, the Co-O-Ti bond generated at the interface availably strikes the charge transport and adjusts the electron structure of the Co active places, thus enhancing the dynamics. Therefore, the optimal CoBDC/MXene shows a low HER overpotential of 76, 41 and 29 mV in neutral, acidic and alkaline electrolytes. Theoretical calculations show that the electron redistribution caused by interface bridging optimizes the free energy of hydrolysis ionization and H* adsorption, thus improving the energy barrier of HER reaction. The above research not only offers a new type catalyst for high efficiency HER under wide pH ranges, but also develops a fire-new way to design highly catalytic activity system.

Conclusion and prospect
Under the background of carbon neutralization, the electrocatalytic reactions have attracted much attention due to the excellent performance in new energy field. The design and fabrication of the electrocatalysts are crucial to the electrocatalytic reactions. We review some latest developments in ultrasound-assisted preparation of 2D material catalysts and classified them according to different types of materials. This review focuses on the cavitation effect and its application in synthesizing inorganic materials, and deeply discusses the representative 2D materials via the ultrasound-assisted method, such as transition metal sulfides, layered bimetal hydroxides (LDH), graphene, MXene, and their catalytic properties as electrocatalysts. Finally, we draw the following conclusions: 1) The use of ultrasonic cavitation is of great significance in the preparation of inorganic materials. Although it is only an auxiliary means for synthesizing inorganic materials, practice shows that the import of ultrasonic cavitation greatly expedites the rate of synthesis and preparation of inorganic materials, and greatly improves the uniformity and dispersion of the separation. As we all know, the ultrasonic cavitation is closely related with the ultrasonic parameters.  [137]. Copyright © 2021, Elsevier.
C. An et al.
However, the relationship between ultrasonic parameters (amplitude, frequency, etc.) and the microstructure and electro-catalytical activities of the material is not clear. Therefore, it is necessary to clarify the relationship between ultrasonic parameters (ultrasonic cavitation) and the structure of catalyst materials for guaranteeing the accurate synthesis of catalyst active places, simplify the process and reduce the cost. Finally, it is necessary to implement the massive commercialized preparation of 2D materials. 2) The appropriate layer distance can enhance the electro-catalytic properties of 2D materials. But the layered structure is easy to collapse in the process of electrocatalytic cycle, resulting in the difficulty of synthesizing highly active 2D materials. The intercalation strategy can effectively alleviate the change of layered structure, so it is easier to prepare 2D materials with high stability.
According to the above description, cheap 2D material catalysts have presented outstanding performance and enormous potential in electrocatalysis area, but some problems have still to be resolved: 1) 2D transition metal sulphide materials are rich and varied. In this review, only a few representative materials are discussed, and there are many unstudied objects. And the prepared 2D molybdenum sulphide nanosheets with lattice distortion show excellent electrocatalytic activity. However, the accurate control of its surface lattice distortion has not been realized. Therefore, the micro-nano processing technologies should be developed to prepared the 2D MoS 2 nanosheets with a particular lattice distortion. 2) Because of the presence of two metals in transition metal-based LDHs system, the selection of metal types, especially the regulation of metal surface electronic structure, directly affects the catalytic activities. However, the precise regulate of its surface electronic structure has not been achieved yet. Therefore, various binary LDHs materials should be synthesized and their surface electronic structures should be characterized, which is helpful to generate the database for guiding the preparation of the electrocatalysts with precise surface electronic structure.
3) Graphene and MXene have low intrinsic activity, which are usually as substrates. They show excellent electrocatalytic activity when compounded with other materials. But their catalytic mechanism is not clear. Therefore, through in-situ analysis technology, machine learning and other new means to further deepen the understanding of the catalyst structure-activities relationship and process mechanism under the condition of dynamic electrochemical reaction, and further guide the intellectually design of the electrocatalysts. 4) The unified standards are lacking for testing the electro-catalytical performances of the prepared 2D material catalysts, such as test conditions, the standards of the commercial Pt/C used for comparison, and parameters which can correctly reflect the performance of the catalyst. Therefore, the uniform standards should be developed by the global researchers in order to better compare the electro-catalytical performances of various electrocatalysts.
In a word, 2D materials synthesized by ultrasound-assisted method have enormous potency in electrocatalytic reaction, which demands us to uncover the distinct micro-structure-activities relationship between the micro-structures and electrocatalytic performance 2D material. Hence, scientists should take into account to prepare electrocatalysts with clear active places, and establish a corresponding model for theoretical calculation according to the characterization results, so as to provide guidance for designing 2D materials with outstanding electrocatalytic properties. It is believed that 2D materials will effectively promote the evolution of novel energy techniques in the immediate future, so as to realize the double carbon goal as soon as possible.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability
No new data were created or analyzed in this study. Data sharing is not applicable to this review article.