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Ultrasound in Nanochemistry: Recent Advances

Oxana V. Kharissovaa; Boris I. Kharisova; Juan Jacobo Ruíz Valdésa; Ubaldo Ortiz Méndeza a Universidad Autónoma de Nuevo León, Monterrey, Mexico Online publication date: 25 May 2011

To cite this Article Kharissova, Oxana V. , Kharisov, Boris I. , Valdés, Juan Jacobo Ruíz and Méndez, Ubaldo Ortiz(2011)

'Ultrasound in Nanochemistry: Recent Advances', Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41: 5, 429 — 448 To link to this Article: DOI: 10.1080/15533174.2011.568424 URL: http://dx.doi.org/10.1080/15533174.2011.568424

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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:429–448, 2011 C Taylor & Francis Group, LLC Copyright ISSN: 1553-3174 print / 1553-3182 online DOI: 10.1080/15533174.2011.568424

Ultrasound in Nanochemistry: Recent Advances Oxana V. Kharissova, Boris I. Kharisov, Juan Jacobo Ru´ız Valdés, and Ubaldo Ortiz Méndez

Downloaded By: [Kharisov, Boris] At: 16:15 27 May 2011

Universidad Autónoma de Nuevo León, Monterrey, Mexico

Applications of ultrasound in the synthesis of nanomaterials, nanocomposites, and nanoforms of chemical compounds are reviewed here. Ultrasonic treatment is widely used to produce nanostructures/nanocomposites containing elemental metals, alloys, carbon nanoforms, metal oxides, salts, and coordination compounds, macrocycles, and polymers, as well as for obtaining nanoemulsions and nanogels. Nanocatalysts can be prepared or function more actively in conditions of ultrasonic irradiation, in particular, in the degradation of organic pollutants. Ultrasonically prepared nanomaterials are also applied for tumor treatments and drug delivery purposes. Keywords

N N

=

Chitosan OH

OH

OH

O O

O

O O OH

CTAB GO

C F2

NH 2

HO

NH 2

n

HO

F C

n O

F2 C

F2 C

y F C

O

F2 C C F2

SO3 H

CF3

N

HO

F2 C

z

ABBREVIATIONS BIBA = 2-bromoisobutyric acid Bibp = 4,4 -bis(1-imidazolyl)biphenyl

HO

MH MIBA MNPs MWCNTs Nafion

catalysis, colloids, composites, drug delivery, nanoparticles, ultrasound

N

= Isobutyric acid = “Mobile crystalline material” (“mobil composition of matter”) on SiO2 basis = Magnesium hydroxide = 2-Methylisobutyric acid = Magnetic nanoparticles = Multi-walled carbon nanotubes =

IBA MCM-41

NH 2

= Cetyltrimethylammonium bromide = Graphene oxide

Received 24 February 2010; accepted 4 February 2011. The authors are grateful to Professors Valentina Belova, Tarun K. Mandal, Ken Cham-Fai Leung, Shu-Lei Chou, Kian-Ping Loh, Yingchun Zhu, Jingsong You, and Tao Gao from a variety of universities worldwide, and to the American Chemical Society for permission to reproduce images and figures from their publications. Address correspondence to Boris I. Kharisov, Universidad Autónoma de Nuevo León, Monterrey, Mexico. E-mail: bkhariss@ mail.ru

P(AMPS-co-MMA) = Poly(2-acrylamido-2methylpropanesulfonic acid Me methacrylate) PANI = Polyaniline PDLLA = Poly(D,L-lactide) PEG = Polyethylene glycol PVP = Poly(vinylpyrrolidone) PZS = Poly(cyclotriphosphazene-co-4,4 sulfonyldiphenol) RhB = Rhodamine B TBAOH = Tetrabutylammonium hydroxide (C4 H9 )4 NOH TMAH = Tetramethylammonium hydroxide TPG = Tetrapropylgermane QD = Quantum dot SNTs = Silica nanotubes US = Ultrasound/ultrasonic ZSM-5 = 2 zeolite sieve of molecular porosity”, an aluminosilicate zeoliteNan Aln Si96-n . 16H2 O (0 < n27). INTRODUCTION Ultrasound (US) is currently a common laboratory tool used to nebulize solutions into fine mixtures, emulsify mixtures, drive

