A power-triggered preparation strategy of nano-structured inorganics: sonosynthesis.

Ultrasound irradiation covers many chemical reactions crucially aiming to design and synthesize various structured materials as an enduring trend in frontier research studies. Here, we focus on the latest progress of ultrasound-assisted synthesis and present the basic principles or mechanisms of sonosynthesis (or sonochemical synthesis) from ultrasound irradiation in a brand new way, including primary sonosynthesis, secondary sonosynthesis, and synergetic sonosynthesis. This current review describes in detail the various sonochemical synthesis strategies for nano-structured inorganic materials and the unique aspects of products including the size, morphology, structure, and properties. In addition, the review points out the probable challenges and technological potential for future advancement. We hope that such a review can provide a comprehensive understanding of sonosynthesis and emphasize the great significance of structured materials synthesis as a power-induced strategy broadening the updated applications of ultrasound. This journal is © The Royal Society of Chemistry.

Introduction

Ultrasound, which is industrially generated from electrical energy by using a magnetostrictive or piezoelectric transducer,[1-4] often serves as a driving force to facilitate many physical, mechanical or chemical processes. Generally, high-frequency ultrasound is typically used in medical applications for diagnosis and therapy, e.g., to aid in drug delivery, to break down blood clots, or even to destroy tumors in surgery.[5-9] By comparison, low-frequency ultrasound is widely used in research and industry to efficiently clean the debris from surfaces, to purify water from degrading contaminants, to initiate reduction, oxidation and hydrolysis reactions, or to accelerate crystallization and polymerizations.[10,11] For example, sonolysis or ultrasonic catalysis, as one of the advanced oxidation processes (AOPs), has been exploited for the degradation of toxic organics in order to protect the aqueous environment, where ultrasound irradiation combined with other AOPs and/or oxidizing agents can enhance the reaction rate, decrease the processing conditions, and reduce the overall energy consumption.[12,13]Fig. 1 lists the general applications of low-frequency ultrasound in chemistry, material, engineering and manufacturing fields, and many processes refer to the chemical effects from ultrasound. As for the chemical effects from ultrasound irradiation, the precise term “sonochemistry” is coined, covering a special research area where powerful ultrasound induces chemical reactions or even merely participates in some chemical processes as a nonclassical method.[14,15] Actually, since ultrasonic equipment came into being, the chemical effects from ultrasound or sonochemistry-based applications have been reported,[16] and especially, more and more ultrasound-assisted methods have emerged in synthetic chemistry, organometallic chemistry, material chemistry, biological chemistry, medical chemistry, etc.[17-23]

Fig. 1

General applications of ultrasound in chemistry, material, and manufacturing processes.

Inorganic materials have attracted much attention owing to their unique structures and/or properties and widespread applications in many fields ranging from catalysis, hydrogen storage, and energy storage to the sensors, electronics, biotechnology and nanomedicine, and correspondingly, numerous methods have been developed for their synthesis, such as the coprecipitation method, sol–gel method, and solvothermal method.[24-27] So far, many sonochemical preparation methods including a few combined strategies with the aid of ultrasound irradiation have been also carried out to create inorganic materials, where ultrasound irradiation seems to be a power-inducing preparation source. Recently, we have published a review paper on the catalytic applications of ultrasound-synthesized inorganic nano-materials, and the content highlighted sonochemistry applications in the inorganic-based catalysis processes including reduction, oxidation, degradation, polymerization, etc.[4] In contrast, this review will provide an overview on the latest developments of the ultrasound-assisted synthesis of nano-structured inorganics, where more emphasis is put on the matters about the sonosynthesis (or sonochemical synthesis), wherein it will provide a fundamental understanding of the basic principles of sonosynthesis including acoustic cavitation, sonochemistry, and physical/mechanical effects and will present the nano-scale inorganic materials produced by the sonochemistry-inducing synthesis, as well as the size, morphology, structure, and properties of products. Moreover, the review will point out the prospective challenges and trends in sonosynthesis-assembled programming of miscellaneous materials, broaden the synthetic methodology, and conclude the significance of sonosynthesis in abundant applications.

Mechanism of sonosynthesis

For either low-frequency or high-frequency ultrasound, sonosynthesis is a conventional representation for chemical reactions. Theoretically, there is no direct interaction at least in a molecular level between ultrasound and chemical species, since some typical index values of sound wave are much larger than the molecular scale in a liquid system, e.g., sound velocity (1000–1500 m s−1) and sound wavelength (10−6 to 10−1 m).[28,29] In comparison to other energy sources such as heat, light and microwave, ultrasonic irradiation can offer some extreme transient conditions (e.g., localized high pressure and temperature) by acoustic cavitation and bring up some interesting chemical effects on the synthesis processes,[30] or taken as a whole, the sonochemistry as well as the derivative sonosynthesis originates from acoustic cavitation primarily.

Acoustic cavitation, namely the continuous changing states of cavitation bubbles including the formation, growth, and violent collapse, is a transient process caused by irradiating a liquid with ultrasound.[31] The ultrasound, as a conventional pressure wave, can cause the liquid medium to experience an alternate compression and rarefaction phase when moving forward, and the particles in the liquid will respond to the acoustic pressure cycles, resulting in sound propagation. During the rarefaction phase (or negative-pressure) cycle, the liquid is pulled apart at some sites containing some gaseous impurities (known as “weak spots”) by overcoming intermolecular interactions and the cavitation bubbles are usually nucleated at the “weak spot” sites.[32] Once formed, the bubbles can keep trapped in the acoustic pressure field and grow through a rectified diffusion process owing to the contributing area and shell effect. The average surface area of cavitation bubbles during the rarefaction phase is larger than during the compression phase, so there is always a gas flow into the bubbles over several cycles. Meanwhile, the gas inflow is always higher during the expansion stage because the diffusion rate into the cavity should be proportional to the concentration gradient of solvent vapor and dissolved gas, so such a concentration gradient in the fluid shell around the bubbles also contributes to the bubble growth in volume. When the bubbles grow up to a critical size range (typically tens of micrometers), they will be no longer stable and undergo a rapid inertial overgrowth by coupling strongly to the acoustic field, and subsequently, a catastrophic collapse occurs at some point. During the cavitation, the compression of oscillating bubbles accumulating ultrasonic energy effectively leads to an enormous energy concentration being stored within a small volume. The concentrated energy will be released from the bubbles within a very short time (estimated 3.5 μs) via an implosive collapse that is a nearly adiabatic process, and thus, acoustic cavitation can result in the formation of a localized micro-scale “hotspot” which experiences high temperature (up to thousands of degrees) and high pressure (over hundreds of atmospheres).[33-36]Fig. 2 gives us the schematic illustration of hotspot formation under acoustic cavitation. The intensity of acoustic cavitation, largely determining the rate and/or productivity of sonosynthesis, can be simultaneously influenced by the ambient conditions (e.g., dissolved gases, solvent, reaction temperature, and hydrostatic pressure) and the device parameters (e.g., ultrasonic frequency and acoustic power), and the inductive description can be presented as follows: (1) the dissolved gases significantly act as the “weak spot” sites in a liquid, but the bubbling gases can produce more cavitation bubbles through the mixture, and the type of bubbling gases is important as well; for instance, the gas with a higher specific heat ratio should give a greater cavitation effect, while the gas with a larger solubility provides more nucleation sites for cavitation.[37,38] (2) The cavities may be more readily formed when a solvent has higher vapor pressure, lower viscosity and surface tension, but the one with opposing characteristics benefits the cavitation intensity, so an optimum solvent is able to dominate the cavitation.[28] (3) In a certain system, an increasing ambient temperature may lead to more favorable results, but has a negative effect on the sonochemical effects, namely, higher ambient temperature results in easier bubble formation, whereas the bubbles containing more vapors receive an overall decrease in the cavitation.[16] (4) Contrary to the temperature, enlarging the ambient pressure of the system generally leads to a predominant increase in acoustic cavitation as a result of the decrease in vapor pressure. (5) Ultrasonic frequency can alter the critical size of the formed cavitation bubbles, and low frequency produces more violent cavitation with higher localized temperature and pressure.[16] (6) When a continuously advancing ultrasonic power is delivered to the reaction system, the acoustic cavitation will increase to a maximum and then decrease probably because the dense cavitation bubbles formed near the probe tip at high power acts to block the energy transmitted from the probe to the fluid.[16]

Fig. 2

Schematic illustration of the hotspot formation under acoustic cavitation.

To date, several competing theories have been postulated to explain the chemical effects created by ultrasound irradiation, but most of them are completely discarded as a valid mechanism of sonoluminescence and sonochemistry.[39,40] Since the acoustic cavitation accounts for the transient characteristic conditions, hotspot theory has become more generally accepted, where it highlights that the extreme conditions from the localized hotspots will form a few active species for the follow-up sonochemistry. In a system, the bulk of the liquid has no direct sonochemical events, but the compressional heat released from collapsing bubbles generates enough high temperatures effectively and locally inducing the dissociation of chemical bonds, and the induced radicals may diffuse into the liquid prior to further reactions. Primary sonochemistry refers to the indiscernible processes that result in the formation of radical species from the volatiles and/or gases inside the collapsing bubble. In the bubble center (Fig. 3), namely at the hotspots, the harsh conditions can cause the sonolysis reaction to occur, where the gases present in the bubbles (Zone 1) or in the interfacial region (Zone 2) between the bubble center and bulk liquid will experience large gradients of temperature, pressure, and shear. As a powerful tool to understand the origin of the drastic conditions created by acoustic cavitation, the analysis of sonoluminescence spectra from water under power ultrasound reveals the overpopulation of OH(A2Σ+) vibrational levels, indicating the formation of non-equilibrium plasma inside the cavitation bubbles.[40] The non-equilibrium plasma is the major state of gas inside violently collapsing cavitation bubbles. Based on a two-site model for the sonochemical regions, most of the radical production is thought to occur in the interior vapor phase when the liquid layer immediately surrounding the cavitation bubbles bursts into the bubbles upon the cavitation collapse. Moreover, the primary sonochemical effect relates to the cavitation bubble types. The transient cavitation bubble often grows to a large size quickly and collapses after one acoustic cycle, which accounts for most of the chemical activity in the system. The stable cavitation bubble can persist for more acoustic cycles and create a large shear gradient when it collapses, where the shear force plays an important role in a sonochemical system, e.g., the emulsification caused by the cavitation is significant to allow an efficient preparation of polymer microspheres.[41]

Fig. 3

Schematic illustration and reaction zones of primary sonochemistry and secondary sonochemistry.

Different from primary sonochemistry, secondary sonochemistry involves that the initial species formed in the primary sonochemical stage migrate into the surrounding liquid and suffer from a variety of reactions with solutes (Zone 3). In the well-studied case of sonicating aqueous liquids (Fig. 4), the fragmentation of water can generate highly active hydrogen radicals (H˙) and hydroxyl radicals (˙OH) (reaction 1), which has been captured photographically by using a luminol solution to enhance the blue sonochemiluminescence generated near the cavitation bubbles.[42] Upon diffusing out, these H˙ and ˙OH radicals can combine to produce hydrogen (H2) and hydrogen peroxide (H2O2) (reactions (2) and (3)) if taking no account of their recombination to be an original form or can also generate secondary radicals (R˙) (reaction 4) by reacting with organic additives (e.g., 2-propanol or ethanol) or produce hydroperoxyl radicals (HO2˙) by combining with dissolved oxygen (O2) (reaction 5). Either the strong oxidants or reductants are used to initiate various chemical reactions in an aqueous or nonaqueous solution including the reductions, oxidation, hydroxylation of organics, etc.[43] Like the acoustic cavitation, the generation of radicals strongly depends on a great number of factors. Take ultrasound frequency as an example, the production of active species stays at a very low level under a low-frequency ultrasound even though the collapse of larger cavitation bubbles creates stronger shear forces, and with the frequency enhancing, the production rates increase quickly up to a maximum at 300–800 kHz and then drop off, where the shear force decreases considerably.

Fig. 4

Sonochemiluminescence (left) observed in a luminol solution (pH 11.0, Na2CO3) under ultrasound irradiation (20 kHz, 33 W), in which the sonolysis of water involves several rapid primary and secondary reactions (right). Reproduced with permission.[42] Copyright 2018 American Chemical Society.

For many materials synthesis or other relevant events, the sonochemical processes of solvents or solutes serve as an excitation source and induce some catalysis or sonosynthesis which is a power-inducing preparation strategy. Besides, ultrasound irradiation often gives rise to numerous physical effects (e.g., simple heating, shock waves and microjets) accompanied by the chemical consequences, and these factors are able to order the sonosynthesis performance in materials synthesis by affecting the frequency and efficiency of chemical reactions.[44] During sonication, the typical ultrasonic horns deliver acoustic energy into the liquid, changing into heat, which can be useful for activating reactive atoms or molecules, accelerating dissolution and facilitating crystallization. The microjets occur due to the asymmetrical collapse of highly energetic bubbles in any heterogeneous system, and they can lead to the modification of surfaces (pitting and erosion) or the generation of surface nanostructures.[36,45] Shock waves will be created if a bubble unperturbed by the surface rebounds rapidly from its minimum spherical radius causing the compression of the surrounding liquid to propagate outwards.[46,47] The shock waves can induce several different mechanical, physical or chemical consequences, e.g., increasing the mass transport due to strong turbulent mixing and acoustic streaming, accelerating solid particles suspended in the liquid, and inducing changes in size distributions, morphologies and surface compositions owing to the enhanced interparticle collisions. Additionally, the mechanical effects (e.g., shear force, interparticle collisions, rapid mixing, and turbulence) from ultrasound irradiation are also important in the chemical consequences. Anyhow, the applications of sonosynthesis almost cover the chemical, physical and/or mechanical effects, for example, the fragmentation for brittle materials, the emulsification for immiscible liquids, the agglomeration for malleable materials, the exfoliation of layered materials into 2D layers, and even the fabrication and delivery of drug systems.[48,49]

Synthesis of inorganic nanomaterials

So far, the synthesis of inorganic nanomaterials has been explored with a view to the nature of precursors, the structures and properties of products and the feasibility and simplification of methods. For instance, Sun et al. successfully prepared a GdMnO3 material using a typical sol–gel method and subsequently in situ generated GdMn2O5/GdMnO3 materials by acid treatment for complete oxidation of oxygenated VOC;[50] Tan et al. synthesized TiO2−/W18O49 with core–shell or double-shelled hollow microspheres through a facile multi-step solvothermal method, where the advanced architecture significantly enhanced the electronic properties of TiO2−/W18O49 for CO2 photo-reduction efficiency;[51] Deng and co-workers fabricated TiO2 supported Pt nano-/single-atom catalyst by using the molten salt method, and 0.2 wt% Pt catalyst showed high catalytic activity for toluene removal.[52] Our team once created superparamagnetic ferroferric oxide nanoparticles by the coprecipitation method and then yielded hydrophilic or hydrophobic targeting biomaterials by a few modifications.[53] As a supplementary choice, ultrasound irradiation provides a facile route for the synthesis of the nanomaterials, mainly attributed to primary sonosynthesis, secondary sonosynthesis and synergetic sonosynthesis, where primary sonosynthesis is based on primary sonochemistry when secondary sonosynthesis widely employs the secondary sonochemical consequences. Many inorganics including metals, alloys, metal compounds (e.g., metal oxides, metal sulfides, and metal nitrides), and non-metals (e.g., carbon materials, silicon materials, and selenium materials) have been yielded from the precursors, widely covering a variety of structures such as solid structures, hollow structures, core–shell structures, porous structures, nanotube structures, nanorod structures, and layered structures (Fig. 5).

Fig. 5

A variety of inorganic nanomaterials based on sonocatalysis.