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chemical reactions, as well as for dispersing nanoparticles and colloids.[1] It consists of acoustic waves with frequencies of more than 20 kHz, interacting with a species, which can cause structural changes and accelerate chemical reactions, disrupting the weak noncovalent interactions or disintegrating aggregated particles, but seldom does it favor assembly formation.[2] On the basis of an ultrasound, the “sonochemistry” (synthesis of materials under non-equilibrium conditions appearing under cavitation, induced by acoustic waves resulting in the creation and collapse of microbubbles, providing extreme synthesis conditions such as: temperatures of ∼5000 K, pressures of ∼1 GPa, and cooling rates of ∼1010 K s−1) studies the synthesis of metals, alloys, oxides, polymers, and a variety of other classes of chemical compounds both in isolated and composite form. As the process duration is extremely short, the resulting particles will be of nanosize. In recent times, several nanochemistry- and nanotechnologyrelated applications of ultrasound have been extensively reviewed. Thus, some preparation methods of nanosized materials, involving ultrasonic precipitation, ultrasonic pyrolysis, ultrasonic reduction, and sonoelectrochemistry have been highlighted in a series of reviews.[3,4] Use of ultrasound for obtaining and functionalization of traditional carbon materials (e.g., carbon black and activated carbon), pigments, adsorbents, and composite components on its basis, as well as carbon nanotubes, graphene, and meso- and macroporous carbons, which are objects of a permanent interest, are described by Skrabalak[5] and Xing,[6] in particular their preparation by both ultrasonic spray pyrolysis and high-intensity ultrasound, or ultrasonic modification of their properties. Additionally, the latest advances in the application of ultrasonic technology in catalytic chemistry, including its application for nanomaterial preparation, active component loading, and heterogeneous chemical reactions, have been generalized by Yan et al.[7] Among other important ultrasound-assisted applications in materials chemistry, the authors have noted the synthesis of ceramic nanoparticles[8] and polymer nanocomposites.[9] In the cases of medical, biomedical, and related areas, new technologies that combined the use of nanoparticles with acoustic power both in drug and gene delivery were summarized by Husseini et al.[10] It was seen that ultrasonic drug and gene delivery from nanocarriers had tremendous potential because of the wide variety of drugs and genes that could be delivered to the targeted tissues by fairly noninvasive means. Other related publications include a discussion on the revolution in cancer treatment,[11] potential uses of nanoparticles in oncology,[12] use of micelles and nanoparticles for ultrasonic drug, and gene delivery.[13,14] Pharmaceutical applications of ultrasound were described by Ishtiag et al.[15] Nanomaterials, in particular those obtained by using the ultrasound, have unusual properties not found in bulk materials, which can be exploited in numerous applications,[16] such as biosensing, electronics, scaffolds for tissue engineering, and diagnostics. Ultrasound can also act as a stimulus to

induce gelation of organic liquids with low-molecular-weight gelators,[17] forming micro- and nanoemulsions,[18] particularly in cosmetic and pharmaceutical products,[19] or for insertion of nanomaterials into the mesostructures.[20] A variety of ultrasonic equipment has been developed for nanotechnological purposes, for instance, an ultrasound reactor has been developed for the fabrication of nanoparticles from the gas phase[21] or an ultrasound flow reactor.[22] In relation to the ultrasound, the most popular objects used were carbon nanotubes, platinum, gold, ZnO, iron oxides, Al2 O3 , SiO2 , TiO2 , polymers, as well as core-shell nanoparticles, on the basis of the said compounds, among many others. We emphasize that ultrasonic applications in nanochemistry and nanotechnology form a very extensive area, so, in the present review we have paid attention to the main recent achievements of ultrasound-assisted preparation of compounds and materials, as well as certain medical applications. ULTRASOUND-ASSISTED SYNTHESIS OF NANOPARTICLES AND NANOCOMPOSITES Metal/Alloy-Containing Nanostructures Manufacture of metal nanoparticles by reduction[23] is now a common procedure for synthesis, leading to different metalcontaining nanostructures and nanocomposites. Ultrasonically prepared elemental metal/alloy-containing nanocomposites on a polymer basis are widespread, in particular those of noble metals, especially Au. Thus, gold-deposited iron oxide/glycol chitosan nanocomposites[24] (chitosan is a classic polysaccharide support in nanotechnology) were obtained from FeCl2 .4H2 O, FeCl3 .6H2 O, and HAuCl4 , as precursors under ultrasonication. Hybrid nanocomposites of carboxyl-terminated generation 4 (G4) poly(amidoamine) dendrimers with gold nanoparticles encapsulated inside them were described by Wei et al.[25] These US-obtained nanocomposites were used to fabricate highly sensitive amperometric glucose biosensors, which exhibited a high and reproducible sensitivity and response time of less than 5 s. The combined application of PEG and ultrasonic irradiation was investigated as a possible method to synthesize intercalated Au/clay nanocomposites (spherical-shaped Au nanoparticles of 6–8 nm diameter spread homogeneously within the nanocomposites).[26] The intercalation method consisted of two steps: (1) intercalation of PEG into the clay matrix by ultrasonic irradiation; and (2) replacement of PEG by gold nanoparticles under ultrasonic treatment. The incorporation (Figure 1) of Au nanoparticles in the clay matrix under ultrasound was twice as efficient with PEG as without it. These new nanocomposites could find practical application in different areas such as in the electronic (computer chips, optoelectronic, information storage, sensors, catalysis, micro-/nanoelectronic devices) and medical fields. Biosynthesis of size-controlled gold nanoparticles was reported[27] using fungus Penicillium sp., which could successfully bioreduce and nucleate AuCl4 − ions, and lead to the assembly and formation of intracellular Au nanoparticles,