Primary sonosynthesis

Primary sonosynthesis derives from the case that the implosive collapse of cavitation bubbles releases the stored energy within a very short time in a nearly adiabatic way so that the surrounding solvent or solutes experience high temperature inducing the breakage of chemical bonds. Some cases on the primary sonosynthesis have been summarized in Table 1. Directly, such sonosynthesis sometimes does not involve the quadratic transformation of active species. A few volatile organometallic compounds in a nonvolatile solvent such as silicone oil or long-chain hydrocarbons can cause the dissociation of organic groups and the release of individual metal atoms under ultrasound irradiation. Under argon gas, the sonication of Fe(CO)5 in an alkane solvent or ionic liquid with a low vapor pressure efficiently yielded a black porous iron powder with traces of residual carbon and oxygen at the freezing point.[54] In the process, the atoms of the organometallic compound were thermally excited to the point where visible light was emitted analogous to the emission from flame excitation. Certainly, the nonvolatile solvents were needed because the additional vapor had to absorb the available energy in the collapsing bubble in order that the achieved conditions were much less extreme. Because the metal atoms were so rapidly cooled that crystallization was prevented within a short-lifetime cavitation event, the obtained product appeared to be an agglomeration of 20 nm amorphous nanoparticles. Other metal precursors with single or multiple ligands like Cr(CO)6, Ni(CO)4 and Co(CO)3NO also have produced amorphous porous metal nanoparticles, even obtaining alloy nanoparticles by varying the composition in the solution.[55]

Synthesis based on the primary sonocatalysis

Precursor	Product	Experimental parameters under ultrasound	Ref.
Fe(CO)5	Porous iron	Decane, Ar atmosphere, 20 kHz, 100 W cm−2, sonication for 3 h	54
Mo(CO)6	Mo2C	Hexadecane (50 mL), Ar atmosphere, 20 kHz, 100 W cm−2, 90 °C, sonication for 3 h	56
W(CO)6	W2C/C	Diphenylmethane (50 mL), 20 kHz, 600 W, 32 W cm−2, 80 ± 2 °C, sonication for 30 min	57
Fe(CO)5	Fe2–3N	Decane, NH3/H2 (3.5 : 1), 20 kHz, 100 W cm−2, 0 °C, sonication for 4 h	58
W(CO)6 or Mo(CO)6	WOx or MoOx	Diphenylmethane and decalin (100 mL), ambient air, 20 kHz, 200 W, 80 °C, sonication for 3 h	59
Fe(CO)5	Iron carbide@iron oxide core–shell nanoparticles	Oleic acid, 1-octanol (5 mL), 40 kHz, room temperature, sonication for 40 min	60
Fe(CO)5	Iron colloid of Fe/FeO	Oleic acid, hexadecane, Ar atmosphere, 40 kHz, 30 °C, sonication for 1 h	61
Mo(CO)6	Clustered MoS2 nanoparticles	Elemental sulfur, 1,2,3,5-tetramethylbenzene, Ar atmosphere, 20 kHz, 80 W cm−2, 80 °C, sonication for 1.5 h	63
Cu(CH3COO)2·H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O	Nano-sized Cu(ii), Co(ii) and Ni(ii) complexes	Ahpv ligand, ethanol (10 mL), 24 kHz, 400 W	64
CuCl2	[Cu(L)(H2O)Cl]	2-((Pyridin-3-ylmethylene)amino)phenol (HL), MeOH (10 mL), 40 kHz, 400 W	65
Pb(NO3)2	[Pb(L)(SCN)2]n	l-Methylimidazole (L), KSCN, methanol, 60 W, 30 °C, sonication for 30 min	66
Gallium	Ga@C-dots	Decane, dodecane, hexadecane or polyethylene glycol, 20 kHz, 750 W, 50 °C or 60 °C, sonication for 3–180 min	67 and 68
Mo(CO)6	MoO3-coated silica nanoparticles, hollow MoS2	Silica spheres, elemental sulfur, isodurene, Ar atmosphere	69
Co4(CO)12	Porous Co3O4	Carbon nanotubes, hexane, sonication for 0.5 h	70
Fe(CO)5	Hollow hematite	Carbon nanoparticles, hexadecane (40 mL), argon flow, 20 kHz, 50 W cm−2, 20 °C, sonication for 3 h	71
Si3H8	Silicon nanoparticles	N2-filled atmosphere with residual traces of H2O and O2 (∼1 ppm), 26 kHz, 32–130 W cm−2, 1–4 W, 25–40 °C, sonication for 2 min	72
p-Xylene	Single-walled or multi-walled carbon nanotubes	Silica powder, ferrocene, p-xylene solution (50 mL), pulsed 65% at 200 W, atmospheric pressure, room temperature, sonication for 20 min	74 and 75
Dopamine hydrochloride	Nitrogen-doped carbon quantum dots	Dimethylformamide (150 mL), 20 kHz, 600 W, sonication for 8 h	76

Under the high-temperature conditions provided by the hotspots, the metal atoms decomposed from volatile organometallic compounds are highly active and can react with dissolved gas in the solvent to form new nanomaterials, for example, Mo(CO)6 in hexadecane and W(CO)6 in diphenylmethane by sonochemically heating under CH4/H2 gas separately led to the formation of nanostructured Mo2C or W2C/C porous aggregates, and the products showed superior activity, selectivity and stability for the dehydrogenation and/or hydrodehalogenation of organic pollutants.[56,57] Alternatively, some metal nitrides or metal oxides were also produced when the sonication of metal carbonyl compounds occurred under a reductive NH3/H2 gas mixture or in the presence of air.[58] Under ambient air of 80 °C, Argirusis et al. used organic solvents containing the precursors W(CO)6 or Mo(CO)6 to sonochemically prepare metal oxide nanoparticles on anode materials[59] and revealed that the size distribution and morphology of nanoparticles responded to ultrasound intensity and solvents when their decoration loading extent on substrates was affected by ultrasound intensity/duration as well as the ratio of precursor and substrate compounds. Interestingly, Zavala-Rivera et al. carried out the sonochemical reaction of Fe(CO)5 in octanol under an inert air condition or pure oxygen condition using low-intensity ultrasound with a frequency of 40 kHz[60] and finally obtained iron carbide@iron oxide nanoparticles composed of an iron carbide core and a magnetite shell. Based on the core–shell structure, researchers concluded a possible sonosynthesis mechanism that the formation of the iron oxide layer occurred via the secondary interaction of iron carbide and oxygen followed by the decarbonization of Fe(CO)5 to produce the nucleation points of iron carbide with the aid of oxygen.

The sonochemically released metal atoms are also able to react with other solutes or even solvent molecules. When the Fe atoms were released from the Fe(CO)5 precursors under ultrasound irradiation, individual smaller iron nanoparticles with 8 nm diameter would be created if oleic acid or other similar surfactants were present in the solvent to stabilize the metal atoms.[61] To prepare the coated iron nanoparticles, Nikitenko et al. reported the sonochemical decomposition of iron carbonyl indiphenylmethane, where an air-stable magnetic powder was formed after annealing the as-prepared sample at 973 K in argon.[62] The nanoparticle structure featured a core–shell geometry consisting of an Fe3C layer (about 3 nm thickness) on the top of a crystallized Fe core, and the ratio of Fe to Fe3C was close to 4 : 1. Other core–shell transition metal nanoparticles have been also created in the similar way, but some syntheses were usually particular and limited.[3] In an ultrasound process reported by Suslick et al., adding elemental sulfur to the 1,2,3,5-tetramethylbenzene containing Mo(CO)6 produced clustered MoS2 nanoparticles with a higher edge surface area under argon gas,[63] and such MoS2 nanoparticles substantially exhibited high catalytic activity for the hydrodesulfurization of thiophene. Abdel-Rahman et al. once synthesized a Schiff base ligand (ahpv) by the condensation of 3-methoxysalicylaldehyde and 2-amino-3-hydroxypyridine and then added it dropwise to the ethanol solutions of different metal salts (i.e., Cu(CH3COO)2·H2O, Co(NO3)2·6H2O, and Ni(NO3)2·6H2O) positioned under high-density ultrasound irradiation, preparing nano-sized Cu(ii), Co(ii) and Ni(ii) complexes of ahpv.[64] Rayan et al. obtained another nano-sized Schiff base ligand copper complex [Cu(L)(H2O)Cl] by using the sonochemical route, where L was 2-((pyridin-3-ylmethylene)amino)phenol prepared by the condensation of 3-pyridinecarboxaldehyde with 2-aminophenol.[65] In a study of Hayati et al.,[66] novel metalorganic coordination polymer compounds were fabricated with the aid of the sonochemical process, in which l-methylimidazole (L), KSCN and Pb(NO3)2 were used to synthesize [Pb(L) (SCN)2] by a branch tube method under high-density ultrasound. Gedanken et al. applied ultrasound irradiation of molten gallium in various organic liquids to produce some identified nanometric gallium particles,[67,68]e.g., gallium spheres coated with a carbon film when gallium was sonicated in decane, dodecane and hexadecane, precipitated gallium particles coated with C-dots and supernatant gallium-doped C-dots during the sonication of gallium in polyethylene glycol. After investigating the nature of the product, they found that the ultrasound irradiation of molten gallium immersed in organic liquids caused the partial decomposition of the hydrocarbon medium prior to the interaction between carbon and gallium.

In addition, a removable template (e.g., SiO2 nanoparticles or microspheres) present can give an aid to the produced free metal atoms to directly deposit onto the support when sonicating the volatile organometallic precursors, forming different structured materials. Under the air, the ultrasound irradiation of Mo(CO)6 and silica nanoparticles in 1,2,3,5-tetramethylbenzene yielded MoO3-coated silica nanoparticles which would leave hollow MoO3 behind by using HF to etch the silica component, and instead, adding elemental sulfur into the system and replacing the air with Ar gas could form hollow MoS2 (Fig. 6).[69] Except the SiO2 materials, carbon nanoparticles and organic ligands can also serve as a sacrificial template to inorganic nanomaterials. The sonochemical product of Co4(CO)12 in the presence of carbon nanotubes would not be amorphous cobalt but porous Co3O4,[70] where the carbon nanotubes as a template spontaneously removed themself via the combustion attributed to the rapid oxidation of elemental Fe upon air exposure. In a similar fashion, the precursor Fe(CO)5 also could produce hollow hematite when carbon nanoparticles were employed as a template.[71]

Fig. 6

TEM micrographs of conventionally prepared silica-templated MoO3 (A, B, C); TEM micrographs of sonochemically prepared hollow MoO3 nanospheres before (D) and after (E) thermal annealing thermal annealing at 350 °C, where insets show higher magnification; TEM micrographs of sonochemically prepared hollow MoS2 nanospheres after thermal annealing at 450 °C, and EDX line analysis across the single MoS2/SiO2 and hollow MoS2 (F). Reproduced with permission.[69] Copyright 2005 American Chemical Society.

In addition, nonmetal precursors are able to facilely and highly produce amorphous nanomaterials by the sonosynthesis effects. Bedini et al. carried out the sonochemical synthesis of silicon nanoparticles,[72] where the trisilane (Si3H8) solution underwent the sonication process in an inert N2 atmosphere with residual traces of H2O and O2. The as-synthesized silicon nanoparticles were discrete, hydrogenated, and non-crystalline. Therefore, the isolated formation of nanoparticles was attributable to the interplay among the extreme conditions during bubble collapse and the high density of active species (e.g., silicon radicals) created at such spatially localized points (Fig. 7), while the amorphous morphology was ascribed to the residual bonded hydrogen in the particles due to incomplete diffusion during the short-lived collapse and the proposal that the silicon matrix did not have enough interaction time to reconstruct into a crystalline state. The silicon nanoparticles had a tunable size in the range of 1.5–50 nm, which could be controlled by varying the ultrasound amplitude or the trisilane concentration. Researchers found that low ultrasound amplitude or low trisilane concentration favored the generation of discrete and monodisperse silicon nanoparticles, which might be attributed to smaller microbubbles and/or less trisilane in-diffusion during bubble growth. Correspondingly, high amplitude and high trisilane concentration produced large amounts of larger and more polydisperse particles. In the absence of any templates or surfactants, the sonication of amorphous selenium colloids in alcohol could thermodynamically form trigonal selenium nanowires through a solid-solution-solid transformation mechanism,[73] where selenium nanocrystals were sonochemically nucleated in a trigonal structure and acted as the seeds of nanowires. Other nonmetal nanomaterials, such as graphene quantum dots, black phosphorus quantum dots and carbon nanodots, can be also prepared from the sonication of the corresponding precursors. When ultrasound was used to irradiate a p-xylene solution containing silica powder and ferrocene, p-xylene would be pyrolyzed to carbon atoms or moieties by the catalytic Fe clusters sonochemically decomposed from the ferrocene and finally produce high-purity, single-walled or multi-walled carbon nanotubes on the nucleation sites provided by silica powder.[74,75] Lately, Zhou et al. fabricated nitrogen-doped carbon quantum dots with temperature-dependent fluorescence by ultrasonic treatment of dopamine in dimethylformamide; moreover, the fluorescence had a sensitive quenching phenomenon towards Fe2+ ions.[76]

Fig. 7

(I) Schematic depiction of the growth and collapse of a bubble and the ensuing sonochemical effects enabling the synthesis of solid-state silicon: (a) during decompression cycles, an expanding bubble is preferentially filled with highly volatile trisilane (Si3H8); (b) upon bubble collapse, high transient temperatures and pressures result in the creation of a “hot spot”; (c) the implosion results in the generation of new silicon-containing species, namely an amorphous silicon (a-Si) particle, Si radicals (Si˙), and Si polymers. (II) Dark field STEM images depicting the effects of sonication conditions on Si-nanoparticle production: (a) monodisperse Si-nanoparticles synthesized with low tip amplitude (A = 48 μm) and high trisilane concentration (47 wt%); (b) larger Si-nanoparticles synthesized with both high A (192 μm) and high trisilane concentration; (c) blend of Si-nanoparticles synthesized with high A and low trisilane concentration (1 wt%). The insets show higher resolution images of the regions enclosed by the dashed boxes. Reproduced with permission.[72] Copyright 2017 Elsevier.

Secondary sonosynthesis

Apart from primary sonosynthesis, secondary sonosynthesis significantly or more assists in the synthesis of inorganic materials as well, because its synthesis mechanism is mainly on the basis that the sonolysis of solvent vapor is capable of producing some active reactants without additional reducing agents,[77] for example, H˙ and ˙OH radicals for water, R˙ radicals for volatile organic vapor, and so on. These radicals are able to act as reactants for some redox chemistry in the production of nanostructured materials. Similar to the volatile organometallic compounds, a few nonvolatile metal precursors dissolved in a volatile solvent can be used to prepare nanostructured metals through ultrasound irradiation, whereas the generated nanometals are usually crystallized well and are not so amorphous as the nanoparticles form volatile precursors. Table 2 summarizes the secondary sonosynthesis of nanostructured metals from the nonvolatile precursors.

Secondary sonocatalysis synthesis of nanostructured metals from the nonvolatile precursors

Precursor	Product	Experimental parameters under ultrasound	Ref.
HAuCl4	Au nanoparticles	Aqueous solution (70 mL or 200 mL), 1-propanol, argon atmosphere, 0.1 ± 0.01 W mL−1, 20–1062 kHz, 21 ± 2 °C	78
HAuCl4	Au nanorods and nanoparticles	Aqueous solution (60 mL), CTAB, AgNO3, ascorbic acid, pH 3.5, argon atmosphere, 200 kHz, 200 W, 27 °C	79
HAuCl4	Au nanodecahedra	PVP/DMF solution (15 mL), NaBH4, platinum nanoparticles (2–3 nm), 20 kHz, 60 W, 100 °C	80
HAuCl4	Au nanobelts	Aqueous solution, α-d-glucose (or β-cyclodextrin), 40 kHz, 100 W, sonication for 10–60 min	81
AgNO3, HAuCl4	Ag nanoplates, Au nanorings	PVP, DMF solution (40 mL), ambient conditions, 20 kHz, 60 W cm−2, sonication for 0.5 h	82
HAuCl4	Au nanocones	2-Ethoxyaniline, hexane, aqueous solution, 35 kHz, 144 W, 45 °C, sonication for 90 min	83
NaAuCl4, PdCl2	Core–shell Au/Pd bimetallic nanoparticles	Aqueous solution (60 mL), sodium dodecyl sulfate, 200 kHz, 200 W, 20 °C	84
HAuCl4, AgNO3	Core–shell Au–Ag bimetallic nanoparticles	Aqueous solution (70 mL), polyethylene glycol, ethylene glycol, pH 3.5, argon atmosphere, 20 kHz, 23–47 W cm−3, room temperature, sonication for 30 min	85
H2PtCl6, HAuCl4	Au–Pt bimetallic nanoparticles	Aqueous solution (6.0 mL), ascorbic acid solution (6.0 mL), sonication for 15 min	86
PtBr4, CuBr2	PtCu3 bimetallic nanoparticles	Tetrahydrofuran (200 mL), 2-propanol (20 mL), lithium metal cuttings, naphthalene, 20 kHz, 15 W cm−2, −20 °C, sonication for 90 min	87
Pd(NO3)2, AgNO3	PdAg nanoparticles	Ethylene glycol/water mixture (60 mL), vulcan carbon XC-72, N2 atmosphere, 20 kHz, sonication for 3 h (20 s/20 s, on/off)	88
Pd(acac)2, Ni(acac)2, Co(acac)2, Fe(acac)3, Mn(acac)2	PdNi, PdCo, PdFe, PdMn nanoparticles	Ethylene glycol (30 mL), pure Ar, 20 kHz, 150 W, room temperature, sonication for 3 h	89
AuCl3	Au nanoclusters	Toluene (4 mL), 30 W, 40 kHz, sonication for 0–180 min	90
AgNO3	Ag nanoclusters	PMAA solutions (30 mL), pH 4.5, Ar flow, 20 kHz, 25 W cm−2, sonication for 10–180 min	91
AgNO3	Ag nanoclusters	Aqueous solution (10 mL), BSA, pH 12, 50 W cm−2, 15 °C, sonication for 4 h (7 : 3, ultrasonic time/intermittent time)	92
AgNO3	Ag nanoclusters	Aqueous solution (25 mL), glutathione, pH 5.0, 40 KHz, 60 W, sonication for 0–170 s	93
Cu(NO3)2	Cu nanoclusters	Reduced glutathione, Cu(NO3)2, deionized water (5 mL), pH 6.0, sonication for 15 min	94