ULTRASOUND IN NANOCHEMISTRY

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FIG. 1. Scheme of ultrasonic intercalation of gold nanoparticles into the clay matrix in the presence of PEG. Reproduced with permission from the American Chemical Society. (Figure is provided in color online.)

with spherical morpholology and good monodispersity, after exposure to HAuCl4 solution. The intracellular gold nanoparticles could be easily separated from the fungal cell lysate by ultrasonication and centrifugation. A new type of core-shell nanostructure Aucore /Ag-PVPshell consisting of Au nanoparticles embedded in the shells of an Ag nanoparticle-filled polymer was ultrasonically fabricated in a two-step procedure, in which the Ag nanoparticles served as the initial reductive agent for Au3+ ions, with the existence of PVP.[28] Solidification and spherulite growth of the polymer was induced around the as-formed Au nanoparticles such that very small Ag nanoparticles were embedded in the crystalline polymer spherulites. The surfactantand reducer-free synthesis of gold nanoparticles from an aqueous HAuCl4 .4H2 O solution, using a high-frequency (950 kHz) ultrasound in the absence of any stabilizing, capping, or reducing agents, was described.[29] It was found that higher AuCl4 − concentration promoted particle growth (size increase) and plate formation, enhanced with the addition of NaCl or HCl (but not NaOH). A one-pot synthesis method to prepare gold nanorods was developed by using the sonochemical reduction of gold ions in aqueous solution.[30] The size and shape of these gold nanoparticles were greatly dependent on the pH of the solu-

tion (3.5–7.7). The mechanism for the sonochemical formation of gold nanorods and nanoparticles is shown in Figure 2. Composites of noble metal nanocrystals and titanium dioxide nanotubes were prepared by ultrasound-assisted pulsed electrodeposition, with titanium dioxide nanotubes growing erect on a titanium substrate as the carrier, based on the high dispersibility of Pd.[31] The composite catalytic electrode had a stable structure, high catalytic activity, many surface catalytic active sites, and a large specific surface area for catalytic reaction. Dispersed and aggregated palladium nanoparticles with large size distribution (centered at 20 nm) were obtained[32] by ultrasonic irradiation of Pd(NO3 )2 solution, in presence of ethylene glycol and PVP. It was seen that sonochemical reduction of Pd(II) ions to Pd(0) atoms depended on the sonication time, and the use of a low quantity of PVP entailed the obtaining of aggregates palladium nanoparticles. The Sn(II→IV)/Pd(II→0) redox couple was used for decorating CNTs with Pd nanoparticles by a displacement reaction, with the aid of ultrasonication,[33] producing nano-sized particles, uniformly dispersed and tightly anchored on the surfaces of CNTs. The constructed heterostructure exhibited a synergistic effect in its hydrogen storage performance. The production of Pt nanoparticles (11–15 nm) from aqueous

FIG. 2. Schematic mechanism for the sonochemical formation of gold nanorods and nanoparticles at pH 3.5. Reproduced with permission from the American Chemical Society. (Figure is provided in color online.)