For the production of nanostructured noble metals, the sonochemical reduction provides an alternative to traditional approaches such as photochemical reduction, controlled chemical reduction, radiolytic reduction, and solvothermal synthesis. A systematic study on the sonochemical synthesis of spherical Au nanoparticles referred to the finding published by Grieser and co-workers:[78] when HAuCl4 aqueous solution was sonicated in the presence of alcohols or other similar organic additives, Au nanospheres would be formed with Au(iii) being reduced to Au(0), and the particle size was not only largely determined by the solvent/surfactant properties but also related to ultrasound frequency. From the point of view of morphology, nonspherical Au nanoparticles have been also prepared by sonochemical reduction. Au(iii) could form Au(0) nanorods in the presence of cetyltrimethylammonium bromide and AgNO3,[79] and alternatively, it would lead to the formation of monodispersed Au(0) nano-decahedra when poly(vinylpyrrolidone) (PVP) is used as a stabilizing polymer.[80] Similarly, Au nanobelts, Au nanocones, Ag nanoplates, Ag nanowires, Ag nanorings, and even bimetallic nanoparticles with a core–shell structure (e.g., Au/Ag, Au/Pt, Au/Pd, and Pt/Ru) have been produced as well by an ultrasound-assisted Ostwald ripening process.[81-89] Significantly, high-intensity ultrasound irradiation was able to extremely create small Au nanoclusters, Ag nanoclusters, and Cu nanoclusters (less than 2 nm) by using some polymer molecules (e.g., polymethylacrylic acid) as a capping agent,[90-94] and such nanoclusters were likely to develop widespread applications in bioimaging, biosensing, and single-molecule studies.

Sonochemical reduction is not limited to the synthesis of nanostructured noble metals, and various metal oxides, metal hydroxides, metal chalcogenides as well as other metal compounds have been also prepared through the treatment of relevant precursors in an analogous manner. For the metal compounds, a typical synthesis way involves the sonication of a solution containing metal salts in the presence of different nonmetal sources, such as air for oxygen, thiourea for sulphur, selenourea for selenium, or others. Kipcak et al. carried out sonochemistry-assisted synthesis of magnesium borate where magnesium chloride hexahydrate and different boron sources (borax, tincalconite, boric acid, and boron oxide) were used as the precursor materials.[95] The synthesis yields were higher than 80%, and the outcomes were MgO(B2O3)3·7H2O with high crystallinity as per XRD analyses. Pinkas et al. obtained insoluble amorphous uranium phosphates from UO22+ precursors in high-boiling trialkyl phosphate solutions by sonication.[96] Sonolysis experiments were carried out in a frequency of 20 kHz and a power of 0.49 W cm−3 under an argon flow at 40 °C, in which trialkyl phosphates served as a phosphate source creating phosphate species by the decomposition of ester molecules and then reacted with uranium precursors. The efficiency of uranium removal from the solution by sonochemical precipitation was higher than 30%, and the obtained amorphous uranium precipitates could change into crystalline uranium diphosphate (UP2O7) when heated to 1000 °C. Hence, for the uranium-containing waste streams, sonication might potentially serve as a remote separation for uranium. As an effective light-absorbing material for thin film solar cells, Cu(InGa1−)Se2 showed high conversion efficiency, but the traditional synthesis of Cu(InGa1−)Se2 compounds involved typically some toxic organic solvents such as hydrazine, ethylenediamine, and polyetheramine, as well as a high reaction temperature. Recently, Park et al. designed a substantially greener procedure to yield Cu(In0.7Ga0.3)Se2 nanoparticles using a slightly modified sonochemical method,[97] where the reaction was carried out at room temperature with ethanol as the only solvent, NaBH4 as a reducing agent, and 5 hour ultrasound irradiation. The proposed reaction scheme revealed the role of ultrasound irradiation in decomposing the intermediate Cu2Se in the reaction pathways, so the sonification time was investigated to form the Cu(InGa1−)Se2 compounds with the expected crystalline structure. The short-period sonification irradiation was not enough to form the expected crystal structure, and even the signs of Cu2Se also appeared. Prolonging the sonification time resulted in much better crystallinity, where 5 hour ultrasound irradiation would yield single-phase tetragonal Cu(In0.7Ga0.3)Se2 nanocrystals without any Cu2Se crystals.

Additionally, secondary sonosynthesis provides a feasible, facial and short-time approach for the synthesis of hybrid inorganic materials. By sonicating sodium chloride, silver nitrate and poly(vinyl pyrrolidone) in ethylene glycol, Ag/AgCl nanocubes with a 115 nm edge length have been produced with silver nanoparticles embedded into AgCl cubic matrices.[98] In the formation of hierarchically structured ZnO/CdS microspheres,[99] CdS nanoparticles were produced by the sonication of a cadmium chloride and aqueous thiourea solution accompanied by the hydrothermal formation of ZnO microspheres. Chen et al. designed a straightforward approach to synthesize Cd(OH)2/Ag nanorods with a core/satellite structure on the basis of the sonochemical method:[100] cadmium chloride would form Cd(OH)2 nanorods via a single sonochemical reaction in a base medium (Fig. 8), followed by the thermal annealing process to obtain highly crystalline CdO nanorods; In the presence of AgNO3 and ammonia, the prepared CdO nanorods would undergo a second ultrasonication to obtain a core/satellite complex. This proposal was feasible, and the Cd(OH)2/Ag core/satellite nanorods were prepared successfully, where the deposition of Ag nanodots was completed on the surface when the cubic CdO transformed into monoclinic crystalline Cd(OH)2. Moreover for the direct surface functionalization of Cd(OH)2 nanorods with Ag nanodots, the microjets, shock waves and turbulent flows generating the cavitation collapse were found to play a crucial role.

Fig. 8

(I) schematic overview the general mechanism and main steps for the synthesis of CdO and Cd(OH)2/Ag core/satellite nanorods. (II) TEM images of (a, b) CdO nanorods and (c, d) Cd(OH)2/Ag nanorods, where the inset shows the HRTEM image of CdO nanorods in Image b. (III) TEM (a) and HRTEM (b) images of Cd(OH)2/Ag nanorods. Reproduced with permission.[100] Copyright 2018 Elsevier.

The assistance of other technologies can improve the performance of secondary sonosynthesis in materials preparation. In order to decrease the sintering temperature to prepare strontium hexaferrite nanoparticles, Jesús and Miró reported the synthesis of strontium hexagonal ferrite (SrFe12O19) by sonochemistry and annealing.[101,102] In the synthesis, the sonochemical process yielded an amorphous phase containing Fe3+, Fe2+ and Sr2+ ions from a complexed polyol solution of metallic acetates and diethylene glycol, where Fe2+ was partially oxidized to Fe3+ due to the formation of H2O2. Subsequently, annealing of the amorphous phase was carried out at temperatures from 300 to 900 °C providing the intermediate maghemite (γ-Fe2O3) at 300 °C, the phase conversion of maghemite to hematite (α-Fe2O3) at 500 °C, and the generation of final SrFe12O19 at 800 °C. High sonication output power and long sonication time (up to 3 h) promoted the crystallinity of amorphous phases, decrease in crystal size, and modification of the morphology of particles. Furthermore, the obtained SrFe12O19 showed a magnetization of >60 emu g−1 consistent with pure hexaferrite obtained by other methods. Similarly, Cavalcante et al. synthesized tungstate (CuWO4) crystals with a triclinic structure based on the sonochemical method and the following heating treatment in a conventional furnace.[103] Copper tungstate hydrate precipitated by the sonochemical reaction would remove the partial water molecules in its lattice as treated at a low temperature from 100 °C to 200 °C, and copper tungstate dihydrate (CuWO4·2H2O) would appear at 300 °C, until the crystals were completely occupied by a single CuWO4 triclinic structure at above 400 °C (Fig. 9). Recently, the simultaneous action of ultrasound and hydrothermal treatment, namely sonohydrothermal treatment, has been developed to accelerate crystallization and improve the catalytic performance of nanocrystalline metal oxides. For the first time, Nikitenko et al. reported the sonohydrothermal synthesis of nanostructured (Ce,Zr)O2 mixed oxides, and the obtained nanocrystal had a higher external mesoporous surface area than that prepared by ultrasonically assisted coprecipitation followed by hydrothermal treatment.[104] Later, they synthesized stable Ti@TiO2 nanoparticles with a core–shell structure by the sonohydrothermal treatment on air-passivated titanium metal nanoparticles in pure water, in which quasi-spherical Ti particles (20–80 nm) were coated by defect-free anatase crystals (5–15 nm) with small amounts of rutile crystals.[105]