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chloroplatinic solutions, in the presence of low-frequency and high-power ultrasound (20 kHz), on Ti alloy electrodes, performed galvanostatically at 298 K using a sonoelectrode, producing ultrasonic pulses triggered and followed immediately by short, applied current pulses.[34] Among the other noble metals used, Ag nanoparticles,[35,36] an Ag/C nanocomposite,[37] Ag/TiO2 nanotube composite material,[38] hydrogel,[39] and solution[40] on the basis of nanoscale Ag, possessing antibacterial properties, are known. As an example, a antibacterial nanopowder, composed of shell powder carriers 80–98 wt.%, and nanoscale antibacterial active component 2–20 wt.% (on the basis of Ag+, Cu2+, Ni2+ or Zn2+ ions) adsorbed in micropores of carriers, was reported.[41] This nanopowder had the advantages of small particle size, good compatibility with materials, good safety, no toxicity, wide antibacterial spectrum, and good stability, and could be applied in plastic, rubber, fiber, coating material, ceramic, and so on. Sonochemical irradiation of Fe(II) acetate aqueous solution, in presence of Ag nanopowder, resulted in the deposition of magnetite nanoparticles on Ag nanocrystals, and imparted them with magnetic properties.[42] The Ag-Fe3 O4 nanocomposite (other magnetite composites are mentioned a little later in the text) was well attracted to a permanent magnet, and demonstrated super paramagnetic behavior typical of nanomaterials in a magnetic field. The strong anchoring of the magnetite to the nanosilver surface was explained by the authors as being the result of the local melting of Ag, when the magnetite nucleus was thrown at the Ag surface by high speed sonochemical microjets. SiC nanoparticles reinforced with magnesium and its alloys including pure Mg, Mg-(2, 4)Al-1Si, Mg-6Zn, and Mg-4Zn, were fabricated by ultrasonic cavitation-based dispersion of SiC nanoparticles in the magnesium melt.[43,44] As compared to the un-reinforced magnesium alloy matrix, the mechanical properties including tensile strength and yield strength improved significantly, and the ductility was retained or even improved. In a related research, magnesium matrix composites reinforced with nano-sized SiC particles (n-SiCp/AZ91D) were fabricated by high-intensity, ultrasonic assisted casting.[45] The dispersion and distribution of n-SiCp in the magnesium alloy melts were significantly improved by ultrasonic processing. The yield strengths were remarkably improved and the yield strength increased by 117% after gravity permanent mold casting. The room temperature preparation of metallic aluminum nanoparticles (10–20 nm) was performed using the pulsed sonoelectrochemical method.[46] The authors noted that the sonoelectrochemical technique was a promising method for the fabrication of air-sensitive metallic nanoparticles that had a high, negative reduction potential. On the contrary, using Al nanoparticles (80 nm), nanothermite powders were prepared by ultrasonic mixing, such as aluminum powders with a variety of metal iodates {bismuth iodate, copper iodate, zinc iodate, molybdenum oxide, and iodine oxide (particle sizes 50 nm–5 µm)} and metal oxides.[47] An interesting feature of these materials (primary explosives) was the production of gaseous products, rapidly converting to condensed phases upon cooling. The major

products of the interaction of the components were as follows: for AgIO3 and Al—nanoparticulate AgI and Al2 O3 , for Bi2 O3 and Al—the bismuth vapor product condensing as Bi metal and/or reacting with oxygen from air to re-form into Bi2 O3 . Using an ultrasonic aerosol pyrolysis approach, diamond cubic Ge nanocrystals with dense, spherical morphologies and sizes ranging from 3 to 14 nm were synthesized at 700◦ C, from an ultrasonically generated aerosol of TPG precursor and toluene solvent.[48] Ni-CeO2 nanocomposites, with high microhardness, were prepared in an ultrasonic field by means of the cavitation effects of the ultrasonic wave and by selecting suitable processing parameters of pulse electrodeposition.[49] The co-deposited CeO2 content in the composites was shown to be markedly affected by the CeO2 particle concentration in the electrolyte. The Ni-TiN composite layer was prepared by the ultrasonic-electrodeposited technology.[50] The surface of the Ni-TiN composite layer was found to be relatively flat and smooth, with an ultrasound-assisted action, and TiN nanoparticles were added. The basic reason for improving the Ni-TiN composite layer wear resistance was that TiN nanoparticles had a dispersion strengthening effect, a fine crystal strengthening effect, and a bearing and lubricating effect. Particles (carbon nanotubes, diamond, graphite, Cu, Al, Fe)-containing liquid metals (Hg, Ga, Pb) with high-heat transfer performance were prepared by mechanical stirring and ultrasonic dispersion of 1:(0.1–99) wt. ratio particle-metal mixtures.[51] Bimetallic and alloy nanoparticles and composites on their basis are widely represented. Thus, the ultrasonic irradiation technique is employed to prepare Cu-Ga/polymethyl methacrylate nanoparticles,[52] in which there are chemical actions between Cu-Ga alloy and polymethylmethacrylate (PMMA). Synthesis of FeCr alloy nanoparticles by using a method that couples electrodeposition of metals with the employment of high power ultrasound has been described.[53] The final product is a suspension of nanoparticles with high purity and high surface/volume ratio, which can be controlled by varying process parameters. Shape-controllable metal nanocrystal/carbon nanotube heterostructures are also known.[54] Synthesis of FeCo nanoparticles by using a method that couples electrodeposition of metals with the employment of high power ultrasound has been described using a titanium alloy horn ultrasound generator.[55] The primary role of the ultrasound in this process is to induce cavitation phenomenon in the electrolyte and to ablate the metallic nuclei from the cathode surface. The final product is a suspension of nanoparticles with high purity and surface/volume ratio, which can be controlled by varying process parameters. Platinum-cobalt (PtCo) alloy nanoparticles were successfully fabricated by the ultrasonic-electrodeposition method, using an inclusion complex film of functionalized cyclodextrin-ionic liquid as a support.[56] The resulting modified glassy carbon electrode showed an excellent catalytic activity for glucose oxidation, therefore, it was promising as a nonenzymic glucose sensor. A non-enzymatic glucose sensor, based on highly dispersed PtM (M = Ru, Pd and Au) nanoparticles on composite films of



multi-walled carbon nanotubes (MWCNTs)-ionic liquid (trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) imide) was fabricated using the ultrasonic-electrodeposition method.[57,58] This novel non-enzyme sensor had potential application in glucose detection. Among other metal pairs, FeNi[59] alloy nanoparticles and nanocomposites Cu-Zn-Al-ZSM-5,[60] have been ultrasonically prepared and characterized. Concluding this section, the authors note that ultrasonic treatment is widely used to produce various metal nanoparticles[61] (in particular, Rieke metals[62]), nanoalloys, and metalcontaining nanocomposites, on different supports, starting from metals and their salts as precursors, in combination with the reduction and electrodeposition methods. At the same time, ultrasound has been applied for the activation of elemental metals in the synthesis of coordination and organometallic compounds.[63]

Carbon Nanotubes, Graphene, Diamond, and Fullerenes Multi-walled carbon nanotubes with minimal defects as templates and facilely fabricated carbon nanotube-polyaniline nanocomposites, with uniform core-shell structures, were prepared by ultrasonic assisted in situ polymerization.[64] By varying the ratio of aniline monomers and carbon nanotubes, the thickness of the polyaniline layers could be effectively controlled. The effective site-selective interaction between the π -bonds in the aromatic ring of the polyaniline and graphitic structure of carbon nanotubes should strongly facilitate the charge-transfer reaction between the two components. CNTs could be ultrasonically filled with magnetic metal iron nanoparticles.[65] A general, rapid, template-free, 1-step, and continuous approach was designed to obtain rattle-type hollow carbon spheres (M@carbon, M = multiple Sn, Pt, Ag, or Fe-FeO nanoparticles) via ultrasonic spray pyrolysis of aqueous solutions containing Na citrate and the corresponding inorganic metal salts,[66] controlling the content of encapsulated nanoparticles in M@carbon via tuning the concentration of metal salts. Sn@carbon exhibited a high capacity and good cycle performance when it was used as anode material for an Li battery. The wet chemical technique to produce carbon nanoscrolls at low temperature, based on the use of readily available acceptor-type graphite intercalation compounds, was reported.[67] It was performed by using the initial graphite intercalation compound, first exfoliated to produce a suspension of graphene monolayers in ethanol and subsequently sonicated, yielding a suspension of carbon nanoscrolls. The C70 nanoscale monocrystal material, composed of granules, rods, or tubes, was prepared from saturated C70 m-xylene solution (as the mother liquor) and linear saturated mono alcohol (as the shape regulator) by ultrasonic vibration, standing, and a series of consequent steps yielding C70 nanoscale crystals.[68] Ultrasonic stability of C70 -gold nanoparticle multilayer films was studied by exposing them to ultrasonic irradiated surroundings, which resulted in partial desorption and a little aggregation of nanoparticles on solid surfaces.[69]