Fig. 9

(I) Schematic representation of the growth mechanism of CuWO4·2H2O and CuWO4 crystals obtained by the sonochemistry method: (a) reaction between complex ions; (b) appearance of the first CuWO4·2H2O nuclei; (c) rotation and alignment of nanocrystals, sharing common crystallographic planes; (d) self-assembly process; (e) aggregation of petal-like crystals; (f) formation of flower-like CuWO4·2H2O microcrystals; (g) crystal growth via heat treatment, (h) irregular stone-like CuWO4·2H2O microcrystals, (i) stone-like CuWO4·2H2O microcrystals and flake-like CuWO4 nanocrystals; (j) formation of aggregated CuWO4 nanocrystals; (k) crystal growth of flake-like CuWO4 nanocrystals. (II) FE-SEM images of CuWO4 crystals heat treated at different temperatures: (a) 100 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C and (e) 500 °C for 1 h, respectively. Reproduced with permission.[103] Copyright 2017 Elsevier.

Uzunov et al. utilized three synthesis methods (i.e., precipitation; mechanochemical treatment and sonochemistry) to prepare nano-sized zinc oxides from different zinc sources,[106] and the as-synthesized samples had various morphologies with a crystallite size below 20 nm. Under UV irradiation, the sonochemistry-assisted ZnO nanorods exhibited the highest photocatalytic activity for the degradation of malachite green while the precipitation-assisted polycrystalline zinc oxide had a better photocatalytic efficiency under visible irradiation, and on the contrary, the mechanochemistry-assisted nano-sized ZnO led to the deterioration of photocatalytic activity due to inhomogeneous distribution in particles size. In a report of Ashiri et al.,[107] both the sonochemical processing and the mechanochemical approach were considered for the synthesis of BaTiO3 nanopowders. The contrasting results demonstrated that the sonochemical synthesis could obtain highly pure tetragonal BaTiO3 at a much lower processing temperature and shorter time span because the reactant ions would homogenously order into the perovskite structure in sonication when dissolved in the solution mixture. If there is a proper structure directing agent or template in sonication, more different nanostructured materials can be obtained. Under ammonia conditions, CdCl2 and NaSeSO3 were sonochemically reacted to form hollow CdSe spheres,[108] where Cd(OH)2 from the hydrolysis acted as an in situ template. To make ZnO nanorods and nanoflakes, vertically aligned nanostructures could be deposited on the substrates such as Zn sheets, Si wafers and glass by sonicating a zinc salt when hexamethylenetetramine was used as a shape directing agent.[109] In the presence of soft templates, ultrasound can result in the formation of unique morphologies during the synthesis of nanostructured metal oxides. For example, sonicating the Pb(CH3COO)2, NaWO4 and P123 (a pluronic block copolymer) mixed solution could prepare hollow PbWO4 spindles while only solid particles were obtained in the absence of ultrasound,[110] and the probable reason was that ultrasound changed the P123 micelles into hollow micelle aggregates forming a hollow spindle structure.

Synergetic sonosynthesis

The synthesis of many materials does not isolate primary sonosynthesis or secondary sonosynthesis, or it goes through the synergy of serial sonosynthesis. Moreover, some physical or mechanical effects from ultrasound irradiation, such as high-speed microjets, intense shock waves, and interparticle collisions, have been frequently applied in preparing emulsions, agglomerating malleable materials, modifying solid surfaces, exfoliating layered materials, and breaking down friable materials, and they also belong to important factors in the formation of nanostructured materials as well as the enhanced mass transfer and the bulk thermal heating. For instance, ultrasonic spray pyrolysis (USP), sonocrystallization, sonofragmentation as well as other technologies on account of the physical or mechanical effects originating from ultrasound have been utilized to make nanostructured materials. Fig. 10 shows that USP creates some nano- and micromaterials.[111-113] As a result, the physical or mechanical effects and the sonochemical effects can play a combined part in many synthesis processes as well, and the notable examples will be described in the latter section.

Fig. 10

(I) The SEM images and size distributions of polydimethylsiloxane microspheres prepared using 4 mg mL−1 (a, b), 20 mg mL−1 (c, d), and 100 mg mL−1 (e, f) polydimethylsiloxane in hexanes, respectively. Reproduced with permission.[111] Copyright 2015 Wiley-VCH. (II) SEM (a) and TEM (b) images showing the hexagonal shape of the NaInS2 nanoplates produced via USP (the inset electron diffraction pattern in (b) demonstrates the single-crystalline nature), SEM image (c) of NaInS2 prepared through a non-USP method for comparison, and X-ray diffraction patterns (d) of the USP and non-USP materials. Reproduced with permission.[112] Copyright 2012 Wiley-VCH. (III) Morphology of MnO2 microspheres produced via USP varies at 150 °C (a–e) and 500 °C (f–j): TEM images (c, d, h, i) where increasing the reaction temperature caused the formation of larger crystals; SEM images (b, g); electron diffraction patterns (e, j). Reproduced with permission.[113] Copyright 2015 Wiley-VCH.

Ultrasound irradiation has become a broadly useful tool to prepare single and few layered materials by overcoming the attractive forces between individual layers to break 3D layer structures down to 2D planar structures. Hammer's method, as a typical synthesis route to graphene oxide (GO), is often used to oxidize pristine graphite to form graphite oxide, where an increased interlayer distance can consequently weaken the van der Waals force relative to graphite[114] and then produce single or few layered GO by a bath or horn sonication (Table 3).[115-130] The direct exfoliation of graphite by sonication in a suitable solvent (e.g. N-methyl-pyrrolidone) is a more straightforward method for the formation of single and few layered graphene, in which the surface energy between the solvent and graphite had to match (about 40 mJ m−2) to obtain high-yield exfoliated graphene.[131] Other layered materials such as MoSe2, MoS2, WS2 and Bi2Te3 can also be used to prepare single-layered nanosheets by direct liquid-phase exfoliation under sonication.[132] In some studies, the physical or mechanical effects of ultrasound dramatically enhance the intercalation of guest substances into layered materials along with the sonochemical synthesis. H2PtCl6 exposure to an H2 gas stream could be ultrasonically applied in graphite layers to prepare Pt-intercalated graphite.[133] K-Intercalated compounds (KC8) also could be prepared by sonicating graphite with potassium in toluene,[134] whereas ultrasonicattion of potassium-intercalated graphite would generate carbon nanoscrolls in ethanol.[135] So far, the detailed reason has been not fully understood. In addition, a few efforts have been paid to deposit the sonochemically produced nanoparticles onto the graphene substrate with a high surface area. Guo et al. prepared the TiO2 nanoparticles deposited on graphene sheets by ultrasonically irradiating GO and TiCl4 in ethanol.[136] Au and Fe3O4 nanoparticles also could be used to couple the GO or the graphene.[137,138]

Sonocatalysis synthesis or exfoliation of graphene derivatives

Product	Ultrasound	Ref.
Graphene nanoribbons	1500 W, 20 kHz	115
Graphene nanosheets	500 W	116
Graphene nanosheets	20 kHz, 750 W	117
Graphene nanosheets	—	118
Graphene dispersions	51–52 W	119
Graphene oxide	600 W	120
Graphene oxide	40 kHz, 157 W	121
Graphite oxide	360 W	122
Graphene oxide nanosheets	—	123
Reduced graphene oxide	20 kHz, 400 W	124
Reduced graphene oxide	—	125
Reduced graphene oxide nanodispersion	20 kHz, 1000 W or 750 W (ultrasound horn); 130–35 kHz, 200 W (ultrasonic bath)	126
Sulfonated graphene	40 W	127
Sulfonated reduced graphene oxide	37 kHz	128
Isocyanate-treated graphene oxide nanoplatelets	150 W	129
Chitosan-modified nano-scale graphene oxide	>360 W	130