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Additionally, ultrasonication at 150 kHz was applied to disperse diamond powders with their primary particle size of 5 nm.[70] The transformation of two-dimensional graphene oxide (GO) nanosheets into carbon nanotubes was achieved by sonicating GO in 70% nitric acid.[71] The main steps of these reactions are shown in Figure 3. This net chemical process (from open-face carbon sheets to curled carbon nanostructures) was, according to the authors opinions, almost magical in its “one-pot” transformation: the open, two-dimensional GO sheets could be readily decomposed into polyaromatics in minutes, which were further reconstituted by acid dehydration reactions to form larger carbon nanoparticles and nanotubes. Oxide and Hydroxide-Containing Nanostructures Among oxide-containing nanoparticles and nanocomposites, the greatest attention has undoubtedly been paid to iron oxides and TiO2 . Thus, talc [Mg3 Si4 O10 (OH)2 ] microparticles were coated with hydrophilic and hydrophobic TiO2 nanoparticles in liquid CO2 .[72] The talc was found to be completely coated with TiO2 nanoparticles, and a smooth coating surface was obtained; the flowability and wettability of the talc was improved by TiO2 nanoparticle coating. A commercially available dry titania nanopowder with a mean primary particle diameter of approximately 30 nm was mixed into an epoxy resin/hardener system to produce nanocomposite samples.[73] Processing techniques, such as ultrasonication and particle surface modification, were used to produce nanocomposites with varying degrees of particle mixture homogeneity. Highly crystalline metal oxide nanoparticles of TiO2 , WO3 , and V2 O5 were synthesized in just a few minutes by reacting transition metal chloride with benzyl alcohol, using ultrasonic irradiation under argon atmosphere in a non-aqueous solvent, at 363 K.[74] The particles’ size and shape revealed: (a) “quasi” zero-dimensional, spherical TiO2 particles (3–7 nm), (b) the V2 O5 particles having a “quasi” one-dimensional ellipsoidal morphology (lengths of 150–200 nm and widths of 40–60 nm), and (c) the WO3 particles as “quasi” two-dimensional platelets with square shapes (facets 30–50 nm). Among the other TiO2 -containing nanocomposites, the authors noted a fluoridated hydroxyapatite/titanium dioxide nanocomposite,[75] an enzyme electrode on the basis of MWCNTs-TiO2 /Nafion composite,[76] and TiO2 nanotubes, prepared by using anodization of Ti foils in H3 PO4 and ethylene glycol, by mechanical stirring and the ultrasonic method, which were found to demonstrate potential in the photoelectrocatalytic degradation (see also the degradation section a little later in the text) of methyl orange dye[77] and higher activity as compared to the stirring method. The addition of oxidants such as oxygen and H2 O2 demonstrated improvement in methyl orange degradation. For TiO2 nanofilms, prepared by using ultrasonic atomization technology, the analytic results showed that the products were uniform TiO2 nanofilms with an anatase 2 crystal phase and particle diameter of 20–80 nm.[78] The sol-gel synthesis technique was suitably modified by incorporating the ultrasound to study the effect of cavitation on the phase transformation,


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FIG. 3. Schematic illustration of the mechanism for transforming GO nanosheets into carbon nanoparticles and nanotubes, following their ultrasonication in acid. (I) Oxidative cutting of graphene oxide produces PAH molecules in concentrated HNO3 . In the dehydrating acidic medium, the polyaromatic fragments fuse and nucleate into (II) carbon nanoparticles or (III) nanotubes via acid-catalyzed intramolecular or intermolecular dehydration reactions. Reproduced with permission from the American Chemical Society. (Figure is provided in color online.)

crystallite size, crystallinity, and morphology, to synthesize nanostructured TiO2 (95% yield vs. 86% by classic sol-gel process) via the sol-gel technique to obtain a 100% rutile polymorph of the nanostructured TiO2 .[79] An enormous number of publications on nanoparticles and nanocomposites, on the basis of iron oxides (mainly Fe3 O4 in particular), recently reviewed Au/Fex Oy core-shell nanoparticles,[80] and testified the permanent interest of nanotechnologists in these non-expensive magnetic nanomaterials with unusual properties (for example shape memory effect), having a series of important applications, for instance in drug delivery systems.[81] Thus, biocompatible PDLLA/magnetite nanocomposites were reported by Zheng et al.[82] Fe3 O4 nanoparticles with an average size of 20 nm were synthesized by chemical co-precipitation and mixed uniformly with a PDLLA matrix, particularly under ultrasonication. These nanocomposites displayed a desirable shape memory effect. Among other nanocomposites, the authors noted magnetic Fe3 O4 /polyphosphazene nanofibers (several microns in length and 50–100 nm in diameter with Fe3 O4 nanoparticles of 5–10 nm attached on the surface through coordination behavior), prepared via a facile approach by ultrasonic irradiation.[83] Magnetic studies showed that the magnetic nanofibers exhibited good superparamagnetic properties with a high magnetization saturation value of about 36 emu/g. In case of closely related (PZS) poly(cyclotriphosphazene-co-4,4 -sulfonyldiphenol) use, magnetic PZS nanotubes, having good thermal stability and superparamagnetic properties, were formed.[84] They were