In addition, ultrasound is a popular method to synthesize other carbon nanomaterials. In traditional carbon nanotube synthesis, sonochemical effects are frequently used to disperse individual single-walled carbon nanotubes in the solution such as N-methyl pyrrolidone.[139] In the cases that volatile organometallic precursors were sonicated to form different structured materials as described above,[74,75]p-xylene acting as a carbon source in the presence of low-concentration ferrocene could produce single-walled carbon nanotubes on the silica powder under ultrasound irradiation. Recently, Ha and Jeong have reported a similar sonochemical method to prepare multi-walled carbon nanotubes, which required a higher amount of ferrocene and a little water added to the reactant suspension.[140] Carbon nanodots with an average diameter of 2–9 nm were made from the sonication of poly(ethylene glycol)-400 (PEG-400) as a carbon source in a water/silicone oil bath[141,142] and when excited at a 390 nm excitation wavelength, the product had 474 nm emission wavelength and 774 A.U. fluorescence intensity. Based on this, Gedanken et al. fabricated an antibacterial material, Ga-doped C-dots on Ga nanoparticles (Ga@C-dots@Ga NPs), where a one-step sonochemical process was applied for the simultaneous fabrication and coating of Ga@C-dots@Ga NPs using PEG-400 and molten gallium (Fig. 11).[143] At present, carbon nanodots have been also sonochemically prepared by using other carbon sources like polyacrylic acid, glucose and citric acid.

Fig. 11

(I) Schematic overview of the sonochemical deposition process for self-assembly of Ga@C-dots and Ga@C-dots@Ga NPs: (a) sonication set-up, (b) formation of Ga@C-dots/Ga@C-dots@Ga NPs, and (c) Ga@C-dots and Ga@C-dots@Ga NPs decorated on a glass substrate. (II) (a) SEM micrograph, (b) TEM images, (c) magnified view of single particles and (d) EDS spectrum of Ga@C-dots@Ga NPs. Reproduced with permission.[143] Copyright 2017 Royal Society of Chemistry.

The combined effects of sonochemical and physical or mechanical processes can promote the preparation of porous materials as an alternative in some cases. Jung and co-workers used 1-methyl-2-pyrrolidone as a solvent to prepare MOF-177 (MOF or metal–organic framework) or Zn4O(BTB)2 (BTB or 4,4′,4′′-benzene-1,3,5-triyl-tribenzoate), within only an hour by the ultrasound-assisted method.[144] Similarly, the sonochemical preparation of spherical Mg-MOF-74,[145] namely Mg2(DHTP)(H2O)2·8H2O (DHTP or 2,5-dihydroxyterephthalate), was adapted from the conventional solvothermal method if triethylamine was added to encourage the deprotonation of DHTP. Morsali et al. synthesized a nano-structured Cu(ii) MOF (namely {Cu(BDT)(DMF)·CH3OH·0.25DMF}) by reacting H2BDT with Cu(NO3)2·6H2O under ultrasound irradiation,[146] in which BDT2− was 1,4-benzeneditetrazolate and DMF was N,N-dimethylformamide, wherein the morphology of the MOF could be altered by ultrasound irradiation at different temperatures, e.g., yielding rice grain-like, pellet-like and laminate-like structures at 10, 40 and 70 °C. In another study, they used such synergetic sonochemical synthesis to prepare a pillared-layer Zn(ii)-based MOF,[147] namely [Zn2(NH2-BDC)2(4-bpdh)]·3DMF (TMU-16-NH2), where NH2-BDC was amino-1,4-benzenedicarboxylate and 4-bpdh was 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene. By the comparison, they found that the sonication time, sonication power and initial reagent concentrations had an important effect on the morphology of TMU-16-NH2, for example, longer sonication time, lower initial reagent concentrations, and higher sonication power could lead to uniform nanocubes. Later, they synthesized a dihydro-tetrazine functionalized MOF via ultrasonic and reflux methods,[148] where [Zn(OBA)(H2DPT)0.5]·DMF (TMU-34) was prepared using 4,4′-oxybis(benzoic acid)(H2OBA) as a dicarboxylate oxygen donor ligand and 3,6-di(pyridin-4-yl)-1,4-dihydro-1,2,4,5-tetrazine (H2DPT) as a pillar spacer. In this synthesis, increasing the sonication time could reduce aggregation morphology, decreasing the initial reagent concentrations could form a uniform plate-like morphology, and enhancing the sonication power could obtain a separate uniform plate-like morphology with a thickness of about 100 nm. The obtained TMU-34 showed a noticeable response to Al(iii) ions; moreover, the nano-sized sample with a uniform separated plate-like morphology had a higher detection limit and speed due to the fast diffusion of Al(iii) inside the framework as well as close contact and strong interaction of Al(iii) with the dihydro-tetrazine group. Additionally, other porous materials such as MOF-5, IRMOF-3 or manganese dioxide octahedral molecular sieves have been also produced by ultrasonic methods.[149-151]

In the preparation of porous nanostructures under ultrasound irradiation, the combined effects can be also utilized to alter the size and distribution of nanomaterials. The ZIF-8 nanoparticles (ZIF or zeolitic imidazolate framework) with an around 200 nm diameter were prepared through the precipitation of zinc nitrate and 2-methylimidazole mixed in methanol,[152] where a bimodal distribution would be caused as an evidence of Ostwald ripening when the nanoparticles were subjected to sonication. Through the sonosynthesis techniques, Kim et al. developed a rapid and facile method for the in situ growth of MOF films over a metal substrate,[153] in which high-intensity ultrasound enhanced the reactivity between metal ion sources and organic ligands after the metal substrate was first treated oxidatively to produce the corresponding metal hydroxide. On the Cu or Zn metal substrates, researchers used such an approach to successfully fabricate four MOF films including Cu-BDC film, Cu-BTC film, ZIF-8 film and MOF-5 film, and found that the crystal size in the films increased with the sonication time except for the MOF-5 film (Fig. 12). On the contrary, MOF-5 had a decrease in the crystallization and growth rate when exposed to long-period ultrasound irradiation, which was likely because the complex formation of several nuclei leaded to a decrease in crystal size.

Fig. 12

(I) Schematic illustration of the two-step method for the fabrication of different thin MOF films. (II) SEM images of Cu-BTC with 0.5 mM BTC under ultrasonic irradiation for (a) 5 min, (b) 15 min, (c) 30 min, and (d) 60 min; SEM images of Cu-BDC with 1 mM BDC under ultrasonic irradiation for (e) 5 min, (f) 15 min, (g) 30 min, and (h) 60 min. (III) SEM images of ZIF-8 with 0.1 mM 2-MIM under ultrasonic irradiation for (a) 5 min, (b) 15 min, (c) 30 min, and (d) 60 min. SEM images of MOF-5 with 1 mM BDC under ultrasonic irradiation for (e) 5 min, (f) 15 min, (g) 30 min, and (h) 60 min. Reproduced with permission.[153] Copyright 2018 Elsevier.