found to have 50–100 nm outer diameter and 5–10 nm inner diameter; the Fe3 O4 nanoparticles with diameter of 5–10 nm were embedded in the walls of the nanotubes. Fe3 O4 MNPs with improved peroxidase-like activity were prepared through an advanced reverse co-precipitation method, under the assistance of ultrasound irradiation.[85] The H2 O2 activating ability of the Fe3 O4 MNPs was evaluated by using RhB as a model compound of organic pollutants to be degraded, showing the ability to activate H2 O2 and remove approximately 90% of the RhB in 60 min. Additionally, magnetite was ultrasonically prepared, not only in the nanoparticles form,[86] but also as nanocrystals and nanofilms. Thus, a magnetite thin film was prepared by one-liquid ultrasonic spray plating using an aqueous solution containing FeCl2 , NaNO2 , and dextran.[87] Using this method, a Fe3 O4 film was deposited at 90◦ C; after post deposition annealing at 850◦ C in N2 , NaCl, coexisting in the film, was evaporated, and crystallinity and the magnetic property of the film was improved. Fe3 O4 magnetic nanocrystals were prepared by the co-precipitation of Fe2+ and Fe3+ ions in ammonia solution, with ultrasonic enhancement and modification by the anionic surfactant sodium dodecyl sulfate.[88] It was shown that the Fe3 O4 magnetic nanoparticles (∼10 nm) were well-crystallized particles with good dispersivity and thermostability, and they were superparamagnetic and could be applied in the biological and medical fields, such as cell or enzyme immobilization. Cobalt and manganese analogs of magnetite, Co3 O4 , and Mn3 O4 were synthesized as uniform sphere-like



or cubic spinel nanocrystals from acetate salts and NaOH or tetramethylammonium hydroxide as precursors.[89] Among a high number of recent reports on the ultrasonic production of SiO2 nanostructures, the authors noted polypropylene/silica nanocomposites,[90] Fe3 O4 @SiO2 coreshell nanoparticles for plasmid DNA purification,[91] and silicalite-1 (colloidal zeolite on the SiO2 basis) nanocrystals with organic functional groups, prepared by a simple ultrasonic treatment in methanol.[92] The organic functionalization enhanced the hydrophobicity of silicalite-1 nanocrystals, which proved to be very useful for fabricating monolayer zeolite films. Other oxides are lesser presented. A study of sonochemical reactions with MSU-X mesoporous alumina in aqueous solutions revealed[93] that sonication (f = 20 kHz, I = 30 W cm−2, W aq = 0.67 W mL−1, T = 36–38◦ C, Ar) caused significant acceleration of m-Al2 O3 dissolution in the pH range of 4–11, forming nanorods and nanofibers of boehmite, {AlO(OH)} at short-time sonication (pH = 4), or at prolonged ultrasonic treatment, as aggregated nanosheets in weak acid solutions, or plated nanocrystals in alkaline solutions {boehmite and small amounts of bayerite, Al(OH)3 }. The authors concluded that the effect of ultrasound on the textural properties of mesoporous alumina, as well as on the transformation of nanosized bayerite to boehmite, could be consistently attributed to the transient strong heating of the liquid shell surrounding the cavitation bubble, which caused chemical processes similar to those that occurred during hydrothermal treatment. The ultrasonic vibration and conventional diamond grinding of Al2 O3 /ZrO2 nanoceramics were performed in order to investigate the effect of workpiece ultrasonic vibration on the brittle-ductile transition mechanism, and the effect of grit size, worktable speed, and grinding depth on the critical depth of the cut.[94] Another classic hot-research object in nanotechnology, ZnO, has been ultrasonically prepared in a variety of nanoforms, from nanowires to nanofilms. Thus, ultrasonic irradiation of a mixture of ZnO nanorods, Ag(NH3 )2+, and formaldehyde in an aqueous medium yielded ZnO nanorod/Ag nanoparticle composites,[95] in which the ZnO nanorods were coated with Ag nanoparticles, with a mean size of several tens nanometers. Doping ZnO nanowires with transition metal[96] was carried out by forming a Zn thin film on a substrate, impregnating this film with an aqueous solution of Zn salt, and reductant and transition metal, and applying ultrasonication to the aqueous solution to grow transition metal-doped ZnO nanowires on the Zn thin film. Oriented zinc nanoparticles in a zinc oxide matrix were obtained by Lu et al.[97] and a nanoscale laminated ZnO nanofilm was synthesized by Cao et al.[98] Zinc oxide nanoparticles (300 nm) were synthesized and deposited on the surface of glass slides using ultrasound irradiation.[99] The antibacterial activities of these ZnO-glass composites were tested against Escherichia coli (Gram neg.) and Staphylococcus aureus (Gram pos.) cultures, demonstrating a significant bactericidal effect, even in a 0.13 wt.% coated glass (the authors noted that the bactericidal effect was obviously obtained for Ag nanoparticle-based com-