Furthermore, the combined effects of ultrasound can also participate in the sonochemical synthesis and/or modification via other approaches, e.g., ultrasound can be applied in promoting the diffusion of dopants into the nanostructured materials. In the study of Xiong et al., high-intensity ultrasound was used to prepare photoluminescence-tunable Mg2+ doped ZnO nanoparticles with no MgO phase existing in the ZnO nanoparticles.[154] In the study of Belova et al., ultrasound irradiation was used to make a slurry containing Au colloids and TiO2 particles to produce an Au nanoparticle-intercalated mesoporous TiO2 structure which had enhanced photocatalytic properties.[155] Recently, Wickleder et al. described a novel ionic liquid-assisted synthesis strategy to produce Eu2+-doped BaFCl nanoparticles by applying sonochemical approaches,[156] where 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4) was used as the fluoride source with EuBr2 or EuSO4 as the europium source. Eu2+-doped BaFCl nanoparticles were stably suspended in BminBF4 presenting a relatively narrow size distribution between 20 and 90 nm, where 80% of the particles were smaller than 40 nm (Fig. 13I). Under daylight, the suspensions with undoped and doped barium fluoride chloride nanoparticles were both colorless because a high reflectance value (above 97%) in the visible range caused nearly no efficient absorbance. The reflectance decreases in the UV spectral range, allowing the undoped BaFCl particles to display weak dark blue emission probably originating from the C-dot present in the ionic liquid while Eu2+-doped BaFCl particles emitted efficient bluish white light (Fig. 13II). By such a combination of ionic liquid and sonication, Eu3+-doped lanthanide nanophosphates were also fabricated from Ln(NO3)3·6H2O and 1-hexyl-3-methylimidazolium dihydric phosphate. Nagabhushana et al. reported the synthesis of Eu3+-doped Y2O3 nanophosphors by a modified sonochemical method using aloe vera gel as a surfactant[157] and investigated the effect of sonication time on the morphology of nanophosphors. Within a short sonication time, the nanophosphors showed agglomerated microstructures, and with the sonication time increasing, the size slightly reduced and began to be uniformly spherical until a prolonged sonication time of over 6 hours caused agglomerated microstructures again because uniform spherical structures were thriving themselves. Previous to Wickleder et al. and Nagabhushana et al., Xu and co-workers had successfully synthesized hexagonal and tetragonal LnPO4:Eu3+ nanostructures (Ln = La, Gd, and Y) through a simple and facile ionic liquid-assisted sonochemical method,[158] where the rapid reaction rate was attributed to the acceleration effect of ionic liquid combined with ultrasound irritation. The obtained LnPO4:Eu3+ displayed four different types of morphologies including nanowires, nanorods, nanowire bundles and nanoparticles, which might be relevant to the period of the sonication process.

Fig. 13

(I) TEM images of BaFCl:Eu2+ nanoparticles which were prepared using EuBr in an ultrasonic bath for 30 min. (II) Eu2+-undoped (a) and Eu2+-doped (b) BaFCl nanoparticles suspended in BminBF4 under UV and daylight. Reproduced with permission.[156] Copyright 2018 Wiley-VCH.

In addition, what is more, the microjets and shock waves originating from the ultrasound near the solid materials can induce a few physical changes on the surface of particles or substrates. For instance, the anticorrosion surface coatings can be improved via the treatment of high-intensity ultrasound in which a newly active oxide layer will form with the existing oxide layer being removed. Ultrasound irradiation can also improve the sol–gel preparation of nanostructured metal oxides by accelerating the hydrolysis process, improving the phase purity, and creating the metal oxides with narrower size distribution and higher surface area. In a study of Yu et al., TiO2 nanoparticles produced by the ultrasound irradiation of the precursor solution had a higher photocatalytic activity,[159] which was attributed to the accelerated hydrolysis in sonication improving the crystallinity of TiO2. To obtain thin zirconium dioxide layers on structured reactors, Jodłowski developed an efficient ultrasound-assisted sol–gel method through the synergistic combination of three processes including the sonochemical synthesis of Zr sol–gel from a Zr(IV) n-propoxide precursor solution,[160] the addition of ethylene glycol and glycerol as stabilizing agents, and the deposition of ZrO2 on FeCr alloy supports using the dip-coating method. The results displayed that the ZrO2 films on the metallic structure were dense and uniform without any impurities. In the study of Sierra-Salazar et al., the hierarchically porous Pt/SiO2 catalyst was prepared in aqueous medium via the combination of latex synthesis, sonochemical reduction and a two-step catalysed sol–gel process, and its mesopore and macropore size ranged as large as 2–15 nm and 90–400 nm, respectively.[161,162] For such a silica-supported Pt catalyst, the noble metal Pt distribution could be enhanced at low loadings by controlling the hierarchical porosity of the support material.[162] Manoharan et al. tailored a hybrid method of the precipitation technique and ultrasonication to accomplish the preparation of polycrystalline zirconia nanophosphors from a zirconium oxalate precursor complex.[163] As a physical aid to the synthesis process, the acoustic wave well controlled the grain size of nano-zirconia where the particles had a uniform spherical shape and narrow size distribution in the range of 15–25 nm, and the crystal structure was found to be predominantly tetragonal at room temperature. By the sonochemical coprecipitation, Zhang et al. prepared catalase-inorganic hybrid microflowers (Cat-HMFs) by using catalase as an organic component and copper phosphate (Cu3(PO4)2) as an inorganic component.[164] Then, Cat-HMFs were dropped onto the glassy carbon electrode to fabricate a H2O2 biosensor. Amperometric detection results indicated that such a biosensor displayed excellent current response stability, selectivity and long-term storage stability toward H2O2 detection.

Certainly, the inorganic-based hybrid nanomaterials such as inorganic/polymer composites and inorganic/biomolecule composites or other multi-level composites have been produced largely based on combined sonosynthesis. For instance, Ashokkumar et al. carried out the sonochemical formation of polystyrene/SiO2 Janus particles on the basis of the phase separation mechanism between the tetraethoxysilane and growing polystyrene particles.[165] In our studies, the magnetic nanoparticles were either fixed on the surface of carrier supporters as a shell or loaded into the interior of carrier supporters as a core (Fig. 14).[166] We have been also reviewed the sonochemical fabrication of inorganic/biomolecule composites for drug delivery and described the multiformity and multifunctionality of products according to the synthetic materials, implementation schemes, or specific demands. In fact, the structure, size, stability and activity of inorganic-based hybrid nanomaterials are important to both the quality of properties and the acceptability of applications, and the relevant research results or viewpoints have been reviewed in detail in other papers.[167-173]

Fig. 14

TEM images of organic/BSA composites with a magnetic shell (A) and with a magnetic core (B); SEM images of organic/BSA composites with a magnetic shell (C) and with a magnetic core (D), where the close-up images lay in the inset of panel (C) and (D). Reproduced with permission.[166] Copyright 2019 Elsevier.

Future outlook

Ultrasound-aided materials synthesis has been developed rapidly and remarkably in the last few decades, constantly in a few novel ways to simplify the synthesis processes or to enhance the properties of products. Indeed, ultrasound irradiation shows potential to assemble nano-structured inorganic materials of defined sizes and with pre-defined functionalities, or together with the recent advances or methods for purpose-specific or site-specific modification, significantly expands its applications in materials synthesis. Thereby, nano-structured inorganic materials in association with sonochemical synthesis are an available future direction. Even so, many challenges and anticipations are still online for the in-depth development of either sonosynthesis or the resultant products. The probable challenges could be as follows: more mechanistic and experimental understanding of the cavitation phenomenon on the nano- and micro-levels is needed to settle a matter in controlling the sonosynthesis processes; the difference of raw materials or a variety of microdevices asks for more designs of ultrasound-based synthesis platforms; sometimes for the ultrasound-assisted preparation of nanostructured materials, controlled morphologies, structures, and compositions are essential but a lot unsolved; the performance, durability, selectivity as well as low cost of product materials are still highly demanding; besides a high mechanical stability and a long shelf life, all the products had to accurately act according to on-site conditions or stimulus; finally, sonosynthesis is an energy consuming process in materials preparation, and although the production of ultrasound from electrical power can be extremely efficient, sonochemistry remains energy inefficient since the coupling of ultrasound into chemically useful cavitation events has low yield, so more effective ways to induce more cavitation in liquids may alleviate the current limitation. Certainly, there are more known and unknown areas meriting further study on ultrasound or sonochemical synthesis, such as the interaction mechanism between the materials at the cavitation interface, the controlled energy distribution of collapsing bubbles, the heterogeneous nucleation onto the solid surfaces with different properties, the in situ observation of the product's macroscopic structures in the formation, and so on.

Conclusion

Ultrasound is appealing and conducive to the preparation or modification of nanomaterials. This review simply gives the view on the chemical, physical and mechanical effects from ultrasound irradiation, presenting the basic principles or mechanisms of primary sonosynthesis and secondary sonosynthesis. Meanwhile, it describes the preparation of inorganic materials from the perspective of sonosynthesis, highly focusing on different inorganics including metals, alloys, metal compounds (e.g., metal oxides, metal sulfides, and metal nitrides), non-metals (e.g., carbon materials, silicon materials, and selenium materials) and hybrid composites, and widely covering a variety of structures such as solid structures, hollow structures, core–shell structures, porous structures, nanotube structures, nanorod structures, and layered structures. In addition, the review also provides the probable challenges and technological potential for the future advancements on the sonochemical synthesis applications. We hope that such a review can provide a comprehensive understanding of sonochemistry as well as the synthesis mechanism and broaden the updated applications in catalysis or other fields.

Conflicts of interest

There are no conflicts to declare.