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posites, see the earlier section). A similar effect, even in a 1 wt.% coated fabric, was found by the authors for copper oxide nanoparticles, synthesized and subsequently deposited on the surface of cotton fabrics, using ultrasound irradiation.[100] Ultrasonically synthesized monodispersed Cu2 O nanoparticles were also reported.[101] The porous WO3 (pore size 2–5 nm) nanoparticles were synthesized using high-intensity ultrasound irradiation of commercially available WO3 nanoparticles (80 nm) in ethanol.[102] These sonochemically modified porous WO3 nanoparticles dispersed more uniformly over the entire volume of the epoxy (without any settlement or agglomeration) as compared to the unmodified WO3 /epoxy nanocomposites. At the same time, WO3 was also ultrasonically prepared as an oriented nanofilm.[103] V2 O5 nanomaterials, including nanoribbons, nanowires, and microflakes (Figure 4), were fabricated by an ultrasonic assisted hydrothermal method and combined with a post-annealing process.[104] The rechargeable lithium battery using the obtained V2 O5 nanoribbons as cathode materials could be the next generation lithium battery with high capacity, safety, and long cycle life. Among other oxides, delamination of layered manganese oxide into colloidal nanosheets occurred[105] when the manganese oxide that intercalated with tetramethylammonium ions was ultrasonically dispersed in acetonitrile. Using SnCl4 .5H2 O and ammonia as raw materials, SnO2 nanoparticles were synthesized by ultrasonic irradiation assisted sol-gel method.[106] The shape of the synthesized 20-nm SnO2 nanoparticles was round and their dispersivity was greatly improved by the anionic surfactant, which was citric acid. Room-temperature ferromagnetism was revealed in Sn1-x Mnx O2 nanocrystalline thin films prepared by ultrasonic spray pyrolysis.[107] Two methods of compacting dry poly- and nanodisperse powders (such as yttria-stabilized zirconia) into compacts of a complicated shape, with uniform density distribution in the volume, were developed by pressing under powerful ultrasonic action and collector pressing by the control of friction force redistribution.[108] Among lanthanide oxides, CeO2 nanoparticles were prepared by an ultrasonic atomization process, using low-cost Ce(NO3 )3 .6H2 O, NH4 HCO3 , and NaOH as starting materials.[109] Mist drops of solutions were generated and used as space-confined microreactors for the nucleation, growth, and crystallization of CeO2 nanoparticles (3 nm) under room temperature conditions. Nano-sized (15 nm) lanthanum oxide, belonging to the hexagonal crystal system, was ultrasonically prepared with LaCl3 and CO(NH2 )2 as raw materials, by hydrolyzation of urea.[110] Hydroxides and their related compounds are represented considerably lesser in the available literature. Thus, nano γ -Ni oxyhydroxide (nano γ -NiOOH), a new cathode material for alkaline Zn/Ni batteries, was synthesized by a sonochemical intercalation method,[111] using NiCl2 solution, NaOH, and NaClO as an oxidant, allowing all four elementary reactions (precipitation, oxidation, cation exchange, and H2 O molecule intercalation). Ni(OH)2 was fabricated as a thin film and also used as an active electrode material.[112] CaSn(OH)6 nanotubes were fabricated


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FIG. 4. SEM images of V2 O5 (a) nanoribbons, (b) nanowires, and (c) microflakes; TEM and HRTEM images of (d, e) nanoribbons and (f, g) nanowires. Reproduced with permission from the American Chemical Society.

in high yield and at low cost by using the sonochemical precipitation method at room temperature,[113] revealing a direct rolling process from nanosheets to nanotubes. The transient CaSn(OH)6 nanosheets were formed as intermediates, produced by the spontaneous self-assembly and transformation of amorphous colloid clusters. Possible applications of these products can be in medicine, pharmaceuticals, and materials science. Metal Salts and Complexes A high number of publications are dedicated to ultrasonic preparation of metal sulfide and selenide nanostructures, very popular and important research objects in current nanotechnology, mainly ZnS,[114] which is the object of thousands of publications. Thus, zinc sulfide nanorods of wurtzite structure were grown using a simple sol-gel method via ultrasonication, in the presence of a capping agent.[115] The photoluminescent spectrum of ZnS nanorods exhibited green emission, which may find applications in optoelectronic devices. The same compound, but in the form of nanocrystallite powder, was synthesized[116] by a sonochemical technique, using zinc chloride and thiacetamide as raw materials. The crystal size of the powder was about 10 nm, and decreased slightly with an increase in ultrasonic irradiation power. Its reaction rate increased in a reaction time of